JPH11335734A - Manufacture of steel material for soft-nitriding, and soft-nitrided parts using the steel material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide soft-nitrided parts having superior fatigue resistance and wear resistance and a method of manufacturing a steel material for soft- nitriding, used as a stock for the parts and excellent in machinability. SOLUTION: A steel, which has a composition consisting of 0.15-0.45% C, >0.10-0.50% Si, 0.2-2.5% Mn, 0.002-0.13% S, 0.5-2.0% Cr, 0.005-0.1% Nd, 0.05-0.5% V, 0.005-0.3% Al, 0-0.2% Ti, 0-0.2% Zr, 0-0.2% Nb, 0-0.35% Pb, 0-0.01% Ca, and the balance Fe with impurities, is subjected to hot working and to spheroidizing annealing to make core hardness to <=Hv180 and then to cold working to make core hardness to >=Hv250 and also make decarburized depth to 0.1 to 0.4 mm from the surface of the steel material. The composition of the steel stock may contain 0.05-0.50% Si and 0.02-0.3% of (Mo+0.5W), while containing the components mentioned above besides Si. The soft-nitrided parts have >=Hv600 surface hardness after soft-nitriding and >=0.1 mm effective case depth and are obtained by using, as a stock, the steel material for soft-nitriding manufactured by either of the above methods.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel material for nitrocarburizing and a nitrocarburized component using the steel material, and more particularly to a steel sheet having excellent fatigue resistance, wear resistance, pitting resistance and spalling resistance. The present invention relates to a nitrocarburized component and a method for producing a nitrocarburized steel material having excellent machinability, which is a material of the nitrocarburized component. In the present specification, among the fatigue phenomena in which the material surface peels off due to the repetitive surface pressure load, those with relatively small peeling are called "pitting", and those with relatively large peeling are called "spalling".

[0002]

2. Description of the Related Art Many parts used in automobiles and industrial machines, such as gears and bearings, generally require large fatigue strength and wear resistance. Therefore, the components have been manufactured by performing a so-called “surface hardening treatment”.

As the surface hardening treatment, generally, carburizing quenching, induction quenching, flame quenching, nitriding and nitrocarburizing are known. Of these, in the treatment of hardening the surface by rapid cooling (quenching) from a high temperature region in an austenitic state such as carburizing quenching, induction quenching, or flame quenching, large quenching distortion occurs in components. Further, in some cases, quenched cracks may occur in the quenched parts.

[0004] For this reason, when a required component is required to have a particularly low strain, nitriding or nitrocarburizing is performed.

However, the general nitriding treatment is a so-called "gas nitriding" treatment of heating at 500 to 550 ° C. for 20 to 100 hours in a stream of ammonia and then gradually cooling, resulting in low productivity and high cost. For this reason, a liquid nitriding method at a nitriding temperature of about 550 ° C. has been developed. However, even in this method, nitridation requires about 12 hours, so that it is not necessarily suitable for efficiently producing mass-produced parts at low cost. I can't say that.
According to the ion nitriding method, nitriding is possible in a short time,
This method is not suitable for the production of mass-produced parts because it is difficult to measure the temperature, and the temperature and nitrided layer become unstable depending on the arrangement, shape, and mass of the part to be treated as the cathode. hard.

On the other hand, the nitrocarburizing treatment is carried out by keeping a salt bath of a cyanide compound at a temperature of about 570 ° C. or a gas obtained by adding ammonia to RX gas (RX gas is a trademark of endothermic modified gas). A method in which N (nitrogen) and O (oxygen) penetrate into the steel from the surface of the steel material to harden the surface layer portion enables short-time processing. Of these, the former method using a salt bath of a cyanide compound increases the cost of waste liquid treatment, so the latter gas nitrocarburizing method using gas is a surface hardening method suitable for mass-produced products requiring low distortion. It is heavily used as a processing method.

Conventionally, as steel for nitrocarburizing, for example, JIS
Chromium molybdenum steel (SC) specified in G 4105
M435) and aluminum chromium molybdenum steel of JIS G 4202 (SACM645) have been widely used.

However, JIS including SCM435
In the case of parts made of chromium molybdenum steel as specified in JIS, the Vickers hardness (H
v) Distance to the position of 500 (hereinafter, "effective hardening depth")
Is as small as about 0.05 mm. Furthermore, the micro Vickers hardness at a position 0.025 mm from the surface (hereinafter, referred to as “surface hardness”) often does not exceed Hv600. For this reason, it was not sufficiently satisfactory in terms of fatigue strength and wear resistance.

On the other hand, in order to improve the above-mentioned disadvantage, SAC
M645 contains a large amount of Al and Cr which are nitriding property improving elements. However, even when SACM645 is used as the material steel, the surface hardness is 8 in Hv by the nitrocarburizing treatment.
Although it is very high as 00 to 1100, the effective hardening depth is as small as about 0.08 mm. Therefore, the hardness gradient from the surface portion to the core portion (hereinafter, the portion that is not surface-hardened after the nitrocarburizing treatment is referred to as “core portion”) is too sharp.
Therefore, in gears and bearings operated under a high load, a peeling phenomenon is likely to occur near the boundary between the hardened portion and the core portion, and the pitting resistance or the spoiling resistance is poor. Furthermore, SACM645 has problems that melting, casting, and hot working are relatively difficult, and that cold workability is poor and that parts having complicated shapes are difficult to press-form.

JP-A-58-71357 discloses JI
"Steel for nitrocarburizing" which solves the problem of S-standard steel is disclosed. If the steel proposed in this publication is used as a material steel, it is possible to obtain a nitrocarburized component having excellent fatigue strength and wear resistance, and also excellent pitting resistance and spalling resistance. However, since the steel is improved in cold workability by reducing the content of elements effective for strengthening such as Si, the surface is hardened by nitrocarburizing, whereas the core is heated during nitrocarburizing. , The core hardness becomes too low after nitrocarburizing, and the fatigue properties are sometimes deteriorated.

Further, chromium molybdenum steel such as SCM435 which is JIS standard steel, SACM645 of aluminum chromium molybdenum steel and the above-mentioned Japanese Patent Application Laid-Open No. 58-7135.
In the case of the steel proposed in Japanese Patent No. 7, the machinability is inferior. Therefore, after forming into a desired nitrocarburized part shape by cold forging or the like, for example, a cutting cost such as drilling a hole in an oil hole or a place that does not affect the strength of the part for the purpose of reducing the weight of the part. Will increase. For this reason, there is an increasing demand for a nitrocarburizing steel material having excellent machinability in order to facilitate cutting and reduce costs.

[0012]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and is excellent only in that a steel material excellent in machinability and cold workability is used as a raw material and only soft nitriding is performed after cold working. An object of the present invention is to provide a nitrocarburized component exhibiting fatigue characteristics, wear resistance, pitting resistance and spalling resistance. Still another object of the present invention is to provide a method for producing a steel material for nitrocarburizing which is excellent in machinability and is used as a material for the nitrocarburized component.

[0013]

The gist of the present invention is to provide a method for producing a steel material for nitrocarburizing excellent in machinability as shown in the following (1) and (2), and using the steel material as shown in (3). It is in nitrocarburized parts.

(1) In weight%, C: 0.15 to 0.45
%, Si: more than 0.10% to 0.50%, Mn:
0.2-2.5%, S: 0.002-0.13%, C
r: 0.5 to 2.0%, Nd: 0.005 to 0.1%,
V: 0.05-0.5%, Al: 0.005-0.3
%, Ti: 0 to 0.2%, Zr: 0 to 0.2%, Nb:
0 to 0.2%, Pb: 0 to 0.35%, and Ca: 0 to 0%
0.01%, the balance being steel having the chemical composition of Fe and unavoidable impurities, after hot working, spheroidizing and annealing to reduce the core hardness to Hv180 or less, and then to cold work to increase the core hardness to Hv250 or more. And producing a steel material for nitrocarburizing excellent in machinability, wherein the decarburization depth is 0.1 to 0.4 mm from the surface of the steel material.

(2) In weight%, C: 0.15 to 0.45
%, Si: 0.05-0.50%, Mn: 0.2-2.
5%, S: 0.002 to 0.13%, Cr: 0.5 to
2.0%, Nd: 0.005 to 0.1%, V: 0.05
-0.5%, Al: 0.005-0.3%, Mo + 0.
5W: 0.02-0.3%, Ti: 0-0.2%, Z
r: 0 to 0.2%, Nb: 0 to 0.2%, Pb: 0 to 0%
0.35% and Ca: 0 to 0.01%, with the balance being F
e and steel having a chemical composition of unavoidable impurities are subjected to spheroidizing annealing after hot working to reduce the core hardness to Hv 180 or less, and then to cold working to increase the core hardness to Hv 250 or more, and the decarburization depth to the steel material. A method for producing a steel material for nitrocarburizing excellent in machinability, wherein the thickness is 0.1 to 0.4 mm from the surface of the steel.

(3) For nitrocarburizing, the surface hardness after nitrocarburizing is Hv600 or more, the effective hardening depth is 0.1 mm or more, and the material is manufactured by the method described in (1) or (2) above. A soft-nitrided part characterized by being a steel material.

The term "steel material for nitrocarburizing" refers to a material formed into a desired shape by cold working or cutting, or a material which has been further polished thereafter. Provided for processing.

As described above, the "effective hardening depth"
Means Vickers hardness (Hv) 5 from the surface after soft nitriding
The term “surface hardness” refers to the Vickers hardness at a position 0.025 mm from the surface. Further, the “core” refers to a part that has not been surface-hardened after the nitrocarburizing treatment.

In the present invention, "decarburization" means that the C content is 0.05% or more in terms of weight% of the C content of the "core".

Hereinafter, the inventions described in the above (1) to (3) are referred to as the inventions (1) to (3), respectively.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have made steel materials excellent in cold workability into a material, and have excellent fatigue resistance, wear resistance, pitting resistance and pitting resistance simply by performing soft nitriding after cold working. Various investigations and studies were conducted with the aim of providing a nitrocarburized component exhibiting spalling properties and providing a method for producing a steel material for nitrocarburizing having excellent machinability as a material of the nitrocarburized component. That is, in order to solve the above-described problems, investigations and studies were repeated on the chemical composition of the steel material used as the material of the nitrocarburized parts, the appropriate microstructure and the mechanical properties in each manufacturing process. As a result, the following findings were obtained.

In order to improve the fatigue resistance and pitting resistance of the nitrocarburized parts, the surface hardness and the effective hardening depth may be increased. Also, to improve wear resistance,
What is necessary is just to increase surface hardness. On the other hand, in order to improve the spalling resistance, the effective hardening depth may be increased.

A soft nitriding treatment is performed, and the surface hardness is set to HV60.
When the hardening depth is 0 or more and the effective hardening depth is 0.1 mm or more, the fatigue resistance, wear resistance, pitting resistance and spalling resistance of the nitrocarburized component can be improved.

The steel material is spheroidized and annealed to have a hardness of Hv180.
If it is reduced below, the cold workability is improved and the mold life can be greatly improved.

If the chemical composition of the base steel is adjusted and the hardness is reduced to Hv180 or lower by spheroidizing annealing, the steel is work-hardened by cold working to have a hardness of Hv250 or higher. The decrease in core hardness is extremely small, and Hv
A value of 250 or more can be maintained.

Unless otherwise specified, the hardness before nitrocarburizing (for example, after spheroidizing annealing or after cold working) means
It refers to the hardness of a portion (for example, “center”) corresponding to the core after soft nitriding.

If the core hardness after nitrocarburizing is Hv250 or more, bending fatigue does not occur from the inside of the component as a starting point even in a component to which a high load is applied, such as a transmission gear of an automobile.

By adding appropriate amounts of Nd and S to steel,
In a relatively high temperature range of the molten steel, Nd ₂ S ₃ is finely dispersed and precipitated, and this Nd ₂ S ₃ acts as a chip breaker, so that the machinability of the steel can be improved.

Next, the present inventors, by weight%, C: 0.3
0%, Si: 0.25%, Mn: 0.6%, and Al:
0.03% as the basic composition, only C content 0.15%
When the nitrocarburizing treatment was performed on the steel material changed from 0.45% to 0.45%, it was found that the effective hardening depth after the nitrocarburizing treatment became larger as the C content was lower. Therefore, the steel material with the C content changed is decarburized to various depths,
Next, a soft nitriding treatment was performed. As a result, the following important findings were obtained.

The surface of the steel material before the nitrocarburizing treatment is 0.1 to
When the C content in the surface layer portion is reduced from the C content in the core portion by decarburizing by 0.4 mm, the effective hardening depth after the nitrocarburizing treatment can be particularly greatly increased.

From the above, the following findings were obtained.

A steel material having excellent cold workability and excellent machinability is used as a material, which is subjected to cold working to secure sufficient hardness by work hardening, and then soft-nitrided to form a hard and deep nitrided layer. The hardness for work hardening (that is, the core hardness) is maintained by heating for soft nitriding, or
If the decrease in hardness can be suppressed to a small value, it is possible to impart large fatigue resistance, wear resistance, pitting resistance and spalling resistance to the nitrocarburized component. In particular, when the decarburization depth before nitrocarburizing is 0.1 to 0.4 mm, the effective hardening depth after nitrocarburizing can be increased, so that the nitrocarburized parts have extremely large fatigue resistance and wear resistance. , Pitting resistance and spalling resistance.

The present invention has been completed based on the above findings.

Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" display of the content of each element is "wt%".
Means

(A) Chemical composition of raw steel C: C is an element necessary for securing static strength. However, if the content is less than 0.15%, the desired static strength (core hardness after nitrocarburizing after cold working, that is, Hv as the core hardness of the nitrocarburized component as the final product).
250 or more) cannot be secured. On the other hand, if it exceeds 0.45%, the ductility and toughness of the core are reduced, and the machinability and the cold workability are deteriorated. Further, the surface hardness and hardening depth after nitrocarburizing are rather reduced. Therefore, the content of C is set to 0.15 to 0.45%.

Si: Si has the effect of improving the hardenability of steel and improving the static strength. But,
In the case of the steel for nitrocarburizing using the steel not containing Mo and W according to the invention of (1) as the base steel, the content of Si is 0.10%.
In the following, the value of Mo + 0.5W, which is the sum of the content of Mo and half the content of W according to the invention (2), is 0.0
In the case of nitrocarburizing steel with 2% or more steel as material steel,
If the content of Si is less than 0.05%, the above-described desired static strength may not be stably secured. On the other hand, in the case of the steel for nitrocarburizing using the steel according to any of the above-mentioned inventions as the base steel, if the content of Si exceeds 0.50%, the toughness is deteriorated, and the cold workability is adversely affected. Exert.

Therefore, in the invention of (1), the Si content of the base steel is set to more than 0.10% and to 0.50%.

Further, in the invention of (2), the S
The i content was 0.05 to 0.50%.

Mn: Mn is an element effective for improving hardenability and ensuring core strength. However, when its content is 0.1.
If it is less than 2%, the effect of the addition is poor. On the other hand, if it exceeds 2.5%, segregation occurs and the cold workability deteriorates. Therefore, the content of Mn is set to 0.2 to 2.5%. Note that the content of Mn is preferably set to 0.5 to 1.5%.

S: S forms Nd ₂ S ₃ together with Nd, acts as a chip breaker, and improves machinability.
However, if the content is less than 0.002%, the desired effect cannot be obtained. On the other hand, if the content exceeds 0.13%, not only the effect is saturated, but also the hot workability and the cold workability deteriorate significantly. Invite. Therefore, the content of S is set to 0.002 to 0.1.
13%.

Cr: Cr is an element that is extremely effective in increasing the surface hardness and increasing the hardening depth by combining with N invading from the steel material surface during soft nitriding. However, if the content is less than 0.5%, the above effects cannot be expected. On the other hand, when Cr is contained in excess of 2.0%,
Since the surface hardness becomes too high due to soft nitriding, the hardness gradient from the surface to the core becomes steep, and the spalling resistance and the pitting resistance are rather deteriorated. Therefore, the content of Cr is set to 0.5 to 2.0.
%.

Nd: Nd combines with S to form Nd ₂ S ₃ and acts as a chip breaker to improve machinability. However, if the content is less than 0.005%, it is difficult to obtain the above-mentioned effects. On the other hand, if the content exceeds 0.1%, the effects are not only saturated but also the fatigue characteristics are deteriorated. Therefore, N
The content of d was set to 0.005 to 0.1%. Note that Nd
Is preferably 0.005 to 0.08%.

V: V combines with N and C invading from the steel material surface during nitrocarburizing treatment and precipitates as fine vanadium carbonitride, thereby increasing the surface hardness.
It has the effect of increasing the curing depth. In the case of V-added steel, the rate of increase in the surface hardness is very small, but the rate of increase in the hardening depth is extremely large, and the carbonitride precipitates to increase the core hardness, as compared with the case of the above Cr addition. Therefore, a hardening curve with a large hardening depth and a gentle hardness gradient from the surface to the core can be obtained. However, when the V content is 0.0
If it is less than 5%, the effect of addition is poor. On the other hand, if it exceeds 0.5%, the above effect is saturated and not only increases the cost but also causes the embrittlement phenomenon. Therefore, the V content is set to 0.05 to 0.5%. In addition,
The V content is preferably 0.1 to 0.3%.

Al: Al has the effect of stabilizing and homogenizing steel deoxidation. Furthermore, it has the effect of increasing the surface hardness by combining with the intrusion N. However, its content is 0.00
If it is less than 5%, the above effects cannot be expected. On the other hand, 0.3
%, The curing depth is reduced. Therefore, the content of Al is set to 0.005 to 0.3%. The Al content is preferably set to 0.005 to 0.15%.

Mo + 0.5W: Mo and W are effective elements for enhancing the hardenability of the steel to increase the softening resistance of the core during nitrocarburizing and to secure the desired static strength described above. or,
It also has the effect of making the structure after normalization a bainite-containing structure.

However, the Si according to the invention of (1) is 0.1%.
In the case of a steel material for nitrocarburizing using steel containing more than 0% as a base steel, the desired static strength described above can be easily imparted to a nitrocarburized component as a final product. It is not necessary to contain it.

Regarding the invention (2), in particular, 0.10%
In the case of a steel material for nitrocarburizing using a steel containing only the following Si as a raw material steel (in the present invention, Si: 0.05 to 0.
In the case of 10%), when the value of Mo + 0.5W, which is the sum of the Mo content and half of the W content, is less than 0.02%, the addition effect is poor. Therefore, 0.0 at Mo + 0.5W
It is necessary to contain 2% or more of Mo and W. In the invention of (2), in the case of a steel material for nitrocarburizing using steel containing more than 0.10% of Si as a material steel, 0.0%
When 2% or more of Mo + 0.5W is contained, it is extremely easy to impart a desired static strength to a final product, a nitrocarburized component, and the structure after normalization is easily changed to a bainite-containing structure. It is possible to

On the other hand, regarding the invention of (2), (a) Si
(B) nitrocarburizing steel using steel containing 0.10% or less as a base steel, and (b) nitrocarburizing steel using steel containing more than 0.10% of steel as a base steel Also, M
Even if o and W are contained in excess of 0.3% at the value of Mo + 0.5 W, the effect of securing the desired static strength and the effect of making the structure after normalization a bainite-containing structure are saturated and the cost increases. Only. Therefore, in the invention of (2), the amount of Mo + 0.5W contained in the material is set to 0.02 to
0.3%. In the invention of (2), when W is added alone, the upper limit of the W content is preferably 0.5%.

Ti: Ti need not be added. If added, they precipitate as nitrides or carbonitrides and have the effect of increasing the softening resistance of the core during nitrocarburizing, so that the above-mentioned desired static strength can be stably secured. In order to ensure the above effects, it is preferable that the content of Ti be 0.01% or more. However, when the content of Ti exceeds 0.2%, toughness is deteriorated. Therefore, the content of Ti is 0 to
0.2%.

Zr: Zr may not be added. If added, they precipitate as nitrides or carbonitrides and have the effect of increasing the softening resistance of the core during nitrocarburizing, so that the above-mentioned desired static strength can be stably secured. To ensure the above effects, it is preferable that the content of Zr be 0.01% or more. However, when Zr exceeds 0.2%, toughness is deteriorated. Therefore, when the content of Zr is 0 to
0.2%.

Nb: Nb may not be added. If it is added, it precipitates as nitride or carbonitride and raises the softening resistance of the core during nitrocarburizing and further refines the structure, so that the above-mentioned desired static strength can be stably secured. In order to ensure the above effects, it is preferable that the content of Nb is 0.01% or more. However, when Nb is contained in excess of 0.2%, toughness is deteriorated. Therefore, the content of Nb is set to 0 to 0.2%.

Pb: Pb may not be added. If it is added, after forming into the desired nitrocarburized part shape by cold forging, etc., it is easier to perform cutting work such as drilling holes that do not affect the strength of the parts for the purpose of reducing oil weight and parts weight. Has the effect of In order to surely obtain this effect, the content of Pb is preferably set to 0.03% or more. However, if the content of Pb exceeds 0.35%, the hot workability is deteriorated, and cracks are caused during hot working such as hot rolling and hot forging. Therefore, the content of Pb is set to 0
-0.35%.

Ca: Ca may not be added. If it is added, after forming into the desired nitrocarburized part shape by cold forging, etc., it is easier to perform cutting work such as drilling holes that do not affect the strength of the parts for the purpose of reducing oil weight and parts weight. Has the effect of In order to surely obtain this effect, the content of Ca is preferably set to 0.001% or more. On the other hand, if Ca is contained in an amount exceeding 0.01%, special smelting techniques and equipment are required, which increases the cost. Therefore, the content of Ca is set to 0 to 0.01%.

(B) Spheroidizing annealing In spheroidizing annealing, after a steel material having the chemical composition shown in the above (A) is hot-worked (for example, hot-rolled), its hardness is reduced to enhance cold workability. At the same time, this is an essential process for greatly improving the mold life and keeping the production cost of the required nitrocarburized component as a final product low.
Further, the spheroidizing annealing is an essential process for imparting a predetermined decarburization depth described later to the steel material.

The hardness after spheroidizing annealing, especially the core hardness is Hv
If it exceeds 180, the life of the mold is greatly reduced, and the production cost of the desired nitrocarburized component as a final product is significantly increased. Therefore, the core hardness after spheroidizing annealing must be Hv180 or less. It is not necessary to particularly limit the lower limit value of the hardness after the spheroidizing annealing.

The spheroidizing annealing of the steel material may be performed as it is after hot working, or may be performed after normalizing after hot working. Further, after the treatment, the spheroidizing annealing has a core hardness of desired Hv 180 or less, and 0.1 mm
As long as the above-mentioned decarburization depth can be obtained, it is not necessary to perform the decarburization by any special method, but may be performed by a normal method. Preliminary tests determine the detailed heat pattern of spheroidizing annealing by obtaining the decarburization depth after spheroidizing annealing in relation to the conditions of hot working performed before spheroidizing annealing and normalizing conditions It is possible to

(C) Cold work The steel material of the above (B), whose core hardness has been adjusted to Hv 180 or less by spheroidizing annealing, is then cold worked to finish into a desired nitrocarburized component shape.

As described above, (1) and (2)
The "steel material for nitrocarburizing" according to the invention of the present invention means a steel material before being subjected to nitrocarburizing treatment.

The above cold working may be performed by a usual method such as cold forging, cold rolling or cold drawing, but the core hardness of the processed part must be Hv250 or more. . The reason is that the steel material of the above (B) whose core hardness is adjusted to Hv180 or less is subjected to cold working, and if the core hardness increases to Hv250 or more, it may be soft-nitrided. This is because the decrease in core hardness due to heating during nitrocarburizing is extremely small, and a value of Hv250 or more can be maintained. If the core hardness after nitrocarburizing is equal to or higher than Hv250, as described above, for example, even in a part to which a high load is applied, such as a transmission gear of an automobile, bending fatigue starts from the inside of the part. Absent.

In order to make the core hardness Hv250 or more by cold working the steel material whose core hardness has been adjusted to Hv180 or less by the spheroidizing annealing shown in the above (B), the reduction in area is 20% or more. The dimensions may be adjusted so that the processing described above is added.

The upper limit of the core hardness after cold working does not need to be particularly limited. In other words, as long as 0.1 to 0.4 mm can be obtained as the decarburization depth from the surface of the steel material described below, processing is performed with the highest surface reduction rate that can be added on equipment, and extremely high hardness and May be. However, for applications requiring extremely good impact properties, it is preferable to limit the upper limit of the core hardness after cold working to Hv400.

(D) Decarburization Depth When the surface of the steel material before the nitrocarburizing treatment is decarburized by 0.1 to 0.4 mm to reduce the C content in the surface layer from the C content in the core, In particular, the effective hardening depth after the nitrocarburizing treatment can be greatly increased. And the decarburization depth before soft nitriding is 0.1-0.
When it is 4 mm and the core hardness is Hv250 or more, extremely large fatigue resistance, wear resistance, pitting resistance and spalling resistance are obtained after soft nitriding.

As described above, "decarburization" in the present invention means that the C content is 0.05% or more lower than the C content of the "core". This "decarburization" can occur during hot working, normalizing and spheroidizing annealing.
The amount of decarburization is measured at a depth of 0.1 mm from the steel surface.
It is preferable that the content is half or less of the C content in the “core”.

If the decarburization depth is less than 0.1 mm from the surface of the steel material, a sufficient effective hardening depth due to soft nitriding cannot be obtained. On the other hand, when the decarburization depth exceeds 0.4 mm from the steel material surface, when the soft nitriding treatment is performed, the softened layer having a low C content remains at the boundary between the hardened nitrided layer and the core. In addition, separation occurs near the boundary. Therefore, the decarburization depth is defined as 0.1 to 0.4 mm from the surface of the steel material.

Since the decarburization depth specified in the present invention is a value before being subjected to the nitrocarburizing treatment, the steel material subjected to the cold working (C) has a predetermined decarburization depth. If so, the steel material may be nitrocarburized without adjusting the decarburization depth. On the other hand, when the decarburization depth of the cold-worked steel material of (C) exceeds 0.4 mm, the steel material may be soft-nitrided after being adjusted to a predetermined decarburization depth by cutting or polishing. . If the dimensions after grinding or polishing are smaller than the dimensions of the desired nitrocarburized part, the dimensions during cold working may be adjusted in consideration of the grinding allowance and polishing allowance. .

As described in the section on spheroidizing annealing in (B), the preliminary test shows that the depth of decarburization after spheroidizing annealing depends on the conditions of hot working performed before spheroidizing annealing and normalizing conditions. Is determined, it is relatively easy to adjust the decarburization depth of the cold-worked steel material (C) to 0.1 to 0.4 mm.

According to the manufacturing method described above,
The "steel material for nitrocarburizing" according to the inventions of (1) and (2) is obtained. This steel material is subjected to the following soft nitriding treatment,
The nitrocarburized component according to the invention of (3) is obtained.

(E) Soft Nitriding The method of nitrocarburizing applied to the steel material for nitrocarburizing is not limited at all, and may be performed by a usual method. By performing nitrocarburizing treatment and setting the surface hardness to Hv600 or more and the effective hardening depth to 0.1 mm or more, the fatigue resistance, wear resistance, pitting resistance and spalling resistance of the nitrocarburized parts can be increased. It is.

In order to nitrocarburize the above-mentioned nitrocarburizing steel to have a surface hardness of at least Hv600 and an effective hardening depth of at least 0.1 mm, for example, the nitrocarburizing steel at a temperature of about 570 ° C. What is necessary is just to hold it in the gas which added ammonia to RX gas for 3 to 9 hours, and then to cool it in oil.

The upper limits of the surface hardness and the effective hardening depth after nitrocarburizing need not be particularly limited.

The nitrocarburized component according to the invention (3) is obtained by melting a steel having the chemical composition of the above (A), which is a raw steel, by, for example, an ordinary method and then hot rolling or forging the steel. And spheroidizing annealing shown in (B), and then cold working and cutting shown in (C) (for example, Forming into the desired part shape by adjusting the depth of decarburization from the surface by oil holes and other parts that do not affect the strength of the part for the purpose of reducing the weight of the part. The surface is cut and polished to adjust the decarburization depth from the surface, and then soft-nitrided, and thereafter, is further subjected to grinding and polishing as necessary.

Here, in the steel material having the chemical composition which is the object of the present invention, if the steel is subjected to normalization after hot working to form a structure containing bainite, the spherical shape of carbide (mainly cementite) after spheroidizing annealing is obtained. Conversion rate is improved. Therefore, the hardness before cold working can be greatly reduced by spheroidizing annealing. Reducing the hardness of the steel material before cold working leads to an improvement in cold workability, extending the life of the mold and reducing the cost of the mold. Further, the spheroidizing annealing time can be shortened, so that productivity can be improved and manufacturing cost can be reduced. For this reason, in the method of manufacturing the steel material for soft nitriding of the inventions of (1) and (2), it is preferable to normalize after hot working and then to perform spheroidizing annealing.

[0073]

EXAMPLES 180 kg of steel having the chemical compositions shown in Tables 1 and 2 was vacuum-melted by a conventional method. Steels 1 to 9 in Table 1 are examples of the present invention in which the chemical composition is within the range specified in the present invention, and Steels 10 to 20 in Table 2 are comparisons in which any of the components is out of the range of the content specified in the present invention. It is an example. Among the steels according to the comparative examples, steel 19 and steel 20 are steels corresponding to JIS-standard SCM435 and SACM645, respectively, to which Nd is added.

[0074]

[Table 1]

[0075]

[Table 2]

Next, these steels were made into billets by a usual method, heated to 1250 ° C.
Hot forging at a temperature of 950 ° C. to a diameter of 32 mm, followed by peeling to remove black scale, and a diameter of 30 mm
Round bar.

Steels 6 and 9 were hot forged under the same conditions as above to produce round bars having a diameter of 40 mm. In addition,
The round bar having a diameter of 40 mm was prepared in order to increase the peeling allowance as described later.

The thus obtained diameters of 30 mm and 40 mm
Was subjected to normalizing at various temperatures of 870 to 925 ° C., and then subjected to spheroidizing annealing with the heat pattern shown in FIG. 1 to change the decarburization depth.

For comparison, steel 2 and steel 9 were hot forged to a diameter of 32 mm and then peeled to a diameter of 30 mm, that is, spheroidized annealing without normalizing was added.

The diameter obtained as described above is 30 mm
The following various investigations were performed using a round bar.

First, from the as-normalized round bar, a diameter of 30
A test piece having a thickness of 20 mm and a thickness of 20 mm was cut out, and the magnification was 400.
At a magnification of x, the central structure was observed with an optical microscope.

On the other hand, from each round bar after spheroidizing annealing, a hardness test piece having a diameter of 30 mm and a thickness of 20 mm and a hardness test piece having a diameter of 15 mm were obtained.
A 22.5 mm long cold working test specimen was prepared.

Using the above-mentioned hardness test piece, the hardness at the center was measured by a micro Vickers hardness meter.

Further, using the test piece for cold working described above,
A cold (room temperature) constrained upsetting test was performed by a normal method using a 00t high-speed press machine, and the limit upsetting ratio was measured.
In addition, three upsetting tests were performed for each condition, and the maximum working rate (area reduction rate) at which cracks did not occur in all three test pieces was evaluated as the limit upsetting rate.

On the other hand, each round bar having a diameter of 30 mm obtained after the spheroidizing annealing obtained as described above was cold-drawn (room temperature) to a draw bench having a diameter of 25 mm (area reduction ratio of 30.6%) by a usual method. Draw out using. Next, the temperature at which ammonia gas was added to the RX gas at a ratio of 1: 1 was 57%.
A soft nitriding treatment was performed by maintaining the gas in a gas at 0 ° C. for 6 hours, and then cooled into oil.

From the as-pulled round bar, the diameter is 25 m.
A hardness test piece having a thickness of 20 mm and a thickness of 20 mm was prepared, and the hardness of the central portion was measured using a micro Vickers hardness meter.
In addition, a test piece having a diameter of 25 mm and a thickness of 5 mm was cut out, and a line analysis of the C element was performed using EPMA to measure a decarburization depth from the surface.

The diameter of the round bar was 25 m
m and a hardness test piece having a thickness of 20 mm were prepared, and the surface hardness (0.025 m from the surface) was measured using a micro Vickers hardness tester.
m), the effective hardening depth (distance from the surface to the position of Hv500), and the center hardness were measured.

The spheroidized annealed steel 6 and steel 9 have a diameter of 40.
The following various investigations using a mm round bar were also performed.

That is, from a round bar having a diameter of 40 mm, a hardness test piece having a diameter of 25 mm and a length of 15 mm and a hardness test piece having a diameter of 15 mm were prepared.
A 22.5 mm long cold working test specimen was prepared.

Using the hardness test piece thus obtained, the hardness at the center was measured by a micro Vickers hardness tester.

Further, using the above-described test piece for cold working,
A cold (room temperature) constrained upsetting test was performed by a normal method using a 00t high-speed press machine, and the limit upsetting ratio was measured.
In addition, three upsetting tests were performed for each condition, and the maximum working rate (area reduction rate) at which cracks did not occur in all three test pieces was evaluated as the limit upsetting rate.

On the other hand, a round bar having a diameter of 40 mm after the spheroidizing annealing was peeled to a diameter of 25 mm, and thereafter, was cold (room temperature) 20.9 mm in diameter (surface reduction rate 30.1%) by a usual method. Until then, drawing was performed using a draw bench. Next, ammonia gas was added to the RX gas at a ratio of 1: 1.
Was maintained in a gas at a temperature of 570 ° C. for 6 hours to perform a soft nitriding treatment, and then cooled into oil.

From the as-pulled round bar, a diameter of 20.
A hardness test piece having a thickness of 9 mm and a thickness of 20 mm was prepared, and the hardness of the center portion was measured using a micro Vickers hardness tester.

[0094] Even from the round bar subjected to the soft nitriding treatment, the diameter is 20.
A hardness test piece having a thickness of 9 mm and a thickness of 20 mm was prepared, and the surface hardness (0.02 from the surface) was measured with a micro Vickers hardness tester.
The Hv hardness at a position of 5 mm), the effective hardening depth (distance from the surface to the position of Hv500), and the center hardness were measured.

For the evaluation of machinability, a drilling test was also performed. That is, the diameter 30 m after spheroidizing annealing already described.
m and a round bar having a diameter of 20.9 mm after drawing were cut into 25 mm lengths, and R / 2 was used.
Through holes were made in the length direction of the portion (R is the radius of the round bar), and the number of through holes when drilling was impossible due to abrasion of the cutting edge was counted to evaluate the machinability. The drilling conditions are JI
Using a φ5 mm straight shank drill made of S high speed tool steel SKH51 and using a water-soluble lubricant, the feed rate is set to 0.
The measurement was performed at 15 mm / rev and a rotation speed of 980 rpm.

Tables 3 and 4 show the results of various tests.

[0097]

[Table 3]

[0098]

[Table 4]

From Table 3, in the case of the present invention example, the core hardness after spheroidizing annealing is less than 180 in Hv, and the limit upsetting ratio is over 80%. And the reduction rate is 3
A core hardness exceeding Hv250 is easily obtained by cold working (drawing) of 0.6%. Furthermore, after the nitrocarburizing treatment, the surface hardness exceeds Hv600 and 0.1
An effective hardening depth of more than 1 mm was obtained, and the center hardness (core hardness) was maintained at a value exceeding Hv250 even after a heat treatment at 570 ° C. for 6 hours for soft nitriding. Further, the machinability is also good.

On the other hand, the following items are evident from Table 4.

That is, even if the steel has a chemical composition according to the present invention (steel 6 and steel 9), the decarburization depth is 0.1 mm from the surface.
If it is less than 1 mm, the effective curing depth has not reached 0.1 mm.

In the case of the steel according to the comparative example in which any of the components is out of the range of the content specified in the present invention,
(A) The core hardness after spheroidizing annealing exceeds Hv180 and the critical upsetting ratio is low. (B) The core hardness after cold working is low, so the core hardness after soft nitriding is low. ) Although the core hardness after cold working exceeds Hv250, the core hardness after nitrocarburizing is greatly reduced. (D) The surface hardness after nitrocarburizing is H
(v) the effective hardening depth after nitrocarburizing is less than 0.1 mm; and (f) the number of through holes in the drilling test is significantly less than 100, resulting in poor machinability. Applicable. For this reason, the life of the mold during the cold working is short, the cost of the mold increases, and the cost of cutting increases, so that the manufacturing cost of the desired nitrocarburized component becomes extremely high. Alternatively, the fatigue resistance, wear resistance, pitting resistance, and spalling resistance of the nitrocarburized parts are inferior even though the manufacturing cost is low.

[0103]

The nitrocarburized parts of the present invention have a core hardness of more than Hv250, a surface hardness of more than Hv600, and a hardness of 0.5%.
Effective cure depths of more than 1 mm are obtained. For this reason, the nitrocarburized component of the present invention is excellent in fatigue resistance, wear resistance, pitting resistance and spalling resistance. Therefore, it can be used as a part for which high fatigue strength and wear resistance are required, such as gears for automobiles and industrial machines. The steel material for nitrocarburizing, which is excellent in machinability, is
It can be manufactured relatively easily by the method of the present invention.

[Brief description of the drawings]

FIG. 1 is a diagram showing a heat pattern of spheroidizing annealing in an example.

Claims (3)

[Claims] C. 0.15 to 0.45% by weight, S
i: more than 0.10% to 0.50%, Mn: 0.2 to
2.5%, S: 0.002 to 0.13%, Cr: 0.5
To 2.0%, Nd: 0.005 to 0.1%, V: 0.0
5 to 0.5%, Al: 0.005 to 0.3%, Ti: 0
-0.2%, Zr: 0-0.2%, Nb: 0-0.2
%, Pb: 0 to 0.35%, and Ca: 0 to 0.01%, the balance being steel having the chemical composition of Fe and unavoidable impurities, after hot working, spheroidizing annealing to obtain a core hardness of Hv180.
And then cold-worked to make the core hardness Hv250 or more, and the decarburization depth from the surface of the steel material to 0.1 to
A method for producing a steel material for nitrocarburizing excellent in machinability, characterized in that the thickness is 0.4 mm. 2. C: 0.15 to 0.45% by weight, S
i: 0.05 to 0.50%, Mn: 0.2 to 2.5%,
S: 0.002 to 0.13%, Cr: 0.5 to 2.0
%, Nd: 0.005 to 0.1%, V: 0.05 to 0.
5%, Al: 0.005 to 0.3%, Mo + 0.5W:
0.02-0.3%, Ti: 0-0.2%, Zr: 0-0
0.2%, Nb: 0 to 0.2%, Pb: 0 to 0.35%
And Ca: 0 to 0.01%, with the balance being steel consisting of the chemical composition of Fe and unavoidable impurities, after hot working, spheroidizing and annealing to reduce the core hardness to Hv 180 or less, and then cold working to form a core. A method for producing a steel material for nitrocarburizing excellent in machinability, wherein the hardness is Hv250 or more and the decarburization depth is 0.1 to 0.4 mm from the surface of the steel material. 3. The surface hardness after nitrocarburizing is Hv600 or more, the effective hardening depth is 0.1mm or more, and the material is
A nitrocarburized component, which is a steel material for nitrocarburizing manufactured by the method according to claim 2.