EP2840159A1 - Method for producing a steel component - Google Patents

Abstract

The inventive method allows in a simple manner, the production of a complex shaped steel component with a tensile strength Rm> 1200 MPa and an elongation at break A50> 6%. For this purpose, according to the invention, a flat steel product is provided which, in addition to iron and unavoidable impurities (in% by weight) C: 0.10-0.60%, Si: 0.4-2.5%, Al: up to 3.0% Mn: 0.4 - 3.0%, Ni: up to 1%, Cu: up to 2.0%, Mo: up to 0.4%, Cr: up to 2%, Co: up to 1.5 %, Ti: up to 0.2%, Nb: up to 0.2%, V: up to 0.5%, the structure of the flat steel product consisting of at least 10% by volume of retained austenite, the globular retained austenite islands having a grain size of at least 1 micron. The flat steel product is heated to 150-400 ° C forming temperature and formed at the forming temperature with a degree of deformation which is at most equal to the uniform strain Ag, to the component. Finally, the steel flat product thus obtained is cooled. Such molded at elevated temperatures component has over the same flat steel product, but molded at room temperature components significantly increased strength.

Description

The invention relates to a method for producing a steel component, which has a tensile strength Rm of more than 1200 MPa and an elongation at break A50 of at least 6%.

Steel components produced according to the invention are distinguished by a very high strength in combination with good elongation properties and, as such, are particularly suitable as components for motor vehicle bodies.

The term "flat steel product" here by a rolling process produced steel sheets or steel strips and divided therefrom boards and the like understood. Steel components of the type according to the invention are produced by a forming process from such flat steel products.

If alloy contents are stated here only in "%", this always means "% by weight", unless expressly stated otherwise.

If the term "elongation at break A50", "elongation at break A80" or "tensile strength Rm" is used, this refers to the mechanical characteristic values determined in accordance with DIN EN 6892-1.

From the US 6,364,968 B1 is a method for producing a hot-rolled steel sheet known in a

Thickness of not more than 3.5 mm should have a uniform distribution of its mechanical properties and a particularly good Lochaufweitungsverhalten. The process envisages that a slab containing (in% by weight) 0.05-0.30% C, 0.03-1.0% Si, 1.5-3.5% Mn, up to 0.02% P, up to 0.005% S, up to 0.150 Al, up to 0.0200% N, and alternatively or in combination 0.003-0.20% Nb or 0.005-0.20% Ti, up to 1200 ° C is heated and then hot rolled at a hot rolling end temperature of at least 800 ° C, in particular 950 - 1050 ° C, to a hot strip. Subsequently, the hot strip obtained is cooled at a cooling rate of 20 - 150 ° C / sec to a reel temperature of 300 - 550 ° C, in which it is wound into a coil. The cooling starts within 2 seconds after the end of the hot rolling. The hot rolled strip thus obtained shall have a fine bainitic structure with a bainite content of at least 90%, the average grain size of which shall not exceed 3,0 μm, the ratio of the longest axis length to the shortest axis length of the grains not exceeding 1, 5 and the length of the longest axis of the grains should not exceed 10 microns. The remainder of the structure not occupied by bainite should consist of tempered martensite, which is very similar in its appearance and properties to bainite. Hot rolled strips produced and produced in this manner have tensile strengths of 850 - 1103 MPa at an elongation of 15 - 23%.

From the EP 2 546 382 A1 In addition, a method for producing a steel sheet having a tensile strength of at least 1470 MPa is known, in which the product of Elongation and tensile strength is at least 29000 MPa%. The steel constituting the steel sheet contains, in addition to iron and unavoidable impurities (in% by weight), 0.30-0.73% C, up to 3.0% Si, up to 3.0 Al, the Sum of the Si and Al contents is at least 0.7%, 0.2-8.0% Cr, up to 10.0% Mn, the sum of the Cr and Mn contents being at least 1.0%, up to 0.1% P, up to 0.07% S and up to 0.010% N. The steel sheet thus composed is processed such that the martensite area fraction in the range of 15-90% based on the total microstructure of the steel and the content of residual austenite of the structure is 10 - 50%. At least 50% of the martensite should be present as tempered martensite and the area fraction of tempered martensite should be at least 10%. If present in the structure, at the same time the area ratio of polygonal ferrites present in the structure should be at most 10%.

To achieve this, according to the EP 2 546 382 A1 first producing a hot rolled steel strip as set forth by heating a steel precursor such as a slab to 1000-1300 ° C and thereafter rolling it to a hot strip at a hot rolling end temperature of 870-950 ° C. The resulting hot strip is then wound at a reel temperature of 350 - 720 ° C to form a coil. After coiling, pickling followed by cold rolling takes place at degrees of deformation of 40-90%. The cold-rolled strip thus obtained is annealed for 15-1000 seconds at a temperature where it has a purely austenitic structure, and then at a cooling rate of at least 3 ° C / s cooled to a temperature ranging from below the martensite start temperature and reaching a temperature lower than 150 ° C lower temperature range to produce tempered martensite in the structure of the steel sheet. Thereafter, the cold rolled steel strip is heated to 340-500 ° C over a period of 15-1000 seconds to stabilize the retained austenite. The cold-rolled steel sheets thus produced reached tensile strengths of more than 1600 MPa at an elongation of up to 27%.

Against the background of the prior art explained above, the object of the invention was to specify a method which makes it possible in a simple way to produce complex shaped components from flat steel products of the type described above.

According to the invention, this object has been achieved by carrying out the operations specified in claim 1 for the production of high-strength steel components having good elongation properties.

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

• Bereitstellen eines Stahlflachprodukts, das neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%):
  • C: 0,10 - 0,60 %,
  • Si: 0,4 - 2,5 %,
  • Al: bis zu 3,0 %
  • Mn: 0,4 - 3,0 %,
  • Ni: bis zu 1 %,
  • Cu: bis zu 2,0 %,
  • Mo: bis zu 0,4 %,
  • Cr: bis zu 2 %,
  • Co: bis zu 1,5 %,
  • Ti: bis zu 0,2 %,
  • Nb: bis zu 0,2 %,
  • V: bis zu 0,5 %,
  enthält, wobei das Gefüge des Stahlflachprodukts zu mindestens 10 Vol.-% aus Restaustenit besteht, der globulare Restaustenitinseln mit einer Korngröße von mindestens 1 µm umfasst,
  • Erwärmen des Stahlflachprodukts auf eine Umformtemperatur, die 150 - 400 °C beträgt,
  • Umformen des auf die Umformtemperatur erwärmten Stahlflachprodukts zu dem Bauteil mit einem höchstens bis zur Gleichmaßdehnung Ag reichenden Umformgrad, in der Praxis auch Umformdehnung oder Verformungsgrad genannt,
  • Abkühlen des erhaltenen Bauteils.

The method according to the invention is suitable for producing a steel component which has a tensile strength Rm of more than 1200 MPa and an elongation at break A50 of at least 6%. For this purpose, the method according to the invention comprises the following steps:

• Providing a flat steel product which, in addition to iron and unavoidable impurities (in% by weight):
  • C: 0.10-0.60%,
  • Si: 0.4-2.5%,
  • Al: up to 3.0%
  • Mn: 0.4 - 3.0%,
  • Ni: up to 1%,
  • Cu: up to 2.0%,
  • Mo: up to 0.4%,
  • Cr: up to 2%,
  • Co: up to 1.5%,
  • Ti: up to 0.2%,
  • Nb: up to 0.2%,
  • V: up to 0.5%,
  wherein the microstructure of the flat steel product consists of at least 10% by volume of retained austenite comprising globular retained austenite islands with a particle size of at least 1 μm,
  • Heating the flat steel product to a forming temperature which is 150-400 ° C,
  • Forming the heated to the forming temperature flat steel product to the component with a maximum extent to the uniform expansion Ag-reaching degree of deformation, in practice also called forming strain or deformation,
  • Cooling of the resulting component.

The invention is based on the recognition that a component which is produced by forming a 150-400 ° C warm flat steel product of the type according to the invention, after a subsequent cooling to room temperature over the strength of the original Stahlflachprodukts significantly increased strength with almost unchanged elongation properties has.

As a result of the heating in the temperature range specified according to the invention, the extensibility of the inventively processed steel flat product increases significantly, so that without special effort and minimized risk of the formation of cracks can be prevented and component shapes can be generated, which have a particularly complex shape. Practical experiments have shown here that flat steel products of the present invention provided in the temperature range in which the deformation is to take place, regularly reach a breaking elongation A50 of at least 30%, whereas the elongation at break A50 of the component at room temperature compared to the starting as a product flat steel product unchanged in the range typically of 22%.

Surprisingly, the elongation properties of a component produced according to the invention thus do not decrease, in spite of the increased strength, compared to a component formed at room temperature. The invention thus results from a pre-deformation at 150 - 400 ° C a significant increase in strength with unchanged extensibility of each component obtained.

No cooling is required to cool down after forming. Thus, the cooling of the flat steel product can take place after forming in still air.

The increase in strength achieved by the forming according to the invention is considerable. Thus, it could be demonstrated that by a component transformation of 15%, which was carried out at elevated temperatures according to the invention, it was possible to regularly increase the tensile strength by about 80-120 MPa compared with the tensile strength of samples which likewise had a degree of deformation of 15%, but have been reformed at room temperature. At the same time, the expansion properties of the component obtained according to the invention correspond to the elongation properties of the component formed at room temperature, so that the component produced according to the invention is particularly suitable for use in automobile bodies due to its deformation behavior.

The reason for the increase in strength achieved by the procedure according to the invention, according to the findings of the invention, is that in the structure of the present invention processed flat steel product present globular retained austenite, which is characterized by a grain size of at least 1 micron, under the load of the forming in accordance with the invention Temperature range of 150 - 400 ° C in film-like retained austenite and bainitic ferrite or below the martensite starting temperature converted into martensite. During the forming in the temperature range concerned, the globular retained austenite present in the steel flat product thus contributes to increasing the elongation at. After the forming and cooling of the component of the steel according to the invention then shows higher tensile strengths due to the additionally formed ferritic bainite or martensite. The parts of film-like retained austenite which remain unchanged over cooling ensure the good residual elongation achieved after the transformation. This effect can be used particularly reliably if the flat steel product for the inventive transformation to the component is heated to 200-400 ° C., in particular 200-300 ° C.

Due to the comparatively low temperatures at which the deformation is carried out according to the invention, the method according to the invention is particularly suitable for converting flat steel products which are provided with a metallic protective coating into components. The metallic protective layer is at most slightly influenced by the invention taking place heating. For example, the protective coating may be a conventional zinc, zinc alloy, aluminum or aluminum alloy, magnesium or magnesium alloy coating.

The composition of a flat steel product processed according to the invention has been chosen taking into account the following aspects:

Carbon in contents of 0.1-0.6% by weight retards conversion to ferrite / pearlite in the steel of the steel flat product processed according to the invention, lowers the martensite start temperature MS and contributes to the increase in hardness. To use these positive effects, the C content of the flat steel product according to the invention to at least 0.25 wt .-%, in particular at least 0.27 wt .-%, at least 0.28 wt .-% or at least 0.3 wt .-%, are set, wherein the be used by the comparably high carbon content effects particularly safe when the C content in the range of> 0.25 to 0.5 wt .-%, in particular 0.27 to 0.4 wt .-% or 0.28 - 0.4 wt .-%, is.

Due to the presence of Si in contents of 0.4-2.5% by weight and Al in contents of up to 3% by weight in the flat steel product processed according to the invention, carbide formation in the bainite can be suppressed and consequently the residual austenite stabilized by dissolved carbon become. In addition, Si contributes to solid solution hardening. In order to avoid potentially harmful influences of Si, the Si content may be limited to 2.0 wt%. In order to use Si as a mixed-crystal former for increasing the strength, it may be expedient for the flat steel product processed according to the invention to contain at least 1% by weight of Si.

Al can partially replace the Si content in the steel processed according to the invention. For this purpose, a minimum content of 0.4 wt .-% Al may be provided. This is especially true when the hardness or tensile strength of the steel is to be set to a lower value in favor of improved ductility by the addition of Al.

The positive effects of the simultaneous presence of Al and Si can be used particularly effectively if the contents of Si and Al within the invention predetermined limits, the condition% Si + 0.8% Al> 1.2 wt .-% or even the condition% Si + 0.8% Al> 1.5 wt .-% (with% Si: respective Si content in % By weight,% Al: respective Al content in% by weight).

Mn in contents of at least 0.4% by weight and up to 3.0% by weight, in particular up to 2.5% by weight or 2.0% by weight, promotes bainitization in the steel processed according to the invention, The optionally additionally present contents of Cu, Cr and Ni also contribute to the formation of bainite. Depending on the respective other constituents of the steel processed according to the invention, it may be expedient to limit the Mn content to a maximum of 1.6% by weight or 1.5% by weight.

By the optional addition of Cr, the martensite start temperature can be lowered and the tendency of the bainite to convert to pearlite or cementite can be suppressed. Furthermore, Cr at levels up to the upper limit of not more than 2% by weight given in accordance with the invention promotes the ferritic transformation, whereby optimum effects of the presence of Cr in a flat steel product according to the invention result if the Cr content is reduced to 1.5% by weight. is limited.

The optional addition of Ti, V or Nb can support the formation of fine-grained microstructures and promote ferritic transformation. In addition, these micro-alloying elements contribute to increasing the hardness by forming precipitates. The positive effects of Ti, V and Nb in the flat steel product processed according to the invention can then be achieved particularly effectively use, if their content is in each case in the range of 0.002 to 0.15 wt .-%, in particular 0.14 wt .-% does not exceed.

erfüllen, wobei mit %Mn der jeweilige Mn-Gehalt in Gew.-%, mit %Cr der jeweilige Cr-Gehalt in Gew.-%, mit %Ni der jeweilige Ni-Gehalt in Gew.-%, mit %Cu der jeweilige Cu-Gehalt in Gew.-% und mit %C der jeweilige C-Gehalt in Gew.-% bezeichnet sind.The formation of the structure provided according to the invention can be ensured, in particular, by the contents of the steel flat product of Mn, Cr, Ni, Cu and C processed according to the invention having the following condition $$\mathrm{1}<\mathrm{0}.\mathrm{5}\%\mathrm{Mn}+\mathrm{0}.\mathrm{167}\%\mathrm{Cr}+\mathrm{0}.\mathrm{125}\%\mathrm{Ni}+\mathrm{0}.\mathrm{125}\%\mathrm{Cu}+\mathrm{1}.\mathrm{334}\%\mathrm{C}<\mathrm{2}$$

meet, where with% Mn the respective Mn content in wt .-%, with% Cr of the respective Cr content in wt .-%, with% Ni of the respective Ni content in wt .-%, with% Cu of the respective Cu content in wt .-% and with% C of the respective C content in wt .-% are designated.

In principle, hot-rolled or cold-rolled flat steel products having a composition corresponding to the specifications according to the invention are suitable as the starting material for the process according to the invention. For this purpose, eligible hot rolled flat steel products and a process for their preparation are the subject of the European patent application EP 12 17 83 30.2 , the content of which is hereby expressly incorporated into the disclosure of the present patent application.

As in the cited European patent application EP 12 17 83 30.2 As explained, the hot-rolled flat steel products produced according to this patent application are characterized by an optimum combination of elongation properties and strength. This combination of properties can be achieved in a particularly reliable way that the structure of flat steel products processed according to the invention, in addition to optionally present fractions of up to 5% by volume of ferrite and up to 10% by volume of martensite, to at least 60% by volume of bainite and the balance of retained austenite, the retained austenite content being at least 10% by volume, at least a portion of the retained austenite is in block form and the blocks of the retained austenite present in block form at least 98% have a mean diameter of less than 5 microns.

One according to the EP 12 17 83 30.2 Accordingly, obtained hot-rolled flat steel product has a two-phase dominated microstructure of which one dominant component is bainite and its second dominant component is retained austenite. In addition to these two main components, small amounts of martensite and ferrite may be present, but their contents are too low to have an influence on the properties of the hot-rolled steel flat product.

In this context, "blocky" retained austenite is referred to as the ratio of length / width, ie longest extent / thickness, of 1 to 5 in the structural components of retained austenite present in the structure. On the other hand, retained austenite is referred to as "film-like" if the ratio of length / width is greater than 5 for the retained austenite accumulations present in the microstructure and the width of the respective microstructure constituents of retained austenite is less than 1 μm. Accordingly, film-like retained austenite is typically present as a finely distributed lamella.

• Bereitstellen eines Vorprodukts in Form einer Bramme, Dünnbramme oder eines gegossenen Bands, das neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%): 0,10 - 0,60 % C, 0,4 - 2,0 % Si, bis zu 2,0 % Al, 0,4 - 2,5 % Mn, bis zu 1 % Ni, bis zu 2,0 % Cu, bis zu 0,4 % Mo, bis zu 2 % Cr, bis zu 0,2 % Ti, bis zu 0,2 % Nb und bis zu 0,5 % V enthält;
• Warmwalzen des Vorprodukts zu einem Warmband in einem oder mehreren Walzstichen, wobei das erhaltene Warmband beim Verlassen des letzten Walzstichs eine Warmwalzendtemperatur von mindestens 880 °C aufweist;
• beschleunigtes Abkühlen des erhaltenen Warmbands mit einer Abkühlrate von mindestens 5 °C/s auf eine Haspeltemperatur, die zwischen der Martensitstarttemperatur MS und 600 °C liegt;
• Haspeln des Warmbands zu einem Coil;
• Abkühlen des Coils, wobei die Temperatur des Coils während der Abkühlung zur Bildung von Bainit solange in einem Temperaturbereich gehalten wird, dessen Obergrenze gleich der Bainitstarttemperatur BS, ab der Bainit im Gefüge des Warmbands entsteht, und dessen Untergrenze gleich der Martensitstarttemperatur MS ist, ab der Martensit im Gefüge des Warmbands entsteht, bis mindestens 60 Vol.-% des Gefüges des Warmbands aus Bainit bestehen.

A method for producing a hot-rolled flat steel product suitable as a starting material for the method according to the invention comprises the following steps:

• Providing a precursor in the form of a slab, thin slab or a cast strip, which in addition to iron and unavoidable impurities (in% by weight): 0.10-0.60% C, 0.4-2.0% Si, up to 2.0% Al, 0.4-2.5% Mn, up to 1% Ni, up to 2.0% Cu, up to 0.4% Mo, up to 2% Cr, up to 0.2% Ti, up to 0.2% Nb and up to 0.5% V;
• Hot rolling the precursor into a hot strip in one or more rolling passes, the resulting hot strip having a hot rolling finish temperature of at least 880 ° C when leaving the last pass;
• accelerated cooling of the obtained hot strip at a cooling rate of at least 5 ° C / s to a coiler temperature which is between the martensite start temperature MS and 600 ° C;
• Coiling the hot strip into a coil;
• Cooling of the coil, wherein the temperature of the coil is maintained during cooling to form bainite in a temperature range whose upper limit equal to the bainite start temperature BS, from the bainite in the structure of the hot strip, and whose lower limit is equal to the martensite start temperature MS, from the Martensite in the structure of the hot strip is produced until at least 60% by volume of the structure of the hot strip of bainite.

A cold-rolled flat steel product suitable for carrying out the process according to the invention as starting material and a process for producing such a cold-rolled steel flat product are the subject of the European patent application 12 17 83 32.8 , the contents of which are hereby expressly included in the disclosure of the present patent application.

In the case of an alloy falling below the steel composition specified in accordance with the invention, the microstructure of the cold-rolled steel flat product preferably consists of at least 20% by volume of bainite, 10 to 35% by volume of retained austenite and the remainder of martensite. It goes without saying that in the structure of the flat steel product technically unavoidable traces of other structural constituents can be present. Accordingly, such a cold-rolled flat steel product suitable for the processing according to the invention has a three-phase structure whose dominant constituent is bainite and which furthermore consists of retained austenite and the remainder of martensite. Optionally, the bainite content is at least 50% by volume, in particular at least 60% by volume, and the residual austenite content is in the range from 10 to 25% by volume, the remainder of the microstructure in each case also being filled with martensite. The optimum martensite content is at least 10% by volume. Such a composite structure in the case of the high tensile strength Rm of typically at least 1400 MPa and an elongation at break A80 of at least 5% required for a cold-rolled flat steel product processed according to the invention produces an optimum product Rm x A80 of elongation and tensile strength. In addition to the main components "Bainite", "retained austenite" and "martensite" may be present in the cold-rolled steel flat product processed according to the invention contents of other microstructure constituents, but their proportions are too low to have an influence on the properties of the cold-rolled steel flat product. The retained austenite is predominantly film-like with small globular islands of blocky retained austenite with a grain size <5 μm in the case of such a flat steel product suitable for processing according to the invention, so that the retained austenite has a high stability and, consequently, a low tendency to undesirable transformation into martensite , The C content of the retained austenite is typically more than 1.0% by weight.

• Bereitstellen eines Vorprodukts in Form einer Bramme, Dünnbramme oder eines gegossenen Bands, das neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%) C: 0,10 - 0,60 %, Si: 0,4 - 2,5 %, Al: bis zu 3,0 %, Mn: 0,4 - 3,0 %, Ni: bis zu 1,0 0 %, Cu: bis zu 2,0 0 %, Mo: bis zu 0,4 %, Cr: bis zu 2 %, Co: bis zu 1,5 %, Ti: bis zu 0,2 %, Nb: bis zu 0,2 %, V: bis zu 0,5 % enthält;
• Warmwalzen des Vorprodukts zu einem Warmband in einem oder mehreren Walzstichen, wobei das erhaltene Warmband beim Verlassen des letzten Walzstichs eine Warmwalzendtemperatur von mindestens 830 °C aufweist;
• Haspeln des erhaltenen Warmbands bei einer Haspeltemperatur, die zwischen der Warmwalzendtemperatur und 560 °C liegt;
• Kaltwalzen des Warmbands zu einem Kaltband mit einem Kaltwalzgrad von mindestens 30 %;
• Wärmebehandeln des erhaltenen Kaltbands, wobei das Kaltband im Zuge der Wärmebehandlung
  • auf eine mindestens 800 °C betragende Glühtemperatur erwärmt wird,
  • optional über eine Glühdauer von 50 - 150 s bei der Glühtemperatur gehalten wird,
  • ausgehend von der Glühtemperatur mit einer mindestens 8 °C/s betragenden Abkühlgeschwindigkeit auf eine Haltetemperatur abgekühlt wird, die in einem Haltetemperaturbereich liegt, dessen Obergrenze 470 °C beträgt und dessen Untergrenze höher ist als die Martensitstarttemperatur MS, ab der Martensit im Gefüge des Kaltbands entsteht, und
  • im Haltetemperaturbereich über einen Zeitraum gehalten wird, der ausreicht, um im Gefüge des Kaltbands mindestens 20 Vol.-% Bainit zu bilden.

A method for producing a cold-rolled flat steel product produced in accordance with the invention and comprises the following steps:

• Providing a precursor in the form of a slab, thin slab or cast strip which, in addition to iron and unavoidable impurities (in% by weight) C: 0.10-0.60%, Si: 0.4-2.5%, Al : up to 3.0%, Mn: 0.4 - 3.0%, Ni: up to 1.0 0%, Cu: up to 2.0 0%, Mo: up to 0.4%, Cr: up to 2%, Co: up to 1.5%, Ti: up to 0.2%, Nb: up to 0.2%, V: up to 0.5%;
• Hot rolling the precursor into a hot strip in one or more rolling passes, the resulting hot strip having a hot rolling end temperature of at least 830 ° C when leaving the last pass;
• Coiling the resulting hot strip at a reel temperature that is between the hot rolling end temperature and 560 ° C;
• Cold rolling the hot strip to a cold strip having a cold rolling degree of at least 30%;
• Heat treating the cold strip obtained, wherein the cold strip during the heat treatment
  • is heated to a minimum of 800 ° C annealing temperature,
  • is held at the annealing temperature over an annealing period of 50 - 150 s,
  • is cooled starting from the annealing temperature with a cooling rate of at least 8 ° C / s to a holding temperature, which is in a holding temperature range whose upper limit is 470 ° C and whose lower limit is higher than the martensite start temperature MS, starting from the martensite in the structure of the cold strip , and
  • held in the holding temperature range for a period sufficient to form at least 20 vol .-% bainite in the structure of the cold strip.

The above-mentioned martensite starting temperature, ie the temperature at which martensite forms in steel processed according to the invention, can be determined in each case according to the article " Thermodynamic Exatrapolation and Martensite Start-Temperature of Substitutionally Alloyed Steels "by H. Bhadeshia, published in Metal Science 15 (1981), pages 178-180 explained procedure can be calculated.

Fig. 1

ein Diagramm, in dem für vier warmgewalzte Stahlflachprodukte derselben Zusammensetzung S1 in erfindungsgemäßer Weise erzeugten Bauteile B1,B2,B3,B4 die Bruchdehnung A50 über die Zugfestigkeit Rm aufgetragen ist;

Fig. 2

eine Abbildung einer Gefügeprobe des Bauteils B4;

Fig. 3a,3b

Abbildungen einer Gefügeprobe des Stahlflachprodukts, aus dem das Bauteil B4 geformt ist, in 20000-facher Vergrößerung und zwar vor (Fig. 3a) und nach (Fig. 3b) der Umformung;

Fig. 4a,4b

Abbildungen einer Gefügeprobe des Stahlflachprodukts, aus dem das Bauteil B4 geformt ist, in 50000-facher Vergrößerung und zwar vor (Fig. 4a) und nach (Fig. 4b) der Umformung.

The invention will be explained below with reference to exemplary embodiments. Show it:

Fig. 1

a diagram in which is plotted for four hot rolled flat steel products of the same composition S1 in accordance with the invention produced components B1, B2, B3, B4, the elongation at break A50 on the tensile strength Rm;

Fig. 2

an illustration of a structural sample of the component B4;

Fig. 3a, 3b

Illustrations of a structural sample of the flat steel product from which the component B4 is formed, magnified 20,000 times before ( Fig. 3a ) and after ( Fig. 3b ) the transformation;

Fig. 4a, 4b

Illustrations of a structural sample of the flat steel product from which the component B4 is formed, magnified 50,000 times before ( Fig. 4a ) and after ( Fig. 4b ) of the transformation.

A steel having the composition shown in Table 1 has been melted.

The molten steel has been conventionally cast into slabs which have subsequently been heated to a reheating temperature TDC in a manner also conventional.

The heated slabs were hot rolled in a likewise conventional hot rolling mill to hot strips W1 - W4 with a thickness of 2.0 mm each.

The hot strips W1-W4 emerging from the hot rolling scale each had a hot rolling end temperature ET, from which they have been accelerated at a cooling rate KR to a coiling temperature HT. At this reel temperature HT, the hot strips W1 - W4 have been wound into coils.

The coils were then each cooled in a temperature range whose upper limit was determined by the respective reel temperature HT and the lower limit by the martensite starting temperature MS calculated for the steel S1. The calculation of the martensite start temperature MS was carried out according to the article " Thermodynamic Exatrapolation and Martensite Start-Temperature of Substituted Alloyed Steels "by H. Bhadeshia, published in Metal Science 15 (1981), pages 178-180 explained procedure.

The duration over which the coil was cooled in the temperature range defined in the manner described above was such that the hot strips thus obtained each had a structure consisting of bainite and retained austenite, in which the proportions of others Structural constituents were present at most in ineffective, against "0" going amounts.

The respective operating parameters reheating temperature TDC, hot rolling end temperature ET, cooling rate KR, reeling temperature HT and martensite starting temperature MS are given in Table 2.

Table 3 also shows the mechanical properties tensile strength Rm, yield strength Rp, elongation at break A80, quality Rm * A80 and the respective retained austenite content RA determined for the individual hot strips W1-W4.

Samples of the steel flat products obtained in the form of the hot strips W1-W4 are then heated to a forming temperature UT lying in the range of 200-250 ° C. and formed into a single component with a degree of deformation of up to 15%. At the temperature UT, the elongation at break A50 of the samples was> 30%, so that it was also possible to image complex shaped elements without the risk of crack formation in the temperature range of the forming process according to the invention.

After forming in the temperature range of 200-250 ° C., the components converted from the samples of the hot strips W1-W4 were air-cooled to room temperature and their breaking elongation A50 and their tensile strength Rm were determined.

For comparison, further samples of the hot strips W1 - W4 at room temperature RT, ie cold, have been converted to the respective components. Also on the so shaped Components, the elongation at break A50 and the tensile strength Rm has been determined.

It was found that, after cooling to room temperature at essentially constant values of the elongation at break A50, the tensile strength Rm of the samples formed according to the invention was 80-120 MPa higher than the samples converted at room temperature.

In Fig. 2 is a section of a structural sample shown, which has been removed at room temperature from the component, which has been formed from the consisting of the steel S1 hot strip W2 in accordance with the invention at temperatures of 200 - 250 ° C. Clearly recognizable there is the residual austenite RAf formed by the transformation in the temperature range mentioned above from the previously globulitic retained austenite islands.

In Fig. 3a, 3b are in each case 20,000-fold magnification cut-outs of a structural sample of steel S1 existing steel component before ( Fig. 3a ) and after ( Fig. 3b ) reproduced the deformation of the invention.

In Fig. 4a, 4b corresponding photographs of the structural samples of the existing steel S1 steel component ( Fig. 4a ) and after ( Fig. 4b ) of the inventive transformation in 50000-fold magnification.

Also the comparison of FIG. 3a with the FIG. 3b and the FIG. 4a with the FIG. 4b clearly show the changes that are caused by a deformation according to the invention.

The method according to the invention thus makes it possible in a simple way to produce a complex-shaped steel component with a tensile strength Rm> 1200 MPa and an elongation at break A50> 6%. For this purpose, according to the invention, a flat steel product is provided which, in addition to iron and unavoidable impurities (in% by weight) C: 0.10-0.60%, Si: 0.4-2.5%, Al: up to 3.0% Mn: 0.4 - 3.0%, Ni: up to 1%, Cu: up to 2.0%, Mo: up to 0.4%, Cr: up to 2%, Co: up to 1.5 %, Ti: up to 0.2%, Nb: up to 0.2%, V: up to 0.5%, the structure of the flat steel product consisting of at least 10% by volume of retained austenite, the globular retained austenite islands having a grain size of at least 1 micron. The flat steel product is heated to 150-400 ° C forming temperature and formed at the forming temperature with a degree of deformation which is at most equal to the uniform strain Ag, to the component. Finally, the steel flat product thus obtained is cooled. Such molded at elevated temperatures component has over the same flat steel product, but molded at room temperature components significantly increased strength. Table 1 stole C Si al Mn Ni Cu Cr other S1 0.48 1.5 0.02 1, 48 0.034 1.51 0.9 In% by weight,
Remaining iron and unavoidable impurities hot strip OT [° C] ET [° C] KR [° C / s] HT [° C] MS [° C] W1 1150 970 20 350 245 W2 1200 1000 10 400 315 W3 1200 1000 20 450 270 W4 1150 1000 20 500 230 hot strip Rm [MPa] Rp [MPa] A80 [%] RM * A80 [MPa *%] RA [vol.%] W1 1357 807 22.2 27387 36 W2 1318 751 17, 8 21328 17 W3 1217 821 25.8 28544 32 W4 1345 889 21.0 25677 30

Claims (10)

A method for producing a steel component having a tensile strength Rm of more than 1200 MPa and an elongation at break A50 of more than 6%, comprising the following steps: Providing a flat steel product containing, in addition to iron and unavoidable impurities (in% by weight): C: 0.10-0.60%, Si: 0.4-2.5%, Al: up to 3.0% Mn: 0.4 - 3.0%, Ni: up to 1%, Cu: up to 2.0%, Mo: up to 0.4%, Cr: up to 2%, Co: up to 1.5%, Ti: up to 0.2%, Nb: up to 0.2%, V: up to 0.5%, wherein the microstructure of the flat steel product consists of at least 10% by volume of retained austenite comprising globular retained austenite islands with a particle size of at least 1 μm, Heating the flat steel product to a forming temperature which is 150-400 ° C, - Forming the heated to the forming temperature flat steel product to the component with a maximum extent to the uniform strain Ag-reaching degree, - Cooling of the formed flat steel product. A method according to claim 1, characterized in that the provided flat steel product is provided with a metallic protective coating. Method according to one of claims 1 or 2,
characterized in that the provided flat steel product is a hot rolled steel strip or sheet. A method according to claim 3, characterized in that the microstructure of the hot-rolled steel flat product contains at least 60% by volume of bainite and at least 10% by volume of retained austenite and optionally up to 5% by volume of ferrite and up to 10% by volume of martensite, and that at least a portion of the retained austenite in blocky form and the blocks of retained austenite present in block form at least 98% have a mean diameter of less than 5 microns. Flat steel product according to claim 4, characterized in that its contents of Mn, Cr, Ni, Cu and C satisfy the following condition: $$\mathrm{1}<\mathrm{0}.\mathrm{5}\%\mathrm{Mn}+\mathrm{0}.\mathrm{167}\%\mathrm{Cr}+\mathrm{0}.\mathrm{125}\%\mathrm{Ni}+\mathrm{0}.\mathrm{125}\%\mathrm{Cu}+\mathrm{1}.\mathrm{334}\%\mathrm{C}<\mathrm{2}$$

with% Mn: respective Mn content in% by weight, % Cr: respective Cr content in% by weight, % Ni: respective Ni content in wt%, % Cu: respective Cu content in% by weight, % C: respective C content in% by weight. Method according to one of the preceding claims,
characterized in that the provided flat steel product is a cold rolled steel strip or sheet. A method according to claim 6, characterized in that the structure of the cold-rolled steel flat product contains at least 20% by volume of bainite, 10-35% by volume of retained austenite and at least 10% by volume of martensite. A method according to claim 7, characterized in that the cold-rolled steel flat product contains at least 50 vol .-% bainite. Method according to one of the preceding claims, characterized in that the sum of the Al and Si contents of the provided flat steel product is at least 1.5 wt .-%. Method according to one of the preceding claims, characterized in that the cooling takes place after the forming of the flat steel product takes place in still air.

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