JPH11181518A - Method of manufacturing steel for nitrocarburizing and nitrocarburized parts using the steel - Google Patents

Abstract

(57) [Problem] To provide a nitrocarburized component exhibiting excellent fatigue resistance and wear resistance, and a method for producing a nitrocarburized steel material having excellent machinability as a material thereof. [Solution] C: 0.15-0.45%, Si: 0.05-0.5%, M
n: 0.2-2.5%, S: 0.002-0.2%, Cu: 0.5-1.5%, N
i: 1.8 ≦ Cu / Ni ≦ 2.2 at 0.25 to 0.75%, Cr: 0.5 to 2%,
V: 0.05-0.5%, Ti: 0.04-1.0%, Al: 0.01-0.3%,
N ≦ 0.008%, Mo: 0 to 0.3%, W: 0 to 0.5%, Pb: 0 to 0.3
5%, Ca: 0-0.01%, the balance is a chemical composition of Fe and impurities. The maximum diameter of Ti carbosulfide in the steel is 10μm or less, and the amount of the steel is 0.05% or more in cleanliness. A method for producing a steel material for nitrocarburizing excellent in machinability, in which spheroidizing annealing is performed after the hot working to make the hardness Hv ≦ 180, and then cold working to make the hardness Hv ≧ 250. A nitrocarburized component whose material is a steel material for nitrocarburizing manufactured by the above method, has a surface hardness after nitrocarburizing of Hv ≧ 600 and an effective hardening depth of 0.1 mm or more.

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. (Note that, among the fatigue phenomena in which the material surface peels off due to repeated surface pressure load, those with relatively small peeling are often called "pitting" and those with relatively large peeling are called "spalling". This also applies to the specification.)

[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 part 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 steel proposed in Japanese Patent Publication No. 7, the machinability is poor,
After this is subjected to hot forging or cold forging, the cost of cutting for forming into a desired nitrocarburized component shape increases. For this reason, there is an increasing demand for a nitrocarburizing steel material having excellent machinability in order to facilitate cutting and reduce costs.

Conventionally, in order to enhance machinability, Pb,
Free-cutting elements such as Te, Bi, Ca and S have been used alone or in combination. However, J
When simply adding the above-mentioned free-cutting elements to IS standard steel or steel proposed in Japanese Patent Application Laid-Open No. 58-71357, it is often impossible to secure desired mechanical properties, especially fatigue strength. .

Iron and steel (vol. 57 (1971) S4)
84) reports that the addition of Ti to deoxidized adjusted free-cutting steel may enhance machinability. But T
It is also described that the addition of a large amount of i increases tool wear due to generation of a large amount of TiN, and is undesirable from the viewpoint of machinability. For example, C: 0.45%, S
i: 0.29%, Mn: 0.78%, P: 0.017
%, S: 0.041%, Al: 0.006%, N: 0.
Steel containing 0087%, Ti: 0.228%, O: 0.004%, and Ca: 0.001%, on the contrary, has a short drill life and poor machinability. Thus, the machinability is not improved simply by adding Ti to steel.

[0014]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and is excellent in that it is made of a steel excellent in machinability and cold workability, and is subjected to nitrocarburizing 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.

[0015]

The gist of the present invention resides in a method for producing a steel material for nitrocarburizing as shown in the following (1) and a nitrocarburized component using the steel material as shown in (2).

(1) In weight%, C: 0.15 to 0.45
%, Si: 0.05 to 0.5%, Mn: 0.2 to 2.5
%, S: 0.002 to 0.2%, Cu: 0.5 to 1.5
%, Ni: 0.25 to 0.75%, and 1.8 ≦ Cu
(%) / Ni (%) ≦ 2.2, Cr: 0.5 to 2%,
V: 0.05-0.5%, Ti: 0.04-1.0%,
Al: 0.01 to 0.3%, N: 0.008% or less, M
o: 0 to 0.3%, W: 0 to 0.5%, Pb: 0 to 0.
35%, Ca: 0 to 0.01%, the balance is a chemical composition composed of Fe and unavoidable impurities. The maximum diameter of Ti carbosulfide in steel is 10 μm or less, and the amount is 0.0% in cleanliness.
Steel having a hardness of 5% or more is subjected to spheroidizing annealing after hot working to a hardness of Hv 180 or less, and then cold worked to a hardness of Hv 180 or less.
A method for producing a steel material for nitrocarburizing excellent in machinability, wherein the material is 250 or more.

(2) The material is a steel material for nitrocarburizing manufactured by the method described in (1) above, and the surface hardness after nitrocarburizing is H.
A nitrocarburized component having a v600 or more and an effective hardening depth of 0.1 mm or more.

It should be noted that the "Ti carbosulfide" in the present invention includes simple Ti sulfide. Further, the “maximum diameter (of the carbosulfide of Ti)” refers to “the longest diameter of the individual carbosulfide of Ti”. The cleanliness of the Ti carbosulfide was determined according to JIS G 0555, with the magnification of the optical microscope set to 400 times.
Means a value measured in 60 visual fields according to the “microscopic test method for nonmetallic inclusions in steel” specified in the above.

Hereinafter, those described in the above (1) and (2) are referred to as the invention of (1) and the invention of (2), respectively.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted investigations and researches on the chemical composition of a steel material used as a material for a nitrocarburized component, and on an appropriate microstructure and mechanical properties in each manufacturing process. As a result, the following findings were obtained.

(A) 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. In order to improve the wear resistance, the surface hardness may be increased. On the other hand, in order to improve the spalling resistance, the effective hardening depth may be increased.

(B) A nitrocarburizing treatment is performed to make the surface hardness Hv
600 or more, if the effective curing depth is 0.1 mm or more,
The fatigue resistance, wear resistance, pitting resistance, and spalling resistance of the nitrocarburized component can be significantly improved.

(C) 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.

(D) A steel material is subjected to spheroidizing annealing to have a hardness of Hv1.
If it is reduced to 80 or less, the cold workability is improved and the mold life can be greatly improved.

(E) A steel material containing appropriate amounts of Cu and Ni is subjected to spheroidizing annealing to reduce the hardness to Hv 180 or less, and if the hardness is increased to Hv 250 or more by work hardening by cold forging, then nitrocarburizing is performed. Even when the treatment is performed, the core is not softened by heating during nitrocarburizing and the hardness of the core does not decrease. That is, the core hardness can be maintained at the value before soft nitriding or further increased. For this reason, since a high core hardness of Hv250 or more can be stably secured in the nitrocarburized component, fatigue resistance, especially bending fatigue resistance, is greatly improved.

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

(F) From the above (a) to (e), steel having excellent cold workability is used as a material steel, which is subjected to cold working to secure sufficient hardness by work hardening. To form a hard and deep nitrided layer. If the hardness for the work hardening (that is, the core hardness) can be maintained or further increased by heating for this soft nitriding, large fatigue resistance characteristics of the nitrocarburized component can be obtained. Abrasion resistance, pitting resistance and spalling resistance can be imparted.

(G) An appropriate amount of Ti is added to steel, and sulfides are changed to Ti carbosulfides to control inclusions in the steel.
If the i-carbosulfide is finely dispersed, the machinability of the steel material is dramatically improved. Therefore, as a result of further research, the following matters were found.

(H) Considering the balance with S, Ti
When Ti is actively added, Ti carbosulfide is formed in the steel.

(I) When the above-mentioned Ti carbosulfide is formed in the steel, the amount of MnS formed is reduced.

(J) When the S content in the steel is the same, T
i Carbosulfide has a greater machinability improving effect than MnS. This is based on the fact that the melting point of Ti carbosulfide is lower than that of MnS, so that the lubricating action on the rake face of the tool during cutting is increased.

(K) In order to sufficiently exert the effect of Ti carbosulfide, it is important to limit the N content to a low level. This is because if the N content is large, Ti is fixed as TiN, and the generation of Ti carbosulfide is suppressed.

(L) Ti carbosulfide generated during steelmaking is
At the heating temperature for normal hot working and the normal heating temperature in normalizing, it does not form a solid solution in the matrix. Therefore, a so-called "pinning action" is exhibited in the austenite region, which is effective in preventing austenite grains from becoming coarse.
Of course, Ti carbosulfide does not form a solid solution in the matrix even at the heating temperature of the nitrocarburizing treatment.

(M) In order to enhance machinability by Ti carbosulfide and to secure high strength, particularly high fatigue strength, the size of Ti carbosulfide and the amount expressed by its cleanliness (hereinafter referred to as “cleanness”) It is important to optimize the “cleanness”).

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, “%” of the component content means “% by weight”.

(A) Chemical composition of base steel C: 0.15 to 0.45% C combines with S together with Ti to form Ti carbosulfide and has an effect of improving machinability. Further, C is an element effective for securing the static strength. However, if the content is less than 0.15%, the desired static strength (core hardness after nitrocarburizing after cold working, ie, Hv250 or more as the core hardness of the nitrocarburized component as the final product) is secured. Can not. On the other hand, when the content exceeds 0.45%, the ductility and toughness of the core decrease, and the cold workability deteriorates. Further, the surface hardness and hardening depth after nitrocarburizing are rather reduced. Therefore, the content of C is 0.1
5 to 0.45%.

Si: 0.05-0.5% Si has an effect of improving the hardenability of steel and improving the static strength. However, its content is 0.05%
If it is less than the above, the desired static strength described above cannot be secured. On the other hand, if it exceeds 0.5%, the toughness is degraded, which adversely affects the cold workability. Therefore, the content of Si is set to 0.1.
05 to 0.5%.

Mn: 0.2 to 2.5% Mn is an element effective for improving hardenability and ensuring core strength. However, if the content is less than 0.2%, the effect of the addition is poor, while if it exceeds 2.5%, segregation occurs and the cold workability is deteriorated. Therefore, M
The content of n was set to 0.2 to 2.5%. Note that the content of Mn is preferably set to 0.5 to 1.5%.

S: 0.002 to 0.2% S combines with C and Ti to form Ti carbosulfide and has an effect of improving machinability. However, if the content is less than 0.002%, the desired effect cannot be obtained.

Conventionally, the purpose of adding S to free-cutting steel is to add Mn
The purpose is to improve the machinability by forming S. However, according to the study of the present inventors, it has been found that the above-described action of improving the machinability of MnS is based on a function of enhancing lubricity between chips and the tool surface during cutting. In addition, MnS increases in size, increases the ground flaw of the steel material main body, and sometimes becomes a defect. In the present invention, the machinability improving action of S is based on an appropriate amount of C
Can be obtained for the first time by forming Ti carbosulfide by complex addition of Ti and Ti. For this purpose, the content of S is required to be 0.002% or more as described above. On the other hand, S
Contains more than 0.2% does not change the effect on machinability, but coarse MnS occurs again in the steel, causing problems such as ground flaws. Further, hot workability is significantly deteriorated, so that hot plastic working becomes difficult, and toughness may decrease. Therefore, the content of S is set to 0.0
02 to 0.2%. The preferred content of S is 0.00
4 to 0.1%.

Cu: 0.5 to 1.5% Cu is an important element in the present invention, and has an effect of finely precipitating during hard nitriding treatment to harden the steel. For this reason, the steel material to be treated can maintain the hardness before the soft nitriding without being softened by heating for the soft nitriding, and in some cases, hardens in reverse. The above-mentioned effect of Cu is particularly exhibited in a steel material in which the hardness is reduced to Hv180 or less by spheroidizing annealing, and the hardness is increased to Hv250 or more by the working effect of cold forging. However, if the content is less than 0.5%, a sufficient amount is not finely precipitated, so that the effect of addition is poor. On the other hand, when the content exceeds 1.5%, not only the above effect is saturated, but also the hot workability is deteriorated. Therefore,
Cu content was set to 0.5 to 1.5%.

Ni: 0.25 to 0.75% Ni has the effect of completely dissolving the above-mentioned Cu in the matrix and sufficiently exhibiting the precipitation hardening effect of Cu during the nitrocarburizing treatment. This effect is remarkable when the ratio of Cu (%) / Ni (%) described later is 1.8 to 2.2. However, Ni
If the content is less than 0.25%, the effect of addition is poor.
Even if the content exceeds 75%, the above effect is saturated. Therefore, the content of Ni is set to 0.25 to 0.75%.

Cu (%) / Ni (%): 1.8 to 2.2 When the value of Cu (%) / Ni (%) is 1.8 to 2.2, an appropriate amount of Cu and Ni By the complex addition, Cu completely dissolves in the matrix, and the effect of Cu that precipitates and hardens during the nitrocarburizing treatment becomes remarkable. Therefore, Cu (%) / N
The value of i (%) was set to 1.8 to 2.2. Note that Cu
(%) / Ni (%) is preferably 1.9 to 2.1.

Cr: 0.5% to 2% Cr is an element that is extremely effective in increasing the surface hardness and increasing the hardening depth by combining with N that enters 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, C
If r is contained in excess of 2%, the surface hardness becomes too high due to soft nitriding, so that the hardness gradient from the surface to the core becomes sharp, and spalling resistance and pitting resistance are rather reduced. Will deteriorate. Therefore, the content of Cr is set to 0.5 to 2%.

V: 0.05-0.5% V combines with N and C invading from the steel material surface during the nitrocarburizing treatment and precipitates as fine vanadium carbonitride, thereby increasing the surface hardness. Further, 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 less than 0.05%, the effect of addition is poor. On the other hand, when the V content exceeds 0.5%, not only the above effect is saturated but the cost is increased but also the embrittlement phenomenon appears. It will be cool. Therefore, the V content is set to 0.05 to 0.5%. The V content is 0.1 to
Preferably, it is 0.3%.

Ti: 0.04 to 1.0% Ti is an important alloying element for controlling inclusions in the present invention. If the content is less than 0.04%, S cannot be sufficiently converted to Ti carbosulfide, so that machinability cannot be enhanced. On the other hand, if the content exceeds 1.0%, not only the machinability improving effect is saturated but the cost is increased, but also the toughness and hot workability are significantly deteriorated. Therefore, the Ti content is set to 0.04 to 1.0%. In order to stably obtain good machinability and toughness, Ti
Is preferably 0.06 to 0.8%.

Al: 0.01 to 0.3% 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, if the content is less than 0.01%, the above effects cannot be expected. On the other hand, if it exceeds 0.3%, the curing depth is reduced. Therefore, the content of Al is set to 0.01 to 0.3%. The Al content is 0.0
It is preferable to set it to 1 to 0.15.

N: 0.008% or less In the present invention, it is extremely important to control the N content to a low level. That is, since N has a large affinity for Ti, it easily bonds with Ti to form TiN and fixes Ti, so that when N is contained in a large amount, N is coated with the above-mentioned Ti carbosulfide. As a result, the effect of improving machinability cannot be sufficiently exhibited. Further, coarse TiN reduces toughness and machinability. Therefore, the N content is 0.008
% Or less. In order to enhance the effect of Ti carbosulfide, the upper limit of the N content is preferably set to 0.006%.

Mo: 0 to 0.3% Mo may not be added. If added, it has the effect of increasing the hardenability of the steel and increasing the softening resistance of the core during nitrocarburizing. To ensure this effect, Mo should be 0.0
The content is preferably 2% or more. However, if the content exceeds 0.3%, the above effect is saturated and the cost is only increased. Therefore, the content of Mo is 0 to
0.3%.

W: 0 to 0.5% W may not be added. If added, it has the effect of increasing the hardenability of the steel and increasing the softening resistance of the core during nitrocarburizing. To ensure this effect, W should be 0.05%
It is preferable to set the content as described above. However, if the content exceeds 0.5%, the above effect is saturated and the cost is increased. Therefore, the content of W is set to 0 to 0.5.
%.

Pb: 0 to 0.35% Pb may not be added. If added, it has the effect of further increasing the machinability of the steel. In order to surely obtain this effect, the content of Pb is preferably set to 0.03% or more. However, when Pb is contained in excess of 0.35%, hot workability is deteriorated, and cracks often occur during hot working such as hot rolling and hot forging. Therefore, P
The content of b was set to 0 to 0.35%.

Ca: 0 to 0.01% Ca may not be added. If added, it has the effect of further increasing the machinability of the steel. 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) Size and cleanliness of Ti carbosulfide In order to increase the machinability of steel having the above chemical composition by Ti carbosulfide and to secure a large strength,
It is important to optimize the size and cleanliness of Ti carbosulfide.

When the maximum particle size of the Ti carbosulfide exceeds 10 μm, the fatigue strength is reduced. It is preferable that the maximum diameter of Ti carbosulfide be 7 μm or less. This Ti
If the maximum diameter of the carbosulfide is too small, the effect of improving machinability is reduced. Therefore, the lower limit of the maximum diameter of the Ti carbosulfide is preferably about 0.5 μm.

If the amount of Ti carbosulfide having a maximum diameter of 10 μm or less is less than 0.05% in cleanliness, the effect of improving the machinability by Ti carbosulfide cannot be exhibited. Preferably, the cleanliness is 0.08% or more. If the value of the cleanliness of the Ti carbosulfide is too large, the fatigue strength may decrease. Therefore, the upper limit of the cleanliness of the Ti carbosulfide is preferably about 2.0%.

In order to set the size and cleanliness of the Ti carbosulfide to the above-mentioned values, it is important to prevent the Ti oxide from being excessively formed. As a steel making method for this purpose, for example, there is a method of sufficiently deoxidizing with Si and Al, and finally adding Ti.

The Ti carbosulfide was obtained by mirror-polishing a test piece taken from a steel material, and setting the polished surface as a test surface to a magnification of 4.
When observed with an optical microscope at a magnification of 00 or more, it can be easily distinguished from other inclusions based on the color and shape. That is, when observed with an optical microscope under the above conditions, the "color" of the Ti carbosulfide is very light gray, and the "shape" is recognized as a granular shape (spherical shape) corresponding to JIS B-based inclusions. The detailed determination of Ti carbosulfide can also be performed by observing the test surface with a microscope having an analysis function such as EDX (energy dispersive X-ray analyzer).

As described above, the cleanliness of the Ti carbosulfide was determined by setting the magnification of the optical microscope to 400 times and setting the JIS G
It refers to the value measured in 60 visual fields by the “microscopic test method for non-metallic inclusions in steel” specified in 0555.

(C) Spheroidizing annealing The spheroidizing annealing is performed by hot working (for example, a steel material having the chemical composition shown in (A) above and the size and cleanliness of Ti carbosulfide shown in (B) above). After hot rolling or hot forging), the hardness is reduced to increase cold workability, thereby significantly improving the mold life and reducing the production cost of the required nitrocarburized parts as final products. This is an essential process to keep it low.

If the hardness after spheroidizing annealing exceeds 180 in Hv, the life of the mold is greatly reduced, and the production cost of the desired nitrocarburized component as the final product is significantly increased. Therefore, the hardness after spheroidizing annealing must be Hv180 or less. The lower limit of the hardness of the spheroidizing annealing does not need to be particularly limited.

The spheroidizing annealing may be performed by a usual method.

(D) Cold work The steel material of (C), whose hardness has been adjusted to Hv 180 or less by spheroidizing annealing, is then cold worked to finish it into a rough shape of a desired nitrocarburized part, and further cut. To form the desired nitrocarburized component. Of course, precision cold working may be performed to finish the desired nitrocarburized component without cutting, or after spheroidizing annealing, cutting may be performed before or after cold working to obtain the desired nitrocarburized component. It may be finished in a shape.

The “soft-nitriding steel material” according to the invention (1) is formed by cold working and cutting (or precision cold working) into a desired shape.
It is the one before soft nitriding.

The above-mentioned cold working may be performed by a usual method such as cold forging, cold rolling or cold drawing, but the hardness of the worked part must be Hv250 or more. This is because the steel material (C) whose hardness has been adjusted to Hv180 or less has a hardness of Hv180 due to cold working.
If the hardness is increased to 250 or more, the hardness of the core portion does not decrease even if the nitrocarburizing treatment is performed, and the hardness before nitrocarburizing can be maintained or the hardness before nitrocarburizing can be increased.

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. There is no.

In order to cold-work a steel material whose hardness has been adjusted to Hv 180 or less by spheroidizing annealing shown in (C) above and to have a hardness of Hv 250 or more, a work of 20% or more in reduction of area is added. The dimensions should be adjusted as follows.

The upper limit of the hardness after cold working does not need to be particularly limited. In other words, processing may be performed with the maximum reduction in area that can be added to the equipment to achieve extremely high hardness.

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

(E) Soft nitriding A part formed into a required shape by performing the cold working of the above (D), or performing the cold working and cutting work before and / or after the cold working (steel material for soft nitriding) Is further subjected to a soft nitriding treatment thereafter. This nitrocarburizing method need not be limited at all, and may be performed by a normal method. Carbide nitriding treatment, surface hardness Hv600 or more, effective hardening depth 0.1
If it is not less than mm, the fatigue resistance, wear resistance, pitting resistance and spalling resistance of the nitrocarburized component can be significantly improved.

The cold working shown in the above (D), or
The part (steel material for nitrocarburizing) that has been subjected to cold working and before or / and after that is nitrocarburized to have a surface hardness of Hv6.
To make the effective hardening depth 0.1 mm or more,
For example, the part may be held in a gas at about 570 ° C. in which ammonia is added to RX gas for 3 to 9 hours, and then cooled in oil.

The upper limits of the surface hardness and the effective hardening depth after soft nitriding need not be particularly limited. However, the upper limit of the surface hardness after soft nitriding is preferably about Hv900.

The nitrocarburized part according to the invention of (2) is a steel having the chemical composition of (A) and the size and cleanliness of the Ti carbosulfide shown in (B), which is a raw steel, for example, is made of ordinary steel. After smelting by the method, hot rolling or forging, normalizing if necessary, spheroidizing annealing shown in (C), and then cold working shown in (D), or (D)
In this case, a desired part shape is formed by the cold working and the cutting work before and / or after the cold working as described in (1), then the nitrocarburizing treatment is performed, and then, if necessary, grinding and polishing are further performed.

Here, in the material steel having the chemical composition targeted by the present invention, the structure in the region from at least the surface layer to a depth of more than 0.5 mm from the surface layer by normalizing after hot working is changed to the structure containing bainite ( A bainite single-phase structure or a mixed structure of at least one of bainite and ferrite, pearlite, and martensite) improves the spheroidization rate of carbide (mainly cementite) after spheroidizing annealing. Therefore, the hardness before cold working can be greatly reduced by spheroidizing annealing. Reducing the hardness of the steel 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 for manufacturing a steel material for soft nitriding of the invention (1), it is preferable to perform normalizing after hot working and then spheroidizing annealing.

[0075]

EXAMPLE 180 kg of steel having the chemical composition shown in Tables 1 and 2 was vacuum-melted by an ordinary method. In addition, steel 18
Excluding Si and A to prevent the formation of Ti oxide
At the end of adding 1 and deoxidizing enough and adding various elements, Ti was added to adjust the size and cleanliness of Ti carbosulfide. For steel 18, Ti was added simultaneously with deoxidation with Si and Al.

Steels 1 to 9 in Table 1 are steels of the examples of the present invention whose chemical compositions are within the range specified in the present invention, and Steels 10 to 20 in Table 2 are steels having a content of any of the components specified in the present invention. It is a steel of a comparative example out of the range. Among the steels of the comparative examples, steels 19 and 20 are steels corresponding to SCM435 and SACM645 of JIS standard, respectively, with Ti added.

[0077]

[Table 1]

[0078]

[Table 2]

Next, these steels were made into billets by a usual method, and then heated to 1250 ° C.
Hot forging at a temperature of 950 ° C., diameter 30 mm and 38
mm round bar. Thereafter, 870 to 870 depending on the C content.
Normalizing was performed at 925 ° C., and then spheroidizing annealing was performed according to the heat pattern shown in FIG.

For comparison, steels 3 and 9 were also prepared as hot forged, that is, spheroidized and annealed without performing normalizing after hot forging.

(Example 1) The following various investigations were carried out using the round bar having a diameter of 30 mm obtained as described above.

That is, from the round bar as hot forged, JI
A test piece was sampled according to FIG. 1 of SG 0555, and a mirror-polished test surface having a width of 15 mm and a height of 20 mm was placed at a magnification of 40.
By observing 60 visual fields with a 0-magnification optical microscope, the cleanliness of the Ti carbosulfide was measured while separating it from other inclusions. Ti
The maximum diameter of the carbosulfide was also investigated by observing 60 visual fields with an optical microscope having a magnification of 400 times.

From the as-normalized round bar, the diameter was 30 mm.
, A test piece having a thickness of 20 mm was cut out, corroded with nital, and observed under an optical microscope with a magnification of 400 times.

From each round bar after spheroidizing annealing, a diameter of 30
A 20 mm thick, 20 mm thick hardness test piece and a 10 mm diameter, 15 mm long cold test piece were prepared.

Using the above-mentioned hardness test piece, the Hv hardness of the central portion was measured by a micro Vickers hardness meter.

Further, using the above-mentioned 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, each round bar having a diameter of 30 mm after the spheroidizing annealing obtained as described above was subjected to peeling processing to a diameter of 25 mm, and thereafter, was subjected to cold (room temperature) by an ordinary method.
To 20.9 mm (30.1% reduction in area) using a draw bench. Next, the temperature at which ammonia gas was added to the RX gas at a ratio of 1: 1 was 570 ° C.
, A soft nitriding treatment was carried out for 6 hours, 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. The diameter is 20.9m from the soft-nitrided round bar.
m and a thickness of 20 mm were prepared as a hardness test piece, and the surface hardness (0.025 m from the surface) was measured with 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.

A drill drilling test was also performed to evaluate the machinability. 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, feed was made 0.
The measurement was performed at 15 mm / rev and a rotation speed of 980 rpm.

Table 3 summarizes the results of various tests.

[0091]

[Table 3]

Table 3 shows that the chemical composition and the maximum diameter were 10 μm.
m, the hardness after spheroidizing annealing is less than 180 in any of the steels 1 to 9 of the present invention in which the cleanliness of Ti carbosulfides of m or less is within the range specified in the present invention. Thus, the limit upsetting ratio exceeds 80%, and the machinability is also good. The hardness exceeding Hv250 is easily obtained by cold working (drawing) with a surface reduction rate of 30.1%, and the machinability after cold drawing is also good. Furthermore, after soft nitriding, a surface hardness exceeding Hv600 and an effective hardening depth exceeding 0.1 mm have been obtained,
In addition, even after a heat treatment at 570 ° C. for 6 hours for nitrocarburizing, the hardness at the center (core hardness) is maintained at a level before nitrocarburizing or is higher than that before nitrocarburizing. .

On the other hand, when the steel of the comparative example is used as a material, (a) the hardness after spheroidizing annealing exceeds Hv180, and (b) the hardness after cold working is low, The core hardness is also low. (C) The hardness after cold working exceeds Hv250, but the core hardness after nitrocarburizing is lower than Hv250. (D) The surface hardness after nitrocarburizing is lower than Hv600.
(E) the effective hardening depth after nitrocarburizing is less than 0.1 mm,
(F) The number of through holes in the drilling test is significantly less than 100 and is inferior in machinability. For this reason, the mold life at the time of cold forging is short, the mold cost increases, and the cutting cost for forming into a desired nitrocarburized part shape also increases. It will be extremely expensive. Alternatively, the fatigue resistance, wear resistance, pitting resistance, and spalling resistance of the nitrocarburized parts are inferior even though the manufacturing cost is low.

(Example 2) The following various investigations were carried out using the round bar having a diameter of 38 mm obtained as described above.

That is, as in the case of Example 1, a test piece was sampled from a hot-forged round bar in accordance with FIG. 1 of JIS G 0555, and the mirror-polished width was 15 mm and the height was 20 m.
The test surface of m was observed with an optical microscope having a magnification of 400 times for 60 visual fields, and the cleanliness was measured while separating Ti carbosulfide from other inclusions. The maximum diameter of Ti carbosulfide was also investigated by observing 60 visual fields with an optical microscope having a magnification of 400 times.

From each round bar after spheroidizing annealing, a diameter of 38
A hardness test piece having a thickness of 20 mm and a thickness of 20 mm was prepared, and the Hv hardness of the center portion was measured using a micro Vickers hardness meter.

Further, each round bar having a diameter of 38 mm after the spheroidizing annealing was peeled to a diameter of 36 mm, and thereafter, was cold (room temperature) with a diameter of 30 mm (area reduction rate of 3) by a usual method.
0.6%) using a draw bench. Thereafter, the rolling fatigue test piece (small roller) shown in FIG.
Ono-type rotating bending fatigue test piece with JIS Z2
274 D = 10 mm, d = 8 mm, ρ = t = 1 mm, D ₀
= 12 mm test piece).

Next, each of the test pieces was heated to a temperature of 570 ° C. by adding ammonia gas to RX gas at a ratio of 1: 1.
, A soft nitriding treatment was carried out for 6 hours, and then cooled into oil. In addition, the above-mentioned process was also performed simultaneously with the thing of 30 mm in diameter x 100 mm in length as it was drawn cold.

The diameter of the round bar as drawn is 30 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.
A hardness test piece having a diameter of 30 mm and a thickness of 20 mm was also prepared from the soft-nitrided round bar, and the surface hardness (Hv hardness at a position of 0.025 mm from the surface) and the effective hardening depth were measured with a micro Vickers hardness meter. Sa (Hv50 from the surface
0) and the hardness at the center.

On the other hand, fatigue characteristics were investigated using the Ono-type rotating bending fatigue test piece and the rolling fatigue test piece subjected to the nitrocarburizing treatment.

That is, an Ono-type rotating bending fatigue test was performed at room temperature (room temperature) and in the air at a rotation speed of 3000 rpm, and the bending fatigue strength (fatigue limit) was determined.

The surface fatigue strength was determined using a roller pitting tester under the conditions of a rotation speed of 1,000 rpm, a lubricating oil temperature of 80 ° C., and a slip ratio of 40%. The hardness of the mating roller is adjusted to 61 by Rockwell C hardness (HRC) using JIS SUJ2, and the outer diameter is 13 mm.
One processed to 0 mm, an inner diameter of 45 mm, and a thickness of 18 mm was used. Then, the surface pressure capable of rotating 10 ⁷ times under the test conditions described above was evaluated as “surface fatigue strength”.

Table 4 summarizes the results of various tests.

[0104]

[Table 4]

From Table 4, it can be seen that the chemical composition and the maximum diameter are 10 μm.
m of Ti carbosulfides having a cleanliness of not more than m is within the range specified in the present invention, the steels 1 to 9 of the present invention as in the case of the spheroidizing annealing as in Example 1 above. The hardness is below 180 in Hv. Then, a hardness exceeding Hv250 is easily obtained by cold working (drawing processing) with a surface reduction rate of 30.6%. Furthermore, after nitrocarburizing, a surface hardness of more than Hv600 and an effective hardening depth of more than 0.1 mm are obtained. (Core hardness) is maintained at a level before nitrocarburizing or higher than that before nitrocarburizing.

Further, the bending fatigue strength is 55 kgf / mm ²
It has the above values, and the surface fatigue strength is also a value exceeding 245 kgf / mm ² .

On the other hand, when the steel of the comparative example is used as the material, (a) the hardness after spheroidizing annealing exceeds Hv180, and (b) the hardness after cold working is low, The core hardness is also low. (C) The hardness after cold working exceeds Hv250, but the core hardness after nitrocarburizing is lower than Hv250. (D) The surface hardness after nitrocarburizing is lower than Hv600.
(E) the effective hardening depth after nitrocarburizing is less than 0.1 mm,
Corresponds to one or more of the above. Furthermore, the bending fatigue strength is at most 46 kgf / mm ² , which is clearly inferior to the case where the steel material of the present invention is used as a material.

[0108]

The nitrocarburized parts of the present invention are excellent in fatigue resistance, wear resistance, pitting resistance and spalling resistance, so that they have great fatigue strength and wear resistance such as gears for automobiles and industrial machines. Is required. In addition, since a high core hardness of Hv250 or more can be stably ensured, it can also be used for parts requiring particularly high bending fatigue strength. The steel material for nitrocarburizing, which is excellent in machinability and is used as the material for the nitrocarburized component, can be produced 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.

FIG. 2 is a view showing the shape of a rolling fatigue test piece used in an example.

──────────────────────────────────────────────────続 き Continued on front page (51) Int.Cl. ⁶ Identification code FI C23C 8/32 C23C 8/32

Claims (2)

[Claims] C. 0.15 to 0.45% by weight, S
i: 0.05 to 0.5%, Mn: 0.2 to 2.5%,
S: 0.002-0.2%, Cu: 0.5-1.5%,
Ni: 0.25 to 0.75%, and 1.8 ≦ Cu
(%) / Ni (%) ≦ 2.2, Cr: 0.5 to 2%,
V: 0.05-0.5%, Ti: 0.04-1.0%,
Al: 0.01 to 0.3%, N: 0.008% or less, M
o: 0 to 0.3%, W: 0 to 0.5%, Pb: 0 to 0.
35%, Ca: 0 to 0.01%, the balance is a chemical composition composed of Fe and unavoidable impurities. The maximum diameter of Ti carbosulfide in steel is 10 μm or less, and the amount is 0.0% in cleanliness.
Steel having a hardness of 5% or more is subjected to spheroidizing annealing after hot working to a hardness of Hv 180 or less, and then cold worked to a hardness of Hv 180 or less.
A method for producing a steel material for nitrocarburizing excellent in machinability, wherein the material is 250 or more. 2. A steel material for nitrocarburizing manufactured by the method according to claim 1, wherein the surface hardness after nitrocarburizing is Hv600.
A nitrocarburized component having an effective hardening depth of 0.1 mm or more.