Classifications

• • 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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • 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
  • C21D8/04—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 to produce plates or strips for drawing, e.g. for deep-drawing
  • C21D8/041—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 to produce plates or strips for drawing, e.g. for deep-drawing involving a particular fabrication or treatment of ingot or slab
• • 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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • 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
  • C21D8/04—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 to produce plates or strips for drawing, e.g. for deep-drawing
  • C21D8/0421—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 to produce plates or strips for drawing, e.g. for deep-drawing characterised by the working steps
  • C21D8/0426—Hot rolling
• • 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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • 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
  • C21D8/04—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 to produce plates or strips for drawing, e.g. for deep-drawing
  • C21D8/0421—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 to produce plates or strips for drawing, e.g. for deep-drawing characterised by the working steps
  • C21D8/0436—Cold rolling
• • 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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • 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
  • C21D8/04—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 to produce plates or strips for drawing, e.g. for deep-drawing
  • C21D8/0447—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 to produce plates or strips for drawing, e.g. for deep-drawing characterised by the heat treatment
  • C21D8/0463—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 to produce plates or strips for drawing, e.g. for deep-drawing characterised by the heat treatment following hot rolling
• • 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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • 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
  • C21D8/04—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 to produce plates or strips for drawing, e.g. for deep-drawing
  • C21D8/0447—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 to produce plates or strips for drawing, e.g. for deep-drawing characterised by the heat treatment
  • C21D8/0473—Final recrystallisation annealing
• • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
  • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • 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
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • 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
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • 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
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • 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
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • 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
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/14—Removing excess of molten coatings; Controlling or regulating the coating thickness
  • C23C2/16—Removing excess of molten coatings; Controlling or regulating the coating thickness using fluids under pressure, e.g. air knives
  • C23C2/18—Removing excess of molten coatings from elongated material
  • C23C2/20—Strips; Plates
• • 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
  • C21D2201/00—Treatment for obtaining particular effects
  • C21D2201/05—Grain orientation

Definitions

• the present invention relates to a hot rolled steel sheet. In a further aspect the present invention relates to a method of manufacturing a hot rolled steel sheet.
• the property of a steel sheet that needs to be there in places in the bodywork with and without deformation, can be expressed by a waviness parameter.
• Steel sheets or strips that are highly formable and that show low waviness after forming into an end shape product are required in the automobile industry. Typically they are suitable for the manufacture of automotive body components such as automotive outer panels. Owing to their high formability, interstitial free steel grades can be the steel grades used for such automotive outer panels.
• improvements in paint baking cycles of vehicles have become important in the automobile industry. This enables realising sublime paint appearance, a global improvement of the environment and a reduction of process cost for the paint baking cycle.
• sheet and strip for a large part can be used interchangeably; in this specification sheet is defined as a part or former part of a strip.
• the state of the art focuses on improving the metal coating process, such as the hot dip coating process, and on skin pass rolling. It is the aim of the present invention to improve the hot rolled steel sheet such that after further processing, such as cold rolling, annealing and galvanizing, the outer surface of a resulting sheet, blank or panel has an optimal surface.
• the present invention seeks to provide a reliable solution to improve the formability and to have good surface properties such as a low waviness after forming and low variance of the waviness due to forming of a final product such as an automotive body panel.
• millipercent or ppm these mean millipercent by weight and ppm by weight respectively.
• the steel sheet is in hot rolled condition and the steel has a composition in millipercent or where so indicated in ppm:
• N max. 100 ppm; wherein ⁇ (Ti+Nb+V) together max. 200; and optionally:
• HRT 0.752 - 0.351 P + 0.004 G + 0.026 T ⁇ 0.10 (Equation A) wherein HRT is a parameter related to the microstructure, texture and final thickness of the steel in its hot rolled condition; wherein P is the total volume fraction of the texture components (112) ⁇ 110> and (554) ⁇ 225>, determined using Electron Back-Scatter Diffraction, EBSD, on a sample representing a cross section from the sheet parallel to the rolling direction with a length of at least 1.5 mm through thickness, whereupon the texture coefficients determined by EBSD are used to calculate the said volume fraction; and wherein G is the average grain size of the steel in hot rolled condition in pm based on the scanned area in EBSD; and wherein T is the final thickness of the steel sheet in hot rolled condition in mm, in order to enable attaining a SEP 1941 waviness value Wsa in the final formed product made from the cold rolled, annealed and optionally metal coated hot rolled steel sheet, of 0.32
• P is the total volume fraction of the texture components (112) ⁇ 110> and (554) ⁇ 225> .
• ⁇ 110> and ⁇ 225> are the directions that are related to the rolling direction.
• the (112) and (554) are related to the normal direction.
• the volume fraction of P is the sum of volume fraction of two orientations; (112) ⁇ 110> and (554) ⁇ 225>.
• the volume fraction of each component is expressed dimensionless. In the calculation of volume fraction, an angular spread of 11 degrees is used. Due to the thickness gradient of the microstructure and the texture, the volume fraction measured at different thickness positions may be different. All the measurements that are carried out here are over the whole thickness.
• the texture is determined using Electron Back-Scatter Diffraction, EBSD, on a sample representing a cross section from the sheet parallel to the rolling direction with a length of at least 1.5 mm through thickness, where upon the texture coefficients determined by EBSD are used to calculate the said volume fraction; and the average grain size of the steel in hot rolled condition in pm based on the scanned area in EBSD.
• An intermediate stage after hot rolling is cold rolling and skin passing, where the surface properties after final cold rolling and after skin passing are important to achieve the desired waviness.
• the steel is cold rolled wherein the roughness Ra as measured with a cut-off threshold of 2.5 mm, herein abbreviated to Ra2.5, of the work roll of the last stand of the cold rolling mill is lower than 4.5 pm but higher than 0.6 pm.
• the skin-pass operation may be carried out using an electric discharge textured work roll (EDT) for which the work surfaces have a roughness Ra2.5 comprised between 2.0 pm and 3.5 pm, preferably between 1.8 pm to 3.5 pm, more preferably between 1.8 pm to 2.5 pm.
• EDT electric discharge textured work roll
• the elongation of the metal sheet during the skin-pass operation is comprised between 0.5% and 2%.
• one of the objectives is to enable production of a strip, sheet or blank that in the cold rolled and annealed condition or in the cold rolled, annealed and metal coated condition has a low waviness value expressed as Wsa (1-5) according to standard SEP 1941 : 2012, ‘Measurement of the waviness characteristic value Wsa (1-5) on cold rolled metallic flat products’, designated hereinafter by the abbreviation Wsa.
• Wsa waviness characteristic value
• Wsa in the final formed product is then 0.30 pm or less, preferably 0.29 pm or less, more preferably of 0.28 pm or less and/or delta waviness is then 0.1 pm or less, preferably 0.08 pm or less, more preferably 0.06 pm or less.
• a suitable P or G has to be chosen to comply with Equation A (herein also referred to as Eqn (A)).
• Mn 10-200, preferably 40-180;
• Si 1-50, preferably 2-15;
• Nb max. 90, more preferably max. 10;
• V max. 90, preferably max. 50, more preferably max. 10;
• N max. 80, preferably max. 60 ppm; wherein ⁇ (Ti+Nb+V) together max. 100, preferably max. 90; and optionally:
• the value of ⁇ (Ti+Nb+V) together is max. 100 or preferably max. 90.
• the advantage of having such a value is to avoid any clogging during continuous casting.
• N 10-60 ppm; and optionally:
• P 2-13, preferably 1-5;
• the present invention in a further aspect relates to a steel sheet, which is characterised in that
• a further embodiment of the present invention relates to a steel sheet where the G is 22 pm or smaller, preferably 20 pm or smaller. If the value of G is lower, then the value of HRT is lower. It allows to achieve a fine graine size, which is favourable for the final product.
• a further aspect of the present invention relates to a method for manufacturing a steel sheet as described above for each of the compositions described.
• the manufacturing method to produce a product from the hot rolled steel sheet according to the present invention embodiments may after the hot rolling step comprise steps such as a pickling step, a cold rolling step, an annealing step, a galvanising step, and a temper rolling or skin pass step.
• the steel with the specified composition according to the embodiments of the present invention is smelted, for example, in a converter and formed into a slab by a continuous casting process or the like.
• the slab to be used is preferably produced by a continuous casting process to prevent macrosegregation of the components.
• the slab to be used may be produced by an ingot-making method or a thin slab casting process.
• an energy saving process such as hot direct rolling or direct rolling, may be applied.
• the energy saving process may comprise placing a slab into a heating furnace whilst keeping the slab temperature without cooling to a room temperature, or performing rolling immediately after keeping the temperature for a short time.
• the slab used in the hot rolling step may be heated.
• the slab heating temperature is preferably as low as possible for the aim of energy saving. If however the heating temperature falls below 1150°C, the carbide is not sufficiently dissolved. In terms of an increase in the scale loss with increasing oxidation weight gains, the slab heating temperature is desirably 1250°C or lower.
• the slab In hot rolling, the slab is rolled at a hot rolling finishing temperature that is at or higher than the Ar3 transformation temperature, then cooled at an average cooling rate of 30°C/s or higher, and then coiled.
• the Ar3 temperature is the temperature at which ferrite transformation starts in the cooling. If the hot rolling finishing temperature falls below the Ar3 temperature, both a and g phase are generated in the rolling, and as a result, the grain size of ferrite becomes large and the grains will be less uniform through the thickness. Therefore, the hot rolling finishing temperature is the Ar3 temperature or higher.
• the method comprises a step of hot rolling the steel sheet; wherein the hot rolling finishing temperature is lower than 960°C.
• the method comprises a step of hot rolling the steel sheet; wherein the hot rolling finishing temperature is lower than 960°C, wherein the finishing temperature is higher than Ar3 transformation temperature.
• the maximum value of hot rolling finishing temperature is preferably Ar3+70°C .
• a further embodiment of the present invention relates to the method for manufacturing a steel sheet as described above, wherein the hot rolling finishing temperature is lower than 945°C.
• the gauge thickness of hot rolled sheet is thicker, the preferred hot rolling finishing temperature is lower.
• a finishing temperature of Ar3+50°C is chosen, while for thickness gauge of 4.7 mm, a finishing temperature of Ar3+30°C is preferred.
• a further embodiment of the present invention relates to a method for manufacturing a steel sheet as described above, wherein reduction in the last stand of the hot rolling finishing mill is more than 15%, preferably more than 20%. By an increased reduction, the grain size G is finer and the texture component P is increased.
• the present invention relates to the method for manufacturing a steel sheet as described above, wherein the steel sheet is cold rolled and wherein the roughness Ra measured with a cut-off threshold at 2.5 mm, herein abbreviated to Ra2.5, of the work roll of the last stand of the cold rolling mill is lower than 4.5 pm but higher than 0.6 pm.
• a total cold rolling reduction is generally comprised between 50% and 85%, so as to obtain a substrate with a thickness which is for example comprised between 0.2 and 2 mm.
• an annealing step may be performed.
• the cold rolled substrate is then subjected to annealing conducted in a conventional way in an annealing furnace under a reducing atmosphere, with a view to recrystallization after the work hardening which it has undergone during the cold rolling operations.
• the annealing step is a step of heating the cold rolled steel sheet to a temperature of 650°C to 900°C.
• the annealing step may be performed preferably in a range of 780°C - 820°C.
• the steel sheet After recrystallization annealing, the steel sheet is cooled down to a temperature close to the bath temperature. After entry in the bath, the steel sheet or the substrate is metal coated on its two sides with a Zn based coating. The coating weight per surface can be 35 to 45 g/m 2 . Then the metal sheet is subjected to wiping by means of nozzles, which project a wiping gas, placed on either side of the metal sheet. The wiping gas is ejected from each nozzle along a direction horizontal and orthogonal to the metal sheet.
• the running speed of the substrate in the production line and therefore in front of the nozzle is between 80 m/min and 160 m/min. This can be preferably greater than 100 m/min, or even 120 m/min.
• the outlet of the nozzle is generally positioned at a distance 6 mm to 12 mm from the metal sheet along the main ejection direction.
• the outlet generally appears as a rectangular slot which extends, perpendicularly to the running direction of steel sheet substrate and over a width at least equal to the width of the metal sheet.
• the distance of the nozzle outlet to the metal sheet is preferably not greater than 10 mm and more preferably below 8 mm.
• the metal sheet When the metal coated steel sheet is completely cooled, the metal sheet may undergo a temper rolling or tension levelling operation for giving texture to the outer surfaces of the metal coating.
• the transferred surface texture to the outer surfaces of the metal coating with sufficient roughness enables the metal sheet to retain sufficient amount of oil applied on the metal sheet, in order for its forming process to be properly carried out.
• the other purpose ofthis transferred surface texture is that it provides the metal sheet with desired low waviness properties.
• the skin pass operation may be carried out using an electric discharge textured work roll (EDT) for which the work surfaces have a roughness Ra2.5 comprised between 2.0 pm and 3.5 pm, preferably between 1.8 pm to 3.5 pm; more preferably between 1.8 pm to 2.5 pm.
• EDT electric discharge textured work roll
• the elongation of the metal sheet during the skin pass operation is comprised between 0.5% and 2%. All steel sheets are given a waviness Wsa of less than 0.35 pm and preferably less than 0.30 pm.
• the steel sheet optionally having been temper rolled or skin passed may then be cut out and undergoes a forming process, for example by stretching, drawing or bending, in order to form a part which may then be painted on each coating.
• the outer surfaces of the metal coating of the part have a waviness Wsa value of less than or equal to 0.32 pm, or even less than or equal to 0.30 pm, or even less than or equal to 0.28 pm.
• This waviness may for example be measured after equi-biaxial deformation of 4.5% in the rolling direction and the transverse direction on the sheet plane, or approximately 9% through thickness direction.
• the present invention relates to a method for manufacturing a steel sheet as described above, wherein the steel is hot dip galvanised and temper rolled and wherein the roughness Ra2.5 of the work roll of the last stand of the temper rolling mill is in a range of 1.0 pm-5.0 pm.
• the present invention relates to a steel sheet wherein the hot rolled sheet gauges are of thickness between 3 mm to 5 mm. Examples
• Table 1 steel compositions all in milli percent except C and N in ppm.
• the I.E. represents invention examples and C.E. represents comparative examples.
• the underlined numbers are out of the scope of the present invention.
• the slab thickness is approximately 225 mm
• the transfer gauge is in the range of 35 mm to 40 mm
• the final gauge thicknesses of the hot rolled strips is in the range of 3 mm to 5 mm.
• the average target cooling rate after the finish rolling is at least 30°C/s, so that the microstructure of the hot rolled steel sheet becomes a more uniform microstructure.
• the coiling temperature is 750°C or lower, but higher than 550°C. Thus the microstructure of the hot rolled steel sheet becomes a more uniform microstructure and the scale loss due to oxidation is reduced. In all of the experiments, a pickling step is performed.
• the pickling step is a step of removing an oxide scale of a surface of a hot rolled steel sheet obtained in the hot rolling step by performing pickling.
• the cold rolling step is a step of cold rolling a pickled sheet after the pickling step.
• at least the last cold rolling pass is carried out with relatively smooth work rolls.
• the surfaces of the rectified and non-etched rolls of the rolling mill directly in contact with the above mentioned sheet for ensuring its deformation have a roughness Ra2.5 less than 0.5 pm.
• the aim for the waviness Wsa (1-5) of the cold rolled sheet is less than 0.40 pm.
• all steel sheets are cold rolled with such a surface characteristic.
• the roughness Ra2.5 of the work surfaces of the EDT work rolls for temper rolling is comprised between 2.50 pm and 3.0 pm.
• the elongation during the temper rolling operation is in the range of 1.0%-1.5%.
• the hot rolling was performed with the process conditions shown in Table 2.
• the rest of process; pickling, cold rolling, annealing, galvanising and temper rolling were conducted as described in the previous section.
• the waviness after temper rolling, and thereafter biaxial stretching using a Marciniak tool at a strain of 4.5% are also shown in Table 2.
• Ra2.5, CR surface roughness of the work roll used for cold rolling CR in the last stand of the cold rolling mill
• TR surface roughness of the work roll used for temper rolling TR of the temper rolling mill
• the waviness Wsa after biaxial stretching turns out to be lower or higher, which leads to a bad (B), good (G) or excellent (E) surface appearance as can be seen in Table 2.
• the waviness values Wsa of the steel sheet surfaces are measured after the skin pass operation (flat) and after stretching (cup). The latter is carried out by equi-biaxial deformation of 4.5% with a Marciniak tool. The results of the measurements of Wsa are grouped in table 2.
• Equation A The use of parameter HRT satisfying Equation A gives the possibility of attaining a waviness Wsa after a skin pass and equi- biaxial deformation as described above of smaller than or equal to 0.30 pm.
• the hot band microstructure and texture do not satisfy the HRT criterion, the waviness becomes considerably higher, as can be seen in Table 2.
• a lower value of finishing temperature can be selected.
• the hot rolled T has a higher value. In such cases it may be possible to lower the finishing temperatures to a lower value to comply with Eqn (A).

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• 2022
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