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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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  • C22C—ALLOYS
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  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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  • C22C—ALLOYS
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• 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.
• 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 ⁇ m 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.
• 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.
• 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.
• the gist of the present invention is as follows:
• 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).
• 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.
• 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.
• 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 MoK ⁇ 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.
• 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 ⁇ m ⁇ 100 ⁇ m 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.
• EDS energy dispersive X-ray spectroscopy
• the image captured by a measurement power of 500X including the 100 ⁇ m ⁇ 100 ⁇ m captured range, is used to find the area fraction of pearlite by the point counting method.
• 500X including the 100 ⁇ m ⁇ 100 ⁇ m 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.
• the ratio of points occupied by pearlite can be calculated as the area fraction of pearlite.
• the average grain interval of the hard structures of martensite is controlled to 2.5 ⁇ m 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.
• the average grain interval of martensite is preferably 2.4 ⁇ m or less, more preferably 2.2 ⁇ m or less, most preferably 2.0 ⁇ m or less or 1.8 ⁇ m or less.
• the lower limit is not particularly prescribed, but, for example, the average grain interval of martensite may be 0.5 ⁇ m or more, 0.8 ⁇ m or more, or 1.0 ⁇ m or more.
• 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 ⁇ m ⁇ 100 ⁇ m 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.
• 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.
• 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.
• 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.
• 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.
• the obtained image is divided into 100 ⁇ m (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.
• the 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.
• the S content 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%.
• the S content may also be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
• the N content is an element becoming a cause of formation of blowholes at the time of welding.
• 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%.
• the N content may also be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
• the O content is an element becoming a cause of formation of blowholes at the time of welding.
• 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%.
• the O content may also be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
• 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.
• 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%, Ce: 0 to 0.0
• Cr 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.
• 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.
• 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.
• 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 like Ti
• 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.
• 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.
• the 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.
• 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 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.
• 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 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.
• 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 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.
• 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 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.
• 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.
• 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%.
• 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.
• 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.
• 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.
• 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.
• the chemical composition of the steel sheet according to the embodiments of the present invention may be measured by a general analysis method.
• 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
• O may be measured using the inert gas fusion-nondispersive infrared absorption method.
• the alloyed plating layer preferably is a plating layer containing 7 mass% or more and 15 mass% of less of Fe.
• the constituents other than zinc and Fe are not particularly limited. Various constitutions can be employed within the usual range.
• the plated layer for example, may be an aluminum plating layer, etc.
• the amount of deposition of the plating layer is not particularly limited and may be a general amount of deposition.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• the maximum heating temperature at the primary annealing step is 950°C or less and the holding time is 500 seconds or less.
• 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.
• 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.
• 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.
• 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.
• it can be produced by a single annealing treatment.
• 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.
• 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.
• TS tensile strength
• El total elongation
• 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 ( ⁇ ).
• Comparative Example 16 the C content was high, therefore the TS became too high and the El fell.
• Comparative Example 17 the Mn content was high, therefore ferrite transformation was suppressed and similarly the El fell.
• 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 ⁇ m.
• 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 ⁇ m 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.
• the microstructure was comprised of an area ratio of 90% or more of martensite.

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