JPH0297618A - Steel member having excellent impact resistance and its production - Google Patents

Abstract

PURPOSE:To easily produce the title steel member having excellent impact resistance at a low cost by applying carburization hardening and shot peening to a steel material, reheating the material to a temp. higher than the austenitizing temp., and then hardening the material. CONSTITUTION:A steel material is carburization-hardened or carbonitriding- hardened, and then shot-peened. The steel material is reheated to a temp. higher than the austenitizing temp., and then hardened. A hardened layer consisting of a mixed structure of martensite and retained austenite and having austenitic crystal grains of >=No.9 crystal grain size is formed on the steel material surface to a depth of >=0.1mm from the surface. By this method, a steel member having high toughness and impact resistance can be inexpensively obtained without using a special steel material such as alloyed steel and without deteriorating the machinability.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a steel member with excellent impact resistance and a method for manufacturing the same.

(Prior Art) Drive system gears in automobiles, especially reverse system and differential system gears, are required to have excellent strength characteristics because they are subjected to high loads. As shown in Japanese Patent Application Laid-Open No. 60-2) 8422, a method is known in which a steel material is carburized and quenched, then tempered, and then shot peened.

However, since gears in reverse and differential systems are subject to shock torque, high impact strength is also required.Recently, materials such as carbon steel or alloy steel for machine structures have been used as a method of manufacturing steel components to improve impact strength. Proposals include a method in which the material is quenched and rephosphorized, and a method in which alloy steel is added with nickel, an alloying element that improves impact strength, and then carburized and quenched. .

(Problem to be Solved by the Invention) However, although the steel members obtained by the former method have higher impact strength than carburized and quenched steel members, they do not have the high impact strength required for drive system gears. Moreover, the wear resistance is not sufficient.

On the other hand, steel members obtained by the latter method have the strength properties required for drive system gears, including impact strength, but the material costs are high, and the steel members that can be easily cut due to the addition of alloying elements. A new problem arises in that productivity decreases due to poor performance, so it cannot be used for drive system gears.

In view of the above, the present invention provides a steel member with excellent toughness and high impact strength without using special steel materials such as alloy steel, that is, without increasing costs or deteriorating machinability. The purpose is to provide. Another object of the present invention is to provide a method for easily manufacturing such a steel member.

(Means for Solving the Problem) In order to achieve the above object, the invention of claim (1) provides a hardened layer consisting of a mixed structure of martensite and retained austenite in a predetermined or more range on the surface of a steel member. At the same time, the austenite in this hardened layer is composed of fine crystal grains of a predetermined size or more.

Specifically, the solution purchased by the invention of claim (1) is to produce a steel member with excellent impact resistance, which is made of a mixed structure of martensite and retained austenite, and has an average grain size of 9 or more. The hardened layer having austenite crystal grains is formed from the surface to a depth of 0.1+nm or more.

In addition, the invention of claim (2) provides that, in obtaining the steel member of the invention of claim (1), shot peening treatment is performed on the steel material after carburizing and quenching or carbonitriding and then predetermined reheating and quenching. It is used for processing.

Specifically, the solution taken by the invention of claim (2) is to provide a method for manufacturing a steel member with excellent impact resistance by applying shot peening treatment to the steel material after carburizing and quenching or carbonitriding and quenching. After that, a reheating and quenching treatment is performed, in which the material is heated above the austenitizing temperature and then quenched.
A hardened layer consisting of a mixed structure of martensite and austenite and having austenite crystal grains having an average grain size of 9 or more is formed from the surface. The structure is such that it is formed over a depth of 1 m11 or more.

(Function) According to the structure of the invention of claim (1), the hardened layer of the steel member is 0.0 mm from the surface. The surface of this steel member has high toughness because it is composed of fine austenite crystal grains with a grain size number of 9 or more over a depth of imi or more.

Further, according to the configuration of the invention of claim (2), the hardened layer of the steel material formed by carburizing and quenching or carbonitriding and quenching is plastically worked by shot peening treatment, and this plastically worked hardened layer is reheated. Since the austenite in the hardened layer becomes fine crystal grains with a grain size number of 9 or more, a steel member with high surface toughness can be obtained.

(Example) Examples of the present invention will be described below.

The steel member with excellent impact resistance according to the present invention has a mixed structure of martensite and retained austenite, and a hardened layer having austenite crystal grains with an average grain size of 9 or more is located 0.1 am or more from the surface. This steel member is manufactured as follows.

First, a steel member that is case hardening steel is carburized and quenched or carbonitrided and quenched to form a hardened layer consisting of a mixed structure of martensite and retained austenite on its surface. The method and conditions for this carburizing and quenching may be general ones, but the depth of the hardened layer should be from the surface to a hardness of Hv550.
The depth up to the point is 0. 2-1. It is preferable to set it in the range of 3+nIII.

Next, shot peening treatment is performed on the steel material that has been subjected to carburizing and quenching or carbonitriding and quenching, to form a plastically worked process-affected layer on the surface of the hardened layer. The depth of this process-affected layer is preferably in the range of 0.1 to 0.2 am from the surface. The reason for this is that the depth is 0. If the depth is less than 111w, the layer thickness of the fine grains formed by the next reheating and quenching process will be insufficient, and if the depth exceeds 0.2), the surface roughness of the steel material will deteriorate. be. In order to form a process-affected layer of such depth, shot hardness HRC 45-65 and shot speed 50-120 m/s.
A peening strength of s is preferred. The reason for this is that as the peening intensity increases, the processing affected layer becomes deeper, and conversely, as the peening intensity decreases, the processing affected layer becomes shallower, and when the shot hardness is less than HRC 45 or the shot speed is less than 50 m/s, the processing affected layer becomes deeper. is less than 0.1 na+, shot hardness exceeds HRC 65 or shot speed 120 m/s
This is because the depth of the machining-affected layer exceeds 0.2 mm.

Next, the steel material subjected to the shot peening treatment is heated to a temperature equal to or higher than the austenitizing temperature and then quenched to perform a reheating quenching treatment to recrystallize the residual austenite in the hardened layer.

Regarding the step of heating above the austenitizing temperature, the heating temperature is preferably 800 to 900°C and the holding time is preferably 20 to 90 minutes. The reason for this is that if the heating temperature is less than 800°C or the holding time is less than 20 minutes, the solid solution of carbides generated during the temperature raising process is insufficient, resulting in poor hardenability.
This is because if the temperature exceeds 0.degree. C. or the holding time exceeds 90 minutes, the fine austenite crystal grains that have been formed will grow again, which is undesirable.

When the reheating treatment is performed after shot peening treatment as described above, the surface portion of the steel member is plastically worked by the shot peening treatment, and the austenite crystal grains during carburizing and quenching or carbonitriding quenching are easily recrystallized. Forms fine crystal grains.

Hereinafter, specific examples and comparative examples of the steel member with excellent impact resistance according to the present invention and its manufacturing method will be described.

As the steel material, case hardened steel of S0M420H was prepared.

Specific example: First, as shown in the heat treatment pattern diagram in Figure 1, a steel material is held at a temperature of 930°C for 3 hours, and then the temperature is lowered to 84°C.
It was held at a temperature of 0° C. for 30 minutes, and then rapidly cooled and carburized and quenched. Carburized and quenched steel material is subjected to shot peening treatment at a shot hardness of HRC 50 to 52 and a shot speed of 60 m/s, and then reheated to 870°C as shown in the heat treatment pattern diagram in Figure 2 (a). After holding at the temperature for 30 minutes, the temperature was lowered and the temperature was kept at 820°C for 30 minutes.
It was held for a minute, and then quenched and reheated and quenched. Further, this steel material was held at a temperature of 170° C. for 2 hours and then cooled in air to perform a tempering treatment, as shown in the heat treatment pattern diagram of FIG. 2(b).

The depth of the processing-affected layer of the steel member thus obtained is 0.15II11 from the surface, and the grain size number of the austenite grains is 10, as shown in the micrograph in Figure 3 (200x magnification). there were.

Comparative Example 1: A steel material was carburized and quenched in the same manner as in the specific example, and then shot peened and reheated and quenched.
After being held at a temperature of 70° C. for 2 hours, it was air cooled and tempered.

As shown in the micrograph of FIG. 4 (200x magnification), the grain size number of the austenite crystal grains in the thus obtained steel member was 7. In this comparative example,
Since shot peening treatment and reheating quenching treatment were not performed, the austenite crystal grains were large.

Comparative Example 2: After carburizing and quenching the steel material in the same manner as in the specific example, shot peening treatment was performed at a shot hardness of HRC 38 to 40 and a shot speed of 60 m/s, and then reheating under the same conditions as in specific example 1. Hardening treatment and tempering treatment were performed.

The depth of the machining-affected layer of the steel member obtained in this way is 0.0 mm from the surface. 04+1ll, the grain size number of the austenite crystal grains was 9.5. In this comparative example, the shot hardness was low, so the peening strength was low, and the depth of the processing affected layer was shallow.

Comparative Example 3: After carburizing and quenching the steel material in the same manner as in the specific example, shot peening treatment was performed at a shot hardness of HRC 50 to 52 and a shot speed of 40 m/s, and then reheating under the same conditions as in specific example 1. Hardening treatment and tempering treatment were performed.

The depth of the work-affected layer of the thus obtained steel member was 0.06 m5 from the surface, and the grain size number of the austenite crystal grains was 9.5. In this comparative example, the shot speed was low, so the peening intensity was low, and the depth of the processing-affected layer was shallow.

Comparative Example 4: A steel material was subjected to carburizing and quenching and shot peening under the same conditions as in the specific example. Next, after being reheated and held at a temperature of 930° C. for 2 hours, it was quenched and subjected to reheating and quenching treatment, followed by tempering treatment under the same conditions as in the specific example.

The depth of the process-affected layer of the thus obtained steel member was 0.16v++ from the surface, and the grain size number of the austenite crystal grains was 6.5. In this comparative example, the heating temperature in the reheating and quenching treatment was high and the holding time was long, so the fine austenite crystal grains grew again and became larger.

Comparative Example 5: A steel material was subjected to carburizing and quenching and shot peening treatment under the same conditions as in the specific example. Next, after being reheated and held at a temperature of 800° C. for 10 minutes, it was quenched and subjected to reheating and quenching treatment, followed by tempering treatment under the same conditions as in the specific example.

The grain size number of the austenite grains is 1 at 0.16 mm from the depth surface of the working affected layer of the steel member thus obtained.
0, but undissolved carbides were precipitated. The reason for this is that the heating temperature in the reheating and quenching treatment was low and the holding time was short.

Hereinafter, in order to evaluate the steel member with excellent impact resistance and the manufacturing method thereof according to the present invention, toughness evaluation tests of steel members of specific examples and comparative examples will be described.

First, as shown in Figure 5 (a) and (b), a width of 10±
0.02)! +, height 10±0.05mm, length 55±
At the center in the length direction on the top surface of a 0.1 mm rectangular parallelepiped,
A test piece 1 having a notch with a semicircular cross section with a radius of 2±0.02 mm is manufactured.

Next, as shown in FIG. 6, this test piece 1 is turned upside down and supported at two points in the length direction, and a load is applied to the back side of the notch by the load cell 2, so that the strain rate at the bottom of the notch is: ε- 0,016
/second, test temperature: A three-point bending test was conducted under the conditions of room temperature.

The test results are as shown in FIG. 7, and it can be seen that the impact strength of the specific example is greatly improved compared to that of Comparative Examples 1 to 5. In this case, Comparative Example 1
Comparative example 2 and 4 have coarse austenite crystal grains.
It is thought that the work-affected layer and the layer of fine austenite grains are thin in Samples 3 and 3, and that the fracture strength of Comparative Example 5 is poor due to the precipitation of undissolved carbides.

(Effects of the Invention) As explained above, in the steel member according to the invention of claim (1), the hardened layer is 0% from the surface. Since it is composed of fine austenite crystal grains with a grain size number of 9 or more over a depth of 1 mm or more, the surface toughness is improved, and thereby it has high impact resistance. Further, since special steel materials such as alloy steel are not used, there is no increase in cost or deterioration of machinability.

In addition, according to the manufacturing method according to the invention of claim 2, the hardened layer of the steel material formed by carburizing and quenching or carbonitriding is plastically worked by shot peening treatment, and the plastically worked hardened layer is reheated. Since austenite forms fine crystal grains, the resulting steel material has improved surface toughness and thus has high impact resistance.

[Brief explanation of the drawing]

1 and 2 are heat treatment pattern diagrams of specific examples of the present invention, FIG. 3 is a micrograph showing the metal structure of the hardened layer of the steel member according to the specific example, and FIG. 4 is a steel sheet according to comparative example 1. Micrograph showing the metal structure of the hardened layer of the member, Figures 5 (a) and (b) are diagrams showing the dimensions and shape of the 3-point bending test piece, Figure 6 is an explanatory diagram of the 3-point bending test method, FIG. 7 is a diagram showing the results of toughness evaluation tests of specific examples and comparative examples. Figure Figure Figure Re 711] #! L, Z filling process (A) Length centering process (B) (A) (B) Figure 5

Claims (2)

[Claims] (1) A hardened layer consisting of a mixed structure of martensite and retained austenite and having austenite crystal grains with an average grain size of 9 or more is 0.1 m from the surface.
A steel member with excellent impact resistance, characterized by being formed over a depth of m or more. (2) Steel materials are subjected to carburizing and quenching or carbonitriding and quenching, then shot peening, and then reheated and quenched to heat above the austenitizing temperature and then quenched to form martensite. Excellent impact resistance characterized by forming a hardened layer with austenite crystal grains having a mixed structure with austenite and having an average grain size of 9 or more over a depth of 0.1 mm or more from the surface. A manufacturing method for steel parts.