EP3330394B1 - High_strength_ hot_rolled steel sheet - Google Patents

High_strength_ hot_rolled steel sheet Download PDF

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Publication number
EP3330394B1
EP3330394B1 EP15900339.1A EP15900339A EP3330394B1 EP 3330394 B1 EP3330394 B1 EP 3330394B1 EP 15900339 A EP15900339 A EP 15900339A EP 3330394 B1 EP3330394 B1 EP 3330394B1
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steel sheet
rolled steel
hot rolled
ferrite
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German (de)
English (en)
French (fr)
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EP3330394A1 (en
EP3330394A4 (en
Inventor
Natsuko Sugiura
Yasuaki Tanaka
Takafumi Yokoyama
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Nippon Steel Corp
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Nippon Steel Corp
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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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • 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
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
    • 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
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon

Definitions

  • the present invention relates to a hot rolled steel sheet, and particularly relates to a high strength hot rolled steel sheet which has excellent hole expansibility and is suitable for chassis components and the like of automobiles formed into various forms through pressing or the like.
  • Hot rolled steel sheets are manufactured at relatively low cost and are widely used for various types of industrial equipment including automobiles. Recently, from a viewpoint on the restriction on carbon dioxide emission entailing the measures against global warming, fuel efficiency of automobiles has been required to be improved. Moreover, for the purpose of reducing weight and ensuring collision safety of vehicle bodies, high strength hot rolled steel sheets have been widely applied to automobile components.
  • steel sheets for automobile components have to satisfy not only the strength but also various types of workability such as press formability and weldability required at the time of forming the components.
  • press formability and weldability required at the time of forming the components.
  • the frequency of use of stretch flange forming and burring forming is extremely high. Therefore, high strength hot rolled steel sheets for the chassis components are required to have excellent hole expansibility.
  • the chassis components from a viewpoint on ensuring safety, many components are required to avoid plastic deformation even in a case where a large load is applied. Therefore, steel sheets for the chassis components are required to have a high yield ratio.
  • the structure is uniformly strengthened by controlling the steel structure to be a single phase structure containing any one of ferrite, bainitic ferrite, bainite, and the like, through solid solution strengthening of Mn, Si, and the like, and/or carbide of Ti, Nb, V, and the like or precipitation strengthening due to Cu.
  • Patent Document 1 discloses a technology that relates to a high strength hot rolled steel sheet having excellent hole expansibility, in which Ti carbide including Mo is dispersed in a substantially single phase structure of ferrite in a uniform and fine manner.
  • Mo which is a very expensive alloying element. Therefore, from an economic viewpoint, the configuration is not suitable for mass production.
  • Patent Document 2 discloses a technology in which elongation and stretch flangeability of a high strength hot rolled steel sheet are improved by appropriately controlling cooling of Ti-added steel containing predetermined amounts of Mn and Si during a period from hot rolling to coiling such that a structure having ferrite and bainite is achieved, and causing TiC to be finely precipitated.
  • the yield ratio which is one of characteristics necessary for a hot rolled steel sheet applied to chassis components.
  • bainite has a low yield ratio compared to ferrite after precipitation strengthening, the technology of Patent Document 2 allows bainite to be included up to 50%, and it is analogized that a high yield ratio cannot be maintained.
  • ferrite includes so-called bainitic ferrite or pseudo-polygonal ferrite which is not polygonal ferrite.
  • a temperature range of 720°C or lower at which polygonal ferrite is not sufficiently formed is also allowed as a first cooling stop temperature.
  • Bainitic ferrite and pseudo-polygonal ferrite have a structure indicating a yield ratio lower than that of polygonal ferrite.
  • Patent Document 3 discloses a Ti-added high strength hot rolled steel sheet of which toughness and hole expansibility are improved by reducing the Mn content and controlling the percentage of C which is precipitated as cementite.
  • a high yield ratio of 75% or more is not obtained in high strength steel of 540 MPa or higher.
  • Patent Document 4 discloses a technology that relates to a high strength hot rolled steel sheet having excellent hole expansibility, in which corsening of TiC is suppressed by reducing the Mn content and the Si content and adding certain amounts of Ti and B.
  • B has an effect of suppressing the recrystallization of austenite from being recrystallized, in a case of being subjected to multiple addition together with Ti having a similar effect, a rolling load during hot rolling increases remarkably, resulting in an increase of a load to a hot rolling mill. Therefore, there is concern that the technology of Patent Document 4 causes operational trouble.
  • the strength of a final product considerably varies when the B content fluctuates only by several ppm, steel essentially containing B is not suitable for mass production.
  • Patent Document 5 discloses a high strength hot rolled steel sheet which has a high yield ratio and excellent hole expansibility and is obtained by cooling steel containing large amounts of Si, Mn, and Ti under an appropriate cooling condition and causing the structure to be a single phase structure of granular bainitic ferrite.
  • Patent Document 5 in order to obtain the granular bainitic ferrite structure, large amounts of Si and Mn need to be contained, thereby leading to a problem of an increase in alloying cost.
  • US 2015/0140358A1 discloses a high strength hot-dip galvannealed hot-rolled steel sheet.
  • An object of the present invention is to provide a high strength hot rolled steel sheet having a high yield ratio and excellent hole expansibility.
  • the high strength in the present invention indicates that tensile strength (TS) is 540 MPa or higher.
  • Ti is relatively inexpensive and exhibits remarkable precipitation strengthening with a minute amount of Ti content.
  • the inventors have examined structures of hot rolled steel sheets on the premise that polygonal ferrite is employed as a main constituent.
  • the inventors have examined utilization of precipitation strengthening of Ti.
  • the inventors have also intensively examined a technique of improving the hole expansibility in a Ti-containing high strength hot rolled steel sheet in which Ti precipitates are precipitated in polygonal ferrite as a main constituent of the structures. As a result, the following knowledge has been acquired.
  • the inventors have measured micro-hardness of each ferrite grain in steel having polygonal ferrite as a main constituent of the structure. As a result, it has been found that the hardness significantly varies depending on each of the measured grains. Furthermore, it has been found that hole expansibility can be remarkably improved by reducing unevenness in the hardness of ferrite grains.
  • the inventors have observed the intragranular state of polygonal ferrite of a sample having inferior hole expansibility, using a transmission electron microscope. As a result, it has been found that a large amount of Ti-based anisometric carbide stretched in a particular orientation of the ferrite is precipitated and this exerts an adverse influence on the hole expansibility. In the related art, there have been few reports on the shape of Ti carbide affecting hole expansibility, and the mechanism of the shape of Ti-based carbide affecting hole expansibility is obscure. However, compared to Ti-based isometric carbide, Ti-based anisometric carbide is highly consistent with matrix phase ferrite, and it is estimated that considerable consistency distortion is accumulated around the Ti-based anisometric carbide. Therefore, it is estimated that this consistency distortion incites cracks to be propagated during hole expanding resulting in deterioration of the hole expansibility.
  • the present invention has been made based on the knowledge described above.
  • the steel sheet according to the aspect of the present invention it is possible to inexpensively manufacture a high strength hot rolled steel sheet having a high yield ratio and excellent hole expansibility.
  • the steel sheet according to the aspect of the present invention also has excellent hole expansibility even during stretch flanges forming frequently employed for automobile components, particularly chassis components and the like. Therefore, the steel sheet particularly contributes to reducing weight and ensuring collision safety of vehicle bodies in automobile fields.
  • a high strength hot rolled steel sheet according to an embodiment of the present invention (hereinafter, will be sometimes referred to as a hot rolled steel sheet according to the present embodiment) will be described in detail.
  • the C content is an element essential to high-strengthening of a steel sheet performed through precipitation strengthening or solid solution strengthening.
  • the C content is set to 0.010% or more, is preferably set to 0.020% or more, and is more preferably set to 0.040% or more.
  • the C content is set to 0.200% or less, is preferably set to 0.130% or less, and is more preferably set to 0.110% or less.
  • Si is a solid solution strengthening element and is an element effective in high-strengthening of a steel sheet.
  • the Si content is set to 0.001% or more, is preferably set to 0.01% or more, and is more preferably set to 0.04% or more. Meanwhile, if there is an excessive amount of Si, island-shaped scale is generated, and surface quality deteriorates. Therefore, the Si content is set to 2.50% or less, is preferably set to 1.30% or less, and is more preferably set to 0.80% or less.
  • Mn is an element effective in improving strength of a steel sheet.
  • Mn is an element which fixes S in steel as MnS and suppresses hot embrittlement caused by a solid solution S.
  • the Mn content is set to 0.001% or more, is preferably set to 0.10% or more, and is more preferably set to 0.45% or more.
  • the Mn content is set to 1.50% or less, is preferably set to 1.00% or less, and is more preferably set to 0.80% or less.
  • P is an element contained as impurities, which cause weldability and toughness of a steel sheet to deteriorate. Therefore, it is preferable to have a small amount of P. However, in a case where the P content exceeds 0.050%, the influence described above becomes prominent. Accordingly, as a range in which deterioration of weldability and toughness is not prominent, the P content is set to 0.050% or less, is preferably set to 0.020% or less, and is more preferably set to 0.010% or less.
  • S is an element contained as impurities, forming MnS in steel and causing hole expansibility of a steel sheet to deteriorate. Therefore, it is preferable to have a small amount of S.
  • the S content exceeds 0.010%, the influence described above becomes prominent. Accordingly, as a range in which deterioration of hole expansibility is not prominent, the S content is set to 0.010% or less, is preferably set to 0.0050% or less, and is more preferably set to 0.0020% or less.
  • N is an element contained as impurities, forming coarse nitride in steel and causing hole expansibility of a steel sheet to deteriorate remarkably. Therefore, it is preferable to have a small amount of N. However, in a case where the N content exceeds 0.0070%, the influence described above becomes prominent. Accordingly, as a range in which deterioration of hole expansibility is not prominent, the N content is set to 0.0070% or less and is preferably set to 0.0050% or less.
  • Al is an element effective in deoxidation of steel.
  • the Al content is set to 0.001% or more.
  • the Al content is set to 0.50% or less, is preferably set to 0.20% or less, and is more preferably set to 0.10% or less.
  • Ti is an element forming carbide in steel and inducing uniform precipitation strengthening of ferrite.
  • Ti is also an element having effects of reducing the amount of a solid solution C by being precipitated as TiC and inhibiting precipitation of cementite which causes deterioration of hole expansibility. Therefore, in the hot rolled steel sheet according to the present embodiment, Ti is a particularly important element.
  • the Ti content is set to 0.050% or more, is preferably set to 0.100% or more, and is more preferably set to 0.130% or more. Meanwhile, if the Ti content exceeds 0.30%, toughness deteriorates remarkably and an unnecessary cost increase is caused. Therefore, the Ti content is set to 0.30% or less, is preferably set to 0.25% or less, and is more preferably set to 0.20% or less.
  • the hot rolled steel sheet according to the present embodiment contains the chemical composition described above and the remainder of Fe and impurities.
  • the hot rolled steel sheet may further include at least one selected from the group consisting of V, Nb, Cr, Ni, Cu, Mo, B, Ca, Mg, and Bi.
  • impurities denote components incorporated due to raw materials such as ores and scraps, or other factors when steel is industrially manufactured.
  • V is an element forming carbide in steel.
  • V is an element of which the solubility product in austenite is greater than that of Ti and which is effective in high-strengthening of a steel sheet. Therefore, although it is expensive compared to Ti, V may be contained as necessary.
  • the V content is set to 0.010% or more, is preferably set to 0.070% or more, and is more preferably set to 0.140% or more. Meanwhile, when there is an excessive amount of V, a cost rise is caused. Therefore, even in a case where V is contained, the V content is set to 0.50% or less.
  • Nb is an element which forms carbide in steel and is effective in high-strengthening of a steel sheet. Therefore, although it is expensive compared to Ti, Nb may be contained as necessary. When the Nb content is less than 0.001%, the effect described above cannot be sufficiently obtained. Accordingly, in a case of obtaining the effect described above, the Nb content is set to 0.001% or more. Meanwhile, when there is an excessive amount of Nb, plastic anisotropy of a steel sheet increases, and hole expansibility deteriorates. Therefore, even in a case where Nb is contained, the Nb content is set to 0.090% or less.
  • All of Cr, Ni, Cu, Mo, and B are elements effective in high-strengthening of a steel sheet. Therefore, as necessary, the elements may be contained independently, or two or more thereof may be contained multiply. In order to achieve the effect described above, there is a need to include Cr: 0.001% or more, Ni: 0.001% or more, Cu: 0.001% or more, Mo: 0.001% or more, and B: 0.0001% or more. Meanwhile, similar to Mn, these elements delay ferritic transformation after hot rolling. Therefore, when there are excessive amounts of the elements, it becomes difficult to obtain, by area ratio, 80% or more of polygonal ferrite in the structure of a hot rolled steel sheet, and hole expansibility of a hot rolled steel sheet deteriorates.
  • the amounts thereof are set to Cr: 0.50% or less, Ni: 0.50% or less, Cu: 0.50% or less, Mo: 0.50% or less, and B: 0.0050% or less respectively; and are preferably set to Cr: 0.20% or less, Ni: 0.20% or less, Cu: 0.20% or less, Mo: 0.09% or less, and B: 0.0040% or less, respectively.
  • Ca and Mg are elements contributing to fine dispersion of inclusions in steel.
  • Bi is an element mitigating micro-segregation of substitutional type alloying elements such as Mn and Si in steel. All of the elements contribute to improvement of hole expansibility of a steel sheet. Therefore, as necessary, the elements may be contained independently, or two or more thereof may be contained multiply. In order to achieve the effect described above, each of the elements needs to be contained 0.0001% or more. Meanwhile, when there are excessive amounts of these elements, ductility deteriorates. Therefore, even in a case where the elements are contained, the amounts of the elements are set to 0.01% or less.
  • Polygonal ferrite has a structure effective in improving hole expansibility.
  • the area ratio of polygonal ferrite is set to 80% or more, is preferably set to 90% or more, and is more preferably set to 95% or more.
  • the area ratio of polygonal ferrite may be 100%. That is, the hot rolled steel sheet according to the present embodiment may be constituted of single phase polygonal ferrite.
  • the total area ratio of the martensite and austenite is set to 5% or less and is preferably set to 2% or less.
  • the total area ratio may be 0% (that is, none of martensite and austenite are contained).
  • the austenite mentioned herein is so-called residual austenite.
  • the total area ratio of the pearlite and cementite is set to 5% or less, is preferably set to 3% or less, and is more preferably set to 1% or less.
  • the total area ratio may be 0% (that is, none of the pearlite and cementite are contained).
  • the structure of the remainder other than those described above includes at least one selected from bainitic ferrite and bainite. However, in a case where the total area ratio of the structure described above is 100%, none of bainitic ferrite and bainite are included.
  • Polygonal ferrite formed by a diffusion mechanism has no internal structures in grains, and its grain boundary is linear or forms an arc. Meanwhile, bainitic ferrite and bainite have an internal structure, have an acicular intergranular shape, and have a structure distinctly different from that of polygonal ferrite. Therefore, polygonal ferrite, bainite, and bainitic ferrite can be determined based on the intergranular shape and the presence or absence of the internal structure from a photograph of the structure obtained by using an optical microscope after etching performed with nital.
  • an image analysis is performed with respect to a photograph of the structure obtained by means of an optical microscope employing a Le Pera-etched sample, so that the total area ratio of residual austenite and martensite can be calculated.
  • a structure of a steel sheet is observed at 1/4 position of the depth in a sheet thickness, in which a representative structure of the steel sheet is shown.
  • hole expansibility of a hot rolled steel sheet can be remarkably improved by reducing unevenness in hardness of ferrite grains.
  • a hardness (micro-hardness) of 50 arbitrary pieces of polygonal ferrite present within a range of ⁇ 100 ⁇ m from a central plane (a face which includes a central portion of the sheet thickness of a steel sheet and is perpendicularly orthogonal to the sheet thickness direction) in the sheet thickness direction is measured, and when a standard deviation of the micro-hardness is the ⁇ HV, excellent hole expansibility can be obtained by setting the ⁇ HV to 30 or smaller. Therefore, the ⁇ HV is set to 30 or smaller. Since the standard deviation is preferred to be small, the lower limit of the ⁇ HV is zero.
  • a specific method of measuring the ⁇ HV will be described below.
  • a steel sheet of which a cross section in a rolling direction is subjected to mirror polishing and in which chemical polishing is performed using colloidal silica in order to remove a worked layer on a surface layer and then the grain boundary is revealed through nital-etching is used.
  • the micro-hardness is measured using a micro-hardness measuring apparatus (brand name: FISCHERSCOPE HM 2000 XYp) by pushing a pyramidal Vickers indenter having an apex angle of 136° into a grain such that its indentation does not overlap the grain boundary of ferrite with respect to randomly selected 50 pieces of polygonal ferrite (grains) which are present within a range of ⁇ 100 ⁇ m from the central plane in the sheet thickness direction.
  • the indentation load is set to 20 N.
  • the standard deviation ⁇ HV of the micro-hardness is obtained from the 50 pieces of obtained data.
  • 5 ⁇ 10 7 pieces/mm 2 or more of Ti-containing carbide are included in a grain of polygonal ferrite.
  • precipitation strengthening is insufficient, thereby resulting in strength deficiency. Meanwhile, there is no need to regulate the upper limit.
  • the number is within the component range described above, the number does not exceed 1 ⁇ 10 11 pieces/mm 2 .
  • the pieces of Ti-containing carbide present in a grain of polygonal ferrite when 50% or more of the carbide, by the number percentage, has the ratio of the length of the short side to the length of the long side (aspect ratio expressed as long side/short side) less than 3, excellent hole expansibility can be obtained. It is preferable to include 2/3 or more of Ti-containing carbide having the aspect ratio of long side/short side less than 3 among the Ti-containing carbide present in a grain of polygonal ferrite. The percentage of the Ti-containing carbide having the aspect ratio less than 3 may be 100%.
  • the percentage of the Ti-containing carbide having the aspect ratio less than 3 is obtained by setting orientation of an electron beam to be parallel to ⁇ 001> of matrix phase ferrite and obtaining carbide having the aspect ratio of long side/short side less than 3 with respect to the total number of pieces of observed Ti-containing carbide when 100 or more pieces of Ti-containing carbide are observed using a transmission electron microscope (magnification: 200,000-fold).
  • the Ti-containing carbide is carbide containing Ti, and the Ti-containing carbide may further contain at least one of V and Nb. That is, the Ti-containing carbide also includes a state where carbide has a crystal structure (NaCl structure) of Ti-containing carbide and some locations of Ti are substituted with V or Nb.
  • the hot rolled steel sheet according to the present embodiment may have a known hot-dip galvanized layer on its surface.
  • the hot-dip galvanized layer may be a galvannealed layer which is alloyed. In a case where a steel sheet has a hot-dip galvanized layer, rust is restrained from being generated, and the corrosion resistance of the hot rolled steel sheet is improved.
  • the claimed steel sheet is specified as their mechanical characteristics, that tensile strength TS is 540 MPa or higher, the ratio (yield ratio (YR)) of the tensile strength TS and 0.2% proof stress YS is 75% or more, and the product (TS ⁇ ) of the tensile strength TS and a hole expanding rate ⁇ regulated by JFST 1001 is 50,000 MPa ⁇ % or higher.
  • the hot rolled steel sheet according to the present embodiment aims to be provided with all the high tensile strength, the high yield ratio, and the high balance between the tensile strength and the hole expansibility (TS ⁇ ) by controlling the chemical composition and the structure.
  • the tensile strength is preferably set to 590 MPa or higher. In addition, if the tensile strength exceeds 1,180 MPa, fatigue properties of a weld portion deteriorate. Accordingly, it is preferable to be 1,180 MPa or lower.
  • the hot rolled steel sheet according to the present embodiment can be stably manufactured in accordance with a manufacturing method including the following processes (A) to (D), and it is preferable.
  • a slab having a chemical composition as described above is heated to approximately 1,200°C.
  • the heating temperature is within the temperature range from 1,150°C to 1,250°C.
  • the heated slab becomes a hot rolled steel sheet via the hot rolling process including the rough rolling process and the finish rolling process.
  • the hot rolled steel sheet according to the present embodiment is manufactured, in each process of rough rolling and finish rolling, it is preferable to control the temperature, the rolling reduction, and the like.
  • the cumulative rolling reduction within the range from 1,050°C to 1,150°C is set to 50% or larger.
  • the cumulative rolling reduction in the present invention is the percentage of the cumulative rolling reduction amount (difference between the inlet sheet thickness before the first pass in rolling and an outlet sheet thickness after the last pass in rolling) with respect to a reference, while the reference is an inlet sheet thickness before a first pass.
  • the cumulative rolling reduction is calculated in each of rough rolling and finish rolling.
  • the cumulative rolling reduction in rough rolling is the percentage of the difference between the inlet sheet thickness before the first pass in rough rolling and the outlet sheet thickness after the last pass in rough rolling.
  • the cumulative rolling reduction in finish rolling is the percentage of the difference between the inlet sheet thickness before the first pass in finish rolling and the outlet sheet thickness after the last pass in finish rolling.
  • the cumulative rolling reduction at 1,050°C or lower ranges from 20% to 80%. if the cumulative rolling reduction at 1,050°C or lower exceeds 80%, the anisotropy of the finally obtained structure of the hot rolled steel sheet is revealed. In this case, there are cases where the ⁇ HV increases and hole expansibility is degraded. The reason is presumed to be the hardness difference which is incited by deviation of the crystal orientation of ferrite grains. Meanwhile, if the cumulative rolling reduction at 1,050°C or lower falls short of 20%, the austenite grain size is coarsened and accumulation of distortion in austenite becomes insufficient. Accordingly, ferritic transformation after finish rolling is suppressed, and the finally obtained polygonal ferrite fraction and standard deviation of micro-hardness of polygonal ferrite deviate from the desired range, and the possibility of deterioration of hole expansibility increases.
  • the rolling reduction of the last pass is from 15% to 35%. If the rolling reduction of the last pass exceeds 35%, the anisotropy of the structure is revealed. As a result, there are cases where the ⁇ Hv increases and hole expansibility is degraded. Therefore, the rolling reduction of the last pass is set to 35% or smaller and is more preferably set to 25% or smaller. Meanwhile, if the rolling reduction of the last pass falls short of 15%, accumulation of distortion in austenite becomes insufficient. Accordingly, ferritic transformation after finish rolling is suppressed, and the finally obtained polygonal ferrite fraction and standard deviation of micro-hardness of polygonal ferrite deviate from the desired range, and the possibility of deterioration of hole expansibility increases.
  • the finishing temperature (temperature of the steel sheet after the last pass of finish rolling) is set to 930°C or higher. If the finishing temperature falls short of 930°C, the anisotropy of the structure is likely to be revealed in the finally obtained hot rolled steel sheet. As a result, the ⁇ Hv increases, and the possibility of deterioration of hole expansibility increases. Meanwhile, in accordance with an increase of the finishing temperature, the austenite grain size is coarsened and accumulation of distortion in austenite becomes insufficient. Accordingly, ferritic transformation after finish rolling is suppressed, and the finally obtained polygonal ferrite fraction and standard deviation of micro-hardness of polygonal ferrite grow, so that the possibility of deterioration of hole expansibility increases. Therefore, it is preferable that the upper limit of the finishing temperature is set to approximately 1,000°C.
  • the hot rolled steel sheet is subjected to cooling.
  • cooling of the hot rolled steel sheet at a low average cooling rate for a desired time which is determined in accordance with the Mn content is effective in promoting ferritic transformation and precipitation of the Ti-containing carbide, and having the finally obtained polygonal ferrite fraction and standard deviation of micro-hardness of polygonal ferrite within the desired range.
  • the hot rolled steel sheet is coiled.
  • the cooling rate is slower than 30°C/s or the coiling temperature exceeds 650°C
  • Ti-containing carbide in the hot rolled steel sheet is excessively coarsened during the cooling or after the coiling, and there are cases where it becomes difficult to ensure the desired strength.
  • the coiling temperature is set to less than 450°C, accuracy of controlling the coiling temperature is degraded, and it is not preferable. Therefore, in order to be effective, the coiling temperature is set to range from 450°C to 650°C, and cooling is performed until the temperature reaches the coiling temperature at a predetermined average cooling rate or faster.
  • cooling is performed under the condition that the average cooling rate within the temperature range from a finish rolling last pass temperature to MT (720°C ⁇ MT ⁇ 830°C) becomes 30°C/s or faster.
  • ii) as secondary cooling cooling is performed for t seconds which is regulated by the following Expression 1 under the cooling condition that the average cooling rate within the temperature range from MT to Tx (720°C ⁇ Tx ⁇ MT) becomes 10°C/s or slower.
  • the hot rolled steel sheet according to the present embodiment is manufactured, as necessary, the following processes may be further provided.
  • a hot-dip galvanizing process for hot-dip galvanizing a hot rolled steel sheet may be provided. It is possible to form a coating layer on a surface of the steel sheet and to improve corrosion resistance of the steel sheet by performing hot-dip galvanizing.
  • a galvannealed layer may be formed on a surface of the steel sheet by performing alloying.
  • the maximum heating temperature during annealing before hot-dip galvanizing dipping is preferably set to 800°C or lower.
  • Other hot-dip galvanizing conditions may comply with routine procedures.
  • pickling may be performed after the hot rolling process.
  • skin pass rolling may be performed for flatness correction or promotion of scale peeling.
  • the elongation rate in a case of performing skin pass rolling is not particularly regulated. However, it is preferable to set to range from 0.1% to less than 3.0%.
  • Pieces of steel respectively having the chemical compositions indicated in Table 1 were each formed into ingot at a laboratory and were cast into slabs. Then, the slabs were subjected to heating, hot rolling, cooling, and coiling in the pattern as shown in FIG. 1 . In this case, the conditions in each process were as indicated in Table 2.
  • SRT, R1, R2, R3, FT, MT, t, and CT indicate the following, respectively.
  • Hot rolled steel sheets obtained as described above were subjected to pickling.
  • JIS No. 5 tensile test pieces were respectively collected from the hot rolled steel sheets in a direction perpendicular to the rolling direction.
  • a tensile test was performed using these test pieces, and the yield strength (YS), the tensile strength (TS), the yield ratio (YR), and the total elongation (EL) were measured.
  • samples each including a cross section of the hot rolled steel sheet in the rolling direction were collected.
  • a surface corresponding to the cross section of each sample in the rolling direction was etched using a nital solution.
  • a photograph of the structure obtained in the visual field of 300 ⁇ m ⁇ 300 ⁇ m at 1/4 position of the depth in the sheet thickness was captured using an optical microscope or an electronic scanning microscope, and the structure was identified.
  • the area ratio of each structure was calculated through a point counting method.
  • Polygonal ferrite, bainite, and bainitic ferrite were determined based on the intergranular shape and the presence or absence of the internal structure.
  • the structure etched in black was discriminated from cementite and pearlite.
  • an image analysis was performed with respect to the photograph of the structure obtained using the optical microscope, and the total area ratio of residual austenite and martensite was thereby calculated.
  • a pellicle sample was collected from each of the hot rolled steel sheets. Then, carbide containing at least one of Ti, V, and Nb precipitated in a grain of ferrite was observed using the transmission electron microscope (magnification: 200,000-fold), and the number density and the percentage of the precipitated element having the aspect ratio of 3 or less were obtained.
  • FIGS. 3A and 3B respectively show the measurement results of micro-hardness of the sample number 14 and the sample number 15, as examples.
  • Tables 3 and 4 show the obtained results.
  • V ⁇ , VP ⁇ , VMA, B, BF, and ⁇ HV indicate the following, respectively.
  • the blank spaces for the structures denote that no observation was performed.
  • JIS No. 5 tensile test pieces were respectively collected in a direction perpendicular to the rolling direction.
  • a tensile test was performed using these test pieces, and the yield strength (YS), the tensile strength (TS), the yield ratio (YR), and the total elongation (EL) were measured.
  • a hole expanding test was performed based on "JFS T 1001 the hole expanding test method" of the Japan Iron and Steel Federation Standard, and the hole expanding rate ( ⁇ ) was measured.
  • a pellicle sample was collected from each of the hot rolled steel sheets. Then, carbide containing at least one of Ti, V, and Nb precipitated in a grain of ferrite was observed using the transmission electron microscope (magnification: 200,000-fold), and the number density and the percentage of the precipitated element having the aspect ratio of 3 or less were obtained. The standard deviation of micro-hardness of the steel from which 80 area% or more polygonal ferrite could be obtained was measured through the method described above.
  • Table 6 shows the obtained results.
  • the sample numbers 39 to 42, and 44 to 47 since all the chemical compositions and the structures were within the range regulated by the present invention, desired mechanical characteristics were obtained. Meanwhile, in the sample number 43, the oHV exceeded the upper limit regulated by the present invention. As a result, desired mechanical characteristics could not be obtained.
  • the sample number 48 the area ratio of polygonal ferrite fell short of the lower limit regulated by the present invention. As a result, desired mechanical characteristics could not be obtained.
  • the steel sheet according to the present invention it is possible to inexpensively manufacture a high strength hot rolled steel sheet having a high yield ratio and excellent hole expansibility.
  • the steel sheet according to the present invention also has excellent hole expansibility even during stretch flanges forming frequently employed for automobile components, particularly chassis components and the like. Therefore, the steel sheet industrially contributes to reducing weight and ensuring collision safety of vehicle bodies particularly in automobile fields.

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CN109047692B (zh) * 2018-08-22 2021-01-26 江阴兴澄特种钢铁有限公司 一种能够在-60℃条件下使用的超薄规格高强钢板及其制造方法
CN114096692A (zh) * 2019-07-09 2022-02-25 杰富意钢铁株式会社 耐硫酸露点腐蚀性优异的无缝钢管及其制造方法
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CN112522568A (zh) * 2019-09-19 2021-03-19 宝山钢铁股份有限公司 一种耐火耐候钢板/带及其制造方法
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WO2017022025A1 (ja) 2017-02-09
CN107849651B (zh) 2019-09-03
KR102079968B1 (ko) 2020-02-21
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BR112018000633A2 (pt) 2018-09-18
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MX2018001140A (es) 2018-04-20
JPWO2017022025A1 (ja) 2018-05-31

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