JPH05156407A - Steel for high-performance rolling die and production thereof - Google Patents

Abstract

PURPOSE:To provide the steel for rolling dies which has the excellent service life to rolling of hardly workable materials, such as stainless steels having a high work hardening property and tempered steels having about 40 HRC and has the hardenability dealing with a vacuum furnace. CONSTITUTION:The material which consists, by weight%, 0.95 to 1.10% C, 0.65 to 1.2O% Si, >=1.0% Mn, 6.80 to 7.80% Cr, 2.50 to 3.50% one or two kinds of Mo and W (Mo+1/2W), 0.20 to 0.60% one or two kinds of V and Nb (V+1/2Nb) or further contg. 1.0 to 4.0% Co in addition thereto and further, 0.2 to 1.0% Ni and the balance Fe, contains respectively carbides of an M7C3 type and MC type, <=20mum max. primary carbide and provides >=64 hardness HRC after high-temp. tempering by specifying the hardening and cooling temp. rate down to 500 deg.C to 25 deg.C/min and the process for production thereof.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tool steel for a rolling die used for rolling various kinds of screws, spline shafts, serration shafts and the like.

[0002]

2. Description of the Related Art The rolling method has been applied not only to screw manufacturing but also to spline shafts and serration shafts for automobile parts due to improvements in rolling mechanism and higher precision. Along with the expansion of such applications, the number of difficult-to-process materials such as stainless steel and heat-resistant steel, which have large work hardening as the work material, and heat-treated steel around HRC40, is increasing. On the other hand, high-speed tool steel and cold tool steel are used as the material for rolling dies, but SKD1 is mainly used for cost performance.
1st class cold work tool steel is often used. But,
SKD11 class cannot secure sufficient tool life against the increase of difficult-to-machine materials. For this reason,
No. 00 and Japanese Patent Application Laid-Open No. 1-201442, that is, a hardness of HRC61 to 63 is obtained in high temperature tempering, and primary carbides are reduced, so that toughness is emphasized over wear resistance. Tool steels have been used.

[0003]

As described above, in response to the increase in difficult-to-machine materials, as a tool material, a high hardness has been obtained, and a cold tool steel with more emphasis placed on toughness has been used. As a result, the tool life is improved as compared with the conventional material, but it is not yet fully satisfactory, and therefore a higher performance material is required. In addition to this, it is required to have high hardenability corresponding to a vacuum heat treatment furnace which has been particularly popular recently. Vacuum heat treatment
It has come to be widely used due to the reduction of man-hours and a good working environment, but it is not possible to obtain a sufficient cooling rate because of gas cooling. Therefore, the processed material is required to have high hardenability. An example can be found in JP-A-3-134136 as a cold tool steel aiming at higher performance. However, according to the knowledge of the present inventor, the steel is not sufficiently defined in terms of the amount and size of primary carbides, and the hardenability which is a problem during actual use is not clear. INDUSTRIAL APPLICABILITY The present invention provides, as a rolling die steel, a rolling die steel which not only secures a long tool life for rolling difficult-to-machine materials but also can be applied to vacuum heat treatment due to sufficient hardenability. The purpose is to

[0004]

The present invention has been made based on the result of detailed analysis of a conventional tool which has been rolled and rolled into a work material of HRC40 level. That is, as the steel for rolling dies, high hardness is required due to fatigue resistance, and toughness is required relative to wear resistance, but especially the amount and distribution of primary carbide crystallized during solidification. It was found that the size has a great influence on the life, and that there is a specific area ratio and grain size range of the carbide that are suitable for maintaining hardenability that facilitates vacuum heat treatment. is there. That is, the first aspect of the present invention is
% By weight C 0.95 to 1.10%, Si 0.65 to 1.20%, Mn 1.0%
Below, Cr 6.80 to 7.80%, one or two kinds of Mo and W are 2.50 to 3.50% at (Mo + 1 / 2W), and one or two kinds of V and Nb are 0.20 to 0.60 at (V + 1 / 2Nb). %, And the balance Fe and unavoidable impurities. After quenching and tempering, M ₇ C ₃ type primary carbides have an area ratio of 4% or less, MC
The area ratio of the type primary carbides is 0.5% or less, and the maximum grain size of the primary carbides is substantially 20 μm or less to form a uniformly distributed microstructure in the matrix, and from the quenching temperature of 1050 ° C to 1100 ° C,
Quenching up to 500 ℃ at a cooling rate of 25 ℃ / min,
This is a high-performance rolling die steel that can obtain a hardness of HRC 64 or higher when tempered at a high temperature. The second aspect of the present invention contains 1.0 to 4.0% of Co in addition to the additive element of the first aspect of the present invention, and the third aspect of the present invention further contains 0.2 to 1.0% of Ni. In addition, P, S, O and N which are inevitable impurities are 0.015% or less for each of the above invention steels,
It is desirable to set it to 0.005% or less, 30 ppm or less, and 300 ppm or less. The fourth invention defines the steps for producing these, and uses a steel ingot by a refining method that goes through a step selected from vacuum melting, vacuum degassing, and electroslag melting. A high-performance rolling die steel manufacturing method characterized by performing high-temperature diffusion treatment.

[0005]

The function, quantity, distribution, size and heat-treatability of the chemical components and carbides of the present invention and the reasons for limiting the numerical values of the manufacturing process will be described below. As is well known, C is an important element that imparts strength to steel. In the case of the present invention, it is defined from two viewpoints. One is the M ₇ C ₃ type (M
Is a carbide-forming element such as Mo or W mainly composed of Cr) and MC
It is related to the amount of carbides of the type (M is a carbide-forming element such as V or Nb). Of these, the M ₇ C ₃ type carbides form a solid solution to some extent by the high temperature diffusion treatment, but the VC type hardly dissolves and remain after the quenching heat treatment. These tend to have a chain distribution and serve as a route for crack development when used as a rolling die, so that the amount of carbide must first be limited by the regulation of the amount of C. From this point, C has an upper limit of 1.10%. Another point is that the hardness after quenching and tempering is set to HRC64 or higher. In the present invention, this is achieved by secondary hardening in high temperature tempering. In the secondary hardening, it is due to precipitated carbides of Mo, W and V. To obtain HRC of 64 or more, at least 0.95% is required from the balance with these carbide forming elements.
You need more than that.

Si is usually added as a deoxidizing agent,
The level contained for this purpose is up to about 0.5%. In the present invention, 0.65% or more is added in order to suppress the agglomeration of precipitated carbide in the tempering process and accelerate the secondary hardening. On the other hand, in the alloy system mainly composed of Cr as in the present invention, Si
Reduces the austenite region and expands the crystallization region of M ₇ C ₃ type carbide, so excessive addition leads to an increase in M ₇ C ₃ type carbide. Therefore, the upper limit was suppressed to 1.2%. Mn is
Like Si, it is added as a deoxidizer, but a large amount of addition impairs the hot workability. In addition, since it is an austenite forming element, the quenching temperature of the steel of the present invention is 1050 ℃ ~ 1
When quenching from 100 ° C, the upper limit was set to 1.0% because excessive austenite is generated. Cr is M ₇ C during solidification
_Although it crystallizes as a ₃ type carbide and contributes to wear resistance, the distribution of this carbide acts as a route for crack propagation as described above, so the upper limit is 7.8% from the balance with the C content.
And On the other hand, Cr is the most effective element for improving hardenability. In the present invention, it is assumed that quenching is performed in a vacuum furnace, and the quenching cooling rate from the quenching heating temperature to 500 ° C. is 25 ° C./min and sufficient quenching is possible. Therefore,
At least 6.8% is required to satisfy this.

Mo and W are important elements that exert a secondary hardening action, and since Mo amount and 1/2 W amount have almost the same effect, they can be selected or added in combination. In this component system, to obtain HRC 64 or higher by high temperature tempering,
(Mo + 1 / 2W) requires at least 2.50% or more.
On the other hand, these elements narrow the austenite region and cause M ₇
The upper limit was set to 3.50% in order to excessively crystallize C ₃ . Both V and Nb crystallize as MC type carbides during solidification,
Contributes to wear resistance. It is also effective in improving the toughness by suppressing the coarsening of crystal grains. To obtain this effect, V
And 1/2 Nb are almost equivalent, and V and Nb can be selected or added in combination. However, since these carbides easily form a striped distribution or a network distribution and have high hardness, they tend to be easily separated from the matrix, that is, they promote the starting point and progress of cracking, and The invention aims at suppressing coarsening of crystal grains at the quenching austenitizing temperature rather than abrasion resistance. For this, (V + 1 / 2Nb)
The required amount is 0.20% or more, but if it exceeds 0.60%, the MC amount is 0.
This is the upper limit because it exceeds 5%. Co is an element effective as a solid solution in the matrix to improve heat resistance and seizure resistance. Also, Co is an element that expands the austenite region,
Since it promotes solid solution of solute elements at the quenching temperature, it is effective for secondary hardening in high temperature tempering. For this purpose, at least 1% or more is necessary, but if it exceeds 4%, this effect becomes small, so this is made the upper limit.

Ni is an element which forms a solid solution in the matrix like Co. It is effective in improving toughness and hardenability, but if it is less than 0.2%, this effect is small. On the other hand, since Ni is an austenite stabilizing element, it promotes the formation of retained austenite and causes a decrease in quenching hardness, so the upper limit was made 1.0%.
P, S, O and N are usually contained in trace amounts as impurity elements. P not only segregates at the crystal grain boundaries and reduces the grain boundary strength, but also promotes matrix segregation during solidification, which causes the directionality of the material. S and O are mainly present in the steel as non-metallic inclusions and adversely affect fatigue strength and the like. Further, N affects the crystallized form of the primary carbide during solidification, and when it exceeds 300 ppm, the primary carbide becomes coarse. By reducing these impurities, the effect on the toughness is particularly large. However, when it is used with high hardness like the steel of the present invention, the effect of reducing one element is small, so four elements are simultaneously limited, and P 0.015% or less, S 0.005% or less, O 30 ppm or less, and N 300 ppm, respectively. Since the improvement effect was obtained by setting it as follows, these were made into the upper limit.

In cold work tool steel, primary M ₇ C ₃ type and MC type carbides that crystallize during solidification are important factors for improving wear resistance. However, in the rolling die, it becomes an important factor affecting the tool life. Especially the work material is HRC40
It was found that in the case of the high hardness material of the class, the wear morphology was not wear, but most of the wear was due to defects. When this is analyzed in detail, it has been confirmed that a crack is generated from the peeling of the large M ₇ C ₃ type carbide or MC type carbide from the matrix and the crack propagates along the carbide array. Therefore, as a result of examining the size, amount, and distribution of these primary carbides from the viewpoint of crack generation and development, the amount of M ₇ C ₃ type carbides is 4% or less in area ratio, and the amount of MC type carbides is area. It has been found that the tool life can be significantly improved by making the ratio 0.5% or less and the size 20 μm or less and by uniformly distributing the particles.

When the material to be processed has a high hardness like a tempered material, the deformation resistance in rolling also becomes large, and the stress applied to the die becomes high. On the other hand, as a result of examining the die material from the viewpoint of hardness, the hardness of the conventional cold tool steel level HRC
It was found that a sufficient life cannot be obtained with 60 to 63, and that a tool life can be significantly improved by setting at least HRC64 or more. In addition, in the actual heat treatment for quenching and tempering, an atmospheric furnace or a salt furnace has been conventionally used, but quenching using a vacuum furnace is increasing due to improvement in process efficiency and working environment. Most of the cooling in a vacuum furnace is N ₂ gas cooling, and the cooling rate is slower than that of conventional oil cooling and the like, so that the tool material is required to have sufficient hardenability.
In the present invention, in order to respond to these trends, the cooling rate in the substance was measured using steel materials of various sizes in a vacuum furnace, and the relationship with the hardness after tempering was examined, and 1050
The cooling rate from the quenching temperature from ℃ to 1100 ℃ to 500 ℃ is 25
It was found that it is necessary to obtain a sufficient heat treatment hardness of HRC64 or higher even at ° C / min. In other words, it was found that at least the hardness of HRC64 should be obtained by quenching under these conditions and performing high temperature tempering. The steel having the composition and carbide distribution (area ratio, grain size) of the present invention makes it possible to use a vacuum heat treatment furnace which has characteristics as a rolling die for the first time and has a relatively slow cooling rate.

To produce the steel of the present invention, S, P, O, N
It is necessary to apply a refining method in a process selected from vacuum melting, vacuum degassing and electroslag melting in order to reduce impurities such as and to adjust non-metallic inclusions to the minimum and to improve toughness. Further, the above-mentioned M ₇ C ₃ type and M
As a means for adjusting the size, amount, and distribution of C-type primary carbide, in the production of the steel of the present invention, at least once at a temperature of 1150 ° C. in the state of a steel ingot or in the process of hot working the steel ingot.
It is necessary to include a step of holding at ~ 1250 ° C and performing high temperature diffusion treatment. If the diffusion temperature is less than 1150 ° C., sufficient diffusion of elements is not carried out, and the size and morphology of primary carbides do not change, so this was made the lower limit. Further, when the temperature exceeds 1250 ° C., partial melting starts, so this is set as the upper limit.

[0012]

EXAMPLES The present invention will be described below based on examples.
As the test material, the steel of the present invention having the composition shown in Table 1 (No. 1 to No. 1).
9), comparative steel (No.10 to No.12), and conventional steel (No.13,1)
4) was melted by the melting method shown in Table 2 with the corresponding No., and for the steel of the present invention and the comparative steel No. 12,
The high temperature diffusion treatment under the conditions shown in Table 2 was performed in the ingot state. After that, hot working and annealing treatment were performed to obtain a test material. No. 9 * indicates that the chemical composition is the same as No. 9 and that high temperature diffusion treatment is not performed. Using these materials,
Quenching was performed using an actual vacuum quenching furnace. The size of the test material is 200 mm, which is the size of a typical round rolling die.
mm, thickness 70 mm, treatment was carried out under the same conditions as in the actual operation, with a charging amount of 300 kg and a cooling N ₂ gas pressure of 1.8 barr. In this case, the average cooling rate from the quenching temperature to 500 ° C was 25 ° C / min. Comparative Example No. 11 was oil-cooled (cooling time in the temperature range ≤ 1 min) because sufficient hardness cannot be obtained by gas cooling. These were tempered at 540 ° C. However, the material equivalent to SKD11 of No. 13 is usually
Since it is used for low temperature tempering, tempering at 200 ° C was performed here. After heat treatment, measure hardness of each test material, primary carbide (M ₇ C ₃ , MC) by image analyzer
The size and area ratio of the In addition, the performance of die materials was compared by simulating the actual rolling state. The method is to set a disc-shaped test piece having blade-shaped projections with an apex angle of 60 ° that are continuous in the circumferential direction so that it can rotate freely and set it on the tool post of a lathe, and support this on a spindle to rotate the mating material. A spiral angle is applied to the outer circumference to apply a predetermined depth of pressure.
Using SCM440 tempered to HRC40, the total number of revolutions before chipping of the test piece was used as the life.

[0013]

[Table 1]

The above results are shown in Table 2. The steel of the present invention has an appropriate area ratio and size of primary carbides, and can obtain HRC64 or more in vacuum heat treatment, so that sufficient strength and toughness are ensured for rolling of high hardness work materials. be able to. That is, it can be seen that the rolling fatigue life test shows a life of 2 to 5 times that of the comparative steel and the conventional steel. No. 9 * is the composition of the steel No. 9 of the present invention, which was manufactured without high temperature diffusion treatment and is a comparative steel. Since the high temperature diffusion treatment step is lacking, sufficient heat treatment hardness can be obtained, but the amount and size of primary carbides are excessive and excessive as can be seen by comparison with No. 9, which impairs the original tool life. Comparative steel No. 10 is satisfactory in hardness,
Since the amounts of C and Cr are inadequate, the amounts and sizes of M ₇ C ₃ type primary carbides become excessive, and the probability of cracking is high, which is inferior to the steel of the present invention. The primary carbides of comparative steel No. 11 are relatively large in hard MC type. Since the MC type carbide has a large difference from the deformation characteristics of the matrix, it easily becomes a stress concentration source and causes a crack. Further, the hardenability is also deteriorated, so that a satisfactory tool life is not obtained. Comparative Steel No. 12
Has an area ratio of primary carbides close to that of the steel of the present invention, and has sufficient heat treatment hardness, but has a large maximum size of primary carbides, and the levels of impurities of P, S and O, N are associated with the melting method. Is high, it has not reached the level of the steel of the present invention. In addition, the conventional steel No. 13 is equivalent to SKD11, and the amount and size of primary carbides are greatly excessive and excessive, and although it has hardenability, it has a chemical composition such as Mo + 1/2 W and V + 1/2 Nb. It is impossible to get HRC64 or higher. Further, in the conventional steel No. 14 of 8Cr system, the level of primary carbides is inadequate although close to that of the steel of the present invention, and HR
It is also difficult to get C64 and above. Therefore, the rolling fatigue life of both the conventional steel types is considerably inferior to that of the steel of the present invention.

[0015]

[Table 2]

Table 3 shows steel Nos. 1 and 9 of the present invention and comparative steel No. 1.
1. The results of investigation of heat treatment characteristics of conventional steel No. 14, especially the influence of cooling time are shown. It is practically important for the tool material to obtain a target hardness under actual heat treatment conditions.
As mentioned above, in vacuum quenching of a typical size rolling die, the cooling rate from the quenching temperature to 500 ° C is about 25 ° C /
min, therefore about 27 minutes from 1180 ° C and about 26 minutes from 1160 ° C. Therefore, within this cooling condition, the hardness after tempering must be HRC 64 or more. From Table 3, it can be seen that No. 1 of the present invention satisfies this, and No. 9 can obtain sufficient hardness even with slow cooling of 30 minutes. On the other hand, comparative steel No. 11
Oil-cooled exceeds HRC64, but hardenability is insufficient, so 2
The hardness decreased considerably under the condition of 7 min, and No. 14 was quenched at 1160 ° C., which is higher than the quenching temperature shown in Table 2. However, even if the cooling time is long, the hardness is not significantly reduced. Secondary curing is limited in terms of chemical composition, and HRC64 can be achieved even by quenching.
It is difficult to obtain the above.

[0017]

[Table 3]

[0018]

As described above, according to the present invention, in the conventional rolling die material, stainless steel or the like having a large work hardening and HRC4 are used.
It was developed in response to the fact that a sufficient tool life could not be obtained for difficult-to-machine materials such as heat-treated steel of about 0, and in consideration of the application of a vacuum furnace, the heat treatment property was added. Yes, by optimizing the microstructure within the optimum component range,
It achieves an excellent tool life.

Claims (5)

[Claims] 1. C. 0.95 to 1.10% by weight, Si 0.65 to 1.10% by weight.
20%, Mn 1.0% or less, Cr 6.80 to 7.80%, 1 or 2 types of Mo and W (Mo + 1 / 2W) 2.50 to 3.50%, 1 or 2 types of V and Nb (V + 1 / 2Nb) Of 0.20 to 0.60%, and the balance Fe and unavoidable impurities. After quenching and tempering, the M ₇ C ₃ type primary carbides have an area ratio of 4% or less and the MC type primary carbides have an area ratio of 0.5% or less. , The primary carbide has a maximum grain size of substantially 20 μm or less and has a uniformly distributed microstructure in the matrix. Further, the quenching cooling rate from the quenching temperature of 1050 ℃ to 1100 ℃ to 500 ℃ is 25 ℃ / min A steel for high-performance rolling dies that can be hardened as above and has a hardness of HRC64 or higher when tempered at high temperature. 2. C 0.95 to 1.10% by weight% and Si 0.65 to 1.10% by weight.
20%, Mn 1.0% or less, Cr 6.80 to 7.80%, 1 or 2 types of Mo and W (Mo + 1 / 2W) 2.50 to 3.50%, 1 or 2 types of V and Nb (V + 1 / 2Nb) 0.20-0.60%, Co
1.0 to 4.0%, balance Fe and unavoidable impurities, and after quenching and tempering, M ₇ C ₃ type primary carbides have an area ratio of 4% or less, MC type primary carbides have an area ratio of 0.5% or less,
The maximum grain size of primary carbide is substantially 20 μm or less, and the microstructure is uniformly distributed in the matrix.
The quenching cooling rate from the quenching temperature of ℃ to 500 ℃ is 25 ℃ / mi
Steel for high-performance rolling dies that can obtain hardness of HRC64 or higher when quenched as n and tempered at high temperature. 3. C 0.95 to 1.10% by weight%, Si 0.65 to 1.10% by weight.
20%, Mn 1.0% or less, Cr 6.80 to 7.80%, 1 or 2 types of Mo and W (Mo + 1 / 2W) 2.50 to 3.50%, 1 or 2 types of V and Nb (V + 1 / 2Nb) 0.20-0.60%, Co
1.0 to 4.0%, Ni 0.2 to 1.0%, balance Fe and unavoidable impurities, and after quenching and tempering, M ₇ C ₃
Area ratio of type primary carbide is 4% or less, MC type primary carbide is 0.5% or less, and maximum grain size of primary carbide is substantially 20μ.
When the microstructure is uniformly distributed in the matrix at m or less, and further quenching is performed at a quenching cooling rate of 1050 ° C to 1100 ° C to 500 ° C at a quenching cooling rate of 25 ° C / min and high temperature tempering. Steel for high performance rolling dies that can obtain hardness of HRC 64 or more. 4. The high-performance rolling die steel according to claim 1, wherein P 0.015% or less, S 0.005% or less, O 30 ppm or less, and N 300 ppm or less are satisfied as unavoidable impurities. 5. A step of producing a steel ingot by a refining method including a step selected from vacuum melting, vacuum degassing and electroslag melting, in the state of the steel ingot, or in the step of hot working the steel ingot, The method for producing a steel for high performance rolling dies according to any one of claims 1 to 4, comprising a step of holding at 1150 ° C to 1250 ° C at least once and performing a high temperature diffusion treatment.