JP2023144674A - Steel for nitriding having excellent cold forging property and cold forged nitrided component - Google Patents

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

The present invention relates to a nitrided steel member suitable for mechanical structural parts that require excellent cold forgeability and hardness after nitriding. Specifically, in addition to having excellent cold forging properties, parts that have been cold forged and nitrided (hereinafter referred to as cold forged nitrided parts) have high deep hardness, surface hardness, and a deep hardened layer depth. The present invention relates to a nitriding steel material that can be used as a material for cold-forged nitrided parts, and to cold-forged nitrided parts using the same.

Steel for mechanical structures used in automobile transmissions such as gears and pulleys for belt-type continuously variable transmissions (CVTs) is subjected to surface hardening treatment with the aim of improving bending fatigue strength and pitching strength. Typical surface hardening treatments include carburizing and hardening, induction hardening, and nitriding.

Among these surface hardening treatments, carburizing and quenching is used for low carbon steels with a carbon content of approximately 0.2%, and is a surface hardening treatment in which C is diffused in the austenite region of A ₃ or more points and then quenched. It is. Carburizing and quenching is excellent in surface hardness and hardened layer depth, but since the process involves quenching, there is a problem in that heat treatment deformation is large. Additionally, they emit a lot of _CO2 during manufacturing, making them unsuitable for the carbon-neutral trend.

Induction hardening is a process of rapidly heating and cooling to a high temperature austenite region of _A3 or higher. Induction hardening makes it easy to adjust the effective hardened layer depth, but it is difficult to apply to large products or complex shapes.

On the other hand, nitriding is a process 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 necessary hardened layer depth. Compared to carburizing and quenching and induction quenching, it has the advantage that the processing temperature is lower and heat treatment deformation is smaller. Low processing temperatures lead to low CO ₂ emissions, so surface hardening is also effective in reducing greenhouse gas emissions.

However, since nitriding is a process that does not involve quenching, it cannot take advantage of the strengthening caused by martensitic transformation, and if alloying components are added to ensure core hardness, cold forgeability may deteriorate. be.

Therefore, it is ideal that the steel material subjected to nitriding is soft during processing, maintains its hardness after nitriding, and is hard to the core.

For example, regarding nitriding, as a nitriding steel for cold forging, for example, in mass %, C: 0.01 to 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%, the remainder consists of Fe and impurities, [399×C+26×Si+123×Mn+30×Cr+32×Mo+19×V≦160], [20 The chemical composition of A cold-forged nitriding steel has been proposed (see Patent Document 1).
This proposal attempts to deal with the problem by limiting other components, since the inclusion of Cr and V reduces cold forgeability, and if the Si content is too high, it becomes hard, so cold forgeability decreases. However, by reducing the Si content, cold forgeability is ensured.

In addition, as parts made of nitriding steel, C: 0.05 to 0.20%, Si: less than 0.30%, Mn: 1.00% or less, Cr: 0.50 to 1.50%, Contains Al: 0.040% or less, N: 0.0100% or less, Ti: 0.50 to 1.50%, and satisfies Ti-4×C-3.4N≧0.20, with the remainder being A short-time product comprising Fe and impurity elements, having a structure after quenching and nitriding consisting of a tempered martensitic structure, and having a surface hardness of Hv650 or more and an internal hardness of Hv150 or more. A nitrided steel component has been proposed that can obtain high surface hardness and deep hardening depth through nitriding treatment (see Patent Document 2).
However, this proposal requires a large amount of Ti in order to obtain a deep hardening depth after nitriding. Further, although Cr improves surface hardness, the amount of Cr is reduced because its inclusion lowers the diffusion rate of nitrogen, making it difficult to obtain a hardening depth.

Japanese Patent Application Publication No. 2013-185186 Japanese Patent Application Publication No. 2004-300472

In the proposals of Patent Documents 1 and 2 mentioned above, cold forging workability is improved by reducing the amount of alloy components that contribute to improving surface hardness during nitriding treatment. However, in order to develop core hardness in such steel, it is assumed that it will be necessary to further appropriately control cold forging conditions and age hardening. However, on the other hand, it is not easy to suppress the decrease in core hardness after nitriding.

Further, in Patent Document 2, there is a concern that the production cost will increase in order to appropriately control the manufacturing conditions, such as the precipitation treatment being performed at a high temperature.

In both Patent Documents 1 and 2, in addition to controlling precipitates and the like, cold forgeability is improved by reducing the amount of alloys such as C and Cr added. Since C and Cr are components related to core hardness, if these components are to be reduced, it seems necessary to devise ways to sufficiently develop core hardness, but this is not clear from the description. There are no particular suggestions regarding how to control cold forging conditions or age hardening.

In line with the recent carbon-neutral trend, there is a demand for nitrided steel that can be applied to high-load parts, so nitrided steel with excellent core hardness is required more than ever before. However, if the amount of alloying elements is increased to ensure core hardness, cold forgeability may be inhibited.

For example, increasing the amount of alloying elements such as Cr and Al may improve nitriding properties, but if the amount is increased too much, cold forgeability will be inhibited.

Therefore, in view of these points, the problem to be solved by the present invention is to provide a material that has excellent workability during cold forging, can be appropriately hardened by cold forging, and can suppress the decrease in core hardness after nitriding. An object of the present invention is to provide a steel material for cold forging and nitriding that has excellent surface hardness and effective hardened layer depth after nitriding, and has an excellent balance between workability during cold forging and nitriding properties.

However, in the case of nitriding treatment, since quenching treatment from the austenite region is not performed, strengthening by martensitic transformation cannot be utilized. Therefore, in order to ensure the desired core hardness in the nitrided parts, it is necessary to contain a large amount of alloying elements, but on the other hand, cold forgeability deteriorates.

Further, in order to cold forge a material containing a large amount of alloy components, a long heat treatment is required, which impedes manufacturability. In order to ensure cold forgeability, if the content of alloy components that contribute to hardness, such as C, is lowered, the amount of nitrides formed during nitriding will be insufficient, resulting in a decrease in surface hardness and hardening of the hardened layer. There is also a risk that there will be a lack of depth. Furthermore, since recrystallization may occur in the nitriding treatment after cold forging, the work hardening obtained by cold forging is also likely to be lost.

Therefore, as a result of intensive study, the inventors found that by combining the optimization of steel components and softening heat treatment before cold forging, even if the alloy composition can ensure sufficient core hardness after nitriding, It has been discovered that heat treatment for a short time and at a relatively low temperature can appropriately promote the spheroidization of carbides in the material and reduce the hardness of the material to a level that allows it to be subjected to cold forging.

Specifically, by using a high Cr component, solid solution C in the ferrite grains stably precipitates as M ₇ C ₃ type (M=mixed component of Fe and Cr) carbide just below the austenitizing temperature. In other words, there is a difference in C diffusion behavior between pearlite grains (structure composed of ferrite and lamellar cementite), bainite grains (structure composed of ferrite and cementite), and ferrite grains (stabilized M ₇ C ₃ carbide). becomes larger. As a result, C diffusion from pearlite grains and bainite grains to ferrite grains is sufficiently promoted. The result is a structure in which spherical carbides are uniformly dispersed within the ferrite grains, resulting in an excellent steel that can simultaneously achieve excellent cold forgeability, shortened softening heat treatment time, and excellent post-nitriding surface hardness and core hardness. I found out that it can be done.

Therefore, the first means for solving the problems of the present invention is, in terms of mass %, C: 0.20 to 0.45%, Si: 0.1 to 0.4%, Mn: 0.2 to 1 .0%, P (as an unavoidable impurity): 0.030% or less, S (as an unavoidable 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 consisting of Fe and inevitable impurities, This is a nitriding steel that has been kept at 730 to 760° C. for 4 to 8 hours as a softening heat treatment and then air cooled, and has a hardness of 87 HRB or less on the Rockwell hardness.

The second means includes, in addition to the chemical components described in the first means, Nb: 0.10% or less, Ti: 0.020 to 0.200%, and B: 0.0030% or less. It is a steel that has one or more of the following types, with the remainder being Fe and unavoidable impurities, and is a state in which it is kept at 730 to 760°C for 4 to 8 hours as a softening heat treatment, and then air cooled. It is a nitriding steel with a Rockwell hardness of 87 HRB or less.

The third means is the nitriding steel described in the first means, which has a Vickers hardness of 270 Hv or more when cold forged at a compression rate of 50% or more. .

The fourth means is the nitriding steel described in the second means, which has a Vickers hardness of 270 Hv or more when cold forged at a compression ratio of 50% or more. .

The fifth means is cold forged using the nitriding steel described in the first or second means, has a surface hardness of 680 Hv or more in Vickers hardness, a core hardness of 220 Hv or more, and has an effective hardened layer. This cold forged nitrided part is characterized by being nitrided to a depth of 0.25 mm or more.

Another means is a cold forged part material for nitriding having a Vickers hardness of 270 Hv or more, which is cold forged using the steel described in any one of the first or second means.

Parts used for machine structural steel etc. have low hardness during machining and are easy to machine, but parts subjected to machining and surface treatment are required to have high hardness and excellent strength. It is desired to achieve both workability and hardness after processing.

Therefore, according to the means of the present invention, nitriding steel has excellent cold forgeability and is easy to process, and furthermore, parts cold forged and nitrided using this nitriding steel have high core hardness. , high surface hardness and excellent effective case depth.
Therefore, it has an excellent balance between workability during cold forging and nitriding properties, and is easy to process while having excellent properties after processing. Therefore, the nitriding steel of the present invention is suitable for use as a material for cold-forged nitrided parts.

The cold forged nitrided parts of the present invention are suitable as mechanical structural parts used in automobile transmissions, such as gears and CVT pulleys.

According to the means of the present invention, although it contains medium carbon and 1.5 to 2.8% Cr, after the softening heat treatment, it becomes a structure in which spherical carbides are uniformly dispersed within the ferrite grains, and the hardness is 87 HRB or less. Therefore, it becomes a nitriding steel that has excellent cold forgeability and is easy to process.

Since it is a nitriding steel with excellent cold forgeability after softening heat treatment, when it is cold forged to a compression rate of 50% or more, the hardness after cold forging is hardened to 270 Hv or more.

Furthermore, if nitriding is performed after cold forging, the outermost surface hardness will be 680 Hv or more, the effective hardened layer depth will be 0.25 mm or more, and the core hardness after nitriding will be 220 Hv or more. That is, reduction in hardness due to work hardening during cold forging due to nitriding treatment is suppressed.

Therefore, by being nitrided, a cold forged product using the nitriding steel of the invention of the present application becomes a cold forged nitrided part with excellent surface hardness and core hardness.

It is a schematic diagram about the derivation|leading-out method of effective hardening layer depth shown in the Example. The steel material after nitriding is cut perpendicular to the longitudinal direction, the hardness is measured from the outermost surface to the core, and the effective hardened layer depth is derived.

Prior to explaining the embodiments of the present invention, we will explain the reasons for specifying the chemical composition of the steel in the means of the present application, the reasons for specifying the heat treatment temperature of the steel, the reasons for specifying the hardness after cold forging, and the reasons for specifying the hardness after cold forging. The reasons for defining the subsequent surface hardness, effective hardened layer depth, and core hardness will be explained. In addition, % in the following chemical components is mass %.

C: 0.20-0.45%
C is a component that increases the hardness of the material. If C is less than 0.20%, the core hardness after nitriding decreases, resulting in insufficient strength. When C exceeds 0.45%, the material hardness increases too much and the workability (machinability, cold workability) decreases. Moreover, when C becomes excessive, diffusion of nitrogen is inhibited, so the depth of the hardened layer is reduced. Therefore, C is set at 0.20 to 0.45%.

C: 0.20-0.45%
C is an element necessary for maintaining core hardness and imparting strength to steel parts after carburizing. However, if C 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 more than necessary, and machinability and cold workability decrease. Therefore, C is set at 0.20 to 0.45%. Preferably C is 0.23-0.43%.

Si: 0.1-0.4%
Si is an effective element for deoxidizing steel during steel manufacturing. However, if Si is less than 0.1%, deoxidation is likely to be insufficient during steel manufacturing, resulting in a decrease in inclusion quality. On the other hand, if Si is more than 0.4%, the material hardness increases and workability decreases. Therefore, Si is set at 0.1 to 0.4%.

Mn: 0.2-1.0%
Mn is an element that improves the hardenability of steel, but if it is less than 0.2%, the hardenability is insufficient. On the other hand, if it exceeds 1.0%, workability will decrease. Therefore, Mn is set to 0.2 to 1.0%.

Cr: 1.50-2.50%
Cr combines with N during nitriding to generate nitrides, improves surface hardness during nitriding, and has the effect of ensuring bending fatigue strength and wear resistance of cold-forged nitrided parts. However, if the Cr content is less than 1.50%, the above effect is small. Further, Cr is an element that stabilizes M ₇ C ₃ type carbide. If the Cr content is less than 1.50%, M ₇ C ₃ type carbides will not precipitate, resulting in insufficient C inflow from pearlite grains to ferrite grains during spheroidizing annealing, resulting in the formation of spheroidized carbides. Since the distribution becomes non-uniform, cold forgeability deteriorates. On the other hand, if Cr exceeds 2.50%, the material hardness increases too much and workability decreases. Therefore, Cr is set to 1.50 to 2.50%. Preferably, Cr is 1.60-2.40%.

Mo: 0.03~0.30%
Mo is a carbide-forming element and has the effect of improving core hardness through age hardening. However, if Mo is less than 0.1%, the core hardness after carburization decreases, resulting in insufficient strength. On the other hand, if Mo is 0.3% or more, the material hardness increases too much and the workability decreases, resulting in poor machinability and cold workability. Therefore, Mo is set at 0.1 to 0.3%.

Al: 0.005-0.300%
Al is an effective element for deoxidizing steel, and has the effect of reacting with N during nitriding to form AlN and improving surface hardness. However, if the Al content is less than 0.005%, not only the above effects cannot be obtained, but also insufficient deoxidation during production is likely to occur, resulting in a decrease in the quality of inclusions. On the other hand, if Al is more than 0.300%, not only will hard and coarse Al2O3 be formed, resulting in poor cold forgeability, but also the effective hardening layer in nitriding will be shallow, resulting in a decrease in bending fatigue strength and pitting strength. occurs. Therefore, Al is set at 0.005 to 0.300%.

N: 0.004-0.030%
N becomes a nitride such as AlN in steel and has the effect of refining crystal grains, and when combined with V, it also contributes to improving the core hardness. Therefore, the content must be 0.0040% or more. On the other hand, when N combines with elements such as V together with C to form carbonitrides, and the hardness increases more than necessary, cold forgeability deteriorates. Furthermore, the effect of improving core hardness due to age hardening at the nitriding temperature cannot be sufficiently obtained. Therefore, it is necessary to limit the N content, which is 0.0030% or less. Therefore, N is set to 0.004 to 0.030%.

V:0.08~0.30%
N combines with C and/or N during nitriding to form carbides, nitrides, and carbonitrides, and has the effect of improving surface hardness. Furthermore, by forming carbides at the nitriding temperature, the core hardness is improved. In order to obtain such hardening, it is necessary to contain V in an amount of 0.08% or more. However, if more V is added than necessary, there is a risk that the cold forgeability will deteriorate. Therefore, the upper limit of V is set to 0.30%. Therefore, V is set to 0.08 to 0.30%.

The remainder of the chemical components defined in the present invention are Fe and unavoidable impurities. Note that among the unavoidable impurities, the upper limits of P and S are defined as follows.

P: 0.030% or less P is an unavoidable impurity. Since P promotes grain boundary segregation, it reduces toughness. Therefore, the unavoidable impurity P is set to 0.030% or less.

S: 0.030% or less S is an unavoidable impurity. If S exceeds 0.030%, a large amount of coarse MnS will be formed, resulting in a decrease in toughness and fatigue strength. Therefore, the unavoidable impurity S is set to 0.030% or less.

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

Nb: 0.10% or less Nb is an effective component for grain refinement. However, if Nb is contained in an amount greater than 0.10%, hardness increases and cold forgeability decreases. Therefore, when Nb is added, it should be 0.10% or less.

Ti: 0.020-0.200%
Ti combines with C and/or N to form fine carbides, nitrides, and carbonitrides to refine crystal grains and has the effect of improving bending fatigue strength. Therefore, in order to obtain these effects, Ti may be contained in an amount of 0.020% or more. However, when the Ti content is high, coarse TiN is produced, which actually reduces the bending fatigue strength. Therefore, an upper limit is set for the amount of Ti when it is included, and it is set to 0.200% or less. The amount of Ti, if included, is preferably 0.100% or less.

B: 0.0030% or less B is an element that contributes to hardenability, so it is a component that can be added arbitrarily. However, if the content exceeds 0.0030%, the hardness of the material increases more than necessary, resulting in a decrease in workability. Therefore, B is set to 0.0030% or less.

Softening heat treatment: Hold at 730 to 760°C for 4 to 8 hours and then air cool.In the present invention, by using a high Cr component, the solid solution C in the ferrite grains becomes M ₇ C ₃ type just below the austenitizing temperature. (M=mixed component of Fe and Cr) is stably precipitated as a carbide, resulting in a structure in which spherical carbides are uniformly dispersed within ferrite grains, and has excellent cold forgeability.
If the treatment temperature or treatment time is excessive, the spheroidization of the carbide is promoted more than necessary, and although the cold forgeability is excellent, the core hardness after nitriding may be insufficient.
On the other hand, if the treatment temperature or treatment time is too low, the spheroidization of the carbide will not be appropriately promoted, which may impair cold forgeability.
Therefore, the softening heat treatment is performed by holding at 730 to 760° C. for 4 to 8 hours, and then air cooling.

Hardness after cold forging: 270 Hv or more In order to ensure sufficient core hardness after nitriding, it is necessary to sufficiently harden the core by cold forging. Therefore, the hardness after cold forging with a compression rate of 50% or more is set to 270Hv or more.

Hardness after nitriding: surface hardness 680Hv or more, core hardness 220Hv or more, effective hardened layer depth: 0.25mm or more,
In order to use steel for nitriding as parts such as gears by cold forging, sufficient pitching fatigue strength and bending fatigue strength are required. Therefore, it is useful to perform nitriding treatment after cold forging, so that it has sufficient nitriding properties, that is, it ensures the surface hardness, effective hardened layer depth, and core hardness after nitriding. I need to do what I can. Therefore, the nitriding properties of the cold forged nitrided parts after nitriding are set to have a surface hardness of 680 Hv or more, an effective hardened layer depth of 0.25 mm or more, and a core hardness of 220 Hv or more.

Next, a mode for carrying out the present invention will be described.
First, the invention steel No. shown in Table 1. 1 to 17 and comparative steel No. Each of the steels having a total chemical composition of 100%, consisting of chemical components Nos. 18 to 23 and the balance Fe and unavoidable impurities, was melted in a 100 kg vacuum induction melting furnace (VIM).

Next, each of these steel specimens was hot-forged into steel bars with a diameter of 40 mm, held at a temperature of 730 to 760° C. for 4 to 8 hours as a softening heat treatment, and then cooled in air. Note that the softening heat treatment was performed using a Kanthal furnace according to the following procedure.

The procedure for softening heat treatment in a Kanthal furnace is to put the above sample material into the furnace set at the above holding temperature, ensure 30 minutes of heating time for the sample material, and then hold for an arbitrary period of time. Then, air or water cooling is performed. In this example, both were air cooled.
In addition, when selecting the holding time for the softening heat treatment, the amount and dimensions of the steel material to be charged into the furnace shall be taken into consideration.

Regarding the properties of the softening heat treated steel, the hardness was confirmed as follows. After the softening heat treatment, cold forging with a compression ratio of 50% or more was performed, and then a nitriding test was performed to check the cross-sectional hardness and core hardness. The results are shown in Table 2.

(About hardness after softening heat treatment)
For evaluation of cold forging workability, inventive steel No. 1 in Table 1 was used. 1 to 17 and comparative steel No. The hardness of each steel No. 18 to No. 23 after softening heat treatment and before cold forging was measured using a Rockwell hardness tester to determine the hardness (HRB). The above-mentioned annealed test material was cut perpendicularly to the rolling direction, the cut surface was surface ground, and then a Rockwell hardness test was conducted at the middle circumferential portion. A case where the HRB was 87 HRB or less was evaluated as having excellent cold forging workability.

(About evaluation of hardness and nitriding properties after cold forging)
Next, the softening heat treated steel was cold forged at a compression ratio of 50% or more, and then nitrided at 400 to 600°C for 2 to 32 hours. The hardness after cold forging was measured, and it was confirmed by hardness measurement whether predetermined nitriding characteristics and core hardness were exhibited when these steels were nitrided.

Regarding the hardness before and after nitriding treatment, a round bar test piece with a diameter of 10 mm was cut across, embedded in resin so that the cut surface became the test surface, and then polished so that the surface had a mirror finish. The hardness after cold forging and the core hardness were measured using a hardness testing machine. Furthermore, the surface hardness and effective hardened layer depth of the nitrided test piece were measured using a micro-Vickers hardness measuring machine. Table 2 summarizes the results of each test in the survey.

The specific measurement procedure is as follows. First, in accordance with JIS (Japanese Industrial Standards) Z2244 (2009), the HV of a total of 5 points, 1 point in the center of a mirror-finished test piece and 4 points in the R/2 part, was tested using a Vickers test force of 9.8N. The hardness was measured using a hardness tester, and the arithmetic mean value of the five points was defined as the "core hardness." A case where the hardness after cold forging was 270 HV or more was evaluated as being appropriately hardened by cold forging. In addition, cases where the core hardness was 220 HV or more were evaluated as being able to suppress the decrease in core hardness after nitriding.

Using the same embedded sample, in accordance with JIS Z2244 (2009) as in the above case, the test force was set to 0.98 N and the depth position was measured at a depth of 0.05 mm from the surface of the test piece using a micro Vickers hardness measuring machine. The HV at 10 arbitrary points was measured, and the values were arithmetic averaged to obtain the "surface hardness." Cases of 680 HV or higher were evaluated as having excellent surface hardness after nitriding.

Furthermore, using the same embedded sample, in accordance with JIS Z2244 (2009), the Vickers hardness was sequentially measured from the surface of the mirror-finished test piece using a micro Vickers hardness measuring machine with a test force of 1.96N, A hardness distribution map was created. The distance from the surface to the position at 400 Hv was defined as the "effective hardened layer depth" (FIG. 1). Cases in which the effective hardened layer depth was 0.25 mm or more were evaluated as excellent in effective hardened depth.

Steel type No. which is the invention steel component. Steels Nos. 1 to 17 all meet the composition stipulated by the invention and have been subjected to softening heat treatment at a predetermined treatment temperature, so they have a hardness of 87 HRB or less after softening heat treatment and excellent cold forgeability. It has been obtained. It was confirmed that the hardness after cold forging was 270 HV or more, and the core hardness after nitriding was 220 HV or more. It was also confirmed that the surface hardness after nitriding was 680 Hv or more, and the effective hardened layer depth was 0.25 mm or more, and that the nitriding properties were excellent. Because it has the above characteristics, the nitriding steel of the present invention has an excellent balance between workability during cold forging and nitriding properties, and can be applied as cold forged nitrided parts that are cold forged and nitrided. It was confirmed that it has excellent properties suitable for various purposes.

On the other hand, when we look at steel types 18 to 23 with comparative steel compositions, when the composition falls outside the specified range of the present invention, a decrease in nitriding properties, a decrease in cold workability, a lack of core hardness, etc. are observed as shown below. Ta.
Steel type 18 has a hardness of 90 HRB after the C and Mn softening heat treatment, which results in poor cold forgeability.
Steel type 19 has too little V, so the effective hardened layer depth after nitriding is reduced.
Since steel type 20 contains too little Cr, the surface hardness after nitriding is reduced.
Steel type 21 has too little C and has a low core hardness after nitriding.
For steel type 22, the temperature of the softening heat treatment was too low, so the hardness was not lowered sufficiently.
For steel type 23, the temperature and time of the softening heat treatment were excessive, so the core hardness after nitriding was reduced.

Claims (5)

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%, V: 0.08 to 0.30%, the balance being Fe and unavoidable It is a steel consisting of impurities, P (as an unavoidable impurity): 0.030% or less, S (as an unavoidable impurity): 0.030% or less, as a softening heat treatment, it is held at 730 to 760 ° C for 4 to 8 hours and then air cooled. A steel for nitriding that is in a hardened state and has a hardness of 87 HRB or less on the Rockwell hardness scale. In addition to the chemical components according to claim 1, any one or two types of Nb: 0.10% or less, Ti: 0.020 to 0.200%, and B: 0.0030% or less, in mass %. It is a steel with the above properties, the balance being Fe and unavoidable impurities, and it is kept at 730 to 760°C for 4 to 8 hours as a softening heat treatment, and then air cooled, and has a Rockwell hardness of 87 HRB. Steel for nitriding which is: The steel for nitriding according to claim 1, having a Vickers hardness of 270 Hv or more when cold forged at a compression ratio of 50% or more. The steel for nitriding according to claim 2, characterized in that the hardness is 270 Hv or more in terms of Vickers hardness when cold forged at a compression ratio of 50% or more. Nitriding which is cold forged using the nitriding steel according to claim 1 or 2, has a Vickers surface hardness of 680 Hv or more, a core hardness of 220 Hv or more, and an effective hardened layer depth of 0.25 mm or more. A cold forged nitrided part, characterized in that it is in a treated state.

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