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 has particularly good aging resistance, and a method for its production.
• Blanks are generally understood to be metal sheets, which can have more complex outlines than the steel strips or steel sheets from which they derive.
• a steel flat product which is formed into a steel component, goes through various production steps. Among other things, it is cold-formed. This can be done, for example, by straightening, cutting or forming. Good cold-forming behavior is reflected, 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 point ideally runs continuously or is only weakly pronounced have proven to be particularly favorable in terms of processing. Continuous yield points are often also referred to as yield points.
• This phenomenon is also referred to as aging and the interstitially dissolved atoms attached to the defects as Cottrell clouds.
• the dislocations are blocked by the carbon, resulting in 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 during blank trimming and 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.
• the end EP 2848709 A1 discloses a steel flat product formed from a steel containing 0.2-0.5 wt% C, 0.5-3.0 wt% Mn, 0.002-0.004 wt% B and optionally one or more elements from 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 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 designed steel flat products are only slightly resistant to aging and have a very pronounced yield point after coating and aging.
• the object of the invention is to provide a coated steel flat product that is suitable for press hardening and has good aging resistance, as well as a method for its production.
• this object is achieved by a flat steel product having the features specified in claim 1 .
• Advantageous and preferred configurations of the flat steel product according to the invention are specified in the claims which refer back to claim 1 .
• the object is achieved by a method having the features specified in claim 10.
• Advantageous and preferred configurations of the method according to the invention are specified in the claims which refer 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 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 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%.
• carbon acts to delay 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 steel flat 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 steel flat product and the tensile strength of the press-hardened product at least 1000 MPa. If a higher level of strength is to be aimed at, preference is given to setting 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 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.
• High C contents can also have a negative impact on weldability.
• the carbon content may preferably be adjusted to at most 0.3% 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 0.05% 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 an adverse effect on the coating behavior, particularly 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 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 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, particularly in areas of greater deformation. 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 a maximum of 3.0% by weight. Above all, the weldability is severely limited, which is why the Mn content is preferably limited to at most 1.6% by weight and in particular to 1.30% by weight. Manganese contents of less than or equal to 1.6% by weight are also preferred for economic reasons.
• Aluminum is used as a deoxidizing agent to bind oxygen. Aluminum also inhibits cementite formation. At least 0.01% by weight, in particular at least 0.02% by weight, of aluminum in the steel is required for reliable binding of oxygen. However, since the Ac3 temperature is also clearly shifted upwards with increasing Al alloy content, the Al content is limited to 0.2% by weight. From a content of 0.2% by weight, Al hinders 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 in order to completely harden the steel to austenitize.
• Chromium is added to the steel of a flat steel product according to the invention in amounts of 0.005-1.0% 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 0.005% by weight, with a Cr content of at least 0.1% by weight, in particular at least 0.18% by weight, for reliable process control, above all for preventing bainite formation, is preferred. If the steel contains more than 1.0% by weight of chromium, the coating behavior deteriorates.
• the Cr content may preferably be limited to at most 0.4% by weight, particularly at most 0.28% by weight.
• Chromium is a carbide former and as such reduces the level of dissolved carbon present in steel flat products. This applies above all to slow cooling of the steel flat product with cooling rates of at most 25 K/s or at most 20 K/s, as occurs during the 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.
• the carbon atoms bound 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 selected in such a way that when a coating process is carried out before the 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 faster binding of the carbon that is still free, 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 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 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 Even the smallest amounts of vanadium of 0.001% by weight can prevent free carbon from adhering to dislocations. From a V salary From 0.2% by weight, vanadium no longer improves the aging resistance.
• the aging-inhibiting effect of vanadium is particularly pronounced at contents of up to 0.009% by weight, with a maximum effect occurring from a preferred content of 0.002% by weight.
• the vanadium content can be restricted to a maximum of 0.004% by weight, in particular to a maximum of 0.003% by weight, in a preferred embodiment. At contents greater than 0.009% by weight, vanadium carbides are increasingly formed.
• vanadium carbides cannot be dissolved at temperatures of 700 to 900° C., which are typical for annealing temperatures in a hot-dip coating plant, for example.
• temperatures of 700 to 900° C. which are typical for annealing temperatures in a hot-dip coating plant, for example.
• vanadium carbides With an increasing vanadium content, there is inevitably more free vanadium available, since the precipitation 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 adjusted to values of at most 0.030% by weight.
• vanadium in addition to reducing aging effects, also contributes to increasing strength through precipitation hardening, higher contents of up to 0.2% by weight can preferably be used to increase strength.
• the vanadium content of the steel of a flat steel product according to the invention is limited to a maximum of 0.2% by weight for cost reasons. On the other hand, higher contents do not bring about any significant improvement in the mechanical properties.
• 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 content or S content.
• the S content of a steel flat 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 amounts in steel due to the steel manufacturing process.
• the N content should be kept as low as possible and should not exceed 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, in particular at most 0.007% by weight. should be.
• Boron, titanium, niobium, nickel, copper, molybdenum and tungsten can optionally be added to the steel of a flat steel product according to the invention, either individually or in combination with one another.
• 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 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 the hardening effect lower again.
• the boron content is limited to at most 0.01% by weight, in particular at most 0.0035% by weight. If boron is alloyed in, titanium is also preferably alloyed in to bind nitrogen. In this case, the Ti content should preferably be at least 3.42 times the nitrogen content.
• 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 adequate binding of nitrogen, with at least 0.001% by weight Ti, preferably at least 0.023% by weight Ti, being added for sufficient availability. Above 0.1% by weight of Ti, the cold-rollability and recrystallizability deteriorate significantly, which is why larger Ti contents should be avoided.
• the Ti content may preferably be limited to 0.038% 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. In order to reduce the risk of deteriorating the recrystallizability, the Nb content may preferably be limited to 0.003% by weight.
• Copper (Cu) can optionally be alloyed to increase hardenability with additions of at least 0.01% by weight.
• copper improves resistance to atmospheric corrosion 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 at most 0.8% by weight, preferably at most 0.10% by weight is.
• 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, 0.01% by weight of nickel can be alloyed with 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 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 an alloy of molybdenum, the content should be at most 1.0% by weight, preferably at most 0.1% by weight.
• a particularly good resistance to aging can be achieved with steel flat products for which the difference ⁇ Re is at most 25 MPa.
• 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 1100 - 1400 °C. If the semi-finished product has cooled down after casting, the semi-finished product is first reheated to 1100 - 1400 °C for thorough heating.
• the through heating temperature should be at least 1100 °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 rough rolling should be at least 1000°C so that the intermediate product contains 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 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. If step c) has been carried out, the intermediate product is 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.
• 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 particle 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 a maximum of 1000 °C in order to prevent coarsening of the austenite grains.
• 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 cut to a within less than 50 s after hot rolling Coiling temperature (T4) cooled.
• T4 hot rolling 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 steel flat 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 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 that have 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 in order to bring vanadium into solution at temperatures of 650-900° C. or to keep already dissolved vanadium in solution, which has a positive effect on the aging resistance of the steel flat product.
• annealing temperatures above 900 °C no improvement in aging resistance is achieved, which is why the annealing temperature is limited to 900 °C for economic reasons.
• 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°C, preferably at least 640°C, particularly 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.
• 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 melt bath which contains the alloy to be applied to the steel flat product in liquid form, typically has a temperature (T7) of 640 - 720 °C.
• Aluminum-based alloys have proven to be particularly suitable for coating age-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 components present being 100% by weight.
• Unavoidable impurities can be, for example, unavoidable proportions 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 supersaturated dissolved carbon can be bound by vanadium. Therefore, the average cooling rate (CR1) in a temperature range, which is optimal for the precipitation kinetics of vanadium, and which in steel flat products with composition according to the invention between 600°C and 450°C is at most 25 K/s, preferably at most 18 K/s, particularly 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, particularly preferably at most 9.5 K/s.
• the free carbon of the steel flat product still has a sufficient diffusion rate 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 preferably nucleate on existing carbides of the micro-alloying elements such as vanadium, niobium or titanium, is particularly high.
• the formation of iron carbides also binds free carbon, 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 preferred, and an average between 220° C. and room temperature Cooling rate set to a maximum of 20 K/s.
• the average cooling rate is preferably at least 0.1 K/s in each of the individual temperature ranges.
• the average 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 divided by the time required for cooling in this temperature range. For example, for a cooldown from a temperature TX to a temperature TY, this is: (TX-TY)/ ⁇ t, where TX is the temperature at the start of the cooldown in K, TY is the temperature at the end of the cooldown in K, and ⁇ t is the duration of the cooldown from TX on TY are 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 age.
• the cooling rate of the entire cooling process i.e. the cooling of the coated flat steel product after leaving the coating bath until it reaches room temperature, can be limited to values of typically at least 0.1 K/s.
• An anti-corrosion coating applied to the steel substrate after cooling has taken place 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 balance aluminum.
• Unavoidable impurities can be, for example, unavoidable proportions of chromium, manganese, calcium or tin.
• the coating composition can be determined, for example, using glow discharge spectroscopy (GDOES).
• 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.
• a steel flat product produced according to the invention is suitable for press hardening and has an anti-corrosion coating, a high uniform elongation Ag of at least 11.5% and a continuous yield point or a pronounced yield point in 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 into an intermediate product with a thickness of 40 mm, with 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 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 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 up 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 grades given in Table 2.
• the cold-rolled flat steel products were heated in a continuous annealing furnace to an annealing temperature T5 and held at the annealing temperature for 100 s before being 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 tapes were blown off in a conventional manner, producing a coating laydown of 150 g/m 2 .
• the strips were first 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 given in Table 2.
• the strips were cooled at a cooling rate of 5 - 15 K/s.
• the following material parameters were determined as part of the tensile test: the type of yield point, which is designated Re for a pronounced yield point and Rp for a continuous yield point, as well as the value for the yield point Rp0.2 in the case of a continuous yield point, and the values for in the case of a pronounced yield point 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 specimens 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 at most 41 MPa and a uniform elongation Ag of at least 11.5%.

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