DE102022132918A1 - Sheet metal part with improved hardness profile - Google Patents

Abstract

The invention relates to a sheet metal part formed from a steel sheet blank, comprising a steel substrate (1) which consists of a steel which has 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B. The sheet metal part has an aluminum-based anti-corrosive coating on at least one side and is characterized in that the amount of the hardness gradient of the anti-corrosive coating and the steel substrate perpendicular to the surface of the steel substrate is less than 1.7 GPa/µm. The invention further relates to a method for producing such a sheet metal part.

Description

The invention relates to a method for producing a sheet metal part by hot forming a steel sheet blank.

“Steel sheet blanks” or “sheet metal blanks” are understood here to mean blanks from flat steel products, such as blanks. When we talk about a “steel flat product” or a “sheet metal product”, we mean rolled products such as steel strips or sheets from which “sheet metal blanks” (also called blanks) are cut for the manufacture of, for example, bodywork components. “Formed sheet metal parts” or “sheet metal components” are made from such sheet metal blanks, whereby the terms “formed sheet metal part” and “sheet metal component” are used synonymously here.

All information on contents of the steel compositions specified in the present application is based on weight, unless expressly stated otherwise. All unspecified "% data" in connection with a steel alloy are therefore to be understood as data in "wt. %". With the exception of the data on the residual austenite content of the structure of a sheet metal molding according to the invention, which is based on the volume (data in "vol. .-%"), data on the contents of the various structural components (e.g. martensite) refer to the area of a ground section of a sample of the respective product (data in area percent "area %"), unless expressly stated otherwise. Information given in this text on the contents of the components of an atmosphere refers to the volume (data in "vol. .-%").

Where formulae or conditions are used in this text in which values are calculated or formed on the basis of contents of certain alloying elements, the relevant contents of alloying elements are entered in these formulae or conditions in % by weight, unless otherwise stated.

From the WO 2022/048990 A1 Flat steel products and sheet metal parts and processes for their production are known. The flat steel products and sheet metal parts have an aluminum-based anti-corrosive coating that is produced by hot-dip coating. The melt used has a Si content of 0.05-3% by weight.

Such flat steel products have an aluminum-based coating and are further processed into sheet metal parts by hot forming. Cut pieces from the flat steel products are heated to a hot forming temperature (e.g. 900°C) for a certain annealing time (e.g. 4 minutes). During this annealing time, iron diffuses from the steel substrate into the aluminum-based coating. This results in a coating that provides very effective protection against corrosion. The hot cut piece is then formed into a sheet metal part in a forming tool and cooled quickly, which forms a hard structure (e.g. martensite) in the steel substrate. The result is a sheet metal part with high strength and a coating that provides very good protection against corrosion.

The object of the present invention is to further develop such sheet metal parts and their manufacturing processes in such a way that improved mechanical properties are achieved. In particular, the formation of cracks during subsequent cold forming (for example in the event of a crash) should be reduced.

1. a) Zurverfügungstellen einer Bramme oder einer Dünnbramme, die aus einem Stahl besteht, der 0,1-3 Gew.-% Mn und optional bis zu 0,01 Gew.-% B aufweist;
2. b) Durcherwärmen der Bramme oder Dünnbramme bei einer Temperatur (T1) von 1000-1400°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) Optionales 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 Eintauchtemperatur (T6), welche 600-800°C, bevorzugt 680-720°C beträgt;
10. j) Beschichten des auf die Eintauchtemperatur abgekühlten Stahlflachprodukts mit einem Überzug durch
    1. i. Eintauchen in ein Schmelzbad mit einer Schmelzentemperatur (T7) 660-800°C, bevorzugt 670-710°C, wobei das Schmelzbad aus 0,5-4 Gew.-% Si, optional 2-4 Gew.-% Fe, optional bis zu 5,0 Gew.-% Alkali- oder Erdalkalimetalle, optional bis zu 10 % Zn und optionalen weiteren Bestandteilen, deren Gehalte in Summe auf höchstens 2,0 Gew.-% beschränkt sind, und als Rest Aluminium besteht;
    2. ii. Abblasen des Stahlflachproduktes nach Austritt aus dem Schmelzbad mittels eines Gasstroms;
11. k) Abkühlen des beschichteten Stahlflachprodukts auf Raumtemperatur, wobei eine mittlere Abkühlrate zwischen 660°C und 570°C mindestens 15 K/s beträgt;
12. l) optionales Dressieren des beschichteten Stahlflachprodukts.

This object is achieved by a method for producing a flat steel product for hot forming with a coating comprising the following work steps:

1. (a) providing a slab or a thin slab consisting of a steel containing 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B;
2. b) heating the slab or thin slab at a temperature (T1) of 1000-1400°C;
3. c) optional pre-rolling of the heated slab or thin slab to an intermediate product with an intermediate product temperature (T2) of 1000-1200°C;
4. d) hot rolling to a hot-rolled flat steel product, the final rolling temperature (T3) being 750-1000°C;
5. (e) optional coiling of the hot-rolled flat steel product, whereby the coiling temperature (T4) is not more than 700°C;
6. f) Optional descaling of the hot-rolled flat steel product;
7. (g) optional cold rolling of the flat steel product, with a cold rolling degree of at least 30 %;
8. h) annealing the flat steel product at an annealing temperature (T5) of 650-900°C;
9. i) cooling the flat steel product to an immersion temperature (T6) which is 600-800°C, preferably 680-720°C;
10. j) Coating the steel flat product cooled to the immersion temperature with a coating by
    1. i. Immersion in a melt bath with a melt temperature (T7) of 660-800°C, preferably 670-710°C, the melt bath consisting of 0.5-4 wt.% Si, optionally 2-4 wt.% Fe, optionally up to 5.0 wt.% alkali or alkaline earth metals, optionally up to 10% Zn and optionally further constituents, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminium;
    2. ii. Blowing off the flat steel product after it has left the molten bath by means of a gas stream;
11. k) cooling the coated flat steel product to room temperature, with an average cooling rate between 660°C and 570°C of at least 15 K/s;
12. l) optional skin passing of the coated flat steel product.

Surprisingly, it has been shown that the Si content of the melt in the range of 0.5-4 wt.% Si, in particular in the range of 0.5-1.5 wt.%, in combination with the special cooling rate in step k) leads to a flat steel product that can be formed into a sheet metal part with improved properties. The mechanism in this regard is explained in detail below.

In step a), a semi-finished product composed according to the alloy specified for the flat steel product according to the invention is provided. This can be a slab produced by conventional continuous slab casting or by thin slab casting.

In step b), the semi-finished product is heated to a temperature (T1) of 1000-1400°C. If the semi-finished product has cooled down after casting, it is first reheated to 1000-1400°C for heating. The heating temperature should be at least 1000°C to ensure good formability for the subsequent rolling process. The heating temperature should not be more than 1400°C to avoid molten phases in the semi-finished product.

In the optional step c), the semi-finished product is pre-rolled to form an intermediate product. Thin slabs are usually not subjected to pre-rolling. Thick slabs that are to be rolled into hot strips can be subjected to pre-rolling if required. In this case, the temperature of the intermediate product (T2) at the end of pre-rolling should be at least 1000°C so that the intermediate product contains sufficient heat for the subsequent step of finish rolling. However, high rolling temperatures can also promote grain growth during the rolling process, which has a detrimental effect on the mechanical properties of the flat steel product. In order to keep grain growth during the rolling process to a minimum, the temperature of the intermediate product at the end of pre-rolling should not exceed 1200°C.

In step d), the slab or thin slab or, if step c) has been carried out, the intermediate product is rolled into a hot-rolled flat steel product. If step c) has been carried out, the intermediate product is typically finish-rolled immediately after rough rolling. Finish rolling typically begins no later than 90 seconds 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 final rolling temperature (T3). The final rolling temperature, i.e. the temperature of the finished hot-rolled flat steel product at the end of the hot rolling process, is 750-1000°C. At final rolling temperatures below 750°C, the amount of free vanadium decreases because larger amounts of vanadium carbides are precipitated. The vanadium carbides precipitated during finish rolling are very large. They typically have an average grain size of 30 nm or more and are no longer dissolved in subsequent annealing processes, such as those carried out before hot-dip coating. The final rolling temperature is limited to values of no more than 1000°C in order to prevent coarsening of the austenite grains. In addition, final rolling temperatures of no more than 1000°C are relevant from a process engineering perspective for setting coiler temperatures (T4) of less than 700°C.

The hot rolling of the steel flat product can be carried out as continuous hot strip rolling or as reversing rolling. In the case of continuous hot strip rolling, step e) provides for a optional coiling of the hot-rolled flat steel product. For this purpose, the hot strip is cooled to a coiling temperature (T4) within less than 50 seconds after hot rolling. The cooling medium used for this can be water, air or a combination of both. The coiling temperature (T4) should not exceed 700°C in order 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 rolling. The coiled hot strip is then cooled to room temperature in air in the conventional manner.

In step f), the hot-rolled flat steel product is optionally 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 cleaned of scale can optionally be subjected to cold rolling before the annealing treatment in step g), for example to meet higher requirements for the thickness tolerances of the flat steel product. The cold rolling degree (KWG) should be at least 30% in order to introduce sufficient deformation energy into the flat steel product for rapid recrystallization. The cold rolling degree KWG is understood to be the quotient of the thickness reduction during cold rolling ΔdKW divided by the hot strip thickness d: $$\text{KWG}=\mathrm{\Delta }\text{dKW}/\text{d}$$

with ΔdKW = thickness reduction during cold rolling in mm and d = hot strip thickness in mm, whereby the thickness reduction 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 of d. The flat steel product after cold rolling is usually also referred to as cold strip. The cold rolling degree can in principle assume very high values of over 90%. However, cold rolling degrees of no more than 80% have proven to be beneficial for preventing strip breaks.

In step h), the flat steel product is subjected to an annealing treatment at annealing temperatures (T5) of 650-900°C. To do this, the flat steel product is first heated to the annealing temperature within 10 to 120s and then held at the annealing temperature for 30 to 600s. The annealing temperature is at least 650°C, preferably at least 720°C. Annealing temperatures above 900°C are undesirable for economic reasons.

In step i), the flat steel product is cooled to an immersion temperature (T6) after annealing in order to prepare it for the subsequent coating treatment. The immersion temperature is lower than the annealing temperature and is adjusted to the temperature of the molten bath. The immersion temperature is 600-800°C, preferably at least 650°C, particularly preferably at least 670°C, particularly preferably at most 700°C. For a particularly homogeneous boundary layer formation, it is important that there is sufficient thermal energy in the boundary layer between the steel substrate and the aluminum melt. This is not the case at temperatures lower than 600°C, so that undesirable compounds can form, the later reconversion of which can lead to pores. From the preferred immersion temperatures, the diffusion rate of iron in aluminum increases significantly again, so that more iron can diffuse into the still liquid boundary layer right at the start of the coating process. The duration of cooling of the annealed flat steel product from the annealing temperature T5 to the immersion temperature T6 is preferably 10-180s. In particular, the immersion temperature T6 deviates from the temperature of the melt bath T7 by no more than 30K, in particular no more than 20K, preferably no more than 10K.

The flat steel product is subjected to a coating treatment in step j). The coating treatment is preferably carried out by means of continuous hot-dip coating. The coating can be applied to just one side, both sides or all sides of the flat steel product. The coating treatment is preferably carried out as a hot-dip coating process, in particular as a continuous process. The flat steel product usually comes into contact with the melt bath on all sides so that it is coated on all sides. The melt bath, which contains the alloy to be applied to the flat steel product in liquid form, typically has a temperature (T7) of 660-800°C, preferably 670-740°C, particularly preferably 670-710°C. Aluminum-based alloys have proven to be particularly suitable for coating ageing-resistant flat steel products with an anti-corrosive coating. In such a case, the melt bath contains 0.5 to 4 wt.% Si, in particular 0.5-1.5 wt.% Si, optionally 2-4 wt.% Fe, optionally 0.1-5.0 wt.% alkali or alkaline earth metals, preferably up to 1.0% by weight of alkali or alkaline earth metals, and optional further constituents whose total contents are limited to a maximum of 2.0% by weight, and the remainder aluminum. In a preferred variant, the optional content of alkali or alkaline earth metals in the melt comprises 0.1-1.0% by weight of Mg, in particular 0.1-0.7% by weight of Mg, preferably 0.1-0.5% by weight of Mg. Furthermore, the optional content of alkali or alkaline earth metals in the melt can comprise in particular at least 0.0015% by weight of Ca, in particular at least 0.01% by weight of Ca. In particular, the optional content of alkali or alkaline earth metals in the melt consists of 0.1-1.0% by weight of Mg, in particular 0.1-0.7% by weight of Mg, preferably 0.1-0.5% by weight of Mg and optionally at least 0.0015% by weight of Ca, preferably at least 0.01% by weight of Ca.

After leaving the molten bath, the flat steel product is blown off using a gas stream to adjust the thickness of the coating.

After the coating treatment, the coated flat steel product is cooled to room temperature in step k). The average cooling rate between 660°C and 570°C is at least 15 K/s, preferably at least 20 K/s. This corresponds to the range between the start of solidification and the end of solidification of the coating. When cooled to 660°C, solidification of the coating begins and when cooled further to 570°C, the coating is completely solidified. The average cooling rate is preferably a maximum of 100 K/s, particularly preferably a maximum of 50 K/s.

During hot-dip coating and cooling, a certain amount of iron diffusion from the steel substrate into the coating already occurs. This diffusion is restarted when the sheet metal blanks of the resulting flat steel product are heated again to temperatures above AC3 before forming. While diffusion during hot-dip coating and cooling takes place at temperatures of around 750°C down to 570°C (at 570°C, diffusion essentially stops due to the solidification of the coating), diffusion takes place at temperatures around 900°C during reheating before forming. Surprisingly, it has been shown that the boundary area between the steel substrate and the coating develops differently depending on the temperatures at which diffusion takes place. In the lower temperature range from 750°C to 570°C, diffusion is comparatively slow and a relatively sharp transition is formed between the steel substrate and the coating. In contrast, the diffusion processes are significantly faster at temperatures of 900°C. In addition, molten phases form in the coating, so that in addition to the uniform diffusion, a certain amount of mixing occurs due to convection. Such convection areas are naturally randomly distributed. As a result, these two effects mean that there is not such a sharp transition between the coating and the steel substrate.

The inventors have recognized that it is advantageous to stop the diffusion processes as quickly as possible after hot-dip coating by setting an average cooling rate between 660°C and 570°C of at least 15 K/s, preferably at least 20 K/s. This means that the diffusion is shifted to the later process step during reheating and takes place there at higher temperatures. The result is a more uneven transition between the steel substrate and the coating in the resulting sheet metal part. This more uneven transition manifests itself as a flatter average hardness gradient in the transition between the steel substrate and the coating. The advantages of this hardness gradient are explained below with reference to the sheet metal part.

The coated flat steel product can optionally be subjected to skin passing with a skin passing degree of up to 2% to improve the surface roughness of the flat steel product.

The steel used in the process for producing a flat steel product, in the process for producing a sheet metal part and in the sheet metal part itself is a steel that contains 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B. The same naturally also applies to the steel of the hot-formed sheet metal part.

In particular, the structure of the steel can be converted into a martensitic or partially martensitic structure by hot forming. The structure of the steel substrate of the sheet metal part is therefore preferably a martensitic or at least partially martensitic structure, since this has a particularly high hardness.

Particularly preferably, the steel substrate is a steel which, in addition to iron and unavoidable impurities (in wt. %), consists of C: 0.04-0.45 wt.%, Yes: 0.02-1.2 wt.%, Mn: 0.5-2.6 wt.%, Al: 0.02-1.0 wt.%, P: ≤ 0.05 wt.%, S: ≤ 0.02 wt.%, N: ≤ 0.02 wt.%, Sn: ≤ 0.03 wt.%, As: ≤ 0.01 wt.%, Approx: ≤ 0.005 wt.%, and optionally one or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V” in the following contents Cr: 0.08-1.0 wt.%, B: 0.001-0.005 wt.%, Mon: ≤0.5 wt.%, No: ≤0.5 wt.%, Cu: ≤0.2 wt.%, N.b.: 0.01-0.08 wt.%, T: 0.01-0.08 wt.%, V: ≤0.3 wt.%, consists.

The elements P, S, N, Sn, As and Ca are impurities that cannot be completely avoided during steel production. Ca is also occasionally deliberately added to alloys to bind sulphur. In such a case, the Ca content is at least 0.001% by weight. The maximum Ca content in this case is also 0.005% by weight.

In addition to these elements, other elements may also be present as impurities in the steel. These other elements are summarized under the "unavoidable impurities". The total content of unavoidable impurities is preferably a maximum of 0.2% by weight, preferably a maximum of 0.1% by weight. The optional alloying elements Cr, B, Nb, Ti, for which a lower limit is specified, can also be present in the steel substrate as unavoidable impurities in contents below the respective lower limit. In this case, they are also counted as unavoidable impurities, the total content of which is limited to a maximum of 0.2% by weight, preferably a maximum of 0.1% by weight. The individual upper limits for the respective contamination of these elements are preferably as follows: Cr: ≤ 0.050 wt.%, B: ≤ 0.0005 wt.% N.b.: ≤ 0.005 wt.%, T: ≤ 0.005 wt.%

These preferred upper limits should be considered alternatively or together. Preferred variants of the steel therefore meet one or more of these four conditions.

In a preferred embodiment, the C content of the steel is a maximum of 0.37 wt.% and/or at least 0.06 wt.%. In particularly preferred embodiments, the C content is in the range of 0.06-0.09 wt.% or in the range of 0.11-0.25 wt.% or in the range of 0.32-0.37 wt.%.

In a preferred embodiment, the Si content of the steel is a maximum of 1.00 wt.% and/or at least 0.06 wt.%.

In a preferred variant, the Mn content of the steel is a maximum of 2.4 wt.% and/or at least 0.75 wt.%. In particularly preferred variants, the Mn content is in the range of 0.75-0.85 wt.% or in the range of 1.0-1.6 wt.%.

In a preferred variant, the Al content of the steel is a maximum of 0.75% by weight, in particular a maximum of 0.5% by weight, preferably a maximum of 0.25% by weight. Alternatively or additionally, the Al content is preferably at least 0.02%.

It has also been shown that it can be helpful if the sum of the contents of silicon and aluminum is limited. In a preferred variant, the sum of the contents of Si and Al (usually referred to as Si+Al) is therefore a maximum of 1.5 wt.%, preferably a maximum of 1.2 wt.%. Additionally or alternatively, the sum of the contents of Si and Al is at least 0.06 wt.%, preferably at least 0.08 wt.%.

The elements P, S and N are typical impurities that cannot be completely avoided during steel production. In preferred variants, the P content is a maximum of 0.03% by weight. Irrespective of this, the S content is preferably a maximum of 0.012%. In addition or in addition, the N content is preferably a maximum of 0.009% by weight.

Optionally, the steel also contains chromium with a content of 0.08-1.0 wt.%. The Cr content is preferably a maximum of 0.75 wt.%, in particular a maximum of 0.5 wt.%.

In the case of an optional alloying of chromium, the sum of the contents of chromium and manganese is preferably limited. The sum is a maximum of 3.3% by weight, in particular a maximum of 3.15% by weight. Furthermore, the sum is at least 0.5% by weight, preferably at least 0.75% by weight.

Preferably, the steel optionally also contains boron in a content of 0.001-0.005 wt.%. In particular, the B content is a maximum of 0.004 wt.%.

Optionally, the steel may contain molybdenum in a content of not more than 0.5% by weight, in particular not more than 0.1% by weight.

Furthermore, the steel can optionally contain nickel with a content of maximum 0.5 wt.%, preferably maximum 0.15 wt.%.

Optionally, the steel may also contain copper with a maximum content of 0.2 wt.%, preferably a maximum of 0.15 wt.%.

In addition, the steel can optionally contain one or more of the microalloying elements Nb, Ti and V. The optional Nb content is at least 0.01 wt.%, in particular at least 0.02 wt.% and a maximum of 0.08 wt.%, preferably a maximum of 0.04 wt.%. The optional Ti content is at least 0.01 wt.% and a maximum of 0.08 wt.%, preferably a maximum of 0.04 wt.%. The optional V content is a maximum of 0.3 wt.%, preferably a maximum of 0.2 wt.%, in particular a maximum of 0.1 wt.%, preferably a maximum of 0.05 wt.%.

In the case of an optional alloying of several of the elements Nb, Ti and V, the sum of the contents of Nb, Ti and V is preferably limited. The sum is a maximum of 0.1 wt.%, in particular a maximum of 0.068 wt.%. Furthermore, the sum is preferably at least 0.015 wt.%.

The above explanations regarding preferred steel substrates naturally also apply to the steel substrate of the flat steel product described below, as well as the steel substrates in the manufacturing processes described.

The flat steel product manufactured in this way includes an aluminum-based anti-corrosive coating. The anti-corrosive coating can be applied to one or both sides of the flat steel product. The two large surfaces of the flat steel product that face each other are referred to as the two sides of the flat steel product. The narrow surfaces are referred to as edges.

As already explained, during hot-dip coating, iron diffuses from the steel substrate into the liquid coating, so that the corrosion protection coating of the flat steel product has, in particular, an alloy layer and an Al base layer when it solidifies.

The alloy layer lies on the steel substrate and is directly adjacent to it. The alloy layer is essentially made of aluminum and iron. The other elements from the steel substrate or the melt composition do not accumulate significantly in the alloy layer. The alloy layer preferably consists of 35-60 wt.% Fe, preferably α-iron, optional further components, the total contents of which are limited to a maximum of 5.0 wt.%, preferably 2.0%, and the remainder aluminum, with the Al content preferably increasing towards the surface. The optional further components include in particular the other components of the melt (i.e. silicon and optionally alkali or alkaline earth metals, in particular Mg or Ca) and the remaining portions of the steel substrate in addition to iron.

The Al base layer lies on the alloy layer and is directly adjacent to it. The composition of the Al base layer preferably corresponds to the composition of the melt of the melt bath. This means that it consists of 0.5-4 wt.% Si, optionally 2-4 wt.% Fe, optionally up to 5.0 wt.% alkali or alkaline earth metals, optionally up to 10% Zn and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder aluminum. Preferred compositions of the Al base layer correspond to the preferred melt compositions.

In a preferred variant of the Al base layer, the optional content of alkali or alkaline earth metals comprises 0.1-1.0 wt.% Mg, in particular 0.1-0.7 wt.% Mg, preferably 0.1-0.5 wt.% Mg. Furthermore, the optional content of alkali or alkaline earth metals can comprise in particular at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca. In particular, the optional content of alkali or alkaline earth metals in the melt consists of 0.1-1.0 wt.% Mg, in particular 0.1-0.7 wt.% Mg, preferably 0.1-0.5 wt.% Mg and optionally at least 0.0015 wt.% Ca, preferably at least 0.01 wt.% Ca.

In a further preferred variant of the corrosion protection coating, the Si content in the alloy layer is lower than the Si content in the Al base layer.

bei beidseitigen Korrosionsschutzüberzügen bzw. $15-180\frac{g}{m^2}$

bei der einseitigen Variante. Bevorzugt beträgt das Auflagengewicht des Korrosionsschutzüberzuges $100-200\frac{g}{m^2}$

bei beidseitigen Überzügen bzw. $50-100\frac{g}{m^2}$

für einseitige Überzüge. Besonders bevorzugt beträgt das Auflagengewicht des Korrosionsschutzüberzuges $120-180\frac{g}{m^2}$

bei beidseitigen Überzügen bzw. $60-90\frac{g}{m^2}$

für einseitige Überzüge.The anti-corrosive coating preferably has a thickness of 5-60 µm, in particular 10 to 40 µm. The coating weight of the anti-corrosive coating is in particular $30-360\frac{G}{m^2}$

with corrosion protection coatings on both sides or $15-180\frac{G}{m^2}$

in the one-sided variant. The coating weight of the corrosion protection coating is preferably $100-200\frac{G}{m^2}$

for double-sided coatings or $50-100\frac{G}{m^2}$

for one-sided coatings. The coating weight of the corrosion protection coating is particularly preferred $120-180\frac{G}{m^2}$

for double-sided coatings or $60-90\frac{G}{m^2}$

for one-sided coverings.

The thickness of the alloy layer is preferably less than 20 µm, particularly preferably less than 16 µm, particularly preferably less than 12 µm, in particular less than 10 µm. The thickness of the Al base layer results from the difference between the thicknesses of the anti-corrosive coating and the alloy layer. The thickness of the Al base layer is preferably at least 1 µm, even with thin anti-corrosive coatings.

In a preferred variant, the flat steel product comprises an oxide layer arranged on the anti-corrosive coating. The oxide layer lies in particular on the Al base layer and preferably forms the outer finish of the anti-corrosive coating.

The oxide layer consists in particular of more than 80% by weight of oxides, with the majority of the oxides (i.e. more than 50% by weight of the oxides) being aluminum oxide. Optionally, in addition to aluminum oxide, hydroxides and/or magnesium oxide are present in the oxide layer alone or as a mixture. Preferably, the remainder of the oxide layer not taken up by the oxides and optionally present hydroxides consists of silicon, aluminum, iron and/or magnesium in metallic form. For the optional embodiment with zinc as a component of the Al base layer, zinc oxide components are also present in the oxide layer.

Preferably, the oxide layer of the flat steel product has a thickness greater than 50 nm. In particular, the thickness of the oxide layer is a maximum of 500 nm.

1. a. Herstellen eines Stahlflachproduktes gemäß des zuvor beschriebenen Verfahrens;
2. b. Abteilen eines Blechzuschnitts von dem Stahlflachprodukt;
3. c. Erwärmen des Blechzuschnitts in einem Ofen mit einer Ofentemperatur T_Ofen während einer Glühzeit t_G derart, dass zumindest teilweise die AC3 Temperatur des Zuschnitts überschritten ist und die Temperatur T_Einlg des Zuschnitts beim Einlegen in ein für ein Warmpressformen vorgesehenes Umformwerkzeug (Arbeitsschritt c)) zumindest teilweise eine Temperatur oberhalb von Ms+100°C aufweist, wobei Ms die der Martensitstarttemperatur bezeichnet;
4. d. Einlegen des erwärmten Blechzuschnitts in ein Umformwerkzeug, wobei die für das Entnehmen aus der Erwärmungseinrichtung und das Einlegen des Zuschnitts benötigte Transferdauer t_Trans höchstens 20 s, bevorzugt höchstens 15 s, beträgt;
5. e. Warmpressformen des Blechzuschnitts zu dem Blechformteil, wobei der Zuschnitt im Zuge des Warmpressformens über eine Dauer t_WZ von mehr als 1s mit einer zumindest teilweise mehr als 30 K/s betragenden Abkühlgeschwindigkeit r_WZ auf die Zieltemperatur T_Ziel abgekühlt und optional dort gehalten wird;
6. f. Entnehmen des auf die Zieltemperatur T_Ziel abgekühlten Blechformteils aus dem Werkzeug.

The invention also relates to a method for producing a sheet metal part. The sheet metal part is designed in particular as follows:

1. a. producing a flat steel product according to the process described above;
2. b. Separating a sheet blank from the flat steel product;
3. c. Heating the sheet metal blank in a furnace with a furnace temperature T _furnace during an annealing time t _G such that the AC3 temperature of the blank is at least partially exceeded and the temperature T _Einlg of the blank when placed in a forming tool intended for hot press forming (working step c)) at least partially has a temperature above Ms+100°C, where Ms denotes the martensite start temperature;
4. d. Inserting the heated sheet metal blank into a forming tool, wherein the transfer time t _Trans required for removing the blank from the heating device and inserting it is at most 20 s, preferably at most 15 s;
5. e. Hot press forming the sheet metal blank to form the sheet metal part, wherein the blank is cooled during the hot press forming over a period t _WZ of more than 1 s at a cooling rate r _WZ which is at least partially more than 30 K/s to the target temperature T _Target and is optionally held there;
6. f. Removing the sheet metal part cooled to the target temperature T _target from the tool.

In the method according to the invention, a flat steel product is first produced as described above. A sheet metal blank is cut from this flat steel product. This blank, which consists of a steel suitably composed in accordance with the above explanations, is then heated in a manner known per se so that the AC3 temperature of the blank is at least partially exceeded and the temperature T _Einlg of the blank when placed in a forming tool intended for hot press forming (work step d)) is at least partially above Ms+100°C. Partially exceeding a temperature (here AC3 or Ms+100°C) is understood in the sense of this application to mean that at least 30%, in particular at least 60%, of the volume of the blank exceeds a corresponding temperature. When placed in the forming tool, at least 30% of the blank therefore has an austenitic structure, i.e. the conversion from the ferritic to the austenitic structure does not have to be complete when placed in the forming tool. Rather, up to 70% of the volume of the blank when placed in the forming tool can consist of other structural components, such as tempered bainite, tempered martensite and/or non- or partially recrystallized ferrite. For this purpose, certain areas of the blank can be kept at a lower temperature level than others during heating. To do this, the heat supply can be directed only at certain sections of the blank, or the parts that are to be heated less can be shielded from the heat supply. In the part of the blank material whose temperature remains lower, no or only significantly less martensite is formed during forming in the tool, so that the structure there is significantly softer than in the other parts that have a martensitic structure. In this way, a softer area can be specifically set in the respective formed sheet metal part, for example by ensuring optimal toughness for the respective intended use, while the other areas of the sheet metal part have maximized strength.

Maximum strength properties of the resulting sheet metal part can be achieved by ensuring that the temperature at least partially reached in the sheet metal blank is between Ac3 and 1000°C, preferably between 850°C and 950°C.

mit %C = jeweiliger C-Gehalt, %Si = jeweiliger Si-Gehalt, %Mn = jeweiliger Mn-Gehalt, %Cr = jeweiliger Cr-Gehalt, %Mo = jeweiliger Mo-Gehalt, %Ni jeweiliger Ni-Gehalt und %V = jeweiliger V-Gehalt des Stahls, aus dem der Zuschnitt besteht, bestimmt.The minimum temperature Ac3 to be exceeded is determined according to the formula given by HOUGARDY, HP. in Werkstoffkunde Stahl Volume 1: Grundlagen, Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229. $$\text{Ac}3=\left(902-225*\%\text{C}+19*\%\text{Si}-11*\%\text{Mn}-5*\%\text{Cr}+13*\%\text{Mon}-20*\%\text{No}+55*\%\text{V}\right)°\text{C}$$

where %C = respective C content, %Si = respective Si content, %Mn = respective Mn content, %Cr = respective Cr content, %Mo = respective Mo content, %Ni respective Ni content and %V = respective V content of the steel from which the blank is made.

An optimally uniform distribution of properties can be achieved by completely heating the blank in step b).

In a preferred embodiment, the average heating rate r _furnace of the sheet metal blank during heating in step b) is at least 3 K/s, preferably at least 5 K/s, in particular at least 10 K/s, preferably at least 15 K/s. The average heating rate r _furnace is to be understood as the average heating rate from 30°C to 700°C.

In a preferred embodiment, the heating takes place in a furnace with a furnace temperature T _furnace of at least 850°C, preferably at least 880°C, particularly preferably at least 900°C, in particular at least 920°C, and a maximum of 1000°C, preferably a maximum of 950°C, particularly preferably a maximum of 930°C.

Preferably, the dew point in the oven is at least -20°C, preferably at least -15°C, in particular at least -5°C, particularly preferably at least 0°C, in particular at least 5°C and a maximum of +25°C, preferably a maximum of +20°C, in particular a maximum of +15°C.

In a special embodiment, the heating in step b) takes place step by step in areas with different temperatures. In particular, the heating takes place in a roller hearth furnace with different heating zones. Here, the heating takes place in a first heating zone with a temperature (so-called furnace inlet temperature) of at least 650°C, preferably at least 680°C, in particular at least 720°C. The maximum temperature in the first heating zone is preferably 900°C, in particular a maximum of 850°C. Furthermore, the maximum temperature of all heating zones in the furnace is preferably a maximum of 1200°C, in particular a maximum of 1000°C, preferably a maximum of 950°C, particularly preferably a maximum of 930°C.

The total time in the furnace t _furnace , which consists of a heating time and a holding time, is preferably at least 2 minutes, in particular at least 3 minutes, preferably at least 4 minutes for both variants (constant furnace temperature, gradual heating). Furthermore, the total time in the furnace for both variants is preferably a maximum of 20 minutes, in particular a maximum of 15 minutes, preferably a maximum of 12 minutes, in particular a maximum of 8 minutes. Longer total times in the furnace have the advantage that uniform austenitization of the sheet metal blank is ensured. On the other hand, holding for too long above Ac3 leads to grain coarsening, which has a negative effect on the mechanical properties.

The blank heated in this way is removed from the respective heating device, which can be, for example, a conventional heating furnace, an induction heating device that is also known per se or a conventional device for keeping steel components warm, and transported into the forming tool so quickly that its temperature when it arrives in the tool is at least partially above Ms+100°C, preferably above 600°C, in particular above 650°C, particularly preferably above 700°C. Here, Ms refers to the martensite start temperature. In a particularly preferred variant, the temperature is at least partially above the AC1 temperature. In all of these variants, the temperature is in particular a maximum of 900°C. These temperature ranges ensure good formability of the material overall.

In step d), the transfer of the austenitized blank from the heating device used to the forming tool is completed within preferably a maximum of 20 seconds, in particular a maximum of 15 seconds. Such rapid transport is necessary to avoid excessive cooling before deformation.

When the blank is inserted, the tool typically has a temperature between room temperature (RT) and 200°C, preferably between 20°C and 180°C, in particular between 50°C and 150°C. Optionally, in a special embodiment, the tool can be tempered at least in some areas to a temperature T _WZ of at least 200°C, in particular at least 300°C, in order to only partially harden the component. Furthermore, the tool temperature T _WZ is preferably a maximum of 600°C, in particular a maximum of 550°C. It is only necessary to ensure that the tool temperature T _WZ is below the desired target temperature T _Ziel . The dwell time in the tool t _WZ is preferably at least 2s, in particular at least 3s, particularly preferably at least 5s. The maximum residence time in the tool is preferably 25s, in particular a maximum of 20s.

The target temperature T _target of the sheet metal part is at least partially below 400°C, preferably below 300°C, in particular below 250°C, preferably below 200°C, particularly preferably below 180°C, in particular below 150°C. Alternatively, the target temperature T _target of the sheet metal part is particularly preferably below Ms-50°C, where Ms denotes the martensite start temperature. Furthermore, the target temperature of the sheet metal part is preferably at least 20°C, particularly preferably at least 50°C.

zu berechnen, wobei hier mit C% der C-Gehalt, mit %Mn der Mn-Gehalt, mit %Mo der Mo-Gehalt, mit %Cr der Cr-Gehalt, mit %Ni der Ni-Gehalt, mit %Cu der Cu-Gehalt, mit %Co der Co-Gehalt, mit %W der W-Gehalt und mit %Si der Si-Gehalt des jeweiligen Stahls in Gew.-% bezeichnet sind.The martensite start temperature of a steel within the scope of the inventive specifications is according to the formula $$\begin{array}{c}\text{Ms}\left[°\text{C}\right]=(490.85-302.6\%\text{C}-30.6\%\text{Mn}-16.6\%\text{No}-8.9\%\text{Cr}+2.4\%\text{Mon}-11.3\%\text{Cu}\\ +8.58\%\text{Co}+7.4\%\text{W}-14.5\%\text{Si})\left[°\text{C}/\text{Gew}.-\%\right]\end{array}$$

to be calculated, where C% is the C content, %Mn is the Mn content, %Mo is the Mo content, %Cr is the Cr content, %Ni is the Ni content, %Cu is the Cu content, %Co is the Co content, %W is the W content and %Si is the Si content of the respective steel in wt.%.

und $$\begin{array}{c}\text{AC3}\left[°\text{C}\right]=\left(902-225*\%\text{C}+19*\%\text{Si}-11*\%\text{Mn}-5*\%\text{Cr}+13*\%\text{Mo}-20*\%\text{Ni}+55*\%\text{V}\right)[°\text{C}/\text{Gew}.-\\ \%]\end{array}$$

zu berechnen, wobei auch hiermit mit %C der C-Gehalt, mit %Si der Si-Gehalt mit %Mn der Mn-Gehalt mit %Cr der Cr-Gehalt, mit %Mo der Mo-Gehalt, mit %Ni der Ni-Gehalt und mit +%V der Vanadium-Gehalt des jeweiligen Stahls bezeichnet sind (Brandis H 1975 TEW-Techn. Ber. 1 8-10)The AC1 temperature and the AC3 temperature of a steel within the scope of the inventive specifications are according to the formulas $$\begin{array}{c}\text{AC1}\left[°\text{C}\right]=\left(739-22*\%\text{C}-7*\%\text{Mn}+2*\%\text{Si}+14*\%\text{Cr}+13*\%\text{Mon}-13*\%\text{Mon}-13*\%\text{No}+20*\%\text{V}\right)[°\text{C}/\text{Gew}.-\\ \%]\end{array}$$

and $$\begin{array}{c}\text{AC3}\left[°\text{C}\right]=\left(902-225*\%\text{C}+19*\%\text{Si}-11*\%\text{Mn}-5*\%\text{Cr}+13*\%\text{Mon}-20*\%\text{No}+55*\%\text{V}\right)[°\text{C}/\text{Gew}.-\\ \%]\end{array}$$

to be calculated, where %C is the C content, %Si is the Si content, %Mn is the Mn content, %Cr is the Cr content, %Mo is the Mo content, %Ni is the Ni content and +%V is the vanadium content of the respective steel (Brandis H 1975 TEW-Techn. Ber. 1 8-10)

In the tool, the blank is not only formed into the sheet metal part, but is also quenched to the target temperature at the same time. The cooling rate in the _tool to the target temperature is in particular at least 20 K/s, preferably at least 30 K/s, in particular at least 50 K/s, in a special design at least 100 K/s.

After the sheet metal part has been removed in step e), the sheet metal part is cooled to a cooling temperature T _AB of less than 100°C within a cooling time t _AB of 0.5 to 600s. This is usually done by air cooling.

The invention further relates to a sheet metal part formed from a steel sheet blank comprising a steel substrate which consists of a steel which has 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B. The sheet metal part has an aluminum-based anti-corrosion coating on at least one side, wherein the amount of the hardness gradient of the anti-corrosion coating and the steel substrate perpendicular to the surface of the steel substrate is less than 1.7 GPa/µm.

Such a sheet metal part can be manufactured using the method described above, among others.

• - Einbetten zumindest eines Teils des Blechformteils in eine Einbettungsmasse und Anfertigen einen Querschliffes des Blechformteils;
• - Polieren des Querschliffs und leichtes Ätzen mit 3% Nital (alkoholische Salpetersäure);
• - Festlegen eines Messfeldes auf dem Querschliff von mindestens 50 µm Breite parallel zur Oberfläche und eines kartesischen Messrasters auf dem Messfeld, wobei die kartesische x-Achse senkrecht zur Oberfläche des Stahlsubstrates verläuft und die kartesische y-Achse parallel zur Oberfläche des Stahlsubstrates verläuft und wobei der Rasterabstand in beide Richtungen jeweils 1,5 µm beträgt;
• - Messung der Nanohärte auf den Gitterpunkten des Messrasters mit einem Nanoindenter mit einer kalibrierten Berkovich-Pyramide als Prüfspritze und mit einer Belastungsfunktion mit einer Maximallast von 2000 µN;
• - Berechnen eines Härteverlaufes als Funktion der kartesischen x-Koordinate, indem jeder x-Koordinate der Mittelwert der Nanohärte über alle Gitterpunkte mit dieser x-Koordinate zugeordnet wird (d.h. die Härtewerte aller Punkte mit dem gleichen Abstand zur Oberfläche des Stahlsubstrates werden gemittelt);
• - Berechnen des Härtegradienten als Differenzenquotient des Härteverlaufes.

The hardness gradient is determined according to the following procedure:

• - Embedding at least part of the sheet metal part in an embedding compound and producing a cross-section of the sheet metal part;
• - Polishing of the cross section and light etching with 3% Nital (alcoholic nitric acid);
• - defining a measuring field on the cross-section of at least 50 µm width parallel to the surface and a Cartesian measuring grid on the measuring field, with the Cartesian x-axis running perpendicular to the surface of the steel substrate and the Cartesian y-axis running parallel to the surface of the steel substrate and the grid spacing being 1.5 µm in both directions;
• - Measurement of the nanohardness on the grid points of the measuring grid using a nanoindenter with a calibrated Berkovich pyramid as a test syringe and with a load function with a maximum load of 2000 µN;
• - Calculating a hardness curve as a function of the Cartesian x-coordinate by assigning to each x-coordinate the mean value of the nanohardness over all grid points with this x-coordinate (ie the hardness values of all points with the same distance to the surface of the steel substrate are averaged);
• - Calculate the hardness gradient as the difference quotient of the hardness curve.

The “Hysitron TI Premier” device from Bruker is used as a nanoindenter, for example. Details about the device can be obtained from Bruker or accessed via the following link: https://www.bruker.com/en/products-and-solutions/test-andmeasurement/nanomechanical-test-systems/hysitron-ti-premier-nanoindenter.html. With the nanoindenter, a specific measuring tip, for example a Berkovich tip (made of diamond), is pressed into a sample to be examined and the hardness can be determined based on the measured force-penetration curve, preferably using the Oliver&Pharr evaluation method (method available via the link: https://www.sciencedirect.com/topics/engineering/oliver-pharrmethod).

A hardness gradient that is smaller than 1.7 GPa/µm means that the hardness averaged parallel to the surface of the steel substrate changes more slowly.

A lower hardness gradient has the advantage that the resistance to cracking is higher. A sharp hardness gradient is always an indicator of a "predetermined breaking point". However, if the transition in hardness is slower, cracks only form in the material when it is deformed (e.g. in the event of a crash) at higher stress intensities. The material can therefore withstand higher external forces before it fails.

Preferably, the corrosion protection coating of the sheet metal part comprises an alloy layer and an Al base layer.

The alloy layer lies on the steel substrate and is directly adjacent to it. The alloy layer of the sheet metal part preferably consists of 35-95 wt.% Fe, preferably 60-95 wt.% Fe, 0.1-4 wt.% Si and optional further components, the total contents of which are limited to a maximum of 3.5 wt.%, preferably 2.0 wt.%, and the remainder being aluminum. The optional further components are preferably the elements present in the steel of the steel substrate in addition to iron and the other elements from the melt such as Zn and alkali or alkaline earth metals. These elements from the melt only accumulate in the alloy layer to a very small extent. The alloy layer preferably has a ferritic structure.

The Al base layer of the sheet metal part lies on the alloy layer of the steel component and is directly adjacent to it. The Al base layer of the steel component preferably consists of 35-55 wt.% Fe, preferably 40-50 wt.% Fe, 0.4 - 4 wt.% Si, in particular 0.4-1.5 wt.% Si, optionally 3 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, optionally up to 10% Zn and optional further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder aluminum. The optional content of alkali or alkaline earth metals is preferably at least 0.1 wt.%.

In a preferred variant of the Al base layer, the optional content of alkali or alkaline earth metals comprises 0.1-1.0 wt.% Mg, in particular 0.1-0.7 wt.% Mg, preferably 0.1-0.5 wt.% Mg. Furthermore, the optional content of alkali or alkaline earth metals in the Al base layer can comprise in particular at least 0.0015 wt.% Ca, in particular at least 0.1 wt.% Ca. Furthermore preferably, the optional content of alkali or alkaline earth metals consists of 0.1-1.0 wt.% Mg, in particular 0.1-0.7 wt.% Mg, preferably 0.1-0.5 wt.% Mg and optionally at least 0.0015 wt.% Ca, in particular at least 0.1 wt.% Ca.

The Al base layer can have a homogeneous element distribution in which the local element contents do not vary by more than 10%. Preferred variants of the Al base layer, on the other hand, have silicon-poor phases and silicon-rich phases. Silicon-poor phases are areas whose average Si content is at least 20% less than the average Si content of the Al base layer. Silicon-rich phases are areas whose average Si content is at least 20% more than the average Si content of the Al base layer.

In a preferred variant, the silicon-rich phases are arranged within the silicon-poor phase. In particular, the silicon-rich phases form a layer that is at least 40% continuous and is bordered by silicon-poor regions. A continuous layer of silicon-rich phases means that in the vertical micrograph a line can be laid parallel to the surface of the steel substrate such that it runs completely through the silicon-rich phases. In contrast, a layer that is at least X% continuous means that in the vertical micrograph a line can be laid parallel to the surface of the steel substrate such that it runs at least X% within the silicon-rich phases. In the present case, the silicon-rich phases are therefore arranged in such a connected manner that in the vertical micrograph a line can be laid parallel to the surface of the steel substrate such that it runs at least 40% within the silicon-rich phases. In an alternative embodiment, the silicon-rich phases are arranged in islands in the silicon-poor phase.

For the purposes of this application, “island-shaped” means an arrangement in which discrete, unconnected areas are enclosed by another material - i.e., “islands” of a particular material are located within another material.

In a preferred variant, the steel component comprises an oxide layer arranged on the anti-corrosive coating. The oxide layer lies in particular on the Al base layer and preferably forms the outer edge of the anti-corrosive coating. The oxide layer of the steel component consists in particular of more than 80% by weight of oxides, with the majority of the oxides (i.e. more than 50% by weight of the oxides) being aluminum oxide. Optionally, in addition to aluminum oxide, hydroxides and/or magnesium oxide are present in the oxide layer alone or as a mixture. Preferably, the remainder of the oxide layer not taken up by the oxides and optionally present hydroxides consists of silicon, aluminum, iron and/or magnesium in metallic form.

The oxide layer preferably has a thickness of at least 50 nm, in particular of at least 100 nm. Furthermore, the thickness is preferably a maximum of 4 µm, in particular a maximum of 2 µm.

The sheet metal part according to the invention is preferably a component for a land vehicle, sea vehicle or aircraft. It is particularly preferably an automobile part, in particular a body part. The component is preferably a B-pillar, longitudinal member, A-pillar, sill or cross member.

• 1 eine rasterelektronemikroskopische Aufnahme des in Versuch 1 mit der Indenterspitze untersuchten Bereichs,
• 2 den Härteverlauf der erfindungsgemäßen Probe,
• 3 den Härtegradienten der erfindungsgemäßen Probe,
• 4 den Härteverlauf der Vergleichsprobe und
• 5 den Härtegradienten der Vergleichsprobe.

The invention is explained in more detail using the figures.

• 1 a scanning electron micrograph of the area examined with the indenter tip in Experiment 1,
• 2 the hardness curve of the sample according to the invention,
• 3 the hardness gradient of the sample according to the invention,
• 4 the hardness curve of the comparison sample and
• 5 the hardness gradient of the comparison sample.

To demonstrate the effect of the invention, several tests were carried out. For this purpose, slabs with the compositions given in Table 1 with a thickness of 240 mm and a width of 1200 mm were produced and heated in a pusher furnace to a temperature T1 of 1200°C. The slabs were then kept at T1 for between 30 and 450 minutes until the temperature T1 in the core of the slabs was reached and the slabs were thus heated through.

The slabs were discharged from the pusher furnace at their respective heating 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 to an intermediate product with a thickness of 40 mm, whereby the intermediate products, which can also be referred to as pre-strips in hot strip rolling, at the end of the rough rolling phase, each had an intermediate product temperature T2 of 1100°C. The pre-strips were fed to the finish rolling immediately after rough rolling, so that the intermediate product temperature T2 corresponds to the initial rolling temperature for the finish rolling phase. The pre-strips were rolled out to hot strips with a final thickness of 4 mm and a final rolling temperature T3 of 890°C, cooled to the respective coiling temperature and wound into coils at a coiling temperature T4 of 580°C and then cooled in still air. The hot strips were descaled in the conventional way by pickling before they were subjected to cold rolling until the thickness given in Table 3 was achieved. The cold-rolled flat steel products were heated in a continuous annealing furnace to an annealing temperature T5 of 870°C and held at annealing temperature for 100s each before being cooled to the immersion temperature T6 of 690°C at a cooling rate of 1 K/s. The cold strips were passed through a molten coating bath with a temperature T7 of 676°C at their respective immersion temperature T6. The strip speed was 76 m/min in all cases. The composition of the coating bath is given in Table 2. After coating, the coated strips were blown off to adjust the coating weights. An air stream was used for this. The temperature of the air stream was 70°C in all cases. The thickness of the coating is given in Table 3. The strips were first cooled to 660°C at an average cooling rate of 10-15 K/s. Between 660°C and 570°C, ie between the beginning of the solidification and the end of the solidification of the coating, the cooling rate in test 1 according to the invention was 21 K/s. In contrast, the cooling rate in reference test 2 between 660°C and 570°C was only 13 K/s. In the further cooling process between 570°C and room temperature, the strips were cooled at a cooling rate of 5 - 12 K/s each.

Blanks were cut from the steel strips produced in this way and used for further tests. In these tests, sheet metal part samples in the form of 200 x 300 mm ² plates were hot-pressed from the respective blanks. For this purpose, the blanks were heated in a heating device, for example a conventional heating furnace, from room temperature at an average heating rate r _furnace (between 30°C and 700°C) in a furnace with a furnace temperature T _furnace of 900°C. The annealing time in the furnace, which includes heating and holding, is designated t _furnace . The dew point of the furnace atmosphere was -5°C in all cases. The blanks were then removed from the heating device and placed in a forming tool which has the temperature T _WZ . When they were removed from the furnace, the blanks had reached the furnace temperature. The transfer time t _Trans, which consists of the removal from the heating device, transport to the tool and insertion into the tool, was 8 s. The temperature T _Einlg of the blanks when inserted into the forming tool was in all cases above the respective martensite start temperature +100 °C. The blanks were formed into the respective sheet metal part in the forming tool, with the sheet metal parts being cooled in the tool at a cooling rate r _WZ to a target temperature T _Target . The residence time in the tool is referred to as t _WZ . Finally, the samples were cooled in air to room temperature. The parameters mentioned are summarized again in Table 4, where "RT" abbreviates room temperature.

Cross sections were made of the sheet metal parts produced in this way. For this purpose, a part of the sheet metal part was embedded in an embedding compound. A cross section of the sheet metal part was then made. This cross section was polished and lightly etched with 3% Nital. A measuring field of at least 50 µm wide was then defined on this cross section parallel to the surface and a Cartesian measuring grid was defined on the measuring field, with the Cartesian x-axis running perpendicular to the surface of the steel substrate and the Cartesian y-axis running parallel to the surface of the steel substrate and the grid spacing being 1.5 µm in both directions. An example is shown in 1 a scanning electron microscope image of the area examined in test 1 with the indenter tip is shown. The section shown corresponds to 70 µm × 70 µm. The steel substrate 1 can be clearly seen. The alloy layer 3 is arranged on the steel substrate. The Al base layer 5 is in turn arranged on the alloy layer 3. Above the alloy layer is the embedding compound 7, which is required to produce the cross-section. The cross-section was also lightly etched with 3% Nital. On the cross-section, a measuring field of at least 50 µm wide was defined parallel to the surface and a Cartesian measuring grid was defined on the measuring field, with the Cartesian x-axis running perpendicular to the surface of the steel substrate and the Cartesian y-axis running parallel to the surface of the steel substrate and the grid spacing in both directions being 1.5 µm. The nanohardness was then determined at the grid points of the measuring grid using a nanoindenter with a Berkovich pyramid as a test tip and a load function with a maximum load of 2000µN. Clearly visible in 1 the impressions left by the probe at the grid points.

A hardness curve was then determined from the measured nanohardness at the grid points as a function of the Cartesian x-coordinate. For this purpose, the mean value of the nanohardness over all grid points with this x-coordinate was assigned to each x-coordinate. This means that the average was calculated over all points with the same distance to the surface of the steel substrate. In the 1 These are all grid points that lie vertically above each other.

2 shows the resulting hardness curve of test 1. The hardness of the coating of about 12-13 GPa can be clearly seen, which drops to 5-6 GPa towards the substrate. The hardness gradient was then determined from this hardness curve by calculating the difference quotient of two adjacent x-values and plotting it in the middle of the interval. The result is shown in 3 shown.

The 4 and 5 show the identical sizes for the comparison sample 2. If one now compares the 3 and 5 It is clear that the hardness gradient is significantly greater in the comparison sample. The amount of the hardness gradient assumes values of more than 2 GPa/µm. In contrast, the maximum amount of the hardness gradient of sample 1 according to the invention is less than 1.5 GPa/µm. Table 1 (steel grades) steel C Si Mn Al Cr Nb T B P S N Sn As Cu Mon Approx No A 0.235 0.3 1.5 0.05 0.28 0.003 0.040 0.0035 0.02 0.005 0.007 0.03 0.01 0.03 0.03 0.005 0.025 Remainder iron and unavoidable impurities. Values in % by weight; Table 2 (Coating variants) Coating variant Melt analysis Si Fe Mg Other Al α 1.0 3.5 0.25 <1% rest β 9.5 3 0.5 <1% rest Table 3 (structure) Construction Steel grade Coating Sheet thickness [mm] Coating thickness [µm] 1 A α 1.5 27 2* A β 1.5 27 * non-inventive reference examples Table 4 (Hot forming parameters) Average heating rate r _oven [30 - 700 °C] [K/s] T _oven [°C] Transfer time [s] Dew point oven [°C] T _Einlg [°C] T _WZ [°C] t _WZ [s] Cooling rate rwz [K/s] T _Target [°C] 8th 900 8th -5 800 RT 15 50 50

QUOTES INCLUDED IN THE DESCRIPTION

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Cited patent literature

• WO 2022/048990 A1 [0005]

Claims (12)

Method for producing a flat steel product for hot forming with a coating, comprising the following steps: a) providing a slab or a thin slab consisting of a steel which has 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B; b) heating the slab or thin slab at a temperature (T1) of 1000-1400°C; c) optionally pre-rolling the heated slab or thin slab to an intermediate product with an intermediate product temperature (T2) of 1000-1200°C; d) hot rolling to a hot-rolled flat steel product, the final rolling temperature (T3) being 750-1000°C; e) optionally coiling the hot-rolled flat steel product, the coiling temperature (T4) being at most 700°C; f) optionally descaling the hot-rolled flat steel product; g) optionally cold rolling the flat steel product, the cold rolling degree being at least 30%; h) annealing the flat steel product at an annealing temperature (T5) of 650-900°C; i) cooling the flat steel product to an immersion temperature (T6) which is 600-800°C, preferably 680-720°C; j) coating the flat steel product cooled to the immersion temperature with a coating by i. Immersion in a melt bath with a melt temperature (T7) of 660-800°C, preferably 670-710°C, the melt bath consisting of 0.5-4 wt.% Si, optionally 2-4 wt.% Fe, optionally up to 5.0 wt.% alkali or alkaline earth metals, optionally up to 10% Zn and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum; ii. Blowing off the flat steel product after it has left the melt bath using a gas stream; k) Cooling the coated flat steel product to room temperature, with an average cooling rate between 660°C and 570°C being at least 15 K/s; l) Optionally skin passing the coated flat steel product. Procedure according to Claim 1 , characterized in that the molten bath consists of 0.5-1.5 wt.% Si, optionally 2-4 wt.% Fe, optionally up to 5.0 wt.% alkali or alkaline earth metals, optionally up to 10% Zn and optionally further constituents, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminium. Method according to one of the Claims 1 until 2 , characterized in that the steel, in addition to iron and unavoidable impurities (in % by weight), consists of C: 0.04-0.45 wt.%, Yes: 0.02-1.2 wt.%, Mn: 0.5-2.6 wt.%, Al: 0.02-1.0 wt.%, P: ≤ 0.05 wt.%, S: ≤ 0.02 wt.%, N: ≤ 0.02 wt.%, Sn: ≤ 0.03 wt.%, As: ≤ 0.01 wt.%, Approx: ≤ 0.005 wt.%, and optionally one or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V” in the following contents Cr: 0.08-1.0 wt.%, B: 0.001-0.005% by weight, Mon: ≤0.5 wt.%, No: ≤0.5 wt.%, Cu: ≤0.2 wt.%, N.b.: 0.01-0.08 wt.%, T: 0.01-0.08 wt.%, V: ≤0.3 wt.%, consists. Sheet metal part formed from a steel sheet blank comprising a steel substrate (1) which consists of a steel which has 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B, wherein the sheet metal part has an aluminum-based corrosion protection coating on at least one side, characterized in that the amount of the hardness gradient of the corrosion protection coating and the steel substrate perpendicular to the surface of the steel substrate is less than 1.7 GPa/µm. Sheet metal part according to Claim 4 the steel, in addition to iron and unavoidable impurities (in % by weight), C: 0.04-0.45 wt.%, Yes: 0.02-1.2 wt.%, Mn: 0.5-2.6 wt.%, Al: 0.02-1.0 wt.%, P: ≤ 0.05 wt.%, S: ≤ 0.02 wt.%, N: ≤ 0.02 wt.%, Sn: ≤ 0.03 wt.%, As: ≤ 0.01 wt.%, Approx: ≤ 0.005 wt.%, and optionally one or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V” in the following contents Cr: 0.08-1.0 wt.%, B: 0.001-0.005% by weight, Mon: ≤0.5 wt.%, No: ≤0.5 wt.%, Cu: ≤0.2 wt.%, N.b.: 0.01-0.08 wt.%, T: 0.01-0.08 wt.%, V: ≤0.3 wt.%, consists. Sheet metal part according to one of the Claims 4 until 5 characterized in that the anti-corrosive coating comprises an alloy layer (3) and an Al base layer (5). Sheet metal part according to Claim 6 , characterized in that the alloy layer (3) consists of 35-95 wt.% Fe, 0.1-4 wt.% Si and optional further components, the total contents of which are limited to a maximum of 3.5 wt.%, and the remainder being aluminum. Sheet metal part according to one of the Claims 6 until 7 , characterized in that the Al base layer (5) consists of 35-55 wt.% Fe, 0.4-4 wt.% Si, optionally 3 wt.% alkali or alkaline earth metals, optionally up to 10% Zn and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder aluminum. Sheet metal part according to one of the Claims 6 until 8th , the Al base layer (5) consists of 35-55 wt.% Fe, 0.4-1.5 wt.% Si, optionally 3 wt.% alkali or alkaline earth metals, optionally up to 10% Zn and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum. Method for producing a sheet metal part, in particular according to one of the Claims 4 until 9 , comprising the following steps: a. Manufacturing a flat steel product according to the process of any of the Claims 1 until 2 ; b. Separating a sheet metal blank from the flat steel product; c. Heating the sheet metal blank in a furnace with a furnace temperature T _furnace for an annealing time t _G such that the AC3 temperature of the blank is at least partially exceeded and the temperature T _Einlg of the blank when inserted into a forming tool intended for hot press forming (work step c)) at least partially has a temperature above Ms+100°C, where Ms denotes the martensite start temperature; d. Inserting the heated sheet metal blank into a forming tool, wherein the transfer time t _Trans required for removing the blank from the heating device and inserting it is at most 20 s, preferably at most 15 s; e. Hot press forming the sheet metal blank to form the sheet metal part, wherein the blank is cooled to the target temperature T _target over a period t _WZ of more than 1 s at a cooling rate r _WZ that is at least partially more than 30 K/s during the hot press forming and is optionally held there; f. Removing the sheet metal part cooled to the target temperature T _target from the tool. Procedure according to Claim 10 , wherein the temperature at least partially reached in the sheet metal blank in step b) is between Ac3 and 1000°C, preferably between 850°C and 950°C. Method according to one of the Claims 10 until 11 , wherein the target temperature Ttarget of the sheet metal part is at least partially below 400°C, preferably below 300°C.