Steel sheet and outer panel member

Abstract

Provided is a steel sheet having a predetermined chemical composition and a microstructure comprising, by area %, ferrite: 75 to 95%, martensite: 5 to 25%, and at least one of bainite, pearlite, and retained austenite: 0 to 10% in total, wherein an amount of Nb in all of Nb carbonitrides is 0.004% or more and an amount of Nb in the Nb carbonitrides with a grain size of 20 nm or more is 60% or more of the amount of Nb in all of the Nb carbonitrides, a mean grain interval of the martensite is 2.5 μm or less, and a standard deviation in an area ratio of martensite in a direction vertical to a rolling direction and sheet thickness direction is 1.5% or less.

Description

FIELD

[0001] The present invention relates to a steel sheet, more particularly relates to a steel sheet excellent in appearance in applications of for example automobiles where external panel members, etc., are mainly used, and relates to such an outer panel member.BACKGROUND

[0002] To keep down the amount of emission of CO2 gas from automobiles, trials are under way to use a high strength steel sheet to secure safety while lightening car bodies. Remarkable progress is being made in such increase of strength of a steel sheet for automobile use in frame parts of automobiles, but in doors, hoods, and other outer panel members, a steel sheet having a tensile of strength 300 MPa or less strength class is mainly being used. Not much progress has been made in increasing the strength. High formability and good appearance are sought from such outer panel members. In general, if raising the strength of a steel sheet, the formability and appearance after forming fall. Therefore, in a high strength steel sheet, achieving both strength and formability and appearance, in particular, appearance after forming, is difficult. In the past, several proposals have been made for solving these problems.

[0003] For example, PTL 1 describes a steel sheet for hot dip galvanization use containing, by mass %, C: 0.02 to 0.3%, Si: 0.1 to 2.0%, Mn: less than 1.0%, Cr: more than 1.0 to 3.0%, P: 0.02% or less, S: 0.02% or less, Al: 0.014% or less, and N: 0.001 to 0.008%, where 2.5≤1.5Mn %+Cr %, 4.1−2.3Mn %−1.2Cr %≤Si % is satisfied, and having a balance of Fe and unavoidable impurities. Further, PTL 1 teaches that by optimizing the amounts of addition of Mn, Cr, and Si, it is possible to achieve both workability of a steel sheet for hot dip galvanization use with a tensile strength of 390 MPa or more and good appearance after forming enabling use as an outer panel for automobile use. Furthermore, PTL 1 teaches that by making the area ratio of the main phase ferrite 70% or more and the area ratio of the hard second phases including martensite 30% or less, it becomes possible to make all of the strength, yield strength, yield ratio, and strength-ductility balance good ranges.

[0004] PTL 2 describes a cold rolled steel sheet having a chemical composition containing, by mass %, C: 0.0005 to 0.01%, Si: 0.2% or less, Mn: 0.1 to 1.5%, P: 0.03% or less, S: 0.005 to 0.03%, Ti: 0.02 to 0.1%, Al: 0.01 to 0.05%, N: 0.005% or less, Sb: 0.03% or less, and Cu: more than 0.005% and 0.03% or less in a range satisfying 0<Ti*<0.02, where Ti* is shown by Ti*=(Ti %)−3.4×(N %)−1.5×(S %)−4×(C %), and further in a range satisfying (Sb %) (Cu %) / 5 and having a balance comprised of Fe and unavoidable impurities, where, at the two sides of the steel sheet, a content (mass %) of Ti element contained in precipitates with a size of less than 20 nm at sheet thickness surface layer parts down to 10 μm from the surfaces is 9% or less of the total Ti content (mass %) in the steel sheet. Further, PTL 2 teaches that by making the content (mass %) of Ti element contained in precipitates with a size of less than 20 nm at sheet thickness surface layer parts down to 10 μm from the surfaces of the two sides of the steel sheet a value of 9% or less of the total Ti content (mass %) in the steel sheet, it is possible to avoid occurrence of uneven appearance caused by such fine Ti-based precipitates, a cold rolled steel sheet excellent in surface properties is obtained, and further the cold rolled steel sheet can be optimally used for parts requiring excellent surface quality after forming such as outer panels of automobiles.CITATION LISTPatent Literature[PTL 1] Japanese Unexamined Patent Publication No. 2009-249737

[0006] [PTL 2] WO2011 / 142473SUMMARYTechnical Problem

[0007] For example, in the case of dual phase structure steel having a microstructure including soft ferrite and hard martensite such as described in PTL 1, at the time of press forming or other working, uneven deformation easily occurs where the soft ferrite and their surroundings deform preferentially. For this reason, if utilizing such dual phase steel comprised of soft structures and hard structures, fine asperities are formed on the surface of the steel sheet after forming whereby sometimes defects in appearance called “ghost lines” occur. Further, in applications for outer panels of automobiles such as described in PTLs 1 and 2, in relatively many cases, thin, broad width steel materials are being sought, but such thin, broad width steel materials have the problem that, in the production process, for example, when run through a continuous annealing processing line (CAPL), buckling called “heat buckling” easily occurs. If continuing operations with heat buckling occurring as is, sometimes this will lead to the sheet fracturing, etc. If sheet fracture, etc., occurs, it becomes necessary to stop the production line and perform reset work. The damage becomes tremendous.

[0008] Therefore, the present invention has as its object to provide, by a novel constitution, a steel sheet able to suppress the occurrence of heat buckling at the production process and able to achieve both strength and good appearance after forming.Solution to Problem

[0009] The inventors engaged in studies to achieve this object focusing on both the chemical composition and microstructure of a steel sheet. As a result, the inventors discovered that by rectifying the chemical composition of the steel sheet and suitably controlling the form and amount of the Nb carbonitrides, it is possible to raise the high temperature strength of a steel sheet and thereby suppress the occurrence of heat buckling at the production process and furthermore that by making the martensite contained in a predetermined ratio in the microstructure uniformly disperse at both of the micro regions and macro regions in the microstructure, it is possible to achieve the desired higher strength due to such hard structures and that even when strain is imparted due to press forming, etc., formation of fine asperities at the steel sheet surface is remarkably suppressed, and thereby completed the present invention.

[0010] The present invention able to achieve the above object is as follows:

[0011] (1) A steel sheet having a chemical composition comprising, by mass %,

[0012] C: 0.030 to 0.100%,

[0013] Mn: 0.70 to 3.00%,

[0014] Si: 0.005 to 1.500%,

[0015] P: 0.100% or less,

[0016] S: 0.0200% or less,

[0017] Al: 1.000% or less,

[0018] N: 0.0010 to 0.0150%,

[0019] O: 0.0100% or less,

[0020] Nb: 0.005 to 0.200%,

[0021] Cr: 0 to 1.00%,

[0022] Mo: 0 to 0.80%,

[0023] B: 0 to 0.0100%,

[0024] Ti: 0 to 0.200%,

[0025] V: 0 to 0.500%,

[0026] Ni: 0 to 1.00%,

[0027] Cu: 0 to 1.00%,

[0028] W: 0 to 1.00%,

[0029] Ta: 0 to 0.10%,

[0030] Co: 0 to 3.00%,

[0031] Sn: 0 to 1.00%,

[0032] Sb: 0 to 0.200%,

[0033] Ca: 0 to 0.0100%,

[0034] Mg: 0 to 0.0100%,

[0035] Zr: 0 to 0.0100%,

[0036] REM: 0 to 0.0100%,

[0037] Bi 0 to 0.0500%,

[0038] As: 0 to 0.10%, and

[0039] balance: Fe and impurities, wherein

[0040] an index A represented by following formula 1 is 0.50% or more, and

[0041] a microstructure comprising, by area %,

[0042] ferrite: 75 to 95%,

[0043] martensite: 5 to 25%, and

[0044] balance structures: 0 to 10% in total, wherein

[0045] an amount of Nb in all of Nb carbonitrides is 0.004% or more and an amount of Nb in the Nb carbonitrides with a grain size of 20 nm or more is 60% or more of the amount of Nb in all of the Nb carbonitrides,

[0046] a mean grain interval of the martensite is 2.5 μm or less, and

[0047] a standard deviation in an area ratio of martensite in a direction vertical to a rolling direction and sheet thickness direction is 1.5% or less.A=[C]-0⁢.1[Si]+0.3⁢([Mn]-0.5)-0⁢.3[Al]+0.1[Cr]+
0.6[Mo]-[Ti]+15[Nb]formula⁢ 1where, [C], [Si], [Mn], [Al], [Cr], [Mo], [Ti], and [Nb] are the contents of the elements [mass %], 0% when the elements are not contained.(2) The steel sheet according to (1), wherein the chemical composition comprises, by mass %, at least one of

[0050] Cr: 0.001 to 1.00%,

[0051] Mo: 0.001 to 0.80%,

[0052] B: 0.0001 to 0.0100%,

[0053] Ti: 0.001 to 0.200%,

[0054] V: 0.001 to 0.500%,

[0055] Ni: 0.001 to 1.00%,

[0056] Cu: 0.001 to 1.00%,

[0057] W: 0.001 to 1.00%,

[0058] Ta: 0.001 to 0.10%,

[0059] Co: 0.001 to 3.00%,

[0060] Sn: 0.001 to 1.00%,

[0061] Sb: 0.001 to 0.200%,

[0062] Ca: 0.0001 to 0.0100%,

[0063] Mg: 0.0001 to 0.0100%,

[0064] Zr: 0.0001 to 0.0100%,

[0065] REM: 0.0001 to 0.0100%,

[0066] Bi 0.0001 to 0.0500%, and

[0067] As: 0.001 to 0.10%.

[0068] (3) The steel sheet according to (1) or (2), wherein a mean crystal grain size of the ferrite is 3.0 to 25.0 μm, a mean crystal grain size of the martensite is 1.0 to 5.0 μm, and a mean aspect ratio of the martensite is 2.5 or more.

[0069] (4) An outer panel member including the steel sheet according to any of (1) to (3).Advantageous Effects of Invention

[0070] According to the present invention, it is possible to provide a steel sheet able to suppress the occurrence of heat buckling at the production process and able to achieve both strength and good appearance after forming.DESCRIPTION OF EMBODIMENTS<Steel Sheet>

[0071] The steel sheet according to an embodiment of the present invention has a chemical composition comprising, by mass %,

[0072] C: 0.030 to 0.100%,

[0073] Mn: 0.70 to 3.00%,

[0074] Si: 0.005 to 1.500%,

[0075] P: 0.100% or less,

[0076] S: 0.0200% or less,

[0077] Al: 1.000% or less,

[0078] N: 0.0010 to 0.0150%,

[0079] O: 0.0100% or less,

[0080] Nb: 0.005 to 0.200%,

[0081] Cr: 0 to 1.00%,

[0082] Mo: 0 to 0.80%,

[0083] B: 0 to 0.0100%,

[0084] Ti: 0 to 0.200%,

[0085] V: 0 to 0.500%,

[0086] Ni: 0 to 1.00%,

[0087] Cu: 0 to 1.00%,

[0088] W: 0 to 1.00%,

[0089] Ta: 0 to 0.10%,

[0090] Co: 0 to 3.00%,

[0091] Sn: 0 to 1.00%,

[0092] Sb: 0 to 0.200%,

[0093] Ca: 0 to 0.0100%,

[0094] Mg: 0 to 0.0100%,

[0095] Zr: 0 to 0.0100%,

[0096] REM: 0 to 0.0100%,

[0097] Bi 0 to 0.0500%,

[0098] As: 0 to 0.10%, and

[0099] balance: Fe and impurities, wherein

[0100] an index A represented by following formula 1 is 0.50% or more, and

[0101] a microstructure comprising, by area %,

[0102] ferrite: 75 to 95%,

[0103] martensite: 5 to 25%, and

[0104] balance structures: 0 to 10% in total, wherein

[0105] an amount of Nb in all of Nb carbonitrides is 0.004% or more and an amount of Nb in the Nb carbonitrides with a grain size of 20 nm or more is 60% or more of the amount of Nb in all of the Nb carbonitrides,

[0106] a mean grain interval of the martensite is 2.5 μm or less, and

[0107] a standard deviation in an area ratio of martensite in a direction vertical to a rolling direction and sheet thickness direction is 1.5% or less.A=[C]-0⁢.1[Si]+0.3⁢([Mn]-0.5)-0⁢.3[Al]+0.1[Cr]+
0.6[Mo]-[Ti]+15[Nb]formula⁢ 1where, [C], [Si], [Mn], [Al], [Cr], [Mo], [Ti], and [Nb] are the contents of the elements [mass %], 0% when the elements are not contained.

[0109] In recent years, there has been a rising need for lightening weight in outer panel members of automobiles (roofs, hoods, fenders, doors, etc.) For this reason, in the same way as the case of frame members, higher strength and reduced thickness are being sought for these outer panel members. On the other hand, from the viewpoint of avoiding surface defects called “surface strain” occurring at the time of press forming in these outer panel members, in many case use is being made of dual phase steel (DP steel) with its relatively low yield strength. However, in the case of DP steel, where soft structures comprised of ferrite and hard structures comprised of martensite are mixed together, at the time of press forming or other working, the soft structures and their surroundings deform preferentially resulting in uneven deformation easily occurring. Fine asperities are formed on the surface of the steel sheet after forming, whereby sometimes defects in appearance called “ghost lines” occur. Explained in more detail, at the time of press forming or other working, the soft structures made of ferrite deform by large amounts and sink into the surface of the steel sheet. On the other hand, the hard structures made of martensite deform in small amounts. For this reason, the hard structures do not sink into the surface of the steel sheet compared with the soft structures but appear rising up like projecting parts. As a result, in particular the amounts of deformation vary in the width direction of the steel sheet (direction perpendicular to rolling direction and sheet thickness direction) resulting in formation of ghost lines in a band shape (streaks). On the other hand, along with the higher strength of a steel sheet, sometimes Mn and other elements are added in relatively large amounts for improving the hardenability of a steel sheet. Mn is an element which easily segregates in streaks in the steel sheet. More particularly, concentrated Mn regions are formed due to center segregation and micro segregation at the time of casting. The concentrated Mn regions are stretched in the rolling direction due to hot rolling and cold rolling resulting in Mn segregating in streaks. For this reason, due to such segregation of Mn, regions with high hardenability and regions with low ones become present in the steel sheet. As a result, a relatively large number of banded hard structures are formed in the microstructure of the steel sheet after hardening. In this case, the occurrence of ghost lines becomes particularly remarkable. As opposed to this, if it were possible to sufficiently suppress Mn segregation in the steel sheet, formation of such banded hard structures could be reduced and the hard structures can be made to more uniformly disperse in the microstructure. In this case, even if strain is imparted due to press forming, etc., it is believed possible to sufficiently reduce the formation of fine asperities at the steel sheet surface and suppress the occurrence of ghost lines. However, along with the demands for higher strength, in particular when the amount of addition of Mn in the steel sheet becomes greater, in practice, it is extremely difficult to reliably and sufficiently suppress Mn segregation. For this reason, achieving both strength and good appearance after forming generally is considerably difficult.

[0110] Further, as explained previously, in outer panel applications of automobiles, thin and broad steel materials are relatively often sought. Such thin and broad steel materials suffer from the problem of easy buckling, called “heat buckling”, in the production process, for example, when run through a continuous annealing processing line (CAPL). Explained in more detail, in a CAPL, in general, hearth rolls provided with crowns with projecting shaped center parts are arranged. For this reason, if running a steel sheet through a CAPL, due to the projecting crowns of the hearth rolls, compressive stress is applied to the steel sheet at the center part in the width direction. On the other hand, continuous annealing is performed at a relatively high temperature. Along with the rise in the sheet temperature, the yield stress of the steel sheet falls. Therefore, in particular, when continuously annealing a thin and broad steel sheet on a CAPL, sometimes the drop in yield stress at the relatively high temperature causes the steel sheet to no longer be able to sufficiently withstand the above compressive stress. In such a case, the phenomenon called “heat buckling” where the sheet bends and wrinkles are formed occurs. If continuing operation in the state where heat buckling occurs, sometimes this leads to sheet fracture. If sheet fracture, etc., occurs, it becomes necessary to stop the production line and perform reset work. The loss becomes tremendous.

[0111] Therefore, first, the inventors studied raising the high temperature strength of a steel sheet from the two viewpoints of the chemical composition and the microstructure of the steel sheet so as to suppress or reduce the occurrence of such heat buckling. As a result, the inventors discovered that, from the viewpoint of the chemical composition of a steel sheet, by controlling the index A represented by the following formula 1 to 0.50% or more, it is possible to improve the high temperature strength of a steel sheet and thereby possible to suppress or reduce the occurrence of heat buckling.A=[C]-0⁢.1[Si]+0.3⁢([Mn]-0.5)-0⁢.3[Al]+0.1[Cr]+
0.6[Mo]-[Ti]+15[Nb]formula⁢ 1where, [C], [Si], [Mn], [Al], [Cr], [Mo], [Ti], and [Nb] are the contents of the elements [mass %], 0% when the elements are not contained.

[0113] While not intending to be bound by any specific theory, by controlling the index A to 0.50% or more, it is believed that not only is it possible to raise the high temperature strength of a steel sheet, but also it is possible to lower the Ac3 point of a steel sheet. As explained later in detail relating to the method of production of a steel sheet, in an embodiment of the present invention, in the primary heat treatment step corresponding to a CAPL after cold rolling, it is necessary to heat the steel sheet to a temperature higher than the Ac3 point where the result becomes an austenite single phase, more specifically the Ac3+10° C. or more. For this reason, it is possible to lower the Ac3 point to thereby lower the heating temperature of the primary heat treatment step and in turn possible to keep down the drop in yield stress of the steel sheet accompanying a rise in sheet temperature at the time of heating. Therefore, by controlling the index A to 0.50% or more, it becomes possible to remarkably improve the resistance of a steel sheet to the compressive stress causing heat buckling based on the positive raising of the high temperature strength of the steel sheet itself and the suppression of the drop in yield stress due to the drop in heating temperature at the primary heat treatment step.

[0114] Furthermore, the inventors discovered that, from the viewpoint of the microstructure of steel, by controlling the form and amount of Nb carbonitrides so that Nb carbonitrides having suitable size are present in a relatively large amount in the steel sheet, more specifically by controlling the form and amount of Nb carbonitrides so that the amount of Nb in all of the Nb carbonitrides is 0.004% or more and the amount of Nb in the Nb carbonitrides with a grain size of 20 nm or more becomes 60% or more of the amount of Nb in all of the Nb carbonitrides, it is possible to raise the high temperature strength of a steel sheet. By controlling the form and amount of Nb carbonitrides to within such ranges, it is possible to ensure the presence in the steel of a sufficient amount of Nb carbonitrides with grain sizes of 20 nm or more effective for raising the high temperature strength, therefore, by combination with the control of the chemical composition by the index A explained above, it is possible to remarkably raise the high temperature strength of a steel sheet and as a result it becomes possible to remarkably suppress or reduce the occurrence of heat buckling of a steel sheet at the production process.

[0115] Next, the inventors studied means for achieving both strength and good appearance after forming by establishing suitable ratios in the microstructure of the soft structures of ferrite and the hard structures of martensite to thereby realize the desired higher strength and furthermore improve the appearance after forming. Specifically, the inventors focused on the state of distribution of the hard structures of martensite in the microstructure, more specifically studied control of the distribution of martensite from another viewpoint different from reduction of segregation of Mn. As a result, as explained in detail later with regard to the method of production of a steel sheet, the inventors discovered that by making the microstructure in the steel sheet before the final annealing by structures mainly comprised of bainite and / or martensite and then final annealing the steel sheet having such a microstructure under predetermined conditions, it is possible to make martensite uniformly disperse in the finally obtained microstructure at both the micro regions and the macro regions without necessarily relying on the presence or degree of Mn segregation. More specifically, the inventors discovered that by final annealing a steel sheet having a microstructure comprised of bainite and / or martensite under predetermined conditions, at the micro regions, it is possible to control the mean grain interval of the martensite to 2.5 μm or less and, at the macro regions, to control the standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and sheet thickness direction to 1.5% or less. By controlling the mean grain interval of the martensite to 2.5 μm or less, it is possible to make the hard structures densely and uniformly disperse in the micro regions. By controlling the standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and sheet thickness direction to 1.5% or less, it is possible to remarkably reduce the variation in the hard structures at the macro regions. By satisfying both of these requirements, it is possible to form a microstructure with the hard structures of martensite finely and uniformly dispersed in the steel sheet as a whole. As a result, according to the steel sheet according to an embodiment of the present invention, it is possible to make the amount of deformation of the steel sheet more uniform, particularly in the width direction, even at the time of press forming or other forming and possible to achieve an excellent appearance after forming with ghost lines and other defects in appearance remarkably suppressed. For example, even if uniformity of martensite is secured at the micro regions, uniformity of martensite at the macro regions must be secured or else a microstructure in which martensite is dispersed finely and uniformly in the steel sheet as a whole cannot be formed. Similarly, even if uniformity of martensite is secured at the macro regions, uniformity of martensite at the micro regions must be secured or else locally martensite will be able to be present unevenly, therefore a microstructure in which martensite is dispersed finely and uniformly in the steel sheet as a whole cannot be formed. Therefore, in the steel sheet according to an embodiment of the present invention, in order to achieve an excellent appearance after forming in which ghost lines or other defects in appearance are remarkably suppressed, it is necessary to satisfy both the requirements of controlling the mean grain interval of martensite to 2.5 μm or less and controlling the standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and sheet thickness direction to 1.5% or less.

[0116] While not intending to be bound by any specific theory, to make martensite finely and uniformly disperse at the steel sheet as a whole in the microstructure of the finally obtained steel sheet, it is believed that it is extremely important to form a large number of austenite nuclei forming sites highly dispersed at the time of heating in the final annealing. In relation to this, martensite structures further have packets, blocks, laths, and other substructures in the retained austenite grains and therefore are structures having many various interfaces at their insides compared with ferrite and other structures. Bainite, in the same way as the case of martensite, are structures having many various interfaces at their insides. Therefore, by making the microstructure at the steel sheet before the final annealing by bainite and / or martensite, it becomes possible to form a very large number of carbides able to serve as austenite nuclei forming sites on these interfaces at the stage of heating such a microstructure in the final annealing. Therefore, by forming a large number of carbides on the interfaces, then further heating to a temperature of a dual phase of ferrite and austenite, it is believed possible to finely and uniformly form austenite on the steel sheet as a whole. Finally, by rapidly cooling the steel sheet having such a microstructure, martensite is formed from the austenite, therefore in the finally obtained microstructure, the mean grain interval of the martensite is controlled to 2.5 μm or less and the standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and sheet thickness direction is controlled to 1.5% or less. That is, it is believed possible to obtain a microstructure in which martensite is uniformly dispersed at both of the micro regions and macro regions. It is believed that by performing such heat treatment, it becomes possible to make martensite finely and uniformly disperse over the steel sheet as a whole enough to cancel out the effects of Mn segregation. In the past, it is believed study of the control of the distribution of hard structures from the viewpoint of reducing Mn segregation itself had been the general practice, therefore the fact that it is possible to make martensite uniformly disperse in the finally obtained microstructure at both the micro regions and macro regions without necessarily depending on the presence or degree of Mn segregation is extremely unexpected and should be surprising.

[0117] According to the steel sheet according to an embodiment of the present invention, in addition to the above findings relating to suppression of the occurrence of heat buckling and ghost lines, by controlling the area ratio of the soft structures of ferrite to 75 to 95%, it is possible to secure excellent formability and by controlling the area ratio of the hard structures of martensite to 5 to 25% and, further, controlling the chemical composition of the steel sheet to within a predetermined range, it is possible to secure a high strength of a tensile strength of 540 MPa or more. As a result, the occurrence of heat buckling at the production process can be suppressed and both strength and good appearance after forming can be realized at a high level.

[0118] Below, the steel sheet according to the present invention will be explained in more detail. In this Description, the units “%” of contents of the element, unless particularly indicated otherwise, mean “mass %”. Further, in this Description, “to”, unless particularly indicated otherwise, is used in a sense including the numerical values described before and after it as the lower limit value and upper limit value.[C: 0.030 to 0.100%]

[0119] C is an element securing a predetermined amount of martensite and raising the strength of a steel sheet. Further, C is also an austenite stabilizing element and is effective for lowering the Ac3 point. To sufficiently obtain these effects, the C content is 0.030% or more. The C content may also be 0.040% or more or 0.050% or more. On the other hand, if excessively including C, the strength becomes too high and sometimes the elongation ability will fall or else the standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and sheet thickness direction cannot be controlled to within a desired range. For this reason, the C content is 0.100% or less. The C content may also be 0.090% or less, 0.080% or less, 0.070% or less, or 0.060% or less.[Mn: 0.70 to 3.00%]

[0120] Mn is an element raising the hardenability and contributing to raising the steel sheet strength. Further, Mn is also an austenite stabilizing element and is effective for lowering the Ac3 point. To sufficiently obtain these effects, the Mn content is 0.70% or more. The Mn content may also be 0.80% or more, 1.00% or more, 1.20% or more, or 1.50% or more. In a preferable method of production of a steel sheet explained later, to make the martensite evenly disperse in the finally obtained microstructure at both the micro regions and macro regions, it is necessary to form the microstructure in the steel sheet before the final annealing by structures mainly comprised of bainite and / or martensite. For this reason, raising the hardenability by addition of Mn also can be said to be important in improving the appearance after forming. On the other hand, if excessively including Mn, it is not possible to sufficiently cancel out the effects due to Mn segregation and sometimes it becomes no longer possible to control the standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and sheet thickness direction to within the desired range. For this reason, the Mn content is 3.00% or less. The Mn content may also be 2.80% or less, 2.50% or less, 2.20% or less, or 2.00% or less.[Si: 0.005 to 1.500%]

[0121] Si is an element raising the strength of a steel sheet by solution strengthening. To sufficiently obtain such an effect, the Si content is 0.005% or more. The Si content may also be 0.010% or more, 0.100% or more, 0.200% or more, 0.300% or more, or 0.400% or more. On the other hand, if excessively including Si, removal of the scale formed by the hot rolling becomes difficult and deterioration of the appearance is sometimes invited. Therefore, the Si content is 1.500% or less. Further, Si is a ferrite stabilizing element, therefore by reducing the Si content, it is possible to lower the Ac3 point. For this reason, the Si content may also be 1.200% or less, 1.000% or less, 0.900% or less, 0.800% or less, 0.700% or less, or 0.600% or less.[P: 0.100% or Less]

[0122] P is an impurity element and an element causing embrittlement of a welded part or deterioration of the plateability. For this reason, the P content is 0.100% or less. The P content may also be 0.060% or less, 0.040% or less, 0.020% or less, or 0.010% or less. The P content is preferably as small as possible. The lower limit is not particularly prescribed and may also be 0%. On the other hand, if reducing the P content to less than 0.0001% in actual steel sheet, the production costs would greatly rise and the result would become economically disadvantageous. For this reason, the P content may also be 0.0001% or more, 0.0002% or more, or 0.0005% or more.[S: 0.0200% or Less]

[0123] S is an impurity element and an element impairing weldability and, further, impairing productivity at the time of casting and at the time of hot rolling. For this reason, the S content is 0.0200% or less. The S content may also be 0.0150% or less, 0.0120% or less, 0.0100% or less, 0.0060% or less, or 0.0030% or less. The S content is preferably as small as possible. The lower limit is not particularly prescribed and may also be 0%. On the other hand, if reducing the S content to less than 0.0001% in actual steel sheet, the production costs would greatly rise and the result would become economically disadvantageous. For this reason, the S content may also be 0.0001% or more, 0.0002% or more, or 0.0005% or more.[Al: 1.000% or Less]

[0124] Al is an element functioning as a deoxidizer and an element effective for raising the strength of steel. The Al content may also be 0%, but to sufficiently obtain these effects, the Al content is preferably 0.001% or more. The Al content may also be 0.005% or more, 0.010% or more, 0.025% or more, or 0.050% or more. On the other hand, if excessively including Al, coarse oxides are formed and sometimes the toughness falls. Therefore, the Al content is 1.000% or less. Further, Al is a stabilizing element of ferrite, therefore by reducing the Al content, it is possible to lower the Ac3 point. For this reason, the Al content may also be 0.800% or less, 0.600% or less, or 0.300% or less.[N: 0.0010 to 0.0150%]

[0125] N is an element effective for forming carbonitrides with Nb and raising the high temperature strength of a steel sheet. To sufficiently obtain such an effect, the N content is 0.0010% or more. The N content may also be 0.0015% or more, 0.0020% or more, 0.0025% or more, or 0.0030% or more. On the other hand, if excessively including N, sometimes this becomes a cause of formation of blow holes at the time of welding. For this reason, the N content is 0.0150% or less. The N content may also be 0.0120% or less, 0.0100% or less, 0.0080% or less, or 0.0060% or less.[O: 0.0100% or Less]

[0126] O is an element becoming a cause of formation of blow holes at the time of welding. For this reason, the O content is 0.0100% or less. The O content may also be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. The O content is preferably as small as possible. The lower limit is not particularly prescribed and may also be 0%. On the other hand, if reducing the O to less than 0.0001% in actual steel sheet, the production costs would greatly rise and the result would become economically disadvantageous. For this reason, the O content may also be 0.0001% or more, 0.0002% or more, or 0.0005% or more.[Nb: 0.005 to 0.200%]

[0127] Nb is an element effective for increasing the index A and raising the high temperature strength of a steel sheet. In particular, by adding Nb and forming a relatively large amount of Nb carbonitrides having suitable sizes, it is possible to further raise the high temperature strength of a steel sheet. To sufficiently obtain these effects, the Nb content is 0.005% or more. The Nb content may also be 0.010% or more, 0.015% or more, 0.020% or more, or 0.040% or more. On the other hand, even if excessively including Nb, the effect would become saturated and a rise in production costs is liable to be invited or sometimes coarse carbonitrides would be formed in the steel and the toughness of the steel sheet would be lowered. For this reason, the Nb content is 0.200% or less. The Nb content may also be 0.150% or less, 0.100% or less, 0.080% or less, or 0.060% or less.

[0128] The basic chemical composition of the steel sheet according to an embodiment of the present invention is as explained above. Furthermore, the steel sheet may, if necessary for the purpose of improving the properties, include at least one of the following optional elements in place of part of the balance of Fe. For example, the steel sheet may contain at least one of Cr: 0 to 1.00%, Mo: 0 to 0.80%, B: 0 to 0.0100%, Ti: 0 to 0.200%, V: 0 to 0.500%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, W: 0 to 1.00%, Ta: 0 to 0.10%, Co: 0 to 3.00%, Sn: 0 to 1.00%, Sb: 0 to 0.200%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, REM: 0 to 0.0100%, Bi: 0 to 0.0500%, and As: 0 to 0.10%. Below, these optional elements will be explained in detail.[Cr: 0 to 1.00%]

[0129] Cr, like Mn, is an element raising the hardenability and contributing to raising the steel sheet strength. The Cr content may also be 0%, but to obtain the above effect, the Cr content is preferably 0.001% or more. The Cr content may also be 0.01% or more, 0.10% or more, or 0.20% or more. On the other hand, even if excessively including Cr, the effect would become saturated and a rise in production costs is liable to be invited. Therefore, the Cr content is preferably 1.00% or less and may also be 0.80% or less, 0.60% or less, or 0.40% or less.[Mo: 0 to 0.80%]

[0130] Mo, like Nb, is an element contributing to raising the high temperature strength of a steel sheet. This effect can be obtained even with a trace amount. The Mo content may also be 0%, but to obtain the above effect, the Mo content is preferably 0.001% or more. The Mo content may also be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more. On the other hand, if excessively including Mo, sometimes the hot workability falls and the productivity falls. For this reason, the Mo content is preferably 0.80% or less. The Mo content may also be 0.60% or less, 0.50% or less, 0.40% or less, or 0.20% or less.[B: 0 to 0.0100%]

[0131] B is an element suppressing the formation of ferrite and pearlite and promoting the formation of martensite from austenite in the cooling process. Further, B is an element advantageous for raising the strength of steel. These effects can be obtained even in a trace amount. The B content may also be 0%, but to obtain the above effect, the B content is preferably 0.0001% or more. The B content may also be 0.0005% or more, or 0.0010% or more. On the other hand, if excessively including B, sometimes the toughness and / or weldability falls. For this reason, the B content is preferably 0.0100% or less. The B content may also be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.[Ti: 0 to 0.200%]

[0132] Ti is an element effective for control of the form of carbides. Due to Ti, an increase of strength of ferrite can be promoted. The Ti content may also be 0%, but to obtain these effects, the Ti content is preferably 0.001% or more. The Ti content may also be 0.002% or more, 0.010% or more, 0.020% or more, or 0.040% or more. On the other hand, even if excessively including Ti, the effect would become saturated and a rise in production costs is liable to be invited. Therefore, the Ti content is preferably 0.200% or less and may also be 0.100% or less, 0.080% or less, or 0.050% or less.[V: 0 to 0.500%]

[0133] V, like Ti, is an element effective for control of the form of the carbides and an element effective for refining the structures and raising the toughness of a steel sheet. The V content may also be 0%, but to obtain the above effect, the V content is preferably 0.001% or more. The V content may also be 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, if excessively including V, a large amount of precipitates are formed and sometimes the toughness is made to fall. For this reason, the V content is preferably 0.500% or less. The V content may also be 0.400% or less, 0.200% or less, or 0.100% or less.[Ni: 0 to 1.00%]

[0134] Ni is an element effective for raising the strength of a steel sheet. The content of Ni may also be 0%, but to obtain the above effect, the Ni content is preferably 0.001% or mor. The Ni content may also be 0.01% or more or 0.05% or more. On the other hand, if excessively including Ni, the weldability of the steel sheet sometimes falls. For this reason, the Ni content is preferably 1.00% or less. The Ni content may also be 0.80% or less, 0.40% or less, or 0.20% or less.[Cu: 0 to 1.00%]

[0135] Cu is an element contributing to raising the strength of a steel sheet. This effect can be obtained even with a trace amount. The Cu content may also be 0%, but to obtain the above effect, the Cu content is preferably 0.001% or more. The Cu content may also be 0.01% or more or 0.05% or more. On the other hand, if excessively including Cu, red shortness is invited and the productivity in the hot rolling is liable to be lowered. For this reason, the Cu content is preferably 1.00% or less. The Cu content may also be 0.80% or less, 0.60% or less, 0.30% or less, or 0.20% or less.[W: 0 to 1.00%]

[0136] W is an element effective for control of the form of the carbides and raising the strength of a steel sheet. The W content may also be 0%, but to obtain these effects, the W content is preferably 0.001% or more. The W content may also be 0.01% or more or 0.05% or more. On the other hand, if excessively including W, sometimes the weldability falls. For this reason, the W content is preferably 1.00% or less. The W content may also be 0.80% or less, 0.40% or less, or 0.20% or less.[Ta: 0 to 0.10%]

[0137] Ta, like W, is an element effective for control of the form of the carbides and raising the strength of a steel sheet. The Ta content may also be 0%, but to obtain these effects, the Ta content is preferably 0.001% or more. The Ta content may also be 0.01% or more or 0.03% or more. On the other hand, even if excessively including Ta, the effect would become saturated and more than the necessary amount would be contained in the steel sheet thereby inviting a rise in the production costs. For this reason, the Ta content is preferably 0.10% or less. The Ta content may also be 0.08% or less, 0.06% or less, or 0.04% or less.[Co: 0 to 3.00%]

[0138] Co, like Ni, is an element effective for raising the strength of a steel sheet. The Co content may also be 0%, but to obtain the above effect, the Co content is preferably 0.001% or more. The Co content may also be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, if excessively including Co, sometimes the hot workability will fall leading also to an increase in the raw material costs. For this reason, the Co content is preferably 3.00% or less. The Co content may also be 2.00% or less, 1.00% or less, 0.50% or less, or 0.20% or less.[Sn: 0 to 1.00%]

[0139] Sn is an element able to be contained in a steel sheet if using scrap as a raw material for the steel sheet. Further, Sn is liable to trigger embrittlement of the ferrite. For this reason, the Sn content is preferably as small as possible or 1.00% or less. The Sn content may also be 0.10% or less, 0.040% or less, or 0.02% or less. The Sn content may also be 0%, but reducing the Sn content to less than 0.001% would invite an excessive increase in the refining costs. For this reason, the Sn content may also be 0.001% or more, 0.005% or more, or 0.01% or more.[Sb: 0 to 0.200%]

[0140] Sb, like Sn, is an element able to be contained in a steel sheet if using scrap as a raw material for the steel sheet. Further, Sb is liable to strongly segregate at the ground boundaries and invite embrittlement of the grain boundaries. For this reason, the Sb content is preferably as small as possible, 0.200% or less. The Sb content may also be 0.100% or less, 0.040% or less, or 0.020% or less. The Sb content may also be 0%, but reducing the Sb content to less than 0.001% would invite an excessive increase in the refining costs. For this reason, the Sb content may also be 0.001% or more, 0.005% or more, or 0.010% or more.[Ca: 0 to 0.0100%][Mg: 0 to 0.0100%][Zr: 0 to 0.0100%][REM: 0 to 0.0100%]

[0141] Ca, Mg, Zr, and REM are elements contributing to raising the formability of a steel sheet. The Ca, Mg, Zr, and REM contents may also be 0%, but to obtain such an effect, the Ca, Mg, Zr, and REM contents are preferably 0.0001% or more and may also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if excessively containing these elements, sometimes the ductility of the steel sheet will fall. Therefore, Ca, Mg, Zr, and REM contents are preferably respectively 0.0100% or less and may also be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less. The “REM” in this Description is the general name of the 17 elements of atomic number 21 scandium (Sc), atomic number 39 yttrium (Y), and the lanthanoid atomic number 57 lanthanum (La) to atomic number 71 lutetium (Lu). The REM content is the total content of these elements.[Bi: 0 to 0.0500%]

[0142] Bi is an element refining the solidified structures and thereby having the action of raising the formability. The Bi content may also be 0%, but to obtain such an effect, the the Bi content is preferably 0.0001% or more and may also be 0.0005% or more, 0.0010% or more, or 0.0030% or more. On the other hand, even if excessively including Bi, the effect would become saturated and more than necessary inclusion in a steel sheet would invite a rise in the production costs. Therefore, the Bi content is preferably 0.0500% or less and may also be 0.0400% or less, 0.0200% or less, 0.0100% or less, or 0.0050% or less.[As: 0 to 0.10%]

[0143] As, like Sn and Sb, is an element able to be contained in a steel sheet if using scrap as a raw material for the steel sheet. Further, As is an element strongly segregating at the grain boundaries. The As content is preferably as small as possible. The As content is preferably 0.10% or less and may also be 0.04% or less or 0.02% or less. The As content may also be 0%, but reducing the As content to less than 0.001% would invite an excessive increase in the refining costs. For this reason, the As content may also be 0.001% or more, 0.005% or more, or 0.010% or more.

[0144] In the steel sheet according to an embodiment of the present invention, the balance besides the above elements is comprised of Fe and impurities. The impurities are elements entering from the steel raw materials and / or steelmaking process and allowed to be present to an extent not impairing the properties of the steel sheet according to an embodiment of the present invention.[Index A: 0.50% or More]

[0145] The chemical composition of the steel sheet according to an embodiment of the present invention has to have an index A represented by the following formula 1 of 0.50% or more:A=[C]-0⁢.1[Si]+0.3⁢([Mn]-0.5)-0⁢.3[Al]+0.1[Cr]+
0.6[Mo]-[Ti]+15[Nb]formula⁢ 1where, [C], [Si], [Mn], [Al], [Cr], [Mo], [Ti], and [Nb] are the contents of the elements [mass %], 0% when the elements are not contained. As explained previously, in the steel sheet according to an embodiment of the present invention, to suppress the occurrence of heat buckling at the production process, raising the high temperature strength of a steel sheet is extremely important. To raise the high temperature strength of a steel sheet, in addition to adding large amounts of Nb and Mo and other elements raising the high temperature strength of a steel sheet, it is effective to lower the heating temperature demanded in the primary heat treatment step explained in detail later with respect to the method of production of the steel sheet (i.e., Ac3+10° C. or more). By lowering the heating temperature demanded in the primary heat treatment step, it becomes possible to suppress the drop in yield stress of a steel sheet accompanying a rise in sheet temperature at the time of heating and remarkably raise the resistance to compressive stress at the center part in the width direction of the steel sheet causing heat buckling. From such a viewpoint, the inventors used experiments to investigate the degree of impact of different elements in steel relating to raising the high temperature strength of the steel sheet itself and the fall in the Ac3 point. More specifically, the inventors discovered that by controlling the index A, defined by coefficients considering the degrees of impact and the contents of these elements, i.e., the index A represented by the above formula 1, to 0.50% or more, it is possible to raise the resistance of the steel sheet to the above compressive stress due to the positive improvement in the high temperature strength of the steel sheet itself and suppression of reduction of the yield stress due to the drop in the heating temperature at the first heat treatment step and thereby possible to remarkably suppress the occurrence of heat buckling at the production process. From the viewpoint of suppression of the occurrence of heat buckling, the larger the index A, the more preferable. For example, it may be 0.55% or more, 0.60% or more, 0.65% or more, 0.70% or more, 0.75% or more, or 0.80% or more. The upper limit of the index A is not particularly limited, but, for example, the index A may also be 2.00% or less, 1.80% or less, 1.50% or less, 1.30% or less, or 1.10% or less.

[0147] The chemical composition of the steel sheet according to an embodiment of the present invention may be measured by a general analysis method. For example, the chemical composition of the steel sheet may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N using the inert gas melting-thermal conductivity method, and O using the inert gas melting-nondispersive type infrared absorption method.[Ferrite: 75 to 95%]

[0148] Ferrite is a soft structure, so easily deforms and contributes to raising the elongation. If the area ratio of ferrite is 75% or more, sufficient formability can be obtained. From the viewpoint of raising the formability, the higher the area ratio of ferrite, the more preferable. For example, it may be 78% or more, 80% or more, 82% or more, or 85% or more. On the other hand, if excessively containing ferrite, sometimes it is not possible to achieve the desired strength at the steel sheet. Therefore, the area ratio of ferrite is 95% or less. The area ratio of ferrite may also be 93% or less, 90% or less, or 87% or less.[Martensite: 5 to 25%]

[0149] Martensite is a hard structure with a high dislocation density, therefore is a structure contributing to raising the tensile strength. By making the area ratio of martensite 5% or more, for example, a tensile strength of 540 MPa or more can be secured. From the viewpoint of enhancing strength, the higher the area ratio of martensite, the more preferable. For example, it may be 7% or more, 10% or more, or 13% or more. On the other hand, if the area ratio of martensite is 25% or less, the formability and good appearance can be secure. The area ratio of martensite may also be 22% or less, 20% or less, 18% or less, or 15% or less In the present invention, “martensite” is not just as-quenched martensite (so-called “fresh martensite”) and also encompasses tempered martensite.[Balance Structures: 0 to 10% in Total]

[0150] The balance structures besides the ferrite and martensite may also be, by area ratio, 0%, but if balance structures are present, the balance structures comprise at least one of bainite, pearlite, and retained austenite. From the viewpoint of securing the above effects due to ferrite and martensite, the area ratio of the balance structures, i.e., at least one of bainite, pearlite, and retained austenite, is 10% or less in total. For example, it may also be 8% or less, 6% or less, 4% or less, or 2% or less. On the other hand, rendering the area ratio of the balance structures 0% requires advanced control in the production process of the steel sheet, therefore a drop in yield would sometimes be invited. Therefore, the area ratio of the balance structures may also be 0.5% or more or 1% or more.[Identification of Microstructure and Calculation of Area Ratios]

[0151] The microstructure is identified and the area ratios are calculated by examination under an FE-SEM (field emission type scan electron microscope) and an optical microscope and X-ray diffraction after corrosion using a Nital reagent (3% nitric acid ethanol solution). The structure is examined under the FE-SEM and the optical microscope at a 100 μm×100 μm region at the steel sheet cross-section in a direction perpendicular to the sheet surface by a 500 to 50000× power. For any microstructure, three locations are measured. The mean value of the measurement values is calculated to determine the area ratio. For example, if the sheet thickness of the steel sheet to be measured is thin and therefore a 100 μm measurement region in the sheet thickness direction cannot be secured, the length in the sheet thickness direction is reduced while securing a measurement region 10000 μm2. For example, a measurement region of 20 μm in the sheet thickness direction and 500 μm in a direction vertical to the sheet thickness direction may also be made the object of measurement. However, if the number of crystal grains included in the sheet thickness direction becomes too small, sometimes the measurement accuracy falls, so the measurement length in the sheet thickness direction is 10 μm or more, preferably 50 μm or more. The same is true of the “100 μm×100 μm region” in the following explanation.

[0152] In this Description, the “sheet thickness x / y position” (here, “x” and “y” are natural numbers satisfying x<y)” means the position when moved from the surface of the steel sheet in the sheet thickness direction (sheet surface) in the sheet thickness direction by exactly a distance of x / y of the sheet thickness “t” (depth) toward the center part of the steel sheet. For example, if the sheet thickness “t” of the steel sheet was 2 mm, the “sheet thickness ⅛ position” means the position becoming a depth of 0.25 mm from the surface of the steel sheet in the sheet thickness direction. Note that if the steel sheet has a plating layer or other coating at its surface, the “surface of the steel sheet” means the interface of the steel sheet and that coating. The “sheet thickness ‘t’” means the sheet thickness of the steel sheet (base material) after removal of the coating.

[0153] The area ratio of ferrite and the area ratio of martensite are found by the following procedure. First, the examined surface of the sample is corroded by a Nital reagent (3% nitric acid ethanol solution), then a 100 μm×100 μm region in the range of sheet thickness ⅛ to ⅜ centered at the sheet thickness ¼ is examined by an FE-SEM (for example, JSM-7200F made by JEOL, measured by an acceleration voltage of 15 kV at power of 500 to 2000×). With Nital corrosion, the martensite and retained austenite are not corroded, therefore the area ratio of the noncorroded regions corresponds to the total area ratio of the martensite and retained austenite. Specifically, the image analysis software Image J (Ver.1.54f) is used to digitalize the microstructure by differences in brightness. The black parts of the image data are ferrite, while the noncorroded white parts are the total structures of martensite and retained austenite. Therefore, the area ratio of ferrite is calculated from the area ratio of the regions of the black parts while the area ratio of the retained austenite measured by the X-ray diffraction method explained later is subtracted from the area ratio of the not corroded regions to calculate the area ratio of martensite. The area ratio of the martensite found by this method also includes the area ratio of the tempered martensite.

[0154] The area ratio of the retained austenite is calculated by the X-ray diffraction method. First, the portion of the sample from the sheet surface to the depth ¼ position in the sheet thickness direction is removed by mechanical polishing and chemical polishing. More specifically, the sample is thinned by mechanical polishing down to near the examined position, then is thinned down to the target position by chemical polishing (fluoric acid). Next, for example, an X-ray diffraction apparatus made by Rigaku (RINT2500, X-ray output 40 kV-200 mA) is used to calculate the structural fraction of the retained austenite at the sheet thickness ¼ position from the integrated intensity ratio of the diffraction peaks of the (200) and (211) of the bcc phase and the (200), (220), and (311) of the fcc phase obtained using MoKα rays. As the method of calculation, the general 5-peak method is utilized. The structural fraction of the retained austenite calculated is determined as the area ratio of retained austenite.

[0155] The bainite, pearlite, and retained austenite of the balance structures may, as explained above, be contained in a total ratio of 0 to 10%. That is, in the steel sheet according to the present embodiment, ferrite and martensite are the main components of the microstructure while bainite, pearlite, and retained austenite of the balance structures are components of the microstructure which be unavoidably formed in production. For this reason, inherently, there is no positive technical significance in identifying and measuring the area ratios of bainite, pearlite, and retained austenite as the balance structures. From the chemical composition and method of production of the steel sheet described in this Description, it is clear that the balance structures in the steel sheet according to the present embodiment consist of bainite, pearlite, retained austenite, or their complexes. Note that, as the methods for identification and measurement of bainite and pearlite as the balance structures, the following methods can be employed. The method of measurement of the area ratio of retained austenite is as explained above.

[0156] The bainite is identified and the area ratio is calculated by the following procedure. First, the examined surface of the sample is corroded by a Nital reagent, then a 100 μm×100 μm region in the range of sheet thickness ⅛ to ⅜ centered at the sheet thickness ¼ is examined by an FE-SEM (for example, JSM-7200F made by JEOL, measured by an acceleration voltage of 15 kV at power of 500 to 2000×). In this examined region, the bainite is identified as follows from the position of the cementite and arrangement of the cementite contained inside the structures in the examined region. Bainite is classified into upper bainite and lower bainite. Upper bainite has cementite or retained austenite present at the interfaces of the lath shaped bainitic ferrite. Lower bainite has cementite present inside the lath shaped bainitic ferrite. There is a single relationship of crystal orientation between bainitic ferrite and cementite. Cementite has the same variants. Upper bainite and lower bainite can be respectively identified by these characterizing points. In the present invention, these are together referred to as “bainite”. The area ratio of the identified bainite is calculated based on image analysis.

[0157] The pearlite is identified and the area ratio is calculated by the following procedure. First, the examined surface of the sample is corroded by a Nital reagent, then a range of sheet thickness ⅛ to ⅜ centered at the sheet thickness ¼ is examined by an SEM (for example, JSM-7200F made by JEOL, measured by an acceleration voltage of 15 kV at power of 500 to 2000×). In the examined image of the SEM, a region where lamellar cementite is observed is identified as pearlite. The area ratio of this region is calculated based on image analysis.[Amount of Nb in all Nb Carbonitrides of 0.004% or More and Amount of Nb in Nb Carbonitrides with Grain Size of 20 nm or More of 60% or More of Nb in all Nb Carbonitrides]

[0158] The steel sheet according to an embodiment of the present invention includes Nb carbonitrides in its microstructure. The amount of Nb in all Nb carbonitrides is 0.004% or more and the amount of Nb in Nb carbonitrides with a grain size of 20 nm or more is controlled to 60% or more of the amount of Nb in all of the Nb carbonitrides. In the present invention, the term “Nb carbonitrides” not only indicates NbCN, but also encompasses NbC and NbN. Furthermore, it includes NbCN, NbC, and NbN where part of the Nb is replaced by one or more elements of Ti, etc. The form and amount of the Nb carbonitrides can be controlled to within the above ranges so as to ensure the presence in the steel of a sufficient amount of Nb carbonitrides with a grain size of 20 nm or more effective for raising the high temperature strength, therefore by combination with the control of the chemical composition by the index A explained above, it is possible to remarkably raise the high temperature strength of a steel sheet. As a result, it becomes possible to reliably suppress or reduce the occurrence of heat buckling of a steel sheet at the production process. From the viewpoint of raising the high temperature strength of a steel sheet, the larger the amount of Nb in all of the Nb carbonitrides and the ratio of the amount of Nb in Nb carbonitrides with a grain size of 20 nm or more with respect to this, the more preferable. For example, the amount of Nb in all of the Nb carbonitrides may be 0.006% or more, 0.008% or more, 0.010% or more, or 0.012% or more. The upper limit is not particularly prescribed, but, for example, the amount of Nb in all of the Nb carbonitrides may also be 0.100% or less, 0.060% or less, 0.040% or less, or 0.030% or less. Similarly, the amount of Nb in Nb carbonitrides with a grain size of 20 nm or more may also be 62% or more, 65% or more, 68% or more, or 70% or more of the amount of Nb in all of the Nb carbonitrides. The upper limit is not particularly prescribed, but, for example, the amount of Nb in Nb carbonitrides with a grain size of 20 nm or more may also be 95% or less, 90% or less, or 85% or less of the amount of Nb in all of the Nb carbonitrides. If the grain size of the Nb carbonitrides is 20 nm or more, the effect of raising the high temperature strength is obtained. Even if the grain size is too large, the effect will not greatly fall. Therefore, the upper limit of the grain size in the Nb carbonitrides is not particularly prescribed, but, for example, the grain size of the Nb carbonitrides may also be 1000 nm or less, i.e., 1 μm or less.[Measurement of Amount of Nb in All Nb Carbonitrides and Ratio of Amount of Nb in Nb Carbonitrides with Grain Size of 20 nm or More with Respect to Same]

[0159] The amount of Nb in all of the Nb carbonitrides and the ratio of the amount of Nb in the Nb carbonitrides with a grain size of 20 nm or more with respect to the same are determined in the following way. First, a test piece is taken from the sheet thickness ½ position. The taken test piece is electrolyzed by constant current in an electrolytic solution (10 vol % acetylacetone-1 mass % tetramethylammonium chloride-methanol). The precipitate deposited on the test piece after electrolysis is made to disperse in a sodium hexametaphosphate aqueous solution, then is recovered by filtration by a pore size 0.02 μmφ (20 nmφ) porous filter. Next, the amount of Nb contained in the precipitate on the filter is measured by ICP atomic emission spectrometry and the content in the steel of the Nb precipitated as Nb precipitates with a grain size of 20 nm or more trapped in the filter is found. Further, the precipitates with a grain size of less than 20 nm contained in the filtrate passing through the filter is measured by ICP atomic emission spectrometry. The amount of Nb of the Nb precipitates with a grain size of 20 nm or more found by these methods and the amount of Nb of the Nb precipitates of less than 20 nm are added together to find the total mass of Nb precipitated as Nb carbonitrides. The obtained value is determined as the amount of Nb in all of the Nb carbonitrides. Further, the amount of Nb precipitating as Nb carbonitrides with a grain size of 20 nm or more is used to calculate the ratio with respect to the total mass of Nb precipitating as Nb carbonitrides. The calculated value is determined as the ratio of the amount of Nb carbonitrides with a grain size of 20 nm or more with respect the amount of Nb in all Nb carbonitrides.[Mean Grain Interval of Martensite: 2.5 μm or Less]

[0160] In an embodiment of the present invention, the mean grain interval of the hard structure martensite is controlled to 2.5 μm or less. The mean grain interval of the martensite is an index showing the uniformity of distribution of hard structures in the micro regions. The smaller the mean grain interval of the martensite, the denser and more uniformly the hard structures are dispersed is meant. Accordingly, the uniformity can be said to be high. The appearance of the steel sheet after press forming becomes better the more uniform the amount of deformation of the steel sheet at the time of press forming, in particular in the width direction of the steel sheet. The amount of deformation of a steel sheet is strongly affected by the state of distribution of hard structures, therefore to make the amount of deformation of the steel sheet uniform in the width direction of the steel sheet, it is necessary to make the distribution of hard structures in the microstructure uniform. By control of the mean grain interval to 2.5 μm or less in addition to control of the standard deviation in the area ratio of martensite explained later, it is possible to make the amount of deformation of the steel sheet more uniform in the width direction even at the time of press forming or other forming and as a result possible to achieve a good appearance after forming. The mean grain interval of the martensite is preferably 2.4 μm or less, more preferably 2.2 μm or less, most preferably 2.0 μm or less or 1.8 μm or less. The lower limit is not particularly prescribed, but, for example, the mean grain interval of the martensite may also be 0.5 μm or more, 0.8 μm or more, or 1.0 μm or more.[Measurement of Mean Grain Interval of Martensite]

[0161] The mean grain interval of the martensite is determined in the following way. First, a sample having a steel sheet cross-section in a direction vertical to the sheet surface is taken and that cross-section is used as the examined surface. In this examined surface, a 100 μm×100 μm region centered at the sheet thickness ¼ position in the range of the sheet thickness ⅛ position to ⅜ position is made the examined region and an FE-SEM (for example, JSM-7200F made by JEOL, acceleration voltage 15 kV, power 1000 to 5000× for measurement) is used to identify the martensite. Specifically, the image analysis software Image J (Ver.1.54f) is used to digitalize the microstructure by the difference in brightness to identify the martensite. Specifically, if using a Nital reagent, the black parts of the image data are ferrite while the not corroded white parts are the total structures of martensite and retained austenite. In the steel sheet according to an embodiment of the present invention, white structures are deemed martensite. Next, the distances between the centers of all adjoining martensite grains (centers of gravity) among the identified martensite are calculated as the grain intervals based on image analysis and the mean value of the calculated grain intervals is found. This operation is performed on another two examined regions, and the mean value of the three values obtained is determined as the mean grain interval of the martensite (strictly speaking, grains including martensite and / or retained austenite).[Standard Deviation in Area Ratio of Martensite in Direction Vertical to Rolling Direction and Sheet Thickness Direction of 1.5% or Less]

[0162] In an embodiment of the present invention, the standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and sheet thickness direction is controlled to 1.5% or less. The standard deviation is an index showing the uniformity of the hard structures in the macro regions. The appearance, which becomes an issue at the time of press-forming, depends on the fine asperities of the steel sheet surface due to the difference in amount of deformation of the steel sheet in the width direction. For this reason, if the fluctuation in the area ratio of the hard structures contained in the sheet thickness in a direction vertical to the rolling direction and sheet thickness direction is large, a difference arises in the amount of deformation of the steel sheet in the width direction and, as a result, fine asperities are formed on the steel sheet surface. Therefore, it is effective to reduce the standard deviation in the area ratio of martensite of the steel sheet in a direction vertical to the rolling direction and sheet thickness direction, i.e., the width direction. More specifically, by control of the standard deviation to 1.5% or less in addition to control of the mean grain interval of martensite explained above, it is possible to reduce the fluctuation of the amount of deformation of the steel sheet in the width direction even at the time of press forming and other forming and as a result possible to achieve good appearance after forming. The standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and sheet thickness direction is preferably 1.4% or less, more preferably 1.2% or less, most preferably 1.0% or less. The lower limit is not particularly prescribed, but, for example, the standard deviation may also be 0.1% or more, 0.3% or more, or 0.5% or more.[Measurement of Standard Deviation in Area Ratio of Martensite in Direction Vertical to Rolling Direction and Sheet Thickness Direction]

[0163] The standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and sheet thickness direction is determined in the following way. First, an image of the microstructure at a cross-section of the steel sheet in a region of 50 mm in the direction vertical to the rolling direction and sheet thickness direction is obtained. In the case of a 10 mm or smaller image, several images may be obtained and stitched together to obtain 50 mm. If the rolling direction is unclear, the cross-section is examined at orientations of 0°, 45°, 90°, and 135° with respect to any direction, the cross-section among these with the highest aspect ratio of precipitates is deemed the cross-section parallel to the rolling direction, and the direction vertical to that and the sheet thickness direction is identified as the direction vertical to the rolling direction and sheet thickness direction. Next, the obtained image is divided into 100 μm (0.1 mm) ranges in a direction vertical to the rolling direction and sheet thickness direction and the area ratio of martensite in the sheet thickness as a whole is calculated in each divided range. The standard deviation in the area ratio of martensite is calculated based on the area ratios of martensite calculated from a total of 500 divided images. The area ratio of martensite in each divided region is calculated in accordance with the procedure explained in the section on “Identification of Microstructure and Calculation of Area Ratio”. Note that for the area ratio of retained austenite, for convenience, instead of the measurement results at the different divided regions, it is also possible to make use of the measurement results at the steel sheet cross-section of the region 50 mm in the direction vertical to the rolling direction and sheet thickness direction.[Mean Crystal Grain Size of Ferrite: 3.0 to 25.0 μm]

[0164] According to a preferred embodiment of the present invention, the mean crystal grain size of the ferrite in the microstructure is 3.0 to 25.0 μm. The mean crystal grain size of the ferrite can be controlled to within such a fine range so as to further enhance the appearance of the steel sheet, in particular the appearance after forming. The mean crystal grain size of the ferrite may also be 5.0 μm or more, 7.0 μm or more, 8.0 μm or more, 9.0 μm or more, or 10.0 μm or more.

[0165] Similarly, the mean crystal grain size of the ferrite may also be 22.0 μm or less, 20.0 μm or less, 16.0 μm or less, 14.0 μm or less, or 12.0 μm or less.

[0166] The mean crystal grain size of the ferrite in the steel sheet is determined in the following way. First, a sample having a steel sheet cross-section in a direction vertical to the sheet surface is taken and that cross-section is used as the examined surface. In this examined surface, a 100 μm×100 μm region centered at the sheet thickness ¼ position in the range of the sheet thickness ⅛ position to ⅜ position is made the examined region. An FE-SEM (for example, JSM-7200F made by JEOL, acceleration voltage 15 kV, power 500 to 2000× for measurement) is used to identify the martensite. Specifically, the image analysis software Image J (Ver.1.54f) is used to digitalize the microstructure by the difference in brightness to identify the martensite. Specifically, if using a Nital reagent, the black parts of the image data are ferrite while the not corroded white parts are the total structures of martensite and retained austenite. Next, the circle equivalent sizes of all of the ferrite identified are calculated. This operation is performed on another two examined regions, the circle equivalent sizes of all of the ferrite obtained in the three examined regions are arithmetically averaged, and the obtained value is determined as the mean crystal grain size of the ferrite.[Mean Crystal Grain Size of Martensite: 1.0 to 5.0 μm]

[0167] According to a preferred embodiment of the present invention, the mean crystal grain size of the martensite in the microstructure is 1.0 to 5.0 μm. By controlling the mean crystal grain size of the ferrite to within such a fine range, it becomes possible to further enhance the appearance of the steel sheet, in particular the appearance after forming. The mean crystal grain size of the martensite may also be 1.2 μm or more, 1.5 μm or more, 1.7 μm or more, or 2.0 μm or more. Similarly, the mean crystal grain size of the martensite may also be 4.7 μm or less, 4.5 μm or less, 4.2 μm or less, 4.0 μm or less, 3.8 μm or less, 3.6 μm or less, or 3.4 μm or less.

[0168] The mean crystal grain size of the martensite is determined in the following way. First, a sample having a steel sheet cross-section in a direction perpendicular to the sheet surface is taken and that cross-section is used as the examined surface. In this examined surface, a 100 μm×100 μm region centered at the sheet thickness ¼ position in the range of the sheet thickness ⅛ position to ⅜ position is made the examined region. An FE-SEM (for example, JSM-7200F made by JEOL, acceleration voltage 15 kV, power 500 to 2000× for measurement) is used to identify the martensite. Specifically, the image analysis software Image J (Ver.1.54f) is used to digitalize the microstructure by the difference in brightness to identify the martensite. Specifically, if using a Nital reagent, the black parts of the image data are ferrite while the not corroded white parts are the total structures of martensite and retained austenite. In the steel sheet according to an embodiment of the present invention, the white structures are deemed martensite. Next, the circle equivalent sizes of all of the martensite identified are calculated. This operation is performed on another two examined regions, the circle equivalent sizes of all of the martensite obtained in the three examined regions are arithmetically averaged, and the obtained value is determined as the mean crystal grain size of the martensite (strictly speaking, grains including martensite and / or retained austenite).[Mean Aspect Ratio of Martensite: 2.5 or More]

[0169] According to a preferred embodiment of the present invention, the mean aspect ratio of the martensite in the microstructure is 2.5 or more. By controlling the mean aspect ratio of the martensite to 2.5 or more, it is possible to obtain a state where greater strain is imparted and the strength of a steel sheet can be raised. The mean aspect ratio of the martensite may also be 2.6 or more, 2.8 or more, or 3.0 or more. The upper limit is not particularly prescribed, but, for example, the mean aspect ratio of the martensite may also be 4.0 or less, 3.8 or less, or 3.6 or less.

[0170] The mean aspect ratio of the martensite is determined in the following way. First, a sample having a steel sheet cross-section in a direction vertical to the sheet surface is taken and that cross-section is used as the examined surface. In this examined surface, a 100 μm×100 μm region centered at the sheet thickness ¼ position in the range of the sheet thickness ⅛ position to ⅜ position is made the examined region and an FE-SEM (for example, JSM-7200F made by JEOL, acceleration voltage 15 kV, power 1000 to 5000× for measurement) is used to identify the martensite. Specifically, the image analysis software Image J (Ver.1.54f) is used to digitalize the microstructure by the difference in brightness to identify the martensite. Specifically, if using a Nital reagent, the black parts of the image data are ferrite while the not corroded white parts are the total structures of martensite and retained austenite. In the steel sheet according to an embodiment of the present invention, white structures are deemed martensite. In the obtained image data, the image analysis software Image J (Ver.1.54f) is used to calculate the aspect ratios of all of the martensite grains. The aspect ratio of a grain (crystal) on an image can be measured by a function mounted in the image analysis software Image J (Ver. 1.54f). Next, this operation is performed on another two examined regions, the aspect ratios of all of the martensite grains obtained in the three examined regions are arithmetically averaged, and the obtained value is determined as the mean aspect ratio of the martensite (strictly speaking, grains including martensite and / or retained austenite).[Sheet Thickness]

[0171] The steel sheet according to an embodiment of the present invention is not particularly limited, but for example has a 0.1 to 2.0 mm sheet thickness. A steel sheet having such a sheet thickness is optimal when used as a material for automobile members such as doors, hoods, and other outer panel members. The sheet thickness may be 0.2 mm or more, 0.3 mm or more, or 0.4 mm or more. Similarly, the sheet thickness may be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, or 1.0 mm or less. For example, by making the sheet thickness 0.2 mm or more, maintaining the shape of a part flat becomes easy and the additional effect can be obtained of improvement of the dimensional precision and shape precision. On the other hand, by making the sheet thickness 1.0 mm or less, the effect of lightening of the member becomes remarkable. The sheet thickness of the steel sheet is measured by a micrometer.[Plating]

[0172] The steel sheet according to an embodiment of the present invention is a cold rolled steel sheet, but may further include a plating layer on its surface for the purpose of raising the corrosion resistance, etc. The plating layer may be either a hot dip coated layer or an electroplated layer. That is, the steel sheet according to an embodiment of the present invention may also be a cold rolled steel sheet having a hot dip coated layer or electroplated layer on its surface. A “hot dip coated layer”, for example, includes a hot dip galvanized layer (GI), hot dip galvannealed layer (GA), hot dip aluminum coated layer, hot dip Zn—Al alloy coated layer, hot dip Zn—Al—Mg alloy coated layer, hot dip Zn—Al—Mg—Si alloy coating layer, etc. An “electroplated layer”, for example, includes an electrogalvanized layer (EG), an electro Zn—Ni alloy plated layer, etc. Preferably, the plating layer is a hot dip galvanized layer, hot dip galvannealed layer, or electro galvanized layer. The amount of deposition of the plating layer is not particularly prescribed and may also be a general amount of deposition.[Mechanical Properties]

[0173] According to the steel sheet having the above chemical composition and microstructure, it is possible to realize a high tensile strength, specifically, a tensile strength of 540 MPa or more. The tensile strength is preferably 570 MPa or more, more particularly 600 MPa or more. The upper limit is not particularly prescribed, but, for example, the tensile strength may also be 980 MPa or less, 900 MPa or less, 850 MPa or less, 830 MPa or less, or 800 MPa or less. By making the tensile strength 850 MPa or less, there is the advantage that formability at the time of press forming the steel sheet is easily secured. The tensile strength is measured by obtaining from the steel sheet a No. 5 tensile test piece of JIS Z2241: 2011 having a direction perpendicular to the rolling direction and sheet thickness direction as its test direction and conducting a tensile test compliant with JIS Z2241: 2011.

[0174] The steel sheet according to an embodiment of the present invention can suppress the occurrence of heat buckling at the production process and can achieve both high strength, for example, a tensile strength of 540 MPa or more, and excellent appearance after press forming or other forming. For this reason, the steel sheet according to an embodiment of the present invention is particularly useful for use in parts in technical fields where realization of both of these properties is sought, etc. In a preferred embodiment, outer panel members including the steel sheet according to an embodiment of the present invention, in particular outer panel members of automobiles, are provided. As examples of outer panel members of automobiles, roofs, hoods, fenders, doors, etc., where high aesthetic sense is sought may be mentioned. These outer panel members, in particular outer panel members of automobiles, need only include the steel sheet according to an embodiment of the present invention in at least parts of these outer panel members. Therefore, in at least parts of these outer panel members, the features of the chemical composition and the microstructure explained above are satisfied. At portions of the steel sheet with relatively low degrees of being worked in press forming and other forming, the features of the microstructure do not particularly change from before to after forming.<Method of Production of Steel Sheet>

[0175] Next, a preferable method of production of the steel sheet according to an embodiment of the present invention will be explained. The following explanation is intended to illustrate the characterizing method for producing the steel sheet according to an embodiment of the present invention and is not intended to limit the steel sheet to one produced by the following method of production.

[0176] The method of production of the steel sheet according to an embodiment of the present invention comprises

[0177] hot rolling including finish rolling of a slab having a chemical composition explained above in relation to the steel sheet, then coiling it, wherein the hot rolling satisfies conditions of following (a) to (d):

[0178] (a) a finish rolling entry side temperature is 1000 to 1080° C.,

[0179] (b) a rolling reduction of a rolling pass at two passes before a last pass is 30% or more,

[0180] (c) a ratio of a rolling reduction of a rolling pass at two passes before the last pass / rolling reduction of the last pass is 1.5 to 2.5, and

[0181] (d) a coiling temperature is 520 to 670° C.,

[0182] cold rolling the obtained hot rolled steel sheet by a rolling reduction of 70% or more,

[0183] primary annealing including heating the obtained cold rolled steel sheet to a temperature of Ac3+10° C. or more, and

[0184] secondary annealing including heating the cold rolled steel sheet and holding it at a temperature of (Ac1+20) to 820° C. for 10 to 500 seconds. Below, each step will be explained in detail.[Hot Rolling Step]

[0185] First, a slab having the chemical composition explained above in relation to the steel sheet is supplied to the hot rolling. The slab used is preferably cast by the continuous casting method from the viewpoint of productivity, but it may also be produced by the ingot making method or thin slab casting method. The slab is preferably heated to 1100° C. or more in advance of the hot rolling. By making the heating temperature 1100° C. or more, in the hot rolling, the rolling reaction force will not become excessively large and the target product thickness can be easily obtained. The upper limit of the heating temperature is not particularly prescribed, but from the economic viewpoint, the heating temperature is preferably 1300° C. or less. Further, the heated slab may optionally be rough rolled before the finish rolling so as to adjust the sheet thickness. Such rough rolling need only secure the desired sheet bar dimensions. The conditions are not particularly limited.[(a) Finish Rolling Entry Side Temperature: 1000 to 1080° C.]

[0186] The heated slab, or the slab additionally rough rolled in accordance with need, is next finish rolled. The finish rolling has to be performed under conditions giving an entry side temperature of the finish rolling becoming 1000 to 1080° C. By controlling the finish rolling entry side temperature to within such a range, in the hot rolling step, it is possible to make Nb carbonitrides suitably precipitate. For this reason, it is possible to sufficiently raise the high temperature strength of a steel sheet by such suitable precipitation of Nb carbonitrides and becomes possible to remarkably suppress the occurrence of heat buckling by the high temperature heat treatment in the subsequent primary annealing step, etc. If the finish rolling entry side temperature is higher than 1080° C., recrystallization easily occurs, strain becomes difficult to accumulate in the latter stage of the finish rolling, and precipitation of Nb carbonitrides having a 20 nm or more grain size no longer can be promoted. As a result, it becomes no longer possible to achieve the desired ratio of the amount of Nb in the Nb carbonitrides having a 20 nm or more grain size with respect to the amount of Nb in all of the Nb carbonitrides. On the other hand, if the finish rolling entry side temperature is lower than 1000° C., strain accumulates in the latter stage of the finish rolling, but precipitation itself of Nb carbonitrides becomes harder to occur and the grain size of Nb carbonitrides becomes smaller. As a result, similarly it becomes no longer possible to achieve the desired ratio of the amount of Nb in the Nb carbonitrides having a 20 nm or more grain size with respect to the amount of Nb in all of the Nb carbonitrides.[(b) Rolling Reduction of Rolling Pass at Two Passes Before Last Pass: 30% or More][(c) Ratio of Rolling Reduction of Rolling Pass at Two Passes Before Last Pass / Rolling Reduction of Last Pass: 1.5 to 2.5]

[0187] In the present method of production, the finish rolling is performed using a tandem rolling mill comprised of a plurality of rolling stands, for example, five or more rolling stands. In the finish rolling of the present method of production, control of the rolling reduction in the latter stage of the finish rolling is important. More specifically, it is important to control the rolling reduction of the rolling pass at two passes before the last pass to 30% or more and control the ratio of the rolling reduction of the rolling pass at two passes before the last pass / rolling reduction of last pass to 1.5 to 2.5. By controlling the rolling reduction in the latter stage of the finish rolling under such conditions, strain is suitably accumulated at the latter stage of the finish rolling and Nb carbonitrides with a grain size of 20 nm or more can be made to precipitate in a desired ratio. For this reason, it becomes possible to remarkably suppress the occurrence of heat buckling even due to high temperature heat treatment in the later primary annealing step, etc. As opposed to this, for example, if the rolling reduction of the rolling pass at two passes before the last pass is low and in turn the ratio of the rolling reduction of the rolling pass at two passes before the last pass / rolling reduction of last pass becomes less than 1.5, strain is insufficiently accumulated after rolling reduction at the rolling pass at two passes before the last pass, and Nb carbonitrides with a grain size of 20 nm or more insufficiently precipitate. On the other hand, if the rolling reduction of the last pass is high and in turn the ratio of the rolling reduction of the rolling pass at two passes before the last pass / rolling reduction of last pass becomes less than 1.5, strain remarkably accumulates after rolling reduction at the last pass and fine Nb carbonitrides precipitating after finish rolling increase, i.e., the ratio of Nb carbonitrides with a grain size of 20 nm or more becomes smaller. Therefore, in either case, it becomes no longer possible to achieve the desired ratio of the amount of Nb in the Nb carbonitride having 20 nm or more grain size to the amount of Nb in all of the Nb carbonitrides.

[0188] On the other hand, for example, if the rolling reduction of the rolling pass at two passes before the last pass is high or the rolling reduction of last pass is low and in turn the ratio of the rolling reduction of the rolling pass at two passes before the last pass / rolling reduction of last pass becomes more than 2.5, strain accumulates too much after rolling in the rolling pass at two passes before the last pass and recrystallization easily occurs. As a result, Nb carbonitrides with a grain size of 20 nm or more become difficult to precipitate and the desired ratio of the amount of Nb in the Nb carbonitrides having a 20 nm or more grain size with respect to the amount of Nb in all of the Nb carbonitrides can no longer be realized.[(d) Coiling Temperature: 520 to 670° C.]

[0189] Next, the finish rolled rolled material is coiled up at a 520 to 670° C. coiling temperature. By suitably controlling the coiling temperature to such a temperature region, it is possible to suppress growth of scale and make the microstructure fine and uniform. This is important for obtaining the desired state of dispersion of martensite in the finally obtained microstructure. If the coiling temperature is more than 670° C., the alloy elements concentrate at the cementite in the microstructure leading to undissolved carbides ending up remaining at the time of heating in the later primary annealing step. As a result, in the primary annealing step, the microstructure cannot be made one made of structures mainly comprised of bainite and / or martensite and the desired state of dispersion of martensite can no longer be obtained even by the later secondary annealing step. More specifically, even by the subsequent secondary annealing step, it is not possible to control the mean grain interval of martensite to 2.5 μm or less and / or no longer becomes possible to control the standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and sheet thickness direction to 1.5% or less, i.e., it becomes no longer possible to obtain a microstructure in which martensite is uniformly dispersed in both of the micro regions and macro region. In this case, it becomes no longer possible to sufficiently suppress the occurrence of ghost lines, etc., and the appearance after forming falls in level.

[0190] On the other hand, if the coiling temperature is lower than 520° C., the amount of dissolved Nb becomes greater and in turn less Nb carbonitrides precipitate. For this reason, it becomes no longer possible to achieve the desired amount of Nb in all of the Nb carbonitrides and / or it becomes no longer possible to achieve the desired ratio of amount of Nb in the Nb carbonitrides having a 20 nm or more grain size with respect to the amount of Nb in all of the Nb carbonitrides. In such a case, it becomes no longer possible to sufficiently raise the high temperature strength of a steel sheet, therefore it becomes no longer possible to sufficiently suppress or reduce the occurrence of heat buckling in the high temperature heat treatment of the subsequent primary annealing step, etc.[Cold Rolling Step]

[0191] The obtained hot rolled steel sheet is suitably pickled to remove scale and then is sent on to the cold rolling step. In the cold rolling step, the hot rolled steel sheet is cold rolled so that the rolling reduction becomes 70% or more. By controlling the rolling reduction to such a range, it is possible to secure the desired sheet thickness and also make the recrystallization at the time of the heating of the primary annealing step finish quickly and make the desired state of precipitation of the Nb carbonitrides more reliable. If the rolling reduction is less than 70%, recrystallization is delayed at the time of heating of the primary annealing step and strain remains until reaching a high temperature. As a result, a large amount of fine Nb carbonitrides precipitate during the heating in the primary annealing step and it becomes no longer possible to the desired ratio of amount of Nb in the Nb carbonitrides having 20 nm or more grain sizes with respect to the amount of Nb of all of the Nb carbonitrides. On the other hand, the rolling reduction of the cold rolling step is preferably 90% or less. By making the rolling reduction 90% or less, it is possible to prevent the rolling load from becoming excessive and rolling becoming difficult. The number of rolling passes and the rolling reduction of each pass are not particularly limited and may be suitably set so that the rolling reduction of the cold rolling as a whole becomes the above range.[Primary Annealing Step]

[0192] The obtained cold rolled steel sheet is heated in the next primary annealing step to the Ac3+10° C. or more in temperature. The Ac3 point (° C.) is found by cutting out a small piece from the cold rolled steel sheet and determining the heat expansion of that small piece while heating from room temperature by 10° C. / s up to 1000° C. By heating the cold rolled steel sheet to the Ac3+10° C. or more temperature, austenization is promoted. By then suitably cooling, for example, by cooling in the temperature region down to 200° C. by a mean cooling speed of 30° C. / s or more, the microstructure in the cooled steel sheet can be reliably made by structures mainly comprised of bainite and / or martensite, for example, full bainite or full martensite. Here, the “structures mainly comprised of bainite and / or martensite” mean structures at least one of bainite and martensite in a total area ratio of 90% or more, “full bainite” means structures compared of an area ratio 100% bainite, and “full martensite” means structures comprised of an area ratio 100% martensite. Bainite and / or martensite structures are structures having a larger number of various interfaces inside them compared with ferrite and other structures. For this reason, by configuring the microstructure in the steel sheet before the secondary annealing step, i.e., before the final annealing step, by structures mainly comprised of bainite and / or martensite, it becomes possible to form an extremely large number of dispersed carbides able to serve as nuclei forming sites of austenite on the interfaces at the stage of heating such a microstructure in secondary annealing. As a result, by forming austenite finely and uniformly in the steel sheet as a whole from the nuclei forming sites dispersed in large numbers in this way and then forming martensite from these austenite, in the microstructure obtained after secondary annealing, the mean grain interval of the martensite is controlled to 2.5 μm or less and the standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and sheet thickness direction is controlled to 1.5% or less. That is, it becomes possible to realize a microstructure where the martensite is evenly dispersed at both of the micro regions and the macro regions.

[0193] If the heating temperature in the primary annealing step is less than Ac3+10° C., austenization will become insufficient and even with subsequent cooling, the microstructure in the steel sheet is no longer able to be formed by structures mainly comprised of bainite and / or martensite, i.e., the total of the area ratios of the bainite and martensite is no longer able to be made 90% or more. As a result, in the finally obtained microstructure, the mean grain interval of the martensite cannot be controlled to 2.5 μm or less and / or the standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and sheet thickness direction can no longer be controlled to 1.5% or less. In this case, occurrence of ghost lines, etc., can no longer be sufficiently suppressed and the appearance after forming falls. On the other hand, heating at a higher temperature would make the productivity fall, therefore the heating temperature in the first annealing step is preferably 1050° C. or less. The holding time at the heating temperature is preferably 10 to 500 seconds.[Secondary Annealing Step (Final Annealing Step)]

[0194] The primary annealed cold rolled steel sheet is again heated in the next secondary annealing step and is held at an (Ac1+20) to 820° C. temperature for 10 to 500 seconds. Here, the Ac1 point (° C.), like the case of the Ac3 point, is found by cutting out a small piece from the cold rolled steel sheet and determining the thermal expansion at the small piece while heating it from room temperature by 10° C. / s to 1000° C. First, at the stage of heating the steel sheet after primary cooling up to the Ac1 to 820° C. temperature, it is possible to form carbides dispersed at a large number of interfaces included inside the bainite and / or martensite in the microstructure. Next, by holding at the Ac1 to 820° C. temperature, corresponding to the dual phase region of ferrite and austenite, for 10 to 500 seconds, it is possible to maintain a state with carbides dispersed on the interfaces while forming austenite from the carbides finely and uniformly at the steel sheet as a whole. Finally, by suitably cooling the steel sheet, for example, by cooling in the temperature region down to 500° C. by a mean cooling speed of 10° C. / s or more, it is possible to suitably form martensite from the finely dispersed austenite and, as a result, the mean grain interval of the martensite is controlled to 2.5 μm or less and the standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and sheet thickness direction is controlled to 1.5% or less. That is, it becomes possible to realize a microstructure with martensite evenly dispersed at both the micro regions and macro regions.

[0195] If the heating temperature at the secondary annealing step is less than Ac1+20° C. or the holding time is less than 10 seconds, the above-mentioned desired microstructure cannot be obtained. On the other hand, if the heating temperature is more than 820° C., the area ratio of austenite becomes too high and the area ratio of ferrite cannot be made 75% or more. Furthermore, it becomes no longer possible to maintain a state where the carbides are dispersed on the interfaces due to the high temperature and it becomes no longer possible to realize uniform dispersion of martensite in the finally obtained microstructure in both of the micro regions and macro regions. Further, if the holding time is more than 500 seconds, the austenite grains coarsen and the martensite grains obtained by subsequent cooling also become relatively coarse. In such a case, it becomes no longer possible to obtain fine martensite structures with mean grain intervals of the martensite of 2.5 μm or less.[Plating Step]

[0196] For the purpose of raising the corrosion resistance, etc., if necessary, the surface of the obtained cold rolled steel sheet may also be plated. The plating may be hot dip coating, alloyed hot dip coating, electroplating, or other treatment. For example, the steel sheet may be hot dip galvanized as plating treatment and may be alloyed after hot dip galvanization. The specific conditions of the plating treatment and alloying treatment are not particularly limited and may be any suitable conditions known to persons skilled in the art. For example, the alloying temperature may be 450 to 600° C.

[0197] Below, examples will be illustrated to more specifically explain the present invention, but the present invention is not limited by these examples in any way.Examples

[0198] In the following examples, steel sheets according to an embodiment of the present invention were produced under various conditions and investigated for the presence of any heat buckling in the production process and for the properties of the tensile strength and appearance after forming of the obtained steel sheets.

[0199] First, slabs having the chemical compositions shown in Table 1 and having thicknesses of 200 to 300 mm were cast by the continuous casting method. The balance besides the constituents shown in Table 1 is comprised of Fe and impurities. Next, these slabs were heated to 1100 to 1300° C. in temperature, then were hot rolled. The hot rolling was performed by rough rolling and finish rolling. More specifically, the rough rolling was performed under the same conditions in all of the examples and comparative examples while the finish rolling was performed using a tandem rolling mill comprised of seven rolling stands. The rolling reduction of the rolling pass (F5 rolling pass) at two passes before the last pass in the finish rolling was 30%.

[0200] Condition I in the hot rolling of the examples: finish rolling entry side temperature 1000 to 1080° C., Condition II: ratio of rolling reduction of rolling pass at two passes before the last pass / rolling reduction of last pass (F5 / F7 rolling reduction ratio) 1.5 to 2.5, and Condition III: whether or not coiling temperature of 520 to 670° C. is satisfied are shown in Table 2. Specifically, in examples satisfying Condition I, the finish rolling entry side temperature was 1050° C. while in examples not satisfying Condition I, the finish rolling entry side temperature was 950° C. (Comparative Example 9) or 1120° C. (Comparative Example 19). Further, in examples satisfying Condition II, the F5 / F7 rolling reduction ratio was 2.0 while in examples not satisfying Condition II, the F5 / F7 rolling reduction ratio was 1.2 (Comparative Example 2) or 3.0 (Comparative Example 20). Further, in examples satisfying Condition III, the coiling temperature was 600° C. while in examples not satisfying Condition III, the coiling temperature was 470° C. (Comparative Example 10) or 700° C. (Comparative Example 21).

[0201] Next, the obtained hot rolled steel sheet was pickled, then cold rolled and primary annealed (at predetermined heating temperature for holding time 100 seconds and mean cooling speed down to 200° C. after annealing of 40° C. / s) and secondary annealed (at 770° C. heating temperature for holding time 100 seconds and mean cooling speed down to 500° C. after annealing of 15° C. / s) to produce sheet thickness 0.4 mm cold rolled steel sheet. The 770° C. heating temperature satisfies the requirement of Ac1+20° C. or more for all of the invention examples and comparative examples. Cases where Condition IV of cold rolling (rolling reduction of 70% or more) and Condition V of primary annealing (heating temperature Ac3+10° C. or more) are satisfied and are not satisfied are shown in Table 2. Specifically, in examples satisfying Condition IV, cold rolling was performed by a rolling reduction of 80%, while in examples not satisfying Condition IV, cold rolling was performed by a rolling reduction of 60% (Comparative Examples 3 and 9). Further, in examples satisfying Condition V, primary annealing was performed while heating to the Ac3+15° C. or more, i.e., 900° C. On the other hand, in examples not satisfying Condition V, the primary annealing was performed while heating to a less than Ac3 temperature (Comparative Examples 4 and 20).

[0202] Finally, the surface of the obtained cold rolled steel sheet was suitably plated and formed with a hot dip galvanized layer (GI), hot dip galvannealed layer (GA), or electrogalvanized (EG) layer. For the hot dip galvannealed layer (GA), the alloying conditions were 550° C. for 20 seconds.TABLE 1Chemical composition (mass %), balance: Fe and impuritiesSteelCMnSiPSAlNONbCrMoBTiOthersIndex ARemarksA0.0462.020.0980.0140.00180.0310.00240.00090.0180.210.00160.0190.75Inv. ex.B0.0482.210.2010.0110.00220.0090.00350.00100.0110.210.250.87Inv. ex.C0.0622.040.1020.0180.00260.0450.00350.00120.0150.100.00200.0120.77Inv. ex.D0.0421.780.0130.0580.00110.0250.00450.00180.0090.00160.031V: 0.0850.52Inv. ex.E0.0562.160.0570.0220.00080.1250.00310.00180.0080.0210.61Inv. ex.F0.0491.560.2920.0120.00310.0340.00390.00120.0420.080.150.0019Cu: 0.18, Ni: 0.091.06Inv. ex.G0.0601.620.1540.0290.00210.0350.00560.00090.0160.550.046W: 0.12, Co: 0.110.62Inv. ex.H0.0811.910.0860.0670.00130.1050.00400.00190.0070.0025Ta: 0.05, Sn: 0.040.57Inv. ex.I0.0552.510.1840.0190.00170.2420.00350.00150.0150.79Inv. ex.J0.0601.720.1510.0200.00280.0540.00210.00150.0220.070.035Zr: 0.00150.73Inv. ex.K0.0601.700.2850.0260.00210.0510.00380.00150.0140.530.050.0100.66Inv. ex.L0.0621.730.1530.0240.00240.0570.00350.00150.0090.480.070.00130.023Ca: 0.00150.60Inv. ex.M0.0681.250.1120.0200.00540.2430.00180.00120.0120.250.340.62Inv. ex.N0.0461.950.1010.0150.00180.0310.00290.00080.0180.180.00160.021Sb: 0.0900.73Inv. ex.O0.0482.210.2010.0110.00220.0090.00350.00100.0110.210.25Mg: 0.0034,0.87Inv. ex.REM: 0.0020P0.0592.020.0540.0090.00260.0450.00350.00120.0180.100.00220.010Bi: 0.0400,0.82Inv. ex.As: 0.03Q0.0581.980.8890.0080.00140.0140.00210.00210.0080.060.00150.0210.54Inv. ex.R0.0501.670.1050.0180.00210.0280.00610.00080.0810.00180.011Cu:0.19, Ni:0.101.59Inv. ex.S0.0462.010.2180.0130.00200.1750.00410.00130.120.00310.0120.42Comp. ex.T0.0511.850.3590.0130.00200.4250.00360.00130.0070.310.00180.015Cu: 0.24, Ni: 0.100.41Comp. ex.U0.1092.240.1530.0250.00280.0340.00350.00150.0150.400.100.93Comp. ex.V0.0613.110.1250.0330.00260.0330.00300.00140.0090.0130.94Comp. ex.W0.0520.660.0570.0150.00130.1050.00410.00110.0270.250.080.00130.018Co: 0.150.52Comp. ex.X0.0261.810.3510.0190.00290.0300.00330.00150.0100.120.00170.60Comp. ex.Underlines indicate outside scope of present invention.TABLE 2Hot rollingColdPrimaryCondition ICondition IIrollingannealingFerriteFinishF5 / F7Condition IIICondition IVCondition VArea ratios ofmeanrolling entryrollingCoilingRollingHeatingmicrostructures (%)crystalSteelside temp.reductiontemp.reductiontemp.Type ofBalancegrain sizeno.Steel(° C.)ratio(° C.)(%)(° C.)platingFerriteMartensitestructures(μm)1A10502.060080900GA881208.42A10501.260080900GA871308.23A10502.060060900GA871218.84A10502.060080820GA8911012.55B10502.060080900GA861319.16C10502.060080900GI831707.37D10502.060080900GA91 816.68E10502.060080900GA851328.29E9502.060060900GA871217.510E10502.047080900GA881208.011F10502.060080900None901008.912G10502.060080900GA881208.413H10502.060080900GA772129.614I10502.060080900EG8713010.615J10502.060080900GI881207.916K10502.060080900GA851419.117L10502.060080900GA851508.618M10502.060080900GA8713011.819M11202.060080900GA8713012.020M10503.060080810GA8613112.321M10502.070080900GA8713011.422N10502.060080900GA891108.423O10502.060080900GA8812010.124P10502.060080900GA861408.925Q10502.060080900GA87 9425.826R10502.060080900GA901002.927S10502.060080900None8911014.828T10502.060080900GA8812011.229U10502.060080900GA8218010.530V10502.060080900GI851509.531W10502.060080900GA89 011 12.832X10502.060080900GA97 3018.5MartensiteNb carbonitridesMeanStandardGraincrystalMeandeviationsize 20BucklingPropertiesgrainMeangrainof areaTotalnm orafterAppearanceSteelsizeaspectintervalratioNb am'tmore NbprimaryTSafterno.(μm)ratio(μm)(%)(%)ratio)annealing(MPa)formingRemarks11.83.12.21.00.01265OK6641Inv. ex.22.02.91.80.90.01452NG6572Comp. ex.31.93.32.00.90.01450NG6702Comp. ex.42.91.84.83.50.01372OK6355Comp. ex.52.12.61.91.20.00870OK6771Inv. ex.61.62.71.70.80.01068OK7883Inv. ex.71.43.02.30.70.00872OK6121Inv. ex.81.92.82.01.10.00766OK6892Inv. ex.91.92.61.81.00.00745NG6782Comp. ex.101.82.81.91.10.00338NG6902Comp. ex.112.92.91.90.80.02962OK6232Inv. ex.122.53.41.71.10.01368OK6491Inv. ex.132.13.51.41.30.00671OK8223Inv. ex.141.82.91.81.00.01175OK6692Inv. ex.151.72.81.80.90.01770OK6592Inv. ex.162.12.61.70.80.01062OK6782Inv. ex.172.22.81.51.10.00764OK6701Inv. ex.182.53.01.80.90.00968OK6692Inv. ex.192.72.71.80.90.00848NG6723Comp. ex.202.81.94.14.20.00944NG6685Comp. ex.212.51.84.62.60.01075OK6664Comp. ex.222.02.92.10.90.01364OK6521Inv. ex.232.83.21.90.80.00865OK6752Inv. ex.242.23.12.01.20.01470OK6801Inv. ex.255.22.82.41.40.01077OK6543Inv. ex.261.52.31.70.90.06865OK6783Inv. ex.273.42.82.41.0——NG6542Comp. ex.282.92.72.21.20.00562NG6592Comp. ex.292.63.01.82.80.01166OK7684Comp. ex.302.43.21.92.30.00768OK7015Comp. ex.31————0.01871OK5045Comp. ex.323.42.82.40.90.00860OK5301Comp. ex.Underlines indicate outside scope of present invention or properties not preferable.The properties of the obtained steel sheets were measured and evaluated by the following methods. Further, for the presence of any occurrence of heat buckling in the production process, the presence of any occurrence of buckling after primary annealing was checked, cases where buckling did not occur were judged as not having heat buckling occurring and evaluated as passing (OK) and cases where buckling did occur were judged as having heat buckling occurring and evaluated as failing (NG).[Tensile Strength (TS)]

[0204] The tensile strength (TS) was measured by obtaining from the steel sheet a No. 5 tensile test piece of JIS Z2241: 2011 having a direction perpendicular to the rolling direction and sheet thickness direction as its longitudinal direction and performing a tensile test compliant with JIS Z2241: 2011.[Appearance After Forming]

[0205] The appearance after forming was evaluated by the extent of ghost lines occurring at the surface of an outer door after forming. As a formed member simulating an outer door, a pressed member obtained by press forming a steel sheet formed by blanking into a 600 mm square shape so that the radius of curvature R of the center part became 1200 mm was used. The surface after the press forming was ground. Stripe patterns formed at the surface at intervals on the order of several mm were judged as ghost lines and were scored as 1 to 5 by the degree of occurrence of the stripe patterns. Any 100 mm×100 mm region was visually examined. Cases where no stripe patterns were confirmed at all were judged as “1”, cases where a maximum length of stripe patterns was 20 mm or less were judged as “2”, cases where a maximum length of stripe patterns was more than 20 mm and 50 mm or less were judged as “3”, cases where a maximum length of stripe patterns was more than 50 mm and 70 mm or less were judged as “4”, and cases where a maximum length of stripe patterns was more than 70 mm were judged as “5”. Cases where the evaluation was “3” or less were deemed as excellent in appearance after forming and judged as passing. On the other hand, cases where the evaluation was “4” or more were deemed as poor in appearance after forming and judged as failing. In the current test, the appearance after forming was evaluated for a pressed member simulating an outer door, but any formed member able to be estimated as having 2.5% strain imparted by press forming may also be evaluated. A test piece taken from the steel sheet and similarly imparted with a 2.5% prestrain may also be evaluated and can be evaluated equivalently be these test methods. In the case of a test piece taken from a steel sheet, a JIS No. 5 test piece having a direction perpendicular to the rolling direction and sheet thickness direction as its longitudinal direction and imparted with 2.5% prestrain can be evaluated.

[0206] Cases where no heat buckling occurred, the tensile strength was 540 MPa or more, and the appearance after forming was evaluated as 3 or less were evaluated as steel sheet where the occurrence of heat buckling at the production process can be suppressed and where both strength and good appearance after forming can be achieved. The results are shown in Table 2. In the microstructure shown in Table 2, the balance structures were at least one of bainite, pearlite, and retained austenite.

[0207] Referring to Tables 1 and 2, in Comparative Example 1, the F5 / F7 rolling reduction ratio in the finish rolling was low, therefore strain remarkably accumulated after rolling reduction at the final pass (F7) and it is believed the fine Nb carbonitrides precipitating after the finish rolling increased. As a result, the ratio of Nb carbonitrides with a grain size of 20 nm or more became smaller and heat buckling occurred at the primary annealing step. In Comparative Example 3, the rolling reduction in the cold rolling step was low, therefore at the time of heating in the subsequent primary annealing step, recrystallization was delayed and strain remained until the temperature became high resulting, it is believed, in a large amount of fine Nb carbonitrides ending up precipitating during the heating of the primary annealing step. As a result, the ratio of grain size 20 nm or more Nb carbonitrides became smaller and heat buckling occurred in the primary annealing step. In Comparative Example 4, the heating temperature of the primary annealing step was low, therefore austenization became insufficient and, it is believed that, even by subsequent cooling, it was not possible to make the microstructure in the steel sheet by structures mainly comprised of bainite and / or martensite. As a result, in the microstructure obtained after the secondary annealing, the mean grain interval of the martensite became more than 2.5 μm or the standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and sheet thickness direction became more than 1.5% and the appearance after forming worsened. In Comparative Example 9, the finish rolling entry side temperature was low, therefore Nb carbonitrides did not sufficiently precipitate. As a result, the ratio of Nb carbonitrides with a grain size of 20 nm or more became smaller and heat buckling occurred in the primary annealing step. In Comparative Example 10, the coiling temperature was low, therefore the amount of solid solution Nb became greater and in turn it is believed that less Nb carbonitrides precipitated. As a result, the amount of Nb in all Nb carbonitrides became smaller or the ratio of Nb carbonitrides with a grain size of 20 nm or more became smaller and heat buckling occurred in the primary annealing step. In Comparative Example 19, the finish rolling entry side temperature was high, therefore recrystallization occurred, strain insufficiently accumulated at the latter stage of the finish rolling, precipitation of Nb carbonitrides with a grain size of 20 nm or more could not be promoted, and it is believed the ratio of the same became smaller. As a result, heat buckling occurred at the primary annealing step.

[0208] In Comparative Example 20, the F5 / F7 rolling reduction ratio at the finish rolling was high, therefore after the rolling reduction at the F5 rolling pass, strain accumulated too much and it is believed recrystallization was promoted. As a result, the ratio of Nb carbonitrides with a grain size of 20 nm or more became smaller and heat buckling occurred at the primary annealing step. Further, in Comparative Example 20, in the same way as the case of Comparative Example 4, the heating temperature of the primary annealing step was low, therefore austenization became insufficient and it is believed that even by subsequent cooling, it was not possible to make the microstructure in the steel sheet by structures mainly comprised of bainite and / or martensite. As a result, in the microstructure obtained after secondary annealing, the mean grain interval of the martensite became more than 2.5 μm, the standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and sheet thickness direction became more than 1.5%, and the appearance after forming deteriorated. In Comparative Example 21, the coiling temperature was high, therefore the ratio of cementite in the microstructure became concentrated, undissolved carbides remained at the time of heating in the subsequent primary annealing step, and it is believed it was not possible to make the microstructure in the steel sheet by structures mainly comprised of bainite and / or martensite in the primary annealing step. As a result, in the microstructure obtained after secondary annealing, the mean grain interval of the martensite became more than 2.5 μm, the standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and sheet thickness direction became more than 1.5%, and the appearance after forming deteriorated. In Comparative Example 27, Nb was not added, therefore Nb carbonitrides with a grain size of 20 nm or more did not precipitate or the value of the index A was also low. As a result, the high temperature strength of the steel sheet could not be sufficiently raised and heat buckling occurred in the primary annealing step. In Comparative Example 28, the value of the index A was low, therefore similarly the high temperature strength of the steel sheet could not be sufficiently raised and heat buckling occurred in the primary annealing step. In each of Comparative Examples 29 and 30, the C or Mn content was high, therefore the standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and sheet thickness direction became more than 1.5% and the appearance after forming deteriorated. In each of Comparative Examples 31 and 32, the C or Mn content was low, therefore sufficient strength could not be obtained.

[0209] In contrast to this, in each of the steel sheets according to all of the invention examples, by having a predetermined chemical composition and microstructure and furthermore by the ratios of ferrite and martensite in the microstructure being suitably controlled, a 540 MPa or more TS was achieved and by the mean grain interval of martensite being controlled to 2.5 μm or less in the micro regions and the standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and sheet thickness direction being controlled to 1.5% or less in the macro regions, even in a cases where strain was imparted by press forming, formation of fine asperities at the steel sheet surface could be suppressed and the occurrence of ghost lines could be remarkably suppressed. When examining in cross-section the microstructure at each of cold rolled steel sheets before secondary annealing according to all of the invention examples, in each case, it was comprised of, by area ratio, 90% or more of martensite. Further, in each of the steel sheets according to all of the invention examples, by controlling the amount of Nb in all of the Nb carbonitrides to 0.004% or more and controlling the amount of Nb in the Nb carbonitrides with a grain size of 20 nm or more to 60% or more of the amount of Nb in all of the Nb carbonitrides, by combination with control of the chemical composition by the index A, the occurrence of heat buckling of the steel sheet at the production process could be remarkably suppressed. Further, the Nb carbonitrides with a grain size of 20 nm or more in the invention examples all had grain sizes of 1000 nm or less, i.e., 1 μm or less.

Claims

1-4. (canceled)5. A steel sheet having a chemical composition comprising, by mass %,C: 0.030 to 0.100%,Mn: 0.70 to 3.00%,Si: 0.005 to 1.500%,P: 0.100% or less,S: 0.0200% or less,Al: 1.000% or less,N: 0.0010 to 0.0150%,0: 0.0100% or less,Nb: 0.005 to 0.200%,Cr: 0 to 1.00%,Mo: 0 to 0.80%,B: 0 to 0.0100%,Ti: 0 to 0.200%,V: 0 to 0.500%,Ni: 0 to 1.00%,Cu: 0 to 1.00%,W: 0 to 1.00%,Ta: 0 to 0.10%,Co: 0 to 3.00%,Sn: 0 to 1.00%,Sb: 0 to 0.200%,Ca: 0 to 0.0100%,Mg: 0 to 0.0100%,Zr: 0 to 0.0100%,REM: 0 to 0.0100%,Bi: 0 to 0.0500%,As: 0 to 0.10%, andbalance: Fe and impurities, whereinan index A represented by following formula 1 is 0.50% or more, and a microstructure comprising, by area %,ferrite: 75 to 95%,martensite: 5 to 25%, andbalance structures: 0 to 10% in total, whereinan amount of Nb in all of Nb carbonitrides is 0.004% or more and an amount of Nb in the Nb carbonitrides with a grain size of 20 nm or more is 60% or more of the amount of Nb in all of the Nb carbonitrides,a mean grain interval of the martensite is 2.5 μm or less, anda standard deviation in an area ratio of martensite in a direction vertical to a rolling direction and sheet thickness direction is 1.5% or less,A=[C]-0⁢.1[Si]+0.3⁢([Mn]-0.5)-0⁢.3[Al]+0.1[Cr]+
0.6[Mo]-[Ti]+15[Nb]formula⁢ 1where, [C], [Si], [Mn], [Al], [Cr], [Mo], [Ti], and [Nb] are the contents of the elements [mass %], 0% when the elements are not contained.

6. The steel sheet according to claim 5, wherein the chemical composition comprises, by mass %, at least one ofCr: 0.001 to 1.00%,Mo: 0.001 to 0.80%,B: 0.0001 to 0.0100%,Ti: 0.001 to 0.200%,V: 0.001 to 0.500%,Ni: 0.001 to 1.00%,Cu: 0.001 to 1.00%,W: 0.001 to 1.00%,Ta: 0.001 to 0.10%,Co: 0.001 to 3.00%,Sn: 0.001 to 1.00%,Sb: 0.001 to 0.200%,Ca: 0.0001 to 0.0100%,Mg: 0.0001 to 0.0100%,Zr: 0.0001 to 0.0100%,REM: 0.0001 to 0.0100%,Bi: 0.0001 to 0.0500%, andAs: 0.001 to 0.10%.

7. The steel sheet according to claim 5, wherein a mean crystal grain size of the ferrite is 3.0 to 25.0 μm, a mean crystal grain size of the martensite is 1.0 to 5.0 μm, and a mean aspect ratio of the martensite is 2.5 or more.

8. The steel sheet according to claim 6, wherein a mean crystal grain size of the ferrite is 3.0 to 25.0 μm, a mean crystal grain size of the martensite is 1.0 to 5.0 μm, and a mean aspect ratio of the martensite is 2.5 or more.

9. An outer panel member including the steel sheet according to claim 5.

10. An outer panel member including the steel sheet according to claim 6.

11. An outer panel member including the steel sheet according to claim 7.

12. An outer panel member including the steel sheet according to claim 8.

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