JPH10202331A - Manufacture of hot working die - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the long life of working die having high toughness and low strain by heating the die stock worked a hot die steel at the quenching temperature, throwing it into coolant to quench, drawing it up from coolant before reaching the martensitic transformation point and cooling it up to the ordinary temperature under the condition that strain is straightened. SOLUTION: The stock 1B strained by heating or cooling is obtained in the case of manufacturing the stock 1A applied die sinking. Although the most direct method for straightening the strain like thin is to apply a mechanical stress to the die stock under air cooling, another method is to apply a thermal stress, about a cooling metal to the strained stack 1B and quickly cool the extended side. For example, the heating temperature of 1000-1050 deg.C at the time of quenching, typically the central temperature of 1030 deg.C is preferably for SKD 61 steel as the temperature condition, and air cooling after throwing it into coolant is preferably executed when the surface temperature of the strained stock 1B is within the range of +200 deg.C to -100 deg.C to the martensitic transformation point. By this way, the generation of cracks is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a method for manufacturing dies for hot working of various metals using steel called hot die steel as a material. The invention also relates to a steel alloy suitable for working in such a method into a hot working die.

[0002]

2. Description of the Related Art For example, a die for hot working used for die casting or hot extrusion of aluminum alloy, hot pressing of steel, etc. is made of hot die steel represented by SKD61 steel, and is engraved. It is manufactured through various processes of cutting, grinding and polishing.

[0003] The heat treatment is optimally at 1000 to 1050 ° C.
This is usually done by air cooling the heated die material for 20 minutes to 1 hour. It is preferable to use oil cooling instead of air cooling to increase the cooling rate, because a tough die is obtained and the mold life is longer, but on the other hand, the amount of distortion caused by cooling increases and cracks may occur. There is. If the distortion increases, a large amount of cutting and grinding must be performed to obtain a product die of a predetermined size, which increases the manufacturing cost and negates the long life benefit. Is not done much.
Attempts have been made to perform engraving with distortion in mind, but there are difficulties in obtaining the desired results. As a means for lowering the cooling rate than usual oil cooling, for example, oil heated to 200 ° C. may be used, which can be said to be effective to some extent but is only an eclectic measure.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to utilize a technique of changing a cooling medium in the course of cooling to obtain a high cooling effect and to maintain distortion caused by cooling in a coolant. An object of the present invention is to remove as much as possible during air cooling so as to achieve both high toughness and low strain, thereby providing a long-life hot working die at low cost. It is also an object of the present invention to provide a steel alloy suitable for this purpose.

[0005]

SUMMARY OF THE INVENTION A method of manufacturing a die for hot working according to the present invention comprises heating a die material obtained by working hot die steel into a die shape to a quenching temperature, and heating the heated die material in a coolant. Quenching by throwing in and cooling from the coolant before reaching the martensitic transformation point, and cooling the quenched die material to room temperature under conditions where distortion caused by cooling is corrected. Consists of This process is illustrated in FIG.

[0006] As the coolant, those capable of obtaining a cooling rate higher than that of air cooling, for example, liquids such as salt, lead, oil, and solvable are preferable. By heating the coolant to a temperature equal to or lower than the Ms point, the cooling capacity can be adjusted.

[0007] The distortion caused by cooling is, for example, in a typical die-casting die, when a steel block is engraved to produce a die material (1A) having a cross-sectional shape as shown in FIG. 2A. This die material is warped (1B) as shown in FIG. 2B when slightly exaggerated by heating and cooling. Correction is to return the warp to a cross-sectional shape (1C) as shown in FIG. 2C. Conditions for correcting such distortion can be realized by various methods. The most direct method is to apply mechanical stress to the die material during air cooling. As shown in FIG. 3, the die material is air-cooled while sandwiched between a press having a top plate (2) and a table (3). If so, the purpose of the correction can be achieved.

Another method is to apply a thermal stress instead of a mechanical stress. For example, as shown in FIG. 4, a chill (4) is applied to a distorted material (1B) to extend it. Cool the side quickly. It is possible to expect the table (3) of the press work to function as a chill. The same effect as the use of the chill can be obtained by applying a heat insulating material (5), for example, a mat made of "kao wool" to the opposite side as shown in FIG. It goes without saying that using both of them is more effective.

In terms of temperature conditions, a steel alloy having a Ms point of 300 ° C. or higher is preferable because cracks during cooling are small. In general, the lower the Ms point, the greater the degree of expansion during the martensitic transformation, which causes cracking. The Ms point of conventional SKD61 is in the range of 280-300C. In addition, the longer the bainite start time Bs, that is, the time from the start of cooling to the start of bainite generation, is that air cooling can be performed while avoiding the formation of bainite, which is preferable. The Bs time of a conventional hot die steel is about 60 minutes. The heating temperature for quenching is 1 which is commonly used for SKD61 steel.
000-1050 ° C, typically 1030 ° C at the center
Is preferred. It is appropriate to transfer to air cooling after being charged into the coolant immediately before the surface temperature of the die material being cooled falls below the martensitic transformation point + 200 ° C., but before reaching the martensitic transformation point −100 ° C. .
If it is pulled out from the coolant at a temperature higher than the above range, baking is insufficient and a strong die is not obtained, and if it is cooled to a temperature significantly below the above range, cooling distortion becomes large and correction becomes difficult. In addition, there is a risk of cracking. The measurement of the surface temperature of the die material in the coolant can be performed relatively easily and accurately by using a contact thermometer.

[0010] Steels suitable as materials for the hot working dies according to the present invention have the following alloy compositions: That is, in weight%, C: 0.25 to 0.50%, Si: 0.1
0% or less, preferably 0.05% or less, Mn: 0.5 to
1.0%, preferably 0.5 to 0.7%, Cr: 5.0
~ 6.0%, preferably 5.4 ~ 5.7%, Mo: 2.5
-3.5%, preferably 2.9-3.1%, and V:
0.8-1.2%, P: 0.010% or less, S:
0.020% or less, O: 0.020% or less, and N:
0.020% or less, and the balance substantially consists of Fe.

The Ms point of this steel is about 310 ° C. as described above.
And the Bs time is about 130 minutes.

The above steel may further contain one or more of Cu, Ni and W in an amount of not more than 2.0%, as long as the bainite start time is not shortened.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
It is thrown into the coolant from a heating temperature of 030 ° C., and is burned by being cooled at a relatively fast cooling rate,
Has moderate hardness and toughness. The martensite transformation point is withdrawn from the coolant in the range of + 200 ° C. to −100 ° C., transferred to air cooling, and the subsequent cooling is performed relatively slowly. During air cooling,
As a result of the application of mechanical or thermal stress against the cooling strain, the strain is almost corrected by the time of cooling to room temperature.

In the above steels, the reasons for selecting the alloy composition are as follows.

C: 0.25 to 0.50%, preferably 0.32 to 0.42% In order to provide the die with appropriate hardness and toughness, the above content is selected particularly in the medium carbon region.

Si: 0.10% or less, preferably 0.0
5% or less The amount specified in the standard of SKD61 steel for ordinary use is Si: 2.
Although it is 0% or less, in the present invention, the amount of Si is kept low from the viewpoint of facilitating solid solution of the primary carbide by soaking and obtaining a uniform structure.

Mn: 0.5 to 1.0%, preferably 0.5 to 0.7% Cr: 2.5 to 6.0%, preferably 5.4 to 5.7% Mo: 2.5 -3.5%, preferably 2.9-3.1% These enhance the hardenability to ensure the toughness required of the die, increase the Ms point, and place the bainite nose on the long side. And each is added in an amount not less than the above lower limit. Even if it exceeds the upper limit, the effect is saturated and the cost is increased.

V: 0.8 to 1.2% Increases tempering resistance and prevents a decrease in hardness during use of a hot die.

P: 0.010% or less S: 0.020% or less O: 0.020% or less N: 0.020% or less These impurities are desired to be reduced as much as possible. P sets the above limit in preventing segregation. S and O are set to fall within the above-mentioned limits in order to prevent the occurrence of ground flaws and to improve the machinability during engraving. N should also not exceed the above limits, since N forms nitrides and impairs die performance.

The optional elements Cu, Ni, and W (all 2.0% or less) are intended to improve weather resistance (Cu), hardenability (Ni, W), and hardness (W), respectively. And add an appropriate amount if desired. If it is added in a large amount, the bainite nose is moved to a short time side, which is not preferable.

[0021]

Example 1 Hot die steel (Ms point 3) having the following alloy composition
(10 ° C., Bs time: about 130 minutes), which was cast and then forged: C Si MnCrMoWVPSON 0.38 0.05 0.61 5.56 3.04 0.89 0.95 79 180 192 150% by weight, provided that P, S, O and N are ppm. The balance Fe.

A block was cut out from the steel material obtained by forging by machining, and was die-cut into a die material for aluminum die casting by electric discharge machining. This mold material weighs about 200
kg. After heating this to 1030 ° C for 1 hour,
It was thrown into a 200 ° C. salt.

When the surface temperature was lower than 450 ° C., the salt was immediately pulled out of the salt, chilled as shown in FIG. 4, and allowed to cool to room temperature while being sandwiched by a press as shown in FIG. The average cooling rate during salt cooling is about 120 ° C./min and during air cooling (up to 150 ° C.) about 3 ° C./min.

Since the obtained mold material had almost the same cavity shape as the intended size, it could be used only by lightly polishing the surface.

[0025]

Example 2 Blocks having various diagonal dimensions in the range of 140 to 650 mm were cut out from the hot die steel forged material produced in Example 1 and processed into a die shape. Some of these die materials were used under the same conditions as in the example, ie, 10
Cooling was carried out under the conditions of 30 ° C. × 1 hour → 200 ° C. salt injection → air cooling, (Example) A part was simply air-cooled (Comparative Example).

FIG. 6 shows the distortion amount (warpage height) of each die material in relation to the diagonal dimension. From this data, even when salt cooling → air cooling is performed according to the present invention, the strain amount can be equal to or smaller than that of simple air cooling, and both high toughness and high hardness due to rapid cooling and small distortion can be achieved. You can see that.

[0027]

According to the present invention, a high-toughness, high-hardness, long-life die for hot working can be manufactured without fear of cracking during manufacturing and without increasing the cost.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing steps of a method for manufacturing a hot working die according to the present invention.

FIG. 2 is a cross-sectional view of a die material for explaining a principle of correcting a cooling strain in the manufacturing method of the present invention. A indicates a stage in which the material is engraved, B indicates a stage in which cooling distortion has occurred, and C indicates a stage in which distortion has been corrected.

FIG. 3 is a diagram showing a case where a press is used as one example of a means for correcting cooling strain.

FIG. 4 is a diagram showing a case where a chill is used as another example of the means for correcting the cooling distortion.

FIG. 5 is a diagram showing a case where a heat insulating material is used as still another example of the means for correcting the cooling distortion.

FIG. 6 is a graph showing the amount of distortion when die materials having various diagonal dimensions are salt-cooled and air-cooled as compared with the case of simple air-cooling, which is data of the example of the present invention.

[Explanation of symbols]

 Reference Signs List 1A Molded material (before quenching) 1B Distorted material (after quenching) 1C Straightened material (same) 2 Top plate (for press) 3 Table (for press) 4 Cold metal 5 Heat insulator

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI C22C 38/46 C22C 38/46 (72) Inventor Sakaichi Ishikawa 3-14-10 Nishinodai, Chita City, Aichi Prefecture

Claims (6)

[Claims] 1. A die material obtained by processing a hot die steel into a die shape is heated to a quenching temperature, and the heated die material is put into a coolant and cooled to reach a martensite transformation point. A method for manufacturing a hot working die, comprising cooling a die material previously pulled up from a coolant and quenched to room temperature under a condition in which distortion generated during cooling is corrected. 2. The method according to claim 1, wherein the conditions for correcting the distortion caused by cooling are realized while applying a mechanical stress to the die material. 3. The method according to claim 1, wherein the conditions for correcting the distortion caused by the cooling are realized by realizing a local cooling, a local warming, or a combination thereof to apply a thermal stress. . 4. A hot die steel having a martensitic transformation point of 300 ° C. or higher is used as a material, and a processed die material is heated to 1000 to 1050 ° C. and put into a cooling material to form a surface of the die material. Temperature is martensitic transformation point +
Immediately after passing through 200 ° C, withdrawing from the coolant before reaching the martensitic transformation point -100 ° C,
4. The method according to claim 1, wherein cooling is performed under a condition in which distortion due to cooling is corrected. 5. C: 0.25 to 0.50% by weight,
Si: 0.10% or less, Mn: 0.5 to 1.0%, C
r: 5.0-6.0%, Mo: 2.5-3.5% and V: 0.8-1.2%, P: 0.010% or less,
S: 0.020% or less, O: 0.020% or less, and N: 0.020% or less, with the balance being substantially Fe, a hot die steel. 6. In addition to the alloy component according to claim 5, C
A hot die steel to which one, two, or three types of u, Ni, and W are added in an amount of 2.0% or less, respectively.