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
• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
  • C21D1/22—Martempering
• • 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
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—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 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
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
• • 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
  • C22C38/001—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
  • C22C38/002—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
  • C22C38/06—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
• • 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
  • 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
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/34—Pretreatment of metallic surfaces to be electroplated
  • C25D5/36—Pretreatment of metallic surfaces to be electroplated of iron or steel
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D7/00—Electroplating characterised by the article coated
  • C25D7/06—Wires; Strips; Foils
  • C25D7/0614—Strips or foils
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
  • 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
• • 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
• • 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
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • 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
  • C21D6/008—Heat treatment of ferrous alloys containing Si
• • 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
• • 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

Definitions

• the invention relates to a high-strength flat steel product suitable for bake hardening treatment and to a method for producing such a flat steel product.
• BH flat steel products suitable for bake hardening (BH) treatment are also referred to as bake hardening flat steel products (BH flat steel products) and are widely used in automotive applications such as body parts.
• this refers to steel strips, steel sheets or blanks produced therefrom, such as blanks.
• Bra steel flat products have a lower level of strength before BH treatment than after BH treatment. This circumstance is used to transform the flat steel products to be formed before the BH treatment and thus at lower yield strengths and with better forming capacity.
• the increase of the strength level is made by the BH treatment in which the material is subjected to a heat treatment.
• the BH treatment is typically carried out for 3 to 40 minutes within a temperature range of 120 to 250 ° C.
• the BH treatment stimulates the diffusion of atoms of interstitially dissolved elements, allowing them to attach to dislocations. The dislocations are thereby hindered in their movement, which leads to an increase in the yield strength.
• This effect of yield strength increase is also called a bake hardening effect (BH effect), and the difference in yield strengths before and after the BH treatment is also called a bake hardening value (BH value).
• BH effect bake hardening effect
• BH value bake hardening value
• yield strength refers to the characteristic value referred to as the upper yield strength ReH meant.
• the yield strength increase or the BH value are mentioned, then for steel flat products which have no pronounced yield strength before the BH treatment but a yield strength, the difference between the yield strength Rp0.2 before the BH treatment and the yield strength ReH understood after BH treatment.
• a high BH value has a positive effect on the dent resistance of components made of BH flat steel products. As a result, it is possible to reduce the component thickness by using steel flat products having a high BH value while maintaining the rigidity of the components.
• the BH effect has hitherto been used on soft steels, which often have a predominantly ferritic matrix, only low martensite fractions and tensile strengths below 700 MPa.
• US 2013/0240094 A1 discloses cold rolled bake hardening sheets for automotive applications.
• the sheets are said to be made of a steel which, besides iron and unavoidable impurities, has 0.0010 - 0.0040 mass% C, 0.005 - 0.05 mass% Si, 0.1 - 0.8 mass% Mn, 0, 01-0.07 mass% P, 0.001-0.01 mass% S, 0.01-0.08 mass% Al, 0.0010-0.0050 mass% N, 0.002-0.02 mass % Nb and 0.005-0.050 mass% Mo, the value of the quotient of the proportions of Mn and P [Mn%] / [P%] being between 1.6 and 45, and the amount of solid solution present Carbon obtained from [C%] - (12/93) x [Nb%] should be between 0.0005 to 0.0025 mass%.
• the cold rolled sheets suitable for bake hardening should satisfy the equation X (222) / ⁇ X (119) + X (200) ⁇ > 3.0.
• X (222), X (110) and X (200) are the integrated intensity of X-ray diffractometry of ⁇ 222 ⁇ plane, ⁇ 110 ⁇ plane and ⁇ 200 ⁇ plane, which are parallel to a plane starting from the sheet surface at k of the sheet thickness.
• the sheets should have a good deep drawability and tensile strengths of 300 to 450 MPa.
• high-strength steel flat products are used to realize low component thickness with good dent resistance.
• High-strength steels are characterized by a high proportion of martensite in the microstructure.
• Martensite is a carbon-rich microstructure constituent from which carbon can diffuse into other microstructural constituents upon thermal activation.
• high martensite fractions are associated with a poor ductility.
• the sheets should be 0.05-0.30 mass% C, 0.5-3.0 mass% Si, 0.2-3.0 mass% Mn, up to 0.10 mass% P, bis to 0.010 mass% S, up to 0.010 mass% N and 0.001-0.0 mass% AI, balance iron and unavoidable impurities.
• the microstructure should contain 50-85 area% martensite, less than 5 area% ferrite and balance bainite and have a dislocation density of at least 5.0 x 10 15 nr 2 and at least 0.08 mass% dissolved carbon.
• the sheets should be suitable for bake hardening and have good bending properties and tensile strengths of 1180 MPa or more.
• the sheets are produced by means of conventional continuous casting, hot rolling and cold rolling.
• the cold-rolled sheets are heated to annealing temperatures of Ac3 + 50 ° C up to 930 ° C, held for 30 to 1200 s at this annealing temperature, then at an average rate of 15 ° C / s or more to a cooling stop temperature between 450 ° C cooled to 550 ° C, then within a maximum of 30 s from reaching the cooling stop temperature for 10 to 60 s at 480 to 525 ° C immersed in a molten bath and then with an average cooling rate of 15 ° C / s or more to 200 ° C are cooled.
• the object of the invention was to specify a very high-strength flat steel product with optimized properties, in particular very good bake hardening properties and very good forming properties both before and after a BH treatment.
• a method for producing such a flat steel product should be specified.
• the object has been achieved in that at least the method steps specified in claim 10 are completed in the production of a flat steel product according to the invention.
• a flat steel product according to the invention consists of a steel which is made of (in parts by weight)
• tempered martensite is at least 75% by volume of tempered martensite.
• a flat steel product according to the invention is characterized in that, prior to BH treatment, it has a yield strength Rp0.2 of more than 700 MPa or a yield strength ReH of more than 700 MPa, a tensile strength Rm of 950-1500 MPa and an elongation A80 of 7 - 25%. and has a high bake hardening potential (BH potential).
• the BH potential is evidenced by the fact that the steel flat product after BH treatment has a yield strength increase of at least 80 MPa and an elongation A80_BH which is at least half the elongation A80 before the BH treatment.
• the carbon content of the steel of a flat steel product according to the invention is 0, 1 - 0.5 wt .-%.
• carbon contributes to the formation and stabilization of austenite.
• C contents of at least 0.1% by weight, preferably at least 0.12% by weight contribute to the stabilization of the austenitic phase, thereby making it possible is, at the time of appropriate flat steel product to ensure a Restaustenitanteil of at least 5% by volume.
• the stabilization of the retained austenite can be carried out particularly reliably if the C content is at least 0.14% by weight.
• the C content has a strong influence on the strength of martensite.
• the C content should be at least 0.1% by weight.
• a minimum content of 0.1% by weight is required in order to provide sufficient C atoms for diffusion to the dislocations present in the material during subsequent BH treatment and thus to ensure a pronounced BH effect.
• Particularly high BH values are obtained when the C content is at least 0.14% by weight.
• the martensite start temperature Ms is also shifted to lower temperatures. A C content above 0.5 wt% could therefore result in insufficient martensite being formed during quenching.
• the processability, in particular the weldability is impaired at higher C contents, for which reason the C content should be at most 0.5% by weight, preferably at most 0.4% by weight.
• Manganese is important as an alloying element for the hardenability of the steel as well as for preventing the formation of the structural constituent perlite during the first quenching.
• the Mn content of the steel of a flat steel product according to the invention is therefore at least 1.0% by weight, preferably at least 1.9% by weight, in order to provide a perlite-free structure for the further process steps after the first quenching.
• Segregation is chemical inhomogeneity of the composition formed during the solidification process in the form of macroscopic or microscopic segregation.
• the Mn content of the steel of a flat steel product according to the invention is limited to at most 3.0% by weight, preferably to at most 2.7% by weight.
• the Si content of the steel of a flat steel product according to the invention is limited to 0.5-2.0% by weight.
• Si as alloying element promotes the suppression of cementite formation.
• Cementite is an iron carbide. Due to the formation of cementite, carbon is set in the form of iron carbide and is no longer available in an atomic form for dissolution in the iron lattice. However, atomic carbon, which is interstitially dissolved in the iron lattice, contributes significantly to the stabilization of retained austenite and to the improvement of the BH effect.
• Remaining austenite helps to improve formability, especially elongation, both before and after BH treatment.
• a similar effect with regard to the stabilization of retained austenite can also be achieved by alloying aluminum.
• the steel contains at least 0.2% by weight of Al, the Si content which is at least required in order to obtain a flat steel product according to the invention can be reduced to 0.5% by weight.
• the Si content should preferably be at least 0.9 wt%.
• the steel should not contain more than 2.0 wt%, preferably not more than 1.6 wt%.
• Aluminum is present in the steel of a flat steel product of the invention at levels of 0.01-1.5% by weight.
• AI is added for deoxidation and grain refining. Grain refining is accomplished by forming AIN clusters and AIN precipitates, which inhibit grain growth during austenitizing annealing, also referred to as austenitizing.
• AIN clusters are generally understood as meaning collections of aluminum and nitrogen atoms, which, however, unlike AIN precipitates, have no sharp phase boundary with the matrix.
• the Al content should be at least 0.01% by weight.
• particularly fine austenite grains can be adjusted by the addition of Al and N to lattice defects and their subsequent clustering or precipitation.
• the finer austenite grain size causes fine martensite with a small lancet length to be formed during the first quench.
• increased AI Contents of at least 0.02 wt .-% particularly advantageous.
• Also advantageous for the formation of AIN clusters and AIN precipitates is a high number of lattice defects that are available during the heating to Austenitmaschinestemperatur (THZ). These lattice defects can be introduced into the material prior to austenitizing, for example in the form of dislocations.
• THZ Austenitmaschinestemperatur
• AI is not as effective as Si in suppressing cementite formation.
• Si adversely affects the scaling and coatability and thus the surface quality of the flat steel products
• Al can be used in the choice of the alloy composition for the substitution of Si.
• Al contents of at least 0.1% by weight have proven to be particularly effective for the steel composition according to the invention. At lower Al contents, the effect of Al on cementite suppression is not significant.
• aluminum contributes to increasing the carbon activity in martensite. This applies both to the after quenching, which takes place after austenitizing, and to the martensite formed after the second quenching, which takes place after the partitioning annealing.
• the increase in carbon activity also has a positive effect on the BH effect.
• a high carbon activity also increases the driving force for the addition of carbon atoms to dislocations, which leads to an increase in the BH value.
• Al contents of at least 0.02 wt .-% have proven to be particularly advantageous. Since aluminum requires the complete austenitizing If the annealing temperature is elevated and complete austenitizing is possible with Al contents above 1.5% by weight, the Al content of the steel of the flat steel product according to the invention is limited to at most 1.5% by weight. If a low austenitizing temperature is to be set to improve the energy efficiency, Al contents of at most 0.2% by weight have proven to be expedient.
• the sum of the Si content and the half of the Al content is at least 0.9 wt%.
• % Al respective Al content of the steel in% by weight.
• Values of less than 0.9% by weight increase the risk of formation of cementite, through which carbon is set and is no longer available for diffusion into the retained austenite and thus no longer for stabilization of the residual austenite in the case of partitioning annealing.
• the N content is limited to 0.001-0.008% by weight in the steel of a flat steel product according to the invention. Nitrogen forms nitrides in the steel of a flat steel product according to the invention, for example with aluminum or titanium.
• nitrogen forms nitrides in the steel of a flat steel product according to the invention, for example with aluminum or titanium.
• AIN clusters or AIN precipitations at least 0.001% by weight of N should be present in the steel.
• a preferred N content of at least 0.002 wt% can be set. Increasing N levels tend to form larger precipitates.
• the N content is limited to at most 0.008 wt .-%. Phosphorus has a negative effect on the weldability in flat steel products according to the invention. Therefore, the P content should be as low as possible and in particular 0.02 wt .-% should not be exceeded.
• Sulfur at sufficiently high levels leads to the formation of sulfides such as MnS or (Mn, Fe) S. These sulfide precipitates deteriorate the elongation of a flat steel product of the present invention, and therefore the S content is limited to at most 0.005 wt%.
• Chromium may optionally be present in the steel at levels up to 1.0% by weight. Chromium is an effective inhibitor of perlite and contributes to strength. This applies in particular to Cr contents of at least 0.01% by weight. At Cr contents of more than 1.0% by weight, however, the risk of pronounced grain boundary oxidation, which leads to deterioration of the surface quality, is increased.
• Molybdenum may also optionally be included in the steel of a flat steel product of the present invention at levels of at least 0.01% by weight to prevent the formation of perlite.
• the Mo content is limited for reasons of cost to levels of up to 0.2 wt .-%.
• Boron may be included as an optional alloying element in amounts of from 0.001 to 0.01 weight percent in the steel of a flat steel product of the invention. Bor sighs on the phase boundaries and thus blocks their movement. This helps to form a fine-grained structure, which improves the mechanical properties of the flat steel product.
• boron is added, sufficient aluminum should be available to form AIN. Therefore, in a preferred embodiment, an Al / B ratio of at least 10 is set. By boron additions of about 0.01 wt .-%, however, no further improvement can be achieved.
• steels of flat steel products according to the invention may also contain one or more micro-alloying elements from the group consisting of Ti, Nb and V.
• Micro-alloying elements can be treated with carbon or nitrogen carbides, Nitrides or carbonitrides. In the form of very finely distributed precipitates, these contribute to a higher strength.
• the sum of the micro-alloying elements should be at least 0.005 wt .-%, so that the precipitation of carbides, nitrides or carbonitrides can lead to the freezing of grain and phase boundaries during Austenit givess and thus can counteract grain coarsening.
• carbon which is favorable in atomic form for the stabilization of the retained austenite, is bound as carbide or carbonitride.
• the concentration of the micro-alloying elements should not amount to more than 0.2% by weight in total. To avoid coarse titanium nitride precipitates, the titanium concentration should not exceed 0.10%.
• a flat steel product according to the invention preferably has a yield strength Rp02 of more than 700 MPa or a yield strength ReH of more than 700 MPa, a tensile strength Rm of 950-1500 MPa and an elongation A80 of 7-25%, the yield strength Rp02 or the yield strength ReH, the tensile strength Rm and the elongation A80 are determined according to DIN EN ISO 6892: 2009.
• a flat steel product according to the invention preferably has a high bake-hardening potential (BH potential).
• a measure of the BH potential is the BH2 value which is determined after a pre-deformation of 2% and a tempering for 20 minutes at 170 ° C according to DIN EN 10325: 2006 and for steel flat products according to the invention is at least 80 MPa.
• the elongation A80_BH present after a BH treatment for 20 minutes at 170 ° C. on 2% pre-formed flat products according to the invention is at least half as high as the elongation A80 before the BH treatment.
• the elongation values A80 and A80_BH are determined according to DIN EN ISO 6892: 2009.
• the flat steel product according to the invention has a structure that does not contain more than 15 area% ferrite in order to ensure the required high strengths.
• the microstructure does not contain more than 5 area% of bainite.
• the microstructure of a flat steel product according to the invention contains at least 5% by volume of retained austenite. Residual austenite has a favorable effect on the formability and elongation of martensitic steels.
• the austenite stabilized to room temperature can be stretched more extensively than other structural components using the TRIP effect with higher solidification. With the limitation of the austenite-stabilizing alloying elements such as C and Mn for reasons of weldability, a residual austenite content of more than 20% by volume is not possible with the described production process.
• the flat steel product according to the invention contains at least 80 area% martensite, of which at least 75 area% is tempered martensite.
• the martensite formed during the process according to the invention after partitioning by the second quenching in step j) is also referred to as non-tempered martensite.
• the martensite resulting from the first quenching after austenitizing, which is subjected to partitioning is also referred to as tempered martensite.
• the total martensite content present in the structure is composed of tempered and unannealed martensite, with the possibility that there is no untempered martensite.
• the total content of martensite ie the sum of tempered and unannealed martensite, should be at least 80 area%, preferably at least 90 area%.
• This high martensite content contributes to high strength of the flat steel product.
• martensite is a carbon-rich microstructure constituent.
• martensite serves as a source of carbon diffusion on the one hand during partitioning annealing and on the other hand during BH treatment.
• the carbon diffusion from the martensite to the austenite during the partitioning annealing stabilizes the retained austenite, which allows the setting of a retained austenite content of at least 5% by volume.
• Carbon diffusion during BH treatment enhances the BH effect, causing BH to increase.
• At least 75% of the martensite present in the steel flat product is tempered martensite because only enough martensite is available for sufficient retained austenite stabilization during the partitioning annealing.
• At least 90% of the martensite lancets have a martensite lancet width of at most 1000 nm. The small lancet width of at most 1000 nm results in partitioning annealing to short diffusion paths, whereby a targeted local stabilization of the retained austenite is possible.
• the Martensitlanzettenilia is limited to at most 7.5 ⁇ to ensure good formability. Since the lancets grow with a defined ratio of length to width, the width is limited, which has an advantageous effect on the diffusion of the carbon.
• the information on the microstructural constituents for the microstructural constituents martensite, ferrite and bainite in the present case refers to area% and for retained austenite to% by volume. Due to the fineness of the microstructures it is recommended to carry out the microstructural investigations including the determination of the martensite lancet length and width on a scanning electron microscope (SEM) at a magnification of 5000x. As a suitable method for the quantitative determination of retained austenite, X-ray diffraction (XRD) testing according to ASTM E975 is recommended.
• SEM scanning electron microscope
• the method according to the invention for producing a high-strength steel flat product suitable for a bake-hardening treatment comprises at least the following working steps: a) providing a hot-rolled steel flat product which consists of a steel which apart from iron and unavoidable impurities consists of (in% by weight) 0, 1 - 0.5% C, 1.0 - 3.0% Mn, 0.5 - 2.0% Si, 0.01 - 1.5% Al, 0.001 - 0.008% N, up to 0.02% P, up to 0.005% S and optionally one or more of the following elements: 0.01-1.0% Cr, 0.01-0.2% Mo, 0.001-0.01% B and optionally 0.005 in total - 0.2% V, Ti and Nb, the Ti content not exceeding 0.10%; b) pickling the hot rolled flat steel product; c) cold rolling the flat steel product with a cold rolling degree of at least 37%; d) heating the cold-rolled steel flat product to a holding zone temperature THZ which is above the A3 temperature
• step a a hot rolled flat steel product is provided, which consists of a steel of the composition mentioned in step a).
• the hot rolled flat steel product is pickled before cold rolling.
• the pickling in step b) is carried out in a conventional manner.
• the cold rolling in step c) should be carried out according to the invention with a cold rolling degree of at least 37%.
• the cold rolling degree KWG is understood to mean the thickness reduction which takes place due to the cold rolling of the flat steel product.
• the KWG can be described with the following relationship:
• KWG (h0 - h l) / h0
• KWG is based on the total cold rolling degree, ie hO is the thickness of the flat steel product before the first cold rolling or cold rolling pass in mm and hl is the thickness of the flat steel product after the last cold rolling or cold rolling pass in mm.
• the heating of the cold-rolled steel flat product in step d) to a holding zone temperature THZ is initially until reaching a turning temperature TW, which is 200 - 400 ° C, with a heating rate ThetaH l of 5 - 50 K / s. Above the turning temperature TW, the heating takes place until reaching the holding zone temperature THZ with a heating rate ThetaH2 of 2 to 10 K / s. The heating can also take place in one stage, i. the heating rates ThetaH l and ThetaH2 are set to the same value.
• the flat steel product is heated to a holding zone temperature THZ which is above the A3 temperature of the steel to allow complete structural transformation into the austenite.
• the A3 temperature is analytically dependent and can be estimated using the following empirical equation:
• the holding zone temperature THZ can also be referred to as austenitizing temperature and the annealing in THZ also as austenitizing.
• the holding zone temperature is limited to 950 ° C for cost reasons.
• the flat steel product is kept at the holding zone temperature THZ for a holding period tHZ of at least 5 seconds in step e) in order to ensure complete austenitization.
• the holding time tHZ should not exceed 15 seconds to avoid the formation of coarse austenite grain and irregular austenite grain growth.
• the goal of austenitizing is the setting of a fine and regular Austenitkorns, since such a structure has a favorable effect on the BH value.
• the flat steel product can optionally be cooled slowly in step f) initially to an intermediate temperature TLK which is 620 ° C. or more.
• TLK is not lower than 620 ° C to avoid phase transition in ferrite.
• the duration tl_K of THZ cooling to TLK is limited to 30 - 300 seconds.
• the steel flat product in step g) After the optional slow cooling of the flat steel product in step f) or after holding the steel flat product at the holding zone temperature THZ in step e), the steel flat product in step g) with a higher compared to the cooling rate in step f) cooling rate
• the cooling rate from the intermediate temperature TLK to the cooling stop temperature TAB is more than 5 K / s in order to avoid both austenite to ferrite and bainite conversion for the steel compositions of the present invention.
• the cooling rate ThetaQ is preferably set to more than 20 K / s.
• the cooling rate ThetaQ is limited in terms of plant technology to values of at most 500 K / s, preferably at most 100 K / s.
• the cooling stop temperature TAB is between the martensite start temperature TMS and a temperature that is less than TMS up to 175 ° C ((TMS-175 ° C) ⁇ TAB ⁇ TMS).
• the martensite start temperature TMS is understood to mean the temperature at which the transformation of austenite into martensite begins.
• the martensite start temperature can be estimated by the following equation:
• the holding time tQ in step h) is at least 10 seconds in order to ensure a sufficient transformation of the austenite into martensite.
• the proportion of martensite produced by the first quenching after austenitizing should be at least 60% by area.
• the holding time tQ should not exceed 60 seconds in order to avoid complete transformation into martensite and to ensure a residual austenite content of at least 5% by volume in the microstructure of the flat steel product at room temperature.
• step i) the steel flat product is heated to a treatment temperature TB at a heating rate ThetaBl and optionally maintained at TB in order to enrich the retained austenite with supersaturated martensite carbon, which was formed by the first quenching, after operation h).
• the redistribution of carbon which can also be referred to as partitioning, takes place during the heating phase on TB.
• partitioning also takes place during the optional isothermal holding. Heating to treatment temperature TB followed by optional holding at the treatment temperature TB are also referred to as partitioning annealing or as partitioning.
• the heating takes place at a heating rate of at least 1 K / s and at most 80 K / s.
• the treatment temperature TB is 350-500 ° C to avoid the formation of carbides and the decomposition of retained austenite.
• the total treatment time tBT is at least 10 and at most 1000 seconds, also to ensure a sufficient redistribution of the carbon.
• the total treatment time tBT is composed of the time required for heating and, optionally, the time used for the optional isothermal hold.
• the steel flat product is cooled to room temperature in step j) with a cooling rate ThetaB2.
• the cooling rate ThetaB2 is more than 5 K / s, preferably more than 20 K / s, to allow the formation of martensite.
• This cooling step may also be referred to as quenching due to the high cooling rate.
• quenching in step j) is also referred to as second quenching.
• the cooling rate ThetaB2 is limited in terms of plant technology to values of at most 500 K / s, preferably at most 100 K / s.
• the flat steel product may optionally be subjected to a coating treatment (step k)).
• the coating treatment can be carried out either as a hot dip coating (step kl)) or as an electrolytic coating (step k2)). If a hot dip coating (step kl)), then the flat steel product undergoes a coating bath with a zinc-based molten bath composition after partitioning in step i) and before cooling in step j).
• the temperature of the molten bath is preferably 450-500 ° C.
• the flat steel product may be subjected to electrolytic coating (step k2)).
• the electrolytic coating is carried out in contrast to Hot dip coating not before, but only after cooling of the flat steel product in step j).
• the coating treatment of steps kl) or k2) is preferably carried out in a continuous process.
• a possible molten bath composition may consist of up to 1% by weight Al, balance zinc and unavoidable impurities.
• Another possible molten bath composition may consist of 1-2 wt% Al, 1-2 wt% Mg, balance zinc and unavoidable impurities.
• the coating treatment applies a corrosion protection coating to the flat steel product on at least one side of the flat steel product.
• the coated flat steel product may also optionally be subjected to galvannealing treatment.
• the process according to the invention can be carried out in continuous operation in conventionally provided annealing plants or strip coating plants.
• the cooling rate ThetaQ of the rapid cooling after austenitizing and the holding time tQ results in a microstructure which has a very fine martensite structure.
• This martensite structure is characterized by a special fine grain with a small lancet width.
• the high degree of cold rolling and the carbidic and nitridic precipitations lead to a fine-grained starting structure for austenitizing annealing.
• grain coarsening during austenitizing is avoided so that a very fine-grained microstructure is present even before the subsequent austenitizing cooling.
• the numerous grain boundaries of the fine structure hinder the growth of martensite lancets.
• the flat steel products made available by the present invention are particularly suitable for further processing processes, which include a cold forming process and a subsequent heat treatment at temperatures below 300 ° C.
• Flat steel products are thereby formed into components, for example by means of cathodic dip coating (KTL) painted and subjected in a further process step, a heat treatment, for example, during paint baking.
• KTL cathodic dip coating
• the heat treatment is usually carried out as heating within a temperature range of typically 120 to 250 ° C for a period of typically 3 to 40 minutes.
• the flat steel products according to the invention are particularly suitable.
• the advantageous properties of the flat steel products according to the invention can also be utilized for products which have not undergone pre-deformation.
• melts AE of the compositions shown in Table 1 were produced, from which 10 hot strips with a thickness of 1.8-2.5 mm were produced in a conventional manner.
• the melts C correspond and E the specifications according to the invention for the steel composition, whereas the melts A, B and D have too low Si contents.
• the hot-rolled strips were pickled in a conventional manner and processed to cold-rolled strips using the cold rolling degrees "KWG” given in Table 2a.
• the further production of the cold strips was carried out according to the information given in Table 2a and Table 2b.
• the cold strips were heated in each case with a first, faster heating rate "ThetaH l” to a turning temperature “TW” and then brought with a second, slower heating rate "ThetaH2" on holding zone temperature "THZ”, on which they held for the duration "tHZ" were.
• the sections were prepared for a scanning electron microscopic (SEM) study and treated with a 3% Nital etch. Due to the fineness of the microstructures, the microstructure was characterized by SEM observation at 5000x magnification. The quantitative determination of retained austenite was carried out by X-ray diffraction (XRD) according to ASTM E975.
• SEM scanning electron microscopic

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Mechanical Engineering (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Electrochemistry (AREA)
• Oil, Petroleum & Natural Gas (AREA)
• Heat Treatment Of Sheet Steel (AREA)
• Manufacturing Cores, Coils, And Magnets (AREA)

Priority Applications (7)

Applications Claiming Priority (1)

Publications (1)

Family

ID=60019885

Family Applications (1)

Country Status (6)

Cited By (12)

Families Citing this family (3)

Citations (6)

Family Cites Families (8)

• 2017
  • 2017-09-28 EP EP17780063.8A patent/EP3688203B1/de active Active
  • 2017-09-28 EP EP22159990.5A patent/EP4043603A1/de not_active Withdrawn
  • 2017-09-28 ES ES17780063T patent/ES2921013T3/es active Active
  • 2017-09-28 CN CN201780095388.XA patent/CN111148853B/zh active Active
  • 2017-09-28 WO PCT/EP2017/074642 patent/WO2019063081A1/de not_active Ceased
  • 2017-09-28 JP JP2020517460A patent/JP7105302B2/ja active Active
  • 2017-09-28 PL PL17780063.8T patent/PL3688203T3/pl unknown

Patent Citations (6)

Also Published As

Similar Documents

Legal Events