JPH11222624A - Production of cold tool steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold tool steel combining wear resistance and tensile compression fatigue strength and capable of a high service life. SOLUTION: A steel having a compsn. contg., by weight, 0.65 to 1.3% C, <=2.0% Si, 0.1 to 2.0% Mn, 5.0 to 11.0% Cr, one or two kinds of Mo and W by 0.7 to 5.0% Mo equivalent (Mo+1/2W), one or two kinds of V and Nb by 0.1 to 2.5% V equivalent (V+1/2Nb), and the balance Fe with inevitable impurities, in which the grain size of M7 C3 type carbide is regulated to 5 to 15 μm and having 1 to 9% area ratio is tempered at 150 to 500 deg.C, preferably at 150 to <450 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a long-life cold work tool steel having excellent fatigue resistance.

[0002]

2. Description of the Related Art Conventionally, JIS-S
KD11 is widely used. However, plastic working technology
Tools to be used in accordance with the progress of
Stress load is large, and 60 HR
Even SKD11, which gives C hardness, has a coarse M₇C_Three
The wear resistance is secured by the type carbide, but M
₇C_ThreeMold carbides contribute to reduced mold life
I have. To solve such a problem, see, for example,
1442, JP-A-2-247357, JP-A-2-247357
JP-A-2-277745, JP-A-3-134136
And the inventions of JP-A-5-156407.
Is being planned.

[0003] Japanese Patent Application Laid-Open No. Hei 1-2201442 discloses that C: 0.90 to 1.35% and Si: 0.70% by weight%.
1.40%, Mn: 1.0% or less, S: 0.004% or less, Cr: 8.0 to 10.0, one or two of Mo and W
A species containing 1.5 to 2.5% of Mo + W / 2 and one or two of V and Nb of 0.15 to 2.5% of V + Nb / 2, the balance being Fe and unavoidable impurities; In a quenched and tempered structure, there is a rolling die steel in which the area ratio of M ₇ C ₃ type carbide is 2% or more and 9% or less, and the area ratio of MC carbide is 2.5% or less. Certainly, the present invention regulates the area ratio and the grain size of carbides, but mainly aims at improving toughness and suppressing crack propagation through a chain distribution of carbides. It is. On the other hand, the present invention has found that factors that cause variation in mold life and extremely low life are major factors such as crack generation and crack propagation due to cracking of M ₇ C ₃ type carbide,
To this end, by setting the M ₇ C ₃ type carbide to 15 μm or less, it is possible to reduce the variation in the life of the mold and the extremely low life mold, thereby improving the average life of the mold.

Further, Japanese Patent Application Laid-Open No. 2-247357 discloses that
Japanese Unexamined Patent Publication (Kokai) No. Hei 1-2201442 further discloses a rolling die steel having a total amount of impurities of As, Sn, Sb, Cu, B, Pb, and Bi of 0.13% or less.
Further, Japanese Patent Application Laid-Open No. 2-277745 discloses that in a quenched and tempered structure, the total area ratio of one or two of MC-type residual carbide and M ₆ C-type residual carbide having a particle size of 2 μm or more is 3% or less, The area ratio of the M ₇ C ₃ type residual carbide having a particle size of 2 μm or more is regulated to 1% or less. In each case, as in JP-A-1-201442, mainly improvement in toughness,
The purpose is to suppress crack propagation through the chain distribution of carbides. In contrast, the present invention has found that, as described above, crack generation and crack propagation due to cracks in M ₇ C ₃ type carbides are major factors, and furthermore, the fracture origin of the M ₇ C ₃ type carbides is It has been found that the particle size is 15 μm or less.

Japanese Unexamined Patent Application Publication No. 3-134136 also discloses the above-mentioned Japanese Unexamined Patent Application Publication No. Hei 1-2201442. Further, among the unavoidable impurities, P is 0.02% or less, S is 0.005% or less, and O is 30 ppm. Hereinafter, N is 300 ppm or less, and further, in the quenched and tempered structure, the area ratio of M ₇ C ₃ type residual carbide having a particle size of 2 μm or more is 8% or less, and the particle size is 2 μm.
A high-hardness, high-toughness cold tool having a total area ratio of one or two types of residual carbides of MC type and M ₆ C type having a m of not less than 3% is 3% or less.
No. 07 discloses that after quenching and tempering, the M ₇ C ₃ type primary carbide has an area ratio of 4.0% or less, the MC type primary carbide has an area ratio of 0.5% or less, and the maximum particle size of the primary carbide is 0.5% or less. Substantially a microstructure uniformly dispersed in the matrix at 20 µm or less, and further from a quenching temperature of 1050 ° C to 1100 ° C,
The hardness when quenching at a quenching cooling rate of up to 500 ° C. of 25 ° C./min and tempering at a high temperature is HRC6.
High performance rolling die steel that can obtain 4 or more.

Further, JP-A-6-212253 discloses that C: 0.75 to 1.75% and Si: 0.5 to 3.0.
%, Mn: 0.1 to 2.0%, Cr: 5.0 to 11.0
%, Mo: 1.3 to 5.0%, V: 0.1 to 5.0%, and the balance of Fe and impurities is 450 ° C.
A method for producing a cold tool steel characterized by tempering at the above temperature. That is, in each of JP-A-3-134136 and JP-A-5-156407,
The main purpose is to improve toughness and to suppress crack propagation through the chain distribution of carbides. In addition, Japanese Unexamined Patent Publication No.
No. 212253 discloses that by performing high-temperature tempering at a temperature of 450 ° C. or more, residual stress during quenching is removed, a stable structure is obtained, secondary hardening hardness is increased, and both hardness and toughness are excellent. When the tool is used as a tool, the tool life is extended and the workability is greatly improved without cracking even when the tool generates heat due to electric discharge machining. However, if the tempering temperature is lower than 450 ° C., it cannot be sufficiently exhibited.

On the other hand, according to the present invention, M ₇ C
Cracking due to cracks in the ₃ -type carbide, and found that crack propagation is a major factor, moreover, since the fracture origin of the M ₇ C ₃ type carbide is the particle size 15 [mu] m, M ₇
While less 15μm to C ₃ -type carbides in tools used in harsh environments where high stress is applied, by low-temperature tempering of 150 to 500 ° C. The tempering temperature,
Compared to high-temperature tempering, the amount of retained austenite is formed more, stress concentration on carbides is alleviated by the retained austenite, carbide cracking is suppressed, and crack generation and crack propagation due to carbide cracking are suppressed, which is more excellent. It is an object of the present invention to reduce the variation in the mold life and the extremely low life mold to improve the average life of the mold.

[0008]

The prior art described above is
The size of the carbide is regulated in terms of toughness and strength. The reason for this is to prevent the occurrence of minute defects due to the lack of the primary carbides and the prevention of cracks from becoming a propagation path. On the other hand, with the recent progress of plastic working technology and the strengthening of the work material, tool steel suitable for molds with more fatigue resistance is required for the purpose of improving wear resistance of tools. Accordingly, an object of the present invention is to provide a method for producing a cold tool steel which has excellent wear resistance and excellent tensile and compression fatigue strength and a long life.

[0009]

SUMMARY OF THE INVENTION The gist of the present invention is as follows: (1) C: 0.65 to 1.3% by weight;
0% or less, Mn: 0.1 to 2.0%, Cr: 5.0 to 1
1.0%, any one or two of Mo or W
o equivalent (Mo + 1 / 2W): 0.7-5.0%, one or two of V or Nb is V equivalent (V + 1/2)
Nb): 0.1 to 2.5%, the balance being Fe and unavoidable impurities, and the particle size of M ₇ C ₃ type carbide is 5 to 15 μm.
m, tempering a steel material having an area ratio of 1 to 9% at a temperature of 150 to 500 ° C. (2) The tempering temperature according to (1) is set to 150 to 450 ° C.
A method for producing cold tool steel, characterized by tempering at less than.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The function of each chemical component of the steel of the present invention and the reasons for limiting the same will be described below. C provides sufficient matrix hardness by quenching and tempering, and Cr,
It is an element that combines with Mo, V, Nb, etc. to form carbides and imparts high-temperature strength and wear resistance. However, if the added amount is too large, coarse carbides are excessively precipitated during solidification and hinder toughness. Therefore, the upper limit of C is set to 1.3%. on the other hand,
If it is less than 0.65%, sufficient secondary hardening hardness cannot be obtained. Therefore, the lower limit is set to 0.65%. However, in order to obtain an optimum balance between strength and toughness, 0.75 to 1.1 is required. % Is desirable.

Si is an element that is mainly added as a deoxidizing agent, is effective in oxidation resistance and hardenability, and suppresses agglomeration of carbides during tempering and promotes secondary hardening. . However, if added in excess of 2.0%, the toughness is reduced, so the upper limit was made 2.0%. Mn
Is an element that is added as a deoxidizing agent similarly to Si to increase the cleanliness of the steel and increase the hardenability. However,
If added in excess of 2.0%, the cold workability is impaired and the toughness is reduced, so the upper limit was made 2.0%.

[0012] Cr is an effective element that enhances hardenability and also enhances tempering softening resistance. To satisfy this effect, at least 5.0% or more is required. Therefore, the lower limit was set to 5.0%. On the other hand, Cr is easily combined with C at the time of solidification to form a giant primary carbide, and excessive addition lowers toughness, so the upper limit is 11.0%.
And

Mo and W are both important elements that form fine carbides and contribute to secondary hardening, and are elements that improve resistance to softening. However, the effect is M
o is twice as strong as W and W is twice as much as Mo to get the same effect. The effect of these two elements can be represented by the Mo equivalent (Mo + ／ W). In the component system of the present invention, the Mo equivalent is required to be at least 0.7% or more. Conversely, excessive addition of Mo equivalents causes toughness to decrease, so the upper limit was made 5.0%.

V and Nb are both effective for secondary curing.
It forms hard carbides with C to greatly contribute to improvement of wear resistance and to refine crystal grains. However, the effect is V
Is twice as strong as Nb, and to obtain the same effect, Nb
Is twice as large as V. The effect of these two elements is V equivalent (V
+ 1 / 2Nb). In the component system of the present invention, in order to obtain a high temperature tempering hardness, at least 0.1% or more in V equivalent is necessary. Excessive addition degrades toughness, so the upper limit was made 2.5%.

Next, in the cold tool steel, eutectic carbide which is crystallized at the time of solidification is conventionally defined in terms of toughness or strength. The reason is that it is regulated in order to prevent the occurrence of minute defects due to the loss of the primary carbides and the prevention of cracks from forming a propagation path. However, as a result of elucidating this point in detail, the greatest feature of the present invention is that an excellent life in tensile and compression fatigue as a factor that affects the tool life of a mold die and the like as a cold tool steel is necessary, In the actual mold, the failure due to fatigue was found to be mainly caused by crack generation and crack propagation due to cracking of M ₇ C ₃ type carbide,
For this purpose, it has been found that when the particle diameter of the M ₇ C ₃ type carbide is 15 μm or less, it is significantly reduced.

FIG. 1 is a graph showing the relationship between the M ₇ C ₃ type carbide size, the number of repetitions of fracture and the wear resistance. According to the figure, according to the results of the tensile compression fatigue test, M ₇ C ₃
It has been found that when the grain size of the type carbide exceeds 15 μm, the number of repetitions of fracture (N) is significantly reduced. On the other hand, according to the results of the Ogoshi type abrasion test, the particle size of the M ₇ C ₃ type carbide was 5 μm.
It has been found that when the amount is less than 1, a marked decrease in wear resistance appears.
As a result, M ₇ C
It has been found that it is optimal to regulate the particle size of the type ₃ carbide in the range of 5 to 15 μm. That is, the particle size of the M ₇ C ₃ type carbide is preferably 15 μm or less in view of tensile compression fatigue and breakage caused by fatigue. Further, from the viewpoint of abrasion resistance, 5 μm or more is desirable. Furthermore, the area ratio of the M ₇ C ₃ type carbide becomes better as the amount of the carbide increases from the viewpoint of wear resistance, and at least 1% or more of the M ₇ C ₃ type carbide is required. On the other hand, from the viewpoint of fatigue resistance, the content is desirably 9% or less in order to disperse carbide as uniformly as possible. Therefore, the area ratio of M ₇ C ₃ type carbide is 1 to 9%.
And

FIG. 2 is a diagram showing the relationship between the M ₇ C ₃ type carbide size and the mold life (number of shots). According to this figure, as a result of testing the mold life from the disposal of the mold due to wear and the disposal due to the cracking of the carbide, the comparative steel E
(Temperature 180 ° C.), the mold was discarded due to abrasion, and in the case of comparative steel F (tempered 300 ° C.), although it was low-temperature tempering, it was scrapped due to cracking of carbide, and further 500 ° C. It can be seen that the number of shots, which is an index as a tool life, is lower in the high-temperature tempering of from super to 550 ° C. than in the low-temperature tempering of 150 to 500 ° C. according to the present invention due to the discard due to carbide cracking. That is, 150 to 500
When comparing the case of low-temperature tempering at 500 ° C with the case of high-temperature tempering exceeding 550 ° C, the life of the mold is longer in the case of low-temperature tempering than in the case of high-temperature tempering. Is clearly understood.

FIG. 3 is a diagram showing the relationship between the tempering temperature and the mold life (number of shots). As shown in FIG.
For example, the mold life (the number of shots) at the tempering temperature of steel A and steel C can be obtained at a tempering temperature of 150 to 500 [deg.] C., although the number of shots can be more than 30,000 with almost the same tendency. On the other hand, when the treatment was performed at a temperature exceeding the tempering temperature of 500 ° C., the number of shots was 30,000 or less, which indicates that the mold life was shortened. As is clear from this, the tempering temperature in the present invention is set to 150
Regulated to ~ 500 ° C. Preferably, the tempering temperature is 150 to less than 450 ° C.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on embodiments. Steel having the chemical composition shown in Table 1 was melted in a vacuum induction melting furnace. Steel types A to D are the steels of the present invention.
F is a comparative steel. 850-1200 ° C
Forging or hot rolling, and annealing at 860 ° C. to obtain test materials. After quenching these test materials from 1040 ° C., they are tempered at the tempering temperatures shown in Table 2 to obtain tool steels of the present invention and comparative steels. Further, the tensile compression fatigue test was performed by processing a test piece having a parallel portion and a diameter of 5 × 15 mm, and then using a hydraulic servo testing machine to obtain a stress amplitude of 1300 MPa, a stress ratio of R = −1,
The test was performed at room temperature.

[0020]

[Table 1]

The Ogoshi abrasion test was performed using SCM420 (86H
RB) as the mating material, wear distance 200m, final load 62
The test results showed that the wear amount of the comparative steel 9 was 10
It was expressed as 0. In addition, the mold test on the actual
A 0 × 100 mm forging die was prepared, and a test was performed using SCM420 as a workpiece. The mold was discarded due to abrasion or cracking, and as a result of investigating the inside of the discarded mold, the cracks of the carbides became the destruction starting point. In addition, as a method for defining the carbide, the measurement surface,
A 1/4 part of the T plane, the particle diameter was measured by an image processing device, and the area ratio was measured by an image processing device. As for M ₇ C ₃ carbide, in the present invention, all of the carbide having a size of 2 μm or more was M ₇ C
_It was considered as type ₃ carbide.

Table 2 shows the results. As shown in Table 2
The steels Nos. 1 to 8 of the present invention all have M ₇C_ThreeCarbide particle size
5 to 15 μm, and M₇C_ThreeCarbide area ratio
(%) Is in the range of 1 to 9%, and the tempering temperature is 150 to
The test was performed within the range of 500 ° C, and the hardness (H
RC), while maintaining a hardness of 59 HRC or more
In, much better than conventional cold tool steel No. 9-12
Extended tensile and compression fatigue life and extended mold life
Was.

[0023]

[Table 2]

[0024]

As described above, the steel of the present invention has a certain range of M ₇ C ₃ carbide particle size and M
_By regulating the area ratio of ₇ C ₃ carbide to a certain range and tempering at the tempering temperature, it is possible to secure an extremely excellent mold life, and it is more economical as a tool steel for molds than conventional steel. Became very advantageous.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the M ₇ C ₃ type carbide size, the number of repeated fractures, and the wear resistance.

FIG. 2 is a diagram showing a relationship between M ₇ C ₃ type carbide size and mold life (number of shots).

FIG. 3 is a diagram showing a relationship between a tempering temperature and a mold life (the number of shots).

Claims (2)

[Claims] C. 0.65 to 1.3%, Si: 2.0% or less, Mn: 0.1 to 2.0%, Cr: 5.0 to 11.0% by weight%. One or two of Mo and W are used at the Mo equivalent (M
o + 1 / 2W): 0.7-5.0%, one or two of V or Nb is V equivalent (V +
ＮNb): 0.1 to 2.5%, the balance being Fe and unavoidable impurities, and the particle size of the M ₇ C ₃ type carbide is 5 to 1
5 to 50% of steel material having an area ratio of 1 to 9%
A method for producing cold tool steel, characterized by tempering at a temperature of ° C. 2. The tempering temperature according to claim 1, wherein the tempering temperature is 150-4.
A method for producing cold tool steel, characterized by tempering at less than 50 ° C.