Steel and soft nitrided parts

Abstract

To provide a steel that exhibits superior cold forgeability and superior fatigue strength after soft nitriding, as well as a soft nitrided component formed from the same.SOLUTION: A steel comprises C: 0.02 mass% or more and less than 0.15 mass%, Si: 0.03 mass% or more and 0.15 mass% or less, Mn: 0.10 mass% or more and 0.95 mass% or less, Cr: 0.50 mass% or more and 1.90 mass% or less, P: 0.1 mass% or less, S: 0.5 mass% or less, Al: 0.005 mass% or more and 0.080 mass% or less, N: 0.0010 mass% or more and 0.0120 mass% or less, V: 0.03 mass% or more and 0.30 mass% or less and Cu: 0.2 mass% or more and 1.5 mass% or less, with the balance being Fe and impurities.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to steel and soft nitrided parts. [Background technology]

[0002] Cold forging allows for near-net-shape forming, which has the advantage of reducing the amount of material removed after forging compared to hot forging, thus suppressing yield reduction. Soft nitriding is a heat treatment that improves the fatigue properties of steel parts while reducing the occurrence of heat treatment distortion compared to carburizing and quenching, and is applied to various industrial parts such as automotive gears.

[0003] Patent Document 1 discloses a structural steel for machine construction with excellent cold workability, a method for manufacturing the structural steel, and a method for manufacturing machined parts using the structural steel. This structural steel has a composition containing C: 0.005-0.06 mass%, Si: 0.01-0.1 mass%, Mn: greater than 1.0-3.0 mass%, P: 0.05 mass% or less, S: 0.005-0.05 mass%, Cr: 0.3-3.0 mass%, Al: 0.005-0.1 mass%, and N: 0.008-0.02 mass%, with the remainder being Fe and unavoidable impurities. The N solid solution content is 0.008-0.02 mass%, the cementite phase fraction in the structure is 2% or less, the remainder is a ferrite phase, and the average grain size of the ferrite phase is 10-100 μm. This structural steel contains a predetermined amount of manganese (Mn), which, through its desulfurization effect, combines with sulfur (S), improving the deformability of the steel while suppressing the thermal softening caused by age-strengthening due to solid-solution nitrogen.

[0004] Patent Document 2 discloses a high-strength steel for soft nitriding with excellent cold forgeability. This steel for soft nitriding has alloying element content in mass percent as follows: C: 0.01% or less (including 0%), Si: 0.01% to 1.5%, Mn: 0.15% to 2%, Cu: 0.5% to 2%, N: limited to less than 0.005%, with the remainder being Fe and unavoidable impurity elements, and has a hardness of HV150 or less in the as-hot-rolled or as-hot-forged state. This high-strength steel for soft nitriding is said to achieve excellent precipitation hardening by precipitating Cu in ferrite, and to obtain high fatigue strength after soft nitriding treatment. Furthermore, this high-strength steel for soft nitriding improves cold forgeability by limiting the amount of C added to 0.01% by mass or less.

[0005] Patent Document 3 discloses a method for manufacturing steel for soft nitriding and soft nitrided parts using this steel. In this method for manufacturing steel for soft nitriding, a steel with a chemical composition of 0.15-0.45% by weight, 0.05-0.50%, 0.2-2.5%, 0.5-1.5%, 0.25-0.75%, with 1.8 ≤ Cu / Ni ≤ 2.2, 0.5-2.0%, 0.05-0.5%, 0.01-0.3%, 0.0 This soft-nitrided component is made from the above-mentioned soft-nitriding steel material, and has a surface hardness of Hv600 or higher after soft nitriding, and an effective hardening depth of 0.1 mm or higher. In this soft-nitrided component, the combined addition of Cu and Ni significantly enhances the action of Cu, which precipitates and hardens during the soft nitriding process.

[0006] Patent Document 4 discloses a method for manufacturing steel for soft nitriding and soft nitrided parts using this steel. In this method for manufacturing steel for soft nitriding, the composition is as follows (by weight%): C: 0.15~0.45%, Si: 0.05~0.50%, Mn: 0.2~2.5%, Cu: 0.5~1.5%, Ni: 0.25~0.75%, and 1.8≦Cu / Ni≦2.2, Cr: 0.5~2.0%, V: 0.05~0.5%, Al: 0.005~0.3%, Mo+0.5W: 0~0.3%, Ti: 0~ Steel with a chemical composition of 0.2%, Zr: 0-0.2%, Nb: 0-0.2%, Pb: 0-0.35%, Ca: 0-0.01%, S: 0.13% or less, with the remainder being Fe and unavoidable impurities, is hot-worked and then spheroidized and annealed to achieve a core hardness of Hv180 or less. Subsequently, it is cold-worked to achieve a core hardness of Hv250 or more, and the decarburization depth is set to 0.1-0.4 mm from the surface of the steel. This soft-nitrided part is made from the above soft-nitrided steel material, and after soft nitriding, the surface hardness is Hv600 or more, and the effective hardening depth is 0.2 mm or more. In this soft-nitrided part as well, the combined addition of Cu and Ni makes the action of Cu, which precipitates and hardens during the soft nitriding treatment, particularly noticeable.

[0007] Non-patent document 1 discloses a method (the so-called Williamson-Hall method) for estimating dislocation density by determining lattice strain caused by non-uniform deformation of the lattice from the half-width of the X-ray diffraction peak. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2011-236449 [Patent Document 2] Japanese Patent Publication No. 2002-69571 [Patent Document 3] Japanese Patent Application Publication No. 9-256045 [Patent Document 4] Japanese Patent Application Publication No. 10-226818 [Non-patent literature]

[0009] [Non-Patent Document 1] GK Williamson and WH Hall, “X-ray Line Broadening from Filed Aluminum and Wolfram,” Acta Metall., Vol. 1, 1953, pp. 22-31. [Overview of the project] [Problems that the invention aims to solve]

[0010] Manufacturing parts by cold forging can reduce costs compared to manufacturing them by hot forging. Furthermore, even greater cost reductions can be achieved by omitting softening annealing before cold forging or by eliminating machining after cold forging through net-shape forming.

[0011] However, in the structural steel for machinery disclosed in Patent Document 1, since Mn is added at a concentration of more than 1.0 mass%, the cold forgeability of the steel is reduced, and it was sometimes difficult to omit softening annealing or to perform net-shape forming.

[0012] Furthermore, in the high-strength steel for soft nitriding disclosed in Patent Document 2, the cold forgeability is improved by reducing the amount of carbon added to 0.01% by mass or less. However, the strength after soft nitriding was sometimes insufficient due to the reduction in the amount of carbon added.

[0013] Furthermore, in the steel materials for soft nitriding disclosed in Patent Documents 3 and 4, the amount of carbon added is 0.15% by weight or more and 0.45% by weight or less, and in some cases, excellent cold forgeability sufficient to omit softening annealing could not be obtained.

[0014] Thus, conventional technology has failed to produce steel that achieves both excellent cold forgeability and fatigue strength after soft nitriding. Therefore, there is a need for steel that achieves both excellent cold forgeability and fatigue strength after soft nitriding.

[0015] The present invention has been made in view of such a situation, and its object is to provide a steel excellent in cold forging property and excellent in fatigue strength after soft nitriding, and a soft nitrided component formed therefrom.

Means for Solving the Problems

[0016] The steel and soft nitrided component according to the present invention for achieving the above object are as follows.

[0017] [1] C: More than 0.02% by mass and less than 0.15% by mass, Si: 0.03% by mass or more and 0.15% by mass or less, Mn: 0.10% by mass or more and 0.95% by mass or less, Cr: 0.50% by mass or more and 1.90% by mass or less, P: 0.1% by mass or less, S: <0.5% by mass, Al: 0.005% by mass or more and 0.080% by mass or less, N: 0.0010% by mass or more and 0.0120% by mass or less, V: 0.03% by mass or more and 0.30% by mass or less and Cu: 0.2% by mass or more and 1.5% by mass or less, A steel having a component composition consisting of the balance being Fe and impurities.

[0018] [2] The above component composition further Ni: 1.5% by mass or less, Mo: 1.0% by mass or less, B: 0.01% by mass or less, Ti: 0.1% by mass or less, Nb: 0.1% by mass or less, Sn: 0.1% by mass or less, Sb: 0.1% by mass or less, Se: 0.3% by mass or less, Ca: 0.1% by mass or less, Pb: 0.3% by mass or less and Bi: 0.3% by mass or less The steel of [1], including one or more selected from the group consisting of. [3] The steel according to [1] or [2], wherein the component composition includes Cu: 0.41% by mass or more and 1.5% by mass or less.

[0019] [4] Formed from one of the steels described in [1] to [3] above, The dislocation density at a depth of 0.2 mm from the surface is 1.5 × 10⁻⁶ 14 (m -2 ) Soft nitride components. [Effects of the Invention]

[0020] According to the present invention, it is possible to provide steel that exhibits excellent cold forging properties and excellent fatigue strength after soft nitriding, as well as soft-nitrided parts formed therefrom. [Brief explanation of the drawing]

[0021] [Figure 1] This is a schematic diagram showing the shape of the test specimen. [Modes for carrying out the invention]

[0022] The steel according to this embodiment will be described below.

[0023] First, an overview of the steel according to this embodiment will be described.

[0024] The steel according to this embodiment has a chemical composition comprising: C (carbon): more than 0.02 mass% and less than 0.15 mass%, Si (silicon): 0.03 mass% or more and 0.15 mass% or less, Mn (manganese): 0.10 mass% or more and 0.95 mass% or less, Cr: 0.50 mass% or more and 1.90 mass% or less, P (phosphorus): 0.1 mass% or less, S: 0.5 mass% or less, Al (aluminum): 0.005 mass% or more and 0.080 mass% or less, N (nitrogen): 0.0010 mass% or more and 0.0120 mass% or less, V (vanadium): 0.03 mass% or more and 0.30 mass% or less, and Cu (copper): 0.2 mass% or more and 1.5 mass% or less, with the remainder being Fe and impurities.

[0025] The steel according to this embodiment exhibits excellent cold forging properties and excellent fatigue strength after soft nitriding.

[0026] The steel according to this embodiment and the soft-nitrided parts realized using this steel will be described in detail below.

[0027] An example of a soft-nitrided part (hereinafter referred to as a soft-nitrided part according to this embodiment) realized by the steel according to this embodiment is a part that forms a vehicle such as an automobile. For example, examples of parts in the automotive field include engine crankshafts, timing gears, etc., transmission gears, ring gears, sun gears, planetary gears, etc., suspension steering pinions, worms, etc., and interior power window worms, etc.

[0028] Nitriding includes both nitriding treatment, which involves impregnating steel with only nitrogen, and soft nitriding treatment, which involves impregnating steel with both nitrogen and carbon simultaneously. Both treatments do not cause the steel to undergo martensitic transformation. In this embodiment, nitriding refers to soft nitriding treatment.

[0029] As described above, the steel according to this embodiment is used after being nitrided. That is, the body of the soft-nitrided part according to this embodiment has a surface layer in which the steel according to this embodiment has been nitrided and has increased hardness.

[0030] The hardness of the surface layer of the soft-nitrided component, i.e., the steel material after nitriding, for example, the hardness at a depth of 0.2 mm from the surface, should be 580 HV or higher. This results in soft-nitrided components with excellent fatigue strength. Here, the hardness at a depth of 0.2 mm from the surface can be measured by the method described in the examples.

[0031] As described above, the fatigue strength of soft-nitrided parts has a strong correlation with the hardness at a depth of 0.2 mm from the surface. Furthermore, when dislocations introduced into the steel during cold forging recover during soft nitriding, a decrease in the hardness of the steel material occurs in proportion to the amount of recovery. In the steel of this embodiment, the dislocation density is 1.5 × 10⁻⁶. 14 (m -2If the hardness falls below 1.5 × 10⁻¹⁰, a decrease in hardness occurs. Therefore, the dislocation density at a depth of 0.2 mm from the surface in the main body of the soft-nitrided part (nitrided steel) is 1.5 × 10⁻¹⁰. 14 (m -2 Preferably, the dislocation density at a depth of 0.2 mm from the surface should be 3.0 × 10⁻⁶ or higher. In order to obtain high surface hardness in the main body of the soft nitrided part (steel material after nitriding), it is more preferable that the dislocation density at a depth of 0.2 mm from the surface is 3.0 × 10⁻⁶. 14 (m -2 It is best to set it to ) or higher. Here, the dislocation density at a depth of 0.2 mm from the surface can be measured by the method described in the examples.

[0032] The soft-nitrided component according to this embodiment may be a body to which a separate structure made of metal or a metal alloy, whose surface is not nitrided, is combined. The soft-nitrided component according to this embodiment may be a body to which a separate structure that is not made of metal is combined.

[0033] The steel according to this embodiment forms a portion with a depth of 0.5 mm or more from the surface of the main body of the soft-nitrided component according to this embodiment. The surface layer of the main body of the soft-nitrided component according to this embodiment may be the steel which has been nitrided according to this embodiment, as described above.

[0034] As described above, the steel according to this embodiment has a composition comprising C, Si, Mn, Cr, P, S, Al, N, V, and Cu, with the remainder being Fe and impurities. This steel may further contain the following optional components. Examples of optional components include the following groups A to E, and may contain one or more of any of these groups.

[0035] Group A: This steel may contain, as an optional component, Ni (nickel): 1.5% by mass or less.

[0036] Group B: This steel may contain one or more elements selected from Mo (molybdenum): 1.0% by mass or less and B (boron): 0.01% by mass or less as optional components.

[0037] Group C: This steel may contain one or more elements selected from Ti (titanium): 0.1% by mass or less and Nb (niobium): 0.1% by mass or less as optional components in its composition.

[0038] Group D: This steel may contain one or more elements selected from among Sn (tin): 0.1% by mass or less and Sb (antimony): 0.1% by mass or less as optional components in its composition.

[0039] Group E: This steel may contain one or more of the following as optional components: Se (selenium): 0.3% by mass or less, Ca (calcium): 0.1% by mass or less, Pb (lead): 0.3% by mass or less, and Bi (bismuth): 0.3% by mass or less.

[0040] The following details the content and effects of each component in steel. In the following explanation, when simply referred to as "content," it refers to the content (mass %) in steel.

[0041] C: More than 0.02% by mass and less than 0.15% by mass The carbon content is between 0.02% by mass and less than 0.15% by mass. The more carbon added, the stronger the steel becomes. However, the more carbon added, the lower the cold forgeability of the steel becomes. If the carbon content is within the above range, the effect on reducing cold forgeability is small, and the dislocations introduced during cold forging do not recover during soft nitriding, work hardening is maintained even after soft nitriding, and high surface hardness can be achieved in the steel material (soft nitrided parts) even after soft nitriding. In order to achieve both cold forgeability and fatigue strength after soft nitriding, the carbon content is preferably between 0.04% by mass and 0.13% by mass.

[0042] Si: 0.03 mass% or more and 0.15 mass% or less The Si content is between 0.03% by mass and 0.15% by mass. In nitrided steel, Si contributes as a deoxidizing agent. Excessive Si content in steel reduces the cold workability of the steel material. The Si content is preferably between 0.05% by mass and 0.10% by mass.

[0043] Mn: 0.10 mass% or more and 0.95 mass% or less The Mn content is between 0.10% by mass and 0.95% by mass. Mn improves hardenability and strengthens the pre-nitrided structure of steel, thereby increasing the strength of the post-nitrided structure (nitrided steel). To obtain sufficient fatigue strength, the Mn content must be 0.10% by mass or more. Excessive Mn content in steel may lead to an increase in deformation resistance. The Mn content is preferably between 0.13% by mass and 0.55% by mass, and more preferably between 0.15% by mass and 0.25% by mass.

[0044] Cr: 0.50 mass% or more and 1.90 mass% or less The Cr content is between 0.50% by mass and 1.90% by mass. Cr has the effect of increasing hardness by forming a Cr nitride layer during soft nitriding, and also has the effect of suppressing the recovery of work hardening introduced in cold forging. Excessive addition of Cr may reduce the amount of N that penetrates into the interior, and may decrease the effective hardened layer depth. The Cr content is preferably between 0.65% by mass and 1.35% by mass, and more preferably between 0.70% by mass and 0.85% by mass.

[0045] P: 0.1% by mass or less The P content is 0.1% by mass or less. Since P segregates at the grain boundaries of nitrided steel and reduces toughness, a lower P content is desirable. A P content of up to 0.1% by mass is acceptable. Preferably, the P content is 0.02% by mass or less. While there is no particular lower limit to the P content, since P content is usually unavoidable, unnecessarily reducing P may increase refining time and refining costs. Therefore, it is reasonable and preferable for the P content to be 0.003% by mass or more.

[0046] S: 0.5% by mass or less The sulfur (S) content is 0.5% by mass or less. S exists as a sulfide inclusion and is an element effective in improving machinability. Excessive S content in steel can lead to a decrease in cold workability. There is no particular lower limit to the S content, but since S content is usually unavoidable, excessively low S content may increase refining costs. Therefore, it is reasonable to set the S content at 0.003% by mass or more. Preferably, the S content is 0.004% by mass or more and 0.3% by mass or less, and more preferably 0.005% by mass or more and 0.09% by mass or less.

[0047] Al: 0.005 mass% or more and 0.080 mass% or less The Al content is between 0.005% by mass and 0.080% by mass. Al forms oxides and is an effective element for deoxidizing nitrided steel. In addition, Al has the effect of suppressing the formation of coarse oxide inclusions in nitrided steel. If the Al content is less than 0.005% by mass, these effects may not be obtained. If Al is included in excess in the steel, it will lead to an increase in inclusions (Al oxides), which may increase the starting points for fatigue fracture and cause low fatigue strength. The Al content is preferably between 0.015% by mass and 0.065% by mass.

[0048] N: 0.0010 mass% or more and 0.0120 mass% or less The N content is between 0.0010% by mass and 0.0120% by mass. Excessive N content in steel can lead to surface cracking of the steel billet after casting. There is no particular lower limit to the N content, but since N content is usually unavoidable, excessively low N content may increase refining costs. The N content is preferably 0.0010% by mass or more. The N content is preferably between 0.0030% by mass and 0.0080% by mass.

[0049] V: 0.03 mass% or more and 0.30 mass% or less The V content is between 0.03% by mass and 0.30% by mass. V combines with carbon and nitrogen and improves the strength of the steel by forming fine precipitates during soft nitriding. In addition, V has the effect of suppressing the recovery of work hardening introduced in cold forging. To obtain this effect of V, it is preferable to include V in the steel at least at a concentration of 0.03% by mass or more. If V is included in the steel in excess, the cold forgeability will decrease. The V content is more preferably between 0.10% by mass and 0.15% by mass.

[0050] Cu: 0.2 mass% or more and 1.5 mass% or less The Cu content is between 0.2% by mass and 1.5% by mass. Cu has the effect of suppressing the recovery of dislocations introduced during cold forging during soft nitriding. Furthermore, this effect contributes to the increase in the hardness of the surface layer of the steel after soft nitriding. The above effect saturates even if the Ni content exceeds 1.5% by mass. Cu also has the effect of embrittlement of steel during hot working, which may reduce the manufacturability of the steel. Considering the effect of Cu on embrittlement of steel during hot working, the Cu content is preferably between 0.2% by mass and 0.8% by mass, and more preferably between 0.3% by mass and 0.7% by mass. If the purpose is to prevent surface cracking during hot rolling, it is preferable to have a Cu content of 0.41% by mass or less.

[0051] The following describes the arbitrary components of steel.

[0052] Ni: 1.5% by mass or less The Ni content may be 1.5% by mass or less. Ni has the effect of mitigating the embrittlement of steel during hot working when Cu is added. Ni may also be added to improve the manufacturability of steel. To obtain this effect from Ni, it is preferable to include Ni in the steel at a concentration of 0.05% by mass or more. The above effect saturates even if the Ni content exceeds 1.5% by mass. The Ni content is preferably 1.0% by mass or less.

[0053] Mo: 1.0% by mass or less The Mo content may be 1.0 mass% or less. Mo improves hardenability and strengthens the post-nitrided structure by strengthening the pre-nitrided structure. However, if the Mo content exceeds 1 mass%, the hardenability becomes excessive, the hardness after rolling increases, and the workability and machinability may decrease. In order to exhibit the effect of Mo on improving the strength of the steel, it is preferable to include Mo in the steel at a concentration of 0.01 mass% or more. The Mo content is more preferably 0.03 mass% to 0.50 mass%, and even more preferably 0.05 mass% to 0.25 mass%.

[0054] B: 0.01% by mass or less The B content may be 0.01% by mass or less. B is effective in improving hardenability by segregating at grain boundaries and suppressing diffusion-type transformation. In addition, it also strengthens grain boundaries, suppresses the occurrence and propagation of fatigue cracks, and improves fatigue strength. To obtain this effect from B, it is preferable to include B in the steel at a concentration of 0.0003% by mass or more. If the B content exceeds 0.01% by mass, the toughness of the steel decreases, so it is preferable to keep the B content at 0.01% by mass or less. More preferably, the B content is 0.0005% by mass or more and 0.005% by mass or less, and even more preferably 0.0007% by mass or more and 0.002% by mass or less.

[0055] Ti: 0.1% by mass or less The Ti content may be 0.1% by mass or less. Ti combines with carbon and nitrogen, and improves the strength of the steel by forming fine precipitates during soft nitriding. However, the effect saturates even if the amount of Ti in the steel exceeds 0.1% by mass. The Ti content is preferably 0.005% by mass or more and 0.08% by mass or less, and more preferably 0.01% by mass or more and 0.06% by mass or less.

[0056] Nb: 0.1% by mass or less The Nb content may be 0.1% by mass or less. Nb combines with carbon and nitrogen, and improves the strength of the steel by forming fine precipitates during soft nitriding. However, the effect saturates even if the Nb content exceeds 0.1% by mass. The Nb content is preferably 0.005% by mass or more and 0.08% by mass or less, and more preferably 0.01% by mass or more and 0.06% by mass or less.

[0057] Sb: 0.1% by mass or less The Sb content may be 0.1% by mass or less. Sb is an effective element for suppressing decarburization on the surface of steel materials and preventing a decrease in surface hardness. To achieve this effect, it is preferable to include 0.0003% by mass or more of Sb in the steel. If the steel contains too much Sb, the workability of the steel will decrease. The Sb content is more preferably 0.001% by mass or more and 0.05% by mass or less, and even more preferably 0.0015% by mass or more and 0.035% by mass or less.

[0058] Sn: 0.1% by mass or less The Sn content may be 0.1% by mass or less. Sn is an effective element for improving the corrosion resistance of the steel surface. From the viewpoint of improving corrosion resistance, it is preferable to include 0.003% by mass or more of Sn in the steel. Including too much Sn in the steel will reduce workability. The Sn content is more preferably 0.0010% by mass or more and 0.050% by mass or less, and even more preferably 0.0015% by mass or more and 0.035% by mass or less.

[0059] Se: 0.3% by mass or less The Se content may be 0.3 mass% or less. Se combines with Mn and Cu and disperses as precipitates in the steel, thereby improving machinability. To obtain this effect, it is preferable to include at least 0.001 mass% or more of Se in the steel. The effect saturates even if the Se content exceeds 0.3 mass%. The Se content is more preferably 0.005 mass% to 0.1 mass%, and even more preferably 0.008 mass% to 0.09 mass%.

[0060] Ca: 0.1% by mass or less The Ca content may be 0.1% by mass or less. Ca combines with S and disperses as sulfides in the steel, improving machinability. To obtain this effect, it is preferable to include at least 0.0005% by mass or more of Ca in the steel. The effect saturates even if the Ca content exceeds 0.1% by mass. The Ca content is more preferably 0.0010% by mass or more and 0.0500% by mass or less, and even more preferably 0.0015% by mass or more and 0.0300% by mass or less.

[0061] Pb: 0.3% by mass or less The Pb content may be 0.3% by mass or less. Pb has the effect of refining chips during cutting. Adding Pb is effective when you want to improve chip handling performance. To obtain this effect, it is preferable to include 0.01% by mass or more of Pb in the steel. If you include too much Pb in the steel, the effect of improving chip handling performance will saturate. The Pb content is preferably 0.01% by mass or more and 0.2% by mass or less, and more preferably 0.01% by mass or more and 0.1% by mass or less.

[0062] Bi: 0.3% by mass or less The Bi content may be 0.3% by mass or less. Bi has the effect of refining chips during cutting. Adding Bi is effective when you want to improve chip handling performance. To obtain this effect, it is preferable to include Bi at a concentration of 0.01% by mass or more. If you include too much Bi in the steel, the effect of improving chip handling performance will saturate. The Bi content is preferably 0.01% by mass or more and 0.2% by mass or less, and more preferably 0.01% by mass or more and 0.1% by mass or less.

[0063] The remaining elements, other than those described above, are Fe and impurities. Impurities are substances that are introduced during the industrial production of steel materials from raw materials such as ore, scrap, or the manufacturing environment, and are acceptable as long as they do not adversely affect the properties of this embodiment.

[0064] The steel according to the present invention may be obtained by melting steel having a predetermined component composition, or it may be a rolled material obtained by hot rolling steel having a predetermined component composition. If the rolled material has a Vickers hardness of 10⁵ HV or less, it can be said to have excellent cold forgeability, and cold forging is possible without softening annealing.

[0065] The steel according to the present invention is suitable for cold forging and nitriding (soft nitriding). In particular, it is especially suitable for cold forging a rolled material obtained by hot rolling a steel having a predetermined composition, followed by nitriding (soft nitriding). The steel according to the present invention can be cold forged without softening annealing of the rolled material, but it may also be cold forged after softening annealing of the rolled material.

[0066] The above methods of hot rolling, cold forging, and nitriding (soft nitriding) are not particularly limited, and known methods may be used. When performing softening annealing, the method is not particularly limited, and known methods may be used.

[0067] The soft-nitrided parts according to the present invention are made of steel according to the present invention and can be manufactured by hot-rolling steel having a predetermined component composition to obtain a rolled material, which may be softened and annealed, then cold-forged and subjected to nitriding treatment (soft nitriding treatment). [Examples]

[0068] The present invention will be described below based on examples, but the present invention is not limited to these examples.

[0069] First, steel (steel No. 1 to 58) with the component compositions shown in Table 1 was melted and formed into 32 mm diameter round bars (rolled material) by hot rolling. Note that the underlined values ​​among the component composition values ​​shown in Table 1 are outside the range specified in this embodiment. Also, in Table 1, "Others" refers to any component other than C, Si, Mn, Cr, P, S, Al, N, V, and Cu.

[0070] [Table 1]

[0071] The Vickers hardness (HV) was measured for each obtained round bar. The load used during Vickers hardness measurement was 1 kgf (9.8 N). Vickers hardness was measured at 10 randomly selected points on the surface of the round bar, and the average of these 10 measurements was obtained. Table 2 shows the average Vickers hardness measurement value of this round bar as the rolled material hardness.

[0072] In addition, the surface of the round bars was visually inspected and evaluated. The evaluation criteria were as follows: A... No cracks B... Minor cracks present C...Significant cracks present.

[0073] [Table 2]

[0074] Next, this round bar was cut to obtain a 17mm diameter round bar. Then, this round bar was cold-worked (drawn) to create a 14mm diameter round bar, and cold-work strain was applied.

[0075] Furthermore, a round bar subjected to cold working strain was machined to obtain a test specimen with the shape shown in the schematic diagram in Figure 1.

[0076] As shown in Figure 1, the test specimen is a straight rod with a circular cross-section perpendicular to its extension direction. The ends of the test specimen along the extension direction of the rod are gripping portions 1,1, which will be described later. A parallel portion 2 is positioned between the gripping portions 1,1. The diameter of the central part of the parallel portion 2 in the axial direction is 10 mm (10 ± 0.05 mm). The diameter of the gripping portions 1,1 is 12 mm (the same diameter as the round bar). The length of the gripping portion 1 in the direction along the axis G is 25 mm. The length between the gripping portions 1,1 including the parallel portion 2 is 30 mm. The ends of the parallel portion 2 in the direction along the axis G are tapered into a curved surface with a radius of R15 from the end of the gripping portion 1 to the central part in the axial direction. A recess 4 is formed in the central part of the parallel portion 2 along the circumferential direction of the test specimen. When viewed in a direction intersecting the extending direction of the test specimen (in a side view), the recess 4 has a shape following a circular arc with a diameter of 1 mm (R1.0), and the maximum depth of this recess from the surface of the test specimen is approximately 1 mm. The width of the recess in the direction along the extending direction of the test specimen (the extending direction of the rod) is approximately 2 mm. The diameter of the recess 4 portion of the parallel section 2 is approximately 8 mm (8 ± 0.02 mm).

[0077] Next, the test specimen was subjected to gas soft nitriding heat treatment (nitriding) at 570°C for 3 hours to obtain a post-treatment test specimen (an example of a soft-nitrided part). The nitriding treatment was carried out in a mixed gas containing ammonia and carbon monoxide. Two or more post-treatment test specimens were prepared for each steel with the component composition shown in Table 1.

[0078] Furthermore, the surface hardness (Vickers hardness) of the recessed portion of the treated test specimen was measured in accordance with the "Method for Measuring Surface Hardness of Nitrided Layers of Steel" specified in JIS G 0563-1993. This measurement was performed at a depth of 0.2 mm from the surface of the bottom of the notch in a cross section overlapping with the axis G of the test specimen. In other words, this measurement was performed at a depth of 0.2 mm below the surface of the test specimen. The dislocation density at this depth of 0.2 mm below the surface of the test specimen was determined using the Williamson-Hall method with X-ray diffraction (see Non-Patent Literature 1). The surface hardness measurement results and dislocation density are shown together in Table 2.

[0079] In addition, a post-treatment test piece different from the one used for measuring the surface hardness was subjected to a rotating bending fatigue test, and the fatigue limit (1×10 7 cycles equivalent stress) was evaluated for each. The evaluation results of this fatigue limit are shown together in Table 2.

[0080] In the rolled material (round bar with a diameter of 32 mm, before nitriding), if the Vickers hardness is 105 HV or less, it has excellent cold forging properties. The rolled materials formed of the steel (No. 1 to 29, No. 45 to 58) according to this embodiment have a Vickers hardness of 105 HV or less and are excellent in cold forging properties.

[0081] In the post-treatment test piece, if the surface hardness at 0.20 mm below the surface layer is 580 HV or more, it has excellent fatigue strength after nitriding. The post-treatment test pieces (soft nitrided parts) formed of the steel (No. 1 to 29, No. 45 to 58) according to this embodiment have a surface hardness of 580 HV or more at 0.20 mm below the surface layer and are excellent in fatigue strength after soft nitriding.

[0082] In the post-treatment test piece, if the fatigue limit is 260 MPa or more, it has excellent fatigue strength after soft nitriding. The post-treatment test pieces (soft nitrided parts) formed of the steel (No. 1 to 2, No. 45 to 58) according to this embodiment have a fatigue limit of 260 MPa or more and are excellent in fatigue strength after soft nitriding.

[0083] That is, the steel (No. 1 to 29, No. 45 to 58) according to this embodiment is excellent in cold forging properties and also excellent in fatigue strength after soft nitriding. Further, the soft nitrided parts formed of the steel according to this embodiment are excellent in fatigue strength.

[0084] Furthermore, in the steel (No. 5, 16, 17, 45 to 58) according to this embodiment, in which the Cu content is 0.2 mass% or more and 41 mass%, surface cracking during hot rolling is prevented.

[0085] As described above, it is possible to provide a steel excellent in cold forging properties and excellent in fatigue strength after soft nitriding, and a soft nitrided part formed therefrom.

[0086] The embodiments disclosed herein are illustrative examples, and the embodiments of the present invention are not limited thereto. They can be modified as appropriate without departing from the purpose of the present invention. [Industrial applicability]

[0087] This invention is applicable to steel and soft-nitrided parts. [Explanation of symbols]

[0088] 1: Grip part 2: Parallel section 4: Recess

Claims

1. C: more than 0.02% by mass and less than 0.15% by mass, Si: 0.03% by mass or more and 0.10% by mass or less, Mn: 0.13 mass% or more and 0.95 mass% or less, Cr: 0.50% by mass or more and 1.90% by mass or less, P: 0.1% by mass or less, S: 0.5% by mass or less, Al: 0.005% by mass or more and 0.080% by mass or less, N: 0.0010% by mass or more and 0.0120% by mass or less, V: 0.03% by mass or more and 0.30% by mass or less, Cu: 0.2% by mass or more and 1.5% by mass or less It contains one or more elements selected from the group consisting of Sn: 0.0010% by mass or more and 0.1% by mass or less, Sb: 0.0003% by mass or more and 0.1% by mass or less, and Se: 0.001% by mass or more and 0.3% by mass or less. The composition consists of Fe and impurities as the remainder. Steel with a Vickers hardness of 105 Hv or less.

2. The aforementioned component composition is further, Ni: 1.5% by mass or less, Mo: 1.0% by mass or less, B: 0.01% by mass or less, Ti: 0.1% by mass or less, Nb: 0.1% by mass or less, Ca: 0.1% by mass or less, Pb: 0.3% by mass or less and Bi: 0.3% by mass or less The steel according to claim 1, comprising one or more selected from the group consisting of the following.

3. The steel according to claim 1 or 2, wherein the aforementioned component composition includes Cu: 0.41% by mass or more and 1.5% by mass or less.

4. Formed from the steel described in claim 1 or 2, The dislocation density at a depth of 0.2 mm from the surface is 1.5 × 10⁻⁶ １４ (m －２ ) Soft nitride components.

5. Formed from the steel described in claim 3, The dislocation density at a depth of 0.2 mm from the surface is 1.5 × 10⁻⁶ １４ (m －２ ) Soft nitride components.

Images