WO2019223854A1

WO2019223854A1 - Shaped sheet-metal part with a high tensile strength formed from a steel and method for the production thereof

Info

Publication number: WO2019223854A1
Application number: PCT/EP2018/063356
Authority: WO
Inventors: Thomas Gerber; Ilse Dr. HECKELMANN; Bernd Linke
Original assignee: Thyssenkrupp Steel Europe Ag; Thyssenkrupp Ag
Priority date: 2018-05-22
Filing date: 2018-05-22
Publication date: 2019-11-28
Also published as: EP3797176A1; CN112585284A; US20210189517A1

Abstract

The invention relates to a shaped sheet-metal part, which has a tensile strength of Rm > 1000 MPa at a bending angle of > 70° and which is formed from a flat steel product, which consists of (in % by weight): C: 0.10 - 0.30%, Si: 0.5 - 2.0%, Mn: 0.5 - 2.4%, Al: 0.01 - 0.2%, Cr: 0.005 - 1.5%, P: 0.01 - 0.1%, and in each case optionally one or more elements of the group "Ti, Nb, V, B, Ni, Cu, Mo, W" where Ti: 0.005 - 0.1%, Nb: 0.005 - 0.1%, V: 0.001 - 0.2%, B: 0.0005 - 0.01%, Ni: 0.05 - 0.4%, Cu: 0.01 - 0.8%, Mo: 0.01 - 1.0%, W: 0.001 - 1.0%, and as the remainder iron and unavoidable impurities, wherein the microstructure of the shaped sheet-metal part consists in a proportion of 40 - 100% by area of plate-like bainite, which consists in a proportion of 70 - 95% of ferrite, in a proportion of 2 - 30% of carbon-rich phases, which are ≥ 70% of a plate-like form, with a plate length PL of ≥ 200 nm in a ratio of plate length PL to plate width PB of the carbon-rich phase of a plate-like form PL/PB of ≥ 1.7 and are arranged at a spacing that is 50 nm to 2 pm, and as the remainder is formed in a proportion of < 5% by other constituents, wherein the microstructure of the shaped sheet-metal part that is not taken up by the bainite of a plate-like form consists in a proportion of ≥ 40% by area of the overall microstructure of bainite that is not of a plate-like form, which is formed in a proportion of 70 - 95% by ferrite, in a proportion of 2 - 30% by carbon-rich phases and in a proportion of < 5% by other constituents, wherein the total of the proportions of the bainite that is in a plate-like form and the bainite that is not in a plate-like form of the microstructure of the shaped sheet-metal part is ≥ 60% by area and the remaining austenite content of the microstructure of the shaped sheet-metal part is 2 - 20% by volume, and the remainder of the microstructure consists of other microstructural constituents. The invention also provides a method for producing such a shaped sheet-metal part.

Description

High tensile steel formed sheet metal molding and method of making the same

The invention relates to a formed from a steel sheet metal part having a high tensile strength Rm of at least 1000 MPa and a bending angle of more than 70 °.

Furthermore, the invention relates to a method for producing such a sheet metal part.

If following from a flat steel product or from a

"Sheet metal product" is mentioned, so are rolling products, such as steel strips or sheets, meant, from which for the production of, for example

Body parts blanks or blanks are divided. "Sheet metal parts" or "sheet metal parts" of the type according to the invention are made of such Stahlflachoder sheet metal products, in which case the terms "sheet metal part" and "sheet metal part" are used interchangeably.

All information on contents of the steel compositions given in the present application are by weight, unless expressly stated otherwise. All unspecified "% figures" in connection with a steel alloy are therefore to be understood as statements in "% by weight". With reference to the volume (in% by volume) information on the retained austenite content of the microstructure of a steel product according to the invention, details of the

Keep the various structural components in each case on the surface of a Grindings of a sample of the product in question (expressed in area percentage "area%"), unless expressly stated otherwise Information on the contents of the constituents of an atmosphere in this text refers to the volume (stated in "Vol .-%") ,

Mechanical properties, such as tensile strength, yield strength, elongation, reported herein have been determined in the tensile test according to DIN EN ISO 6892-1: 2009, unless expressly stated otherwise.

The microstructure was determined on cross sections which had been subjected to etching with 3% Nital (alcoholic nitric acid). The texture was determined in a scanning electron microscope at 5000 magnifications for the determination of the proportion of plate-like and other non-plate-like bainite and at 20,000- to 50,000-fold magnification for the determination of plate length, width and plate spacing. The proportion of retained austenite was determined by X-ray diffractometry.

From EP 2 719 786 B1 a sheet metal part and a method for

Producing such a sheet steel molding known, which has a tensile strength of at least 980 MPa. The sheet metal part consists of a steel, in addition to iron and unavoidable impurities from (in mass%) 0.15 - 0.4% C, 0.5 - 3% Si, 0.5 - 2% Mn, up to 0.05% P, to to 0.05%

S, 0.01 - 0.1% Al, 0.01 - 1% Cr, 0.0002 - 0.01% B, 0.001 - 0.01% N and TI is composed with the proviso that the Ti content is at least four times the N content and not more than 0.1 % is. According to the

As is known in the art, a sheet metal blank produced from a composite steel is heated to a temperature not lower than the Ac3 temperature of the respective steel and not higher than 1000 ° C, and then hot-worked in a press tool to form the hot-press formed sheet metal blank to shape. During molding, the sheet molding is cooled at an average cooling rate of at least 20 ° C / sec or higher in the die. This is called the target temperature range This cooling called a span 100 ° C below the

Bainite start temperature "BS", ie 100 ° C below the temperature at which bainite forms in the structure of the steel, begins and ends at the martensite start temperature MS, ie the temperature at which martensite forms in the structure of the steel. In this temperature range, the sheet metal part is held for at least 10 s to adjust the properties of the molded part. The holding may include isothermal holding, cooling or reheating, as long as it takes place in said temperature range. The thus obtained

Stahipnodukt is said to have a structure consisting (in area%) of 70-97% of bainitic ferrite, up to 27% of martensite and 3 - 20% of retained austenite, with the remaining structural constituents occupying at most 5%.

Against the background of the prior art, the object was to name a sheet metal part which can be produced by hot forming, such as press hardening, which has an optimized strength in combination with an optimal energy absorption capacity in the event of sudden deformation load, as occurs in a crash of an automobile.

In addition, a method should be specified, with which such sheet metal parts can be divided in practice.

The invention has achieved this object on the one hand by a sheet metal part having the features specified in claim 1.

On the other hand proposes the invention for the preparation of such sheet metal components, the method specified in claim 8.

Advantageous embodiments of the invention are in the dependent

Claims are given below and will be like the general one

Invention idea explained in detail. A sheet metal part according to the invention accordingly has a tensile strength Rm of at least 1000 MPa and a bending angle of more than 70 ° and is formed from a flat steel product which consists of (in% by weight):

and in each case optionally additionally from one or more elements

Elements from the group "Ti, Nb, V, B, Ni, Cu, Mo, W" with the proviso

and the remainder being iron and unavoidable impurities, including less than 0.05% S and less than 0.01% N,

- The structure of the sheet metal part to 40 - 100 area%

consists of plate-shaped pronounced bainite, the

to 70% to 95% of ferrite, to 2% to 30% of carbon rich phases which are at least 70% plate - shaped with a plate length PL of at least 200 nm at a ratio of plate length PL to plate width PB of the plate - shaped high - carbon phase PL / PB of at least 1, 7 are formed and arranged at a distance which is 50 nm to 2 mm, as well as - the remainder consists of less than 5% of other constituents,

- Where not by the plate-shaped bainite

occupied residual structure of the sheet metal part up to 40 area% consists of non-plate-shaped bainite,

- to 70 - 95% of ferrite, to 2 - 30% of carbon rich phases and

- less than 5% is composed of other ingredients,

- where the sum of the parts of the plate-shaped and not

plate-shaped bainits in the structure of the sheet metal part is at least 60 area%,

- Wherein the retained austenite content of the structure of the sheet metal part

2 - 20% by volume. and

- Wherein the remainder of the microstructure of the sheet metal part not occupied by the bainitic constituents consists of one or more constituents of the following group: martensitic or austenitic

Constituents, proeutectoid ferrite, elsene carbides, iron nitrides,

Transition metal carbides, transition metal nitrides, non-metal carbides, non-metal nitrides, metallic or non-metallic inclusions, sulfides and other unavoidable impurities. "

The non-ferrite and carbon-rich phase containing less than 5% of the plate-shaped bainite includes, for example, nitrides of micro-alloying elements or other inclusions.

By "bainite" is meant here the conversion product, which is in the microstructure during the cooling of the steel from which the invention

Sheet metal part consists of the austenite forms. These are the Bainit not a single phase. Rather, bainite always consists at least of bainitic ferrite and one or more carbon-rich

Phases.

In the present case, a "plate-shaped bainite" is understood as meaning a mixture of ferrite and carbon-rich phases, which are at least 70% plate-shaped, and up to 5% of remaining constituents.

In the present context, the term "high-carbon phases" is understood to mean austenite, cementite and other carbides.

Ferrite can be easily visualized in the micrograph of a sample of the respective sheet metal part by etching with 3% strength nital solution.

The carbon-rich phases are also identified after etching with 3% strength nital solution in the micrograph and are by means of a

Rasterelektonenmikroskops determinable. While ferrite is strongly removed by the etchant, the carbon-rich phases remain largely in their original form as they are hardly etched. When quantifying the carbon-rich phases according to shape, size and position to each other, only the phases that have remained standing after the etching are considered - ie those that were ground before the etching. Any larger depth carbides that have been exposed by ferrite etching away are not included. Otherwise, the result would depend on the depth of the etched ferrite.

The residual austenite content of the total structure is usually through

Microdiffractometry determined. The cementite is the most stable and important iron carbide with the stoichiometric composition FeaC.

The cementite, as part of the carbon-rich phases, is not determined separately, but is co-determined in the entirety of the carbon-rich phases. The microstructure of a sheet metal part according to the invention consists of: i) plate-shaped embossed bainite (proportion of the total structure 40-100

Area%), wherein 70-95% of the respective portion of the plate-shaped bainite of ferrite, 2-30% of the respective portion of the plate-shaped bainite of carbon-rich phases which at least 70% plate-shaped with a plate length of at least 200 nm Ratio of plate length to plate width of at least 1, 7 and arranged at a distance which is 50 nm to 2 mm, are formed, and the remainder to less than 5% of the proportion of the plate-shaped bainite of other constituents, which are to nitrides of micro-alloying elements or other

Inclusions can act, be taken; ii) other bainite, ie bainite, which is not plate-shaped, such as

Globularer bainite, wherein this other non-plate-shaped bainite can occupy up to 40 area% of the total structure and here again 70-95% of the non-plate-shaped bainite of ferrite, 2-30% of the non-plate-shaped bainite of

carbon-rich phases and the balance less than 5% of the respective proportion of the plate-shaped bainite be taken by other ingredients, such as nitrides of the micro-controllers or other inclusions; and iii) remainder of martensitic or austenitic constituents, including tempered martensite, tempered martensite or austenite, and, as further remainder, proeutectoid ferrite, iron carbides,

Iron nitrides, transition metal carbides, transition metal nitrides,

May include non-metal carbides, non-metal nitrides such as boron carbonitride, metallic inclusions, non-metallic inclusions, sulfides, and unavoidable impurities even understands that the proportion of the remainder in the total structure in the technical sense can also be "0", so practically undetectable or so low that it has no technical effect.

The bainitic proportions defined above under i) and ii) (proportion of

plate-shaped pronounced bainite and portion of the other, not plate-shaped bainite) on the structure of a sheet metal part according to the invention are adjusted so that they amount to a total of at least 60 area% of the structure of the sheet metal part. In addition to the invention

given bainite content martensite shares of up to 30 area% in the microstructure of the sheet metal part according to the invention can be tolerated, the

Martensitanteil optimally is as low as possible, ie in particular less than 20 area% or less than 5 area%

Essential to the invention is thus that of the structure of a

bainite according to the invention present sheet metal to a substantial part, optimally to more than 50%, plate-like pronounced. This means that the constituents of the respective bainite are present as plates of bainitic ferrite as well as of carbon-rich phases, such as retained austenite and cementite.

To explain the basic structure of the structure of a

sheet metal part according to the invention is made to the accompanying figures 1 a and 1 b reference. In these are possible configurations of

carbon-rich phases each shown in black. The white area between the black carbon-rich phases represents the ferrite. In the white area as many as you can

Excretions are located whose maximum length in the polished section is 200 nm.

As can be seen with reference to FIGS. 1a and 1b, according to the invention, for the carbon-rich phases of the plate-shaped bainite, at least 70% are plate-shaped. This 70% plate shape pronounced carbon-rich phases have a length PL of

at least 200 nm and a ratio of the length PL to the width PB that is at least 1.7 times larger than the width PB of the respective disk (PL / PB> 1.7). The size of the plates of the carbon-rich phases of the plate-shaped bainite is determined so that the ferrite plates lying between them are sufficiently far apart to avoid a simple bypass by dislocations. Elongated plates (PL / PB> 1, 7, Figures 1 a, 1 b) are further necessary to obtain the ductility. A blocky expression (PL / PB <1, 7) would lead to an increased crack sensitivity at shear stresses.

The latter would be disadvantageous especially under bending load. The distance PA between two mutually adjacent and parallel aligned plates of the carbon-rich phase must be at least 50 nm, preferably at least 100 nm, and at most 2 mm. The distance PA represents the effective grain size of the bainitic ferrite. The smaller the grain size, the higher the

Resistance to deformation and, consequently, the strength of the structural component in question. For sufficient strength, the distance may not be more than 2 mm, preferably not more than 1, 2 mm. If the distance PA were below 50 nm, the strength would be so strong that this area hardly deforms, since the critical crack stress in the overall structure is achieved. This would result in a brittle material failure, which should just be avoided. In this case, two plates K are considered to be "aligned in parallel" if the orientation of the longest side of the respective plates in question deviates from each other by less than 25 °.

The structure of the invention has several advantages that lead to an extraordinary combination of strength and flexibility: i. The high strength of at least 1000 MPa is achieved by the fineness of the structure and not by brittle components such as martensite. After the Hall-Petch relationship, the strength increases with decreasing grain size. In the sheet metal component according to the invention, the maximum orthogonal distance between two nearest carbon-rich plates represents the effective grain size. The two necessary constituents of the microstructure are austenite and bainitic ferrite, both of which have a high deformability. If cementite also forms, it is even finer than austenite. As a result, it hardly deteriorates the bending properties, although cementite itself represents a very hard and brittle phase. Over a larger area (> 100mm), the microstructure is again very homogeneous, which is crucial for good flexibility. FIG. 2 shows a photomicrograph of a sample of a sample processed according to the invention and FIG

composite steel in 1000x magnification. Clearly the very good macroscopic homogeneity can be recognized. ii. The function of austenite in the microstructure of an inventive

Sheet metal part is mainly to prevent the free movement of dislocations through the ferrite and thus to produce a higher resistance to deformation (= strength). At the same time, cracks are absorbed by the layer structure during deformation, which thus can not grow to a critical crack length and can lead to premature failure in bending. iii. In addition to the high carbon content, which is set by the partitioning of the carbon at 350 - 450 ° C, the high fineness of the austenite also ensures that it is mechanically stabilized against deformation-induced martensite formation. The formation of coarse, brittle

Martensite would significantly worsen the flexibility. The risk of the formation of larger amounts of martensite in the microstructure of a sheet metal component according to the invention is limited by the particular fineness of the retained austenite twice: firstly, the small grain size leads to a further reduction in Ms temperature, so that less austenite at deforming into martensite. If, however, martensite still forms, then on the other hand it is still so fine that the negative influence on the mechanical properties remains limited.

For the plate-like ferrite in the microstructure of an inventive

Sheet metal part applies that this so regularly from plates of the

carbon-rich phase is interrupted, that at each point a carbon-rich plate is at most 1 mm, preferably at most 0.6 mm away. This requirement limits the range of dislocations in the ferrite far enough so that sets in response to the very fine effective grain size, the exceptionally high strength of inventive sheet metal parts.

Due to its relatively low Si contents of up to 2 wt .-%, preferably up to 1, 4 wt .-%, particularly preferably up to 1 wt .-%, which allows

Invention a hot dip coating of the sheet metal part in particular with an aluminum-based protective coating. In the case that such a coating is applied, the forming process can be carried out reliably in atmospheric air, without thereby a

Scaling and the associated problems are triggered.

The composition of the flat steel product, from the one

According to the invention shaped sheet metal part is formed, is chosen so that with optimum deformability of the flat steel product, a tensile strength of at least 1000 MPa, in particular at least 1100 MPa, can be obtained, with tensile strengths of 1200 MPa and more are achieved regularly.

At the same time sheet metal parts according to the invention have a

VDA 238-100 determined bending angles of more than 70 °. Such a high bending angle stands for being used on an example as a body component in a passenger or transport vehicle

sheet metal part according to the invention by a high energy absorption

Bending occurs when the sheet metal part of a sudden strong Deformation load as it occurs when hitting an obstacle and the same, ie in a typical accident situation occurs.

The above-described property combination can be

in particular achieve that an inventive component is hardened, so, as explained in detail below, it has been withdrawn in a form so quickly heat that the

According preset according to the structure by which the

Condition is created for the properties achieved according to the invention.

Their special properties make sheet metal parts according to the invention in particular for use as part of a body or a

Suspension of a vehicle, especially a land-based

Vehicle, suitable.

Specifically, the steel of a sheet metal part according to the invention contains contents as a basis for this combination of properties

Compulsory ingredients (C, Sl, Mn, Al, Cr, P, Fe) and optionally added, i. unnecessary optional components (Ti, Nb, V, B, Ni, Cu, Mo, W). The contents of the individual components of the steel constituting a sheet metal part according to the invention are determined according to the invention as follows:

Carbon ("C") is in the steel, consist of the sheet metal parts according to the invention, in contents of 0.10 to 0.30 wt .-%. Such set C contents contribute to the hardenability of the steel by the ferrite and

To delay bainite formation and to stabilize the retained austenite in the microstructure. A carbon content of at least 0.10% by weight is required to achieve sufficient hardenability and high strength. From a C content above 0.30 wt .-%, however, the bainite formation is delayed too much, so that a sufficient Degree of conversion during the inventively provided holding time or the air cooling is not guaranteed. For a particularly high strength of bainite, a low transition temperature is necessary. This in turn is limited downwards by the martensitic transformation, which in turn can be shifted by C to lower temperatures. C in the contents provided according to the invention reduces the Ac3 transformation temperature and the martensite start temperature MS. In order to be able to use the positive effects of the presence of C particularly reliably, C-casings of at least 0.13% by weight, in particular at least 0.15% by weight, can be provided. With these contents, it is possible to safely achieve strengths of at least 1000 MPa, in particular at least 1100 MPa, while observing the further provisos of the invention. If negative influences of the presence of high C content on the properties of a sheet metal part according to the invention are to be avoided, this can be achieved by limiting the C content to at most 0.25% by weight,

in particular at most 0.20 wt .-% can be achieved. Maintaining the lower C content limits is particularly useful in improving weldability since greater C content is achieved at lower C levels

Hardened differences between the weld lens and the surrounding material of the sheet metal component can be avoided.

Silicon ("Si") is used in the steel of a flat steel product according to the invention in amounts of 0.5-2.0% by weight for suppressing cementite precipitation. Si is practically insoluble in cementite, so that in the presence of sufficient Si contents nucleation is significantly reduced. An Si content of less than 0.5% by weight would not be sufficient for the

To suppress the precipitation of cementite from bainitic ferrite at the holding temperatures prescribed according to the invention. By the

According to the invention provided Si content of at least 0.5 wt .-% can also stabilize the retained austenite. This effect can be achieved by increasing the Si content to at least 0.6 wt .-%, in particular at least 0.7 wt .-%, further reinforce. Contents of at least 0.7% by weight of Si open up a larger process window in hot forming by significantly slowing down the rest of Austenitzerfall. However, with a silicon content exceeding 2.0% by weight, the surface quality and coatability of a sheet metal part obtained according to the invention would decrease too much. If an inventive sheet metal part or the

Flat steel product from which the sheet metal part is formed, with a

To avoid coating problems, it may be expedient to limit the Si content to at most 1.4% by weight, in particular at most 1.0% by weight. This is especially true when the hot dip coating is to be carried out with an Si-containing melt based on Al. At the same time, lower Si contents, a flat steel product from which the sheet metal component according to the invention is to be formed, allow for lower temperatures

austenitized. Surprisingly, it has been shown here that it succeeds in a composite according to the invention, low alloy steel, at Si contents of less than 1 wt .-%, significant amounts of austenite to

stabilize.

Manganese ("Mn") is present in the sheet metal part according to the invention in contents of 0.5-2.4% by weight. Mn serves as a hardening element by greatly retarding ferrite and bainite formation. In addition, it stabilizes retained austenite (austenite former) and inhibits downstream of bainite transformation

Decomposition of retained austenite in cementite and ferrite. With a manganese content of less than 0.5% by weight, the austenite would not be sufficiently stabilized, so that the Si contents used would lead to a subsequent decomposition of the austenite. By raising the Mn levels on

At least 0.9% by weight, in particular at least 1.1% by weight, the austenite stability can once again be markedly increased, because in this way

Combination with the other inventively provided

Alloy elements can be prevented from getting larger amounts of microstructures form above the inventively provided maximum holding temperature. If the Mangangehait raised to more than 2.4 wt .-%, but the Bainitumwandlung slows down so clearly that during the process according to the invention the

Preserve predetermined holding temperature would be too long to the inventively desired conversion of the structure of a

Sheet metal molding according to the invention in a preferably more than 60

Area% according to the invention to achieve bainitic structure. If an optimized weldability is to be achieved at the same time, this can be achieved by limiting the Mn content to at most 2.0% by weight,

in particular at most 1, 8 wt .-%, accomplish. Mn contents of not more than 1.6% by weight, in particular less than 1.6% by weight, prove to be particularly favorable, since the bainitic transformation then proceeds so rapidly that the ensuing recalescence after removal from the

Press mold the effort for additional heating of the

Workpiece is unnecessary, which may be necessary to the sheet metal component after the press molding sufficiently long on the bainitic

To maintain the transformation temperature. In many applications, with such adjusted Mn content on the additional heating by selecting a suitable C content of at most 0.2 wt .-% and at a suitable

Sheet thickness of at least 1, 2 mm even completely waived.

Aluminum ("Al") is used in amounts of from 0.01 to 0.2% by weight in the production of the steel constituting a sheet metal part according to the invention

Deoxidizer. For safe setting of the oxygen contained in the molten steel at least 0.01 wt .-% AI are needed. AI can also in addition to setting the in the inventive

Sheet metal product undesirable, but production-related unavoidable levels of N can be used. At the same time, AI inhibits the formation of cementite in the microstructure of the sheet metal part. However, too high levels of AI would also significantly increase the Ac3 temperature. From At a level of more than 0.2% by weight, Al would hinder austenitization too much. In order to safely avoid in practice the negative effects of Al in the steel of the sheet metal part according to the invention, the Al content can be limited to at most 0.1 wt .-%.

Chromium ("Cr") contributes to the hardness of the steel of an invention

Sheet metal molding, by slowing diffusive conversions during cooling to the inventively predetermined holding temperature, thus ensuring a stable process in the Wannumformung. This beneficial effect arises from a content of 0.005 wt .-%, with a content of at least 0.15 wt .-% has proven in practice for safe process management. Too high levels of Cr, however, affect the coatability of the steel. Therefore, the Cr content of the steel of a present invention

Sheet metal part to at most 1, 5 wt .-%, in particular 0.75 wt .-%, limited.

Phosphorus ("P") is required at levels of 0.01-0.1 wt% in the steel of a sheet metal part according to the invention to compensate for the reduced Si content to suppress the nucleation of cementite. P leads to grain boundaries, lattice defects, and other sites that typically function as nucleation sites for cementite. In this way, P displaces the carbon present at the sites concerned and decreases

accordingly locally the carbon concentration of possible

Cementite germination sites with the consequence that there the thermodynamic driving force is lowered to the excretion with the result that the cementite excretion is suppressed. This effect is from a P content of at least 0.01 wt .-% and increases with increasing P content. Too high a phosphorus content, however, would affect the weldability, coatability and impact energy of the steel from which a sheet metal component according to the invention is formed. Its maximum P content is therefore

limited to 0.1 wt .-% according to the invention. Titanium ("Ti") is optionally contained in the steel of a sheet metal part according to the invention in amounts of 0.005-0.1% by weight in order to liberate nitrogen and thus allow the boron, which is also optionally present in effective levels, to promote the formation of ferrite strong inhibitory effect unfold. At the same time, Ti contributes to grain refining as a micro-alloying element. To take advantage of these positive effects, a Ti content of at least 0.005 wt% may be provided, with the Ti content optimally adjusted to be at least 3.42 times the N content of the steel. However, Ti also tends to form coarse TIN and can significantly lower the cold rollability as well as the recrystallizability. Therefore, the Ti content, if any, is limited to at most 0.1% by weight.

Like Ti, niobium (Nb) can also be added to the steel of an invention

Sheet metal part optionally in amounts of 0.005 - 0.1 wt .-% for grain refining and reduction of Zementitausscheidung be added. However, at levels greater than 0.1% by weight, Nb also degrades the

Rekristallisierbarkeit.

Optionally, vanadium (V) can also be added to the steel of a shaped sheet metal part according to the invention in amounts of 0.001-0.2% by weight for an additional increase in strength. V also contributes to the stabilization of the

Restaustenits at. In the case of cold strip production, however, V forms

Vanadium carbides, which must dissolve during austenitization of the material before hot working. This is ensured by the fact that the V content is limited to max. 0.2 wt .-% is limited. The vanadium in solution becomes small during bainite formation

Excreted nanometer and thus contributes to the strength by means of precipitation hardening. For a sufficient driving force V-contents of at least 0.001 wt .-%, in particular more than 0.01 wt .-%, required.

Also optionally, boron ("B") may be present in the steel of the component of the invention at levels of from 0.0005 to 0.01 percent by weight to enhance the hardenability of the invention To raise steel. B lays on the grain boundaries and thus reduces their energy. This suppresses the nucleation of ferrite. For a significant effect, B contents of at least 0.0005 wt% are needed. at

Contents of more than 0.01 wt .-%, however, increasingly boron carbides, boron nitrides or Bomitrokarbide form, which in turn represent preferred nucleation sites for the nucleation of ferrite and lower the hardening effect again.

Nickel ("Ni"), which is also optionally present in the steel of a component according to the invention, is an austenite former which improves the stability of austenite and thus the process stability with longer hold times during bainite formation. In the case where copper is present in the steel of the component according to the invention, the simultaneous presence of Ni can negate the negative influence of copper on the hot rollability. This already helps small amounts of Ni of at least 0.05 wt .-%. At Ni contents of more than 0.4% by weight, on the other hand, a slowing down of bainite formation may occur.

Optionally, copper ("Cu") can also be added to the steel of a component according to the invention in order to increase the hardenability. For this purpose, at least 0.01 wt .-% Cu are sufficient. Cu also improves the resistance to atmospheric corrosion on uncoated sheets. However, Cu contents above 0.8% by weight significantly impair the hot rolling market due to the low-melting copper phases on the surface.

Molybdenum ("Mo") may optionally be present at levels of 0.01-1.0% by weight in the steel of a sheet metal part according to the present invention in order to provide the

Improve process stability. Mo slows down the ferrite formation significantly and has only a minor effect on the formation of bainite in the temperature window intended according to the invention. From contents of at least 0.01% by weight, molybdenum-carbon clusters form dynamically up to ultrafines

Molybdenum carbides on the grain boundaries, which control the mobility of the

Grain boundary and thus effectively prevent diffusive phase transformations. In addition, the limited energy and the associated

Nucleation rate of ferrite reduced. At levels greater than 1.0% by weight, there is no significant increase in the effects of Mo used here.

Also tungsten ("W") can optionally in the steel of a

Sheet metal components may be present. It works similar to Mo here, but is effective even at lower levels. Thus, there is a positive effect on the hardenability even at a W content of 0.001 wt .-%. From a content of 1.0% by weight, no substantial increase in the effectiveness of W on the properties in question here can be observed.

Nitrogen ("N") and sulfur ("S") are basically undesirable because they adversely affect the properties of the steel of a sheet metal part according to the invention. However, due to manufacturing, N and S inevitably get into the steel. They are therefore assigned to the unavoidable impurities of the steel, which per se are kept so low (N content <0.01% by weight, S content <0.05% by weight) that they do not have the properties of Steel have negative impact.

As explained above, in a steel constituting the flat steel product of which the sheet molding is formed, the contents of C, Si, Mn, Al and Cr are set to be as follows

Salary ranges are (in% by weight):

In the method according to the invention for producing a sheet-metal component according to the invention produced in the above-described manner, at least the following working steps are performed: a) Provision of a sheet-metal blank, which is made of steel

Composition consists (in% by weight): C: 0.10-0.30%, Si: 0.5-2.0%, Mn: 0.5-2.4%, Al: 0.01-0 , 2%, Cr. 0.005 - 1, 5%, P: 0.01 - 0.1%, and in each case optionally additionally from one or more elements from the group "Ti, Nb, V, B, Ni, Cu, Mo, W" with the Ti: 0.005-0.1%, Nb: 0.005-0.1%, V: 0.001-0.2%, B: 0.0005-0.01%, Ni: 0.05-0.4%, Cu: 0.01 - 0.8%, Mo: 0.01 - 1, 0%, W: 0.001 - 1, 0%, and the remainder being iron and unavoidable impurities, the impurities being less than 0.05 % S and less than 0.01% N; b) heating the blank so that at least 30% of the blank volume when placed in a hot forming die (step c)) is at a temperature T_Aust above the Ac1 temperature wherein the Ac1 temperature according to the formula

Ac1 = [739 - 22 *% C - 7 *% Mn + 2 *% Si + 14 *% Cr + 13 *% Mo + 13 *% NI] ° C with% C = C content,% Si = Si Content,% Mn = Mn content,% Cr = Cr content,% Mo = Mo content and% Ni = Ni content of the respective steel of the blank;

c) inserting the heated blank into the forming tool conditioned to a tool temperature T_WZ of 200-430 ° C., wherein the transfer time t_Trans required for the removal and insertion of the blank is at most 20 s; d) hot press forming the blank to the sheet metal part, wherein the

Blank in the course of hot press forming over a duration t_WZ of 1-50 s with a cooling rate of more than 10 K / s r_WZ cooled to the cooling stop temperature T_Kühlstopp and optionally held there; e) removing the cooled to the cooling stop temperature T_Kühlstopp sheet metal molding from the tool; fl) optional: holding the sheet metal part at a holding temperature T_Halt of 300 - 450 ° C for a holding period t_Halt of up to 100 s; f2) optional: heating the sheet metal part within 1 - 10 s on a

Homogenization temperature of 380 - 500 ° C; f3) optionally: further shaping of the sheet-metal shaped part, wherein the shaping can be carried out in particular as a calibration step for improving the dimensional accuracy of the sheet-metal shaped part; g) optional: trimming the sheet metal part; h) cooling the sheet metal part to a cooling temperature T_AB of less than 200 ° C within a cooling time t_AB of 0.5 to 200 s.

In the method according to the invention, therefore, a blank consisting of a steel composed in a suitable manner in accordance with the above explanations is provided (step a)), which is then heated in a manner known per se such that at least 30%,

in particular at least 60% of its volume during subsequent insertion into the respective forming tool has an austenitic structure (step b)). This means that the transformation from ferritic to austenitic microstructure does not have to be completed when it is inserted into the forming tool his. Rather, up to 70% of the volume of the blank can be

Insertion into the forming tool from other structural components, such as

tempered bainite, tempered martensite and / or non-or partially recrystallized ferrite. For this purpose, certain areas of the blank may be targeted at a lower temperature during heating

Temperature level are kept as others. For this purpose, the heat can be targeted only targeted to certain sections of the blank or the parts that are to be heated less, are shielded against the heat. In the part of the blank material whose temperature remains below the minimum temperature specified for the temperature T_Aust, no or only significantly less bainite is formed during the forming in the tool, so that the microstructure there is significantly softer than in the respective other parts, in which bainitic structure is present. In this way, in each case

shaped sheet metal part targeted a softer range can be set, in which, for example, an optimal for the particular application

Toughness voriiegt, while the other areas of the sheet metal part a

have maximized strength.

Maximum strength properties of the obtained sheet metal part can be made possible by the fact that the blank in step b) on a

Austenitizing temperature T_Aust is heated, for which applies

wherein in this variant of the temperature T_Aust to be exceeded minimum temperature Ac3 according to the HOUGARDY, HP in Materials Science, Volume 1: Basics, Verlag Stahleisen GmbH, Dusseldorf, 1984, p. 229., given formula

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

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

For this purpose, the duration of the performed in step b)

Austenitization treatment be adjusted so that on the one hand by a limitation to 1000 s, a Komvergröberung is avoided, but on the other hand, the speed of the austenitic transformation is taken into account, which increases significantly in particular at a beyond the Ac3 temperature heating the steel blank. When heated above the Ac3 temperature, the austenitizing temperature is T_Aust

optimally at least 30 ° C, in particular at least 50 ° C, above the respective Ac3 temperature of the steel from which the blank to be formed in each case consists. At such a high austenitizing temperature, the austenitic transformation proceeds so rapidly that after reaching the relevant temperature, no holding at this temperature is required in order to achieve the complete transformation of the structure into austenite.

Instead, the cut can be made immediately after reaching the

Austenitizing be sent for further processing.

The thus heated blank is taken from the respective heating device, which may be, for example, a conventional heating furnace, an induction heating device known per se or a conventional device for keeping steel components warm, and transported so quickly into the forming tool that its temperature during Arriving in the tool with 600 - 900 ° C still clearly above the critical for the invention temperature of at least 450 ° C.

In step c), the transfer of the austenitized blank from the respectively used heating device for

Forming tool completed within preferably less than 20 s. Such rapid transport is required to avoid excessive cooling before deformation.

When inserting the blank, the mold is tempered to a temperature of 200-430 ° C., preferably 300-400 ° C., particularly preferably 320 380 ° C., which lies below the cooling stop temperature T_cool stop. In this case, the particular when selecting the blank concrete to be selected tool temperature

T_WZ as a function of the cooling stop temperature T_Kühlstopp, with which the sheet metal part is removed from the tool, and the sheet thickness D of the reshaped to form the sheet metal part blank are determined as follows:

During the subsequent transformation of the blank to the

Shaping takes place cooling a cooling rate of at least 10 K / s, in particular at least 20 K / s or at least 30 K / s, required to a conversion of austenite before reaching the

Hold temperature to prevent. Undesirable is both a transformation into ferrite, and in bainite at a temperature which is more than 100 ° C above the target holding temperature T_Halt of 300-450 ° C, in particular 320-430 ° C, in particular 320-400 ° C, because the conversion products would have a much lower strength. This would be less

Overall strength as well as lead to a reduced flexibility. In the tool, the blank is thus not only formed into the sheet metal part, but at the same time also lying in a range of 450-300 ° C, preferably 430-320 ° C, more preferably 400-320 ° C amount

Cooling stop temperature quenched. For this purpose, the tool is tempered to a tool temperature T_WZ of 200 - 430 ° C. At the cooling stop temperature, some martensite may already have formed in the microstructure of the sheet metal part, which may act as a germination site. However, at this point most of the structure still consists of unstable austenite, which subsequently turns extremely fast to fine bainite. The alloying according to the invention of Si, Al, P slows down the formation of carbides, so that either no or only fine carbides separate out. In this case, the conversion, made possible by the alloy specifically determined by the invention, so fast that no long holding time in

Temperature range of 450-300 ° C, especially 430-320 ° C,

especially 400 - 320 ° C, is necessary. Specifically, the invention looks for the

Cooling and optionally held in the forming tool in the temperature reached after cooling T_Kühlstopp in the still closed tool a duration t_WZ of 1 - 50 s before. Of the

As such, manufacturing process according to the invention can easily be incorporated into a short-cycle duty cycle.

After cooling, the sheet metal part obtained is optionally maintained over a sufficient for the conversion into the desired structure time t_Halt at a holding temperature of 300-450 ° C, in particular 320-430 ° C, in particular 320-400 ° C. This holding can be carried out both in the forming tool prior to removal or in a separate device after removal from the forming tool.

A particular advantage of the combination of the material according to the invention with the method according to the invention is the shortness of the holding time t_Halt practically required for the formation of the bainitic structure. Experiments have shown that after just a few seconds more than 50% of austenite are converted. This leads to a high dimensional accuracy of the sheet metal part produced according to the invention with short process times and excellent mechanical properties. A longer holding t_Halt than 100 s would be uneconomical and on the other hand also disadvantageous for the constitution of the structure. With too long hold times t_Halt, there would be an increased conversion of retained austenite to cementite, which would especially worsen the tensile properties by reducing the elongation at break.

In particular, in case that step f1) is performed, it may

be expedient, optionally as an additional step an additional

To carry out forming, for example, to improve the

Dimensional accuracy of the sheet metal part contributes.

In the event that no separate holding takes place, the step f1) is thus not completed (t_Halt = 0 seconds), a first part of the conversion already takes place in the tool, whereas a second part of the conversion takes place during the cooling in step h), the in this case preferred as

Air cooling is performed.

The invention thus provides a method with which

Produce sheet metal parts whose structure is characterized by a plate-like structure. The so plate-like structure results in a combination of high tensile strength (> 1000 MPa, in particular> 1100 MPa) with a very high bending angle (uncoated> 80 °). This sheet metal components can be through

Use of the method according to the invention in a particularly short time ago. Thus, in accordance with the invention, sheet metal components of high strength and optimum energy absorption capacity can be produced within

Generate total times that are less than a minute.

In order to reliably initiate the conversion processes used according to the invention, the composition of the steel can be adjusted so that the activation energy for bainite formation Qb is sufficiently low. To do this within the limits specified above according to the invention, the C content% C, the Mn content% Mn, the Mo content% Mo, the Cr content% Cr, the Ni content% Ni and the Cu content% Cu depending on the B content of the steel in each case in wt .-% set such that the activation energy Qb the

Bainite formation is less than 45 kJ, especially less than 40 kJ, more preferably less than 35 kJ, to sufficiently fast

Bainite conversion to achieve. Qb can be used for B contents of up to 0.0005 wt .-% according to the formula

Qb [kJ] = (90 *% C + 10 * (% Mn +% Mo) + 2 * (% Cr +% Ni) + 1 *% Cu) [kJ / wt%], and at B contents of more 0.0005 wt .-% according to the formula

Calculate Qb [kJ] = (90 *% C + 10 * (% Mn +% Mo) + 2 * (% Cr +% Ni) + 1 *% Cu +2) [kJ / wt%], where the C -Content% C, Mn content% Mn, Mo content

% Mo, the Cr content% Cr, the Ni content% Ni and the Cu content% Cu each in wt .-% are used in these formulas.

For sheet thicknesses greater than 2 mm, Qb values less than 40 kJ, for

Sheet thicknesses of 1-2 mm have Qb values less than 35 kJ and for

Sheet thicknesses smaller than 1 mm, Qb values less than 34 kJ have proved favorable.

A low activation energy Qb is particularly helpful when the

Hold time t_Halt is to be kept low, especially if the hold time tJHalt should be 0 seconds. In particular, when Qb is less than 38 kJ, the heat output, that is, the specific heat of transformation per unit time, is significant and counteracts cooling. This

Heat dissipation is sufficient even at low values of Qb to keep the component at temperature by the heat of transformation. It is also possible that the component gains slightly in temperature and cools only after conversion, without heat is introduced from the outside. The lower limit of the invention according to the holding temperature T_Halt predetermined range is 300 ° C, because at underlying

Temperatures the martensite start temperature MS even at maximum

Utilization of the alloying possibilities lying within the scope of the invention would be significantly lower. However, since as much bainite is to be formed, the formation of martensite must be avoided as far as possible. Martensite contents of more than 30 area% would lead to a significant deterioration of the properties of a sheet metal part according to the invention. Therefore, the process control should be chosen so that the martensite portion of the microstructure of a sheet metal part according to the invention is as small as possible. At the top, the range of the holding temperature T_Halt is limited to 450 ° C, because at higher temperatures, the strength of the bainite would fall too much.

According to CAPDEVILA, C. et al. Determination of Ms. Temperature in Steels: A Bayesian Neural Network Model. ISIJ International, 42: 8, August 2002, 894-902, the Martensitstafttemperatur one in the context of

according to the invention lying steel according to the formula:

Here, too, with C% the C content, with% Mn the Mn content, with% Mo the Mo content, with% Cr the Cr content, with% Ni the Ni content, with% Cu the Cu Content, with% Co of the Co content (not in accordance with the invention

% W is the W content and% Si is the Si content of the respective steel in% by weight.

Does it come in the tool during the forming to the sheet metal part

different local contact pressures, so may be present in the sheet metal part in its removal from the tool an inhomogeneous temperature distribution. In order to ensure a uniform and complete bainitic transformation in such a case, can be provided by an optional additional Heating the temperature distribution are homogenized so that the temperature over the entire sheet metal part in a temperature range of Ms-20 to Ms + 100 ° C, in particular Ms to Ms + 80 ° C, is.

At particularly low holding temperatures T_Halt, the transformation may take place rapidly, which is desirable, but the retained austenite does not remain stable up to room temperature due to an inhomogeneous distribution of the carbons it contains. In order to remedy this and to ensure a homogeneous C distribution in the retained austenite, the resulting sheet metal part can be heated to a temperature of 380-500 ° C. after the holding time required for the formation of the Bainltic microstructure, possibly going to "0" (step f2). ). At the onset after reaching the respective homogenization temperature cooling the carbon has enough time and thermal energy for redistribution. The upper limit of 500 ° C for the homogenization temperature results from the fact that at higher temperatures, too much softening would occur. At homogenization temperatures of less than 380 ° C, the diffusion rate would be too low. As particularly favorable for the

Homogenization temperature has a temperature range of 420 - 470 ° C proved.

Also, following the optionally performed step f2), it may be expedient to carry out an additional forming forming, which can be completed, for example, as a calibration step in order to further improve the dimensional accuracy of the sheet metal part.

A major cost burden in press hardening is the laser cutting of the press-hardened components. Conventional press-hardening requires that the component is not lowered until it reaches the required strength

Martensite finish temperature, which is generally below 200 ° C is removed from the tool. In contrast, at

inventive method, the component above 300 ° C, preferably at at least 350 ° C, are removed from the tool. Due to the reduced hardness and higher ductility at the higher temperature, the workpiece can also be hot cut at this point of the method according to the invention (step g)).

The latter is very cost-effective compared to laser cutting and leads to a significantly simplified logistics. Thus, cycle times of 1 to 20 s, in particular 1 to 10 s, can be achieved with the procedure according to the invention. If more than 20 s are required for temperature homogenization (steps d) - f2)), the relevant work steps can be subdivided into several steps, which are executed one after the other in different units.

During the cooling in the concluding step h) of the method according to the invention, the obtained sheet metal part is cooled within a duration t_AB of 0.5-200 s to a cooling temperature T_AB which is less than 200.degree. In this case, the time required for the cooling t_AB and, consequently, the speed at which the cooling takes place in

Dependence on the progress of microstructural transformation set in the previous steps. In the event that a part of the conversion is to follow during the air cooling, the cooling is under a

Medium, such as static or moving air, made at the comparatively low cooling rates can be achieved. It should be noted that the final cooling is crucial for a homogeneous

Carbon distribution in the retained austenite is. While the Bainitumwandlung in the steel according to the invention runs so fast that a very good

Dimensional accuracy results, the dwell time t_WZ often is not sufficient on average to achieve a homogeneous carbon distribution. At local

Carbon content <0.8% in the retained austenite is rather a risk of martensitic transformation before reaching room temperature. This martensite then in the structure is very hard due to its carbon content, ensures a very poor bending behavior and must therefore be prevented become. Therefore, the invention here preferably sees a comparably slow, within a cooling time of at least 5 seconds

ongoing cooling down. The slower, at least 5 s amounting cooling times t_AB also contribute to the dimensional accuracy of the component, in which distortion is avoided. The upper limit of 200 seconds for the range over which the cooling time t_AB extends ensures acceptable cycle time in component manufacturing.

In order to prevent scale formation during austenitisation and to protect the shaped sheet metal part according to the invention against corrosion, a metallic corrosion protection coating can be applied to the flat steel product from which the sheet metal component is formed. Particularly suitable for this purpose are coatings based on aluminum, such coatings typically having contents of Si in order to optimize their protective effect (so-called "AS coatings"). Al-based protective coatings are particularly suitable

economically apply to a processed according to the invention flat steel product by hot dip coating. One for the protection of one

According to the invention processed flat steel product and associated therewith a sheet metal part according to the invention particularly suitable

Aluminum-based coating has, for example, the following composition (in% by weight): 3-15% Si, 1-15% Fe, optionally up to 40% Zn, in particular up to 10% Zn, optionally up to 1% Mg, preferably 0 , 11-0.5% Mg, as well as the balance AI and unavoidable impurities. Typical platen thicknesses of such a coating are 2 mm - 40 mm, in particular 10 mm - 35 mm, in the blank formed according to the invention into the sheet metal part prior to thermoforming.

The plate-like structure results in a combination of high

Tensile strength (> 1100MPa) with very high bending angle (uncoated clearly> 80 °, see application examples). While plate-like bainltl-like structures are already well described in the prior art, it is absolutely new that this results in very good bending properties. Besides that is absolutely new, that a cementite-poor, plate-like bainite structure with tensile strengths> 1000 MPa and excellent bending properties in very short holding times or only in air cooling and can be set within times <1 minute.

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

Fig. 1a - 1b each derived from a micrograph schematic

Representation of the microstructure of a sample according to the invention

Fig. 2 is a light micrograph of a section of a sample of an inventively processed and

composite steel in 1000 times magnification;

3 is a scanning electron micrograph of a

Micrograph of a sample produced according to the invention;

Fig. 4 is a light microscope image of a micrograph of a

sample produced according to the invention;

Fig. 5 is a diagram showing the time course of the

Austenite bainite transformation of a

alloy according to the invention at 400 ° C in the dilatometer shows.

To test the invention, melts A-0 have been produced, each of which has been formulated in accordance with the provisos of the invention and whose composition is given in Table 1.

From the melts thus composed, cold rolled steel strips have been conventionally produced. Part of the steel bands is In also conventional manner with a so-called AS coating

hot-dip coated. The AS coating each consisted of 3-15% by weight of Si, 3% by weight of Fe and the remainder being AI and unavoidable

Impurities at a coating thickness of 22 mm per side of the blank.

From the steel strips each blanks have been divided, which have been used for further experiments. In these tests, sheet metal part samples 1 to 24 in the form of 200 x 300 mm ² large plates were thermoformed from the respective blanks. For this purpose, the blanks in a heating device, such as a conventional

Heating oven, have been heated from room temperature to an austenitizing temperature T_Aust at which they have been heated and maintained for an austenitizing time t_Aust. Subsequently, the blanks are removed from the heating device and placed in a heated to a tool temperature T_WZ forming tool. The resulting from the removal of the heating device, the transport to the tool and the insertion into the tool composed transfer time was 7 s each.

In the forming tool, the blanks have been reshaped to the respective sheet metal part.

Except for the samples 5, 22 and 23, the resulting Blechfbrmteile are then removed from the forming tool and tempered in a tempering to a holding temperature T_Halt and kept at the temperature T_Halt over a holding period t_Halt to homogenize the

Temperature distribution and a uniform Bainitumwandlung too

guarantee.

The sample 5 is not a temperature homogenization (step f1 of the method according to the invention), but after a within a

Transfer time t_Trans carried out transfer into Fast heating tool has been subjected to a rapid heating, in which they at a heating rate HR on a

Homogenization temperature T_HOM has been heated.

Finally, the samples have been cooled to room temperature. Cooling took place either in still air at a cooling rate of 7 K / s or in compressed air at 30 K / s.

Samples 22 and 23 were removed from the mold after reaching the cooling stop temperature and cooled in still air.

In addition, some of the samples have undergone cathodic dip coating (KTL) to test their paintability on the one hand and to investigate whether the mechanical properties change as a result of the KTL treatment. A comparison of Samples 3 and 4 shows that the KTL has hardly any influence on the mechanical properties even in the case of samples cooled in still air

The parameters "coating", austenitizing temperature "T_Aust" provided or set during the processing of samples 1 to 24,

Austenitizing time "t_Aust", tool temperature "T_WZ", duration of the cooling process "t_WZ" in the forming tool, cooling stop temperature

T_cool stop, holding temperature "T_Halt", holding time "tJHalt", transfer time "t_Trans", heating rate "HR", homogenizing temperature "T_HOM", "air cooling" and cathodic dip painting "KTL" are given in table 2.

In addition, Table 3 shows the sheet thickness D of the blanks from which the individual samples 1 to 24 were produced, as well as those determined on the obtained samples 1 to 24 according to DIN EN ISO 6892-1: 2009

Yield strength Rp02, tensile strength Rm and elongation at break A50, the direction of the tensile test relative to the rolling direction or the bending axis relative to the rolling direction ("Q" = transverse), the determined according to VDA standard 238-100 bending angle BW_Fmax, which has been calculated in each case according to the formula specified in the standard of the stamp path (the angle BW_Fmax is the bending angle at which the force in the bending test her Maximum has), and the corrected bending angle BW specified. The corrected bending angle BW_korr is calculated according to the formula:

and takes into account that the bending angle depends very much on the thickness. At the corrected bending angle, the influence of the thickness is eliminated.

In Table 4, finally, given in the obtained samples 1 - 24 certain microstructural proportions are given, in the column "bainite total" the entire bainite, in the column "plate-like bainite" the proportion of the plate-like bainite in the context of the invention, in the column "Martensite" is the proportion of martensitic constituents and column RA is the proportion of total retained austenite in the total structure.

Figures 1 a, 1 b and 2 have already been explained above.

In the example of FIG. 3, which is a scanning electron micrograph of a section of a microstructure of a sample obtained from the alloy of the melt A, exemplary embodiment 1, the areas removed by etching are bainitic ferrite (bF). , The overhanging areas are one of the carbon rich phases Restaustenite (RA) or Cementite (Z). These have in common that they are dating at least

85 wt .-% Fe and at least 0.6 wt .-% C exist. Due to the high carbon content, they can hardly be etched and are still high almost to the polishing level. Both carbon-rich phases hinder

Displacement movements in bainitic ferrite and thus cause a

Increase in strength. At even higher resolution, retained austenite and Cementite differ in that cementite by its higher

Carbon content has an even higher resistance to the etchant than the retained austenite, so that the Zementitanteile have a smooth surface, while the surface of austenite appear rather rough.

From the example of the alloy "O", it can be seen from FIG. 5 that in the case of a steel alloyed according to the alloy concept according to the invention, the transformation from austenite to bainite proceeds particularly rapidly. This not only has process engineering advantages, but also makes it possible to use very low Si contents.

The latter allows an AS coating that fits well on the particular

Steel substrate adheres, as shown in Fig. 4 using a light microscope

Illustration of a micrograph derived from the sheet metal blank from which the sheet metal part sample no. 7 has been generated.

FIG. 5 shows the dilatometer curve of the bainitic transformation at 400 ° C., which is determined using the example of the alloy "0 ^u . According to this, the bainitic transformation is already 25% advanced after 10s and already 66% after another 10s.

Table 2

Table 3

Claims

1. sheet metal part having a tensile strength Rm of at least 1000 MPa and a bending angle of more than 70 ° and that of a

Is formed of flat steel product which consists of (in% by weight):

and in each case optionally additionally from one or more elements from the group "Ti, Nb, V, B, Ni, Cu, Mo, W" with the proviso

- The structure of the sheet metal part to 40 - 100 area%

consists of plate-shaped pronounced bainite, the - to 70 - 95% of ferrite,

- To 2 - 30% of carbon-rich phases, which formed at least 70% plate-shaped with a plate length PL of at least 200 nm at a ratio of plate length PL to plate width PB of the plate-shaped carbon-rich phase PL / PB of at least 1, 7 and in one Distance are arranged, which is 50 nm to 2 mm, as well

- the remainder consists of less than 5% of other constituents,

- Where not by the plate-shaped bainite

occupied residual structure of the sheet metal part up to 40 area% of the total structure consists of non-plate-shaped bainite,

- to 70 - 95% of ferrite, to 2 - 30% of carbon rich phases and

- less than 5% is composed of other ingredients,

- where the sum of the parts of the plate-shaped and not

2 - 20% by volume. and

wherein the remainder of the microstructure of the sheet metal part not occupied by the bainitic constituents consists of one or more constituents of the following group: martensitic or austenitic constituents, proeutectoid ferrite, iron carbides, iron nitrides,

Transition metal carbides, transition metal nitrides, non-metal carbides, Non-metallic, metallic or non-metallic inclusions, sulphides and other unavoidable impurities ".

2. Sheet metal molding according to claim 1, characterized

the C content is% C, the Mn content is% Mn, the Mo content is% Mo, the Cr content is% Cr, the Ni content is% Ni, and the Cu content is% Cu depending on the B content of the steel each in wt% such that the activation energy Qb of bainite formation is <45 kJ, with Qb at B contents of up to 0.0005 wt%

3. Sheet metal molding according to one of the preceding claims, characterized

in that the steel constituting the flat steel product from which the sheet metal part is formed comprises (in% by weight) 0.13-0.25% C, 0.6-1.4% Si, 0.9 - 1, 8% Mn, 0.01 - contains 0.1% Al and 0.15 - 0.75% Cr.

4. Sheet metal molding according to one of the preceding claims, characterized

in that the steel constituting the flat steel product of which the sheet metal part is formed comprises (in% by weight) 0.15-0.20% C, 0.7-1.0% Si and 1.1. Contains 1.6% Mn.

5. Sheet metal molding according to one of the preceding claims, characterized in that it is a metallic

Korroslonsschutzüberzug is provided.

6. Blechformtell according to claim 5, characterized in that the corrosion protection coating of (in wt .-%) 3 - 15% Si, 1 - 3.5 Fe, optionally up to 40% Zn, up to 0.5 of one or more alkali and / or alkaline earth metals, optionally containing up to 1% Mg, balance AI and unavoidable impurities.

7. Sheet metal molding according to one of the preceding claims, characterized in that it is hardened.

8. A method for producing a sheet metal part comprising the following

Steps: a) Provision of a sheet metal blank consisting of a steel of the following composition (in% by weight):

W: 0.001 - 1, 0%,

and as residual iron and unavoidable impurities, with impurities less than 0.05% S and less than 0.01% N; b) heating the blank such that at least 30% of the

Volume of the blank when inserting into one for a

Hot press forming provided forming tool (step c)) have a lying above the Ac1 temperature T_Aust temperature, wherein the Ac1 temperature according to the formula

Cr content,% Mo = Mo content and% Ni = Ni content of the respective steel of the blank;

c) placing the heated blank in the on one

Tool temperature T_WZ tempered from 200 - 430 ° C

Forming tool, wherein the transfer time required for the removal and the insertion of the blank t_Trans is at most 20 s; d) hot press forming the blank to the sheet metal part, wherein the

Blank in the course of hot press forming over a duration t_WZ of 1-50 s with a cooling rate of more than 10 K / s r_WZ cooled to the cooling stop temperature T_Kühlstopp and optionally held there; e) removing the cooled to the cooling stop temperature T_Kühlstopp sheet metal molding from the tool; fl) optional: holding the sheet metal part at a holding temperature T_Halt of 300-450 ° C for a holding period tj-ialt of up to 100 s; f2) optional: heating the sheet metal part within 1 - 10 s to a homogenization temperature of 380 - 500 ° C; f3) optional: further forming of the sheet metal part; g) optional: trimming the sheet metal part; h) cooling the sheet metal part to a cooling temperature T_AB of

less than 200 ° C within a cooling time t_AB of 0.5 - 200 s.

9. The method according to claim 8, characterized in that for the temperature reached in step b) T_Aust applies

in which

10. The method according to claim 8 or 9, characterized in that the blank in step b) is durcherwärmt to the temperature T_Aust.

11. The method according to any one of claims 9 and 10, characterized

characterized in that for the heating in step b) a heating time t_Aust of

is complied with.

12. The method according to any one of claims 8-11, d ad u rch characterized in that the cooling rate r_WZ in step d) is more than 20 K / s.

13. The method according to any one of claims 8 or 12, characterized

characterized in that the duration t_WZ in step d) is at most 20 s.

14. The method according to any one of claims 8 to 13, d a d u r c h

in that the temperature T_WZ of the tool during insertion of the blank as a function of the cooling stop temperature T_cool stop and the thickness D of the blank to be formed into the sheet metal part is determined as follows:

15. The method according to any one of claims 8 to 14, d a d u rc h

characterized in that the temperature T_Kühlstopp the

Sheet metal part at the time of removal from the tool is 300 - 450 ° C.

16. The method according to any one of claims 8 to 15, characterized

characterized in that the cooling in step h) takes place in air.

17. Use of a procured according to one of claims 1 -7

Sheet metal part as part of a body or a chassis of a

Vehicle.