EP3330394B1 - High_strength_ hot_rolled steel sheet - Google Patents
High_strength_ hot_rolled steel sheet Download PDFInfo
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- 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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- European Patent Office
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- steel sheet
- rolled steel
- hot rolled
- ferrite
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- 229910000831 Steel Inorganic materials 0.000 title claims description 126
- 239000010959 steel Substances 0.000 title claims description 126
- 229910001568 polygonal ferrite Inorganic materials 0.000 claims description 55
- 229910000859 α-Fe Inorganic materials 0.000 claims description 34
- 238000000034 method Methods 0.000 claims description 21
- 229910001566 austenite Inorganic materials 0.000 claims description 20
- 239000000126 substance Substances 0.000 claims description 16
- 239000000203 mixture Substances 0.000 claims description 15
- 229910001567 cementite Inorganic materials 0.000 claims description 13
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 claims description 13
- 229910001563 bainite Inorganic materials 0.000 claims description 12
- 229910052758 niobium Inorganic materials 0.000 claims description 11
- 229910052720 vanadium Inorganic materials 0.000 claims description 11
- 229910000734 martensite Inorganic materials 0.000 claims description 10
- 229910001562 pearlite Inorganic materials 0.000 claims description 10
- 239000012535 impurity Substances 0.000 claims description 7
- 229910052802 copper Inorganic materials 0.000 claims description 6
- 229910052750 molybdenum Inorganic materials 0.000 claims description 6
- 229910052759 nickel Inorganic materials 0.000 claims description 6
- 229910052796 boron Inorganic materials 0.000 claims description 5
- 229910052791 calcium Inorganic materials 0.000 claims description 4
- 229910052804 chromium Inorganic materials 0.000 claims description 4
- 229910052749 magnesium Inorganic materials 0.000 claims description 4
- 229910052797 bismuth Inorganic materials 0.000 claims description 2
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- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 description 2
- FAPWRFPIFSIZLT-UHFFFAOYSA-M Sodium chloride Chemical group [Na+].[Cl-] FAPWRFPIFSIZLT-UHFFFAOYSA-M 0.000 description 2
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- JEIPFZHSYJVQDO-UHFFFAOYSA-N iron(III) oxide Inorganic materials O=[Fe]O[Fe]=O JEIPFZHSYJVQDO-UHFFFAOYSA-N 0.000 description 1
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Images
Classifications
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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
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
-
- 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
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—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
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
-
- C—CHEMISTRY; METALLURGY
- 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
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—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
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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- 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
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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- 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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Description
- 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.
- Needless to mention, 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. For example, when chassis components are press-formed, 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. In addition, in 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.
- Generally, in high strength hot rolled steel sheets, in order to achieve both the high yield ratio and the excellent hole expansibility, it is examined that 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.
- For example, 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. However, in the technology of Patent Document 1, it is essential to add 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. However, in Patent Document 2, there is no consideration for the yield ratio which is one of characteristics necessary for a hot rolled steel sheet applied to chassis components. In addition, even though 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. Moreover, the definition of ferrite defined in Patent Document 2 is unclear, and it is assumed that the ferrite includes so-called bainitic ferrite or pseudo-polygonal ferrite which is not polygonal ferrite. As the reason, in Patent Document 2, 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. However, in the hot rolled steel sheet of Patent Document 3, in a case of postulating application to chassis components, for example, a high yield ratio of 75% or more is not obtained in high strength steel of 540 MPa or higher.
- In addition, 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. However, since 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. In addition, since 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. However, in the technology of 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. -
- [Patent Document 1] Japanese Unexamined Patent Application, First Publication No.
2002-322540 - [Patent Document 2] Japanese Unexamined Patent Application, First Publication No.
2007-009322 - [Patent Document 3] Japanese Unexamined Patent Application, First Publication No.
H10-287949 - [Patent Document 4] Japanese Unexamined Patent Application, First Publication No.
2012-026032 - [Patent Document 5] Japanese Unexamined Patent Application, First Publication No.
2004-307919 - The present invention has been made in consideration of the current circumstances described above. 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. In order to achieve excellent hole expansibility, the inventors have examined structures of hot rolled steel sheets on the premise that polygonal ferrite is employed as a main constituent. Moreover, in order to improve the strength of the structure having excellent hole expansibility and containing polygonal ferrite 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.
- In addition, 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.
- 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. In addition, 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.
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FIG. 1 is a schematic view showing an example of a treatment pattern of hot rolling. -
FIG. 2 is a schematic view showing an example of a heat treatment pattern in a galvannealing line employed in Example 2. -
FIG. 3A is a view showing an example of micro-hardness distribution of polygonal ferrite measured in Example 1. -
FIG. 3B is a view showing another example of micro-hardness distribution of polygonal ferrite measured in Example 1. - Hereinafter, 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.
- There is provided a hot rolled steel sheet according to the present embodiment.
- (a) The hot rolled steel sheet according to the present embodiment includes, as a chemical composition, by mass%, C: 0.010% to 0.200%, Si: 0.001% to 2.50%, Mn: 0.001% to 1.50%, P: 0.050% or less, S: 0.010% or less, N: 0.0070% or less, Al: 0.001% to 0.50%, and Ti: 0.050% to 0.30%. Furthermore, as necessary, the hot rolled steel sheet includes at least one selected from the group consisting of V: 0.50% or less, Nb: 0.090% or less, Cr: 0.50% or less, Ni: 0.50% or less, Cu: 0.50% or less, Mo: 0.50% or less, B: 0.0050% or less, Ca: 0.01% or less, Mg: 0.01% or less, and Bi: 0.01% or less; and a remainder of Fe and impurities.
- (b) A structure includes, by area ratio, 80% or more of polygonal ferrite, a total of 5% or less of martensite and austenite, and a total of 5% or less of pearlite and cementite, and the remainder is at least one selected from bainitic ferrite and bainite.
- (c) When a standard deviation of micro-hardness of 50 arbitrary pieces of the polygonal ferrite present within a range of ±100 µm from a central plane in a sheet thickness direction is σHV, the σHV is 30 or smaller.
- (d) A grain of the polygonal ferrite contains 5×107 pieces/mm2 or more of Ti-containing carbide, and in 50% or more of the Ti-containing carbide, an aspect ratio which is a ratio of the length of a long side to the length of a short side is less than 3.
- (e) Tensile strength is 540 MPa or higher, wherein the yield ratio of tensile strength and 0.2% proof stress is 75% or more and
- First, the reasons for limiting the chemical composition of the hot rolled steel sheet according to the present embodiment will be described. Hereinafter, all the percentage signs "%" regulating the chemical composition indicate "mass%".
- C is an element essential to high-strengthening of a steel sheet performed through precipitation strengthening or solid solution strengthening. In order to achieve this effect, 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. Meanwhile, when there is an excessive amount of C, forming of polygonal ferrite is suppressed and cementite is likely to be formed. In addition, the hardness difference among the grains of polygonal ferrite tends to increase. As a result, hole expansibility deteriorates. In addition, weldability also deteriorates remarkably. Therefore, 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. In order to achieve this effect, 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. In addition, Mn is an element which fixes S in steel as MnS and suppresses hot embrittlement caused by a solid solution S. In order to achieve these effects, 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. Meanwhile, when there is an excessive amount of Mn, ferritic transformation from austenite is delayed, so that it becomes difficult to obtain 80 area% or more of polygonal ferrite, and hole expansibility deteriorates. Therefore, 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. However, in a case where 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. In order to achieve this effect, the Al content is set to 0.001% or more. Meanwhile, although the Al content exceeds 0.50%, not only the effect is saturated but also a cost increase is caused. Therefore, 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. In addition, 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. When the Ti content is less than 0.050%, the effect is not sufficient. Accordingly, 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.
- Basically, the hot rolled steel sheet according to the present embodiment contains the chemical composition described above and the remainder of Fe and impurities. However, in order to improve strength and hole expansibility, in place of a part of Fe, within the range described below, 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. However, since these elements are not necessarily contained, their lower limits are 0%. Here, impurities denote components incorporated due to raw materials such as ores and scraps, or other factors when steel is industrially manufactured.
- Similar to Ti, V is an element forming carbide in steel. In addition, 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. When the V content is less than 0.010%, the effect described above cannot be sufficiently obtained. Accordingly, in a case of obtaining the effect described above, 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.
- Similar to Ti, 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. Therefore, even in a case where the elements are contained, 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.
- Next, the reasons for limiting the structure of the hot rolled steel sheet according to the present embodiment will be described.
- Polygonal ferrite has a structure effective in improving hole expansibility. In order to ensure 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.
- If the area ratio of martensite and austenite exceeds 5% in total, hole expansibility deteriorates remarkably. Therefore, the total area ratio of the martensite and austenite is set to 5% or less and is preferably set to 2% or less. In addition, the total area ratio may be 0% (that is, none of martensite and austenite are contained). In addition, the austenite mentioned herein is so-called residual austenite.
- If the area ratio of pearlite and cementite exceeds 5% in total, hole expansibility deteriorates remarkably. Therefore, 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. In addition, 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.
- After a structure of a sample cut out from a hot rolled steel sheet is revealed through etching, the structure described above can be identified from a photograph of the structure.
- 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. In a case where the internal structure is not distinctly revealed and a structure having an acicular intergranular shape (pseudo-polygonal ferrite) is present, it is counted as bainitic ferrite. In addition, since cementite and pearlite are etched in black, their structures can be distinctly discriminated.
- In addition, 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.
- In the present embodiment, 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.
- As described above, hole expansibility of a hot rolled steel sheet can be remarkably improved by reducing unevenness in hardness of ferrite grains. Specifically, when a hardness (micro-hardness) of 50 arbitrary pieces of polygonal ferrite present within a range of ±100 µm 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 σHV, excellent hole expansibility can be obtained by setting the σHV to 30 or smaller. Therefore, the σHV is set to 30 or smaller. Since the standard deviation is preferred to be small, the lower limit of the σHV is zero.
- A specific method of measuring the σHV will be described below. As a sample for measuring hardness, 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 µm from the central plane in the sheet thickness direction. The indentation load is set to 20 N. The standard deviation σHV of the micro-hardness is obtained from the 50 pieces of obtained data.
- In the hot rolled steel sheet according to the present embodiment, 5×107 pieces/mm2 or more of Ti-containing carbide are included in a grain of polygonal ferrite. When there are 5×107 pieces/mm2 or less, precipitation strengthening is insufficient, thereby resulting in strength deficiency. Meanwhile, there is no need to regulate the upper limit. Generally, when the number is within the component range described above, the number does not exceed 1×1011 pieces/mm2.
- In addition, among 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).
- In the present embodiment, 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.
- [Ratio (Yield Ratio) of Tensile Strength (TS) and 0.2% Proof Stress (YS): 75% or More]
- In order to satisfy strict performance required in recent high strength hot rolled Steel sheets for automobiles, 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·λ) of the tensile strength TS and a hole expanding rate λ 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·λ) 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.
- Next, a preferable manufacturing method for obtaining the hot rolled steel sheet according to the present embodiment will be described. 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) A slab obtained from molten steel having the chemical composition within the range described above is heated to approximately 1,200°C.
- (B) The heated slab is subjected to rough rolling such that the cumulative rolling reduction within a range from 1,050°C to 1,150°C becomes 50% or larger.
- (C) The steel sheet after rough rolling is subjected to finish rolling such that the cumulative rolling reduction at 1,050°C or lower ranges from 20% to 80%, the rolling reduction of the last pass ranges from 15% to 35%, and the temperature of the last pass (finishing temperature) becomes 930°C or higher.
- (D) Thereafter, with respect to the hot rolled steel sheet, i) as primary cooling, 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. Thereafter, ii) as secondary cooling, cooling is performed for t seconds which is regulated by t(sec)=5·(Mn)2 under the condition that the average cooling rate within the temperature range from MT to Tx (720°C≤Tx<MT) (here, (Mn) is the Mn content by unit mass%) becomes 10°C/s or slower. Subsequently, iii) as third cooling, cooling is performed under the condition that the average cooling rate within the temperature range from Tx to CT (450°C≤CT≤650°C) which is a secondary cooling end temperature becomes 30°C/s or faster. Then, after being cooled to CT, the hot rolled steel sheet is coiled.
- Hereinafter, the reasons will be described.
- In the heating process, a slab having a chemical composition as described above is heated to approximately 1,200°C. From viewpoints on affecting the precipitation density of Ti-containing carbide in a grain of polygonal ferrite, and the solid solution states of carbide forming elements such as Ti, Nb, and V; and restraining coarse carbide from being formed, in order to obtain desired performance, it is preferable that 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. When 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.
- In the rough rolling process of hot rolling, it is preferable that the cumulative rolling reduction within the range from 1,050°C to 1,150°C is set to 50% or larger. When the cumulative rolling reduction within the range from 1,050°C to 1,150°C falls short of 50%, the structure becomes inhomogeneous, and there are cases where the σHV increases and hole expansibility is degraded. 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. In addition, the cumulative rolling reduction is calculated in each of rough rolling and finish rolling. That is, 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.
- In the finish rolling process of hot rolling, it is preferable that 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 σHV 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.
- It is preferable that 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 σHv 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.
- It is preferable that 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 σHv 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.
- After the finish rolling, the hot rolled steel sheet is subjected to cooling.
- Within the temperature range from the finish rolling last pass temperature to 720°C, i), a change in density of the Ti-containing carbide in a grain of polygonal ferrite due to the growing (coarsening) of Ti-containing carbide precipitated in ferrite, and ii) a change in aspect ratio of long side/short side of the Ti-containing carbide present in a grain of polygonal ferrite increase. Therefore, in order to obtain the desired performance, it is effective that the average cooling rate within the temperature range from the finish rolling last pass temperature to 720°C is set to 30°C/s.
- Furthermore, after the cooling, within the temperature range from 830°C to 720°C, 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.
- Thereafter, cooling is further performed, and then the hot rolled steel sheet is coiled. In this case, if 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. Meanwhile, in a case where 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.
- That is, in the cooling process after finish rolling, with respect to the hot rolled steel sheet after finish rolling, it is preferable that i) as primary cooling, 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. Thereafter, 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. Subsequently, iii) as third cooling, cooling is performed under the cooling condition that the average cooling rate becomes 30°C/s or faster within the temperature range from the secondary cooling end temperature (Tx) to CT (450°C≤CT≤650°C). Then, coiling is performed within the temperature range from the 450°C to 650°C.
(t(sec)=5·(Mn)2) Expression 1
Here, (Mn) is the Mn content by unit mass%. - In a case where the hot rolled steel sheet according to the present embodiment is manufactured, as necessary, the following processes may be further provided.
- After the coiling process, 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. In addition, after hot-dip galvanizing, a galvannealed layer may be formed on a surface of the steel sheet by performing alloying. In addition, in this case, in order to suppress degradation of strength of the steel sheet, 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.
- In the hot rolled steel sheet according to the present embodiment, in accordance with the routine procedure, after the hot rolling process, pickling may be performed. In addition, before pickling or after pickling, 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%.
- Hereinafter, Examples of the present invention will be described.
- 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. In Table 2, SRT, R1, R2, R3, FT, MT, t, and CT indicate the following, respectively. - SRT: slab heating temperature
- R1: cumulative rolling reduction within range from 1,050°C to 1,150°C
- R2: cumulative rolling reduction at 1,050°C or lower
- R3: rolling reduction at last finish pass
- FT: finish rolling temperature
- MT: primary cooling stop temperature
- t: secondary cooling time
- CT: coiling temperature
- Hot rolled steel sheets obtained as described above were subjected to pickling. In regard to the condition indicated as plating in the spaces for treatment in Table 3, after hot-dip galvanizing was performed, 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.
- In addition, 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 (λ) was measured.
- In addition, 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. Thereafter, a photograph of the structure obtained in the visual field of 300 µm×300 µm 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. From the photograph of the obtained structure, 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. In addition, by means of a Le Pera-etched sample, 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.
- In addition, 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.
- In addition, 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.
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. In Tables 3 and 4, Vα, VPθ, VMA, B, BF, and σHV indicate the following, respectively. The blank spaces for the structures denote that no observation was performed.
- Vα: area ratio of ferrite
- VPθ: total area ratio of pearlite and cementite
- VMA: total area ratio of martensite and austenite
- B, BF: bainite and bainitic ferrite
- σHV: standard deviation of micro-hardness of ferrite
- In the sample numbers 1 to 3, 5, 6, 11, 17 to 19, 22, and 25 to 34, 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 numbers 4, 10, 12 to 16, 20 to 21, 24, and 36, the σHV exceeded the upper limit regulated by the present invention. As a result, desired mechanical characteristics could not be obtained. In the sample numbers 7, 8, 18, and 36, 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. In the sample number 9, the total area ratio of martensite and austenite overtook the upper limit regulated by the present invention. As a result, desired mechanical characteristics could not be obtained. In the sample numbers 36 and 38, the total area ratio of pearlite and cementite overtook the upper limit regulated by the present invention. As a result, desired mechanical characteristics could not be obtained.
- In addition, in the sample numbers 7, 8, 12, 23, 24, 35, and 38, the number density of carbide was low. In addition, in the sample numbers 7, 8, 12, 23, 24, and 36, the percentage of the Ti-containing carbide having the aspect ratio of 3 or less increased, so that desired mechanical characteristics could not be obtained.
- In the sample number 37, toughness was low and breaking occurred at the time of test piece processing. Accordingly, no test could be performed.
[Table 2] Condition SRT R1 R2 R3 FT Primary average cooling rate MT t Secondary average cooling rate Third average cooling rate CT (°C) (%) (%) (%) (°C) (°C/s) (°C) (sec) (°C/s) (°C/s) (°C) a 1,200 82 64 20 950 40 780 10 5 20 570 b 1,200 82 64 20 950 40 730 10 5 30 570 c 1,200 82 64 20 950 50 620 10 5 30 550 d 1,200 82 64 20 950 50 780 1 5 30 570 e 1,200 82 64 20 950 40 730 10 15 20 260 f 1,200 93 12 12 950 40 730 10 5 40 590 g 1,200 82 64 20 950 60 770 10 8 20 430 h 1,200 82 64 20 950 50 680 1 8 30 570 i 1,200 82 64 20 950 30 820 1 8 30 700 j 1,200 37 64 20 950 40 770 10 8 30 570 k 1,200 82 90 20 950 40 780 10 10 30 570 l 1,200 82 64 42 950 35 770 10 10 30 570 m 1,200 82 64 20 880 50 780 10 10 30 570 [Table 3] Sample No. Kind of steel Condition Mechanical characteristics Steel structure Treatment Remarks TS YS YR EL λ TS·λ Vα VPθ VMA Remainder σHV Number density of carbide Proportion of Ti-containing carbide having aspect ratio of 3 or less (%) (MPa) (MPa) (%) (%) (%) (MPa·%) (%) (%) (%) (×107 pieces/mm2) 1 A a 630 535 84.9 24.2 124 78120 98 1 B,BF 15 11.0 87 Example of invention 2 B a 746 599 80.3 19.6 102 76241.2 98 B,BF 17 11.3 89 Plating Example of invention 3 B b 814 670 82.3 18.2 89 72446 96 B,BF 24 12.3 90 Example of invention 4 B c 801 654 81.6 18.2 60 48060 75 B,BF 34 3.2 47 Comparative Example 5 C a 748 600 80.2 20.0 100 74800 98 B,BF 23 9.2 78 Example of invention 6 C b 891 765 85.9 19.6 73 65043 96 B,BF 27 10.2 89 Example of invention 7 C c 935 857 91.7 17.6 52 48620 69 B,BF - 2.7 29 Comparative Example 8 C d 890 815 91.6 18.0 87 48060 60 B,BF - 3.9 33 Comparative Example 9 C e 765 569 74.4 19.0 60 46053 87 10 B,BF 34 5.6 78 Comparative Example 10 C f 877 774 88.3 17.9 57 49989 82 B,BF 34 7.9 82 Comparative Example 11 D a 791 650 82.2 19.0 101 79891 99 B,BF 18 8.9 89 Example of invention 12 D i 712 603 84.7 18.0 66 46992 96 B,BF 34 4.3 35 Comparative Example 13 D j 802 703 87.7 17.2 61 48922 95 B,BF 32 10.2 85 Comparative Example 14 D k 821 673 82.0 18.9 52 42692 97 B,BF 33 10.5 89 Comparative Example 15 D 1 803 666 82.9 18.6 59 47377 98 B,BF 32 9.5 82 Comparative Example 16 D m 780 642 82.3 17.9 52 40560 91 B,BF 31 9.3 76 Comparative Example 17 E a 842 698 82.9 18.9 93 78306 98 B,BF 20 9.2 85 Example of invention 18 F a 773 671 86.8 18.6 116 89668 99 B,BF 18 9.0 90 Plating Example of invention 19 G a 863 696 80.6 18.2 94 81122 99 B,BF 20 12.1 95 Example of invention [Table 4] Sample No. Kind of steel Condition Mechanical characteristics Steel structure Treatment Remarks TS YS YR EL λ TS·λ Vα VPθ VMA Remainder σHV Number density of carbide Proportion of Ti-containing carbide having aspect ratio of 3 or less (%) (MPa) (MPa) (%) (%) (%) (MPa·%) (%) (%) (%) (×107 pieces/mm2) 20 G c 939 784 83.5 18.0 50 46950 65 B,BF 34 4.2 23 Comparative Example 21 G f 920 745 81.0 18.4 54 49680 78 B,BF 35 10.2 78 Comparative Example 22 G g 875 739 84.5 18.1 99 86625 98 B,BF 19 10.5 89 Example of invention 23 H a 910 835 91.8 17.6 47 42770 30 B,BF - 5.2 53 Comparative Example 24 H h 935 871 93.2 17.9 43 40205 42 B,BF 35 4.2 38 Comparative Example 25 I a 790 654 82.8 19.1 96 75840 99 B,BF 17 11.9 90 Example of invention 26 J a 784 639 81.5 19.7 98 76832 99 B,BF 18 10.3 91 Plating Example of invention 27 K a 798 656 82.2 20.0 91 72618 98 B,BF 21 8.7 87 Example of invention 28 L a 763 626 82.0 20.1 104 79352 98 B,BF 19 9.2 86 Example of invention 29 M a 777 643 82.8 19.5 92 71484 99 B,BF 21 12.1 85 Example of invention 30 N a 780 665 85.3 18.4 89 69420 98 B,BF 23 10.3 83 Example of invention 31 O a 742 601 81.0 19.7 118 87556 99 B,BF 17 11.5 92 Example of invention 32 P a 741 599 80.8 19.8 116 85956 99 B,BF 17 9.5 90 Example of invention 33 Q a 743 597 80.3 19.4 117 86931 98 B,BF 17 10.0 87 Example of invention 34 R a 762 618 81.1 19.7 98 74676 99 B,BF 16 11.0 88 Example of invention 35 T a 482 367 76.1 31.0 83 40006 100 B,BF 2 0.01 100 Plating Comparative Example 36 U a 859 758 88.2 15.0 18 15462 47 9 BF 35 6.3 51 Comparative Example 37 V a Breaking occurred at time of test piece processing Comparative Example 38 S a 593 449 75.7 24.7 71 42103 84 8 B,BF 15 3.8 83 Comparative Example - Next, among the pieces of steel having the chemical composition indicated in Table 1, five kinds of steel (Ato C, G, and H) were subjected to hot rolling and cooling shown in
FIG. 1 . Thereafter, descaling was performed. Then, without performing cold rolling, heat treatment simulating the galvannealing line having the pattern shown inFIG. 2 was performed using a continuous heat treatment simulator. In this case, the conditions in each process were as indicated in Table 5. In Table 5, RA, LTH, DIP, and GA indicate the following, respectively. - RA: maximum heating temperature
- LTH: low-temperature retention temperature
- DIP: Zn bath temperature
- GA: galvannealing temperature
- From the hot rolled steel sheets obtained as described above, 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. In addition, 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 (λ) was measured.
- In addition, samples each including a cross section of the steel sheet in the rolling direction was collected, and the area ratio of each structure was calculated through the same method as that in Example 1.
- In addition, 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. In 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. In 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.
[Table 5] Condition Hot rolling conditions Galvannealing conditions SRT R1 R2 R3 FT MT Primary average cooling rate t Secondary average cooling rate Third average cooling rate CT RA LTH DIP GA (°C) (%) (%) (%) (°C) (°C) (°C/s) (sec) (°C/s) (°C/s) (°C) (°C) (°C) (°C) (°C) a' 120C 82 64 20 950 780 40 10 5 20 570 740 490 460 Absent b' 1200 82 64 20 950 780 40 10 5 30 570 740 490 460 530 c' 1200 82 64 20 950 620 50 10 5 30 550 740 490 460 530 [Table 6] Sample No. Kind of steel Condition Mechanical characteristics Steel structure Remarks TS YS YR EL λ TS·λ Vα VPβ VMA Remainder σHV Number density of carbide Proportion of Ti-containing carbide having aspect ratio of 3 or less (%) (MPa) (MPa) (%) (%) (%) (MPa·%) (%) (%) (%) (×107 pieces/mm2) 39 A a' 615 541 88.0 24.1 124 76260 98 B,BF 14 10.2 89 Example of invention 40 A b' 617 539 87.4 24.2 130 80210 98 B,BF 15 11 90 Example of invention 41 B a' 738 620 84.0 19.3 114 84132 98 B,BF 15 9.8 88 Example of invention 42 B b' 733 623 85.0 19.6 110 80630 98 B,BF 16 11.2 92 Example of invention 43 B c' 796 663 83.3 17.8 58 46168 95 B,BF 34 4.3 46 Comparative Example 44 C a' 740 639 86.4 20.1 113 83620 98 B,BF 22 8.9 88 Example of invention 45 C b' 741 642 86.6 20.2 111 82251 99 B,BF 23 9.2 86 Example of invention 46 G a' 850 725 85.3 18.3 99 84150 99 B,BF 20 9.2 93 Example of invention 47 G b' 844 729 86.4 18.2 100 84400 99 B,BF 22 8.9 89 Example of invention 48 H b' 899 833 92.7 18.4 50 44950 27 4 B,BF - 3.2 48 Comparative Example - 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. In addition, 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.
Claims (4)
- A high strength hot rolled steel sheet comprising, as a chemical composition, by mass%,
C: 0.010% to 0.200%,
Si: 0.001% to 2.50%,
Mn: 0.001% to 1.50%,
P: 0.050% or less,
S: 0.010% or less,
N: 0.0070% or less,
Al: 0.001% to 0.50%,
Ti: 0.050% to 0.30%,
V: 0% to 0.50%,
Nb: 0% to 0.090%,
Cr: 0% to 0.50%,
Ni: 0% to 0.50%,
Cu: 0% to 0.50%,
Mo: 0% to 0.50%,
B: 0% to 0.0050%,
Ca: 0% to 0.01%,
Mg: 0% to 0.01%,
Bi: 0% to 0.01%, and
a remainder of Fe and impurities,
wherein a structure includes, by area ratio, 80% or more of a polygonal ferrite, a total of 5% or less of a martensite and an austenite, and a total of 5% or less of a pearlite and a cementite, and the remainder is at least one selected from a bainitic ferrite and a bainite,
wherein when a standard deviation of micro-hardness of 50 arbitrary pieces of the polygonal ferrite present within a range of ±100 µm from a central plane in a sheet thickness direction is σHV, the σHV is 30 or smaller,
wherein a grain of the polygonal ferrite contains 5×107 pieces/mm2 or more of Ti-containing carbide, and in 50% or more of the Ti-containing carbide, an aspect ratio which is a ratio of a length of a long side to a length of a short side is less than 3, and
wherein a tensile strength is 540 MPa or higher,
wherein the yield ratio of tensile strength and 0.2% proof stress is 75% or more and wherein the product of tensile strength and hole expanding rate is 50,000 MPa·% or higher,
wherein the micro-hardness is determined in accordance with the method disclosed in the description. - The high strength hot rolled steel sheet according to Claim 1,
wherein the chemical composition includes, by mass%, at least one selected from the group consisting of V: 0.010% to 0.50%, Nb: 0.001% to 0.090%, Cr: 0.001% to 0.50%, Ni: 0.001% to 0.50%, Cu: 0.001% to 0.50%, Mo: 0.001% to 0.50%, and B: 0.0001% to 0.0050%. - The high strength hot rolled steel sheet according to Claim 1 or 2, wherein the chemical composition includes, by mass%, at least one selected from the group consisting of Ca: 0.0001% to 0.01%, Mg: 0.0001% to 0.01%, and Bi: 0.0001% to 0.01%.
- The high strength hot rolled steel sheet according to any one of Claims 1 to 3, further comprising:
a hot-dip galvanized layer on a surface.
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CN110656292A (en) * | 2018-06-28 | 2020-01-07 | 上海梅山钢铁股份有限公司 | 440 MPa-level tensile strength low-yield-ratio high-hole-expansibility hot-rolled steel plate |
CN109047692B (en) * | 2018-08-22 | 2021-01-26 | 江阴兴澄特种钢铁有限公司 | Ultrathin high-strength steel plate capable of being used at-60 ℃ and manufacturing method thereof |
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US9273370B2 (en) * | 2010-07-28 | 2016-03-01 | Nippon Steel & Sumitomo Metal Corporation | Hot-rolled steel sheet, cold-rolled steel sheet, galvanized steel sheet, and methods of manufacturing the same |
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