EP4298255A1 - High-strength hot-rolled flat steel product having high local cold formability and a method of producing such a flat steel product - Google Patents

Abstract

It is an object of the present invention to provide a high-strength hot-rolled flat steel product and a method of producing such a flat steel product, and hence to achieve, based on the steel, a combination of high strength with simultaneously high local cold formability and high economic viability. This is achieved by a high-strength hot-rolled flat steel product with high local cold formability, having a tensile strength Rm of at least 760 MPa, a yield point ratio of at least 0.8 and a hole expansion ratio of at least 30%, advantageously at least 40%, particularly advantageously at least 50%, an elongation at break of at least 10%, preferably at least 16%, a measure of cold formability of at least 0.12, advantageously at least 0.17, and a ratio of local and global cold formability of at least 5 and at most 13, and a microstructure consisting of more than 50% by volume of bainite and up to 10% by volume, advantageously up to 5% by volume, of carbon-rich microstructure constituents such as martensite, residual austenite, perlite, residual precipitation-hardened ferrite, with the following chemical composition of the steel (in % by weight): C: 0.04 to 0.08; Si: 0.1 to 0.6; Mn: 1.0 to 2.0; P: max. 0.06; S: max. 0.01; N: max. 0.012; Al: up to 0.06; Ti: up to 0.18 and/or Nb: up to 0.08; Mo: up to 0.35; with Ti+Nb more than 0.06, where there is a superstoichiometric proportion of carbon and nitrogen according to the following formula: 1.0 < (C/12+N/14) / (Ti/48+Nb/93+Mo/96), balance: iron including unavoidable steel-accompanying elements.

Description

High-strength, hot-rolled flat steel product with high local cold workability and a method for producing such a flat steel product

The invention relates to a high-strength, hot-rolled flat steel product with high local cold workability. Furthermore, the invention relates to a method for producing such a flat steel product.

In the present case, cold formability is understood to mean formability at a temperature in the range from 10° C. to 700° C., preferably between 10° C. and 200° C., particularly preferably between 10 and 80° C. and particularly preferably at room temperature between 15 and 40° C .

In particular, the invention relates to a high-strength, micro-alloyed, predominantly bainitic hot strip with an optimized alloy composition and microstructure, which is used, for example, as a chassis component in the automotive industry.

The invention also relates to high-strength hot strip with tensile strengths of at least 760 MPa and at the same time high cold workability.

The description of cold formability is complex and can only be adequately quantified by an AND-linked combination of characteristic values.

The following characteristic values are therefore used to describe the cold formability of the flat steel product according to the invention:

1. Elongation at break (A)

2. Hole Expansion Ratio (LA)

3. Degree of cold formability (FL)

4. Ratio of local to global cold formability (LFR)

Parameters such as elongation at break, uniform elongation and hole expansion ratio are established parameters for describing cold formability.

In order to quantify high global cold workability and high local cold workability, it is necessary to use both a local and a global cold workability parameter. In the context of the invention, the The characteristic value hole expansion ratio was selected as a representative of the local formability and the characteristic value uniform elongation as a representative of the global cold formability. In this case, the true sizes are given and not the technical (percentage) sizes, the determination of which is given in the description of the exemplary embodiments.

The invention particularly includes flat steel products made of steels with a multi-phase structure, which essentially contains, ie a proportion of more than 50% by volume, bainite and which have a yield point ratio of at least 0.8. In addition to a high tensile strength of at least 760 MPa and an elongation at break A of at least 10%, the flat steel product also has a high hole expansion capacity with a hole expansion ratio LA of at least 30%, a degree of cold workability FL of at least 0.12 and a local to global ratio Cold workability LFR in the range of at least 5 and at most 13.

As is known, bainitic steels are steels which are characterized by a comparatively high yield point and tensile strength with a sufficiently high elongation for cold forming processes. Good weldability is given due to the chemical composition. The microstructure typically consists of bainite as the predominant component with portions of ferrite. The microstructure may occasionally contain small amounts of other phases, such as martensite and retained austenite.

Such a steel is disclosed, for example, in the published application DE 102012 002 079 A1. The disadvantage here, however, is that the hole expansion capacity is not yet sufficiently high.

The highly competitive automotive market is forcing manufacturers to constantly find solutions to reduce fleet consumption and CO2 emissions while maintaining the greatest possible level of comfort and passenger protection. On the one hand, the weight reduction of all vehicle components plays a decisive role, but on the other hand, the best possible behavior of the individual components under the high static and dynamic loads both during use of an automobile and in the event of a crash.

By providing high-strength to ultra-high-strength steels with strengths of up to 1050 MPa or more and due to the reduction in sheet thickness that can be achieved with these steels, the weight of the vehicles can be reduced while at the same time improving the forming behavior of the steels used during production and operation.

High-strength steels must therefore meet comparatively high requirements in terms of their strength, ductility and energy absorption, without their processing, such as stamping, hot and cold forming, thermal hardening (e.g. air hardening, press hardening), welding and/or surface treatment, e.g. a metallic refinement, organic coating or painting, disadvantages occur compared to conventional steels.

Newly developed steels therefore have to meet the increasing material requirements for yield strength, tensile strength, hardening behavior and elongation at break with good processing properties such as formability and weldability, in addition to the required weight reduction through reduced sheet thicknesses.

To ensure the required reduction in sheet thickness, a high-strength steel with a single- or multi-phase structure must therefore be used to ensure sufficient strength of the motor vehicle components and to meet the high forming and component requirements in terms of toughness, edge crack resistance, improved bending angle, bending radius and energy absorption.

Improved joint suitability in the form of better general weldability, expressed by a larger usable welding area in resistance spot welding and improved failure behavior of the weld seam (fracture pattern) under mechanical stress, as well as sufficient resistance to delayed cracking due to hydrogen embrittlement, is also increasingly required.

The hole expansion capacity is a material property that describes the resistance of the material to crack initiation and crack propagation during forming operations in areas close to edges and previously sheared, such as when collaring.

The hole expansion test is normatively regulated in ISO 16630, for example. Accordingly, holes punched in sheet metal are widened by means of a dome. The measured variable is the change in the hole diameter, based on the initial diameter, to the diameter at which the first crack through the sheet occurs at the edge of the hole.

Improved resistance to edge cracks means an increased formability of the sheet edges and is described by an increased hole expansion capacity. This fact is known under the synonyms "Low Edge Crack" (LEC) or under "High Hole Expansion" (HHE) as well as xpand®.

Patent specification EP 3516 085 B1 discloses a method for producing a high-strength hot-rolled steel strip with a tensile strength of at least 570 MPa, preferably at least 780 MPa, with which good cold-formability of the steel strip is to be achieved. The procedure comprises the following steps:

- casting a slab, followed by the step of reheating the solidified slab to a temperature between 1050 and 1260°C;

- hot rolling of the steel slab with an entry temperature in the last finishing stand between 980 and 1100 °C;

- finish rolling with a finish rolling temperature between 950 and 1080 °C;

- Cooling of the hot-rolled steel strip with a primary cooling rate between 50 and 150 °C/s to an intermediate temperature on the ROT (outlet roller conveyor) between 600 and 720 °C and followed by:

- mild heating of the steel between 0 and +10 °C/s by latent heat resulting from the phase transformation from austenite to ferrite or;

- holding the steel isothermally, or;

- gentle cooling of the steel, resulting in an overall temperature change rate in the secondary stage of the ROT of -20 to 0 °C/s; - to reach the coiling temperature between 580 and 660 °C.

The steel known from this consists of (in weight %): between 0.015 and 0.15 C; at most 0.5 Si; between 1.0 and 2.0 Mn; at most 0.06 P; at most 0.008S; at most 0.1 Al sol; at most 0.02N; between 0.02 and 0.45V; optionally one or more of: at least 0.05 and/or at most

0.7 Mo; at least 0.15 and/or at most 1.2 Cr; at least 0.01 and/or at most 0.1 Nb; optionally Ca in an amount consistent with calcium treatment inclusion control matches; remainder Fe and unavoidable impurities; and wherein the steel has a substantially single-phase ferritic microstructure containing a mixture of polygonal ferrite (PF) and acicular/bainitic ferrite (AF/BF) and wherein the total volume fraction of the sum of the ferrite constituents is at least 95% and wherein the ferrite constituents are mixed with fine Composite carbides and/or carbo-nitrides of V and optionally of Mo and/or Nb are precipitation strengthened.

However, it has been found that the cold-formability, in particular the local cold-formability, of this flat steel product is not yet sufficiently large. In addition, the alloy concept is comparatively expensive.

Furthermore, from the patent specification EP 3492 611 B1 a method for the production of hot-rolled steel is known, with a tensile strength of at least 950 MPa and a microstructure which comprises bainite with an area ratio of 70% or more, the difference being one or both of the following is: martensite with an area ratio of 30% or less, and optionally ferrite with an area ratio of 20% or less, the method comprising the following steps:

- heating steel with the chemical composition to a temperature of at least 1250 °C,

- hot rolling of the steel at a final rolling temperature of 850 - 930 °C,

- quenching the steel to a coiling temperature of 450 - 575 °C,

- steel coiling at coiling temperature,

- cooling the steel, and

- skin pass rolling

The steel has the following alloy composition in % by mass,

C 0.07-0.10, Si 0.01-0.25, Mn 1.5-2.0, Cr 0.5-1.0, Ni 0.1-0.5, Cu 0.1- 0.3, Mo 0.01-0.2, Al 0.01-0.05, Nb 0.015-0.04, V 0-0.1, Ti 0-0.1, difference Fe and unavoidable impurities. The steel used is comparatively expensive due to the addition of chromium, copper and nickel and also does not yet have sufficiently high cold workability. Local cold formability is not discussed.

Proceeding from this, the present invention is based on the object of creating a high-strength, hot-rolled flat steel product and a method for producing such a flat steel product, and so, based on the steel, a To achieve a combination of high strength with high local cold formability and high cost-effectiveness.

This object is achieved by a high-strength, hot-rolled flat steel product having the features of claim 1 and a method for producing a flat steel product having the features of claim 13. Advantageous refinements of the invention are specified in the dependent claims.

According to the invention, a high-strength, hot-rolled flat steel product with high local cold formability, having a tensile strength Rm of at least 760 MPa, a yield point ratio of at least 0.8 and a hole expansion ratio of at least 30%, advantageously at least 40%, particularly advantageously at least 50%, has an elongation at break of at least 10%, preferably at least 16%, a degree of cold workability of at least 0.12, advantageously at least 0.17 and a ratio of local to global cold workability of at least 5 and at most 13, and a structure consisting of more than 50% by volume Bainite, up to 10% by volume, advantageously up to 5% by volume, high-carbon microstructural components such as martensite, retained austenite, pearlite, retained austenite, remainder precipitation-hardened ferrite, with the following chemical composition of the steel (in percent by weight):

C: 0.04 to 0.08 Si: 0.1 to 0.6 Mn: 1.0 to 2.0 P: 0.06 max. S: 0.01 max. N: 0.012 max. AI: up to 0.06

Ti: up to 0.18 and/or Nb: up to 0.08 Mo: up to 0.35 with Ti+Nb more than 0.06% by weight, with a superstoichiometric proportion of carbon and nitrogen according to formula 1 below being present :

1.0 < (C/12+N/14) / (Ti/48+Nb/93+Mo/96),

Rest iron including unavoidable elements accompanying steel, with optional addition of one or more elements of Cr, Ni, V, B or Ca, whereby the structure distribution over the thickness of the steel flat product in the three areas close to the surface, 1/4 thickness and ¹ 2 thickness of the Steel flat product is characterized by - an absolute deviation of a maximum of 12, advantageously a maximum of 7% by volume of the proportion of ferrite in the areas close to the surface or 1/4 thickness of the flat steel product, in relation to the area 1/2 thickness of the flat steel product, and/or

- the deviation in the aspect ratio in the rolling direction in the three areas of the steel flat product, from the mean, is less than 0.3 in each of the three areas, and/or

- in the three areas the hardness difference HV0.1 is a maximum of 20 HV 0.1, advantageously a maximum of 15 HV 0.1, even more advantageously a maximum of 10 HV 0.1 compared to the mean value over the entire thickness of the steel flat product, an excellent combination of Strength, elongation and forming properties.

For the measurements of microstructure distribution and hardness across the thickness of the steel flat product, it is irrelevant from which side of the surface these are carried out.

In the case of the optional addition of one or more of the elements Cr, Ni, V, B and Ca, it is provided in particular that Cr up to 0.6% by weight, Ni up to 0.6% by weight, V up to 0.2 wt%, B: up to 0.01 wt% and Ca up to 0.01 wt%. The microstructure preferably consists of more than 50% by volume of bainite and the remainder of precipitation-strengthened ferrite.

In particular, the flat steel product is characterized by a combination of high strength and excellent cold formability. In addition, the production of this flat steel product according to the invention based on the alloying elements C, Si, Mn, Nb and/or Ti is comparatively inexpensive.

The steel flat product according to the invention is specifically characterized by a high elongation at break A of at least 10%, a high hole expansion ratio (LA) of at least 30%, advantageously at least 40%, particularly advantageously at least 50%, a measure of cold workability (FL) of at least 0, 12, advantageously at least 0.17 and a ratio of local to global cold workability (LFR) of at least 5 and at most 13, while at the same time having a high tensile strength of at least 760 MPa.

To set optimized combinations of properties, the steel alloy has a advantageous further development of the invention optionally one or more elements of Cr, Ni, V or B with the following contents in % by weight: Cr: more than 0.1 up to 0.6, Ni: more than 0.1 up to 0 ,6, V: more than 0.01 up to 0.2 and B: more than 0.0005 up to 0.01, where there is a superstoichiometric proportion of carbon and nitrogen according to the following formula 2:

1.0 < (C/12+ N/14) / ( P/48+ N b/93+M 0/96+ V/51 )

In a further advantageous development of the invention, the steel is alloyed with Ca for inclusion control. As a result, the inclusions of MnS and Al ₂ O ₃ , which are unfavorable with regard to the final properties, are replaced by inclusions containing Ca that are less harmful, in particular with regard to the morphology. The addition to the steel is a maximum of 0.01% by weight.

In a further advantageous development of the invention, the flat steel product contains the following alloy composition in % by weight in order to achieve particularly favorable combinations of properties:

Ti: at least 0.02, advantageously at least 0.04, more advantageously at least 0.06, Nb: at least 0.01, Mo: at least 0.05 and Ti + Nb: up to 0.2.

The microstructure consists mainly of bainite and, to a lesser extent, of ferrite. The bainite is a mixture of constituents characterized by a main constituent of at least 50% by volume and minor constituents, the main constituent being bainitic ferrite formed by precipitations of (Ti, Nb, Mo)(C,N) or V( C,N) is hardened and the secondary components consist of carbon-rich components such as martensite, retained austenite, lower bainite and pearlite. The structure advantageously consists of more than 75% by volume of bainite.

In addition, the structure may contain carbon-rich structural components.

Particularly favorable properties are only achieved if the microstructure contains a maximum of 10%, preferably a maximum of 5%, carbon-rich microstructure components (e.g. martensite, retained austenite, pearlite).

It has also proven to be advantageous if the grain elongation of all structural components in the rolling direction, measured at a position at 1/2 the thickness of the flat steel product below the surface of the flat steel product, is characterized by the area-average aspect ratio of all microstructure components in the rolling direction of at most 2.0, and/or the mean value over the three areas close to the surface, 1/4 thickness and 1/2 thickness of the steel flat product is at most 2.0, advantageously 1.6.

It has also proven to be advantageous for high cold workability if, on average, half of the precipitations of (Ti, Nb, Mo)(C,N) or V(C,N), which strengthen the ferrite and the main component of bainitic ferrite a diameter of less than 10 nm and/or the precipitates have a mean distance of less than 750 nm.

It is also advantageous if the ratio of shear texture components to rolling texture components increases towards the surface and has the following values:

- near the surface: at least 0.9

- 1/2 thickness of the steel flat product: maximum 0.1

The hot-rolled flat steel product according to the invention can be provided with a metallic or non-metallic coating. The metallic coating can be applied to the flat steel product electrolytically or by hot dipping and is advantageously zinc-based.

Such a hot-rolled flat steel product is advantageously used in the automobile industry for the production of components, in particular chassis components. Hot-rolled flat steel products according to the invention have thicknesses of 1.6 to 6.0 mm. However, thicknesses less than 1.6 mm or greater than 6.0 mm are also covered by the invention.

The flat steel product according to the invention advantageously has a tensile strength Rm of at least 760 MPa along the rolling direction, a yield point ratio of at least 0.8, an elongation at break A of at least 10%, preferably at least 16%, a hole expansion ratio of at least 30%, advantageously at least 40% or even at least 50%. The degree of cold workability is at least 0.12, advantageously at least 0.17, with a ratio of local and global cold workability of at least 5 and at most 13. Alloying elements are usually added to the steel in order to specifically influence certain properties. An alloying element can influence different properties in different steels. The effect and interaction generally depends heavily on the amount, the presence of other alloying elements and the state of the solution in the material. The connections are varied and complex. The effect of the alloying elements in the alloy according to the invention will be discussed in more detail below.

In the case of the figures given below and in the claims for alloying element contents and all other figures, the figures should be included as basic values. The use of the term "up to" in the content ranges, e.g. 0.01 to 1% by weight means that the basic values, here 0.01 and 1 are included.

Carbon C: Needed for the formation of carbides, especially in connection with the so-called micro-alloying elements Nb, V and Ti, promotes the formation of martensite and bainite, stabilizes austenite and generally increases strength. Higher contents of C deteriorate the welding properties and lead to the deterioration of the elongation and toughness properties, which is why a maximum content of at most 0.08% by weight is specified. In order to achieve sufficient material strength, a minimum addition of 0.04% by weight is required.

Manganese Mn: Stabilizes austenite, increases strength and toughness. Higher contents of > 2.0% by weight Mn increase the risk of central segregation, which significantly reduces ductility and thus product quality. Lower contents <1.0% by weight do not allow the required strength and toughness to be achieved at the desired moderate analysis costs. Therefore, the content of Mn is specified to be 1.0 to 2.0% by weight.

Aluminum AI: Used for deoxidation in the steelworks process. The amount of AI used depends on the process. Therefore, no minimum Al content is specified. An Al content of more than 0.06% by weight significantly impairs the casting behavior in continuous casting. This results in greater effort when casting. Therefore, the content of Al is set to 0.06% by weight at maximum Silicon Si: Is one of the elements that enable steel to be strengthened by solid solution strengthening in a cost-effective way. However, Si reduces the surface quality of the hot strip by promoting firmly adhering scale on the reheated slabs, which can only be removed with great effort or only insufficiently if the Si content is high. This is particularly disadvantageous during subsequent galvanizing. Therefore, the Si content is limited to a maximum of 0.6% by weight. For the effectiveness of Si, a lower limit of 0.1% by weight is to be regarded as reasonable.

Calcium Ca: is added to the steel for inclusion control to prevent the formation of unfavorable inclusions of MnS and Al203 and to form, with these elements, less harmful Ca-containing inclusions in terms of morphology. The addition to the steel is a maximum of 0.01% by weight.

Micro-alloying elements are usually only added in very small amounts (<0.2% by weight per element). In contrast to the alloying elements, they mainly act through the formation of precipitates, but can also influence the properties in the dissolved state. Despite the small amounts added, micro-alloying elements have a strong influence on the targeted manufacturing conditions as well as the processing and end properties of the product.

Typical micro-alloying elements are, for example, niobium and titanium. These elements can be dissolved in the iron lattice and form carbides, nitrides and carbonitrides with carbon and nitrogen. Since the micro-alloying elements are comparatively expensive, the alloyed proportion is kept as low as possible. On the other hand, the carbon that is over-stoichiometric and therefore not bound in precipitations of the micro-alloying elements in carbon-rich structural components contributes to the cost-effective and necessary increase in strength. Therefore, the over-stoichiometric proportion of carbon and nitrogen calculated according to formula 1: (C/12+N/14) / (Ti/48+Nb/93+Mo/96) is set to > 1.

The effect of Nb and Ti depends in particular on the process control during hot rolling and the subsequent cooling process. With the addition of micro-alloying elements, the aim is to achieve grain refinement in the course of the process and to produce precipitations in the nanometer size range. Hence an Nb+Ti content of more than 0.06% by weight is a prerequisite for achieving the desired strength and good elongation properties. A cumulative value of more than 0.2% by weight, on the other hand, no longer improves the properties of the steel, since contents above the specified cumulative value in the specified analysis and when using conventional furnaces can no longer be dissolved when the slabs are reheated and thus have no positive effect.

Niobium Nb: The addition of niobium has a grain-refining effect, in particular due to the formation of carbides in the rolling process, which at the same time improves strength, toughness and elongation properties. In addition, very fine Nb-containing precipitations can be formed after the phase transformation, which contribute significantly to the strength of the product. At contents of more than 0.08% by weight, saturation occurs, which is why a maximum content of less than or equal to 0.08% by weight. -% is provided. A minimum content of 0.01% by weight is provided for sufficient effectiveness.

Titanium Ti: As a carbide former, it has a grain-refining effect, which improves strength, toughness and elongation properties at the same time. Contents of Ti of more than 0.18% by weight impair the ductility and the hole expansion capacity by the formation of very coarse, primary TiN precipitates, which is why a maximum content of 0.18% by weight is specified. A minimum content of 0.02, advantageously 0.04, even more advantageously 0.06% by weight is provided for sufficient effectiveness.

Molybdenum Mo: Increases hardenability and reduces the critical cooling rate, thus promoting the formation of fine, bainitic structures. In addition, even the use of small amounts of Mo delays the coarsening of fine precipitates, which should be as fine as possible to increase the strength of micro-alloyed structures. A minimum content of 0.05% by weight is provided for sufficient effectiveness and is limited to a maximum of 0.35% by weight for cost reasons.

Phosphorus P: Is a trace element from iron ore and is dissolved in the iron lattice as a substitution atom. Phosphorus increases hardness through solid solution strengthening and improves hardenability. However, attempts are usually made to reduce the phosphorus content as much as possible, since it is, among other things, very susceptible to segregation and greatly reduced toughness. The accumulation of phosphorus at the grain boundaries can cause cracks to appear along the grain boundaries during hot rolling. In addition, phosphorus increases the transition temperature from tough to brittle behavior by up to 300 °C. However, the use of small amounts of P can also be used to increase the strength at low cost through targeted measures that are precisely controlled on the process side. For the above reasons, the phosphorus content is limited to a maximum of 0.06% by weight.

Sulfur S: Like phosphorus, it is bound as a trace element in iron ore. It is generally undesirable in steel because it leads to undesirable inclusions of MnS, which degrades elongation and toughness properties. Attempts are therefore made to achieve the lowest possible amounts of sulfur in the melt and, if necessary, to convert the elongated inclusions into a more favorable geometric shape by means of a so-called Ca treatment. For the above reasons, the sulfur content is limited to a maximum of 0.01% by weight.

Nitrogen N: Is also a secondary element from steel production. Steels with free nitrogen tend to have a strong aging effect. Even at low temperatures, the nitrogen diffuses at dislocations and blocks them. It thus causes an increase in strength combined with a rapid loss of toughness. Binding of the nitrogen in the form of nitrides is possible, for example, by alloying aluminum, niobium or titanium. As a result, however, the alloying elements mentioned are no longer available for the new formation of small precipitations, which are very efficient in terms of strength, in the later process. For the above reasons, the nitrogen content is limited to a maximum of 0.012% by weight.

Chromium Cr: As an optional alloyed element, Cr improves strength and reduces corrosion rate and retards ferrite and pearlite formation. The maximum content is set at a maximum of 0.6% by weight, since higher contents result in a deterioration in ductility. A content of more than 0.1% by weight is intended for sufficient effectiveness.

Nickel Ni: The optional use of even small amounts of Ni promotes ductility while maintaining strength. Because of the comparatively high cost, the content of Ni is limited to at most 0.6% by weight. For a sufficient Effectiveness, a level of more than 0.1% by weight is contemplated.

Vanadium V: With the present alloy concept, the addition of vanadium is not absolutely necessary. For cost reasons, the vanadium content is limited to a maximum of 0.2% by weight. If V is nevertheless intended to be added, the over-stoichiometric proportion of carbon and nitrogen is calculated using formula 2: (C/12+N/14) / (Ti/48+Nb/93+Mo/96+V/51) to > 1 fixed. A V content of more than 0.01% by weight is then also provided for sufficient effectiveness.

Boron B: B is an effective hardenability enhancing element that is effective in very small amounts. The martensite start temperature remains unaffected. To be effective, boron must be in solid solution. Since it has a high affinity for nitrogen, the nitrogen must first be bound, preferably with the stoichiometrically required amount of titanium. Due to its low solubility in iron, the dissolved boron tends to accumulate at the austenite grain boundaries. There it partially forms Fe-B carbides, which are coherent and lower the grain boundary energy.

Both effects delay the formation of ferrite and pearlite and thus increase the hardenability of the steel. Excessively high levels of boron are harmful, however, since iron boride can form, which has a negative effect on the hardenability, formability and toughness of the material.

For the above reasons, the boron content for the alloy concept according to the invention is limited to values of a maximum of 0.01% by weight. A content of more than 0.0005% by weight is intended for sufficient effectiveness.

A method according to the invention for producing a hot-rolled flat steel product with high local cold formability, having a tensile strength Rm of at least 760 MPa, a yield point ratio of at least 0.8 and a hole expansion ratio of over 30%, advantageously at least 40%, particularly advantageously at least 50%, a measure the cold formability of at least 0.12, advantageously at least 0.17 and a ratio of local and global cold formability of at least 5 and at most 13, comprising the steps:

- Melting of a steel melt containing (in weight %):

C: 0.04 to 0.08 Si: 0.1 to 0.6

Mn: 1.0 to 2.0

P: 0.06 max

S: 0.01 max

N: 0.012 or less

AI: up to 0.06

Ti: up to 0.18 and/or

Nb: up to 0.08

Mo: up to 0.35 with Ti+Nb greater than 0.06 and with a superstoichiometric ratio of carbon and nitrogen according to Formula 1 below: 1.0<(C/12+N/14)/(Ti/48+ Nb/93+Mo/96) with optional alloying of one or more elements of Cr, Ni, V, B or Ca, balance iron including unavoidable steel-accompanying elements

- Casting of the molten steel into a slab or thin slab by means of a horizontal or vertical slab or thin slab casting process,

- Reheating of the slab or thin slab to 1100 °C to 1270 °C and subsequent hot rolling of the slab or thin slab with the following consecutive steps:

- Rolling in the last rolling pass to a hot strip to the required final thickness at a final rolling temperature EWT where the following applies:

EWT ≥ EWTmin = 682 °C + 464 C + 6445 Nb - 644 x Nb0.5 + 732 V - 230 V0.5 + 890 Ti +363 AI - 36 Si (formula 3)

- Cooling with an average cooling speed of 30 K/s to 150 K/s

- Coiling the hot strip into a coil at a coiling temperature HT that is low enough to set the advantageous structural components with

HT < HTmax = 761 °C - 217 x C - 77 x Mn + 97 x Si - 47 x Mo - 53 x Cr - 34 x Ni - 21 x V (formula 4) and on the other hand is suitable for sufficient precipitation strengthening in the subsequent time-dependent Cooling process T(t) defined by (Formula 5) 17000 < HP < 18800 with HP(T, t) = T(t) x (ln(t) + 20), where the temperature T is in K and the duration t is specified in h.

- Cooling in a cooling process T(t) with an average cooling rate of 5 K/h to 50 K/h between coiling temperature and 100°C, with subsequent cooling in still air to room temperature. In the case of the optional addition of one or more of the elements Cr, Ni, V, B and Ca, it is provided in particular that Cr up to 0.6% by weight, Ni up to 0.6% by weight, V up to 0.2 % by weight, B: up to 0.01% by weight and Ca up to 0.01% by weight.

The idea on which the invention is based is explained below and described in more detail using examples.

Today, thermomechanical rolling is usually used to produce high-strength, micro-alloyed hot strip. The finish rolling takes place in a low temperature range of less than EWTmin, in which the austenite no longer recrystallizes and as a result the accumulated dislocations lead to a high nucleus density at the beginning of the phase transformation and thus produce a fine hot strip structure. A key goal of thermomechanical rolling is to increase strength and ductility by reducing the grain size of the hot strip structure.

In alloys with a superstoichiometric ratio of carbon and nitrogen to microalloy elements, the carbon, in contrast to nitrogen, is not completely precipitated in the form of strength-increasing microalloy precipitates. The carbon that is not precipitated in microalloy precipitates leads to the formation of carbon-rich structural components and different carbon-rich components of bainite. It is crucial for cold workability that the carbon-rich structural components and the carbon-rich components of the bainite are advantageously present in terms of size and distribution. Advantageously means that the size is small and the distribution is as uniform as possible.

In addition to different alloy compositions, different processes are also used to achieve a balanced relationship between local and global cold workability.

There are essentially three process paths that can be distinguished in which the type and distribution of the carbon-rich structural components and the state of precipitation of the micro-alloying elements are adjusted. The type and distribution of carbon-rich structural components affect the cold formability and the state of precipitation of the micro-alloying elements affects the strength.

The process paths are

1. Low coiling temperatures of e.g. 450 < HT < 550 °C lead to the setting of a low-temperature bainite with components of carbon-rich components that are very finely distributed, e.g. lower bainite. The resulting product has high cold formability with a pronounced proportion of local cold formability (“high local cold formability”). However, the strength is comparatively low, since at the low coiling temperatures only a small proportion of micro-alloying elements are precipitated and the contribution to precipitation hardening is correspondingly small.

2. High coiling temperature of e.g. HT > 650 °C to set a ferritic structure. The carbon is in the form of hard structural components such as carbides, pearlite or martensite. The resulting product has high cold formability with a lower proportion of local cold formability. The strength is higher because a higher proportion of micro-alloying elements are precipitated.

3. An average coiling temperature of e.g. 550 < HT < 650 °C to produce a mixed structure consisting of high-temperature bainite (e.g. upper bainite and granular bainite) and ferrite with both high local cold workability and high strength due to a high proportion of precipitation has not been expedient up to now. Either only high cold formability was achieved or only high strength was achieved through a high proportion of precipitation.

Within the scope of the present investigations, it was found to be essential that the predominantly bainitic, micro-alloyed hot strip exhibits both high strength and high local cold formability when, in combination with the alloy composition and a super-stoichiometric ratio of 1.0 < (C/12+N /14) / (Ti/48+Nb/93+ Mo/96), the steel flat product is finish-rolled with a final rolling temperature of at least EWTmin according to formula 3 and then coiled and cooled in a temperature-time window that is characterized by a maximum coiling temperature HTmax according to formula 4 and characterized by 17000 < HP < 18800, where HP is calculated by Formula 5. During investigations, it was found that the hot-rolled steel flat product has a strength contribution to the tensile strength of at least 80 MPa or more due to precipitation formation when coiled and cooled in a temperature time window characterized by 17000 < HP < 18800 compared to a temperature time window , which is characterized by HP < 15990. The strength contribution is necessary to cost-effectively achieve high tensile strength and high yield strength ratio. At the same time, if HTmax is observed within the temperature-time window, the structure that is favorable for local formability and strength is formed.

The flat steel product according to the invention is characterized by compliance with the temperature-time window mentioned in that half of the precipitations of (Ti, Nb, Mo) (C,N) and / or V (C,N), the ferrite and the main component of bainitic Reinforce ferrite, have a diameter of less than 10 nm and/or the precipitates have an average spacing of less than 750 nm.

Surprisingly, it was found that when the specified temperature-time window is observed in combination with a final rolling temperature of at least EWTmin, the local cold workability is high, while when the specified temperature-time window is observed in combination with a final rolling temperature of less than EWTmin, the local cold workability is comparatively low.

The flat steel product produced according to the invention has, in addition to a cost-effective alloy concept, high strength and, at the same time, high local cold workability. In addition, the manufacturing method according to the invention is characterized by high process stability.

In contrast to ferrite, bainite usually consists of different components. The different components of the bainite are formed from the austenitic phase with decreasing temperature during the production of the hot strip after the final rolling. Compared to ferrite, bainite forms at lower temperatures and bainite has a higher average dislocation density. Both the high strength and the high local cold formability can only be achieved with a predominantly bainitic structure. The reason is that the bainitic structure has a high density of dislocations and a small grain size.

With a predominantly ferritic structure, the high local cold formability is not achieved. The reason is that the grain size of the ferrite is comparatively large and the over-stoichiometric carbon is precipitated in the form of very hard and comparatively coarse carbides at phase boundaries. During cold forming, these carbides lead to early material failure due to local stress concentrations.

With previously known solutions, hot strip can be achieved with either the property combination of comparatively low strength with comparatively high local cold workability or the property combination of comparatively high strength with comparatively low local cold workability.

The invention, on the other hand, allows the property combination of high strength and high local cold workability to be achieved.

The reason is that the accumulated local damage during the forming process, especially with large differences in strain, is only adequately taken into account when the true values are considered.

- Definition of true uniform elongation: In (1+Ag/100), where Ag is the technical uniform elongation

- Definition of true hole expansion ratio: In (1+LA/100), where LA is the technical hole expansion ratio

The degree of cold formability is described by the geometric mean of local and global formability:

- Definition of degree of cold formability (Formability Level "FL"): (true uniform elongation x true hole expansion ratio) ^0.5

The ratio of local and global cold formability is defined as: Local Formability Ratio (LFR) = (true hole expansion ratio / true uniform strain) The following criteria are required for high local cold formability, in particular for the area of application of the flat steel product according to the invention:

- A ≥ 10%

- LA ≥ 30%

- Degree of cold formability ≥ 0.12

- 5 < ratio of local and global cold workability < 13

The test results presented in the appendix extend to exemplary embodiments with a tensile strength of at least 760 MPa. For the exemplary embodiments, the following criteria are required for high local cold formability, especially for the area of application:

- A ≥ 16%

LA ≥ 50%

- Degree of cold formability ≥ 0.17

- 5 < ratio of local to global cold workability < 13

As part of the investigations, the mechanical-technological properties and the microstructure of the hot-rolled steel flat products were examined. In addition to tensile tests according to ISO 6892-1 to determine the tensile strength Rm, yield point Rp0.2 and elongation at break A and uniform elongation Ag, hole expansion tests according to ISO 16630 were carried out.

The following abbreviations are used in the tables in the appendix and in the following description:

EWT = final rolling temperature HT = coiler temperature MW = average leg. = alloy

GOS = Grain Orientation Spread

KAM = Kernel Average Misorientation

IQ = Image Quality (German: diffraction pattern quality)

AR = Aspect Ratio

SGV= yield ratio

S _P = Contribution to strength through the formation of precipitates S _M = strength in a predominantly bainitic structure that does not show any precipitates due to the low value of the HP parameter

For hot strip thicknesses > 3 mm, the proportional specimen shape with the designation A for elongation at break was used. In the case of hot strip thicknesses of < 3 mm, the non-proportional specimen shape with an initial gauge length of 80 mm was used. For better comparability, when using the non-proportional specimen shape, the values for the elongation at break were converted from the uniform elongation according to A=AG×b, b being previously determined using comparison specimens as 2.254. In the case of the hole expansion test, the mean value from at least 3 individual tests is always given.

For the metallographic assessment of the structure of the hot strip, the following areas were defined in different thickness ranges of the sample:

- Near-surface: measuring field with 100 μm x 100 μm at a distance of 0.1 mm from the sample surface

- 1/4 thickness: measuring field with 100 μm x 100 μm in the middle between the surface and the center of the sample - 1/2 thickness: measuring field with 100 μm x 100 μm with a distance of 0.1 mm from the center of the sample The positioning of the measuring fields can be seen in the sketch in see Figure 2.

The metallographic investigations were carried out on samples along the direction of rolling.

To characterize the microstructure, electron backscatter images (EBSD) were taken in the measuring fields defined above. For this purpose, longitudinal sections were created which were ground mechanically and polished to 1 μm. The samples were then polished with OP-S for approx. 10 minutes in order to create a surface that was as free of deformation as possible. An EDAX DigiView 5 EBSD camera with a binning of 10 x 10 and a recording rate of 140 Hz was used for the measurements, the acceleration voltage was 15 kV. The increment between the individual measuring points was 0.1 μm in each case. The parameters relevant to the invention were determined as follows: GOS (Grain Orientation Spread): Mean disorientation of all measuring points within a grain to the mean orientation of the grain. A segmentation angle of 15° is used to determine the grains.

KAM (Kernel Average Misorientation) and GKAM: To calculate the KAM values, the average misorientation of an EBSD measuring point to its next-but-one neighboring measuring points is determined. The maximum permissible misorientation is 4°. For the GKAM, the KAM values of all measuring points of a grain are averaged, whereby a segmentation angle of 15° is used to determine the grains.

The types of structure that occur are defined metallographically as follows:

- Ferrite consists of polygonal and quasi-polygonal ferrite and the grains are delimited by grain boundaries with misorientation angles > 15°. There are no small-angle grain boundaries < 15° in the grain interior of the ferrite, the values of the grain orientation spread (GOS) are < 2° and the values of the grain kernel average misorientation (GKAM) are typically < 0.4°. TEM images show a high density of (Ti, Nb, Mo)(C,N) precipitations in the interior of the grain. (Fe.Mn)-carbides can be present in particular in the area of the grain interstices.

- The granular bainite grains are delimited by grain boundaries of > 15°. Due to the displacive phase transformation of austenite into bainite, small-angle grain boundaries occur inside the grain, the GOS values are ≥ 2° and the GKAM values are typically ≥ 0.4°. In the EBSD IPF (Inverse Pole Figure) map, lancets of different orientation can typically be seen in the interior of the grain. Lancets that do not show a second phase in the EBSD Image Quality Map are referred to as "bainitic ferrite" in the following. Interspersed between the grains of bainitic ferrite is a carbon-rich second phase in the form of martensite, MA phase, lower bainite, or pearlite. (Fe,Mn) carbides can be present in particular in the area of the interstices of the grain. The surface area of the second phase decreases with increasing coiling temperature and can be 0 - 10%.

Aspect ratio: For the EBSD measurement in the electron microscope, the samples were aligned in such a way that the rolling direction corresponds to the Y-direction of the measuring field. With the help of the Matlab Toolbox MTEX, ellipses were adapted to the shape of the individual grains (segmentation angle 15°) and parameterized via their long and short semi-axes, as well as the orientation of the long axis. The ellipses was made from these parameters are calculated for each grain and then the intersection of these ellipses with the X and Y axes of the coordinate system is determined. The ratio of the X-axis intersections of the grain ellipses to the Y-axis intersections of the grain ellipses corresponds to the aspect ratio of the grains in the rolling direction to the sheet normal. This calculation method ensures that the stretching of grains whose long axis does not point exactly in the direction of rolling is only determined in the normal direction of rolling or sheet metal.

The HV0.1 hardness test was carried out on the microsection in points at different distances from the surfaces. No measurement is made at a distance from the surfaces and the center of 0.1 mm. In addition:

The hardness values are given as the average of 6 individual measurements.

3 hardness indentations for the near-surface position are positioned between 0% and 10% and 90% and 100% distance from the surface based on the thickness of the sheet.

3 hardness impressions each for the 1/4 position are positioned between 20% and 30% and 70% and 80% distance from the surface based on the thickness of the sheet. 3 hardness impressions each for the 1/2 position are positioned between 40% and 50% and 50% and 60% distance from the surface based on the thickness of the sheet.

The positioning of the hardness indentations can be seen in the sketch in Figure 2.

The alloy compositions of two exemplary embodiments are summarized in Table 1. Alloys A and B are single casts, so all examples A1-A14 and B1-B20 have the same compositions. Table 1 also shows the calculated values for the hyper-stoichiometric ratio of carbon and nitrogen to micro-alloying elements (formula 2), i.e.

1.0 < (C/12+N/14) / (Ti/48+Nb/93+Mo/96+V/51)

Table 1 shows the alloy compositions of two examples.

Tables 2 and 3 show the results from different exemplary embodiments. Also shown is an evaluation of the results with regard to achieving the required characteristic values with J (achieved) and N (not achieved). If the specifications according to the invention, according to the 2nd line in the tables, are not met, is marked with an underscore. The listed values are commercially rounded.

For alloys A and B, Table 2 lists the results for the mechanical characteristics with different process conditions. The underlined values are outside the required mechanical properties or outside the targeted process conditions.

With regard to the final rolling temperature (EWT), it must be ensured that complete recrystallization is achieved across the strip thickness in every thickness range. This is the case if EWT - EWTmin ≥ 0, where EWTmin = 682 °C + 464 C + 6445 Nb - 644 x Nb ^0.5 + 732 V - 230 V ⁰ ' ⁵ + 890 Ti +363 AI - 36 Si (formula 3). All element data are given in % by weight.

During the subsequent coiling of the strip, it must be ensured that a structure consisting of more than 50% by volume of bainite is created. This is the case when the HT - HTmax < 0, where HTmax = 761 °C - 217 x C - 77 x Mn + 97 x Si - 47 x Mo - 53 x Cr - 34 x Ni - 21 x V (Formula 4) . All elements are given in weight percent. If the conditions for EWT and HT are met, the characteristic values for formability given in Table 2 can be achieved. It turns out that a cost-effective increase in strength can only be achieved if a sufficient contribution to the tensile strength Rm is made by precipitation hardening _SP after coiling during the subsequent cooling process. For this it is necessary that during the cooling process T(t) a suitable temperature T prevails for a suitable duration t. This is given (Formula 5) when 17000 < HP < 18800 with HP(T, t) = T(t) x (ln(t) + 20), where when computing HP, T is always in K and t in h is specified.

The procedure for calculating the HP parameter is as follows:

1. The cooling process T(t) is divided into n equal periods of time t, with the associated temperatures T , where n must be selected sufficiently large so that the result remains almost the same when divided into significantly more periods of time.

2. Calculation of the individual parameters HP,=HP,(t _i ,T _i )=T _i ,x(In(t _i )+20). 3. Calculation of the time periods t* = exp (HPi/T*-20), where T* represents any temperature, for example the coiler temperature HT.4. Calculation of the parameter HP with HP = T* x (ln(ti*+t _2* +...+t _n* )+20)

The strength contribution S _P due to the formation of precipitates is determined in the following steps:

1. Determination of the strength S _M in a predominantly bainitic structure which, due to the low value of the parameter HP, does not exhibit any precipitations. With the help of TEM investigations, it was found that the state exists at HP = 15990, regardless of the alloy. As a first step, the Rm data is plotted against HP in the range 16080 < HP < 18000 for all application examples and then a linear regression is carried out. In the second step, the strength at HP = 15990 is determined using the regression lines. In the present case, this is the strength contribution S _M,A (15990) = 804 MPa for alloy A and the strength contribution S _M,B (15990) = 762 MPa for alloy B.

2. Calculation of the theoretical strength of a precipitation-free structure as a function of HP by SM (HP) = SM ( ₁₅₉₉₀ ) - _0.0495 (HP-15990)

3. Calculation of the strength contribution S _P (HP) by S _P (HP) = Rm (HP) - S _M (HP)

In the case of alloy compositions A and B, high strengths Rm are achieved in a particularly cost-effective manner, since the cooling process with the specified ranges of HP allows a strength contribution S _P through precipitation of ≥ 80 MPa.

A high local cold formability with a high strength can only be observed at 17000 < HP < 18800, but not at HP > 18800 or HP < 17000 (Table 2).

The material-related causes for the different local cold formability at high strength were analyzed using microstructure components and features in longitudinal sections.

The bainitic structure of the hot strip produced according to the invention consists of a main component of ≧50% and secondary components, the main component being bainitic ferrite, which is strengthened by precipitations of (Ti, Nb, Mo)(C,N). Transmission electronic investigations of the excretions Individual representative samples have shown that half of the precipitates of (Ti,Nb,Mo)(C,N) strengthening the main bainitic ferrite constituent have a diameter of <10 nm and/or the precipitates have an average spacing of less than have 750 nm. The minor components consist of higher carbon components such as martensite, MA phase, lower bainite, and pearlite. Since the main component has a higher formability than the secondary components, a minimum proportion of the main component of ≧50% is advantageous.

Table 3 shows the results of the microstructure investigations for alloy A for different final rolling temperatures according to formula 3, different coiling temperatures according to formula 4 and HP values according to formula 5.

The following applies to samples not processed according to the invention: The structure of the hot strip samples is inhomogeneous and anisotropic across the strip thickness. The inhomogeneity and the anisotropy can be described as follows for the two samples A2 and A6 with the HP values 17232 and 18380: a. The samples consist of a ferritic-bainitic structure. The ferrite content is 48% and 66%. b. The deviation in the proportion of ferrite in the near-surface position and the %thickness position with respect to the 1/2thickness position is at most 59% and 17%. c. The stretching of the structure is comparatively pronounced. This is especially true for item 14 thickness; here the aspect ratio is 2.9 and 2.5. i.e. The hardness over thickness varies comparatively strongly. The hardness on the surface in particular is lower and deviates by -24 HV0.1 and -26 HV0.1 from the mean value over the sample thickness. e. The shear texture components vary relatively widely, being 0.92 and 0.96 in the near-surface position and 0.01 and 0.01 in the 14 thickness position.

It is known that the local material behavior during cold forming with its high demands on local cold forming capacity is negatively influenced by microstructural inhomogeneities, since the damage is localized early and leads to material failure. In the present case, the in a. - i.e. listed features directly and indirectly to elongated areas of reduced formability, eg areas with an increased proportion of carbon-rich second phase of the granular bainite. The influence of the in e. However, the feature listed on the local cold formability is not known. The different completed recrystallization of the austenite over the thickness of the strip immediately after the last rolling step and before cooling was identified as the cause of the structural inhomogeneity across the hot strip thickness. With an increased temperature range for finish rolling to more than EWTmin but with the average coiling temperature remaining the same, it has been possible within the scope of the invention to achieve complete recrystallization across the strip thickness in every thickness range and thus to set a ferritic-bainitic structure that is homogeneous across the strip thickness.

The result is surprising, since the absence of recrystallization in the last passes of the rolling process with over-stoichiometric bainitic hot strip grades leads to a coarser structure, as expected, but the local cold formability is positively influenced, contrary to expectations.

An EWT of at least EWTmin according to formula 3 is necessary for complete recrystallization across the strip thickness in every thickness range.

The material-related causes for the high local cold formability were analyzed using microstructure components and features in longitudinal sections. The result of the structural analysis of samples A7 and A9 according to Table 3, which were processed according to the invention, is:

- The structure of the hot-rolled strip samples is comparatively homogeneous and isotropic across the strip thickness. The homogeneity and the isotropy can be described as follows for the two samples with the HP values 18380 (sample A7) and 17232 (sample A9): a. The samples consist of a predominantly bainitic structure. The ferrite content is 22% and 49%. b. The deviation in the proportion of ferrite in the near-surface position and the % thickness position with respect to the 1/2 thickness position is at most 7% and -2%. c. The structural elongation is comparatively low. This is especially true for item 14 thickness; here the aspect ratio is 1.5 and 1.5. i.e. Hardness over thickness varies comparatively little. This applies in particular to the position close to the surface. The hardness deviates from the mean value by -10 HV0.1 and 0 HV0.1 on the surface. e. The shear texture components are 0.98 and 0.98 in the near surface position and 0.01 and 0.03 in the 14 thickness position. Through further investigations, the target-oriented structure was further characterized in the following point:

At least half of the precipitates of (Ti,Nb,Mo)(C,N) strengthening the main bainitic ferrite component are <10 nm in diameter and/or the precipitates have an average spacing of less than 750 nm .

FIG. 1 shows a summary of the range of cold workability claimed according to the invention, which is limited by the data for FL and LFR.

FIG. 2 shows the positioning of the hardness indentations at a distance from the surfaces (0% and 100%): 0.1 mm and a distance from the center (50%): 0.1 mm, and the EBSD measuring fields at a distance from the surface (0%): 0.1mm Distance from center (50%): 0.1mm.

Claims

patent claims

1. High-strength, hot-rolled flat steel product with high local cold formability, having a tensile strength Rm of at least 760 MPa, a yield point ratio of at least 0.8 and a hole expansion ratio of at least 30%, advantageously at least 40%, particularly advantageously at least 50%, an elongation at break of at least 10%, preferably at least 16%, a degree of cold workability of at least 0.12, advantageously at least 0.17 and a ratio of local and global cold workability of at least 5 and at most 13, and a structure consisting of more than 50% by volume bainite and up to 10% by volume, advantageously up to 5% by volume, high-carbon microstructure components such as martensite, retained austenite, pearlite, remainder ferrite, with the following chemical composition of the steel (in % by weight):

C: 0.04 to 0.08 Si: 0.1 to 0.6 Mn: 1.0 to 2.0 P: 0.06 max. S: 0.01 max. N: 0.012 max. AI: up to 0.06

Ti: up to 0.18 and/or Nb: up to 0.08 Mo: up to 0.35 with Ti+Nb more than 0.06, with a superstoichiometric proportion of carbon and nitrogen according to the following formula:

1.0 < (C/12+N/14) / (Ti/48+Nb/93+Mo/96),

Remainder iron including unavoidable elements accompanying steel, with optional

Addition of one or more elements of Cr, Ni, V, B or Ca, whereby the structure distribution over the thickness of the steel flat product in the three areas close to the surface, 1/4 thickness and 1/2 thickness of the steel flat product, is characterized by an absolute Deviation of a maximum of 12% by volume, advantageously a maximum of 7% by volume, of the proportion of ferrite in the areas near the surface or 1/4 thickness of the flat steel product, in relation to the area 1/2 thickness of the flat steel product and/or the deviation in the aspect ratio of the Grains in the rolling direction in the three Areas of the steel flat product is less than 0.3 on average in each of the three areas, and/or in the three areas the hardness difference HV0.1 is a maximum of 20 HV 0.1, advantageously a maximum of 15 HV 0.1, even more advantageously a maximum of 10 HV 0.1 compared to the mean value over the entire thickness of the steel flat product.

2. Flat steel product according to claim 1, characterized by an addition of Ca of max. 0.01% by weight.

3. Steel flat product according to claim 1 or 2, characterized in that the steel contains (in weight %):

Ti + Nb: 0.2 max

4. Steel flat product according to at least one of claims 1 to 3, characterized in that the steel contains (in weight %):

Ti min: 0.02, preferably 0.04, more preferably 0.06 Nb min: 0.01 Mo min: 0.05

5. Steel flat product according to at least one of claims 1 to 4, characterized in that the steel contains (in weight %):

Cr: up to 0.6 Ni: up to 0.6 V: up to 0.2 B: up to 0.01 where there is a superstoichiometric proportion of carbon and nitrogen according to the following formula:

1.0 < (C/12+ N/14) / (TΪ/48+ N b/93+M 0/96+ V/51 )

6. Steel flat product according to claim 5, characterized in that the steel contains (in weight %):

Cr: more than 0.1 Ni: more than 0.1 V: more than 0.01 B: more than 0.0005

7. Steel flat product according to at least one of claims 1 to 6, characterized in that the bainite is a mixture of components, which is characterized by a main component of at least 50% by volume and secondary components, the main component consisting of bainitic ferrite, which consists of precipitations of (Ti, Nb, Mo)(C,N) and/or V(C;N) and the minor components consist of higher carbon components such as martensite, retained austenite, lower bainite and pearlite.

8. Flat steel product according to at least one of claims 1 to 7, characterized in that the structure consists of more than 50% by volume of bainite and the remainder is ferrite.

9. Flat steel product according to claim 8, characterized in that the structure consists of more than 75% by volume of bainite and the remainder is ferrite.

10. Flat steel product according to at least one of Claims 1 to 9, characterized in that the grain elongation of all structural components in a position 1/2 the thickness of the flat steel product is characterized by the aspect ratio of all structural components in the rolling direction of at most 2.0 and/or the mean value over the three areas close to the surface, 1/4 thickness and 1/2 thickness of the steel flat product is at most 2.0, advantageously 1.6.

11. Flat steel product according to at least one of claims 1 to 10, characterized in that half of the precipitates of (Ti, Nb, Mo)(C,N) and/or V(C,N) which contain the ferrite and the main component bainitic ferrite, have a diameter of less than 10 nm and/or the precipitations have an average spacing of less than 750 nm.

12. Flat steel product according to at least one of claims 1 to 11, characterized in that the ratio of shear texture components to rolling texture components increases towards the surface and has the following values:

- close to the surface: min. 0.9

- 1/2 thickness: 0.1 max

13. A method for producing a hot-rolled flat steel product with high local cold formability, having a tensile strength Rm of at least 760 MPa, a yield point ratio of at least 0.8 and a hole expansion ratio of over 30%, advantageously at least 40%, particularly advantageously at least 50%, a measure the cold formability of at least 0.12, advantageously at least 0.17 and a ratio of local and global cold formability of at least 5 and at most 13, comprising the steps:

- Melting of a steel melt containing (in weight %):

C: 0.04 to 0.08

Si: 0.1 to 0.6 Mn: 1.0 to 2.0 P: 0.06 max. S: 0.01 max. N: 0.012 max. AI: up to 0.06

Ti: up to 0.18 and/or Nb: up to 0.08 Mo: up to 0.35 with Ti+Nb more than 0.06 and wherein there is a superstoichiometric ratio of carbon and nitrogen according to the following formula: 1.0 < (C/12+N/14) / (Ti/48+Nb/93+Mo/96) with optional addition of one or more elements of Cr, Ni, V, B or Ca, remainder iron including unavoidable steel accompanying elements elements

- Casting of the molten steel into a slab or thin slab by means of a horizontal or vertical slab or thin slab casting process,

- Reheating of the slab or thin slab to 1100 °C to 1270 °C and subsequent hot rolling of the slab or thin slab with the following consecutive steps:

- Rolling in the last rolling pass to a hot strip to the required final thickness at a final rolling temperature EWT where the following applies:

EWT ≥ EWTmin = 682 °C + 464 C + 6445 Nb - 644 x Nb ^0.5 + 732 V - 230 V ^0.5 + 890 Ti +363 AI - 36 Si

- Cooling with an average cooling speed of 30 K/s to 150 K/s

- Coiling the hot strip into a coil at a coiling temperature HT that is low enough to set the advantageous structural components with HT < HTmax = 761 °C - 217 x C - 77 x Mn + 97 x Si - 47 x Mo - 53 x Cr - 34 x Ni - 21 x V and on the other hand is suitable for sufficient precipitation hardening in the subsequent cooling process T(t) to provide, defined by

17000 < HP < 18800 with HP(T, t) = T(t) x (ln(t) + 20), where T is given in K and t in h,

- Cooling in a cooling process T(t) with an average cooling rate of 5 K/h to 50 K/h between coiling temperature and 100 °C, with subsequent cooling in still air to room temperature.

14. The method according to claim 13, characterized in that the slab is hot-rolled into a steel flat product with a thickness of 1.6 mm to 6.0 mm.

15. The method according to claim 13 and 14, characterized in that the flat steel product is provided with a metallic coating electrolytically or by means of hot dipping.

16. The method according to claim 15, characterized in that the metallic coating is zinc-based.

17. The method according to at least one of claims 13 to 16 for the production of a steel flat product according to claims 1 to 12.

18. Use of a steel flat product according to at least one of claims 1 to 12 for the production of components in the motor vehicle industry.

19. Use of a steel flat product according to claim 18 for the production of chassis components.