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  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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Definitions

• the present invention relates to a hot-rolled steel sheet and a method of manufacturing the same. Specifically, the present, invention relates to a hot-rolled steel sheet having high strength and excellent ductility, hole expansibility, and toughness, and a method of manufacturing the same.
• a steel sheet having both high strength and excellent formability is strongly desired.
• a steel sheet having excellent ductility and hole expansibility particularly among the formability is desired.
• a steel sheet applied to a vehicle body of a vehicle is also required to have excellent toughness in order to sufficiently absorb impact at the time of a collision.
• Patent Document 1 discloses a high strength hot-rolled steel sheet which has excellent fatigue properties and stretch flangeability and in which when a bainite fraction is 80% or more, an average grain size r (nm) of a precipitation satisfies an expression of (r ⁇ 207/(27.4 ⁇ (V)+23.5 ⁇ (Nb)+31.4 ⁇ (Ti)+17.6 ⁇ (Mo)+25.5 ⁇ (Zr)+23.5 ⁇ (W)), and the average grain size rand a precipitation fraction f satisfies an expression of (r/f ⁇ 12000).
• Patent Document 2 discloses a hot-rolled steel sheet in which a steel structure at a position at a depth of 1 ⁇ 4 of a sheet thickness from a surface of the steel sheet contains, by area %, 60% or more, of bainite, 5% or more and less than 30% of polygonal ferrite, less than 3% of residual austenite, and 10% or less of a remainder excluding the bainite, the residual austenite, and the polygonal ferrite, and a polygonal ferrite area ratio at, a position at a depth of 100 ⁇ m from the surface of the steel sheet and a polygonal ferrite area ratio at a position at a depth of 1 ⁇ 4 of the sheet thickness satisfy an expression of (V ⁇ s>1.5 V ⁇ q, where V ⁇ s is an area ratio (%) of the polygonal ferrite a a position at a depth of 100 ⁇ m from the surface of the steel sheet, and V ⁇ q is an area ratio of the polygonal ferrite at a position of a depth of 1
• Patent Document 1 Japanese Unexamined Patent Application, First Publication No. 2009-84637
• Patent Document 2 Japanese Unexamined Patent Application, First Publication No. 2016-50335
• Patent Documents 1 and 2 toughness is not considered.
• the present inventors have found that it is necessary not only to improve ductility and hole expansibility but also to secure toughness, in order to achieve both weight reduction, of a vehicle body and collision properties.
• the present invention has been made in view of the above problems, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and excellent ductility, hole expansibility, and toughness, and a method of manufacturing the same.
• an object of the present invention is, preferably, to provide a hot-rolled steel sheet having excellent punching properties in addition to the above-mentioned properties and a method of manufacturing the same.
• the gist of the present invention made based on the above findings is as follows.
• a hot-rolled steel sheet according to an aspect of the present invention includes, as a chemical composition, by mass %.
• sol. Al 0.001% to 2.000%
• Nb 0% to 0.200%
• V 0% to 1.00%
• a microstructure contains, by area %
• a total density of a length L 7 of a grain boundary having a crystal orientation difference of 7° and a length L 68 of a grain boundary having a crystal orientation difference of 68° about a ⁇ 110> direction in the bainite is 0.35 to 0.60 ⁇ m/ ⁇ m 2 , and
• a tensile strength is 780 MPa or more.
• the hot-rolled steel sheet according to (1) may include, as a chemical composition, by mass %, one or two or more selected from the group consisting of:
• Nb 0.005% to 0.200%
• V 0.005% to 1.00%
• Ni 0.005% to 1.00%
• a method of manufacturing a hot-rolled steel, sheet according to another aspect of the present invention includes:
• The-hot-rolled steel sheet according to the present embodiment includes, by mass % C: 0.030% to 0.200%, Si: 0.05% to 2.50%, Mn: 1.00% to 4.00%, sol. Al: 0.001% to 2.000%, Ti: 0.030% to 0.200, P: 0.020% or less, S: 0.020% or less, N: 0.010% or less, and a remainder: Fe and impurities.
• mass % C 0.030% to 0.200%
• Si 0.05% to 2.50%
• Mn 1.00% to 4.00%
• Al 0.001% to 2.000%
• Ti: 0.030% to 0.200 P: 0.020% or less
• S 0.020% or less
• N 0.010% or less
• a remainder Fe and impurities.
• C is an element that promotes formation of bainite by improving a strength of a hot-rolled steel sheet and also improving hardenability.
• a C content is set to 0.030% or more.
• the C content is preferably 0.040% or more.
• the C content is set to 0.200% or less.
• the C content is preferably 0.180% or less.
• Si is an element that contributes to solid solution strengthening and is an element that contributes to improving the strength of the hot-rolled steel sheet.
• Si has an action of making steel soundness by deoxidation (suppressing an occurrence of a defect such as blow holes in the steel).
• the Si content is set to 0.05% or more.
• the Si content is preferably 0.50% or more and more preferably 1.00% or more.
• Si is an element that promotes formation of a mixture (MA) of full hard martensite (hereinafter, when simply referred to as martensite, this martensite means fresh martensite) and residual austenite.
• MA full hard martensite
• this martensite means fresh martensite
• residual austenite residual austenite
• Mn dissolves in steel to contribute to an increase in the strength of the hot-rolled steel sheet, promotes the formation of bainite by improving the hardenability and improves the hole, expansibility of the hot-rolled steel sheet.
• a Mn content is set to 1.00% or more.
• the Mn content is preferably 1.30% or more.
• the Mn content is set to 4.00% or less.
• the Mn content is preferably 3.50% or less.
• Al has an action of deoxidizing steel to make the steel soundness.
• a sol. Al content is less than 0.001%, an effect by the action cannot be obtained. Therefore, the sol. Al content is set to 0.001% or more.
• the sol. Al content is preferably 0.010% or more.
• the sol. Al content is set to 2.000% or less.
• the sol. Al content is preferably 1,500% or less and more preferably 1.300% or less.
• the sol. Al in the present embodiment means acid-soluble Al, and refers to solid solution Al present in steel in a solid solution state.
• Ti precipitates as a carbide or a nitride in steel, and has an action of refining the microstructure by an austenite pinning effect and improving the strength of the hot-rolled steel sheet.
• a Ti content is less than 0.030%, an effect by the action cannot be obtained. Therefore, the Ti content is set to 0.030% or more.
• the Ti content is preferably 0.050% or more and more preferably 0.080% or more.
• the Ti content is set to 0.200% or less.
• the Ti content is preferably 0.170% or less, and more preferably 0.150% or less.
• P is an element that dissolves in steel and contributes to an increase of the strength of the hot-rolled steel sheet.
• P is also an element that segregates at a grain boundary, particularly at a prior austenite grain boundary, and promotes a grain boundary fracture, due to a boundary segregation, thereby causing a decrease in the workability of the hot-rolled steel sheet.
• a P content is preferably as low as possible, and containing of P is acceptable up to 0.020%. Therefore, the P content is set to 0.020% or less.
• the P content is preferably 0.015% or less.
• the P content is preferably set to 0%. However, when the P content is reduced to less than 0.0001%, the manufacturing costs increase. Therefore, the P content may be 0.0001% or more.
• S is an element that adversely affects weldability and manufacturability during casting and hot rolling.
• S combines with Mn to form coarse MnS. This MnS deteriorates the bendability and hole expansibility of the hot-rolled steel sheet, and promotes an initiation of a delayed fracture.
• a S content is preferably as low as possible, and containing of S is acceptable up to 0.020%. Therefore, the S content is set to 0.020% or less.
• the S content is preferably 0.015% or less.
• the S content is preferably set to 0%. However, when the S content is reduced to less than 0.0001%, the manufacturing cost increases and it is economically disadvantageous. Therefore, the S content may be set to 0.0001% or more.
• N is an element that forms a coarse nitride in steel. This nitride deteriorates the bendability and the hole expansibility of the hot-rolled steel sheet. Therefore, a N content is set to 0.010% or less. The N content is preferably 0.008% or less.
• the N content When the N content is reduced to less than 0.0001%, a significant increase in manufacturing cost is caused. Therefore, the N content may be set to 0.0001% or more.
• a remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment consists of Fe and impurities.
• the impurities mean those mixed from ore as a raw material, scrap, manufacturing environment, and the like, and/or those allowed within a range that does not adversely affect the hot-rolled steel sheet according to the present embodiment.
• the hot-rolled steel sheet according to the present embodiment may contain the following elements as an optional element in addition to a part of Fe.
• a lower limit of a content thereof is 0%.
• Nb is an element that forms a carbide during hot rolling and contributes to improvement in the strength of hot-rolled steel sheet by precipitation hardening.
• a Nb content is preferably set to 0.005% or more.
• the Nb content is set to 0.200% or less.
• B is an element that segregates into the prior austenite grain boundary, suppresses the formation and growth of ferrite, and contributes to improvement in the strength and hole expansibility of the hot-rolled steel sheet.
• a B content is preferably set to 0.001% or more.
• the B content is set to 0.010% or less.
• V 0% to 1.00%
• V is an element that forms a carbonitride during hot rolling and contributes to improvement in the strength of hot-rolled steel sheet by precipitation hardening.
• a V content is preferably set to 0.005% or more.
• the V content is set to 1.00% or less.
• Mo is an element, that promotes the formation of bainite by improving the hardenability of steel and contributes to the improvement in the strength and the hole expansibility of the hot-rolled steel sheet.
• a Mo content is preferably set to 0.005% or more.
• the Mo content is set to 1.00% or less.
• Cu is an element that has an effect for stably securing the strength of the hot-rolled steel sheet. Therefore, Cu may also be contained. However, even when containing Cu in an amount more than 1.00%, the effect of the action is likely to be saturated and may be economically disadvantageous in some cases. Therefore, the Cu content is set to 1.00% or less.
• the Cu content is preferably 0.80% or less and more preferably 0.50% or less. In order to more reliably obtain the effect by the action, the Cu content is preferably 0.005% or more.
• W is an element that is effective in improving the strength of the hot-rolled steel sheet by solid or precipitation.
• a W content is set to 1.00% or less.
• the W content is preferably 0.80% or less and more preferably 0.50% or less. In order to more reliably obtain the effect by the action, the W content is preferably 0.005% or more.
• Cr is an element that is effective in improving the hardenability and improving the strength of the hot-rolled steel sheet.
• a Cr content is set to 1.00% or less.
• the Cr content is preferably 0.80% or less and more preferably 0.50% or less. In order to more reliably obtain the effect by the action, the Cr content is preferably 0.005% or more.
• Ni is an element that is effective in improving the hardenability and improving the strength of the hot-rolled steel shed.
• a Ni content is set to 1.00% or less.
• the Ni content is preferably 0.80% or less and more preferably 0.50% or less. In order to more reliably obtain the effect by the action, the Ni content is preferably 0.005% or more.
• Co is an element that is effective in improving the strength of the hot-rolled steel sheet by solid solution strengthening.
• a Co content is set to 1.00% or less.
• the Co content is preferably 0.80% or less and more preferably 0.50% or less. In order to more reliably obtain the effect by the action, the Co content is preferably 0.005% or more.
• All of calcium (Ca), magnesium (Mg), a rare earth element (REM), and zirconium (Zr) are elements that contribute to inclusion control, especially fine dispersion of an inclusion, and has an action of enhancing the toughness of the hot-rolled steel sheet. Therefore, these elements may be contained. However, when each of the elements is contained in an amount of more than 0.010%, deterioration of surface properties may become apparent in some cases. Therefore, the amount of each of these elements is set to 0.010% or less. Each amount of these elements is preferably 0.005% or less and, more preferably 0.003% or less. In order to obtain the effect by the action more reliably, each amount of the elements is preferably 0.0005% or more.
• REM in the present embodiment refers to a total of 17 elements including , Y, and lanthanoid, and the REM content refers to a total amount of these elements.
• lanthanoid is industrially added in the form of misch metal.
• the chemical composition of the hot-rolled steel sheet may be measured by a general analytical method.
• the chemical composition may be measured using inductively coupled plasma-atomic emission spectroscopy (ICP-AES) or optical emission spectroscopic (OES).
• ICP-AES inductively coupled plasma-atomic emission spectroscopy
• OES optical emission spectroscopic
• C and S may be measured by using a combustion-infrared absorption method
• N may be measured by using the inert, gas melting-thermal conductivity method.
• a microstructure contains, by area %, bainite: 80.0% or more, ferrite: 10.0% or less, and a remainder in the microstructure: 10.0% or less, and a total density of a length L 7 of a grain boundary having a crystal orientation difference of 7° and a length L 68 of a grain boundary having a crystal orientation difference of 68° about a ⁇ 110> direction in the bainite is 0.35 to 0.60 ⁇ m/ ⁇ m 2 .
• an average grain size of prior austenite grains is 10 to 30 ⁇ m, and a ratio I d /S d between a long axis I d and a short axis S d of the prior austenite grains may be 2.0 or less.
• the microstructure is defined at a depth of 1 ⁇ 4 of the sheet thickness from a surface and a center position in a sheet width direction in a cross section parallel to a rolling direction. The reason is that the microstructure, at this position is a typical microstructure of a steel sheet.
• Bainite 80.0% ⁇ or more
• Bainite means a lath-shaped bainitic ferrite and a structure having a Fe-based carbide between and/or inside the bainitic ferrite. Unlike polygonal ferrite, the bainitic ferrite has a lath shape and has a relatively high dislocation density inside, and therefore can be easily distinguished from other structures using SEM or TEM.
• the area ratio of the bainite is set to 80.0% or more.
• the area ratio of the bainite is preferably 85.0% or more and more preferably 90.0% or more. The higher the area ratio of the bainite, the more preferable.
• a practical upper limit may be 97.5%.
• the bainitic ferrite is not included in ferrite.
• the area ratio of the ferrite is set to 10.0% or less.
• the area ratio of the ferrite is preferably 5.0% or less. From the viewpoint of securing ductility, the area ratio of the ferrite may be 1.0% or more.
• All of Cementite, pearlite, martensite, tempered martensite, and residual austenite are starting points of voids during distortion, and are structures that deteriorate the hole expansibility of the hot-rolled steel sheet.
• the area ratio of the remainder in the microstructure is set to 10.0% or less. The area ratio thereof is preferably 5.0% or less.
• the area ratio of the remainder in the microstructure may be 1.0% or more.
• the total area ratio of the martensite and the tempered martensite in the remainder in the microstructure is preferably 5.0% or less.
• the total area fraction thereof is more preferably 3.0% or less.
• a test piece is taken from the hot-rolled steel sheet so that a microstructure at a depth of 1 ⁇ 4 of the sheet thickness from the surface and a center position in a sheet width direction in the cross section parallel to the rolling direction can be observed.
• polishing After polishing the cross section of the test piece with silicon carbide paper of #600 to #1500, finishing is performed to a mirror surface using a diamond powder having a grain size of 1 to 6 ⁇ m using a diluted solution such as alcohol or a liquid dispersed in pure water. Next, polishing is performed with colloidal silica without containing an alkaline solution at a room temperature to remove a strain introduced into a surface layer of a sample..
• a region with a length of 50 ⁇ m and between a depth of 1 ⁇ 8 of the sheet thickness from the surface to a depth of 3 ⁇ 8 of the sheet thickness from the surface is measured by electron backscatter diffraction at a measurement interval of 0.1 ⁇ m, so that a position at the depth of 1 ⁇ 4 of the sheet thickness from the surface is the center in a random position of the sample cross section in a longitudinal direction, to obtain crystal orientation information.
• an EBSD analyzer configured of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used.
• the EBSD analyzer is set such that the degree of vacuum inside is 9.6 ⁇ 10 ⁇ 5 Pa or less, an acceleration voltage is 15 kV, an irradiation current level is 13, and an electron beam irradiation level is 62.
• the obtained crystal orientation information is used to calculate the area ratio of the residual austenite using a “Phase Map” function installed in the software “OIM Analysis (registered trademark)” attached to the EBSD analyzer. Those having a crystal structure of fcc are determined to be residual austenite.
• those having a crystal structure of bcc are determined to be bainite, ferrite, and the “remainder in the microstructure (cementite, pearlite, martensite, and tempered martensite) other than residual austenite”.
• a region where the “Grain Orientation Spread” is 1° or less is extracted as ferrite, by using a “Grain Orientation Spread” function installed in the software “OIM Analysis (registered trademark)” attached to the EBSD analyzer, under the condition, in which 15° grain boundary is defined as the grain boundary.
• OIM Analysis registered trademark
• the area ratio of the extracted “remainder in the microstructure (cementite, pearlite, martensite, and tempered martensite) other than residual austenite” is calculated, and the area ratio of the above residual austenite is added to obtain the area ratio of the remainder in the microstructure (cementite, pearlite, martensite, tempered martensite, and residual austenite).
• cementite, pearlite, martensite, and tempered martensite can be distinguished by the following method. First, in order to observe the same region as the EBSD measurement region by SEM, a Vickers indentation is imprinted in the vicinity of an observation position. Thereafter, a contamination on the surface layer is removed by polishing, leaving the structure of the observed section, and nital etching is performed. Next, the same visual field as the EBSD observed section is observed by SEM at a magnification of 3000 times.
• a region having a substructure in the grain and where cementite precipitates with a plurality of variants is determined to be tempered martensite.
• a region where the cementite precipitates in a lamellar shape is determined to be pearlite.
• Spherical particles with high brightness and grain size circle equivalent diameter) of 2 ⁇ m or less are determined to be cementite.
• a region where the brightness is high and the substructure is not exposed by etching is determined as “martensite and residual austenite”.
• a method such as buffing using alumina particles having a particle diameter of 0.1 ⁇ m or less or Ar ion sputtering may be used.
• the total density of L 7 and L 68 is set to 0.35 ⁇ m/ ⁇ m 2 or more.
• the total density thereof is preferably 0.40 ⁇ m/ ⁇ m 2 or more.
• the total density of L 7 and L 68 is set to 0.60 ⁇ m/ ⁇ m 2 or less.
• the total density thereof is preferably 0.55 ⁇ m/ ⁇ m 2 or less.
• the grain boundary having a crystal orientation difference of X° about the ⁇ 110> direction refers to a grain boundary having a crystallographic relationship in which the crystal orientations of the crystal grain A and, the crystal grain B are the same by rotating one crystal grain B by X° about the ⁇ 110> axis, when two adjacent crystal grain A and crystal grain B are specified at a certain grain boundary.
• an orientation difference of ⁇ 4° is allowed from the matching orientation relationship.
• the length L 7 of a grain boundary and the length L 68 as above are measured by using the electron back scatter diffraction pattern-orientation image microscopy (EBSP-OIM) method.
• EBSP-OLM electron back scatter diffraction pattern-orientation image microscopy
• a crystal orientation of an irradiation point can be measured for a short time period in such manner that a highly inclined sample in a scanning electron microscope (SEM) is irradiated with electron beams, a Kikuchi pattern formed by back scattering is photographed by a high sensitive camera, and the photographed image is processed by a computer.
• SEM scanning electron microscope
• the EBSP-OIM method is performed using a device in which a scanning electron microscope and an EBSP analyzer are combined and an OIM Analysis (registered trademark) manufactured by AMETEK Inc.
• the analyzable area of the EBSP-OIM method is a region that can be observed by the SEM.
• the EBSP-OIM method makes it possible to analyze a region with a minimum resolution of 20 nm, which varies depending on the resolution of the SEM.
• an analysis is performed in at least 5 visual fields of a region of 50 ⁇ m ⁇ 50 ⁇ m at a magnification of 1000 times and an average value of the lengths of the grain boundary having a crystal orientation difference of 7° about the ⁇ 110> direction in the bainite is calculated to obtain L 7 .
• an average value of the lengths of the grain boundary having a crystal orientation difference of 68° about the ⁇ 110> direction in the bainite is calculated to obtain L 68 .
• the orientation difference of ⁇ 4° is allowed.
• the total density of the length L 7 of the grain boundary having the crystal orientation difference of 7° and the length L 68 of the grain boundary having the crystal orientation difference of 68° about the ⁇ 110> direction in the bainite is obtained.
• a region exceeding I ⁇ /2 may be extracted as the bainite, as in the case of determining the area ratio of the bainite.
• Average grain size of prior austenite grains 10 to 30 ⁇ m
• the average grain size of the prior austenite grains is 10 to 30 ⁇ m, and the ratio I d /S d between the long axis I d and the short axis S d of the prior austenite grains may be 2.0 or less.
• a test piece is taken from the hot-rolled steel sheet so that a microstructure at a depth of 1 ⁇ 4 of the sheet thickness from the surface and a center position in a sheet width direction in the cross section parallel to the rolling direction can be observed.
• the prior austenite grain boundary is exposed by corroding the observed section with a saturated aqueous solution of picric acid.
• a magnified photograph of a cross section parallel to the rolling direction that has been corroded, at a depth of 1 ⁇ 4 of the sheet thickness from the surface and at the center position, in the sheet width direction is photographed with a scanning electron microscope (SEM) at a magnification of 1000 times and 5 or more visual fields.
• SEM scanning electron microscope
• the equivalent circle diameters (diameters) of at least 20 prior austenite grains having an equivalent circle diameter (diameter) of 2 ⁇ m or more, which are included in each SEM photograph, are determined by image processing, and an average value thereof is calculated to obtain the average grain size of the prior austenite grains.
• the above measurement is performed by excluding these grains.
• the long axis and the short axis of at least 20 prior austenite grains having an equivalent circle diameter (diameter) of 2 ⁇ m or more, which are included in each of the above SEM photographs, are measured.
• the long axis id and the short axis S d of the prior austenite grain are obtained.
• the ratio I d /S d between the long axis I d and the short axis S d of the prior austenite rains is obtained.
• The-hot-rolled steel sheet according to the present embodiment has a tensile (maximum) strength of 780 MPa or more.
• tensile strength is less than 780 MPa, an applicable component is limited, and the contribution of weight reduction of the vehicle body is small.
• the tensile strength is preferably 980 MPa or more.
• An upper limit is not particularly limited, and may be 1800 MPa from the viewpoint of suppressing wearing of a die.
• the hot-rolled steel sheet according to the present embodiment may have a total elongation of 14.0% or more.
• An upper limit of the total elongation is not particularly limited, and may be 30.0% or less or 25.0% or less.
• the tensile strength and the total elongation are measured according to JIS Z 2241: 2011 using a No. 5 test piece of JIS Z 2241: 2011.
• a sampling position of the tensile test piece is set to the center position in the sheet width direction, and the direction perpendicular to the rolling direction may be the longitudinal direction.
• the cross-head speed is set to 3 mm/min.
• the hot-rolled steel sheet according to the present embodiment may have a hole expansion rate of 50% or more. It is not necessary to particularly limit an upper limit of the hole expansion rate, and the upper limit thereof may be 90% or less or 85% or less.
• the hole expansion rate is obtained, by performing a hole expanding test in accordance with JIS Z 2256: 2010.
• the hot-rolled steel sheet according to the present embodiment may have an impact value of 60 or more at ⁇ 40° C. It is not necessary to particularly limit an upper limit of the impact value at ⁇ 40° C., and the upper limit thereof may be 180 J/cm 2 or less or 175 J/cm 2 or less.
• a sub-sized Charpy impact test piece is taken from a predetermined position of the hot-rolled steel sheet, and the impact value at ⁇ 40° C. is determined in accordance, with a test method described in JIS Z 2242: 2005.
• the sheet thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited and may be (16 to 8.0 mm.
• the sheet thickness of the steel sheet When the sheet thickness of the steel sheet is less than 0.6 mm, it becomes difficult to secure the rolling completion temperature and the rolling force becomes excessive, which may make hot rolling difficult. Therefore, the sheet thickness of the steel sheet according to the present embodiment may be set to 0.6 mm or more.
• the sheet thickness is preferably 1.2 mm or more or 1.4 mm or more.
• the sheet thickness when the sheet thickness is more than 8.0 mm, it becomes difficult to refine the microstructure, particularly, the prior austenite grains, and it may become difficult to secure the microstructure described above from the viewpoint of the microstructural fraction. Therefore, the sheet thickness may be set to 8.0 mm or less.
• the sheet thickness is preferably 6.0 mm or less.
• The-hot-rolled steel sheet according to the present embodiment having the above-described chemical composition and microstructure may be a surface-treated steel sheet provided with a plating layer on the surface for the purpose of improving corrosion resistance and the like.
• the plating layer may be an electro plating layer or a hot-dip plating layer.
• the electro plating layer include electrogalvanizing and electro Zn—Ni alloy plating.
• the hot-dip plating layer include hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminum plating, hot-dip Zn—Al alloy plating, hot-dip Zn—Al—Mg alloy plating, and hot-dip Zn—Al—Mg—Si alloy plating.
• the plating adhesion amount is not particularly limited and may be the same as before. Further, it is also possible to further enhance the corrosion resistance by applying an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment liquid) after plating.
• an appropriate chemical conversion treatment for example, application and drying of a silicate-based chromium-free chemical conversion treatment liquid
• the preferred method of manufacturing the hot-rolled steel sheet according to the present embodiment includes the following steps.
• the temperature of the slab and the temperature of the steel sheet in the present embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet.
• a total rolling reduction in the rough rolling is set to 70% or more, and the finish rolling may be performed so that all rolling reductions of final three stages of the finish rolling are less than 25%.
• the slab having the above-mentioned chemical composition is heated to a heating temperature of 1200° C. or higher and retained for 1.0 hour. Since a coarse precipitate present at a slab stage causes cracking during rolling and a decrease in material properties, it is preferable to heat a steel material before the hot rolling to dissolve the coarse carbide. Therefore, the heating temperature is set to 1200° C. or higher, and the retention time is set to 1.0 hour or longer. The preferred heating temperature is 1230° C. or higher, and the preferred retention time is 3.0 hours or longer.
• the heating temperature may be set to 1400° C. or lower, and the retention time may be set to 20.0 hours or shorter.
• the slab to be heated is preferably produced by continuous casting from the viewpoint of manufacturing cost, but may be produced by another casting method (for example, ingot-making method).
• the rough rolling completion temperature is 1000° C. or higher.
• the rough rolling completion temperature is preferably 1050° C. or higher.
• the rough rolling completion temperature may be 1300° C. or lower.
• the total rolling reduction in the rough rolling is set to more than 65%.
• the total rolling reduction in the rough rolling is preferably 68% or more, more preferably 70% or more, and even more preferably 80% or more.
• An upper limit of the total rolling reduction in the rough rolling is not particularly limited, and may be set to 90% or less.
• the total rolling, reduction in the rough rolling is represented by (1 ⁇ t r /t s ) ⁇ 100 (%) using the slab thickness: t s and the sheet thickness t r at the end of the rough rolling.
• the average grain size and an aspect ratio of the prior austenite grains described above can be realized.
• the finish rolling completion temperature is set to 860° C. or higher.
• the finish rolling completion temperature is preferably set to 900° C. or higher.
• the finish rolling completion temperature is set to 980° C. or lower.
• the finish rolling completion temperature is preferably 950° C. or lower.
• the total rolling reduction in the rough rolling and rolling reduction of final three stages of the finish rolling may be strictly controlled. Specifically, as described above, the total rolling reduction in the rough, rolling may be set to 70% or more, and the rolling reduction in the final three stages of the finish rolling may be set to less than 25%.
• all of the rolling reductions of the final three stages of the finish rolling may be set to less than 25%. All of the rolling reductions are 20% or less.
• the rolling reduction can be represented by (1 ⁇ h/h 0 ) ⁇ 100 (%) when the sheet thickness after rolling in one pass is h and the sheet thickness before rolling is h 0 .
• cooling is performed to a temperature range of 570° C. to 620° C. at an average cooling rate of 20° C./s or higher.
• the average cooling rate is a value obtained by dividing the temperature difference between the start point and the end point of the set range by the elapsed time from the start point to the end point.
• the average cooling rate is set to 20° C./s or higher.
• the average cooling rate is preferably 30° C./s or higher, and more preferably 50° C./s or higher. From the viewpoint of suppressing the increase in cooling equipment, the average cooling rate may be 200° C./s or lower.
• Cooling at the average cooling rate of 20° C./s or higher is performed to a temperature range of 570° C. to 620° C.
• the cooling stop temperature is set to 620° C. or lower.
• the cooling stop temperature may be any temperature as long as it can be retained in a temperature range of 620° C. or lower and 500° C. to 580° C., and in order to retain in the temperature range of 500° C. to 580° C. for 2.0 hours or more, the cooling stop temperature is preferably set to 550° C. or higher.
• the cooling stop temperature is preferably set to 570° C. or higher.
• the cooling stop temperature is lower than 500° C. and retention is performed in the temperature range of 500° C. to 580° C. by reheating, a desired amount of bainite cannot be obtained. Therefore, heating after the stop of cooling is not desirable.
• the retention temperature is set to a temperature range of 500° C. to 580° C., and the retention time is set to 2.0 to 12.0 hours.
• a lower limit of the retention temperature is preferably 530° C.
• An upper limit of the retention temperature is preferably 560° C.
• a lower limit of the retention time is preferably 4.0 hours, and more preferably 6.0 hours.
• An upper limit of the retention time is preferably 10.0 hours, and more preferably 8.0 hours.
• the steel sheet temperature may be fluctuated or be constant in the temperature range of 500° C. to 580° C.
• the cooling stop temperature of cooling having an average cooling rate of 20° C./s or higher is lower than 580° C., it is sufficient that the retention time of 2.0 to 12.0 hours can be secured in the temperature range of 500° C. to 580° C.
• cooling is performed to a room temperature.
• any method may be used, and cooling may be performed by an appropriate method such as mist cooling or rapid cooling using a water cooling tank, in addition to air cooling.
• the room temperature referred to here is a temperature range of 20° C. to 30° C.
• the microstructural fraction, the total density of L 7 and L 68 , the average grain size of the prior austenite grains, and the ratio I d /S d between the long axis I d and the short, axis S d of the prior austenite grains were determined.
• the results obtained are shown in Tables 5 to 7.
• the tensile strength (TS) and total elongation (EI) were measured by using a test piece No 5 of JIS Z 2241: 2011, in accordance with JIS Z 2241: 2011.
• a sampling position of the tensile test piece was set to the center position, in the sheet width direction, and the direction perpendicular to the rolling direction was set to the longitudinal direction.
• the cross-head speed was set to 3 mm/min.
• TS tensile strength
• EI total elongation
• the hole expansion rate ( ⁇ ) was evaluated by performing a hole expanding test in accordance with JIS Z 2256: 2010.
• the toughness was evaluated by performing a Charpy impact test at ⁇ 40° C. and determining the impact value.
• a sub-sized Charpy impact test piece was taken from a predetermined position of the hot-rolled steel sheet, and the impact value at ⁇ 40° C. was determined in accordance with a test method described in JIS Z 2242: 2005 to evaluate the toughness.
• the toughness was determined excellent which was pass, and in a case where the impact value (vE 40 ) was less than 60 J/cm 2 , the toughness was determined poor which was fail.
• the punching properties were evaluated by performing a punching test and observing the properties of the punched end surface.
• a punched hole was prepared with a hole diameter of 10 mm, a clearance of 12.5%, and a punching speed of 80 mm/s.
• a cross section of the punched hole perpendicular to the direction was embedded in a resin, and, the punched end surface was imaged with a scanning electron microscope.
• “E (Excellent)” was noted in Tables 5 to 7 as having particularly good punching properties.
• Invention Examples have high strength and excellent ductility, hole expansibility, and toughness.
• Invention Examples in which the average grain size of the prior austenite grains is 10 to 30 ⁇ m, and the, ratio I d /S d between the long axis I d and the short axis S d of the prior austenite grains is 2.0 or less have particularly good punching properties.
• Comparative Example is poor in any one or more of the strength, the ductility, the hole expansibility and toughness.
• Air cooling Comparative Example 2 80 596 580 500 3.1 Air cooling Invention Example 3 87 589 580 500 5.3 Air cooling Invention Example 4 100 596 580 500 11.1 Air cooling Invention Example 5 100 599 580 500 14.2 Air cooling Comparative Example 6 122 582 580 500 3.5 Air cooling Comparative Example 7 125 592 580 500 4.3 Air cooling Comparative Example 8 100 584 580 500 4.2 Air cooling Comparative Example 9 45 593 580 500 2.2 Air cooling Comparative Example 10 41 581 580 500 8.0 Air cooling Comparative Example 11 67 595 580 500 4.6 Air cooling Comparative Example 12 98 583 580 500 2.6 Air cooling Comparative Example 13 80 585 580 500 3.5 Air cooling Comparative Example 14 85 589 580 500 4.1 Air cooling Invention Example 15 151 581 580 500 5.2 Air cooling Invention Example 16 120 594 580 500 3.4
• Air cooling Invention Example 52 100 596 580 500 3.5 Air cooling Invention Example 53 120 587 580 500 4.6 Air cooling Invention Example 54 80 590 580 500 4.8 Air cooling Invention Example 55 101 587 580 500 6.2 Air cooling Invention Example 56 130 586 580 500 5.1 Air cooling Comparative Example 58 130 580 580 500 3.3 Air cooling Comparative Example 59 89 598 580 500 4.2 Air cooling Comparative Example 60 15 595 580 500 6.0 Air cooling Comparative Example 61 35 596 580 500 3.7 Air cooling Invention Example 62 80 630 580 500 7.2 Air cooling Comparative Example 63 78 381* 568 500 3.5 Air cooling Comparative Example 64 120 573 573 500 3.7 Air cooling Invention Example 65 100 577 577 513 2.3 Water cooling Invention Example 66 80 520 520 500 0.7 Air cooling Comparative Example 67 150 5
• the present invention it is possible to provide a hot-rolled steel sheet having high strength, and excellent ductility, hole expansibility, and toughness, and a method of manufacturing the same. According to the above preferred aspect according to the present invention, it is possible to provide a hot-rolled steel sheet having excellent punching properties in addition to the above-mentioned properties and a method of manufacturing the same.

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