Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0242—Flattening; Dressing; Flexing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite

Definitions

• the present disclosure relates to an austenitic stainless steel and a method for manufacturing same, and more particularly, to a low-cost austenitic stainless steel having high strength and high formability and a method for manufacturing same.
• Vehicle market trends are changing from conventional internal combustion engine-based automotive industry toward battery-based eco-friendly vehicle markets. That is, conventional internal combustion engine vehicle markets which are of high interest in middle-sized or large-sized vehicles are changing toward battery-based vehicle markets which prefer small-sized or lightweight vehicles.
• Structural materials protecting batteries are required to have high strength in order to protect the batteries from the risk of safety accidents such as explosions or from external impact and for the safety of passengers, and the structural materials are also required to be lightweight to prevent weight of small-sized or lightweight vehicles from increasing.
• structural materials for protecting batteries general structural materials have become smaller in size and higher in strength to comply with environmental regulations. Accordingly, there is a need to develop materials with high productivity, excellent stability, high strength, and excellent formability applicable throughout the industry.
• Stainless steels are materials applicable throughout the industry due to excellent corrosion resistance. Particularly, austenitic stainless steels with excellent elongation have no problem in forming complex shapes to meet various needs of customers and are advantageous in terms of aesthetic appearance.
• austenitic stainless steels have lower yield strength compared to common carbon steels and are economically disadvantageous because expensive alloying elements are used therein. Therefore, there is a need to develop stainless steels for structural materials having high levels of yield strength and proper tensile strength with excellent formability maintained.
• a low-cost austenitic stainless steel having high strength and high formability includes, in percent by weight (wt%), greater than 0% and at most 0.08% of C, 0.2 to 0.25% of N, 0.8 to 1.5% of Si, 8.0 to 9.5% of Mn, 15.0 to 16.5% of Cr, greater than 0% and at most 1.0% of Ni, 0.8 to 1.8% of Cu, and the remainder of Fe and other unavoidable impurities and satisfies Expressions (1) to (4) below: (1) Ni+0.47Mn+15N ⁇ 7.5 (2) 23(C+N)+1.3Si+0.24(Cr+Ni+Cu)+0.1Mn ⁇ 12 (3) 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu) ⁇ 70 (4) 11 ⁇ 1+45C-5Si+0.09Mn+2.2Ni-0.28Cr-0.67Cu+88.6N
• a yield strength of a cold-rolled, annealed steel sheet may be 400 MPa or more.
• an elongation of a cold-rolled, annealed steel sheet may be 55% or more.
• a yield strength of a skin pass-rolled steel sheet may be 800 MPa or more.
• an elongation of the skin pass-rolled steel sheet may be 25% or more.
• a method for manufacturing a low-cost austenitic stainless steel having high strength and high formability includes: preparing a slab including, in percent by weight (wt%), greater than 0% and at most 0.08% of C, 0.2 to 0.25% of N, 0.8 to 1.5% of Si, 8.0 to 9.5% of Mn, 15.0 to 16.5% of Cr, greater than 0% and at most 1.0% of Ni, 0.8 to 1.8% of Cu, and the remainder of Fe and other unavoidable impurities and satisfying Expressions (1) to (4) below; hot rolling the slab to prepare a hot-rolled steel sheet and hot annealing the hot-rolled steel sheet to prepare a hot-rolled, annealed steel sheet; cold rolling the hot-rolled, annealed steel sheet to prepare a cold-rolled steel sheet and cold annealing the cold-rolled steel sheet at a temperature of 1050°C or higher to prepare a cold-rolled, annealed steel sheet; and skin
• the skin pass rolling may be performed at a reduction ratio of 20% or more.
• the slab may have a reduction of area of 50% or more at a high temperature of 800°C or higher.
• an austenitic stainless steels having excellent yield strength in which a cold-rolled, annealed steel sheet prepared by cold annealing at a temperature of 1050°C or higher after cold rolling has excellent yield strength and excellent elongation sufficient for forming may be obtained after skin pass rolling performed to further increase strength. Also, a low-cost austenitic stainless steel having high strength and high formability with high productivity even using reduced amounts of expensive alloying elements may be provided.
• a low-cost austenitic stainless steel having high strength and high formability includes, in percent by weight (wt%), greater than 0% and at most 0.08% of C, 0.2 to 0.25% of N, 0.8 to 1.5% of Si, 8.0 to 9.5% of Mn, 15.0 to 16.5% of Cr, greater than 0% and at most 1.0% of Ni, 0.8 to 1.8% of Cu, and the remainder of Fe and other unavoidable impurities and satisfies Expressions (1) to (4) below.
• a low-cost austenitic stainless steel having high strength and high formability includes, in percent by weight (wt%), greater than 0% and at most 0.08% of C, 0.2 to 0.25% of N, 0.8 to 1.5% of Si, 8.0 to 9.5% of Mn, 15.0 to 16.5% of Cr, greater than 0% and at most 1.0% of Ni, 0.8 to 1.8% of Cu, and the remainder of Fe and other unavoidable impurities.
• Carbon (C) as an element effective on stabilizing an austenite phase, is added to obtain a yield strength of an austenitic stainless steel.
• an excess of C may not only deteriorate cold workability due to solid strengthening effect but also may induce grain boundary precipitation of a Cr carbide thereby adversely affecting ductility, toughness, corrosion resistance, and the like and deteriorating welding properties among the elements. Therefore, an upper limit thereof may be set to 0.08 wt%.
• Nitrogen (N) is the most important element in the present disclosure. Nitrogen is a strong austenite-stabilizing element effective on enhancing corrosion resistance and yield strength of an austenitic stainless steel. However, an excess of N may cause occurrence of defects such as nitrogen pores while a slab is made and deteriorate cold workability due to solid solution strengthening effect. Therefore, an upper limit thereof may be set to 0.25 wt%.
• Si acting as a deoxidizer during a steelmaking process, is an element effective for improving corrosion resistance. Also, Si is an effective element for increasing yield strength of steel materials among substitutional elements. In consideration of these effects, Si may be added in an amount of 0.8 wt% or more in the present disclosure. However, an excess of Si, as a ferrite phase-stabilizing element, may promote formation of delta ( ⁇ ) ferrite in a cast slab, thereby not only deteriorating hot workability but also deteriorating ductility and impact characteristics of steel materials. Therefore, an upper limit of the Si content may be set to 1.5 wt%.
• Manganese (Mn) as an austenite phase-stabilizing element added as a Ni substitute, may be added in an amount of 8.0 wt% or more to enhance cold workability by inhibiting formation of strain-induced martensite.
• MnS S-based inclusions
• Mn fumes an excess of Mn
• an upper limit of the Mn content may be set to 9.5 wt%.
• Chromium (Cr) is a ferrite-stabilizing element but effective on suppressing formation of a martensite phase.
• Cr may be added in an amount of 15% or more.
• an excess of Cr, as a ferrite-stabilizing element may promote formation of delta ( ⁇ ) ferrite in a slab in large quantity resulting in deterioration of hot workability and adverse effects on material characteristics. Therefore, an upper limit thereof may be set to 16.5 wt%.
• Ni is an expensive element, costs of raw materials may increase in the case of adding a large amount of Ni. Therefore, an upper limit of the Ni content may be set to 1.0% in consideration of both costs and efficiency of steel materials.
• Cu Copper
• Ni nickel
• Cu copper
• Cu as an austenite phase-stabilizing element added instead of nickel (Ni) in the present disclosure.
• Cu as an element improving corrosion resistance of steel materials under a reducing environment, may be added in an amount of 0.8 wt% or more.
• an excess of Cu not only increases costs of steel materials but also causes liquefaction and embrittlement at a low temperature.
• an excess of Cu may be segregated in edges of a slab, thereby deteriorating hot workability of steel materials.
• an upper limit of the Cu content may be set to 1.8 wt% in consideration of costs, efficiency, and properties of steel materials.
• the remaining component of the composition of the present disclosure is iron (Fe).
• the composition may include unintended impurities inevitably incorporated from raw materials or surrounding environments.
• addition of other unintended alloying elements in addition to the above-described alloying elements is not excluded.
• the impurities are not specifically mentioned in the present disclosure, as they are known to any person skilled in the art.
• unavoidable impurities examples include phosphorus (P) and sulfur (S), and at least one of P (at most 0.035 wt%) and S (at most 0.01 wt%) may be contained according to an embodiment of the present disclosure.
• Phosphorus (P) at most 0.035 wt%
• Phosphorus (P) as an impurity that is inevitably contained in steels, is a major causative element of grain boundary corrosion of steel materials or deterioration of hot workability, and therefore, it is preferable to control the P content as low as possible.
• an upper limit of the P content may be set to 0.035 wt%.
• S Sulfur
• an upper limit of S may be set to 0.01 wt%.
• the composition of alloying elements may further be limited to satisfy Expressions (1) to (4) in addition to the above-described composition.
• Mn, Ni, and N denote contents (wt%) of the elements, respectively.
• the austenitic stainless steel may include delta ferrite in an amount of 5% or more or phase transformation into martensite phase occurs during cold rolling. As a result, elongation of the austenitic stainless steel may deteriorate, and thus a lower limit of the value of Expression (1) may be set to 7.5 in the present disclosure to obtain a sufficient elongation.
• Expression (2) has been derived in the present disclosure in consideration that the yield strength is improved by a stress field of a steel material.
• C, N, Si, Mn, Cr, Ni, and Cu represent contents (wt%) of the elements, respectively.
• Expression (2) As the value of Expression (2) increases, a stress field between lattices increases due to size difference among the alloying elements so that a limit to withstand plastic deformation against external stress increases. When the value of Expression (2) is less than 12, it is difficult to obtain a yield strength required in the present disclosure. Therefore, a lower limit of the value of Expression (2) may be set to 12 in the present disclosure to obtain high strength characteristics.
• Expression (3) has been derived in the present disclosure.
• C, N, Si, Mn, Cr, Ni, and Cu represent contents (wt%) of the elements, respectively.
• the austenite phase is easily transformed by an external stress.
• the value of Expression (3) exceeds 70, the austenitic stainless steel exhibits a rapid strain-induced martensite transformation behavior, causing non-uniform plastic processing. As a result, a problem of deteriorating elongation of the austenitic stainless steel may occur, and thus a lower limit of the value of Expression (3) may be set to 70.
• C, N, Si, Mn, Cr, Ni, and Cu represent contents (wt%) of the elements, respectively.
• Expression (4) As the value of Expression (4) decreases, expression of cross slip of an austenite phase by an external stress becomes difficult.
• the value of Expression (4) is less than 11, the austenitic stainless steel exhibits only a planar slip behavior with respect to deformation and dislocation is rapidly piled up by an external stress. As a result, problems of non-uniform plastic processing and high work hardening may occur. Accordingly, the elongation of the austenitic stainless steel may deteriorate, it may be difficult to perform the skin pass rolling, and hot rolling defects such as edge cracks may occur during deformation at a high temperature, thereby causing a problem of decreasing productivity.
• a lower limit of Expression (4) may be set to 11.
• an upper limit of the value of Expression (4) may be set to 17.
• the slab having the above-described composition of alloying elements may have a reduction of area of 50% or more at a high temperature of 800°C or higher.
• a yield strength of a cold-rolled, annealed steel sheet may be 400 MPa.
• an elongation of the cold-rolled, annealed steel sheet may be 55% or more.
• the "cold-rolled, annealed steel sheet” refers to a steel material prepared by treating a slab by hot rolling, annealing, cold rolling, and annealing.
• a yield strength of a skin pass-rolled steel sheet may be 800 MPa or more.
• a yield strength may be 800 MPa or more and an elongation may be 25% or more.
• the "skin pass-rolled steel sheet” refers to a steel material prepared by skin pass rolling the above-described cold-rolled, annealed steel sheet.
• the method for manufacturing the low-cost austenitic stainless steel having high strength and high formability includes: preparing a slab including, in percent by weight (wt%), greater than 0% and at most 0.08% of C, 0.2 to 0.25% of N, 0.8 to 1.5% of Si, 8.0 to 9.5% of Mn, 15.0 to 16.5% of Cr, greater than 0% and at most 1.0% of Ni, 0.8 to 1.8% of Cu, and the remainder of Fe and other unavoidable impurities and satisfying Expressions (1) to (4); hot rolling the slab to prepare a hot-rolled steel sheet and hot annealing the hot-rolled steel sheet to prepare a hot-rolled, annealed steel sheet; cold rolling the hot-rolled, annealed steel sheet to prepare a cold-rolled steel sheet and cold annealing the cold-rolled steel sheet at a temperature of 1050°C or higher to prepare a cold-rolled, annealed steel sheet, and skin pass rolling the cold-rolled, annealed steel sheet to prepare
• the slab having the above-described composition of alloying elements may be hot-rolled at a temperature of 1000 to 1300°C to prepare a hot-rolled steel sheet, and then annealed at a temperature of 1000 to 1100°C to prepare a hot-rolled, annealed steel sheet.
• annealing heat treatment may be performed for 10 seconds to 10 minutes.
• the hot-rolled, annealed steel sheet is cold-rolled to prepare a cold-rolled steel sheet and then annealed to prepare a cold-rolled, annealed steel sheet.
• low-temperature annealing heat treatment was performed at a low temperature of 1000°C or below after cold rolling as described above.
• the low-temperature annealing heat treatment is a method for increasing strength using energy accumulated in the steel material during cold rolling without completing recrystallization.
• under pickling may occur during a subsequent picking process or aesthetic appearance may not be obtained as well as the possibility of non-uniform quality.
• the hot-rolled, annealed steel sheet is cold-rolled to prepare a cold-rolled steel sheet, and then annealed at a temperature of 1050°C or higher to prepare a cold-rolled, annealed steel sheet.
• the annealing heat treatment may be performed for 10 seconds to 10 minutes.
• excellent elongation may be obtained because low-temperature annealing is not performed after cold rolling, and an appropriate level of yield strength may be obtained by designing the composition of alloying elements.
• the cold-rolled, annealed steel sheet according to the present disclosure may have a yield strength of 400 MPa or more.
• the cold-rolled, annealed steel sheet according to the present disclosure may have an elongation of 55% or more.
• a cold-rolled, annealed steel sheet may have an appropriate yield strength without performing low-temperature annealing treatment via a process which does not cause loads on production.
• high yield strength may be obtained via adjustment of the composition of alloying elements and subsequent skin pass rolling without performing low-temperature annealing treatment after cold rolling.
• the yield strength of the skin pass-rolled steel sheet may be 800 MPa or more.
• the skin pass rolling may be performed at a reduction ratio 20% or more according to the present disclosure.
• Skin pass rolling may increase strength by using a high work hardening phenomenon while the austenite phase is transformed into strain-induced martensite during cold deformation or using dislocation pile-up of a steel material.
• elongation of the steel material may rapidly deteriorate by skin pass rolling.
• a rapid decrease in elongation of a steel material, which is caused by skin pass rolling may be prevented by appropriately controlling phase transformation and dislocation behavior by designing the composition of alloying elements as described above.
• a low-cost austenitic stainless steel having high strength and high formability in which a skin pass-rolled steel sheet has a yield strength of 800 MPa or more and an elongation of 25% or more, may be provided according to an embodiment of the present disclosure.
• Slabs having compositions of allying elements shown in Table 1 below were prepared by ingot melting, heated at 1250°C for 2 hours, and hot-rolled to prepare hot-rolled steel sheets. Then, the hot-rolled steel sheets were subjected to annealing heat treatment at 1100°C for 90 seconds to prepare hot-rolled, annealed steel sheets. Subsequently, the steel materials were cold-rolled at a reduction ratio of 70% to prepare cold-rolled steel sheets and subjected to annealing heat treatment at 1100°C for 10 seconds to prepare cold-rolled, annealed steel sheets.
• Yield strength, tensile strength, and elongation of the each of the cold-rolled, annealed steel sheets of the inventive examples and comparative examples were measured. Also, yield strength, tensile strength, and elongation of skin pass-rolled steel sheets respectively prepared by skin pass rolling the cold-rolled, annealed steel sheets according to the inventive examples and comparative examples by 20% were measured.
• the measurement of the yield strength, tensile strength, and elongation was carried out according to the ASTM standards, and the measured yield strength (YS, MPa), tensile strength (TS, MPa) and elongation (EL, %) are shown in Table 2 below. Also, occurrence of cracks in annealed materials was measured after a 180° adhesion bending test and results are shown in Table 2 below.
• the steel material according to Comparative Example 1 as a commercially available standard austenitic stainless steel, had a low yield strength because the steel material did not satisfy the composition of alloying elements of the present disclosure and Expressions (2), (3), and (4). Also, the commercial austenitic stainless steel of Comparative Example 1 had inferior price competitiveness due to the high Ni content of 8.1 wt% which is far higher than that of the Ni content according to the present disclosure.
• Comparative Example 2 does not satisfy Expression (1), a considerable amount of initial delta ferrite remains in the steel material after cold rolling and annealing. Cracks easily occur at an interface between delta ferrite phase and austenite phase during a forming process such as bending a steel material due to a phase difference, and thus a low value of Expression (1) involves cracks when bent. As a result, although Comparative Example 2 exhibited a high yield strength due to the high Si content and a high elongation, cracks occurred by the bending test due to the remaining delta ferrite indicating inferior formability including bending characteristics.
• All of the steel materials according to Comparative Examples 3 to 5 are steel types not satisfying Expressions (1) to (4). Because Expression (1) was not satisfied, considerable amounts of initial delta ferrite remained in the steel materials after cold rolling and annealing, and thus formability including bending characteristics was inferior. In addition, because Expression (2) was not satisfied, low yield strengths were obtained. In addition, because the value of Expression (3) exceeds 100, plastic non-uniformity easily occurs due to phase transformation into strain-induced martensite. In addition, due to the too low value of Expression (4), serious dislocation pile-up occurred by planar slip. As a result, elongation deteriorated. Particularly, elongations of Comparative Examples 3 to 5, which deteriorate because Expressions (3) and (4) were not satisfied, further deteriorated after skin pass rolling, so that physical properties of the steel materials were not suitable as skin pass-rolled steel sheets.
• Comparative Example 6 inferior formability including bending characteristics was obtained because Expression (1) was not satisfied and thus a considerable amount of initial delta ferrite remained in the steel material after cold rolling and annealing. In addition, although the steel material of Comparative Example 6 had the high yield strength due to the high Si content and Expression (2), the elongation was not sufficient due to effects of Expressions (3) and (4).
• the steel material of Comparative Example 7 had inferior formability including bending characteristics because Expression (1) was not satisfied and thus a considerable amount of initial delta ferrite remained in the steel material after cold rolling and annealing. Also, plastic non-uniformity easily occurs during deformation due to phase transformation into strain-induced martensite because the value of Expression (3) was over 100, which did not satisfy Expression (3). Therefore, the cold-rolled, annealed steel sheet and the skin pass-rolled steel sheet had inferior elongation.
• Comparative Example 8 satisfied the contents of the alloying elements except for Cu and satisfied Expressions (1) to (4).
• the cold-rolled, annealed steel sheet had excellent yield strength and elongation.
• Comparative Example 8 had inferior hot workability due to an excessive Cu content. Evaluation thereof will be described below in more detail with reference to Table 3.
• the steel materials according to Comparative Examples 9 and 10 had inferior hot workability due to excessive amounts of Si and Cu. Evaluation thereof will be described below in more detail with reference to Table 3.
• the steel materials according to Comparative Examples 11 and 12 had inferior formability including bending characteristics due to a considerable amount of initial delta ferrite remaining in the steel material after cold rolling and annealing because Expression (1) was not satisfied. Also, plastic non-uniformity, in which stress is concentrated on weak parts of the steel materials, increased due to frequent cross slip in Comparative Examples 11 and 12 because the values of Expression (4) were too high. As a result, the cold-rolled, annealed steel sheet and the skin pass-rolled steel sheet had inferior elongation. Although effects of the stress concentrated by cross slip on elongation are negligible in commercial steel materials, elongation significantly deteriorate in high-strength steel materials having too high values of Expression (2) as in Comparative Examples 11 and 12.
• the austenitic stainless steel according to the present disclosure has excellent price competitiveness due to high productivity and high actual yield due to excellent hot workability.
• reduction of area was measured in slabs of several comparative examples with high elongation and the inventive examples at different temperatures. Measurement of the reduction of area was performed according to the ASTM standards by a high-temperature tensile test, and results are shown in Table 3.
• the steel material according to Comparative Example 1 had excellent hot workability due to low amounts of Cu and N added to reduce the amounts of Si and Ni, which are required to increase strength.
• a large amount of Ni, which is an expensive element is contained in the commercial 300 series austenitic stainless steels, the 300 series austenitic stainless steels have considerably low price competitiveness.
• the steel material had inferior yield strength because the composition of alloying elements and Expressions (2), (3), and (4) were not satisfied.
• Comparative Examples 2, 6, 9, and 10 excessive amounts of Si were added to improve yield strength of the cold-rolled, annealed steel sheets and excessive amounts of Cu replacing Ni were added for price competitiveness.
• the steel materials according to Comparative Examples 2, 6, 9, and 10 had low hot workability due to excessive amounts of Si and Cu.
• the steel material according to Comparative Example 7 had excellent hot workability. However, as evaluated in Table 2, the steel material had inferior formability because Expression (1) was not satisfied and had inferior elongation of the cold-rolled, annealed steel sheet and the skin pass-rolled steel sheet because Expression (3) was not satisfied.
• Comparative Example 8 The Cu content of Comparative Example 8 exceeded the range suggested by the present disclosure. Excessive Cu was segregated on edges or surface of slabs causing liquid metal embrittlement, thereby deteriorating hot workability of Comparative Example 8. In Comparative Example 8, due to inferior hot workability, actual yield may decrease due to edge cracks occurring after hot rolling, correcting costs therefor may increase, or investment for additional equipment to reduce edge cracks may be required.
• a low-cost austenitic stainless steel having high strength and high formability applicable throughout various industrial fields may be provided.

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• 2020
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