High-strength austenitic stainless steel with excellent productivity and cost reduction effect and production method thereof

Abstract

The present invention relates to a high-strength austenitic stainless steel and a production method thereof, wherein the austenitic stainless steel has high productivity due to its excellent hot workability, has an excellent cost reduction effect due to a greatly reduced content of nickel (Ni) which is a high-price element, and has a yield strength of 450 MPa or more and an elongation of 45% or more after cold rolling and annealing, and has an ultra-high strength of 1800 MPa or more even after temper rolling.

Description

Technical Field

[0001] The present disclosure relates to a high-strength austenitic stainless steel having excellent productivity and cost reduction effects and a method for producing the same. Background Art

[0002] For the stability and reliability of products, it is conventionally required that structural steel materials constituting frames and exterior panels of automobiles, buildings, etc. and used to prevent personal injuries and physical damages caused by external stresses or impacts have high-strength characteristics.

[0003] Meanwhile, recent trends in the automotive and construction markets pursue complex and unique appearances, and thus excellent formability as well as high-strength characteristics are required in structural steel materials.

[0004] In other words, to meet market needs, it is required that structural steel materials have excellent formability in the annealed state to be easily deformed into various shapes and have high-strength characteristics after a forming process or a final process such as skin pass rolling.

[0005] However, conventional steel materials having excellent formability tend to have poor strength characteristics after forming, and conventional steel materials having high-strength characteristics tend to have poor formability, and thus it is difficult to meet recent market trends in many cases. Even when these conditions are satisfied, price competitiveness is generally poor due to the use of a large amount of high-price alloy elements contained therein.

[0006] At the same time, since no separate investment in equipment is required for stainless steels having excellent corrosion resistance, these steels are suitable for mass production of small types required in the recent battery-based eco-friendly automotive market and are also suitable for buildings in environments where corrosion is relatively accelerated, such as beaches or city centers.

[0007] In particular, since austenitic stainless steel basically has a high elongation rate, a complex and unique appearance can be obtained, thereby meeting various needs of customers and having an aesthetically excellent appearance.

[0008] However, due to a high content of high-price alloy elements, austenitic stainless steel has poor yield strength and low price competitiveness compared with ordinary structural carbon steel. In particular, the disadvantage is that the supply price is unstable and the price is high due to nickel (Ni) with a wide range of raw material price fluctuations resulting in unstable supply, and the price competitiveness of austenitic stainless steel is significantly reduced.

[0009] Therefore, there is a need to develop an austenitic stainless steel for structural materials that has high yield strength in a final product maintaining high formability and has high price competitiveness by significantly reducing the content of high-price alloy elements such as nickel (Ni). Summary of the Invention

[0010] Technical problem

[0011] There are also provided a high-strength austenitic stainless steel having a high yield strength of 1800 MPa or more in a final product maintaining high formability and a production method thereof.

[0012] There are also provided an austenitic stainless steel having excellent price competitiveness by significantly reducing the content of high-cost alloying elements such as nickel (Ni) and a production method thereof.

[0013] There are also provided an austenitic stainless steel having a high yielding percentage and excellent productivity in which no cracks occur even after hot rolling even when the content of high-cost alloying elements is reduced and a production method thereof.

[0014] However, the technical problems to be solved by the present disclosure are not limited to the foregoing problems, and those skilled in the art to which the present disclosure pertains will clearly understand any other technical problems not mentioned herein from the following description.

[0015] Technical solution

[0016] According to one aspect of the present disclosure for achieving the above object, the high-strength austenitic stainless steel contains, by weight percentage (% by weight): 0.1% to 0.2% of C, 0.2% to 0.3% of N, 0.8% to 1.5% of Si, 7.0% to 8.5% of Mn, 15.0% to 17.0% of Cr, 0.5% or less (excluding 0) of Ni, 1.0% or less (excluding 0) of Cu, 0% to 0.2% of Nb, and the balance of Fe and inevitable impurities, and satisfies the following Expression (1).

[0017] Expression (1): 14 ≤ 23(C + N) + 1.3Si + 0.24(Cr + Ni + Cu) + 0.1Mn

[0018] (In Expression (1), C, N, Si, Mn, Cr, Ni, and Cu respectively represent the contents (wt%) of the elements.)

[0019] In the above aspect, the high-strength austenitic stainless steel satisfies the following Expression (2).

[0020] Expression (2): 30 ≤ 551 - 462(C + N) - 9.2Si - 8.1Mn - 13.7Cr - 29(Ni + Cu) - 68Nb ≤ 80

[0021] (In Expression (2), C, N, Si, Mn, Cr, Ni, Cu, and Nb respectively represent the contents (wt%) of the elements.)

[0022] In this aspect, the high-strength austenitic stainless steel satisfies the following expression (3).

[0023] Expression (3): 16 ≤ 1 + 45C - 5Si + 0.09Mn + 2.2Ni - 0.28Cr - 0.67Cu + 88.6N ≤ 20

[0024] (In Expression (3), C, N, Si, Mn, Cr, Ni, and Cu respectively represent the contents (wt%) of the elements).

[0025] In this aspect, the high-strength austenitic stainless steel satisfies the following expression (4).

[0026] Expression (4):

[0027]

[0028] (In Expression (4), C, N, Si, Mn, Cr, Ni, Cu, and Nb respectively represent the contents (wt%) of the elements).

[0029] In this aspect, the high-strength austenitic stainless steel may have a yield strength of 450 MPa or more after cold rolling and annealing, and a yield strength of 1,800 MPa or more after skin pass cold rolling.

[0030] In this aspect, the high-strength austenitic stainless steel may have an elongation of 45% or more after cold rolling and annealing, and an elongation of 3% or more after skin pass cold rolling.

[0031] Furthermore, according to one aspect of the present disclosure, a method for producing a high-strength austenitic stainless steel includes: heating and hot rolling a steel slab, the steel slab containing, by weight percentage (wt%): more than 0.1% to 0.2% of C, 0.2% to 0.3% of N, 0.8% to 1.5% of Si, 7.0% to 8.5% of Mn, 15.0% to 17.0% of Cr, 0.5% or less (excluding 0) of Ni, 1.0% or less (excluding 0) of Cu, 0% to 0.2% of Nb, and the balance of Fe and inevitable impurities; performing hot annealing on the hot-rolled steel plate; performing cold rolling on the hot-annealed steel plate; and performing cold annealing on the cold-rolled steel plate, wherein the steel slab satisfies the following expression (1).

[0032] Expression (1): 14 ≤ 23(C + N) + 1.3Si + 0.24(Cr + Ni + Cu) + 0.1Mn

[0033] (In Expression (1), C, N, Si, Mn, Cr, Ni, and Cu respectively represent the contents (wt%) of the elements).

[0034] In this aspect, the steel billet may satisfy the following expression (2).

[0035] Expression (2): 30 ≤ 551 - 462(C + N) - 9.2Si - 8.1Mn - 13.7Cr - 29(Ni + Cu) - 68Nb ≤ 80

[0036] (In Expression (2), C, N, Si, Mn, Cr, Ni, Cu, and Nb respectively represent the contents (wt.%) of the elements.)

[0037] In this aspect, the steel billet may satisfy the following expression (3).

[0038] Expression (3): 16 ≤ 1 + 45C - 5Si + 0.09Mn + 2.2Ni - 0.28Cr - 0.67Cu + 88.6N ≤ 20

[0039] (In Expression (3), C, N, Si, Mn, Cr, Ni, and Cu respectively represent the contents (wt.%) of the elements.)

[0040] In this aspect, the steel billet may satisfy the following expression (4).

[0041] Expression (4):

[0042]

[0043] (In Expression (4), C, N, Si, Mn, Cr, Ni, Cu, and Nb respectively represent the contents (wt.%) of the elements.)

[0044] Advantageous Effects

[0045] Since the austenitic stainless steel according to the present disclosure satisfies the composition and content of alloying elements and satisfies Expression (1), a yield strength of 450 MPa or greater can be obtained after cold rolling and annealing, and a yield strength of 1,800 MPa or greater can be obtained after skin pass cold rolling, and high formability is maintained. The austenitic stainless steel can have excellent price competitiveness by reducing the content of high-cost elements such as nickel (Ni) as low as possible to 0.5 wt.% or less, and has a high production rate percentage and excellent productivity due to no cracks occurring during hot rolling. Detailed Embodiments

[0046] One aspect of the present disclosure provides a high-strength austenitic stainless steel, which contains, by weight percentage (% by weight): 0.1% to 0.2% of C, 0.2% to 0.3% of N, 0.8% to 1.5% of Si, 7.0% to 8.5% of Mn, 15.0% to 17.0% of Cr, 0.5% or less (excluding 0) of Ni, 1.0% or less (excluding 0) of Cu, 0% to 0.2% of Nb, and the balance of Fe and inevitable impurities, and satisfies the following expression (1).

[0047] Expression (1): 14 ≤ 23(C + N) + 1.3Si + 0.24(Cr + Ni + Cu) + 0.1Mn

[0048] (In Expression (1), C, N, Si, Mn, Cr, Ni, and Cu respectively represent the contents of the elements (% by weight).)

[0049] Embodiments of the Invention

[0050] Hereinafter, the high-strength austenitic stainless steel according to the present disclosure and its production method will be described in detail. In this regard, unless otherwise defined, technical terms or scientific terms used herein have an obvious meaning to those of ordinary skill in the art, and detailed descriptions of known functions or structures incorporated herein will be omitted when they may obscure the subject matter of the present disclosure.

[0051] Throughout the specification, unless otherwise stated, the term "comprising" an element does not exclude other elements, but also includes additional elements.

[0052] According to an embodiment of the present disclosure, there is provided a high-strength austenitic stainless steel, which contains, by weight percentage (% by weight): 0.1% to 0.2% of C, 0.2% to 0.3% of N, 0.8% to 1.5% of Si, 7.0% to 8.5% of Mn, 15.0% to 17.0% of Cr, 0.5% or less (excluding 0) of Ni, 1.0% or less (excluding 0) of Cu, 0% to 0.2% of Nb, and the balance of Fe and inevitable impurities, and satisfies the following expression (1).

[0053] Expression (1): 14 ≤ 23(C + N) + 1.3Si + 0.24(Cr + Ni + Cu) + 0.1Mn

[0054] (In Expression (1), C, N, Si, Mn, Cr, Ni, and Cu respectively represent the contents of the elements (% by weight).)

[0055] As described above, the austenitic stainless steel according to the present disclosure can have a high yield strength of 450 MPa after cold rolling and annealing and 1,800 MPa after skin pass cold rolling, and maintain high formability by not only satisfying the composition and content of the above alloying elements but also satisfying Expression (1). In addition, the austenitic stainless steel has excellent price competitiveness by reducing the content of high-price elements such as nickel (Ni) as low as possible to 0.5 wt% or less and has a high production yield percentage and excellent productivity due to no cracks occurring during hot rolling.

[0056] Hereinafter, the reasons for the numerical limits of the alloying element contents in the embodiments of the present disclosure will be described. Hereinafter, unless otherwise specified, the unit is wt%.

[0057] In a high-strength austenitic stainless steel according to an embodiment of the present disclosure, the content of C can be 0.1% to 0.2%, preferably 0.15% to 0.2%.

[0058] C can be added as an element effective for stabilizing the austenite phase to obtain the yield strength of the austenitic stainless steel. Since an insufficient C content cannot satisfy the sufficient yield strength required in the present disclosure, its lower limit can be set to 0.1%, preferably set to 0.15%. On the contrary, since an excessive C content may not only deteriorate the cold workability due to the solid solution strengthening effect but also deteriorate the hot workability by inducing grain boundary precipitation of Cr carbides during hot working, and also adversely affect the properties of the steel such as ductility, toughness, and corrosion resistance, its upper limit can be set to 0.2%.

[0059] In a high-strength austenitic stainless steel according to an embodiment of the present disclosure, the content of nitrogen (N) can be 0.2% to 0.3%, preferably 0.2% to 0.25%.

[0060] N is one of the most important elements in the present disclosure. N, as a strong austenite stabilizing element, is effective for improving the corrosion resistance and yield strength of austenitic stainless steel. Since an insufficient N content cannot satisfy the sufficient yield strength required in the present disclosure, its lower limit can be set to 0.2%. On the contrary, since an excessive N content may cause defects such as holes during the production of steel billets and deteriorate the cold workability due to the solid solution strengthening effect, its upper limit can be set to 0.3%, more preferably set to 0.25%.

[0061] In a high-strength austenitic stainless steel according to an embodiment of the present disclosure, the content of silicon (Si) can be 0.8% to 1.5%, more preferably 0.8% to 1.2%.

[0062] Si, which is used as a deoxidizer during the steelmaking process, is effective in enhancing corrosion resistance. In addition, Si, which is an element effective in improving the yield strength of steel among substitution elements, is added to improve the yield strength in the present disclosure. Since an insufficient Si content cannot satisfy the sufficient corrosion resistance and yield strength required in the present disclosure, its lower limit can be set to 0.8%. On the contrary, an excessive Si content may not only deteriorate the hot workability by promoting the formation of δ-ferrite in the cast steel slab, but also adversely affect the ductility and impact properties of the material, and its upper limit can be set to 1.5%, more preferably, to 1.2%.

[0063] In a high-strength austenitic stainless steel according to an embodiment of the present disclosure, the content of manganese (Mn) can be 7.0% to 8.5%, more preferably, 7% to 8%.

[0064] Mn, which is an austenite phase stability element added as a Ni substitute in the present disclosure, can be added in an amount of 7.0% or more to enhance the cold rollability by suppressing the formation of strain-induced martensite. However, since an excessive Mn content may deteriorate the ductility and toughness of the austenitic stainless steel by excessively forming S-based inclusions (MnS) and increase the manufacturing risk by generating Mn soot during the steelmaking process. In addition, since excessive Mn may rapidly deteriorate the corrosion resistance of the product, its upper limit can be set to 8.5%, more preferably, to 8%.

[0065] In a high-strength austenitic stainless steel according to an embodiment of the present disclosure, the content of chromium (Cr) can be 15.0% to 17.0%, more preferably, 15.5% to 16.5%.

[0066] Although Cr is a ferrite stability element, Cr is an element effective in suppressing the formation of the martensite phase and can be added in an amount of 15% or more as a basic element for obtaining the required corrosion resistance in stainless steel. However, as a ferrite stability element, an excessive Cr content may promote the formation of a large amount of δ-ferrite in the steel slab, resulting in deterioration of the hot workability and adverse effects on the steel properties, so its upper limit can be set to 17.0%, more preferably, to 16.5%.

[0067] In a high-strength austenitic stainless steel according to an embodiment of the present disclosure, the content of nickel (Ni) may be greater than 0% and 0.5% or less, more preferably, 0.01% to 0.3%. As a strong austenite phase stability element, Ni is essential for obtaining excellent hot workability and cold workability. However, since Ni is a high-cost element, adding a large amount of Ni may lead to an increase in manufacturing cost. Therefore, considering the cost and benefit of the steel, its upper limit can be set to 0.5%, more preferably, set to 0.3%.

[0068] In a high-strength austenitic stainless steel according to an embodiment of the present disclosure, the content of copper (Cu) may be greater than 0% and 1.0% or less, more preferably, 0.1% to 1%.

[0069] In the present disclosure, Cu as an austenite phase stability element is added as a Ni substitute. Cu can be added to enhance the corrosion resistance in a reducing environment. However, an excessive Cu content not only increases the cost of raw materials but also causes embrittlement and liquefaction at low temperatures. In addition, adding an excessive amount of Cu may cause a problem of deteriorating hot workability due to Cu segregation into the edges of the steel billet. Therefore, considering the cost-benefit and properties of the steel, its upper limit can be set to 1.0%.

[0070] In addition, the high-strength austenitic stainless steel according to an embodiment of the present disclosure may also contain niobium (Nb) in an amount of 0.2% or less.

[0071] Nb, which has a high affinity for carbon and nitrogen, forms precipitates during heat treatment to improve the grain refinement of the material and increase the yield strength. However, an excessive amount of Nb may not only deteriorate the hot workability of the material as a ferrite stability element but also increase the cost of raw materials as a high-cost element. Therefore, considering the cost-benefit and properties of the steel, its upper limit can be set to 0.2%, more preferably, set to 0.15%.

[0072] In addition, the high-strength austenitic stainless steel according to an embodiment of the present disclosure may also contain at least one of P of 0.035% or less and S of 0.01% or less as inevitable impurities.

[0073] Phosphorus (P), which is an impurity inevitably contained in steel, is a main causative element of intergranular corrosion or deteriorated hot workability. Therefore, it is preferable to control the P content as low as possible. In the present disclosure, the upper limit of the P content is adjusted to 0.035%.

[0074] Sulfur (S), which is inevitably contained as an impurity in steel, is a main causative element for deterioration of hot workability due to segregation at grain boundaries. Therefore, it is preferable to control the S content as low as possible. In the present disclosure, the upper limit of the S content is adjusted to 0.01%.

[0075] The remaining component of the present disclosure is iron (Fe). However, in ordinary manufacturing processes, undesirable impurities from raw materials or the manufacturing environment may inevitably be mixed therewith, and this cannot be excluded. Such impurities are well known to those of ordinary skill in the art, and thus, specific descriptions thereof will not be given in the present disclosure.

[0076] In recent years, enhancement of the yield strength of steel has been considered an important factor for the light weight and stability of steel. In particular, in order to manufacture structural materials having various shapes including automotive structural materials, sufficient elongation should be obtained in the annealed state. In addition, since the final products used as structural materials require a significantly high level of yield strength after skin pass cold rolling and forming, a high level of yield strength is required after skin pass cold rolling or forming.

[0077] In addition, the content of high-cost austenite stability elements such as Ni should be reduced to obtain price competitiveness of austenitic stainless steel, and the amounts of Mn, N, and Cu that can compensate for it must be predicted. However, reducing the Ni content and adding Mn, N, and Cu as described above to obtain price competitiveness has the following risks: rapidly increasing work hardening and thus deteriorating the elongation of the steel, or causing a decrease in resistance to hot deformation and thus deteriorating productivity. Therefore, considering the coordination of elements, it is necessary to estimate the amounts of the elements.

[0078] Therefore, in order to obtain the following high-strength austenitic stainless steel, it is preferable to satisfy the following Expression (1) and satisfy the composition and content of alloy elements: The high-strength austenitic stainless steel has a high yield strength of 450 MPa or more after cold rolling and annealing and a high yield strength of 1,800 MPa after skin pass cold rolling, has high formability, and has excellent price competitiveness by reducing the content of high-cost alloy elements such as nickel (Ni) as low as possible to 0.5 wt% or less, has an excellent yield percentage and productivity by preventing cracks during hot rolling, and has high formability.

[0079] Expression (1): 14 ≤ 23(C + N) + 1.3Si + 0.24(Cr + Ni + Cu) + 0.1Mn

[0080] (In Expression (1), C, N, Si, Mn, Cr, Ni, and Cu respectively represent the contents (wt%) of the elements.)

[0081] In the present disclosure, in order to obtain a high yield strength of austenitic stainless steel, Expression (1) is derived considering the increase in the yield strength caused by the stress field of the steel material.

[0082] As the value of Expression (1) increases, due to the difference in atomic size between alloying elements, the stress field between the lattices increases, making the limit of plastic deformation against external stress increase. Specifically, when the value of Expression (1) is less than 14, it is difficult to obtain the required yield strength in the present disclosure. However, when the value of Expression (1) is too high, the yield strength may instead decrease after skin pass cold rolling. Preferably, the upper limit of Expression (1) can be 16.5. Therefore, when the value of Expression (1) satisfies the range of 14 to 16.5, a high-strength austenitic stainless steel with a yield strength of 450 MPa or more after cold rolling and annealing and a yield strength of 1,800 MPa or more after skin pass cold rolling can be obtained.

[0083] In addition, the high-strength austenitic stainless steel according to an embodiment of the present disclosure can satisfy the following Expression (2).

[0084] Expression (2): 30 ≤ 551 - 462(C + N) - 9.2Si - 8.1Mn - 13.7Cr - 29(Ni + Cu) - 68Nb ≤ 80

[0085] (In Expression (2), C, N, Si, Mn, Cr, Ni, Cu, and Nb respectively represent the contents (wt.%) of the elements).

[0086] Expression (2) is derived considering the phase transformation represented by the deformation of austenitic stainless steel. When the value of Expression (2) exceeds 80, the austenitic stainless steel exhibits rapid strain-induced martensite transformation behavior with respect to deformation and plastic inhomogeneity may occur, leading to the problem of deterioration of the elongation of the austenitic stainless steel. On the contrary, when the value of Expression (2) is less than 30, it is difficult for the austenitic stainless steel to exhibit strain-induced martensite transformation behavior with respect to deformation, resulting in the inability to obtain a martensite phase and thus the inability to obtain ultra-high strength after skin pass cold rolling.

[0087] In addition, the high-strength austenitic stainless steel according to an embodiment of the present disclosure can satisfy the following Expression (3).

[0088] Expression (3): 16 ≤ 1 + 45C - 5Si + 0.09Mn + 2.2Ni - 0.28Cr - 0.67Cu + 88.6N ≤ 20

[0089] (In Expression (3), C, N, Si, Mn, Cr, Ni, and Cu respectively represent the contents (wt.%) of the elements).

[0090] Expression (3) is obtained by considering the dislocation slip behavior of austenitic stainless steel with respect to deformation. When the value of Expression (3) is less than 16, austenitic stainless steel strongly exhibits planar slip behavior with respect to deformation, resulting in plastic inhomogeneity and high work hardening due to the accumulation of dense dislocations caused by external stress. Therefore, there may be problems of deterioration of the elongation rate of austenitic stainless steel and difficulty in skin pass cold rolling. In addition, when hot deformation is carried out at high temperature, hot rolling defects such as edge cracks occur, increasing the risk of deterioration of productivity. On the contrary, when the value of Expression (3) exceeds 20, cross slip usually occurs, reducing the dislocation accumulation in the steel, or forming dislocation clusters and dislocation cells through deformation, thereby reducing the strength of the material. Since the effect of forming such dislocation clusters and dislocation cells increases with the increase in the number of skin pass cold rolling processes, in the case of steel according to the present disclosure where the number of skin pass cold rolling is high and ultra-high strength is to be obtained, the desired strength cannot be obtained. More preferably, the upper limit of Expression (3) can be 19. When the value of Expression (3) exceeds 19, since the yield strength and tensile strength of the cold-rolled material are similar to each other, the strength characteristics of austenitic stainless steel may deteriorate.

[0091] In addition, the high-strength austenitic stainless steel according to an embodiment of the present disclosure can satisfy the following Expression (4).

[0092] Expression (4):

[0093]

[0094] (In Expression (4), C, N, Si, Mn, Cr, Ni, Cu, and Nb respectively represent the contents (wt.%) of the elements).

[0095] Expression (4) is obtained by considering the hot workability and considering the fraction of δ-ferrite that significantly affects the hot workability. When the value of Expression (4) is less than 2, the fraction of δ-ferrite significantly decreases at high temperature, causing the material to exist as single-phase austenite during hot working, grain boundary growth, and segregation of S and P at the grain boundaries, thereby causing cracks in the material. The cracks generated as described above reduce the output percentage of the material, resulting in a problem of deterioration of productivity. On the contrary, when the value of Expression (4) exceeds 10, the fraction of δ-ferrite with poor workability increases excessively and the austenite-ferrite phase boundary that is easily affected by deformation increases, deteriorating the hot workability and deteriorating the productivity. More preferably, the lower limit of Expression (4) can be set to 3. When the value of Expression (4) is less than 3, the yield strength and tensile strength of the cold-rolled material are similar to each other, deteriorating the strength characteristics of austenitic stainless steel.

[0096] Therefore, by satisfying the composition and content ranges of the above alloying elements and satisfying all of Expressions (1) to (4), the high-strength austenitic stainless steel according to the present disclosure can have high yield strength, tensile strength, and elongation and maintain high formability, and can have excellent price competitiveness and productivity.

[0097] Specifically, the high-strength austenitic stainless steel according to an embodiment of the present disclosure can have a yield strength of 450 MPa or greater after cold rolling and annealing and a yield strength of 1,800 MPa or greater after skin pass cold rolling. In this regard, the upper limit of the yield strength after cold rolling and annealing can be set to 1,000 MPa and the upper limit of the yield strength after skin pass cold rolling can be set to 2,500 MPa, but is not limited thereto.

[0098] In addition, the high-strength austenitic stainless steel according to an embodiment of the present disclosure can have an elongation of 45% or greater after cold rolling and annealing and an elongation of 3% or greater after skin pass cold rolling. In this regard, the upper limit of the elongation after cold rolling and annealing can be, for example, 70%, and the upper limit of the elongation after skin pass cold rolling can be 10%, but is not limited thereto.

[0099] Hereinafter, a method for producing the above high-strength austenitic stainless steel will be described.

[0100] Conventionally, as a method for improving the yield strength of austenitic stainless steel, a method of performing final annealing at a low temperature below 1,000 °C has been introduced. Low-temperature annealing is a method of utilizing the energy accumulated in the steel during cold rolling without completing recrystallization. However, austenitic stainless steel to which low-temperature annealing has been applied may have the following disadvantages: non-uniform distribution of elements, insufficient pickling effect during subsequent pickling processes, and poor surface appearance aesthetically.

[0101] Therefore, the present disclosure provides a highly ductile and high-strength austenitic stainless steel having high yield strength and high yield ratio even after cold annealing at a temperature of 1,000 °C or higher.

[0102] Specifically, a method for producing a high-strength austenitic stainless steel according to an embodiment of the present disclosure includes: heating and hot rolling a steel billet, the steel billet containing, by weight percentage (wt%): C greater than 0.1% to 0.2%, N of 0.2% to 0.3%, Si of 0.8% to 1.5%, Mn of 7.0% to 8.5%, Cr of 15.0% to 17.0%, Ni of 0.5% or less (excluding 0), Cu of 1.0% or less (excluding 0), Nb of 0% to 0.2%, and the balance being Fe and inevitable impurities; performing hot annealing on the hot-rolled steel plate; performing cold rolling on the hot-rolled and annealed steel plate; and performing cold annealing on the cold-rolled steel plate, wherein the steel billet satisfies the following expression (1).

[0103] Expression (1): 14 ≤ 23(C + N) + 1.3Si + 0.24(Cr + Ni + Cu) + 0.1Mn

[0104] (In Expression (1), C, N, Si, Mn, Cr, Ni, and Cu respectively represent the contents (wt%) of the elements).

[0105] As described above, according to the method of the present disclosure, by using a steel billet that satisfies the composition and content range of alloy elements and satisfies Expression (1), a high-strength austenitic stainless steel having a high yield strength of 1,800 MPa or greater in a final product with high formability can be produced.

[0106] In addition, the advantages are that since no cracks occur, the yield percentage and productivity are excellent, and high price competitiveness is obtained by reducing the content of high-price alloy elements such as nickel (Ni) as low as possible.

[0107] Hereinafter, a method for producing a high-strength austenitic stainless steel according to an embodiment of the present disclosure will be described in more detail.

[0108] First, an operation of heating and hot rolling a steel billet is performed, the steel billet containing, by weight percentage (wt%): C greater than 0.1% to 0.2%, N of 0.2% to 0.3%, Si of 0.8% to 1.5%, Mn of 7.0% to 8.5%, Cr of 15.0% to 17.0%, Ni of 0.5% or less (excluding 0), Cu of 1.0% or less (excluding 0), Nb of 0% to 0.2%, and the balance being Fe and inevitable impurities. In this regard, the reasons for the numerical limitations of the alloy element contents and the reasons for satisfying Expression (1) are as described above, so the repeated description thereof will be omitted. As described above, the steel billet according to an embodiment of the present disclosure can satisfy Expression (2), Expression (3), and Expression (4), and the reasons for satisfying them are also as described above, so the repeated description thereof will be omitted.

[0109] In this regard, the temperature conditions for heating the steel billet can be the temperatures commonly used for rolling. For example, the steel billet can be heated at a temperature of 1,100°C to 1,300°C for 1 hour to 3 hours and then hot-rolled.

[0110] Subsequently, the hot-rolled steel plate can be subjected to thermal annealing. This process can also be carried out using ordinary methods. For example, the hot-rolled steel plate can be annealed at a temperature in the range of 1000°C to 1,150°C for 10 seconds to 10 minutes.

[0111] Subsequently, the thermally annealed steel plate can be cold-rolled to produce a thin plate. In this case, a cooling process can be carried out before the rolling process, and the cooling process can be carried out by water quenching. The cold rolling can be carried out under ordinary conditions, for example, at a reduction rate of 50% or more, but is not limited thereto.

[0112] Subsequently, the cold-rolled steel plate can be subjected to cold annealing. Specifically, the cold annealing can be carried out at a temperature of 1000°C or higher for 10 seconds to 10 minutes. Different from the conventional low-temperature annealing method that is carried out at a temperature below 1000°C to improve the yield strength of austenitic stainless steel and results in uneven distribution of elements, insufficient pickling during the subsequent pickling process, and poor surface appearance aesthetically, although the cold annealing is carried out at a temperature higher than 1000°C, the austenitic stainless steel according to the present disclosure has a yield strength of 450 MPa or more and an elongation of 45% or more.

[0113] As described above, even when using ordinary cold annealing conditions instead of low-temperature annealing, high strength can be obtained through a process of adjusting alloy elements without causing a load in terms of production and distribution, so that the price competitiveness can be further improved.

[0114] Additionally, the method for producing high-strength austenitic stainless steel according to an embodiment of the present disclosure may further include skin pass rolling the cold-annealed steel plate, and a higher level of strength can be obtained through the skin pass rolling.

[0115] Conventional skin pass rolling is a method that utilizes the phenomenon of increased work hardening when the austenite phase transforms into strain-induced martensite during cold deformation or a method that utilizes the dislocation pile-up of steel. Excellent strength can be obtained by appropriately utilizing phase transformation and dislocation pile-up. On the contrary, in the case of austenitic stainless steel that satisfies the above alloy elements and relational expressions, by appropriately controlling phase transformation and dislocation behavior, after skin pass rolling, the yield strength can be 1800 MPa or more. In this case, the skin pass rolling can be carried out at a reduction rate of 60% to 85%, but is not limited thereto.

[0116] The high-strength austenitic stainless steel according to the present disclosure can be used for general products for forming, for example, and can also be produced into products such as steel billets, blooms, billets, coils, strips, sheets, plates, bars, wire rods, sections, pipes, or tubes.

[0117] Hereinafter, the high-strength austenitic stainless steel according to the present disclosure and a method for producing the same will be described in detail with reference to the following examples and comparative examples. However, the following examples are only used as references for describing the present disclosure in detail, and the present disclosure is not limited to the exemplary embodiments described below, but can be embodied in many different forms.

[0118] In addition, unless otherwise defined, all technical terms and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art. The terms used herein are used to effectively describe specific examples and are not intended to limit the scope of the present disclosure. In addition, unless otherwise defined, the unit of the additive can be wt%.

[0119] [Examples 1 to 3 and Comparative Examples 1 to 19]

[0120] The compositions (wt%) of the elements of the steel grades used in Examples 1 to 3 and Comparative Examples 1 to 19 and the values of Expressions (1) to (4) are shown in Table 1 below.

[0121] Steel billets having the compositions of alloy elements shown in Table 1 below were prepared by ingot melting, heated at 1,250 °C for 2 hours, and hot-rolled. After hot-rolling, hot annealing was performed at 1,100 °C for 90 seconds. Then, cold rolling was performed at a reduction rate of 70% and cold annealing was performed at 1,100 °C for 10 seconds to obtain cold-rolled and annealed materials.

[0122] In addition, the cold-annealed samples were skin-passed at a reduction rate of 70% to prepare skin-passed materials, respectively.

[0123] [Table 1]

[0124]

[0125]

[0126] [Evaluation of Physical Properties]

[0127] The physical properties of the samples prepared in Examples 1 to 3 and Comparative Examples 1 to 19 were measured, respectively. Specifically, tensile tests were performed at room temperature according to ASTM standards, and the yield strength (YS, MPa), tensile strength (TS, MPa), and elongation (EL, %) and the occurrence of cracks during hot rolling of the cold-rolled and annealed materials are shown in Table 2 below.

[0128] [Table 2]

[0129]

[0130]

[0131] Referring to Table 2, in the case of Examples 1 to 3, since the composition of alloying elements provided in the present disclosure and the ranges of the values of Expressions (1), (2), (3), and (4) are satisfied, a yield strength of 450 MPa or greater and an elongation of 45% or greater are achieved after cold annealing. Based on such high yield strength and elongation, it is determined that the austenitic stainless steel of the present disclosure can be used as a structural material with a complex shape and has high utility value.

[0132] In addition, in Examples 1 to 3, the skin-passed cold-rolled material obtained after skin-pass cold rolling of the cold-annealed sample at a reduction rate of 70% has a high-strength property of 1800 MPa or greater. Such a high yield strength after deformation means that the stability of the structural steel as the final product can be further improved.

[0133] In addition, Examples 1 to 3 have an increased percentage of output rate and improved productivity due to no occurrence of cracks during hot rolling by obtaining sufficient hot workability, and have excellent price competitiveness by significantly reducing the content of nickel (Ni).

[0134] On the contrary, the austenitic stainless steels according to Comparative Examples 1 and 2, which are commercially available austenitic stainless steels, are steel grades that do not satisfy the composition of alloying elements according to the present disclosure. Comparative Examples 1 and 2 have a low yield strength of less than 300 MPa due to not satisfying Expression (1), and have a low yield strength after skin-pass cold rolling due to the value of Expression (2) being lower than the value of Expression (2) provided in the present disclosure. In addition, the commercially available austenitic stainless steel has a problem of poor price competitiveness due to the addition of a large amount of nickel (Ni).

[0135] Comparative Example 3 exhibits a low yield strength of about 400 MPa due to not satisfying Expression (1) and has a problem of poor price competitiveness due to the addition of a large amount of nickel (Ni).

[0136] Comparative Example 4 has a poor elongation due to the value of Expression (3) being lower than the value of Expression (3) provided in the present disclosure, and thus severe plastic non-uniformity occurs during deformation. In addition, although the amount of δ-ferrite is appropriate during hot working due to the satisfactory value of Expression (4), cracks are observed during hot working due to the low value of Expression (3) and the high C content, resulting in a problem of deteriorating productivity.

[0137] Comparative Example 5 has poor elongation because a martensite phase is excessively formed during deformation due to a high value of Expression (2), and Comparative Example 6 has poor elongation because plastic non-uniformity is severe during deformation due to a low value of Expression (3).

[0138] Although Comparative Examples 7 to 9 exhibit excellent yield strength after cold annealing due to a high value of Expression (1), a high-level yield strength of 1800 MPa or more cannot be obtained after skin pass cold rolling because the value of Expression (2) is far lower than 30 and the value of Expression (3) is far greater than 20. In addition, due to a low value of Expression (4) and a high C content, hot workability is poor in Comparative Examples 7 to 9 and a large number of cracks appear during hot rolling.

[0139] Comparative Examples 10 and 11 have the following problems: it is difficult to obtain sufficient yield strength after annealing due to a low value of Expression (1), and the elongation of the cold-rolled and annealed material is poor because the value of Expression (2) is far higher than 80 and the value of Expression (3) is far lower than 16.

[0140] Comparative Example 12 not only has low ductility and toughness due to the formation of a large amount of S-based inclusions (MnS) due to excessive manganese (Mn), but also increases the manufacturing risk due to Mn soot generated during the steelmaking process.

[0141] Comparative Example 13 has excellent strength and elongation by adding nickel (Ni) in an amount of 1.1% by weight, but has a problem of slightly reducing its cost reduction effect.

[0142] Comparative Example 14 has poor productivity because excessive copper (Cu) causes the formation of a large amount of δ-ferrite in the steel billet, resulting in deterioration of hot workability and adverse effects on material properties, leading to the appearance of cracks.

[0143] The value of Expression (1) in Comparative Example 15 is slightly higher than the value of Expression (1) provided in the present disclosure, the value of Expression (2) in Comparative Example 16 is slightly lower than the value of Expression (2) provided in the present disclosure, and the value of Expression (3) in Comparative Example 17 is higher than the value of Expression (3) provided in the present disclosure. Therefore, a high-level yield strength higher than 1800 MPa cannot be obtained after skin pass cold rolling.

[0144] Comparative Example 18 has poor hot workability because the value of Expression (4) is lower than the value of Expression (4) provided in the present disclosure and a large number of cracks appear during hot rolling. Comparative Example 19 has poor hot workability because excessive δ-ferrite is caused by the value of Expression (4) exceeding the range provided in the present disclosure.

[0145] Although the present disclosure has been specifically described with reference to exemplary embodiments, those skilled in the art should understand that various changes and modifications in form and detail can be made without departing from the spirit and scope of the present disclosure.

[0146] Therefore, the spirit of the present disclosure should not be construed as being limited to the described embodiments, and all equivalent and equivalent solutions of the claims, as well as the appended claims, fall within the scope of the present disclosure.

[0147] Industrial Applicability

[0148] The present disclosure is applicable to various industrial fields such as automobiles and construction.

Claims

1. A high-strength austenitic stainless steel, comprising, by weight percentage (% by weight): 0.1% to 0.2% of C, 0.2% to 0.3% of N, 0.8% to 1.5% of Si, 7.0% to 8.5% of Mn, 15.0% to 17.0% of Cr, 0.3% or less but not including 0 of Ni, 1.0% or less but not including 0 of Cu, 0% to 0.2% of Nb, and the balance being Fe and inevitable impurities, and the high-strength austenitic stainless steel satisfies the following expressions (1) to (4): Expression (1): 14 ≤ 23(C + N) + 1.3Si + 0.24(Cr + Ni + Cu) + 0.1Mn ≤ 15.16 Expression (2): 30 ≤ 551 - 462(C + N) - 9.2Si - 8.1Mn - 13.7Cr - 29(Ni + Cu) - 68Nb ≤ 80 Expression (3): 16 ≤ 1 + 45C - 5Si + 0.09Mn + 2.2Ni - 0.28Cr - 0.67Cu + 88.6N ≤ 20 Expression (4): wherein in Expressions (1) to (4), C, N, Si, Mn, Cr, Ni, Cu, and Nb respectively represent the weight % content of the elements.

2. The high-strength austenitic stainless steel according to claim 1, wherein the high-strength austenitic stainless steel has a yield strength of 450 MPa or more after cold rolling and annealing and a yield strength of 1,800 MPa or more after skin pass cold rolling.

3. The high-strength austenitic stainless steel according to claim 1, wherein the high-strength austenitic stainless steel has an elongation of 45% or more after cold rolling and annealing and an elongation of 3% or more after skin pass cold rolling.

4. A method for producing the high-strength austenitic stainless steel according to claim 1, the method comprising: The steel billet is heated and hot-rolled, and the steel billet contains, by weight percentage (wt%), greater than 0.1% to 0.2% C, 0.2% to 0.3% N, 0.8% to 1.5% Si, 7.0% to 8.5% Mn, 15.0% to 17.0% Cr, 0.3% or less but not including 0 Ni, 1.0% or less but not including 0 Cu, 0% to 0.2% Nb, and the balance of Fe and unavoidable impurities; The hot-rolled steel plate is subjected to hot annealing in the temperature range of 1000 °C to 1150 °C; The hot-annealed steel plate is cold-rolled; and The cold-rolled steel plate is subjected to cold annealing at a temperature of 1000 °C or higher, wherein the steel billet satisfies the following expressions (1) to (4): Expression (1): 14 ≤ 23(C + N) + 1.3Si + 0.24(Cr + Ni + Cu) + 0.1Mn ≤ 15.16 Expression (2): 30 ≤ 551 - 462(C + N) - 9.2Si - 8.1Mn - 13.7Cr - 29(Ni + Cu) - 68Nb ≤ 80 Expression (3): 16 ≤ 1 + 45C - 5Si + 0.09Mn + 2.2Ni - 0.28Cr - 0.67Cu + 88.6N ≤ 20 Expression (4): where in expressions (1) to (4), C, N, Si, Mn, Cr, Ni, Cu, and Nb respectively represent the wt% content of the elements.

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