WO2021213647A1

WO2021213647A1 - Hot-rolled flat steel product and method for the production thereof

Info

Publication number: WO2021213647A1
Application number: PCT/EP2020/061200
Authority: WO
Inventors: Nicholas WINZER; Ekaterina Bocharova; Roland Sebald
Original assignee: Thyssenkrupp Steel Europe Ag
Priority date: 2020-04-22
Filing date: 2020-04-22
Publication date: 2021-10-28
Also published as: CN115427589A; EP4139492A1; US20230151468A1

Abstract

The invention relates to a hot-rolled flat steel product of a thickness of < 1.5 mm which has optimized mechanical properties and is particularly suitable for application of a Zn-based corrosion protection layer by hot-dip coating. For this purpose, the flat steel product consists of, in % by mass, C: 0.04 - 0.23 %, Si: 0.04 - 0.54 %, Mn: 1.4 - 2.9 %, Ti + V , wherein the sum of %Ti+%V of the contents in Ti and V is such that 0.005 % < %Ti+%V < 0.15 %, and, in each case, optionally one or more elements of the group „AI, Cr, Mo, B" with contents that are, if applicable, as follows: AI: 0.01 - 1.5 %, sum of %Cr+%Mo of the contents in Cr and M: 0.02 < %Mo+%Cr < 1.4 %, B: 0.0005 - 0.005 %, the remainder consisting of iron and inevitable impurities, among these inevitable impurities being < 0.02 % P, < 0.005 % S, < 0.01 % N and < 0.005 % Nb. The structure of the flat steel product consists of, in percent by area, in sum, 50 - 90 % ferrite and bainite ferrite, 5 - 50 % martensite, 2 - 15 % residual austenite and < 10 % other structure elements. At the same time, the flat steel product has a yield point Rp0,2 > 290 MPa, a tensile strength Rm > 490 MPa and an elongation at break A80 which is calculated according to the following formula (1): A80 [%] = B - Rm / 37 with 31 < B < 51. To at least one surface of the flat steel product a Zn coating is applied by hot-dip coating. The invention also relates to a method for producing a flat steel product of this kind.

Description

HOT-ROLLED STEEL FLAT PRODUCT AND PROCESS FOR

ITS MANUFACTURING

The invention relates to a hot-rolled flat steel product which comprises a steel substrate and a zinc anti-corrosion layer applied thereon by hot-dip coating.

The invention also relates to a method for producing such a flat steel product.

In the present text, “flat steel products” are understood to mean rolled products, the length and width of which are each significantly greater than their thickness. These include, in particular, steel strips and steel sheets.

In the present text, unless explicitly stated otherwise, information on the content of alloy components is always given in% by mass.

The proportions of certain components in the structure of the steel substrate of a flat steel product are given in area%, unless otherwise noted.

In the present text, “impurities” of a steel, zinc or other alloy are technically unavoidable steel components that get into the steel during production or cannot be completely removed from it, but whose contents are in any case so low that they have no influence on the properties of the steel. The image analysis for quantitative structure determination is carried out optically by means of light microscopy ("LOM") with 200 to 2,000 times and with a scanning electron microscope ("SEM") with 2,000 to 20,000 times magnification.

The distribution of manganese (Mn) in the structure of the steel substrate of a flat steel product according to the invention has been determined by wavelength-dispersive X-ray micro-range analysis (WDX) of the structure, which was published, for example, by Reimer L. (1998) in "Elemental Analysis and Imaging with X-Rays" in Scanning Electron Microscopy, Springer Series in Optica! Sciences, Vol. 45. Springer, Berlin, Heidelberg.

The strength and elongation properties mentioned here, such as tensile strength Rm, yield point Rp0.2, uniform elongation Ag, elongation A50 and elongation A80 of flat steel products were determined in tensile tests according to DIN-EN 6892-1: 2017, unless otherwise noted.

Highly stressed passenger and truck components, such as crash structures and chassis of automobile bodies, require a hot-dip galvanized steel sheet with a thickness of more than 1.5 mm and a tensile strength of more than 590 MPa.

Hot-rolled flat steel products, which are made of complex phase steels (CP-W), the structure of which consists largely of bainite, are often used for the production of such components. CP-W steels, however, suffer from a relatively lower deformability, which prevents the design of geometrically complex components.

Dual-phase steels (DP), which consist of a combination of hard (e.g. martensite or bainite) and soft (e.g. ferrite) phases, are suitable for complex components due to their combination of high strength and good ductility. Cold rolled dual phase steels (DP-K) with thicknesses of more than 1.5 mm, however, have a higher sensitivity for surface defects, such as non-galvanized areas. That is why the maximum sheet thickness of hot-dip galvanized DP-K steels is usually limited to 2 mm.

The direct galvanizing of hot-rolled dual-phase steels (DP-W) is also not possible. For galvanizing, the sheet must be heated to temperatures of more than 460 ° C (the zinc bath temperature). At these temperatures, however, the hard component of the structure, in particular martensite, is tempered, with the result that the DP characteristics are lost.

One possibility would be to anneal and then galvanize a hot strip in a hot-dip coating plant with a typical DP-K annealing cycle (i.e. partial austenitization in the intercritical temperature range, i.e. in the temperature range between the Ac1 and Ac3 temperatures of the respective steel, in the a- and g-Fe arise in equilibrium). This procedure is similar to the manufacturing process of a DP-K steel with the exception of the cold rolling step. Here, however, there is the risk that omitting the cold rolling step will lead to poorer mechanical properties compared to those of a DP-K steel.

A high-strength multiphase steel with minimum tensile strengths of 580 MPa is known from DE 102012013 113 A1. The steel should preferably have a dual-phase structure and make it possible to produce cold-rolled or hot-rolled steel strips with improved forming properties, from which parts for lightweight vehicle construction in particular can be produced. The known multiphase steel consists of, in mass%, 0.075% ≤ C ≤ 0.105%, 0.600% ≤ Si ≤ 0.800%, 1,000% ≤ Mn ≤ 2.250%, 0.280% ≤ Cr ≤ 0.480%, 0.010% ≤ AI ≤ 0.060%, ≤ 0.020% P, ≤ 0.0100% N, ≤ 0.0150% S and the remainder of iron and impurities. Another high-strength multiphase steel with a minimum tensile strength of 580 MPa is the steel known from DE 102012006017 A1. This should also preferably have a dual-phase structure and be suitable for the production of cold-rolled or hot-rolled steel strips that have good forming properties. As such, parts for lightweight vehicle construction in particular should be able to be formed from these steel strips. For this purpose, the known steel consists of, in% by mass, 0.075% C 0.105%; 0.200% ≤ Si ≤ 0.300%, 1.000% ≤ Mn ≤ 2.000%, 0.280% ≤ Cr ≤ 0.480%, 0.010% ≤ Al ≤ 0.060%, up to 0.020% P, 0.005% ≤ Nb ≤ 0.025%, up to 0, 0 100% N, up to 0.0050% S and the remainder of iron and technically unavoidable impurities.

The steel known from DE 102013013067 A1 also belongs to the type of known multiphase steels explained above, which preferably have a dual-phase structure and are intended to be suitable for cold-rolled or hot-rolled steel strip with improved forming properties. This known steel should have a maximum yield strength ratio of 73% and, in% by mass, 0.075% ≤ C ≤ 0.105%, 0.600% ≤ Si ≤ 0.800%, 1,000% ≤ Mn ≤ 1, 900%, 0.100% ≤ Cr ≤ 0.700%, 0.010% ≤ AI ≤ 0.060%, 0.0020% ≤ N ≤ 0.0120%, ≤ 0.0030% S, 0.005% ≤ Nb ≤ 0.050%,

0.005% ≤ Ti ≤ 0.050%, 0.0005% ≤ B ≤ 0.0040%, ≤ 0.200% Mo, ≤ 0.040% Cu, ≤ 0.040% Ni and the remainder of iron and unavoidable impurities.

Against the background of the prior art explained above, the task arose of developing a flat steel product that not only has optimized mechanical properties, but is also particularly well suited for the application of a Zn-based corrosion protection layer by hot-dip coating. The invention has achieved this object by means of a flat steel product which has at least the features specified in claim 1.

In addition, the invention should specify a method with which the production of flat steel products obtained according to the invention is operationally reliable.

To solve this problem, the invention has proposed the method specified in claim 8. It goes without saying that when carrying out the method according to the invention, the person skilled in the art not only completes the method steps mentioned in the claims and explained here, but also carries out all other steps and activities that regularly occur in the practical implementation of such methods in the prior art be carried out if the need arises.

Advantageous embodiments of the invention are specified in the dependent claims and, like the general inventive concept, are explained in detail below.

The invention thus provides a hot-rolled flat steel product which comprises a steel substrate and a zinc (Zn) -based anti-corrosion layer applied thereon by hot-dip coating.

The steel of the steel substrate of a flat steel product according to the invention consists of, in% by mass,

C: 0.04-0.23%,

Si: 0.04-0.54%,

Mn: 1.4-2.9%,

Ti + V, where for the sum% Ti +% V of the contents of Ti and V the following applies:

0.005% ≤% Ti +% V ≤ 0.15%, as well as optionally one or more of the elements from the group "AI, Cr, Mo, B" with the proviso that their contents, if present, are measured as follows:

AI: 0.01 - 1.5%

Sum of the contents of Cr + Mo: 0.02 - 1.4%

B: 0.0005-0.005

. and the balance iron and unavoidable impurities, the unavoidable impurities including less than 0.02% P, less than 0.005 S, less than 0.01% N and less than 0.005% Nb.

The steel substrate of a flat steel product according to the invention is at least 1.5 mm thick and has a structure that consists of, in area%, in total 50-90% ferrite and bainitic ferrite, 5-50% martensite, 2-15% residual austenite and up to 10% unavoidable other structural components due to the manufacturing process.

At the same time, a flat steel product according to the invention has a yield point Rp0.2 of at least 290 MPa, a tensile strength Rm of at least 490 MPa and an elongation at break A80, which is determined according to the following formula (1):

A80 [%] = B - Rm / 37 with 31 ≤ B ≤ 51.

A flat steel product according to the invention can be produced by going through at least the following work steps:

A) Production of a hot-rolled steel substrate in the form of a steel strip in at least the following sub-steps:

A.1) melting a steel composed according to the invention; A.2) Pouring the molten steel into a preliminary product, which is a slab or thin slab;

A.3) preheating of the preliminary product at a preheating temperature which is at least 1150 ° C and at most 1350 ° C;

A.4) Hot rolling of the preliminary product into a hot-rolled steel strip, the final temperature of the hot-rolling being at least 840-980 ° C and the thickness of the hot-rolled steel strip being 1.5-10 mm;

A.5) cooling the hot-rolled steel strip to a coiling temperature which is 510-640 ° C;

A.6) Coiling of the hot rolled one cooled to the coiling temperature

Steel belts.

B) Coating the steel substrate in the form of a hot-rolled steel strip with an anti-corrosion coating based on zinc in at least the following sub-steps completed in a continuous cycle:

B.1) optional pickling of the hot rolled steel strip;

B.2) heating the hot-rolled steel strip at a heating rate of 0.5-100 ° C./s to an annealing temperature of 750-950 ° C. and holding the hot-rolled steel strip at the annealing temperature for an annealing period of 10-1000 s;

B.3) Cooling the hot-rolled steel strip at a cooling rate of 0.5-100 ° C / s to a bath inlet temperature BET, for which BT ≤ BET ≤ (BT + 20 ° C), where BT denotes the temperature of the molten zinc bath which is 450-480 ° C; B.4) Passing the hot-rolled steel strip cooled to the bath inlet temperature BET through the molten zinc bath, which consists of up to 5% by mass of Mg, up to 10% by mass of Al, the remainder Zn and unavoidable impurities;

B.5) cooling of the flat steel product obtained with a 0.5 to 100 ^p C / s cooling rate forming amount;

B.6) Optional skin pass rolling of the flat steel product with a skin pass degree of 0.3 - 2.0%.

A preheating temperature of at least 1150 ° C is required in step A.1 in order to completely homogenize the structure of the preliminary product.

At lower temperatures, the microstructure of the preliminary product would be passed on to the subsequently produced hot strip, so that the Mn segregations aimed at according to the invention could not be formed. Likewise, at lower preheating temperatures, the alloying elements would remain bound in precipitates, so that their effects on the mechanical properties of a flat steel product according to the invention could not develop.

A final hot-rolling temperature of at least 840 ° C is required in order to be able to reliably roll the intermediate product alloyed according to the invention into a hot-rolled steel strip. At lower final hot-rolling temperatures, the rolling forces would be too high and the associated risk of damage to the rolls of the rolling stands used for hot rolling would increase disproportionately. In order to minimize these risks, a hot rolling end temperature of at least 880 ° C can be provided. The final hot rolling temperature should not exceed 980 ° C., since final hot rolling temperatures above this upper limit cannot be achieved in practice. The steel strip hot-rolled according to the invention must be at least 1.5 mm thick so that the Mn segregations aimed at according to the invention can arise in the structure after hot rolling. With smaller strip thicknesses, the hot-rolled steel strip would experience excessive deformations during hot-rolling, which in turn would result in an undesirable homogenization of the Mn distribution in the structure of the hot-rolled steel strip. A steel belt with a thickness of more than 10 mm cannot be used for the intended purpose.

Therefore, the maximum tape thickness is limited to 10 mm.

The coiling temperature at which the hot-rolled steel strip, which forms the steel substrate of the flat steel product according to the invention, is reeled, is at least 510 ° C. in order to ensure that Mn segregation occurs during the cooling of the hot-rolled steel strip in the coil. Higher reel temperatures can promote this process, so that reel temperatures of at least 530 ° C., in particular at least 550 ° C., are particularly advantageous. If the coiling temperatures are too low, an undesirably homogeneous Mn distribution would result, with which the mechanical properties aimed for according to the invention would not be achieved. Too high a coiling temperature would trigger the risk of pronounced grain boundary oxidation. To prevent this, the coiling temperature is limited to 640 ° C, preferably 620 ° C.

After cooling in the coil, the hot-rolled steel strip can, if necessary, be pickled in a conventional manner in order to remove any scale present on the steel strip or to prepare the surface of the steel strip for the subsequent work steps.

For hot-dip coating, the hot-rolled steel strip is first heated to an annealing temperature in a preheating stage at a rate of 0.5-100 ° C per second. The heating rate must lie within this window in order to ensure sufficient transformation of the structure, in particular its complete recrystallization. For the same reason is a Annealing temperature of 750 - 950 ° C and a holding time of 10 - 1000 seconds required. If the annealing temperatures are too low or the holding times are too short, the structure would not completely recrystallize, with the result that there would not be enough austenite available during the subsequent cooling to form the desired martensite content of the structure. An unrecrystallized steel substrate would also result in pronounced anisotropy of the mechanical properties of a flat steel product according to the invention.

The cooling from the annealing temperature to the zinc bath inlet temperature BET also takes place at a cooling rate of 0.5 to 100 ° C. per second. The bath inlet temperature BET is at least the same and at most 20 ° C. higher than the melt bath temperature in order to prevent the melt bath temperature from changing significantly due to the introduction of the hot-rolled steel strip.

Optionally, the hot-dip coating can be followed by a further heat treatment ("galvannealing") in which the hot-dip coated steel flat product is heated to up to 550 ° C in order to burn in the previously applied corrosion protection layer.

Either immediately after leaving the zinc bath or after the additional heat treatment, the flat steel product obtained is cooled to room temperature at a cooling rate of 0.5-100 ° C./s.

The flat steel product produced in this way can optionally also be subjected to conventional skin pass rolling in order to optimize its dimensional accuracy and surface quality. The skin-pass degree set is typically at least 0.3% and at most 2.0%, with skin-pass degrees of at least 0.5% having proven to be particularly practical. A skin pass degree of less than 0.3% leads to a lower surface roughness of the corrosion protection layer, which would have a negative impact on the formability of the flat steel product. at If the degree of skin pass is more than 2.0%, the yield strength Rp0.2 is increased and the elongation at break A80 is reduced, so that an elongation at break according to formula 1 could not be achieved.

Surprisingly, it has been found that a flat steel product which comprises a steel substrate alloyed according to the invention and exhibiting a structure according to the invention achieves high elongation at break values in the hot-rolled state which are comparable to the elongations at break A80 which conventional cold-rolled flat steel products of the type explained at the beginning have (“DP-K Steels "), which have similar strengths. In practice, elongation at break values A80 can regularly be achieved for which parameter B in formula (1) is at least in the range 31-51, preferably 36-46.

The combination of high strength and high elongation at break results from the proportion of 2-15% retained austenite in the steel substrate of a flat steel product according to the invention, with retained austenite proportions of at least 5% by area regularly being present in the structure of the steel substrate of a flat steel product according to the invention and having a positive effect affect the mechanical properties of the flat steel product. The retained austenite contents that can be determined in the flat steel product according to the invention are thus significantly higher than in a cold-rolled flat steel product with a comparable alloy.

According to the findings of the invention, the presence of larger residual austenite fractions in the structure is a consequence of the inheritance of Mn segregations, which are present in the hot-rolled steel substrate of a flat steel product according to the invention and are retained via the annealing treatment that the flat steel product undergoes for its hot-dip coating. It was thus possible to show that in the method according to the invention of producing a flat steel product according to the invention after coiling (sub-step A.6 of the method according to the invention) and before hot-dip coating (step B of the method according to the invention), the hot-rolled steel substrate is very has anisotropic and inhomogeneous structure with a high pearlite content, which is in line form. Wavelength-dispersive X-ray micro-range analyzes (WDX) of the structure have shown that Mn segregates in the pearl cords and that the Mn segregations are very anisotropic and inhomogeneous distribution after reeling and before hot-dip coating.

In the continuous hot dip coating, the steel substrate of a flat steel product according to the invention undergoes annealing (substep B.2 of the method according to the invention) before entering the molten bath, during which it is kept at the annealing temperature for a period of time. According to the invention, the annealing temperature and the annealing duration are coordinated with one another in such a way that there is no redistribution of the Mn segregations. For this reason, even in the steel flat product according to the invention with a finished hot-dip coating, there is an anisotropic and inhomogeneous Mn distribution in the steel substrate despite the annealing treatment required for the preparation of the Zn anti-corrosion coating, which as such has been "inherited" from the final structure present after the hot-rolled steel substrate of the steel flat product has been reeled.

Because Mn makes a very strong contribution to the stability of the austenite during annealing in the intercritical range, both the transformation temperature and the residual austenite content after cooling are more inhomogeneously distributed compared to hot-rolled flat steel products which, contrary to the provisions of the invention, have been coiled at lower temperatures . In a steel flat product produced according to the invention, the structural areas of the steel substrate in which there is a higher Mn concentration convert more easily and thus retain more austenite after cooling than the microstructural areas in which a lower Mn concentration is present. These convert at higher temperatures or not at all, which means that a higher proportion of the original ferrite is retained there. The inhomogeneity of the Mn distribution in the steel substrate of a completely processed flat steel product according to the invention can be quantified by the total area of the structure of the steel substrate in which there is an Mn concentration (in% by mass) that is more than 15% higher than the mean value of the Mn concentrations in the entire structure of the steel flat product. The sum of the surface fractions of the structure of the steel substrate of a steel flat product according to the invention which have an Mn concentration that is more than 15% higher than the mean value of the Mn concentration in the entire structure is expressed as “X “In a flat steel product according to the invention, X is at least 10%, in particular at least 12%, advantageously at least 15% of the total structure. The area proportions forming the sum X can be evaluated using a WDX measurement, with the Mn concentration typically being over a measuring area of at least 200 x 200 μm with a step size of 0.5 μm is determined.

The steel of the steel substrate of a steel flat product according to the invention, which is present as a hot-rolled steel strip in the course of production according to the invention, is composed as follows:

Carbon (C) is present in the steel substrate of a flat steel product according to the invention in contents of 0.04-0.23% by mass. C is an essential element for the formation of martensite and austenite, which are required in order to achieve the strength properties required of a steel flat product according to the invention. So that this effect occurs to a sufficient extent, the steel according to the invention contains at least 0.04% by mass, the desired effect being achieved particularly reliably with C contents of at least 0.07% by mass. Too high a C content would have a negative effect on the welding behavior of the flat steel product. The general rule here is that the weldability of a steel decreases with the level of its C content. In order to avoid negative influences of the C content on its processability, the C- Content limited to a maximum of 0.23% by mass, in particular a maximum of 0.20% by mass, with contents of at most 0.17% by mass being able to avoid the negative effects of the presence of C with particular certainty.

Silicon (Si) is present in the steel substrate of a flat steel product according to the invention in contents of 0.04-0.54% by mass. Si is required to suppress the formation of pearlite in the structure during annealing, which would have a negative effect on the mechanical properties of the end product. A minimum Si content of 0.04% by mass is required for this. An excessively high Si content also prevents the formation of pearlite during coiling and the associated segregation of Mn in the structure of the steel substrate. Substantial segregation of Mn during coiling is required to achieve a high sum X and the desired mechanical properties. Too high a Si content would also impair the surface quality of a flat steel product according to the invention. For these reasons, the upper limit of the Si content is limited to 0.54 mass%.

Aluminum (Al) can optionally be added to the steel substrate of a flat steel product according to the invention in contents of 0.01-1.5% by mass in order to help suppress the formation of pearlite. Even if Al is used in the usual way to deoxidize the melt, the minimum Al content is 0.01% by mass. However, too high an Al content can have a negative effect on the castability of the steel and worsen the coating behavior during hot-dip coating. These negative influences of the presence of Al in the steel of the substrate of a flat steel product according to the invention can be avoided particularly reliably by limiting the Al content to a maximum of 1.0 mass%, in particular a maximum of 0.5 mass%. Manganese (Mn) is present in the steel substrate of a flat steel product according to the invention in contents of 1.4-2.9% by mass. Mn is a mixed crystal element that contributes to the strength of the material. The presence of Mn in the steel of the substrate of a flat steel product according to the invention also stabilizes the austenite in the structure of the substrate. The peculiarity of the alloy concept according to the invention in combination with the production according to the invention of a flat steel product according to the invention is that a flat steel product according to the invention is an optimal combination of high tensile strength and high elongation at break as a result of the segregation of Mn in the bead strands of the steel substrate after coiling, which is also then remain intact when the flat steel product for the hot-dip coating has been annealed and has passed through the hot-dip bath. So that Mn is sufficiently enriched in the pearl cords by segregation, Mn contents of at least 1.4% by mass are required, with regard to the reliability with which the positive influence of Mn on the properties of a flat steel product according to the invention sets, it is favorable if the Mn content is at least 1.5 mass%. However, too high a Mn concentration would also have a negative effect on weldability. Therefore, the upper limit of the Mn content of the steel substrate of a flat steel product according to the invention is limited to 2.9% by mass, preferably 2.5% by mass, the amount of Mn being added to the properties of a flat steel product according to the invention at Mn contents of up to 2.2 mass% can be used particularly effectively.

Chromium (Cr) and molybdenum (Mo) can be added to the steel of the steel substrate of a flat steel product according to the invention as optional elements for increasing strength. In addition, the presence of Cr and / or Mo increases the formation of martensite compared to pearlite when the flat steel product is cooled from the intercritical area in a continuous coating system. If these effects are to be used, so are for this Contents of Cr and Mo required which total at least 0.02% by mass, in particular at least 0.05% by mass. If the Cr content is too high, however, the risk of pronounced grain boundary oxidation would be increased. Too high a Mo content should also be avoided for reasons of cost. In order to be able to effectively use the effects of Cr and Mo in the steel of the steel substrate of a flat steel product according to the invention, the upper limit of the total content of Cr and Mo is therefore set to 1.4% by mass, preferably 1.0% by mass. Cr and Mo do not necessarily have to occur in combination, but can also be added to the steel alone in the contents of 0.02-1.4% by weight, in particular 0.05-1.0% by weight, specified according to the invention, to achieve the effects explained. However, particularly beneficial effects result when Cr and Mo are present together in effective contents as long as the sum of these contents is within the limits according to the invention.

At least one of the elements titanium (Ti) and vanadium (V) is present as a mandatory component in the steel of the steel substrate of a flat steel product according to the invention in a content of 0.005-0.15% by mass, whereby it is also true here that an optimal effect of these elements occurs, when Ti and V are present together in effective contents. Ti and V are micro-alloy elements that cause fine precipitates to form in steel. Such precipitates prevent the austenite grains from becoming coarser at temperatures which are higher than the Ar1 temperature of the steel, and in this way lead to a refinement of the structure. A finer structure favors the segregation of Mn, which is aimed at according to the invention, during the reeling carried out in the course of the production of a flat steel product according to the invention, because the distance over which Mn diffuses is reduced by the presence of Ti and / or V. Precipitations containing Ti and V also contribute to the strength of a flat steel product according to the invention through dispersion hardening. To achieve these effects of Ti and V are Ti and / or V contents of at least 0.005 mass% in total is required. At contents above 0.15% by mass, the presence of Ti and / or V im results. With regard to the properties aimed for according to the invention, there is no longer any particular increase. Rather, Ti and V can be used particularly effectively when the sum of their contents does not exceed 0.1% by mass.

According to the invention, the content of niobium (Nb) is limited to less than 0.005% by mass, so that, if niobium is present at all, it is one of the impurities that are technically ineffective. Higher Nb contents would lead to the formation of fine Nb precipitates, which would make them susceptible to cracking during continuous casting or during slab cooling or reheating. Therefore, the Nb content is preferably limited to less than 0.003 mass%, in particular less than 0.002 mass%.

Boron (B) can also optionally be added to the steel of the steel substrate of a flat steel product according to the invention in contents of 0.0005-0.005% by mass in order to prevent the formation of ferrite during the cooling from the intercritical range during the production of the flat steel product. In this way, B promotes the formation of bainite, which increases strength. A minimum content of 0.0005 mass% B is required for this. However, too high a B content can lead to undesirable embrittlement. Therefore, according to the invention, the upper limit of the ^' B content, if B is added, is set to at most 0.005% by mass, in particular 0.002% by mass.

Phosphorus (P) is one of the undesirable but technically generally unavoidable impurities in the steel of the steel substrate of a flat steel product according to the invention and should therefore be as low as possible. P proves particularly unfavorable in terms of weldability. In order to safely avoid its unfavorable influence, the P content is according to the invention to less than 0.02 mass%, preferably less than 0.01 mass%, in particular less than 0.005 mass%.

Sulfur (S) is also one of the undesirable but technically generally unavoidable impurities in the steel of the steel substrate of a flat steel product according to the invention and should therefore be as low as possible. At higher concentrations, S leads to the formation of MnS or (Mn, Fe) S, which would have a negative effect on the elongation behavior of a flat steel product according to the invention. In order to avoid these unfavorable effects, the S content is limited according to the invention to less than 0.005% by mass, preferably less than 0.002% by mass.

Nitrogen (N) is also one of the undesirable, but technically generally unavoidable impurities in the steel of the steel substrate of a flat steel product according to the invention and should therefore be as low as possible. For example, N forms nitrides with aluminum or titanium. In the case of higher N contents, this would lead to coarse precipitations, which could be detrimental to the formability of the flat steel product. According to the invention, the N content is therefore limited to less than 0.01% by mass, preferably less than 0.005% by mass.

Calcium (Ca) also gets into the steel during conventional steelmaking because it is added both for deoxidation and desulphurisation and to improve castability. Too high a concentration of Ca can lead to the formation of undesirable inclusions, which have a negative effect on the mechanics and rollability. Therefore, the upper limit of the Ca content is limited to at most 0.005 mass%, preferably at most 0.002 mass%.

Copper (Cu), nickel (Ni), tin (Sn), arsenic (As), cobalt (Co), zircon (Zr), lanthanum (La) and / or cerium (Ce) are alloying elements that also contribute to the impurities of the Steel of the steel substrate of an inventive Flat steel product, the presence of which is undesirable per se. In order to reliably prevent influences of these elements on the properties of a flat steel product according to the invention, the Cu content in the steel of the steel substrate of a flat steel product according to the invention is at most 0.2% by mass, the Ni content at at most 0.1% by mass, the Sn Content to a maximum of 0.05% by mass, the As content to a maximum of 0.02% by mass, the Co content to a maximum of 0.02% by mass, the Zr content to a maximum of 0.0002% by mass, the La content is limited to a maximum of 0.0002% by mass and the Ce content to a maximum of 0.0002% by mass.

Oxygen (O) is also an undesirable impurity, since the presence of larger amounts of O causes oxide deposits that have a negative effect both on the mechanical properties of the flat steel product and on the castability and rollability of the steel on its steel substrate. The oxygen content is therefore limited to at most 0.005% by mass, preferably 0.002% by mass.

Hydrogen (H) is also one of the undesirable impurities in the steel of the steel substrate of a flat steel product according to the invention. As the smallest atom, H is very mobile in interstitial spaces in steel and can lead to tears in the core when it cools down from hot rolling, especially in high-strength steels. The content of H in the steel of the steel substrate of a flat steel product according to the invention is therefore to a maximum of 0.001% by mass, preferably a maximum of 0.0006% by mass, more preferably a maximum of 0.0004% by mass, most preferably a maximum of 0.0002% by mass, reduced.

No special requirements are placed on the composition of the anti-corrosion coating and the associated melt bath that the flat steel product passes through during its hot-dip coating. The corrosion protection coating of a flat steel product according to the invention consists in its main part of zinc (Zn) and can otherwise be composed in a conventional manner. Accordingly, in addition to Zn and unavoidable impurities, the corrosion protection layer can contain up to 20% by mass Fe, up to 5% by mass

Contain Mg and up to 10% by mass of Al. Typically, if present, at least 5% by mass of Fe, at least 1% by mass of Mg and / or at least 1% by mass of Al are provided in order to achieve optimal usage properties of the corrosion protection.

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

To test the invention, steels A - I were melted and cast into slabs, the composition of which is given in Table 1. Contents of an alloy element that are so low that they are “0” in the technical sense, that is, so low that they have no influence on the properties of the steel, are listed in Table 1 by the entry

designated.

The slabs were thoroughly heated in a preheating furnace in which a preheating temperature VT prevailed.

The preheated slabs were then hot-rolled in a conventional manner into hot-rolled steel strips W1-W35, the hot-rolling having been terminated at a final rolling temperature ET.

The hot-rolled steel strips W1-W35 obtained in this way have, starting from a coiling temperature HT, been reeled into a coil in an equally conventional manner. For this purpose, if necessary, they have been cooled in a conventional manner to the reel temperature HT prior to reeling.

To demonstrate the effect of the invention, one of the combinations I-VIII given in Table 2 is required in the production of the hot-rolled steel strips W1-W35, which each consisted of one of the steels A-I of preheating furnace temperature VT, hot rolling end temperature ET and coiling temperature HT. The preheating furnace temperatures VT, hot rolling end temperatures ET and coiling temperatures HT associated with each of the combinations I - VIII are given in Table 2. Those preheating furnace temperatures VT, hot rolling end temperatures ET and coiling temperatures HT, which in each case did not correspond to the requirements of the invention, are emphasized by underlining.

After cooling in the coiler, the hot-rolled steel strips W1 - W35 have been coated with a Zn-based corrosion protection layer by hot-dip coating. For this purpose, they were each subjected to one of six variants a - f of an annealing treatment and a melt application, in which they were heated in a preheating stage at a heating rate HR to an annealing temperature GT, at which they were then annealed for an annealing period of 40 s to 100 s have been held. The hot-rolled steel strips W1-W35 were then cooled at a cooling rate KR1 to a bath inlet temperature BET which was the same as the bath temperature of the melt bath through which the hot strips were passed after the respective annealing treatment a-f. The molten bath consisted of at least 99% Zn by mass. The now finished steel flat products emerging from the molten bath and produced on the basis of the hot-rolled steel strips W1 - W35 were then cooled to room temperature at a cooling rate KR2 Annealing treatment and the parameters associated with the melt application: heating rate HR, annealing temperature GT, cooling rate KR1, bath inlet temperature BET and cooling rate KR2 are shown in Table 3.

The mechanical properties and structural components have been determined on the flat steel products obtained in the manner explained above. The results of these investigations yield strength Rp0.2, tensile strength Rm, elongation at break A80, parameter "B" from formula (1), ferrite content F of the structure, martensite content M of the structure, austenite content A of the structure, proportion SO of Other components of the structure and sum X of the surface fractions of the structure of the steel substrate in which there is an Mn concentration that is more than 15% above the mean value of the Mn concentration in the structure are summarized in Table 4, where for the based on the flat steel products produced by hot-rolled steel strips W1 - W35, which of the steels A - I consisted of the steel substrate of the respective flat steel product, which of the combinations I - VIII of the hot strip production (column "HAZ") and which of the variants af the annealing treatment and the melt application has passed through the respective steel substrate (column "GS").

The flat steel products produced from the hot-rolled steel strips W1, W3, W6, W7, W8 and W27 have not been produced in the manner according to the invention:

In the case of the flat steel product produced from the hot-rolled steel strip W1, the slab was heated with a preheating temperature VT that was too low, so that the slab was not completely annealed. As a result, the alloying elements and the manufacturing process did not affect the mechanical properties.

The hot-rolled steel strip W3 contains too little Mn, so that Mn has not segregated to a sufficient extent in the pearl cords of the hot-rolled strip structure. This led to a lower residual austenite content and therefore to a relatively low elongation at break A80 of the flat steel product produced from the hot-rolled steel strip W3. As a result, parameter B was below 31 there.

In the production of the hot-rolled steel strips W6, W7 and W8, the coiling temperatures set were too low. This led to a similar effect on the Mn segregations and therefore to insufficient mechanical Properties as those produced from the hot-rolled steel strip W3

Flat steel product.

During the annealing treatment of the hot-rolled steel strip W27, the GT set was too low, so that the structure was not completely recrystallized. This resulted in a low austenite content in the structure of the steel substrate of the flat steel product obtained and therefore in a low elongation at break A80.

*) Parameters not according to the invention are underlined

Claims

PATENT CLAIMS

1. Hot rolled steel flat product that

- a steel substrate at least 1.5 mm thick,

- that from, in mass%,

C: 0.04-0.23%,

Si: 0.04-0.54%,

Mn: 1.4-2.9%,

Ti + V, where the following applies to the total% Ti +% V of the contents of Ti and V: 0.005% ≤% Ti +% V ≤0.15%, as well as optionally one or more of the elements from the group "AI, Cr, Mo, B "with the proviso that their contents, if available, are measured as follows:

AI: 0.01 - 1.5%,

Cr and Mo, where the following applies to the sum of the% Cr +% Mo content of Cr and Mo: 0.02 ≤% Mo +% Cr ≤ 1.4%,

B: 0.0005-0.005%, and the balance of iron and unavoidable impurities, the unavoidable impurities including less than 0.02% P, less than 0.005 S, less than 0.01% N and less than 0.005% Nb , consists,

- which has a structure that consists of, in area%, in total 50 - 90% ferrite and bainitic ferrite, 5 - 50% martensite, 2-15% Residual austenite and up to 10% other structural components unavoidable due to production, and

- which has a yield point Rp0.2 of at least 290 MPa, a tensile strength Rm of at least 490 MPa and an elongation at break A80, which is determined according to the following formula (1):

A80 [%] = B - Rm / 37 with 31 ≤ B ≤ 51, and

- comprises a corrosion protection layer based on zinc, which is applied to at least one of its surfaces by hot dip coating.

2. Flat steel product according to claim 1, characterized in that the structure of the steel substrate contains at least 5 area% retained austenite.

3. Flat steel product according to one of the preceding claims, characterized in that the following applies to parameter B of formula (1): 36 B 46.

4. Flat steel product according to one of the preceding claims, characterized in that the sum X of those area-related fractions of the structure of the steel substrate in which there is an Mn concentration that is more than 15% above the mean value of the Mn concentration in the structure is at least 10 % of the total structure.

5. Flat steel product according to claim 4, characterized in that the sum X is at least 12%.

6. Flat steel product according to claim 5, characterized in that the sum X is at least 15%.

7. Flat steel product according to one of the preceding claims, characterized in that the corrosion protection layer consists of at least 75% by mass of Zn.

8. A method for producing a flat steel product procured according to any one of claims 1-7, in which at least the following work steps are carried out:

A.1) Melting of a steel melt consisting of, in% by mass, C: 0.04-0.23%, Si: 0.04-0.54%, Mn: 1.4-2.9%, Ti + V, where the following applies to the total% Ti +% V of the contents of Ti and V: 0.005%% Ti +% V 0.15%, as well as optionally one or more of the elements from the group “AI, Cr, Mo , B "with the proviso that their contents, if available, are measured as follows: AI: 0.01 - 1.5%, Cr and Mo, whereby the following applies to the sum of the contents% Cr +% Mo of Cr and Mo: 0.02 ≤% Mo +% Cr ≤ 1.4%, B: 0.0005 - 0.005%, and the remainder consists of iron and unavoidable impurities, with less than 0.02% P and less than 0.005% of the unavoidable impurities Count S, less than 0.01% N and less than 0.005% Nb; A.2) Pouring the molten steel into a preliminary product, which is a slab or thin slab;

A.3) Preheating the preliminary product at a preheating temperature which is at least 1150 ° C and at most 135 ° C;

A.6) Coiling of the hot-rolled steel strip cooled to the coiling temperature.

B.1) optional pickling of the hot rolled steel strip;

B.5) cooling the flat steel product obtained at a cooling rate of 0.5-100 ° C./s;

9. The method according to claim 8, characterized in that the coiling temperature is at least 530 ° C.

10. The method according to claim 9, characterized in that the coiling temperature is at least 550 ° C.

11. The method according to any one of claims 8-9, characterized in that the coiling temperature is at most 620 ° C.