JPS6227515A - Method for strengthening surface - Google Patents

Abstract

PURPOSE:To strengthen the surface effectively and to retain the excellent toughness of the core part and the moldability by applying surface treatment at a temp. in the austenite region which is higher than the A1 transformation point of steel, then working the austenite and cooling the austenite. CONSTITUTION:Surface treatment such as high-temp. carburization, ordinary carburization and carbonitriding are applied at a temp. in the austenite region which is higher than the A1 transformation point of steel. The plastic working such as forging is applied to the austenite by utilizing the heat. The worked steel as such or the steel which is cooled to a temp. lower than the Ms point is quenched and hardened. Consequently, the surface of the steel material is effectively strengthened, the hardness of the surface is improved and a material having high toughness of the core part, high fatigue strength, resistance to long-period loaded use and excellent moldability is obtained.

Description

[Detailed Description of the Invention] [Object of the Invention] (Field of Industrial Application) This invention relates to a method for strengthening the surface of steel, which is used to strengthen the surface of steel materials and steel parts (products). .

(Prior Art) Conventionally, carburizing and quenching, nitriding, nitrocarburizing, and the like are well known as methods for strengthening the surface of steel materials and steel parts (products).

Among these, carburizing and quenching is a very effective surface strengthening method and is widely used, accounting for about 25% of all heat treatments. However, this degree-of-immersion hardening has the disadvantage of a long processing time, so shortening the carburizing time has been an issue for a long time. (Technology described in Publication No. 101328)
is being industrialized. However, the drawback of high-temperature carburizing is that the toughness decreases due to coarsening of crystal grains, and as a countermeasure to this problem, anti-corrosion prevention elements An and Nb are used to increase the crystal Rffi.

It is necessary to use steel to which Ti, Zr, etc. are added, or to undergo treatment to raise or lower the A or transformation point after immersion, which is a factor in increasing costs.

On the other hand, one of the techniques for toughening steel is the heat treatment method, which can be divided into three types depending on the timing of processing: 1) processing before transformation, 2) processing during transformation, and 2) processing after transformation.

In the conventional working heat treatment method, forging and quenching (Journal of the Institute of Metals, Vol. 32, No. 11) involves plastic working and quenching at a stable austenite region temperature.
1968) p. 1052, Journal of the Japan Institute of Metals VoJL3.1
, No. 2 (1967) p. 126, Journal of the Japan Institute of Metals V
oJ131. No. 4 (1967) p. 347), ausforming in which the process is rapidly cooled at a temperature in the metastable austenite range, and controlled rolling in which it is gradually cooled belong to the process heat treatment before the transformation described in (2) above, and processing is performed during the pearlite or bainite transformation. isoforming, which rapidly cools the
Sub-zero processing, which involves processing during martensite transformation, belongs to the above-mentioned heat treatment during transformation, while patenting, which involves processing after pearlite or bainite transformation, and marforming, which involves processing martensite, are post-transformation processing. Belongs to processing.

Among these, the processing heat treatment before transformation mentioned above is a processing technology that significantly improves the strength and toughness of steel by improving hardenability and refining crystal grains and precipitates. Widely used mainly for rough shaped materials.

On the other hand, the above-mentioned processing heat treatment during the transformation has been studied as a treatment method that significantly improves toughness through the refinement of subgrains and precipitates, but it has not yet been put to practical use.

By the way, gears manufactured by rolling or forging are said to have the following advantages over gears manufactured by conventional cutting.

(1) Loading capacity is increased because extremely excellent fiber flow is formed at the root of the tooth.

(The sides of the two teeth can be easily crowned, resulting in improved load conditions.

(3) It can be manufactured at a lower cost compared to mechanical processing such as gear cutting machines.

etc.

A method for manufacturing gears by rolling that has such advantages is already known (for example, Japanese Patent Application Laid-Open No. 59-225838
In addition, methods for manufacturing gears by precision forging are already known, including the West German BLW method and cold and warm forging methods.

Among these, the concept of precision forging of gears using the BLW method is that the tooth profile of the gear is precisely V! Etlj-made and not to be used as black leather, BLW's unique technology has succeeded in forging precision gears, such as dovetails, bevel gears,
It is widely used in spur gears, etc.

(Problem to be solved by the invention) However, since the gears manufactured by precision forging described above are used with the black skin intact, such gears have a limited wear resistance and are not suitable for use in low load ranges. Therefore, carburizing and quenching is required.Therefore, there is a problem that heat treatment distortion occurs due to this carburizing and quenching, which diminishes the meaning of precision Wi construction.

On the other hand, cold forging makes it possible to manufacture gears with high precision, but in order to ensure the wear resistance of the tooth surface and the strength of the tooth root, surface hardening such as carburizing and quenching or carburizing/nitriding treatment is usually required. is necessary. However, when a material that has been plastically worked for strength by cold forging is heated to a carburizing temperature (approximately 920° C.), crystal grains are enlarged due to recrystallization. At this time, if the processing rate is one pair, the recrystallized grain size will be constant, but
In the case of a gear shape, there is a slight change in the machining rate from the tooth tip to the tooth root, so the recrystallized grains are not constant and become mixed grains.
This may cause pitching, scoring, etc.
There are cases where it cannot be used as a gear, resulting in lower yields and higher costs.

Furthermore, the manufacturing technology for precision gears using warm forging has progressed considerably, and some examples of practical use have been announced. but,
Even for gears manufactured by warm forging, surface hardening treatments such as carburizing and quenching or carbonitriding are required when used in a high load range, which can result in heat treatment distortion for gears that have been formed with high precision. There was a problem.

Furthermore, another way to manufacture high-strength gears is to use medium-ψ high carbon steel (0.4 to 0.6% C) as a material and perform gear cutting, followed by carburizing or carbonitriding. Post-bainite transformation temperature range (e.g. 235-275℃
) is a well-known technology to make the core of the gear a bainite structure and the surface a martensite structure by performing so-called austempering treatment (Heat Treatment Technology Association; Proceedings of the 20th Jijitsu Lecture Conference (Showa)). May 23, 1960
(Japanese) page 49).

However, this gear manufacturing method has a problem in that the machinability of the material in the gear cutting process is very poor, making it difficult to apply to mass-produced gears.

This invention was made by focusing on the conventional problems as described above, and has a large surface treatment effect, such as a large surface hardness, a large toughness in the 6th part, and a large fatigue strength. A steel material that can be used for a long time and has significantly improved formability.

The purpose is to provide a method for surface strengthening of steel parts (products).

[Structure of the Invention] (Means for Solving the Problems) The method for surface hardening of steel according to the present invention provides
It is characterized in that after surface treatment is performed at an austenite range temperature above the transformation point, the austenite is cooled while being processed or after being processed.

A surface strengthening method according to an embodiment of the present invention is a method for strengthening steel.
, After performing surface treatment such as high temperature carburizing, ordinary carburizing, carbonitriding, etc. on the whole or in part at an austenite range temperature higher than the transformation point, using the heat, plastic working such as forging is applied to the austenite, The material is quenched as it is being processed or immediately thereafter cooled to below the Ms point.

In addition, a surface strengthening method according to another embodiment of the present invention includes:
After surface treatment such as high-temperature carburizing, ordinary carburizing, carbonitriding, etc. is carried out on the whole or in part at an austenite region temperature higher than the A1 transformation point of steel, the austenite is rapidly cooled to about 400 to 700 ° C. It is preferable that the plastic working such as forging be applied to a temperature of 50% before the transformation is completed.
% Rapid cooling before the metamorphosis is completed.

A surface strengthening method according to yet another embodiment of the present invention includes:
After surface treatment such as high-temperature soaking, normal carburizing, carbonitriding, etc. is performed in whole or in part at an austenite range temperature higher than the Al deterioration point of steel, the austenite is forged immediately or after cooling to a predetermined temperature. The material is subjected to plastic working such as, and after the processing is performed or after processing, it is held in the bainite temperature range to undergo bainite transformation, and then cooled.

Next, the surface strengthening method according to the present invention will be explained in more detail based on the isothermal transformation diagram (T, T, T, diagram) shown in FIG.

Figure 1 is a isothermal transformation diagram of JIS SNCM42OH material.

The pattern ■ shown in Figure 1 is for 10 steel parts (products), etc.
After heating to 00°C or higher (approximately 1040°C in the figure) and performing high-temperature carburizing or high-temperature carbonitriding on the whole or in part at this temperature, forging is performed at a processing rate of 50%, resulting in approximately 90%
After forging is completed at 0°C, it is rapidly cooled (for example, placed in oil at 60°C) and quenched.

In addition, pattern ■ shown in Figure 1 involves heating steel parts (products), etc. to 1000°C or higher (approximately 1040°C in the figure), and applying high-temperature soaking or high-temperature carburizing/nitriding to the whole or part at this temperature. After this, the entire body is placed in a constant temperature holding furnace such as a fluidized bed furnace or a neutral salt bath furnace at a temperature of about 500 to 700°C (about 600°C in the figure). ) when the temperature reaches
The quenching process is performed by placing the steel in oil or water).

Furthermore, pattern ■ shown in Figure 1 is a steel part (product)
After normal carburizing or immersion nitriding at 800 to 960° C., pattern (1) is subjected to constant temperature heating and plastic working, and then rapidly cooled.

Furthermore, the pattern ■ shown in FIG. It is then rapidly cooled to convert 6 parts into bainite.

The surface strengthening method according to the present invention is classified into the four basic patterns described above, and FIG. 2 is a diagram showing the relationship between processing rate and crystal grain size at high temperatures.

As shown in Figure 2, for example, carburization at 1040°C (10~
30 minutes], the crystal grain size becomes coarse to about 2.5 JISG. When this is forged at a processing rate of 50%, the coarse grains are refined to about JIS G 6, making it possible to significantly improve toughness.

The surface strengthening method according to the present invention includes performing carburizing or carbonitriding not only on the entire surface but also partially at required locations, and also including performing the immersion or carbonitriding excessively.

In addition, the steel to be surface strengthened is a commonly used steel material suitable for case hardening (S-C material).

5-GK material, SNC material, SNCM material, SCr material.

SCM material, SMn material, SMnC material) are used,
CD carburizing steel or two-phase carburizing steel can also be used.

(Example 1) Pipe made of JIS 520C steel (outer diameter 25 mm
The pattern shown in Figure 1 is
Accordingly, as shown in Figure 3, 5 with 104 CL"O
After vacuum carburizing for 0 minutes (carburizing depth 1.0 mm), processing was performed using a flat rotary forging machine at a processing rate of 30%. Subsequently, the piston pin was quenched from a temperature of about 900°C to 60°C in oil, tempered at 180°C, and polished to a depth of 0.1 mm on one side to produce a piston pin.

The carburizing depth of the piston pin obtained here is 0.6 m.
m, the hardness of the carburized layer is Hv800, the hardness of the core is Hv2
80, the hardness was improved by about 15% in the core and about 50% in the carburized layer compared to the normally quenched product, making it possible to obtain a high-strength piston pin.

By the way, a carburized layer containing about 1.0% carbon and a carburized layer containing about 0.2% C
The elongation at 1000℃ of each core is the same, about 85%,
The deformation resistance is also the same, approximately 20 kgf/mm', and it is known from literature (for example, Molding Processing, 1975, October 1977, page 53.54) that the carburized layer and the core have almost the same deformation rate. Therefore, the required finishing immersion layer depth J must be determined in advance by taking into consideration the machining rate and polishing allowance.

Further, since dimensional changes due to phase transformation during quenching and tempering are unavoidable, there will be no problem if the finished forging dimensions are determined in advance through preliminary experiments.

(Example 2) A round bar made of JIS SN0M42OH steel (diameter 50
The round bar was carburized at 930°C for 5 hours (depth 1.2mm) according to the pattern shown in Fig. 1, as shown in Fig. 4, and then carburized at 600°C. After putting it into a fluidized bed furnace and holding it for about 1 minute, it was forged into a gear shape using the tooth profile warm forging method (mold temperature 200°C), and then immediately cooled with oil to form module 3. A spur gear with 17 teeth was manufactured.

(Example 3) A round bar (diameter 50 mm) made of JIS 520C steel was used as the material, and r3 was made into a third piece according to the pattern ■ shown in the figure.
As shown in the figure, the round bar was subjected to high-temperature carburization at 1040°C for 25 minutes and diffusion treatment (depth 1.2mm) for 50 minutes, and then shaped into a gear shape using a flat rolling die (dice temperature 200°C). After rolling, it was oil-cooled from about 900°C to produce a spur gear with 3 modules and 17 teeth.

(Example 4) A round bar (diameter 50 m) made of JIS S0M42OH steel
m) as the material and according to the pattern ■ shown in Figure 1,
As shown in FIG. 5, the round bar was subjected to high-temperature carburizing at 1040°C for 25 minutes and diffusion treatment (depth 1.2mm) for 50 minutes, and then placed in a neutral salt bath furnace at 600°C. After holding for about 3 minutes, it was rolled into a gear shape using a round rolling die (dice temperature: 200° C.), and immediately cooled in oil to produce a spur gear with a 3° module and 17 teeth.

(Evaluation Example 1) Each of the gears manufactured in Examples 2 to 4 above was tested using a hydraulic gear bending fatigue tester to examine the root fatigue strength of each gear, as well as tooth surface hardness and The hardness of the tooth core was investigated.

Furthermore, the Charpy impact value was measured using a 5R Charpy impact test piece treated under the same conditions. These values are shown in Table 1 in comparison with the characteristics when gas carburizing and vacuum carburizing were performed.

As shown in Table 1, the gears manufactured in Examples 2 to 4 all had high tooth surface hardness and excellent tooth surface wear resistance, and also had considerably high root fatigue strength. It is clear that the material can withstand long-term use, and the impact value has increased considerably, indicating that it can sufficiently withstand the impact load applied during use.

Also, the surface carbon concentration is 0.9%, and the effective carburized layer depth is 0.6m.
When comparing the energies when obtaining m, it was found that in the conventional surface strengthening method, in - example, forging heating is 120
In order to perform heating at 0°C for 5 minutes and carburizing at 930°C for 2.5 hours, a heat source of approximately 0.98 KWH per 1 kg was required. 1'040℃ for 25 minutes, diffusion for 60 minutes,
Because forging uses the heat generated during this process, 0 per kg
．． Only requires a heat source of 89KWH, approximately 90W per kg
We were able to achieve energy savings of H.

(Example 5) A preform 1 for a ring gear of an automobile differential device made of JIS 30M42OH steel and having the shape shown in FIG. 6 was used. Then, for this preform 1, 1040°C
High temperature carburizing for 25 minutes and diffusion treatment for 50 minutes (depth 1.
2 mm), the gear material 4 having the tooth profile 2 and the pilot hole 3 for the screw is precision forged as shown in FIG. , HRC20 in the pilot hole 3 for the screw to a depth of approximately 1.5 mm using a high frequency induction coil.
By tempering the front and back, followed by drilling to remove the carburized layer, and then tapping and finishing the product, it has the characteristics of precision and 25% higher root strength and 15% higher pitting strength than conventional products. I got angry and angry.

(Example 6) 0.55%C, 0.28%Si, 1.03%Mn, 1.
Using steel whose main components are 0% Ni, 1.0% Cr, and 0.2% MO, gas was heated at 930°C for 2 hours as shown in Figure 8 according to the pattern ■ shown in Figure 1. After carburizing, it was kept in a fluidized bed furnace at 600°C for about 1 minute, and then forged into a gear shape using a tooth profile warm forging method (mold temperature 200°C). Immediately thereafter, module 3. A spur gear with 17 teeth was manufactured. This gear has a total immersion hardening depth of 0.4 mm.

The surface hardness is Hv800, the core hardness is Hv630, and the tooth surface pitting life is about 5.
Improved by 5 times.

Effects of the Invention As explained above, in the surface strengthening method according to the present invention, after surface treatment is performed at a temperature in the austenite region above the A, transformation point of steel, the austenite is processed to a temperature below the Ms point. The surface treatment effect is great, for example, the surface hardness due to carburization or immersion nitriding is large, and the hardness of 6 parts and It is a surface strengthening method for steel materials and steel parts (products) that have high toughness and fatigue strength, can withstand long-term load use, and has excellent formability during processing. A significant effect is brought about when it is applied to the manufacture of mechanical structural parts such as shafts, bottles, rods, retainers, hubs, etc., which are used after surface treatment.

4. Explanation of the drawings Fig. 1 is a isothermal transformation diagram of SN0M420 steel showing an embodiment of the present invention, Fig. 2 is a graph showing the influence of forging temperature and processing rate on grain size, Fig. 3, Fig. 4 and 5 respectively show embodiments 1.3. of the present invention. % Example 2 and Example 4
An explanatory diagram of the surface strengthening process adopted in Figure 6 (IL
) (b) is a left half front view and a cross-sectional view of the preform for the ring gear used in Example 5 of this invention, and FIG. 7 (fiL) (b) is a gear material processed in Example 5 of this invention FIG. 8 is an explanatory view of the surface strengthening process employed in Example 6 of the present invention.

Figure 1 F3 force F=’l (SeO) Force labor rate Sig

Claims (1)

[Claims] (1) A surface strengthening method characterized by performing surface treatment at an austenite range temperature of steel A_1 transformation point or higher, and then processing the austenite and cooling it.