EP2553133B1 - Steel, flat steel product, steel component and method for producing a steel component - Google Patents

Description

The invention relates to a steel, a flat steel product, a steel component produced therefrom and a method for producing a steel component.

The demands on the automotive industry by the legislator have increased in recent years. On the one hand, increased passenger safety is demanded in the event of a crash, and on the other hand, lightweight construction is an important prerequisite for minimizing CO ₂ emissions and fuel consumption. At the same time, user demands for comfort increase, leading to an increase in weight of the automobile due to the increased share of electronic components leads. In order to meet these contradictory requirements, the automotive industry and the flat steel industry are relying heavily on vehicle lightweight construction in the field of body structure.

Hot-formed, press-hardened components made of manganese-boron steels are particularly suitable for crash-relevant automotive components. A typical example of this steel quality is the MnB steel known under the name "22MnB5" (material number 1.5528). Possible uses of MnB steels, Press-hardened components are z. B. B-pillar, B-pillar reinforcement and bumper of car bodies. Combined hot forming and press hardening can be used to produce components with complex geometries and highest strengths (R _m : approx. 1500 MPa, R _{p 0.2} : approx. 1100 MPa).

The components thus obtained are characterized by a predominantly martensitic structure. Their high strength basically allows a significant reduction in wall thickness and thus a significantly reduced weight of the component. However, hot-press hardened components typically have only a low ductility from MnB steels (A ₈₀ : approx. 5 - 6%). In order to avoid failure in the event of a crash, the sheet thickness of hot-press-hardened components is therefore designed in practice much more pronounced for safety reasons, as would normally be necessary taking into account their strength in practice.

On the one hand to exploit the lightweight construction potential of components made of steels of the type in question, but on the other hand also to ensure the deformation behavior required in a crash, body components are made of so-called "tailored blanks". These are sheet metal blanks, which are composed of blanks of different grades of steel. Thus, for example, a "tailored blank" is made available for the production of a B pillar of a car body, the area of which associated with the upper part of the B pillar consists of a 22MnB5 steel. In the area of the tailored blanks assigned to the base of the B-pillar, a steel grade is then provided which also after hot-press hardening indicates a higher ductility. A suitable steel is known under the name H340LAD (material number 1.0933).

Although the use of tailored blanks can achieve significant weight savings while simultaneously optimizing the performance properties of the components produced therefrom, the areas made of the ductile material in the critical area of the respective component generally have to have a higher sheet thickness in order to achieve the same in normal operation To be able to absorb component loadings. This in turn has a correspondingly higher weight for the overall component.

There is therefore a general desire to manufacture highly stressed components, such as those used in motor vehicle bodies, from a sheet steel material in which high strengths are combined with good elongation properties.

A first development direction to meet this requirement is aimed at optimizing the manufacturing process. Thus, by controlling the cooling rate, a steel grade with a martensitic structure and improved elongation at break should be able to be produced. An example of this procedure is in the EP 1 642 991 B1 described and provides for reaching the martensite stop temperature before a high and then a slower cooling rate. In this way, a self-tempered martensite is produced, which has an improved elongation at break. An alternative development direction is the optimization of the process for producing a quality with a multi-phase structure by means of the so-called "semi-hot forming". In this method, the flat steel product to be formed to the respective component is heated to a temperature lying between the A _c1 and the A _c3 temperature at which the steel has a two-phase structure. When the component so heated is hot press-hardened, the finished component has a lower martensite content and higher proportions of more ductile phases, such as ferrite or austenite, after cooling compared to conventionally austenitized and hardened components. At the same time, the components still have a comparably high strength. For semi-thermoformed components, tensile strengths R _m of 800 - 1000 MPa are achieved with only slightly reduced elongation at break values (A ₈₀ approx. 10-20%) compared to the initial condition. Such a procedure is for example in the WO 2007/034063 A1 described.

A similar concept, but with particular emphasis on the formation of a coating applied for protection against corrosion pursues the patent application WO 2008/102012 , In this prior art, it is merely specified that the heating temperature is above the A _c1 temperature and should be selected taking into account a possible grain growth and the evaporation of the Zn-based coating of the flat steel product from which the component is formed. The processed flat steel product is composed according to different alloying concepts. Thus, the steel in question (in wt .-%) 0.15 - 0.25% C, 1, 0 - 1.5% Mn, 0.1 - 0.35% Si, max. 0.8% Cr, in particular 0.1-0.4% Cr, max. 0.1% Al, up to 0.05% Nb, in particular max. 0.03% Nb, up to 0.01% N, 0.01-0.07% Ti, <0.05% P, especially <0.03% P, <0.03% S,> 0.0005 to <0.008% B, in particular at least 0.0015% B, and the balance unavoidable impurities and iron, wherein the Ti content is 3.4 times greater than the N content.

Against the background of the prior art described above, the object of the invention was to provide a steel in which it is ensured with high reliability that a component manufactured from it has in each case high strength values and an increased elongation at break. Likewise, a steel flat product made using this steel, a steel component made therefrom, and a method suitable for producing such a steel component should be given.

With respect to the steel, this object has been achieved according to the invention by a steel alloyed according to claim 1.

With regard to the flat steel product, the above-mentioned object has been achieved according to the invention in that such a flat steel product according to claim 6 is formed.

With regard to the steel component, the solution according to the invention of the abovementioned object is that such a steel component is designed according to claim 9.

Finally, the above object is related to the method of manufacturing a steel component According to the invention have been solved by the method specified in claim 13.

Advantageous embodiments of the invention are set forth in the dependent claims and, like the subject matter of the independent claims, are explained in detail below.

The invention is based on the recognition that, by selecting a suitable alloy and setting a suitable microstructure composition, it is possible to provide a steel having a high strength of at least 1000 MPa and an elongation at break A ₈₀ after austenitization, thermoforming and hardening each safely above 6%. The steel according to the invention contains (in% by weight) 0.15-0.40% C, 1.0-2.0% Mn, 0.2-1.6% Al, up to 1.4% Si, wherein the sum of the contents of Si and Al is 0.25 - 1.6%, up to 0.10% P, 0 - 0.03% S, up to 0.5% Cr, up to 1.0% Mo , up to 0.01% N, up to 2.0% Ni, 0.012-0.04 Nb, up to 0.40% Ti, 0.0015-0.0050% B and up to 0.0050% by weight. % Ca and the remainder iron and unavoidable impurities.

Accordingly, a flat steel product according to the invention has at least one region which consists of a steel according to the invention. Thus, a flat steel product according to the invention can be designed as a tailored blank, in which one region is produced from a steel according to the invention, while another region is produced from a different steel. The area produced by the steel according to the invention of the tailored blanks according to the invention forms on the finished, from the steel flat product produced steel component then a high-strength area in which a high strength combined with a good elongation at break. Likewise, it is of course also possible to manufacture a flat steel product according to the invention in the form of a blank cut from a sheet steel or steel strip uniformly from the steel according to the invention. A steel component produced from such a flat steel product according to the invention then has at each point the advantageous combination of high strength and good extensibility achieved by the steel alloy according to the invention.

A steel component according to the invention is correspondingly characterized in that it consists of a steel according to the invention at least in one region and that its structure in the region of the high-strength steel according to the invention is composed of martensite, austenite and up to 20 area% ferrite.

In the course of a process according to the invention for producing a steel component according to the invention, a flat steel product according to the invention is accordingly first provided. This flat steel product is then heated to a temperature of 780-950 ° C. The austenite content is thus adjusted to at least 80% in order to produce, after thermoforming, a steel according to the invention with a structure consisting of martensite, austenite and up to 20 area% of ferrite. The required hold time is typically 2 to 10 minutes.

Subsequently, the flat steel product is usually transported to a thermoforming mold to be thermoformed there to become. In order to avoid excessive cooling during transport, the transport time should be limited to 5 - 12 seconds. The thermoforming itself can be carried out in a conventional manner as compression molding.

Following the hot forming, the steel component is cooled down so rapidly that the steel component obtained after cooling has a structure consisting of martensite, austenite and up to 20 area% of ferrite. The typically required cooling rates are in the range of at least 25 ° C / s. The thermoforming and cooling can be carried out in one stage or two stages. In single-stage hot press hardening, hot forming and hardening are performed in one go together in one tool. In contrast, in the two-stage process, cold forming takes place first (up to 100%) and only then is the final hot forming, including the production of the hardened structure.

When the respective processed flat steel product has been austenitized within the abovementioned temperatures, the component according to the invention, after hot forming and accelerated cooling in the region consisting of a steel according to the invention, has a structure which is characterized by a combination of a hard phase ( Martensite) and at least one more ductile phase (austenite and ferrite). The ferrite content is limited by the inventively given composition of the processed steel to 20 area%, since an improvement in the elongation values and a Increasing the energy absorption by austenite are preferred. By the combination of martensite, austenite and a maximum of 20 area% ferrite, the mechanical technological properties of components according to the invention over the entire temperature range of the inventively at 780-950 ° C, especially 850-950 ° C, Austenitisierung performed reliably.

The stability of the mechanical-technological properties of the component produced according to the invention is ensured by the analysis concept according to the invention. The structure consisting of a combination of hard (martensite) and ductile (austenite and ferrite) phases of a component according to the invention ensures optimum behavior in the event of a crash load. The austenite to martensite phase transformation that occurs during the deformation of the hot formed component causes the component to subsequently harden when deformed at high kinetic energy in the event of a crash.

The combination of high strength, good elongation at break and optimum crash behavior in the region of its high-strength region which is the aim of the invention is achieved with particular certainty if the martensite content of the microstructure in the high-strength region in question is at least 75 area% in a component according to the invention. The required high elongation at break can be ensured by the fact that the austenite content of the structure of the component according to the invention is at least 2 area%.

The tensile strength of a component made from steel according to the invention should not be below 1000 MPa in the region of its high-strength range. In order for the martensite hardness necessary for this purpose to be achieved, the steel alloy according to the invention contains a C content of at least 0.15% by weight. At the same time to ensure sufficient weldability for the practice, the C content of the steel according to the invention is limited to 0.4 wt .-% upwards.

With regard to the adjustment of the structure according to the invention, the alloying elements Mn, Si and Al of a steel used according to the invention are of particular importance, since they stabilize the austenite at room temperature.

The Mn present in amounts of at least 1.0% by weight in the steel of the invention serves as an austenite former by lowering the Ac ₃ temperature of the steel. The result is a microstructure consisting essentially of austenite and martensite after hot working. In order to simultaneously ensure optimum welding suitability for the respective use, the Mn content is limited to a maximum of 2% by weight.

Silicon is present in the steel of the present invention at levels of up to 1.4% by weight. It affects the hardenability and serves in the melting of the steel of the component according to the invention as a deoxidizer. At the same time, Si increases the yield strength, stabilizes the ferrite and austenite at room temperature, and prevents unwanted carbide precipitation in austenite during cooling. However, too high an Si content causes surface defects. Therefore, the Si content of a steel of the present invention is limited to 1.4% by weight.

Aluminum contributes to the stabilization of the ferrite and the austenite at room temperature in the steel according to the invention, similar to Si, and effects a grain size control. These effects are certainly achieved when the contents of Al in the inventive manner to 0.2 to 1.6 wt .-% are limited, with Al contents of at least 0.4 wt .-% particularly positive on the properties of an inventive Impact component. By above 0.4 wt .-% lying Al content, the carbide formation is suppressed during the heat treatment and thus the inventively provided proportion of austenite of preferably at least 2 area% stabilized in the thermoformed structure.

Due to the phase constellation according to the invention, a reduction of the scattering of the mechanical properties of a steel according to the invention after its austenitization, hot working and cooling is achieved. Surprisingly, it has been found here that the mechanical properties of a component produced according to the invention can be achieved with high reliability over a comparatively large temperature range of the temperatures to which the steel flat products are heated during their processing according to the invention. Thus, despite the tolerances inevitably occurring in practice when setting the relevant heating temperature, the desired properties of components according to the invention can be ensured with high safety and stability of the work result.

Negative influences on the surface properties, which could have Si and Al, are avoided by limiting the sum of the Al and Si contents of a steel according to the invention or of a component produced therefrom to 0.25-1.6% by weight , In order to simultaneously use the positive effects of the combined presence of Al and Sibesonders safely, the sum of the Al and Si contents of a steel component according to the invention can be increased to at least 0.5 wt .-%.

Mo may be present in a steel of the invention at levels of up to 1.0% by weight. The presence of Mo promotes martensite formation and improves the toughness of the steel. However, an excessive Mo content may cause cold cracking.

By adding Cr in amounts of up to 0.5% by weight to alloy a steel according to the invention, the hardenability can be increased. However, the Cr content should not be higher to avoid surface defects. Certainly, these effects can be achieved when the Cr content is limited to 0.1 wt%.

P can be alloyed in amounts of up to 0.10 wt .-% to increase the yield strength and thus to secure the mechanical properties. Too high a P content, however, damages the ductility and toughness of a steel made in accordance with the present invention.

erfüllt, wobei mit %Ti sein jeweiliger Ti-Gehalt und mit %N sein jeweiliger N-Gehalt bezeichnet ist.Ti in levels of up to 0.40 wt% increases the yield strength both dissolved and by precipitation formation (eg of Ti carbonitrides). Ti binds N to TiN promoting the efficiency of B in terms of conversion behavior. This effect can be ensured by the Ti content of the steel according to the invention being the condition $$\%\mathrm{Ti}-\left(\mathrm{3}.\mathrm{42}\mathrm{x}\%\mathrm{N}\right)>\mathrm{0}.\mathrm{005}\mathrm{weight}\mathrm{,}-\%$$

where% Ti is its respective Ti content and% N is its respective N content.

By 0.0010 - 0.0050 wt% B, the hardenability of a steel according to the invention is improved by retarding the ferrite transformation during cooling towards longer transformation times. At the same time, the boron present in the steel according to the invention stabilizes the mechanical properties for a wide temperature range of the hot forming process.

Up to 0.01% by weight of N stabilizes the austenite and increases the yield strength of a steel according to the invention. If the nitrogen present in the alloy steel according to the invention is not completely bound by Ti, it reacts in combination with boron to give boron nitrides. These boron nitrides cause a grain refining of the initial microstructure and thus a refining of the martensitic thermoformed microstructure. As a result, the susceptibility to cracking of a steel processed according to the invention is thus reduced. At the same time, the boron nitrides contribute significantly to increasing the strength of the steel according to the invention.

die Bedingung $$0,0015\le \%\mathrm{N}-\%\mathrm{Ti}/3,42\le 0,0060\mathrm{Gew}\mathrm{.}-\%$$

erfüllt ist, wobei mit %Ti sein jeweiliger Ti-Gehalt und mit %N sein jeweiliger N-Gehalt bezeichnet ist.If N is to be used in combination with B by the formation of boron nitrides for grain refining and strength increase, If necessary, the N content which is not bound to Ti can be adjusted in a targeted manner such that, in the case that applies to its Ti content $$\%\mathrm{Ti}-\left(\mathrm{3}.\mathrm{42}\mathrm{x}\%\mathrm{N}\right)\le \mathrm{0}.\mathrm{005}\mathrm{weight}\mathrm{,}-\%.$$

the condition $$0.0015\le \%\mathrm{N}-\%\mathrm{Ti}/3.42\le 0.0060\mathrm{weight}\mathrm{,}-\%$$

is satisfied, wherein denoted by% Ti its respective Ti content and% N its respective N content.

The additional addition of Nb at levels of 0.012-0.04% by weight in a steel alloyed according to the invention promotes the combination of high tensile strength values with increased elongation at break, resulting in an overall increase in the energy absorption capacity of steel components according to the invention. Nb in composite steel according to the invention increases the yield strength by means of carbide precipitation and, due to austenitic grain refinement, causes a fine martensite structure which has a high stability against crack propagation. In addition, Nb precipitates may act as hydrogen traps, thereby reducing susceptibility to hydrogen-induced cracking.

Ni in amounts of up to 2.0% by weight contributes to increasing the yield strength and the elongation at break.

The S content of the steel of a component according to the invention is limited to max. 0.03 wt .-% limited because S a strong negative impact on the weldability and the Possibilities of surface refinement has. Also, this limitation is intended to prevent the formation of harmful, elongated MnS excretions.

Ca may be added to the steel of this invention at levels of up to 0.0050 wt.% To effect sulfide form control. Thus, in the presence of Ca in the course of rolling, Ca sulfides are formed which, in contrast to the otherwise possibly arising elongate MnS precipitates, promote a higher isotropy of the properties of the steel according to the invention.

The steel component according to the invention may be coated on its free surface with a protective coating against oxidation. This is preferably already present on the flat steel product from which the component is thermoformed. The protective cover can be designed so that it protects against scale formation during heating and thermoforming and / or corrosion during processing or in practical use. For this purpose, metallic, organic or inorganic coatings as well as combinations of these coatings can be used.

The coating of the flat steel product can be carried out by conventional methods. A surface refinement in the hot-dip process is preferred. The optional metallic coatings are based on the Zn, Al, Zn-Al, Zn-Mg, Al-Mg, Al-Si and Zn-Al-Mg systems and their unavoidable impurities. Coatings on an Al-Si basis have proven to be particularly useful.

To improve the surface quality and the connection of the coating to the steel surface, the pre-oxidation can advantageously be preceded by the hot-dip process. In this case, an oxide layer which is 10-1000 nm thick is specifically produced on the steel surface product, with particularly good coating qualities resulting when the oxide layer is 70-500 nm thick. The adjustment of the oxide layer thickness takes place in an oxidation chamber, as for example from the WO 2007/124781 A1 is known. Before immersion in the melt or before surface refinement, the iron oxide layer is reduced by hydrogen of the annealing atmosphere. In this case, oxides of the alloying elements can be present on the surface and up to a depth of 10 μm.

Furthermore, it is possible to anneal the flat steel product processed according to the invention in continuous annealing plants or in a bell annealing plant and to coat it by means of a downstream offline surface finishing plant. For this purpose, different methods can be used.

The electrolytic coating is particularly suitable for applying the respective coating. Particularly good results are obtained when Zn, ZnFe, ZnMn, ZnNi systems or their combination are used as the coating material.

However, it is also possible to apply the coating by PVD (Physical Vapor Deposition) or CVD (CVD) coating methods. Similarly, an electroless or chemical deposition of metallic (alloy) coatings based on Zn, Zn-Ni, Zn-Fe and their combinations as well as organic / organometallic / inorganic coatings in coil coating systems in the coil coating, spraying or dipping method may be useful. Typical thicknesses of the coatings which can be produced by the methods described here are in the range from 1 to 15 μm.

The invention will be explained in more detail by means of exemplary embodiments.

From steels E1-E6 whose compositions are shown in Table 1, cold-rolled steel sheets have been conventionally produced. Of these steel sheets, a larger number of sheet metal blanks were divided, which consisted uniformly of the respective steel E1 - E6.

For comparison, a steel sheet is produced in a similar manner from comparison steel V with a composition likewise indicated in Table 1, and a larger number of sheet metal plates have been divided from this steel sheet, which also consisted uniformly of comparative steel V.

The blanks consisting of the steels E1-E6 and V are in the uncoated state in each case heated to a temperature in the range of 880-925 ° C temperature, then placed in a thermoforming mold and then hot-formed into a component. After thermoforming, each of the boards thermoformed components are each with A cooling rate of at least 25 ° C / s has been cooled to room temperature so quickly that hardened structures have formed in it. After the actual hot forming conditioning, the samples were additionally subjected to a cathodic dip coating treatment including a 20 minute bake at 170 ° C.

The mechanical properties yield strength R _p0.2 , tensile strength R _m and elongation A _{80 were} determined for the components obtained. The respective average values R _p0,2 , R _m and A ₈₀ as well as the associated standard deviations σR _p0,2 , σR _m and σA ₈₀ are given in Table 2 for the steel components produced from the steels E1 - E6 and V. Furthermore, the product of tensile strength R _m and elongation A ₈₀ as well as the result of a 3-point bending test are entered in Table 2 for the steel components consisting of steels E1-E6 and V, in which the respective test specimen is placed on two mutually spaced supports positioned and loaded in the middle with a test stamp. The "energy intake in the 3-point bending test" mentioned above is the energy uptake until it breaks. Similarly, in Table 2 for the components produced from the steels E1, E2 and V, the microstructural compositions are mentioned.

It can be seen that the components consisting of the steels E1-E6 according to the invention have a consistently high residual deformation capacity characterized by a high value of the product of tensile strength R _m and elongation A ₈₀ and, consequently, high energy absorption capacity. At the same time, the results of the experiments that the mechanical properties R _p0,2 , R _m and A _{80 of} the components produced from the steels E1-E6 _according to the invention can be reproduced with a significantly higher reliability characterized by small values of the respective standard deviation than that produced by the comparison steel V. Components is the case. Table 1 (in% by weight) stole C Si Mn P S al Cr Not a word N Ni Nb Ti B Ca E1 0.217 0.39 1.63 0,003 <0.001 1.08 0,038 0.0016 0.0011 0,014 0,025 0,036 0.0030 <0.001 E2 0.217 0.41 1.64 0.005 0,002 0.62 0.027 0.0016 0.0023 0,008 0,029 0,022 0.0024 <0.001 E3 0,205 0,203 1.64 ≤0,10 ≤0,10 0,690 <0.1 0.0041 0,012 0.0010 0.0029 <0.001 E4 0.211 0,203 1.65 ≤0,10 ≤0,10 0.662 <0.1 0.0024 0,013 0.0020 0.0032 <0.001 E5 0.237 0.48 1.74 0,012 0.001 0.93 0,039 0,002 0.0023 0,012 0.027 0.033 0.0026 0.0019 E6 0.352 0.25 1.26 0013 0,002 0.25 0.12 0,002 0.0044 0,015 0,012 0.028 0.0026 0.0011 V 0.214 0.14 1.62 0.005 0,002 1,386 0.086 <0.002 0.0015 0,006 0,006 0.0030 0.0004 <0.001 stole R _p0.2 [MPa] σR _p0.2 [MPa] R _m [MPa] σR _m [MPa] A ₈₀ [%] σA ₈₀ [%] R _m x A ₈₀ [MPa x%] Energy absorption in the 3-point bending test [J] Ferrite [Fl. -%] Austenite [Fl .-%] Martensite [Fl .-%] E1 966 81 1467 29 8.5 1.1 12470 80.4 10 4 86 E2 1225 12 1525 5 8.1 0.4 12353 83.3 0 3 97 E3 1128 22 1443 8th 6.7 0.6 10101 73 0 2 98 E4 1156 32 1479 12 6.5 0.5 10353 74 0 2 98 E5 1162 91 1558 24 7.1 0.5 11062 77.4 1 3 96 E6 1393 23 1864 19 4.2 0.9 7829 60.8 0 2 98 V1 688 121 1231 55 9.6 2.6 11818 83.3 22 3 75

Claims (14)

1. Steel for producing a steel part by hot forming with subsequent hardening, containing (in % wt.) C: 0.15 - 0.40 %, Mn: 1.0 - 2.0 %, Al: 0.2 - 1.6 %, Si: 0 - 1.4 %, total of the contents of Si and Al: 0.25 - 1.6 %, P: 0 - 0.10 %, S : 0 - 0.03 %, Cr: 0 - 0.5 %, Mo: 0 - 1.0 %, N: 0 - 0.01 %, Ni: 0 - 2.0 %, Nb: 0.012 - 0.04 %, Ti 0 - 0.40 %, B: 0.0010 - 0.0050 %, Ca: 0 - 0.0050 %, remainder iron and unavoidable impurities.
2. Steel according to Claim 1, characterised in that the total of its Al and Si contents is at least 0.5 % wt.
3. Steel according to either of the preceding claims, characterised in that its Al content is at least 0.4 % wt.
4. Steel according to any one of the preceding claims, characterised in that its Ti content satisfies the condition $$\%\mathrm{Ti}-\left(\mathrm{3.42}\mathrm{x}\%\mathrm{N}\right)>\mathrm{0.005}\%\mathrm{wt}\mathrm{.},$$

   

   
   wherein %Ti indicates its respective Ti content and %N indicates its respective N content.
5. Steel according to any one of Claims 1 to 3, characterised in that, if $$\%\mathrm{Ti}-\left(\mathrm{3.42}\mathrm{x}\%\mathrm{N}\right)\le \mathrm{0.005}\%\mathrm{wt}\mathrm{.}$$

   

   
   applies for its Ti content,
   the condition $$0.0015\le \%\mathrm{N}-\%\mathrm{Ti}/3.42\le 0.0060\%\mathrm{wt}\mathrm{.}$$

   

   
   is satisfied, wherein %Ti indicates its respective Ti content and %N indicates its respective N content.
6. Steel flat product for producing a steel part, characterised in that it has at least one area which consists of high-strength steel obtained according to any one of Claims 1 to 5.
7. Steel flat product according to Claim 6, characterised in that it uniformly consists of the high-strength steel.
8. Steel flat product according to any one of the preceding claims, characterised in that at least one of its surfaces is coated with a coating protecting against oxidation.
9. Steel part produced from a steel flat product obtained according to any one of Claims 6 to 8, wherein its microstructure consists of martensite, austenite and up to 20 % by area of ferrite in the area of the high-strength steel obtained according to any one of Claims 1 to 6.
10. Steel part according to Claim 9, characterised in that the martensite content of its microstructure in the area of the high-strength steel is at least 75 % by area.
11. Steel part according to either of Claims 9 and 10, characterised in that the austenite content of its microstructure in the area of the high-strength steel is at least 2 % by area.
12. Steel part according to any one of Claims 9 to 11, characterised in that its surface is coated with a coating protecting against oxidation.
13. Method for producing a steel part obtained according to any one of Claims 9 to 12, comprising the following production steps:

    - providing a steel flat product formed according to any one of Claims 7 to 9,

    - heating the steel flat product through to a temperature of 780 - 950 °C,

    - hot forming the steel flat product into the steel part,

    - accelerated cooling of the steel part, so that the steel part obtained after cooling, at least in the area of the high-strength steel, has a microstructure which consists of martensite, austenite and up to 20 % by area of ferrite.
14. Method according to Claim 13, characterised in that the cooling rate during cooling of the steel part is at least 25 °C/s.