JPH08269632A - High strength and high corrosion resistant nitrogen-containing austenitic stainless steel - Google Patents

Abstract

PURPOSE: To produce an austenitic stainless steel in which nitrides are allowed to enter into solid solution by an ordinary solid solution heat treating furnace, having corrosion resistance more excellent than that of SUS XM7, SUS 304 and SUS 304 N and capable of obtaining high strength. CONSTITUTION: This stainless steel has a compsn. contg., by weight, <=0.10% C, <=1.0% Si, 5 to 10% Mn, <=0.01% S, 8 to 15% Ni, 15 to 25% Cr, 0.5 to 4% Mo, 0.3 to 1.0% N, and the balance substantial Fe. Furthermore, for the purpose of improving its cold workability, Cu may be incorporated therein by <=3%, for the purpose of improving its hot workability, B may be incorporated therein by <=0.01% and for the purpose of improving its strength, one or >= two kinds among <=1.0% Nb, <=1.0% V and <=1.0% W may be incorporated therein.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is applicable to, for example, outdoor, industrial,
The present invention relates to stainless steel used as a material for parts such as self-tapping used in coastal areas, drill screws and bolts, stern shafts, and the like that require high strength and high corrosion resistance.

[0002]

2. Description of the Related Art Austenitic stainless steels such as SUS XM7 and SUS304 are used as materials for parts such as self-tapping, drill screws and bolts that are required to have strength and corrosion resistance used outdoors, in industry, in coastal areas, etc. ing. However, SUSXM7, SUS304
In such cases, the required strength cannot be obtained, and corrosion resistance has been confirmed, especially in coastal areas where the environment is harsh.

[0003]

For example, in recent years, self-drilling screws have been increasing in demand because they do not require a prepared hole in a material to be fastened and have excellent on-site workability. However, in the conventional steels SUS XM7 and SUS 304, the drill edge hardness is HV ≤ 400, and its use is limited to Al plate, thin steel plate, etc., and the hardness is increased by cold working, so the corrosion resistance decreases. I also have it. Currently,
As stainless steels for seawater resistance, various inventions have been invented on high hardness and high corrosion resistant stainless steels to which a large amount of nitrogen is added, but all of them contain a large amount of expensive alloying elements such as Cr, Ni and Mo, and cold working. In order to form a solid solution of a nitride that adversely affects the properties, a high temperature solution heat treatment at about 1200 ° C. is required and the cost is high, and it is not suitable as a material for screws and bolts. Further, although SUS 304N in which nitrogen is added to SUS 304 to improve strength and corrosion resistance has been developed, the hardness is still insufficient.

Therefore, there is a demand for the development of a material that has corrosion resistance that can withstand use in coastal areas and the like, high strength that can be used for thick steel plates, and that suppresses cost increases as much as possible. The present invention has been made to solve such a problem, in which a nitride is solid-solved in an ordinary solution heat treatment furnace, and S
It is an object of the present invention to provide an austenitic stainless steel that is superior in corrosion resistance to US XM7, SUS 304, and SUS 304N and that can obtain high strength.

[0005]

The stainless steel according to the present invention for achieving the above object has a C content of C: 0.10.
% Or less, Si: 1.0% or less, Mn: 5 to 10%, S:
0.01% or less, Ni: 8-15%, Cr: 15-25
%, Mo: 0.5 to 4%, N: 0.3 to 1.0%, and the balance is substantially Fe.

The present invention aims to improve the cold workability by using Cu,
B for the purpose of improving hot workability, N for the purpose of improving strength
Cu: 3% or less, B: 0.01% or less, Nb: 1.0% or less, V:
It may be contained in an amount of 1.0% or less and W: 1.0% or less.

[0007]

ACTIONS AND EFFECTS OF THE INVENTION The reasons for limiting the component range of the present invention will be described.

C: 0.10% or less C is an interstitial element and contributes to the improvement of strength. However, addition of a large amount lowers the solid solution amount of N, and
It is necessary to set the upper limit to 0.1% in order to combine with r to form a carbide and reduce the amount of solid solution Cr in the matrix to deteriorate the corrosion resistance.

Si: 1.0% or less Si is an element mainly acting as a deoxidizing agent during melting and refining. However, if contained in a large amount, the manufacturability is deteriorated, so the upper limit was made 1.0%.

Mn: 5-10% Mn is an austenite-forming element and significantly increases the solid solution amount of nitrogen. In order to obtain an austenite single phase in the phase balance, it is necessary to add 5% or more. Further, if it exceeds 10%, the hot workability is deteriorated, so the range is made 5 to 10%.

S: 0.01% or less% S becomes MnS, which becomes a starting point of cracks during cold working, so that cold workability is significantly deteriorated. Therefore, the upper limit is set to 0.01%.

Ni: 8-15% Ni is an austenite forming element and contributes to the austenite phase stability. Further, although the deformation resistance is lowered and the cold workability is improved, the addition of a large amount conversely causes a reduction in strength, so the range is set to 8 to 15%.

Cr: 15 to 25% Cr is effective as an element for increasing the amount of nitrogen dissolved in stainless steel and improving the corrosion resistance, and is a ferrite forming element. If it is less than 15%, the corrosion resistance is not sufficient,
The nitrogen solid solution amount and corrosion resistance increase with the increase. However, if it exceeds 25%, ferrite-austenite 2
Since it becomes a phase structure, the range was made 15% to 25%.

Mo: 0.5-4% Mo is effective as an element for increasing the amount of nitrogen dissolved in stainless steel and improving corrosion resistance, and is a ferrite-forming element. If it is 0.5% or less, the effect of improving the characteristics is small and increases with the increase. However, if it exceeds 4%, the hot workability is deteriorated and the cost is increased,
The range was 0.5% to 4%.

N: 0.3 to 1.0% N is an interstitial element and contributes to the improvement of strength, stability of austenite phase and corrosion resistance, and strength improvement by aging treatment after working. If it is less than 0.3%, the effect is low, and if it is more than 1%, the nitride does not completely form a solid solution.
Since the cold workability and the toughness and ductility deteriorate as the temperature rises, the range is set to 0.3% to 1.0%.

Cu is an austenite-forming element and has the effect of improving cold workability and corrosion resistance, so it may be added if necessary. However, if the content exceeds 3%, the hot workability deteriorates, so the upper limit was made 3%.

B: 0.01 or less B contributes to the improvement of hot workability, so B may be added if necessary. However, if added in an amount of 0.01% or more, it bonds with nitrogen and conversely reduces the hot workability, so the content must be 0.01% or less.

Nb: 1.0% or less, V: 1.0% or less,
W: 1.0% or less Nb, V, and W contribute to refinement of crystal grains and improvement in strength, and thus may be added alone or in combination of two or more, if necessary. However, if added in a large amount, it binds to nitrogen and reduces the amount of solid solution N, so the upper limit was made 1.0% or less.

[0019]

EXAMPLES Next, examples will be described in detail below in order to further clarify the characteristics of the present invention. The chemical composition shown in Table 1 was melted in a 50 kg high-frequency induction furnace, cast into a 50 kg steel ingot, and then forged into a 20 mm round bar.
After heating at 100 ° C. for 1 hr, water-cooled solution heat treatment was performed.
After that, the test piece is cut out from each round bar and the tensile test, CPT
A test and a limit cracking test were conducted. The comparative steel is JIS
SUS304N, SUSXM7, SUS316L, S
It is US317J4L.

In the room temperature tensile test, JIS No. 4 test piece was used to measure 0.2% proof stress and tensile strength. C
In the PT test, a corrosive liquid was prepared in accordance with JIS G 0578, and the temperature was raised at 2.5 ° C. increments for measurement. In the limit cracking test, an end face restraint compression test was performed to measure the area reduction rate at which cracks can be visually confirmed. The results are shown in Table 2. In addition, according to the place confirmed by a microscope (× 100),
It was also found that undissolved nitrides could hardly be confirmed in the solution heat treatment state.

[0021]

[Table 1]

[0022]

[Table 2]

As shown in Table 2, the steels 1 to 13 of the present invention are
Compared with SUS304N and SUS317J4L, 0.2% proof stress and tensile strength at room temperature are excellent, pitting corrosion resistance is far superior to SUS316L, and it has properties similar to seawater-resistant stainless steel. The workability is SUS.
Although slightly inferior to XM7 etc., SUS317J
Much better than 4L.

[0024]

As described above, the present invention is 1100.
By completely dissolving nitrogen in solution heat treatment at ℃ or below, we can provide austenitic stainless steel with improved strength and corrosion resistance while suppressing cost increase and workability deterioration. Suitable for self-tapping, drill screws, bolts, stern shafts, etc.

Claims (2)

[Claims] 1. By weight%, C: 0.10% or less Si: 1.0% or less Mn: 5-10% S: 0.01% or less Ni: 8-15% Cr: 15-25% Mo: 0 0.5 to 4% N: 0.3 to 1.0% A high-strength, high-corrosion nitrogen-containing austenitic stainless steel having a composition consisting essentially of Fe. 2. In addition to the components of claim 1, Cu, Nb,
Any one or more of B, V and W Cu: 3% or less B: 0.01% or less Nb: 1.0% or less V: 1.0% or less W: 1.0% or less A high-strength, high-corrosion-resistant nitrogen-containing austenitic stainless steel.