DE102022132907A1 - Flat steel product with colour change - Google Patents

Abstract

The present invention is a flat steel product for producing a sheet metal part by hot forming, comprising: a steel substrate consisting of a steel containing 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B, and b. an aluminum-based coating arranged on at least one side of the steel substrate, wherein the coating has an Al base layer consisting of 1.0-15 wt.% Si, optionally 2-4 wt.% Fe, 0.1-5.0 wt.% alkali or alkaline earth metals, and optional further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder aluminum, and wherein the alkali or alkaline earth metals in the Al base layer have a distribution such that the flat steel product in the cooled state after annealing in a furnace with a furnace temperature of 900° for an annealing time of 9 minutes experiences a color change compared to the cooled state after a reference annealing time of 4 minutes, wherein the color change has a color dynamic of greater than 100.

Description

The invention relates to a flat steel product for producing a shaped sheet metal part by hot forming, a method for producing such a flat steel product, a method for checking a shaped sheet metal part and a method for producing a shaped sheet metal part.

When we refer to a "flat steel product" or a "sheet metal product" below, 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" of the type according to the invention 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 the contents of the steel compositions specified in this 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. %". Information provided in this text on the contents of the components of an atmosphere refers to the volume (data in "vol. %").

From the WO 2022/048990 A1 and the EP 2 993 248 B1 Flat steel products with similar aluminium-based coatings and processes for their production are known.

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 hardened 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 problem, however, is that the desired corrosion protection is only achieved if the processing conditions, especially the annealing time, are adhered to very precisely. An annealing time that is too long leads to more diffusion of iron into the coating, which seriously disrupts the layer structure. In industrial production, however, deviations can occasionally occur due to disruptions in the operating process. It is therefore necessary to identify and sort out sheet metal parts that were accidentally manufactured under incorrect conditions. In principle, this is possible, for example, by making a metallographic section or by chemically analyzing the surface. However, such examinations are very complex and not non-destructive.

Therefore, the object of the present invention is to further develop a flat steel product in such a way that it can be determined without great effort on the formed sheet metal part whether the desired manufacturing conditions have been met.

1. a. ein Stahlsubstrat, das aus einem Stahl, der 0,1-3 Gew.-% Mn und optional bis zu 0,01 Gew.-% B aufweist, besteht und
2. b. einen auf mindestens einer Seite des Stahlsubstrats angeordneten Überzugs auf Aluminium-Basis,

wobei der Überzug eine AI-Basisschicht aufweist, die aus 1,0-15 Gew.-% Si, optional 2-4 Gew.-% Fe, 0,1-5,0 Gew.-% Alkali- oder Erdalkalimetalle, und optionalen weiteren Bestandteilen, deren Gehalte in Summe auf höchstens 2,0 Gew.-% beschränkt sind, und als Rest Aluminium besteht.This object is achieved by a flat steel product for producing a sheet metal part by hot forming, comprising

1. a steel substrate consisting of a steel containing 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B, and
2. b. an aluminium-based coating on at least one side of the steel substrate,

wherein the coating has an Al base layer consisting of 1.0-15 wt.% Si, optionally 2-4 wt.% Fe, 0.1-5.0 wt.% alkali or alkaline earth metals, and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum.

The alkali or alkaline earth metals in the Al base layer have a distribution such that the steel flat product in the cooled state after annealing in a furnace with a furnace temperature of 900° for an annealing time of 9 minutes experiences a color change compared to the cooled state after a reference annealing time of 4 minutes, whereby the color change has a color dynamic of greater than 100.

Typically, coatings of this type are produced by hot-dip coating. Surprisingly, it has been shown that the addition of 0.1-5.0 wt.% alkali or alkaline earth metals to the melt and certain cooling conditions, which are explained in detail later, produce a coating with a certain distribution of the alkali or alkaline earth metals in the Al base layer. This distribution leads to a color change in the flat steel product when annealing times exceed 4 minutes. Further diffusion processes change the concentration of alkali and alkaline earth metals near the surface, which in turn causes the color change. Comparative tests clearly show that this color change is much stronger than with coatings without the addition of alkali or alkaline earth metals. It is therefore possible to determine whether the sheet metal part has undergone the desired annealing process by measuring color values on the formed sheet metal part and comparing them with reference color values of a cooled reference sheet metal part. The reference sheet metal part was annealed for 4 minutes in a furnace with a furnace temperature of 900°C. The cooled sheet metal part (or reference sheet metal part) is understood to mean that the sheet metal part (or reference sheet metal part) has reached room temperature.

• Mittels eines Spektralphotometers und einer Lichtquelle mit der CIE-Normlichtart A (kann mit hinreichender Genauigkeit durch eine gasgefüllte Wolfram-Glühlampe der Verteilungstemperatur TV≈2856 K realisiert werden) und einem 10° Sichtfeld gemäß der Konvention CIE 1964 wird die Farbe des Blechformteils im CIE-Lab Farbraum erstellt. Dieser ist gemäß EN ISO 11664-4 „Colorimetry - Part 4: CIE 1976 L*a*b* Colour space“ genormt. Aus dem CIE-Lab Farbraum werden die Werte in den sRGB₈-Farbraum umgerechnet. Bei dem verwendeten sRGB₈-Farbraum handelt es sich um die 8-Bit-Variante, bei der die Farbwerte Integerzahlen von 0-255 annehmen. Die Umrechnung ist dem Fachmann geläufig und beispielsweise auf https://convertingcolors.com/cielab-color-1.00_1.00_1.00.html online durchführbar.

The color dynamics of a color change is determined as follows:

• Using a spectrophotometer and a light source with CIE standard illuminant A (can be achieved with sufficient accuracy using a gas-filled tungsten lamp with a distribution temperature TV≈2856 K) and a 10° field of view in accordance with the CIE 1964 convention, the color of the sheet metal part is created in the CIE Lab color space. This is standardized in accordance with EN ISO 11664-4 “Colorimetry - Part 4: CIE 1976 L*a*b* Color space”. The values from the CIE Lab color space are converted to the sRGB ₈ color space. The sRGB ₈ color space used is the 8-bit variant, in which the color values take on integers from 0-255. The conversion is familiar to the expert and can be used, for example, on https://convertingcolors.com/cielab-color-1.00_1.00_1.00.html can be carried out online.

The result is unique RGB values for the sheet metal part. The reference color values (R _ref G _ref B _ref ) of a correctly processed sheet metal part (reference sheet metal part) are determined in a similar way.

A color dynamic D of a color change from the reference sheet metal part to the sheet metal part is now defined as the sum of the amounts of the differences of the RGB values, ie: $$D=\left|R-R_{ree}\right|+\left|G-G_{ree}\right|+\left|B-B_{ree}\right|$$

It has been shown that the color dynamics defined in this way are particularly well suited to quantify the color change.

• 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;
• b) Durcherwärmen der Bramme oder Dünnbramme bei einer Temperatur (T1) von 1000-1400°C;
• c) Optionales Vorwalzen der durcherwärmten Bramme oder Dünnbramme zu einem Zwischenprodukt mit einer Zwischenprodukttemperatur (T2) von 1000-1200°C;
• d) Warmwalzen zu einem warmgewalzten Stahlflachprodukt, wobei die Endwalztemperatur (T3) 750-1000°C beträgt;
• e) Optionales Haspeln des warmgewalzten Stahlflachprodukts, wobei die Haspeltemperatur (T4) höchstens 700°C beträgt;
• f) Optionales Entzundern des warmgewalzten Stahlflachprodukts;
• g) Optionales Kaltwalzen des Stahlflachprodukts, wobei der Kaltwalzgrad mindestens 30 % beträgt;
• h) Glühen des Stahlflachprodukts bei einer Glühtemperatur (T5) von 650-900°C;
• i) Abkühlen des Stahlflachprodukts auf eine Eintauchtemperatur (T6), welche 650-800°C, bevorzugt 670-720°C beträgt;
• 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;
  2. ii. Abblasen des Stahlflachproduktes nach Austritt aus dem Schmelzbad mittels eines Gasstroms;
• k) Abkühlen des beschichteten Stahlflachprodukts auf Raumtemperatur, wobei eine Abkühlrate zwischen 660°C und 570°C mindestens 15 K/s beträgt;
• l) optionales Dressieren des beschichteten Stahlflachprodukts.

Such a flat steel product according to the invention is produced using the method according to the invention described below. The method according to the invention for producing a flat steel product for hot forming with a coating comprises the following work steps:

• (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;
• b) heating the slab or thin slab at a temperature (T1) of 1000-1400°C;
• 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;
• d) hot rolling to a hot-rolled flat steel product, the final rolling temperature (T3) being 750-1000°C;
• (e) optional coiling of the hot-rolled flat steel product, the coiling temperature (T4) being not more than 700°C;
• f) Optional descaling of the hot-rolled flat steel product;
• (g) optional cold rolling of the flat steel product, with a cold rolling degree of 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 650-800°C, preferably 670-720°C;
• 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) 660-800°C, preferably 670-710°C;
  2. ii. Blowing off the flat steel product after it has left the molten bath by means of a gas stream;
• k) cooling the coated flat steel product to room temperature, with a cooling rate between 660°C and 570°C of at least 15 K/s;
• l) optional skin passing of the coated flat steel product.

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 flat steel 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 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 50s after hot rolling. The cooling medium used for this can be, for example, 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.

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

Where Δd _KW = thickness reduction during cold rolling in mm and d = hot strip thickness in mm, where the thickness reduction Δd _KW 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 in 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 10-120s and then held at the annealing temperature for 30-600s. 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 1.0-15 wt.% Si, optionally 2-4 wt.% Fe, optionally 0.1-5.0 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, and optional further constituents, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder aluminum. In a preferred variant, the Si content of the melt is 1.0-3.5 wt.% or 7-12 wt.%, in particular 8-10 wt.%. In a preferred variant, the optional content of alkali or alkaline earth metals in the melt 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 melt can comprise in particular at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% 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 begins of the coating and with further cooling 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. It has been shown that this rapid cooling results in a certain distribution of the alkali or alkaline earth metals in the Al base layer of the coating, which leads to the advantageous color change described during the subsequent forming process. Basically the alkali or alkaline earth metals tend to concentrate close to the surface. While the alkali or alkaline earth metals are still evenly distributed in the melt, the alkali or alkaline earth metals in the coating diffuse towards the surface. This is possible as long as sufficient thermal energy is available for diffusion processes. If the cooling rate between the start and end of solidification is too slow, the majority of the alkali or alkaline earth metals have already diffused to the surface. During the subsequent annealing for the forming process, there is therefore no longer any significant diffusion of the alkali or alkaline earth metals to the surface. Therefore, in such a case, no significant color change of the surface occurs. The diffusion must therefore be stopped so quickly after the coating treatment that during the subsequent annealing for the forming process sufficient diffusion processes can still take place to cause the color change. This is achieved by an average cooling rate of between 660°C and 570°C, i.e. between the start of solidification and the end of solidification, which is at least 15 K/s. However, the diffusion must not be stopped too quickly either. A certain proportion of the alkali or alkaline earth metals must already be concentrated on the surface. The purpose of adding the alkali or alkaline earth metals is that during the subsequent annealing process before forming, oxides of the alkali or alkaline earth metals form on the surface of the coating instead of aluminum oxides. This has the advantage that less free hydrogen is produced than when Al ₂ O ₃ is formed. (The oxygen bound during oxidation usually comes from water molecules in the atmosphere, so that the remaining hydrogen is inevitably released during oxidation). The free hydrogen diffuses into the steel substrate and leads to undesirable hydrogen embrittlement. The addition of alkali or alkaline earth metals leads to a reduction in free hydrogen and thus to a reduction in hydrogen embrittlement. However, the alkali or alkaline earth metals can only bring about this effect if they are close to the surface of the substrate during the annealing process before forming. It is therefore advantageous if a certain proportion of the alkali or alkaline earth metals has already diffused to the surface during solidification. Then a sufficient proportion of alkali or alkaline earth metals is available for the oxidation process described immediately from the start of the annealing process before forming. It is therefore advantageous if the cooling rate is a maximum of 100 K/s, preferably a maximum of 50 K/s. This ensures that a sufficient proportion of the alkali or alkaline earth metals has diffused to the surface of the coating. This targeted choice of cooling process achieves a distribution of the alkali or alkaline earth metals in the coating (especially in the Al base layer of the coating) which, on the one hand, has sufficient alkali or alkaline earth metals close to the surface to reduce the free hydrogen immediately from the start of the subsequent annealing and, on the other hand, still allows sufficient diffusion which leads to the color change according to the invention.

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 flat steel product and in the process for producing a flat steel product 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.

• C: 0,04 - 0,45 Gew.-%
• Si: 0,02 -1,2 Gew.-%
• Mn: 0,5 - 2,6 Gew.-%
• AI: 0,02 - 1,0 Gew.-%
• P: ≤ 0,05 Gew.-%
• S: ≤ 0,02 Gew.-%
• N: ≤ 0,02 Gew.-%
• Sn: ≤ 0,05 Gew.-%
• As: ≤ 0,01 Gew.-%
• Ca: ≤ 0,005 Gew.-%

sowie optional einem oder mehreren der Elemente „Cr, B, Mo, Ni, Cu, Nb, Ti, V“ in folgenden Gehalten:

• Cr: 0,08 - 1,0 Gew.-%
• B: 0,001 - 0,005Gew.-%
• Mo: <_0,5 Gew.-%
• Ni: <_0,5 Gew.-%
• Cu: ≤0,2 Gew.-%
• Nb: 0,02 - 0,08 Gew.-%,
• Ti: 0,01- 0,08 Gew.-%
• V: ≤0,2 Gew.-%

Particularly preferably, in addition to iron and unavoidable impurities (in % by weight), it consists of:

• C: 0.04 - 0.45 wt.%
• Si: 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: ≤ 0.05 wt.%
• As: ≤ 0.01 wt.%
• Ca: ≤ 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.%
• Mo: <_0.5 wt.%
• Ni: <_0.5 wt.%
• Cu: ≤0.2 wt.%
• Nb: 0.02 - 0.08 wt.%,
• Ti: 0.01- 0.08 wt.%
• V: ≤0.2 wt.%

• Cr: ≤ 0,050 Gew.-%,
• B: ≤ 0,0005 Gew.-%
• Nb: ≤ 0,005 Gew.-%,
• Ti: ≤ 0,005 Gew.-%

The elements P, S, N, Sn, As, Ca are impurities that cannot be completely avoided during steel production. In addition to these elements, other elements can 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 wt.%, preferably a maximum of 0.1 wt.%. 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 wt.%, preferably a maximum of 0.1 wt.%. The individual upper limits for the respective contamination of these elements are preferably as follows:

• Cr: ≤ 0.050 wt.%,
• B: ≤ 0.0005 wt.%
• Nb: ≤ 0.005 wt.%,
• Ti: ≤ 0.005 wt%

These preferred upper limits should be considered as alternatives 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 0.12-0.25 wt.% or in the range of 0.33-0.37 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 embodiments, 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 0.02 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 0.2 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 as well as to the steel substrates in the described manufacturing processes and also to the method for checking a sheet metal part described below.

The flat steel product according to the invention comprises an aluminum-based coating arranged on at least one side of the steel substrate, the coating having an Al base layer consisting of 1.0-15 wt.% Si, optionally 2-4 wt.% Fe, 0.1-5.0 wt.% alkali or alkaline earth metals, and optionally further constituents whose total contents are limited to a maximum of 2.0 wt.%, and the remainder being aluminum.

Such a coating serves to protect the steel substrate from oxidation and corrosion during hot forming and when the steel component produced is used. The coating is therefore also referred to as a corrosion protection 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. The narrow surfaces are referred to as the edges.

Such a corrosion protection coating is preferably produced by hot-dip coating the flat steel product. The flat steel product is passed through a liquid melt which consists of 0.1-15 wt.% Si, preferably more than 1.0 wt.% Si, optionally 2-4 wt.% Fe, 0.1 up to 5.0 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum.

In a preferred variant, the Si content of the melt is 1.0-3.5 wt.% or 7-12 wt.%, in particular 8-10 wt.%.

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

In a preferred embodiment, the 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.

In a preferred embodiment, the 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 at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca.

During hot-dip coating, iron diffuses from the steel substrate into the liquid coating, so that the anti-corrosive 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% by weight of Fe, preferably α-iron, optional further components, the total contents of which are limited to a maximum of 5.0% by weight, 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 components 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 1.0-15 wt.% Si, optionally 2-4 wt.% Fe, 0.1-5.0 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being 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 in the Al base layer can comprise in particular at least 0.0015 wt.% Ca, in particular at least 0.1 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.

The anti-corrosive coating preferably has a thickness of 5-60 µm, in particular 10-40 µm. The coating weight of the anti-corrosive coating is in particular 30-360 g/m^2 for anti-corrosive coatings on both sides or 15-180 g/m^2 for the one-sided variant. The coating weight of the anti-corrosive coating is preferably 100-200 g/m^2 for anti-corrosive coatings on both sides or 50-100 g/m^2 for one-sided coatings. The coating weight of the anti-corrosive coating is particularly preferably 120-180 g/m^2 for anti-corrosive coatings on both sides or 60-90 g/m^2 for one-sided coatings.

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, the majority of the oxides (i.e. more than 50% by weight of the oxides) being aluminum oxide. In particular, 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.

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

1. a. Bereitstellen eines Blechzuschnitts aus einem zuvor beschriebenen Stahlflachprodukt;
2. b. Erwärmen des Blechzuschnitts 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;
3. c. 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 20s, bevorzugt höchstens 15s, beträgt;
4. d. 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;
5. e. Entnehmen des auf die Zieltemperatur T_Ziel abgekühlten Blechformteils aus dem Werkzeug;
6. f. Abkühlen des Blechformteils auf Raumtemperatur

The invention further relates to a method for producing a sheet metal part, comprising the following work steps:

1. a. Providing a sheet metal blank from a previously described flat steel product;
2. b. Heating the sheet metal blank in such a way 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;
3. c. 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;
4. d. 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 which is at least partially more than 30 K/s and is optionally held there during the hot press forming;
5. e. Removing the sheet metal part cooled to the target temperature T _target from the tool;
6. f. Cooling the sheet metal part to room temperature

In the method according to the invention, a blank is thus provided which consists of a steel suitably composed in accordance with the above explanations (work step a)), which 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 c)) is at least partially above Ms+100°C. Partially exceeding a temperature (here AC3 or Ms+100°C) is understood in the context 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 the 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 in accordance with HOUGARDY, HP. in “Material Science Steel Volume 1: Basics”, Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229 ., given formula $$\text{Ac}3=\left(902-225*\%\text{C}+19*\%Si-11*\%\text{Mn}-5*\%\text{Cr}+13*\%\text{Mon}-20*\%\text{No}+55*\%\text{V}\right)°\text{C}$$

with %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 c), 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 only has to be ensured that the tool temperature T _WZ is below the desired target temperature T _Target . The residence 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 $$\begin{array}{l}\text{Ms}\left[°\text{C}\right]=(490.85-302.6\%\text{C}-30.6\%\text{Mn}-16.6\%\text{No}-8.9\%\text{Cr}+2.4\%\text{Mon}-11.3\%\text{Cu}\\ +8.58\%\text{Co}+7.4\%\text{V}+7.4\%\text{W}-14.5\%\text{Si})\left[°\text{C}/\text{Gew}.-\%\right]\end{array}$$

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 $$\begin{array}{c}\text{AC}3\left[°\text{C}\right]=\left(902-225*\%\text{C}+19*\%\text{Si}-11*\%\text{Mn}+13*\%\text{Mo}-20*\%\text{Ni}+55*\%\text{V}\right)[°\text{C}/\text{Gew}.-\\ \%]\end{array}$$

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 AC5 temperature of a steel within the scope of the inventive specifications are according to the formulas $$\begin{array}{c}\text{AC}1\left[°\text{C}\right]=\left(739-22*\%\text{C}-7*\text{Mn}+2*\%\text{Si}+14*\%\text{Cr}+13*\%\text{Mon}-13*\%\text{No}+20*\%\text{V}\right)[°\text{C}/\text{Gew}.-\\ \%]\end{array}$$

and $$\begin{array}{c}\text{AC}3\left[°\text{C}\right]=\left(902-225*\%\text{C}+19*\%\text{Si}-11*\%\text{Mn}+13*\%\text{Mon}-20*\%\text{No}+55*\%\text{V}\right)[°\text{C}/\text{Gew}.-\\ \%]\end{array}$$

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 50 K/s, in particular at least 50 K/s, in a special design at least 100 K/s.

In step f) after the sheet metal part has been removed in step e), the sheet metal part is cooled to room temperature within a cooling time t _AB of 0.5-600s. This is usually done by air cooling.

• Bei einer speziellen Ausführungsform umfasst das Blechformteil bevorzugt einen Korrosionsschutzüberzug auf Aluminium-Basis. Bevorzugt umfasst der Korrosionsschutzüberzug des Blechformteils eine Legierungsschicht und eine Al-Basisschicht.

During the heating of the flat steel product before hot forming, further diffusion of iron into the anti-corrosive coating occurs. This results in an anti-corrosive coating alloyed with iron, which has an Fe content of at least 35% by weight, within a short heating period. In addition, a special structure of the anti-corrosive coating of the sheet metal part is preferably obtained, which is described below:

• In a specific embodiment, the sheet metal part preferably comprises an aluminum-based anti-corrosive coating. The anti-corrosive coating of the sheet metal part preferably comprises an alloy layer and an Al-base layer.

Preferably, the corrosion protection coating of the steel component 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 35-90 wt.% Fe, 0.1-12 wt.% Si, optionally up to 0.5 wt.% Mg and optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder aluminum. The further diffusion of iron into the alloy In the coating layer, the proportions of Si and Mg are correspondingly lower than their respective proportions in the melt of the molten bath.

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, 0.4-10 wt.% Si, 0.1-5.0 wt.% alkali or alkaline earth metals and optional other components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum.

In a preferred variant, the 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 content of alkali or alkaline earth metals in the melt can comprise in particular at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca.

In a preferred embodiment, the 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.

In a preferred embodiment, the 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 at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca.

The Al base layer can have a homogeneous silicon distribution in which the local silicon 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 at least a 40% continuous layer that is delimited by silicon-poor regions. In an alternative embodiment, the silicon-rich phases are arranged in the form of 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 finish 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.

1. 1. Bestimmen eines Farbwertes oder mehrerer Farbwerte des Überzuges
2. 2. Vergleichen des einen Farbwertes oder der mehreren Farbwerte mit einem oder mehreren Referenzwerten
3. 3. Aussortieren des Blechformteils auf Basis eines Ergebnisses des Vergleichs

A further aspect of the invention is the inspection of a hot-formed sheet metal part with an aluminum-based coating, in particular a sheet metal part that is further developed as above and/or was produced according to the method explained above. The inspection method comprises at least the following steps:

1. 1. Determine one or more color values of the coating
2. 2. Comparing one or more color values with one or more reference values
3. 3. Sorting out the sheet metal part based on the result of the comparison

A simple process, namely determining one or more color values and comparing them with corresponding reference values, can be used to automatically determine whether the component has undergone the desired production process. In this way, every component can be easily tested in series production. This increases the security that no faulty components are reused. Previous testing methods required chemical analysis and/or the production of a metallographic section. Such methods are therefore not non-destructive. Consequently, only samples of the components could be tested. Using the method according to the invention, it is therefore possible to test the entire component production.

In a preferred development of the method, the one or more reference values are one or more color values of a cooled reference sheet metal part that has been annealed in a furnace with a furnace temperature of 900°C for a reference annealing time of 4 minutes. Tests have shown that annealing at 900°C for 4 minutes in many cases produces a component with the desired properties. It is therefore advisable to use this as a reference process and to use the color values that are present on such a reference component as reference values.

In a special embodiment of the method, the determination of at least one color value or several color values is carried out using a photometer, in particular using a spectrophotometer. The use of photometers has proven to be particularly useful because they work very reliably and can be easily integrated into an industrial series process. For example, it is easily possible to use a photometer to measure many components one after the other at short intervals.

Particularly preferably, determining the one color value or the multiple color values of the coating comprises determining at least three color values in a color space. In particular, determining the one color value or the multiple color values of the coating consists of determining at least three color values in a color space. The three color values in a color space can, for example, be the values L*a*b in the standardized CIE-Lab color space. Alternatively, a selected RGB color space can also be used. In this case, the three color values are the corresponding RGB values. The three color values in a color space are of course three independent color values, so that the color in this color space can be clearly identified by specifying the three color values. By recording at least three color values in a color value, the color change can be better characterized because more information is available and the color can be clearly identified in the color space.

Versuche haben gezeigt, dass eine solche Farbdynamik D besonders sensitiv auf die hier auftretenden Farbveränderungen ist. Daher eignet sich eine solche Farbdynamik gut, um das Blechformteil auf Basis der, im Rahmen des Vergleichs, berechneten Farbdynamik auszusortieren, beispielsweise wenn die Farbdynamik größer ist als 270.In a particularly preferred variant, comparing the one color value or the several color values with one or more reference values comprises the calculation of a color dynamic. In particular, the color values and the reference values are RGB values and the color dynamic D is calculated as the sum of the amounts of the differences between the RGB values and the RGB reference values: $$D=\left|R-R_{ree}\right|+\left|G-G_{ree}\right|+\left|B-B_{ree}\right|$$

Tests have shown that such a color dynamic D is particularly sensitive to the color changes that occur here. Therefore, such a color dynamic is well suited to sorting out the sheet metal part based on the color dynamic calculated as part of the comparison, for example if the color dynamic is greater than 270.

• g. Überprüfen des abgekühlten Blechformteils mit dem erläuterten Verfahren zum Überprüfen eines Blechformteils

In a preferred embodiment of the method for producing a sheet metal part explained above, the described method steps a) - f are followed, in particular immediately, by step g):

• g. Checking the cooled sheet metal part using the procedure described for checking a sheet metal part

The manufactured sheet metal parts are therefore checked as part of a quality control process to ensure that they have undergone the desired manufacturing process.

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 thoroughly heated. The slabs were discharged from the pusher furnace at their respective thorough 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 in hot strip rolling can also be referred to as pre-strips, each had an intermediate product temperature T2 of 1100°C at the end of the pre-rolling phase. The pre-strips were fed to the finish rolling immediately after pre-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 a conventional manner 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 they were cooled at a cooling rate of 1 K/s to the immersion temperature T6 of 690°C. 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 initially cooled to 660°C at an average cooling rate of 10-15 K/s. Between 660°C and 570°C, ie between the start of solidification and the end of solidification of the coating, the cooling rate was 21 K/s. During 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 × 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 with 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 as 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.

Tabelle 2 (Beschichtungsvarianten) Beschichtungsvariante Schmelzenanalyse Si Fe Mg Sonstig e Al α 9,5 3 <0,0 1 <1% Rest β 10 3 0,5 <1% Rest Tabelle 3 (Aufbau) Aufbau Stahls orte Besch ichtun 9 Blech dicke [mm] Überzugs dicke [µm] 1* A α 1,5 27 2 A β 1,5 27 * nicht erfindungsgemäße Referenzbeispiele Tabelle 4 (Warmumformparameter) Mittlere Aufheizgeschwindigkeit r_Ofen [30 - 700 °C] [K/s] T_Ofen [°C] Transferzeit [s] Taupunkt Ofen [°C] T_Einlg [°C] T_WZ Z[°C] t_WZ [s] Abkühlgeschwindigkeit rwz [K/s] T_Ziel [°C] 8 900 8 -5 800 RT 15 50 50 Tabelle 5 (Ergebnisse) Aufbau Glühdauer tOfen [Minuten] L a b R G B D 1* 4 39,94 1,35 3,09 99 93 89 0 (Referenz) 1* 5 37,22 1,58 0,39 91 87 87 16 1* 6 34,02 0,33 -5,74 76 80 89 40 1* 7 34,46 -1,16 -7,99 72 82 94 51 1* 8 34,14 -2,93 -10,77 65 82 97 61 1* 9 35,70 -5,00 -12,61 61 87 104 77 1* 10 37,23 -6,33 -13,00 61 92 108 86 1* 11 37,27 -4,63 -14,67 63 91 111 92 2 4 38,41 -0,78 -6,38 84 91 101 0 (Referenz) 2 5 36,70 -3,78 -11,66 68 89 105 22 2 6 38,65 -1,61 -4,82 84 92 99 47 2 7 42,00 -5,76 -10,23 78 103 116 81 2 8 53,68 -5,76 -9,13 108 132 144 168 2 9 45,29 -4,77 -5,68 93 110 116 233 2 10 45,31 -0,46 6,10 111 107 97 273 2 11 50,79 -2,87 6,99 121 122 109 310 In the process described, the coating was varied. Sheet metal types 1 and 2 were used, which only differed in the coating type α or β (see Table 3). The annealing time in the furnace was also varied between 4 minutes and 11 minutes. The color in the CIE Lab color space was determined for the sheet metal parts produced in this way using a spectrophotometer and a light source with the CIE standard illuminant A and a 10° field of view. The color values in the RGB space were calculated from this data. From this, the color dynamics D were determined. The results are shown in Table 5.

Table 2 (Coating variants) Coating variant Melt analysis Si Fe Mg Other e Al α 9.5 3 <0.0 1 <1% rest β 10 3 0.5 <1% rest Table 3 (structure) Construction Stahl's places Coating 9 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 Z[°C] t _WZ [s] Cooling rate rwz [K/s] T _Target [°C] 8th 900 8th -5 800 RT 15 50 50 Table 5 (Results) Construction Annealing time tOven [minutes] L a b R G B D 1* 4 39.94 1.35 3.09 99 93 89 0 (reference) 1* 5 37.22 1.58 0.39 91 87 87 16 1* 6 34.02 0.33 -5.74 76 80 89 40 1* 7 34.46 -1.16 -7.99 72 82 94 51 1* 8th 34.14 -2.93 -10.77 65 82 97 61 1* 9 35.70 -5.00 -12.61 61 87 104 77 1* 10 37.23 -6.33 -13,00 61 92 108 86 1* 11 37.27 -4.63 -14.67 63 91 111 92 2 4 38.41 -0.78 -6.38 84 91 101 0 (reference) 2 5 36.70 -3.78 -11.66 68 89 105 22 2 6 38.65 -1.61 -4.82 84 92 99 47 2 7 42.00 -5.76 -10.23 78 103 116 81 2 8th 53.68 -5.76 -9.13 108 132 144 168 2 9 45.29 -4.77 -5.68 93 110 116 233 2 10 45.31 -0.46 6.10 111 107 97 273 2 11 50.79 -2.87 6.99 121 122 109 310

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• WO 2022048990 A1 [0004]
• EP 2993248 B1 [0004]

Cited non-patent literature

• https://convertingcolors.com/cielab-color-1.00_1.00_1.00.html [0011]
• HOUGARDY, HP. in “Material Science Steel Volume 1: Basics”, Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229 [0069]

Claims (11)

A flat steel product for producing a sheet metal part by hot forming, comprising a. a steel substrate consisting of a steel containing 0.1-3 wt.% Mn and optionally up to 0.01 wt.% B, and b. an aluminum-based coating arranged on at least one side of the steel substrate, the coating having an Al base layer consisting of 1.0-15 wt.% Si, optionally 2-4 wt.% Fe, 0.1-5.0 wt.% alkali or alkaline earth metals, and optional further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum, characterized in that the alkali or alkaline earth metals in the Al base layer have a distribution such that the flat steel product in the cooled state after annealing in a furnace with a furnace temperature of 900° for an annealing time of 9 minutes experiences a color change compared to the cooled state after a reference annealing time of 4 minutes, the color change having a color dynamic of greater than 100. Flat steel product according to Claim 1 , characterized in that the steel, in addition to iron and unavoidable impurities (in wt.%), consists of C: 0.04-0.45 wt.%, Si: 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: ≤ 0.05 wt.%, As: ≤ 0.01 wt.%, Ca: ≤ 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.%, Mo: <_0.5 wt.%, Ni: <_0.5 wt.%, Cu: ≤0.2 wt.%, Nb: 0.02-0.08 wt.%, Ti: 0.01-0.08 wt.%, V: ≤0.2 wt.%. 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) Optional descaling of the hot-rolled flat steel product; g) Optional cold rolling of 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. Immersing it in a melt bath with a melt temperature (T7) of 660-800°C, preferably 670-710°C; ii. Blowing off the flat steel product after it has emerged from 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 of at least 15 K/s; l) optionally skin passing the coated flat steel product. Method for checking a hot-formed sheet metal part with an aluminum-based coating, comprising at least the following steps: a. determining one or more color values of the coating; b. comparing the one or more color values with one or more reference values; c. sorting out the sheet metal part based on a result of the comparison. Procedure according to Claim 3 , characterized in that the one or more reference values are one or more color values of a cooled reference sheet metal part which has undergone annealing in a furnace with a furnace temperature of 900°C for a reference annealing time of 4 minutes. Method according to one of the Claims 3 until 4 , characterized in that the determination of the at least one color value or the plurality of color values is carried out by means of a photometer, in particular by means of a spectrophotometer. Method according to one of the Claims 3 until 5 , characterized in that determining the one or more color values of the coating comprises determining at least three color values in a color space. Procedure according to Claim 6 , characterized in that comparing the one color value or the several color values with one or more reference values comprises the calculation of a color dynamic. Method for producing a sheet metal part comprising the following steps: a. Providing a sheet metal blank from a flat steel product according to one of the Claims 1 until 2 ; b. Heating the sheet metal blank in such a way 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; c. 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; d. Hot press forming the sheet metal blank to form the sheet metal part, wherein the blank is cooled to the target temperature T _Ziel over a duration t _WZ of more than 1 s at a cooling rate r _WZ which is at least partially more than 30 K/s and is optionally held there; e. Removing the sheet metal part cooled to the target temperature T _target from the tool; f. Cooling the sheet metal part to room temperature; g. Checking the cooled sheet metal part using the method according to one of the Claims 4 until 8th . Procedure according to Claim 9 , 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 9 until 10 , wherein the target temperature T _target of the sheet metal part is at least partially below 400°C, preferably below 300°C.