EP3964602A1 - Method for manufacturing a sheet component by hot forming a flat steel product with an anti-corrosion coating - Google Patents

Abstract

The invention makes it possible to produce a sheet metal component from a flat steel product provided with an anti-corrosion coating and having sections of different thicknesses without the risk of H ₂ penetrating, the transition between the sections being abrupt. For this purpose, a) a flat steel product is provided with a steel substrate made of a steel which, in wt % Si, ≤ 0.03% P, ≤ 0.01% S, ≤ 0.1% Al, ≤ 0.15% Ti, ≤ 0.6% Nb, ≤ 0.005% B, ≤ 0.5% Cr, ≤ 0.5% Mo, where the sum of Cr and Mo is ≤ 0.5%, the balance being Fe and unavoidable impurities, and with an anti-corrosion coating of, in % by weight, ≤ 15% Si, ≤ 5 %, Fe, ≤ 5% of at least one alkaline earth metal or transition metal and the balance being Al and unavoidable impurities. If the anti-corrosion coating contains ≦0.1% by weight of the alkaline earth metal or transition metal, b) a solution containing at least one alkaline earth metal or transition metal is applied to the anti-corrosion coating of the flat steel product. Then, c) the flat steel product is flexibly cold-rolled to produce sections of different thicknesses. It is then d) heated to 800-1000°C in an atmosphere with >15% by volume O ₂ until a quantity of thermal energy Js of >44,000 kJs and ≤400,000 kJs is introduced. Thereafter, the anti-corrosion coating is densely covered with a layer consisting of a primary oxide of at least one alkaline earth metal or transition metal. Finally, the steel flat product is hot formed into the sheet metal component,

Description

The invention relates to a method for producing a sheet metal component by hot forming a flat steel product, which is provided with an anti-corrosion coating in particular by hot-dip coating and which is given at least one section by flexible cold rolling, which has a different thickness than another section of the flat steel product adjoining it. the transition between the sections of the flat steel product having different thicknesses being abrupt.

"Flat steel products" are understood here to mean rolled products whose length and width are each significantly greater than their thickness. These include, in particular, steel strips and steel sheets.

In the present text, unless explicitly stated otherwise, information on the content of alloying components is always given in % by weight.

On the other hand, the proportions of certain components in an atmosphere, in particular an annealing atmosphere, are given in vol. %, unless otherwise stated.

From the JP 2004-083988 A a process is known with which a component is formed from a hot-dip galvanized sheet steel intended for use at high temperatures of 450 - 650 °C with an aluminum-based anti-corrosion coating, which is said to have improved oxidation resistance at the high operating temperatures. The anti-corrosion coating of the sheet consists of up to 13% by weight Si, 0.5 - 8% by weight Mg and, if necessary, one or more metals from the group "0.001 - 1% by weight Sr, 0.001 - 1 wt% Ca, 0.0001 - 0.1 wt% Be, 0.001 - 1 wt% Ba". When the components formed in this way are used at high temperatures, an alloy layer is formed between the steel substrate and the anti-corrosion coating of the flat steel product. The Mg present in the anti-corrosion coating causes Mg or Mg oxides to accumulate on the exposed surfaces of the coating in the area of cracks that develop in the anti-corrosion coating. At the same time, up to 50% by volume of iron oxides can be found in a transitional layer between the anti-corrosion coating and the steel substrate.

From the EP 2 993 248 A1 another method is known in which flat steel products of the type in question are hot-formed. A steel flat product is used as the starting product for this process, the steel substrate of which consists of so-called "MnB steel". Steels of this type are standardized in DIN EN 10083-3 and have good hardenability. They allow safe process control during hot pressing, which makes it possible in an economical way to bring about martensite hardening in the tool without additional cooling during the course of hot forming. A typical example of such a steel is the steel known under the designation 22MnB5, which can be found in the Steel Code 2004 under the material number 1.5528. Typically, fully killed 22MnB5 steel available on the market contains, in addition to iron and unavoidable impurities, in % by weight, 0.10 - 0.250% C, 1.0 - 1.4% Mn, 0.35 - 0.4% Si, up to 0.03% P, up to 0.01% S, up to 0.040% Al, up to 0.15% Ti, up to 0.1% Nb, in total up to 0.5% Cr + Mo, and up to 0.005% B. In order to protect the steel flat products made of such a composition of steel against corrosive attacks and at the same time to minimize the risk of hydrogen absorption during the heating required for hot forming, the steel flat products are coated according to the known method with an anti-corrosion coating on Al -Base which contains effective amounts of 0.005 - 0.7% by weight of at least one alkaline earth metal or transition metal as an additional alloying component. In addition, Si contents of 3-15% by weight and Fe contents of up to 5% by weight can also be present in the coating. Mg in contents of 0.1-0.5% by weight is preferably used as the at least one alkaline earth metal or transition metal of the protective coating, with calcium, strontium, sodium or barium also being possible as a substitute or in addition. The Al-based protective coating can be applied to the steel substrate by hot-dip coating, also known as "hot-dip aluminizing" in technical jargon, or by a gas deposition process, e.g. the well-known PVD (Physical Vapor Deposition) or CVD (Chemical Vapor Deposition) process.

Particular requirements for the manner in which the anti-corrosion coating is applied to the steel substrate consisting of an MnB steel are not mentioned in the prior art explained above. Due to the presence of the alkaline earth metal or transition metal in the coating, if a circuit board coated in the manner described above is heated in a conventional manner under a normal atmosphere for a period of 360-800 s to a temperature of 900 °C, at most a minimal oxygen uptake in the steel substrate.

"Flexible rolling" is a process for the production of metal strips with different strip thicknesses defined over their length. As for example in the DE 198 46 900 A1 or the DE 100 41 280 C2 explained, for this purpose the height of the roll gap provided between two work rolls of a roll stand, through which the flat steel product to be rolled has to pass, is usually varied during rolling. In this way, sections of greater thickness (wider roll gap) and less thickness (narrower roll gap) can be produced on the flat steel product in succession over the length of the flat steel product.

Due to the possibility of producing specific thicknesses on a flat steel product, flexible rolling is ideally suited for producing a flat steel product whose properties are adapted, for example, to the locally limited loads acting on it during use or to the requirements placed on its deformation behavior. For example, flexible rolling can be used to form flat steel products in such a way that a component obtained from such a flat steel product by forming has different sheet thicknesses at the required locations, which enable the component to withstand high loads while having a minimized weight.

If, as in the case of the present invention, flat steel products that are provided with an anti-corrosion coating are to be processed by flexible cold rolling, i.e. flexible rolling that is carried out on a flat steel product that has not been separately preheated, damage to the flat steel products can occur due to the high rolling forces that regularly occur Anti-corrosion coating come in the form of detachments. Diffusible hydrogen, which can be triggered by cracks in the steel substrate, can get into the steel flat product through the holes that form in the anti-corrosion coating. In order to avoid this danger, in today's operational practice, the degree of rolling, ie over a The relative reduction in thickness achieved during the rolling step is limited to certain maximum values at which experience has shown that the anti-corrosion coating is not damaged.

Against this background, the task arose of specifying a method that makes it possible to flexibly hot-roll a flat steel product of the type explained above with high rolling degrees without having to accept the risk of hydrogen penetrating the steel substrate.

In order to solve this problem, the invention proposes that at least the method steps specified in claim 1 be completed during the flexible cold rolling of a flat steel product provided with an anti-corrosion coating.

It goes without saying that when carrying out the method according to the invention, the person skilled in the art not only completes the method steps mentioned in the claims and explained here, but also carries out all other steps and activities that are routine in the practical implementation of such methods in the prior art be carried out if the need arises.

Advantageous refinements of the invention are specified in the dependent claims and, like the general inventive concept, are explained in detail below.

1. a) Bereitstellen eines Stahlflachprodukts, das ein Stahlsubstrat, das aus einem Stahl erzeugt ist, der, in Gew.-%, aus 0,07 - 0,4 % C, 1,0 - 2,5 % Mn, 0,06 - 0,9 % Si, bis zu 0,03 % P, bis zu 0,01 % S, bis zu 0,1 % Al, bis zu 0,15 % Ti, bis zu 0,6 % Nb, bis zu 0,005 % B, bis zu 0,5 % Cr, bis zu 0,5 % Mo, wobei die Summe der Gehalte an Cr und Mo höchstens 0,5 % beträgt, und als Rest aus Eisen und unvermeidbaren Verunreinigungen besteht, und eine auf das Stahlsubstrat applizierte Korrosionsschutzbeschichtung umfasst, die aus, in Gew.-%, bis zu 15 % Si, bis zu 5 % Fe, optional bis zu 5 Gew.-% mindestens eines Erdalkali- oder Übergangsmetalls und als Rest aus AI und unvermeidbaren Verunreinigungen gebildet ist, sodann
2. b) im Fall, dass die Korrosionsschutzbeschichtung kein oder weniger als 0,1 Gew.-% an dem mindestens Erdalkali- oder Übergangsmetall enthält: Applizieren einer mindestens ein Erdalkali- oder Übergangsmetall enthaltenden Lösung auf die Korrosionsschutzbeschichtung des Stahlflachprodukts,
3. c) flexibles Kaltwalzen des Stahlflachprodukts, um an dem Stahlflachprodukt die Abschnitte unterschiedlicher Dicke zu erzeugen, sodann
4. d) Erwärmen des flexibel kaltgewalzten Stahlflachprodukts auf eine 800 - 1000 °C betragende Warmformtemperatur unter einer Atmosphäre, die mehr als 15 Vol.-% Sauerstoff enthält, über eine Haltedauer, die ausreicht, um in das Stahlflachprodukt eine Wärmeenergiemenge Js von mehr als 44.000 kJs und höchstens 400.000 kJs einzubringen, so dass nach dem Erwärmen die Oberfläche der Korrosionsschutzbeschichtung des Stahlflachprodukts dicht mit einer Schicht belegt ist, die aus einem primären Oxid des mindestens einen in der Korrosionsschutzschicht enthaltenen und/oder im Arbeitsschritt b) optional zusätzlich aufgetragenen Erdalkali- oder Übergangsmetalls besteht, sodann
5. e) Warmumformen des Stahlflachprodukts zu dem Blechbauteil.

According to the invention, during the production of a sheet metal component by hot forming a flat steel product that is provided with an anti-corrosion coating and that is given at least one section by flexible cold rolling that has a different thickness than another section of the flat steel product, with the transition between the sections of the flat steel product having different thicknesses being abrupt, the following work steps are completed:

1. a) Providing a steel flat product which comprises a steel substrate which is produced from a steel which, in % by weight, consists of 0.07 - 0.4% C, 1.0 - 2.5% Mn, 0.06 - 0.9% Si, up to 0.03% P, up to 0.01% S, up to 0.1% Al, up to 0.15% Ti, up to 0.6% Nb, up to 0.005% B, up to 0.5% Cr, up to 0.5% Mo, the sum of the contents of Cr and Mo being at most 0.5%, and the balance being iron and unavoidable impurities, and one applied to the steel substrate comprises an anti-corrosion coating, which is formed from, in wt
2. b) in the event that the anti-corrosion coating contains no or less than 0.1% by weight of at least one alkaline earth metal or transition metal: applying a solution containing at least one alkaline earth metal or transition metal to the anti-corrosion coating of the flat steel product,
3. c) flexible cold rolling of the steel flat product to produce the sections of different thickness on the steel flat product, then
4. d) heating the flexible cold-rolled steel flat product to a hot forming temperature of 800-1000 °C in an atmosphere containing more than 15% by volume of oxygen for a holding time sufficient to impart into the steel flat product a quantity of thermal energy Js greater than 44,000 kJs and to introduce at most 400,000 kJs, so that after heating the surface of the anti-corrosion coating of the steel flat product is densely covered with a layer consisting of a primary Oxide of the at least one alkaline earth metal or transition metal contained in the anti-corrosion layer and/or optionally additionally applied in step b), then
5. e) hot forming of the flat steel product to form the sheet metal component.

According to the invention, a flat steel product is thus provided which comprises an MnB steel substrate composed in a specific way and an Al-based anti-corrosion coating applied thereto, in particular by hot-dip coating. When hot-dip coating is carried out in a conventional manner for the purposes of the invention, the flat steel product is passed through a molten bath alloyed according to the invention and the coating layer thickness of the protective layer is adjusted from the flat steel product emerging from the molten bath by means of wiping nozzles. Air is used as the wiping medium. As a result of the impact of the air jet and the associated rapid drop in temperature, the oxide layer on the anti-corrosion layer is "frozen", i.e. it cannot form according to the chemical equilibrium rules.

The anti-corrosion coating of the flat steel product contains at least one alkaline earth metal or transition metal or is wetted with a solution containing at least one such alkaline earth metal or transition metal in step b), which is carried out if necessary. The solution used for this purpose according to the invention is preferably an aqueous solution whose solvent “water” is easily mastered in terms of process technology and is harmless with regard to the environment.

Step b) is carried out when the anti-corrosion coating contains too little of the at least one alkaline earth metal or transition metal. However, wetting with the aqueous solution containing the at least one alkaline earth metal or transition metal, of course, also take place as a supplementary measure if the anti-corrosion coating contains a fundamentally sufficient amount of alkaline earth metal or transition metal, but other amounts of the at least alkaline earth metal or transition metal are on the surface of the Anti-corrosion coating are to be applied in order to ensure the occurrence of the effect used according to the invention of the presence of these metals in or on the anti-corrosion layer.

The alkaline earth metals and transition metals alloyed with the anti-corrosion coating and/or applied in the form of a solution to the surface of the anti-corrosion coating for the purposes of the invention include, in particular, magnesium ("Mg") and calcium ("Ca"), but also beryllium ("Be"), strontium ("Sr") and barium ("Ba").

The application of the solution containing the at least one alkaline earth metal or transition metal, which may be necessary or optional, can take place before or after the flexible rolling. It is essential that before heating to the hot-forming temperature, there is a sufficient quantity of the respective alkaline earth metal or transition metal in or on the anti-corrosion coating.

In step c), the steel flat product provided and optionally coated with the layer containing at least one alkaline earth metal or transition metal is flexibly cold-rolled at room temperature in a conventional manner in order to give it sections of different thicknesses.

With flexible rolling, the steel flat product is rolled with rolling degrees W, which are 0.1 to 80%. The degree of rolling W is calculated according to the formula W = ((U / Xn) - 1) ^∗ 100%, in which "U" denotes the initial thickness of the respective rolled section n before rolling and Xn denotes the thickness of the relevant section n after rolling. That is, with an initial thickness U of 2.75 mm in each case to produce a first section with a thickness X1 of 1.85 mm, a degree of rolling W of 48.64% is required to produce a second section with a thickness X2 of 2.5 mm a rolling degree W of 10.00%, to produce a third section with a thickness X3 of 2.15 mm a rolling degree W of 27.90% and to produce a fourth section with a thickness X4 of 2.25 mm requires a degree of rolling W of 22.22%. Particularly practical rolling degrees W are 0.1 - 60%, in particular 0.1 - 50%. By varying the degree of rolling W in the ranges mentioned, sections of different thicknesses are produced on the flat steel product. The degree of rolling W specifically set in each case depends on the desired extent of reduction in the thickness of the flat steel product compared to the initial state. The range specified here for the degree of rolling W thus only defines the limits within which the degree of rolling set in each case is set according to the invention.

Through flexible rolling, the thickness of the steel flat product is specifically reduced in limited length sections. Due to the constant volume, this reduction in thickness is inevitably accompanied by an elongation of the flat steel product. The aluminum alloy of the anti-corrosion coating on a flat steel product processed according to the invention is so ductile that it can follow the deformation of the flat steel product occurring in the longitudinal and thickness direction, even in the border areas where the sections of different thickness meet.

However, the protective oxide layer on the anti-corrosion coating is much more brittle, with the result that it locally cracks due to the deformation of the steel flat product. The resulting cracks are quickly closed again by newly forming oxides. Since this process takes place in the ambient atmosphere and without separate temperature supply or removal, the new oxide layer can form in such a way that it corresponds to the chemical equilibrium at the location of the crack, taking into account the respective ambient conditions. Damage to the oxide layer originally present during flexible rolling is closed by new oxides formed in the course of cold rolling, so that the finished flexibly rolled flat steel product has a tightly closed oxide layer again. This is characterized by areas where the original oxide layer has remained and areas where a new oxide layer has been formed.

According to the findings of the invention, there is a direct connection between the degree of rolling W set in each case and the proportions in which the flat steel product obtained after flexible cold rolling is covered by original and newly formed oxide layers. In this way, the percentage area A that is occupied by the original oxide can be estimated with an accuracy of ± 5% according to the formula A = 100% - W. Correspondingly, the percentage area B of the surface of the steel flat product obtained after flexible rolling occupied by the newly formed oxide is B=100%−A±5%. If, for example, a rolling degree W of 15% has been rolled, the surface of a flat steel product that has been flexibly hot-rolled according to the invention is 80-90% covered with the original oxide layer formed before the flexible rolling, while the remaining surface is covered with the oxide layer formed in the course of the flexible rolling itself new oxide layer is covered.

In the case of a flat steel product flexibly rolled in the manner according to the invention, the ratio of the Si and Al content of the oxide layer and the ratio of the Al, Si and Mg content of the oxide layer also depend on the values set by flexible rolling Degree of rolling W. For example, in the case where Mg, as the at least one alkaline earth metal or transition element, is alloyed to the anti-corrosion coating of the flat steel product processed according to the invention or has been applied to this anti-corrosion coating, the total oxide layer present on the flat steel product after flexible cold rolling is %Al/ %Si ≥ 6.4 x W ^-0.1 , while the ratio %Al/%Mg ≥ (2.66xW ^0.11 ) ± 1 applies (with %AI = Al content of the total oxide layer in atomic %, %Si = Si content of the entire oxide layer in atomic %, %Mg = Mg content of the entire oxide layer in atomic %.

The original oxide layer present on the flat steel product processed according to the invention before flexible cold rolling typically consists of silicon, magnesium and aluminum oxides, the proportion of Si being significantly smaller than the proportion of Mg, which in turn is smaller than the proportion of Al. Typically, given in atomic %, the oxide layer contains 10-40% C, 30-60% O, 4-30% Al, 0-5% Si and 1-20% of at least one alkaline earth metal or transition metal, in particular mg, before In addition, small amounts of Fe of up to 10 atom % can be present in the oxide layer. This applies in particular if the anti-corrosion coating has been applied by hot dip coating. The thickness of the original oxide layer is typically 5-600 nm, in particular 5-300 nm, particularly preferably 5-150 nm. The original oxide layer covers the surface of the anti-corrosion coating completely, ie 100%.

The oxide layer newly formed by the flexible cold rolling, which can form in equilibrium, also essentially consists of oxides of silicon, magnesium and aluminum. The quantity distribution of the Si, Mg and Al oxides corresponds to their distribution in the primary oxide layer. The secondary oxide layer typically consists of, in atom %, 10 - 40 % C, 40-60% O, 20-30% Al, 0-5% Si and 1-20% of at least one alkaline earth metal or transition metal, in particular Mg, with small traces of iron of up to 10 atoms also in the secondary oxide layer -% may be included. The thickness of the secondary oxide layer is 1-100 nm, in particular 1-80 nm or 1-50 nm, with thicknesses of up to 30 nm having turned out to be particularly favorable. The percentage area F _ox of the secondary oxide layer in the total oxide layer covering the anti-corrosion coating of the steel flat product processed according to the invention after flexible cold rolling is related to the degree of rolling W, where F _ox < W.

The compositions of the oxide layers can be determined using X-ray photoelectron spectroscopy (XPS). For this purpose, the sample of the steel flat product to be examined, for which the composition and thickness are to be determined, is degreased with n-heptane, rinsed with propanol and blown off in air. The sample is then attached to a sample carrier, introduced into the measuring chamber of the X-ray photoelectron spectroscope and examined in a high vacuum. The boiler pressure is typically less than 5x10 ⁸ mbar. Argon is typically used as the bombardment gas. The radiation was excited as Al K with a bombardment voltage of 2 or 4 kV. At least one measurement regarding the composition and oxide layer thickness is carried out on each sample. Typically, several samples of a board are examined and the results of all samples of the relevant board are arithmetically averaged. The composition and thickness determined in this way of the oxide layer present on the circuit board examined is therefore also referred to as "average composition" or "average thickness".

After flexible rolling, the steel flat product is heated to a hot forming temperature, if necessary from that previously For example, in the form of a steel strip or larger sheet steel flat product, at least one is separated, which is then further processed according to the invention as a steel flat product.

The composition of the anti-corrosion coating selected according to the invention and/or the additional application of the alkaline earth metal or transition metal to the anti-corrosion coating by means of the aqueous solution ensures that a primary oxide layer formed from the at least one alkaline earth metal or transition metal is formed as a result of the heat treatment carried out before hot forming of the anti-corrosion coating.

The invention is based on the finding that on a steel flat product that is provided with an aluminum-based ("Al-based") anti-corrosion coating, which is doped with at least one alkaline earth metal or transition metal according to the invention, during the hot forming carried out heating on the anti-corrosion coating forms an oxide layer ("primary oxide layer"), which protects the underlying layers of the anti-corrosion coating and thus the steel substrate of the steel flat product against exposure to the ambient atmosphere. The primary oxide layer in question is formed in such a way that it is in chemical equilibrium under the conditions prevailing during heating, in particular those determined by the respective hot-forming temperature. This process also continues during and after hot forming. Injuries to the oxide layer present before heating and hot forming are closed very quickly. Due to the oxygen affinity of the elements Al, Mg and Si of the anti-corrosion layer, an oxide layer forms immediately as soon as the surface of the anti-corrosion layer is exposed to even the smallest amounts of oxygen. The reactivity of the alkaline earth metals or transition metals provided according to the invention in and/or on the anti-corrosion layer, so that the oxides of the newly formed oxide layer form within such a short time that the penetration of harmful substances from the environment is reliably prevented.

In this way, in the case of a steel flat product heated to the respective hot-forming temperature according to the invention, not only is its steel substrate generally protected against corrosive attack. The oxide layer present on the anti-corrosion coating, formed in particular from the alkaline earth metals or transition metals provided according to the invention, covers the underlying aluminum of the anti-corrosion coating, so that contact of the Al with the ambient moisture and the associated elimination of larger amounts of hydrogen during heating to the hot-forming temperature or the hot forming itself can be prevented. The penetration of relatively large amounts of hydrogen into the anti-corrosion coating and the steel substrate of a flat steel product processed according to the invention can thus be effectively suppressed.

The effects used by the invention occur particularly reliably when the alkaline earth metal or transition metal additionally present in the anti-corrosion coating or additionally applied to the anti-corrosion coating is magnesium ("Mg"), i.e. when Mg alone or in combination with others elements belonging to the group of alkaline earth metals or transition metals are present in the levels provided according to the invention in the anti-corrosion coating provided according to the invention of a flat steel product processed according to the invention or is additionally applied by means of the aqueous solution if the alkaline earth metal or transition metal content in the anti-corrosion coating is too low.

The method according to the invention is suitable for processing flat steel products with a wide range of thicknesses. With the method according to the invention, flat steel products can be processed with a thickness of 0.6-7 mm.

The flat steel product provided in step a) can be produced in any manner known from the prior art. The method according to the invention is particularly suitable for processing flat steel products with a thickness of 0.8-4 mm, in particular 0.8-3 mm.

For the method according to the invention, flat steel products can also be provided in step a), which are formed from a stack of metal sheets comprising, for example, three to five metal layers, which have been connected in a manner known per se, for example by roll cladding, to form a uniform flat steel product. Likewise, in step a) for the method according to the invention, in the manner of Taylored blanks from different sheet metal blanks welded to one another or similarly assembled flat steel products and steel strips, which are welded to one another and together form the flat steel product to be processed, can be provided for the process according to the invention.

The respective flat steel product provided according to the invention consists of a steel which has a composition typical of MnB steels. Such steels typically have yield strengths of 250 - 580 MPa and tensile strengths of 400 - 720 MPa in the as-delivered condition.

Due to their property profile, in particular their potential for developing high strength, they are of particular interest in practice Flat steel products, the steel substrate of which consists, in a manner known per se, of 0.07 - 0.4% by weight C, 1.0 - 2% by weight Mn, 0.06 - 0.4% by weight Si, up to 0 .03 wt% P, up to 0.01 wt% S, up to 0.1 wt% Al, up to 0.15 wt% Ti, up to 0.6 wt% Nb, up to 0.005% by weight B, up to 0.5% by weight Cr, up to 0.5% by weight Mo, the sum of the contents of Cr and Mo being at most 0.5% by weight is, the remainder consists of iron and unavoidable impurities.

This includes steels that are already in series use and consist of 0.07-0.4% by weight C, 1.0-1.5% by weight Mn, 0.3-0.4% by weight Si, bis up to 0.03% by weight P, up to 0.01% by weight S, up to 0.05% by weight Al, up to 0.15% by weight Ti, up to 0.6% by weight -% Nb, up to 0.005% by weight B, up to 0.5% by weight Cr, up to 0.5% by weight Mo, the sum of the Cr and Mo contents not exceeding 0.5% by weight. -%, the balance being iron and unavoidable impurities. Steels composed in this way achieve tensile strengths of up to 2000 MPa after hot forming and cooling.

The prerequisite for the effects achieved according to the invention is the presence of at least one alkaline earth metal or transition metal in or on the Al-based anti-corrosion coating provided according to the invention. A sufficient amount of alkaline earth metal or transition metal can be alloyed with the anti-corrosion coating. The minimum required alkaline earth metal or transition metal content in the anti-corrosion coating is 0.1% by weight and can reach up to 5% by weight. Alkaline earth or transition metal contents of at least 0.11 wt permit. If the alkaline earth metal or transition metal content is more than 5% by weight, the Oxide layer and thus dust formation, which should be avoided. In order to avoid this consequence in a particularly reliable manner, the alkaline earth metal or transition metal content of the anti-corrosion coating applied in step a) can be limited to a total of at most 1.5% by weight, in particular at most 0.6% by weight. If the alloy of the anti-corrosion coating present on the steel substrate of a flat steel product processed according to the invention contains alkaline earth metals or transition metals that are sufficiently effective for the purposes according to the invention, these amounts to 0.1-5% by weight, in particular 0.11- 1.5% by weight or, especially, 0.11-0.6% by weight.

The optional application of the solution containing the respective alkaline earth metal or transition metal (step b)) can take place directly after the application of the anti-corrosion layer inline by spraying and squeezing or by conventional coil coating. In practice, salt solutions with up to 200 g/l are used for this purpose.

The alkaline earth or transition metals can be present as sulfates, phosphates and nitrates or in oxidic form as a dispersion of alkaline earth metal or transition metal oxide particles. Chlorides should not be used due to the potential for corrosive attack. Silicates can also find application. However, it should be noted here that these connections can impede further processing due to possible silicon connections. Fluorine compounds are not suitable because they can react to form hydrofluoric acid when heated to the hot forming temperature. Mixtures formed from compounds of the type discussed herein and/or different alkaline earth or transition metals can also be used. In order to support the formation of the oxide layer to be produced according to the invention, the solution applied according to the invention, if necessary, to the surface of the anti-corrosion layer can additionally a network former such as bismuth nitrate and/or a wetting agent such as a surfactant.

A separate drying treatment ("baking") is not normally necessary.

Preferably, the drying of the solution applied if necessary takes place by utilizing the process heat. If, for example, work step b) required according to the invention is to be carried out inline in a hot-dip coating system, the aqueous solution containing the at least one alkaline earth metal or transition metal can be applied at one point after the flat steel product has emerged from the molten bath and the coating thicknesses have been adjusted , at which the treated flat steel product is still warm enough for the solvent in the solution to evaporate quickly after contact with the surface of the flat steel product, i.e. the applied layer dries quickly.

As an alternative to an application that is integrated into the process, the solution can also be applied in an additional process step on a conventional coil coating system.

A separate drying treatment can be useful if you want to ensure that the solution is dry before further processing. This applies in particular when water is used as the solvent.

If water is used as a solvent, before reeling or stacking the flat steel product treated according to the invention, care should be taken that no residual water remains on the surface. On the one hand, residual water could initiate corrosion processes. In addition, there would be There is a risk that water coming into intensive contact with the aluminum surface will be split into oxygen and hydrogen, which would increase the risk of hydrogen absorption.

In order to bring about effective drying, either the flat steel product itself can be 100-250° C., in particular 100-180° C., warm when the at least one alkaline earth metal or transition metal-containing solution is applied, or it can be subjected to a drying treatment at these temperatures. Typical drying times are 0-300 s, in particular 10-60 s. Drying times of "0 s" are achieved when the steel flat product or its surroundings are so hot when the solution is applied that the respective solvent when it hits the The surface of the anti-corrosion layer evaporates spontaneously, ie without waiting.

In practice, it will be the rule that at least work steps a) and c) are completed by the producer of the steel flat product and work steps d) and e) of the method according to the invention by the end processor, ie the customer of the producer of the steel flat product, with before or after work step c), work step b) can also be carried out in the factory of the manufacturer of the flat steel product. In terms of process economy, it can be expedient to apply the solution containing the at least one alkaline earth metal or transition metal immediately before the flat steel product enters the furnace provided for heating to the hot-forming temperature. With this variant, care should be taken to ensure that no solvent, especially water, gets into the oven. It should therefore be ensured that the flat steel product coated according to the invention is completely dry when it enters the furnace. Otherwise, the humidity brought into the oven by the water could lead to an excessive increase in the humidity of the oven atmosphere and thus to an unwanted Increase the dew point, which in turn would entail the risk of increased hydrogen absorption during the hot forming process.

The anti-corrosion coating of the flat steel product provided according to the invention can optionally contain silicon ("Si") in amounts of up to 15% by weight, in particular up to 11% by weight, in order to reduce the formation of an iron-aluminum phase. Si contents of at least 3 wt. 11% by weight, specifically 8.5-11% by weight, allow the positive influences of Si to be used particularly reliably in practice. With Si contents of at least 3% by weight, it is ensured that the alloy layer between the steel substrate and the anti-corrosion layer of a steel flat product according to the invention does not become too thick and optimal further processing properties are retained.

Likewise optionally, Fe can be present in the anti-corrosion coating provided on a flat steel product provided according to the invention in contents of up to 5% by weight, in particular up to 4% by weight, especially up to 3.5% by weight. The Fe content is mainly due to the diffusion of Fe from the steel substrate and contributes to the optimal adhesion of the protective layer to the substrate. Fe contents of at least 1 wt. 5% by weight, the positive influences of the presence of Fe can be used particularly reliably in practice.

The anti-corrosion coating can be applied to the steel substrate of a flat steel product according to the invention in any known manner. Hot-dip coating, also known as "hot-dip aluminizing", is particularly suitable for this purpose, in which the respective flat steel product is a suitably heated molten bath composed in accordance with the provisions of the invention with regard to the composition of the anti-corrosion coating is passed. Such a hot-dip coating is particularly suitable for strip-shaped flat steel products with a thickness of up to 3 mm. In the case of greater thicknesses, one of the vapor deposition processes (PVD, CVD) already mentioned at the beginning can also have been used in order to apply the anti-corrosion coating.

The application weight of an anti-corrosion coating present on a flat steel product processed according to the invention is typically 30-100 g/m ² , in particular 40-80 g/m ² , per side of the flat steel product.

As already mentioned, from the group of alkaline earth metals or transition metals, Mg in particular has proven to be suitable for the purposes according to the invention. Mg can be present alone or in combination with other alkaline earth metals or transition metals, such as the elements beryllium, calcium, strontium and/or barium already mentioned, in the coating applied according to the invention in order to use the effects aimed at according to the invention.

mit

T2:

Endtemperatur des Bauteils am Ende der Erwärmung in K

T1:

Starttemperatur des Bauteils zu Beginn der Erwärmung in K

c:

Wärmekapazität Stahl (typischerweise 460 J/kgK)

t:

Haltezeit des Stahlflachprodukts oder der Platine auf der Endtemperatur in s

m:

Masse des Stahlflachprodukts oder der Platine in kg

In step c), the flat steel product provided according to the invention is heated to a hot-forming temperature of 800-1000° C., in particular 850-950° C., and is kept at this temperature until a sufficient amount of heat has been introduced into the flat steel product or a blank separated from it. Hot forming temperatures of 850 - 930 °C have proven to be particularly favourable. The specifically required holding time and annealing temperature can be estimated based on the proviso that the amount of heat energy Js introduced into the steel flat product or the blank in step c) is more than 40,000 kJs and should be at most 400,000 kJs, where Js can be calculated according to the following well-known equation: $$\mathrm{js}\left[\mathrm{kJs}\right]=\left[\left(\mathrm{T}\mathrm{2}-\mathrm{T}\mathrm{1}\right)\times \mathrm{c}\times \mathrm{t}\times \mathrm{m}\right]/\mathrm{1000};$$

with

T2:

Final temperature of the component at the end of heating in K

T1:

Start temperature of the component at the start of heating in K

c:

Heat capacity of steel (typically 460 J/kgK)

t:

Holding time of the flat steel product or blank at the final temperature in s

m:

Mass of the steel flat product or blank in kg

The heating can be carried out in any suitable way. If a conventional continuous furnace is used for this purpose, in which the steel flat product or the blank is heated by radiant heat, the suitable holding time is typically 100-900 s, in particular 100-600 s or, particularly practical, 180-600 s If a hot forming temperature of 850 - 930 °C is selected, holding times of 180 - 600 s are usually sufficient in practice.

Optionally, a pre-alloying of the anti-corrosion layer can be carried out before hot forming in combination with heating to the hot forming temperature or as a separate treatment step. For this purpose, the flat steel product can be kept at temperatures of 650-1100° C. for a period of 10-240 s, in particular 30-90 s.

The flat steel product heated in the manner according to the invention is fed within a transfer time customary in practice to a hot-forming device in which the flat steel product is hot-formed into the component (step e)).

The invention is explained below using exemplary embodiments.

Conventionally alloyed MnB steel sheets A - F were made available for nine tests V1 - V9, the compositions of which are given in Table 1.

Steel sheets each had a thickness D and were provided with an Al-based anti-corrosion coating in a conventional manner by hot-dip coating. Five variants Z1 - Z5 of such an anti-corrosion coating were used, the compositions of which are given in Table 2. Each of the anti-corrosion coatings Z1-Z5 contained the Mg content shown in Table 2 as the alkaline earth metal or transition metal added in accordance with the provisions of the invention.

The steel sheets AF, each provided with one of the anti-corrosion coatings Z1-Z5, have been flexibly cold-rolled in a conventional manner, with a rolling degree W being achieved in each case via this cold-rolling.

After flexible rolling, the steel sheets A - F, each provided with one of the anti-corrosion coatings Z1 - Z5, were heated in a conventional continuous furnace to a hot forming temperature of 850 - 930 °C in each case, with the holding time at the respective hot forming temperature being varied so that a sufficient amount of energy EE has been introduced into the respective sheet. In the tests V4 and V6, the heating was carried out in two stages in order to first bring about a pre-alloying of the anti-corrosion coating. All other tests V1 - V3, V5 and V7 - V9 were heated in one stage.

The sheet metal samples AF heated in this way to the respective hot forming temperature have been hot formed in a conventional manner in a tool provided for this purpose to form a sheet metal component.

After the hot forming, the steel sheets obtained were cooled to room temperature at a cooling rate of 20-1000 K/s.

Table 3 shows the steel of the steel substrate of the steel sheet used in the tests V1 - V9, the respective coating applied to the steel sheet in question, the thickness D of the sheet metal samples examined, the coating weight of the coating before heating on the Hot forming temperature, the amount of heat introduced during heating to the hot forming temperature and the degree of rolling W achieved via flexible cold rolling are given.

On the steel sheets obtained after flexible cold rolling, the percentage of area %OB of the newly formed oxide layer OB, which was formed in the course of flexible cold rolling on the anti-corrosion coating of the steel sheet processed in each case, is determined by means of XPS analysis on the oxide layer densely covering the surface of the steel sheet overall been. The rest of the oxide layer on the samples consisted of the original oxide layer OA that was already present before the flexible cold rolling, the area percentage of which %OA on the entire surface of samples AF densely covered by the oxide layer was thus %OA = 100% - %OB.

Likewise, the thicknesses D_OA of the original oxide layers OA present before the flexible rolling, the thicknesses D_OB of the oxide layers OB newly formed via the flexible rolling and present after the flexible rolling, and the thickness D_OP present after the hot forming are the same as the XPS measurement during heating the oxide layer formed on the hot-forming temperature and present on the component obtained after hot-forming has been determined. The relevant measurement results are summarized in Table 4.

The compositions of the oxide layer present on the anti-corrosion coating before flexible rolling, between flexible rolling and heating to the hot forming temperature and after hot forming were also determined on samples A - F using XPS measurements.

Finally, the increase in the hydrogen content, which got into the steel sheet during heating and cold rolling, was also determined using XPS analysis.

The results of these investigations are summarized in Table 5. The slight increase in the content of diffusible hydrogen proves the effectiveness of the oxide layer, which is set as a result of the doping of the Al-based anti-corrosion coating with Mg provided according to the invention, on the one hand during flexible rolling and on the other hand with heating to the hot forming temperature, and damage spots that are caused by the flexible Rollers or hot forming result, close again within a very short time due to the immediate renewed reaction of the Mg with the ambient oxygen, so that at most minimal amounts of hydrogen can penetrate the anti-corrosion coating. Table 1 stole C si Mn P S Al Nb Ti B A 0.08 0.33 0.95 0.025 0.020 0.013 0.09 0.010 0.005 B 0.23 0.38 1.3 0.020 0.007 0.013 - 0.03 0.004 C 0.38 0.37 1.38 0.020 0.008 0.013 - 0.10 0.005 D 0.20 0.35 1.35 0.020 0.008 0.012 - 0.02 0.004 E 0.14 0.25 1.07 0.010 0.001 0.08 0.025 0.010 0.002 f 0.24 0.30 1.3 0.022 0.008 0.012 - 0.02 0.004 Data in % by weight, remainder Fe and unavoidable impurities Anti-corrosion coating before hot forming mg si feet Z1 0.3 9.5 3 Z2 0.5 8th 3.5 Z3 0.1 10 3 Z4 2 8th 2.0 Z5 0.8 8th 3 Data in % by weight, remainder Al and unavoidable impurities attempt stole Thick D anti-corrosion coating Print run weight per page rolling grade W Amount of energy EE brought in [mm] [g/m ² ]* [%] [kJs/kg] V1 A 2.95 Z3 69 30 74.106 v2 B 1.5 Z2 70 30 123,510 V3 C 2.95 Z3 75 50 247,020 V4 D 1.5 Z5 65 30 79,350 + 49680 V5 E 1.5 Z1 70 50 74.106 V6 f 2.95 Z4 71 30 262,200 + 40,365 V7 B 2.95 Z1 65 50 247,020 V8 B 1.5 Z3 72 50 123,510 V9 D 1.5 Z2 71 30 123,510 attempt D_OA D_OB D_OP %IF [nm] [%] V1 280 10 12 15 v2 300 20 22 10 V3 150 7 8th 24 V4 130 9 10 17 V5 150 8th 9 27 V6 130 8th 9 21 V7 250 18 20 21 V8 130 7 8th 27 V9 130 9 10 22 attempt Composition of the oxide layer before flexible rolling Composition of the oxide layer between flexible rolling and heating to the hot working temperature Composition of the oxide layer after hot working Increase in diffusible hydrogen O Al si mg C O Al si mg C O Al si mg C Diffusible hydrogen [Atom-%] [Atom-%] [Atom-%] [ppm] V1 35 18.40 4 5 rest 35 21 4.5 4.0 rest 49 28 8.0 1.0 rest 0.22 v2 38 16:10 3.5 4 38 25 4.9 4.7 50 26 9.0 0.5 0.28 V3 45 12.00 2.7 3 45 23 4.8 4.5 51 25 8.5 2.0 0.27 V4 55 14.50 3.1 4 55 29 6.0 6.2 45 30 10.0 1.0 0.21 V5 54 17.90 4.1 4 54 27 4.7 5.4 44 31 10.0 1.0 0.31 V6 37 7.50 1.5 2 37 28 4.9 5.8 51 27 8.0 1.5 0.24 V7 41 14.80 3.3 4 41 29 5.0 5.1 49 28 8.0 0.5 0.31 V8 38 21.40 4.9 5 38 22 4.8 4.2 48 29 7.0 2.0 0.29 V9 40 10.00 2.1 3 40 21 4.1 3.8 53 24 8.0 1.0 0.24

Claims (12)

Method for producing a sheet metal component by hot forming a steel flat product, which is provided with an anti-corrosion coating and which is given at least one section by flexible cold rolling, which has a different thickness than another section of the steel flat product adjoining it, the transition between the different thicknesses Sections of the steel flat product takes place abruptly, comprising the following work steps: a) Providing a steel flat product which comprises a steel substrate which is produced from a steel which, in % by weight, consists of 0.07 - 0.4% C, 1.0 - 2.5% Mn, 0.06 - 0.9% Si, up to 0.03% P, up to 0.01% S, up to 0.1% Al, up to 0.15% Ti, up to 0.6% Nb, up to 0.005% B, up to 0.5% Cr, up to 0.5% Mo, the sum of the contents of Cr and Mo being at most 0.5%, and the balance being iron and unavoidable impurities, and one applied to the steel substrate comprises an anti-corrosion coating formed from, in % by weight, up to 15% Si, up to 5% Fe, optionally up to 5% of at least one alkaline earth metal or transition metal and the balance Al and unavoidable impurities, then b) in the event that the anti-corrosion coating contains no or less than 0.1% by weight of at least one alkaline earth metal or transition metal: applying a solution containing at least one alkaline earth metal or transition metal to the anti-corrosion coating of the flat steel product, c) flexible cold rolling of the steel flat product to produce the sections of different thickness on the steel flat product, then d) heating the flexible cold-rolled steel flat product to a hot forming temperature of 800-1000 °C in an atmosphere containing more than 15% by volume of oxygen for a holding time sufficient to impart into the steel flat product a quantity of thermal energy Js greater than 44,000 kJs and a maximum of 400,000 kJs, so that after heating the surface of the anti-corrosion coating of the steel flat product is densely covered with a layer consisting of a primary oxide of at least one alkaline earth metal or transition metal contained in the anti-corrosion layer and/or optionally additionally applied in step b). exists, then e) hot forming of the flat steel product to form the sheet metal component. Method according to Claim 1, characterized in that the thickness of the flat steel product provided in step a) is 0.6 - 7 mm. Method according to one of the preceding claims, characterized in that the Si content of the anti-corrosion coating of the flat steel product is at least 3% by weight. The method according to any one of the preceding claims, characterized in that the Fe content of Anti-corrosion coating of the steel flat product is at least 1% by weight. Method according to one of the preceding claims, characterized in that the anti-corrosion coating of the flat steel product contains a total content of at least 0.1% by weight of alkaline earth metals or transition metals. Method according to Claim 5, characterized in that the anti-corrosion coating of the flat steel product contains a total content of at least 0.11% by weight of alkaline earth metals or transition metals. Method according to Claim 6, characterized in that the anti-corrosion coating of the flat steel product contains a total content of at most 1.5% by weight of alkaline earth metals or transition metals. Method according to Claim 7, characterized in that the anti-corrosion coating of the flat steel product contains a total content of at most 0.6% by weight of alkaline earth metals or transition metals. Method according to one of the preceding claims, characterized in that the anti-corrosion coating of the flat steel product or the solution applied according to step b) contains magnesium as the at least one alkaline earth metal or transition metal. Method according to one of the preceding claims, characterized in that the coating weight of the anti-corrosion coating of the flat steel product is 30 - 100 g/m ² per coated side of the flat steel product. Method according to one of the preceding claims, characterized in that the anti-corrosion coating is applied to the steel substrate of the flat steel product by hot-dip coating. Method according to one of the preceding claims, characterized in that the flat steel product is heated in step c) in a continuous furnace by radiant heat and the holding time is 100 - 900 s.