High strength hot rolled steel sheet

Abstract

Provided is a high strength hot rolled steel sheet having a predetermined chemical composition, wherein when a total width in a direction perpendicular to a rolling direction and sheet thickness direction is W, a microstructure at a ¼ position in sheet thickness at all positions of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in a width direction comprises, by area %, tempered martensite: 95% or more, fresh martensite: 5% or less, and at least one of ferrite, upper bainite, and pearlite: 5% or less in total, and a difference between a maximum value and minimum value in tensile strength at all the positions in the width direction is 30 MPa or less.

Description

FIELD

[0001] The present invention relates to a high strength hot rolled steel sheet.BACKGROUND

[0002] In recent years, in the auto industry, steel sheet has been made higher in strength from the viewpoint of reducing the environmental load and securing safety of occupants. Along with the higher strength of steel sheet, the main structures forming the microstructure of steel sheet have been becoming martensite structures.

[0003] For example, PTL 1 describes high strength hot rolled steel sheet excellent in low temperature toughness having a composition containing, by mass %, C: 0.08% or more and less than 0.16%, Si: 0.01 to 1.0%, Mn: 0.8 to 2.0%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.002 to 0.006%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, Cr: 0.01 to 1.0%, and B: 0.0005 to 0.0050% and having a balance of Fe and unavoidable impurities and having structures mainly comprising martensite phases or tempered martensite phases, having the main phases present in a volume ratio with respect to the structure as a whole of 90% or more, having an average grain size of the prior austenite grains of 20 μm or less in a cross-section parallel to the rolling direction and 15 μm or less in a cross-section perpendicular to the rolling direction, and having an aspect ratio of the prior austenite grains in the cross-section parallel to the rolling direction of 18 or less. Further, PTL 1 describes that, according to the above constitution, it is easy to produce hot rolled steel sheet having a high strength of a yield strength YS: 960 MPa or more and a high toughness of a vE−40 of 40 J or more without containing expensive Mo and further being excellent in bendability and delayed fracture resistance, further having a surface hardness of a Brinell hardness of 360 HB or more, being excellent in wear resistance, and being optimal for use as a structural member of a construction machine or industrial machine and that a remarkable effect is exhibited in industry.

[0004] PTL 2 describes high strength hot rolled steel sheet having a chemical composition containing, by mass %, C: 0.05 to 0.14%, Si: 0.01 to 1.0%, Mn: 0.50 to 2.0%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.002 to 0.006%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, Cr: 0.01 to 1.0%, and B: 0.0005 to 0.0050% and having a balance of Fe and unavoidable impurities and having a steel structure comprised of at least one of martensite phases and tempered martensite phases as main phases, having those main phases in an area ratio with respect to the steel structure as a whole of 95% or more, containing cementite with an average grain size of 0.5 μm or less in the laths of the martensite phases and / or tempered martensite phases, and having a content of cementite by mass % of 0.01 to 0.08%. Further. PTL 2 describes that, according to this constitution, it is possible to provide high strength hot rolled steel sheet high in toughness and excellent in punchability and punch bending fatigue strength even without containing the expensive alloying element Mo, specifically high strength hot rolled steel sheet having a high strength of a tensile strength TS: 980 MPa or more and having a high toughness of an absorption energy vE−40 of 40 J or more in a Charpy impact test at a test temperature of-40° C. and, furthermore, excellent in punchability and punch bending fatigue strength.

[0005] PTL 3 describes high strength hot rolled steel sheet excellent in uniformity of strength in the sheet width direction having a chemical composition containing, by mass %, C: 0.10 to 0.25%, Si: 0.10% or less, Mn: 1.0 to 2.0%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, Nb: 0.01 to 0.05%, Ti: 0.005 to 0.05%, Cr: 0.05 to 1.0%, and B: 0.0005 to 0.0050% and having a balance of Fe and unavoidable impurities, having a structure with tempered martensite phases comprising 95% or more by volume ratio with respect to the structure as a whole and with an average grain size of prior austenite grains of 20 μm or less in the cross-section parallel to the rolling direction and 15 μm or less in the cross-section perpendicular to the rolling direction, and having a yield strength (YS) of 960 MPa or more. Further, PTL 3 teaches that by making the structure of the steel sheet over the entire region in the width direction a structure having main phases of tempered martensite, having an average grain size of prior austenite (Y) grains in the cross-section parallel to the rolling direction of 20 μm or less, and having an average grain size of prior austenite grains in the cross-section perpendicular to the rolling direction of 15 μm or less, high strength hot rolled steel sheet with a high strength of a yield strength YS: 960 MPa or more uniformly in the width direction is obtained.CITATIONS LISTPatent Literature

[0006] [PTL 1] Japanese Unexamined Patent Publication No. 2016-211073

[0007] [PTL 2] Japanese Unexamined Patent Publication No. 2018-188675

[0008] [PTL 3] Japanese Unexamined Patent Publication No. 2016-183414SUMMARYTechnical Problem

[0009] In the hot rolled steel sheet such as described in PTLs 1 to 3, if making the microstructure a structure of just martensite phases or closer to just martensite phases so as to raise the strength, sometimes the steel sheet is defective in shape due to uneven cooling at the time of cooling or transformation plasticity at the time of transformation. In such a case, it is difficult to maintain a sufficient flatness in the obtained hot rolled steel sheet. For example, PTL 3 studies a high strength hot rolled steel sheet with uniform yield strength YS in the width direction, but does not specifically study anything from the viewpoint of improving the flatness of the high strength hot rolled steel sheet.

[0010] Therefore, the present invention has as its object the provision of a high strength hot rolled steel sheet having improved flatness by a novel constitution.Solution to Problem

[0011] The inventors studied how to achieve the above object while focusing in particular on the microstructure in the width direction of a hot rolled steel sheet. As a result, the inventors discovered that by making the microstructure of a hot rolled steel sheet having a predetermined chemical composition a structure mainly comprised of tempered martensite, it is possible to secure high strength and that by making the structure uniform in the width direction, it is possible to reduce the variation of strength in the width direction. In this way, by reducing the variation in strength in the width direction, it is possible to provide a hot rolled steel sheet remarkably improved in flatness in the width direction of the hot rolled steel sheet.

[0012] The present invention able to achieve this object is as follows:

[0013] (1) A high strength hot rolled steel sheet comprising a chemical composition comprising, by mass %,

[0014] C: 0.050 to 0.100%,

[0015] Si: 0.010 to 0.200%,

[0016] Mn: 1.00 to 2.50%,

[0017] Ti: 0.001 to 0.120%,

[0018] Al: 0.001 to 0.050%,

[0019] B: 0.0005 to 0.0050%,

[0020] P: 0.100% or less,

[0021] S: 0.050% or less,

[0022] N: 0.0050% or less,

[0023] 0:0 to 0.0050%,

[0024] Cu: 0 to 0.20%,

[0025] Ni: 0 to 0.20%,

[0026] Sn: 0 to 0.10%,

[0027] Cr: 0 to 0.40%,

[0028] Mo: 0 to 0.20%,

[0029] Nb: 0 to 0.05%,

[0030] V: 0 to 0.10%,

[0031] As: 0 to 0.100%,

[0032] Zr: 0 to 0.100%,

[0033] Ca: 0 to 0.0050%,

[0034] Mg: 0 to 0.100%,

[0035] Bi: 0 to 0.020%,

[0036] Co: 0 to 0.20%,

[0037] W: 0 to 0.20%,

[0038] Zn: 0 to 0.20%,

[0039] REM: 0 to 0.1000%, and

[0040] balance: Fe and impurities, wherein

[0041] when a total width in a direction perpendicular to a rolling direction and sheet thickness direction is W, a microstructure at a ¼ position in sheet thickness at all positions of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in a width direction comprises, by area %,

[0042] tempered martensite: 95% or more,

[0043] fresh martensite: 5% or less, and

[0044] at least one of ferrite, upper bainite, and pearlite: 5% or less in total, and

[0045] a difference between a maximum value and minimum value in tensile strength at all the positions in the width direction is 30 MPa or less.

[0046] (2) The high strength hot rolled steel sheet according to the above (1), wherein the chemical composition contains, by mass %, at least one of

[0047] O: 0.0001 to 0.0050%,

[0048] Cu: 0.001 to 0.20%,

[0049] Ni: 0.001 to 0.20%,

[0050] Sn: 0.001 to 0.10%,

[0051] Cr: 0.001 to 0.40%,

[0052] Mo: 0.001 to 0.20%,

[0053] Nb: 0.001 to 0.05%,

[0054] V: 0.001 to 0.10%,

[0055] As: 0.001 to 0.100%,

[0056] Zr: 0.0001 to 0.100%,

[0057] Ca: 0.0001 to 0.0050%,

[0058] Mg: 0.0001 to 0.100%,

[0059] Bi: 0.0001 to 0.020%,

[0060] Co: 0.001 to 0.20%,

[0061] W: 0.001 to 0.20%,

[0062] Zn: 0.001 to 0.20%, and

[0063] REM: 0.0001 to 0.1000%.

[0064] (3) The high strength hot rolled steel sheet according to the above (1) or (2), wherein a prior austenite grain size in the microstructure is 40 μm or less.Advantageous Effects of Invention

[0065] According to the present invention, it is possible to provide a high strength hot rolled steel sheet having improved flatness.DESCRIPTION OF EMBODIMENTS<High Strength Hot Rolled Steel Sheet>

[0066] The high strength hot rolled steel sheet according to the embodiments of the present invention is characterized by having a chemical composition comprising, by mass %,

[0067] C: 0.050 to 0.100%,

[0068] Si: 0.010 to 0.200%,

[0069] Mn: 1.00 to 2.50%,

[0070] Ti: 0.001 to 0.120%,

[0071] Al: 0.001 to 0.050%,

[0072] B: 0.0005 to 0.0050%,

[0073] P: 0.100% or less,

[0074] S: 0.050% or less,

[0075] N: 0.0050% or less,

[0076] O: 0 to 0.0050%,

[0077] Cu: 0 to 0.20%,

[0078] Ni: 0 to 0.20%,

[0079] Sn: 0 to 0.10%,

[0080] Cr: 0 to 0.40%,

[0081] Mo: 0 to 0.20%,

[0082] Nb: 0 to 0.05%,

[0083] V: 0 to 0.10%,

[0084] As: 0 to 0.100%,

[0085] Zr: 0 to 0.100%,

[0086] Ca: 0 to 0.0050%,

[0087] Mg: 0 to 0.100%,

[0088] Bi: 0 to 0.020%,

[0089] Co: 0 to 0.20%,

[0090] W: 0 to 0.20%,

[0091] Zn: 0 to 0.20%,

[0092] REM: 0 to 0.1000%, and

[0093] balance: Fe and impurities, wherein

[0094] when a total width in a direction perpendicular to a rolling direction and sheet thickness direction is W, a microstructure at a ¼ position in sheet thickness at all positions of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in a width direction comprises, by area %,

[0095] tempered martensite: 95% or more,

[0096] fresh martensite: 5% or less, and

[0097] at least one of ferrite, upper bainite, and pearlite: 5% or less in total, and

[0098] a difference between a maximum value and minimum value in tensile strength at all the positions in the width direction is 30 MPa or less.

[0099] As explained above, if increasing the strength in hot rolled steel sheet by making the microstructure a structure of just martensite phases or close to just martensite phases, sometimes the steel sheet becomes defective in shape due to uneven cooling at the time of cooling or transformation plasticity at the time of transformation. In such a case, it becomes difficult to maintain a sufficient flatness in the obtained hot rolled steel sheet. Regarding the defective shape of the steel sheet, specifically sometimes warpage occurs in the width direction of the steel sheet (direction perpendicular to the rolling direction and the sheet thickness direction). If warpage occurs in the width direction of the steel sheet, when utilizing such steel sheet to form a long member, sometimes this becomes a cause of shape defects or cracking at the time of shaping. As an existing technique for alleviating such warpage, use of a leveler, etc., for correction (flattening) may be mentioned. However, if the strength of steel sheet becomes higher, sometimes sufficient correction is not necessarily possible by a leveler, etc. Further, if ending up performing preworking by correction by a leveler, etc., such preworking causes part of the ductility inherently possessed by steel sheet to be consumed and the residual ductility falls. If the residual ductility falls, defects in shaping easily arise at the time of pressing the steel sheet and as a result the productivity falls. For this reason, good flatness is sought in the state of steel sheet as hot rolled even without correction by a leveler, etc. As reasons for steel sheet to become defective in shape, mainly a fast cooling speed of finish rolling may be mentioned. If the cooling speed after finish rolling is fast, the controllability of the amount of water used for cooling deteriorates and locally overcooled parts are formed, etc., causing remarkable unevenness of cooling. As a result, due to the uneven temperature in the width direction of the steel sheet, a heat stress occurs and warpage occurs in the width direction of the steel sheet. For this reason, from the viewpoint of securing flatness, it is not necessarily suitable to excessively increase the cooling speed after finish rolling. On the other hand, to obtain a structure of just martensite phases or close to just martensite phases from the viewpoint of higher strength, it is necessary to cool by a cooling speed of the critical cooling speed or more.

[0100] Therefore, the inventors engaged in studies to achieve higher strength while improving the flatness of steel sheet by selecting suitable steel compositions, in particular focusing on the microstructure in the width direction of the hot rolled steel sheet. First, the inventors discovered that by making the microstructure of hot rolled steel sheet having a predetermined chemical composition a structure mainly comprised of tempered martensite, it is possible to achieve high strength, more specifically 980 MPa or more high strength. Next, the inventors thought that making such a structure mainly comprised of tempered martensite uniform in the width direction would be effective and conducted further studies accordingly. First, to make the microstructure uniform in the width direction, it is necessary to suitably control it after finish rolling. Further, to form a structure mainly comprised of tempered martensite, it is necessary to adroitly utilize autotempering at the time of cooling. Therefore, the inventors discovered that limiting the Si, which is able to delay or suppress autotempering, to a content of 0.200 mass % or less in the steel sheet, it is possible to promote autotempering at the time of cooling while by making the amount of cooling water sprayed at the steel sheet at the time of cooling equal at the top surface and bottom surface of the steel sheet from after finish rolling to a temperature corresponding to the end temperature of martensite transformation as explained in detail later regarding the method of production of hot rolled steel sheet, i.e., by evenly cooling the top and bottom surfaces of the steel sheet by cooling water from after finish rolling to about 200° C. it is possible to make a structure comprised of tempered martensite in 95 area % or more uniform over the entire width direction of the hot rolled steel sheet. More specifically, the inventors discovered in particular that by making the Si content of the hot rolled steel sheet 0.200 mass % or less and suitably controlling the cooling after finish rolling as explained above, when a total width in a direction perpendicular to a rolling direction and sheet thickness direction is W, it is possible to make a microstructure at a ¼ position in sheet thickness at all positions of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction a structures comprised of, by area %, tempered martensite: 95% or more. As a result, the inventors discovered that it is possible to sufficiently reduce the variation in strength in the width direction of hot rolled steel sheet at a level so that a difference between a maximum value and minimum value in tensile strength at all the positions in the width direction becomes 30 MPa or less and that, along with the uniformity of the microstructure in the width direction as a whole and reduction of such variation in strength, it is possible to remarkably improve the flatness of the hot rolled steel sheet regardless of the high strength.

[0101] In the past, in cooling after the finish rolling, in general the amount of water sprayed on the top surface of steel sheet was greater than the amount of water sprayed on the bottom surface of the steel sheet. Therefore, by performing the cooling at the top and bottom surfaces of the steel sheet evenly from after the finish rolling until a temperature corresponding to the martensite transformation end temperature, it is possible to achieve uniformity of the microstructure and reduce the variation in strength in the width direction such as explained above and thereby possible to remarkably improve the flatness of hot rolled steel sheet despite the high strength. This fact was first made clear this time by the inventors. Further, according to the high strength hot rolled steel sheet according to the embodiments of the present invention, since there is sufficient flatness in the state as hot rolled, there is no need for preworking by a leveler, etc., and therefore part of the ductility inherently possessed by steel is not consumed by such preworking either. Along with this, it is possible to reduce the risk of defects in shaping at the time of press-forming such high strength hot rolled steel sheet and becomes possible to remarkably improve the productivity as well. Therefore, the high strength hot rolled steel sheet according to the embodiments of the present invention is only naturally particularly useful in use in the automotive field and can also be extremely effectively used in other fields as well.

[0102] Below, the high strength hot rolled steel sheet according to the embodiments of the present invention will be explained in more detail. In the following explanation, the “%” of the units of contents of the elements, unless otherwise indicated, means “mass %”. Further, in the Description, “to” showing a numerical range, unless otherwise indicated, is used in the sense including the numerical values entered before and after it as a lower limit value and upper limit value.[C: 0.050 to 0.100%]

[0103] C is an element effective for raising the strength of steel sheet. To sufficiently obtain such an effect, the C content is 0.050% or more. The C content may also be 0.055% or more, 0.060% or more, 0.065% or more, or 0.070% or more. On the other hand, if excessively containing C, due to an excessive increase in strength, it becomes difficult to control the variation in strength in the width direction to within a predetermined range. Therefore, the C content is 0.100% or less. The C content may also be 0.095% or less, 0.090% or less, 0.085% or less, or 0.080% or less.[Si: 0.010 to 0.200%]

[0104] Si is an element effective for raising strength as a solution strengthening element. To sufficiently obtain such an effect, the Si content is 0.010% or more. The Si content may also be 0.020% or more, 0.040% or more, 0.060% or more, 0.080% or more, or 0.100% or more. On the other hand, if excessively containing Si, sometimes the autotempering at the time of cooling the steel sheet is delayed or inhibited. In such a case, hot rolled steel sheet having the desired microstructure can no longer be obtained. Therefore, the Si content is 0.200% or less. The Si content may also be 0.180% or less, 0.160% or less, 0.140% or less, or 0.120% or less.[Mn: 1.00 to 2.50%]

[0105] Mn is an element effective for raising the hardenability and the strength as a solution strengthening element. If the Mn content is low, the hardenability becomes insufficient, therefore ferrite and other soft phases are formed in relatively large amounts at the time of cooling and it becomes no longer possible to make a structure having tempered martensite as its main phase uniform in the width direction. Further, due to the expansion of the steel sheet accompanying such transformation, warpage occurs in the width direction and sometimes the steel sheet is defective in shape. Therefore, the Mn content is 1.00% or more. The Mn content may also be 1.20% or more, 1.40% or more, 1.60% or more, or 1.80% or more. On the other hand, if excessively containing Mn, due to the improvement in the hardenability, the martensite is not sufficiently tempered even by the autotempering at the time of cooling of the steel sheet and sometimes fresh martensite cannot be sufficiently reduced in the final microstructure. Therefore, the Mn content is 2.50% or less. The Mn content may also be 2.40% or less, 2.20% or less, 2.00% or less, or 1.90% or less.[Ti: 0.001 to 0.120%]

[0106] Ti is an element contributing to improvement of strength by precipitation strengthening, etc. Further, Ti bonds with N to form titanium nitride (TiN) and thereby consumes the dissolved N in the steel and has the action of suppressing the reduction in the amount of dissolved B due to the formation of BN. To sufficiently obtain these effects, the Ti content is 0.001% or more. The Ti content may also be 0.010% or more, 0.020% or more, 0.040% or more, or 0.060% or more. On the other hand, Ti is also an element suppressing recrystallization of austenite, therefore if excessively containing Ti, due to the presence of nonrecrystallized austenite including relatively large numbers of dislocations, the force driving ferrite transformation, etc., at the time of cooling is raised, ferrite and other soft phases are easily formed from the nonrecrystallized austenite, and sometimes the desired microstructure cannot be obtained. Therefore, the Ti content is 0.120% or less. The Ti content may also be 0.110% or less, 0.100% or less, 0.090% or less, or 0.080% or less.[Al: 0.001 to 0.050%]

[0107] Al is an element acting as a deoxidizer. To sufficiently obtain such an effect, the Al content is 0.001% or more. The Al content may also be 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if excessively containing Al, coarse oxides are formed and sometimes the toughness is lowered. Therefore, the Al content is 0.050% or less. The Al content may also be 0.045% or less or 0.040% or less.[B: 0.0005 to 0.0050%]

[0108] B is an element raising the hardenability of steel and contributing to improvement of the strength. To sufficiently obtain such effects, the B content is 0.0005% or more. The B content may also be 0.0008% or more, 0.0010% or more, 0.0015% or more, or 0.0020% or more. On the other hand, if excessively containing B, sometimes the toughness and / or weldability fall. Therefore, the B content is 0.0050% or less. The B content may also be 0.0045% or less, 0.0040% or less, 0.0030% or less, or 0.0025% or less.[P: 0.100% or Less]

[0109] P, if excessively contained, sometimes affects weldability, etc., disadvantageously. Therefore, the P content is 0.100% or less. The P content may also be 0.080% or less, 0.050% or less, 0.030% or less, or 0.020% or less. The lower limit of the P content is not particularly prescribed and may also be 0%, but excessive reduction invites a rise in costs. Therefore, the P content may also be 0.0001% or more, 0.0005% or more, or 0.001% or more.[S: 0.050% or Less]

[0110] S, if excessively contained, forms a large amount of MnS and sometimes lowers the toughness. Therefore, the S content is 0.050% or less. The S content may also be 0.020% or less, 0.010% or less, or 0.005% or less. The lower limit of the S content is not particularly prescribed and may also be 0%, but excessive reduction invites a rise in costs. Therefore, the S content may also be 0.0001% or more, 0.0005% or more, or 0.001% or more.[N: 0.0050% or Less]

[0111] N, if excessively contained, forms coarse nitrides and sometimes causes the toughness to fall. Further, N bonds with the B in the steel to form boronitrides (BN) and thereby reduce the amount of dissolved B and sometimes lowers the effect of improvement of hardenability due to the addition of B. Therefore, the N content is preferably as low as possible and is 0.0050% or less. The N content may also be 0.0045% or less, 0.0040% or less, or 0.0035% or less. The lower limit of the N content is not particularly prescribed and may also be 0%, but excessive reduction invites a rise in costs. Therefore, the N content may also be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0112] The basic chemical composition of the high strength hot rolled steel sheet according to the embodiments of the present invention is as described above. Furthermore, the high strength hot rolled steel sheet may also contain at least one of the following optional elements in place of part of the Fe of the balance in accordance with need. For example, the high strength hot rolled steel sheet may contain at least one of O: 0 to 0.0050%, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Sn: 0 to 0.10%, Cr: 0 to 0.40%, Mo: 0 to 0.20%, Nb: 0 to 0.05%, V: 0 to 0.10%, As: 0 to 0.100%, Zr: 0 to 0.100%, Ca: 0 to 0.0050%, Mg: 0 to 0.100%, Bi: 0 to 0.020%, Co: 0 to 0.20%, W: 0 to 0.20%, Zn: 0 to 0.20%, and REM: 0 to 0.1000%. Below, these optional elements will be explained in detail.[O: 0 to 0.0050%]

[0113] O is an element entering in the process of production. The O content may also be 0%. However, to reduce the O content to less than 0.0001%, time is required for refining and a drop in productivity is invited. Therefore, the O content may also be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if excessively containing O, coarse inclusions are formed and sometimes the toughness of the steel sheet is lowered. Therefore, the O content is preferably 0.0050% or less. The O content may also be 0.0040% or less, 0.0035% or less, or 0.0030% or less.[Cu: 0 to 0.20%]

[0114] Cu is an element contributing to improvement of strength and / or corrosion resistance. The Cu content may also be 0%, but to obtain these effects, the Cu content is preferably 0.001% or more. The Cu content may also be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if excessively containing Cu, deterioration of the toughness and weldability is sometimes invited. Therefore, the Cu content is preferably 0.20% or less. The Cu content may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, 0.08% or less, or 0.06% or less.[Ni: 0 to 0.20%]

[0115] Ni is an element raising the hardenability of steel and contributing to improvement of the strength and / or corrosion resistance. The Ni content may also be 0%, but to obtain these effects, the Ni content is preferably 0.001% or more. The Ni content may also be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, even if excessively including Ni, the effects become saturated and a rise in production costs is invited. Therefore, the Ni content is preferably 0.20% or less. The Ni content may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, 0.08% or less, or 0.06% or less.[Sn: 0 to 0.10%]

[0116] Sn is an element effective for improvement of the corrosion resistance. The Sn content may also be 0%, but to obtain such an effect, the Sn content is preferably 0.001% or more and may also be 0.005% or more, 0.01% or more, or 0.02% or more. On the other hand, if excessively containing Sn, sometimes a fall in the toughness is invited. Therefore, the Sn content is preferably 0.10% or less. The Sn content may also be 0.08% or less, 0.06% or less, or 0.04% or less.[Cr: 0 to 0.40%]

[0117] Cr is an element raising the hardenability of steel and contributing to improvement of the strength and / or corrosion resistance. The Cr content may also be 0%, but to obtain these effects, the Cr content is preferably 0.001% or more. The Cr content may also be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, even if excessively including Cr, the effects become saturated and a rise in production costs is invited. Therefore, the Cr content is preferably 0.40% or less. The Cr content may also be 0.30% or less, 0.20% or less, 0.15% or less, or 0.12% or less.[Mo: 0 to 0.20%]

[0118] Mo is an element raising the hardenability of steel and contributing to improvement of the strength and an element contributing also to improvement of corrosion resistance. The Mo content may also be 0%, but to obtain these effects, the Mo content is preferably 0.001% or more. The Mo content may also be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if excessively containing Mo, the deformation resistance at the time of hot working increases and sometimes the load on the facilities becomes greater. Therefore, the Mo content is preferably 0.20% or less. The Mo content may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.[Nb: 0 to 0.05%]

[0119] Nb is an element forming carbides, nitrides, and / or carbonitrides in the steel and contributes to refinement of the structure and in turn higher strength of the steel sheet by a pinning effect. The Nb content may also be 0%, but to obtain such an effect, the Nb content is preferably 0.001% or more. The Nb content may also be 0.005% or more or 0.01% or more. On the other hand, if excessively containing Nb, coarse carbides, etc., are formed in the steel and sometimes the toughness of the steel sheet is lowered. Therefore, the Nb content is 0.05% or less. The Nb content may also be 0.04% or less, 0.03% or less, or 0.02% or less.[V: 0 to 0.10%]

[0120] V is an element contributing to improvement of strength by precipitation strengthening, etc. The V content may also be 0%, but to obtain such an effect, the V content is preferably 0.001% or more. The V content may also be 0.005% or more, 0.01% or more, or 0.02% or more. On the other hand, if excessively containing V, a large amount of precipitates is formed and sometimes the toughness is lowered. Therefore, the V content is preferably 0.10% or less. The V content may also be 0.08% or less, 0.06% or less, or 0.04% or less.[As: 0 to 0.100%]

[0121] As is an element effective for improvement of the corrosion resistance. The As content may also be 0%, but to obtain such an effect, the As content is preferably 0.001% or more and may be 0.005% or more, 0.008% or more, or 0.010% or more. On the other hand, even if excessively containing As, the effect become saturated and a rise in the production costs is invited. Therefore, the As content is preferably 0.100% or less. The As content may also be 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.[Zr: 0 to 0.100%]

[0122] Zr is an element able to control the form of sulfides. The Zr content may also be 0%, but to obtain such an effect, the Zr content is preferably 0.0001% or more. The Zr content may also be 0.0005% or more, 0.001% or more, or 0.010% or more. On the other hand, even if excessively containing Zr, the effect become saturated and a rise in the production costs is invited. Therefore, the Zr content is preferably 0.100% or less. The Zr content may also be 0.050% or less, 0.030% or less, or 0.020% or less.[Ca: 0 to 0.0050%]

[0123] Ca is an element able to control the form of sulfides. The Ca content may also be 0%, but to obtain such an effect, the Ca content is preferably 0.0001% or more. The Ca content may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if excessively containing Ca, the effect becomes saturated and a rise in production costs is invited. Therefore, the Ca content is preferably 0.0050% or less. The Ca content may also be 0.0040% or less, 0.0030% or less, or 0.0020% or less.[Mg: 0 to 0.100%]

[0124] Mg is an element able to control the form of sulfides. The Mg content may also be 0%, but to obtain such an effect, the Mg content is preferably 0.0001% or more and may be 0.001% or more, 0.005% or more, or 0.008% or more. On the other hand, even if excessively containing Mg, the effect becomes saturated and a rise in production costs is invited. Therefore, the Mg content is preferably 0.100% or less. The Mg content may also be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.[Bi: 0 to 0.020%]

[0125] Bi is an element effective for improvement of the corrosion resistance. The Bi content may also be 0%, but to obtain such an effect, the Bi content is preferably 0.0001% or more. The Bi content may also be 0.0005% or more, 0.001% or more, or 0.003% or more. On the other hand, if excessively containing Bi, the effect becomes saturated and a rise in production costs is invited. Therefore, the Bi content is preferably 0.020% or less. The Bi content may also be 0.010% or less, 0.008% or less, or 0.005% or less.[Co: 0 to 0.20%]

[0126] Co is an element contributing to improvement of the hardenability and / or heat resistance. The Co content may also be 0%, but to obtain these effects, the Co content is preferably 0.001% or more. The Co content may also be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if excessively containing Co, sometimes the hot workability falls and an increase in raw material cost results. Therefore, the Co content is preferably 0.20% or less. The Co content may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.[W: 0 to 0.20%]

[0127] W is an element raising the hardenability of steel and contributing to improvement of the strength. The W content may also be 0%, but to obtain such an effect, the W content is preferably 0.001% or more. The W content may also be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if excessively containing W, sometimes the weldability falls. Therefore, the W content is preferably 0.20% or less. The W content may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.[Zn: 0 to 0.20%]

[0128] Zn is an element effective for controlling the form of inclusions. To obtain such an effect, the Zn content is preferably 0.001% or more. The Zn content may also be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, even if excessively containing Zn, the effect becomes saturated and a rise in production costs is invited. Therefore, the Zn content is preferably 0.20% or less. The Zn content may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.[REM: 0 to 0.1000%]

[0129] REM (rare earth metals) are elements able to control the form of sulfides. The REM content may also be 0%, but to obtain such an effect, the REM content is preferably 0.0001% or more. The REM content may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if excessively containing REM, the effect becomes saturated and a rise in production costs is invited. Therefore, the REM content is preferably 0.1000% or less. The REM content may also be 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. In this Description, “REM” is the general name of the 17 elements of atomic number 21 scandium (Sc), atomic number 39 yttrium (Y), and the lanthanoids of atomic number 57 lanthanum (La) to atomic number 71 lutetium (Lu). The REM content is the total content of these elements.

[0130] In the high strength hot rolled steel sheet according to embodiments of the present invention, the balance besides the above elements is comprised of Fe and impurities. The “impurities” are constituents, etc., entering the high strength hot rolled steel sheet due to the ore, scrap, or other raw materials and other various factors in the production process when industrially producing it.

[0131] The chemical composition of the high strength hot rolled steel sheet according to embodiments of the present invention may be measured by a general analysis method. For example, the chemical composition of the high strength hot rolled steel sheet may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S may be measured using combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-nondispersive infrared absorption method.[Microstructure]

[0132] In the high strength hot rolled steel sheet according to embodiments of the present invention, when the total width in the direction perpendicular to the rolling direction and the sheet thickness direction of the high strength hot rolled steel sheet is W, the microstructure at a ¼ position in sheet thickness at all positions of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction is comprised of, by area %, tempered martensite (tM): 95% or more, fresh martensite (fM): 5% or less, and at least one of ferrite (α), upper bainite (B), and pearlite (P): total 5% or less. In the present invention, the “total width” means the length of the steel sheet in the direction perpendicular to the rolling direction and the sheet thickness direction of the high strength hot rolled steel sheet (for example, coiled high strength hot rolled steel sheet).

[0133] Here, if the rolling direction of the hot rolled steel sheet is not clear, the rolling direction of the hot rolled steel sheet can be identified by the following method. The sheet thickness cross-section of the hot rolled steel sheet is finished by polishing to a mirror surface, then the S concentration is measured by an electron probe micro analyzer (EPMA). The measurement conditions are an acceleration voltage of 15 kV. The image of distribution in a range of 500 μm square at the sheet thickness center part is measured using a measurement pitch of 1 μm. At this time, a stretched region with a high S concentration is judged to be an inclusion of MnS, etc. At the time of observation, several fields may also be observed. Next, based on the sheet thickness cross-section first examined by the above method, surfaces parallel to surfaces rotated by 5° increments about the sheet thickness direction in 0° to 180° in range are observed by the above method. The average value of the lengths of the long axes of the plurality of inclusions at the obtained cross-sections are calculated for each cross-section and the cross-section where the average value of the lengths of the long axes of the inclusions becomes maximum is identified. The direction parallel to the long axis direction of the inclusions at the cross-section is judged to be the rolling direction.

[0134] Below, the different structures will be explained in more detail.[Tempered Martensite: 95% or More]

[0135] By making the tempered martensite in the microstructure at a ¼ position in sheet thickness at all positions of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction an area % of 95% or more, it is possible to achieve high strength due to the structure mainly comprised of martensite while it is possible to reliably control the difference between a maximum value and minimum value in tensile strength at all the positions in the width direction to 30 MPa or less and therefore possible to remarkably reduce the variation in strength in the width direction. At all the positions in the width direction, the area ratio of tempered martensite may be 96% or more. 97% or more, or 98% or more. The upper limit of the area ratio of the tempered martensite is not particularly prescribed and may also be 100%.[Fresh Martensite: 5% or Less]

[0136] In the high strength hot rolled steel sheet according to embodiments of the present invention, the area ratio of fresh martensite has to be controlled to 5% or less at all the positions in the width direction. If the area ratio of fresh martensite is more than 5% at even one position among them, the strength becomes too high at that position and sometimes the variation in strength in the width direction can no longer be sufficiently reduced. Therefore, from the viewpoint of reducing the variation in strength, at all the positions in the width direction, the lower the area ratio of fresh martensite, the more preferable. For example, it may be 4% or less, 3% or less, 2% or less, or 1% or less. The lower limit of area ratio of fresh martensite is not particularly prescribed and may also be 0%.[At Least One of Ferrite, Upper Bainite, and Pearlite: 5% or Less in Total]

[0137] In the high strength hot rolled steel sheet according to embodiments of the present invention, the remaining structure other than the tempered martensite and fresh martensite is comprised of at least one of ferrite, upper bainite, and pearlite. The at least one of the ferrite, upper bainite, and pearlite also similarly has to be controlled to 5% or less in total at all the positions in the width direction. If at even one position among these the area ratio of the at least one of ferrite, upper bainite, and pearlite becomes more than 5% in total, the strength falls too much at that position and sometimes the variation in strength in the width direction can no longer be sufficiently reduced. Therefore, from the viewpoint of reducing the variation in strength, the lower the area ratio of the at least one of ferrite, upper bainite, and pearlite at all the positions in the width direction, the more preferable. For example, it may be 4% or less, 3% or less, 2% or less, or 1% or less in total. The lower limit of the at least one of ferrite, upper bainite, and pearlite is not particularly prescribed and may also be 0% in total. For example, the total area ratio of the fresh martensite, ferrite, upper bainite, and pearlite may also be 5% or less, 4% or less. 3% or less, 2% or less, or 1% or less. Similarly, the total area ratio of fresh martensite, ferrite, upper bainite, and pearlite may also be 0) %.[Identification of Microstructure and Calculation of Area Ratio]

[0138] The microstructure is identified and the area ratio is calculated after corrosion using a Nital reagent or Le Pera solution by an FE-SEM (field emission scan electron microscope) and optical microscope and X-ray diffraction. The structure is observed by the FE-SEM and optical microscope at a 100 μm×100 μm region in the steel sheet cross-section in a direction parallel to the rolling direction and perpendicular to the sheet surfaces by a 1000 to 50000× power. In each microstructure, three locations are measured at each position of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction (direction perpendicular to rolling direction and sheet thickness direction) and the average value of the three measured values is calculated to determine the area ratio at each respective position.

[0139] The area ratio of the ferrite is found by observing a region of 100 μmx 100 μm in the range of the sheet thickness ⅛ position to ⅜ position centered about the ¼ position in sheet thickness in an electron channeling contrast image by an FE-SEM (field emission scan electron microscope). More specifically, it is possible to identify parts shown by even contrast in the region as ferrite and calculate the area ratio of the same by the image analysis software Image J.

[0140] The area ratio of fresh martensite (martensite as hardened) is found by the following procedure. The examined surface of the sample is etched by a Le Pera solution, then a region of 100 μmx 100 μm in the range of the sheet thickness ⅛ position to ⅜ position centered about the ¼ position in sheet thickness is examined by an FE-SEM. Fresh martensite and retained austenite are not corroded by Le Pera corrosion, therefore in an SEM image, appear as flat regions with brighter contrast than other parts. The area ratio of the not corroded regions corresponds to the total area ratio of the fresh martensite and any retained austenite which is present. The area ratio of the fresh martensite is calculated by subtracting the area ratio of the retained austenite measured by the X-ray diffraction method explained later from the area ratio of the not corroded regions.

[0141] The area ratio of retained austenite is calculated by the X-ray diffraction method. First, the part from a surface of the sample down to a depth ¼ position in the sheet thickness direction is removed by mechanical polishing and chemical polishing. Next, at the ¼ position in sheet thickness, the structure fraction of retained austenite is calculated from the integrated intensity ratios of the diffraction peaks of (200), (211) of the bcc phase and (200), (220), (311) of the fcc phase obtained using MoKα rays. As this method of calculation, the general 5-peak method is utilized. The structure fraction of retained austenite calculated is determined as the area ratio of the retained austenite.

[0142] The upper bainite and tempered martensite are identified and the area ratios are calculated by the following procedure. First, the observed surface of a sample is corroded by a Nital reagent, then a region of 100 mx 100 μm in the range of the sheet thickness ⅛ position to ⅜ position centered about the ¼ position in sheet thickness is examined by an FE-SEM. The upper bainite and tempered martensite are identified in the following way from the positions of cementite and arrangement of cementite contained inside the structure in this observed region. Upper bainite is comprised of laths of bainitic ferrite at the interfaces of which cementite or retained austenite are present. Based on such features, upper bainite is identified. The value obtained by dividing the regions of the identified bainite by the area of the examined field is calculated as the area ratio of the upper bainite. On the other hand, in tempered martensite, there is cementite at the inside of the martensite laths, but there are two or more types of crystal orientation of martensite laths and cementite and cementite comes in several variants. therefore it is possible to identify tempered martensite. The value obtained by dividing the regions of the thus identified tempered martensite by the area of the examined field is calculated as the area ratio of the tempered martensite.

[0143] The pearlite is identified and the area ratio is calculated by the following procedure. First, the observed surface of a sample is corroded by a Nital reagent, then a range of the sheet thickness ⅛ position to ⅜ position centered about the ¼ position in sheet thickness is examined by an optical microscope. In the observed image of the optical microscope, regions where carbides and ferrite are present in lamellar shapes area identified as pearlite. The value of these regions divided by the area of the observed field is calculated as the area ratio of pearlite.[Difference Between Maximum Value and Minimum Value of Tensile Strength at All Positions of 1 / 10W Position, 3 / 10W Position, 5 / 10W Position, 7 / 10W Position, and 9 / 10W Position From End Part in Width Direction of 30 MPa or Less]

[0144] In the high strength hot rolled steel sheet according to embodiments of the present invention, as explained previously, by making the tempered martensite in the microstructure at a ¼ position in sheet thickness at all positions of the 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction an area % of 95% or more, it is possible to achieve high strength due to the structure mainly comprised of martensite while it is possible to reliably control the difference between a maximum value and minimum value in tensile strength at all the positions in the width direction to 30 MPa or less and therefore possible to remarkably reduce the variation in strength in the width direction. Further, in relation to the uniformity of microstructure and reduction of the variation in strength in the width direction as a whole, regardless of the high strength, it is possible to remarkably improve the flatness of the hot rolled steel sheet. From the viewpoint of improving the flatness of hot rolled steel sheet, the smaller the difference between the maximum value and minimum value of the tensile strength, the more preferable. For example, it may be 28 MPa or less, 25 MPa or less, 22 MPa or less, 20 MPa or less, 17 MPa or less, or 15 MPa or less. The lower limit is not particularly prescribed, but, for example, a difference between the maximum value and minimum value of the tensile strength of 5 MPa or more, 8 MPa or more, or 10 MPa or more may be allowed.

[0145] The difference between the maximum value and minimum value of the tensile strength is determined in the following way. First, No. 5 tensile test pieces of JIS Z2241:2011 having directions parallel to the rolling direction as test directions were taken at the positions of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction of the hot rolled steel sheet. Next, these tensile test pieces were used for tensile tests compliant with JIS Z2241:2011 so as to obtain five tensile strengths. Finally, the difference between the maximum value and minimum value of these was calculated.[Tensile Strength]

[0146] The minimum value among the above five values of tensile strength is determined as the tensile strength of the high strength hot rolled steel sheet according to embodiments of the present invention. In the high strength hot rolled steel sheet according to embodiments of the present invention, by having the above explained chemical composition and microstructure, a high tensile strength, specifically a tensile strength of 980 MPa or more, can be achieved. The tensile strength is preferably 1000 MPa or more, 1050 MPa or more, or 1100 MPa or more. According to the high strength hot rolled steel sheet according to embodiments of the present invention, despite having such an extremely high tensile strength, along with uniformity of the microstructure and reduction of variation of strength in the width direction as a whole, it is possible to achieve extremely excellent flatness. The upper limit of the tensile strength is not particularly prescribed, but, for example, the tensile strength of the high strength hot rolled steel sheet may be 1300 MPa or less, 1250 MPa or less, 1200 MPa or less, or 1180 MPa or less.[Prior Austenite Grain Size in Microstructure: 40 μm or Less]

[0147] According to preferable embodiments of the present invention, the prior austenite grain size in the microstructure is 40 μm or less. As explained above, in the high strength hot rolled steel sheet according to embodiments of the present invention, it is possible to achieve extremely excellent flatness along with the uniformity of the microstructure and reduction in variation in strength in the width direction as a whole, but in addition to this, by controlling the prior austenite grain size to such a fine range, it becomes possible to further improve the toughness and other additional properties. From the viewpoint of improvement of the toughness, the smaller the prior austenite grain size, the more preferable. For example, it may be 37 μm or less, 35 μm or less. 32 μm or less, 30 μm or less, 27 μm or less, or 25 μm or less. The lower limit is not particularly prescribed, but, for example, the prior austenite grain size may be 10 μm or more. 12 μm or more, 15 μm or more, 18 μm or more, or 20 μm or more.

[0148] The prior austenite grain size in the microstructure is determined in the following way. First, a region of 200 μm×200 μm in an L-cross-section of a steel slab taken at the position of sheet thickness ¼ from a surface of hot rolled steel sheet was analyzed by SEM / EBSD (scan electron microscope / electron backscatter diffraction). More specifically, the crystal orientation data obtained by SEM / EBSD is processed by predetermined crystal orientation conversion (see “Study For Increasing Precision of Method of Reconstruction of Austenite Structures of Steel”, Kengo Hata, Masayuki Wakita, Tomoya Fujiwara, Kaori Kawano, Nippon Steel & Sumikin Engineering Co., Ltd. Technical Review, no. 404 (2016), p. 24-30) to obtain an image of reconstructed prior austenite grains. From a prior austenite grain of the image, the diameter of a circle having the same area, i.e., the circle equivalent diameter, is sought. This operation is performed for a total of 10 prior austenite grains and the obtained circle equivalent diameters are averaged to determine the prior austenite grain size.[Total Width W]

[0149] The high strength hot rolled steel sheet according to embodiments of the present invention can have any total width W. While not particularly limited, for example, in the case of the total width W at hot rolled steel sheet in the coiled state (coil), the total width W may be 700 mm or more, 800 mm or more, 900 mm or more, or 1000 mm or more. The upper limit is not particularly prescribed, but from the viewpoint of more reliably improving the flatness, the total width is preferably 2500 mm or less and may also be 2200 mm or less, 2000 mm or less, 1800 mm or less, 1600 mm or less, 1500 mm or less, 1400 mm or less, or 1300 mm or less.[Sheet Thickness]

[0150] The high strength hot rolled steel sheet according to embodiments of the present invention is not particular limited, but in general has a 1.0 to 6.0 mm sheet thickness. For example, the sheet thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more and / or may be 5.0 mm or less, 4.0 mm or less, or 3.0 mm or less.<Method of Production of High Strength Hot Rolled Steel Sheet>

[0151] Next, the preferable method of production of the high strength hot rolled steel sheet according to embodiments of the present invention will be explained. The following explanation is intended to illustrate the characterizing method for producing the high strength hot rolled steel sheet according to embodiments of the present invention and is not intended to limit the high strength hot rolled steel sheet to one produced by a method of production such as explained below.

[0152] The method of production of the high strength hot rolled steel sheet according to embodiments of the present invention is characterized by including

[0153] hot rolling including heating a slab having the chemical composition explained above in relation to the high strength steel sheet to a temperature of 1220 to 1300° C. and rough rolling and finish rolling it, wherein an exit side temperature of the rough rolling is 1100 to 1200° C., an entry side temperature (F0) of the finish rolling is 1000 to 1100° C., an exit side temperature (FT) of the finish rolling is 940 to 1000° C., and a total rolling reduction of the finish rolling is 85 to 95%,

[0154] cooling of primary cooling the finished rolled steel sheet in a temperature region from the exit side temperature (FT) of the finish rolling to the martensite transformation start temperature Ms+50° C. by an average cooling speed of a critical cooling speed Vc+10° C. / s or more and 60° C. / s or less, then secondary cooling it in a temperature region from Ms+50° C. to 200° C. by an average cooling speed of 50 to 120° C. / s, wherein the top-bottom cooling ratio of the top surface of the steel sheet to the bottom surface in the primary cooling is 0.8 to 1.2 and the top-bottom cooling ratio of the top surface of the steel sheet to the bottom surface in the secondary cooling is 0.8 to 1.2, and

[0155] coiling the secondary cooled steel sheet at 50 to 100° C. Below, each step will be explained in detail.[Hot Rolling Step][Heating of Slab]

[0156] First, a slab having the chemical composition explained above in relation to the hot rolled steel sheet is heated. The slab used is preferably cast by the continuous casting method from the viewpoint of productivity, but may also be produced by the ingot making method or thin slab casting method. The slab used contains relatively large amounts of alloying elements for obtaining high strength steel sheet. For this reason, it is also necessary to heat the slab before sending it on to hot rolling to make the alloying elements dissolve in the slab. If the heating temperature is less than 1220° C., the alloying elements will not sufficiently dissolve in the slab but will remain as coarse alloy carbides and sometimes brittle cracking will occur during the hot rolling. For this reason, the heating temperature is 1220° C. or more, preferably 1230° C. or more. The upper limit of the heating temperature is not particularly prescribed, but is preferably 1300° C. or less from the viewpoint of the capacity of the heating facility and the productivity.[Rough Rolling]

[0157] In this method, the heated slab is rough rolled before the finish rolling so as to adjust the sheet thickness, etc. In order for the rough rolling to secure the desired sheet bar dimensions and enable the total rolling reduction in the 940° C. or more temperature region in the finish rolling to be adjusted to within the desired range, the exit side temperature of the rough rolling is 1100 to 1200° C. and is preferably 1150 to 1200° C. If the exit side temperature of the rough rolling is less than 1100° C., in the finish rolling continuing after the rough rolling, it becomes difficult to obtain a 940° C. or more exit side temperature. Further, if the exit side temperature of the rough rolling becomes more than 1200° C., the crystal grains become coarser and sometimes the obtained hot rolled steel sheet falls in toughness.[Finish Rolling]

[0158] The rough rolled slab is next finish rolled. As explained above, the slab used contains relatively large amounts of alloying elements, therefore at the time of hot rolling, the rolling load has to be made greater. For this reason, the hot rolling is performed at a high temperature under a high pressure. Specifically, the entry side temperature (F0) of the finish rolling is 1000 to 1100° C., the exit side temperature (FT) of the finish rolling is 940 to 1000° C., and the total rolling reduction of the finish rolling is 85 to 95%. In particular, the exit side temperature of the finish rolling is important on the point of control of the microstructure of the steel sheet. In more detail, if the exit side temperature of the finish rolling is low, the microstructure will become uneven and sometimes the formability will fall. For this reason, the exit side temperature of the finish rolling is 940° C. or more. On the other hand, to suppress coarsening of the austenite, the exit side temperature of the finish rolling is 1000° C. or less.[Cooling Step][Primary Cooling]

[0159] In the next cooling step, the finish rolled steel sheet is first primary cooled in the temperature region from the exit side temperature (FT) of the finish rolling to the martensite transformation start temperature Ms+50° C. by an average cooling speed of a critical cooling speed Vc+10° C. / s or more and 60° C. / s or less. By primary cooling in this temperature region by an average cooling speed of the critical cooling speed Vc+10° C. / s or more and 60° C. / s or less, it is possible to promote martensite transformation and reduce the at least one of ferrite, upper bainite, and pearlite in the finally obtained microstructure to 5 area % or less in total and make the microstructure uniform in the width direction. If the average cooling speed of the primary cooling is less than the critical cooling speed Vc+10° C. / s, the at least one of the ferrite, upper bainite, and pearlite becomes more than 5 area % in total and sometimes the desired strength cannot be achieved. On the other hand, if the average cooling speed of the primary cooling is more than 60° C. / s, the cooling speed is fast, therefore it becomes difficult to evenly cool the steel sheet in the width direction and uneven cooling occurs in the width direction. In this case, in the finally obtained hot rolled steel sheet, the desired microstructure cannot be obtained in the width direction and / or the variation in tensile strength in the width direction becomes greater. Along with this, the hot rolled steel sheet becomes defective in shape, warpage occurs in the width direction of the steel sheet, and sufficient flatness can no longer be achieved. Therefore, the average cooling speed of the primary cooling is the critical cooling speed Vc+10° C. / s or more and 60° C. / s or less, preferably the critical cooling speed Vc+12° C. / s or more and 60° C. / s or less.

[0160] In the present method of production, the Ms point (° C.) is found by the following formula 1:Ms=823-350[C]-4⁢0[M⁢n]-3⁢5[V]-2⁢0[C⁢r]-1⁢7[N⁢i]-1⁢0[C⁢u]-1⁢0[M⁢o]-1⁢0[W]+1⁢5[C⁢o]+3⁢0[A⁢l]-2⁢7⁢3formula⁢ 1where, [C], [Mn], [V], [Cr], [Ni], [Cu], [Mo], [W], [Co], and [Al] are the contents (mass %) of the elements in the steel. If an element is not included, 0 is entered. Further, the critical cooling speed Vc (° C.) is also an indicator of the hardenability where the martensite area ratio becomes 90% or more and can be expressed by the following formulas 2 and 3:

[0162] when amount of dissolved B≥0.0005 mass %,log⁢Vc=2.94-0.75×(2.7[C]+0.4[Si]+[M⁢n]+0.4⁢5[N⁢i]+0.8[C⁢r]+2[M⁢o])formula⁢ 2when amount of dissolved B<0.0005 mass %,log⁢Vc=3.69-0.75×(2.7[C]+0.4[S⁢i]+[M⁢n]+0.4⁢5[N⁢i]+0.8[C⁢r]+[M⁢o])formula⁢ 3where, [C], [Si], [Mn], [Ni], [Cr], and [Mo] are the contents (mass %) of the elements in the steel. If an element is not included, 0 is entered. Further, the amount of dissolved B (mass %) corresponds to the amount of the B content contained in the steel minus the amount of B consumed for forming boronitrides (BN). On the other hand, the amount of dissolved N (mass %) able to form BN can be reduced by including Ti in the steel and fixing it as TiN. Therefore, the amount of dissolved B can be calculated by the following formulas 4 and 5:Amount⁢ of⁢ dissolved⁢ B=10.81×([B] / 10.81-amount⁢ of⁢ dissolved⁢ N / 14.01)formula⁢ 4where, when [N] / 14.01−[Ti] / 47.88>0,Amount⁢ of⁢ dissolved⁢ N=1⁢4.0⁢1×([N] / 14.01-[Ti] / 47.88)formula⁢ 5when [N] / 14.01−[Ti] / 47.88≤0, the amount of dissolved N is 0).where, [B], [N], and [Ti] are the contents (mass %) of the elements in the steel. If an element is not included, 0) is entered.In the primary cooling, in addition to control of the average cooling speed, evenly cooling the steel sheet at the top surface and bottom surface is extremely important. Such cooling is performed so that the top-bottom cooling ratio of the top surface of the steel sheet to the bottom surface becomes 0.8 to 1.2, more specifically, is performed so that the amount of cooling water sprayed on the top surface of the steel sheet becomes 0.8 to 1.2 times the amount of cooling water sprayed on the bottom surface of the steel sheet. By evenly performing cooling at the top and bottom surfaces of the steel sheet in this way, it becomes possible to remarkably suppress or reduce the occurrence of uneven cooling. As a result, it is possible to achieve uniformity of the microstructure and reduction of variation in strength in the width direction as a whole. Along with this, it becomes possible to prevent the occurrence of warpage in the width direction of the hot rolled steel sheet and achieve sufficient flatness. If the top-bottom cooling ratio is less than 0).8 or is more than 1.2, due to the occurrence of uneven cooling, it becomes no longer possible to obtain a uniform microstructure in the width direction, i.e., it becomes no longer possible to make the microstructure at a ¼ position in sheet thickness at all positions of the 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction an area % of a structure of tempered martensite: 95% or more. As a result, it becomes no longer possible to sufficiently reduce the variation in strength in the width direction to a level such as one where a difference between a maximum value and minimum value in tensile strength at all the positions in the width direction become 30 MPa or less.Here, the “top-bottom cooling ratio” does not mean the ratio of the amount of cooling water of the entire top surface and the amount of cooling water of the entire bottom surface in the range of FT to (Ms+50)° C. More specifically, in the method of production, the range of FT to (Ms+50)° C. is divided into sections of 10 meters each, the top-bottom cooling ratio is calculated from the amount of cooling water of the top surface and the amount of cooling water of the bottom surface for each section, and the top-bottom cooling ratios of the different sections calculated in this way are all controlled to ranges of 0.8 to 1.2. With control of the top-bottom cooling ratio of the range as a whole rather than each divided section, for example, it is extremely difficult to sufficiently suppress the occurrence of uneven cooling due to local overcooling, etc. However, by realizing control of the top-bottom cooling ratio for each section in this way, it becomes possible to reduce local overcooling, etc., and reliably suppress the occurrence of uneven cooling. Further, top-bottom cooling ratio for each section can be controlled by any suitable means. While not particularly limited, for example, each section has several cooling water nozzles arranged along the direction of advance of the steel sheet at the top side and bottom side of the steel sheet, therefore by suitably spraying water by on / off control of these cooling water nozzles, it is possible to relatively easily control the top-bottom cooling ratio of each section to within 0.8 to 1.2 in range.[Secondary Cooling]

[0170] The primary cooled steel sheet is next secondary cooled in the temperature region of Ms+50° C. to 200° C. by an average cooling speed of 50 to 120° C. / s. By secondary cooling in the temperature region from Ms+50° C. to 200° C. possibly corresponding to the end temperature of martensite transformation by an average cooling speed of 50 to 120° C. / s, it is possible to sufficiently secure the dwell time in the temperature region in which autotempering progresses, therefore it becomes possible to promote autotempering. Therefore, the structure transformed to martensite by the above primary cooling can be sufficiently tempered and 95 area % or more of tempered martensite can be made uniform in the width direction in the final microstructure. If the average cooling speed of the secondary cooling is less than 50° C. / s, it is not possible to obtain the desired microstructure in the width direction of the steel sheet and sometimes the variation in strength in the width direction cannot be sufficiently reduced. On the other hand, if the average cooling speed of the secondary cooling is more than 120° C. / s, it becomes no longer possible to promote autotempering and sometimes in the final microstructure, fresh martensite remains in more than 5 area %. In addition to or in place of this, the controllability of the amount of water used for cooling deteriorates due to such rapid cooling, locally overcooled parts occur, etc., and uneven cooling occurs in the width direction. In this case, in the finally obtained hot rolled steel sheet, the variation in tensile strength in the width direction becomes greater. In relation to this, the hot rolled steel sheet becomes defective in shape, warpage occurs in the width direction of the steel sheet, and sufficient flatness can no longer be achieved.

[0171] In the secondary cooling as well, in the same way as the case of the primary cooling, in addition to control of the average cooling speed, evenly cooling the steel sheet at the top surface and bottom surface is extremely important. Such cooling, in the same way as the case of the primary cooling, is performed so that the top-bottom cooling ratio of the top surface of the steel sheet to the bottom surface becomes 0).8 to 1.2, more specifically, is performed so that the amount of cooling water sprayed on the top surface of the steel sheet becomes 0.8 to 1.2 times the amount of cooling water sprayed on the bottom surface of the steel sheet. By evenly performing cooling at the top and bottom surfaces of the steel sheet in this way, it becomes possible to remarkably suppress or reduce the occurrence of uneven cooling. As a result, it is possible to achieve uniformity of the microstructure and reduction of variation in strength in the width direction as a whole. Along with this, it becomes possible to prevent the occurrence of warpage in the width direction of the hot rolled steel sheet and achieve sufficient flatness. If the top-bottom cooling ratio is less than 0.8 or is more than 1.2, due to the occurrence of uneven cooling, it becomes no longer possible to make the microstructure uniform in the width direction, i.e., it becomes no longer possible to make the microstructure at a ¼ position in sheet thickness at all positions of the 1 / 10W position. 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction an area % of structures of tempered martensite: 95% or more. As a result, it becomes no longer possible to sufficiently reduce the variation in strength in the width direction to a level such as one where a difference between a maximum value and minimum value in tensile strength at all the positions in the width direction become 30 MPa or less.

[0172] Here, the above top-bottom cooling ratio does not mean the ratio of the amount of cooling water of the entire top surface and the amount of cooling water of the entire bottom surface in the range of (Ms+50)° C. to 200° C. More specifically, in the method of production, the range of (Ms+50)° C. to 200° C. is divided into sections of 10 meters each, the top-bottom cooling ratio is calculated from the amount of cooling water of the top surface and the amount of cooling water of the bottom surface for each section, and the top-bottom cooling ratios of the different sections calculated in this way are all controlled to ranges of 0.8 to 1.2. With control of the top-bottom cooling ratio of the range as a whole rather than each divided section, for example, it is extremely difficult to sufficiently suppress the occurrence of uneven cooling due to local overcooling, etc. However, by realizing control of the top-bottom cooling ratio for each section in this way, it becomes possible to reduce the local overcooling, etc., and reliably suppress the occurrence of uneven cooling. Further, the control of the top-bottom cooling ratio for each such section can be performed by any suitable means. While not particularly limited to this, in the same way as the case of primary cooling, for example, several cooling water nozzles are arranged along the advancing direction of the steel sheet at the top side and bottom side of the steel sheet at each section, therefore by suitably spraying based on on / off control of these cooling water nozzles, it becomes possible to relatively easily control the top-bottom cooling ratio of the individual sections to 0.8 to 1.2 in range.[Coiling Step]

[0173] The secondary cooled steel sheet is finally coiled in the coiling step at 50 to 100° C. If the coiling temperature is too low; sometimes the hot rolled steel sheet becomes hard and embrittled. Further, excessive water cooling, etc., becomes required and the productivity falls. Therefore, the coiling temperature is 50° C. or more and preferably is 80° C. or more.

[0174] According to the hot rolled steel sheet produced by the above method of production, when the total width in a direction perpendicular to the rolling direction and the sheet thickness direction is W, it is possible to make the tempered martensite at the ¼ position in sheet thickness at all positions of the 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction 95% or more and achieve a high strength due to the structure mainly comprised of martensite, more specifically a tensile strength of 980 MPa or more while reliably controlling the difference between the maximum value and minimum value of the tensile strength at all the positions in the width direction to 30 MPa or less. Therefore, the variation in strength in the width direction can be remarkably reduced and along with this, it becomes possible to prevent the occurrence of warpage in the width direction of the hot rolled steel sheet and achieve sufficient flatness. Therefore, the high strength hot rolled steel sheet produced by the above method of production, despite being high strength, has homogeneous properties in the width direction. The flatness is extremely good. Further, for example, in hot rolled steel sheet not flattened using a leveler, etc., for example, hot rolled steel sheet right after production, the above high strength and flatness can be achieved, therefore part of the ductility inherently possessed by steel sheet is not consumed by such flattening (preworking) either. For this reason, it is possible to reduce the risk of a defect occurring in the shaping at the time of pressing the steel sheet and possible to remarkably improve the productivity. Therefore, the high strength hot rolled steel sheet is naturally particularly useful for use in the automotive field and can also be used extremely effectively in other fields.

[0175] Below, examples will be used to explain the present invention in more detail, but the present invention is not limited to these examples in any way.EXAMPLES

[0176] In the following examples, the high strength hot rolled steel sheet according to embodiments of the present invention was produced under various conditions. The obtained high strength hot rolled steel sheets were investigated for properties in the width direction and the flatness.

[0177] First, molten steels were cast by the continuous casting method to form slabs having the various chemical compositions shown in Table 1. Each of these slabs was heated under the conditions shown in Table 2 and then hot rolled. The hot rolling was performed by rough rolling and finish rolling. The exit side temperature of the rough rolling and the entry side temperature (F0), exit side temperature (FT), and total rolling reduction of the finish rolling were as shown in Table 2. Next, under the conditions shown in Table 2, the finish rolled steel sheet was subjected to primary cooling first in the temperature region from the exit side temperature (FT) of the finish rolling to the martensite transformation start temperature Ms+50° C., then was subjected to secondary cooling in the temperature region from Ms+50° C. to 200° C.

[0178] At the primary cooling and secondary cooling, the range of FT to (Ms+50)° C. and the range of (Ms+50)° C. to 200° C. was divided into 10 meter sections. The top-bottom cooling ratio was calculated from the amount of cooling water of the top surface and the amount of cooling water of the bottom surface for each of these sections. Cooling was performed so that the top-bottom cooling ratios of the different sections calculated in this way were controlled to within predetermined ranges. The top-bottom cooling ratios at the primary cooling and secondary cooling of Table 2 show the ratios where the absolute values of the differences from the cooling ratio 1 in the top-bottom cooling ratios of the different sections in the primary cooling and the secondary cooling are the largest. Finally, the secondarily cooled steel sheet was coiled under the conditions shown in Table 2 to obtain hot rolled steel sheet having approximately 2.3 to 3.2 mm sheet thickness and 1200 mm total width.TABLE 1SteelChemical composition (mass %) balance: Fe and impuritiesno.CSiMnTiAlBPSNOCuNiSnA0.0740.0512.120.0220.0270.00150.0100.0030.00420.00280.050.04B0.0520.1982.480.0140.0310.00210.0110.0020.00350.0025C0.0980.0501.980.0010.0290.00160.0090.0030.00320.00270.200.15D0.0910.2001.050.0310.0310.00190.0120.0020.00330.00310.20E0.0810.0321.980.0510.0330.00210.0110.0020.00480.0021F0.0670.1102.340.0150.0210.00250.0120.0030.00340.00230.01G0.0710.0522.210.0190.0250.00190.0120.0030.00440.00280.01H0.0720.0611.950.0210.0310.00220.0120.0020.00410.0031I0.0810.0511.830.1180.0410.00150.0110.0020.00450.0033J0.0930.1121.310.0150.0330.00200.0120.0020.00450.00410.030.04K0.0830.0511.800.0310.0250.00210.0110.0020.00450.00310.040.03L0.1090.1112.410.0170.0270.00210.0110.0020.00380.00280.090.05M0.0440.0512.500.0210.0280.00250.0120.0020.00430.0032N0.0910.2502.200.0220.0310.00280.0110.0030.00330.0031O0.0730.1102.600.0010.0320.00300.0110.0020.00310.00310.050.10P0.0980.1100.980.0210.0320.00310.0110.0030.00310.00320.050.03Q0.0850.0501.800.1300.0320.00240.0120.0030.00440.0031SteelChemical composition (mass %) balance: Fe and impuritiesno.CrMoNbVAsZrCaMgBiCoWZnREMA0.200.0021B0.100.090.020.050.01C0.350.200.05D0.400.200.020.011EF0.0050.150.010.0017G0.100.0031H0.050.020.0110.0210.00310.009I0.30J0.380.110.0025K0.310.150.0021M0.00230.020N0.23O0.400.10Q0.0021Underlines indicate outside scope of present invention.TABLE 2Cooling stepHot rolling stepPrimary coolingSecondary coolingRoughFinish rollingFT to (Ms +FT to (Ms +(Ms +(Ms +rollingEntryExit50)° C.50)° C.50)° C. to50)° C. toCoilingHeatingExitsidesideTotalaveragetop-bottom200° C.200° C.stepHeatingsidetemp.temp.rollingcoolingcoolingaveragetop-bottomCoilingTestSteeltemp.temp.F0FTreductionVc + 10speedratiocooling speedcooling ratiotemp.no.no.° C.° C.° C.° C.%° C. / s° C. / s—° C. / s—° C.Remarks 1A1230118110429448721521.1511.190Inv. ex. 2A1230118210419418721511.2143 1.785Comp. ex. 3B1250118810489619015601.2631.280Inv. ex. 4C1250118510539519148541.2551.190Inv. ex. 5D1240118310549528830510.9501.180Inv. ex. 6E1250117110519448829551.1550.880Inv. ex. 7F1250116110439549020510.8561.180Inv. ex. 8G1240115110419428719911.6119 1.570Comp. ex. 9H1240114810369628729122 3.1501.180Comp. ex.10I1260113010459458726421.1131 2.180Comp. ex.11J1230117110419468831320.7320.780Comp. ex.12K1240116010259418820551.0551.080Inv. ex.13L1240117810419538817451.2541.270Comp. ex.14M1230117610429409119410.9511.170Comp. ex.15N1230117710529508718461.2891.280Comp. ex.16O1250117010449528812521.2901.280Comp. ex.17P12301181104694391102521.2521.280Comp. ex.18Q1270118110429419135551.2551.280Comp. ex.19D1235118510519508730191.2501.180Comp. ex.20G1240115310529438719651.1119 1.170Comp. ex.21G1240115510479448719510.6511.180Comp. ex.22G1240116110499458719521.3821.185Comp. ex.23G1240115010459438719501.1451.185Comp. ex.24G1240115110439458719511.1125 1.285Comp. ex.25G1240115310429468719491.1620.680Comp. ex.26G1240115410419428719531.1611.385Comp. ex.Underlines indicate outside scope of present invention or properties which are not preferable.The properties of the obtained hot rolled steel sheet were measured and evaluated by the following methods:[Prior Austenite Grain Size in Microstructure]

[0180] The prior austenite grain size in the microstructure was determined as follows: First, a region of 200 μm×200 μm in an L-cross-section of a steel slab taken at the position of sheet thickness ¼ from a surface of hot rolled steel sheet was analyzed by SEM / EBSD. More specifically, the martensite structure obtained by SEM / EBSD was transformed to a predetermined crystal orientation and the prior austenite grains were reconstructed in an image, next the circle equivalent diameter was sought from a prior austenite grain of the image. This operation was performed for a total of 10 austenite grains. The 10 circle equivalent diameters obtained were averaged to determine the prior austenite grain size.[Variation in Strength in Width Direction]

[0181] First, No. 5 tensile test pieces of JIS Z2241:2011 having directions parallel to the rolling direction as test directions were taken at the positions of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction of the hot rolled steel sheet. Next, these tensile test pieces were used for tensile tests compliant with JIS Z2241:2011 so as to obtain five tensile strengths. Finally, the difference between the maximum value and minimum value of these was calculated to determine the variation in strength in the width direction.[Tensile Strength of Hot Rolled Steel Sheet]

[0182] The minimum value in the above five values of tensile strength is determined as the tensile strength of the hot rolled steel sheet.[Evaluation of Flatness]

[0183] The flatness was evaluated in the following way. First, the obtained hot rolled steel sheet was placed on a surface plate so that at least part of one of the sheet surfaces (bottom surface) contacted the surface plate, then the height of the hot rolled steel sheet from the surface plate at the position with the highest height to the bottom surface of the hot rolled steel sheet was measured, and the obtained measured value was determined as the maximum warpage height H (mm) of the hot rolled steel sheet. As the evaluation of the flatness, cases where the maximum warpage height H was within 10 mm were deemed passing and cases where the maximum warpage height H was more than 10 mm were deemed as failing.

[0184] Cases where the tensile strength of the hot rolled steel sheet was 980 MPa or more and the flatness was evaluated as passing were evaluated as high strength hot rolled steel sheet having improved flatness. The results are shown in Table 3.TABLE 3PropertiesMicrostructureMin. tensiletMα + B + PtMfMα + B + Pstrength inMax.VariationPrior γfractionfMfractionfractionfractionfractionwidth directiontensilein tensileMax.grainof min.fractionof minof max.of max.of max(tensile strengthstrengthstrengthwarpagesizestrengthof min.strengthstrengthstrengthstrengthof hot rolledin widthin widthheightTestSteeldgportionstrengthportionportionportionportionsteel sheet)directiondirectionHno.no.μm%portion%%%%MPaMPaMPammRemarks 1A32980299101022 104119 6Inv. ex. 2A31100 0093701025 10704515Comp. ex. 3B21100 009820981100827 8Inv. ex. 4C23961398111072 109119 9Inv. ex. 5D28980299101055 107116 7Inv. ex. 6E33981198201045 106116 6Inv. ex. 7F3496139730991102130 9Inv. ex. 8G349217928097010437317Comp. ex. 9H269118991098110375614Comp. ex.10I2299108812 01043 10854216Comp. ex.11J339316961399010334314Comp. ex.12K34990199101031 104514 3Inv. ex.13L3096409010 01105 11504511Comp. ex.14M2896139631910 94030 9Comp. ex.15N32981193701050 11015111Comp. ex.16O3195509010 01020 10513111Comp. ex.17P—40555 60535 772 8124011Comp. ex.18Q189028952399110404911Comp. ex.19D—60337 70327 862 8922810Comp. ex.20G349316982097810507216Comp. ex.21G339217971197910345512Comp. ex.22G349316981197810416313Comp. ex.23G349127961397110215011Comp. ex.24G339136946097510618614Comp. ex.25G349415971297410154112Comp. ex.26G349415972198110133211Comp. ex.Underlines indicate outside scope of present invention or properties which are not preferable.

[0185] Referring to Tables 1 to 3, in each of Comparative Examples 2 and 10, the average cooling speed of the secondary cooling was high and, further, the top-bottom cooling ratio of the secondary cooling was not suitable, therefore the martensite could not be sufficiently tempered by autotempering at the time of cooling, the ratio of fresh martensite (IM) became higher and, further, the microstructure could not be made uniform in the width direction due to the occurrence of uneven cooling and the variation in tensile strength in the width direction became remarkable. As a result, sufficient flatness could not be achieved. In Comparative Example 8, the average cooling speed of the primary cooling was high and, further, the top-bottom cooling ratio of the primary cooling and the secondary cooling was not suitable, therefore the desired microstructure could not be made uniform in the width direction due to the occurrence of uneven cooling. In relation to this, a tensile strength of 980 MPa or more could not be achieved and, further, the variation in tensile strength in the width direction became remarkable. As a result, sufficient flatness could not be achieved. In Comparative Example 9, the average cooling speed of the primary cooling was high and, further, the top-bottom cooling ratio of the primary cooling was not suitable, therefore it was not possible to make the desired microstructure uniform in the width direction due to the occurrence of uneven cooling and the variation in tensile strength in the width direction became remarkable. As a result, sufficient flatness could not be achieved. In Comparative Example 11, the top-bottom cooling ratio of the primary cooling and secondary cooling was not suitable, therefore it was not possible to make the desired microstructure uniform in the width direction due to the occurrence of uneven cooling and along with this, the variation in tensile strength in the width direction became remarkable. As a result, sufficient flatness could not be achieved. In Comparative Example 13, the C content was high, therefore, variation in tensile strength in the width direction could not be suppressed to within a predetermined range, and the flatness fell. In Comparative Example 14, the C content was low, therefore the desired tensile strength could not be achieved. In Comparative Example 15, the Si content was high, therefore it is believed that the autotempering at the time of cooling of the steel sheet was suppressed. As a result, the ratio of fresh martensite (fM) in the microstructure became higher and, along with this, the variation in tensile strength in the width direction could not be 30) sufficiently reduced and the flatness fell. In Comparative Example 16, the Mn content was high, therefore, it is believed martensite could not be sufficiently tempered even due to the autotempering at the time of cooling of the steel sheet due to the improvement in hardenability. As a result, the ratio of fresh martensite (fM) in the microstructure became higher. Along with this, the variation in tensile strength in the width direction could not be sufficiently reduced and the flatness fell. In Comparative Example 17, the Mn content was low, therefore the desired tensile strength could not be achieved. Further, the Mn content was low, therefore it is believed the hardenability was insufficient and relatively large amounts of ferrite and other soft phases were formed at the time of cooling. As a result, it was not possible to make a structure mainly comprised of tempered martensite uniform in the width direction, further, it is believed the steel sheet became defective in shape due to the expansion of the steel sheet accompanying transformation to ferrite, etc., and the flatness fell. In Comparative Example 18, the Ti content was high, therefore relatively large amounts of ferrite and other soft phases were formed from the nonrecrystallized austenite at the time of the cooling step. In relation to this, it was not possible to form the desired microstructure in the width direction and as a result it was not possible to sufficiently reduce variation in the tensile strength in the width direction and the flatness fell.

[0186] In Comparative Example 19, the average cooling speed of the primary cooling was low, therefore the at least one of ferrite, upper bainite, and pearlite becomes more than 5 area % in total and the desired tensile strength could not be achieved. In Comparative Example 20, the average cooling speed of the primary cooling was high, therefore it was not possible to make the desired microstructure uniform in the width direction due to the occurrence of uneven cooling and, in relation to this, the variation in tensile strength in the width direction also became conspicuous. As a result, it was not possible to achieve a sufficient flatness. In each of Comparative Examples 21 and 22, the top-bottom cooling ratio of the primary cooling was not suitable, therefore the desired microstructure could not be made uniform in the width direction due to the occurrence of uneven cooling and along with this, the variation in tensile strength in the width direction also became conspicuous. As a result, sufficient flatness could not be achieved. In Comparative Example 23, the average cooling speed of the secondary cooling was low, therefore the desired microstructure could not be obtained in the width direction of the steel sheet and along with this, the variation in tensile strength in the width direction also became conspicuous. As a result, sufficient flatness could not be achieved. In Comparative Example 24, the average cooling speed of the secondary cooling was high, therefore it was not possible to sufficiently temper the martensite by autotempering at the time of cooling, the ratio of fresh martensite (fM) became higher, further, the microstructure could not be made uniform in the width direction due to the occurrence of uneven cooling, and the variation in tensile strength in the width direction became conspicuous. As a result, sufficient flatness could not be achieved. In each of Comparative Examples 25 and 26, the top-bottom cooling ratio of the secondary cooling was not suitable, therefore the desired microstructure could not be made uniform in the width direction due to the occurrence of uneven cooling, and along with this, the variation in tensile strength in the width direction also became conspicuous. As a result, sufficient flatness could not be achieved.

[0187] In contrast to this, in the hot rolled steel sheets according to all of the invention examples, by having a predetermined chemical composition and in particular suitably controlling the conditions of the cooling step in the method of production, it was possible to make the tempered martensite in the microstructure at a ¼ position in sheet thickness at all positions of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction an area % of 95% or more and achieve a tensile strength of 980 MPa or more due to the structure mainly comprised of tempered martensite. In addition, it became possible to reliably control the difference between the maximum value and minimum value in the tensile strengths at all the positions in the width directions to 30 MPa or less. Therefore, it became possible to remarkably reduce the variation of strength in the width direction. Along with this, it was possible to keep warpage from occurring in the width direction of the hot rolled steel sheet and to achieve sufficient flatness. In Table 3, only the microstructures of portions at the 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction where the minimum tensile strength was obtained and portions where the maximum tensile strength was obtained are shown. However, in each of Invention Examples 1, 3 to 7, and 12, at all of these positions, the microstructure at a ¼ position in sheet thickness contained, by area ratio, tempered martensite: 95% or more, fresh martensite: 5% or less, and at least one of ferrite, upper bainite, and pearlite: 5% or less in total.

Examples

examples

[0176]In the following examples, the high strength hot rolled steel sheet according to embodiments of the present invention was produced under various conditions. The obtained high strength hot rolled steel sheets were investigated for properties in the width direction and the flatness.

[0177]First, molten steels were cast by the continuous casting method to form slabs having the various chemical compositions shown in Table 1. Each of these slabs was heated under the conditions shown in Table 2 and then hot rolled. The hot rolling was performed by rough rolling and finish rolling. The exit side temperature of the rough rolling and the entry side temperature (F0), exit side temperature (FT), and total rolling reduction of the finish rolling were as shown in Table 2. Next, under the conditions shown in Table 2, the finish rolled steel sheet was subjected to primary cooling first in the temperature region from the exit side temperature (FT) of the finish rolling to the martensite transf...

FIELD

[0001] The present invention relates to a high strength hot rolled steel sheet.BACKGROUND

[0002] In recent years, in the auto industry, steel sheet has been made higher in strength from the viewpoint of reducing the environmental load and securing safety of occupants. Along with the higher strength of steel sheet, the main structures forming the microstructure of steel sheet have been becoming martensite structures.

[0003] For example, PTL 1 describes high strength hot rolled steel sheet excellent in low temperature toughness having a composition containing, by mass %, C: 0.08% or more and less than 0.16%, Si: 0.01 to 1.0%, Mn: 0.8 to 2.0%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.002 to 0.006%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, Cr: 0.01 to 1.0%, and B: 0.0005 to 0.0050% and having a balance of Fe and unavoidable impurities and having structures mainly comprising martensite phases or tempered martensite phases, having the main phases present in a volume ratio with respect to the structure as a whole of 90% or more, having an average grain size of the prior austenite grains of 20 μm or less in a cross-section parallel to the rolling direction and 15 μm or less in a cross-section perpendicular to the rolling direction, and having an aspect ratio of the prior austenite grains in the cross-section parallel to the rolling direction of 18 or less. Further, PTL 1 describes that, according to the above constitution, it is easy to produce hot rolled steel sheet having a high strength of a yield strength YS: 960 MPa or more and a high toughness of a vE−40 of 40 J or more without containing expensive Mo and further being excellent in bendability and delayed fracture resistance, further having a surface hardness of a Brinell hardness of 360 HB or more, being excellent in wear resistance, and being optimal for use as a structural member of a construction machine or industrial machine and that a remarkable effect is exhibited in industry.

[0004] PTL 2 describes high strength hot rolled steel sheet having a chemical composition containing, by mass %, C: 0.05 to 0.14%, Si: 0.01 to 1.0%, Mn: 0.50 to 2.0%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.002 to 0.006%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, Cr: 0.01 to 1.0%, and B: 0.0005 to 0.0050% and having a balance of Fe and unavoidable impurities and having a steel structure comprised of at least one of martensite phases and tempered martensite phases as main phases, having those main phases in an area ratio with respect to the steel structure as a whole of 95% or more, containing cementite with an average grain size of 0.5 μm or less in the laths of the martensite phases and / or tempered martensite phases, and having a content of cementite by mass % of 0.01 to 0.08%. Further. PTL 2 describes that, according to this constitution, it is possible to provide high strength hot rolled steel sheet high in toughness and excellent in punchability and punch bending fatigue strength even without containing the expensive alloying element Mo, specifically high strength hot rolled steel sheet having a high strength of a tensile strength TS: 980 MPa or more and having a high toughness of an absorption energy vE−40 of 40 J or more in a Charpy impact test at a test temperature of-40° C. and, furthermore, excellent in punchability and punch bending fatigue strength.

[0005] PTL 3 describes high strength hot rolled steel sheet excellent in uniformity of strength in the sheet width direction having a chemical composition containing, by mass %, C: 0.10 to 0.25%, Si: 0.10% or less, Mn: 1.0 to 2.0%, P: 0.025% or less, S: 0.005% or less, Al: 0.005 to 0.10%, Nb: 0.01 to 0.05%, Ti: 0.005 to 0.05%, Cr: 0.05 to 1.0%, and B: 0.0005 to 0.0050% and having a balance of Fe and unavoidable impurities, having a structure with tempered martensite phases comprising 95% or more by volume ratio with respect to the structure as a whole and with an average grain size of prior austenite grains of 20 μm or less in the cross-section parallel to the rolling direction and 15 μm or less in the cross-section perpendicular to the rolling direction, and having a yield strength (YS) of 960 MPa or more. Further, PTL 3 teaches that by making the structure of the steel sheet over the entire region in the width direction a structure having main phases of tempered martensite, having an average grain size of prior austenite (Y) grains in the cross-section parallel to the rolling direction of 20 μm or less, and having an average grain size of prior austenite grains in the cross-section perpendicular to the rolling direction of 15 μm or less, high strength hot rolled steel sheet with a high strength of a yield strength YS: 960 MPa or more uniformly in the width direction is obtained.CITATIONS LISTPatent Literature

[0006] [PTL 1] Japanese Unexamined Patent Publication No. 2016-211073

[0007] [PTL 2] Japanese Unexamined Patent Publication No. 2018-188675

[0008] [PTL 3] Japanese Unexamined Patent Publication No. 2016-183414SUMMARYTechnical Problem

[0009] In the hot rolled steel sheet such as described in PTLs 1 to 3, if making the microstructure a structure of just martensite phases or closer to just martensite phases so as to raise the strength, sometimes the steel sheet is defective in shape due to uneven cooling at the time of cooling or transformation plasticity at the time of transformation. In such a case, it is difficult to maintain a sufficient flatness in the obtained hot rolled steel sheet. For example, PTL 3 studies a high strength hot rolled steel sheet with uniform yield strength YS in the width direction, but does not specifically study anything from the viewpoint of improving the flatness of the high strength hot rolled steel sheet.

[0010] Therefore, the present invention has as its object the provision of a high strength hot rolled steel sheet having improved flatness by a novel constitution.Solution to Problem

[0011] The inventors studied how to achieve the above object while focusing in particular on the microstructure in the width direction of a hot rolled steel sheet. As a result, the inventors discovered that by making the microstructure of a hot rolled steel sheet having a predetermined chemical composition a structure mainly comprised of tempered martensite, it is possible to secure high strength and that by making the structure uniform in the width direction, it is possible to reduce the variation of strength in the width direction. In this way, by reducing the variation in strength in the width direction, it is possible to provide a hot rolled steel sheet remarkably improved in flatness in the width direction of the hot rolled steel sheet.

[0012] The present invention able to achieve this object is as follows:

[0013] (1) A high strength hot rolled steel sheet comprising a chemical composition comprising, by mass %,

[0014] C: 0.050 to 0.100%,

[0015] Si: 0.010 to 0.200%,

[0016] Mn: 1.00 to 2.50%,

[0017] Ti: 0.001 to 0.120%,

[0018] Al: 0.001 to 0.050%,

[0019] B: 0.0005 to 0.0050%,

[0020] P: 0.100% or less,

[0021] S: 0.050% or less,

[0022] N: 0.0050% or less,

[0023] 0:0 to 0.0050%,

[0024] Cu: 0 to 0.20%,

[0025] Ni: 0 to 0.20%,

[0026] Sn: 0 to 0.10%,

[0027] Cr: 0 to 0.40%,

[0028] Mo: 0 to 0.20%,

[0029] Nb: 0 to 0.05%,

[0030] V: 0 to 0.10%,

[0031] As: 0 to 0.100%,

[0032] Zr: 0 to 0.100%,

[0033] Ca: 0 to 0.0050%,

[0034] Mg: 0 to 0.100%,

[0035] Bi: 0 to 0.020%,

[0036] Co: 0 to 0.20%,

[0037] W: 0 to 0.20%,

[0038] Zn: 0 to 0.20%,

[0039] REM: 0 to 0.1000%, and

[0040] balance: Fe and impurities, wherein

[0041] when a total width in a direction perpendicular to a rolling direction and sheet thickness direction is W, a microstructure at a ¼ position in sheet thickness at all positions of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in a width direction comprises, by area %,

[0042] tempered martensite: 95% or more,

[0043] fresh martensite: 5% or less, and

[0044] at least one of ferrite, upper bainite, and pearlite: 5% or less in total, and

[0045] a difference between a maximum value and minimum value in tensile strength at all the positions in the width direction is 30 MPa or less.

[0046] (2) The high strength hot rolled steel sheet according to the above (1), wherein the chemical composition contains, by mass %, at least one of

[0047] O: 0.0001 to 0.0050%,

[0048] Cu: 0.001 to 0.20%,

[0049] Ni: 0.001 to 0.20%,

[0050] Sn: 0.001 to 0.10%,

[0051] Cr: 0.001 to 0.40%,

[0052] Mo: 0.001 to 0.20%,

[0053] Nb: 0.001 to 0.05%,

[0054] V: 0.001 to 0.10%,

[0055] As: 0.001 to 0.100%,

[0056] Zr: 0.0001 to 0.100%,

[0057] Ca: 0.0001 to 0.0050%,

[0058] Mg: 0.0001 to 0.100%,

[0059] Bi: 0.0001 to 0.020%,

[0060] Co: 0.001 to 0.20%,

[0061] W: 0.001 to 0.20%,

[0062] Zn: 0.001 to 0.20%, and

[0063] REM: 0.0001 to 0.1000%.

[0064] (3) The high strength hot rolled steel sheet according to the above (1) or (2), wherein a prior austenite grain size in the microstructure is 40 μm or less.Advantageous Effects of Invention

[0065] According to the present invention, it is possible to provide a high strength hot rolled steel sheet having improved flatness.DESCRIPTION OF EMBODIMENTS<High Strength Hot Rolled Steel Sheet>

[0066] The high strength hot rolled steel sheet according to the embodiments of the present invention is characterized by having a chemical composition comprising, by mass %,

[0067] C: 0.050 to 0.100%,

[0068] Si: 0.010 to 0.200%,

[0069] Mn: 1.00 to 2.50%,

[0070] Ti: 0.001 to 0.120%,

[0071] Al: 0.001 to 0.050%,

[0072] B: 0.0005 to 0.0050%,

[0073] P: 0.100% or less,

[0074] S: 0.050% or less,

[0075] N: 0.0050% or less,

[0076] O: 0 to 0.0050%,

[0077] Cu: 0 to 0.20%,

[0078] Ni: 0 to 0.20%,

[0079] Sn: 0 to 0.10%,

[0080] Cr: 0 to 0.40%,

[0081] Mo: 0 to 0.20%,

[0082] Nb: 0 to 0.05%,

[0083] V: 0 to 0.10%,

[0084] As: 0 to 0.100%,

[0085] Zr: 0 to 0.100%,

[0086] Ca: 0 to 0.0050%,

[0087] Mg: 0 to 0.100%,

[0088] Bi: 0 to 0.020%,

[0089] Co: 0 to 0.20%,

[0090] W: 0 to 0.20%,

[0091] Zn: 0 to 0.20%,

[0092] REM: 0 to 0.1000%, and

[0093] balance: Fe and impurities, wherein

[0094] when a total width in a direction perpendicular to a rolling direction and sheet thickness direction is W, a microstructure at a ¼ position in sheet thickness at all positions of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in a width direction comprises, by area %,

[0095] tempered martensite: 95% or more,

[0096] fresh martensite: 5% or less, and

[0097] at least one of ferrite, upper bainite, and pearlite: 5% or less in total, and

[0098] a difference between a maximum value and minimum value in tensile strength at all the positions in the width direction is 30 MPa or less.

[0099] As explained above, if increasing the strength in hot rolled steel sheet by making the microstructure a structure of just martensite phases or close to just martensite phases, sometimes the steel sheet becomes defective in shape due to uneven cooling at the time of cooling or transformation plasticity at the time of transformation. In such a case, it becomes difficult to maintain a sufficient flatness in the obtained hot rolled steel sheet. Regarding the defective shape of the steel sheet, specifically sometimes warpage occurs in the width direction of the steel sheet (direction perpendicular to the rolling direction and the sheet thickness direction). If warpage occurs in the width direction of the steel sheet, when utilizing such steel sheet to form a long member, sometimes this becomes a cause of shape defects or cracking at the time of shaping. As an existing technique for alleviating such warpage, use of a leveler, etc., for correction (flattening) may be mentioned. However, if the strength of steel sheet becomes higher, sometimes sufficient correction is not necessarily possible by a leveler, etc. Further, if ending up performing preworking by correction by a leveler, etc., such preworking causes part of the ductility inherently possessed by steel sheet to be consumed and the residual ductility falls. If the residual ductility falls, defects in shaping easily arise at the time of pressing the steel sheet and as a result the productivity falls. For this reason, good flatness is sought in the state of steel sheet as hot rolled even without correction by a leveler, etc. As reasons for steel sheet to become defective in shape, mainly a fast cooling speed of finish rolling may be mentioned. If the cooling speed after finish rolling is fast, the controllability of the amount of water used for cooling deteriorates and locally overcooled parts are formed, etc., causing remarkable unevenness of cooling. As a result, due to the uneven temperature in the width direction of the steel sheet, a heat stress occurs and warpage occurs in the width direction of the steel sheet. For this reason, from the viewpoint of securing flatness, it is not necessarily suitable to excessively increase the cooling speed after finish rolling. On the other hand, to obtain a structure of just martensite phases or close to just martensite phases from the viewpoint of higher strength, it is necessary to cool by a cooling speed of the critical cooling speed or more.

[0100] Therefore, the inventors engaged in studies to achieve higher strength while improving the flatness of steel sheet by selecting suitable steel compositions, in particular focusing on the microstructure in the width direction of the hot rolled steel sheet. First, the inventors discovered that by making the microstructure of hot rolled steel sheet having a predetermined chemical composition a structure mainly comprised of tempered martensite, it is possible to achieve high strength, more specifically 980 MPa or more high strength. Next, the inventors thought that making such a structure mainly comprised of tempered martensite uniform in the width direction would be effective and conducted further studies accordingly. First, to make the microstructure uniform in the width direction, it is necessary to suitably control it after finish rolling. Further, to form a structure mainly comprised of tempered martensite, it is necessary to adroitly utilize autotempering at the time of cooling. Therefore, the inventors discovered that limiting the Si, which is able to delay or suppress autotempering, to a content of 0.200 mass % or less in the steel sheet, it is possible to promote autotempering at the time of cooling while by making the amount of cooling water sprayed at the steel sheet at the time of cooling equal at the top surface and bottom surface of the steel sheet from after finish rolling to a temperature corresponding to the end temperature of martensite transformation as explained in detail later regarding the method of production of hot rolled steel sheet, i.e., by evenly cooling the top and bottom surfaces of the steel sheet by cooling water from after finish rolling to about 200° C. it is possible to make a structure comprised of tempered martensite in 95 area % or more uniform over the entire width direction of the hot rolled steel sheet. More specifically, the inventors discovered in particular that by making the Si content of the hot rolled steel sheet 0.200 mass % or less and suitably controlling the cooling after finish rolling as explained above, when a total width in a direction perpendicular to a rolling direction and sheet thickness direction is W, it is possible to make a microstructure at a ¼ position in sheet thickness at all positions of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction a structures comprised of, by area %, tempered martensite: 95% or more. As a result, the inventors discovered that it is possible to sufficiently reduce the variation in strength in the width direction of hot rolled steel sheet at a level so that a difference between a maximum value and minimum value in tensile strength at all the positions in the width direction becomes 30 MPa or less and that, along with the uniformity of the microstructure in the width direction as a whole and reduction of such variation in strength, it is possible to remarkably improve the flatness of the hot rolled steel sheet regardless of the high strength.

[0101] In the past, in cooling after the finish rolling, in general the amount of water sprayed on the top surface of steel sheet was greater than the amount of water sprayed on the bottom surface of the steel sheet. Therefore, by performing the cooling at the top and bottom surfaces of the steel sheet evenly from after the finish rolling until a temperature corresponding to the martensite transformation end temperature, it is possible to achieve uniformity of the microstructure and reduce the variation in strength in the width direction such as explained above and thereby possible to remarkably improve the flatness of hot rolled steel sheet despite the high strength. This fact was first made clear this time by the inventors. Further, according to the high strength hot rolled steel sheet according to the embodiments of the present invention, since there is sufficient flatness in the state as hot rolled, there is no need for preworking by a leveler, etc., and therefore part of the ductility inherently possessed by steel is not consumed by such preworking either. Along with this, it is possible to reduce the risk of defects in shaping at the time of press-forming such high strength hot rolled steel sheet and becomes possible to remarkably improve the productivity as well. Therefore, the high strength hot rolled steel sheet according to the embodiments of the present invention is only naturally particularly useful in use in the automotive field and can also be extremely effectively used in other fields as well.

[0102] Below, the high strength hot rolled steel sheet according to the embodiments of the present invention will be explained in more detail. In the following explanation, the “%” of the units of contents of the elements, unless otherwise indicated, means “mass %”. Further, in the Description, “to” showing a numerical range, unless otherwise indicated, is used in the sense including the numerical values entered before and after it as a lower limit value and upper limit value.[C: 0.050 to 0.100%]

[0103] C is an element effective for raising the strength of steel sheet. To sufficiently obtain such an effect, the C content is 0.050% or more. The C content may also be 0.055% or more, 0.060% or more, 0.065% or more, or 0.070% or more. On the other hand, if excessively containing C, due to an excessive increase in strength, it becomes difficult to control the variation in strength in the width direction to within a predetermined range. Therefore, the C content is 0.100% or less. The C content may also be 0.095% or less, 0.090% or less, 0.085% or less, or 0.080% or less.[Si: 0.010 to 0.200%]

[0104] Si is an element effective for raising strength as a solution strengthening element. To sufficiently obtain such an effect, the Si content is 0.010% or more. The Si content may also be 0.020% or more, 0.040% or more, 0.060% or more, 0.080% or more, or 0.100% or more. On the other hand, if excessively containing Si, sometimes the autotempering at the time of cooling the steel sheet is delayed or inhibited. In such a case, hot rolled steel sheet having the desired microstructure can no longer be obtained. Therefore, the Si content is 0.200% or less. The Si content may also be 0.180% or less, 0.160% or less, 0.140% or less, or 0.120% or less.[Mn: 1.00 to 2.50%]

[0105] Mn is an element effective for raising the hardenability and the strength as a solution strengthening element. If the Mn content is low, the hardenability becomes insufficient, therefore ferrite and other soft phases are formed in relatively large amounts at the time of cooling and it becomes no longer possible to make a structure having tempered martensite as its main phase uniform in the width direction. Further, due to the expansion of the steel sheet accompanying such transformation, warpage occurs in the width direction and sometimes the steel sheet is defective in shape. Therefore, the Mn content is 1.00% or more. The Mn content may also be 1.20% or more, 1.40% or more, 1.60% or more, or 1.80% or more. On the other hand, if excessively containing Mn, due to the improvement in the hardenability, the martensite is not sufficiently tempered even by the autotempering at the time of cooling of the steel sheet and sometimes fresh martensite cannot be sufficiently reduced in the final microstructure. Therefore, the Mn content is 2.50% or less. The Mn content may also be 2.40% or less, 2.20% or less, 2.00% or less, or 1.90% or less.[Ti: 0.001 to 0.120%]

[0106] Ti is an element contributing to improvement of strength by precipitation strengthening, etc. Further, Ti bonds with N to form titanium nitride (TiN) and thereby consumes the dissolved N in the steel and has the action of suppressing the reduction in the amount of dissolved B due to the formation of BN. To sufficiently obtain these effects, the Ti content is 0.001% or more. The Ti content may also be 0.010% or more, 0.020% or more, 0.040% or more, or 0.060% or more. On the other hand, Ti is also an element suppressing recrystallization of austenite, therefore if excessively containing Ti, due to the presence of nonrecrystallized austenite including relatively large numbers of dislocations, the force driving ferrite transformation, etc., at the time of cooling is raised, ferrite and other soft phases are easily formed from the nonrecrystallized austenite, and sometimes the desired microstructure cannot be obtained. Therefore, the Ti content is 0.120% or less. The Ti content may also be 0.110% or less, 0.100% or less, 0.090% or less, or 0.080% or less.[Al: 0.001 to 0.050%]

[0107] Al is an element acting as a deoxidizer. To sufficiently obtain such an effect, the Al content is 0.001% or more. The Al content may also be 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if excessively containing Al, coarse oxides are formed and sometimes the toughness is lowered. Therefore, the Al content is 0.050% or less. The Al content may also be 0.045% or less or 0.040% or less.[B: 0.0005 to 0.0050%]

[0108] B is an element raising the hardenability of steel and contributing to improvement of the strength. To sufficiently obtain such effects, the B content is 0.0005% or more. The B content may also be 0.0008% or more, 0.0010% or more, 0.0015% or more, or 0.0020% or more. On the other hand, if excessively containing B, sometimes the toughness and / or weldability fall. Therefore, the B content is 0.0050% or less. The B content may also be 0.0045% or less, 0.0040% or less, 0.0030% or less, or 0.0025% or less.[P: 0.100% or Less]

[0109] P, if excessively contained, sometimes affects weldability, etc., disadvantageously. Therefore, the P content is 0.100% or less. The P content may also be 0.080% or less, 0.050% or less, 0.030% or less, or 0.020% or less. The lower limit of the P content is not particularly prescribed and may also be 0%, but excessive reduction invites a rise in costs. Therefore, the P content may also be 0.0001% or more, 0.0005% or more, or 0.001% or more.[S: 0.050% or Less]

[0110] S, if excessively contained, forms a large amount of MnS and sometimes lowers the toughness. Therefore, the S content is 0.050% or less. The S content may also be 0.020% or less, 0.010% or less, or 0.005% or less. The lower limit of the S content is not particularly prescribed and may also be 0%, but excessive reduction invites a rise in costs. Therefore, the S content may also be 0.0001% or more, 0.0005% or more, or 0.001% or more.[N: 0.0050% or Less]

[0111] N, if excessively contained, forms coarse nitrides and sometimes causes the toughness to fall. Further, N bonds with the B in the steel to form boronitrides (BN) and thereby reduce the amount of dissolved B and sometimes lowers the effect of improvement of hardenability due to the addition of B. Therefore, the N content is preferably as low as possible and is 0.0050% or less. The N content may also be 0.0045% or less, 0.0040% or less, or 0.0035% or less. The lower limit of the N content is not particularly prescribed and may also be 0%, but excessive reduction invites a rise in costs. Therefore, the N content may also be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0112] The basic chemical composition of the high strength hot rolled steel sheet according to the embodiments of the present invention is as described above. Furthermore, the high strength hot rolled steel sheet may also contain at least one of the following optional elements in place of part of the Fe of the balance in accordance with need. For example, the high strength hot rolled steel sheet may contain at least one of O: 0 to 0.0050%, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Sn: 0 to 0.10%, Cr: 0 to 0.40%, Mo: 0 to 0.20%, Nb: 0 to 0.05%, V: 0 to 0.10%, As: 0 to 0.100%, Zr: 0 to 0.100%, Ca: 0 to 0.0050%, Mg: 0 to 0.100%, Bi: 0 to 0.020%, Co: 0 to 0.20%, W: 0 to 0.20%, Zn: 0 to 0.20%, and REM: 0 to 0.1000%. Below, these optional elements will be explained in detail.[O: 0 to 0.0050%]

[0113] O is an element entering in the process of production. The O content may also be 0%. However, to reduce the O content to less than 0.0001%, time is required for refining and a drop in productivity is invited. Therefore, the O content may also be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if excessively containing O, coarse inclusions are formed and sometimes the toughness of the steel sheet is lowered. Therefore, the O content is preferably 0.0050% or less. The O content may also be 0.0040% or less, 0.0035% or less, or 0.0030% or less.[Cu: 0 to 0.20%]

[0114] Cu is an element contributing to improvement of strength and / or corrosion resistance. The Cu content may also be 0%, but to obtain these effects, the Cu content is preferably 0.001% or more. The Cu content may also be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if excessively containing Cu, deterioration of the toughness and weldability is sometimes invited. Therefore, the Cu content is preferably 0.20% or less. The Cu content may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, 0.08% or less, or 0.06% or less.[Ni: 0 to 0.20%]

[0115] Ni is an element raising the hardenability of steel and contributing to improvement of the strength and / or corrosion resistance. The Ni content may also be 0%, but to obtain these effects, the Ni content is preferably 0.001% or more. The Ni content may also be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, even if excessively including Ni, the effects become saturated and a rise in production costs is invited. Therefore, the Ni content is preferably 0.20% or less. The Ni content may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, 0.08% or less, or 0.06% or less.[Sn: 0 to 0.10%]

[0116] Sn is an element effective for improvement of the corrosion resistance. The Sn content may also be 0%, but to obtain such an effect, the Sn content is preferably 0.001% or more and may also be 0.005% or more, 0.01% or more, or 0.02% or more. On the other hand, if excessively containing Sn, sometimes a fall in the toughness is invited. Therefore, the Sn content is preferably 0.10% or less. The Sn content may also be 0.08% or less, 0.06% or less, or 0.04% or less.[Cr: 0 to 0.40%]

[0117] Cr is an element raising the hardenability of steel and contributing to improvement of the strength and / or corrosion resistance. The Cr content may also be 0%, but to obtain these effects, the Cr content is preferably 0.001% or more. The Cr content may also be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, even if excessively including Cr, the effects become saturated and a rise in production costs is invited. Therefore, the Cr content is preferably 0.40% or less. The Cr content may also be 0.30% or less, 0.20% or less, 0.15% or less, or 0.12% or less.[Mo: 0 to 0.20%]

[0118] Mo is an element raising the hardenability of steel and contributing to improvement of the strength and an element contributing also to improvement of corrosion resistance. The Mo content may also be 0%, but to obtain these effects, the Mo content is preferably 0.001% or more. The Mo content may also be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if excessively containing Mo, the deformation resistance at the time of hot working increases and sometimes the load on the facilities becomes greater. Therefore, the Mo content is preferably 0.20% or less. The Mo content may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.[Nb: 0 to 0.05%]

[0119] Nb is an element forming carbides, nitrides, and / or carbonitrides in the steel and contributes to refinement of the structure and in turn higher strength of the steel sheet by a pinning effect. The Nb content may also be 0%, but to obtain such an effect, the Nb content is preferably 0.001% or more. The Nb content may also be 0.005% or more or 0.01% or more. On the other hand, if excessively containing Nb, coarse carbides, etc., are formed in the steel and sometimes the toughness of the steel sheet is lowered. Therefore, the Nb content is 0.05% or less. The Nb content may also be 0.04% or less, 0.03% or less, or 0.02% or less.[V: 0 to 0.10%]

[0120] V is an element contributing to improvement of strength by precipitation strengthening, etc. The V content may also be 0%, but to obtain such an effect, the V content is preferably 0.001% or more. The V content may also be 0.005% or more, 0.01% or more, or 0.02% or more. On the other hand, if excessively containing V, a large amount of precipitates is formed and sometimes the toughness is lowered. Therefore, the V content is preferably 0.10% or less. The V content may also be 0.08% or less, 0.06% or less, or 0.04% or less.[As: 0 to 0.100%]

[0121] As is an element effective for improvement of the corrosion resistance. The As content may also be 0%, but to obtain such an effect, the As content is preferably 0.001% or more and may be 0.005% or more, 0.008% or more, or 0.010% or more. On the other hand, even if excessively containing As, the effect become saturated and a rise in the production costs is invited. Therefore, the As content is preferably 0.100% or less. The As content may also be 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.[Zr: 0 to 0.100%]

[0122] Zr is an element able to control the form of sulfides. The Zr content may also be 0%, but to obtain such an effect, the Zr content is preferably 0.0001% or more. The Zr content may also be 0.0005% or more, 0.001% or more, or 0.010% or more. On the other hand, even if excessively containing Zr, the effect become saturated and a rise in the production costs is invited. Therefore, the Zr content is preferably 0.100% or less. The Zr content may also be 0.050% or less, 0.030% or less, or 0.020% or less.[Ca: 0 to 0.0050%]

[0123] Ca is an element able to control the form of sulfides. The Ca content may also be 0%, but to obtain such an effect, the Ca content is preferably 0.0001% or more. The Ca content may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if excessively containing Ca, the effect becomes saturated and a rise in production costs is invited. Therefore, the Ca content is preferably 0.0050% or less. The Ca content may also be 0.0040% or less, 0.0030% or less, or 0.0020% or less.[Mg: 0 to 0.100%]

[0124] Mg is an element able to control the form of sulfides. The Mg content may also be 0%, but to obtain such an effect, the Mg content is preferably 0.0001% or more and may be 0.001% or more, 0.005% or more, or 0.008% or more. On the other hand, even if excessively containing Mg, the effect becomes saturated and a rise in production costs is invited. Therefore, the Mg content is preferably 0.100% or less. The Mg content may also be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.[Bi: 0 to 0.020%]

[0125] Bi is an element effective for improvement of the corrosion resistance. The Bi content may also be 0%, but to obtain such an effect, the Bi content is preferably 0.0001% or more. The Bi content may also be 0.0005% or more, 0.001% or more, or 0.003% or more. On the other hand, if excessively containing Bi, the effect becomes saturated and a rise in production costs is invited. Therefore, the Bi content is preferably 0.020% or less. The Bi content may also be 0.010% or less, 0.008% or less, or 0.005% or less.[Co: 0 to 0.20%]

[0126] Co is an element contributing to improvement of the hardenability and / or heat resistance. The Co content may also be 0%, but to obtain these effects, the Co content is preferably 0.001% or more. The Co content may also be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if excessively containing Co, sometimes the hot workability falls and an increase in raw material cost results. Therefore, the Co content is preferably 0.20% or less. The Co content may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.[W: 0 to 0.20%]

[0127] W is an element raising the hardenability of steel and contributing to improvement of the strength. The W content may also be 0%, but to obtain such an effect, the W content is preferably 0.001% or more. The W content may also be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if excessively containing W, sometimes the weldability falls. Therefore, the W content is preferably 0.20% or less. The W content may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.[Zn: 0 to 0.20%]

[0128] Zn is an element effective for controlling the form of inclusions. To obtain such an effect, the Zn content is preferably 0.001% or more. The Zn content may also be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, even if excessively containing Zn, the effect becomes saturated and a rise in production costs is invited. Therefore, the Zn content is preferably 0.20% or less. The Zn content may also be 0.18% or less, 0.15% or less, 0.12% or less, 0.10% or less, or 0.08% or less.[REM: 0 to 0.1000%]

[0129] REM (rare earth metals) are elements able to control the form of sulfides. The REM content may also be 0%, but to obtain such an effect, the REM content is preferably 0.0001% or more. The REM content may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, even if excessively containing REM, the effect becomes saturated and a rise in production costs is invited. Therefore, the REM content is preferably 0.1000% or less. The REM content may also be 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. In this Description, “REM” is the general name of the 17 elements of atomic number 21 scandium (Sc), atomic number 39 yttrium (Y), and the lanthanoids of atomic number 57 lanthanum (La) to atomic number 71 lutetium (Lu). The REM content is the total content of these elements.

[0130] In the high strength hot rolled steel sheet according to embodiments of the present invention, the balance besides the above elements is comprised of Fe and impurities. The “impurities” are constituents, etc., entering the high strength hot rolled steel sheet due to the ore, scrap, or other raw materials and other various factors in the production process when industrially producing it.

[0131] The chemical composition of the high strength hot rolled steel sheet according to embodiments of the present invention may be measured by a general analysis method. For example, the chemical composition of the high strength hot rolled steel sheet may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S may be measured using combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-nondispersive infrared absorption method.[Microstructure]

[0132] In the high strength hot rolled steel sheet according to embodiments of the present invention, when the total width in the direction perpendicular to the rolling direction and the sheet thickness direction of the high strength hot rolled steel sheet is W, the microstructure at a ¼ position in sheet thickness at all positions of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction is comprised of, by area %, tempered martensite (tM): 95% or more, fresh martensite (fM): 5% or less, and at least one of ferrite (α), upper bainite (B), and pearlite (P): total 5% or less. In the present invention, the “total width” means the length of the steel sheet in the direction perpendicular to the rolling direction and the sheet thickness direction of the high strength hot rolled steel sheet (for example, coiled high strength hot rolled steel sheet).

[0133] Here, if the rolling direction of the hot rolled steel sheet is not clear, the rolling direction of the hot rolled steel sheet can be identified by the following method. The sheet thickness cross-section of the hot rolled steel sheet is finished by polishing to a mirror surface, then the S concentration is measured by an electron probe micro analyzer (EPMA). The measurement conditions are an acceleration voltage of 15 kV. The image of distribution in a range of 500 μm square at the sheet thickness center part is measured using a measurement pitch of 1 μm. At this time, a stretched region with a high S concentration is judged to be an inclusion of MnS, etc. At the time of observation, several fields may also be observed. Next, based on the sheet thickness cross-section first examined by the above method, surfaces parallel to surfaces rotated by 5° increments about the sheet thickness direction in 0° to 180° in range are observed by the above method. The average value of the lengths of the long axes of the plurality of inclusions at the obtained cross-sections are calculated for each cross-section and the cross-section where the average value of the lengths of the long axes of the inclusions becomes maximum is identified. The direction parallel to the long axis direction of the inclusions at the cross-section is judged to be the rolling direction.

[0134] Below, the different structures will be explained in more detail.[Tempered Martensite: 95% or More]

[0135] By making the tempered martensite in the microstructure at a ¼ position in sheet thickness at all positions of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction an area % of 95% or more, it is possible to achieve high strength due to the structure mainly comprised of martensite while it is possible to reliably control the difference between a maximum value and minimum value in tensile strength at all the positions in the width direction to 30 MPa or less and therefore possible to remarkably reduce the variation in strength in the width direction. At all the positions in the width direction, the area ratio of tempered martensite may be 96% or more. 97% or more, or 98% or more. The upper limit of the area ratio of the tempered martensite is not particularly prescribed and may also be 100%.[Fresh Martensite: 5% or Less]

[0136] In the high strength hot rolled steel sheet according to embodiments of the present invention, the area ratio of fresh martensite has to be controlled to 5% or less at all the positions in the width direction. If the area ratio of fresh martensite is more than 5% at even one position among them, the strength becomes too high at that position and sometimes the variation in strength in the width direction can no longer be sufficiently reduced. Therefore, from the viewpoint of reducing the variation in strength, at all the positions in the width direction, the lower the area ratio of fresh martensite, the more preferable. For example, it may be 4% or less, 3% or less, 2% or less, or 1% or less. The lower limit of area ratio of fresh martensite is not particularly prescribed and may also be 0%.[At Least One of Ferrite, Upper Bainite, and Pearlite: 5% or Less in Total]

[0137] In the high strength hot rolled steel sheet according to embodiments of the present invention, the remaining structure other than the tempered martensite and fresh martensite is comprised of at least one of ferrite, upper bainite, and pearlite. The at least one of the ferrite, upper bainite, and pearlite also similarly has to be controlled to 5% or less in total at all the positions in the width direction. If at even one position among these the area ratio of the at least one of ferrite, upper bainite, and pearlite becomes more than 5% in total, the strength falls too much at that position and sometimes the variation in strength in the width direction can no longer be sufficiently reduced. Therefore, from the viewpoint of reducing the variation in strength, the lower the area ratio of the at least one of ferrite, upper bainite, and pearlite at all the positions in the width direction, the more preferable. For example, it may be 4% or less, 3% or less, 2% or less, or 1% or less in total. The lower limit of the at least one of ferrite, upper bainite, and pearlite is not particularly prescribed and may also be 0% in total. For example, the total area ratio of the fresh martensite, ferrite, upper bainite, and pearlite may also be 5% or less, 4% or less. 3% or less, 2% or less, or 1% or less. Similarly, the total area ratio of fresh martensite, ferrite, upper bainite, and pearlite may also be 0) %.[Identification of Microstructure and Calculation of Area Ratio]

[0138] The microstructure is identified and the area ratio is calculated after corrosion using a Nital reagent or Le Pera solution by an FE-SEM (field emission scan electron microscope) and optical microscope and X-ray diffraction. The structure is observed by the FE-SEM and optical microscope at a 100 μm×100 μm region in the steel sheet cross-section in a direction parallel to the rolling direction and perpendicular to the sheet surfaces by a 1000 to 50000× power. In each microstructure, three locations are measured at each position of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction (direction perpendicular to rolling direction and sheet thickness direction) and the average value of the three measured values is calculated to determine the area ratio at each respective position.

[0139] The area ratio of the ferrite is found by observing a region of 100 μmx 100 μm in the range of the sheet thickness ⅛ position to ⅜ position centered about the ¼ position in sheet thickness in an electron channeling contrast image by an FE-SEM (field emission scan electron microscope). More specifically, it is possible to identify parts shown by even contrast in the region as ferrite and calculate the area ratio of the same by the image analysis software Image J.

[0140] The area ratio of fresh martensite (martensite as hardened) is found by the following procedure. The examined surface of the sample is etched by a Le Pera solution, then a region of 100 μmx 100 μm in the range of the sheet thickness ⅛ position to ⅜ position centered about the ¼ position in sheet thickness is examined by an FE-SEM. Fresh martensite and retained austenite are not corroded by Le Pera corrosion, therefore in an SEM image, appear as flat regions with brighter contrast than other parts. The area ratio of the not corroded regions corresponds to the total area ratio of the fresh martensite and any retained austenite which is present. The area ratio of the fresh martensite is calculated by subtracting the area ratio of the retained austenite measured by the X-ray diffraction method explained later from the area ratio of the not corroded regions.

[0141] The area ratio of retained austenite is calculated by the X-ray diffraction method. First, the part from a surface of the sample down to a depth ¼ position in the sheet thickness direction is removed by mechanical polishing and chemical polishing. Next, at the ¼ position in sheet thickness, the structure fraction of retained austenite is calculated from the integrated intensity ratios of the diffraction peaks of (200), (211) of the bcc phase and (200), (220), (311) of the fcc phase obtained using MoKα rays. As this method of calculation, the general 5-peak method is utilized. The structure fraction of retained austenite calculated is determined as the area ratio of the retained austenite.

[0142] The upper bainite and tempered martensite are identified and the area ratios are calculated by the following procedure. First, the observed surface of a sample is corroded by a Nital reagent, then a region of 100 mx 100 μm in the range of the sheet thickness ⅛ position to ⅜ position centered about the ¼ position in sheet thickness is examined by an FE-SEM. The upper bainite and tempered martensite are identified in the following way from the positions of cementite and arrangement of cementite contained inside the structure in this observed region. Upper bainite is comprised of laths of bainitic ferrite at the interfaces of which cementite or retained austenite are present. Based on such features, upper bainite is identified. The value obtained by dividing the regions of the identified bainite by the area of the examined field is calculated as the area ratio of the upper bainite. On the other hand, in tempered martensite, there is cementite at the inside of the martensite laths, but there are two or more types of crystal orientation of martensite laths and cementite and cementite comes in several variants. therefore it is possible to identify tempered martensite. The value obtained by dividing the regions of the thus identified tempered martensite by the area of the examined field is calculated as the area ratio of the tempered martensite.

[0143] The pearlite is identified and the area ratio is calculated by the following procedure. First, the observed surface of a sample is corroded by a Nital reagent, then a range of the sheet thickness ⅛ position to ⅜ position centered about the ¼ position in sheet thickness is examined by an optical microscope. In the observed image of the optical microscope, regions where carbides and ferrite are present in lamellar shapes area identified as pearlite. The value of these regions divided by the area of the observed field is calculated as the area ratio of pearlite.[Difference Between Maximum Value and Minimum Value of Tensile Strength at All Positions of 1 / 10W Position, 3 / 10W Position, 5 / 10W Position, 7 / 10W Position, and 9 / 10W Position From End Part in Width Direction of 30 MPa or Less]

[0144] In the high strength hot rolled steel sheet according to embodiments of the present invention, as explained previously, by making the tempered martensite in the microstructure at a ¼ position in sheet thickness at all positions of the 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction an area % of 95% or more, it is possible to achieve high strength due to the structure mainly comprised of martensite while it is possible to reliably control the difference between a maximum value and minimum value in tensile strength at all the positions in the width direction to 30 MPa or less and therefore possible to remarkably reduce the variation in strength in the width direction. Further, in relation to the uniformity of microstructure and reduction of the variation in strength in the width direction as a whole, regardless of the high strength, it is possible to remarkably improve the flatness of the hot rolled steel sheet. From the viewpoint of improving the flatness of hot rolled steel sheet, the smaller the difference between the maximum value and minimum value of the tensile strength, the more preferable. For example, it may be 28 MPa or less, 25 MPa or less, 22 MPa or less, 20 MPa or less, 17 MPa or less, or 15 MPa or less. The lower limit is not particularly prescribed, but, for example, a difference between the maximum value and minimum value of the tensile strength of 5 MPa or more, 8 MPa or more, or 10 MPa or more may be allowed.

[0145] The difference between the maximum value and minimum value of the tensile strength is determined in the following way. First, No. 5 tensile test pieces of JIS Z2241:2011 having directions parallel to the rolling direction as test directions were taken at the positions of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction of the hot rolled steel sheet. Next, these tensile test pieces were used for tensile tests compliant with JIS Z2241:2011 so as to obtain five tensile strengths. Finally, the difference between the maximum value and minimum value of these was calculated.[Tensile Strength]

[0146] The minimum value among the above five values of tensile strength is determined as the tensile strength of the high strength hot rolled steel sheet according to embodiments of the present invention. In the high strength hot rolled steel sheet according to embodiments of the present invention, by having the above explained chemical composition and microstructure, a high tensile strength, specifically a tensile strength of 980 MPa or more, can be achieved. The tensile strength is preferably 1000 MPa or more, 1050 MPa or more, or 1100 MPa or more. According to the high strength hot rolled steel sheet according to embodiments of the present invention, despite having such an extremely high tensile strength, along with uniformity of the microstructure and reduction of variation of strength in the width direction as a whole, it is possible to achieve extremely excellent flatness. The upper limit of the tensile strength is not particularly prescribed, but, for example, the tensile strength of the high strength hot rolled steel sheet may be 1300 MPa or less, 1250 MPa or less, 1200 MPa or less, or 1180 MPa or less.[Prior Austenite Grain Size in Microstructure: 40 μm or Less]

[0147] According to preferable embodiments of the present invention, the prior austenite grain size in the microstructure is 40 μm or less. As explained above, in the high strength hot rolled steel sheet according to embodiments of the present invention, it is possible to achieve extremely excellent flatness along with the uniformity of the microstructure and reduction in variation in strength in the width direction as a whole, but in addition to this, by controlling the prior austenite grain size to such a fine range, it becomes possible to further improve the toughness and other additional properties. From the viewpoint of improvement of the toughness, the smaller the prior austenite grain size, the more preferable. For example, it may be 37 μm or less, 35 μm or less. 32 μm or less, 30 μm or less, 27 μm or less, or 25 μm or less. The lower limit is not particularly prescribed, but, for example, the prior austenite grain size may be 10 μm or more. 12 μm or more, 15 μm or more, 18 μm or more, or 20 μm or more.

[0148] The prior austenite grain size in the microstructure is determined in the following way. First, a region of 200 μm×200 μm in an L-cross-section of a steel slab taken at the position of sheet thickness ¼ from a surface of hot rolled steel sheet was analyzed by SEM / EBSD (scan electron microscope / electron backscatter diffraction). More specifically, the crystal orientation data obtained by SEM / EBSD is processed by predetermined crystal orientation conversion (see “Study For Increasing Precision of Method of Reconstruction of Austenite Structures of Steel”, Kengo Hata, Masayuki Wakita, Tomoya Fujiwara, Kaori Kawano, Nippon Steel & Sumikin Engineering Co., Ltd. Technical Review, no. 404 (2016), p. 24-30) to obtain an image of reconstructed prior austenite grains. From a prior austenite grain of the image, the diameter of a circle having the same area, i.e., the circle equivalent diameter, is sought. This operation is performed for a total of 10 prior austenite grains and the obtained circle equivalent diameters are averaged to determine the prior austenite grain size.[Total Width W]

[0149] The high strength hot rolled steel sheet according to embodiments of the present invention can have any total width W. While not particularly limited, for example, in the case of the total width W at hot rolled steel sheet in the coiled state (coil), the total width W may be 700 mm or more, 800 mm or more, 900 mm or more, or 1000 mm or more. The upper limit is not particularly prescribed, but from the viewpoint of more reliably improving the flatness, the total width is preferably 2500 mm or less and may also be 2200 mm or less, 2000 mm or less, 1800 mm or less, 1600 mm or less, 1500 mm or less, 1400 mm or less, or 1300 mm or less.[Sheet Thickness]

[0150] The high strength hot rolled steel sheet according to embodiments of the present invention is not particular limited, but in general has a 1.0 to 6.0 mm sheet thickness. For example, the sheet thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more and / or may be 5.0 mm or less, 4.0 mm or less, or 3.0 mm or less.<Method of Production of High Strength Hot Rolled Steel Sheet>

[0151] Next, the preferable method of production of the high strength hot rolled steel sheet according to embodiments of the present invention will be explained. The following explanation is intended to illustrate the characterizing method for producing the high strength hot rolled steel sheet according to embodiments of the present invention and is not intended to limit the high strength hot rolled steel sheet to one produced by a method of production such as explained below.

[0152] The method of production of the high strength hot rolled steel sheet according to embodiments of the present invention is characterized by including

[0153] hot rolling including heating a slab having the chemical composition explained above in relation to the high strength steel sheet to a temperature of 1220 to 1300° C. and rough rolling and finish rolling it, wherein an exit side temperature of the rough rolling is 1100 to 1200° C., an entry side temperature (F0) of the finish rolling is 1000 to 1100° C., an exit side temperature (FT) of the finish rolling is 940 to 1000° C., and a total rolling reduction of the finish rolling is 85 to 95%,

[0154] cooling of primary cooling the finished rolled steel sheet in a temperature region from the exit side temperature (FT) of the finish rolling to the martensite transformation start temperature Ms+50° C. by an average cooling speed of a critical cooling speed Vc+10° C. / s or more and 60° C. / s or less, then secondary cooling it in a temperature region from Ms+50° C. to 200° C. by an average cooling speed of 50 to 120° C. / s, wherein the top-bottom cooling ratio of the top surface of the steel sheet to the bottom surface in the primary cooling is 0.8 to 1.2 and the top-bottom cooling ratio of the top surface of the steel sheet to the bottom surface in the secondary cooling is 0.8 to 1.2, and

[0155] coiling the secondary cooled steel sheet at 50 to 100° C. Below, each step will be explained in detail.[Hot Rolling Step][Heating of Slab]

[0156] First, a slab having the chemical composition explained above in relation to the hot rolled steel sheet is heated. The slab used is preferably cast by the continuous casting method from the viewpoint of productivity, but may also be produced by the ingot making method or thin slab casting method. The slab used contains relatively large amounts of alloying elements for obtaining high strength steel sheet. For this reason, it is also necessary to heat the slab before sending it on to hot rolling to make the alloying elements dissolve in the slab. If the heating temperature is less than 1220° C., the alloying elements will not sufficiently dissolve in the slab but will remain as coarse alloy carbides and sometimes brittle cracking will occur during the hot rolling. For this reason, the heating temperature is 1220° C. or more, preferably 1230° C. or more. The upper limit of the heating temperature is not particularly prescribed, but is preferably 1300° C. or less from the viewpoint of the capacity of the heating facility and the productivity.[Rough Rolling]

[0157] In this method, the heated slab is rough rolled before the finish rolling so as to adjust the sheet thickness, etc. In order for the rough rolling to secure the desired sheet bar dimensions and enable the total rolling reduction in the 940° C. or more temperature region in the finish rolling to be adjusted to within the desired range, the exit side temperature of the rough rolling is 1100 to 1200° C. and is preferably 1150 to 1200° C. If the exit side temperature of the rough rolling is less than 1100° C., in the finish rolling continuing after the rough rolling, it becomes difficult to obtain a 940° C. or more exit side temperature. Further, if the exit side temperature of the rough rolling becomes more than 1200° C., the crystal grains become coarser and sometimes the obtained hot rolled steel sheet falls in toughness.[Finish Rolling]

[0158] The rough rolled slab is next finish rolled. As explained above, the slab used contains relatively large amounts of alloying elements, therefore at the time of hot rolling, the rolling load has to be made greater. For this reason, the hot rolling is performed at a high temperature under a high pressure. Specifically, the entry side temperature (F0) of the finish rolling is 1000 to 1100° C., the exit side temperature (FT) of the finish rolling is 940 to 1000° C., and the total rolling reduction of the finish rolling is 85 to 95%. In particular, the exit side temperature of the finish rolling is important on the point of control of the microstructure of the steel sheet. In more detail, if the exit side temperature of the finish rolling is low, the microstructure will become uneven and sometimes the formability will fall. For this reason, the exit side temperature of the finish rolling is 940° C. or more. On the other hand, to suppress coarsening of the austenite, the exit side temperature of the finish rolling is 1000° C. or less.[Cooling Step][Primary Cooling]

[0159] In the next cooling step, the finish rolled steel sheet is first primary cooled in the temperature region from the exit side temperature (FT) of the finish rolling to the martensite transformation start temperature Ms+50° C. by an average cooling speed of a critical cooling speed Vc+10° C. / s or more and 60° C. / s or less. By primary cooling in this temperature region by an average cooling speed of the critical cooling speed Vc+10° C. / s or more and 60° C. / s or less, it is possible to promote martensite transformation and reduce the at least one of ferrite, upper bainite, and pearlite in the finally obtained microstructure to 5 area % or less in total and make the microstructure uniform in the width direction. If the average cooling speed of the primary cooling is less than the critical cooling speed Vc+10° C. / s, the at least one of the ferrite, upper bainite, and pearlite becomes more than 5 area % in total and sometimes the desired strength cannot be achieved. On the other hand, if the average cooling speed of the primary cooling is more than 60° C. / s, the cooling speed is fast, therefore it becomes difficult to evenly cool the steel sheet in the width direction and uneven cooling occurs in the width direction. In this case, in the finally obtained hot rolled steel sheet, the desired microstructure cannot be obtained in the width direction and / or the variation in tensile strength in the width direction becomes greater. Along with this, the hot rolled steel sheet becomes defective in shape, warpage occurs in the width direction of the steel sheet, and sufficient flatness can no longer be achieved. Therefore, the average cooling speed of the primary cooling is the critical cooling speed Vc+10° C. / s or more and 60° C. / s or less, preferably the critical cooling speed Vc+12° C. / s or more and 60° C. / s or less.

[0160] In the present method of production, the Ms point (° C.) is found by the following formula 1:Ms=823-350[C]-4⁢0[M⁢n]-3⁢5[V]-2⁢0[C⁢r]-1⁢7[N⁢i]-1⁢0[C⁢u]-1⁢0[M⁢o]-1⁢0[W]+1⁢5[C⁢o]+3⁢0[A⁢l]-2⁢7⁢3formula⁢ 1where, [C], [Mn], [V], [Cr], [Ni], [Cu], [Mo], [W], [Co], and [Al] are the contents (mass %) of the elements in the steel. If an element is not included, 0 is entered. Further, the critical cooling speed Vc (° C.) is also an indicator of the hardenability where the martensite area ratio becomes 90% or more and can be expressed by the following formulas 2 and 3:

[0162] when amount of dissolved B≥0.0005 mass %,log⁢Vc=2.94-0.75×(2.7[C]+0.4[Si]+[M⁢n]+0.4⁢5[N⁢i]+0.8[C⁢r]+2[M⁢o])formula⁢ 2when amount of dissolved B<0.0005 mass %,log⁢Vc=3.69-0.75×(2.7[C]+0.4[S⁢i]+[M⁢n]+0.4⁢5[N⁢i]+0.8[C⁢r]+[M⁢o])formula⁢ 3where, [C], [Si], [Mn], [Ni], [Cr], and [Mo] are the contents (mass %) of the elements in the steel. If an element is not included, 0 is entered. Further, the amount of dissolved B (mass %) corresponds to the amount of the B content contained in the steel minus the amount of B consumed for forming boronitrides (BN). On the other hand, the amount of dissolved N (mass %) able to form BN can be reduced by including Ti in the steel and fixing it as TiN. Therefore, the amount of dissolved B can be calculated by the following formulas 4 and 5:Amount⁢ of⁢ dissolved⁢ B=10.81×([B] / 10.81-amount⁢ of⁢ dissolved⁢ N / 14.01)formula⁢ 4where, when [N] / 14.01−[Ti] / 47.88>0,Amount⁢ of⁢ dissolved⁢ N=1⁢4.0⁢1×([N] / 14.01-[Ti] / 47.88)formula⁢ 5when [N] / 14.01−[Ti] / 47.88≤0, the amount of dissolved N is 0).where, [B], [N], and [Ti] are the contents (mass %) of the elements in the steel. If an element is not included, 0) is entered.In the primary cooling, in addition to control of the average cooling speed, evenly cooling the steel sheet at the top surface and bottom surface is extremely important. Such cooling is performed so that the top-bottom cooling ratio of the top surface of the steel sheet to the bottom surface becomes 0.8 to 1.2, more specifically, is performed so that the amount of cooling water sprayed on the top surface of the steel sheet becomes 0.8 to 1.2 times the amount of cooling water sprayed on the bottom surface of the steel sheet. By evenly performing cooling at the top and bottom surfaces of the steel sheet in this way, it becomes possible to remarkably suppress or reduce the occurrence of uneven cooling. As a result, it is possible to achieve uniformity of the microstructure and reduction of variation in strength in the width direction as a whole. Along with this, it becomes possible to prevent the occurrence of warpage in the width direction of the hot rolled steel sheet and achieve sufficient flatness. If the top-bottom cooling ratio is less than 0).8 or is more than 1.2, due to the occurrence of uneven cooling, it becomes no longer possible to obtain a uniform microstructure in the width direction, i.e., it becomes no longer possible to make the microstructure at a ¼ position in sheet thickness at all positions of the 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction an area % of a structure of tempered martensite: 95% or more. As a result, it becomes no longer possible to sufficiently reduce the variation in strength in the width direction to a level such as one where a difference between a maximum value and minimum value in tensile strength at all the positions in the width direction become 30 MPa or less.Here, the “top-bottom cooling ratio” does not mean the ratio of the amount of cooling water of the entire top surface and the amount of cooling water of the entire bottom surface in the range of FT to (Ms+50)° C. More specifically, in the method of production, the range of FT to (Ms+50)° C. is divided into sections of 10 meters each, the top-bottom cooling ratio is calculated from the amount of cooling water of the top surface and the amount of cooling water of the bottom surface for each section, and the top-bottom cooling ratios of the different sections calculated in this way are all controlled to ranges of 0.8 to 1.2. With control of the top-bottom cooling ratio of the range as a whole rather than each divided section, for example, it is extremely difficult to sufficiently suppress the occurrence of uneven cooling due to local overcooling, etc. However, by realizing control of the top-bottom cooling ratio for each section in this way, it becomes possible to reduce local overcooling, etc., and reliably suppress the occurrence of uneven cooling. Further, top-bottom cooling ratio for each section can be controlled by any suitable means. While not particularly limited, for example, each section has several cooling water nozzles arranged along the direction of advance of the steel sheet at the top side and bottom side of the steel sheet, therefore by suitably spraying water by on / off control of these cooling water nozzles, it is possible to relatively easily control the top-bottom cooling ratio of each section to within 0.8 to 1.2 in range.[Secondary Cooling]

[0170] The primary cooled steel sheet is next secondary cooled in the temperature region of Ms+50° C. to 200° C. by an average cooling speed of 50 to 120° C. / s. By secondary cooling in the temperature region from Ms+50° C. to 200° C. possibly corresponding to the end temperature of martensite transformation by an average cooling speed of 50 to 120° C. / s, it is possible to sufficiently secure the dwell time in the temperature region in which autotempering progresses, therefore it becomes possible to promote autotempering. Therefore, the structure transformed to martensite by the above primary cooling can be sufficiently tempered and 95 area % or more of tempered martensite can be made uniform in the width direction in the final microstructure. If the average cooling speed of the secondary cooling is less than 50° C. / s, it is not possible to obtain the desired microstructure in the width direction of the steel sheet and sometimes the variation in strength in the width direction cannot be sufficiently reduced. On the other hand, if the average cooling speed of the secondary cooling is more than 120° C. / s, it becomes no longer possible to promote autotempering and sometimes in the final microstructure, fresh martensite remains in more than 5 area %. In addition to or in place of this, the controllability of the amount of water used for cooling deteriorates due to such rapid cooling, locally overcooled parts occur, etc., and uneven cooling occurs in the width direction. In this case, in the finally obtained hot rolled steel sheet, the variation in tensile strength in the width direction becomes greater. In relation to this, the hot rolled steel sheet becomes defective in shape, warpage occurs in the width direction of the steel sheet, and sufficient flatness can no longer be achieved.

[0171] In the secondary cooling as well, in the same way as the case of the primary cooling, in addition to control of the average cooling speed, evenly cooling the steel sheet at the top surface and bottom surface is extremely important. Such cooling, in the same way as the case of the primary cooling, is performed so that the top-bottom cooling ratio of the top surface of the steel sheet to the bottom surface becomes 0).8 to 1.2, more specifically, is performed so that the amount of cooling water sprayed on the top surface of the steel sheet becomes 0.8 to 1.2 times the amount of cooling water sprayed on the bottom surface of the steel sheet. By evenly performing cooling at the top and bottom surfaces of the steel sheet in this way, it becomes possible to remarkably suppress or reduce the occurrence of uneven cooling. As a result, it is possible to achieve uniformity of the microstructure and reduction of variation in strength in the width direction as a whole. Along with this, it becomes possible to prevent the occurrence of warpage in the width direction of the hot rolled steel sheet and achieve sufficient flatness. If the top-bottom cooling ratio is less than 0.8 or is more than 1.2, due to the occurrence of uneven cooling, it becomes no longer possible to make the microstructure uniform in the width direction, i.e., it becomes no longer possible to make the microstructure at a ¼ position in sheet thickness at all positions of the 1 / 10W position. 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction an area % of structures of tempered martensite: 95% or more. As a result, it becomes no longer possible to sufficiently reduce the variation in strength in the width direction to a level such as one where a difference between a maximum value and minimum value in tensile strength at all the positions in the width direction become 30 MPa or less.

[0172] Here, the above top-bottom cooling ratio does not mean the ratio of the amount of cooling water of the entire top surface and the amount of cooling water of the entire bottom surface in the range of (Ms+50)° C. to 200° C. More specifically, in the method of production, the range of (Ms+50)° C. to 200° C. is divided into sections of 10 meters each, the top-bottom cooling ratio is calculated from the amount of cooling water of the top surface and the amount of cooling water of the bottom surface for each section, and the top-bottom cooling ratios of the different sections calculated in this way are all controlled to ranges of 0.8 to 1.2. With control of the top-bottom cooling ratio of the range as a whole rather than each divided section, for example, it is extremely difficult to sufficiently suppress the occurrence of uneven cooling due to local overcooling, etc. However, by realizing control of the top-bottom cooling ratio for each section in this way, it becomes possible to reduce the local overcooling, etc., and reliably suppress the occurrence of uneven cooling. Further, the control of the top-bottom cooling ratio for each such section can be performed by any suitable means. While not particularly limited to this, in the same way as the case of primary cooling, for example, several cooling water nozzles are arranged along the advancing direction of the steel sheet at the top side and bottom side of the steel sheet at each section, therefore by suitably spraying based on on / off control of these cooling water nozzles, it becomes possible to relatively easily control the top-bottom cooling ratio of the individual sections to 0.8 to 1.2 in range.[Coiling Step]

[0173] The secondary cooled steel sheet is finally coiled in the coiling step at 50 to 100° C. If the coiling temperature is too low; sometimes the hot rolled steel sheet becomes hard and embrittled. Further, excessive water cooling, etc., becomes required and the productivity falls. Therefore, the coiling temperature is 50° C. or more and preferably is 80° C. or more.

[0174] According to the hot rolled steel sheet produced by the above method of production, when the total width in a direction perpendicular to the rolling direction and the sheet thickness direction is W, it is possible to make the tempered martensite at the ¼ position in sheet thickness at all positions of the 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction 95% or more and achieve a high strength due to the structure mainly comprised of martensite, more specifically a tensile strength of 980 MPa or more while reliably controlling the difference between the maximum value and minimum value of the tensile strength at all the positions in the width direction to 30 MPa or less. Therefore, the variation in strength in the width direction can be remarkably reduced and along with this, it becomes possible to prevent the occurrence of warpage in the width direction of the hot rolled steel sheet and achieve sufficient flatness. Therefore, the high strength hot rolled steel sheet produced by the above method of production, despite being high strength, has homogeneous properties in the width direction. The flatness is extremely good. Further, for example, in hot rolled steel sheet not flattened using a leveler, etc., for example, hot rolled steel sheet right after production, the above high strength and flatness can be achieved, therefore part of the ductility inherently possessed by steel sheet is not consumed by such flattening (preworking) either. For this reason, it is possible to reduce the risk of a defect occurring in the shaping at the time of pressing the steel sheet and possible to remarkably improve the productivity. Therefore, the high strength hot rolled steel sheet is naturally particularly useful for use in the automotive field and can also be used extremely effectively in other fields.

[0175] Below, examples will be used to explain the present invention in more detail, but the present invention is not limited to these examples in any way.EXAMPLES

[0176] In the following examples, the high strength hot rolled steel sheet according to embodiments of the present invention was produced under various conditions. The obtained high strength hot rolled steel sheets were investigated for properties in the width direction and the flatness.

[0177] First, molten steels were cast by the continuous casting method to form slabs having the various chemical compositions shown in Table 1. Each of these slabs was heated under the conditions shown in Table 2 and then hot rolled. The hot rolling was performed by rough rolling and finish rolling. The exit side temperature of the rough rolling and the entry side temperature (F0), exit side temperature (FT), and total rolling reduction of the finish rolling were as shown in Table 2. Next, under the conditions shown in Table 2, the finish rolled steel sheet was subjected to primary cooling first in the temperature region from the exit side temperature (FT) of the finish rolling to the martensite transformation start temperature Ms+50° C., then was subjected to secondary cooling in the temperature region from Ms+50° C. to 200° C.

[0178] At the primary cooling and secondary cooling, the range of FT to (Ms+50)° C. and the range of (Ms+50)° C. to 200° C. was divided into 10 meter sections. The top-bottom cooling ratio was calculated from the amount of cooling water of the top surface and the amount of cooling water of the bottom surface for each of these sections. Cooling was performed so that the top-bottom cooling ratios of the different sections calculated in this way were controlled to within predetermined ranges. The top-bottom cooling ratios at the primary cooling and secondary cooling of Table 2 show the ratios where the absolute values of the differences from the cooling ratio 1 in the top-bottom cooling ratios of the different sections in the primary cooling and the secondary cooling are the largest. Finally, the secondarily cooled steel sheet was coiled under the conditions shown in Table 2 to obtain hot rolled steel sheet having approximately 2.3 to 3.2 mm sheet thickness and 1200 mm total width.TABLE 1SteelChemical composition (mass %) balance: Fe and impuritiesno.CSiMnTiAlBPSNOCuNiSnA0.0740.0512.120.0220.0270.00150.0100.0030.00420.00280.050.04B0.0520.1982.480.0140.0310.00210.0110.0020.00350.0025C0.0980.0501.980.0010.0290.00160.0090.0030.00320.00270.200.15D0.0910.2001.050.0310.0310.00190.0120.0020.00330.00310.20E0.0810.0321.980.0510.0330.00210.0110.0020.00480.0021F0.0670.1102.340.0150.0210.00250.0120.0030.00340.00230.01G0.0710.0522.210.0190.0250.00190.0120.0030.00440.00280.01H0.0720.0611.950.0210.0310.00220.0120.0020.00410.0031I0.0810.0511.830.1180.0410.00150.0110.0020.00450.0033J0.0930.1121.310.0150.0330.00200.0120.0020.00450.00410.030.04K0.0830.0511.800.0310.0250.00210.0110.0020.00450.00310.040.03L0.1090.1112.410.0170.0270.00210.0110.0020.00380.00280.090.05M0.0440.0512.500.0210.0280.00250.0120.0020.00430.0032N0.0910.2502.200.0220.0310.00280.0110.0030.00330.0031O0.0730.1102.600.0010.0320.00300.0110.0020.00310.00310.050.10P0.0980.1100.980.0210.0320.00310.0110.0030.00310.00320.050.03Q0.0850.0501.800.1300.0320.00240.0120.0030.00440.0031SteelChemical composition (mass %) balance: Fe and impuritiesno.CrMoNbVAsZrCaMgBiCoWZnREMA0.200.0021B0.100.090.020.050.01C0.350.200.05D0.400.200.020.011EF0.0050.150.010.0017G0.100.0031H0.050.020.0110.0210.00310.009I0.30J0.380.110.0025K0.310.150.0021M0.00230.020N0.23O0.400.10Q0.0021Underlines indicate outside scope of present invention.TABLE 2Cooling stepHot rolling stepPrimary coolingSecondary coolingRoughFinish rollingFT to (Ms +FT to (Ms +(Ms +(Ms +rollingEntryExit50)° C.50)° C.50)° C. to50)° C. toCoilingHeatingExitsidesideTotalaveragetop-bottom200° C.200° C.stepHeatingsidetemp.temp.rollingcoolingcoolingaveragetop-bottomCoilingTestSteeltemp.temp.F0FTreductionVc + 10speedratiocooling speedcooling ratiotemp.no.no.° C.° C.° C.° C.%° C. / s° C. / s—° C. / s—° C.Remarks 1A1230118110429448721521.1511.190Inv. ex. 2A1230118210419418721511.2143 1.785Comp. ex. 3B1250118810489619015601.2631.280Inv. ex. 4C1250118510539519148541.2551.190Inv. ex. 5D1240118310549528830510.9501.180Inv. ex. 6E1250117110519448829551.1550.880Inv. ex. 7F1250116110439549020510.8561.180Inv. ex. 8G1240115110419428719911.6119 1.570Comp. ex. 9H1240114810369628729122 3.1501.180Comp. ex.10I1260113010459458726421.1131 2.180Comp. ex.11J1230117110419468831320.7320.780Comp. ex.12K1240116010259418820551.0551.080Inv. ex.13L1240117810419538817451.2541.270Comp. ex.14M1230117610429409119410.9511.170Comp. ex.15N1230117710529508718461.2891.280Comp. ex.16O1250117010449528812521.2901.280Comp. ex.17P12301181104694391102521.2521.280Comp. ex.18Q1270118110429419135551.2551.280Comp. ex.19D1235118510519508730191.2501.180Comp. ex.20G1240115310529438719651.1119 1.170Comp. ex.21G1240115510479448719510.6511.180Comp. ex.22G1240116110499458719521.3821.185Comp. ex.23G1240115010459438719501.1451.185Comp. ex.24G1240115110439458719511.1125 1.285Comp. ex.25G1240115310429468719491.1620.680Comp. ex.26G1240115410419428719531.1611.385Comp. ex.Underlines indicate outside scope of present invention or properties which are not preferable.The properties of the obtained hot rolled steel sheet were measured and evaluated by the following methods:[Prior Austenite Grain Size in Microstructure]

[0180] The prior austenite grain size in the microstructure was determined as follows: First, a region of 200 μm×200 μm in an L-cross-section of a steel slab taken at the position of sheet thickness ¼ from a surface of hot rolled steel sheet was analyzed by SEM / EBSD. More specifically, the martensite structure obtained by SEM / EBSD was transformed to a predetermined crystal orientation and the prior austenite grains were reconstructed in an image, next the circle equivalent diameter was sought from a prior austenite grain of the image. This operation was performed for a total of 10 austenite grains. The 10 circle equivalent diameters obtained were averaged to determine the prior austenite grain size.[Variation in Strength in Width Direction]

[0181] First, No. 5 tensile test pieces of JIS Z2241:2011 having directions parallel to the rolling direction as test directions were taken at the positions of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction of the hot rolled steel sheet. Next, these tensile test pieces were used for tensile tests compliant with JIS Z2241:2011 so as to obtain five tensile strengths. Finally, the difference between the maximum value and minimum value of these was calculated to determine the variation in strength in the width direction.[Tensile Strength of Hot Rolled Steel Sheet]

[0182] The minimum value in the above five values of tensile strength is determined as the tensile strength of the hot rolled steel sheet.[Evaluation of Flatness]

[0183] The flatness was evaluated in the following way. First, the obtained hot rolled steel sheet was placed on a surface plate so that at least part of one of the sheet surfaces (bottom surface) contacted the surface plate, then the height of the hot rolled steel sheet from the surface plate at the position with the highest height to the bottom surface of the hot rolled steel sheet was measured, and the obtained measured value was determined as the maximum warpage height H (mm) of the hot rolled steel sheet. As the evaluation of the flatness, cases where the maximum warpage height H was within 10 mm were deemed passing and cases where the maximum warpage height H was more than 10 mm were deemed as failing.

[0184] Cases where the tensile strength of the hot rolled steel sheet was 980 MPa or more and the flatness was evaluated as passing were evaluated as high strength hot rolled steel sheet having improved flatness. The results are shown in Table 3.TABLE 3PropertiesMicrostructureMin. tensiletMα + B + PtMfMα + B + Pstrength inMax.VariationPrior γfractionfMfractionfractionfractionfractionwidth directiontensilein tensileMax.grainof min.fractionof minof max.of max.of max(tensile strengthstrengthstrengthwarpagesizestrengthof min.strengthstrengthstrengthstrengthof hot rolledin widthin widthheightTestSteeldgportionstrengthportionportionportionportionsteel sheet)directiondirectionHno.no.μm%portion%%%%MPaMPaMPammRemarks 1A32980299101022 104119 6Inv. ex. 2A31100 0093701025 10704515Comp. ex. 3B21100 009820981100827 8Inv. ex. 4C23961398111072 109119 9Inv. ex. 5D28980299101055 107116 7Inv. ex. 6E33981198201045 106116 6Inv. ex. 7F3496139730991102130 9Inv. ex. 8G349217928097010437317Comp. ex. 9H269118991098110375614Comp. ex.10I2299108812 01043 10854216Comp. ex.11J339316961399010334314Comp. ex.12K34990199101031 104514 3Inv. ex.13L3096409010 01105 11504511Comp. ex.14M2896139631910 94030 9Comp. ex.15N32981193701050 11015111Comp. ex.16O3195509010 01020 10513111Comp. ex.17P—40555 60535 772 8124011Comp. ex.18Q189028952399110404911Comp. ex.19D—60337 70327 862 8922810Comp. ex.20G349316982097810507216Comp. ex.21G339217971197910345512Comp. ex.22G349316981197810416313Comp. ex.23G349127961397110215011Comp. ex.24G339136946097510618614Comp. ex.25G349415971297410154112Comp. ex.26G349415972198110133211Comp. ex.Underlines indicate outside scope of present invention or properties which are not preferable.

[0185] Referring to Tables 1 to 3, in each of Comparative Examples 2 and 10, the average cooling speed of the secondary cooling was high and, further, the top-bottom cooling ratio of the secondary cooling was not suitable, therefore the martensite could not be sufficiently tempered by autotempering at the time of cooling, the ratio of fresh martensite (IM) became higher and, further, the microstructure could not be made uniform in the width direction due to the occurrence of uneven cooling and the variation in tensile strength in the width direction became remarkable. As a result, sufficient flatness could not be achieved. In Comparative Example 8, the average cooling speed of the primary cooling was high and, further, the top-bottom cooling ratio of the primary cooling and the secondary cooling was not suitable, therefore the desired microstructure could not be made uniform in the width direction due to the occurrence of uneven cooling. In relation to this, a tensile strength of 980 MPa or more could not be achieved and, further, the variation in tensile strength in the width direction became remarkable. As a result, sufficient flatness could not be achieved. In Comparative Example 9, the average cooling speed of the primary cooling was high and, further, the top-bottom cooling ratio of the primary cooling was not suitable, therefore it was not possible to make the desired microstructure uniform in the width direction due to the occurrence of uneven cooling and the variation in tensile strength in the width direction became remarkable. As a result, sufficient flatness could not be achieved. In Comparative Example 11, the top-bottom cooling ratio of the primary cooling and secondary cooling was not suitable, therefore it was not possible to make the desired microstructure uniform in the width direction due to the occurrence of uneven cooling and along with this, the variation in tensile strength in the width direction became remarkable. As a result, sufficient flatness could not be achieved. In Comparative Example 13, the C content was high, therefore, variation in tensile strength in the width direction could not be suppressed to within a predetermined range, and the flatness fell. In Comparative Example 14, the C content was low, therefore the desired tensile strength could not be achieved. In Comparative Example 15, the Si content was high, therefore it is believed that the autotempering at the time of cooling of the steel sheet was suppressed. As a result, the ratio of fresh martensite (fM) in the microstructure became higher and, along with this, the variation in tensile strength in the width direction could not be 30) sufficiently reduced and the flatness fell. In Comparative Example 16, the Mn content was high, therefore, it is believed martensite could not be sufficiently tempered even due to the autotempering at the time of cooling of the steel sheet due to the improvement in hardenability. As a result, the ratio of fresh martensite (fM) in the microstructure became higher. Along with this, the variation in tensile strength in the width direction could not be sufficiently reduced and the flatness fell. In Comparative Example 17, the Mn content was low, therefore the desired tensile strength could not be achieved. Further, the Mn content was low, therefore it is believed the hardenability was insufficient and relatively large amounts of ferrite and other soft phases were formed at the time of cooling. As a result, it was not possible to make a structure mainly comprised of tempered martensite uniform in the width direction, further, it is believed the steel sheet became defective in shape due to the expansion of the steel sheet accompanying transformation to ferrite, etc., and the flatness fell. In Comparative Example 18, the Ti content was high, therefore relatively large amounts of ferrite and other soft phases were formed from the nonrecrystallized austenite at the time of the cooling step. In relation to this, it was not possible to form the desired microstructure in the width direction and as a result it was not possible to sufficiently reduce variation in the tensile strength in the width direction and the flatness fell.

[0186] In Comparative Example 19, the average cooling speed of the primary cooling was low, therefore the at least one of ferrite, upper bainite, and pearlite becomes more than 5 area % in total and the desired tensile strength could not be achieved. In Comparative Example 20, the average cooling speed of the primary cooling was high, therefore it was not possible to make the desired microstructure uniform in the width direction due to the occurrence of uneven cooling and, in relation to this, the variation in tensile strength in the width direction also became conspicuous. As a result, it was not possible to achieve a sufficient flatness. In each of Comparative Examples 21 and 22, the top-bottom cooling ratio of the primary cooling was not suitable, therefore the desired microstructure could not be made uniform in the width direction due to the occurrence of uneven cooling and along with this, the variation in tensile strength in the width direction also became conspicuous. As a result, sufficient flatness could not be achieved. In Comparative Example 23, the average cooling speed of the secondary cooling was low, therefore the desired microstructure could not be obtained in the width direction of the steel sheet and along with this, the variation in tensile strength in the width direction also became conspicuous. As a result, sufficient flatness could not be achieved. In Comparative Example 24, the average cooling speed of the secondary cooling was high, therefore it was not possible to sufficiently temper the martensite by autotempering at the time of cooling, the ratio of fresh martensite (fM) became higher, further, the microstructure could not be made uniform in the width direction due to the occurrence of uneven cooling, and the variation in tensile strength in the width direction became conspicuous. As a result, sufficient flatness could not be achieved. In each of Comparative Examples 25 and 26, the top-bottom cooling ratio of the secondary cooling was not suitable, therefore the desired microstructure could not be made uniform in the width direction due to the occurrence of uneven cooling, and along with this, the variation in tensile strength in the width direction also became conspicuous. As a result, sufficient flatness could not be achieved.

[0187] In contrast to this, in the hot rolled steel sheets according to all of the invention examples, by having a predetermined chemical composition and in particular suitably controlling the conditions of the cooling step in the method of production, it was possible to make the tempered martensite in the microstructure at a ¼ position in sheet thickness at all positions of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction an area % of 95% or more and achieve a tensile strength of 980 MPa or more due to the structure mainly comprised of tempered martensite. In addition, it became possible to reliably control the difference between the maximum value and minimum value in the tensile strengths at all the positions in the width directions to 30 MPa or less. Therefore, it became possible to remarkably reduce the variation of strength in the width direction. Along with this, it was possible to keep warpage from occurring in the width direction of the hot rolled steel sheet and to achieve sufficient flatness. In Table 3, only the microstructures of portions at the 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in the width direction where the minimum tensile strength was obtained and portions where the maximum tensile strength was obtained are shown. However, in each of Invention Examples 1, 3 to 7, and 12, at all of these positions, the microstructure at a ¼ position in sheet thickness contained, by area ratio, tempered martensite: 95% or more, fresh martensite: 5% or less, and at least one of ferrite, upper bainite, and pearlite: 5% or less in total.

Examples

examples

[0176]In the following examples, the high strength hot rolled steel sheet according to embodiments of the present invention was produced under various conditions. The obtained high strength hot rolled steel sheets were investigated for properties in the width direction and the flatness.

[0177]First, molten steels were cast by the continuous casting method to form slabs having the various chemical compositions shown in Table 1. Each of these slabs was heated under the conditions shown in Table 2 and then hot rolled. The hot rolling was performed by rough rolling and finish rolling. The exit side temperature of the rough rolling and the entry side temperature (F0), exit side temperature (FT), and total rolling reduction of the finish rolling were as shown in Table 2. Next, under the conditions shown in Table 2, the finish rolled steel sheet was subjected to primary cooling first in the temperature region from the exit side temperature (FT) of the finish rolling to the martensite transf...

Claims

1. A high strength hot rolled steel sheet comprising a chemical composition comprising, by mass %,C: 0.050 to 0.100%,Si: 0.010 to 0.200%,Mn: 1.00 to 2.50%,Ti: 0.001 to 0.120%,Al: 0.001 to 0.050%,B: 0.0005 to 0.0050%,P: 0.100% or less,S: 0.050% or less,N: 0.0050% or less,O: 0 to 0.0050%,Cu: 0 to 0.20%,Ni: 0 to 0.20%,Sn: 0 to 0.10%,Cr: 0 to 0.40%,Mo: 0 to 0.20%,Nb: 0 to 0.05%,V: 0 to 0.10%,As: 0 to 0.100%,Zr: 0 to 0.100%,Ca: 0 to 0.0050%,Mg: 0 to 0.100%,Bi: 0 to 0.020%,Co: 0 to 0.20%,W: 0 to 0.20%,Zn: 0 to 0.20%,REM: 0 to 0.1000%, andbalance: Fe and impurities, whereinwhen a total width in a direction perpendicular to a rolling direction and sheet thickness direction is W, a microstructure at a ¼ position in sheet thickness at all positions of a 1 / 10W position, 3 / 10W position, 5 / 10W position, 7 / 10W position, and 9 / 10W position from an end in a width direction comprises, by area %,tempered martensite: 95% or more,fresh martensite: 5% or less, andat least one of ferrite, upper bainite, and pearlite: 5% or less in total, anda difference between a maximum value and minimum value in tensile strength at all the positions in the width direction is 30 MPa or less.

2. The high strength hot rolled steel sheet according to claim 1, wherein the chemical composition contains, by mass %, at least one ofO: 0.0001 to 0.0050%,Cu: 0.001 to 0.20%,Ni: 0.001 to 0.20%,Sn: 0.001 to 0.10%,Cr: 0.001 to 0.40%,Mo: 0.001 to 0.20%,Nb: 0.001 to 0.05%,V: 0.001 to 0.10%,As: 0.001 to 0.100%,Zr: 0.0001 to 0.100%,Ca: 0.0001 to 0.0050%,Mg: 0.0001 to 0.100%,Bi: 0.0001 to 0.020%,Co: 0.001 to 0.20%,W: 0.001 to 0.20%,Zn: 0.001 to 0.20%, andREM: 0.0001 to 0.1000%.

3. The high strength hot rolled steel sheet according to claim 1, wherein a prior austenite grain size in the microstructure is 40 μm or less.

4. The high strength hot rolled steel sheet according to claim 2, wherein a prior austenite grain size in the microstructure is 40 μm or less.

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