EP3856936A1

EP3856936A1 - Method for producing a coated flat steel product and coated flat steel product

Info

Publication number: EP3856936A1
Application number: EP18789005.8A
Authority: EP
Inventors: Richard Georg THIESSEN; Manuela Irnich; Jan-hendrik RUDOLPH; Bernd Linke; Rainer FECHTE-HEINEN
Original assignee: ThyssenKrupp Steel Europe AG; ThyssenKrupp AG
Current assignee: ThyssenKrupp Steel Europe AG; ThyssenKrupp AG
Priority date: 2018-09-26
Filing date: 2018-09-26
Publication date: 2021-08-04
Anticipated expiration: 2038-09-26
Also published as: EP4083236A1; ES2927204T3; CN112789358A; PL3856936T3; JP2021530624A; EP3856936B1; JP7029574B2; WO2020064096A1; CN112789358B

Abstract

The present invention relates to a method for producing a super-high-strength flat steel product provided with a metal coating and also to a coated flat steel product. The method comprises providing a hot-rolled flat steel product, which comprises a steel which consists of (in % by weight) 0.1-0.5% C, at least one element selected from the group consisting of Mn and Si, where the Mn content is 1.0-3.0% and the Si content is 0.7-2.5%, 0.05-1% Cr, up to 0.020% P, up to 0.005% S, up to 0.008% N, optionally one or more of the following elements 0.01-1.5% Al, 0.05-0.5% Mo, 0.0004-0.001% B and optionally in total 0.001-0.3% V, Ti and Nb, as the remainder iron and unavoidable impurities. The method also comprises pickling, cold rolling, heat-treating and hot-dip coating of the flat steel product with a zinc-based corrosion protection coating. The steel substrate has a microstructure which contains 5-20% by volume residual austenite, less than 5% by area bainite, less than 10% by area ferrite and at least 80% by area martensite, of which at least 75% by area is tempered martensite and less than 25% by area is untempered martensite. The coated flat steel product has in the boundary layer between the corrosion protection coating and the steel substrate a ratio of the sum of Si and Mn to Cr of at least 1.7 and at most 15. The ratio of the sum of Si+Mn to Cr is smaller in the boundary layer than in the base material.

Description

Process for producing a coated steel flat product and coated

Flat steel product

The present application relates to a method for producing a high-strength flat steel product provided with a metallic coating and to a coated flat steel product.

When we are talking about flat steel products, this means steel strips, steel sheets or blanks produced from them, such as blanks. When metallic coatings are mentioned here, they are understood to mean in particular metallic protective coatings and metallic anti-corrosion coatings.

High-strength steels are characterized by a high proportion of alloying elements that increase the strength of the material, such as silicon, manganese and chromium. Often, a surface-finishing layer is required to use high-strength steels, such as in automotive engineering, to avoid material corrosion. A surface-finishing layer can, for example, be applied electrolytically or by means of hot-dip coating, which is also referred to as hot-dip coating. Zinc-based coatings which are applied by means of hot-dip coating are of particular technical importance for corrosion protection.

When producing ultra-high-strength steels by means of fire coating, silicon, manganese and chromium accumulate in the area of the transition between the corrosion protection layer and the steel substrate, which can also be called the base material. In the present case, the boundary layer between the corrosion protection layer and the steel substrate or the base material is understood to be the layer that begins with the position between the corrosion protection layer and base material, in which the zinc and iron content have the same value in% by weight, to a depth of 300 nm into the base material. Enrichment of one or more of the elements silicon, manganese and chromium in the boundary layer has a negative effect on the performance properties of the coated steel flat product. For example, the adhesion of the corrosion protection layer to the base material deteriorates. But the formability of the coated flat steel product is also limited. Since the production of coated high-strength steels, which are alloyed with silicon, manganese or chrome, leads to problems with the adhesion of the coating and the formability of the coated steel flat product via a hot-dip coating system, these steels have so far only been electrolytically galvanized.

From EP2540854B1 an ultra high-strength, cold-rolled steel sheet is known which in mass% 0.15-0.30% C, 0.01-1.8% Si, 1.5-3.0% Mn, not more than 0, 05% P, not more than 0.005% S, 0.005-0.05% AI and not more than 0.005% N, optionally one or more elements made of 0.001-0, 10% Ti, 0.001-0, 10% Nb, 0.01-0.50% V, 0.0001-0.005% B, 0.01-0.50% Cu, 0.01-0.50% Ni, 0.01-0.50% Mo and 0, 01-0.50% Cr, and which has a soft surface portion containing at least 90% tempered martensite. The steel sheet has a tensile strength of not less than 1270 MPa. To soften the surface section, the steel sheet is decarburized in an atmosphere which has a high dew point of 30 ° C for 15-60 min at 700-800 ° C. The decarburizing annealing in an atmosphere with a high dew point over a relatively long period of time leads to a decarburized, ductile surface layer, which is then subjected to a coating treatment.

From US2016 / 230259A1 hot-dip coated steel sheets are known, which in mass% 0.08-0.20% C, 0.0-3.0% Si, 0.5-3.0% Mn, 0.001-0.10% P, not more than 0.200% S, 0.01-3.00% AI included. The sheets are decarburized. When annealing in an atmosphere containing 3-25 vol.% Hydrogen and 0.070% or less water vapor, an oxide layer of up to 5 pm thick forms in the interior of the steel sheet. The steel strip is specifically heated in an oven with direct flame heating in order to achieve a targeted oxidation of the surface. The disadvantage of this oxide layer, which consists only of iron, manganese and silicon, is that, due to the lack of chromium and a thickness of the oxide layer of up to 5 pm, the adhesion of metallic coatings can be impaired. In addition, a deterioration in the local formability is to be expected between the soft, decarburized and easily formable ferrite layer and the harder and brittle oxide layer.

Against this background, the object of the invention was to provide a method for producing a high-strength steel flat product coated by means of a fire coating system, which had good adhesion of the metallic coating to the Steel substrate and good formability of the coated steel flat product is guaranteed.

In addition, a high-strength, coated steel flat product should be specified, which has good adhesion of the metallic coating to the steel substrate and good forming properties.

With regard to the method, the object was achieved in that at least the process steps specified in claim 1 are carried out in the manufacture of a high-strength, coated steel flat product.

With regard to the flat steel product, the object was achieved by a product which has at least the features specified in claim 5.

The invention is based on the knowledge that the distribution of the main alloy elements silicon, manganese and chromium in the boundary layer has a significant influence on the adhesion of the corrosion protection coating. This applies in particular to zinc-based corrosion protection coatings. Silicon, manganese and chromium are strong oxide formers. Theoretically, silicon has a higher affinity for oxygen than manganese, manganese has a higher affinity for oxygen than chromium, and chromium has a higher affinity for oxygen than iron. Accordingly, it would be expected that, depending on the respective proportion of the element under consideration in the boundary layer, silicon oxides first form before manganese oxides and before chromium oxides. This applies on the assumption of equilibrium states and ideal conditions that can only be achieved theoretically, according to which all phases are pure phases and the formation of mixed phases is excluded, while the reaction kinetics and diffusion processes are not taken into account.

It was recognized that the distribution of silicon, manganese and chromium in the boundary layer can vary greatly and that the distribution can be influenced via the manufacturing parameters, such as the set temperatures and gas atmosphere.

The method according to the invention for producing a high-strength flat steel product provided with a metallic protective coating comprises at least the following working steps: a) Providing a hot-rolled flat steel product which comprises a steel which consists of (in% by weight)

0.1 - 0.5% C,

at least one element selected from the group consisting of Mn and Si, the Mn content being 1.0-3.0% and the Si content being 0.7-2.5%,

0.05-1% Cr,

up to 0.020% P,

up to 0.005% S,

up to 0.008% N,

and optionally one or more of the following elements

0.01 - 1.5% AI,

0.05 - 0.5% Mo,

0.0004 - 0.001% B

and optionally consists of a total of 0.001 - 0.3% V, Ti and Nb, and the remainder consists of iron and unavoidable impurities; b) pickling and cold rolling the hot-rolled flat steel product, the hot-rolled flat steel product undergoing a thickness reduction of at least 37%; c) two-stage heating of the cold-rolled flat steel product to a holding zone temperature THZ, which is above the A3 temperature of the steel, the heating first with a first heating rate Theta_H l of 5 - 50 K / s up to a 200 - 400 ° C Turning temperature TW and above the turning temperature TW with a second heating rate Theta_H2 of 2 - 10 K / s up to the holding zone temperature THZ; d) holding the flat steel product at the holding zone temperature THZ for a duration tHZ of 5-15 s in an oven atmosphere which contains 3-7% by volume hydrogen and the remainder nitrogen-wetted with water vapor and unavoidable impurities, the dew point of the oven atmosphere is between -22 ° C and 0 ° C; e) cooling the flat steel product from the holding zone temperature THZ to a temperature TLK which is not lower than 150 ° C. below the A3 temperature of the steel of the flat steel product, the duration for the cooling from THZ to TLK being at least 50 s and at most 300 s amounts to; f) cooling the flat steel product from the temperature TLK with a cooling rate ThetaQ of at least 30 K / s to a cooling stop temperature TAB, which lies between the start temperature of the Martian site and a temperature which is up to 175 ° C lower than TMS; g) holding the flat steel product at the cooling stop temperature TAB for a period of 10-60 s; h) heating the flat steel product with a heating rate ThetaBl, which is a maximum of 80 K / s, to a 450 - 500 ° C treatment temperature TB and optionally isothermally keeping the flat steel product at the treatment temperature TB, the total treatment time tBT for the heating and the optional isothermal hold is 10-1000 s; i) hot-dip coating the flat steel product with a zinc-based corrosion protection coating; j) optional tempering of the coated steel flat product at a temperature TGA of 500-565 ° C. for a duration tGA of 10s-60s; k) cooling the coated steel flat product to room temperature with a cooling rate ThetaB2 of at least 5 K / s.

In step a), a hot-rolled flat steel product produced using conventional casting and hot rolling processes is made available. The hot-rolled flat steel product provided in step a) is uncoated, which means that it has no metallic corrosion protection coating. The uncoated flat steel product forms the steel substrate or the base material for the metallic corrosion protection coating, which is applied in step i). The uncoated flat steel product comprises a steel, in particular it consists of a steel, the composition explained in more detail below.

The carbon content of the steel of a flat steel product according to the invention is 0.1-1.5% by weight. Carbon (C) influences the formation and stabilization of austenite. During the quenching, which is carried out to form martensite, and during the subsequent annealing treatment, the residual austenite which may be present is replaced by C stabilized. In addition, the C content has a strong influence on the strength of the martensite, which is formed during the cooling in step f) with a cooling rate ThetaQ, and on the strength of the martensite, which during the last cooling step in step k) with a cooling rate ThetaB2 is formed. The C content should be at least 0.1% by weight in order to ensure the austenite-stabilizing and strength-increasing effects. In a preferred embodiment, the C content is at least 0.12% by weight in order to be able to use the austenite-stabilizing and strength-increasing effects of the carbon particularly effectively. As the C content increases, the martensite start temperature is shifted to ever lower temperatures, so that if the C content is too high, no or only a too small proportion of martensite can be formed. Furthermore, the weldability of the flat steel product deteriorates with increasing carbon content. In order to ensure the formation of a sufficient proportion of martensite and good weldability, the C content of the steel of a flat steel product according to the invention is limited to at most 0.5% by weight, preferably at most 0.4% by weight.

The steel of a flat steel product according to the invention contains either manganese or silicon or both manganese and silicon.

If the steel of a flat steel product according to the invention is alloyed with manganese or with manganese and silicon, the manganese content is 1.0-3.0% by weight. Manganese (Mn) influences the hardenability of the steel and helps to avoid undesired pearlite formation during cooling. These requirements enable the formation of a suitable structure of martensite and residual austenite after quenching in step f) with cooling rates of less than 100 K / s. In order to reliably prevent the formation of perlite, the steel of a flat steel product according to the invention contains at least 1.0% by weight, preferably at least 1.9% by weight, of Mn. Since too high a concentration of Mn has a negative effect on weldability and increases the risk of strong segregations, which are chemical inhomogeneities in the structure that occur during solidification, the Mn content is at most 3 , 0 wt .-%, preferably at most 2.7 wt .-% limited. Manganese contents that are too high also lead to excessive accumulation of manganese in the boundary layer between the corrosion protection coating and the steel substrate and thus lead to poor adhesion. For this reason too, the Mn content is limited to at most 3.0% by weight, preferably at most 2.7% by weight. If the steel of a steel flat product according to the invention is alloyed with silicon or with silicon and manganese, the silicon content is 0.7-2.5% by weight, preferably at least 0.9% by weight. Silicon (Si) helps suppress the formation of cementite. When cementite is formed, carbon is bound in the form of carbides. By suppressing the formation of cementite, free carbon is available, which helps to stabilize the remaining austenite and thus to improve the elongation. This effect can also be partially achieved by alloying aluminum. If the Si content is too high, silicon can accumulate in the boundary layer between the corrosion protection coating and the base material, which leads to poor adhesion of the corrosion protection coating. In order to ensure good adhesion, the Si content is limited to a maximum of 2.5% by weight, in particular to less than 2.5% by weight. In a preferred embodiment, the Si content is limited to a maximum of 1.5% by weight in order to additionally reduce the risk of red scale formation which can occur during hot strip production.

The chromium content of the steel in a flat steel product according to the invention is 0.05-1% by weight. Chromium (Cr) helps to increase strength and is an effective inhibitor of pearlite. In addition, the accumulation of Cr in the boundary layer between the corrosion protection coating and the base material leads to improved adhesion. In order to ensure good adhesion properties, the Cr content is at least 0.05% by weight, preferably at least 0.1% by weight. At levels higher than 1.0% by weight, Cr increases the risk of pronounced grain boundary oxidation, which has an adverse effect on weldability and surface quality. To avoid pronounced grain boundary oxidation, the Cr content is limited to a maximum of 1.0% by weight. In a preferred embodiment, the Cr content is limited to a maximum of 0.6% by weight for cost reasons, which additionally contributes to further minimizing the risk of grain boundary oxidation.

Aluminum (AI) can optionally be contained in the steel of a flat steel product according to the invention with 0.01-1.5% by weight. AI can be used to deoxidize and set any nitrogen present. AI can also be used to suppress cementite. The addition of Al increases the austenitizing temperature of the steel. If higher annealing temperatures can be set, AI can be added with up to 1.5% by weight. Since aluminum increases the annealing temperature required for complete austenitization and complete austenitization is difficult with Al contents above 1.5% by weight, the Al content of the steel is one Flat steel product according to the invention limited to at most 1.5% by weight, preferably at most 1.0% by weight. In a preferred embodiment, the Al content to limit the austenitizing temperature is limited to a maximum of 0.1% by weight, in particular to 0.01-0.1% by weight.

Phosphorus (P), sulfur (S) and nitrogen (N) have a negative effect on the mechanical-technological properties of flat steel products according to the invention, which is why their presence in flat steel products according to the invention should be avoided as far as possible. Phosphorus (P) has an unfavorable effect on weldability, which is why the P content should be at most 0.02% by weight, preferably less than 0.02% by weight. Sulfur (S) leads to the formation of MnS or to the formation of (Mn, Fe) S at higher concentrations, which has a negative effect on the elongation. The S content is therefore restricted to values of at most 0.005% by weight, preferably to less than 0.005% by weight.

Nitrogen (N) leads to embrittlement of the steel, both in interstitially dissolved form and as nitride, for example in combination with titanium, niobium or vanadium, which can have a negative effect on the formability, which is why the N content does not exceed 0.008 % By weight, preferably to less than 0.008% by weight.

Steels of steel flat products according to the invention can optionally contain molybdenum (Mo) in contents of 0.05-0.5% by weight. Mo promotes the suppression of pearlite formation and for this purpose can be contained in the steel at least 0.05% by weight. For cost reasons, the Mo content is limited to a maximum of 0.5% by weight, in particular less than 0.5% by weight.

Steels of steel flat products according to the invention can optionally contain boron (B) in contents of 0.0004-0.001% by weight. Boron segregates to the phase boundaries and blocks their movement. This supports the formation of a fine-grained structure, which improves the mechanical properties of the flat steel product. In order to improve the mechanical properties, boron can be added in a content of at least 0.0004% by weight. When alloying with boron, there should preferably be enough Ti or Nb available to set N, which prevents the formation of harmful boron nitrides. In order to prevent the formation of boron nitrides, it has proven to be advantageous if a titanium content is chosen which is greater than 3.42 times the N content, or if a niobium content is selected which is greater than this Is 3.42 times the N content. The positive effect of B is saturated at a content of around 0.001% by weight, which is why the steel contains at most 0.001% by weight of B.

Steels of flat steel products according to the invention can optionally contain one or more microalloying elements in total contents of 0.001 to 0.3% by weight. In the present case, microalloying elements are understood to mean the elements titanium (Ti), niobium (Nb) and vanadium (V). Titanium or niobium or a combination of both are preferably used. The microalloying elements can form carbides with carbon, which in the form of very finely divided precipitates contribute to greater strength. With a total content of microalloying elements of at least 0.001% by weight, preferably at least 0.005% by weight, precipitates can occur which lead to the freezing of grain and phase boundaries during austenitizing. At the same time, however, carbon, which is favorable in atomic form for the stabilization of the remaining austenite, is bound as carbide. In order to ensure adequate stabilization of the residual austenite by carbon present in atomic form, the total concentration of the microalloying elements should be at most 0.3% by weight, preferably at most 0.2% by weight.

If information on alloy contents and compositions is given here, these refer to the weight or the mass, unless expressly stated otherwise.

In step b), the hot-rolled flat steel product is first pickled in a conventional manner and then subjected to cold rolling. Cold rolling causes the flat steel product to have a thickness reduction of at least 37%, in particular more than 37%. The thickness reduction refers to the difference between the initial thickness of the flat steel product before the first cold rolling pass and the final thickness of the flat steel product after the last cold rolling pass. Cold rolling with a thickness reduction of at least 37% brings about a mechanical homogenization of the material and leads to a particularly fine-grained structure with an average grain size of less than 30 pm in the cold-rolled state. The very fine-grained microstructure created by cold rolling provides many germ sites for the formation of austenite grains for the subsequent austenitizing annealing, which consequently also leads to a very fine-grained austenite. The grain-refining effect can be increased if a thickness reduction of preferably at least 42% is set during cold rolling. In addition, due to the fact that takes place during cold rolling Mechanical homogenization of the material facilitates the subsequent setting of the targeted ratio of Si, Mn and Cr in the boundary layer between the corrosion protection coating and the steel substrate.

In step c), the cold-rolled flat steel product is heated to an annealing temperature THZ above the Ar temperature of the steel, which can also be referred to as the holding zone temperature, in order to enable a complete structural transformation into the austenite. The A3 temperature of the steel depends on the analysis and can be estimated using the following empirical equation:

A3 [° C] = 910-15, 2% Ni + 44.7% Si + 31.5% Mo-21, l% Mn-203 * V% C with% C = C content of the steel in weight %,% Ni = Ni content of the steel in% by weight,% Si = Si content of the steel in% by weight,% Mo = Mo content of the steel in% by weight,% Mn = Mn content of the steel in% by weight.

In a preferred embodiment, the holding zone temperature THZ can be limited to a maximum of 950 ° C. in order to save operating costs.

Heating to THZ takes place in two stages. The flat steel product is first heated up to a turning temperature TW, which is 200 - 400 ° C, with a heating rate Theta_H l of 5 - 50 K / s. Above the turning temperature T_W the heating takes place until the holding zone temperature THZ is reached with a heating speed Theta_H2 of 2 - 10 K / s. The first heating rate Theta_H l is not equal to the second heating rate Theta_H2. In a preferred embodiment, Theta_H2 is smaller than Theta_H1.

In a preferred embodiment, the flat steel product is heated in a continuous furnace. In a particularly preferred embodiment, the flat steel product is heated in an oven which is equipped with ceramic radiant tubes, which is particularly advantageous for reaching strip temperatures above 900 ° C. Indirect heating also prevents undesirable strong oxidation of the steel surface combined with the formation of an oxide layer, since the oxygen components required for combustion do not come into contact with the material. Here, a gas mixture in one closed burner burned and the heat transfer takes place in this case by radiation. Such a furnace is also known as a Radiant Tube Furnace or RTF.

In step d), the flat steel product is held at the holding zone temperature THZ for a holding time tHZ of 5-15 s. The holding time tHZ should not exceed 15 seconds in order to avoid the formation of a coarse austenite grain as well as irregular austenite grain growth and thus negative effects on the formability of the flat steel product. The holding time should last at least 5 s in order to achieve a complete transformation into austenite and a homogeneous C distribution in austenite.

The atmosphere in which the flat steel product is kept contains 3 - 7 vol .-% hydrogen. The rest of the atmosphere is composed of nitrogen dampened with water vapor and unavoidable impurities, with a nitrogen content of 93-97% by volume being aimed for and the sum of all the components giving 100% by volume. In the present case, the information on the furnace atmosphere composition relates to atmospheric compositions totaling 100% by volume. For example, the atmosphere during holding consists of 3 - 7% by volume of hydrogen and the remainder of nitrogen dampened with water vapor and avoidable impurities. The proportion of water vapor in the atmosphere is regulated via the dew point. The dew point is set to values from -22 ° C to 0 ° C, preferably to values of at most -5 ° C, in particular to values from -22 ° C to -5 ° C, and particularly preferably to values of at least -20 ° C and / or at most -15 ° C, in particular set to values from -20 ° C to -15 ° C. The concentration curve of the elements Si, Mn and Cr in the boundary layer can be controlled by the dew point and concentration profiles of the elements Si, Mn and Cr in the boundary layer can be obtained.

The water vapor content is described via the dew point. The dew point corresponds to the temperature at which the water condenses in a gas volume. With low dew point values, the water content in the gas mixture is low. As the dew point increases, the water content in the gas mixture increases. The moistened gas mixture in the furnace atmosphere, in combination with the easier diffusion during the annealing, initially leads to an enrichment of the elements Mn, Si and Cr, which are less affine to oxygen than iron, on the surface of the base material. Due to the small size difference between manganese and iron, Mn diffuses faster in the iron lattice than Cr or Si. Chromium has a slightly slower diffusion than Mn, while silicon diffuses much slower. Of the The diffusion of the elements from the base material during the annealing in step d) counteracts enrichment. The out-diffusion is particularly pronounced for Mn, but can also be observed for Si. Cr, on the other hand, passivates near the surface through the formation of oxides. Cr is therefore enriched in the range of up to 300 nm below the surface of the base material. However, if the dew point of the gas mixture is less than -22 ° C, or if the oversupply of the elements Mn and Si, which are more affine to oxygen than Cr in the range of up to 300 nm below the surface of the base material, Cr also diffuses through the surface, which has a negative impact on the adhesion of the corrosion protection coating and on the formability.

In a preferred embodiment, the water vapor content in the furnace atmosphere, in particular while holding in step d), is more than 0.070% by volume, particularly preferably at least 0.080% by volume. The water vapor content in the furnace atmosphere is typically at most 1.0% by volume, preferably at most 0.8% by volume.

The gas composition can be controlled, for example, using an automated system. For this purpose, dry and moist gas components can be mixed with one another, nitrogen being used as the carrier gas for the water vapor. The nitrogen, moistened with water vapor, can be fed into the annealing furnace, for example, below the deflection roller. The annealing furnaces in which the flat steel product is subjected to an annealing treatment can be designed vertically or horizontally. The strip is passed through the furnace during the annealing process. For example, the direction of movement of the flat steel product in a vertical furnace is changed from downwards to upwards and vice versa by means of so-called deflection rollers.

By observing the annealing time, annealing temperature and atmospheric composition according to the invention with a dew point of -22 ° C to 0 ° C during annealing in working step d), it is ensured that the elements Si, Mn and Cr in the after performing step i ) coated steel flat product in the boundary layer between the corrosion protection coating and the steel substrate have the following ratio of the sum of Si and Mn to Cr:

1.7 <[(Si + Mn) / Cr] _GS <15 with Si: Si content in% by weight in the boundary layer; Mn: Mn content in% by weight in the boundary layer; Cr: Cr content in% by weight in the boundary layer.

One finding of the present invention consists in the fact that high Si and Mn contents in the boundary layer impair the coatability, whereas Cr does not have a negative influence, but instead has a positive influence on the adhesion of the corrosion protection coating if the above-mentioned ratio is observed. Maintaining the ratio of the oxide-forming elements Si, Mn and Cr in the boundary layer leads not only to an excellent adhesion of the corrosion protection coating but also to a good formability of the coated steel flat product.

By observing the annealing time, annealing temperature and atmospheric composition with a dew point of -22 ° C to 0 ° C during annealing in step d), it is further ensured that the elements Si, Mn and Cr have the following concentration gradient in the boundary layer:

[(Si + Mn) / Cr] _GS <[(Si + Mn) / Cr] _GW with [(Si + Mn) / Cr] _GS: ratio of the sum of the Si content in% by weight and the Mn content in% by weight to the Cr content in% by weight in the boundary layer;

[(Si + Mn) / Cr] _GW: ratio of the sum of the Si content in% by weight and the Mn content in% by weight to the Cr content in% by weight in the base material.

The element contents of the base material typically relate to a layer which is at a third of the thickness of the steel substrate.

By adjusting the concentration gradient from [(Si + Mn) / Cr] _GS to [(Si + Mn) / Cr] _GW, both the adhesion of the corrosion protection coating and the formability of the coated steel flat product can be improved.

In a preferred embodiment, the heating of the flat steel product takes place in work step c) and / or the holding in work step d) in a radiant tube furnace. In the furnace equipped with ceramic blasting tubes, the oxygen-containing combustion gases do not come into contact with the flat steel product because the gas mixture to be burned is in a closed burner is burned and the heat is transferred by radiation. Decarburization of the surface and strong oxidation of the surface of the uncoated steel flat product and the formation of a covering oxide layer can thereby be reduced and preferably avoided.

In step e) the flat steel product is cooled to a temperature TLK. The cooling starts after work in step d). In particular, cooling begins immediately after stopping, and thus at the latest after the maximum holding time of 15 s. The temperature TLK is not lower than 150 ° C below the A3 temperature of the steel of the flat steel product in order to avoid the formation of ferrite. The cooling time from THZ to TLK is at least 50 s and at most 300 s. The cooling carried out in step e) can also be referred to as controlled and slow cooling.

In step f) the flat steel product is cooled further from the temperature TLK to a cooling stop temperature TAB. The cooling from TLK to TAB takes place with a cooling rate ThetaQ, which is at least 30 K / s. The cooling can also be referred to as rapid cooling. The cooling rate ThetaQ is at least 30 K / s in order to avoid the formation of ferrite and the formation of bainite. Cooling can preferably be carried out at up to 120 K / s, which can be achieved, for example, by using modern gas jet cooling.

The cooling stop temperature TAB lies between the martensite start temperature TMS, that is to say the temperature at which a martensitic transformation begins, and a temperature which is up to 175 ° C. lower than TMS. The following applies:

(TMS-175 ° C) <TAB <TMS.

The martensite start temperature can be estimated using the following equation:

TMS [° C] = 539 ° C + (- 423% C - 30.4% Mn - 17.7% Ni - 12, l% Cr - ll% Si - 7% Mo) * ° C /% by weight with % C = C content of the steel in% by weight,% Mn = Mn content of the steel in% by weight,% Ni = Ni content of the steel in% by weight,% Cr = Cr content of the steel in% by weight,% Si = Si content of the steel in% by weight,% Mo = Mo content of the steel in% by weight. In step g), the flat steel product is kept at the cooling stop temperature TAB for a holding time tQ, which is between 10 and 60 seconds. tQ is used as a parameter for setting the structure, in particular the martensite content. By holding the flat steel product at the TAB temperature for 10 to 60 s, a very fine martensite structure with a small package size and a small lancet width can be set. In the subsequent treatment step of heating, this leads to short diffusion paths, which enables targeted local stabilization of the residual austenite.

In step h), the flat steel product is heated to a treatment temperature TB of 450-500 ° C. with a heating rate ThetaBl of at most 80 K / s in order to enrich residual austenite with carbon from the supersaturated martensite. The formation of carbides and the decay of residual austenite are avoided by observing a total treatment time for this work step of 10 - 1000 s. In addition, the treatment temperature TB is matched to the subsequent hot dip coating treatment. At 450 - 500 ° C, TB is also a suitable temperature for immersion in a zinc-based melt bath. Heating takes place at a heating rate of at most 80 K / s, in particular less than 80 K / s, to ensure adequate redistribution to ensure the carbon. In a preferred embodiment, the heating can be achieved, for example, by using jet pipes or by using a booster.

The total treatment time tBT is at least 10 and at most 1000 s in order to ensure a sufficient redistribution of the carbon. The total treatment time tBT is made up of the time tBR required for the heating and the time tBI during which the flat steel product is optionally kept isothermal.

In step i), the flat steel product is subjected to a coating treatment, in particular hot-dip coating. The flat steel product passes through a coating bath with a zinc-based molten bath composition. The temperature of the molten bath is preferably 450-500 ° C. A suitable molten bath composition can contain, for example, up to 2% by weight of AI, up to 2% by weight of Mg, the rest of zinc and unavoidable impurities, in particular of up to 2% by weight of AI, up to 2% by weight of Mg, Remaining zinc and unavoidable impurities exist. In a further preferred embodiment, a suitable molten bath composition can contain, for example, up to 1% by weight Al, balance zinc and unavoidable impurities contain, in particular consist of up to 1 wt .-% Al, balance zinc and unavoidable impurities. In a particularly preferred embodiment, a molten bath composition can contain 1-2% by weight Al, 1-2% by weight Mg, balance zinc and unavoidable impurities, in particular from 1-2% by weight Al, 1-2% by weight .-% Mg, balance zinc and unavoidable impurities exist. The coating treatment applies a corrosion protection coating to the flat steel product on at least one side of the flat steel product.

Immediately after step i), the flat steel product can be subjected to a galvannealing treatment in an optional step j). To do this, it is left on for a duration tGA of 10 s - 60 s at a temperature TGA of 500 - 565 ° C.

In step k), the coated flat steel product is cooled to room temperature with a cooling rate ThetaB2 of at least 5 K / s, preferably of more than 5 K / s. In the present case, the martensite formed in the course of the method according to the invention by the second quenching in step k) is referred to as non-tempered martensite. The martensite created by the first quenching after austenitizing, which is subjected to heating in step h), is also referred to as tempered martensite.

In a preferred embodiment, the atmospheric compositions which the flat steel product passes through in the further working steps, in particular in working steps e) to k), can be adapted to the furnace atmosphere of the holding process of working step d). An atmosphere is preferably set in at least one further working step, which further prefers 3-7% by volume of hydrogen and the rest with water vapor, preferably with at least 0.070% by volume, particularly preferably with at least 0.080% by volume with at most 1.0 vol .-%, particularly preferably with at most 0.8 vol .-% water vapor, moistened nitrogen and unavoidable impurities.

In a preferred embodiment, the method according to the invention for producing a high-strength flat steel product provided with a metallic corrosion protection coating does not comprise any further work steps and thus exclusively the work steps mentioned under a) - k). A product according to the invention comprises a steel substrate which comprises a steel, preferably consists of a steel which consists of (in% by weight): 0.1-1.5% C, at least one element selected from the group consisting of Mn and Si , wherein the Mn content is 1.0-3.0% and the Si content is 0.7-2.5%, 0.05-1% Cr, up to 0.020% P, up to 0.005% S, to to 0.008% N, as well as optionally from one or more of the following elements 0.01 - 1.5% AI, 0.05 - 0.5% Mo, 0.0004 - 0.001% B and optionally from a total of 0.001 - 0, 3% V, Ti and Nb, and the remainder consists of iron and unavoidable impurities.

The steel substrate has a structure that is 5-20 vol.% Residual austenite, less than 5 area% bainite, less than 10 area% ferrite and at least 80 area% martensite, of which at least 75 area% tempered martensite and less than 25% by area of non-tempered martensite. In a preferred embodiment, the structure of the product according to the invention consists of 5-20% by volume of residual austenite, less than 5 area% of bainite, less than 10 area% of ferrite and the rest of martensite, the martensite component of the total structure being at least 80 %, of which at least 75% by area is tempered martensite and less than 25% by area is tempered martensite.

A high proportion of martensite is used to achieve the desired strength. The ductility can be influenced by the proportion of tempered martensite. The entire martensite component present in the structure is composed of tempered and non-tempered martensite, with the possibility that there is no non-tempered martensite.

Unless otherwise stated, the information on the structural proportions for residual austenite is based on% by volume and for other structural components such as martensite, ferrite and bainite, based on area%.

The structure is particularly fine-grained and preferably has an average grain size of less than 30 pm. Due to the fineness of the microstructures, it is advisable to carry out the microstructural examinations on a scanning electron microscope (SEM) with at least 5000x magnification. A suitable method for the quantitative determination of the residual austenite is an investigation using X-ray diffraction (XRD) according to ASTM E975. The product according to the invention further comprises a metallic protective coating, preferably a Zn-based corrosion protection coating. A suitable corrosion protection coating contains up to 2% by weight of AI, up to 2% by weight of Mg, the rest of zinc and unavoidable impurities, in particular the corrosion protection coating consists of up to 2% by weight of AI and up to 2% by weight of Mg , Balance Zn and unavoidable impurities. In a particularly preferred embodiment, the corrosion protection coating has 1-2% by weight Al, 1-2% by weight Mg, balance zinc and unavoidable impurities, in particular it consists of 1-2% by weight Al, 1-2% by weight .-% Mg, balance zinc and unavoidable impurities. In an alternative preferred embodiment, the corrosion protection coating has up to 1% by weight of Al, the rest of zinc and unavoidable impurities, in particular it consists of up to 1% by weight of Al, the rest of zinc and unavoidable impurities.

The coated steel flat product according to the invention has a ratio of the sum of Si and Mn to Cr of at least 1.7 and at most 15 in the boundary layer between the corrosion protection coating and the steel substrate in accordance with the following relationship:

1.7 <[(Si + Mn) / Cr] _GS <15 with Si: Si content in% by weight in the boundary layer, Mn: Mn content in% by weight in the boundary layer, Cr: Cr content in% by weight in the boundary layer.

One finding of the present invention consists in the fact that high Si and Mn contents in the boundary layer have a negative influence on the coatability, whereas Cr has no negative influence, but instead has a positive influence on the adhesion of the corrosion protection coating if the above-mentioned ratio is observed. Studies have shown that the adhesion of the corrosion protection coating deteriorates when Si and Mn are enriched in the boundary layer, whereas the adhesion is significantly improved if chromium is also present. However, the addition of Cr is limited to a maximum of 1.0% by weight, preferably a maximum of 0.6% by weight, due to its negative effect on the grain boundary oxidation and to economic considerations, while minimum levels of Si and / or Mn are required to achieve the desired mechanical properties are required. A relatively strong enrichment of Si and / or Mn in the boundary layer leads locally to a pronounced oxide formation. These oxides lead to problems with the hot-dip coating and result in inadequate adhesion of the corrosion protection coating to the base material. However, the risk of liability errors is low if that Ratio of the sum of Si + Mn to Cr is at most 15, preferably at most 13. The risk of liability errors is also low if the ratio of the sum of Si + Mn to Cr is at least 1.7, preferably at least 2.5.

Cr enrichment in the boundary layer with a ratio of the sum of Si + Mn to Cr of at most 15, preferably at most 13, also has a positive effect on the forming behavior of the coated steel flat product. This is due to the fact that Cr counteracts the formation of Si and Mn oxides. Si and Mn oxides are brittle, which promotes the formation of cracks during forming. By maintaining the ratio of the oxide-forming elements Si, Mn and Cr at the boundary layer, hole expansion of over 25% can be set even for steels with very high tensile strengths of, for example, 1180 MPa and more.

According to the invention, the ratio of the sum of Si + Mn to Cr in the boundary layer is smaller than in the base material. The coated steel flat product has a concentration gradient between the boundary layer and the steel substrate or the base material, which can be represented by the following relationship:

[(Si + Mn) / Cr] _GS <[(Si + Mn) / Cr] _GW with [(Si + Mn) / Cr] _GS: ratio of the sum of the Si content in% by weight and the Mn content in% by weight to the Cr content in% by weight in the boundary layer,

[(Si + Mn) / Cr] _GW: ratio of the sum of the Si content in% by weight and the Mn content in% by weight to the Cr content in% by weight in the base material. The specification of the element contents of the base material typically relates to the composition at a third of the thickness of the steel substrate.

The fact that [(Si + Mn) / Cr] _GS is smaller than [(Si + Mn) / Cr] _GW ensures that the flat steel product has good adhesion of the metallic coating to the steel substrate and good forming properties. This effect can be achieved particularly safely if [(Si + Mn) / Cr] _GS is preferably less than 0.9 * [(Si + Mn) / Cr] _GW, particularly preferably less than 0.6 * [(Si + Mn ) / Cr] _GW is. The coated flat steel products preferably have a tensile strength Rm of at least 600 MPa, an elastic limit Rp02 of at least 400 MPa and an elongation A80 of at least 7%, in particular of more than 7%. Tensile strengths of 950 to 1500 MPa are typically achieved. The yield strength values are typically at least 700 MPa. The yield strength is below the tensile strength achieved. The yield strength is typically below 950 MPa. Furthermore, the coated steel flat products have excellent adhesion of the corrosion protection coating, preferably a level 1 adhesion determined according to the impact test according to SEP 1931, on the steel substrate and very good formability. For example, the hole expansion can be used as a measure of the formability. The hole expansion is typically at least 25%. The product of tensile strength and hole expansion can also be used as a measure of the formability. In a preferred embodiment, the product of tensile strength and hole expansion is at least 20,000 MPa *%, preferably at least 25,000 MPa *%.

The tensile strength, yield strength and elongation were determined in accordance with DIN EN ISO 6892, sample form 2, the adhesion was determined using a ball impact test KST in accordance with SEP 1931 and the hole expansion was determined in accordance with ISO 16630. The element distribution in the boundary layer as well as in the areas adjacent to the boundary layer can be carried out by means of the glow discharge spectroscopy (GDOES) method. For example, a GDOES measuring device from Leco can be used for this. With GDOES it is possible to carry out the quantitative determination of elements in layer structures along the layer thickness. The start of the boundary layer can be determined using GDOES by using the intersection of the curve of the Zn content and the Fe content as the starting point of the boundary layer, which extends from this intersection from 300 nm into the base material.

In a further preferred embodiment, the flat steel product according to the invention is produced by the method according to the invention explained above.

The invention is explained in more detail below on the basis of exemplary embodiments.

For testing, seven melts AG of the compositions given in Table 1 were produced, from which 11 hot strips with a thickness of 1.8 to 2.5 mm were produced in a conventional manner. The melts C, E, F and G correspond to the Requirements according to the invention for the steel composition, whereas melts A and B have too low Si contents and melt D has too low Si contents and too high Al contents.

The hot strips were pickled in a conventional manner and processed further with the production parameters given in Table 2. The hot strips were each rolled into cold strips with the cold rolling degree "KWG" given in Table 2, the cold strips were each heated to a turning temperature "TW" at a first, faster heating rate "ThetaFI l" and then with a second, slower heating rate "ThetaFI2" brought to the folded zone temperature "TFIZ", at which they were kept for a period of "tFIZ" from 5 to 15s in an atmosphere with a dew point "TP". The cold strips were then slowly cooled to an intermediate temperature "TLK" within a period of "tLK" from 50 to 300 s, then quickly quenched from the intermediate temperature "TLK" to a cooling stop temperature "TAB" at a cooling rate "ThetaQ" which they were held for a duration "tQ" of 10 to 60s. The flat steel products were then heated to a treatment temperature "TB" at a heating rate "ThetaBl" of at most 80K / s. The steel flat products were not kept at the treatment temperature. The flat steel products were then subjected to hot dip coating in a melt bath in an otherwise conventional manner, with the following composition: up to 2% by weight Al, up to 2% by weight Mg, balance zinc and unavoidable impurities. The flat steel products of the melts A - F were finally quenched to room temperature with a "ThetaB2" cooling rate of at least 5 K / s. The steel flat products of the melt G were initially tempered at a temperature TGA for a period of tGA after hot-dip coating and only quenched to room temperature after tempering at a cooling rate of at least 5 K / s.

Samples were taken from the flat steel products of tests A1-G12, on which the structure was examined and the mechanical properties were checked. The letters in the sample designation indicate from which of the melts shown in Table 1 the sample material originates. The results of the structural investigations are given in Table 3 and the results of the tests of the mechanical properties are given in Table 4. "MA" denotes the proportion of tempered martensite in the overall structure, "M" the proportion of non-tempered martensite in the overall structure, "F" the proportion of ferrite, "B" the proportion of bainite, "RA" the proportion of residual austenite. The structural investigations were carried out on cross-sections at 1 / 3t position, ie on sections which were removed at a third of the sheet thickness of the steel substrate. The sections were prepared for scanning electron microscopic (SEM) examination and treated with a 3% nital etch. Due to the fineness of the microstructures, the microstructure was characterized by SEM observation at 5000x magnification. The quantitative determination of the residual austenite was carried out by means of X-ray diffraction (XRD) according to ASTM E975. The GDOES analysis of the element distribution in the boundary layer and in the areas adjacent to the boundary layer was carried out on a further sample that was taken alongside the ground section. The elemental content of the base material was determined using the ICP-OES (inductively coupled plasma optical emission spectrometry) combustion analysis in a l / 3t position. The mechanical properties of the yield strength "Rp02", tensile strength "Rm" and elongation "A80" were tested in accordance with DIN EN ISO 6892: 2009, sample form 2, on longitudinal samples taken from the middle of the flat steel products. The adhesion of the zinc-based corrosion protection coating was determined as KST according to SEP 1931 and the hole expansion was determined according to IS016630.

The tests show that the samples C4, C5, E8 and F10 produced according to the invention have very low values for the ratio [(Si + Mn) / Cr] _GS of at most 15. At the same time, these samples show an excellent adhesion of the corrosion protection coating of less than 1.5 and a very good hole expansion of over 25%. In comparison, samples of steels of the same strength class, which, however, have a value higher than 15 for [(Si + Mn) / Cr] _GS, show poorer formability and poorer coating adhesion. Sample E9 shows that if the nitrogen in the gas mixture with water vapor is too humid and the dew point too low, it is still possible to achieve sufficient values for the product of tensile strength and hole expansion (tensile strength * hole expansion), but liability is corrosion protection coating impaired. In the samples of tests Al, B2 and Fl l it can be seen that the increasing difference between the yield strength and the tensile strength in the annealed material means that the product of tensile strength * hole expansion no longer achieves sufficient values. stated in% by weight, balance iron and unavoidable impurities. Underlined values lie outside the specifications according to the invention. table 1

underlined values lie outside the requirements of the invention. belle 2

Underlined values lie outside the specifications according to the invention. table 3

A = hole expansion

Underlined values lie outside the specifications according to the invention. table 4

Claims

l. Method for producing a high-strength flat steel product provided with a metallic protective coating, comprising at least the following steps: a) providing a hot-rolled flat steel product which comprises a steel which consists of (in% by weight)

0.1 - 0.5% C,

0.05-1% Cr,

up to 0.020% P,

up to 0.005% S,

up to 0.008% N,

and optionally one or more of the following elements

0.01 - 1.5% AI,

0.05 - 0.5% Mo,

0.0004 - 0.001% B

and optionally consists of a total of 0.001 - 0.3% V, Ti and Nb, and the remainder consists of iron and unavoidable impurities;

b) pickling and cold rolling the hot-rolled flat steel product, the

hot-rolled flat steel products experience a thickness reduction of at least 37%;

c) two-stage heating of the cold-rolled flat steel product to one

Holding zone temperature THZ, which is above the A3 temperature of the steel, the heating first with a first heating rate Theta_H l of 5 - 50 K / s up to a turning temperature TW of 200 - 400 ° C and above the turning temperature TW with a second heating rate Theta_H2 from 2 - 10 K / s up to the holding zone temperature THZ;

d) holding the flat steel product at the holding zone temperature THZ for a duration tHZ of 5-15 s in an oven atmosphere which contains 3-7% by volume hydrogen, and the remainder containing nitrogen dampened with water vapor and unavoidable impurities, the dew point of the oven atmosphere being between Is -22 ° C and 0 ° C; e) cooling the flat steel product from the holding zone temperature THZ to

Temperature TLK, which is not lower than 150 ° C below the A3 temperature of the Steel of the flat steel product, the time for cooling from THZ to TLK being at least 50 s and at most 300 s;

f) cooling the flat steel product from the temperature TLK with a cooling rate

ThetaQ of at least 30 K / s to a cooling stop temperature TAB, which lies between the martensite starting temperature TMS and a temperature which is up to 175 ° C. lower than TMS;

g) holding the flat steel product at the cooling stop temperature TAB for a period of 10-60 s;

h) heating the flat steel product with a heating rate ThetaBl, which is a maximum of 80 K / s, to a 450 - 500 ° C treatment temperature TB and optional isothermal holding of the flat steel product on the

Treatment temperature TB, the total treatment time tBT for the heating and the optional isothermal holding being 10-1000 s;

i) Hot dip coating the flat steel product with a zinc-based

Anti-corrosion coating;

j) optionally tempering the coated steel flat product at a temperature of 500-565 ° C for a period of 10s-60s;

k) cooling the coated steel flat product to room temperature with a

Cooling rate ThetaB2 of at least 5 K / s.

2. The method according to claim 1, characterized in that the hot dip coating is carried out in a molten bath which contains up to 2 wt .-% Al, up to 2 wt .-% Mg, the rest zinc and unavoidable impurities.

3. The method according to claim 1 or 2, characterized in that the dew point of the furnace atmosphere is between -22 ° C and -5 ° C.

4. The method according to any one of the preceding claims, characterized in that the heating of the flat steel product in step c) and / or the holding in step d) takes place in a radiant tube furnace.

5. Highest strength flat steel product provided with a metallic protective coating, characterized in that it comprises a steel substrate which comprises a steel which consists of (in% by weight): 0.1 - 0.5% C,

0.05-1% Cr,

up to 0.020% P,

up to 0.005% S,

up to 0.008% N,

and optionally one or more of the following elements

0.01 - 1.5% AI,

0.05 - 0.5% Mo,

0.0004 - 0.001% B

as well as optionally consisting of a total of 0.001 - 0.3% V, Ti and Nb, and the rest consisting of iron and unavoidable impurities,

wherein the flat steel product has a structure that

- 5 - 20% by volume of residual austenite,

- less than 5 area% bainite,

- less than 10 area% ferrite,

- at least 80 area% martensite, of which at least 75 area% is tempered martensite,

contains, wherein the flat steel product in the boundary layer between the corrosion protection coating and the steel substrate has a ratio of the sum of Si and Mn to Cr according to the following relationship:

1.7 <[(Si + Mn) / Cr] _GS <15 and the ratio of the sum of Si + Mn to Cr in the boundary layer is smaller than in the base material, so that:

6. Flat steel product according to claim 5, characterized in that there is a concentration gradient of between the base material and the boundary layer

[(Si + Mn) / Cr] _GS <0.9 * [(Si + Mn) / Cr] _GW.

7. Flat steel product according to claim 5, characterized in that it has a concentration gradient of between the base material and the boundary layer

[(Si + Mn) / Cr] _GS <0.6 * [(Si + Mn) / Cr] _GW.

8. Flat steel product according to one of claims 5 to 7, characterized in that it has a tensile strength Rm of at least 600 MPa, an elastic limit Rp02 of at least 400 MPa and an elongation A80 of at least 7%.

9. Flat steel product according to one of claims 5 to 8, characterized in that it has a hole expansion of at least 25%, a product of tensile strength and hole expansion of at least 20,000 MPa *% and / or a very good adhesion of the corrosion protection coating on the steel substrate.

10. Flat steel product according to one of claims 5 to 9, characterized in that the metallic protective coating contains up to 2 wt .-% Al, up to 2 wt .-% Mg, the rest zinc and unavoidable impurities.

11. Flat steel product according to claim 10, characterized in that the metallic protective coating contains 1-2 wt .-% Al, 1-2 wt .-% Mg, the rest zinc and unavoidable impurities.

12. Flat steel product according to one of claims 5 to 9, characterized in that the metallic protective coating contains up to 1 wt .-% Al, balance zinc and unavoidable impurities.

13. Flat steel product according to one of claims 5 to 12, characterized in that the Ti content of the steel substrate is more than 3.42 times the N content of the steel substrate or that the Nb content of the steel substrate is more than 3.42- times the N content of the steel substrate.

14. Flat steel product according to one of claims 5 to 13, characterized in that the flat steel product in the boundary layer between the corrosion protection coating and the steel substrate has a ratio of the sum of Si and Mn to Cr [(Si + Mn) / Cr] _GS of at most 13.

15. Flat steel product according to one of claims 5 to 14, characterized in that the flat steel product in the boundary layer between the corrosion protection coating and the steel substrate has a ratio of the sum of Si and Mn to Cr [(Si + Mn) / Cr] _GS of at least 2.5 having.