WO2006085549A1

WO2006085549A1 - High-concentration carburized/low-strain quenched member and process for producing the same

Info

Publication number: WO2006085549A1
Application number: PCT/JP2006/302161
Authority: WO
Inventors: Isao Machida; Hisashi Abe; Toshio Fukushima; Koji Horikiri
Original assignee: Parker Netsushori Kogyo K.K.
Priority date: 2005-02-08
Filing date: 2006-02-08
Publication date: 2006-08-17
Also published as: JPWO2006085549A1; KR20070095390A; CN101115859B; EP1847630A4; EP1847630A1; JP4627776B2; CA2594838A1; KR100898679B1; CN101115859A; US20080156399A1; CA2594838C; EP1847630B1

Abstract

A high-concentration carburized/low-strain quenched member that in the field of high-concentration carburization quenching characterized by high strength, high surface pressure, etc., simultaneously attains realization of high performance and minimization of heat treatment strain. As the process for producing the same, there is provided a process for producing a high-concentration carburized/low-strain quenched member, characterized by including a primary treatment in which according to vacuum carburization (low-pressure carburization) technique, a steel member for mechanical structure is heated to temperature within the austenite region, subsequently a solid dissolution of carbon amounting to ≥ eutectoid carbon concentration is effected in the surface layer portion of the member and thereafter the member is quenched at a cooling rate of 3° to 15°C/sec from the temperature within the austenite region to temperature of ≤ A1 transformation point so that in the surface layer portion of the member, there are generated fine carbide and/or nuclei thereof; and a secondary treatment in which continuously the member is subjected to temperature raising to temperature within the austenite region and soaking and thereafter the member is rapidly quenched so that in the outermost surface layer portion, fine carbide is precipitated in an effective hardening depth ratio ranging from 10 to 30%.

Description

Specification

High-concentration carburizing · Low strain quenching member and its manufacturing method

Technical field

[0001] The present invention relates to a high-concentration carburizing and quenching member having features such as high temper softening resistance, high strength and high surface pressure, among carburizing and quenching processes that are widely used as a means for strengthening mechanical structural members. High-concentration carburization that achieves both high performance and the opposite characteristics of heat treatment strain • Conventional technology related to low-strain quenched parts (hereinafter sometimes referred to simply as “members”) and their manufacturing methods

[0002] Carburized and quenched members are widely used as various components for transportation equipment, industrial machinery, etc. due to their excellent durability and wear resistance. Many developments related to carburized and quenched members have been made from the viewpoints of 匕. In recent years, a vacuum carburizing (low pressure carburizing) process has been developed, which is more environmentally friendly than conventional gas carburizing processes, prevents intergranular oxidation of the carburized layer, and enables high-temperature carburizing treatment. Has excellent features such as easy control of carburization and carbon diffusion, and is expected to become more widespread from the standpoint of further improving the performance and quality of components and improving productivity of components.

[0003] As a method for carburizing and quenching mechanical structural members such as gears and shaft members and improving the anti-pitting performance of the members, there is a carbonitriding process. This simultaneously diffuses carbon and nitrogen into the matrix (matrix), and improves the temper softening resistance of the member. In addition, a high-concentration carburizing process has been developed to precipitate carbide on the surface layer of the member and improve the temper softening resistance of the member. In recent years, many studies have been conducted in conjunction with the evolution of low-pressure carburizing equipment.

[0004] As a representative example of the high-concentration carburizing method, there is a carburizing method for members disclosed in Patent Document 1. According to Patent Document 1, spherical carbide is precipitated on the surface layer portion of a steel material, and precarburizing is performed with a carbon amount that is equal to or greater than the eutectoid concentration of steel and carbon when the carbon concentration of the surface layer portion is Acm or less. After that, the treated member is gradually cooled or rapidly cooled, and the surface layer portion thereof is bainite, norlite. Alternatively, after forming a martensite structure, the temperature is raised to a temperature range of Acl point strength of 750 to 950 ° C at a heating rate of 20 ° CZ or less, carburizing and quenching is performed, and a volume ratio in a depth range of 0.4 mm A method for producing 30% or more pseudo-spherical or spherical carbide has been proposed.

[0005] However, in the above method, although the characteristics such as the anti-pitting performance of the member are improved by precipitating carbide on the surface layer portion of the member, this method is effective for 30% of the carbide on the surface layer portion. The resulting member has problems such as heat treatment deformation and strain due to the high-concentration carburizing that precipitates.

[0006] With regard to a method for finely depositing carbide on the surface layer of a member by a high-concentration carburizing method, many heating and cooling methods have been studied. Generation) or quenching (a martensite structure is generated), and in the next carbide generation process, heating is performed from the Acl transformation temperature to 750 to 950 ° C at a slow rate of 20 ° C / min and directly quenching or air cooling. Later, it is said that the reheating quenching method is good.

[0007] In addition, Patent Document 2 and Patent Document 3 also propose that slow cooling (or 30 ° C / Hr or less) is optimal after preliminary carburization or primary carburization.

[0008] However, in the method shown in Patent Documents 1, 2, and 3, when the quenching after preliminary carburization or primary carburization is by air cooling or slow cooling, along the grain boundary within the surface layer portion of the member Retardation of reticulated carbides It is difficult to decompose the reticulated carbides in a short time and disperse and precipitate in the surface layer in the next carbide generation process, and there are examples in which heating and cooling are performed multiple times. .

[0009] On the other hand, in Patent Document 1, after preliminary carburizing of a member, there is quenching aimed at a martensite structure by increasing the cooling rate, but there is a possibility that carbide nuclei in the surface layer dissolve and disappear. There is a concern that deformation or distortion such as expansion or contraction of the material increases due to the supersaturated quenching of carbon and high carbon martensite transformation.

Patent Document 4 describes a method for producing a high-concentration carburized member by a low-pressure carburizing method. The carbon concentration of primary carburization is 0.5 to 0.7% by mass, and the carbon concentration of secondary carburization is 0. Although there is a description of carbide refinement, such as 7 ~:!% By mass and primary cooling 1 ~: 10 ° C / min, etc. Preferred as in 1, 2 and 3 Not expected.

[0011] For reference, the following points show that the low-pressure carburizing method, which has been expanding recently, is superior to the normal gas carburizing method.

a) The atmosphere conditions in the furnace can be changed easily and quickly, and it is easy to switch from a carburizing atmosphere to a carbon diffusing atmosphere.

b) High-temperature treatment is possible and quick carburization is possible.

c) Since there is no grain boundary oxidation in the surface layer of the member, the occurrence of cracks in the treated member starting from these is suppressed.

d) There is no occurrence of uneven carburization due to sooting.

Patent Document 1: Japanese Patent Publication No. 62-24499

Patent Document 2: Japanese Patent No. 2787455

Patent Document 3: Japanese Patent No. 2808621

Patent Document 4: Japanese Patent Laid-Open No. 2002-348615

Disclosure of the invention

Problems to be solved by the invention

[0012] However, even in the high-concentration carburizing process by the conventional low-pressure carburizing method, the optimum balance between the formation process of carbide in the surface layer of the member to be processed and the microstructure of the surface layer part cannot be obtained. The problem of deformation and distortion still remains. For this reason, it is essential to polish the parts after the carburizing process and finish the parts to correct the distortion, which reduces the high surface pressure performance of the high-concentration carburization that should be obtained, and further improves the productivity. Reductions and cost increases hinder the spread of high-concentration carburizing treatment. Means for solving the problem

[0013] In the present invention, various controls such as the carbon concentration of the member, repeated carburizing / diffusion treatment, various temperature conditions such as temperature rise, soaking, carburizing, and quenching of the member, heating conditions, and cooling rate (quenching) conditions. Using the low-pressure carburizing equipment that can respond quickly and with high accuracy, we have developed an optimal process that can achieve both high surface pressure and low distortion of the parts, and solved the above problems.

[0014] The above-described object is achieved by the present invention described below.

1.By using vacuum carburizing (low pressure carburizing) method, steel for machine structural use And the surface layer portion of the member is solid-dissolved with carbon having a eutectoid carbon concentration or higher, and then the member is heated from the temperature of the austenite region to a temperature not higher than the A transformation point.

Tempering is performed at a cooling rate of 1 ° CZ second to generate fine carbides and / or nuclei of the carbides on the surface layer of the member, and then the member is heated and soaked to the temperature of the austenite region. After that, high-concentration carburizing and low-distortion quenching, characterized by consisting of a secondary treatment that precipitates fine carbides in the range of 10 to 30% of effective hardening depth ratio in the outermost layer by rapid quenching A method for manufacturing the insert member.

[0015] 2. The method for manufacturing a member, wherein in the secondary treatment, additional carburization is performed on a surface layer portion of the member.

3. In the secondary treatment, fine carbides are precipitated on the surface layer portion of the member, and a structure mainly composed of martensite including a mixed structure such as troostite and retained austenite is formed on the surface layer portion. The order of fineness of the austenite grain size of the outermost layer part (part A), the layer part inside part A (part B) and the layer part inside part B (part C) is given as A≥C≥ B The manufacturing method of the said member.

[0016] The surface layer is a structure mainly composed of martensite including a mixed structure such as troostite and retained austenite, and the order of the fineness of the austenite grain size of the layer. A high-concentration carburizing / low-distortion quenching member characterized in that A ≥ C≥B in the layer inside part A (part B) and the layer inside part B (part C).

The invention's effect

[0017] The method of the present invention uses a low-pressure carburizing equipment for the processing of the member, a primary treatment in which the member is quenched at an appropriate high-concentration carburizing and an optimal cooling rate, and a simple and efficient fine carbide that is subsequently performed. In combination with the secondary treatment to be precipitated, the heat treatment deformation and distortion of the treated member can be minimized. This method greatly reduces the number of complicated grinding and distortion correction processes after processing, such as bending of the shaft and deformation of the tooth profile, which were the biggest concerns of conventional high-concentration carburization. It has the effect of making significant improvements in productivity, quality, and cost of high-concentration carburized components.

[0018] Further, in the method of the present invention, in the secondary treatment, the surface of the member is subjected to additional carburization to increase the hardness of the matrix (base), and the outermost layer of the member. The crystal grain size of the part can be made ultrafine, which is extremely effective for increasing the strength and toughness of the member. According to the method of the present invention, it is possible to easily achieve high strength, high toughness, high surface pressure, etc. of members such as shafts and gears, which have conventionally been difficult to apply high-concentration carburizing. Therefore, the method of the present invention can be widely applied to fields where such needs are high, and has the effect of being able to make a significant contribution to the improvement of the performance of members and the small and light weight. .

BEST MODE FOR CARRYING OUT THE INVENTION

Next, the present invention will be described in more detail with reference to the best mode for carrying out the invention. The technical background and knowledge leading to the present invention are as follows.

For the purpose of developing a high-concentration carburizing process for depositing fine carbides on the surface layer of a member using low-pressure carburizing equipment, the present inventors have developed a carbon concentration in the above-mentioned surface layer, various heating and cooling conditions, and fineness in the surface layer. A thorough investigation was conducted on the precipitation form of carbides and the relationship with the microstructure of the base. In addition, research and development has been conducted from various fields to improve heat treatment distortion assuming members such as gears and axles, and the conflicting characteristics between high-performance carburizing and high-performance carburizing and deformation and distortion of parts. The aim was to establish a new high-concentration carburizing and low-distortion quenching method that can balance both of these at a high level.

[0020] An important point in carrying out high-concentration carburizing on the surface layer of steel (member) is to disperse and precipitate how much fine carbide is distributed on the surface layer of the member in the optimal combination of primary treatment and secondary treatment. However, in order to control the formation of fine carbides, the carburizing and quenching equipment used is greatly involved. In the present invention, compared with conventional carburizing equipment, various controls such as carbon concentration, repeated carburizing Z diffusion, temperature rise, soaking, carburizing, quenching, and other temperature conditions, heating conditions and cooling rate conditions are quicker and higher. Various developments were carried out using low-pressure carburizing equipment that can be used with precision.

[0021] Specifically, various investigations such as temperature rise, soaking, high-concentration carburization, diffusion and cooling (quenching) conditions of the member during the primary treatment were conducted, and first, deformation and strain of the member during the primary treatment stage. In addition, carburizing and quenching (cooling) conditions that can precipitate fine carbides and adjust the austenite grain size of the carburized layer are important in the secondary treatment of the next process. In other words, in the secondary treatment, the precipitation area of fine carbides on the surface layer of the member is changed to the effective hardening of the carburized layer. It has been found that the heat treatment deformation and strain of the member can be minimized by setting the depth ratio to 10 to 30% and further making the outermost layer part an ultrafine crystal structure.

[0022] Here, the effective hardening depth ratio means the carburized effective hardening depth (T) after completion of the secondary treatment of the member (including 180 ° C tempering treatment) and the finest surface layer portion of the member. It means the ratio (t / T) to the carbide precipitation depth (t). The carburizing effective hardening depth is the Vickers hardness (HV) from the surface of the hardened layer as it is quenched or tempered at a temperature not exceeding 200 ° C according to the JIS G0557 steel carburized hardened layer depth measurement method. The distance to the limit depth position of 550. Next, the precipitation depth of fine carbide is analyzed by an optical microscope or an electron microscope, and the maximum depth at which fine carbide exists from the outermost layer portion of the member is measured. Here, in order to make it easy to distinguish fine carbides, the member is analyzed in an etching state using a corrosive solution such as 5% nitrate alcohol.

[0023] The vacuum carburizing (low pressure carburizing) equipment used in the present invention is equipment having a processing furnace in which a carburizing heating chamber can control partial pressure to 200-2, OOOPa, and the equipment is commercially available. Any equipment can be used in the present invention. In the present invention, as a primary treatment, after raising the temperature of the material to a predetermined temperature and soaking in the furnace of the equipment, the carbon concentration of the surface layer of the member is set to a carbon concentration equal to or higher than the eutectoid carbon concentration. After alternately repeating diffusion and diffusion, quench the member at an appropriate cooling rate. In the subsequent secondary treatment, carbides are finely deposited on the surface layer of the member, and additional carburization is performed as necessary.

[0024] In the primary treatment in the method of the present invention, the steel material (member) to be treated is heated to an austenite region at 900 to 1,100 ° C and soaked, and the carbon concentration in the surface layer portion is preferably 0. Carburize to 8% by mass or higher, and then quench from that state at the optimal cooling rate. Here, the optimum cooling condition is from the carburizing temperature (temperature in the austenite region) to the A transformation point or less, preferably

1

The temperature range up to 400 ° C is 3 to: The member is cooled uniformly at a cooling rate of 15 ° C / sec. By this cooling, fine carbides are deposited on the surface layer of the member, and a structure mainly composed of martensite is formed on the surface layer. Here, fine carbide means Fe C (Cementite

Three

) And carbide-forming elements such as Cr and Mo in steel materials mean MC type carbides produced by combining with supersaturated solid solution carbon.

23 6

[0025] Next, in the secondary treatment, the non-carburized part (inside) of the member is austenitized at + 80 ° C or higher. Of these, the temperature is preferably raised and soaked in the range of austenite temperature +10 to 70 ° C., and then rapidly cooled, so that the carbon concentration in the surface layer is preferably 0.8% by mass or more, more preferably 1.0. Precipitate fine carbides to make ~ 2.0% by mass. In parallel with the precipitation of fine carbides in the secondary treatment, additional carburizing treatment is performed on the surface layer to promote the precipitation of fine carbides in the surface layer, and the carbon concentration of the matrix (base) is set appropriately. It is preferable to perform rapid quenching from the adjusted state.

[0026] As for the final quenching temperature after the secondary treatment, the pre-treatment conditions are as follows. The temperature may be increased or the temperature may be raised or lowered with respect to those temperatures. That is, the final quenching temperature after the secondary treatment can be set in accordance with the heat treatment quality such as hardness and microstructure required for the member.

[0027] In order to establish the optimum conditions for high-concentration carburization, the inventors have performed a primary treatment in which high-concentration carburization is performed on the surface layer portion of the material using low-pressure carburizing equipment, and fine particles of carbide are precipitated in the surface layer portion. A detailed study was conducted on the secondary treatment to be performed, including temperature rise, soaking, carbon concentration and diffusion during carburization, and various cooling (quenching) conditions. As a result, the carbon concentration is preferably 0.8% by mass or more, more preferably 1.0 to 2.0% by mass in the range of 10 to 30% in effective curing depth ratio (t / T) at the outermost layer. %, And succeeded in obtaining a high-concentration carburized and quenched member having a three-layer structure consisting of an ultrafine-grained layer with an austenite grain size of 10 or more than the outermost layer, followed by a fine-grained layer and a fine-grained layer. . This high-concentration carburizing and quenching member was found to have minimal deformation and distortion after processing, and it was found that the correction of distortion, which was unavoidable with conventional high-concentration carburizing, is unnecessary or can be handled more easily than conventional methods.

Example

Next, the present invention will be described in more detail with reference to examples.

Using steel materials for machine structure shown in Table 1 below, post-standardizing the material at 900 ° C, and then creating a stepped round bar test piece of φ30 / φ25 / φ20 X L300mm by machining did. The carburizing and quenching of the test piece was performed in the high concentration carburizing process of the present invention using equipment that can be heated and carburized under a low pressure and that can be oil-quenched and cooled by pressurized gas. Here, steel type symbols 1 and 2 are JIS carburizing and quenching steel materials, symbol 1 is chromium-molybden steel SCM420, and symbol 2 is chromium steel SCr415. MA C14 with steel grade code 3 is a product code developed by a steel material manufacturer. Compared to the above two steel grades, the Cr content is increased, and Mo element is added to the MC, and during high-concentration carburizing (primary and secondary treatment), MC Fine type

23 6 Steel developed for the purpose of precipitation of fine carbides.

[0030]

Table 1: Steel materials used and chemical composition (mass%)

[0031] Table 2 summarizes various effects of the cooling rate on the precipitation state of carbide in the surface layer portion of the specimen and the heat treatment deformation of the specimen in the primary treatment of the present invention. . Here, as a primary treatment condition, the test piece was subjected to high-concentration carburization with the target of an effective hardening depth of 0.5 mm after heating and soaking in the heat cycle shown in FIG. Specifically, high-concentration carburization and diffusion treatment were alternately carried out at a temperature of 950 ° C for about 70 minutes so that the carbon concentration of the surface layer of the test piece in the final state was about 1.5% by mass, When the carbon concentration of the surface layer of the specimen is supersaturated, the specimen is quenched at each cooling rate condition shown in Table 2, and the shape and size of carbide in the surface layer of the specimen and the microstructure of the surface layer are investigated. did.

[0032] Further, in order to investigate the heat treatment deformation and strain of the test piece by the primary treatment, a stepped round bar test piece (30 / φ25 / φ20 X L300mm) was used as the test piece, and the test piece with both ends supported was used. The amount of runout at the center of the shaft was analyzed, and the relationship between the cooling rate during quenching of the specimen and the amount of runout of the specimen was investigated. [0033]

Table 2: Relationship between primary treatment cooling conditions, carbide precipitation and axial runout

Here, symbols and analysis methods shown in the table will be described below.

1) Cooling rate represents the average cooling rate at the axial center of the specimen from the quenching temperature of 950 ° C after completion of carburizing and diffusion of the specimen to 400 ° C.

2) The shape and size of the carbide were observed with a scanning electron microscope.

3) Microstructure abbreviation

F: Ferrite, P: Pearlite, B: Bainite, T: Truthite, M: Martensite, γ: Residual austenite

4) Shaft run-out represents the run-out at the center of the shaft measured with a dial gauge with the test piece attached to a run-out measuring instrument supported at both ends.

[0035] Here, in the comparative examples shown in test pieces Nos. 1, 4, and 6 in Table 2, the cooling rate during cooling is as low as 1 ° C / sec. Precipitation of continuous network carbides is the main, and the base becomes an incompletely quenched structure of ferrite, pearlite, and bainite. As a result, axial runout and deformation become large. In addition, the comparative example shown in test piece No. 3 was obtained by rapid cooling equivalent to a typical oil quenching (20 ° C / sec), and the amount of carbide precipitation was very small and high carbon with supersaturated carbon. It becomes a quenched structure from the state, and the shaft runout and deformation are large. [0036] Next, when the cooling rate of the examples shown in test pieces 2, 5, and 7 is 4 to 12 ° C / sec (the present invention), a large amount of fine carbides are precipitated, and the fine particles that form the core The appearance of the structure improved the deformation and strain (shaft runout) of the specimen, which was a major concern for high-concentration carburization. In other words, compared to slow cooling, which is slow cooling, or conversely rapid rapid quenching, in the present invention, the shaft runout amount of the test piece is approximately half that of the other examples, and the shaft runout amount can be significantly reduced. It was. Based on these results, the cooling rate during quenching of the primary treatment is optimally 3 to 15 ° C / sec.

[0037] Table 3 uses a representative test piece of the primary treatment shown in Table 2 and performs a secondary treatment for the purpose of finally precipitating fine carbides on the surface layer portion. The results of various investigations such as concentration, carbide precipitation state, microstructure, grain size, and axial runout of the specimen are shown. As conditions for the secondary treatment, the soaking temperature is set to three levels of 800 ° C, 850 ° C and 900 ° C above the A transformation in the heat cycle shown in Fig. 2, and the surface layer is obtained by secondary treatment.

1

As a method of increasing the carbon concentration of the steel part and further increasing the precipitation of fine carbides, additional carburization that exceeded the eutectoid carbon concentration was also performed simultaneously after heating and soaking.

Figures:! ~ 3 (Carburizing / Diffusion) n and (Additional Carburizing / Diffusion) n of “n” means the number of repetitions of carburizing and diffusion in each process, the number of “n” is required for each member Set according to quality. For example, n = 8 in the case of Example No. 2 shown in Table 2, and n = 5 in the case of Example No. 2_2 shown in Table 3.

Table 3: Relationship between secondary treatment conditions, carbide precipitation and shaft runout

"Analyzing method of carbon concentration in surface layer"

Using a test piece (307 () 25/20 and 300111111), cut chips from the surface layer of φ25mm to a depth of 0.05mm by turning and obtain the carbon concentration of the surface layer by wet analysis. The [0040] From Table 3, the specimen No. 2 series shows the carbide precipitation form and other effects when the secondary treatment temperature is changed, and the No. 5 and 7 series show secondary effects. The effect of the presence or absence of additional carburization in the treatment on the precipitation of fine carbides and the final carbon concentration in the surface layer is shown.

[0041] Here, regarding the secondary treatment temperature (herein referred to as the tempered carburizing temperature), at a temperature of 900 ° C using test piece No. 2_1, the carbide in the surface layer was dissolved and the entire carbide There is a problem that the precipitation of particles is small and the axial runout of the test piece is also large. In addition, at the 800 ° C secondary treatment temperature used for test piece No. 2-3, flake carbides precipitated at the grain boundaries of the surface layer, and the inside of the member (unhardened part) was incompletely quenched. Variations in the axial runout of the test piece appear. From these results, the optimum processing temperature for precipitating fine carbides on the surface layer in the secondary treatment is determined by the composition of the member (before carburizing treatment), and a temperature corresponding to the A transformation point temperature + 10 to 70 ° C is preferable. .

[0042] Next, regarding the presence or absence of additional carburizing treatment in the secondary treatment, test pieces 5-1 and 7

By performing the additional carburizing treatment as apparent from the result of 1, the effect of the fine precipitation of carbides as well as the improvement of the carbon concentration in the surface layer is recognized. This is because the carbon in the surface layer precipitates as carbides and the carbon concentration of the matrix (base) becomes dilute. In addition, the formation of fine carbides such as Fe C and MC and their nuclei may be promoted.

[0043] Further, it was found that the austenite grain size of the outermost layer portion of the additional-force carburized member was an ultrafine grain size as shown in FIG. Here, the ultra-fine grain size and carburized grain size test method in the austenite grain size test method of IS-G0551 steel, the austenite crystal grain size corresponds to No. 10 or more, and further consists of fine grains and fine grains toward the internal direction. We have discovered a major feature that forms the layer structure. Also, looking at the relationship between the austenite grain size and the carburized layer, the outermost layer where the amount of fine carbide precipitates is the heel part, the carburized layer part (fine grain part) inside is the B part, and If the fine part inside (non-hardened part) is part C, the grain size of these three layers is in the relationship of A≥C≥B. Incidentally, the austenite grain size of the surface layer in normal carburizing is generally equivalent to No. 7-8, and in the present invention, the conventional carburizing treatment is performed. It has a distinctive three-layered grain structure that does not appear in theory.

Here, as an effect of these ultrafine particle size layers, it becomes possible to improve the toughness of the hardened surface layer, which has been a concern with conventional carburized members, and with the increase in surface pressure that is a feature of the present invention, The carburized layer itself has a great feature that can be imparted with high toughness, and is extremely effective for further strengthening carburized parts in the future.

[0045] Table 4 shows the effect of the effective hardening depth ratio of the carbide precipitation layer on various characteristics in the high-concentration carburization of the present invention. Here, various test pieces were prepared by machining after pre-normalizing the material at 900 ° C using SCM420 of JIS machine structural steel as a material. The high-concentration carburization treatment of the test piece was performed in the heat cycle of the primary treatment and the secondary treatment shown in Fig. 3. The treated specimens were analyzed for pitting life, impact strength, heat treatment strain, etc. In addition, the effect of the carbon concentration in the outermost layer of the test piece shown in Table 5 on the strength durability and heat treatment deformation of the test piece was also treated in the heat cycle shown in Fig. 3, as in the case of the various test pieces in Table 4. The carbon concentration of the treated specimen was examined.

Here, the adjustment of the precipitation depth of the carbides in Table 4 is mainly performed by controlling the carburizing time and the carbon concentration, and the adjustment of the carbon concentration in the outermost layer in Table 5 is performed by the primary treatment or the secondary treatment. The amount of process gas and time during repeated carburizing and diffusion of treatment were controlled by a pre-calculated program. There are propane, acetylene, ethylene, etc. as low-pressure carburizing process gas, but the most popular and inexpensive propane gas was used, and nitrogen gas was used as the inert gas during diffusion. In addition, the rapid quenching in the secondary treatment is the force performed by oil cooling.

2

It can also be carried out by pressurized gas cooling in which gases such as He, H, etc. are used alone or in combination.

[0047]

Table 4: Effect of effective cure depth ratio on strength, durability and heat treatment deformation

Table ⁵ : Effect of carbon concentration on outermost layer on strength, durability and heat treatment deformation

[0049] 1) The effective cure depth ratio represents the ratio (t / T) of the fine carbide layer depth (t) to the cure depth (T) having a micro Vickers hardness of 550 HMV or more.

2) Rolling fatigue life indicates the number of repetitions until the occurrence of pitting under the following conditions.

Surface pressure: 3GPa, rotation speed: 1500i "pm, slip rate: 40%, hydraulic pressure: 80 ° C

3) Impact strength shows the fracture energy by Charpy specimen.

4) Roundness is the test piece shape: φ 100 (> 80) XI 5t ring is used, and the shape measuring instrument shows the deformation amount in the XY direction of the inner diameter of the ring. [0050] Regarding the effect of the effective hardening depth ratio on the rolling fatigue life, in the comparative example of symbol A, when the effective hardening depth ratio is shallow at 5%, high-concentration carburization with less precipitation of fine carbide itself It can be said that the pitching toughness, which is the characteristic of tempering soft resistance, is low. On the other hand, in the comparative example of symbol E having an effective hardening depth ratio of 40%, there is a problem that the impact strength is lowered because the range of high hardness is widened, and the heat treatment deformation as seen from the roundness is also distorted. Tends to increase. From these results, the depth ratio of the carbide precipitation layer to the effective hardening depth is optimally in the range of 10-30%.

[0051] Next, regarding the effect of the carbon concentration of the outermost layer shown in Table 5 on the pitching life, the symbols H, J, and K, which have a higher carbon concentration in the outermost layer, are superior, and the carbon concentration is higher than that of the former. In the case of symbols G and I, which are as low as 1%, it can be said that the pitching life is partially inferior. When the carbon content of the outermost layer is less than 0.8% by mass as shown by reference symbol F, the test piece has a significantly poor pitching toughness. That is, the higher the carbon concentration, the better the finer carbides deposited on the outermost layer. Therefore, in the present invention, the carbon concentration of the high-concentration carburization is set to 0.8 mass% or more.

[0052] The upper limit of the carbon concentration to be carburized was not particularly problematic up to 2.0% by mass. In addition, when the carbon concentration is further higher than 2.0% by mass, flake carbides are likely to precipitate, and there is a concern that the impact strength of the test piece and the heat treatment deformation are disadvantageous in the direction of disadvantage. For this purpose, it is necessary to set the carbon concentration of the outermost layer according to the required characteristics of the member (test piece).

[0053] Next, regarding the effects on the pitching life, impact strength, and heat treatment deformation (strain) when the additional carburizing treatment is applied in the secondary treatment of symbols I, J, and K, the carbon concentration is equivalent and the additional carburizing is performed. Compared to the symbols “N” and “G”, both characteristics are good with little variation. The reason for this is that additional carburization treatment stabilizes the carbon concentration of the base, and further helps to generate fine carbides in the outermost layer, making the carburized layer itself a dense and balanced structure, and the overall heat treatment quality is improved. It seems to be stabilized.

[0054] From the various analysis results described above, the optimum processing condition in the method of the present invention is to perform high-concentration carburization combining primary treatment and secondary treatment by low-pressure carburization using steel for machine structure as a member, After the optimal heating and cooling conditions, It is desirable to control the precipitation depth to be within the range of 10 to 30% of the effective hardening depth ratio and the carbon concentration of the surface layer portion to be 0.8 mass% or more.

Industrial applicability

[0055] From the above series of results, according to the present invention, it is possible to increase the strength and surface pressure of gears and axle members, which are mechanical structural members, as well as members and bearing structures that require low distortion. High-strength and high-performance of various members such as reciprocating and sliding members with high contact pressure, members that require contact fatigue under high surface pressure, and wear resistance, lightweight compactness, etc. It is possible to provide a completely new high-concentration carburizing / low-distortion quenching member that can realize the needs of low-strain and a manufacturing method thereof.

Brief Description of Drawings

[0056] [Fig. 1] Heat treatment for primary processing

[Figure 2] Heat treatment for secondary treatment

[Fig. 3] Example of heat cycle

[Figure 4] Optical micrograph of specimen No. 2-2 in Table 3 (magnification X 100)

Claims

The scope of the claims

[1] A steel member for machine structural use is heated to a temperature in the austenite region by vacuum carburizing (low pressure carburizing) method, and carbon having a eutectoid carbon concentration or more is dissolved in the surface layer portion of the member. From the austenite temperature to the temperature below the A transformation point 3 to: Bake at a cooling rate of 15 ° C / sec.

1

First, a primary treatment for generating fine carbides and / or nuclei of the carbides on the surface layer of the member, and then heating and soaking the member to the temperature of the austenite region, followed by rapid quenching. A method for producing a high-concentration carburized / low-distortion hardened member, characterized by comprising a secondary treatment for precipitating fine carbides in the range of 10 to 30% in effective hardening depth ratio in the surface layer portion.

[2] The method for manufacturing a member according to [1], wherein in the secondary treatment, additional carburization is performed on a surface layer portion of the member.

[3] In the secondary treatment, fine carbides are precipitated on the surface portion of the member, and a structure mainly composed of martensite including a mixed structure such as troostite and retained austenite is formed on the surface layer portion. The order of fineness of the austenite grain size of the outermost layer part (part A), the layer part inside part A (part B) and the layer part inside part B (part C) is A≥C≥ The method for producing the member according to claim 2, wherein B is used.

[4] The surface layer is a structure mainly composed of martensite including a mixed structure such as troostite and retained austenite, and the order of the fineness of the austenite grain size of the layer. A high-concentration carburizing / low-distortion quenching member characterized in that A ≥ C≥B in the layer inside part A (part B) and the layer inside part B (part C).