WO2020187419A1

WO2020187419A1 - Method for producing a hot-rolled flat steel product with different properties, a correspondingly hot-rolled flat steel product, and a corresponding use

Info

Publication number: WO2020187419A1
Application number: PCT/EP2019/057028
Authority: WO
Inventors: Dr. Rainer FECHTE-HEINEN; Christian Müller; Dr. Karina SCHUCK; Michael Bössler
Original assignee: Thyssenkrupp Steel Europe Ag
Priority date: 2019-03-21
Filing date: 2019-03-21
Publication date: 2020-09-24

Abstract

The invention relates to a method for producing a hot-rolled flat steel product with different properties. The invention additionally relates to a hot-rolled flat steel product with different properties and to the use of the hot-rolled flat steel product with different properties.

Description

Process for the production of a hot-rolled flat steel product with different properties, a corresponding hot-rolled flat steel product and a corresponding one

use

Technical Field

The invention relates to a method for producing a hot-rolled flat steel product with different properties. The invention also relates to a hot-rolled flat steel product with different properties. The invention also relates to a use of the hot-rolled with different properties.

Technical Background (Background Art)

In the industrial sector, there are many applications in which the components made from one material are subject to extreme abrasive wear, for example in flushing pipes which transport abrasive media inside. Another example are baffle plates, on which abrasive media slide along. Components that require increased resistance to abrasive wear are of increasing importance in industrial engineering areas such as construction and agricultural machinery or knife construction as well as for the production of pipes for the transport of media with abrasive components.

In order that acceptable service life of the components can be achieved under operating conditions, the highest possible hardness is required, which, however, is associated with a greatly restricted ductility for previous wear-resistant steels. However, a ductile component is also necessary for the intended use. In this case, the toughness of the overall system is a particularly decisive factor, which is required in order to achieve better performance than with a conventional wear / safety material in the case of occasional sudden loads.

In the field of security steel applications, a high level of hardness increases the penetration resistance against projectiles of any kind. However, steels with high hardness tend to break out on the side facing away from the load, particularly when subjected to shock loads. In the prior art, this is achieved, for example, by means of multi-layer materials, one layer having a high hardness and another the necessary toughness, cf. for example the statements in the European publication EP 2 123 447 A1. The production of such multi-layer material composites is very costly. For both areas of application (safety and wear), sheets are suitable which have a high hardness on a first surface and have increased toughness in the core and / or near the opposite surface or second surface. In addition to high strength and wear resistance, the steels produced for the purposes mentioned above have to meet high requirements in terms of behavior during cold forming and suitability for welding. Furthermore, in the construction machinery sector and for armoring civil vehicles, it would be advantageous if steels of this type had increased formability at the same time, so that smaller or narrower bending radii could be achieved during final processing and thus the degree of freedom in component design could be increased.

For example, from the international publication WO 2017/153265 A1, it is known to heat a flat product consisting of steel on one or both sides by means of a high-frequency heat source and then to cool it so that a hardness structure is established on the surface or surfaces. This type of heat treatment leads to sharp transitions between the different material properties within the material, which can reflect shock waves at the inner interfaces when subjected to shock loads, which can have negative effects and lead to early failure, which reduces the service life of the component. Furthermore, at least one further step (hardening, for example using a high-frequency heat source in conjunction with suitable coolants) with a corresponding investment in the process chain is required to set the desired properties.

There is still a need for optimization with regard to the cost-effectiveness of the production of steels with corresponding properties, in particular for use as safety or wear steel.

Summary of Invention

The invention is thus based on the object of providing a method for producing a hot-rolled flat steel product with which the required properties within the flat steel product can be adjusted more economically than in the prior art, specifying a correspondingly produced hot-rolled flat steel product and a corresponding use.

According to a first aspect of the invention, this object is achieved by a method having the features of claim 1. According to the invention, a method for producing a hot-rolled flat steel product is provided, comprising the steps:

Pouring a melt consisting of Fe and unavoidable impurities (in% by weight)

C: 0.05 to 0.9%, in particular 0.1 to 0.6%, preferably 0.2 to 0.5%, preferably 0.2 to 0.4%, optionally one or more alloy elements from the group ( Mn, AI, P, S, N, Si, Cr, Mo, Ni, Ti, V, Nb, W, Zr, Ca, B, Cu, Co, Be, Sb, Sn, SEM) with

Mn: 0.1 to 3%, in particular 0.5 to 2%, preferably 1.0 to 1.6%,

AI: 0.01 to 2%, in particular 0.015 to 1%, preferably 0.02 to 0.15%, preferred

0.02 to 0.05%,

P: up to 0.15%, in particular 0.003 to 0.05%, preferably 0.007 to 0.02%,

S: up to 0.02%, in particular 0.0002 to 0.01%, preferably 0.0005 to 0.008%, preferably 0.0008 to 0.005%, particularly preferably 0.001 to 0.003%,

N: up to 0.015%, in particular 0.0005 to 0.01%, preferably 0.001 to 0.008%, preferably 0.002 to 0.006%,

Si: up to 1.5%, in particular 0.01 to 0.8%, preferably 0.1 to 0.5%, preferably 0.15 to

0.35%,

Cr: up to 2%, in particular 0.02 to 1.5%, preferably 0.1 to 0.75%, preferably 0.15 to

0.45%,

Mo: up to 1.5%, in particular 0.01 to 0.8%, preferably 0.1 to 0.5%,

Ni: up to 2.5%, in particular 0.02 to 2.5%, preferably 0.05 to 2%, preferably 0.05 to

1.5%, particularly preferably 0.1 to 0.8%,

Ti: up to 0.2%, in particular 0.005 to 0.15%, preferably 0.01 to 0.1%, preferred

0.015 to 0.05%,

V: up to 0.2%, in particular 0.005 to 0.2%, preferably 0.01 to 0.15%, preferred

0.015 to 0.10%,

Nb: up to 0.2%, in particular 0.005 to 0.2%, preferably 0.01 to 0.15%, preferred

0.015 to 0.10%, W: up to 0.2%, in particular 0.005 to 0.2%, preferably 0.01 to 0.15%, preferred

0.015 to 0.10%,

Zr: up to 0.2%, in particular 0.005 to 0.2%, preferably 0.01 to 0.15%, preferred

0.015 to 0.10%,

Ca: up to 0.015%, in particular 0.0005 to 0.010%, preferably 0.0005 to 0.005%, preferably 0.001 to 0.004%,

B: up to 0.01%, in particular 0.0005 to 0.01%, preferably 0.001 to 0.005%, preferably 0.0015 to 0.004%,

Cu: up to 0.5%, in particular 0.01 to 0.3%, preferably 0.05 to 0.25%,

Co: up to 1%, in particular 0.01 to 1%, preferably 0.05 to 0.7%, preferably 0.1 to 0.5

%,

Be: up to 0.1%, in particular 0.002 to 0.05%, preferably 0.005 to 0.02%,

Sb: up to 0.3%; in particular 0.001 to 0.2%, preferably 0.002 to 0.1%, preferred

0.005 to 0.05%,

Sn: up to 0.3%, in particular 0.001 to 0.25%, preferably 0.005 to 0.2%, preferred

0.01 to 0.1%,

SEM: up to 0.05%, in particular 0.0005 to 0.02%, preferably 0.001 to 0.01%, the impurities including:

0: up to 0.005%, in particular up to 0.002%,

H: up to 0.001%, in particular up to 0.0004%, preferably up to 0.0002%,

As: up to 0.02%, in particular where the alloying elements specified as optional can alternatively also be tolerated as impurities in contents below the specified minimum limits without affecting the properties of the flat steel product, preferably not worsening them, to a preliminary product;

Reheating the intermediate product to a temperature or holding the intermediate product at a temperature at which the structure of the intermediate product consists essentially entirely of austenite, in particular a temperature of 1300 ° C. not being exceeded; Hot rolling of the intermediate product to a flat steel product in one or more roll stands with a final rolling temperature between 600 and 1200 ° C;

Cooling of the hot-rolled flat steel product to a temperature between room temperature RT and 500 ° C, the hot-rolled flat steel product being cooled asymmetrically, in such a way that at least in sections, different properties are set over the thickness and / or width and / or length of the hot-rolled flat steel product, at least A hardness difference of at least 50 HV is set from parts of a first to parts of a second surface of the hot-rolled flat steel product;

Coiling or storage of the asymmetrically cooled flat steel product and optionally cooling of the reeled or stored flat steel product to ambient temperature.

According to a second aspect of the invention, this object is achieved by a hot-rolled flat steel product having the features of claim 7.

According to the invention, a hot-rolled flat steel product is provided which, in addition to Fe and impurities that are unavoidable due to the manufacturing process, consists in% by weight

C: 0.05 to 0.9%, in particular 0.1 to 0.6%, preferably 0.2 to 0.5%, preferably 0.2 to 0.4

%, optionally one or more alloy elements from the group (Mn, Al, P, S, N, Si, Cr, Mo, Ni, Ti, V, Nb, W, Zr, Ca, B, Cu, Co, Be, Sb , Sn, SEM) with

Mn: 0.1 to 3%, in particular 0.5 to 2%, preferably 1.0 to 1.6%,

AI: 0.01 to 2%, in particular 0.015 to 1%, preferably 0.02 to 0.15%, preferably 0.02 to

0.05%,

P: up to 0.15%, in particular 0.003 to 0.05%, preferably 0.007 to 0.02%,

S: up to 0.02%, in particular 0.0002 to 0.01%, preferably 0.0005 to 0.008%, preferred

0.0008 to 0.005%, particularly preferably 0.0005 to 0.003%,

N: up to 0.015%, in particular 0.0005 to 0.01%, preferably 0.001 to 0.008%, preferably 0.002 to 0.006%, Si: up to 1.5%, in particular 0.01 to 0.8%, preferably 0.1 to 0.5%, preferably 0.15 to 0.35%,

Cr: up to 2%, in particular 0.02 to 1.5%, preferably 0.1 to 0.75%, preferably 0.15 to 0.45%,

Mo: up to 1.5%, in particular 0.01 to 0.8%, preferably 0.1 to 0.5%,

Ni: up to 2.5%, in particular 0.02 to 2.5%, preferably 0.05 to 2%, preferably 0.05 to 1.5%, particularly preferably 0.1 to 0.8%,

Ti: up to 0.2%, in particular 0.005 to 0.15%, preferably 0.01 to 0.1%, preferably 0.015 to

0.05%,

V: up to 0.2%, in particular 0.005 to 0.2%, preferably 0.01 to 0.15%, preferably 0.015 to

0, 10%,

Nb: up to 0.2%, in particular 0.005 to 0.2%, preferably 0.01 to 0.15%, preferably 0.015 to

0, 10%,

W: up to 0.2%, in particular 0.005 to 0.2%, preferably 0.01 to 0.15%, preferably 0.015 to

0, 10%,

Zr: up to 0.2%, in particular 0.005 to 0.2%, preferably 0.01 to 0.15%, preferably 0.015 to

0, 10%,

Ca: up to 0.015%, in particular 0.0005 to 0.010%, preferably 0.0005 to 0.005%, preferred

0.001 to 0.004%,

B: up to 0.01%, in particular 0.0005 to 0.01%, preferably 0.001 to 0.005%, preferred

0.0015 to 0.004%,

Cu: up to 0.5%, in particular 0.01 to 0.3%, preferably 0.05 to 0.25%,

Co: up to 1%, in particular 0.01 to 1%, preferably 0.05 to 0.7%, preferably 0.1 to 0.5%,

Be: up to 0.1%, in particular 0.002 to 0.05%, preferably 0.005 to 0.02%,

Sb: up to 0.3%; in particular from 0.001 to 0.2%, preferably from 0.002 to 0.1%, preferably from 0.005 to

0.05%,

Sn: up to 0.3%, in particular 0.001 to 0.25%, preferably 0.005 to 0.2%, preferably 0.01 to

0.1%,

0: up to 0.005%, in particular up to 0.002%,

H: up to 0.001%, in particular up to 0.0004%, preferably up to 0.0002%,

As: up to 0.02%, In particular, the alloying elements specified as optional can also alternatively be tolerated as impurities in contents below the specified minimum limits without affecting, preferably not worsening, the properties of the flat steel product, the hot-rolled flat steel product on a first surface, which in particular in later use is facing a load, at least partially a hardness of at least 200 HV, in particular at least 300 HV, preferably at least 380 HV, preferably at least 430 HV, more preferably at least 490 HV, particularly preferably at least 600 HV and at least in sections over its thickness and / or width and / or length has different properties, the hot-rolled flat steel product at least from parts of its first to parts of its second surface having a hardness difference of at least 50 HV, in particular of at least 100 HV, preferably of at least 150 HV , preferably of at least 200 HV.

If the hot-rolled flat steel product is subjected to a tensile test in its entire thickness, the result is a yield strength of at least 500 MPa, in particular of at least 650 MPa, preferably of at least 800 MPa, particularly preferably of at least 950 MPa. The tensile strengths can be between 800 and 1500 MPa and higher, the yield point as well as the tensile strength being determined as an integral variable over the (total) thickness of the hot-rolled flat steel product in the tensile test according to DIN EN ISO 6892-1.

This object is achieved according to a third aspect of the invention through the use of a hot-rolled flat steel product with the features of claim 12, as a flat or shaped component in the area of wear or safety applications.

Due to the interaction of the different properties, at least in sections over the thickness and / or width and / or length of the hot-rolled flat steel product, an economically manufactured flat steel product with high toughness and high hardness compared to the prior art can be provided, so that the flat product is preferably used as safety steel or Wear steel can be used.

After casting a melt with an alloy composition within the specified ranges to form a pre-product, for example in a continuous caster or casting and rolling plant, the pre-product can be further processed directly, i.e. coming directly from the casting heat, for example in the case of the casting and rolling plant, so that the pre-product is kept at one temperature or, if necessary, is reheated to a temperature, for example in a compensation or re-heating Heating furnace in which the most complete possible homogenization is guaranteed and in which any precipitates that may have formed dissolve as completely as possible. If, for example, the melt is cast into a preliminary product in a continuous caster, the cast and completely solidified strand is cut into slabs of finite dimensions and finally allowed to cool down to ambient temperature by natural cooling. Alternatively, to avoid hot cracks, complete cooling can either be completely avoided or significantly delayed with the aid of insulating storage or oven storage. The preliminary product or the slab is then reheated, for example in a walking beam furnace or by other suitable means, to a temperature at which the structure of the preliminary product consists essentially entirely of austenite, in particular at a temperature above Ac3, preferably above Ac3 + 50K ensure that the structure is completely austenitic. In order to reduce the deformation resistance, a temperature of at least 1000 ° C. is preferably selected; in order to ensure that any precipitates that may be present are dissolved as completely as possible, a temperature of at least 1100 ° C. is particularly preferred. The holding or reheating temperature should not exceed a temperature of 1300 ° C in order to avoid partial melting of the surface of the preliminary product. For ecological and economic reasons, the holding or reheating temperature is preferably limited to a maximum of 1260 ° C.

The warm preliminary product with a preferably essentially completely austenitic structure is hot-rolled in one or more roll stands with a final rolling temperature of between 600 and 1200 ° C. to form a flat steel product.

If the pre-product is to be hot-rolled, for example, into a sheet with a thickness between 3 and 150 mm, in particular between 8 and 150 mm, to ensure the most continuous and non-abrupt transition in hardness, preferably between 15 and 150 mm, hot rolling is carried out in one or more steps Roll stands, preferably in a four-high roll stand known to those skilled in the art, carried out at a final rolling temperature between 600 and 1200 ° C. In particular, a rolling end temperature of at least 800 ° C is selected for rolling sheet metal, preferably on a four-high roll stand, in order not to let the deformation resistance increase too much, in particular at least 900 ° C, preferably at least 950 ° C, is set as the rolling end temperature in order to use the grain-refining effect of the recrystallization after the roller passes as reliably as possible. For reasons of energy efficiency, a roller end temperature of a maximum of 1100 ° C. is selected. In order to avoid undesirable coarse grain formation, the final rolling temperature is preferably limited to a maximum of 1050 ° C. Alternatively, if the intermediate product is to be hot-rolled, for example, into a hot strip with a thickness between 1.5 and 25 mm, to ensure the desired hardness gradient, preferably over the thickness of the flat steel product, in particular between 6 and 25 mm, preferably between 8 and 20 mm, this is the case Hot rolling is carried out in several roll stands, in particular in a multi-stand finishing relay at a final rolling temperature between 600 and 1200 ° C, in particular between 750 and 1000 ° C. The final rolling temperature during rolling in a multi-stand finishing relay is preferably set to at least 850 ° C. to ensure the highest possible austenite content, and preferably to at least 880 ° C. to ensure recrystallization. To limit the amount of coolant required, final rolling temperatures of a maximum of 950 ° C. are particularly preferred, and a maximum of 930 ° C. is more preferably selected to avoid the undesirable formation of coarse grains.

Optionally, in an intermediate step, the hot-rolled flat steel product can be cooled, in particular to room temperature, for example in still air, and reheated, in particular to a temperature at which the structure of the hot-rolled flat steel product consists essentially entirely of austenite. In particular, in the manufacture of flat steel products in sheet form, first hot-rolled, then the rolled sheet cooled and (subsequently) the rolled sheet re-heated, for example completely austenitized in a roller hearth furnace.

According to the invention, a targeted heat treatment to set different properties takes place at least in sections over the thickness and / or width and / or length in a monolithic flat steel product either in the course of or after a hot rolling process, quasi in situ after the last rolling pass, or after an additional one Warming. If the targeted heat treatment takes place directly in the course of or after a hot rolling process, the hot-rolled flat steel product has, at the beginning of the cooling, in particular a temperature equal to or slightly below the rolling end temperature, which is referred to as the cooling start temperature. If the targeted heat treatment takes place following additional heating, the cooling start temperature is specifically set as part of this heating process.

The cooling start temperature is above an alloy-dependent Acl temperature to set an at least partially austenitic starting structure, for carbon contents up to 0.60% by weight, in particular at least at the Ac3 temperature, preferably at a temperature of at least Ac3 + 50 ° C to ensure a completely austenitic initial structure, preferably at at least 900 ° C or above in order to dissolve the highest possible proportion of any precipitations. In particular, for reasons of energy saving, the cooling start temperature is a maximum of 1000 ° C; cooling start temperatures of a maximum of 950 ° C are preferred to limit the amount of coolant required, and a maximum of 930 ° C is particularly preferred to avoid the undesired formation of coarse grains. For carbon contents above 0.6% by weight, the cooling start temperature is in particular between Acl and Acl + 100 ° C.

Starting from the cooling start temperature, the hot-rolled flat steel product is heat-treated or cooled by at least partially asymmetrical cooling in such a way that at least parts of a first to parts of a second surface of the hot-rolled flat steel product have a hardness difference of at least 50 HV. HV corresponds to the Vickers hardness and is determined according to DIN EN ISO 6507-1: 2016. The more strongly cooled side of the hot-rolled flat steel product is referred to as the first surface. Optionally, the cooling of the first surface is acted upon with locally differently strong cooling, which results in different cooling speeds, or the cooling is interrupted or reduced at a different temperature. The area of the first surface hardened to the greatest extent by the cooling is referred to as the hardness area. As a result, the hardness range is at least partially or partially available. Alternatively, the first side can be cooled uniformly and essentially completely, so that the hardness range is set essentially completely on the first surface. The surface opposite the first surface is referred to as the second surface and can also be cooled, but to a lesser extent or only up to a higher temperature than the first surface. Optionally, the cooling of the second surface is acted upon with locally differently strong cooling, which results in different cooling speeds, or the cooling is interrupted or reduced at a different temperature. The area of the second surface that is least hardened by the optional cooling is referred to as the ductile area.

The level of hardness can be influenced in particular as a function of the cooling rate and in particular also due to the alloy elements used, so that the hot-rolled flat steel product is preferably active at least on one side with a cooling rate of at least 30 K / s, in particular of at least 40 K / s, preferably of at least 60 K / s is cooled in order to be able to set a higher hardness on the first surface or in the hardness range of the first surface within the hot-rolled flat steel product compared to the second surface or the ductile range of the second surface. A liquid is preferred for cooling, in particular their water used. The active cooling takes place on the first surface, whereas the second surface of the hot-rolled flat product can be actively cooled, with a lower cooling rate compared to the cooling on the first surface, or passively cooled or not cooled at all, so that on the second surface, respectively in the ductile region of the second surface, a structure that is less hard and, above all, tougher, can set in comparison to the first surface or the hardness region of the first surface.

The local toughness of the flat steel product can be determined through complex preparation and subsequent testing, for example, of impact specimens in different layers across the thickness. However, it has proven to be sufficient for the application to only set and test the hardness in a targeted manner in order to be able to guarantee the toughness properties desired on the component.

To achieve an at least partially hardened structure, the asymmetrical cooling of the flat steel product is ended or at least significantly slowed down at a defined cooling stop temperature. In particular, after the cooling stop temperature has been reached, further cooling takes place in still or moving air, the flat steel product optionally being wound up beforehand into a coil. The cooling stop temperature is measured on the first surface or in the hardness range of the first surface and is below the alloy-dependent bainite start temperature Bs, preferably below the alloy-dependent martensite start temperature Ms, more preferably 50 ° C or more below Ms, particularly preferably 100 ° C or more below Ms.

In one embodiment, the hot-rolled flat steel product has a structure of martensite with at least 50 area%, in particular at least 60 area%, preferably at least 70 area%, preferably at least 80 area%, on its first surface or in the hardness area on the first surface, particularly preferably at least 90% by area, other or remaining structural components in the form of bainite, austenite, retained austenite, cementite, pearlite and / or ferrite can be present. In particular, in this embodiment the remaining non-martensitic structural component consists for the most part of bainite, with pearlite and / or ferrite preferably being present with up to 10 area%, preferably with up to 5 area%. The first surface or the hardness range of the first surfaces within the hot-rolled flat steel product preferably has a structure consisting of 100% martensite, whereby the highest possible hardness can be provided, in particular in connection with the corresponding alloying elements used. In an alternative embodiment, the hot-rolled flat steel product has a structure of bainite with at least 50 area%, in particular at least 60 area%, preferably at least 70 area%, preferably at least 80 area%, particularly on its first surface or in the hardness range on the first surface. %, particularly preferably at least 90% by area, it being possible for other or remaining structural components to be present in the form of austenite, retained austenite, martensite, cementite, pearlite and / or ferrite. In particular, in this embodiment the remaining non-bainitic structural component consists for the most part of martensite, with pearlite and / or ferrite preferably being present with up to 10 area%, preferably up to 5 area%.

On its second surface or in the ductile area of the second surface, the hot-rolled flat steel product has a structure of at least one of the structural components ferrite, pearlite, bainite and / or martensite (in total) with at least 70 area%, in particular with at least 80 area%, preferably with at least 90% by area, preferably at least 95% by area, with other structural components in the form of austenite, retained austenite and / or cementite, with martensite being less present in the structure of the second surface than in the structure of the first surface, wherein structural constituents in the form of martensite can be present in particular with a maximum of 40 area%, preferably with a maximum of 20 area%, preferably with a maximum of 5 area%, or particularly preferably no martensite can be present.

If there is a bainite content of at least 50 area% on the first surface or in the hardness area of the first surface, the bainite content on the second surface or in the ductile area of the second surface is to be set so that the bainite content of the second surface or the ductile area of the second surface is less than the summed up proportions of bainite and martensite on the first surface or in the hardness range of the first surface. The proportion of martensite on the second surface or in the ductile region of the second surface is in particular limited to a maximum of 10 area%, preferably to a maximum of 5 area%, or is preferably not present.

By setting the above-mentioned structural proportions or constituents, a tough area can be set on the second surface or in the ductile area of the second surface within the hot-rolled flat steel product. The level of toughness depends in particular on whether and with what content martensite is present, in particular also in connection with the corresponding alloying elements. In the context of the invention, both surfaces or, in their hardness or ductile range, up to a maximum of 5 area% production-related, unavoidable structural components such as cementite or other precipitates such as carbides, nitrides and / or oxides and their mixed forms are permitted.

Compared to the state of the art, the hardness curve between the first and second surfaces of the hot-rolled flat steel product has a curved (harmonic) curve, so that shock waves occurring in the event of impact loads can be diffusely scattered within the flat steel product / component, thereby essentially preventing premature failure can be. The curved (harmonic) course is characterized, for example, in that in a range of at least 5%, in particular of at least 10%, preferably of at least 20%, preferably of at least 30% of the thickness of the flat steel product, there is a local hardness Hl for which applies :

Hmin + (Hmax - Hmin) / HH <= Hl <= Hmax - (Hmax - Hmin) / HH, where Hmax is the maximum hardness on the first surface or in the hardness range on the first surface, Hmin is the minimum hardness on the second surface, respectively in the ductile range on the second surface and HH corresponds to a value of 20, in particular 10, preferably 5, preferably 3. HH is a factor that describes the degree of differentiation between the local hardness in the transition area compared to the maximum and minimum hardness. This factor is used to clearly delimit the hardness value of the transition area from the hardness values of the hardness area and the ductile area. If, in the ductile and / or hardness range, there is a structure made up of different components with significantly different hardnesses, an HH value of 10 or less should be used, in particular for reliable metrological delimitation of the areas.

The occurrence of a strong property gradient (sharp transition) can also be avoided according to the invention, as has been observed in the prior art, due to the chemical analyzes of the steel material composites or through the use of a high-frequency heat source with which heat treatment can be carried out specifically over the thickness . Due to the different properties or microstructures on the two surfaces, in particular due to the second surface with the tougher microstructure, the service life of the components can be increased in the event of occasional, sudden loads. Depending on the design of the hot-rolled and, in particular, asymmetrically cooled flat steel product, it is reeled as hot strip or stored as sheet metal, for example, with the reeled or stored flat steel product optionally being further cooled to ambient temperature or being allowed to cool to ambient temperature. Depending on the design, it is made available for further processing, for example for the production of components.

All information on the contents of the alloying elements specified in the present application are based on weight, unless expressly stated otherwise. All contents are therefore to be understood as figures in% by weight. The specified structural components are determined by evaluating light or electron microscopic examinations and are therefore to be understood as area proportions in area%, unless expressly stated otherwise. An exception to this is the structural component austenite or retained austenite, which is specified as a volume percentage in% by volume, unless expressly stated otherwise.

At the temperature Acl, the structure begins to transform into austenite and, in particular, is completely austenitic when the temperature Ac3 is exceeded. Bs bainite start, Bf bainite finish, Ms martensite start and Mf martensite finish indicate the temperatures at which a transformation into bainite or martensite begins or is completed. Acl, Ac3, Bs, Bf, Ms and Mf are characteristic values which depend on the composition (alloying elements) of the steel material used and can be taken from so-called ZTA or ZTU diagrams. Austenite can be converted into a hard structure, which in particular can predominantly have martensite and / or bainite, by means of suitable cooling speeds. The required cooling rates can also be taken from the ZTU diagrams, depending on the desired structure.

As the flat steel product is hot-rolled essentially in the horizontal plane, the active cooling is preferably carried out from the underside, which corresponds to the first surface or first side of the hot-rolled flat steel product. As a result of the asymmetrical cooling, on the cooled first side, for example, a transformation of austenite into bainite and / or martensite takes place in the structure, which is associated in particular with a greater volume increase, in particular on one side, which can lead to a deformation out of the plane of the flat product. If the active cooling were to take place exclusively on the upper side, which should not be excluded here, the deformation from the plane could have a negative effect on the transport properties. Furthermore, because the preferred active cooling takes place from the underside, measuring means, In particular, contactless measuring means, which are usually aimed at the upper side of the flat steel products produced, operate more precisely, since no stray coolant can negatively influence the measuring accuracy and thus better process control can be achieved.

According to one embodiment, the hot-rolled flat steel product or the component made therefrom can be subjected to a heat treatment, for example a tempering treatment, which can be carried out at a temperature below Acl, so that a renewed conversion into austenite is prevented. Depending on the tempering temperature, the hardness on the first surface or in the edge area of the first surface or on the first side can be reduced, but this can lead to an increase in toughness in the entire flat steel product or component and thus the susceptibility to cracks and / or breakouts can be reduced in the event of an abrasive and / or impact load.

In contrast to the production of a conventional, monolithic wear or safety steel with high hardenability, in particular a steel material is used according to the invention which has a high initial hardness with low hardenability. The alloying elements of the melt or the steel material are specified as follows:

Carbon (C) takes on several important functions in the flat steel product according to the invention. First and foremost, C is a martensite former and therefore essential for setting the desired hardness on the first surface or first side of the flat steel product in connection with the asymmetrical cooling, so that at least a content of 0.05% by weight, in particular for safe Setting a greater hardness difference at least a content of 0.1% by weight, preferably at least a content of 0.2% by weight is present. Furthermore, C contributes to a large extent to a higher CEV value (CEV = carbon equivalent), which has a negative impact on the weldability, so that a content of up to 0.9% by weight, to reduce the tendency to cracks, in particular up to a maximum of 0 , 6% by weight, preferably up to a maximum of 0.5% by weight, preferably up to a maximum of 0.4% by weight. The difference in hardness between the first and second surfaces of the flat steel product according to the invention is specifically adjusted by means of C and, in particular, the desired reinforcement of the difference in strength between the two surfaces or sides is specifically influenced. Furthermore, the specified upper limit can prevent negative influences on the toughness properties, the forming properties and the suitability for welding. Depending on the required formability and toughness, the C content can be added individually within the specified ranges. The steel flat product can optionally contain one or more alloy elements from the group (Mn, Al, P, S, N, Si, Cr, Mo, Ni, Ti, V, Nb, W, Zr, Ca, B, Cu, Co, Be, Sb, Sn, SEM) included.

Manganese (Mn) is an optional alloying element that can contribute to hardenability. At the same time, Mn reduces the tendency for undesired formation of pearlite during cooling and lowers the critical cooling rate, whereby the hardenability is increased. In addition, Mn can be used to set S, so that in particular a content of at least 0.1% by weight is present. Too high an Mn concentration, on the other hand, has a negative effect on weldability, so that Mn is limited to a maximum of 3% by weight. To ensure the desired formability, the content is limited in particular to a maximum of 2% by weight, and to improve the toughness properties, preferably to a maximum of 1.6% by weight. In order to reduce the pearlite content, a content of at least 0.5% by weight, preferably at least 1.0% by weight, is used in particular to set the desired strength properties.

Aluminum (Al) can be used as an optional alloying element in contents of at least 0.01% by weight. In particular, Al can be used to bind any nitrogen that may be present, so that optionally alloyed boron can develop its strength-increasing effect. A content of at least 0.015% by weight, preferably of at least 0.02% by weight, is therefore set in particular. AI can also be alloyed to reduce density and to suppress undesired cementite formation. To avoid problems with casting technology, the content is limited to a maximum of 2% by weight, in particular to a maximum of 1% by weight. If a reduction in density is not necessary and the formation of cementite is avoided by other measures, the maximum content of 0.15% by weight should not be exceeded in order to substantially reduce undesired precipitations in the material, in particular in the form of non-metallic oxidic inclusions / or to avoid, which can negatively influence the material properties. The preferred range is between 0.02 and 0.05% by weight in order not to reduce the hardenability too much due to the ferrite-stabilizing effect of Al, for example.

Phosphorus (P) is an optional alloying element that can be added in amounts of up to 0.15% by weight to increase the strength level. To ensure the desired increase in strength, in particular at least 0.003% by weight, preferably at least 0.007% by weight, are used. However, P has a strong toughness-reducing effect and therefore has an unfavorable effect on formability. P can also due to its low diffusion rate when solidifying lead to strong segregation of the melt. Negative influences on the formability can be safely excluded if the content is limited in particular to a maximum of 0.05% by weight, and preferably to a maximum of 0.02% by weight to further reduce the segregation effects.

Sulfur (S) can be used as an optional alloying element in contents of a maximum of 0.02% by weight in order to bring about a reduction in heat conduction through the formation of sulphides with Mn and / or Fe, in particular in the core of the hot-rolled flat steel product, which results in the desired Hardness difference in the material can be set particularly reliably. In order to achieve the desired effect, a content of at least 0.0002% by weight, preferably at least 0.0005% by weight, preferably at least 0.0008% by weight, particularly preferably at least 0.001% by weight, is required. -% used. However, S in steel has a strong tendency to segregate and can impair formability as a result of the excessive formation of FeS, MnS or (Mn, Fe) S. The content is therefore limited in particular to a maximum of 0.01% by weight, preferably to a maximum of 0.008% by weight, preferably to a maximum of 0.005% by weight, particularly preferably to a maximum of 0.003% by weight.

Nitrogen (N) can be used as an optional alloying element in contents of up to 0.015% by weight to form nitride and / or improve hardenability. In order to achieve this effect, contents of at least 0.0005% by weight, preferably of at least 0.001% by weight, preferably of at least 0.002% by weight, are used in particular. However, especially in connection with Al and / or Ti, N leads to the formation of coarse nitrides, which can have a negative effect on formability. The content is therefore limited to a maximum of 0.015% by weight, in particular to a maximum of 0.01% by weight, preferably to a maximum of 0.008% by weight, preferably to a maximum of 0.006% by weight.

As an optional alloying element, silicon (Si) can act as a deoxidation element in addition to Al and can therefore be added with a maximum content of 1.5% by weight. In particular, a content of at least 0.01% by weight is used to ensure effectiveness. However, it can also contribute to increasing the strength, so that a content of at least 0.1% by weight, preferably of at least 0.15% by weight, is preferably added. If too much Si is added to the steel, this can have a negative impact on the toughness properties, formability and weldability. The content is therefore limited in particular to a maximum of 0.8% by weight, preferably to a maximum of 0.5% by weight, preferably to a maximum of 0.35% by weight, to improve the surface quality. In an alternative embodiment, the Si content can be limited to a maximum of 0.03% by weight in order to ensure particularly good batch galvanizing capability for reasons of corrosion protection. Chromium (Cr), as an optional alloying element, can contribute to setting the difference in hardness, in particular to increasing the difference in strength between the two surfaces or sides of the flat steel product, in particular with a content of at least 0.02% by weight, since like C the conversion ( in austenite). For reasons of cost, the upper limit is defined as 2% by weight. If the content is too high, the weldability can be adversely affected, so that the content is limited in particular to a maximum of 1.5% by weight. In order to avoid a deterioration in the toughness of the second surface of the flat steel product, a maximum of 0.75% by weight, preferably a maximum of 0.45% by weight, is preferably alloyed. In order to reduce carbon diffusion and thus promote a non-equilibrium conversion, contents of at least 0.02% by weight, preferably at least 0.1% by weight, preferably at least 0.15% by weight, are alloyed. For components that do not have to be welded, Cr can be added up to the specified upper limit and, if the corresponding costs are less relevant, also beyond that. The use of the optional alloy element Cr is preferred, for example, when the flat steel product has a thickness of 15 mm and more, in particular of 20 mm and more, in order to ensure greater hardening.

Molybdenum (Mo) can be used as an optional alloying element to increase strength and hardness. Since it can contribute to strengthening the effectiveness of Cr or can replace the use of this alloying element, it can optionally be used with a content of up to 1.5% by weight, in particular between 0.01 and 0.8% by weight To achieve the greatest possible hardness difference and to reduce carbon diffusion, preferably between 0.1 and 0.5% by weight are added. Mo is particularly preferably alloyed together with Cr.

As an optional alloying element, nickel (Ni), like Cr, can improve the conversion (into austenite) and increase the strength, so that a content of up to 2.5% by weight can optionally be set. To ensure effectiveness, a content of at least 0.02% by weight is set in particular. To promote the desired phase transition, contents of at least 0.05% by weight are preferably used, and to increase the toughness it is particularly preferred to use at least 0.1% by weight. To improve the weldability, the content is preferably limited to a maximum of 2% by weight, for cost reasons preferably to a maximum of 1.5% by weight, particularly preferably to a maximum of 0.8% by weight.

As an optional alloying element, titanium (Ti) can increase strength through the formation of carbides, nitrides and / or carbonitrides and act as a micro-segregation element. In addition, the formation of a coarse austenite structure can be suppressed, in particular during reheating. There it can also contribute to increasing the effectiveness of Cr, it can optionally be added with a content of up to 0.2% by weight. For reasons of cost, the content is limited in particular to a maximum of 0.1% by weight, to reliably avoid the formation of excessively large titanium nitrides, preferably to a maximum of 0.1% by weight, preferably to a maximum of 0.05% by weight. To ensure effectiveness, a content of in particular at least 0.005% by weight is added. To utilize the strength-increasing effect, contents of at least 0.01% by weight, preferably of at least 0.015% by weight, are preferably used.

Vanadium (V), niobium (Nb), tungsten (W) and / or zirconium (Zr) can be added as optional alloying elements individually or in combination for grain refinement. These optional alloying elements, like Ti, can be used as micro-alloying elements in order to form strength-increasing carbides, nitrides and / or carbonitrides. To ensure their effectiveness, V, Nb, W and / or Zr can be used in particular with contents of (each) at least 0.005% by weight, preferably at least 0.01% by weight, preferably at least 0.015% by weight . For Nb, W and Zr, the minimum content, individually or in total, is particularly preferably at least 0.02% by weight, and for V, particularly preferably at least 0.04% by weight. The optional alloy elements are limited by weight to a maximum of 0.2% by weight, in particular to a maximum of 0.15% by weight, preferably to a maximum of 0.1% by weight, since higher contents have a detrimental effect on the material properties, in particular can have a negative effect on the toughness properties of the flat steel product.

Calcium (Ca) can be used as an optional alloying element of the melt as a desulphurisation agent and for targeted sulphide influence in contents of up to 0.015% by weight, in particular up to a maximum of 0.01% by weight, preferably up to a maximum of 0.005% by weight, preferably up to a maximum of 0.004 wt .-% can be added, which can lead to a changed plasticity of the sulfides during hot rolling. In addition, the cold forming behavior can also be improved by adding Ca. The effects described can be effective from a content of in particular at least 0.0005% by weight, preferably of at least 0.001% by weight.

As an optional alloying element, boron (B) can segregate on the phase boundaries and prevent their movement. This can lead to a fine-grain structure, which can have beneficial effects on the mechanical properties. To ensure the effectiveness of these effects, a content of up to 0.01% by weight, in particular for cost reasons up to a maximum of 0.005% by weight, and to reliably avoid embrittlement at grain boundaries, preferably up to a maximum of 0.004% by weight, as well as in particular to guarantee the safe effectiveness even in the presence of N, for example in the form of technically unavoidable contamination of the steel melt with N, in particular of at least 0.0005% by weight, preferably of at least 0.0010% by weight, preferably of at least 0.0015% by weight, to increase the fine grain size will. With the optional alloying of B, sufficient Ti should also be alloyed for the setting of N.

Copper (Cu) can be alloyed as an optional alloying element to increase strength and weather resistance with a content of up to 0.5% by weight. In order to ensure the strength-increasing effect, contents of in particular at least 0.01% by weight are added; contents of at least 0.05% by weight are preferably used to increase the weathering resistance. Levels in excess of 0.5% by weight provide no additional benefit. The content is limited in particular to a maximum of 0.3% by weight, preferably to a maximum of 0.25% by weight, in order to avoid negative influences on the weldability and the toughness properties in the heat-affected zone of a weld made on the flat steel product.

Cobalt (Co) can be used as an optional alloying element with contents in particular of at least 0.01% by weight to increase the hardness. To utilize this effect, contents of at least 0.05% by weight, preferably of at least 0.1% by weight, can be used. Contents below 0.01% by weight show no noticeable effect, but can be tolerated. For cost reasons, the Co content is limited to a maximum of 1% by weight, preferably to a maximum of 0.7% by weight, preferably to a maximum of 0.5% by weight.

Beryllium (Be) can be used as an optional alloying element in contents of up to 0.1% by weight in order to increase the wear resistance through the formation of high-strength carbides and / or oxides. To set the effectiveness reliably, contents of at least 0.002% by weight, preferably of at least 0.005% by weight, are used. For reasons of cost, the content is limited in particular to a maximum of 0.05% by weight, preferably to a maximum of 0.02% by weight. It is particularly preferred not to use Be because of its toxicity.

Antimony (Sb) can be added as an optional alloying element in contents of up to 0.3% by weight in order to reduce the susceptibility to grain boundary oxidation and, if higher contents are used, to increase the corrosion resistance in acidic media by segregating and there the tendency to generate hydrogen and thus to hydrogen-induced cracking is reduced or completely prevented. In order to achieve a reliable effect of the alloying, a content of at least 0.001% by weight, preferably of at least 0.002% by weight, is preferred of at least 0.005% by weight are used. For reasons of cost, the maximum content is limited in particular to 0.2% by weight, preferably to 0.1% by weight, preferably to 0.05% by weight. If there are no special requirements for corrosion resistance in acidic media, a maximum of 0.02% by weight is particularly preferably used to avoid embrittlement, particularly at grain boundaries. If an improved corrosion resistance in acidic media is required, in an alternative embodiment the minimum content is particularly preferably set at 0.02% by weight.

Tin (Sn) can be added as an optional alloying element in order to increase the corrosion resistance in acidic media and can be used for this purpose with a content of up to 0.3% by weight. To ensure at least a slight effectiveness, a content of at least 0.001% by weight is used. In order to ensure this effect to an increased extent, a content of at least 0.005% by weight, preferably of at least 0.01% by weight, is used. In order to avoid a deterioration in the toughness of the material, the upper limit is restricted in particular to a maximum of 0.25% by weight, preferably to a maximum of 0.2% by weight, preferably to a maximum of 0.1% by weight.

Rare earth metals such as cerium, lanthanum, neodymium, praseodymium, yttrium and others, which are individually or collectively abbreviated as SEM, can be added as optional alloying elements to bind S, P and / or O and the formation of oxides and / or sulfides as well To reduce or avoid phosphorus segregations at grain boundaries and thus increase toughness. In order to achieve a recognizable effect, when using SEM, a content of in particular at least 0.0005% by weight, preferably at least 0.001% by weight, is added. The SEM content is limited to a maximum of 0.05% by weight, in particular to a maximum of 0.02% by weight, preferably to a maximum of 0.01% by weight, in order not to form too many additional precipitates, which affects the toughness can negatively affect. For reasons of cost, preference is given to adding up to a maximum of 0.005% by weight of SEM.

In addition to iron, the flat steel product may contain one or more of the elements from the group (0, H, As) as unavoidable impurities, which are not specifically alloyed as alloy elements.

Oxygen (0) is an undesirable, but for technical reasons usually unavoidable impurity. The maximum content for 0 is given as up to 0.005% by weight, in particular up to 0.002% by weight. Hydrogen (H), as the smallest atom in interstitial spaces in steel, can be very mobile and, especially in high-strength steels, can lead to tears in the flat steel product when it cools down after hot rolling. The possible impurity H is therefore reduced to a content of a maximum of 0.001% by weight, in particular of a maximum of 0.0004% by weight, preferably of a maximum of 0.0002% by weight.

Arsenic (As) is an impurity that can be present in the hot-rolled flat steel product, the content being limited to a maximum of 0.02% by weight in order to avoid negative influences.

The alloying elements specified as optional can in particular alternatively also be tolerated as impurities in contents below the specified minimum limits without influencing the properties of the flat steel product, preferably not impairing them.

Since, according to an embodiment of the invention, no complete or uniform hardening of the flat steel product is sought, cost-intensive alloying elements such as Ni, Cr, Mo and / or other alloying elements, the upper limits of which have been set for economic reasons, can only be used optionally and in small amounts. The low or no content of these alloying elements in comparison to conventional through-hardening material concepts therefore enables alloying agent costs to be saved.

The hot-rolled flat steel product is available as sheet metal with a thickness between 3 and 150 mm, in particular between 8 and 150 mm, preferably between 15 and 150 mm, or as hot strip with a thickness between 1.5 and 25 mm, in particular between 6 and 25 mm, preferably carried out between 8 and 20 mm. For the production of components, blanks or blanks can be worked out or cut or cut from the sheet metal or hot strip, which can be flat or shaped, depending on the component design, also with narrow bending radii, by cold forming, especially in wear or safety applications.

If, according to one embodiment, the hot-rolled flat steel product has a completely or predominantly martensitic microstructure on its first surface or in the hardness range of the first surface, the production process according to the invention initially results in a curvature of the flat steel product due to the increase in volume during martensite formation, if the flat steel product is not geometrically fixed during cooling. If the aforementioned curvature occurs, this is compensated in particular by deformation at a lower temperature, preferably by bending back and forth in a straightening machine, with the strength of the more ductile material of the second surface or in the ductile area of the second surface also being increased by strain hardening can increase. In particular, the occurrence of the aforementioned curvature can be reduced or preferably completely prevented by geometric fixation, with the geometric fixation reducing or compensating for the curvature resulting from the volume increase through deformation directly in a corresponding device, for example in a quartet using suitable fixing means. This allows any stresses that may arise on the second surface of the flat steel product to be completely or partially reduced directly, since the temperature on the second surface (second side) is higher than that of the first surface (first side), while on the first surface it is preferably due to cooling Martensite is formed.

Components for agricultural or construction machinery are subject to very high abrasive wear due to their intended use. At the same time, the design aspect is becoming increasingly important for these components too. Conventional, conventional, hardened steels are unsuitable here, especially when implementing narrow beige radii, because they have the required strength but lack the necessary formability. Due to the different properties, in particular the different microstructure, at least in sections over the thickness and / or width and / or length, preferably only over the thickness of the flat steel product hot-rolled according to the invention, both high strengths and improved formability compared to the conventional design are achieved. In addition, in addition to its high hardness on its first surface or in the hardness range, the hot-rolled flat steel product according to the invention also has high toughness on its second surface or in the ductile range. As a result, in contrast to steels with a consistently high hardness on the first and second surface, it is also significantly better suited for use with a combination of abrasive and shock loads.

A preferred use of the hot-rolled flat steel product according to the invention can be used in planar or shaped design as a component in security applications, for example as bullet-proof security steel. Here, for example, the high hardness on one side can be aligned in the direction of a load case, with an impacting projectile on the high hardness side breaks and the tough part on the opposite side of the safety steel can absorb the kinetic energy of the projectile.

Another possible use of the hot-rolled steel product according to the invention are knives, in particular self-sharpening knives, such as, for example, delta knives or cutting strips for the agricultural or construction machine sector.

The method of production as well as the hot-rolled flat steel product according to the invention can contribute both to increasing the service life of the end product (component) and to reducing costs by eliminating the additionally necessary process steps of hardening, for example thermochemical hardening.

Brief Description of Drawing

The invention is explained in more detail below with the aid of a drawing showing an exemplary embodiment. In the single FIG. 1), a measured hardness curve and a calculated tensile strength curve within a hot-rolled flat steel product according to an exemplary embodiment are shown.

Description of the Preferred Embodiments (Best Mode for Carrying out the Invention)

In a first embodiment, a melt was made up of, in% by weight: C = 0.214%, Mn = 1.3%, Al = 0.03%, P = 0.014%, S = 0.002%, N = 0.0025% , Si = 0.243%, Cr = 0.179%, Mo = 0.038%, Ni = 0.14%, Ti = 0.023%, Ca = 0.001%, B = 0.003%, Cu = 0.074%, the balance Fe and unavoidable impurities , for example in a continuous caster, cast into a preliminary product and divided in the form of slabs. Alloying elements not specified here were not present in measurable contents or only as unavoidable impurities. The slabs were allowed to cool to ambient temperature. One of the slabs was provided and, for example, reheated or thoroughly heated in a walking beam furnace to a temperature of 1250 ° C., so that the structure of the preliminary product consisted entirely of austenite. After reheating, the slab was fed to a rolling train, in which it was first rolled in a reversing manner in a stand and then seven roll stands acted on the slab to reduce its thickness, and from this a flat steel product with a final thickness of 19 mm was hot-rolled, the final rolling temperature being 920 ° C. Immediately after the last rolling pass, the hot-rolled flat steel product was cooled asymmetrically, in particular in the form of a hot strip, a coolant in the form of water, in particular at high pressure, acted on the underside or on the first surface or the first side (hardness range) of the flat steel product and was therefore only actively cooled on one side at a cooling rate of 40 K / s, in particular up to was cooled down to a temperature of 400 ° C and then, in particular into a coil, coiled. The coil was then cooled to ambient temperature. After cooling, a blank was cut off from the coil as a sample and hardness tests were carried out to determine the Vickers hardness (HV10) at different points across the thickness of the sample. The results and a tensile strength calculated from the results are shown in the graph in FIG. The hardness profile shows that the active, asymmetrical cooling took place from the left side, which is the first surface or first side of the hot-rolled and asymmetrically cooled flat steel product. It is also evident that there is a hardness difference between the first surface (hardness range) and the second surface (ductile range) of the flat steel product of at least 50 HV.

In a second embodiment, a melt was made up of, in% by weight: C = 0.5%, Mn = 1.04%, Al = 0.02%, P = 0.009%, S = 0.003%, N = 0.005% , Si = 0.27%, Cr = 0.96%, Mo = 0.02%, Ni = 0.1%, V = 0.097%, Ca = 0.001%, B = 0.0015%, Cu = 0.024% , Remainder Fe and unavoidable impurities, for example in a continuous caster, cast into a preliminary product and separated in the form of slabs. Alloying elements not specified here were not present in measurable contents or only as unavoidable impurities. The slabs were allowed to cool to ambient temperature. One of the slabs was provided and, for example, reheated or warmed through in a pusher furnace to a temperature of 1150 ° C., so that the structure of the preliminary product consisted entirely of austenite. After reheating, the slab was fed to a four-high roll stand and hot-rolled in reversing motion to form a sheet with a thickness of 25 mm, the final rolling temperature being 820 ° C. The metal sheet was then stored in air and cooled to room temperature in the process. The sheet metal was then reheated in a roller hearth furnace to a temperature of 905 ° C. and fed directly to a quench. In the quette, it was geometrically fixed in several places with stamps in order to prevent excessive distortion due to the one-sided cooling, and then subjected to water from below until there was a surface on its underside or on the first surface (hardness range) of the flat steel product Temperature of 100 ° C had reached. The metal sheet was then cooled in air to room temperature. Since the one-sided cooling caused a slight curvature in spite of the geometric fixation, the sheet metal was then fed to a straightening machine to set the desired evenness. After cooling down and the Straightening, a blank was cut off from the sheet metal as a sample and hardness tests were carried out to determine the Vickers hardness (HV10) at different points across the thickness of the sample. The hardness profile showed that there was a hardness difference between the first surface (hardness range) and the second surface (ductile range) of the flat steel product of at least 100 HV.

In a third exemplary embodiment, a melt was made up of, in% by weight: 0 = 0.23%, Mn = 1.23%, Al = 0.03%, P = 0.015%, S = 0.001%, N = 0.005% , Si = 0.22%, Cr = 0.16%, Ti = 0.032%, Ca = 0.001%, B = 0.002%, the remainder Fe and unavoidable impurities, for example in a casting and rolling plant, cast into a preliminary product in the form of a cast strand. Alloying elements not specified here were not present in measurable contents or only as unavoidable impurities. Coming from the casting heat, the temperature of the cast strand was insufficient and the cast strand passed through a reheating furnace installed in a line and was reheated or warmed through to a temperature of 1200 ° C so that the structure of the preliminary product consisted entirely of austenite. After reheating, the pre-product ran through a rolling line connected in a line, in which seven roll stands acted to reduce the thickness of the pre-product and this was hot-rolled to a flat steel product with a final thickness of 20 mm, the final rolling temperature being 900 ° C. Immediately after the last rolling pass, the hot-rolled flat steel product, for example in the form of a hot strip, was asymmetrically cooled, with a coolant in the form of water, in particular at high pressure, acting on the underside or on the first surface or the first side of the flat steel product and thus only on one side was actively cooled at a cooling rate of 40 K / s, in particular cooled down to a temperature of 300 ° C. and then reeled, in particular into a coil. The coil was then cooled to ambient temperature. After cooling, a blank was cut off from the coil as a sample and hardness tests were carried out to determine the Vickers hardness (HV10) at different points across the thickness of the sample. Here, in a similar way to the previous exemplary embodiments, a hardness difference was determined which was more than 80 HV.

Further melts, with alloy elements as indicated in Table 1, in addition to Fe and unavoidable impurities, were cast into preliminary products. For the production of hot-rolled and asymmetrically cooled flat steel products, the procedure was chosen analogously to the second exemplary embodiment, but with a final rolling temperature of 900 ° C. The thickness of the flat steel products was in each case 25 mm. After cooling, each circuit board was used as a sample cut off and a hardness test to determine the Vickers hardness (HV10) was carried out 1 mm below the first and second surface on both sides of the respective flat steel product or the respective samples. In addition, the structural components were determined with a light microscope 1 mm below the two surfaces on both sides. The results for the respective samples are shown in Table 2. From Table 2 it can be seen that the hardness difference between the two surfaces or the hardness and ductile area on the first and second surface is at least 50 HV, in particular between 70 and 350 HV, preferably between 80 and 250 HV.

For example C, only part of the sheet metal was strongly cooled on one side, the remainder of the first surface or first side of the sheet metal being shielded from splashing water. Table 2 shows the hardness values of the hardness range and the second surface. For example D, part of the sheet metal on the second surface or side was shielded from the cooling water during the cooling, as a result of which this part of the sheet metal cooled down more slowly and thus became the ductile area. The rest of the sheet was greatly cooled on both sides. Table 2 shows the hardness values of the first surface and the ductile area.

Table 1

Table 2

Claims

1. A method for producing a hot-rolled flat steel product comprising the steps:

a) Casting a melt consisting of Fe and unavoidable impurities (in% by weight)

C: 0.05 to 0.9%, optionally one or more alloy elements from the group (Mn, Al, P, S, N, Si, Cr, Mo, Ni,

Ti, V, Nb, W, Zr, Ca, B, Cu, Co, Be, Sb, Sn, SEM) with

Mn: 0.1 to 3%,

AI: 0.01 to 2%,

P: up to 0.15%,

S: up to 0.02%,

N: up to 0.015%,

Si: up to 1.5%,

Cr: up to 2%,

Mon: up to 1.5%,

Ni: up to 2.5%,

Ti: up to 0.2%,

V: up to 0.2%,

Nb: up to 0.2%,

W: up to 0.2%,

Zr: up to 0.2%,

Ca: up to 0.015%,

B: up to 0.01%,

Cu: up to 0.5%,

Co: up to 1%,

Be: up to 0.1%,

Sb: up to 0.3%,

Sn: up to 0.3%,

SEM: up to 0.05%, where the impurities include:

0: up to 0.005%,

H: up to 0.001%,

As: up to 0.02%, to an intermediate product; b) reheating the intermediate product to a temperature or holding the intermediate product at a temperature at which the structure of the intermediate product consists essentially entirely of austenite; c) hot rolling of the intermediate product to form a flat steel product in one or more roll stands at a final rolling temperature between 600 and 1200 ° C .; d) cooling the hot-rolled flat steel product to a temperature between room temperature RT and 500 ° C, the hot-rolled flat steel product being cooled asymmetrically, in such a way that at least in sections, different properties are established over the thickness and / or width and / or length of the hot-rolled flat steel product, a hardness difference of at least 50 HV is set at least from parts of a first to parts of a second surface of the hot-rolled flat steel product; e) reeling or storing the asymmetrically cooled flat steel product and optionally cooling the reeled or stored flat steel product to ambient temperature.

2. The method according to claim 1, wherein in step c) the preliminary product is hot rolled in one or more roll stands at a final rolling temperature between 600 and 1200 ° C to form a sheet with a thickness between 3 and 150 mm.

3. The method according to claim 1, wherein in step c) the intermediate product is hot rolled in several roll stands at a final rolling temperature between 600 and 1200 ° C to form a hot strip with a thickness between 1.5 and 25 mm.

4. The method according to any one of the preceding claims, wherein after step c) and before step d) the hot-rolled flat steel product is cooled and reheated.

5. The method according to any one of the preceding claims, wherein in step d) the hot-rolled flat steel product is actively cooled at least on one side at a cooling rate of at least 30 K / s.

6. The method according to any one of the preceding claims, wherein a liquid is used for cooling.

7. Hot-rolled flat steel product, in particular produced according to one of the preceding claims, which, in addition to Fe and impurities which are unavoidable due to the manufacturing process, in% by weight

C: 0.05 to 0.9%, optionally one or more alloy elements from the group (Mn, Al, P, S, N, Si, Cr, Mo, Ni, Ti, V, Nb, W, Zr, Ca, B, Cu, Co, Be, Sb, Sn, SEM) with

Mn: 0.1 to 3%,

AI: 0.01 to 2%,

P: up to 0.15%,

S: up to 0.02%,

N: up to 0.015%,

Si: up to 1.5%,

Cr: up to 2%,

Mon: up to 1.5%,

Ni: up to 2.5%,

Ti: up to 0.2%,

V: up to 0.2%,

Nb: up to 0.2%,

W: up to 0.2%,

Zr: up to 0.2%,

Ca: up to 0.015%,

B: up to 0.01%, Cu: up to 0.5%,

Co: up to 1%,

Be: up to 0.1%,

Sb: up to 0.3%,

Sn: up to 0.3%,

SEM: up to 0.05%, whereby the impurities include:

0: up to 0.005%,

H: up to 0.001%,

As: up to 0.02%, the hot-rolled flat steel product at least partially has a hardness of at least 200 HV on a first surface and at least in sections different properties over its thickness and / or width and / or length, the hot-rolled flat steel product at least from Parts of its first to parts of its second surface has a hardness difference of at least 50 HV.

8. Hot-rolled flat steel product according to claim 7, wherein the hot-rolled flat steel product has a structure of martensite with at least 50 area% on its first surface, with other structural components in the form of bainite, austenite, retained austenite, cementite, pearlite and / or ferrite may be present , and has a structure of at least one of the structural components ferrite, pearlite, bainite and / or martensite with at least 70 area% on its second surface, with other structural components in the form of austenite, retained austenite and / or cementite, with martensite in the The structure of the second surface is less present than in the structure of the first surface.

9. Hot-rolled flat steel product according to claim 7, wherein the hot-rolled flat steel product has a structure of bainite with at least 50 area% on its first surface, with other structural components in the form of austenite, retained austenite, martensite, cementite, pearlite and / or ferrite may be present , and on its second surface has a structure of at least one of the structural components ferrite, pearlite, bainite and / or martensite with at least 70 area%, with other structural components in the form of austenite, retained austenite and / or cementite may be present, wherein the bainite content on the second surface is less than the summed up proportions of bainite and martensite on the first surface.

10. Hot-rolled flat steel product according to one of claims 7 to 9, wherein the hardness profile between the first and second surfaces of the hot-rolled flat steel product has a curved profile.

11. Hot-rolled flat steel product according to one of claims 7 to 10, wherein the hot-rolled flat steel product is in the form of a sheet metal with a thickness between 3 and 150 mm or in the form of a hot strip with a thickness between 1.5 and 25 mm.

12. Use of a hot-rolled flat steel product according to one of claims 7 to 11 as a flat or shaped component in the area of wear or safety applications.