Classifications

• • 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
  • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
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  • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
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  • 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 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
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips 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 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
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
  • C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips 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 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
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips 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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with 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
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • 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
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • 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
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
  • C23C2/0224—Two or more thermal pretreatments
• • 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
  • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
• • 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
  • C23C2/26—After-treatment
  • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
  • C23C2/29—Cooling or quenching
• • 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
  • C21D2211/002—Bainite
• • 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
  • C21D2211/005—Ferrite
• • 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
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a high strength steel sheet product and a method of manufacturing the same.
• the invention relates to a high strength steel sheet product with ultimate tensile strength of at least 950 MPa and having improved flatness and gauge uniformity.
• AHSS advanced high strength steel
• AHSS grades with ultimate tensile strength levels of 950 MPa and above have been developed. Achieving such high strength requires quite high levels of alloying elements in the chemical composition, in particular when the steel is designed for processing in conventional continuous galvanizing lines (CGLs) with relatively low cooling capacity.
• the steel compositions also need to be fine-tuned for specific CGL heat treatment cycles in order to meet the desired mechanical property values e.g. strength, toughness, formability and weldability; and other practical customer requirements related to products exterior qualities e.g. dimensions, shape tolerance, flatness, gauge uniformity and surface quality.
• Producing AHSS grades with ultimate tensile strength of 950 MPa and above requires complex alloying to increase hardenability of the steel, which severely suppresses transformation of austenite on hot rolling mill runout table such that the low-temperature microstructure is formed mainly during coil cooling.
• the formed microstructures can range from ferritic-pearl itic to bainitic to even martensitic within a single coil, even when a constant coiling temperature has been achieved over the length of the strip.
• WO2017/108251 A1 concerning a high strength galvannealed steel sheet with an ultimate tensile strength of 1180 - 1300 MPa, recognizes that the hot rolled sheet needs to be subjected to batch annealing before cold rolling in order to obtain a microstructure which is necessary for cold-rolling with narrow thickness deviation.
• the thickness deviation is diminished or the problem of cold gauge hashing can be solved by the extra step of batch annealing.
• An extra step of batch annealing would inevitably enhance production costs and reduce productivity.
• the present invention is intended to improve the production yield of high strength steel sheet products with ultimate tensile strength of at least 950 MPa, and to obtain a wide dimensional range for the respective final products. This is achieved by preventing the above-mentioned problem of cold gauge hashing and alleviating other issues related to strength variations or flatness defects in the hot rolled strip.
• the object of the present invention is to solve the problem of providing a high strength steel sheet with ultimate tensile strength of at least 950 MPa and having improved flatness and gauge uniformity.
• the problem is solved by the combination of specific alloy designs and hot rolling mill process which attenuates the phenomenon of cold gauge hashing.
• the present invention provides a high strength steel sheet with an ultimate tensile strength (R m ) of at least 950 MPa and a thickness accuracy better than 1/5 of EN 10143:2006 normal thickness tolerances, which steel sheet has a composition consisting of, in terms weight percentages (wt. %):
• Si 0.1 - 0.3 preferably more than 0.1 and less than 0.3
• Nb is not added in order for the steel product to have a wide
• the steel product is alloyed with essential alloying elements such as C, Si, Mn, Cr, Ti, B, Ca, and Al. Other elements such as Mo, V, Cu, and Ni may be present as residual contents that are not purposefully added.
• the present invention provides a high strength steel sheet with an ultimate tensile strength (R m ) of at least 980 MPa and a thickness accuracy better than 1/5 of EN 10143:2006 normal thickness tolerances, which steel sheet has a composition consisting of, in terms weight percentages (wt. %):
• the present invention provides a high strength steel sheet with an ultimate tensile strength (R m ) of at least 1180 MPa and a thickness accuracy better than 1/5 of EN 10143:2006 normal thickness tolerances, which steel sheet has a composition consisting of, in terms weight percentages (wt. %):
• the steel sheet has a microstructure comprising a matrix consisting of, in terms of volume percentages (vol. %):
• the microstructure has an average grain size of less than 10 pm.
• the bainite and/or fine-grained ferrite have an average grain size of less than 5 pm, preferably in the range of 1 pm to 5 pm.
• the steel sheet further has at least one of the following mechanical properties:
• the high strength steel sheet with an ultimate tensile strength (R m ) of at least 980 MPa has a minimum bending radius (Ri) of 2.5 1 or less.
• the high strength steel sheet with an ultimate tensile strength (R m ) of at least 980 MPa has a total elongation (A 8 o) of at least 8 %.
• the high strength steel sheet with an ultimate tensile strength (R m ) of at least 1 180 MPa has a total elongation (A 80 ) of at least 5 %.
• the steel sheet is a strip having a thickness in the range of 0.8 - 2.2 mm.
• the present invention provides a method for manufacturing the steel sheet comprising the following steps of
• (slow) cooling at an average rate of 10 °C/s or below to a
• final cooling at an average rate in the range of typically 1 °C/s to 20 °C/s.
• the optional step of hot-dip coating is preferably hot-dip galvanizing or
• the method further comprises steps of temper rolling and/or levelling.
• the method further comprises a step of extra batch annealing at a temperature in the range of 200 to 400 °C, e.g. 270 °C.
• Figure 1 illustrates the microstructures.
• Figure 2 illustrates the flatness maps.
• Figure 3 illustrates the thickness deviations along the length of the tested steel sheets.
• steel is defined as an iron alloy containing carbon (C).
• flatness is used to indicate deviations from a horizontal flat surface in a steel strip.
• the term“thickness accuracy” is used to indicate deviations from a target thickness in a steel strip in length direction.
• gauge refers generally to a measure of the thickness of a metal sheet.
• EN 10143:2006 refers to European Standard tolerances on dimensions and shape, which is applicable to continuously hot-dip coated steel sheet and strip.
• the term“ultimate tensile strength (UTS, R m )” refers to the limit, at which the steel fractures under tension, thus the maximum tensile stress.
• yield strength (YS, Rpo . 2) refers to 0.2 % offset yield strength defined as the amount of stress that will result in a plastic strain of 0.2 %.
• total elongation refers to the percentage by which the material can be stretched before it breaks; a rough indicator of formability, usually expressed as a percentage over a fixed gauge length of the measuring extensometer. Two common gauge lengths are 50 mm (A 50 ) and 80 mm (A 8 o).
• minimum bending radius (Ri) is used to refer to the minimum radius of bending that can be applied to a test sheet without occurrence of cracks.
• hole expansion ratio (l) is a key indicator to evaluate stretch flanging performance of steel sheets, which is usually obtained by hole expanding test using cylindrical or conical punch.
• Carbon C is used in the range of 0.07 wt. % to 0.13 wt. %.
• C alloying increases strength of steel by solid solution strengthening, and hence C content determines the strength level.
• C content less than 0.07 wt. % may lead to insufficient tensile strength below 950 MPa.
• C also functions as an austenite stabilizer and delays transformation of austenite, which inhibits the formation of ferritic-pearl itic microstructures.
• C content needs to be set to not more than 0.13 wt. % to prevent excessive strengthening within the hot rolled coil in parts that are locally cooled too fast to form the desired bainitic microstructures.
• Silicon Si is used in the range of 0.1 wt. % to 0.3 wt. %.
• Si is effective as a deoxidizing or killing agent that can remove oxygen from the melt during a steelmaking process.
• Si alloying enhances strength by solid solution strengthening, and enhances hardness by increasing austenite hardenability.
• the presence of Si favours good balance of strength and elongation since Si
• the Si content is more than 0.1 wt. % and less than 0.3 wt. %.
• Manganese Mn is used in the range of 2.3 wt. % to 3.0 wt. %.
• Mn alloying enhances strength by solid solution strengthening, and enhances hardness by increasing austenite hardenability.
• Mn stabilizes the remaining austenite at later stages of the bainitic transformation thereby inhibiting transformation of the carbon-rich constituent into different microstructures in different parts of the hot rolled coil.
• Mn is beneficial to the preferred carbon-rich second phase being martensite or autotempered martensite. Transformation to degenerate pearlite is inhibited in the presence of Mn.
• Mn content needs to be set to not more than 3.0 wt. % to prevent excessive strengthening and hardenability.
• Chromium Cr is used in the range of 0.2 wt. % to 0.5 wt. %.
• At least 0.2 wt. % of chromium is required for sufficient hardenability at the HDG line.
• Cr also functions as an austenite stabilizer in the same manner as Mn. Cr alloying promotes the formation of the preferred carbon-rich second phase being martensite or autotempered martensite, while inhibiting the transformation to degenerate pearl ite.
• Titanium Ti is used in the range of 0.02 wt. % to 0.05 wt. %.
• Ti is added to bind free N that is harmful to toughness by forming stable TiN, which can efficiently prevent austenite grain growth in the reheating stage at high temperatures. TiN formation also suppresses BN precipitation, thereby leaving B free to make its contribution to hardenability. Thus, Ti is usually required to ensure the effectiveness of B in the case of B alloying.
• Boron B is used in the range of 0.002 wt. % to 0.01 wt. %.
• B is a transformation-retarding element that suppresses formation of diffusional transformation products such as polygonal ferrite, thereby promoting formation of low carbon bainitic structures.
• diffusional transformation products such as polygonal ferrite
• B prevents formation of soft ferritic-pearlitic microstructures in the slow-cooling core of the hot rolled coil.
• Effective B alloying would require the presence of Ti to prevent formation of BN. Toughness is rapidly deteriorated when B content exceeds 0.01 wt. %.
• Molybdenum Mo is used in a content of 0.2 wt. % or less.
• Mo is a transformation-retarding element that has the effects of promoting the formation of low carbon bainitic structure.
• the presence of Mo also enhances strength and hardness by increasing austenite hardenability.
• Mo is optionally required to ensure the effectiveness of B.
• Mo is not an economically acceptable alloying element. Mo content should not exceed 0.2 wt. %. Excessive amount of Mo may impose limitations to the achievable dimensional range due to excessive strengthening of the bainitic microstructure beyond the desired level. If Mo is used in an amount above 0.2 wt. % toughness may be deteriorated thereby increasing risk of brittleness, and also the effect of B may be reduced.
• Vanadium V is used in a content of 0.2 wt. % or less.
• V is a strong carbide and nitride former, but V(C,N) can also form and its solubility in austenite is high.
• V alloying has potential for dispersion and precipitation strengthening, because large quantities of V are dissolved and available for precipitation in ferrite.
• Cu promotes low carbon bainitic structures, causes solid solution strengthening and contributes to precipitation strengthening.
• the upper limit of Cu content is set to 0.2 wt. % to prevent excessive strengthening. When added in excessive amount Cu also deteriorates toughness.
• Nickel Ni is used in a content of 0.5 wt. % or less.
• Ni is an alloying element that improves austenite hardenability and increases strength without any loss of toughness.
• Ni contents of above 0.5 wt. % would increase alloying costs too much without significant technical improvement. Excess amount of Ni may produce high viscosity iron oxide scales, which deteriorate surface quality of the steel product.
• Calcium Ca is used in a content of 0.005 wt. % or less.
• Ca is not used as alloying element due to its low solubility in steel and high vapor pressure.
• the optional Ca addition during a steelmaking process is for refining, deoxidation, desulphurization, and control of shape, size and distribution of oxide and sulphide inclusions.
• Ca is used in the range of 0.001 wt. % to 0.005 wt. %.
• Aluminum Al is used in a content of 0.1 wt. % or less. Al is effective as a deoxidizing or killing agent that can remove oxygen from the melt during a steelmaking process. Al also removes N by forming stable AIN particles and provides grain refinement, which has the effects of promoting high toughness. Also, Al stabilizes ferrite and residual austenite.
• Al may increase non-metallic inclusions thereby deteriorating cleanliness if used in excessive amount above 0.1 wt. %.
• Al is used in the range of 0.01 wt. % to 0.1 wt. %.
• Niobium Nb is considered as a major grain refining element. Nb contributes to the strengthening and toughening of steels. Specifically, Nb is not added in the composition according to the present invention in order to allow the steel product to have a wide dimensional range since excessive strengthening may decrease hot and cold reliability of the hot-rolled sheet.
• the steel product is alloyed with essential alloying elements such as C, Si, Mn, Cr, Ti, B, Ca, and Al.
• Other elements such as Mo, V, Cu, and Ni may be present as residual contents that are not purposefully added.
• residual contents are controlled quantities of alloying elements, which are not considered to be impurities.
• a residual content as normally controlled by an industrial process does not have an essential effect upon the alloy.
• Unavoidable impurities can be phosphor P, sulfur S, nitrogen N. Their contents are preferably limited as follows:
• the method for producing the intermediate hot rolled product comprises the steps of: - providing a steel slab with
• composition (I) consisting of, in terms weight percentages (wt. %):
• Si 0.1 - 0.3 preferably more than 0.1 and less than 0.3
• composition (II) consisting of, in terms weight percentages (wt. %):
• composition (III) consisting of, in terms weight percentages (wt. %): C 0.1 -0.13, e.g.0.12
• the microstructure of the intermediate hot-rolled steel sheet consists of less than 5% of pearlite and polygonal ferrite with a grain size above 10 pm, and the rest majority being bainite and/or fine-grained ferrite, and martensite, at all positions along the full length of the steel sheet.
• Fine-grained ferrite refers here to ferritic transformation products with a grain size in the order of typically 1 pm, which may nucleate above the bainitic temperature range, but are indistinguishable from low carbide containing bainitic ferrite by visual identification from a secondary electron microscope image.
• the martensite having variable carbon contents may be tempered and/or auto- tempered to variable degrees.
• (slow) cooling at an average rate of 10 °C/s or below to a
• final cooling at an average rate in the range of typically 1 °C/s to 20 °C/s.
• the step of cooling from the annealing temperature in the range of 780 °C to 860 °C to the holding temperature in the range of 440 °C to 525 °C is continuous cooling at an average rate in the range of typically 5 °C/s to 50 °C/s.
• the step of cooling from the annealing temperature in the range of 780 °C to 860 °C to the holding temperature in the range of 440 °C to 525 °C is a two-step cooling comprising
• a first slow cooling at an average rate of 10 °C/s or below to a temperature in the range of 720 °C to 780 °C;
• the first cooling step is not crucial. If omitted, the yield strength will increase slightly due to smaller amount of fine-grained ferrite in the microstructure.
• the optional step of hot-dip coating is preferably hot-dip galvanizing or
• the method further comprises steps of temper rolling and/or levelling which are not crucial, but may improve yield strength of the final product.
• the method further comprises a step of extra batch annealing at a temperature in the range of 200 to 400 °C, e.g. 270 °C.
• the step of extra batch annealing results in bake hardening and tempering of the microstructure, which increases the yield strength of the steel while improving local formability parameters such as the hole expansion ratio.
• the microstructure of the final steel sheet comprises a matrix consisting of, in terms of volume percentages (vol. %):
• the microstructure has an average grain size of less than 10 pm.
• the bainite and/or fine-grained ferrite have an average grain size of less than 5 pm, preferably in the range of 1 pm to 5 pm.
• the bainite may comprise or consist of low carbide containing bainitic ferrite which is indistinguishable from fine-grained ferrite with a grain size of less than 2 pm.
• the martensite having variable carbon contents may be tempered and/or auto-tempered to variable degrees.
• the final steel sheet has a thickness in the range of 0.8 - 2.2 mm, and a thickness accuracy better than 1/5 of EN 10143:2006 normal thickness tolerances at all positions along the full length of the steel sheet, excluding threading and tail-out sections in batch type cold rolling mills.
• Tensile strength is determined mainly by the chemical composition of steel.
• the steel composition (I) is used for manufacturing a steel product with an ultimate tensile strength (Rm) of at least 950 MPa.
• the steel composition (II) is used for manufacturing a steel product with an ultimate tensile strength (Rm) of at least 980 MPa.
• the steel composition (III) is used for manufacturing a steel product with an ultimate tensile strength (Rm) of at least 1 180 MPa. It is preferable that the steel sheet further has at least one of the following mechanical properties:
• the high strength steel sheet with an ultimate tensile strength (Rm) of at least 980 MPa has a minimum bending radius (Ri) of 2.5 t or less.
• the high strength steel sheet with an ultimate tensile strength (Rm) of at least 980 MPa has a total elongation (A 8 o) of at least 8 %.
• the high strength steel sheet with an ultimate tensile strength (Rm) of at least 1 180 MPa has a total elongation (A 80 ) of at least 5 %.
• EXAMPLE 1 A steel slab having the composition grade A was prepared by conventional steel metallurgy and continuous casting. The slab was then hot rolled with a finish rolling temperature of 920 °C to produce a sheet of 2.5 mm thickness, then water cooled to a coiling temperature of 640 °C, and the coil was thereafter allowed to cool freely in coil storage. The hot-rolled steel sheet was then pickled and cold rolled with a thickness reduction of 40 % to a final thickness of 1.5 mm. Finally, the cold-rolled steel sheet was processed in a continuous galvanizing line, including steps of
• the first cooling step is not crucial. If omitted, the yield strength will increase slightly due to smaller amount of fine-grained ferrite in the microstructure.
• temper rolling and levelling are not crucial, but may improve yield strength of the final product.
• Microstructures of the inventive steel Ex. 1 were investigated by scanning electron microscopy after preparation of cross-section samples by grinding, polishing, and etching with nital reagent. The constituent phases were visually identified and their relative fractions were determined based on surface proportions.
• Figure 1 (a) shows that the microstructure in the hot rolled condition comprises mainly bainite (bainitic ferrite - dark areas), and martensite with variable carbon contents or degrees of (auto)tempering (light areas).
• Figure 1 (b) shows that the microstructure in the cold-rolled and hot-dip galvanized condition comprises mainly bainite (bainitic ferrite - dark areas) and martensite (light areas).
• the flatness of cold rolled steel strips was measured by BFI flatness measurement roll with 54 piezoelectric encoders distributed across the barrel and expressed in I- units (IU) as 2D color map.
• the determination of the flatness distribution is realized by the measurement of the local deflection forces of the strip with the help of a measurement roll. This process is based on the fact that a strip which is under longitudinal tension, when deflected, exerts a radial force on the deflection roll.
• the center line thickness of cold rolled steel strips was measured by a non-contact X-ray thickness gauge.
• Figure 3 shows that the cold rolled steel strip 2.5 1.51 x 1241 mm (a) according to the invention Ex. 1 has less thickness deviation along the body length than the comparative steel strip 2.8 1.51 x 1221 mm (b). High variations in the head and tail are due to threading and tail-out sections of the batch type cold rolling mill. Extra variation in the body of comparative strip (b) is due to the cold gauge hashing phenomenon, which is absent in the cold rolled steel strip (a) according to the invention Ex. 1. The dashed lines indicate 1/5 of EN 10143:2006 normal thickness tolerances.
• the mechanical properties were determined by tensile testing of longitudinal test pieces according to EN ISO 6892-1 :2009.
• the minimum bending radius was determined by performing a 90° bending test with bend parallel to the longitudinal (rolling) direction, and measuring the minimum radii on approved bends.
• the hole expansion test was carried out according to ISO 16630:2017.
• the tested steel sheet has an ultimate tensile strength of 1032 MPa (Table 3).
• a steel slab having the composition grade B was prepared by conventional steel metallurgy and continuous casting.
• the slab was then hot rolled with a finish rolling temperature of 920 °C to produce a sheet of 2.5 mm thickness, then water cooled to a coiling temperature of 670 °C, and the coil was thereafter allowed to cool freely in coil storage.
• the hot-rolled steel sheet was then pickled and cold rolled with a thickness reduction of 40 % to a final thickness of 1.5 mm.
• the cold-rolled steel sheet was processed in a continuous galvanizing line, including steps of
• the first cooling step is not crucial. If omitted, the yield strength will increase slightly due to smaller amount of fine-grained ferrite in the microstructure.
• temper rolling and/or levelling are not crucial, but may improve yield strength of the final product. Mechanical properties
• the minimum bending radius was determined by performing a 90° bending test with bend parallel to the longitudinal (rolling) direction, and measuring the minimum radii on approved bends.
• the hole expansion test was carried out according to ISO 16630:2017.
• the tested steel sheet has an ultimate tensile strength of 1230 MPa (Table 3).
• a steel slab having the composition grade B was prepared by conventional steel metallurgy and continuous casting.
• the slab was then hot rolled with a finish rolling temperature of 920 °C to produce a sheet of 2.5 mm thickness, then water cooled to a coiling temperature of 670 °C, and the coil was thereafter allowed to cool freely in coil storage.
• the hot-rolled steel sheet was then pickled and cold rolled with a thickness reduction of 40 % to a final thickness of 1.5 mm.
• the cold-rolled steel sheet was processed in a continuous galvanizing line, including steps of
• the first cooling step is not crucial. If omitted, the yield strength will increase slightly due to smaller amount of fine-grained ferrite in the microstructure.
• the steps of temper rolling and/or levelling are not crucial, but may improve yield strength of the final product.
• the minimum bending radius was determined by performing a 90° bending test with bend parallel to the longitudinal (rolling) direction, and measuring the minimum radii on approved bends.
• the hole expansion test was carried out according to ISO 16630:2017.
• the tested steel sheet has an ultimate tensile strength of 1203 MPa (Table 3).
• the hot-dip galvanized steel sheet of Example 3 was further batch annealed in a laboratory furnace at 270 °C with a holding time of 12 hours and tested after cooling to room temperature.
• the minimum bending radius was determined by performing a 90° bending test with bend parallel to the longitudinal (rolling) direction, and measuring the minimum radii on approved bends.
• the hole expansion test was carried out according to ISO 16630:2017.
• the tested steel sheet has an ultimate tensile strength of 1190 MPa (Table 3).
• Table 1 Chemical composition (wt. %) of the tested steel sheets

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