US20230279531A1 - Austenitic stainless steel and manufacturing method thereof - Google Patents

Austenitic stainless steel and manufacturing method thereof Download PDF

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US20230279531A1
US20230279531A1 US18/019,570 US202118019570A US2023279531A1 US 20230279531 A1 US20230279531 A1 US 20230279531A1 US 202118019570 A US202118019570 A US 202118019570A US 2023279531 A1 US2023279531 A1 US 2023279531A1
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stainless steel
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Sangseok Kim
Mi-nam Park
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Posco Holdings Inc
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Posco Co Ltd
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • 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
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • 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
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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    • 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
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
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    • 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
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • 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 which may be utilized as various materials for an outer panel of a vehicle, a component for construction, and the like and a manufacturing method thereof.
  • Austenitic stainless steels have been applied for various uses to manufacture components for transportation and construction due to excellent formability, work hardenability, and weldability.
  • 304 series stainless steels or 301 series stainless steels which are representative general-use austenitic stainless steels, have low yield strengths of 200 to 350 MPa, and thus there are limits to apply these stainless steels to structural materials due to low strength.
  • a skin pass rolling process may additionally be performed as a method for increasing yield strength of austenitic stainless steels, this method causes problems in increasing manufacturing costs due to the additional process and significantly deteriorating elongation of materials.
  • Patent Document 1 discloses a method for manufacturing a 300 series stainless steel having a small curvature even after half etching by performing stress relief (SR) heat treatment twice after skin pass rolling a cold-rolled, annealed material for a laser metal mask for photoetching.
  • SR stress relief
  • Patent Document 1 relates to a manufacturing method to control etchability and curvature after etching. Because an austenitic stability parameter (ASP) value is from 30 to 50, strain-induced martensite transformation rapidly occurs during formation, resulting in deterioration of elongation.
  • ASP austenitic stability parameter
  • Patent Document 2 relates a nuclear power component.
  • heat treatment was performed for a long time over 48 hours at a temperature of 600 to 700° C. According to Patent Document 2, heat treatment is required for a long time, productivity decreases in the case of being implemented in a real production line, and manufacturing costs increase.
  • an ultrafine-grained 300 series stainless steel having simultaneously satisfying high yield strength and excellent elongation.
  • an austenitic stainless steel includes, in percent by weight (wt %), 0.005 to 0.03% of C, 0.1 to 1% of Si, 0.1 to 2% of Mn, 6 to 9% of Ni, 16 to 19% of Cr, 0.2% or less of N, and the remainder being Fe and unavoidable impurities, wherein assuming that a total thickness of the steel material is t, a value of an average grain size d in a thickness range of 1 ⁇ 4t to 3 ⁇ 4t is 5 ⁇ m or less, an ASP value represented by Formula (1) below is from 10 to 25, a value represented by Formula (2) below is 435 or more, and a value represented by Formula (3) below is 6000 or more.
  • [C], [N], [Si], [Mn], [Cr], [Ni], [Cu], [Mo], [Nb], and [V] represent weight percentages (wt %) of respective elements
  • YS represents a yield strength (MPa)
  • EL represents an elongation (%).
  • a value of Formula (4) below may be 200 or more.
  • Hv represents a Vickers hardness (Hv) and [Ni] and [Cr] represent weight percentages (wt %) of respective elements.
  • the austenitic stainless steel according to the present disclosure may further include, in percent by weight (wt %), at least one of 0.4% or less of Cu, 0.2% or less of Mo, 0.25% or less of Nb, and 0.25% or less of V.
  • the t may be from 0.4 to 2.0 mm.
  • a pitting potential measured by dipping in a 3.5% NaCl solution at 30° C., may be 250 mV or more.
  • a method of manufacturing the austenitic stainless steel includes: hot rolling a slab comprising, in percent by weight (wt %), 0.005 to 0.03% of C, 0.1 to 1% of Si, 0.1 to 2% of Mn, 6 to 9% of Ni, 16 to 19% of Cr, 0.2% or less of N, and the remainder being Fe and unavoidable impurities, and cold rolling the hot-rolled steel material at room temperature with a reduction ratio of 40% or more; and annealing the cold-rolled steel material at a temperature of 700 to 850° C.
  • the slab may be hot-rolled and then cold-rolled without being annealed.
  • the method of manufacturing the austenitic stainless steel according to the present disclosure may further include skin pass rolling with a reduction ratio of 60% or more.
  • the present disclosure may provide an ultrafine-grained 300 series stainless steel simultaneously satisfying high strength and high ductility.
  • the present disclosure provides an austenitic stainless steel simultaneously satisfying high strength and high ductility with high economic feasibility by reducing the contents of Ni and Cr which are high-priced alloying elements.
  • the present disclosure provides an austenitic stainless steel simultaneously satisfying high strength and high ductility with excellent corrosion resistance.
  • FIG. 1 is a graph showing ranges of values of Formulae (2) and (3) of inventive examples and comparative examples.
  • FIGS. 2 A to 2 C are views for comparison of average grain size d among the inventive examples and comparative examples.
  • FIGS. 3 A to 3 C are graphs showing Strain (%)-Stress (MPa) curves of inventive examples.
  • FIGS. 4 A to 4 C are graphs showing Strain (%)-Stress (MPa) curves of comparative examples.
  • An austenitic stainless steel includes, in percent by weight (wt %), 0.005 to 0.03% of C, 0.1 to 1% of Si, 0.1 to 2% of Mn, 6 to 9% of Ni, 16 to 19% of Cr, 0.2% or less of N, and the remainder being Fe and unavoidable impurities, wherein assuming that a total thickness of the steel material is t, a value of an average grain size d in a thickness range of 1 ⁇ 4t to 3 ⁇ 4t is 5 ⁇ m or less, an ASP value represented by Formula (1) below is from 10 to 25, a value represented by Formula (2) below is 435 or more, and a value represented by Formula (3) below is 6000 or more.
  • austenitic stainless steels have been used for various uses due to excellent formability, work hardenability, and weldability, yield strength thereof is low. In the case of performing skin pass rolling to increase yield strength, a problem of deteriorating elongation may occur.
  • the present inventors have focused on an ultrafine austenitic stainless steel as a steel material simultaneously satisfying high yield strength and excellent elongation. However, not all of ultrafine stainless steels satisfy both high yield strength and excellent elongation.
  • Ultrafine grained austenitic stainless steels may have considerably different yield strength and elongation, because the Ni content and the Cr content vary according to steel types, the amount of martensite transformation during cold working may vary according to the stability of the austenite phase, and tensile curve characteristics may vary according to the transformation induced plasticity (TRIP) transformation behavior.
  • TRIP transformation induced plasticity
  • An austenitic stainless steel includes, in percent by weight (wt %), 0.005 to 0.03% of C, 0.1 to 1% of Si, 0.1 to 2% of Mn, 6 to 9% of Ni, 16 to 19% of Cr, 0.2% or less of N, and the remainder being Fe and unavoidable impurities.
  • the austenitic stainless steel may further include at least one of 0.4% or less of Cu, 0.2% or less of Mo, 0.25% or less of Nb, and 0.25% or less of V.
  • the content of carbon (C) is from 0.005 to 0.03 wt %.
  • C is an austenite phase-stabilizing element.
  • C is added in an amount of 0.005 wt % or more.
  • the C content is controlled to 0.03 wt % or less in the present disclosure.
  • the content of silicon (Si) is from 0.1 to 1 wt %.
  • Si is an element added as a deoxidizer during a steel-making process and has an effect on improving corrosion resistance of a steel by forming an Si oxide in a passivated layer in the case of performing a bright annealing process.
  • Si is added in an amount of 0.1 wt % or more in the present disclosure.
  • the Si content is controlled to 1.0 wt % or less in the present disclosure.
  • the content of manganese (Mn) is from 0.1 to 1.0 wt %.
  • Mn is an austenite phase-stabilizing element.
  • Mn is added in an amount of 0.1 wt % or more in the present disclosure.
  • the Mn content is controlled to 1.0 wt % or less in the present disclosure.
  • the content of nickel (Ni) is from 6.0 to 9.0 wt %.
  • Ni as an austenite phase-stabilizing element, has an effect on softening a steel material.
  • Ni is added in an amount of 6.0 wt % or more in the present disclosure.
  • the Ni content is controlled to 9.0 wt % or less in the present disclosure.
  • the content of chromium (Cr) is from 16.0 to 19.0 wt %.
  • Cr is a major element for improving corrosion resistance of a stainless steel.
  • Cr is added in an amount of 16.0 wt % or more in the present disclosure.
  • the Cr content is controlled to 19.0 wt % or less in the present disclosure.
  • the content of nitrogen (N) is 0.2 wt % or less.
  • N is an austenite phase-stabilizing element and improves strength of a steel material.
  • the N content is controlled to 0.2 wt % or less in the present disclosure.
  • the content of copper (Cu) may be from 0.4 wt % or less.
  • Cu is an austenite phase-stabilizing element.
  • the Cu content is controlled to 0.4 wt % or less in the present disclosure.
  • the content of molybdenum (Mo) may be 0.2 wt % or less.
  • Mo has an effect on improving corrosion resistance and workability. However, since an excess of Mo causes a problem of increasing costs, the Mo content is controlled to 0.2 wt % or less in the present disclosure.
  • niobium (Nb) or vanadium (V) may be 0.25 wt % or less.
  • Nb and V have an effect on suppressing the growth of grains by forming (Nb,V)(C,N) precipitates.
  • Nb,V contents of Nb and V
  • a problem of increasing manufacturing coast may occur, and thus the contents of Nb and V are controlled to 0.25 wt % or less, respectively, in the present disclosure.
  • 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 alloying elements 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.
  • An Austenitic Stability Parameter (ASP) value represented by Formula (1) below may be from 10 to 25.
  • [C], [N], [Si], [Mn], [Cr], [Ni], [Cu], [Mo], [Nb], and [V] represent weight percentages (wt %) of respective elements. For an element not added, 0 wt % is substituted into Formula (1).
  • Formula (1) indicates a temperature at which 50% of austenite is transformed into martensite when a stainless steel is deformed with a true strain of 0.3 and is used as an index of stability of the austenite phase.
  • a lower value of Formula (1) means higher stability of the austenite phase indicating a smaller amount of strain-induced martensite during transformation.
  • a value of Formula (2) below may be 435 or more.
  • [N] and [Ni] represent weight percentages (wt %) of respective elements.
  • ASP means the ASP value of Formula (1). Assuming that a total thickness of the steel material is t, d indicates an average grain size in a thickness range of 1 ⁇ 4t to 3 ⁇ 4t.
  • t may be from 0.4 to 2.0 mm.
  • a material having a thickness of 0.4 to 2.0 mm has been widely applied to structural components.
  • an austenitic stainless steel having high strength and high ductility may be provided within the above-described thickness range.
  • d may be 5 ⁇ n or less.
  • Formula (2) is a parameter to obtain high strength, derived in consideration of factors affecting strength, such as the N content, the Ni content, grain size, and austenitic stability parameter. When the value of Formula (2) is less than 435, sufficient strength cannot be obtained.
  • a value of Formula (3) below may be 6000 or more.
  • YS represents yield strength (MPa)
  • EL represents elongation (%)
  • [Ni] and [Cr] represent weight percentages (wt %) of respective elements.
  • the YS*EL value is considerably affected by the contents of Cr and Ni.
  • the contents of Cr and Ni are small, TRIP transformation easily occurs and the YS*EL value tends to increase.
  • the value of Formula (3) is less than 6000, high strength and high ductility cannot be satisfied simultaneously.
  • a value of Formula (4) below may be 200 or more.
  • Hv Vickers hardness (Hv) and [Ni] and [Cr] represent weight percentages (wt %) of respective elements.
  • the Hv value is considerably affected by the contents of Cr and Ni.
  • the contents of Cr and Ni are small, TRIP transformation easily occurs, and thus an amount of strain-induced martensite transformation increases during cold deformation. As a result, hardness of a steel material increases.
  • the value of Formula (4) is less than 200, sufficient hardness cannot be obtained.
  • the austenitic stainless steel according to the present disclosure has not only high strength and high ductility but also excellent corrosion resistance.
  • the austenitic stainless steel according to an embodiment may have a pitting potential of 250 mV or more when measured by dipping in an 3.5% NaCl solution at 30° C.
  • the austenitic stainless steel according to an embodiment of the present disclosure may have a tensile strength of 1750 MPa or more after skin pass rolling.
  • a method of manufacturing an austenitic stainless steel according to an embodiment of the present disclosure includes: hot rolling a slab including, in percent by weight (wt %), 0.005 to 0.03% of C, 0.1 to 1% of Si, 0.1 to 2% of Mn, 6 to 9% of Ni, 16 to 19% of Cr, 0.2% or less of N, and the remainder being Fe and unavoidable impurities, and cold rolling the hot-rolled steel material at room temperature with a reduction ratio of 40% or more; and annealing the cold-rolled steel material at a temperature of 700 to 850° C.
  • the austenitic stainless steel of the present disclosure having an ASP value of 10 to 25 has a too low amount of TRIP transformation, resulting in a decrease in the fraction of martensite in the cold-rolled steel material and an increase in the fraction of the retained austenite phase.
  • the ratio of the reverted austenite phase during the subsequent low-temperature annealing decreases, and the fraction of the retained austenite phase without being transformed into martensite increases, making it difficult to obtain ultrafine grains.
  • the hot-rolled material is cold-rolled and annealed at a low temperature of 700 to 850° C.
  • a low temperature 700 to 850° C.
  • recrystallization from the strain-induced martensite phase into the reverted austenite phase does not occur.
  • the temperature of the low-temperature annealing is higher than 850° C., the grain size of the reverted austenite may increase, thereby decreasing the yield strength.
  • the slab may be hot-rolled and then cold-rolled without conducting annealing.
  • the method may further include skin pass rolling with a reduction ratio of 60% or more to further increase strength.
  • ASP refers to a value obtained by substituting the contents of alloying elements of Table 1 into Formula (1) below indicating stability of an austenite phase.
  • [C], [N], [Si], [Mn], [Cr], [Ni], [Cu], [Mo], [Nb], and [V] represent weight percentages (wt %) of respective elements.
  • Formula (3) of Table 2 means a value obtained by Formula (3) below.
  • the value of Formula (3) was obtained by substituting a yield strength (YS, MPa) and an elongation (EL, %) measured under the conditions described below and the contents (wt %) of [Ni] and [Cr] of Table 1 thereinto.
  • the yield strength (YS, MPa) and the elongation (EL, %) were measured after preparing a sample of the cold-rolled, annealed material according to the JIS13B standards and conducting a tensile test at room temperature in a crosshead range of 10 mm/min to 20 mm/min.
  • Formula (4) of Table 2 represents a value obtained by Formula (4) below.
  • the value of Formula (4) was derived by substituting a Vickers hardness (Hv) measured under the conditions described below and the contents (wt %) of [Ni] and [Cr] of Table 1 thereinto.
  • the Vickers hardness (Hv) is a value measured using a Vickers hardness tester with a load of 2 kgf.
  • Pitting potential (mV) of Table 2 is a value measured after dipping a cold-rolled, annealed material in a 3.5% NaCl solution at 30° C.
  • Inventive Examples 1 to 8 satisfied the ASP values of 10 to 25, the d value of 5 ⁇ m or less, the value of Formula (2) of 435 or more, and the value of Formula (3) of 6000 or more, and thus high strength, high ductility, and excellent corrosion resistance were obtained. Also, referring to Table 2, in Inventive Examples 1 to 8, the values of Formula (4) were at least 200 and pitting potentials were at least 250 mV. In Comparative Examples 6 to 18, the ASP values were out of the range defined in the present disclosure. In Comparative Examples 6 to 13 and 18 in which the ASP values exceeded 25, low elongation was observed due to too high TRIP transformation rates during formation. In Comparative Examples 14 to 17 in which the ASP values were less than 10, ultrafine grains could not be obtained due to high fractions of the retained austenite phase.
  • FIG. 1 is a graph showing ranges of values of Formulae (2) and (3) of inventive examples and comparative examples. Referring to FIG. 1 , it may be confirmed that all inventive examples satisfied the ranges of Formulae (2) and (3) defined in the present disclosure, and thus austenitic stainless steels simultaneously satisfying the ranges of high strength and high ductility may be provided. On the contrary, in the case of the comparative examples, the values of Formula (2) and/or Formula (3) were not satisfied, and in this case it is difficult to simultaneously satisfy high strength and high ductility characteristics.
  • FIGS. 2 A to 2 C are views for comparison of average grain size d among the inventive examples and comparative examples.
  • FIGS. 2 A and 2 B show samples according to the present disclosure
  • FIG. 2 C shows a sample according to a comparative example.
  • the austenitic stainless steel according to the present disclosure are in the form of ultrafine grains with a d value of 5 ⁇ m or less.
  • FIGS. 3 A to 3 C are graphs showing Strain (%)-Stress (MPa) curves of the inventive examples.
  • FIGS. 4 A to 4 C are graphs showing Strain (%)-Stress (MPa) curves of the comparative examples.
  • the austenitic stainless steel according to an embodiment of the present disclosure may have both high strength and high ductility, be economically feasible by reducing the contents of Ni and Cr which are high-priced alloying elements, have excellent corrosion resistance. Therefore, the austenitic stainless steel may be applied to outer panels of vehicles, components for construction, and the like, as various materials.

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