Steel material

Abstract

A steel material according to the present disclosure has a chemical composition containing, in terms of mass%, 0.10-0.40% C, 0.01-0.45% Si, 0.30-0.95% Mn, not more than 0.030% P, not more than 0.025% S, more than 0.50 and up to 1.80% Cr, 0.005-0.100% Al, not more than 0.0250% N, and not more than 0.0050% O with the remainder being Fe and impurities, wherein: the total area ratio of ferrite and pearlite in the microstructure is not less than 95%; an average equivalent circle diameter DN of nitrides having an equivalent circle diameter of not less than 20 nm is not more than 70 nm; and a sample standard deviation SDN of the equivalent circle diameter of the nitrides is not more than 45 nm.

Description

steel material

[0001] The present disclosure relates to a steel material, and more particularly to a steel material suitable for use as a material for machine structural parts manufactured by cold forging and cutting.

[0002] High fatigue strength is required for machine structural parts, such as automobile parts, industrial machinery parts, and construction machinery parts.

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

[0004] The steel material disclosed in Patent Document 1 has a chemical composition, in mass %, of C: 0.10 to 0.35%, Si: 0.60 to 1.50%, Mn: 0.20 to 1.30%, P: 0.030% or less, S: 0.050% or less, Ni: 0.01 to 0.20%, Cr: 0.65 to 1.50%, Mo: 0.01 to 0.60%, Al: 0.045% or less, and N: 0.0250% or less, with the balance being Fe and impurities, and satisfies formulas (1) to (4). In Patent Document 1, by satisfying formulas (1) to (4), fatigue strength is increased in mechanical structural parts manufactured using the steel material as a raw material. 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 contains, by mass%, 0.10 to 0.60% C, 0.05 to 2.0% Si, 0.3 to 2.5% Mn, 0.02 to 0.25% S, 0.002 to 0.030% Al, 0.0005 to 0.01% Ca, 0.0005 to 0.008% O, and 0.02% or less N, with a mass% ratio of Ca / Al of 0.1 to 1.0 and a Ca / O ratio of more than 0.2, with the balance being Fe and inevitable impurities. In Patent Document 2, 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 components manufactured using the steel material is improved.

[0006] JP 2022-080369 A JP 2005-240120 A

[0007] Now, a method for manufacturing a mechanical structural part is, for example, as follows: A steel material is hot forged to produce a preform having the rough shape of the mechanical structural part; The preform is cut; The preform after cutting is heat treated; Examples of heat treatments include overall quenching, induction hardening, carburizing, and carbonitriding; The heat-treated preform is finished to produce the final product, the mechanical structural part.

[0008] Here, the heat treatment causes the surface layer of the mechanical structural component to have 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, in mechanical structural components, steel materials are required to have a property that suppresses coarsening of the prior austenite grains (grain coarsening resistance). However, Patent Documents 1 and 2 do not discuss specific means for suppressing coarsening of the prior austenite grains.

[0009] Recently, in order to improve productivity, efforts have been made to switch from hot forging to cold forging in the above manufacturing process. By adopting cold forging, the shape of the preform after cold forging can be made to be near net shape (a shape almost identical to the final shape). This reduces the amount of cutting in the subsequent cutting process, which in turn reduces manufacturing costs. Therefore, steel materials are required to have excellent cold forgeability.

[0010] An object of the present disclosure is to provide a steel material that can provide excellent cold forgeability and excellent resistance to grain coarsening during the production of machine structural parts.

[0011] The steel material of the present disclosure has a chemical composition, in mass%, of C: 0.10 to 0.40%, Si: 0.01 to 0.45%, Mn: 0.30 to 0.95%, P: 0.030% or less, S: 0.025% or less, Cr: over 0.50 to 1.80%, Al: 0.005 to 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%, The alloy contains Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, with the balance being Fe and impurities; the total area ratio of ferrite and pearlite in the microstructure is 95% or more; nitrides having an equivalent circle diameter of 20 nm or more have an average equivalent circle diameter DN of 70 nm or less, and the sample standard deviation SDN of the equivalent circle diameters of the nitrides is 45 nm or less.

[0012] The steel material of the present disclosure provides excellent cold forgeability and excellent resistance to grain coarsening during the production of machine structural parts.

[0013] FIG. 1 is a schematic diagram for explaining a product rolling step in an example of a method for producing a steel material according to this embodiment.

[0014] The present inventors first investigated steel materials with excellent cold forgeability from the viewpoints of chemical composition and microstructure, and found that the chemical composition was, in mass %, C: 0.10 to 0.40%, Si: 0.01 to 0.45%, Mn: 0.30 to 0.95%, P: 0.030% or less, S: 0.025% or less, Cr: over 0.50 to 1.80%, Al: 0.005 to 0.100%, N: 0.0250% or less, O: 0.0050% or less, Mo: 0 to 0.60%, B: 0 to 0.0050%, and Ti: 0 to 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 balance being Fe and impurities, and in which the total area ratio of ferrite and pearlite in the microstructure is 95% or more, the cold forgeability of the steel is found to be improved.

[0015] Next, the present inventors investigated means for improving the grain coarsening resistance of steel materials. Mechanical structural components manufactured using steel materials may experience coarsening of austenite grains during heat treatment. The present inventors focused on fine nitrides precipitated in steel materials as a means for suppressing coarsening of prior austenite grains during the manufacture of mechanical structural components. Fine nitrides in steel materials suppress coarsening of prior austenite grains during the manufacture of mechanical structural components through a pinning effect. The present inventors believed that if a steel material has the above-described chemical composition and microstructure, the fine nitrides may be able to suppress coarsening of prior austenite grains. However, even with steel materials having the above-described chemical composition and microstructure, there have been cases where coarsening of prior austenite grains during the manufacture of mechanical structural components could not be suppressed. Therefore, the present inventors investigated the cause of this problem. As a result, the following findings were discovered.

[0016] It was found that in steels that did not exhibit coarse grain resistance, fine nitrides aggregated to form clustered nitrides. The clustered aggregates of fine nitrides can be regarded as a single coarse nitride. In this case, the pinning effect of the nitrides is not fully achieved. Therefore, it was thought that if the clustering of nitrides could be suppressed, the coarse grain resistance of the steel could be improved.

[0017] Therefore, the present inventors have considered that an effective means for improving the grain coarsening resistance of steel materials is to appropriately adjust (I) the size of nitrides and (II) the variation of nitrides. As a result, the present inventors have obtained the following findings.

[0018] (I) As described above, clustered nitrides can be regarded as a single coarse nitride. When clustered nitrides are present in a steel material, the average circle-equivalent diameter of the nitrides becomes excessively large. Therefore, in order to improve the grain coarsening resistance of the steel material, it is effective to reduce the average circle-equivalent diameter of the nitrides.

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

[0020] The present inventors have conducted studies based on the findings of (I) and (II) above, and as a result, have found that in a steel material having the above-mentioned chemical composition and microstructure, if nitrides having an equivalent circle diameter of 20 nm or more have an average equivalent circle diameter DN of 70 nm or less and a sample standard deviation SDN of the equivalent circle diameters of the nitrides of 45 nm or less, then clustering of the nitrides is suppressed and the steel material can have excellent resistance to grain coarsening.

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

[0022] The steel material of the first configuration has a chemical composition, in mass%, of C: 0.10 to 0.40%, Si: 0.01 to 0.45%, Mn: 0.30 to 0.95%, P: 0.030% or less, S: 0.025% or less, Cr: over 0.50 to 1.80%, Al: 0.005 to 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%, The alloy contains Ca: 0 to 0.0050%, and Mg: 0 to 0.0050%, with the balance being Fe and impurities; the total area ratio of ferrite and pearlite in the microstructure is 95% or more; nitrides having an equivalent circle diameter of 20 nm or more have an average equivalent circle diameter DN of 70 nm or less, and the sample standard deviation SDN of the equivalent circle diameters of the nitrides is 45 nm or less.

[0023] The steel material of the second configuration is the steel material of the first configuration, wherein the chemical composition contains, in mass%, one or more elements selected from the group consisting of 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 to 0.150%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Sn: 0.01 to 0.10%, Ca: 0.0001 to 0.0050%, and Mg: 0.0001 to 0.0050%.

[0024] The steel material of this embodiment will be described in detail below. In the following description, the "%" designation for the content of each element means "mass %" unless otherwise specified. Furthermore, in this specification, "ferrite" refers to a microstructure with no substructure within the grains, and does not include ferrite that constitutes pearlite or bainite.

[0025] [Features of the Steel Material of the Present Embodiment] The steel material of the present embodiment satisfies the following features: (Feature 1) The chemical composition, in mass %, is: C: 0.10 to 0.40%, Si: 0.01 to 0.45%, Mn: 0.30 to 0.95%, P: 0.030% or less, S: 0.025% or less, Cr: over 0.50 to 1.80%, Al: 0.005 to 0.100%, N: 0.0250% or less, O: 0.0050% or less, Mo: 0 to The alloy contains 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%, with the balance consisting of Fe and impurities. (Feature 2) The total area ratio of ferrite and pearlite in the microstructure is 95% or more. (Feature 3) The average equivalent circle diameter DN of nitrides having an equivalent circle diameter of 20 nm or more is 70 nm or less, and the sample standard deviation SDN of the equivalent circle diameters of the nitrides is 45 nm or less. Each feature is described below.

[0026] [(Feature 1) Chemical Composition] The chemical composition of the steel material of this embodiment contains the following elements.

[0027] C: 0.10 to 0.40% Carbon (C) increases the strength of mechanical structural components manufactured using steel as a raw material. If the C content is less than 0.10%, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.40%, the strength of the steel becomes excessively high. In this case, the cold forgeability of the steel decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.10 to 0.40%. The preferred lower limit of the C content is 0.16%, more preferably 0.18%, and even more preferably 0.20%. The preferred upper limit of the C content is 0.38%, more preferably 0.36%, and even more preferably 0.34%.

[0028] Si: 0.01 to 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 sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 0.45%, the strength of the steel material becomes excessively high. In this case, the cold forgeability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.01 to 0.45%. The preferred lower limit of the Si content is 0.02%, more preferably 0.03%, and even more preferably 0.04%. The preferred upper limit of the Si content is 0.40%, more preferably 0.35%, and even more preferably 0.30%.

[0029] Mn: 0.30 to 0.95% Manganese (Mn) increases the strength of mechanical structural components manufactured using steel as a raw material. If the Mn content is less than 0.30%, the above effect cannot be sufficiently achieved even if the contents of other elements are within the ranges 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, the cold forgeability of the steel decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.30 to 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 of 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 and reduces the cold forgeability of the steel. If the P content exceeds 0.030%, the cold forgeability of the steel significantly decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the P content is 0.030% or less. The P content is preferably as low as possible. 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 of 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. In other words, the S content is greater than 0%. S combines with Mn to form MnS, improving the machinability of the steel. Even if even a small amount of S is contained, the above-mentioned effects are fully achieved, provided that the contents of other elements are within the ranges of this embodiment. On the other hand, if the S content exceeds 0.025%, excessive coarse MnS is formed. These coarse MnS may become the starting point for cracks during cold forging. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel is significantly reduced. Therefore, the S content is 0.025% or less. The S content is preferably as low as possible. 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 upper limit of the S content is preferably 0.022%, more preferably 0.020%, and even more preferably 0.018%.

[0032] Cr: More than 0.50% to 1.80% Chromium (Cr) improves the hardenability of steel and increases the strength of mechanical structural components. If the Cr content exceeds 0.50%, the above effects can be effectively achieved, provided that the contents of other elements are within the ranges 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 contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel decreases. Therefore, the Cr content is more than 0.50% to 1.80%. The preferred lower limit of the Cr content is 0.51%, more preferably 0.55%, even more preferably 0.58%, and even more preferably 0.60%. The preferred upper limit of the Cr content is 1.75%, even more preferably 1.70%, and even more preferably 1.60%.

[0033] Al: 0.005 to 0.100% Aluminum (Al) deoxidizes steel during the steelmaking stage of the steel manufacturing process. Furthermore, Al combines with N in the steel to form AlN. This suppresses coarsening of austenite grains during heat treatment in the manufacturing process of mechanical structural components. If the Al content is less than 0.005%, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content exceeds 0.100%, the generated fine AlN is likely to cluster. In this case, coarsening of austenite grains during heat treatment in the manufacturing process of mechanical structural components cannot be suppressed even if the contents of other elements are within the ranges of this embodiment. Therefore, the Al content is 0.005 to 0.100%. The preferred lower limit of the Al content is 0.008%, more preferably 0.010%, and even more preferably 0.012%. The upper limit of the Al content is preferably 0.090%, more preferably 0.080%, even more preferably 0.070%, and still more preferably 0.067%. The Al content in this embodiment means the content of total Al (Total-Al).

[0034] N: 0.0250% or less Nitrogen (N) is an impurity. That is, the N content is greater than 0%. N forms nitrides and suppresses coarsening of austenite grains during heat treatment in the manufacturing process of mechanical structural components. Even if even a small amount of N is contained, the above effect can be sufficiently obtained, provided that the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content exceeds 0.0250%, clustered nitrides are likely to form. In this case, even if the contents of other elements are within the ranges of this embodiment, coarsening of austenite grains during heat treatment in the manufacturing process of mechanical structural components cannot be suppressed. Therefore, the N content is 0.0250% or less. The N content is preferably as low as possible. 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 upper limit of the N content is preferably 0.0243%, more preferably 0.0200%, even more preferably 0.0180%, and still more preferably 0.0150%.

[0035] O: 0.0050% or less Oxygen (O) is an impurity. In other words, the O content exceeds 0%. If the O content exceeds 0.0050%, excessive coarse oxides are generated. In this case, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material decreases. Therefore, the O content is 0.0050% or less. The O content is preferably as low as possible. However, excessive reduction of the O content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the O content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The preferred upper limit of 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 balance of the chemical composition of the steel material of this embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the steel material, and are acceptable within a range that does not adversely affect the steel material of this embodiment.

[0037] [Regarding optional elements] The chemical composition of the steel material of 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%. All of these elements are optional elements and may not be contained. These optional elements will be described below.

[0038] [First Group: Mo and B] The chemical composition of the steel material of the present embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Mo and B. Any of these elements improves the hardenability of the steel material and increases the strength of mechanical structural parts.

[0039] Mo: 0 to 0.60% Molybdenum (Mo) is an optional element and does not necessarily need to be contained. That is, the Mo content may be 0%. When contained, that is, when the Mo content exceeds 0%, Mo improves the hardenability of the steel material and increases the strength of the mechanical structural parts. Even if even a small amount of Mo is contained, the above effects can be obtained to some extent. On the other hand, if the Mo content exceeds 0.60%, the strength of the steel material becomes excessively high. In this case, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material decreases. Therefore, the Mo content is 0 to 0.60%. The preferred lower limit of the Mo content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit of the Mo content is 0.50%, more preferably 0.40%, and even more preferably 0.30%.

[0040] B: 0 to 0.0050% Boron (B) is an optional element and does not necessarily need to be contained. In other words, the B content may be 0%. When contained, that is, when the B content exceeds 0%, B improves the hardenability of the steel material and increases the strength of mechanical structural parts. Even if even a small amount of B is contained, the above effect can be obtained to a certain extent. On the other hand, when the B content exceeds 0.0050%, the above effect saturates. Therefore, the B content is 0 to 0.0050%. The preferred lower limit of the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the B content is 0.0045%, more preferably 0.0035%, and even more preferably 0.0025%.

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

[0042] Ti: 0 to 0.050% Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When titanium is contained, that is, when the Ti content exceeds 0%, Ti forms Ti precipitates (carbides, carbonitrides, or nitrides) during the manufacturing process of a mechanical structural component made from steel. These Ti precipitates increase the strength of the mechanical structural component. Even if even a small amount of Ti is contained, the above effect can be achieved to some extent. On the other hand, when the Ti content exceeds 0.050%, excessive coarse Ti precipitates are formed. In this case, the toughness of the mechanical structural component decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ti content is 0 to 0.050%. The preferred lower limit of the Ti content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Ti content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0043] Nb: 0 to 0.050% Niobium (Nb) is an optional element and does not necessarily need to be contained. That is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content exceeds 0%, Nb forms Nb precipitates (carbides, carbonitrides, or nitrides) during the manufacturing process of a mechanical structural component made from steel. These Nb precipitates increase the strength of the mechanical structural component. Even if even a small amount of Nb is contained, the above effect can be achieved to some extent. On the other hand, when the Nb content exceeds 0.050%, excessive coarse Nb precipitates are formed. In this case, even if the contents of other elements are within the ranges of this embodiment, the toughness of the mechanical structural component decreases. Therefore, the Nb content is 0 to 0.050%. The preferred lower limit of the Nb content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Nb content is preferably 0.040%, more preferably 0.030%, and even more preferably 0.020%.

[0044] V: 0 to 0.150% Vanadium (V) is an optional element and does not necessarily need to be contained. That is, the V content may be 0%. When vanadium (V) is contained, that is, when the V content exceeds 0%, V forms V precipitates (carbides or carbonitrides) during the manufacturing process of a mechanical structural component made from steel. These V precipitates increase the strength of the mechanical structural component. Even if even a small amount of V is contained, the above effect can be achieved to some extent. On the other hand, when the V content exceeds 0.150%, excessive V precipitates are formed in the steel. In this case, even if the contents of other elements are within the ranges of this embodiment, the toughness of the mechanical structural component decreases. Therefore, the V content is 0 to 0.150%. A preferred lower limit of the V content is 0.001%, more preferably 0.010%, and even more preferably 0.020%. A preferred upper limit of the V content is 0.130%, more preferably 0.110%, and even more preferably 0.090%.

[0045] [Third Group: Cu, Ni, and Sn] The chemical composition of the steel material according to the present 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 to 0.40% Copper (Cu) is an optional element and does not necessarily need to be contained. That is, the Cu content may be 0%. When contained, that is, when the Cu content exceeds 0%, Cu enhances the corrosion resistance of the steel material. Even if even a small amount of Cu is contained, the above effect can be obtained to some extent. On the other hand, when the Cu content exceeds 0.40%, the steel material becomes embrittled. In this case, even if the contents of other elements are within the ranges of this embodiment, the hot workability of the steel material decreases. Therefore, the Cu content is 0 to 0.40%. The preferred lower limit of the Cu content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit of the Cu content is 0.36%, more preferably 0.32%, and even more preferably 0.28%.

[0047] Ni: 0 to 0.30% Nickel (Ni) is an optional element and does not necessarily need to be contained. That is, the Ni content may be 0%. When contained, that is, when the Ni content is greater than 0%, Ni enhances the corrosion resistance of the steel material. Even if even a small amount of Ni is contained, the above effect can be obtained to some extent. On the other hand, if the Ni content exceeds 0.30%, the strength of the steel material becomes excessively high. In this case, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel material decreases. Therefore, the Ni content is 0 to 0.30%. The preferred lower limit of the Ni content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit of the Ni content is 0.26%, more preferably 0.22%, and even more preferably 0.18%.

[0048] Sn: 0 to 0.10% Tin (Sn) is an optional element and does not necessarily need to be contained. In other words, the Sn content may be 0%. When contained, that is, when the Sn content exceeds 0%, Sn enhances the corrosion resistance of the steel material. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. On the other hand, if the Sn content exceeds 0.10%, the steel material becomes embrittled. In this case, even if the contents of other elements are within the ranges of this embodiment, the hot workability of the steel material decreases. Therefore, the Sn content is 0 to 0.10%. The preferred lower limit of the Sn content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Sn content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0049] [Fourth Group: Ca and Mg] The chemical composition of the steel material of 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 improve the cold forgeability of the steel material.

[0050] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and does not necessarily need to be contained. In other words, the Ca content may be 0%. When contained, that is, when the Ca content exceeds 0%, Ca fixes the S in the steel as spherical CaS. This improves the cold forgeability of the steel. Even if even a small amount of Ca is contained, the above effect can be achieved to some extent. On the other hand, if the Ca content exceeds 0.0050%, excessive coarse oxides are formed. In this case, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel will be reduced. Therefore, the Ca content is 0 to 0.0050%. The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the Ca content is 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0051] Mg: 0 to 0.0050% Magnesium (Mg) is an optional element and does not necessarily need to be contained. In other words, the Mg content may be 0%. When magnesium is contained, that is, when the Mg content exceeds 0%, Mg spheroidizes sulfides in the steel. This improves the cold forgeability of the steel. Even if even a small amount of Mg is contained, the above effect can be achieved to some extent. On the other hand, if the Mg content exceeds 0.0050%, excessive coarse oxides are generated. In this case, even if the contents of other elements are within the ranges of this embodiment, the cold forgeability of the steel will be reduced. Therefore, the Mg content is 0 to 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 of the Mg content is 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0052] [(Feature 2) Regarding the Microstructure] The microstructure of the steel material of 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 is, for example, 55 to 80%. The total area ratio of ferrite and pearlite may be 100%. When the total area ratio of ferrite and pearlite is less than 100%, the remainder of the microstructure other than ferrite and pearlite is, for example, one or more types selected from the group consisting of bainite and martensite.

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

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

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

[0056] The observation surface of the test piece is mirror-polished. The mirror-polished observation surface is etched using 2% nitric acid alcohol (Nital etching solution). An observation region of the etched observation surface, centered on the R / 2 portion, is observed using an optical microscope at 400x magnification. The area of ​​the observation region is 500 μm × 500 μm.

[0057] First, ferrite in the observation region is identified. In the observation field of an optical microscope, ferrite is observed with a higher brightness than structures other than ferrite. Therefore, ferrite in the observation region can be identified based on contrast. Next, the observation region is observed at a magnification of 2000 times using a scanning electron microscope (SEM). Of the structures other than ferrite in the observation region, a structure having a lamellar structure is identified as pearlite.

[0058] The total area of ​​ferrite and pearlite identified in any three observation regions is calculated. The total area of ​​ferrite and pearlite is divided by the total area of ​​the observation regions (500 μm × 500 μm × 3) to calculate the total area ratio (%) of ferrite and pearlite. The total area ratio of ferrite and pearlite is an integer value obtained by rounding the calculated value to one decimal place.

[0059] [(Feature 3) Average Equivalent Circular Diameter DN and Sample Standard Deviation SDN of Equivalent Circular Diameters of Nitrides] In the steel material of this embodiment, the average equivalent circular diameter DN of nitrides having an equivalent circular diameter of 20 nm or more is 70 nm or less, and the sample standard deviation SDN of the equivalent circular diameters of nitrides is 45 nm or less. In this specification, nitrides having an equivalent circular diameter of 20 nm or more are also simply referred to as nitrides.

[0060] For example, when the chemical composition of the nitride is composed 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. When the chemical composition is composed 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 described above, fine nitrides in steel have a pinning effect that suppresses coarsening of austenite grains during heat treatment. However, when nitrides are clustered, the number of fine nitrides decreases. In this case, the pinning effect of the nitrides is reduced. Furthermore, clustered nitrides have a lower pinning effect than fine nitrides.

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

[0063] When the mean equivalent circular diameter DN exceeds 70 nm or the sample standard deviation SDN exceeds 45 nm, the amount of clustered nitrides is too large, and therefore coarsening of austenite grains cannot be suppressed during heat treatment in the manufacturing process of mechanical structural components. As a result, the steel does not have sufficient resistance to coarsening.

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

[0065] The upper limit of the sample standard deviation SDN is preferably 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, taking into account normal industrial production, it is, for example, 20 nm, and even more preferably 25 nm.

[0066] [Method of Measuring the Average Equivalent Circular Diameter DN of Nitrides and the Sample Standard Deviation SDN of the Equivalent Circular Diameters] The average equivalent circular diameter DN of nitrides and the sample standard deviation SDN of the equivalent circular diameters are determined by the following method.

[0067] A test piece for preparing an extraction replica is taken from the R / 2 part of the steel material. The cross section of the surface of the test piece perpendicular to the axial direction of the steel material is used as the observation surface. The test piece is prepared so that the R / 2 part is located at the center of the observation surface.

[0068] The observation surface of the test piece is mirror-polished. The mirror-polished observation surface micro-test piece is immersed in a 3.0% nital etching solution for 600 seconds to etch the observation surface. This allows for clear distinction between ferrite and pearlite in subsequent transmission electron microscope (TEM) observation. The test piece including the etched observation surface is covered with a carbon vapor deposition film (replica film). The test piece whose surface is covered with the vapor deposition film is immersed in a 5.0% nital etching solution for 1200 seconds to facilitate peeling of the vapor deposition film from the test piece. The vapor deposition film is peeled from the immersed test piece. The vapor deposition film peeled from the test piece is washed with ethanol, then scooped up with a Cu sheet mesh and dried.

[0069] This vapor-deposited film (replica film) is observed using a TEM. Specifically, the R / 2 portion within the observation surface is observed, and a region consisting of a structure that does not have a lamellar structure within the grains is identified as a ferrite region. Observation fields are set at any 10 points within the identified ferrite region. Each observation field is 3 μm × 3 μm. Observation is performed at a magnification of 40,000 times and an acceleration voltage of 200 kV, and photographic images of each observation field are generated. In each observation field, particles of 20 nm or larger are identified. Nitrides smaller than 20 nm cannot be identified as particles, so they are considered noise and excluded from the particle count.

[0070] Particles in the observation field of view can be identified from contrast. Here, when multiple particles are clustered, the clustered particles (hereinafter also referred to as clustered particles) are identified as a single particle. Clustered particles are determined to be clustered when multiple particles overlap or contact each other. Whether multiple particles overlap or contact each other can be determined from the contrast of a photographic image. When particles are not in contact with each other, the brightness between the particles increases. In this case, the particles are not in contact with each other, so they are not determined to be clustered particles. Furthermore, clustering can be determined by analysis using well-known image processing software. An example of well-known image processing software is ImageJ (trade name). It is well known to those skilled in the art that similar analysis is possible using image processing software other than ImageJ.

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

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

[0073] [Effects of the Steel Material of the Present Embodiment] As described above, the steel material of the present embodiment satisfies Features 1 to 3. Therefore, the steel material of the present embodiment has excellent cold forgeability. Furthermore, it has excellent resistance to grain coarsening during the production of mechanical structural parts.

[0074] The steel material of this embodiment, which provides excellent cold forgeability, has a Vickers hardness of 190 HV or less. It is a technical matter well known to those skilled in the art that the lower the hardness of a steel material, the higher the cold forgeability. A 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, or for example 130 HV.

[0075] [Method for measuring Vickers hardness] The Vickers hardness of steel can be measured by the following method. A test piece including the R / 2 part is taken from the steel. A cross section of the surface of the test piece perpendicular to the axial direction of the steel is used as the measurement surface. The measurement surface is mirror-polished. A Vickers hardness test in accordance with JIS Z 2244-1:2020 is carried out at any three points on the R / 2 part of the measurement surface after mirror polishing to determine the Vickers hardness (HV). The test force for the Vickers hardness test is 9.807 N. The arithmetic mean value of the results obtained at the three points is taken as the Vickers hardness (HV) of the steel.

[0076] [Shape of Steel Material of the Present Embodiment] The steel material of the present embodiment is a steel bar or a wire rod. The steel bar or wire rod is a steel material that extends in a rod shape. The steel material may be wound in a coil shape or may be 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 forgeability and resistance to grain coarsening 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. Note that the steel material of this embodiment may also be used for applications other than the above applications.

[0078] [Method for manufacturing steel material] An example of a method for manufacturing steel material according to this embodiment will be described. The method for manufacturing steel material described below is one example for manufacturing the steel material according to this embodiment. Therefore, steel material having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing steel material according to this embodiment.

[0079] An example of the method for producing a steel material according to this embodiment includes the following steps: (Step 1) Refining step, (Step 2) Continuous casting step, and (Step 3) Hot rolling step.

[0080] Each step will be described below. [Step 1: Refining step] In the refining step, molten steel having a chemical composition that satisfies the above-mentioned feature 1 is produced. The refining method is not particularly limited, and any well-known method may be used. For example, molten pig iron produced by a well-known method is subjected to refining (primary refining) in a converter. Molten steel tapped from the converter is subjected to well-known secondary refining. Note that primary refining may be performed using, for example, an electric furnace. Through the above steps, molten steel having a chemical composition that satisfies feature 1 is produced.

[0081] [(Step 2) Continuous Casting Step] In the continuous casting step, blooms are produced by continuous casting using the molten steel produced in the refining step.

[0082] [(Step 3) Hot Rolling Step] In the hot rolling step, the bloom is hot rolled to produce a steel material. The hot rolling step includes a blooming step and a product rolling step.

[0083] [(Step 31) Blooming Process] In the blooming process, the bloom is hot-rolled (blooming) using a blooming mill to produce a billet. If a continuous rolling mill with multiple rolling stands arranged in a line is located downstream of the blooming mill, the billet may be hot-rolled using the continuous rolling mill to further reduce the size of the billet. In the blooming process, the bloom is heated in a heating furnace. The heating temperature of the bloom in the heating furnace is 1100 to 1250°C, and the holding time at the heating temperature is 60 to 300 minutes. The heating temperature here refers to the temperature (°C) of the heating furnace. The billet produced in the blooming process is cooled to room temperature before the product rolling process.

[0084] [(Step 32) Product Rolling Step] In the product rolling step, hot rolling (product rolling) is performed on the billet to produce the steel material of this embodiment. Specifically, the billet after the blooming step is heated using a heating furnace. The heating temperature is not particularly limited, but is, for example, 950 to 1150°C. The heated billet is then hot rolled (product rolling) 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 on the outlet side of the 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 is set to 2°C / sec or less. As a result, the total area ratio of ferrite and pearlite in the steel material becomes 95% or more, provided that the product rolling conditions described below are met. The product rolling process further meets the product rolling conditions shown below.

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

[0087] FIG. 1 is a schematic diagram for explaining the product rolling process in this manufacturing method. Referring to FIG. 1 , a group of rolling stands arranged continuously from the most upstream rolling stand among the multiple rolling stands of a continuous rolling mill is referred to as a "roughing train (RT)." A group of rolling stands arranged continuously downstream of the roughing train is referred to as an "intermediate train (IT)." A group of rolling stands arranged one by one downstream of the intermediate train is referred to as a "finishing train (FT)." In short, for convenience, the continuous rolling mill is divided into three rolling stand groups, from upstream to downstream: a roughing train RT, an intermediate train IT, and a finishing train FT. The number of rolling stands in the roughing mill train RT, the number of rolling stands in the intermediate mill train IT, and the number of rolling stands in the final mill train FT are not particularly limited. For the purpose of explaining the product rolling conditions in the roughing mill train RT, the intermediate mill train IT, and the final 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 mill train IT, and final mill train FT).

[0088] In the product rolling process, the cross-sectional area perpendicular to the longitudinal direction of the billet produced in the blooming process and before entering the continuous rolling mill (hereinafter also simply referred to as the cross-sectional area) is defined as S Ｏ The cross-sectional area of ​​the billet when it passes through the roughing mill train RT is S ＲＴ The cross-sectional area of ​​the billet when it passes through the intermediate rolling mill train IT is S ＩＴ The cross-sectional area of ​​the billet when it passes through the final rolling mill train FT, that is, the cross-sectional area of ​​the steel material produced by the product rolling process, is defined as S ＦＴ The total area reduction rate R (%) in the product rolling process is defined by the following formula: R = (1 - (S ＦＴ / S Ｏ )) x 100

[0089] Here, one time when a billet is subjected to a rolling force (external force) from a pair of work rolls of one rolling stand is called a "pass." When n is a positive integer, the cross-sectional area of ​​the billet immediately after the nth pass is S ｎThe cumulative reduction rate R from the first pass when the billet is first reduced by the continuous rolling mill immediately after coming out of the heating furnace to the nth pass is defined as ｎ ac (%) is defined by the following formula: ｎ ac = (1 - (S ｎ / S Ｏ )) × 100 At this time, the cumulative area reduction rate R from the first pass to the time of passing through the rough rolling mill train RT ＲＴ ac (%), the cumulative reduction in area R from the first pass to the time of passing through the intermediate rolling mill train IT ＩＴ ac (%), and the cumulative area reduction rate R from the first pass to the time of passing through the final rolling mill train FT ＦＴ ac (%) is expressed by the following formula: ＲＴ ac = (1 - (S ＲＴ / S Ｏ )) x 100 R ＩＴ ac = (1 - (S ＩＴ / S Ｏ )) x 100 R ＦＴ ac = (1 - (S ＦＴ / S Ｏ )) x 100 = R

[0090] In addition, the cumulative reduction in area R up to the nth pass with respect to the total reduction in area R in the product rolling process ｎ Ratio of ac R ｎ pr(%) is defined by the following formula: ｎ pr=R ｎ ac / R×100=(1-(S ｎ / S Ｏ )) / (1-(S ＦＴ / S Ｏ )) × 100 The roughing mill train RT, the intermediate mill train IT, and the finishing mill train FT are R ｎ Specifically, from the most upstream rolling stand, ｎ The rolling stand up to the first pass where pr (%) is 72% or more is classified as the roughing mill train RT. ｎThe rolling stand up to the first pass where pr(%) is 95% or more is classified as the intermediate rolling mill train IT. The rolling stand next to the intermediate rolling mill train IT to the most downstream rolling stand is classified as the final rolling mill train FT. The cumulative reduction in area R from the first pass to the time of passing through the roughing mill train RT relative to the total reduction in area R is ＲＴ Ratio of ac R ＲＴ pr (%), the cumulative area reduction rate R from the first pass to the time of passing through the intermediate rolling mill train IT relative to the total area reduction rate R ＩＴ Ratio of ac R ＩＴ pr (%), and the cumulative area reduction rate R from the first pass to the time of passing through the final rolling mill train FT relative to the total area reduction rate R ＦＴ Ratio of ac R ＦＴ pr (%) is expressed by the following formula: ＲＴ pr=R ＲＴ ac / R×100=(1-(S ＲＴ / S Ｏ )) / (1-(S ＦＴ / S Ｏ )) x 100 R ＩＴ pr=R ＩＴ ac / R×100=(1-(S ＩＴ / S Ｏ )) / (1-(S ＦＴ / S Ｏ )) x 100 R ＦＴ pr=R ＦＴ ac / R×100=100

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

[0092] Referring to FIG. 1, the inlet temperature in the roughing mill train RT is T ＲＴ(°C), and the entry temperature at the intermediate rolling mill train IT is T ＩＴ (°C), and the inlet temperature at the final rolling mill train FT is T ＦＴ (°C). In this embodiment, the following formula (1) is satisfied in 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 suppression of nitride clustering is affected by the reduction amounts in the roughing mill train RT, the intermediate mill train IT, and the finishing mill train FT, and the temperatures at which the reductions are applied.

[0094] In the product rolling process, strain is imparted to the billet by rolling in the roughing mill train RT, the intermediate rolling mill train IT, and the final rolling mill train FT. This strain acts as a driving force to promote the formation of fine nitrides. Here, the inlet temperature T ＲＴ , entrance temperature T ＩＴ and entrance temperature T ＦＴ If the rolling pressure is too high, the strain imparted by the rolling process disappears early, and the nitrides that are formed tend to cluster.

[0095] If Fn1 is 0.50 or more, the area reduction rate RＲＴ , area reduction rate R ＩＴ and reduction rate R ＦＴ is suitable, and the inlet temperatures T ＲＴ , entrance temperature T ＩＴ and entrance temperature T ＦＴ is the appropriate temperature. Therefore, in the product rolling process, fine nitrides are generated and clustering of the nitrides is suppressed. As a result, the average circle-equivalent diameter DN of the nitrides becomes 70 nm or less, and the sample standard deviation SDN of the circle-equivalent diameter becomes 45 nm or less.

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

[0097] The effects of the steel material of this embodiment will be explained more specifically using examples. The conditions in the following examples are 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.

[0098] Steel materials (steel bars) having the chemical compositions shown in Table 1 (Table 1A and Table 1B) were produced by the following method.

[0099]

[0100]

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

[0102] The manufactured billet was subjected to a product rolling process. The heating temperature in the heating furnace in the product rolling process was 1100 to 1150°C. The 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 / sec or less.

[0103] Through the above steps, a steel material (steel bar) having a diameter of 60 mm was produced.

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

[0105]

[0106] [Evaluation Tests] The following evaluation tests were carried out using the steel materials with each test number: (Test 1) Test for measuring the total area ratio of ferrite and pearlite (Test 2) Test for measuring the average circle-equivalent diameter DN of nitrides and the sample standard deviation of circle-equivalent diameter SDN (Test 3) Vickers hardness test (Test 4) Test for evaluating grain coarsening resistance Each test will be described below.

[0107] [(Test 1) Test for Measuring Total Area Ratio of Ferrite and Pearlite] Based on the method described in the above-mentioned [Method for Measuring Total Area Ratio of Ferrite and Pearlite in Microstructure], the total area ratio (%) of ferrite and pearlite for the steel material of 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. Note that the area ratio of ferrite was 55 to 80% for all test numbers.

[0108]

[0109] [(Test 2) Measurement test of average equivalent circle diameter DN of nitrides and sample standard deviation SDN of equivalent circle diameter] Based on the method described in the above-mentioned [Method for measuring average equivalent circle diameter DN of nitrides and sample standard deviation SDN of equivalent circle diameter], the average equivalent circle diameter DN (nm) of nitrides and the sample standard deviation SDN (nm) of equivalent circle diameter were determined for the steel material of each test number. The average equivalent circle diameter DN (nm) of nitrides is shown in the "DN (nm)" column in Table 3. The sample standard deviation SDN (nm) of equivalent circle diameter of nitrides is shown in the "SDN (nm)" column in Table 3.

[0110] In Test 2, 20 random particles identified as nitrides by TEM observation were subjected to point analysis using an energy dispersive X-ray spectrometry (EDS) device attached to the TEM. As a result, N and one or more of Cr, Al, Ti, Nb, and V were detected in all particles. In other words, it was confirmed that the particles identified as nitrides in Test 2 were actually nitrides.

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

[0112] [(Test 4) Coarsening Resistance Evaluation Test] The coarsening resistance of each test number was evaluated using the following method. Specifically, the steel material of each test number was cut perpendicular to the axial direction and further machined to obtain test specimens with a diameter of 30 mm and a length of 30 mm. The centers of the test specimens were approximately aligned with the centers of the steel bars of each test number. The obtained 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, the specimens were held at 850°C for 0.5 hours. After the above steps, the test specimens were oil-cooled and quenched.

[0113] Prior austenite grains were observed at a depth of 0.2 mm from the surface. Specifically, the observation surface was a cut surface perpendicular to the longitudinal direction of the carburized steel part. The observation surface was mirror-polished and then etched with a saturated aqueous solution of picric acid. A field of view (300 μm × 300 μm) of the etched observation surface, including a depth of 0.2 mm from the surface, was observed with an optical microscope (400x magnification) to identify the prior austenite grains. The grain size of each identified prior austenite grain was determined in terms of its equivalent circle diameter (μm) in accordance with JIS G 0551:2020. When no prior austenite grains had an equivalent circle diameter exceeding the JIS grain size number 5 (62.5 μm), the prior austenite grains were rated "E (Excellent)" and were judged to have excellent coarse-graining resistance (represented by "E" in the "Coarse-graining resistance" column in Table 3). When even one crystal grain was present whose equivalent circle diameter exceeded the equivalent circle diameter (62.5 μm) corresponding to the grain size number 5 specified in the JIS, the sample was rated as "B (Bad)" and it was determined that excellent resistance to coarsening of grains was not obtained ("B" is entered in the "Resistance to coarsening of grains" column in Table 3).

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

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

[0116] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. Chemical composition, in mass%, is: C: 0.10 to 0.40%, Si: 0.01 to 0.45%, Mn: 0.30 to 0.95%, P: 0.030% or less, S: 0.025% or less, Cr: over 0.50 to 1.80%, Al: 0.005 to 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%, A steel material comprising: Ca: 0 to 0.0050%; and Mg: 0 to 0.0050%, with the balance being Fe and impurities; a total area ratio of ferrite and pearlite in the microstructure is 95% or more; nitrides having an equivalent circle diameter of 20 nm or more have an average equivalent circle diameter DN of 70 nm or less, and a sample standard deviation SDN of the equivalent circle diameters of the nitrides of 45 nm or less.

2. A steel material according to claim 1, wherein the chemical composition contains, in mass%, one or more elements selected from the group consisting of 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 to 0.150%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, Sn: 0.01 to 0.10%, Ca: 0.0001 to 0.0050%, and Mg: 0.0001 to 0.0050%.

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