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 suitable for a press-hardened coated flat steel product, which has a particularly good aging resistance, and a process for its preparation.
• flat steel products refers to steel strips, steel sheets or blanks obtained therefrom and the like.
• Under sinkers are usually understood metal sheets that can have more complex contours than the steel strips or steel sheets from which they emerge.
• a flat steel product which is formed into a steel component, undergoes various manufacturing steps. Among other things, it is cold formed. This can be done for example by straightening, cutting or forming. A good cold forming behavior is shown, inter alia, in a good dimensional stability, quality of the cut edges and more even surface of the cold-formed parts. Good cold working behavior is favored by steels with a low yield strength and a high uniform elongation. Steels whose yield strength ideally runs continuously or only weakly are found to be particularly favorable in processing. Continuous 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 sets a certain free carbon content.
• Temperatures above 300 ° C are usually achieved in coating processes such as in hot dip coating. Carbon can thus diffuse in the steel in the temperature and time curves typical for coating processes.
• the proportion of free carbon at room temperature is then significantly higher than the equilibrium content, since the approach to the 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 supersaturated with carbon. When However, even at room temperature, carbon can still diffuse very slowly interstitially, and deposits on defects, including but not limited to dislocations.
• This phenomenon is also referred to as aging and the interstitially dissolved atoms deposited at the defects as Cottrell clouds.
• the dislocations are blocked by the carbon, resulting in a pronounced yield strength, which is very undesirable for cold forming.
• straightening the flat steel product is made more difficult by the discontinuous deformation behavior.
• the increased resistance to deformation leads to increased tool wear in board trimming and a possible subsequent deep-drawing cold forming leads to an uneven, uneven surface.
• aging of the steel by free carbon should be prevented or at least mitigated where possible.
• a steel flat product which is formed from a steel, the 0.2-0.5 wt .-% C, 0.5-3.0 wt .-% Mn, 0.002-0.004 wt .-% B and optionally one or more elements of the group "Si, Cr, Al, Ti" in the following contents: 0, 1-0.3 wt.% Si, 0, 1-0.5 wt.% Cr, 0, 02-0.05 wt.% Al, 0.025-0.04 wt.% Ti.
• the flat steel product is coated with a corrosion protection coating formed from an aluminum-zinc alloy.
• the coated flat steel product is intended for the production of a component by means of press hardening.
• Correspondingly manufactured flat steel products are only slightly resistant to aging and have a very pronounced yield strength after coating and aging.
• the object of the invention is to make available a press-hardened, coated flat steel product with a good aging resistance and a process for its production.
• this object is achieved by a flat steel product having the features specified in claim 1.
• Advantageous and preferred embodiments of the flat steel product according to the invention are specified in the claims dependent on claim 1.
• the object is achieved by a method having the features mentioned in claim 10.
• Advantageous and preferred embodiments of the method according to the invention are specified in the claims back to claim 10.
• the steel of a flat steel product according to the invention consists, in addition to iron and unavoidable impurities (in% by weight), of 0, 10-0.4% C, 0.05-0.5% Si, 0.5-3.0% Mn, 0 , 01-0.2% Al, 0.005-1.0% Cr, 0.001-0.2% V, ⁇ 0, 1% P, ⁇ 0.05% S, ⁇ 0.02% N and optionally one or more of the elements "B, Ti, Nb, Ni, Cu, Mo, W" in the following contents: B: 0.0005-0.01%, Ti: 0.001-0.1%, Nb: 0.001-0.1%, Ni: 0.01 - 0.4%, Cu: 0.01 - 0.8%, Mo: 0.002 - 1.0%, W: 0.001 - 1.0%.
• the carbon content of the steel of a flat steel product according to the invention is limited to 0, 10 and 0.4 wt .-%.
• 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-cured product at least 1000 MPa. If a higher strength level is desired, it is preferred to set C contents of at least 0.15% 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 0.4% by weight have an adverse effect on the mechanical properties of the flat steel product, since C contents of greater than 0.4% by weight during press-hardening promote the formation of brittle martensite.
• high C contents can adversely affect the weldability.
• the carbon content may preferably be set to at most 0.3 wt%.
• the weldability can be significantly improved again and additionally a good ratio of power and maximum bending angle in the bending test according to VDA238-100 be achieved in the press-hardened state.
• Silicon is used to further increase the hardenability of the flat steel product as well as the strength of the press-hardened product via solid solution strengthening. Silicon also allows the use of ferro-silizio-manganese as an alloying agent, which has a favorable effect on the production costs. From a Si content of 0.05 wt .-% already sets a hardening effect. From a Si content of at least 0.15% by weight, in particular at least 0.20% by weight, a significant increase in strength occurs. Si contents above 0.5% by weight have an adverse effect on the coating behavior, in particular in the case of Al-based coatings. Si contents of at most 0.4% by weight, especially at most 0.30% by weight are preferably adjusted to improve the surface quality of the coated flat steel product.
• Manganese acts as a hardening element, greatly delaying ferrite and bainite formation. At manganese contents 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, in particular in areas of greater transformation. Manganese contents of more than 3.0% by weight have an adverse effect on the processing properties, which is why the Mn content of flat steel products according to the invention is limited to not more than 3.0% by weight.
• the Mn content is preferably limited to at most 1.6 wt%, and more preferably, to 1.30 wt%.
• manganese contents less than or equal to 1.6% by weight are also preferred for economic reasons.
• Aluminum is used as a deoxidizer for binding oxygen.
• aluminum inhibits cementite formation.
• For the secure setting of oxygen at least 0.01% by weight, in particular at least 0.02% by weight, of aluminum in the steel is required.
• the Al content is limited to 0.2 wt%. From a content of 0.2% by weight, Al hinders conversion into austenite before press-hardening too much, so that austenitization can not be performed more efficiently in time and energy.
• an Al content of at most 0.1% by weight, in particular at most 0.05% by weight, is preferably set in order to obtain the Austenitizing steel completely.
• Chromium is added to the steel of a flat steel product of the present invention at levels of 0.005-1.0 weight percent. Chromium influences the hardenability of the flat steel product by slows down the diffusive transformation during press hardening. Chromium acts in the flat steel products according to the invention at a content of 0.005 wt .-% favorable to the hardenability, wherein a Cr content of at least 0, 1 wt .-%, in particular at least 0, 18 wt .-% for a safe process, especially 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 may preferably be limited to at most 0.4% by weight, in particular to at most 0.28% by weight.
• Chromium is a carbide former and, as such, lowers the level of dissolved carbon present in the steel flat product. This is especially true in a slow cooling of the flat steel product with cooling rates of at most 25 K / s or at most 20 K / s, such as during cooling of the coated steel flat 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 takes place.
• the carbon atoms bonded by chromium do not diffuse to dislocations and do not block them, so that the formation of a pronounced yield point is reduced or completely suppressed.
• the Cr content is chosen so that when performing a coating process prior to coating, only a small amount of carbon is bound by chromium and the formation of chromium carbides takes place above all during the cooling occurring after coating.
• the chromium carbides are 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 strength 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 an unbound or dissolved state, it can supersaturate dissolved carbon in the form of carbides or clusters, or at least reduce its rate of diffusion. It is crucial that V is present in a dissolved state. Surprisingly, in particular very low V contents have proved to be particularly favorable for aging resistance. At 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 of hot-dip coating systems.
• vanadium Even the smallest amounts of vanadium from 0.001% by weight may already hinder free carbon during the addition to dislocations. From a V content of 0.2 wt .-% occurs no improvement in aging resistance by vanadium.
• the aging-inhibiting effect of vanadium is particularly pronounced at levels up to 0.009 wt .-%, with a maximum effect from a preferred content of 0.002 wt .-% sets.
• the vanadium content in a preferred embodiment can be limited to at most 0.004% by weight, in particular to at most 0.003% by weight. At contents greater than 0.009 wt .-% vanadium carbides are increasingly formed.
• Vanadium carbides can not be dissolved at temperatures of 700 to 900 ° C., which are typical for annealing temperatures in a hot-dip coating plant, for example, from a vanadium content in the steel of 0.009% by weight. With increasing vanadium content, more free vanadium is not inevitably available, since the precipitation kinetics of vanadium carbides is accelerated more and more, so that although the vanadium carbides are larger and more stable, but the proportion of dissolved vanadium does not increase further. This effect occurs in particular at contents of more than 0.030 wt .-%, which is why the content is preferably set to values of at most 0.030% by weight.
• vanadium also contributes to increasing the strength by precipitation strengthening in addition to the reduction of aging effects
• higher contents of up to 0.2 wt .-% can preferably be adjusted to increase the strength.
• the vanadium content of the steel of a flat steel product according to the invention is limited on the one hand for cost reasons to at most 0.2 wt .-%. On the other hand, higher contents do not significantly improve the mechanical properties.
• Phosphorus (P) and sulfur (S) are elements that are introduced into the steel as iron ore impurities that can not be completely eliminated in the large scale steelmaking process.
• the P-content and the S-content should be kept as low as possible, because the mechanical properties such as the impact energy deteriorate with increasing P content or S content. From P contents of 0.1% by weight, there is also an increasing embrittlement of the martensite, 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 is.
• the S content of a flat steel product according to the invention is limited to at most 0.05% by weight, in particular at most 0.003% by weight.
• Nitrogen (N) is present in small quantities in the steel due to the steelmaking process.
• the N content should be kept as low as possible and should not exceed 0.02% by weight.
• nitrogen is detrimental because it prevents the conversion-retarding effect of boron by the formation of boron nitrides, and therefore the nitrogen content in this case is preferably at most 0.01 wt%, especially at most 0.007 wt%. should be.
• Boron, titanium, niobium, nickel, copper, molybdenum and tungsten may optionally be added to the steel of a flat steel product according to the invention individually or in combination with each other.
• Boron may optionally be added to improve the hardenability of the flat steel product by reducing boron atoms or boron precipitates deposited on the austenite grain boundaries to reduce the grain boundary energy, thereby suppressing the nucleation of ferrite during press hardening.
• a significant effect on the hardenability occurs at levels of at least 0.0005 wt .-%, in particular at least 0.0020 wt .-% on.
• levels above 0.01 wt .-% however, increasingly boron carbides, boron nitrides or Bornitro- carbides, which in turn represent preferred nucleation sites for the nucleation of ferrite and lower the curing effect again.
• the boron content is limited to at most 0.01% by weight, especially at most 0.0035% by weight.
• the Ti content in this case should preferably be at least 3.42 times the content of nitrogen.
• Titanium (Ti) is a micro-alloying element which can optionally be added to contribute to grain refining.
• titanium forms coarse titanium nitrides with nitrogen, which is why the Ti content should be kept comparatively low.
• Titanium breaks down nitrogen and allows boron to develop its strong ferrite-inhibiting action.
• For a sufficient setting of nitrogen at least 3.42 times the nitrogen content is needed, wherein at least 0.001 wt .-% Ti, preferably at least 0.023 wt .-% Ti, should be added for sufficient availability. From 0, 1 wt .-% Ti, the cold rollability and recrystallization deteriorated significantly, which is why larger Ti contents should be avoided.
• the Ti content may preferably be limited to 0.038 wt%.
• Niobium (Nb) may optionally be added to contribute to grain refining from a level of 0.001% by weight. However, niobium degrades the recrystallization of the steel. At an Nb content of more than 0.1 wt%, the steel can no longer be recrystallized in conventional continuous furnaces before the fire coating.
• the content of Nb may preferably be limited to 0.003 wt%.
• Copper (Cu) may optionally be alloyed in order to increase hardenability with additions of at least 0.01% by weight.
• copper improves resistance to atmospheric corrosion of uncoated sheets or cut edges. From a content of 0.8 wt .-%, the hot rollability deteriorates significantly due to low-melting Cu phases on the surface, which is why the Cu content is limited to at most 0.8 wt .-%, preferably at most 0, 10 wt .-% is.
• Nickel (Ni) stabilizes the austenitic phase and can optionally be alloyed to reduce the Ac3 temperature and suppress the formation of ferrite and bainite. Nickel also has a positive effect on hot rollability, especially if the steel contains copper. Copper worsens hot rollability. In order to counteract the negative influence of copper on the hot rollability, 0.01 wt .-% nickel can be added to the steel. For economic reasons, the nickel content should be limited to at most 0.4% by weight, in particular at most 0-10% by weight.
• Molybdenum can optionally be added to improve process stability as it significantly slows ferrite formation. From a content of 0.002% by weight, molybdenum-carbon clusters form dynamically up to ultrafine 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 grain boundary energy, which reduces the nucleation rate of ferrite. Due to the high cost associated with an alloy of molybdenum, the content should be at most 1.0 wt%, preferably at most 0.1 wt%.
• Tungsten (W) may optionally be added to levels of 0.001-1.0 wt% to retard ferrite formation. A positive effect on the hardenability already results at W contents of at least 0.001 wt .-%. For cost reasons, a maximum of 1.0% by weight of tungsten is added.
• a flat steel product according to the invention after coating has a high uniform elongation Ag of at least 11.5%.
• the yield strength of a flat steel product according to the invention has a continuous course or only a small extent.
• continuous course means that there is no pronounced yield strength.
• a yield strength with a continuous course can also be called the yield strength Rp0,2.
• a yield strength with a low degree of expression is understood as meaning a pronounced yield strength at which the difference ARe between the upper yield strength value ReH and the lower yield strength value ReL is at most 45 MPa. The following applies:
• a particularly good aging resistance can be achieved in flat steel products for which the difference ARe is at most 25 MPa.
• the process according to the invention for producing a coated steel flat product suitable for press-hardening, which has particularly good aging resistance comprises the following working steps: a) providing a slab or thin slab consisting of (in% by weight) 0, 10 - 0.4% C, 0.05-0.5% Si, 0.5-3.0% Mn, 0.01-0.2% Al, 0.005-1.0% Cr, 0.001-0.2% V, ⁇ 0 , 1% P, ⁇ 0.05% S, ⁇ 0.02% N and optionally one or more of the elements "B, Ti, Nb, Ni, Cu, Mo, W" in the following contents B: 0.0005-0 , 01%, Ti: 0.001-0.0%, Nb: 0.001-0.0%, Ni: 0.01-0.4%, Cu: 0.01-0.8 ° / o, Mo: 0.002-1.0 ° / o, W: 0.001-1.0% and balance iron and unavoidable impurities; b) heating the slab or thin slab at a temperature (TI) of 1100 - 1400
• a semifinished product composed in accordance with the invention for the alloy steel flat product is made available.
• This can be a slab produced in conventional continuous slab casting or in thin slab continuous casting.
• the semi-finished product is through-heated at a temperature (Tl) of 1100-1400 ° C. If the semifinished product has cooled down after casting, the semi-finished product is first reheated to 1100 - 1400 ° C for heating.
• the heating temperature should be at least 1100 ° C to ensure good ductility for the subsequent rolling process.
• the heating temperature should not exceed 1400 ° C in order to avoid portions of molten phases in the semifinished product.
• the semi-finished product is pre-rolled to an intermediate product.
• Thin slabs are usually not subjected to rough rolling.
• Thick slabs, which are to be rolled out to hot strips, can be subjected to a pre-rolling if necessary.
• the temperature of the intermediate (T2) at the end of roughing should be at least 1000 ° C in order for the intermediate to contain enough heat for the subsequent finish-rolling step.
• high rolling temperatures can also promote grain growth during the rolling process, which adversely affects the mechanical properties of the flat steel product.
• the temperature of the intermediate product at the end of the rough rolling should not exceed 1200 ° C.
• the slab or thin slab or, if operation c) has been carried out, the intermediate product is rolled into a hot rolled flat steel product. If step c) was carried out, the intermediate product is finish rolled immediately after roughing. Typically, finish rolling begins no later than 90 seconds after the end of rough rolling.
• the slab, the thin slab or, if operation c) was carried out, the intermediate product are rolled out at a final rolling temperature (T3).
• the final rolling temperature that is 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 of less than 750 ° C, the amount of free vanadium decreases as larger amounts of vanadium carbides are precipitated.
• the vanadium carbides precipitated during finish rolling are very large. They typically have a mean grain size of 30 nm or more and are no longer dissolved in subsequent annealing processes, such as performed prior to hot dip coating.
• the final rolling temperature is limited to values of at most 1000 ° C to prevent coarsening of the austenite grains.
• final rolling temperatures of at most 1000 ° C. are technically relevant for setting reel temperatures (T4) of less than 700 ° C.
• the hot rolling of the flat steel product can be carried out as a continuous hot strip rolling or as a reversing rolling.
• Operation e) provides for the case of continuous hot strip rolling an optional coiling of the hot rolled flat steel product.
• the hot strip is after hot rolling cooled within less than 50 s to a reel temperature (T4).
• T4 a reel temperature
• the reel temperature (T4) should be at most 700 ° C to avoid the formation of large vanadium carbides.
• the reel temperature is not limited in principle down. However, reel temperatures of at least 500 ° C have proven to be favorable for the cold rollability.
• the coiled hot strip is cooled in a conventional manner in air to room temperature.
• the hot rolled flat steel product is descaled in a conventional manner by pickling or other suitable treatment.
• the hot-rolled flat steel product cleaned of scale can optionally be subjected to cold rolling prior to 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 amount to at least 30% in order to introduce sufficient deformation energy into the flat steel product for rapid recrystallization.
• the cold rolling degree KWG is understood here to mean the quotient of the reduction in thickness in the case of cold rolling AdKW due to the hot strip thickness d:
• the flat steel product before cold rolling is usually a hot strip of hot strip thickness d.
• the flat steel product after cold rolling is commonly referred to as cold strip.
• the degree of cold rolling can in principle be very high values of more than 90%. to take. However, cold rolling degrees of at most 80% have proven to be favorable for avoiding tape breaks.
• 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 to keep vanadium in solution.
• vanadium carbide precipitates 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 at temperatures of 650-900 ° C vanadium in solution or to keep already dissolved vanadium in solution, which has a positive effect on the aging resistance of the flat steel product.
• annealing temperatures above 900 ° C no improvement in the aging resistance is achieved, which is why the annealing temperature is also limited to 900 ° C for economic reasons.
• step i) the steel flat product after annealing is cooled to a pre-cooling temperature (T6) to prepare it for 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 ° C, preferably at least 640 ° C, more preferably at most 700 ° C.
• 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 in contact with the melt bath on all sides, so that it is coated on all sides.
• the melt bath which contains the alloy to be applied to the flat steel product in liquid form, has typically a temperature (T7) of 640 - 720 ° C. Alloys based on aluminum have proven to be particularly suitable for coating age-resistant flat steel products with a corrosion protection coating.
• the melt bath which contains the applied to the flat steel product corrosion protection coating in liquid form, typically contains in addition to aluminum 3-15 wt .-% silicon, preferably 9-12 wt .-% silicon, up to 5 wt .-% iron and up to 0 , 5 wt .-% unavoidable impurities, wherein the sum of the present components is 100 wt .-%.
• Unavoidable impurities may be unavoidable proportions of chromium, manganese, calcium or tin, for example.
• the coated flat steel product is cooled to room temperature in step k).
• the cooling rate is adjusted such that the largest possible proportion of supersaturated dissolved carbon can be bound by vanadium. Therefore, the mean cooling rate (CRI) in a temperature range which is optimal for the precipitation kinetics of vanadium, and which lies between 600 ° C and 450 ° C in flat steel products with inventive composition, at most 25 K / s, preferably at most 18 K / s , more preferably at most 12 K / s.
• the average cooling rate (CR2) should therefore be between 400 ° C and 220 ° C at most 20 K / s, preferably 14 K / s, more preferably at most 9.5 K / s.
• the free carbon of the flat steel product still has a sufficient diffusion rate for recombination with vanadium, which promotes the setting free carbon.
• the driving force for the growth of vanadium carbides is particularly high in this temperature range, which also free carbon is bound. This applies in particular to V contents of 0.002-0.009% by weight.
• the driving force for the formation of iron carbides which germinate preferably on existing carbides of the micro-alloying elements such as vanadium, niobium or titanium, particularly high.
• the formation of iron carbides also binds free carbon, which has a favorable effect on the aging behavior.
• the cooling rate has no significant impact on the aging resistance. For process-technical reasons, an average cooling rate of at most 25 K / s and between 220 ° C.
• an average cooling rate of at most 20 K / s is preferably set between the annealing temperature and 600 ° C. and between 450 ° C. and 400 ° C.
• the average cooling rate is preferably at least 0.1 K / s in the individual temperature ranges.
• the average cooling rate is understood to mean the average cooling rate, which results purely mathematically from the quotient of the temperature difference of the cooling temperature range considered by the time required for cooling in this temperature range.
• TX is the temperature at the beginning of the cooling in K
• TY the temperature at the end of the cooling in K
• At the duration of the cooling of TX on TY in s are.
• the cooling can be carried out arbitrarily slowly, since the proportion of free carbon decreases continuously, which improves the tendency to age. Due to technical conditions and for economic reasons, the cooling rate of the entire cooling process, that is the cooling of the coated steel flat product after leaving 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 resting on the steel substrate after cooling typically contains 3-15% by weight of silicon, preferably 9-12% by weight of silicon, particularly preferably 9-10% by weight of silicon, up to 5% by weight of iron. up to 0.5% by weight of unavoidable impurities and balance aluminum. Unavoidable impurities may be unavoidable proportions of chromium, manganese, calcium or tin, for example.
• the coating composition can be determined, for example, by means of glow discharge spectroscopy (GDOES).
• the coated flat steel product may optionally be subjected to a skin pass-through of up to 2% 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 strength or a pronounced yield strength at which the difference between the upper and lower yield strength is at most 45 MPa ,
• the continuous yield strength or the lower yield strength is at least 380 M Pa, 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, more preferably at least 550 MPa, and most preferably at least 560 MPa.
• slabs were produced with the compositions shown in Table 1 with a thickness of 200-280 mm and width of 1000-1200 mm, heated in a blast furnace to a respective temperature Tl and kept at Tl between 30 and 450 min until the temperature Tl was reached in the core of the slabs and the slabs were thus warmed through.
• the production parameters are given in Table 2.
• the slabs were discharged from the blast furnace with their respective heating temperature Tl and subjected to hot rolling.
• the experiments were carried out as continuous hot strip rolling.
• the slabs were first pre-rolled to an intermediate of thickness 40 mm, wherein the intermediate products, which can also be referred to as pre-bands in the hot strip rolling, each had an intermediate temperature T2 at the end of the rough rolling.
• the pre-belts were fed to the finish rolling immediately after the pre-rolling, so that the intermediate product temperature T2 corresponds to the initial rolling temperature for the finish rolling phase.
• the pre-ribbons were rolled into hot strips having a final thickness of 3-7 mm and the respective final rolling temperatures T3 given in Table 2, cooled to the respective coiler temperature and wound up into coils at the respective reeling 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 shown in Table 2.
• the cold-rolled steel flat products were heated to a respective annealing temperature T5 in a continuous annealing furnace and kept at annealing temperature for 100 s each, before being cooled to their respective pre-cooling temperature T6 at a cooling rate of 1 K / s.
• the cold strips were passed through their respective precooling temperature T6 through a molten coating bath of temperature T7.
• the composition of the coating bath was as follows: 9% by weight of Si, 2.9% by weight of Fe, 87.8% by weight of aluminum and the remainder unavoidable impurities.
• the coated tapes were blown off in a conventional manner, whereby a circulation of the coating of 150 g / m 2 was produced.
• the strips were first cooled to 600 ° C at an average cooling rate of 10-15 K / s.
• the bands were each cooled with the cooling rates CR1 and CR2 shown in Table 2.
• the strips were cooled at a cooling rate of 5 - 15 K / s each.
• the yield strength type which is denoted by Re for a pronounced yield strength and Rp for a continuous yield strength
• the value for the yield strength Rp0.2 with a pronounced yield strength Values for the lower yield strength ReL, the upper yield strength ReH and the difference between the upper and lower yield strength ARe, the tensile strength Rm, the uniform elongation Ag and the elongation at break A80.
• All samples have a continuous yield strength Rp or only a slight yield strength with a difference ARe between upper and lower yield strength of at most 41 MPa and a uniform elongation Ag of at least 11.5%.

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