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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • 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
  • 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
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
• • 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
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • 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
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
• • 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
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips 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
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
• • 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
• • 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
  • C22C38/02—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
  • C22C38/12—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
  • C22C38/18—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
  • 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
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • 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
  • C23C2/04—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
  • C23C2/12—Aluminium 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
• • 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
  • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
  • C23C2/36—Elongated material
  • C23C2/40—Plates; Strips
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/22—Electroplating: Baths therefor from solutions of zinc
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/42—Electroplating: Baths therefor from solutions of light metals
  • C25D3/44—Aluminium

Definitions

• This invention relates to a cold-rolled and annealed bake-hardenable steel hot-dip galvanised strip or sheet and a method for producing the strip or sheet.
• Various strengthening means are at the steel industries' disposal such as, but not limited to, increasing the temper rolling reduction after annealing, changing the chemical composition, changing processing conditions such as the cold-rolling reductions imposed on the strip or sheet, or the annealing conditions of the cold-rolled steel strip or sheet.
• the various means affect each other and may improve some property and adversely affect another. For instance, increasing the temper rolling reduction improves the yield strength, but reduces the uniform elongation values.
• JP06-322441 discloses a method for producing a high strength cold rolled steel plate having formability and baking hardenability based on a steel composition having 0.003 to 0.010% C.
• One or more of the objects is reached with a cold-rolled and annealed bake- hardenable steel hot-dip galvanised strip or sheet having a chemical composition comprising, in wt.%:
• Nb 0.004 to 0.028
• V 0.005 or less
• Ni 0.045 or less
• the steel is preferably produced on the basis of a steel melt produced in a Basic Oxygen Steelmaking (BOS) process, preferably also using vacuum degassing technology. This process is preferable over the other commonly used steelmaking process, the Electric Arc Furnace (EAF) process, because the BOS-process is able to produce steels with a low impurity level.
• BOS Basic Oxygen Steelmaking
• EAF Electric Arc Furnace
• the EAF-process is based on scrap melting whereas the BOS-process starts from virgin pig iron to which only small amounts and carefully selected scrap is added as a coolant during the refining process of the pig iron into a steel melt ready to be cast into a thick slab (t: 150-350 mm) or a thin slab (t: 50-150 mm) in a continuous casting process, or into a strip (t ⁇ 20 mm) in a strip casting process.
• the cast slab or strip is the input for the hot rolling process, of which the resulting hot-rolled strip is the input for the subsequent cold-rolling process according to the invention.
• Mn (wt.%) + 10 * P (wt.%) + Si (wt.%)) is at most 1.825 and/or at least 1.525 wt.%.
• the inventors have found that using a careful balance between the elements Mn, Si and P, in combination with a carefully balanced process, it is possible to produce a Ti stabilized Ultra Low Carbon steel with a minimum yield strength of 270 MPa //RD, a minimum tensile strength of 360 MPa //RD, a minimum elongation A80 of at least 28% // RD a minimum bake hardening response BH2 (according to EN10325:2002) of at least 30 MPa // RD.
• the width of the rolled coil can be up to 2000 mm.
• Rp, Rm, A80, nO, rO as claimed are determined according to VDA239-100:2016, and the BH2 according to EN10325:2002.
• the notation // RD means that the property is measured on a tensile specimen with its longitudinal direction being parallel to the rolling direction. The 0 behind n and r denote the same (0 being the angle to the rolling direction).
• A80 is the total elongation, measured over a gauge length of 80 mm.
• the BH2 was determined by measuring the increase in Rp after heating the cold-rolled, annealed and hot-dip galvanised material after a paint-baking cycle of 20 minutes at 170 °C, according to EN10325:2006. The tensile tests were performed according to EN10002-1:2001.
• A80 // RD is at least 30% and preferably at least 32%.
• nO is at least 0.15 and preferably at least 0.16.
• the rO- value is at least 1.20 and preferably at least 1.30. In case of rO and nO both parameters higher values indicate better formability.
• BH2 // RD is at least 40 MPa, preferably at least 45 MPa.
• Rp // RD is at least 280 MPa, more preferably at least 290 MPa.
• titanium (Ti) is added mainly to make sure all N has precipitated as TiN. Free N is detrimental for the mechanical properties. Any excess Ti might precipitate as TiC or T14C2S2.
• niobium (Nb) is added in such a way that most of the C precipitates as NbC. However, a small amount of C in solid solution is necessary to get the required BH response.
• aluminium (Al) is used as a deoxidizing agent.
• Al is also added as an alloying element to promote the formation of aluminium nitride in order to improve the deep- drawing.
• Al can be found in steel as aluminium oxide (remnants from the deoxidation) metallic Al and AIN.
• Al and AIN can both be dissolved in acid and is therefore jointly referred to as AI_sol.
• the P-content is at least 0.054 and preferably at least 0.065 wt.%.
• the P-content is at most 0.099 and more preferably at most 0.085 wt.%.
• a preferred range is therefore 0.065-0.085 wt.%.
• the Mn-content is at least 0.610, and preferably at least 0.660 wt.%.
• the Mn-content is below 0.880 and more preferably below 0.800 wt.%.
• the Si-content is at least 0.148. In an embodiment the Si-content is at least 0.165 wt.%. In the steel according to the invention the Si-content is at most 0.248. In an embodiment the Si-content is at most 0.225 wt.%.
• the S-content is at least 0.010, more preferably at most 0.005 wt.%.
• the maximum waviness Wsa(l-5) after deformation of 5% is 0.37 pm. This value is obtained in a reproducible and robust way.
• the value of the Wsa(l-5) depends to a significant extent on the roughness of the work roll in the last cold rolling stand of the cold-rolling mill.
• the steel strip or sheet has a roughness value (Ra) of between 0.80 and 1.50 pm and a waviness value (Wsa(l-5)) after 5% deformation of at most 0.37 pm.
• the waviness value Wsa(l-5) after 5% deformation is at most 0.35 pm, and more preferably at most 0.34 pm and/or the Ra is at most 1.45 pm.
• the steel strip or sheet according to the invention is provided with a metallic coating layer after cold-rolling and annealing, such as zinc layer which can contain a small amount of Al to allow the formation of a very thin ternary alloy layer composed of approximately 45% Al, 35% Fe and 20-35% Zn (Fe Al -xZnx) to ensure proper adhesion of the zinc layer to the substrate. For this reason between 0.10 to 1.0 wt.% Al may be added to the zinc bath.
• a metallic coating layer after cold-rolling and annealing such as zinc layer which can contain a small amount of Al to allow the formation of a very thin ternary alloy layer composed of approximately 45% Al, 35% Fe and 20-35% Zn (Fe Al -xZnx) to ensure proper adhesion of the zinc layer to the substrate. For this reason between 0.10 to 1.0 wt.% Al may be added to the zinc bath.
• the metallic coating layer may be a zinc alloy coating layer comprising 0.3-4.0 wt.% Mg and 0.3-6.0 wt.% Al; optionally at most 0.2 wt.% of one or more additional elements, unavoidable impurities; the remainder being zinc.
• the alloying element contents in the zinc-alloy coating layer shall be 1.0 - 2.0 wt.% Magnesium and 1.0 -3.0 wt.% Aluminium, optionally at most 0.2 wt.% of one or more additional elements, unavoidable impurities and the remainder being zinc.
• the zinc alloy coating comprises at most 1.0 to 2.0 wt.% Mg and between 1.5 and 2.5 wt.% Al, optionally at most 0.2 wt.% of one or more additional elements, unavoidable impurities and the remainder being zinc.
• the invention is also embodied in the method of providing the steel strip or sheet according to the invention wherein the strip is continuously annealed in accordance with the invention and subsequently electro-galvanised.
• the steel strip or sheet is provided with a metallic coating layer such as a (commercially pure) aluminium layer or an aluminium alloy layer.
• a metallic coating layer such as a (commercially pure) aluminium layer or an aluminium alloy layer.
• Commercially pure aluminium contains at least 99 wt.% Al (AA lXXX-series).
• a typical metal bath for a hot-dip coating with an aluminium alloy layer comprises of aluminium alloyed with silicon e.g. aluminium alloyed with 8 to 11 wt.% of silicon and at most 4 wt.% of iron, optionally at most 0.2 wt.% of one or more additional elements such as calcium, unavoidable impurities, the remainder being aluminium.
• Silicon is present in order to prevent the formation of a thick iron-intermetallic layer which reduces adherence and formability.
• Iron is preferably present in amounts between 1 and 4 wt.%, more preferably at least 2 wt.%.
• the thickness of the cold-rolled strip or sheet is not particularly limiting, it is still preferable that the thickness of the steel strip or sheet is between 0.50 and 1.20 mm.
• the grain size of the ferrite grains in the cold-rolled steel strip and sheet according to the invention is between 9 and 13 ASTM, and preferably between 9.5 and 12 ASTM, more preferably at least 10 ASTM.
• one, more or all of Cr, V, Cu, Ni and/or Mo are present only as inevitable impurities in the steel according to the invention, and more preferably one, more or all of Cr, V, Cu, Ni and/or Mo are completely absent.
• the invention is also embodied in the method for producing a cold-rolled and annealed bake-hardenable steel hot-dip galvanised strip or sheet having a chemical composition in wt.% comprising:
• Nb 0.004 to 0.028
• the temper-rolled steel strip such as coiling, cutting into sheets, forming, etc.; wherein the yield strength (Rp) is between 270 and 330 MPa, the tensile strength (Rm) is between 360 and 460, the total strain (A80) is at least 28%, the n0-value is at least 0.13, the rO-value is at least 1.10, and a bake-hardening index (BH2) of at least 30 MPa, all measured // RD.
• the yield strength (Rp) is between 270 and 330 MPa
• the tensile strength (Rm) is between 360 and 460
• the total strain (A80) is at least 28%
• the n0-value is at least 0.13
• the rO-value is at least 1.10
• BH2 bake-hardening index
• process conditions of the method according to the invention according to the invention are selected as follows: ⁇ heating or reheating the slab to a temperature of at least 1100 °C or even at least 1150 °C;
• the finish rolling temperature is between 900 and 945 °C; • the coiling temperature of the middle of the strip is not lower than 710 °C and/or not higher than 750 °C;
• the cold-rolling reduction is at least 68%, preferably 70% and more preferably at least 75% and/or at most 90%;
• temper rolling the hot-dip galvanised steel strip between 0.2 and 2.5%, preferably between 1.25 and 2.10%.
• process conditions can be selected independent of each other, so that the process according to the invention embodies one, more or all of these process conditions.
• the coiling temperature of the head end and the tail end may be higher than the coiling temperature of the middle of the strip to compensate for the faster cooling rate of the head and tail after coiling. This compensation is important to promote certain metallurgical reactions such as precipitation of AIN or NbC.
• a so-called U-type cooling may be employed wherein the head and tail are coiled at a higher temperature (see figure 3).
• the temperature difference between the head/tail and middle section of such a cooling that is required to compensate for the tail/head effects can be easily determined by the skilled person, but as a general indication the coiling temperature of the head and tail may be about 30 °C higher than the middle of the strip.
• the head and tail ends are relatively short as compared to the middle section.
• the optimal length of the head and tail ends in comparison to the middle section, the length of the transfer between head/tail and the middle and the temperature differential over the length of the strip can be easily determined with a thermal model that calculates the cooling of every point in a coil and coupled with a metallurgical model for precipitation of e.g. AIN the optimum temperature path can be derived (Metallurgy for Hoogovens' new CA line for tin plate, Th.M. Hoogendoorn, A.J. van den Hoogen, 1991 TMS Fall Meeting, Cincinnati (OH), October 22-24, 1991).
• Table 1 chemical composition and main process settings of test materials.
• Alloys A to J are according the invention. Alloys 0 to T are outside the claims of this invention and are shown to demonstrate the effect of the chemistry and processing.
• the steels have all been produced in a Basic Oxygen Steelmaking-plant and continuously cast as thick slabs ( ⁇ 225 mm). It should be noted that the steels could also be produced by thin slab casting and direct hot rolling, in which case the thickness of the cast slab is usually between 50 and 100 mm. This does not affect the properties of the final inventive product.
• a non-limiting schematic overview of the production is depicted in figure 1.
• the steel slabs were reheated in the slab reheating furnace (SRF) to a temperature of at least 1050 °C and subsequently hot-rolled to thicknesses of 2.8, 3.1 and 3.5 mm.
• the finish hot rolling temperature (FRT) was above Ac3, so the hot-rolling was performed in the austenitic state.
• a target FRT of between about 910 °C to 930 °C was used.
• the hot-rolled coils were subsequently pickled and cold-rolled to a thickness of 0.70 mm, resulting in a cold rolling reduction of 75, 77 and 80% (depending on the thickness of the HRC).
• the cold-rolled coils were subsequently continuously annealed to get full recrystallisation, and 1.4 to 2.0% temper rolled. In between the annealing and the temper rolling the coils are hot-dip galvanised, to provide the strip with a metal coating layer to produce a cold- rolled metal coated coil (CRCC).
• microstructures of the cold-rolled metal coated coils show a fully recrystallised microstructure with a grainsize between 10 and 10.5 ASTM.
• the mechanical properties are given in table 2.
• Table 2 Mechanical properties of test materials. An analysis of the results shows that the strength does indeed depend on the levels of Mn, Si and P. Coils having a composition outside the claimed ranges do not guarantee the required minimum strength of 270 MPa.
• the cold-rolled metal coated coils were produced as standard Full Finish (FF).
• the roughness of the rolling cylinders in the last stand of the cold mill (#5) was about 5,8 pm EDT.
• Roughness and waviness were measured at the middle of the coil (over the length) according to SEP1941 (May 2012) after deformation as described in SEP1942 (draft, May 2017). The results are summarized in table 3 which also includes some relevant process settings.
• Table 3 Coiling temperature, roughness and waviness values.
• FIG 1 Schematic production process.
• FIG 3 U-type run-out table cooling.
• the production process of the steel according to the invention is schematically shown. The process starts with the casting and ends with the cold-rolled galvanised coil (CRGC). The intermediary steps are schematically shown.
• CRGC cold-rolled galvanised coil
• Figure 2 shows the relation between one of the mechanical properties (Rp in MPa) as a function of ⁇ (Mn+Si+ 10 ⁇ P) (in wt.%) which shows the improvement that the steel and the process according to the invention offers.
• Figure 3 shows a schematic drawing of the U-type cooling, and a specification of the head section, middle section and tail section of a strip.
• the head section is the section that is hot-rolled first and therefore end up as the inner wraps in the "eye" of the coil and the tail section is the section that is hot-rolled last and therefore end up as the outer wraps of the coil.
• the length of the middle section depends on the thickness of the strip, the width of the strip and the weight of the coil.

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• 2022
  • 2022-04-21 WO PCT/EP2022/060599 patent/WO2022223719A1/en active Application Filing
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