EP3655560B1 - Flat steel product with a high degree of aging resistance, and method for producing same - Google Patents

Description

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.

When "flat steel products" are mentioned in the present case, this refers to steel strips, steel sheets or blanks obtained therefrom and the like. 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.

Continuously running yield strengths are often referred to as yield strengths.

The aging of steel is caused by free carbon in the ferrite. At temperatures of over 300 ° C, 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. At room temperature, 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. Among other things, 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. In this respect, aging of the steel due to free carbon should be prevented or at least mitigated as far as possible.

the end EP 2848709 A1 a flat steel product is known 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.

From the EP 2 631 307 A1 In addition, a steel sheet is known 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%. It holds that a ratio of Cr _Θ / Cr _M 2, where Cr _{Θ is} a concentration of Cr which is in a solid solution in iron carbide, and Cr _{M is} a concentration of Cr in a solid solution in a base material, or that Mn _Θ / Mn _M Mn 10, where Mn _{Θ is} a concentration of Mn that is present in a solid solution in an iron carbide, and Mn _{M is} a concentration of Mn that is exposed to a solid solution in a base material.

Likewise from D2: 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 ¼ of the thickness of the steel sheet should be 0.8 mm. To protect against corrosion, the sheets can be provided with an AlSi protective layer applied in the process.

Finally from the JP 53-87073 B2 a steel sheet for hot pressing is known which has sufficient strength and high ductility and is therefore said to be suitable for machine structural components such as body structural components and frame trim parts. For this purpose, 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. At the same time, it has a structure in which carbides are dispersed in ferrite, the average grain diameter D (µm) of the ferrite being 3 to 13 µm, the average opening intervals λ (µm) of the dispersed carbides being ≤ 5 µm and at the same time the condition D. <90 λ ^{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.

With regard to the flat steel product, this object is achieved by a flat steel product with the features specified in claim 1. Advantageous and preferred embodiments of the flat steel product according to the invention are specified in the claims which refer back to claim 1.

With regard to the method, the object is achieved by a method having the features mentioned in claim 5. Advantageous and preferred embodiments of the method according to the invention are specified in the claims referring back to claim 5.

If information on alloy contents and compositions is given here, these relate to weight or mass, unless otherwise expressly stated.

In flat steel products according to the invention, 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. If the C content is increased further to values of at least 0.19% by weight, in particular at least 0.205% by weight, the hardenability can also be improved so that the flat steel product has a very good combination of hardenability and strength. However, 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. In order to improve weldability, the carbon content can preferably be set to 0.3 wt% or less. With C contents of at most 0.25% by weight, in particular at most 0.235% by weight, the weldability can again be significantly improved and, in addition, a good one Ratio of force absorption and maximum bending angle can be achieved in the bending test according to VDA238-100 in the press-hardened state.

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. In addition, aluminum inhibits the formation of cementite. For reliable binding of oxygen, at least 0.01% by weight, in particular at least 0.02% by weight, of aluminum is required in the steel. However, since the Ac3 temperature is also shifted upwards significantly with increasing Al alloy content, 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. For normal furnace temperatures between 850 and 950 ° C., which are set for austenitizing before press hardening, 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. Even the smallest amounts of 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. In order to use the aging-inhibiting effect of vanadium particularly reliably, 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. From a vanadium content of 0.009% by weight in the steel, 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. 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. Since 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. In the case of alloys containing boron in particular, 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. In contrast, at contents above 0.01% 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. For this reason, the boron content is limited to at most 0.01% by weight, in particular at most 0.0035% by weight. When adding boron as an alloy, titanium is also preferred to bind nitrogen alloyed. In this case, 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. In addition, 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 (Nb) 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. In addition, 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. In the present case, a low yield strength is understood to mean a pronounced yield strength at which the difference ΔRe between the upper yield limit value ReH and the lower yield limit value ReL is at most 45 MPa. The following applies: $$\begin{array}{l}\Delta \mathrm{re}=\left(\mathrm{Deer}-\mathrm{ReL}\right)\le 45\mathrm{MPa}\mathrm{with}\mathrm{Deer}=\mathrm{upper}\mathrm{Stretch\; limit}\mathrm{in}\mathrm{MPa}\\ \mathrm{and}\mathrm{ReL}=\mathrm{lower}\mathrm{Stretch\; limit}\mathrm{in}\mathrm{MPa}\mathrm{.}\end{array}$$

Particularly good aging resistance can be achieved with flat steel products for which the difference ΔRe is at most 25 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.

In work step a), 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.

In 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.

In the optional work step c), 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. In this case, 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. However, 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. In order to keep the grain growth low during the rolling process, the temperature of the intermediate product should not be more than 1200 ° C. at the end of the roughing process.

In 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. In addition, 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. In the case of continuous hot strip rolling, work step e) provides for an optional coiling of the hot-rolled flat steel product. For this purpose, the hot strip is cooled to a coiling temperature (T4) within less than 50 s after hot rolling. For example, water, air or a combination of both can be used as the cooling medium. 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.

In step f), the hot-rolled flat steel product is descaled in a conventional manner by pickling or by another suitable treatment.

mit ΔdKW = Dickenabnahme beim Kaltwalzen in mm und d = Warmbanddicke in mm, wobei sich die Dickenabnahme ΔdKW aus der Differenz der Dicke des Stahlflachprodukts vor dem Kaltwalzen zur Dicke des Stahlflachprodukts nach dem Kaltwalzen ergibt. Beim Stahlflachprodukt vor dem Kaltwalzen handelt es sich üblicherweise um ein Warmband der Warmbanddicke d. Das Stahlflachprodukt nach dem Kaltwalzen wird üblicherweise auch als Kaltband bezeichnet. Der Kaltwalzgrad kann prinzipiell sehr hohe Werte von über 90 % annehmen. Allerdings haben sich Kaltwalzgrade von höchstens 80 % als günstig zur Vermeidung von Bandrissen erwiesen.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. Under the cold rolling degree KWG is understood as the quotient of the thickness decrease during cold rolling ΔdKW through the hot strip thickness d: $$\mathrm{KWG}=\Delta \mathrm{dKW\; /\; d}$$

with ΔdKW = thickness decrease during cold rolling in mm and d = hot strip thickness in mm, whereby the thickness decrease ΔdKW results from the difference between the thickness of the flat steel product before cold rolling and the thickness of the flat steel product after cold rolling. 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.

In step h), the flat steel product is subjected to an annealing treatment at annealing temperatures (T5) of 650 - 900 ° C. For this purpose, 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. From a thermodynamic point of view, 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. However, 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. At 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.

In 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.

After the coating treatment, 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. In the temperature range between 400 ° C and 220 ° C, 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. In addition, 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.

In addition, in the temperature range between 400 ° C and 220 ° C, 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. Through the formation of iron carbides free carbon is also bound, which has a positive effect on aging behavior.

In the temperature range between the annealing temperature and 600 ° C, between 450 ° C and 400 ° C and between 220 ° C and room temperature, the cooling rate has no significant influence on the aging resistance. For process engineering reasons, 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. For economic reasons, the mean cooling rate is preferably at least 0.1 K / s in each of the individual temperature ranges. In the present case, 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) / Δt, where TX is the temperature at the beginning of cooling in K, TY is the temperature at the end of cooling in K and Δt is the duration of cooling from TX are on TY in s.

In principle, 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 .

In a preferred embodiment, 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.

In a further preferred embodiment, 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 invention is explained in more detail below with the aid of exemplary embodiments.

Several tests were carried out to demonstrate the effect of the invention. For this purpose, slabs with the compositions given in Table 1 with a thickness of 200-280 mm and a width of 1000-1200 mm were produced, heated in a pusher furnace to a respective temperature T1 and held at T1 for between 30 and 450 minutes until temperature T1 was reached in the core of the slabs and the slabs were thus heated through. The production parameters are given in Table 2. The slabs were discharged from the pusher furnace at their respective soaking temperatures T1 and subjected to hot rolling. The tests were carried out as continuous hot strip rolling. For this purpose, 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. After coating, 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. In the further course of cooling between 600 ° C. and 450 ° C. and between 400 ° C. and 220 ° C., 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.

After cooling to room temperature, samples were taken transversely to the rolling direction from the cooled steel strips in accordance with DIN EN ISO 6892-1: 2009-12. The samples were subjected to a tensile test in accordance with DIN EN ISO 6892-1: 2009-12. Table 3 shows the results of the tensile test. In the course of the tensile test, the following material parameters were determined: 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 ΔRe, 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 ΔRe between the upper and lower yield point of no more than 41 MPa and a uniform elongation Ag of at least 11.5%. There is a continuous yield point Rp for samples 8, 12-17, 19, 21, 22 and 24 and a pronounced yield point Re for samples 1-7, 9-11, 18, 20 and 23. The yield strength value given in Table 3 for samples 1 - 7, 9 - 11, 18, 20 and 23 with a pronounced yield point is the value for the lower yield point ReL determined in the course of the tensile test. The table 3 for samples 8, 12 - 17, 19, 21, 22 and 24 with continuous yield strength is the value determined in the course of the tensile test for the yield strength Rp0.2. <b><u> Table 1, Part 1 </u></b> stole C. Si Mn P. S. Al Cr Ti N V B. Other A. 0.21 0.22 1.11 0.015 0.0029 0.038 0.19 0.026 0.0045 0.002 0.0022 - B. 0.21 0.22 1.12 0.011 0.0044 0.044 0.19 0.029 0.0032 0.002 0.0022 - C. 0.22 0.23 1.12 0.014 0.0022 0.044 0.23 0.030 0.0039 0.001 0.0027 - D. 0.22 0.23 1.14 0.018 0.0024 0.041 0.20 0.031 0.0036 0.003 0.0029 - E. 0.22 0.25 1.12 0.012 0.0022 0.038 0.22 0.027 0.0044 0.001 0.0024 - F. 0.24 0.27 1.15 0.015 0.0034 0.039 0.23 0.036 0.0044 0.001 0.0026 - G 0.21 0.24 1.11 0.013 0.0015 0.036 0.20 0.033 0.0051 0.001 0.0024 Mo: 0.0036 H 0.22 0.24 1.13 0.011 0.0030 0.037 0.21 0.028 0.0047 0.002 0.0032 W: 0.002 I. 0.22 0.22 1.15 0.011 0.0025 0.032 0.19 0.030 0.0042 0.002 0.0028 - J 0.21 0.23 1.12 0.015 0.0028 0.036 0.19 0.027 0.0051 0.002 0.0025 - K 0.21 0.22 1.14 0.018 0.0025 0.038 0.19 0.029 0.0027 0.001 0.0026 - L. 0.22 0.23 1.13 0.017 0.0030 0.036 0.19 0.030 0.0033 0.002 0.0027 Ni: 0.023 Cu: 0.015 Remainder iron and unavoidable impurities. Details in% by weight stole C. Si Mn P. S. Al Cr Ti N V B. Other M. 0.21 0.22 1.11 0.015 0.0026 0.035 0.19 0.026 0.0024 0.002 0.0023 - N 0.21 0.24 1.11 0.016 0.0020 0.036 0.20 0.026 0.0031 0.002 0.0024 - O 0.22 0.22 1.19 0.017 0.0014 0.039 0.19 0.029 0.0038 0.001 0.0025 - P. 0.21 0.22 1.11 0.013 0.0030 0.034 0.19 0.028 0.0033 0.001 0.0026 - Q 0.24 0.27 1.15 0.015 0.0034 0.039 0.23 0.036 0.0044 0.001 0.0026 - R. 0.22 0.23 1.13 0.016 0.0020 0.039 0.20 0.030 0.0031 0.003 0.0029 - S. 0.23 0.25 1.14 0.016 0.0029 0.037 0.21 0.033 0.0056 0.002 0.0034 - T 0.22 0.23 1.14 0.014 0.0032 0.049 0.21 0.028 0.0035 0.003 0.0025 - U 0.22 0.23 1.16 0.015 0.0032 0.037 0.20 0.031 0.0048 0.003 0.0029 - V 0.22 0.23 1.13 0.017 0.0021 0.043 0.22 0.026 0.0033 0.003 0.0027 - W. 0.22 0.23 1.12 0.015 0.0026 0.037 0.20 0.027 0.0044 0.002 0.0027 - X 0.21 0.23 1.13 0.012 0.0024 0.044 0.22 0.028 0.0042 0.001 0.0030 - Remainder iron and unavoidable impurities. Details in% by weight Attempt no. stole T1 [° C] T2 [° C] T3 [° C] T4 [° C] KWG [%] T5 [° C] T6 [° C] T7 [° C] CR1 [K / s] CR2 [K / s] 1 A. 1250 1100 850 600 50 750 700 675 16.43 11.19 2 B. 1250 1100 850 600 60 750 700 675 14.87 10.12 3 C. 1250 1150 800 600 60 750 700 675 15.45 10.52 4th D. 1250 1100 850 600 60 775 700 675 15.45 10.52 5 E. 1250 1100 850 600 50 750 700 675 16.43 11.19 6th F. 1250 1100 850 600 50 750 700 675 16.43 11.19 7th G 1250 1100 800 600 55 750 700 675 16.65 10.66 8th H 1250 1100 800 650 50 750 700 675 6.99 3.11 9 I. 1250 1100 850 600 60 750 700 675 14.87 10.12 10 J 1250 1100 800 650 60 750 700 675 14.87 10.12 11 K 1250 1100 800 600 60 750 700 675 8.23 3.66 12th L. 1300 1100 800 600 60 750 700 675 10.41 4.63 Attempt no. stole T1 [° C] T2 [° C] T3 [° C] T4 [° G] KWG [%] T5 [° C] T6 [° C] T7 [° C] CR1 [K / s] CR2 [K / s] 13th M. 1300 1150 800 650 60 750 700 675 10.88 4.84 14th N 1300 1100 800 650 60 750 700 675 12.74 5.67 15th O 1250 1100 850 600 50 750 700 675 13.67 6.08 16 P. 1250 1100 800 650 50 775 700 675 9.98 6.79 17th Q 1250 1100 825 600 50 775 700 675 11.54 7.86 18th R. 1250 1100 850 600 55 725 700 675 15.84 10.79 19th S. 1250 1100 825 600 50 750 700. 675 11.54 7.86 20th T 1250 1100 850 600 50 750 675 675 11.54 7.86 21 U 1250 1100 825 600 60 725 675 675 12.71 8.66 22nd V 1250 1150 850 600 60 725 675 675 13.11 8.92 23 W. 1250 1100 850 600 60 725 675 675 14.67 9.99 24 X 1250 1100 850 600 50 725 700 675 13.89 9.46 Attempt no. Yield strength type Rp0.2 or ReL [MPa] ReH [MPa] ΔRe [MPa] Rm [MPa] Uniform elongation Ag [MPa] Elongation at break A80 [MPa] 1 re 425 445 20th 592 14.5 24 2 re 401 413 12th 577 14.5 26th 3 re 492 533 41 594 12.9 19.6 4th re 439 445 6th 578 13.8 23 5 re 446 473 27 600 14.2 25.6 6th re 455 474 19th 609 12.8 24.8 7th re 418 427 9 581 13.4 24.7 8th Rp 422 624 12.7 20.5 9 re 420 423 3 627 13.8 21 10 re 409 414 5 542 15.3 25th 11 re 436 460 24 598 14.4 24.2 12th Rp 399 636 12.5 19.7 Rp = continuous yield point, Re = pronounced yield point Attempt no. Yield strength type Rp0.2 or ReL [MPa] ReH [MPa] ΔRe [MPa] Rm [MPa] Uniform elongation Ag [MPa] Elongation at break A80 [MPa] 13th Rp 403 611 13.1 22.7 14th Rp 402 597 12.9 24.7 15th Rp 400 623 12.2 19th 16 Rp 413 617 13.6 21.3 17th Rp 419 618 11.5 17.8 18th re 446 463 17th 588 12.6 21 19th Rp 413 601 13th 19.4 20th re 417 426 9 581 14.4 24.5 21 Rp 406 586 13.4 22nd 22nd Rp 392 596 13.5 20.5 23 re 422 433 11 560 14th 24 24 Rp 389 585 13.1 21.5 Rp = continuous yield point, Re = pronounced yield point

Claims (10)

1. Flat steel product suitable for press hardening and coated with an aluminium-based alloy,

   - wherein the steel substrate of the flat steel product consists of a steel, which, in % by weight, consists of C: 0.10 - 0.4%, Si: 0.05 - 0.5%, Mn: 0.5 - 3.0%, Al: 0.01 - 0.2%, Cr: 0.005 - 1.0%, V: 0.002 - 0.009%, Ti: 0.023 - 0.038%, P: ≤ 0.1%, S: ≤ 0.05%, N: ≤ 0.02%, as well as optionally one or a plurality of the elements "B, Nb, Ni, Cu, Mo, W" in the following contents B: 0.0005 - 0.01%, 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%, and as the remainder consists of iron and unavoidable impurities, and

   - wherein the flat steel product has a yield strength with continuous course (Rp0.2) or a yield strength with a difference (ΔRe) between upper yield strength value (ReH) and lower yield strength value (ReL) of at most 45 MPa according to DIN EN ISO 6892-1:2009-12.
2. Flat steel product according to claim 1, characterised in that the flat steel product has a uniform elongation Ag of at least 11.5% according to DIN EN ISO 6892-1:2009-12.
3. Flat steel product according to any one of the preceding claims, characterised in that the carbon content of the steel of the flat steel product is at most 0.3% by weight.
4. Flat steel product according to any one of the preceding claims, characterised in that the corrosion protection coat applied on the steel substrate contains 3 to 15% by weight silicon, up to 5% by weight iron, up to 0.5% by weight unavoidable impurities, and remainder aluminium.
5. Method for producing a flat steel product suitable for hot forming, comprising the following work steps:

   a. providing a slab or a 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, 0.002 - 0.009% V, 0.023 - 0.038% Ti, ≤ 0.1% P, ≤ 0.05% S, ≤ 0.02% N as well as optionally one or a plurality of the elements "B, Nb, Ni, Cu, Mo, W" in the following contents B: 0.0005 - 0.01%, 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% and remainder of iron and unavoidable impurities;

   b. heating the slab or thin slab at a temperature (T1) of 1100 - 1400°C;

   c. optionally pre-rolling the heated slab or thin slab into an intermediate product with an intermediate product temperature (T2) of 1000 - 1200°C;

   d. hot rolling into a hot-rolled flat steel product, wherein the final rolling temperature (T3) is 750 - 1000°C;

   e. optionally coiling the hot-rolled flat steel product, wherein the coiling temperature (T4) is at most 700°C;

   f. igniting the hot-rolled flat steel product;

   g. optionally cold rolling the flat steel product, wherein the cold rolling degree is at least 30%;

   h. annealing the flat steel product at an annealing temperature (T5) of 650 - 900°C;

   i. cooling the flat steel product to a pre-cooling temperature (T6), which is 600 - 800°C;

   j. coating the flat steel product with a corrosion protection coat by means of continuous hot dip coating with an aluminium-based alloy;

   k. cooling the coated flat steel product to ambient temperature, wherein the cooling in the temperature range of between 600°C and 450°C takes place at an average cooling rate (CR1) of at most 25 K/s and in the temperature range of between 400°C and 220°C at an average cooling rate (CR2) of at most 20 K/s and the cooling in the temperature range of between 400°C and 220°C takes place at a lower cooling rate than in the temperature range of between 600°C and 450°C;

   l. optionally dressing the coated flat steel product.
6. Method according to claim 5, characterised in that the annealing temperature (T5) in work step h) is at least 720°C.
7. Method according to claim 5 or 6, characterised in that the average cooling rate (CR1) between 600°C and 450°C is at most 18 K/s.
8. Method according to any one of claims 5 to 7, characterised in that the average cooling rate (CR2) between 400°C and 220°C is at most 14 K/s.
9. Method according to claim 8, characterised in that the average cooling rate (CR2) between 400°C and 220°C is at most 9.5 K/s.
10. Method according to any one of claims 5 to 9, characterised in that the melt bath containing the corrosion protection in fluid form to be applied to the flat steel product contains, in addition to aluminium, 3 - 15% by weight silicon, up to 5% by weight iron and up to 0.5% by weight unavoidable impurities, wherein the sum of the constituents present is 100% by weight.