JPH055159A - High strength and high corrosion resistant stainless steel excellent in cold workability - Google Patents

Abstract

PURPOSE:To manufacture a martensitic stainless steel having high strength and high corrosion resistance and furthermore excellent in cold workability. CONSTITUTION:A high strength and high corrosion resistant stainless steel contg., by weight, 0.6 to O.75% C, <=0.3% Si, <=0.3% Mn, <0.10% Ni, 12 to 16% Cr, <=0.03% Al and 0.01 to 0.1% Ti, furthermore contg., at need, 0.3 to 3.0% Mo and the balance Fe with inevitable impurities, in which the content of P and S as impurities is regulated to <=0.2% and <=0.005% and its hardness as annealed is regulated to <=94 HRB and excellent in cold workability is prepd.

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.

SUS440C, which is a high C-Cr material, is a hyper-eutectoid steel, so it has a high hardness (HRC) in the quenched and low temperature tempered state.
(58 or more) is obtained, but a significantly hard eutectic carbide (HV2000 or more) crystallizes coarsely during production and is annealed.
Since the hardness in the (annealed) state is high, the cold workability is extremely poor. 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. In addition, this SUS440C has a low Cr content in the base material because a large amount of large eutectic carbide is formed, and thus the corrosion resistance is not necessarily good.

Medium C-Cr material SUS420J2 and low C
Since SUS410, which is a Cr material, has a lower C content than SUS440C, it does not have a very high hardness even in a quenched and low temperature tempered state, and these stainless steels also have a low Cr content, so that 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 described 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,
Mn that hinders cold workability and also deteriorates corrosion resistance
It is specified to be 1.0% by weight or less, or in addition to these, Ni that causes pitting corrosion in a special corrosive environment is specified to be 1.5% by weight or less, but such a specification is insufficient, and Si, Mn If the contents of Ni and Ni are high, cold workability and corrosion resistance may not be improved. On the other hand, JP-A-1
In the steel described in -275737, the content of C is 0.3 to 0.
Since it is as low as 6% by weight, the hardness after quenching and tempering cannot be sufficiently obtained, and the upper limit of Ni is 0.6% by weight in this steel as well.
Therefore, 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]

The inventors of the present invention firstly adjusted the amounts of C and Cr in steel and suppressed Si, Mn, and Ni to a low level, and were excellent in cold workability and had high strength and high corrosion resistance stainless steel. Invented steel (Japanese Patent Application No. 2-401896). The invention of this prior application was based on the following findings.

Coarse M ₇ C ₃ is used to improve cold workability.
It is effective to reduce type eutectic carbides, improve the shape and change the composition. By adjusting the amounts of C and Cr in the steel, the amount of these carbides is reduced and the shape is also reduced.
Changed to ₂₃ C ₃ type carbide to improve cold workability.

Further, the amount of Si and the amount of Mn in steel are respectively
When it is 0.3% by weight or less, cold workability is further improved, and N
When the i content is less than 0.10% by weight, the corrosion resistance is remarkably improved.

The steel of the prior invention based on the above knowledge has a hardness after quenching and tempering that is equal to or higher than that of the conventional SUS440C, and has greatly improved corrosion resistance and cold workability. As a result of further study, they newly found that if Ti is added in an appropriate amount, the crystal grains become finer and the toughness and corrosion resistance are greatly improved.

The present invention based on this new finding has as its gist the following stainless steels.

(I) C: 0.6 to 0.75% by weight, Si:
0.3% or less, Mn: 0.3% or less, Ni: less than 0.10%, Cr: 12
~ 16%, Al: 0.03% or less, Ti: 0.01 to 0.1%,
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 strength and corrosion resistance stainless steel.

(Ii) In addition to the above components, 0.3 to 3.0
The high-strength and high-corrosion-resistant stainless steel excellent in cold workability according to (i) above, which contains Mo in a weight percentage.

[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, the upper limit of C is 0.75
%.

Si: 0.3% or less Si is added as a deoxidizing agent for steel. However, a large amount of Si
The addition deteriorates the toughness of 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 deoxidizer 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 it causes pitting corrosion in a special environment, so 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 was set to less than 0.10%.

Cr: 12 to 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 content is made 0.03%. .

Ti: 0.01 to 0.1% Ti is an element effective for refining the crystal grains of the steel and improving the toughness and corrosion resistance of the steel. To fully exhibit the above effects, 0.01% or more of the content is required,
If the content exceeds 0.1%, not only the cleanliness of the steel is deteriorated and the machinability is deteriorated, but also the crystal grains of the steel are coarsened and the toughness is deteriorated. Therefore, the content is set to 0.01 to 0.1%.

In the present invention, in addition to the above components, Mo
Can be added as required.

Mo: 0.3-3.0% Mo has the function of enhancing the corrosion resistance of steel. But,
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, which lowers the strength. It is recommended to set it in the range of 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. 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. Further, since S impairs the hot workability of steel, its content was suppressed to 0.005% or less.

[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 this invention becomes parts such as shafts and bearings by cold forging, cold rolling, etc. after annealing, but if the hardness of the material steel after 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 crystallization state of carbides was examined by an electron microscope, Comparative Steel No. 16 had coarse M ₇ C ₃ type eutectic carbides with a size of 20 μm or more, and the shape was also square. All of the steels of the present invention are low hardness M ₂₃ C ₆ type carbides, and their size is as small as 10 μm or less.
The shape was also rounded, and the results were consistent 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.
Satisfies the hardness of C (58 or more in HRC), has a small corrosion weight loss, and is superior in corrosion resistance to SUS440C of comparative steel No. 16. Comparative steel Nos. 10 to 15 are
The chemical composition is out of the range specified in the present invention.
As can be seen from these examples, those having a chemical composition outside the range defined by the present invention are inferior in any one or more of cold workability, corrosion resistance and hardness.

[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 (1)

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%, Cr: 12 to 16
%, Al: 0.03% or less, Ti: 0.01 to 0.1%, the balance Fe and unavoidable impurities, and P as an impurity.
A high-strength, high-corrosion-resistant stainless steel with excellent cold workability, which has a hardness of 0.02% or less, an S content of 0.005% or less, and a hardness of HRB94 or less in the annealed state. 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.