JP2000234148A - Cold tool steel excellent in high cycle fatigue life and machinability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel excellent in fatigue characteristics and moreover excellent in machinability without the remarkable increase of the cost and without deteriorating the toughness of a die by well balancing SKD 11 series steel with S without requiring the addition of expensive REM and the remarkable change of the components of steel kinds. SOLUTION: This steel is the one having a compsn. contg., as essential components, by weight, 0.65 to 1.50% C, <=2.0% Si, 5.0 to 13.0% Cr, 0.030 to 0.20% S and <=1.50% Mn, contg., as selective components, one or two kinds of Mo and W of 1.0 to 5.0% by Mo equivalent (Mo+1/2W) and one or two kinds of V on Nb of 0.05 to 1.0% by V equivalent (V+1/2Nb), and the balance Fe with inevitable impurities. Or, the steel is furthermore mixed with 0.20 to 3.0% Co.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold tool steel excellent in high cycle fatigue life and machinability.

[0002]

2. Description of the Related Art Conventionally, JIS-S
KD11 is widely used. However, in applications where repeated stress is applied, there is a problem that cracks originating from coarse Cr-based carbides are generated, resulting in poor fatigue characteristics. On the other hand, for example, inventions such as Japanese Patent Publication No. 57-59298 and Japanese Patent Publication No. 3-36897 have been proposed. Further, as an invention for improving the machinability of SKD11,
JP-B-63-66384 and JP-A-8-12033
No. 3 has been proposed.

[0003]

Problems to be Solved by the Invention
No. 59298 discloses that, for an alloy composition almost corresponding to a conventionally used mold steel, Ca: 0.0005 to 0.5.
The life of the mold is improved by adding 010%. However, there is no description about the improvement of the fatigue limit. In the present invention, the addition of a machinability improving element other than Ca has some effect on the life of the mold. It has been shown to act negatively. Japanese Patent Publication No. 3-36897 discloses a SKD.
Although the strength, toughness and fatigue properties were improved by lowering the C-Cr content and significantly reducing the amount of coarse Cr-based carbides, no improvement in machinability was shown.

Further, Japanese Patent Publication No. 63-66384 discloses a method in which non-metallic inclusions are greatly granulated by adding REM to improve machinability. V in V and Nb addition
There is no regulation of + 1 / 2Nb, and there is a problem that the cost is increased by adding REM and the economic efficiency is poor. Japanese Patent Application Laid-Open No. Hei 8-120333 is intended to improve machinability by adding S. However, there is no description about improvement of fatigue limit, and Mo + 1 / 2W when Mo and W are added.
The fact is that there are no regulations and these have not been elucidated.

[0005]

Means for Solving the Problems As a result of the inventor's intensive development to solve the above-mentioned problems, as a result of adding S to steel and forming MnS in a dispersed manner, the steel has a higher fatigue strength than S-free steel. On the other hand, as the S content increases, the machinability also improves. The gist of the invention is as follows: (1) C: 0.65 to 1.50% by weight, Si:
2.0% or less, Cr: 5.0 to 13.0%, S: 0.0
30 to 0.20%, Mn: 1.50% or less as a main component, and any one or two of Mo and W are Mo equivalent (Mo + 1 / 2W): 1.0 to 5.0%, V or Nb
Of any one or two of them is V equivalent (V + ／ N
b): containing one or more selected components of 0.05 to 1.0%,
A cold tool steel excellent in high cycle fatigue life and machinability, characterized by the balance consisting of Fe and unavoidable impurities. (2) The steel according to (1), further comprising Co: 0.20 to 0.20.
A cold tool steel excellent in high cycle fatigue life and machinability characterized by adding 3.0%.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting various components in the present invention will be described below. C: 0.65 to 1.50% C gives sufficient matrix hardness by quenching and tempering, and combines with Cr, Mo, V, etc. to form carbides and give wear resistance. However, if added in excess, a large amount of coarse carbides precipitate during solidification, which impairs toughness. On the other hand, in order to obtain a sufficient secondary hardening hardness, 0.65% is required. Therefore, the range is set to 0.65 to 1.50%.
And

Si: 2.0% or less Si is added as a deoxidizing agent and has oxidation resistance,
Effective for hardenability. Further, it suppresses agglomeration of carbides during the tempering process and promotes secondary hardening. However, if it exceeds 2.0%, cold workability is impaired, and toughness is deteriorated. Therefore, the upper limit is set to 2.0%. Cr: 5.0 to 13.0% Cr increases the hardenability and the tempering softening resistance. To obtain this effect, 5.0% or more is required. However, when solidified, it is likely to combine with C to form a large temporary carbide, and if added excessively, it deteriorates toughness. Therefore, the range was set to 5.0 to 13.0%.

S: 0.030% to 0.20% S combines with Mn to form MnS. MnS in steel
In the case where the cyclic stress is relatively low, MnS suppresses slippage in the matrix and delays the propagation of fatigue fracture. This effect is not exhibited unless 0.03% or more is added, and excessive addition deteriorates toughness. Therefore, the range is set to 0.030 to 0.20%. Further, as the amount of S added increases, the machinability also improves. In order to obtain sufficient machinability and not reduce the hot workability, the content is desirably 0.040 to 0.10%.

Mn: 1.50% or less Mn combines with S to improve fatigue characteristics and machinability. Further, it is an element that increases the cleanliness of steel and enhances hardenability as a deoxidizing agent. If it exceeds 1.5%, the cold workability is impaired and the toughness is degraded, so the upper limit is made 1.50%.
One or two of Mo and W are used at the Mo equivalent (M
o + 1 / 2W): 1.0 to 5.0% Mo, W is an element that forms fine carbides, contributes to secondary hardening, and improves softening resistance. However, the effect of Mo is twice as strong as that of W, and W is twice as large as Mo to obtain the same effect. Mo equivalent (Mo + ／)
W) requires at least 1.0%. However, since excessive addition causes a decrease in toughness, the upper limit is set to 5.0%.

One or two of V or Nb is V equivalent (V + ／ Nb): 0.05 to 1.0% V, Nb is effective for secondary hardening and forms a hard carbide with C This contributes to wear resistance and refines crystal grains. At least 0.05% in V equivalent (V + ／ Nb)
is necessary. However, since excessive addition causes a decrease in toughness, the upper limit is set to 1.0%. Co: 0.20 to 3.0% Co suppresses carbide agglomeration due to tempering and is effective for softening resistance. In order to obtain these effects, 0.1.
20% is required, and if added in excess of 3.0%, toughness decreases. Therefore, the range is 0.20-3.
0%.

Hereinafter, the characteristics of the steel of the present invention will be specifically described based on examples.

EXAMPLES Table 1 shows the chemical compositions of the test steels and the test results of quenching and tempering hardness, fatigue limit, machinability and toughness of the test steels. As for the fatigue limit is a load stress value of 10 ⁷ times repeated stress times in fatigue test Ono-type rotating bending, machinability, the annealed material of the steel type, SKH5
1 and the time required to drill 10 mm with a φ5 mm drill, and the toughness was taken from the rolling direction.
The evaluation was performed using a 10R-C notch Charpy test piece.

[0012]

[Table 1]

As shown in Table 1, Nos. 1 to 5 are steels of the present invention, and Nos. 6 to 8 are comparative steels. Invention steel No. 1
Is compared with comparative steel No. 6 without lowering toughness.
Improves fatigue limit and machinability. Invention Steel Nos. 2-5
Improves the fatigue limit and machinability as compared with comparative steel No. 7, and the tendency increases as the S content increases. The toughness of the steels Nos. 2 to 5 of the present invention is almost the same as that of the comparative steel No. 7. Comparative steel No. 8 is a steel to which more S was added than inventive steels Nos. 2 to 5, and although the fatigue limit and machinability were superior to the inventive steel, the toughness was deteriorated.

[0014]

As described above, according to the present invention, it is not necessary to add expensive REM or to largely change the steel type components.
By properly balancing S with SKD11-based steel, it is possible to provide a steel having excellent fatigue properties and excellent machinability without a large increase in cost and without reducing the toughness of a mold. It has excellent effects.

Claims (2)

[Claims] C: 0.65 to 1.50%, Si: 2.0% or less, Cr: 5.0 to 13.0%, S: 0.030 to 0.20% by weight% Mn: 1.50% or less, and one or two of Mo and W are Mo equivalents (Mo + 1 / 2W): 1.0 to 5.0%, V
Or one or two of Nb at V equivalent (V + 1
/ 2Nb): A cold tool steel excellent in high cycle fatigue life and machinability, characterized in that it contains at least one selected component of 0.05 to 1.0% and is composed of the balance of Fe and unavoidable impurities. 2. The steel according to claim 1, further comprising Co: 0.
A cold tool steel excellent in high cycle fatigue life and machinability characterized by being added in an amount of 20 to 3.0%.