Steel material

Abstract

A steel material in which excellent cold forgeability is obtained, and with which excellent fatigue strength is obtained when the steel material is subjected to cold forging and an aging heat treatment and made into a steel component is provided. A steel material according to the present disclosure contains, in mass %, C: 0.03 to 0.25%, Si: 0.05 to 0.50%, Mn: more than 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less, Al: 0.001 to 0.100%, Cr: 0.05 to 0.70%, V: more than 0.10 to 0.40%, and N: 0.002 to 0.015%. In the microstructure of the steel material, an area fraction of polygonal ferrite is 30 to 90%, an area fraction of martensite is 5% or less, and the balance is bainite and / or pearlite. In the polygonal ferrite, the number density of VCr precipitates having an equivalent circular diameter of 5 to 100 nm is 1000 to 5000 pieces / μm3.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a steel material, and more particularly relates to a steel material that can be utilized as a starting material for a steel component to be produced by performing cold forging and an aging heat treatment.BACKGROUND ART

[0002] Steel components for machine structural use as typified by automobile components, industrial machinery components, and construction machinery components and the like are required to have a shape and high fatigue strength for performing a prescribed function. Steel materials, especially steel materials for machine structural use, are widely used as starting materials for satisfying requirements with respect to shape and fatigue strength. Examples of steel materials for machine structural use include carbon steel materials for machine structural use and alloy steel materials for machine structural use.

[0003] In a process for producing a steel component of a predetermined shape using a steel material as a starting material, the steel material is subjected to plastic deformation and machining. In the plastic deformation, the steel material is plastically deformed by hot forging or cold forging or the like to produce an intermediate component. In the machining, unwanted portions of the steel material are removed by cutting or the like using a lathe or a drill or the like.

[0004] When comparing plastic deformation and machining, the accuracy of finishing of machining is higher than the accuracy of finishing of plastic deformation. Therefore, machining is used to finish the shape of steel components. On the other hand, because machining involves removing a portion of the steel material, the amount of steel material loss is large, and there is also a large amount of wear of the working tools. For this reason, the processing costs of machining are higher in comparison to plastic deformation. If the shape accuracy of plastic deformation is improved, the portion of the steel material that is removed in the finishing machining can be reduced. Therefore, there is a demand to improve the shape accuracy of plastic deformation in order to reduce the production cost of steel components.

[0005] As mentioned above, the main kinds of plastic deformation are hot forging and cold forging. In hot forging, a steel material is subjected to forging in a state in which the steel material has been heated to a high-temperature austenite temperature range. The strength of a steel material decreases at a high temperature. Therefore, in hot forging, a large amount of plastic deformation can be applied to a steel material with a lower load in comparison to cold forging. On the other hand, in the process of cooling a steel material to room temperature after being plastically deformed at a high temperature, the steel material undergoes thermal contraction and the shape of the steel material changes due to phase transformation. For this reason, the shape accuracy of an intermediate component that is produced is low. As a result, the portion of the steel material to be removed by machining that is the next process is increased, which increases costs.

[0006] On the other hand, in cold forging, a steel material is forged at room temperature. In cold forging, since thermal strain is less likely to occur, the shape of the produced intermediate component can be made close to the shape of the final component. Hence, the machining costs can be reduced. On the other hand, because the strength of a steel material is high at room temperature, cold forging requires the use of forging machines and dies that can withstand high loads. Therefore, a steel material for cold forging is required to have low strength (deformation resistance) with respect to plastic deformation at room temperature, in other words, is required to have excellent cold forgeability.

[0007] In this connection, to increase the fatigue strength of a steel component, in some cases the steel material serving as the starting material (intermediate component after forging) is subjected to a heat treatment. In one method for carrying out a heat treatment, the intermediate component after forging is subjected to quenching and tempering. However, quenching and tempering increase the production costs. In addition, because quenching and tempering generate thermal strain in the intermediate component, the shape accuracy of the intermediate component decreases, resulting in an increase in the machining cost. Therefore, performing quenching and tempering after cold forging will compromise the advantage of cold forging that can provide high shape accuracy.

[0008] Therefore, as a heat treatment method that is an alternative to quenching and tempering, a technique has been proposed in which an aging heat treatment is performed after cold forging to thereby increase the fatigue strength of the steel component. In the aging heat treatment, heat treatment is performed in a low temperature range where phase transformation does not occur to thereby form precipitates and increase the fatigue strength of the steel component by age hardening. Therefore, when producing a steel component by using a steel material as a starting material and performing cold forging and an aging heat treatment, the shape accuracy can be increased in the production process to thereby reduce the production cost, and furthermore, the fatigue strength of the steel component can also be increased by age hardening.

[0009] A steel material that is suitable for use as a starting material for a steel component to be produced by performing cold forging and an aging heat treatment is proposed in Japanese Patent Application Publication No. 2019-173168 (Patent Literature 1).

[0010] The steel material proposed in Patent Literature 1 has a chemical composition that consists of, in mass %, C: 0.02 to 0.25%, Si: 0.005 to 0.50%, Mn: more than 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less, Al: 0.005 to 0.050%, Cr: 0.02 to 0.70%, V: 0.02 to 0.30%, N: 0.003 to 0.030%, Nb: 0 to 0.10%, B: 0 to 0.005%, Ca: 0 to 0.005%, Bi: 0 to 0.10%, Pb: 0 to 0.20%, and the balance: Fe and impurities, in which the total content of Cu, Ni, and Mo among the impurities is 0.05% by mass or less and the content of Ti among the impurities is 0.005% by mass or less, and that satisfies Formula (1). Formula (1) is as follows: [V precipitates] / [content of V]≤0.50. In this steel material, the microstructure contains polygonal ferrite, bainite and / or pearlite. The area fraction of polygonal ferrite in the microstructure is 10 to 90%.

[0011] According to Patent Literature 1, a steel material which contains V but in which precipitation of V carbides is suppressed (that is, a steel material in which the amount of dissolved V is large) is used as a starting material, and an aging heat treatment is performed after cold forging. By this means, V carbides are formed in the steel component by the aging heat treatment, and the endurance ratio (fatigue strength / tensile strength) of the steel component is increased by age hardening.

[0012] In Patent Literature 1, importance is placed on the cold forgeability of the steel material, and it is assumed that the hardness of the steel material is low. Therefore, although the endurance ratio of the steel component is high, in some cases the fatigue strength may not be so high. Therefore, the steel material disclosed in Patent Literature 1 is effective as a starting material for steel components with a complex shape for which high cold forgeability is required. On the other hand, in some cases the steel material disclosed in Patent Literature 1 may not be an appropriate starting material for steel components with shapes that are not very complex and for which high fatigue strength is required.

[0013] Further, a steel near-net-shape material that increases the strength of a steel component subjected to cold forging and an aging heat treatment is proposed in International Application Publication No. WO2021 / 210557 (Patent Literature 2).

[0014] The steel near-net-shape material proposed in Patent Literature 2 consists of, in mass %, C: 0.03 to 0.25%, Si: 0.02 to 0.50%, Mn: more than 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less, Al: 0.005 to 0.050%, V: more than 0.10 to 0.40%, N: 0.003 to 0.030%, Cr: 0 to 0.70%, Nb: 0 to 0.100%, B: 0 to 0.0100%, Cu: 0 to 0.30%, Ni: 0 to 0.30%, Ca: 0 to 0.0050%, Bi: 0 to 0.10%, Pb: 0 to 0.090%, Mo: 0 to 0.50%, Ti: 0 to 0.005%, Zr: 0 to 0.010%, Se: 0 to 0.10%, Te: 0 to 0.10%, rare earth metal: 0 to 0.010%, Sb: 0 to 0.10%, Mg: 0 to 0.0050%, W: 0 to 0.0050%, and the balance: Fe and impurities. In this steel near-net-shape material, in addition, fine V precipitates are formed in the steel near-net-shape material, and [V in precipitates] / [V]≥0.30. Further, a diffusible hydrogen content when charged with hydrogen by a cathodic hydrogen charging method is 0.10 ppm or more. According to Patent Literature 2, in order to obtain such fine V precipitates, in a cold forging process, the steel near-net-shape material is subjected to cold working from a first direction in which a working strain amount is 0.05 or more, and is then subjected to cold working from a second direction, which is different from the first direction, in which a working strain amount is 0.05 or more. By performing this kind of cold forging process, the strength of the steel near-net-shape material on which an aging heat treatment is performed after the cold forging increases.

[0015] However, the fatigue strength of a steel component may also be increased by means that is different from the means described in Patent Literature 2.CITATION LISTPatent Literature

[0016] Patent Literature 1: Japanese Patent Application Publication No. 2019-173168

[0017] Patent Literature 2: International Application Publication No. WO2021 / 210557SUMMARY OF INVENTIONTechnical Problem

[0018] An objective of the present disclosure is to provide a steel material in which excellent cold forgeability is obtained, and with which excellent fatigue strength is obtained when the steel material is subjected to cold forging and an aging heat treatment and made into a steel component.Solution to Problem

[0019] A steel material according to the present disclosure has a chemical composition consisting of, in mass %,

[0020] C: 0.03 to 0.25%,

[0021] Si: 0.05 to 0.50%,

[0022] Mn: more than 0.70 to 2.50%,

[0023] P: 0.035% or less,

[0024] S: 0.050% or less,

[0025] Al: 0.001 to 0.100%,

[0026] Cr: 0.05 to 0.70%,

[0027] V: more than 0.10 to 0.40%,

[0028] N: 0.002 to 0.015%,

[0029] Nb: 0 to 0.100%,

[0030] Ti: 0 to 0.050%,

[0031] B: 0 to 0.0100%,

[0032] Cu: 0 to 0.30%,

[0033] Ni: 0 to 0.30%,

[0034] Mo: 0 to 0.30%,

[0035] Ca: 0 to 0.0050%,

[0036] Bi: 0 to 0.100%,

[0037] Pb: 0 to 0.090%,

[0038] Zr: 0 to 0.010%,

[0039] Se: 0 to 0.010%,

[0040] Te: 0 to 0.010%,

[0041] rare earth metal: 0 to 0.010%,

[0042] Mg: 0 to 0.0050%,

[0043] W: 0 to 0.050%, and

[0044] Sn: 0 to 0.100%,

[0045] with the balance being Fe and impurities, and

[0046] has a microstructure in which an area fraction of polygonal ferrite is 30 to 90%, an area fraction of martensite is 5% or less, and the balance is bainite and / or pearlite, wherein

[0047] in the polygonal ferrite, a number density of VCr precipitates, which are precipitates containing V and Cr, having an equivalent circular diameter of 5 to 100 nm is 1000 to 5000 pieces / μm3.Advantageous Effects of Invention

[0048] In the steel material according to the present disclosure, excellent cold forgeability is obtained, and excellent fatigue strength is obtained when the steel material is made into a steel component by being subjected to cold forging and an aging heat treatment.DESCRIPTION OF EMBODIMENTS

[0049] The present inventors have conducted various studies to solve the problem described above, and obtained the following findings.

[0050] First, the present inventors have conducted studies from the viewpoint of the chemical composition with respect to a steel material in which excellent cold forgeability is obtained, and excellent fatigue strength is obtained when subjected to cold forging and an aging heat treatment as a starting material and made into a steel component. As a result, the present inventors have considered that if the chemical composition consists of, in mass %, C: 0.03 to 0.25%, Si: 0.05 to 0.50%, Mn: more than 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less, Al: 0.001 to 0.100%, Cr: 0.05 to 0.70%, V: more than 0.10 to 0.40%, N: 0.002 to 0.015%, Nb: 0 to 0.100%, Ti: 0 to 0.050%, B: 0 to 0.0100%, Cu: 0 to 0.30%, Ni: 0 to 0.30%, Mo: 0 to 0.30%, Ca: 0 to 0.0050%, Bi: 0 to 0.100%, Pb: 0 to 0.090%, Zr: 0 to 0.010%, Se: 0 to 0.010%, Te: 0 to 0.010%, rare earth metal (REM): 0 to 0.010%, Mg: 0 to 0.0050%, W: 0 to 0.050%, and Sn: 0 to 0.100%, with the balance being Fe and impurities, there is a possibility that excellent cold forgeability and excellent fatigue strength can be obtained.

[0051] Therefore, the present inventors have conducted further studies from the viewpoint of the microstructure with respect to means for obtaining excellent cold forgeability and excellent fatigue strength.

[0052] To improve cold forgeability, it is effective to reduce the hardness of the steel material. If the area fraction of polygonal ferrite in the microstructure is high, the hardness of the steel material can be reduced. On the other hand, if the area fraction of polygonal ferrite is too high, the hardness of the steel material will be too low. In such case, sufficient fatigue strength cannot be obtained in a steel component after cold forging and an aging heat treatment. Further, because martensite is extremely hard, if martensite is formed in the microstructure, the cold forgeability will markedly decrease. On the other hand, although bainite and pearlite are hard structures, the influence of bainite and pearlite on cold forgeability is low compared to martensite.

[0053] Taking the above matters into consideration, in the microstructure of a steel material satisfying the chemical composition described above, if the area fraction of polygonal ferrite is 30 to 90%, the area fraction of martensite is 5% or less, and the balance is bainite and / or pearlite, sufficient cold forgeability will be obtained, and in a case where the steel material is made into a steel component by being subjected to cold forging and an aging heat treatment, the fatigue strength of the steel component can be increased.

[0054] In addition, the present inventors have investigated factors that affect the fatigue strength of a steel component in a case where a steel material is used as a starting material and subjected to cold forging and an aging heat treatment and made into a steel component. Fatigue strength exhibits a positive correlation with hardness. For this reason it is considered that if the hardness of a steel component is high, the fatigue strength can also be increased. Therefore, the present inventors have investigated the factors that affect the hardness of a steel component produced by the production process described above. As a result, the present inventors have considered that the hardness of a steel component is affected by the following three factors.

[0055] (1) Hardness of steel material that serves as a starting material

[0056] (2) Work hardening by cold forging

[0057] (3) Age hardening by aging heat treatment

[0058] In the steel material according to Patent Literature 1, attention is focused on age hardening by an aging heat treatment, and in order to ensure the amount of V carbides formed during an aging heat treatment, formation of V carbides in the steel material is suppressed.

[0059] On the other hand, the present inventors have considered that by also utilizing work hardening by cold forging and not just age hardening by an aging heat treatment to increase the hardness of a steel component, the hardness (fatigue strength) of the steel component will be further increased.

[0060] In order to utilize work hardening by cold forging, it is necessary to increase the dislocation density in the steel material by cold forging. To increase the dislocation density, it is necessary to increase the amount of sites that act as dislocation multiplication sources during cold forging. Precipitates that have a certain size, such as cementite, function as dislocation multiplication sources. On the other hand, V carbides (VC) do not function as dislocation multiplication sources due to their fine size.

[0061] Bainite and pearlite in the microstructure of the steel material described above contain cementite, which acts as a dislocation multiplication source. Therefore, work hardening occurs in the bainite and pearlite during cold forging. On the other hand, polygonal ferrite does not contain cementite. Consequently, it is difficult for sufficient work hardening to occur in polygonal ferrite.

[0062] Therefore, in the present embodiment, a large number of VCr precipitates, which are precipitates containing V and Cr, are formed in the polygonal ferrite of the steel material that is the starting material. VCr precipitates have a larger particle size than V carbides. If the VCr precipitates have an equivalent circular diameter of 5 to 100 nm, the VCr precipitates will function as dislocation multiplication sources in the polygonal ferrite during cold forging. Furthermore, if VCr precipitates are utilized instead of V carbides, a certain amount of dissolved V can be secured even if VCr precipitates form to a certain extent in a steel material having the aforementioned chemical composition. Therefore, V carbides or VCr precipitates can be formed utilizing dissolved V during an aging heat treatment, and thus not only work hardening but also age hardening can be utilized.

[0063] Based on the above findings, the present inventors have conducted studies regarding an appropriate amount of VCr precipitates that can cause work hardening and age hardening to occur in the process of producing a steel component described above. As a result, it has been revealed that in a steel material having the chemical composition described above, if the number density of VCr precipitates having an equivalent circular diameter of 5 to 100 nm is made 1000 to 5000 pieces / μm3, in a steel component produced by subjecting the steel material to cold forging and an aging heat treatment, not only age hardening but also work hardening occurs, and excellent fatigue strength is obtained.

[0064] The gist of a forged steel component according to an embodiment of present invention, which has been completed based on the findings described above, is as follows.[1]

[0065] A steel material that has a chemical composition consisting of, in mass %,

[0066] C: 0.03 to 0.25%,

[0067] Si: 0.05 to 0.50%,

[0068] Mn: more than 0.70 to 2.50%,

[0069] P: 0.035% or less,

[0070] S: 0.050% or less,

[0071] Al: 0.001 to 0.100%,

[0072] Cr: 0.05 to 0.70%,

[0073] V: more than 0.10 to 0.40%,

[0074] N: 0.002 to 0.015%,

[0075] Nb: 0 to 0.100%,

[0076] Ti: 0 to 0.050%,

[0077] B: 0 to 0.0100%,

[0078] Cu: 0 to 0.30%,

[0079] Ni: 0 to 0.30%,

[0080] Mo: 0 to 0.30%,

[0081] Ca: 0 to 0.0050%,

[0082] Bi: 0 to 0.100%,

[0083] Pb: 0 to 0.090%,

[0084] Zr: 0 to 0.010%,

[0085] Se: 0 to 0.010%,

[0086] Te: 0 to 0.010%,

[0087] rare earth metal: 0 to 0.010%,

[0088] Mg: 0 to 0.0050%,

[0089] W: 0 to 0.050%, and

[0090] Sn: 0 to 0.100%,

[0091] with the balance being Fe and impurities, and

[0092] has a microstructure in which an area fraction of polygonal ferrite is 30 to 90%, an area fraction of martensite is 5% or less, and the balance is bainite and / or pearlite, wherein

[0093] in the polygonal ferrite, a number density of VCr precipitates, which are precipitates containing V and Cr, having an equivalent circular diameter of 5 to 100 nm is 1000 to 5000 pieces / μm3.[2]

[0094] The steel material according to [1], wherein the chemical composition contains one or more kinds of element selected from a group consisting of, in mass %:

[0095] Nb: 0.001 to 0.100%,

[0096] Ti: 0.001 to 0.050%,

[0097] B: 0.0001 to 0.0100%,

[0098] Cu: 0.01 to 0.30%,

[0099] Ni: 0.01 to 0.30%,

[0100] Mo: 0.01 to 0.30%,

[0101] Ca: 0.0001 to 0.0050%,

[0102] Bi: 0.001 to 0.100%,

[0103] Pb: 0.001 to 0.090%,

[0104] Zr: 0.001 to 0.010%,

[0105] Se: 0.001 to 0.010%,

[0106] Te: 0.001 to 0.010%,

[0107] rare earth metal: 0.001 to 0.010%,

[0108] Mg: 0.0001 to 0.0050%,

[0109] W: 0.001 to 0.050%, and

[0110] Sn: 0.001 to 0.100%.

[0111] Hereunder, a steel for cold forging of the present embodiment is described in detail. The symbol “%” in relation to an element means “mass percent”.FEATURES OF STEEL MATERIAL OF PRESENT EMBODIMENT

[0112] The steel material of the present embodiment satisfies the following feature 1 to feature 3.Feature 1

[0113] The chemical composition consists of, in mass %, C: 0.03 to 0.25%, Si: 0.05 to 0.50%, Mn: more than 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less, Al: 0.001 to 0.100%, Cr: 0.05 to 0.70%, V: more than 0.10 to 0.40%, N: 0.002 to 0.015%, Nb: 0 to 0.100%, Ti: 0 to 0.050%, B: 0 to 0.0100%, Cu: 0 to 0.30%, Ni: 0 to 0.30%, Mo: 0 to 0.30%, Ca: 0 to 0.0050%, Bi: 0 to 0.100%, Pb: 0 to 0.090%, Zr: 0 to 0.010%, Se: 0 to 0.010%, Te: 0 to 0.010%, rare earth metal (REM): 0 to 0.010%, Mg: 0 to 0.0050%, W: 0 to 0.050%, and Sn: 0 to 0.100%, with the balance being Fe and impurities.Feature 2

[0114] In the microstructure, an area fraction of polygonal ferrite is 30 to 90%, an area fraction of martensite is 5% or less, and the balance is bainite and / or pearlite.Feature 3

[0115] In the polygonal ferrite, the number density of VCr precipitates, which are precipitates containing V and Cr, having an equivalent circular diameter of 5 to 100 nm is 1000 to 5000 pieces / μm3.

[0116] Feature 1 to feature 3 are described hereunder.(Feature 1) Regarding Chemical Composition

[0117] The chemical composition of the steel material according to an embodiment of the present invention contains the following elements.C: 0.03 to 0.25%

[0118] Carbon (C) increases the fatigue strength of a steel component produced using the steel material as a starting material. Specifically, C forms bainite and / or pearlite that are hard structures in the microstructure and thereby increases the fatigue strength of the steel component. In addition, C forms VCr precipitates and increases the fatigue strength of the steel component. If the content of C is less than 0.03%, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

[0119] On the other hand, if the content of C is more than 0.25%, even if the contents of other elements are within the range of the present embodiment, the strength of the steel material will excessively increase and cold forgeability will decrease.

[0120] Therefore, the content of C is 0.03 to 0.25%.

[0121] A preferable lower limit of the content of C is 0.05%, more preferably is 0.07%, and further preferably is 0.09%.

[0122] A preferable upper limit of the content of C is 0.23%, more preferably is 0.21%, and further preferably is 0.20%.Si: 0.05 to 0.50%

[0123] Silicon (Si) increases the fatigue strength of a steel component produced using the steel material as a starting material. If the content of Si is less than 0.05%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

[0124] On the other hand, if the content of Si is more than 0.50%, the strength of the steel material will excessively increase. Therefore, cold forgeability will decrease even if the contents of other elements are within the range of the present embodiment.

[0125] Therefore, the content of Si is 0.05 to 0.50%.

[0126] A preferable lower limit of the content of Si is 0.08%, more preferably is 0.10%, and further preferably is 0.12%.

[0127] A preferable upper limit of the content of Si is 0.45%, more preferably is 0.43%, and further preferably is 0.41%.Mn: more than 0.70 to 2.50%

[0128] Manganese (Mn) increases hardenability of the steel material, and forms bainite and pearlite that are hard structures in the microstructure. Therefore, Mn increases the fatigue strength of a steel component produced using the steel material as a starting material. If the content of Mn is 0.70% or less, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

[0129] On the other hand, if the content of Mn is more than 2.50%, hardenability of the steel material will be excessively increased and martensite will form. In such case, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0130] Therefore, the content of Mn is more than 0.70 to 2.50%.

[0131] A preferable lower limit of the content of Mn is 0.80%, more preferably is 0.90%, and further preferably is 1.00%.

[0132] A preferable upper limit of the content of Mn is 2.30%, more preferably is 2.20%, and further preferably is 2.10%.P: 0.035% or Less

[0133] Phosphorus (P) is an impurity. P segregates to grain boundaries and decreases the cold forgeability of the steel material. Therefore, the content of P is 0.035% or less.

[0134] The content of P is preferably as low as possible. However, excessively reducing the content of P will raise the production cost. Therefore, when ordinary industrial productivity is taken into consideration, a preferable lower limit of the content of P is more than 0%, more preferably is 0.001%, further preferably is 0.005%, and further preferably is 0.010%.

[0135] A preferable upper limit of the content of P is 0.030%, more preferably is 0.025%, and further preferably is 0.020%.S: 0.050% or Less

[0136] Sulfur(S) is an impurity. S combines with Mn to form MnS, which increases the machinability of the steel material. However, if the content of S is more than 0.050%, coarse MnS will form. The coarse MnS will decrease the cold forgeability. Therefore, the content of S is 0.050% or less.

[0137] The content of S is preferably as low as possible. However, excessively reducing the content of S will raise the production cost. Therefore, when ordinary industrial productivity is taken into consideration, a preferable lower limit of the content of S is more than 0%, more preferably is 0.001%, further preferably is 0.005%, and further preferably is 0.010%.

[0138] A preferable upper limit of the content of S is 0.045%, more preferably is 0.040%, further preferably is 0.030%, and further preferably is 0.020%.Al: 0.001 to 0.100%

[0139] Aluminum (Al) deoxidizes the steel. If the content of Al is less than 0.001%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

[0140] On the other hand, if the content of Al is more than 0.100%, coarse Al-based inclusions such as Al oxides will form in the steel material. The coarse Al-based inclusions will decrease the cold forgeability of the steel material.

[0141] Therefore, the content of Al is 0.001 to 0.100%.

[0142] A preferable lower limit of the content of Al is 0.005%, more preferably is 0.010%, and further preferably is 0.015%.

[0143] A preferable upper limit of the content of Al is 0.090%, more preferably is 0.070%, further preferably is 0.050%, and further preferably is 0.040%. Note that, in the steel material of the present embodiment, the term “content of Al” means the total content of Al.Cr: 0.05 to 0.70%

[0144] Chromium (Cr) increases hardenability of the steel material and promotes formation of bainite and / or pearlite in the microstructure. Therefore, Cr increases the fatigue strength of a steel component produced using the steel material as a starting material. Furthermore, Cr forms VCr precipitates in the steel material and, in addition, forms VCr precipitates in an aging heat treatment process during the process of producing the steel component. By this means, Cr increases the fatigue strength of the steel component. If the content of Cr is less than 0.05%, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

[0145] On the other hand, if the content of Cr is more than 0.70%, hardenability of the steel material will be excessively high, the area fraction of polygonal ferrite will be too low, and the area fraction of martensite will be too high. In such case, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0146] Therefore, the content of Cr is 0.05 to 0.70%.

[0147] A preferable lower limit of the content of Cr is 0.06%, more preferably is 0.08%, and further preferably is 0.10%.

[0148] A preferable upper limit of the content of Cr is 0.60%, more preferably is 0.50%, and further preferably is 0.30%.V: More than 0.10 to 0.40%

[0149] Vanadium (V) forms VCr precipitates in the steel material, and also forms VCr precipitates and / or V carbides in an aging heat treatment process during the process of producing a steel component. By this means, V increases the fatigue strength of the steel component. If the content of V is 0.10% or less, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

[0150] On the other hand, if the content of V is more than 0.40%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0151] Therefore, the content of V is more than 0.10 to 0.40%.

[0152] A preferable lower limit of the content of V is 0.12%, more preferably is 0.14%, and further preferably is 0.15%.

[0153] A preferable upper limit of the content of V is 0.38%, more preferably is 0.35%, and further preferably is 0.30%.N: 0.002 to 0.015%

[0154] Nitrogen (N) combines with V in the steel material to form V nitrides. V nitrides are more thermodynamically stable than VCr precipitates, and form at high temperatures. Therefore, if V nitrides form, the amount of V available for formation of VCr precipitates will decrease. If the content of N is more than 0.015%, V nitrides will excessively form. Consequently, even if the contents of other elements are within the range of the present embodiment, formation of VCr precipitates will be insufficient and the fatigue strength of the steel component will decrease.

[0155] On the other hand, a certain amount of V can dissolve in the steel material at high temperatures. Therefore, it is not necessary to reduce the content of N to less than 0.002%.

[0156] Therefore, the content of N is 0.002 to 0.015%.

[0157] A preferable lower limit of the content of N is 0.003%, and more preferably is 0.004%.

[0158] A preferable upper limit of the content of N is 0.012%, more preferably is 0.010%, and further preferably is 0.008%.

[0159] The balance of the chemical composition of the steel material according to the present embodiment is Fe and impurities. Here, the term “impurities” in the chemical composition means substances which are mixed in from ore and scrap used as raw material or from the production environment or the like when industrially producing the steel material, and which are not intentionally contained but are permitted within a range that does not adversely affect the steel material of the present embodiment. The content of O (oxygen) in the impurities is, for example, 0.0040% or less.[Regarding Optional Elements]

[0160] The chemical composition of the steel material of the present embodiment may further contain one or more kinds of element selected from a group consisting of the following first group to third group in lieu of a part of Fe.[First Group]

[0161] One or more kinds of element selected from the group consisting of:

[0162] Nb: 0 to 0.100%,

[0163] Ti: 0 to 0.050%,

[0164] B: 0 to 0.0100%,

[0165] Cu: 0 to 0.30%,

[0166] Ni: 0 to 0.30%, and

[0167] Mo: 0 to 0.30%.[Second Group]

[0168] One or more kinds of element selected from the group consisting of:

[0169] Ca: 0 to 0.0050%,

[0170] Bi: 0 to 0.100%, and

[0171] Pb: 0 to 0.090%.[Third Group]

[0172] One or more kinds of element selected from the group consisting of:

[0173] Zr: 0 to 0.010%,

[0174] Se: 0 to 0.010%,

[0175] Te: 0 to 0.010%,

[0176] rare earth metal: 0 to 0.010%,

[0177] Mg: 0 to 0.0050%,

[0178] W: 0 to 0.050%, and

[0179] Sn: 0 to 0.100%.

[0180] The first group to third group are described hereunder.[First Group (Nb, Ti, B, Cu, Ni, and Mo)]

[0181] The steel material of the present embodiment may contain one or more kinds of element selected from the group consisting of Nb: 0 to 0.100%, Ti: 0 to 0.050%, B: 0 to 0.0100%, Cu: 0 to 0.30%, Ni: 0 to 0.30%, and Mo: 0 to 0.30% in lieu of a part of Fe. These elements are optional elements, and each of these elements increases the fatigue strength of a steel component produced using the steel material as a starting material.Nb: 0 to 0.100%

[0182] Niobium (Nb) is an optional element and does not have to be contained. That is, the content of Nb may be 0%.

[0183] When contained, that is, when the content of Nb is more than 0%, Nb combines with C and / or N in the steel material to form Nb precipitates. The Nb precipitates increase the fatigue strength of the steel component by precipitation strengthening. If even a small amount of Nb is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0184] However, if the content of Nb is more than 0.100%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0185] Therefore, the content of Nb is 0 to 0.100%.

[0186] A preferable lower limit of the content of Nb is 0.001%, more preferably is 0.010%, and further preferably is 0.020%.

[0187] A preferable upper limit of the content of Nb is 0.080%, more preferably is 0.060%, and further preferably is 0.050%.Ti: 0 to 0.050%

[0188] Titanium (Ti) is an optional element and does not have to be contained. That is, the content of Ti may be 0%.

[0189] When contained, that is, when the content of Ti is more than 0%, Ti combines with N or C in the steel material to form Ti precipitates, and thereby increases the fatigue strength of the steel component. If even a small amount of Ti is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0190] However, if the content of Ti is more than 0.050%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0191] Therefore, the content of Ti is 0 to 0.050%.

[0192] A preferable lower limit of the content of Ti is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.

[0193] A preferable upper limit of the content of Ti is 0.045%, more preferably is 0.040%, and further preferably is 0.030%.B: 0 to 0.0100%

[0194] Boron (B) is an optional element and does not have to be contained. That is, the content of B may be 0%.

[0195] When contained, that is, when the content of B is more than 0%, B strengthens the crystal grain boundaries of the steel material, and thereby increases the fatigue strength of the steel component. If even a small amount of B is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0196] However, if the content of B is more than 0.0100%, the aforementioned advantageous effect will be saturated. In addition, if the content of B is more than 0.0100%, the raw material cost will increase and the producibility will also decrease. Therefore, the content of B is 0 to 0.0100%.

[0197] A preferable lower limit of the content of B is 0.0001%, more preferably is 0.0005%, further preferably is 0.0010%, and further preferably is 0.0020%.

[0198] A preferable upper limit of the content of B is 0.0080%, more preferably is 0.0060%, and further preferably is 0.0050%.Cu: 0 to 0.30%

[0199] Copper (Cu) is an optional element and does not have to be contained. That is, the content of Cu may be 0%.

[0200] When contained, that is, when the content of Cu is more than 0%, Cu increases hardenability of the steel material and increases the fatigue strength of the steel component. If even a small amount of Cu is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0201] However, if the content of Cu is more than 0.30%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0202] Therefore, the content of Cu is 0 to 0.30%.

[0203] A preferable lower limit of the content of Cu is 0.01%, more preferably is 0.05%, and further preferably is 0.10%.

[0204] A preferable upper limit of the content of Cu is 0.28%, more preferably is 0.25%, and further preferably is 0.20%.Ni: 0 to 0.30%

[0205] Nickel (Ni) is an optional element and does not have to be contained. That is, the content of Ni may be 0%.

[0206] When contained, that is, when the content of Ni is more than 0%, Ni increases hardenability of the steel material and increases the fatigue strength of the steel component. If even a small amount of Ni is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0207] However, if the content of Ni is more than 0.30%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0208] Therefore, the content of Ni is 0 to 0.30%.

[0209] A preferable lower limit of the content of Ni is 0.01%, more preferably is 0.05%, and further preferably is 0.10%.

[0210] A preferable upper limit of the content of Ni is 0.28%, more preferably is 0.25%, and further preferably is 0.20%.Mo: 0 to 0.30%

[0211] Molybdenum (Mo) is an optional element and does not have to be contained. That is, the content of Mo may be 0%.

[0212] When contained, that is, when the content of Mo is more than 0%, Mo increases hardenability of the steel material and increases the fatigue strength of the steel component. If even a small amount of Mo is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0213] However, if the content of Mo is more than 0.30%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0214] Therefore, the content of Mo is 0 to 0.30%.

[0215] A preferable lower limit of the content of Mo is 0.01%, more preferably is 0.05%, and further preferably is 0.10%.

[0216] A preferable upper limit of the content of Mo is 0.28%, more preferably is 0.25%, and further preferably is 0.20%.[Second Group (Ca, Bi, and Pb)]

[0217] The chemical composition of the steel material of the present embodiment may further contain one or more kinds of element selected from the group consisting of Ca: 0 to 0.0050%, Bi: 0 to 0.100%, and Pb: 0 to 0.090% in lieu of a part of Fe. These elements are optional elements, and each of these elements increases the machinability of the steel material.Ca: 0 to 0.0050%

[0218] Calcium (Ca) is an optional element and does not have to be contained. That is, the content of Ca may be 0%.

[0219] When contained, that is, when the content of Ca is more than 0%, Ca increases the machinability of the steel material. If even a small amount of Ca is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0220] However, if the content of Ca is more than 0.0050%, coarse CaO will form. In such case, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0221] Therefore, the content of Ca is 0 to 0.0050%.

[0222] A preferable lower limit of the content of Ca is 0.0001%, more preferably is 0.0005%, further preferably is 0.0010%, and further preferably is 0.0020%.

[0223] A preferable upper limit of the content of Ca is 0.0048%, more preferably is 0.0045%, and further preferably is 0.0040%.Bi: 0 to 0.100%

[0224] Bismuth (Bi) is an optional element and does not have to be contained. That is, the content of Bi may be 0%.

[0225] When contained, that is, when the content of Bi is more than 0%, Bi increases the machinability of the steel material. If even a small amount of Bi is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0226] However, if the content of Bi is more than 0.100%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0227] Therefore, the content of Bi is 0 to 0.100%.

[0228] A preferable lower limit of the content of Bi is 0.001%, more preferably is 0.010%, further preferably is 0.020%, and further preferably is 0.030%. A preferable upper limit of the content of Bi is 0.090%, more preferably is 0.080%, and further preferably is 0.060%.

[0229] Pb: 0 to 0.090%

[0230] Lead (Pb) is an optional element and does not have to be contained. That is, the content of Pb may be 0%.

[0231] When contained, that is, when the content of Pb is more than 0%, Pb increases the machinability of the steel material. If even a small amount of Pb is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0232] However, if the content of Pb is more than 0.090%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0233] Therefore, the content of Pb is 0 to 0.090%.

[0234] A preferable lower limit of the content of Pb is 0.001%, more preferably is 0.010%, further preferably is 0.020%, and further preferably is 0.040%.

[0235] A preferable upper limit of the content of Pb is 0.080%, more preferably is 0.075%, and further preferably is 0.070%.[Third Group (Zr, Se, Te, Rare Earth Metal (REM), Mg, W, and Sn)]

[0236] The chemical composition of the steel material of the present embodiment may further contain one or more kinds of element selected from the group consisting of Zr: 0 to 0.010%, Se: 0 to 0.010%, Te: 0 to 0.010%, rare earth metal (REM): 0 to 0.010%, Mg: 0 to 0.0050%, W: 0 to 0.050%, and Sn: 0 to 0.100% in lieu of a part of Fe. These elements are optional elements, and each of these elements can be contained in the steel material as an impurity.Zr: 0 to 0.010%

[0237] Zirconium (Zr) is an impurity, and does not have to be contained. That is, the content of Zr may be 0%.

[0238] When contained, that is, when the content of Zr is more than 0%, if the content of Zr is more than 0.010%, Zr will form coarse inclusions. In such case, the fatigue strength of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0239] Therefore, the content of Zr is 0 to 0.010%.

[0240] The content of Zr is preferably as low as possible. However, excessively reducing the content of Zr will raise the production cost. Therefore, a preferable lower limit of the content of Zr is 0.001%.

[0241] A preferable upper limit of the content of Zr is 0.008%, more preferably is 0.005%, and further preferably is 0.002%.Se: 0 to 0.010%

[0242] Selenium (Se) is an impurity, and does not have to be contained. That is, the content of Se may be 0%.

[0243] When contained, that is, when the content of Sn is more than 0%, if the content of Se is more than 0.010%, Se will embrittle the steel material and reduce the fatigue strength of the steel component even if the contents of other elements are within the range of the present embodiment.

[0244] Therefore, the content of Se is 0 to 0.010%.

[0245] The content of Se is preferably as low as possible. However, excessively reducing the content of Se will raise the production cost. Therefore, a preferable lower limit of the content of Se is 0.001%.

[0246] A preferable upper limit of the content of Se is 0.008%, more preferably is 0.005%, and further preferably is 0.002%.Te: 0 to 0.010%

[0247] Tellurium (Te) is an impurity, and does not have to be contained. That is, the content of Te may be 0%.

[0248] When contained, that is, when the content of Te is more than 0%, if the content of Te is more than 0.010%, Te will embrittle the steel material and reduce the fatigue strength of the steel component even if the contents of other elements are within the range of the present embodiment.

[0249] Therefore, the content of Te is 0 to 0.010%.

[0250] The content of Te is preferably as low as possible. However, excessively reducing the content of Te will raise the production cost. Therefore, a preferable lower limit of the content of Te is 0.001%.

[0251] A preferable upper limit of the content of Te is 0.008%, more preferably is 0.005%, and further preferably is 0.002%.

[0252] Rare earth metal (REM): 0 to 0.010%

[0253] Rare earth metal (REM) is an impurity, and does not have to be contained. That is, the content of REM may be 0%.

[0254] When contained, that is, when the content of REM is more than 0%, if the content of REM is more than 0.010%, REM will form coarse inclusions and will thereby reduce the fatigue strength of the steel material even if the contents of other elements are within the range of the present embodiment.

[0255] Therefore, the content of REM is 0 to 0.010%.

[0256] The content of REM is preferably as low as possible. However, excessively reducing the content of REM will raise the production cost. Therefore, a preferable lower limit of the content of REM is 0.001%.

[0257] A preferable upper limit of the content of REM is 0.008%, more preferably is 0.005%, and further preferably is 0.002%.

[0258] Note that, in the present description the term “REM” means one or more kinds of element selected from the group consisting of scandium (Sc) which is the element with atomic number 21, yttrium (Y) which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 that are lanthanoids. In the present description, the term “content of REM” means the total content of these elements.Mg: 0 to 0.0050%

[0259] Magnesium (Mg) is an impurity, and does not have to be contained. That is, the content of Mg may be 0%.

[0260] When contained, that is, when the content of Mg is more than 0%, if the content of Mg is more than 0.0050%, Mg will form coarse inclusions and reduce the fatigue strength of the steel material even if the contents of other elements are within the range of the present embodiment.

[0261] Therefore, the content of Mg is 0 to 0.0050%.

[0262] The content of Mg is preferably as low as possible. However, excessively reducing the content of Mg will raise the production cost. Therefore, a preferable lower limit of the content of Mg is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%.

[0263] A preferable upper limit of the content of Mg is 0.0040%, more preferably is 0.0030%, and further preferably is 0.0020%.W: 0 to 0.050%

[0264] Tungsten (W) is an impurity, and does not have to be contained. That is, the content of W may be 0%.

[0265] When contained, that is, when the content of W is more than 0%, if the content of W is more than 0.050%, W will decrease the cold forgeability of the steel material even if the contents of other elements are within the range of the present embodiment.

[0266] Therefore, the content of W is 0 to 0.050%.

[0267] The content of W is preferably as low as possible. However, excessively reducing the content of W will raise the production cost. Therefore, a preferable lower limit of the content of W is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.

[0268] A preferable upper limit of the content of W is 0.040%, more preferably is 0.030%, and further preferably is 0.020%.Sn: 0 to 0.100%

[0269] Tin (Sn) is an impurity, and does not have to be contained. That is, the content of Sn may be 0%.

[0270] When contained, that is, when the content of Sn is more than 0%, if the content of Sn is more than 0.100%, Sn will decrease the cold forgeability of the steel material even if the contents of other elements are within the range of the present embodiment.

[0271] Therefore, the content of Sn is 0 to 0.100%.

[0272] The content of Sn is preferably as low as possible. However, excessively reducing the content of Sn will raise the production cost. Therefore, a preferable lower limit of the content of Sn is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.

[0273] A preferable upper limit of the content of Sn is 0.060%, more preferably is 0.040%, and further preferably is 0.020%.(Feature 2) Regarding Microstructure

[0274] In the microstructure of the steel material of the present embodiment, an area fraction of polygonal ferrite is 30 to 90%, an area fraction of martensite is 5% or less, and the balance is bainite and / or pearlite.

[0275] Polygonal ferrite is a soft phase, and is an effective phase for improving cold forgeability. However, there is a difference in strength between polygonal ferrite and hard structures such as bainite and pearlite. When structures with large strength differences are intermixed in the microstructure, cracks may occur during cold forging due to the strength difference.

[0276] However, in the steel material of the present embodiment, VCr precipitates are formed in the microstructure, and VCr precipitates are also formed in the polygonal ferrite. Therefore, in the steel material of the present embodiment, a strength difference between the polygonal ferrite and the hard structures can be suppressed by the VCr precipitates formed in the steel material. As a result, even if the area fraction of polygonal ferrite is high, cold forgeability can be improved.

[0277] If the area fraction of polygonal ferrite in the microstructure of the steel material is 30 to 90%, the area fraction of martensite is 5% or less, and the balance is bainite and / or pearlite, the aforementioned advantageous effect will be sufficiently obtained.

[0278] A preferable lower limit of the area fraction of polygonal ferrite is 32%, more preferably is 34%, and further preferably is 36%.

[0279] A preferable upper limit of the area fraction of polygonal ferrite is 85%, more preferably is 80%, further preferably is 75%, further preferably is 68%, further preferably is 66%, and further preferably is 64%.

[0280] In the microstructure, the balance is bainite and / or pearlite. The strength of bainite and pearlite is higher than the strength of polygonal ferrite. Therefore, bainite and pearlite increase the fatigue strength of a steel component produced using the steel material as a starting material. Furthermore, bainite and pearlite do not markedly reduce the cold forgeability as in the case of martensite.

[0281] On the other hand, although martensite is a structure that has high strength, as mentioned above, martensite markedly reduces the cold forgeability of a steel material. Therefore, in the microstructure of the present embodiment, the area fraction of martensite is preferably as low as possible. Accordingly, in the present embodiment, the area fraction of martensite is 5% or less.

[0282] A preferable upper limit of the area fraction of martensite is 4%, more preferably is 3%, further preferably is 2%, further preferably is 1%, and further preferably is 0%.[Method for Measuring Area Fraction of Each Structure in Microstructure]

[0283] The polygonal ferrite area fraction, bainite area fraction, and pearlite area fraction in the microstructure of the steel material are measured by the following method.

[0284] A cross section that is parallel to the axial direction of the steel material and that includes the central axis of the steel material is adopted as an observation surface. A test specimen that includes the observation surface is taken from the steel material. The observation surface of the test specimen is mirror-polished. The observation surface after mirror-polishing is subjected to etching with 3% Nital (ethanol+3% nitric acid solution) to reveal the microstructure.

[0285] On the etched observation surface, observation fields that each includes an R / 2 depth position in the radial direction from the surface of the steel material are observed with an optical microscope. The observation magnification is to be set to ×400, and five fields are to be set as observation fields. In the present description, the term “R / 2 position” of the steel material means the center position of the radius (=R) in a cross section (circular shape) perpendicular to the axial direction of the steel material. Further, the center position of the radius means a position from a 3R / 8 depth position to a 5R / 8 depth position from the surface of the steel material. The area of each observation field is to be 58800 μm2 (280 μm×210 μm).

[0286] In each observation field, polygonal ferrite has a different contrast and morphology to the hard structures (bainite, pearlite, and martensite). Therefore, the respective structures are recognized as follows based on contrast and morphology. White polygonal grains in which there is no internal structure are recognized as polygonal ferrite. A structure in which white regions with high brightness and black regions with low brightness are intermixed in a striped pattern is recognized as pearlite. A structure having a narrow plate-like or acicular structure and in which black precipitates are present inside or at the boundaries of the narrow plate-like or acicular structure is recognized as bainite.

[0287] In each of the five observation fields, the area fraction of polygonal ferrite, and the area fractions (%) of bainite and pearlite are determined. For each structure, the arithmetic average value of the values obtained in the five observation fields is defined as the polygonal ferrite area fraction (%), pearlite area fraction (%), and bainite area fraction (%), respectively.

[0288] Note that, in the steel material of the present embodiment that satisfies feature 1, martensite forms in gaps between bainite. Therefore, an area fraction of martensite that is described later is also included in the bainite area fraction determined by the aforementioned observation with an optical microscope at a magnification of ×400. Accordingly, with respect to the bainite area fraction, a value that is determined by, furthermore, subtracting an area fraction of martensite obtained by a method described later from the area fraction of bainite that is the arithmetic average value for the five observation fields determined by the method described above is defined as the bainite area fraction (%) of the steel material of the present embodiment.

[0289] Here, the martensite area fraction in the microstructure of the steel material is measured by the following method.

[0290] A cross section that is parallel to the axial direction of the steel material and that includes the central axis of the steel material is adopted as an observation surface. A test specimen that includes the observation surface is taken from the steel material. The observation surface of the test specimen is mirror-polished. The observation surface after mirror-polishing is subjected to etching with 3% Nital (ethanol+3% nitric acid solution) to reveal the microstructure.

[0291] On the observation surface after etching, observation fields that each includes an R / 2 depth position in the radial direction from the surface of the steel material are observed using a scanning electron microscope (SEM) equipped with a backscattered electron image detector. The observation magnification is to be set to ×1000, and five fields are to be set as observation fields. The area of each observation field is to be 10800 μm2 (120 μm×90 μm).

[0292] In each observation field, martensite is identified based on contrast. Specifically, in a backscattered electron image, martensite exhibits brighter contrast than polygonal ferrite, bainite, and pearlite. The area fraction (%) of martensite in each of the five observation fields is determined based on the contrast. The arithmetic average value of the area fractions of martensite in the five observation fields is defined as the area fraction (%) of martensite.

[0293] As mentioned above, a value obtained by subtracting the martensite area fraction from the bainite area fraction is defined as the bainite area fraction (%) in the microstructure of the steel material of the present embodiment.(Feature 3) Regarding VCr Precipitates Number Density

[0294] In the steel material of the present embodiment, in addition, in the polygonal ferrite, the number density of VCr precipitates, which are precipitates containing V and Cr, that have an equivalent circular diameter of 5 to 100 nm is 1000 to 5000 pieces / μm3.

[0295] Generally speaking, in the microstructure of steel material, although cementite, which acts as a dislocation multiplication source, is present in bainite and pearlite, polygonal ferrite does not, in principle, contain cementite. Therefore, although work hardening occurs in bainite and pearlite as a result of working such as cold forging, work hardening does not occur in polygonal ferrite since there are no dislocation multiplication sources present therein.

[0296] On the other hand, as mentioned above, in the steel material of the present embodiment, VCr precipitates are formed in polygonal ferrite in the steel material. The VCr precipitates are coarse in comparison to V carbides (VC). The VCr precipitates which are coarser than V carbides act as sites of dislocation multiplication sources during working such as cold forging. Therefore, during cold forging, together with the occurrence of work hardening in bainite and pearlite, work hardening also occurs in polygonal ferrite because of the presence of the VCr precipitates. Therefore, when a steel component is produced by performing cold forging and an aging heat treatment on the steel material of the present embodiment as a starting material, two hardening mechanisms, namely, work hardening in a cold forging process by VCr precipitates, and age hardening by precipitates (VCr precipitates and / or V carbides) that form during an aging heat treatment, act to increase the hardness of the steel component. As a result, excellent fatigue strength is obtained in the steel component.

[0297] Specifically, if the VCr precipitates have an equivalent circular diameter of 5 to 100 nm, the VCr precipitates will function as dislocation multiplication sources. Here, the number density of VCr precipitates having an equivalent circular diameter of 5 to 100 nm is referred to as “VCr precipitates number density ND”. If the VCr precipitates number density ND is less than 1000 pieces / μm3, the formed amount of VCr precipitates will not be sufficient. In this case, sufficient fatigue strength will not be obtained in a steel component produced by performing a cold forging process and an aging heat treatment process on the steel material as a starting material. On the other hand, if the VCr precipitates number density ND is more than 5000 pieces / μm3, sufficient cold forgeability will not be obtained in the steel material. Therefore, the VCr precipitates number density ND is 1000 to 5000 pieces / μm3.

[0298] A preferable lower limit of the VCr precipitates number density ND is 1100 pieces / μm3, more preferably is 1200 pieces / μm3, further preferably is 1300 pieces / μm3, further preferably is 1400 pieces / μm3, and further preferably is 1500 pieces / μm3.

[0299] A preferable upper limit of the VCr precipitates number density ND is 4900 pieces / μm3, more preferably is 4500 pieces / μm3, further preferably is 4000 pieces / μm3, further preferably is 3800 pieces / μm3, and further preferably is 3600 pieces / μm3.[Method for Measuring VCr Precipitates Number Density ND]

[0300] The VCr precipitates number density ND in polygonal ferrite can be measured by the following method using a scanning transmission electron microscope (STEM-EDS).

[0301] A circular disc test specimen that has a diameter of 3 mm and a thickness of approximately 2 mm and that has the R / 2 position of the steel material as the center of the circle is taken from the steel material. The axial direction of the circular disc test specimen is to be parallel to the axial direction of the steel material. Both sides (front surface and rear surface) of the circular disc are polished using emery paper. At such time, the circular disc is polished so that the front surface is parallel with the rear surface. One of the front surface and rear surface of the circular disc is adopted as an observation surface. The observation surface is further subjected to mirror polishing. The mirror-polished observation surface is then subjected to further polishing using colloidal silica as an abrasive.

[0302] The observation surface after polishing is subjected to electron back scatter diffraction (EBSD) analysis to identify the crystal orientation of the parent phase. Based on the identified crystal orientation of the parent phase, focused ion beam processing (FIB processing) is then performed on the circular disc test specimen in a manner so that the direction (observation direction) perpendicular to the observation surface of a thin film is the <001> crystal orientation of the parent phase to thereby prepare a thin film sample for a STEM.

[0303] In the FIB processing, the thin film sample for a STEM is prepared by a lift-out method using a gallium (Ga) ion beam with an acceleration voltage of 30 kV.

[0304] Dislocation loops and amorphous regions are present in the outer layer of the thin film sample for a STEM prepared using the ion beam with an acceleration voltage of 30 kV, and consequently the outer layer is not suitable for observing VCr precipitates with a size of several nm. Therefore, the surface of the thin film sample for a STEM is polished using an ion beam with a low acceleration voltage of 1 kV or less. A thin film sample for a STEM having a thickness of about 100 nm is prepared by the above production process.

[0305] The prepared thin film sample is observed with the optical system of a STEM. Specifically, the thin film sample for a STEM is tilted so that the zone-axis incidence is the crystal orientation <001> of the polygonal ferrite of the parent phase in the thin film sample for a STEM. The observation magnification is to be set to ×200000, and the acceleration voltage is to be set to 300 kV. The detector is to be set to known high-angle annular dark-field (HAADF) conditions, known low-angle annular dark-field (LAADF) conditions, and known bright field (BF) conditions, and an STEM image of one observation field is taken at each of an arbitrary five polygonal ferrite grains (STEM images of observation fields at a total of five locations). Note that, there is a difference in dislocation density between polygonal ferrite grains and other structures. Therefore, polygonal ferrite grains can be easily identified in the observation with the STEM.

[0306] The area of each observation field is to be 1000 nm×1000 nm. In all of the observation fields, the thickness of the thin film sample for a STEM is measured using the log-ratio method of electron energy loss spectroscopy (EELS).

[0307] Quantitative analysis of the chemical composition of three or more representative particles in the five observation fields is performed using an EDS-ZAF method. In the chemical composition of the respective particles, when the total content of metallic elements excluding Fe is taken as 100% in percent by mass, if the content of V is 20% by mass or more, the content of Cr is 5% by mass or more, and the total content of V and Cr is 60% or more, the relevant particle is recognized as a VCr precipitate.

[0308] The contrast of the particles identified as VCr precipitates is identified. In the observation fields, particles with the same contrast as the identified contrast are recognized as VCr precipitates. The equivalent circular diameter of each recognized VCr precipitate is determined by a well-known image processing method. Among the identified VCr precipitates, the number of VCr precipitates having an equivalent circular diameter of 5 to 100 nm is determined. The VCr precipitates number density (pieces / μm3) is determined based on the number of VCr precipitates having an equivalent circular diameter of 5 to 100 nm in each observation field, and the area (1000 nm×1000 nm) and thickness of the observation field. The arithmetic average value of the VCr precipitates number density values obtained in the respective observation fields is defined as a VCr precipitates number density ND (pieces / μm3).Advantageous Effects of Steel Material of Present Embodiment

[0309] The steel material of the present embodiment satisfies feature 1 to feature 3. Therefore, in the steel material of the present embodiment, excellent cold forgeability is obtained. In addition, excellent fatigue strength is obtained in a steel component produced by performing a cold forging process and an aging heat treatment process on the steel material as a starting material.Shape and Uses of Steel Material of Present Embodiment

[0310] The steel material of the present embodiment is, for example, a steel material in which a cross section perpendicular to the axial direction is a circular shape, and for example is a steel bar or a wire rod. The steel material of the present embodiment can be widely applied to applications that require cold forgeability and fatigue strength. In particular, the steel material of the present embodiment can be applied as a starting material for steel components to be produced by performing a cold forging process and an aging heat treatment process on the steel material as a starting material. However, the steel material of the present embodiment is not limited to the applications described above.Production Method

[0311] An example of a method for producing the steel material of the present embodiment will now be described. The method for producing the steel material described hereunder is one example for producing the steel material of the present embodiment. Accordingly, a steel material satisfying feature 1 to feature 3 may also be produced by a production method other than the production method described hereunder. However, the production method described hereunder is a preferable example of a method for producing the steel material of the present embodiment.

[0312] The method for producing the steel material of the present embodiment includes the following processes.

[0313] (Process 1) Starting material preparation process

[0314] (Process 2) Hot working process

[0315] Each of these processes is described hereunder.[Starting Material Preparation Process]

[0316] In the starting material preparation process, a starting material satisfying feature 1 is produced by the following method. Specifically, a molten steel that satisfies feature 1 is produced using a converter and an electric furnace or the like. The molten steel is used to produce a bloom by a continuous casting process. Alternatively, the molten steel is used to produce an ingot by an ingot-making process. A starting material (a bloom or an ingot) is prepared by the above process.[Hot Working Process]

[0317] In the hot working process, the starting material is subjected to hot working to produce a steel material. The hot working, for example, may be hot rolling or may be hot forging.

[0318] In a case where the hot working is hot rolling, the hot working process, for example, includes a rough rolling process of subjecting the starting material (bloom or ingot) to rough rolling to obtain a billet, and a finish rolling process of subjecting the billet to finish rolling to obtain a steel material.

[0319] In the rough rolling process, for example, the following processes are performed. The starting material is heated, and thereafter is subjected to blooming using a blooming mill. As necessary, after blooming, the starting material is further subjected to rolling using a continuous mill to produce a billet. The heating temperature in the rough rolling process is a well-known temperature, and for example is 1050 to 1300° C.

[0320] In the finish rolling process, the billet is subjected to hot rolling to produce a steel material. In the finish rolling process, for example, the following processes are performed. The billet produced in the rough rolling process is charged into a heating furnace and heated. The heated billet is then used to perform finish rolling (hot rolling) with a finish rolling mill to obtain a steel material. The steel material after the finish rolling is cooled to obtain the steel material of the present embodiment. The heating temperature in the finish rolling process is a well-known temperature, and for example is 1050 to 1250° C. Further, when the steel material temperature (surface temperature of the steel material) on the exit side of the roll stand at which rolling was last performed in the finish rolling mill train in the finish rolling process is taken as the rolling finishing temperature (° C.), the finishing temperature is, for example, 900 to 1100° C.[Production Conditions in Hot Working Process]

[0321] In the hot working process, the following conditions are satisfied in a cooling process for cooling the steel material after the finish rolling.(Condition 1)

[0322] A holding time t at 900 to 800° C. after the finish rolling is to be 25 to 300 seconds.(Condition 2)

[0323] An average cooling rate CR from 800 to 600° C. is to be 0.6 to 4.0° C. / sec.

[0324] Each condition is described hereunder.[(Condition 1) Regarding Holding Time t]

[0325] The steel material is cooled after finish rolling of the steel material is completed, and at such time, nuclei of VCr precipitates form in the temperature range of 900 to 800° C. In other words, the temperature range of 900 to 800° C. after finish rolling is a VCr precipitates formation temperature range. Therefore, in order to make the number density of VCr precipitates an appropriate amount, the steel material is held for an appropriate holding time t in the temperature range of 900 to 800° C. (VCr precipitates formation temperature range) after the finish rolling. If the holding time t is less than 25 seconds, a sufficient amount of nuclei of VCr precipitates will not form. In such case, a sufficient VCr precipitates number density ND will not be obtained in the produced steel material. On the other hand, if the holding time t is more than 300 seconds, the nuclei of VCr precipitates will grow and coarsen, and as a result the number of formed nuclei will decrease. In such case, a sufficient VCr precipitates number density ND will not be obtained in the produced steel material.

[0326] If the holding time t is 25 to 300 seconds, the steel material will be held for an appropriate time in the temperature range of 900 to 800° C. (VCr precipitates formation temperature range). As a result, a sufficient amount of nuclei of VCr precipitates will form, and the VCr precipitates number density ND in the produced steel material will be 1000 to 5000 pieces / μm3.[(Condition 2) Regarding Average Cooling Rate CR]

[0327] If the average cooling rate CR in the temperature range of 800 to 600° C. after the holding time t elapses is less than 0.6° C. / sec, the cooling rate will be too slow. In such case, the VCr precipitates will coarsen. As a result, in the produced steel material, the VCr precipitates number density ND having an equivalent circular diameter of 5 to 100 nm will be less than 1000 pieces / μm3.

[0328] On the other hand, if the average cooling rate CR in the temperature range of 800 to 600° C. is more than 4.0° C. / sec, the cooling rate will be too fast. In such case, the formed amount of polygonal ferrite may be too small, the formed amount of bainite and / or pearlite may be too large, and martensite may excessively form or the like. In addition, the VCr precipitates will not be able to grow sufficiently, and as a result, the VCr precipitates number density ND having an equivalent circular diameter of 5 to 100 nm will be less than 1000 pieces / μm3 in the produced steel material.

[0329] If the average cooling rate CR is 0.6 to 4.0° C. / sec, an appropriate amount of polygonal ferrite will form and an appropriate amount of bainite and / or pearlite will form, and formation of martensite can be sufficiently suppressed. As a result, in the microstructure of the produced steel material, the area fraction of polygonal ferrite will be 30 to 90%, the area fraction of martensite will be 5% or less, and the balance will be bainite and / or pearlite.

[0330] Further, the nuclei of VCr precipitates that form in the temperature range of 900 to 800° C. grow in the temperature range of 800 to 600° C. If the average cooling rate CR in the temperature range of 800 to 600° C. is 0.6 to 4.0° C. / sec, the VCr precipitates will grow to an appropriate size. Therefore, in the produced steel material, the VCr precipitates number density ND having an equivalent circular diameter of 5 to 100 nm will be 1000 to 5000 pieces / μm3.

[0331] In a case where the hot working is hot forging, the hot forging is performed on the starting material to produce a steel material. The heating temperature in the hot forging is 1050 to 1300° C. When the temperature at the time that the final process (pressing) in the hot forging is completed is taken as the finishing temperature, the finishing temperature is 900 to 1100° C. In the cooling after hot forging, the aforementioned condition 1 and condition 2 are satisfied. The steel material of the present embodiment is produced by the above production method.[Method for Producing Steel Component Using Steel Material of Present Embodiment as a Starting Material]

[0332] In the case of producing a steel component by performing a cold forging process and an aging heat treatment process on the steel material of the present embodiment as a starting material, it suffices to use a well-known production method as the method for producing the steel component. The method for producing the steel component includes a cold forging process and an aging heat treatment process.

[0333] In the cold forging process, the steel material is subjected to cold forging to produce an intermediate steel material. In a case where the steel material is a steel bar or a wire rod, wire drawing may be performed before the cold forging process. It suffices to perform the cold forging and wire drawing under well-known conditions.

[0334] In the aging heat treatment process, the intermediate steel material is subjected to a heat treatment. It suffices to adopt well-known conditions as the conditions for the heat treatment temperature and holding time in the aging heat treatment process. For example, the heat treatment temperature is 550 to 700° C., and the holding time at the heat treatment temperature is 20 to 120 minutes.EXAMPLES

[0335] Hereunder, the advantageous effects of one aspect of the steel material of the present embodiment are described more specifically by way of examples. The conditions adopted in the following examples are one example of conditions adopted for confirming the feasibility and advantageous effects of the steel material of the present embodiment. Accordingly, the steel material of the present embodiment is not limited to this one example of conditions.

[0336] Steel materials having the chemical compositions shown in Table 1A to Table 1D were produced by the following method.TABLE 1AChemical CompositionTest(unit is mass %; balance is Fe and impurities)NumberCSiMnPSAlCrVN 10.170.191.600.0160.0120.0210.110.220.006 20.140.201.500.0160.0120.0250.150.200.005 30.100.201.810.0160.0120.0240.120.250.005 40.240.211.250.0130.0250.0220.120.150.006 50.210.201.200.0150.0140.0300.210.220.007 60.070.281.790.0220.0190.0180.240.200.006 70.040.322.010.0120.0150.0310.250.290.005 80.050.441.800.0080.0120.0260.310.220.006 90.130.481.800.0140.0160.0240.130.160.004100.180.081.190.0160.0150.0290.210.280.006110.080.192.350.0160.0130.0300.090.120.005120.190.250.750.0150.0220.0250.260.330.009130.150.261.490.0320.0170.0220.190.200.012140.140.171.520.0140.0470.0320.200.190.006150.150.181.600.0130.0130.0490.210.220.008160.210.121.510.0150.0160.0080.200.250.005170.090.201.250.0150.0180.0310.680.250.006180.200.261.500.0140.0210.0260.060.220.010190.160.201.080.0130.0190.0270.240.390.011200.140.091.020.0110.0130.0250.520.380.009210.190.211.190.0140.0180.0300.150.110.006220.140.151.490.0160.0190.0260.180.240.014230.160.201.890.0120.0160.0290.160.200.004240.120.211.800.0150.0160.0310.150.220.006250.140.101.790.0140.0130.0230.110.120.005260.110.191.510.0120.0150.0280.200.250.006270.110.191.510.0120.0150.0280.200.250.006280.110.191.510.0120.0150.0280.200.250.006290.150.211.880.0130.0160.0280.180.210.005300.150.251.490.0160.0150.0280.210.190.006310.160.191.480.0150.0160.0250.150.270.005320.150.191.500.0140.0180.0240.140.200.005330.140.201.590.0120.0160.0320.150.230.011340.120.131.210.0170.0110.0370.150.190.007350.090.352.010.0130.0140.0330.150.170.005TABLE 1BTestNum-Chemical Composition (unit is mass %; balance is Fe and impurities)berNbTiBCuNiMoCaBiPbZrSeTeREMMgWSn 1———————————————— 2———————————————— 3———————————————— 4———————————————— 5———————————————— 6———————————————— 7———————————————— 8———————————————— 9————————————————10————————————————11————————————————12————————————————13————————————————14————————————————15————————————————16————————————————17————————————————18————————————————19————————————————20————————————————21————————————————22————————————————23————————————————24————————————————25————————————————26————————————————27————————————————28————————————————290.096———————————————30—0.047——————————————31——0.0096—————————————32———0.28————————————33————0.29———————————34—————0.28——————————35——————0.0048—————————TABLE 1CChemical CompositionTest(unit is mass %; balance is Fe and impurities)NumberCSiMnPSAlCrVN360.120.251.800.0130.0200.0200.200.220.009370.160.201.480.0160.0110.0160.190.250.012380.200.111.320.0140.0150.0260.200.210.005390.160.121.490.0140.0190.0270.250.160.006400.130.381.820.0150.0180.0260.210.270.007410.140.201.500.0150.0210.0290.160.200.006420.190.191.680.0130.0170.0310.140.200.006430.160.191.410.0160.0150.0310.140.190.005440.140.201.490.0140.0100.0340.150.210.007450.290.121.500.0140.0200.0280.200.150.006460.020.211.510.0150.0180.0290.150.190.007470.160.601.490.0150.0160.0280.160.190.006480.140.031.500.0160.0170.0300.150.200.006490.150.193.020.0130.0150.0250.150.180.005500.150.180.510.0160.0170.0270.140.210.005510.140.341.120.0140.0130.0260.890.200.005520.160.181.310.0150.0160.0250.030.190.004530.150.111.520.0130.0210.0250.150.500.006540.160.191.700.0150.0190.0230.160.050.004550.120.211.800.0150.0160.0310.150.220.006560.120.211.800.0150.0160.0310.150.220.006570.090.251.500.0130.0120.0240.090.150.005580.140.101.790.0140.0130.0230.110.120.005590.110.191.510.0120.0150.0280.200.250.006600.120.211.800.0150.0160.0310.150.220.006610.140.101.790.0140.0130.0230.110.120.005620.110.191.510.0120.0150.0280.200.250.006630.120.211.800.0150.0160.0310.150.220.006640.090.251.500.0130.0120.0240.090.150.005650.140.101.790.0140.0130.0230.110.120.005660.110.191.510.0120.0150.0280.200.250.006670.100.211.490.0130.0120.0220.190.220.007680.110.191.510.0120.0150.0280.200.250.006690.100.211.490.0130.0120.0220.190.220.007TABLE 1DTestNum-Chemical Composition (unit is mass %; balance is Fe and impurities)berNbTiBCuNiMoCaBiPbZrSeTeREMMgWSn36———————0.096————————37————————0.088———————38—————————0.009——————39——————————0.010—————40———————————0.008————41————————————0.009———42—————————————0.0049——43——————————————0.048—44———————————————0.09045————————————————46————————————————47————————————————48————————————————49————————————————50————————————————51————————————————52————————————————53————————————————54————————————————55————————————————56————————————————57————0.29———————————58————————————————59————————————————60————————————————61————————————————62————————————————63————————————————64————0.29———————————65————————————————66————————————————670.010——0.130.080.01——————————68————————————————690.010——0.130.080.01——————————The symbol “-” in the column “Chemical Composition” in Table 1B and Table 1D means that the content of the corresponding element was at the level of an impurity or less. Note that, in the steel of each test number shown in the tables, the content of O was 0.0040% or less.Molten steels of each test number were produced by vacuum melting, and 50 kg ingots were produced. Each of the produced ingots was used to produce a steel material. Specifically, each ingot was subjected to hot working (hot forging). The heating temperature for the hot forging was 1200° C., and the finishing temperature was 1000° C. In a cooling process for cooling the steel material after the hot forging, a holding time t (secs) in the temperature range of 900 to 800° C., and an average cooling rate CR (° C. / sec) in the temperature range of 800 to 600° C. were as shown in Table 2A and Table 2B. Steel materials (round bar material) with a diameter of 24 mm were produced by the above production process.TABLE 2AMicrostructurePolygonalVCrSteelAverageFerrite MartensitePrecipitatesComponentHoldingCoolingAreaAreaNumberVickersTest Time tRate CRFractionFractionDensity NDColdHardnessNumber(secs)(° C. / s)(%)(%)Balance(pieces / μm3)Forgeability(HV)Remarks 1801.2480521660E294Inventive Example 2801.2480522570E286Inventive Example 3801.2520482040E280Inventive Example 4801.2362621420E308Inventive Example 5801.2650352840E312Inventive Example 6801.2560441560E286Inventive Example 7801.2520481210E270Inventive Example 8801.2580421060E272Inventive Example 9801.2460541580E302Inventive Example10801.2401593880E278Inventive Example11801.2350651280E306Inventive Example12801.2580423580E269Inventive Example13801.2460542240E280Inventive Example14801.2510492090E273Inventive Example15801.2553422670E284Inventive Example16801.2490512840E299Inventive Example17801.2640361750E307Inventive Example18801.2530473000E284Inventive Example19801.2720284280E318Inventive Example20801.2670334910E310Inventive Example21801.2520481470E268Inventive Example22801.2630372480E272Inventive Example23801.2421572020E276Inventive Example24800.8540461290E274Inventive Example25800.8820181420E270Inventive Example26804.0400601250E289Inventive Example27801.8620383110E295Inventive Example28800.6790212230E274Inventive Example29801.2480522440E285Inventive Example30801.2680322380E278Inventive Example31801.2580422440E310Inventive Example32801.2382601580E286Inventive Example33801.2401591740E288Inventive Example34801.2332651490E304Inventive Example35801.2450551880E291Inventive ExampleTABLE 2BMicrostructurePolygonalVCrSteelAverageFerriteMartensitePrecipitatesComponentHoldingCoolingAreaAreaNumberVickersTest Time tRate CRFractionFractionDensity NDColdHardnessNumber(secs)(° C. / s)(%)(%)Balance(pieces / μm3)Forgeability(HV)Remarks36801.2480522000E278Inventive Example37801.2442542110E286Inventive Example38801.2523452360E303Inventive Example39801.2560442240E288Inventive Example40801.2510491900E280Inventive Example41801.2570431770E276Inventive Example42801.2530472740E284Inventive Example43801.2381611550E294Inventive Example44801.2440561720E281Inventive Example45801.2324644890B—Comparative Example46801.29604360E116Comparative Example47801.2525431420B—Comparative Example48801.2780221600E233Comparative Example49801.21512731940B—Comparative Example50801.2760242040E242Comparative Example51801.2263712250B—Comparative Example52801.265035460E239Comparative Example53801.2620383680B—Comparative Example54801.255045240E166Comparative Example55800.262038650E230Comparative Example56800.182018520E180Comparative Example57800.275025620E192Comparative Example58800.487013850E198Comparative Example59800.288012740E215Comparative Example608020.008614310B—Comparative Example618015.0123256290B—Comparative Example62808.018379540B—Comparative Example63806.034264820E252Comparative Example64805.040060850E240Comparative Example65806.034462490E262Comparative Example66101.268032670E254Comparative Example67101.258042520E260Comparative Example686001.275025340E232Comparative Example696001.266034280E249Comparative Example[Evaluation Tests]The following evaluation tests were carried out on the round bar material (steel material) serving as a starting material for a steel near-net-shape material of each test number, and on a steel near-net-shape material of each test number.(Test 1) Microstructure observation test(Test 2) VCr precipitates number density ND measurement test(Test 3) Cold forgeability evaluation test

[0343] (Test 4) Test to measure Vickers hardness of steel component

[0344] Each test is described hereunder.[(Test 1) Microstructure Observation Test]

[0345] The structures in the microstructure of the steel material of each test number were identified in accordance with the method described above in the section [Method for measuring area fraction of each structure in microstructure]. As a result, in each test number, polygonal ferrite and bainite and / or pearlite were confirmed in the microstructure. In addition, martensite was also confirmed in some test numbers. Furthermore, in each test number, the area fraction (%) of polygonal ferrite, area fraction (%) of pearlite, area fraction (%) of bainite, and area fraction (%) of martensite were determined. As the result of the test, it was found that in the steel material of each test number, in the microstructure, a structure other than polygonal ferrite and martensite was bainite and / or pearlite. The area fraction (%) of polygonal ferrite, area fraction (%) of martensite, and total area fraction (%) of the balance (bainite and pearlite) are shown in Table 2A and Table 2B.[(Test 2) VCr Precipitates Number Density ND Measurement Test]

[0346] The VCr precipitates number density ND (pieces / μm3) in the steel material of each test number was determined in accordance with the method described above in the section [Method for measuring VCr precipitates number density ND]. The VCr precipitates number density ND (pieces / μm3) of each test number is shown in Table 2A and Table 2B.[(Test 3) Cold Forgeability Evaluation Test]

[0347] A plurality of cylindrical test specimens of 14 mm in diameter and 21 mm in length were taken from the steel material of each test number. The central axis of each cylindrical test specimen was coaxial with the central axis of the steel material. A compression test (cold forging) was conducted at room temperature (25° C.) using the cylindrical test specimens. Specifically, a compression test was conducted until a working ratio defined by Formula (1) became 70%. The compression test was conducted on five cylindrical test specimens for each test number. The presence or absence of a crack in the cylindrical test specimens after the test was confirmed. In general, the working ratio of cold forging processing is 70% or less. Therefore, if a crack was not generated in the steel material by the compression test at a working ratio of 70%, it was determined that excellent cold forgeability was obtained.Working ratio=(1−(length of cylindrical test specimen after compression test / length of cylindrical test specimen before compression test))×100  (1)

[0348] The presence or absence of a crack in the steel material after conducting the compression test at a working ratio of 70% was confirmed by the following method. The presence or absence of a crack in the five cylindrical test specimens after the compression test was confirmed by observation using a magnifying glass with a magnification of ×5. If a crack of 0.5 mm or more in length was not observed in any of the five cylindrical test specimens, it was determined that a crack did not occur and excellent cold forgeability was obtained (indicated by “E (Excellent)” in the column “Cold Forgeability” in Table 2A and Table 2B). On the other hand, if a crack of 0.5 mm or more in length was confirmed in any one or more of the five cylindrical test specimens, it was determined that sufficient cold forgeability was not obtained (indicated by “B (Bad)” in the column “Cold Forgeability” in Table 2A and Table 2B). Note that, for those test numbers in which sufficient cold forgeability was not obtained, Test 4 was not performed (indicated by “-” in the column “Steel Component Vickers Hardness (HV)” in Table 2A and Table 2B).[(Test 4) Test to Measure Vickers Hardness of Steel Component]

[0349] The Vickers hardness of a steel component obtained by performing a cold forging process and an aging heat treatment process on the steel material of each test number as a starting material was determined by the following method.

[0350] A steel material with a diameter of 24 mm of each test number was subjected to cold drawing that simulated a cold forging process by the following method. A test specimen for cold drawing of 20 mm in diameter and in which the diameter of a pointed part was 16 mm was prepared from the steel material. The central axis of the test specimen for cold drawing was coaxial with the central axis of the steel material. The test specimen for cold drawing was subjected to a bonderizing treatment to form a zinc phosphate coating film for lubrication on the surface of the test specimen. Thereafter, the test specimen for cold drawing was subjected to cold drawing using a die with a diameter of 17 mm. The cold drawing was performed in one pass, and the reduction of area was 28%. An aging heat treatment was performed on the test specimen after the cold drawing. The heat treatment temperature in the aging heat treatment was set to 600° C., and the holding time at the heat treatment temperature was set to 1 hour (60 minutes). Steel components for which the steel materials of the respective test numbers were used as starting materials were produced by the above production process.

[0351] As a simple evaluation of the fatigue strength of each steel component, the Vickers hardness, which has a positive correlation with fatigue strength, was measured. Specifically, for each test number, a test specimen was taken in which a cross section perpendicular to the central axis of the steel component was adopted as the test surface. The test surface of the test specimen was subjected to a Vickers hardness test in accordance with JIS Z 2244-1:2020. The test force was set to 9.8 N. An arbitrary three points near the central axis of the test surface were adopted as measurement positions. The arithmetic average value of the values obtained at the three measurement positions was defined as the Vickers hardness (Hv) of the steel component of the relevant test number. The obtained Vickers hardness (Hv) of each steel component is shown in Table 2A and Table 2B. If the Vickers hardness was 265 HV or more, the relevant test number was regarded as having excellent fatigue strength. If the Vickers hardness was less than 265 HV, the relevant test number was regarded as not having sufficient fatigue strength.[Test Results]

[0352] The test results are shown in Table 1A to Table 1D, Table 2A, and Table 2B. The steel materials of Test Nos. 1 to 44 satisfied feature 1 to feature 3. Therefore, excellent cold forgeability was obtained. In addition, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was 265 HV or more and thus excellent fatigue strength was obtained.

[0353] On the other hand, in Test No. 45, the content of C was too high. Therefore, sufficient cold forgeability was not obtained.

[0354] In Test No. 46, the content of C was too low. Consequently, the area fraction of polygonal ferrite was too high. In addition, the number density of VCr precipitates was too low. Therefore, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was less than 265 HV and thus sufficient fatigue strength was not obtained.

[0355] In Test No. 47, the content of Si was too high. Therefore, sufficient cold forgeability was not obtained.

[0356] In Test No. 48, the content of Si was too low. Therefore, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was less than 265 HV and thus sufficient fatigue strength was not obtained.

[0357] In Test No. 49, the content of Mn was too high. Consequently, the area fraction of polygonal ferrite was less than 30%, and the area fraction of martensite was more than 5%. Therefore, sufficient cold forgeability was not obtained.

[0358] In Test No. 50, the content of Mn was too low. Therefore, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was less than 265 HV and thus sufficient fatigue strength was not obtained.

[0359] In Test No. 51, the content of Cr was too high. Consequently, the area fraction of polygonal ferrite was less than 30%. Therefore, sufficient cold forgeability was not obtained.

[0360] In Test No. 52, the content of Cr was too low. Consequently, the number density of VCr precipitates was too low. Therefore, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was less than 265 HV and thus sufficient fatigue strength was not obtained. In Test No. 53, the content of V was too high. Therefore, sufficient cold forgeability was not obtained.

[0361] In Test No. 54, the content of V was too low. Consequently, the number density of VCr precipitates was too low. Therefore, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was less than 265 HV and thus sufficient fatigue strength was not obtained.

[0362] In Test Nos. 55 to 59, the average cooling rate CR from 800 to 600° C. after finish rolling was too slow. As a result, the number density of VCr precipitates was too low. Therefore, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was less than 265 HV and thus sufficient fatigue strength was not obtained.

[0363] In Test Nos. 60 to 62, the average cooling rate CR from 800 to 600° C. after finish rolling was too fast. Therefore, in Test Nos. 60 to 62, the area fraction of polygonal ferrite was too low, and in Test Nos. 60 and 61, in addition, the area fraction of martensite was too high. As a result, sufficient cold forgeability was not obtained in these test numbers. Furthermore, in these test numbers, the number density of VCr precipitates was too low.

[0364] In Test Nos. 63 to 65, the average cooling rate CR from 800 to 600° C. after finish rolling was too fast, although it was not as fast as in Test Nos. 60 to 62. Consequently, in Test Nos. 63 to 65, the number density of VCr precipitates was too low. Therefore, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was less than 265 HV and thus sufficient fatigue strength was not obtained.

[0365] In Test Nos. 66 and 67, the holding time t in the temperature range of 900 to 800° C. after finish rolling was too short. Consequently, in these test numbers, the number density of VCr precipitates was too low. Therefore, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was less than 265 HV and thus sufficient fatigue strength was not obtained.

[0366] In Test Nos. 68 and 69, the holding time t in the temperature range of 900 to 800° C. after finish rolling was too long. Consequently, in these test numbers, the number density of VCr precipitates was too low. Therefore, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was less than 265 HV and thus sufficient fatigue strength was not obtained.

[0367] An embodiment of the present disclosure has been described above. However, the embodiment described above is merely an example for carrying out the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment within a range that does not depart from the gist of the present disclosure.

Examples

examples

[0335]Hereunder, the advantageous effects of one aspect of the steel material of the present embodiment are described more specifically by way of examples. The conditions adopted in the following examples are one example of conditions adopted for confirming the feasibility and advantageous effects of the steel material of the present embodiment. Accordingly, the steel material of the present embodiment is not limited to this one example of conditions.

[0336]Steel materials having the chemical compositions shown in Table 1A to Table 1D were produced by the following method.

TABLE 1AChemical CompositionTest(unit is mass %; balance is Fe and impurities)NumberCSiMnPSAlCrVN 10.170.191.600.0160.0120.0210.110.220.006 20.140.201.500.0160.0120.0250.150.200.005 30.100.201.810.0160.0120.0240.120.250.005 40.240.211.250.0130.0250.0220.120.150.006 50.210.201.200.0150.0140.0300.210.220.007 60.070.281.790.0220.0190.0180.240.200.006 70.040.322.010.0120.0150.0310.250.290.005 80.050.441.800.0080.0120.02...

TECHNICAL FIELD

[0001] The present disclosure relates to a steel material, and more particularly relates to a steel material that can be utilized as a starting material for a steel component to be produced by performing cold forging and an aging heat treatment.BACKGROUND ART

[0002] Steel components for machine structural use as typified by automobile components, industrial machinery components, and construction machinery components and the like are required to have a shape and high fatigue strength for performing a prescribed function. Steel materials, especially steel materials for machine structural use, are widely used as starting materials for satisfying requirements with respect to shape and fatigue strength. Examples of steel materials for machine structural use include carbon steel materials for machine structural use and alloy steel materials for machine structural use.

[0003] In a process for producing a steel component of a predetermined shape using a steel material as a starting material, the steel material is subjected to plastic deformation and machining. In the plastic deformation, the steel material is plastically deformed by hot forging or cold forging or the like to produce an intermediate component. In the machining, unwanted portions of the steel material are removed by cutting or the like using a lathe or a drill or the like.

[0004] When comparing plastic deformation and machining, the accuracy of finishing of machining is higher than the accuracy of finishing of plastic deformation. Therefore, machining is used to finish the shape of steel components. On the other hand, because machining involves removing a portion of the steel material, the amount of steel material loss is large, and there is also a large amount of wear of the working tools. For this reason, the processing costs of machining are higher in comparison to plastic deformation. If the shape accuracy of plastic deformation is improved, the portion of the steel material that is removed in the finishing machining can be reduced. Therefore, there is a demand to improve the shape accuracy of plastic deformation in order to reduce the production cost of steel components.

[0005] As mentioned above, the main kinds of plastic deformation are hot forging and cold forging. In hot forging, a steel material is subjected to forging in a state in which the steel material has been heated to a high-temperature austenite temperature range. The strength of a steel material decreases at a high temperature. Therefore, in hot forging, a large amount of plastic deformation can be applied to a steel material with a lower load in comparison to cold forging. On the other hand, in the process of cooling a steel material to room temperature after being plastically deformed at a high temperature, the steel material undergoes thermal contraction and the shape of the steel material changes due to phase transformation. For this reason, the shape accuracy of an intermediate component that is produced is low. As a result, the portion of the steel material to be removed by machining that is the next process is increased, which increases costs.

[0006] On the other hand, in cold forging, a steel material is forged at room temperature. In cold forging, since thermal strain is less likely to occur, the shape of the produced intermediate component can be made close to the shape of the final component. Hence, the machining costs can be reduced. On the other hand, because the strength of a steel material is high at room temperature, cold forging requires the use of forging machines and dies that can withstand high loads. Therefore, a steel material for cold forging is required to have low strength (deformation resistance) with respect to plastic deformation at room temperature, in other words, is required to have excellent cold forgeability.

[0007] In this connection, to increase the fatigue strength of a steel component, in some cases the steel material serving as the starting material (intermediate component after forging) is subjected to a heat treatment. In one method for carrying out a heat treatment, the intermediate component after forging is subjected to quenching and tempering. However, quenching and tempering increase the production costs. In addition, because quenching and tempering generate thermal strain in the intermediate component, the shape accuracy of the intermediate component decreases, resulting in an increase in the machining cost. Therefore, performing quenching and tempering after cold forging will compromise the advantage of cold forging that can provide high shape accuracy.

[0008] Therefore, as a heat treatment method that is an alternative to quenching and tempering, a technique has been proposed in which an aging heat treatment is performed after cold forging to thereby increase the fatigue strength of the steel component. In the aging heat treatment, heat treatment is performed in a low temperature range where phase transformation does not occur to thereby form precipitates and increase the fatigue strength of the steel component by age hardening. Therefore, when producing a steel component by using a steel material as a starting material and performing cold forging and an aging heat treatment, the shape accuracy can be increased in the production process to thereby reduce the production cost, and furthermore, the fatigue strength of the steel component can also be increased by age hardening.

[0009] A steel material that is suitable for use as a starting material for a steel component to be produced by performing cold forging and an aging heat treatment is proposed in Japanese Patent Application Publication No. 2019-173168 (Patent Literature 1).

[0010] The steel material proposed in Patent Literature 1 has a chemical composition that consists of, in mass %, C: 0.02 to 0.25%, Si: 0.005 to 0.50%, Mn: more than 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less, Al: 0.005 to 0.050%, Cr: 0.02 to 0.70%, V: 0.02 to 0.30%, N: 0.003 to 0.030%, Nb: 0 to 0.10%, B: 0 to 0.005%, Ca: 0 to 0.005%, Bi: 0 to 0.10%, Pb: 0 to 0.20%, and the balance: Fe and impurities, in which the total content of Cu, Ni, and Mo among the impurities is 0.05% by mass or less and the content of Ti among the impurities is 0.005% by mass or less, and that satisfies Formula (1). Formula (1) is as follows: [V precipitates] / [content of V]≤0.50. In this steel material, the microstructure contains polygonal ferrite, bainite and / or pearlite. The area fraction of polygonal ferrite in the microstructure is 10 to 90%.

[0011] According to Patent Literature 1, a steel material which contains V but in which precipitation of V carbides is suppressed (that is, a steel material in which the amount of dissolved V is large) is used as a starting material, and an aging heat treatment is performed after cold forging. By this means, V carbides are formed in the steel component by the aging heat treatment, and the endurance ratio (fatigue strength / tensile strength) of the steel component is increased by age hardening.

[0012] In Patent Literature 1, importance is placed on the cold forgeability of the steel material, and it is assumed that the hardness of the steel material is low. Therefore, although the endurance ratio of the steel component is high, in some cases the fatigue strength may not be so high. Therefore, the steel material disclosed in Patent Literature 1 is effective as a starting material for steel components with a complex shape for which high cold forgeability is required. On the other hand, in some cases the steel material disclosed in Patent Literature 1 may not be an appropriate starting material for steel components with shapes that are not very complex and for which high fatigue strength is required.

[0013] Further, a steel near-net-shape material that increases the strength of a steel component subjected to cold forging and an aging heat treatment is proposed in International Application Publication No. WO2021 / 210557 (Patent Literature 2).

[0014] The steel near-net-shape material proposed in Patent Literature 2 consists of, in mass %, C: 0.03 to 0.25%, Si: 0.02 to 0.50%, Mn: more than 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less, Al: 0.005 to 0.050%, V: more than 0.10 to 0.40%, N: 0.003 to 0.030%, Cr: 0 to 0.70%, Nb: 0 to 0.100%, B: 0 to 0.0100%, Cu: 0 to 0.30%, Ni: 0 to 0.30%, Ca: 0 to 0.0050%, Bi: 0 to 0.10%, Pb: 0 to 0.090%, Mo: 0 to 0.50%, Ti: 0 to 0.005%, Zr: 0 to 0.010%, Se: 0 to 0.10%, Te: 0 to 0.10%, rare earth metal: 0 to 0.010%, Sb: 0 to 0.10%, Mg: 0 to 0.0050%, W: 0 to 0.0050%, and the balance: Fe and impurities. In this steel near-net-shape material, in addition, fine V precipitates are formed in the steel near-net-shape material, and [V in precipitates] / [V]≥0.30. Further, a diffusible hydrogen content when charged with hydrogen by a cathodic hydrogen charging method is 0.10 ppm or more. According to Patent Literature 2, in order to obtain such fine V precipitates, in a cold forging process, the steel near-net-shape material is subjected to cold working from a first direction in which a working strain amount is 0.05 or more, and is then subjected to cold working from a second direction, which is different from the first direction, in which a working strain amount is 0.05 or more. By performing this kind of cold forging process, the strength of the steel near-net-shape material on which an aging heat treatment is performed after the cold forging increases.

[0015] However, the fatigue strength of a steel component may also be increased by means that is different from the means described in Patent Literature 2.CITATION LISTPatent Literature

[0016] Patent Literature 1: Japanese Patent Application Publication No. 2019-173168

[0017] Patent Literature 2: International Application Publication No. WO2021 / 210557SUMMARY OF INVENTIONTechnical Problem

[0018] An objective of the present disclosure is to provide a steel material in which excellent cold forgeability is obtained, and with which excellent fatigue strength is obtained when the steel material is subjected to cold forging and an aging heat treatment and made into a steel component.Solution to Problem

[0019] A steel material according to the present disclosure has a chemical composition consisting of, in mass %,

[0020] C: 0.03 to 0.25%,

[0021] Si: 0.05 to 0.50%,

[0022] Mn: more than 0.70 to 2.50%,

[0023] P: 0.035% or less,

[0024] S: 0.050% or less,

[0025] Al: 0.001 to 0.100%,

[0026] Cr: 0.05 to 0.70%,

[0027] V: more than 0.10 to 0.40%,

[0028] N: 0.002 to 0.015%,

[0029] Nb: 0 to 0.100%,

[0030] Ti: 0 to 0.050%,

[0031] B: 0 to 0.0100%,

[0032] Cu: 0 to 0.30%,

[0033] Ni: 0 to 0.30%,

[0034] Mo: 0 to 0.30%,

[0035] Ca: 0 to 0.0050%,

[0036] Bi: 0 to 0.100%,

[0037] Pb: 0 to 0.090%,

[0038] Zr: 0 to 0.010%,

[0039] Se: 0 to 0.010%,

[0040] Te: 0 to 0.010%,

[0041] rare earth metal: 0 to 0.010%,

[0042] Mg: 0 to 0.0050%,

[0043] W: 0 to 0.050%, and

[0044] Sn: 0 to 0.100%,

[0045] with the balance being Fe and impurities, and

[0046] has a microstructure in which an area fraction of polygonal ferrite is 30 to 90%, an area fraction of martensite is 5% or less, and the balance is bainite and / or pearlite, wherein

[0047] in the polygonal ferrite, a number density of VCr precipitates, which are precipitates containing V and Cr, having an equivalent circular diameter of 5 to 100 nm is 1000 to 5000 pieces / μm3.Advantageous Effects of Invention

[0048] In the steel material according to the present disclosure, excellent cold forgeability is obtained, and excellent fatigue strength is obtained when the steel material is made into a steel component by being subjected to cold forging and an aging heat treatment.DESCRIPTION OF EMBODIMENTS

[0049] The present inventors have conducted various studies to solve the problem described above, and obtained the following findings.

[0050] First, the present inventors have conducted studies from the viewpoint of the chemical composition with respect to a steel material in which excellent cold forgeability is obtained, and excellent fatigue strength is obtained when subjected to cold forging and an aging heat treatment as a starting material and made into a steel component. As a result, the present inventors have considered that if the chemical composition consists of, in mass %, C: 0.03 to 0.25%, Si: 0.05 to 0.50%, Mn: more than 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less, Al: 0.001 to 0.100%, Cr: 0.05 to 0.70%, V: more than 0.10 to 0.40%, N: 0.002 to 0.015%, Nb: 0 to 0.100%, Ti: 0 to 0.050%, B: 0 to 0.0100%, Cu: 0 to 0.30%, Ni: 0 to 0.30%, Mo: 0 to 0.30%, Ca: 0 to 0.0050%, Bi: 0 to 0.100%, Pb: 0 to 0.090%, Zr: 0 to 0.010%, Se: 0 to 0.010%, Te: 0 to 0.010%, rare earth metal (REM): 0 to 0.010%, Mg: 0 to 0.0050%, W: 0 to 0.050%, and Sn: 0 to 0.100%, with the balance being Fe and impurities, there is a possibility that excellent cold forgeability and excellent fatigue strength can be obtained.

[0051] Therefore, the present inventors have conducted further studies from the viewpoint of the microstructure with respect to means for obtaining excellent cold forgeability and excellent fatigue strength.

[0052] To improve cold forgeability, it is effective to reduce the hardness of the steel material. If the area fraction of polygonal ferrite in the microstructure is high, the hardness of the steel material can be reduced. On the other hand, if the area fraction of polygonal ferrite is too high, the hardness of the steel material will be too low. In such case, sufficient fatigue strength cannot be obtained in a steel component after cold forging and an aging heat treatment. Further, because martensite is extremely hard, if martensite is formed in the microstructure, the cold forgeability will markedly decrease. On the other hand, although bainite and pearlite are hard structures, the influence of bainite and pearlite on cold forgeability is low compared to martensite.

[0053] Taking the above matters into consideration, in the microstructure of a steel material satisfying the chemical composition described above, if the area fraction of polygonal ferrite is 30 to 90%, the area fraction of martensite is 5% or less, and the balance is bainite and / or pearlite, sufficient cold forgeability will be obtained, and in a case where the steel material is made into a steel component by being subjected to cold forging and an aging heat treatment, the fatigue strength of the steel component can be increased.

[0054] In addition, the present inventors have investigated factors that affect the fatigue strength of a steel component in a case where a steel material is used as a starting material and subjected to cold forging and an aging heat treatment and made into a steel component. Fatigue strength exhibits a positive correlation with hardness. For this reason it is considered that if the hardness of a steel component is high, the fatigue strength can also be increased. Therefore, the present inventors have investigated the factors that affect the hardness of a steel component produced by the production process described above. As a result, the present inventors have considered that the hardness of a steel component is affected by the following three factors.

[0055] (1) Hardness of steel material that serves as a starting material

[0056] (2) Work hardening by cold forging

[0057] (3) Age hardening by aging heat treatment

[0058] In the steel material according to Patent Literature 1, attention is focused on age hardening by an aging heat treatment, and in order to ensure the amount of V carbides formed during an aging heat treatment, formation of V carbides in the steel material is suppressed.

[0059] On the other hand, the present inventors have considered that by also utilizing work hardening by cold forging and not just age hardening by an aging heat treatment to increase the hardness of a steel component, the hardness (fatigue strength) of the steel component will be further increased.

[0060] In order to utilize work hardening by cold forging, it is necessary to increase the dislocation density in the steel material by cold forging. To increase the dislocation density, it is necessary to increase the amount of sites that act as dislocation multiplication sources during cold forging. Precipitates that have a certain size, such as cementite, function as dislocation multiplication sources. On the other hand, V carbides (VC) do not function as dislocation multiplication sources due to their fine size.

[0061] Bainite and pearlite in the microstructure of the steel material described above contain cementite, which acts as a dislocation multiplication source. Therefore, work hardening occurs in the bainite and pearlite during cold forging. On the other hand, polygonal ferrite does not contain cementite. Consequently, it is difficult for sufficient work hardening to occur in polygonal ferrite.

[0062] Therefore, in the present embodiment, a large number of VCr precipitates, which are precipitates containing V and Cr, are formed in the polygonal ferrite of the steel material that is the starting material. VCr precipitates have a larger particle size than V carbides. If the VCr precipitates have an equivalent circular diameter of 5 to 100 nm, the VCr precipitates will function as dislocation multiplication sources in the polygonal ferrite during cold forging. Furthermore, if VCr precipitates are utilized instead of V carbides, a certain amount of dissolved V can be secured even if VCr precipitates form to a certain extent in a steel material having the aforementioned chemical composition. Therefore, V carbides or VCr precipitates can be formed utilizing dissolved V during an aging heat treatment, and thus not only work hardening but also age hardening can be utilized.

[0063] Based on the above findings, the present inventors have conducted studies regarding an appropriate amount of VCr precipitates that can cause work hardening and age hardening to occur in the process of producing a steel component described above. As a result, it has been revealed that in a steel material having the chemical composition described above, if the number density of VCr precipitates having an equivalent circular diameter of 5 to 100 nm is made 1000 to 5000 pieces / μm3, in a steel component produced by subjecting the steel material to cold forging and an aging heat treatment, not only age hardening but also work hardening occurs, and excellent fatigue strength is obtained.

[0064] The gist of a forged steel component according to an embodiment of present invention, which has been completed based on the findings described above, is as follows.[1]

[0065] A steel material that has a chemical composition consisting of, in mass %,

[0066] C: 0.03 to 0.25%,

[0067] Si: 0.05 to 0.50%,

[0068] Mn: more than 0.70 to 2.50%,

[0069] P: 0.035% or less,

[0070] S: 0.050% or less,

[0071] Al: 0.001 to 0.100%,

[0072] Cr: 0.05 to 0.70%,

[0073] V: more than 0.10 to 0.40%,

[0074] N: 0.002 to 0.015%,

[0075] Nb: 0 to 0.100%,

[0076] Ti: 0 to 0.050%,

[0077] B: 0 to 0.0100%,

[0078] Cu: 0 to 0.30%,

[0079] Ni: 0 to 0.30%,

[0080] Mo: 0 to 0.30%,

[0081] Ca: 0 to 0.0050%,

[0082] Bi: 0 to 0.100%,

[0083] Pb: 0 to 0.090%,

[0084] Zr: 0 to 0.010%,

[0085] Se: 0 to 0.010%,

[0086] Te: 0 to 0.010%,

[0087] rare earth metal: 0 to 0.010%,

[0088] Mg: 0 to 0.0050%,

[0089] W: 0 to 0.050%, and

[0090] Sn: 0 to 0.100%,

[0091] with the balance being Fe and impurities, and

[0092] has a microstructure in which an area fraction of polygonal ferrite is 30 to 90%, an area fraction of martensite is 5% or less, and the balance is bainite and / or pearlite, wherein

[0093] in the polygonal ferrite, a number density of VCr precipitates, which are precipitates containing V and Cr, having an equivalent circular diameter of 5 to 100 nm is 1000 to 5000 pieces / μm3.[2]

[0094] The steel material according to [1], wherein the chemical composition contains one or more kinds of element selected from a group consisting of, in mass %:

[0095] Nb: 0.001 to 0.100%,

[0096] Ti: 0.001 to 0.050%,

[0097] B: 0.0001 to 0.0100%,

[0098] Cu: 0.01 to 0.30%,

[0099] Ni: 0.01 to 0.30%,

[0100] Mo: 0.01 to 0.30%,

[0101] Ca: 0.0001 to 0.0050%,

[0102] Bi: 0.001 to 0.100%,

[0103] Pb: 0.001 to 0.090%,

[0104] Zr: 0.001 to 0.010%,

[0105] Se: 0.001 to 0.010%,

[0106] Te: 0.001 to 0.010%,

[0107] rare earth metal: 0.001 to 0.010%,

[0108] Mg: 0.0001 to 0.0050%,

[0109] W: 0.001 to 0.050%, and

[0110] Sn: 0.001 to 0.100%.

[0111] Hereunder, a steel for cold forging of the present embodiment is described in detail. The symbol “%” in relation to an element means “mass percent”.FEATURES OF STEEL MATERIAL OF PRESENT EMBODIMENT

[0112] The steel material of the present embodiment satisfies the following feature 1 to feature 3.Feature 1

[0113] The chemical composition consists of, in mass %, C: 0.03 to 0.25%, Si: 0.05 to 0.50%, Mn: more than 0.70 to 2.50%, P: 0.035% or less, S: 0.050% or less, Al: 0.001 to 0.100%, Cr: 0.05 to 0.70%, V: more than 0.10 to 0.40%, N: 0.002 to 0.015%, Nb: 0 to 0.100%, Ti: 0 to 0.050%, B: 0 to 0.0100%, Cu: 0 to 0.30%, Ni: 0 to 0.30%, Mo: 0 to 0.30%, Ca: 0 to 0.0050%, Bi: 0 to 0.100%, Pb: 0 to 0.090%, Zr: 0 to 0.010%, Se: 0 to 0.010%, Te: 0 to 0.010%, rare earth metal (REM): 0 to 0.010%, Mg: 0 to 0.0050%, W: 0 to 0.050%, and Sn: 0 to 0.100%, with the balance being Fe and impurities.Feature 2

[0114] In the microstructure, an area fraction of polygonal ferrite is 30 to 90%, an area fraction of martensite is 5% or less, and the balance is bainite and / or pearlite.Feature 3

[0115] In the polygonal ferrite, the number density of VCr precipitates, which are precipitates containing V and Cr, having an equivalent circular diameter of 5 to 100 nm is 1000 to 5000 pieces / μm3.

[0116] Feature 1 to feature 3 are described hereunder.(Feature 1) Regarding Chemical Composition

[0117] The chemical composition of the steel material according to an embodiment of the present invention contains the following elements.C: 0.03 to 0.25%

[0118] Carbon (C) increases the fatigue strength of a steel component produced using the steel material as a starting material. Specifically, C forms bainite and / or pearlite that are hard structures in the microstructure and thereby increases the fatigue strength of the steel component. In addition, C forms VCr precipitates and increases the fatigue strength of the steel component. If the content of C is less than 0.03%, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

[0119] On the other hand, if the content of C is more than 0.25%, even if the contents of other elements are within the range of the present embodiment, the strength of the steel material will excessively increase and cold forgeability will decrease.

[0120] Therefore, the content of C is 0.03 to 0.25%.

[0121] A preferable lower limit of the content of C is 0.05%, more preferably is 0.07%, and further preferably is 0.09%.

[0122] A preferable upper limit of the content of C is 0.23%, more preferably is 0.21%, and further preferably is 0.20%.Si: 0.05 to 0.50%

[0123] Silicon (Si) increases the fatigue strength of a steel component produced using the steel material as a starting material. If the content of Si is less than 0.05%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

[0124] On the other hand, if the content of Si is more than 0.50%, the strength of the steel material will excessively increase. Therefore, cold forgeability will decrease even if the contents of other elements are within the range of the present embodiment.

[0125] Therefore, the content of Si is 0.05 to 0.50%.

[0126] A preferable lower limit of the content of Si is 0.08%, more preferably is 0.10%, and further preferably is 0.12%.

[0127] A preferable upper limit of the content of Si is 0.45%, more preferably is 0.43%, and further preferably is 0.41%.Mn: more than 0.70 to 2.50%

[0128] Manganese (Mn) increases hardenability of the steel material, and forms bainite and pearlite that are hard structures in the microstructure. Therefore, Mn increases the fatigue strength of a steel component produced using the steel material as a starting material. If the content of Mn is 0.70% or less, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

[0129] On the other hand, if the content of Mn is more than 2.50%, hardenability of the steel material will be excessively increased and martensite will form. In such case, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0130] Therefore, the content of Mn is more than 0.70 to 2.50%.

[0131] A preferable lower limit of the content of Mn is 0.80%, more preferably is 0.90%, and further preferably is 1.00%.

[0132] A preferable upper limit of the content of Mn is 2.30%, more preferably is 2.20%, and further preferably is 2.10%.P: 0.035% or Less

[0133] Phosphorus (P) is an impurity. P segregates to grain boundaries and decreases the cold forgeability of the steel material. Therefore, the content of P is 0.035% or less.

[0134] The content of P is preferably as low as possible. However, excessively reducing the content of P will raise the production cost. Therefore, when ordinary industrial productivity is taken into consideration, a preferable lower limit of the content of P is more than 0%, more preferably is 0.001%, further preferably is 0.005%, and further preferably is 0.010%.

[0135] A preferable upper limit of the content of P is 0.030%, more preferably is 0.025%, and further preferably is 0.020%.S: 0.050% or Less

[0136] Sulfur(S) is an impurity. S combines with Mn to form MnS, which increases the machinability of the steel material. However, if the content of S is more than 0.050%, coarse MnS will form. The coarse MnS will decrease the cold forgeability. Therefore, the content of S is 0.050% or less.

[0137] The content of S is preferably as low as possible. However, excessively reducing the content of S will raise the production cost. Therefore, when ordinary industrial productivity is taken into consideration, a preferable lower limit of the content of S is more than 0%, more preferably is 0.001%, further preferably is 0.005%, and further preferably is 0.010%.

[0138] A preferable upper limit of the content of S is 0.045%, more preferably is 0.040%, further preferably is 0.030%, and further preferably is 0.020%.Al: 0.001 to 0.100%

[0139] Aluminum (Al) deoxidizes the steel. If the content of Al is less than 0.001%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

[0140] On the other hand, if the content of Al is more than 0.100%, coarse Al-based inclusions such as Al oxides will form in the steel material. The coarse Al-based inclusions will decrease the cold forgeability of the steel material.

[0141] Therefore, the content of Al is 0.001 to 0.100%.

[0142] A preferable lower limit of the content of Al is 0.005%, more preferably is 0.010%, and further preferably is 0.015%.

[0143] A preferable upper limit of the content of Al is 0.090%, more preferably is 0.070%, further preferably is 0.050%, and further preferably is 0.040%. Note that, in the steel material of the present embodiment, the term “content of Al” means the total content of Al.Cr: 0.05 to 0.70%

[0144] Chromium (Cr) increases hardenability of the steel material and promotes formation of bainite and / or pearlite in the microstructure. Therefore, Cr increases the fatigue strength of a steel component produced using the steel material as a starting material. Furthermore, Cr forms VCr precipitates in the steel material and, in addition, forms VCr precipitates in an aging heat treatment process during the process of producing the steel component. By this means, Cr increases the fatigue strength of the steel component. If the content of Cr is less than 0.05%, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

[0145] On the other hand, if the content of Cr is more than 0.70%, hardenability of the steel material will be excessively high, the area fraction of polygonal ferrite will be too low, and the area fraction of martensite will be too high. In such case, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0146] Therefore, the content of Cr is 0.05 to 0.70%.

[0147] A preferable lower limit of the content of Cr is 0.06%, more preferably is 0.08%, and further preferably is 0.10%.

[0148] A preferable upper limit of the content of Cr is 0.60%, more preferably is 0.50%, and further preferably is 0.30%.V: More than 0.10 to 0.40%

[0149] Vanadium (V) forms VCr precipitates in the steel material, and also forms VCr precipitates and / or V carbides in an aging heat treatment process during the process of producing a steel component. By this means, V increases the fatigue strength of the steel component. If the content of V is 0.10% or less, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

[0150] On the other hand, if the content of V is more than 0.40%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0151] Therefore, the content of V is more than 0.10 to 0.40%.

[0152] A preferable lower limit of the content of V is 0.12%, more preferably is 0.14%, and further preferably is 0.15%.

[0153] A preferable upper limit of the content of V is 0.38%, more preferably is 0.35%, and further preferably is 0.30%.N: 0.002 to 0.015%

[0154] Nitrogen (N) combines with V in the steel material to form V nitrides. V nitrides are more thermodynamically stable than VCr precipitates, and form at high temperatures. Therefore, if V nitrides form, the amount of V available for formation of VCr precipitates will decrease. If the content of N is more than 0.015%, V nitrides will excessively form. Consequently, even if the contents of other elements are within the range of the present embodiment, formation of VCr precipitates will be insufficient and the fatigue strength of the steel component will decrease.

[0155] On the other hand, a certain amount of V can dissolve in the steel material at high temperatures. Therefore, it is not necessary to reduce the content of N to less than 0.002%.

[0156] Therefore, the content of N is 0.002 to 0.015%.

[0157] A preferable lower limit of the content of N is 0.003%, and more preferably is 0.004%.

[0158] A preferable upper limit of the content of N is 0.012%, more preferably is 0.010%, and further preferably is 0.008%.

[0159] The balance of the chemical composition of the steel material according to the present embodiment is Fe and impurities. Here, the term “impurities” in the chemical composition means substances which are mixed in from ore and scrap used as raw material or from the production environment or the like when industrially producing the steel material, and which are not intentionally contained but are permitted within a range that does not adversely affect the steel material of the present embodiment. The content of O (oxygen) in the impurities is, for example, 0.0040% or less.[Regarding Optional Elements]

[0160] The chemical composition of the steel material of the present embodiment may further contain one or more kinds of element selected from a group consisting of the following first group to third group in lieu of a part of Fe.[First Group]

[0161] One or more kinds of element selected from the group consisting of:

[0162] Nb: 0 to 0.100%,

[0163] Ti: 0 to 0.050%,

[0164] B: 0 to 0.0100%,

[0165] Cu: 0 to 0.30%,

[0166] Ni: 0 to 0.30%, and

[0167] Mo: 0 to 0.30%.[Second Group]

[0168] One or more kinds of element selected from the group consisting of:

[0169] Ca: 0 to 0.0050%,

[0170] Bi: 0 to 0.100%, and

[0171] Pb: 0 to 0.090%.[Third Group]

[0172] One or more kinds of element selected from the group consisting of:

[0173] Zr: 0 to 0.010%,

[0174] Se: 0 to 0.010%,

[0175] Te: 0 to 0.010%,

[0176] rare earth metal: 0 to 0.010%,

[0177] Mg: 0 to 0.0050%,

[0178] W: 0 to 0.050%, and

[0179] Sn: 0 to 0.100%.

[0180] The first group to third group are described hereunder.[First Group (Nb, Ti, B, Cu, Ni, and Mo)]

[0181] The steel material of the present embodiment may contain one or more kinds of element selected from the group consisting of Nb: 0 to 0.100%, Ti: 0 to 0.050%, B: 0 to 0.0100%, Cu: 0 to 0.30%, Ni: 0 to 0.30%, and Mo: 0 to 0.30% in lieu of a part of Fe. These elements are optional elements, and each of these elements increases the fatigue strength of a steel component produced using the steel material as a starting material.Nb: 0 to 0.100%

[0182] Niobium (Nb) is an optional element and does not have to be contained. That is, the content of Nb may be 0%.

[0183] When contained, that is, when the content of Nb is more than 0%, Nb combines with C and / or N in the steel material to form Nb precipitates. The Nb precipitates increase the fatigue strength of the steel component by precipitation strengthening. If even a small amount of Nb is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0184] However, if the content of Nb is more than 0.100%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0185] Therefore, the content of Nb is 0 to 0.100%.

[0186] A preferable lower limit of the content of Nb is 0.001%, more preferably is 0.010%, and further preferably is 0.020%.

[0187] A preferable upper limit of the content of Nb is 0.080%, more preferably is 0.060%, and further preferably is 0.050%.Ti: 0 to 0.050%

[0188] Titanium (Ti) is an optional element and does not have to be contained. That is, the content of Ti may be 0%.

[0189] When contained, that is, when the content of Ti is more than 0%, Ti combines with N or C in the steel material to form Ti precipitates, and thereby increases the fatigue strength of the steel component. If even a small amount of Ti is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0190] However, if the content of Ti is more than 0.050%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0191] Therefore, the content of Ti is 0 to 0.050%.

[0192] A preferable lower limit of the content of Ti is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.

[0193] A preferable upper limit of the content of Ti is 0.045%, more preferably is 0.040%, and further preferably is 0.030%.B: 0 to 0.0100%

[0194] Boron (B) is an optional element and does not have to be contained. That is, the content of B may be 0%.

[0195] When contained, that is, when the content of B is more than 0%, B strengthens the crystal grain boundaries of the steel material, and thereby increases the fatigue strength of the steel component. If even a small amount of B is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0196] However, if the content of B is more than 0.0100%, the aforementioned advantageous effect will be saturated. In addition, if the content of B is more than 0.0100%, the raw material cost will increase and the producibility will also decrease. Therefore, the content of B is 0 to 0.0100%.

[0197] A preferable lower limit of the content of B is 0.0001%, more preferably is 0.0005%, further preferably is 0.0010%, and further preferably is 0.0020%.

[0198] A preferable upper limit of the content of B is 0.0080%, more preferably is 0.0060%, and further preferably is 0.0050%.Cu: 0 to 0.30%

[0199] Copper (Cu) is an optional element and does not have to be contained. That is, the content of Cu may be 0%.

[0200] When contained, that is, when the content of Cu is more than 0%, Cu increases hardenability of the steel material and increases the fatigue strength of the steel component. If even a small amount of Cu is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0201] However, if the content of Cu is more than 0.30%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0202] Therefore, the content of Cu is 0 to 0.30%.

[0203] A preferable lower limit of the content of Cu is 0.01%, more preferably is 0.05%, and further preferably is 0.10%.

[0204] A preferable upper limit of the content of Cu is 0.28%, more preferably is 0.25%, and further preferably is 0.20%.Ni: 0 to 0.30%

[0205] Nickel (Ni) is an optional element and does not have to be contained. That is, the content of Ni may be 0%.

[0206] When contained, that is, when the content of Ni is more than 0%, Ni increases hardenability of the steel material and increases the fatigue strength of the steel component. If even a small amount of Ni is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0207] However, if the content of Ni is more than 0.30%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0208] Therefore, the content of Ni is 0 to 0.30%.

[0209] A preferable lower limit of the content of Ni is 0.01%, more preferably is 0.05%, and further preferably is 0.10%.

[0210] A preferable upper limit of the content of Ni is 0.28%, more preferably is 0.25%, and further preferably is 0.20%.Mo: 0 to 0.30%

[0211] Molybdenum (Mo) is an optional element and does not have to be contained. That is, the content of Mo may be 0%.

[0212] When contained, that is, when the content of Mo is more than 0%, Mo increases hardenability of the steel material and increases the fatigue strength of the steel component. If even a small amount of Mo is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0213] However, if the content of Mo is more than 0.30%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0214] Therefore, the content of Mo is 0 to 0.30%.

[0215] A preferable lower limit of the content of Mo is 0.01%, more preferably is 0.05%, and further preferably is 0.10%.

[0216] A preferable upper limit of the content of Mo is 0.28%, more preferably is 0.25%, and further preferably is 0.20%.[Second Group (Ca, Bi, and Pb)]

[0217] The chemical composition of the steel material of the present embodiment may further contain one or more kinds of element selected from the group consisting of Ca: 0 to 0.0050%, Bi: 0 to 0.100%, and Pb: 0 to 0.090% in lieu of a part of Fe. These elements are optional elements, and each of these elements increases the machinability of the steel material.Ca: 0 to 0.0050%

[0218] Calcium (Ca) is an optional element and does not have to be contained. That is, the content of Ca may be 0%.

[0219] When contained, that is, when the content of Ca is more than 0%, Ca increases the machinability of the steel material. If even a small amount of Ca is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0220] However, if the content of Ca is more than 0.0050%, coarse CaO will form. In such case, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0221] Therefore, the content of Ca is 0 to 0.0050%.

[0222] A preferable lower limit of the content of Ca is 0.0001%, more preferably is 0.0005%, further preferably is 0.0010%, and further preferably is 0.0020%.

[0223] A preferable upper limit of the content of Ca is 0.0048%, more preferably is 0.0045%, and further preferably is 0.0040%.Bi: 0 to 0.100%

[0224] Bismuth (Bi) is an optional element and does not have to be contained. That is, the content of Bi may be 0%.

[0225] When contained, that is, when the content of Bi is more than 0%, Bi increases the machinability of the steel material. If even a small amount of Bi is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0226] However, if the content of Bi is more than 0.100%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0227] Therefore, the content of Bi is 0 to 0.100%.

[0228] A preferable lower limit of the content of Bi is 0.001%, more preferably is 0.010%, further preferably is 0.020%, and further preferably is 0.030%. A preferable upper limit of the content of Bi is 0.090%, more preferably is 0.080%, and further preferably is 0.060%.

[0229] Pb: 0 to 0.090%

[0230] Lead (Pb) is an optional element and does not have to be contained. That is, the content of Pb may be 0%.

[0231] When contained, that is, when the content of Pb is more than 0%, Pb increases the machinability of the steel material. If even a small amount of Pb is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0232] However, if the content of Pb is more than 0.090%, the cold forgeability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0233] Therefore, the content of Pb is 0 to 0.090%.

[0234] A preferable lower limit of the content of Pb is 0.001%, more preferably is 0.010%, further preferably is 0.020%, and further preferably is 0.040%.

[0235] A preferable upper limit of the content of Pb is 0.080%, more preferably is 0.075%, and further preferably is 0.070%.[Third Group (Zr, Se, Te, Rare Earth Metal (REM), Mg, W, and Sn)]

[0236] The chemical composition of the steel material of the present embodiment may further contain one or more kinds of element selected from the group consisting of Zr: 0 to 0.010%, Se: 0 to 0.010%, Te: 0 to 0.010%, rare earth metal (REM): 0 to 0.010%, Mg: 0 to 0.0050%, W: 0 to 0.050%, and Sn: 0 to 0.100% in lieu of a part of Fe. These elements are optional elements, and each of these elements can be contained in the steel material as an impurity.Zr: 0 to 0.010%

[0237] Zirconium (Zr) is an impurity, and does not have to be contained. That is, the content of Zr may be 0%.

[0238] When contained, that is, when the content of Zr is more than 0%, if the content of Zr is more than 0.010%, Zr will form coarse inclusions. In such case, the fatigue strength of the steel material will decrease even if the contents of other elements are within the range of the present embodiment.

[0239] Therefore, the content of Zr is 0 to 0.010%.

[0240] The content of Zr is preferably as low as possible. However, excessively reducing the content of Zr will raise the production cost. Therefore, a preferable lower limit of the content of Zr is 0.001%.

[0241] A preferable upper limit of the content of Zr is 0.008%, more preferably is 0.005%, and further preferably is 0.002%.Se: 0 to 0.010%

[0242] Selenium (Se) is an impurity, and does not have to be contained. That is, the content of Se may be 0%.

[0243] When contained, that is, when the content of Sn is more than 0%, if the content of Se is more than 0.010%, Se will embrittle the steel material and reduce the fatigue strength of the steel component even if the contents of other elements are within the range of the present embodiment.

[0244] Therefore, the content of Se is 0 to 0.010%.

[0245] The content of Se is preferably as low as possible. However, excessively reducing the content of Se will raise the production cost. Therefore, a preferable lower limit of the content of Se is 0.001%.

[0246] A preferable upper limit of the content of Se is 0.008%, more preferably is 0.005%, and further preferably is 0.002%.Te: 0 to 0.010%

[0247] Tellurium (Te) is an impurity, and does not have to be contained. That is, the content of Te may be 0%.

[0248] When contained, that is, when the content of Te is more than 0%, if the content of Te is more than 0.010%, Te will embrittle the steel material and reduce the fatigue strength of the steel component even if the contents of other elements are within the range of the present embodiment.

[0249] Therefore, the content of Te is 0 to 0.010%.

[0250] The content of Te is preferably as low as possible. However, excessively reducing the content of Te will raise the production cost. Therefore, a preferable lower limit of the content of Te is 0.001%.

[0251] A preferable upper limit of the content of Te is 0.008%, more preferably is 0.005%, and further preferably is 0.002%.

[0252] Rare earth metal (REM): 0 to 0.010%

[0253] Rare earth metal (REM) is an impurity, and does not have to be contained. That is, the content of REM may be 0%.

[0254] When contained, that is, when the content of REM is more than 0%, if the content of REM is more than 0.010%, REM will form coarse inclusions and will thereby reduce the fatigue strength of the steel material even if the contents of other elements are within the range of the present embodiment.

[0255] Therefore, the content of REM is 0 to 0.010%.

[0256] The content of REM is preferably as low as possible. However, excessively reducing the content of REM will raise the production cost. Therefore, a preferable lower limit of the content of REM is 0.001%.

[0257] A preferable upper limit of the content of REM is 0.008%, more preferably is 0.005%, and further preferably is 0.002%.

[0258] Note that, in the present description the term “REM” means one or more kinds of element selected from the group consisting of scandium (Sc) which is the element with atomic number 21, yttrium (Y) which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 that are lanthanoids. In the present description, the term “content of REM” means the total content of these elements.Mg: 0 to 0.0050%

[0259] Magnesium (Mg) is an impurity, and does not have to be contained. That is, the content of Mg may be 0%.

[0260] When contained, that is, when the content of Mg is more than 0%, if the content of Mg is more than 0.0050%, Mg will form coarse inclusions and reduce the fatigue strength of the steel material even if the contents of other elements are within the range of the present embodiment.

[0261] Therefore, the content of Mg is 0 to 0.0050%.

[0262] The content of Mg is preferably as low as possible. However, excessively reducing the content of Mg will raise the production cost. Therefore, a preferable lower limit of the content of Mg is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%.

[0263] A preferable upper limit of the content of Mg is 0.0040%, more preferably is 0.0030%, and further preferably is 0.0020%.W: 0 to 0.050%

[0264] Tungsten (W) is an impurity, and does not have to be contained. That is, the content of W may be 0%.

[0265] When contained, that is, when the content of W is more than 0%, if the content of W is more than 0.050%, W will decrease the cold forgeability of the steel material even if the contents of other elements are within the range of the present embodiment.

[0266] Therefore, the content of W is 0 to 0.050%.

[0267] The content of W is preferably as low as possible. However, excessively reducing the content of W will raise the production cost. Therefore, a preferable lower limit of the content of W is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.

[0268] A preferable upper limit of the content of W is 0.040%, more preferably is 0.030%, and further preferably is 0.020%.Sn: 0 to 0.100%

[0269] Tin (Sn) is an impurity, and does not have to be contained. That is, the content of Sn may be 0%.

[0270] When contained, that is, when the content of Sn is more than 0%, if the content of Sn is more than 0.100%, Sn will decrease the cold forgeability of the steel material even if the contents of other elements are within the range of the present embodiment.

[0271] Therefore, the content of Sn is 0 to 0.100%.

[0272] The content of Sn is preferably as low as possible. However, excessively reducing the content of Sn will raise the production cost. Therefore, a preferable lower limit of the content of Sn is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.

[0273] A preferable upper limit of the content of Sn is 0.060%, more preferably is 0.040%, and further preferably is 0.020%.(Feature 2) Regarding Microstructure

[0274] In the microstructure of the steel material of the present embodiment, an area fraction of polygonal ferrite is 30 to 90%, an area fraction of martensite is 5% or less, and the balance is bainite and / or pearlite.

[0275] Polygonal ferrite is a soft phase, and is an effective phase for improving cold forgeability. However, there is a difference in strength between polygonal ferrite and hard structures such as bainite and pearlite. When structures with large strength differences are intermixed in the microstructure, cracks may occur during cold forging due to the strength difference.

[0276] However, in the steel material of the present embodiment, VCr precipitates are formed in the microstructure, and VCr precipitates are also formed in the polygonal ferrite. Therefore, in the steel material of the present embodiment, a strength difference between the polygonal ferrite and the hard structures can be suppressed by the VCr precipitates formed in the steel material. As a result, even if the area fraction of polygonal ferrite is high, cold forgeability can be improved.

[0277] If the area fraction of polygonal ferrite in the microstructure of the steel material is 30 to 90%, the area fraction of martensite is 5% or less, and the balance is bainite and / or pearlite, the aforementioned advantageous effect will be sufficiently obtained.

[0278] A preferable lower limit of the area fraction of polygonal ferrite is 32%, more preferably is 34%, and further preferably is 36%.

[0279] A preferable upper limit of the area fraction of polygonal ferrite is 85%, more preferably is 80%, further preferably is 75%, further preferably is 68%, further preferably is 66%, and further preferably is 64%.

[0280] In the microstructure, the balance is bainite and / or pearlite. The strength of bainite and pearlite is higher than the strength of polygonal ferrite. Therefore, bainite and pearlite increase the fatigue strength of a steel component produced using the steel material as a starting material. Furthermore, bainite and pearlite do not markedly reduce the cold forgeability as in the case of martensite.

[0281] On the other hand, although martensite is a structure that has high strength, as mentioned above, martensite markedly reduces the cold forgeability of a steel material. Therefore, in the microstructure of the present embodiment, the area fraction of martensite is preferably as low as possible. Accordingly, in the present embodiment, the area fraction of martensite is 5% or less.

[0282] A preferable upper limit of the area fraction of martensite is 4%, more preferably is 3%, further preferably is 2%, further preferably is 1%, and further preferably is 0%.[Method for Measuring Area Fraction of Each Structure in Microstructure]

[0283] The polygonal ferrite area fraction, bainite area fraction, and pearlite area fraction in the microstructure of the steel material are measured by the following method.

[0284] A cross section that is parallel to the axial direction of the steel material and that includes the central axis of the steel material is adopted as an observation surface. A test specimen that includes the observation surface is taken from the steel material. The observation surface of the test specimen is mirror-polished. The observation surface after mirror-polishing is subjected to etching with 3% Nital (ethanol+3% nitric acid solution) to reveal the microstructure.

[0285] On the etched observation surface, observation fields that each includes an R / 2 depth position in the radial direction from the surface of the steel material are observed with an optical microscope. The observation magnification is to be set to ×400, and five fields are to be set as observation fields. In the present description, the term “R / 2 position” of the steel material means the center position of the radius (=R) in a cross section (circular shape) perpendicular to the axial direction of the steel material. Further, the center position of the radius means a position from a 3R / 8 depth position to a 5R / 8 depth position from the surface of the steel material. The area of each observation field is to be 58800 μm2 (280 μm×210 μm).

[0286] In each observation field, polygonal ferrite has a different contrast and morphology to the hard structures (bainite, pearlite, and martensite). Therefore, the respective structures are recognized as follows based on contrast and morphology. White polygonal grains in which there is no internal structure are recognized as polygonal ferrite. A structure in which white regions with high brightness and black regions with low brightness are intermixed in a striped pattern is recognized as pearlite. A structure having a narrow plate-like or acicular structure and in which black precipitates are present inside or at the boundaries of the narrow plate-like or acicular structure is recognized as bainite.

[0287] In each of the five observation fields, the area fraction of polygonal ferrite, and the area fractions (%) of bainite and pearlite are determined. For each structure, the arithmetic average value of the values obtained in the five observation fields is defined as the polygonal ferrite area fraction (%), pearlite area fraction (%), and bainite area fraction (%), respectively.

[0288] Note that, in the steel material of the present embodiment that satisfies feature 1, martensite forms in gaps between bainite. Therefore, an area fraction of martensite that is described later is also included in the bainite area fraction determined by the aforementioned observation with an optical microscope at a magnification of ×400. Accordingly, with respect to the bainite area fraction, a value that is determined by, furthermore, subtracting an area fraction of martensite obtained by a method described later from the area fraction of bainite that is the arithmetic average value for the five observation fields determined by the method described above is defined as the bainite area fraction (%) of the steel material of the present embodiment.

[0289] Here, the martensite area fraction in the microstructure of the steel material is measured by the following method.

[0290] A cross section that is parallel to the axial direction of the steel material and that includes the central axis of the steel material is adopted as an observation surface. A test specimen that includes the observation surface is taken from the steel material. The observation surface of the test specimen is mirror-polished. The observation surface after mirror-polishing is subjected to etching with 3% Nital (ethanol+3% nitric acid solution) to reveal the microstructure.

[0291] On the observation surface after etching, observation fields that each includes an R / 2 depth position in the radial direction from the surface of the steel material are observed using a scanning electron microscope (SEM) equipped with a backscattered electron image detector. The observation magnification is to be set to ×1000, and five fields are to be set as observation fields. The area of each observation field is to be 10800 μm2 (120 μm×90 μm).

[0292] In each observation field, martensite is identified based on contrast. Specifically, in a backscattered electron image, martensite exhibits brighter contrast than polygonal ferrite, bainite, and pearlite. The area fraction (%) of martensite in each of the five observation fields is determined based on the contrast. The arithmetic average value of the area fractions of martensite in the five observation fields is defined as the area fraction (%) of martensite.

[0293] As mentioned above, a value obtained by subtracting the martensite area fraction from the bainite area fraction is defined as the bainite area fraction (%) in the microstructure of the steel material of the present embodiment.(Feature 3) Regarding VCr Precipitates Number Density

[0294] In the steel material of the present embodiment, in addition, in the polygonal ferrite, the number density of VCr precipitates, which are precipitates containing V and Cr, that have an equivalent circular diameter of 5 to 100 nm is 1000 to 5000 pieces / μm3.

[0295] Generally speaking, in the microstructure of steel material, although cementite, which acts as a dislocation multiplication source, is present in bainite and pearlite, polygonal ferrite does not, in principle, contain cementite. Therefore, although work hardening occurs in bainite and pearlite as a result of working such as cold forging, work hardening does not occur in polygonal ferrite since there are no dislocation multiplication sources present therein.

[0296] On the other hand, as mentioned above, in the steel material of the present embodiment, VCr precipitates are formed in polygonal ferrite in the steel material. The VCr precipitates are coarse in comparison to V carbides (VC). The VCr precipitates which are coarser than V carbides act as sites of dislocation multiplication sources during working such as cold forging. Therefore, during cold forging, together with the occurrence of work hardening in bainite and pearlite, work hardening also occurs in polygonal ferrite because of the presence of the VCr precipitates. Therefore, when a steel component is produced by performing cold forging and an aging heat treatment on the steel material of the present embodiment as a starting material, two hardening mechanisms, namely, work hardening in a cold forging process by VCr precipitates, and age hardening by precipitates (VCr precipitates and / or V carbides) that form during an aging heat treatment, act to increase the hardness of the steel component. As a result, excellent fatigue strength is obtained in the steel component.

[0297] Specifically, if the VCr precipitates have an equivalent circular diameter of 5 to 100 nm, the VCr precipitates will function as dislocation multiplication sources. Here, the number density of VCr precipitates having an equivalent circular diameter of 5 to 100 nm is referred to as “VCr precipitates number density ND”. If the VCr precipitates number density ND is less than 1000 pieces / μm3, the formed amount of VCr precipitates will not be sufficient. In this case, sufficient fatigue strength will not be obtained in a steel component produced by performing a cold forging process and an aging heat treatment process on the steel material as a starting material. On the other hand, if the VCr precipitates number density ND is more than 5000 pieces / μm3, sufficient cold forgeability will not be obtained in the steel material. Therefore, the VCr precipitates number density ND is 1000 to 5000 pieces / μm3.

[0298] A preferable lower limit of the VCr precipitates number density ND is 1100 pieces / μm3, more preferably is 1200 pieces / μm3, further preferably is 1300 pieces / μm3, further preferably is 1400 pieces / μm3, and further preferably is 1500 pieces / μm3.

[0299] A preferable upper limit of the VCr precipitates number density ND is 4900 pieces / μm3, more preferably is 4500 pieces / μm3, further preferably is 4000 pieces / μm3, further preferably is 3800 pieces / μm3, and further preferably is 3600 pieces / μm3.[Method for Measuring VCr Precipitates Number Density ND]

[0300] The VCr precipitates number density ND in polygonal ferrite can be measured by the following method using a scanning transmission electron microscope (STEM-EDS).

[0301] A circular disc test specimen that has a diameter of 3 mm and a thickness of approximately 2 mm and that has the R / 2 position of the steel material as the center of the circle is taken from the steel material. The axial direction of the circular disc test specimen is to be parallel to the axial direction of the steel material. Both sides (front surface and rear surface) of the circular disc are polished using emery paper. At such time, the circular disc is polished so that the front surface is parallel with the rear surface. One of the front surface and rear surface of the circular disc is adopted as an observation surface. The observation surface is further subjected to mirror polishing. The mirror-polished observation surface is then subjected to further polishing using colloidal silica as an abrasive.

[0302] The observation surface after polishing is subjected to electron back scatter diffraction (EBSD) analysis to identify the crystal orientation of the parent phase. Based on the identified crystal orientation of the parent phase, focused ion beam processing (FIB processing) is then performed on the circular disc test specimen in a manner so that the direction (observation direction) perpendicular to the observation surface of a thin film is the <001> crystal orientation of the parent phase to thereby prepare a thin film sample for a STEM.

[0303] In the FIB processing, the thin film sample for a STEM is prepared by a lift-out method using a gallium (Ga) ion beam with an acceleration voltage of 30 kV.

[0304] Dislocation loops and amorphous regions are present in the outer layer of the thin film sample for a STEM prepared using the ion beam with an acceleration voltage of 30 kV, and consequently the outer layer is not suitable for observing VCr precipitates with a size of several nm. Therefore, the surface of the thin film sample for a STEM is polished using an ion beam with a low acceleration voltage of 1 kV or less. A thin film sample for a STEM having a thickness of about 100 nm is prepared by the above production process.

[0305] The prepared thin film sample is observed with the optical system of a STEM. Specifically, the thin film sample for a STEM is tilted so that the zone-axis incidence is the crystal orientation <001> of the polygonal ferrite of the parent phase in the thin film sample for a STEM. The observation magnification is to be set to ×200000, and the acceleration voltage is to be set to 300 kV. The detector is to be set to known high-angle annular dark-field (HAADF) conditions, known low-angle annular dark-field (LAADF) conditions, and known bright field (BF) conditions, and an STEM image of one observation field is taken at each of an arbitrary five polygonal ferrite grains (STEM images of observation fields at a total of five locations). Note that, there is a difference in dislocation density between polygonal ferrite grains and other structures. Therefore, polygonal ferrite grains can be easily identified in the observation with the STEM.

[0306] The area of each observation field is to be 1000 nm×1000 nm. In all of the observation fields, the thickness of the thin film sample for a STEM is measured using the log-ratio method of electron energy loss spectroscopy (EELS).

[0307] Quantitative analysis of the chemical composition of three or more representative particles in the five observation fields is performed using an EDS-ZAF method. In the chemical composition of the respective particles, when the total content of metallic elements excluding Fe is taken as 100% in percent by mass, if the content of V is 20% by mass or more, the content of Cr is 5% by mass or more, and the total content of V and Cr is 60% or more, the relevant particle is recognized as a VCr precipitate.

[0308] The contrast of the particles identified as VCr precipitates is identified. In the observation fields, particles with the same contrast as the identified contrast are recognized as VCr precipitates. The equivalent circular diameter of each recognized VCr precipitate is determined by a well-known image processing method. Among the identified VCr precipitates, the number of VCr precipitates having an equivalent circular diameter of 5 to 100 nm is determined. The VCr precipitates number density (pieces / μm3) is determined based on the number of VCr precipitates having an equivalent circular diameter of 5 to 100 nm in each observation field, and the area (1000 nm×1000 nm) and thickness of the observation field. The arithmetic average value of the VCr precipitates number density values obtained in the respective observation fields is defined as a VCr precipitates number density ND (pieces / μm3).Advantageous Effects of Steel Material of Present Embodiment

[0309] The steel material of the present embodiment satisfies feature 1 to feature 3. Therefore, in the steel material of the present embodiment, excellent cold forgeability is obtained. In addition, excellent fatigue strength is obtained in a steel component produced by performing a cold forging process and an aging heat treatment process on the steel material as a starting material.Shape and Uses of Steel Material of Present Embodiment

[0310] The steel material of the present embodiment is, for example, a steel material in which a cross section perpendicular to the axial direction is a circular shape, and for example is a steel bar or a wire rod. The steel material of the present embodiment can be widely applied to applications that require cold forgeability and fatigue strength. In particular, the steel material of the present embodiment can be applied as a starting material for steel components to be produced by performing a cold forging process and an aging heat treatment process on the steel material as a starting material. However, the steel material of the present embodiment is not limited to the applications described above.Production Method

[0311] An example of a method for producing the steel material of the present embodiment will now be described. The method for producing the steel material described hereunder is one example for producing the steel material of the present embodiment. Accordingly, a steel material satisfying feature 1 to feature 3 may also be produced by a production method other than the production method described hereunder. However, the production method described hereunder is a preferable example of a method for producing the steel material of the present embodiment.

[0312] The method for producing the steel material of the present embodiment includes the following processes.

[0313] (Process 1) Starting material preparation process

[0314] (Process 2) Hot working process

[0315] Each of these processes is described hereunder.[Starting Material Preparation Process]

[0316] In the starting material preparation process, a starting material satisfying feature 1 is produced by the following method. Specifically, a molten steel that satisfies feature 1 is produced using a converter and an electric furnace or the like. The molten steel is used to produce a bloom by a continuous casting process. Alternatively, the molten steel is used to produce an ingot by an ingot-making process. A starting material (a bloom or an ingot) is prepared by the above process.[Hot Working Process]

[0317] In the hot working process, the starting material is subjected to hot working to produce a steel material. The hot working, for example, may be hot rolling or may be hot forging.

[0318] In a case where the hot working is hot rolling, the hot working process, for example, includes a rough rolling process of subjecting the starting material (bloom or ingot) to rough rolling to obtain a billet, and a finish rolling process of subjecting the billet to finish rolling to obtain a steel material.

[0319] In the rough rolling process, for example, the following processes are performed. The starting material is heated, and thereafter is subjected to blooming using a blooming mill. As necessary, after blooming, the starting material is further subjected to rolling using a continuous mill to produce a billet. The heating temperature in the rough rolling process is a well-known temperature, and for example is 1050 to 1300° C.

[0320] In the finish rolling process, the billet is subjected to hot rolling to produce a steel material. In the finish rolling process, for example, the following processes are performed. The billet produced in the rough rolling process is charged into a heating furnace and heated. The heated billet is then used to perform finish rolling (hot rolling) with a finish rolling mill to obtain a steel material. The steel material after the finish rolling is cooled to obtain the steel material of the present embodiment. The heating temperature in the finish rolling process is a well-known temperature, and for example is 1050 to 1250° C. Further, when the steel material temperature (surface temperature of the steel material) on the exit side of the roll stand at which rolling was last performed in the finish rolling mill train in the finish rolling process is taken as the rolling finishing temperature (° C.), the finishing temperature is, for example, 900 to 1100° C.[Production Conditions in Hot Working Process]

[0321] In the hot working process, the following conditions are satisfied in a cooling process for cooling the steel material after the finish rolling.(Condition 1)

[0322] A holding time t at 900 to 800° C. after the finish rolling is to be 25 to 300 seconds.(Condition 2)

[0323] An average cooling rate CR from 800 to 600° C. is to be 0.6 to 4.0° C. / sec.

[0324] Each condition is described hereunder.[(Condition 1) Regarding Holding Time t]

[0325] The steel material is cooled after finish rolling of the steel material is completed, and at such time, nuclei of VCr precipitates form in the temperature range of 900 to 800° C. In other words, the temperature range of 900 to 800° C. after finish rolling is a VCr precipitates formation temperature range. Therefore, in order to make the number density of VCr precipitates an appropriate amount, the steel material is held for an appropriate holding time t in the temperature range of 900 to 800° C. (VCr precipitates formation temperature range) after the finish rolling. If the holding time t is less than 25 seconds, a sufficient amount of nuclei of VCr precipitates will not form. In such case, a sufficient VCr precipitates number density ND will not be obtained in the produced steel material. On the other hand, if the holding time t is more than 300 seconds, the nuclei of VCr precipitates will grow and coarsen, and as a result the number of formed nuclei will decrease. In such case, a sufficient VCr precipitates number density ND will not be obtained in the produced steel material.

[0326] If the holding time t is 25 to 300 seconds, the steel material will be held for an appropriate time in the temperature range of 900 to 800° C. (VCr precipitates formation temperature range). As a result, a sufficient amount of nuclei of VCr precipitates will form, and the VCr precipitates number density ND in the produced steel material will be 1000 to 5000 pieces / μm3.[(Condition 2) Regarding Average Cooling Rate CR]

[0327] If the average cooling rate CR in the temperature range of 800 to 600° C. after the holding time t elapses is less than 0.6° C. / sec, the cooling rate will be too slow. In such case, the VCr precipitates will coarsen. As a result, in the produced steel material, the VCr precipitates number density ND having an equivalent circular diameter of 5 to 100 nm will be less than 1000 pieces / μm3.

[0328] On the other hand, if the average cooling rate CR in the temperature range of 800 to 600° C. is more than 4.0° C. / sec, the cooling rate will be too fast. In such case, the formed amount of polygonal ferrite may be too small, the formed amount of bainite and / or pearlite may be too large, and martensite may excessively form or the like. In addition, the VCr precipitates will not be able to grow sufficiently, and as a result, the VCr precipitates number density ND having an equivalent circular diameter of 5 to 100 nm will be less than 1000 pieces / μm3 in the produced steel material.

[0329] If the average cooling rate CR is 0.6 to 4.0° C. / sec, an appropriate amount of polygonal ferrite will form and an appropriate amount of bainite and / or pearlite will form, and formation of martensite can be sufficiently suppressed. As a result, in the microstructure of the produced steel material, the area fraction of polygonal ferrite will be 30 to 90%, the area fraction of martensite will be 5% or less, and the balance will be bainite and / or pearlite.

[0330] Further, the nuclei of VCr precipitates that form in the temperature range of 900 to 800° C. grow in the temperature range of 800 to 600° C. If the average cooling rate CR in the temperature range of 800 to 600° C. is 0.6 to 4.0° C. / sec, the VCr precipitates will grow to an appropriate size. Therefore, in the produced steel material, the VCr precipitates number density ND having an equivalent circular diameter of 5 to 100 nm will be 1000 to 5000 pieces / μm3.

[0331] In a case where the hot working is hot forging, the hot forging is performed on the starting material to produce a steel material. The heating temperature in the hot forging is 1050 to 1300° C. When the temperature at the time that the final process (pressing) in the hot forging is completed is taken as the finishing temperature, the finishing temperature is 900 to 1100° C. In the cooling after hot forging, the aforementioned condition 1 and condition 2 are satisfied. The steel material of the present embodiment is produced by the above production method.[Method for Producing Steel Component Using Steel Material of Present Embodiment as a Starting Material]

[0332] In the case of producing a steel component by performing a cold forging process and an aging heat treatment process on the steel material of the present embodiment as a starting material, it suffices to use a well-known production method as the method for producing the steel component. The method for producing the steel component includes a cold forging process and an aging heat treatment process.

[0333] In the cold forging process, the steel material is subjected to cold forging to produce an intermediate steel material. In a case where the steel material is a steel bar or a wire rod, wire drawing may be performed before the cold forging process. It suffices to perform the cold forging and wire drawing under well-known conditions.

[0334] In the aging heat treatment process, the intermediate steel material is subjected to a heat treatment. It suffices to adopt well-known conditions as the conditions for the heat treatment temperature and holding time in the aging heat treatment process. For example, the heat treatment temperature is 550 to 700° C., and the holding time at the heat treatment temperature is 20 to 120 minutes.EXAMPLES

[0335] Hereunder, the advantageous effects of one aspect of the steel material of the present embodiment are described more specifically by way of examples. The conditions adopted in the following examples are one example of conditions adopted for confirming the feasibility and advantageous effects of the steel material of the present embodiment. Accordingly, the steel material of the present embodiment is not limited to this one example of conditions.

[0336] Steel materials having the chemical compositions shown in Table 1A to Table 1D were produced by the following method.TABLE 1AChemical CompositionTest(unit is mass %; balance is Fe and impurities)NumberCSiMnPSAlCrVN 10.170.191.600.0160.0120.0210.110.220.006 20.140.201.500.0160.0120.0250.150.200.005 30.100.201.810.0160.0120.0240.120.250.005 40.240.211.250.0130.0250.0220.120.150.006 50.210.201.200.0150.0140.0300.210.220.007 60.070.281.790.0220.0190.0180.240.200.006 70.040.322.010.0120.0150.0310.250.290.005 80.050.441.800.0080.0120.0260.310.220.006 90.130.481.800.0140.0160.0240.130.160.004100.180.081.190.0160.0150.0290.210.280.006110.080.192.350.0160.0130.0300.090.120.005120.190.250.750.0150.0220.0250.260.330.009130.150.261.490.0320.0170.0220.190.200.012140.140.171.520.0140.0470.0320.200.190.006150.150.181.600.0130.0130.0490.210.220.008160.210.121.510.0150.0160.0080.200.250.005170.090.201.250.0150.0180.0310.680.250.006180.200.261.500.0140.0210.0260.060.220.010190.160.201.080.0130.0190.0270.240.390.011200.140.091.020.0110.0130.0250.520.380.009210.190.211.190.0140.0180.0300.150.110.006220.140.151.490.0160.0190.0260.180.240.014230.160.201.890.0120.0160.0290.160.200.004240.120.211.800.0150.0160.0310.150.220.006250.140.101.790.0140.0130.0230.110.120.005260.110.191.510.0120.0150.0280.200.250.006270.110.191.510.0120.0150.0280.200.250.006280.110.191.510.0120.0150.0280.200.250.006290.150.211.880.0130.0160.0280.180.210.005300.150.251.490.0160.0150.0280.210.190.006310.160.191.480.0150.0160.0250.150.270.005320.150.191.500.0140.0180.0240.140.200.005330.140.201.590.0120.0160.0320.150.230.011340.120.131.210.0170.0110.0370.150.190.007350.090.352.010.0130.0140.0330.150.170.005TABLE 1BTestNum-Chemical Composition (unit is mass %; balance is Fe and impurities)berNbTiBCuNiMoCaBiPbZrSeTeREMMgWSn 1———————————————— 2———————————————— 3———————————————— 4———————————————— 5———————————————— 6———————————————— 7———————————————— 8———————————————— 9————————————————10————————————————11————————————————12————————————————13————————————————14————————————————15————————————————16————————————————17————————————————18————————————————19————————————————20————————————————21————————————————22————————————————23————————————————24————————————————25————————————————26————————————————27————————————————28————————————————290.096———————————————30—0.047——————————————31——0.0096—————————————32———0.28————————————33————0.29———————————34—————0.28——————————35——————0.0048—————————TABLE 1CChemical CompositionTest(unit is mass %; balance is Fe and impurities)NumberCSiMnPSAlCrVN360.120.251.800.0130.0200.0200.200.220.009370.160.201.480.0160.0110.0160.190.250.012380.200.111.320.0140.0150.0260.200.210.005390.160.121.490.0140.0190.0270.250.160.006400.130.381.820.0150.0180.0260.210.270.007410.140.201.500.0150.0210.0290.160.200.006420.190.191.680.0130.0170.0310.140.200.006430.160.191.410.0160.0150.0310.140.190.005440.140.201.490.0140.0100.0340.150.210.007450.290.121.500.0140.0200.0280.200.150.006460.020.211.510.0150.0180.0290.150.190.007470.160.601.490.0150.0160.0280.160.190.006480.140.031.500.0160.0170.0300.150.200.006490.150.193.020.0130.0150.0250.150.180.005500.150.180.510.0160.0170.0270.140.210.005510.140.341.120.0140.0130.0260.890.200.005520.160.181.310.0150.0160.0250.030.190.004530.150.111.520.0130.0210.0250.150.500.006540.160.191.700.0150.0190.0230.160.050.004550.120.211.800.0150.0160.0310.150.220.006560.120.211.800.0150.0160.0310.150.220.006570.090.251.500.0130.0120.0240.090.150.005580.140.101.790.0140.0130.0230.110.120.005590.110.191.510.0120.0150.0280.200.250.006600.120.211.800.0150.0160.0310.150.220.006610.140.101.790.0140.0130.0230.110.120.005620.110.191.510.0120.0150.0280.200.250.006630.120.211.800.0150.0160.0310.150.220.006640.090.251.500.0130.0120.0240.090.150.005650.140.101.790.0140.0130.0230.110.120.005660.110.191.510.0120.0150.0280.200.250.006670.100.211.490.0130.0120.0220.190.220.007680.110.191.510.0120.0150.0280.200.250.006690.100.211.490.0130.0120.0220.190.220.007TABLE 1DTestNum-Chemical Composition (unit is mass %; balance is Fe and impurities)berNbTiBCuNiMoCaBiPbZrSeTeREMMgWSn36———————0.096————————37————————0.088———————38—————————0.009——————39——————————0.010—————40———————————0.008————41————————————0.009———42—————————————0.0049——43——————————————0.048—44———————————————0.09045————————————————46————————————————47————————————————48————————————————49————————————————50————————————————51————————————————52————————————————53————————————————54————————————————55————————————————56————————————————57————0.29———————————58————————————————59————————————————60————————————————61————————————————62————————————————63————————————————64————0.29———————————65————————————————66————————————————670.010——0.130.080.01——————————68————————————————690.010——0.130.080.01——————————The symbol “-” in the column “Chemical Composition” in Table 1B and Table 1D means that the content of the corresponding element was at the level of an impurity or less. Note that, in the steel of each test number shown in the tables, the content of O was 0.0040% or less.Molten steels of each test number were produced by vacuum melting, and 50 kg ingots were produced. Each of the produced ingots was used to produce a steel material. Specifically, each ingot was subjected to hot working (hot forging). The heating temperature for the hot forging was 1200° C., and the finishing temperature was 1000° C. In a cooling process for cooling the steel material after the hot forging, a holding time t (secs) in the temperature range of 900 to 800° C., and an average cooling rate CR (° C. / sec) in the temperature range of 800 to 600° C. were as shown in Table 2A and Table 2B. Steel materials (round bar material) with a diameter of 24 mm were produced by the above production process.TABLE 2AMicrostructurePolygonalVCrSteelAverageFerrite MartensitePrecipitatesComponentHoldingCoolingAreaAreaNumberVickersTest Time tRate CRFractionFractionDensity NDColdHardnessNumber(secs)(° C. / s)(%)(%)Balance(pieces / μm3)Forgeability(HV)Remarks 1801.2480521660E294Inventive Example 2801.2480522570E286Inventive Example 3801.2520482040E280Inventive Example 4801.2362621420E308Inventive Example 5801.2650352840E312Inventive Example 6801.2560441560E286Inventive Example 7801.2520481210E270Inventive Example 8801.2580421060E272Inventive Example 9801.2460541580E302Inventive Example10801.2401593880E278Inventive Example11801.2350651280E306Inventive Example12801.2580423580E269Inventive Example13801.2460542240E280Inventive Example14801.2510492090E273Inventive Example15801.2553422670E284Inventive Example16801.2490512840E299Inventive Example17801.2640361750E307Inventive Example18801.2530473000E284Inventive Example19801.2720284280E318Inventive Example20801.2670334910E310Inventive Example21801.2520481470E268Inventive Example22801.2630372480E272Inventive Example23801.2421572020E276Inventive Example24800.8540461290E274Inventive Example25800.8820181420E270Inventive Example26804.0400601250E289Inventive Example27801.8620383110E295Inventive Example28800.6790212230E274Inventive Example29801.2480522440E285Inventive Example30801.2680322380E278Inventive Example31801.2580422440E310Inventive Example32801.2382601580E286Inventive Example33801.2401591740E288Inventive Example34801.2332651490E304Inventive Example35801.2450551880E291Inventive ExampleTABLE 2BMicrostructurePolygonalVCrSteelAverageFerriteMartensitePrecipitatesComponentHoldingCoolingAreaAreaNumberVickersTest Time tRate CRFractionFractionDensity NDColdHardnessNumber(secs)(° C. / s)(%)(%)Balance(pieces / μm3)Forgeability(HV)Remarks36801.2480522000E278Inventive Example37801.2442542110E286Inventive Example38801.2523452360E303Inventive Example39801.2560442240E288Inventive Example40801.2510491900E280Inventive Example41801.2570431770E276Inventive Example42801.2530472740E284Inventive Example43801.2381611550E294Inventive Example44801.2440561720E281Inventive Example45801.2324644890B—Comparative Example46801.29604360E116Comparative Example47801.2525431420B—Comparative Example48801.2780221600E233Comparative Example49801.21512731940B—Comparative Example50801.2760242040E242Comparative Example51801.2263712250B—Comparative Example52801.265035460E239Comparative Example53801.2620383680B—Comparative Example54801.255045240E166Comparative Example55800.262038650E230Comparative Example56800.182018520E180Comparative Example57800.275025620E192Comparative Example58800.487013850E198Comparative Example59800.288012740E215Comparative Example608020.008614310B—Comparative Example618015.0123256290B—Comparative Example62808.018379540B—Comparative Example63806.034264820E252Comparative Example64805.040060850E240Comparative Example65806.034462490E262Comparative Example66101.268032670E254Comparative Example67101.258042520E260Comparative Example686001.275025340E232Comparative Example696001.266034280E249Comparative Example[Evaluation Tests]The following evaluation tests were carried out on the round bar material (steel material) serving as a starting material for a steel near-net-shape material of each test number, and on a steel near-net-shape material of each test number.(Test 1) Microstructure observation test(Test 2) VCr precipitates number density ND measurement test(Test 3) Cold forgeability evaluation test

[0343] (Test 4) Test to measure Vickers hardness of steel component

[0344] Each test is described hereunder.[(Test 1) Microstructure Observation Test]

[0345] The structures in the microstructure of the steel material of each test number were identified in accordance with the method described above in the section [Method for measuring area fraction of each structure in microstructure]. As a result, in each test number, polygonal ferrite and bainite and / or pearlite were confirmed in the microstructure. In addition, martensite was also confirmed in some test numbers. Furthermore, in each test number, the area fraction (%) of polygonal ferrite, area fraction (%) of pearlite, area fraction (%) of bainite, and area fraction (%) of martensite were determined. As the result of the test, it was found that in the steel material of each test number, in the microstructure, a structure other than polygonal ferrite and martensite was bainite and / or pearlite. The area fraction (%) of polygonal ferrite, area fraction (%) of martensite, and total area fraction (%) of the balance (bainite and pearlite) are shown in Table 2A and Table 2B.[(Test 2) VCr Precipitates Number Density ND Measurement Test]

[0346] The VCr precipitates number density ND (pieces / μm3) in the steel material of each test number was determined in accordance with the method described above in the section [Method for measuring VCr precipitates number density ND]. The VCr precipitates number density ND (pieces / μm3) of each test number is shown in Table 2A and Table 2B.[(Test 3) Cold Forgeability Evaluation Test]

[0347] A plurality of cylindrical test specimens of 14 mm in diameter and 21 mm in length were taken from the steel material of each test number. The central axis of each cylindrical test specimen was coaxial with the central axis of the steel material. A compression test (cold forging) was conducted at room temperature (25° C.) using the cylindrical test specimens. Specifically, a compression test was conducted until a working ratio defined by Formula (1) became 70%. The compression test was conducted on five cylindrical test specimens for each test number. The presence or absence of a crack in the cylindrical test specimens after the test was confirmed. In general, the working ratio of cold forging processing is 70% or less. Therefore, if a crack was not generated in the steel material by the compression test at a working ratio of 70%, it was determined that excellent cold forgeability was obtained.Working ratio=(1−(length of cylindrical test specimen after compression test / length of cylindrical test specimen before compression test))×100  (1)

[0348] The presence or absence of a crack in the steel material after conducting the compression test at a working ratio of 70% was confirmed by the following method. The presence or absence of a crack in the five cylindrical test specimens after the compression test was confirmed by observation using a magnifying glass with a magnification of ×5. If a crack of 0.5 mm or more in length was not observed in any of the five cylindrical test specimens, it was determined that a crack did not occur and excellent cold forgeability was obtained (indicated by “E (Excellent)” in the column “Cold Forgeability” in Table 2A and Table 2B). On the other hand, if a crack of 0.5 mm or more in length was confirmed in any one or more of the five cylindrical test specimens, it was determined that sufficient cold forgeability was not obtained (indicated by “B (Bad)” in the column “Cold Forgeability” in Table 2A and Table 2B). Note that, for those test numbers in which sufficient cold forgeability was not obtained, Test 4 was not performed (indicated by “-” in the column “Steel Component Vickers Hardness (HV)” in Table 2A and Table 2B).[(Test 4) Test to Measure Vickers Hardness of Steel Component]

[0349] The Vickers hardness of a steel component obtained by performing a cold forging process and an aging heat treatment process on the steel material of each test number as a starting material was determined by the following method.

[0350] A steel material with a diameter of 24 mm of each test number was subjected to cold drawing that simulated a cold forging process by the following method. A test specimen for cold drawing of 20 mm in diameter and in which the diameter of a pointed part was 16 mm was prepared from the steel material. The central axis of the test specimen for cold drawing was coaxial with the central axis of the steel material. The test specimen for cold drawing was subjected to a bonderizing treatment to form a zinc phosphate coating film for lubrication on the surface of the test specimen. Thereafter, the test specimen for cold drawing was subjected to cold drawing using a die with a diameter of 17 mm. The cold drawing was performed in one pass, and the reduction of area was 28%. An aging heat treatment was performed on the test specimen after the cold drawing. The heat treatment temperature in the aging heat treatment was set to 600° C., and the holding time at the heat treatment temperature was set to 1 hour (60 minutes). Steel components for which the steel materials of the respective test numbers were used as starting materials were produced by the above production process.

[0351] As a simple evaluation of the fatigue strength of each steel component, the Vickers hardness, which has a positive correlation with fatigue strength, was measured. Specifically, for each test number, a test specimen was taken in which a cross section perpendicular to the central axis of the steel component was adopted as the test surface. The test surface of the test specimen was subjected to a Vickers hardness test in accordance with JIS Z 2244-1:2020. The test force was set to 9.8 N. An arbitrary three points near the central axis of the test surface were adopted as measurement positions. The arithmetic average value of the values obtained at the three measurement positions was defined as the Vickers hardness (Hv) of the steel component of the relevant test number. The obtained Vickers hardness (Hv) of each steel component is shown in Table 2A and Table 2B. If the Vickers hardness was 265 HV or more, the relevant test number was regarded as having excellent fatigue strength. If the Vickers hardness was less than 265 HV, the relevant test number was regarded as not having sufficient fatigue strength.[Test Results]

[0352] The test results are shown in Table 1A to Table 1D, Table 2A, and Table 2B. The steel materials of Test Nos. 1 to 44 satisfied feature 1 to feature 3. Therefore, excellent cold forgeability was obtained. In addition, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was 265 HV or more and thus excellent fatigue strength was obtained.

[0353] On the other hand, in Test No. 45, the content of C was too high. Therefore, sufficient cold forgeability was not obtained.

[0354] In Test No. 46, the content of C was too low. Consequently, the area fraction of polygonal ferrite was too high. In addition, the number density of VCr precipitates was too low. Therefore, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was less than 265 HV and thus sufficient fatigue strength was not obtained.

[0355] In Test No. 47, the content of Si was too high. Therefore, sufficient cold forgeability was not obtained.

[0356] In Test No. 48, the content of Si was too low. Therefore, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was less than 265 HV and thus sufficient fatigue strength was not obtained.

[0357] In Test No. 49, the content of Mn was too high. Consequently, the area fraction of polygonal ferrite was less than 30%, and the area fraction of martensite was more than 5%. Therefore, sufficient cold forgeability was not obtained.

[0358] In Test No. 50, the content of Mn was too low. Therefore, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was less than 265 HV and thus sufficient fatigue strength was not obtained.

[0359] In Test No. 51, the content of Cr was too high. Consequently, the area fraction of polygonal ferrite was less than 30%. Therefore, sufficient cold forgeability was not obtained.

[0360] In Test No. 52, the content of Cr was too low. Consequently, the number density of VCr precipitates was too low. Therefore, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was less than 265 HV and thus sufficient fatigue strength was not obtained. In Test No. 53, the content of V was too high. Therefore, sufficient cold forgeability was not obtained.

[0361] In Test No. 54, the content of V was too low. Consequently, the number density of VCr precipitates was too low. Therefore, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was less than 265 HV and thus sufficient fatigue strength was not obtained.

[0362] In Test Nos. 55 to 59, the average cooling rate CR from 800 to 600° C. after finish rolling was too slow. As a result, the number density of VCr precipitates was too low. Therefore, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was less than 265 HV and thus sufficient fatigue strength was not obtained.

[0363] In Test Nos. 60 to 62, the average cooling rate CR from 800 to 600° C. after finish rolling was too fast. Therefore, in Test Nos. 60 to 62, the area fraction of polygonal ferrite was too low, and in Test Nos. 60 and 61, in addition, the area fraction of martensite was too high. As a result, sufficient cold forgeability was not obtained in these test numbers. Furthermore, in these test numbers, the number density of VCr precipitates was too low.

[0364] In Test Nos. 63 to 65, the average cooling rate CR from 800 to 600° C. after finish rolling was too fast, although it was not as fast as in Test Nos. 60 to 62. Consequently, in Test Nos. 63 to 65, the number density of VCr precipitates was too low. Therefore, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was less than 265 HV and thus sufficient fatigue strength was not obtained.

[0365] In Test Nos. 66 and 67, the holding time t in the temperature range of 900 to 800° C. after finish rolling was too short. Consequently, in these test numbers, the number density of VCr precipitates was too low. Therefore, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was less than 265 HV and thus sufficient fatigue strength was not obtained.

[0366] In Test Nos. 68 and 69, the holding time t in the temperature range of 900 to 800° C. after finish rolling was too long. Consequently, in these test numbers, the number density of VCr precipitates was too low. Therefore, in the steel component obtained by performing cold forging and an aging heat treatment, the Vickers hardness was less than 265 HV and thus sufficient fatigue strength was not obtained.

[0367] An embodiment of the present disclosure has been described above. However, the embodiment described above is merely an example for carrying out the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment within a range that does not depart from the gist of the present disclosure.

Examples

examples

[0335]Hereunder, the advantageous effects of one aspect of the steel material of the present embodiment are described more specifically by way of examples. The conditions adopted in the following examples are one example of conditions adopted for confirming the feasibility and advantageous effects of the steel material of the present embodiment. Accordingly, the steel material of the present embodiment is not limited to this one example of conditions.

[0336]Steel materials having the chemical compositions shown in Table 1A to Table 1D were produced by the following method.

TABLE 1AChemical CompositionTest(unit is mass %; balance is Fe and impurities)NumberCSiMnPSAlCrVN 10.170.191.600.0160.0120.0210.110.220.006 20.140.201.500.0160.0120.0250.150.200.005 30.100.201.810.0160.0120.0240.120.250.005 40.240.211.250.0130.0250.0220.120.150.006 50.210.201.200.0150.0140.0300.210.220.007 60.070.281.790.0220.0190.0180.240.200.006 70.040.322.010.0120.0150.0310.250.290.005 80.050.441.800.0080.0120.02...

Claims

1. A steel material that has a chemical composition consisting of, in mass %,C: 0.03 to 0.25%,Si: 0.05 to 0.50%,Mn: more than 0.70 to 2.50%,P: 0.035% or less,S: 0.050% or less,Al: 0.001 to 0.100%,Cr: 0.05 to 0.70%,V: more than 0.10 to 0.40%,N: 0.002 to 0.015%,Nb: 0 to 0.100%,Ti: 0 to 0.050%,B: 0 to 0.0100%,Cu: 0 to 0.30%,Ni: 0 to 0.30%,Mo: 0 to 0.30%,Ca: 0 to 0.0050%,Bi: 0 to 0.100%,Pb: 0 to 0.090%,Zr: 0 to 0.010%,Se: 0 to 0.010%,Te: 0 to 0.010%,rare earth metal: 0 to 0.010%,Mg: 0 to 0.0050%,W: 0 to 0.050%, andSn: 0 to 0.100%,with the balance being Fe and impurities, andhas a microstructure in which an area fraction of polygonal ferrite is 30 to 90%, an area fraction of martensite is 5% or less, and the balance is bainite and / or pearlite, whereinin the polygonal ferrite, a number density of VCr precipitates, which are precipitates containing V and Cr, having an equivalent circular diameter of 5 to 100 nm is 1000 to 5000 pieces / μm3.

2. The steel material according to claim 1, wherein the chemical composition contains one or more kinds of element selected from a group consisting of, in mass %:Nb: 0.001 to 0.100%,Ti: 0.001 to 0.050%,B: 0.0001 to 0.0100%,Cu: 0.01 to 0.30%,Ni: 0.01 to 0.30%,Mo: 0.01 to 0.30%,Ca: 0.0001 to 0.0050%,Bi: 0.001 to 0.100%,Pb: 0.001 to 0.090%,Zr: 0.001 to 0.010%,Se: 0.001 to 0.010%,Te: 0.001 to 0.010%,rare earth metal: 0.001 to 0.010%,Mg: 0.0001 to 0.0050%,W: 0.001 to 0.050%, andSn: 0.001 to 0.100%.