JP2023002842A - Machine component for automobiles made of steel material for carburization excellent in static torsional strength and torsional fatigue strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a machine component for automobiles having a shot-peening layer in the induction hardened/tempered state of the machine component for automobiles made of a steel material for carburization hardening, and excellent in static torsional strength and torsional fatigue strength.

SOLUTION: Provided is a machine component for automobiles that contains, by mass%, C:0.33 to 0.43%, Si:0.45 to 0.65%, Mn:0.20 to 0.40%, P:0.030% or less, S:0.030% or less, Ni:0.25% or less, Cr:1.70 to 2.00%, Al:0.010 to 0.040% and N:0.0200% or less, and the balance Fe with inevitable impurities. In the case of a torsional test specimen of Fig. 1 for the machine component for automobiles in a carburization hardened/tempered state, the surface hardness is 650 Hv or more, the core hardness is 450 Hv or more, the hardened layer depth is 1.0 to 2.6 mm, the cross-sectional average hardness is 550 Hv or more, the grain size number of the surface is 7.0 or more, and the surface C concentration is 0.60 to 1.00%.

SELECTED DRAWING: Figure 1

COPYRIGHT: (C)2023,JPO&INPIT

Description

TECHNICAL FIELD The present invention relates to a carburizing steel material excellent in static torsional strength and torsional fatigue strength, and more particularly to mechanical parts for automobiles such as automobile shaft members.

2. Description of the Related Art In recent years, there has been a trend toward higher torque and higher output in automobile engines. Along with this, there is a demand for increasing the strength of the shaft member, which is a shaft component that outputs the input torque from the engine. Furthermore, since the shaft member is subjected to torsional stress, static torsional strength and torsional fatigue strength are required.
Conventionally, SCr420 and SCM420, for example, are used as case hardening steels for shaft members, and carburizing treatment is sometimes performed as a carburizing, quenching and tempering material. However, with the increasing torque and output of automobiles as described above, there is a demand for the development of mechanical parts for automobiles, such as shaft members, which are even more excellent in static torsional strength and torsional fatigue strength.

Accordingly, techniques have been proposed for increasing the strength of shaft members manufactured by carburizing case-hardened steel.
For example, one or more of Te, Ca, Zr, Mg, Y, and rare earth elements are included as chemical components of steel, and the torsional fatigue strength is improved by controlling the morphology of MnS and the rolling structure. It has been proposed (see Patent Document 1, for example).
However, there is a problem that operating conditions are restricted in the steelmaking and rolling processes.

In addition, a carburizing steel having excellent torsional fatigue properties has been proposed by adding an appropriate amount of Co, Ni, and Cu, which suppress the formation of carbides, and Cr, which forms carbides, in the carburizing steel (see, for example, Patent Document 2).
However, further static torsional strength and torsional fatigue strength are desired.

Further, there has been proposed a carburizing steel that is excellent in torsional impact strength and torsional fatigue strength without a significant increase in part cost (see, for example, Patent Document 3).
However, this imposes restrictions on operating conditions, such as the need for quenching in quenching oil at a temperature lower than the temperature T calculated by the formula T (°C) = 40 x [Mn%] + 75 during carburizing and quenching. end up

In addition, due to restrictions on the cooling rate during steel casting and the operating conditions during hot rolling, the average aspect ratio of inclusion particles with a major axis of 3 μm or more is 6.0 or more, and the area ratio of inclusion particles is 0.6%. A carburizing steel material having excellent torsional fatigue strength has been proposed (see, for example, Patent Document 4).
However, there is a problem that operating conditions are restricted in the steelmaking and rolling processes.

Furthermore, by using Mo as an essential element in addition to B and optimizing the depth of the carburized layer after carburizing, quenching and tempering, there is no significant increase in the cost of parts, excellent manufacturability, and excellent torsional fatigue strength. A shaft has been proposed (see Patent Document 5, for example).
However, further static torsional strength and torsional fatigue strength are desired.

In addition, there has been proposed a case-hardening steel for shafts that is excellent in low-cycle torsional fatigue strength by specifying only the chemical composition and the hardness obtained by the Jominy single-end quenching method (see, for example, Patent Document 6). However, further static torsional strength and torsional fatigue strength are desired.

In addition, a steel material made of carburizing steel with excellent torsional fatigue properties has been proposed by specifying the chemical composition to be 6.0%≧2C+5Si+Cr-3Mn≧2.0% (see, for example, Patent Document 7). ). However, the chemical composition alone is not sufficient to exhibit the characteristics as a machine part, and further static torsional strength and torsional fatigue strength are desired.

JP-A-2002-069573 Japanese Patent Application Laid-Open No. 2003-183772 Japanese Patent Application Laid-Open No. 2003-239039 JP 2004-107694 A JP 2006-152330 A JP 2009-293070 A JP 2013-028860 A

In addition to carburizing, induction hardening is also used to manufacture machine parts. However, since it is necessary to work the steel into a desired shape before the quenching treatment, the C content of the steel for induction hardening is generally high in many cases, so the machinability is sometimes inferior. In addition, mechanical parts made by induction hardening generally do not necessarily have high bending fatigue strength. Therefore, it cannot be compared with induction hardening only in terms of improvement in torsional fatigue strength, and it is desired to improve torsional fatigue characteristics of machine parts in carburizing treatment.

Therefore, the problem to be solved by the present invention is to optimize the chemical composition of steel materials, and to define the hardness of steel parts in the carburized, quenched and tempered state, the depth of the hardened layer, and the C concentration on the surface of the steel parts. Thus, it is an object of the present invention to provide an automotive mechanical part of a shaft member which is superior in static torsional strength and torsional fatigue strength as compared with conventionally used automotive mechanical parts.

ａ：試験片の半径、Ｈｖ（ｒ）：ビッカース硬さ、ｒ：中心からの距離 Means for solving the above problems are, in the first means, as chemical components, in mass%, C: 0.33 to 0.43%, Si: 0.45 to 0.65%, Mn: 0 .20-0.40%, P: 0.030% or less, S: 0.030% or less, Ni: 0.25% or less, Cr: 1.70-2.00%, Al: 0.010-0 .040%, N: 0.0200% or less, the balance being Fe and unavoidable impurities, and the surface hardness of the steel part in the carburized, quenched and tempered state is 650 Hv or more, the core hardness is 450 Hv or more, and the full hardening The layer depth is 1.0 to 2.6 mm, the average cross-sectional hardness represented by the following formula is 550 Hv or more, the surface grain size number is 7.0 or more, and the surface C concentration is 0.60 to 1.0. 00%.

a: radius of test piece, Hv (r): Vickers hardness, r: distance from center

ａ：試験片の半径、Ｈｖ（ｒ）：ビッカース硬さ、ｒ：中心からの距離 In the second means, in addition to the chemical components of the first means, in mass%, Mo: 0.10 to 0.30%, Nb: 0.02 to 0.07%, the balance Fe and unavoidable The surface hardness of the steel part in the carburized, quenched and tempered state is 650 Hv or more, the core hardness is 450 Hv or more, the depth of the fully hardened layer is 1.0 to 2.6 mm, and is expressed by the following formula: An automotive mechanical part characterized by having an average cross-sectional hardness of 550 Hv or more, a surface grain size number of 7.0 or more, and a surface C concentration of 0.60 to 1.00%.

a: radius of test piece, Hv (r): Vickers hardness, r: distance from center

ａ：試験片の半径、Ｈｖ（ｒ）：ビッカース硬さ、ｒ：中心からの距離 In the third means, in addition to the chemical components of the first or second means, in mass%, Ti: 0.010 to 0.050%, B: 0.0003 to 0.0030%, the balance The surface hardness of the steel part in the carburized, quenched and tempered state, which is composed of Fe and inevitable impurities, is 650 Hv or more, the core hardness is 450 Hv or more, the depth of the hardened layer is 1.0 to 2.6 mm, and the following formula 550 Hv or more, a surface grain size number of 7.0 or more, and a surface C concentration of 0.60 to 1.00%.

a: radius of test piece, Hv (r): Vickers hardness, r: distance from center

In the fourth means, the automotive mechanical part has a shot peening layer in the carburized, quenched and tempered automotive mechanical part, the surface hardness of the shot peening layer is 700 Hv or more, and the compressive residual stress on the surface is is a mechanical part for automobiles according to any one of the first to third means, characterized in that has a value of 1000 MPa or more.

A fifth means is the automobile mechanical part according to any one of the first to fourth means, characterized in that the automobile mechanical part has a shaft member.

According to the present invention, carburizing, quenching, and tempering the steel for carburizing and quenching, which has the chemical composition specified in the present invention, in automobiles and other industrial machinery, the mechanical parts for automobiles have a surface hardness of 650 Hv. Above, the core hardness is 450 Hv or more, the hardened layer depth is 1.0 to 2.6 mm, the cross-sectional average hardness is 550 Hv or more, the surface grain size number is 7.0 or more, and the surface C concentration is 0.60. ∼1.00%, resulting in automotive mechanical parts with excellent static torsional strength and excellent torsional fatigue strength.
In addition, the steel parts obtained by shot peening after carburizing, quenching and tempering have a surface hardness of 700 Hv or more and a compressive residual stress on the surface of 1000 MPa or more, so static torsional strength and torsional fatigue strength , it has become a much more excellent steel part.
As described above, the present invention has static torsional strength and torsional fatigue characteristics suitable for a shaft member having a shaft portion, among mechanical parts for automobiles.

It is a figure which shows the shape of a torsion test piece. It is a figure which shows hardening and tempering conditions, (a) is a figure which shows hardening conditions, (b) is a figure which shows tempering conditions.

Before describing the embodiments of the present invention in detail, first, the reasons for limiting the chemical composition of the steel for carburizing and quenching in the present invention will be explained. The reasons for limiting the ranges of the surface hardness, core hardness, hardened layer depth, cross-sectional average hardness, surface grain size number, and surface C concentration in the tempered state will be explained.

First, the reason for limiting the chemical composition of the steel for carburizing and quenching in the present invention will be explained below. In addition, % in these reasons for limitation is all % by mass.

C: 0.33-0.43%
C needs to be 0.33% or more in order to secure a cross-sectional average hardness of 550 Hv or more of the steel part in the carburized, quenched and tempered state.
However, when C exceeds 0.43%, the machinability and cold forgeability of the steel material deteriorate, and the fracture of the steel material during the torsion test becomes brittle fracture, which rather reduces the torsional strength.
Therefore, C is set to 0.33 to 0.43%.

Si: 0.45-0.65%
Si is an element necessary for deoxidation, an element effective for improving the hardenability and strength of steel materials, an element for strengthening grain boundaries, and an element effective for improving torsional strength. For this purpose, Si needs to be 0.45% or more.
However, when Si exceeds 0.65%, the hardness of the ferrite matrix after annealing is increased, cracks are likely to occur during cold forging, and machinability is further reduced.
Therefore, Si should be 0.45 to 0.65%.

Mn: 0.20-0.40%
Mn is an element necessary for deoxidation and is an element effective for improving the hardenability and strength of steel. For this purpose, Mn must be 0.20% or more.
However, when Mn exceeds 0.40%, the hardness of the ferrite matrix after annealing is increased, cracks are likely to occur during cold forging, and machinability is reduced. Furthermore, it reduces the torsional strength of steel parts by promoting grain boundary segregation of embrittlement elements such as P.
Therefore, Mn is set to 0.20 to 0.40%.

P: 0.030% or less P is an element that segregates at grain boundaries to lower the torsional fatigue strength, increases the hardness of the ferrite matrix after annealing, and makes cracks more likely to occur during cold forging. . Therefore, P is set to 0.030% or less.

S: 0.030% or less S is an element that improves machinability, but if it combines with Mn to form a large amount of MnS, it is an element that reduces cold forgeability and torsional strength. Therefore, S is set to 0.030% or less.

Ni: 0.25% or less Ni is an element that increases the hardness of the ferrite matrix after annealing, makes cracks more likely to occur during cold forging, and reduces machinability. Therefore, Ni should be 0.25% or less.

Cr: 1.70-2.00%
Cr is an element effective in improving the hardenability of steel. For this purpose, Cr must be 1.70% or more.
However, when the Cr content exceeds 2.00%, coarse carbides and reticulated carbides are formed during carburizing of the steel member, which reduces the torsional strength.
Therefore, Cr is set to 1.70 to 2.00%.

Al: 0.010-0.040%
Al is an element necessary for deoxidation. Furthermore, Al is an element that suppresses coarsening of crystal grains during carburizing by forming AlN in combination with solute N. For this purpose, it is necessary to contain 0.010% or more of Al.
In addition, Al increases alumina-based oxides in steel and reduces the torsional fatigue strength of steel parts. Therefore, Al needs to be 0.040% or less.
Therefore, Al is set to 0.010 to 0.040%.

N: 0.0200% or less N is an element that suppresses grain coarsening by combining with Al and Nb in steel to form AlN and NbCN. However, if the N content exceeds 0.0200%, the hot workability of the steel material deteriorates, and furthermore, by forming nitrides in the steel material, the torsional fatigue strength of the steel parts is reduced. Adversely affect. Therefore, N is set to 0.0200% or less.

Mo: 0.10-0.30%
Mo is an element that is effective for improving the hardenability and strength of steel, strengthens grain boundaries and reduces brittle fracture surfaces, and is an element that is effective for improving the torsional strength of steel members. For this purpose, Mo should be 0.10% or more.
However, when Mo is more than 0.30%, the hardness of the ferrite matrix after annealing increases, cracking occurs during cold forging, machinability decreases, and the cost of steel increases.
Therefore, Mo is set to 0.10 to 0.30%.

Nb: 0.02-0.07%
Nb is an element that suppresses grain coarsening by forming nano-order carbonitrides, but this effect cannot be obtained if the Nb content is less than 0.02%.
However, when Nb is more than 0.07%, the amount of Nb carbonitride in the steel material becomes excessive and the workability of the steel material is lowered, and furthermore, it becomes difficult for C to diffuse into the surface during carburizing.
Therefore, Nb is set to 0.02 to 0.07%.

Ti: 0.010-0.050%
Ti is an element that suppresses the coarsening of crystal grains during carburizing heating by combining with C to form TiC. Furthermore, Ti combines with N to prevent B from becoming BN. do. For that purpose, Ti needs to be 0.010% or more.
On the other hand, when Ti is more than 0.050%, the machinability and cold forgeability are deteriorated due to excessive formation of TiC and TiN.
Therefore, Ti should be 0.010 to 0.050%.

B: 0.0003 to 0.0030%
B is an element that significantly improves the hardenability of steel when added in a small amount, and the addition of B can reduce the amount of other alloying elements added. Furthermore, B is an element that strengthens grain boundaries and reduces brittle fracture surfaces, and is an element that is effective in improving the torsional strength of steel materials. For these, B needs to be 0.0003% or more.
However, even if the B content exceeds 0.0030%, the effect of improving hardenability and strength is saturated.
Therefore, B is set to 0.0003 to 0.0030%.

The mechanical parts for automobiles of the present invention are formed by cold forging, hot forging, cutting, etc., of carburizing and quenching steel materials having the chemical composition specified in the present invention, into shaft members of predetermined shapes and mechanical parts for automobiles. It can be obtained by subjecting the surface of the component to carburizing, quenching, tempering, and, if necessary, subjecting it to shot peening. Therefore, the surface hardness, core hardness, hardening layer depth, cross-sectional average The reasons for limiting the ranges of the hardness, surface grain size number, and surface C concentration will be explained.

Surface hardness: 650 Hv or more Surface hardness increases the cross-sectional average hardness and improves the torsional strength of the steel member. For this purpose, a surface hardness of 650 Hv or more is required.

Core Hardness (ie, Non-Hardened Layer): 450 Hv or More The core hardness (that is, non-hardened layer) increases the cross-sectional average hardness and improves the torsional strength of the steel member. For this purpose, the hardness of the core (that is, the non-hardened layer) must be 450 Hv or more.

Hardened layer depth: 1.0 to 2.6 mm
The hardened layer depth increases the cross-sectional average hardness and improves the torsional strength of the steel member. For this purpose, the hardened layer depth must be 1.0 mm or more.
However, if the hardening layer depth is greater than 2.6 mm, long carburization is required, and coarse carbides and reticulated carbides are likely to be formed, and the torsional strength of the steel member is reduced.
Therefore, the total hardened layer depth is set to 1.0 to 2.6 mm.

Cross-sectional average hardness: 550 Hv or more The cross-sectional average hardness improves the torsional strength of the steel member. For this purpose, a cross-sectional average hardness of 550 Hv or more is required.

Surface grain size number: 7.0 or more The larger the surface grain size number, the more effective it is in improving the torsional strength of the steel member. Acts as a volume reducer. For this purpose, the crystal grain size number of the surface must be 7.0 or more.

Surface C concentration: 0.60 to 1.00%
The surface C concentration affects the surface hardness of the steel member. Therefore, if the surface C concentration is less than 0.60%, the surface hardness will be 650 Hv or less.
However, when the surface C concentration exceeds 1.00%, the torsional strength of the steel member decreases due to the formation of coarse carbides and network carbides. will lead to a decline in Therefore, the surface C concentration is set to 0.60 to 1.00%.

Next, the reasons for limiting each surface hardness and compressive residual stress of a steel member which is shot peened to a carburized, quenched and tempered steel member will be explained below.

Surface hardness: 700 Hv or more If the surface hardness is less than 700 Hv, the average cross-sectional hardness of the steel member is low, and the torsional strength of the steel member is low. Therefore, in order to increase the cross-sectional average hardness of the steel member and improve the torsional strength of the steel member, the surface hardness is set to 700 Hv or more.

Compressive residual stress: 1000 MPa or more The compressive residual stress on the surface improves the torsional strength of the steel member, and the compressive residual stress delays the generation and propagation of cracks. For that purpose, the compressive residual stress on the surface needs to be 1000 MPa or more.

Here, in order to describe an embodiment using the steel for carburizing, quenching and tempering of the present invention, the chemical composition of the steel material will be described with reference to the No. of the test material corresponding to the specified range of the present invention. 1 to 8, and No. of the test material outside the specified range of components. 9-12 are shown. The content of the chemical components in Table 1 is in mass %, and Fe and unavoidable impurities are excluded from Table 1.
Moreover, test material No. 10 is SCr420 of JIS (Japanese Industrial Standards), test material No. 11 is SCM420 of JIS, test material No. 12 is JIS SNCM420.

Next, Tables 2 and 3 describe the characteristics of Examples and Comparative Examples using test pieces subjected to carburizing treatment or the like. The "No." values of the examples and comparative examples in the table correspond to the "No." The test material No. in Table 1 is used as the steel material. It shows which of 1 to 12 it was.
Furthermore, in Table 2, when each test material is (a) carburized, quenched and tempered (carburized),
Table 3 shows the surface hardness, core hardness, Hardened layer depth, cross-sectional average hardness, surface grain size number, surface C concentration, surface hardness of carburized hardened steel and shot peening layer, compressive residual stress of carburized hardened steel and shot peened layer, static torsional strength ratio, torsion Fatigue strength ratios are listed and shown.

Examples shown in Tables 2 and 3 are test material Nos. 1 to No. 3, test material No. 5 to No. Example no. 1(a), Example No. 1(b), Example No. 2(a), Example No. 2(b), Example No. 3(a), Example No. 3(b), Example No. 5(a), Example No. 5(b), Example No. 6(a), Example No. 6(b), Example 7 No. (a), Example No. 7(b).
Tables 2 and 3 also show test material No. 1 as a comparative example. 4, test material No. 8 to No. Comparative Example No. 12 using 4(a), Comparative Example No. 4(b), Comparative Example No. 8(a), Comparative Example No. 8(b), Comparative Example No. 9(a), Comparative Example No. 9(b), Comparative Example No. 10(a), Comparative Example No. 10(b), Comparative Example No. 11(a), Comparative Example No. 11(b), Comparative Example No. 12(b).
Note that the static torsional strength ratio and the torsional fatigue strength ratio in Tables 2 and 3 are both for Comparative Example No. 2 in Table 2. 10(a) shows the ratio to 1.00, the strength of the carburized, quenched and tempered steel part of 10(a).

(Regarding procedures for Tables 2 and 3)
Now, a test material having the chemical composition shown in Table 1 and having the balance of Fe and unavoidable impurities is melted in a 100 kg vacuum melting furnace, hot forged to a diameter of 45 mm, and then allowed to cool. After normalizing and holding at 720° C. for 4 hours as low temperature annealing, it was air-cooled and processed into a torsion test piece 1 shown in FIG.
Then, this torsion test piece 1 was subjected to carburizing, quenching, and tempering treatment under the carburizing, quenching, and tempering conditions shown in FIG. That is, it is heated to 930°C and held as preheat for 0.5 hours, then carburized at 930°C for 3 hours, subsequently held at 930°C for 2.5 hours for diffusion, and then held at 850°C for 0.5 hours, followed by It was oil quenched in oil at 60°C. After that, as tempering, the temperature was raised to 180° C., held for 1.5 hours, and then air-cooled.
Shot peening was applied following the above carburizing treatment step to provide a shot peening layer.
Furthermore, this torsional test piece 1 was tested for static torsional strength and torsional fatigue strength shown in Tables 2 and 3.

The shape of the torsion test piece 1 is shown in FIG. The torsion test piece 1 has a length of 150 mm and a size of 30 mm square, and has a through hole of φ7 mm along the axial direction and a through hole of φ4 mm in the vertical direction perpendicular to the axial direction in the center of the longitudinal direction.

The static torsional strengths in Tables 2 and 3 are based on proportional limit values obtained from load torque-test angle data by a hydraulic servo type torsional fatigue tester.
Also, the torsional fatigue strength is based on the value of 10 ⁵ cycle fatigue strength under the conditions of both swings and a frequency of 5 Hz.
In addition, the value of the ratio in these is comparative example No. 10(a) carburized, quenched and tempered to 1.00.

The static torsional strength ratios shown in Tables 2 and 3 are the same as those of Comparative Example No. 10(a) after carburizing, quenching and tempering is taken as 1.00. 1(a) is 1.69, Example No. 2(a) is 1.79, Example No. 3(a) has a static torsional strength ratio of 1.81, and Example No. 5(a) is 1.67, Example No. 6(a) is 1.74, Example No. Since 7(a) is 1.83, it can be said that the steel member in the carburized, quenched and tempered state has excellent properties in the example of the present invention.

Further, as shown in Table 3, in the example (b) in which shot peening was applied after the carburizing treatment to form a shot peening layer, the static torsional strength ratio was lower than that of Example No. 3. 1(b) is 1.90, Example No. 2(b) is 1.99, Example No. 3(b) is 2.05, Example No. 5(b) is 2.16, Example No. 6(b) is 2.07, Example No. 7(b) was 2.31, and these were shot peened even better than (a).

The torsional fatigue strength ratios shown in Tables 2 and 3 are for Comparative Example No. The torsional fatigue strength ratio in Table 2 is the ratio when the value in the case of carburizing, quenching and tempering of 10(a) is taken as 1.00. 1(a) is 1.05, Example No. 2(a) is 1.06, Example No. 3(a) has a static torsional strength ratio of 1.07, and Example No. 5(a) is 1.05, Example No. 6(a) is 1.07, Example No. 7(a) is 1.10, which is better than the comparative examples in Table 2.

Further, as shown in Table 3, in Example (b), shot peening was applied after carburizing treatment to form a shot peening layer, and the torsional fatigue strength ratio was lower than that of Example No. 3. 1(b) is 1.52, Example No. 2(b) is 1.54, Example No. 3(b) is 1.65, Example No. 5(b) is 1.62, Example No. 6(b) is 1.63, Example No. 7(b) is 1.73, and Comparative Example No. 7 (b) is all made of high-strength SNCM420. Even compared with the torsional fatigue strength ratio of 1.47 in 12(b), the strength is higher. Also, Comparative Example No. 4(b), Comparative Example No. 8(b), Comparative Example No. 9(b), Comparative Example No. 10(b), Comparative Example No. Compared to 11(b), it is excellent in torsional fatigue strength ratio.

Thus, the reason why the static torsional strength and torsional fatigue strength of the examples are excellent is that the chemical composition of the steel and the heat treatment quality after carburizing, quenching and tempering are optimized, and the brittle fracture surface ratio of the surface after fracture is reduced. It is mentioned that it was reduced.

By the way, test material No. Comparative example no. 4(a) and comparative example no. 4(b) reduces the equilibrium carbon concentration at the time of carburizing to 2/3 compared to other conditions, and the surface C concentration is 0.55% (in the means of the present invention, the surface C concentration is 0.60 to 1 .00%), the surface hardness and cross-sectional average hardness are low, and the test material No. 1, which has almost the same chemical composition, has a low surface hardness and a low cross-sectional average hardness. Example No. 3 (surface C concentration is 0.85%). 3(a) and no. Compared to 3(b), the torsional strength is low.

By the way, test material No. Comparative example no. 8(a) and comparative example no. 8(b) reduces the equilibrium carbon concentration at the time of carburizing to 2/3 compared to other conditions, and the surface C concentration is 0.55% (in the means of the present invention, the surface C concentration is 0.60 to 1 .00%), the surface hardness and cross-sectional average hardness are low, and the test material No. 1, which has almost the same chemical composition, has a low surface hardness and a low cross-sectional average hardness. Example No. 7 (surface C concentration is 0.90%). 3(a) and no. Compared to 3(b), the torsional strength is low.

Moreover, test material No. Comparative example no. 9(a) and comparative example no. 9(b) had a surface crystal grain size number of 6.8, which was lower than 7.0 of the present invention, and therefore had low static torsional strength and low torsional fatigue strength.
This is the test material No. In No. 9, Mo was 0.32% (In the means of the present invention, Mo is 0.10 to 0.30%.), so the structure before carburization is bainite + martensite, which is disadvantageous in grain size characteristics. This is because the crystal grain size number fell below 7.0 specified in the present invention due to the formation of a texture and the absence of Nb that contributes as pinning particles.

Next, the average cross-sectional hardness is a value obtained by determining the hardness distribution from the surface to the core, performing weighted integration corresponding to the area, and dividing by the total cross-sectional area, and is expressed in Hv. ing. The next paragraph gives the formula for this weighted integral.

ａ：捩り試験片１の半径、Ｈｖ（ｒ）：ビッカース硬さ、ｒ：中心からの距離

a: radius of torsion test piece 1, Hv (r): Vickers hardness, r: distance from center

The crystal grain size number of the surface shown in Table 2 was calculated by the cutting method of JIS G 0551 from two fields of view taken with an optical microscope at a magnification of 400 or 1000. Furthermore, the C concentration on the surface was measured with an EPMA (Electron Probe Micro Analyzer).

In addition, the compressive residual stress of the shot peening layer was measured by electropolishing to 100 μm from the surface and using X-ray diffraction. Table 2 shows a high compressive residual stress of 1300 Pa.

1 torsion test piece 2 φ7mm through hole 3 φ4mm through hole

Claims (4)

As chemical components, in mass%, C: 0.33 to 0.43%, Si: 0.45 to 0.65%, Mn: 0.20 to 0.40%, P: 0.030% or less, S : 0.030% or less, Ni: 0.25% or less, Cr: 1.70 to 2.00%, Al: 0.010 to 0.040%, N: 0.0200% or less, and the balance Fe and unavoidable impurities, the surface hardness of the steel part in the carburized, quenched and tempered state is 650 Hv or more, the core hardness is 450 Hv or more, the depth of the fully hardened layer is 1.0 to 2.6 mm, and the following formula A steel part having an average cross-sectional hardness of 550 Hv or more, a surface grain size number of 7.0 or more, and a surface C concentration of 0.60 to 1.00%.

a: radius of torsion test piece 1, Hv (r): Vickers hardness, r: distance from center In addition to the chemical components described in claim 1, it contains Mo: 0.10 to 0.30% and Nb: 0.02 to 0.07% in mass%, and the balance is Fe and unavoidable impurities, carburizing The steel part in the quenched/tempered state has a surface hardness of 650 Hv or more, a core hardness of 450 Hv or more, a depth of the hardened layer of 1.0 to 2.6 mm, and an average cross-sectional hardness represented by the following formula. is 550 Hv or more, the surface grain size number is 7.0 or more, and the surface C concentration is 0.60 to 1.00%.

a: radius of torsion test piece 1, Hv (r): Vickers hardness, r: distance from center In addition to the chemical components of claim 1 or claim 2, it contains Ti: 0.010 to 0.050%, B: 0.0003 to 0.0030%, and the balance is Fe and unavoidable impurities. , The surface hardness of the steel part in the carburized, quenched and tempered state is 650 Hv or more, the core hardness is 450 Hv or more, the depth of the entire hardened layer is 1.0 to 2.6 mm, and the cross-sectional average represented by the following formula A steel part having a hardness of 550 Hv or more, a surface grain size number of 7.0 or more, and a surface C concentration of 0.60 to 1.00%.

a: radius of torsion test piece 1, Hv (r): Vickers hardness, r: distance from center A carburized, quenched and tempered automobile mechanical part has a shot peening layer, and the shot peening layer has a surface hardness of 700 Hv or more and a surface compressive residual stress of 1000 MPa or more. Steel component according to any one of the preceding claims.

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