JPH08225830A - Quenching of die made of hot die steel - Google Patents

Abstract

PURPOSE: To improve toughness, heat check resistance, etc., of die by executing quenching while subjecting the die to slow cooling and rapid cooling sequentially under a specific condition after the die made of hot die steel is heated up to austenitizing temp. CONSTITUTION: The die made of hot die steel is heated to the austenitizing temp. C step by step. Successively, the heated die is relatively slowly cooled at the cooling rate directing to the nose (n) of bainite region B with air-blast cooling to softly cool down to the blend cooling changing region G. Further, the die is rapidly cooled at the cooling rate avoiding the nose (n) of bainite region B by oil cooling, etc., down to the martensite region M. By this method, dimensional change due to heat treatment is improved, thermal strain and finishing margine are reduced and die life is prolonged.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment for hot die steel, and more particularly to a quenching method with a small heat treatment distortion and improved toughness.

[0002]

2. Description of the Related Art For example, a die casting die and a hot forging die are made of hot die steel because they are used at a high temperature (for example, 500 to 1000 ° C.). In the case of die casting mold, the shape is complicated, the wall thickness is not uniform,
In addition, many products have a small finishing allowance, but in such products, quenching and quenching distortion occur when rapid cooling is performed, so slow cooling is performed to prevent such defects. As a slow cooling method, it is usual to perform slow cooling by air cooling or air blow cooling in the cooling process from the quenching temperature.

[0003]

According to the slow cooling method described above, the occurrence of quench cracks and strains can be suppressed, but the cooling rate is slow, which causes the following problems. (1) The toughness of the mold, especially the Charpy value, is insufficient, resulting in poor durability. The reality is that this problem is addressed by means such as early replacement of the mold.

(2) Depending on the thickness of the mold, an incompletely hardened structure results, so that the life of the mold is shortened. This is solved by the above-mentioned means such as early replacement of the mold. (3) The heat check resistance is poor. That is, the mold gradually cooled below the quenching temperature has an unstable internal structure and is used with low hardness, so it is vulnerable to thermal fatigue, and it is possible to repeat rapid heating and quenching, which is the original method of using a hot mold. A heat check occurs early and it becomes unusable.

As mentioned above, when slow cooling is used for quenching the die, both toughness and hardness are lowered, resulting in poor durability and inevitably requiring early replacement of the die. There was a problem. Therefore, when rapid cooling such as oil cooling is performed from the austenitizing temperature, hardness and toughness are simultaneously improved,
It is advantageous in improving durability. However, quenching by rapid cooling such as oil cooling often causes large heat treatment distortion and remarkable quench cracking as described above. When the distortion is large in the quenching process, of course, there is a problem that machining is required to correct the distortion of this mold after the quenching, or the defect rate increases due to the uncorrectability, In order to avoid this, quenching and quenching can be applied only to extremely small molds.

For the above-mentioned reason, in the general die casting mold, the heat treatment has to be carried out by air cooling or blast cooling. Specific publicly known techniques include JP-B-60-37851 and JP-A-54-103732. The former invention relates to a heat treatment method for a roll die for a cold Pilger rolling mill, which comprises heating a roll die crude material made of a high-strength, high-toughness die steel material to an austenizing temperature of about 500 ° C. Rapid heat treatment is performed with a 1st-stage medium-temperature heat bath using a salt bath and a 2nd-stage low-temperature heat bath using a salt bath at approximately 250 ° C. During this 1st-stage medium-temperature heat bath treatment, the nose of the pearlite transformation region The cooling is performed at a speed that does not affect the temperature, and during the second-stage low-temperature hot bath treatment, the temperature is immediately above the martensite transformation region so that the nose of the bainite transformation region is not affected.

However, this heat treatment method cannot be used for heat treatment of a die manufactured using hot die steel. The reason for this is that rapid cooling with a salt bath is adopted for the first-stage medium-temperature heat bath treatment, and the temperature difference between the surface portion and the core portion of the crude material is made as small as possible.
From this, it is inevitable that a die such as a die-casting die that is relatively large and has a large heat capacity will generate a large amount of thermal strain, especially a plate-shaped or block-shaped die made of hot die steel. It cannot be applied to heat treatment of the mold.

The latter invention has a carbon content of 0.10% or less and an Mn of about 1.
8-4.%, And hardly cause ferrite or pearlite be allowed to cool in air from A _C3 transformation point or higher temperature region, so as to form a bainite structure containing bainite structure or a part martensite hardening state The present invention relates to quenching of ultra-low carbon toughness steel to which a hardenable element is added. The present invention is premised on the use of an ultra-low carbon toughness steel material containing a special component in order to omit the tempering step, and performs quenching so as to positively enter the bainite region.

However, when this bainite quenching is applied to a die made of hot die steel having a carbon content of 0.32 to 0.42%, the hot strain is small but the toughness is remarkably reduced. It has a drawback that it cannot be used and is easily broken. The present invention was obtained as a result of conducting a large amount of experiments in order to solve the problem of heat treatment of the hot die steel used in the conventional die casting mold or hot forging, and its purpose and In order to provide a quenching method that improves toughness, improves heat check resistance, reduces thermal strain, improves dimensional change of heat treatment, reduces finishing allowance, and further extends die life. Is.

[0010]

Means for Solving the Problems A quenching method for hot die steel according to the present invention for achieving the above object is (1) heating the hot die steel to an austenitizing temperature, and the temperature and the bainite region. It is cooled to the combined cooling change temperature by air cooling or air blast cooling at the cooling rate passing through the path toward the nose.

(2) Then, quenching is performed from the combined cooling change temperature. More specifically, in the conventional quenching method for special steels, the mold heated to the quenching temperature is gradually cooled by air cooling or blow wind cooling in the sense of preventing quenching distortion and large cracks while sacrificing improvement in toughness. The method of cooling was adopted. On the other hand, in the present invention, the gradual cooling range is set to the "combined cooling change temperature", and the gradual cooling rate is set to the cooling rate passing through the path toward the bainite region.

The invention is characterized in that the temperature is rapidly cooled from the composite cooling change temperature to the martensitic temperature. In order to perform the steps (1) and (2), the positions of the noses n of the pearlite region P and the bainite region B are confirmed using the continuous transformation curve of the hot die steel that has already been obtained. Is good. Further, this confirmation work can obtain a better result by using a material similar to the mold and a test piece of a size.

In general, by gradually cooling the die steel having poor heat conductivity from the austenitizing temperature to the composite cooling change temperature, it is possible to minimize a large thermal strain generated by being cooled from the corner portion of the mold. . Then, oil cooling or the like is adopted from the combined cooling change temperature to perform rapid cooling so as not to pass through the bainite region that most affects deterioration of toughness, thereby preventing generation of bainite in the structure. By configuring as described above, it is possible to improve the toughness of the mold, improve the heat check, and manufacture the mold with less heat distortion.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION First, in order to help understanding of the present invention, the quenching method of the present invention will be described in comparison with a conventional quenching method which employs blast cooling in all steps. Figure 1 shows die steel (S
KD-61) is a heating / cooling curve represented on the isothermal transformation curve diagram of a mold, wherein (a) -diagram is according to the present invention and (b) -diagram is according to a conventional method. Are shown respectively.

In the figure, P indicates a pearlite region, B indicates a bainite region, and M indicates a martensite region. F indicates the temperature of the center of the mold, and E indicates the temperature of the surface. (1) The quenching heat treatment diagram is created in advance by the continuous cooling transformation curve, and the pearlite region P,
Check the bainite area B.

(2) The relationship between the time required for the mold to cool from the nose n in the bainite region B and the temperature drop is determined from experimental values, and based on this data, quenching is started from the extended war of the austenitizing temperature C. Check point D. Specifically, a large number of test pieces of the same material as the mold material and having substantially the same dimensions are prepared, a thermocouple is attached to the central portion, and the cooling rate is measured. By adopting such a method, the composite cooling change temperature is changed variously to determine an appropriate temperature.

This cooling rate gradient is important so that the cooling rate gradient curves E'and F'on the surface and on the inside of the test piece do not come into contact with the nose n of the bainite region B. The composite cooling change temperature G is a temperature named for convenience of the present inventor, and soft cooling is performed from the austenitizing temperature C, then this soft cooling is stopped, and then the cooling means is changed to hard cooling, that is, rapid cooling. It means the temperature range to operate. This combined cooling change temperature G must be within a temperature range where the cooling means can be changed easily by confirming it in a normal quenching process.

For example, assuming that the austenitizing temperature is X ° C. (eg 1020 ° C.), the composite cooling change temperature G is a temperature of approximately X / 2 (eg 500 ° C.), and the temperature range is about ± 50 ° C. is there. Within this temperature range, the cooling means can be changed sufficiently. However, it is a sufficiently permissible range to change the temperature range slightly depending on the material and size of the mold. The combined cooling change temperature G
Often corresponds to a semi-cooling temperature in normal quenching.

In FIGS. 1- (a) and (b), A, B,
C indicates the temperature step of raising the temperature of the mold for heat treatment, and C indicates the austenitizing temperature. (A) -In the figure, the cooling line E above the combined cooling change temperature G is the surface temperature of the mold that has been softly cooled such as blast cooling from the austenitizing temperature C, and similarly the cooling line F is the mold. The temperature of the inner surface of each is shown.

Further, E'-F 'on the extension line of the cooling lines E and F below the combined cooling change temperature G indicates the surface temperature and the center temperature of the mold in the hard quenching mainly in oil quenching. It represents. As can be understood by comparing FIGS. 1- (a) and (b), according to the conventional quenching method of (b) -diagram, from the quenching start point D on the austenitizing temperature C to the room temperature. Soft cooling such as blast cooling is adopted in all the areas of B, and as a result, cooling lines E ″ and F ″ enter the nose n portion of the bainite area B, and the structure of the quenched die has a bainite structure. It is obvious that the above-mentioned various drawbacks occur.

On the other hand, according to the method of the present invention shown in (a)-(b)-, since both the inner surface temperature F'and the surface temperature E'of the mold are cooled while avoiding the bainite region B, (b)- No defects occur as in the conventional method shown in the figure. Now, the temperature of the die (300 mm long, 200 mm wide, 150 mm thick) obtained by machining die steel is started in a vacuum furnace.

B) 600 ° C. for 120 minutes (Temperature A), b) then 850 ° C. for 120 minutes (Temperature B), c) 1020 ° C. to 1030 ° C.
For 120 minutes, the temperature is gradually increased to the final austenitizing temperature C of 1020 ° C to 1030 ° C. Cooling is relatively soft from the austenitizing temperature C to a combined cooling change temperature G of 500 ± 50 ° C., which is a temperature approximately midway between the austenitizing temperature C and room temperature. As the cooling means, air blow cooling is continuously performed on a heated mold by a blower or the like.

The blast cooling is constituted by cooling lines E'and F'which go from the quenching start temperature D, which is the end point of the austenitizing temperature C region, to the bainite region B.
In the present invention, as shown in FIG. 7A, the extension line between the surface temperature E of the mold and the inner surface temperature F of the bainite region B is formed by soft cooling while moving from the austenitizing temperature C to the composite cooling change temperature G. Towards the nose portion n, this soft cooling is stopped on the combined cooling change temperature G line. Then, the soft-cooled mold is immersed in oil or the like to be rapidly cooled to the martensite forming temperature MS, and thereafter to the martensite completion temperature MF (not shown) to complete the quenching heat treatment.

The above-mentioned embodiment is carried out by one mold model, and the respective temperature ranges in quenching of a mold having a thickness of 100 mm to 250 mm are as follows. A) Temperature A: 600 ° C. to 650 ° C., time to raise temperature A:
Hold time at 150 ℃ to 1Hr, temperature A: 120 minutes b) Temperature B: 800 ℃ to 850 ℃, time to heat to temperature B:
Hold time at 150 ℃ to 1Hr, temperature B: 120 minutes c) Temperature C: 1010 ℃ to 1030 ℃, Time to heat to temperature C:
Holding time of 150 ° C / 1Hr, temperature C: 120 minutes d) Time to reach temperature G from temperature C: within 15 minutes to 30 minutes About "cooling rate" Fig. 2 shows the center of the mold during quenching of the present invention FIG. 3 shows a cooling diagram in FIG. 3, and FIG. 3 shows a cooling diagram in the center portion of the mold in the case of performing conventional air flow cooling. As can be seen by comparing the above two diagrams, the temperature of the mold at the semi-cooling temperature g or less in the conventional method is cooled for a considerably long time.

On the other hand, according to the heat treatment method of the present invention, it is understood that the temperature is rapidly cooled by oil cooling or the like at a temperature equal to or lower than the composite cooling change temperature G. FIG. 4 is a diagram showing the heat check resistance, and shows the results of the following experiment in which a large number of 40 × 40 × 20 mm test pieces were prepared from the test material heat-treated in the same manner as described above. It is a thing. The test piece is heated to about 670 ° C. with a gas burner, submerged in water and cooled. This heating and cooling is performed 1000 times.

In the conventional method (indicated by a and b in FIG. 4), the number of cracks generated (a) is about 100 and the average depth of cracks (b) is about 0.5 mm. According to the method of the present invention (indicated by c and d in FIG. 4), it was confirmed that the number of cracks generated (c) was about 50 and the average depth of cracks (d) was about 0.3 mm. There is. FIG. 5 is a comparison of toughness (Charpy value) according to the present invention and the conventional method. It can be seen that the method of the present invention shows an improvement of 30 to 40% as compared with the conventional method. it can.

When the heat treatment method of the present invention is compared with the conventional heat treatment method, the following can be said. It can be used with a Rockwell hardness of 2-3 higher than that of conventional heat treatment (H _RC 48-
50). Charpy value is about 30-40% compared to conventional heat treatment
Improvement is seen. Compared with the conventional heat treatment method, the heat check showed a significant improvement of 1/3 in the depth and about 1/2 in the number of occurrences.

Table 1 shows data comparing the durability of the molds treated by the quenching method of the hot die steel of the present invention and the conventional quenching method, which is described in the evaluation section. As can be seen, the heat-treated mold of the present invention has a durability improved by about 1.3 to 2 times as compared with the conventional mold.

[0029]

[Table 1]

[0030]

EFFECTS OF THE INVENTION In general, die steel having poor thermal conductivity is gradually cooled from the austenitizing temperature to a composite cooling change temperature, thereby suppressing a large thermal strain generated by cooling from a corner portion of a mold. You can At the composite cooling change temperature, the mold cooled to a temperature midway from the austenitizing temperature C is rapidly cooled by oil cooling or the like so as not to pass through the bainite region B, particularly the nose portion n.

The present invention eliminates the cause of distortion at high temperature by adopting soft cooling such as blow wind cooling from the austenitizing temperature G which is the heat treatment temperature to the composite cooling change temperature G which is the temperature during quenching. When the composite cooling change temperature G is reached, the following effects can be obtained by rapidly cooling with oil cooling or the like on the cooling curves E'and F'that can pass through the bainite region B.

(1) Since the structure of the mold does not pass through the bainite region B during the quenching treatment, its internal structure is stable,
High hardness and strong in thermal fatigue. In particular, even if the conditions of being heated to a high temperature and being cooled to near room temperature are repeated, the heat check is less likely to occur as compared with the conventional method. (2) As shown in FIG. 4, as a result of the heat check resistance test, the number of cracks generated and the average depth of the cracks are about half in the method of the present invention as compared with the conventional method, and 1 /
A significant improvement was recognized in No. 3. Further, the mold is reliable, and the mold can be used for a long period of time. (3) As shown in Table 1, the life of the mold according to the present invention is about twice as long as that of the conventional mold.

[Brief description of drawings]

FIG. 1 is a diagram showing a quenching heat treatment diagram of hot working steel, wherein (a) -the figure shows the method of the present invention, and (b) -the figure shows the conventional method.

FIG. 2 is a cooling rate diagram of the mold of the present invention.

FIG. 3 is a cooling rate diagram of a conventional method.

FIG. 4 is a diagram showing a heat check resistance test result, showing the number of cracks generated and the average depth of the cracks in comparison with the method of the present invention and the conventional method.

FIG. 5 shows a toughness comparison table between a mold according to the present invention and a conventional mold.

[Explanation of symbols]

P Pearlite area B Bainite area M
Martensite region C Austenitizing temperature F Center temperature of mold E Surface temperature of mold G Complex cooling change temperature
n nose

Claims (1)

[Claims] 1. A hot austenitizing temperature C of die steel
And a step of cooling the heated hot die steel to a composite cooling change temperature (G) by air cooling or air blast cooling at a cooling rate toward the nose (n) of the bainite region (B), Nose (n) in the bainite region (B) is obtained by using hot die steel that has been air-cooled or blow-cooled.
A method of quenching a die made of hot die steel, which comprises a step of quenching with oil cooling or the like at a cooling rate that avoids