JPH11350092A - Stainless steel having high strength and high corrosion resistance and excellent in cold workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stainless steel having high strength, high corrosion resistance, and excellent cold workability and suitably used as a stock for parts to be formed by cold working. SOLUTION: This stainless steel has a composition consisting of, by weight, 0.27-0.4% C, <=0.3% Si, <=0.3% Mn, >0.005-0.030% S, 15.0-18.0% Cr, 0.5-2.0% Mo, <=0.0030% O, and the balance Fe with inevitable impurities and satisfying inequality 1.2 2[%Si]+[%Cr]+1.5[%Mo]}-18<=30[%C]+0.5[%Mn]<=-0.8 2[% Si]+[%Cr]+1.5[%MO]}+30 and has excellent cold workability in an annealed state.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a martensitic stainless steel used as a material for parts requiring high strength and high corrosion resistance in various industrial machines, automobiles, electronic devices, chemical devices and the like. The present invention relates to a high-strength, high-corrosion-resistant stainless steel excellent in cold workability, which is suitable as a material for components formed by cold working.

[0002]

2. Description of the Related Art Conventionally, in the above-mentioned industrial fields, SUS410 and SUS420 have been used for materials requiring high strength and corrosion resistance, such as various shafts and corrosion-resistant bearings.
Martensitic stainless steels such as J2 and SUS440C are used. On the other hand, Japanese Patent Application Laid-Open No. 1-275737 discloses a high-strength stainless steel excellent in cold workability. However, since the machinability is improved by the addition of Ca, there is a problem in that the production is more difficult than that of steel to which S is added, such as the steel of the present invention.

[0003]

SUS410, which is a low-C low-Cr material, described above, has a low C content, so that a very high hardness cannot be obtained even in a quenched and tempered state. In addition, SUS410 and the like of low C and low Cr materials and SUS420J2 and the like of low Cr material have a low Cr content, so that their corrosion resistance is not so good. On the other hand, SUS440C and the like of high C and high Cr can obtain high hardness in a quenched and annealed state, but have poor cold workability because they contain coarse carbides and have high hardness in an annealed state. . For this reason, when processing parts using high C and high Cr materials such as SUS440C, most parts are cut in the annealed state to form a shape close to the product, and after quenching and tempering, grinding to finish the product shape There are many. In addition, SUS
Although a high C and high Cr material such as 440C has a high Cr content, a large amount of carbides precipitate, so that the amount of Cr in the base material is reduced, and the corrosion resistance is not always good.

In order to improve the corrosion resistance, Cr, Mo
When only the ferrite forming element such as
-Ferrite is formed, resulting in a decrease in strength. In addition, SU
Precipitation hardening stainless steels such as S630 are used for applications requiring high strength and high corrosion resistance, but they are expensive, do not provide very high hardness even in the precipitation hardening treatment state, and have very good cold workability. Absent. As described above, the conventional martensitic stainless steel generally has poor corrosion resistance and has problems in cold workability and hardness. Further, precipitation hardening stainless steel is expensive and has a problem in cold workability.
However, at present, there is no relatively inexpensive material that can be used, and it is unavoidable to use it.

On the other hand, Japanese Patent Application Laid-Open No. 1-275737 mentioned above proposes a high-strength stainless steel excellent in cold workability. However, the machinability is improved by the addition of Ca, and it is more difficult to manufacture compared to steel to which S is added, such as the steel of the present invention. An object of the present invention is to solve the above-mentioned problems of cold workability, strength and corrosion resistance, and to provide a high-strength and high corrosion-resistant stainless steel which is inexpensive and has excellent cold workability.

[0006]

The martensitic stainless steel according to the present invention has a C content of 0.27-0.
4%, Si: 0.3% or less, Mn: 0.3% or less, S:
More than 0.005% and 0.030% or less, Cr: 15.0 to
18.0%, Mo: 0.5 to 2.0%, O: 0.003
This is a high-strength and high-corrosion-resistant stainless steel containing 0% or less, the balance being Fe and inevitable impurities, satisfying the formula (1), and exhibiting excellent cold workability in an annealed state. 1.2 {2 [% Si] + [% Cr] +1.5 [% Mo]}-18 ≦ 30 [% C] +0.5 [% Mn] ≦ −0.8 {2 [% Si] + [ % Cr] +1.5 [% Mo]｝ +30 (1)

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION C: 0.27 to 0.4%, C is an element necessary for obtaining the strength of martensitic stainless steel. In the quenched state, the structure is changed to a martensite structure, which is solid-dissolved to increase the strength. In addition, it forms carbides by tempering and shows effective strengthening. In order to obtain sufficient strength, it is necessary to contain 0.27% or more. However, when a large amount of C is added, coarse carbides are generated,
In addition, the annealing hardness increases, which impairs cold workability. Also in the corrosion resistance, the precipitation of Cr carbide due to the addition of a large amount of C has an adverse effect since the amount of Cr in the base material is reduced. Further, C lowers the martensitic transformation point, so that an austenite phase remains during quenching, leading to a decrease in strength. For these reasons, the upper limit of C is set to 0.4%.

[0008] Si: 0.3% or less, Si is an effective deoxidizing agent in steel production. However, since Si reduces toughness, increases annealing hardness, and deteriorates cold workability, the upper limit is set to 0.3%. Mn: 0.3% or less Mn is added as a deoxidizing and desulfurizing agent, but degrades toughness like Si and increases annealing hardness, impairs cold workability and further deteriorates corrosion resistance. 0.3% upper limit
And

S: more than 0.005% and not more than 0.030%,
When S is dispersed as sulfide in steel, it acts as a chip breaker and improves machinability.
The above addition was made. However, the addition of a large amount deteriorates the hot workability, so is set to 0.030% or less. Cr: 15.0 to 18.0% Cr is a basic component of stainless steel and is a very important element that imparts corrosion resistance to steel. In order to obtain sufficient corrosion resistance, the amount of Cr added was set to 15.0% or more. A lot of C
Since the addition of r generates δ-ferrite and lowers the strength, the content is set to 18.0% or less.

Mo: 0.5-2.0% Mo is a very effective element for stabilizing the passivation film and thus for improving the corrosion resistance. In order to obtain sufficient corrosion resistance, the amount of Mo added was set to 0.5% or more. But M
Since o is expensive and causes a decrease in strength by forming δ-ferrite, it is set to 2.0% or less. O: 0.0030% or less Furthermore, in the present invention steel, since O generates nonmetallic inclusions at the time of solidification, it impairs the cleanliness of the steel and deteriorates cold workability and corrosion resistance. I found it. Therefore, the content of O is set to 0.0030% or less.

The amount of δ-ferrite produced after forging to reduce the strength is defined by the relationship between Ni equivalent and Cr equivalent.
The inventors have found from the experimental results the formula (1) that shows the interrelation of each component that does not produce δ-ferrite. for that reason,
Each component is adjusted so as to satisfy the formula (1) within the above-described component ranges so that δ-ferrite is not generated. 1.2 {2 [% Si] + [% Cr] +1.5 [% Mo]}-18 ≦ 30 [% C] +0.5 [% Mn] ≦ −0.8 {2 [% Si] + [ % Cr] +1.5 [% Mo]｝ +30 (1)

[0012]

EXAMPLES Steel No. 1 of the present invention having the chemical components shown in Table 1 was used. 1-9
And comparative steel No. 10 to 15 are melted and forged to a diameter of 30
mm material. In order to soften these materials, spheroidizing annealing was performed, and the cold workability was evaluated using the spheroidizing annealing materials. The cold workability was evaluated by measuring the hardness of the spheroidized annealed material and performing a cold upsetting test. In addition,
The critical upsetting ratio, which is a measured value of the cold upsetting test, is a value indicating the ratio (%) of the upsetting amount when cracking occurs with respect to the test piece height before the test. The steel No. of the present invention. All of the hardness after spheroidizing annealing of Nos. 1 to 9 are sufficiently low annealing hardness, and the critical upsetting ratio shows a high value of 75% or more. 15 shows better cold workability than SUS440C.

Next, the spheroidized annealed material was subjected to quenching and low-temperature tempering to evaluate hardness and corrosion resistance. The corrosion resistance was evaluated by a salt spray test (temperature: 35 ° C., 5% aqueous NaCl solution, 16 hours). The evaluation was performed on a four-point scale, with A indicating no rusting, B indicating slight rusting,
C indicates that considerable rust was observed, and D indicates that rust was observed on the entire surface.

According to the steel No. 1 of the present invention. Hardnesses after quenching and low-temperature tempering of Nos. 1 to 9 are all 50 HRC or more, indicating sufficiently high hardness. The steel No. of the present invention. 1 to 9 show good corrosion resistance in the salt spray test. Comparative steel No. No. 10 is corrosion resistance, comparative steel No. No. 11 is strength, comparative steel No. No. 12 is annealing hardness, comparative steel No. 13-15
Is inferior to the steel of the present invention in cold workability and corrosion resistance.

[0015]

[Table 1]

[0016]

As described above, the steel of the present invention has low hardness in an annealed state and has excellent cold workability. There is an advantage that parts can be manufactured at low cost by processing. further,
Since it has high hardness in the quenched and tempered state, high strength, and is superior in corrosion resistance to conventional steel, it is very suitable as a material for components requiring high strength and high corrosion resistance.

Claims (1)

[Claims] 1. In weight%, C: 0.27 to 0.4%, Si: 0.3% or less, Mn: 0.3% or less, S: 0.005% to 0.030%, Cr : 15.0 to 18.0%, Mo: 0.5 to 2.0%, O: 0.0030% or less, the balance being Fe and unavoidable impurities, and the formula (1)
A high-strength, high-corrosion-resistant stainless steel excellent in cold workability, characterized by satisfying the above conditions and having excellent cold workability in an annealed state. 1.2 {2 [% Si] + [% Cr] +1.5 [% Mo]}-18 ≦ 30 [% C] +0.5 [% Mn] ≦ −0.8 {2 [% Si] + [ % Cr] +1.5 [% Mo]｝ +30 (1)