EP4375389A1 - Cold-rolled steel sheet and manufacturing method thereof - Google Patents

Cold-rolled steel sheet and manufacturing method thereof Download PDF

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Publication number
EP4375389A1
EP4375389A1 EP22845844.4A EP22845844A EP4375389A1 EP 4375389 A1 EP4375389 A1 EP 4375389A1 EP 22845844 A EP22845844 A EP 22845844A EP 4375389 A1 EP4375389 A1 EP 4375389A1
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EP
European Patent Office
Prior art keywords
steel sheet
cold
rolled steel
less
rolling
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP22845844.4A
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German (de)
English (en)
French (fr)
Inventor
Arisa IKEDA
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nippon Steel Corp
Original Assignee
Nippon Steel Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nippon Steel Corp filed Critical Nippon Steel Corp
Publication of EP4375389A1 publication Critical patent/EP4375389A1/en
Pending legal-status Critical Current

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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
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—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")
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • 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
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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
    • 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
    • C21D8/0242—Flattening; Dressing; Flexing
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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
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
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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
    • 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
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
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    • 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
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    • C22C—ALLOYS
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    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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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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    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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    • 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
    • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • 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
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a cold-rolled steel sheet and a manufacturing method thereof.
  • steel sheets for a vehicle are required to have high strength in order to improve fuel efficiency by reducing a weight of a vehicle body in consideration of the global environment.
  • a desired strength can be imparted to the vehicle body while reducing a sheet thickness of the steel sheet and reducing the weight of the vehicle body.
  • Patent Document 1 discloses, as a high strength steel sheet used for a vehicle component or the like, a high strength steel sheet having a predetermined composition and having a predetermined steel sheet structure primarily containing martensite and bainite, in which an average number of inclusions having an average grain size of 5 ⁇ m or more in a cross section perpendicular to a rolling direction is 5.0 /mm 2 or less, and the high strength steel sheet has an excellent delayed fracture resistance property, and a tensile strength of 1,470 MPa or more.
  • Patent Document 2 discloses a thin steel sheet having a steel structure in which an area ratio of ferrite is 30% or less (including 0%), an area ratio of bainite is 5% or less (including 0%), and an area ratio of martensite and tempered martensite is 70% or more (including 100%), an area ratio of retained austenite is 2.0% or less (including 0%), a ratio of a dislocation density in a range of 0 to 20 ⁇ m from a surface of the steel sheet to a dislocation density of a sheet thickness center portion is 90% or more and 110% or less, and an average of the top 10% of cementite particle sizes from the surface of the steel sheet to a depth of 100 ⁇ m is 300 nm or less, in which a maximum warpage amount of the steel sheet when sheared at a length of 1 m in a longitudinal direction of the steel sheet is 15 mm or less.
  • Patent Document 2 discloses that this thin steel sheet has a tensile strength of 980 MPa or more and can also obtain a ten
  • Patent Document 3 discloses a high strength steel sheet in which a chemical composition (C, Si, Mn, Al, P, and S) satisfies a specified range, a remainder is iron and unavoidable impurities, and martensite occupies 95 area% or more in the entire structure, a structure from a position at a depth of 10 ⁇ m from a surface the steel sheet in a sheet thickness direction to a position at a 114 thickness depth satisfies a predetermined relation, and the steel sheet has a tensile strength of 1,180 MPa or more and an excellent delayed fracture resistance property.
  • a chemical composition C, Si, Mn, Al, P, and S
  • high strength steel sheets having a tensile strength of 1,310 MPa or more have been proposed.
  • high strength steel sheets include martensite and/or tempered martensite as a primary structure.
  • An object of the present invention is to provide a cold-rolled steel sheet having a structure primarily including martensite and tempered martensite, and being capable of, in a case where a load is applied, the steel sheet is left for a certain period of time after removing the load, and a load is applied again, suppressing a decrease in a flow stress when the load is applied again from a flow stress when the initial load is applied (suppressing a decrease in flow stress).
  • the present inventors examined the cause of the above-described decrease in the flow stress. As a result, it was found that even if a structure is martensite and/or tempered martensite throughout the entire sheet thickness direction, in a case where there is a difference in dislocation density in the structure depending on the position in the sheet thickness direction, the flow stress decreases.
  • the present invention has been made in view of the above findings.
  • the gist of the present invention is as follows.
  • a cold-rolled steel sheet having a structure primarily including martensite and tempered martensite, and being capable of, in a case where a load is applied, the steel sheet is left for a certain period of time after removing the load, and a load is applied again, suppressing a decrease in a flow stress when the load is applied again from a flow stress when the initial load is applied, and a manufacturing method thereof.
  • a cold-rolled steel sheet according to an embodiment of the present invention (a cold-rolled steel sheet according to the present embodiment) and a manufacturing method for obtaining the cold-rolled steel sheet will be described.
  • the cold-rolled steel sheet according to the present embodiment has a predetermined chemical composition, in which, in a case where a range of 1/8 to 3/8 of a sheet thickness from a surface in a sheet thickness direction is defined as a t/4 portion and a range of 20 ⁇ m from the surface in the sheet thickness direction is defined as a surface layer portion, a microstructure (metallographic structure) at the t/4 portion includes, by volume percentage, 0% or more and 10.0% or less of retained austenite and 90.0% or more and 100% or less of one or two of martensite and tempered martensite, a ratio of a dislocation density of the surface layer portion to a dislocation density of the t/4 portion is 0.80 or more, and a ratio of a hardness of the surface layer portion to a hardness of the t/4 portion is 0.90 or more.
  • a tensile strength of the cold-rolled steel sheet is 1,310 MPa or more.
  • a range indicated with “to” in between includes, in principle, the values at both ends of the range as a lower limit and an upper limit. However, numerical values indicated as “more than” or “less than” are not included in the range.
  • % of an amount of each element means mass%.
  • C is related to a hardness of martensite and tempered martensite and is an element necessary for increasing a strength of the steel sheet.
  • a C content needs to be at least 0.150% or more. Therefore, the C content is set to 0.150% or more.
  • the C content is preferably 0.180% or more, and more preferably 0.200% or more.
  • the C content is set to 0.500% or less.
  • the C content is preferably 0.350% or less, and more preferably 0.300% or less.
  • Si is a solid solution strengthening element and is an effective element for high-strengthening of the steel sheet.
  • a Si content is set to 0.01% or more.
  • the Si content is set to preferably 0.10% or more, and more preferably 0.20% or more.
  • the Si content is set to 2.00% or less. Therefore, the Si content is set to preferably 1.80% or less and more preferably 1.70% or less.
  • Mn is an element that improves hardenability and is an element that promotes the generation of martensite.
  • the Mn content is set to 0.50% or more.
  • the Mn content is set to 3.00% or less.
  • the Mn content is preferably 2.80% or less.
  • a P content is preferably as small as possible and may be 0%. However, in consideration of a time and a cost for removing P, the P content is set to 0.0200% or less.
  • the P content is preferably 0.0150% or less, and more preferably 0.0100% or less.
  • the P content may be set to 0.0001% or more.
  • a S content is preferably as small as possible and may be 0%. However, in consideration of a time and a cost for removing S, the S content is set to 0.0200% or less.
  • the S content is preferably 0.0100% or less, more preferably 0.0050% or less, and even more preferably 0.0030% or less.
  • the S content may be set to 0.0001% or more.
  • Al is an element having an action of deoxidizing molten steel.
  • A1 does not necessarily have to be contained, and an Al content may be 0%.
  • Al may be contained for the purpose of deoxidation. Therefore, the Al content is preferably set to 0.001% or more.
  • Al has an action of enhancing the stability of austenite like Si, and thus may be contained in order to obtain retained austenite.
  • the Al content is set to 0.100% or less.
  • the Al content is preferably 0.050% or less, more preferably 0.040% or less, and even more preferably 0.030% or less.
  • N is an element that can be contained in steel as an impurity and is an element that forms coarse precipitates and deteriorates the formability. Therefore, a N content is set to 0.0200% or less.
  • the N content is preferably 0.0100% or less, and more preferably 0.0060% or less.
  • the N content is preferably as small as possible and may be 0%. However, from the viewpoint of a refining cost or the like, the N content may be set to 0.0001% or more.
  • O is an element that is contained as an impurity.
  • an O content is more than 0.020%, coarse oxides are formed in steel, and the formability decreases. Therefore, the O content is set to 0.020% or less.
  • the O content is set to preferably 0.010% or less, and more preferably 0.005% or less.
  • the O content may be 0%. However, from the viewpoint of refining cost or the like, the O content may be set to 0.0001% or more or 0.001% or more.
  • the remainder excluding the above elements is basically Fe and impurities.
  • the impurities are incorporated from steel raw materials and/or in a steelmaking process and are elements that are allowed to be present in a range in which the characteristics of the cold-rolled steel sheet according to the present embodiment are not clearly deteriorated.
  • the chemical composition of the cold-rolled steel sheet according to the present embodiment may contain, instead of a portion of Fe, one or two or more selected from the group consisting of Ni, Mo, Cr, B, As, Co, Ti, Nb, V, Cu, W, Ta, Ca, Mg, La, Ce, Y, Zr, Sb, and Sn in the following ranges. Since these elements may not be contained, lower limits thereof are 0%. In addition, even if these elements are contained as impurities, the effects of the cold-rolled steel sheet according to the present embodiment are not impaired as long as the amounts of the elements are within the ranges described below.
  • Ni, Mo, Cr, B, and As are elements that improve the hardenability and contribute to the high-strengthening of the steel sheet. Therefore, these elements may be contained.
  • a Ni content, a Mo content, and a Cr content are set to 0.010% or more
  • a B content is set to 0.0001% or more
  • an As content is set to 0.001% or more. More preferably, the Ni content, the Mo content, and the Cr content are 0.050% or more
  • the B content is 0.001% or more
  • the As content is 0.005% or more. It is not essential to obtain the above effects. Therefore, it is not necessary to particularly limit lower limits of the Ni content, the Mo content, the Cr content, the B content, and the As content, and the lower limits thereof are 0%.
  • the Ni content and the Mo content are set to 1.000% or less, the Cr content is set to 2.000% or less, the B content is set to 0.010% or less, and the As content is set to 0.050% or less.
  • the Ni content and the Mo content are each preferably 0.500% or less, the Cr content is preferably 1.000% or less, the B content is preferably 0.0060% or less, and the As content is 0.030% or less.
  • Co is an element effective in improving the strength of the steel sheet.
  • a Co content may be 0%. However, in order to obtain the above effect, the Co content is preferably 0.010% or more, and more preferably 0.100% or more.
  • the Co content is set to 0.500% or less.
  • Ti, Nb, V, Cu, W, and Ta are elements having an action of improving the strength of the steel sheet by precipitation hardening. Therefore, these elements may be contained. In order to sufficiently obtain the above effect, it is preferable that one or more of Ti, Nb, V, Cu, W, and Ta are contained and the amount of each element is 0.001% or more.
  • the Ti content, the Nb content, the V content, and the Cu content are each set to 0.500% or less.
  • the W content and the Ta content are each set to 0.100% or less.
  • Ca, Mg, La, Ce, Y, Zr, and Sb are elements that contribute to the fine dispersion of inclusions in steel, and are elements that contribute to the improvement of the formability of the steel sheet by this fine dispersion. Therefore, these elements may be contained. In order to obtain the above effects, it is preferable that one or more of Ca, Mg, La, Ce, Y, Zr, and Sb are contained and the amount of each element is set to 0.001% or more.
  • the amounts of Ca, Mg, La, Ce, Y, Zr, and Sb are each set to 0.050% or less.
  • Sn is an element that suppresses the coarsening of grains and contributes to the improvement in the strength of the steel sheet. Therefore, Sn may be contained.
  • Sn is an element that may cause a decrease in cold formability of the steel sheet attributed to the embrittlement of ferrite.
  • Sn content is set to 0.050% or less.
  • the Sn content is preferably 0.040% or less.
  • the chemical composition of the cold-rolled steel sheet according to the present embodiment can be obtained by the following method.
  • the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES) for chips according to JIS G 1201 (2014).
  • ICP-AES inductively coupled plasma-atomic emission spectrometry
  • the chemical composition is an average content in the entire sheet thickness.
  • C and S may be measured using a combustion-infrared absorption method
  • N may be measured using an inert gas fusion-thermal conductivity method
  • O may be measured using an inert gas fusion-non-dispersive infrared absorption method.
  • the chemical composition may be analyzed after removing the coating layer by mechanical grinding or the like.
  • the coating layer may be removed by dissolving the plating layer in an acid solution containing an inhibitor that suppresses the corrosion of the steel sheet.
  • a range of a 1/8 thickness position to a 3/8 thickness position from the surface centered at a 1/4 thickness position from the surface in the sheet thickness direction will be described as a t/4 portion ((1/4)t portion), and a range from the surface to 20 ⁇ m in the sheet thickness direction will be described as a surface layer portion.
  • Retained austenite contributes to the improvement in the formability of the steel sheet by improving uniform elongation of the steel sheet through a TRIP effect. Therefore, retained austenite (retained y) may be contained.
  • the volume percentage of retained austenite is preferably set to 1.0% or more.
  • the volume percentage of retained austenite is more preferably 2.0% or more, and even more preferably 3.0% or more.
  • the volume percentage of retained austenite is set to 10.0% or less.
  • the volume percentage of retained austenite is preferably 8.0% or less, and more preferably 7.0% or less.
  • one or two of martensite and tempered martensite are contained.
  • Martensite (so-called fresh martensite) and tempered martensite are aggregates of lath-shaped grains and greatly contribute to the improvement in strength. Therefore, the cold-rolled steel sheet according to the present embodiment contains martensite and tempered martensite in a total volume percentage of 90.0% to 100%.
  • tempered martensite is a hard structure containing fine iron-based carbides inside by tempering.
  • Tempered martensite is a structure that contributes less to the improvement in strength than martensite but is not brittle and has ductility. Therefore, in a case where it is desired to further increase the formability, it is preferable to increase the volume percentage of tempered martensite.
  • the volume percentage of tempered martensite is 85.0% or more.
  • the microstructure may contain bainite in addition to retained austenite, martensite, and tempered martensite. It is preferable that ferrite and pearlite are not contained.
  • the volume percentage of each structure in the microstructure of the 114 portion of the cold-rolled steel sheet according to the present embodiment is measured as follows.
  • the volume percentages of ferrite, bainite, martensite, tempered martensite, and pearlite are obtained by collecting a test piece from an arbitrary position in a rolling direction and in a width direction of the steel sheet, polishing a longitudinal section parallel to the rolling direction (cross section parallel to the sheet thickness direction), and observing a structure revealed by Nital etching in the range (t/4 portion) of 1/8 to 3/8 of the sheet thickness from the surface using a scanning electron microscope (SEM).
  • SEM scanning electron microscope
  • a region with no substructure revealed and a low luminance is defined as ferrite.
  • a region with no substructure revealed and a high luminance is defined as martensite or retained austenite.
  • a region in which a substructure is revealed is defined as tempered martensite or bainite.
  • Bainite and tempered martensite can be distinguished from each other by further carefully observing carbides in grains.
  • tempered martensite includes martensite laths and cementite generated within the laths.
  • cementite included in the tempered martensite has a plurality of variants.
  • bainite is classified into upper bainite and lower bainite.
  • Upper bainite includes lath-shaped bainitic ferrite and cementite generated at the interface between the laths and can be easily distinguished from tempered martensite.
  • Lower bainite includes lath-shaped bainitic ferrite and cementite generated within the laths.
  • the volume percentage of martensite is calculated by subtracting the volume percentage of retained austenite calculated by a method described later from a volume percentage of a structure determined to be martensite or retained austenite.
  • the volume percentage of retained austenite is obtained as described below: a test piece is collected from an arbitrary position in the steel sheet, a rolled surface is chemically polished from the surface of the steel sheet to a 114 thickness position, and the volume percentage of retained austenite is quantified from integrated intensities of (200) and (210) planes of ferrite and integrated intensities of (200), (220), and (311) planes of austenite by MoK ⁇ radiation.
  • the microstructure primarily includes martensite and/or tempered martensite obtained by tempering martensite.
  • Martensite can be obtained by holding the steel sheet in an austenite single phase region and then rapidly cooling the steel sheet.
  • martensite has a difference in structural characteristics (for example, dislocation density contained) depending on the position in the steel sheet in the sheet thickness direction. This difference is attributed to a difference in timing of transformation. That is, during cooling, a temperature of a region close to the surface of the steel sheet first decreases, and then a temperature inside the steel sheet decreases. Therefore, the transformation from austenite to martensite occurs first on a surface layer side of the steel sheet.
  • the martensite generated on the surface layer side is held at a high temperature for a longer period of time than the martensite inside, and undergoes tempering.
  • the tempering causes a decrease in dislocation density in martensite.
  • the ratio ( ⁇ s / ⁇ t/4 ) of the dislocation density ( ⁇ s ) of the surface layer portion to the dislocation density ( ⁇ t/4 ) of the t/4 portion is set to 0. 80 or more.
  • ( ⁇ s / ⁇ t/4 ) is preferably 0.85 or more, and more preferably 0.90 or more.
  • the dislocation density of the t/4 portion is preferably 5.2 ⁇ 10 15 M -2 or more. Therefore, in consideration of a preferable range of ⁇ s / ⁇ t/4 , the dislocation density of the surface layer portion is preferably 4.2 ⁇ 10 15 M -2 or more.
  • the dislocation density at each position is obtained by the following method.
  • a position 20 ⁇ m away from the surface of the steel sheet is a representative structure of the surface layer portion and a 114 thickness position from the surface is a representative structure of the 114 portion
  • a sample obtained by grinding 20 ⁇ m from the surface and a sample obtained by grinding 114 of the sheet thickness from the surface are prepared, strain is removed by performing chemical polishing on each ground surface, and then X-ray diffraction is performed.
  • a dislocation density at the position 20 ⁇ m away from the surface and a dislocation density at the 114 thickness position from the surface are obtained from an X-ray diffraction profile obtained by the X-ray diffraction using a modified Williamson-Hall method and a modified Warren-Averbach method.
  • the dislocation density is obtained according to a method described in ISIJ Int. vol. 50 (2010) p. 875-882 .
  • the dislocation density at the position 20 ⁇ m away from the surface is defined as the dislocation density of the surface layer portion, and the dislocation density at the 114 thickness position from the surface is defined as the dislocation density of the 114 portion.
  • dislocations in the surface layer portion are particularly immobilized.
  • the ratio of the hardness of the surface layer portion to the hardness of the t/4 portion is used as an index of whether or not dislocations are immobilized.
  • ( ⁇ s / ⁇ t/4 ) is 0.80 or more and the ratio of the hardness of the surface layer portion to the hardness of the t/4 portion is 0.90 or more, dislocations are immobilized, and a decrease in the flow stress can be prevented.
  • the ratio of the hardness is preferably 0.92 or more, more preferably 0.94 or more, and even more preferably 0.95 or more.
  • Immobilization of dislocations can be achieved by performing skin pass rolling on a steel sheet with an intentionally provided difference in sheet thickness, which will be described later.
  • the hardness of the t/4 portion is preferably 360 Hv or more. Therefore, in consideration of a preferable range of the ratio of the hardness of the surface layer portion to the hardness of the t/4 portion, the hardness of the surface layer portion is preferably 324 Hv or more.
  • the hardness is obtained by the following method.
  • a cut surface perpendicular to the rolling direction of the steel sheet and parallel to the sheet thickness direction is formed and mirror-polished.
  • Vickers hardness measurement is performed at four points each at the position 20 ⁇ m away from the surface of the steel sheet and at the 1/4 thickness position from the surface on the cut surface based on JIS Z 2244-1 (2020).
  • a load in the Vickers hardness measurement is set to 2 kgf.
  • An average value of hardness measurement values at the position 20 ⁇ m away from the surface of the steel sheet is defined as the hardness of the surface layer portion
  • an average value of hardness measurement values at the 114 thickness position from the surface is defined as the hardness of the 114 portion.
  • the cold-rolled steel sheet according to the present embodiment may have, on the surface (one or both), zinc, aluminum, or magnesium, or an alloy of one or more of these metals, or coating layer made of zinc, aluminum, or magnesium, or an alloy of one or more of these metals (Containing of impurities and the like are permitted).
  • Corrosion resistance is improved by providing the coating layer on the surface.
  • the steel sheet cannot be thinned to a certain sheet thickness or less even if the high-strengthening is achieved.
  • One of the purposes of the high-strengthening of the steel sheet is to reduce the weight by thinning. Therefore, even if a high strength steel sheet is developed, an application range of a steel sheet with low corrosion resistance is limited. As a method for solving these problems, it is conceivable to form a coating layer on the surface in order to improve corrosion resistance.
  • the coating layer is, for example, a hot-dip galvanized layer, a hot-dip galvannealed layer, an electrogalvanized layer, an aluminum plating layer, a Zn-Al alloy plating layer, an Al-Mg alloy plating layer, or a Zn-Al-Mg alloy plating layer.
  • a surface used as a reference for the t/4 portion and the like described above is a surface of base metal excluding the coating layer.
  • an object is to achieve a tensile strength (TS) of 1,310 MPa or more as a strength that contributes to a reduction in weight of a vehicle body of a vehicle.
  • TS tensile strength
  • the tensile strength of the steel sheet is preferably 1,400 MPa or more and more preferably 1,470 MPa or more.
  • the tensile strength may be set to 2,000 MPa or less.
  • the cold-rolled steel sheet according to the present embodiment can be stably manufactured according to the following manufacturing method, although the effects can be obtained as long as the cold-rolled steel sheet has the above-described characteristics regardless of the manufacturing method.
  • the cold-rolled steel sheet according to the present embodiment can be manufactured by a manufacturing method including the following steps (I) to (VI):
  • a difference between a sheet thickness of the center portion in the width direction and a sheet thickness of the edge portion of the cold-rolled steel sheet after the cold rolling step is set to 10 ⁇ m or more.
  • a slab having the same chemical composition as the cold-rolled steel sheet according to the present embodiment is hot-rolled to obtain a hot-rolled steel sheet.
  • the hot rolling is preferably performed under conditions in which a finish rolling completion temperature is Ac3°C or higher in order to satisfy the temperature at the time of coiling, which will be described later.
  • An upper limit of the finish rolling completion temperature is not particularly limited, but is generally 950°C or lower.
  • This hot-rolled steel sheet is coiled in a state in which the temperature of the center portion in the width direction is higher than 600°C and 700°C or lower and the temperature of the edge portion at the position 20 mm from the end portion in the width direction is 600°C or lower.
  • the edge portion In order to cause a coiling temperature of the edge portion to be lower than that of the center portion in the width direction, the edge portion is cooled at a cooling rate faster than that of the center portion. For example, in a case where only the edge portion of the steel sheet after hot rolling is subjected to water cooling or the entire steel sheet is subjected to water cooling, the amount of cooling water for the edge portion may be set to be larger than that for the center portion in the width direction.
  • the edge portion After the edge portion is subjected to water cooling, the edge portion is tempered by heat transfer from the center portion in the width direction with a higher temperature during the coiling and thus becomes softer than the center portion in the width direction. As a result, in a state of being cooled to near room temperature, a strength of the edge portion is lower than a strength of the center portion in the width direction.
  • the center portion in the width direction When the coiling temperature of the center portion in the width direction is higher than 700°C, the center portion in the width direction is softened. In addition, when the coiling temperature of the center portion in the width direction is 600°C or lower, difference in temperature from the edge portion becomes small, or the edge portion cannot be sufficiently tempered.
  • the coiling temperature of the center portion is preferably 620°C or higher.
  • the coiling temperature of the edge portion is higher than 600°C, a softening effect by the tempering cannot be sufficiently obtained.
  • the coiling temperature of the edge portion is 400°C or lower, tempering is performed by heat transfer from the center portion in the width direction.
  • the coiling temperature of the edge portion is preferably higher than 400°C, and more preferably 450°C or higher.
  • a difference in coiling temperature between the center portion in the width direction and the edge portion is preferably 50°C or higher, more preferably 75°C or higher, and even more preferably 100°C or higher.
  • a manufacturing method of the slab that is subjected to the hot rolling is not particularly limited.
  • a steel having the above-described chemical composition is melted by a known method, thereafter made into a steel ingot by a continuous casting method, or made into a steel ingot by any casting method, and then made into a steel piece by a blooming method or the like.
  • an external additional flow such as electromagnetic stirring to occur in molten steel in a mold.
  • the steel ingot or steel piece may be reheated after being cooled once and subjected to hot rolling, or the steel ingot in a high temperature state after the continuous casting or the steel piece in a high temperature state after the blooming may be subjected to hot rolling as it is, after being kept hot, or after being subjected to auxiliary heating.
  • the steel ingot and the steel piece are collectively referred to as a "slab" as a material of hot rolling.
  • the hot-rolled steel sheet after the hot rolling step is pickled and cold-rolled at a rolling reduction of 30% to 90% to obtain a cold-rolled steel sheet.
  • Pickling conditions are not particularly limited and may be known conditions.
  • the cold rolling step by performing cold rolling on the steel sheet having a difference in strength in the width direction, a steel sheet (cold-rolled steel sheet) having a difference in sheet thickness in the width direction is obtained.
  • the cold-rolled steel sheet in which the difference between the sheet thickness of the center portion in the width direction of the cold-rolled steel sheet and the sheet thickness of the edge portion is 10 ⁇ m or more after the cold rolling step can be obtained.
  • the difference in sheet thickness is preferably 15 ⁇ m or more.
  • An upper limit of the difference in sheet thickness is not limited. However, when the difference in sheet thickness is large, there are cases where cracks are initiated from a portion with a small sheet thickness and hole expansibility decreases. Therefore, from the viewpoint of formability, the difference in sheet thickness may be set to 55 ⁇ m or less.
  • the sheet thickness of the center portion in the width direction and the sheet thickness of the edge portion can be measured by installing a scanning sheet thickness meter on an outlet side of a cold rolling mill.
  • Width trimming may be performed to cut off any width from the end portion in the width direction of the steel sheet as long as the difference between the sheet thickness of the center portion in the width direction and the sheet thickness of the edge portion is 10 ⁇ m or more after cutting.
  • the portion is cut off, whereby the steel sheet can be provided for a subsequent step, which is preferable in terms of cost and yield.
  • the cold-rolled steel sheet after the cold rolling step is performed or further after the width trimming step is performed if necessary is heated to an annealing temperature of higher than Ac3°C and is held at this annealing temperature.
  • the annealing temperature is Ac3°C or lower, the structure does not sufficiently undergo austenitic transform, and a desired microstructure primarily containing martensite cannot be obtained after the annealing step.
  • the annealing temperature is preferably set to 900°C or lower.
  • a temperature (°C) at the Ac3 point can be obtained by the following method.
  • Ac3 910 - (203 ⁇ C 1/2 ) + 44.7 ⁇ Si - 30 ⁇ Mn + 700 ⁇ P - 20 ⁇ Cu - 15.2 ⁇ Ni - 11 ⁇ Cr + 31.5 ⁇ Mo + 400 ⁇ Ti + 104 ⁇ V + 120 ⁇ Al
  • a holding time at the annealing temperature is preferably 40 to 135 seconds.
  • the cold-rolled steel sheet after the holding is cooled to a cooling stop temperature so that an average cooling rate to 400°C is 10 °C/sec or faster and an average cooling rate from 400°C to a cooling stop temperature of 100°C or lower is 15 °C/sec or faster.
  • a coating layer made of zinc, aluminum, or magnesium, or an alloy of one or more of these metals may be formed on the surface (one or both surfaces) of the cold-rolled steel sheet.
  • the steel sheet in the case of forming the coating layer, for example, in a case of hot-dip plating, in a range in which the average cooling rate to 400°C is 10 °C/sec or faster and the average cooling rate from 400°C to a cooling stop temperature of 100°C or lower is 15 °C/sec or faster, the steel sheet may be immersed in a plating bath during the cooling to form a hot-dip plating on the surface and held in a temperature range of 450°C to 470°C for 10 to 40 seconds.
  • an alloying treatment is applied to the hot-dip plating layer
  • 470°C melting temperature
  • 550°C melting temperature
  • alloying temperature is more preferably 480°C or higher.
  • the alloying temperature is more preferably 540°C or lower.
  • Such a cold-rolled steel sheet is subjected to a heat treatment by heating the cold-rolled steel sheet to a temperature range of 200°C to 350°C and holding the cold-rolled steel sheet in this temperature range.
  • the holding time is preferably set to 1 second or longer.
  • a heating temperature is lower than 200°C, there are cases where martensite is not sufficiently tempered and satisfactory changes in microstructure and mechanical properties cannot be achieved. In a case where the heating temperature is higher than 350°C, a dislocation density in tempered martensite decreases, which may lead to a decrease in tensile strength.
  • the cold-rolled steel sheet after the heat treatment step is subjected to skin pass rolling at a rolling reduction of 0.1% or more.
  • the cold-rolled steel sheet after the heat treatment step has a difference in sheet thickness of 10 ⁇ m or more between the center portion in the width direction and the edge portion.
  • any rolling reduction may be selected by settings a skin pass rolling mill.
  • the amount of strain introduced into the surface layer portion can be even higher than the amount of strain assumed to be introduced by the rolling reduction set in a case where the sheet thickness is uniform.
  • the rolling reduction is set to 0.1% or more.
  • An upper limit of the rolling reduction is not limited. However, when the upper limit thereof is more than 1.5%, the productivity significantly decreases. Therefore, the upper limit is preferably set to less than 1.5%.
  • skin pass rolling with a rolling reduction of 0.1% or more is not performed on a steel sheet having a tensile strength of 1,310 MPa or more.
  • skin pass rolling with a rolling reduction of 0.1% or more is performed based on the above-described new findings found by the present inventors.
  • Slabs (kinds of steel A to W) having the chemical composition shown in Tables 1-1 and 1-2 (unit: mass%, remainder: Fe and impurities) were manufactured by continuous casting.
  • hot rolling was performed such that a finish rolling completion temperature reached Ac3°C or higher, and coiling was performed under the conditions shown in Table 2-1 by changing cooling conditions between a center portion and an edge portion, thereby obtaining hot-rolled steel sheets.
  • hot-dip galvanizing was performed on some of the cold-rolled steel sheets during the annealing.
  • a holding temperature After immersion in a plating bath, some of plating layers were alloyed.
  • a holding time was set to 10 to 40 seconds.
  • GI indicates that a hot-dip galvanized layer is formed
  • GA indicates that a hot-dip galvannealed layer is formed.
  • Table 3-1 shows measurement results of the volume percentages of martensite, tempered martensite, bainite, and retained austenite. Although not shown in the table, in Nos. 38 and 39, ferrite was generated in addition to martensite, tempered martensite, bainite, and retained austenite.
  • Results are shown in Table 3-2. However, regarding the notation of the dislocation density, "5.0E + 15" in the table means 5.0 ⁇ 10 15 , and YE + X also means Y ⁇ 10 X in the same manner.
  • the tensile strength was obtained by the following method.
  • the tensile strength (TS) was measured by collecting a JIS No. 5 test piece from an orientation in which a longitudinal direction of the test piece was parallel to an orthogonal-to-rolling direction of the steel sheet and conducting a tensile test in accordance with JIS Z 2241 (2011).
  • a change in flow stress was evaluated by collecting a JIS No. 5 test piece from an orientation in which a longitudinal direction of the test piece was parallel to the orthogonal-to-rolling direction of the steel sheet, and comparing a stress in a case where a prestrain of 0.1% was applied in accordance with JIS Z 2241 (2011), the steel sheet was left for one day after removing the prestrain, and the test piece was pulled again, to a stress when a prestrain of 0.1% was applied.

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EP22845844.4A 2021-07-21 2022-07-14 Cold-rolled steel sheet and manufacturing method thereof Pending EP4375389A1 (en)

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JP5662920B2 (ja) 2011-11-11 2015-02-04 株式会社神戸製鋼所 耐遅れ破壊性に優れた高強度鋼板およびその製造方法
JP6294197B2 (ja) * 2014-09-19 2018-03-14 株式会社神戸製鋼所 熱延鋼板及びその製造方法
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