WO2023001693A1

WO2023001693A1 - Method for producing a cold-rolled flat steel product having a bainitic basic structure and cold-rolled flat steel product having a bainitic basic structure

Info

Publication number: WO2023001693A1
Application number: PCT/EP2022/069774
Authority: WO
Inventors: Bernd Linke; Michael BELDE
Original assignee: Thyssenkrupp Steel Europe Ag
Priority date: 2021-07-22
Filing date: 2022-07-14
Publication date: 2023-01-26
Also published as: CN117677718A; DE102021119047A1

Abstract

The invention relates to a method for producing a cold-rolled flat steel product having a bainitic basic structure and to a correspondingly cold-rolled flat steel product having a bainitic basic structure.

Description

Process for producing a cold-rolled flat steel product with a bainitic matrix and cold-rolled flat steel product with a bainitic structure

basic structure

Technical Field

The invention relates to a method for producing a cold-rolled flat steel product with a bainitic matrix and a corresponding cold-rolled flat steel product with a bainitic matrix.

Technical background

Methods for producing bainitic sheet steel are known from the prior art, see EP 2 707 514 B1, EP 3 024951 B1.

Steels with a bainitic matrix with proportions of martensite and retained austenite are characterized by a particularly good combination of strength and elongation at break, especially with higher carbon contents. At the same time, for high proportions of residual austenite, an average of about 1.5% by weight of silicon is alloyed, which increases the transformation temperature into austenite and thus changes the temperatures for setting the structure, or the temperature, during heat treatment, for example inline in a hot-dip coating system would have to be increased compared to the conventional temperature and thus higher costs would be caused by the throughput. Furthermore, high silicon contents can significantly impair the surface quality, coatability and weldability.

There is a need for optimization with regard to the economics of manufacturing flat steel products with a bainitic matrix, in particular with high tensile strength and high elongation at break.

Summary of the Invention

The invention is therefore based on the object of providing a method for producing a cold-rolled flat steel product with a bainitic matrix, with which the disadvantages mentioned in the prior art can be overcome and a correspondingly produced cold-rolled flat steel product with a bainitic matrix can be specified. According to a first aspect of the invention, this object is achieved by a method having the features of patent claim 1.

According to the invention, there is a method for producing a cold-rolled flat steel product with a bainitic matrix, comprising the steps: a) Casting a melt consisting of Fe and unavoidable impurities (in % by weight) from

C: 0.10 to 0.30%, in particular 0.15 to 0.25%, preferably 0.18 to 0.22%,

Si: 0.40 to 1.20%, in particular 0.60 to 1.10%, preferably 0.80 to 1.10%,

Mn: 1.00 to 2.00%, in particular 1.10 to 1.80%, preferably 1.10 to 1.60%,

Cr: 0.50 to 1.50%, in particular 0.60 to 1.20%, preferably 0.70 to 1.00%,

N: 0.0030 to 0.040%, in particular 0.0070 to 0.030%, preferably 0.0090 to 0.025%,

P: up to 0.10%, in particular up to 0.050%, preferably up to 0.030%;

S: up to 0.050%, in particular up to 0.020%, preferably up to 0.0080%, where P and S can be among the impurities, optionally one or more alloying elements from the group (Al, V, Ti, Nb, Ni, Mo, W , Ca) with

Al: up to 0.050%, in particular 0.0010 to 0.015%,

V: up to 0.20%, in particular 0.0010 to 0.20%,

Ti: up to 0.010%, in particular 0.0010 to 0.010%,

Nb: up to 0.10%, in particular 0.0010 to 0.10%,

Ni: up to 0.40%, in particular 0.010 to 0.40%,

Cu: up to 0.80%, in particular 0.010 to 0.80%,

Mo: up to 1.00%, in particular 0.0020 to 1.00%,

W: up to 1.00%, in particular 0.0010 to 1.00%,

Ca: up to 0.0050%, in particular 0.0001 to 0.0050%, to a precursor; b) reheating the precursor to a temperature and/or maintaining the precursor at a temperature between 1100 and 1350°C; c) optional intermediate hot rolling of the preliminary product into an intermediate flat product in one or more roll stands at an intermediate final rolling temperature between 950 and 1250 °C; d) hot rolling of the preliminary product or the optional intermediate flat product to form a hot-rolled flat steel product in one or more roll stands at a final rolling temperature of between 800 and 1000°C; e) coiling the hot-rolled flat steel product at a coiling temperature between 400 and 650 °C; f) optional annealing of the hot-rolled steel flat product at an annealing temperature between 500 and 900 °C; g) cold rolling of the hot-rolled and optionally annealed flat steel product in one or more rolling stands with a total degree of cold rolling of at least 30%; h) Heat treatment of the cold-rolled flat steel product with austenitizing at a temperature T_A between above 800 and 950 °C, quenching to a temperature T_B between 300 and 580 °C, such that a bainitic basic structure is established in the cold-rolled flat steel product.

According to a second aspect of the invention, this object is achieved by a cold-rolled

Steel flat product with the features of patent claim 6.

According to the invention, a cold-rolled flat steel product is provided with a bainitic matrix which, in addition to Fe and impurities that are unavoidable due to production

wt% off

C: 0.10 to 0.30%, in particular 0.15 to 0.25%, preferably 0.18 to 0.22%,

Si: 0.40 to 1.20%, in particular 0.60 to 1.10%, preferably 0.80 to 1.10%,

Mn: 1.00 to 2.00%, in particular 1.10 to 1.80%, preferably 1.10 to 1.60%,

Cr: 0.50 to 1.50%, in particular 0.60 to 1.20%, preferably 0.70 to 1.00%,

P: up to 0.10%, in particular up to 0.050%, preferably up to 0.030%, S: up to 0.050%, in particular up to 0.020%, preferably up to 0.0080%, where P and S can be among the impurities, optionally one or more alloying elements from the group (Al, Ti, Nb, Ni, Mo, W, Ca ) with

Al: up to 0.050%, in particular 0.0010 to 0.015%,

V: up to 0.20%, in particular 0.0010 to 0.20%,

Ti: up to 0.010%, in particular 0.0010 to 0.010%,

Nb: up to 0.10%, in particular 0.0010 to 0.10%,

Ni: up to 0.40%, in particular 0.010 to 0.40%,

Cu: up to 0.80%, in particular 0.010 to 0.80%,

Mo: up to 1.00%, in particular 0.0020 to 1.00%,

W: up to 1.00%, in particular 0.0010 to 1.00%,

Ca: up to 0.0050%, in particular 0.0001 to 0.0050%, the structure of the cold-rolled flat steel product having the following proportions: at least 0.5% martensite, at least 5% residual austenite,

Rest bainite and unavoidable structural components.

A bainitic matrix is therefore to be understood as meaning a structure which has bainite with a proportion which is greater than the proportion of martensite and also greater than the proportion of retained austenite. In particular, the proportion of bainite is greater than the sum of the proportions of martensite and retained austenite. The proportion of bainite in the structure is preferably greater than 50%.

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

The cold-rolled flat steel product according to the invention has a tensile strength of at least 1000 MPa, in particular at least 1100 MPa, preferably at least 1200 MPa. The elongation at break A50 of the cold-rolled flat steel product according to the invention is at least 9%, in particular at least 11%, preferably at least 13%. The tensile strength and the elongation at break A50 can be determined in a tensile test according to DIN EN ISO 6892-1.

After casting a melt with an alloy composition within the specified ranges to form a preliminary product, for example in a continuous casting plant or cast-rolling plant, the preliminary product can be further processed directly, i. H. coming directly from the casting heat, for example in the case of the cast-rolling plant, so that the preliminary product is kept at a temperature or, if necessary, reheated to a temperature, for example in a soaking or reheating furnace, in which the most complete possible homogenization is ensured and in which any precipitates that may have formed dissolve as completely as possible. For example, if the melt is cast into a preliminary product in a continuous casting plant, the cast and completely solidified strand is cut into several slabs of finite size and the slabs are then allowed to cool down to ambient temperature through natural cooling. The preliminary product or the slab is (subsequently) reheated to a temperature, for example in a walking beam furnace or by other suitable means. Alternatively, the still hot preliminary product or the still hot slab can also be transferred into a soaking or reheating furnace, for example, without intermediate cooling (allowing).

The temperature during reheating and/or when holding the preliminary product is at least 1100° C., in particular at least 1140° C., preferably at least 1180° C., in order to ensure that any precipitations present in the preliminary product are dissolved as completely as possible. The temperature for reheating and/or holding should not exceed 1350°C in order to avoid partial melting and/or excessive scaling of the pre-product. For ecological and economic reasons, the temperature for reheating and/or holding is limited in particular to a maximum of 1280°C.

Depending on the available equipment and/or processing options, it can and is optionally stated that intermediate hot rolling of the preliminary product to form an intermediate flat product can be carried out in one or more roll stands at a final rolling temperature of between 950 and 1250°C. A final rolling temperature for the optional production of the intermediate flat product of at least 950° C. is selected in order to utilize a grain-refining effect of recrystallization after the rolling pass(es) as reliably as possible. For reasons of energy efficiency, a maximum rolling temperature of 1250° C. is selected in particular for the optional production of the intermediate flat product.

The preliminary product or the optional intermediate flat product is hot-rolled in one or more roll stands with a final rolling temperature between 800 and 1000°C to form a hot-rolled flat steel product.

A final rolling temperature for producing the hot-rolled flat steel product of at least 800° C. is selected so that the deformation resistance does not increase too much. In order to avoid undesirable coarse grain formation, the final rolling temperature for producing the hot-rolled flat steel product is limited to a maximum of 1000°C. In particular, the final rolling temperature for producing the hot-rolled flat steel product in a multi-stand hot-rolling/finishing line is set to at least 850° C. to ensure the highest possible austenite content, and preferably to at least 880° C. to ensure recrystallization. To limit the amount of coolant required, rolling end temperatures of up to a maximum of 950° C. are selected, in order to minimize recrystallization and grain growth between the end rolls and coilers, preferably up to a maximum of 930° C. The degree of hot rolling in the last pass or on the last hot rolling stand is preferably at least 10% in order to be able to set a fine structure in the hot-rolled flat steel product.

The hot-rolled steel flat product is coiled at a coiling temperature between 400 and 650°C. The coiling temperature must be at least 400°C to prevent martensite formation. In order to limit the diffusion of oxygen-affinity alloying elements to the surface during the coiling process, the coiling temperature is limited to a maximum of 650°C. In particular, the coiling temperature can be at least 500° C. in order to produce (considerable) ferrite components in the microstructure, which enable good cold-rollability, preferably with high degrees of cold-rolling. In order to create a particularly good surface which, in conjunction with the low Si content within the aforementioned limits, can ensure a particularly wide range of joining (welding area), the coiling temperature is selected up to a maximum of 570°C. The hot-rolled flat steel product (hot strip) can have a thickness between 1.5 and 10 mm.

The hot-rolled flat steel product can optionally be subjected to annealing at an annealing temperature of between 500 and 900°C, in particular up to a maximum of 800°C, preferably up to a maximum of 700°C. The optional annealing essentially corresponds to a standard annealing process for hot-rolled steel flat products and can lead in particular to better cold-rollability.

The hot-rolled and optionally annealed flat steel product is subjected to cold rolling, the cold rolling being carried out in one or more rolling stands with a total cold rolling degree of at least 30%. A total degree of rolling of at least 30% is required in order to provide targeted nucleation sites in the structure for the subsequent heat treatment, at which austenite nuclei can advantageously develop. The total degree of cold rolling can be at least 38%, preferably at least 45%, in order to break longer pearlite lines within the structure, if present in the structure, which means that existing cementite/ferrite interfaces can be further distributed within the structure, at which during the subsequent heat treatment austenite can germinate particularly well. The total degree of cold rolling can be a maximum of 80%, in particular a maximum of 70%.

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

According to the invention, a targeted heat treatment of the cold-rolled flat steel product takes place with austenitizing at a temperature T_A between 800 and 950° C., quenching to a temperature T_B between 300 and 580° C. such that a bainitic basic structure is established in the cold-rolled flat steel product. At a temperature T_A above 800°C, the thermodynamic driving force for the formation of austenite from cementite and ferrite is already extremely high, which can contribute to a rapid and desired austenitization. In order to achieve an energy-efficient process, the temperature T_A must be set to less than or equal to 950°C. In particular, the temperature T_A can be set to be less than or equal to 900° C. in order to prevent severe austenite grain coarsening. The temperature T_A can preferably be set to be less than or equal to 875° C., at which the carbon in the structure is preferably not yet 100% homogeneous distributed, which can lead to faster bainite nucleation and thus to a faster bainitic transformation rate in the subsequent quenching. After austenitizing, quenching takes place to a temperature T_B between 300 and 580°C. The temperature T_B of at least 300°C must be set so that the carbon content in the retained austenite can be redistributed and no more than 40% martensite is formed. The temperature T_B can in particular be at least 340° C., preferably at least 380° C., in order to reduce inhomogeneities in the carbon distribution to such an extent that carbon hardly accumulates at the bainite/residual austenite interfaces. The temperature T_B is set to a maximum of 580°C in order to reliably prevent ferrite/pearlite formation. In particular, the temperature T_B can be a maximum of 550° C., preferably a maximum of 510° C., in order to ensure high strength of the bainite.

For example, a first quenching of T_A can take place with a cooling rate dT_AB of at least 10 K/s to a temperature below the martensite start, in order to enable a particularly simple nucleation of the bainite and to produce a correspondingly fine bainite. The temperature is then raised again to a temperature of at least 380° C., in particular at least 450° C., in order to ensure particularly rapid redistribution of the carbon in the austenite.

The obligatory elements chromium and nitrogen in the aforementioned contents can specifically support the establishment of the desired structure during the heat treatment and in combination with carbon in the aforementioned contents, the nitrogen leads to a significant increase in the speed of the bainitic transformation, e.g. due to the formation of very fine chromium nitride, which acts as a nucleating agent. In addition, nitrogen can significantly reduce carbon segregation at the grain boundary. Since carbon slows down the nucleation of bainite, an increased rate of nucleation at the grain boundaries is assumed.

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

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

Carbon (C) contributes to hardness and, depending on the content, can retard ferrite and bainite formation, stabilize retained austenite and reduce the Ac3 temperature. A content of at least 0.10% is required to achieve sufficient hardenability/hardness and strength. Above a content of 0.30%, bainite formation is too slow. For improved weldability and setting a good ratio of force absorption and maximum bending angle in the bending test and for rapid bainite formation, the content can be adjusted to a maximum of 0.25%, preferably a maximum of 0.22%. In order to achieve a higher level of strength, the content can be adjusted to at least 0.15% in particular and to at least 0.18% in order to set a very good combination of hardenability and strength.

Silicon (Si) contributes to a further increase in hardenability/hardness and strength via solid solution strengthening. Furthermore, the use of Ferro Silizio Manganese as an alloying agent, which has a beneficial effect on production costs, can also be made possible. Depending on the content, it is also possible to suppress cementite and thus stabilize retained austenite. The use of chromium and nitrogen as mandatory elements means that very high silicon contents of around 1.50%, which are common in steels of this type, can be dispensed with. A content of at least 0.40% results in an initial hardening effect, with a content of at least 0.60% in particular being set for a significant increase in strength. A content of at least 0.80% is preferably set in order to almost completely suppress cementite formation and possibly also to avoid excessive martensite formation. A good surface can be produced up to a maximum content of 1.20%, which can be coated with a coating, in particular with a zinc-based coating, without any problems and if necessary. Especially with contents up to a maximum of 1.10%, the weldability can be ensured and/or improved in addition to an improved surface quality.

Manganese (Mn) contributes to the hardness and, depending on the content, can greatly delay the formation of ferrite. In order to suppress ferrite during heat treatment, a content of at least 1.00% set, In order not to restrict the weldability, the content is set to a maximum of 2.00%. In order to avoid proeutectoid ferrite formation and to stabilize the retained austenite, the content can be adjusted to at least 1.10%. To improve the elongation at break, the content can be adjusted in particular to a maximum of 1.80%, preferably to a maximum of 1.60%, in order to ensure rapid bainite formation.

Chromium (Cr) contributes to clarification and can slow down diffusive phase transformations during quenching, especially ferrite. Chromium has a significantly lower influence on bainite formation at lower temperatures. It is therefore optimally suited to ensure a low critical cooling rate on the one hand, but at the same time not to impede the formation of bainite too much at low temperatures. In order to achieve a critical cooling rate low enough to avoid unwanted ferrite formation, a content of at least 0.50% is set. A good surface can be produced up to a maximum content of 1.50%, which can be coated with a coating, in particular with a zinc-based coating, without any problems and if necessary. For good surface quality and improved weldability, the content can in particular be adjusted to a maximum of 1.20%, preferably to a maximum of 1.00%. In order to stabilize the retained austenite even with very long holding times in the bainite area, the content can be adjusted to at least 0.60%, preferably at least 0.70%.

As an austenite former, nitrogen (N) slows down the critical cooling rate, since nitrogen can suppress the diffusive formation of ferrite. When kept in the bainite area, very fine clusters and/or precipitations, especially chromium nitrides, can then form, which accelerate the formation of bainite at low temperatures. Furthermore, nitrogen reduces the carbon supersaturation at grain boundaries and thus reduces the unwanted formation of chromium carbides, which can become very coarse, form particularly along the grain boundaries and thus significantly impair toughness. A content of at least 0.0030% is set for a significant effect. The content is limited to a maximum of 0.040% to ensure good and problem-free castability of the melt/steel. If a content of in particular at least 0.0070% is set, the retained austenite can be stabilized against cementite formation and, with a content of preferably at least 0.0090%, preferably at least 0.011%, the formation of bainite can be accelerated. In order to achieve improved weldability, the content can in particular be limited to a maximum of 0.030% and preferably set to a maximum of 0.025%, so that chromium nitrides produced, for example, can form very finely.

In the broadest sense, phosphorus (P) is an impurity that is carried 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, with the content being limited to a maximum of 0.10%. Negative influences on formability can be ruled out with certainty if the content is limited in particular to a maximum of 0.050% by weight, and preferably to a maximum of 0.030% by weight to additionally reduce the segregation effects.

Sulfur (S) is also an impurity in the broadest sense and can be adjusted to a maximum content of 0.050% in order to avoid a strong tendency to segregation and a negative influence on formability as a result of excessive formation of sulfides (FeS; MnS; (Mn, Fe )S) to avoid. The content is therefore limited in particular to a maximum of 0.020% by weight, preferably to a maximum of 0.0080%. As a rule, calcium is alloyed for desulfurization and adjustment of the S content depending on the Ca content.

The steel flat product can optionally contain one or more alloying elements from the group (Al, V, Ti, Nb, Ni, Mo, W, Ca).

Aluminum (Al) can be added as an optional alloying element, in particular as a deoxidizing agent, with a maximum content of 0.050%, with a content of at least 0.0010% being able to be added in particular for the reliable binding of any oxygen (0) present. Above a content of 0.050%, however, there is an increased risk that (coarser) aluminum nitride will form, thus unintentionally binding nitrogen and thus also deteriorating cold-rollability. In particular, the content is limited to a maximum of 0.015% in order to be able to reliably rule out the formation of aluminum nitride.

Vanadium (V) can be added as an optional alloying element for grain refinement with a maximum content of 0.20%, so that in particular there is no negative influence on the elongation at break. In order to achieve a desired effect of grain refinement, a content of at least 0.0010% can be alloyed in particular. Titanium (Ti) can be added as an optional alloying element as a micro-segregation element with a maximum content of 0.010%, so that in particular undesirable binding with nitrogen can be ruled out, which would form very hard and coarse titanium nitrides, which could lead to embrittlement. In order to set the free nitrogen content precisely, a content of in particular at least 0.0010% can be added.

Niobium (Nb) can be added as an optional alloying element for grain refinement with a maximum content of 0.10%, in particular to prevent nitrogen from binding to form niobium nitride. In order to achieve a desired effect of grain refinement, a content of at least 0.0010% can be alloyed in particular.

As an optional alloying element, nickel (Ni), like chromium, can improve the transformation into austenite, increase strength and improve process stability with longer holding times during bainite formation, so that a maximum content of 0.40% can be added to prevent bainite formation in particular to slow down. For example, nickel is alloyed in conjunction with copper, since when copper is added, nickel essentially suppresses the negative influence of copper on hot-rollability. For example, a copper content of between 0.3*nickel and 0.7*nickel can be added to this in order to prevent the iron-copper eutectic and thus the formation of a liquid phase on the surface during hot rolling. In order to achieve a desired effect of the aforementioned improvement, a content of at least 0.010% can be added in particular.

Copper (Cu) can be added as an optional alloying element to increase hardness and strength with a maximum content of 0.80%, in particular in order not to impair weldability and hot-rollability due to low-melting Cu phases on the surface. In order to ensure the strength-increasing effect, but also to improve the resistance to atmospheric corrosion in uncoated steel flat products, a content of at least 0.010% can be added.

As an optional alloying element, molybdenum (Mo) can increase strength and hardness, in particular to improve process stability, since molybdenum significantly slows down ferrite formation and has hardly any effect on bainite formation in the temperature range between 300 and 580°C, so that a maximum content of 1 .00% can be added. In particular, a content of at least 0.0020% can be added, at which dynamic Molybdenum-carbon clusters up to ultra-fine molybdenum carbides can form on the grain boundaries, which significantly slow down the mobility of the grain boundary and thus diffusive phase transformations. In addition, the grain boundary energy can be reduced, which in turn can reduce the nucleation rate of ferrite.

Tungsten (W) can act as an optional alloying element in a similar way to molybdenum, with a content that can be alloyed up to a maximum of 1.00%. In order to be able to have a positive effect on the hardness/hardenability, a content of at least 0.0010% can be added.

Calcium (Ca) can be added as an optional alloying element to the melt as a desulfurizing agent and to specifically influence the sulfide in contents of up to a maximum of 0.0050%, which can lead to a change in the plasticity of the sulfides during hot rolling. In addition, the cold forming behavior can also be improved by adding Ca. The effects described can be effective from a content of in particular at least 0.0001%, preferably at least 0.0003%.

In addition to iron, the flat steel product can contain one or more of the elements from the group (O, H, As) as unavoidable impurities due to the manufacturing process, which are not specifically alloyed with it.

Oxygen (0) is an undesirable impurity, but usually unavoidable for technical reasons. The oxygen content is limited to a maximum of 0.0050%, in particular to a maximum of 0.0020%.

As the smallest atom, hydrogen (H) can be very mobile on interstitial sites in the steel and lead to cracks in the steel flat product. The possible impurity hydrogen is therefore reduced to a maximum content of 0.0010%, in particular a maximum of 0.0004%, preferably a maximum of 0.0002%.

Arsenic (As) is an impurity that can be present in steel flat products, but its content is limited to a maximum of 0.020% to avoid negative influences.

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

According to one embodiment of the method according to the invention, austenitizing to T_A is carried out with a heating rate dTA of at least 1.0 K/s between 600 and 800° C. in step h). The targeted setting of the heating rate dTA in the range between 600 and 800°C has i.a. Influence on the formation of the austenite grain size, which is important insofar as it influences the bainitic transformation rate but also the final properties in the structure after heat treatment. The faster this area is heated, the more austenite nuclei can form, which block each other, slow down their growth and thus lead to an overall fine austenite grain. Below 600°C, for example, there are no relevant recrystallization processes in continuous heat treatment plants, and there is hardly any coarsening of cementite, so that the heating rate below 600°C can simply follow the technical conditions. In particular, the heating rate dTA is at least 2 K/s, preferably at least 2.5 K/s, so that a particularly fine austenite grain can form. A heating rate dTA of up to 50 K/s is helpful for uniform heating, but higher heating rates dTA can generally also be selected.

According to one embodiment of the method according to the invention, in step h) after the temperature T_A has been reached, the cold-rolled flat steel product is held at the temperature T_A for a holding period t_A of between 1 and 300 s. To prevent grain coarsening, the holding time t_A should not exceed 300s. In particular, the holding time t_A can be selected up to a maximum of 200s, for example to limit the diffusion of undesirable accompanying elements such as phosphorus to the austenite grain boundaries.

According to one embodiment of the method according to the invention, in step h) the quenching takes place in two stages, so that first to an intermediate temperature T_Z between 640 and 800° C. with a cooling rate dTZ of at least 0.50 K/s and then to the temperature T_B between 300 and 580°C with a cooling rate dTB of at least 10 K/s, where dTB is greater than dTZ. Thus, the cooling rate dTZ can correspond to pre-cooling and the cooling rate dTB to rapid cooling. Even if quenching in two stages is not absolutely necessary for the final properties in the microstructure, it can still be economical to provide for two-stage quenching for process engineering reasons. On the one hand, the flat steel product can be pre-cooled be cooled more slowly and more homogeneously. On the other hand, the area or the section with rapid cooling can often be limited in terms of system technology, so that if pre-cooling to an intermediate temperature is provided, rapid cooling from an intermediate temperature to the temperature T_B can be implemented much more easily. The intermediate temperature T_Z and the cooling rate dTZ for the intermediate temperature must be high enough so that the pre-cooling does not lead to an undesired formation of ferrite. The intermediate temperature is therefore at least 640° C. in order to prevent coarse ferrite formation, in particular at least 700° C. in order, for example, to be able to completely suppress proeutectoid ferrite formation. The cooling rate dTZ is at least 0.5 K/s in order to prevent coarse ferrite formation, in particular at least 1.5 K/s in order, for example, to be able to completely suppress proeutectoid ferrite formation. The cooling rate dTZ can be selected up to a maximum of 10 K/s or more if required. The cooling rate dTB is at least 10 K/s to prevent ferrite formation. In particular, the cooling rate dTB can be at least 20 K/s in order, for example, to also be able to suppress complete bainite formation in the upper temperature range. For economic reasons, the cooling rate dTB can be limited to a maximum of 200 K/s, in particular to a maximum of 150 K/s.

According to one embodiment of the method according to the invention, in step h) after the temperature T_B has been reached, the cold-rolled flat steel product is held at the temperature T_B for a holding time t_B of at least 15 s. The holding time t_B at the temperature T_B is at least 15s. The longer the holding period is selected, the more completely the austenite can transform into bainite, with stabilized residual austenite quite possibly being part of the bainite. The holding time t_B can be chosen to be at least 25s, for example to be able to stabilize larger austenite areas, and preferably at least 35s, for example to minimize the formation of fresh martensite, which would lead to embrittlement. The holding time t_B can be limited to a maximum of 100 s, for example, although it can also be longer, particularly if required and depending on the design of the system.

So that no excess martensite can form in the structure, and in particular to prevent a martensite proportion in the structure exceeding 40%, a temperature T_B_min of Ms−50° C., in particular of Ms−25° C., is optionally not allowed fall below A slight drop below the Ms temperature within the specified limits can facilitate bainite nucleation, for example on martensite lancets, and thus lead to a general acceleration of bainite formation. So it's entirely possible that Allow bainite formation to begin for a few seconds, then cool briefly under Ms or cool directly under Ms without prior bainite formation and then return to the temperature range T_B between at least 300 and a maximum of 580°C, in particular between at least 340 and a maximum of 550 °C, preferably between 380 and 510°C, to complete bainite formation. Depending on the selection of the alloying elements, it can happen with particularly low carbon contents that the Ms temperature is greater than at least 300°C, in particular at least 340°C, preferably at least 380°C, so that in this case during the holding time t_B the temperature should not fall below at least 300°C, in particular at least 340°C, preferably at least 380°C.

According to one embodiment, the cold-rolled flat steel product according to the invention has a structure of martensite with a proportion of between 0.5 and 40%, in particular between 3 and 33%, preferably between 5 and 28%, retained austenite with a proportion of between 5 and 22%, and a remainder Bainite and unavoidable structural components. The proportion of bainite in the structure is in particular at least 55%, preferably at least 60%. The proportion of martensite in the microstructure can in particular be at least 8%, preferably at least 10%, and preferably be limited to a maximum of 26%, particularly preferably to a maximum of 24%. More preferably, the martensite can be completely or partially tempered. The proportion of retained austenite in the structure can be in particular at least 8%, preferably at least 10%, and in particular can be limited to a maximum of 20%, preferably to a maximum of 18%. Finely distributed austenite and/or also carbides can be part of the bainite and/or tempered martensite. The unavoidable structural components can be proportions in the form of ferrite, pearlite and/or cementite apart from bainite and martensite, which are permitted up to a maximum of 10%, in particular up to a maximum of 8%, preferably up to a maximum of 6%, preferably up to a maximum 4% Precipitations in the form of chromium nitrides can also be present in the structure.

According to one embodiment of the cold-rolled flat steel product according to the invention, bainite is also present in the form of lancet bainite and the ratio of the proportions of lancet bainite to bainite is at least 60%, in particular at least 65%, preferably at least 70%. As a result, a finer and/or more fracture-resistant structure can be achieved. Furthermore, due to its fineness, this can particularly contribute to an increase in strength and also allows accelerated stabilization of retained austenite due to the particularly short diffusion paths of the carbon from bainitic ferrite to austenite. Description of the Preferred Embodiments

In a first exemplary embodiment, a melt A consisting of (in % by weight) C=0.217%, Si=0.98%, Mn=1.56%, Cr=0.8%, N=0.019%, P= 0.01%, S = 0.003%, Al = 0.01%, Ti = 0.003%, Nb = 0.001%, Mo = 0.005%, the rest Fe and unavoidable impurities generated, cast into a precursor, and after solidification in the form of slabs divided. Alloying elements not specified here were not present in measurable amounts or only as unavoidable impurities. The slabs were allowed to cool to ambient temperature. The slabs were reheated or throughheated in a walking beam furnace to a temperature of 1250°C so that the structure of the preliminary product consisted entirely of austenite and all precipitations that had formed in the structure during the continuous casting could dissolve. After reheating, the slab was fed to a rolling train in which the slab was first reversingly hot-rolled in a (roughing) stand at a final rolling temperature of 1100° C. to form an intermediate flat product and the intermediate flat product was then rolled in a seven-stand finishing/hot-rolling group, for example, to form one hot-rolled steel flat product (hot strip) to a thickness of 3mm was finish/hot-rolled, the final rolling temperature being 890°C and the degree of hot-rolling in the last rolling pass being 15%. Immediately after the last rolling pass, the hot-rolled flat steel product was actively cooled with water along a cooling section to a coiling temperature of 560°C. A coil cooling to ambient temperature followed. The hot-rolled flat steel product was cold-rolled to a thickness of 1.5 mm in a five-stand cold-rolling set, for example, with a total degree of cold-rolling of 50% to form a cold-rolled flat steel product (cold strip). Ten specimens in the form of blanks, specimens 1 to 10, were cut from the cold-rolled flat steel product and subjected to further investigations.

In a second embodiment, the same conditions as in the first embodiment were taken into account for the production of a cold-rolled steel flat product, but with the difference that a melt B consisting of (in wt %) C = 0.221%, Si = 1.0%, Mn = 1.58%, Cr = 0.8%, N = 0.0092%, P = 0.011%, S = 0.004%, Al = 0.032%, Ti = 0.004%, Nb = 0.001%, Mo = 0.003%, Rest Fe and unavoidable impurities was cast. From this cold-rolled flat steel product, two specimens were cut off in the form of blanks, specimens 11 and 12, and subjected to further investigations.

Table 1

Table 2 Samples 1 to 12 were subjected to a heat treatment on a laboratory scale using the specifications defined in step h) in order to set the desired bainitic matrix in the cold-rolled flat steel product. The individual parameters in step h) are listed in Table 1. Table 2 shows the structure and the associated properties. The samples marked with an asterisk * are according to the invention.

Since the residual austenite is measured by volume diffractometry, for example by means of XRD, it can also be a component of bainite and/or martensite, so that the addition of the structural components can sometimes result in more than 100%. Depending on how coarse the retained austenite is, it can also be evaluated as a separate structural component, n.b. means not determinable.

Claims

patent claims

1. A method for producing a cold-rolled flat steel product with a bainitic matrix comprising the steps: a) Casting a melt consisting of Fe and unavoidable impurities (in % by weight) from

C: 0.10 to 0.30%,

Si: 0.40 to 1.20%,

Mn: 1.00 to 2.00%,

Cr: 0.50 to 1.50%,

N: 0.0030 to 0.040%,

P: up to 0.10%,

S: up to 0.050%, where P and S can be among the impurities, optionally with one or more alloying elements from the group (Al, V, Ti, Nb, Ni, Mo, W, Ca).

AI: up to 0.050%,

V: up to 0.20%,

Ti: up to 0.010%,

Nb: up to 0.10%,

Ni: up to 0.40%,

Cu: up to 0.80%,

Mon: up to 1.00%,

W: up to 1.00%,

Ca: up to 0.0050%, to a precursor; b) reheating the precursor to a temperature and/or maintaining the precursor at a temperature between 1100 and 1350°C; c) optional hot rolling of the preliminary product into an intermediate flat product in one or more roll stands at a final rolling temperature between 950 and 1250°C; d) hot rolling of the preliminary product or the optional intermediate flat product into a hot-rolled steel flat product in one or more roll stands at a final rolling temperature of between 800 and 1000°C; e) coiling the hot-rolled flat steel product at a coiling temperature between 400 and 650°C; f) optional annealing of the hot-rolled steel flat product at an annealing temperature between 500 and 900 °C; g) cold rolling of the hot-rolled and optionally annealed flat steel product in one or more rolling stands with a total degree of cold rolling of at least 30%; h) Heat treatment of the cold-rolled flat steel product with austenitizing at a temperature T_A between above 800 and 950° C., quenching to a temperature T_B between 300 and 580° C. such that a bainitic basic structure is established in the cold-rolled flat steel product.

2. The method according to claim 1, wherein in step h) the austenitizing to T_A is carried out with a heating rate dTA of at least 1.0 K/s between 600 and 800 °C.

3. The method according to claim 1, wherein in step h) after reaching the temperature T_A, the cold-rolled flat steel product is held at the temperature T_A for a holding time t_A between 1 and 300 s.

4. The method according to claim 1, wherein in step h) the quenching takes place in two stages, so that first to an intermediate temperature T_Z between 640 and 800 ° C with a cooling rate dTZ of at least 0.50 K / s and then to the temperature T_B between 300 and 580 °C with a cooling rate dTB of at least 10 K/s, where dTB is greater than dTZ.

5. The method according to claim 1 or 3, wherein in step h) after reaching the temperature T_B, the cold-rolled flat steel product is held at the temperature T_B for a holding time t_B of at least 15 s.

6. Cold-rolled flat steel product with a bainitic basic structure, in particular made according to one of the preceding claims, which, in addition to Fe and unavoidable impurities in wt

C: 0.10 to 0.30%,

Si: 0.40 to 1.20%,

Mn: 1.00 to 2.00%,

Cr: 0.50 to 1.50%,

N: 0.0030 to 0.040%,

P: up to 0.10%,

AI: up to 0.050%,

V: up to 0.20%,

Ti: up to 0.010%,

Nb: up to 0.10%,

Ni: up to 0.40%,

Cu: up to 0.80%,

Mon: up to 1.00%,

W: up to 1.00%,

Ca: up to 0.0050%, where the microstructure of the cold-rolled flat steel product has the following proportions: at least 0.5% martensite, at least 5% retained austenite,

Rest bainite and unavoidable structural components.

7. Cold-rolled flat steel product according to claim 6, wherein bainite is also present in the form of lancet bainite and the ratio of the proportions of lancet bainite to bainite is at least 60%.

8. Cold-rolled flat steel product according to claim 6, wherein the proportion of bainite is at least 60%.

9. Cold-rolled flat steel product according to claim 6, wherein the proportion of retained austenite is at least 8%.

10. Cold-rolled flat steel product according to claim 6, wherein the proportion of martensite is at least 10%.

11. Cold-rolled flat steel product according to claim 6, wherein the unavoidable structural components can be up to a maximum of 10% and in particular proportions in the form of ferrite, pearlite and/or cementite apart from bainite and martensite can be present.

12. Cold-rolled flat steel product according to any one of claims 6 to 11, wherein the cold-rolled steel sheet has a tensile strength Rm of at least 1000 MPa.