Nitriding steel excellent in cold forgeability and nitriding properties, and cold-forged nitrided component

Abstract

To provide a nitriding steel for cold forging that is excellent in cold forgeability and excellent in hardness after nitriding.SOLUTION: A nitriding steel for cold forging contains, in mass%, C: 0.15 to 0.30%, Si: 0.15 to 0.60%, Mn: 0.10 to 1.50%, Cr: 0.15 to 2.20%, Mo: 0.02 to 0.30%, Al: 0.015 to 0.300%, V: 0.05 to 0.30%, N: 0.004 to 0.030%, with the balance being Fe and impurity elements, where the value of a formula A: 0.10×[Cr]+0.67×[Al]+0.24×[V] is more than 0.15 and less than 0.4, the value of a formula B: Pd=N / L×(1-Sα) is 2.10 or less, the value of a formula C: 217.2×[Mo]+61.3×Pd is 61.0 or more, and the Vickers hardness is 180 Hv or less.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a nitriding steel for cold forging, which is suitable as a material for steel parts of cold forging nitriding parts used after cold forging and subjected to surface hardening treatment such as gas nitriding or gas soft nitriding, for example, gears of automobiles, construction machinery, machine tools, etc., and has excellent cold forging properties and nitriding properties for mechanical structures.

Background Art

[0002] So far, as a steel for cold forging nitriding, it contains 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, and the balance consists of Fe and impurities. Further, the value of 399×C + 26×Si + 123×Mn + 30×Cr + 32×Mo + 19×V is 160 or less, and 669.3×log e C - 1959.6×log e N - 6983.3)×(0.067×Mo + 0.147×V) is 20 to 80, and a cold forging nitriding steel having a chemical composition in which the value of 140×Cr + 125×Al + 235×V is 160 or more has been proposed (see Patent Document 1).

[0003] Also, as another nitriding steel, a method for producing a nitriding steel having an alloy composition containing C: 0.15 to 0.30%, Si: 0.2% or less, Mn: 0.4 to 1.5%, Cr: 0.6 to 1.5%, s - Al: 0.05 to 0.20% and V: 0.05 to 0.30%, and the balance being substantially composed of Fe has been proposed (see Patent Document 2).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0005] The cold forging nitriding steel of Patent Document 1 promotes precipitation hardening due to carbide precipitation during nitriding by adding Mo or V, and aims to secure the core hardness. However, since it is necessary to keep the nitrogen content low to promote precipitation hardening during nitriding, the production cost may increase.

[0006] The invention of Patent Document 2 aims to secure the core hardness by adding V to precipitate carbides during nitriding and making it difficult for recrystallization to occur by utilizing this. However, the production cost may increase to appropriately control the manufacturing conditions such as the precipitation treatment being at a high temperature.

[0007] In cold forging nitriding steel, due to the relationship of performing nitriding treatment after cold forging a steel for mechanical structures, it is necessary to be excellent in both cold forging properties and nitriding properties. Based on this, an object of the present invention is to provide a steel material that is excellent in cold forging properties and can suppress the reduction of the hardness of the part after nitriding.

[0008] However, since the heat treatment in nitriding treatment does not perform quenching treatment from the temperature range of the austenite region, there is a situation where the strengthening by martensite transformation cannot be utilized in nitriding treatment. Therefore, in order to secure the desired core hardness for nitrided parts, it is necessary to contain a large amount of alloying elements, but adding them deteriorates the cold forging properties.

[0009] On the other hand, if an attempt is made to reduce the alloy content to secure cold forging properties, there is a risk that the amount of nitrides formed during nitriding will be insufficient and the surface hardness and the depth of the hardened layer will be insufficient.

[0010] Also, in cold forging, work hardening due to forging can be obtained, but there is also a concern that the work hardening obtained by cold forging will be lost during the nitriding treatment after cold forging.

[0011] Therefore, an object of the present invention is to provide a steel for nitriding for cold forging that has excellent cold forging properties and excellent hardness after nitriding by appropriately controlling the structure and alloy element content before forging.

Means for Solving the Problems

[0012] A first means for solving the problems of the present invention is, by mass%, C: 0.15 to 0.30%, Si: 0.15 to 0.60%, Mn: 0.10 to 1.50%, Cr: 0.15 to 2.20%, Mo: 0.02 to 0.30%, Al: 0.015 to 0.300%, V: 0.05 to 0.30%, N: 0.004 to 0.030%, the balance being composed of Fe and impurity elements, the value of formula A: 0.10×[Cr]+0.67×[Al]+0.24×[V] is more than 0.15 and less than 0.4, the value of formula B: Pd = N / L×(1 - Sα) is 2.10 or less, the value of formula C: 217.2[Mo]+61.3×Pd is 61.0 or more, and it is a steel for nitriding for cold forging with a Vickers hardness of 180 Hv or less. However, for [ ] in formula A and formula C, the value of the mass% of the corresponding alloy component is substituted, Sα in formula B is the value of the ferrite area ratio in%, N is the total number of cementite grain boundaries intersecting a straight line arbitrarily drawn on the microstructure observation surface, and L is the value of the total length (μm) of a straight line arbitrarily drawn on the microstructure observation surface.

[0013] A second means thereof is that, in the chemical composition described in the first means, as additional optional components, it contains any one or more of Nb: 0.10% or less, Ti: 0.010 to 0.200%, B: 0.0030% or less, the balance being composed of Fe and impurity elements, the value of 0.10×[Cr]+0.67×[Al]+0.24×[V] in formula A is more than 0.15 and less than 0.4, the value of Pd = N / L×(1 - Sα) in formula B is 2.10 or less, the value of 217.2[Mo]+61.3×Pd in formula C is 61.0 or more, and it is a steel for nitriding for cold forging with a hardness of 180 Hv or less in terms of Vickers hardness. However, for the [ ] in Formula A and Formula C, the value of the mass % of the corresponding alloy component shall be substituted, Sα in Formula B is the value of the ferrite area ratio in %, N is the total number of cementite grain boundaries intersecting a straight line arbitrarily drawn on the microstructure observation surface, and L shall be substituted with the value of the total length (μm) of the straight line arbitrarily drawn on the microstructure observation surface.

[0014] The third means is the nitriding steel for cold forging according to any one of the first and second means, characterized in that the hardness when cold forged at a reduction rate of 60% or more is 250 Hv or more in Vickers hardness.

[0015] The fourth means is a cold forged nitrided part in a state where a cold forged part made of the steel according to any one of the first to third means is nitrided, the surface hardness is 680 Hv or more, the core hardness is 230 Hv or more, and the hardness from the surface to a depth of 0.25 mm or more is 400 Hv or more.

Advantages of the Invention

[0016] The nitriding steel for cold forging according to the means of the present invention is excellent in cold forging property and also excellent in hardness after nitriding. That is, the nitriding steel for cold forging of the present invention has excellent workability with a hardness of 180 Hv or less before cold forging and nitriding treatment. After cold forging, when cold forged at a reduction rate of 60% or more, hardening of 250 Hv or more can be obtained, and it is easy to ensure the required mechanical strength. Further, when the nitriding steel of the present invention is nitrided, the surface can be hardened to 680 HV or more, and the hardened layer depth (the depth at which a hardness of 400 Hv or more can be ensured) is also 0.25 mm or more, obtaining a sufficient hardened depth. On the other hand, the core hardness after nitriding treatment is 230 Hv or more, ensuring the material hardness, so the hardness as a mechanical part can be maintained even after nitriding treatment.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Figure 3

DETAILED DESCRIPTION OF THE INVENTION

[0018] Prior to describing embodiments of the present invention, the reasons for defining the chemical components of the steel used for nitriding steel for cold forging and the reasons for defining the values of Formula A, Formula B, and Formula C will be described. Note that the % of the chemical components is mass %.

[0019] C: 0.15 to 0.30% C is a component that increases the hardness of the material. If C is too little, the ferrite area ratio increases, leading to a decrease in the core hardness after nitriding, resulting in insufficient strength of the steel. Therefore, from these viewpoints, C is set to 0.15% or more. If C is too much, the material hardness rises too much, the workability deteriorates, and the machinability and cold workability become inferior. Also, if C is too much, the diffusion of nitrogen is inhibited, so the hardened layer depth decreases. From these viewpoints, C is set to 0.30% or less.

[0020] Si: 0.15 to 0.60% Si is a component useful for deoxidation and improves the hardness of the material. If Si is too little, deoxidation deficiency is likely to occur during manufacturing, and the inclusion grade is likely to decrease. From these viewpoints, Si is set to 0.15% or more. If Si becomes too much, the material hardness rises too much and the workability deteriorates. Therefore, Si is set to 0.60% or less.

[0021] Mn: 0.10 to 1.50% Since Mn is a component that improves toughness, if Mn is too little, the toughness decreases, so Mn is set to 0.10% or more. If Mn is too much, the workability deteriorates, so Mn is set to 1.50% or less.

[0022] Cr: 0.15 - 2.20% Cr is a component that increases the hardness of the material. If the amount of Cr is too small, the hardness after nitriding will be insufficient. Therefore, the amount of Cr should be 0.15% or more. Preferably, the amount of Cr is 0.80% or more. On the other hand, if the amount of Cr is too large, the hardness of the material will increase too much, resulting in a decrease in workability. In addition, it will inhibit the diffusion of nitrogen, resulting in a reduction in the hardened layer depth. Therefore, the amount of Cr should be 2.20% or less. Preferably, the amount of Cr is 1.50% or less.

[0023] Mo: 0.02 - 0.30% Since Mo is a component that increases the hardness of the material, if the amount of Mo is too small, the hardness in the deep part after nitriding is likely to decrease, leading to insufficient strength. Therefore, the amount of Mo should be 0.02% or more. On the other hand, if the amount of Mo is too large, the hardness of the material will increase, resulting in a decrease in workability, and thus the machinability and cold workability will deteriorate. Therefore, the amount of Mo should be 0.30% or less.

[0024] Al: 0.015 - 0.300% Since Al is a component useful for deoxidation during manufacturing, if it is insufficient, it is likely to cause insufficient deoxidation and the inclusion grade is likely to decrease. In addition, since Al is also a component useful for the surface hardness and the hardened layer depth after nitriding, if it is insufficient, the surface hardness after nitriding will decrease or the hardened layer depth will be insufficient. Therefore, the amount of Al should be 0.015% or more. Preferably, the amount of Al is 0.050% or more. On the other hand, if the amount of Al is too large, coarse nitrides AlN will be formed, resulting in a decrease in fatigue characteristics and workability. Therefore, the amount of Al should be 0.300% or less. Preferably, the amount of Al is 0.210% or less.

[0025] V: 0.05 - 0.30% V is a component useful for obtaining the hardened layer depth. If the amount of V is too small, the hardened layer depth will be insufficient, so the amount of V should be 0.05% or more. On the other hand, if the amount of V is too large, the workability will deteriorate and the cost will increase due to the component. Therefore, the amount of V should be 0.30% or less.

[0026] N: 0.004 - 0.030% N is a component that forms carbonitrides. If the amount of N is too small, fine carbonitrides will be insufficient, resulting in coarsening of crystal grains and causing a decrease in toughness and fatigue properties. Therefore, N should be 0.004% or more. If the amount of N becomes excessive, coarse carbonitrides will be formed, leading to a decrease in fatigue properties and workability. In addition, the reduction of nitrides with pinning hardening will cause coarsening of crystal grains. Therefore, N should be 0.030% or less.

[0027] In addition, the nitriding steel for cold forging of the present invention may contain any one or more of Nb, Ti, and B as selective additional components within the following ranges. The reasons for defining these components are as follows.

[0028] Nb: 0.10% or less Nb can be added as a component to increase hardness. However, if too much Nb is added, the hardness will increase and the workability will deteriorate. Therefore, when adding Nb, it should be 0.10% or less.

[0029] Ti: 0.010 - 0.200% Ti is a component that improves the bending fatigue strength. If the amount of Ti is too small, the amount of fine nitrides will be insufficient, resulting in insufficient strengthening of the bending fatigue strength. Therefore, when adding Ti, it should be 0.010% or more. On the other hand, if the amount of Ti becomes excessive, coarse carbonitrides will increase and the bending fatigue strength will decrease. Therefore, when adding Ti, it should be 0.200% or less.

[0030] B: 0.0030% or less B is a component that increases the hardness of the material. However, if the amount of B becomes excessive, the hardness of the material will increase too much and the workability will deteriorate. In addition, it is likely to become brittle due to the formation of boron carbide. Therefore, when adding B, it should be 0.0030% or less.

[0031] The balance of the chemical components of the steel is Fe and inevitable impurities.

[0032] Among the inevitable impurities, P tends to promote grain boundary segregation and reduce toughness. Therefore, it is desirable that P be 0.030% or less.

[0033] Among inevitable impurities, S tends to form a large amount of coarse MnS, which will reduce toughness and fatigue strength. Therefore, it is desirable that S be 0.030% or less.

[0034] Next, the reasons for defining the numerical ranges for Formula A, Formula B, and Formula C will be explained.

[0035] Formula A: 0.15 < 0.10×[Cr] + 0.67×[Al] + 0.24×[V] < 0.4 For [ ] in 0.10×[Cr] + 0.67×[Al] + 0.24×[V] of Formula A, substitute the value of % of the corresponding elemental component. The index of this Formula A is an index related to nitriding hardness and case depth. If the value of Formula A is 0.15 or less, the nitriding hardness and case depth will be insufficient. If the value of Formula A is 0.43 or more, the surface hardness will be excessive and pitching etc. will easily occur. Therefore, the value of Formula A that satisfies the requirements of the present invention is more than 0.15 and less than 0.43.

[0036] Formula B: Pd = N / L×(1 - Sα) ≦ 2.10 The structure parameter Pd of Formula B is an index related to the core hardness and material after nitriding, and is calculated by the formula Pd = N / L×(1 - Sα). Note that Sα in Formula B is the value of % of the ferrite area ratio. Also, N in Formula B means the total number of cementite grain boundaries intersecting a straight line arbitrarily drawn on the image of the microstructural observation surface. For L in Formula B, substitute the value of the total length (μm) of a straight line arbitrarily drawn on the image of the microstructural observation surface. Note that N is the total number of evaluated numbers obtained by counting and converting the number of cementite grain boundaries intersecting the straight line as follows. That is, the counting of the grain boundaries intersecting the straight line is i) When the straight line completely intersects the grain boundary, count it as 1. ii) When the straight line is in contact with the cementite grain boundary, count it as 0.5. iii) When the straight line exists inside the cementite crystal and is not in contact with the cementite grain boundary, count it as 1.5. Now, when the value of Expression B obtained by substituting each of these values is 2.10 or less, the requirements of Expression B of the present invention are satisfied. On the other hand, when the value of Expression B exceeds 2.10, the material hardness becomes too high and the workability of the parts deteriorates.

[0037] Expression C: 217.2[Mo] + 61.3×Pd ≥ 61.0 The expression 217.2[Mo] + 61.3×Pd of Expression C is an index related to the core hardness after nitriding and the material, Pd is the value of the structure parameter calculated by the formula Pd = N / L×(1 - Sα), and [Mo] is the value of the contained elemental component Mo substituted in %. If the value of Expression C is 61.0 or more, the requirements of Expression C of the present invention are satisfied. When the value of Expression C is less than 61.0, the core hardness after nitriding is insufficient.

[0038] For each of Invention Steels No. 1 to 23 and Comparative Steel No. 24 to 33, which are composed of the chemical components described in Table 1 and the balance Fe and inevitable impurities, 100 kg was melted in a vacuum melting furnace. Next, a bar with a diameter of 40 mm was produced by hot forging, and as shown in Table 2, isothermal annealing of the procedure shown in FIG. 1 as softening heat treatment A or spheroidizing annealing of the procedure shown in FIG. 2 as softening heat treatment B was performed using a Kanthal furnace. Next, from these softened bars, cylindrical test pieces of φ14×21 mm were taken and compressed as cold forging at a compression rate of 10 mm / min to a compression ratio of 70%. The test pieces cold-worked at a compression ratio of 70% were nitrided at 570°C for 8 hours.

[0039]

Table 1

[0040] <Evaluation Items and Evaluation Methods> The value Sα of the ferrite area ratio in %, the total number N of the cementite grain boundaries intersecting a straight line arbitrarily drawn on the microstructure observation surface, and the value L of the total length (μm) of the straight line arbitrarily drawn on the microstructure observation surface are obtained by the following procedure. The structure parameter Pd is obtained from Pd = N / L×(1 - Sα).

[0041] Regarding the ferrite area ratio Sα The Sα, which is the % value of the ferrite area ratio, was calculated by mirror-polishing each test piece after softening heat treatment, etching with nital solution, observing with an optical microscope, and counting the number of pixels of the captured microscope image using image processing software.

[0042] Regarding the structure parameter Pd The structure parameter Pd was obtained by mirror-polishing each test piece after softening heat treatment, etching with nital solution, observing with a scanning electron microscope (SEM) at a magnification of 5000 to 10,000 times, and following the procedures a to c below. a. First, draw an arbitrary line on the non-ferrite part such as the pearlite part of the observation surface. In the case of Fig. 3, a total of 10 straight lines were drawn in a grid pattern. Since the length of one line is 5 μm, the total length L of 10 lines is 50 μm. b. Count the total number N of cementite grain boundaries intersecting the straight line. The total number N of grain boundaries is the total number of the counted and converted numbers of the number of cementite grain boundaries intersecting the straight line as follows. i) When the straight line completely intersects the grain boundary, it is counted as 1, ii) When the straight line touches the cementite grain boundary, it is counted as 0.5, iii) When the straight line exists inside the cementite crystal and does not touch the cementite grain boundary, it is counted as 1.5. In the example of Fig. 3, the number N of cementite grain boundaries was 189.5. c. Substitute the value of the ferrite area ratio Sα obtained in (1) and the value of N obtained above into the formula Pd = N / L×(1 - Sα) to obtain Pd.

[0043] Regarding the surface hardness The surface hardness is the hardness at a depth of 0.05 mm from the surface, and was measured with a Vickers hardness tester according to the procedure conforming to Japanese Industrial Standard JIS Z2244. The results are shown in Table 2.

[0044] Regarding the surface hardened layer depth (400 Hv hardness depth) The hardness distribution from the surface after nitriding treatment was measured with a Vickers hardness tester according to the procedure specified in JIS Z2244, and the region maintaining a hardness of 400 Hv or more was defined as the hardened layer depth. The results are shown in Table 2.

[0045] Regarding the hardness after cold forging and the core hardness after nitriding Regarding the hardness after cold forging and after nitriding, the core hardness of the cross-section of the test piece was measured with a Vickers hardness tester according to the procedure specified in JIS Z2244. The results are shown in Table 2.

[0046]

Table 2

[0047] The inventive steels Nos. 1 to 23 of the present invention have a hardness of 180 Hv or less after softening heat treatment before nitriding, which is not too hard and suitable for cold forging, and the steel for nitriding has excellent workability. After cold forging, hardening to 250 Hv or more can be obtained by cold forging, so it is easy to secure the required strength. Also, the surface can be appropriately hardened to 680 HV or more by nitriding, and a sufficient hardened layer depth (depth at which a hardness of 400 Hv or more can be ensured) of 0.25 mm or more is obtained. On the other hand, the core hardness after nitriding treatment is 230 Hv or more, and it is not overly softened by the high temperature in the nitriding treatment process, has appropriate softening resistance, and the material hardness is ensured, so the hardness as a mechanical part can be maintained even after nitriding treatment.

[0048] Comparative steel No. 24 has a low carbon C component, and the value of formula C is outside the specified range of the present invention, resulting in insufficient core hardness after nitriding. Comparative steel No. 25 has an excessive Si component, the material hardness before nitriding has increased too much, and the workability has decreased. Comparative steel No. 26 has a too small Cr component, the value of formula A is less than 0.15, and the surface hardness after nitriding is insufficient. Comparative steel No. 27 contains an excessive amount of Cr, which increases the material hardness and reduces the workability. In addition, the diffusion of nitrogen is inhibited, resulting in a reduced hardened layer depth. In comparative steel No. 28, the Mo content is too low, the value of formula C is outside the range specified by the present invention, and the deep hardness after nitriding is reduced. Comparative steel No. 29 contains an excessive amount of Mo, which increases the material hardness and reduces the workability. In comparative steel No. 30, the Al content is too low, the value of formula A is below 0.15, the surface hardness after nitriding is insufficient, and the depth of the hardened layer due to nitriding is not sufficient. In the comparative steel No. 31, the V content is insufficient, and the hardened layer depth due to nitriding is not sufficient. In the case of comparative steel No. 32, the overall balance of Cr, Al, and V is insufficient, so the value of formula A is less than 0.15, the surface hardness after nitriding is insufficient, and the hardened layer depth due to nitriding is not sufficient. For comparative steel No. 33, the softening heat treatment was performed according to procedure B of the conventional spheroidizing annealing in FIG. 2, and the value was outside the range of formula C, resulting in low core hardness after nitriding heat treatment.

Claims

1. In mass percent, C: 0.15-0.30%, Si: 0.15-0.60%, Mn: 0.10 to 1.50%, Cr: 0.15-2.20%, Mo: 0.02-0.30%, Al: 0.015-0.300%, V: 0.05-0.30%, N: 0.004-0.030%, The balance is Fe and impurity elements, Formula A: The value of 0.10 × [Cr] + 0.67 × [Al] + 0.24 × [V] is greater than 0.15 and less than 0.4, The value of formula B: Pd = N / L × (1 - Sα) is 2.10 or less, The value of formula C: 217.2[Mo] + 61.3 × Pd is 61.0 or more, A nitriding steel for cold forging having a Vickers hardness of 180 Hv or less. However, the values ​​of the mass percentages of the corresponding alloy components shall be substituted into the brackets [ ] in Formula A and Formula C. In formula B, Sα is the percentage value of the ferrite area ratio, N is the total number of cementite grain boundaries that intersect with a straight line drawn arbitrarily on the microstructure observation surface, and L is the total length (μm) of the straight line drawn arbitrarily on the microstructure observation surface.

2. The chemical composition according to claim 1 further contains, as selective components, one or more of Nb: 0.10% or less, Ti: 0.010 to 0.200%, and B: 0.0030% or less, with the balance being Fe and impurity elements; The value of 0.10 × [Cr] + 0.67 × [Al] + 0.24 × [V] in formula A is greater than 0.15 and less than 0.4, The value of Pd = N / L × (1-Sα) in formula B is 2.10 or less, The value of 217.2[Mo]+61.3×Pd in ​​formula C is 61.0 or more, A nitriding steel for cold forging having a Vickers hardness of 180 Hv or less. However, the mass percentage of the corresponding alloy components is substituted into the brackets [ ] in Formula A and Formula C, Sα in Formula B is the percentage of the ferrite area ratio, N is the total number of cementite grain boundaries that intersect with a straight line drawn arbitrarily on the microstructure observation surface, and L is the total length (μm) of the straight line drawn arbitrarily on the microstructure observation surface.

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

4. A cold forged nitrided part, which is made of the steel for nitriding for cold forging according to any one of claims 1 to 3 and is in a nitrided state, has a surface hardness of 680 Hv or more, a core hardness of 230 Hv or more, and a hardness to a depth of 0.25 mm or more from the surface of the cold forged nitrided part.

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