steel

Abstract

The present invention provides a steel material that can suppress the occurrence of cracks during cold forging, even when cold forging is performed without spheroidizing annealing. [Solution] The steel material of this embodiment has a chemical composition in mass%, consisting of C: 0.30-0.60%, Si: 0.01-0.10%, Mn: 0.30-2.00%, P: less than 0.050%, S: 0.001-0.050%, Al: 0.001-0.200%, N: less than 0.020%, Cr: 0.01-0.40%, O: 0.0030% or less, and the remainder being Fe and impurities. The total area ratio of ferrite and perlite is 90% or more, the area ratio of perlite is 40% or more, the average particle size dP of perlite grains is 10.0 to 40.0 μm, the average particle size dP of perlite grains and the standard deviation σP of perlite grain size satisfy equation (1), the average particle size dF of ferrite grains is 25.0 μm or less, and the difference between the hardness of ferrite and the hardness of perlite is 140 HV or less. σP / dP<0.80 (1)

Description

Technical Field

[0001] The present disclosure relates to steel materials, and more particularly to steel materials suitable as materials for steel parts manufactured by cold forging.

Background Art

[0002] Steel parts for mechanical structural applications typified by automotive parts, industrial machine parts, and construction machine parts are manufactured by plastic working of steel materials as the material. The plastic working mainly includes hot forging and cold forging. In hot forging, the steel material is forged in a state where it is heated to a high-temperature austenite temperature range. At high temperatures, the strength of the steel material decreases. Therefore, in hot forging, compared with cold forging, a large plastic deformation can be imparted to the steel material with a lower load. On the other hand, in the process of cooling the steel material from a high temperature to room temperature after plastic deformation, the steel material thermally shrinks and changes its shape due to a phase transformation. Therefore, the dimensional accuracy of the steel material (intermediate part) after hot forging is low. As a result, the amount of steel material removed by machining in the next process increases, and the cost increases.

[0003] On the other hand, in cold forging, the steel material is forged at room temperature. In cold forging, thermal strain is unlikely to occur. Therefore, the shape of the steel material (intermediate part) after cold forging can be made closer to the final part. Therefore, the cost of machining can be suppressed.

[0004] However, in cold forging, since the steel material is forged at room temperature, it is necessary to suppress the occurrence of cracks in the steel material during cold forging. That is, excellent cold forging properties are required for the steel material. In order to obtain excellent cold forging properties, usually, spheroidizing annealing is performed on the steel material before cold forging. In spheroidizing annealing, the steel material is held in a temperature range just below the A c1 point for a long time and then slowly cooled. As a result, the lamellar cementite in the pearlite of the steel material is segmented, and the cementite becomes spheroidized. Therefore, the hardness of the steel material decreases, and the cold forging property increases.

[0005] However, spheroidizing annealing increases manufacturing costs. Therefore, in order to reduce manufacturing costs associated with reducing man-hours, there is a need for steel materials that suppress crack formation during cold forging even when spheroidizing annealing is omitted.

[0006] The steel material disclosed in Japanese Patent Publication No. 2005-133153 (Patent Document 1) contains, by mass%, C: 0.05~0.3%, Si: 0.01~0.35%, Mn: 0.2~2%, P: 0.001~0.02%, S: 0.001~0.02%, and also contains at least one selected from Ni: 0.01~2%, Cr: 0.01~2%, and Mo: 0.01~0.5%, and further contains Ti: 0.005~0.02%, Al: 0.020~0.1%, and N: 0.005~0.02%, with the remainder being Fe and unavoidable impurities, and further satisfies Al / (N-Ti / 3.4)≧4 for Ti, Al, and N. Patent Document 1 states that this steel material has excellent cold forging properties by satisfying the above formula. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2005-133153 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, even if cold forging is performed by means other than those disclosed in Patent Document 1, spheroidizing annealing may be omitted, and the occurrence of cracks during cold forging may still be suppressed.

[0009] The purpose of this disclosure is to provide a steel material that can suppress the occurrence of cracks during cold forging, even when cold forging is performed without spheroidizing annealing. [Means for solving the problem]

[0010] The steel material disclosed herein has a chemical composition, in mass%, of C: 0.30-0.60%, Si: 0.01-0.10%, Mn: 0.30-2.00%, P: less than 0.050%, S: 0.001-0.050%, Al: 0.001-0.200%, N: less than 0.020%, Cr: 0.01-0.40%, O: 0.0030% or less, with the remainder being Fe and impurities. The total area ratio of ferrite and pearlite is 90% or more, and the area ratio of pearlite is 40% or more. The average particle size dP of pearlite grains is 10.0-40.0 μm. The average particle size dP of pearlite grains and the standard deviation σP of the particle size of pearlite grains satisfy equation (1). The average particle size dF of ferrite grains is 25.0 μm or less. The difference between the hardness of ferrite and the hardness of pearlite is 140 HV or less. σP / dP<0.80 (1)

[0011] The steel material of this disclosure has a chemical composition, in mass%, containing C: 0.30-0.60%, Si: 0.01-0.10%, Mn: 0.30-2.00%, P: less than 0.050%, S: 0.001-0.050%, Al: 0.001-0.200%, N: less than 0.020%, Cr: 0.01-0.40%, and O: 0.0030% or less, and further containing one or more selected from the groups consisting of Group 1 and Group 2, with the remainder being Fe and impurities. [Group 1] One or more elements selected from the group consisting of Mo: 0.20% or less, V: 0.150% or less, Nb: 0.050% or less, Ti: 0.050% or less, Cu: 0.40% or less, Ni: 0.40% or less, Sn: 0.100% or less, and B: 0.0050% or less, in mass percent. [Group 2] One or two selected from the group consisting of Ca: 0.0050% or less and Mg: 0.0050% or less. The steel material of this disclosure further has a total area ratio of ferrite and pearlite of 90% or more, and a pearlite area ratio of 40% or more. The average particle size dP of pearlite grains is 10.0 to 40.0 μm. The average particle size dP of pearlite grains and the standard deviation σP of the pearlite grain size satisfy equation (1). The average particle size dF of ferrite grains is 25.0 μm or less. The difference between the hardness of ferrite and the hardness of pearlite is 140 HV or less. σP / dP<0.80 (1) [Effects of the Invention]

[0012] With the steel material disclosed herein, even if spheroidizing annealing is omitted and cold forging is performed, the occurrence of cracks during cold forging can be suppressed. [Modes for carrying out the invention]

[0013] The inventors first investigated steel materials suitable for cold forging applications from the perspective of chemical composition. As a result, in mass%, the following composition was found: C: 0.30-0.60%, Si: 0.01-0.10%, Mn: 0.30-2.00%, P: less than 0.050%, S: 0.001-0.050%, Al: 0.001-0.200%, N: less than 0.020%, Cr: 0.01-0.40%, O: 0.0030% or less, Mo: 0-0.20%, V: 0-0. The inventors believe that a chemical composition consisting of 150%, Nb: 0-0.050%, Ti: 0-0.050%, Cu: 0-0.40%, Ni: 0-0.40%, Sn: 0-0.100%, B: 0-0.0050%, Ca: 0-0.0050%, Mg: 0-0.0050%, and the remainder being Fe and impurities, is suitable for cold forging applications.

[0014] Therefore, the inventors further investigated, from the perspective of microstructure, a means to sufficiently suppress the occurrence of cracks even when cold forging is performed without spheroidizing annealing when manufacturing steel parts by cold forging using steel materials having the above-mentioned chemical composition as the raw material.

[0015] In the steel manufacturing process, it is possible to soften steel to a degree equivalent to that achieved by spheroidizing annealing by adjusting the rolling temperature and cooling rate. However, even softened steel can crack during cold forging. Therefore, the inventors investigated the mechanism of cracking during cold forging. As a result, the following was discovered.

[0016] Cracking during cold forging is caused by ductile fracture. In ductile fracture, voids are formed in areas of the steel where there are local differences in strain distribution. These voids expand, or multiple voids connect with each other, causing the crack to propagate. Repeated expansion and connection of these cracks leads to fracture.

[0017] In the case of steel materials with the chemical composition described above, a microstructure consisting mainly of ferrite and pearlite is formed. Generally, pearlite is harder than ferrite. When cold forging is performed on such steel materials, strain concentrates near the interface between ferrite grains and pearlite grains, causing localized stress concentration in the ferrite. As a result, it is thought that the ferrite deforms excessively, causing voids to form. Furthermore, if there is a large variation in the pearlite grains, excessive deformation occurs locally in some of the ferrite. In this case as well, it is thought that voids will form.

[0018] Taking the above into consideration, the inventors conducted further investigations. As a result, the inventors found that by increasing the pearlite area ratio in the microstructure of steel with the above chemical composition, deformation during cold forging becomes predominantly pearlite-based, suppressing excessive deformation in ferrite. Furthermore, the inventors found that excessive deformation in ferrite can also be suppressed by suppressing the variation in pearlite grains.

[0019] Furthermore, as mentioned above, the generation of voids during cold forging is also due to the large difference in hardness ΔHV between pearlite and ferrite. Therefore, the inventors of this invention considered that the generation of voids during cold forging could be further suppressed by reducing the difference in hardness between pearlite and ferrite.

[0020] Based on the above findings, the inventors further investigated. As a result, in the steel material having the above chemical composition, the total area ratio of ferrite and pearlite is 90% or more, the pearlite area ratio is 40% or more, the average grain size dP of pearlite grains is 10.0 to 40.0 μm, the average grain size dP of pearlite grains and the standard deviation σP of the grain size of pearlite grains satisfy the formula (1), the average grain size dF of ferrite grains is 25.0 μm or less, and the difference between the hardness of ferrite and the hardness of pearlite is 140 HV or less. It was found that even if spheroidizing annealing is omitted and cold forging is carried out, the occurrence of cracks can be sufficiently suppressed. σP / dP < 0.80 (1)

[0021] The steel material of the present embodiment is completed according to the above technical idea and has the following configuration.

[0022] The steel material of the first configuration has a chemical composition in mass %, C: 0.30 to 0.60%, Si: 0.01 to 0.10%, Mn: 0.30 to 2.00%, P: less than 0.050%, S: 0.001 to 0.050%, Al: 0.001 to 0.200%, N: less than 0.020%, Cr: 0.01 to 0.40%, O: 0.0030% or less, and the balance consists of Fe and impurities. The total area ratio of ferrite and pearlite is 90% or more, and the pearlite area ratio is 40% or more. The average grain size dP of pearlite grains is 10.0 to 40.0 μm. The average grain size dP of pearlite grains and the standard deviation σP of the grain size of pearlite grains satisfy the formula (1). The average grain size dF of ferrite grains is 25.0 μm or less. The difference between the hardness of ferrite and the hardness of pearlite is 140 HV or less. σP / dP < 0.80 (1)

[0023] The steel material of the second composition has a chemical composition, in mass%, of C: 0.30-0.60%, Si: 0.01-0.10%, Mn: 0.30-2.00%, P: less than 0.050%, S: 0.001-0.050%, Al: 0.001-0.200%, N: less than 0.020%, Cr: 0.01-0.40%, and O: 0.0030% or less, and further contains one or more selected from the group consisting of the first and second groups, with the remainder being Fe and impurities. [Group 1] One or more elements selected from the group consisting of Mo: 0.20% or less, V: 0.150% or less, Nb: 0.050% or less, Ti: 0.050% or less, Cu: 0.40% or less, Ni: 0.40% or less, Sn: 0.100% or less, and B: 0.0050% or less, in mass percent. [Group 2] One or two selected from the group consisting of Ca: 0.0050% or less and Mg: 0.0050% or less. The steel material of this disclosure further has a total area ratio of ferrite and pearlite of 90% or more, and a pearlite area ratio of 40% or more. The average particle size dP of pearlite grains is 10.0 to 40.0 μm. The average particle size dP of pearlite grains and the standard deviation σP of the pearlite grain size satisfy equation (1). The average particle size dF of ferrite grains is 25.0 μm or less. The difference between the hardness of ferrite and the hardness of pearlite is 140 HV or less. σP / dP<0.80 (1)

[0024] The third component of the steel material is the same as the second component of the steel material, and its chemical composition contains the first group.

[0025] The fourth component of the steel material is the second or third component of the steel material, and its chemical composition contains the second group.

[0026] The steel material of this embodiment will be described below. In the following description, unless otherwise specified, percentages for elements refer to mass percentages.

[0027] [Features of the steel material of this embodiment] The steel material of this embodiment satisfies the following characteristics. (Feature 1) The chemical composition, in mass%, is as follows: C: 0.30-0.60%, Si: 0.01-0.10%, Mn: 0.30-2.00%, P: less than 0.050%, S: 0.001-0.050%, Al: 0.001-0.200%, N: less than 0.020%, Cr: 0.01-0.40%, O: 0.0030% or less, Mo The composition is as follows: 0-0.20%, V: 0-0.150%, Nb: 0-0.050%, Ti: 0-0.050%, Cu: 0-0.40%, Ni: 0-0.40%, Sn: 0-0.100%, B: 0-0.0050%, Ca: 0-0.0050%, Mg: 0-0.0050%, and the remainder consists of Fe and impurities. (Feature 2) The total area ratio of ferrite and pearlite is 90% or more, and the area ratio of pearlite is 40% or more. (Feature 3) The average particle size dP of the perlite grains is 10.0 to 40.0 μm, and the average particle size dP of the perlite grains and the standard deviation σP of the particle size of the perlite grains satisfy equation (1). σP / dP<0.80 (1) (Feature 4) The average particle size dF of the ferrite grains is 25.0 μm or less. (Feature 5) The difference in hardness between ferrite and pearlite is 140 HV or less. Features 1 through 5 are explained below.

[0028] [(Feature 1) Regarding chemical composition] The chemical composition of the steel material in this embodiment contains the following elements:

[0029] C: 0.30~0.60% Carbon (C) enhances the hardenability of steel, thereby increasing the strength of steel parts manufactured using steel as a material. If the C content is less than 0.30%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the carbon content exceeds 0.60%, the strength of the steel material will increase excessively, even if the content of other elements is within the range of this embodiment. As a result, the cold forgeability of the steel material will decrease. Therefore, the C content is 0.30-0.60%. The preferred lower limit for the C content is 0.32%, more preferably 0.34%, and even more preferably 0.36%. The preferred upper limit for the C content is 0.55%, more preferably 0.50%, and even more preferably 0.45%.

[0030] Si: 0.01~0.10% Silicon (Si) deoxidizes steel during the steelmaking process in the manufacturing of steel materials. If the Si content is less than 0.01%, the above effect cannot be fully obtained. On the other hand, if the Si content exceeds 0.10%, the strength of the steel material will increase excessively, even if the content of other elements is within the range of this embodiment. As a result, the cold forgeability of the steel material will decrease. Therefore, the Si content is 0.01 to 0.10%. The preferred lower limit for the Si content is 0.02%, and more preferably 0.03%. The preferred upper limit for the Si content is 0.09%, and more preferably 0.08%.

[0031] Mn: 0.30~2.00% Manganese (Mn) is A c3 The point is reduced to increase the amount of pearlite produced. Mn further enhances the strength of ferrite through solid solution strengthening. If the Mn content is less than 0.30%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content exceeds 2.00%, coarse Mn sulfides are excessively produced. As a result, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel material decreases. Therefore, the Mn content is 0.30-2.00%. The preferred lower limit of the Mn content is 0.35%, more preferably 0.40%, even more preferably 0.45%, and even more preferably 0.50%. The preferred upper limit for the Mn content is 1.90%, more preferably 1.80%, and even more preferably 1.70%.

[0032] P: Less than 0.050% Phosphorus (P) is an unavoidable impurity. In other words, the P content is greater than 0%. If the P content is 0.050% or more, even if the content of other elements is within the range of this embodiment, P will excessively segregate at the grain boundaries, reducing the grain boundary strength. As a result, the cold forgeability of the steel material will decrease. Therefore, the P content is less than 0.050%. A low phosphorus (P) content is preferable. However, excessively reducing the P content increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit for the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the P content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0033] S: 0.001~0.050% Sulfur (S) combines with manganese (Mn) to form Mn sulfides. Mn sulfides improve the machinability of steel. If the S content is less than 0.001%, the above effect cannot be fully obtained. On the other hand, if the sulfur content exceeds 0.050%, even if the content of other elements is within the range of this embodiment, excessive coarse Mn sulfides will be formed. These coarse Mn sulfides become the starting point for cracks during cold forging. As a result, the cold forgeability of the steel material decreases. Therefore, the sulfur content is between 0.001% and 0.050%. The preferred lower limit for the S content is 0.002%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit for the S content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0034] Al: 0.001~0.200% Aluminum (Al) deoxidizes steel. If the Al content is less than 0.001%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Al content exceeds 0.200%, coarse Al-based inclusions will form even if the content of other elements is within the range of this embodiment. These coarse Al-based inclusions become the starting points for cracks during cold forging. As a result, the cold forgeability of the steel material decreases. Therefore, the Al content is between 0.001% and 0.200%. The preferred lower limit for the Al content is 0.003%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Al content is 0.150%, more preferably 0.100%, and even more preferably 0.080%. In this embodiment, the Al content refers to the total Al (Total-Al) content.

[0035] N: Less than 0.020% Nitrogen (N) is an unavoidable impurity. In other words, the N content is greater than 0%. N combines with Al, Ti, B, etc. to form nitrides. If the N content is 0.020% or more, even if the content of other elements is within the range of this embodiment, an excess of coarse nitrides will be formed. Coarse nitrides become the starting point for cracks during cold forging. Therefore, the cold forgeability of the steel material decreases. Therefore, the N content is less than 0.020%. A low nitrogen (N) content is preferable. However, excessively reducing the N content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit for the N content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the N content is 0.018%, more preferably 0.016%, and even more preferably 0.014%.

[0036] Cr: 0.01~0.40% Chromium (Cr) enhances the hardenability of steel and increases the hardness of steel parts after quenching. If the Cr content is less than 0.01%, the above effects cannot be fully obtained. On the other hand, if the chromium content exceeds 0.40%, the lamellar spacing in pearlite becomes excessively narrow. In this case, the hardness of pearlite increases excessively, and the hardness difference between pearlite and ferrite becomes excessively large. As a result, the cold forgeability of the steel decreases. Therefore, the Cr content is 0.01-0.40%. The preferred lower limit for the Cr content is 0.02%, and more preferably 0.03%. The preferred upper limit for the Cr content is 0.35%, and more preferably 0.30%.

[0037] O: 0.0030% or less Oxygen (O) is an unavoidable impurity. In other words, the O content is greater than 0%. O combines with other elements in the steel to form oxides. If the O content exceeds 0.0030%, even if the content of other elements is within the range of this embodiment, coarse oxides will be excessively formed. Coarse oxides become the starting point for cracks during cold forging. Therefore, the cold forgeability of the steel decreases. Therefore, the O content is 0.0030% or less. A low oxygen content is preferable. However, excessively reducing the oxygen content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit for oxygen content is 0.0001%, and more preferably 0.0002%. The preferred upper limit for the O content is 0.0027%, more preferably 0.0024%, and even more preferably 0.0020%.

[0038] The remainder of the chemical composition of the steel material according to this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during the industrial production of steel material, and are acceptable within a range that does not adversely affect the steel material according to this embodiment.

[0039] [About Optional Elements] The chemical composition of the steel material in this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Mo: 0.20% or less, V: 0.150% or less, Nb: 0.050% or less, Ti: 0.050% or less, Cu: 0.40% or less, Ni: 0.40% or less, Sn: 0.100% or less, B: 0.0050% or less, Ca: 0.0050% or less, and Mg: 0.0050% or less. These elements are arbitrary elements. Each element will be described below.

[0040] [Group 1 (Mo, V, Nb, Ti, Cu, Ni, Sn and B)] Mo, V, Nb, Ti, Cu, Ni, Sn, and B are optional elements and may not be present. All of these elements increase the strength of steel parts manufactured using steel as the raw material.

[0041] Mo: 0.20% or less Molybdenum (Mo) is an optional element and does not need to be included. In other words, the Mo content may be 0%. When present, Mo enhances the hardenability of steel, thereby increasing the strength of steel parts manufactured using steel as a material. Even a small amount of Mo will provide some degree of the above effect. On the other hand, if the Mo content exceeds 0.20%, the strength of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. As a result, the cold forgeability of the steel decreases. Therefore, the Mo content is 0-0.20%, and if present, the Mo content is 0.20% or less. The preferred lower limit for the Mo content is 0.01%, more preferably 0.05%, and even more preferably 0.08%. The preferred upper limit for the Mo content is 0.18%, more preferably 0.15%, and even more preferably 0.12%.

[0042] V:0.150% or less Vanadium (V) is an optional element and does not need to be present. In other words, the V content may be 0%. When V is present, that is, when the V content is greater than 0%, V produces V precipitates such as carbides and carbonitrides. These V precipitates enhance the strength of steel parts manufactured using steel as a material through precipitation strengthening. Even if only a small amount of V is present, the above effect can be obtained to some extent. However, if the V content exceeds 0.150%, coarse V precipitates are formed. These coarse V precipitates become the starting points for cracks during cold forging. Therefore, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel material decreases. Therefore, the V content is between 0 and 0.150%, and if present, the V content is 0.150% or less. The preferred lower limit for the V content is 0.001%, more preferably 0.010%, and even more preferably 0.030%. The preferred upper limit for the V content is 0.130%, more preferably 0.100%, and even more preferably 0.080%.

[0043] Nb: 0.050% or less Niobium (Nb) is an optional element and does not need to be included. In other words, the Nb content may be 0%. When Nb is present, that is, when the Nb content is greater than 0%, Nb forms Nb precipitates such as carbides, nitrides, and carbonitrides. These Nb precipitates enhance the strength of steel parts manufactured from steel materials through precipitation strengthening. Even a small amount of Nb can provide some degree of the above effect. However, if the Nb content exceeds 0.050%, coarse Nb precipitates will form. These coarse Nb precipitates become the starting points for cracks during cold forging. Therefore, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel material will decrease. Therefore, the Nb content is 0-0.050%, and if present, the Nb content is 0.050% or less. The preferred lower limit for the Nb content is 0.001%, more preferably 0.002%, and even more preferably 0.005%. The preferred upper limit for the Nb content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0044] Ti: 0.050% or less Titanium (Ti) is an optional element and does not need to be included. In other words, the Ti content may be 0%. When Ti is present, that is, when the Ti content is greater than 0%, Ti forms Ti precipitates such as carbides and nitrides. These Ti precipitates enhance the strength of steel fittings manufactured using steel as a material through precipitation strengthening. Even if only a small amount of Ti is present, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.050%, coarse Ti precipitates will form. These coarse Ti precipitates become the starting points for cracks during cold forging. Therefore, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel material will decrease. Therefore, the Ti content is between 0 and 0.050%, and if present, the Ti content is 0.050% or less. The preferred lower limit for the Ti content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Ti content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0045] Cu: 0.40% or less Copper (Cu) is an optional element and does not need to be included. In other words, the Cu content may be 0%. When present, i.e., when the Cu content is greater than 0%, Cu enhances the hardenability of steel, thereby increasing the strength of steel parts manufactured using steel as a material. Even a small amount of Cu will provide some degree of the above effect. However, if the Cu content exceeds 0.40%, the strength of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. As a result, the cold forgeability of the steel decreases. Therefore, the Cu content is 0-0.40%, and if present, the Cu content is 0.40% or less. The preferred lower limit for the Cu content is 0.01%, more preferably 0.02%, and even more preferably 0.04%. The preferred upper limit for the Cu content is 0.37%, more preferably 0.34%, and even more preferably 0.30%.

[0046] Ni: 0.40% or less Nickel (Ni) is an optional element and does not need to be included. In other words, the Ni content may be 0%. When present, i.e., when the Ni content is greater than 0%, Ni enhances the hardenability of steel and increases the strength of steel parts manufactured using steel as a material. Even a small amount of Ni will provide some degree of the above effect. However, if the Ni content exceeds 0.40%, the strength of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. As a result, the cold forgeability of the steel decreases. Therefore, the Ni content is 0-0.40%, and if present, the Ni content is 0.30% or less. The preferred lower limit for the Ni content is 0.01%, more preferably 0.02%, and even more preferably 0.04%. The preferred upper limit for the Ni content is 0.39%, more preferably 0.30%, and even more preferably 0.20%.

[0047] Sn: 0.100% or less Tin (Sn) is an optional element and does not need to be included. In other words, the Sn content may be 0%. When sn is present, that is, when the sn content is greater than 0%, sn suppresses grain coarsening and increases the strength of the steel. Even if only a small amount of sn is present, the above effect can be obtained to some extent. However, if the Sn content exceeds 0.100%, excessive segregation of Sn occurs, causing the steel to become brittle. Therefore, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel decreases. Therefore, the Sn content is between 0 and 0.100%, and if present, the Sn content is 0.100% or less. The preferred lower limit for the Sn content is 0.001%, more preferably 0.002%, and even more preferably 0.004%. The preferred upper limit for the Sn content is 0.090%, more preferably 0.080%, and even more preferably 0.070%.

[0048] B: 0.0050% or less Boron (B) is an optional element and does not need to be included. In other words, the B content may be 0%. When present, i.e., when the B content is greater than 0%, B enhances the hardenability of steel. As a result, it increases the strength of steel parts manufactured using steel as a material. Even a small amount of B can provide the above effect to some extent. However, if the B content exceeds 0.0050%, coarse B nitrides are formed. These coarse B nitrides become the starting points for cracks during cold forging. Therefore, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel material will decrease. Therefore, the B content is 0-0.0050%, and if present, the B content is 0.0050% or less. The preferred lower limit for the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit for the B content is 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%.

[0049] [Group 2: Ca and Mg] The chemical composition of the steel material in this embodiment may further contain one or more elements selected from the group consisting of Ca and Mg in place of a portion of Fe. These elements are arbitrary and all of them refine the Mn sulfide in the steel material, thereby improving the cold forgeability of the steel material.

[0050] Ca: 0.0050% or less Calcium (Ca) is an optional element and does not need to be included. In other words, the Ca content may be 0%. When calcium is present, i.e., when the calcium content is greater than 0%, the calcium refines the manganese sulfides. This improves the cold forgeability of the steel. Even a small amount of calcium can provide some degree of this effect. However, if the Ca content exceeds 0.0050%, coarse Ca oxides will be formed, even if the content of other elements is within the range of this embodiment. Coarse Ca oxides become the starting point for cracks during cold forging. As a result, the cold forgeability of the steel material decreases. Therefore, the Ca content is between 0 and 0.0050%, and if present, the Ca content is 0.0050% or less. The preferred lower limit for the Ca content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The preferred upper limit for the Ca content is 0.0040%, and more preferably 0.0030%.

[0051] Mg: 0.0050% or less Magnesium (Mg) is an optional element and does not need to be included. In other words, the Mg content may be 0%. When Mg is present, that is, when the Mg content is greater than 0%, Mg refines Mn sulfides. This improves the cold forgeability of the steel. Even a small amount of Mg can provide some degree of this effect. However, if the Mg content exceeds 0.0050%, coarse Mg oxides will be formed, even if the content of other elements is within the range of this embodiment. Coarse Mg oxides become the starting point for cracks during cold forging. As a result, the cold forgeability of the steel material decreases. Therefore, the Mg content is between 0 and 0.0050%, and if present, the Mg content is 0.0050% or less. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The preferred upper limit for the Mg content is 0.0040%, and more preferably 0.0030%.

[0052] [(Feature 2) About Microorganisms] In the steel material of this embodiment, the total area ratio of ferrite and pearlite is 90% or more, and the area ratio of pearlite is 40% or more.

[0053] As mentioned above, when cold forging is performed on steel materials that have not undergone spheroidizing annealing, it becomes necessary to suppress the occurrence of cracks during cold forging. Cracks that occur during cold forging are mainly caused by ductile fracture. In ductile fracture, strain concentrates at the interface between ferrite and pearlite in the steel, creating voids. Multiple voids that are created connect, causing cracks to occur and propagate.

[0054] In the microstructure of the steel material of this embodiment, the total area ratio of ferrite and pearlite is 90% or more, and furthermore, the area ratio of pearlite is 40% or more. By increasing the total area ratio of ferrite and pearlite in the microstructure, the occurrence of cracks during cold forging can be suppressed. Furthermore, by increasing the area ratio of pearlite in the microstructure to 40% or more, the proportion of pearlite in the deformation of the steel material during cold forging can be increased. This makes it possible to suppress excessive deformation of ferrite during cold forging.

[0055] In the microstructure of steel, if other structures besides ferrite and pearlite are present, the remaining structures other than ferrite and pearlite consist of one or more selected from the group consisting of, for example, bainite and martensite.

[0056] The preferred lower limit for the total area ratio of ferrite and pearlite is 91%, more preferably 93%, and even more preferably 95%. The total area ratio of ferrite and pearlite may be 100%.

[0057] The preferred lower limit for the perlite area ratio is 42%, more preferably 44%, and even more preferably 46%. The preferred upper limit for the perlite area ratio is 90%, more preferably 85%, and even more preferably 80%.

[0058] [Method for measuring ferrite area ratio and pearlite area ratio] The ferrite area ratio and pearlite area ratio in the microstructure of steel materials are measured by the following method.

[0059] The observation surface is a cross-section parallel to the axial direction (longitudinal direction) of the steel material, and which includes the central axis of the steel material. A test piece containing the observation surface is taken from the steel material. The observation surface of the test piece is mirror-polished. After mirror polishing, the observation surface is etched with 3% nital (ethanol + 3% nitric acid solution) to reveal the microstructure.

[0060] Observe the etched observation surface, focusing on the R / 2 depth position radially from the surface of the steel material, using an optical microscope. The magnification is 1000x, and the size of the observation field is 120 μm radially and 160 μm axially. Five observation fields are used. In this specification, the R / 2 position of the steel material refers to the central position of the radius (=R) in a cross-section (circular shape) perpendicular to the axial direction of the steel material.

[0061] Within the observation field, each tissue (ferrite, pearlite, bainite, and martensite) differs in contrast and morphology. Therefore, each tissue is identified as follows based on its contrast and morphology: White, polygonal grains without internal structure are identified as ferrite. A structure in which bright white areas and dark black areas are mixed in a striped pattern is identified as pearlite. A structure with narrow plate-like or needle-like structures, accompanied by black precipitates inside or at the boundaries of these narrow plate-like or needle-like structures, is identified as bainite and martensite. Note that in the microstructure of the steel material of this embodiment, it is not necessary to strictly distinguish between bainite and martensite.

[0062] Determine the total area of ​​ferrite and the total area of ​​pearlite in each of the five observation fields. Based on the total area of ​​ferrite, the total area of ​​pearlite, and the total area of ​​the five observation fields, calculate the ferrite area percentage (%) and the pearlite area percentage (%). The ferrite area percentage (%) is rounded to the nearest integer by rounding the obtained value to the first decimal place. The pearlite area percentage (%) is rounded to the nearest integer by rounding the obtained value to the nearest integer by rounding the obtained value to the first decimal place.

[0063] [(Feature 3) Regarding the average particle size dP and the standard deviation σP of perlite particles] In this embodiment, the average particle size dP of the perlite grains is 10.0 to 40.0 μm, and the average particle size dP (μm) and the standard deviation σP (μm) of the perlite grains satisfy the following equation (1). σP / dP<0.80 (1)

[0064] If the average particle size dP of the pearlite grains is between 10.0 and 40.0 μm, the pearlite grains are sufficiently small. Therefore, during cold forging, localized stress concentration can be alleviated, and uniform deformation can be promoted.

[0065] Here, a perlite grain refers to a continuous region of perlite tissue. When adjacent perlite colonies and pseudo-perlites are in contact with each other, they are considered a single perlite grain. Pseudo-perlite refers to perlite in which fragmented lamellar cementite and / or spheroidized cementite are arranged in a dotted pattern. The preferred lower limit for the average particle size dP of perlite grains is 11.0 μm, more preferably 12.0 μm, even more preferably 12.5 μm, even more preferably 13.0 μm, and even more preferably 14.0 μm. The preferred upper limit for the average particle size dP of perlite grains is 38.0 μm, more preferably 36.0 μm, even more preferably 34.0 μm, and even more preferably 32.0 μm.

[0066] Furthermore, if the average particle size dP (μm) of the pearlite grains and the standard deviation σP (μm) of the pearlite grain size satisfy equation (1), then the variation in pearlite grains within the steel material is sufficiently small. Therefore, localized stress concentration during cold forging caused by the variation in pearlite grains can be mitigated, and uniform deformation can be promoted.

[0067] F1 is defined as follows: F1 = σP / dP A preferred upper limit for F1 is 0.78, more preferably 0.75, and even more preferably 0.70. The lower limit of F1 is not particularly limited. For example, the lower limit of F1 could be 0.10, or for example, 0.20.

[0068] [Method for measuring the average particle size dP and the standard deviation σP of perlite particles] The average particle size dP (μm) and the standard deviation σP (μm) of pearlite grains in steel materials are measured by the following method.

[0069] In the five observation fields selected in the above-described [Method for Measuring Ferrite Area Ratio and Perlite Area Ratio], the perlite grains to be measured are identified based on the contrast and morphology described above.

[0070] Perlite grains are identified as follows: Perlite colonies and pseudo-perlite are identified in the observation field based on contrast and morphology. When focusing on one perlite colony or pseudo-perlite, if the focused perlite colony or pseudo-perlite is separated from other adjacent perlite colonies or pseudo-perlite without contact, each of those perlite colonies or pseudo-perlite is considered a single perlite grain. If the focused perlite colony or pseudo-perlite is in contact with other adjacent perlite colonies or pseudo-perlite, the multiple contacting perlite colonies and pseudo-perlite are considered a single perlite grain.

[0071] For example, if the first and second perlite colonies are in contact with each other, and the second and third perlite colonies are in contact with each other, then the first to third perlite colonies are considered as a single perlite grain, regardless of whether the third perlite colony is in contact with the first perlite colony or not. Also, if the first and second perlite colonies are in contact with each other, and the third perlite colony is not in contact with either the first or second perlite colonies, then the first and second perlite colonies are considered as a single perlite grain, and the third perlite colony is considered as a single perlite grain.

[0072] In this process, perlite grains whose entire surface is contained within the rectangular field of view are selected for measurement, while those in which part of the grain extends outside the field of view are excluded. The equivalent circle diameter (μm) of each perlite grain selected for measurement is then determined. Here, the equivalent circle diameter refers to the diameter (μm) of a circle with the same area.

[0073] The average particle size dP (μm) of the perlite grains is defined as the arithmetic mean of the equivalent circle diameters of all perlite grains identified as the measurement target in the five observation fields. In this case, the average particle size dP (μm) is the value obtained by rounding the second decimal place of the arithmetic mean to the first decimal place.

[0074] Furthermore, the sample standard deviation of the equivalent circle diameter of all perlite grains identified as the measurement target in the five observation fields is defined as the standard deviation σP(μm). In this case, the standard deviation σP(μm) is the value rounded to the second decimal place after rounding the third decimal place of the calculated value.

[0075] [(Feature 4) Regarding the average particle size dF of ferrite grains] In this embodiment, the average particle size dF of the ferrite grains is 25.0 μm or less.

[0076] If the average particle size dF of the ferrite grains is 25.0 μm or less, the ferrite grains are sufficiently small. Therefore, during cold forging, localized stress concentration can be alleviated, and uniform deformation can be promoted. The lower limit of the average particle size dF of ferrite grains is not particularly limited. However, excessive refinement of ferrite grains increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the average particle size dF of ferrite grains is 1.0 μm, more preferably 3.0 μm, more preferably 5.0 μm, more preferably 8.0 μm, more preferably 10.0 μm, more preferably 11.0 μm, and more preferably 12.0 μm. The preferred upper limit for the average particle size dF of the ferrite grains is 24.0 μm, more preferably 22.0 μm, even more preferably 20.0 μm, and even more preferably 18.0 μm.

[0077] [Method for measuring the average particle size dF of ferrite grains] The average particle size dF (μm) of ferrite grains in steel is measured by the following method.

[0078] In the five observation fields identified in the above-described [Method for Measuring Ferrite Area Ratio and Pearlite Area Ratio], the ferrite grains to be measured are identified based on the contrast and morphology described above. At this time, ferrite grains whose entire body is contained within the rectangle of the observation field are to be measured, while ferrite grains whose part extends outside the observation field are not to be measured. The equivalent circular diameter (μm) of each ferrite grain identified as the target of measurement is determined. Here, the equivalent circular diameter refers to the diameter (μm) when the same area is made into a circle.

[0079] The arithmetic mean of the equivalent circle diameters of all ferrite grains identified as the target of measurement in the five observation fields is defined as the average particle size dF (μm). In this case, the average particle size dF (μm) is rounded to the first decimal place by rounding the second decimal place of the value obtained by the arithmetic mean. σP / dP is rounded to the second decimal place by rounding the third decimal place of the obtained value.

[0080] [(Feature 5) Regarding the difference in hardness ΔHV between ferrite and pearlite] In the steel material of this embodiment, the difference ΔHV between the hardness of ferrite and the hardness of pearlite (hereinafter also referred to as hardness difference ΔHV) is 140HV or less. The hardness difference ΔHV is defined by the following formula. Hardness difference ΔHV = |Hardness of pearlite HVP - Hardness of ferrite HVF| Pearlite is harder than ferrite. If a steel material has a structure mainly composed of ferrite and pearlite, and the hardness of the ferrite is excessively lower than that of the pearlite, that is, if the hardness difference ΔHV is excessively large, stress concentration in the ferrite is likely to occur during cold forging. In this case, the ferrite deforms excessively. As a result, cracks are more likely to occur in the excessively deformed ferrite.

[0081] When the hardness difference ΔHV is 140HV or less, the difference in hardness between pearlite and ferrite in the steel is sufficiently suppressed. Therefore, excessive stress concentration in the ferrite is less likely to occur during cold forging, and uniform deformation is promoted. As a result, excellent cold forgeability can be obtained even without spheroidizing annealing.

[0082] The lower limit of the hardness difference ΔHV is not particularly limited. For example, the lower limit of the hardness difference ΔHV is 30HV, more preferably 20HV, and even more preferably 10HV. The preferred upper limit of the hardness difference ΔHV is 135HV, more preferably 130HV, even more preferably 125HV, and even more preferably 120HV.

[0083] [Method for measuring hardness difference ΔHV] The hardness difference ΔHV is measured by the following method. In the five observation fields identified in the above-mentioned [Method for Measuring Ferrite Area Ratio and Perlite Area Ratio], the perlite and ferrite grains to be measured are identified based on the contrast and morphology described above. At this time, perlite and ferrite grains that are entirely contained within the rectangle of the observation field are to be measured, while perlite and ferrite grains in which part of the grain extends outside the observation field are not to be measured.

[0084] From the perlite grains designated for measurement, five arbitrary perlite grains are selected. A Vickers hardness test is performed on each selected perlite grain in accordance with JIS Z 2244-1:2020. At this time, the indenter is pressed in such a way that an indentation is formed inside the perlite grain and does not extend beyond the grain boundary. The test force is set to 0.098 N. The arithmetic mean of the Vickers hardness of the five obtained perlite grains is defined as the Vickers hardness HVP (HV) of the perlite. The value of the Vickers hardness HVP is an integer value obtained by rounding the arithmetic mean to the first decimal place.

[0085] From the ferrite grains designated for measurement, five ferrite grains are selected. A Vickers hardness test is performed on each selected ferrite grain in accordance with JIS Z 2244-1:2020. At this time, the indenter is pressed in such a way that an indentation is formed inside the ferrite grain and does not extend beyond the grain boundary. The test force is set to 0.098 N. The arithmetic mean of the Vickers hardness of the five obtained ferrite grains is defined as the Vickers hardness HVF (HV) of the ferrite. The value of Vickers hardness HVF is an integer value obtained by rounding the arithmetic mean to the first decimal place.

[0086] Based on the obtained Vickers hardness HVP of pearlite and Vickers hardness HVF of ferrite, the hardness difference ΔHV is calculated using the above formula.

[0087] [Effects of the steel material in this embodiment] The steel material of this embodiment satisfies features 1 to 5. Therefore, when manufacturing steel parts by cold forging using the steel material of this embodiment as the raw material, even if spheroidizing annealing of the steel material is omitted, the occurrence of cracks during cold forging can be sufficiently suppressed.

[0088] [Shape of the steel material in this embodiment] The steel material in this embodiment is a steel bar or wire. The steel bar or wire is a steel material whose cross-section perpendicular to the axial direction (longitudinal direction) is circular and extends in the longitudinal direction. The steel material may be wound into a coil or cut to a predetermined length.

[0089] [Applications of the steel material of this embodiment] The steel material of this embodiment can be widely used as a material for steel parts manufactured by cold forging. In particular, the steel material of this embodiment is suitable as a material for steel parts manufactured by cold forging without spheroidizing annealing before cold forging.

[0090] [Method for manufacturing steel material according to this embodiment] An example of a method for manufacturing the steel material of this embodiment will be described. The steel material having the above-described structure may be manufactured by methods other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing the steel material of this embodiment.

[0091] An example of a method for manufacturing steel materials according to this embodiment includes the following steps. (Process 1) Steelmaking process (Process 2) Blooming rolling process (Process 3) Finish rolling process The following describes each step.

[0092] [(Process 1) Steelmaking Process] In the steelmaking process, molten steel having a chemical composition satisfying characteristic 1 described above is produced. The refining method is not particularly limited, and any well-known method may be used. For example, molten iron produced by a well-known method is refined in a converter (primary refining). The molten steel tapped from the converter is then refined using a well-known secondary refining method. Through these steps, molten steel having a chemical composition satisfying characteristic 1 is produced. Using the produced molten steel, blooms (cast slabs) are manufactured by a continuous casting method.

[0093] [(Process 2) Blooming rolling process] In the bract rolling process, a bract rolling mill is used to hot-roll (bract rolling) the bloom to produce a billet. If a continuous rolling mill with multiple rolling stands arranged in a row is located downstream of the bract rolling mill, the billet may be further hot-rolled using the continuous rolling mill to further reduce its size. The heating temperature in the bract rolling process can be within a well-known temperature range. For example, the heating temperature is 1100 to 1300°C. The billet produced in the bract rolling process is allowed to cool to room temperature (air-cooled) before the finish rolling process.

[0094] [(Process 3) Finish Rolling Process] In the finishing rolling process, the billet is subjected to hot rolling (finishing rolling) to produce the steel material of this embodiment. Specifically, the billet after the bloc rolling process is heated using a heating furnace. The heating temperature is, for example, 1000 to 1300°C. After heating, the billet is subjected to hot rolling (finishing rolling) using a finishing rolling mill in which multiple rolling stands are arranged in a row. The steel material of this embodiment is produced through the above manufacturing process.

[0095] [Regarding conditions during the manufacturing process] In the manufacturing process described above, the following conditions 1 to 4 must be met. (Condition 1) The solidification and cooling rate CR1 at a depth of 1 / 4 of the short side of the bloom, from the bloom surface corresponding to the long side in a cross-section perpendicular to the longitudinal direction of the bloom during casting, shall be 8.0°C / min or higher. (Condition 2) The finishing rolling temperature (FT) should be kept below 850°C. (Condition 3) The average cooling rate CR2 from the finishing rolling temperature FT until the steel surface temperature reaches 500°C shall be less than 0.05 to 1.00°C / second. (Condition 4) The FA defined by formula (A) shall be 5.0 or greater. FA=10000×CR1 / {(CR2×60)×FT} (A) Conditions 1 through 4 will be explained below.

[0096] [(Condition 1) Regarding the solidification and cooling rate CR1] In the steelmaking process, the average cooling rate from the liquidus temperature to the solidus temperature within the temperature range of steel during casting is defined as the solidification cooling rate CR1 (°C / min). In the steelmaking process, the solidification cooling rate CR1 at a depth of 1 / 4 of the short side of the bloom, from the bloom surface corresponding to the long side of the cross section perpendicular to the longitudinal direction of the bloom during casting, is defined as 8.0°C / min or higher.

[0097] If the solidification and cooling rate CR1 is too slow, the crystal grains will become coarse, and the variation in crystal grain size will increase. In this case, the average particle size dP of the pearlite grains will become coarse, or the average particle size dP of the pearlite grains and the standard deviation σP of the pearlite grain size will not satisfy equation (1). Furthermore, the average particle size dF of the ferrite grains will also become coarse. Therefore, the solidification and cooling rate CR1 should be set to 8.0°C / min or higher.

[0098] [Method for measuring the solidification cooling rate CR1] The solidification and cooling rate CR1 can be determined by the following method: Take a specimen from a rectangular cross-section perpendicular to the longitudinal direction of a bloom produced by continuous casting, including a position 1 / 4 of the way down from the long side of the bloom surface to the short side of the bloom. More specifically, take a specimen from the cross-section that has a 5 mm × 5 mm observation area on its surface, centered on a position 1 / 4 of the way down from the width center of the long side of the bloom surface to the short side of the bloom. One side of the observation area is parallel to the long side of the bloom, and the other pair of sides of the observation area is parallel to the short side of the bloom.

[0099] In the observation area, the spacing between 10 dendrite secondary arms is measured, and the arithmetic mean is taken as λ² (μm). Using the measured dendrite secondary arm spacing λ² (μm), the solidification and cooling rate CR1 (°C / min) is calculated using the following formula. CR1 = (λ² / 770) -1 / 0.41

[0100] [(Condition 2) Regarding the finishing rolling temperature FT] In the finishing rolling process, the surface temperature of the steel material at the exit of the rolling stand that last rolled the steel material down in the continuous rolling mill is defined as the finishing rolling temperature FT (°C). The finishing rolling temperature FT (°C) can be measured using a thermometer installed at the exit of the rolling stand. If the finishing rolling temperature FT is 850°C or higher, the rolling temperature in the finishing rolling process is too high. In this case, the crystal grains become coarse, and the variation in crystal grain size increases. As a result, the average particle size dP of the pearlite grains becomes coarse, or the average particle size dP of the pearlite grains and the standard deviation σP of the pearlite grains do not satisfy equation (1). Furthermore, the average particle size dF of the ferrite grains becomes coarse. Therefore, the finishing rolling temperature should be set to less than 850°C. The lower limit of the finishing rolling temperature FT is, for example, 750°C.

[0101] [(Condition 3) Regarding the average cooling rate CR2] The average cooling rate from the finishing rolling temperature FT until the surface temperature of the steel reaches 500°C is defined as the average cooling rate CR2 (°C / second). In this embodiment, the average cooling rate CR2 is set to 0.05 to less than 1.00°C / second.

[0102] If the average cooling rate CR2 is too slow, the crystal grains become coarse, and the grain variability increases. As a result, the average particle size dP of the pearlite grains becomes coarse, or the average particle size dP and the standard deviation σP of the pearlite grains do not satisfy equation (1). Furthermore, the average particle size dF of the ferrite grains becomes coarse. In addition, the ferrite area fraction increases, and the pearlite area fraction becomes excessively small. On the other hand, if the average cooling rate CR2 is too fast, the hardness of the pearlite increases excessively. In this case, the hardness difference ΔHV becomes excessively large. Therefore, the average cooling rate CR2 should be set to less than 0.05 to 1.00 °C / second.

[0103] [(Condition 4) Regarding FA] In the manufacturing process of this embodiment, the FA defined by formula (A) is further set to 5.0 or higher. FA=10000×CR1 / {(CR2×60)×FT} (A) Even if conditions 1 to 3 are met, if FA is less than 5.0, the average particle size dP of the pearlite grains will be coarse, or the average particle size dP of the pearlite grains and the standard deviation σP of the pearlite grain size will not satisfy equation (1). Furthermore, the average particle size dF of the ferrite grains will be coarse. In addition, the ferrite area ratio will increase and the pearlite area ratio will become excessively small. Therefore, FA must be 5.0 or greater. Note that FA is the value of the first decimal place obtained by rounding the second decimal place of the calculated value.

[0104] The steel material of this embodiment can be manufactured through the above manufacturing process. [Examples]

[0105] The effects of one embodiment of the steel material of this embodiment will be further explained in detail by the following examples. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effects of the steel material of this embodiment. Therefore, the steel material of this embodiment is not limited to this one example of conditions.

[0106] Steel materials having the chemical compositions shown in Table 1 (Tables 1A and 1B) were manufactured by the following method.

[0107] [Table 1A]

[0108] [Table 1B]

[0109] Blooms were produced by continuous casting using molten steel. The solidification and cooling rate CR1 (°C / min) was determined using a portion of the produced blooms based on the method described in [Method for Measuring Solidification and Cooling Rate CR1] above. The obtained solidification and cooling rates CR1 are shown in the "CR1 (°C / min)" column of the "Condition 1" column in Table 2.

[0110] [Table 2]

[0111] The manufactured bloom was subjected to a bract rolling process to produce billets. The heating temperature of the bloom during the bract rolling process was 1100-1300°C. Using the billets, a finish rolling process was performed to produce steel materials with a diameter of 60 mm. The heating temperature during the finish rolling process was 1000-1300°C. The finish rolling temperature FT(°C) during the finish rolling process is shown in the "FT(°C)" column of the "Condition 2" column in Table 2. The average cooling rate CR2 during the finish rolling process is shown in the "CR2(°C / sec)" column of the "Condition 3" column in Table 2. The FA is shown in the "FA" column of the "Condition 4" column in Table 2.

[0112] [Evaluation Test] The following tests were performed on the steel material for each test number. (Test 1) Measurement test of ferrite area ratio and pearlite area ratio (Test 2) Measurement test of the average particle size dP and the standard deviation σP of perlite particles (Test 3) Measurement test of average particle size dF of ferrite grains (Test 4) Hardness difference ΔHV measurement test (Test 5) Cold forging performance evaluation test The following explains Exams 1 through 5.

[0113] [(Test 1) Measurement test of ferrite area ratio and pearlite area ratio] Based on the method described in [Measurement Method for Ferrite Area Ratio and Pearlite Area Ratio] above, the ferrite area ratio (%) and pearlite area ratio (%) of the steel material for each test number were determined. The obtained ferrite area ratio is shown in the "F Area Ratio (%)" column of Table 3. The pearlite area ratio (%) is shown in the "P Area Ratio (%)" column of Table 3. The total area ratio (%) of ferrite and pearlite is shown in the "F+P Area Ratio (%)" column of Table 3. Note that if the total area ratio of ferrite and pearlite is less than 100%, the remaining microstructure for all test numbers was bainite and / or martensite.

[0114] [Table 3]

[0115] [(Test 2) Measurement test of average particle size dP and standard deviation σP of perlite particle size] Based on the method described in the above-mentioned [Method for measuring the average particle size dP and the standard deviation σP of pearlite grains], the average particle size dP (μm) and the standard deviation σP (μm) of pearlite grains were determined for each test number of steel material. The obtained average particle size dP is shown in the "dP (μm)" column of Table 3. The obtained standard deviation σP is shown in the "σP (μm)" column of Table 3. In addition, F1 is shown in the "F1" column of Table 3.

[0116] [(Test 3) Measurement test of average particle size dF of ferrite grains] Based on the method described in [Method for Measuring the Average Particle Size dF of Ferrite Grains] above, the average particle size dF (μm) of ferrite grains in the steel material for each test number was determined. The obtained average particle size dF is shown in the "dF (μm)" column of Table 3.

[0117] [(Test 4) Hardness difference ΔHV measurement test] Based on the method described in the above-mentioned [Method for Measuring Hardness Difference ΔHV], the hardness difference ΔHV(HV) of the steel material for each test number was determined. The obtained hardness difference ΔHV is shown in the "ΔHV(HV)" column of Table 3.

[0118] [(Test 5) Cold Forgeability Evaluation Test] Multiple cylindrical test specimens, 10 mm in diameter and 15 mm in length, were taken from the steel material of each test number. The central axis of the cylindrical test specimens was coaxial with the central axis of the steel material. Assuming cold forging without spheroidizing annealing, a compression test (cold forging) was performed on the cylindrical test specimens at room temperature (25°C). Specifically, the compression test was performed until the processing rate, defined by formula (A), reached 70%. The compression test was performed on five cylindrical test specimens for each test number. The presence or absence of cracks in the cylindrical test specimens after the test was checked. In industrial production, the processing rate for cold forging is generally 70% or less. Therefore, if no cracks occurred in the steel material during the compression test at a processing rate of 70%, it was judged that excellent cold forgeability had been obtained. Processing rate = (1 - (Length of cylindrical specimen after compression test / Length of cylindrical specimen before compression test)) × 100 (A)

[0119] The presence or absence of cracks in the steel material after a compression test at a processing rate of 70% was checked using the following method. The presence or absence of cracks was observed on five cylindrical test pieces after the compression test using a 5x magnifying glass. If no cracks of 0.5 mm or longer were observed in any of the five cylindrical test pieces, it was determined that excellent cold forgeability was obtained without crack formation (indicated as "E (Excellent)" in the "Cold Forgeability" column in Table 3). On the other hand, if cracks of 0.5 mm or longer were found in one or more of the five cylindrical test pieces, it was determined that sufficient cold forgeability was not obtained (indicated as "B (Bad)" in the "Cold Forgeability" column in Table 3).

[0120] [Evaluation Results] Referring to Tables 1 to 3, the steel materials for test numbers 1 to 14 satisfied characteristics 1 to 5. Therefore, in a compression test at a processing rate of 70%, simulating cold forging without spheroidizing annealing, crack occurrence was sufficiently suppressed, and excellent cold forgeability was obtained.

[0121] In test number 15, the carbon content was too low. As a result, the pearlite area ratio was excessively low. Consequently, cracks were observed in a compression test at a processing rate of 70%, simulating cold forging without spheroidizing annealing, and sufficient cold forgeability could not be obtained.

[0122] In test number 16, the carbon content was too high. As a result, cracks were observed in a compression test at a processing rate of 70%, simulating cold forging without spheroidizing annealing, and sufficient cold forgeability could not be obtained.

[0123] In test number 17, the Si content was too high. As a result, cracks were observed in a compression test at a processing rate of 70%, which simulates cold forging without spheroidizing annealing, and sufficient cold forgeability could not be obtained.

[0124] In test number 18, the Mn content was too low. As a result, the pearlite area ratio was excessively low. In addition, the F1 value was excessively large, and the hardness difference ΔHV between pearlite and ferrite became excessively large. Consequently, in a compression test at a processing rate of 70%, which simulates cold forging without spheroidizing annealing, cracks were observed, and sufficient cold forgeability could not be obtained.

[0125] In test number 19, the Mn content was too high. As a result, cracks were observed in a compression test at a processing rate of 70%, simulating cold forging without spheroidizing annealing, and sufficient cold forgeability could not be obtained.

[0126] In test number 20, the P content was too high. As a result, cracks were observed in a compression test at a processing rate of 70%, simulating cold forging without spheroidizing annealing, and sufficient cold forgeability could not be obtained.

[0127] In test number 21, the sulfur content was too high. As a result, cracks were observed in a compression test at a processing rate of 70%, simulating cold forging without spheroidizing annealing, and sufficient cold forgeability could not be obtained.

[0128] In test number 22, the Al content was too high. As a result, cracks were observed in a compression test at a processing rate of 70%, simulating cold forging without spheroidizing annealing, and sufficient cold forgeability could not be obtained.

[0129] In test number 23, the nitrogen content was too high. As a result, cracks were observed in a compression test at a processing rate of 70%, simulating cold forging without spheroidizing annealing, and sufficient cold forgeability could not be obtained.

[0130] In test number 24, the Cr content was too high. As a result, the hardness difference ΔHV between pearlite and ferrite became excessively large. Consequently, cracks were observed in a compression test at a processing rate of 70%, simulating cold forging without spheroidizing annealing, and sufficient cold forgeability could not be obtained.

[0131] In tests 25 and 26, the solidification and cooling rate CR1 was too slow. As a result, the average particle size dP of the pearlite grains was excessively large, and F1 was also excessively large. Consequently, cracks were observed in the compression test at a processing rate of 70%, which simulates cold forging without spheroidizing annealing, and sufficient cold forgeability was not achieved.

[0132] In tests 27 and 28, the finishing rolling temperature FT was too high. As a result, the average particle size dP of pearlite grains and the average particle size dF of ferrite grains became excessively large, the F1 value became excessively large, and the hardness difference ΔHV also became excessively large. Consequently, in a compression test at a processing rate of 70%, which simulates cold forging without spheroidizing annealing, cracks were observed, and sufficient cold forgeability could not be obtained.

[0133] In tests 29 and 30, the average cooling rate CR2 was too slow. As a result, the average particle size dP of pearlite grains and the average particle size dF of ferrite grains became excessively large. Furthermore, in test 30, the F1 value became excessively large. Consequently, cracks were observed in the compression test at a processing rate of 70%, simulating cold forging without spheroidizing annealing, and sufficient cold forgeability could not be obtained.

[0134] In tests 31 and 32, the average cooling rate CR2 was too fast. As a result, the hardness difference ΔHV became excessively large. Consequently, cracks were observed in the compression test at a processing rate of 70%, which simulates cold forging without spheroidizing annealing, and sufficient cold forgeability could not be obtained.

[0135] In tests 33 and 34, the FA value was too low, resulting in an excessively large F1 value. Furthermore, in test 33, the average particle size dF of the ferrite grains was excessively large. In addition, in test 34, the pearlite area ratio was excessively small, and the average particle size dP of the pearlite grains was excessively large. As a result, cracks were observed in the compression test at a processing rate of 70%, simulating cold forging without spheroidizing annealing, and sufficient cold forgeability could not be obtained.

[0136] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

1. The chemical composition is expressed in mass percent. C: 0.30-0.60%, Si: 0.01 to 0.10%, Mn: 0.30-2.00%, P: Less than 0.050% S: 0.001-0.050%, Al: 0.001-0.200%, N: Less than 0.020% Cr: 0.01-0.40%, O: 0.0030% or less, and, The remainder consists of Fe and impurities. The total area ratio of ferrite and pearlite is 90% or more, and the area ratio of pearlite is 40% or more. The average particle size dP of the perlite grains is 10.0 to 40.0 μm. The average particle size dP of the perlite particles and the standard deviation σP of the particle size of the perlite particles satisfy equation (1), The average particle size dF of the ferrite grains is 25.0 μm or less. The difference between the hardness of the ferrite and the hardness of the pearlite is 140 HV or less. Steel material. σP / dP<0.80 (1)

2. The chemical composition is expressed in mass percent. C: 0.30-0.60%, Si: 0.01 to 0.10%, Mn: 0.30-2.00%, P: Less than 0.050% S: 0.001-0.050%, Al: 0.001-0.200%, N: Less than 0.020% Cr: 0.01-0.40%, and, O: Contains 0.0030% or less, Furthermore, it contains one or more types selected from the group consisting of Group 1 and Group 2, The remainder consists of Fe and impurities. The total area ratio of ferrite and pearlite is 90% or more, and the area ratio of pearlite is 40% or more. The average particle size dP of the perlite grains is 10.0 to 40.0 μm. The average particle size dP of the perlite particles and the standard deviation σP of the particle size of the perlite particles satisfy equation (1), The average particle size dF of the ferrite grains is 25.0 μm or less. The difference between the hardness of the ferrite and the hardness of the pearlite is 140 HV or less. Steel material. σP / dP<0.80 (1) [Group 1] Mo: 0.20% or less V: 0.150% or less, Nb: 0.050% or less, Ti: 0.050% or less, Cu: 0.40% or less, Ni: 0.40% or less, Sn: 0.100% or less, and, B: One or more selected from the group consisting of 0.0050% or less. [Group 2] Ca: 0.0050% or less, One or two selected from the group consisting of Mg: 0.0050% or less.

3. The steel material according to claim 2, The aforementioned chemical composition contains the first group, Steel material.

4. The steel material according to claim 2, The aforementioned chemical composition contains the second group, Steel material.

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