JP2710941B2 - Rolling die steel - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a tool steel for a rolling die used for rolling various screws, spline shafts, serration shafts and the like.

[Conventional technology]

Conventionally, rolling processing has been mainly used for screw production. However, with the improvement of the rolling mechanism and higher precision, it has also been applied to the production of spline shafts and serration shafts of automobile parts. As for the material of the rolling dies, high-speed tool steel may be used when the rolling conditions are partly severe. Tool steel is used.

[Problems to be solved by the invention]

With the application issues of rolled products, these materials, that is, the materials of the workpieces have also been diversified. In the case of automobile parts and the like, the case where a tempered material having a hardness of about HRC40 is rolled has also appeared. Rolling of stainless steel, which is a difficult-to-process material, is also increasing. Under these circumstances, the use conditions of rolling dies have become severe, and it has become difficult for conventional rolling dies to maintain a sufficient tool life due to chipping or wear of the ridges. ing. For this reason, as a result of investigating the cause of scrapping of the rolling dies in detail, it was found that the properties of the residual carbides were the main factors affecting the tool life. In other words, in the conventional material, M ₇ C ₃
The structure was such that a large primary carbide of 30 μm or more of the mold type or MC type was distributed in a large amount from 10% to several tens%. However, defects and abrasion were caused by the chain-like distribution of these large primary carbides. Or the accumulation of microscopic defects due to the lack of carbide units caused excessive primary carbides to act as an adverse effect. In addition, conventional steel has been used by low-temperature tempering because high-temperature tempering does not provide sufficient hardness.However, in the rolling of difficult-to-process materials, there is a considerable rise in temperature. In addition, a material that can secure sufficient hardness and has high softening resistance is required.

The present invention has been made based on these facts, and by optimizing the chemical components and adjusting the microstructure, the primary carbide is appropriately quantified, and the material is emphasized in toughness. An object of the present invention is to provide a high-performance rolling die material having a hardness of HRC62 or more. Furthermore, in order to minimize sulfide-based non-metallic inclusions that are likely to cause microcracks in the rolling dies, it is effective to regulate the S content together with the area ratio of carbides, and the P content is also specified. It has been found that when the value is equal to or less than the value, the toughness is further improved and it is effective to prevent chipping of the rolling die.

[Means for solving the problem]

In the present invention, 0.90 to 1.35% by weight of C, 0.70 to 1.40% of Si
%, Mn 1.0% or less, S 0.004% or less, Cr 6.0 to 10.0%, M
One or two of o and W are 1.5% to 2.5% by Mo + W / 2, and V and Nb
Of 0.15 to 2.5% by V + Nb / 2, the balance being Fe and unavoidable impurities. In the quenched and tempered structure, the area ratio of M ₇ C ₃ type carbide is 2% or more.
% Or less, and the area ratio of MC carbide is 2.5 or less. Rolled die steel, and C 0.90 to 1.35 by weight%.
%, Si 0.70-1.40%, Mn 1.0% or less, S 0.004% or less,
Cr 6.0 to 10.0%, Mo + W / 2 with one or two of Mo and W
1.5 to 2.5%, one or two of V and Nb are 0.15 in V + Nb / 2
~ 2.5%, one or two of Ni and Co in Ni + Co 0.3 ~ 1.5
%, The balance consists of Fe and inevitable impurities, and in the quenched and tempered structure, 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. Features rolling steel for die and by weight
C 0.90-1.35%, Si 0.70-1.40%, Mn 1.0% or less, S
0.004% or less, P 0.015% or less, Cr 6.0 to 10.0%, Mo and W
One or two of Mo + W / 2 1.5-2.5%, one of V and Nb
Species or two containing 0.15 to 2.5% by V + Nb / 2 and the balance Fe
A rolling die comprising an unavoidable impurity and a quenched and tempered structure having an area ratio of M ₇ C ₃ type carbide of 2% or more and 9% or less and an area ratio of MC carbide of 2.5% or less. For steel, and C 0.90-1.35% by weight,
Si 0.70 to 1.40%, Mn 1.0% or less, S 0.004% or less, P 0.
015% or less, Cr 6.0 to 10.0%, one or two of Mo and W are 1.5 to 2.5% by Mo + W / 2, and one or two of V and Nb are V
0.15 to 2.5% at + Nb / 2, one or two of Ni and Co are Ni + C
o 0.3-1.5%, the balance consists of Fe and unavoidable impurities. In the quenched and tempered structure, 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
Rolling die steel characterized by being 2.5% or less, and a step of producing a steel ingot by one or more refining methods selected from vacuum melting, vacuum degassing, and electroslag melting; 5. The method for producing rolling die steel according to any one of claims 1 to 4, further comprising a step of hot-working the ingot and holding the steel ingot at least once at 1150 to 1250 ° C and performing a high-temperature diffusion treatment. It is.

 The reasons for limiting the chemical components are described below.

C is an element that plays a major role in the strength and wear resistance of tool steel, and is required to be 0.9% or more to obtain sufficient heat treatment hardness. C forms carbides with Cr, Mo, and V, and contributes to wear resistance. However, if it becomes excessive, the primary carbides at the time of solidification increase, which causes deterioration of toughness. Therefore, the upper limit is set to 1.35%.

Although Si is usually added as a deoxidizing agent, in the present invention, it is assumed that it is used in low-temperature tempering (180 to 250 ° C.), and is specified to exhibit softening resistance in this temperature range. In order to fully demonstrate this effect, at least
0.70% or more is required, but if it exceeds 1.40%, the toughness is reduced.

Cr crystallizes as M ₇ C ₃ type carbide during solidification. This is the main factor that demonstrates the wear resistance of tool steel,
The amount greatly affects the toughness of the material. For this reason, 6.0% or more is required to maintain the wear resistance of tool steel from the balance with the C content, and if it exceeds 10%, the primary carbide becomes an appropriate amount and the toughness deteriorates. Was set as the upper limit.

Mo and W are elements that form a solid solution with Cr carbide to enhance wear resistance, improve hardenability, and precipitate as carbides during tempering and exhibit strong secondary hardening. In the case of a rolled die, the type of the material to be processed, particularly in the case of a difficult-to-process material, is heated during processing, so that the die may require high-temperature tempering. Therefore, the hardness due to high temperature tempering and HRC6
It was set to 2 or more, and the Mo and W amounts were specified.

Mo is effective for improving toughness, and W is effective for improving wear resistance.
Since the atomic weight of W is about twice that of Mo, it is set as (Mo + W / 2) and set to 1.5 to 2.5% from the setting of hardness.

V and Nb both crystallize as MC-type carbides during solidification and play an important role in wear resistance as hard carbides, and have an effect of refining crystal grains and are effective in improving toughness. Further, since it is a secondary hardening element, it is advantageous for securing hardness by high-temperature tempering. To exhibit these effects, one or two of V and Nb must be at least 0.15% (V + Nb / 2). However, carbides of this type are distributed in a chain form when the amount is large, not only deteriorating the toughness in the distribution direction, but also hardly changing by the heat treatment. For this reason, the upper limit is set to 2.5% from the provision of the amount of MC type carbide described later.

Both Ni and Co are dissolved in the matrix to improve toughness and seizure. Ni also contributes to the improvement of hardenability. For this purpose, one or two of Ni and Co must be at least 0.3% or more (Ni + Co), but the upper limit is set to 1.5% because excessive addition lowers the heat treatment hardness.

Mn is usually added as a deoxidizing agent, but if it exceeds 1%, it not only impairs hot workability but also deteriorates machinability, so the upper limit is set. S is particularly Mn and nonmetallic inclusions (Mn
S) is formed and tends to be distributed in a state of extending in the rolling direction. In this case, the toughness particularly in the rolling direction decreases. In the rolling die, if the non-metallic inclusions are arranged in the direction of the forging of the blade portion, it will lead to chipping of the blade. After examining this point, we found 0.
An improvement effect was observed in S of 004% or less, so the upper limit was set.

P is preferably low in order to withstand the impact force acting on the blade portion of the rolling die. In particular, when the content is 0.015% or less, the anisotropy in the vertical and horizontal directions of the impact value can be reduced, which contributes to the improvement of the life of the rolling die.

As described above, the primary carbide that crystallizes during solidification determines the life of the rolled die material. Therefore,
In order to sufficiently exert the effects of the present invention, it is necessary to further define the amount of the primary carbide in the component range of the present invention. In conventional SKD11 level, the area ratio of the M ₇ C ₃ type primary carbides was 10 to 12%. It was a M ₇ C ₃ type primary carbides to investigate the relationship between toughness for materials showing various area ratio, since the improvement is observed below 9%, which was the upper limit. However, wear resistance as a tool is also necessary and cannot be completely eliminated. In terms of wear resistance, at least 2% is required. The MC type carbide not only has a great effect on wear resistance, but also has an effect of crystallizing prior to M ₇ C ₃ type carbide during solidification and improving the distribution state of the M ₇ C ₃ type carbide. However, if the area ratio exceeds 2.5%, the toughness decreases, so the upper limit was set.

In order to manufacture the steel of the present invention, one or more selected from vacuum melting, vacuum degassing, and electroslag melting in order to reduce impurities such as S and P and minimize nonmetals to improve toughness. It is desirable to apply two or more refining methods. Further, as a means for adjusting the amount of primary carbides of the above-mentioned M ₇ C ₃ type and MC type, at least one step is performed during the hot working of the steel ingot or the steel ingot in producing the steel of the present invention.
It is desirable to include a step of performing the high-temperature diffusion treatment while maintaining the temperature at 1150 to 1250 ° C. or more. If the diffusion temperature is less than 1150 ° C, sufficient diffusion and homogenization of elements will not be performed, so the lower limit is 1150 ° C, and if it exceeds 1250 ° C, some carbides melt and the upper limit is 1250 ° C to lower the mechanical strength. And

〔Example〕

Hereinafter, the present invention will be described based on examples. Table 1 shows the chemical composition of the steel of the present invention produced together with the conventional steel and the comparative steel to evaluate the steel of the present invention. Test steel
Among Nos, No. 1 is a conventional steel, No. 2 is within the composition range of the present invention except for P and S, but contains many M ₇ C ₃ type carbides,
No. 3 has the same composition as the steel of the present invention, but has more M ₇ C ₃ type carbides than the steel of the present invention. High frequency melting for test steel No.1 and No.2, vacuum induction melting for test steel No.3 to No.7, ESR melting for test steel No.8 and No.9, and test steel No. 10 to No. 12 have been subjected to vacuum degassing. For the ingots of the test steels No. 4 to No. 12 of the present invention, high-temperature diffusion treatment was performed at 1170 ° C. for 20 hours in order to adjust primary carbides, especially M ₇ C ₃ type carbides. .

Since the temperature of the high-temperature diffusion treatment is desirably as high as possible without melting the primary carbide, the temperature is optimally in the range of 1150 to 1250 ° C. All ingots were forged and rolled into slabs having a cross-sectional dimension of 100 mm x 100 mm.
This steel piece was subjected to an annealing treatment to obtain a test material.

The bending test specimen is the rolling direction of the steel slab (hereinafter referred to as L direction).
From the direction perpendicular to the rolling direction (hereinafter referred to as the T direction)
A test piece of × 70 mml was sampled, subjected to a predetermined quenching and tempering, and subjected to a test. In addition, the tempering was performed at a low temperature for sample steel No. 1, and at a high temperature within the range showing secondary hardening. The bending test is performed at two points and the support point interval is 50mm.
The quantification of the carbide in the microstructure was expressed as the average value of the samples in the L direction and the T direction. These results are summarized in Table 2.

The steel of the present invention has a high hardness of HRC62 or more, and has a high bending strength and a large deflection as compared with the conventional material by adjusting the primary carbide, indicating that the steel is excellent in toughness.

Further, among the comparative steels similar to the steel of the present invention, those having a large amount of primary carbides have a low transverse rupture strength, and L / L of the transverse rupture strengths of the test steels No. 4 to No. The ratio of T is
The test steels No. 1 and No. 2 containing a lot of impurities are larger than those of the test steels.

The results of the Ogoshi type abrasion test were also shown for the abrasion resistance. The test was performed under the conditions of SKD61 (HRC40) with a friction speed of 1.37 m / s, a friction distance of 400 m, and a final load of 6.3 kg, and the wear volume was measured. It is an index. The comparative steel has a large amount of M ₇ C ₃ type carbide and MC type carbide and has excellent wear resistance but poor toughness. The steel of the present invention is a material that emphasizes toughness,
It can be seen that the abrasion resistance is not inferior to that of the conventional steel, and that the test steels No. 6 and No. 12 having a large amount of MC type carbide are superior to the conventional steel.

FIG. 1 shows the microstructures of the test steels No. 1 and No. 5 for comparison.

〔The invention's effect〕

The present invention has been made based on the analysis of the causes of scrapping conventional rolling dies, and has developed a steel for rolling dies having high hardness and high toughness by optimizing the primary carbide. It is. According to the present invention, a rolling die having a long life can be manufactured even under severe rolling conditions.

[Brief description of the drawings]

FIG. 1 is a microstructure photograph of test steel No. 1 (conventional steel) and test steel No. 5 (inventive steel) subjected to a bending test.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-61-23747 (JP, A)

Claims (5)

(57) [Claims] C. 0.90 to 1.35% by weight, Si 0.70 to 1.40
%, Mn 1.0% or less, S 0.004% or less, Cr 6.0 to 10.0%, M
One or two of o and W are 1.5% to 2.5% by Mo + W / 2, and V and Nb
Of 0.15 to 2.5% by V + Nb / 2, the balance being Fe and unavoidable impurities. In the quenched and tempered structure, the area ratio of M ₇ C ₃ type carbide is 2% or more.
% Or less, and the area ratio of MC carbide is 2.5% or less. 2. C 0.90 to 1.35% by weight, Si 0.70 to 1.40 by weight%
%, Mn 1.0% or less, S 0.004% or less, Cr 6.0 to 10.0%, M
One or two of o and W are 1.5% to 2.5% by Mo + W / 2, and V and Nb
Contains 0.15 to 2.5% of V + Nb / 2 in one or two kinds, and 0.3 to 1.5% of one or two kinds of Ni and Co in Ni + Co, and the balance consists of Fe and unavoidable impurities. , M ₇ C ₃ type and the area ratio of the carbides than 9% 2%, steel rolling die, characterized in that the area ratio of MC carbides and 2.5% or less. 3. C 0.90-1.35% by weight, Si 0.70-1.40% by weight
%, Mn 1.0% or less, S 0.004% or less, P 0.015% or less, Cr
6.0 to 10.0%, Mo + W / 2 1.5% for one or two types of Mo and W
2.5%, one or two of V and Nb are 0.15 to V + Nb / 2
Containing 2.5%, the balance being Fe and unavoidable impurities,
Further, in a quenched and tempered structure, 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. 4. A composition wherein C is 0.90 to 1.35% by weight and Si is 0.70 to 1.40.
%, Mn 1.0% or less, S 0.004% or less, P 0.015% or less, Cr
6.0 to 10.0%, Mo + W / 2 1.5% for one or two types of Mo and W
2.5%, one or two of V and Nb are 0.15 to V + Nb / 2
2.5%, one or two of Ni and Co are 0.3 to 1.5% with Ni + Co
And the balance consists of Fe and unavoidable impurities, and in the quenched and tempered structure, 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. And rolling die for steel. 5. A step of producing a steel ingot by one or more refining methods selected from vacuum melting, vacuum degassing and electroslag melting, and hot-working the steel ingot or the steel ingot, The method for producing steel for a rolling die according to any one of claims 1 to 4, further comprising a step of holding at 1150 to 1250 ° C at least once and performing a high-temperature diffusion treatment.