JPH0539546A - High strength and high corrosion resistance stainless steel excellent in cold workability - Google Patents

Abstract

PURPOSE:To improve the cold workability of a stainless steel by regulating the content of C and Cr in a steel into optimum one and reducing the amt. of carbides. CONSTITUTION:The compsn. of the objective stainless steel is formed of a one constituted of, by weight, 0.6 to 0.75% C, <=0.3% Si, <=0.3% Mn, <0.10% Ni, 12 to 16% Cr, <=0.03% Al, 0.01 to 0.10% Nb and the balance Fe with inevitable impurities. Furthermore, the content of P as impurities is regulated to <=0.2% and that of S to <=0.05% as well as its hardness as annealed is regulated to <=94 HRB. If required, this components are incorporated with 0.3 to 3.0% Mo. This stainless steel has low hardness as annealed as well as excellent cold workability and in which its hardness as hardened and tempered is equal to or above that of the existing one.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to martensitic stainless steel used as a material for parts requiring corrosion resistance and high strength in automobiles, industrial machines, electronic devices, chemical devices, etc. It relates to a high-strength and high-corrosion-resistant stainless steel excellent in cold workability that can be easily processed into these parts in between.

[0002]

2. Description of the Related Art Conventionally, in the above industrial fields, SUS440C, SUS420J are used as materials for parts, such as shafts and bearings, which require corrosion resistance and high strength.
2. Martensitic stainless steel such as SUS410 is used. However, in reality, these stainless steels have not always been satisfactory in production and use.

Since SUS440C, which is a high C-Cr material, is a hyper-eutectoid steel, it is possible to obtain a high hardness of HRC58 or higher in the quenched and low-temperature tempered state, but a eutectic carbide of extremely high hardness of HV2000 or higher is coarsely crystallized during manufacture. Cold workability is extremely poor due to high hardness in the as-annealed and annealed state. For this reason, when SUS440C is used as a material to be processed into a predetermined part shape, it is similar to the product shape after the material steel is annealed regardless of cold working such as cold forging and cold rolling. It employs a method in which it is cut into a shape, and then it is quenched and tempered and then finish-ground to the final product shape.
Further, this SUS440C has a low Cr content in the base material because a large amount of large eutectic carbides are produced, and thus the corrosion resistance is not necessarily good.

SUS420J2, which is a medium C-Cr material, and SUS410, which is a low C-Cr material, have a lower C content than SUS440C, so that a very high hardness cannot be obtained even in a quenched and low temperature tempered state. Cr
Since the amount is low, the corrosion resistance is not so good.

As described above, conventional martensitic stainless steels generally have poor corrosion resistance, and also have problems in cold workability and hardness. However, at present, there is no relatively inexpensive material superior to these stainless steels, and it is unavoidable that the material is used.

JP-B-57-2266 and JP-A-1-27
Japanese Patent No. 5737 proposes a martensitic stainless steel having the above-mentioned properties improved. However, even if stainless steel having the chemical composition disclosed in these publications is manufactured, cold workability and corrosion resistance may not be improved. That is, in the steel disclosed in Japanese Examined Patent Publication No. 57-2266, 0.6% by weight or less of Si, which has a significant adverse effect on cold workability, inhibits cold workability and deteriorates corrosion resistance.
Although n is specified to 1.0% by weight or less, or in addition to these, Ni that causes pitting corrosion in a special corrosive environment is specified to 1.5% by weight or less, but such a specification is not sufficient. If the contents of Mn, Ni and Ni are high, the cold workability and corrosion resistance may not be improved. On the other hand, in the steel described in JP-A-1-275737, since the C content is as low as 0.3 to 0.6% by weight, the hardness after quenching / tempering cannot be sufficiently obtained, and this steel also has a Ni content. Since the upper limit of is as high as 0.6% by weight, pitting corrosion may occur in a special corrosive environment.

[0007]

The object of the present invention is to overcome the problems of the existing martensitic stainless steel as described above and to endure cold working such as cold forging and cold rolling. An object of the present invention is to provide an inexpensive, high-strength, high-corrosion-resistant stainless steel having workability.

[0008]

Means for Solving the Problems The inventors of the present invention have earnestly studied in order to achieve the above-mentioned objects, and as a result, have obtained the following findings.

As described above, the martensitic stainless steel is inferior in cold workability because it has a hard M _{7 of} HV 2000 or more.
This is because a C ₃ -type eutectic carbide is generated, which grows and cracks occur at the interface with the base material, or the carbide itself cracks. Therefore, in order to improve the cold workability, the above-mentioned coarse M ₇
It is effective to reduce the C ₃ -type eutectic carbide, improve the shape, or change the structure.

When the content of C and Cr in steel is adjusted to an appropriate content, the amount of the above-mentioned carbides decreases, the shape also becomes smaller, and the carbides also change to low hardness M ₂₃ C ₃ type carbides, which causes cold working. Workability is improved.

In addition to the above countermeasures, if the contents of Si and Mn in steel are each 0.3 wt% or less,
Cold workability is further improved, and when Mn is suppressed in this way and the Ni content is kept below 0.10% by weight, the corrosion resistance is remarkably improved.

The present invention based on the above findings is summarized in the following stainless steel.

(I) C: 0.6 to 0.75% by weight, S
i: 0.3% or less, Mn: 0.3% or less, Ni: less than 0.10%, Cr: 12 to 16%, Al: 0.03% or less, Nb: 0.01 to
0.10% is contained, the balance is Fe and unavoidable impurities, P as impurities is 0.02% or less, S is 0.005% or less, and the hardness in the annealed state is HRB94 or less. Excellent high strength and corrosion resistant stainless steel.

(Ii) In addition to the above components, 0.3 to 3.0
High strength and corrosion resistant stainless steel with excellent cold workability containing Mo by weight.

[0015]

The chemical composition (% by weight) of the present invention and the reason why the hardness in the annealed state is limited as described above will be explained below.

[Composition of Steel] C: 0.6 to 0.75% C is an important element for ensuring the strength required for martensitic stainless steel. C makes the structure martensite in the quenched state, and forms a solid solution in this to enhance the strength. To obtain hardness equal to or higher than SUS440C
A C content of 0.6% or more is required. However, if C is contained in an amount of more than 0.75%, coarse M ₇ C ₃ -type eutectic carbide crystallizes in the base material after spheroidizing annealing, and cracks occur at the interface between the carbide and the base material during cold working. Cold workability is seriously impaired by the generation of carbides or cracking of the carbides themselves. Further, C lowers the martensite transformation point and leaves an austenite phase during quenching, leading to a reduction in strength and the occurrence of quench cracks. From these things, the upper limit of C was set to 0.75%.

Si: 0.3% or less Si is added as a deoxidizing agent for steel. However, addition of a large amount of Si deteriorates the toughness of the steel and significantly impairs cold workability, so the upper limit content was made 0.3%.

Mn: 0.3% or less Mn is also added as a deoxidizing agent like Si, but a large amount of addition impairs the corrosion resistance and cold workability of steel, so the upper limit content was made 0.3%.

Ni: less than 0.10% Ni has the effect of increasing the toughness of steel, but since it causes pitting corrosion in a special environment, it is desirable that the amount be small. If Ni is contained in an amount of 0.10% or more, pitting corrosion is likely to occur, so the content is set to less than 0.10%.

Cr: 12-16% Cr is an important element that imparts corrosion resistance to steel. In order to secure sufficient corrosion resistance as stainless steel, a Cr content of 12% or more is required, but if Cr is contained in excess of 16%, M ₇ C ₃ type eutectic carbide will be coarsely crystallized and cold The content was set to 12 to 16% to impair the workability.

Al: 0.03% or less Al is added as a deoxidizer, but if added excessively, it remains as an oxide in the steel and deteriorates the cold workability, so the upper limit was made 0.03%.

Nb: 0.01 to 0.10% Nb has the function of refining the crystal grains of steel to improve toughness. However, if the content is less than 0.01%, the above-mentioned action is not sufficiently obtained, and if it exceeds 0.10%, not only the machinability during machining of parts is impaired, but also the crystal grains of steel become coarse and the toughness is Since it will deteriorate, its content is 0.
It was set to 01 to 0.10%.

In addition to the above components, Mo can be added to the stainless steel of the present invention as required.

Mo: 0.3-3.0% Mo has the function of increasing the corrosion resistance of steel. However, if the content is less than 0.3%, the effect of improving the corrosion resistance is small, and if it exceeds 3.0%, δ-ferrite is formed and the strength is reduced, so when adding for the purpose of further increasing the corrosion resistance, the content is It is recommended to set it in the range of 0.3 to 3.0%. In the steel of the present invention, in addition to the above-mentioned components, the balance is Fe and inevitable impurities. Typical impurities are P and S
Is. Since P segregates at the grain boundaries and impairs the toughness and corrosion resistance of the steel, its content was suppressed to 0.02% or less. Also, S
Impairs the hot workability of steel, its content is 0.005%.
I kept it below.

[Hardness in Annealed State] The hardness in the annealed state is set to HRB94 or less. If the hardness exceeds this value, it becomes difficult to perform cold working after annealing. Or because cold working itself becomes impossible. The steel of the present invention, after annealing, becomes parts such as shafts and bearings by cold forging, cold rolling, etc., but if the hardness of the material steel after the annealing is too high, cold working Cracks will occur inside and cold working will not be possible. Hardness capable of cold working without such a problem is HRB94 or less.

[0026]

EXAMPLES Steels of the present invention having chemical compositions shown in Table 1 (Nos. 1 to 9)
And comparative steels (No.10-16) were melted and forged to a diameter of 30mm
Made of material. Next, spheroidizing annealing was performed to soften these materials. A test piece was cut out from the material after annealing, and the hardness and cold workability in the annealed state were examined.

The cold workability was evaluated by carrying out a compression test at room temperature and obtaining a crack limit compression rate. The crack limit compression ratio is the ratio (%) of the minimum compression amount in which cracking occurs to the height of the test piece before the test.

Next, the material after annealing was subjected to quenching and low temperature tempering, and then the hardness and corrosion resistance were examined.

The corrosion resistance was examined by a salt spray test established in JIS standard, and evaluated by the corrosion weight loss after 200 hours from the start of the test. These results are summarized and shown in Table 2.

[0030]

[Table 1]

[0031]

[Table 2]

From Table 2, all of the invention steels Nos. 1 to 9 have a hardness in the annealed state of HRB94 or less, a high crack compressibility, and are more cold workable than the comparative steel No. 16, SUS440C. It turns out to be excellent. When the crystallized state of carbides was examined by an electron microscope, in Comparative Steel No. 16, coarse M ₇ C ₃ type eutectic carbides with a size of 20 μm or more were formed and the shape was also square. All of the steels of the present invention have low hardness M
_{It was a 23} C ₆ type carbide with a size as small as 10 μm or less and a rounded shape, which was in agreement with the evaluation of cold workability.

Further, the hardness of the steel of the present invention after quenching and low temperature tempering is SUS440 defined in JIS standard.
The hardness of C (58 or more in HRC) is satisfied, and the corrosion weight loss is as small as 5.1 mg / cm ² or less.
It has better corrosion resistance than S440C (corrosion loss 7.2 mg / cm ² ). The comparative steels No. 10 to No. 15 have chemical compositions outside the range defined by the present invention. As can be seen from these examples, when the chemical composition is out of the range defined by the present invention, any one of cold workability, corrosion resistance and hardness is
One or more are inferior.

[0034]

As is clear from the examples, the stainless steel of the present invention has a low hardness in the annealed state and excellent cold workability, and the hardness in the quenched / tempered state is the same as that of the existing SUS440C. Is equivalent to or higher than, and the corrosion resistance is further excellent. INDUSTRIAL APPLICABILITY The stainless steel of the present invention is suitable as a material for parts used in automobiles, industrial machines, electronic devices, chemical devices, etc. that require corrosion resistance and high strength, and can manufacture these parts by cold working. Therefore, the processing cost is significantly reduced, and the effect is practically great.

Claims (2)

[Claims] 1. By weight%, C: 0.6 to 0.75%, Si:
0.3% or less, Mn: 0.3% or less, Ni: less than 0.10%, C
r: 12-16%, Al: 0.03% or less, Nb: 0.01-0.10%
And the balance is Fe and inevitable impurities, P as impurities is 0.02% or less, S is 0.005% or less, and the hardness in the annealed state is HRB94 or less. High strength and corrosion resistant stainless steel with excellent workability. 2. The high-strength and high-corrosion-resistant stainless steel excellent in cold workability according to claim 1, further comprising 0.3 to 3.0% by weight of Mo in addition to the above components.