KR101830563B1 - Austenitic stainless steel - Google Patents

Abstract

The present invention relates to austenitic high manganese stainless steels having high strength and ductility. Stainless steels may comprise 0.03-0.1% carbon, 0.08-1.0% silicon, 14-26% manganese, 10.5-18% chromium, less than 0.8% nickel, 0.05-0.6% copper, 0.8% of nitrogen and 0.0008-0.005% of boron, the balance being inevitable impurities arising from iron and stainless steel, and the stainless steel can be cold deformed using a TWIP (Twinin Induced Plasticity) mechanism.

Description

[0001] AUSTENITIC STAINLESS STEEL [0002]

The present invention relates to a high strength and high ductility austenitic high manganese stainless steel utilizing TWIN (Twinning Induced Plasticity) mechanism during the modification process.

Austenitic stainless steels such as the most widely used and widely used 304 austenitic stainless steels have a significantly lower strength combined with a relatively high residual elongation after cold deformation such as cold rolling. Ferrite austenitic duplex stainless steels, such as 2304 ferrite austenitic duplex stainless steels, provide high strength but also provide loose ductility with low cold strain.

Austenitic manganese stainless steels with high carbon content are susceptible to intergranular corrosion after welding in heat affected zones and welds due to chrome depletion in this area. Also, typical manganese stainless steels are generally fragile to form martensite phase after cold deformation, which can lead to delayed cracking.

FR patent application 2071667 discloses that as an indispensable component to have an austenitic phase after annealing, it is required to contain 0.02-0.3% C, 0.1-3.0% Si, 8.0-17.0% manganese, 12.0-16.0% Cr, 0.05-0.3% N and optionally 0.1 - 3.0% copper and 0.1 - 3.0% Mo, the remainder being iron and austenitic stainless steels. The purpose of FR patent application 2071667 is to have improved ductility and good molding properties. However, in the chemical composition of FR patent application 2071667, the manganese (Mn) content should not exceed 17 wt.%, Because higher contents produce less austenite phase.

U.S. Patent No. 6,454,879 discloses an alloy having a maximum of 0.1 C in weight percent, 0.21-0.60 Si, 20-30% Mn, 17-24% Cr, 2.5% Ni up to 1.9% Mo, 0.6-1.4% 0.002% B, up to 0.8% carbide-forming elements and the remainder being iron and the microstructure having substantially no waste light content. Titanium, zirconium, vanadium and niobium are said to be strong carbides and nitrides and / or carbon nitride formers, the content of which is less than 0.8% by weight. According to the method, the material is hot formed to a temperature of at least 850 DEG C to a degree of deformation of at least about 3.5 times and is aggressively cooled. In the second step, it is cold-formed with a strain of 5 - 20% below the temperature of the deposits of the nitrides to an elevated temperature of > Thus, to prevent the formation of martensite, at every process step, the deformation is carried out at elevated temperatures, which increases the manufacturing cost of the manufacturing material.

The twisting of the microstructure of the metal material is generally defined as two separate crystals sharing a portion of the same crystal lattice. Twined Induced Plasticity (TWIP) stainless steels are used in conjunction with a face-centered cubic grating (FCC) with relatively low stacking fault energy (SFE) to facilitate the operation of twinning deformation mechanisms such as mechanical induction twining in a crystal lattice, ≪ / RTI > having an austenitic microstructure. The term TWIP indicates that the twinning is often accompanied by resolidification through lattice dislocation.

It is an object of the present invention to overcome some of the disadvantages of the prior art and also to provide a process for the production of weld metal in cold deformations in order to have a good corrosion resistance and a high machining rate with low susceptibility to intergranular corrosion and delayed cracking and stress corrosion cracking after welding To provide an improved austenitic high manganese stainless steel that utilizes a TWIN (TWINING Induced Plasticity) mechanism. Essential features of the austenitic stainless steels are listed in the appended claims.

According to the present invention, an austenitic high manganese stainless steel comprises 0.03 - 0.1% carbon, 0.08 - 1.0% silicon, 14 - 26% manganese, 10.5 - 18% chromium, less than 0.8% Nickel, 0.05 - 0.6% copper, 0.1 - 0.8% nitrogen and 0.0008 - 0.005% boron, with the balance being inevitable impurities arising from iron and stainless steel. The austenitic stainless steels of the present invention additionally optionally comprise 0.001 - 0.02% titanium and optionally less than 0.04% aluminum. The austenitic stainless steels of the present invention have excellent ductility and high strength after plastic deformation utilizing a twinning induced plasticity (TWIP) mechanism in cold deformation. Thus, the austenitic stainless steels of the present invention combine high strength and high work hardening rates in the initial annealing state with high elongation after cold deformation such as cold rolling in conjunction with low nickel content.

Austenitic high Mn stainless steel, the yield strength in the annealed condition according to the invention R _p0.2, a tensile strength R _m, and the elongation at break A ₈₀ is 1000 - after the annealing at temperatures of 1150 ℃ range, R _p0.2 Is 470 - 600 MPa, R _m is 800 - 930 MPa, and A ₈₀ is 40 - 60%. The austenitic stainless steel according to the present invention is further subjected to cold deformation such as cold rolling. The effect of the TWIP (TWINING Induced Plasticity) mechanism is obtained by cold rolling after 10% and 20% It can be expressed by a strength R _p0.2, a tensile strength R _m, and the breaking elongation for each of the ranges a _80:

The austenitic high manganese stainless steel of the present invention has a high work hardening rate of at least 20% when the cold deformation is 10% and a high work hardening rate of at least 40% when the cold deformation is 20% with respect to the yield strength R _p0.2 . In addition, the elongation at break A ₈₀ is 25 - 35% when the strain is 10% and 10 - 20% when strain is 20%, showing good ductility.

The effects of the major elements in the chemical composition of the austenitic high manganese stainless steels according to the present invention are described, and unless otherwise stated, the range is% by weight.

Carbon (C) is a valuable austenite forming and stabilizing element and allows for reduced use of expensive elemental nickel and copper. The upper limit for carbon alloying (less than 0.1%) is set by the risk of carbide precipitation, which degrades the corrosion resistance of stainless steels. The carbon content is low enough to maintain good corrosion resistance. Decreasing the carbon content to a low level by the decarburization process is uneconomical and therefore the carbon content should not be less than 0.03%.

Silicon (Si) is added to the stainless steel for deoxidation purposes at the time of melting, and should not be less than 0.08%. Since silicon is a ferrite-forming element, its content should be limited to less than 1%.

Manganese (Mn) is a key element of the austenitic stainless steel of the present invention, ensuring a stable austenitic crystal structure, and enabling a twinning mechanism and, consequently, a reduced use of expensive nickel. Manganese also increases the solubility of nitrogen in stainless steel. The plastic deformation associated with the twinning deformation occurs readily in the case of a deformed structure, i.e., a quantity of manganese of greater than 14% without strain-induced martensite. The high manganese content makes the steel decarburization process more difficult, impairs surface quality and reduces corrosion resistance of the steel. Therefore, the manganese content should be less than 26%. Preferably, the manganese content is in the range of 17.5 - 26.0%, more preferably 19 - 23%.

Chromium (Cr) is responsible for ensuring corrosion resistance of stainless steel. Therefore, the chromium content in this stainless steel is at least 10.5%. Chromium is important in terms of prevention of delayed cracking. By increasing the content at this level, the corrosion resistance of the steel can be improved. However, since chromium is a ferrite-forming element, an increase in chromium content increases the need for expensive austenite formers such as nickel and manganese, or requires a high carbon and nitrogen content impractically. Therefore, the chromium content should be lower than 18%. Chromium also increases the solubility of nitrogen. Preferably, the chromium content is in the range of 12 - 16.0%, more preferably 12.5 - 14%.

Nickel (Ni) is a powerful austenitizing agent and stabilizer, but nickel is an expensive element. However, very low nickel content necessitates high austenitic formation and impractical high alloying with stabilizing elements. Therefore, the nickel content should be less than 0.8%, preferably less than 0.5%.

Copper (Cu) is present as a residue of 0.05 - 0.6%, preferably 0.3 - 0.6%. Copper is a weak stabilizer of austenite, but has a strong effect on resistance to martensite formation. Copper also has positive effects on ductility and molding properties.

Nitrogen (N) is a powerful austenitizing agent and stabilizer. Thus, nitrogen alloying improves the cost efficiency of steel by enabling low use of nickel and copper. In order to ensure a comparatively low use of the above-described alloying elements, the nitrogen content should be at least 0.1%. The high nitrogen content increases the strength of the steel and thus makes the molding operation more difficult. Also, the risk of nitride precipitation increases with increasing nitrogen content. For this reason, the nitrogen content should not exceed 0.8%, preferably the nitrogen content should be less than 0.6%. Nitrogen increases the stacking defect energy (SFE) used to predict the TWIP effect, thus enabling and facilitating the TWIP effect.

The austenitic stainless steels according to the present invention do not form any modified martensite at room temperature or above during cold rolling. Therefore, the stainless steel of the present invention has high ductility. Further, the austenitic stainless steel according to the present invention does not have stress corrosion cracks and delayed cracks even after aging in air and in a 5% sodium chloride (NaCl) environment.

Yes

The austenitic stainless steels of the invention were melted on a production scale and then cast into slabs of chemical composition in weight percent.

The slabs were further hot rolled to a thickness of 4.0 mm and then annealed at a temperature of 1080 캜. In addition, the austenitic stainless steels of the present invention were cold rolled to a thickness of 2.0 mm at 50% rolling and annealed at a temperature of 1080 占 폚. The annealed strip product was then tested by determining the yield strength R _p0.2 , the tensile strength R _m , and the elongation at break A ₈₀ .

In order to utilize the TWIP effect in the austenitic stainless steels of the present invention, the stainless steel strip was cold deformed by a reduction of 10%, and then the yield strength R _p0.2 , tensile strength R _m , and fracture elongation A ₈₀ were determined . In addition, each of the cold-deformed strips with a 20% reduction was also performed. The results for the test results are shown in the following table:

The results show that the austenitic stainless steels according to the present invention have a high work hardening rate with respect to the yield strength R _p0.2 . In addition, the elongation at break A ₈₀ is 28% at 10% strain and 14% at 20% strain, indicating that it still has good ductility at high strength after cold rolling.

The austenitic stainless steels according to the present invention can be used as slabs, blooms, billets, flat products such as coils, strips, plates, sheets, elongated products such as bars, rods, wires, profiles and sections, tubular products, And can also be applied to, for example, automotive structures, tanks, crash-related parts, construction and railway vehicles.

The high manganese austenitic stainless steels according to the present invention can be produced by hot working, i.e. annealed strips after hot rolling, or cold working, for example, in order to utilize the TWIP effect for higher yield strength and tensile strength values, E., Annealed strip after cold rolling, or annealed after hot working and cold working, followed by cold deformation in the state of cold deformed strip.

Claims (15)

Austenitic high manganese stainless steel having high strength and ductility,
Wherein the composition comprises 0.03-0.1% carbon, 0.08-1.0% silicon, 14-26% manganese, 10.5-18% chromium, less than 0.8% nickel, 0.05-0.6% copper, 0.1-0.8% Nitrogen, 0.0008 to 0.005% of boron, 0.001 to 0.02% of titanium and less than 0.04% of aluminum, the _{balance being} inevitable impurities arising in iron and stainless steel, and in the annealed state, the yield strength R _p0.2 and a tensile strength R _m, and the elongation at break a _80, R _p0.2 is 470 - 600 MPa, and, R _m is 800 - 930 MPa, and, a ₈₀ is 40 - 60% of the above stainless steel, the yield strength R _{p0 .} Austenitic high manganese stainless steel which can be cold deformed using a TWIP (Twinin Induced Plasticity) mechanism so that the work hardening rate based on the cold deformation for the _.2 is at least 20% when the cold deformation is 10%. The method according to claim 1,
_{Wherein the} work hardening rate based on cold strain to yield strength R _p0.2 is at least 40% when cold strain is 20%. The method according to claim 1,
Wherein the elongation at break A ₈₀ is 25 - 35% when the cold deformation is 10%. The method according to claim 1,
Austenitic high manganese stainless steel characterized in that the elongation at break A ₈₀ is 10-20% when the cold strain is 20%. The method according to claim 1,
Wherein the manganese content is in the range of 19 - 23%. The method according to claim 1,
Wherein the chromium content is in the range of 12 - 16.0%. The method according to claim 1,
Wherein the chromium content is in the range of 12.5 - 14%. Austenitic high manganese stainless steel. The method according to claim 1,
Wherein the copper content is in the range of 0.3 - 0.6%. 9. The method according to any one of claims 1 to 8,
Wherein said stainless steel can be cold deformed for TWIP effect in the state of an annealed strip after hot working. 9. The method according to any one of claims 1 to 8,
Wherein said stainless steel can be cold deformed for TWIP effect in the state of an annealed strip after cold working. 9. The method according to any one of claims 1 to 8,
Wherein said stainless steel is annealed after hot working and cold working and then cold deformable for TWIP effect in the state of cold deformed strip. 9. The method according to any one of claims 1 to 8,
Wherein said austenitic stainless steels are manufactured as slabs, blooms, billets, coils, strips, plates, sheets, bars, rods, wires, profiles and sections, pipes, tubes. 9. The method according to any one of claims 1 to 8,
Wherein the austenitic stainless steel is applied to an automobile structure, a tank, a crash-related part, a building, and a railway vehicle. delete delete