STEEL PLATE

Description

FIELD

[0001] The present invention relates to a steel sheet, more particularly relates to a steel sheet having a tensile strength of 400 MPa or more excellent in appearance in, for example, applications of mainly exterior panel members of automobiles, etc.

BACKGROUND

[0002] To keep down the amount of emission of CO₂ gas from automobiles, attempts have been made to use high strength steel sheet to secure safety while lightening the weight of automobile bodies. There has been remarkable progress in increasing strength in steel sheet for automobile use, but in doors, hoods, and other exterior panel members, steel sheet of a tensile strength of the 300 MPa or less strength class is mainly being used. Strength is not being made higher. Higher formability and good appearance are being sought from such exterior panel members. In general, if increasing the strength of steel sheet, the formability and appearance after forming fall. Therefore, in a high strength steel sheet, strength and formability plus good appearance, in particular appearance after forming, are difficult to simultaneously achieve. In the past, several means have been proposed to solve these issues.

[0003] For example, PTL 1 describes 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%, satisfying 2.5≤1.5Mn%+Cr%, 4.1-2.3Mn%-1.2Cr%≤Si%, 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 and good appearance after working in steel sheet for hot dip galvanization use with a tensile strength of 390 MPa or more. Furthermore, PTL 1 teaches that by making the area ratio of the main phase of ferrite 70% or more and making the area ratio of a hard second phase containing 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 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% to 0.03% or less in a range where Ti* shown by Ti*=(Ti%)-3.4×(N%)-1.5×(S%)-4×(C%) satisfies 0<Ti*<0.02 and furthermore in a range where (Sb%)≥(Cu%)/5 is satisfied, and having a balance of Fe and unavoidable impurities, wherein at the two surfaces of the steel sheet, a content (mass%) of the 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 the 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 surfaces of the steel sheet 9% or less of the total Ti content (mass%) in the steel sheet, the occurrence of uneven appearance due to such fine Ti-based precipitates is avoided and cold rolled steel sheet excellent in surface properties is obtained and furthermore that such cold rolled steel sheet can be optimally used for parts such as exterior panels of automobiles requiring excellent surface quality after forming.

[CITATIONS LIST]

[PATENT LITERATURE]

[0005] 

[PTL 1] Japanese Unexamined Patent Publication No. 2009-249737

[PTL 2] WO2011/142473

SUMMARY

[TECHNICAL PROBLEM]

[0006] For example, in the case of dual phase steel having a microstructure containing soft ferrite and hard martensite such as described in PTL 1, uneven deformation easily occurs where the soft ferrite and its surroundings deform more readily at the time of press-forming and other working. For this reason, if utilizing dual phase steel comprised of such soft structures and hard structures, sometimes fine asperities are formed at the surfaces of the steel sheet after forming and defects in appearance called "ghost lines" are formed. In relation to this, for example, PTL 1 studies improvement of the formability and appearance after forming mainly from the viewpoint of chemical composition, but does not necessarily sufficiently study this from the viewpoint of making the microstructure a suitable one. Therefore, in the prior art steel sheet, there was still room for improvement in regard to improving the formability and appearance after forming.

[0007] Therefore, the present invention has as its object the provision of a steel sheet able to achieve both strength and formability plus good appearance after forming by a novel constitution.

[SOLUTION TO PROBLEM]

[0008] The inventors engaged in studies to achieve the above object focusing on the state of distribution of martensite in addition to finding the suitable ratio of the hard structures of martensite in the microstructure. As a result, the inventors discovered that by making the martensite contained in a predetermined ratio in the microstructure uniformly disperse in both micro-regions and macro-regions in the microstructure, the desired higher strength and formability are achieved based on such hard structures and formation of fine asperities at the steel sheet surfaces is remarkably suppressed even when strain is imparted by press-forming, etc., and thereby completed the present invention.

[0009] The gist of the present invention is as follows:

(1) A steel sheet comprising a chemical composition comprising, by mass%,

C: 0.03 to 0.08%,

Si: 0.01 to 1.00%,

Mn: 0.50 to 3.00%,

P: 0.1000% or less,

S: 0.0200% or less,

Al: 1.000% or less,

N: 0.0200% or less,

O: 0 to 0.020%,

Cr: 0 to 2.000%,

Mo: 0 to 1.000%,

Ti: 0 to 0.500%,

Nb: 0 to 0.500%,

B: 0 to 0.0100%,

Cu: 0 to 1.000%,

Ni: 0 to 1.00%,

W: 0 to 0.100%,

V: 0 to 1.000%,

Ta: 0 to 0.100%,

Co: 0 to 3.000%,

Sn: 0 to 1.000%,

Sb: 0 to 0.500%,

As: 0 to 0.050%,

Mg: 0 to 0.050%,

Zr: 0 to 0.050%,

Ca: 0 to 0.0500%,

Y: 0 to 0.0500%,

La: 0 to 0.0500%,

Ce: 0 to 0.0500%,

Bi: 0 to 0.0500%, and

balance: Fe and impurities, and

a microstructure comprising, by area ratio,

ferrite: 80 to 95%,

martensite: 5 to 20%, and

at least one of bainite, pearlite, and retained austenite: 0 to 10% in total, wherein

an average grain interval of martensite is 2.5 µm or less, and

a standard deviation in area ratio of martensite in a direction vertical to a rolling direction and a sheet thickness direction is 1.5% or less.

(2) The steel sheet according to the above (1), wherein the chemical composition contains, by mass%, at least one of

Cr: 0.001 to 2.000%,

Mo: 0.001 to 1.000%,

Ti: 0.001 to 0.500%,

Nb: 0.001 to 0.500%,

B: 0.0001 to 0.0100%,

Cu: 0.001 to 1.000%,

Ni: 0.001 to 1.00%,

W: 0.001 to 0.100%,

V: 0.001 to 1.000%,

Ta: 0.001 to 0.100%,

Co: 0.001 to 3.000%,

Sn: 0.001 to 1.000%,

Sb: 0.001 to 0.500%,

As: 0.001 to 0.050%,

Mg: 0.0001 to 0.050%,

Zr: 0.0001 to 0.050%,

Ca: 0.0001 to 0.0500%,

Y: 0.0001 to 0.0500%,

La: 0.0001 to 0.0500%,

Ce: 0.0001 to 0.0500%, and

Bi: 0.0001 to 0.0500%.

[ADVANTAGEOUS EFFECTS OF INVENTION]

[0010] According to the present invention, it is possible to provide a steel sheet able to achieve both strength and formability plus good appearance after forming.

DESCRIPTION OF EMBODIMENTS

<Steel Sheet>

[0011] The steel sheet according to the embodiments of the present invention has a chemical composition comprising, by mass%,

C: 0.03 to 0.08%,

Si: 0.01 to 1.00%,

Mn: 0.50 to 3.00%,

P: 0.1000% or less,

S: 0.0200% or less,

Al: 1.000% or less,

N: 0.0200% or less,

O: 0 to 0.020%,

Cr: 0 to 2.000%,

Mo: 0 to 1.000%,

Ti: 0 to 0.500%,

Nb: 0 to 0.500%,

B: 0 to 0.0100%,

Cu: 0 to 1.000%,

Ni: 0 to 1.00%,

W: 0 to 0.100%,

V: 0 to 1.000%,

Ta: 0 to 0.100%,

Co: 0 to 3.000%,

Sn: 0 to 1.000%,

Sb: 0 to 0.500%,

As: 0 to 0.050%,

Mg: 0 to 0.050%,

Zr: 0 to 0.050%,

Ca: 0 to 0.0500%,

Y: 0 to 0.0500%,

La: 0 to 0.0500%,

Ce: 0 to 0.0500%,

Bi: 0 to 0.0500%, and

balance: Fe and impurities, and

a microstructure comprising, by area ratio,

ferrite: 80 to 95%,

martensite: 5 to 20%, and

at least one of bainite, pearlite, and retained austenite: 0 to 10% in total, wherein

an average grain interval of martensite is 2.5 µm or less, and

a standard deviation in area ratio of martensite in a direction vertical to a rolling direction and a sheet thickness direction is 1.5% or less.

[0012] In a door or hood or other exterior panel member, from the viewpoint of avoiding surface defects called "surface deflection" occurring at the time of press-forming, etc., in many cases dual phase steel (DP steel) with a relatively low yield strength is being used. However, in the case of DP steel where soft structures comprised of ferrite and hard structures comprised of martensite are mixed, at the time of press-forming and other working, uneven deformation easily occurs where the soft structures and their surroundings deform more readily and fine asperities are formed at the steel sheet surfaces after forming, sometimes leading to the defects in appearance called "ghost lines". Explained in more detail, at the time of press-forming and other working, the amount of deformation of the soft structures comprised of ferrite is great causing them to become recessed, while the amount of deformation of the hard structures is small. Therefore, the hard structures do not become recessed compared with the soft structures, but are built up and project out. As a result, variations occur in amount of deformation in particular in the width direction of steel sheet and ghost lines appear in band shapes (streaks). On the other hand, along with the higher strength of steel sheet, Mn and other elements are sometimes added in relatively large amounts so as to improve the hardenability of steel sheet. Mn is an element which easily segregates in streak shapes in the steel sheet. In more detail, concentrated Mn regions of center segregation or microsegregation are formed at the time of casting. Due to the hot rolling and cold rolling, the concentrated regions are stretched in the rolling direction whereby the Mn segregates in streak shapes. For this reason, due to such segregation of Mn, there are regions with high hardenability and regions with low ones in the steel sheet. As a result, in the microstructure of steel sheet after hardening, a relatively large amount of banded hard structures are formed. In this case, the formation of ghost lines becomes particularly prominent. As opposed to this, if possible to sufficiently suppress Mn segregation in the steel sheet, it becomes possible to reduce the formation of such banded hard structures and make the hard structures more uniformly disperse in the microstructure. In this case, even if strain is imparted by press-forming, etc., it may be possible to sufficiently reduce the formation of fine asperities at the steel sheet surfaces and possible to suppress the formation of ghost lines. However, along with the demands for higher strength, in particular if the amount of addition of Mn in the steel sheet becomes greater, in actuality, reliably and sufficiently suppressing Mn segregation is extremely difficult. In addition, along with such higher strength, the formability itself falls, therefore achieving both strength and formability plus good appearance after forming is generally extremely difficult.

[0013] Therefore, first, the inventors studied means for optimizing the chemical composition of steel sheet and optimizing the ratio of the soft structures of ferrite and the hard structures of martensite in the microstructure so as to realize the desired higher strength and formability while further improving the appearance after forming. Specifically, the inventors took note of the state of distribution of the hard structures of martensite in the microstructure, in more detail, studied control of the distribution of martensite from another viewpoint different from reduction of segregation of Mn. As a result, as explained later in detail regarding the method of production of steel sheet, the inventors discovered that by forming the microstructure in the steel sheet before the final annealing from bainite and/or martensite and then final annealing the steel sheet having such a microstructure under predetermined conditions, it is possible to make the martensite uniformly disperse in both the micro-regions and macro-regions in the finally obtained microstructure without necessarily depending on the presence and extent of Mn segregation. More specifically, the inventors discovered that by final annealing steel sheet having a microstructure comprised of bainite and/or martensite under predetermined conditions, it is possible to control the average grain interval of martensite to 2.5 µm or less in the micro-regions and possible to control the standard deviation in the area ratio in a direction vertical to the rolling direction and the sheet thickness direction to 1.5% or less in the macro-regions. By controlling the average grain interval of martensite to 2.5 µm or less, it is possible to make the hard structures be dispersed densely and uniformly in the micro-regions. By controlling the standard deviation in the area ratio in a direction vertical to the rolling direction and the sheet thickness direction to 1.5% or less, it is possible to remarkably reduce the variation in hard structures in the macro-regions. By satisfying these two requirements, it is possible to form a microstructure in which the hard structures of martensite are finely and uniformly dispersed in the steel sheet as a whole. As a result, according to the steel sheet according to the embodiments of the present invention, it is possible to make the amount of deformation the steel sheet more uniform, particularly in the width direction, even at the time of press-forming or other forming and becomes possible to achieve excellent appearance after forming where ghost lines and other defects in appearance are remarkably suppressed. For example, even if uniformity of martensite at the micro-regions is secured, if uniformity of martensite at the macro-regions is not secured, it is not possible to form a microstructure where martensite is dispersed finely and uniformly in the steel sheet as a whole. Similarly, even if uniformity of martensite at the macro-regions is secured, if uniformity of the martensite in the micro-regions is not secured, locally martensite can be present unevenly, therefore it is not possible to form a microstructure where martensite is finely and uniformly dispersed in the steel sheet as a whole. Therefore, in the steel sheet according to the embodiments of the present invention, to achieve excellent appearance after forming in which ghost lines and other defects in appearance are remarkably suppressed, it becomes necessary to satisfy the two requirements of controlling the average grain interval of martensite to 2.5 µm or less and controlling the standard deviation in the area ratio in a direction vertical to the rolling direction and the sheet thickness direction to 1.5% or less.

[0014] While not intending to be bound to any specific theory, to make the martensite finely and uniformly disperse in the steel sheet as a whole in the microstructure of the finally obtained steel sheet, it may be that forming a large number of austenite nucleation sites highly dispersed at the time of heating in the final annealing is extremely important. In relation to this, martensite structures have packets, blocks, las, and other substructures in the prior austenite grains. For this reason, compared with ferrite and other structures, they are structures having numerous various interfaces inside. Bainite also, in the same way as the case of martensite, forms structures having numerous various interfaces inside. Therefore, by forming the microstructure in the steel sheet before final annealing by bainite and/or martensite, it becomes possible to form carbides able to act as austenite nucleation sites on these interfaces at the stage of heating such a microstructure in the final annealing in an extremely large number. Therefore, it is believed that by forming numerous carbides on the interfaces, then further heating the temperature at the dual phase region of ferrite and austenite, it becomes possible to form austenite finely and uniformly in the steel sheet as a whole. Finally, by rapidly cooling steel sheet having such a microstructure, martensite is formed from the austenite, therefore in the finally obtained microstructure, the average grain interval of martensite is controlled to 2.5 µm or less and the standard deviation in the area ratio in a direction vertical to the rolling direction and the sheet thickness direction is controlled to 1.5% or less. That is, it is believed that a microstructure where martensite is uniformly dispersed in both of the micro-regions and macro-regions can be obtained. It is believed that by performing such heat treatment, it becomes possible to make the martensite finely and uniformly disperse over the steel sheet as a whole to an extent cancelling out the effects of Mn segregation. In the past, it is believed that studying control of the distribution of hard structures from the viewpoint of reducing Mn segregation itself was the general practice, therefore the fact that it is possible to make the martensite uniformly disperse in both the micro-regions and macro-regions in the finally obtained microstructure without necessarily depending on the presence and extent of Mn segregation is extremely unexpected and further is surprising.

[0015] According to the steel sheet according to the embodiments of the present invention, in addition to the above discovery, by controlling the area ratio of the soft structures of ferrite to 80 to 95%, good formability can be secured and by controlling the area ratio of hard structures of martensite to 5 to 20% and, furthermore, controlling the chemical composition of the steel sheet to within a predetermined range, a high strength of a tensile strength of 400 MPa or more can be secured. As a result, it becomes possible to realize both strength and formability plus good appearance after forming at a high level. Below, the component elements of the steel sheet according to the embodiments of the present invention will be explained in more detail.

[0016] First, the microstructure of the steel sheet according to the embodiments of the present invention will be explained. Below, the structure fractions are displayed by area ratio. The unit "%" of the structure fractions means the area%. Further, as explained later, the microstructure is controlled at the sheet thickness 1/4 part of the steel sheet. The "sheet thickness 1/4 part of the steel sheet" means the region between the plane at 1/8 depth and the plane at 3/8 depth of sheet thickness from the rolled surface of the steel sheet. Below, unless particularly indicated otherwise, the "structure fractions" all mean the values at the sheet thickness 1/4 part.

[Ferrite: 80 to 95%]

[0017] Ferrite forms soft structures and contributes to improvement of the elongation. If the area ratio of ferrite is 80% or more, it is possible to obtain sufficient formability. From the viewpoint of improvement of the formability, the area ratio of ferrite is preferably as high as possible. For example, it may be 82% or more, 85% or more, 87% or more, or 90% or more. On the other hand, if excessively containing ferrite, sometimes the desired strength cannot be achieved in the steel sheet. Therefore, the area ratio of ferrite is 95% or less. The area ratio of ferrite may also be 94% or less or 92% or less.

[Martensite: 5 to 20%]

[0018] Martensite forms hard structures with high dislocation density, therefore forms structures contributing to tensile strength. By making the area ratio of martensite 5% or more, it is possible to secure a tensile strength of 400 MPa or more. From the viewpoint of improvement of the strength, the area ratio of martensite is preferably as high as possible. 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 20% or less, formability and good appearance can be secured. The area ratio of martensite may also be 17% or less or 15% or less. In the present invention, "martensite" includes not only martensite as hardened (so-called "fresh martensite"), but also tempered martensite.

[At Least One of Bainite, Pearlite, and Retained Austenite: 0 to 10% in Total]

[0019] The remaining structures aside from ferrite and martensite may be in an area ratio of 0% as well, but if there are remaining structures present, the remaining structures comprise at least one of bainite, pearlite, and retained austenite. From the viewpoint of securing the above effects based on ferrite and martensite, the area ratio of the remaining structures, i.e., the at least one of bainite, pearlite, and retained austenite, is 10% or less in total. For example, it may be 8% or less, 6% or less, 4% or less, 3% or less, or 2% or less in total. In particular, the area ratio of retained austenite may be 0 to 3%. For example, the area ratio of retained austenite may also be 2% or less, 1% or less, 0.5% or less, 0.3% or less, or 0.1% or less. On the other hand, to make the area ratio of the remaining structures 0%, advanced control would be required in the process of production of the steel sheet, therefore sometimes a drop in yield would be invited. Therefore, the area ratio of the remaining structures is 0.5% or more or 1% or more.

[Identification of Microstructure and Calculation of Area Ratio]

[0020] The microstructure is identified and the area ratio is calculated after corrosion using a Nital reagent or Le Pera solution by an FE-SEM (field emission scan electron microscope, for example, JSM-7200F made by JEOL, measured by an acceleration voltage of 15 kV) and optical microscope and X-ray diffraction. The structure is observed by the FE-SEM and optical microscope for a 100 µm×100 µm region in the steel sheet cross-section in a direction parallel to the rolling direction and vertical to the sheet surfaces by a 1000 to 50000X power. In each microstructure, three locations are measured and the average value of these measured values is calculated to determine the area ratio. For example, if the sheet thickness of the steel sheet covered by measurement is thin, if not possible to secure a measurement region of 100 µm in the sheet thickness direction, the length in the sheet thickness direction is decreased and a measurement region of 10000 µm² is secured. For example, a measurement region of 20 µm in the sheet thickness direction and 500 µm in the rolling direction may also be covered by measurement. However, if the number of crystal grains contained in the sheet thickness direction becomes too small, sometimes the measurement precision falls, therefore the measurement length in the sheet thickness direction is 10 µm or more, preferably 50 µm or more. The same is true for the "100 µm×100 µm region" in the following explanation.

[0021] The area ratio of the ferrite is found by observing a region of 100 µm×100 µm in the range of the sheet thickness 1/8 position to 3/8 position centered about the sheet thickness 1/4 position in an electron channeling contrast image by an FE-SEM (field emission scan electron microscope). More specifically, the image analysis software Image J is used for calculation by image analysis.

[0022] The area ratio of the martensite is found by the following procedure. First, the examined surface of the sample is etched by a Le Pera solution, then a region of 100 µm×100 µm is examined by an FE-SEM in the range of the sheet thickness 1/8 position to 3/8 position centered about the sheet thickness 1/4 position. The martensite and retained austenite are not corroded by Le Pera etching, therefore the area ratio of the noncorroded region corresponds to the total area ratio of martensite and retained austenite. Specifically, the image analysis software Image J is used to binarize the microstructure based on the brightness. The black parts of the image data are ferrite and the white parts not corroded by Le Pera solution are the total structures of martensite and retained austenite. Therefore, the area ratio of martensite is calculated by subtracting from the area ratio of the not corroded regions the area ratio of retained austenite measured by the later explained X-ray diffraction method. The martensite area ratio found by this method also includes the tempered martensite area ratio.

[0023] The area ratio of austenite is calculated by the X-ray diffraction method. First, the part from a surface of the sample down to a depth 1/4 position in the sheet thickness direction is removed by mechanical polishing and chemical polishing. Next, at the sheet thickness 1/4 position, the structure fraction of retained austenite is calculated from the integrated intensity ratios of the diffraction peaks of (200), (211) of the bcc phase and (200), (220), (311) of the fcc phase obtained using MoKα rays. As this method of calculation, the general 5-peeak method is utilized. The structure fraction of retained austenite calculated is determined as the area ratio of the retained austenite.

[0024] The bainite is identified and the area ratio is calculated by the following procedure. First, the observed surface of a sample is corroded by a Nital reagent, then a region of 100 µm×100 µm in the range of the sheet thickness 1/8 position to 3/8 position centered about the sheet thickness 1/4 position is examined by an FE-SEM. The bainite is identified in the following way from the positions of cementite and arrangement of cementite contained inside the structures in this observed region. Bainite is classified into upper bainite and lower bainite. Upper bainite is comprised of laths of bainitic ferrite at the interfaces of which cementite or retained austenite are present. Lower bainite is comprised of laths of bainitic ferrite at the inside of which cementite is present. There is one type of the crystal orientation relationship of bainitic ferrite. Cementite has the same variants. Upper bainite and lower bainite can be identified based on these features. In the present invention, these are together called bainite. The area ratio of the identified bainite is calculated based on image analysis. Note that, cementite is observed as regions with high brightness on the SEM image. Cementite can be identified by using energy dispersive X-ray spectroscopy (EDS) to analyze the chemical composition and thereby confirm carbonitrides comprised mainly of iron.

[0025] The pearlite is identified and the area ratio is calculated by the following procedure. First, the observed surface of a sample is corroded by a Nital reagent, then a range of the sheet thickness 1/8 position to 3/8 position centered about the sheet thickness 1/4 position is examined by an optical microscope. The image observed through the optical microscope is binarized by the differences in brightness, the regions with black parts and white parts dispersed in lamellar forms are identified as pearlite, and the area ratio of the regions are calculated by image analysis. More specifically, the image analysis software Image J is used to binarize the microstructure based on the differences of brightness. The image captured by a measurement power of 500X, including the 100 µm×100 µm captured range, is used to find the area fraction of pearlite by the point counting method. In the above captured range, eight lines are drawn parallel to the rolling direction at equal intervals and eight are drawn vertical to the rolling direction at equal intervals. Among the 64 cross points comprised of these lines, the ratio of points occupied by pearlite can be calculated as the area fraction of pearlite.

[Average Grain Interval of Martensite: 2.5 µm or Less]

[0026] In the embodiments of the present invention, the average grain interval of the hard structures of martensite is controlled to 2.5 µm or less. The average grain interval of martensite is an indicator expressing the uniformity of distribution of the hard structures in the micro-regions. The smaller the average grain interval of martensite, the denser and more uniform the hard structures are distributed is meant. Accordingly, the uniformity can be said to be high. The appearance of steel sheet after press-forming becomes more excellent the more uniform the amount of deformation of the steel sheet, in particular in the width direction of the steel sheet, at the time of press-forming. The amount of deformation of steel sheet is greatly affected by the state of distribution of the hard structures, therefore to make the amount of deformation of steel sheet uniform in the width direction of steel sheet, it is necessary to make the distribution of hard structures in the microstructure uniform. In addition to control of the standard deviation in the area ratio of martensite explained later, by controlling the average grain interval of martensite to 2.5 µm or less, 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 it is possible to achieve excellent appearance after forming. The average grain interval of 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 average grain interval of martensite may be 0.5 µm or more, 0.8 µm or more, or 1.0 µm or more.

[Measurement of Average Grain Interval of Martensite]

[0027] The average grain interval of martensite is determined in the following way. First, a sample having a steel sheet cross-section in a direction parallel to the rolling direction and vertical to the sheet surface is taken and that cross-section is examined. In that examined surface, a region of 100 µm×100 µm in the range of the sheet thickness 1/8 position to 3/8 position centered about the sheet thickness 1/4 position is used as the examined region. An FE-SEM is used to identify the martensite. Specifically, the image analysis software Image J is used to binarize the microstructure based on the brightness and identify the martensite. If using a Le Pera solution, the black parts of the image data are ferrite and the white parts not corroded by the Le Pera solution are the total structures of martensite and retained austenite. However, in the steel sheet according to the embodiments of the present invention, the area ratio of the retained austenite is sufficiently lower compared with the area ratio of martensite, therefore it is possible to deem white structures as martensite. Next, in the identified martensite, the distances between the centers (centers of gravity) of all adjoining martensite grains are calculated as the grain intervals based on image analysis. The average value of the grain intervals calculated is determined as the average grain interval of the martensite (strictly speaking, grains including martensite and/or retained austenite).

[Standard Deviation in Area Ratio in Direction Vertical to Rolling Direction and Sheet Thickness Direction of 1.5% or Less]

[0028] In the embodiments of the present invention, the standard deviation in the area ratio in a direction vertical to the rolling direction and the sheet thickness direction is controlled to 1.5% or less. This standard deviation is an indicator expressing the uniformity of hard structures in the macro-regions. At the time of press-forming, the appearance in question depends on the fine asperities of the steel sheet surfaces due to the difference in amounts of deformation in the width direction of the steel sheet. For this reason, if the variation in the area ratio of the hard structures contained in a sheet thickness in the direction vertical to the rolling direction and the sheet thickness direction is large, a difference arises in the amount of deformation in the width direction of the steel sheet and, as a result, fine asperities are formed at the steel sheet surfaces. Therefore, it is effective to reduce the standard deviation in the area ratio of martensite in a direction vertical to the rolling direction and the sheet thickness direction, i.e., the width direction of the steel sheet. More specifically, in addition to the above-mentioned control of the average grain interval of martensite, by controlling the standard deviation to 1.5% or less, it is possible to further reduce the variation in the amount of deformation in the width direction of steel sheet even at the time of press-forming or other forming and, as a result, achieve good appearance after forming. The standard deviation in the area ratio in a direction vertical to the rolling direction and the 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 be 0.1% or more, 0.3% or more, or 0.5% or more.

[Measurement of Standard Deviation in Area Ratio in Direction Vertical to Rolling Direction and Sheet Thickness Direction]

[0029] The standard deviation in the area ratio in a direction vertical to the rolling direction and the sheet thickness direction is determined in the following way. First, an image of the microstructure at the steel sheet cross-section in a region of 50 mm in a direction vertical to the rolling direction is obtained. In the case of images of 10 mm or less, it is also possible to obtain several images and stitch them together to 50 mm. Next, the obtained image is divided into 100 µm (0.1 mm) sections in the direction vertical to the rolling direction and the area ratios of martensite in the sheet thickness as a whole are calculated in the divided sections. The standard deviation in the area ratio of martensite is calculated based on the martensite area ratios calculated from the total of 500 divided images. This operation is performed on three regions with different positions in the rolling direction. The average value of the respectively found standard deviations is determined as the standard deviation in the area ratio in a direction vertical to the rolling direction and the sheet thickness direction.

[0030] If the rolling direction of the steel sheet is not clear, as the method of identifying the rolling direction of the steel sheet, for example, the following method is employed. The sheet thickness cross-section of the steel sheet is finished by polishing to a mirror surface, then the S concentration is measured by an electron probe micro analyzer (EPMA). The measurement conditions are an acceleration voltage of 15 kV. The image of distribution in a range of 500 µm square at the sheet thickness center part using a measurement pitch of 1 µm is measured. At this time, a stretched region with a high S concentration is judged an inclusion of MnS, etc. At the time of observation, several fields may also be observed. Next, based on the sheet thickness cross-section first examined by the above method, surfaces parallel to surfaces rotated by 5° increments about the sheet thickness direction in 0° to 180° in range are observed by the above method. The average value of the lengths of the long axes of the plurality of inclusions at the obtained cross-sections are calculated for each cross-section and the cross-section where the average value of the lengths of the long axes of the inclusions becomes maximum is identified. The direction parallel to the long axis direction of the inclusions at the cross-section is judged to be the rolling direction. Note that, for example, the sheet thickness of the steel sheet being measured is thin, therefore if a 500 µm square measurement region cannot be secured, the length in the sheet thickness direction is reduced and a measurement region of 250000 µm² is secured.

[0031] Next, the reasons for limitation of the chemical composition of the steel sheet according to the embodiments of the present invention will be explained. Below, the % relating to the chemical composition will mean mass%.

[C: 0.03 to 0.08%]

[0032] C is an element for securing a predetermined amount of martensite and improving the strength of steel sheet. To sufficiently obtain such an effect, the C content is 0.03% or more. The C content may also be 0.04% or more or 0.05% or more. On the other hand, if excessively containing C, the strength becomes too high and sometimes the stretchability falls. For this reason, the C content is 0.08% or less. The C content may also be 0.07% or less or 0.06% or less.

[Si: 0.01 to 1.00%]

[0033] Si is an element for raising the strength of steel sheet by solution strengthening. To sufficiently obtain such an effect, the Si content is 0.01% or more. The Si content may also be 0.05% or more, 0.10% or more, 0.20% or more, 0.30% or more, or 0.40% or more. On the other hand, if excessively containing Si, removal of the scale formed at the hot rolling becomes difficult and sometimes deterioration of the appearance is invited. For this reason, the Si content is 1.00% or less. The Si content may also be 0.90% or less, 0.80% or less, 0.70% or less, or 0.60% or less.

[Mn: 0.50 to 3.00%]

[0034] Mn is an element for raising the hardenability and contributing to improvement of the steel sheet strength. To sufficiently obtain such an effect, the Mn content is 0.50% or more. The Mn content may also be 0.70% or more, 1.00% or more, 1.20% or more, or 1.50% or more. In a preferred method of production of the steel sheet explained later, to make the martensite be uniformly distributed in both of the micro-regions and macro-regions in the finally obtained microstructure, it is necessary to form the microstructure in the steel sheet before the final annealing by bainite and/or martensite. For this reason, improvement of the hardenability by addition of Mn can be said to be important in improving the appearance after forming as well. On the other hand, if excessively containing Mn, ferrite transformation is excessively suppressed, the desired amount of ferrite cannot be secured, and sometimes the stretchability falls. 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.

[P: 0.1000% or Less]

[0035] P is an impurity element and an element causing embrittlement of the weld zone and deterioration of the plateability. For this reason, the P content is 0.1000% or less. The P content may also be 0.0600% or less, 0.0200% or less, 0.0150% or less, or 0.0100% 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, in practical steel sheet, if reducing the P content to less than 0.0001%, the production costs greatly rise and 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]

[0036] S is an impurity element and an element which detracts from weldability and further detracts from producibility at the time of casting and 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, or 0.0080% or less. The S content is preferably as small as possible, The lower limit is not particularly prescribed and may be 0%. On the other hand, in practical steel sheet, if reducing the S content to less than 0.0001%, the production costs greatly rise and 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]

[0037] Al is an element functioning as a deoxidizer and an element effective for improving 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 containing Al, coarse oxides are formed and sometimes the toughness is made to fall. Therefore, the Al content is 1.000% or less. The Al content may also be 0.800% or less, 0.600% or less, or 0.300% or less.

[N: 0.0200% or Less]

[0038] N is an element becoming a cause of formation of blowholes at the time of welding. For this reason, the N content is 0.0200% or less. The N content may also be 0.0180% or less, 0.0150% or less, 0.0100% or less, 0.0080% or less, or 0.0060% or less. The N content is preferably as small as possible. The lower limit is not particularly prescribed and may also be 0%. On the other hand, in practical steel sheet, if reducing the N content to less than 0.0001%, the production costs greatly rise and become economically disadvantageous. For this reason, the N content may also be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[O: 0 to 0.020%]

[0039] O is an element becoming a cause of formation of blowholes at the time of welding. For this reason, the O content is 0.020% or less. The O content may also be 0.018% or less, 0.015% or less, 0.010% or less, or 0.008% 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, in practical steel sheet, if reducing O to less than 0.0001%, the production costs greatly rise and 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.

[0040] The basic chemical composition of the steel sheet according to the embodiments of the present invention is as explained above. Furthermore, the steel sheet may contain at least one of the following optional elements in place of part of the balance of Fe as necessary for the improvement of the properties. For example, the steel sheet may contain at least one of Cr: 0 to 2.000%, Mo: 0 to 1.000%, Ti: 0 to 0.500%, Nb: 0 to 0.500%, B: 0 to 0.0100%, Cu: 0 to 1.000%, Ni: 0 to 1.00%, W: 0 to 0.100%, V: 0 to 1.000%, Ta: 0 to 0.100%, Co: 0 to 3.000%, Sn: 0 to 1.000%, Sb: 0 to 0.500%, As: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.050%, Ca: 0 to 0.0500%, Y: 0 to 0.0500%, La: 0 to 0.0500%, Ce: 0 to 0.0500%, and Bi: 0 to 0.0500%. Below, these optional elements will be explained in detail.

[Cr: 0 to 2.000%]

[0041] Cr, like Mn, is an element raising the hardenability and contributing to improvement of the steel sheet strength. The Cr content may also be 0%, but to obtain the above effects, the Cr content is preferably 0.001% or more. The Cr content may also be 0.010% or more, 0.100% or more, or 0.200% or more. On the other hand, even if excessively including Cr, the effects become saturated and a rise in production costs is liable to be invited. Therefore, the Cr content is preferably 2.000% or less and may also be 1.500% or less, 1.000% or less, or 0.500% or less.

[Mo: 0 to 1.000%]

[0042] Mo, like Cr, is an element contributing to higher strength of steel sheet. This effect can be obtained even in trace amounts. 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 0.010% or more, 0.020% or more, 0.050% or more, or 0.100% or more. On the other hand, if excessively containing Mo, sometimes the hot workability falls and the productivity falls. For this reason, the Mo content is preferably 1.000% or less. The Mo content may also be 0.800% or less, 0.400% or less, or 0.200% or less.

[Ti: 0 to 0.500%]

[0043] Ti is an element effective for control of the form of the carbides. Due to Ti, an increase in 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 0.050% or more. On the other hand, even if excessively containing Ti, the effects become saturated and a rise in the production costs is invited. Therefore, the Ti content is preferably 0.500% or less and may also be 0.400% or less, 0.200% or less, or 0.100% or less.

[Nb: 0 to 0.500%]

[0044] Nb, like Ti, is an element effective for control of the form of the carbides and is an element also effective for refining the structure and improving the toughness of the steel sheet. These effects can also be obtained by trace amounts. The Nb content may also be 0%, but to obtain the above effects, the Nb content is preferably 0.001% or more. The Nb content may also be 0.005% or more or 0.010% or more. On the other hand, if excessively containing Nb, coarse carbides, etc., are formed in the steel and sometimes cause the toughness of the steel sheet to fall. For this reason, the Nb content is preferably 0.500% or less. The Nb content may also be 0.200% or less, 0.100% or less, or 0.060% or less.

[B: 0 to 0.0100%]

[0045] B is an element suppressing the formation of ferrite and pearlite and promoting the formation of martensite in the process of cooling from austenite. Further, B is an element beneficial for raising the strength of steel. These effects can be obtained even in trace amounts. The B content may also be 0%, but to obtain the above effects, 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 containing 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.

[Cu: 0 to 1.000%]

[0046] Cu is an element contributing to improvement of the strength of steel sheet. This effect can be obtained even in 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 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, if excessively containing Cu, red shortage is invited and the productivity in hot rolling is liable to be lowered. For this reason, the Cu content is preferably 1.000% or less. The Cu content may also be 0.800% or less, 0.600% or less, 0.300% or less, or 0.100% or less.

[Ni: 0 to 1.00%]

[0047] Ni is an element effective for improving the strength of steel sheet. The Ni content may also be 0%, but to obtain the above effect, the Ni content is preferably 0.001% or more. The Ni content may also be 0.005% or more or 0.010% or more. On the other hand, if excessively containing Ni, the weldability of 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.

[W: 0 to 0.100%]

[0048] W is an element effective for control of the form of carbides and improvement of the strength of 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 0.005% or more or 0.010% or more. On the other hand, if excessively containing W, sometimes the weldability falls. For this reason, the W content is preferably 0.100% or less. The W content may also be 0.080% or less, 0.040% or less, or 0.020% or less.

[V: 0 to 1.000%]

[0049] V, like Ti and Nb, is an element effective for control of the form of carbides and an element effective for refinement of the structure and improvement of the toughness of steel sheet. The V content may also be 0%, but to obtain the above effects, 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 containing V, a large amount of precipitates is formed and sometimes the toughness is made to drop. For this reason, the V content is preferably 1.000% or less. The V content may also be 0.400% or less, 0.200% or less, or 0.100% or less.

[Ta: 0 to 0.100%]

[0050] Ta, like W, is an element effective for control of the form of carbides and improvement of the strength of 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 0.005% or more or 0.010% or more. On the other hand, even if excessively including Ta, the effects become saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. For this reason, the Ta content is preferably 0.100% or less. The Ta content may also be 0.080% or less, 0.040% or less, or 0.020% or less.

[Co: 0 to 3.000%]

[0051] Co, like Ni, is an element effective for improvement of the strength of 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 0.005% or more, 0.010% or more, or 0.100% or more. On the other hand, if excessively containing Co, sometimes the hot workability falls and an increase in the cost of the raw materials is also led to. For this reason, the Co content is preferably 3.000% or less. The Co content may also be 2.000% or less, 1.000% or less, 0.500% or less, or 0.200% or less.

[Sn: 0 to 1.000%]

[0052] Sn is an element able to be included in steel sheet when using scrap as a raw material of the steel sheet. Further, Sn is liable to trigger embrittlement of ferrite. For this reason, the Sn content is preferably as small as possible. It is preferably 1.000% or less. The Sn content may also be 0.100% or less, 0.040% or less, or 0.020% or less. The Sn content may also be 0%, but reducing the Sn content to less than 0.001% invites an excessive increase in the refining cost. For this reason, the Sn content may also be 0.001% or more, 0.005% or more, or 0.010% or more.

[Sb: 0 to 0.500%]

[0053] Sb, like Sn, is an element able to be included in steel sheet when using scrap as a raw material of the steel sheet. Further, Sb strongly segregates at the grain boundaries and is liable to invite embrittlement of the grain boundaries. For this reason, the Sb content is preferably as small as possible and may be 0.500% 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% invites an excessive increase in the refining cost. For this reason, the Sb content may also be 0.001% or more, 0.005% or more, or 0.010% or more.

[As: 0 to 0.050%]

[0054] As, like Sn and Sb, is an element able to be included in steel sheet when using scrap as a raw material of 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 0.050% or less and may also be 0.040% or less or 0.020% or less. The As content may also be 0%, but reducing the As content to less than 0.001% invites an excessive increase in the refining cost. For this reason, the As content may also be 0.001% or more, 0.005% or more, or 0.010% or more.

[Mg: 0 to 0.050%]

[0055] Mg controls the form of the sulfides or oxides and contributes to improvement of the bendability of steel sheet. This effect can be obtained even by a trace amount. The Mg content may also be 0%, but to obtain the above effect, the Mg content is preferably 0.0001% or more. The Mg content may also be 0.0005% or more, 0.001% or more, or 0.005%. On the other hand, even if excessively including Mg, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. For this reason, the Mg content is preferably 0.050% or less. The Mg content may also be 0.040% or less, 0.020% or less, or 0.010% or less.

[Zr: 0 to 0.050%]

[0056] Zr is an element able to control the form of sulfides in a trace amount. The Zr content may also be 0%, but to obtain the above effect, the Zr content is preferably 0.0001% or more. The Zr content may also be 0.0005% or more, 0.001% or more, or 0.005% or more. On the other hand, even if excessively including Zr, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. For this reason, the Zr content is preferably 0.050% or less. The Zr content may also be 0.040% or less, 0.020% or less, or 0.010% or less.

[Ca: 0 to 0.0500%]

[Y: 0 to 0.0500%]

[La: 0 to 0.0500%]

[Ce: 0 to 0.0500%]

[0057] Ca, Y, La, and Ce are elements able to control the form of sulfides by trace amounts. The Ca, Y, La, and Ce contents may also be 0%, but to obtain the above effect, the Ca, Y, La, and Ce contents are preferably respectively 0.0001% or more and may be 0.0005% or more, 0.0010% or more, 0.0020% or more, or 0.0030% or more. On the other hand, even if excessively including these elements, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the Ca, Y, La, and Ce contents are preferably respectively 0.0500% or less and may also be 0.0200% or less, 0.0100% or less, or 0.0060% or less.

[Bi: 0 to 0.0500%]

[0058] Bi is an element having the action of raising the formability by refinement of the solidified structure. The Bi content may also be 0%, but to obtain such an effect, the Bi content is preferably 0.0001% or more and may also be 0.0005% or more, 0.0010% or more, or 0.0050% or more. On the other hand, even if excessively including Bi, the effect becomes saturated. Inclusion in the steel sheet more than necessary invites a rise in production costs. Therefore, the Bi content is preferably 0.0500% or less and may also be 0.0400% or less, 0.0200% or less, or 0.0100% or less.

[0059] In the steel sheet according to the embodiments of the present invention, the balance besides the above elements is comprised of Fe and impurities. The "impurities" are elements which enter from the steel raw materials and/or at the steelmaking process and whose presence is allowed in a range not obstructing the properties of the steel sheet according to the embodiments of the present invention.

[0060] The chemical composition of the steel sheet according to the embodiments 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 may be measured using combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-nondispersive infrared absorption method.

[Sheet Thickness]

[0061] The steel sheet according to the embodiments of the present invention is not particularly limited, but, for example, has a 0.2 to 2.0 mm sheet thickness. Steel sheet having such a sheet thickness is optimal in the case of use as a material for a door or hood of an automobile or other panel member. The sheet thickness is 0.3 mm or more or 0.4 mm or more. Similarly, the sheet thickness may also be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, or 1.0 mm or less. The sheet thickness of the steel sheet is measured by a micrometer.

[Plating]

[0062] The steel sheet according to the embodiments of the present invention may further include, for the purpose of improvement of the corrosion resistance, a plating layer. The plating layer may be any suitable plating layer. For example, it may be either a hot dip coating layer and electroplating layer. The hot dip coating layer may be, for example, a hot dip galvanized layer, hot dip galvannealed layer (hot dip coating layer comprised of an alloy of zinc and Si, Al, or other additional element) or a hot dip galvannealed layer obtaining by alloying these (alloyed plating layer). The hot dip galvanized layer and the hot dip galvannealed layer are preferably plating layers containing less than 7 mass% of Fe. Further, the alloyed plating layer preferably is a plating layer containing 7 mass% or more and 15 mass% of less of Fe. In the hot dip galvanized layer, hot dip galvannealed layer, and alloyed plating layer, the constituents other than zinc and Fe are not particularly limited. Various constitutions can be employed within the usual range. Further, the plated layer, for example, may be an aluminum plating layer, etc. Further, the amount of deposition of the plating layer is not particularly limited and may be a general amount of deposition.

[Mechanical Properties]

[0063] According to the steel sheet according to the embodiments of the present invention, it is possible to achieve a high tensile strength, specifically a tensile strength of 400 MPa or more. The tensile strength is preferably 440 MPa or more or 480 MPa or more, more preferably 540 MPa or more or 600 MPa or more. The upper limit is not particularly prescribed, but, for example, the tensile strength may be 980 MPa or less or 900 MPa or less. Similarly, according to the steel sheet according to the embodiments of the present invention, excellent formability can be achieved, more specifically 20% or more total elongation can be achieved. The total elongation is preferably 22% or more, more preferably 25% or more or 30% or more. The upper limit is not particularly prescribed, but, for example, the total elongation may be 50% or less or 45% or less. The tensile strength and total elongation are measured by conducting a tensile test compliant with JIS Z 2241: 2011 based on a JIS No. 5 test piece taken from an orientation in which the longitudinal direction of the test piece becomes parallel with the perpendicular direction to rolling of the steel sheet.

[0064] The steel sheet according to the embodiments of the present invention, despite having a high strength, specifically a tensile strength of 400 MPa or more, can maintain formability and excellent appearance even after press-forming, etc. For this reason, the steel sheet according to the embodiments of the present invention is extremely useful for use for example as a roof, hood, fender, door, or other exterior panel member in an automobile in which a high design sense is sought.

<Method of Production of Steel Sheet>

[0065] Next, a preferable method of production of the steel sheet according to the embodiments of the present invention will be explained. The following explanation is intended to illustrate the characteristic method for producing the steel sheet according to the embodiments of the present invention and is not intended to limit the steel sheet to ones produced by the method of production such as explained below.

[0066] The method of production of the steel sheet according to the embodiments of the present invention is characterized by including

hot rolling including heating a slab having the chemical composition explained above in relation to the steel sheet to a temperature of 1100 to 1400°C, finish rolling it, then coiling it at a temperature of 500 to 700°C, wherein an end temperature of the finish rolling is 800 to 1350°C,

pickling the obtained hot rolled steel sheet,

cold rolling the pickled hot rolled steel sheet by a rolling reduction of 20 to 90%,

primary annealing the obtained cold rolled steel sheet, wherein the primary annealing includes heating the cold rolled steel sheet, holding it at a maximum heating temperature of Ac3 to 950°C for 10 to 500 seconds, then controlling the average cooling speed in a temperature region of 500 to 700°C to 50°C/s or more and cooling until a cooling stop temperature of 350°C or less, and

secondary annealing the cold rolled steel sheet after the primary annealing, wherein the secondary annealing includes heating the cold rolled steel sheet, holding it at a maximum heating temperature of (Ac1+20) to 820°C for 10 to 500 seconds, then controlling the average cooling speed in the temperature region of 500 to 700°C to 30°C/s or more and further controlling an average cooling speed in the temperature region of 200 to 500°C to 40°C/s or more. Below, each step will be explained in detail.

[Hot Rolling Step]

[Heating of Slab]

[0067] First, a slab having the chemical composition explained above in relation to the steel sheet is heated. The slab used is preferably cast by the continuous casting method from the viewpoint of productivity, but may also be produced by the ingot making method or the thin slab casting method. The slab used contains relatively large amounts of alloy elements for obtaining high strength steel sheet. For this reason, it is necessary to heat the slab before sending it on to hot rolling so as to make the alloy elements dissolve in the slab. If the heating temperature is less than 1100°C, the alloy elements will not sufficiently dissolve in the slab and coarse alloy carbides will remain resulting sometimes in brittle cracks during hot rolling. For this reason, the heating temperature is preferably 1100°C or more. The upper limit of the heating temperature is not particularly limited, but from the viewpoint of the capacity of the heating facilities and productivity is preferably 1400°C or less.

[Rough Rolling]

[0068] In the present method, for example, the heated slab may be rough rolled before the finish rolling so as to adjust the sheet thickness, etc. The rough rolling need only be able to secure the desired sheet bar dimensions. The conditions are not particularly limited.

[Finish Rolling and Coiling]

[0069] The heated slab or the slab additionally rough rolled according to need is next finish rolled. The slab used in the above way contains relatively large amounts of alloy elements, therefore it is necessary to increase the rolling load at the time of hot rolling. For this reason, the hot rolling is preferably performed at a high temperature. In particular, the end temperature of the finish rolling is important in control of the microstructure of the steel sheet. If the end temperature of the finish rolling is low, the microstructure becomes uneven and the formability sometimes falls. For this reason, the end temperature of the finish rolling is preferably 800°C or more. On the other hand, to suppress coarsening of the austenite, the end temperature of the finish rolling is preferably 1350°C or less. Next, the finish rolled hot rolled steel sheet is coiled by a 500 to 700°C coiling temperature. By the coiling temperature being 500 to 700°C, growth of oxide scale can be suppressed.

[Pickling Step]

[0070] Next, the obtained hot rolled steel sheet is pickled to remove the oxide scale formed on the surfaces of the hot rolled steel sheet. The pickling should be performed under conditions suitable for removing the oxide scale. This may be done one time or may be done divided into several times so as to reliably remove the oxide scale.

[Cold Rolling Step]

[0071] The pickled hot rolled steel sheet is cold rolled in the cold rolling step by a rolling reduction of 20 to 90%. By the rolling reduction of the cold rolling being 20% or more, it is possible to keep the shape of the cold rolled steel sheet flat and keep down a drop in ductility in the final product. On the other hand, by the rolling reduction of the cold rolling being 90% or less, it is possible to prevent the rolling load from becoming excessive and the rolling from becoming difficult. The number of rolling passes and the rolling reduction of each pass are not particularly limited and need only be suitably set so that the rolling reduction of the cold rolling becomes the above range.

[Primary Annealing Step]

[0072] The obtained cold rolled steel sheet is heated at the next primary annealing step, held at the maximum heating temperature of Ac3 to 950°C for 10 to 500 seconds, then cooled down to the cooling stop temperature of 350°C or less while controlling the average cooling speed of the temperature region of 500 to 700°C to 50°C/s or more. Here, the Ac3 point (°C) is found by cutting out a small piece from the cold rolled steel sheet and examining the thermal expansion during heating from room temperature to 1000°C by 10°C/s in the small piece. By holding at the Ac3 point or more temperature for a sufficient time, austenizing is promoted. By cooling down to the 350°C or less temperature by subsequent rapid cooling, it becomes possible to reliably make the microstructure in the steel sheet after cooling a structure mainly comprised of bainite and/or martensite, for example, make it full bainite or full martensite. Here, the "structure mainly comprised of bainite and/or martensite" means a structure containing at least one of bainite and martensite in a total area ratio of 90% or more. "Full bainite" means a structure comprised of an area ratio of 100% of bainite, while "full martensite" means a structure comprised of an area ratio of 100% of martensite. A bainite and/or martensite structure is a structure having numerous various interfaces inside compared with ferrite and other structures. For this reason, by making the microstructure in the steel sheet before the secondary annealing step, i.e., the final annealing step, by bainite and/or martensite, it becomes possible to form carbides able to act as nucleation sites of austenite dispersed extremely numerously on these interfaces at the stage of heating the microstructure at the secondary annealing. As a result, by forming austenite finely and uniformly in the steel sheet as a whole from the numerous dispersed nucleation sites in this way and then forming martensite from the austenite, in the microstructure obtained after the secondary annealing, the average grain interval of martensite is controlled to 2.5 µm or less and the standard deviation in the area ratio in a direction vertical to the rolling direction and the sheet thickness direction is controlled to 1.5% or less. That is, it becomes possible to achieve a microstructure with martensite uniformly dispersed in both of the micro-regions and macro-regions.

[0073] If not performing the primary annealing step, only naturally the microstructure in the steel sheet before the final annealing (secondary annealing) step cannot be comprised of structures mainly consisting of bainite and/or martensite. Further, if the maximum heating temperature in the primary annealing step is less than the Ac3 point or the holding time is less than 10 seconds even if performing the primary annealing step, the austenizing becomes insufficient and the microstructure in the steel sheet cannot be formed by structures mainly consisting of bainite and/or martensite even by the subsequent cooling. That is, the total of the area ratios of bainite and martensite cannot be made 90% or more. On the other hand, heating and holding at higher temperatures and longer times cause the productivity to fall, therefore the maximum heating temperature at the primary annealing step is 950°C or less and the holding time is 500 seconds or less.

[0074] Further, if the average cooling speed of the temperature region of 500 to 700°C in the primary annealing step is less than 50°C/s or the cooling stop temperature is more than 350°C, ferrite is formed during cooling and the microstructure in the steel sheet cannot be made one with a total of the area ratios of bainite and martensite of 90% or more. Therefore, the average cooling speed has to be 50°C/s or more. The upper limit is preferably 300°C/s. On the other hand, the lower limit of the cooling stop temperature is not particularly prescribed. For example, it may be room temperature (25°C) and is preferably 200°C.

[Secondary Annealing Step (Final Annealing Step)]

[0075] The cold rolled steel sheet after the primary annealing is again heated in the next secondary annealing step, is held at the maximum heating temperature of (Ac1+20) to 820°C for 10 to 500 seconds, then is cooled while controlling the average cooling speed in the temperature region of 500 to 700°C to 30°C/s or more and, furthermore controlling the average cooling speed in the temperature region of 200 to 500°C to 40°C/s or more. Here, the Ac1 point (°C), in the same way as the case of the Ac3 point, is found by cutting out a small piece from the cold rolled steel sheet and examining the thermal expansion during heating from room temperature to 1000°C by 10°C/s in the small piece. First, at the stage of heating the steel sheet after the primary cooling to the maximum heating temperature of (Ac1+20) to 820°C, carbides can be formed dispersed on the many interfaces contained inside the bainite and/or martensite in the microstructure. Next, by holding in the maximum heating temperature corresponding to the dual phase region of ferrite and austenite for 10 to 500 seconds, it is possible to maintain a state where carbides are dispersed on the interfaces while forming austenite from the carbides finely and uniformly in the steel sheet as a whole. Finally, by controlling the average cooling speed in the temperature region of 500 to 700°C to 30°C/s or more and furthermore controlling the average cooling speed in the temperature region of 200 to 500°C to 40°C/s or more, it is possible to suitably form martensite from the finely dispersed austenite. As a result, the average grain interval of martensite is controlled to 2.5 µm or less and the standard deviation in the area ratio in a direction vertical to the rolling direction and the sheet thickness direction is controlled to 1.5% or less. That is, it becomes possible to achieve a microstructure where martensite is uniformly dispersed in both of the micro-regions and macro-regions.

[0076] If the maximum 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, in particular, martensite cannot be suitably formed. On the other hand, if the maximum heating temperature is more than 820°C, the area ratio of austenite becomes too high and the area ratio of ferrite cannot be made 80% or more. Furthermore, it becomes no longer possible to maintain a state where carbides are dispersed at the interfaces due to the high temperature and becomes no longer possible to achieve uniform dispersion of martensite in the finally obtained microstructure at both the micro-regions and macro-regions. Further, if the holding time is more than 500 seconds, the austenite grains become coarser. The martensite grains obtained by subsequent cooling also become relatively coarse. In such a case, it is not possible to obtain fine martensite structures with an average grain interval of martensite controlled to 2.5 µm or less.

[0077] Further, if the average cooling speed of the temperature region of 500 to 700°C at the secondary annealing step is less than 30°C/s, transformation from austenite to bainite, etc., is promoted. Even if suitably cooling after that, sometimes the desired amount of martensite cannot be obtained. In this case, the desired strength can no longer be achieved and/or uniform dispersion of the martensite in particular at the micro-regions can no longer be achieved. Therefore, the average cooling speed of the temperature region of 500 to 700°C has to be 30°C/s or more. The upper limit is, for example, 200°C/s or less, preferably 60°C/s or less. On the other hand, if the average cooling speed of the temperature region of 200 to 500°C is less than 40°C/s, transformation from austenite to martensite cannot be promoted and similarly the amount of formation of bainite and other structures becomes greater. Therefore, the average cooling speed of the temperature region of 200 to 500°C has to be 40°C/s or more. The upper limit is, for example, 200°C/s or less, preferably 80°C/s or less.

[0078] The above method produces the steel sheet according to the embodiments of the present invention by two annealing treatments, including primary annealing and secondary annealing, but the steel sheet according to the embodiments of the present invention is not necessarily limited to one produced by such a method. For example, it can be produced by a single annealing treatment. More specifically, by making the microstructure of the steel sheet after the hot rolling step by full bainite or full martensite, it is possible to omit the previously explained primary annealing. However, in this case, it is necessary to suitably control the cooling conditions, coiling temperature, etc., after the hot rolling. Further, control of the rolling reduction in the subsequent cold rolling is also important. That is to say, if the rolling reduction of the cold rolling becomes higher, recrystallization occurs at the time of heating in the subsequent annealing step and it becomes no longer possible to maintain the microstructure formed at the hot rolling step.

[Plating Step]

[0079] For the purpose of improving the corrosion resistance, etc., the surfaces of the obtained cold rolled steel sheet may also be plated. The plating treatment may be hot dip coating, alloyed hot dip coating, electroplating, and other treatment. For example, as plating treatment, the steel sheet may be hot dip galvanized and may be alloyed after hot dip galvanization. The specific conditions of the plating treatment and the alloying treatment are not particularly limited and may be any suitable conditions known to persons skilled in the art. For example, in hot dip galvanization, the plating bath immersion sheet temperature (temperature of steel sheet at time of immersion in a hot dip galvanization bath) is preferably a temperature range from a temperature lower by 40°C from the hot dip galvanization bath temperature (hot dip galvanization bath temperature-40°C) to a temperature higher by 50°C than the hot dip galvanization bath temperature (hot dip galvanization bath temperature+50°C). If alloying the hot dip galvanized layer, the steel sheet formed with the hot dip galvanized layer is preferably heated in a temperature range of 400 to 600°C.

[0080] Below, examples will be used to explain the present invention in more detail, but the present invention is not limited to these examples in any way.

EXAMPLES

[0081] In the following examples, steel sheets according to the embodiments of the present invention were produced under various conditions. The obtained steel sheets were investigated for the properties of tensile strength, formability, and appearance after forming.

[0082] First, molten steels were cast by the continuous casting method to form slabs having the various chemical compositions shown in Table 1. Each of these slabs was heated to a 1100 to 1400°C predetermined temperature and hot rolled. The hot rolling was performed by rough rolling and finish rolling. The end temperature of the finish rolling and the coiling temperature were as shown in Table 2. Next, the obtained hot rolled steel sheet was pickled, then was cold rolled by a rolling reduction shown in Table 2 to obtain cold rolled steel sheet having a 0.4 mm sheet thickness. Next, the obtained cold rolled steel sheet was subjected to primary annealing and secondary annealing under the conditions shown in Table 2. Finally, as plating treatment, hot dip galvanization was suitably performed. Furthermore, several of these were alloyed at the alloying temperature shown in Table 2.

[Table 1-1]

[0083] 

Table 1-1

Steel no.	Chemical composition (mass%), balance: Fe and impurities
	C	Si	Mn	P	S	Al	N	O	Cr	Mo	Ti	Nb	B	Cu	Ni	W
A	0.04	0.59	2.25	0.0820	0.0014	0.060	0.0013	0.010
B	0.04	0.40	1.42	0.0068	0.0008	0.110	0.0009	0.002		0.100			0.0015
C	0.06	0.97	1.11	0.0292	0.0029	0.150	0.0107	0.004					0.0017			0.009
D	0.07	0.31	1.81	0.0084	0.0021	0.080	0.0168	0.006
E	0.04	0.88	0.59	0.0099	0.0018	0.090	0.0010	0.002	0.892		0.123
F	0.05	0.61	2.02	0.0103	0.0101	0.050	0.0034	0.001						0.546	0.10
G	0.06	0.69	2.27	0.0041	0.0009	0.080	0.0016	0.001
H	0.06	0.79	1.93	0.0059	0.0034	0.050	0.0021	0.002		0.073					0.82
I	0.03	0.48	1.87	0.0079	0.0015	0.510	0.0024	0.016			0.049	0.035	0.0025
J	0.07	0.14	2.90	0.0546	0.0052	0.190	0.0051	0.002
K	0.03	0.41	0.74	0.0167	0.0147	0.750	0.0151	0.002	1.036
L	0.04	0.27	1.57	0.0061	0.0015	0.070	0.0023	0.003
M	0.05	0.07	1.39	0.0088	0.0010	0.080	0.0014	0.002				0.051
N	0.05	0.16	2.46	0.0774	0.0163	0.270	0.0010	0.015
O	0.05	0.82	2.73	0.0140	0.0022	0.810	0.0024	0.001
P	0.11	0.42	2.62	0.0071	0.0009	0.010	0.0029	0.008
Q	0.05	0.15	3.61	0.0062	0.0037	0.025	0.0024	0.002

Bold underlines indicate outside scope of present invention.

[Table 1-2]

[0084] 

Table 1-2

Steel no.	Chemical composition (mass%), balance: Fe and impurities													Ac1	Ac3
	V	Ta	Co	Sn	Sb	As	Mg	Zr	Ca	Y	La	Ce	Bi
A														716	856
B		0.009									0.0061			719	850
C			0.356				0.005		0.0058					739	864
D	0.132									0.0053				713	844
E				0.069										757	869
F												0.0053	0.0366	717	850
G								0.007						719	851
H						0.003								712	845
I														717	856
J														696	823
K		0.013												745	853
L					0.038									714	841
M														710	828
N														701	832
O														718	861
P														707	821
Q														689	831

Bold underlines indicate outside scope of present invention.

[Table 2]

[0085] 

Table 2

Ex. no.	Steel no.	Hot rolling step		Cold rolling step	Primary annealing step				Secondary annealing step				Plating step			Remarks
		Finish rolling end temp. (°C)	Coiling temp. (°C)	Rolling reduction (%)	Max. heating temp. (°C)	Holdin g time (s)	Coolin g stop temp. (°C)	Average cooling speed of 500 to 700°C temp. region (°C/s)	Max. heating temp. (°C)	Holdin g time (s)	Average cooling speed of 500 to 700°C temp. region (°C/s)	Average cooling speed of 200 to 500°C temp. region (°C/s)	Plating (Yes /No)	Alloying (Yes/No)	Alloying temp. (°C)
1	A	890	687	68	885	34	133	53	745	383	95	48	Yes	Yes	519	Inv. ex.
2	B	1226	670	81	853	105	341	83	758	86	43	145	Yes	No	-	Inv. ex.
3	C	1090	552	23	920	237	302	238	774	341	54	162	No	No	-	Inv. ex.
4	D	989	607	51	844	142	231	131	763	472	45	129	No	No	-	Inv. ex.
5	E	895	503	89	934	83	271	107	792	243	44	109	No	No	-	Inv. ex.
6	F	1044	586	29	898	288	170	215	769	35	45	189	No	No	-	Inv. ex.
7	G	933	525	46	874	373	196	294	745	282	45	80	No	No	-	Inv. ex.
8	H	1341	573	58	910	185	105	189	755	194	43	98	No	No	-	Inv. ex.
9	I	1164	633	73	943	445	37	263	745	427	165	175	Yes	Yes	582	Inv. ex.
10	J	1288	503	41	890	170	78	72	737	66	43	116	No	No	-	Inv. ex.
11	K	859	507	28	867	280	236	74	776	241	45	177	No	No	-	Inv. ex.
12	L	1247	503	65	930	107	301	73	744	273	164	55	No	No	-	Inv. ex.
13	M	1025	510	28	886	261	58	60	745	119	51	51	Yes	No	-	Inv. ex.
14	N	858	511	52	883	470	235	71	748	469	40	86	No	No	-	Inv. ex.
15	O	1288	502	72	932	436	164	60	768	422	40	58	Yes	Yes	490	Inv. ex.
16	P	854	514	31	908	205	95	74	762	213	42	58	No	No	-	Comp. ex.
17	Q	845	510	76	900	126	273	253	752	94	45	53	Yes	No	-	Comp. ex.
18	A	853	514	29	827	135	157	262	805	16	36	51	No	No	-	Comp. ex.
19	A	855	508	34	912	343	460	135	783	299	59	66	No	No	-	Comp. ex.
20	B	1191	502	25	862	173	86	36	756	156	43	176	No	No	-	Comp. ex.
21	B	857	512	58	855	214	55	238	845	350	176	55	No	No	-	Comp. ex.
22	B	857	512	58	855	214	55	238	780	650	176	55	No	No	-	Comp. ex.
23	C	1298	508	80	902	486	44	64	776	287	24	65	No	No	-	Comp. ex.
24	C	1234	510	69	870	124	305	83	798	206	173	34	No	No	-	Comp. ex.
25	D	989	607	51	None				763	472	45	129	No	No	-	Comp. ex.
26	D	989	607	51	844	142	231	131	710	472	45	129	No	No	-	Comp. ex.

Bold underlines indicated outside preferred scope.

[0086] The properties of the obtained steel sheet were measured and evaluated by the following methods.

[Tensile Strength (TS) and Total Elongation (El)]

[0087] The tensile strength (TS) and the total elongation (El) were measured by conducting a tensile test compliant with JIS Z 2241: 2011 based on a JIS No. 5 test piece taken from an orientation in which the longitudinal direction of the test piece becomes parallel with the perpendicular direction to rolling of the steel sheet.

[Appearance After Forming]

[0088] The appearance after forming was evaluated by the extent of ghost lines formed at the surface of an outer door member given an approximately 5% strain by press-forming. A surface after press-forming was rubbed by a grindstone. Straight line shaped streak patterns formed at the surface extending substantially parallel to the rolling direction were evaluated judged to be ghost lines. Any 100 mm×100 mm region was visually checked. Cases where no streak patterns at all could be confirmed were judged as passing (○) and cases where streak patterns were confirmed were judged as failing (×). In the test this time, an outer door member was actually press-formed so as to evaluate the appearance after forming, but it is also possible to evaluate a shaped member able to be estimated to have been given an approximately 5% strain by press-forming and possible to evaluate a test piece taken from the steel sheet similarly given a 5% prestrain. Similar results are obtained by such test methods as well. Note that, in the case of a test piece taken from steel sheet, it is possible to evaluate a JIS No. 5 test piece having a direction perpendicular to the rolling direction and the sheet thickness direction as a long direction given a 5% prestrain.

[0089] Cases where the tensile strength (TS) was 400 MPa or more, the total elongation (E1) was 20% or more, and the appearance after forming was evaluated as passing were evaluated as steel sheets achieving both strength and formability and appearance after forming. The results are shown in Table 3.

[Table 3]

[0090] 

Table 3

Ex. no.	Steel no.	Area ratios of microstructure (%)						Average grain interval of martensite (µm)	Standard deviation in area ratio of martensite (%)	TS (MPa)	El (%)	Appearance after forming	Remarks
		Ferrite	Martensite	Bainite	Pearlite	Retained austenite	Remaining structure total
1	A	89	9	0	2	0	2	2.0	1.2	551	33	○	Inv. ex.
2	B	81	13	1	5	0	6	2.1	0.9	401	40	○	Inv. ex.
3	C	80	20	0	0	0	0	1.7	0.9	696	23	○	Inv. ex.
4	D	81	19	0	0	0	0	2.4	1.5	769	21	○	Inv. ex.
5	E	83	17	0	0	0	0	1.7	1.0	484	34	○	Inv. ex.
6	F	86	14	0	0	0	0	2.4	1.3	613	28	○	Inv. ex.
7	G	91	9	0	0	0	0	1.8	1.4	787	23	○	Inv. ex.
8	H	83	17	0	0	0	0	1.8	0.8	725	20	○	Inv. ex.
9	I	90	10	0	0	0	0	2.1	1.4	425	38	○	Inv. ex.
10	J	83	17	0	0	0	0	1.7	0.9	856	20	○	Inv. ex.
11	K	84	16	0	0	0	0	2.3	1.0	401	38	○	Inv. ex.
12	L	87	13	0	0	0	0	1.8	0.8	455	38	○	Inv. ex.
13	M	81	15	0	4	0	4	2.0	1.4	480	34	○	Inv. ex.
14	N	80	20	0	0	0	0	1.9	1.3	661	24	○	Inv. ex.
15	O	86	7	1	6	0	7	1.6	1.2	512	34	○	Inv. ex.
16	P	81	11	8	0	0	8	1.9	1.1	1020	15	○	Comp. ex.
17	Q	70	26	0	4	0	4	1.9	1.2	626	19	○	Comp. ex.
18	A	50	50	0	0	0	0	10.6	3.2	567	18	×	Comp. ex.
19	A	64	36	0	0	0	0	9.4	3.6	587	22	×	Comp. ex.
20	B	81	19	0	0	0	0	8.9	4.2	454	36	×	Comp. ex.
21	B	31	25	36	8	0	44	9.6	2.4	762	17	×	Comp. ex.
22	B	87	13	0	0	0	0	15.3	1.1	435	37	×	Comp. ex.
23	C	80	4	16	0	0	16	3.6	1.4	693	23	×	Comp. ex.
24	C	82	3	15	0	0	15	4.8	1.2	629	26	×	Comp. ex.
25	D	84	16	0	0	0	0	8.8	5.2	732	25	×	Comp. ex.
26	D	100	0	0	0	0	0	-	-	320	46	○	Comp. ex.

Bold underlines indicate outside scope of present invention.

[0091] Referring to Tables 1 to 3, in Comparative Example 16, the C content was high, therefore the TS became too high and the El fell. In Comparative Example 17, the Mn content was high, therefore ferrite transformation was suppressed and similarly the El fell. In Comparative Example 18, the maximum heating temperature of the primary annealing step was low, therefore it is believed that the austenizing became insufficient and the microstructure in the steel sheet could not be made a structure mainly comprised of bainite and/or martensite even by subsequent cooling. As a result, in the microstructure obtained after secondary annealing, the average grain interval of martensite became more than 2.5 µm. Further, the standard deviation in the area ratio in a direction vertical to the rolling direction and the sheet thickness direction became more than 1.5% and the appearance after forming fell. In Comparative Example 19, the cooling stop temperature of the primary annealing step was high, therefore, it is believed that transformation from austenite to bainite and/or martensite could not be made to sufficiently proceed. As a result, similarly in the microstructure obtained after secondary annealing, the average grain interval of martensite became more than 2.5 µm, the standard deviation in the area ratio in a direction vertical to the rolling direction and the sheet thickness direction became more than 1.5%, and the appearance after forming fell. In Comparative Example 20, the average cooling speed in the temperature region of 500 to 700°C at the primary annealing step was slow, therefore similarly it may be that transformation from austenite to bainite and/or martensite could not be made to sufficiently proceed. As a result, in the microstructure obtained after secondary annealing, the average grain interval of martensite became more than 2.5 µm, the standard deviation in the area ratio in a direction vertical to the rolling direction and the sheet thickness direction became more than 1.5%, and the appearance after forming fell. In Comparative Example 21, the maximum heating temperature of the secondary annealing step was high, therefore the austenizing proceeded too much and the desired amount of ferrite could not be obtained in the microstructure after cooling. Further, a large amount of remaining structures were formed and uniform dispersion of the martensite at both of the micro-regions and macro-regions could not be achieved. As a result, both of the formability and the appearance after forming fell. In Comparative Example 22, the holding time at the secondary annealing step was long, therefore it may be that the austenite grains became coarser. As a result, in the microstructure obtained after secondary annealing, the average grain interval of martensite became more than 2.5 µm and the appearance after forming fell. In Comparative Example 23, the average cooling speed in the temperature region of 500 to 700°C in the secondary annealing step was low, therefore transformation from austenite to bainite, etc., was promoted and the desired amount of martensite could not be obtained. As a result, in the microstructure obtained after secondary annealing, the average grain interval of martensite became more than 2.5 µm and the appearance after forming fell. In Comparative Example 24, the average cooling speed of the temperature region of 200 to 500°C in the secondary annealing step was low, therefore transformation from austenite to martensite could not be promoted and similarly a large amount of bainite was formed. As a result, similarly, in the microstructure obtained after secondary annealing, the average grain interval of martensite became more than 2.5 µm and the appearance after forming fell. In each of Comparative Examples 22 to 24, the standard deviation in the area ratio in a direction vertical to the rolling direction and the sheet thickness direction was 1.5% or less, but the average grain interval of martensite was more than 2.5 µm, i.e., uniformity of martensite in the macro-regions was secured, but uniformity of martensite in the micro-regions was not secured. For this reason, in these comparative examples, it is believed that the appearance after forming fell due to the presence of locally uneven martensite. In Comparative Example 25, the primary annealing step was not performed, therefore it was not possible to form a microstructure in the steel sheet before the secondary annealing step by structures mainly comprised of bainite and/or martensite. As a result, in the microstructure obtained after secondary annealing, the average grain interval of martensite became more than 2.5 µm, the standard deviation in the area ratio in a direction vertical to the rolling direction and the sheet thickness direction became more than 1.5%, and the appearance after forming dropped. In Comparative Example 26, the maximum heating temperature in the secondary annealing step was low, therefore it was not possible to suitably form martensite in the microstructure after cooling and the desired TS could not be obtained.

[0092] In contrast to this, in the steel sheet according to all of the invention examples, by having a predetermined chemical composition and furthermore suitably controlling the ratios of ferrite and martensite in the microstructure, a 400 MPa or more TS and a 20% or more El were achieved and by controlling the average grain interval of martensite in the micro-regions to 2.5 µm or less and on the other hand controlling the standard deviation in the area ratio of martensite in a direction vertical to a rolling direction and a sheet thickness direction in the macro-regions to 1.5% or less, even in the case where strain is imparted by press-forming, it was possible to suppress the formation of fine asperities at the steel sheet surfaces and remarkably suppress formation of ghost lines. When examining by cross-section the microstructures in the cold rolled steel sheets before secondary annealing in all of the invention examples, in each case, the microstructure was comprised of an area ratio of 90% or more of martensite.

Claims

1. A steel sheet comprising a chemical composition comprising, by mass%,

C: 0.03 to 0.08%,

Si: 0.01 to 1.00%,

Mn: 0.50 to 3.00%,

P: 0.1000% or less,

S: 0.0200% or less,

Al: 1.000% or less,

N: 0.0200% or less,

O: 0 to 0.020%,

Cr: 0 to 2.000%,

Mo: 0 to 1.000%,

Ti: 0 to 0.500%,

Nb: 0 to 0.500%,

B: 0 to 0.0100%,

Cu: 0 to 1.000%,

Ni: 0 to 1.00%,

W: 0 to 0.100%,

V: 0 to 1.000%,

Ta: 0 to 0.100%,

Co: 0 to 3.000%,

Sn: 0 to 1.000%,

Sb: 0 to 0.500%,

As: 0 to 0.050%,

Mg: 0 to 0.050%,

Zr: 0 to 0.050%,

Ca: 0 to 0.0500%,

Y: 0 to 0.0500%,

La: 0 to 0.0500%,

Ce: 0 to 0.0500%,

Bi: 0 to 0.0500%, and

balance: Fe and impurities, and

a microstructure comprising, by area ratio,

ferrite: 80 to 95%,

martensite: 5 to 20%, and

at least one of bainite, pearlite, and retained austenite: 0 to 10% in total, wherein

an average grain interval of martensite is 2.5 µm or less, and

a standard deviation in area ratio of martensite in a direction vertical to a rolling direction and a sheet thickness direction is 1.5% or less.

2. The steel sheet according to claim 1, wherein the chemical composition contains, by mass%, at least one of

Cr: 0.001 to 2.000%,

Mo: 0.001 to 1.000%,

Ti: 0.001 to 0.500%,

Nb: 0.001 to 0.500%,

B: 0.0001 to 0.0100%,

Cu: 0.001 to 1.000%,

Ni: 0.001 to 1.00%,

W: 0.001 to 0.100%,

V: 0.001 to 1.000%,

Ta: 0.001 to 0.100%,

Co: 0.001 to 3.000%,

Sn: 0.001 to 1.000%,

Sb: 0.001 to 0.500%,

As: 0.001 to 0.050%,

Mg: 0.0001 to 0.050%,

Zr: 0.0001 to 0.050%,

Ca: 0.0001 to 0.0500%,

Y: 0.0001 to 0.0500%,

La: 0.0001 to 0.0500%,

Ce: 0.0001 to 0.0500%, and

Bi: 0.0001 to 0.0500%.