EP2690183A1 - Hot-rolled steel flat product and method for its production - Google Patents

Abstract

Hot-rolled flat steel product comprises 0.1-0.6 wt.% carbon, 0.4-2 wt.% silicon, up to 2 wt.% aluminum, 0.4-2.5 wt.% manganese, up to 1 wt.% nickel, up to 2 wt.% copper, up to 0.4 wt.% molybdenum, up to 2 wt.% chromium, up to 0.2 wt.% titanium, up to 0.2 wt.% niobium, up to 0.5% vanadium, iron and unavoidable impurities. The structure of the flat steel product comprises optionally up to 5 vol.% ferrite, up to 10 vol.% martensite, at least 60 vol.% bainite and balance of residual austenite. At least a part of the residual austenite is present in block form. Hot-rolled flat steel product comprises 0.1-0.6 wt.% carbon, 0.4-2 wt.% silicon, up to 2 wt.% aluminum, 0.4-2.5 wt.% manganese, up to 1 wt.% nickel, up to 2 wt.% copper, up to 0.4 wt.% molybdenum, up to 2 wt.% chromium, up to 0.2 wt.% titanium, up to 0.2 wt.% niobium, up to 0.5% vanadium, iron and unavoidable impurities. The structure of the flat steel product comprises optionally up to 5 vol.% ferrite, up to 10 vol.% martensite, at least 60 vol.% bainite and balance of residual austenite. At least a part of the residual austenite in block form and blocks of the austenite present in block form to at least 98% exhibits an average diameter of less than 5 mu m. The flat steel product exhibits a product of tensile strength and elongation of at least 18000 Mpa.%. An independent claim is also included for producing the flat steel product, comprising providing an intermediate product in the form of a slab, thin slab or cast strip, which comprises 0.1-0.6 wt.% carbon, 0.4-2 wt.% silicon, up to 2 wt.% aluminum, 0.4-2.5 wt.% manganese, up to 1 wt.% nickel, up to 2 wt.% copper, up to 0.4 wt.% molybdenum, up to 2 wt.% chromium, up to 0.2 wt.% titanium, up to 0.2 wt.% niobium, up to 0.5% vanadium, iron and unavoidable impurities, hot rolling the intermediate product to form a hot strip in at least one roll stitch, accelerating cooling of the resulting hot strip at a cooling rate of at least 5[deg] C/second to a coiling temperature, which lies in the region between the martensite starting temperature and 600[deg] C, coiling the hot strip to form a coil, and cooling the coils, where (a) the temperature of the coil during cooling to form bainite is maintained at a temperature range with upper limit and lower limit until at least 60 vol.% structure of the hot strip is made of bainite, (b) the upper limit is equal to the bainite starting temperature for producing bainite in the structure of the hot strip, and lower limit is equal to the martensite starting temperature for producing martensite in the structure of the hot strip, and (c) the resulting hot strip on leaving the last roll stitch, exhibits a final hot-rolling of at least 880[deg] C.

Description

The invention relates to a hot-rolled flat steel product with a product of tensile strength Rm and elongation A80 of at least 18000 MPa *%. Flat steel products of this type are characterized by a very high strength in combination with good elongation properties and are suitable as such, in particular for the production of components for motor vehicle bodies.

The invention likewise relates to a method for producing a flat steel product according to the invention.

The term "flat steel product" here by a rolling process produced steel sheets or steel strips and divided therefrom boards and the like understood.

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

The product of tensile strength Rm and elongation A80 is also referred to in technical jargon as "quality".

From the EP 1 466 024 B1 ( DE 603 15 129 T2 ) discloses a method for producing a flat steel product which is to have tensile strengths of significantly more than 1000 MPa. To achieve this, a molten steel containing (in% by weight) 0.0005-1% C, 0.5-10% Cu, up to 2% Mn, up to 5% Si, up to 0.5 % Ti, up to 0.5% Nb, up to 5% Ni, up to 2% Al and the remainder iron and inevitable impurities due to production. The melt is poured into a tape whose thickness is max. 10 mm and which is rapidly cooled by spraying with water or a water-air mixture to a temperature of at most 1000 ° C. Subsequently, the cast strip is hot rolled at a reduction rate of at least 10%. The hot rolling is terminated at a final temperature at which all the copper is still in solid solution in the ferrite and / or austenite matrix. Then, the strip is subjected to a rapid cooling step to hold the copper in supersaturated solid solution in the ferrite and / or austenite solution. The thus cooled tape is finally wound into a coil. The copper precipitates cause precipitation hardening to achieve the desired strength level of the steel. At the same time, the copper content should increase the corrosion and embrittlement resistance of the steel by forming a protective oxide layer.

A hot strip with a tensile strength exceeding 1200 MPa and an elongation of up to 10% and a method for its production are known from US 2009/0107588 A1 known. The well-known hot strip exists of a steel containing not only iron and unavoidable impurities (in% by weight) but also 0.10-0.25% C, 1-3% Mn, more than 0.015% Al, up to 1.985% Si, up to 0, 30% Mo, up to 1.5% Co, up to 0.005% B, where 1% ≤% Si +% Al ≤ 2% (% Al = respective Al content,% Si = respective Si content) and % Cr + (3 x% Mo) ≥ 0.3% (% Cr = respective Cr content,% Mo = respective Mo content). At the same time, the steel should have a microstructure consisting of at least 75% bainite, at least 5% retained austenite and at least 2% martensite. For the production of the hot strip, a suitably composed melt is cast into a precursor, which is then heated to more than 1150 ° C and then hot rolled at a hot rolling end temperature at which the steel is still fully austenitic. The resulting hot strip is then cooled in three stages. In the first stage, the cooling takes place from a temperature which is above the Ar3 temperature of the steel, with a cooling rate of at least 70 ° C / s to a first intermediate temperature above 650 ° C. Starting from this first intermediate temperature, cooling then takes place to a second intermediate temperature which is between the bainite start temperature, ie at which bainite begins to form in the steel, and below the limit temperature, which is 50 ° C. higher than the martensite start temperature, ie the temperature from which martensite forms in steel. The cooling rate in this second stage of cooling is 20 - 90 ° C / s. This is followed by a third cooling step, in which the hot strip is cooled to room temperature. The temperature at which this third stage of cooling starts becomes determined as a function of the respective cooling rate.

Another method, also based on the strength-increasing effect of Cu precipitates, for producing a high-strength and easily deformable hot strip is disclosed in US Pat US 6,190,469 B1 described. In this process, a steel is cast into slabs containing (in% by weight) 0.15-0.3% C, 1.5-2.5% Si, 0.6-1.8% Mn, O, 02 - 0.10% Al, 0.6-2.0% Cu, 0.6-2.0% Ni and the balance iron and unavoidable impurities. The slabs are rolled to hot strip with the final hot rolling temperature of 750-880 ° C. The obtained hot strip is then cooled starting from a starting temperature of 680-740 ° C. by means of water to a coiling temperature which is at least equal to the temperature calculated according to the formula 240 × (% Mn +% Ni) - 140 (with% Mn = respective Mn). Salary,
% Ni = respective Ni content) and not higher than 540 ° C. Subsequently, the cooled to the reel temperature hot strip is wound into a coil. The resulting hot strip has a microstructure containing 5-20% of retained austenite and 20-50% of bainite besides ferrite, with copper precipitates present in the structure which contribute to the strength of the resulting hot strip by precipitation hardening. The hot strip produced and produced in this way has an elongation of up to 23% at strengths which lie in the range of 1000 MPa, so that altogether high quality values of more than 20,000 MPa *% are achieved.

Against the background of the prior art explained above, the object of the invention was to provide a hot rolled flat steel product which can be produced in a simple and reliable manner and has an optimized combination of particularly high strength and good deformability. In addition, a method for producing such a flat steel product should be mentioned.

With respect to the hot strip, this object has been achieved by the specified in claim 1 hot rolled flat steel product.

With regard to the method, the solution according to the invention of the abovementioned object consists in that at least the working steps specified in claim 8 are run through to produce a hot-rolled flat steel product according to the invention.

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

• C: 0,10 - 0,60 %,
• Si: 0,4 - 2,0 %,
• Al: bis zu 2,0 %,
• Mn: 0,4 - 2,5 %,
• Ni: bis zu 1 %,
• Cu: bis zu 2,0 %,
• Mo: bis zu 0,4 %,
• Cr: bis zu 2 %,
• Ti: bis zu 0,2 %,
• Nb: bis zu 0,2 %,
• V: bis zu 0,5 %,

enthält. Dabei besteht das Gefüge des Stahlflachprodukts neben optional vorhandenen Anteilen von bis zu 5 Vol.-% Ferrit und bis zu 10 Vol.-% Martensit zu mindestens 60 Vol.-% aus Bainit und als Rest aus Restaustenit, wobei zumindest ein Teil des Restaustenits in blockiger Form und die Blöcke des in blockiger Form vorliegenden Restaustenits zu mindestens 98 % einen mittleren Durchmesser von weniger als 5 µm aufweisen.The hot-rolled flat steel product according to the invention is characterized in that in addition to iron and unavoidable impurities (in% by weight)

• C: 0.10-0.60%,
• Si: 0.4-2.0%,
• Al: up to 2.0%,
• Mn: 0.4-2.5%,
• Ni: up to 1%,
• Cu: up to 2.0%,
• Mo: up to 0.4%,
• Cr: up to 2%,
• Ti: up to 0.2%,
• Nb: up to 0.2%,
• V: up to 0.5%,

contains. The structure of the flat steel product consists not only of optionally present proportions 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 remainder of retained austenite, at least a portion of the retained austenite in block form and the blocks of austenite present in block form at least 98% have a mean diameter of less than 5 microns.

• 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.

The method according to the invention for producing a flat steel product 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 flat steel product according to the invention has a structure dominated by two phases, 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.

The invention is based on the finding that it is favorable for the required properties of the hot-rolled steel flat product, if the retained austenite is in block form, as long as the diameter of the austenite blocks Restestenten not exceed 5 microns. In the prior art, it has hitherto been assumed that block-like retained austenite basically belongs to avoid, since block-shaped retained austenite has been interpreted as the cause of instability of the structure and a concomitant tendency to form undesirable martensite. Accordingly, the highest possible proportions of film-like retained austenite in the structure of a steel of the type in question have always been sought in the prior art (see FIG. HKDH Bhadeshia and DV Edmond's "Bainite in silicon steels: new composition-property approach" published in Metal Science Vol. 17, September 1983, pp. 411-419 ("Part 1") and Pp. 420-425 ("Part 2")).

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

The effort still required in accordance with the prior art for avoiding block-retained residual austenite can thus be circumvented in the production of a flat steel product according to the invention by keeping the retained austenite blocks present in the microstructure of the obtained flat steel product according to the invention small, ie in its center Diameter expressed extent are limited to less than 5 microns. Surprisingly, it has been found that block-shaped retained austenite with a diameter which is smaller than 5 microns, has a positive effect on the elongation properties of a steel of the type according to the invention. The retained austenite blocks present in this size are more stable than coarser block-shaped retained austenite. At the same time, they are not as stable as retained austenite in the form of films, thus enabling the TRIP effect. The positive influence of residual austenite present in block form can be used particularly reliably if the blocky retained austenite in the expansion measures at most 4 μm, in particular at most 3 μm. In practice, it has been found in practice that, in the case of flat steel products assembled and produced according to the invention, the maximum extent of the retained austenite present in block form is regularly in the range of 1 to 3 μm, the maximum extent of the retained austenite blocks being typically limited to 2 μm on average. A complex, multi-stage temperature control during the production of the flat steel product is not required to surprisingly.

Accordingly, it is possible to produce a hot-rolled flat steel product according to the invention without any special effort while maintaining the parameters predetermined according to the invention for the production process. In particular, it is no longer necessary to have cooling strategies which require complex cooling or high cooling power, as have been considered to be unavoidable in the prior art.

Hot-rolled flat steel products produced according to the invention regularly reach tensile strengths Rm of more than 1000 MPa, in particular at least 1200 MPa, at elongations A80, which likewise regularly exceed 17%, in particular above 19%. Accordingly, the quality Rm * A80 of hot strips according to the invention is regularly in the range of 18000-30,000 MPa *%. In particular, it is regularly at least 20000 MPa *%. As such, a flat steel product according to the invention has an optimum combination of extreme strength and good formability.

Also in a hot-rolled flat steel product according to the invention, the strength-increasing effect of copper can be used. For this purpose, a minimum content of 0.15% by weight of Cu may be present in the hot-rolled flat steel product according to the invention.

Carbon retards the conversion to ferrite / pearlite in the steel according to the invention, lowers the martensite starting temperature MS and contributes to increasing the hardness. To take advantage of these positive effects, the C content of the flat steel product according to the invention can be set to at least 0.3% by weight.

Mn in contents of up to 2.5% by weight, in particular up to 2.0% by weight, promotes bainite formation in the steel processed according to the invention, with the optionally additionally present contents of Cu, Cr and Ni also leading to the formation of bainite contribute. Depending on the respective other constituents of the invention processed It may be useful in this case to limit the Mn content to a maximum of 1.6% by weight.

In addition, the optional addition of Cr can also lower the martensite start temperature and suppress the tendency of the bainite to convert to perlite or cementite. Furthermore, Cr at contents up to the upper limit of not more than 2% by weight specified in 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 helps to promote 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 according to the invention can be used particularly effectively if their content is in each case in the range from 0.002 to 0.15% by weight, in particular not exceeding 0.14% by weight.

Due to the presence of Si and Al, carbide formation in the bainite can be suppressed and consequently the residual austenite can be stabilized by dissolved carbon. In addition, Si in particular contributes to solid solution hardening. Al can replace the Si content to a part 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 if the addition of Al should set the hardness or tensile strength of the steel to a lower value in favor of improved ductility.

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 limits prescribed by the invention satisfy the condition% Si + 0.8% Al> 1.2% by weight or even the Condition% Si + 0.8% Al> 1.5 wt% (with% Si: respective Si content in wt%,% Al: respective Al content in wt%).

The formation of the structure according to the invention can be ensured, in particular, by the contents of the steel processed according to the invention and, accordingly, the contents of the flat steel product according to the invention of Mn, Cr, Ni, Cu and C having the following condition

1 <0.5% Mn + 0.167% Cr + 0.125% Ni + 0.125% Cu + 1.334% C <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 order to produce a flat steel product according to the invention, the precursor cast from a composite steel according to the invention is first brought to a temperature or kept at a temperature sufficient to produce the hot rolling carried out from this temperature To end hot rolling temperature, in which the hot strip obtained has a fully recrystallized, austenitic structure, which provides optimal conditions for bainite formation. This is the case when the hot strip obtained at leaving the last pass a hot rolling end temperature of at least 880 ° C, the inventive method can run particularly reliable when the hot rolling end temperature is set to at least 900 ° C and 1100 ° C, in particular 1050 ° C, does not exceed. Typically, for this purpose, the precursor is heated to a temperature in the range of 1100 - 1300 ° C temperature before hot rolling. If the hot rolling end temperature is lower than 900 ° C, austenite softening can be achieved as much as possible by the main forming of the hot strip in the final passes of hot rolling. The resulting hot strip also has a microstructure with Restaustenitanteilen that meet the requirements of the invention.

wobei tp die Pausenzeit nach der letzten Umformung in Sekunden und T die Temperatur in °C sind. Die Formel gibt die Mindestzeit an, nach der 50 - 60 % entfestigter Austenit vorliegt. Daraus berechnete Pausenzeiten sind: T [°C] t [s] 850 1,21 900 0,59 950 0,30 1000 0,16 Following hot rolling, the hot strip is accelerated at a cooling rate of at least 5 ° C / sec to a coiler temperature which is in the range of 350-600 ° C. Cooling is optimally started when 50-60% of the austenite has softened. In practice, for this purpose, a break of approximately up to 2 s is provided between the end of the hot rolling and the beginning of the cooling. The minimum pause time tp can be calculated using the following empirical formula: $$\mathrm{tp}=\mathrm{5}\cdot {\mathrm{10}}^{+\mathrm{36}}\cdot {\mathrm{T}}^{-\mathrm{12}.\mathrm{5}}.$$

where tp is the pause time after the last transformation in seconds and T is the temperature in ° C. The formula indicates the minimum time after which 50-60% of debonded austenite is present. The calculated break times are: T [° C] t [s] 850 1.21 900 0.59 950 0.30 1000 0.16

The cooling to the coiler temperature is carried out in such a way that there is no conversion of the austenite until reeling. As a result, bainite formation takes place exclusively in the coil for a sufficiently long time. After the hot strip cooled in the above-described manner has been wound into a coil, for this purpose, this coil is cooled in a temperature range whose upper limit is equal to the temperature at which bainite is formed from austenite and whose lower limit is above the temperature the martensite is created in the microstructure of the hot strip. The duration over which the coil is held in this temperature range is chosen so that the bainite content sought according to the invention of at least 60% by volume is achieved. In practice, a duration of at least 0.5 h is regularly sufficient for this, with longer bainite contents being established over a longer period.

Practical studies have shown that a structural transformation between the end of the hot rolling and the coiling can be particularly safely avoided when the cooling rate is at least 10 ° C / s, with practical cooling rates in the range of up to 150 ° C / s, in particular 10 - 50 ° C / s.

The formation of undesirable martensite can thereby be avoided with particular certainty that the lower limit of the reel temperature by at least 10 ° C., in particular at least 20 ° C., is higher than the martensite starting temperature.

At the same time, the desired course of bainite formation in practice can be ensured by setting the upper limit of the coiler temperature to 550 ° C.

An optimum course of the bainit formation taking place in the coil according to the invention results when the reel temperature corresponds at least to the temperature HTopt determined according to the following formula: $$\mathrm{HTMin}=\mathrm{MS}+\left(\mathrm{BS}-\mathrm{MS}\right)/3$$

It is understood that under the operating conditions compliance with this temperature is always subject to a certain tolerance, that is usually not exactly met, but is maintained with a tolerance of typically +/- 20 ° C. The invention will be explained in more detail by means of exemplary embodiments.

Seven steels S1-S7 were melted, the composition of which is given in Table 1.

The correspondingly assembled molten steel was cast into slabs in a conventional manner and then heated in a conventional manner to a reheating temperature TDC.

The heated slabs were hot rolled in a conventional hot rolling mill to hot strips W1 - W10 with a thickness of 2.0 mm.

The hot strips W1-W10 emerging from the hot rolling scale each had a hot rolling end temperature ET, from which they have been acceleratedly cooled at a cooling rate KR to a coiling temperature HT. At this coiler temperature HT, the hot strips W1 - W10 have been wound into coils.

The coils have then each been cooled in a temperature range whose upper limit was determined by the respective reel temperature HT and the lower limit thereof by the martensite starting temperature MS determined for the respective steel S1-S7. 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 has been 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 other structural components were at most ineffective against "0" outgoing quantities were present.

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 determined for the individual hot strips: tensile strength Rm, yield strength Rp, elongation A80, quality Rm * A80 and the respective residual austenite content RA.

It was found that the hot strip W3 produced from steel S3, which had a comparably low Si content, did not reach the tensile strength of at least 1200 MPa desired here.

When due to the low hot rolling end temperature ET not produced according to the invention, consisting of the steel S4 hot strip W5 up to 12 vol .-% block-shaped, coarse retained austenite and coarse martensite were present in the structure, resulting in a significantly deteriorated elongation A80.

By contrast, the hot strip W4, which was also produced from steel S4 but in compliance with the specifications according to the invention, had only up to 1% by volume of coarse blocky retained austenite with an average expansion of more than 5 μm. The remaining retained austenite was in filmy and fine blocky form, with the result that a high elongation A80 was achieved.

The hot-rolled strip W7 produced from the steel S5 and the hot-rolled strip W10 produced from the steel S7 also did not reach the minimum tensile strength of 1200 MPa which was aimed for here. The reason was in these cases in each case too high reel temperature HT.

In the attached figure, the cross-section of a cold strip is shown as an example of the RA excretions discussed. Here, by way of example, residual austenite blocks RA-b are marked and a point is highlighted by an encircling, on which film-like retained austenite RA-f is present in a lamellar layering. 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 S2 0.51 1.5 0.02 1.58 0,015 1.53 0, 9 Ti: 0,013 V: 0,099 S3 0.52 0, 4 1.40 1.48 0,030 1.51 0.9 V: 0.09 S4 0.30 1, 4 0.02 1.46 0,021 1.47 0, 9 Ti: 0,014 V: 0.09 S5 0.51 1.5 0.01 0.40 0, 63 0.60 1.3 Ti: 0.011 V: 0.098 Not a word: 0.3 S6 0.49 1.5 0.01 0.41 0, 60 0, 61 1.5 Ti: 0,014 V: 0.1 S7 0.38 2.0 0.02 0.41 0.59 0.57 1, 4 Not a word: 0.30 In% by weight,
Remaining iron and unavoidable impurities hot strip stole OT [° C] ET [° C] KR [° C / s] HT [° C] MS [° C] According to the invention? W1 S1 1150 970 20 350 245 YES W2 S2 1150 1000 20 500 230 YES W3 S3 1150 1000 10 450 275 YES W4 S4 1150 900 10 400 320 YES W5 S4 1150 850 10 400 320 NO W6 S5 1200 1000 10 400 270 YES W7 S5 1200 1000 10 500 270 YES W8 S6 1200 1000 20 450 270 YES W9 S7 1200 1000 10 400 315 YES W10 S7 1200 1000 10 500 315 YES hot strip stole Rm [MPa] Rp [MPa] A80 [%] RM * A80 [MPa *%] RA [vol.%] W1 S1 1357 807 22.2 27387 36 W2 S2 1345 889 21.0 25677 30 W3 S3 1137 807 23.7 24497 32 W4 S4 1346 878 16.5 20190 20 W5 S4 1593 887 6.4 9268 17 W6 S5 1291 778 22.7 26642 29 W7 S5 1166 830 29.1 30846 30 W8 S6 1217 821 25.8 28544 32 W9 S7 1318 751 17.8 21328 17 W10 S7 1164 812 23.4 24761 17

Claims (15)

Hot rolled flat steel product, having a product of tensile strength Rm and elongation A80 of at least 18000 MPa *%, containing in addition to iron and unavoidable impurities (in% by weight): C: 0.10-0.60%, Si: 0.4-2.0%, Al: up to 2.0% Mn: 0.4-2.5%, Ni: up to 1%, Cu: up to 2.0%, Mo: up to 0.4%, Cr: up to 2%, Ti: up to 0.2%, Nb: up to 0.2%, V: up to 0.5%, wherein the structure of the flat steel product consists, in addition to optionally present proportions of up to 5% by volume of ferrite and up to 10% by volume of martensite, of at least 60% by volume of bainite and the balance of retained austenite, at least a portion of the retained austenite being block form and the blocks of austenite present in block form at least 98% have a mean diameter of less than 5 microns. Flat steel product according to claim 1, characterized in that its Cu content is at least 0.15 wt .-%. Flat steel product according to one of the preceding claims, characterized in that its C content is at least 0.3 wt .-%. Flat steel product according to one of the preceding claims, characterized in that its Mn content is at most 2.0% by weight. Flat steel product according to one of the preceding claims, characterized
characterized in that its contents of Mn, Cr, Ni, Cu and C satisfy the following condition:

1 <0.5% Mn + 0.167% Cr + 0.125% Ni + 0.125% Cu + 1.334% C <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. Flat steel product according to one of the preceding claims, characterized
characterized in that its contents of Si and Al satisfy the following condition:

% Si + 0.8% Al> 1.2 wt%

with% Si: respective Si content in% by weight,
% Al: respective Al content in% by weight. Flat steel product according to one of the preceding claims, characterized in that the diameter of the blocky retained austenite is 1 to 3 μm. A method for producing a flat steel product according to any one of claims 1 to 7, comprising the following steps: - Providing a precursor in the form of a slab, thin slab or a cast strip, in addition to iron and unavoidable impurities (in wt .-%): 0.10 - 0.60% C, 0.4 - 2.0% Si, to 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 of the precursor into a hot strip in one or more rolling passes, wherein the hot strip obtained when leaving the last Walzstichs has a hot rolling end temperature of at least 880 ° C; accelerated cooling of the obtained hot strip at a cooling rate of at least 5 ° C / s to a coiler temperature which is in the range 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 while 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 method according to claim 8, characterized in that the final temperature of the hot rolling is at least 900 ° C. Method according to one of claims 8 or 9,
characterized in that the cooling rate is at least 10 ° C / s. Method according to one of claims 8 to 10,
characterized in that the cooling rate is at most 150 ° C / s. Method according to one of claims 8 - 11,
characterized in that the cooling rate is at most 50 ° C / s. Method according to one of claims 8 - 12,
characterized in that the lower limit of the coiler temperature, at which the cooling in the coil begins, is 20 ° C higher than the martensite start temperature MS. Method according to one of claims 8 - 11,
characterized in that the upper limit of the reel temperature, at which the cooling in the coil begins, is 550 ° C. Method according to one of claims 8 - 12,
characterized in that the reel temperature corresponds at least to the temperature HTopt determined according to the following formula: $$\mathrm{HTopt}=\mathrm{MS}+\left(\mathrm{BS}-\mathrm{MS}\right)/3$$

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