WO2022085708A1 - Acier inoxydable ferritique et procédé de fabrication d'acier inoxydable ferritique - Google Patents

Acier inoxydable ferritique et procédé de fabrication d'acier inoxydable ferritique Download PDF

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WO2022085708A1
WO2022085708A1 PCT/JP2021/038703 JP2021038703W WO2022085708A1 WO 2022085708 A1 WO2022085708 A1 WO 2022085708A1 JP 2021038703 W JP2021038703 W JP 2021038703W WO 2022085708 A1 WO2022085708 A1 WO 2022085708A1
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stainless steel
hot
steel strip
cold rolling
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PCT/JP2021/038703
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Japanese (ja)
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祐太 吉村
直樹 平川
詠一朗 石丸
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日鉄ステンレス株式会社
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Priority to KR1020227045272A priority Critical patent/KR20230015982A/ko
Priority to CN202180044813.9A priority patent/CN115917029A/zh
Priority to JP2022557573A priority patent/JP7374338B2/ja
Publication of WO2022085708A1 publication Critical patent/WO2022085708A1/fr

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    • 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
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties 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
    • 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
    • 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
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • 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
    • 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
    • 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
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • 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
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • 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
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • 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
    • C21D2211/005—Ferrite

Definitions

  • the present invention relates to a ferritic stainless steel and a method for manufacturing a ferritic stainless steel.
  • Ferritic stainless steel has excellent corrosion resistance and heat resistance, and is used in various fields such as home appliances, cooking utensils, and construction applications. On the other hand, ferritic stainless steel is inferior in ductility to austenitic stainless steel. Further, the ferritic stainless steel has a problem that rigging occurs during the molding process, and this rigging impairs the surface quality of the molded product and the polishability of the ferritic stainless steel after the forming process.
  • rigging is a surface defect that occurs on the surface of ferritic stainless steel, and specifically refers to striped or streaky undulations that occur on the surface of ferritic stainless steel in a direction parallel to the processing direction.
  • the "machining direction” is a direction in which a steel strip of ferritic stainless steel is stretched by forming. Further, as the forming process that causes rigging, press working, tensile processing, drawing processing and the like can be exemplified.
  • Patent Document 1 a ferritic stainless steel sheet having an improved r value and excellent rigging resistance is manufactured by adding an amount of Ti satisfying a predetermined condition to control the amount of precipitate deposited.
  • the technology to be used is disclosed.
  • the r value (Rankford value) is a characteristic value indicating the general anisotropy of the plate material, and is an index indicating the superiority or inferiority of the deep drawing property of the ferritic stainless steel. It is said that the larger the r value, the better the deep drawing property of the ferritic stainless steel.
  • Patent Document 2 discloses a technique for producing a ferritic stainless thin steel sheet having a large r value by cold rolling using a work roll having a predetermined roll diameter.
  • the work roll is a component of a cold rolling mill that comes into direct contact with a metal plate to be rolled during cold rolling.
  • Patent Document 1 adds Ti, which is an expensive element, so that the manufacturing cost of the ferritic stainless steel sheet is high.
  • the technique disclosed in Patent Document 2 does not specify the manufacturing conditions and the component composition for improving the rigging resistance, the ferritic stainless steel sheet manufactured based on this technique has the rigging resistance. It cannot be said that it is sufficient in terms of.
  • One aspect of the present invention has been made in view of the above-mentioned problems, and is to realize a ferritic stainless steel having excellent both deep drawing property and rigging resistance at a lower cost than the conventional one.
  • the ferritic stainless steel according to one aspect of the present invention has a mass% of C: 0.12% or less, Si: 1.0% or less, Mn: 1.0% or less, Ferritic stainless steel containing 1.0% or less, Cr: 12.0% or more and 18.0% or less, N: 0.10% or less and Al: 0.50% or less, and the balance is Fe and unavoidable impurities.
  • the average height of the waviness curve element in the rigging formed on the surface of the ferritic stainless steel is 15 ⁇ m or less
  • the r value is 0.9 or more, and in the rolling direction.
  • a martensite phase having an area ratio of 0% or more and less than 1.0% in a cross section cut in a plane that is parallel and perpendicular to the width direction is included.
  • the method for producing ferritic stainless steel according to one aspect of the present invention is, in terms of mass%, C: 0.12% or less, Si: 1.0% or less, Mn: 1.0. % Or less, Ni: 1.0% or less, Cr: 12.0% or more and 18.0% or less, N: 0.10% or less and Al: 0.50% or less, and the balance is Fe and unavoidable impurities.
  • a hot rolling process that hot-rolls a steel slab made of ferritic stainless steel to produce a hot-rolled steel strip.
  • the martensite phase in the cross section cut in a plane parallel to the rolling direction and perpendicular to the width direction of the hot-rolled hardened steel strip.
  • a cold rolling step of cold-rolling the hot-rolled annealed steel strip produced in the softening annealing step to produce a cold-rolled steel strip is included.
  • the total cold-rolling ratio which is the ratio of the difference between the thickness of the hot-rolled annealed steel strip and the thickness of the cold-rolled steel strip to the thickness of the hot-rolled annealed steel strip, is set to 60% or more.
  • the hot-rolled annealed steel strip (I) Using the first work roll having a roll diameter of 200 mm or more, the cold rolling rate per pass is 15% or more, and the cold rolling rate after the completion of all passes is 50% or more of the cold rolling rate. After cold rolling to (Ii) Further cold rolling is performed using a second work roll having a roll diameter of less than 200 mm.
  • a ferritic stainless steel having excellent both deep drawing resistance and rigging resistance can be realized at a lower cost than before.
  • the "rolling direction” is a direction in which the strip is passed through the rolling apparatus when the strip of stainless steel is rolled.
  • the martensite phase is excessively dispersed, the amount of the martensite phase will be excessively increased in the stainless steel strip after softening and annealing. Since the martensite phase has a hard and high-strength structure and the ferrite phase has a soft and excellent ductility structure, the ductility of stainless steel cannot be improved. It also causes edge cracking and coil (steel strip) breakage when the stainless steel strip after softening and annealing is cold-rolled.
  • the present inventors have appropriately dispersed the martensite phase in order to improve the ductility and rigging resistance of stainless steel while maintaining the strength at the conventional level. It was found that it is effective to cause it. Specifically, it was found that it is effective to cause the dispersion of the martensite phase so that the area ratio of the martensite phase in the stainless steel strip after softening and annealing is 5.0 to 30.0%. rice field.
  • Area ratio of martensite phase after softening annealing is the ratio of the total area of the martensite phase region contained in the cut cross section to the area of the cut cross section of the stainless steel strip after softening annealing.
  • This cut cross section is a cross section formed when the stainless steel strip after softening and annealing is cut in a plane parallel to the rolling direction and perpendicular to the width direction of the stainless steel strip.
  • first martensite area ratio the area ratio of the martensite phase after softening and annealing.
  • the first martensite area ratio can be calculated using, for example, EBSD (electron back scattering diffraction) crystal orientation analysis.
  • EBSD electron back scattering diffraction
  • an EBSD detector mounted on a scanning electron microscope (SEM) is used to acquire an EBSD pattern on the measurement surface of a stainless steel strip after softening and annealing.
  • SEM scanning electron microscope
  • the acquisition conditions are set as follows, for example.
  • an IQ (Image Quality) image is generated from the acquired EBSD pattern using OIM (Orientation Imaging Microscopy) analysis software.
  • the IQ image is an image map showing the high and low sharpness of each structure formed on the measurement surface of the hot-rolled annealed steel strip.
  • the martensite phase has a more complicated internal structure and lower sharpness than the ferrite phase. Therefore, the martensite phase region on the measurement surface appears relatively dark in the IQ image.
  • the ferrite phase has a simpler internal structure and higher sharpness than the martensite phase. Therefore, the ferrite phase region on the measurement surface appears relatively bright in the IQ image.
  • the first martensite area ratio can be calculated by binarizing this IQ image and dividing the total area of the martensite phase region by the area of the measurement surface.
  • the present inventors By setting the first martensite area ratio to 5.0% or more, the present inventors have made the waviness height of the rigging formed on the surface of the stainless steel (details will be described later) lower than before. It has been found that the surface texture of stainless steel is improved and the molding process becomes easier. Further, by setting the first martensite area ratio to 30.0% or less, the present inventors do not reduce the ductility of the stainless steel strip after softening and annealing, and edge cracks and coils during cold rolling. It was found that poor cold ductility such as breakage is less likely to occur.
  • the r value which is an index of superiority or inferiority of deep drawability in stainless steel, is the crystal orientation (hereinafter, " ⁇ 111 ⁇ crystal orientation") produced in stainless steel and having a Miller index of ⁇ 111 ⁇ . It is known that the larger the number of (abbreviation), the larger the value. Since the ⁇ 111 ⁇ crystal orientation tends to be generated at a place where rolling strain occurs, it is likely to be generated at a grain boundary where rolling strain is concentrated in stainless steel.
  • the concentration of rolling strain is concentrated on the stainless steel after cold-rolling. It tends to stay at the end of the steel strip in the plate thickness direction.
  • the concentration of rolling strain is unlikely to occur and the grain boundaries tend to be difficult to be generated.
  • the central portion of the stainless steel strip in the plate thickness direction is referred to as a "plate thickness center portion”
  • the end portion in the plate thickness direction is referred to as a "plate thickness surface layer portion”.
  • the present inventors tend to make the roll diameter larger than that of the above-mentioned general work roll, so that the concentration of rolling strain is likely to occur even in the central portion of the plate thickness, and the crystal grain boundaries are likely to be generated.
  • ⁇ 111 ⁇ I came up with the idea that many crystal orientations might be generated. The idea is that if the distance between the surface of the stainless steel strip and the axis of rotation is the same for the general work roll and the work roll with a larger roll diameter, the work roll with a larger roll diameter is the plate. It is based on the fact that it can be rolled to a part closer to the thick center.
  • the roll diameter of the work roll is set to 200 mm or more, the desired number of ⁇ 111 ⁇ is obtained in the entire portion from the plate thickness surface layer portion to the plate thickness center portion. It turned out that it may be possible to generate a crystal orientation.
  • the cold rolling rate per pass in cold rolling is low, and cold rolling using the above-mentioned work roll is performed. It has been found that if the cold rolling ratio is low, it is difficult to generate a desired number of ⁇ 111 ⁇ crystal orientations at the center of the plate thickness.
  • the cold spreading rate per pass is the thickness of the stainless steel strip one pass before and the thickness of the stainless steel strip after one pass with respect to the thickness of the stainless steel strip one pass before for any pass. It is the ratio of the difference from the thickness.
  • the present inventors further studied.
  • the size of the roll diameter of the work roll is set to 200 mm or more
  • the cold rolling rate per pass is set to 15% or more, and all. It was found that it is effective to set the cold rolling rate after the end of the pass to 50% or more of the total cold rolling rate.
  • the total cold-rolled ratio is the ratio of the difference between the thickness of the hot-rolled annealed steel strip and the thickness of the cold-rolled steel strip to the thickness of the hot-rolled annealed steel strip before cold rolling. ..
  • the cold-rolled steel strip which is the basis for calculating the total cold-rolling ratio, is after all the processing in cold rolling is completed (in this embodiment, after the first cold rolling and the second cold rolling described later are completed). Refers to the steel strip of. The cold rolling rate after the completion of all passes will be described later.
  • the size of the roll diameter of the work roll is set to 200 mm or more, the cold rolling rate per pass is set to 15% or more, and the cold rolling rate after the completion of all passes is the total cold rolling rate. It was found that by setting it to 50% or more, a desired number of ⁇ 111 ⁇ crystal orientations can be generated in the entire portion from the surface layer portion of the plate thickness to the center portion of the plate thickness.
  • a work roll having a roll diameter of 200 mm or more is referred to as a "large diameter roll”
  • a work roll having a roll diameter of less than 200 mm is referred to as a "small diameter roll”.
  • a work roll having a roll diameter of 50 to 100 mm, which is generally used in cold rolling belongs to a "small diameter roll”.
  • the cold-rolled steel strip is heated to 800 ° C. or higher and less than Ac1 at a heating rate of 50 ° C./s or lower.
  • Ac1 is a measure of the temperature at which the formation of the austenite phase is started.
  • the disappearance of the martensite phase starts when the cold-rolled steel strip reaches a certain temperature (about 700 ° C.).
  • the martensite phase in the cold-rolled steel strip is substantially eliminated by soaking the heated cold-rolled steel strip in a temperature range of 800 ° C. or higher and lower than Ac1 for 5 seconds or longer.
  • the disappearance of the martensite phase in the cold-rolled steel strip is aimed at.
  • the "disappearance of the martensite phase” in the present specification basically means that the area ratio of the martensite phase contained in the stainless steel as the final product is 0%, that is, the martensite in the stainless steel as the final product. It means that the site phase has completely disappeared.
  • the "area ratio of martensite phase contained in stainless steel as a final product” is the ratio of the total area of the martensite phase region contained in the cut cross section to the area of the cut cross section of stainless steel as a final product.
  • This cut cross section is a cross section formed when stainless steel as a final product is cut in a plane parallel to the rolling direction and perpendicular to the width direction of the stainless steel.
  • the "disappearance of the martensite phase" in the present specification is actually an area ratio of stainless steel as a final product as a result of finish annealing aiming at complete disappearance of the martensite phase (area ratio 0%). It is a concept that allows less than 1.0% of the martensite phase to remain. When the remaining martensite phase has an area ratio of less than 1.0%, the corrosion resistance and workability of stainless steel as a final product are both excellent.
  • the area ratio of the martensite phase contained in stainless steel as a final product is referred to as "second martensite area ratio”.
  • the present inventors applied the above-mentioned finish annealing to the cold-rolled steel strip to complete the recrystallization of the ferrite phase, which is the parent phase, and the martensite phase (first martensite phase) intentionally produced in the softening annealing. It was found that the site area ratio (5.0 to 30.0%) can also be eliminated. Recrystallization refers to the formation of new grain grains free of dislocations in the cold-rolled steel strip. Dislocations are an example of lattice defects that occur inside a crystal.
  • the present inventors have also found that by applying the above-mentioned finish annealing to the cold-rolled steel strip, it is possible to prevent the formation of a new martensite phase in the cold-rolled steel strip.
  • the series of methods from softening annealing to finish annealing described above is different from the general stainless steel manufacturing method aiming at the positive disappearance of the martensite phase from the time of softening annealing.
  • the stainless steel according to the embodiment of the present invention has a mass% of C: 0.12% or less, Si: 1.0% or less, Mn: 1.0% or less, Ni: 1.0% or less, Cr: 12.0 to 18.0%, N: 0.10% or less, Al: 0.50% or less, Mo: 0.50% or less, Cu: 1.0% or less, O: 0.01% or less, V : 0.15% or less, B: 0.10% or less, Ti: 0.50% or less, Co: 0.01 to 0.50%, Zr: 0.01 to 0.10%, Nb: 0.01 ⁇ 0.10%, Mg: 0.0005 ⁇ 0.003%, Ca: 0.0003 ⁇ 0.003%, Y: 0.01% ⁇ 0.20%, rare earth metals (REM) excluding Y: total 0.01 to 0.10%, Sn: 0.001 to 0.50%, Sb: 0.001 to 0.50%, Pb: 0.01 to 0.10% and W: 0.01 to 0. .Contains 50%.
  • REM rare earth metals
  • the rest of this stainless steel consists of Fe and unavoidable impurities.
  • Each of Mo, Cu, O, V, B, Ti, Co, Zr, Nb, Mg, Ca, Y, REM, Sn, Sb, Pb, and W is not an essential element of this stainless steel.
  • Each of these elements is an arbitrary element as long as it contains at least one of these elements, if necessary.
  • each element contained in this stainless steel will be described.
  • C is an important element that forms a carbide with Cr to form an interface that is a source of dislocations when the stainless steel is deformed. However, if C is added in excess, the martensite phase is excessively generated, and the ductility of the present stainless steel is lowered. Therefore, the content of C is set to 0.12% or less.
  • Si has an effect as a deoxidizing agent in the melting stage. However, if Si is added in excess, the stainless steel is hardened and the ductility is lowered. Therefore, the Si content is set to 1.0% or less.
  • Mn has an effect as a deoxidizing agent. However, if Mn is excessively added, the amount of MnS produced increases and the corrosion resistance of the present stainless steel decreases. Therefore, the Mn content is set to 1.0% or less.
  • Ni is an austenite-forming element and is an effective element for controlling the first martensite area ratio and the strength of the present stainless steel.
  • the austenite phase is stabilized more than necessary, the ductility of the present stainless steel is lowered, and the raw material cost of the present stainless steel is increased. Therefore, the Ni content is set to 1.0% or less.
  • the strip of this stainless steel after softening and annealing is referred to as "hot-rolled annealed strip".
  • the steel strip of the present stainless steel after softening and annealing is an example of the hot-rolled annealed steel strip according to the present invention.
  • ⁇ Cr: 12.0 to 18.0%> Cr is required to form a passivation film on the surface of the steel strip of this stainless steel after cold rolling to improve corrosion resistance.
  • Cr is added in excess, the ductility of the present stainless steel is lowered. Therefore, the Cr content is set to 12.0 to 18.0%.
  • the steel strip of this stainless steel after cold rolling is referred to as "cold rolled steel strip".
  • the steel strip of the present stainless steel after cold rolling is an example of the cold-rolled steel strip according to the present invention.
  • N is an important element that forms a nitride with Cr to form an interface that is a source of dislocations when the stainless steel is deformed.
  • the content of N is set to 0.10% or less.
  • Al is an element effective for deoxidation and can reduce A2 inclusions which adversely affect press workability. However, if Al is added in excess, the surface defects of the present stainless steel increase. Therefore, the Al content is set to 0.50% or less.
  • Mo is an element effective for improving corrosion resistance. However, if Mo is added in excess, the raw material cost of this stainless steel increases. Therefore, the Mo content is preferably set to 0.50% or less.
  • Cu is an element effective for improving corrosion resistance.
  • the Cu content is preferably set to 1.0% or less.
  • O produces non-metal inclusions, which reduces the impact value and fatigue life of the present stainless steel. Therefore, the content of O is preferably set to 0.01% or less.
  • V is an element effective for improving hardness and strength. However, if V is added in excess, the raw material cost of the present stainless steel increases. Therefore, the V content is preferably set to 0.15% or less.
  • B is an element effective for improving toughness. However, this effect saturates above 0.10%. Therefore, the content of B is preferably set to 0.10% or less.
  • Ti is an element that forms a carbonitride, and suppresses grain boundary precipitation of Cr carbonitride during heat treatment to improve the corrosion resistance of this stainless steel. Further, by fixing the solid solution C and the solid solution N in the stainless steel as carbonitride, the content of the solid solution C and the solid solution N is reduced and the r value of the stainless steel is improved. Further, by fixing the solid solution C and the solid solution N in the stainless steel as carbonitrides, the ductility of the stainless steel can be improved and the stretcher strain can be reduced. Stretcher strains are tiny irregularities formed on the surface of stainless steel that occur due to the yield elongation of several percent that occurs during stamping of stainless steel.
  • the Ti content is preferably set to 0.50% or less.
  • Co is an element effective for improving corrosion resistance and heat resistance. However, if Co is added in excess, the raw material cost of this stainless steel increases. Therefore, the Co content is preferably set to 0.01 to 0.50%.
  • Zr is an element effective for denitrification, deoxidation and desulfurization. However, if Zr is excessively added, the raw material cost of the present stainless steel increases. Therefore, the Zr content is preferably set to 0.01 to 0.10%.
  • Nb reduces the content of solid solution C and solid solution N by fixing the solid solution C and solid solution N in the stainless steel as carbonitride, and reduces the r value of the stainless steel. Improve. Further, by fixing the solid solution C and the solid solution N in the stainless steel as carbonitrides, the ductility of the stainless steel can be improved and the stretcher strain can be reduced. However, since Nb is an expensive element, if Nb is excessively added, the raw material cost of the present stainless steel increases. Therefore, the Nb content is preferably set to 0.01 to 0.10%.
  • Mg forms Mg oxide together with Al in molten steel and acts as a deoxidizing agent.
  • the Mg content is preferably set to 0.0005 to 0.003%, more preferably 0.002% or less.
  • Ca is an element effective for degassing.
  • the Ca content is preferably set to 0.0003 to 0.003%.
  • Y is an element effective for improving hot workability and oxidation resistance. However, these effects saturate above 0.20%. Therefore, the Y content is preferably set to 0.01 to 0.20%.
  • ⁇ REM preferably 0.01 to 0.10% in total> REMs (Rare Earth Metals) such as Sc and La are effective in improving hot workability and oxidation resistance, as in Y. However, these effects saturate above 0.10%. Therefore, the total content of REM is preferably set to 0.01 to 0.10%.
  • Sn is an element effective for improving corrosion resistance. However, if Sn is added in excess, the hot workability and tenacity are lowered. Therefore, the Sn content is preferably set to 0.001 to 0.50%.
  • Sb is effective in improving workability by promoting the formation of a deformed zone during rolling.
  • the Sb content is preferably set to 0.001 to 0.50%, more preferably 0.20% or less.
  • Pb is an element effective for improving free-cutting property.
  • the Pb content is preferably set to 0.01 to 0.10%.
  • W is an element effective for improving high temperature strength. However, if W is added in excess, the raw material cost of the present stainless steel increases. Therefore, the W content is preferably set to 0.01 to 0.50%.
  • the balance other than the above-mentioned components is Fe and unavoidable impurities.
  • the unavoidable impurities are impurities that are mixed from the raw materials and the manufacturing process, and are mixed within a range that does not affect the characteristics of each of the above-mentioned components.
  • the waviness height of the rigging formed on the surface is 15 ⁇ m or less.
  • the undulation height of the rigging formed on the surface of the present stainless steel hereinafter, abbreviated as” the undulation height of the present stainless steel " means the undulation of the rigging measured by the method shown below. Means height.
  • first tensile test piece a JIS No. 5 tensile test piece (hereinafter abbreviated as "first tensile test piece") specified in JIS Z 2201 is collected from the final product of this stainless steel.
  • first tensile test piece is pulled so that the direction parallel to the rolling direction is the tensile direction, with the distance between the gauge points being 50 mm.
  • a tensile strain of 16% is applied to the first tensile test piece.
  • the measurement length in the direction orthogonal to the rolling direction (in other words, the width direction of the first tensile test piece) in the portion between the gauge points of the first tensile test piece was set to 18 mm. Measure the swell height.
  • the swell height is the average height of the swell curve element measured by the surface texture measurement specified in JIS B 0601: 2001 or the like.
  • the average height of the waviness curve element of the first tensile test piece is measured by the surface texture measurement specified in JIS B 0601: 2001.
  • the average height of the waviness curve elements measured by this method is the waviness height of this stainless steel.
  • the undulation height of the rigging formed on the surface is measured by the above method, the undulation height is 20 to 50 ⁇ m, which is higher than the undulation height of this stainless steel. From this, it can be said that this stainless steel has improved rigging resistance as compared with the conventional stainless steel.
  • the JIS13B tensile test piece specified in JIS Z2201 is collected from the final product of this stainless steel. Specifically, from the above-mentioned final product, a second tensile test piece whose tensile direction is parallel to the rolling direction, a third tensile test piece whose tensile direction is a direction forming an angle of 45 ° with the rolling direction, and rolling. Each of the fourth tensile test pieces whose direction orthogonal to the direction is the tensile direction is collected. Next, each of the second to fourth tensile test pieces is pulled using an Instron type tensile tester with the distance between the gauge points set to 20 mm. Then, by this tensile test, a tensile strain of 14.4% is applied to each of the second to fourth tensile test pieces.
  • the r value of each of the second to fourth tensile test pieces is calculated using the following equation (1).
  • r ln (W / W1) / ln (t / t1) ...
  • W is the width before the tensile test
  • W1 is the width after the tensile test
  • t is the thickness before the tensile test
  • t1 is the thickness after the tensile test.
  • the "width” is the width of the portion between the gauge points in each of the second to fourth tensile test pieces.
  • the "thickness” is the thickness of the portion between the gauge points in each of the second to fourth tensile test pieces.
  • the average r value obtained by averaging the r values of the second to fourth tensile test pieces is calculated.
  • This average r value is the r value of this stainless steel.
  • Average r value (r0 + 2r45 + r90) / 4 ... (2)
  • r0 is the r value of the second tensile test piece
  • r45 is the r value of the third tensile test piece
  • r90 is the r value of the fourth tensile test piece.
  • the r value is 0.6 to 0.8, which is smaller than the r value of this stainless steel. From this, it can be said that this stainless steel has improved deep drawing property as compared with the conventional stainless steel.
  • This stainless steel has a second martensite area ratio of 0% or more and less than 1.0%.
  • the second martensite area ratio is preferably 0% from the viewpoint of improving the corrosion resistance and workability of the present stainless steel. However, if the second martensite area ratio is less than 1.0%, even if the area ratio is higher than 0%, this stainless steel has not only rigging resistance and deep drawing resistance but also corrosion resistance and workability. Excellent.
  • the second martensite area ratio can be calculated using EBSD crystal orientation analysis in the same manner as the first martensite area ratio.
  • index value In this stainless steel, the index value represented by the following equation (3) is 15 to 50. This index value is an index showing the maximum amount of austenite phase produced by annealing. In the following equation (3), each element symbol represents the mass% concentration of the element.
  • FIG. 1 A method for manufacturing stainless steel according to an embodiment of the present invention will be described with reference to FIG. 1.
  • the present stainless steel is manufactured by going through each of the melting step S1, the hot rolling step S2, the softening annealing step S3, the cold rolling step S4, and the baking step S5.
  • each step will be described, but the method for manufacturing the present stainless steel is not limited to the method shown in FIG.
  • ⁇ Melting process S1 and hot rolling process S2> In order to produce this stainless steel, first, in the melting step S1, the stainless steel containing each of the above-mentioned components is melted to produce a steel slab. In the melting step S1, a general melting apparatus for stainless steel can be used, and general melting conditions can be set. Next, in the hot rolling step S2, the hot rolled steel strip is manufactured by hot rolling the steel slab produced in the melting step S1. This hot-rolled steel strip is an example of the hot-rolled steel strip according to the present invention. In the hot rolling step S2, a general hot rolling apparatus and hot rolling conditions for stainless steel can be used.
  • the hot-rolled annealed steel strip produced in the hot rolling step S2 is softened and annealed to produce a hot-rolled annealed steel strip.
  • the softening annealing is a heat treatment in which the hot-rolled steel strip is annealed by setting the maximum temperature during the soaking process to Ac1 or higher in order to soften the hot-rolled steel strip.
  • the amount of austenite phase in the steel strip begins to increase. Then, when the temperature of the softening annealing becomes higher, the amount of the austenite phase in the steel strip increases to the peak amount and then starts to decrease. Since the austenite phase can be transformed into a martensite phase during the cooling process in the softening annealing, the first martensite area ratio is affected by the austenite phase that is increased by the softening annealing.
  • the maximum annealing temperature is the maximum temperature during the soaking process in softening annealing.
  • the maximum annealing temperature is set to 0.76 ⁇ Ac1 + 201 ° C. or higher and 1.10 ⁇ Ac1-56 ° C. or lower to obtain the first martensite area. It was found that the ratio can be set to 5.0 to 30.0%.
  • the peak amount of the austenite phase itself is small, so even if the total amount of the peak amount is transformed from the austenite phase to the martensite phase, the first martensite area ratio is 5.0 to 30. I was able to set it to 0.0%. Therefore, when Ac1 is 921 or more, it is not necessary to set the upper limit of the maximum annealing temperature to 1.10 ⁇ Ac1-56 ° C.
  • the upper limit of the maximum annealing temperature is set to 1050 ° C. I decided to set it.
  • the rate of temperature rise in the temperature rise process in softening annealing is preferably set to 10 ° C./sec or higher. If the temperature rise rate is 10 ° C./sec or more, the temperature rise time in the temperature rise process can be shortened to a practically meaningful degree, so that the total time required for manufacturing this stainless steel is also to a practically meaningful degree. Can be shortened. Therefore, the productivity of this stainless steel can be improved.
  • the heat equalization time in the heat equalization process in the softening annealing is preferably set to 5 seconds or more. If the heat soaking time is 5 seconds or more, the austenite phase can be reliably generated during the heat soaking process. Since the austenite phase transforms into the martensite phase during the cooling process after the soaking process, the first martensite area ratio is set to 5.0 to 30.0% by setting the soaking time to 5 seconds or longer. It will be easier to manage.
  • the cooling rate in the cooling process in softening annealing is set to 5.0 ° C./sec or higher. If the cooling rate is less than 5.0 ° C./sec, the cooling time in the cooling process becomes longer than necessary, and the austenite phase is transformed into a stable ferrite phase. Therefore, the area ratio of the first martensite is lowered to less than 5.0%, and the rigging resistance of the present stainless steel is lowered to be lower than that of the conventional stainless steel. For this reason, the cooling rate is set to 5.0 ° C./sec or higher in order to maintain good rigging resistance of the present stainless steel.
  • the hot-rolled annealed steel strip produced in the softening annealing step S3 is cold-rolled to produce a cold-rolled steel strip.
  • the total cold rolling ratio after the completion of the cold rolling step S4 is set to 60% or more.
  • the first cold rolling is performed by passing a hot-rolled annealed steel strip through a large-diameter roll having a roll diameter of 200 mm or more.
  • a large-diameter roll having a roll diameter of 200 mm or more is an example of a first work roll according to the present invention.
  • the cold rolling rate per pass is set to 15% or more, and the cold rolling rate after the completion of the first cold rolling (after the completion of all the passes in the first cold rolling). Is set to 50% or more of the total cold rolling ratio.
  • the cold rolling ratio after the completion of the first cold rolling is the hot rolling annealing with respect to the thickness of the hot-rolled annealed steel strip before the first cold rolling. It is the ratio of the difference between the thickness of the steel strip and the thickness of the steel strip after the completion of all passes.
  • the steel strip passed through the large diameter roll is subjected to the second cold rolling through the small diameter roll having a roll diameter of 50 to 100 mm.
  • a small diameter roll having a roll diameter of 50 to 100 mm is an example of a second work roll according to the present invention.
  • the second cold rolling only the remaining band thickness that could not be rolled in the first cold rolling is rolled.
  • the steel strip after the completion of the second cold rolling becomes a cold-rolled steel strip.
  • the reason for performing the second cold rolling after the completion of the first cold rolling will be described. That is, when comparing the first cold rolling and the second cold rolling, assuming that both cold rollings have the same rolling ratio, the first cold rolling using a large diameter roll is better. A larger rolling load is required than in the second cold rolling using a small diameter roll.
  • stainless steel is harder than ordinary steel, and in cold rolling, work hardening progresses in the latter half of the treatment, and the strength of the steel strip increases. From these facts, if cold rolling is performed only with a large diameter roll in the cold rolling step S4, the rolling load that must be applied to the steel strip in order to obtain a cold rolled steel strip having a desired strip thickness is the present stainless steel.
  • the first cold rolling is performed on the large diameter roll, and then the second cold rolling is performed on the small diameter roll.
  • a work roll having a roll diameter of 50 to 100 mm which is generally used as a small diameter roll, is used as the second work roll.
  • the second cold rolling is performed after the first cold rolling, but the first cold rolling is performed after the second cold rolling.
  • stainless steel is generally harder than ordinary steel. Further, in cold rolling, work hardening progresses in the latter half of the treatment, and the strength of the steel strip increases. Therefore, when the first cold rolling is performed after the second cold rolling, the first cold rolling is performed in order to cause the concentration of rolling strain at the center of the plate thickness of the steel strip after the second cold rolling. A larger rolling load is required than in the case of performing the second cold rolling after performing the above. From these facts, considering the manufacturability and productivity of the present stainless steel, it is preferable to perform the first cold rolling and then the second cold rolling as in the present embodiment.
  • the cold-rolled steel strip produced in the cold rolling step S4 is annealed at a temperature equal to or higher than the start temperature of recrystallization and lower than Ac1.
  • the annealing performed in the annealing step S5 is a finish annealing for the purpose of achieving both the completion of recrystallization of the ferrite phase in the cold-rolled steel strip and the disappearance of the martensite phase.
  • the finish annealing performed in the annealing step S5 is composed of a temperature raising process, a soaking process, and a cooling process, similarly to the softening annealing in the softening annealing step S3.
  • the cold-rolled steel strip is heated to a temperature equal to or higher than the start temperature of recrystallization and lower than Ac1 at a heating rate of 50 ° C./s or less.
  • the temperature rise rate to 50 ° C./s or less
  • the martensite phase can be eliminated in the process of temperature rise.
  • the cold-rolled steel strip after the temperature raising process is heated at a temperature equal to or higher than the start temperature of recrystallization and lower than Ac1 for 5 seconds or longer.
  • the start temperature of recrystallization is set to 800 ° C. By setting the recrystallization start time to 800 ° C., the recrystallization of the ferrite phase is completed in a short soaking time.
  • the start temperature of recrystallization is not limited to 800 ° C., and the start temperature of recrystallization may be set to a temperature lower than, for example, 800 ° C.
  • the upper limit of the soaking temperature is set to a temperature of Ac1 or less, the martensite phase remaining in the cold-rolled steel strip is substantially eliminated while preventing the formation of a new martensite phase in the cold-rolled steel strip. be able to.
  • the cold-rolled steel strip after the soaking process is cooled at a cooling rate of 50 ° C./s or less.
  • the martensite phase can be eliminated even in the course of the cooling process.
  • the finish annealing composed of each of these treatments to the cold-rolled steel strip, it is possible to efficiently achieve both the completion of recrystallization of the ferrite phase in the cold-rolled steel strip and the disappearance of the martensite phase in the annealing step S5. can do.
  • the annealing step S5 is completed, the present stainless steel as a final product is obtained, and the production of the present stainless steel is completed.
  • the ferritic stainless steel according to one aspect of the present invention has Mo: 0.50% or less, Cu: 1.0% or less, O: 0.01% or less, V: 0.15% or less, B in mass%. It may further contain one or more selected from: 0.10% or less and Ti: 0.50% or less.
  • the ferritic stainless steel according to one aspect of the present invention has Co: 0.01% or more and 0.50% or less, Zr: 0.01% or more and 0.10% or less, Nb: 0.01% or more in mass%. 0.10% or less, Mg: 0.0005% or more and 0.003% or less, Ca: 0.0003% or more and 0.003% or less, Y: 0.01% or more and 0.20% or less, rare earth metals excluding Y : Total 0.01% or more and 0.10% or less, Sn: 0.001% or more and 0.50% or less, Sb: 0.001% or more and 0.50% or less, Pb: 0.01% or more and 0.10 % Or less and W: One or more selected from 0.01% or more and 0.50% or less may be further contained.
  • the steel slab has Mo: 0.50% or less, Cu: 1.0% or less, O: 0.01% or less, V in mass%. It may further contain one or more selected from: 0.15% or less, B: 0.10% or less, and Ti: 0.50% or less.
  • the steel slab has Co: 0.01% or more and 0.50% or less and Zr: 0.01% or more and 0.10% or less in terms of mass%.
  • Nb 0.01% or more and 0.10% or less
  • Mg 0.0005% or more and 0.003% or less
  • Ca 0.0003% or more and 0.003% or less
  • Y 0.01% or more and 0.20 % Or less
  • Sn 0.001% or more and 0.50% or less
  • Sb 0.001% or more and 0.50% or less
  • Pb One or more selected from 0.01% or more and 0.10% or less and W: 0.01% or more and 0.50% or less may be further contained.
  • the ferritic stainless steel according to the embodiment of the present invention will be referred to as "invention steel”
  • the ferritic stainless steel according to the comparative example of the present invention will be referred to as "comparative steel”.
  • the content of each element constituting the compositions A to E was within the numerical range of the content of each element contained in the ferritic stainless steel according to one aspect of the present invention.
  • Table 1 also shows the numerical value of Ac1 in each case of composition A to E.
  • the index value of the invention steel and the comparative steel produced based on the steel slab having the composition E was 85 as shown in Table 1 above, which exceeded the upper limit value 50 of the preferable numerical range in the present invention. It is presumed that such a result was obtained because the content of Cr in the composition E, which affects the index value, was 12.5%.
  • hot-rolled steel strips of each composition having a plate thickness of 3 mm and a plate width of 150 mm were manufactured.
  • the hot-rolled steel strips of each composition are subjected to softening annealing and cold rolling under the "actual conditions" shown in Table 2 below, so that the hot-rolled steel strips of each composition having a plate thickness of 1 mm and a plate width of 150 mm are cold-rolled.
  • the cold-rolled steel strips having each composition were finish-annealed to produce the 1st to 7th invention steels and the 1st to 18th comparative steels.
  • Table 2 above also shows "recommended conditions” for softening annealing and cold rolling.
  • the conditions described in the “Recommended conditions” column in Table 2 above are the same as those in the present embodiment.
  • the "lower limit temperature” in the “softening annealing” column of the “recommended conditions” indicates the lower limit of the maximum annealing temperature
  • the "upper limit temperature” in the same column indicates the upper limit of the maximum annealing temperature.
  • the numerical values described in the "lower limit temperature” and “upper limit temperature” columns are 0.76 ⁇ Ac1 + 201 ° C. or higher and 1050 ° C. or lower for each invention steel and each comparative steel of composition A. It was calculated by substituting the numerical value (942).
  • each of the invention steels of compositions B to E and each comparative steel the numerical values of each Ac1 of compositions B to E (811, 855,) in the formula of 0.76 ⁇ Ac1 + 201 ° C. or higher and 1.10 ⁇ Ac1-56 ° C. or lower. It was calculated by substituting 920 and 710).
  • Table 2 above also shows "characteristic evaluation” and "comprehensive evaluation” for each of the 1st to 7th invention steels and the 1st to 18th comparative steels.
  • the column of “waviness height” of “characteristic evaluation” shows the measurement result of the waviness height of rigging.
  • the column of “r value” of “characteristic evaluation” shows the calculation result of r value.
  • the method for measuring the swell height of the rigging and the method for calculating the r value were the same as those in the present embodiment.
  • “Comprehensive evaluation” is “ ⁇ " when the swell height of the rigging is 15 ⁇ m or less, the r value is 0.9 or more, and the second martensite area ratio is 0% or more and less than 1.0%. And said. On the other hand, if the swell height of the rigging is higher than 15 ⁇ m, the r value is less than 0.9, or the second martensite area ratio is 1.0% or more, it is evaluated as “x
  • the underlined numerical values in Table 2 above indicate numerical values outside the preferable numerical range in the present embodiment. Further, the underlined “x” in Table 2 above indicates that the cold rolling rate after the completion of all passes was less than 50% of the total cold rolling rate.
  • the swell height of the rigging was higher than 15 ⁇ m, so the overall evaluation was “x”. It became.
  • the swell height of the rigging was higher than 15 ⁇ m because the first martensite area ratio was less than 5.0% for all of the 1st, 9th, 11th, 13th, and 16th comparative steels. It is inferred that. That is, in all of the 1st, 9th, 11th, 13th, and 16th comparative steels, the amount of increase in colonies in the steel exceeded the permissible range for improving the rigging resistance, so that the swell height of the rigging was high. It is presumed that the height was higher than 15 ⁇ m.
  • the r value was less than 0.9, so that the overall evaluation was “x”. ..
  • the reason why the r value is less than 0.9 is presumed to be explained below.
  • the total cold rolling ratio is less than 60%, and the cold rolling ratio per pass is high. It is presumed that this is because it corresponds to at least one of less than 15% and the cold rolling rate after the completion of all passes is less than 50% of the total cold rolling rate.
  • the rolling strain is concentrated in the center of the plate thickness of these comparative steels. It is presumed that the r value was less than 0.9 because it did not occur sufficiently.
  • the 1st martensite area ratio was larger than 30.0% for the 14th and 15th comparative steels. That is, it is presumed that the r value of both the 14th and 15th comparative steels was less than 0.9 because the martensite phase increased more than necessary and the ductility decreased.
  • the total cold rolling ratio was less than 60%, and the first martensite area ratio was larger than 30.0%, so the r value was less than 0.9. It is presumed that it has become.
  • the characteristic evaluations of the 3rd and 4th invention steels were the best overall results among all the invention steels.
  • the steel of the third invention had the third lowest swell height of rigging among all the steels of the invention (2.39 ⁇ m). It is presumed that such a result was obtained because the first martensite area ratio was the third highest among all the invention steels (9.44%).
  • the steel of the third invention had the largest r value among all the steels of the invention (1.12). It is presumed that such a result was obtained because the total cold rolling ratio was the highest among all the invention steels (85%).
  • the steel of the third invention had the fourth lowest ratio of the area of the second martensite among all the steels of the invention (0.17%).
  • the swell height of the rigging of the 4th invention steel was the lowest among all the invention steels (2.28 ⁇ m).
  • the steel of the fourth invention had the fourth largest r value among all the steels of the invention (0.93). It is presumed that such a result was obtained because the total cold rolling ratio was the fourth highest (69%) of all invention steels.
  • the 4th invention steel had the third lowest (0.15%) of the 2nd martensite area ratio among all the invention steels.
  • the present invention can be used for manufacturing ferritic stainless steel and ferritic stainless steel.

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Abstract

Grâce à la présente invention, un acier inoxydable ferritique qui est supérieur à la fois dans l'aptitude à l'emboutissage profond et la résistance au striage est réalisé à un coût inférieur par rapport à l'état de la technique.　Dans cet acier inoxydable ferritique, la hauteur moyenne d'éléments de courbe d'ondulation dans le striage formé sur la surface de l'acier inoxydable ferritique est de 15 µm ou moins et sa valeur r est supérieure ou égale à 0,9 et le rapport de surface d'une phase martensitique dans une coupe transversale dans un plan parallèle à une direction de laminage et perpendiculaire à une direction transversale est de 0 % à moins de 1,0 %.
PCT/JP2021/038703 2020-10-23 2021-10-20 Acier inoxydable ferritique et procédé de fabrication d'acier inoxydable ferritique WO2022085708A1 (fr)

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