MOLDED SHEET-METAL PART WITH IMPROVED HARDNESS CURVE

Abstract

A shaped sheet metal part formed from a sheet steel blank includes a steel substrate which consists of a steel which includes 0.1-3% by weight of Mn and optionally up to 0.01% by weight of B. The shaped sheet metal part has an aluminum-based anticorrosion coating on at least one side and is characterized in that the magnitude of the hardness gradient of anticorrosion coating and steel substrate perpendicular to the surface of the steel substrate is less than 1.7 GPa/μm. A process for producing such a shaped sheet metal part.

Description

The invention relates to a process for producing a shaped sheet metal part by means of hot forming a sheet steel blank.

“Sheet steel blanks” or “sheet metal blanks” are understood here to be trimmed blanks of flat steel products. Where a “flat steel product” or else a “sheet metal product” is discussed, this means rolled products, such as steel strips or sheets, from which “sheet metal blanks” (also called just “blanks”) are divided for the production of bodywork components, for example. “Shaped sheet metal parts” or “sheet metal components” are produced from such sheet metal blanks; the terms “shaped sheet metal part” and “sheet metal component” are used synonymously here.

All FIGURES relating to contents of the steel compositions that are reported in the present application are based on weight, unless explicitly stated otherwise. All indeterminate percentage FIGURES associated with a steel alloy should therefore be regarded as FIGURES in “% by weight”. With the exception of the FIGURES relating to the residual austenite content of the microstructure of a shaped sheet metal part of the invention that are based on volume (reported in “% by volume”), FIGURES relating to the contents of the various microstructure constituents (e.g., martensite) are each based on the area of a section of a sample of the respective product (reported in area percent, “area %”), unless explicitly stated otherwise. Figures given in this text for the contents of the constituents of an atmosphere are based on volume (reported in “% by volume”).

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

WO 2022/048990 A1 discloses flat steel products and also shaped sheet metal parts and processes for their production. These flat steel products and shaped sheet metal parts have an aluminum-based anticorrosion coating which is produced by hot dip coating. The melt used in this case has an Si content of 0.05-3% by weight.

Such flat steel products have an aluminum-based coating and are further processed to give shaped sheet metal parts by means of hot forming. During this operation, blanks of the flat steel products are heated to a hot forming temperature (e.g., 900° C.) over a certain annealing period (e.g., 4 minutes). During this annealing period, iron diffuses from the steel substrate into the aluminum-based coating. This results in a coating that protects against corrosion very effectively. The hot blank is then formed in a forming tool to give a shaped sheet metal part and quickly cooled, resulting in formation of a hardness structure (e.g., martensite) in the steel substrate. The outcome is a shaped sheet metal part with high strength and a coating that protects very well against corrosion.

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

This object is achieved by a process for producing a flat steel product for hot forming with a coating, comprising operating steps as follows:

• • a) providing a slab or a thin slab consisting of a steel which includes 0.1-3% by weight of Mn and optionally up to 0.01% by weight of B;
  • b) through-heating the slab or thin slab at a temperature (T1) of 1000-1400° C.;
  • c) optionally pre-rolling the through-heated slab or thin slab to give an intermediate product having an intermediate product temperature (T2) of 1000-1200° C.;
  • d) hot rolling to give a hot-rolled flat steel product, where the final rolling temperature (T3) is 750-1000° C.;
  • e) optionally coiling the hot-rolled flat steel product, where the coiling temperature (T4) is not more than 700° C.;
  • f) optionally descaling the hot-rolled flat steel product;
  • g) optionally cold-rolling the flat steel product, where the degree of cold rolling is at least 30%;
  • h) annealing the flat steel product at an annealing temperature (T5) of 650-900° C.;
  • i) cooling the flat steel product to a dipping temperature (T6) of 600-800° C., preferably 680-720° C.;
  • j) coating the flat steel product cooled to the dipping temperature with a coating by
    • i. immersing the product in a melt bath with a melt temperature (T7) of 660-800° C., preferably 670-710° C., where the melt bath consists of 0.5-4% by weight of Si, optionally 2-4% by weight of Fe, optionally up to 5.0% by weight of alkali metals or alkaline earth metals, optionally up to 10% of Zn, and optional further constituents, the contents of which are limited to a total of not more than 2.0% by weight, and aluminum as the balance;
    • ii. blowing off the flat steel product after discharge from the melt bath by means of a gas stream;
  • k) cooling the coated flat steel product to room temperature, where an average cooling rate between 660° C. and 570° C. is at least 15 K/s;
  • l) optionally skin pass rolling the coated flat steel product.

Surprisingly, it has emerged that the Si content of the melt in the range of 0.5-4% by weight of Si, more particularly in the range of 0.5-1.5% by weight, in combination with the specific cooling rate in step k), leads to a flat steel product which can be formed into a shaped sheet metal part having improved properties. The relevant mechanism is explained in more detail later on below.

In step a), a semifinished product with a composition in accordance with the alloy specified in accordance with the invention for the flat steel product is provided. This may be a slab produced by conventional continuous slab casting or by continuous thin slab casting.

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

In the optional step c), the semifinished product is pre-rolled to an intermediate product. Thin slabs are typically not subjected to any pre-rolling. Thick slabs that are to be rolled out to hot strips can be subjected to pre-rolling if required. In that case, the temperature of the intermediate product (T2) at the end of the pre-rolling should be at least 1000° C. in order 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 operation, which has an adverse effect on the mechanical properties of the flat steel product. In order to minimize grain growth during the rolling operation, the temperature of the intermediate product at the end of the pre-rolling should not be more than 1200° C.

In step d), the slab or thin slab or, if step c) has been performed, the intermediate product is rolled to give a hot-rolled flat steel product. If step c) has been performed, the intermediate product is typically finish-rolled immediately after the pre-rolling. The finish-rolling typically commences no later than 90 s after the end of the pre-rolling. The slab, the thin slab or, if step c) has been performed, the intermediate product are rolled to completion at a final rolling temperature (T3). The final rolling temperature, i.e., the temperature of the fully hot-rolled flat steel product at the end of the hot rolling process, is 750-1000° C. At final rolling temperatures of less than 750° C., the amount of free vanadium decreases because larger amounts of vanadium carbides are precipitated. The vanadium carbides that precipitate in the course of 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, as conducted prior to hot dip coating, for example. The final rolling temperature is limited to values of not more than 1000° C. in order to prevent coarsening of the austenite grains. Moreover, final rolling temperatures of not more than 1000° C. are of relevance for process technology purposes in order to establish coiling temperatures (T4) of less than 700° C.

The hot rolling of the flat steel product can be effected in the form of a continuous hot strip rolling operation or of a reversing rolling operation. Step e) in the case of continuous hot strip rolling provides for optional coiling of the hot-rolled flat steel product. For this purpose, the hot strip, after the hot rolling, is cooled down to a coiling temperature (T4) within less than 50 s. The cooling medium used for this purpose may, for example, be water, air or a combination of the two. The coiling temperature (T4) should be not more than 700° C. in order to avoid the formation of large vanadium carbides. There is in principle no lower limit to the coiling temperature. However, coiling temperatures of at least 500° C. have been found to be favorable for cold rollability. Subsequently, the coiled hot strip is cooled down to room temperature in a conventional manner under air.

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

The descaled hot-rolled flat steel product, prior to the annealing treatment in step g), may optionally be subjected to cold rolling in order, for example, to meet higher demands on the thickness tolerances of the flat steel product. The degree of cold rolling (DCR) should be at least 30% in order to introduce sufficient deformation energy into the flat steel product for rapid recrystallization. The degree of cold rolling DCR is understood to mean the quotient of the decrease in thickness on cold rolling ΔdCR divided by the hot strip thickness d:

$\mathrm{DCR}=\Delta \mathrm{dcR}/d$

with ΔdCR=decrease in thickness on cold rolling in mm and d=hot strip thickness in mm, where the decrease in thickness ΔdCR is calculated from the difference in thickness of the flat steel product before cold rolling relative to the thickness of the flat steel product after cold rolling. The flat steel product before cold rolling is usually a hot strip of thickness d. The flat steel product after cold rolling is usually also referred to as cold strip. The degree of cold rolling may in principle assume very high values of more than 90%. However, degrees of cold rolling of not more than 80% have been found to be favorable for avoidance of strip cracks.

In step h), the flat steel product is subjected to an annealing treatment at annealing temperatures (T5) of 650-900° C. For this purpose, the flat steel product is first heated to the annealing temperature within 10 to 120 s and then kept at the annealing temperature for 30 to 600 s. The annealing temperature is at least 650° C., preferably at least 720° C. Annealing temperatures above 900° C. are not desirable, for economic reasons.

In step i), the flat steel product, after the annealing, is cooled down to a dipping temperature (T6) in order to prepare it for the subsequent coating treatment. The dipping temperature is lower than the annealing temperature and is matched to the temperature of the melt bath. The dipping temperature is 600-800° C., preferably at least 650° C., particularly preferably at least 670° C., particularly preferably at most 700° C. For particularly homogeneous boundary layer formation, it is important that there is sufficient thermal energy in the boundary layer between steel substrate and aluminum melt. This is not the case at temperatures lower than 600° C., and so unwanted compounds may form whose later reconversion can lead to pores. Over and above the preferred dipping temperatures, there is another significant increase in the diffusion rate of iron into aluminum, such that more iron can diffuse into the still-liquid interfacial layer even at the start of the coating process. The duration of the cooling of the annealed flat steel product from the annealing temperature T5 to the dipping temperature T6 is preferably 10-180 s. In particular, the dipping temperature T6 differs from the temperature of the melt bath T7 by not more than 30 K, especially not more than 20 K, preferably not more than 10 K.

The flat steel product is subjected to a coating treatment in step j). The coating treatment is preferably effected by continuous hot dip coating. The coating can be applied only on one side, on both sides or on all sides of the flat steel product. The coating treatment is preferably effected as a hot dip coating process, especially as a continuous process. The flat steel product typically comes into contact with the melt bath on all sides, such that it is coated on all sides. The melt bath containing in liquid form the alloy to be applied to the flat steel product is typically at a temperature (T7) of 660-800° C., preferably 670-740° C., more preferably 670-710° C. Aluminum-based alloys have been found to be particularly suitable for coating of aging-resistant flat steel products with an anticorrosion coating. In such a case, the melt bath contains 0.5% to 4% by weight of Si, more particularly 0.5-1.5% by weight of Si, optionally 2-4% by weight of Fe, optionally 0.1-5.0% by weight of alkali metals or alkaline earth metals, preferably up to 1.0% by weight of alkali metals or alkaline earth metals, and optional further constituents, the total contents of which are limited to not more than 2.0% by weight, and aluminum as the balance. In a preferred variant, the optional content of alkali metals or alkaline earth metals in the melt comprises 0.1-1.0% by weight of Mg, especially 0.1-0.7% by weight of Mg, preferably 0.1-0.5% by weight of Mg. In addition, the optional content of alkali metals or alkaline earth metals in the melt may especially comprise at least 0.0015% by weight of Ca, especially at least 0.01% by weight of Ca. More particularly, the optional content of alkali metals or alkaline earth metals in the melt consists of 0.1-1.0% by weight of Mg, especially 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 exiting the melt bath, the flat steel product is blown off by a gas stream to adjust the thickness of the coating.

After the coating treatment, the coated flat steel product is cooled down to room temperature in step k). An average cooling rate between 660° C. and 570° C. here is at least 15 K/s, preferably at least 20 K/s. This corresponds to the range between the beginning of solidification and the end of solidification of the coating. On cooling to 660° C., solidification of the coating begins, and on further cooling to 570° C., the coating is completely solidified. Preferably, the average cooling rate is a maximum of 100 K/s, especially preferably a maximum of 50 K/s.

A certain diffusion of iron from the steel substrate into the coating occurs even during the hot dip coating and cooling. This diffusion starts up again when the sheet metal blanks of the resulting flat steel product are reheated to temperatures above AC3 before forming. While diffusion during hot dip coating and cooling takes place at temperatures from about 750° C. down to 570° C. (at 570° C. diffusion stops mainly due to the solidification of the coating), diffusion during reheating before forming takes place at temperatures around 900° C. Surprisingly, it has emerged that the boundary region between the steel substrate and the coating develops differently depending on the temperatures at which diffusion takes place. In the lower temperature range of 750° C. to 570° C., diffusion is comparatively slow and a relatively sharp transition is formed between steel substrate and coating. At temperatures of 900° C., conversely, the diffusion processes are much faster. In addition, liquid-melt phases are formed in the coating, so that in addition to the uniform diffusion, a certain mixing occurs due to convection. Such convection regions are naturally randomly distributed. As a result, these two effects result in not so sharp a transition forming between coating and steel substrate.

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

The coated flat steel product can optionally be subjected to a skin pass rolling operation with a degree of skin pass rolling of up to 2%, in order 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 shaped sheet metal part and in the shaped sheet metal part itself is a steel which includes 0.1-3% by weight of Mn and optionally up to 0.01% by weight of B. The same applies, of course, to the steel of the hot-formed shaped sheet metal part as well.

In particular, the microstructure of the steel is convertible by hot forming to a martensitic or partly martensitic microstructure. The microstructure of the steel substrate of the shaped sheet metal part is thus preferably a martensitic or at least partly martensitic microstructure, since this has particularly high hardness.

The steel substrate is more preferably a steel consisting of, as well as iron and unavoidable impurities, (in % by weight)

• • C: 0.04-0.45% by weight,
  • Si: 0.02-1.2% by weight,
  • Mn: 0.5-2.6% by weight,
  • Al: 0.02-1.0% by weight,
  • P: ≤0.05% by weight,
  • S: ≤0.02% by weight,
  • N: ≤0.02% by weight,
  • Sn: ≤0.03% by weight,
  • As: ≤0.01% by weight,
  • Ca: ≤0.005% by weight,

And Optionally One or More of the Elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V” in Amounts as Follows:

• • Cr: 0.08-1.0% by weight,
  • B: 0.001-0.005% by weight,
  • Mo: ≤0.5% by weight,
  • Ni: ≤0.5% by weight,
  • Cu: ≤0.2% by weight,
  • Nb: 0.01-0.08% by weight,
  • Ti: 0.01-0.08% by weight,
  • V: ≤0.3% by weight.

The elements P, S, N, Sn, As, Ca are impurities that cannot be entirely avoided in steelmaking. Occasionally, Ca is also added intentionally for the binding of sulfur. In such a case, the content of Ca is at least 0.001% by weight. The maximum Ca content in this case as well is 0.005% by weight.

As well as these elements, further elements may also be present in the steel as impurities. These further elements are combined under the “unavoidable impurities”. The content of unavoidable impurities preferably adds up to not more than 0.2% by weight, preferably not more than 0.1% by weight. The optional alloy constituents Cr, B, Nb, Ti for which a lower limit is reported may also occur in contents below the respective lower limit as unavoidable impurities in the steel substrate. In that case, they are likewise counted among the unavoidable impurities, the total content of which is limited to not more than 0.2% by weight, preferably not more than 0.1% by weight. The individual upper limits for the respective contamination by these elements are as follows:

• • Cr: ≤0.050% by weight,
  • B: ≤0.0005% by weight
  • Nb: ≤0.005% by weight,
  • Ti: ≤0.005% by weight

These preferred upper limits should be considered alternatively or collectively. Preferred variants of the steel thus fulfill one or more of these four conditions.

In a preferred embodiment, the C content of the steel is not more than 0.37% by weight and/or at least 0.06% by weight. In particularly preferred variant embodiments, the C content is in the range of 0.06-0.09% by weight or in the range of 0.11-0.25% by weight or in the range of 0.32-0.37% by weight.

In a preferred embodiment, the Si content of the steel is not more than 1.00% by weight and/or at least 0.06% by weight.

The Mn content of the steel in a preferred variant is not more than 2.4% by weight and/or at least 0.75% by weight. In particularly preferred variant embodiments, the Mn content is in the range of 0.75-0.85% by weight or in the range of 1.0-1.6% by weight.

The Al content of the steel in a preferred variant is not more than 0.75% by weight, especially not more than 0.5% by weight, preferably not more than 0.25% by weight. Alternatively or additionally, the Al content is preferably at least 0.02% by weight.

In addition, it has been found that it can be helpful when the sum total of the contents of silicon and aluminum is limited. In a preferred variant, the sum total of the contents of Si and Al (typically referred to as Si+Al) is therefore not more than 1.5% by weight, preferably not more than 1.2% by weight. Supplementarily or alternatively, the sum total of the contents of Si and Al is at least 0.06% by weight, preferably at least 0.08% by weight.

The elements P, S and N are typical impurities that cannot be entirely avoided in steelmaking. In preferred variants, the P content is not more than 0.03% by weight. Independently thereof, the S content is preferably not more than 0.012%. Additionally or supplementarily, the N content is preferably not more than 0.009% by weight.

Optionally, the steel additionally contains chromium with a content of 0.08-1.0% by weight. The Cr content is preferably not more than 0.75% by weight, especially not more than 0.5% by weight.

In the case of optional inclusion of chromium in the alloy, the sum total of the contents of chromium and manganese is preferably limited. The sum total is not more than 3.3% by weight, especially not more than 3.15% by weight. In addition, the sum total is at least 0.5% by weight, preferably at least 0.75% by weight.

The steel preferably optionally additionally contains boron with a content of 0.001-0.005% by weight. In particular, the B content is not more than 0.004% by weight.

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

In addition, the steel may optionally contain nickel with a content of not more than 0.5% by weight, preferably not more than 0.15% by weight.

Optionally, the steel may additionally contain copper with a content of not more than 0.2% by weight, preferably not more than 0.15% by weight.

In addition, the steel may optionally contain one or more of the microalloy elements Nb, Ti and V. The optional Nb content here is at least 0.01% by weight, more particularly at least 0.02% by weight and at most 0.08% by weight, preferably at most 0.04% by weight. The optional Ti content is at least 0.01% by weight and at most 0.08% by weight, preferably at most 0.04% by weight. The optional V content is at most 0.3% by weight, preferably at most 0.2% by weight, in particular at most 0.1% by weight, preferably at most 0.05% by weight.

In the case of optional inclusion of two or more of the elements Nb, Ti and V in the alloy, the sum total of the contents of Nb, Ti and V is preferably limited. The sum total is not more than 0.1% by weight, especially not more than 0.068% by weight. In addition, the sum total is preferably at least 0.015% by weight.

The above explanations for preferred steel substrates also apply, of course, to the steel substrate of the flat steel product described below, as well as to the steel substrates in the production processes described.

The flat steel product thus produced comprises an aluminum-based anticorrosion coating. This anticorrosion coating may have been applied on one or both sides of the flat steel product. “Both sides of the flat steel product” refer to the two opposite large faces of the flat steel product. The narrow faces are referred to as edges.

In the case of hot dip coating, as already explained, iron diffuses out of the steel substrate into the liquid coating, so that the anticorrosion coating of the flat steel product on solidification comprises an alloy layer and an Al-base layer in particular.

The alloy layer lies atop and directly adjoins the steel substrate. The alloy layer is formed essentially from 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 constituents, the contents of which are limited to a total of not more than 5.0% by weight, preferably 2.0% by weight, and aluminum as the balance, where the Al content preferably increases in the direction of the surface. The optional further constituents especially include the other constituents of the melt (i.e. silicon and optionally alkali metals or alkaline earth metals, especially Mg and Ca) and the other components of the steel substrate in addition to iron.

The Al-base layer lies atop and directly adjoins the alloy layer. The composition of the Al-base layer preferably corresponds to the composition of the melt in the melt bath. This means that this layer consists of 0.5-4% by weight of Si, optionally 2-4% by weight of Fe, optionally up to 5.0% by weight of alkali metals or alkaline earth metals, optionally up to 10% of Zn, and optional further constituents, the contents of which are limited to a total of not more than 2.0% by weight, and aluminum as the balance. 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 metals or alkaline earth metals comprises 0.1-1.0% by weight of Mg, especially 0.1-0.7% by weight of Mg, preferably 0.1-0.5% by weight of Mg. In addition, the optional content of alkali metals or alkaline earth metals may comprise especially at least 0.0015% by weight of Ca, especially at least 0.01% by weight of Ca. More particularly, the optional content of alkali metals or alkaline earth metals in the melt consists of 0.1-1.0% by weight of Mg, especially 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.

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

The anticorrosion coating preferably has a thickness of 5-60 μm, especially of 10-40 μm. The coat weight of the anticorrosion coating is especially

$30-360\frac{g}{m^2}$

in the case of double-sided anticorrosion coatings or

$15-180\frac{g}{m^2}$

in the case of the single-sided variant. The coat weight of the anticorrosion coating is preferably

$100-200\frac{g}{m^2}$

in the case of double-sided coatings or

$50-100\frac{g}{m^2}$

for single-sided coatings. The coat weight of the anticorrosion coating is more preferably

$120-180\frac{g}{m^2}$

in the case of double-sided coatings or

$60-90\frac{g}{m^2}$

for single-sided coatings.

The thickness of the alloy layer is preferably less than 20 μm, more preferably less than 16 μm, more preferably less than 12 μm, especially less than 10 μm. The thickness of the Al-base layer is found from the difference in the thicknesses of anticorrosion coating and alloy layer. The thickness of the Al-base layer even in the case of thin anticorrosion coatings is preferably at least 1 μm.

In a preferred variant, the flat steel product comprises an oxide layer disposed atop the anticorrosion coating. The oxide layer lies in particular atop the Al-base layer and preferably forms the outer conclusion of the anticorrosion coating.

The oxide layer consists especially to an extent of more than 80% by weight of oxides, where the majority of the oxides (i.e. more than 50% by weight of the oxides) is aluminum oxide. The oxide layer optionally includes, in addition to aluminum oxide, hydroxides and/or especially magnesium oxide alone or as a mixture. The remainder of the oxide layer not accounted for by the oxides and optionally present hydroxides preferably consists of silicon, aluminum, iron and/or magnesium in metallic form. For the optional embodiment with zinc as a constituent of the Al-base layer, zinc oxide constituents are also present in the oxide layer.

The oxide layer of the flat steel product preferably has a thickness greater than 50 nm. In particular, the thickness of the oxide layer is not more than 500 nm.

The invention also relates to a process for producing a shaped sheet metal part. This shaped sheet metal part is configured in particular as described in detail in the following text. This process comprises the following operating steps:

• • a. producing a flat steel product according to the process described above;
  • b. dividing a sheet metal blank from the flat steel product;
  • c. heating the sheet metal blank in a furnace having a furnace temperature T_furnace for an annealing time t_G such that at least to some degree the AC3 temperature of the blank is exceeded and the temperature T_ins of the blank on insertion into a forming tool provided for a hot press forming operation (step c)) is at least partly at a temperature above Ms+100° C., where Ms denotes the martensite start temperature;
  • d. inserting the heated sheet metal blank into a forming tool, where the transfer time t_trans required for the removal from the heating device and the insertion of the blank is not more than 20 s, preferably not more than 15 s;
  • e. hot press forming the sheet metal blank to give the shaped sheet metal part, where the blank in the course of hot press forming is cooled to the target temperature T_target over a period t_tool of more than 1 s at a cooling rate r_tool of at least partly more than 30 K/s and optionally held at that temperature;
  • f. removing the shaped sheet metal part that has been cooled to the target temperature T_target from the tool.

In the process according to the invention, a flat steel product is thus first produced as previously described. From this flat steel product, a sheet metal blank is divided. This blank, consisting of a steel of suitable composition in accordance with the elucidations above, is then heated in a manner known per se such that the AC3 temperature of the blank is exceeded at least to some degree and the temperature T_ins of the blank on insertion into a forming tool provided for a hot press forming operation (step d)) is at least partly at a temperature above Ms+100° C. What is meant in the context of this application by “exceedance of a temperature to some degree” (here, AC3 or Ms+100° C.) is that at least 30%, especially at least 60%, of the volume of the blank exceeds a corresponding temperature. On insertion into the forming tool, at least 30% of the blank thus has an austenitic microstructure, meaning that the transformation from the ferritic to austenitic microstructure need not be complete on insertion into the forming tool. Instead, up to 70% of the volume of the blank on insertion into the forming tool may consist of other microstructure constituents, such as annealed bainite, annealed martensite and/or non-recrystallized or partly recrystallized ferrite. For this purpose, particular regions of the blank may be specifically kept at a lower temperature level during the heating than others. For this purpose, the supply of heat may be directed specifically only to particular sections of the blank, or the parts that are to be heated to a lesser degree may be shielded against the supply of heat. In the part of the blank material wherein the temperature remains lower, only a distinctly smaller amount of martensite, if any, is formed in the course of forming in the tool, such that the microstructure there is much softer than in the respective other parts in which there is a martensitic microstructure. In this way, in the respectively formed shaped sheet metal part, it is possible to specifically establish a softer region in which, for example, toughness is optimal for the respective end use, while the other regions of the shaped sheet metal part have maximized strength.

Maximum strength properties of the resultant shaped sheet metal part may be enabled in that the temperature attained at least to some degree in the sheet metal blank is between Ac3 and 1000° C., preferably between 850° C. and 950° C.

The minimum temperature Ac3 to be exceeded is determined in accordance with the formula specified by HOUGARDY, H P. in Werkstoffkunde Stahl Band 1: Grundlagen, Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229:

$\mathrm{Ac}3=\left(902-{225}^{*}\%C+{19}^{*}\%\mathrm{Si}-{11}^{*}\%\mathrm{Mn}-5^{*}\%\mathrm{Cr}+13^{*}\%\mathrm{Mo}-{20}^{*}\%\mathrm{Ni}+55^{*}\%V\right)°C.$

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 of which the blank consists.

An optimally uniform distribution of properties can be achieved in that the blank is fully through-heated in step b).

In a preferred variant embodiment, the average heating rate r_furnace of the sheet metal blank on heating in step b) is at least 3 K/s, preferably at least 5 K/s, especially at least 10 K/s, preferably at least 15 K/s. The average heating rate r_furnace here is the average heating rate from 30° C. to 700° C.

In a preferred variant embodiment, heating takes place in a furnace having a furnace temperature T_furnace of at least 850° C., preferably at least 880° C., more preferably at least 900° C., especially at least 920° C., and not more than 1000° C., preferably not more than 950° C., more preferably not more than 930° C.

Preferably, the dewpoint in the furnace here is at least −20° C., preferably at least −15° C., especially at least −5° C., more preferably at least 0° C., in particular at least 5° C. and not more than +25° C., preferably not more than +20° C., especially not more than +15° C.

In a specific execution variant, the heating in step b) is effected stepwise in regions with different temperature. In particular, the heating is effected in a roller hearth furnace with different heating zones. In this case, the heating takes place in a first heating zone with a temperature (so-called furnace entry 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, composed of a heating time and a hold time, in both variants (constant furnace temperature, stepwise heating), is preferably at least 2 minutes, especially at least 3 minutes, preferably at least 4 minutes. In addition, the total time in the furnace in both variants is preferably not more than 20 minutes, especially not more than 15 minutes, preferably not more than 12 minutes, especially not more than 8 minutes. Prolonged total times in the furnace have the advantage that uniform austenitization of the sheet metal blank is assured. On the other hand, holding above Ac3 for an excessively long period leads to grain coarsening, which has an adverse effect on mechanical properties.

The blank thus heated is removed from the respective heating device, which may, for example, be a conventional heating furnace, an induction heating device which is likewise known per se, or a conventional device for keeping steel components hot, and is transported into the forming tool with sufficient speed that its temperature on arrival in the tool is at least partly above Ms+100° C., preferably above 600° C., in particular above 650° C., more preferably above 700° C. Ms here denotes the martensite start temperature. In a particularly preferred variant, the temperature is at least partly above the AC1 temperature. In all these variants, the temperature is especially not more than 900° C. These temperature ranges ensure a good formability of the material overall.

In step d), the transfer of the austenitized blank from the respectively used heating device to the forming tool is completed within preferably not more than 20 s, especially not more than 15 s. Such rapid transport is required in order to avoid excessive cooling prior to shaping.

The tool on insertion of the blank typically has a temperature between room temperature (RT) and 200° C., preferably between 20° C. and 180° C., more particularly between 50° C. and 150° C.

Optionally, the tool in one particular embodiment may be at least regionally heated to a temperature T_tool of at least 200° C., in particular at least 300° C., to only partially harden the component. Furthermore, the tool temperature T_tool is preferably a maximum of 600° C., in particular a maximum of 550° C. It must only be ensured that the tool temperature T_tool is below the desired target temperature T_target. The dwell time in the tool t_tool is preferably at least 2 s, especially at least 3 s, more preferably at least 5 s. The dwell time in the tool is preferably not more than 25 s, especially not more than 20 s.

The target temperature T_target of the shaped 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 shaped sheet metal part is particularly preferably below Ms−50° C., where Ms denotes the martensite start temperature. In addition, the target temperature of the shaped sheet metal part is preferably at least 20° C., more preferably at least 50° C.

The martensite start temperature of a steel within the provisions of the invention should be calculated by the formula

$\mathrm{Ms}\left[°C.\right]=\left(490.85-32.6\%C-30.6\%\mathrm{Mn}-16.6\%\mathrm{Ni}-8.9\%\mathrm{Cr}+2.5\%\mathrm{Mo}--11.3\%\mathrm{Cu}+8.58\%\mathrm{Co}+7.4\%W-14.5\%\mathrm{Si}\right)\left[°C./\%\mathrm{by}\mathrm{weight}\right]$

where here C % denotes the C content, % Mn the Mn content, % Mo the Mo content, % Cr the Cr content, % Ni the Ni content, % Cu the Cu content, % Co the Co content, % W the W content and % Si the Si content of the respective steel in % by weight.

The AC1 temperature and the AC3 temperature of a steel within the provisions of the invention should be calculated by the formulae

$AC1\left[°C.\right]=\left(739-{22}^{*}\%C-7^{*}\%\mathrm{Mn}+2^{*}\%\mathrm{Si}+{14}^{*}\%\mathrm{Cr}+{13}^{*}\%\mathrm{Mo}-{13}^{*}\%\mathrm{Ni}+{20}^{*}\%V\right)\left[°C./\%\mathrm{by}\mathrm{weight}\right]$ $\mathrm{and}$ $AC3\left[°C.\right]=\left(902-{225}^{*}\%C+{19}^{*}\%\mathrm{Si}-{11}^{*}\%\mathrm{Mn}-5^{*}\%\mathrm{Cr}+13^{*}\%\mathrm{Mo}-{20}^{*}\%\mathrm{Ni}+55^{*}\%V\right)\left[°C./\%\mathrm{by}\mathrm{weight}\right]$

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

In the tool, the blank is thus not just shaped to give the shaped sheet metal part, but simultaneously also quenched to the target temperature. The cooling rate in the tool r_tool to the target temperature is especially at least 20 K/s, preferably at least 30 K/s, especially at least 50 K/s, in a particular execution at least 100 K/s.

The removal of the shaped sheet metal part in step e) is followed by cooling of the shaped sheet metal part to a cooling temperature T_cool of less than 100° C. within a cooling period T_cool of 0.5 to 600 s. This is generally accomplished by air cooling.

The invention further relates to a shaped sheet metal part formed from a sheet steel blank comprising a steel substrate consisting of a steel which includes 0.1-3% by weight of Mn and optionally up to 0.01% by weight of B. In this case, the shaped sheet metal part has an aluminum-based anticorrosion coating on at least one side, where the magnitude of the hardness gradient of the anticorrosion coating and steel substrate perpendicular to the surface of the steel substrate is less than 1.7 GPa/μm.

A shaped sheet metal part of this kind can be produced by the process described above, among others.

The Hardness Gradient is Determined According to the Following Process:

• • embedding at least a part of the shaped sheet metal part in an embedding compound and preparing a section of the shaped sheet metal part;
  • polishing the section and light etching with 3% Nital (alcoholic nitric acid);
  • defining a measuring field on the section of at least 50 μm width parallel to the surface and a Cartesian measuring grid on the measuring field, where the Cartesian x-axis is perpendicular to the surface of the steel substrate and the Cartesian y-axis is parallel to the surface of the steel substrate, and where the grid spacing is 1.5 μm in both directions;
  • measuring the nanohardness on the grid points of the measuring grid using a nanoindenter with a calibrated Berkovich pyramid as a test tip and using a load function with a maximum load of 2000 μN;
  • calculating a hardness curve as a function of the Cartesian x-coordinate, by assigning each x-coordinate the average value of the nanohardness across all grid points with this x-coordinate (i.e., the hardness values of all points with the same distance from the surface of the steel substrate are averaged);
  • calculating the hardness gradient as the difference quotient of the hardness curve.

For example, the “Hysitron TI Premier” device from the company Bruker is used as the nanoindenter. Details of the device can be sourced from Bruker or can be accessed, for example, via the following link: https://www.bruker.com/en/products-and-solutions/test-and-measurement/nanomechanical-test-systems/hysitron-ti-premier-nanoindenter.html. With the nanoindenter, a defined measuring tip, for example a Berkovich tip (consisting of diamond), is pressed into a sample to be examined and the measured force-penetration curve, preferably using the evaluation method according to Oliver & Pharr (method accessible via the following link: https://www.sciencedirect.com/topics/engineering/oliver-pharr-method), can be used to determine a hardness.

A hardness gradient of less than 1.7 GPa/μm in magnitude means that the hardness averaged parallel to the surface of the steel substrate changes relatively 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 of hardness is slower, the material when it is deformed (e.g., in the event of a crash) will not crack until higher stress intensities are reached. The material can thus tolerate higher external forces before it fails.

The anticorrosion coating of the shaped sheet metal part preferably comprises an alloy layer and an Al base layer.

The alloy layer here lies atop and directly adjoins the steel substrate. The alloy layer of the shaped sheet metal part preferably consists of 35-95% by weight of Fe, preferably 60-95% by weight of Fe, 0.1-4% by weight of Si, and optional further constituents, the contents of which are limited to a total of not more than 3.5% by weight, preferably 2.0% by weight, and aluminum as the balance. The optional further components are preferably the elements present in addition to iron in the steel of the steel substrate and the other elements from the melt such as Zn and alkali metals or alkaline earth metals. These elements from the melt accumulate only to a very small extent in the alloy layer. The alloy layer preferably has a ferritic microstructure.

The Al-base layer of the shaped sheet metal part lies atop and directly adjoins the alloy layer of the steel component. Preferably, the Al-base layer of the steel component consists of 35-55% by weight of Fe, preferably 40-50% by weight of Fe, 0.4-4% by weight of Si, in particular 0.4-1.5% by weight of Si, optionally up to 3% by weight of alkali metals or alkaline earth metals, preferably up to 1.0% by weight of alkali metals or alkaline earth metals, optionally up to 10% of Zn, and optional further constituents, the total content of which is limited to not more than 2.0% by weight, and aluminum as the balance. Preferably, the optional content of alkali metals or alkaline earth metals is at least 0.1% by weight.

In a preferred variant of the Al-base layer, the optional content of alkali metals or alkaline earth metals comprises 0.1-1.0% by weight of Mg, especially 0.1-0.7% by weight of Mg, preferably 0.1-0.5% by weight of Mg. In addition, the optional content of alkali metals or alkaline earth metals in the Al-base layer may especially comprise at least 0.0015% by weight of Ca, especially at least 0.1% by weight of Ca. Further preferably, the optional content of alkali metals or alkaline earth metals consists of 0.1-1.0% by weight of Mg, especially 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, especially at least 0.1% by weight of Ca.

The Al-base layer may have a homogeneous element distribution in which the local element contents vary by not more than 10%. Preferred variants of the Al-base layer, by contrast, have low-silicon phases and silicon-rich phases. Low-silicon phases here are regions wherein the average Si content is at least 20% less than the average Si content of the Al-base layer. Silicon-rich phases here are regions wherein the 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 disposed within the low-silicon phase. In particular, the silicon-rich phases form an at least 40% continuous layer bounded by low-silicon regions. A continuous layer of silicon-rich phases is understood to mean that in the vertical section a line can be laid parallel to the surface of the steel substrate in such a way that it runs completely through the silicon-rich phases. By contrast, an at least X % continuous layer is understood to mean that in the vertical section a line can be laid parallel to the surface of the steel substrate in such a way that it runs to an extent of at least X % within the silicon-rich phases. In the present case, therefore, the silicon-rich phases are so contiguous that in the vertical section a line can be laid parallel to the surface of the steel substrate in such a way that it runs to an extent of at least 40% within the silicon-rich phases. In an alternative variant embodiment, the silicon-rich phases are arranged in island form in the low-silicon phase.

What is meant by “in island form” in the context of this application is an arrangement in which discrete noncoherent regions are surrounded by another material—i.e. there are “islands” of a particular material in another material.

In a preferred variant, the steel component comprises an oxide layer disposed atop the anticorrosion coating. The oxide layer lies in particular atop the Al-base layer and preferably forms the outer conclusion of the anticorrosion coating. The oxide layer of the steel component consists especially to an extent of more than 80% by weight of oxides, where the majority of the oxides (i.e. more than 50% by weight of the oxides) is aluminum oxide. The oxide layer optionally includes, in addition to aluminum oxide, hydroxides and/or especially magnesium oxide alone or as a mixture. The remainder of the oxide layer not accounted for by the oxides and optionally present hydroxides preferably consists of silicon, aluminum, iron and/or magnesium in metallic form.

The oxide layer preferably has a thickness of at least 50 nm, especially of at least 100 nm. In addition, the thickness is preferably not more than 4 μm, especially not more than 2 μm.

The shaped sheet metal part of the invention is preferably a component for a land vehicle, nautical vessel or aircraft. It is more preferably an automobile component, especially a bodywork component. The component is preferably a B pillar, longitudinal beam, A pillar, sill or transverse beam.

The invention is explained in more detail with reference to the FIGURES. In the drawings:

FIG. 1 shows a scanning electron microscopy image of the region examined in experiment 1 with the indenter tip,

FIG. 2 shows the hardness curve of the sample according to the invention,

FIG. 3 shows the hardness gradient of the sample according to the invention,

FIG. 4 shows the hardness curve of the comparative sample, and

FIG. 5 shows the hardness gradient of the comparative sample.

The effect of the invention was demonstrated by conducting a number of experiments. For this purpose, slabs with the compositions specified in Table 1 with a thickness of 240 mm and a width of 1200 mm were produced and heated in a blast furnace to a temperature T1 of 1200° C. Subsequently, the slabs were held at T1 for between 30 and 450 minutes until the temperature T1 was reached in the core of the slabs and the slabs were thus heated through. The slabs with their respective through-heating temperature T1 were discharged from the blast furnace and subjected to hot rolling. The experiments were performed in the form of a continuous hot strip rolling operation. For this purpose, the slabs were first pre-rolled to an intermediate product of thickness 40 mm, and the intermediate products, which can also be referred to as preliminary strips in the hot strip rolling operation, each had an intermediate product temperature T2 of 1100° C. at the end of the preliminary rolling phase. Immediately after preliminary rolling, the preliminary strips were sent to finish rolling, such that the intermediate product temperature T2 corresponds to the initial rolling temperature for the finish rolling phase. The preliminary strips were rolled out to hot strips having a final thickness of 4 mm and a final rolling temperature T3 of 890° C., cooled down to the respective coiling temperature and wound up to coils at a coiling temperature T4 of 580° C. and then cooled in stationary air. The hot strips were descaled in a conventional manner by pickling, before being subjected to cold rolling to the thickness specified in Table 3. The cold-rolled flat steel products were heated in a tunnel annealing furnace to an annealing temperature T5 of 870° C. and held at annealing temperature for 100 s in each case, before being cooled down to the dipping temperature T6 of 690° C. at a cooling rate of 1 K/s. The cold strips with their respective dipping temperature T6 were conducted through a liquid-melt coating bath at the temperature T7 of 676° C. The strip speed here was 76 m/min in all cases. The composition of the coating bath is reported in Table 2. After coating, the coated strips were blown off to adjust the coat weights. A stream of air was used for this purpose. In all cases, the temperature of the airstream was 70° C. The thickness of the coating is reported in Table 3. The strips were first cooled down to 660° C. at an average cooling rate of 10-15 K/s. Between 660° C. and 570° C., i.e., between the beginning of solidification and the end of solidification of the coating, the cooling rate in the inventive experiment 1 was 21 K/s. In contrast, the cooling rate between 660° C. and 570° C. in the reference experiment 2 was only 13 K/s. In the further course of cooling, between 570° C. and room temperature, the strips were cooled down at a cooling rate of 5-12 K/s in each case.

Blanks were divided from each of the steel strips thus produced, and used for the further experiments. In these experiments, shaped sheet metal part samples in the form of sheets of size 200×300 mm² were hot press formed from the respective blanks. For this purpose, the blanks were heated in a heating device, for example in 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 period in the furnace, comprising heating and holding, is designated T_furnace. The dewpoint of the furnace atmosphere was in all cases −5° C.

Subsequently, the blanks were taken from the heating device and inserted into a forming tool, which has the temperature T_tool. At the time of removal from the furnace, the blanks had assumed the furnace temperature. The transfer period t_trans, composed of the duration for removal from the heating device, transport to the tool and insertion into the tool, was 8 s. In all cases, the temperature Tin, of the blanks on insertion into the forming tool was above the respective martensite start temperature +100° C. In the forming tool, the blanks were formed into the respective shaped sheet metal part; the shaped sheet metal parts in the tool were cooled to a target temperature T_target at a cooling rate r_tool. The dwell time in the tool is designated t_tool. Finally, the samples have been cooled to room temperature under air. Table 4 is a further collation of the stated parameters, where “RT” is an abbreviation of room temperature.

Sections were prepared from the shaped sheet metal parts produced in this way. For this purpose, part of the shaped sheet metal part was embedded in an embedding compound. A section of the shaped sheet metal part was then prepared. This section was polished and lightly etched with 3% Nital. On this section, a measuring field of at least 50 μm width parallel to the surface and a Cartesian measuring grid were then defined on the measuring field, where the Cartesian x-axis is perpendicular to the surface of the steel substrate and the Cartesian y-axis is parallel to the surface of the steel substrate, and where the grid spacing is 1.5 μm in both directions. Illustratively, a scanning electron microscopy image of the region examined with the indenter tip in experiment 1 is shown in FIG. 1. The detail shown corresponds to 70 μm×70 μm. The steel substrate 1 is clearly visible. The alloy layer 3 is arranged on the steel substrate. On the alloy layer 3, in turn, the Al-base layer 5 is arranged. Above the alloy layer is the embedding compound 7, which is required for the preparation of the section. The section was also lightly etched with 3% Nital. On the section, a measuring field of at least 50 μm width parallel to the surface and a Cartesian measuring grid were defined on the measuring field, where the Cartesian x-axis is perpendicular to the surface of the steel substrate and the Cartesian y-axis is parallel to the surface of the steel substrate, and where the grid spacing is 1.5 μm in both directions. At the grid points of the measuring grid, the nanohardness was then determined with a nanoindenter having a Berkovich pyramid as test tip and having a load function with a maximum load of 2000 μN. The impressions left by the test tip at the grid points are clearly visible in FIG. 1.

From the measured nanohardnesses at the grid points, a hardness curve as a function of the Cartesian x-coordinates was then determined. For this purpose, each x-coordinate was assigned the average value of the nanohardness over all grid points with this x-coordinate. Averaging thus took place over all points having the same distance from the surface of the steel substrate. In FIG. 1, these are all grid points that are perpendicular to each other.

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

FIGS. 4 and 5 show the identical quantities for the comparative sample 2. Comparing FIGS. 3 and 5, it becomes clear that the hardness gradient is significantly greater in the comparative sample. The magnitude of the hardness gradient assumes values of more than 2 GPa/μm. In contrast, the maximum magnitude of the hardness gradient of the inventive sample 1 is less than 1.5 GPa/μm.

TABLE 1
(steel types)
Steel	C	Si	Mn	Al	Cr	Nb	Ti	B	P	S	N	Sn	As	Cu	Mo	Ca	Ni
A	0.235	0.3	1.3	0.05	0.28	0.003	0.040	0.0035	0.02	0.003	0.007	0.03	0.01	0.03	0.03	0.005	0.025
balance: iron and unavoidable impurities figures each in % by weight;

TABLE 2
(coating variants)
	Coating	Melt analysis
	variant	Si	Fe	Mg	Others	Al
	α	1.0	3.5	0.25	<1%	Balance
	β	9.5	3	0.3	<1%	Balance

TABLE 3
(structure)
			Sheet
			thickness	Coating thickness
Structure	Steel type	Coating	[mm]	[μm]
1	A	α	1.5	27
2*	A	β	1.5	27
*noninventive reference examples

TABLE 4
(hot forming parameters)
Average			Fur-				Cool-
heating		Trans-	nace				ing
rate rfurnace		fer	dew-				rate
[30-700° C.]	Tfurnace	time	point	Tins	Ttool	ttool	rtool	Ttarget
[K/s]	[° C.]	[s]	[° C.]	[° C.]	[° C.]	[s]	[K/s]	[° C.]
8	900	8	−5	800	RT	15	50	50

Claims

1-12. (canceled)

13. A process for producing a flat steel product for hot forming with a coating, comprising operating steps as follows:

providing a slab or a thin slab comprising a steel which includes 0.1-3% by weight of Mn and optionally up to 0.01% by weight of B;

through-heating the slab or thin slab at a temperature (T1) of 1000-1400° C.;

hot rolling to give a hot-rolled flat steel product, where the final rolling temperature (T3) is 750-1000° C.;

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

cooling the flat steel product to a dipping temperature (T6) of 600-800° C., preferably 680-720° C.;

coating the flat steel product cooled to the dipping temperature with a coating by: immersing the product in a melt bath with a melt temperature (T7) of 660-800° C., preferably 670-710° C., where the melt bath comprises 0.5-4% by weight of Si, optionally 2-4% by weight of Fe, optionally up to 5.0% by weight of alkali metals or alkaline earth metals, optionally up to 10% by weight of Zn, and optional further constituents, the contents of which are limited to a total of not more than 2.0% by weight, and aluminum as the balance; and blowing off the flat steel product after discharge from the melt bath by means of a gas stream; and

cooling the coated flat steel product to room temperature, where an average cooling rate between 660° C. and 570° C. is at least 15 K/s.

14. The process of claim 13 comprises pre-rolling the through-heated slab or thin slab to give an intermediate product having an intermediate product temperature (T2) of 1000-1200° C.

15. The process of claim 13 comprises coiling the hot-rolled flat steel product, where the coiling temperature (T4) is not more than 700° C.

16. The process of claim 13 comprises descaling the hot-rolled flat steel product.

17. The process of claim 13 comprises cold-rolling the flat steel product, where the degree of cold rolling is at least 30%.

18. The process of claim 13 comprises skin pass rolling the coated flat steel product.

19. The process of claim 13, wherein the melt bath comprises 0.5-1.5% by weight of Si, optionally 2-4% by weight of Fe, optionally up to 5.0% by weight of alkali metals or alkaline earth metals, optionally up to 10% of Zn, and optional further constituents, the total contents of which are limited to not more than 2.0% by weight, and aluminum as the balance.

20. The process of claim 13, wherein the steel, besides iron and unavoidable impurities, comprises (in % by weight):

C: 0.04-0.45% by weight,

Si: 0.02-1.2% by weight,

Mn: 0.5-2.6% by weight,

Al: 0.02-1.0% by weight,

P: ≤0.05% by weight,

S: ≤0.02% by weight,

N: ≤0.02% by weight,

Sn: ≤0.03% by weight,

As: ≤0.01% by weight, and

Ca: ≤0.005% by weight.

21. The process of claim 20, wherein the steel further comprises one or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V” in amounts as follows:

Cr: 0.08-1.0% by weight,

B: 0.001-0.005% by weight,

Mo: ≤0.5% by weight,

Ni: ≤0.5% by weight,

Cu: ≤0.2% by weight,

Nb: 0.01-0.08% by weight,

Ti: 0.01-0.08% by weight, and

V: ≤0.3% by weight.

22. The process of claim 13, further comprising producing a shaped sheet metal part formed by operating steps as follows:

producing the flat steel product;

dividing a sheet metal blank from the flat steel product;

heating the sheet metal blank in a furnace having a furnace temperature Tfurnace for an annealing time tG such that at least to some degree the AC3 temperature of the blank is exceeded and the temperature Tins of the blank on insertion into a forming tool provided for a hot press forming operation is at least partly at a temperature above Ms+100° C., where Ms denotes the martensite start temperature;

inserting the heated sheet metal blank into a forming tool, where the transfer period ttrans required for the removal from the heating device and the insertion of the blank is not more than 20 s, preferably not more than 15 s;

hot press forming the sheet metal blank to give the shaped sheet metal part, where the blank in the course of hot press forming is cooled to the target temperature Ttarget over a period ttool of more than 1 s at a cooling rate rtool of at least partly more than 30 K/s and optionally held at that temperature; and

removing the shaped sheet metal part that has been cooled to the target temperature Ttarget from the tool.

23. The process of claim 22, wherein the temperature attained in the sheet metal blank at least to some degree is between Ac3 and 1000° C., preferably between 850° C. and 950° C.

24. The process of claim 22, wherein the target temperature Ttarget of the shaped sheet metal part is at least partly below 400° C., preferably below 300° C.

25. A shaped sheet metal part, comprising:

a sheet metal blank comprising a steel substrate comprising a steel comprising 0.1-3% by weight of Mn and optionally up to 0.01% by weight of B, wherein the shaped sheet metal part on at least one side has an aluminum-based anticorrosion coating, characterized in that the magnitude of the hardness gradient of anticorrosion coating and steel substrate perpendicular to the surface of the steel substrate is less than 1.7 GPa/μm.

26. The shaped sheet metal part of claim 25, wherein the steel, besides iron and unavoidable impurities, comprises (in % by weight):

C: 0.04-0.45% by weight,

Si: 0.02-1.2% by weight,

Mn: 0.5-2.6% by weight,

Al: 0.02-1.0% by weight,

P: ≤0.05% by weight,

S: ≤0.02% by weight,

N: ≤0.02% by weight,

Sn: ≤0.03% by weight,

As: ≤0.01% by weight, and

Ca: ≤0.005% by weight.

27. The shaped sheet metal of claim 26, wherein the steel further comprises one or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V” in amounts as follows:

Cr: 0.08-1.0% by weight,

B: 0.001-0.005% by weight,

Mo: ≤0.5% by weight,

Ni: ≤0.5% by weight,

Cu: ≤0.2% by weight,

Nb: 0.01-0.08% by weight,

Ti: 0.01-0.08% by weight, and

V: ≤0.3% by weight.

28. The shaped sheet metal part of claim 26, wherein the aluminum-based anticorrosion coating comprises an alloy layer and an Al-base layer.

29. The shaped sheet metal part of claim 28, wherein the alloy layer comprises 35-95% by weight of Fe, 0.1-4% by weight of Si, and optional further constituents, the contents of which are limited to a total of not more than 3.5% by weight, and aluminum as the balance.

30. The shaped sheet metal part of claim 28, wherein the Al-base layer comprises 35-55% by weight of Fe, 0.4-4% by weight of Si, optionally up to 3% by weight of alkali metals or alkaline earth metals, optionally up to 10% of Zn, and optional further constituents, the contents of which are limited to a total of not more than 2.0% by weight, and aluminum as the balance.

31. The shaped sheet metal part of claim 28, wherein the Al-base layer comprises 35-55% by weight of Fe, 0.4-1.5% by weight of Si, optionally up to 3% by weight of alkali metals or alkaline earth metals, optionally up to 10% of Zn, and optional further constituents, the contents of which are limited to a total of not more than 2.0% by weight, and aluminum as the balance.

32. The shaped sheet metal part of claim 25 is formed by steps comprising:

producing a flat steel product, which is formed by steps comprising: providing a slab or a thin slab comprising a steel which includes 0.1-3% by weight of Mn and optionally up to 0.01% by weight of B; through-heating the slab or thin slab at a temperature (T1) of 1000-1400° C.; hot rolling to give a hot-rolled flat steel product, where the final rolling temperature (T3) is 750-1000° C.; annealing the flat steel product at an annealing temperature (T5) of 650-900° C.; cooling the flat steel product to a dipping temperature (T6) of 600-800° C., preferably 680-720° C.; coating the flat steel product cooled to the dipping temperature with a coating by: immersing the product in a melt bath with a melt temperature (T7) of 660-800° C., preferably 670-710° C., where the melt bath comprises 0.5-4% by weight of Si, optionally 2-4% by weight of Fe, optionally up to 5.0% by weight of alkali metals or alkaline earth metals, optionally up to 10% by weight of Zn, and optional further constituents, the contents of which are limited to a total of not more than 2.0% by weight, and aluminum as the balance; and blowing off the flat steel product after discharge from the melt bath by means of a gas stream; and cooling the coated flat steel product to room temperature, where an average cooling rate between 660° C. and 570° C. is at least 15 K/s;

dividing a sheet metal blank from the flat steel product;

heating the sheet metal blank in a furnace having a furnace temperature Tfurnace for an annealing time tG such that at least to some degree the AC3 temperature of the blank is exceeded and the temperature Tins of the blank on insertion into a forming tool provided for a hot press forming operation is at least partly at a temperature above Ms+100° C., where Ms denotes the martensite start temperature;

inserting the heated sheet metal blank into a forming tool, where the transfer period ttrans required for the removal from the heating device and the insertion of the blank is not more than 20 s, preferably not more than 15 s;

hot press forming the sheet metal blank to give the shaped sheet metal part, where the blank in the course of hot press forming is cooled to the target temperature Ttarget over a period ttool of more than 1 s at a cooling rate rtool of at least partly more than 30 K/s and optionally held at that temperature; and

removing the shaped sheet metal part that has been cooled to the target temperature Ttarget from the tool.