US20220177995A1 - High strength steel sheet - Google Patents

High strength steel sheet Download PDF

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US20220177995A1
US20220177995A1 US17/601,799 US202017601799A US2022177995A1 US 20220177995 A1 US20220177995 A1 US 20220177995A1 US 202017601799 A US202017601799 A US 202017601799A US 2022177995 A1 US2022177995 A1 US 2022177995A1
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steel sheet
less
residual austenite
high strength
ferrite
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Genki ABUKAWA
Hiroshi Shuto
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Nippon Steel Corp
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Nippon Steel Corp
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Definitions

  • the present invention relates to a high strength steel sheet having excellent tensile strength, elongation, stretch flangeability, and bendability, and being excellent in terms of material quality stability.
  • the temperature history may vary in the width direction and the longitudinal direction due to unevenness in a method for applying cooling water in the width direction, unevenness in the cooling rate depending on positions in a wound coil, or the like. Therefore, in the manufacture of high strength steel sheets, there is a need for a technique for stabilizing material qualities such as the use of a manufacturing method in which the temperature history is reduced as much as possible or a material design that decreases the influence of the temperature history as much as possible.
  • TRIP steel As a technique for improving the ductility of high strength steel sheets, there is TRIP steel in which the transformation induced plasticity (TRIP) effect is used by leaving austenite in the steel structure (for example, refer to Patent Document 1). TRIP steel has higher ductility than DP steel.
  • TRIP steel has higher ductility than DP steel.
  • Non-Patent Document 1 discloses that the use of a double annealing method, in which a steel sheet is annealed twice, improves the elongation and hole expansibility of steel sheets.
  • Patent Document 2 reports a technique in which, in a hot-rolled steel sheet having a tensile strength of 780 MPa or more, the amount of Ti and V added is controlled to be within a certain range, whereby fine carbides are uniformly precipitated during hot rolling and coiling and, consequently, the material quality of the hot-rolled steel sheet is stabilized.
  • the present inventors carried out searches to obtain a steel sheet satisfying both elongation and hole expansibility.
  • annealing is carried out twice, there is a problem in that the fuel cost or the like increases compared with a manufacturing method in which annealing is carried out once. Therefore, in order to build the same planar structure (that is, a structure in which the aspect ratio of austenite is large) without carrying out annealing twice, the present inventors made an attempt of a manufacturing method in which a TRIP steel sheet is built by annealing a hot-rolled steel sheet. Specifically, the present inventors studied a manufacturing method in which a hot-rolled steel sheet is coiled at low temperatures of 450° C.
  • Coiling at low temperatures is capable of turning the structure of the hot-rolled steel sheet into a structure in which a low temperature transformation structure is the main component.
  • the present inventors considered that a planar structure can be obtained by single annealing by annealing a hot-rolled steel sheet having a structure in which a low temperature transformation structure is the main component.
  • An object of the present invention is to provide a high strength hot-rolled steel sheet having excellent tensile strength, elongation, stretch flangeability, and bendability and being excellent in terms of material quality stability.
  • the material quality stability means that the variation in tensile strength and total elongation is small in each portion in a steel sheet.
  • a high strength steel sheet contains, as a chemical composition, by mass %, C: 0.030% to 0.280%, Si: 0.50% to 2.50%, Mn: 1.00% to 4.00%, sol. Al: 0.001% to 2.000%, P: 0.100% or less, S: 0.0200% or less, N: 0.01000% or less, O: 0.0100% or less, B: 0% to 0.010%, Ti: 0% to 0.20%, Nb: 0% to 0.20%, V: 0% to 1.000%, Cr: 0% to 1.000%, Mo: 0% to 1.000%, Cu: 0% to 1.000%, Co: 0% to 1.000%, W: 0% to 1.000%, Ni: 0% to 1.000%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, REM: 0% to 0.0100%, Zr: 0% to 0.0100%, and a remainder including Fe and impurities, a metallographic structure
  • a standard deviation of surface roughness Ra may be 0.5 ⁇ m or less.
  • the high strength steel sheet according to (1) or (2) may contain, as the chemical composition, by mass %, at least one from the group consisting of B: 0.001% to 0.010%, Ti: 0.01 to 0.20%, Nb: 0.01% to 0.20%, V: 0.005% to 1.000%, Cr: 0.005% to 1.000%, Mo: 0.005% to 1.000%, Cu: 0.005% to 1.000%, Co: 0.005% to 1.000%, W: 0.005% to 1.000%, Ni: 0.005% to 1.000%, Ca: 0.0003% to 0.0100%, Mg: 0.0003% to 0.0100%, REM: 0.0003% to 0.0100%, and Zr: 0.0003% to 0.0100%.
  • FIG. 1 is a conceptual diagram showing an observed section for evaluating a metallographic structure.
  • FIG. 2 is a conceptual diagram showing an observed section for evaluating residual austenite.
  • FIG. 3 is a conceptual diagram showing an observed section for evaluating the standard deviation of the area ratios of ferrite.
  • the present inventors repeated intensive studies regarding causes for impairing material quality stability in steel sheets that were annealed once.
  • the present inventors found that a variation in the surface properties of hot-rolled steel sheets before annealing affects the material quality stability of the steel sheets after annealing.
  • a variation in the surface properties (surface roughness) of hot-rolled steel sheets tends to be larger than that of cold-steel sheets.
  • surface roughness is uneven, in a process of heating for annealing, the unevenness in the surface roughness causes an unevenness in emissivity, and a resulting temperature variation is brought to steel sheets.
  • a variation in the amount of ferrite in annealed steel sheets increases. It was clarified for the first time by the present inventors' finding that the control of the surface properties of hot-rolled steel sheets contributes to the stabilization of the material quality of hot-rolled annealed sheets.
  • the present inventors also found a hot rolling method effective for suppressing the variation in the surface properties of steel steels (hot-rolled steel sheets) before annealing.
  • the present inventors discovered that a phenomenon in which, during hot rolling, surface layer scale is pressed against a steel sheet with a hot rolling roll significantly characterizes the surface properties of the hot-rolled steel sheet.
  • the control of scale growth during hot rolling is important, and this can be achieved by spraying a water film onto the surfaces of the steel sheets under specific conditions during rolling.
  • the present invention is not limited only to a constitution disclosed in the present embodiment and can be modified in a variety of manners within the scope of the gist of the present invention.
  • numerical limiting ranges described below includes the lower limits and the upper limits in the ranges. Numerical values expressed with ‘more than’ or ‘less than’ are not included in the numerical ranges. “%” regarding the amount of each element means “mass %”.
  • a rolling direction RD means a direction in which the steel sheet is moved by a rolling roll during rolling.
  • the thickness direction TD is a direction perpendicular to a rolled surface 11 of the steel sheet.
  • the width direction WD is a direction perpendicular to the rolling direction RD and the thickness direction TD.
  • the rolling direction RD can be easily specified based on the stretching direction of the crystal grain of the steel sheet. Therefore, the rolling direction RD can be specified even for a steel sheet cut out from a rolled material steel sheet.
  • the amount of ferrite and the like in the metallographic structure are regulated.
  • the metallographic structure is evaluated in a cross section 12 parallel to the rolling direction RD and perpendicular to the rolled surface 11 (refer to FIG. 1 ).
  • the cross section 12 parallel to the rolling direction RD and perpendicular to the rolled surface 11 is simply referred to as the cross section parallel to the rolling direction RD.
  • a detailed method for evaluating the metallographic structure will be described below.
  • the number proportion of residual austenite having an aspect ratio of 2.0 or more with respect to the number of all residual austenite is previously defined. Residual austenite is evaluated in a cross section parallel to the rolling direction RD and the thickness direction TD (refer to FIG. 2 ). A detailed method for evaluating residual austenite will be described below.
  • the standard deviation of the area ratios of ferrite is regulated.
  • the area ratio of ferrite is measured at a sheet thickness 1 ⁇ 4 position 121 of the cross section 12 parallel to the rolling direction RD and perpendicular to the rolled surface 11 (refer to FIG. 3 ).
  • Ten cross sections 12 parallel to the rolling direction RD and perpendicular to the rolled surface 11 are produced every 50 mm along the width direction WD, and the standard deviation of the 10 area ratios of ferrite measured on these surfaces is regarded as the standard deviation of the area ratios of ferrite according to the present embodiment.
  • the sheet thickness 1 ⁇ 4 position is a position at a depth of 1 ⁇ 4 of the thickness of the steel sheet 1 from the rolled surface 11 of the steel sheet 1 .
  • FIG. 1 and FIG. 2 only the position at a depth of 1 ⁇ 4 of the thickness of the steel sheet 1 from the upper rolled surface 11 of the steel sheet 1 is shown as the sheet thickness 1 ⁇ 4 position.
  • the position at a depth of 1 ⁇ 4 of the thickness of the steel sheet 1 from the lower rolled surface 11 of the steel sheet 1 can also be treated as the sheet thickness 1 ⁇ 4 position.
  • FIG. 3 shows only some of the 10 measurement surfaces. Furthermore, FIG.
  • FIG. 3 merely conceptually shows the measurement points of the area ratios of ferrite, and there is no need to form the number density of measurement surfaces as shown in FIG. 3 as long as a predetermined requirement is satisfied.
  • a detailed method for evaluating the standard deviation of the area ratios of ferrite will be described below.
  • the high strength steel sheet according to the present embodiment contains, as a chemical composition, by mass %,
  • sol. Al 0.001% to 2.000%
  • Nb 0% to 0.20%
  • a metallographic structure includes, by an area ratio,
  • a number proportion of residual austenite having an aspect ratio of 2.0 or more with respect to all residual austenite is 50% or more
  • a standard deviation of area ratios of ferrite measured at 10 points every 50 mm in a width direction is less than 10%
  • a tensile strength is 780 MPa or more.
  • the high strength steel sheet according to the present embodiment contains, as a chemical composition, basic elements and an optional element as necessary, and the remainder includes Fe and impurities.
  • the C is an important element for ensuring the strength of the steel sheet.
  • the C content is set to 0.030% or more, preferably 0.050% or more, 0.100% or more, 0.120% or more, or 0.140% or more.
  • the upper limit is set to 0.280%.
  • the C content is preferably 0.260% or less or 0.250% or less and more preferably 0.200% or less, 0.180% or less, or 0.160% or less.
  • Si is an important element for suppressing the precipitation of an iron-based carbide and stabilizing residual ⁇ .
  • the Si content is set to 0.50% or more.
  • the Si content is preferably 0.80% or more, 1.00% or more, or 1.20% or more.
  • the Si content when the Si content is more than 2.50%, since the deterioration of the surface properties is caused, the Si content is set to 2.50% or less.
  • the Si content is preferably 2.00% or less, more preferably 1.80% or less, 1.50% or less, or 1.30% or less.
  • Mn is an effective element for increasing the mechanical strength of the steel sheet.
  • the Mn content is set to 1.00% or more.
  • the Mn content is preferably 1.50% or more and more preferably 1.80% or more, 2.00% or more, or 2.20% or more.
  • the Mn content is set to 4.00% or less, preferably 3.00% or less, more preferably 2.80% or less, 2.60% or less, or 2.50% or less.
  • Al is an element having an action of deoxidizing steel to make the steel sheet sound.
  • the sol. Al content is set to 0.001% or more.
  • 0.010% or more of sol. Al is desirably added.
  • the sol. Al content is more desirably 0.020% or more, 0.030% or more, or 0.050% or more.
  • the sol. Al content is set to 2.000% or less and is preferably 1.500% or less, more preferably 1.000% or less, or 0.700% or less and most preferably 0.090% or less, 0.080% or less, or 0.070% or less.
  • Sol. Al means acid-soluble Al that does not turn into an oxide such as Al 2 O 3 and is soluble in acids.
  • the high strength steel sheet according to the present embodiment contains, as the chemical composition, impurities.
  • impurities refer to, for example, elements that are contained by accident from ore or scrap that is a raw material or from the manufacturing environments or the like at the time of industrially manufacturing steel.
  • the impurities mean, for example, elements such as P, S, and N. These impurities are preferably limited as described below in order to make the effect of the present embodiment sufficiently exhibited.
  • the amount of the impurities is preferably small, it is not necessary to limit the lower limit, and the lower limit of impurities may be 0%.
  • P is ordinarily an impurity that is contained in steel, but has an action of increasing the tensile strength, and thus P may be positively contained.
  • the P content is limited to 0.100% or less.
  • the P content is preferably limited to 0.080% or less, 0.070% or less, or 0.050% or less. In order to more reliably obtain the effect of the above-described action, the P content may be set to 0.001% or more, 0.002% or more, or 0.005% or more.
  • the S content is an impurity that is contained in steel, and the S content is preferably as low as possible from the viewpoint of the weldability.
  • the S content is more than 0.0200%, the weldability significantly deteriorates, the amount of MnS precipitated increases, and the low temperature toughness deteriorates. Therefore, the S content is limited to 0.0200% or less.
  • the S content is preferably 0.0100% or less and more preferably limited to 0.0080% or less, 0.0070% or less, or 0.0050% or less. From the viewpoint of the desulfurization cost, the S content may be set to 0.0010% or more, 0.0015% or more, or 0.0020% or more.
  • N is an impurity that is contained in steel, and the N content is preferably as low as possible from the viewpoint of the weldability.
  • the N content is limited to 0.01000% or less and may be preferably 0.00900% or less, 0.00700% or less, or 0.00500% or less.
  • the lower limit of the N content is not particularly limited, but the N content may be set to, for example, 0.00005% or more, 0.00010% or more, or 0.00020% or more.
  • the O content is an impurity that is contained in steel, and the O content is preferably as low as possible from the viewpoint of the weldability.
  • the O content is limited to 0.0100% or less and is preferably 0.0090% or less, 0.0070% or less, or 0.0050% or less.
  • the lower limit of the O content is not particularly limited, but the O content may be set to, for example, 0.0005% or more, 0.0008% or more, or 0.0010% or more.
  • the high strength steel sheet according to the present embodiment may contain an optional element in addition to the basic elements and the impurities described above.
  • an optional element instead of some of Fe that is the remainder described above, B, Ti, Nb, V, Cr, Mo, Cu, Co, W, Ni, Ca, Mg, REM, and Zr may be contained as the optional elements.
  • These optional elements may be contained according to the purpose. Therefore, it is not necessary to limit the lower limits of these optional elements, and the lower limits may be 0%. In addition, even when these optional elements are contained as impurities, the above-described effects are not impaired.
  • B, Ti, Nb, V, Cr, Mo, Cu, Co, W, and Ni are all effective elements for stably ensuring the strength. Therefore, these elements may be contained. However, even when more than 0.010% of B, more than 0.20% of each of Ti and Nb, and more than 1.000% of each of V, Cr, Mo, Cu, Co, W, and Ni are contained, the effect of the above-described action is likely to be saturated, and there are cases where containing such elements becomes economically disadvantageous.
  • the B content is set to 0.010% or less, the amount of each of Ti and Nb is set to 0.20% or less, and the amount of each of V, Cr, Mo, Cu, Co, W, and Ni is set to 1.0% or less or 1.000% or less.
  • the B content may be set to 0.008% or less, 0.007% or less, or 0.005% or less.
  • the upper limit of the amount of each of Ti and Nb may be set to 0.18%, 0.15%, or 0.10%.
  • the upper limit of the amount of each of V, Cr, Mo, Cu, Co, W, and Ni may be set to 0.500% or less, 0.300% or less, or 0.100% or less.
  • Nb 0.01% or more, 0.02% or more, or 0.05% or more
  • V 0.005% or more, 0.008% or more, or 0.010% or more
  • Mo 0.005% or more, 0.008% or more, or 0.010% or more
  • Cu 0.005% or more, 0.008% or more, or 0.010% or more
  • Co 0.005% or more, 0.008% or more, or 0.010% or more
  • W 0.005% or more, 0.008% or more, or 0.010% or more
  • Ni 0.005% or more, 0.008% or more, or 0.010% or more
  • Ca, Mg, REM, and Zr are all elements that contribute to the control of an inclusion, particularly, the fine dispersion of an inclusion and have an action of enhancing toughness. Therefore, one or more of these elements may be contained. However, when the content is more than 0.0100% for any of the elements, there are cases where the deterioration of surface properties becomes apparent. Therefore, the amount of each of Ca, Mg, REM, and Zr is preferably set to 0.01% or less or 0.0100% or less. The upper limit of the amount of each of Ca, Mg, REM, and Zr may be set to 0.0080%, 0.0050%, or 0.0030%. In order to more reliably obtain the effect of the above-described action, the amount of at least one of these elements is preferably set to 0.0003% or more, 0.0005% or more, or 0.0010% or more.
  • REM refers to a total of 17 elements of Sc, Y and lanthanoids and is at least one of them.
  • the REM content means the total amount of at least one of these elements.
  • lanthanoids are added in a mischmetal form.
  • the high strength steel sheet according to the present embodiment preferably contains, as the chemical composition, by mass %, at least one of Ca: 0.0003% or more and 0.0100% or less, Mg: 0.0003% or more and 0.0100% or less, REM: 0.0003% or more and 0.0100% or less, and Zr: 0.0003% or more and 0.0100% or less.
  • the above-described steel composition may be measured by an ordinary analysis method of steel.
  • the steel composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES).
  • ICP-AES inductively coupled plasma-atomic emission spectrometry
  • C and S may be measured using an infrared absorption method after combustion
  • N may be measured using an inert gas melting-thermal conductivity method
  • O may be measured using an inert gas fusion-nondispersive infrared absorption method.
  • the metallographic structure includes, by the area ratio, ferrite: 20% to 70%, residual austenite: 5% to 40%, fresh martensite: 0% to 30%, tempered martensite and bainite: 20% to 75% in total, and pearlite and cementite: 0% to 10% in total.
  • Ferrite is a structure that is relatively soft and contributes to forming. Having ferrite improves elongation, hole expansibility, and bendability. In order to obtain this effect, 20% or more of ferrite needs to be included. Therefore, the area ratio of ferrite in the metallographic structure is set to 20% or more. The area ratio of ferrite may be set to 25% or more, 30% or more, or 35% or more.
  • the area ratio of ferrite in the metallographic structure is set to 70% or less.
  • the area ratio of ferrite may be set to 65% or less, 60% or less, or 50% or less.
  • Residual austenite is a structure that contributes to elongation. In order to obtain this effect, 5% or more of residual austenite needs to be included. Therefore, the area ratio of residual austenite in the metallographic structure is set to 5% or more and is preferably 8% or more, 10% or more, or 15% or more.
  • the upper limit of the area ratio of residual austenite in the metallographic structure is 40%.
  • the area ratio of residual austenite may be set to 35% or less, 30% or less, or 25% or less.
  • Fresh martensite is a structure that impairs formability instead of contributing to strength. Therefore, fresh martensite may not be included, and the lower limit thereof is set to 0%.
  • the area ratio of fresh martensite in the metallographic structure is set to 30% or less.
  • the area ratio of fresh martensite is preferably 20% or less and more preferably 15% or less or 10% or less.
  • Tempered martensite and bainite are structures that contribute to strength. In order to obtain a tensile strength of 780 MPa or more, a total of 20% or more of tempered martensite and bainite need to be included. Therefore, in the metallographic structure of the high strength steel sheet according to the present embodiment, the total area ratio of tempered martensite and bainite is set to 20% or more and is preferably 30% or more, 40% or more, or 50% or more.
  • the metallographic structure of the steel sheet according to the present embodiment includes 20% or more of ferrite and 5% or more of residual austenite, and all of the remainder may be tempered martensite and bainite.
  • the total area ratio of tempered martensite and bainite can be set to a maximum of 75%.
  • the total area ratio of tempered martensite and bainite may be 70% or less, 60% or less, or 55% or less.
  • Pearlite and cementite are structures that impair formability.
  • the total area ratio of pearlite and cementite is set to 10% or less in total.
  • the total area ratio of pearlite and cementite may be set to 8% or less, 5% or less, or 3% or less. Since pearlite and cementite are not required to solve the problem of the present invention, the lower limit of the total area ratio thereof is 0%. However, the total area ratio of pearlite and cementite may be 0.5% or more, 1% or more, or 2% or more.
  • a cross section parallel to the rolling direction (that is, a cross section parallel to the rolling direction and perpendicular to the surface) is corroded using a Nital reagent and a reagent disclosed in Japanese Unexamined Patent Application, First Publication No. S59-219473.
  • a solution prepared by dissolving 1 to 5 g of picric acid in 100 ml of ethanol is used as a solution A
  • a solution prepared by dissolving 1 to 25 g of sodium thiosulfate and 1 to 5 g of citric acid in 100 ml of water is used as a solution B
  • the solution A and the solution B are mixed at a proportion of 1:1 to prepare a liquid mixture
  • nitric acid is further added and mixed at a proportion of 1.5% to 4% with respect to the total amount of this liquid mixture, thereby preparing a pretreatment liquid.
  • the above-described pretreatment liquid is added to and mixed with a 2% Nital liquid at a proportion of 10% with respect to the total amount of the 2% Nital liquid, thereby preparing a post-treatment liquid.
  • the cross section parallel to the rolling direction (that is, the cross section parallel to the rolling direction and perpendicular to the surface) is immersed in the pretreatment solution for 3 to 15 seconds, washed with an alcohol, dried, then, immersed in the post-treatment solution for 3 to 20 seconds, then, washed with water, and dried, thereby corroding the cross section.
  • the width direction central position is a position that is substantially equidistant from both ends of the steel sheet 1 in the width direction WD.
  • the total area fraction of “bainite and tempered martensite” is obtained by measuring the area fractions of “upper bainite” and “lower bainite or tempered martensite”.
  • Upper bainite is an aggregate of laths and a structure containing a carbide between the laths.
  • Lower bainite is a structure containing iron-based carbides having major axes of 5 nm or more and extending in the same direction.
  • Tempered martensite is an aggregate of lath-shaped crystal grains and a structure containing iron-based carbides having major axes of 5 nm or more and extending in different directions.
  • Ferrite is a region in which the brightness is low and no sub-microstructure is observable.
  • a region in which the brightness is high and a sub-microstructure is not exposed by etching is determined as fresh martensite or residual austenite. Therefore, the area fraction of fresh martensite can be obtained as a difference between the area fraction of a non-corroded region that is observed with FE-SEM and the area fraction of residual austenite measured with X-rays described below.
  • Pearlite means a region in which planar cementite and planar ferrite are alternately arranged. In the observation with an FE-SEM, it is possible to clearly distinguish pearlite and the above-described structures (ferrite, bainitic ferrite, bainite, and tempered martensite).
  • the area fraction of residual austenite As a method for measuring the area fraction of residual austenite, there are methods in which X-ray diffraction, electron back scattering diffraction pattern (EBSP) analysis, or magnetic measurement is used, and there are cases where measurement values vary with measurement methods.
  • the area fraction of residual austenite is measured by X-ray diffraction.
  • the integrated intensities of a total of six peaks of ⁇ (110), ⁇ (200), ⁇ (211), ⁇ (111), ⁇ (200), and ⁇ (220) are obtained using Co-K ⁇ rays, and then the area fraction of residual austenite is obtained by calculation using the strength averaging method.
  • the number proportion of residual austenite having an aspect ratio of 2.0 or more needs to be 50% or more with respect to all residual austenite. Therefore, the number proportion is set to 50% or more and preferably set to 70% or more. When the number proportion is less than 50%, it becomes difficult to satisfy all of excellent elongation, excellent hole expansibility, and excellent bendability, which is not preferable.
  • the aspect ratios and major axes of residual austenite grains that are included in the steel structure inside the steel sheet are evaluated by observing the crystal grains using an FE-SEM and carrying out high-resolution crystal orientation analysis by the electron back scattering diffraction method (EBSD method).
  • EBSD method electron back scattering diffraction method
  • a cross section parallel to the rolling direction and the thickness direction of the steel sheet is regarded as an observed section 13 and collected as a sample, and the observed section is finished to a mirror surface by polishing.
  • the crystal structure is analyzed by the EBSD method for an area of a total of 2.0 ⁇ 10 ⁇ 9 m 2 or more (possibly in any of a plurality of visual fields or the same visual field).
  • austenite having a major axis length of 0.1 ⁇ m or more is extracted from the crystal orientation of the residual austenite grains measured by the above-described method, and a crystal orientation map is drawn.
  • a boundary from which a crystal orientation difference of 100 or more is generated is regarded as a crystal grain boundary between the residual austenite grains.
  • the aspect ratio is defined as a value obtained by dividing the major axis length of a residual austenite grain by the minor axis length.
  • the major axis is defined as the major axis length of a residual austenite grain.
  • “OIM Analysis 6.0” manufactured by TSL Solutions is used for the analysis of data obtained by the EBSD method.
  • the distance between evaluation points (step) is set to 0.01 to 0.20 m. From the observation results, a region that is determined as FCC iron is defined as residual austenite. From this result, the number proportion of residual austenite having an aspect ratio of 2.0 or more in all residual austenite in the range of a 1 ⁇ 8 thickness to a 3 ⁇ 8 thickness is obtained.
  • ferrite is important in order to secure elongation and hole expansibility.
  • strength, elongation, or hole expansibility changes depending on the microstructural fraction of ferrite. Therefore, it is important for the microstructure fraction of ferrite to be uniformly distributed in the hot rolling width direction in terms of obtaining material quality stability.
  • the area ratio of ferrite at the sheet thickness 1 ⁇ 4 position 121 of a cross section parallel to the rolling direction is measured at 10 points every 50 mm along the width direction (that is, a direction at a right angle with respect to the rolling direction RD) WD as shown in FIG. 3
  • the standard deviation of the area ratios of ferrite is 10% or more, a variation in mechanical properties is caused, and material quality stability cannot be obtained. Therefore, the above-described standard deviation of the area ratio of ferrite is set to less than 10% and is preferably 8% or less, less than 5%, or 4% or less.
  • the measurement points for the standard deviation of the area ratios of ferrite may be disposed on one straight line along the width direction.
  • the measurement points for the standard deviation of the area ratios of ferrite may be disposed on two or more straight lines along the width direction.
  • the measurement points can be disposed as described above.
  • the steel sheet according to the present embodiment is not particularly limited as long as the chemical composition, the metallographic structure, and the tensile strength described below are within predetermined ranges.
  • the standard deviation of the surface roughness Ra may be set to 0.5 ⁇ m or less.
  • the standard deviation is preferably set to 0.5 ⁇ m or less.
  • processing for changing the surface roughness such as hairline processing may be carried out on this high strength steel sheet.
  • setting the standard deviation of the surface roughness Ra within the above-described range is not essential.
  • a roughness curve that is 5 mm long in the width direction is acquired at each measurement position using a contact type roughness meter (SURFTEST SJ-500 manufactured by Mitutoyo Corporation), and the arithmetic average roughness Ra is obtained by the method described in JIS B0601: 2001.
  • the standard deviation of the surface roughness Ra is obtained using the value of the arithmetic average roughness Ra at each measurement position obtained as described above.
  • the “surface roughness Ra of the steel sheet” means the surface roughness that is measured after removing the surface treatment membrane from the steel sheet. That is, the surface roughness Ra of the steel sheet is the surface roughness of the base metal.
  • the method for removing the surface treatment membrane can be appropriately selected according to the type of the surface treatment membrane to an extent that the surface roughness of the base metal is not affected. For example, in a case where the surface treatment membrane is a zinc plating, it is necessary to dissolve the galvanized layer using dilute hydrochloric acid to which an inhibitor is added. This makes it possible to exfoliate only the galvanized layer from the steel sheet.
  • the inhibitor is an additive that is used to suppress a change in roughness attributed to the prevention of the excessive dissolution of the base metal.
  • a substance obtained by adding a corrosion inhibitor for hydrochloric acid pickling “IBIT No. 700BK” manufactured by Asahi Chemical Co., Ltd. to hydrochloric acid diluted 10 to 100 times such that the concentration reaches 0.6 g/L can be used as exfoliation means for the galvanized layer.
  • the high strength steel sheet according to the present embodiment has, as a sufficient strength that contributes to the weight reduction of vehicles, a tensile strength (TS) of 780 MPa or more.
  • the tensile strength of the steel sheet may be 800 MPa or more, 900 MPa or more, or 1000 MPa or more. Meanwhile, it is assumed that it is difficult to obtain a tensile strength of more than 1470 MPa with the configuration of the present embodiment. Therefore, it is not necessary to particularly specify the upper limit of the tensile strength, but the substantial upper limit of the tensile strength in the present embodiment can be set to 1470 MPa.
  • the tensile strength of the steel sheet may be set to 1400 MPa or less, 1300 MPa or less, or 1200 MPa or less.
  • a tensile test may be carried out in the following order in accordance with JIS Z 2241 (2011).
  • JIS No. 5 test pieces are collected from 10 positions in the high strength steel sheet at intervals of 50 mm in the width direction.
  • the width direction of the steel sheet and the longitudinal direction of the test pieces are made to coincide with each other.
  • individual test pieces are collected at positions shifted in the rolling direction of the steel sheet such that the collection positions of the individual test pieces do not interfere with each other.
  • Tensile tests are carried out on these test pieces in accordance with the regulations of JIS Z 2241 (2011), tensile strengths TS (MPa) are obtained, and the average value thereof is calculated. This average value is regarded as the tensile strength of the high strength steel sheet.
  • the high strength steel sheet according to the present embodiment may have the following characteristics in terms of elongation and hole expansibility as an index of formability. These mechanical properties are obtained due to a variety of properties of the high strength steel sheet according to the present embodiment described above.
  • the high strength steel sheet according to the present embodiment may have a total elongation of 14% or more in the tensile test as an index of elongation. Meanwhile, it is difficult to obtain a total elongation of more than 35% with the configuration of the present embodiment. Therefore, the substantial upper limit of the total elongation may be set to 35%.
  • the high strength steel sheet according to the present embodiment may have a hole expansion rate of 25% or more as an index of hole expansibility. Meanwhile, it is difficult to obtain a hole expansion rate of more than 80% with the configuration of the present embodiment. Therefore, the substantial upper limit of the hole expansion rate may be set to 80%.
  • the hole expansion rate can be evaluated by a hole expansion test in accordance with the test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996.
  • the high strength steel sheet according to the present embodiment may have R/t of 2.0 or less. Meanwhile, it is difficult to set the index R/t of the bendability to 0.1 or less with the configuration of the present embodiment. Therefore, the substantial lower limit of the index R/t of the bendability may be set to 0.1.
  • the limit bend radius R is obtained by repeatedly carrying out bending tests to which a variety of bend radii are applied. In the bending test, bending is carried out in accordance with JIS Z 2248 (V block 90° bending test). The bend radius (to be exact, the inner radius of bending) changes at pitches of 0.5 mm. As the bend radius in the bending test decreases, cracks and other defects are more likely to be generated in the steel sheet. The minimum bending at which cracks and other defects are not generated in the steel sheet, which has been obtained in this test, is regarded as the limit bend radius R. In addition, a value obtained by dividing this limit bend radius R by the thickness t of the steel sheet is used as the index R/t for evaluating the bendability.
  • the standard deviation of TS may be 50 MPa or less
  • the standard deviation of EL may be 1% or less.
  • the method for obtaining the TS standard deviation and the EL standard deviation is the same as the above-described tensile test method for obtaining the average value of the tensile strengths.
  • the TS standard deviation and the EL standard deviation can be obtained by obtaining the standard deviation of the results of 10 tensile tests by the above-described method.
  • the standard deviation of R/t (limit bend radius R (mm), sheet thickness t (mm)) measured at 10 points every 50 mm along the width direction may be set to 0.2 or less.
  • the method for manufacturing the high strength steel sheet according to the present embodiment is not particularly limited. Any steel sheet that satisfies the above-described requirements is regarded as the steel sheet according to the present embodiment regardless of manufacturing methods therefor.
  • a manufacturing step preceding hot rolling is not particularly limited. That is, subsequent to melting with a blast furnace, an electric furnace, or the like, a variety of secondary smelting is carried out, and then casting may be carried out by a method such as ordinary continuous casting, casting by an ingot method, or thin slab casting.
  • a cast slab may be hot-rolled after being once cooled to a low temperature and then heated again or the cast slab may be hot-rolled as it is after being cast without being cooled to a low temperature.
  • Scrap may be used as a raw material.
  • a heating step is carried out on the cast slab.
  • the slab is preferably heated to a temperature of 1100° C. or higher and 1300° C. or lower. Since there is a possibility that a coarse precipitate precipitated in the slab (an iron-based carbide, a carbonitride of an alloying element, or the like) impairs material quality stability, the slab is preferably heated to 1100° C. or higher in order to dissolve the coarse precipitate. On the other hand, from the viewpoint of preventing a scale loss, the slab heating temperature is preferably 1300° C. or lower.
  • conditions therefor are not particularly limited as long as the slab is made into a desired dimension and a desired shape.
  • the thickness of the rough rolled sheet affects the amount of the temperature lowered from the tip to the tail of the hot-rolled steel sheet during the beginning of the rolling to the completion of the rolling in a finish rolling step and is thus preferably determined in consideration of such a fact.
  • Finish rolling is carried out on the rough rolled sheet.
  • multi-stage finish rolling is carried out.
  • finish rolling is carried out within a temperature range of 850° C. to 1200° C. under conditions that satisfy the following formula (1).
  • Si* is set to 140 ⁇ Si
  • Si* is set to 80.
  • Si represents the Si content (mass %) of the steel sheet.
  • K′ in the formula (1) is represented by the following formula (2).
  • D is the amount sprayed per hour (m 3 /min) of hydraulic descaling before the start of the finish rolling
  • DT is the steel sheet temperature (° C.) at the time of the hydraulic descaling before the start of the finish rolling
  • FT n is the steel sheet temperature (° C.) in the n th stage of the finish rolling
  • S n is the amount sprayed per hour (m 3 /min) at the time of spraying water to the steel sheet on spray between the n ⁇ 1 th stage and the n th stage of the finish rolling.
  • Si* is a parameter relating to a steel sheet component that indicates the easiness in the generation of unevenness attributed to scale.
  • scale that is generated on the surface layer during hot rolling changes from wustite (FeO) to fayalite (Fe 2 SiO 4 ), in which the wustite is relatively easily descaled and is unlikely to produce unevenness on the steel sheet and the fayalite grows so as to lay down roots in the steel sheet and is likely to produce unevenness. Therefore, as the amount of Si increases, that is, as the Si* increases, unevenness on the surface layer is more likely to be formed.
  • Si to facilitate the formation of unevenness on the surface layer becomes significantly effective particularly when 0.35 mass % or more of Si is added. Therefore, when 0.35 mass % or more of Si is added, Si* acts as a function of Si; however, when 0.35 mass % or less of Si is added, Si acts as a constant.
  • K′ is a parameter of a manufacturing condition that indicates the difficulty in forming unevenness.
  • the first item of the formula (2) indicates that, when hydraulic descaling is carried out before the start of the finish rolling in order to suppress the formation of unevenness, as the amount sprayed per hour of the hydraulic descaling increases and as the steel sheet temperature increases, the hydraulic descaling becomes more effective from the viewpoint of descaling.
  • the value of descaling that is closest to the finish rolling is used.
  • the second item of the formula (2) is an item that indicates the effect of descaling, during finish rolling, scale that has not been completely exfoliated by descaling before finishing or scale that is formed again during the finish rolling and indicates that spraying a large amount of water onto spray to the steel sheet at high temperatures facilitates descaling.
  • K′/Si* is set to 2.5 or more, preferably 3.0 or more, and more preferably 3.5 or more.
  • K′/Si* is preferably set to 3.0 or more (K′/Si* ⁇ 3.0).
  • cooling is carried out at an average cooling rate of 50° C./s or faster, and coiling is carried out at a coiling temperature of 450° C. or lower.
  • the average cooling rate is a value obtained by dividing the difference in temperature between the start of the cooling and before the coiling by the time therebetween.
  • the coiling temperature is set to 450° C. or lower, preferably set to 400° C. or lower, and more preferably set to 200° C. or lower.
  • setting the coiling temperature to 450° C. or lower also has an effect of suppressing the formation of an internal oxide on the surface of the steel sheet after the coiling and an increase in the roughness of the surface layer.
  • Pickling is carried out on the high strength steel sheet manufactured in this manner for the purpose of removing an oxide on the surface of the steel sheet.
  • the pickling may be carried out, for example, with hydrochloric acid having a concentration of 3% to 10% at a temperature of 85° C. to 98° C. for 20 seconds to 100 seconds.
  • soft reduction with a rolling reduction of 20% or smaller may be carried out on the manufactured hot-rolled steel sheet for the purpose of shape correction.
  • the rolling reduction of the soft reduction becomes larger than 20%, recrystallization occurs in an annealing process, and it becomes impossible to obtain the effect of the morphology control that can be obtained from the low temperature transformation structure during annealing, and thus the rolling reduction is set to 20% or smaller even in a case where soft reduction is carried out.
  • the soft reduction may be carried out before the pickling or carried out after the pickling.
  • the soft reduction being carried out after the pickling has an effect of further reducing the roughness of the surface layer.
  • An annealing treatment is carried out on the obtained steel sheet.
  • the heating temperature is set to A c1 point to A c3 point—10° C. that is calculated by the following formula.
  • ferrite-austenite transformation occurs from a carbide formed between the laths of the low temperature transformation structure, and planar austenite is generated.
  • a region that has not undergone austenitic transformation can also be considered as a low temperature transformation structure that has been tempered at high temperatures (tempered martensite or tempered bainite).
  • tempered martensite or tempered bainite since the dislocation density is significantly reduced by tempering, and the sub-microstructure is also not clear, the region is a region that is evaluated as ferrite in the structural observation after annealing. Therefore, here, the region will be referred to as ferrite.
  • a region that is evaluated as tempered martensite or bainite in the structural observation after annealing mainly refers to a region generated by the bainitic transformation or martensitic transformation of austenite formed by heating during the retention of the austenite at 150° C. to 550° C., which will be described below.
  • the reason for the heating temperature being set to A c1 point to A c3 point ⁇ 10° C. is that an appropriate ferrite-austenite transformation fraction is set in order to set the area ratio of ferrite to 20% to 70%.
  • the heating time is set to 10 seconds to 1000 seconds.
  • cementite remains undissolved in steel and there is a concern that the properties of the steel sheet may deteriorate.
  • the retention time is longer than 1000 seconds, this effect is saturated, the productivity deteriorates, and thus the upper limit of the retention time is set to 1000 seconds.
  • the steel sheet is retained at 150° C. to 550° C. for 10 seconds to 1000 seconds.
  • austenite In this temperature range, some of austenite is caused to undergo bainitic transformation or martensitic transformation, and solid solution carbon is emitted to austenite along with the bainitic transformation or solid solution carbon is emitted to austenite along with the tempering of martensite, thereby stabilizing austenite.
  • the steel sheet When the steel sheet is retained at 150° C. or lower, the majority of austenite undergoes martensitic transformation, and a sufficient amount of residual austenite cannot be obtained.
  • the steel sheet is retained at 550° C. or higher, pearlitic transformation occurs, and it is not possible to sufficiently stabilize residual austenite.
  • the retention time is shorter than 10 seconds, carbon does not sufficiently diffuses, and it is not possible to sufficiently stabilize residual austenite.
  • the retention time When the retention time is longer than 1000 seconds, the effect of stabilizing residual austenite is saturated, and the productivity deteriorates.
  • the steel sheet While the steel sheet is retained in the temperature range, the steel sheet may be heated or cooled in this temperature range. For example, when the steel sheet is once cooled to a temperature range of 250° C. or lower to transform some of residual austenite into martensite and then reheated to a temperature range of approximately 400° C., martensite acts as a nucleation site of bainitic transformation, and an effect of accelerating bainitic transformation is obtained.
  • hot-dip galvanizing or hot-dip galvannealing may be carried out in this temperature range.
  • plating conditions such as the hot-dip galvanizing bath temperature and the hot-dip galvanizing bath composition in the hot-dip galvanizing step
  • ordinary conditions can be used, and there is no particular limitation.
  • the plating bath temperature may be 420° C. to 500° C.
  • the intrusion sheet temperature of the steel sheet may be 420° C. to 500° C.
  • the immersion time may be five seconds or shorter.
  • the plating bath is preferably a plating bath containing 0.08% to 0.2% of Al and may additionally contain Fe, Si, Mg, Mn, Cr, Ti, Pb, or the like, which is an impurity.
  • the basis weight of hot-dip galvanizing is preferably controlled by a well-known method such as gas wiping.
  • the basis weight may be 5 g/m 2 or more per surface, and is preferably set to 25 to 75 g/m 2 and more preferably set to 20 to 120 g/m 2 .
  • the alloying treatment may be carried out according to a normal method, but the alloying treatment temperature is preferably set to 460° C. to 550° C.
  • the alloying treatment temperature is preferably set to 460° C. or higher.
  • the alloying treatment temperature exceeds 550° C., pearlitic transformation occurs, and it is not possible to sufficiently stabilize residual austenite.
  • the alloying treatment is preferably carried out under conditions that the iron concentration in a hot-dip galvanized layer reaches 6.0 mass % or more.
  • an electrogalvanized layer may be formed on the steel sheet manufactured as described above.
  • the electrogalvanized layer can be formed by a well-known conventional method.
  • the high strength steel sheet according to the present embodiment can be manufactured by the above-described manufacturing method.
  • the high strength steel sheet according to the present invention will be described more specifically with reference to examples.
  • the following examples are examples of the high strength steel sheet of the present invention, and the high strength steel sheet of the present invention is not limited to the following aspects.
  • Conditions in examples to be described below are exemplary conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these exemplary conditions.
  • the present invention is capable of adopting a variety of conditions within the scope of the gist of the present invention as long as the object of the present invention is achieved.
  • the steel sheets were in a temperature range of 400° C. to 520° C.
  • the metallographic structures of the obtained high strength steel sheets were observed by the following method.
  • a cross section parallel to the rolling direction and perpendicular to the surface was corroded using a Nital reagent and a reagent disclosed in Japanese Unexamined Patent Application, First Publication No. S59-219473.
  • a solution prepared by dissolving 1 to 5 g of picric acid in 100 ml of ethanol was used as a solution A
  • a solution prepared by dissolving 1 to 25 g of sodium thiosulfate and 1 to 5 g of citric acid in 100 ml of water was used as a solution B
  • the solution A and the solution B were mixed at a proportion of 1:1 to prepare a liquid mixture
  • nitric acid was further added and mixed at a proportion of 1.5% to 4% with respect to the total amount of this liquid mixture, thereby preparing a pretreatment liquid.
  • the above-described pretreatment liquid was added to and mixed with a 2% Nital liquid at a proportion of 10% with respect to the total amount of the 2% Nital liquid, thereby preparing a post-treatment liquid.
  • the cross section parallel to the rolling direction and perpendicular to the surface was immersed in the pretreatment solution for 3 to 15 seconds, washed with an alcohol, dried, then, immersed in the post-treatment solution for 3 to 20 seconds, then, washed with water, and dried, thereby corroding the cross section.
  • the total area fraction of “bainite and tempered martensite” was obtained by measuring the area fractions of “upper bainite” and “lower bainite or tempered martensite”.
  • a region in which the brightness was low and no sub-microstructure was observed was determined as ferrite.
  • a region in which the brightness was high and a sub-microstructure was not exposed by etching was determined as fresh martensite or residual austenite.
  • the area fraction of fresh martensite was obtained as the difference between the area fraction of a non-corroded region that is observed with FE-SEM and the area fraction of residual austenite measured with X-rays.
  • the area fraction of residual austenite was measured by X-ray diffraction.
  • the integrated intensities of a total of six peaks of ⁇ (110), ⁇ (200), ⁇ (211), ⁇ (111), ⁇ (200), and ⁇ (220) were obtained using Co-K ⁇ rays, and the area fraction of residual austenite was obtained by calculation using the strength averaging method.
  • the aspect ratios and major axes of residual austenite grains that were included in the steel structure in the steel sheet were evaluated by observing the crystal grains using an FE-SEM and carrying out high-resolution crystal orientation analysis by the electron back scattering diffraction method (EBSD method).
  • EBSD method electron back scattering diffraction method
  • a cross section parallel to the rolling direction and the thickness direction of the steel sheet was regarded as an observed section and collected as a sample, and the observed section was finished to a mirror surface by polishing.
  • the crystal structure was analyzed by the EBSD method for an area of a total of 2.0 ⁇ 10 ⁇ 9 m 2 or more (possibly in any of a plurality of visual fields or the same visual field).
  • austenite having a major axis length of 0.1 ⁇ m or more was extracted from the crystal orientation of the residual austenite grains measured by the above-described method, and a crystal orientation map was drawn.
  • a boundary from which a crystal orientation difference of 100 or more was generated was regarded as a crystal grain boundary between the residual austenite grains.
  • the aspect ratio was defined as a value obtained by dividing the major axis length of a residual austenite grain by the minor axis length.
  • the major axis was defined as the major axis length of a residual austenite grain.
  • “OIM Analysis 6.0” manufactured by TSL Solutions was used for the analysis of data obtained by the EBSD method.
  • the distance between evaluation points (step) was set to 0.01 to 0.20 km. From the observation results, a region that was determined as FCC iron was defined as residual austenite. From this result, the number proportion of residual austenite having an aspect ratio of 2.0 or more in all residual austenite in the range of a 1 ⁇ 8 thickness to a 3 ⁇ 8 thickness was obtained.
  • the area ratio of ferrite at a sheet thickness 1 ⁇ 4 position of the cross section parallel to the rolling direction and perpendicular to the surface was obtained by the above-described method.
  • the area ratios of ferrite were obtained by the same method at 10 points at intervals of 50 mm in the width direction, and the standard deviation of the area ratios was calculated.
  • the standard deviation of the surface roughness Ra that was measured at 10 positions at intervals of 50 mm in the width direction was obtained in the following order.
  • a roughness curve that was 5 mm long in the width direction was acquired at each measurement position using a contact type roughness meter (SURFTEST SJ-500 manufactured by Mitutoyo Corporation), and the arithmetic average roughness Ra was obtained by the method described in JIS B0601: 2001.
  • the standard deviation of the surface roughness Ra was obtained using the value of the arithmetic average roughness Ra at each measurement position obtained as described above.
  • a tensile test was carried out in accordance with the regulations of JIS Z 2241 (2011) using a JIS No. 5 test piece collected from the high strength steel sheet in a manner that the width direction was along the longitudinal direction, and the tensile strength TS (MPa) and the butt elongation (total elongation) EL (%) were obtained.
  • the samples were collected from 10 positions in the steel sheet at intervals of 50 mm in the width direction.
  • the average value of the tensile strengths of the 10 test pieces was regarded as the tensile strength TS of the steel sheet, and, in a case where TS ⁇ 780 MPa was satisfied, the steel sheet was determined as a high strength hot-rolled steel sheet and evaluated as pass.
  • the hole expansion rate was evaluated by a hole expansion test in accordance with the test method described in the Japan Iron and Steel Federation Standard JFS T 1001-1996.
  • a bending test was carried out in accordance with JIS Z 2248 (V block 90° bending test), and the bend R (mm) was tested at pitches of 0.5 mm.
  • Comparative Example 11 the proportion of residual austenite having an aspect ratio of 2.0 or more was insufficient, and the hole expansibility was impaired. This is assumed to be because the average cooling rate after the finish rolling was insufficient.
  • Comparative Example 14 the proportion of residual austenite having an aspect ratio of 2.0 or more was insufficient, and the hole expansibility was impaired. This is assumed to be because the rolling reduction of the soft reduction that was carried out on the steel sheet before the annealing of the steel sheet was excessive.
  • Comparative Example 16 the amount of residual austenite was insufficient, and the total elongation and the hole expansibility were impaired. This is assumed to be because the retention pattern in the annealing step was inappropriate, that is, the retention temperature was too low.
  • Comparative Example 31 and Comparative Example 32 the amount of Si was insufficient. Therefore, in Comparative Example 31 and Comparative Example 32, the amount of residual austenite was insufficient, and the total elongation and the hole expansibility were impaired.

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