EP3974554A1 - Flat steel product with good ageing resistance and method for producing the same - Google Patents

Abstract

The invention relates to a steel flat product which is suitable for press hardening and is coated with an aluminum-based alloy. The steel of the steel flat product consists of, in % by weight, C: 0.10 - 0.4%, Si: 0.05 - 0.5%, Mn: 0.5 - 3.0%, Al: 0, 01 - 0.2%, Cr: 0.18 - 1.0%, V: 0.002 - 0.009%, P: ≤ 0.1%, S: ≤ 0.05%, N: ≤ 0.02% and optional one or more of the elements "B, Ti, Nb, Ni, Cu, Mo, W" in amounts of 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%, balance Fe and unavoidable impurities. The steel flat product has a continuous yield point (Rp0.2) or a yield point with a difference (ΔRe) between the upper yield point value (ReH) and the lower yield point value (ReL) of at most 45 MPa and is characterized by good aging resistance. For the production of a flat steel product according to the invention, a slab or thin slab composed according to the invention is provided, which is then thoroughly heated at 1100-1400° C. and then, after an optional pre-rolling, with a final rolling temperature (T3) of 750-1000° C., is hot-rolled to form a hot-rolled flat steel product . After optional coiling and descaling, the flat steel product can optionally be cold-rolled with a degree of cold rolling ≥ 30%. The hot or cold rolled steel flat product is annealed at 650 - 900 °C, cooled to 600 - 800 °C and then hot dip coated with an aluminium-based alloy. The coated flat steel product is then cooled to room temperature, with the cooling taking place in the range from 600 - 450 °C at an average cooling rate (CR1) ≤ 25 K/s and in the range from 400 - 220 °C at an average cooling rate (CR2) ≤ 20 K/s occurs. The cooling rate in the range between 400 - 220 °C is lower than in the range between 600 - 450 °C. Finally, the coated flat steel product can optionally be skin pass rolled.

Description

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.

When "flat steel products" are mentioned here, this means steel strips, steel sheets or blanks made from them and the like. 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.

In body construction, steels are used that are subject to high demands in terms of their mechanical properties and processing behavior. 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.

Steel aging is caused by free carbon in ferrite. At temperatures above 300°C, the solubility of carbon in ferrite is significantly higher than at room temperature, so that a certain free carbon content is established. Temperatures of over 300°C are usually reached in coating processes such as hot dip coating. With the temperature and time profiles typical for coating processes, carbon can therefore 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 strongly supersaturated with carbon. However, as an interstitial alloying element, carbon can still diffuse very slowly even at room temperature and accumulates at defects, such as dislocations, among other things. 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. 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 during blank trimming and 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 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.

With regard to the flat steel product, 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 . With regard to the method, 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%.

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

In steel flat products according to the invention, 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. If the C content is further increased 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. High C contents can also have a negative impact on weldability. In order to improve weldability, the carbon content may preferably be adjusted to at most 0.3% by weight. With C contents of at most 0.25% by weight, in particular at most 0.235% by weight, the weldability can be significantly improved again and, in addition, a good ratio of force absorption and maximum bending angle in the bending test according to VDA238-100 can be achieved in the press-hardened state.

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. For usual 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 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. In flat steel products according to the invention, 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. In order to obtain a good surface quality, 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. 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. In order to use the aging-inhibiting effect of vanadium in a particularly reliable manner, 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. From a vanadium content in the steel of 0.009% by weight, vanadium carbides cannot be dissolved at temperatures of 700 to 900° C., which are typical for annealing temperatures in a hot-dip coating plant, for example. 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. Since 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. Above P contents of 0.1% by weight, increasing embrittlement of the martensite also occurs, 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 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. In particular in the case of alloys that contain boron, 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. At contents above 0.01% by weight, on the other hand, boron carbides, boron nitrides or boron nitrocarbides are increasingly formed, which in turn represent preferred nucleation sites for the nucleation of ferrite and the hardening effect lower again. For this reason, 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. In addition, 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. In order to improve cold rollability, 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. In addition, 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 (Mo) 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.

• ΔRe = (ReH - ReL) ≤ 45 MPa mit ReH = obere Streckgrenze in MPa und
• ReL = untere Streckgrenze in MPa.

Tungsten (W) can optionally be added in amounts of 0.001-1.0% by weight to slow down the formation of ferrite. A positive effect on hardenability is obtained even with W contents of at least 0.001% by weight. For cost reasons, a maximum of 1.0% by weight of tungsten is added. After coating, a flat steel product according to the invention has a high uniform elongation Ag of at least 11.5%. The yield point of a flat steel product according to the invention shows a continuous course or only a small extent. For the purposes of the invention, continuous progression means that there is no pronounced yield point. A continuous yield point can also be referred to as a yield point Rp0.2. In the present case, a low yield point is understood to mean a pronounced yield point in which the difference ΔRe between the upper yield point value ReH and the lower yield point value ReL is at most 45 MPa. The following applies:

• ΔRe = (ReH - ReL) ≤ 45 MPa with ReH = upper yield point in MPa and
• ReL = lower yield point in MPa.

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

1. a) Zurverfügungstellen einer Bramme oder einer Dünnbramme, die aus (in Gew.-%) 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 sowie optional einem oder mehreren der Elemente "B, Ti, Nb, Ni, Cu, Mo, W" in folgenden Gehalten 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 % und Rest Eisen und unvermeidbaren Verunreinigungen besteht;
2. b) Durcherwärmen der Bramme oder Dünnbramme bei einer Temperatur (T1) von 1100 - 1400 °C;
3. c) optionales Vorwalzen der durcherwärmten Bramme oder Dünnbramme zu einem Zwischenprodukt mit einer Zwischenprodukttemperatur (T2) von 1000 - 1200 °C;
4. d) Warmwalzen zu einem warmgewalzten Stahlflachprodukt, wobei die Endwalztemperatur (T3) 750 - 1000 °C beträgt;
5. e) optionales Haspeln des warmgewalzten Stahlflachprodukts, wobei die Haspeltemperatur (T4) höchstens 700 °C beträgt;
6. f) Entzundern des warmgewalzten Stahlflachprodukts;
7. g) optionales Kaltwalzen des Stahlflachprodukts, wobei der Kaltwalzgrad mindestens 30 % beträgt;
8. h) Glühen des Stahlflachprodukts bei einer Glühtemperatur (T5) von 650 - 900 °C;
9. i) Abkühlen des Stahlflachprodukts auf eine Vorkühltemperatur (T6), welche 600 - 800 °C beträgt;
10. j) Beschichten des Stahlflachprodukts mit einem Korrosionsschutzüberzug;
11. k) Abkühlen des beschichteten Stahlflachprodukts auf Raumtemperatur, wobei die Abkühlung im Temperaturbereich zwischen 600 °C und 450 °C mit einer mittleren Abkühlrate (CR1) von höchstens 25 K/s und im Temperaturbereich zwischen 400 °C und 220 °C mit einer mittleren Abkühlrate (CR2) von höchstens 20 K/s erfolgt;
12. l) optionales Dressieren des beschichteten Stahlflachprodukts.

The method according to the invention for producing a coated flat steel product that is suitable for press hardening and has particularly good aging resistance comprises the following work steps:

1. 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, 0.001 - 0.2% V, ≤ 0.1% P, ≤ 0.05% S, ≤ 0.02% N and optionally one or more of 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% and balance iron and unavoidable impurities;
2. b) through heating of the slab or thin slab at a temperature (T1) of 1100 - 1400 °C;
3. c) optional pre-rolling of the through-heated slab or thin slab into an intermediate product with an intermediate product temperature (T2) of 1000 - 1200 °C;
4. d) hot-rolling into a hot-rolled steel flat product, the finish rolling temperature (T3) being 750 - 1000 °C;
5. e) optional coiling of the hot-rolled steel flat product, the coiling temperature (T4) being at most 700 °C;
6. f) descaling the hot-rolled flat steel product;
7. g) optional cold rolling of the flat steel product, the degree of cold rolling being at least 30%;
8. h) annealing of the steel flat product at an annealing temperature (T5) of 650 - 900 °C;
9. i) cooling the flat steel product to a pre-cooling temperature (T6) which is 600 - 800 °C;
10. j) coating the flat steel product with an anti-corrosion coating;
11. k) Cooling of the coated flat steel product to room temperature, the cooling taking place in the temperature range between 600 °C and 450 °C with an average cooling rate (CR1) of at most 25 K/s and im temperature range between 400 °C and 220 °C with a mean cooling rate (CR2) of at most 20 K/s;
12. l) optional skin-passing of the coated steel flat product.

In 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 continuous slab casting or in thin slab continuous casting.

In 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.

In the optional work step c), 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. In this case, 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. However, high rolling temperatures can also promote grain growth during the rolling process, which adversely affects the mechanical properties of the flat steel product. to grain growth during the rolling process, the temperature of the intermediate product at the end of rough rolling should not be more than 1200 °C.

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

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 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 degree of cold rolling KWG is the quotient of the reduction in thickness during cold rolling ΔdKW divided by the hot strip thickness d: $$\mathrm{KWG}=\Delta \mathrm{dKW}/\mathrm{i.e}$$

with ΔdKW = reduction in thickness during cold rolling in mm and d = hot strip thickness in mm, where the reduction in thickness 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 hot strip thickness d. The steel flat product after cold rolling is usually also referred to as cold strip. In principle, 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.

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 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 that have 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 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. At 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.

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°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.

In step j), 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.

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 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 extent to which free carbon is bound by vanadium increases when cooling occurs at a lower cooling rate in a temperature range between 400°C and 220°C than in the temperature range between 600°C and 450°C. 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. In the temperature range between 400°C and 220°C, 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. 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 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.

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, between the annealing temperature and 600° C. and between 450° C. and 400° C., 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. For economic reasons, the average cooling rate is preferably at least 0.1 K/s in each of the individual temperature ranges. In the present case, 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)/Δt, 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 Δt is the duration of the cooldown from TX on TY are 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 age. For technical reasons and for economic reasons, 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 .

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 using 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 to a respective temperature T1 in a pusher furnace and held at T1 for between 30 and 450 minutes until the temperature T1 was reached in the core of the slabs and the slabs were thus heated through were. The manufacturing parameters are given in Table 2. The slabs were discharged from the pusher furnace at their respective soaking temperature 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 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. After coating, 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. 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 given in Table 2. Between 450 °C and 400 °C and below 220 °C, the strips were cooled at a cooling rate of 5 - 15 K/s.

After cooling to room temperature, samples were taken from the cooled steel strips in accordance with DIN EN ISO 6892-1:2009-12 transversely to the rolling direction. The samples were subjected to a tensile test in accordance with DIN EN ISO 6892-1:2009-12. Table 3 gives the results of the tensile test. 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 ΔRe, 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 ΔRe between the upper and lower yield point of at most 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 point value given in Table 3 for specimens 1 - 7, 9 - 11, 18, 20 and 23 with a pronounced yield point is the value determined in the course of the tensile test for the lower yield point ReL. The value given in Table 3 for samples 8, 12 - 17, 19, 21, 22 and 24 with continuous yield point is the value for the yield point Rp0.2 determined in the course of the tensile test.

1. a. Zurverfügungstellen einer Bramme oder einer Dünnbramme, die aus, in Gew.-%, 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 sowie optional einem oder mehreren der Elemente "B, Ti, Nb, Ni, *Cu, Mo, W" in folgenden Gehalten 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 % und Rest Eisen und unvermeidbaren Verunreinigungen besteht;
2. b. Durcherwärmen der Bramme oder Dünnbramme bei einer Temperatur (T1) von 1100 - 1400 °C;
3. c. optionales Vorwalzen der durcherwärmten Bramme oder Dünnbramme zu einem Zwischenprodukt mit einer Zwischenprodukttemperatur (T2) von 1000 - 1200 °C;
4. d. Warmwalzen zu einem warmgewalzten Stahlflachprodukt, wobei die Endwalztemperatur (T3) 750 - 1000 °C beträgt;
5. e. optionales Haspeln des warmgewalzten Stahlflachprodukts, wobei die Haspeltemperatur (T4) höchstens 700 °C beträgt;
6. f. Entzundern des warmgewalzten Stahlflachprodukts;
7. g. optionales Kaltwalzen des Stahlflachprodukts, wobei der Kaltwalzgrad mindestens 30 % beträgt;
8. h. Glühen des Stahlflachprodukts bei einer Glühtemperatur (T5) von 650 - 900 °C;
9. i. Abkühlen des Stahlflachprodukts auf eine Vorkühltemperatur (T6), welche 600 - 800 °C beträgt;
10. j. Beschichten des Stahlflachprodukts mit einem Korrosionsschutzüberzug;
11. k. Abkühlen des beschichteten Stahlflachprodukts auf Raumtemperatur, wobei die Abkühlung im Temperaturbereich zwischen 600 °C und 450 °C mit einer mittleren Abkühlrate (CR1) von höchstens 25 K/s und im Temperaturbereich zwischen 400 °C und 220 °C mit einer mittleren Abkühlrate (CR2) von höchstens 20 K/s erfolgt;
12. l. optionales Dressieren des beschichteten Stahlflachprodukts.

xi) Das Verfahren nach Satz x) kann dadurch gekennzeichnet sein, dass die Glühtemperatur (T5) in Arbeitsschritt h) mindestens 720 °C beträgt.
xii) Das Verfahren nach Satz x) oder xi) kann dadurch gekennzeichnet sein, dass die mittlere Abkühlrate (CR1) zwischen 600 °C und 450 °C höchstens 18 K/s beträgt.
xiii) Das Verfahren nach einem der Sätze x) bis xii) kann dadurch gekennzeichnet sein, dass die mittlere Abkühlrate (CR2) zwischen 400 °C und 220 °C höchstens 14 K/s beträgt.
xiv) Das Verfahren nach einem der Sätze x) bis xiii) kann dadurch gekennzeichnet sein, dass die mittlere Abkühlrate (CR2) zwischen 400 °C und 220 °C höchstens 9,5 K/s beträgt.
xv) Das Verfahren nach einem der Sätze x) bis xiv) kann dadurch gekennzeichnet sein, dass das Schmelzenbad, das den auf das Stahlflachprodukt aufzubringenden Korrosionsschutz in flüssiger Form enthält, neben Aluminium 3 - 15 Gew.-% Silizium, bis zu 5 Gew.-% Eisen und bis zu 0,5 Gew.-% unvermeidbare Verunreinigungen enthält, wobei die Summe der vorliegenden Bestandteile 100 Gew.-% beträgt. Rest Eisen und unvermeidbare Verunreinigungen. Angaben jeweils in Gew.-% Tabelle 1, Teil 1 Stahl C Si Mn P S Al Cr Ti N V B Andere 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 Rest Eisen und unvermeidbare Verunreinigungen. Angaben jeweils in Gew.-% Tabelle 1, Teil 2 Stahl C Si Mn P S Al Cr Ti N V B Andere 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 - Tabelle 2, Teil 1 Versuch Nr. Stahl T1 T2 T3 T4 KWG T5 T6 T7 CR1 CR2 [°C] [°C] [°C] [°C] [%] [°C] [°C] [°C] [K/s] [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 4 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 6 F 1250 1100 850 600 50 750 700 675 16,43 11,19 7 G 1250 1100 800 600 55 750 700 675 16,65 10,66 8 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 12 L 1300 1100 800 600 60 750 700 675 10,41 4,63 Tabelle 2, Teil 2 Versuch Nr. Stahl T1 T2 T3 T4 KWG T5 T6 T7 CR1 CR2 [°C] [°C] [°C] [°C] [%] [°C] [°C] [°C] [K/s] [K/s] 13 M 1300 1150 800 650 60 750 700 675 10,88 4,84 14 N 1300 1100 800 650 60 750 700 675 12,74 5,67 15 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 17 Q 1250 1100 825 600 50 775 700 675 11,54 7,86 18 R 1250 1100 850 600 55 725 700 675 15,84 10,79 19 S 1250 1100 825 600 50 750 700 675 11,54 7,86 20 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 22 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 Rp=kontinuierliche Streckgrenze, Re= ausgeprägte Streckgrenze Tabelle 3, Teil 1 Versuch Nr. Streckgrenzenart Rp0,2 oder ReL [MPa] ReH [MPa] ΔRe [MPa] Rm [MPa] Gleichmaßdehnung Ag [MPa] Bruchdehnung A80 [MPa] 1 Re 425 445 20 592 14,5 24 2 Re 401 413 12 577 14,5 26 3 Re 492 533 41 594 12,9 19,6 4 Re 439 445 6 578 13,8 23 5 Re 446 473 27 600 14,2 25,6 6 Re 455 474 19 609 12,8 24,8 7 Re 418 427 9 581 13,4 24,7 8 Rp 422 624 12,7 20,5 9 Re 420 423 3 627 13,8 21 10 Re 409 414 5 542 15,3 25 11 Re 436 460 24 598 14,4 24,2 12 Rp 399 636 12,5 19,7 Rp=kontinuierliche Streckgrenze, Re= ausgeprägte Streckgrenze Tabelle 3, Teil 2 Versuch Nr. Streckgrenzenart Rp0,2 oder ReL [MPa] ReH [MPa] ΔRe [MPa] Rm [MPa] Gleichmaßdehnung Ag [MPa] Bruchdehnung A80 [MPa] 13 Rp 403 611 13,1 22,7 14 Rp 402 597 12,9 24,7 15 Rp 400 623 12,2 19 16 Rp 413 617 13,6 21,3 17 Rp 419 618 11,5 17,8 18 Re 446 463 17 588 12,6 21 19 Rp 413 601 13 19,4 20 Re 417 426 9 581 14,4 24,5 21 Rp 406 586 13,4 22 22 Rp 392 596 13,5 20,5 23 Re 422 433 11 560 14 24 24 Rp 389 585 13,1 21,5 The invention disclosed here is characterized by the following sentences:
i) It is a coated steel flat product suitable for press hardening, the steel of the steel flat product consisting of iron and unavoidable impurities in % by weight 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: 0.001 - 0.2%, P: ≤ 0.1%, S: ≤ 0.05%, N: ≤ 0.02%, 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%, Mon: 0.002 - 1.0%, W: 0.001 - 1.0% consists.
ii) The flat steel product according to sentence i) can be characterized in that the flat steel product has a continuous yield point (Rp0.2) or a yield point with a difference (ΔRe) between the upper yield point value (ReH) and the lower yield point value (ReL) of at most 45 MPa.
iii) The flat steel product according to sentence i) or ii) can also be characterized in that the flat steel product has a uniform elongation Ag of at least 11.5%.
iv) The flat steel product according to one of sentences i) to iii) can also be characterized in that the carbon content of the steel of the flat steel product is at most 0.3% by weight.
v) The flat steel product according to one of sentences i) to iv) can also be characterized in that the vanadium content of the steel of the flat steel product is at least 0.002% by weight.
vi) The flat steel product according to one of sentences i) to v) can also be characterized in that the vanadium content of the steel of the flat steel product is at most 0.03% by weight.
vii) The flat steel product according to one of sentences i) to vi) can also be characterized in that the vanadium content of the steel of the flat steel product is at most 0.009% by weight.
viii) The flat steel product according to one of sentences i) to vii) can also be characterized in that it has an anti-corrosion coating on at least one side.
ix) The flat steel product according to sentence viii) can also be characterized in that the anti-corrosion coating on the steel substrate contains 3-15% by weight silicon, up to 5% by weight iron, up to 0.5% by weight unavoidable impurities , and balance aluminum.
x) A method according to the invention for producing a steel flat product suitable for hot forming comprises the following work steps:

1. 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, 0.001 - 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% and balance iron and unavoidable impurities;
2. b. through heating of the slab or thin slab at a temperature (T1) of 1100 - 1400 °C;
3. c. optional pre-rolling of the soaked slab or thin slab into an intermediate product with an intermediate product temperature (T2) of 1000 - 1200 °C;
4. i.e. hot rolling into a hot-rolled steel flat product, wherein the finish rolling temperature (T3) is 750 - 1000 °C;
5. e. optionally coiling the hot-rolled steel flat product, the coiling temperature (T4) being at most 700 °C;
6. f. descaling the hot-rolled flat steel product;
7. G. optional cold rolling of the steel flat product, wherein the degree of cold rolling is at least 30%;
8. H. annealing of the steel flat product at an annealing temperature (T5) of 650 - 900 °C;
9. i. cooling the steel flat product to a pre-cooling temperature (T6) which is 600 - 800 °C;
10. j. Coating the flat steel product with an anti-corrosion coating;
11. k. Cooling of the coated steel flat product to room temperature, with the cooling taking place in the temperature range between 600 °C and 450 °C with an average cooling rate (CR1) of at most 25 K/s and in the temperature range between 400 °C and 220 °C with an average cooling rate (CR2 ) of no more than 20 K/s;
12. l. optional skin-passing of the coated steel flat product.

xi) The method according to sentence x) can be characterized in that the annealing temperature (T5) in step h) is at least 720 °C.
xii) The method according to sentence x) or xi) can be characterized in that the mean cooling rate (CR1) between 600 °C and 450 °C is at most 18 K/s.
xiii) The method according to one of clauses x) to xii) can be characterized in that the average cooling rate (CR2) between 400°C and 220°C is at most 14 K/s.
xiv) The method according to one of clauses x) to xiii) can be characterized in that the average cooling rate (CR2) between 400°C and 220°C is at most 9.5 K/s.
xv) The method according to one of sentences x) to xiv) can be characterized in that the melt bath, which is on the The corrosion protection to be applied to the steel flat product in liquid form contains, in addition to aluminum, 3-15% by weight silicon, up to 5% by weight iron and up to 0.5% by weight unavoidable impurities, the sum of the components present being 100% by weight -% amounts to. remainder iron and unavoidable impurities. Data in % by weight <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 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 - attempt no. stole T1 T2 T3 T4 KWG T5 T6 T7 CR1 CR2 [°C] [°C] [°C] [°C] [%] [°C] [°C] [°C] [K/s] [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 4 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 6 f 1250 1100 850 600 50 750 700 675 16:43 11:19 7 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 12 L 1300 1100 800 600 60 750 700 675 10:41 4.63 attempt no. stole T1 T2 T3 T4 KWG T5 T6 T7 CR1 CR2 [°C] [°C] [°C] [°C] [%] [°C] [°C] [°C] [K/s] [K/s] 13 M 1300 1150 800 650 60 750 700 675 10.88 4.84 14 N 1300 1100 800 650 60 750 700 675 12.74 5.67 15 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 17 Q 1250 1100 825 600 50 775 700 675 11.54 7.86 18 R 1250 1100 850 600 55 725 700 675 15.84 10.79 19 S 1250 1100 825 600 50 750 700 675 11.54 7.86 20 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 22 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 type Rp0.2 or ReL [MPa] ReH [MPa] ΔRe [MPa] Rm [MPa] Elongation Ag [MPa] Elongation at break A80 [MPa] 1 re 425 445 20 592 14.5 24 2 re 401 413 12 577 14.5 26 3 re 492 533 41 594 12.9 19.6 4 re 439 445 6 578 13.8 23 5 re 446 473 27 600 14.2 25.6 6 re 455 474 19 609 12.8 24.8 7 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 25 11 re 436 460 24 598 14.4 24.2 12 Rp 399 636 12.5 19.7 attempt no. yield type Rp0.2 or ReL [MPa] ReH [MPa] ΔRe [MPa] Rm [MPa] Elongation Ag [MPa] Elongation at break A80 [MPa] 13 Rp 403 611 13.1 22.7 14 Rp 402 597 12.9 24.7 15 Rp 400 623 12.2 19 16 Rp 413 617 13.6 21.3 17 Rp 419 618 11.5 17.8 18 re 446 463 17 588 12.6 21 19 Rp 413 601 13 19.4 20 re 417 426 9 581 14.4 24.5 21 Rp 406 586 13.4 22 22 Rp 392 596 13.5 20.5 23 re 422 433 11 560 14 24 24 Rp 389 585 13.1 21.5

Claims (10)

Aluminum-based alloy coated steel flat product suitable for press hardening, - where the steel of the steel flat product, 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: 0.18 - 1.0%, V: 0.002 - 0.009%, P: ≤ 0.1%, S: ≤ 0.05%, N: ≤ 0.02%, 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%, Mon: 0.002 - 1.0%, W: 0.001 - 1.0% and than consists of iron and unavoidable impurities and - the flat steel product having a continuous yield point (Rp0.2) or a yield point with a difference (ΔRe) between the upper yield point value (ReH) and the lower yield point value (ReL) of at most 45 MPa. Flat steel product according to Claim 1, characterized in that the flat steel product has a uniform elongation Ag of at least 11.5%. Flat steel product according to one of the preceding claims, characterized in that the carbon content of the steel of the flat steel product is at most 0.3% by weight. Flat steel product according to one of the preceding claims, characterized in that the anti-corrosion coating on the steel substrate contains 3-15% by weight silicon, up to 5% by weight iron, up to 0.5% by weight unavoidable impurities and the remainder aluminium contains. Process 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, 0.18 - 1.0% Cr, 0.002 - 0.009% V, ≤ 0.1% P, ≤ 0.05% S, ≤ 0.02% N and optionally one or more of 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% and balance iron and unavoidable impurities; b) through heating of the slab or thin slab at a temperature (T1) of 1100 - 1400 °C; c) optional pre-rolling of the through-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 steel flat product, the finish rolling temperature (T3) being 750 - 1000 °C; e) optional coiling of the hot-rolled steel flat product, the coiling temperature (T4) being at most 700 °C; f) descaling the hot-rolled flat steel product; g) optional cold rolling of the flat steel product, the degree of cold rolling being at least 30%; h) annealing of the steel flat 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 an anti-corrosion coating by means of continuous hot-dip coating with an aluminum-based alloy; k) Cooling of the coated flat steel product to room temperature, with the cooling taking place in the temperature range between 600 °C and 450 °C with an average cooling rate (CR1) of at most 25 K/s and in the temperature range between 400 °C and 220 °C with an average cooling rate (CR2) of at most 20 K/s and the cooling takes place in the temperature range between 400 °C and 220 °C a lower cooling rate than in the temperature range between 600 °C and 450 °C; l) optional skin-passing of the coated steel flat product. Method according to Claim 5, characterized in that the annealing temperature (T5) in step h) is at least 720°C. Method according to Claim 5 or 6, characterized in that the mean cooling rate (CR1) between 600°C and 450°C is at most 18 K/s. Method according to one of Claims 5 to 7, characterized in that the mean cooling rate (CR2) between 400°C and 220°C is at most 14 K/s. Method according to one of Claims 5 to 8, characterized in that the mean cooling rate (CR2) between 400°C and 220°C is at most 9.5 K/s. Method according to one of Claims 5 to 9, characterized in that the molten bath, which contains the corrosion protection to be applied to the flat steel product in liquid form, in addition to aluminum contains 3-15% 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.