JPH1136016A - Quenching method for case hardening steel capable of preventing heat treatment strain - Google Patents

Abstract

PROBLEM TO BE SOLVED: To unnecessitate the grinding correction of a shape by applying carburizing treatment and diffusion treatment to a case hardening steel containing a specific composition of alloy elements while specifying the temp. and the medium to reduce carbon concn. on the surface layer part to a specific range, holding it at the specific temp. to generate the specific ratio of ferrite in the non-carburized part, and quenching it. SOLUTION: The case hardening steel material containing C, Si and Mn, or e.g. a gear using the case hardening steel containing C, Si, Mn and one or more kinds among Cr, Ni and Mo is decarbonized at A3 +50 deg.C temp. in the medium having 0.8-1.4 wt.% carbon potential. Successively, after lowering the carbon concn. on the surface layer part of the gear to 0.6-0.9 wt.% by applying the heating and diffusion treatment, the gear is held at A1 +10 deg.C to A3 -10 deg.C for >=10 min. After generating the ferrite in the micro structure of the non- carburized part in the case hardening steel material of the gear in the range of 10-50% the area ratio, the quenching is executed. The gear having high hardness, such as the transmission gear for car, is obtd. without needing the specific alloy elements.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for carbonitriding a case-hardened steel, and more particularly to a method for carbonitriding a case-hardened steel suitable for producing a gear for a transmission of an automobile having a low distortion during quenching and a low noise. The present invention relates to a heat treatment method.

[0002]

2. Description of the Related Art Generally, SCr420, gears for automobiles are used in order to guarantee fatigue strength and surface wear resistance.
Case hardening steels such as SCM420 and SNCM420 are used. In manufacturing the gear, the case hardened steel bar is hot forged into a shape close to the gear, and then cut to form a gear. To cure the surface. However, when carburizing and quenching are performed, the gears are distorted due to expansion accompanying martensitic transformation.

That is, in order to infiltrate carbon or nitrogen into the surface of a low-alloy steel having a relatively low carbon concentration and then to perform quenching and tempering to convert the tooth surface into martensite and harden, and to secure the toughness of the gear. Then, the gear core is made to have a ferrite + pearlite structure partially containing martensite or bainite. Here, FIG. 7 is a schematic perspective view illustrating the inside of the gear teeth (the inside of the tooth root and the gear core) and the surface.
The gear core portion is the central portion of the gear indicated by reference numeral 8 in FIG. 7, and the regions indicated by reference numerals 6 and 7 in FIG. 7 are referred to as a tooth surface portion and a tooth root inside, respectively. Since the cooling rate during the quenching is higher inside the tooth root than in the gear core, the microstructure inside the tooth root is a mixed structure of martensite, which is a quenched structure, and a part of bainite. At this time, transformation stress occurs due to expansion during transformation from austenite to martensite. Therefore, distortion occurs in the gear, and the gear accuracy cannot be maintained. In particular, the distortion of the gears of the transmission system of an automobile is the largest cause of gear noise in an actual vehicle. Therefore, it is common to perform grinding for correcting the tooth profile after quenching, which causes a significant cost increase.

As a countermeasure against distortion as described above, Japanese Patent Application Laid-Open No. 2-277744 discloses a method of adding Nb, Al and Ti to make crystal grains fine (prior art 1). However, this method can prevent distortion due to coarsening of crystal grains during carburizing heating, but cannot prevent distortion due to martensitic transformation stress. Therefore, distortion cannot be suppressed sufficiently small.

Japanese Patent Application Laid-Open No. 60-161942 discloses that C: 0.03 to 0.2 wt.%, Si: 1.5 to 3.0.
wt.%, Mn: 0.2 to 2.0 wt.%, Ti: 0.03 to
A method has been disclosed in which a core is formed into a two-phase structure of austenite and ferrite when a steel containing 0.30 wt.% Is subjected to high-temperature carburizing treatment (prior art 2). However, in the method of securing a ferrite phase by increasing the concentration of Si, which is a ferrite-forming element, since Si is an element that impedes the intrusion of C from the surface of a steel material, it adversely affects carburization, thereby improving To increase carburizing time,
A large amount of Mo or Cr must be added, leading to an increase in cost.

On the other hand, in order to avoid the occurrence of distortion due to carburizing and quenching, Japanese Patent Application Laid-Open No. 55-152175 discloses a method of performing a soft nitriding treatment (prior art 3). However, in nitrocarburizing, the depth of the hardened layer is only as small as about 0.2 mm, and pitting is likely to occur on the gear surface, making it unsuitable for large gears with large surface pressure.
Only applicable to small gears. In the nitriding process,
It takes a long time of 20 to 40 hours to obtain a deep cured layer, which is also a major cause of cost increase.

[0007]

As described above, the prior arts 1 to 3 have the following problems. In other words, carburizing with a large carburizing depth required for large gears requires a long-term treatment, requires the addition of large amounts of expensive alloying elements such as Mo and Cr, and still requires Hardening distortion is not sufficiently suppressed, and the problem of distortion correction grinding has not been solved.

[0008] Therefore, the present inventors have found that, when steel is cut into a predetermined gear shape using steel having an inexpensive composition which is generally used in the past, and then carburized and quenched, the strain absorption capacity due to this quenching is obtained. The task was to find a quenching method that would result in a steel material having a large microstructure. Thus, an object of the present invention is to provide a case hardening method for a case hardened steel material that does not require correction grinding of a steel shape such as a tooth shape after carburizing and quenching a steel material such as a gear without using steel having a special component composition. It is in.

[0009]

Means for Solving the Problems From the above-mentioned viewpoints, the present inventors have made intensive studies to develop a low strain case hardening steel material. As a result, the case hardened steel material processed into a required shape such as a gear shape is subjected to a carburizing treatment at a high temperature equal to or higher than a certain temperature, and this is held in a two-phase temperature range of austenite + ferrite to precipitate ferrite. By quenching and changing the microstructure to an appropriate ferrite + martensite mixed structure, the strain accompanying the expansion of the martensitic transformation can be absorbed by soft ferrite, and the modified grinding of the original processed shape such as tooth shape Was found to be unnecessary.

[0010] The present invention has been made based on the above findings, and has the following features. The method for preventing quenching of heat treatment of case hardened steel according to claim 1 includes a component composition of a conventionally used case hardened steel, that is, contains C, Si and Mn, or contains C, Si and Mn, and C.
r, when carburizing-hardened steel having a component composition comprising one or more alloying elements selected from the group consisting of Ni and Mo, in A ₃ + 50 ° C. or higher, from 0.8 wt.% 1 The case hardening steel is carburized in a medium having a carbon potential in the range of up to 0.4 wt.% (Denoted as 0.8-1.4 wt.%; Otherwise equivalent) and then carburized. The case-hardened steel material thus obtained is subjected to a diffusion treatment to reduce the carbon concentration in the surface layer to a range of 0.6 to 0.9 wt.%, And then the case-hardened steel material is heated to a temperature range of A ₁ +10 to A ₃ -10 ° C. After the ferrite is generated at an area ratio in the range of 10 to 50% with respect to the microstructure of the non-carburized portion by maintaining the temperature for 10 minutes or more, quenching is performed.

The method for preventing and quenching a case-hardened steel material according to the second aspect of the present invention is the same as that of the first embodiment, except that the case-hardened steel having the component composition of the conventionally used case-hardened steel is carburized and quenched. In addition, at a temperature of A ₃ + 50 ° C. or more, 0.6 to
Carburizing the case hardened steel in a medium having a carbon potential within the range of 0.9 wt.
After produced within 10 to 50% in the ₁ + _{10~A 3} noncarburized area percentage of ferrite with respect to microstructure of holding the temperature at 10 minutes or more in the range of -10 ° C., characterized in that quenching It has.

However, the component composition of the conventionally used case hardening steel material is so-called case hardening steel, wherein C is 0.12 to 0.12.
0.24 wt.%, A carbon steel containing an appropriate amount of Si and Mn, and a low alloy steel or a boron steel containing an appropriate amount of an alloying element such as Ni, Cr, Mo, V and B in addition to the carbon steel, and Refers to those conforming to these, for example, JIS G40
The “type symbol” of the chemical component in “Structural steel material (H steel) with guaranteed hardenability” specified in 52 is SCM
420H (chrome molybdenum steel), SCr420H (chrome steel), SCM822H (chrome molybdenum steel), S
Mn420H (manganese steel) and SNCM420H (nickel chromium molybdenum steel) correspond to them, and other steel materials specified in JIS G4102-G4106 are also applicable. The above carbon steel and low alloy steel or boron steel of the case hardening steel which is the object of the present invention further contains N
A crystal grain refinement element such as b, Ti, and Al may be included. Further, Pb, Bi, S, Se, Te, or the like may be included as a free-cutting property imparting element in steel materials having any of the above component compositions.

[0013] The case hardening steel material to which the present invention is applied is to form a surface layer having a high carbon and / or nitrogen concentration by infiltrating carbon and / or nitrogen from the surface of a material having a relatively low carbon concentration. And then quenching it,
A steel material having a chemical composition as described above, which is suitable for hardening the surface layer portion and improving the internal toughness, and a molded product obtained by processing such a steel material into a predetermined shape, for example, a gear. The present invention is applied not only to carburizing and quenching, but also to carbonitriding and quenching.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The method of the present invention is for carburizing, diffusing and heat treating steel materials (including mechanical parts such as gears) processed into a required shape from case hardened steel under the above conditions. And generating the above-mentioned microstructure on the way. The reasons for limiting various conditions in the present invention will be described below.

(1) Carburizing temperature: A ₃ + 50 ° C. or higher If the carburizing temperature is lower than A ₃ + 50 ° C., the diffusion rate of carbon in steel is slow, and it takes a long time to enter the inside. ₃ Set to + 50 ° C or higher. In addition, the heating time of carburizing is determined according to a desired carburizing depth using a known equation for estimating the relationship between the heating time and the carburizing depth.

(2) Carbon concentration in the surface layer: 0.6 to 0.9
wt.% and carbon potential at the time of carburization: 0.8-1.
4 wt.%, Or 0.6-0.9 wt.% The carbon concentration of the surface layer of the case hardened steel
The reason for adjusting the content within the range of wt.% is to transform the entire surface layer portion (for example, the tooth surface portion of the gear) after quenching into martensite and to obtain a microstructure having a required surface hardness (HV = 600 or more). This is because it is necessary to obtain an organization.

In the first aspect, the carbon potential at the time of carburizing is set to 0.8 to 1.4. This is because, when obtaining a deep hardened layer depth such that the carburized depth exceeds 1.0 mm in a large part, after carburizing with a high carbon potential to increase the carbon concentration on the surface, 0.6 to 0. 9wt.%
Diffusion treatment is performed so that the surface carbon concentration becomes. By doing so, the carburizing time can be shortened. However, if the depth of the hardened layer is relatively shallow, about 0.5 mm, carburization may be performed at a carbon potential of 0.6 to 0.9 wt.% From the beginning. Therefore, in claim 2, the carbon potential during carburization is set to 0.6 to 0.9 watts.
Within the range of t.%.

(3) Holding temperature before quenching: A ₁ +10
A ₃ -10 ° C If the holding temperature before quenching is higher than A ₃ -10 ° C, it is difficult to form a sufficient amount of ferrite inside the tooth root. On the other hand, if the holding temperature is lower than A ₁ + 10 ° C.,
Conversely, the amount of ferrite becomes too large, and the strength of the steel becomes insufficient. Therefore, the steel material temperature before quenching is A ₁ +10 to A
₃ maintained at a temperature in the range of -10 ° C..

(4) Holding time before quenching: 10 minutes or more In order to precipitate ferrite with an area ratio of 10 wt.% Or more while maintaining the temperature in the range of A ₁ +10 to A ₃ -10 ° C. If the holding time at the temperature is less than 10 minutes, the precipitation of ferrite is not sufficient. Therefore, the above holding time should be secured for 10 minutes or more.

(5) Microstructure: ferrite area ratio =
It is extremely important in the present invention to make the microstructure in the non-carburized portion (for example, the microstructure in the tooth root and in the gear core portion in a gear) into a ferrite in the present invention, and as described above, carburizing and quenching. This is because the stress (transformation stress) due to volume expansion accompanying the martensitic transformation at that time is absorbed in the ferrite formation region to prevent the occurrence of distortion. In this case, when the ferrite area ratio is less than 10%, the transformation stress generated in the steel material such as the gear cannot be sufficiently absorbed, and the generation of the strain cannot be sufficiently suppressed. If the content of ferrite exceeds 50%, the strength of the material becomes insufficient, so the ferrite area ratio needs to be 50% or less. Further, the optimum amount of ferrite is in the range of 20 to 30% in area ratio.
Then, the microstructure after quenching is a two-phase structure of ferrite + martensite. At this time, a part of the retained austenite or bainite may be mixed in the martensite. After the quenching, tempering is usually performed at a temperature in the range of 150 to 200 ° C.

[0021]

Next, the present invention will be described in more detail with reference to examples. Table 1 shows the steel types (symbols) and chemical composition of the eight case hardened steel materials used in this test. However, all of the chemical composition shown in the table are included in the chemical composition of the conventionally used case hardening steel. For each of these eight steel subjected to transformation point measurement test, the resulting A ₁ and A ₃ temperature are also shown in Table 1.

[0022]

[Table 1]

Each of the case hardened steel materials shown in Table 1 was used in two types: a US navy test piece for measuring the quenching strain shown in FIGS. 4 and 5, and a spur gear for measuring the microstructure and hardness of the gear shown in FIG. Processed to.

FIG. 4 is a front view of the navy C test piece, 2 is an opening, 3 is a circular space, o is the center of the navy C test piece 1, a is 60 mm, c is 34.8 mm, and d is 6 mm. FIG. 5 is a side view of FIG. 4, and the thickness b of the navy C test piece 1 is 12 mm. FIG.
It is a perspective view which shows the external appearance of a spur gear, and 4 and 5 are sparger large and sparger small, respectively.

Various heat treatment methods (Examples 1 to 10) within the scope of the present invention and various heat treatment methods outside the scope of the present invention were applied to each of the navy C test piece and sparger having the chemical composition shown in Table 1. A test was performed by a heat treatment method (Comparative Examples 1 to 3). Examples 1 to 4, 6, 7, and 1
0 is within the scope of the invention described in claim 1, and Examples 5, 8, and 9 are within the scope of the invention described in claim 2.

Tables 2 and 3 show treatment conditions such as carburizing treatment, case diffusion treatment, and holding temperature before quenching of the case hardened steel material in Examples and Comparative Examples.

[0027]

[Table 2]

[0028]

[Table 3]

The test materials described above were subjected to the following test measurements. Measurement of heat treatment strain: Using a navy C test piece. Gear surface carbon content: tooth surface 6 of spur gear 5
(See FIG. 7) C concentration. Amount of ferrite inside the root: Ferrite area ratio of the inside 7 of the spur gear 5 (see FIG. 7) Hardness inside the root: Vickers hardness (HV) of the inside 7 of the spur gear 5 (see FIG. 6) ) The measurement results are also shown in Tables 2 and 3 above.

(1) Comparative Example 1, Examples 1 to 3, and Comparative Example 2 All used steel type SCM420, and carburizing conditions were constant (carbon potential: 1.0 wt.%, After carburizing at 920 ° C. for 4 hours,
(Diffusion treatment for 1 hour), and the test was conducted by changing only the temperature and time before quenching, followed by oil cooling. Comparative Example 1 is a conventional method, and FIG.
Shows carburizing and quenching patterns. The temperature before quenching is
After holding at 850 ° C. in the austenite single phase region for 30 minutes, oil cooling was performed. No ferrite was observed in the structure of the non-carburized portion, and the structure was a martensite structure partially including retained austenite. In addition, the amount of distortion at the opening of the US navy test piece is as large as 2.5, and the deformation of the gear is large. If it was used as it was, there was a concern that the surface pressure would locally increase, causing pitching to occur early and increasing noise, so the tooth profile was ground and modified.

In Example 1, the holding temperature before quenching was set to A ₃
It is kept at 800 ° C. lower than −10 ° C. for 65 minutes. The non-carburized portion contained 12% ferrite, so the strain amount was remarkably small at 0.35%, and there was no need to correct the tooth profile.

In Example 2, the holding temperature before quenching was 78
The temperature was kept at 0 ° C. and kept for 35 minutes. Figure 1 shows the case of carburizing
3 shows a quenching pattern. The non-carburized portion contained 32% ferrite, so that the distortion amount was as small as 0.02%, there was almost no distortion, and there was no need to modify the tooth profile.

In Example 3, the holding temperature before quenching was set to 76.
It was kept at 0 ° C. and kept for 25 minutes. 2 for non-carburized part
5% ferrite, so that the strain amount was 0.05%
% And did not need to be modified.

In Comparative Example 2, the holding temperature before quenching was A ₁
The temperature was set to 730 ° C. lower than the temperature and maintained for 20 minutes. For this reason, the ferrite area ratio is as large as 56%,
The internal hardness is as low as 260 in Vickers hardness, which is insufficient for the hardness of the gear. In addition, pearlite is also mixed in the structure of the carburized portion, which is an unfavorable structure from the viewpoint of wear resistance.

(2) Example 4 and Comparative Example 3 In Example 4, the steel type SCM421 having a slightly higher C and Mn concentration than the steel type SCM420 was treated with the carburizing and quenching pattern shown in FIG. Things. In Example 2, the hardness inside the tooth root is HV of 300, which is slightly lower than the hardness HV330 of Comparative Example 1. As described above, the hardness HV330 is reduced by using the steel type SCM421. can get.

Comparative Example 3 is a case in which steel type SCM420 was processed into a test piece or after being processed into a gear, normalizing was performed to refine the structure, and then gas soft nitriding was performed for 5 hours. It belongs to the law. The distortion was small, and there was no need to correct the tooth profile. However, the maximum problem of the present invention has not been solved in that the hardness inside the tooth root is extremely low as HV200 and two heat treatments are required.

(3) Further, tests were carried out using case hardening steel materials of each steel type shown in Table 1 and variously changing carburizing conditions and the like (Examples 5 to 10). In Example 5, the carbon potential was 0.85 wt.
Carburizing at 00 ° C for 3 hours without diffusion annealing
Thereafter, it was kept at 770 ° C. for 30 minutes and then quenched. FIG. 3 shows the carburizing / quenching pattern. Also in this example, the distortion amount was as small as 0.16%, and there was no need to correct the tooth profile.

In all of Examples 6 to 10, the holding conditions before quenching are appropriate, the amount of ferrite is appropriate, and the distortion is small. As a result, there was no need to modify the tooth profile of the quenched gear.

[0039]

As described above, according to the present invention,
Without using a special steel component, a high-strength gear with low distortion equivalent to that of normal nitrocarburizing and higher internal hardness of the tooth root by nitrocarburizing becomes possible. The present invention
As described above, it is possible to provide a method of preventing and quenching heat treatment of a case hardened steel material, which has an industrially useful effect.

[Brief description of the drawings]

FIG. 1 shows an example (Example 2) of a carburizing / quenching pattern according to the present invention.

FIG. 2 shows an example of a conventional carburizing and quenching pattern (Comparative Example 1).

FIG. 3 shows another example (Example 5) of the carburizing / quenching pattern of the present invention.

FIG. 4 is a front view of a navy C test piece for measuring a quenching distortion amount.

FIG. 5 is a side view of FIG. 4;

FIG. 6 is a perspective view showing an appearance of a spur gear.

FIG. 7 is a schematic perspective view illustrating the inside of the teeth (the inside of the root and the gear core) and the surface of the gear.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Navy C test piece 2 Opening 3 Circular space 4 Large spur gear 5 Small spur gear 6 Tooth surface part (carburized part) 7 Inside tooth root 8 Gear core part

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI F16H 55/06 F16H 55/06

Claims (2)

[Claims] 1. A case hardening steel material containing C, Si and Mn, or a case hardening steel material containing one or more alloying elements selected from the group consisting of C, Si and Mn, and Cr, Ni and Mo. a method of carburizing and quenching, by a ₃ + 50 ° C. or higher temperatures, 0.8~1.4wt.% of carburizing the hardened steel in Baizai having carbon potential in the range, then in this way is carburized The case hardening steel material is subjected to a diffusion treatment to reduce the carbon concentration of the surface layer within a range of 0.6 to 0.9 wt.%, And then the case hardening steel material is heated to a temperature range of A ₁ +10 to A ₃ -10 ° C. At a temperature of 10 minutes or more, and ferrite in an area ratio of 10 to 10% with respect to the microstructure of the non-carburized portion of the case hardened steel material.
A method for preventing quenching of a case hardened steel material by heat treatment, wherein the quenching is performed after being formed within a range of 50%. 2. A case hardening steel material containing C, Si and Mn, or a case hardening steel material containing one or more alloying elements selected from the group consisting of C, Si and Mn, and Cr, Ni and Mo. a method of carburizing and quenching, by a ₃ + 50 ° C. or higher temperatures, 0.6~0.9wt.% of carburizing the hardened steel in Baizai having carbon potential in the range, then the hardened steel Was maintained at a temperature in the range of A ₁ +10 to A ₃ -10 ° C. for 10 minutes or more to produce a ferrite in an area ratio of 10 to 50% with respect to the microstructure of the non-carburized portion of the case hardened steel material. A method for preventing hardening of a case hardened steel material by heat treatment, which is followed by quenching.