JP2023037454A - Carburized part and manufacturing method thereof - Google Patents

Abstract

To provide a carburized part with excellent low cycle fatigue strength and a method of manufacturing the same.SOLUTION: A core contains, in mass%, C: 0.10-0.30%, Si: 0.03-0.80%, Mn: 0.40-1.30%, P: 0.005-0.020%, S: 0.003-0.060%, Cr: 0.10-2.00%, Al: 0.050% or less, N: 0.0030-0.0300%, and O: 0.0020% or less, the remainder is Fe and impurities, and a ratio of the grain boundary P concentration Pb, which is P content of the grain boundary at a depth of 50 μm below the surface of the part, to the steel P concentration Pm, which is the P content of the core, Pb/Pm is 30.00 or less, the Vickers hardness at a depth of 0.10 mm below the surface of the part is 650 HV or more, and the Vickers hardness at a depth of 1.5 mm below the surface of the part is 300 HV or more.SELECTED DRAWING: None

Description

The present invention relates to carburized parts and methods of making same.

In recent years, from the viewpoint of improving the fuel efficiency of automobiles, there has been an increasing demand for downsizing and weight reduction of parts for mechanical structures such as gear parts (gear parts). In particular, gear parts such as differential gears and transmission gears of automobiles are often subjected to shock loads when the vehicle suddenly starts or stops or runs over a step on the road surface. Therefore, such gear components may fail due to low cycle fatigue, leading to failure after a very small number of repetitions, such as tens to thousands of cycles. Therefore, parts used for these applications are required to have improved strength, particularly strength against low-cycle fatigue fracture.

Many of the above parts are manufactured by machining a steel material into a predetermined shape and then performing a carburizing and quenching treatment. Most of the steel materials used in this case are alloy steel materials for machine structures specified in JIS G 4053:2008. In general, for example, by using case-hardening steel such as SCr420 or SCM420 as the steel material, the toughness of the core of the part is secured, and the surface layer of the part is reduced to C: 0.8 by carburizing and quenching and low-temperature tempering at around 180 ° C. It has a tempered martensite structure of about 10%, and the bending fatigue strength and wear resistance are enhanced.

Various gear steels, gear parts, and manufacturing methods thereof have been proposed for the purpose of improving low-cycle bending fatigue strength. Low-cycle fatigue fractures of gear parts occur at grain boundaries in the surface layer as fracture initiation points. For this reason, for example, steel for gears in which impurity elements such as P and S are reduced in order to strengthen the austenite grain boundaries of the carburized hardened layer, gear parts with improved impact resistance due to the refinement of the austenite grain size, and methods for manufacturing the same is proposed.

For example, in Patent Document 1, after carburizing and quenching, the entire steel is reheated to the austenite region and then quenched to refine the austenite grain size of the carburized hard layer, thereby increasing the impact resistance. In addition, Patent Document 1 proposes adding Ni, Mo and B to increase the grain boundary strengthening effect.

Further, in Patent Document 2, a first step of carburizing the material, a second step of cooling below the austenitizing temperature after the first step, and a surface carburized portion of the material cooled in the second step and a carburizing and quenching method comprising a third step of rapidly heating the inside of the material immediately above the austenitizing temperature, and a fourth step of performing quenching treatment following the third step, to make the austenite grain size #10 or more, In addition, a carburizing and quenching method is proposed to prevent the segregation of P, S, carbides, etc., which lower the grain boundary strength.

In addition, in Patent Document 3, the amount of grain boundary segregation per unit volume is reduced by specifying the amount of P in the steel material to be more than 0.015% and 0.030% or less, and further refining the austenite grain size by induction hardening. A steel material and a method for manufacturing the same have been proposed.

JP-A-8-92690 JP 2005-048292 A JP-A-9-241798

However, the conventional technique described above has the following problems.
In Patent Document 1, it is mentioned that grain boundary strength is improved by refining crystal grains by reheating and quenching and short-time heat treatment. However, in Patent Document 1, reheating and quenching after carburizing are performed by oil cooling. As a result, the P concentration at the grain boundaries of the obtained parts increases during oil cooling, and the grain boundary strength may decrease. In other words, the part described in Patent Document 1 is insufficient to significantly improve the low cycle fatigue strength compared to the conventional parts.

Patent Literature 2 mentions that the short-time heat treatment improves the grain boundary strength of the resulting parts. However, in Patent Document 2, since oil cooling is used for cooling during quenching, the P concentration in the grain boundaries of the obtained parts may increase and the grain boundary strength may decrease. In other words, the component described in Patent Document 2 is insufficient to significantly improve the low cycle fatigue strength over the conventional one.

In Patent Document 3, it is mentioned that grain boundary segregation amount per unit volume is reduced by refining crystal grains by induction hardening. However, with the technique of Patent Document 3, the reduction of the grain boundary P concentration is insufficient, and the low cycle fatigue strength cannot be significantly improved as compared with the conventional technique.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a carburized component having excellent low-cycle fatigue strength and a method for manufacturing the same.

As a result of intensive research to solve the above problems, the inventors have found the following new findings.

(a) In order to increase the low cycle bending fatigue strength of carburized parts, the grain boundary P concentration (hereinafter referred to as the grain boundary P concentration It is effective to reduce the ratio of the P concentration in the core (hereinafter referred to as the steel P concentration) to 30.00 or less.

(b) In order to increase the low cycle bending fatigue strength of the carburized part, the Vickers hardness at 0.10 mm below the surface is 650 HV or more, the Vickers hardness at 1.5 mm below the surface is 300 HV or more, and the surface layer It is effective that the structure of is composed of tempered martensite and retained austenite.

(c) In order to reduce the grain boundary P concentration/steel material P concentration to 30.00 or less, the following equations (1) and (2) are satisfied for parts having a predetermined steel composition after carburizing treatment. It is important to carry out induction hardening.

1000<T1 (1)
50<y (2)
However, T1 in the above formulas (1) and (2) is the quenching temperature (°C) on the surface of the part, and y is the average cooling rate (°C/sec) from the highest heating temperature to 500°C during the induction hardening process. .

Here, the finding of (c) above will be described in more detail.
In the academic field of grain boundary segregation, it is generally known that in simple compositions such as Fe—P and Fe—C, the amount of grain boundary segregation tends to decrease as the temperature increases, and the amount tends to increase as the temperature decreases. It is However, it was not clear whether a similar grain boundary segregation tendency with respect to temperature is observed in practical steels such as carburized parts and steels for carburizing, which contain a large amount of alloying elements in addition to grain boundary segregation elements.

Therefore, as a result of intensive research using the practical steel, the present inventors found that the higher the quenching temperature, the lower the grain boundary P concentration, and that even if the quenching temperature is increased, the average cooling up to 500 ° C. We found that the grain boundary P concentration in the austenite grain boundaries increases when the rate is lower than 50°C/sec.

Carburized parts are generally subjected to low-temperature tempering at about 180° C. for about 2 hours after quenching. That is, the inventors have found that in order to reduce the grain boundary P concentration/steel material P concentration, it is important to increase the temperature of the quenching process and to perform high-speed cooling by water cooling.

However, it is industrially difficult to raise the temperature of the carburizing process and cool it with water. The reason for this is that the carburizing process and temperature rise take a long time, and in addition, if the carburizing process is held at a high temperature, the austenite grains become coarser and the fatigue strength decreases. Furthermore, in the first place, it is difficult to heat the carburizing process to 1000° C. or higher, and it is difficult to perform water cooling. Therefore, the present inventors focused on "induction hardening treatment", which is considered to be industrially feasible for hardening from a high temperature and water cooling with a sufficiently high cooling rate, which is performed after the carburizing process.

According to the studies of the present inventors, the grain boundary P concentration of the austenite grain boundaries in the steel material is reduced by heating to a high temperature exceeding 1000° C. in the step of induction hardening. Furthermore, by setting the average cooling rate to more than 50 ° C./sec when quenching from a high temperature of more than 1000 ° C., it is possible to suppress the increase in the grain boundary P concentration of the austenite grain boundary during quenching, and as a result, the grain boundary strength is high. state can be maintained. Further, quenching is more preferably water cooling, which can relatively easily achieve an average cooling rate of more than 50°C/sec. In the induction hardening process after carburizing, if hardening from high temperature is not performed or if the average cooling rate is slowed, the grain boundary P concentration in the austenite grain boundaries increases and the grain boundary strength decreases.

The present invention was obtained as a result of further detailed studies based on the above findings, and the gist thereof is as follows.

(1) A carburized part according to an aspect of the present invention has a core composition of, in mass %,
C: 0.10 to 0.30%,
Si: 0.03 to 0.80%,
Mn: 0.40-1.30%,
P: 0.005 to 0.020%,
S: 0.003 to 0.060%,
Cr: 0.10 to 2.00%,
Al: 0.050% or less,
N: 0.0030 to 0.0300% and O: 0.0020% or less, the balance being Fe and impurities,
The ratio P _b /P _{m of the grain boundary P concentration P b} , which is the amount of P in the grain boundary at a position 50 μm below the surface of the part, to the P concentration P _m of the steel material, which is the amount of P in the core, is 30.00 or _less . ,
The Vickers hardness at a depth of 0.10 mm below the surface of the part is 650 HV or more,
The Vickers hardness at a depth of 1.5 mm below the part surface is 300 HV or more.
(2) In the carburized part of (1) above, the composition of the core further comprises, in mass %,
Mo: 0 to 1.00%,
V: 0 to 0.50%,
Cu: 0-0.50%,
Ni: 0 to 1.00%,
Bi: 0 to 0.10%,
Nb: 0 to 0.100%,
Ti: 0-0.200%, and B: 0-0.005%
You may contain 1 type(s) or 2 or more types among.

(3) A method for manufacturing a carburized component according to an aspect of the present invention is the method for manufacturing a carburized component according to (1) above,
The composition, in mass %,
C: 0.10 to 0.30%,
Si: 0.03 to 0.80%,
Mn: 0.40-1.30%,
P: 0.005 to 0.020%,
S: 0.003 to 0.060% or less,
Cr: 0.10 to 2.00%,
Al: 0.050% or less,
N: 0.003 to 0.030% or less,
A forming step of forming a steel material containing O: 0.0020% or less and the balance being Fe and impurities into a part shape;
a carburizing step of performing carburizing treatment at a carburizing temperature of 870° C. to 1050° C. and then cooling from the austenite region;
An induction hardening step of performing induction hardening treatment under conditions that satisfy the following formulas (1) and (2);
Then, a tempering step of tempering at 130 to 200 ° C.;
have
1000<T1 (1)
50<y (2)
However, T1 in the above formulas (1) and (2) is the quenching temperature (°C) on the surface of the part, and y is the average cooling rate (°C/sec) from the highest heating temperature to 500°C during the induction hardening process. .
(4) The method for manufacturing a carburized part according to (3) above, wherein the composition is, in mass%,
Mo: 0 to 1.00%,
V: 0 to 0.50%,
Cu: 0-0.50%,
Ni: 0 to 1.00%,
Bi: 0 to 0.10%,
Nb: 0 to 0.10%,
Ti: 0-0.20%, and B: 0-0.0050%
You may contain 1 type(s) or 2 or more types among.

(5) The carburized part of (1) or (2) above may be a machine structural part.

With the carburized part according to the present embodiment, it is possible to provide a carburized part having excellent low cycle fatigue strength and a manufacturing method thereof. Therefore, the carburized part of the present invention is suitable, for example, as a gear for an automobile or an industrial machine, especially a machine powered by an electric motor.

FIG. 1 shows a cantilever fatigue test piece. The unit of dimensions in the drawing is mm. FIG. 2A is a diagram showing the locations for taking Auger specimens from cantilever fatigue specimens. FIG. 2B is a diagram showing an Auger test strip. The unit of dimensions in the drawing is mm.

A carburized component and a manufacturing method thereof according to an embodiment of the present invention will be described in detail below. In addition, "%" of the content of each component element means "% by mass" unless otherwise specified.

The carburized component (hereinafter, sometimes simply referred to as “component”) according to the present embodiment includes a core portion (hereinafter, sometimes simply referred to as “core portion”), which is the central portion in the depth direction of the component, and a stiffening layer located on the surface of the part.
Here, the core portion refers to a portion where carbon penetration did not reach due to the carburizing treatment and a portion where the structure did not undergo martensite transformation due to induction hardening. In other words, the core part is a region where there is no change in the chemical composition and metallographic structure, or the change is negligibly small, despite the carburizing and induction hardening treatments, and has the same chemical composition as the base material of the part. It is a part with The composition of the core can also be said to be, for example, the composition at a depth of 1.5 mm from the part surface.

[Chemical composition]
The chemical composition of the core of the carburized component of this embodiment will be described. Normally, the composition of the core of a carburized part is the same as the composition of the material (steel) of the part. In other words, the chemical composition of the core described below can also be said to be the chemical composition of the component material.

C: 0.10-0.30%
Carbon (C) has the effect of increasing the hardenability of steel and increasing the hardness of the core. This can increase the low cycle fatigue strength of the carburized component. If the C content is less than 0.10%, this effect cannot be obtained. On the other hand, if the C content exceeds 0.30%, the machinability and cold forgeability of the raw material steel deteriorate. Therefore, the C content is 0.10-0.30%. A preferable lower limit of the C content is 0.15% or more. A preferable upper limit of the C content is 0.28% or less.

Si: 0.03% to 0.80%
Silicon (Si) has the effect of deoxidizing steel. In addition, Si has the effect of increasing the hardenability of steel, and also has the effect of increasing the strength of steel through solid solution strengthening. These can increase the hardness of the core and increase the low cycle fatigue strength of the carburized part. This effect cannot be obtained if the Si content is less than 0.10%. On the other hand, if the Si content exceeds 0.80%, the machinability and cold workability of the raw material steel deteriorate. Therefore, the Si content is 0.03-0.80%. A preferable lower limit of the Si content is 0.20% or more. A preferable upper limit of the Si content is 0.50% or less.

Mn: 0.40-1.30%
Manganese (Mn) has the effect of deoxidizing steel. Moreover, since Mn has the effect of increasing the hardenability and strength of steel, it is possible to increase the hardness of the core and the low cycle fatigue strength of carburized parts. This effect cannot be obtained if the Mn content is less than 0.40%. On the other hand, if the Mn content exceeds 1.30%, the machinability and cold workability of the raw material steel deteriorate. Therefore, the Mn content is 0.40-1.30%. A preferable lower limit of the Mn content is 0.60% or more. A preferable upper limit of the Mn content is 1.00% or less.

P: 0.005 to 0.020%
Phosphorus (P) is an impurity. P segregates at the austenite grain boundaries during carburization, and has the effect of lowering the grain boundary strength of the carburized layer. This reduction in grain boundary strength reduces the low cycle fatigue strength. In this embodiment, by optimizing the conditions of the induction hardening process, the concentration of P segregated at grain boundaries during carburization can be reduced, and the grain boundary P concentration/steel material P concentration can be reduced. However, if the P content of the material is excessively high, even if the conditions of the induction hardening process are optimized to reduce the grain boundary P concentration/steel material P concentration, the grain boundary P content will reach a level that contributes to the improvement of the low cycle fatigue strength. It may not be possible to sufficiently reduce the concentration. Therefore, the P content is set to 0.020% or less. If the P content is 0.020% or less, the P content of not only the core portion but also the surface layer is low, so the toughness of the surface layer is enhanced and the occurrence of intergranular cracks is suppressed. As a result, low cycle fatigue strength increases. Therefore, the P content is 0.020% or less. The lower the P content is, the better. However, excessive reduction of P leads to an increase in cost for removing P. Therefore, the P content is set to 0.005% or more in consideration of the economy of refining. Preferably, it is 0.006% or more.

S: 0.003% to 0.060%
Sulfur (S) is an impurity. S remains at grain boundaries and has the effect of lowering the grain boundary strength of the carburized layer. S also has the effect of forming coarse MnS at grain boundaries to reduce the low cycle fatigue strength. Therefore, the S content is 0.060% or less. A preferable upper limit of the S content is 0.030% or less, and a more preferable upper limit is 0.015% or less. It is preferable that the S content is as low as possible. However, excessively reducing the amount of S leads to an increase in cost for removing S. Therefore, the S content is set to 0.003% or more in consideration of the economy of refining. The S content is preferably 0.005% or more.

Cr: 0.10-2.00%
Chromium (Cr) has the effect of increasing the hardenability of steel. Therefore, the hardness of the core can be increased, and the low cycle fatigue strength can be increased. If the Cr content is less than 0.10%, this effect cannot be sufficiently obtained. On the other hand, if the Cr content exceeds 2.00%, the machinability and cold workability of the raw material steel deteriorate. Therefore, the Cr content is 0.10-2.00%. A preferable lower limit of the Cr content is 0.50% or more. A preferable upper limit of the Cr content is 1.50% or less.

Al: 0.050% or less Aluminum (Al) has the effect of deoxidizing steel. Further, Al combines with N in steel to form AlN, and has the effect of suppressing coarsening of austenite grains during carburizing. This can increase the low cycle fatigue strength of the carburized component. If the Al content exceeds 0.050%, coarsening of austenite grains cannot be suppressed due to coarsening of inclusions, and low cycle fatigue strength may deteriorate. Therefore, the Al content is 0.050% or less. A preferable upper limit of the Al content is 0.035% or less. The Al content is preferably as low as possible. Although the lower limit of the Al content is not particularly limited, it may be 0.005% or more in order to enjoy the deoxidizing action.

N: 0.0030 to 0.0300%
Nitrogen (N) combines with Ti, Al, V, and Nb in steel to form nitrides and carbonitrides, thereby suppressing coarsening of austenite grains during carburizing. This can increase the low cycle fatigue strength of the carburized component. This effect cannot be obtained if the N content is less than 0.0030%. On the other hand, if the N content exceeds 0.0300%, the above effects are saturated. Therefore, the N content is 0.0030-0.0300%. A preferable upper limit of the N content is 0.0200% or less. A preferable lower limit of the N content is 0.0035% or more.

O: 0.0020% or less Oxygen (O) is an element that is inevitably contained and segregates at grain boundaries to easily cause grain boundary embrittlement. O is an element that easily forms hard oxide inclusions that cause brittle fracture in steel materials. In order to prevent such grain boundary embrittlement and brittle fracture, O should be 0.0020% or less. A preferable upper limit of the O content is 0.0018% or less. It is preferable that the O content is as low as possible. Although the lower limit of the amount of O is not particularly limited, it may be, for example, 0.0001% or more.

The chemical composition of the core of the carburized component of the present embodiment contains the above elements, and the balance consists of Fe and impurities. The term "impurities" as used herein refers to elements that are mixed from raw materials such as ores, scraps, or the manufacturing environment when industrially manufacturing carburized parts, and includes elements that are not intentionally included. . The term "impurities" as used herein means those that are permissible within a range that does not adversely affect the carburized component of the present embodiment.

[Regarding arbitrary elements]
The core of the carburized component of the present embodiment further contains one or more selected from the group consisting of Mo, V, Cu, Ni, Bi, Nb, Ti and B in place of part of Fe. You may These elements are optional elements. In other words, the carburized component according to the present embodiment can solve the problem without including the optional elements exemplified below. Therefore, the lower limit of the contents of the elements exemplified below is 0%.

Mo: 0 to 1.00% or less Molybdenum (Mo) is an optional element and may not be contained. When Mo is contained, Mo has the effect of increasing the hardenability of the steel, so that it can increase the core hardness and the low cycle fatigue strength of the carburized part. Mo also has the effect of increasing the toughness of the carburized layer. These effects can be obtained if even a small amount of Mo is contained. However, if the Mo content exceeds 1.00%, these effects become saturated and raw material costs increase. Therefore, the Mo content is 0-1.00%. A preferable lower limit of the Mo content for stably obtaining the above effects is 0.03% or more. A preferable upper limit of the Mo content is 0.50% or less.

V: 0 to 0.50% or less Vanadium (V) is an optional element and may not be contained. When V is contained, V combines with C and N in the steel to form V carbonitride (V(CN)), thereby suppressing coarsening of austenite grains during carburizing. This can increase the low cycle fatigue strength of the carburized component. This effect can be obtained if even a small amount of V is contained. However, if the V content exceeds 0.50%, the carburizability is lowered. Therefore, the V content is 0-0.50%. A preferable lower limit of the V content for stably obtaining the above effects is 0.01% or more. A preferable upper limit of the V content is 0.48% or less.

Cu: 0-0.50%
Copper (Cu) is an optional element and may not be contained. When Cu is contained, since Cu has the effect of increasing the hardenability of steel, it is possible to increase the core hardness and the low cycle fatigue strength of the carburized part. This effect can be obtained if even a small amount of Cu is contained. On the other hand, if the Cu content exceeds 0.50%, the hot workability deteriorates. Therefore, the Cu content is 0-0.50%. A preferable lower limit of the Cu content for stably obtaining the above effects is 0.01% or more. A preferable upper limit of the Cu content is 0.48% or less.

Ni: 0-1.00%
Nickel (Ni) is an optional element and may not be contained. When Ni is contained, Ni has the effect of increasing the hardenability of steel, so that the hardness of the core can be increased. This can enhance the low cycle fatigue strength properties of the carburized component. Ni also has the effect of increasing the toughness of the carburized layer. These effects can be obtained if even a small amount of Ni is contained. However, if the Ni content exceeds 1.00%, the amount of retained austenite in the surface layer after carburizing treatment increases, the surface layer hardness decreases, and as a result, the low cycle bending fatigue strength may decrease. . Therefore, the Ni content is 0-1.00%. A preferable lower limit of the Ni content for stably obtaining the above effect is 0.01% or more. A preferable upper limit of the Ni content is 0.80% or less.

Bi: 0-0.10%
Bismuth (Bi) is an optional element and may not be contained. When Bi is contained, the machinability of the steel can be enhanced by Bi. This effect can be obtained if even a small amount of Bi is contained. However, if the Bi content exceeds 0.10%, the above effects are saturated. Therefore, the Bi content is 0-0.10%. A preferable lower limit of the Bi content for stably obtaining machinability is 0.001% or more. A preferable upper limit of the Bi content is 0.08% or less.

Nb: 0-0.100%
Niobium (Nb) is an optional element and may not be contained. When Nb is contained, Nb combines with C and N in the steel to form Nb carbonitride (Nb(CN)), thereby suppressing coarsening of austenite grains during carburizing. This can increase the low cycle fatigue strength of the carburized component. This effect can be obtained if even a small amount of Nb is contained. However, if the Nb content exceeds 0.100%, the carburizability is lowered. Therefore, the Nb content is 0-0.100%. A preferable lower limit of the Nb content for stably obtaining the above effect is 0.001% or more. A preferable upper limit of the Nb content is 0.060% or less.

Ti: 0-0.200%
Titanium (Ti) is an optional element and may not be contained. When Ti is contained, Ti combines with C and S in steel to form fine TiC and TiS, thereby suppressing coarsening of austenite grains during carburizing. This can increase the low cycle fatigue strength of the carburized component. This effect can be obtained if even a small amount of Ti is contained. However, if the Ti content exceeds 0.200%, TiC coarsens and the toughness of the steel decreases. In this case, the low cycle fatigue strength of the carburized part is reduced. Therefore, the Ti content is 0-0.200%. A preferable lower limit of the Ti content for stably obtaining the above effects is 0.001% or more. A preferable upper limit of the Ti content is 0.150% or less.

B: 0-0.005%
Boron (B) is an optional element and may not be contained. When B is contained, since B has the effect of increasing the hardenability of steel, it can increase the core hardness and the low cycle fatigue strength of the carburized part. These effects can be obtained if even a small amount of B is contained. However, if the B content exceeds 0.005%, these effects are saturated. Therefore, the B content is 0-0.005%. A preferable lower limit of the B content for stably obtaining the above effect is 0.0001% or more. A preferable upper limit of the B content is 0.003% or less.

Elements that can be mixed into the component as impurities include, for example, Pb, Ca, Mg, W, Sb, Co, Ta, and REM. Even when these elements are included, their contents are respectively Pb: 0.10% or less, Ca: 0.001% or less, Mg: 0.001% or less, W: 0.10% or less, With Sb: 0.005% or less, Co: 0.10% or less, Ta: 0.10% or less, and REM: 0.001% or less, the present invention can be carried out without problems. You can enjoy the effect.

Next, the surface layer hardness, core hardness, grain boundary P concentration and metallographic structure of the carburized part of the present embodiment will be described.

[Surface hardness: 650HV or more]
The part has a Vickers hardness (surface layer hardness) of 650 HV or more at a depth of 0.10 mm from the surface.
The hardness of the depth region (surface layer) from the part surface to a depth of 0.10 mm affects the low cycle fatigue strength of the part. That is, the higher the hardness of the surface layer, the lower the amount of plastic strain in the surface layer and the higher the low-cycle fatigue strength. However, when the Vickers hardness at a depth of 0.10 mm from the surface is less than 650 HV, the plastic strain amount of the surface layer increases, low cycle fatigue strength decreases, and wear resistance deteriorates. Therefore, the surface layer hardness is 650HV or more. In addition, in the present embodiment, "a position at a depth of 0.10 mm from the surface" is located within the hardened layer.

Since the part of this embodiment is a carburized part, the surface layer of the part includes a part where carbon penetrates by carburizing treatment and a hardened layer (for example, a A region up to a depth of about 1.0 mm) is formed. However, as described above, the hardness region that affects the low cycle fatigue strength is the depth region (surface layer) from the surface of the part to a depth of 0.10 mm. Define hardness. In the present embodiment, the Vickers hardness at a depth of 0.10 mm from the surface is defined as a representative of the surface layer hardness. Therefore, a position shallower than the depth of 0.10 mm is naturally 650 HV or more.

[Core hardness: 300HV or more]
The part should have a Vickers hardness (core hardness) of 300 HV or more at a depth of 1.5 mm from the surface. The internal region (core) at a depth of 1.5 mm or more from the part surface is a region where there is no change in the chemical composition and metal structure despite undergoing carburizing and induction hardening. It is a part having the same component composition as the material (base material). If the core hardness is low, sub-surface originating fractures are exhibited and the low cycle fatigue strength is reduced. Therefore, the core hardness is 300HV or more.

Vickers hardness in the present embodiment refers to Vickers hardness (HV) conforming to JIS Z 2244:2009 "Vickers hardness test - test method".

The surface hardness and core hardness can be calculated by the following methods.
For both surface layer hardness and core hardness, first, the carburized part after induction hardening is cut perpendicularly to the main axis direction or longitudinal direction, and the resulting cross section is mirror-polished. After that, the surface layer hardness is measured at 3 arbitrary points on the plane corresponding to a depth position of 0.10 mm (100 μm) from the part surface (position in the direction perpendicular to the part surface) with a measurement load of 300 gf. The hardness of the core portion is measured at three arbitrary points on a plane corresponding to a position 1.5 mm deep from the part surface with a measurement load of 300 gf. By calculating the average value of the Vickers hardness obtained respectively, the surface layer hardness and the core hardness are obtained.

[Grain boundary P concentration P _b (%)/steel material P concentration P _m (%): 30.00 or less]
In the part, the ratio P _b /P of the amount of P in the grain boundary at a depth of 50 μm below the surface of the part (grain boundary P concentration P _b , mass %) to the amount of P in the core (steel material P concentration P _m , mass %) _m is 30.00 or less. The lower the grain boundary P concentration _Pb is, the more the grain boundary strength is improved and the low cycle fatigue strength is improved. Further, it is known that there is a correlation between the steel material P concentration _Pm and the grain boundary P concentration _Pb , and if the steel material P concentration _Pm is reduced, the grain boundary P concentration _Pb can be naturally reduced. However, there was a limit to the improvement in low-cycle fatigue strength by simply reducing the amount of P in the material. It has been found that the boundary P concentration _Pb can be further reduced. That is, the carburized part of the present embodiment can significantly reduce the grain boundary P concentration as compared with the conventional carburized part having the same amount of steel material P. Specifically, when P _b /P _m is 30.00 or less, the intergranular strength is high and the low cycle fatigue strength is greatly improved as compared with conventional carburized parts with the same steel material P content. P _b /P _m is preferably 26.00 or less. Since the grain boundary P concentration _Pb is preferably as low as possible, _Pb / _Pm is also preferably low. However, P _b /P _m may be 2.00 or more, 8.00 or more, 12.00 or more, or 15.00 or more as a practically achievable level.

Further, it is the grain boundary P concentration _Pb that directly affects the properties of the carburized part, and from the viewpoint of improving the grain boundary strength, the lower the grain boundary P concentration _Pb , the better. Therefore, the grain boundary P concentration _Pb is preferably 0.45% by mass or less, more preferably 0.35% by mass.

The grain boundary P concentration P _b (% by mass) can be determined by the Auger analysis described below.
First, an Auger test piece having the shape shown in FIG. 2 is produced from the surface of the carburized component. Each test piece thus obtained is stored in an Auger electron spectrometer ("PHI-700", manufactured by ULVAC-Phi, FE type), and cooled and fractured inside the apparatus. After that, spectral analysis is performed on the intergranular fracture surface up to a depth of 50 μm from the carburized surface. Since the grain boundary concentration varies to some extent, the grain boundary P concentration _Pb is obtained by measuring the concentration for at least seven grain boundaries and calculating the average value of the obtained concentrations. For calculation of the grain boundary concentration, a narrow scan is used in which the measurement energy range is narrowed and the number of accumulations in each measurement is increased to 10 times. Also, the grain boundary P concentration _Pb is calculated from the relative sensitivity coefficients of the elements detected by the above method. Test conditions for Auger analysis are as shown in Table 3.

[Metal structure]
The metal structure of the surface layer of the carburized part of the present embodiment is mainly a tempered martensite structure. Specifically, it is mainly composed of tempered martensite and retained austenite. The tempered martensite structure is an acicular metal structure, and it is difficult to measure the ratio of structures other than tempered martensite and retained austenite. Even if structures other than tempered martensite and retained austenite are clearly included, the effects of the present invention can be obtained as long as the ratio of structures other than tempered martensite and retained austenite is 20% or less. Preferably, the proportion of structures other than tempered martensite and retained austenite is 10% or less.

The carburized part of the present embodiment has been described above, but the surface layer hardness, core hardness, grain boundary P concentration, and metallographic structure described above are applied to the steel material having the above chemical composition, for example, as follows. It can be obtained by applying heat treatment under certain conditions.

[Production method]
The manufacturing method according to the present embodiment includes a forming step of forming a component shape such as a gear using steel having the above chemical composition as a material, and a carburizing step of carburizing and quenching the obtained formed body (intermediate product). , an induction hardening step of performing induction hardening on the compact after carburizing, and a tempering step of tempering at 130 to 200°C. However, the method for manufacturing a carburized component according to this embodiment is not limited to this aspect.

[Molding process]
First, a steel material that satisfies the chemical composition described above is produced. For example, molten steel having the above chemical composition is produced, and the molten steel is used to produce a cast piece (slab or bloom) by a continuous casting method. Alternatively, an ingot (steel ingot) may be produced by an ingot casting method using molten steel. The slab or ingot is then hot worked to produce a billet. The billet is hot worked to produce a steel bar or wire. Hot working may be hot rolling or hot forging.

In addition, for the purpose of homogenizing the grain size of the structure after hot working, normalizing in accordance with JIS B 6911: 2010 "Normalizing and annealing of steel" before working described later Isothermal annealing (IA) may be performed to reduce hardness. The structure after hot working or after normalizing is a mixed structure of ferrite + pearlite or ferrite + pearlite + bainite, and preferably has an average Vickers hardness of 130 to 220 HV.

The produced steel bar or wire rod is cold forged or machined to produce an intermediate product having a predetermined shape. Machining is, for example, cutting or drilling. The shape of the intermediate product is formed by well-known methods. For example, when the part is a gear, it is processed by broaching or the like.

[Carburizing process]
The produced intermediate product is carburized at a carburizing temperature of 870° C. to 1050° C. and cooled from the austenite region. The carburizing method may be vacuum carburizing or gas carburizing. Nitriding is not performed in this embodiment. Carburizing conditions may be treated according to well-known conditions that are generally carried out. Further, in the subsequent induction hardening process, when the core portion at a depth of 1.5 mm below the surface is reheated to the austenite region, the cooling in the carburizing process may be quenching or slow cooling. Moreover, if the induction hardening process cannot be performed immediately after the carburizing process, so-called placement cracks may occur due to residual stress generated during the carburizing heat treatment. In such cases, it is desirable to carry out low temperature tempering at 130-200°C. Further, in this embodiment, other treatments such as graphitization are not performed between the carburizing process and the induction hardening process.

[Induction hardening process]
After the carburizing process, induction hardening is performed one or more times under conditions that satisfy the following formulas (1) and (2), and then a tempering process of tempering at 130 to 200° C. is performed.

1000<T1 (1)
50<y (2)
However, T1 in the above formulas (1) and (2) is the quenching temperature (°C) on the surface of the part, and y is the average cooling rate (°C/sec) from the maximum heating temperature during the induction hardening process to 500°C. be.

The reasons for defining the hardening temperature T1 during induction hardening and the average cooling rate y will be described below.

<Quenching temperature T1: over 1000°C>
As the quenching temperature T1 during induction hardening increases, the grain boundary P concentration _Pb during heat treatment decreases and the grain boundary strength improves, so the low cycle fatigue strength improves. However, if the quenching temperature is less than 1000° C., the grain boundary P concentration P _b exceeds 0.8 at %, the reduction of P _b /P _m becomes insufficient, and it is difficult to sufficiently improve the low cycle fatigue strength. becomes. Therefore, the hardening temperature T1 during induction hardening is over 1000°C. It is preferably 1020° C. or higher. More preferably, it is 1030° C. or higher. Moreover, the upper limit of the quenching temperature T1 may be equal to or lower than the liquidus temperature, which is the upper limit of the austenite single-phase region. However, in practice, the upper limit of the quenching temperature T1 is preferably about 1100° C. in order to suppress coarsening of the austenite grain size.

<Average cooling rate: over 50°C/sec>
As the average cooling rate from the maximum heating temperature to 500° C. during the induction hardening process increases, the diffusion of P on the grain boundaries during cooling is suppressed, and an increase in grain boundary P concentration can be avoided. In order to obtain this effect, the average cooling rate from the highest heating temperature to 500°C should be over 50°C/sec. Therefore, in the present embodiment, even if the grain boundary P concentration _Pb can be sufficiently reduced by sufficiently increasing the quenching temperature T1, when the average cooling rate is 50° C./sec or less, P diffusion occurs during cooling. It occurs actively, the grain boundary P concentration increases again, and the low cycle fatigue strength decreases. Therefore, in order to sufficiently reduce P _b /P _m , it is important to achieve both an increase in the quenching temperature T1 and an increase in the average cooling rate. The temperature range in which the cooling is controlled is from the maximum heating temperature to 500.degree. Therefore, in the present embodiment, the average cooling rate up to 500° C. during induction hardening is defined using this 500° C. as a guideline.

Note that the average cooling rate for water cooling and oil cooling varies depending on the dimensions of the part. For example, in the case of a small component with a diameter of about Φ20 mm, the average cooling rate up to 500° C. is about 30° C./second for oil cooling and about 150° C./second for water cooling. In the case of a large member having a diameter of about 95 mm, the average cooling rate up to 500° C. is about 10° C./second for oil cooling and about 50° C./second for water cooling. A member smaller than Φ20 mm may have a cooling rate of 50° C./sec or more even with oil cooling, but water cooling is desirable in order to stably maintain an average cooling rate up to 500° C. of more than 50° C./sec.

In addition, the number of times the induction hardening treatment is performed may be one time or a plurality of times. Crystal grains can be refined by performing the induction hardening treatment multiple times. When the induction hardening treatment is performed a plurality of times, it is sufficient that the last treatment satisfies the above conditions, and the other treatments may be performed under general conditions.

The carburized component of the present embodiment can be manufactured by the above steps.

As described above, the carburized part and the manufacturing method thereof according to the present embodiment have been described. According to the present embodiment, grain boundary It is possible to reduce the P concentration and improve the grain boundary strength. As a result, it is possible to provide a carburized part having significantly improved low-cycle fatigue strength compared to the conventional one. Therefore, the carburized part of the present embodiment is suitable for gears of automobiles and industrial machines, especially machines powered by electric motors.

Next, examples of the present invention will be described. The conditions in the examples are one example of conditions adopted for confirming the feasibility and effect of the present invention, and the present invention is based on this one example of conditions. It is not limited.
Various conditions can be adopted in the present invention as long as the objects of the present invention are achieved without departing from the gist of the present invention.

(Manufacture of steel materials for carburized parts)
Steels a to z having chemical compositions shown in Table 1 were melted in a 50 kg vacuum melting furnace to produce molten steel, and the molten steel was cast to produce ingots. Steels a to o in Table 1 are steels having chemical compositions specified in the present invention. On the other hand, steels p to z are comparative example steels that deviate from the chemical composition specified in the present invention by at least one element. The underlines in Table 1 indicate compositions outside the scope of the present invention, and the blanks indicate that alloying elements are not intentionally added. Further, of the compositions of steels a to z shown in Table 1, the components (remainder) other than those shown in Table 1 are Fe and impurities. Here, the chemical composition of steels a and b corresponds to SCM420 defined in JIS G 4052:2008.

The obtained ingot was hot forged into a steel bar with a diameter of 60 mm. In the hot forging, the ingot was first held at a temperature between 1100° C. and 1200° C. for 5 hours in a heating furnace, and then worked into the steel bar described above by forging. After forging, it was allowed to cool in the air. After that, normalizing treatment was performed on the steel bar (vacuum melting material). The heating temperature in the normalizing treatment was 925° C., and the holding time was 2 hours. The steel bar was then allowed to cool in the air.

(Preparation of test piece)
A steel bar having a diameter of 60 mm after the normalizing treatment was machined to prepare a cantilever beam fatigue test piece having the shape shown in FIG. Each test piece was taken from the position of R/2 (R: radius of steel bar). When producing the cantilever beam fatigue test piece from the steel bar, the test piece was taken so that the longitudinal direction coincided with the longitudinal direction of the steel bar.
However, all of the four steel types w to z (test numbers 28 to 31) had high hardness after normalizing (HV230 or higher) and poor cold workability due to their chemical compositions. Therefore, they were judged to be unsuitable for processing and were not processed further.

(Carburizing treatment)
Gas carburizing and quenching treatments were performed on the prepared test pieces under the following two conditions.
<1> Cp value 0.8% target: 930 ° C. × 80 min (Cp value 1.0%) → 930 ° C. × 60 min (Cp value 0.8%) → 830 ° C. × 30 min (Cp value 0.8%) → Oil cooling (oil temperature: 80°C)
<2> Cp value 1.0% target: 930 ° C. × 80 min (Cp value 1.0%) → 930 ° C. × 60 min (Cp value 1.0%) → 830 ° C. × 30 min (Cp value 1.0%) → Oil cooling (oil temperature: 80°C)

The cooling rate to 500°C during quenching by oil cooling was 30°C/sec. Table 2 shows the correspondence between test numbers and carburizing conditions. It was confirmed by EPMA measurement that the carbon concentration in the surface layer after carburization was about 0.65 to 0.80%. In addition, in this example, since induction hardening could not be performed within 24 hours after carburizing and quenching, low temperature tempering was performed at 200° C. for 120 minutes after carburizing and quenching. If induction hardening is performed immediately after carburizing and quenching, this low-temperature tempering need not be performed.

(Induction hardening treatment)
The gas carburizing and quenching test pieces were subjected to induction hardening treatment once under the conditions in Table 2 (IH conditions). The heating holding time during high-frequency heating was set to 10 seconds or less in order to prevent grain growth of γ grains and decarburization. When the cooling method was water cooling, the average cooling rate from the maximum heating temperature (quenching temperature) during quenching to 500°C was 100°C/sec. Also, when the cooling method was oil cooling, the average cooling rate was 30° C./sec.

(Tempering treatment)
Tempering was performed on the test piece after the induction hardening treatment. The tempering temperature was 200°C and the holding time was 120 minutes.

Carburized parts (cantilever beam fatigue test specimens) of test numbers 1 to 27 were produced through the above manufacturing process. In any test piece, the metal structure of the carburized surface layer was composed of tempered martensite and retained austenite, and the residual structure was not confirmed or was 10 area % or less.

(Evaluation test)
(Hardness measurement)
The cantilever beam fatigue test piece treated as described above was cut perpendicularly to the longitudinal direction, and the exposed cross section was mirror-polished. After that, on the cross section, the hardness was measured at three points at a position 0.10 mm below the surface of the test piece, and the average value was calculated as the Vickers hardness (surface layer hardness) at 0.10 mm below the surface. A measurement load was 300 gf.
Further, on the same cross section, hardness measurements were performed at three locations 1.5 mm below the surface, and the average value was calculated as the Vickers hardness (core hardness) at 1.5 mm below the surface.

(Grain boundary P concentration measurement)
An Auger test piece having a shape shown in FIG. 2B was sampled from the surface of the cantilever beam fatigue test piece subjected to the above treatment from the sampling position shown in FIG. 2A. Specifically, the 3.7×18 mm back surface of the notch portion of the Auger specimen includes the 20×100 mm surface of the cantilever fatigue test piece, and the back surface of the Auger specimen (surface opposite the notch portion)3. A test piece was prepared so that the center of the 7×18 mm surface was aligned with the center of the 20×100 mm surface of the cantilever fatigue test piece. Note that the unit of numerical values in FIGS. 2A and 2B is mm. Details of the Auger test conditions are shown in Table 3.
Next, each Auger test piece thus obtained was stored in an Auger electron spectrometer (“PHI-700”, manufactured by ULVAC-Phi, FE type), and cooled and broken inside the device. After that, the spectral analysis was performed on the intergranular fractured surface from the carburized surface to a depth of 50 μm among the fractured surfaces. The temperature of the sample when cooled was about -120°C. In addition, since the grain boundary concentration varies to some extent, seven grain boundaries were measured and the average value was obtained. For grain boundary concentration calculation, a narrow scan was used in which the measurement energy range was narrowed and the number of accumulations in each measurement was increased to 10 times. The grain boundary P concentration (at %) was calculated from the relative sensitivity coefficients of the elements detected by the above method, and converted to mass %. Table 2 shows the grain boundary P concentration converted to mass %.

(Low cycle bending fatigue test)
For the low-cycle bending fatigue strength, a cantilever beam bending fatigue test was performed using a tension-compression hydraulic servo type fatigue tester (manufactured by Shimadzu Corporation, Servo Pulser "EHF-UM50kN-10L"). Fig. 1 shows the shape of the cantilever beam fatigue test piece. The cantilever fatigue specimens are 20×14(10)×210 mm as shown in FIG. 1 and have a notch of R2.0 110 mm from the end. Also, the power point is 10 mm from the end. The unit of dimension in FIG. 1 is "mm". The central axis of the cantilever fatigue specimen was coaxial with the central axis of the steel bar.

A fatigue test shown in Table 4 was performed at room temperature in an air atmosphere using the above cantilever beam fatigue test piece. The test was conducted under pulsating load control, and the number of fractures was measured. In this example, assuming application to gear parts, evaluation was made according to the following criteria.
First, using steel that meets the SCM420 standard of JIS G 4053: 2016, a standard test piece is prepared by the general manufacturing process, that is, "normalizing → test piece processing → eutectoid carburizing in a gas carburizing furnace → low temperature tempering". A certain cantilever beam fatigue test piece is prepared, and the 100-time breaking strength of this reference test piece (test numbers 26 and 27) is used as a standard, and if the fatigue limit of the target test number exceeds this standard by 15% or more, it passes ( ○ mark), and when it was less than 15%, it was rejected (× mark). In addition, it is known that when the amount of steel P is reduced, the grain boundary P concentration also decreases concomitantly. Test No. 26 was used as the standard for the breaking strength, and Test No. 27 was used as the standard for the 100-cycle breaking strength of steel b having a P concentration of less than 0.010%. The 100 times breaking strength of Test No. 26 was 4800N, and the 100 times breaking strength of Test No. 27 was 5200N.

(Test results)
Table 2 shows the test results. In test numbers 1 to 15, the chemical composition, surface layer hardness, and core hardness were within the scope of the present invention, so that the steel bars after normalizing had sufficient machinability and could be used as carburized parts. had a low grain boundary P concentration and excellent low cycle bending fatigue properties.

On the other hand, in test number 16, the amount of P in the steel composition of the part was excessive. As a result, the P _b /P _m value was within the range of the invention, but the grain boundary P concentration was not sufficiently reduced, resulting in a decrease in the grain boundary strength. there were.

In Test No. 17, the target low cycle fatigue strength was not achieved due to excessive S content in the steel composition of the part and coarsening of sulfides.

In Test No. 18, the Al content of the steel component of the part was excessive, and the low cycle fatigue strength was not achieved due to coarsening of inclusions.

In Test No. 19, the amount of C in the steel composition of the part was insufficient, and the core hardness was not within the target range, so fatigue fracture originating from the inside occurred, and the target low cycle fatigue strength was not achieved. .

In Test No. 20, the amount of Si in the steel composition of the part was insufficient, and the core hardness was not within the target range, so fatigue fracture originating from the inside occurred, and the target low cycle fatigue strength was not achieved. .

In Test No. 21, the amount of Mn in the steel composition of the part was insufficient, and the core hardness was not within the target range, so fatigue fracture originating from the inside occurred, and the target low cycle fatigue strength was not achieved. .

In Test No. 22, the amount of Cr in the steel composition of the part was insufficient, and the core hardness was not within the target range, so fatigue fracture originating from the inside occurred, and the target low cycle fatigue strength was not achieved. .

In Test No. 23, the quenching temperature during induction quenching was out of the range, and the quenching temperature was too low and the grain boundary P concentration was high, so the grain boundary strength decreased, and the target low cycle fatigue strength was not achieved. .

In Test No. 24, the quenching temperature during induction quenching was out of the range, and the quenching temperature was too low and the grain boundary P concentration was high, so the grain boundary strength decreased, and the target low cycle fatigue strength was not achieved. .

In Test No. 25, the cooling rate during induction hardening was out of the range, the grain boundary P concentration increased during cooling, and the grain boundary strength decreased. As a result, the target low cycle fatigue strength was not achieved.

Test No. 26 is a reference specimen for 100 times breaking strength of steel a and steel c to steel z, and Test No. 27 is a reference specimen for 100 times breaking strength of steel b. Both Test Nos. 26 and 27 are comparative examples in which induction hardening was not performed after the carburizing treatment.

Here, when comparing Test No. 1 and Test No. 26 using the same steel a (steel material P concentration: 0.015%), in Test No. 1 in which induction hardening was performed under appropriate conditions, the grain boundary P concentration was sufficient. It can be seen that the low cycle fatigue strength can be greatly improved. Similarly, in the case of Test Nos. 2 and 27, in Test No. 2, in which induction hardening was performed under appropriate conditions, the grain boundary P concentration was sufficiently reduced, and the low cycle fatigue strength was significantly improved. . In addition, since Steel b is a steel type with a relatively low steel material P concentration, in Test No. 27 using Steel b, the grain boundary P concentration is reduced to some extent without performing the induction hardening treatment. However, in Test No. 1 using Steel a, which is a comparative example steel material with a high P concentration, steel b with a low P concentration was obtained by applying the induction hardening of the present invention under appropriate conditions despite the high P concentration. It can be seen that the grain boundary P concentration can be reduced more than in Test No. 27 used. That is, according to the present invention, the grain boundary P concentration can be significantly reduced by optimizing the heat treatment conditions without the need to excessively reduce the P content of the material.

In addition, in test numbers 28 to 31, the Si content, Mn content, Cr content, or Mo content of the steel components of the parts was excessive, and the hardness after normalizing was high, so the machinability and cold workability decreased. . It was judged that the workability as a steel material was inferior, and subsequent tests were not conducted.

As described above, the use of the carburized steel parts of the present invention having excellent low-cycle bending fatigue strength can significantly reduce the size and weight of gears such as differential gears and transmission gears for automobiles. As a result, it becomes possible to increase the fuel efficiency of automobiles and reduce _CO2 emissions. Moreover, by using the manufacturing process of the present embodiment, the grain boundary P concentration can be reduced without reducing the amount of steel material P to the limit in the steelmaking process, resulting in significant cost reduction. Therefore, the effects of the present invention are extremely remarkable, and the present invention has great industrial applicability.

Claims (5)

The composition of the core is % by mass,
C: 0.10 to 0.30%,
Si: 0.03 to 0.80%,
Mn: 0.40-1.30%,
P: 0.005 to 0.020%,
S: 0.003 to 0.060%,
Cr: 0.10 to 2.00%,
Al: 0.050% or less,
N: 0.0030 to 0.0300% and O: 0.0020% or less, the balance being Fe and impurities,
The ratio P _b /P _{m of the grain boundary P concentration P b} , which is the amount of P in the grain boundary at a position 50 μm below the surface of the part, to the P concentration P _m of the steel material, which is the amount of P in the core, is 30.00 or _less . ,
The Vickers hardness at a depth of 0.10 mm below the surface of the part is 650 HV or more,
A carburized part having a Vickers hardness of 300 HV or more at a depth of 1.5 mm below the surface of the part. Furthermore, the composition of the core is, in mass%,
Mo: 0 to 1.00%,
V: 0 to 0.50%,
Cu: 0-0.50%,
Ni: 0 to 1.00%,
Bi: 0 to 0.10%,
Nb: 0 to 0.100%,
Ti: 0-0.200%, and B: 0-0.005%
2. The carburized part according to claim 1, characterized in that it contains one or more of A method for manufacturing a carburized component according to claim 1,
The composition, in mass %,
C: 0.10 to 0.30%,
Si: 0.03 to 0.80%,
Mn: 0.40-1.30%,
P: 0.005 to 0.020%,
S: 0.003 to 0.060% or less,
Cr: 0.10 to 2.00%,
Al: 0.050% or less,
N: 0.003 to 0.030% or less,
A forming step of forming a steel material containing O: 0.0020% or less and the balance being Fe and impurities into a part shape;
a carburizing step of performing carburizing treatment at a carburizing temperature of 870° C. to 1050° C. and then cooling from the austenite region;
An induction hardening step of performing induction hardening treatment under conditions that satisfy the following formulas (1) and (2);
Then, a tempering step of tempering at 130 to 200 ° C.;
A method for manufacturing a carburized part, comprising:
1000<T1 (1)
50<y (2)
However, T1 in the above formulas (1) and (2) is the quenching temperature (°C) on the surface of the part, and y is the average cooling rate (°C/sec) from the highest heating temperature to 500°C during the induction hardening process. . The composition, in % by mass,
Mo: 0 to 1.00%,
V: 0 to 0.50%,
Cu: 0-0.50%,
Ni: 0 to 1.00%,
Bi: 0 to 0.10%,
Nb: 0 to 0.10%,
Ti: 0-0.20%, and B: 0-0.0050%
4. The method for manufacturing a carburized component according to claim 3, characterized in that it contains one or more of 3. The carburized part according to claim 1, which is a mechanical structural part.

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