EP2439290A1 - Multiphase steel, cold rolled flat product produced from this multiphase steel and method for producing same - Google Patents

Abstract

Multi-phase steel comprises (in wt.%): carbon (0.14 - 0.25), manganese (1.7-2.5), silicon (0.2-0.7), chromium (less than 0.1), molybdenum (less than 0.05), niobium (0.02-0.06), sulfur (up to 0.01), phosphorus (up to 0.02), nitrogen (up to 0.01), optionally at least one of titanium (up to 0.1), boron (up to 0.002) or vanadium (up to 0.15), and remaining iron and unavoidable impurities. The microstructure of the steel comprises at least 10 vol-% of ferrite and at least 6 vol-% of residual austenite and the steel has a tensile strength of at least 950 MPa. Multi-phase steel comprises (in wt.%): carbon (0.14 - 0.25), manganese (1.7-2.5), silicon (0.2-0.7), chromium (less than 0.1), molybdenum (less than 0.05), niobium (0.02-0.06), sulfur (up to 0.01), phosphorus (up to 0.02), nitrogen (up to 0.01), optionally at least one of titanium (up to 0.1), boron (up to 0.002) or vanadium (up to 0.15), and remaining iron and unavoidable impurities. The microstructure of the steel comprises at least 10 vol-% of ferrite and at least 6 vol-% of residual austenite and the steel has a tensile strength of at least 950 MPa, a yield strength of at least 500 MPa, and an elongation of at least 15% measured in the transverse direction. Independent claims are also included for: (1) a cold-flat product produced from the multi-phase steel; and (2) producing the cold-flat product, comprising melting and casting the multi-phase steel to an intermediate product, hot rolling the precursor starting from an initial temperature of 1100-1300[deg] C ending at a final temperature of 820-950[deg] C to a hot strip, coiling the hot band to a coiling temperature of 400 -750[deg] C, optional annealing the hot bands for improving the cold rollability, cold rolling the hot bands to the cold-rolled product, after coiling at a cold rolling degree of 30-80%, continuously annealing the cold-rolled product at an annealing temperature of 750-900[deg] C, optionally acceleratedly cooling the continuously annealed cold-rolled product, and overaging the cold-rolled product at an aging temperature of 350-500[deg] C.

Description

The invention relates to a multiphase steel, a Kaltwalzflachprodukt produced by such a multi-phase steel by cold rolling and a process for its preparation. The "flat products" according to the invention may be sheets, strips, blanks obtained therefrom or comparable products. If this is referred to as "cold flat products", it means flat products produced by cold rolling.

Particularly in the field of vehicle body construction, there is a demand for materials which, on the one hand, have high strengths but, on the other hand, are also so easily deformable that complex components can be formed from them by simple means.

A multiphase steel, which should have a balanced property profile in this respect, is from the EP 1 367 143 A1 known. In addition to a comparable high strength and good ductility of the known steel should also have a particularly good weldability.

The known steel contains to 0.03 - 0.25 wt .-% C, by its presence in combination with the other alloying elements tensile strengths of at least 700 MPa to be achieved. In addition, the strength of the known steel is to be supported by Mn in contents of 1.4-3.5% by weight. Al is used in the melting of the known steel as the oxidizing agent and may be present in the steel in amounts of up to 0.1% by weight. The known steel may also have up to 0.7% by weight of Si, the presence of which stabilizes the ferritic-martensitic structure of the steel. Cr is added to the known steel in amounts of 0.05-1% by weight in order to reduce the influence of the heat introduced by the welding process in the region of the weld. For the same purpose 0.005 - 0.1 wt .-% Nb are present in the known steel. Nb should additionally have a positive influence on the deformability of the steel, since its presence brings about a thinning of the ferrite grain. For the same purpose, 0.05 to 1% by weight of Mo, 0.02 to 0.5% by weight of V, 0.005 to 0.05% by weight of Ti and 0.0002 to 0.002% by weight of the known steel can be used. % B are added. Mo and V contribute to the hardenability of the known steel, while Ti and B should additionally have a positive effect on the strength of the steel.

Another, also made of a high strength multiphase steel, well malleable steel sheet is from the EP 1 589 126 B1 known. This known steel sheet contains 0.10-0.28 wt% C, 1.0-2.0 wt% Si, 1.0-3.0 wt% Mn, 0.03-0.10 Wt% Nb, up to 0.5 wt% Al, up to 0.15 wt% P, up to 0.02 wt% S. Optionally, in the steel sheet, up to 1.0 wt% Mo, up to 0.5 wt% Ni, up to 0.5 wt% Cu, to 0.003 wt% Ca, up to 0.003 Wt .-% rare earth metals, up to 0.1 wt .-% Ti or up to 0.1 wt .-% V be present. The structure of the known steel sheet, based on its overall structure, has a retained austenite content of 5 to 20% and at least 50% bainitic ferrite.

At the same time, the proportion of polygonal ferrite in the structure of the known steel sheet should be at most 30%. By limiting the proportion of polygonal ferrite in the known steel sheet bainite to form the matrix phase and Restautenit shares be present, which contribute to the balance of tensile strength and ductility. Here, too, the presence of Nb should ensure that the retained austenite content of the microstructure is fine-grained.

To ensure this effect is in the course of the production of the EP 1 589 126 B1 known steel sheet a particularly high initial temperature of hot rolling 1250-1350 ° C chosen. In this temperature range, Nb is completely solidified, so that during the hot rolling of the steel, a large number of fine Nb carbides are formed in the polygonal ferrite or bainite. It is said in the EP 1 589 126 B1 in that although the high initial temperature of hot rolling is the prerequisite for the fineness of the retained austenite, it does not alone have the desired effect. Rather, to a final annealing at temperatures above the Ac ₃ temperature, a subsequent controlled cooling with a Cooling rate of at least 10 ° C / s up to a lying in the range of 300 - 450 ° C temperature at which the bainite conversion is completed, and finally a holding at this temperature for a sufficiently long time may be required.

Against the background of the prior art described above, the object of the invention was to provide a multiphase steel having a further increased strength, which at the same time has a high elongation at break. Likewise, a flat product with a further optimized combination of high strength and good ductility and a method for producing such a flat product should be specified.

With regard to the steel, the above-mentioned object has been achieved according to the invention by a procured according to claim 1 steel.

With respect to the flat product, the solution of the above-mentioned object consists of a cold flat product formed according to claim 13.

With regard to the method, the above-mentioned object has finally been achieved according to the invention in that the working steps specified in claim 14 are run through.

Advantageous embodiments of the invention are specified in the dependent claims and will be explained in detail together with the general inventive concept.

A multiphase steel according to the invention contains (in% by weight) C: 0.14-0.25%, Mn: 1.7-2.5%, Si: 0.2-0.7%, Al: 0.5- 1.5%, Cr: <0.1%, Mo: <0.05%, Nb: 0.02-0.06%, S: up to 0.01%, especially up to 0.005%, P: to to 0.02%, N: up to 0.01%, and optionally at least one element from the group "Ti, B, V", and the remainder being iron and unavoidable impurities, with the contents of the optional elements provided being in that Ti: ≦ 0.1%, B: ≦ 0.002%, V: ≦ 0.15% and wherein at least 10% by volume of ferrite and at least 6% by volume of retained austenite are present in the structure of the steel.

A steel assembled and obtained _{according to the} invention achieves a tensile strength R _m of at least 950 MPa, a yield strength R _eL of at least 500 MPa and an elongation at break A ₈₀ in the transverse direction of at least 15%.

Carbon increases the amount and stability of retained austenite. Therefore, in the steel of the present invention, at least 0.14 wt% of carbon is present to stabilize the austenite to room temperature and to prevent complete conversion of the austenite formed in an annealing treatment into martensite, ferrite or bainite, and bainitic ferrite, respectively. However, over 0.25 wt .-% lying carbon contents have a negative effect on the weldability.

Like C, Mn contributes to the strength and increase the amount and stability of the retained austenite. However, excessive Mn levels increase the risk of segregation. They also have a negative effect on the elongation at break, since the ferrite and bainite conversions be greatly delayed and as a result comparable high levels of martensite remain in the structure. The Mn content of a steel according to the invention is set at 1.7-2.5% by weight.

In a steel according to the invention, Al are present in amounts of 0.5-1.5% by weight and Si in contents of 0.2-0.7% by weight in order to overcome the overaging treatment carried out in the course of the inventive processing of the steel to avoid carbide formation in the bainite stage. The bainite transformation does not proceed completely due to the presence of Al and Si, so that only bainitic ferrite is formed and carbide formation does not occur. In this way, the present invention desired stability of carbon-enriched retained austenite is achieved. This effect can be ensured particularly reliably by limiting the Si content to 0.6% by weight or the Al content to 0.7-1.4% by weight, with Si contents of more than are set as 0.2 wt .-% and less than 0.6 wt .-% and the Al contents between 0.7 wt .-% and 1.4 wt .-% are. In the combined presence of Si and Al, optimum properties of the multiphase steel according to the invention result when the sum of its Al and Si contents is 1.2-2.0% by weight.

Cr and Mo are undesirable in a steel according to the invention and should therefore be present only in ineffective amounts, as they delay the bainitic transformation and hinder the stabilization of the retained austenite. Therefore, according to the invention, the Cr content is on less than 0.1 wt .-% and the Mo content of a steel according to the invention to less than 0.05 wt .-%, in particular less than 0.01 wt .-%, limited.

A steel according to the invention contains Nb in amounts of 0.02-0.06% by weight and optionally one or more of the elements "Ti, V, B" in order to increase the strength of the steel according to the invention. Nb, Ti, V and B form very fine precipitates with the C and N present in the steel according to the invention. These precipitates increase strength and yield strength by particle hardening and grain refining. The grain refining is also of great advantage for the forming properties of the steel.

Ti still binds N during solidification or at very high temperatures, so that possible negative effects of this element on the properties of the steel according to the invention are reduced to a minimum. In order to utilize these effects, up to 0.1% by weight of Ti and up to 0.15% by weight of V can be added to a steel according to the invention in addition to the ever present Nb.

Exceeding the inventively given upper limits of the contents of micro-alloying elements would lead to the delay of the recrystallization during annealing, so that they either can not be realized in real production or would require an additional furnace performance.

wobei mit "%N" der jeweilige N-Gehalt des Mehrphasenstahls bezeichnet ist und diese Bedingung insbesondere dann einzuhalten ist, wenn der Ti-Gehalt 0,01 - 0,03 Gew.-% beträgt.The positive influence of the presence of Ti in relation to the setting of the N content can then be particularly used purposefully if the Ti content "% Ti" of a multiphase steel according to the invention fulfills the following condition [3]: $$\mathrm{\%}\mathrm{Ti}\mathrm{\ge }\mathrm{3}\mathrm{.}\mathrm{4}\mathrm{x}\mathrm{\%}\mathrm{N}\mathrm{.}$$

where "% N" denotes the respective N content of the multiphase steel and this condition is to be observed, in particular, when the Ti content is 0.01-0.03 wt%.

The positive effect of Ti in a steel according to the invention occurs particularly reliably when its Ti content is at least 0.01% by weight.

The addition of up to 0.002 wt .-% boron, the ferrite formation can be delayed upon cooling, so that a larger amount of austenite is present in the bainite. As a result, the amount and the stability of the retained austenite can be increased. In addition, bainitic ferrite is formed instead of normal ferrite, which contributes to increasing the yield strength.

Practical variants of the steel according to the invention which are particularly advantageous with regard to the costs and the property profile of the steel according to the invention result if the Ti content is limited to 0.02% by weight and B in contents of 0.0005-0.002% by weight. % or V are present at levels of 0.06 - 0.15% by weight.

At least 10% by volume of ferrite, in particular at least 12% by volume of ferrite, and at least 6% by volume of retained austenite are present in the structure of a steel according to the invention, in order to ensure the desired high strength on the one hand and good ductility on the other hand. Depending on the amount of the remaining structural constituents, up to 90% by volume of the structure of ferrite and up to a maximum of 20% by volume of retained austenite may exist for this purpose. Contents of at least 5 vol.% Martensite in the structure of the steel according to the invention contribute to its strength, wherein the martensite content to max. 40 vol .-% should be limited to ensure sufficient extensibility of the steel according to the invention. In this case, optionally 5 to 40% by volume of bainite can be present in the microstructure of a steel according to the invention.

mit a_γ: 0,3578 nm (Gitterkonstante des Austenits); a_RA: jeweiliger Gitterparameter des Restaustenits nach der Endabkühlung in nm am fertigen Kaltband gemessen.The retained austenite of a steel according to the invention is preferably enriched in carbon such that it is present in accordance with the method described in the article of A. Zarei Hanzaki et al. in ISIJ Int. Vol. 35, No 3, 1995, pp. 324 - 331 published formula [1] calculated C _inRA content is more than 0.6 Ges .-%. $${\mathrm{C}}_{\mathrm{INRA}}\mathrm{=}\left({\mathrm{a}}_{\mathrm{RA}}\mathrm{-}{\mathrm{a}}_{\mathrm{\gamma }}\right)\mathrm{/}\mathrm{0}\mathrm{.}\mathrm{0044}$$

with a _γ : 0.3578 nm (lattice constant of austenite); a _RA : respective lattice parameter of the retained austenite after final cooling in nm measured on the finished cold strip.

The amount of carbon present in the retained austenite substantially affects the TRIP properties and ductility of a steel according to the invention. Accordingly, it is advantageous if the C _inRA content is as high as possible.

mit
%RA: Restaustenit-Gehalt des Mehrphasenstahls in Vol.%;
C_inRA:C-Gehalt des Restaustenits berechnet gemäß Formel [1].With regard to the desired high stability of the retained austenite, it is furthermore advantageous if it has a quality G _{RA of} the retained austenite ("retained austenite quality") of more than 6, in particular more than 8, calculated according to the formula [2]. $${\mathrm{G}}_{\mathrm{RA}}=\%{\mathrm{RA\; x\; C}}_{\mathrm{INRA}}$$

With
% RA: retained austenite content of the multiphase steel in% by volume;
C _inRA : C content of _retained austenite calculated according to formula [1].

A cold-rolled flat product of the type according to the invention can be produced according to the invention by melting an inventive multi-phase steel in the first working step and casting it into a preliminary product. This precursor may be a slab or thin slab.

The precursor is then, if necessary, reheated to a temperature of 1100-1300 ° C, from which the precursor is then hot rolled into a hot strip. The final temperature of the hot rolling is according to the invention 820-950 ° C. The resulting hot strip is wound into a coil at a reel temperature of 400-750 ° C., in particular 530-600 ° C.

In order to improve the cold rolling capability of the hot strip, the hot strip may be subjected to annealing after being reeled and before being cold rolled. This can be advantageously carried out as a bell annealing or completed in a continuous flow annealing. The annealing temperatures set in the annealing preparatory to cold rolling are typically 400-700 ° C.

After coiling, the hot strip at cold rolling degrees of 30 - 80%, especially 50 - 70%, cold rolled to a cold rolled product, with cold rolling degrees of 30 - 75%, especially 50 - 65% lead particularly safe to the desired result. The resulting cold-rolled product is then subjected to a heat treatment in which it undergoes a continuous annealing at a 750 - 900 ° C, in particular 800 - 830 ° C, amounting annealing temperature, then at 350 - 500 ° C, especially 370 - 460 ° C. to be subjected to an overaging treatment. The annealing time over which the cold flat product is annealed in the course of continuous annealing at the annealing temperature is typically 10 - 300 s, while the duration of the overaging treatment after annealing can be up to 800 s, with the minimum annealing time generally being 10 s becomes.

Optionally, the annealed cold rolled product may be quenched between annealing and overaging to obtain a return to ferrite and to suppress the formation of perlite. Starting from the Annealing temperature up to an intermediate temperature of 500 ° C, the set cooling rate can be at least 5 ° C / s. Thereafter, if necessary, the cold flat product is held at the intermediate temperature over a period sufficient for the formation of the desired microstructure, to which the cold flat product is then further cooled.

The annealing of the cold flat product can be carried out in the course of a fire coating, in which the cold flat product is provided with a metallic protective coating.

It is likewise possible to provide the cold strip produced according to the invention with a protective layer after the heat treatment by electrolytic coating or another deposition method.

Additionally or alternatively, it may also be expedient to cover the cold-rolled product with an organic protective layer.

Optionally, the cold strip obtained can also be subjected to re-rolling at degrees of deformation of up to 10% in order to improve its dimensional stability, surface finish and mechanical properties.

To demonstrate the properties of sheets produced and produced according to the invention, those shown in Table 1 are shown Melted melts S1 to S13 melted and processed into cold rolled products K1 - K41.

• Erschmelzen und Vergießen der Schmelzen S1 - S13 zu jeweils einer Dünnbramme;
• Warmwalzen der Dünnbramme des Vorprodukts ausgehend von einer Anfangstemperatur WAT und endend bei einer Endtemperatur WET zu einem Warmband;
• Haspeln des Warmbands bei einer Haspeltemperatur HT;
• Kaltwalzen des Warmbands nach dem Haspeln bei Kaltwalzgraden KWG zum jeweiligen Kaltflachprodukt K1 - K41;
• Durchlaufglühen des Kaltflachprodukts bei einer Glühtemperatur GT innerhalb einer Glühdauer Gt;
• Überaltern des Kaltflachprodukts bei einer betragenden Überalterungstemperatur UA T über eine Überalterungsdauer UA t.

The production of cold flat products K1 - K41 included the following steps:

• Melting and casting of the melts S1 - S13 into one thin slab each;
• Hot rolling the thin slab of the precursor from an initial temperature WAT and ending at a final temperature WET to a hot strip;
• Coiling the hot strip at a reel temperature HT;
• Cold rolling of the hot strip after reeling with cold rolling degrees KWG to the respective cold flat product K1 - K41;
• Continuous annealing of the cold flat product at an annealing temperature GT within an annealing time Gt;
• Overaging of the cold-rolled product at an excess aging temperature UA T over an overaging period UA t.

Table 2 shows the respectively set parameters "annealing temperature GT", "annealing time Gt", "cooling rate V after annealing", "aging temperature UA T" and "aging time UA t" for annealing and overaging cycles 1-15.

The cold-rolled products K1 - K41, which are cold strips or sheets, are incidentally produced The set parameters, the selected annealing cycle as well as the properties of the obtained cold strips K1 - K41 are listed in Table 3. Table 1 (content by weight in%, balance iron and unavoidable impurities) melt C Si Mn al Nb V Ti P S N B According to the invention? S1 0.210 0.41 1.82 1,020 0,041 0,004 0.005 0,004 0,003 0.0015 0.0005 YES S2 0,250 0.42 1.79 0.970 0,044 0,006 0,003 0.005 0,004 0.0041 0.0004 YES S3 0.230 0.42 2.48 0.980 0,042 0.005 0,015 0,006 0.005 0.0016 0.0004 YES S4 0,220 0.42 2.27 0.98 0,040 0.011 0,015 0,004 0,003 0.0016 0.0016 YES S5 0.231 0.70 1.83 1,020 0,044 0,120 0,006 0,004 0,003 0.0015 0.0005 YES S6 0,220 0.40 1.83 1.03 0,045 0,006 0,003 0,004 0.005 0.0011 0.0006 YES S7 0.231 0.40 1.90 1,400 0,025 0,100 0,007 0,004 0,004 0.0013 0.0004 YES S8 0.215 0.41 2.23 0.970 0.058 0.005 0,004 0,003 0,004 0.0014 0.0005 YES S9 0.222 0.40 1.80 1.01 0,045 0.10 0,003 0,004 0,004 0.0017 0.0005 YES S10 0,220 0.65 1.95 1,250 0,029 0,006 0.019 0.005 0,003 0.0016 0.0013 YES S11 0.215 0.41 2.24 0.91 0,041 0.11 0,004 0.005 0,003 0.0016 0.0005 YES S12 0,220 0.35 2.50 1,230 0.027 0.005 0,017 0.005 0,003 0.0016 0.0010 YES S13 0.226 0.41 1.81 1.03 0,003 0.005 0.001 0,003 0.005 0.0013 0.0006 NO Annealing cycle no. GT [° C] Gt [s] V [° C / s] UA T [° C] UA t [s] 1 820 60 15 375 60 2 820 60 15 375 120 3 820 60 15 375 360 4 820 60 15 425 30 5 820 60 15 425 60 6 820 60 15 425 120 7 820 60 15 450 30 8th 820 60 15 450 60 9 820 60 15 450 120 10 820 60 50 425 30 11 820 60 50 425 60 12 820 60 50 425 120 13 820 60 100 425 120 14 840 60 100 425 120 15 860 60 100 425 120 melt Incandescent. No. WAT [° C] WET [° C] HT [° C] KWG [%] R _el [MPa] R _m [MPa] A ₈₀ [%] RA [Vol%] C in _RA [% by weight] Goodness RA a _Ra [nm] According to the invention? K1 S1 1 1250 940 600 65 512 975 23.1 18.0 0.76 13.68 .3611 Yes K2 S1 2 1260 940 610 68 550 1002 23.7 17.0 0.78 13.26 .3612 Yes K3 S1 3 1250 930 620 63 561 963 24, 6 16.5 0.81 13.37 .3614 Yes K4 S2 13 1300 930 700 63 614 1070 18.2 15.0 0, 91 13.65 .3618 Yes K5 S2 14 1140 950 690 55 603 1050 23.1 15.5 0.93 14.42 .3619 Yes K6 S2 15 1250 870 400 56 580 1020 23.6 17.0 0, 94 15, 98 .3619 Yes K7 S3 10 1160 860 430 52 552 1103 15.5 15.0 0, 65 9.75 .3607 Yes K8 S3 11 1180 870 420 55 584 1070 17.1 17.5 0.74 12.95 .3611 Yes K9 S3 12 1180 920 560 54 570 1007 18.2 18.0 0.78 14.04 .3612 Yes K10 S4 10 1190 920 560 63 509 964 16.1 15.5 0.73 11.32 .3610 Yes K11 S4 11 1170 910 550 75 592 990 18.5 18.0 0.82 14.76 .3614 Yes K12 S4 12 1260 910 530 73 548 1050 21.4 19.0 0.80 15.20 .3613 Yes K13 S4 14 1240 820 450 30 517 1035 25, 6 13.0 0, 95 12.35 .3620 Yes K14 S5 7 1300 940 560 54 503 981 18.1 16.5 0.78 12.87 .3612 Yes K15 S5 8th 1250 830 450 45 524 968 19.3 17.5 0.83 14.53 .3615 Yes K16 S5 9 1140 850 460 50 563 1003 20.8 18.0 0.85 15,30 .3615 Yes K17 S6 4 1150 900 500 50 532 1010 25.9 18.0 0.84 15,12 .3615 Yes K18 S6 5 1300 900 530 56 575 986 26.6 16.5 0.91 15.02 .3618 Yes K19 S6 6 1290 930 530 53 584 978 28, 0 16.5 0.95 15.68 .3620 Yes K20 S7 4 1280 920 540 54 520 965 22.1 17.5 0.76 13,30 .3611 Yes K21 S7 5 1280 930 700 56 536 954 22.5 18.0 0.81 14.58 .3614 Yes K22 S7 6 1290 910 650 58 587 992 21.4 18.5 0.84 15.54 .3615 Yes K23 S8 13 1150 880 430 60 571 997 20.7 14.5 0.91 13,20 .3618 Yes K24 S8 14 1150 870 460 65 525 981 22.4 15.0 0.95 14.25 .3620 Yes K25 S8 15 1100 880 460 45 521 962 24.1 15.5 0, 94 14.57 .3619 Yes K26 S9 4 1160 930 660 63 511 1009 18.7 17.0 0.77 13,09 .3612 Yes K27 S9 5 1230 950 650 62 526 1021 19.5 17.5 0.82 14.35 .3614 Yes K28 S9 6 1230 950 650 70 574 1019 21.2 18.5 0.86 15.91 .3616 Yes K29 S10 10 1170 940 680 75 510 1003 20.1 17.5 0.79 13.83 .3613 Yes K30 S10 11 1240 930 560 64 564 997 21.6 18.5 0.84 15.54 .3615 Yes K31 S10 12 1200 850 490 55 589 1011 22.2 18.5 0.88 16.28 .3617 Yes K32 S11 10 1190 860 470 46 545 1130 15.5 15.0 0.70 10.50 .3609 Yes K33 S11 11 1190 870 470 35 529 1062 16.7 17.0 0.82 13, 94 .3614 Yes K34 S11 12 1150 910 530 49 602 1018 18.1 18.0 0, 80 14,40 .3613 Yes K35 S11 14 1160 920 520 51 608 993 23.4 13.5 0.93 12.56 .3619 Yes K36 S12 10 1140 910 520 52 542 1089 15.9 15.5 0.65 10.08 0, 3607 Yes K37 S12 11 1200 920 530 50 583 1054 18.1 17.0 0, 63 10.71 .3606 Yes K38 S12 12 1210 930 560 49 589 1023 19.4 18.5 0, 67 12.40 .3607 Yes K39 S13 7 1210 940 700 70 404 796 30.0 19.5 0.91 17.75 .3618 No K40 S13 8th 1220 860 410 45 440 763 27.0 18.0 0.93 16.74 .3619 No K41 S13 9 1230 870 420 60 453 775 25.4 17.5 0, 95 16, 63 .3620 No

Claims (16)

Multiphase steel containing (in% by weight)
C: 0.14-0.25%
Mn: 1, 7 - 2.5%
Si: 0.2 - 0.7%
Al: 0.5 - 1.5%
Cr: <0.1%
Mo: <0.05%
Nb: 0.02-0.06%
S: up to 0.01%
P: up to 0.02%
N: up to 0.01%
and optionally at least one element from the group "Ti, B, V" according to the following proviso: Ti: up to 0.1% B: up to 0.002% V: up to 0.15% The remainder being iron and unavoidable impurities, wherein at least 10% by volume of ferrite and at least 6% by volume of retained austenite are present in the structure of the steel and the steel has a tensile strength R _m of at least 950 MPa, a yield strength R _eL of at least 500 MPa and a in the transverse direction measured breaking elongation A ₈₀ of at least 15%. Multiphase steel according to claim 1, characterized in that the C _inRA content of the retained austenite calculated according to the formula [1] is more than 0.6% by weight: $${\mathrm{C}}_{\mathrm{INRA}}\mathrm{=}\left({\mathrm{a}}_{\mathrm{RA}}\mathrm{-}{\mathrm{a}}_{\mathrm{\gamma }}\right)\mathrm{/}\mathrm{0}\mathrm{.}\mathrm{0044}$$

With
a _γ : 0.3578 nm (lattice constant of austenite);
a _RA : lattice parameters of retained austenite in the finished multiphase steel after final cooling in nm. Multiphase steel according to Claim 2, characterized in that it has a quality G _RA of retained austenite calculated according to the formula [2], for which G _RA > 6: $${\mathrm{G}}_{\mathrm{RA}}=\%{\mathrm{RA\; x\; C}}_{\mathrm{INRA}}$$

With %
RA: retained austenite content of the multiphase steel in% by volume;
C _inRA : C content of _retained austenite calculated according to formula [1]. Polyphase steel according to one of the preceding claims, characterized in that the sum of its Al and Si contents is 1.2-2.0% by weight. Multiphase steel according to one of the preceding claims, characterized in that its Si content is less than 0.6 wt .-%. Multiphase steel according to one of the preceding claims, characterized in that its Al content is 0.7-1.4% by weight. Multiphase steel according to one of the preceding claims, characterized in that its Ti content is up to 0.02 wt .-%. Multi-phase steel according to one of the preceding claims, characterized in that its Ti content% Ti satisfies the condition [3]: $$\mathrm{\%}\mathrm{Ti}\mathrm{\ge }\mathrm{3}\mathrm{.}\mathrm{4}\mathrm{x}\mathrm{\%}\mathrm{N}$$

with% N: N content of the multiphase steel. Multi-phase steel according to one of the preceding claims, characterized in that it contains at least 0.0005 wt .-% B. Multiphase steel according to one of the preceding claims, characterized in that it contains at least 0.06% by weight of V. Multi-phase steel according to one of the preceding claims, characterized in that its structure has a martensite content of at least 5% by volume. Multi-phase steel according to one of the preceding claims, characterized in that its structure has a bainite content of 5 to 40% by volume. Cold flat product produced from a multi-phase steel obtained according to one of claims 1 to 11. Method for producing a cold-rolled product according to claim 13, in which the following work steps are completed: Melting and casting of a multiphase steel obtained according to any one of claims 1 to 11 into a precursor; - Hot rolling of the precursor starting from a 1100 - 1300 ° C amounting initial temperature and ending at a final temperature of 820-950 ° C to form a hot strip; - Coiling of the hot strip at a 400 - 750 ° C reel temperature; optional annealing of the hot strip to improve cold rollability; after cold-rolling, from 30 to 80% cold-rolling of the hot strip to the cold-rolled product; Continuous annealing of the cold-rolled product at a calcination temperature of 750 to 900 ° C; optionally accelerated cooling of the pass-annealed cold-rolled product; - Overaging of the cold-rolled product at an over-aging temperature of 350 - 500 ° C. A method according to claim 14, characterized in that the coiling temperature 530-600 ° C, the cold rolling degree 50-70%, the annealing temperature 800-830 ° C or the overaging temperature 370-460 ° C. Method according to one of claims 14 or 15, characterized in that the optionally carried out after the reeling and before the cold rolling annealing as Haubenglühung or as Continuous annealing is carried out at an annealing temperature of 400 - 700 ° C.