Classifications

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  • 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B32—LAYERED PRODUCTS
  • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
  • B32B15/00—Layered products comprising a layer of metal
  • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
  • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/185—Hardening; Quenching with or without subsequent tempering from an intercritical temperature
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
  • C21D1/20—Isothermal quenching, e.g. bainitic hardening
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
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  • C21D6/00—Heat treatment of ferrous alloys
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  • C21D6/00—Heat treatment of ferrous alloys
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  • 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
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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
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • 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
  • C21D8/0273—Final recrystallisation annealing
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  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing 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
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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  • 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
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
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  • 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
  • C21D2211/002—Bainite
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  • 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
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present disclosure relates to a high-strength steel sheet with excellent formability which is suitable mainly for automobile structural members and a production method therefor, and in particular to provision of a high-strength steel sheet having a tensile strength (TS) of 780 MPa or more, not only excellent ductility but also excellent stretch flangeability, and excellent in-plane anisotropy of TS.
• TS tensile strength
• high-strength steel sheets having a TS of 780 MPa or more and reduced in thickness have been increasingly applied to automobile structural members. Further, in recent years, examination has been made of applications of ultra-high-strength steel sheets with 980 MPa and 1180 MPa grade TS.
• JP 2014-189868 A discloses a high-strength steel sheet that has a chemical composition containing, in mass %, C: 0.15% to 0.40%, Si: 1.0% to 2.0%, Mn: 1.5% to 2.5%, P: 0.020% or less, S: 0.0040% or less, Al: 0.01% to 0.1%, N: 0.01% or less, and Ca: 0.0020% or less, with the balance being Fe and inevitable impurities, and has a microstructure in which, in area fraction to the whole microstructure, ferrite phase and bainite phase in total are 40% to 70%, martensite phase is 20% to 50%, and retained austenite phase is 10% to 30%.
• Such a high-strength steel sheet has a tensile strength of 900 MPa or more, and excellent elongation, stretch flangeability, and bendability.
• JP 5454745 B2 discloses a high-strength steel sheet that has a steel component composed of a composition containing, in mass %, C: 0.10% or more and 0.59% or less, Si: 3.0% or less, Mn: 0.5% or more and 3.0% or less, P: 0.1% or less, S: 0.07% or less, Al: 3.0% or less, and N: 0.010% or less where [Si %]+[Al %] ([X %] is mass % of element X) satisfies 0.7% or more, with the balance being Fe and inevitable impurities, and has a steel sheet microstructure in which, in area fraction to the whole steel sheet microstructure, the area fraction of martensite is 5% to 70%, the amount of retained austenite is 5% to 40%, the area fraction of bainitic ferrite in upper bainite is 5% or more, the total of the area fraction of martensite, the area fraction of retained austenite, and the area fraction of bainitic
• JP 5728115 B2 discloses a high-strength steel sheet that contains, in mass %, C: 0.10% to 0.5%, Si: 1.0% to 3.0%, Mn: 1.5% to 3%, Al: 0.005% to 1.0%, P: more than 0% and 0.1% or less, and S: more than 0% and 0.05% or less with the balance being iron and inevitable impurities, and has a metal microstructure that includes polygonal ferrite, bainite, tempered martensite, and retained austenite and in which the area fraction a of the polygonal ferrite to the whole metal microstructure is 10% to 50%, the bainite has a multi-phase of high-temperature-induced bainite in which the average center position distance between adjacent retained austenite grains, between adjacent carbide particles, and between adjacent retained austenite grains and carbide particles is 1 ⁇ m or more and low-temperature-induced bainite in which the average center position distance between adjacent retained austenite grains, between adjacent carbide particles, and between
• PTL 1 to PTL 3 disclose high-strength steel sheets excellent in elongation, stretch flangeability, and bendability as workability, in-plane anisotropy of TS is not considered in any of PTL 1 to PTL 3.
• excellent ductility i.e. “excellent El (total elongation)” denotes that the value of TS ⁇ El is 19000 MPa ⁇ % or more.
• excellent stretch flangeability denotes that the value of X, which is an index of stretch flangeability, is 20% or more regardless of the strength of the steel sheet.
• excellent in-plane anisotropy of TS denotes that the value of
• is calculated according to the following equation (1):
• TS L , TS D , and TS C are TS values measured by performing a tensile test at a crosshead speed of 10 mm/min in accordance with JIS Z 2241 (2011) respectively using JIS No. 5 test pieces collected in three directions: the rolling direction (L direction) of the steel sheet, the direction (D direction) of 45° with respect to the rolling direction of the steel sheet, and the direction (C direction) orthogonal to the rolling direction of the steel sheet.
• An appropriate amount of fine retained austenite can be contained in the microstructure after final annealing, by heating a slab having an appropriately adjusted chemical composition, then subjecting the slab to hot rolling and optionally hot band annealing to soften the hot-rolled sheet, thereafter subjecting the hot-rolled sheet to cold rolling, heating the obtained cold-rolled sheet and subjecting the cold-rolled sheet to first annealing in an austenite single phase region and then controlled cooling, to suppress ferrite transformation and pearlite transformation and cause the microstructure before second annealing to be mainly composed of martensite single phase, bainite single phase, or martensite and bainite mixed phase.
• a high-strength steel sheet having a TS of 780 MPa or more, not only excellent ductility but also excellent stretch flangeability, and excellent in-plane anisotropy of TS can thus be produced.
• a high-strength steel sheet comprising:
• reheating the cold-rolled sheet to a reheating temperature range that is (the T 2 temperature ⁇ 50° C.) or more (i.e. 50° C. below the T 2 temperature or more) and (the T 2 temperature+50° C.) or less (i.e. 50° C. above the T 2 temperature or less) and is (the cooling end temperature+5° C.) or more (i.e. 5° C. above the cooling end temperature or more); and holding the cold-rolled sheet in the reheating temperature range for a time of 10 s or more, wherein
• [% X] denotes a content of an element X in the steel sheet in mass %, and is 0 for any element not contained in the steel sheet.
• Bainite in particular upper bainite, is described below. Transformation from austenite to bainite occurs over a wide temperature range of approximately 150° C. to 550° C., and various types of bainite form in this temperature range. Although these various types of bainite are often simply defined as “bainite” with regard to conventional techniques, upper bainite and lower bainite are separately defined herein because of the need to precisely specify bainite microstructure in order to achieve desired workability.
• the inverse intensity ratio of ⁇ -fiber to ⁇ -fiber can be calculated as follows: Using wet polishing and buffing with a colloidal silica solution, the surface of a cross section (L-cross section) of the steel sheet taken in the sheet thickness direction parallel to the rolling direction is smoothed. The resultant sample surface is then etched with 0.1 vol. % nital so as to reduce irregularities on the surface as much as possible and completely remove the work affected layer. Following this, crystal orientation at a position of sheet thickness ⁇ 1 ⁇ 4 of the steel sheet (a position at a depth of one-fourth of the sheet thickness from the steel sheet surface) is measured using SEM-EBSD (Electron Backscatter Diffraction). Using OIM Analysis available from AMETEK EDAX, the inverse intensity of each of ⁇ -fiber and ⁇ -fiber is determined from the obtained data, to calculate the inverse intensity ratio of ⁇ -fiber to ⁇ -fiber.
• the finisher delivery temperature in the hot rolling needs to be 800° C. or more and 1000° C. or less.
• the finisher delivery temperature is preferably 820° C. or more and 950° C. or less.
• Finish rolling may be performed continuously by joining rough-rolled sheets in the hot rolling.
• Rough-rolled sheets may be coiled on a temporary basis.
• At least part of finish rolling may be conducted as lubrication rolling to reduce the rolling load in the hot rolling.
• Such lubrication rolling is effective from the perspective of making the shape and material properties of the steel sheet uniform.
• the coefficient of friction in the lubrication rolling is preferably in a range of 0.10 to 0.25.
• Second Average Cooling Rate is Higher than Second Average Cooling Rate
• This cooling to (T 2 temperature ⁇ 10° C.) or less is intended to increase the degree of undercooling of upper bainite transformation in the holding after the reheating. If the lower limit of the cooling end temperature after the second annealing treatment is less than 150° C., non-transformed austenite is almost entirely transformed into martensite at this point, so that desired amounts of upper bainite and retained austenite cannot be ensured. If the upper limit of the cooling end temperature after the second annealing treatment is more than (T 2 temperature ⁇ 10° C.), the amounts of upper bainite and retained austenite defined in the present disclosure cannot be ensured. The cooling end temperature after the second annealing treatment is therefore 150° C. or more and (T 2 temperature ⁇ 10° C.) or less.
• the reheating temperature is less than (cooling end temperature+5° C.), the driving force for upper bainite transformation cannot be obtained, and desired amounts of upper bainite and retained austenite cannot be ensured.
• the reheating temperature is therefore (cooling end temperature+5° C.) or more.
• the temperature difference between the reheating temperature and the cooling end temperature has no upper limit, as long as the reheating temperature is not more than (T 2 temperature+50° C.) which is the upper limit temperature.
• the holding time in the reheating temperature range is less than 10 s, the time for the concentration of C into austenite to progress is insufficient, making it difficult to obtain a desired volume fraction of retained austenite in the end.
• the holding time in the reheating temperature range is therefore 10 s or more. If the holding time is more than 1000 s, the volume fraction of retained austenite does not increase and ductility does not improve significantly, where the effect is saturated.
• the holding time in the reheating temperature range is therefore preferably 1000 s or less.
• Cooling after the holding is not limited, and any method may be used to cool the steel sheet to a desired temperature.
• the desired temperature is preferably around room temperature.
• the steel sheet subjected to the above-described annealing treatment is immersed in a galvanizing bath at 440° C. or more and 500° C. or less for hot-dip galvanizing, after which coating weight adjustment is performed using gas wiping or the like.
• a galvanizing bath with a Al content of 0.10 mass % or more and 0.23 mass % or less is preferably used.
• the alloying treatment is performed on the galvanized layer in a temperature range of 470° C. to 600° C. after the hot-dip galvanizing treatment.
• the alloying treatment is preferably performed on the galvanized layer in a temperature range of 470° C. to 600° C. Electrogalvanization may be performed.
• the coating weight is preferably 20 g/m 2 to 80 g/m 2 per side (in the case of both-sided coating).
• a galvannealed steel sheet (GA) is preferably subjected to alloying treatment so that the Fe concentration in the coated layer is 7 mass % to 15 mass %.
• the skin pass rolling is preferably performed with a rolling reduction of 0.1% or more and 2.0% or less.
• a rolling reduction of less than 0.1% is not very effective and complicates control, and hence 0.1% is the lower limit of the favorable range.
• a rolling reduction of more than 2.0% significantly decreases productivity, and thus 2.0% is the upper limit of the favorable range.
• hot-dip galvanized steel sheets GI
• galvannealed steel sheets GA
• electrogalvanized steel sheets EG
• hot-dip galvanizing baths were a zinc bath containing 0.14 mass % or 0.19 mass % of Al for GI and a zinc bath containing 0.14 mass % of Al for GA, and in each case the bath temperature was 470° C.
• the coating weight per side was 72 g/m 2 or 45 g/m 2 in GI (in the case of both-sided coating), and 45 g/m 2 in GA (in the case of both-sided coating).
• the Fe concentration in the coated layer of each hot-dip galvannealed steel sheet (GA) was 9 mass % or more and 12 mass % or less.
• T 1 temperature (° C.) 946 ⁇ 203 ⁇ [% C] 1/2 +45 ⁇ [% Si] ⁇ 30 ⁇ [% Mn]+150 ⁇ [% Al] ⁇ 20 ⁇ [% Cu]+11 ⁇ [% Cr]+400 ⁇ [% Ti].
• the T 1 temperature denotes the Ac 3 point
• the T 2 temperature denotes the upper bainite transformation start temperature
• the tensile test was performed in accordance with JIS Z 2241 (2011) to measure TS (tensile strength) and El (total elongation), using JIS No. 5 test pieces collected so that the longitudinal direction of each tensile test piece coincided with three directions: the rolling direction (L direction) of the steel sheet, the direction (D direction) of 45° with respect to the rolling direction of the steel sheet, and the direction (C direction) orthogonal to the rolling direction of the steel sheet.
• ductility, i.e. El was determined as excellent in the case where the value of TS ⁇ El was 19000 MPa ⁇ % or more.
• the in-plane anisotropy of TS was determined as excellent in the case where the value of
• F ferrite
• UB upper bainite
• M martensite
• TM tempered martensite
• RA retained austenite
• P pearlite
• LB lower bainite
• ⁇ cementite
• AF acicular ferrite

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