CN112585284A

CN112585284A - Sheet metal formed part made of steel and having high tensile strength, and method for producing same

Info

Publication number: CN112585284A
Application number: CN201880093705.9A
Authority: CN
Inventors: 托马斯·格伯; 伊尔斯·赫克曼; 贝恩德·林克
Original assignee: ThyssenKrupp Steel Europe AG; ThyssenKrupp AG
Current assignee: ThyssenKrupp Steel Europe AG; ThyssenKrupp AG
Priority date: 2018-05-22
Filing date: 2018-05-22
Publication date: 2021-03-30
Also published as: US20210189517A1; EP3797176A1; WO2019223854A1

Abstract

The invention relates to a sheet metal profile having a tensile strength Rm of > 1000MPa and a bending angle of more than 70 DEG and formed from a flat steel product consisting of (in wt%): 0.10 to 0.30% of C, 0.5 to 2.0% of Si, 0.5 to 2.4% of Mn, 0.01 to 0.2% of Al, 0.005 to 1.5% of Cr, 0.01 to 0.1% of P, and optionally one or more elements selected from the group consisting of "Ti, Nb, V, B, Ni, Cu, Mo, W", wherein Ni is 0.005 to 0.1% of Nb, 0.005 to 0.1% of V, 0.001 to 0.2% of B, 0.0005 to 0.01% of B, 0.05 to 0.4% of Ni, 0.01 to 0.8% of Cu, 0.01 to 1.0% of Mo, 0.001 to 1.0% of W, and the balance of iron and unavoidable impurities, wherein 40 to 100% by area of the structure of the sheet-shaped article consists of plate-like bainite composed of 70 to 95% of PB, 2 to 30% of ferrite, 70% or more of carbon rich phase, and wherein PL forms a plate-1.7% of plate-like carbon phase width/plate width ratio, the plate length PL is greater than or equal to 200nm and is provided with a spacing of 50nm to 2 [ mu ] m, and the balance is less than 5% and is composed of other components, wherein more than 40% by area of the structure of the plate-shaped part not occupied by the plate-shaped bainite is composed of non-plate-shaped bainite composed of 70 to 95% ferrite, 2 to 30% carbon-rich phase and less than 5% of other components, wherein the proportion and more than 60% by area of the plate-shaped and non-plate-shaped bainite on the structure of the plate-shaped part, wherein the residual austenite content of the structure of the plate-shaped part is 2 to 20% by volume, and the rest of the structure is composed of other structure components. The invention also provides a method for producing such a sheet metal profile.

Description

Sheet metal formed part made of steel and having high tensile strength, and method for producing same

Technical Field

The invention relates to a sheet metal profile made of steel, having a high tensile strength Rm of at least 1000MPa and a bending angle of more than 70 deg.

The invention further relates to a method for producing such a sheet metal profile.

Background

When reference is made below to flat steel products or "sheet metal products", this is to be understood as meaning rolled products, such as steel strips or sheets, from which cut pieces or slabs are cut for the production of, for example, bodywork components. "sheet metal profiles" or "sheet metal components" of the type according to the invention are produced from such flat steel or sheet metal products, wherein the terms "sheet metal profile" and "sheet metal component" are used synonymously herein.

All steel component content data given in this application are on a weight basis unless explicitly stated otherwise. All "% -data relating to the steel alloy, which are not further specified, are therefore to be understood to be given in" wt% ". In addition to the data on the residual austenite content of the structure of the steel product according to the invention on a volume basis (given in "volume%"), the data on the content of the different structure components are each based on the respective slice area (given in area percent "area%") of the sample of the respective product, unless explicitly stated otherwise. In this context, the values for the contents of the atmosphere components are made on a volume basis (given in vol%).

Unless otherwise specifically stated, the mechanical properties reported here, such as tensile strength, yield strength, elongation, are in the range according to DIN EN ISO 6892-1: 2009 in the tensile test.

The tissue is determined at the cross-sectional section, which is subjected to 3% Nital solution (Nital) etching. In a scanning electron microscope, texture measurements are performed at 5000 x magnification to determine the fraction of plate-like and other non-plate-like bainite, and the length, width and spacing of the plates are determined at 20000 to 50000 x magnification. The proportion of retained austenite is determined by X-ray diffraction.

EP 2719786B 1 discloses a sheet metal profile and a method for producing such a sheet metal profile, which has a tensile strength of at least 980 MPa. Here, the sheet shaped part consists of steel which, in addition to iron and unavoidable impurities, consists of (in mass%) 0.15-0.4% C, 0.5-3% Si, 0.5-2% Mn, up to 0.05% P, up 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, with the proviso that the Ti content is at least 4 times the N content and at most 0.1%. According to this known method, a slab made of the steel thus composed is heated to a temperature not lower than the Ac3 temperature of the corresponding steel and not higher than 1000 ℃, and then hot-formed in a press tool to form a hot-formed sheet shaped part. During forming, the sheet formed part is cooled in the press tool at an average cooling rate of at least 20 ℃/s or higher. Here, the target temperature range of the cooling is referred to as a span, which starts from 100 ℃ below the bainite start temperature "BS", i.e., 100 ℃ below a certain temperature from which bainite is formed in the steel structure, and ends at the martensite start temperature MS, i.e., a certain temperature from which martensite is formed in the steel structure. The sheet profile is held for at least 10s in this temperature range to adjust the properties of the profile. The holding may include isothermal holding, cooling, or reheating, as long as it occurs within the temperature range. The steel product thus obtained should have a structure which consists of 70-97% (area%) bainitic ferrite, up to 27% martensite and 3-20% retained austenite, the remaining structure constituents occupying up to 5%.

Disclosure of Invention

Against the background of the prior art, the object of the invention is to provide a sheet metal shaped part which can be produced by hot forming, for example press hardening, and which has an optimized strength combined with an optimized energy absorption capacity in the event of sudden deformation loads, for example in the event of a vehicle collision.

Furthermore, a method should be specified with which such a sheet metal profile can be produced in practice.

The object of the invention is achieved on the one hand by a sheet metal profile having the features of claim 1.

On the other hand, for the manufacture of such a sheet element, the invention proposes a method as claimed in claim 8.

Advantageous embodiments of the invention are set forth in the dependent claims and are explained in detail below as well as the general inventive concept.

The sheet profile according to the invention therefore has a tensile strength Rm of at least 1000MPa and a bending angle of more than 70 ° and is formed from a flat steel product consisting of (in wt.%):

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 each optionally additionally comprising one or more elements from the group of "Ti, Nb, V, B, Ni, Cu, Mo, W",

Ni: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 consisting of iron and unavoidable impurities also including less than 0.05% of S and less than 0.01% of N,

-wherein 40-100 area% of the structure of the sheet shaped part is formed by the sheet shape

The bainite formed in a plate shape is formed by,

-70% to 95% ferrite;

-2-30% of a carbon-rich phase, at least 70% of which is structured in a plate-like manner, wherein when the ratio PL/PB of the plate length PL to the plate width PB of the plate-like formed carbon-rich phase is at least 1.7, the plate length PL is at least 200nm and an interval of 50 nm-2 μm is arranged, and

-the balance being less than 5% of other components;

wherein up to 40 area% of the remaining structure of the sheet shaped part, which is not occupied by plate-shaped bainite, consists of non-plate-shaped bainite consisting of

-70-95% of ferrite,

-2-30% of a carbon-rich phase and,

-less than 5% of other components;

-wherein the sum of the fractions of the platelike and non-platelike bainite in the structure of the sheet-shaped part is at least 60 area%;

-wherein the residual austenite content of the structure of the sheet shaped part is 2-20% by volume; and is

-wherein the remaining structure of the sheet shaped part not occupied by the bainite component is composed of one or more components of the following group: martensite or austenite components, proeutectoid ferrite, iron carbide, iron nitride, transition metal carbide, transition metal nitride, non-metal carbide, non-metal nitride, metallic or non-metallic inclusion, sulfide, and other unavoidable impurities.

The remainder, which does not consist of ferrite and carbon-rich phases, which occupies less than 5% of the bainite formed in the plate shape, for example comprises nitrides or other inclusions of microalloying elements.

Here, "bainite" refers to the transformation product of the steel constituting the sheet shaped part according to the invention, formed from austenite in the structure upon cooling. Here, bainite is not a single phase. In contrast, bainite always consists at least of bainitic ferrite and one or more carbon-rich phases.

By "platelike bainite" is understood here a mixture of ferrite and a carbon-rich phase (at least 70% platelike structure) and up to 5% of the remaining components.

In the present application, the term "carbon-rich phase" is to be understood as austenite, cementite and other carbides.

Ferrite can be readily demonstrated in the section images of samples of the corresponding sheet metal shaped parts by etching with a 3% nital solution.

The carbon-rich phase is likewise recognizable in the section after etching with a 3% ethanolic nitrate solution and can be determined by means of scanning electron microscopy. When ferrite is strongly removed by an etchant, the carbon-rich phase maintains its original shape as much as possible because it is hardly etched. In quantifying the carbon-rich phases in terms of shape, size and relative position, only phases remaining after etching, i.e. those sliced before etching, are considered. Any carbides at deeper depths that are exposed by corrosion of the ferrite are not taken into account. Otherwise, the result will depend on the depth of ferrite etched away.

The residual austenite fraction of the entire structure is usually determined by means of micro-diffraction. The cementite being of stoichiometric composition Fe₃The most stable and important iron carbide of C.

The cementite as part of the carbon-rich phase is not determined separately, but is determined together in the overall carbon-rich phase.

The structure of the sheet metal profile according to the invention consists of:

i) platelike bainite (40-100 area% in the overall structure), wherein 70-95% of the corresponding proportion of platelike bainite is occupied by ferrite, 2-30% of the corresponding proportion of platelike bainite is occupied by a carbon-rich phase which is at least 70% platelike structure, wherein the plate length is at least 200nm, the ratio of the plate length to the plate width is at least 1.7 and a spacing of 50nm to 2 μm is arranged, and the remaining proportion of platelike bainite, which is less than 5%, is occupied by other components, which may be nitrides of microalloying elements or other inclusions;

ii) further bainitics, i.e. non-platelike bainite, such as globular bainite, wherein these further non-platelike bainite can occupy up to 40 area% of the overall structure, and wherein here 70-95% of the non-platelike bainite is also occupied by ferrite, 2-30% of the non-platelike bainite is occupied by a carbon-rich phase, and the corresponding share of less than 5% of the platelike bainite as the remainder is occupied by other components, such as nitrides or other inclusions of microalloying elements;

and

iii) as a balance martensite or austenite constituents, including tempered martensite, untempered martensite or austenite, and as a further balance eutectoid ferrite, iron carbide, iron nitride, transition metal carbide, transition metal nitride, non-metal carbide, non-metal nitride (e.g. boron carbonitride), metal inclusion, non-metal inclusion, sulfide and unavoidable impurities, wherein it is understood that in the industrial sense the share of the balance involved in the overall structure may also be "0", i.e. practically undetectable, or as small as not having a technical effect.

The bainite portions defined above in i) and ii) (the portion of bainite formed in plate form and the portion of additional bainite not formed in plate form) are arranged in the structure of the sheet metal shaped part according to the invention in such a way that they amount to at least 60 area% of the structure of the sheet metal shaped part. In addition to the bainite component provided according to the invention, a martensite component of up to 30 area% can be tolerated in the structure of the sheet metal shaped part according to the invention, the martensite component being optimally as small as possible, i.e. in particular less than 20 area% or less than 5 area%.

It is therefore essential to the invention that the bainite content in the structure of the sheet metal shaped part according to the invention is optimally formed in the form of plates of more than 50%. This means that the bainite component concerned is present as a plate of bainitic ferrite and of carbon-rich phases such as retained austenite and cementite.

To illustrate the basic structure of the sheet metal profile according to the invention, reference is made to fig. 1a and 1 b. In these figures, the possible configuration (konconfiguration) of the carbon-rich phase is shown in black each. The white areas between the black carbon-rich phases represent ferrite. In the white region, there may be any number of additional precipitates, and the maximum length thereof in the slice is 200 nm.

As can be seen from fig. 1a and 1b, it is true for the carbon-rich phase of the bainite formed in plate form according to the invention that at least 70% of the bainite is formed in plate form. The 70% plate-formed carbon-rich phase has a length PL of at least 200nm and the ratio of the length PL to the width PB is at least 1.7 times greater than the length PB of each plate (PL/PB > 1.7). The dimensions of the carbon-rich phase of the bainite formed in plate form are determined in such a way that the ferrite plates between them are sufficiently far apart from one another to avoid a simple bypass (Umgehung) due to dislocations. In addition, in order to obtain ductility, a stretched sheet is required (PL/PB > 1.7; FIG. 1a, FIG. 1 b). Lump formation (PL/PB < 1.7) will lead to an increase in crack sensitivity under shear stress.

The latter is particularly disadvantageous under bending loads. The spacing PA between two carbon-rich phase plates oriented adjacent to each other and parallel must be at least 50nm, preferably at least 100nm, and at most 2 μm. The interval PA represents the effective grain size of bainitic ferrite. The smaller the grain size, the greater the resistance to deformation and, consequently, the greater the strength of the associated structural component. For sufficient strength, the distance is not allowed to be greater than 2 μm, preferably not greater than 1.2 μm. If the distance PA is less than 50nm, the intensity is so strong that the region is hardly deformed, since a critical crack stress is reached in the entire tissue. This will lead to brittle material failure, which should be avoided. Here, two plates K are considered to be "oriented in parallel" when the orientations of the longest sides of the respective observed plates deviate from each other by less than 25 °.

The tissue properties according to the invention have a number of advantages which lead to an extraordinary combination of strength and bendability:

i. high strength of at least 1000MPa is obtained by the fineness of the structure and not by brittle components such as martensite. According to the hall-peck relationship, the strength increases with decreasing grain size. In the plate member according to the invention, the maximum vertical distance between the two closest carbon-rich plates represents the effective grain size. Two essential components of the structure are austenite and bainitic ferrite, both of which have high deformability. If cementite is to be additionally formed, it is finer than austenite. Therefore, even if the cementite itself is a very hard and brittle phase, it hardly deteriorates the bending property. The tissue is again very homogeneous, which is decisive for good bendability, observed over a large range (> 100 μm). Figure 2 shows an optical micrograph at 1000 times magnification of a sample section of steel processed and composed according to the invention. Very good macroscopic homogeneity can clearly be seen.

The function of the austenite in the structure of the sheet metal shaped part according to the invention is mainly to prevent the free movement of dislocations due to ferrite and thus to generate a higher resistance to deformation (═ strength). At the same time, during deformation, cracks are absorbed by the layer structure, so that they do not grow to the critical crack length and do not lead to premature failure on bending.

The high fineness of the austenite ensures its mechanical stability against deformation-induced martensite formation, in addition to the high carbon content regulated by the carbon partitioning at 350-450 ℃. The formation of coarse, brittle martensite will significantly deteriorate the flexibility. The risk of a large martensite fraction forming in the structure of the sheet metal component according to the invention is doubly limited by the particular fineness of the retained austenite: on the one hand, the small grain size leads to a further reduction of the Ms temperature, so that less austenite transforms into martensite upon deformation. If martensite is still formed, on the other hand, the martensite should also be so fine that the adverse effect on the mechanical properties remains limited.

For the plate-like ferrite in the structure of the sheet shaped part according to the invention, it applies that the plate-like ferrite is regularly interrupted by the carbon-rich phase plate in such a way that at any point the carbon-rich plate is removed by at most 1 μm, preferably at most 0.6 μm. This condition is far enough to limit the travel distance of the dislocations in the ferrite, thereby setting an exceptionally high strength of the sheet profile according to the invention due to the very fine effective grain size.

The invention allows hot-dip coating of sheet metal profiles, in particular application of aluminum-based protective coatings, due to its relatively low Si content of at most 2 wt.%, preferably at most 1.4 wt.%, particularly preferably at most 1 wt.%. With the application of such a coating, the forming process can be carried out in an operationally reliable manner in the atmosphere without scale formation and the associated problems arising therefrom.

The composition of the flat steel product from which the sheet metal profile according to the invention is formed is selected such that a tensile strength of at least 1000MPa, in particular at least 1100MPa, can be achieved with optimum deformability of the flat steel product, wherein a tensile strength of 1200MPa or more is generally achieved.

Meanwhile, the sheet member according to the present invention has a bending angle of more than 70 ° measured according to VDA 238-. Such a high bending angle means that, when the sheet metal molding is subjected to a sudden, intense deformation load, as occurs in the event of a crash into an obstacle or the like, i.e. in a typical accident situation, a high energy absorption is achieved by bending on the sheet metal molding according to the invention, which is used, for example, as a body component in a passenger car or a transport vehicle.

The above-described combination of properties can be achieved in particular in that the component according to the invention is quenched by a die

As explained in detail below, the heat is removed from the component in a mold so rapidly that the structure defined according to the invention is adjusted, by means of which the requirements of the properties achieved according to the invention are provided.

The special properties of the sheet metal profile according to the invention make it particularly suitable for use as part of the body or chassis of a vehicle, in particular a land vehicle.

In particular, as a basis for this combination of properties, the steel of the sheet metal shaped part according to the invention comprises the essential components (C, Si, Mn, Al, Cr, P, Fe) and optionally additional, i.e. optionally present, optional components (Ti, Nb, V, B, Ni, Cu, Mo, W). The contents of the individual components of the steel forming the sheet metal profile according to the invention are determined according to the invention as follows:

carbon ("C") is present in the steel constituting the sheet profile according to the invention in an amount of 0.10-0.30% by weight. The C content thus set contributes to the quenchability of the steel by delaying the formation of ferrite and bainite and stabilizing the residual austenite in the structure. A carbon content of at least 0.10 wt.% is required to obtain sufficient quenchability and concomitant high strength. However, starting from a C content of more than 0.30 wt.%, bainite formation is significantly delayed, so that a sufficient conversion cannot be ensured during the holding time or air cooling provided according to the invention. In order to obtain particularly high strength of bainite, low transformation temperatures are required. This is in turn limited downwards by the martensitic transformation, which in turn can be transferred to lower temperatures by C. C lowers the Ac3 transformation temperature and the martensite start temperature MS by an amount set according to the invention. In order to be able to use the positive effect of the presence of C particularly reliably, the C content can be set to at least 0.13% by weight, in particular at least 0.15% by weight. At these contents, a strength of at least 1000MPa, in particular at least 1100MPa, can be reliably achieved under other conditions in which the invention is considered. If it is to be avoided that a high C content has an adverse effect on the properties of the sheet metal shaped part according to the invention, this can be achieved by limiting the C content to a maximum of 0.25 wt.%, in particular to a maximum of 0.20 wt.%. Maintaining a lower upper limit for the C content is particularly helpful for improving weldability, since at lower C contents a greater hardness difference between the weld nugget and the material surrounding the sheet member is avoided.

In the steel of the flat steel product according to the invention, silicon ("Si") is used in a content of 0.5-2.0 wt.%, for suppressing cementite precipitation. Si is practically insoluble in cementite, so that nucleation is significantly reduced in the presence of sufficient Si content. An Si content of less than 0.5 wt.% will not be sufficient to suppress cementite precipitation from bainitic ferrite at the holding temperature specified according to the present invention. The retained austenite can furthermore be stabilized by the Si content of at least 0.5 wt.% specified according to the invention. This effect can be further enhanced by increasing the Si content to at least 0.6 wt.%, in particular to at least 0.7 wt.%. In this case, a Si content of at least 0.7 wt.% opens up a large processing window during hot forming, since it significantly slows down the decomposition of the retained austenite. However, if the Si content exceeds 2.0% by weight, the surface quality and coatability of the sheet shaped part obtained according to the invention will be too strongly reduced. If the sheet metal profile or the flat steel product forming the sheet metal profile according to the invention is to be provided with a hot dip coating, the Si content can be expediently limited to a maximum of 1.4 wt.%, in particular to a maximum of 1.0 wt.%, in order to avoid coating problems. This applies in particular to the case where the Al base is to be hot dip coated with a Si-containing melt. At the same time, the lower Si content allows the flat steel product constituting the plate member according to the invention to be austenitized at lower temperatures. It has surprisingly been shown here that in low-alloy steels composed according to the invention, it is possible to stabilize large amounts of austenite at Si contents of less than 1 wt.%.

Manganese ("Mn") is present in the sheet metal shaped part according to the invention in a content of 0.5 to 2.4 wt.%. Mn acts as a strengthening element by strongly delaying ferrite and bainite formation. In addition, it stabilizes the residual austenite (austenitbldner) and inhibits the decomposition of the residual austenite into cementite and ferrite after bainite transformation. For manganese contents below 0.5 wt.%, the austenite is not sufficiently stabilized, so that later decomposition of the austenite occurs for the Si content used. By increasing the Mn content to at least 0.9 wt.%, in particular to at least 1.1 wt.%, the austenite stability can once again be increased significantly, since in combination with the other alloying elements provided according to the invention, the formation of a greater microstructure content above the maximum holding temperature provided according to the invention can be prevented. However, if the manganese content is increased to more than 2, 4% by weight, the bainite transformation is slowed down considerably, so that during the method according to the invention the holding temperature specified according to the invention must be maintained for too long a time in order to achieve the transformation of the structure of the sheet metal shaped part according to the invention to preferably a bainite structure according to the invention of more than 60 area%. If at the same time optimum weldability is to be achieved, this can be achieved by limiting the Mn content to at most 2.0 wt.%, in particular to at most 1.8 wt.%. It has proven to be particularly advantageous for the Mn content to be at most 1.6 wt.%, in particular less than 1.6 wt.%, since the bainite transformation proceeds so rapidly that, due to the subsequent recalescence, after removal from the press tool, the expenditure for additional heating of the workpiece, which is necessary if necessary, is eliminated in order to keep the sheet metal component at the bainite transformation temperature for a sufficiently long time after press forming. In many applications, additional heating can even be dispensed with completely by selecting a suitable C content of up to 0.2% by weight for the Mn content thus set and at a suitable plate thickness of at least 1.2 mm.

In the production of the steel which forms the sheet metal profile according to the invention, aluminum ("Al") is used as a deoxidizing agent in a content of 0.01 to 0.2% by weight. In order to reliably bind the oxygen contained in the steel melt, at least 0.01% by weight of Al is required. Furthermore, Al can additionally also be used for incorporation of N contents which are not desired in the sheet product according to the invention, but which are unavoidable as a result of manufacturing. At the same time, Al suppresses the generation of cementite in the structure of the sheet formed part. However, with too high an Al content, the Ac3 temperature also shifts significantly upward. Starting from a content of more than 0.2 wt.%, Al will excessively hinder austenitization. In order to reliably avoid the adverse effect of Al in the steel of the sheet metal profile according to the invention in practice, the Al content can be limited to a maximum of 0.1 wt.%.

Chromium ("Cr") contributes to the hardness of the steel of the sheet metal profile according to the invention in that the transition to diffusion is slowed down during cooling to the holding temperature predetermined according to the invention and thus a stable process in hot forming is ensured. This advantageous effect arises from a content of 0.005% by weight, wherein a content of at least 0.15% by weight has proven effective in practice for reliable process control. However, too high Cr content affects coatability of the steel. The Cr content of the steel of the sheet metal profile according to the invention is therefore limited to a maximum of 1.5 wt.%, in particular 0.75 wt.%.

Phosphorus ("P") in an amount of 0.01 to 0.1 wt.% is required in the steel of the sheet metal shaped part according to the invention to compensate for the reduced Si content in order to suppress cementite nucleation. P isolates grain boundaries, lattice defects, and other sites that are commonly used as nucleation sites for cementite. In this way, P substitutes for the carbon present at the site of interest and correspondingly locally reduces the carbon concentration at the potential cementite nucleation sites, resulting in a reduced thermodynamic driving force for precipitation there, with consequent inhibition of cementite precipitation. This effect occurs starting from a P content of at least 0.01% by weight and increases with increasing P content. However, too high a phosphorus content may impair the weldability, coatability and notch impact work of the steel forming the sheet member according to the invention. Therefore, according to the invention, its maximum P content is limited to 0.1% by weight.

Titanium ("Ti") is optionally contained in the steel of the sheet metal shaped part according to the invention in a content of 0.005 to 0.1 wt.%, in order to bind nitrogen and in this way to enable boron, which is also optionally present in an effective content, to exert its strong ferrite formation inhibiting effect. Meanwhile, Ti as a microalloying element contributes to grain refinement. In order to utilize these positive effects, a Ti content of at least 0.005 wt.% can be provided, wherein the Ti content is optimally adjusted to be at least equal to 3.42 times the N content of the steel. However, Ti also tends to form coarse TiN, and can significantly reduce cold rollability and recrystallizability. Therefore, the Ti content, if present, is limited to at most 0.1 wt.%.

Niobium (Nb) may also be added, optionally in amounts of 0.005-0.1 wt.%, to the steel of the sheet shaped part according to the invention, like Ti, for grain refinement and reduction of cementite precipitation. However, at a content of more than 0.1% by weight, Nb also deteriorates the re-crystallizability.

Optionally, vanadium (V) may also be added to the steel of the sheet metal profile according to the invention in a content of 0.001 to 0.2 wt.%, in order to additionally increase the strength. In addition, V contributes to stabilization of retained austenite. However, in cold strip manufacturing, V forms vanadium carbides, which must be dissolved during austenitization of the material prior to hot forming. This is ensured by limiting the V content to a maximum of 0.2 wt.%. During the formation of bainite, vanadium in the solution precipitates in the size of a few nanometers, thus contributing to the strength by precipitation hardening. In order to obtain a sufficient driving force, a V content of at least 0.001 wt.%, in particular more than 0.01 wt.%, is required.

Also optionally, boron ("B") may be present in the steel of the component according to the invention in an amount of 0.0005-0.01 wt.%, in order to improve the quenchability of the steel. B is located on the grain boundaries and thus lowers its energy. Thereby inhibiting the nucleation of ferrite. To obtain a significant effect, a B content of at least 0.0005 wt.% is required. However, at contents of more than 0.01 wt.%, boron carbide or boron carbonitride are increasingly formed, which in turn are preferred nucleation sites for ferrite nucleation and again reduce the hardening effect.

Nickel ("Ni"), which is also optionally present in the steel of the component according to the invention, is an austenite constituent, which improves the stability of the austenite and thus the process stability under longer holding times during the formation of bainite. In the case of copper in the steel of the component according to the invention, the negative effect of copper on the hot-rollability can be eliminated by the simultaneous presence of Ni. Already helpful here is a small amount of Ni of at least 0.05% by weight. In contrast, at a Ni content of more than 0.4 wt%, a delay in bainite formation may occur.

Optionally, copper ("Cu") may also be added to the steel of the component according to the invention to increase the quenchability. For this purpose, at least 0.01% by weight of Cu is sufficient. Cu also improves the barrier of uncoated sheet material to atmospheric corrosion. However, a Cu content higher than 0.8 wt% significantly deteriorates the hot rolling property due to the low melting point Cu phase at the surface.

Molybdenum ("Mo") may optionally be present in the steel of the sheet profile according to the invention in an amount of 0.01 to 1.0 wt.%, in order to improve the process stability. Mo significantly retards the formation of ferrite and has only a slight effect on the formation of bainite in the temperature window proposed by the present invention. Starting from a content of at least 0.01 wt.%, molybdenum-carbon clusters are formed dynamically on the grain boundaries up to ultra-fine molybdenum carbides, which effectively prevents mobility of the grain boundaries and thus diffuse phase transitions. In addition, the grain boundary energy is reduced and the ferrite nucleation rate is consequently reduced. At contents above 1.0 wt.%, the significant increase in the Mo action utilized here no longer occurs.

Tungsten ("W") may also optionally be present in the steel of the plate member according to the invention. It acts here like Mo, but is already effective at smaller contents. Therefore, already at a W content of 0.001 wt.% a positive influence on the quenchability is produced. Starting from a content of 1.0 wt.%, no significant increase in the effect of W on the key properties is observed.

Nitrogen ("N") and sulfur ("S") are in principle undesirable, since they adversely affect the properties of the steel of the sheet metal profile according to the invention. However, N and S inevitably enter the steel due to manufacturing condition limitations. They therefore belong to the inevitable impurities of the steel, which should themselves be kept so low (N content < 0.01 wt%; S content < 0.05 wt%) that they have no negative effect on the properties of the steel.

According to the above description, in the steel constituting the flat steel product forming the sheet shaped part, the contents of C, Si, Mn, Al and Cr are set within the following content ranges (in wt%):

c0.13-0.25%, especially 0.15-0.20%,

0.6 to 1.4 percent of Si, especially 0.7 to 1.0 percent of Si,

0.9 to 1.8 percent of Mn, especially 1.1 to 1.6 percent,

Al:0.01–0.1％,

Cr:0.15–0.75％。

in the method according to the invention for producing a sheet metal component according to the invention obtained in the above-described manner, at least the following working steps are carried out:

a) providing a slab consisting of steel having the following composition (in weight%): 0.10 to 0.30% of C, 0.5 to 2.0% of Si, 0.5 to 2.4% of Mn, 0.01 to 0.2% of Al, 0.005 to 1.5% of Cr, 0.01 to 0.1% of P, and optionally additionally consisting of one or more elements selected from the group of "Ti, Nb, V, B, Ni, Cu, Mo, W", respectively, with the proviso that Ti is 0.005 to 0.1%, Nb is 0.005 to 0.1%, V is 0.001 to 0.2%, B is 0.0005 to 0.01%, Ni is 0.05 to 0.4%, Cu is 0.01 to 0.8%, Mo is 0.01 to 1.0%, W is 0.001 to 1.0%, the balance consisting of iron and unavoidable impurities including less than 0.05% of S and less than 0.01% of N;

b) heating the blank in such a way that at least 30% of the volume of the blank has a temperature T _ Aust above the temperature Ac1, when the blank is placed in a forming tool provided for hot-forming (step C)), wherein the temperature Ac1 is determined according to the formula,

Ac1＝[739–22*％C-7*％Mn+2*％Si+14*％Cr+13*％Mo+13*％Ni]℃

wherein% C-Si-Mn-Cr-Mo-Ni-Mn-Cr-Ni-Si-content of the respective steel of the blank;

c) placing the heated blank into a forming tool which is adjusted to a tool temperature T _ WZ of 200-430 ℃, wherein the transfer time T _ Trans required for taking out and placing in the blank is at most 20 s;

d) hot-press forming the blank into a sheet shaped part, wherein the blank is cooled during the hot-press forming at a cooling rate r _ WZ of more than 10K/s to a cooling stop temperature T _ Kklstopp in a duration T _ WZ of 1 to 50s and optionally held there;

e) removing the sheet metal shaped part from the tool, which has been cooled to a cooling stop temperature T _ Kklstopp;

f1) optionally: keeping the plate forming part at the holding temperature T _ Halt of 300-450 ℃ for the duration T _ Halt to 100 seconds;

f2) optionally: heating the plate forming part to the homogenizing temperature of 380-500 ℃ within 1-10 seconds;

f3) optionally: other shaping of the sheet metal profile, wherein the shaping can be carried out in particular as a calibration step for improving the dimensional stability of the sheet metal profile;

g) optionally: cutting a plate forming part;

h) cooling the sheet metal formed part to a cooling temperature T _ AB of less than 200 ℃ within a cooling duration T _ AB of 0.5 to 200 s.

In the method according to the invention, a blank is thus provided which consists of a steel which consists in a suitable manner according to the above statements (step a)), which is then heated in a manner known per se so that at least 30%, in particular at least 60%, of its volume has an austenitic structure when subsequently placed in a corresponding forming tool (step b)). I.e. the transformation from ferritic to austenitic structure does not have to be completed yet when being placed in the forming tool. Conversely, at most 70% of the volume of the blank may consist of other structural elements, such as tempered bainite, tempered martensite and/or ferrite without or partial recrystallization, when placed in the forming tool. For this purpose, certain regions of the blank can be held specifically at a lower temperature level during heating than other regions. For this purpose, the heat supply can be directed specifically only at specific sections of the blank or the parts which are to be heated to a lesser extent can be shielded from the heat supply. In one part of the blank material, the temperature of which remains below the minimum temperature predetermined for the temperature T _ Aust, no or only significantly less bainite is produced during the forming process in the tool, so that the structure there is significantly softer than in the corresponding other part, in which the bainite structure is present. In this way, a softer region can be provided in the respectively shaped sheet metal profile in a targeted manner, in which region, for example, optimum toughness for the respective purpose of use is present, while the other regions of the sheet metal profile have the greatest strength.

The maximum strength properties of the sheet metal profile obtained can be achieved in that the blank is heated in working step b) to the austenitizing temperature T _ Aust, for which the austenitizing temperature applies

Ac3＜T_Aust≤1250℃

Wherein, in this variant, the minimum temperature Ac3 exceeded by the temperature T _ Aust is determined by the formula according to HOUGARDY, H.P. materials science: Steel, Vol.1: introduction, Stahleissen GmbH Press, Dusseldorf, 1984, page 229,

Ac3＝(902-225*％C+19*％Si-11*％Mn-5*％Cr+13*％Mo-20*％Ni+55*％V)℃

where% C is the respective C content of the steel constituting the billet,% Si is the respective Si content of the steel constituting the billet,% Mn is the respective Mn content of the steel constituting the billet,% Cr is the respective Cr content of the steel constituting the billet,% Mo is the respective Mo content of the steel constituting the billet,% Ni is the respective Ni content of the steel constituting the billet and% V is the respective V content of the steel constituting the billet.

By completely heating the blank sufficiently in working step b), an optimally uniform property distribution can be achieved.

For this purpose, the duration of the austenitizing treatment carried out in working step b) can be set such that, on the one hand, particle coarsening is avoided by limiting to 1000s, and, on the other hand, the rate of the austenite transformation is also taken into account, which increases significantly, in particular when the steel blank is heated above the Ac3 temperature. The austenitizing temperature T _ Aust is optimally situated at least 30 ℃, in particular at least 50 ℃ above the respective Ac3 temperature of the steel respectively constituting the blank to be deformed when heated above the Ac3 temperature. At such high austenitizing temperatures, the austenite transformation proceeds so rapidly that after reaching the relevant temperature no longer has to be maintained at this temperature to achieve a complete transformation of the structure into austenite. Instead, the blank may be transported to further processing immediately after the austenitizing temperature is reached.

The blanks thus heated are removed from the respective heating device, which may be, for example, a conventional heating furnace, an induction heating device likewise known per se or a conventional device for the thermal retention of steel components, and are transferred into the forming tool so quickly that their temperature, when reaching the tool with 600-.

In working step c), the transport of the austenitized blank from the respectively used heating device to the forming tool is preferably completed in less than 20 s. Such rapid transport is necessary to avoid excessive cooling prior to deformation.

The tool is tempered to a temperature of 200-. The tool temperature T _ WZ, which is selected in each case during the insertion of the blank, is determined as a function of the cooling stop temperature T _ kuhlstopp (at which the sheet metal profile is removed from the tool) and the sheet metal thickness D of the blank to be formed into the sheet metal profile as follows:

when cooling is carried out during the subsequent shaping of the blank into a sheet shaped part, a cooling rate of at least 10K/s, in particular at least 20K/s or at least 30K/s, is required in order to prevent the transformation of austenite before the holding temperature is reached. It is not desirable to convert to ferrite and bainite at temperatures above 100 ℃ above the targeted holding temperature T _ halt of 300-450 ℃, especially 320-430 ℃, especially 320-400 ℃, because the transformation products have significantly lower strength. This will result in lower overall strength and lower flexibility.

The blank is thus not only shaped in the tool as a sheet profile but is also simultaneously quenched to a cooling stop temperature in the range from 450-. For this purpose, the tool is tempered to a tool temperature T _ WZ of 200-430 ℃. At the cooling stop temperature, some martensite may have formed in the structure of the sheet shaped part, which may serve as nucleation sites. However, at this point, the majority of the structure still consists of unstable austenite, which then transforms very rapidly into fine bainite. By alloying Si, Al, P according to the invention, the formation of carbides is delayed, so that no or only fine carbides are precipitated. The transformation effected by the alloys specifically defined by the invention proceeds so rapidly that no long holding times are required in the temperature range from 450-. In particular, for the holding during cooling and, if necessary, in the forming tool, the invention sets a duration T _ WZ of 1 to 50s in the still closed tool at the temperature T _ Kuhlstopp reached after cooling. The production process according to the invention can be integrated into the short-stroke (kurz getektet) working cycle without any problems.

After cooling, the sheet shaped part obtained is optionally kept at a holding temperature of 300-. This holding can be carried out not only in the forming tool before removal from the forming tool, but also in a separate device after removal from the forming tool.

A particular advantage of the combination of the material according to the invention and the method according to the invention is here the shortening of the holding time t _ halt which is actually required for the formation of the bainite structure. Tests have shown that after a few seconds more than 50% of the austenite has transformed. This results in a high dimensional accuracy of the sheet metal shaped parts produced according to the invention with short processing times and excellent mechanical properties. A holding time t _ Halt of longer than 100 seconds is not economical on the one hand and is also disadvantageous for the structure of the tissue (Konstitution) on the other hand. At too long a holding time t _ Halt, an increased transition from retained austenite to cementite will occur, which deteriorates the tensile properties, in particular by reducing the elongation at break.

In particular in the case of the execution of work step f1), it can be expedient if additional shaping is optionally carried out as a further work step, which, for example, contributes to improving the dimensional stability of the sheet-metal profile.

Without a separate hold, i.e. without completing the working step f1) (t _ Halt of 0 seconds), a first part of the transition is already carried out in the tool, while a second part of the transition takes place during the cooling in the working step h), which in this case is preferably carried out as air cooling.

The invention therefore provides a method by means of which sheet-metal shaped parts can be produced whose structure is characterized by a plate-like structure. This plate-like structure results in a combination of high tensile strength (> 1000MPa, in particular > 1100MPa) and very high bending angles (> 80 ℃ uncoated). In this case, the sheet metal shaped part can be produced in a particularly short time by using the method according to the invention. In the method according to the invention, therefore, sheet metal shaped parts of high strength and optimum energy absorption capacity can be produced in a total time of less than one minute.

In order to start the transformation process used according to the invention in a operationally reliable manner, the composition of the steel can be adjusted in such a way that the activation energy Qb for bainite formation is sufficiently small. For this purpose, within the above-specified limits according to the invention, the C content% C, Mn content% Mn, the Mo content% Mo, the Cr content% Cr, the Ni content% Ni and the Cu content% Cu can each be adjusted in weight% depending on the B content of the steel in such a way that the activation energy Qb for bainite formation is less than 45kJ, in particular less than 40kJ, particularly preferably less than 35kJ, in order to achieve a sufficiently fast bainite transformation. Here, for B contents of up to 0.0005% by weight, Qb can be calculated according to the formula,

qb [ kJ ] (90 × C +10 (% Mn +% Mo) +2 (% Cr +% Ni) +1 × Cu) [ kJ/wt%) ],

for B contents of greater than 0.0005 wt.%, Qb can be calculated according to the formula,

qb [ kJ ] (90 × C +10 (% Mn +% Mo) +2 (% Cr +% Ni) +1 × Cu +2) [ kJ/wt%) ],

wherein Mn in the amount of C content% C, Mn, Mo in the amount of Mo, Cr in the amount of Cr, Ni in the amount of Ni and Cu in the amount of Cu are substituted in these formulae in weight%.

For sheet thicknesses of more than 2mm, a Qb value of less than 40kJ, for sheet thicknesses of 1-2mm, a Qb value of less than 35kJ, and for sheet thicknesses of less than 1mm, a Qb value of less than 34kJ have proved advantageous.

A lower activation energy Qb is particularly helpful when the holding time t _ hash should be kept low, in particular when the holding time t _ hash should be 0 seconds. In particular, if Qb is less than 38kJ, the thermal power, i.e. the specific heat converted per unit time, is significant and the cooling is counteracted. This heat release is sufficient to keep the component at temperature by the heat of conversion even at small Qb values. It is also possible for the component to acquire a slight temperature and to cool only after the transition has taken place, without heat being introduced from the outside.

The lower limit of the range specified for the holding temperature T _ Halt according to the invention is 300 ℃, since at temperatures below this lower limit the martensite start temperature MS itself is clearly not reached when the alloy possibilities within the scope of the invention are maximally utilized. However, since bainite should be formed as much as possible, it is necessary to avoid the formation of martensite as much as possible. A martensite fraction of more than 30 area% leads to a marked deterioration in the properties of the sheet metal shaped part according to the invention. The process control should therefore be selected such that the martensite fraction of the structure of the sheet metal shaped part according to the invention is as small as possible. The range of the holding temperature T _ halt is limited upwards to 450 c, since at higher temperatures the bainite strength will drop too much.

According to CAPDEVILA, C. et al, the Ms temperature in steel was determined by a Bayesian neural network model, ISIJ International,42:8,2002.8 months, 894-902, and the martensite start temperature of steel within the specified range according to the present invention can be calculated according to the following formula:

ms [ ° C ] (490.85-302.6% C-30.6% Mn-16.6% Ni-8.9% Cr + 2.4% Mo-11.3% Cu + 8.58% Co + 7.4% W-14.5% Si) [ ° C/wt. ]

Here, too, the C content of each steel is represented by C%, the Mn content of each steel is represented by% Mn, the Mo content of each steel is represented by% Mo, the Cr content of each steel is represented by% Cr, the Ni content of each steel is represented by% Ni, the Cu content of each steel is represented by% Cu, the Co content of each steel (not included in the steel having the composition according to the present invention) is represented by% Co, the W content of each steel is represented by% W, and the Si content of each steel is represented by% Si.

If different local contact pressures occur in the tool during the forming of the sheet metal shaped part, an uneven temperature distribution occurs in the sheet metal shaped part during its removal from the tool. In order to ensure a uniform and complete bainite transformation also in this case, the optional additional heating can homogenize the temperature distribution in such a way that the temperature over the entire sheet metal shaped part lies in the temperature range from Ms-20 to Ms +100 ℃, in particular from Ms to Ms +80 ℃.

At a particularly low holding temperature T _ halt, it may happen that, although this transformation proceeds rapidly (which is desirable), the retained austenite cannot remain stable up to room temperature due to the inhomogeneous distribution of the carbon contained therein. In order to solve this and ensure a uniform C distribution in the retained austenite, the sheet shaped part obtained can be heated to a temperature of 380-500 ℃ after the holding time required for the formation of the bainite structure, if appropriate tending to "0" (step f 2)). In the cooling after reaching the homogenization temperature involved, the carbon has sufficient time and heat energy for redistribution. The upper limit of 500 c for the homogenization temperature is due to the fact that too intense softening will occur at higher temperatures. At homogenization temperatures below 380 c, the diffusion rate will be too low. The temperature range of 420-470 ℃ has proven to be particularly advantageous for homogenizing the temperature.

After the optionally performed work step f2), an additional shaping can also be carried out, which can be carried out, for example, as a calibration step in order to further improve the dimensional stability of the sheet-metal profile.

A large cost burden in press quenching is laser cutting of the press quenched member. In conventional press quenching, the component needs to be removed from the tool below the martensite finish temperature (typically below 200 ℃) in order to achieve strength. In contrast, in the method according to the invention, the component can be removed from the tool at a temperature of more than 300 ℃, preferably at least 350 ℃. The workpiece can also be hot cut at this point of the method according to the invention (step g)) due to the reduced hardness and higher ductility at higher temperatures.

The latter is very cost-effective compared to laser cutting and leads to a significantly simplified logistics. Thus, cycle times of 1 to 20s, in particular 1 to 10s, can be achieved by the method according to the invention. If more than 20s are required for temperature homogenization (working steps d) -f2)), the relevant working step can be divided into a plurality of steps, which are carried out in succession in different units.

In the cooling of the last step h) of the method according to the invention, the obtained sheet shaped part is cooled down to a cooling temperature T _ AB of less than 200 ℃ over a duration T _ AB of 0.5 to 200 s. The duration t _ AB required for cooling and the accompanying cooling rate for cooling are set as a function of the progression of the tissue transformation in the preceding working step. In the case that a partial change should also follow during the air cooling, the cooling takes place under a medium, for example stationary or moving air, in which a relatively low cooling rate is achieved. It is to be noted here that the final cooling is decisive for a uniform carbon distribution in the retained austenite. The bainite transformation in the steel according to the invention proceeds so quickly that very good dimensional stability is obtained, while the residence time t _ WZ in the cut is often insufficient to achieve a uniform carbon distribution. In contrast, for local carbon contents in the retained austenite of < 0.8%, there is a risk of martensitic transformation before room temperature is reached. This martensite present in the structure is very hard due to its carbon content, provides very poor bending properties, and must therefore be prevented. The invention is therefore preferably provided here with a comparatively slow cooling which takes place over a cooling duration of at least 5 seconds. The slower cooling time t _ AB of at least 5s also contributes to the dimensional stability of the component, in which deformations are avoided. The upper limit of the range of the cooling time t _ AB is 200s, which ensures an acceptable cycle time in the manufacture of the component.

In order to prevent the formation of scale during austenitization and to protect the sheet metal profile according to the invention from corrosion, a metallic corrosion protection coating can be applied to the flat steel product forming the sheet metal component. Particularly suitable for this purpose are aluminum-based coatings, wherein such coatings typically have a Si content in order to optimize their protective action (so-called "AS coatings"). An Al-based protective coating can be applied particularly economically by hot dip coating to a flat steel product processed according to the invention. The aluminum-based coating, which is particularly suitable for protecting the flat steel product processed according to the invention and the accompanying sheet metal profile according to the invention, has, for example, the following composition (in% by weight): 3-15% Si, 1-15% Fe, optionally up to 40% Zn, especially up to 10% Zn, optionally up to 1% Mg, preferably 0.11-0.5% Mg, and the balance Al and unavoidable impurities. Typical application thicknesses of such coatings in the blank formed into a sheet shaped part according to the invention are 2 μm to 40 μm, in particular 10 μm to 35 μm, before thermoforming.

The combination of higher tensile strength (> 1100MPa) and very high bending angles (no coating clearly > 80 ℃ C., cf. application examples) is produced by the plate-shaped microstructure. Although the plate-like bainite structure has been fully described in the prior art, it is absolutely novel to result in very good bending properties. Further, it is absolutely novel that a plate-like bainite structure having a tensile strength of > 1000MPa and excellent bending properties and little carburization can be provided within a short holding time or only under air cooling for a period of < 1 minute.

Drawings

The invention is further illustrated by the following examples. The figures show:

figures 1a-1b respectively show schematic views of a tissue of a sample according to the invention obtained from a slice,

FIG. 2 shows an optical micrograph at 1000 times magnification of a slice of a sample of steel processed and composed according to the present invention;

FIG. 3 shows a scanning electron micrograph of a slice of a sample produced according to the present invention;

FIG. 4 shows an optical micrograph of a section of a sample produced according to the present invention;

fig. 5 shows a diagram which shows the time course of the bainite transformation from austenite for an alloy composed according to the invention at 400 ℃ in a dilatometer.

Detailed Description

For testing the invention, melts A-0 were produced, which were each composed according to the conditions of the invention and whose composition is given in Table 1.

From the melt thus composed, cold-rolled steel strips are produced in a conventional manner. A part of the steel strip is hot dip coated in the same conventional manner with a so-called AS-coating. The AS coating consists of 3-15 wt.% Si, 3 wt.% Fe and the balance Al and unavoidable impurities, respectively, and the coating thickness is 22 μm on each side of the blank.

Blanks were individually divided from the strip and used for further testing. In these tests, sheet-shaped part samples 1 to 24 were measured at 200X300 mm²The form of the large sheet is hot-formed from a corresponding blank. For this purpose, the blank is heated in a heating device, for example a conventional furnace, from room temperature to an austenitizing temperature T _ Aust, at which the blank is heated and held for an austenitizing time T _ Aust. The blank is then removed from the heating device and placed into a forming tool heated to a tool temperature T _ WZ. The transfer times consisting of removal from the heating device, transport to the tool and insertion into the tool were 7 seconds each.

In the forming tool, the blank is formed into a corresponding sheet metal profile.

The sheet metal shaped parts obtained, with the exception of samples 5, 22 and 23, were then taken out of the forming tool and tempered in a tempering tool to a holding temperature T _ Halt and held at the temperature T _ Halt for a duration T _ Halt to ensure homogenization of the temperature distribution and uniform bainite transformation.

The sample 5 is not temperature-homogenized (working step f1 of the method according to the invention), but is subjected to rapid heating after a transfer to a rapid heating tool carried out within a transfer time T _ Trans, in which the sample 5 is heated to a homogenization temperature T _ HOM at a heating speed HR.

Finally, the sample was cooled to room temperature. The cooling takes place here either in still air at a cooling rate of 7K/s or in compressed air at 30K/s.

After reaching the cooling stop temperature, samples 22 and 23 were removed from the tool and cooled in still air.

Furthermore, some samples were subjected to cathodic dip coating (KTL) to demonstrate their paintability on the one hand and to check whether the mechanical properties were altered by the KTL treatment on the other hand. Comparison of samples 3 and 4 shows that KTL itself has little effect on the mechanical properties of the samples cooled in still air.

The parameters "coating", austenitizing temperature "T _ Aust", austenitizing time "T _ Aust", tool temperature "T _ WZ", duration "T _ WZ" of the cooling process in the forming tool, cooling stop temperature T _ khlstpp, holding temperature "T _ Halt", holding time "T _ Halt", transfer time "T _ Trans", heating rate "HR", homogenization temperature "T _ HOM", "air cooling" and cathode dip coating "KTL" provided or set when processing samples 1 to 24 are given in table 2.

In addition, the sheet thickness D of the blanks from which the respective samples 1 to 24 were produced is given in Table 3, and the thickness D of the sheet material at the respective samples 1 to 24 obtained is in accordance with DIN EN ISO 6892-1: 2009, yield limit Rp02, tensile strength Rm and elongation at break a50, the direction of the tensile specimen relative to the rolling direction or the direction of the bending axis relative to the rolling direction ("Q" ═ transverse), the bending angle BW _ Fmax determined according to VDA-standard 238-100, which was calculated from the punch path in accordance with the formula given in the standard, respectively (angle BW _ Fmax is the bending angle at which the force was greatest in the bending test), and the corrected bending angle BW. The corrected bend angle BW _ korr is calculated according to the following equation:

and the bending angle is believed to depend strongly on the thickness. For the corrected bend angle, the effect of thickness is eliminated.

Finally, the structure fractions determined on the samples 1 to 24 obtained are given in table 4, wherein the total bainite fraction is given in the column "total bainite", the fraction of bainite formed in the plate form in the sense of the invention is given in the column "plate bainite", the fraction of the martensite component is given in the column "martensite", and the fraction of the total retained austenite in the entire structure is given in the column RA.

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

In the example of fig. 3, which is a scanning electron micrograph of a portion of a histological section of a specimen obtained from the alloy of melt a (example 1), the area removed by etching is bainitic ferrite (bF). The still shown region is one of carbon-rich phase Retained Austenite (RA) or cementite (Z). These have the common feature that they consist of at least 85% by weight of Fe and at least 0.6% by weight of C. Due to the high carbon content they are hardly etched and still remain high to almost polished levels. Both of these carbon-rich phases impede the movement of dislocations in the bainitic ferrite and thus lead to an increase in strength. At higher solubility, the retained austenite and cementite differ because the cementite, through its higher carbon content, has a higher resistance to the etchant than the retained austenite, so that the cementite portion has a smooth surface, while the surface of the austenite appears rougher.

As can be seen from fig. 5 by way of example of the alloy "O", the transformation from austenite to bainite takes place particularly rapidly in the steel alloyed according to the alloying concept of the invention. This not only has advantages in terms of process technology, but also enables the use of very low Si contents.

The latter allows good adhesion of the AS coating to the respective steel substrate, AS can be seen from fig. 4 by means of an optical microscopic representation of a slice from the slab from which the sheet-shaped part No. 7 was produced.

For this purpose, FIG. 5 shows the dilatometer curve of the bainite transformation at 400 ℃ measured for the alloy "O". Accordingly, the bainite transformation has proceeded to 25% after 10 seconds and to 66% after another 10 seconds.

Claims

1. A sheet profile having a tensile strength Rm of at least 1000MPa and a bending angle of more than 70 ° and being formed from a flat steel product consisting of (in 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 optionally additionally one or more elements from the group of "Ti, Nb, V, B, Ni, Cu, Mo, W", respectively, with the proviso that:

Ni: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％,

-wherein 40-100 area% of the structure of the sheet shaped part consists of platelike formed bainite consisting of,

-70% to 95% ferrite;

-2-30% of a carbon-rich phase, at least 70% of which is structured in the form of plates, wherein the carbon-rich phase is formed in the form of plates with a ratio PL/PB of plate length PL to plate width PB of at least 1.7, a plate length PL of at least 200nm and with an interval of 50 nm-2 μm arranged, and

-the balance being less than 5% of other components,

-wherein the remaining structure of the sheet shaped part not occupied by the plate-like formed bainite consists of up to 40 area% of the overall structure of non-plate-like formed bainite

-70-95% consists of ferrite,

2-30% of a carbon-rich phase, and

less than 5% of other components,

-wherein the sum of the fractions of the platelike and non-platelike formed bainite over the structure of the sheet shaped part is at least 60 area%,

-wherein the residual austenite content of the structure of the sheet metal shaped part is 2 to 20% by volume, and

2. The sheet forming section of claim 1,

depending on the B content of the steel, 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 are each configured in% by weight such that the activation energy for bainite formation Qb is < 45kJ, where for Qb at B contents up to 0.0005% by weight, this applies

Qb [ kJ ] (90 × C +10 (% Mn +% Mo) +2 (% Cr +% Ni) +1 × Cu) [ kJ/wt%) ],

for Qb at B contents of greater than 0.0005 wt.%, the application applies

Qb [ kJ ] (90 × C +10 (% Mn +% Mo) +2 (% Cr +% Ni) +1 × Cu +2) [ kJ/wt%).

3. Sheet profile according to any one of the preceding claims, characterised in that the sheet profile is a profile with a cross-sectional profile with a cross

The steel of the flat steel product forming the sheet profile comprises (in weight%) 0.13-0.25% C, 0.6-1.4% Si, 0.9-1.8% Mn, 0.01-0.1% Al and 0.15-0.75% Cr.

4. Sheet profile according to any one of the preceding claims, characterised in that the steel of the flat steel product forming the sheet profile comprises (in% by weight) 0.15-0.20% C, 0.7-1.0% Si and 1.1-1.6% Mn.

5. A sheet profile according to any one of the preceding claims, characterised in that the sheet profile is provided with a corrosion protection coating of metal.

6. Sheet shaped body according to claim 5, characterized in that the corrosion protection coating contains (in weight%) 3-15% Si, 1-3.5Fe, optionally up to 40% Zn, up to 0.5 of one or more alkali and/or alkaline earth metals, optionally up to 1% Mg, the balance being Al and unavoidable impurities.

7. The sheet shaped part according to any one of the preceding claims, characterized in that the sheet shaped part is die quenched.

8. A method of manufacturing a sheet profile comprising the steps of:

a) providing a slab consisting of steel having the following composition (in 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 each optionally additionally consisting of one or more elements from the group of "Ti, Nb, V, B, Ni, Cu, Mo, W", with the proviso that

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％，

The balance consisting of iron and unavoidable impurities including less than 0.05% S and less than 0.01% N;

Ac1＝[739–22*％C-7*％Mn+2*％Si+14*％Cr+13*％Mo+13*％Ni]℃

wherein% C-the respective C content of the steel of the billet,% Si-the respective Si content of the steel of the billet,% Mn-the respective Mn content of the steel of the billet,% Cr-the respective Cr content of the steel of the billet,% Mo-the respective Mo content of the steel of the billet,% Ni-the respective Ni content of the steel of the billet;

f1) optionally: keeping the plate forming part at a holding temperature T _ Halt of 300-450 ℃ for a duration T _ Halt until 100 seconds;

f3) optionally: other forming of the sheet forming part;

g) optionally: cutting a plate forming part;

9. Method according to claim 8, characterized in that the temperature T _ Aust reached in working step b) applies

Ac3＜T_Aust≤1250℃，

Wherein

Ac3＝(902-225*％C+19*％Si-11*％Mn-5*％Cr+13*％Mo-20*％Ni+55*％V)℃

Where% C is the C content of the respective steel of the billet,% Si is the Si content of the respective steel of the billet,% Mn is the Mn content of the respective steel of the billet,% Cr is the Cr content of the respective steel of the billet,% Mo is the Mo content of the respective steel of the billet,% Ni is the Ni content of the respective steel of the billet and% V is the V content of the respective steel of the billet.

10. Method according to claim 8 or 9, characterized in that the blank is heated to a temperature T _ Aust sufficiently in working step b).

11. Method according to either of claims 9 and 10, characterized in that, for sufficient heating in working step b), the heating time t _ Aust is maintained,

1000s/(T_Aust/℃-Ac3/℃+10)^2≤t_Aust≤1000s。

12. method according to any of claims 8-11, characterized in that the cooling rate r _ WZ in working step d) is greater than 20K/s.

13. Method according to claim 8 or 12, characterized in that the duration t _ WZ in working step d) is at most 20 s.

14. Method according to any one of claims 8 to 13, characterized in that the temperature T _ WZ of the tool is determined at the moment of introduction of the blank as a function of the cooling stop temperature T _ khlsopp and the sheet thickness D of the blank to be formed into a sheet profile as follows:

15. method according to any one of claims 8 to 14, characterized in that the temperature T _ khlsupp of the sheet profile when taken out of the tool is 300-.

16. Method according to any one of claims 8 to 15, characterized in that the cooling in working step h) is carried out in air.

17. Use of a sheet profile according to any one of claims 1 to 7 as a component of the body or chassis of a vehicle.