Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
  • B21D22/02—Stamping using rigid devices or tools
  • B21D22/022—Stamping using rigid devices or tools by heating the blank or stamping associated with 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
  • 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
• • 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
  • C21D1/62—Quenching devices
  • C21D1/673—Quenching devices for die 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
  • C21D1/76—Adjusting the composition of the atmosphere
• • 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
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
• • 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
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0242—Flattening; Dressing; Flexing
• • 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
  • 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
  • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/02—Alloys based on aluminium with silicon as the next major constituent
• • 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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
• • 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
  • 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
  • C23C30/00—Coating with metallic material characterised only by the composition of the metallic material, i.e. not characterised by the coating process
• • 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 invention relates to a flat steel product for hot forming and a method for producing such a flat steel product. Furthermore, the invention relates to a shaped sheet metal part with improved processing properties and a method for producing such a shaped sheet metal part from a flat steel product.
• the structure was determined on longitudinal sections that had been etched with 3% Nital (alcoholic nitric acid). The proportion of retained austenite was determined by X-ray diffractometry.
• press hardening also referred to as hot forming
• a protective gas atmosphere must be used to avoid excessive oxidation during hot forming.
• the surface quality, in particular the coefficient of friction, of uncoated material leads to increased tool wear on the hot forming tools.
• Another problem is the additional logistics, since uncoated material has to go through a different hot stamping process, as described.
• the object of the present invention is to provide an inexpensive material for applications with low susceptibility to corrosion for hot forming, which material has improved processing properties.
• the coating has a coating weight of 15-30 g/m ⁇ 2.
• the coating has an Al base layer consisting of 1.0-15% by weight Si, optionally 2-4% by weight Fe, 0.1-5.0% by weight alkali or alkaline earth metals, and optional further components, the total content of which is limited to a maximum of 2.0% by weight, and the remainder is aluminum.
• alkali and alkaline earth metals leads to an optimized oxide layer formation with >50% alkali and alkaline earth metal oxides, particularly preferably >60%, most preferably >70%. These oxide layers result, for example, in improved coefficients of friction. In addition, this results in a changed roughness, which is advantageous with regard to paint adhesion and adhesive adhesion.
• the flat steel product described can be further processed particularly efficiently.
• the total time in the furnace during production of the shaped sheet metal part can be chosen to be particularly low.
• the finishing process is not overly sensitive to deviations in oven temperature and total time in the oven - so there is a relatively generous process window that simplifies finishing.
• the object according to the invention is therefore also achieved in particular by the use of an aluminum-based coating on at least one side of a steel substrate of a flat steel product to reduce tool wear when producing a sheet metal component by hot forming, in particular with high and very high degrees of forming, which are necessary in the production of bodywork components, such as tunnel, B-pillar or A-pillar.
• the steel substrate consists of a steel which has 0.1-3% by weight Mn and optionally up to 0.01% by weight B.
• the cover has a coating weight of 15-30 g/m ⁇ 2.
• the coating has an Al base layer consisting of 1.0-15% by weight Si, optionally 2-4% by weight Fe, 0.1-5.0% by weight alkali or alkaline earth metals, and optional further components, the total content of which is limited to a maximum of 2.0% by weight, and the remainder is aluminum.
• the coating is arranged on at least one side of the steel substrate and has a coating weight of 15-30 g/m ⁇ 2.
• the coating weight always refers to one side of the steel flat product. In other words, the coating weight is 15-30 g/m ⁇ 2 per side on which the cover is placed.
• the total coating weight is 30-60 g/m ⁇ 2.
• the two opposite large surfaces of the flat steel product are referred to as the two sides of the flat steel product. The narrow faces are called edges.
• the coating has an alloy layer which lies on the steel substrate and on which the Al base layer is arranged, the alloy layer consisting of 35-60% by weight Fe, optional further components, their total content are limited to at most 5.0% by weight, and the remainder is aluminum.
• the alloy layer rests on the steel substrate and is directly adjacent to it.
• the alloy layer is essentially made up of aluminum and iron.
• the remaining elements from the steel substrate or the melt composition do not enrich significantly in the alloy layer.
• the alloy layer preferably consists of 35-60% by weight Fe, preferably - iron, optional further components, the total content of which is limited to a maximum of 5.0% by weight, preferably 2.0%, and the remainder aluminum, with the Al content preferably increases towards the surface.
• the optional further components include in particular the other components of the melt used in the production process (see below) (i.e. silicon and optionally alkali or alkaline earth metals, in particular Mg or Ca) and the other components of the steel substrate in addition to iron.
• the Al base layer lies on top of the alloy layer and is immediately adjacent to it.
• the composition of the Al base layer preferably corresponds to the composition of the melt in the molten bath. That is, it consists of 0.1 - 15 wt% Si, optionally 2-4 wt% Fe, 0.1 - 5 wt% alkali or alkaline earth metals, preferably up to 1.0% wt. -% Alkaline or alkaline earth metals and optional other components, the total content of which is limited to a maximum of 2.0% by weight, and the remainder aluminum.
• the content of alkali metals or alkaline earth metals is 0.1-1.0% by weight Mg, in particular 0.1-0.7% by weight Mg, preferably 0.1-0. 5 wt% Mg.
• the content of alkali metals or alkaline earth metals in the Al base layer can include, in particular, at least 0.0015% by weight of Ca, in particular at least 0.1% by weight of Ca.
• the steel substrate has a proportion of diffusible hydrogen H diff of at most 0.15 ppm by weight.
• the diffusible hydrogen of the steel substrate within the meaning of this application is to be determined within 48 hours after application of the coating.
• the steel substrate is of a steel containing 0.1-3 wt% Mn and optionally up to 0.01 wt% B.
• the microstructure of the steel can be converted into a martensitic or partially martensitic microstructure by hot forming.
• the microstructure of the steel substrate of the steel component is therefore preferably a martensitic or at least partially martensitic microstructure, since this has a particularly high degree of hardness.
• the carbon content of the steel is at most 0.37% by weight and/or at least 0.06% by weight. In particularly preferred variants, the C content is in the range from 0.06 to 0.09% by weight or in the range from 0.12 to 0.25% by weight or in the range from 0.33 to 0.37% by weight %.
• carbon acts to delay the formation of ferrite and bainite. At the same time, retained austenite is stabilized and the Ac3 temperature is reduced.
• a carbon content of at least 0.06% by weight is advantageous in order to ensure the hardenability of the steel flat product and the tensile strength of the press-hardened product of at least 1000 MPa. If a higher level of strength is to be aimed at, preference is given to setting C contents >0.12% by weight. If the C content is further increased to values of at least 0.19% by weight, the hardenability can also be improved, so that the flat steel product has a very good combination of hardenability and strength.
• carbon contents greater than 0.45% by weight have an adverse effect on the mechanical properties of the flat steel product, since C contents greater than 0.45% by weight promote the formation of brittle martensite during press hardening.
• High C contents can also have a negative impact on weldability.
• the carbon content can preferably be adjusted to values below 0.40% by weight, in particular 0.3% by weight.
• the weldability can be significantly improved and a good ratio of force absorption and maximum bending angle in the bending test according to VDA238-100 in the press-hardened state can also be achieved.
• the Si content of the steel is at most 1.00% by weight and/or at least 0.06% by weight.
• Silicon is used to further increase the hardenability of the steel flat product as well as the strength of the press-hardened product via solid solution strengthening. Silicon also enables the use of ferro-silizio-manganese as an alloying agent, which has a beneficial effect on production costs.
• a hardening effect occurs from an Si content of 0.06% by weight. From an Si content of >0.15% by weight, there is a significant increase in strength. Si contents above 0.5% by weight have an adverse effect on the coating behavior, particularly in the case of Al-based coatings. Si contents of less than 0.4% by weight are preferably set in order to improve the surface quality of the coated flat steel product.
• the Mn content of the steel is at most 2.4% by weight and/or at least 0.75% by weight. In particularly preferred embodiment variants, the Mn content is in the range of 0.75-0.85% by weight or in the range of 1.0-1.6% by weight.
• Manganese acts as a hardening element by greatly retarding the formation of ferrite and bainite. With manganese contents of less than 0.5% by weight, ferrite and bainite are formed during press hardening, even with very rapid cooling rates, which should be avoided. Mn contents greater than 0.75% by weight, in particular 0.9% by weight, are preferred if a martensitic structure is to be ensured, particularly in areas of greater deformation. Manganese contents greater than 3.0% by weight have an adverse effect on the processing properties. The weldability in particular is severely limited, which is why the Mn content of flat steel products according to the invention is limited to a maximum of 2.4% by weight, in particular a maximum of 1.6% by weight. Manganese contents of less than 1.6% by weight are also preferred for economic reasons.
• the Al content of the steel is at most 0.75% by weight, in particular at most 0.5% by weight, preferably at most 0.25% by weight.
• the Al content is preferably at least 0.02%.
• Aluminum is used as a deoxidizing agent to bind oxygen. Aluminum also inhibits cementite formation. At least 0.01% by weight of Al in the steel is required to reliably bind oxygen. However, since the Ac3 temperature is also clearly shifted upwards with increasing Al alloy content, the Al content is limited to 0.25% by weight. From a content of 0.25% by weight, Al hinders the transformation into austenite too much before press hardening, so that the austenitization can no longer be carried out efficiently in terms of time and energy. For usual furnace temperatures between 850 and 950° C. in hot forming, an Al content of at most 0.1% by weight is preferably maintained in order to nevertheless fully austenitize the steel.
• the sum of the contents of Si and Al (usually referred to as Si+Al) is therefore at most 1.5% by weight, preferably at most 1.2% by weight. Additionally or alternatively, the sum of the contents of Si and Al is at least 0.06% by weight, preferably at least 0.08% by weight.
• the elements P, S, N are typical impurities that cannot be completely avoided in steel production.
• the P content is maximum 0.03% by weight.
• the S content is preferably at most 0.012%.
• the N content is preferably at most 0.009% by weight.
• Phosphorus (P) and sulfur (S) are elements that are introduced into steel as impurities from iron ore and cannot be completely eliminated in the large-scale steelworks process.
• the P content and the S content should be kept as low as possible since the mechanical properties such as notched bar impact work deteriorate with increasing P or S content.
• P content of a steel flat product according to the invention is limited to ⁇ 0.1% by weight, preferably a maximum of 0.03% by weight .
• the S content of a flat steel product according to the invention is limited to ⁇ 0.05% by weight, preferably a maximum of 0.012% by weight.
• Nitrogen (N) is present in small amounts in steel due to the steel manufacturing process.
• the N content should be kept as low as possible and should be less than 0.02% by weight.
• nitrogen is harmful because it prevents the transformation-retarding effect of boron through the formation of boron nitrides, which is why the nitrogen content in this case is preferably at most 0.01% by weight, preferably at most 0.009% by weight. should be.
• the steel also contains chromium with a content of 0.08 - 1.0% by weight.
• the Cr content is preferably at most 0.75% by weight, in particular at most 0.5% by weight.
• Chromium is added to the steel of a flat steel product according to the invention in amounts of 0.08-1.0% by weight. Chromium affects the hardenability of the flat steel product by slowing down the diffusive transformation during press hardening.
• chromium has a beneficial effect on the hardenability from a content of 0.08% by weight, with a Cr content >0.1% by weight being preferred for reliable process control, above all to prevent the formation of bainite. If the steel contains more than 1.0% by weight of chromium, the coating behavior deteriorates.
• the Cr content can preferably be reduced to a maximum of 0.75% by weight, in particular a maximum of 0.5% by weight. be limited.
• the sum of the chromium and manganese contents is preferably limited.
• the total is at most 3.3% by weight, in particular at most 3.15 wt%.
• the sum is at least 0.5% by weight, preferably at least 0.75% by weight.
• the steel preferably also optionally contains boron with a content of 0.001-0.005% by weight.
• the B content is at most 0.004% by weight.
• Boron can be optionally alloyed to improve the hardenability of the flat steel product by having boron atoms or boron precipitates attached to the austenite grain boundaries reduce the grain boundary energy, thereby suppressing the nucleation of ferrite during press hardening.
• a clear effect on hardenability occurs with B contents of at least 0.001% by weight.
• B contents above 0.01% by weight on the other hand, boron carbides, boron nitrides or boron nitrocarbides are increasingly formed, which in turn represent preferred nucleation sites for the nucleation of ferrite and reduce the hardening effect again. For this reason, the B boron content is limited to at most 0.01% by weight.
• titanium is also preferably alloyed in to bind nitrogen.
• the Ti content should preferably be at least 3.42 times the content in wt. % of nitrogen.
• the steel can optionally contain molybdenum with a content of at most 0.5% by weight, in particular at most 0.1% by weight.
• Molybdenum can optionally be added to improve process stability as it significantly slows down ferrite formation. From contents of 0.002% by weight, dynamic molybdenum-carbon clusters form up to ultra-fine molybdenum carbides on the grain boundaries, which significantly slow down the mobility of the grain boundary and thus diffusive phase transformations. In addition, molybdenum reduces the grain boundary energy, which reduces the nucleation rate of ferrite. Due to the high costs associated with alloying molybdenum, the Mo content should be at most 1.0% by weight, preferably at most 0.5% by weight.
• the steel can also contain copper with a content of at most 0.2% by weight, preferably at most 0.15% by weight.
• Copper (Cu) can optionally be alloyed to increase hardenability with additions of at least 0.01% by weight.
• copper improves the resistance to atmospheric corrosion of uncoated sheet metal or cut edges. From a content of 0.8% by weight, the hot-rollability deteriorates significantly due to low-melting Cu phases on the surface.
• the steel can optionally contain nickel with a content of at most 0.5% by weight, preferably at most 0.15% by weight.
• Nickel (Ni) stabilizes the austenitic phase and can optionally be alloyed to lower the Ac3 temperature and suppress the formation of ferrite and bainite. Nickel also has a positive effect on hot-rollability, especially when the steel contains copper. Copper degrades hot-rollability. To counteract the negative influence of copper on hot-rollability, at least 0.01% by weight of nickel can be alloyed with the steel. For economic reasons, the nickel content should be limited to a maximum of 0.5% by weight, preferably a maximum of 0.4% by weight.
• the steel can optionally contain one or more of the micro-alloying elements Nb, Ti and V.
• the optional Nb content is at least 0.02% by weight and at most 0.08% by weight, preferably at most 0.04% by weight.
• the optional Ti content is at least 0.01% by weight and at most 0.08% by weight, preferably at most 0.04% by weight.
• the optional V content is at most 0.1% by weight, preferably at most 0.05% by weight.
• Niobium (Nb) can optionally be alloyed to contribute to grain refinement from a content of 0.001% by weight. However, niobium degrades the recrystallizability of the steel. With an Nb content of more than 0.1% by weight, the steel can no longer be recrystallized in conventional continuous furnaces before hot-dip coating.
• Titanium (Ti) is a micro-alloying element that can optionally be alloyed to contribute to grain refinement.
• titanium forms coarse titanium nitrides with nitrogen, which is why the Ti content should be kept comparatively low.
• Titanium binds nitrogen and thus enables boron to unfold its strong ferrite-inhibiting effect.
• At least 3.42 times the nitrogen content is required for sufficient nitrogen fixation, with at least 0.001 wt.% Ti being added for sufficient availability. Deteriorated from 0.1 wt% Ti cold-rollability and recrystallizability are significantly improved, which is why larger Ti contents should be avoided.
• the sum of the contents of Nb, Ti and V is preferably limited.
• the total is at most 0.1% by weight, in particular at most 0.068% by weight. Furthermore, the sum is preferably at least 0.015% by weight.
• Vanadium (V) is an element with a very high affinity for carbon. When vanadium is free, that is, in an unbound or dissolved state, it can bind to supersaturated dissolved carbon in the form of carbides or clusters, or at least reduce its rate of diffusion. It is crucial that V is in the dissolved state. Surprisingly, very low V contents in particular have proven to be particularly favorable for resistance to aging. With higher V contents, larger vanadium carbides can precipitate even at higher temperatures, which then no longer dissolve at temperatures of 800-900°C, which are typical for continuous annealing in hot-dip coating systems. Even the smallest amounts of vanadium of 0.001% by weight can prevent free carbon from adhering to dislocations.
• vanadium no longer improves the aging resistance.
• the aging-inhibiting effect of vanadium is particularly pronounced with V contents of up to 0.009% by weight, with a maximum effect occurring from a preferred V content of 0.002% by weight.
• V contents greater than 0.009% by weight vanadium carbides are increasingly formed. From a vanadium content in the steel of 0.009% by weight, vanadium carbides cannot be dissolved at temperatures of 860°C, which are typical for annealing temperatures in a hot-dip coating plant, for example.
• the vanadium content of the steel of a flat steel product according to the invention is limited to a maximum of 0.1% by weight for cost reasons. On the other hand, higher V contents do not bring about any significant improvement in the mechanical properties.
• Tungsten (W) can optionally be alloyed in amounts of 0.001 - 1.0% by weight to slow down the formation of ferrite. A positive effect on hardenability is obtained even with W contents of at least 0.001% by weight. For cost reasons, a maximum of 1.0% by weight of tungsten is added.
• a semi-finished product composed according to the alloy specified according to the invention for the flat steel product is made available.
• This can be a slab produced in conventional continuous slab casting or in thin slab continuous casting.
• step b) the semi-finished product is heated through at a temperature (T1) of 1000 - 1400°C. If the semi-finished product has cooled down after casting, the semi-finished product is first reheated to 1000 - 1400°C for thorough heating.
• the through heating temperature should be at least 1000°C to ensure good formability for the subsequent rolling process.
• the heating temperature should not exceed 1400°C in order to avoid molten phases in the semi-finished product.
• the semi-finished product is pre-rolled into an intermediate product.
• Thin slabs are usually not subjected to pre-rolling.
• Thick slabs that are to be rolled into hot strip can be pre-rolled if necessary.
• the temperature of the intermediate product (T2) at the end of the rough rolling should be at least 1000°C in order that the intermediate product contains enough heat for the subsequent finish rolling step.
• high rolling temperatures can also cause grain growth during of the rolling process, which has an adverse effect on the mechanical properties of the steel flat product.
• the temperature of the intermediate product at the end of rough rolling should not be more than 1200°C.
• step d) the slab or thin slab or, if step c) has been carried out, the intermediate product is rolled to form a hot-rolled flat steel product.
• step c) the intermediate product is typically finish-rolled immediately after rough-rolling. Typically, finish rolling begins no later than 90 s after the end of rough rolling.
• the slab, the thin slab or, if step c) has been carried out, the intermediate product are rolled at a finish rolling temperature (T3).
• the final rolling temperature i.e. the temperature of the finished hot-rolled steel flat product at the end of the hot-rolling process, is 750 - 1000°C. At final rolling temperatures below 750°C, the amount of free vanadium decreases because larger amounts of vanadium carbides are precipitated.
• the vanadium carbides precipitated during finish rolling are very large. They typically have an average grain size of 30 nm or more and are no longer dissolved in subsequent annealing processes, such as those carried out before hot-dip coating.
• the final rolling temperature is limited to values not exceeding 1000°C in order to prevent the austenite grains from coarsening.
• final rolling temperatures of no more than 1000°C are process-technically relevant for setting coiling temperatures (T4) below 700°C.
• the hot rolling of the steel flat product can take place as continuous hot strip rolling or as reversing rolling.
• step e) provides for an optional coiling of the hot-rolled flat steel product.
• the hot strip is cooled to a coiling temperature (T4) within less than 50 s after hot rolling.
• T4 a coiling temperature
• the coiling temperature (T4) should not exceed 700°C to avoid the formation of large vanadium carbides. In principle, there is no lower limit on the coiling temperature. However, coiling temperatures of at least 500°C have proven to be favorable for cold-rollability.
• the coiled hot strip is then cooled in air to room temperature in a conventional manner.
• step f the hot-rolled flat steel product is descaled in a conventional manner by pickling or by another suitable treatment.
• the hot-rolled flat steel product that has been cleaned of scale can optionally be subjected to cold rolling before the annealing treatment in step g), in order, for example, to meet higher requirements for the thickness tolerances of the flat steel product.
• the degree of cold rolling (KWG) should be at least 30% in order to introduce sufficient deformation energy into the steel flat product for rapid recrystallization.
• the flat steel product before cold rolling is usually a hot strip with a hot strip thickness d.
• the flat steel product after cold rolling is usually also referred to as cold strip.
• the degree of cold rolling can assume very high values of over 90%. However, degrees of cold rolling of at most 80% have proven to be beneficial for avoiding strip cracks.
• step h) the flat steel product is subjected to an annealing treatment at annealing temperatures (T5) of 650-900°C.
• T5 annealing temperatures
• the flat steel product is first heated to the annealing temperature within 10 to 120 s and then held at the annealing temperature for 30 to 600 s.
• the annealing temperature is at least 650°C, preferably at least 720°C. Annealing temperatures above 900°C are not desirable for economic reasons.
• the flat steel product is cooled to an immersion temperature (T6) after annealing in order to prepare it for the subsequent coating treatment.
• the pre-cooling temperature is lower than the annealing temperature and is adjusted to the temperature of the melt pool.
• the immersion temperature is 600-800°C, preferably at least 650°C, particularly preferably at least 680°C, particularly preferably at most 700°C.
• the immersion temperature T6 is preferably at most 750°C, in particular at most 720°C.
• the duration of the cooling of the annealed flat steel product from the annealing temperature T5 to the immersion temperature T6 is preferably 10-180 s.
• the immersion temperature T6 deviates from the temperature of the molten bath T7 by no more than 30K, in particular no more than 20K, preferably no more than 10 K off
• the flat steel product is subjected to a coating treatment.
• the coating treatment is preferably carried out by continuous hot dip coating.
• the coating can be applied to only one side, to both sides or to all sides of the steel flat product.
• the coating treatment preferably takes place as a hot-dip coating process, in particular as a continuous process.
• the steel flat product usually comes into contact with the molten bath on all sides, so that it is coated on all sides.
• the molten bath which contains the alloy to be applied to the flat steel product in liquid form, typically has a temperature (T7) of 660-800°C, preferably 670-710°C.
• the melt temperature T7 is preferably at least 670°C, in particular at least 680°C.
• the melt temperature is preferably at most 750°C, in particular at most 730°C, preferably at most 710°C.
• the molten bath preferably contains up to 15% by weight Si, preferably more than 1.0%, optionally 2-4% by weight Fe, 0.1-5.0% by weight alkali or alkaline earth metals, preferably up to 1, 0% by weight alkali metals or alkaline earth metals, and optional further components, the total content of which is limited to a maximum of 2.0% by weight, and the remainder aluminum.
• the Si content of the melt is 7-12% by weight, in particular 8-10% by weight.
• the optional content of alkali or alkaline earth metals in the melt comprises 0.1-1.0% by weight Mg, in particular 0.1-0.7% by weight Mg, preferably 0.1-0. 5% by weight Mg.
• the optional content of alkali metals or alkaline earth metals in the melt can include in particular at least 0.0015% by weight Ca, in particular at least 0.01% by weight Ca.
• the flat steel product After exiting the molten pool, the flat steel product is blown off by means of a gas stream.
• a first cooling time t mT in the temperature range between 600°C and 450°C is more than 10s, in particular more than 14s
• a second cooling time t nT in the temperature range between 400°C and 300°C is more than 8s, especially more than 12s.
• the first cooling time t mT can be realized in the temperature range between 600° C. and 450° C. (average temperature range mT) by slow, continuous cooling or by holding at a temperature in this temperature range for a certain time. Even intermediate heating is possible. The only important thing is that the flat steel product remains in the temperature range between 600°C and 450°C for at least a cooling period t mT . In this temperature range, on the one hand, there is a significant rate of diffusion of iron in aluminum and, on the other hand, the diffusion of aluminum in steel is inhibited because the temperature is below half the melting point of steel. This allows diffusion of iron into the coating without extensive diffusion of aluminum into the steel substrate.
• the diffusion of iron into the coating has several advantages: On the one hand, the melting of the coating during austenitizing before press hardening is delayed. On the other hand, the thermal expansion coefficients of the coating and substrate are homogenized. This means that the transition area between the coefficient of thermal expansion of the substrate and the surface becomes wider, which reduces the thermal stresses during reheating.
• the iron concentration in the transition boundary layer increases to such an extent that the activity of aluminum in the coating directly at the substrate boundary is further reduced. This then leads to an even further reduced aluminum absorption in the substrate during austenitization before press hardening, with the associated advantages described above.
• the second cooling time t nT in the temperature range between 400° C. and 300° C. can also be realized by slow, continuous cooling or by holding at a temperature in this temperature range for a certain time. Even intermediate heating is possible. It is only important that the flat steel product remains in the temperature range between 400°C and 300°C for at least a cooling period tnT.
• transition carbides very fine iron carbides
• the coated steel flat product can optionally be skin-passed with a skin-pass degree of up to 2% in order to improve the surface roughness of the steel flat product.
• the gas flow is an air flow, which preferably has a temperature of from room temperature to 130°C, preferably from 50-90°C.
• the temperature ranges mentioned have proven to be particularly useful for suitably influencing the surface temperature of the coating.
• the surface is prevented from solidifying too quickly, so that sufficient diffusion can still take place.
• the surface of the coating is cooled down a little in order to prevent adhesion to subsequent rolls by the thin phases of the coating.
• the dew point temperature TP during the thorough heating completed in step b) is 30-80° C. due to the flames of the burners used for thorough heating.
• the lambda value of an annealing atmosphere during the thorough heating completed in step b) is 0.95-1.1.
• the lambda value (also combustion air ratio) describes the ratio of the masses of air to fuel introduced into the continuous furnace.
• the invention further relates to a shaped sheet metal part, in particular formed from a flat steel product as described above, comprising a steel substrate as explained above and an aluminum-based coating arranged on at least one side of the steel substrate.
• the coating has a coating weight of 15-30 g/m ⁇ 2 and consists of 1-15% by weight Si, 15-35% by weight Fe, 0.1-5% by weight alkali or alkaline earth metals , and optional further components, the total content of which is limited to a maximum of 2.0% by weight, and the remainder aluminum.
• the composition leads to increased surface hardness, which is already noticeable in the lower tool wear of the forming tool during hot forming.
• the Si content of the coating is at least 6% by weight, in particular at least 7.0% by weight. Furthermore, the Si content of the coating is preferably at most 9% by weight, preferably at most 8.0% by weight.
• the Fe content is preferably at least 20% by weight, preferably at least 23% by weight, in particular at least 25% by weight. Furthermore, the Fe content is at most 30% by weight, preferably at most 29% by weight, in particular at most 28.0% by weight.
• the content of alkali or alkaline earth metals 0.1-1.0% by weight Mg, in particular 0.1-0.7% by weight Mg, preferably 0.1-0.5% by weight Mg. Furthermore, the content of alkali or alkaline earth metals in the Al base layer, in particular at least 0.0015% by weight of Ca, in particular at least 0.1% by weight of Ca.
• the coating of the sheet metal part has an Al base layer and an alloy layer, with the alloy layer overlying the steel substrate and the Al base layer overlying the alloy layer.
• the alloy layer of the shaped sheet metal part preferably consists of 35-90% by weight Fe, 0.1-10% by weight Si, optionally up to 0.5% by weight Mg and optional other components, the total content of which is at most 2 .0% by weight, and the remainder aluminum.
• the proportions of Si and Mg are correspondingly lower than their respective proportions in the melt of the molten bath.
• the alloy layer preferably has a ferritic structure.
• the alloy layer of the shaped sheet metal part preferably has a thickness which corresponds to 60-95% of the thickness of the coating, in particular 70-90% of the thickness of the coating.
• the aluminum base layer of the shaped sheet metal part lies on the alloy layer of the steel component and is directly adjacent to it.
• the Al base layer of the steel component preferably consists of up to 55% by weight Fe, 0.4-10% by weight Si, optionally up to 0.5% by weight Mg and optional other components, the total content of which is at most 2.0% by weight, and the remainder aluminum.
• the optional content of Mg is preferably more than 0.1% by weight.
• the Al base layer can have a homogeneous element distribution in which the local element contents vary by no more than 10%.
• preferred variants of the Al base layer have low-silicon phases and high-silicon phases.
• Low-silicon phases are areas whose average Si content is at least 20% less than the average Si content of the Al base layer.
• Silicon-rich phases are areas whose average Si content is at least 20% more than the average Si content of the Al base layer.
• the silicon-rich phases are arranged within the silicon-poor phase.
• the silicon-rich phases form at least a 40% continuous layer bounded by silicon-poor regions.
• the silicon-rich phases are arranged in islands in the silicon-poor phase.
• island-shaped is understood to mean an arrangement in which discrete, non-connected areas are surrounded by another material—that is, “islands” of a specific material are located in another material.
• the steel component comprises an oxide layer arranged on the coating.
• the oxide layer lies in particular on the Al base layer and preferably forms the outer end of the coating.
• the oxide layer of the steel component consists in particular of more than 80% by weight of oxides, with the majority of the oxides (i.e. more than 50% by weight of the oxides, in particular more than 70%, preferably more than 80%) being alkali and alkaline earth metal oxides (preferably Mg oxides) are.
• alkali and alkaline earth metal oxides preferably Mg oxides
• hydroxides and/or aluminum oxide are present alone or as a mixture in the oxide layer in addition to the oxides of the alkali and alkaline earth metals.
• the remainder of the oxide layer not occupied by the oxides and optionally present hydroxides consists of silicon, aluminum, iron and/or the alkali and alkaline earth metals (preferably magnesium) in metallic form.
• the oxide layer preferably has a thickness of at least 50 nm, in particular at least 100 nm. Furthermore, the thickness is at most 4 ⁇ m, in particular at most 2 ⁇ m.
• the Al base layer has a nanohardness of at least 1.3 GPa (gigapascal), in particular at least 1.4 GPa. To minimize tool wear, a maximum nanohardness of 1.8 GPa should preferably not be exceeded.
• the Al base layer preferably also has a penetration modulus of at least 79 GPa, preferably at least 82 GPa, in particular at least 84 GPa. The penetration modulus is preferably at most 105 GPa, in particular at most 100 GPa.
• the nanohardness and the penetration modulus are determined using a nano-indenter with a Berkovich pyramid as a test syringe. A cross-section is made for this purpose and then the corresponding layer is identified.
• the measurement is carried out using a load function with a maximum load of 200 ⁇ N.
• the measurement is carried out with a load function with a maximum load of 2000 ⁇ N.
• the alloy layer has a nanohardness of at least 10.9 GPa, preferably at least 11.0 GPa, in particular at least 11.5 GPa.
• the maximum nanohardness should preferably not exceed 16 GPa.
• the alloy layer preferably also has a penetration modulus of at least 175 GPa, preferably at least 180 GPa, in particular at least 185 GPa.
• the penetration modulus is preferably at most 250 GPa, in particular at most 230 GPa.
• the area of the steel substrate close to the surface has a nanohardness of at least 10.9 GPa, preferably at least 11.0 GPa, in particular at least 12.0 GPa.
• the maximum nanohardness should not exceed 17 GPa, preferably 16 GPa.
• the near-surface area of the steel substrate preferably also has a penetration modulus of at least 205 GPa, preferably at least 180 GPa, in particular at least 185 GPa.
• the penetration modulus is preferably at most 280 GPa, in particular at most 260 GPa.
• the strip with a thickness of 20 ⁇ m, which is directly adjacent to the alloy layer, is located under the near-surface area of the steel substrate.
• the near-surface area of the steel substrate means the top 20 ⁇ m of the steel substrate.
• the steel substrate of the shaped sheet metal part has a structure with at least partially more than 80% martensite, preferably at least partially more than 90% martensite, in particular at least partially more than 95%, particularly preferably at least partially more than 99%.
• partially having is to be understood as meaning that there are areas of the shaped sheet metal part that have the structure mentioned.
• the shaped sheet metal part therefore has the above-mentioned structure in sections or in regions.
• the shaped sheet metal part according to the invention is preferably a component for a land vehicle, sea vehicle or aircraft. It is particularly preferably an automobile part, in particular a body part;
• the component is preferably a B-pillar, side member, A-pillar, rocker panel or cross member.
• a blank is thus provided (work step a)) which consists of a flat steel product assembled in a suitable manner in accordance with the explanations above, which is then heated in a manner known per se in such a way that the AC3 temperature of the steel is at least partially exceeded and the temperature of the blank when it is placed in a forming tool provided for hot pressing (work step c)) is at least partially a temperature above Ms+100° C., preferably AC1.
• partially exceeding a temperature here AC3 or Ms+100°C means that at least 30%, in particular at least 60%, of the volume of the blank exceeds a corresponding temperature.
• At least 30% of the blank has an austenitic structure, i.e. the transformation from ferritic to austenitic structure does not have to be completed when it is placed in the forming tool. Rather, up to 70% of the volume of the blank when it is placed in the forming tool can consist of other structural components, such as tempered bainite, tempered martensite and/or non-recrystallized or partially recrystallized ferrite. For this purpose, certain areas of the blank can be kept at a lower temperature level than others during heating. For this purpose, the supply of heat can be directed only to specific sections of the blank, or the parts that are to be heated less can be shielded from the supply of heat.
• Maximum strength properties of the formed sheet metal part obtained can be made possible by the temperature at least partially reached in the sheet metal blank being between Ac3 and 1000°C, preferably between 850°C and 950°C.
• An optimally uniform distribution of properties can be achieved by completely heating the blank in step b).
• the mean heating rate rOven of the sheet metal blank during heating in step b) is at least 3 K/s, preferably at least 5 K/s, in particular at least 10 K/s, preferably at least 15 K/s.
• the average heating rate rOven is to be understood as the average heating rate from 30°C to 700°C.
• the heating takes place in an oven with an oven temperature T oven of at least 850° C., preferably at least 880° C., particularly preferably at least 900° C., in particular at least 920° C., and at most 1000° C., preferably at most 950° C, particularly preferably a maximum of 930°C.
• T oven of at least 850° C., preferably at least 880° C., particularly preferably at least 900° C., in particular at least 920° C., and at most 1000° C., preferably at most 950° C, particularly preferably a maximum of 930°C.
• the dew point in the oven is preferably at least -20°C, preferably at least -15°C, in particular at least -5°C, preferably at least 0°C, particularly preferably at least 5°C and at most +25°C, preferably at most +20° C in particular a maximum of +15°C.
• the heating in step b) takes place in stages in areas with different temperatures.
• the heating takes place in a roller hearth furnace with different heating zones.
• the heating takes place in a first heating zone at a temperature (so-called furnace inlet temperature) of at least 650°C, preferably at least 680°C, in particular at least 720°C.
• the maximum temperature in the first heating zone is preferably 900°C, in particular a maximum of 850°C.
• the maximum temperature of all heating zones in the furnace is preferably at most 1200°C, in particular at most 1000°C, preferably at most 950°C, particularly preferably at most 930°C.
• the total time in the oven t oven which is made up of a heating time and a holding time, is preferably at least 1 minute, preferably at least 2 minutes, for both variants (constant oven temperature, gradual heating) for sheet metal thicknesses of 1.5 mm or less. Furthermore, the total time in the oven for such sheets in both variants is preferably a maximum of 10 minutes, in particular a maximum of 8 minutes, preferably a maximum of 6 minutes, particularly preferably a maximum of 4 minutes.
• the total time in the oven t oven is in particular at least 1.5 minutes, preferably at least 2 minutes, preferably at least 3 minutes. Furthermore, the total time in the oven for such sheets in both variants is preferably a maximum of 12 minutes, in particular a maximum of 10 minutes, preferably a maximum of 8 minutes, particularly preferably a maximum of 6 minutes.
• the blank heated in this way is removed from the respective heating device, which can be, for example, a conventional heating furnace, an induction heating device that is also known per se, or a conventional device for keeping steel components warm, and transported into the forming tool so quickly that its temperature during arrival in the tool is at least partially above Ms+100°C, preferably above 600°C, in particular above 650°C, particularly preferably above 700°C.
• Ms denotes the martensite start temperature.
• the temperature is at least partially above the AC1 temperature.
• the temperature is in particular a maximum of 900°C. Overall, these temperature ranges ensure good formability of the material.
• the austenitized blank is transferred from the heating device used in each case to the forming tool within preferably a maximum of 20 s, in particular a maximum of 15 s. Such rapid transport is necessary to avoid excessive cooling before deformation.
• the tool When inserting the blank, the tool typically has a temperature between room temperature (RT) and 200°C, preferably between 20°C and 180°C, in particular between 50°C and 150°C.
• the tool can optionally be tempered at least in regions to a temperature T WZ of at least 200° C., in particular at least 300° C., in order to only partially harden the component.
• the tool temperature Twz is preferably at most 600°C, in particular at most 550°C. It is only necessary to ensure that the tool temperature Twz is below the desired target temperature T target .
• the residence time in the tool twz is preferably at least 2s, in particular at least 3s, particularly preferably at least 5s.
• the maximum residence time in the tool is preferably 25 s, in particular a maximum of 20 s.
• the target temperature T target of the sheet metal part is at least partially below 400°C, preferably below 300°C, in particular below 250°C, preferably below 200°C, particularly preferably below 180°C, in particular below 150°C.
• the target temperature T target of the shaped sheet metal part is particularly preferably below Ms ⁇ 50° C., with Ms denoting the martensite start temperature.
• the target temperature of the sheet metal part is preferably at least 20°C, particularly preferably at least 50°C.
• AC 3 ° C 902 ⁇ 225 * % C + 19 * % si ⁇ 11 * % Mn ⁇ 5 * % Cr + 13 * % Mon ⁇ 20 * % no + 55 * % V ° C / weight .
• the blank is not only formed into the shaped sheet metal part, but is also quenched to the target temperature at the same time.
• the cooling rate in the tool rwz to the target temperature is in particular at least 20 K/s, preferably at least 30 K/s, in particular at least 50 K/s, in a particular embodiment at least 100 K/s.
• the shaped sheet metal part is cooled to a cooling temperature TAB of less than 100° C. within a cooling time t AB of 0.5 to 600 s. This is usually done by air cooling.
• the intermediate products which can also be referred to as pre-strips in hot strip rolling, each had an intermediate product temperature T2 of 1100° C. at the end of the pre-rolling phase.
• the pre-strips were fed to finish-rolling immediately after rough-rolling, so that the intermediate product temperature T2 corresponds to the rolling start temperature for the finish-rolling phase.
• the pre-strips were rolled out to form hot strip with a final thickness of 4 mm and a final rolling temperature T3 of 890° C respective coiling temperature and wound up into coils at a coiling temperature T4 of 580°C and then cooled in still air.
• the hot strip was descaled in a conventional manner by pickling before being subjected to cold rolling until the thickness given in Table 4 was obtained.
• the cold-rolled flat steel products were heated to an annealing temperature T5 of 870° C. in a continuous annealing furnace and held at the annealing temperature for 100 s before being cooled at a cooling rate of 1 K/s to the immersion temperature T6 given in Table 3.
• the cold strips were passed through a molten coating bath at temperature T7 at their respective immersion temperature T6.
• the belt speed was 76 m/min in all cases.
• the composition of the coating bath is given in Table 2.
• the coated ribbons were blown off to set the lay weights. For this purpose, an air stream with a flow pressure that is given in Table 3 was used. The temperature of the air stream was 70°C in all cases.
• the strips were first cooled to 600°C at an average cooling rate of 10-15 K/s. In the further course of cooling between 600°C and 450°C and between 400°C and 300°C, the strips were cooled over the cooling times TmT of 18s and TnT of 15s. Between 450°C and 400°C and below 220°C, the strips were cooled at a cooling rate of 5 - 15 K/s each.
• Table 6 summarizes which steel grade (see Table 1) was combined with which coating variant (see Table 2), which production variant (see Table 3), which preheating variant (see Table 5) and which dimensions (see Table 4).
• Table 6 also shows the proportion of diffusible hydrogen in the steel substrate of the flat steel product produced in this way. This proportion is given in ppm. 1 ppm corresponds to a proportion of 0.0001% by weight.
• the production variant E4 and thus the test T5 is a reference example that is not according to the invention.
• the steel strips produced in this way were each cut into blanks which were used for further tests.
• sheet metal part samples 1 - 9 in the form of 200 x 300 mm2 large plates were hot-press formed from the respective blanks.
• the blanks are heated from room temperature at a medium heating rate in a heating device, for example in a conventional heating oven of 6 K/s (in the temperature range between 30°C and 700°C) in a furnace with a furnace temperature T furnace of 920°C.
• the total time in the furnace, including heating and holding, is denoted t furnace and is given in Table 7.
• the blanks were then removed from the heating device and placed in a forming tool that had been heated to room temperature RT.
• the blanks When removed from the oven, the blanks had taken on the oven temperature.
• the transfer time made up of the removal from the heating device, transport to the tool and insertion into the tool was around 10 s.
• the temperature of the blanks when they were placed in the forming tool was above the respective AC1 temperature.
• the forming tool had a temperature Twz of 60° C.
• the blanks were formed into the respective shaped sheet metal part in the forming tool, the shaped sheet metal parts being cooled in the tool at a cooling rate rwz of 50 K/s. Finally, the samples were cooled to room temperature. The cooling took place in still air at a cooling rate of 7 K/s.
• Table 7 also gives the properties of the sheet metal parts obtained in this way.
• the coating weights were between 20.0 and 22.0 g/m ⁇ 2 for all variants according to the invention.
• reference example T5 has a significantly lower Fe content, but higher proportions of Si and Mg. Based on the higher coating weight and thus the layer thickness, the diffusion of Fe into the coating in reference example T5 is much less advanced, although the time in the oven (tofen) was significantly increased compared to the examples according to the invention.
• the reference example T10 also has a lower coating weight of 20.3 g/m ⁇ 2, but no alkali or alkaline earth metals in the coating. The resulting lower hardness values in the coating are clearly visible.
• the coating was also analyzed in the cross section. All tests showed a structure with an Al base layer and an alloy layer, with the alloy layer lying on the steel substrate and the Al base layer lying on the alloy layer.
• the cuts produced were also specifically examined for their hardness.
• the Al base layer, the alloy layer and the substrate area close to the surface were sampled separately.
• the nano-hardness and the penetration modulus were determined with a nano-indenter.
• a Berkovich pyramid was used as a test syringe.
• the measurement was carried out using a load function with a maximum load of 200 ⁇ N.
• the measurement was carried out with a load function with a maximum load of 2000 ⁇ N. It can be clearly seen that in all three areas harder structures have been set in the samples according to the invention than in the reference example.
• the structure of the steel substrate was also determined using the cross-sections. In all cases, a martensite content of more than 95% by area was found.

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• 2021
  • 2021-11-02 EP EP21205912.5A patent/EP4174207A1/fr active Pending
• 2022
  • 2022-11-01 US US17/978,246 patent/US11891676B2/en active Active

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