Nitriding steel and cold-forged nitrided parts with excellent cold-forgeability.

Abstract

To provide a steel for nitriding having excellent nitriding property that has both cold forgeability and core part hardness.SOLUTION: Provided is a steel for nitriding, which is a steel that has, in mass%, C:0.20 to 0.45%, Si:0.1 to 0.4%, Mn:0.2 to 1.0%, Cr:1.50 to 2.80%, Mo:0.03 to 0.30%, Al:0.005 to 0.300%, N:0.004 to 0.030, and V: 0.08 to 0.30%, and the balance is Fe and unavoidable impurities, and the steel for nitriding has P (as an unavoidable impurity): 0.030% or less, S (as an unavoidable impurity): 0.030% or less, and has a hardness in a state of air-cooled after being held at 730 to 760°C for 4 to 8 hours as a softening heat treatment of 87HRB or less in Rockwell hardness.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a nitrided steel member suitable for mechanical structural parts that require excellent cold forging properties and hardness after nitriding. Specifically, it has excellent cold forging properties and can provide high deep hardness, surface hardness, and in addition, a deep hardened layer depth to parts (hereinafter referred to as cold forging nitrided parts) that have been subjected to cold forging and nitriding treatment. It relates to a steel material for nitriding that is suitable for use as a material for cold forging nitrided parts, and cold forging nitrided parts using the same.

Background Art

[0002] Steel for mechanical structures used in automotive transmissions such as gears and pulleys for belt-type continuously variable transmissions (CVTs) is subjected to surface hardening treatment in order to improve bending fatigue strength and pitching strength. Typical surface hardening treatments include carburizing and quenching, high-frequency quenching, nitriding, etc.

[0003] Among these surface hardening treatments, carburizing and quenching is a surface hardening treatment that is used for low-carbon steel with a carbon content of about 0.2%, and after diffusing C in the austenite region above the A3 point, quenching is carried out. Carburizing and quenching is excellent in surface hardness and hardened layer depth, but has a problem that the heat treatment deformation is large because the treatment involves quenching. In addition, the CO2 emissions during manufacturing are also large, which is not in line with the trend towards carbon neutrality.

[0004] High-frequency quenching is a treatment in which rapid heating and cooling are carried out in the high-temperature austenite region above the A3 point for quenching. High-frequency quenching is easy to adjust the effective hardened layer depth, but it is difficult to apply to large parts and complex shapes.

[0005] On the other hand, nitriding treatment is a treatment in which N is diffused at a temperature of about 400 to 600 °C below the A1 point to obtain high surface hardness and the required hardened layer depth. Compared with carburizing and quenching and high-frequency quenching, it has the advantage that the treatment temperature is low and the heat treatment deformation is small. The low treatment temperature leads to a small amount of CO2 emissions, so it is also an effective surface hardening treatment in terms of reducing greenhouse gas emissions.

[0006] However, since nitriding is a process that does not involve quenching, it cannot utilize the strengthening effect of martensitic transformation, and if alloy components are added to ensure core hardness, it may lead to a deterioration in cold forgeability.

[0007] Therefore, ideally, the steel material subjected to nitriding should be soft during processing, maintain its hardness after nitriding, and be hard all the way to the core.

[0008] For example, regarding nitriding, as a nitriding steel for cold forging, there have been, for example, steels containing, by mass%, C: 0.01~0.15%, Si < 0.10%, Mn: 0.10~0.50%, P ≤ 0.030%, S ≤ 0.050%, Cr: 0.80~2.0%, V: 0.03% or more and less than 0.10%, Al: 0.01~0.10%, N ≤ 0.0080%, and O ≤ 0.0030%, with the remainder being Fe and A cold forging and nitriding steel has been proposed that consists of impurities and has a chemical composition of [399×C+26×Si+123×Mn+30×Cr+32×Mo+19×V≦160], [20≦(669.3×logeC-1959.6×logeN-6983.3)×(0.067×Mo+0.147×V)≦80] and [140×Cr+125×Al+235×V≧160] (see Patent Document 1). This proposal attempts to address the decrease in cold forgeability caused by the inclusion of Cr and V by limiting other components. It also addresses the fact that too much Si content makes the material hard and reduces cold forgeability, thus ensuring cold forgeability by reducing the Si content.

[0009] Furthermore, a nitrided steel component has been proposed that, as a component made of nitrided steel, contains, by mass%, C: 0.05~0.20%, Si: less than 0.30%, Mn: 1.00% or less, Cr: 0.50~1.50%, Al: 0.040% or less, N: 0.0100% or less, and Ti: 0.50~1.50%, satisfying Ti-4×C-3.4N≧0.20, with the remainder being Fe and impurity elements, and the structure after nitriding treatment following quenching treatment is a tempered martensitic structure, and has a surface hardness of Hv650 or higher and an internal hardness of Hv150 or higher, thereby achieving high surface hardness and deep hardening depth through a short nitriding treatment (see Patent Document 2). However, this proposal requires a large amount of Ti to obtain a deep hardening depth after nitriding. Also, although Cr improves surface hardness, its inclusion reduces the diffusion rate of nitrogen, making it difficult to obtain a deep hardening depth, so the amount of Cr is reduced. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2013-185186 [Patent Document 2] Japanese Patent Publication No. 2004-300472 [Overview of the project] [Problems that the invention aims to solve]

[0011] The proposals in the aforementioned Patent Documents 1 and 2 improve cold forging properties by reducing the amount of alloying components added during the nitriding process, which contributes to improving surface hardness. However, it is presumed that in order to achieve core hardness with such steel, it will be necessary to further appropriately control the cold forging conditions and age hardening. Therefore, while ensuring cold forging properties is important, it is not easy to suppress the reduction in core hardness after nitriding.

[0012] Furthermore, Patent Document 2 raises concerns about increased production costs in order to properly control manufacturing conditions, such as the high temperature required for the precipitation treatment.

[0013] Patent Documents 1 and 2 both improve cold forgeability by reducing the amount of alloy additives such as C and Cr, in addition to controlling precipitates. Since C and Cr are components related to core hardness, if these components are reduced, it seems necessary to devise ways to sufficiently bring out the core hardness, but this is not clear from the descriptions, and there are no particular suggestions regarding ways to control cold forging conditions or age hardening.

[0014] In line with the recent trend towards carbon neutrality, the need for nitrided layers that can withstand high loads in components necessitates nitrided steel with superior core hardness. However, increasing the amount of alloying elements to ensure core hardness could potentially impair cold forgeability.

[0015] For example, increasing the amount of alloying elements such as Cr and Al can improve nitriding properties, but increasing them too much can hinder cold forgeability.

[0016] Therefore, taking these points into consideration, the problem that the present invention aims to solve is to provide a cold forging and nitriding steel material that exhibits excellent workability during cold forging, hardens appropriately by cold forging, suppresses reduction in core hardness after nitriding, and has excellent surface hardness and effective hardened layer depth after nitriding, thus having an excellent balance between workability during cold forging and nitriding characteristics.

[0017] However, since nitriding does not involve quenching from the austenite region, it is not possible to utilize strengthening through martensitic transformation. Therefore, in order to ensure the desired core hardness in nitrided parts, it is necessary to include a large amount of alloying elements, but on the other hand, this will worsen cold forgeability.

[0018] Furthermore, cold forging materials containing a large amount of alloy components requires prolonged heat treatment, hindering manufacturability. If the content of alloy components that contribute to hardness, such as carbon, is reduced to ensure cold forgeability, the amount of nitride formed during nitriding may be insufficient, potentially resulting in insufficient surface hardness and hardened layer depth. Moreover, recrystallization can occur during the nitriding treatment after cold forging, making it easy to lose the work hardening achieved through cold forging. [Means for solving the problem]

[0019] As a result of diligent research, the inventors discovered that by optimizing the steel composition and combining it with a softening heat treatment before cold forging, it is possible to appropriately promote the spheroidization of carbides in the material with a short-time, relatively low-temperature heat treatment, even with alloy compositions that can sufficiently ensure core hardness after nitriding, thereby reducing the material's hardness to a level suitable for cold forging.

[0020] Specifically, by increasing the Cr content, the solid-solution carbon within the ferrite grains stably precipitates as M7C3 type carbides (M = mixed components of Fe and Cr) just below the austenitization temperature. In other words, the difference in carbon diffusion behavior between pearlite grains (structure composed of ferrite and lamellar cementite) and bainite grains (structure composed of ferrite and cementite) and ferrite grains (where M7C3 carbides are stable) becomes significant. As a result, carbon diffusion from pearlite and bainite grains to ferrite grains is sufficiently promoted. Consequently, a structure is formed in which spherical carbides are uniformly dispersed within the ferrite grains, and we have found that this results in a superior steel that achieves excellent cold forgeability, reduced softening heat treatment time, and excellent surface and core hardness after nitriding simultaneously.

[0021] Therefore, a first means for solving the problems of the present invention is a steel having, by mass%, C: 0.20 to 0.45%, Si: 0.1 to 0.4%, Mn: 0.2 to 1.0%, P (as an inevitable impurity): 0.030% or less, S (as an inevitable impurity): 0.030% or less, Cr: 1.50 to 2.80%, Mo: 0.03 to 0.30%, Al: 0.005 to 0.300%, N: 0.004 to 0.030%, V: 0.08 to 0.30%, with the balance being Fe and inevitable impurities, and is in a state of being held at 730 to 760 °C for 4 to 8 hours as a softening heat treatment and then air-cooled, and is a nitriding steel having a hardness of 87 HRB or less in Rockwell hardness.

[0022] A second means thereof is a steel having, in addition to the chemical components described in the first means, any one or two or more of Nb: 0.10% or less, Ti: 0.020 to 0.200%, B: 0.0030% or less, by mass%, with the balance being Fe and inevitable impurities, and is in a state of being held at 730 to 760 °C for 4 to 8 hours as a softening heat treatment and then air-cooled, and is a nitriding steel having a hardness of 87 HRB or less in Rockwell hardness.

[0023] A third means thereof is the nitriding steel described in the first means, characterized in that the hardness when cold-forged with a compression ratio of 50% or more is 270 Hv or more in Vickers hardness.

[0024] A fourth means thereof is the nitriding steel described in the second means, characterized in that the hardness when cold-forged with a compression ratio of 50% or more is 270 Hv or more in Vickers hardness.

[0025] A fifth means thereof is a cold-forged nitrided part, characterized in that it is in a nitrided state using the nitriding steel described in the first or second means and cold-forged, having a surface hardness of 680 Hv or more and a core hardness of 220 Hv or more in Vickers hardness, and an effective hardened layer depth of 0.25 mm or more.

[0026] As another means, a cold-forged part material for nitriding is obtained by cold-forging using the steel described in either the first or second means, and has a Vickers hardness of 270 Hv or higher. [Effects of the Invention]

[0027] Parts used in machine structural steel and similar materials are required to have low hardness and be easy to process during the initial processing stage, but to have high hardness and excellent strength after processing and surface treatment. Therefore, it is desirable to achieve both good processability before processing and high hardness after processing.

[0028] Therefore, according to the means of the present invention, nitriding steel is easy to process because it has excellent cold forging properties, and furthermore, parts that are cold forged using this nitriding steel and subjected to nitriding treatment can be made to have high core hardness, high surface hardness, and excellent effective hardened layer depth. Therefore, it offers an excellent balance between machinability during cold forging and nitriding properties, making it easy to process while maintaining excellent properties after processing. Thus, the nitriding steel of the present invention is suitable for use as a material for cold-forged and nitrided parts.

[0029] Furthermore, the cold-forged nitrided parts of the present invention are suitable as mechanical structural parts used in automobile transmissions, such as gears and pulleys for CVTs.

[0030] According to the present invention, although the material has a moderate carbon content and contains 1.5-2.8% Cr, after softening heat treatment, it develops a structure in which spherical carbides are uniformly dispersed within the ferrite grains, resulting in a hardness of 87 HRB or less. This makes it an easily processable nitriding steel with excellent cold forging properties.

[0031] Because it is a nitriding steel with excellent cold forgeability after softening heat treatment, when it is cold forged to a compressibility of 50% or more, the hardness after cold forging hardens to 270 Hv or more.

[0032] Furthermore, when nitriding is performed after cold forging, the hardness of the outermost surface becomes 680 Hv or higher, the effective hardened layer depth becomes 0.25 mm or higher, and the core hardness after nitriding also becomes 220 Hv or higher. In other words, the reduction in hardness due to work hardening during cold forging is suppressed by the nitriding treatment.

[0033] Therefore, the cold-forged product using the nitriding steel of the present invention becomes a cold-forged and nitrided part with excellent surface hardness and core hardness after nitriding. [Brief explanation of the drawing]

[0034] [Figure 1] This is a schematic diagram of the method for deriving the effective hardened layer depth as shown in the example. The nitrided steel material is cut perpendicular to the longitudinal direction, and the hardness is measured from the outermost surface to the core to derive the effective hardened layer depth. [Modes for carrying out the invention]

[0035] Prior to describing embodiments for carrying out the present invention, the reasons for specifying the chemical composition of the steel in the means of this application, the reasons for specifying the heat treatment temperature of the steel, the reasons for specifying the hardness after cold forging, the surface hardness after nitriding, the effective hardened layer depth, and the core hardness will be explained. Note that the % in the following chemical composition refers to mass %.

[0036] C: 0.20~0.45% Carbon (C) is a component that increases the hardness of the material. If the C content is less than 0.20%, the hardness of the core after nitriding will decrease, leading to insufficient strength. If the C content exceeds 0.45%, the material hardness will increase too much, reducing workability (machinability, cold workability). Also, if there is too much C, nitrogen diffusion will be inhibited, reducing the depth of the hardened layer. Therefore, the C content should be between 0.20% and 0.45%.

[0037] C: 0.20~0.45% Carbon (C) is an essential element for maintaining core hardness and providing strength to steel parts after carburization. However, if the C content is less than 0.20%, the core hardness of the part may be insufficient depending on the cold forging conditions, leading to insufficient strength. On the other hand, if the C content is more than 0.45%, the material hardness increases excessively, reducing machinability and cold workability. Therefore, the C content should be between 0.20% and 0.45%. Preferably, the C content is between 0.23% and 0.43%.

[0038] Si: 0.1~0.4% Si is an effective element for deoxidation during steelmaking. However, if the Si content is less than 0.1%, it tends to lead to insufficient deoxidation during steelmaking, and the position of intervening materials decreases. On the other hand, if the Si content is more than 0.4%, the hardness of the material increases, and the workability decreases. Therefore, the Si content should be set between 0.1% and 0.4%.

[0039] Mn: 0.2~1.0% Mn is an element that improves the hardenability of steel, but if the amount is less than 0.2%, the hardenability becomes insufficient. On the other hand, if it is more than 1.0%, the workability decreases. Therefore, the amount of Mn should be set to 0.2 to 1.0%.

[0040] Cr: 1.50~2.50% Cr combines with N during nitriding to form nitrides, improving surface hardness in cold forging. It is effective in ensuring the bending fatigue strength and wear resistance of nitrided parts. However, the Cr content If the Cr content is less than 1.50%, the aforementioned effects are minimal. Furthermore, Cr is an element that stabilizes M7C3 type carbides. If the Cr content is less than 1.50%, M7C3 type carbides will not precipitate, resulting in insufficient carbon inflow from pearlite grains to ferrite grains during spheroidizing annealing. This leads to an uneven distribution of spheroidized carbides, resulting in reduced cold forgeability. On the other hand, if the Cr content exceeds 2.50%, the hardness of the material increases too much, reducing workability. Therefore, the Cr content should be between 1.50% and 2.50%. Preferably, the Cr content is between 1.60% and 2.40%.

[0041] Mo: 0.03~0.30% Mo is a carbide-forming element that improves core hardness through age hardening. However, if the Mo content is less than 0.1%, the core hardness after carburizing decreases, leading to insufficient strength. On the other hand, if the Mo content is 0.3% or more, the material hardness increases too much, reducing workability and resulting in poor machinability and cold workability. Therefore, the Mo content should be set between 0.1% and 0.3%.

[0042] Al: 0.005~0.300% Al is an effective element for deoxidizing steel, and it reacts with N during nitriding to form AlN, which improves surface hardness. However, if the Al content is less than 0.005%, not only is the above effect not obtained, but it also easily leads to insufficient deoxidation during manufacturing, resulting in a decrease in the amount of intervening material. On the other hand, if the Al content is more than 0.300%, it forms hard, coarse Al2O3, which not only reduces cold forgeability but also causes problems such as a shallower effective hardened layer during nitriding, resulting in reduced bending fatigue strength and pitting strength. Therefore, the Al content should be set between 0.005% and 0.300%.

[0043] N: 0.004~0.030% N forms nitrides such as AlN in steel, which have the effect of refining the crystal grains, and by bonding with V, it also contributes to improving core hardness. Therefore, a content of 0.0040% or more is necessary. On the other hand, when N, together with C, bonds with elements such as V to form carbonitrides, and the hardness becomes excessively high, the cold forgeability decreases. Also, the effect of improving core hardness through age hardening at the nitriding temperature is not sufficiently obtained. For this reason, the N content needs to be limited to 0.0030% or less. Therefore, the N content is set to 0.004-0.030%.

[0044] V: 0.08~0.30% During nitriding, N combines with C and / or N to form carbides, nitrides, and carbonitrides, thereby improving surface hardness. Furthermore, the formation of carbides at the nitriding temperature improves core hardness. To achieve this hardening, V must be present at a concentration of 0.08% or more. However, excessive V addition may reduce cold forgeability. Therefore, the upper limit for V is set at 0.30%. Thus, V should be between 0.08% and 0.30%.

[0045] The remainder of the chemical components defined in this invention consists of Fe and unavoidable impurities. Of the unavoidable impurities, the upper limits for P and S are defined as follows.

[0046] P:0.030% or less P is an unavoidable impurity. Since P promotes grain boundary segregation, it reduces toughness. Therefore, the amount of P, an unavoidable impurity, should be kept below 0.030%.

[0047] S: 0.030% or less S is an unavoidable impurity. If the amount of S exceeds 0.030%, a large amount of coarse MnS will be formed, leading to a decrease in toughness and fatigue strength. Therefore, the amount of S, an unavoidable impurity, should be kept below 0.030%.

[0048] Furthermore, in the present invention, one or more of the following Nb, Ti, and B may be selectively added.

[0049] Nb: 0.10% or less Nb is an effective component for grain refinement. However, if the Nb content exceeds 0.10%, the hardness increases and the cold forgeability decreases. Therefore, when adding Nb, the amount should be kept below 0.10%.

[0050] Ti: 0.020~0.200% Ti, when combined with C and / or N, forms fine carbides, nitrides, and carbonitrides, thereby refining the crystal grains and improving bending fatigue strength. Therefore, to obtain these effects, Ti may be included at a concentration of 0.020% or more. However, if the Ti content is too high, coarse TiN is formed, which actually reduces bending fatigue strength. For this reason, an upper limit should be set on the amount of Ti included, to 0.200% or less. Preferably, the amount of Ti included should be 0.100% or less.

[0051] B: 0.0030% or less Since element B contributes to hardenability, it is an element that can be added at will. However, if it is included in a quantity greater than 0.0030%, the hardness of the material will increase excessively, leading to a decrease in workability. Therefore, the amount of B should be kept below 0.0030%.

[0052] Softening heat treatment: Hold at 730-760°C for 4-8 hours, then air cool. In this invention, by using a high Cr component, the solid-solution carbon within the ferrite grains stably precipitates as M7C3 type (M = mixed components of Fe and Cr) carbides just below the austenitization temperature, resulting in a structure in which spherical carbides are uniformly dispersed within the ferrite grains, and exhibiting excellent cold forgeability. If the processing temperature or processing time is excessive, the spheroidization of the carbides will be promoted more than necessary, which may result in excellent cold forgeability but insufficient core hardness after nitriding. On the other hand, if the processing temperature or processing time is too low, the spheroidization of the carbides may not be properly promoted, potentially impairing the cold forgeability. Therefore, the softening heat treatment is performed by holding the material at 730-760°C for 4-8 hours, followed by air cooling.

[0053] Hardness after cold forging: 270 Hv or higher To ensure sufficient core hardness after nitriding, it is necessary to harden the material sufficiently through cold forging. Therefore, the hardness after cold forging with a compression ratio of 50% or more is set to 270 Hv or higher.

[0054] Hardness after nitriding: Surface hardness 680 Hv or higher, core hardness 220 Hv or higher, effective hardened layer depth: 0.25 mm or higher. To use nitrided steel as a component such as a gear after cold forging, sufficient pitting fatigue strength and bending fatigue strength are required. Therefore, it is useful to perform nitriding treatment after cold forging, and it is necessary to ensure sufficient nitriding properties, that is, to ensure surface hardness, effective hardened layer depth, and core hardness after nitriding. Accordingly, the nitriding properties of cold-forged nitrided parts after nitriding are set to surface hardness: 680 Hv or higher, effective hardened layer depth: 0.25 mm or higher, and core hardness: 220 Hv or higher.

[0055] Next, embodiments for carrying out the present invention will be described. First, each of the steels No. 1 to 17 and comparative steels No. 18 to 23, as shown in Table 1, with their respective chemical compositions, and the remainder consisting of Fe and unavoidable impurities, were melted in a 100 kg vacuum induction melting furnace (VIM) to obtain a total chemical composition of 100%.

[0056] [Table 1]

[0057] Next, each of these steel test specimens was hot forged into 40 mm diameter steel bars, and then subjected to a softening heat treatment at a temperature of 730-760°C for 4-8 hours, followed by air cooling. The softening heat treatment was carried out using a Kanthal furnace according to the following procedure.

[0058] The procedure for softening heat treatment in a Kanthal furnace involves placing the test material into the furnace set to the above-mentioned holding temperature, allowing 30 minutes for the material to heat up, then holding it for an arbitrary period of time before air cooling or water cooling. In this example, air cooling was used in both cases. Furthermore, the selection of the holding time for the softening heat treatment should take into consideration the quantity and dimensions of the steel material being charged into the furnace.

[0059] Regarding the properties of the steel subjected to softening heat treatment, the hardness was confirmed as follows. Furthermore, after the softening heat treatment, cold forging with a compressibility of 50% or more was performed, followed by a nitriding test to confirm the cross-sectional hardness and core hardness. These results are shown in Table 2.

[0060] (Regarding hardness after softening heat treatment) To evaluate the cold forgeability, the hardness (HRB) of each of the steels No. 1 to 17 of the present invention and No. 18 to 23 of the comparative steels shown in Table 1, after softening heat treatment and before cold forging, was measured using a Rockwell hardness tester. The annealed test material was cut perpendicular to the rolling direction, the cut surface was surface-ground, and then the Rockwell hardness test was performed at the middle circumference. A hardness of 87 HRB or less was evaluated as having excellent cold forgeability.

[0061] (Evaluation of hardness and nitriding properties after cold forging) Next, the steel, which had undergone softening heat treatment, was cold-forged to a compressibility of 50% or more, and then nitrided at 400-600°C for 2-32 hours. The hardness after cold forging was measured, and hardness measurements were used to confirm whether the steel exhibited the specified nitriding characteristics and core hardness after nitriding.

[0062] To assess the hardness before and after nitriding, a 10 mm diameter round bar specimen was cut in half, embedded in resin so that the cut surface was the test surface, and then polished to a mirror finish. The hardness after cold forging and the core hardness were measured using a Vickers hardness tester. Furthermore, the surface hardness and effective hardened layer depth of the specimen after nitriding were measured using a micro-Vickers hardness tester. Table 2 summarizes the results of each test in this investigation.

[0063] The specific measurement procedure is as follows: First, in accordance with JIS (Japanese Industrial Standards) Z2244 (2009), the HV (Hardness Value) of five points—one at the center and four at the R / 2 section—of a mirror-finished test specimen was measured using a Vickers hardness tester with a test force of 9.8 N. The arithmetic mean of the five points was defined as the "core hardness." A hardness of 270 HV or higher after cold forging was evaluated as indicating that the material hardened appropriately by cold forging. Furthermore, a core hardness of 220 HV or higher was evaluated as indicating that the reduction in core hardness after nitriding was suppressed.

[0064] Using the same embedded sample, and in accordance with JIS Z2244 (2009) as described above, the HV was measured at 10 arbitrary points at a depth of 0.05 mm from the surface of the specimen using a micro-Vickers hardness analyzer with a test force of 0.98 N, and the arithmetic mean of these values ​​was taken as the "surface hardness." A value of 680 HV or higher was evaluated as having excellent surface hardness after nitriding.

[0065] Furthermore, using the same embedded sample, the Vickers hardness was measured sequentially from the surface of a mirror-finished specimen with a test force of 1.96 N using a micro-Vickers hardness analyzer in accordance with JIS Z2244 (2009), and a hardness distribution map was created. The distance from the surface to the position where the hardness reached 400 Hv was defined as the "effective hardened layer depth" (Figure 1). A specimen with an effective hardened layer depth of 0.25 mm or more was evaluated as having excellent effective hardened layer depth.

[0066] [Table 2]

[0067] The steel grades No. 1 to 17, which are the components of the invented steel, all satisfy the components specified in the invention and undergo softening heat treatment at a predetermined treatment temperature. As a result, a hardness of 87 HRB or less after softening heat treatment and excellent cold forgeability are obtained. The hardness after cold forging is 270 HV or higher, and the core hardness after nitriding is confirmed to be 220 HV or higher. The surface hardness after nitriding is 680 HV or higher, and the effective hardened layer depth is 0.25 mm or higher, confirming excellent nitriding properties. Possessing these characteristics, the nitriding steel of the present invention has an excellent balance between workability during cold forging and nitriding properties, and has been confirmed to possess excellent properties that make it suitable for application as a cold-forged and nitrided part.

[0068] On the other hand, when examining steel grades 18 to 23 of the comparative steel composition, when they fell outside the specified range of the present invention, the following issues were observed: a decrease in nitriding properties, a decrease in cold workability, and insufficient core hardness. Steel grade 18 has a hardness of 90 HRB after heat treatment to soften C and Mn, indicating reduced cold forgeability. Steel grade 19 has an insufficient V, resulting in a reduced effective hardened layer depth after nitriding. Steel grade 20 has a low chromium content, resulting in reduced surface hardness after nitriding. Steel grade 21 has a low carbon content, resulting in reduced core hardness after nitriding. Steel grade 22 has not had its hardness reduced sufficiently because the temperature during the softening heat treatment was insufficient. Steel grade 23 exhibits reduced core hardness after nitriding due to excessive temperature and duration during the softening heat treatment.

Claims

1. A steel having, by mass%, C: 0.20-0.45%, Si: 0.1-0.4%, Mn: 0.2-1.0%, Cr: 1.50-2.80%, Mo: 0.03-0.30%, Al: 0.005-0.300%, N: 0.004-0.030%, V: 0.08-0.30%, with the remainder being Fe and unavoidable impurities, wherein P (as an unavoidable impurity): 0.030% or less, S (as an unavoidable impurity): 0.030% or less, and after being held at 730-760°C for 4-8 hours as a softening heat treatment and then air-cooled, the hardness is 87 HRB or less on a Rockwell hardness scale.

2. In addition to the chemical composition described in claim 1, the steel comprises, by mass%, one or more of the following: Nb: 0.10% or less, Ti: 0.020 to 0.200%, and B: 0.0030% or less, with the remainder being Fe and unavoidable impurities, and is a nitriding steel having been held at 730 to 760°C for 4 to 8 hours as a softening heat treatment, followed by air cooling, and having a hardness of 87 HRB or less on a Rockwell hardness scale.

3. The nitriding steel according to claim 1, characterized in that when cold forged with a compression ratio of 50% or more, its hardness is 270 Hv or more on the Vickers hardness scale.

4. The nitriding steel according to claim 2, characterized in that when cold forged with a compression ratio of 50% or more, its hardness is 270 Hv or more on the Vickers hardness scale.

5. A cold-forged nitrided part, characterized in that it is cold-forged using the nitriding steel described in claim 1 or 2, and is in a nitrided state having a surface hardness of 680 Hv or more and a core hardness of 220 Hv or more on a Vickers hardness scale, and an effective hardened layer depth of 0.25 mm or more.

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