steel

Abstract

The steel material disclosed herein has a chemical composition in mass%, containing C: 0.10-0.40%, Si: 0.01-0.45%, Mn: 0.30-0.95%, P: 0.030% or less, S: 0.025% or less, Cr: greater than 0.50% to 1.80%, Al: 0.005-0.100%, N: 0.0250% or less, and O: 0.0050% or less, with the remainder being Fe and impurities. The total area ratio of ferrite and pearlite in the microstructure is 95% or more, the average equivalent circle diameter DN of nitrides with an equivalent circle diameter of 20 nm or more is 70 nm or less, and the sample standard deviation SDN of the equivalent circle diameter of nitrides is 45 nm or less.

Description

[Technical Field]

[0001] This disclosure relates to steel materials, and more particularly to steel materials suitable for use as materials for mechanical structural components manufactured by cold forging and machining. [Background technology]

[0002] Mechanical structural components, such as those found in automobiles, industrial machinery, and construction machinery, require high fatigue strength.

[0003] Techniques for increasing the fatigue strength of mechanical structural components are proposed in Japanese Patent Publication No. 2022-080369 (Patent Document 1) and Japanese Patent Publication No. 2005-240120 (Patent Document 2).

[0004] The steel material disclosed in Patent Document 1 has a chemical composition, in mass%, containing C: 0.10-0.35%, Si: 0.60-1.50%, Mn: 0.20-1.30%, P: 0.030% or less, S: 0.050% or less, Ni: 0.01-0.20%, Cr: 0.65-1.50%, Mo: 0.01-0.60%, Al: 0.045% or less, and N: 0.0250% or less, with the remainder being Fe and impurities, satisfying formulas (1) to (4). Patent Document 1 increases the fatigue strength of machine structural parts manufactured using steel material by satisfying formulas (1) to (4). 2.5≦(2Mn+5Cr+Mo) / Si≦7.5 (1) Mn × Ni ≤ 0.05 (2) 1.7 ≤ Al / N ≤ 2.4 (3) 22 ≤ Mn / S ≤ 65 (4)

[0005] The steel material disclosed in Patent Document 2 consists of, by mass%, C: 0.10~0.60%, Si: 0.05~2.0%, Mn: 0.3~2.5%, S: 0.02~0.25%, Al: 0.002~0.030%, Ca: 0.0005~0.01%, O: 0.0005~0.008%, N: 0.02% or less, with a mass% ratio of Ca / Al: 0.1~1.0 and Ca / O: greater than 0.2, with the remainder being Fe and unavoidable impurities. In Patent Document 2, the sulfides are controlled to an appropriate form by adjusting the Ca / Al and Ca / O ratios. As a result, the fatigue strength of mechanical structural parts manufactured using the steel material is increased. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2022-080369 [Patent Document 2] Japanese Patent Publication No. 2005-240120 [Overview of the project] [Problems that the invention aims to solve]

[0007] By the way, the manufacturing method for mechanical structural parts is as follows: Hot forging is performed on the raw material, steel, to produce a raw material having the rough shape of a mechanical structural part. Machining is performed on the raw material. Heat treatment is performed on the raw material after machining. Examples of heat treatments include overall hardening, high-frequency induction hardening, carburizing, and carbonitriding. Finishing is performed on the raw material after heat treatment to produce the final product, the mechanical structural part.

[0008] In this process, heat treatment causes the surface of the mechanical structural component to develop a microstructure mainly composed of hard structures (martensite and bainite). This increases the fatigue strength of the mechanical structural component. However, if the prior austenite grains of the mechanical structural component are coarse, the fatigue strength of the mechanical structural component decreases. Therefore, steel materials are required to have properties that suppress the coarsening of prior austenite grains (coarsening resistance properties) in mechanical structural components. However, Patent Documents 1 and 2 do not examine specific means for suppressing the coarsening of prior austenite grains.

[0009] Furthermore, in recent years, there has been a shift from hot forging to cold forging in the manufacturing process described above in order to improve productivity. By adopting cold forging, the shape of the raw material after cold forging can be made to a near-net shape (almost the same shape as the final shape). This reduces the amount of machining required in the subsequent machining process. As a result, manufacturing costs can be reduced. Therefore, excellent cold forgeability is required for steel materials.

[0010] The purpose of this disclosure is to provide a steel material that exhibits excellent cold forgeability and excellent resistance to grain coarseness during the manufacturing of mechanical structural components. [Means for solving the problem]

[0011] The steel material disclosed herein is The chemical composition is expressed in mass percent. C: 0.10~0.40%, Si: 0.01~0.45%, Mn: 0.30~0.95%, P: 0.030% or less, S: 0.025% or less, Cr: over 0.50~1.80%, Al: 0.005~0.100%, N: 0.0250% or less, O: 0.0050% or less, Mo: 0~0.60%, B: 0~0.0050%, Ti: 0~0.050%, Nb: 0~0.050%, V: 0~0.150%, Cu: 0~0.40%, Ni: 0~0.30%, Sn: 0~0.10%, Ca: 0 ~ 0.0050%, and, It contains Mg: 0-0.0050%, The remainder consists of Fe and impurities. The total area ratio of ferrite and pearlite in the microstructure is 95% or more. The nitride has an equivalent circle diameter of 20 nm or more, and its average equivalent circle diameter DN is 70 nm or less, and the sample standard deviation SDN of the equivalent circle diameter of the nitride is 45 nm or less. [Effects of the Invention]

[0012] The steel material disclosed herein provides excellent cold forgeability and excellent resistance to grain coarseness during the manufacture of mechanical structural components. [Brief explanation of the drawing]

[0013] [Figure 1] Figure 1 is a schematic diagram illustrating the product rolling process in an example of the steel manufacturing method of this embodiment. [Modes for carrying out the invention]

[0014] The inventors first investigated steel materials with excellent cold forging properties from the perspective of chemical composition and microstructure. As a result, the chemical composition was found to be as follows (by mass%): C: 0.10~0.40%, Si: 0.01~0.45%, Mn: 0.30~0.95%, P: 0.030% or less, S: 0.025% or less, Cr: greater than 0.50~1.80%, Al: 0.005~0.100%, N: 0.0250% or less, O: 0.0050% or less, Mo: 0~0.60%, B: 0~0.0050%, Ti: 0~0.050% We found that steel containing Nb: 0-0.050%, V: 0-0.150%, Cu: 0-0.40%, Ni: 0-0.30%, Sn: 0-0.10%, Ca: 0-0.0050%, and Mg: 0-0.0050%, with the remainder being Fe and impurities, and having a total area ratio of ferrite and pearlite of 95% or more in its microstructure, exhibits improved cold forgeability.

[0015] Next, the inventors investigated means to improve the grain coarsening resistance of steel materials. Mechanical structural parts manufactured using steel materials may experience grain coarsening of austenite crystals during heat treatment. The inventors focused on fine nitrides precipitated in steel materials as a means to suppress grain coarsening of prior austenite crystals during the manufacture of mechanical structural parts. Fine nitrides in steel materials suppress grain coarsening of prior austenite crystals during the manufacture of mechanical structural parts through a pinning effect. The inventors considered that if a steel material has the above-mentioned chemical composition and microstructure, it may be possible to suppress grain coarsening of prior austenite crystals by fine nitrides. However, even with steel materials having the above-mentioned chemical composition and microstructure, there were cases where grain coarsening of prior austenite crystals during the manufacture of mechanical structural parts could not be suppressed. Therefore, the inventors investigated the cause of this. As a result, the following was found.

[0016] In steel materials that did not exhibit resistance to grain coarseness, it was found that fine nitrides aggregated to form clustered nitrides. These clustered fine nitride aggregates can be considered as a single coarse nitride. In this case, the pinning effect by the nitrides is not sufficiently obtained. Therefore, we hypothesized that suppressing the clustering of nitrides could potentially improve the resistance to grain coarseness of the steel.

[0017] Therefore, the inventors considered that (I) the size of the nitrides and (II) the variation of the nitrides would be effective means of improving the resistance to grain coarseness of steel materials. As a result, the inventors obtained the following findings.

[0018] (I) As described above, clustered nitrides can be considered as a single coarse nitride. When clustered nitrides are present in steel, the average equivalent diameter of the nitrides becomes excessively large. Therefore, reducing the average equivalent diameter of the nitrides is effective in improving the resistance of steel to grain coarseness.

[0019] (II) When nitride clustering is suppressed and fine nitrides are generated, not only is the average equivalent diameter of the nitrides small, but the variation in the equivalent diameter of the nitrides is also small. On the other hand, when clustered nitrides are present in the steel, the variation in the equivalent diameter of the nitrides is large. In this case, the sample standard deviation of the equivalent diameter of the nitrides becomes large. Therefore, to improve the resistance of steel to grain coarseness, it is effective to reduce the sample standard deviation of the equivalent diameter of the nitrides.

[0020] Based on the findings in (I) and (II) above, the inventors conducted an investigation. As a result, the inventors found that in steel materials having the above-mentioned chemical composition and microstructure, if the average equivalent circle diameter DN is 70 nm or less and the sample standard deviation SDN of the equivalent circle diameter of the nitride is 45 nm or less, then the clustering of the nitride is suppressed, and excellent resistance to grain coarseness of the steel material can be obtained.

[0021] The steel material of this embodiment was completed based on the above technical concept and has the following configuration.

[0022] The steel material of the first component is The chemical composition is expressed in mass percent. C: 0.10~0.40%, Si: 0.01~0.45%, Mn: 0.30~0.95%, P: 0.030% or less, S: 0.025% or less, Cr: over 0.50~1.80%, Al: 0.005~0.100%, N: 0.0250% or less, O: 0.0050% or less, Mo: 0~0.60%, B: 0~0.0050%, Ti: 0~0.050%, Nb: 0~0.050%, V: 0~0.150%, Cu: 0~0.40%, Ni: 0~0.30%, Sn: 0~0.10%, Ca: 0 ~ 0.0050%, and, It contains Mg: 0-0.0050%, The remainder consists of Fe and impurities. The total area ratio of ferrite and pearlite in the microstructure is 95% or more. The nitride has an equivalent circle diameter of 20 nm or more, and its average equivalent circle diameter DN is 70 nm or less, and the sample standard deviation SDN of the equivalent circle diameter of the nitride is 45 nm or less.

[0023] The steel material of the second component is, A steel material of the first composition, The aforementioned chemical composition, in mass%, Mo: 0.01~0.60%, B: 0.0001~0.0050%, Ti: 0.001~0.050%, Nb: 0.001~0.050%, V: 0.001~0.150%, Cu: 0.01~0.40%, Ni: 0.01~0.30%, Sn: 0.01~0.10%, Ca: 0.0001~0.0050%, and, It contains one or more elements selected from the group consisting of Mg: 0.0001 to 0.0050%.

[0024] The steel material of this embodiment will be described in detail below. In the following description, the percentage (%) for the content of each element means "mass %" unless otherwise specified. Furthermore, "ferrite" in this specification refers to a microstructure without substructure within the grain, and does not include ferrite that constitutes pearlite or bainite.

[0025] [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: 0.10~0.40%, Si: 0.01~0.45%, Mn: 0.30~0.95%, P: 0.030% or less, S: 0.025% or less, Cr: greater than 0.50~1.80%, Al: 0.005~0.100%, N: 0.0250% or less, O: 0.0050% or less, Mo: 0~ It contains 0.60% of the following: B: 0-0.0050%, Ti: 0-0.050%, Nb: 0-0.050%, V: 0-0.150%, Cu: 0-0.40%, Ni: 0-0.30%, Sn: 0-0.10%, Ca: 0-0.0050%, and Mg: 0-0.0050%, with the remainder being Fe and impurities. (Feature 2) The total area ratio of ferrite and pearlite in the microstructure is 95% or more. (Feature 3) For nitrides with an equivalent circle diameter of 20 nm or more, the average equivalent circle diameter DN is 70 nm or less, and the sample standard deviation SDN of the equivalent circle diameter of the nitride is 45 nm or less. The following describes each of its features.

[0026] [(Feature 1) Regarding chemical composition] The chemical composition of the steel material in this embodiment contains the following elements:

[0027] C: 0.10~0.40% Carbon (C) increases the strength of mechanical structural components manufactured using steel as a material. If the C content is less than 0.10%, 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 0.40%, the strength of the steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel decreases. Therefore, the C content is 0.10-0.40%. The preferred lower limit for the C content is 0.16%, more preferably 0.18%, and even more preferably 0.20%. The preferred upper limit for the C content is 0.38%, more preferably 0.36%, and even more preferably 0.34%.

[0028] Si: 0.01~0.45% Silicon (Si) deoxidizes steel during the steelmaking stage of the manufacturing process. If the Si content is less than 0.01%, 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 Si content exceeds 0.45%, the strength of the steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel decreases. Therefore, the Si content is 0.01 to 0.45%. The preferred lower limit for the Si content is 0.02%, more preferably 0.03%, and even more preferably 0.04%. The preferred upper limit for the Si content is 0.40%, more preferably 0.35%, and even more preferably 0.30%.

[0029] Mn: 0.30~0.95% Manganese (Mn) enhances the strength of mechanical structural components manufactured from steel. If the Mn 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 Mn content exceeds 0.95%, the strength of the steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel decreases. Therefore, the Mn content is 0.30-0.95%. The preferred lower limit of the Mn content is 0.35%, more preferably 0.40%, and even more preferably 0.45%. The preferred upper limit for the Mn content is 0.90%, more preferably 0.85%, and even more preferably 0.80%.

[0030] P:0.030% or less Phosphorus (P) is an impurity. In other words, the P content is greater than 0%. P tends to segregate in steel, reducing its cold forgeability. If the P content exceeds 0.030%, the cold forgeability of the steel will be significantly reduced, even if the content of other elements is within the range of this embodiment. Therefore, the P content is 0.030% or less. A low phosphorus (P) content is preferable. However, excessive reduction of the P content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the P content is 0.028%, more preferably 0.025%, and even more preferably 0.020%.

[0031] S: 0.025% or less Sulfur (S) is an impurity; therefore, the S content is greater than 0%. S combines with Mn to form MnS, which improves the machinability of the steel. Assuming that the content of other elements is within the range of this embodiment, even a small amount of S is present, the above effects can be sufficiently obtained. On the other hand, if the sulfur content exceeds 0.025%, coarse MnS is produced in excess. Coarse MnS can become the starting point 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 be significantly reduced. Therefore, the sulfur content is 0.025% or less. A low sulfur (S) content is preferable. However, excessive reduction of the S content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the S content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit for the S content is 0.022%, more preferably 0.020%, and even more preferably 0.018%.

[0032] Cr: More than 0.50~1.80% Chromium (Cr) enhances the hardenability of steel materials, thereby increasing the strength of mechanical structural components. Assuming that the Cr content exceeds 0.50%, the above effects can be effectively obtained, provided that the content of other elements is within the range of this embodiment. On the other hand, if the Cr content exceeds 1.80%, the strength of the steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel decreases. Therefore, the Cr content is between 0.50% and 1.80%. The preferred lower limit for the Cr content is 0.51%, more preferably 0.55%, more preferably 0.58%, and still more preferably 0.60%. The preferred upper limit for the Cr content is 1.75%, more preferably 1.70%, and even more preferably 1.60%.

[0033] Al: 0.005~0.100% Aluminum (Al) deoxidizes steel during the steelmaking stage of the steel manufacturing process. Al further combines with nitrogen in the steel to form AlN. This suppresses the coarsening of austenite grains during heat treatment in the manufacturing process of mechanical structural parts. If the Al content is less than 0.005%, 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 0.100%, the fine AlN particles generated tend to cluster. In this case, even if the content of other elements is within the range of this embodiment, it is not possible to suppress the coarsening of austenite crystal grains during heat treatment in the manufacturing process of mechanical structural parts. Therefore, the Al content is between 0.005% and 0.100%. The preferred lower limit for the Al content is 0.008%, more preferably 0.010%, and even more preferably 0.012%. The preferred upper limit for the Al content is 0.090%, more preferably 0.080%, more preferably 0.070%, and still more preferably 0.067%. In this embodiment, the Al content refers to the total Al (Total-Al) content.

[0034] N: 0.0250% or less Nitrogen (N) is an impurity; that is, the N content is greater than 0%. N forms nitrides, which suppress the coarsening of austenite crystal grains during heat treatment in the manufacturing process of mechanical structural parts. Assuming that the content of other elements is within the range of this embodiment, even a small amount of N is present, the above effect can be sufficiently obtained. On the other hand, if the N content exceeds 0.0250%, clustered nitrides are likely to form. In this case, even if the content of other elements is within the range of this embodiment, it is not possible to suppress the coarsening of austenite crystal grains during heat treatment in the manufacturing process of mechanical structural parts. Therefore, the N content is 0.0250% or less. A low N content is preferable. However, excessive reduction of the N content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the N content is 0.0001%, more preferably 0.0030%, even more preferably 0.0060%, and even more preferably 0.0080%. The preferred upper limit for the N content is 0.0243%, more preferably 0.0200%, even more preferably 0.0180%, and even more preferably 0.0150%.

[0035] O: 0.0050% or less Oxygen (O) is an impurity. In other words, the O content is greater than 0%. If the O content exceeds 0.0050%, coarse oxides are excessively produced. In this case, 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 O content is 0.0050% or less. A low oxygen content is preferable. However, excessive reduction of the oxygen content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the oxygen content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The preferred upper limit for the O content is 0.0045%, more preferably 0.0040%, even more preferably 0.0033%, even more preferably 0.0030%, and even more preferably 0.0025%.

[0036] The remainder of the chemical composition of the steel material in 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 in this embodiment.

[0037] [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: 0-0.60%, B: 0-0.0050%, Ti: 0-0.050%, Nb: 0-0.050%, V: 0-0.150%, Cu: 0-0.40%, Ni: 0-0.30%, Sn: 0-0.10%, Ca: 0-0.0050%, and Mg: 0-0.0050%. These elements are all optional and do not need to be included. The following describes these optional elements.

[0038] [Group 1: Mo and B] The chemical composition of the steel material in this embodiment may further include one or more elements selected from the group consisting of Mo and B in place of a portion of Fe. All of these elements enhance the hardenability of the steel material and increase the strength of the mechanical structural components.

[0039] Mo: 0~0.60% Molybdenum (Mo) is an optional element and does not need to be included. In other words, the Mo content may be 0%. When present, i.e., when the Mo content is greater than 0%, Mo enhances the hardenability of steel and increases the strength of mechanical structural components. Even a small amount of Mo will provide some degree of the above effect. On the other hand, if the Mo content exceeds 0.60%, the strength of the steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel decreases. Therefore, the Mo content is 0-0.60%. The preferred lower limit for the Mo content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit for the Mo content is 0.50%, more preferably 0.40%, and even more preferably 0.30%.

[0040] B: 0~0.0050% Boron (B) is an optional element and does not need to be included. In other words, the B content may be 0%. If present, i.e., if the B content is greater than 0%, B enhances the hardenability of steel and increases the strength of mechanical structural components. Even if only a small amount of B is present, the above effect can be obtained to some extent. On the other hand, if the B content exceeds 0.0050%, the above effect will saturate. Therefore, the B content is 0-0.0050%. 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 0.0045%, more preferably 0.0035%, and even more preferably 0.0025%.

[0041] [Group 2: Ti, Nb, and V] The chemical composition of the steel material in this embodiment may further contain one or more elements selected from the group consisting of Ti, Nb, and V in place of a portion of Fe. All of these elements form precipitates that increase the strength of the mechanical structural components.

[0042] Ti: 0~0.050% 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 generates Ti precipitates (carbides, carbonitrides, or nitrides) during the manufacturing process of mechanical structural parts made from steel. These Ti precipitates increase the strength of the mechanical structural parts. Even if only a small amount of Ti is present, the above effect can be obtained to some extent. On the other hand, if the Ti content exceeds 0.050%, coarse Ti precipitates will be excessively formed. In this case, even if the content of other elements is within the range of this embodiment, the toughness of the mechanical structural component will decrease. Therefore, the Ti content is 0-0.050%. 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%.

[0043] Nb: 0~0.050% 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 generates Nb precipitates (carbides, carbonitrides, or nitrides) during the manufacturing process of mechanical structural parts made from steel. These Nb precipitates increase the strength of the mechanical structural parts. Even if only a small amount of Nb is present, the above effect can be obtained to some extent. On the other hand, if the Nb content exceeds 0.050%, excessive amounts of coarse Nb precipitates are formed. In this case, even if the content of other elements is within the range of this embodiment, the toughness of the mechanical structural component will decrease. Therefore, the Nb content is 0-0.050%. The preferred lower limit of the Nb content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Nb content is 0.040%, more preferably 0.030%, and even more preferably 0.020%.

[0044] V: 0~0.150% 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 generates V precipitates (carbides or carbonitrides) during the manufacturing process of mechanical structural parts made from steel. These V precipitates increase the strength of the mechanical structural parts. Even if only a small amount of V is present, the above effect can be obtained to some extent. On the other hand, if the V content exceeds 0.150%, excessive V precipitates will be formed in the steel. In this case, even if the content of other elements is within the range of this embodiment, the toughness of the mechanical structural component will decrease. Therefore, the V content is 0-0.150%. The preferred lower limit of the V content is 0.001%, more preferably 0.010%, and even more preferably 0.020%. The preferred upper limit for the V content is 0.130%, more preferably 0.110%, and even more preferably 0.090%.

[0045] [Group 3: Cu, Ni, and Sn] The chemical composition of the steel material in this embodiment may further contain one or more elements selected from the group consisting of Cu, Ni, and Sn in place of a portion of Fe. All of these elements enhance the corrosion resistance of the steel material.

[0046] Cu: 0~0.40% 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 corrosion resistance of steel. Even a small amount of Cu will provide some degree of the above effect. On the other hand, if the Cu content exceeds 0.40%, the steel becomes brittle. In this case, even if the content of other elements is within the range of this embodiment, the hot workability of the steel decreases. Therefore, the copper content is 0-0.40%. The preferred lower limit for the Cu content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit for the Cu content is 0.36%, more preferably 0.32%, and even more preferably 0.28%.

[0047] Ni: 0~0.30% 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 corrosion resistance of steel. Even a small amount of Ni will provide some degree of the above effect. On the other hand, if the Ni content exceeds 0.30%, the strength of the steel becomes excessively high. In this case, even if the content of other elements is within the range of this embodiment, the cold forgeability of the steel decreases. Therefore, the Ni content is 0-0.30%. The preferred lower limit for the Ni content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit for the Ni content is 0.26%, more preferably 0.22%, and even more preferably 0.18%.

[0048] Sn: 0~0.10% Tin (Sn) is an optional element and does not need to be included. In other words, the Sn content may be 0%. When present, i.e., when the Sn content is greater than 0%, Sn enhances the corrosion resistance of steel. Even a small amount of Sn present will provide some degree of the above effect. On the other hand, if the Sn content exceeds 0.10%, the steel becomes brittle. In this case, even if the content of other elements is within the range of this embodiment, the hot workability of the steel decreases. Therefore, the Sn content is 0-0.10%. The preferred lower limit for the Sn content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the Sn content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0049] [Group 4: 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. All of these elements enhance the cold forgeability of the steel material.

[0050] Ca: 0~0.0050% 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, that is, when the calcium content is greater than 0%, the calcium fixes the sulfur in the steel as spherical CaS. This improves the cold forgeability of the steel. Even a small amount of calcium can provide some degree of this effect. On the other hand, if the Ca content exceeds 0.0050%, coarse oxides are produced in excess. In this case, 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 Ca content is 0-0.0050%. The preferred lower limit for the Ca content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit for the Ca content is 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0051] Mg: 0~0.0050% 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 spheroidizes sulfides in the steel. This improves the cold forgeability of the steel. Even a small amount of Mg can provide some degree of this effect. On the other hand, if the Mg content exceeds 0.0050%, coarse oxides are excessively produced. In this case, 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 Mg content is 0-0.0050%. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit for the Mg content is 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0052] [(Feature 2) About Microorganisms] The microstructure of the steel material in this embodiment is a mixed structure of ferrite and pearlite. More specifically, the total area ratio of ferrite and pearlite is 95% or more. The area ratio of ferrite is not particularly limited, but for example, it is 55-80%. The total area ratio of ferrite and pearlite may be 100%. If the total area ratio of ferrite and pearlite is less than 100%, the remainder of the microstructure other than ferrite and pearlite consists of one or more materials selected from the group consisting of, for example, bainite and martensite.

[0053] If the total area ratio of ferrite and pearlite in the microstructure is less than 95%, sufficient cold forgeability of the steel material cannot be obtained. If the total area ratio of ferrite and pearlite in the microstructure is 95% or more, excellent cold forgeability can be obtained, provided that characteristics 1 and 3 are satisfied.

[0054] The preferred lower limit for the total area ratio of ferrite and pearlite is 96%, more preferably 97%, and even more preferably 98%.

[0055] [Regarding the method for measuring the total area ratio of ferrite and pearlite in microstructures] The total area ratio of ferrite and pearlite in the microstructure is determined by the following method. A test specimen is taken from a cross-section of the steel material perpendicular to its axial direction, including the R / 2 portion. Here, the R / 2 portion refers to the part corresponding to the center of the radius (R) in a circular cross-section perpendicular to the axial direction of the steel material. The surface of the test specimen with a cross-section perpendicular to the axial direction of the steel material is used as the observation surface.

[0056] The observation surface of the test specimen is polished to a mirror finish. Etching is performed on the mirror-polished observation surface using 2% nitric acid alcohol (Nital etching solution). The observation area centered on the R / 2 portion of the etched observation surface is observed using a 400x optical microscope. The area of ​​the observation region is 500 μm × 500 μm.

[0057] First, ferrite is identified within the observation area. In the field of view of an optical microscope, ferrite is observed with higher brightness than other tissues. Therefore, ferrite within the observation area can be identified based on contrast. Next, the observation area is observed using a scanning electron microscope (SEM) at a magnification of 2000x. Among the tissues other than ferrite within the observation area, tissues with a lamellar structure are identified as pearlite.

[0058] Determine the total area of ​​ferrite and pearlite identified in any three observation regions. Divide the total area of ​​ferrite and pearlite by the total area of ​​the observation region (500 μm × 500 μm × 3) to obtain the percentage of the total area of ​​ferrite and pearlite. The percentage of the total area of ​​ferrite and pearlite is an integer value obtained by rounding the calculated value to the first decimal place.

[0059] [(Feature 3) Regarding the mean equivalent circle diameter DN and the sample standard deviation SDN of the equivalent circle diameter of nitrides] In the steel material of this embodiment, the average equivalent circle diameter DN of nitrides with an equivalent circle diameter of 20 nm or more is 70 nm or less, and the sample standard deviation SDN of the equivalent circle diameter of the nitrides is 45 nm or less. In this specification, nitrides with an equivalent circle diameter of 20 nm or more are also simply referred to as nitrides.

[0060] For example, if the chemical composition consists only of essential elements, the nitride is one or more selected from the group consisting of CrN, AlN, and composite nitrides containing Cr and Al. If the chemical composition consists of essential elements and optional elements, the nitride is one or more selected from the group consisting of CrN, AlN, TiN, NbN, VN, and composite nitrides containing one or more of Cr, Al, Ti, Nb, and V.

[0061] As mentioned above, fine nitrides in steel suppress the coarsening of austenite grains during heat treatment through a pinning effect. However, when nitrides cluster, the number of fine nitrides decreases. In this case, the pinning effect of nitrides decreases. Furthermore, clustered nitrides have a lower pinning effect compared to fine nitrides.

[0062] If clustered nitrides are present in the steel, the average equivalent circle diameter DN of the nitrides becomes excessively large. Furthermore, the sample standard deviation SDN of the equivalent circle diameter of the nitrides also becomes excessively large. If the average equivalent circle diameter DN of the nitrides is 70 nm or less, and the sample standard deviation SDN of the equivalent circle diameter of the nitrides is 45 nm or less, assuming that the steel satisfies characteristics 1 and 2, the clustering of nitrides is suppressed, and the pinning effect by fine nitrides is enhanced. As a result, the resistance to grain coarseness of the steel is improved.

[0063] If the average equivalent circle diameter DN exceeds 70 nm, or if the sample standard deviation SDN exceeds 45 nm, there are too many clustered nitrides. Therefore, grain coarsening of austenite crystals cannot be suppressed during heat treatment in the manufacturing process of mechanical structural parts. As a result, sufficient grain coarsening resistance of the steel material cannot be obtained.

[0064] The preferred upper limit for the average circle equivalent diameter DN is 65 nm, more preferably 60 nm, and even more preferably 55 nm. The lower limit of the average circle equivalent diameter DN is not particularly limited. However, considering typical industrial production, it is, for example, 20 nm, and more preferably 30 nm.

[0065] The preferred upper limit for the sample standard deviation SDN is 40 nm, more preferably 35 nm, and even more preferably 30 nm. The lower limit of the sample standard deviation SDN is not particularly limited. However, considering typical industrial production, it is, for example, 20 nm, and more preferably 25 nm.

[0066] [Regarding the measurement method for the mean equivalent circle diameter DN and the sample standard deviation SDN of nitrides] The mean equivalent circle diameter DN and the sample standard deviation SDN of the nitride are determined by the following method.

[0067] A test specimen for creating an extraction replica is taken from the R / 2 portion of the steel material. The cross section of the test specimen perpendicular to the axial direction of the steel material is used as the observation plane. The test specimen is prepared so that the R / 2 portion is located in the center of the observation plane.

[0068] The observation surface of the test specimen is polished to a mirror finish. After mirror polishing, the micro-test specimen with the observation surface is immersed in a 3.0% nital etching solution for 600 seconds to etch the observation surface. This makes it easier to distinguish between ferrite and pearlite in subsequent transmission electron microscope (TEM) observation. The test specimen, including the etched observation surface, is covered with a carbon vapor-deposited film (replica film). The test specimen with the vapor-deposited film covering its surface is immersed in a 5.0% nital etching solution for 1200 seconds to facilitate the removal of the vapor-deposited film from the test specimen. The vapor-deposited film is removed from the immersed test specimen. The vapor-deposited film removed from the test specimen is washed with ethanol, then scooped up with a copper sheet mesh and dried.

[0069] This deposited film (replica film) is observed using a TEM. Specifically, the R / 2 region within the observation plane is observed, and the area consisting of a structure without lamellar structure within the grains is identified as the ferrite region. Observation fields are set at 10 arbitrary points within the identified ferrite region. Each observation field is 3 μm × 3 μm. Observation is performed at a magnification of 40,000x and an acceleration voltage of 200 kV, and photographic images are generated for each observation field. Particles larger than 20 nm are identified in each observation field. Nitrides smaller than 20 nm cannot be identified as particles, so they are considered noise and excluded from the particle count.

[0070] Particles within the observation field can be identified by their contrast. Here, if multiple particles are clustered, the clustered particles (hereinafter referred to as clustered particles) are identified as a single particle. Clustering is determined when multiple particles overlap or touch. Overlapping or touching particles can be determined by the contrast of the photographic image. If particles are not touching, the brightness between them increases. In this case, since the particles are not touching, they are not determined to be clustered particles. Furthermore, clustering can be analyzed using well-known image processing software. An example of well-known image processing software is ImageJ (product name). It is well known to those skilled in the art that similar analysis is possible with image processing software other than ImageJ.

[0071] The particles in the observation field are identified, and their equivalent circular diameter is determined. The equivalent circular diameter refers to the diameter of a circle with the same area as the particle. In the steel material of this embodiment, the particles identified in the observation field are mainly nitrides. Among the particles present in the steel material of this embodiment, particles other than nitrides are, for example, MnS. In the steel material of this embodiment, the equivalent circular diameter of MnS is much larger than the equivalent circular diameter of clustered nitrides. Therefore, in the observation field, nitrides can be easily identified based on the equivalent circular diameter of the particles. Specifically, among the identified particles, all particles with an equivalent circular diameter of less than 700 nm can be considered nitrides.

[0072] The arithmetic mean of the equivalent circle diameter of each nitride identified in all observation fields is calculated and defined as the average equivalent circle diameter DN (nm). Furthermore, the sample standard deviation SDN (nm) of the equivalent circle diameter of the nitrides is calculated based on the equivalent circle diameter of each nitride in all observation fields and the average equivalent circle diameter DN of the nitrides. Note that the average equivalent circle diameter DN and the sample standard deviation SDN are integer values ​​obtained by rounding the calculated values ​​to the first decimal place.

[0073] [Effects of the steel material in this embodiment] As described above, the steel material of this embodiment satisfies features 1 to 3. Therefore, the steel material of this embodiment provides excellent cold forging properties. Furthermore, it provides excellent resistance to grain coarseness during the manufacture of mechanical structural parts.

[0074] Furthermore, the steel material of this embodiment, which provides excellent cold forgeability, has a Vickers hardness of 190 HV or less. It is a well-known fact to those skilled in the art that the lower the hardness of the steel material, the higher its cold forgeability. The preferred upper limit of the Vickers hardness of the steel material of this embodiment is 180 HV, more preferably 170 HV, and even more preferably 160 HV. The lower limit of the Vickers hardness of the steel material of this embodiment is not particularly limited. However, considering that the steel material of this embodiment satisfies features 1 to 3, the lower limit of the Vickers hardness is, for example, 120 HV, and for example, 130 HV.

[0075] [Regarding the measurement method of Vickers hardness] The Vickers hardness of steel can be measured by the following method. A test specimen including the R / 2 section is taken from the steel material. The surface of the test specimen, perpendicular to the axial direction of the steel material, is used as the measurement surface. The measurement surface is mirror-polished. A Vickers hardness test is performed at three arbitrary points on the R / 2 section of the mirror-polished measurement surface, in accordance with JIS Z 2244-1:2020, to determine the Vickers hardness (HV). The test force for the Vickers hardness test is 9.807 N. The arithmetic mean of the results obtained at the three points is taken as the Vickers hardness (HV) of the steel material.

[0076] [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 that extends in a rod shape. The steel material may be wound in a coil shape or cut to a predetermined length. The cross-section of the steel material perpendicular to the axial direction is circular.

[0077] [Applications of the steel material of this embodiment] The steel material of this embodiment is widely applicable to applications requiring cold forging properties and resistance to grain coarseness during the manufacture of mechanical structural parts. For example, the steel material of this embodiment is suitable as a material for mechanical structural parts manufactured by cold forging and heat treatment. However, the steel material of this embodiment may also be used for applications other than those mentioned above.

[0078] [Methods for manufacturing steel materials] An example of a method for manufacturing the steel material of this embodiment will be described. The method for manufacturing the steel material described below is just one example for manufacturing the steel material of this embodiment. Therefore, the steel material having the above-described structure may be manufactured by other manufacturing methods other than the method described below. However, the method described below is a preferred example of a method for manufacturing the steel material of this embodiment.

[0079] An example of a method for manufacturing steel materials according to this embodiment includes the following steps. (Process 1) Refining process (Process 2) Continuous casting process (Process 3) Hot rolling process

[0080] The following describes each step. [(Process 1) Refining Process] In the refining 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 subjected to refining in a converter (primary refining). The molten steel tapped from the converter is subjected to a well-known secondary refining. Primary refining may be carried out using, for example, an electric furnace. Through the above process, molten steel having a chemical composition satisfying characteristic 1 is produced.

[0081] [(Process 2) Continuous casting process] In the continuous casting process, bloom is produced by continuous casting using molten steel manufactured in the refining process.

[0082] [(Process 3) Hot Rolling Process] In the hot rolling process, the bloom is hot-rolled to produce steel. The hot rolling process includes the bract rolling process and the product rolling process.

[0083] [(Step 31) 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 hot-rolled using the continuous rolling mill to further reduce its size. In the bract rolling process, the bloom is heated in a heating furnace. The heating temperature of the bloom in the heating furnace is set to 1100-1250°C, and the holding time at the heating temperature is set to 60-300 minutes. The heating temperature here refers to the temperature of the heating furnace (°C). The billets produced by the bract rolling process are cooled to room temperature before the product rolling process.

[0084] [(Process 32) Product Rolling Process] In the product rolling process, the steel material of this embodiment is manufactured by hot rolling (product rolling) of the billet. Specifically, the billet after the bloc rolling process is heated using a heating furnace. The heating temperature is not particularly limited, but for example, it is set to 950 to 1150°C. Hot rolling (product rolling) is performed on the heated billet using a continuous rolling mill. The continuous rolling mill includes multiple rolling stands arranged in a line from upstream to downstream. In product rolling, the surface temperature of the billet at the exit rolling stand of the continuous rolling mill that applies the final reduction to the billet is defined as the product rolling temperature. The product rolling temperature is, for example, 830°C or higher.

[0085] The average cooling rate CR in the temperature range from the product rolling temperature to 650°C shall be 2°C / second or less. As a result, assuming that the product rolling conditions described later are met, the total area ratio of ferrite and pearlite in the steel material shall be 95% or more. The product rolling process further satisfies the following product rolling conditions.

[0086] [Regarding product rolling conditions] As described above, a continuous rolling mill includes multiple rolling stands arranged in a line from upstream to downstream. Each stand includes multiple rolls arranged around a pass line. A die is formed in the rolling rolls of each stand. Hot rolling is performed by passing a billet through the die formed by the multiple rolls of each stand, thereby gradually reducing the cross-section of the billet and producing steel material.

[0087] Figure 1 is a schematic diagram illustrating the product rolling process in this manufacturing method. Referring to Figure 1, among the multiple rolling stands of a continuous rolling mill, the group of rolling stands arranged continuously from the upstream rolling stand is called the "Roughing Train (RT)". The group of rolling stands arranged continuously downstream of the Roughing Train is called the "Intermediate Train (IT)". The group of rolling stands arranged one or continuously downstream of the Intermediate Train is called the "Finishing Train (FT)". In short, for convenience, the continuous rolling mill is divided into three groups of rolling stands from upstream to downstream: the Roughing Train RT, the Intermediate Train IT, and the Finishing Train FT. The number of rolling stands in the Roughing Train RT, the Intermediate Train IT, and the Finishing Train FT is not particularly limited. To explain the product rolling conditions in the roughing mill train RT, intermediate rolling mill train IT, and final rolling mill train FT, which will be described later, the continuous rolling mill will be conveniently divided into three rolling stand groups (roughing mill train RT, intermediate rolling mill train IT, and final rolling mill train FT).

[0088] In the product rolling process, the cross-sectional area perpendicular to the longitudinal direction of the billet produced in the bract rolling process and before entering the continuous rolling mill (hereinafter also simply referred to as the cross-sectional area) is S O Let S be the cross-sectional area of ​​the billet at the point when it has passed through the roughing mill train RT. RT Let S be the cross-sectional area of ​​the billet at the point when it has passed through the intermediate rolling mill train IT. IT Let S be the cross-sectional area of ​​the billet at the point when it has passed through the final rolling mill train FT, that is, the cross-sectional area of ​​the steel material produced by the product rolling process. FTLet it be so. The total reduction ratio R(%) in the product rolling process is defined by the following formula. R=(1-(S FT / S O ))×100

[0089] Here, the situation where a billet receives one reduction (external force) from a pair of work rolls of a single rolling stand is called a "pass". When n is a positive integer, the cross-sectional area of the billet immediately after the n-th pass is defined as S n . The cumulative reduction ratio R n ac(%) from the time when the billet just out of the heating furnace first receives reduction by the continuous rolling mill (the 1st pass) to the n-th pass is defined by the following formula. R n ac=(1-(S n / S O ))×100 At this time, the cumulative reduction ratio R RT ac(%) from the 1st pass to the time when passing through the rough rolling mill train RT, the cumulative reduction ratio R IT ac(%) from the 1st pass to the time when passing through the intermediate rolling mill train IT, and the cumulative reduction ratio R FT ac(%) from the 1st pass to the time when passing through the final rolling mill train FT are respectively expressed by the following formulas. R RT ac=(1-(S RT / S O ))×100 R IT ac=(1-(S IT / S O ))×100 R FT ac=(1-(S FT / S O ))×100=R

[0090] Also, the ratio R n pr(%) of the cumulative reduction ratio R n ac up to the n-th pass with respect to the total reduction ratio R in the product rolling process is defined by the following formula. R n pr=R n ac / R×100=(1-(S n / S O )) / (1-(S FT / S O )) × 100 The rough rolling mill train RT, the intermediate rolling mill train IT, and the final rolling mill train FT are R n It is classified based on pr(%). Specifically, from the upstream rolling stand, R n The rolling mill train RT is defined as the first pass up to the rolling stand where the pr(%) is 72% or higher. From the rolling stand following the rough rolling mill train RT, R n The rolling mill train IT is defined as the section from the first pass where pr(%) exceeds 95% to the final rolling mill. The rolling mill train FT is defined as the section from the next rolling mill after intermediate rolling mill train IT to the final rolling mill. The cumulative reduction ratio R from the first pass to the point where it passes through the rough rolling mill train RT is defined as the total reduction ratio R. RT Ratio of ac RT pr (%), cumulative reduction ratio R from the first pass to the point where it passes through the intermediate rolling mill train IT relative to the total reduction ratio R. IT Ratio of ac IT pr (%), and the cumulative reduction ratio R from the first pass to the point where it passes through the final rolling mill train FT relative to the total reduction ratio R. FT Ratio of ac FT pr(%) is expressed by the following formulas. R RT pr=R RT ac / R × 100 = (1 - (S RT / S O )) / (1-(S FT / S O )) × 100 R IT pr=R IT ac / R × 100 = (1 - (S IT / S O )) / (1-(S FT / S O )) × 100 R FT pr=R FT ac / R × 100 = 100

[0091] Furthermore, the reduction ratio R in the roughing mill train RT RT (%), reduction ratio R in the intermediate rolling mill train IT IT (%) and the reduction ratio R at the final rolling mill train FT FT (%) is defined by the following formula: R RT =(1-(S RT / S O )) × 100 = R RT ac R IT =(1-(S IT / S RT )) × 100 R FT =(1-(S FT / S IT )) × 100

[0092] Referring to Figure 1, the entry temperature at the roughing mill train RT is T RT Let (°C) be the temperature at the entry side of the intermediate rolling mill train IT be T IT Let (°C) be the temperature at the entry side of the final rolling mill train FT, and T be the temperature at the entry side of the final rolling mill train. FT Let the temperature be (°C). In this embodiment, the following equation (1) is satisfied during the product rolling process. R RT ×exp{-(T RT -900) 2 / 1800} / 75.2+R IT ×exp{-(T IT -900) 2 / 1800} / 75.2+R FT ×exp{-(T FT -900) 2 / 1800} / 75.2≧0.50 (1)

[0093] Fn1 is defined as follows: Fn1=R RT ×exp{-(T RT -900) 2 / 1800} / 75.2+R IT ×exp{-(T IT -900) 2 / 1800} / 75.2+R FT ×exp{-(T FT -900) 2 / 1800} / 75.2 Fn1 is an index for suppressing nitride clustering. The amount of reduction applied in the roughing mill RT, intermediate rolling mill IT, and final rolling mill FT, as well as the temperature at which the reduction is applied, all influence the suppression of nitride clustering.

[0094] In the rough rolling mill train RT, intermediate rolling mill train IT, and final rolling mill train FT in the product rolling process, strain due to rolling reduction is imparted to the billet. This strain serves as a driving force and promotes the formation of fine nitrides. Here, the inlet temperature T in the rough rolling mill train RT, intermediate rolling mill train IT, and final rolling mill train FT RT , the inlet temperature T IT and the inlet temperature T FT If they are too high, the strain imparted by rolling reduction disappears early. In this case, the generated nitrides tend to cluster.

[0095] If Fn1 is 0.50 or more, the reduction ratios R in the rough rolling mill train RT, intermediate rolling mill train IT, and final rolling mill train FT RT , the reduction ratio R IT and the reduction ratio R FT are appropriate, and the inlet temperatures T in the rough rolling mill train RT, intermediate rolling mill train IT, and final rolling mill train FT RT , the inlet temperature T IT and the inlet temperature T FT are appropriate temperatures. Therefore, in the product rolling process, fine nitrides are formed and the clustering of nitrides is suppressed. As a result, the average equivalent circle diameter DN of the nitrides becomes 70 nm or less, and the specimen standard deviation SDN of the equivalent circle diameter becomes 45 nm or less.

[0096] By satisfying the above manufacturing process, the steel material of the present embodiment that satisfies Features 1 to 3 is manufactured. Note that the steel material of the present embodiment may be manufactured by a manufacturing method other than the above-described manufacturing method.

Example

[0097] The effects of the steel material of the present embodiment will be further specifically described by examples. The conditions in the following examples are one example of the conditions adopted to confirm the feasibility and effects of the steel material of the present embodiment. Therefore, the steel material of the present embodiment is not limited to this one example of conditions.

[0098] Steel materials (bar steels) having the chemical compositions shown in Table 1 (Table 1A and Table 1B) were manufactured by the following method.

[0099] [Table 1A]

[0100] [Table 1B]

[0101] Specifically, using molten steel produced in the refining process, a bloom was manufactured in the continuous casting process using the continuous casting method. The manufactured bloom was then subjected to a bloc rolling process to produce a billet. The holding temperature of the heating furnace in the bloc rolling process was 1200°C, and the holding time at this temperature was 120 minutes. The manufactured billet was allowed to cool to room temperature.

[0102] The manufactured billets underwent a product rolling process. The heating temperature in the furnace during the product rolling process was 1100-1150°C. The final product rolling temperature was 830°C or higher. The average cooling rate CR in the temperature range from the product rolling temperature to 650°C was 2°C / second or less.

[0103] Through the above process, a steel bar with a diameter of 60 mm was manufactured.

[0104] In the manufacturing process, the entry temperature T of the rough rolling mill train in the product rolling process. RT (°C), R reduction ratio in the roughing mill. RT (%), entry temperature T in the intermediate rolling mill train IT (°C), reduction ratio R in the intermediate rolling mill. IT (%), entry temperature T at the final rolling mill train FT (°C), reduction ratio R at the final rolling mill. FT (%) and Fn1 were as shown in Table 2.

[0105] [Table 2]

[0106] [Evaluation Test] The following evaluation tests were conducted using the steel materials for each test number. (Test 1) Measurement test of total area ratio of ferrite and pearlite (Test 2) Measurement test of the mean equivalent circle diameter DN and the sample standard deviation SDN of the equivalent circle diameter of nitrides. (Test 3) Vickers hardness test (Test 4) Evaluation test for resistance to coarse grain formation The following describes each test.

[0107] [(Test 1) Measurement test of total area ratio of ferrite and perlite] Based on the method described in [Measurement Method for the Total Area Ratio of Ferrite and Pearlite in Microstructure] above, the total area ratio (%) of ferrite and pearlite in the steel material for each test number was determined. The total area ratio (%) of ferrite and pearlite is shown in the "F+P Total Area Ratio (%)" column in Table 3. In all test numbers, the area ratio of ferrite was between 55% and 80%.

[0108] [Table 3]

[0109] [(Test 2) Measurement test of the mean equivalent circle diameter DN and the sample standard deviation SDN of the equivalent circle diameter of nitrides] Based on the method described above in "[Measurement Method for Average Equivalent Circular Diameter DN and Sample Standard Deviation SDN of Equivalent Circular Diameter of Nitrides]", the average equivalent circular diameter DN (nm) and the sample standard deviation SDN (nm) of the nitrides for each test number of steel material were determined. The "DN (nm)" column in Table 3 shows the average equivalent circular diameter DN (nm) of the nitrides. The "SDN (nm)" column in Table 3 shows the sample standard deviation SDN (nm) of the equivalent circular diameter of the nitrides.

[0110] In Test 2, point analysis was performed on 20 randomly selected particles identified as nitrides by TEM observation using an Energy Dispersive X-ray Spectrometry (EDS) instrument attached to the TEM. As a result, one or more of the elements Cr, Al, Ti, Nb, and V, along with N, were detected in each particle. In other words, it was confirmed that the particles identified as nitrides in Test 2 were indeed nitrides.

[0111] [(Test 3) Vickers hardness test] In this example, the Vickers hardness (HV) of the steel material for each test number was used as an indicator of cold forgeability. Specifically, the Vickers hardness (HV) of the steel material for each test number was determined based on the method described in [Regarding the Measurement Method of Vickers Hardness] above. The obtained Vickers hardness (HV) is shown in the "Vickers Hardness (HV)" column of Table 3. If the Vickers hardness of the steel material was 190 HV or less, it was judged that excellent cold forgeability was obtained. On the other hand, if the Vickers hardness of the steel material exceeded 190 HV, it was judged that excellent cold forgeability was not obtained.

[0112] [(Test 4) Evaluation test for resistance to coarse granulation] The coarse grain resistance characteristics of each test number were evaluated using the following method. Specifically, the steel material for each test number was cut perpendicular to the axial direction and then machined to produce test specimens with a diameter of 30 mm and a length of 30 mm. The center of each test specimen approximately coincided with the center of the steel bar for each test number. The collected test specimens were subjected to a heat treatment simulating carburizing (gas carburizing). In the gas carburizing treatment, the carbon potential was set to 0.8%, and the specimens were held at 940°C for 5 hours. Subsequently, they were held at 850°C for 0.5 hours. After these steps, the test specimens were oil-cooled and quenched.

[0113] The prior austenite grains were observed at a depth of 0.2 mm from the surface. Specifically, the cross-section perpendicular to the longitudinal direction of the carburized steel part was used as the observation surface. After mirror polishing the observation surface, etching was performed with a saturated picric acid solution. The field of view (300 μm × 300 μm) including the position 0.2 mm from the surface of the etched observation surface was observed with an optical microscope (400x magnification) to identify the prior austenite grains. For the identified prior austenite grains, the grain size of each prior austenite grain was determined in terms of equivalent circle diameter (μm) in accordance with JIS G 0551:2020. If there were no prior austenite grains whose equivalent circle diameter exceeded the equivalent circle diameter corresponding to grain size number 5 (62.5 μm) as defined in the above JIS standard, the evaluation was set to "E (Excellent)", and it was determined that excellent resistance to grain coarsening was obtained (indicated as "E" in the "Resistance to Grain Coarsening" column in Table 3). If even one crystal grain has an equivalent diameter exceeding the equivalent diameter of grain size number 5 (62.5 μm) specified in the above JIS standard, the evaluation is set to "B (Bad)," and it is determined that excellent resistance to grain coarsening was not obtained (indicated as "B" in the "Resistance to Grain Coarsening" column in Table 3).

[0114] [Evaluation Results] Referring to Tables 1 to 3, in tests 1 to 26, the steel material met characteristics 1 to 3. Therefore, the Vickers hardness was 190 HV or less, and it was determined that excellent cold forgeability was obtained. Furthermore, in the grain coarseness resistance evaluation test, it was determined that grain coarseness of the prior austenite crystal grains was suppressed, and excellent grain coarseness resistance was obtained. In addition, the remainder of the microstructure of test number 24, other than ferrite and pearlite, consisted of bainite and martensite.

[0115] On the other hand, in test numbers 27 and 28, Fn1 during the product rolling process was too low. As a result, the average equivalent circle diameter DN of the nitride exceeded 70 nm, and the sample standard deviation SDN of the equivalent circle diameter of the nitride exceeded 45 nm. Consequently, excellent resistance to grain coarseness could not be obtained.

[0116] 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: 0.10-0.40%, Si: 0.01-0.45%, Mn: 0.30-0.95%, P: 0.030% or less, S: 0.025% or less, Cr: more than 0.50 to 1.80%, Al: 0.005-0.100%, N: 0.0250% or less, O: 0.0050% or less, Mo: 0 to 0.60%, B: 0 to 0.0050%, Ti: 0 to 0.050%, Nb: 0 to 0.050%, V: 0 to 0.150%, Cu: 0 to 0.40%, Ni: 0 to 0.30%, Sn: 0 to 0.10%, Ca: 0-0.0050%, and, It contains Mg: 0-0.0050%, The remainder consists of Fe and impurities. The total area ratio of ferrite and pearlite in the microstructure is 95% or more. When an observation field of 3 μm × 3 μm is set to any 10 points within the ferrite, the average equivalent circle diameter DN of the nitride with an equivalent circle diameter of 20 nm or more that can be identified in all of the observation fields is 70 nm or less, and the sample standard deviation SDN of the equivalent circle diameter of the nitride with an equivalent circle diameter of 20 nm or more that can be identified in all of the observation fields is 45 nm or less. Steel material.

2. The steel material according to claim 1, The aforementioned chemical composition is, in mass%, Mo: 0.01 to 0.60%, B: 0.0001 to 0.0050%, Ti: 0.001 to 0.050%, Nb: 0.001 to 0.050%, V: 0.001-0.150%, Cu: 0.01-0.40%, Ni: 0.01 to 0.30%, Sn: 0.01 to 0.10%, Ca: 0.0001 to 0.0050%, and, It contains one or more elements selected from the group consisting of Mg: 0.0001 to 0.0050%, Steel material.

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