Steel for cold forging / nitriding having excellent cold forgeability and excellent nitriding properties, and cold forged / nitrided component using same

Abstract

The purpose of the present invention is to provide: a steel for cold forging / nitriding, the steel having excellent cold forgeability and excellent hardness after nitriding; and a cold forged / nitrided component using the same. The present invention provides a steel for cold forging / nitriding, which contains, in mass%, 0.15-0.30% of C, 0.15-0.60% of Si, 0.10-1.50% of Mn, 0.15-2.20% of Cr, 0.02-0.30% of Mo, 0.015-0.300% of Al, 0.05-0.30% of V, 0.004-0.030% of N, 0.00-0.10% of Nb, 0.000-0.200% of Ti, and 0.0000 to 0.0030% of B, with the balance being made up of Fe and inevitable impurities, wherein: the value of 0.10 × [Cr] + 0.67 × [Al] + 0.24 × [V] is more than 0.15 and less than 0.40; the value of Pd = N / L × (1-Sα) is 2.10 or less; the value of 217.2 [Mo] + 61.3 × Pd is 61.0 or more; and the surface hardness is 180 Hv or less in terms of Vickers hardness. The present invention also provides a cold forged / nitrided component using this steel for cold forging / nitriding.

Description

Cold forged nitriding steel with excellent cold forgeability and nitriding properties, and cold forged nitrided parts made using the same

[0001] The present invention relates to a cold-forged nitriding steel having excellent cold forgeability and nitriding property, and a cold-forged nitrided part using the same. Specifically, the present invention relates to a cold-forged nitriding steel suitable as a raw material for cold-forged nitrided parts obtained by cold forging and subjecting a cold-forged part obtained by cold forging to a surface hardening treatment in which N penetrates, such as gas nitriding or gas soft nitriding, for example, for machine structures such as gears for automobiles, construction machinery, machine tools, etc., and a cold-forged nitrided part using the same.

[0002] In the present invention, "nitriding" includes not only nitriding treatment in which N penetrates and diffuses, but also soft nitriding treatment in which N and C penetrate and diffuse.

[0003] In the present invention, "cold forging nitriding" means that steel is subjected to cold forging and then nitriding.

[0004] In the present invention, "steel for cold forging nitriding" means steel before being subjected to cold forging nitriding, and steel intended to be subjected to cold forging nitriding.

[0005] In the present invention, "cold forged nitrided part" means a steel part obtained by cold forging nitriding of steel, that is, a cold forged part in a nitrided state obtained by nitriding a cold forged part (a steel part obtained by cold forging steel).

[0006] Patent Document 1 proposes a steel for cold forging nitriding containing, in mass%, C: 0.01 to 0.15%, Si: less than 0.10%, Mn: 0.10 to 0.50%, P: 0.030% or less, S: 0.050% or less, Cr: 0.80 to 2.0%, V: 0.03% or more but less than 0.10%, Al: 0.01 to 0.10%, N: 0.0080% or less, and O: 0.0030% or less, with the balance being Fe and impurities, in which Fn1 represented by the following formula (1) is 160 or less, Fn2 represented by the following formula (2) is 20 to 80, and Fn3 represented by the following formula (3) is 160 or more. Fn1=399×C+26×Si+123×Mn+30×Cr+32×Mo+19×V...(1) Fn2=(669.3×log ｅC-1959.6×log ｅ N-6983.3) × (0.067 × Mo + 0.147 × V) (2) Fn3 = 140 × Cr + 125 × Al + 235 × V (3) In the above formulas (1) to (3), C, Si, Mn, Cr, Mo, V, N, and Al each represent the content of the element in mass%.

[0007] Patent Document 2 proposes a method for producing a nitrided steel having an alloy composition containing C: 0.15 to 0.30%, Si: 0.2% or less, Mn: 0.4 to 1.5%, Cr: 0.6 to 1.5%, s-Al: 0.05 to 0.20%, V: 0.05 to 0.30%, and the balance essentially consisting of Fe.

[0008] JP 2013-185186 A JP 2006-063378 A

[0009] Patent Document 1 aims to ensure core hardness by promoting precipitation hardening through carbide precipitation during nitriding by adding Mo and V. However, since the nitrogen content must be kept low to promote precipitation hardening during nitriding, there is a possibility that production costs will increase.

[0010] Patent Document 2 aims to ensure core hardness by adding V to precipitate carbides during nitriding, which makes recrystallization less likely to occur, but this requires appropriate control of manufacturing conditions, such as the need to carry out the precipitation treatment at high temperatures, which may increase production costs.

[0011] Steel for cold forging and nitriding is required to have excellent cold forging properties and nitriding properties because it is nitrided after cold forging.

[0012] Nitriding does not involve quenching from the austenite temperature range, so strengthening by martensitic transformation cannot be utilized. To ensure the desired core hardness of nitrided parts, it is necessary to include a large amount of alloying elements, but adding too many alloying elements will deteriorate cold forgeability.

[0013] On the other hand, if the content of alloying elements is reduced in order to ensure cold forgeability, the amount of nitrides formed during nitriding may be insufficient, resulting in insufficient surface hardness and depth of the hardened layer.

[0014] Furthermore, cold forging provides work hardening through forging, but there is a concern that the work hardening obtained through cold forging may be lost in the nitriding treatment that follows cold forging.

[0015] Therefore, an object of the present invention is to provide a cold forged nitriding steel that has excellent cold forgeability and excellent hardness after nitriding by appropriately controlling the structure and alloy element contents before cold forging, and a cold forged nitrided part made using the same.

[0016] In order to achieve the above object, the present invention provides the following invention: [1] A steel sheet comprising, in mass%, C: 0.15 to 0.30%, Si: 0.15 to 0.60%, Mn: 0.10 to 1.50%, Cr: 0.15 to 2.20%, Mo: 0.02 to 0.30%, Al: 0.015 to 0.300%, V: 0.05 to 0.30%, N: 0.004 to 0.030%, Nb: 0.00 to 0.10%, Ti: 0.000 to 0.200%, B: 0.0000 to 0.0030%, and the balance consisting of Fe and unavoidable impurities, wherein the formula A is 0.10 × [Cr] + 0.67 × [Al] + 0.24 × [V]. a value of formula B: Pd = N / L x (1 - Sα) [where Sα represents the ferrite area fraction (%), N represents the total number of cementite grain boundaries intersecting with a straight line drawn arbitrarily on the microstructure observation surface, and L represents the total length (μm) of the straight lines drawn arbitrarily on the microstructure observation surface] is 2.10 or less; a value of formula C: 217.2[Mo] + 61.3 x Pd [where [Mo] represents the Mo content (% by mass), and Pd has the same meaning as defined above] is 61.0 or more; and a surface hardness of 180 Hv or less in Vickers hardness. [2] The steel for cold forging nitriding according to [1], containing, by mass%, one or more of Nb: more than 0.00% to 0.10%, Ti: 0.010 to 0.200%, and B: more than 0.0000% to 0.0030%. [3] The steel for cold forging nitriding according to [1] or [2], characterized in that the hardness when cold forged at a compression ratio of 60% or more is 250 Hv or more in Vickers hardness. [4] A cold forged nitrided part obtained using the steel for cold forging nitriding according to any one of [1] to [3], wherein the cold forged nitrided part has a surface hardness of 680 Hv or more, a core hardness of 230 Hv or more, and a hardened layer depth of 0.25 mm or more in a surface layer having a hardness of 400 Hv or more.

[0017] According to the present invention, there are provided a cold forged nitriding steel having excellent cold forgeability and excellent hardness after nitriding, and a cold forged nitrided part using the same. The cold forged nitriding steel of the present invention has excellent cold forgeability because it has a surface hardness of 180 Hv or less in Vickers hardness. Furthermore, when the cold forged nitriding steel of the present invention is cold forged at a compression ratio of 60% or more, a Vickers hardness of 250 Hv or more is obtained, thereby providing excellent material hardness and easily ensuring the required mechanical strength. Furthermore, the cold forged nitrided part obtained using the cold forged nitriding steel of the present invention has a surface hardness of 680 Hv or more, a core hardness of 230 Hv or more, and a hardened layer depth of the surface layer having a hardness of 400 Hv or more (the depth of the region maintaining a hardness of 400 Hv or more) of 0.25 mm or more, thereby providing excellent hardness after nitriding and easily ensuring the mechanical strength required for the mechanical part. Therefore, the cold-forged nitrided parts of the present invention are suitable as cold-forged nitrided parts for mechanical structures such as gears for automobiles, construction machinery, machine tools, etc., and the cold-forged nitriding steel of the present invention is suitable as the material for such parts.

[0018] Fig. 1 shows the procedure of isothermal annealing according to the present invention, which was carried out as softening heat treatment A. Fig. 2 shows the procedure of conventional spheroidizing annealing, which was carried out as softening heat treatment B. Fig. 3 is a diagram showing an example of lines drawn on a scanning electron microscope (SEM) observation surface of a pearlite structure, in order to explain how to determine the structural parameter Pd according to the present invention. In Fig. 3, ten lines (white lines) arranged in a lattice pattern are drawn to count the total number N of cementite grain boundaries that intersect with the lines.

[0019] <Steel for cold forging and nitriding> Hereinafter, the steel for cold forging and nitriding of the present invention (hereinafter referred to as "steel of the present invention") will be described.

[0020] <Chemical Composition> The reasons for specifying the chemical composition of the steel of the present invention will be explained below. Note that "%" in the chemical composition means mass %.

[0021] C: 0.15 to 0.30% C is a component that increases the hardness of the material. Insufficient C content increases the ferrite area ratio, resulting in a decrease in core hardness after nitriding, and thus insufficient strength of the steel. Therefore, the C content is set to 0.15% or more. On the other hand, excessive C content increases the material hardness too much, reducing workability and resulting in poor machinability and cold workability. Furthermore, excessive C inhibits nitrogen diffusion, reducing the hardened layer depth. Therefore, the C content is set to 0.30% or less. The C content may be, for example, 0.17% or more, 0.19% or more, 0.21% or more, 0.22% or more, or 0.23% or more. The C content may be, for example, 0.29% or less, 0.28% or less, 0.27% or less, 0.26% or less, or 0.25% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0022] Si: 0.15 to 0.60% Si is a useful component for deoxidation and improves material hardness. Insufficient Si content can easily lead to insufficient deoxidation during production, resulting in a decrease in the inclusion quality. Therefore, the Si content is set to 0.15% or more. On the other hand, excessive Si content can excessively increase material hardness and reduce workability. Therefore, the Si content is set to 0.60% or less. The Si content may be, for example, 0.16% or more, 0.25% or more, 0.31% or more, 0.33% or more, 0.34% or more, or 0.37% or more. The Si content may be, for example, 0.58% or less, 0.56% or less, 0.55% or less, 0.54% or less, or 0.51% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0023] Mn: 0.10 to 1.50% Mn is a component that improves toughness. If the Mn content is too low, toughness decreases. Therefore, the Mn content is set to 0.10% or more. On the other hand, if the Mn content is excessive, workability decreases. Therefore, the Mn content is set to 1.50% or less. The Mn content may be, for example, 0.50% or more, 0.54% or more, 0.59% or more, 0.62% or more, or 0.64% or more. The Mn content may be, for example, 1.36% or less, 1.34% or less, 1.33% or less, 1.31% or less, or 0.99% or less. Each of the above lower limit values ​​may be combined with any of the above upper limit values.

[0024] Cr: 0.15 to 2.20% Cr is a component that increases the hardness of the material. If the Cr content is too low, the hardness after nitriding will be insufficient. Therefore, the Cr content is set to 0.15% or more. The Cr content is preferably 0.80% or more. On the other hand, if the Cr content is excessive, the material hardness will increase too much, resulting in reduced workability. Furthermore, if the Cr content is excessive, the diffusion of nitrogen will be inhibited, resulting in a reduced hardened layer depth. Therefore, the Cr content is set to 2.20% or less. The Cr content is preferably 1.50% or less. The Cr content may be, for example, 0.22% or more, 0.25% or more, 0.28% or more, 0.44% or more, or 0.78% or more. The Cr content may be, for example, 2.18% or less, 2.02% or less, 1.97% or less, 1.75% or less, or 1.67% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0025] Mo: 0.02 to 0.30% Mo is a component that increases the hardness of the material. If the Mo content is too low, the deep hardness after nitriding is likely to decrease, resulting in insufficient strength. Therefore, the Mo content is set to 0.02% or more. On the other hand, if the Mo content is excessive, the material hardness increases and workability decreases, resulting in poor machinability and cold workability. Therefore, the Mo content is set to 0.30% or less. The Mo content may be, for example, 0.03% or more, 0.04% or more, 0.05% or more, 0.07% or more, or 0.08% or more. The Mo content may be, for example, 0.27% or less, 0.26% or less, 0.25% or less, 0.22% or less, or 0.19% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0026] Al: 0.015 to 0.300%. Since Al is a useful component for deoxidation during manufacturing, a deficiency of Al can easily lead to insufficient deoxidation and a decrease in the inclusion quality. Furthermore, since Al is also a useful component for surface hardness and hardened layer depth after nitriding, a deficiency of Al can lead to a decrease in surface hardness after nitriding and an insufficient hardened layer depth. Therefore, the Al content is set to 0.015% or more. The Al content is preferably 0.050% or more. On the other hand, excessive Al can form coarse nitrides, AlN, which can degrade fatigue properties and workability. Therefore, the Al content is set to 0.300% or less. The Al content is preferably 0.210% or less. The Al content may be, for example, 0.018% or more, 0.036% or more, 0.047% or more, 0.058% or more, 0.066% or more, or 0.70% or more. The Al content may be, for example, 0.298% or less, 0.231% or less, 0.222% or less, 0.218% or less, 0.215% or less, or 0.214% or less. Each of the above lower limit values ​​may be combined with any of the above upper limit values.

[0027] V: 0.05 to 0.30% V is a component useful for obtaining a hardened layer depth. If the V content is too low, the hardened layer depth will be insufficient. Therefore, the V content is set to 0.05% or more. On the other hand, if the V content is excessive, workability will deteriorate and costs will increase due to the component. Therefore, the V content is set to 0.30% or less. The V content may be, for example, 0.09% or more, 0.10% or more, 0.13% or more, 0.14% or more, or 0.16% or more. The V content may be, for example, 0.29% or less, 0.27% or less, 0.25% or less, 0.23% or less, or 0.21% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0028] N: 0.004 to 0.030% N is a component that forms carbonitrides. Insufficient N content results in a lack of fine carbonitrides, resulting in coarsening of crystal grains and reduced toughness and fatigue properties. Therefore, the N content is set to 0.004% or more. On the other hand, excessive N content results in the formation of coarse carbonitrides, which reduces fatigue properties and workability. Furthermore, the reduction in pinning-hardening nitrides leads to coarsening of crystal grains. Therefore, the N content is set to 0.030% or less. The N content may be, for example, 0.006% or more, 0.007% or more, 0.010% or more, 0.013% or more, or 0.014% or more. The N content may be, for example, 0.029% or less, 0.028% or less, 0.027% or less, 0.025% or less, or 0.024% or less. Each of the above lower limits may be combined with any of the above upper limits.

[0029] The steel of the present invention may contain one or more of Nb, Ti and B as optional additional components.

[0030] Nb: 0.00 to 0.10% Nb can be added as a component that increases hardness. However, adding excessive Nb increases hardness and deteriorates workability. Therefore, the Nb content is set to 0.10% or less. The lower limit may be 0.00% or more. The Nb content may be, for example, 0.02% or more.

[0031] Ti: 0.000 to 0.200% Ti can be added as a component that improves bending fatigue strength. However, if Ti is excessive, coarse carbonitrides increase, resulting in a decrease in bending fatigue strength. Therefore, the Ti content is set to 0.200% or less. The Ti content may be, for example, 0.170% or less. The lower limit may be 0.000% or more. If Ti is too little, the amount of fine nitrides will be insufficient, resulting in insufficient strengthening of bending fatigue strength. Therefore, the Ti content is preferably 0.010% or more. The Ti content may be, for example, 0.015% or more.

[0032] B: 0.0000 to 0.0030% B can be added as a component to increase the material hardness. However, if there is an excessive amount of B, the material hardness increases too much, resulting in a decrease in workability. Furthermore, if there is an excessive amount of B, the material is likely to become embrittled due to the formation of boron carbide. Therefore, the B content is set to 0.0030% or less. The lower limit may be 0.0000% or more. The B content may be, for example, 0.0010% or more.

[0033] The balance of the steel of the present invention is Fe and unavoidable impurities, such as P and S.

[0034] Among the unavoidable impurities, P tends to promote grain boundary segregation and reduce toughness. Therefore, the P content is preferably 0.030% or less. The lower limit may be 0.000% or more. The P content may be, for example, 0.014% or more.

[0035] Among the inevitable impurities, S tends to form a large amount of coarse MnS, which reduces toughness and fatigue strength. Therefore, the S content is preferably 0.030% or less. The lower limit may be 0.000% or more. The S content may be, for example, 0.011% or more.

[0036] <Values ​​of Formulas A, B, and C> The reasons for specifying the values ​​of Formulas A, B, and C will be explained below.

[0037] Formula A: 0.10 × [Cr] + 0.67 × [Al] + 0.24 × [V] In Formula A, [Cr], [Al], and [V] represent the contents (mass%) of Cr, Al, and V, respectively. The value of Formula A is an index related to nitrided hardness and hardened layer depth. If the value of Formula A is 0.15 or less, the nitrided hardness and hardened layer depth will be insufficient. If the value of Formula A is 0.40 or more, the surface hardness will be excessive, making pitting and other problems more likely to occur. Therefore, the value of Formula A is set to be greater than 0.15 and less than 0.40. The value of Formula A may be, for example, 0.16 or more, 0.19 or more, 0.20 or more, 0.21 or more, or 0.24 or more. The value of Formula A may be, for example, 0.37 or less, 0.34 or less, 0.32 or less, 0.31 or less, or 0.30 or less. The above lower limit may be combined with any of the above upper limits.

[0038] Formula B: Pd = N / L × (1 - Sα) The structural parameter Pd is an index relating to the core hardness and material hardness after nitriding, and is defined by formula B: Pd = N / L × (1 - Sα), where Sα represents the ferrite area fraction (%), N represents the total number of cementite grain boundaries that intersect with a straight line drawn arbitrarily on the microstructure observation surface, and L represents the total length (μm) of the straight lines drawn arbitrarily on the microstructure observation surface.

[0039] The ferrite area ratio Sα (%) can be determined according to the method described in the Examples.

[0040] The total number N of cementite grain boundaries intersecting with a line drawn arbitrarily on the microstructure observation surface, and the total length L (μm) of the line drawn arbitrarily on the microstructure observation surface can be determined as follows.

[0041] First, the steel is cut, embedded in resin so that the cut surface becomes the test surface, and the test surface is mirror-polished and etched with nital solution. The microstructure of the test surface after etching is observed at a magnification of 5,000 to 10,000 times using a scanning electron microscope (SEM).

[0042] Next, any straight lines are drawn on the microstructure observation surface. The part of the microstructure observation surface on which the straight lines are drawn, the total number of straight lines, the direction of each straight line, and the length of each straight line are all arbitrary.

[0043] In one embodiment, a straight line is drawn in a non-ferrite portion (e.g., pearlite portion) of the microstructure observation surface.

[0044] In one embodiment, the total number of lines drawn on the microstructure observation surface is 10-30.

[0045] In one embodiment, the total number of lines drawn on the microstructure observation surface is ten.

[0046] In one embodiment, the straight lines drawn on the microstructure observation surface include a first group of straight lines consisting of a plurality of straight lines parallel to one another, and a second group of straight lines consisting of a plurality of straight lines parallel to one another.

[0047] In one embodiment, the number of lines constituting the first group of lines is between 5 and 15, and the number of lines constituting the second group of lines is between 5 and 15.

[0048] In one embodiment, the number of lines constituting the first group of lines is five, and the number of lines constituting the second group of lines is five.

[0049] In one embodiment, the direction of the first group of lines and the direction of the second group of lines are not parallel.

[0050] In one embodiment, the direction of the first group of lines is perpendicular to the direction of the second group of lines, i.e., the first group of lines and the second group of lines form a checkerboard pattern.

[0051] In one embodiment, the length of each straight line drawn on the microstructure observation surface is independently 5 to 10 μm. The lengths of the straight lines drawn on the microstructure observation surface may be the same or different.

[0052] In one embodiment, the length of each straight line drawn on the microstructure observation surface is 5 μm.

[0053] Two or more of the above embodiments may be combined.

[0054] When determining the total number N of cementite grain boundaries intersecting with a straight line drawn arbitrarily on the microstructure observation surface, the number of cementite grain boundaries intersecting with the straight line is counted according to the following evaluation criteria. i) When a straight line completely intersects with a cementite grain boundary, the number of cementite grain boundaries intersecting with the straight line is counted as 1. "A straight line completely intersects with a cementite grain boundary" means that the line crosses the cementite grain boundary. ii) When a straight line is tangent to a cementite grain boundary, the number of cementite grain boundaries intersecting with the straight line is counted as 0.5. "When a straight line is tangent to a cementite grain boundary" includes cases where a point other than the end point of the line is tangent to the cementite grain boundary and cases where the end point of the line is tangent to the cementite grain boundary. iii) When a straight line is inside a cementite crystal and does not tangent to a cementite grain boundary, the number of cementite grain boundaries intersecting with the line is counted as 1.5.

[0055] When the line drawn on the microstructure observation surface is composed of one line, the total length L (μm) means the length (μm) of that one line. When the line drawn on the microstructure observation surface is composed of two or more lines, the total length L (μm) means the sum of the lengths (μm) of the two or more lines. For example, when the line drawn on the microstructure observation surface is composed of 10 lines, each of which is 5 μm long, the total length L is 50 μm.

[0056] If the value of formula B exceeds 2.10, the material hardness becomes too high, and the workability of the part deteriorates. Therefore, the value of formula B is set to 2.10 or less. The value of formula B may be, for example, 0.53 or more, 0.74 or more, 0.92 or more, 0.98 or more, or 0.99 or more. The value of formula B may be, for example, 2.04 or less, 2.01 or less, 1.72 or less, 1.70 or less, or 1.64 or less. The above lower limit value may be combined with any of the above upper limit values.

[0057] Formula C: 217.2 [Mo] + 61.3 × Pd In Formula C, Pd represents the value of the structural parameter Pd defined by Formula B: Pd = N / L × (1 - Sα), and [Mo] represents the Mo content (mass%). The value of Formula C is an index related to the core hardness and material hardness after nitriding. If the value of Formula C is less than 61.0, the core hardness after nitriding will be insufficient. Therefore, the value of Formula C is set to 61.0 or more. The value of Formula C may be, for example, 84 or more, 87 or more, 91 or more, 95 or more, or 98 or more. The value of Formula C may be, for example, 141 or less, 139 or less, 136 or less, 133 or less, or 132 or less. The above lower limit value may be combined with any of the above upper limit values.

[0058] <Surface Hardness> The surface hardness of the steel of the present invention is 180 Hv or less in Vickers hardness. The surface hardness of the steel of the present invention may be, for example, 168 Hv or less, 166 Hv or less, 165 Hv or less, or 164 Hv or less in Vickers hardness. The lower limit of the surface hardness of the steel of the present invention is not particularly limited. The surface hardness of the steel of the present invention may be, for example, 130 Hv or more in Vickers hardness. Here, "surface hardness" means the surface hardness after softening heat treatment. The surface hardness of the steel of the present invention is 180 Hv or less in Vickers hardness, and the steel has excellent cold forgeability.

[0059] The surface hardness of the steel of the present invention can be measured according to the method described in the Examples.

[0060] <Hardness when cold forged at a compression ratio of 60% or more> The hardness of the steel of the present invention when cold forged at a compression ratio of 60% or more is preferably 250 Hv or more in Vickers hardness. The hardness of the steel of the present invention when cold forged at a compression ratio of 60% or more may be, for example, 276 Hv or more, 278 Hv or more, 279 Hv or more, or 281 Hv or more in Vickers hardness. There is no particular upper limit to the hardness of the steel of the present invention when cold forged at a compression ratio of 60% or more. The hardness of the steel of the present invention when cold forged at a compression ratio of 60% or more may be, for example, 320 Hv or less in Vickers hardness. The fact that the hardness of the steel of the present invention when cold forged at a compression ratio of 60% is 250 Hv or more in Vickers hardness indicates that a material hardness sufficient for practical use is obtained after cold forging, and that there is no shortage of strength in cold forged nitrided parts, for example, cold forged nitrided parts for mechanical structures such as gears for automobiles, construction machinery, machine tools, etc.

[0061] The hardness of the steel of the present invention when cold forged at a compression ratio of 60% or more can be measured according to the method described in the Examples.

[0062] <Method for producing steel> The steel of the present invention can be produced, for example, as follows. A steel satisfying a predetermined chemical composition is produced. The produced steel is hot rolled or hot forged, and then softened to obtain the steel of the present invention. An example of the softening heat treatment is isothermal annealing according to the procedure shown in Figure 1. The softening heat treatment can be carried out, for example, using a Kanthal furnace.

[0063] <Cold-Forged Nitrided Component> Hereinafter, the cold-forged nitrided component of the present invention (hereinafter referred to as "the component of the present invention") will be described.

[0064] The part of the present invention can be obtained by cold forging the steel of the present invention and then nitriding it, i.e., the part of the present invention is a cold-forged part in a nitrided state.

[0065] The nitriding treatment may be performed at 500 to 600°C for 2 to 80 hours, for example. The type of nitriding treatment is not particularly limited, and examples include gas nitriding, salt bath nitriding, and ion nitriding. The gas introduced into the furnace during nitriding is NH ３It may be a gas, such as NH ３ and N ２ and / or H ２ The gas introduced into the furnace during nitriding may also contain a carburizing gas.

[0066] <Surface Hardness> A sufficiently hard surface is an important characteristic of a part. Therefore, the surface hardness of the part of the present invention is set to 680 Hv or more. The surface hardness of the part of the present invention may be, for example, 718 Hv or more, 724 Hv or more, 735 Hv or more, or 745 Hv or more. The upper limit of the surface hardness of the part of the present invention is not particularly limited. The surface hardness of the part of the present invention may be, for example, 860 Hv or less.

[0067] The surface hardness of the part of the present invention can be measured according to the method described in the Examples.

[0068] <Core Hardness> If the core hardness after nitriding is insufficient, the strength of the entire part will be insufficient. Therefore, the core hardness of the part of the present invention is set to 230 Hv or more. The core hardness of the part of the present invention may be, for example, 231 Hv or more, 234 Hv or more, 236 Hv or more, or 239 Hv or more. The upper limit of the core hardness of the part of the present invention is not particularly limited. The core hardness of the part of the present invention may be, for example, 290 Hv or less.

[0069] The core hardness of the parts of the present invention can be measured according to the method described in the Examples.

[0070] <Hardened layer depth of surface layer portion> In the part of the present invention, the hardened layer depth of the surface layer portion having a hardness of 400 Hv or more is 0.25 mm or more. The hardened layer depth of the surface layer portion having a hardness of 400 Hv or more may be, for example, 0.31 mm or more, 0.32 mm or more, 0.33 mm or more, or 0.34 mm or more. If the hardened layer depth of the surface layer portion having a hardness of 400 Hv or more is 0.25 mm or more, the part will be resistant to wear and have durability. There is no particular upper limit for the hardened layer depth of the surface layer portion having a hardness of 400 Hv or more. The hardened layer depth of the surface layer portion having a hardness of 400 Hv or more may be, for example, 0.50 mm or less.

[0071] The depth of the hardened layer can be measured according to the method described in the Examples, and the depth of the region where a hardness of 400 Hv or more is maintained (the distance from the surface to the position where the hardness becomes 400 Hv) (mm) is defined as the depth of the hardened layer (mm).

[0072] 100 kg of each of the invention steels Nos. 1 to 23 and the comparative steels Nos. 24 to 33 was melted in a vacuum melting furnace. Each steel had the chemical composition shown in Table 1, with the balance being Fe and unavoidable impurities.

[0073]

[0074] Next, steel bars having a diameter of 40 mm were produced by hot forging and subjected to softening heat treatment A or B. As shown in Table 2, softening heat treatment A was performed for inventive steels Nos. 1 to 23 and comparative steels Nos. 24 to 32, and softening heat treatment B was performed for comparative steel No. 33. For softening heat treatment A, isothermal annealing was performed using a Kanthal furnace according to the procedure shown in Figure 1. For softening heat treatment B, spheroidizing annealing was performed using a Kanthal furnace according to the procedure shown in Figure 2.

[0075] Next, cylindrical test pieces of φ14×21 mm were taken from the softened heat treated steel bars and cold forged by compressing them at a compression rate of 10 mm / min to a compression ratio of 70%.

[0076] The test pieces were cold forged at a compression ratio of 70% and then nitrided at 570°C for 8 hours.

[0077] <Evaluation Items and Evaluation Method> The ferrite area ratio Sα (%), the total number N of cementite grain boundaries intersecting with a line drawn arbitrarily on the microstructure observation surface, and the total length L (μm) of the line drawn arbitrarily on the microstructure observation surface were determined by the following procedure. In addition, the structural parameter Pd was determined from the formula: Pd = N / L × (1 - Sα).

[0078] [Ferrite Area Ratio Sα (%)] The steel bar after softening heat treatment was cross-sectioned and embedded in resin so that the cut surface was the test surface, and the test surface was mirror-polished and etched with nital solution. After etching, the test surface was observed under an optical microscope (400x magnification, 5 fields of view), the ferrite structure was identified, and an image was taken. Each image was processed with image processing software, and the number of pixels of the ferrite structure was counted. The ratio (%) of the number of pixels of the ferrite structure to the total number of pixels of each image was calculated, and the average value was taken as the ferrite area ratio Sα (%).

[0079] [Structural parameter Pd] The steel bar after softening heat treatment was cross-sectioned and embedded in resin so that the cut surface was the test surface. The test surface was mirror-polished and etched with nital solution. After etching, the test surface was observed using a scanning electron microscope (SEM) at magnifications of 5,000 to 10,000 times. The total number N of cementite grain boundaries intersecting with lines drawn arbitrarily on the microstructure observation surface, the total length L (μm) of the lines drawn arbitrarily on the microstructure observation surface, and the structural parameter Pd were determined according to the following procedures a to c. The results are shown in Table 2.

[0080] a. First, draw any line in the non-ferrite portion (e.g., pearlite portion, etc.) of the observation surface. The total number of lines, the direction of each line, and the length of each line are all arbitrary. When the line drawn on the microstructure observation surface consists of one line, the total length L (μm) means the length (μm) of that one line. When the line drawn on the microstructure observation surface consists of two or more lines, the total length L (μm) means the sum of the lengths (μm) of those two or more lines. In the case of Figure 3, a total of 10 lines are drawn in a grid pattern. In the case of Figure 3, each line is 5 μm long, so the total length L of the 10 lines is 50 μm.

[0081] b. The total number N of cementite grain boundaries intersecting with the straight line is calculated. When calculating the total N, the number of cementite grain boundaries intersecting with the straight line is counted according to the following evaluation criteria: i) When a straight line completely intersects with a cementite grain boundary, the number of cementite grain boundaries intersecting with the straight line is counted as 1. "A straight line completely intersects with a cementite grain boundary" means that the line crosses the cementite grain boundary in a crisscross pattern. ii) When a straight line is tangent to a cementite grain boundary, the number of cementite grain boundaries intersecting with the straight line is counted as 0.5. "When a straight line is tangent to a cementite grain boundary" includes cases where a point other than the end point of the line is tangent to the cementite grain boundary and cases where the end point of the line is tangent to the cementite grain boundary. iii) When a straight line is inside a cementite crystal and is not tangent to a cementite grain boundary, the number of cementite grain boundaries intersecting with the line is counted as 1.5.

[0082] In the example of FIG. 3, the total N was 189.5.

[0083] c) The value of the ferrite area ratio Sα, the value of the total N, and the value of the total length L are substituted into the formula: Pd = N / L × (1 - Sα) to determine the value of Pd.

[0084] [Surface Hardness after Softening Heat Treatment and Surface Hardness after Nitriding] The softening heat treated steel bar was cross-sectioned and embedded in resin so that the cut surface was the test surface, and the test surface was then mirror-polished. The Vickers hardness (Hv) of 10 randomly selected points at a depth of 0.05 mm from the mirror-finished test surface was measured using a Vickers hardness tester at a test force of 1.96 N in accordance with JIS Z 2244 (2009). The average of the measured Vickers hardness values ​​was taken as the surface hardness (Hv) after softening heat treatment. The Vickers hardness (Hv) was measured in the same manner, except that the test specimens used were those after nitriding treatment. The average of the measured Vickers hardness values ​​was taken as the surface hardness (Hv) after nitriding. The results are shown in Table 2.

[0085] [Hardness Depth] The nitrided test specimen was cross-sectioned and embedded in resin so that the cut surface was the test surface, and the test surface was then mirror-polished. The hardness distribution from the mirror-finished test surface was measured using a Vickers hardness tester at a test force of 1.96 N in accordance with JIS Z 2244 (2009). The depth (mm) of the region maintaining a hardness of 400 Hv or more (the distance from the surface to the position where the hardness reaches 400 Hv) was taken as the hardness depth (mm). The results are shown in Table 2.

[0086] [Hardness after cold forging and core hardness after nitriding] The test piece after cold forging and before nitriding was cross-sectioned and embedded in resin so that the cut surface was the test surface, and then the test surface was mirror-polished. The Vickers hardness (Hv) of any five points (one point at the center of the test surface and any four points at R / 2 positions on the test surface, for a total of five points) in the core of the mirror-finished test surface was measured using a Vickers hardness tester with a test force of 1.96 N in accordance with JIS Z 2244 (2009), and the average value of the measured Vickers hardness was taken as the hardness (Hv) after cold forging. R represents the diameter of the test surface, and the R / 2 position represents the intermediate position between the center and the outer periphery of the test surface. The Vickers hardness (Hv) was measured in the same manner, except that the test piece after nitriding was used, and the average value of the measured Vickers hardness was taken as the core hardness (Hv) after nitriding. The results are shown in Table 2.

[0087]

[0088] Invention Steel Nos. 1 to 23 have a surface hardness of 180 Hv or less before nitriding and after softening heat treatment, which is not too hard and suitable for cold forging, and have excellent workability. Invention Steel Nos. 1 to 23 can achieve a hardness of 250 Hv or more by cold forging, making it easy to ensure the required strength. Invention Steel Nos. 1 to 23 have a surface hardness of 680 HV or more because the surface is appropriately hardened by nitriding, and the hardened layer depth of the surface layer with a hardness of 400 Hv or more (the depth of the region maintaining a hardness of 400 Hv or more (the distance from the surface to the position where the hardness becomes 400 Hv)) is 0.25 mm or more, thereby achieving a sufficient hardened depth. On the other hand, Invention Steel Nos. Nos. 1 to 23 have a core hardness of 230 Hv or more after nitriding (i.e., they are not excessively softened by the high temperatures in the nitriding process, have appropriate softening resistance, and ensure material hardness), and therefore can maintain the hardness required for mechanical parts even after nitriding.

[0089] Comparative steel No. 24 has a low carbon (C) content and the value of formula C is outside the specification of the present invention, resulting in insufficient core hardness after nitriding. Comparative steel No. 25 has an excessive Si content, which causes the material hardness before nitriding to increase too much and reduces workability. Comparative steel No. 26 has an insufficient Cr content and the value of formula A is below 0.15, resulting in insufficient surface hardness after nitriding. Comparative steel No. 27 has an excessive Cr content, which increases the material hardness and reduces workability, and the diffusion of nitrogen is hindered, resulting in a reduced hardened layer depth. Comparative steel No. 28 has an insufficient Mo content and the value of formula C is outside the specification of the present invention, resulting in a reduced deep hardness after nitriding. Comparative steel No. 29 has an excessive Mo content and causes the material hardness to increase and reduces workability. Steel No. 30 had an insufficient Al content, resulting in a value of formula A below 0.15, which meant that the surface hardness after nitriding was insufficient and the hardened layer depth due to nitriding was insufficient. Comparative Steel No. 31 had an insufficient V content, resulting in a value of formula A below 0.15, which meant that the surface hardness after nitriding was insufficient and the hardened layer depth due to nitriding was insufficient. Comparative Steel No. 32 had an insufficient overall balance of Cr, Al, and V, resulting in a value of formula A below 0.15, which meant that the surface hardness after nitriding was insufficient and the hardened layer depth due to nitriding was insufficient. Comparative Steel No. 33 underwent softening heat treatment according to the conventional spheroidizing annealing procedure B shown in Figure 2, and the value of formula C was not met, resulting in low core hardness after nitriding heat treatment.

Claims

1. In mass%, it consists of C: 0.15-0.30%, Si: 0.15-0.60%, Mn: 0.10-1.50%, Cr: 0.15-2.20%, Mo: 0.02-0.30%, Al: 0.015-0.300%, V: 0.05-0.30%, N: 0.004-0.030%, Nb: 0.00-0.10%, Ti: 0.000-0.200%, B: 0.0000-0.0030%, and the balance consisting of Fe and unavoidable impurities. Formula A: 0.10 x [Cr] + 0.67 x [Al] + 0.24 x [V] wherein [Cr], [Al], and [V] respectively represent the Cr, Al, and V contents (% by mass).] has a value of more than 0.15 and less than 0.40; wherein the value of Formula B: Pd = N / L x (1 - Sα) [where Sα represents the ferrite area ratio (%), N represents the total number of cementite grain boundaries intersecting with a straight line arbitrarily drawn on the microstructure observation surface, and L represents the total length (μm) of the straight lines arbitrarily drawn on the microstructure observation surface] is 2.10 or less; wherein the value of Formula C: 217.2[Mo] + 61.3 x Pd [where [Mo] represents the Mo content (% by mass), and Pd has the same meaning as above] is 61.0 or more; and wherein the surface hardness is 180 Hv or less in Vickers hardness.

2. The steel for cold forging nitriding according to claim 1, containing, by mass%, one or more of Nb: more than 0.00% and not more than 0.10%, Ti: 0.010 to 0.200%, and B: more than 0.0000% and not more than 0.0030%.

3. A steel for cold forging nitriding as set forth in claim 1 or 2, characterized in that the hardness when cold forged at a compression rate of 60% or more is 250 Hv or more in Vickers hardness.

4. A cold forged nitrided part obtained by using the steel for cold forging nitriding according to any one of claims 1 to 3, wherein the surface hardness is 680 Hv or more, the core hardness is 230 Hv or more, and the hardened layer depth of the surface layer having a hardness of 400 Hv or more is 0.25 mm or more.

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