JP2000042675A - Method of hot forging steel for die or tool steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a forging method in which the energy loss for heating can be eliminated, the forging efficiency is improved, and the heavy working can be effected even with a limited capacity of a press. SOLUTION: In hot forging a steel for dies or a tool steel, the press reduction is stopped for a time specified by the formula t=1.75.10-20.ε-1.04e×p(64700/T) [t: standing time (s), ε: work strain, T: forging temperature [material temperature] (K)] when the forging load to be increased along with the press production reaches the value close to the load capacity of a forging machine, and then, the press reduction is continued again.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for hot forging of mold steel or tool steel.

[0002]

2. Description of the Related Art When plastic working such as forging is performed on steel, FIG.
As shown in (1), the material is work-hardened, and the deformation resistance increases with an increase in strain. For this reason, the processing load increases as the processing rate increases. On the other hand, when upsetting is performed on a cylindrical ingot A, for example, as shown in FIG. 2, the cross-sectional area of the ingot A increases as the height of the ingot A decreases, and the material does not work harden. Also, as the rolling reduction increases, the processing load increases. In a general forging process, these two processing load increasing factors have a synergistic effect on the forging load increase.

Under such a load increasing characteristic at the time of forging, when a large steel ingot is forged by a limited press machine capacity (for example, when upsetting is performed), conventional compression is performed at once. In the processing method of (1), when a certain processing amount (reduction amount) is reached, the forging load reaches the limit of the forging load capacity of the press, and there is a problem that further processing (reduction) cannot be performed.

Therefore, in the conventional processing method, when further processing is necessary, the steel is inserted and held in a heating furnace to reduce the deformation resistance of the steel ingot (recrystallization of austenite crystal grains in the steel ingot). After that, forging is performed again, or as in the invention disclosed in Japanese Patent Publication No. 7-39017, the temperature drop during forging of the steel ingot is minimized to prevent an increase in deformation resistance. There is also a way to do it.

[0005]

However, the former processing method requires a lot of labor such as energy loss for heating, heating time, preparation time for re-forging, and has a problem that the cost is increased. Also, Tokuho 7-3901
In the case of the invention disclosed in Japanese Patent Publication No. 7, a high-temperature insulating member or the like is required, and there is a problem that the forging operation becomes complicated due to heating and handling of the insulating member and the cost increases.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and is intended to eliminate the energy loss for heating and improve the forging efficiency. The purpose of the present invention is to provide a forging method that can perform strong working even with a limited press machine capacity. When the load reaches the load capacity of the forging machine, the equation t = 1.75 · 10 ⁻²⁰ · ε ^−1.04 · e × p (6470
0 / T) where t: standing time (s), ε: working strain, T:
A hot forging method for die steel or tool steel, characterized in that the reduction is interrupted for a time according to the forging temperature [material temperature] (K), the steel is once allowed to stand, and then the reduction is continued again.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. As described in the above prior art, in general forging, an increase in deformation resistance due to an increase in strain caused by work hardening of a material. This increases the processing load due to the increase in the processing load due to the increase in the cross-sectional area of the material during rolling.
The two processing load increase factors affect the forging load increase as a synergistic effect. Therefore, if rolling is performed continuously during forging, the processing load increases due to the rolling, and eventually reaches the forging load capacity of the forging machine, and further reduction cannot be performed. This applies to both hot forging and cold forging.

[0008] The steel for mold and tool steel is generally forged by hot working after melting and casting to improve the internal quality and mechanical properties. Hot working of large steel materials is mainly performed by forging, but since mold steel or tool steel contains more alloying components than structural steel, the deformation resistance during hot is higher than that of structural steel. . Therefore, when forging die steel or tool steel, the limit processing amount is reduced due to the limitation of the load capacity of the forging machine as compared with the case of forging structural steel.

[0009] In hot forging of mold steel or tool steel, the inventors have effectively utilized the phenomenon that austenite crystal grains are recrystallized, thereby canceling the strain caused by the processing accumulated up to that time. The present inventors have completed the present invention with the following findings as a result of intensive studies.

[0010] When austenite crystal grains are recrystallized by heating, they are divided into dynamic recrystallization in which recrystallization occurs during processing and static recrystallization in which recrystallization occurs after processing. It is known that the processing load is reduced by the crystal. However, in hot forging of large ingots, it is difficult to obtain a large amount of strain at a time necessary for dynamic recrystallization, and the amount of load reduction due to dynamic recrystallization is the amount of load reduction when all processing strains are cancelled. At present, it is not possible to expect a decrease in load due to dynamic recrystallization in hot forging of large ingots and the like, because they are much smaller.

On the other hand, in the case of low- and medium-carbon steel, etc., after the first stage of processing, the isothermal holding is performed, and then the second stage of processing is performed. It is known that at the start of the second stage processing, softening from the first stage processing has occurred. From these facts, in these steel types, the processing load increases gradually due to reduction,
Eventually, the forging machine will reach the forging load capacity limit and further reduction will be difficult.However, if the reduction is temporarily suspended and left for a certain period of time, the austenite crystal grains will recrystallize and the material from the previous reduction It is anticipated that the work hardening of the material will be canceled, the deformation resistance of the material will be reduced, and the processing load required for reduction will be reduced.

However, in the case of mold steel or tool steel,
Because there are more alloying components than these steel types, it is unknown how the solid solution elements and carbides affect the recrystallization behavior and work hardening and softening of austenite crystal grains, and the reduction in working load required under rolling is practical. It was completely unknown what the value would be.

[0013] The inventors have conducted intensive experiments, and as a result, even when hot forging die steel or tool steel, the working load increased by the reduction reached near the forging load capacity limit of the forging machine. At this point, if the reduction is interrupted for a required time and left to stand once, and then reduced again after the austenite crystal grains have recrystallized, it is possible to apply more reduction than before the standing. I found something.

That is, in hot forging of die steel or tool steel, the working load is gradually increased due to the reduction, and eventually the forging machine reaches the forging load capacity limit and further reduction is difficult. However, if the reduction is temporarily suspended and left for a required time, the austenite crystal grains are recrystallized and the work hardening of the material due to the previous reduction is canceled, and the deformation resistance of the material is reduced and the reduction is performed. It has been found that the processing load required for cutting is reduced.

[0015] However, from the viewpoint of industrial production, the time period for temporarily stopping the reduction must be the minimum necessary for each of the various forging conditions. There has been no way to determine the time. First, the recrystallization completion time differs depending on the steel type, reduction amount, and forging temperature, and the forging temperature (material temperature) changes successively during forging, so that the temporary suspension time for reduction could not be determined uniquely. Further, if the interruption time of reduction is too short, recrystallization is insufficient and sufficient reduction of deformation resistance does not occur, and a mixed grain structure deteriorates the mechanical properties of the forged product. If the time is much longer than the recrystallization completion time, not only does the forging time increase unnecessarily and the forging efficiency deteriorates, but also the material temperature decreases and the deformation resistance increases. It is.

As a result of various experiments on the recrystallization of austenite grains during hot forging of mold steel or tool steel, the inventors have found that the austenite grains are recrystallized, and the reduction of the austenitic grains is reduced. It has been found that the temporary interruption time (rest time) required for canceling the work hardening of the material due to the above can be obtained by the following equation (1). That is, when t: standing time (s), ε: working strain, and T: forging temperature (material temperature) (K), the standing time t was obtained by the following equation (1). t = 1.75 · 10 ^-20 · ε ^-1.04 · e × p (64700 / T) ·· (1)

Here, the processing strain ε and the forging temperature T are factors closely related to the recrystallization start time and the completion time after the processing, and the recrystallization completion time increases as the processing strain increases and the forging temperature increases. Will be shorter.

The effects of the contained alloy components on the recrystallization time are as follows: Cr, Mo, and V form C and carbides, and themselves act as a solid solution in the matrix iron to delay recrystallization. It is thought that Cr forms carbides with C, but most of them become huge eutectic carbides,
It was also thought that this giant eutectic carbide did not significantly affect the recrystallization delay.

Therefore, various effects of the above-mentioned alloy components on the recrystallization of austenite crystal grains after working were investigated. C: 0.15 to 1.60%, Cr: 1.00 to 1
3.00%, Mo: 0.18 to 1.60%, V: 0.0
Within the range of 8 to 1.20%, it was confirmed that the temporary interruption time (resting time) t under rolling was represented by the above equation.

According to the above formula (1), for example, C:
0.35%, Cr: 5.0%, Mo: 1.2%, V:
A steel containing 0.8% is strained at 1150 ° C. with a strain of 0.55
The temporary suspension time (resting time) of the reduction when processing to the maximum is 1.8 seconds, and even if the reduction is temporarily suspended for this length of time, the forging load increases due to the deterioration of the forging efficiency and the decrease in the material temperature. It turns out that hardly occurs.

The time t obtained from the above equation (1)
Is the lower limit of the required standing time.In actual production, it is difficult to completely adjust to the time of the above formula, and it will be somewhat longer.However, when hot forging die steel or tool steel, When the processing load increased by the forging reaches the limit of the forging load capacity of the forging machine, the reduction is interrupted, and the steel sheet is allowed to stand still for a time t or more obtained by the above equation (1), and the austenite crystal grains are recrystallized. Then, the rolling is performed again successively, thereby succeeding in applying a larger amount of rolling than at a stretch without lowering the forging efficiency, thereby completing the present invention.

It is to be noted that the reduction is interrupted for a necessary time when the processing load increased by the reduction reaches the limit of the forging load capacity of the forging machine, and the steel is once allowed to stand. It has been found that when rolling is performed, it is possible to apply a larger amount of rolling than before standing, but after rolling down a certain part of the steel ingot, that part is interrupted and the austenite crystal grains are recrystallized. Even if the other parts are reduced while being allowed to stand still, if the part where the reduction is interrupted is reduced again after the austenite crystal grains are recrystallized, more parts are reduced compared to before the standing. It is the same that it is possible to apply the reduction, and at least two parts of the ingot are reduced during resting after the reduction,
Further, when the original reduced portion is alternately reduced while the portion is still standing, forging can be performed more efficiently.

[0023]

FIG. 4 shows an embodiment of the method of the present invention. This is JIS steel type SKD11 diameter 8mm × height 12mm
Was heated to 1100 ° C. and compression-processed at a strain rate of 0.1 s ⁻¹ in the height direction (axial direction). As shown in FIG. 3, the maximum load of the forging machine was limited to 600 kgf, and compression was performed by the conventional method of processing at once. As a result, as shown in FIG. 3, 1.4 mm when the processing load reached the upper limit of 600 kgf of the forging machine. I was only able to reduce it.

As described above, the maximum load of the forging machine is set to 6
The forging method of the present invention is limited to 00 kgf, that is, when the processing load increased by the reduction reaches near the forging load capacity of the forging machine, the reduction is interrupted for a time in accordance with the above-mentioned formula (1) and temporarily settled. After that, compression was performed by a processing method in which the reduction was continued again. As a result, as shown in FIG. 4, a total reduction amount of 2.3 mm was obtained, which was much larger than that in the conventional method.

The embodiment of the present invention will be described in more detail with reference to FIG. 5. First, in the first stage of compression working, the working load gradually increases with the reduction as in the conventional working method, and the upper limit load of the forging machine is increased. It gradually approaches 600 kgf (). And
When the processing load reaches the load capacity limit (600 kgf) of the forging machine, that is, when the reduction is interrupted and allowed to stand at a reduction amount of 1.4 mm similar to the reduction limit by the conventional processing method, the processing load temporarily decreases. (). The standing time was set to 20 seconds obtained from the above-described equation (1) for the main processing conditions.

Then, when the rolling is resumed again, the material is deformed at the time of the first stage of compression working, and the working load increases sharply because the cross-sectional area is increased from the initial shape.
However, the material is softened by the standing after the first-stage processing, and the deformation of the material starts from a load lower than the final processing load of 600 kgf of the first stage (). For this reason, the total reduction amount 2.3 which reaches the upper limit load of 600 kgf of the forging machine is 2.3.
mm, further reduction was possible. In the present embodiment, the number of times of standing is set to 1 and the number of times of reduction is set to 2. However, it is obvious that a great effect can be obtained even if the forging is performed by repeating the number of times of setting and the number of times of reduction.

FIG. 6 is a view similar to FIG.
A 12 mm high cast structure material was heated to 950 ° C. and subjected to compression processing in the height direction (axial direction) at a strain rate of 0.1 s ⁻¹ , but the maximum load of the forging machine was 1250 k.
gf, when the processing load increased by the reduction reaches near the forging load capacity of the forging machine, the reduction is interrupted for 30 seconds, which is shorter than the time obtained by the above-mentioned formula (1), and the system is once settled. This is a result of compression performed by a working method of continuing the reduction after the compression.

As described above, when the forging method of the present invention, that is, the actual standing time is shorter than the temporary suspension time (resting time) under the rolling obtained by the above-mentioned formula (1), as shown in FIG. It can be seen that only a reduction amount that is almost the same as that obtained by compression by the conventional method of processing at once can be obtained. Therefore, it is clear that the time t obtained from the above equation (1) is the lower limit of the required standing time.

[0029]

The hot forging method according to the present invention has a structure as described above. Therefore, when forging a large steel ingot with a limited press machine capacity, conventional hot forging is performed at once. According to the method, when the forging load reaches the forging load capacity limit of the press when a certain processing amount (reduction amount) is reached, the processing (reduction) cannot be performed any more. On the other hand, the method of the present invention requires more reduction. It is possible to add. In the case where further processing is required in the conventional processing method, once the austenite crystal grains are recrystallized in the steel ingot by once inserting and holding in a heating furnace, and then forging is performed again, heating is required. However, the method of the present invention requires a lot of labor such as energy loss, heating time, preparation time for re-forging, etc., and there is a problem that the cost increases. However, according to the method of the present invention, the energy loss of heating and the deterioration of the forging efficiency are markedly reduced. There are various effects such as being able to decrease.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between strain and deformation resistance.

FIG. 2 is an explanatory view at the time of upsetting forging.

FIG. 3 is a diagram showing a relationship between a reduction amount and a processing load when compressed by a conventional method of processing at once.

FIG. 4 is a diagram showing the relationship between the amount of reduction and the processing load according to the method of the present invention.

FIG. 5 is an explanatory diagram of FIG.

FIG. 6 is a view showing the relationship between the amount of reduction and the processing load when the reduction is continued again after standing for a shorter time than the method of the present invention.

FIG. 7 is a diagram showing a relationship between a reduction amount and a processing load when compressed by a conventional method of processing at once.

Continued on the front page (72) Inventor Sonoko Kitagawa 3-10-15, Yawata-cho, Shinminato-shi, Toyama Japan High-frequency Steel Industry Co., Ltd. Toyama Works F-term (reference) 4E087 AA01 AA10 BA02 CA32 CB01 CB11 DB11 DB15 DB22 DB23 EA12 EC22 ED04 HA71

Claims (1)

[Claims] In the hot forging of mold steel or tool steel, when the working load increased by rolling approaches the load capacity of a forging machine, the formula t = 1.75 · 10 ⁻²⁰ · ε ^-1.04.e × p (6470
0 / T) where t: standing time (s), ε: working strain, T:
The forging temperature [material temperature] (K) is shown. A method for hot forging die steel or tool steel, wherein the reduction is interrupted for a time longer than or equal to and the steel is once left to stand still and then reduced again.