WO2024046913A1

WO2024046913A1 - Method for producing a cold-rolled flat steel product

Info

Publication number: WO2024046913A1
Application number: PCT/EP2023/073402
Authority: WO
Inventors: Nicholas WINZER
Original assignee: Thyssenkrupp Steel Europe Ag
Priority date: 2022-08-29
Filing date: 2023-08-25
Publication date: 2024-03-07
Also published as: DE102022121780A1

Abstract

The present invention relates to a method for producing a cold-rolled flat steel product having a ferritic primary structure and carbide precipitates based on Ti, Nb and/or V embedded in the ferritic primary structure.

Description

Process for producing a cold-rolled flat steel product

The invention relates to a method for producing a cold-rolled flat steel product with a ferritic basic structure and carbide precipitates based on Ti, Nb and/or V embedded in the ferritic basic structure.

Energy consumption represents by far the largest contribution to the economic and ecological costs of steel production. In large-scale steel production, energy is predominantly provided in the form of fossil fuels (e.g. coal and natural gas). Steel production is therefore associated with very high CO ₂ emissions. The constantly rising costs of CO ₂ certificates (so-called “carbon offset credits”), especially within the EU, mean that the steel industry's dependence on fossil fuels will no longer be sustainable in the future. For this reason, alternatives to the consumption of fossil fuels in steel production are being urgently sought.

In sheet metal production, fossil fuels are consumed in hot-dip coating plants and continuous annealing plants to heat cold-rolled flat steel products to very high temperatures. Such systems consist of several ovens heated with natural gas-fired burners through which the flat products pass continuously. Annealing the cold-rolled flat steel product is necessary to convert the structure into desired phases, to eliminate the negative influences of cold rolling and, if necessary, to prepare the flat steel product for hot-dip coating.

Cold rolling is typically performed to achieve both lower thickness and high surface quality and dimensional tolerance on the flat product. However, cold rolling results in both severe work hardening and the formation of strong texture (anisotropy) in terms of grain orientation and mechanical properties. These effects generally have a negative impact on the mechanical-technological properties of the end product and are eliminated by annealing at high temperatures (typically in the range 650 - 950 °C). At these temperatures, recovery and recrystallization or transformation of the structure occur. Recovery involves the annihilation of crystallographic defects (e.g. dislocations) in the structure, which otherwise cause strain hardening. This typically occurs at lower temperatures compared to recrystallization and conversion. While During recrystallization, new ferrite grains form instead of old work-hardened and anisotropic ferrite grains. This can begin at temperatures below Acl. During the transformation, the ferrite converts into austenite; Depending on the temperature (between Acl and Ac3) and the chemical analysis of the flat steel product, the structure can be partially or completely transformed. In multi-phase steels, the newly formed austenite transforms into hard carbon-rich phases such as martensite and/or bainite when subsequently cooled to low temperatures. By combining hard phases, together with ferrite and retained austenite, the mechanical properties of multi-phase steels can be adjusted.

Cold-rolled multi-phase steels, especially with a zinc-based anti-corrosion coating, are used in the construction of automotive components. Multiphase steels offer a number of advantages for such applications. Due to their low alloying elements, multiphase steels are characterized by lower costs, good reproducibility, good recyclability and good suitability for processing processes (e.g. welding) compared to other materials. Furthermore, a wide range of structural components and consequently mechanical properties can be achieved with multi-phase steels by adjusting the chemical analysis and process conditions. This flexibility enables the use of multi-phase steels for components that have different requirements in terms of mechanical properties.

For many multiphase steels, the diffusion of alloying elements, especially carbon (C), during the annealing cycle is essential for the formation of the structural components and therefore the adjustment of the mechanical properties. C has different solubilities in different iron allotropes. Accordingly, C atoms must be redistributed between the newly formed phases during the structural transformation. The redistribution of C has a decisive influence on the formation of the new phases upon cooling and consequently the mechanical properties of the final product. In contrast, the diffusion of alloying elements (including C) is a relatively slow process. For this reason, the cold-rolled flat steel product must be aged for an extended period of time (usually several minutes) at a specified annealing temperature.

DE 10 2021 105 357 A1 describes a method for producing a generic cold-rolled flat steel product. The object of the invention is to provide a generic method which reduces or essentially avoids the use of fossil fuels.

The teaching of the invention relates to a method for producing a cold-rolled flat steel product with a ferritic basic structure and carbide precipitates based on Ti, Nb and / or V embedded in the ferritic basic structure, comprising the steps: a) melting a steel consisting of Fe and unavoidable impurities in % by weight

C: 0.020 to 0.20%,

Mn: 0.10 to 4.00%,

P: up to 0.020%,

S: up to 0.010%,

N: up to 0.010%, with at least one or more microalloy elements from the group (Ti, Nb, V):

Ti: at least 0.040%,

Nb: at least 0.040%,

V: at least 0.040%, subject to the following conditions:

I) 0.04% <= X <= 0.3% with X = Ti + V/1.06 + Nb/1.94

II) 0.3 <= Y <= 1.0 with Y = 0.25 * X / (C + 0.86 *N) optionally one or more alloying elements from the group (Si, Al, Cr, Mo, W, Cu) with

Si: up to 1.50%,

AI: up to 1.50%,

Cr: up to 1.50%,

Mon: up to 0.50%,

W: up to 0.50%,

Cu: up to 0.10%; b) casting the melt into a preliminary product; c) preheating the preliminary product to a temperature and/or maintaining the preliminary product at a temperature between 1150 and 1350 ° C; d) hot rolling the preliminary product into a hot-rolled flat steel product with a final hot rolling temperature between 850 and 980 ° C; e) cooling the hot-rolled flat steel product obtained at a cooling rate of between 20 and 400 ° C/s to a coiling temperature of between 400 and 700 ° C; f) coiling the hot-rolled flat steel product cooled to the coiling temperature into a coil; g) uncoiling the coil and cold rolling with a degree of cold rolling between 5 and 80% to form a cold-rolled flat steel product; h) coiling the cold-rolled flat steel product into a coil; i) Uncoiling the coil and annealing the cold-rolled flat steel product in a continuous flow comprising the steps:

11) Heating at an average heating rate between > 100 and 1000 °C/s to a temperature between 800 and 900 °C and holding at 800 to 900 °C for a period between 0.1 and 18 s;

12) Cooling at a mean cooling rate between 100 and 1000 °C/s to a temperature of not more than 550 °C and optionally holding at this temperature for a period of not more than 100 s; j) Coiling the cold-rolled flat steel product into a coil.

The molten steel with an alloy composition within the ranges specified above is cast into a preliminary product, which in the classic production route can be a slab of standard dimensions. However, a preliminary product of a thin slab can also be produced from the steel by direct hot rolling of a continuous casting in a casting rolling plant or a preliminary product of a cast strip in a strip casting plant. For example, in a casting-rolling plant or strip casting plant, the preliminary product can be further processed directly, that is, coming directly from the casting heat, so that the preliminary product is kept at a temperature or, if necessary, preheated to a temperature, for example in an equalization or preheating furnace, in which one The most complete homogenization possible is ensured and in which any precipitates that may have formed during casting dissolve (again) as completely as possible. If, for example, the melt is cast into a preliminary product in a continuous casting plant, the cast and completely solidified strand is separated into several slabs of finite dimensions and finally the slabs are allowed to cool down to ambient temperature, in particular through natural cooling. The preliminary product or slab is used for further processing, for example in a walking beam furnace or using other suitable means Means reheated to a temperature. Otherwise, after casting, the slab is placed more slowly under a hood or directly in a walking beam furnace.

The temperature when preheating and/or holding the preliminary product is at least 1150 ° C, in particular at least 1200 ° C, in order to ensure the most complete possible dissolution of any undesirable precipitates in the form of carbides/carbonitrides and/or nitrides in the preliminary product. The temperature for preheating and/or holding should not exceed 1350 °C in order to avoid partial melting and/or excessive scaling of the preliminary product. For ecological and economic reasons, the temperature for preheating and/or holding is limited in particular to a maximum of 1275 °C.

The preliminary product is hot-rolled into a hot-rolled flat steel product in one or more rolling stands (hot rolling mill) with a final hot rolling temperature between 850 and 980 °C. A hot rolling final temperature for producing the hot-rolled flat steel product of at least 850 ° C, in particular at least 870 ° C, is selected in order not to allow the forming resistance to increase too much. In order to avoid unwanted coarse grain formation, the final rolling temperature for producing the hot-rolled flat steel product is limited to a maximum of 980 °C.

The hot-rolled flat steel product obtained is cooled to a coiling temperature of between 400 and 700 °C at a cooling rate of between 20 and 400 °C/s. The cooling rate of at least 20 °C/s is required to largely avoid the formation of pearlite and cementite and the formation of coarse precipitates that cannot be dissolved in the later process steps. A cooling rate above 400 °C/s does not bring any further advantages. The coiling temperature is at least 400 ° C, in particular at least 410 ° C, in order to prevent martensite formation and to promote the formation of a structure of bainite, bainitic ferrite and / or ferrite in the hot-rolled flat steel product. Martensite in the structure of the hot-rolled flat steel product would be transferred to the structure of the cold-rolled and annealed flat steel product and would be an undesirable phase in the structure of the cold-rolled flat steel product. In addition, the martensite in the structure of the hot-rolled flat steel product has a negative effect on both the cold-rollability of the hot-rolled flat steel product and the isotropy of the structure of the cold-rolled and annealed flat steel product. About the diffusion of oxygen-affinous alloy elements to the surface During the reeling process, the reel temperature is limited to a maximum of 700 °C, in particular a maximum of 660 °C. At a coiling temperature above 700 °C, the precipitates containing Ti, Nb and/or V would become coarser, meaning that the desired precipitate size in the cold-rolled flat steel product and consequently a high tensile strength and a high hole expansion ratio would not be achieved. In the case of hot-rolled flat steel products, which consist of ferrite and/or bainitic ferrite reinforced with precipitates containing Ti, Nb and/or V, see, among others, WO 2020/048599 Al and EP 1 338 665 Al, the coiling temperature is usually at least 550 up to 600 °C so that carbides containing Ti, Nb and/or V can precipitate during the cooling of the coiled hot strip product. At a lower coiling temperature, the carbides do not precipitate, meaning that the desired mechanical properties cannot be achieved. In the present case, it does not matter whether the precipitates already form in the hot-rolled flat steel product. In the event that no precipitates form in the hot-rolled flat steel product during the cooling of the hot strip product, they will form during the subsequent annealing of the cold-rolled flat steel product. The desired mechanical-technological properties can therefore also be achieved by coiling at a coiling temperature of at least 400 ° C, in particular at least 410 ° C.

The hot-rolled flat steel product, cooled to the coiling temperature, is coiled into a coil.

The hot-rolled flat product is uncoiled from the coil and cold-rolled into a cold-rolled flat steel product with a degree of cold rolling between 5 and 80%. Cold rolling is necessary for high surface quality and dimensional tolerance, which is necessary for the intended use of the cold-rolled flat steel product for thin-walled components (e.g. body-in-white components). However, cold rolling leads to work hardening, which negatively affects the ductility and hole expansion ratio of the steel. In addition, cold rolling results in a dominant rolling texture, which leads to a pronounced anisotropy in mechanical properties and consequently a reduction in the hole expansion ratio. The influence of work hardening and rolling texture on the mechanical-technological properties cannot be fully recovered by subsequent annealing. The cold rolling degree KWG is calculated according to the formula: KWG = 100 * (LWB - LKB) / LWB, where LWB is the thickness of the hot-rolled flat steel product (hot strip) and LKB is the thickness of the cold-rolled flat steel product (cold strip). When cold rolling with If the degree of cold rolling is too low, the surface quality and dimensional tolerance required for the target application will not be achieved. For this reason, the degree of cold rolling is at least 5%, in particular at least 10%. When cold rolling with a degree of cold rolling that is too high, the influences of work hardening and rolling texture are so high that the required mechanical-technological properties cannot be achieved. For this reason, the degree of cold rolling is limited to a maximum of 80%, in particular a maximum of 70%, preferably a maximum of 50%.

The cold-rolled flat steel product is coiled into a coil.

The cold-rolled steel flat product (cold strip) can have a thickness between 0.5 and 4 mm.

The cold-rolled flat steel product is uncoiled from the coil and annealed in a continuous process. The annealing can take place, for example, in a multi-stage continuous annealing system, which includes the following steps:

Heating at a mean heating rate between > 100 and 1000 °C/s to a temperature between 750 and 900 °C and holding at 750 to 900 °C for a period between 0.1 and 18 s;

Cooling at an average cooling rate of between 100 and 1000 °C/s to a maximum temperature of 550 °C.

The annealing of the cold-rolled flat steel product has a significant influence on the setting of the mechanical-technological properties of the end product. In general, the mechanical-technological properties of the end product are influenced by two changes in the structure during annealing: on the one hand, the recovery or recrystallization of the ferrite and/or bainitic ferrite grains and, on the other hand, the coarsening of those containing Ti, Nb and/or V excretions. The recovery or recrystallization and the coarsening of the precipitate have opposite effects on the hole expansion ratio. The recovery or recrystallization of the structure is necessary to at least partially eliminate the hardening caused by cold rolling, which would otherwise affect the hole expansion ratio and increase the tensile strength. The recovery or recrystallization only takes place at such high temperatures at which the Excretions can also become coarser quickly. This has a negative effect on both the hole expansion ratio and the tensile strength.

What is essential to the invention is that the annealing of the cold-rolled flat steel product takes place faster and in a significantly shorter period of time compared to the annealing of a conventional multi-phase steel. The short glow time takes advantage of the different kinetics of recovery or recrystallization and coarsening of the precipitate. Depending on the temperature, recovery or recrystallization takes place in a few seconds, whereas the coarsening of the precipitation would require a comparatively longer time. When producing the cold-rolled flat steel product, the annealing time is sufficiently long to largely recover or recrystallize the structure, but short enough to prevent excessive coarsening of the precipitates.

For a short annealing time, both a fast average heating and cooling rate as well as a short holding time are required. In the first stage of the annealing process, the cold-rolled flat steel product is heated to a temperature in the range between 800 to 900 °C at an average heating rate in the range > 100 to 1000 °C/s. Such a high heating rate can be achieved, for example, by inductive heating. Inductive heating is powered by electricity, which can preferably be generated by renewable energies (wind, water, sun) and thus has an advantageous effect on the CO ₂ footprint. Much higher heating rates can be achieved through inductive heating compared to those which can be achieved through gas fired burner heating. The heating rate is at least > 100 °C/s, 120 °C/s, 150 °C/s, in particular at least 200 °C/s, 220 °C/s, 250 °C/s, preferably at least 300 °C/s , 320 °C/s, 350 °C/s to avoid excessive coarsening of the Ti, Nb and/or V-containing precipitates. Heating rates of more than 1000 °C/s have no advantage. A restriction to in particular a maximum of 900 °C/s, 800 °C/s, preferably a maximum of 700 °C/s, 600 °C/s, 500 °C/s would be possible.

The “average” heating or cooling rate is to be understood as being related to the difference between an initial temperature (actual temperature) and a target temperature (target temperature) for the time required between the initial temperature and reaching the target temperature. As a rule, the heating and cooling rate is not a constant value. As already noted, existing annealing processes in steel production are very energy-intensive and involve high CO ₂ emissions. Alternatives that do not use fossil fuels are therefore being pursued at high speed. A possible alternative for annealing thin sheet metal is inductive heating. This process occurs very quickly: the strip is heated at rates that are significantly higher than the heating rates that occur in conventional systems. In contrast to conventional annealing processes, the strip is held at the annealing temperature for a few seconds instead of minutes. Such a short annealing time is sufficient for the recovery or recrystallization of the structure, but leaves insufficient time for the diffusion of alloying elements. Therefore, inductive heating is not suitable for annealing many multiphase steels consisting of Fe- and C-based phases such as martensite, bainite and austenite.

The use of inductive heating in steel production requires steel concepts that can achieve mechanical properties at a similar level to modern multi-phase steels through annealing over very short times. One possibility is ferritic steels, which are reinforced by very fine precipitates such as carbides or carbonitrides or intermetallic particles. The carbides or carbonitrides form either during the cooling of the coiled hot strip product or in the first stage of annealing the cold-rolled flat steel product. The creation of the final structure and consequently the setting of the mechanical-technological properties is therefore not dependent on slower diffusion processes. Due to the very short glow time, excessive coarsening of the precipitates can be avoided. During annealing in conventional annealing or hot-dip coating systems, however, the annealing time is so long that the precipitates can become coarser. The coarsening of the precipitates has a negative effect on the mechanical properties of the end product. Due to their very isotropic or homogeneous structure, such concepts have both high tensile strength and high ductility as well as very high resistance to edge cracks. Edge cracks can occur when forming stamped sheets and are therefore a criterion that should not be ignored in automobile construction. In the laboratory, the sensitivity of a steel to edge cracks is assessed using the so-called “hole expansion test”, in which a hole punched into a sheet metal sample is expanded with a mandrel until the first crack appears, see ISO 16630:2017.

After heating, the cold-rolled flat steel product is in the second stage of the annealing process at a temperature in the range between 800 to 900 ° C for a period of 0.1 held until 18 s. The setting of the annealing temperature and time in the second step is crucial both for sufficient recovery or recrystallization of the structure and for minimizing the coarsening of the precipitation. At an annealing temperature of less than 800 °C or a duration of less than 0.1 s, the structure would not be sufficiently recovered or recrystallized. Sufficient recovery or recrystallization is required to eliminate work hardening and anisotropy resulting from cold rolling. For this reason, the annealing temperature is at least 800 °C, in particular at least 810 °C, 820 °C, preferably at least 830 °C, 840 °C, 850 °C. A temperature of more than 900 °C and a duration of more than 18 s would in turn lead to an excessive coarsening of the Ti, Nb and/or V-containing precipitates and consequently to a deterioration of the mechanical-technological properties. Therefore, the duration of the annealing after heating can be limited to in particular a maximum of 16 s, 15 s, preferably a maximum of 13 s, 11 s, preferably a maximum of 10 s, 8 s.

In the third stage of the annealing process, the cold-rolled flat steel product is cooled to a maximum temperature of 550 °C at an average cooling rate of 100 to 1000 °C/s. The cooling rate can be limited in particular to 900 °C/s, 800 °C/s, preferably to 700 °C/s, 600 °C/s. Such a high cooling rate can be achieved, for example, by water quenching. Conventionally, air blowers are used for cooling, which have poorer efficiency compared to water quenching. In a similar way to the average heating rate in the first stage, an average cooling rate that is too slow would lead to excessive coarsening of the Ti, Nb and/or V-containing precipitates, so that the desired precipitate size and consequently the required mechanical-technological properties are not achieved can be achieved. For this reason, the average cooling rate is at least 100 °C/s, 120 °C/s, 150 °C/s, in particular at least 200 °C/s, 220 °C/s, 250 °C/s, preferably at least 300 ° C/s, 320°C/s, 350°C/s.

At temperatures of less than 550 °C, there is neither a recovery or recrystallization of the structure nor a coarsening of the Ti, Nb and/or V-containing precipitates. For this reason, there are no special requirements for further cooling after the end of the third stage. The cold-rolled flat steel product can possibly be cooled to a maximum temperature of 100 °C directly after the third stage at an average cooling rate of 100 - 1000 °C/s. Alternatively, the cold-rolled flat steel product can be cooled to an average cooling rate of 100 - 1000 °C/s Temperature of less than 550 °C and higher than 100 °C for a maximum period of 1000 s and then continued to be cooled at an average cooling rate of 100 - 1000 °C/s to a temperature of maximum 100 °C.

According to one embodiment, the hot-rolled flat steel product can be pickled, in particular, directly before cold rolling. Alternatively, the hot-rolled flat steel product can be coiled into a coil after pickling and unwound before cold rolling.

According to one embodiment, the cold-rolled flat steel product can be coated with a Zn-based anti-corrosion coating after step i2). The Zn-based anti-corrosion coating can be applied either during cooling or holding, in particular during or after the third stage of the annealing process in the temperature range 100 - 550 ° C, or subsequently to the cold-rolled flat steel product. The Zn-based anti-corrosion coating can be applied either by immersing it in a melt bath (fire coating) or following annealing in an electro-galvanizing system.

Optionally, the hot-dip coating can be followed by a further heat treatment (“galvannealing”), in which the fire-coated flat steel product is heated to up to 550 °C in order to burn in the zinc layer. Either immediately after emerging from the melt pool or following further heat treatment, the cold-rolled flat steel product obtained can be cooled to a temperature of less than 100 °C at a cooling rate of between 0.5 and 1000 °C/s.

There are no special requirements for the composition of the corrosion protection coating and the associated melt pool through which the cold-rolled flat steel product passes during its hot-dip coating. In particular, the corrosion protection coating consists primarily of zinc (Zn) and can otherwise be composed in a conventional manner. Accordingly, the anti-corrosion coating can contain up to 20% Fe, up to 5% Mg and up to 10% Al in addition to Zn and unavoidable impurities (in wt.%). Typically, at least 5% Fe, at least 1% Mg and/or at least 1% Al are provided, if present, in order to achieve optimal performance properties of the corrosion protection. The coated or uncoated cold-rolled flat steel product obtained in this way can optionally be subjected to conventional tempering in order to optimize its dimensional stability and surface quality. The degree of skin passance set is typically at least 0.1% and at most 2.0%, with a degree of skin pass of at least 0.3% and at most 1.0% being set as particularly preferred. A degree of tempering of less than 0.1% would have no significant effect on the dimensional accuracy and surface quality and would lead to a too low surface roughness in a cold-rolled flat steel product optionally coated with a metal coating, which would have a negative influence on the formability of the flat steel product. With a temper degree of more than 2.0%, both the mechanical properties (yield strength and elongation at break) and the hole expansion ratio would be negatively influenced.

The alloying elements of the melt or steel (flat steel product) are given as follows:

Carbon (C) is mainly sequestered in excretions. The concentration of C dissolved in the mixed crystal is minimized in order to avoid the formation of undesirable iron-based phases. A content of at least 0.020%, in particular at least 0.030%, preferably at least 0.040% is required in order to achieve a high precipitate density and thus achieve the required tensile strength. Too high a content would in turn lead to the formation of undesirable phases such as martensite, bainite, austenite, retained austenite, pearlite and/or cementite in the structure, which would reduce the ductility and increase the sensitivity to edge cracks. The content is therefore limited to a maximum of 0.20%, in particular a maximum of 0.150%, preferably a maximum of 0.120%, whereby negative influences of the presence of C can be particularly reliably avoided if the content is preferably a maximum of 0.110%.

Manganese (Mn) contributes to the increase in strength through solid solution strengthening of the ferrite. In addition, Mn suppresses the formation of pearlite and in this way promotes the formation of precipitates containing Ti, Nb and/or V. In order to achieve these effects of Mn, a Mn content of at least 0.10%, in particular at least 0.20%, preferably at least 0.40% is required. However, too high a content has a negative effect on weldability and increases the risk of dominant segregation occurring (chemical inhomogeneities in the structure that arise during solidification). Therefore, the upper limit of the salary is set to a maximum of 4.0%, with lower contents of in particular a maximum of 3.0%, preferably at most 2.50%, can avoid possible negative effects of the presence of Mn.

In the broadest sense, phosphorus (P) is a contaminant that is introduced into the steel by iron ore and cannot be completely eliminated in the large-scale steelworks process. The content should be set as low as possible, whereby the content should be at most 0.020%, in particular at most 0.010%, for reliable weldability. The lower limit can in particular be 0.0002%.

In the broadest sense, sulfur (S) is also an impurity and must therefore be adjusted to a maximum content of 0.010% in order to avoid a strong tendency to segregation and a negative influence on formability or elongation as a result of excessive formation of sulfides (FeS; MnS; (Mn, Fe)S), in particular at most 0.0050%. As a rule, calcium can be added for desulfurization and adjustment of the S content depending on the Ca content. The lower limit can in particular be 0.0002%.

Nitrogen (N) is also an unavoidable impurity due to production. If N is present, Ti, Nb and/or V preferably form nitrides or carbonitrides with N in the simultaneous presence of C. Therefore, in practice, under the technically and economically feasible conditions, the uptake of N in the excretions is unavoidable. In principle, however, the lowest possible content should be aimed for, since N-dominated carbonitrides are often very coarse and angular, which is why they do not contribute to solidification, but rather act as crack initiators. In order to avoid the formation of N-dominated carbonitrides, the content must be limited to a maximum of 0.010%, in particular a maximum of 0.0050%. The lower limit can in particular be 0.0002%.

Titanium (Ti), niobium (Nb) and vanadium (V) are considered microalloying elements and are alloyed either individually or in combination (Ti and Nb; or Ti and V; or Nb and V; or Ti, Nb and V). The microalloy elements are essential for the formation of the “carbide” precipitates. The required density of the precipitates can be achieved if at least one of the microalloy elements from the group (Ti, Nb, V) is alloyed with a content of at least 0.040% each. In addition, condition I) must be fulfilled with 0.04% <= X <= 0.3%, where X can be calculated using the formula X = Ti + V/1.06 + Nb/1.94. If the contents of Ti, Nb and/or V are too low, which would result in a value of X below 0.04%, the required density of the precipitates would be and consequently the required tensile strength is not achieved. X is therefore at least 0.04% and can in particular be at least 0.05%, preferably at least 0.07%. In order to avoid negative effects of high levels of microalloy elements, X is limited to a maximum of 0.3%. In this way, it is avoided that, for example in the presence of Nb, increased Nb contents lead to crack formation during (continuous) casting or during slab cooling and/or preheating. At the same time, only a certain content of microalloy elements is required to achieve the desired strength. If this is exceeded, only a slight further increase in strength occurs. In addition, the average diffusion distances decrease, which increases the risk of undesirably large precipitates forming. For these reasons, Ti, Nb and/or V contents are added in such a way that X is at most 0.3%, in particular at most 0.25%, preferably at most 0.2%. X in % means % by weight.

When setting or alloying the microalloy elements, it must be noted that condition II) is still fulfilled with 0.3 <= Y <= 1.0, where Y is given by the formula Y = 0.25 x x / (C + 0 .86 x N) can be calculated. A Y < 0.3 would result from too high a C or N content. An excess of C would lead to excess formation of undesirable phases such as martensite, bainite, austenite, retained austenite, pearlite and/or cementite, which would have a negative effect on the hole expansion ratio. Too high a N content would lead to the formation of coarse N-dominated carbonitrides, which would also have a negative effect on the hole expansion ratio. For these reasons, the contents of the alloying elements determining condition II) are adjusted so that Y is at least 0.3, in particular at least 0.35, preferably at least 0.4. A Y > 1.0 results from an excessively high content of Ti, Nb and V. An excess of Ti, Nb and V does not in any case contribute to a further increase in strength, but increases the risk of the formation of undesirable ones Excretions that have a negative impact on the mechanical-technological properties. For this reason, the contents of the alloying elements determining condition b) are adjusted so that Y is at most 1.0, in particular at most 0.8, preferably at most 0.7. Y has no units of measurement.

Furthermore, one or more alloy elements from the group (Si, Al, Cr, Mo, W, Ca, B, Cu, Ni, Sn, As, Co, Zr, La, Ce, Nd, Pr, 0, H) can optionally be used. be included: Silicon (Si) can be added as an optional alloying element to suppress the formation of pearlite in the structure. In addition to Mn, Si contributes to the increase in strength through solid solution strengthening of the ferrite. To achieve these effects of Si, a content of at least 0.050% is required. If the Si content is too high, the rollability of the steel would be negatively affected and growth could occur on the rolls in the cold rolling mill or in the cold rolling stand during rolling processing. Furthermore, too high a Si content can affect the surface quality of the flat steel product with optional hot-dip galvanizing. In order to avoid these negative influences of Si, the Si content is limited to a maximum of 1.50%, with contents of in particular a maximum of 1.00%, preferably a maximum of 0.70% with a view to avoiding negative influences of the presence of Si prove to be particularly favorable. If there are special requirements for the ability to be galvanized in pieces, it is preferable to avoid alloying with Si and allow a maximum content of 0.03%.

Aluminum (AI) can be added as an optional alloying element to suppress pearlite. Because AI is usually used to deoxidize the melt, a content of at least 0.010% is unavoidable when producing steel normally. However, too high a content can have a negative effect on castability. Therefore, the upper limit of the Al content is limited to at most 1.50%, in particular at most 1.00%, preferably at most 0.70%.

Chromium (Cr), molybdenum (Mo) and/or tungsten (W) can be alloyed as optional alloying elements either individually or in combination (Cr and Mo; or Cr and W; or Mo and W; or Cr, Mo and W). If Cr, Mo or W are present, the precipitates containing Ti, Nb and/or V are partially or completely bound and slow down or prevent their coarsening. The fineness of the precipitates is essential for achieving the desired mechanical-technological properties of the cold-rolled flat steel product, in particular the tensile strength and the hole expansion ratio. With very short glow times, the coarsening of the precipitates can be avoided or reduced. In such cases, alloying with Cr, Mo and/or W is not absolutely necessary to slow down the coarsening of the precipitate. For slightly longer annealing times, alloying with Cr, Mo and/or W can prevent the excessive coarsening of the precipitates and consequently the impairment of the tensile strength and the hole expansion ratio. In order to achieve these effects of Cr, Mo and/or W, a content of at least 0.050%, in particular at least 0.08% by weight, preferably at least 0.10 is required % by weight required. If the Cr, Mo and/or W content is too high, there is an increased risk that undesirable phases will form, which can impair the mechanical-technological properties. In addition, an excess of Cr would cause undesirable, pronounced grain boundary oxidation. For these reasons, the Cr content is limited to a maximum of 1.5% by weight, in particular a maximum of 1.2% by weight, preferably a maximum of 1.0% by weight. Mo and W are among the most expensive alloying elements, so high Mo and W contents should be avoided for economic reasons. For these reasons, the contents of Mo and W are each limited to 0.5% by weight, in particular at most 0.3% by weight, preferably at most 0.2% by weight.

Copper (Cu) can precipitate in the form of coarse particles, which have a negative effect on the mechanical properties. Cu also has a negative impact on castability. In order to avoid any influence of Cu, the content is limited to a maximum of 0.10%, preferably a maximum of 0.05%. The lower limit can in particular be 0.0002%.

Calcium (Ca), Boron (B), Nickel (Ni), Tin (Sn), Arsenic (As), Cobalt (Co), Zirconium (Zr), Lanthanum (La), Cerium (Ce), Neodymium (Nd), Praseodymium (Pr), oxygen (0) and hydrogen (H), like all other conceivable alloying elements not explicitly listed here, are to be attributed to the impurities that are unavoidable in manufacturing technology and that are present as components of the starting material from which the steel is produced, or as a result of the process during steel melting and processing Steel can reach. The contents of these elements must be kept so low that they have no technical effect on the properties of the underlying steel.

Ca is usually added to the melt during steel production for deoxidation and desulfurization as well as to improve castability. Too high a content can lead to the formation of undesirable inclusions, which have a negative effect on the mechanical properties and rollability. Therefore, the upper limit is limited to a maximum of 0.0050%, in particular a maximum of 0.0020%.

Carbides form in the cold-rolled flat steel product at moving phase boundaries. The movement of phase boundaries can be slowed down by B segregated at them. This can prevent the formation of carbides. To avoid this effect, the content of B is limited to a maximum of 0.0010%, in particular a maximum of 0.0006%, preferably a maximum of 0.0004%.

Ni, Sn, As, Co, Zr, as well as rare earths such as La, Ce, Nd and Pr, are also optional alloying elements and are not required and are considered unavoidable impurities if they are nevertheless detectable. Accordingly, the Ni content is at most 0.10%; the Sn content to a maximum of 0.050%; the As content to a maximum of 0.020%; the Co content to a maximum of 0.020%; the Zr content to a maximum of 0.0002%; the La content to a maximum of 0.0002%; the Ce content to a maximum of 0.0002%; the Nd content to a maximum of 0.0002%; the Pr content is limited to a maximum of 0.0002%.

0 is also undesirable in the melt or in the steel, as an oxide coating would have a negative effect on both the mechanical properties and the castability and rollability. The maximum permissible content is therefore set at a maximum of 0.0050%, in particular a maximum of 0.0020%.

As the smallest atom in interstitial spaces in steel, H is very mobile and can lead to cracks in the core, especially in high-strength steels, when cooling from hot rolling. The content should therefore be as low as possible, in any case at most 0.0010%, in particular at most 0.0006%, preferably at most 0.0004%, with levels preferably at most 0.0002% being aimed for.

The information in % in connection with the aforementioned alloying elements refers to % by weight.

The cold-rolled flat steel product has a ferritic basic structure. A ferritic basic structure is therefore to be understood as a structure which contains ferrite in a proportion of at least 90%. The proportion of the ferritic structure can in particular be at least 92%, preferably at least 94%, preferably at least 96%, more preferably at least 98%. The main components of the structure can be determined using light optical microscopy (LOM) at a magnification of 200 to 2000 times. Fine “carbide” precipitates based on Ti, Nb and/or V are embedded in the ferritic structure. The precipitates have an average precipitate diameter of at most 10 nm, in particular at most 7 nm, preferably at most 5 nm. If there are precipitates, the average precipitate diameter is > 0 nm. Due to their fineness, these are measured using LOM cannot be recognized, but can only be determined using transmission electron microscopy (TEM) at a magnification of 50,000 to 500,000 times. Hard iron-based phases such as martensite, bainite, austenite, retained austenite, pearlite and/or cementite are detrimental to the desired mechanical-technological properties, in particular for the hole expansion ratio, and are therefore undesirable phases, but depending on the alloying elements in the above-mentioned ranges in total with less than 10%, in particular less than 7%, preferably less than 5%, preferably less than 3%. Furthermore, the structure can have other structural components that are unavoidable during production, at most up to 1%, in particular at most up to 0.5%.

Precipitates mean “carbides” with a NaCl (Bl) crystal structure, which consist predominantly of C and at least one of the alloying elements from the group Ti, Nb and V. If one or more of the alloying elements from the group Mo, W or Cr are present, they may also be contained in the precipitates. In addition, the excreta may contain a low N content. Precipitates with a significant proportion of N are often referred to as “carbonitrides”, but have the same crystal structure and effect on the mechanical-technological properties of the steel as carbides. The sum of the Ti, Nb and V contents (each in atomic%) based on the chemical analysis of the precipitates is therefore at least 20 atomic%. To convert from wt% to atomic%, the usual formula KAT = 100 * (KG / mK) / sum of (iG / mi) is used, where KAT and KG are the concentration of element K in atomic% and % by weight; mK is the atomic mass of the element K; and iG is the content (in% by weight) and mi is the atomic mass of component i in the mixture of components. If the contents of Ti, Nb and/or V in the precipitates are too low, they are not “carbides”, but rather other precipitates that are significantly coarser than the Ti, Nb and/or V-based precipitates do not meet the requirements regarding the size of the excretions. These requirements are essential for achieving the desired mechanical-technological properties; If the precipitates are too coarse, the requirements regarding tensile strength R _m and hole expansion ratio X cannot be achieved.

The cold-rolled flat steel product has a tensile strength R _m of at least 550 MPa, in particular at least 580 MPa, preferably at least 610 MPa, preferably at least 650 MPa, although tensile strengths of at least 780 MPa or even 960 MPa can also be achieved. The maximum tensile strength can be, for example, 1300 MPa. The cold-rolled flat steel product has a low yield strength ratio. The yield strength ratio is defined by the ratio of the yield strength R _p0 .2 TO the tensile strength R _m . The yield strength ratio is at least 0.6, in particular at least 0.7 and at most 0.9, in particular at most 0.85.

The elongation at break A ₅₀ in the cold-rolled flat steel product is at least 9%, in particular at least 11%, preferably at least 12%.

The tensile strength R _m , the yield strength R _p0 , ₂ and the elongation at break A ₅₀ can be determined in tensile tests according to DIN EN ISO 6892-1:2017.

The hole expansion ratio It has been found that the cold-rolled flat steel product has a particularly favorable ratio of hole expansion ratio X to tensile strength R _m . In a cold-rolled flat steel product according to the invention, high hole expansion ratios are achieved even with high tensile strengths, which are expressed in high values for the product of tensile strength R _m and hole expansion ratio X. Values of at least 30,000 MPa*%, in particular at least 40,000 MPa*%, preferably at least 45,000 MPa*%, preferably at least 50,000 MPa*% are therefore achieved.

The invention is explained in more detail below using exemplary embodiments. These are summarized in Tables 1 - 4. Table 1 shows both the chemical compositions as well as the sum X and the quantitative ratio Y of the exemplary embodiments. The production specifications with regard to hot and cold rolling as well as annealing and galvanizing are given in Tables 2 and 3, respectively. Table 4 shows both the mechanical-technological properties and structural characteristics of the exemplary embodiments.

To test the invention, the melts A - AE alloyed in accordance with the compositions given in Table 1 were produced and cast into slabs. The melts not according to the invention and their contents of certain alloying elements that deviate from the specifications of the invention are underlined in Table 1 highlighted. Contents of an alloy element that are so low that they are “0” in the technical sense, i.e. so low that they have no influence on the properties of the steel, are designated in Table 1 by the entry “-”.

The slabs produced from steels A - AE were heated through in a preheating furnace in which a preheating temperature (“VWO”) prevailed. The preheated slabs were then hot-rolled in a conventional manner into a hot-rolled flat steel product (hot strip). The hot-rolled steel strip obtained in each case left the hot rolling train with a final hot rolling temperature (“WET”) and was then cooled at a cooling rate (“KR1”) to a coiling temperature (“HT”), at which they were each coiled into a coil. After the coil has cooled to room temperature, the hot-rolled flat steel product has been pickled in a conventional manner and then cold-rolled using a cold rolling grade ("KWG") into a cold-rolled flat steel product.

To demonstrate the effect of the invention, one of the combinations I - XII of VWO, WET, KR1, HT and KWG given in Table 2 was selected when producing the cold-rolled flat steel products. The combinations of I - XII that are not according to the invention and the specifications that do not correspond to the requirements of the invention are highlighted in Table 2 by underlining.

After cold rolling, the cold-rolled flat steel product was annealed continuously in an induction annealing system. During annealing, the cold-rolled steel strip is heated at a medium heating rate (“HR”) to a (medium) annealing temperature (“GT”), at which it is held for a holding time (“HZ”). The cold-rolled steel strip was then cooled at a medium cooling rate (“KR2”) to a maximum temperature of 550 °C. The flat steel product produced in this way may have been tempered with a temper grade (“DG”) and/or coated with a Zn-based corrosion protection coating.

In the exemplary embodiments, the annealing and, if necessary, the galvanizing takes place according to a combinations a - i of HR, GT, HZ, KR2 and DG given in Table 3. For combinations a - i it is also stated whether the cold-rolled flat steel product has been coated with a Zn coating. The combinations of a - i and which are not according to the invention the specifications that did not correspond to the requirements of the invention are highlighted in Table 3 by underlining.

The mechanical-technological properties and structural characteristics of the flat steel products produced in this way were determined. The results of these investigations such as the yield strength R _p0 , ₂ , the tensile strength R _m , the yield strength ratio R _p o.2 / R _m , the elongation at break A ₅₀ , the hole expansion ratio X, the product Rm * X, the average precipitation diameter DM and the ferrite content F of the structure are summarized in Table 4. For flat steel products produced on the basis of the hot-rolled flat steel products Al - AE1, it is also stated which of the steels A -AE the steel substrate of the respective flat steel product consisted of (column "Analysis"), which of the combinations I - XII of hot strip production (column "Rolls") and which of the variants a - i of the annealing treatment and the melt application the respective steel substrate has undergone (column “Annealing”).

Example Al is an example according to the invention, which consists of a steel substrate with the chemical composition A and was produced according to rolling specification I and annealing specification a. This resulted in an optimal combination of mechanical-technological properties and structural characteristics.

Examples A2 - A4 are comparable to Example Al, except that the hot rolling conditions differ from the specifications required according to the invention. In this sense, they serve as counterexamples. In example A2, the slab was heated with a preheating temperature VWO that was too low, so that the slab was not completely annealed. As a result, the alloying elements and manufacturing methods did not affect the mechanical properties. In example A3, a final rolling temperature WET was set that was too low, so that the desired isotropy of the material was lost due to the effects of thermomechanical rolling. In example A4, the hot-rolled steel strip was cooled at a cooling rate KR1 that was too low, so that coarse precipitates were formed in front of the coiler. As a result, the required precipitate size and, consequently, the required mechanical-technological properties could not be achieved.

Chemical analyzes B - G are variations of analysis A, with Ti replaced by various combinations of Ti, Nb and V, respectively. The chemical analyzes H - J are also variations of analysis A, which additionally contain Cr, Mo or Mo. For all analyzes B - J Both other alloy components as well as the sum X and the quantitative ratio Y were kept the same as in analysis A. The resulting examples B1 - J1 were processed using the same conditions as example A1. This resulted in mechanical-technological properties on the same level as Example Al.

The examples Kl - Ml are counterexamples that are also based on the example Al. In example K1, sum X and consequently the quantitative ratio Y is too low. Example LI has a C content that is too low and therefore a quantitative ratio Y that is too high. In contrast, Example Ml has a very high content of C and consequently a quantitative ratio Y that is too low. Examples Kl - Ml are with the same process conditions as Example Al produced, but as a result of the different chemical analyzes have mechanical-technological properties outside the target range.

The steels N and 0 are low-alloy concepts that only differ in their C content and therefore in the proportions Y. Using the steel N, the influence of the cold rolling grade KWG was examined in the examples NI - N4. This showed that as the KWG decreases, the tensile strength Rm decreases and the hole expansion ratio X increases. Steel 0 has a slightly higher C content and consequently a lower Y compared to steel N. This resulted in an increased R _m and a reduced X compared to example N2, which was processed under the same conditions.

The steels P and Q are very low-alloy concepts in which high proportions Y were achieved by adjusting the C and Nb contents. The resulting examples PI and Ql were processed under the same conditions as examples N2 and 01. In comparison to Examples N2 and 01, Examples PI and Ql had lower but still acceptable tensile strengths R _m .

The influence of Mn on the mechanical properties was investigated using the steels R - U. The steels R - U have lower Mn contents compared to steel A. In addition, steels R - U, in contrast to steel A, are alloyed with Cr and Mo. The resulting examples RI - Ul were processed under the same conditions as the examples N2 and 01 - Ql. This showed that the tensile strength increases with increasing Mn content. The steel U was used in the examples Ul - U5 to determine the influence of the heating rate HR; the holding time HZ and the cooling rate KR2 should be examined. This shows that as HR and KR2 decrease and HZ increase, the excretory size increases and consequently the R _m and the X decrease. In example U2, the HR and KR2 were set too low and the HR was too long. This resulted in too high a precipitate size and, consequently, an R _m and an X outside the required ranges. The example U2 therefore serves as a counterexample.

Steels V and W are medium-strength variants that have higher Al and Si contents, respectively. These were used in Examples VI - W3 to investigate the influence of the reel temperature HT. This shows that as HT increases, the excretory size increases and consequently the R _m and X decrease. In the counterexamples V3 and W3, the HT was set too high. This led to a precipitation size that was too high and consequently to a product R _m x X that was too low.

The steels X and Y are higher-strength variants that have even higher Al and Si contents. These were hot rolled in Examples XI - Y4 with a low final rolling temperature WET, cooling rate KR1 and coiling temperature HT. The influence of the annealing temperature GT was examined in Examples XI - Y4. This shows that as GT increases, R _m decreases and X increases. In counterexamples XI and Y1, the GT was set too low. This led to insufficient recovery or recrystallization of the structure and consequently to an X that was too low.

The steels Z and AA are extremely high-strength variants that have very high Si and Al contents, respectively. The resulting examples ZI and AA1 had a high X with a very high R _m . Steel AB is similar to steel Z, except that the C content is too high. The resulting example ABI was processed with the same conditions as the example ZI. Due to the excess of C, example ABI contained too low a proportion of ferrite. This led to a sharp deterioration in the mechanical and technological properties. Therefore, example ABI serves as a counterexample.

The steels AC - AE are comparable to reference example A, but have a different concentration of impurities P, S, N and Cu. These were processed in example AC1 - AE1 with the same conditions as example Al. AC steel has a very low concentration of impurities. The resulting example AC1 has mechanical-technological properties comparable to those of example Al. Compared to steel A, steel AD has a higher but still acceptable concentration of impurities. The resulting example ADI has poorer mechanical technological properties as an example of Al, but they were still within the required ranges. The steel AE has too high a concentration of impurities. As a result, the resulting example AE1 has a ferrite content that is too low and a precipitate diameter that is too high and therefore an X that is too low.

Table 1

Table 2

Table 3

Table 4

Claims

Claims Process for producing a cold-rolled flat steel product with a tensile strength R _m of at least 550 MPa, an elongation at break A ₅₀ of at least 9%, R _m and A ₅₀ determined according to DIN EN ISO 6892-1:2017, with a hole expansion ratio X of at least 40% determined according to DIN EN ISO16630:2017, with a ferritic base structure with at least 90% ferrite and carbide precipitates based on Ti, Nb and/or V embedded in the ferritic base structure with an average precipitate diameter of at most 10 nm, comprising the steps: a) Melting of a steel consisting of Fe and unavoidable impurities in wt.%

C: 0.020 to 0.20%,

Mn: 0.10 to 4.00%,

P: up to 0.020%,

S: up to 0.010%,

Ti: at least 0.040%,

Nb: at least 0.040%,

V: at least 0.040%, subject to the following conditions:

I) 0.04% <= X <= 0.3% with X = Ti + V/1.06 + Nb/1.94

Si: up to 1.50%,

AI: up to 1.50%,

Cr: up to 1.50%,

Mon: up to 0.50%,

W: up to 0.50%,

Cu: up to 0.10%, b) casting the melt into a preliminary product; c) preheating the preliminary product to a temperature and/or maintaining the preliminary product at a temperature between 1150 and 1350 ° C; d) hot rolling the preliminary product into a hot-rolled flat steel product with a final hot rolling temperature between 850 and 980 ° C; e) cooling the hot-rolled flat steel product obtained at a cooling rate of between 20 and 400 ° C / s to a coiling temperature of between 400 and 700 ° C; f) coiling the hot-rolled flat steel product cooled to the coiling temperature into a coil; g) uncoiling the coil and cold rolling with a degree of cold rolling between 5 and 80% to form a cold-rolled flat steel product; h) coiling the cold-rolled flat steel product into a coil; i) Uncoiling the coil and annealing the cold-rolled flat steel product in a continuous flow comprising the steps:

11) Heating at an average heating rate between > 100 and 1000 ° C / s to a temperature between 800 and 900 ° C and holding at 800 to 900 ° C for a period between 0.1 and 18 s;

12) Cooling at a mean cooling rate between 100 and 1000 °C/s to a temperature of not more than 550 °C and optionally holding at this temperature for a period of not more than 100 s; j) Coiling the cold-rolled flat steel product into a coil. A method according to claim 1, wherein the hot-rolled flat steel product is pickled between steps f) and g). Method according to claim 1 or 2, wherein after step i2) the cold-rolled steel flat product is fire-coated with a Zn-based anti-corrosion coating. A method according to claim 3, wherein the fire-coated steel flat product is heated to a temperature of up to 550°C. Method according to claim 1 or 2, wherein after step i2) the cold-rolled steel flat product is electrolytically coated with a Zn-based anti-corrosion coating. Method according to one of the preceding claims, wherein the cold-rolled flat steel product is tempered with a degree of tempering between 0.1 and 2.0%. A cold-rolled flat steel product produced according to one of the preceding claims with a yield strength ratio R _p0 , ₂ / R _m of at least 0.6 and at most 0.9. Flat steel product according to claim 7 with an yield strength ratio R _p0 , ₂ / R _m of at least 0.7 and at most 0.85. Flat steel product according to claim 7 or 8, wherein the product of tensile strength R _m and hole expansion ratio X reaches at least 30,000 MPa*%. Flat steel product according to one of claims 7 to 9, wherein the ferrite in the structure is at least 94%. Flat steel product according to one of claims 7 to 10, wherein the average precipitation diameter is at most 7 nm.