JP2000063944A - Heat treating method for cold-warm plastic working stock and cold-warm plastic worked article - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an effective means for recovering carbide. cracking caused at the time of cold-warm plastic working for tool steel stocks large in the quantity of carbide in the structure as the objects and to provide cold-warm plastic worked articles excellent in mechanical properties. SOLUTION: In the method for heat-treating cold-warm plastic worked stocks, a tool steel stock contg., by weight, 14.0 to 42.0% (Cr+13C) and moreover a tool steel stock contg. one or more kinds of Mo and W by <=15.0% (Mo+1/2W) and <=5.0% V is subjected to cold or warm plastic working in >=5% upsetting ratio at <=800 deg.C, is thereafter held at 700 to 950 deg.C and is subjected to cooling in which >=1 hr is secured for the time required for cooling in continuous cooling to 600 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold / warm type including a die itself for plastic working such as forging, in addition to parts formed / manufactured by cold / warm plastic working such as drive system parts of automobiles, home electric appliances and the like. The present invention relates to a plastically worked product and a heat treatment method for cold-warm plastically worked material used for its production.

[0002]

2. Description of the Related Art Conventionally, cold-warm plastically worked parts which are formed and manufactured by cold or warm forging, drawing, etc., are made of structural steel containing a low alloy containing a relatively small amount of carbide in the structure. Things were the main subjects. However, for example, in the case of cold-warm plastically processed parts used in the engine and the engine peripheral part, with the recent increase in environmental problems, high heat resistance,
As the demand for high wear resistance is increasing, the material conversion from structural steel to tool steel with high wear resistance has begun. As a technique related to this, for example, a technical report has been announced that enables cold forging of JIS SKD11, which has been impossible in the past, for the first time industrially.

In recent years, in the parts processing industry, in response to the globalization of the world, cost reduction for survival has been promoted in order to meet international costs. An important issue in cost reduction is the conversion from "cutting" to "plastic forming" such as forging and drawing, and also the tool used as the material in the manufacture of the mold itself used for plastic forming. Attempts to convert steel cutting and electrical discharge machining into forging have begun to be studied. In this way, it is considered that the material used for cold and warm plastic working products will be changed from structural steel to tool steel, and that the die manufacturing means will be changed from cutting to plastic working. At present, it is becoming an increasingly important task to improve the processing technology of tool steel, especially high-alloy tool steel having a large amount of carbide in the structure, by cold-warm plastic working.

[0004]

As described above, the improvement of cold or warm plastic working technology of tool steel for manufacturing cold-warm plastic worked products is an important subject for improving the characteristics of parts and reducing costs. Is. However, regarding the change from cutting to plastic working, the advantage of improving fatigue strength by forming grain flows (fiber flow), which was said at the structural steel level in terms of strength, is that the amount of carbides in the structure is high. There is a circumstance that alloy tool steel cannot be used. Natsume et al. "Proceedings of the Japan Society of Mechanical Engineers, Material Mechanics Conference '90 P. 32.
3 ”, it is pointed out that when cold plastic working is performed on the alloy tool steel, carbides existing in the steel are cracked by the cold plastic working, and variations in fatigue strength increase. This is a fatal weak point in the above-mentioned automobile engine peripheral parts and molds, so that the current situation is that high-alloy tool steels have not been widely used in items requiring high workability and material strength. .

Therefore, the present invention is intended for a tool steel material having a large amount of carbides in the structure, provides an effective means for repairing carbide cracks generated during cold-warm plastic working, and has a cold temperature excellent in mechanical properties. The object is to provide an inter-plastic processed product.

[0006]

Means for Solving the Problems The present inventor has studied a method of repairing voids formed by cracking of carbides generated in cold or warm plastic working, and found that the cold warm plastic working was repeated several times. It has been found that optimally adjusting the heat treatment conditions after the cold-warm plastic working including the heat treatment to be performed is an effective means for obtaining a cold-warm plastic worked product with few carbide cracks at low cost.

That is, according to the present invention, the content of (Cr +
13C): Tool steel material containing 14.0 to 42.0%, and one or more of Mo and W (Mo + 1/2).
W): A tool steel material containing 15.0% or less and V: 5.0% or less is subjected to cold or warm plastic working with an upsetting ratio of 5% or more at 800 ° C or less, and then 700 to 950 ° C
The time required for continuous cooling up to 600 ° C is maintained at 1
It is intended to provide a cold-warm plastically worked product having few carbide cracks by a heat treatment method of continuously cooling for securing time or more. According to the heat treatment method of the present invention, it is possible to repair carbide cracks sufficient to obtain excellent mechanical properties by securing a cooling time for continuous cooling up to 600 ° C. for at most 10 hours.

The method of the present invention can also be applied to a plurality of cold and warm plastic workings. That is, a predetermined heat treatment is carried out between a plurality of cold-warm plastic workings, and the above-mentioned cold or warm plastic workings to cooling is repeated twice or more. Among them, if at least one time of repeated cooling, the cooling time for continuous cooling up to 600 ° C. is secured for at least 10 hours, it is possible to repair carbide cracks sufficient to obtain excellent mechanical properties. .

According to the method of the present invention, the voids formed by the cracking of carbides existing in the structure have an area ratio of 2% or less, or the voids caused by the cracking of carbides having a maximum length of 5 μm or more existing in the structure. A cold-warm plastically worked product having a forming rate of 30% or less can be provided.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The most important feature of the present invention is that in a cold-warm plastically worked product made of a tool steel containing a large amount of carbide in the structure, it is necessary to suppress carbide cracking in the structure, which was difficult to achieve in the past. We have found a solution to this problem, and achieved a cold-warm plastically processed product with excellent mechanical properties.

First, the inventor of the present invention prefers to obtain the effects of the present invention by using (Cr + 13
C): Tool steel material containing 14.0 to 42.0%, and one or more of Mo and W (Mo + 1 / 2W):
A tool steel material containing 15.0% or less and V: 5.0% or less was selected. That is, the "carbide crack repair effect" of the present invention is remarkably achieved in the above tool steel in which a large amount of carbide is present in the structure, and particularly, the maximum length is 5 μm.
In tool steel containing carbides of m or more, the effectiveness of the means becomes clearer. In other words, even if the present invention is applied to a low-alloy steel having a small amount of carbide, only the effect of ordinary annealing can be obtained. In addition to the above, the tool steel material to which the present invention is applied is S in addition to the above in order to impart various characteristics depending on the melting process and use of the material.
At least one of i, Mn, Ni, Co, Nb, and Ti may be contained, and this does not hinder the effects of the present invention.

Then, the present inventor has conceived to solve by heat treatment a method of repairing voids formed by cracking of carbide generated in cold or warm plastic working in the above tool steel material, The inventors have reached the point that appropriately adjusting the heat treatment conditions after cold-warm plastic working is an effective means for obtaining a cold-warm plastic worked product having excellent mechanical properties at low cost.

That is, various tests were conducted by conducting experiments and various simulations so that the metal phase could flow into the voids of the carbide cracks in the heat treatment after the cold-warm plastic working. Then, as a result of studying to obtain the above effect in a temperature range as low as possible, it was confirmed that the object of the present invention can be achieved in a temperature range where a metal plastic flow occurs even at a low stress, that is, a creep phenomenon occurs. did. The basic requirements necessary for the rapid occurrence of the creep phenomenon during heat treatment are (1) low deformation resistance (2) high creep driving force stress (3) conditions under which substitutional elements are likely to diffuse .

Among the above conditions, (1) is better as the heat treatment temperature is raised, while transformation is required to generate stress commensurate with the condition (2), and the A ₁ transformation point is used. Become. Condition (3) is higher the general temperature in a single phase, but the diffusion rate is increased, if the same temperature, than the austenite, since the more the ferrite diffusion rate faster, the still A ₁ point vicinity It will be used. However, since the creep phenomenon tends to become remarkable even if the time is maintained, it is important to control the cooling time. The plastic working and heat treatment conditions of the present invention will be described below in view of these conditions.

First, in the present invention, the cold-warm plastic working temperature performed on a predetermined tool steel material is limited to 800 ° C. or lower.
It is a well-known phenomenon that in the hot plastic working conventionally performed by steel makers, the metal phase efficiently flows into the voids of carbide cracks. That is, according to the present invention, which is intended to repair carbide cracks that occur after plastic working, as opposed to hot plastic working that originally eliminates voids during plastic working, the metal phase is less likely to flow into the voids of carbide cracks at 800 ° C. or less. The effect is demonstrated by the tool steel material that is subjected to "cold warm plastic working".

Next, in the present invention, the upsetting rate of cold-warm plastic working performed on a predetermined tool steel material is limited to 5% or more. If the upsetting ratio is less than 5%, the occurrence of carbide cracking is small, and therefore, even if the heat treatment of the present invention is carried out, almost no effect is recognized. That is, the present invention, which aims at repairing carbide cracks generated after plastic working, exerts its effect by setting the upsetting rate of cold-warm plastic working to 5% or more, and the upsetting rate of 5% or more. With regard to the cold-warm plastically worked product obtained through the cold-warm plastic working, the effect that the variation in fatigue strength is small is achieved. The upsetting ratio of the present invention is defined as follows. Here, S ₀ is the cross-sectional area before cold-warm plastic working, Se is the cross-sectional area after cold-warm plastic working, and e is the upsetting rate (%).

[0017]

[Equation 1]

What is important in the present invention is the heat treatment condition after cold and warm plastic working. The present invention relates to the heat treatment after cold and warm plastic working, and the holding temperature is 700 ° C.
It was set to 950 ° C. That is, in the present invention, the temperature at which the above-mentioned transformation occurs and the creep effect is recognized is set to 700 ° C. or higher, preferably 760 ° C. or higher, and set to 950 ° C. or lower, when the holding temperature exceeds 950 ° C. This is because the cooling rate becomes high and sufficient time cannot be secured to cause creep deformation.

According to the present invention, the above holding temperature is 6
The cooling is performed so that the time required for continuous cooling up to 00 ° C. is 1 hour or more. First, the reason why cooling to 600 ° C. is continuously performed is to prevent the stress generated by the transformation from being relaxed by recrystallization and to generate the transformation stress successively. Then, the lower limit temperature of the continuous cooling is 600
The reason why the temperature is set to ° C is that "diffusion of substitutional element", which is a basic mechanism causing the creep phenomenon, hardly occurs below that.

Securing the time required for continuous cooling is important for the present invention utilizing the creep phenomenon, which has a large time dependence, and is set to 1 hour or more in order to obtain the effect of the present invention. In this case, by ensuring the cooling time for continuous cooling up to 600 ° C. for at least 10 hours, it is possible to repair carbide cracks sufficient to obtain excellent mechanical properties.

As described above, the above-described method of the present invention can be applied to a plurality of cold / warm plastic workings, that is, from the cold or warm plastic workings of the present invention. This is a method in which the processing up to is repeated twice or more.
At least one of the repeated coolings requires at least 10 hours of continuous cooling up to 600 ° C.
If sufficient time is secured, it is possible to repair carbide cracks sufficient to obtain excellent mechanical properties.

According to the above conditions, the voids formed by the cracking of carbides existing in the structure have a cold-warm plastic processed product having an area ratio of 2% or less, or carbides having a maximum length of 5 μm or more existing in the structure. The void formation rate due to cracking is 3
It is possible to achieve a cold-warm plastically worked product of 0% or less, and it is possible to achieve excellent mechanical properties.

[0023]

EXAMPLES (Example 1) First, sample No. 3 where carbide cracks are likely to occur during cold and hot plastic working. The tool steels 1 to 4 were selected as shown in Table 1, and 50% of them were cumulatively cold drawn from room temperature.

[0024]

[Table 1]

Next, the tool steel samples subjected to the cold drawing were held at 850 ° C. for 1 hour, and then “up to 600 ° C. for 5 hours”.
After cooling at 00 ° C / h (that is, the required time is 30 minutes),
Air-treatment A, “cooling to 600 ° C. at 250 ° C./h (that is, the required time is 1 hour), then air-cooling B treatment”, and cooling to 600 ° C. at 25 ° C./h (the required time 10 After that, the air-cooled C treatment was performed (the heat pattern is shown in FIG. 1). With respect to the cold drawn product subjected to the above treatment, the void formation ratio before and after the heat treatment in the center of the material and the difference A certain void reduction rate was investigated, and Table 2 shows the void reduction rate and the void formation rate after heat treatment.

[0026]

[Table 2]

The method for measuring the void formation rate is as follows. Observing an arbitrary visual field with a 400x optical microscope, the number of carbides with a maximum length of 5 μm or more that can be confirmed in the visual field, and
Among the carbides having a maximum length of 5 μm or more, the number of carbides having cracks and voids is counted. Then, the number of carbides having a maximum length of 5 μm or more and the number of void-forming carbides are integrated and counted in the same manner while changing to an arbitrary visual field. When the count reaches 100, the measurement is stopped and the maximum length is 5μ.
Number of carbides forming voids of m or more) / (maximum length 5μ
The void formation rate was calculated from (the number of carbides of m or more) × 100. Then, (void formation rate before heat treatment)-(void formation rate after heat treatment) is the void reduction rate.

From Table 2, sample No. which is the object tool steel of the present invention. Regarding 1 to 4, the effect of reducing voids is achieved by the B and C treatments which are the heat treatment conditions satisfying the present invention.
Since the void formation rate after the heat treatment is also 30% or less, the effect of the present invention is exhibited when the target steel type and the heat treatment conditions simultaneously satisfy the present invention. On the other hand, sample No. which is out of the compositional regulation of the present invention. 5 (SKD61) does not show the void reduction effect of the present invention. Figure 2
Is the sample No. 1 is a microstructure photograph before and after C treatment of 1 (SKD11), showing that the voids formed in the texture after heat treatment of the present invention are 2% or less in area% and at the same time the voids are reduced. .

(Example 2) SKD11 used in Example 1
Using the above material, a cumulative 50% cold drawing was performed from room temperature. Next, each of the cold-drawn samples was
After holding for 1 hour at each holding temperature from 0 to 1000 ° C,
The cooling rate was controlled so that continuous cooling up to 600 ° C. took 1 hour. Subsequent cooling is performed by air cooling, and no special cooling management is performed. Table 3 shows the void reduction rate before and after the heat treatment under each cooling condition.

[0030]

[Table 3]

The holding temperature of the present invention is from 700 ° C. to 95
It can be seen that when the temperature is 0 ° C., the reduction of voids is recognized, whereas when the holding temperature of the present invention is not satisfied, the void reducing effect of the present invention does not occur. Further, if the holding temperature is 760 ° C to 950 ° C, a better void reduction effect can be achieved.

Next, each of the heat-treated samples was cold-worked, then rough-worked, quenched 1020 ° C. × 1 hour and tempered 520 ° C. × 2 hours twice, and then finished. The fatigue test piece shape was adopted. The test piece had a parallel part diameter of 7 mmφ and a parallel part length of 30 mm, and an Ono-type rotary bending fatigue test was performed. Evaluation of variation in fatigue strength is J
This was done using the Staircase method in SME S002-1981. The step stress was 50 MPa, and the number of test pieces was 12 times, which was twice the number of JSME specified test pieces. Since the 10 7th power fatigue strength of this material is generally 700 MPa for a non-cold plastic work material, the starting stress amplitude is 700
Set to MPa and rupture less than 10 7 times 50 MPa
If the life is 10 7 times, the lowering test is performed, then 50
The method of increasing the MPa was performed. The fatigue strength width was defined as the standard deviation of the stress amplitude obtained from the 12 pieces.

The results are also shown in Table 3. The variation of the fatigue strength is half when the retention time of the present invention is applied as compared with when the retention time of the present invention is not applied. It has fallen to ⅓. When the standard deviation of 10 7th power fatigue strength of this material was measured as a non-cold plastic working material, it was 53 MPa. Therefore, comparing with the variation in fatigue strength of the heat treated material to which the holding time of 800 and 850 ° C. was applied, It can be seen that the fatigue properties are improved to be almost the same as those of the non-cold plastically worked material.

[0034]

EFFECTS OF THE INVENTION According to the present invention, variations in mechanical properties due to carbide cracking caused by cold and warm plastic working of a tool steel containing a large amount of carbide in the structure can be eliminated at low cost. The quality of cold and warm plastic processed products can be made equal to that of cut and electrical discharge processed products. Further, since the heat treatment of the present invention is at a relatively low temperature, there is little thermal deformation and surface alteration,
It is a process that can be easily inserted during the cold-warm plastic working process.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of heat patterns of the present invention and a comparative example.

FIG. 2 is a metal microstructure photograph showing an example of the carbide of the present invention.

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Claims (8)

[Claims] 1. In weight%, (Cr + 13C): 14.0
Tool steel material containing ~ 42.0%, cold or warm plastic working with an upset rate of 5% or more at 800 ° C or less, then held at 700 to 950 ° C and continuously up to 600 ° C. A method for heat treating a cold-warm plastic working material, which comprises performing cooling so as to secure a cooling time for one hour or more. 2. The heat treatment method for cold / warm plastic working material according to claim 1, wherein the cooling time for continuous cooling up to 600 ° C. is 1 hour or more and 10 hours or less. 3. The heat treatment method for cold / warm plastic working material according to claim 1, wherein the processing from cold or warm plastic working to cooling according to claim 1 is repeated twice or more. 4. At least one of the repeated coolings,
The heat treatment method for cold and warm plastic working material according to claim 3, wherein the cooling time for continuous cooling up to 600 ° C is 1 hour or more and 10 hours or less. 5. A tool steel material containing at least one of Mo and W (Mo + 1 / 2W): 15.0% or less and V: 5.0% or less by weight. Item 5. A heat treatment method for cold and warm plastic working material according to any one of items 1 to 4. 6. The weight percentage of (Cr + 13C): 14.0.
A cold-warm plastically worked product, characterized in that the void ratio formed by carbide cracking existing in the structure is 2% or less in an area ratio of 2% or less. 7. The weight percentage of (Cr + 13C): 14.0.
~ 42.0%, the maximum length present in the tissue is 5
A cold-warm plastically worked product having a void formation rate of 30% or less due to cracking of carbides of μm or more. 8. The method according to claim 6, wherein the content of at least one of Mo and W is (Mo + 1 / 2W): 15.0% or less and V: 5.0% or less by weight. Cold and warm plastic processed products.