Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • 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
  • C21D1/26—Methods of 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
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—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
  • 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
  • C22C38/02—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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • 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
  • C23C2/29—Cooling or quenching

Definitions

• the invention relates to a coated flat steel product which is suitable for press hardening and which has particularly good aging resistance, as well as a method for its production.
• Blanks are generally understood to mean sheet metal, which can have more complex outlines than the steel strips or steel sheets from which they emerge.
• Steels are used in body construction, which are subject to high requirements in terms of their mechanical properties but also in terms of their processing behavior.
• a flat steel product which is formed into a steel component, goes through various manufacturing steps. Among other things, it is cold-formed. This can be done, for example, by straightening, cutting or reshaping. Good cold forming behavior can be seen, among other things, in good dimensional accuracy, quality of the cut edges and a more even surface of the cold-formed parts. Good cold forming behavior is favored by steels with a low yield point and high uniform elongation. Steels whose yield strength is ideally continuous or only weakly pronounced have proven to be particularly good for processing.
• yield strengths are often referred to as yield strengths.
• the aging of steel is caused by free carbon in the ferrite.
• the solubility of carbon in ferrite is significantly greater than at room temperature, so that a certain free carbon content is established.
• Temperatures of over 300 ° G are usually reached in coating processes such as hot dip coating. With the temperature and time profiles typical for coating processes, carbon can diffuse in the steel. The proportion of free carbon at room temperature is then significantly greater than the equilibrium content, since the approach to thermodynamic equilibrium requires a longer period of time than is available during the cooling to room temperature following the coating.
• the ferrite is then very heavily oversaturated with carbon. As an interstitial alloying element, however, carbon can still diffuse very slowly even at room temperature and attach to defects such as dislocations.
• This phenomenon is also known as aging and the interstitially dissolved atoms attached to the imperfections as Cottrell clouds.
• the dislocations are blocked by the carbon, so that there is a pronounced yield point, which is very undesirable for cold forming.
• straightening the flat steel product is made more difficult by the discontinuous deformation behavior.
• the increased deformation resistance leads to increased tool wear when cutting blanks and a possible subsequent deep-drawing cold forming leads to an uneven, uneven surface.
• aging of the steel due to free carbon should be prevented or at least mitigated as far as possible.
• a flat steel product which is formed from a steel containing 0.2-0.5% by weight of C, 0.5-3.0% by weight of Mn, 0.002-0.004% by weight of B and optionally one or contains several elements of the group "Si, Cr, Al, Ti" in the following contents: 0.1-0.3% by weight Si, 0.1-0.5% by weight Cr, 0.02-0, 05% by weight Al, 0.025-0.04% by weight Ti.
• the flat steel product is coated with an anti-corrosion coating made from an aluminum-zinc alloy.
• the coated flat steel product is intended for the production of a component by means of press hardening.
• Correspondingly designed flat steel products are only slightly resistant to aging and have a very pronounced yield point after coating and aging.
• a steel sheet which, in% by mass, consists of 0.18-0.35% C, 1.0-3.0% Mn, 0.01-1.0% Si, 0.001-0.02 % P, 0.0005-0.01% S, 0.001-0.01% N, 0.01-1.0% Al, 0.005-0.2% Ti, 0.0002-0.005% B and 0.002-2 , 0% Cr and the remainder of Fe and unavoidable impurities, with a proportion of ferrite in the structure of this flat steel product being 50% or more and a proportion of non-recrystallized ferrite being 30% or less in volume%.
• EP 2 703 511 A1 a steel sheet for a component obtained by hot press molding is known.
• This steel sheet consists of, in% by mass, 0.10 - 0.35% C, 0.01 - 1.0% Si, 0.3 - 2.3% Mn, 0.01 - 0.5% Al, ⁇ 0.03% P, ⁇ 0.02% S, ⁇ 0.1% N, remainder Fe and unavoidable Impurities.
• the standard deviation of the diameter of iron carbides that are present in the structure of the steel sheet in an area that extends from the surface to 1 ⁇ 4 of the thickness of the steel sheet should be 0.8 mm.
• the sheets can be provided with an AlSi protective layer applied in the process.
• this steel sheet contains, in% by mass, 0.15 to 0.45% C, 0.5 to 3.0% Mn + Cr, 0.05% P, 0.03% S, 0.5 % Si and ⁇ 1% Al.
• this steel sheet has a structure in which carbides are dispersed in ferrite, the average grain diameter D ( ⁇ m) of the ferrite being 3 to 13 ⁇ m, the average opening intervals ⁇ ( ⁇ m) of the dispersed carbides being ⁇ 5 ⁇ m and at the same time the condition D. ⁇ 90 ⁇ 2 is fulfilled.
• the steel sheet produced in this way should have a yield strength Rp0.2 of 310 to 400 MPa, a tensile strength ⁇ 400 MPa, a uniform elongation ⁇ 12% and a total elongation ⁇ 20%.
• the invention is based on the object of providing a coated flat steel product which is suitable for press hardening and has good aging resistance, as well as a method for its production.
• carbon has a retarding effect on the formation of ferrite and bainite. At the same time, austenite is stabilized and the Ac3 temperature is reduced.
• the carbon content of the steel of a flat steel product according to the invention is limited to 0.10 and 0.4% by weight. A carbon content of at least 0.10% by weight is required to ensure the hardenability of the flat steel product and the tensile strength of the press-hardened product at least 1000 MPa. If a higher level of strength is to be aimed for, C contents of at least 0.15% by weight are preferred.
• 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 0.4% by weight have a disadvantageous effect on the mechanical properties of the flat steel product, since C contents greater than 0.4% by weight promote the formation of brittle martensite during press hardening.
• the weldability can also be adversely affected by high carbon contents.
• the carbon content can preferably be set to 0.3 wt% or less.
• Silicon is used to further increase the hardenability of the flat steel product and the strength of the press-hardened product via solid solution strengthening. Silicon also enables ferro-silicon-manganese to be used as an alloying agent, which has a positive effect on production costs.
• a hardening effect occurs from an Si content of 0.05% by weight. From an Si content of at least 0.15% by weight, in particular at least 0.20% by weight, there is a significant increase in strength. Si contents above 0.5% by weight have a disadvantageous effect on the coating behavior, in particular in the case of Al-based coatings. Si contents of at most 0.4% by weight, in particular at most 0.30% by weight, are preferably set in order to improve the surface quality of the coated flat steel product.
• Manganese acts as a hardening element by greatly delaying the formation of ferrite and bainite. If the manganese content is less than 0.5% by weight, ferrite and bainite are formed during press hardening, even at very fast cooling rates, which should be avoided. Mn contents of at least 0.9% by weight, in particular at least 1.10% by weight, are preferred if a martensitic structure is to be ensured, especially in areas of greater deformation. Manganese contents of more than 3.0% by weight have a disadvantageous effect on the processing properties, which is why the Mn content of flat steel products according to the invention is limited to a maximum of 3.0% by weight. Above all, the weldability is severely restricted, which is why the Mn content is preferably limited to a maximum of 1.6% by weight and in particular to 1.30% by weight. Manganese contents less than or equal to 1.6% by weight are also preferred for economic reasons.
• Aluminum is used as a deoxidizer to bind oxygen.
• aluminum inhibits the formation of cementite.
• at least 0.01% by weight, in particular at least 0.02% by weight, of aluminum is required in the steel.
• the Al content is limited to 0.2% by weight. From a content of 0.2% by weight, Al hampers the transformation into austenite too much before press hardening, so that austenitizing can no longer be carried out in a time and energy-efficient manner.
• an Al content of at most 0.1% by weight, in particular at most 0.05% by weight, is preferably set to completely cover the steel to austenitize.
• Chromium is added to the steel of a flat steel product according to the invention in contents of 0.005-1.0% by weight. Chromium influences the hardenability of the flat steel product by slowing down the diffusive transformation during press hardening. Chromium has a favorable effect on hardenability in steel flat products according to the invention from a content of 0.005% by weight, with a Cr content of at least 0.1% by weight, in particular at least 0.18% by weight, for reliable process management, in particular to prevent bainite formation, is preferred. If the steel contains more than 1.0% by weight of chromium, the coating behavior deteriorates. In order to obtain a good surface quality, the Cr content can preferably be limited to a maximum of 0.4% by weight, in particular to a maximum of 0.28% by weight.
• Chromium is a carbide former and as such lowers the amount of dissolved carbon present in the flat steel product. This applies above all to slow cooling of the flat steel product with cooling rates of at most 25 K / s or at most 20 K / s, as occurs during cooling of the coated flat steel product to room temperature in the temperature range between 600 ° C and 450 ° C or in the temperature range between 400 ° C and 220 ° C.
• the carbon atoms bound by chromium do not diffuse into dislocations and do not block them, so that the formation of a pronounced yield point is reduced or completely suppressed.
• the Cr content is selected in such a way that when a coating process is carried out before coating, only a small amount of carbon is bound by chromium and the formation of chromium carbides takes place primarily during the cooling that takes place after the coating.
• the chromium carbides represent preferred nucleation sites for other precipitates such as vanadium carbides and vice versa. This leads to a faster setting of the still free carbon, so that the formation of a pronounced yield point is further reduced or completely suppressed.
• Vanadium (V) is of particular importance in the steel of a flat steel product according to the invention. Vanadium is a very carbon-affine element. When vanadium is free, that is, in the unbound or dissolved state, it can bind supersaturated dissolved carbon in the form of carbides or clusters or at least reduce its diffusion rate. It is crucial that V is in a dissolved state. Surprisingly, very low V contents in particular have proven to be particularly favorable for the aging resistance. With higher V contents, larger vanadium carbides can form even at higher temperatures, which then no longer dissolve at temperatures of 650-900 ° C, which are typical for continuous annealing in hot-dip coating systems.
• vanadium of 0.001% by weight can prevent free carbon from attaching to dislocations. From a V content of 0.2% by weight, there is no longer any improvement in the aging resistance due to vanadium.
• the anti-aging effect of vanadium is particularly pronounced at contents of up to 0.009% by weight, with a maximum effect starting from a preferred content of 0.002% by weight.
• the vanadium content can in a preferred embodiment be restricted to a maximum of 0.004% by weight, in particular to a maximum of 0.003% by weight.
• Vanadium carbides are increasingly formed at contents greater than 0.009% by weight.
• vanadium carbides cannot be dissolved at temperatures of 700 to 900 ° C., which are typical for annealing temperatures in a hot-dip coating system, for example.
• temperatures of 700 to 900 ° C. which are typical for annealing temperatures in a hot-dip coating system, for example.
• vanadium carbides With an increasing vanadium content, there is not inevitably more free vanadium available, since the elimination kinetics of vanadium carbides are accelerated further and further, so that the vanadium carbides become larger and more stable, but the proportion of dissolved vanadium does not increase any further. This effect occurs in particular with contents of more than 0.030% by weight, which is why the content is preferably set to values of at most 0.030% by weight.
• vanadium in addition to reducing aging effects, also contributes to increasing strength through precipitation strengthening, higher contents of up to 0.2% by weight can preferably be set to increase strength.
• the vanadium content of the steel of a flat steel product according to the invention is limited to 0.002 to 0.009% by weight.
• Phosphorus (P) and sulfur (S) are elements that are introduced into the steel as impurities by 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 the impact energy, deteriorate with increasing P content or S content. From P content of 0.1% by weight, the martensite becomes increasingly brittle, which is why the P content of a flat steel product according to the invention is limited to at most 0.1% by weight, in particular at most 0.02% by weight.
• the S content of a flat steel product according to the invention is limited to a maximum of 0.05% by weight, in particular a maximum of 0.003% by weight.
• Nitrogen (N) is present in steel in small amounts due to the steel making process.
• the N content is to be kept as low as possible and should be at most 0.02% by weight.
• nitrogen is harmful, since it prevents the conversion-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, in particular at most 0.007% by weight, should be.
• Boron, niobium, nickel, copper, molybdenum and tungsten can optionally be added to the steel of a flat steel product according to the invention individually or in combination with one another.
• Boron can optionally be added to the alloy in order to improve the hardenability of the flat steel product, in that boron atoms or boron precipitates deposited on the austenite grain boundaries reduce the grain boundary energy, whereby the nucleation of ferrite is suppressed during press hardening.
• a clear effect on the hardenability occurs at contents of at least 0.0005% by weight, in particular at least 0.0020% by weight.
• 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.
• the boron content is limited to at most 0.01% by weight, in particular at most 0.0035% by weight.
• titanium is also preferred to bind nitrogen alloyed.
• the Ti content should preferably be at least 3.42 times the nitrogen content.
• Titanium (Ti) is an essential micro-alloy element 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 enables boron to develop its strong ferrite-inhibiting effect.
• For sufficient binding of nitrogen at least 3.42 times the nitrogen content is required, with at least 0.023% by weight of Ti being added according to the invention for sufficient availability. From 0.1% by weight Ti, the cold-rollability and recrystallizability deteriorate significantly, which is why larger Ti contents should be avoided. In order to improve cold rollability, the Ti content is limited to 0.038 wt%.
• Niobium can optionally be added in order to contribute to grain refinement from a content of 0.001% by weight. However, niobium impairs the recrystallizability of the steel. If the Nb content exceeds 0.1% by weight, the steel can no longer be recrystallized in conventional continuous furnaces prior to hot-dip coating. In order to reduce the risk of deterioration in recrystallizability, the Nb content can preferably be restricted to 0.003% by weight.
• Copper (Cu) can optionally be added in order to increase the 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, which is why the Cu content is limited to a maximum of 0.8% by weight, preferably a maximum of 0.10% by weight is.
• Nickel (Ni) stabilizes the austenitic phase and can optionally be added to it in order to reduce the Ac3 temperature and suppress the formation of ferrite and bainite. Nickel also has a positive influence on hot rollability, especially if the steel contains copper. Copper deteriorates hot rollability. To counteract the negative influence of copper on hot rollability, 0.01% by weight of nickel can be added to the steel. For economic reasons, the nickel content should remain limited to a maximum of 0.4% by weight, in particular a maximum of 0.10% by weight.
• Molybdenum (Mo) can optionally be added to improve process stability, as it significantly slows down the formation of ferrite. From a content of 0.002% by weight, molybdenum-carbon clusters up to ultra-fine molybdenum carbides form dynamically on the grain boundaries, which significantly slow down the mobility of the grain boundary and thus diffusive phase transformations. In addition, molybdenum lowers the grain boundary energy, which lowers the rate of nucleation of ferrite. Because of the high costs associated with an alloy of molybdenum, the content should be at most 1.0% by weight, preferably at most 0.1% by weight.
• Tungsten (W) can optionally be added in contents of 0.001-1.0% by weight to slow down the formation of ferrite. A positive effect on the hardenability already results 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 flat steel product according to the invention has a high uniform elongation Ag of at least 11.5% after coating.
• the yield point of a flat steel product according to the invention has a continuous profile or is only slightly pronounced. In the context of the invention, continuous course means that there is no pronounced yield point is present.
• a yield point with a continuous curve can also be referred to as the yield point Rp0.2.
• a low yield strength is understood to mean a pronounced yield strength at which the difference ⁇ Re between the upper yield limit value ReH and the lower yield limit value ReL is at most 45 MPa.
• the method according to the invention for producing a coated flat steel product which is suitable for press hardening and which has particularly good aging resistance comprises the working steps mentioned in claim 5.
• 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 slab continuous casting or thin slab continuous casting.
• step b) the semi-finished product is heated through at a temperature (T1) of 1100 - 1400 ° C. If the semifinished product has cooled down after casting, the semifinished product is first reheated to 1100 - 1400 ° C to heat it through.
• the through-heating temperature should be at least 1100 ° C in order to ensure good deformability for the subsequent rolling process.
• the soak temperature should not be more than 1400 ° C in order to avoid fractions of 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 roughing.
• Thick slabs that are to be rolled into hot strip can, if necessary, be subjected to pre-rolling.
• the temperature of the intermediate product (T2) at the end of the rough rolling should be at least 1000 ° C, so that the intermediate product contains enough heat for the subsequent work step of finish rolling.
• high rolling temperatures can also promote grain growth during the rolling process, which has a detrimental effect on the mechanical properties of the flat steel product.
• the temperature of the intermediate product should not be more than 1200 ° C. at the end of the roughing process.
• step d) the slab or thin slab or, if step c) has been carried out, the intermediate product is rolled into a hot-rolled flat steel product. If step c) has been carried out, the intermediate product is finish-rolled immediately after roughing. Typically, finish rolling begins no later than 90 seconds after the end of roughing.
• the slab, the thin slab or, if work step c) has been carried out, the intermediate product are rolled out at a final rolling temperature (T3).
• the final rolling temperature i.e. the temperature of the finished hot-rolled flat steel 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, for example.
• the final rolling temperature is limited to a maximum of 1000 ° C in order to prevent coarsening of the austenite grains.
• final rolling temperatures of at most 1000 ° C are process-related relevant for setting reel temperatures (T4) below 700 ° C.
• the flat steel product can be hot rolled as continuous hot strip rolling or as reversing rolling.
• work 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 in order to avoid the formation of large vanadium carbides. In principle, the coiling temperature is not restricted below. However, coiling temperatures of at least 500 ° C have proven to be favorable for cold rollability.
• the coiled hot strip is then cooled to room temperature in the conventional manner in air.
• 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 which 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 demands on the thickness tolerances of the flat steel product.
• the degree of cold rolling (KWG) should be at least 30% in order to introduce enough deformation energy into the flat steel product for rapid recrystallization.
• the flat steel product before cold rolling is usually a hot strip with a thickness of d.
• the flat steel product after cold rolling is usually also referred to as cold strip.
• the degree of cold rolling can in principle assume very high values of over 90%. However, degrees of cold rolling of at most 80% have proven to be beneficial for avoiding strip tears.
• 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., in order to keep vanadium in solution.
• vanadium carbide separates out at V contents of 0.002% by weight and temperatures above 650 ° C. or vanadium carbides already formed no longer dissolve.
• very fine vanadium carbides are thermodynamically unstable due to their high surface energy.
• This effect is used in the present invention to bring vanadium into solution at temperatures of 650-900 ° C. or to keep vanadium already dissolved in solution, which has a positive effect on the aging resistance of the flat steel product.
• annealing temperatures above 900 ° C there is no improvement in the Achieved aging resistance, which is why the annealing temperature is limited to 900 ° C for economic reasons.
• step i) the flat steel product is cooled to a pre-cooling 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 coating bath.
• the pre-cooling temperature is 600-800.degree. C., preferably at least 640.degree. C., particularly preferably at most 700.degree.
• the duration of the cooling of the annealed flat steel product from the annealing temperature T5 to the pre-cooling temperature T6 is preferably 10-180 s.
• the flat steel product is subjected to a coating treatment in step j).
• the coating treatment is preferably carried out by means of continuous hot dip coating.
• the coating can only be applied on one side, on both sides or on all sides of the flat steel product.
• the coating treatment is preferably carried out as a hot-dip coating process, in particular as a continuous process.
• the flat steel 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 640 - 720 ° C. Alloys based on aluminum have proven to be particularly suitable for coating aging-resistant flat steel products with an anti-corrosion coating.
• the molten bath which contains the anti-corrosion coating to be applied to the flat steel product in liquid form, typically contains, in addition to aluminum, 3-15% by weight silicon, preferably 9-12% by weight silicon, up to 5% by weight iron and up to 0% , 5% by weight of unavoidable impurities, the sum of the constituents present being 100% by weight.
• Inevitable Impurities can be, for example, unavoidable amounts of chromium, manganese, calcium or tin.
• the coated flat steel product is cooled to room temperature in step k).
• the cooling rate is set in such a way that the largest possible proportion of oversaturated, dissolved carbon can be bound by vanadium.
• the mean cooling rate (CR1) should therefore be at most 25 K / s, preferably at most 18 K / s, in a temperature range which is optimal for the precipitation kinetics of vanadium and which for flat steel products with a composition according to the invention is between 600 ° C and 450 ° C , particularly preferably not more than 12 K / s.
• the extent to which free carbon is bound by vanadium increases if the cooling takes place in a temperature range between 400 ° C and 220 ° C with a lower cooling rate than in the temperature range between 600 ° C and 450 ° C.
• the mean cooling rate (CR2) is therefore between 400 ° C. and 220 ° C. at most 20 K / s, preferably 14 K / s, particularly preferably at most 9.5 K / s.
• the free carbon of the flat steel product still has a diffusion rate sufficient for recombination with vanadium, which promotes the setting of free carbon.
• the driving force for the growth of vanadium carbides is particularly high in this temperature range, which also binds free carbon. This applies in particular to V contents of 0.002-0.009% by weight.
• the driving force for the formation of iron carbides which preferentially germinate on existing carbides of the micro-alloying elements such as vanadium, niobium or titanium, is particularly high.
• iron carbides free carbon is also bound, which has a positive effect on aging behavior.
• the cooling rate has no significant influence on the aging resistance.
• an average cooling rate of at most 25 K / s is preferably set between the annealing temperature and 600 ° C and between 450 ° C and 400 ° C and an average cooling rate of at most 20 K / s between 220 ° C and room temperature.
• the mean cooling rate is preferably at least 0.1 K / s in each of the individual temperature ranges.
• the mean cooling rate is understood to mean the average cooling rate, which results purely arithmetically from the quotient of the temperature difference of the cooling temperature range under consideration by the time required for cooling in this temperature range. For example, for cooling from a temperature TX to a temperature TY: (TX-TY) / ⁇ t, where TX is the temperature at the beginning of cooling in K, TY is the temperature at the end of cooling in K and ⁇ t is the duration of cooling from TX are on TY in s.
• the cooling can be carried out as slowly as desired, since the proportion of free carbon decreases continuously, which improves the tendency to aging. Due to technical conditions and for economic reasons, the cooling rate of the entire cooling process, i.e. the cooling of the coated flat steel product after exiting the coating bath until it reaches room temperature, can be limited to values of typically at least 0.1 K / s.
• a corrosion protection coating lying on the steel substrate after cooling down typically contains 3-15% by weight silicon, preferably 9-12% by weight silicon, particularly preferably 9-10% by weight silicon, up to 5% by weight iron, up to 0.5% by weight unavoidable impurities and the remainder aluminum.
• Unavoidable impurities can be, for example, unavoidable proportions of chromium, manganese, calcium or tin.
• the coating composition can be determined, for example, with the aid of glow discharge spectroscopy (GDOES).
• the coated flat steel product can optionally be subjected to skin passaging with a skin pass degree of up to 2% in order to improve the surface roughness of the flat steel product.
• a flat steel product produced according to the invention is suitable for press hardening and has a corrosion protection coating, a high uniform elongation Ag of at least 11.5% and a continuous yield point or a pronounced yield point at which the difference between the upper and lower yield point is at most 45 MPa .
• the continuous yield point or the lower yield point is at least 380 MPa, preferably at least 400 MPa, in particular more than 400 MPa, and particularly preferably at least 410 MPa and very particularly preferably at least 420 MPa.
• the flat steel product has a tensile strength of at least 540 MPa, particularly preferably at least 550 MPa and very particularly preferably at least 560 MPa.
• the slabs were first pre-rolled to an intermediate product with a thickness of 40 mm, the intermediate products, which can also be referred to as pre-strips in hot strip rolling, each having an intermediate product temperature T2 at the end of the pre-rolling phase.
• the preliminary strips were fed to finish rolling immediately after roughing, so that the intermediate product temperature T2 corresponds to the rolling start temperature for the finish rolling phase.
• the pre-strips were rolled into hot strips with a final thickness of 3-7 mm and the respective final rolling temperatures T3 given in Table 2, cooled to the respective coiling temperature and wound into coils at the respective coiling temperatures T4 and then cooled in still air.
• the hot strips were descaled in a conventional manner by means of pickling before they were subjected to cold rolling with the cold rolling degrees indicated in Table 2.
• the cold-rolled flat steel products were heated in a continuous annealing furnace to a respective annealing temperature T5 and held at annealing temperature for 100 s each before they were cooled to their respective pre-cooling temperature T6 at a cooling rate of 1 K / s.
• the cold strips were passed through a molten coating bath at temperature T7 at their respective pre-cooling temperature T6.
• the composition of the coating bath was as follows: 9% by weight Si, 2.9% by weight Fe, 87.8% by weight aluminum and the remainder unavoidable Impurities.
• the coated strips were blown off in a conventional manner, whereby an application of the coating of 150 g / m 2 was produced.
• the strips were initially cooled to 600 ° C. at an average cooling rate of 10-15 K / s.
• the strips were each cooled at the cooling rates CR1 and CR2 indicated in Table 2.
• the strips were cooled between 450 ° C. and 400 ° C. and below 220 ° C. at a cooling rate of 5-15 K / s.
• the type of yield strength which is designated Re for a pronounced yield point and Rp for a continuous yield point, as well as the value for the yield strength Rp0.2 for a continuous yield point and the values for the lower yield point ReL, the upper yield point ReH and the difference between the upper and lower yield point ⁇ Re, the tensile strength Rm, the uniform elongation Ag and the elongation at break A80.
• All samples have a continuous yield point Rp or an only slightly pronounced yield point with a difference ⁇ Re between the upper and lower yield point of no more than 41 MPa and a uniform elongation Ag of at least 11.5%.

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• 2017
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