EP3964591A1 - Hot-rolled steel sheet product and method for producing a hot-rolled steel sheet product - Google Patents

Abstract

The invention relates to a hot-rolled flat steel product consisting of a steel with the following composition (in % by weight) - C: 0.03-0.3 % by weight - Mn: 0.4-2.5 % by weight. -%- balance iron and unavoidable impurities, and has a yield strength R_e of at least 680 MPa. Optionally, the steel includes other elements. In addition, the flat steel product does not have a pronounced yield point, i.e. the stress-strain curve is continuous. Furthermore, the invention relates to a method for producing the hot-rolled flat steel product.

Description

The invention relates to a high-strength flat steel product with a minimum yield point of 680 MPa with improved further processing properties.

Since the 1980s, the so-called HSLA steels (High Strength Low Alloy) have been used for vehicle, crane and infrastructure construction and many other applications. HSLA steels are characterized in particular by a combination of high strength and formability with relatively low alloy contents. They achieve their high strength through the addition of micro-alloying elements such as titanium, niobium or vanadium in connection with a controlled rolling and cooling process. Due to their low content of alloying elements, they also have excellent weldability and can be produced at low cost.

Especially in the field of commercial vehicle and mobile crane construction, efforts to reduce vehicle weight have increased significantly in recent decades. This is due in particular to the fact that fuel consumption should be reduced to a minimum for economic and ecological reasons. In addition to the reduction in vehicle weight, the demands on the load-bearing capacity of structures are also increasing, while at the same time the greatest possible freedom of design.

In order to meet the challenges mentioned, structural steels with increasingly higher yield strengths of up to 1300 MPa and at the same time good formability have been developed since the beginning of the 1990s.

For the production of such high-strength steels, metal sheets are usually first rolled and tempered in an additional production step. For this purpose, the sheet metal is regularly first reheated and quenched (hardened) and then tempered.

In order to reduce the number of production steps required and the associated effort and costs, ways of avoiding them have been sought in recent years. An efficient solution here is the rolling of a hot strip with quenching or hardening from the rolling heat, the so-called direct hardening. A rapid cooling takes place of a hot strip immediately after leaving the last rolling stand in the cooling section of the hot strip mill. If this production step is combined with an upstream thermomechanical rolling, high strength can be achieved in combination with excellent notched impact strength.

In a downstream stripping process, high-strength strip plates can be produced from such a directly hardened hot strip in a conventional cut-to-length line. The strip sheets produced in this way can either be delivered to the end customer in an exclusively hardened condition and processed further. In practice, the depaneling process is often followed by an optional tempering of the sheets. This enables fine adjustment of the mechanical-technological properties. Furthermore, the scattering of the mechanical properties over the strip length and strip width can be reduced.

In the EP2576848 this procedure is described for classic water treatment concepts. The analyzes of these classic water treatment concepts have high chromium and molybdenum contents, which results in a high carbon equivalent CEV, which has a negative effect on weldability. Due to the high content of the elements mentioned, high tempering temperatures or high heat inputs are also required during tempering, which cause a sharp increase in the yield strength ratio Re/Rm. This high yield strength ratio is disadvantageous against the background of production reliability.

In addition, it has been shown that the steel flat products produced in this way have disadvantages in bending and folding processes due to the pronounced yield point and the resulting uneven flow of the material during deformation.

The object of the present invention is to provide a high-strength flat steel product that eliminates these disadvantages. A further object is to provide an efficient method for producing this flat steel product.

• C : 0,03-0,3 Gew.-%
• Mn: 0,4-2,5 Gew.-%
• Rest Eisen und unvermeidbare Verunreinigungen,

und eine Streckgrenze R_e von mindestens 680 MPa aufweist. Optional umfasst der Stahl weitere Elemente, die folgenden noch ausführlich erläutert werden. Zudem weist das Stahlflachprodukt keine ausgeprägte Streckgrenze auf, das heißt die Spannungs-Dehnungs-Kurve hat einen stetigen Verlauf.This object is achieved by a hot-rolled flat steel product which consists of a steel with the following composition (in % by weight)

• C : 0.03-0.3% by weight
• Mn: 0.4-2.5 wt%
• remainder iron and unavoidable impurities,

and has a yield strength _Re of at least 680 MPa. Optionally, the steel includes other elements, which will be explained in detail below. In addition, the flat steel product does not have a pronounced yield point, i.e. the stress-strain curve is continuous.

Due to the not pronounced yield point, a uniform hardening in forming processes, such as bending and folding processes, is guaranteed during the subsequent further processing, since there is no uneven flow behavior during the forming.

For the purposes of this application, the yield point R _e of a flat steel product is understood to mean the upper yield point R _eH if the flat steel product has a pronounced yield point. Otherwise (that is, for flat steel products without a pronounced yield point), the yield point of the flat steel product is understood to mean the yield point R _p02 within the meaning of this application. Flat steel products according to the invention do not have a pronounced yield point, so that the yield point R _e for these is to be understood as the yield point R _p02 .

The yield point R _e is basically to be understood as parallel to the rolling direction.

The yield point of the flat steel product according to the invention, which is determined according to DIN EN ISO 6892, is at least 680 MPa in order to ensure sufficient strength for structural and wear-resistant applications. In order to achieve the desired potential for lightweight construction, the microstructure of the hot-rolled flat steel product is preferably adjusted in such a way that its yield point Re is at least 890 MPa, particularly preferably at least 960 MPa.

A flat steel product is understood here as a strip sheet, ie a sheet cut from a hot strip, the length and width of which are each significantly greater than its thickness. Therefore, when a flat steel product or a "sheet metal product" is mentioned below, this means a rolled product in the form of a sheet metal strip, from which, for example, vehicle, crane or infrastructure components as well as supporting structures or shields can be used in the upper or lower section Underground mining blanks or blanks are divided.

“Shaped sheet metal parts” or “sheet metal components” are made from such a flat steel product or sheet metal product, the terms “shaped sheet metal part” and “sheet metal component” being used synonymously here. Infrastructure construction is understood to mean the manufacture of buildings, bridges, ships and aircraft. Vehicle construction here refers in particular to the construction of commercial vehicles, buses and trailers. Crane construction refers here in particular to the construction of mobile cranes, especially to the construction of crane booms. Components subject to wear and tear, such as e.g. B. troughs of dump trucks can be made of flat steel products according to the invention.

If information on alloy contents is given here, this refers to the weight or mass, unless otherwise expressly stated. If element contents are given in formulas, this also means the corresponding alloy content in % by mass. Element contents in formulas are indicated by the symbol "%" followed by the element symbol, i.e. %C denotes the element content of carbon in % by mass. Information on the content of microstructure components refers to the surface (area %, FI%) observed in the metallographic section, unless otherwise stated, with the exception of the volume fraction of retained austenite determined by X-ray in % by volume (% by volume).

In particular, the hot-rolled flat steel product has a thickness dW of 1.5 mm to 25 mm, in particular up to 20 mm. Because of the intended use profile, the thickness is preferably at least 2.0 mm, in particular at least 3.0 mm, in order to enable sufficiently rigid constructions. The maximum thickness is preferably 15 mm since a weight reduction is possible in this way.

In particular, the hot-rolled flat steel product has a tensile strength R _m that is greater than 730 MPa and less than 1700 MPa. The tensile strength is also determined in a tensile test according to DIN EN ISO 6892. Tensile strength is also generally understood to be parallel to the rolling direction. To increase component safety, a tensile strength of at least 930 MPa, particularly preferably at least 980 MPa, is preferably set. Since too high a tensile strength is associated with too little toughness, the tensile strength is preferably limited to a maximum of 1550 MPa.

The tempering treatment of the hot-rolled and directly hardened hot strip as a coil in the bell-type annealer, explained later, serves not only to specifically adjust the yield point and tensile strength level, but also to homogenize the mechanical properties across strip length, strip width and strip thickness. This process step reduces the spread of the yield point and the tensile strength over the strip length and strip width to a maximum of 100 MPa each. In the flat steel product according to the invention, the yield strength R _e and tensile strength R _m over the length and width consequently vary by no more than 100 MPa (difference between the maximum value and the minimum value).

In particular, the hot-rolled flat steel product has a ratio of yield point _Re to tensile strength _Re / _Rm that is at least 0.84 and at most 0.95. This has the advantage that a flat steel product according to the invention can be used for bending and edging without any special pretreatment. This can be used, for example, to produce highly rigid structural components by roll profiling. To avoid excessive hardening, which is associated with high forming forces, yield point ratios R _e /R _m of at least 0.88 are preferably set. In order to ensure improved production safety and component safety, the yield strength ratio R _e /R _m is preferably limited to a maximum of 0.94.

The restricted yield strength ratio of 0.84 to 0.94 offers an ideal combination of high safety through sufficient strong strain hardening in case of overloading and limited deflection during bending and folding by avoiding over hardening.

The elongation at break A according to DIN EN ISO 6892, proportional sample, of the flat steel product according to the invention is between 5% and 25% parallel to the rolling direction. The minimum elongation at break is preferably 7% in order to ensure good formability.

In particular, the hot-rolled flat steel product has a Brinell hardness (hereafter HB) for which the following applies: $$47+0,2\cdot R_e\le \mathrm{hb}\le 0,4\ast {\mathrm{R}}_e$$

The Brinell hardness (HB) is determined as Brinell hardness HBW 5/750 according to DIN EN ISO 6506-1 1 mm below the surface as the mean value from at least 3 individual measurements of the flat steel product according to the invention.

In a preferred variant, the relation applies: $$47+0,25\cdot R_e\le \mathrm{hb}\le 0,35\ast {\mathrm{R}}_e$$

In an even more preferred variant, the relation applies: $$47+0,287\cdot R_e\le \mathrm{hb}\le 0,323\ast {\mathrm{R}}_e$$

In particular, the hot-rolled flat steel product has a notched impact strength
which is at least 25J/cm2 at a test temperature of -60°C
and or
which is at least 35J/cm2 at a test temperature of -40°C
and or
which is at least 50J/cm2 at a test temperature of -20°C.

The specified minimum notched impact strength is determined as a Charpy V notch according to DIN EN ISO 148-1 parallel to the direction of rolling.

Due to the good notched impact strengths, especially in the heat-affected zone of weld seams, high-strength welded constructions and structural components can be produced inexpensively with flat steel products according to the invention.

In a preferred variant of the hot-rolled flat steel product, the yield point is less than 890 MPa. In addition, the steel has a microstructure which comprises more than 50% by area of bainite, the remainder being martensite, ferrite, pearlite and retained austenite, with the proportion of ferrite and pearlite in particular being at most 10% by area, preferably at most 5% by area. is, ie the sum of the proportions of ferrite and pearlite is at most 10% by area, preferably at most 5% by area. In addition, the proportion of retained austenite is preferably at most 5% by volume.

The structural components specified in this application are basically to be understood as being 1/3 of the sheet thickness. In the core area (1/2 sheet thickness), deviating structural components can occur due to segregation, and it can also occur on the sheet surface (e.g in an edge area of a maximum of 50 µm, in particular a maximum of 40 µm), deviations can occur due to edge decarburization.

Alternatively, the yield point is at least 890MPa. In addition, the steel has a structure which comprises more than 50 FI% martensite, the remainder being bainite, ferrite, pearlite and residual austenite, with the proportion of ferrite and pearlite being at most 10 FI%, preferably at most 5 FI%. is, ie the sum of the proportions of ferrite and pearlite is at most 10% by area, preferably at most 5% by area. In addition, the proportion of retained austenite is preferably at most 5% by volume.

In the preferred embodiment with a yield point ≥ 890 MPa and in the particularly preferred embodiment with a yield point ≥ 960 MPa, the martensite has fine needles with a maximum needle width of 200 nm. Due to the tempering, the predominantly martensitic structure has a large number of finely distributed tempered carbides, whose size is preferably at most 200 nm. The density X _Z of the tempering carbides is at least 10 ¹¹ m ^-2 , preferably at least 10 ¹² m ^-2 . This ensures good notched impact strength. The density of the tempering carbides is at most 10 ¹⁴ m ^-2 , preferably at most 10 ¹³ m ^-2 , in order to avoid excessive softening of the martensite.

The flat steel product has, in particular, grain elongation of the former austenite grains in the rolling direction, with the ratio V _γ of austenite grain length in the rolling direction to austenite grain height in the normal direction of the sheet being between 1.3 and 5.0. The austenite grain length and the austenite grain height are determined on the former austenite grains by means of EBSD investigations.

In a preferred variant, the minimum folding radius of the flat steel product for folding by 90° is five times the sheet thickness d _W when the bending line is parallel and perpendicular to the rolling direction. For increased freedom of design when bending components, the minimum bending radius is preferably four times the sheet thickness d _W , also when the bending line is parallel and perpendicular to the rolling direction.

The hot-rolled flat steel product according to the invention has good tempering resistance and is distinguished by high notched impact strength in the heat-affected zone of weld seams. In addition, it is excellently suited for edging and has good wear properties due to the high surface hardness.

The combination of high strength and excellent formability combined with high notched impact strength achieved in a hot-rolled flat steel product according to the invention makes the flat steel product according to the invention particularly suitable for use in welded constructions for crane booms in telescopic crane construction, in vehicle construction, in infrastructure construction and for equipment used in surface or underground mining, such as supporting structures of shields and the like. A significant weight saving can also be achieved through the use of flat steel products according to the invention in the construction of commercial vehicles, such as semi-trailers, also known as “trailers”, for articulated lorries or trailers for trucks, in the manufacture of chassis parts and in the manufacture of vehicle wheels. These advantages can also be used in the construction of rail vehicles or in shipbuilding.

A hot-rolled flat steel product according to the invention can be provided for further processing in the unpickled, pickled or blasted state. To protect against corrosive attacks, it can be coated with a metallic protective layer, with the zinc-based protective layers known from the prior art being particularly suitable for this purpose. Such Zn-based coatings can be applied in a practical manner to a flat steel product according to the invention, in particular by electrolytic galvanizing.

• Si: 0,05 - 1,5 Gew.-%
• Al: 0,01 - 0,3 Gew.-%
• B: 0,0005 - 0,007 Gew.-%
• Nb: 0,002 - 0,2 Gew.-%
• Cr: 0,05 - 2,5 Gew.-%
• Mo: 0,01 - 1,0 Gew.-%
• Mg: 0,0005 -0,005 Gew.-%
• H: bis zu 0,001 Gew.-%
• Ti: 0,002 - 0,2 Gew.-%
• V: 0,002 - 0,15 Gew.-%
• Ni: 0,05 - 10 Gew.-%
• Cu: 0,01- 1,0 Gew.-%
• Ca: 0,0005 - 0,005 Gew.-%
• REM: 0,001- 0,05 Gew.-%
• N: bis zu 0,01 Gew.-%
• P: 0,003 - 0,08 Gew.-%
• Sn: bis zu 0,05 Gew.-%
• As: bis zu 0,05 Gew.-%
• 0: bis zu 0,03 Gew.-%
• Co: bis zu 0,2 Gew.-%
• W: bis zu - 0,2 Gew.-%
• Be: 0,001 - 0,1 Gew.-%
• Sb: 0,001- 0,3 Gew.-%
• S: bis zu 0,01 Gew.-%
• Pb: bis zu 0,02 Gew.-%

In preferred variants, the flat steel product includes one or more of the following elements with the weight percentage specified below:

• Si: 0.05 - 1.5 wt%
• Al: 0.01 - 0.3% by weight
• B: 0.0005 - 0.007% by weight
• Nb: 0.002 - 0.2% by weight
• Cr: 0.05 - 2.5 wt%
• Mo: 0.01 - 1.0% by weight
• Mg: 0.0005 -0.005 wt%
• H: up to 0.001% by weight
• Ti: 0.002 - 0.2 wt%
• V: 0.002 - 0.15% by weight
• Ni: 0.05 - 10% by weight
• Cu: 0.01-1.0 wt%
• Ca: 0.0005 - 0.005% by weight
• REM: 0.001-0.05 wt%
• N: up to 0.01% by weight
• P: 0.003 - 0.08% by weight
• Sn: up to 0.05 wt%
• As: up to 0.05% by weight
• 0: up to 0.03% by weight
• Co: up to 0.2% by weight
• W: up to - 0.2% by weight
• Be: 0.001 - 0.1% by weight
• Sb: 0.001-0.3 wt%
• S: up to 0.01% by weight
• Pb: up to 0.02% by weight

The various components of the steel are explained in detail below, with the respective preferred minimum and maximum contents for their addition being specified.

Carbon (C) is primarily added to increase tensile strength and yield point. It exerts its effect on these two mechanical parameters through different mechanisms. On the one hand, up to a certain proportion of carbon can be present interstitially in dissolved form both in the body-centered cubic and in the face-centered cubic iron lattice and in this way cause an increase in strength. However, this mechanism is of secondary importance for the invention claimed here. The most important task of the carbon here is to enable a martensitic and/or bainitic structural transformation, which results in a significant increase in strength. The martensitic microstructural transformation is ensured by the greatly differing solubility of the carbon in the fcc and bcc lattice in conjunction with an increased cooling rate. The austenite-stabilizing effect of carbon ensures that the cooling rate required for martensite formation is reduced. In addition, however, it also causes a reduction in the martensite start temperature, so that lower temperatures have to be set for martensite formation. In order to bring about a defined increase in strength through martensite formation, a minimum carbon content is always required. Therefore, the minimum carbon content is set at 0.03% here. For safe compliance To ensure the required minimum values for yield point Re and tensile strength Rm even with unavoidable fluctuations in the process parameters during the manufacturing process, the minimum carbon content is preferably set at 0.04%. In order to achieve sufficiently high yield points and tensile strengths for the desired lightweight design potential, the minimum C content is particularly preferably set at 0.06%. An increasing carbon content causes an increase in strength and yield point. However, too high a C content also causes a very high carbon equivalent (CEV), since the C content has the greatest influence on this characteristic value. An increase in the CEV, in turn, significantly reduces suitability for welding. For this reason, the C content is limited to a maximum of 0.3%. In order to avoid excessive strength and thus an inevitable reduction in toughness, the C content is preferably set at a maximum of 0.25%. In order to ensure good toughness, a maximum C content of 0.20% is particularly preferably set.

Manganese (Mn) fulfills three essential tasks. On the one hand, manganese forms a substitution mixed crystal with iron, which causes an increase in strength. Furthermore, manganese has an austenite-stabilizing effect and thus enables a martensitic transformation even at lower cooling rates. In addition, manganese has a high affinity for sulfur and binds it to form MnS. In this way, the formation of brittle phases such as FeS can be avoided. In order to ensure solid solution strengthening through manganese, a minimum manganese content of 0.1% is added. Furthermore, in order to promote the martensitic transformation, a manganese content of at least 0.5% is preferably adjusted. In order to ensure martensitic transformation even at relatively low cooling rates, a minimum manganese content of 0.7% is particularly preferred. On the other hand, high manganese contents lead to an unfavorable martensite structure and to increasing element segregation over the thickness of the material, which can have a negative impact on the mechanical properties. In order to keep the proportion of coarse plate martensite low, the manganese content is limited to a maximum of 2.5%, in order to avoid coarse plate martensite, the manganese content is preferably limited to a maximum of 1.7%. In order to additionally avoid segregation over the thickness of the material, the manganese content is particularly preferably limited to a maximum of 1.5%.

C and Mn are always present in steel flat products according to the invention as obligatory elements in the contents explained above.

In order to particularly develop the properties of a flat steel product according to the invention or to optimize its processability, further alloying elements can be added to the flat steel product according to the invention, which are explained below. However, the properties of flat steel products according to the invention are also achieved without these elements, so that the alloying elements specified below as "optional" present in the flat steel product according to the invention can also be missing, so these elements can also be deleted from the alloy specification defined by the invention. In addition, one or more of these elements may be present as impurities in lower concentrations than the minimum values specified below. In this case, they are not effective in the way indicated, but they do not impair the indicated properties of the flat steel product according to the invention and can therefore be tolerated.

The optional addition of the alloying element boron and of microalloying elements such as aluminum, niobium, titanium or vanadium—either individually or in combination with one another—is advantageous for adjusting the material properties of the present invention. Although all of the elements mentioned have a positive effect on the material properties when alloyed alone, the interactions between the individual micro-alloying elements are of overriding importance here. This is explained in more detail below: Boron (B) plays a major role in increasing the hardenability of steels. Coming from the rolling heat, it segregates to the austenite grain boundaries and suppresses the nucleation of ferrite there. In this way, the ferritic-pearlitic transformation is shifted to longer cooling times and a martensitic transformation can be achieved at lower cooling rates. For this mechanism it is imperative that boron is present in dissolved form. To avoid boron nitride formation, the free nitrogen content must be controlled below 0.0007%. For technical reasons, such a low content can only be achieved by binding nitrogen in other compounds.

If boron is optionally used to improve hardenability, a minimum B content is required. Therefore, in this case, the boron content is specified to be at least 0.0005%. A reliable improvement in the hardenability through free boron should also be guaranteed if the nitrogen is not completely bound. Therefore, the boron content is preferably set to be at least 0.0010%, more preferably at least 0.0015%. Since the strength-increasing effect initially increases with increasing boron content and falls again above a maximum, the boron content of the steel according to the invention is limited to a maximum of 0.007%. From a determine However, with the boron content, no significant improvement in hardenability is achieved, but toughness at the grain boundaries is reduced. Therefore, the boron content is preferably set to not more than 0.005%. In order to achieve the optimum properties in the flat steel product according to the invention, the boron content is limited to a maximum of 0.0035% in a particularly preferred variant.

Nitrogen (N) forms nitrides with B, Al and the micro-alloying elements Nb, Ti and V as well as carbonitrides due to the presence of C. If boron is optionally used to increase hardenability, as already explained, the formation of boron nitrides should be avoided. For this purpose, appropriate alloying measures for nitrogen binding, e.g. B. by the targeted alloying of the elements Al, Ti, Nb and / or V, individually or in combination. Taking into account the practical conditions in economic steel production, nitrogen contents of up to 0.01% can be accepted as unavoidable contamination. In order to keep the use of the nitrogen-binding elements low and to ensure process-reliable production of the flat steel product according to the invention, N contents of at most 0.008%, particularly preferably at most 0.006%, are preferably set. Contents of less than 0.002% can technically only be avoided with great effort, which is why a content of at least 0.002% is tolerated for economic reasons.

In steel production, aluminum usually fulfills the task of deoxidizing or "calming down" the molten steel. The high affinity of aluminum for oxygen is used here. By binding the oxygen to form Al ₂ O ₃ , the rising of oxygen bubbles ("boiling" of the melt) during steel production is avoided. An Al content of at least 0.01% is generally added to cool steel melts. In order to reduce austenite grain growth via fine AlN particles that are present in sufficient numbers, an Al content of at least 0.02% is preferably added. Excessive aluminum alloying leads to the formation of coarse Al ₂ O ₃ particles. The Al content is limited to a maximum of 0.3% in order to avoid problems when pouring the molten steel due to clogging of the dip tube with Al ₂ O ₃ particles. In order to essentially reduce and/or avoid undesired precipitations in the material, in particular in the form of non-metallic oxidic inclusions, which can adversely affect the material properties, the maximum Al content is preferably limited to 0.2%.

Another optional alloying element is titanium (Ti). In order to inhibit austenite grain growth through the formation of fine TiN particles, a titanium content of at least 0.002% can optionally be added. A Ti content of at least 0.005% is preferably alloyed in order to reliably avoid austenite grain growth through fine TiN particles that are present in sufficient numbers. To utilize the grain-refining effect of Ti during hot rolling, a Ti content of at least 0.008% is preferably set. Since the formation temperature of titanium nitrides is significantly higher than that of boron nitrides, a titanium content of at least 0.015% can preferably be set in the case of a boron alloy for the purpose of binding nitrogen. In order to utilize an increase in strength through the formation of titanium carbonitrides, a maximum Ti content of 0.2% can be added to the flat steel product according to the invention. In order to avoid an excessive drop in toughness due to the formation of a large number of coarse titanium nitrides or titanium carbonitrides, the titanium content is preferably limited to a maximum of 0.04%, in particular to a maximum of 0.1%. In order to also reliably avoid the formation of less coarse titanium nitrides or titanium carbonitrides, the titanium content is particularly preferably limited to a maximum of 0.025%.

Austenite grain growth is inhibited particularly reliably if the titanium content is sub-stoichiometrically alloyed with respect to nitrogen. Therefore, in a particularly preferred variant, the ratio %Ti/%N is limited to a maximum of 3.42.

Another optional alloying element is niobium (Nb). The formation of niobium carbides and nitrides or carbonitrides at relatively high temperatures impedes grain growth before, after and during a rolling process, which results in grain refinement and thus an increase in toughness. A niobium content of at least 0.002% is required for this. In order to use this effect reliably, at least 0.005% Nb is preferably alloyed with the flat steel product according to the invention. In addition, in order to effectively use the precipitation strengthening potential of niobium, it is preferable to set a niobium content of at least 0.010%. Both the grain refinement effect and the precipitation hardening no longer increase significantly above a certain Nb content. If the mentioned effects are utilized, the niobium content is preferably restricted to a maximum of 0.1%. The effects described are particularly effective with a preferred maximum niobium content of 0.05%.
In the case of a boron alloy, in a particularly preferred variant, the binding of nitrogen is ensured by alloying in a combination of aluminum (Al) and niobium (Nb), optionally in combination with titanium. The elements Al and Nb are also strong nitride and carbide or carbonitride formers. A combination of Al and Nb leaves a higher content of free nitrogen in the material than when only titanium (Ti) is alloyed with it, but this content is sufficiently low to avoid the formation of boron nitrides and thus to keep boron in solution. For this purpose, a niobium content of at least 0.02% is required with a simultaneous addition of at least 0.09% Al. If the focus is only on binding nitrogen, the niobium content in this particularly preferred variant can be limited to a maximum of 0.03% and the aluminum content to a maximum of 0.15%.

Chromium (Cr) can be included as an optional alloying element to increase strength and improve hardenability at levels of 0.05% to 2.5%. In order to achieve an effective increase in strength through precipitation and solid solution strengthening, a minimum Cr content of 0.1% is preferably set. Cr also suppresses the formation of ferrite and pearlite during the cooling process, allowing full martensite and/or bainite formation even at slower cooling rates. This hardenability-increasing effect is particularly effective with a Cr content of at least 0.2%. Therefore, in the case of an optional alloying of Cr, a content of at least 0.2% is particularly preferably alloyed. In order to avoid a negative influence on the weldability, the Cr content is preferably limited to a maximum of 1.2%. In order to reduce an adverse influence due to the formation of coarse Cr-containing precipitates, particularly carbides, the Cr content is preferably set to 0.8% or less. To avoid coarse precipitation containing Cr, the Cr content is particularly preferably limited to a maximum of 0.5%.

As a further optional alloying element, molybdenum (Mo) can be contained in the steel flat product according to the invention in amounts of 0.01% to 1% to increase strength and to improve hardenability. In order to achieve an effective increase in strength through precipitation and solid solution strengthening, a minimum Mo content of 0.1% is preferably set. Like Cr, Mo effectively suppresses the formation of ferrite and pearlite during the cooling process and thus enables complete martensite and/or bainite formation even at lower cooling rates (hardenability increase). This hardenability-increasing effect is particularly effective with Mo contents of at least 0.20%. Therefore, in the case of an optional Mo alloy, a Mo content of at least 0.20% is particularly preferably added. In order to avoid a negative influence on the suitability for welding, the Mo content is preferably limited to a maximum of 0.5%, particularly preferably to a maximum of 0.3%.

Both Cr and Mo reduce weldability with increasing levels. For this reason, the sum of the proportions by weight of chromium (Cr) and molybdenum (Mo) is at most 1.2% by weight, preferably at most 0.85% by weight, particularly preferably at most 0.65% by weight.

Also optionally, one element or more elements from the group “Si, Ni, Cu, V, Ca, Mg, REM, Be, Sb” can be present in the steel flat product according to the stipulations explained below.

Thus, the flat steel product according to the invention can optionally have silicon (Si) in contents of 0.05% to 1.5%. Si forms a substitution mixed crystal with the iron lattice and thus causes a significant increase in strength. For this purpose, an Si content of at least 0.05% is alloyed. In addition, Si suppresses cementite formation from the austenite, leaving more carbon dissolved in the austenite, which in turn promotes the martensitic transformation. In order to utilize this effect, an Si content of at least 0.07% is preferably set. In addition, Si reduces the risk of undesired subsequent cementite formation in the martensite and thereby increases the resistance to an undesired reduction in strength in the heat-affected zone during welding and tempering. If this effect is desired, contents of at least 0.10% are particularly preferred. To ensure rollability, the Si content is limited to a maximum of 1.5%. However, high levels of silicon can lead to the formation of red scale. Rotzunder has an insulating effect on the surface of the material and can therefore significantly reduce the effect of the cooling water used to cool it down. This in turn has negative effects on the martensitic transformation. For this reason, the content of Si, if present at all in effective amounts, is preferably limited to a maximum of 0.6%. Here, negative effects of the optional presence of Si can be avoided in a particularly reliable manner in that the optional Si content of the flat steel product according to the invention is particularly preferably limited to at most 0.35%.

At particularly low cooling stop temperatures, there is a risk that residues of cooling water will remain on the directly hardened hot strip, since the residual heat is not sufficient for complete evaporation. These cooling water residues are wrapped between the windings when the hot strip is wound into a coil and have a negative effect on the surface quality through the formation of porous scale, which can flake off during further processing. Experiments have shown that the optional Si contents mentioned above increase this effect. For this reason, in an alternative preferred embodiment, the Si content is maximized 0.08% limited. In an alternative, particularly preferred variant, the Si content is limited to a maximum of 0.030% in order to ensure batch galvanization according to class 1.

As a further optional alloying element, nickel (Ni) can be provided in the flat steel product according to the invention in contents of 0.05% to 10%. In order to ensure an improvement in hardenability by reducing the critical cooling rate, a minimum Ni content of 0.05% is required. To ensure through-hardening even with greater sheet thicknesses, a minimum Ni content of 0.15% is preferably added. In addition to increasing hardenability, Ni can also be alloyed to improve toughness. If this is desired, a minimum Ni content of 0.3% Ni is particularly preferred. A toughness-enhancing effect is effectively achieved up to a Ni content of 10%. Therefore, the Ni content is limited to a maximum of 10%. In order to ensure sufficient hardenability and toughness also from an economical point of view, Ni is preferably limited to a maximum of 5%. Since the addition of Ni results in an increase in the carbon equivalent CEV and as a result weldability is adversely affected, the Ni content is preferably limited to a maximum of 1%, particularly preferably limited to a maximum of 0.5%, to ensure weldability.

Furthermore, the flat steel product according to the invention can optionally contain copper (Cu) in a content of 0.01% to 1%. If the hardenability-increasing effect of Cu is to be exploited, a Cu content of at least 0.01% is added. A minimum Cu content of 0.03% is preferably set to ensure through-hardenability even with greater sheet thicknesses. In addition, Cu can improve weather resistance. In order to utilize this effect, a minimum Cu content of 0.1% is particularly preferred. In order to avoid a negative influence on the castability of the steel melt, the Cu content is limited to a maximum of 1%. In order to reduce a negative impact on toughness due to the formation of coarse Cu carbides, the Cu content is preferably limited to a maximum of 0.5%. In order to reliably avoid coarse Cu carbides, the Cu content is particularly preferably restricted to a maximum of 0.3%.

As a further optional alloying element, vanadium (V) can be added in contents of 0.002% to 0.2%. A minimum of 0.002% V must be added to increase yield and tensile strength through precipitation hardening due to the formation of vadium carbides and vanadium carbonitrides. If grain refinement is to be achieved at the same time, a minimum V content of 0.005% is preferably set. A drop in yield point and tensile strength during welding in the heat-affected zone due to the dissolution and re-precipitation of vadium carbonitrides to counteract this, a minimum content of 0.01% V is particularly preferably added. In order to ensure the effects described above from an economic point of view as well, the V content is limited to a maximum of 0.15%. Since too high a V content may lower toughness, the V content is preferably limited to a maximum of 0.07%. In order to avoid an excessive increase in the carbon equivalent CEV by vanadium and a reduced suitability for welding as a result, the preferred V content is set at a maximum of 0.05%, particularly preferably at a maximum of 0.03%.

With the optional addition of vanadium in the amounts described above, vanadium carbides are formed as a result of tempering at temperatures above 180°C. The carbon bound in vanadium carbides is no longer available for sufficient strengthening of the martensite. For this reason, in an alternative embodiment, the V content is limited to a maximum of 0.008%, which is particularly preferred compared to the V contents mentioned above.

Calcium (Ca) is used in steel for deoxidation and for forming non-metallic inclusions, especially manganese sulfides. The rounded indentation significantly reduces the negative effect of the inclusions on hot workability, fatigue strength and toughness. In order to also utilize these effects in a flat steel product according to the invention, a flat steel product according to the invention can optionally contain 0.0005% to 0.005% Ca. In order to guarantee a particularly reliable effect, contents of at least 0.001% are preferably added; for reasons of resource efficiency, the Ca content is preferably limited to a maximum of 0.004%.

If Ca is used as an optional alloying element, a calcium to sulfur ratio (Ca/S, proportions in weight percent) of 0.5-2.5 should be set in a preferred embodiment. In a particularly preferred embodiment, the Ca/S ratio should be at most 2.0.

Like calcium, magnesium (Mg) can be used in steel for deoxidation and desulfurization. For this purpose, a flat steel product according to the invention can optionally contain 0.0005% to 0.005% Mg.
In the production of the flat steel product according to the invention, the addition of metals from the group of rare earths (REM—rare-earth metal), such as cerium, lanthanum and neodymium, is optionally possible. To increase the strength and use the grain-refining effect, a content of at least 0.001%. In addition, the binding effect of the rare earths on sulfur, phosphorus and oxygen can reduce the segregation of these elements at grain boundaries, which makes it possible to increase toughness. At levels above 0.05% there is a risk of the formation of toughness-reducing precipitates. Therefore, the rare earth content is limited to a maximum of 0.05%.

Beryllium (Be) can be used as an optional alloying element in levels from 0.001% up to 0.1% to increase wear resistance through the formation of high strength carbides and/or oxides. To reliably set the effectiveness, a Be content of at least 0.002% is preferably set. Since the toughness is greatly reduced if the content is too high, which is undesirable in the present case, the content is preferably limited to a maximum of 0.05%. Due to its toxicity, it is particularly preferred not to use Be as an optional alloying element.

Antimony (Sb) can be added as an optional alloying element in contents of 0.001 to 0.3% in order to reduce the susceptibility to grain boundary oxidation and, when using higher contents, also to increase the corrosion resistance in acidic media by segregating at grain boundaries and there the tendency for hydrogen generation and thus for hydrogen-induced cracking is reduced or completely prevented. In order to achieve a reliable alloying effect, an Sb content of at least 0.002%, particularly preferably at least 0.005%, is alloyed. For reasons of cost, the maximum content is preferably limited to 0.1%. In order to avoid embrittlement, in particular at grain boundaries, the content is particularly preferably set at a maximum of 0.05%.

Phosphorus (P) can be added as an optional alloying element to increase strength in amounts of 0.003% to 0.08%. In order not to adversely affect the weldability of the flat steel product according to the invention, the maximum P content is preferably set at 0.05%. In order to avoid a negative influence on the toughness, the P content in the flat steel product according to the invention is preferably limited to at most 0.02%.
Cobalt (Co) has a negative impact on hardenability and toughness. For technical reasons, however, traces of cobalt usually always remain in steel. Since the negative effects of cobalt generally only occur above 0.2%, its content is limited to a maximum of 0.2%.

Tungsten (W) forms a Laves phase with molybdenum above certain levels. This can have a negative effect on the notched impact strength. For technical reasons, however, the tungsten content cannot usually be reduced to any desired extent, but in order to avoid negative influences it may not exceed 0.2% in accordance with the invention.
Arsenic (As) and tin (Sn) can accumulate at grain boundaries at temperatures around 500 °C, causing embrittlement. In order to prevent these negative effects, the content of As and Sn in the steel flat product according to the invention should be limited to a maximum of 0.05%, preferably a maximum of 0.03%, in the usual way.

If the concentration is sufficient, sulfur (S) forms sulfides with iron or manganese (FeS or MnS). These have a negative impact on deformability and toughness. Therefore, the sulfur content is limited to at most 0.01%, preferably at most 0.008%, and more preferably at most 0.006%.

If the content is too high, hydrogen (H) can lead to the formation of cracks in the material. In order to avoid this, its content in the flat steel product according to the invention is limited to a maximum of 0.001%, preferably to a maximum of 0.0005%, particularly preferably to a maximum of 0.0003%. Oxygen (O) combines in particular with aluminum to form oxides (Al ₂ O ₃ ). These reduce both toughness and fatigue strength. Therefore, the oxygen content is limited to a maximum of 0.03%, preferably a maximum of 0.02%, particularly preferably a maximum of 0.01%.

Lead (Pb) is an unwanted by-element, so its content is limited to a maximum of 0.02%.

All of the above-mentioned optional alloying elements can be present in small amounts as process-related unavoidable impurities, but are then not effective for the purposes of the present invention.

In a preferred variant of the flat steel product according to the invention, the carbon equivalent CEV is at most 0.7, in particular at most 0.6. The carbon equivalent CEV is calculated using the formula: $$\mathit{CEV}=\%C+\frac{\%\mathit{Mn}}{6}+\frac{\%\mathit{Cu}+\%\mathit{no}}{15}+\frac{\%\mathit{Cr}+\%\mathit{Mon}+\%V}{5}$$

The specified maximum values for the carbon equivalent CEV result in better weldability. Here, %C, %Mn, %Cu, %Ni, %Cr, %Mo and %V designate the respective content of this alloying element in percent by weight.
In order to avoid excessive hardening in the heat-affected zone and to further reduce the sensitivity to cold cracking, the carbon equivalent CEV is limited to a maximum of 0.5 in a preferred variant.

In a further preferred embodiment, the carbon equivalent CET is at most 0.7, in particular at most 0.5, preferably at most 0.35. The carbon equivalent CET is calculated using the formula: $$\mathit{CET}=\%C+\frac{\%\mathit{Mn}+\%\mathit{Mon}}{10}+\frac{\%\mathit{Cr}+\%\mathit{Cu}}{20}+\frac{\%\mathit{no}}{40}$$

Here, %C, %Mn, %Mo, %Cr, %Cu and %Ni indicate the respective content of this alloying element in percent by weight. The specified maximum values for the carbon equivalent CET result in better weldability.

In a preferred variant of the flat steel product, the flat steel product has cementite particles, the proportion of cementite particles with a diameter of 20 nm to 250 nm being greater than 98%. The mean value of the diameter of the cementite particles is in particular between 50 nm and 150 nm. The expression of the cementite particles is determined in a microstructure area characteristic of the material at approximately 1/3 of the sheet metal thickness.

With the optional alloying of the elements Cr or Mo, individually or in combination, the majority (i.e. more than 50%) of Cr and Mo is built into the cementites and is not dissolved in the matrix.

In the corresponding preferred variant, a hot-rolled flat steel product is thus available which has a high yield point R _e without discontinuity in the stress-strain diagram, a high tensile strength R _m and a high elongation at break A in combination with good bending ability. In addition, this preferred hot-rolled steel flat product excels characterized by a low yield strength ratio R _e /R _m which is advantageous for further processing.

Due to its low tendency to edge cracking, which is guaranteed by the production process and a structure consisting predominantly of martensite or bainite, a flat steel product according to the invention is outstandingly suitable for stamping and mechanical cutting.

Thermal cutting processes such as laser or plasma cutting can also be used without any problems when processing flat steel products according to the invention.

• Erzeugen einer Stahlschmelze mit der nachfolgend angegebenen Zusammensetzung (in Gew.-%):
  • C : 0,03-0,3 Gew.-%
  • Mn: 0,4-2,5 Gew.-%
  • Rest Eisen und unvermeidbare Verunreinigungen,
• Vergießen der Stahlschmelze zu einem Vorprodukt in Form eines Blocks, einer Bramme, einer Dünnbramme oder eines gegossenen Bandes
• Durcherwärmen des Vorproduktes auf eine Austenitisierungstemperatur T_WE zwischen 1100°C und 1350°C
• Warmwalzen des Vorproduktes zu dem warmgewalzten Stahlflachprodukt bei einer Warmwalzendtemperatur T_E von mindestens 770 °C, insbesondere mindestens Ar3+20K
• Erstes Abkühlen des erhaltenen warmgewalzten Stahlflachproduktes mit einer 20 - 400 °C/s betragenden Abkühlrate auf eine Kühlstopptemperatur
• Haspeln des auf die Kühlstopptemperatur abgekühlten, warmgewalzten Stahlflachproduktes zu einem Coil
• Optionales zweites Abkühlen des erhaltenen Coils auf Raumtemperatur
• Anlassen des zu dem Coil aufgehaspelten Stahlflachproduktes bei einer Glühtemperatur T_G, wobei die Aufheizrate auf die Glühtemperatur maximal 500 K/h beträgt, und wobei eine Haltezeit t_G des Coils auf der Glühtemperatur T_G größer oder gleich einer Mindesthaltezeit t_G,min ist, für die gilt: $$t_{G,\mathit{min}}=150\cdot B_C\cdot \sqrt{D_C^2-d_C^2}\cdot T_G^{-\frac{2}{3}}$$
  
  mit

  BC

  Breite des zum Coil aufgehaspelten Stahlflachproduktes in Metern

  DC

  Außendurchmesser des Coils in Metern

  dC

  Innendurchmesser des Coils in Metern

  TG

  Glühtemperatur in °C

  tG,min

  Mindesthaltezeit in Stunden

• Abhaspeln und Richten des Stahlflachproduktes

The object according to the invention is also achieved by a method for producing a flat steel product as described above. The procedure includes the following work steps:

• Production of a steel melt with the following composition (in % by weight):
  • C : 0.03-0.3% by weight
  • Mn: 0.4-2.5 wt%
  • remainder iron and unavoidable impurities,
• Casting of the molten steel into a preliminary product in the form of an ingot, a slab, a thin slab or a cast strip
• Thorough heating of the preliminary product to an austenitization temperature T _WE between 1100°C and 1350°C
• Hot rolling of the preliminary product to form the hot-rolled flat steel product at a final hot-rolling temperature T _E of at least 770° C., in particular at least Ar3+20K
• First cooling of the obtained hot-rolled flat steel product at a cooling rate of 20 - 400 °C/s to a cooling stop temperature
• Coiling the hot-rolled flat steel product, which has been cooled to the cooling stop temperature, into a coil
• Optional second cooling of the coil obtained to room temperature
• Tempering the steel flat product coiled into the coil at an annealing temperature T _G , the heating rate to the annealing temperature being a maximum of 500 K/h, and a holding time t _G of the coil at the annealing temperature T _G being greater than or equal to a minimum holding time t _G,min , for which applies: $$t_{G,\mathit{at\; least}}=150\cdot B_C\cdot \sqrt{D_C^2-{\mathrm{i.e}}_C^2}\cdot T_G^{-\frac{2}{3}}$$
  
  with

  B.C

  Width of the steel flat product coiled in meters

  DC

  Outside diameter of the coil in meters

  DC

  Inner diameter of the coil in meters

  TG

  Annealing temperature in °C

  tG, min

  Minimum holding time in hours

• Uncoiling and straightening the steel flat product

Said molten steel can preferably also contain one or more optional elements which have been explained in detail in relation to the flat steel product. Likewise, the content of C and Mn can be within the preferred ranges discussed.

The process engineering production of the flat steel product according to the invention takes place via hot rolling with direct hardening in the cooling section of the hot strip mill and subsequent tempering of the hot strip as a coil in a bell annealer, optionally under H2 or N2 protective atmosphere or a protective atmosphere consisting of a mixture of H2 and N2. First, a molten steel is produced with the above analysis, in order to set or shape specific properties of the hot-rolled flat steel product to be produced according to the invention.

This melt is then cast in a conventional manner to form a preliminary product with a thickness _dV . This preliminary product is typically a slab. Casting into thin slabs, cast strips or blocks is also possible. The pre-product produced in this way has a thickness d _V between 2.5 mm and 350 mm.

For hot rolling, the preliminary product is heated to the austenitization temperature T _WE , also referred to as the reheating temperature, with the analysis according to the invention. The reheating temperature of the steels of the present invention should be between 1100°C and 1350°C. However, the reheating temperature is preferably at least 1220° C. in order to reduce hardening in the subsequent rolling process due to the reheating temperature being too low and preferably at most 1320° C. in order to avoid melting of the slab surface and excessive austenite coarsening and to enable economical production. A homogeneous initial structure is also established in this temperature range. In addition, precipitates from the specifically alloyed micro-alloying elements are reliably dissolved.

abschätzen.During the manufacturing process, the preliminary product is hot-rolled to form the hot-rolled flat steel product at a final hot-rolling temperature T _E of at least 770° C., in particular at least Ar3+20K. During this hot rolling step, the temperature of the rolled flat steel product decreases continuously with each pass down to the final rolling temperature T _E , at which the hot-rolled flat steel product leaves the last pass. To prevent ferrite formation during hot rolling, the final rolling temperature must be at least 770 °C. If the final rolling temperature T _E is at least 20° C. above the Ar3 temperature of the flat steel product according to the invention, the formation of ferrite is avoided in a particularly reliable manner. The Ar3 temperature can be calculated according to " Mathematical Model for Predictions of Austenite and Ferrite Microstructures in Hot Rolling Processes", IRSID Report, St. Germain-en-Laye, 1985, p. 7 about the equation $$\mathrm{are}\mathrm{3}\left[\mathrm{°C}\right]=\mathrm{902}-\mathrm{527}\cdot \%\mathrm{C}-\mathrm{62}\cdot \%\mathrm{Mn}+\mathrm{60}\cdot \%\mathrm{si}$$

estimate.

In a preferred variant, the hot rolling of the preliminary product comprises at least two hot rolling passes, which are carried out at a temperature above a recrystallization temperature T _NR , where the following applies to the recrystallization temperature T _NR : $${\mathrm{T}}_{\mathrm{NO}}\left[\mathit{°C}\right]=\mathrm{887}+\mathrm{464}\cdot \mathrm{C}+\mathrm{6445}\cdot \mathrm{Nb}-\mathrm{644}\cdot \sqrt{\mathrm{Nb}}+\mathrm{732}\cdot \mathrm{V}-\mathrm{230}\cdot \sqrt{\mathrm{V}}+\mathrm{890}\cdot \mathrm{Ti}+\mathrm{363}\cdot \mathrm{Al}-\mathrm{357}\cdot \mathrm{si}$$

The at least two hot rolling passes above the recrystallization temperature have the advantage that a fine, multiple recrystallized austenite structure results, since above this temperature the austenite recrystallizes completely in the structure of the steel flat product. The approximate calculation of the recrystallization temperature is carried out according to the Effect of Chemical Compostion on Critical Temperatures of Microalloyed Steels", Boratto et al., THERMEC '88, Proceedings, Iron and Steel Institute of Japan, Tokyo, 1988, pp. 383-390 specified method.

It should preferably be ensured that the pass reduction above T _NR in the individual passes is at least 6% in each case in order to introduce sufficient deformation so that grain refinement is achieved by recrystallization. In order to ensure sufficient deformation energy over the entire cross section, especially in the core area with thicker pre-products, in a further preferred embodiment the pass reduction above T _NR in the individual passes should be at least 8%. This ensures sufficient notched impact strength in the flat steel product according to the invention even at low tempering temperatures and with low alloy contents of Cr, Mo and Ni and thus low CEV and CET compared to classic analyzes for water tempering. During hot rolling above this temperature T _NR , the austenite in the structure of the steel flat product recrystallizes completely.

wobei d_V die Dicke des Vorproduktes und d_W die Dicke des warmgewalzten Stahlflachproduktes ist. Diese Mindestanzahl n_W an Walzstichen oberhalb T_NR hat den Vorteil, dass sich durch Rekristallisation ein optimal feinkörniges Gefüge ergibt.In a preferred development, the hot rolling of the preliminary product includes a minimum number n _W of hot rolling passes, which are carried out at a temperature above the recrystallization temperature T _NR , the minimum number n _W corresponding to the result n _W' rounded to a whole number $$n_{\mathit{W\text{'}}}=2\cdot \sqrt{\frac{{\mathrm{i.e}}_V}{6\cdot {\mathrm{i.e}}_W}}$$

where d _V is the thickness of the pre-product and d _W is the thickness of the hot-rolled flat steel product. This minimum number n _W of rolling passes above T _NR has the advantage that recrystallization results in an optimally fine-grained structure.

In a special development, the hot rolling of the preliminary product includes at least one hot rolling pass, which is carried out at a temperature below the recrystallization temperature T _NR . The final hot rolling temperature T _E is therefore lower than the recrystallization temperature T _NR . As the temperature decreases successively during the rolling passes, this means that the last rolling pass or passes will be performed at a temperature below the recrystallization temperature. This suppresses the recrystallization of the austenite during the last rolling pass (or the last rolling passes in the case of several rolling passes below the recrystallization temperature).

wobei d_W die Dicke des warmgewalzten Stahlflachproduktes bezeichnet und d_ENR die Dicke bezeichnet, die das Stahlflachprodukt nach dem letzten bei einer Temperatur oberhalb der Temperatur T_NR durchgeführten Walzstiche erreicht hat. Der Umformgrad ist als Absolutbetrag des natürlichen Logarithmus vom Verhältnis dieser beiden Dicken definiert.The degree of deformation φ is preferably at least 0.25 over all hot rolling passes that are carried out at a temperature below the recrystallization temperature T _NR . The degree of deformation is defined as follows: $$\phi =\left|\ln \left(\frac{{\mathrm{i.e}}_W}{{\mathrm{i.e}}_{\mathit{ENR}}}\right)\right|$$

where d _W denotes the thickness of the hot-rolled flat steel product and d _ENR denotes the thickness which the flat steel product has reached after the last rolling pass carried out at a temperature above the temperature T _NR . The degree of deformation is defined as the absolute value of the natural logarithm of the ratio of these two thicknesses.

The described selection of the final hot-rolling temperature T _E and the degree of deformation ϕ in the framework explained above ensures that the cooling that takes place after hot-rolling transforms the non-recrystallized austenite into a fine structure, resulting in good formability of the structure and at the same time high strength is ensured.

The obtained hot-rolled flat steel product is cooled immediately after the hot-rolling. Due to the design of hot rolling mills known from the prior art and the associated cooling devices, the term "immediately" describes cooling that begins a maximum of 8 s after the flat steel product emerges from the last rolling pass. The cooling rate for this first cooling to a cooling stop temperature is 20-400°C/s, preferably at least 40°C/s, particularly preferably at least 60°C/s. Water, which is applied to the flat steel product in a conventional manner in a conventional cooling section, is particularly suitable as a coolant.

The cooling stop temperature T _KS is preferably at least 250° C. lower than the hot rolling end temperature T _E , with cooling stop temperatures T _KS of at most 550° C., in particular 500° C., being practical, provided they are not above T _E −250° C.

The microstructure of the flat steel product according to the invention and, associated therewith, its yield point R _e and its other mechanical-technological properties explained above are adjusted via the selection of the cooling stop temperature T _KS .

mit

BS

Bainit-Starttemperatur in °C

MS

Martensit-Starttemperatur in °C

In a first preferred variant, the hot-rolled flat steel product has a yield strength _Re less than 890 MPa. In this case, the hot-rolled steel flat product obtained is first cooled to a cooling stop temperature of T _KS , where the following applies to the cooling stop temperature T _KS : $$\left({\mathrm{M}}_{\mathrm{S}}-\mathrm{30}\mathrm{°C}\right)\le {\mathrm{T}}_{\mathrm{KS}}<{\mathrm{B}}_{\mathrm{S}}$$

with

B.S

Bainite start temperature in °C

MS

Martensite start temperature in °C

In this variant, the flat steel product has a microstructure that comprises more than 50% by area of bainite, the remainder being martensite, ferrite and retained austenite. The proportion of bainite in the structure can be determined by setting a cooling stop temperature T _KS . For example, at a cooling stop temperature T _KS of about 50°C below the bainite start temperature BS (T _KS ≈ B _S - 50°C) with a microstructure of 50% by area bainite. A bainite proportion of 100% by area, ie a completely bainitic structure, can be achieved, for example, by choosing a cooling stop temperature T _KS that is about 120 °C below the bainite start temperature BS (T _KS ≈ B _S - 120 °C). . In addition to bainite, the other microstructure components are martensite, up to 10% by area ferrite, preferably up to 5% by area ferrite and up to 5% by volume residual austenite, with the proportions of ferrite, martensite and residual austenite at correspondingly high proportions of the other Structural component can also be "0". At relatively high cooling stop temperatures T _KS , part of the martensite can also be present in a tempered form after cooling.

mit

MS

Martensit-Starttemperatur in °C

In a second preferred variant, the flat steel product has a yield strength of at least 890 MPa. In this case, the hot-rolled steel flat product obtained is first cooled to a cooling stop temperature of T _KS , where the following applies to the cooling stop temperature T _KS : $${\mathrm{T}}_{\mathrm{KS}}<\left({\mathrm{M}}_{\mathrm{S}}-\mathrm{100}\mathrm{°C}\right)$$

with

MS

Martensite start temperature in °C

In this variant, the flat steel product has a microstructure that comprises more than 50 FI% martensite, the remainder being bainite, ferrite and retained austenite. The proportion of martensite can be adjusted by selecting the cooling stop temperature T _KS . In order to produce a completely martensitic structure, for example, a cooling stop temperature T _KS is required which is about 380 °C below the martensite start temperature MS ( _{T KS ≈MS} - 380 °C).

In addition to martensite, the other microstructure components are bainite, up to 10% by area ferrite, preferably up to 5% by area ferrite and up to 5% by area retained austenite, with the proportions of ferrite, bainite and retained austenite at correspondingly high proportions of the other Structural component can also be "0".

abschätzen.The bainite start temperature B _S can be calculated according to Kirkaldy in JS Kirkaldy et al.: "Prediction of Microstructure and Hardenability in Low Alloy Steels", Phase Transformations in Ferrous Alloys, AIME, Philadelphia, 1983, 125-148 [4] published formula: $$B_S\left[\mathit{°C}\right]=656-57,7\cdot \%C-35\cdot \%\mathit{Mn}-75\cdot \%\mathit{si}-15,3\cdot \%\mathit{no}-34\cdot \%\mathit{Cr}-41,2\cdot \%\mathit{Mon}$$

estimate.

abschätzen.The martensite start temperature M _S can be calculated according to Andrews in KW ANDREWS: "Empirical Formulae for the Calculation of Some Transformations, Temperatures". Journal of the Iron and Steel Institute, 203, Part 7, July 1965, 721-727 , [5] published formula: $$M_S\left[\mathit{°C}\right]=539-423\cdot \%C-30,4\cdot \%\mathit{Mn}-17,7\cdot \%\mathit{no}-12,1\cdot \%\mathit{Cr}-11\cdot \%\mathit{si}-7,5\cdot \%\mathit{Mon}$$

estimate.

In a next step, the hot-rolled flat steel product, which has been cooled to the cooling stop temperature, is coiled into a coil.

Optionally, the coil obtained is cooled a second time to room temperature. Room temperature is understood to mean a temperature in the range of 20°C - 50°C. The second cooling takes place slowly with a cooling rate which is at most 0.1 K/s, preferably at most 0.05 K/s. This avoids strong internal stresses after cooling in the coil.

The coil obtained is tempered as a coil to set the mechanical-technological properties. This tempering process preferably takes place in a hood annealer. Pure hydrogen is preferably used as the protective gas because of the excellent heat transfer and the resulting improved temperature control during the tempering process. Alternatively, however, nitrogen or a mixture of hydrogen and nitrogen can also be used as protective gas.

mit

BC

Breite des zum Coil aufgehaspelten Stahlflachproduktes in Metern

DC

Außendurchmesser des Coils in Metern

dC

Innendurchmesser des Coils in Metern

TG

Glühtemperatur in °C

tG,min

Mindesthaltezeit in Stunden

The heating rate to the annealing temperature T _G should not exceed 500 K/h. In order to avoid local overheating of the coil, the heating rate should preferably be a maximum of 300 K/h. To ensure uniform heating and thus a homogeneous structure and homogeneous mechanical and technological properties over the length and bandwidth of the tape, the holding time t _G should be greater than or equal to a minimum holding time t _G,min , for which the following applies: $$t_{G,\mathit{at\; least}}=150\cdot B_C\cdot \sqrt{D_C^2-{\mathrm{i.e}}_C^2}\cdot T_G^{-\frac{2}{3}}$$

with

B.C

Width of the steel flat product coiled in meters

DC

Outside diameter of the coil in meters

DC

Inner diameter of the coil in meters

TG

Annealing temperature in °C

tG, min

Minimum holding time in hours

The annealing temperature T _G during tempering must be selected depending on the analysis used and the mechanical properties to be set depending on the holding time. In particular, the annealing temperature T _G is at least 170°C, preferably at least 200°C and at most 600°C, preferably at most 450°C.

mit
t_G   Haltezeit in Stunden
C  Kohlenstoffgehalt in Gew.-%
$T_G^{ʹ}$

  Glühtemperatur in KThe steel flat product according to the invention must not be over-tempered in order to set the special microstructure with a large number of extremely fine cementite precipitations. The tempering state can be controlled very well using the Hollomon-Jaffe parameter (H _P ), which is calculated using the following formula: $$H_P=\frac{T_G^{ʹ}}{1000}\cdot \left(17,7-5,\mathrm{8th}\cdot \%C+{\log t}_G\right)$$

with
t _G holding time in hours
C Carbon content in % by weight
$T_G^{ʹ}$

annealing temperature in K

The Hollomon-Jaffe parameter is preferably limited to a maximum of 20, in particular a maximum of 15, preferably a maximum of 13.3, particularly preferably a maximum of 12.1.

Tempering of the flat steel product coiled into the coil is preferably followed by a third cooling to room temperature, the cooling rate being at least 10 K/h, preferably at least 20 K/h. When using a cooling hood, the cooling rate can be up to 500 K/h.

A coil manufactured in this way is then typically processed on a cut-to-length line to form flat strips.

The special feature of this invention lies in the method described with tempering of the coil before further processing into strip sheets on a cut-to-length line. The yield point and tensile strength level can be adjusted and the scattering of the mechanical properties over the length of the strip can be reduced by tempering. Normally, the tempering step results in a pronounced yield point R _eH instead of the yield point R _p0.2 in the non-tempered state. This pronounced yield point is disadvantageous for bending and edging processes, as already explained. In addition, the yield point ratio R _e /R _m increases to values > 0.95 as a result of tempering and is therefore at an unfavorably high level for processing by the end customer and for the necessary production reliability and component safety.

In the method described in the present invention, the processing of the coil into sheet metal takes place only after tempering. To do this, the coil is unwound and straightened and then divided into sheets. During the straightening process, a plastic deformation takes place, whereby the pronounced yield point R _eH , which was formed during the tempering process, is eliminated again. In addition, the yield strength ratio R _e /R _m is reduced to < 0.95, which ensures problem-free processing by the end customer.

The invention is explained in more detail using the following exemplary embodiments.

To demonstrate the effect of the invention, steel melts A - H according to the invention and, for comparison, melts I and J not according to the invention with the compositions given in Table 1 were melted and formed into slabs, thin slabs or strips with a thickness d _V of 2.5 mm to 260 mm been shed.

The temperature T _NR , the A _r3 temperature, the bainite start temperature B _S and the martensite start temperature M _S have been calculated for the steel melts A - J in the manner explained above. The result of these calculations is listed in Table 2.

The slabs or blocks cast from melts A - D and F - J have each been reheated to an austenitizing temperature T _WE , with which they entered a conventional reversing stand and then a conventional rolling mill to form a hot-rolling final temperature T _ET to a To be hot-rolled steel strip with a thickness d _W between 4 and 8 mm. A strip having a thickness of 3 mm was cast from melt E, in order to then be hot-rolled to a thickness of 1.5 mm. Tests with different thicknesses d _V (and d _W ) resulted in similar properties and are therefore not shown in detail here.

In the course of hot rolling, the flat steel products were first rolled over a minimum number n _W of rolling passes at a temperature that was above the temperature T _NR . The number n _W was determined in the manner explained above from the thickness d _V of the slabs and the final thickness d _W of the respective hot-rolled flat steel product in the tests. After passing through the rolling passes completed at temperatures above the temperature T _NR , the respective steel flat product, with the exception of example 10, was hot-rolled in at least one further rolling pass at a temperature below the temperature T _NR . Immediately after the last hot-rolling pass, the hot-rolled steel strips obtained by hot-rolling were accelerated at a cooling rate Θ _Q to a cooling stop temperature T _KS . After reaching the respective cooling stop temperature T _KS , the steel strips were slowly cooled to room temperature at a cooling rate Θ _Q '.

In Table 3, for each of the tests 1 - 16, the steel (A - J) from which the steel flat product processed in the respective test consisted, the thickness of the preliminary product d _V , the final thickness of the hot strip produced d _W , the number n _W of rolling passes , which has passed through the respective steel flat product at temperatures above the temperature T _NR , the degree of deformation φ _NR , the at hot rolling has been achieved at temperatures below the temperature T _NR , the austenitizing temperature T _WE , the hot rolling end temperature T _ET , the cooling stop temperature T _KS , the first cooling rate Θ _Q and the second cooling rate Θ _Q ' are indicated.

For each of the tests 1 - 16, Table 4 shows the steel (A - J) from which the steel flat product processed in the respective test was made, the width B _C and the outer diameter D _C and inner diameter d _C of the coil produced, the selected annealing temperature T _G in °C, the required minimum annealing time t _G,min and the actual annealing time t _G , the annealing temperature T _G ' in K, the third cooling rate Θ _QHG after annealing in the hood annealer and the Hollomon-Jaffe parameter H _P .

The structures of the hot-rolled flat steel products obtained in the tests were also examined. The result of this investigation is listed in Table 5. Here, M is the structural proportion of tempered martensite, B is the structural proportion of bainite, F + P is the structural proportion of ferrite and pearlite, RA is the structural proportion of retained austenite and D _Z the mean value of the diameter of the individual cementite particles and the proportion A _Z in area % of cementite particles with a diameter between 20 nm and 250 nm based on the total proportion of cementite particles.

According to DIN EN ISO 6892, the yield strength R _e , the tensile strength R _m , the elongation A ₅ , according to DIN EN ISO _148-1 the notched impact strength AV -20°C at a test temperature of -20 °C, A _V -40°C at a test temperature of -40 °C and A _V -40°C at a test temperature of -60 °C. In addition, the hardness Brinell HBW 5/750 was determined according to DIN EN ISO 6506-1. The results of these tests are summarized in Table 6. No pronounced yield point was found in any of the examples according to the invention, so that the yield point R _p02 for _Re is given in Table 6. It is also evident that the hot-rolled flat steel products produced from the steel analyzes (analyses I and J) not composed according to the invention and those produced with production parameters not according to the invention (experiments 3 and 7) have significantly different mechanical-technological properties compared to the flat steel products according to the invention. The reason for this are unfavorable analysis compositions or structural characteristics, which is why either the required strength values or the required minimum notched impact strengths are not achieved. <b>Table 1: Contents of the required alloying elements of this invention and proven analyses. Counterexamples are marked with (*). Elements not listed are included only as unavoidable impurities.</b> analysis C Mn Al si Nb Ti V Cr Mon no Cu B Approx A 0.091 1.42 0.092 0.201 0.025 0.007 0.337 0.198 0.0019 0.0014 B 0.087 0.89 0.087 0.023 0.011 0.223 0.0020 0.0017 C 0.133 1.43 0.088 0.177 0.024 0.529 0.205 0.0020 D 0.148 1:13 0.109 0.331 0.014 0.012 0.276 0.0023 0.0015 E 0.175 1.04 0.089 0.334 0.023 0.013 0.221 0.451 1,039 0.0015 f 0.182 0.86 0.034 0.003 0.016 0.026 0.440 0.352 0.256 G 0.210 1.02 H 0.096 0.84 0.131 0.230 0.029 0.013 0.016 0.215 0.243 0.147 0.146 0.0029 0.0039 I* 0.026* 1.36 0.041 0.190 0.023 0.325 0.212 0.0013 Y* 0.094 2.93* 0.051 0.228 0.025 0.243 0.165 analysis mg Sb Be SEM P S N Cr+Mo ca/s Ti/N CEV CET A 0.010 0.002 0.0037 0.535 0.93 1.89 0.43 0.27 B 0.009 0.003 0.0041 0.223 0.57 2.68 0.28 0.20 C 0.0082 0.009 0.005 0.0042 0.734 0.52 0.32 D 0.010 0.002 0.0044 0.276 0.79 3:18 0.39 0.27 E 0.011 0.002 0.0056 0.672 0.83 2.32 0.55 0.36 f 0.013 0.003 0.0061 0.440 2.62 0.46 0.31 G 0.38 0.31 H 0.0011 0.0072 0.0035 0.071 0.009 0.0070 0.458 0.44 1.86 0.35 0.23 I* 0.011 0.002 0.0039 0.537 0.65 0.36 0.20 Y* 0.009 0.003 0.0054 0.408 4.63 0.66 0.41 analysis T _NO A _r3 B _S M _S A 956 778 566 450 B 1019 801 611 473 C 972 754 559 429 D 873 774 574 435 E 943 765 542 405 f 964 753 595 424 G 984 728 608 419 H 968 813 584 463 I* 897 815 573 479 Y* 890 686 517 404 attempt analysis d _v d _w n _w ϕ T _WE T _{ET TKS} Θ _{Q ΘQ} ' [mm] [mm] [1] [1] [°C] [°C] [°C] [°C/s] [°C/s] 1 A 260 5 12 0.35 1298 873 76 60 0.003 2 A 260 6 10 0.29 1284 881 122 65 0.003 3* A 260 6 10 0.36 1256 758* 87 55 0.003 4 B 215 8th 8th 0.37 1249 854 265 60 0.002 5 B 215 4 8th 0.43 1275 883 86 55 0.002 6 C 250 6 10 0.26 1297 865 126 55 0.002 7* C 250 6 11 0.28 1278 869 132 20* 0.003 8th D 260 8th 10 0.31 1302 838 106 60 0.001 9 D 260 5.5 10 0.29 1281 861 93 60 0.001 10 E 3 1.5 1 0 1297 1136 78 44 0.003 11 f 215 5 10 0.26 1287 794 118 50 0.003 12 f 215 5 10 0.29 1276 874 81 55 0.002 13 G 180 4 8th 0.33 1297 878 97 60 0.003 14 H 180 4 10 0.30 1283 865 86 55 0.001 15 I* 260 6 12 0.35 1297 867 103 60 0.003 16 Y* 260 6 10 0.34 1297 878 73 60 0.003 attempt analysis B _{C DC dc} T _G t _{G, min} t _{G tg} ' Θ _{QHG HP} [m] [m] [m] [°C] [H] [H] [K] [°C/s] 1 A 1,531 1,462 0.76 350 5.78 8.2 623 35 11:27 2 A 1,527 1,468 0.76 325 6.09 8.2 598 30 10.82 3* A 1,536 1,466 0.76 350 5.82 8.2 623 40 11:27 4 B 1,262 1,475 0.76 350 4.82 7.6 623 35 11:26 5 B 1,267 1,439 0.76 365 4.55 7.6 638 35 11.53 6 C 1,334 1,824 0.76 320 7.09 9.5 593 25 10.62 7* C 1,329 1,813 0.76 345 6.67 9.8 618 25 11.07 8th D 1,326 1,451 0.76 290 5.61 9.5 563 40 10.03 9 D 1,329 1,517 0.76 410 4.74 7.6 683 45 12:10 10 E 1,025 1.213 0.61 360 3:19 4.1 633 35 10.95 11 f 1,536 1,487 0.76 320 6.29 8.9 593 15 10:43 12 f 1,533 1,512 0.76 320 6.42 8.9 593 30 10:43 13 G 0.803 1.216 0.61 330 2.65 4.1 603 15 10:31 14 H 0.808 1.194 0.61 335 2.58 4.1 608 35 10.80 15 I* 1,326 1,513 0.76 350 5.24 6.8 623 30 11.45 16 Y* 1,325 1,536 0.76 370 5:15 8.9 643 35 11.64 attempt analysis M B F+P RA D _Z ** _AZ ** X _Z ** [area %] [Volume-%] nm [area %] number/m ² 1 A 100 < 1 < 1 < 1 103 98.7 6.2 • 10 ¹² 2 A 100 < 1 < 1 < 1 83 99.3 8.1 • 10 ¹² 3* A 65 < 1 35 < 1 4 B 80 20 < 1 < 1 84 98.4 1.7 • 10 ¹² 5 B 100 < 1 < 1 < 1 88 99.2 1.3 • 10 ¹³ 6 C 95 5 < 1 < 1 72 99.6 6.5 • 10 ¹² 7* C < 1 5 95 < 1 8th D 95 5 < 1 < 1 77 99.6 7.6 • 10 ¹² 9 D 95 5 < 1 < 1 108 98.2 4.4 • 10 ¹² 10 E 98 < 1 < 1 < 2 65 98.8 7.3 • 10 ¹² 11 f 95 5 < 1 < 1 69 99.7 8.2 • 10 ¹² 12 f 100 < 1 < 1 < 1 68 99.8 8.5 • 10 ¹² 13 G 90 8th < 1 < 2 72 99.8 8.1 • 10 ¹³ 14 H 100 < 1 < 1 < 1 94 99.7 7.4 • 10 ¹² 15 I* 5 70 25 < 1 16 Y* 95 < 1 < 1 5 98 98.4 6.3 • 10 ¹² ** D _Z , A _Z and X _Z were only determined for exemplary embodiments with a minimum proportion of martensite + bainite of 90% attempt analysis _Rp0.2 R _{m Rp0.2} / _Rm A _{5 AV} - 20°C _AV - 40°C _AV - 60°C HBW [MPa] [MPa] [1] [%] [J/cm ² ] [J/cm ² ] [J/cm ² ] [1] 1 A 10 ¹³ 1095 0.93 9.2 105 81 63 345 2 A 1037 1124 0.92 10.3 97 68 59 368 3* A 659 713 0.92 12.7 45 30 18 232 4 B 1026 1137 0.90 10.1 112 86 71 357 5 B 1043 1132 0.92 10.8 104 83 69 362 6 C 1146 1286 0.89 8.3 98 76 58 406 7* C 562 765 0.73 12.2 130 113 86 231 8th D 1202 1319 0.91 7.4 85 59 47 412 9 D 1178 1257 0.94 8.4 79 63 46 409 10 E 1206 1305 0.92 9.0 56 48 43 398 11 f 1186 1321 0.90 8.9 118 93 76 426 12 f 1225 1345 0.91 8.7 98 81 77 432 13 G 1298 1451 0.89 7.5 56 47 42 447 14 H 1018 1116 0.91 11.3 126 107 76 353 15 I* 614 698 0.88 13.4 78 66 52 219 16 Y* 1023 1156 0.88 6.2 39 28 18 363

Claims (14)

Hot-rolled flat steel product - consists of a steel with the following composition (in % by weight) - C : 0.03-0.3% by weight - Mn: 0.4-2.5% by weight - Optionally, one or more of the following elements in the proportion by weight indicated below: - Si: 0.05-1.5% by weight - Al: 0.01 - 0.3% by weight - B: 0.0005 - 0.007% by weight - Nb: 0.002 - 0.2% by weight - Cr: 0.05 - 2.5% by weight - Mo: 0.01 - 1.0% by weight - Mg: 0.0005 -0.005% by weight - H: up to 0.001% by weight - Ti: 0.002 - 0.2% by weight - V: 0.002 - 0.15% by weight - Ni: 0.05 - 10% by weight - Cu: 0.01-1.0% by weight - Ca: 0.0005 - 0.005% by weight - REM: 0.001-0.05 wt% - N: up to 0.01% by weight - P: 0.003 - 0.08% by weight - Sn: up to 0.05% by weight - As: up to 0.05% by weight - O: up to 0.03% by weight - Co: up to 0.2% by weight - W: up to - 0.2% by weight - Be: 0.001 - 0.1% by weight - Sb: 0.001-0.3 wt% - S: up to 0.01% by weight - Pb: up to 0.02% by weight - remainder iron and unavoidable impurities, and has a yield strength R _e of at least 680 MPa,
and the steel flat product does not have a pronounced yield point. Hot-rolled flat steel product according to Claim 1, characterized in that the ratio of yield point to tensile strength R _e /R _m is at least 0.84 and at most 0.95. Hot-rolled flat steel product according to one of Claims 1 to 2, characterized in that the sum of the proportions by weight of chromium (Cr) and molybdenum (Mo) is at most 1.2% by weight, preferably at most 0.85% by weight, particularly preferably at most is 0.65% by weight. Hot-rolled flat steel product according to one of Claims 1 to 3, characterized in that the carbon equivalent C _EV is at most 0.7, in particular at most 0.6, preferably at most 0.5. Hot-rolled flat steel product according to one of Claims 1 to 4, characterized in that the carbon equivalent C _ET is at most 0.7, in particular at most 0.5, preferably at most 0.35. Hot-rolled flat steel product according to one of Claims 1 to 5, characterized in that the flat steel product has a notched impact strength which is at least 25J/cm ² at a test temperature of -60°C and/or which at a test temperature of -40°C is at least 35J/cm ² and/or which is at least 50J/cm ² at a test temperature of -20°C. Hot-rolled flat steel product according to one of claims 1 to 6, characterized in that - the yield point is less than 890 MPa and the steel has a microstructure which comprises more than 50% by area of bainite, the remainder being martensite, ferrite, pearlite and retained austenite, with the maximum proportion of ferrite and pearlite being preferably 10% by area is at most 5 FI% and the proportion of retained austenite is preferably at most 5% by volume
or - the yield point is at least 890 MPa and the steel has a microstructure that comprises more than 50 FI% martensite, the remainder bainite, ferrite, pearlite and retained austenite, with the proportion of ferrite and pearlite being at most 10 FI%, preferably at most 5 FI% and the proportion of retained austenite is preferably at most 5% by volume. Hot-rolled flat steel product according to one of Claims 1 to 7, characterized in that the flat steel product has tempering carbides whose density is at least 10 ¹¹ m ^-2 , preferably at least 10 ¹² m ^-2 and at most 10 ¹⁴ m ^-2 , preferably at most 10 ¹³ m ^-2 amounts to. Hot-rolled flat steel product according to one of Claims 1 to 8, characterized in that the flat steel product has cementite particles, the proportion of cementite particles with a diameter of 20 nm to 250 nm being greater than 98% and in particular the mean value of the diameter of the cementite particles being between 50 nm and 150 nm. Method for producing a hot-rolled flat steel product designed according to one of the preceding claims, comprising the following work steps - Production of a steel melt with the following composition (in % by weight): - C : 0.03-0.3% by weight - Mn: 0.4-2.5% by weight - Optionally, one or more of the following elements in the proportion by weight indicated below: - Si: 0.05 - 1.5% by weight - Al: 0.01 - 0.3% by weight - B: 0.0005 - 0.007% by weight - Nb: 0.002 - 0.2% by weight - Cr: 0.05 - 2.5% by weight - Mo: 0.01 - 1.0% by weight - Mg: 0.0005 -0.005% by weight - H: up to 0.001% by weight - Ti: 0.002 - 0.2% by weight - V: 0.002 - 0.15% by weight - Ni: 0.05 - 10% by weight - Cu: 0.01-1.0% by weight - Ca: 0.0005 - 0.005% by weight - REM: 0.001-0.05 wt% - N: up to 0.01% by weight - P: 0.003 - 0.08% by weight - Sn: up to 0.05% by weight - As: up to 0.05% by weight - O: up to 0.03% by weight - Co: up to 0.2% by weight - W: up to - 0.2% by weight - Be: 0.001 - 0.1% by weight - Sb: 0.001-0.3 wt% - S: up to 0.01% by weight - Pb: up to 0.02% by weight - remainder iron and unavoidable impurities, - Casting of the molten steel into a preliminary product in the form of an ingot, a slab, a thin slab or a cast strip - Thorough heating of the preliminary product to an austenitization temperature T _WE between 1100°C and 1350°C - Hot rolling of the preliminary product to form the hot-rolled flat steel product at a final hot-rolling temperature T _E of at least 770 °C, in particular at least A _r3 +20K - First cooling of the obtained hot-rolled steel flat product with a cooling rate of 20 - 400 °C/s to a cooling stop temperature - Coiling the hot-rolled flat steel product, which has been cooled to the cooling stop temperature, into a coil - Optional second cooling of the coil obtained to room temperature - Tempering of the steel flat product coiled into the coil at an annealing temperature T _G , with the heating rate to the annealing temperature being a maximum of 500 K/h, and with a holding time t _G of the coil at the annealing temperature T _G greater than or equal to a minimum holding time t _G,min is, for which applies: $$t_{G,\mathit{at\; least}}=150\cdot B_C\cdot \sqrt{D_C^2-{\mathrm{i.e}}_C^2}\cdot T_G^{-\frac{2}{3}}$$

with B _C Width of the steel flat product coiled in meters D _C outside diameter of the coil in meters d _C inner diameter of the coil in meters T _G annealing temperature in °C t _G,min minimum holding time in hours - Uncoiling and straightening of the steel flat product Process according to Claim 10, characterized in that the annealing temperature T _G is at least 170°C, preferably at least 200°C and at most 600°C, preferably at most 450°C. The method according to claim one of claims 10 to 11, characterized in that the Hollomon-Jaffe parameter H _P during starting is a maximum of 20, in particular a maximum of 15, preferably a maximum of 13.3, particularly preferably a maximum of 12.1, the Hollomon-Jaffee -Parameter H _P is defined as: $$H_P=\frac{T_G^{ʹ}}{1000}\cdot \left(17,7-5,\mathrm{8th}\cdot \%C+{\log t}_G\right)$$

with t _G holding time in hours C Carbon content in % by weight $T_G^{ʹ}$

annealing temperature in K Method according to one of Claims 10 to 12, characterized in that the tempering of the flat steel product coiled into the coil is followed by a third cooling to room temperature, the cooling rate being at least 10 K/h, preferably at least 20 K/h. A method according to any one of claims 10 to 13, wherein the hot rolled steel flat product has a yield point and wherein - the yield point is less than 890MPa and the first cooling of the hot-rolled flat steel product obtained takes place at a cooling stop temperature of T _KS , where the following applies to the cooling stop temperature T _KS : $$\left({\mathrm{M}}_{\mathrm{S}}-\mathrm{30}\mathrm{°C}\right)\le {\mathrm{T}}_{\mathrm{KS}}<{\mathrm{B}}_{\mathrm{S}}$$

with B _S Bainite start temperature in °C M _S Martensite start temperature in °C or - the yield strength is at least 890MPa and the first cooling of the hot-rolled flat steel product obtained takes place at a cooling stop temperature of T _KS , where the following applies to the cooling stop temperature T _KS : $${\mathrm{T}}_{\mathrm{KS}}<\left({\mathrm{M}}_{\mathrm{S}}-\mathrm{100}\mathrm{°C}\right)$$

with M _S Martensite start temperature in °C