WO2020017609A1 - Tôle d'acier - Google Patents

Tôle d'acier Download PDF

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WO2020017609A1
WO2020017609A1 PCT/JP2019/028357 JP2019028357W WO2020017609A1 WO 2020017609 A1 WO2020017609 A1 WO 2020017609A1 JP 2019028357 W JP2019028357 W JP 2019028357W WO 2020017609 A1 WO2020017609 A1 WO 2020017609A1
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steel sheet
retained austenite
martensite
content
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PCT/JP2019/028357
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English (en)
Japanese (ja)
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宏志 海藤
林　宏太郎
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日本製鉄株式会社
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Priority to JP2019562021A priority Critical patent/JP6737411B2/ja
Priority to CN201980019592.2A priority patent/CN111868282B/zh
Publication of WO2020017609A1 publication Critical patent/WO2020017609A1/fr

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    • 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
    • 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/20—Recycling

Definitions

  • the present disclosure relates to a steel sheet having excellent formability and ultra-high strength characteristics, and specifically relates to a steel sheet having excellent local ductility and high tensile strength and having a high content of Mn.
  • Patent Document 1 a so-called TRIP steel utilizing the transformation-induced plasticity of retained austenite has been proposed (for example, Patent Document 1).
  • Retained austenite is obtained by enriching C in austenite so that austenite does not transform into another phase even at room temperature.
  • a technique for stabilizing austenite it has been proposed to include a carbide precipitation suppressing element such as Si and Al in a steel sheet and to enrich C in austenite during bainite transformation occurring in the steel sheet in a steel sheet manufacturing stage. I have.
  • the austenite is further stabilized, the amount of retained austenite can be increased, and as a result, a steel sheet excellent in both strength and elongation can be produced.
  • welding is often performed on the steel sheet.
  • Non-Patent Document 1 As a steel sheet having a C content lower than that of the TRIP steel, a retained austenite amount larger than that of the TRIP steel, and a strength and ductility exceeding the TRIP steel, a steel to which Mn of more than 4.0% by mass is added is proposed.
  • the steel requires a long heating process such as box annealing. For this reason, material design in a short-time heating process such as continuous annealing suitable for manufacturing a high-strength steel sheet to be used as a member for an automobile has not been sufficiently studied, and a requirement for increasing local ductility in that case was not clear.
  • Patent Document 2 discloses a steel sheet to which 3.5% by mass or more of Mn is added and which has excellent tensile strength and elongation by controlling ferrite to 30% to 80%.
  • Patent Document 3 discloses that in a steel containing 3 to 7% by mass of Mn, local ductility is improved by suppressing the generation of retained austenite to less than 20% by volume. Specifically, local elongation is 8%. The above steel sheet is disclosed.
  • Patent Document 4 discloses a high-strength steel sheet containing 2.0 to 6.0% by mass of Mn and having a retained austenite amount of 20% by volume or more.
  • Patent Document 5 discloses a high-strength steel sheet containing 1.68 to 3.8% by mass of Mn, having excellent local ductility, and having a local elongation of 4% or more.
  • a steel sheet having a high content of Mn as disclosed in, for example, Patent Document 2 and Non-Patent Document 1 contains a large amount of unrecrystallized ferrite and thus does not work harden and has low workability and formability. That is, a steel sheet having a high content of Mn having such a structure containing a large amount of unrecrystallized ferrite cannot have both the tensile strength and the workability and formability required for a steel sheet for automobiles.
  • the steel sheet as an example of the invention described in Patent Document 3 has a low C content of less than 0.2% by mass, and thus has a tensile strength of 1090 MPa or less and maintains the C content at less than 0.2% by mass.
  • a comparative example in which the tensile strength is 1233 MPa and the local elongation is 1.3% is shown, and the local ductility is reduced.
  • the steel sheet described in Patent Document 4 is obtained by preheating a hot-rolled sheet or a cold-rolled sheet to 800 ° C. or higher, austenitizing and cooling, and then performing annealing twice at an annealing temperature of 650 to 750 ° C. This is to promote the concentration of alloy elements therein to generate retained austenite of 20% by volume or more.
  • sufficient local ductility was not obtained.
  • the present inventors have set the metal structure of the steel sheet to have an area ratio (area%) of 25% or more and 90% or more as a matrix.
  • the following tempered martensite and 3% or less of ferrite, and 10% or more and 50% or less of retained austenite and 25% or less of fresh martensite are contained as other phases. It has been found that it is effective to set the interface density between retained austenite and fresh martensite to 2.7 ⁇ m ⁇ 1 or more.
  • Local ductility is a property that a material can withstand crack propagation generated in a local deformation region of stretch flange forming or bending, and its index is local elongation obtained by a uniaxial tensile test.
  • voids may be generated in a constricted region after the start of local deformation.
  • the voids are connected and propagated in the subsequent deformation process, thereby generating and propagating a crack, leading to fracture.
  • the present inventors believe that it is effective to improve the local ductility to have a microstructure with little variation with respect to crack propagation from any direction, and as a result of earnest study, the present inventors have found that the following microstructure By doing so, a steel sheet having high strength and excellent local ductility can be obtained.
  • the matrix of the metal structure of the steel sheet is tempered martensite.
  • Martensite is a low-temperature transformation phase containing many dislocations in its phase, and is a useful phase for increasing the strength of a steel sheet. Further, by using tempered martensite, the balance between strength and local ductility is improved.
  • the ductility is improved by containing retained austenite in the microstructure.
  • the ductility improving effect of soft austenite but also the TRIP effect is exhibited by the work-induced transformation to martensite during working, and the balance of strength-ductility-local ductility can be further improved.
  • Fresh martensite partially transformed during quenching may be adjacent to the retained austenite.
  • Fresh martensite and martensite formed by process-induced transformation are harder phases than the tempered martensite of the parent phase, and not only increase the strength of the steel, but also reduce the propagation of cracks that propagate from the soft phase. It also has the role of stopping at the interface.
  • the metal structure at a position 1/4 of the thickness from the surface is tempered martensite having an area ratio of 25% or more and 90% or less, 3% or less ferrite, and 10% or more and 50% or less. Contains retained austenite and up to 25% fresh martensite.
  • the L section refers to a surface cut parallel to the sheet thickness direction and the rolling direction so as to pass through the central axis in the rolling direction of the steel sheet.
  • FIG. 1 is a scanning electron microscope (SEM) image obtained by observing the L section of a steel sheet according to an embodiment of the present invention after mirror polishing and nital treatment.
  • FIG. 2 is a two-tone image obtained by binarizing the SEM image of FIG.
  • the metal structure of the steel sheet is controlled such that the interface density between the retained austenite and the fresh martensite with respect to the matrix mainly composed of tempered martensite is 2.7 ⁇ m ⁇ 1 or more.
  • the interface between the retained austenite and the fresh martensite with respect to the parent phase which is an obstacle to any crack propagation direction, exists at a fixed ratio, so that the interface strength is improved and excellent local ductility is obtained.
  • the steel plate of the present disclosure has been made based on the above findings, and the gist is as follows. (1) In mass%, C: more than 0.20% and less than 0.55%, Si: 0.001% or more and less than 3.50%, Mn: more than 4.00% and less than 9.00%, sol.
  • the interface density which is a value obtained by dividing the total length of the boundary between the second region and the second region by the total area of the first region and the second region, is 2.7 ⁇ m ⁇ 1 or more.
  • steel plate (2) In mass%, B: The steel sheet according to the above (1), which contains 0.0003% or more and less than 0.010%.
  • a steel sheet according to claim 1. In mass%, Cr: 0.01% or more and less than 2.00%, Mo: 0.01% or more and 2.00% or less, W: 0.01% or more and 2.00% or less, (1) or (2), characterized by containing one or more of Cu: 0.01% or more and 2.00% or less, and Ni: 0.01% or more and 2.00% or less.
  • a steel sheet according to claim 1. (4) In mass%, Ti: 0.005% or more and 0.300% or less, (1) to (3), wherein one or more of Nb: 0.005% to 0.300%, and V: 0.005% to 0.300% are contained. The steel sheet according to any one of the above.
  • FIG. 1 is a scanning electron microscope (SEM) image obtained by mirror-polishing and nital-treating the L section of the steel sheet obtained in the example.
  • FIG. 2 is a two-tone image obtained by binarizing the SEM image of FIG.
  • FIG. 3 is an SEM image obtained by observing the L section of the steel sheet obtained in the comparative example after mirror polishing and nital treatment.
  • C is a very important element for increasing the tensile strength of steel.
  • a C content of more than 0.20% is required.
  • the upper limit of the C content is set to less than 0.55%.
  • the lower limit of the C content is preferably 0.24% or more, more preferably 0.28% or more.
  • the upper limit of the C content is preferably 0.40% or less, more preferably 0.35% or less.
  • Si is an element effective for strengthening tempered martensite, homogenizing the structure, and improving local ductility. Further, Si also has an effect of suppressing precipitation and coarsening of cementite and facilitating control of austenite generated during annealing. In order to obtain the above effects, a Si content of 0.001% or more is required.
  • the lower limit of the Si content is preferably 0.01% or more, more preferably 0.30% or more, and still more preferably 0.50% or more. By setting the lower limit of the Si content to the above range, the local ductility of the steel sheet can be further improved.
  • the upper limit of the Si content is set to less than 3.50%. Further, the upper limit of the Si content is preferably 3.00% or less, more preferably 2.50% or less.
  • Mn is an element that stabilizes austenite and enhances hardenability. Further, in the steel sheet of the present disclosure, Mn is distributed in austenite to further stabilize austenite. To stabilize austenite at room temperature, more than 4.00% Mn is required. On the other hand, if the steel sheet excessively contains Mn, ductility and local ductility are impaired, so the upper limit of the Mn content is set to less than 9.00%.
  • the lower limit of the Mn content is preferably at least 4.30%, more preferably at least 4.80%.
  • the upper limit of the Mn content is preferably 8.00% or less, more preferably 7.50% or less.
  • Al is a deoxidizing agent and needs to be contained at 0.001% or more.
  • Al also has the effect of increasing the material stability in order to widen the two-phase temperature range during annealing. The effect increases as the content of Al increases, but if the content of Al is excessive, deterioration of surface properties, paintability, weldability, and the like is caused.
  • the upper limit of Al was set to less than 1.00%. sol.
  • the lower limit of the Al content is preferably 0.005% or more, more preferably 0.010% or more, and still more preferably 0.020% or more. sol.
  • the upper limit of the Al content is preferably 0.80% or less, more preferably 0.60% or less. sol.
  • sold. Al means “acid-soluble Al”.
  • P 0.100% or less
  • the upper limit of the P content is set to 0.100% or less.
  • the upper limit of the P content is preferably 0.050% or less, more preferably 0.030% or less, and still more preferably 0.020% or less. Since the steel sheet according to the present embodiment does not require P, it may not substantially contain P, and the lower limit of the P content is 0.000%.
  • the lower limit of the P content may be more than 0.000% or 0.001% or more, but the smaller the P content, the more preferable.
  • S (S: 0.010% or less) S is an impurity, and if the steel sheet excessively contains S, MnS elongated by hot rolling is generated, which causes deterioration of toughness. Therefore, the upper limit of the S content is set to 0.010% or less.
  • the upper limit of the S content is preferably 0.007% or less, more preferably 0.003% or less. Since the steel sheet according to the present embodiment does not require S, it does not need to substantially contain S, and the lower limit of the S content is 0.000%.
  • the lower limit of the S content may be more than 0.000% or 0.001% or more, but the lower the S content, the more preferable.
  • N is an impurity, and if the steel sheet contains 0.050% or more of N, the toughness is impaired. Therefore, the upper limit of the N content is set to less than 0.050%.
  • the upper limit of the N content is preferably 0.010% or less, more preferably 0.006% or less. Since the steel sheet according to the present embodiment does not require N, it need not substantially contain N, and the lower limit of the N content is 0.000%.
  • the lower limit of the N content may be more than 0.000% or 0.001% or more, but the smaller the N content, the more preferable.
  • O is an impurity, and when the steel sheet contains 0.020% or more of O, ductility is deteriorated. Therefore, the upper limit of the O content is set to less than 0.020%.
  • the upper limit of the O content is preferably 0.010% or less, more preferably 0.005% or less, and still more preferably 0.003% or less. Since the steel sheet according to the present embodiment does not require O, it does not need to substantially contain O, and the lower limit of the O content is 0.000%.
  • the lower limit of the O content may be more than 0.000% or 0.001% or more, but the lower the O content, the more preferable.
  • B is not an essential element in the steel sheet according to the present embodiment, and thus may not be included.
  • the content is 0.000% or more.
  • B has a large effect of strengthening the interface between retained austenite and fresh martensite with respect to the matrix mainly composed of tempered martensite and further improving local ductility. Therefore, the steel sheet of the present embodiment preferably contains B. .
  • a B content of more than 0.0003% is required.
  • the upper limit of the B content is set to 0.010% or less.
  • the lower limit of the B content is preferably 0.0005% or more, more preferably 0.0008% or more, and the upper limit of the B content is preferably 0.0050% or less, more preferably 0.0030%. It is as follows.
  • the steel sheet of the present embodiment further includes one or two selected from the group consisting of Cr, Mo, W, Cu, Ni, Ti, Nb, V, Ca, Mg, Zr, REM, Sb, Sn, and Bi.
  • the above may be contained.
  • the steel sheet according to the present embodiment does not require Cr, Mo, W, Cu, Ni, Ti, Nb, V, Ca, Mg, Zr, REM, Sb, Sn, and Bi, Cr, Mo, W, It may not contain Cu, Ni, Ti, Nb, V, Ca, Mg, Zr, REM, Sb, Sn and Bi, that is, the lower limit of the content may be 0%.
  • REM refers to a total of 17 elements of Sc, Y, and lanthanoid
  • the REM content refers to the content of REM when one kind is used and the total content of REM when two or more kinds are used.
  • Point to. REM is also generally supplied as misch metal, which is an alloy of a plurality of types of REM. For this reason, one or more individual elements may be added so that the REM content falls within the above range, or, for example, the REM content may be added in the form of a misch metal so that the REM content falls within the above range. May be contained.
  • Cr 0.00% or more and less than 2.00%
  • Mo 0.00% or more and 2.00% or less
  • W 0.00% or more and 2.00% or less
  • Cu 0.00% or more and 2.00% or less
  • Ni 0.00% or more and 2.00% or less
  • Cr, Mo, W, Cu, and Ni may be included because they are elements that improve the strength of the steel sheet.
  • the steel sheet may contain 0.01% or more of each of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni.
  • the upper limit of the content of each of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni is set to 2.00% or less.
  • Ti, Nb, and V are not essential elements in the steel sheet according to the present embodiment, and thus may not be included, and each content is 0.000% or more. However, since Ti, Nb, and V are elements that generate fine carbides, nitrides, or carbonitrides, they are effective in improving the strength of a steel sheet. Therefore, the steel sheet may contain one or more elements selected from the group consisting of Ti, Nb, and V.
  • the lower limit of the content of each of one or more elements selected from the group consisting of Ti, Nb, and V is preferably set to 0.005% or more.
  • the upper limit of the content of each of one or more elements selected from the group consisting of Ti, Nb, and V is set to 0.300% or less.
  • Ca, Mg, Zr, and REM are not essential elements in the steel sheet of the present disclosure, they may not be included, and their contents are 0.000% or more.
  • Ca, Mg, Zr, and REM improve the toughness of the steel sheet.
  • the lower limit of the content of each of one or more elements selected from the group consisting of Ca, Mg, Zr, and REM is preferably 0.0001% or more, more preferably 0.001% or more.
  • the upper limit of the content of each of these elements is set to 0.010% or less, and 1 selected from the group consisting of Ca, Mg, Zr, and REM is used. It is preferred that the total content of the species or two or more elements be 0.030% or less.
  • Sb, Sn, and Bi are not essential elements in the steel sheet of the present disclosure, and thus may not be included, and each content is 0.000% or more.
  • Sb, Sn, and Bi suppress oxidizable elements such as Mn, Si, and / or Al in the steel sheet from diffusing to the steel sheet surface to form oxides, and reduce the surface properties and plating properties of the steel sheet.
  • the lower limit of the content of each of one or more elements selected from the group consisting of Sb, Sn, and Bi is preferably 0.0005% or more, more preferably 0.001% or more. % Or more.
  • the upper limit of the content of each of these elements is set to 0.050% or less.
  • the balance is Fe and impurities.
  • Impurities are components that are mixed in due to various factors in the ore, scrap, and other raw materials and manufacturing processes when steel products are manufactured industrially, and are acceptable within a range that does not adversely affect the present invention.
  • % representing a fraction of each phase means an area ratio (%) unless otherwise specified.
  • the metal structure at a quarter position (also referred to as a 1/4 t portion) of the thickness from the surface is 25% or more and 90% or less of tempered martensite and 3% or less as a matrix. It contains 10% to 50% of retained austenite and 25% or less of fresh martensite as ferrite and other phases. Further, the interface density between the retained austenite and the fresh martensite with respect to the matrix mainly composed of tempered martensite is 2.7 ⁇ m ⁇ 1 or more.
  • the fraction of each phase changes depending on the annealing conditions, and affects the material such as strength and local ductility.
  • the L section refers to a plane cut in such a manner as to pass through the central axis in the rolling direction of the steel sheet in parallel with the sheet thickness direction and the rolling direction.
  • the area ratio of tempered martensite and ferrite is calculated from the observation of the structure with a scanning electron microscope (SEM). After the L section of the steel plate is mirror-polished, it is corroded with 3% nital (3% nitric acid-ethanol solution), and the metallographic structure at a quarter position from the surface is observed with a scanning electron microscope at a magnification of 5000 times. The area ratio of each of the returned martensite and the ferrite is measured.
  • the area ratio of retained austenite is measured by the X-ray diffraction method. Since it is difficult to distinguish between retained austenite and fresh martensite (that is, untempered martensite) in the observation with a scanning electron microscope, the area ratio of tempered martensite and fresh martensite is measured by the following method. After the L section of the steel sheet is mirror-polished, it is corroded with 3% nital (3% nitric acid-ethanol solution), and the microstructure at a quarter position of the thickness from the surface of the steel sheet is observed with a scanning electron microscope with a magnification of 5000 times. Then, the total area ratio of retained austenite and fresh martensite is measured. Next, the area ratio of fresh martensite is calculated by subtracting the area ratio of retained austenite measured by X-ray diffraction from the total area ratio of retained austenite and fresh martensite.
  • the metal structure at a 1 / 4t part of the thickness from the surface includes tempered martensite having an area ratio of 25% or more and 90% or less.
  • Tempered martensite is a parent phase of the steel sheet according to the present embodiment that improves local ductility. Therefore, in order to improve local ductility, tempered martensite of 25% or more is required.
  • the lower limit of the area ratio of tempered martensite is preferably 40% or more.
  • the upper limit of the area ratio of tempered martensite is preferably 80% or less.
  • the metal structure at a 1 / 4t part of the thickness from the surface includes retained austenite in an area ratio of 10% or more and 50% or less.
  • Retained austenite is a phase that enhances the tensile strength and ductility of a steel sheet by transformation induced plasticity. Retained austenite can be transformed into martensite by overhanging, drawing, stretch-flanging, or bending with tensile deformation, and thus contributes to an improvement in the strength of a steel material obtained by processing a steel sheet. To obtain these effects, the area ratio of retained austenite needs to be 10% or more. The lower limit of the area ratio of retained austenite is preferably 15% or more, more preferably 20% or more, and further preferably 30% or more.
  • the area ratio of retained austenite is preferably 45% or less, and more preferably 40% or less, in view of hydrogen embrittlement.
  • the metal structure at a 1 / 4t portion of the thickness from the surface includes fresh martensite having an area ratio of 25% or less.
  • the fresh martensite in the metal structure is martensite that has not been tempered, and is a phase that strengthens the steel sheet according to the present embodiment and increases the tensile strength of the steel sheet.
  • fresh martensite itself is a hard phase, it is also a phase having an effect of deteriorating local ductility.
  • the area ratio of fresh martensite in the metal structure is set to 25% or less, preferably 15% or less, and more preferably 10% or less.
  • the metallographic structure at a 1 / 4t part of the thickness from the surface includes ferrite of 3% or less in area ratio.
  • the content of ferrite in the metal structure is small. This is because when the ferrite content in the metal structure increases, the ductility decreases.
  • the area ratio of ferrite in the metal structure is set to 3% or less, more preferably 1% or less, and further preferably substantially 0%.
  • Interface density of retained austenite and fresh martensite with respect to a matrix mainly composed of tempered martensite in a metal structure of a tt portion of a steel sheet is 2.7 ⁇ m ⁇ 1 or more
  • the first region which is either retained austenite or fresh martensite
  • the interface density which is a value obtained by dividing the sum of the lengths of the boundaries between the first region and the second region by the total area of the first region and the second region, is 2. 7 ⁇ m ⁇ 1 or more. More preferably, the interface density is 3.15 ⁇ m ⁇ 1 or more.
  • the retained austenite and the fresh martensite form an interface with a matrix mainly composed of tempered martensite.
  • the steel sheet according to the present embodiment has an interface density between retained austenite and fresh martensite with respect to a matrix mainly composed of tempered martensite, that is, an interface density of 2.7 ⁇ m ⁇ 1 or more.
  • the measurement of the interface density between the retained austenite and the fresh martensite with respect to the matrix mainly composed of tempered martensite can be carried out based on the microstructure image by the above-mentioned scanning electron microscope.
  • the interface density between the retained austenite and the fresh martensite with respect to the matrix mainly composed of tempered martensite greatly affects the local ductility.
  • the density is 2.7 ⁇ m ⁇ 1 or more, the retained austenite and the fresh martensite are fine.
  • the interface distribution becomes uniform, and good local ductility is obtained.
  • the interface density between the retained austenite and the fresh martensite with respect to the parent phase mainly composed of tempered martensite is less than 2.7 ⁇ m ⁇ 1
  • the retained austenite and the fresh martensite have a coarse and interconnected structure, and therefore have a homogeneous structure. And the effect of suppressing crack propagation is impaired.
  • the interface density between the retained austenite and the fresh martensite with respect to the matrix mainly composed of tempered martensite is derived as follows.
  • the distinction between the first region and the second region is based on the fact that retained austenite and fresh martensite are regions that are relatively brighter with respect to other phases in a microstructure image by a scanning electron microscope. Do it.
  • the interface density between the retained austenite and the fresh martensite with respect to the parent phase shows, for example, an area of about 24 ⁇ m ⁇ 18 ⁇ m in a metal structure at a 1 / 4t portion from the surface in the L cross section of the steel sheet at a magnification of 5000 times with a scanning electron microscope (SEM) photograph. There can be.
  • the interface density is derived as follows.
  • the interface density between the retained austenite and the fresh martensite with respect to the parent phase mainly composed of tempered martensite is measured using image analysis software ImageJ.
  • a target tissue is observed at a magnification of 5000 times using an SEM to obtain an SEM image (24 ⁇ m ⁇ 18 ⁇ m).
  • 1280 ⁇ 960 divided regions are formed on the SEM image using ImageJ.
  • binarization processing is performed so that a region that is either retained austenite or fresh martensite is black, and the other regions are white, to obtain a two-tone image.
  • the measurement of the interface density between the retained austenite and the fresh martensite with respect to the tempered martensite was performed by measuring the length of the interface with respect to the tempered martensite between all the isolated residual austenite and the fresh martensite in the obtained two-tone image.
  • the analysis is performed sequentially, and the total value is divided by the area (24 ⁇ m ⁇ 18 ⁇ m) of the entire area of the image.
  • the sequential analysis refers to measuring the interface density of each of retained austenite and fresh martensite without overlapping the interface.
  • the tensile strength (TS) of the steel sheet according to the present embodiment is preferably 1200 MPa or more, more preferably 1320 MPa or more. This is because when a steel sheet is used as a material for an automobile, the sheet thickness is reduced by increasing the strength, thereby contributing to weight reduction.
  • the upper limit of the tensile strength of the steel sheet is not particularly defined, but may be, for example, 1600 MPa.
  • the upper limit of the total elongation is not particularly limited, but may be, for example, 35% or less.
  • the local elongation is preferably at least 2%, more preferably at least 3%, further preferably at least 4%, still more preferably at least 5%.
  • the upper limit of the local elongation is not particularly defined, but may be, for example, 6% or less.
  • the steel sheet of the present disclosure has high strength, good local ductility, and excellent formability, and is therefore most suitable for structural parts of automobiles such as cross members. Furthermore, since the steel sheet of the present disclosure has a high content of Mn, which contributes to the reduction of the weight of automobiles, the industrial contribution is extremely remarkable.
  • the steel sheet according to the present embodiment is prepared by melting a steel having the above-described chemical composition by a conventional method, casting to produce a slab or a steel ingot, and heating and hot rolling the obtained steel sheet to obtain a hot-rolled steel sheet. Is pickled, cold rolled, and annealed to produce.
  • Hot rolling may be performed in a normal continuous hot rolling line.
  • Annealing may be performed in either an annealing furnace or a continuous annealing line as long as the conditions described later are satisfied, but preferably, the first annealing and the second annealing described below are both performed using the continuous annealing line. Therefore, productivity can be improved.
  • the first annealing and the second annealing are preferably performed in a reducing atmosphere, for example, in a reducing atmosphere of 98% nitrogen and 2% hydrogen. By performing the heat treatment in a reducing atmosphere, the scale can be prevented from adhering to the surface of the steel sheet, and can be directly sent to the plating step without requiring acid cleaning. Further, skin pass rolling may be performed on the steel sheet after cold rolling.
  • the molten steel may be one produced by the ordinary blast furnace method, and the raw material may generate a large amount of scrap, such as steel produced by the electric furnace method. May be included.
  • the slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting.
  • the above-mentioned slab or steel ingot is heated and hot-rolled. It is preferable that the temperature of the steel material to be subjected to hot rolling is 1100 ° C. or more and 1300 ° C. or less. By setting the temperature of the steel material subjected to hot rolling to 1100 ° C. or higher, the deformation resistance during hot rolling can be further reduced. On the other hand, by setting the temperature of the steel material to be subjected to hot rolling to 1300 ° C. or lower, it is possible to suppress a decrease in yield due to an increase in scale loss. In this specification, the temperature is a temperature measured at a central position on the surface of the steel sheet.
  • the time for maintaining the temperature in the temperature range of 1100 ° C. or more and 1300 ° C. before hot rolling is not particularly limited, but is preferably 30 minutes or more, and more preferably 1 hour or more, in order to improve the bendability. . Further, in order to suppress excessive scale loss, the time for maintaining the temperature in a temperature range of 1100 ° C. to 1300 ° C. is preferably 10 hours or less, more preferably 5 hours or less. When direct rolling or direct rolling is performed, hot rolling may be performed as it is without performing heat treatment.
  • the finish rolling start temperature is preferably set to 700 ° C. or more and 1000 ° C. or less. By setting the finish rolling start temperature to 700 ° C. or higher, it becomes possible to reduce deformation resistance during rolling. On the other hand, by setting the finish rolling start temperature to 1000 ° C. or lower, deterioration of the surface properties of the steel sheet due to grain boundary oxidation can be suppressed.
  • the hot-rolled steel sheet obtained by performing finish rolling can be cooled, wound, and made into a coil.
  • the winding temperature after cooling is preferably set to 700 ° C. or lower. By setting the winding temperature to 700 ° C. or lower, internal oxidation is suppressed, and subsequent pickling is facilitated.
  • the winding temperature is more preferably 650 ° C or lower, and further preferably 600 ° C or lower.
  • the lower limit of the winding temperature is not particularly specified, but may be room temperature, for example.
  • the hot rolled sheet may be tempered at a temperature of 300 ° C. or more and 600 ° C. or less before being cold-rolled after being cooled to room temperature.
  • cold rolling is performed to obtain a cold-rolled steel sheet.
  • the shape is corrected by performing light rolling of more than 0% to about 5% before or after pickling and before or after cold rolling because it is advantageous in terms of securing flatness. Further, by performing light rolling before pickling, pickling properties are improved, removal of surface-concentrating elements is promoted, and there is an effect of improving chemical conversion property and plating property.
  • the structure after annealing can be homogenized, that is, the interface distribution between the retained austenite and the fresh martensite in the matrix mainly composed of tempered martensite is homogenized. Is done. As a result, more interfaces between the retained austenite and the fresh martensite can be present in the metal structure.
  • the upper limit of the rolling reduction of the cold rolling is 50% or less, preferably 20% or less, more preferably 18% or less, and even more preferably 15% or less.
  • the lower limit of the rolling reduction of the cold rolling is 0% or more, and preferably 5% or more. Making the rolling reduction of the cold rolling 50% or less is an important requirement for satisfying the interface density condition specified in the present invention. By setting the rolling reduction of the cold rolling to preferably 18% or less, it is possible to obtain a larger interface density of 3.15 ⁇ m ⁇ 1 or more and obtain a larger local elongation of 3.0% or more. Can be.
  • the cold-rolled steel sheet obtained through the hot rolling step and the cold rolling step is heated to 740 ° C. or more and less than 800 ° C. to perform the first annealing.
  • the temperature is held at a temperature range of 740 ° C or more and less than 800 ° C for 10 seconds or more, and thereafter, from the above-mentioned holding temperature of 740 ° C or more and less than 800 ° C, a temperature of 500 ° C or less at an average cooling rate of 2 ° C / sec or more and 2000 ° C / sec or less. It is preferable to cool to a region and cool to room temperature.
  • a second annealing is performed by heating again to 600 ° C. or higher and lower than the Ac 3 point.
  • the first annealing temperature after cold rolling is 740 ° C. or more and less than 800 ° C.
  • the annealing temperature is preferably Ac 3 or more.
  • austenite grain growth is suppressed, and the interface density between residual austenite and fresh martensite with respect to tempered martensite increases due to the refining effect of old austenite grains, and local ductility increases. Contribute to improvement.
  • the holding time in a temperature range of 740 ° C. or more and less than 800 ° C. be 10 seconds or more.
  • the holding time in a temperature range of 740 ° C. or more and less than 800 ° C. is preferably 300 seconds or less.
  • the steel material is cooled at a rate higher than the critical cooling rate, and the entire steel material after cooling can have a structure mainly composed of martensite.
  • the water quenching cooling method or the mist injection cooling method it is difficult to control the average cooling rate to 2000 ° C./sec or more, so the practical upper limit of the average cooling rate is 2000 ° C./sec.
  • the steel is cooled to the martensite transformation start temperature or lower, and the entire steel material after cooling can have a martensite-based structure.
  • the steel sheet is preferably cooled to room temperature.
  • the tempering treatment may be performed by holding the material in a temperature range of 100 ° C. or more and 500 ° C. or less for 10 seconds or more and 1000 seconds or less.
  • the second annealing temperature after the first annealing is 600 ° C. or more and less than Ac 3 points.
  • the second annealing temperature below 3 points 600 ° C. or higher Ac, martensite tempering a can control a desired area ratio, it is possible to increase the tensile strength and the local ductility.
  • the holding time in a temperature range of 600 ° C. or more and less than the Ac 3 point is preferably 5 seconds or more. Further, from the viewpoint of productivity, the holding time in a temperature range of 600 ° C. or more and less than the three Ac points is preferably 300 seconds or less.
  • cooling is performed from the above-mentioned holding temperature of 600 ° C. or more and less than Ac 3 points to a temperature range of 300 ° C. or less at an average cooling rate of 10 ° C./sec or more.
  • it is kept at a temperature range of 200 ° C. or more and 450 ° C. or less for 30 seconds or more.
  • the practical upper limit of the average cooling rate in the cooling after the second annealing is 2000 ° C./sec.
  • diffusion of supersaturated carbon in fresh martensite into retained austenite is promoted, and the generation of retained austenite is promoted and the interface density is high.
  • C diffusion is promoted, the effect of uniforming C inside retained austenite is obtained, and local ductility is enhanced.
  • the cooling after holding for 30 seconds or more in a temperature range of 200 ° C. or more and 450 ° C. or less may be performed as it is to room temperature if the steel sheet is not plated. Moreover, when plating on a steel plate, it is manufactured as follows.
  • the steel sheet held at a temperature range of 200 ° C. to 450 ° C. for 30 seconds or more is heated again to a temperature range of 430 to 500 ° C.
  • the steel sheet is dipped in a hot-dip galvanizing bath to perform hot-dip galvanizing.
  • the conditions of the plating bath may be within a normal range. After the plating process, it may be cooled to room temperature.
  • An alloying process of hot-dip galvanizing is performed at a temperature. Alloying treatment conditions may be within a normal range.
  • the steel sheet according to the present embodiment can be obtained.
  • the obtained slab was hot-rolled under the conditions shown in Table 2 to produce a hot-rolled steel sheet having a thickness of 2.6 mm.
  • the obtained hot-rolled steel sheet was pickled and cold-rolled at a cold-rolling rate shown in Table 2 to produce cold-rolled steel sheets having various thicknesses shown in Table 2.
  • the obtained cold-rolled steel sheet was subjected to a heat treatment under the conditions shown in Table 3 to produce an annealed cold-rolled steel sheet.
  • the heat treatment of the cold rolled steel sheet was performed in a reducing atmosphere of 98% nitrogen and 2% hydrogen.
  • Some examples of the annealed cold-rolled steel sheets were subjected to a hot dip galvanizing treatment, and then to an alloying treatment without cooling to room temperature.
  • the alloy was heated to 520 ° C. and held at 520 ° C. for 5 seconds to perform an alloying treatment, and then cooled to room temperature at an average cooling rate of 10 ° C./sec.
  • the area ratios of tempered martensite and ferrite were calculated from observation of the structure by a scanning electron microscope.
  • the area ratio of the retained austenite and fresh martensite was calculated from the structure observation by a scanning electron microscope and the X-ray diffraction measurement.
  • the L section of the steel sheet cut in parallel to the rolling direction is mirror-polished, and then the microstructure is revealed with 3% nital.
  • the microstructure at a quarter position from the surface is obtained with a scanning electron microscope at a magnification of 5000 times.
  • the interface density between the retained austenite and the fresh martensite with respect to the matrix mainly composed of tempered martensite was measured using image analysis software ImageJ.
  • a target tissue was observed at a magnification of 5000 times using an SEM to obtain an SEM image (24 ⁇ m ⁇ 18 ⁇ m).
  • 1280 ⁇ 960 divided regions were formed in the SEM image using ImageJ. Each of the divided regions was subjected to a binarization process so that a region that was either retained austenite or fresh martensite was black, and the other regions were white, to obtain a two-tone image.
  • the measurement of the interface density between the retained austenite and the fresh martensite with respect to the tempered martensite was performed by measuring the length of the interface with respect to the tempered martensite between all the isolated residual austenite and the fresh martensite in the obtained two-tone image.
  • the analysis was performed sequentially, and the total value was divided by the area (24 ⁇ m ⁇ 18 ⁇ m) of the entire area of the image.
  • the sequential analysis refers to measuring the interface density of each of retained austenite and fresh martensite without overlapping the interface.
  • JIS No. 5 tensile test pieces were sampled from a direction perpendicular to the rolling direction of the steel sheet, and tensile strength (TS), total elongation (EL), and local elongation (LEL) were measured.
  • the tensile test was carried out using a JIS No. 5 tensile test piece by a method specified in JIS Z2241: 2011.
  • the total elongation was measured using a JIS No. 5 test piece according to the method specified in JIS Z2241: 2011.
  • the local elongation was calculated by subtracting the value of the elongation at the maximum load point (uniform elongation) from the value of the total elongation when the broken test pieces were butted.
  • Evaluation results Table 4 shows the results of the above evaluations.
  • a steel sheet exhibiting an interface density of 2.7 ⁇ m ⁇ 1 or more, a tensile strength of 1200 MPa or more, and a local elongation of 2.0% or more was evaluated as a steel sheet having excellent local ductility and high strength.
  • FIG. 2 shows a two-tone image obtained by binarizing the SEM image of FIG.
  • FIG. 4 shows an SEM image of the L section of the steel sheet No. 4 observed by mirror polishing and nital treatment.
  • the interface density measured from the two-tone image of FIG. 2 was 3.35 ⁇ m ⁇ 1 .
  • the interface density measured from the binary image obtained by binarizing the SEM image in FIG. 3 was 1.50 ⁇ m ⁇ 1 .

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Abstract

L'invention concerne une tôle d'acier qui présente une concentration élevée en Mn, et qui possède une excellente ductilité locale et une résistance élevée. Cette feuille d'acier est caractérisée en ce qu'elle contient des quantités prédéfinies de C, Si et Al soluble dans l'acide, et en % en masse, plus de 4 % mais moins de 9 % de Mn, des quantités limitées de P, S, N et O, et un élément arbitrairement sélectionné, le reste étant constitué de fer et d'impuretés, la structure métallique au niveau d'une partie 1/4t dans une section transversale L de celle-ci contenant, dans une proportion surfacique, 25 à 90 % de martensite revenue, 3 % ou moins de ferrite, 10 à 50 % d'austénite résiduelle, et 25 % ou moins de martensite fraîche, et ayant une densité d'interface supérieure ou égale à 2,7 μm-1 déterminée en divisant, par la surface totale des première et seconde régions, la somme totale des longueurs de limites entre des premières régions qui contiennent soit de l'austénite résiduelle, soit de la martensite fraîche, et des secondes régions qui sont des régions obtenues en éliminant les premières régions de la structure métallique au niveau de la position 1/4.
PCT/JP2019/028357 2018-07-18 2019-07-18 Tôle d'acier WO2020017609A1 (fr)

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WO2022264585A1 (fr) 2021-06-15 2022-12-22 Ｊｆｅスチール株式会社 Tôle d'acier galvanisé à et élément et procédé de fabrication associé
KR20240007934A (ko) 2021-06-15 2024-01-17 제이에프이 스틸 가부시키가이샤 고강도 아연 도금 강판 및 부재 그리고 그들의 제조 방법

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JP2017053001A (ja) * 2015-09-09 2017-03-16 新日鐵住金株式会社 溶融亜鉛めっき鋼板および合金化溶融亜鉛めっき鋼板、並びにそれらの製造方法
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WO2022264585A1 (fr) 2021-06-15 2022-12-22 Ｊｆｅスチール株式会社 Tôle d'acier galvanisé à et élément et procédé de fabrication associé
KR20240007934A (ko) 2021-06-15 2024-01-17 제이에프이 스틸 가부시키가이샤 고강도 아연 도금 강판 및 부재 그리고 그들의 제조 방법

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TW202012649A (zh) 2020-04-01

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