EP3292228B1 - Flat steel product and methode for production of said product - Google Patents

Description

The invention relates to a flat steel product which has an optimized combination of strength and elongation.

The invention also relates to a method for producing such a product.

When we talk about flat steel products, we mean steel strips, sheets or sheet blanks obtained from them, such as blanks.

Unless expressly stated otherwise, in the present text and in the claims the contents of certain alloying elements are indicated in% by weight and the proportions of certain structural components in area%.

From the CA 2 734 976 A1 ( WO 2010/029983 A1 ) a steel with good toughness and ductility is known, which should have a tensile strength of at least 980 MPa. In addition to iron and unavoidable impurities (in% by weight), the steel contains 0.17 - 0.73% C, up to 3.0% Si, 0.5 - 3.0% Mn, up to 0.1% P, up to 0.07% S, up to 3.0% Al and up to 0.010% N. The sum of the Al and Si contents should be at least 0.7%. At the same time, in relation to the totality of all microstructure components, the martensite content in the structure of the steel should be at least 10 - 90%, the content of retained austenite in the range of 5 - 50% and the proportion of ferritic bainite that comes from "upper bainite" 5%. "Upper bainite" is a bainite in which fine carbide grains are evenly distributed, such as cannot be found in "lower bainite". Higher contents of upper bainite of 17% and more are regarded as advantageous in order to produce the desired high residual austenite contents in the structure.

From the EP 2 524 970 A1 Furthermore, a flat steel product is known which has a tensile strength R _m of at least 1200 MPa and consists of a steel which, in addition to Fe and unavoidable impurities (in% by weight), C: 0.10-0.50%, Si: 0 , 1 - 2.5%, Mn: 1.0 - 3.5%, Al: up to 2.5%, P: up to 0.020%, S: up to 0.003%, N: up to 0.02% , and optionally one or more of the elements "Cr, Mo, V, Ti, Nb, B and Ca" in the following contents: Cr: 0.1-0.5%, Mo: 0.1-0.3%, V : 0.01-0.1%, Ti: 0.001-0.15%, Nb: 0.02-0.05%. For the sum Σ (V, Ti, Nb) of the contents of V, Ti and Nb, Σ (V, Ti, Nb) ≤ 0.2%, B: 0.0005 - 0.005%, Ca: up to 0 applies .01%. At the same time, the flat steel product has a structure with (in area%) less than 5% ferrite, less than 10% bainite, 5 - 70% untempered martensite, 5 - 30% residual austenite and 25 - 80% tempered martensite, whereby at least 99% of the iron carbides contained in the tempered martensite have a size of less than 500 nm. Due to its minimized proportion of over-tempered martensite, a flat steel product made in this way has an optimized formability.

Likewise from the EP 2 524 970 A1 a method for producing a flat steel product of the type explained above is known. In this procedure, first a flat steel product with the above-mentioned composition is heated at a heating rate θ _H1 , θ _H2 of at least 3 ° C / s to an austenitizing temperature T _HZ above the A ₃ temperature of the steel of the flat steel product and not exceeding 960 ° C. The flat steel product is held there for an austenitizing period t _HZ of 20-180 s in order to be subsequently cooled to a cooling stop temperature. This is greater than the martensite stop temperature and less than the martensite start temperature, the cooling taking place at a cooling rate which is at least equal to a minimum cooling rate determined as a function of the alloy content of the steel. The flat steel product is then held at the cooling stop temperature for 10-60 s, in order then to be heated to a partitioning temperature of 400-500 ° C. at a heating rate of 2-80 ° C / s. This can be followed by isothermal holding of the flat steel product at the partitioning temperature for up to 500 s. The flat steel product is then cooled at a cooling rate of 3 - 25 ° C / s.

In the known method explained above, the heating and the optionally additionally carried out holding at the partitioning temperature enriches the retained austenite in the structure of the flat steel product with carbon from the supersaturated martensite. This process is also known as "partitioning the carbon" or "partitioning". The partitioning can already be carried out during the heating up as so-called "ramped partitioning" holding at the partitioning temperature (so-called "isothermal" partitioning) or a combination of isothermal and ramped partitioning, carried out after heating. The slower heating rate aimed for in ramped partitioning compared to isothermal partitioning allows a particularly precise control of the given partitioning temperature with reduced energy consumption. The steels procured and processed in the manner explained above belong to the so-called "AHSS steels" (Advanced High Strength Steel).

Modern variants of these steels and flat steel products made from them have a very high strength with high elongation at the same time and are therefore particularly suitable for the production of safety-relevant components of automobile bodies that are intended to absorb deformation energy in the event of a crash. However, it has been shown in practice that high residual austenite contents in the structure of such steels can improve their uniaxial elongation through the well-known TRIP effect, but that they do not reliably achieve deformability in all directions, as is the case, for example, with a good hole expansion behavior is characterized.

From the JP3374659 B2 and the JP2013 227657 A Examples of flat steel products are known which have a similar element and structural composition as well as a combination of high strength and ductility.

Against this background, the task arose of creating a flat steel product that not only has an optimized combination of high strength and elongation, but also with improved performance properties, such as good weldability, surface properties and suitability for coating with a metallic one Protective coating, also has a structure that ensures optimized deformability regardless of the orientation of the deformation.

A method for producing such a flat steel product should also be specified.

With regard to the flat steel product, the invention has achieved this object in that a flat steel product according to the invention has at least the features specified in claim 1.

With regard to the method, the solution according to the invention to the above-mentioned object is that at least the work steps mentioned in claim 9 are carried out in the production of a flat steel product according to the invention.

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

A flat steel product according to the invention is defined by the features contained in claim 1.

The invention is based on the knowledge that by choosing a suitable alloy, a flat steel product can be obtained in which a high strength is characterized by a structure that includes minimal residual austenite content and a high proportion of tempered martensite and finely distributed, non-tempered martensite is paired with very good formability.

Typical tensile strengths Rm of flat steel products according to the invention are 950-1300 MPa with a yield strength which is at least 800 MPa and can reach the respective tensile strength. The elongation A ₅₀ of flat steel products according to the invention is typically 8-20%. At the same time, a flat steel product according to the invention regularly achieves hole expansion ratios of at least 30% in the hole expansion test according to ISO 16630.

According to the invention, these combinations of properties are achieved through the precisely measured addition of inexpensive alloy components. These are coordinated in such a way that the desired mechanical properties are reliably achieved and the flat steel product obtained shows good weldability and coatability at the same time.

The setting of a suitable ratio between the elements which influence the austenite formation and hardenability of the steel and the elements which suppress the carbide formation is of essential importance here. This ratio is set in an alloy according to the invention using the factor ψ in which the respective C, Mn, Cr, Al and Si content of the steel is incorporated. The factor ψ should not be less than 1.5. Too high a content of silicon or aluminum would have a negative effect on the coatability (silicon) or the castability (aluminum) of the steel. If the carbon, manganese or chromium content is insufficient, the required strength would not be achieved. According to the invention, the values for the factor ψ are at least 1.6. Values for the factor ψ of at least 1.6 have proven to be advantageous for setting a stable production process, with values for the factor ψ of at least 1.8 having proven to be particularly advantageous for production stability. Too much carbon and manganese can lead to an increased retained austenite content, which in turn would result in lower formability. This is avoided by setting the value to 3.0 as the upper limit for the range in which the ψ factor of a steel according to the invention lies.

Carbon has several important functions in the steel of the invention. On the one hand, the C content plays a major role in the formation of the austenite and the setting of the A ₃ temperature. A sufficient C content enables full austenitization at temperatures of less than 930 ° C. During the subsequent quenching, the retained austenite is stabilized by carbon. This stabilization can be supported by an additional heat treatment step, as the invention provides in the method according to the invention. The strength of the martensite is also strongly influenced by the carbon content of the steel. On the other hand, the martensite start temperature is shifted to lower and lower temperatures with increasing C content, which leads to production challenges. For these reasons, the invention provides a C content of 0.05-0.2% by weight, in particular at least 0.065% by weight of C, in the steel of a flat steel product according to the invention, the positive effect of C in the invention in practice Steel can then be used particularly safely if the C content is 0.07-0.19% by weight.

• mit %C: jeweiliger C-Gehalt des Stahls
• %Si: jeweiliger Si-Gehalt des Stahls
• %Mn: jeweiliger Mn-Gehalt des Stahls
• %Cr: jeweiliger Cr-Gehalt des Stahls
• %Mo: jeweiliger Mo-Gehalt des Stahls

The so-called carbon equivalent “CE”, the value of which is decisively influenced by the C content, can also be used for the specific measurement of the respective C content within the limits provided according to the invention. To calculate the carbon equivalent CE, the American Welding Society has proposed the following formula: $$\mathrm{CE}=\%\mathrm{C.}+\left(\%\mathrm{Si}+\%\mathrm{Mn}\right)/5+\left(\%\mathrm{Cr}+\%\mathrm{Mon}\right)/6$$

• with% C: the respective C content of the steel
• % Si: respective Si content of the steel
• % Mn: respective Mn content of the steel
• % Cr: the respective Cr content of the steel
• % Mo: the respective Mo content of the steel

According to the invention, the carbon equivalent CE should be at most 1.1% by weight in order to ensure good weldability. Particularly good weldability can be ensured by limiting the CE value to a maximum of 1.0% by weight. However, the CE value should be not less than 0.254% by weight and in particular not less than 0.29% by weight in order to have the effect of the invention provided in the calculation of the Carbon equivalents CE inflowing alloying elements.

The presence of silicon in the steel of a flat steel product according to the invention suppresses the formation of cementite, which would bind carbon, which would then no longer be available for stabilizing the retained austenite and which would worsen the elongation. The same effect can also be achieved by alloying with Al. However, a minimum of 0.2 wt% Si should be present in the steel contemplated by the invention. However, Si contents of more than 1.5% by weight would have a negative effect on the surface quality of a flat steel product according to the invention. In a flat steel product according to the invention, the Si content is therefore 0.2-1.5% by weight, with Si contents of at least 0.25% by weight or at most 0.95% by weight being particularly useful in practice have shown favorable and of at most 0.63 wt .-% as very particularly favorable.

In steel production, aluminum is added to the steel of a flat steel product according to the invention for deoxidation and for binding any nitrogen that may be present. Al can also be used to suppress cementite. However, if higher contents of Al are present, the austenitizing temperature also rises. The Al content of a steel intended for a flat steel product according to the invention is therefore limited to 0.01-1.5% by weight. If low austenitizing temperatures are to be guaranteed, it can be expedient to limit the Al content to a maximum of 0.44% by weight, in particular to 0.1% by weight. In addition, higher Al contents have a negative effect on castability in steel production. Al contents of at most 1.0% by weight, in particular at most 0.44% by weight, have proven to be favorable for ensuring particularly good castability. In addition, aluminum can be bound to aluminum nitride by nitrogen. Aluminum nitride precipitates present in the flat steel product can have an unfavorable effect on the formability of the flat steel product. For example, with a view to optimizing the formability, it can be useful to limit the Al content to a maximum of 1.0% by weight, in particular to a maximum of 0.44% by weight.

In order to exclude any negative effects of Si and Al in the flat steel product according to the invention, the sum of the Al and Si contents in the steel of a flat steel product according to the invention can be limited to a maximum of 1.7% by weight, with upper limits of at most 1.5% by weight. -%, in particular at most 1.0% by weight, in particular with regard to an optimization of the weldability have proven to be particularly favorable. With a view to optimizing the formability, upper limits for the sum of the Al and Si contents of at most 1.0% by weight, in particular at most 0.4% by weight, have also proven advantageous. In the flat steel product according to the invention, the upper limits for Si and Al are additionally limited by the factor ψ.

Manganese is important for the hardenability of the steel of a flat steel product according to the invention and also prevents the formation of undesired pearlite during cooling. The presence of Mn thus enables the formation of an initial structure (martensite and retained austenite) that is suitable for the formation of the structure prescribed according to the invention. However, too high a Mn concentration would have a negative effect on the elongation and weldability of the steel. Hence for the Mn content according to the invention is provided in a range of 1.0-3.0% by weight, in particular at least 1.5% by weight or at most 2.4% by weight.

Phosphorus has an unfavorable effect on the weldability of a flat steel product according to the invention. The P content should be as low as possible, in any case not exceed 0.02% by weight, in particular be less than 0.02% by weight or less than 0.018% by weight.

The presence of effective contents of sulfur would lead to the formation of sulfides, in particular MnS or (Mn, Fe) S, in the steel of a flat steel product according to the invention, which would have a negative effect on the elongation. To avoid this, the S content of the steel should be kept as low as possible, but in any case not higher than 0.005% by weight, in particular less than 0.005% by weight or less than 0.003% by weight.

In order to avoid the formation of nitrides, which could be detrimental to formability, the N content of the steel of a flat steel product according to the invention is limited to at most 0.008% by weight. To avoid any negative influence, the N content should advantageously be below 0.008% by weight, in particular less than 0.006% by weight.

Chromium in contents of up to 1.0% by weight can optionally be used as an effective inhibitor of pearlite in the steel provided according to the invention and also contributes to strength. With contents of more than 1.0 wt.% Cr, there is a risk of pronounced grain boundary oxidation. To be able to use the positive effects of Cr are at least 0.05% by weight is required. The presence of Cr in the steel of a flat steel product according to the invention has a particularly favorable effect if at least 0.15% by weight of Cr is present, an optimal effect being achieved with contents of up to 0.8% by weight.

Optionally, the steel of a flat steel product according to the invention can also contain molybdenum in contents of 0.05-0.2% by weight. Mo in these contents also suppresses the formation of undesired pearlite particularly effectively.

The steel of a flat steel product according to the invention can furthermore optionally contain contents of one or more micro-alloying elements in order to support the strength through the formation of very finely divided carbides. Contents of Ti and Nb have proven to be particularly suitable for this.

Ti contents of at least 0.005% by weight and Nb contents of at least 0.001% by weight, either alone or in combination with one another, lead to the freezing of the grain and phase boundaries during the heat treatment that a flat steel product according to the invention undergoes during its production according to the invention. Ti can also be used to bind the nitrogen present in the steel in order to allow other alloying elements, in particular boron, to have an effect. Ti contents of at least 0.02% by weight have proven to be particularly advantageous. However, too high a concentration of micro-alloy elements would lead to oversized carbides, which could initiate cracks at high degrees of deformation. Hence the Ti content of the steel of a steel flat product according to the invention is limited to a maximum of 0.2% by weight and its Nb content to a maximum of 0.05% by weight, whereby the presence of micro-alloying elements has proven to be advantageous in order to avoid negative influences, when the sum of the contents of Nb and Ti does not exceed 0.2% by weight.

The boron which is also optionally present in the steel of a flat steel product according to the invention segregates on the phase boundaries and brakes their movement. This leads to a fine-grain structure, which has an advantageous effect on the mechanical properties. As mentioned above, Ti can be added to the steel so that the effect of B can be used. In order to be able to use the positive effect of B, the steel provided according to the invention must contain at least 0.0001% B by weight. At contents of more than 0.005% by weight, no increase in the positive effect of B can be determined.

In order to protect it against corrosive attacks, the flat steel product according to the invention can be provided with a metallic protective coating. This can in particular be applied by hot-dip coating. Zn-based coatings are suitable for a flat steel product according to the invention.

The method according to the invention for producing a flat steel product according to the invention comprises the working steps defined in claim 5.

The principle of the procedure according to the invention is in the as Fig. 1 attached diagram.

In step a) a steel flat product is provided which consists of a steel with the composition explained above. The flat steel product provided can in particular be a cold-rolled flat steel product. However, it is also conceivable to use a hot-rolled To process flat steel product in the manner according to the invention.

For the heating of the flat steel product to the austenitizing temperature T _HZ (step b)), two uninterrupted successive steps are possible in principle, with the flat steel product being heated in the first step at a heating rate Θ _H1 of 5 - 25 K / s up to a turning point temperature T _W , which is 200 - 400 ° C. Values for Θ _H1 of at least 5 K / s have proven to be favorable for the productivity of the process, while a heating rate Θ _H1 of more than 25 K / s has proven to be very energy-intensive and cost-intensive. In the second step, the heating is then continued at a heating rate Θ _H2 of 2-10 K / s until the austenitizing temperature T _{HZ is} reached. In the second heating step, the alloying elements present in the flat steel product can diffuse in the flat steel product during the heating process. As the heating rate increases, the time available for the diffusion process and thus for the homogenization of the alloying element distribution of the flat steel product decreases. Unevenly distributed alloying elements can lead to locally different structural changes. To set a uniform structure, it has proven to be advantageous to limit the heating rate Θ _H2 to a maximum of 10 K / s. Values for the heating rate Θ _H2 of less than 2 K / s have proven to be unfavorable for the economy of the process. Since the If the heating speeds Θ _H1 , Θ _H2 overlap, the heating to the austenitizing temperature can also take place in one go with a constant heating rate of 5 - 10 K / s. The heating speeds θ _H1 and θ _H2 in step b) are then the same.

• mit %C: C-Gehalt des Stahls,
• %Ni: Ni-Gehalt des Stahls,
• %Si: Si-Gehalt des Stahls,
• %Mo: Mo-Gehalt des Stahls,
• %Mn: Mn-Gehalt des Stahls.

The austenitizing temperature T _HZ must be above the A ₃ temperature. The A ₃ temperature depends on the analysis and can be estimated using the following empirical equation (alloy content used in% by weight): $${\mathrm{A}}_{\mathrm{3rd}}\left[°\mathrm{C.}\right]=910-203\sqrt{\%\mathrm{C.}}-15.2\%\mathit{Ni}+44.7\%\mathit{Si}+31.5\%\mathit{Mon}-21.1\%\mathit{Mn}$$

• with% C: C content of the steel,
• % Ni: Ni content of the steel,
• % Si: Si content of the steel,
• % Mo: Mo content of the steel,
• % Mn: Mn content of the steel.

The alloy of the steel selected according to the invention makes it possible to limit the austenitizing temperature T _HZ to a maximum of 950 ° C. and thus to keep the operating costs incurred for carrying out the method according to the invention limited.

In order to prevent large austenite grains from forming, which would have a detrimental effect on the formability, the austenitizing time t _HZ , over which the flat steel product in work step c) is Austenitizing temperature T _{HZ is} kept, limited to 5-15 seconds, the austenitizing time t _{HZ can be} less than 15 s in order to avoid any undesirable grain growth.

In step d), the austenitizing duration t _HZ is followed by a controlled and slow cooling of the flat steel product. This cooling can take 50-300 seconds and must end at an intermediate temperature T _K that is not lower than 680 ° C. in order to avoid the undesired formation of ferrite. Upwards the intermediate temperature T _K is preferably to temperatures that exceed A _3, and typically limited to 775 ° C because the cooling capacity required for the subsequent cooling at higher intermediate temperature T _K and, consequently, found disproportionately high, the economics of the process in question is.

After the slow cooling in step d), the flat steel product is quenched in step e) at a high cooling rate θ _Q to an analysis-dependent cooling stop temperature T _Q. The high cooling rate θ _Q can be achieved, for example, with modern gas jet cooling.

The minimum cooling rate θ _Q , which is necessary to avoid the ferritic and bainitic transformation, is more than 30 K / s. The cooling rate θ _Q is typically limited upwards due to the system and is typically not more than 200 K / s. The range in which the cooling stop temperature T _Q lies is upward through the martensite start temperature T _MS , and downwardly about a horizontal to 175 ° C below the martensite start temperature T _MS limited ((T _MS -175 ° C) <T _Q <T _MS).

• mit %C: C-Gehalt des Stahls,
• %Mn: Mn-Gehalt des Stahls,
• %Si: Si-Gehalt des Stahls,
• %Al: Al-Gehalt des Stahls.

The martensite starting temperature can be estimated using the following equation (alloy content used in% by weight): $${\mathrm{T}}_{\mathrm{MS}}\left(°\mathrm{C.}\right)=539°\mathrm{C.}+\left(-423\%\mathrm{C.}-30.4\%\mathrm{Mn}-7.5\%\mathrm{Si}+\mathrm{30th}\%\mathrm{Al}\right)°\mathrm{C.}/\mathrm{Weight}.-\%$$

• with% C: C content of the steel,
• % Mn: Mn content of the steel,
• % Si: Si content of the steel,
• % Al: Al content of the steel.

In step f), the flat steel product is held at the cooling stop temperature T _Q for a holding time t _Q of 10-60 seconds in order to set the structure. In the course of this step, a martensitic structure with up to 30% retained austenite is obtained. How much martensite is generated in this step is essentially dependent on how much the cooling stop temperature is below the martensite start temperature T _MS . The holding time t _Q is at least 10 seconds in order to ensure a homogenization of the temperature in the flat steel product and thus a uniform structure. If the holding time is longer than 60 seconds, the homogenization of the temperature is complete. The holding period t _Q is at most 60 seconds in order to increase the productivity of the method.

In contrast to the prior art set out at the beginning, the invention does not seek to stabilize Retained austenite down to room temperature. Rather, the aim of the heat treatment of the flat steel product carried out in step g) is a controlled redistribution of the carbon so that the structure of the flat steel product obtained after completion of the process consists essentially of two different types of martensite, namely a tempered martensite and a non-tempered martensite.

According to the invention, step g) comprises two process variants g.1) and g.2), of which the first variant g.1) to an uncoated steel flat product according to the invention and the second variant g.2) to a steel flat product according to the invention provided with a Zn coating leads.

The temperature control in both variants g.1), g.2) of work step g) is selected in such a way that the retained austenite present in the structure up to that point is enriched with carbon from the supersaturated martensite. The formation of carbides and the disintegration of retained austenite are specifically suppressed by the inventive limitation of the total treatment duration t _BT . This takes 10 - 1000 seconds to allow sufficient redistribution of the carbon.

For the first process variant g.1), the treatment of the flat steel product in step g) comprises holding the flat steel product at a treatment temperature T _B that is at least equal to the cooling stop temperature T _Q and not higher than 550 ° C. over the entire treatment time t _BT , a cooling stop temperature T _Q of a maximum of 500 ° C. has proven to be particularly favorable.

In variant g.1), the treatment temperature T _{B can} also be higher than the cooling stop temperature T _Q. In this case, the flat steel product is heated starting from the cooling stop temperature T _Q to the respective treatment temperature T _B , the heating being carried out at a heating rate Θ _{B1 of} less than 80 K / s.

According to the second alternative of step g), however, the flat steel product is brought to a treatment temperature T _B of 400-500 ° C at a heating rate Θ _B1 of less than 80 K / s in order to enrich the retained austenite with carbon from the supersaturated martensite. The formation of carbides and the disintegration of retained austenite is specifically suppressed by the inventive limitation of the total treatment time t _BT , which in this variant g.2) of work step g) is made up of the heating time t _BR required for the heating and the holding time t _BI , Via which the flat steel product is kept isothermally at the temperature T _B. If the heating rate Θ _{B1 is} sufficiently slow, the isothermal holding can also be dispensed with, the holding time t _BI therefore being equal to "0".

In the second variant g.2) of work step g), following the heating and the optional holding at the treatment temperature T _{B, the} flat steel product passes through a hot-dip coating in which it is coated with a Zn coating. For this purpose, the treatment temperature T _{B can} be selected so that it corresponds to the inlet temperature at which the flat steel product enters the respective melt bath Treatment temperatures T _B in the range from 450 to 500 ° C. In addition to zinc and unavoidable impurities, the molten bath typically contains up to 3.0% by weight of one or more elements from the group consisting of Al, Mg, Si, Pb, Ti, Ni, Cu, B and Mn.

Regardless of which variant has been selected, the flat steel product is cooled in a controlled manner at a cooling rate θ _B2 of more than 5 K / s after the completion of work step g) for the renewed production of martensite, the cooling rates typically being at most 50 K / s. θ _B2 is more than 5 K / s in order to avoid the formation of pearlite and ferrite.

The process according to the invention can be carried out continuously in conventional annealing systems or strip coating systems which are usually provided for this purpose.

• zu mindestens 90 Flächen-% aus Martensit, von dem : mindestens 50 Flächen-% angelassener Martensit aus dem ersten Abkühlschritt (Arbeitsschritt f)) ist,
• zu höchstens 5 Flächen-% aus Bainit,
• zu höchstens 2 Volumen-% aus Restaustenit und
• zu höchstens 5 Flächen-% aus polygonalem Ferrit besteht.

The flat steel product according to the invention has a structure that

• at least 90% by area of martensite, of which: at least 50% by area is tempered martensite from the first cooling step (step f)),
• a maximum of 5% bainite area,
• to a maximum of 2% by volume of retained austenite and
• to a maximum of 5 area% made of polygonal ferrite consists.

The structure of a flat steel product according to the invention is very fine with an average grain size of less than 2 μm and can hardly be assessed by means of conventional light-optical microscopy. Therefore, an assessment using scanning electron microscopy (SEM) and a minimum magnification of 5000x is recommended.

The maximum permissible residual austenite content can only be determined with difficulty using a light microscope or a scanning electron microscope, even at high magnification. A quantitative determination of the retained austenite by means of X-ray diffraction (XRD) is therefore recommended (according to ASTM E975), according to which the retained austenite content is given in% by volume.

The distortion of the crystal lattice can also be used as a measure of the quality of the mechanical properties of a flat steel product according to the invention. This lattice distortion is very important for the initial resistance to plastic deformation. A suitable method for measuring and quantifying lattice distortion is electron backscatter diffraction (EBSD). With the EBSD method, a sample is scanned at points in the SEM, whereby a diffraction pattern is recorded at each measuring point, from which the crystallographic orientation can be determined. Details on the measurement and the various evaluation methods can be found in the manuals. A useful EBSD evaluation method is the so-called kernel average misorientation (KAM - further description in the manual "OIM Analysis v5.31" from EDAX Inc., 91 McKee Drive, Mahwah, NJ 07430, USA) Measuring point is compared with the neighboring points. Below a threshold value, typically 5 °, neighboring points belong to the same (deformed) grain. Above the threshold value, the neighboring points belong to different (sub) grains. Because the structure is so fine, a maximum step size of 100 nm is recommended for the EBSD. For the assessment of the structure of the flat steel products according to the invention, the KAM is evaluated from the third neighboring points. A flat steel product according to the invention must have a KAM mean value from a measuring range of at least 75 μm × 75 μm of more than 1.20 °, preferably more than 1.25 °.

The invention is explained in more detail below on the basis of exemplary embodiments.

In order to test the invention, samples of conventionally produced steel sheets were provided, which consisted of steels A - I with the compositions given in Table 1.

und $$\mathrm{CE}=\%\mathrm{C}+\left(\%\mathrm{Si}+\%\mathrm{Mn}\right)/5+\left(\%\mathrm{Cr}+\%\mathrm{Mo}\right)/6$$

mit %C der jeweilige C-, mit %Si der jeweilige Si-, mit %Mn der jeweilige Mn-, mit %Cr der jeweilige Cr-, mit %Mo der jeweilige Mo- und mit %Al der jeweilige Al-Gehalt der Stähle A - I berechnet worden sind.In Table 1, the factor ψ and the carbon equivalent CE are also given for each of the steels A - I, according to the formulas already explained above $$\mathrm{\psi }=\left(\%\mathrm{C.}+\%\mathrm{Mn}/5+\%\mathrm{Cr}/6\right)/\left(\%\mathrm{Al}+\%\mathrm{Si}\right)$$

and $$\mathrm{CE}=\%\mathrm{C.}+\left(\%\mathrm{Si}+\%\mathrm{Mn}\right)/5+\left(\%\mathrm{Cr}+\%\mathrm{Mon}\right)/6$$

with% C the respective C-, with% Si the respective Si-, with% Mn the respective Mn-, with% Cr the respective Cr-, with% Mo the respective Mo and with% Al the respective Al content of steels A - I have been calculated.

The steels E, F and G therefore did not meet the requirements determined according to the invention by the factor ψ for the coordination of the alloying elements essential for austenite formation and hardenability.

Samples 1 - 7, 11, 12, 16 - 23, 28 - 31, 33 - 35, 39, 40, 43 - 60 made from steels A - I have the in Figure 1 process sequence shown. They were initially heated at a heating rate θ _H1 to an inflection point temperature T _W and then at a heating rate θ _H2 to an austenitizing temperature T _HZ , which was above the A ₃ temperature of the respective steel, but lower than 950 ° C. The samples heated in this way are then kept at the austenitizing temperature T _HZ for an austenitizing period t _HZ and then cooled to an intermediate temperature T _K over a cooling period t _K. When the intermediate temperature T _{K is reached} , accelerated cooling has commenced at a cooling rate θ _Q at which the samples 1-7, 11, 12, 16-23, 28-31, 33-35, 39, 40, 43-60 are reduced to one Cooling stop temperature T _{Q have been} cooled down, which for samples 1 - 7, 11, 12, 16, 17, 19-23, 28-31, 33-35, 39, 40, 43-60 are each up to 175 ° C lower and for sample 18 was higher than the martensite start temperature T _{MS of} the respective steel A-I of samples 1-7, 11, 12, 16-23, 28-31, 33-35, 39, 40, 43-60. At the cooling stop temperature T _Q are samples 1 - 7, 11, 12, 16 - 23, 28 - 31, 33 - 35, 39, 40, 43-60 for a holding period t _Q of 10-60 s. Samples 1-7, 11, 12, 16, 17, 19-23, 28-31, 33-35, 39, 40, 43-48 were then heated to a treatment temperature T _B at a heating rate θ _B1 over a heating time t _BR heated, on which they have been held for an additional holding period t _BI in some attempts. Sample 18 was cooled to the treatment temperature T _B analogously. It was then cooled to room temperature at a cooling rate θ _B2 . After cooling to the cooling stop temperature T _Q and holding at T _Q, the samples 49-60 were kept isothermally at the treatment temperature T _B for the holding period t _Q without heating for a holding period t _BI . Samples 49-60 were then also cooled to room temperature at a cooling rate θ _B2 .

The above-mentioned parameters used in the tests are given in Table 2. Of the samples 1 - 7, 11, 12, 16 - 23, 28 - 31, 44 - 55 consisting of the steels A - D, H and I according to the invention, samples 3 (θ _Q <30 K / s), 11 are accordingly (T _HZ <A _3), 18 (TQ> 500 ° C), 19 (θ _Q <30 K / s), 28 (T _HZ <A _3), 29 (t _HZ> 15s) and 48 (θ _B2 < 5 K / s) has not been treated according to the invention.

In the course of the last cooling, the samples would have 1-7, 11, 12, 16-23, 28-31, 33-35, 39, 40, 43-60 in those cases where the treatment temperature T _{B is} on one for entry were in a Zn melt bath at a sufficient level of approx. 450 ° C Can pass through the melt bath. In the course of the tests, however, this was not done without affecting the test results.

On the samples obtained after the heat treatment, the mechanical properties are the yield strength R _p0.2 , tensile strength R _m , the ratio R _p0.2 / R _m , the elongation at break A ₅₀ (according to DIN EN ISO 6892, sample form 1), the product R _m ^∗ A ₅₀ , and the hole expansion ratios λ1, λ2 (according to ISO 16630) have been determined. The structural proportions of ferrite "F", tempered martensite "AM", retained austenite "RA", non-tempered martensite "M" and bainite "B" as well as the value "KAM" determined according to the kernel average misorientation were determined. The respective property values are given in Table 3 for each of the samples.

The mechanical properties achieved in the annealed material with a quantification of the structure can be found in Table 3. In the samples that meet both the specifications of the invention with regard to the alloy of the respective steel, as well as the conditions of the heat treatment according to the invention, _elongation limits R _p0.2 of more than 800 MPa, tensile strengths R _m of more than 950 MPa, Elongation at break values A50 of more than 8% with hole expansion ratios λ1, λ2 of regularly more than 30%.

Comparative examples B11 and D28, on the other hand, illustrate the effect of an inadequate austenitizing temperature T _HZ . In these examples, the structure has not been completely austenitized, so that too much ferrite forms in the structure. This leads to extremely localized damage and premature failure during the forming process.

Comparative example D29 shows how too long austenitizing at high temperatures can also have a negative impact on formability.

Comparative examples A3 and C19 show that if the cooling speeds θ _{Q are} too low, the desired yield strength is not achieved, which is due to the fact that the formation of ferrite could not be prevented sufficiently.

Comparative example C18, which was produced with a cooling stop temperature T _Q that was too high, shows that the yield point was below the desired yield strength and that the hole expansion ratios were low. These are due to an increased proportion of ferrite and bainite in the structure.

The comparative examples E33-E35 and E56-E58 show that the yield point and strength are below the desired yield point, which is due to the composition not according to the invention and too high a proportion of ferrite in the structure obtained. The high proportion of ferrite is due to insufficient prevention of carbide formation due to a too low silicon content and too low a content of aluminum and silicon in relation to carbon, manganese and chromium and thus an excessively high Faktor factor.

The comparative examples F39, F40, F59 and F60 finally show the effects of an excessively low ψ factor, which also leads to deviations from the desired structure. The minimum strength was partially achieved, but the yield strength and the hole expansion are not in the target area here.

Comparative example G43 makes it clear that too high a ψ factor leads to too high residual austenite proportions and reduced formability, which is expressed in poor hole expansion values λ1, λ2.

Comparative example I48 makes it clear that too low a cooling rate θ _B2 leads to increased ferrite formation and thus to low elongation limits. Table 1 C. Si Mn Al P S. N Cr Mon Ti Nb B. CE ψ According to the invention? A 0.066 0.29 2.54 0.037 0.009 0.003 0.005 0.666 0.000 0.071 0.001 0.0013 0.74 2.09 YES B. 0.085 0.30 2.75 0.030 0.000 0.003 0.005 0.750 0.100 0.070 0.000 0.0000 0.84 2.3 YES C. 0.159 0.29 1.82 0.041 0.015 0.003 0.004 0.422 0.101 0.047 0.001 0.0010 0.67 1.79 YES D 0.180 0.30 1.95 0.030 0.010 0.003 0.003 0.300 0.000 0.050 0.000 0.0000 0.68 1.88 YES E 0.075 0.10 1.52 0.035 0.010 0.001 0.004 0.530 0.050 0.025 0.000 0.0030 0.50 3.46 NO F 0.164 0.72 1.90 0.041 0.012 0.001 0.005 0.370 0.010 0.114 0.001 0.0000 0.75 0.8 NO G 0.190 0.23 2.97 0.030 0.008 0.004 0.006 0.801 0.050 0.060 0.001 0.0009 0.97 3.53 NO H 0.186 0.40 2.20 0.029 0.009 0.001 0.005 0.350 0.080 0.000 0.000 0.0000 0.78 1.6 YES I. 0.190 0.25 2.85 0.210 0.008 0.003 0.005 0.000 0.110 0.000 0.000 0.0000 0.83 1.65 YES Figures in% by weight, remainder Fe and unavoidable impurities
Underlined and bold values denote values outside the specifications according to the invention Ψ = (% C +% Mn / 5 +% Cr / 6) / (% Si +% Al)
% C = C content,% Mn = Mn content,% Cr = Cr content,% Si = Si content,% Al = Al content steel Serial No. Θ _H1 T _W Θ _H2 A ₃ T _HZ t _HZ T _K t _K Θ _Q T _Q T _MS t _Q Θ _B1 t _BR t _BI T _B Θ _B2 [K / s] [° C] [K / s] [° C] [° C] [s] [° C] [s] [K / s] [° C] [° C] [s] [K / s] [s] [s] [° C] [K / s] A 1 10th 300 5 817 860 10th 760 105 -31 310 433 50 3rd 46.7 0 450 -11 A 2nd 11 270 4th 817 860 12th 760 100 -47 310 433 50 3rd 46.7 0 450 -11 A 3rd 11 270 4th 817 860 12th 760 100 -16 370 433 40 2nd 40.0 0 450 -11 A 4th 5 270 5 817 860 10th 775 100 -42 350 433 50 3rd 33.3 0 450 -10 A 5 5 270 5 817 860 10th 775 100 -39 370 433 50 1.75 45.7 0 450 -9 A 6 5 270 5 817 860 12th 775 120 -36 370 433 50 1.75 45.7 15th 450 -20 A 7th 5 270 5 817 860 12th 775 120 -36 370 433 50 1 55.0 20th 425 -20 B. 11 5 300 2nd 809 780 8th 760 135 -21 350 418 15th 3rd 33.3 15th 450 -10 B. 12th 5 300 2nd 809 840 10th 760 110 -35 290 418 12th 2nd 80.0 15th 450 -12 B. 16 8th 300 2nd 809 860 10th 740 120 -32 300 418 12th 25 7.6 15th 490 -15 B. 17th 5 300 2nd 809 840 12th 740 120 -45 325 418 10th 4th 31.3 15th 450 -15 C. 18th 9 255 3rd 807 860 10th 740 105 -32 510 415 10th -1 60.0 16 450 -20 C. 19th 9 255 3rd 807 860 12th 740 105 -15 350 415 10th 3rd 33.3 0 450 -20 C. 20th 20th 295 3rd 807 860 10th 740 105 -49 290 415 50 3rd 53.3 22nd 450 -20 C. 21st 5 270 5 807 860 14 760 95 -42 350 415 50 3rd 33.3 0 450 -20 C. 22nd 14 310 5 807 860 14 715 125 -39 350 415 50 3rd 33.3 0 450 -10 C. 23 10th 270 3rd 807 860 12th 700 125 -39 350 415 50 1.5 50.0 0 425 -10 D 28 5 270 5 796 775 10th 750 120 -32 290 402 11 3rd 45.0 0 425 -10 D 29 5 270 5 796 840 25 750 120 -44 290 402 10th 3rd 53.3 25 450 -10 D 30th 5 270 5 796 840 10th 750 135 -38 250 402 12th 3.5 57.1 0 450 -10 D 31 5 270 5 796 840 12th 700 70 -50 350 402 15th 3.5 28.6 0 450 -10 E 33 5 270 5 828 860 10th 700 120 -54 300 461 50 3rd 50.0 5 450 -12 E 34 11 270 3rd 828 860 12th 685 140 -49 300 461 50 3rd 50.0 5 450 -12 E 35 11 270 3rd 828 860 12th 700 165 -42 350 461 50 3rd 33.3 5 450 -20 F 39 5 270 4th 820 860 12th 700 120 -31 310 408 50 3rd 46.7 5 450 -16 F 40 5 270 5 820 860 10th 685 125 -33 310 408 20th 3rd 46.7 0 450 -16 G 43 5 340 4th 771 850 10th 720 100 -21 325 368 25 8th 40.0 25 465 -11 H 44 21st 375 7th 796 835 12th 695 135 -37 230 391 14 3.5 57.1 0 430 -15 I. 45 11 350 3rd 776 860 9 720 75 -41 295 376 11 2.8 53.6 0 445 -12 I. 46 11 270 4th 776 840 12th 800 75 -35 290 376 10th 0.012 833.3 0 300 -15 I. 47 13 325 3.5 776 860 12th 745 65 -45 240 376 13 6 36.7 0 460 -12 I. 48 10th 340 4th 776 860 10th 730 70 -31 350 376 15th 3rd 46.7 0 450 -2 A 49 10th 270 4th 817 840 12th 740 120 -32 300 433 10th 0 0 420 300 -20 A 50 11 300 5 817 840 12th 740 120 -32 325 433 10th 0 0 420 325 -20 A 51 5 270 5 817 860 12th 740 120 -31 325 433 10th 0 0 420 325 -20 C. 52 10th 270 3rd 807 840 10th 760 100 -32 300 415 12th 0 0 420 300 -10 C. 53 15th 290 5 807 840 10th 780 80 -32 325 415 12th 0 0 470 325 -10 C. 54 5 270 5 807 860 12th 750 140 -31 325 415 12th 0 0 470 325 -16 C. 55 20th 300 3rd 807 860 12th 775 135 -32 350 415 12th 0 0 380 350 -16 E 56 5 270 5 828 840 14 700 135 -22 300 461 15th 0 0 410 300 -10 E 57 5 270 5 828 840 14 700 135 -20 325 461 15th 0 0 460 325 -10 E 58 5 270 5 828 860 8th 735 135 -21 325 461 15th 0 0 460 325 -10 F 59 10th 300 3rd 820 840 10th 720 140 -22 350 408 13 0 0 770 350 -9 F 60 8th 270 4th 820 840 12th 720 80 -22 300 408 13 0 0 420 300 -9 Underlined and bold values denote values outside the specifications according to the invention steel Serial No. R _P02 R _m R _P02 / R _m A ₅₀ R _m ^∗ A ₅₀ λ ₁ λ ₂ F AT THE RA M. B. CAME According to the invention? [MPa] [%] [MPa *%] [%] [Area%] [Vol-%] [Area%] [°] A 1 1050 1063 0.99 9.3 9885.9 80 62 - 80 1 19th - 1.43 YES A 2nd 1090 1093 1.00 8.0 8744 64 80 - 90 0 10th - 1.45 YES A 3rd 661 952 0.69 11.2 10662 35 28 10 45 1 43 Sp. 1.19 NO A 4th 989 1072 0.92 9.9 10613 61 41 - 75 1 24 - 1.37 YES A 5 890 1063 0.84 10.2 10843 60 62 - 70 0.5 29 Sp. 1.35 YES A 6 873 1056 0.83 10.7 11299 47 40 - 70 1.5 28 Sp. 1.36 YES A 7th 866 1071 0.81 8.8 9425 44 32 - 70 0 29 Sp. 1.34 YES B. 11 565 1197 0.47 11.2 13406 26th 32 10 50 3.5 30th 6.5 1.03 NO B. 12th 1030 1255 0.82 10.8 13554 54 49 - 75 0.5 24 Sp. 1.29 YES B. 16 980 1183 0.83 8.3 9819 38 41 - 60 1 38 Sp. 1.32 YES B. 17th 1077 1292 0.83 10.5 13566 31 32 - 70 0.5 27 Sp. 1.3 YES C. 18th 630 1056 0.60 12.7 13411 15th 18th 15th 0 0 55 30th 1.01 NO C. 19th 695 992 0.70 13 12896 35 29 20th 65 1 14 - 1.09 NO C. 20th 1120 1123 1.00 8.3 9321 55 51 - 85 0 15th - 1.42 YES C. 21st 1026 1119 0.92 8.4 9400 48 47 - 75 0.5 23 Sp. 1.4 YES C. 22nd 927 1074 0.86 9.9 10633 46 43 - 75 1 23 Sp. 1.34 YES C. 23 908 1074 0.85 9.5 10203 45 40 Sp. 65 0.5 33 Sp. 1.31 YES D 28 701 1231 0.57 13.4 16495 24 17th 20th 30th 2nd 40 8th 1.03 NO D 29 979 1290 0.76 9.1 11739 31 29 Sp. 50 4th 45 Sp. 1.38 NO D 30th 1138 1366 0.83 8.9 12157 45 39 - 75 0.5 24 Sp. 1.47 YES D 31 1091 1204 0.91 11.1 13364 31 34 Sp. 65 1 33 - 1.45 YES E 33 416 616 0.68 10.5 6468 71 71 30th 15th 1 45 9 1.22 NO E 34 277 538 0.51 19.8 10652 76 78 45 10 0.5 40 4.5 1.03 NO E 35 283 540 0.52 23.4 12636 77 61 40 10 1 45 4th 0.98 NO F 39 428 931 0.46 17.4 16199 20th 23 35 30th 1 30th 4th 1.02 NO F 40 442 977 0.45 17.4 17000 17th 16 35 30th 0.5 34 Sp. 1.05 NO G 43 873 1253 0.70 14.3 17918 26th 23 10 40 5 40 5 1.11 NO H 44 812 1079 0.75 16.7 18019 56 62 Sp. 75 1.5 20th 3rd 1.32 YES I. 45 823 1156 0.65 15.9 18380 35 42 - 65 1 30th 4th 1.42 YES I. 46 917 1109 0.83 16.2 17966 62 57 - 75 0.5 20th 4.5 1.42 YES I. 47 890 1047 0.85 13.1 13716 73 68 - 85 1.5 12th Sp. 1.38 YES I. 48 690 978 0.71 18.3 17897 14 12th 10 60 2.0 20th 8th 1.21 NO A 49 861 1049 0.82 9 9441 82 75 - 60 0.5 39 Sp. 1.34 YES A 50 816 1019 0.80 10.7 10903 62 53 - 50 1.5 48 Sp. 1.29 YES A 51 875 1052 0.83 9.1 9573 72 76 - 60 1 38 Sp. 1.31 YES C. 52 893 1139 0.78 9.1 10365 35 35 Sp. 65 0 34 - 1.29 YES C. 53 862 1091 0.79 9 9819 47 36 Sp. 70 0.5 27 Sp. 1.27 YES C. 54 959 1153 0.83 8.1 9339 55 38 - 50 0 50 - 1.41 YES C. 55 1038 1144 0.91 8.4 9610 43 36 - 55 1 44 - 1.46 YES E 56 429 661 0.65 13.6 8990 78 76 25 25 1.5 45 3.5 1.17 NO E 57 398 628 0.63 15.9 9985 65 81 30th 15th 0 55 - 1.13 NO E 58 521 695 0.75 8.5 5908 71 72 15th 35 0 50 - 1.26 NO F 59 491 841 0.58 19.7 16568 30th 26th Sp. 64 6 28 Sp. 1.26 NO F 60 405 961 0.42 16.6 15953 21st 20th 15th 35 0 40 10 1.18 NO "Sp." = Proportion <2 area%;
Underlined and bold values denote values outside the specifications according to the invention

Claims (7)

1. Flat steel product with tensile strength R_m of at least 950 MPa, yield strength of at least 800 MPa and elongation at break A₅₀ of at least 8%, wherein the tensile strength, yield strength and elongation at break are determined according to DIN EN ISO 6892, sample form 1 and wherein the flat steel product consists of a steel containing, in addition to iron and inevitable impurities expressed in percentage by weight:

   C: 0.05-0.20%,

   Si: 0.2-1.5%,

   Al: 0.01-1.5%,

   Mn: 1.0-3.0%,

   P: up to 0.02%,

   S: up to 0.005%,

   N: up to 0.008%,

   and optionally one or more of the elements of the group "Cr, Mo, Ti, Nb, B" with the respective contents:

   Cr: 0.05-1.0%,

   Mo: 0.05-0.2%,

   Ti: 0.005-0.2%,

   Nb: 0.001-0.05%,

   B: 0.0001-0.005%,

   wherein for $$\mathrm{\psi }=\left(\%\mathrm{C}+\%\mathrm{Mn}/5+\%\mathrm{Cr}/6\right)/\left(\%\mathrm{Al}+\%\mathrm{Si}\right)$$

   

   with %C: respective C content of the steel

   %Mn: respective Mn content of the steel

   %Cr: respective Cr content of the steel

   %Al: respective Al content of the steel

   %Si: respective Si content of the steel

   $$1.6\le \mathrm{\psi }\le 3$$

   

   shall apply and wherein the flat steel product possesses a microstructure consisting of

   - not more than 5% by area bainite,

   - not more than 5% by area polygonal ferrite,

   - not more than 2% by volume retained austenite,
   and

   - at least 90% by area martensite, wherein at least half of the martensite is tempered martensite, and wherein for the carbon equivalent

   $$\mathrm{CE}=\%\mathrm{C}+\left(\%\mathrm{Si}+\%\mathrm{Mn}\right)/5+\left(\%\mathrm{Cr}+\%\mathrm{Mo}\right)/6$$

   

   with

   %C: respective C content of the steel

   %Si: respective Si content of the steel

   %Mn: respective Mn content of the steel

   %Cr: respective Cr content of the steel

   %Mo: respective Mo content of the steel,

   0.254 ≤ CE ≤ 1.1% by weight shall apply.
2. Flat product according to claim 1, characterised in that the carbon equivalent CE is at most 1.0% by weight.
3. Flat steel product according to one of the preceding claims, characterised in that the sum of the contents of Ti and Nb is at most equal to 0.2% by weight.
4. Flat steel product according to one of the preceding claims, characterised in that it is provided with a protective Zn-based metallic coating applied by hot-dip galvanizing.
5. Method for manufacturing a flat steel product according to one of claims 1 to 4, comprising the following operational steps:

   a) Providing an uncoated flat steel product that consists of a steel containing, in addition to iron and inevitable impurities expressed in percentage by weight:

   C: 0.05-0.20%,

   Si: 0.2-1.5%,

   Al: 0.01-1.5%,

   Mn: 1.0-3.0%,

   P: up to 0.02%,

   S: up to 0.005%,

   N: up to 0.008%,

   and optionally one or more of the elements of the group "Cr, Mo, Ti, Nb, B" with the respective contents:

   Cr: 0.05-1.0%,

   Mo: 0.05-0.2%,

   Ti: 0.005-0.2%,

   Nb: 0.001-0.05%,

   B: 0.0001-0.005%

   wherein for $$\mathrm{\psi }=\left(\%\mathrm{C}+\%\mathrm{Mn}/5+\%\mathrm{Cr}/6\right)/\left(\%\mathrm{Al}+\%\mathrm{Si}\right)$$

   

   with %C: respective C content of the steel

   %Mn: respective Mn content of the steel

   %Cr: respective Cr content of the steel

   %Al: respective Al content of the steel

   %Si: respective Si content of the steel

   1.6 ≤ ψ ≤ 3 shall apply
   and wherein for the carbon equivalent $$\mathrm{CE}=\%\mathrm{C}+\left(\%\mathrm{Si}+\%\mathrm{Mn}\right)/5+\left(\%\mathrm{Cr}+\%\mathrm{Mo}\right)/6$$

   

   with %C: respective C content of the steel

   %Si: respective Si content of the steel

   %Mn: respective Mn content of the steel

   %Cr: respective Cr content of the steel

   %Mo: respective Mo content of the steel

   0.254 ≤ CE ≤ 1.1% by weight shall apply;

   b) Heating of the flat steel product to an austenitising temperature T_HZ that is above the A₃ temperature of the steel of the flat steel product and does not exceed 950°C, wherein the flat steel product is heated to an inflection point temperature Tw of up to 200-400°C at a heating rate θ_H1 of 5-25 K/s and subsequently up to an austenitising temperature T_HZ at a heating rate θ_H2 of at least 2-10 K/s

   with T_HZ > A_c3[°C] = 910-203 √(%C) - 15.2% Ni + 44.7% Si + 31.5% Mo - 21.1% Mn

   with %C: C content of the steel

   %Ni: Ni content of the steel

   %Si: Si content of the steel

   %Mo: Mo content of the steel

   %Mn: Mn content of the steel;

   c) Holding the flat steel product at an austenitising temperature T_HZ for an austenitising time t_Hz of 5-15 s;

   d) Initial cooling of the flat steel product for a cooling time t_k of 50-300 s to an intermediate temperature T_k of not less than 680°C;

   e) Quenching of the flat steel product starting from the intermediate temperature T_k at a cooling rate 9_Q of more than 30 K/s to a cooling stop temperature T_Q wherein: $$\left({\mathrm{T}}_{\mathrm{MS}}-175°\mathrm{C}\right)<{\mathrm{T}}_{\mathrm{Q}}<{\mathrm{T}}_{\mathrm{MS}}$$

   

   with $${\mathrm{T}}_{\mathrm{MS}}\left(°\mathrm{C}\right)=539°\mathrm{C}+\left(-423°\mathrm{C}-\mathrm{30,4}\%\mathrm{Mn}-\mathrm{7,5}\%\mathrm{Si}+30\%\mathrm{Al}\right)°\mathrm{C}/\%\mathrm{by}\mathrm{wt}.$$

   

   with %C: C content of the steel

   %Mn: Mn content of the steel

   %Si: Si content of the steel

   %Al: Al content of the steel;

   f) Holding the flat steep product at the cooling stop temperature T_Q for a holding time t_Q of 10-60 s;

   g) Treating the flat steel product quenched to the cooling stop temperature T_Q,

   g.1) wherein the flat steel product is held for a total treatment time t_B of 100-1000 s at a treatment temperature T_B that is at least equal to the cooling stop temperature T_Q and not higher than 550°C,
   or

   g.2) wherein, starting from the cooling stop temperature T_Q, the flat steel product is heated to a treatment temperature T_B of 450-500°C, wherein subsequently the flat steel product is optionally held at a constant treatment temperature T_B for a holding time t_BI, wherein the flat steel product is heated to the treatment temperature T_B at a heating rate θ_B1 of less than 80 K/s and the total treatment time t_BT resulting from the sum of the required heating time t_BR and holding time t_BI is 10-1000 s, and wherein the flat steel product is passed through a bath smelter after treatment for purposes of applying a protective Zn-based metallic coating;

   h) Cooling the flat steel product starting from the treatment temperature T_B at a cooling rate θ_B2 of more than 5 K/s.
6. Method according to claim 5, characterised in that the heating rates θ_H1 and θ_H2 in step b) are equal.
7. Method according to claim 5 or 6, characterised in that the flat steel product in step g.1), starting from the cooling stop temperature T_Q, is heated to the treatment temperature T_B at a heating rate θ_B1 of less than 80 K/s.

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