Steel material

Abstract

To provide a steel material with reduced distortion caused by heat treatment during quenching and tempering.SOLUTION: A steel material of the present embodiment comprises, in mass%, C: 0.31-0.65%, Si: 0.01-0.73%, Mn: 0.65-1.72%, P: 0.015% or less, S: 0.025% or less, Cr: 0.50% or less, Mo: 0.21% or less, Al: 0.005-0.100%, N: 0.0020-0.0300%, and O: 0.0015% or less, with the balance being Fe and impurities, wherein DA defined by formula (1A) is 133 or more and formula (2A) is satisfied. DA=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo) (1A). 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DA≥-62.0 (2A).SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates to a steel material suitable for use as a material for machine structural parts manufactured by carrying out quenching and tempering treatment. [Background technology]

[0002] Machine structural parts are used in automobiles, construction vehicles, mining machines, etc. Examples of machine structural parts include gears, shafts, and gears. In recent years, with the electrification of automobiles and the like, machine structural parts have become lighter and smaller. For this reason, machine structural parts are required to have an excellent balance of strength and toughness. For example, alloy steel materials for machine structures, such as SCM440 specified in JIS G 4053 (2016), are used as materials for machine structural parts having an excellent balance of strength and toughness.

[0003] Machine structural components made from these steel materials are manufactured, for example, by the following manufacturing process. The steel material is hot worked (hot forged), and then cut as necessary to produce an intermediate product of the desired shape. The intermediate product is heat treated to adjust the hardness and microstructure of the intermediate product. The heat treatment is, for example, a quenching and tempering process. Through the above manufacturing process, machine structural components having an excellent balance of strength and toughness are manufactured.

[0004] However, when quenching and tempering is performed, machine structural parts are easily deformed. In this specification, the deformation caused in the machine structural parts by the influence of heat during quenching and tempering is called heat treatment distortion. The shape of the machine structural parts is distorted by the heat treatment distortion. The distortion of the shape of the machine structural parts causes noise and vibration when an automobile or the like is operated.

[0005] In the case of conventional automobiles and the like powered by an internal combustion engine, the sound emitted by the internal combustion engine was louder than the sound emitted by mechanical structural parts such as gears, cogwheels, and shafts. Therefore, the sound of mechanical structural parts has not been given much attention. However, with the recent electrification of automobiles and the like, the sound emitted from the power source has been greatly reduced, and as a result, the sound emitted by mechanical structural parts such as gears has become more noticeable. Therefore, recently, there has been a demand for steel materials that can suppress heat treatment distortion when quenching and tempering treatment is performed.

[0006] A technique for suppressing heat treatment distortion is proposed in International Publication No. 2014 / 038548 (Patent Document 1).

[0007] The steel material disclosed in Patent Document 1 contains, by mass%, C: 0.20-0.30%, Si: 0.10-1.50%, Mn: 0.10-1.20%, P: 0.030% or less, S: 0.030% or less, Cr: 1.30-2.50%, Cu: 0.30% or less, Al: 0.008-0.300%, O: 0.0030% or less, N: 0.0020-0.0300% and the balance being Fe and unavoidable impurities. This steel material has a martensitic transformation start temperature (Ms point) of 460°C or less. Furthermore, when the hardness at a position 1.5 mm away from the quenched end of a steel material measured by the Jominy end quenching method is defined as J1.5, the hardness at a position 9 mm away from the quenched end as J9, and the hardness at a position 11 mm away from the quenched end as J11, (J9 / J1.5) is 0.70 to 0.85, and (J11 / J1.5) is 0.67 to 0.78. Patent Document 1 describes that by adjusting the Ms point and hardness of this steel material, deformation during heat treatment of the steel material is suppressed. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2014 / 038548 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the heat treatment distortion may be suppressed in the quenching and tempering treatment in the manufacturing process of the machine structural component by means other than that of Patent Document 1.

[0010] An object of the present disclosure is to provide a steel material that can be used as a material and that can provide a sufficient strength-toughness balance in machine structural parts manufactured by carrying out quenching and tempering treatment, and that can suppress heat treatment distortion during the quenching and tempering treatment in the manufacturing process of the machine structural parts. [Means for solving the problem]

[0011] The steel material of the present disclosure comprises, in mass%, C: 0.31-0.65%, Si: 0.01 to 0.73%, Mn: 0.65-1.72%, P: 0.015% or less, S: 0.025% or less, Cr: 0.50% or less, Mo: 0.21% or less Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, the balance being Fe and impurities, DA defined by formula (1A) is 133 or more, Satisfies formula (2A). DA=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo) (1A) 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DA≧-62.0 (2A) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1A) and formula (2A).

[0012] The steel material of the present disclosure comprises, in mass%, C: 0.31-0.65%, Si: 0.01 to 0.73%, Mn: 0.65-1.72%, P: 0.015% or less, S: 0.025% or less, Cr: 0.50% or less, Mo: 0.21% or less Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 5, the balance being Fe and impurities, DB defined by formula (1B) is 133 or more, Satisfies equation (2B). [Group 1] Cu: 0.50% or less, and Ni: 0.50% or less, one or more selected from the group consisting of [Group 2] Co:0.50% or less [Group 3] V: 0.50% or less, Nb: 0.200% or less, Ti: 0.200% or less, and W: 0.50% or less, one or more selected from the group consisting of [Group 4] Ca: 0.0100% or less, and Mg: 0.015% or less, one or more selected from the group consisting of [Group 5] Te: 0.080% or less, Bi: 0.500% or less, Pb: 0.09% or less, Sn: 0.150% or less, and Sb: 0.150% or less, one or more selected from the group consisting of DB=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo)×(1+0.35×Cu)×(1+0.36×Ni) (1B) 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DB≧-62.0 (2B) Here, the content of the corresponding element in mass% is substituted for each element symbol in formula (1B) and formula (2B), and when an element is not contained, "0" is substituted for the corresponding element symbol. Effect of the Invention

[0013] The steel material disclosed herein is used as a material and subjected to quenching and tempering treatment to manufacture machine structural components, which provides a sufficient balance of strength and toughness, and the heat treatment distortion is suppressed during the quenching and tempering treatment in the manufacturing process of the machine structural components. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a perspective view of a test piece for evaluating heat treatment strain used in a heat treatment strain evaluation test. [Diagram 2] FIG. 2 is a view showing a surface (measurement surface) perpendicular to the longitudinal direction of the test piece for evaluating heat treatment distortion shown in FIG. [Diagram 3] FIG. 3 is a schematic diagram in which a circle is approximated by the least squares method based on the measurement points in FIG. [Figure 4] FIG. 4 is a schematic diagram for explaining a method for determining the amount of bending based on the center positions obtained on three measurement surfaces in a heat treatment distortion evaluation test. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The present inventors first conducted a study from the viewpoint of chemical composition on a steel material that can be used as a material and manufactured by quenching and tempering a machine structural part, and that can obtain a sufficient strength-toughness balance equal to or greater than that of a machine structural part manufactured by quenching and tempering an alloy steel material for machine structures, such as SCM440, as a material. As a result, the steel material contains, by mass%, C: 0.31-0.65%, Si: 0.01-0.73%, Mn: 0.65-1.72%, P: 0.015% or less, S: 0.025% or less, Cr: 0.50% or less, Mo: 0.21% or less, Al: 0.005-0.100%, N: 0.0020-0.0300%, and O: 0.0015% or less, with the balance being Fe and impurities, and is quenched according to the formula (1A) It was believed that a steel material having a DA, which is an index of toughness, of 133 or more, or a steel material which contains one or more elements selected from the group consisting of the above-mentioned Groups 1 to 5 instead of a part of the Fe in the above-mentioned chemical composition and has a DB, which is an index of hardenability, defined by formula (1B), of 133 or more, would be used as a material and be manufactured into a machine structural part by carrying out quenching and tempering treatment, thereby obtaining an excellent balance of strength and toughness. DA=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo) (1A) DB=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo)×(1+0.35×Cu)×(1+0.36×Ni) (1B) Here, the content of the corresponding element in mass% is substituted for each element symbol in formula (1A) and formula (1B), and when an element is not contained, "0" is substituted for the corresponding element symbol.

[0016] Therefore, the present inventors further investigated a technique capable of suppressing the heat treatment distortion during the quenching and tempering treatment in the manufacturing process of machine structural parts in a steel material having the above-mentioned chemical composition.

[0017] First, the present inventors attempted to reduce the heat treatment strain by lowering the Ms point of the steel material, similar to Patent Document 1. However, in the steel material having the above-mentioned chemical composition, there were cases where the heat treatment strain could not be sufficiently suppressed even if the contents of each element were adjusted so as to lower the Ms point.

[0018] Therefore, the present inventors attempted to suppress the heat treatment distortion from a different perspective, rather than suppressing the heat treatment distortion by lowering the Ms point.

[0019] First, the inventors of the present invention investigated the main factors of the heat treatment strain caused by quenching and tempering. As a result of the investigation, it was found that the main factors among various strains constituting the heat treatment strain caused by quenching and tempering are transformation plastic strain and transformation expansion strain. Therefore, the inventors of the present invention thought that if the transformation plastic strain and transformation expansion strain could be reduced, the heat treatment strain could be suppressed.

[0020] The transformation plastic strain ε is expressed by the following formula, where K is the transformation plasticity coefficient and σ is the stress applied by the transformation during quenching and tempering. Note that the formula below shows the transformation plastic strain ε after complete transformation (i.e., after 100% of the phase transformation has progressed). ε=Kσ

[0021] Therefore, if the transformation plasticity coefficient can be reduced, the transformation plastic strain can be reduced, and as a result, the heat treatment strain can also be suppressed. Therefore, the present inventors have investigated means for reducing the transformation plasticity coefficient from the viewpoint of chemical composition. As a result, the present inventors have obtained the following findings.

[0022] In the above chemical composition, C, Si, Mn, and N act to reduce the transformation plasticity coefficient. On the other hand, Cr and Mo act to increase the transformation plasticity coefficient. Therefore, by appropriately adjusting the C, Si, Mn, and N contents and the Cr and Mo contents within the above chemical composition range, the transformation plasticity coefficient can be reduced.

[0023] The transformation expansion strain is caused by volume expansion due to martensitic transformation during quenching. Therefore, by reducing the hardenability of the steel material, the transformation expansion strain is reduced, and as a result, the heat treatment strain is also suppressed.

[0024] Based on the above findings, the inventors have investigated the relationship between the C, Si, Mn, N, Cr and Mo contents, the hardenability parameters DA or DB, and the heat treatment distortion. As a result, the inventors have found that if the chemical composition is made of essential elements and satisfies formula (2A), and if the chemical composition contains essential elements and optional elements and satisfies formula (2B), the heat treatment distortion can be suppressed in the quenching and tempering process during the manufacturing process of a machine structural part. 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DA≧-62.0 (2A) 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DB≧-62.0 (2B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (2A) and formula (2B).

[0025] The steel material of this embodiment has been completed based on the above technical concept, and has the following configuration.

[0026] [1] In mass%, C: 0.31-0.65%, Si: 0.01 to 0.73%, Mn: 0.65-1.72%, P: 0.015% or less, S: 0.025% or less, Cr: 0.50% or less, Mo: 0.21% or less Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, the balance being Fe and impurities, DA defined by formula (1A) is 133 or more, Satisfying formula (2A), Steel material. DA=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo) (1A) 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DA≧-62.0 (2A) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1A) and formula (2A).

[0027] [2] In mass%, C: 0.31-0.65%, Si: 0.01 to 0.73%, Mn: 0.65-1.72%, P: 0.015% or less, S: 0.025% or less, Cr: 0.50% or less, Mo: 0.21% or less Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 5, the balance being Fe and impurities, DB defined by formula (1B) is 133 or more, Satisfying formula (2B), Steel material. [Group 1] Cu: 0.50% or less, and Ni: 0.50% or less, one or more selected from the group consisting of [Group 2] Co:0.50% or less [Group 3] V: 0.50% or less, Nb: 0.200% or less, Ti: 0.200% or less, and W: 0.50% or less, one or more selected from the group consisting of [Group 4] Ca: 0.0100% or less, and Mg: 0.015% or less, one or more selected from the group consisting of [Group 5] Te: 0.080% or less, Bi: 0.500% or less, Pb: 0.09% or less, Sn: 0.150% or less, and Sb: 0.150% or less, one or more selected from the group consisting of DB=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo)×(1+0.35×Cu)×(1+0.36×Ni) (1B) 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DB≧-62.0 (2B) Here, the content of the corresponding element in mass% is substituted for each element symbol in formula (1B) and formula (2B), and when an element is not contained, "0" is substituted for the corresponding element symbol.

[0028] [3] The steel material according to [2], Containing the first group, Steel material.

[0029] [4] The steel material according to [2] or [3], Containing the second group, Steel material.

[0030] [5] The steel material according to any one of [2] to [4], Containing the third group, Steel material.

[0031] [6] The steel material according to any one of [2] to [5], Containing the fourth group, Steel material.

[0032] [7] The steel material according to any one of [2] to [6], Containing the fifth group Steel material.

[0033] The steel material according to this embodiment will be described in detail below. Note that "%" for elements means mass % unless otherwise specified.

[0034] [Features of the steel material of this embodiment] The steel material of this embodiment includes the following features. (Feature 1) Chemical composition, by mass%, is C: 0.31-0.65%, Si: 0.01-0.73%, Mn: 0.65-1.72%, P: 0.015% or less, S: 0.025% or less, Cr: 0.50% or less, Mo: 0.21% or less, Al: 0.005-0.100%, N: 0.0020-0.0300%, O: 0.0015% or less, Cu: 0-0.50%, Ni: 0-0.50%. , Co: 0-0.50%, V: 0-0.50%, Nb: 0-0.200%, Ti: 0-0.200%, W: 0-0.50%, Ca: 0-0.0100%, Mg: 0-0.015%, Te: 0-0.080%, Bi: 0-0.500%, Pb: 0-0.09%, Sn: 0-0.150%, and Sb: 0-0.150%, with the balance being Fe and impurities. (Feature 2) When the chemical composition consists of essential elements, DA defined by formula (1A) is 133 or more, and when the chemical composition contains essential elements and optional elements, DB defined by formula (1B) is 133 or more. DA=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo) (1A) DB=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo)×(1+0.35×Cu)×(1+0.36×Ni) (1B) Here, the content of the corresponding element in mass% is substituted for each element symbol in formula (1A) and formula (1B), and when an element is not contained, "0" is substituted for the corresponding element symbol. (Feature 3) When the chemical composition consists of essential elements, it satisfies formula (2A), and when the chemical composition contains essential elements and optional elements, it satisfies formula (2B). 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DA≧-62.0 (2A) 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DB≧-62.0 (2B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (2A) and formula (2B). Features 1 to 3 will be explained below.

[0035] [(Feature 1) Chemical composition] The chemical composition of the steel material of this embodiment contains the following elements.

[0036] C: 0.31-0.65% Carbon (C) improves the hardenability of steel and increases the strength of machine structural parts manufactured using the steel as a raw material. If the C content is 0.31% or more, the above effects can be sufficiently obtained. On the other hand, if the C content is 0.65% or less, the formation of coarse precipitates is suppressed, and therefore the toughness of machine structural parts is improved. Therefore, the C content is 0.31 to 0.65%. The lower limit of the C content is preferably 0.33%, more preferably 0.37%, and further preferably 0.40%. The upper limit of the C content is preferably 0.62%, more preferably 0.59%, more preferably 0.56%, more preferably 0.53%, and even more preferably 0.50%.

[0037] Silicon: 0.01 to 0.73% Silicon (Si) improves the hardenability of steel and increases the strength of machine structural parts manufactured using steel as a material. Si also increases the temper softening resistance of machine structural parts manufactured using steel as a material. If the Si content is less than 0.01%, the above effects 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 0.73%, the grain boundary oxide layer after quenching and tempering becomes excessively deep, and in this case, the toughness of the machine structural part decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.01 to 0.73%. The lower limit of the Si content is preferably 0.05%, more preferably 0.10%, and further preferably 0.20%. The upper limit of the Si content is preferably 0.70%, more preferably 0.65%, and further preferably 0.60%.

[0038] Mn: 0.65-1.72% Manganese (Mn) improves the hardenability of steel and increases the strength of machine structural parts manufactured using the steel. Mn also suppresses heat treatment distortion. If the Mn content is less than 0.65%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 1.72%, the formation of Mn oxides is promoted on the surface layer of the steel material during quenching and tempering. In this case, the grain boundary oxide layer of the machine structural part becomes excessively deep. Therefore, even if the contents of other elements are within the ranges of this embodiment, the toughness of the machine structural part decreases. Therefore, the Mn content is 0.65 to 1.72%. The lower limit of the Mn content is preferably 0.70%, more preferably 0.80%, and further preferably 0.90%. The upper limit of the Mn content is preferably 1.65%, more preferably 1.60%, further preferably 1.55%, and further preferably 1.50%.

[0039] P:0.015% or less Phosphorus (P) is an impurity. If the P content exceeds 0.015%, P will segregate excessively at grain boundaries, lowering grain boundary strength. As a result, even if the contents of other elements are within the ranges of this embodiment, the toughness of machine structural parts made of the steel material will decrease. Therefore, the P content is 0.015% or less. The P content is preferably as low as possible. However, excessive reduction in the P content increases the production cost. Therefore, in consideration of normal industrial production, the lower limit of the P content is preferably more than 0%, more preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%. The upper limit of the P content is preferably 0.012%, more preferably 0.010%, and further preferably 0.008%.

[0040] S: 0.025% or less Sulfur (S) is an impurity. If the S content exceeds 0.025%, coarse sulfide-based inclusions are generated. Therefore, even if the contents of other elements are within the ranges of this embodiment, the toughness of machine structural parts made of the steel material decreases. Therefore, the S content is 0.025% or less. The S content is preferably as low as possible. However, excessive reduction in the S content increases the production cost. Therefore, in consideration of normal industrial production, the lower limit of the S content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.002%. The upper limit of the S content is preferably 0.020%, more preferably 0.010%, and further preferably 0.005%.

[0041] Cr:0.50% or less Chromium (Cr) improves the hardenability of steel and increases the strength of machine structural parts manufactured from steel. Even if the steel contains even a small amount of Cr, the above effects can be obtained to a certain extent. On the other hand, Cr increases the transformation plasticity coefficient. If the Cr content exceeds 0.50%, even if the contents of other elements are within the ranges of this embodiment, the transformation plasticity coefficient of the steel material increases excessively, and the heat treatment strain during quenching and tempering increases. Therefore, the Cr content is 0.50% or less. The lower limit of the Cr content is preferably more than 0%, more preferably 0.01%, further preferably 0.05%, and further preferably 0.10%. The upper limit of the Cr content is preferably 0.47%, more preferably 0.44%, more preferably 0.41%, more preferably 0.38%, more preferably 0.35%, more preferably 0.32%, and more preferably 0.29%.

[0042] Mo: 0.21% or less Molybdenum (Mo) improves the hardenability of steel and increases the strength of machine structural parts manufactured from steel. Even if the steel contains even a small amount of Mo, the above effects can be obtained to a certain extent. However, if the Mo content exceeds 0.21%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mo content is 0.21% or less. The lower limit of the Mo content is preferably more than 0%, more preferably 0.01%, even more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Mo content is preferably 0.18%, more preferably 0.15%, and further preferably 0.12%.

[0043] Al: 0.005 to 0.100% Aluminum (Al) combines with N in the steel material to form AlN, and the pinning effect suppresses the coarsening of crystal grains during heating in the quenching and tempering process. This improves the toughness of machine structural parts made of steel material. If the Al content is less than 0.005%, 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 Al content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, coarse oxide clusters are generated, and the coarse oxide clusters reduce the toughness of machine structural parts made of steel. Therefore, the Al content is 0.005 to 0.100%. The lower limit of the Al content is preferably 0.008%, and more preferably 0.010%. The upper limit of the Al content is preferably 0.080%, more preferably 0.070%, and further preferably 0.060%.

[0044] N: 0.0020~0.0300% Nitrogen (N) reduces the transformation plasticity coefficient of steel material. Therefore, the heat treatment strain of the steel material during quenching and tempering is reduced. If the N content is less than 0.0020%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content exceeds 0.0300%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the N content is 0.0020 to 0.0300%. The lower limit of the N content is preferably 0.0021%, and more preferably 0.0022%. The upper limit of the N content is preferably 0.0280%, more preferably 0.0250%, and further preferably 0.0200%.

[0045] O: 0.0015% or less Oxygen (O) is an impurity. O combines with other elements in the steel material to form coarse oxide-based inclusions. The coarse oxide-based inclusions reduce the toughness of machine structural parts made of the steel material. If the O content exceeds 0.0015%, the toughness of the machine structural parts is significantly reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the O content is 0.0015% or less. The O content is preferably as low as possible. However, excessive reduction in the O content increases the production cost. Therefore, in consideration of normal industrial production, the lower limit of the O content is preferably more than 0%, more preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the O content is preferably 0.0013% or less, more preferably 0.0011%, and further preferably 0.0009%.

[0046] The balance of the chemical composition of the steel material of the present 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 from the manufacturing environment, during industrial production of the steel material, and are permissible within a range that does not adversely affect the steel material of the present embodiment.

[0047] [About optional elements] The chemical composition of the steel material of this embodiment may further contain one or more elements selected from the group consisting of first to fifth groups, in place of a portion of Fe. [Group 1] Cu: 0.50% or less, and Ni: 0.50% or less, one or more selected from the group consisting of [Group 2] Co:0.50% or less [Group 3] V: 0.50% or less, Nb: 0.200% or less, Ti: 0.200% or less, and W: 0.50% or less, one or more selected from the group consisting of [Group 4] Ca: 0.0100% or less, and Mg: 0.015% or less, one or more selected from the group consisting of [Group 5] Te: 0.080% or less, Bi: 0.500% or less, Pb: 0.09% or less, Sn: 0.150% or less, and Sb: 0.150% or less, one or more selected from the group consisting of Each of the elements in the first to fifth groups will be explained below.

[0048] [Group 1: Cu and Ni] The chemical composition of the steel material of this embodiment may further contain elements of the first group instead of a part of Fe. These elements are optional elements, and all of them improve the hardenability of the steel material. Each element of the first group will be described below.

[0049] Cu: 0.50% or less Copper (Cu) is an optional element and may not be contained, that is, the Cu content may be 0%. When contained, that is, when the Cu content is more than 0%, Cu improves the hardenability of the steel material and increases the strength of machine structural parts. Even if even a small amount of Cu is contained, the above effects can be obtained to a certain extent. However, if the Cu content exceeds 0.50%, the hardness of the steel material becomes excessively high, and therefore the machinability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0 to 0.50%, and if contained, it is 0.50% or less. The lower limit of the Cu content is preferably 0.01%, more preferably 0.02%, and further preferably 0.05%. The upper limit of the Cu content is preferably 0.20%, more preferably 0.10%, and further preferably 0.05%.

[0050] Ni: 0.50% or less Nickel (Ni) is an optional element and may not be contained, that is, the Ni content may be 0%. When contained, that is, when the Ni content is more than 0%, Ni increases the hardenability of the steel material and increases the strength of machine structural parts. Even if even a small amount of Ni is contained, the above effects can be obtained to a certain extent. However, if the Ni content exceeds 0.50%, the hardness of the steel material becomes excessively high, and therefore the machinability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is 0 to 0.50%, and if contained, it is 0.50% or less. The lower limit of the Ni content is preferably 0.01%, more preferably 0.02%, and further preferably 0.05%. The upper limit of the Ni content is preferably 0.40%, more preferably 0.20%, more preferably 0.12%, more preferably 0.10%, and even more preferably 0.05%.

[0051] [Group 2: Co] The chemical composition of the steel material of the present embodiment may further contain Co instead of a portion of Fe. Co:0.50% or less Cobalt (Co) is an optional element and may not be contained, that is, the Co content may be 0%. When contained, that is, when Co exceeds 0%, Co enhances the corrosion resistance of the steel material. Even if even a small amount of Co is contained, the above effect can be obtained to some extent. However, if the Co content exceeds 0.50%, the above effects become saturated and the production cost increases. Therefore, the Co content is 0 to 0.50%, and if Co is contained, it is 0.50% or less. The lower limit of the Co content is preferably 0.01%, more preferably 0.02%, further preferably 0.05%, and further preferably 0.07%. The upper limit of the Co content is preferably 0.49%, more preferably 0.48%, further preferably 0.47%, and further preferably 0.45%.

[0052] [Group 3: V, Nb, Ti, and W] The chemical composition of the steel material of this embodiment may further contain elements of the third group instead of a part of Fe. These elements are optional elements, and all of them form precipitates to increase the strength of machine structural parts made of the steel material. Each element of the third group will be described below.

[0053] V: 0.50% or less Vanadium (V) is an optional element and may not be contained, that is, the V content may be 0%. When V is contained, that is, when the V content is more than 0%, V forms V precipitates such as V carbides and V carbonitrides. The V precipitates have a pinning effect and suppress the coarsening of grains in steel during quenching and tempering. This increases the strength of machine structural parts. Even if even a small amount of V is contained, the above effects can be obtained to some extent. However, if the V content exceeds 0.50%, the hardness of the steel material becomes excessively high, and therefore the machinability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the V content is 0 to 0.50%, and if V is contained, the V content is 0.50% or less. The lower limit of the V content is preferably 0.01%, more preferably 0.03%, further preferably 0.05%, and further preferably 0.07%. The upper limit of the V content is preferably 0.47%, more preferably 0.45%, still more preferably 0.42%, and still more preferably 0.38%.

[0054] Nb: 0.200% or less Niobium (Nb) is an optional element and may not be contained, that is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content is more than 0%, Nb forms Nb precipitates such as Nb carbides and Nb carbonitrides. The Nb precipitates have a pinning effect and suppress the coarsening of crystal grains in steel during quenching and tempering. Therefore, the strength of machine structural parts is increased. Even if even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.200%, the Nb precipitates become coarse. In this case, the coarsening of crystal grains during quenching and tempering cannot be sufficiently suppressed. Therefore, even if the contents of other elements are within the ranges of this embodiment, the toughness of the machine structural part decreases. Therefore, the Nb content is 0 to 0.200%, and if Nb is contained, it is 0.200% or less. The lower limit of the Nb content is preferably 0.001%, more preferably 0.010%, further preferably 0.030%, and further preferably 0.050%. The upper limit of the Nb content is preferably 0.190%, more preferably 0.175%, still more preferably 0.150%, and still more preferably 0.125%.

[0055] Ti:0.200% or less Titanium (Ti) is an optional element and may not be contained, that is, the Ti content may be 0%. When Ti is contained, that is, when the Ti content is more than 0%, Ti forms Ti precipitates such as Ti carbides and Ti carbonitrides. The Ti precipitates have a pinning effect and suppress the coarsening of crystal grains in steel during quenching and tempering. This increases the strength of machine structural parts. Even if even a small amount of Ti is contained, the above effects can be obtained to a certain extent. However, if the Ti content exceeds 0.200%, the Ti precipitates become coarse. In this case, the coarsening of crystal grains during quenching and tempering cannot be sufficiently suppressed. Therefore, even if the contents of other elements are within the ranges of this embodiment, the toughness of the machine structural part decreases. Therefore, the Ti content is 0 to 0.200%, and if contained, it is 0.200% or less. The lower limit of the Ti content is preferably 0.001%, more preferably 0.010%, further preferably 0.030%, and further preferably 0.050%. The upper limit of the Ti content is preferably 0.190%, more preferably 0.175%, further preferably 0.150%, and further preferably 0.125%.

[0056] W: 0.50% or less Tungsten (W) is an optional element and may not be contained, that is, the W content may be 0%. When W is contained, that is, when the W content is more than 0%, W forms W precipitates such as W carbides and W carbonitrides. W precipitates have a pinning effect and suppress the coarsening of crystal grains in steel during quenching and tempering. This increases the strength of machine structural parts. Even if even a small amount of W is contained, the above effects can be obtained to some extent. However, if the W content exceeds 0.50%, the W precipitates become coarse. In this case, the grains cannot be sufficiently prevented from becoming coarse during quenching and tempering. Therefore, even if the contents of other elements are within the ranges of this embodiment, the toughness of the machine structural part decreases. Therefore, the W content is 0 to 0.50%, and if W is contained, it is 0.50% or less. The lower limit of the W content is preferably 0.01%, more preferably 0.05%, and further preferably 0.10%. The upper limit of the W content is preferably 0.45%, more preferably 0.40%, and further preferably 0.35%.

[0057] [Group 4: Ca and Mg] The chemical composition of the steel material of this embodiment may further contain elements of the fourth group instead of part of Fe. These elements are optional elements, and all of them refine and spheroidize sulfides to improve the toughness of machine structural parts made of the steel material. Each element of the fourth group will be described below.

[0058] Ca:0.0100% or less Calcium (Ca) is an optional element and may not be contained, that is, the Ca content may be 0%. When contained, that is, when the Ca content is more than 0%, Ca refines the sulfides in the steel. Furthermore, Ca promotes the spheroidization of the sulfides in the steel. Therefore, the toughness of machine structural parts made of steel is improved. Even if even a small amount of Ca is contained, the above effects can be obtained to a certain extent. However, if the Ca content exceeds 0.0100%, the formation of coarse Ca oxides in the steel material is promoted, and in this case, the toughness of the machine structural part is reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ca content is 0 to 0.0100%, and if contained, it is 0.0100% or less. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0005%, further preferably 0.0010%, and further preferably 0.0015%. The upper limit of the Ca content is preferably 0.0085%, more preferably 0.0070%, and further preferably 0.0050%.

[0059] Mg: 0.015% or less Magnesium (Mg) is an optional element and may not be contained, that is, the Mg content may be 0%. When Mg is contained, that is, when the Mg content is more than 0%, Mg refines the sulfides in the steel. Furthermore, Mg promotes the spheroidization of the sulfides in the steel. Therefore, the toughness of machine structural parts made of steel is improved. Even if even a small amount of Mg is contained, the above effects can be obtained to some extent. However, if the Mg content exceeds 0.015%, the formation of coarse Mg oxides in the steel material is promoted, and in this case, the toughness of the machine structural part is reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mg content is 0 to 0.015%, and if contained, it is 0.015% or less. The lower limit of the Mg content is preferably 0.001%, more preferably 0.003%, and further preferably 0.005%. The upper limit of the Mg content is preferably 0.013%, more preferably 0.011%, and further preferably 0.008%.

[0060] [Group 5: Te, Bi, Pb, Sn, and Sb] The chemical composition of the steel material of this embodiment may further contain elements of Group 5 instead of a part of Fe. These elements are optional elements, and all of them improve the machinability of the steel material. Each element of Group 5 will be described below.

[0061] Te: 0.080% or less Tellurium (Te) is an optional element and may not be contained, that is, the Te content may be 0%. When contained, that is, when the Te content is more than 0%, Te improves the machinability of the steel material. Even if even a small amount of Te is contained, the above effect can be obtained to a certain extent. However, if the Te content exceeds 0.080%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Te content is 0 to 0.080%, and if contained, it is 0.080% or less. The lower limit of the Te content is preferably 0.001%, more preferably 0.003%, and further preferably 0.010%. The upper limit of the Te content is preferably 0.075%, more preferably 0.070%, further preferably 0.065%, and further preferably 0.050%.

[0062] Bi:0.500% or less Bismuth (Bi) is an optional element and may not be contained, that is, the Bi content may be 0%. When contained, that is, when the Bi content exceeds 0%, Bi improves the machinability of the steel material. Even if even a small amount of Bi is contained, the above effect can be obtained to a certain extent. However, if the Bi content exceeds 0.500%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Bi content is 0 to 0.500%, and if contained, it is 0.500% or less. The lower limit of the Bi content is preferably 0.001%, more preferably 0.002%, further preferably 0.005%, and further preferably 0.010%. The upper limit of the Bi content is preferably 0.470%, more preferably 0.440%, more preferably 0.400%, more preferably 0.370%, more preferably 0.340%, and more preferably 0.300%.

[0063] Pb: 0.09% or less Lead (Pb) is an optional element and may not be contained, i.e., the Pb content may be 0%. When contained, that is, when the Pb content is more than 0%, Pb improves the machinability of the steel material. Even if even a small amount of Pb is contained, the above effect can be obtained to some extent. However, if the Pb content exceeds 0.09%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Pb content is 0 to 0.09%, and if contained, the Pb content is 0.09% or less. The lower limit of the Pb content is preferably 0.01%, more preferably 0.02%, and further preferably 0.03%. The upper limit of the Pb content is preferably 0.08%, more preferably 0.07%, further preferably 0.06%, and further preferably 0.05%.

[0064] Sn: 0.150% or less Tin (Sn) is an optional element and may not be contained, that is, the Sn content may be 0%. When contained, that is, when the Sn content is more than 0%, Sn improves the machinability of the steel material. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. However, if the Sn content exceeds 0.150%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0 to 0.150%, and if Sn is contained, the Sn content is 0.150% or less. The lower limit of the Sn content is preferably 0.001%, more preferably 0.010%, further preferably 0.030%, and further preferably 0.050%. The upper limit of the Sn content is preferably 0.140%, more preferably 0.130%, further preferably 0.110%, and further preferably 0.090%.

[0065] Sb: 0.150% or less Antimony (Sb) is an optional element and may not be contained, that is, the Sb content may be 0%. When contained, that is, when the Sb content is more than 0%, Sb improves the machinability of the steel material. Even if even a small amount of Sb is contained, the above effect can be obtained to some extent. However, if the Sb content exceeds 0.150%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sb content is 0 to 0.150%, and if Sb is contained, the Sb content is 0.150% or less. The lower limit of the Sb content is preferably 0.001%, more preferably 0.010%, further preferably 0.030%, and further preferably 0.050%. The upper limit of the Sb content is preferably 0.145%, more preferably 0.130%, further preferably 0.110%, and further preferably 0.090%.

[0066] [(Feature 2) Regarding DA defined by formula (1A) and DB defined by formula (1B)] Furthermore, in the steel material of this embodiment, when the chemical composition consists of essential elements, DA defined by formula (1A) is 133 or more, and when the chemical composition contains essential elements and optional elements, DB defined by formula (1B) is 133 or more. DA=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo) (1A) DB=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo)×(1+0.35×Cu)×(1+0.36×Ni) (1B) Here, the content of the corresponding element in mass% is substituted for each element symbol in formula (1A) and formula (1B), and when an element is not contained, "0" is substituted for the corresponding element symbol.

[0067] DA defined by formula (1A) and DB defined by formula (1B) are indicators of the hardenability of a steel material. Even if a steel material satisfies characteristic 1, if DA or DB is less than 133, martensitic transformation does not proceed sufficiently during quenching treatment. In this case, a sufficient balance of strength and toughness is not obtained in a machine structural part manufactured by carrying out quenching and tempering treatment using the steel material as a raw material.

[0068] If DA or DB is 133 or more, the hardenability of the steel material is sufficiently ensured. Therefore, a sufficient balance of strength and toughness is obtained in machine structural parts manufactured by carrying out quenching and tempering treatment using the steel material as a raw material.

[0069] The lower limits of DA and DB are preferably 140, more preferably 150, even more preferably 161, even more preferably 170, and even more preferably 180. The upper limits of DA and DB are not particularly limited. When the steel material satisfies the characteristics 1 and 3, the upper limits of DA and DB are, for example, 185. In addition, DA and DB shall be integer values ​​obtained by rounding off the obtained numerical values ​​to one decimal place.

[0070] [(Feature 3) Regarding formula (2A) and formula (2B)] The steel material of this embodiment further satisfies formula (2A) when the chemical composition consists of essential elements, and satisfies formula (2B) when the chemical composition contains essential elements and optional elements. 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DA≧-62.0 (2A) 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DB≧-62.0 (2B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (2A) and formula (2B).

[0071] FA and FB are defined as follows: FA=40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DA FB=40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DB FA and FB are indicators that show the degree of suppression of heat treatment strain caused by quenching and tempering. As mentioned above, if the transformation plasticity coefficient decreases, the heat treatment strain is suppressed. Among the above elements in the chemical composition, C, Si, Mn, Cr, Mo, and N affect the transformation plasticity coefficient. Specifically, C, Si, Mn, and N have the effect of lowering the transformation plasticity coefficient. On the other hand, Cr and Mo have the effect of increasing the transformation plasticity coefficient. In FA and FB, the coefficients of each element are different depending on the degree of influence of each element on the transformation plasticity coefficient. In addition, the heat treatment strain is also suppressed by the decrease in the hardenability of the steel. Therefore, if DA and DB, which are indicators of hardenability, become smaller, FA and FB become larger.

[0072] If FA or FB is -62.0 or more, the transformation plastic strain and transformation expansion strain, which are the main causes of heat treatment strain, can be sufficiently reduced, and therefore the heat treatment strain can be suppressed in the quenching and tempering process in the manufacturing process of machine structural parts.

[0073] The lower limits of FA and FB are preferably −60.0, more preferably −50.0, more preferably −40.0, more preferably −30.0, and even more preferably −20.0. The upper limits of FA and FB are not particularly limited. When the steel material satisfies the characteristics 1 and 2, the upper limits of FA and FB are, for example, −15.7. In addition, FA and FB shall be values ​​obtained by rounding off the obtained numbers to one decimal place.

[0074] [Effects of the steel material of this embodiment] The steel material of this embodiment satisfies Features 1 to 3. Therefore, the steel material of this embodiment is used as a material and a machine structural part manufactured by carrying out quenching and tempering treatment has a sufficient strength-toughness balance, and further, heat treatment distortion is suppressed in the quenching and tempering treatment during the manufacturing process of the machine structural part.

[0075] [About the microstructure of steel] The microstructure of the steel material according to the present embodiment is not particularly limited. Heat treatment strain occurs when phase transformation occurs. For example, in the quenching process in the manufacturing process of a machine structural part made of steel material, the steel material is A c3 The steel material is heated to or above this temperature point and then rapidly cooled. At this time, the microstructure of the steel material undergoes a phase transformation from austenite to martensite, which makes it easy for heat treatment strain to occur. As described above, the heat treatment strain does not depend on the microstructure of the steel material itself, which is the raw material, and is sufficiently suppressed as long as Features 1 to 3 are satisfied. Therefore, the microstructure of the steel material of this embodiment is not particularly limited.

[0076] [Shape of steel material in this embodiment] The steel material in this 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, for example, a round steel bar. The steel material may be wound in a coil shape or may be cut to a predetermined length.

[0077] [Use of the steel material according to this embodiment] The steel material of this embodiment can be used as a material for machine structural parts used in automobiles, industrial machines, etc. In particular, the steel material of this embodiment can be applied as a material for machine structural parts manufactured by carrying out quenching and tempering treatment. The machine structural parts are not particularly limited, but examples thereof include gears, cogwheels, and shafts. The steel material of this embodiment can also be applied to applications other than machine structural parts.

[0078] [Steel manufacturing method] 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 an example for manufacturing the steel material of this embodiment. Therefore, the steel material having the above-mentioned 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 the steel material of this embodiment. In this embodiment, a method for manufacturing a steel bar will be described as an example of a steel material.

[0079] An example of a method for manufacturing a steel material according to this embodiment includes the following steps. (Process 1) Material preparation process (Process 2) Hot processing process Each step will be described below.

[0080] [(Process 1) Material preparation process] In the material preparation step, a material for the steel material of this embodiment is prepared. Specifically, molten steel having a chemical composition that satisfies Features 1 to 3 is produced. The refining method is not particularly limited, and any known method may be used. For example, molten iron produced by a known method is subjected to refining (primary refining) in a converter. The molten steel tapped from the converter is subjected to known secondary refining. In the secondary refining, alloy elements are added to the molten steel to adjust the composition, and molten steel having a chemical composition that satisfies Features 1 to 3 is produced.

[0081] The molten steel produced by the above-mentioned refining method is used to produce a material by a known casting method. For example, an ingot is produced by an ingot casting method using the molten steel. Also, a bloom may be produced by a continuous casting method using the molten steel. By the above-mentioned method, a material (ingot or bloom) is produced.

[0082] [(Process 2) Hot processing process] The produced material is hot worked to produce a steel material. In the hot working step, one or more hot workings are usually performed. When hot working is performed multiple times, the first hot working may be, for example, blooming or hot forging, and the next and subsequent hot workings may be rolling using a continuous rolling mill. The continuous rolling mill has a plurality of rolling stands arranged in a line. A billet may be produced by blooming and rolling using a continuous rolling mill, and then the billet may be reheated and further finished rolling using a continuous rolling mill to produce a steel material of a desired size. Alternatively, the steel material may be produced from the material only by hot forging. The heating temperature of the material during hot working is not particularly limited, but is, for example, 1000 to 1300°C.

[0083] [About machine structural parts] The machine structural component made of the steel material of the present embodiment is manufactured by a known manufacturing method, for example, the following manufacturing method.

[0084] Known manufacturing methods for machine structural components include, for example, hot working steps and quenching and tempering steps. In the hot working step, hot working is performed on the steel material of this embodiment to manufacture an intermediate product having a predetermined shape. The hot working is, for example, hot forging. In the hot working step, the steel material is c3 After heating to a temperature above the melting point, the steel material is processed. Therefore, the microstructure of the steel material is reset during the heating in the hot processing step. The heating temperature is, for example, 1000 to 1300°C. The intermediate product after hot processing is cooled to room temperature. If necessary, cutting may be performed on the intermediate product after hot working. In other words, cutting is an optional step.

[0085] The manufactured intermediate products are subjected to the well-known quenching and tempering treatment to manufacture machine structural parts. c3 The quenched intermediate product is held at or above that temperature for a predetermined time, and then rapidly cooled. In tempering, for example, the quenched intermediate product is held at 150 to 200°C for a predetermined time.

[0086] Through the above steps, a machine structural part is manufactured using the steel material of the present embodiment as a raw material. Note that the steel material after the hot working step may be subjected to normalizing treatment or spheroidizing annealing treatment as necessary. In a machine structural part manufactured by a manufacturing process including a quenching and tempering process using the steel material of this embodiment as a raw material, a sufficient balance of strength and toughness is obtained, and heat treatment distortion is sufficiently suppressed. EXAMPLES

[0087] Steel materials having the chemical compositions shown in Tables 1-1 and 1-2 were manufactured. The steel material with test number REF was a reference steel material and had a chemical composition equivalent to SCM440 specified in JIS G 4053 (2016).

[0088] [Table 1-1]

[0089] [Table 1-2]

[0090] Molten steel was used to carry out continuous casting to produce a slab. The produced slab was heated and subjected to blooming and subsequent continuous rolling to produce a billet of 160 mm x 160 mm. The heating temperature of the slab during blooming was 1000°C to 1300°C. The produced billet was then heated again and hot rolled to produce a steel material (steel bar) with a diameter of 50 mm. The heating temperature of the billet at this time was 1000°C to 1300°C. The produced steel material was air-cooled to room temperature. Through the above production process, steel materials with a diameter of 50 mm and each test number were produced.

[0091] [About the evaluation test] The following evaluation tests were carried out on the manufactured steel materials. (Test 1) Strength-toughness balance evaluation test (Test 2) Heat treatment distortion evaluation test Test 1 and Test 2 will be described below.

[0092] [(Test 1) Strength-toughness balance evaluation test] A machine structural part simulant was manufactured from the steel material of each test number. Specifically, the steel material of each test number (steel bar with a diameter of 50 mm) was heated at a heating temperature of 1200°C and a holding time of 30 minutes. After that, hot processing (hot forging) was performed with a finishing temperature of 950°C or higher to manufacture a steel bar with a diameter of 35 mm. The steel bar with a diameter of 35 mm was subjected to quenching and tempering treatment to manufacture a machine structural part simulant of each test number.

[0093] Quenching and tempering were carried out in the following manner. Intermediate products with each test number were quenched. The quenching temperature was 870°C, the holding time at the quenching temperature was 30 minutes, and the products were cooled in oil at 130°C. After oil cooling, the intermediate products were tempered at a tempering temperature of 180°C and a holding time of 120 minutes.

[0094] The strength-toughness balance of the machine structural part simulants of each test number was evaluated by carrying out the following tests. (Test 1A) Tensile test (Test 1B) Charpy impact test Tests 1A and 1B will be described below.

[0095] [(Test 1A) Tensile test] From the machine structural part simulants of each test number, No. 4 test pieces as specified in JIS Z 2241 (2011) were prepared as tensile test pieces. The central axis of the tensile test piece corresponded to the central axis of the steel material. Using the prepared tensile test pieces, a tensile test conforming to JIS Z 2241 (2011) was carried out at room temperature (25°C) in air to obtain the tensile strength (MPa) of the machine structural part simulants of each test number.

[0096] [(Test 1B) Charpy impact test] A V-notch test piece was prepared from the R / 2 position of the machine structural part simulant of each test number. The R / 2 position corresponds to the center position of the radius R connecting the surface and the central axis of the machine structural part simulant. The size of the V-notch test piece was 10 mm × 10 mm × 55 mm. The longitudinal direction of the V-notch test piece was parallel to the central axis of the steel material. A V-notch was formed at the center position of one of the surfaces of the V-notch test piece that was parallel to the longitudinal direction and in a direction perpendicular to the longitudinal direction. The V-notch had a depth of 2 mm, a V-notch angle of 45°, and a V-notch tip R of 0.25 mm. Using the prepared V-notch test piece, a tensile test was performed in air at room temperature (25 ° C) in accordance with JIS Z 2242 (2018) to obtain the absorbed energy (J) of the machine structural part simulant of each test number.

[0097] Based on the tensile strength of the machine structural part simulant of each test number obtained in Test 1A and the absorbed energy of the machine structural part simulant of each test number obtained in Test 1B, the strength-toughness balance ratio based on the machine structural part simulant of Test No. REF was calculated using the following formula. Strength-toughness balance ratio = (tensile strength of the test number (MPa) × absorbed energy of the test number (J)) / (tensile strength of test number REF (MPa) × absorbed energy of test number REF (J)) If the strength-toughness balance ratio was 1.0 or more, it was determined that a sufficient strength-toughness balance was obtained (indicated by "〇" in the "Strength-toughness balance" column in Table 2). On the other hand, if the strength-toughness balance ratio was less than 1.0, it was determined that a sufficient strength-toughness balance was not obtained (indicated by "×" in the "Strength-toughness balance" column in Table 2).

[0098] [Table 2]

[0099] [(Test 2) Heat treatment distortion evaluation test] The heat treatment strain of the steel material of each test number was obtained by the following method. First, a heat treatment strain evaluation test piece simulating a machine structural part was prepared from the steel material of each test number (steel bar with a diameter of 50 mm). The shape of the heat treatment strain evaluation test piece is shown in Figure 1. The numbers in Figure 1 indicate the dimensions (unit: mm). "φ" in Figure 1 means the diameter. In the following explanation, it is assumed that the heat treatment strain evaluation test piece is arranged in a three-dimensional orthogonal coordinate system (xyz coordinate system). The longitudinal direction of the heat treatment strain evaluation test piece is defined as the y direction, the height direction of the heat treatment strain evaluation test piece is defined as the z direction, and the direction perpendicular to the z direction and y direction is defined as the x direction.

[0100] The steel material with each test number was heated at a heating temperature of 1200°C for a holding time of 30 minutes. It was then hot worked (hot forged) with a finishing temperature of 950°C or higher to produce a steel bar with a diameter of 25 mm. The 25 mm diameter steel bar was machined (cut) to produce an intermediate product for the heat treatment strain evaluation test piece with a diameter of 10 mm and a length of 100 mm. A groove with a depth of 2.5 mm, a width of 4 mm, and a length of 100 mm was machined (cut) on the side of the intermediate product to produce the heat treatment strain evaluation test piece shown in Figure 1.

[0101] The straightness of the test pieces for evaluating heat treatment distortion before quenching and tempering was measured by the following method. With reference to FIG. 1, the outer peripheral shape of the test piece was obtained on a plane (plane on the xz plane) perpendicular to the longitudinal direction (y direction) at measurement positions A1 and A2 at both ends in the longitudinal direction and at measurement position A3, which is the central position in the longitudinal direction. Specifically, a CNC three-dimensional measuring machine (product name: Crysta-Apex) manufactured by Mitutoyo Corporation was used as the three-dimensional measuring machine. As shown in FIG. 2, the heat treatment strain evaluation test piece was placed so that the groove of the test piece was located below on a plane (hereinafter referred to as the measurement surface) perpendicular to the longitudinal direction of the heat treatment strain evaluation test piece. With the CNC three-dimensional measuring machine, the coordinates (x, y, z) of measurement points P1 to P7 at 45° pitches around the outer circumference were obtained at each of measurement positions A1 to A3, with the apex of the measurement surface set to 0°. Note that the 180° position when the apex of the measurement surface is set to 0° corresponds to the groove, so the coordinates at the 180° position were not measured. That is, the coordinates were obtained at seven measurement points P1 to P7 on the outer periphery of each measurement surface.

[0102] As shown in Fig. 3, the circle C0 was approximated by the least squares method using the coordinates of the seven measurement points P1 to P7 obtained on each measurement surface. From the obtained circle C0, the three-dimensional coordinates (x, y, z) of the center position CP of the circle C0 were obtained. In the following explanation, the center position of the circle C0 at the measurement position A1 is defined as CP1. Similarly, the center position of the circle C0 at the measurement position A2 is defined as CP2, and the center position of the circle C0 at the measurement position A3 is defined as CP3.

[0103] 4, based on the coordinates of the obtained three center positions CP1 to CP3, the distance D in the x direction between the line segment SG connecting the center positions CP1 and CP2 and the center position CP3 was obtained, and the obtained distance D was defined as the amount of bending D. The amount of bending D is an index showing the degree of bending of the test piece.

[0104] After determining the amount of bending D of the test piece for evaluating heat treatment strain before quenching and tempering, the test piece for evaluating heat treatment strain was subjected to quenching and tempering under the same conditions as in Test 1. The amount of bending D of the test piece for evaluating heat treatment strain after quenching and tempering was determined in the same manner as the test piece for evaluating heat treatment strain before quenching and tempering. A difference value ΔD between the amount of bending D of the test piece for evaluating heat treatment strain after quenching and tempering and the amount of bending D of the test piece for evaluating heat treatment strain before quenching and tempering was determined. The obtained difference value ΔD is an index showing the amount of heat treatment strain introduced by quenching and tempering.

[0105] Using the difference value ΔD obtained for the reference steel material with test number REF and the difference value ΔD for each test number, the bending amount ratio of the heat treatment strain evaluation test piece with each test number was calculated by the following formula. Bending ratio = difference value ΔD of the test number / difference value ΔD of test number REF If the obtained bending ratio was 0.8 or less, it was determined that the heat treatment distortion was sufficiently suppressed (indicated by "○" in the "Heat treatment distortion" column in Table 2). On the other hand, if the bending ratio was more than 0.8, it was determined that the heat treatment distortion was not sufficiently suppressed (indicated by "×" in the "Heat treatment distortion" column in Table 2).

[0106] [Test Results] The test results are shown in Table 2. Referring to Table 1 (Table 1-1 and Table 1-2) and Table 2, the steel materials of test numbers 1 to 23 satisfied characteristics 1 to 3. Therefore, a sufficient balance of strength and toughness was obtained in the machine structural part simulants manufactured using the steel materials. Furthermore, the heat treatment strain was sufficiently suppressed.

[0107] On the other hand, in test number 24, the DA defined by formula (1A) was too low, and therefore a sufficient balance of strength and toughness was not obtained in the machine structural part simulant manufactured using the material.

[0108] In test number 25, DB defined by formula (1B) was too low. Therefore, a sufficient balance of strength and toughness was not obtained in the machine structural part simulant manufactured using the material.

[0109] In test number 26, the FA was too low, so the heat treatment distortion was not sufficiently suppressed.

[0110] In test number 27, FB was too low, so the heat treatment distortion was not sufficiently suppressed.

[0111] 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 without departing from the spirit of the present disclosure.

Claims

1. In mass percent, C: 0.31-0.65%, Si: 0.01 to 0.73%, Mn: 0.65-1.72%, P: 0.015% or less, S: 0.025% or less, Cr: 0.50% or less, Mo: 0.21% or less, Al: 0.005-0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less; The balance is Fe and impurities, DA defined by formula (1A) is 133 or more, Satisfies formula (2A), Steel material. DA=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo) (1A) 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DA≧-62.0 (2A) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1A) and formula (2A).

2. In mass percent, C: 0.31-0.65%, Si: 0.01 to 0.73%, Mn: 0.65-1.72%, P: 0.015% or less, S: 0.025% or less, Cr: 0.50% or less, Mo: 0.21% or less, Al: 0.005-0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less; Further, it contains one or more selected from the group consisting of Groups 1 to 5, The balance is Fe and impurities, DB defined by formula (1B) is 133 or more, Satisfies formula (2B), Steel material. [Group 1] Cu: 0.50% or less, and Ni: 0.50% or less, one or more selected from the group consisting of [Group 2] Co: 0.50% or less [Group 3] V: 0.50% or less, Nb: 0.200% or less, Ti: 0.200% or less, and W: 0.50% or less, one or more selected from the group consisting of [Group 4] Ca: 0.0100% or less, and Mg: 0.015% or less, one or more selected from the group consisting of [Group 5] Te: 0.080% or less, Bi: 0.500% or less, Pb: 0.09% or less, Sn: 0.150% or less, and Sb: 0.150% or less, one or more selected from the group consisting of DB=8.59×√C×(1+0.64×Si)×(1+4.1×Mn)×(1+2.33×Cr)×(1+3.14×Mo)×(1+0.35×Cu)×(1+0.36×Ni) (1B) 40.0×C+10.0×Si+30.0×Mn-10.0×Cr-0.5×Mo+200.0×N-0.8×DB≧-62.0 (2B) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1B) and formula (2B), and when an element is not contained, "0" is substituted for the corresponding element symbol.

3. The steel material according to claim 2, Containing the first group, Steel material.

4. The steel material according to claim 2, Containing the second group, Steel material.

5. The steel material according to claim 2, Containing the third group, Steel material.

6. The steel material according to claim 2, Containing the fourth group Steel material.

7. The steel material according to claim 2, Containing the fifth group Steel material.

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