Nitriding steel with excellent cold forgeability and nitriding properties and cold forged nitrided parts

Abstract

To provide a steel for nitriding and a cold forged / nitrided component which prevent a decrease in core hardness after nitriding while exhibiting excellent cold forgeability.SOLUTION: A nitriding steel for cold forging comprises, in mass%, C: 0.15-0.30%, Si: 0.15-0.40%, Mn: 0.50-1.50%, Cr: 0.50-2.00%, Mo: 0.02-0.30%, Al: 0.025-0.300%, V: 0.05-0.30%, N: 0.004-0.030%, with the balance being Fe and inevitable impurities, and satisfies formula 1: 0.15<0.10×[Cr]+0.67×[Al]+0.24×[V]<0.43 and formula 2: 0.25<0.80×[C]+0.10×[Cr]+0.36×[Mo]<0.50. The steel structure before nitriding comprises ferrite and perlite, or ferrite and perlite as well as bainite. The area ratio of pearlite is 10 to 60% and the area ratio of bainite is 0 to 10%. The steel has a hardness of 173 Hv or less in terms of Vickers hardness.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to cold forged nitrided parts which are used by subjecting cold forged steel parts to a surface hardening treatment such as gas nitriding or gas soft nitriding in which nitrogen (N) penetrates, for example, cold forged nitrided parts such as gears for automobiles, construction machinery, machine tools, etc., and to a nitriding steel for cold forging suitable as a material for these parts. [Background technology]

[0002] As a steel for cold forging and nitriding, for example, a steel containing, by 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 and 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, and satisfying the values ​​of formulas (1) to (3) has been proposed (see Patent Document 1).

[0003] This steel is designed to improve cold forgeability by reducing the amount of C and Si added. In order to achieve core hardness after cold forging, Further refinement of cold forging conditions and age hardening is required.

[0004] Also proposed is a nitrided steel part which can obtain high surface hardness and deep hardening depth by short nitriding treatment, characterized in that it contains 0.05 to 0.20% C, less than 0.30% Si, 1.00% or less Mn, 0.50 to 1.50% Cr, 0.040% or less Al, 0.0100% or less N, 0.50 to 1.50% Ti, satisfies Ti-4×C-3.4N≧0.20, with the balance being Fe and impurity elements, has a structure after nitriding treatment that is a tempered martensite structure, has a surface hardness of Hv650 or more, and an internal hardness of Hv150 or more (see Patent Document 2).

[0005] However, in the case of parts using this steel, precipitation treatment needs to be performed at high temperatures, and there is a concern that production costs will increase in order to appropriately control the manufacturing conditions. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2013-185186 A [Patent Document 2] JP 2004-300472 A Summary of the Invention [Problem to be solved by the invention]

[0007] In nitriding steel for cold forging, it is necessary to obtain excellent nitriding characteristics, i.e., sufficient surface hardness and hardness depth after nitriding, as well as core hardness and cold forgeability. However, when the nitriding characteristics are improved by adding an additional component, the workability is reduced on the other hand, so that a steel with an excellent balance of these characteristics has not been obtained stably.

[0008] For example, when nitriding, since quenching from the austenite region is not performed, it is difficult to utilize the strengthening caused by martensitic transformation. In this case, in order to ensure the desired core hardness in the nitrided part, it is necessary to include a large amount of alloying elements, but in this case, cold forgeability is deteriorated.

[0009] On the other hand, if the content of alloy elements that contribute to hardness, such as C, is reduced in order to ensure cold forgeability, the amount of nitrides formed during nitriding will also be insufficient, which may result in insufficient surface hardness and insufficient depth of the hardened layer.

[0010] In addition, in cold forging, work hardening is obtained by forging, but there is a concern that the work hardening obtained by cold forging may be lost in the nitriding treatment after cold forging.

[0011] Therefore, an object of the present invention is to provide a nitriding steel that has excellent cold forgeability while suppressing a decrease in core hardness after nitriding, and a cold forged nitrided part using this steel. [Means for solving the problem]

[0012] Therefore, the inventors have developed a steel with excellent cold forgeability and excellent hardness after nitriding by optimizing the composition balance of Al, V, and Cr and appropriately controlling the structure before forging. Since it is effective to add alloy elements such as Cr, Al, and V in combination to increase and deepen the hardness by nitriding, the inventors have conceived of adding Al and V, which have a strong effect on the hardness and depth of nitriding, in the present invention. However, excessive addition of Al and V leads to deterioration of workability. As a result of intensive research, the inventors have focused on the combination of Al and V and have invented a nitriding steel with a composition balance that exhibits excellent nitriding and workability.

[0013] The first means for solving the problems of the present invention is In mass%, it is composed of C: 0.15-0.30%, Si: 0.15-0.40%, Mn: 0.50-1.50%, Cr: 0.50-2.00%, Mo: 0.02-0.30%, Al: 0.025-0.300%, V: 0.05-0.30%, N: 0.004-0.030%, and the balance being Fe and unavoidable impurities, A steel satisfying the following formula 1: 0.15<0.10×[Cr]+0.67×[Al]+0.24×[V]<0.43 and the following formula 2: 0.25<0.80×[C]+0.10×[Cr]+0.36×[Mo]<0.50, The steel structure before nitriding is a structure containing ferrite and pearlite, or ferrite, pearlite and bainite, the proportion of pearlite being 10 to 60% by area and the proportion of bainite being 0 to 10% by area, and the hardness being 173 Hv or less in Vickers hardness. However, the element symbols in [ ] in Formula 1 and Formula 2 are substituted with the mass % value of the corresponding component.

[0014] The second means includes, in mass %, the chemical components described in the first means, namely C: 0.15-0.30%, Si: 0.15-0.40%, Mn: 0.50-1.50%, Cr: 0.50-2.00%, Mo: 0.02-0.30%, Al: 0.025-0.300%, V: 0.05-0.30%, and N: 0.004-0.030%, in addition to the above. As optional additional components, the alloy contains one or more of Nb: 0.10% or less, Ti: 0.020 to 0.200%, and B: 0.0030% or less, with the balance being Fe and unavoidable impurities; A steel satisfying the following formula 1: 0.15<0.10×[Cr]+0.67×[Al]+0.24×[V]<0.43 and the following formula 2: 0.25<0.80×[C]+0.10×[Cr]+0.36×[Mo]<0.50, The steel structure before nitriding is a structure containing ferrite and pearlite, or ferrite, pearlite and bainite, the proportion of pearlite being 10 to 60% by area and the proportion of bainite being 0 to 10% by area, and the hardness being 173 Hv or less in Vickers hardness. However, the element symbols in [ ] in Formula 1 and Formula 2 are substituted with the mass % value of the corresponding component.

[0015] The third means is the nitriding steel for cold forging according to the first or second means, characterized in that the hardness when cold forged at a compression rate of 60% or more is 250 Hv or more in Vickers hardness.

[0016] The fourth means is a cold forged product that has been nitrided using the nitriding steel for cold forging described in the first or second means, and has a surface hardness of 680 Hv or more, a core hardness of 230 to 400 Hv, and a hardened layer depth of the surface layer having 400 Hv or more and a depth of 0.25 mm or more.

[0017] The fifth means is a cold forged product that has been nitrided using the nitriding steel for cold forging described in the third means, and is a cold forged nitrided part having a surface hardness of 680 Hv or more, a core hardness of 230 to 400 Hv, and a hardened layer depth of the surface layer having 400 Hv or more and a depth of 0.25 mm or more. Effect of the Invention

[0018] According to the present invention, a nitriding steel having a good balance between workability, surface hardness and core hardness after nitriding can be obtained. That is, the nitriding steel of the present invention has a hardness of 173Hv or less after softening heat treatment and is excellent in cold forgeability. In addition, since the hardness after cold forging is 250Hv or more when the compression rate in cold working is 60% or more, it has excellent hardness as a material and is practical in that it does not lack strength even when cold forged into parts. In addition, the cold forged parts that have been nitrided have a surface hardness of 680Hv or more, a core hardness of 230 to 400Hv, and a hardened layer depth of the surface layer that is 400Hv or more is 0.25mm or more. Thus, the cold forged nitrided parts according to the present invention have excellent nitriding properties while suppressing a reduction in the core hardness after nitriding. Therefore, according to the present invention, the nitriding steel has excellent cold forgeability, excellent nitriding characteristics, and can suppress a reduction in the core hardness after nitriding. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Before describing the embodiments of the present invention, the reasons for specifying the chemical composition of the steel for nitriding for cold forging will be described. Note that % of the chemical composition is mass %.

[0020] C: 0.15-0.30% C is a useful component for ensuring core strength. If the C content is too low, the core hardness after nitriding will decrease, resulting in insufficient strength, so the C content is set to 0.15% or more. On the other hand, if the C content is too high, the material hardness will increase too much, decreasing workability (machinability, cold workability), and it will also hinder the diffusion of nitrogen, reducing the hardened layer depth, so the C content is set to 0.30% or less. Therefore, the C content is set to 0.15 to 0.30%.

[0021] Silicon: 0.15 to 0.40% Silicon is a useful component for deoxidation during manufacturing. If there is too little silicon, it is likely to cause insufficient deoxidation and the inclusion grade will decrease, so the silicon content is set to 0.15% or more. On the other hand, if there is too much silicon, the material hardness increases and workability decreases, so the silicon content is set to 0.40% or less. Therefore, the silicon content is set to 0.15 to 0.40%.

[0022] Mn: 0.50 to 1.50% Mn is an ingredient that improves core hardness (material hardness). If there is too little Mn, the core hardness will be insufficient, so it is set to 0.50% or more. On the other hand, if there is too much Mn, it will hinder workability such as machinability and forgeability, resulting in a decrease in workability. Therefore, Mn is set to 0.50 to 1.50%.

[0023] Cr: 0.50~2.00% Cr is a component that improves the core hardness of steel and also improves the hardness when the steel is nitrided. If the Cr content is too low, the hardness after nitriding will be insufficient, and the core hardness will also be insufficient, so the Cr content is set to 0.50% or more. On the other hand, if the Cr content is too high, the material hardness will increase, resulting in a decrease in workability, and the diffusion of nitrogen will be hindered, reducing the depth of the hardened layer and making the nitriding shallow, so the Cr content is set to 2.00% or less. Therefore, the Cr content is set to 0.50 to 2.00%. Preferably, the Cr content is set to 0.80 to 1.50%.

[0024] Mo: 0.02 to 0.30% Mo is a component that improves the hardness of the material. If the Mo content is less than 0.02%, the core hardness after nitriding is reduced, resulting in insufficient strength. On the other hand, if the Mo content exceeds 0.30%, the workability is reduced due to the increase in the material hardness, resulting in poor machinability and cold workability. Therefore, the Mo content is set to 0.02 to 0.30%.

[0025] Al: 0.025~0.300 Al is a useful component for deoxidation during manufacturing, and is effective in improving the surface hardness after nitriding. Since Al does not show a tendency to make the nitriding depth shallow, it can be added proactively. If Al is too little, it is likely to cause insufficient deoxidation during manufacturing, and the inclusion quality will decrease. In addition, the surface hardness and hardened layer depth of the steel part after nitriding will also be insufficient. From these viewpoints, Al is set to 0.025% or more. On the other hand, if Al is too much, coarse nitrides (AlN) are formed, which reduces fatigue properties and workability, and thus manufacturability deteriorates. From these viewpoints, Al is set to 0.300% or less. Therefore, Al is set to 0.025 to 0.300%. Desirably, Al is set to 0.050 to 0.150%.

[0026] V: 0.05~0.30% V is a useful component for increasing the hardened layer depth, and since it has little effect in inhibiting nitrogen diffusion, it is also useful for preventing the hardened layer depth from decreasing. In addition, Al and V act in combination, forming clusters with N to ensure the diffusion of N, and the generation of nitrides of Al and N there greatly increases the hardened layer depth. If V is too little, the hardened layer depth will be insufficient, so V is set to 0.05% or more. On the other hand, if V is too much, in addition to deteriorating workability, the cost will increase due to the V component, so V is set to 0.30% or more. Therefore, V is set to 0.05 to 0.30%.

[0027] N: 0.004~0.030% N is a component that forms carbonitrides. If there is too little N, there will be a shortage of carbonitrides and the crystal grains will become coarse, which will lead to a decrease in toughness and fatigue properties. On the other hand, if there is too much N, coarse carbonitrides will be formed, which will result in a decrease in fatigue properties and workability, and the pinning effect will not be exerted due to a decrease in fine carbonitrides that have a pinning effect, leading to coarsening of the crystal grains. Therefore, the N content is set to 0.004 to 0.030%.

[0028] Remainder: Fe and unavoidable impurities The balance is Fe and inevitable impurities. Note that, since both P and S may be contained as inevitable impurities, it is preferable to specify the upper limits as follows.

[0029] P (unavoidable impurities): 0.030% or less Since P promotes grain boundary segregation and reduces toughness, it is desirable to reduce the content of P, an unavoidable impurity, to 0.030% or less.

[0030] S (unavoidable impurities): 0.030% or less If S is excessive, a large amount of coarse MnS is formed, which reduces toughness and fatigue strength. Therefore, it is preferable to reduce the content of S, an unavoidable impurity, to 0.030% or less.

[0031] Optional additional ingredients The optional additional components in the present invention will now be described. The components of the steel of the present invention can further contain one or more of Nb, Ti and B in the following ranges in place of a portion of Fe.

[0032] Nb: 0.10% or less Nb is an ingredient that improves hardness, but if there is too much, the workability deteriorates as the hardness increases. Therefore, if Nb is contained, the content is limited to 0.10% or less.

[0033] Ti: 0.020~0.200% Ti is a component that improves hardness by precipitating nitrides and carbonitrides. If Ti is less than 0.020%, the amount of fine nitrides is insufficient, and Ti does not sufficiently improve bending fatigue strength. On the other hand, if Ti is excessive, the amount of coarse carbonitrides increases, and bending fatigue strength decreases. Therefore, when Ti is added, the amount is set to 0.020 to 0.200%.

[0034] B: 0.0030% or less B is an element that increases the hardness of the material, but if it exceeds 0.0030%, the workability decreases as the hardness of the material increases. It also leads to embrittlement due to the formation of carboborides. Therefore, if B is added, it should be 0.0030% or less.

[0035] Formula 1: 0.15<0.10×[Cr]+0.67×[Al]+0.24×[V]<0.43 Formula 2: 0.25<0.80×[C]+0.10×[Cr]+0.36×[Mo]<0.50 Next, we will explain the reasons for defining formulas 1 and 2. Note that the element symbols in [ ] in formulas 1 and 2 are substituted with the mass% values ​​of the corresponding steel components. First, formula 1 is an index using Cr, Al, and V, and is an index related to nitriding hardness and hardened layer depth. If the value of formula 1 is 0.15 or less, the nitriding hardness and hardened layer depth will be insufficient. If it is 0.43 or more, the surface hardness will be excessive, making it more likely to cause pitting, etc.

[0036] Formula 2 is an index using C, Cr, and Mo, and is an index related to the core hardness and material hardness after nitriding. If the value of Formula 2 is 0.25 or less, the core hardness is insufficient, resulting in insufficient strength. If the value of Formula 2 is 0.50 or more, the material hardness is too high, resulting in poor workability (machinability and cold workability) of the part.

[0037] (The structure of the steel before nitriding is a ferrite and pearlite structure, with the pearlite ratio being 10 to 60% by area, or A structure that contains bainite in addition to ferrite and pearlite, with the area ratio of pearlite being 10-60% and the area ratio of bainite being 10% or less. The steel for nitriding of the present invention has a ferrite + pearlite structure or a structure in which ferrite + pearlite further contains bainite, in which the proportion of pearlite in terms of area ratio is 10 to 60%, and the proportion of bainite in terms of area ratio is 0 to 10% or less. If the proportion of pearlite is less than 10%, the structure will be mainly soft ferrite, which will make it difficult to cut chips and therefore reduce machinability.If the proportion of pearlite is more than 60%, the hardness will be excessive and the machinability will decrease. Furthermore, if the proportion of bainite exceeds 10%, the core hardness increases and workability deteriorates, so bainite may be 0%, and even if bainite is contained, its proportion is set to 10% or less in terms of area ratio.

[0038] The nitriding steel of the present invention has a Vickers hardness of 173 Hv or less after softening heat treatment and is excellent in workability. If the material hardness is too high, the workability is poor, which leads to poor workability in cold forging.

[0039] (When cold forged with a compression ratio of 60% or more, the Vickers hardness must be 250Hv or more.) When the nitriding steel of the present invention is processed by cold forging at a compression ratio of 60%, the material hardness obtained by cold working is practically sufficient after processing, which means that the overall strength of parts such as gears is not insufficient.

[0040] (Surface hardness must be 680Hv or more) The surface hardness of parts that have been nitrided after cold forging shall be 680Hv or more. A sufficiently hard surface is an important characteristic for cold forged nitrided parts such as gears.

[0041] (Core hardness is 230~400Hv) If the core hardness after nitriding is insufficient, the strength of the entire part such as gears may be insufficient. Therefore, the core hardness is set to 230 to 400 Hv.

[0042] (The surface layer has a hardness of 400Hv or more and a depth of 0.25mm or more.) When the hardness distribution from the surface is measured using a Vickers hardness tester in accordance with JIS Z 2244 and the area where a hardness of 400 Hv or more is maintained is taken as the hardened layer depth, if the hardened layer depth is 0.25 mm or more, the cold forged nitrided parts such as gears will be resistant to wear and durable.

[0043] 100 kg of each of developed steels No. 1 to No. 30, which are examples of the present invention and have the chemical compositions shown in Table 1 with the balance being Fe and unavoidable impurities, and comparative steels No. 1 to No. 5, which have the chemical compositions shown in Table 2 with the balance being Fe and unavoidable impurities, were melted in a vacuum melting furnace. Next, steel bars with a diameter of 40 mm were produced by hot forging, and then softening heat treatment was performed by holding the bars at temperatures of 760 to 840°C for 2 to 13 hours, followed by furnace cooling to about 600°C and then air cooling.

[0044] The softening heat treatment was carried out using a Kanthal furnace according to the following procedure. First, each test material with the above composition is placed in a furnace set to a specified holding temperature, and the test material is allowed to heat up for 30 minutes. After that, it is held for a desired period of time, and then furnace cooling, air cooling, or water cooling is performed. The holding time is selected taking into consideration the amount and dimensions of the steel material to be charged into the furnace.

[0045] [Table 1]

[0046] [Table 2]

[0047] Tables 3 and 4 show the area ratio (%) of pearlite, the area ratio (%) of bainite, and the hardness (Hv) of the steel after softening treatment.

[0048] Next, cylindrical test pieces of φ14×21 mm were cold forged and compressed to a compression ratio of 70% at a compression speed of 10 mm / min. After cold working at a compression ratio of 70%, they were subjected to nitriding treatment at 570°C for 8 hours.

[0049] The hardness (Hv) after cold forging, the surface hardness (Hv) after nitriding, the hardened layer depth (mm) after nitriding, and the core hardness (Hv) were measured according to the following procedure, and are shown in Tables 3 and 4.

[0050] [Table 3]

[0051] [Table 4]

[0052] <Evaluation method> The hardness distribution from the surface of each steel in Tables 1 and 2 was measured using a Vickers hardness tester in accordance with JIS Z 2244. The surface hardness after softening heat treatment, the hardness after cold forging, the hardness after nitriding, and the depth of the effective hardened layer are shown in Tables 3 and 4. (1) Surface hardness The surface hardness was measured at a depth of 0.05 mm from the surface using a Vickers hardness tester in accordance with JIS Z 2244. (2) Depth of hardened surface layer The hardness distribution from the surface after nitriding was measured using a Vickers hardness tester in accordance with JIS Z 2244, and the area maintaining a hardness of 400 Hv or more was taken as the hardened layer depth. (3) Hardness after cold forging, core hardness after nitriding Regarding the hardness after cold forging and after nitriding, each test piece was cut and the core hardness of the cross section was measured using a Vickers hardness tester in accordance with JIS Z 2244.

[0053] In addition, the structure of each steel after softening heat treatment was mirror-polished, etched with nital solution, and then observed under an optical microscope to identify and distinguish each structure. For each identified structure, the ratio of each structure was calculated by counting the number of pixels in the captured microscope image using image processing software, and the area ratio of pearlite and bainite in the total structure are shown in Tables 3 and 4 in percentages.

[0054] Each of the developed steels No. 1 to 30 is a steel that satisfies the composition of the present invention and the values ​​of formula 1 and formula 2, and the pearlite ratio after softening heat treatment was 10 to 60% in area ratio, the bainite ratio was 0 to 10% or less in area ratio, and the hardness was 173 Hv or less. Therefore, it has excellent cold forgeability. Furthermore, the hardness after cold forging is 265 Hv or more, and the strength of the entire part can be ensured. In addition, the surface hardness after nitriding is 700 Hv or more, and the hardened layer depth is 0.30 mm or more, and a nitrided layer with practical strength is formed. In addition, the core hardness after nitriding is 230 or more, and the decrease in the core hardness due to the nitriding treatment is suppressed compared to the hardness after cold forging. As described above, the nitriding steel of the present invention has excellent cold forgeability, and nitrided parts cold forged using this nitriding steel do not experience an excessive decrease in core hardness and an effective nitrided layer is sufficiently formed, resulting in parts with an excellent balance between surface hardness and core hardness after nitriding.

[0055] In Comparative Example 1, C is excessive and the formula 2 is higher than 0.50, so that the material hardness increases too much and the workability (machinability and cold workability) decreases. In Comparative Example 2, the C, Al and V contents are too small, so that the depth of the hardened layer is insufficient. In Comparative Example 3, the amount of C is too little and the values ​​of Formulas 1 and 2 are also low and outside the ranges specified, so that the hardness after cold forging is insufficient, resulting in insufficient strength, and the surface hardness after nitriding is also insufficient. In Comparative Example 4, the amount of V is too small, and therefore the depth of the hardened layer after nitriding is insufficient. In Comparative Example 5, the value of formula 1 is low and deviates from the range, so that the surface hardness after nitriding is insufficient.

Claims

1. In mass percent, C: 0.15-0.30%, Si: 0.15-0.40%, Mn: 0.50 to 1.50%, Cr: 0.50-2.00%, Mo: 0.02-0.30%, Al: 0.025-0.300%, V: 0.05-0.30%, N: 0.004-0.030%, The balance is Fe and unavoidable impurities, Formula 1: 0.15<0.10×[Cr]+0.67×[Al]+0.24×[V]<0.43, Formula 2: 0.25<0.80×[C]+0.10×[Cr]+0.36×[Mo]<0.50 A steel material made of a nitriding steel that satisfies the above requirements, The structure of the steel before nitriding is a structure containing ferrite and pearlite or ferrite and pearlite and further containing bainite, The area ratio of pearlite is 10 to 60%, and the area ratio of bainite is 0 to 10% or less, Furthermore, the hardness is 173 Hv or less in Vickers hardness. Steel for cold forging and nitriding. However, the element symbols in [ ] in Formula 1 and Formula 2 are substituted with the mass % values ​​of the corresponding components.

2. In addition to the chemical components according to claim 1, the alloy contains, in mass%, one or more of Nb: 0.10% or less, Ti: 0.020 to 0.200%, and B: 0.0030% or less as selective additional components; The balance is Fe and unavoidable impurities, Formula 1: 0.15<0.10×[Cr]+0.67×[Al]+0.24×[V]<0.43, Formula 2: 0.25<0.80×[C]+0.10×[Cr]+0.36×[Mo]<0.50 A steel material made of a nitriding steel that satisfies the above requirements, The structure of the steel before nitriding is a structure containing ferrite and pearlite or ferrite and pearlite and further containing bainite, The area ratio of pearlite is 10 to 60%, and the area ratio of bainite is 0 to 10% or less, Furthermore, the hardness is 173 Hv or less in Vickers hardness. Steel for cold forging and nitriding. However, the element symbols in [ ] in Formula 1 and Formula 2 are substituted with the mass % values ​​of the corresponding components.

3. The hardness when cold forged at a compression rate of 60% or more is 250 Hv or more in Vickers hardness. The steel material for nitriding for cold forging according to claim 1 or 2.

4. A cold forged product in a nitriding treatment state using the steel material for nitriding for cold forging according to claim 1 or 2, wherein the surface hardness is 680 Hv or more, the core hardness is 230 to 400 Hv, and the hardened layer depth of the surface layer having 400 Hv or more is 0.25 mm or more.

5. A cold forged product in a nitriding treatment state using the steel material for nitriding for cold forging according to claim 3, wherein the surface hardness is 680 Hv or more, the core hardness is 230 to 400 Hv, and the hardened layer depth of the surface layer having 400 Hv or more is 0.25 mm or more.

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