WO2014016420A1

WO2014016420A1 - Hot-rolled flat steel product and method for the production thereof

Info

Publication number: WO2014016420A1
Application number: PCT/EP2013/065836
Authority: WO
Inventors: Brigitte Hammer; Thomas Heller; Frank Hisker; Rudolf Kawalla; Grzegorz Korpala
Original assignee: Thyssenkrupp Steel Europe Ag
Priority date: 2012-07-27
Filing date: 2013-07-26
Publication date: 2014-01-30
Also published as: CN104662179B; EP2690183A1; EP2690183B1; KR20150038426A; US20150203946A1; JP2015528064A; BR112015001456A2; JP6154010B2; CN104662179A

Abstract

The invention relates to a hot rolled flat steel product, with the product of Rm and A80 ≥ 18000 MPa*%, a composition consisting of (in percent by weight): C: 0.10 - 0.60 %, Si: 0.4 - 2.0 %, Al: ≤ 2.0%, Mn: 0.4 - 2.5 %, Ni: ≤ 1%, Cu: ≤ 2.0%, Mo: ≤ 0.4%, Cr: ≤ 2%, Ti: ≤ 0.2%, Nb: ≤ 0.2%, V: ≤ 0.5%, the remainder being composed of Fe and unavoidable impurities, and a structure dominated by two phases, in which bainite is one dominant component and residual austenite is the second dominant component, wherein the structure consists of ≥ 60 percent by volume of bainite and the remainder of residual austenite, wherein, optionally, (in percent by volume) ≤ 5% of ferrite and ≤ 10% of martensite may be present in the structure, at least a part of the residual austenite being present in the form of a block and ≥ 98% of the residual austenite blocks having a size of ≤ 5 μm. In order to produce the steel flat product, a slab, a thin slab or a cast strip having said composition are provided. The pre-product is hot-rolled with a final hot-rolling temperature of ≥ 880 °C, then hot-rolled into a hot strip, and the obtained hot strip is cooled with a cooling rate of ≥ 5 °C/s to a temperature lying between the initial martensite temperature MS and the coiling temperature of 600 °C. The hot strip is coiled into a coil and is cooled in the coil, wherein the temperature of the coil is maintained between the initial bainite temperature BS and the initial martensite temperature MS until ≥ 60 percent by volume of the hot strip microstructure consist of bainite.

Description

Hot rolled steel flat product and

Process for its preparation

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

Stretching properties and are as such

in particular for the production of components for

Motor vehicle bodies suitable.

Likewise, the invention relates to a method for

Production of a flat steel product according to the invention.

By the term "flat steel product" here are produced by a rolling process 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 EP 1 466 024 Bl (DE 603 15 129 T2) is a

A process for producing a flat steel product is known, the tensile strengths of significantly more than

1000 MPa should have. 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 with a

Reduction rate of at least 10% hot rolled. The

Hot rolling is terminated at a final temperature at which all copper is still in solid solution in the ferrite and / or austenite matrix. The strip is then subjected to a rapid cooling step to remove the supersaturated solid solution copper in the strip

Ferrite and / or austenite solution to keep. That so

cooled tape is finally a coil

wound. The copper precipitates cause a

Precipitation hardening, by which the desired

Strength level of the steel is to be achieved.

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 over 1200 MPa

Tensile strength and elongation of up to 10% and a method for its production are known from

US 2009/0107588 AI known. The well-known hot strip exists of a steel containing, in addition to iron and unavoidable impurities (in% by weight): 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 +% A1 <2% (% A1 = respective Al content,% Si = respective Si content) and % Cr + (3x% 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, too

at least 5% of retained austenite and at least 2% of 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 starting 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 then cooling to a second intermediate temperature, which takes place between the bainite start temperature, d. b., at which bainite begins to form in the steel, and one below limit temperature, which is 50 ° C higher than that

Martensite start temperature, ie the temperature at which martensite forms in the 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 readily deformable material

Hot strip is described in US 6,190,469 Bl. 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 remainder iron and unavoidable impurities

having. The slabs are rolled to hot strip with the final hot rolling temperature of 750-880 ° C. The hot strip obtained is then cooled starting from a starting temperature of 680-740 ° C by means of water to a reeling temperature which is at least equal to that calculated by the formula 240 × (% Mn +% Ni) -140

Temperature (with% Mn = respective n-content,

% 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

obtained hot strip has a microstructure containing in addition to ferrite 5 - 20% retained austenite and 20 - 50% bainite, wherein in the microstructure copper precipitates

that exist by precipitation hardening to

Strength of the resulting hot strip contribute. 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 is this task

According to the invention has been solved by the hot rolled flat steel product specified in claim 1.

With regard to the method, the solution according to the invention of the above-mentioned object is that for the production of a hot-rolled according to the invention

Flat steel product at least in claim 8

be passed through the specified steps.

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

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%,

AI: up to 2.0%,

Mn: 0.4 - 2 _f 5%

Ni: up to 1%, Cu: up to 2.0 o.

Mo: up to 0.4 g,

o,

Cr: up to 9 9-

Ti: up to 0.2 o.

°

Nb: up to 0.2 o.

o,

V: up to 0.5 g. A flat steel product according to the invention in this case has a structure dominated by two phases, of which one dominant component is bainite and the second dominant component is retained austenite. In addition to these two main components can be low levels of

Martensite and ferrite are present, but their contents are too low to have an influence on the properties of the hot-rolled steel flat product.

Accordingly, the microstructure of the flat steel product according to the invention 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 50% by volume, in particular at least 60% by volume, of bainite and the remainder from retained austenite, wherein at least a portion of the retained austenite is in block form and the blocks are in block form

at least 98% of retained austenites have an average diameter of less than 5 μm.

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, Contains 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 pass 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 one

Reel temperature 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 the cooling to form bainite while in a temperature range whose

Upper limit equal to the bainite start temperature BS, from which bainite is produced in the microstructure of the hot strip, and whose lower limit is equal to the martensite start temperature MS, from which martensite is produced in the microstructure of the hot strip, up to 50% by volume, in particular at least 60% by volume, of

Structure of the hot strip made of bainite.

The invention is based on the recognition that it is for the required properties of the hot-rolled

Flat steel product is favorable, if the retained austenite is in block form, as long as the diameter of the retained austenite blocks does not exceed 5 m. In the prior art, it has hitherto been assumed that block-like retained austenite basically belongs to is avoided, since block-shaped retained austenite as

Causes of instabilities of the structure and a concomitant tendency to form

unwanted martensite has been interpreted.

Accordingly, the highest possible proportion of film-like retained austenite in the structure of a steel of the type in question has always been sought in the prior art (see 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. In contrast, retained austenite is referred to as "film-like", if existing in the structure

Restaustenitansammlungen the ratio length / width is greater than 5 and the width of the respective structural constituents of retained austenite is less than 1 μηα.

Film-like retained austenite is accordingly

typically present as a finely dispersed lamella.

The effort still required in accordance with the prior art for avoiding block-retained residual austenite can thus be used in the production of a

Steel flat product according to the invention are bypassed by the existing in the structure of the resulting flat steel product invention Restaustenitblöcke are kept small, ie in their by their middle Diameter expressed extent are limited to less than 5 μπι. Surprisingly it has

proved that block-shaped

Residual austenite with a diameter which is smaller than 5 μτα, 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

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 measures at most 4 μιτι, in particular at most 3 μιη, in the expansion. It has been found in practice that present in accordance with the invention and produced steel flat products the maximum extent of present in block form

Restaustenits regularly in the range of 1 - 3 μηα, wherein the maximum extent of Restaustenitblöcke is typically limited to 2 μιτι. An elaborate, multistage temperature control during the production of the flat steel product is required

not 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, there are no complex or high cooling power

requiring cooling strategies more necessary, as they have been held in the prior art still unavoidable. The positive influence of retained austenite contents in the microstructure of a flat steel product according to the invention then occurs particularly reliably when the residual austenite content

at least 10 vol .-% is, at levels of at least 15 vol .-% Restaustenit particularly safe favorable effects are expected.

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 *%. One

As such, flat steel product according to the invention has an optimum combination of extreme strength and good formability.

Also in a hot rolled according to the invention

Flat steel product, the strength-increasing effect of copper can be used. This can be done in the

Hot rolled flat steel product according to the invention, a minimum content of 0.15 wt .-% Cu be present.

Carbon retards the conversion to ferrite / perlite in the steel according to the invention, lowers the

Martensite starting temperature MS and contributes to increase 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 bainitiation in the steel processed according to the invention, with the optionally additionally present contents of Cu, Cr and Ni also contributing to

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.

In addition, the optional addition of Cr also lowers the martensite start temperature and the

Tendency of bainite to transform into perlite or

Cementite be suppressed. Furthermore, Cr in contents promotes up to the present invention

Upper limit of maximum 2% by weight ferritic

Conversion, wherein optimum effects of the presence of Cr in a flat steel product according to the invention result when the Cr content is limited to 1.5 wt .-%.

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 bainite can be suppressed and consequently the Restaustenite be stabilized by dissolved carbon. In addition, especially contributes to Si

Solid solution hardening at. AI can do it in the

According to the invention processed steel replace the Si content to a part. For this purpose, a minimum content of 0.4 wt .-% AI can 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 then be used particularly effectively if the contents of Si and Al within the limits specified in accordance with the invention are the condition

% Si + 0.8% Al> 1.2% by weight or even the condition% Si + 0.8% Al> 1.5% by weight (with% Si: respective Si content in% by weight % Al: respective Al content in% by weight).

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, where with% Mn the respective Mn content in

% By weight, with% Cr the respective Cr content in% by weight, with% Ni the respective Ni content in% by weight, with% Cu the respective Cu content in% by weight and with% C the respective C content in wt .-% are designated. For the preparation of an inventive

In a flat steel product, the precursor cast from a composite steel according to the invention is first brought to a temperature or kept at a temperature which is sufficient for the hot rolling carried out from this temperature to be at a temperature

Finish hot rolling, wherein the obtained hot strip a fully recrystallized,

Austenitic structure possesses the optimal

Conditions for bainite formation offers. This is the case when the hot strip obtained has a hot rolling end temperature of at least 880 ° C when leaving the last pass, wherein the

process according to the invention 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 hot strip thus obtained also has a structure

Restaustenitanteilen that meet the requirements of the invention.

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. The cooling is thereby

optimally started when 50 - 60% of austenite are 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: tp = 5 · io ^{+ 36} -r ¹² ' 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:

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 is achieved for a sufficiently long time

takes place exclusively in the coil. 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 in the structure of

Hot strip is created. The duration over which the coil is held in this temperature range, is chosen so that the invention sought after Bainitanteil of at least 60 vol .-% is achieved. In practice this is regularly a duration of

sufficient for at least 0.5 h, with longer bainite levels occurring 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 be particularly reliably avoided that the lower limit of the reel temperature by at least 10 ° C, in particular at least 20 ° C, higher than that

Martensite start temperature.

At the same time, the desired course of the

Bainit formation can be ensured in practice 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:

HTMin = MS + (BS-MS) / 3 It goes without saying that among the

operating conditions compliance with this

Temperature is always subject to a certain tolerance, so usually not exactly met, but with a tolerance of typically +/- 20 ° C is maintained.

The invention is based on

Embodiments explained in more detail.

Seven steels Sl-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 are in one too

conventional hot rolling mill to hot strip Wl - W10 was hot rolled to 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 are then each in one

Temperature range has been cooled, the upper limit by the respective reel temperature HT and its

Lower limit by the for the respective steel Sl - S7 certain martensite start temperature MS was determined. The martensite start temperature MS was calculated according to the procedure described in the article "Thermodynamic Exatrapolation and Martensite Start Temperature of Substitutionally Alloyed Steels" by H. Bhadeshia, published in Metal Science 15 (1981), pages 178-180.

The duration over which the coil has been cooled in the temperature range defined in the above-described manner was such that the hot strips thus obtained each have a bainite and

Residual austenite had existing structure in which the proportions of other structural components at most in

ineffective, against "0" going quantities were present.

The respective operating parameters

Reheating temperature OT, hot rolling end temperature ET, cooling rate KR, reeling temperature HT and

Martensite starting temperature MS are given in Table 2.

In addition, Table 3 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

indicated.

It was found that the hot strip W3, which had a comparably low Si content and was produced from steel S3, had the desired tensile strength of

at least 1200 MPa has not been achieved. 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 strip W7 produced from the steel S5 and the hot strip W10 produced from the steel S7 is here

the desired minimum tensile strength of 1200 MPa has also not been achieved. The reason was in these cases in each case too high reel temperature HT.

Steel C Si Al Mn Ni Cu Cr Other

Sl 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 Mo: 0.3

56 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 Mo: 0.30

In% by weight,

Remaining iron and unavoidable impurities

Table 1

Table 2 Warm steel Rm Rp A80 R * A80 RA band. [MPa] [MPa] [%] [MPa *%] [Vol. -%]

WI Sl 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

Table 3

Claims

P A T E N T A N S P R E C H E

1. Hot rolled flat steel product, having a product of tensile strength Rm and elongation A80 of at least 18000 MPa *%, in addition to iron and

unavoidable impurities (in% by weight):

C: 0.10-0.60 g

O

Si: 0.4-2.0 g,

O

AI: up to 2.0 o,

0

Mn: 0.4-2.5 0,

o,

Ni: up to 11 2o,

Cu: up to 2.0,

O

Mo: up to 0.4 g,

o _r

Cr: up to 9 9-

Ti: up to 0.2 0,

O

Nb: up to 0.2 g,

o,

V: up to 0.5 0,

0,

wherein the flat steel product has a dominated by two phases structure, one of

Bainit dominant component and its second dominant ingredient Restaustenit is, the structure of the flat steel product to at least

Bainite and the balance of retained austenite, optionally up to 5% by volume of ferrite and up to 10% by volume of martensite in the structure of

Steel flat product can be present and where

SI / cs 120791WO at least a portion of the retained austenite is in block form and the blocks of retained austenite present in block form at least 98% have a mean diameter of less than 5 pm

exhibit .

2. Flat steel product according to claim 1, characterized in that its structure contains at least 10% by volume of retained austenite.

3. Flat steel product according to one of the preceding claims, d a d u r c h

be e c e n c e s, s e s

Cu content is at least 0.15 wt .-%.

4. Flat steel product according to one of the preceding claims, d a d u r c h

be e c e n c e s, s e s

C content is at least 0.3 wt .-%.

5. Flat steel product according to one of the preceding claims, d a d u r c h

Let's say that its contents of Mn, Cr, Ni, Cu and C are as follows

Fulfill condition:

1 <0, 5% Mn + 0, 167% Cr + 0, 125% Ni + 0, 125% Cu + l, 334% C <2 with% n: 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, d a d u r c

In other words, 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,

% A1: respective Al content in% by weight.

Flat steel product according to one of the preceding claims, d a d u r c h

g e n c i n e s, the s

Diameter of the blocky retained austenite is 1 - 3 pm.

A method for producing a flat steel product according to any one of claims 1 to 7, comprising the following steps:

- Provide a precursor in the form of a slab, thin slab or a cast strip, 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 of the precursor into a hot strip in one or more rolling passes, the

hot strip obtained leaving the last pass a Hotwalzendtemperatur of

at least 880 ° C;

accelerated cooling of the resulting 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 during cooling to form bainite while kept 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 which martensite is produced in the microstructure of the hot strip, until at least 50% by volume of the microstructure of the hot strip consists of bainite.

9. The method of claim 8, d a d u r c h

e n c ia n, the s

Final temperature of hot rolling is at least 900 ° C.

10. The method according to any one of claims 8 or 9,

The cooling rate is at least 10 ° C / s

is.

11. The method according to any one of claims 8 to 10,

The cooling rate is 150 ° C / s or less

is.

12. The method according to any one of claims 8 - 11,

The cooling rate is 50 ° C / s or less.

13. The method according to any one of claims 8 - 12,

The lower limit of the coiling temperature, at which cooling begins in the coil, is 20 ° C higher than the martensite start temperature MS.

14. The method according to any 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,

d a d u r c h e k e n c i n e s the reel temperature is at least equal to the temperature HTopt determined according to the following formula:

HTopt = MS + (BS-MS) / 3