JPH0615687B2 - Surface strengthening method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention (Industrial field of application) The present invention relates to a steel surface strengthening method used to strengthen the surfaces of steel materials and steel parts (products). .

(Prior Art) Carburizing and quenching, nitriding, and nitrocarburizing are well known as methods for strengthening the surfaces of steel materials and steel parts (products).

Of these, carburizing and quenching is a very effective surface strengthening method, and accounts for about 25% of the total heat treatment and is widely used. However, since this carburizing and quenching has a drawback that the processing time is long, it has long been a problem to shorten the carburizing time, and high-temperature carburizing such as high frequency carburizing and vacuum carburizing (for example, JP-A-48-101328). The technology described in the publication) is being industrialized. However, the disadvantage of high-temperature carburization is that the toughness is lowered due to the coarsening of the crystal grains, and as a countermeasure against this, Al, Nb, Ti, Zr which are crystal grain coarsening preventing elements.
It is necessary to use steel added with the above, or to raise or lower the A ₁ transformation point after carburizing, which is a factor of cost increase.

On the other hand, there is a thermomechanical treatment method as one of steel toughening techniques, and it is classified into three types, that is, pre-transformation machining, mid-transformation machining, and post-transformation machining depending on the timing of the machining.
And, in the conventional thermomechanical treatment method, forging and quenching in which plastic working is applied at a stable austenite region temperature and quenching (Metallurgical Journal Vol 32, No. 11 (1968) No. 10)
52 pages, Journal of the Institute of Metals, Vol 31, No. 2 (1967) 126 pages, Journal of the Institute of Metals, Vol 31, No. 4 (1967)
(Page 347) and ausforming for quenching working at metastable austenite temperature and controlled rolling for slow cooling belong to the above-mentioned thermomechanical treatment before transformation, and isoforming for quenching by working during pearlite or bainite transformation. Sub-zero processing, which adds processing during martensitic transformation, belongs to thermomechanical processing during the above-mentioned transformation, and patenting for processing after completion of pearlite or bainite transformation and marform for processing martensite are processing after transformation. Belong to.

Among these, the thermomechanical treatment before transformation is a treatment technique that significantly improves the strength and toughness of steel by improving hardenability and refining crystal grains and precipitates. It is widely used mainly for shape materials.

On the other hand, the thermomechanical treatment during the transformation has been studied as a treatment method in which the toughness is remarkably improved due to the refinement of subgrains and precipitates, but it has not yet been put to practical use at present.

By the way, it is said that a gear manufactured by rolling or forging has the following advantages as compared with a gear manufactured by conventional cutting.

(1) The load capacity is increased because a very good fiber flow is formed at the root of the tooth.

(2) The side surface of the tooth can be easily crowned, and as a result, the load condition can be improved.

(3) It can be manufactured at a lower cost than machining such as gear cutting machines.

And so on.

A method of manufacturing a gear by rolling which has such advantages is already known (for example, Japanese Patent Laid-Open No. 59-225838).
Issue). Further, a method of manufacturing a gear by precision forging is already known, and among them, the BLW method of West Germany, the cold and warm forging method, etc. are known.

Of these, the concept of precision gear forging by the BLW method is to forge the tooth profile of the gear and use it as it is as a black skin, and to forge precision gear with BLW's unique technology. It was successful and has been put to practical use in bevel gears, spur gears, etc.

(Problems to be solved by the invention) However, in the gear manufactured by the above precision forging, since it is used as it is as a black skin, such a gear has a limit to wear resistance, and in a low load range Since it can only be used, it requires carburizing and quenching. Therefore, there is a problem that heat treatment strain is generated by the carburizing and quenching, and the meaning of precision forging is diminished.

On the other hand, cold forging can manufacture gears with high accuracy, but in order to secure the wear resistance and root strength of the tooth surface, surface hardening such as carburizing and quenching or carburizing / nitriding is usually required. is necessary. However, when a material that has been plastically processed by cold forging is heated to a carburizing temperature (about 920 ° C.), recrystallization causes coarsening of crystal grains. At this time, if the processing rate is constant, the recrystallized grain size is constant,
In the case of a gear shape, there is a slight change in the processing rate from the tooth tip to the tooth root, so the recrystallized grains are not constant, and they become mixed grains.
As it causes pitching and scoring,
In some cases, it cannot be used as a gear, which causes a decrease in yield and an increase in cost.

Furthermore, the technology for manufacturing precision gears by warm forging has advanced considerably, and some examples of practical applications have been announced. But,
Even in gears manufactured by this warm forging, surface hardening treatment such as carburizing and carbonitriding is necessary when used in a high load range, and heat treatment distortion is applied to gears molded with high precision. Had the problem.

Further, as another way of thinking of manufacturing high-strength gears, medium and high carbon steel (0.4 to 0.6% C) was used as a material, which was subjected to gear cutting, and then carburized or carbonitrided. Post bainite transformation temperature range (for example, 235 to 275)
It is known that a so-called austempering treatment for holding the gear at a temperature of ℃) makes the gear core have a bainite structure and the surface has a martensite structure (Heat Treatment Technology Association; Proc. May 60, 2
3) page 49).

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

The present invention has been made by paying attention to the conventional problems as described above, and has a large surface treatment effect, a large surface hardness, a large core toughness, and a large fatigue strength. It is an object of the present invention to provide a steel material and a method for strengthening the surface of a steel part (product) that enables long-term use and is also extremely excellent in formability.

[Structure of the Invention] (Means for Solving the Problems) The method for strengthening the surface of steel according to the present invention is A _{1 of} the above-mentioned steel.
After carburizing or carbonitriding at austenite temperature above the transformation point, the material containing at least a portion of the austenite is processed by forging or rolling or while being processed and then cooled. I am trying.

The surface strengthening method according to one embodiment of the present invention is
_After surface treatment such as high temperature carburization, normal carburization, carbonitriding etc. is carried out wholly or partially at austenite region temperature of ₁ transformation point or higher, the austenite is subjected to plastic working such as forging by utilizing the heat, Quenching with or without processing, or immediately thereafter, quenching to below the Ms point.

Further, a surface strengthening method according to another embodiment of the present invention is
In steel A ₁ transformation point or more austenite region temperature, high temperature carburizing, ordinary carburizing, carburizing After whole or partially to surface treatment such as nitriding, the austenite transformation middle by quenching to about 400 to 700 ° C. 50% more than before the transformation is completed by adding plastic working such as forging
Quench before the transformation ends.

A surface strengthening method according to still another embodiment of the present invention is
After surface treatment such as high temperature carburizing, normal carburizing, carbonitriding etc. wholly or partially at austenite temperature above the A ₁ transformation point of steel, immediately or after cooling to a predetermined temperature, forging to the austenite The plastic working is applied, and the bainite transformation is carried out while the working is applied or after the working, and then the bainite is transformed, and then cooled.

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

FIG. 1 is a diagram of the isothermal transformation of JIS SNCM420H material.

The pattern shown in FIG. 1 is used for steel parts (products), etc.
After heating to over 00 ℃ (about 1040 ℃ in the figure) and performing high temperature carburization or high temperature carbonitriding at this temperature wholly or partially, forging is performed at a processing rate of 50% to about 900
After the forging is finished at ℃, it is quenched (for example, put in oil at 60 ℃) and quenched.

In the pattern shown in FIG. 1, steel parts (products) are heated to 1000 ° C. or higher (about 1040 ° C. in the figure) and subjected to high temperature carburizing or high temperature carburizing / nitriding at this temperature in whole or in part. After that, it is charged into a constant temperature holding furnace such as a fluidized bed furnace or a neutral salt bath furnace at about 500 to 700 ° C (about 600 ° C in the figure), and the whole temperature is about 500 to 700 ° C (about 600 ° C in the figure). In this case, warm forging is carried out to continue working during transformation, and immediately after working, quenching is carried out by rapid cooling (for example, in oil or water at 60 ° C.).

Furthermore, the pattern shown in FIG. 1 is a steel part (product).
Ordinary carburizing or carbonitriding is performed at 800 to 960 ° C., followed by constant temperature heating and plastic working in the same manner as the pattern, and then rapid cooling.

Furthermore, the pattern shown in FIG. 1 is subjected to warm forging in the same manner as the above pattern or the like, and after warm forging, it is held in a bainite transformation region of 200 to 300 ° C. (about 270 ° C. in the figure) and then rapidly cooled. Then, the core is made into bainite.

The surface strengthening method according to the present invention is distinguished into the above-mentioned four basic patterns. FIG. 2 is a diagram showing the relationship between the processing rate and the crystal grain size at high temperature. As shown in FIG. For example, when carburizing (10 to 30 minutes) at 1040 ° C., the crystal grain size is coarsened to JIS G about 2.5. When this is subjected to forging with a working rate of 50%, the coarsened crystal grains become JIS.
G can be refined to about 6 and the toughness can be remarkably enhanced.

In the surface strengthening method according to the present invention, in addition to performing carburization or carbonitriding as a whole, it is also possible to partially perform it at a required position and also to perform the carburization or carbonitriding excessively.

In addition, as the steel to be surface-reinforced, steel materials (SC material, SC material, SN material, SN material, and SN that are generally used and suitable for case hardening are used.
C material, SNCM material, SCr material, SCM material, SMn material, S
In addition to the MnC material), CD carburizing steel and two-phase carburizing steel can also be used.

(Example 1) Using a pipe made of JIS S20C steel (outer diameter 25 mm x inner diameter 15 mm) as a material, according to the pattern shown in FIG. 1, as shown in FIG. 3, vacuum carburizing treatment at 1040 ° C. for 50 minutes ( After the carburizing depth of 1.0 mm), the flattened rotary forging machine was used for processing at a processing rate of 30%. Then, quench at about 900 ℃ in oil at 60 ℃, 180 ℃
After being tempered in, a 0.1 mm polishing finish was performed on one surface to manufacture a piston pin.

The carburizing depth of the piston pin obtained here is 0.6 m
m, carburized layer hardness is Hv800, core hardness is Hv28
The hardness was 0, and the hardness was improved by about 15% in the core portion and about 50% in the carburized layer as compared with the ordinary quenched product, and a high-strength piston pin could be obtained.

By the way, a carburized layer containing about 1.0% carbon and 0.2% C
The core of each has the same elongation at 1000 ° C of about 85%,
It is well known in the literature (for example, forming process theory October 1976, pages 53 and 54) that the carburized layer and the core have substantially the same deformation rate as the deformation resistance is also about 20 kgf / mm ² . Therefore, it is necessary to determine the required carburized layer depth in advance in consideration of the working rate and the polishing allowance.

Further, since it is difficult to avoid dimensional changes due to phase transformation during quenching and tempering, there is no problem if the forged finished dimensions are determined in advance by preliminary experiments.

(Example 2) A round bar made of JIS SNCM420H steel (diameter 50 m
m) as a material and according to the pattern shown in FIG.
As shown in FIG. 4, the round bar was subjected to carburizing treatment (depth 1.2 mm) at 930 ° C. for 5 hours, then placed in a fluidized bed furnace at 600 ° C. and held for about 1 minute, and then tooth profile was overhanged. It was forged into a gear shape by a warm forging method (mold temperature: 200 ° C.), and immediately thereafter was oil-cooled to manufacture a module 3 and a spur gear having 17 teeth.

(Example 3) Using a round bar (diameter 50 mm) made of JIS S20C steel as a material, according to the pattern shown in FIG. 1, as shown in FIG. 3, the round bar was subjected to high temperature carburization at 1040 ° C. for 25 minutes and Diffusion treatment (depth: 1.2 mm) for 50 minutes, rolling with a flat die for rolling (die temperature: 200 ° C) into a gear shape, then oil cooling from about 900 ° C, module 3,
A spur gear having 17 teeth was manufactured.

(Example 4) A round bar made of JIS SCM420H steel (diameter 50 m
m) as a material and according to the pattern shown in FIG.
As shown in FIG. 5, the round bar was subjected to high temperature carburization at 1040 ° C. for 25 minutes and diffusion treatment for 50 minutes (depth 1.2 mm), and then placed in a neutral salt bath furnace at 600 ° C. After holding it for about 3 minutes, it is rolled into a gear shape with a rolling die (die temperature 200 ° C), then immediately oil cooled, and the module 3,
A spur gear having 17 teeth was manufactured.

(Evaluation Example 1) The gears manufactured in the above Examples 2 to 4 are subjected to a test by a hydraulic gear bending fatigue tester to examine the root fatigue strength of each gear and the tooth surface hardness and The hardness of the tooth core was examined. Further, the Charpy impact value was measured using a 5R Charpy impact test piece treated under the same conditions. Then, these values are shown in Table 1 in comparison with the characteristics in the case of gas carburizing and vacuum carburizing.

As shown in Table 1, all of the gears manufactured in Examples 2 to 4 had a large gear hardness and excellent wear resistance on the tooth surface, and the tooth root fatigue strength was considerably large, and It is possible to withstand the use for a certain period of time, and the impact value is considerably increased, and it is clear that it can withstand the impact load applied during use.

The surface carbon concentration is 0.9% and the effective carburized layer depth is 0.6 m.
When energy comparison was performed when m was obtained, in the conventional surface strengthening method, forging heating was 120 in one example.
In order to perform heating at 0 ° C. for 5 minutes and carburizing at 930 ° C. for 2.5 hours, a heat source of about 0.98 KWH was required per 1 kg, whereas in the method of the present invention, in one example, carburizing heating was 1040 ° C. It takes 25 minutes and the diffusion time is 60 minutes. Since the forging uses the heat at this time, 0.89 per kg
Only with the heat source of KWH, it was possible to realize energy saving of about 90WH / Kg.

(Example 5) A preform 1 for a ring gear of an automotive differential having a shape shown in Fig. 6 and made of JIS SCM420H steel was used. Then, this preform 1 was subjected to high temperature carburization at 1040 ° C. for 25 minutes and diffusion treatment for 50 minutes (depth 1.2).
mm), a gear material 4 having a tooth profile 2 and a screw pilot hole 3 as shown in FIG. 7 was precision forged by a forging press, and quenched by a quenching press at a forging end temperature of 900 ° C. Then, tempering the part of the pilot hole 3 screw back and forth H _R C20 to a depth of about 1.5mm at a high frequency induction coil, by finishing with a threaded by subsequently tapped after removing the carburized layer in drilling,
A ring gear was obtained which was more precise and had characteristics of being 25% higher in root strength and 15% higher in pitching strength than conventional products.

(Example 6) 0.55% C, 0.26% Si, 1.03% Mn, 1.
Using steel containing 0% Ni, 1.0% Cr, and 0.2% Mo as main components, according to the pattern shown in FIG. 1, gas carburizing was performed at 930 ° C. for 2 hours as shown in FIG. After that, it was held in a fluidized bed furnace at 600 ° C. for about 1 minute, and then forged into a gear shape by a warm tooth forging method (die temperature 200 ° C.). Immediately thereafter, it was kept in oil at 230 ° C. for 60 minutes to transform it to bainite, and then water-cooled to manufacture a spur gear having three modules and 17 teeth. This gear has a total carburizing hardening depth of 0.4 mm, a surface hardness of Hv800,
The core hardness is Hv630, and the tooth surface pitting life is about 5.5 times longer than that of the SCM420 carburized product.

[Effects of the Invention] As described above, in the surface strengthening method according to the present invention, after the carburizing or carbonitriding treatment is performed at the austenite region temperature of the A ₁ transformation point or higher of the steel, the material containing at least a part of the austenite. Is processed by forging or rolling to quench the temperature below the Ms point, or quenching after holding in the bainite generation temperature range, the surface treatment effect is large, and the surface hardening by carburizing or carbonitriding is performed. Steel with a large core and a large core hardness and toughness, large fatigue strength that can withstand long-term use under load, and remarkably excellent formability during processing by forging or rolling. Surface strengthening method for materials and steel parts (products), including various gears, shafts, pins,
It has a great effect that it is effective when applied to the manufacture of machine structural parts used for carburizing or carbonitriding rods, retainers, hubs, etc.

[Brief description of drawings]

FIG. 1 is an isothermal transformation diagram of SNCM420 steel showing an embodiment of the present invention, FIG. 2 is a graph showing the effects of forging temperature and working rate on the crystal grain size, and FIGS. 3, 4, and 5 are Examples 1, 3, 2 and 4 of the present invention, respectively
Of the surface strengthening process adopted in Fig. 6, Fig. 6 (a)
FIG. 7B is a left half front view and a sectional view of a ring gear preform used in Embodiment 5 of the present invention.
(A) and (b) are a left half front view and a sectional view of a gear material machined in Example 5 of the invention, and FIG. 8 is an explanatory view of a surface strengthening step adopted in Example 6 of the invention. .

Claims (1)

[Claims] 1. A method of carrying out carburizing or carbonitriding at a temperature of austenite above the A ₁ transformation point of steel, and then forging or rolling the material containing at least a portion of said austenite and cooling it. And surface strengthening method.