JPH11217653A - Martensitic stainless steel excellent in machinability and having high corrosion resistance and high strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a martensitic stainless steel excellent in machinability and having high corrosion resistance and high strength, which is used for parts requiring strength and corrosion resistance, such as high strength shafts, sliding part parts, and bearings used under a wet environment, and particularly for members to be subjected to machining. SOLUTION: This martensitic stainless steel has a composition consisting of, by weight, 0.25-0.50% C, >1.0-2.5% Si, <=1.0% Mn, 10-16% Cr, 0.05-0.2% N, <=20 ppm O, and the balance Fe with inevitable impurities or further containing, besides the above components, one or >=2 kinds among <=1.0% Mo, <=1.0% V, and <=1.0% W.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a high strength shaft, a sliding part, a bearing or the like which is required to have high strength and corrosion resistance in a wet environment, and particularly to a steel used for a member to be machined. It is about.

[0002]

2. Description of the Related Art Conventionally, martensitic stainless steel has been able to obtain high strength, but its corrosion resistance has been somewhat insufficient, such as rusting in a salt spray test. In order to improve this, a method of increasing the amount of chromium in the matrix has been used, but when the corrosion resistance is increased by this method, the strength decreases,
It was difficult to achieve both high strength and high corrosion resistance. On the other hand, steel grades intended to increase the strength and improve the corrosion resistance by increasing the nitrogen content have been developed, for example, in Japanese Patent Application Laid-Open No. Hei 7-233442. It is necessary to suppress free-cutting elements such as S to a very small amount even in a nitrogenized steel type. Therefore, the conventional high nitrogen martensitic stainless steel has a problem that the machinability is inferior. Therefore, a well-balanced martensitic stainless steel having high corrosion resistance, high strength, and good machinability is demanded.

[0003]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems and to provide a steel type having both high corrosion resistance and high strength and also improved machinability.

[0004]

Means for Solving the Problems While increasing the effective Cr amount for the purpose of securing corrosion resistance, the C ratio is reduced by decreasing C in order to reduce the eutectic carbide, and the ratio of C / Cr is increased.
Means for improving the corrosion resistance while compensating for the strength decrease due to the decrease in C by the addition of C are the same, but the inventors generally increase the hardness at the time of annealing and reduce the Si which is said to deteriorate the machinability. It has been found that by adding a certain amount or more, although the annealing hardness increases, the machinability can be significantly improved. This makes it possible to improve the machinability even at the impurity level without adding S, which has an adverse effect on the corrosion resistance, so that both corrosion resistance and machinability can be achieved. Also,
By adding Mo, V, W to change the composition of the carbide from chromium-based carbide to Mo, V, W-based carbide, it has been found that the amount of chromium in the matrix can be increased and the corrosion resistance can be further improved. Was.

[0005] Based on the above findings, C: 0.25
0.50%, Si: more than 1.0%, 2.5% or less, M
n: 1.0% or less, Cr: 10 to 16%, N: 0.0
A high-corrosion-resistant high-strength martensitic stainless steel excellent in machinability, characterized by containing 5 to 0.2% and O: 20 ppm or less, with the balance being Fe and unavoidable impurities.

Alternatively, Mo:
High corrosion resistance and high strength martensitic stainless steel excellent in machinability, containing one or more of 1.0% or less, V: 1.0% or less, and W: 1.0% or less. It is in.

[0007]

The reasons for limiting the components of the present invention will be described below. C: C forms a solid solution in the matrix to secure necessary material hardness after quenching and tempering. In this component system, if the content is less than 0.25%, even if nitrogen within the range of the present invention is added, a quenching and tempering hardness of 55 HRC or more required as a high strength material cannot be secured. 0.5
If it exceeds 0%, coarse chromium carbides are crystallized even at the C / Cr ratio of this component, and the machinability and corrosion resistance are deteriorated. Therefore, C was set to 0.25 to 0.50%.

[0008] Si: Si is usually added for deoxidation, and excessive addition is usually limited to about 0.5% to increase the annealing hardness by solid solution strengthening. However, it was found that the addition of more than 1.0% of the component in the present invention increased the annealing hardness, but significantly improved machinability and other machinability. In addition, the addition of Si strengthens the matrix, thereby improving the fatigue life. However, if it exceeds 2.5%, the hardness of the annealing increases significantly, and the carbides become coarse, so that the workability tends to deteriorate. for that reason,
Si is set to more than 1.0% and 2.5%.

Mn: Mn is added as a deoxidizing agent and enhances the tendency to form austenite, promotes solid solution of carbide during quenching, and secures quenching hardness. However, if it exceeds 1.0%, the amount of retained austenite increases during quenching, and it is not possible to secure 55 HRC or more necessary for bearing applications.
Therefore, Mn is set to 1.0% or less. Cr: Cr is an essential element for ensuring the corrosion resistance of stainless steel. In order to ensure the corrosion resistance of stainless steel, it is usually necessary to add 11% or more. However, since the components of the present invention are highly nitrogenized, the addition of 10% or more is almost equivalent to conventional martensitic stainless steel. Corrosion resistance can be secured. However, when it exceeds 16%, the component system of the present invention has a large ferrite phase, and it is impossible to secure high hardness. Therefore, Cr is set to 10 to 16%.

N: N is an element necessary to secure hardness by dissolving in the matrix together with C, and N is essential in the present invention because it can reduce carbon by the amount added and is effective in improving corrosion resistance. Element. The effect is remarkable when added at 0.05% or more. However, since it is an element having a large tendency to form austenite, if added in excess of 0.2%, the amount of retained austenite increases during quenching, and a hardness of 55 HRC or more cannot be secured. Therefore, N does not exceed 0.2%.

Mo: Mo is an element that contributes to the improvement of corrosion resistance by further strengthening the Cr oxide film that maintains the corrosion resistance of stainless steel, and has a greater tendency to form carbides than Cr, so that Mo-based carbides are formed, and accordingly Cr is formed. Promotes solid solution in the matrix to improve corrosion resistance. However, if it exceeds 1.0%, the carbide becomes coarse and the machinability, machinability and fatigue life deteriorate. Therefore, Mo is set to 1.0% or less. V: V has a greater tendency to form carbides than Cr, so that V forms V-based carbides, which promotes the solid solution of Cr into the matrix and improves corrosion resistance. However, if it exceeds 1.0%, the carbide becomes coarse and the machinability, machinability and fatigue life deteriorate. Therefore, V is set to 1.0% or less.

W: Since W has a greater tendency to form carbides than Cr, it forms W-based carbides, which promotes the solid solution of Cr into the matrix, thereby improving corrosion resistance. However, if it exceeds 1.0%, the carbide becomes coarse and the machinability, machinability and fatigue life deteriorate. Therefore, W is set to 1.0% or less. O: O forms oxide-based inclusions with almost no solid solution in the matrix. Since this oxide is very hard, it reduces tool life and deteriorates machinability. Also,
It becomes a source of stress concentration during use and easily becomes the starting point of fatigue fracture. Therefore, it is desirable that O is small,
ppm or less.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The features of the present invention will be described with reference to embodiments. Table 1 shows the chemical components of the steel of the present invention and the comparative steel used in the experiment. These were melted in a 100 kg vacuum induction melting furnace. The steel ingot of each component was forged to φ20 by hot forging and subjected to the following experiment. (1) Machinability: Each material was annealed at 870 ° C., and a drilling test was performed. The machinability was measured using a 8φ high-speed steel drill with a thrust of 696 N and a rotation speed of 925 RPM, using a drill hole, and measuring the time required to drill a 10 mm depth.
25 seconds or less were evaluated as having good machinability.

(2) Hardening and tempering hardness: In the above-mentioned applications, the hardness in the quenching and tempering treatment needs to be 55 HRC or more. Therefore, it was evaluated whether 55HRC was obtained. The quenching condition is oil cooling after holding at 1000 ° C. for 30 minutes, and the tempering condition is air cooling after holding at 150 ° C. for 1 hour. (3) Corrosion resistance: Since rusting impairs performance and appearance, it is necessary to have corrosion resistance. Therefore, a test piece of φ12 × L21 subjected to quenching and tempering under the above conditions was subjected to a salt spray test to evaluate corrosion resistance. The test was performed by spraying a 5% sodium chloride solution at 35 ° C. for 16 hours. The evaluation was as follows: A where no rust was observed, A which slightly rusted (the area ratio of the rusted portion to the entire surface area of the test piece was 5% or less) B, and a slightly rusted one (the test piece at the rusted portion). The area ratio with respect to the entire surface area was not more than 20%) and C was strongly rusted. And what was more than B judgment was evaluated as favorable.

[0015]

[Table 1]

[0016]

[Table 2]

Table 2 shows the results of each test. Regarding the machinability, the steel of the present invention has an excellent drill piercing property of 20.1 to 23.2 seconds, but has a large amount of C and precipitates huge carbides. No. 8 having a low Si content. 9. N with high oxygen content
o. No. 13 and No. No. 15 requires a perforation time exceeding 25 seconds, and the machinability is not good. Regarding the quenching and tempering hardness, the steels of the present invention were all 55HRC.
Although the above hardness can be obtained, the No. 2 with a low C content is obtained. No. 7, the amount of Si was too high, and No. 10, the amount of Cr was too high, and No. 12 shows that sufficient hardness was not obtained. With regard to corrosion resistance, the steel of the present invention was evaluated as B or higher, and in particular, Mo, V, and W
The ones with good corrosion resistance in the A judgment. However, the C content was large and the Cr content effective for corrosion resistance was reduced. No. 8, the amount of Si is too high, and No. 1
0, a low Cr content. No. 11 having a low N content. 14
Do not have good corrosion resistance.

[0018]

From the above, it can be seen that the present invention provides a high-strength stainless steel excellent in machinability and corrosion resistance as compared with conventional high-strength stainless steel.

Claims (2)

[Claims] C .: 0.25 to 0.50%, Si: more than 1.0% and 2.5% or less, Mn: 1.0% or less, Cr: 10 to 16% by weight%. A high-corrosion-resistant high-strength martensitic stainless steel excellent in machinability, characterized by containing 0.05 to 0.2% of N: and 20 ppm or less of O: and the balance being Fe and unavoidable impurities. 2. In% by weight, C: 0.25 to 0.50%, Si: more than 1.0%, 2.5% or less, Mn: 1.0% or less, Cr: 10 to 16%, N: 0.05 to 0.2%, O: 20 ppm or less, the balance being Fe and unavoidable impurities, Mo: 1.0% or less, V: 1.0% or less, W: High corrosion resistance and high strength martensitic stainless steel excellent in machinability, containing one or more of 1.0% or less.