JPH09316601A - Cold tool steel suitable for surface treatment, die and tool for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a cold tool steel, die and tool easily treatable in heat treatment and working and suitable for surface hardening treatment with >=400 deg.C treating temp. at low cost. SOLUTION: This cold tool steel suitable for the surface treatment is the one having a compsn. contg., by weight, 0.70 to 1.20% C, <=2.0% Si, <=1.5% Mn, 5.0 to 11.0% Cr, one or two kinds of Mo and W by >3.0 to 5.0% by Mo equivalent (1/2W+Mo), and the balance Fe with inevitable impurities, and in which high hardness of 63 to 66HR can be obtd. by high temp. tempering at >=500 deg.C. In addition to the above components, one or >= two kinds among 0.5 to 4.0% Co, 0.01 to 0.30% S and <=60ppm N are furthermore added.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to nitriding, PVD, T
Processing temperature typified by D processing and CVD is 400
The present invention relates to a cold work tool steel suitable for a surface hardening treatment at a temperature of ℃ or higher, a mold thereof, and a tool.

[0002]

2. Description of the Related Art Conventionally, JIS-S
KS3, SKD11 and SKH51 are widely used. In particular, nitriding for improving the wear resistance of tools, P
SKD11 or SKH51 is applied as a base material for surface hardening treatment such as VD, TD treatment and CVD.
This is because the surface treatment temperature is generally a high temperature of 400 ° C. or higher, so in steels that do not undergo secondary hardening such as SKS3, the hardness of the base material is significantly reduced, and the tool itself is deformed due to insufficient strength. This is because the surface treatment layer is easily peeled off.

However, with the progress of the plastic working technology and the higher strength of the work material, the stress load on the tool to be used becomes large, and the SKD11 which can obtain the hardness of 60 HRC by tempering at 500 ° C. is used for the surface treatment. Increasingly, delamination of layers occurs. Further, in SKH51 which is a high speed tool steel, although the base metal hardness of 64 HRC or more can be secured, the material unit price is high and the appropriate quenching temperature is 1200.
It is very high at around ℃, and there are many restrictions in terms of workability and cost of heat treatment. Further, since SKH51 contains a large amount of hard MC carbide, it has poor machinability and grindability.

For such a problem, for example, Japanese Patent Publication No. 6
No. 1-1310, Japanese Patent Publication No. 3-36897,
JP-B-64-5100 and JP-A-5-1564
Inventions such as the '97 publication have been proposed. This special public Sho 61
Japanese Patent No. 11310 relates to a work roll for cold rolling, which contains 0.4% to 3.0% of Mo and has a surface hardness of 720 to 800 HV (61 to 64 HRC).
Although the tempering temperature is 150
It is a low temperature of about ℃ and cannot be applied as a material for high temperature tempering, which is the goal of this development.

Japanese Patent Publication No. 3-36897 discloses SK.
The present invention relates to a cold work tool steel which has improved D11 and is returned to a high temperature of about 520 ° C. to achieve high hardness and high toughness and is suitable for a carbide coating treatment. Also in this invention steel, Mo is 0.7
It is as low as 5 to 1.95%, contains V in an amount of 0.5 to 1.0%, and has a hardness of 62 HRC level, which cannot stably achieve a high hardness of 64 HRC or more. Further, as an invention similar to this, there is Japanese Patent Publication No. Sho 64-5100, but none of these can sufficiently achieve the development target.

On the other hand, JP-A-5-156497 further improves the inventions of Japanese Patent Publication No. 3-36897 and Japanese Patent Publication No. 64-5100. This patent relates to a high-performance rolling die steel which contains one or two kinds of V and Nb, and which can obtain a hardness after high temperature tempering of 64 HRC or more, and a method for producing the same. However, in order to achieve the goal, not only the regulation of alloy composition,
Special melting method in the manufacturing process, application of high temperature diffusion treatment,
It is necessary to limit the composition of the primary carbide of the product and its grain size and the amount of precipitation, which is quite complicated.

[0007]

With the recent progress in plastic working technology and higher strength of work materials, hard surface treatment is being applied for the purpose of improving wear resistance of tools. As a material suitable for such an application, a tool steel having a base material hardness as high as possible and high toughness is required. The present invention
It is an object of the present invention to provide an economical cold tool steel suitable for a surface hardening treatment having a treatment temperature of 400 ° C. or higher and easy to handle in heat treatment and processing, and a die and a tool thereof.

[0008]

In order to solve the above-mentioned problems, the present invention eliminates the addition of V, which is generally accepted as an essential component of cold-work tool steel, to obtain a temperature of 500 ° C or higher. 63 ~ 66HRC by high temperature tempering
The high hardness of is easily obtained. The gist of the invention is (1) by weight, C: 0.70 to 1.20%, Si: 2.
0% or less, Mn: 1.5% or less, Cr: 5.0-11.
0% Mo One or two kinds of W or Mo is equivalent to (1 /
2W + Mo) is more than 3.0% to 5.0%, the balance is Fe and inevitable impurities, and a high hardness of 63 to 66 HRC is obtained by high temperature tempering at 500 ° C. or higher. Suitable cold work tool steel.

(2) By weight, C: 0.70 to 1.20
%, Si: 2.0% or less, Mn: 1.5% or less, Cr:
5.0 to 11.0% Any one or two kinds of Mo or W is equivalent to Mo (1 /
2W + Mo) is more than 3.0% to 5.0% and further C
o: 0.5 to 4.0% S: 0.01 to 0.30% N: 6
Any one or two or more of 0 ppm or less, the balance F
e) and unavoidable impurities, and a high hardness of 63 to 66 HRC is obtained by high temperature tempering at 500 ° C. or higher, which is a cold work tool steel suitable for surface treatment. (3) Nitriding to the cold work tool steel according to (1) or (2) above,
A mold and tool characterized by being subjected to high temperature surface hardening treatment at 400 ° C. or higher such as PVD, TD treatment and CVD.

The action of each chemical component of the steel of the present invention and the reason for limiting the chemical components will be described below. C gives sufficient matrix hardness by quenching and tempering, and at the same time Cr, Mo, V,
It is an element that combines with Nb and the like to form a carbide, which gives high-temperature strength and wear resistance. However, in the case of the present invention,
Since V is not added and the secondary hardening by Mo carbide is basically used, it is not necessary to add C excessively, and if the addition amount is too large, coarse carbide excessively precipitates during solidification and hinders toughness. , C was set to the upper limit of 1.20%. on the other hand,
If it is less than 0.70%, sufficient secondary hardening hardness cannot be obtained, so the lower limit was made 0.70%.

Si is mainly added as a deoxidizer, is an element that is advantageous for oxidation resistance and hardenability, and is an element that suppresses agglomeration of carbides in the tempering process and promotes secondary hardening. However, if added in excess of 2.0%, the toughness decreases, so the upper limit was made 2.0%. Mn is S
Similar to i, it is an element added as a deoxidizer to improve the cleanliness of steel and hardenability. However, 1.5
If added in excess of%, the hot workability is impaired and the toughness is lowered, so the upper limit was made 1.5%.

[0012] Cr is an effective element that enhances the hardenability and the resistance to temper softening. To satisfy this effect, at least 5.0% or more is necessary. Therefore,
The lower limit was 5.0%. On the other hand, Cr is likely to combine with C to form a giant primary carbide during solidification, and excessive addition thereof lowers toughness, so the upper limit was made 11.0%. 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 of Mo is twice as strong as that of W. To obtain the same effect, W is 2 times higher than that of Mo.
You need twice. The effect of both these elements is Mo equivalent (1/2 W
+ Mo). In the component diameter of the present invention,
In order to obtain a high hardness of 63 to 66 HRC by returning to a high temperature of 500 ° C. or higher, Mo equivalent of at least 3.0% is required. On the contrary, since the excessive increase of the Mo equivalent causes a decrease in toughness, the upper limit was made 5.0%.

V is an effective element that forms a carbide that is difficult to form a solid solution and enhances wear resistance and softening resistance.
As in the present invention, a material premised on a hard surface treatment does not necessarily need the wear resistance required for a tool because the surface treatment layer can secure the wear resistance. V is also an element that promotes striped microsegregation and is not preferable from the viewpoint of toughness and heat treatment distortion. Furthermore, by not adding V, the C originally captured as VC can be effectively used, and the low C composition which is an alloy composition effective for toughness.
It has a great advantage that the amount of Mo or W carbide that is effective for oxidization and secondary hardening can be increased. Therefore, the present invention is characterized by not adding V.

Further, in addition to the above-mentioned elements, Co, S, and N are selectively added to or regulated in the steel of the present invention. That is, Co is a case where higher hardness is required in the above component range, and when Co is added, it forms a solid solution in the matrix, promotes the solid solution of the solute element at the quenching temperature, and also tempers at high temperature. It is an element effective in promoting secondary hardening because it suppresses the agglomeration of carbides. This gives 6
A hardness of 5 HRC or more can be stably obtained. To satisfy this effect, at least 0.5% or more is necessary.
However, excessive addition lowers the toughness, so the upper limit was made 4.0%.

[0015] S is an element necessary for ensuring free-cutting property, because the effect of improving the machinability of steel increases as the amount of S added increases. To satisfy this effect, at least 0.01% or more is required. However, 0.30
%, The toughness and hot workability deteriorate, so the upper limit was made 0.30%. N is necessary when importance is placed on toughness. However, when N exceeds 60 ppm, the toughness is significantly reduced, so that the steel according to the present invention has 60 pp.
m or less.

[0016]

【Example】

(Example 1) 200 kg of each of the test materials (A) to (G) of the present invention steel and the test materials (H) and (I) of the comparative steels were tapped in a vacuum induction melting furnace. Table 1 shows the chemical composition of the 9-heat steel that was tapped. A 200 kg steel ingot (average diameter 250 mm) of steel having a total of 9 heats was forged and stretched to a corner of 45 mm, and a test piece having a corner of 35 mm and a length of 50 mm was produced by machining. This test piece was held at 1050 ° C. for 30 minutes, air-cooled and hardened, held at 525 ° C. for 60 minutes, air-cooled twice, and then surface-treated. Surface treatment is plasma CV
Performed at a processing temperature of 500 ° C. and a thickness of Ti of 3 μm.
The CN layer was coated. From the center of the surface-treated test piece, a bending test piece having a length of 4 × width of 8 × length of 40 mm was processed, and after measuring the hardness on a Rockwell C scale at room temperature, the bending strength and the amount of deflection were measured. . As shown in Table 2, all of the steels of the present invention (A) to (G) maintain a hardness of 63 HRC or more, and are far more than the conventional cold work tool steels (H) and (I). It can be seen that it has excellent strength. In particular, the steel of the present invention having N of 60 ppm or less (D)
~ (G) maintains higher hardness than the conventional steels (H) and (I), and has a toughness (deflection amount) superior to that of the conventional steel (H) which is SKD11. It is understood that there is. Further, the present invention steels (B) and (F) containing Co
It can be seen that and (G) have higher hardness and stable characteristics.

Further, the invention steels and comparative steels shown in Table 1 were tapped, annealed after forging and subjected to a cutting test. In the cutting test, an end mill of φ12 was used with an NC milling cutter to make a cut with a shaft diameter of 15 mm and a radius of 1.2 mm, and the amount of wear of the end mill after processing for 5 m at a rotation speed of 900 rpm and a feed speed of 34 m / min was measured. As shown in Table 2,
The steels (A), (B), (D) and (F) of the present invention which do not contain S showed end mill wear comparable to those of the conventional steels (H) and (I), but the present invention in which S was added Steel (C),
It can be seen that (E) and (G) have improved machinability with almost no reduction in strength and toughness. Especially,
It can be seen that the steel (G) of the present invention containing Co and S and having N of 60 ppm or less has excellent machinability and has an excellent balance of strength and toughness.

[0018]

[Table 1]

[0019]

[Table 2]

(Example 2) Steel of the present invention (G) shown in Table 1
A cold forging die is manufactured with 1050 ° C. quenching, 525
After the ° C tempering treatment, a 2.4 µm thick TiCN coating was carried out by plasma PVD at 500 ° C. Using this mold, a forging test was conducted with an actual machine. The work piece was a 4 mm thick SCM420. The SKD11 mold, which was a conventional steel and subjected to the same surface treatment, had a life due to cracking, and its average life was about 4,660 shots. On the other hand, the life of the mold made of the steel of the present invention (G) is 1
The number of shots was 5,543 shots, which was about 3.3 times as long as the conventional mold. Since seizure and chipping were the causes of die disposal at that time, conventional materials could easily cause early peeling of the coating layer due to insufficient hardness of the base material and cracks due to insufficient toughness starting from that. On the other hand, it is presumed that the steel of the present invention has a sufficiently high base metal hardness, the coating layer does not separate, and only wear progresses and the life is long.

[0021]

As described in detail above, the steel of the present invention has a high hardness and toughness, especially after high temperature surface treatment, by optimally balancing the C and Mo equivalents in the V-free additive system in the cold work tool steel. Therefore, it is economical and extremely superior to the conventional one as a tool steel for molds used in cold.

Claims (3)

[Claims] 1. By weight, C: 0.70 to 1.20%, Si: 2.0% or less, Mn: 1.5% or less, Cr: 5.0 to 11.0% Mo or W. 1 or 2 kinds of Mo equivalent (1 /
2W + Mo) is more than 3.0% to 5.0%, the balance is Fe and inevitable impurities, and a high hardness of 63 to 66 HRC is obtained by high temperature tempering at 500 ° C. or higher. Suitable cold work tool steel. 2. By weight, C: 0.70 to 1.20%, Si: 2.0% or less, Mn: 1.5% or less, Cr: 5.0 to 11.0% Mo or W. 1 or 2 kinds of Mo equivalent (1 /
2W + Mo) is more than 3.0% to 5.0%, and further Co: 0.5 to 4.0% S: 0.01 to 0.30% N: 60 ppm or less. And a balance of Fe and unavoidable impurities, and a high hardness of 63 to 66 HRC is obtained by high temperature tempering at 500 ° C. or higher, which is a cold work tool steel suitable for surface treatment. 3. A mold and a tool obtained by subjecting the cold work tool steel according to claim 1 or 2 to high temperature surface hardening treatment at 400 ° C. or higher such as nitriding, PVD, TD treatment and CVD.