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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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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  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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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  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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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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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
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  • C21D2211/008—Martensite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/009—Pearlite

Definitions

• 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.
• 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 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.
• 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 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.
• 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 steel sheet according to the embodiments of the present invention has a chemical composition comprising, by mass %,
• 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.
• 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.
• 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.
• 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.
• 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.
• 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 ⁇ m 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.
• the average grain interval of martensite to 2.5 ⁇ m or less, it is possible to make the hard structures be dispersed densely and uniformly in the micro-regions.
• the standard deviation in the area ratio in a direction vertical to the rolling direction and the sheet thickness direction it is possible to remarkably reduce the variation in hard structures in the macro-regions.
• 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.
• 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.
• the component elements of the steel sheet according to the embodiments of the present invention will be explained in more detail.
• the microstructure of the steel sheet according to the embodiments of the present invention will be explained.
• the structure fractions are displayed by area ratio.
• the unit “%” of the structure fractions means the area %.
• 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.
• the “structure fractions” all mean the values at the sheet thickness 1 ⁇ 4 part.
• the area ratio of 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 forms hard structures with high dislocation density, therefore forms structures contributing to tensile 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.
• 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.
• “martensite” includes not only martensite as hardened (so-called “fresh martensite”), but also tempered martensite.
• 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.
• 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.
• the area ratio of retained austenite may be 0 to 3%.
• the area ratio of retained austenite may also be 2% or less.
• the area ratio of the remaining structures is 0.5% or more or 1% or more.
• 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 ⁇ m ⁇ 100 ⁇ m 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 50000 ⁇ power.
• three locations are measured and the average value of these measured values is calculated to determine the area ratio.
• the length in the sheet thickness direction is decreased and a measurement region of 10000 ⁇ m 2 is secured.
• a measurement region of 20 ⁇ m in the sheet thickness direction and 500 ⁇ m in the rolling direction may also be covered by measurement.
• the measurement length in the sheet thickness direction is 10 ⁇ m or more, preferably 50 ⁇ m or more. The same is true for the “100 ⁇ m ⁇ 100 ⁇ m region” in the following explanation.
• the area ratio of the ferrite is found by observing 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 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.
• 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 ⁇ m ⁇ 100 ⁇ m 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.
• 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 Moka rays. As this method of calculation, the general 5-peak method is utilized. The structure fraction of retained austenite calculated is determined as the area ratio of the retained austenite.
• 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 pearlite is identified and the area ratio is calculated by the following procedure.
• 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 500 ⁇ including the 100 ⁇ m ⁇ 100 ⁇ m captured range, is used to find the area fraction of pearlite by the point counting method.
• 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.
• 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.
• 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).
• 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 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 ⁇ m square at the sheet thickness center part using a measurement pitch of 1 ⁇ m 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.
• the reasons for limitation of the chemical composition of the steel sheet according to the embodiments of the present invention will be explained.
• the % relating to the chemical composition will mean mass %.
• 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 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 is an element for raising the hardenability and contributing to improvement of the steel sheet strength.
• 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.
• 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.
• the P content 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%.
• the P content may also be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
• 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.
• 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.
• 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.
• 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.
• Mo 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.
• 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 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.
• 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.
• 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.
• 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.
• Co 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.
• 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 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 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 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 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.
• the 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.
• 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. 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.
• 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.
• 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.
• 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 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.
• 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.
• 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.
• 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.
• 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 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.
• the hot rolling is preferably performed at a high temperature.
• 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.
• the end temperature of the finish rolling is preferably 1350° C., or less.
• 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.
• 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.
• the pickled hot rolled steel sheet is cold rolled in the cold rolling step by a rolling reduction of 20 to 90%.
• 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.
• 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.
• 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.
• 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.
• 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
• “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.
• the average grain interval of martensite is controlled to 2.5 ⁇ m 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.
• 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 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.
• 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.
• carbides can be formed dispersed on the many interfaces contained inside the bainite and/or martensite in the microstructure.
• carbides can be formed dispersed on the many interfaces contained inside the bainite and/or martensite in the microstructure.
• 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.
• the average grain interval of martensite is controlled to 2.5 ⁇ m 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.
• 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.
• 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.
• 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 ⁇ m or less.
• 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 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.
• 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.
• 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.
• 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.
• 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.).
• the steel sheet formed with the hot dip galvanized layer is preferably heated in a temperature range of 400 to 600° C.
• 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.
• 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.
• 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.
• the obtained cold rolled steel sheet was subjected to primary annealing and secondary annealing under the conditions shown in Table 2.
• hot dip galvanization was suitably performed. Furthermore, several of these were alloyed at the alloying temperature shown in Table 2.
• the properties of the obtained steel sheet were measured and evaluated by the following methods.
• 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 (O) and cases where streak patterns were confirmed were judged as failing (x).
• 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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