EP4177368A1 - Austenitischer rostfreier stahl mit verbesserter tiefziehfähigkeit - Google Patents

Austenitischer rostfreier stahl mit verbesserter tiefziehfähigkeit Download PDF

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EP4177368A1
EP4177368A1 EP21861894.0A EP21861894A EP4177368A1 EP 4177368 A1 EP4177368 A1 EP 4177368A1 EP 21861894 A EP21861894 A EP 21861894A EP 4177368 A1 EP4177368 A1 EP 4177368A1
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Prior art keywords
stainless steel
austenitic stainless
less
work
hardening
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English (en)
French (fr)
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EP4177368A4 (de
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Kyung-Hun Kim
Jisoo Kim
Jongjin JEON
Mi-Nam Park
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Posco Holdings Inc
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Posco Co Ltd
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Definitions

  • the present disclosure relates to an austenitic stainless steel with improved deep drawability, and more particularly, to an austenitic stainless steel in which cracks do not occur during a deep drawing process applied for transforming a plate into three-dimensional parts.
  • Deep drawing is an efficient method for reducing manufacturing costs by omitting additional processes such as welding and stress-removing heat treatment. Meanwhile, in the case of involving formation of cylindrical shapes such as a cup or a battery, materials having excellent deep drawability are required.
  • Austenitic stainless steel materials may be used to form in complex shapes without causing any problem due to high elongation and have excellent work-hardening ability, as a steel type applied to various fields involving deep drawing.
  • austenitic stainless steels are deformed by work-hardening occurring during cold working. In this case, it has been known that austenitic stainless steels having excellent work-hardening ability are easily formed.
  • an austenitic stainless steel capable of obtaining forming processibility when applied to deep drawing by minimizing an increase in strength caused by work hardening.
  • an austenitic stainless steel with improved deep drawability includes, in percent by weight (wt%), 0.01 to 0.05% of C, 0.01 to 0.25% of N, 1.5% or less of Si (excluding 0), 0.3 to 3.5% of Mn, 17.0 to 22.0% of Cr, 9.0 to 14.0% of Ni, 2.0% or less of Mo (excluding 0), 0.2 to 2.5% of Cu, and the balance of Fe and inevitable impurities, and satisfying Expression (1) below: Cr + Si + 2 * Mo + 3 * Ni + Cu + 50 * C + N ⁇ 63 wherein Cr, Si, Mo, Ni, Cu, C, and N represent the content (wt%) of each element.
  • the austenitic stainless steel may satisfy Expression (2) below: 0 ⁇ 2.4 * Cr + 1.7 * Mo + 3.9 * Si ⁇ 2.1 * Ni ⁇ Mn ⁇ 0.4 * Cu ⁇ 58 * C ⁇ 64 * N ⁇ 13 ⁇ 5.5 wherein Cr, Mo, Si, Ni, Mn, Cu, C, and N represent the content (wt%) of each element.
  • the austenitic stainless steel may further include at least one of 0.04% or less of Al (excluding 0), 0.003% or less of Ti (excluding 0), 0.0025% or less of B (excluding 0), 0.035% or less of P, and 0.0035% or less of S.
  • a difference between a true strain value at the maximum work-hardening exponent and a true strain value at a work-hardening exponent of 0 may be 0.11 or more.
  • an elongation may be 35% or more.
  • a tensile strength may be 360 MPa or more.
  • cracks do not occur until a fifth stage in the case of multi-stage formation at a drawing ratio of 1.7 to 4.3.
  • an austenitic stainless steel applicable as a deep drawing material may be provided because intermediate heat treatment may be omitted and an increase in strength caused by work hardening may be minimized during deep drawing.
  • An austenitic stainless steel with improved deep drawability includes, in percent by weight (wt%), 0.01 to 0.05% of C, 0.01 to 0.25% of N, 1.5% or less of Si (excluding 0), 0.3 to 3.5% of Mn, 17.0 to 22.0% of Cr, 9.0 to 14.0% of Ni, 2.0% or less of Mo (excluding 0), 0.2 to 2.5% of Cu, and the balance of Fe and inevitable impurities and satisfies Expression (1) below.
  • Cr, Si, Mo, Ni, Cu, C, and N represent the content (wt%) of each element.
  • An austenitic stainless steel is a steel type used in products having various shapes due to high elongation and excellent formability. Under a stress, the austenitic stainless steel is deformed by transformation, i.e., transformation induced plasticity, from an unstable austenite phase to a martensite phase, at room temperature.
  • the generated martensite phase has a high strength
  • strength of the material also increases.
  • the work-hardening ability is represented using a work-hardening exponent, and the work-hardening exponent varies according to strain.
  • Austenitic stainless steels having excellent work-hardening ability have been known to easy formed.
  • work-hardening of the austenitic stainless steel is related to the degree of stability of an austenite phase.
  • the work-hardening of the austenitic stainless steel may be inhibited by increasing the degree of stability by controlling elements.
  • the present inventors have made various studies to enhance elongation of an austenitic stainless steel and inhibit an increase in strength caused by work-hardening during a deep drawing process and have found those described below.
  • An austenitic stainless steel with improved deep drawability include, in percent by weight (wt%), 0.01 to 0.05% of C, 0.01 to 0.25% of N, 1.5% or less of Si (excluding 0), 0.3 to 3.5% of Mn, 17.0 to 22.0% of Cr, 9.0 to 14.0% of Ni, 2.0% or less of Mo (excluding 0), 0.2 to 2.5% of Cu, and the balance of Fe and inevitable impurities.
  • the content of C is from 0.01 to 0.05%.
  • Carbon (C) is an element effective on stabilization of an austenite phase and may be added in an amount of 0.01% or more to inhibit formation of martensite and obtain strength during deformation.
  • an excess of C may bind to Cr to induce grain boundary precipitation of a Cr carbide, thereby deteriorating corrosion resistance. Therefore, an upper limit the C content may be controlled to 0.05%.
  • the content of N is from 0.01 to 0.25%.
  • the content of Si is 1.5% or less (excluding 0).
  • Si is an element serving as a deoxidizer during a steelmaking process and used to obtain strength and corrosion resistance of an austenitic stainless steel.
  • an excess of Si as a ferrite phase-stabilizing element, may promote martensite transformation and precipitate intermetallic compounds such as a ⁇ phase to deteriorate mechanical properties and corrosion resistance.
  • an upper limit of the Si content may be controlled to 1.5%.
  • the content of Mn is from 0.3 to 3.5%.
  • MnS S-based inclusions
  • an upper limit of the Mn content may be controlled to 3.5%.
  • the content of Cr is from 17.0 to 22.0%.
  • Chromium (Cr) stabilizes ferrite as a basic element contained in stainless steels in the largest amount among the elements used to improve corrosion resistance.
  • Cr may be added in an amount of 17.0% or more to obtain corrosion resistance by forming a passivated layer that inhibits oxidation.
  • an upper limit of the Cr content may be controlled to 22.0%.
  • the content of Ni is from 9.0 to 14.0%.
  • Nickel (Ni) is the strongest austenite phase-stabilizing element. As the Ni content increases, an austenite phase is stabilized to soften a material, and it is essential to include 9% or more of Ni to inhibit work-hardening caused by deformation-induced martensite. However, use of a large amount of Ni, which is a high-priced element, cases an increase in costs of raw materials. Therefore, an upper limit of the Ni content may be controlled to 14.0% in consideration of costs and efficiency of steel materials.
  • the content of Mo is 2.0% or less (excluding 0).
  • Molybdenum (Mo) is an element effective on obtaining corrosion resistance.
  • an excess of molybdenum, as a ferrite phase-stabilizing element may decrease stability of an austenite phase making it difficult to obtain deep drawability, and precipitate intermetallic compounds such as a ⁇ phase to deteriorate mechanical properties and corrosion resistance. Therefore, an upper limit of the Mo content may be controlled to 2.0%.
  • the content of Cu is from 0.2 to 2.5%%.
  • Copper (Cu) as an austenite phase-stabilizing element added instead of the high-priced nickel (Ni), may be added in an amount of 0.2% or more to enhance price competitiveness and deep drawability.
  • Cu content when the Cu content is excessive, ⁇ -Cu precipitates with a low-melting point are formed to deteriorate the surface quality.
  • an upper limit of the Cu content may be controlled to 2.5%.
  • the austenitic stainless steel may further include at least one of 0.04% or less of Al (excluding 0), 0.003% or less of Ti (excluding 0), 0.0025% or less of B (excluding 0), 0.035% or less of P, and 0.0035% or less of S.
  • the content of Al is 0.04% or less (excluding 0).
  • Aluminum (Al) as a strong deoxidizer, reduces a content of oxygen in molten steels.
  • Al Aluminum
  • an excess of Al may cause sleeve defects of a cold-rolled strip due to an increase in nonmetallic inclusions, and therefore an upper limit of the Al content may be controlled to 0.04%.
  • the content of Ti is 0.003% or less (excluding 0).
  • Titanium (Ti) is an element effective on corrosion resistance of a steel because Ti preferentially binds to interstitial elements such as carbon (C) or nitrogen (N) to form precipitates (carbonitrides), thereby reducing amounts of solute C and solute N in the steel and inhibits formation of a Cr depletion region.
  • interstitial elements such as carbon (C) or nitrogen (N) to form precipitates (carbonitrides), thereby reducing amounts of solute C and solute N in the steel and inhibits formation of a Cr depletion region.
  • an excess of Ti may form Ti-based inclusions causing a problem in a manufacturing process and a surface defect such as scabs, and therefore an upper limit of the Ti content may be controlled to 0.003%.
  • the content of B is 0.0025% or less (excluding 0).
  • B Boron
  • BN nitride
  • the content of P is 0.035% or less.
  • Phosphorus (P) as an impurity that is inevitably contained in steels, is a major element causing grain boundary corrosion 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 is controlled to 0.035%.
  • the content of S is 0.0035% or less.
  • S Sulfur
  • an upper limit of the S content is controlled to 0.0035% or less.
  • 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, and thus addition of other alloy components is not excluded. These impurities are known to any person skilled in the art of manufacturing and details thereof are not specifically mentioned in the present disclosure.
  • work-hardening of the austenitic stainless steel is caused by transformation of an austenite phase unstable at room temperature into a martensite phase by a stress generated by plastic deformation.
  • Expression (1) was derived in consideration of phase transformation occurring due to deformation of the austenitic stainless steel.
  • Cr, Si, Mo, Ni, Cu, C, and N represent the content (wt%) of each element.
  • the austenitic stainless steel with improved deep drawability satisfies a value, represented by Expression (1), of 63 or more.
  • the present inventors have found that a variation in strength during deformation caused by an external stress increases as the value of Expression (1) decreases. Specifically, when the value of Expression (1) is less than 63, the austenitic stainless steel including the above-described alloying elements exhibits a rapid deformation-induced martensite transformation behavior by external deformation, or plastic non-uniformity due formation of twin crystals. Accordingly, due to problems of deterioration in elongation of the austenitic stainless steel and reduction in deep drawability in multi-stage formation, a lower limit of Expression (1) is controlled to 63.
  • FIG. 1 is a graph for describing the relationship between stress and strain in a tensile test of a material.
  • a work-hardening exponent (n) indicating the degree of work-hardening ability may be represented by an equation below.
  • ⁇ K ⁇ n
  • ⁇ is a stress
  • K is a strength coefficient
  • ⁇ is a strain
  • the work-hardening exponent n corresponds to the slope of the graph, and a higher slope means a more increase in strength of a material during plastic deformation.
  • Cr, Mo, Si, Ni, Mn, Cu, C, and Nr represent the content (wt%) of each element.
  • the austenitic stainless steel with improved deep drawability satisfies a value, represented by Expression (2), of 0 or more and 5.5 or less.
  • the present inventors have found that as the value of Expression (2) increases, transformation into martensite more easily occurs by an external stress resulting in an excessive increase in a strength, thereby deteriorating formability. Specifically, when the value of Expression (2) is 5.5 or more, the strength continuously increases from tensile deformation to immediately before fracture resulting in a problem of occurrence of rapid fracture. Therefore, elongation cannot be obtained, and thus an upper limit of the value of Expression (2) is controlled to 5.5.
  • FIG. 2 is a graph illustrating the relationship between stress and strain together with work-hardening exponent in a tensile test of an austenitic stainless steel according to the disclosed embodiment.
  • the austenitic stainless steel with improved deep drawability may have a true strain value of 0.2 or less in the case where the work-hardening exponent is a maximum value.
  • a point where the work-hardening exponent is a maximum value is indicated as A, and a point where the work hardening exponent is 0 is indicated as B.
  • the work-hardening exponent decreases after point A although deformation proceeds. That is, it may be confirmed that the strength gradually increase from point A to point B.
  • the austenitic stainless steel with improved deep drawability has a true strain value of 0.2 or less when the work-hardening exponent is the maximum value.
  • a difference between a true stain value at the maximum work-hardening exponent and a true stain value at a work-hardening exponent of 0 is 0.11 or more.
  • the austenitic stainless steel having improved deep drawability according to the disclosed embodiment satisfying the composition ratio of alloying elements and the above-described relationship may have an elongation of 35% or more and a tensile strength of 360 MPa or more.
  • Slabs having compositions of alloying elements shown in Table 1 below and having a thickness of 200 mm were prepared by a continuous casting process, heated at 1,250°C for 2 hours, and hot-rolled to a thickness of 6 mm. After hot rolling, hot annealing was performed at 1,150°C and wound. Then, the hot-rolled coil was cold-rolled and cold-annealed twice to a thickness of 1 mm. The cold rolling was performed with a reduction ratio of 30 to 70% per pass, and the cold annealing was performed in a furnace at a temperature of 1100 to 1200°C within 5 minutes.
  • the number of multi-stage formation and the work-hardening exponent of each steel sheet were measured. Specifically, deep drawing formation was performed in five stages using a blank having a diameter of 85 mm with a first-stage punch diameter of 50 mm, a second-stage punch diameter of 38 mm, a third-stage punch diameter of 30 mm, a fourth-stage punch diameter of 24 mm, and a fifth-stage punch diameter of 20 mm. Drawing ratios of the respective stages were 1.7 at the first stage, 2.2 at the second stage, 2.8 at the third stage, 3.5 at the fourth stage, and 4.3 at the fifth stage.
  • Example 2 tensile strength (MPa) and elongation (%) measured during the tensile test are shown in Table 2 below.
  • Table 2 Example Maximum number of processing (a) (b) (c) (b)-(c) Tensile strength Elongation
  • Example 1 5 0.37 0.17 0.29 0.12 450 37.4
  • Example 2 5 0.36 0.18 0.29 0.12 451 37.5
  • Example 3 5 0.36 0.18 0.29 0.12 450 37.5
  • Example 4 5 0.30 0.17 0.30 0.14 441 42.2
  • Example 5 5 0.27 0.17 0.30 0.13 467 41.0
  • Example 6 5 0.35 0.17 0.32 0.15 401 46.0
  • Example 7 5 0.35 0.18 0.32 0.15 404 46.7
  • Example 8 5 0.35 0.18 0.32 0.14 402 46.2
  • Example 9 5 0.36 0.17 0.32 0.15 386 45.6
  • Example 10 5 0.36 0.17 0.32 0.15 387 45.4
  • Example 11 5 0.36 0.17 0.32 0.15 388 45.5
  • an austenitic stainless steel having an elongation of 35% or more and a tensile strength of 360 MPa or more without having cracks until the fifth formation in the case of forming at the second or more formation at a drawing ratio of 1.7 to 4.3 may be manufactured by controlling the alloying elements and the relationship therebetween.
  • the present disclosure is applicable to various industrial fields involving deep drawing.

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  • Organic Chemistry (AREA)
  • Crystallography & Structural Chemistry (AREA)
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EP21861894.0A 2020-08-31 2021-07-23 Austenitischer rostfreier stahl mit verbesserter tiefziehfähigkeit Pending EP4177368A4 (de)

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PCT/KR2021/009569 WO2022045595A1 (ko) 2020-08-31 2021-07-23 심가공성이 향상된 오스테나이트계 스테인리스강

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JPH0853738A (ja) * 1994-08-10 1996-02-27 Nisshin Steel Co Ltd 抗菌性に優れたコイン用オーステナイト系ステンレス鋼
JP3464297B2 (ja) * 1994-08-31 2003-11-05 日新製鋼株式会社 高速温間絞り成形用オーステナイト系ステンレス鋼板およびその温間絞り成型法
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JP5116265B2 (ja) * 2006-07-13 2013-01-09 新日鐵住金ステンレス株式会社 強度及び延性に優れたオーステナイト系ステンレス圧延鋼板及びその製造方法
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KR102120700B1 (ko) * 2018-09-13 2020-06-09 주식회사 포스코 확관가공성 및 내시효균열성이 우수한 오스테나이트계 스테인리스강
KR102173302B1 (ko) * 2018-11-12 2020-11-03 주식회사 포스코 비자성 오스테나이트계 스테인리스강 및 그 제조방법

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