steel

Abstract

To provide a steel that exhibits superior cold forgeability and superior fatigue strength after soft nitriding.SOLUTION: A steel comprises C: 0.01 mass% or more and 0.18 mass% or less, Si: 0.03 mass% or more and 0.15 mass% or less, Mn: 0.10 mass% or more and 0.35 mass% or less, Cr: 0.50 mass% or more and 1.10 mass% or less, P: 0.1 mass% or less, S: 0.5 mass% or less, Al: 0.005 mass% or more and 0.050 mass% or less, N: 0.0010 mass% or more and 0.0120 mass% or less and V: 0.03 mass% or more and 0.20 mass% or less, with the balance being Fe and impurities.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to steel. [Background technology]

[0002] Cold forging allows for near-net-shape forming, which has the advantage of reducing the amount of cutting required after forging compared to hot forging, thereby suppressing yield declines. In addition, 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 rolled steel material for cold forging and nitriding. This rolled steel for cold forging and nitriding contains C: 0.06-0.15%, Si: 0.02-0.35%, Mn: 0.10-0.90%, S≦0.030%, Cr: 0.50-2.0%, V: 0.10-0.50%, Al: 0.010-0.090%, and the balance being Fe and impurities, where the P, N, and O impurities are P≦0.030%, N≦0.0080%, and O≦0.0030%. The chemical composition is 5.0≦35(C+N)+5(Si+Mn)+Cr+3(Cu+Ni)+4(Mo+V)≦10.0 and 1.8≦{V+(9 / 20)Mo} / C≦9.0, and the microstructure is a ferrite-pearlite structure with an average ferrite grain size≦50μm. This rolled steel for cold forging and nitriding can be cold forged without prior heat treatment, has low surface roughness after cold forging, has excellent machinability after cold forging, and is said to be able to give parts that have been cold forged and nitrided high core hardness.

[0004] Patent Document 2 describes a steel for cold forging and nitriding. This steel for cold forging and nitriding contains C: 0.01 to 0.15%, Si<0.10%, Mn: 0.10 to 0.50%, P≦0.030%, S≦0.050%, 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%, and O≦0.0030%, with the balance being Fe and impurities. The chemical composition is [399 × C + 26 × Si + 123 × Mn + 30 × Cr + 32 × Mo + 19 × V ≦ 160], [20 ≦ (669.3 × log e C - 1959.6 × log e N - 6983.3) × (0.067 × Mo + 0.147 × V) ≦ 80], and [140 × Cr + 125 × Al + 235 × V ≧ 160]. This cold forging and nitriding steel has excellent cold forgeability and machinability after cold forging, and is said to be able to provide cold forged and nitrided parts with high core hardness, high surface hardness, and a deep effective case depth. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-159794 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-185186 Summary of the Invention [Problem to be solved by the invention]

[0006] If parts can be manufactured by cold forging, costs can be reduced compared to manufacturing parts by hot forging. Furthermore, if die life can be improved, further cost reductions can be achieved in cold forging. However, in the above-mentioned conventional techniques, assuming that fatigue strength after soft nitriding is ensured, the deformation resistance of the steel during cold forging remains unchanged, and the die life is equivalent to that of conventional techniques, and therefore costs could not be reduced sufficiently. Therefore, it is desirable to provide a steel that combines excellent cold forgeability and fatigue strength after soft nitriding.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a steel that has excellent cold forgeability and excellent fatigue strength after soft nitriding. [Means for solving the problem]

[0008] In order to achieve the above object, the steel according to the present invention is as follows.

[0009] [1] The component composition is: C: 0.01% by mass or more and 0.18% by mass or less, Si: 0.03 mass% or more and 0.15 mass% or less, Mn: 0.10 mass% or more and 0.35 mass% or less, Cr: 0.50 mass% or more and 1.10 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 0.050% by mass or less, N: 0.0010 mass% or more and 0.0120 mass% or less and V: 0.03% by mass or more and 0.20% by mass or less, The balance is steel containing Fe and impurities.

[0010] [2] The component composition is: Mo: 1.0% by mass or less, Ni: 1.0 mass% or less, and B: 0.01% by mass or less, The steel according to the above [1], further comprising one or more selected from the following:

[0011] [3] The component composition is: Ti: 0.1 mass% or less; Nb: 0.1% by mass or less The steel according to the above [1] or [2], further comprising one or more selected from the following:

[0012] [4] The component composition is: Sn: 0.1 mass% or less; Sb: 0.1% by mass or less, The steel according to any one of [1] to [3] above, further comprising one or more selected from the following:

[0013] [5] The component composition is: 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 according to any one of [1] to [4] above, further comprising one or more selected from the following:

[0014] [6] The steel according to any one of [1] to [5] above, having a Vickers hardness of 105 HV or less. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide a steel that has excellent cold forgeability and excellent fatigue strength after soft nitriding. DETAILED DESCRIPTION OF THE INVENTION

[0016] The steel according to this embodiment will be described below.

[0017] First, an overview of the steel according to this embodiment will be described.

[0018] The steel according to this embodiment has a chemical composition (chemical composition) of C (carbon): 0.01% by mass or more and 0.18% by mass or less, Si (silicon): 0.03% by mass or more and 0.15% by mass or less, Mn (manganese): 0.10% by mass or more and 0.35% by mass or less, Cr: 0.50% by mass or more and 1.10% by mass or less, P (phosphorus): 0.1% by mass or less, S: 0.5% by mass or less, Al (aluminum): 0.005% by mass or more and 0.050% by mass or less, N (nitrogen): 0.0010% by mass or more and 0.0120% by mass or less, and V (vanadium): 0.03% by mass or more and 0.20% by mass or less, with the remainder containing Fe and impurities.

[0019] The steel according to this embodiment has excellent cold forgeability and excellent fatigue strength after soft nitriding.

[0020] The steel according to this embodiment and the soft nitrided parts realized by this steel will be described in detail below.

[0021] An example of a soft-nitrided part realized by the steel according to this embodiment (hereinafter referred to as the soft-nitrided part according to this embodiment) is a part that forms a vehicle such as an automobile. 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.

[0022] Nitriding includes both nitriding, in which only nitrogen penetrates steel, and soft nitriding, in which nitrogen and carbon penetrate steel simultaneously, and both refer to treatments that do not cause martensitic transformation of steel. Of these, nitriding in this embodiment refers to soft nitriding.

[0023] As described above, the steel according to this embodiment is nitrided before use, that is, the body of the soft-nitrided component according to this embodiment has a surface layer of nitrided steel.

[0024] The soft-nitrided component according to the present embodiment may be a component obtained by combining with the main body another structure that is made of a metal or a metal alloy and has a non-nitrided surface. The soft-nitrided component according to the present embodiment may be a component obtained by combining with the main body another structure that is not made of metal.

[0025] The surface layer of the soft nitrided part according to this embodiment is, for example, about 0.5 mm from the surface of the main body. This embodiment also includes cases where the thickness of this surface layer is greater than 0.5 mm.

[0026] As described above, the steel according to this embodiment contains, as its chemical composition, C, Si, Mn, Cr, P, S, Al, N, and V, with the balance being Fe and impurities. It is not excluded that this steel may further contain optional elements, which will be described later.

[0027] This steel may contain, as optional components, one or more selected from the group consisting of 1.0 mass % or less of Mo (molybdenum), 1.0 mass % or less of Ni (nickel), and 0.01 mass % or less of B (boron).

[0028] Furthermore, this steel may contain, as an optional component, one or more selected from the group consisting of 0.1 mass % or less of Ti (titanium) and 0.1 mass % or less of Nb (niobium).

[0029] Furthermore, this steel may contain, as optional components in its composition, one or more selected from the group consisting of 0.1 mass % or less of Sn (tin) and 0.1 mass % or less of Sb (antimony).

[0030] Furthermore, this steel may contain, as optional components, one or more selected from the group consisting of 0.3 mass% or less of Se (selenium), 0.1 mass% or less of Ca (calcium), 0.3 mass% or less of Pb (lead), and 0.3 mass% or less of Bi (bismuth).

[0031] The content and effects of each component in the steel will be described in detail below. In the following description, when simply referring to a content, it refers to the content (mass %) in the steel.

[0032] C: 0.01 mass% or more and 0.18 mass% or less The C content is 0.01% by mass or more and 0.18% by mass or less. In order to increase the central hardness after nitriding, the C content must be 0.01% by mass or more. If the C content exceeds 0.18% by mass, the load during cold working increases, which may shorten the life of the mold. The C content is preferably 0.04% by mass or more and 0.15% by mass or less.

[0033] Si: 0.03 mass% or more and 0.15 mass% or less The Si content is 0.03% by mass or more and 0.15% by mass or less. In nitrided steel, Si serves as a deoxidizer. Excessive Si content in steel reduces the cold workability of the steel. The Si content is preferably 0.05% by mass or more and 0.10% by mass or less.

[0034] Mn: 0.10 mass% or more and 0.35 mass% or less The Mn content is 0.10% by mass or more and 0.35% by mass or less. Mn improves hardenability and strengthens the pre-nitriding structure of steel, thereby strengthening the post-nitriding structure (nitrided steel). In order to obtain sufficient fatigue strength, the Mn content must be 0.10% by mass or more. If excessive Mn is added to steel, it may result in an increase in deformation resistance. The Mn content is preferably 0.15% by mass or more and 0.25% by mass or less.

[0035] Cr: 0.50 mass% or more and 1.10 mass% or less The Cr content is 0.50% by mass or more and 1.10% by mass or less. Cr has the effect of forming a Cr nitride layer during soft nitriding, increasing hardness, and inhibiting recovery of work hardening introduced by cold forging. Excessive addition of Cr may reduce the amount of N penetrating into the interior, resulting in a decrease in the effective hardened layer depth. The Cr content is preferably 0.70% by mass or more and 0.85% by mass or less.

[0036] P: 0.1% by mass or less The P content is 0.1% by mass or less. P segregates at the grain boundaries of nitrided steel and reduces toughness, so the lower the P content, the better. The P content is allowed up to 0.1% by mass. The P content is preferably 0.02% by mass or less. There is no problem if there is no particular lower limit for the P content, but since the inclusion of P is usually unavoidable, unnecessary reduction in P content may increase refining time and refining costs. Therefore, it is reasonable and preferable to set the P content to 0.003% by mass or more.

[0037] S: 0.5% by mass or less The S content is 0.5% by mass or less. S exists as sulfide-based inclusions 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 for the S content, but since the inclusion of S is usually unavoidable, excessively reducing the S content can increase refining costs. Therefore, it is reasonable to set the S content to 0.003% by mass or more. The S content is preferably 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.

[0038] Al: 0.005 mass% or more and 0.050 mass% or less The Al content is 0.005% by mass or more and 0.050% by mass or less. Al forms oxides and is an element effective in deoxidizing nitrided steel. Al also has the effect of suppressing the formation of coarse oxide-based inclusions in nitrided steel. If the Al content is less than 0.005% by mass, these effects may not be obtained. Excessive Al content in steel may increase the number of inclusions (Al oxides), which may increase the initiation points of fatigue fracture and cause low fatigue strength.

[0039] N: 0.0010 mass% or more and 0.0120 mass% or less The N content is 0.0010% by mass or more and 0.0120% by mass or less. Excessive N content in steel may cause surface cracks in the slab after casting. There is no particular lower limit for 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 0.0030% by mass or more and 0.0080% by mass or less.

[0040] V: 0.03 mass% or more and 0.20 mass% or less The V content is 0.03% by mass or more and 0.20% by mass or less. V combines with carbon and nitrogen to form fine precipitates during soft nitriding, thereby improving the strength of the steel. V also has the effect of suppressing the recovery of work hardening introduced by cold forging. To obtain this effect of V, it is preferable to include at least 0.03% by mass or more of V in the steel. If excessive V is included in the steel, the amount of N that penetrates into the steel during nitriding will decrease, resulting in a shallower effective hardened layer depth. The V content is more preferably 0.10% by mass or more and 0.15% by mass or less.

[0041] The optional components of the steel will be explained below.

[0042] Mo: 1.0% by mass or less The Mo content may be 1.0% by mass or less. Mo improves hardenability and strengthens the pre-nitriding structure, thereby strengthening the post-nitriding structure. However, if the Mo content exceeds 1.0% by mass, the hardenability becomes excessive, the hardness after rolling increases, and workability and machinability may decrease. In order to realize the effect of Mo in improving the strength of the steel material, it is preferable to contain Mo in an amount of 0.01% by mass or more in the steel. The Mo content is more preferably 0.03% by mass or more and 0.50% by mass or less, and even more preferably 0.05% by mass or more and 0.25% by mass or less.

[0043] Ni: 1.0% by mass or less The Ni content may be 1.0 mass% or less. Ni is an element useful for improving toughness. To obtain these effects, it is preferable to contain 0.01 mass% or more of Ni in the steel. Even if the Ni content in the steel exceeds 1.0 mass%, the above effects will saturate. The Ni content is more preferably 0.015 mass% or more and 0.5 mass% or less, and even more preferably 0.03 mass% or more and 0.3 mass% or less.

[0044] B: 0.01% by mass or less The B content may be 0.01% by mass or less. B segregates at grain boundaries and suppresses diffusional transformation, thereby effectively improving hardenability. In addition, B strengthens grain boundaries, suppresses the initiation and propagation of fatigue cracks, and improves fatigue strength. To obtain this effect of B, it is preferable to include 0.0003% by mass or more of B in the steel. If the B content exceeds 0.01% by mass, the toughness of the steel decreases, so the B content is preferably 0.01% by mass or less. The B content is more preferably 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.

[0045] Ti: 0.1% by mass or less The Ti content may be 0.1% by mass or less. Ti combines with carbon and nitrogen to form fine precipitates during soft nitriding, thereby improving the strength of the steel. However, even if the Ti content in the steel exceeds 0.1% by mass, the effect saturates. 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.

[0046] Nb: 0.1% by mass or less The Nb content may be 0.1 mass% or less. Nb bonds with carbon and nitrogen to form fine precipitates during soft nitriding, thereby improving the strength of the steel. However, even if the Nb content in the steel exceeds 0.1 mass%, the effect is saturated. The Nb content is preferably 0.005 mass% or more and 0.08 mass% or less, and more preferably 0.01 mass% or more and 0.06 mass% or less.

[0047] Sb: 0.1% by mass or less The Sb content may be 0.1% by mass or less. Sb is an element that is effective in suppressing decarburization of the steel surface and preventing a decrease in surface hardness. To achieve this effect, it is preferable to contain 0.0003% by mass or more of Sb in the steel. If an excessive amount of Sb is contained in the steel, 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.

[0048] Sn: 0.1% by mass or less The Sn content may be 0.1% by mass or less. Sn is an element effective for improving the corrosion resistance of the steel surface. From the viewpoint of improving corrosion resistance, it is preferable that the Sn content in the steel is 0.003% by mass or more. Excessive Sn content in the steel reduces 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.

[0049] Se: 0.3% by mass or less The Se content may be 0.3% by mass or less. Se improves machinability by combining with Mn and Cu and dispersing as precipitates in the steel. To achieve this effect, it is preferable that the steel contains at least 0.001% by mass of Se. Even if the Se content exceeds 0.3% by mass, the effect saturates. The Se content is more preferably 0.005% by mass to 0.1% by mass, and even more preferably 0.008% by mass to 0.09% by mass.

[0050] Ca: 0.1% by mass or less The Ca content may be 0.1% by mass or less. Ca improves machinability by combining with S and dispersing as sulfides in the steel. To achieve this effect, it is preferable to include at least 0.0005% by mass of Ca in the steel. Even if the Ca content in the steel exceeds 0.1% by mass, the effect saturates. 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.

[0051] 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 improving chip disposability. To achieve this effect, it is preferable to include 0.01% by mass or more of Pb in the steel. Even if an excessive amount of Pb is included in the steel, the effect of improving chip disposability 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.

[0052] 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 improving chip disposability. To achieve this effect, it is preferable to contain Bi at 0.01% by mass or more. Even if excessive Bi is contained in steel, the effect of improving chip disposability 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.

[0053] The balance other than the elements described above is Fe and impurities. The impurities are substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment during industrial production of steel, and are allowed to a degree that does not adversely affect the properties of this embodiment.

[0054] Next, the hardness of the steel will be explained. In the rolled material obtained by hot rolling the steel having the above-mentioned composition, i.e., the rolled material to be subjected to nitriding, if the Vickers hardness is 105 HV or less, it will have excellent cold forgeability and can be cold forged without softening annealing. Furthermore, if it is less than 100 HV, it is even more preferable, as it allows softening annealing to be omitted and can improve the life of the die during cold forging. [Example]

[0055] The steel according to this embodiment will be described below based on examples, but the steel according to this embodiment is not limited to these examples.

[0056] First, steels (Steel Nos. 1 to 52) having the chemical compositions shown in Table 1 were melted and formed into round bars (rolled materials) having a diameter of 32 mm by hot rolling. Among the chemical composition values ​​shown in Table 1, underlined values ​​are outside the ranges specified in this embodiment. In Table 1, "Others" denotes optional elements other than C, Si, Mn, Cr, P, S, Al, N, and V.

[0057] [Table 1]

[0058] The Vickers hardness (HV) of each of the obtained round bars was measured. The load used for measuring Vickers hardness was 1 kgf. The Vickers hardness was measured at 10 points randomly selected from the surface of the round bar, and the average value of the measurements at these 10 points was obtained. Table 1 shows the average value of the Vickers hardness measurements of the round bars as the hardness of the rolled material.

[0059] Next, this round bar was cut to obtain a cylinder with a diameter of 15 mm and a length of 22.5 mm, which was then cold compressed (cold forged) to an upsetting ratio of 50% to prepare a test piece for gas soft nitriding heat treatment.

[0060] Next, this test piece was subjected to gas soft-nitriding heat treatment (nitriding treatment) at 570°C for 3 hours to obtain a post-treatment test piece (an example of a soft-nitrided part). The nitriding treatment was carried out in a mixed gas containing ammonia and carbon monoxide.

[0061] The Vickers hardness (HV) of the treated test pieces was measured. The load used for measuring the Vickers hardness was 0.3 kgf. The Vickers hardness was measured on the subsurface of the treated test piece after mirror polishing a cross section passing through the center of the test piece and parallel to the axial direction. The Vickers hardness was measured from a position 0.1 mm deep below the surface of the test piece, and then every 0.1 mm (at 0.1 mm pitch) in the depth direction.

[0062] The effective case depth of the treated test specimen was determined as the depth below the surface of the test specimen where the Vickers hardness reached 550 HV. From the measured Vickers hardness at each depth below the surface of the test specimen, the depth below 550 HV closest to 550 HV and the depth over 550 HV closest to 550 HV were linearly approximated to determine the depth where the Vickers hardness reached 550 HV, and this was designated as the effective case depth (mm) of the treated test specimen. These effective case depths are also shown in Table 1.

[0063] In the test specimen (rolled material, before nitriding), if the Vickers hardness is 105 HV or less, the cold forgeability is excellent. The test specimens formed from the steels according to this embodiment (Nos. 1 to 41) have a Vickers hardness of 105 HV or less and are excellent in cold forgeability.

[0064] In the treated test specimen, if the effective case depth is 0.22 mm or more, the fatigue strength after soft nitriding is excellent. The treated test specimens formed from the steels according to this embodiment (Nos. 1 to 41) have an effective case depth of 0.22 mm or more and are excellent in fatigue strength after soft nitriding.

[0065] That is, the steels according to this embodiment (Nos. 1 to 41) have excellent cold forgeability and excellent fatigue strength after soft nitriding. More specifically, the steel according to this embodiment can reduce the deformation resistance of the steel during cold forging, provided that fatigue strength after soft nitriding is ensured, thereby extending the life of the die. As a result, the manufacturing costs of soft-nitrided parts made from the steel according to this embodiment can be significantly reduced.

[0066] In this manner, a steel having excellent cold forgeability and excellent fatigue strength after soft nitriding can be provided.

[0067] It should be noted that the embodiments disclosed in this specification are merely examples, and the present invention is not limited to these embodiments, and can be modified as appropriate within the scope of the purpose of the present invention. [Industrial Applicability]

[0068] The present invention is applicable to steel.

Claims

1. The component composition is as follows: C: 0.01% by mass or more and 0.18% by mass or less, Si: 0.03% by mass or more and 0.15% by mass or less, Mn: 0.10% by mass or more and 0.25% by mass or less, Cr: 0.50% by mass or more and 1.10% 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 0.050% by mass or less, N: 0.0010% by mass or more and 0.0120% by mass or less, and V: 0.03% by mass or more and 0.20% by mass or less, The balance is Fe and impurities.

2. The component composition is Mo: 1.0% by mass or less, Ni: 1.0 mass% or less; B: 0.01% by mass or less, The steel according to claim 1, further comprising one or more selected from the group consisting of:

3. The component composition is Ti: 0.1% by mass or less; Nb: 0.1% by mass or less The steel according to claim 1, further comprising one or more selected from the group consisting of:

4. The component composition is Ti: 0.1% by mass or less; Nb: 0.1% by mass or less The steel according to claim 2, further comprising one or more selected from the group consisting of:

5. The component composition is Sn: 0.1 mass% or less; and Sb: 0.1% by mass or less, The steel according to any one of claims 1 to 4, further comprising one or more selected from the following:

6. The component composition is 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 according to any one of claims 1 to 4, further comprising one or more selected from the following:

7. The component composition is 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 according to claim 5, further comprising one or more selected from the group consisting of:

8. 5. The steel according to claim 1, having a Vickers hardness of 105 HV or less.

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