JP2000129349A - Method for hot-forging steel for die or tool steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a steel ingot which generates fine austenitic crystal grains in the steel ingot after forging and has excellent mechanical properties of tensile strength, impact value, etc. by stilly standing a worked portion for a specific time or longer after applying the working to a blank. SOLUTION: The time for stilly standing, is shown in the equation [wherein, (t) is the stilly standing time (s), ε is the working strain, T is the forging temp. (material temp.) (K)]. Desirably, after applying the working to the blank, the working is interrupted for the time or longer in the equation which uses the working strain ε and the forging temp. T to the worked portion as the factors, and after stilly standing, the working and the stilly standing having the time or longer in the equation are again applied to this portion. After working a portion in the blank, the portion is stilly stood until the austenitic crystal grains is recrystallized by interrupting the working. During this period, the other position is worked, and after recrystallizing the austenitic crystal grains at the portion interrupting the working, the working and the stilly standing to the portion are again applied and then, the structure which well regulates the grain diameter having no mixed grain structure, is obtd. to improve the forging efficiency.

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 die steel or tool steel capable of obtaining fine austenite crystal grains in a forged steel ingot.

[0002]

2. Description of the Related Art When a plastic material is subjected to hot plastic working, the original austenite crystal grains undergo strain due to the working, and thereafter, recovery and recrystallization occur so as to cancel the strain in the grains. Austenite transforms into various structures during the cooling process after processing, and the transformation behavior strongly depends on the structure state of austenite before transformation.

[0003] In the field of hot rolling of steel, in order to increase the strength and toughness of the rolled steel, controlled rolling of low carbon steel is performed to refine the recrystallized austenite structure. Rolling is performed in the non-recrystallized region to refine the ferrite structure. On the other hand, for medium carbon steels, controlled rolling is mainly performed to refine the recrystallized austenite structure.

As described above, in the field of hot rolling of steel materials, in order to increase the strength and toughness of a low-carbon steel and a medium-carbon steel by refining the recrystallized austenite structure, the austenite structure during working is predicted and controlled. Various technologies have been developed, and the austenite grains during hot working can reduce the recrystallization completion time and the crystal grain size after recrystallization by appropriately selecting hot working conditions such as working temperature and working strain. It is known that it can be controlled.

[0005]

On the other hand, steels for mold steel and tool steel are generally subjected to forging by hot working after melting and casting to improve internal quality and mechanical properties. I have. Hot working of large steel materials is mainly performed by forging, but the main purpose is to block void defects in the steel ingot, disperse carbides, etc., reduce the size, or reduce segregation zones. Has not been actively controlled.

[0006] Further, mold steel or tool steel is
Since the amount of alloying components contained in the low-carbon steel and medium-carbon steel is larger than that in the low-carbon steel and medium-carbon steel, it is unknown how the solid solution elements and carbides affect the recrystallization behavior of austenite crystal grains. The austenitic structure refinement technology cannot be applied as it is. Therefore, there has been no method of reliably refining the recrystallized austenite structure at the time of hot working to achieve high strength and high toughness in mold steel or tool steel.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in order to solve the above-mentioned conventional problems, and is for hot forging a die steel or tool steel ingot. It is an object of the present invention to provide a forging method in which fine austenite crystal grains can be obtained in a later steel ingot.

In order to achieve the above object, a first invention of the present application is to hot forging die steel or tool steel. ε) and forging temperature (T) as factors, time t according to the following equation (1): t = 1.84 · 10 ⁻²⁰ · ε ^−1.04 · e × p (64700 / T) ·· (1) [ Here, t: standing time (s), ε: working strain, T:
Shows forging temperature (material temperature) (K)] This is a hot forging method for die steel or tool steel, characterized by standing still.

According to a second aspect of the present invention, in hot forging die steel or tool steel, after working a raw material, a working strain (ε) and a forging temperature (T) are determined for the processed portion. The mold is characterized in that the machining is interrupted for at least the time according to the above formula (1), the machining is temporarily stopped, and then the machining and resting for the time of the formula (1) are applied to this portion again. This is a hot forging method for steel or tool steel.
By adopting such a forging method, it is possible to obtain an internal structure of a steel ingot in which the austenite crystal grains are finer than before the working and have a uniform grain size without exhibiting a mixed grain structure.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The constitution of the present invention will be described in detail below. As described in the above prior art, steel for mold steel and tool steel is generally hot worked after melting and casting. To improve internal quality and mechanical properties. Hot working of large steel materials is mainly carried out by forging, but the main purpose is to increase the crystal grain size by closing void defects in the steel ingot, dispersing carbides, etc., refining or reducing segregation zones. Control has not been done so far.

[0011] Mold steel or tool steel contains a large amount of alloying components, and the recrystallization behavior of austenite crystal grains differs from that of low-carbon steel and medium-carbon steel due to the effects of solid-solution elements and carbides. Can be considered. Therefore, the austenitic structure refinement technology in the controlled rolling of low-carbon steel and medium-carbon steel cannot be applied as it is, and in the case of mold steel or tool steel, hot working for high strength and high toughness is required. There was no reliable method for refining the recrystallized austenite structure in.

Therefore, the inventors have conducted intensive experiments and have found that, even when hot forging die steel or tool steel, after processing the material, the required time for the processed portion is interrupted. After the material is processed, the material is processed, and then the processed part is allowed to stand for a certain required time and then cooled, so that the austenite crystal grains recrystallize and become finer than the austenite crystal grains before processing. I confirmed that. In addition, during forging, it was confirmed that the austenite crystal grains were recrystallized again by being left for a necessary period of time after reworking the portion, and the austenite crystal grain size was smaller than that before the forging.

However, the standing time after processing must be the minimum necessary for various forging conditions from the viewpoint of industrial production, but the standing time when hot forging die steel or tool steel is required. There was no way to decide.

First, since the recrystallization completion time varies depending on the type of steel, the amount of reduction, and the forging temperature, and the forging temperature (material temperature) changes successively during forging, it is necessary to uniquely determine the standing time after processing. If the standing time was too short, recrystallization would not occur at all or would be insufficient, resulting in a mixed-grain structure in the material and deterioration of the mechanical properties of the forged product. If the crystallization completion time is much longer than the crystallization completion time, not only does the forging time unnecessarily increase, but also the forging efficiency is deteriorated due to an increase in deformation resistance due to a decrease in the material temperature.

As a result of various studies on the recrystallization of austenite crystal grains during hot forging of mold steel or tool steel, the inventors have found that the austenite recrystallized structure does not show a mixed grain structure. It has been found that the standing time (t) after processing required for recrystallization can be obtained by the following equation (1). t = 1.84 · 10 ⁻²⁰ · ε ^−1.04 · e × p (64700 / T) ·· (1) [t: standing time (s), ε: working strain, T: forging temperature (material temperature) (K)]

Here, the processing strain and the forging temperature are factors closely related to the recrystallization start time and the completion time after the processing. The larger the processing strain and the higher the forging temperature, the shorter the recrystallization completion time. Become.

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

Thus, 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
In the range of 8 to 1.20%, the standing time after processing (t)
Was confirmed to be represented by the above formula (1).

According to the above equation (1), for example, 0.35%
When a steel containing C, 5.0% Cr, 1.2% Mo and 0.8% V was processed at 1150 ° C. to a strain of 0.55, the standing time after the processing was 2 seconds, which was about this level. It can be seen that even if the processing for a period of time is temporarily suspended, the forging efficiency deteriorates and the forging load hardly increases due to a decrease in the material temperature.

The time obtained from the above formula (1) is the lower limit of the required standing time, and it is difficult to completely match the time of the above formula in actual production, but it is somewhat longer. Alternatively, in hot forging of tool steel, after processing the material, the processed part is allowed to stand for at least the time required by the above formula, and the austenite crystal grains are completely recrystallized to obtain austenite. The present invention succeeded in obtaining an internal structure in which the crystal grains were finer than before processing and did not exhibit a mixed grain structure and had a uniform particle size, and completed the present invention.

After completion of the recrystallization, the austenite crystal grains which have become finer due to the recrystallization gradually become coarser due to grain growth, so that the standing time increases the time (t) obtained by the above equation (1). It is desirable not to exceed.

As described above, after the material has been processed, the processed part is suspended for at least the time in accordance with the above formula (1) and once settled, so that the austenite crystal grains are finer than before the processing. However, when this part is further processed and allowed to stand still for the time (t) or more in the equation (1), the austenite crystal grains become finer than the structure after the first processing. As described above, even if the combination of the processing and the standing for more than the time of the formula (1) is repeated at the same place, it is possible to obtain an internal structure that is fine and has a uniform particle size that does not exhibit a mixed particle structure.

After processing a part of the material, the processing is interrupted and the part is allowed to stand until the austenite crystal grains are recrystallized. In the meantime, other parts are processed, and after the austenite crystal grains are recrystallized at the place where the processing is interrupted, if this part is processed and settled again, it does not exhibit a fine and mixed grain structure. It is the same that an internal structure with a uniform particle size can be obtained. For two or more parts of the material, other parts are processed during standing after processing, and The processing can be performed efficiently by alternately repeating the processing of the original processed portion again.

[0024]

Next, one embodiment of the present invention will be described. A columnar material of JIS steel type SKD61 having a diameter of 8 mm and a height of 12 mm is heated and held at 1150 ° C. for 600 seconds, and then in the height direction (axial direction). The material is subjected to compression processing at a strain rate of 0.1 s ^-1 until the strain at the center of the material becomes 0.55, and then kept at a constant temperature of 1150 ° C. for a predetermined time. Was frozen.

The photographs shown in FIGS. 1 to 3 show the vertical cross-sectional microstructures of the central part of the raw material in which the former austenite crystal grain boundary was frozen at a high temperature under the above processing conditions.
According to the hot working conditions in the present embodiment, from the above-mentioned formula (1), the austenite crystal grains are fine and the static after working necessary to obtain the internal structure of a material having a uniform grain size that does not exhibit a mixed grain structure. The placement time is 2 seconds.

FIG. 1 shows the structure of a material which was heated and held at 1150 ° C. for 600 seconds and then water-cooled, and shows the state of austenite crystal grains before processing. The average crystal grain size at this time is 187 μm. Figure 2 shows 600 at 1150 ° C.
This shows the structure of the material which was subjected to compression as described above after heating and holding for 2 seconds, then kept at a constant temperature for 1 second, and then water-cooled. The old austenite crystal grains have a mixed grain structure and a sound microstructure. Was not obtained.

Similarly to the above, compression processing is performed after heating and holding at 1150 ° C. for 600 seconds. FIG. 3 shows the structure of the obtained material. The isothermal holding (resting) time here was 4 seconds. The austenite crystal grains in the material do not have a mixed grain structure, and the average crystal grain size is 55 μm, and a much finer microstructure is obtained with respect to the austenite crystal grains before processing. As described above, according to the present invention, it is possible to obtain an internal structure of a raw material in which old austenite crystal grains during heating are finer than before processing and have a uniform particle size that does not exhibit a mixed grain structure.

[0028]

According to the present invention having the above-described structure, it is possible to obtain an internal structure of a steel ingot having a fine grain size in which hot austenite crystal grains are fine and do not exhibit a mixed grain structure. In addition, a steel ingot excellent in mechanical properties such as tensile strength and impact value can be manufactured. Further, according to the present invention, since the standing time required for completing the recrystallization of the austenite crystal grains is obtained, when obtaining the internal structure of the steel ingot, the forging and the standing time can be optimized, and the forging can be performed. There are various effects such that deterioration of efficiency can be significantly reduced.

[Brief description of the drawings]

FIG. 1 is a photomicrograph showing the structure of a raw material that has been heated and held at 1150 ° C. for 600 seconds and then water-cooled before processing.

FIG. 2 After heating and holding at 1150 ° C. for 600 seconds, compression processing is performed in the height direction (axial direction) at a strain rate of 0.1 s ⁻¹ until the strain at the center of the material becomes 0.55, and then for 1 second It is a microscope picture which shows the structure of the material which carried out water cooling after isothermal holding.

FIG. 3 After heating and holding at 1150 ° C. for 600 seconds, compression processing is performed in the height direction (axial direction) at a strain rate of 0.1 s ⁻¹ until the strain at the center of the material becomes 0.55. It is a microscope picture which shows the structure of the material which carried out isothermal holding | maintenance (standing still) for more than time according to the said Formula (1), and water-cooled.

 ────────────────────────────────────────────────── ─── Continuing 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 CB01 ED01 GA01 GA09 4K032 AA05 AA06 AA07 AA12 AA13 AA19 AA20 AA36 BA02 CA02 CD06

Claims (2)

[Claims] In the hot forging of mold steel or tool steel, a material is worked, and then, for the worked portion, a working strain (ε) and a forging temperature (T) are used as factors. 1) t = 1.84 · 10 ⁻²⁰ · ε ^−1.04 · e × p (64700 / T) (1) where t: standing time (s), ε: working strain T: forging temperature ( A hot forging method for die steel or tool steel, wherein the steel is left standing for a time corresponding to (material temperature) (K). 2. The hot forging of mold steel or tool steel, wherein after working a raw material, the processed portion is made to have a working strain (ε) and a forging temperature (T) as factors. A steel or tool for a mold, characterized in that machining is interrupted for a period of time according to the formula (1) described above, and the workpiece is once settled and then subjected to machining and resting for a time equal to or longer than the formula (1). Hot forging method for steel.