US20150218684A1

US20150218684A1 - Cold-Rolled Flat Steel Product and Method for the Production Thereof

Info

Publication number: US20150218684A1
Application number: US14/417,659
Authority: US
Inventors: Brigitte Hammer; Thomas Heller; Frank Hisker; Rudolf Kawalla; Grzegorz Korpala
Original assignee: ThyssenKrupp Steel Europe AG
Current assignee: ThyssenKrupp Steel Europe AG
Priority date: 2012-07-27
Filing date: 2013-07-26
Publication date: 2015-08-06
Also published as: EP2690184B1; EP2690184A1; JP6202579B2; JP2015528065A; WO2014016421A1; CN104641008A

Abstract

A cold-rolled flat steel product where Rm≧1400 MPa and A80≧5% and also a method for producing such product. The product includes, in addition to Fe and unavoidable impurities (in wt. %), 0.10-0.60% C, 0.4-2.5% Si, ≦3.0% Al, 0.4-3.0% Mn, ≦1.0% Ni, ≦2.0% Cu, ≦0.4% Mo, ≦% Cr, ≦1.5% Co, ≦0.2% Ti, ≦0.2% Nb, ≦0.5% V. The microstructure includes (in vol. %) ≧20% bainite, 10-35% residual austenite and martensite as the remainder. A slab, thin slab or a cast strip having said composition, is hot-rolled to form hot strip with a hot-rolling end temperature ≧830° C., coiled at a coiling temperature ≦560° C., cold-rolled at ≧30% reduction and heat-treated by firstly being heated to an annealing temperature ≧800° C., then being cooled at a cooling rate of ≧8° C./s to a holding temperature of 470° C. to greater than the martensite start temperature and then being held at the holding temperature until at least 20 vol. % bainite is present in the microstructure.

Description

The invention relates to a cold-rolled flat steel product having a tensile strength Rm of at least 1400 MPa and an elongation A80 of at least 5%. Products of this type are distinguished 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 similarly relates to a method for producing a flat steel product according to the invention.
The term “flat steel product” is to be understood here as meaning steel sheets or steel strips produced by a rolling process and also sheet bars and the like separated therefrom.
Where alloy contents are stated here merely in “%”, this always means “% by weight”, unless expressly stated otherwise.
EP 1 466 024 B1 (DE 603 15 129 T2) discloses a method for producing a flat steel product which is intended to have tensile strengths of considerably more than 1000 MPa. In order to achieve this, a steel melt comprising (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 as remainder iron and impurities which are unavoidable for production-related reasons is produced. The melt is cast to form a strip, the thickness of which is at most 10 mm and which is cooled rapidly to a temperature of at most 1000° C. by sprinkling with water or a water-air mixture. Then, the cast strip is hot-rolled with a conventional reduction rate. The hot-rolling is ended at an end temperature at which all of the copper is still in a solid solution in the ferrite and/or austenite matrix. Then, the strip is subjected to a step of rapid cooling, in order to keep the copper in a supersaturated solid solution in the ferrite and/or austenite solution. After coiling to form a coil, a cold strip can be rolled from the hot strip thus obtained with a degree of cold-rolling amounting to 40-80%. This cold strip is then subjected to recrystallization annealing, during which it is brought as rapidly as possible to an annealing temperature lying in the region of 840° C. and held at said temperature, in order to bring the greatest possible proportion of the copper present in the steel into solution. This is followed by rapid cooling to a temperature amounting to 400-700° C., at which Cu precipitations form once again. In this way, precipitation hardening is intended to achieve the desired strength level of the steel. At the same time, the copper content is intended to increase the corrosion and embrittlement resistance of the steel through the formation of a protective oxide layer.
A further method for producing a cold strip of extreme strength is known from U.S. Pat. No. 7,591,977 B2. According to this method, a hot strip comprising (in % by weight) 0.1-0.25% C, 1.0-2.0% Si and 1.5-3.0% Mn is rolled with a degree of cold-rolling of 30-70% to form a cold strip, which is then subjected to a heat treatment completed in a continuous pass. In this heat treatment, the cold strip is heated, in a first annealing step, to a first annealing temperature lying above the Ar3 temperature thereof, in order to bring carbides present in the cold strip into solution. This is followed by cooling, proceeding from the first annealing temperature and being effected at a cooling rate of at least 10° C./s, to a second annealing temperature. This temperature is selected such that bainite forms in the cold strip, and typically lies in the range of 300-450° C. This second annealing step carried out to form bainite is performed until the microstructure of the cold strip consists of bainite to an extent of at least 60% and of residual austenite to an extent of at least 5% and also of polygonal ferrite as remainder. The aim here is for the microstructure to be bainitic to the fullest possible extent and for other microstructure constituents to be present at most in traces. The cold strip thus provided achieves tensile strengths of up to 1180 MPa combined with an elongation of at least 9% and can be coated, if required, with a metallic layer affording protection against corrosion.
Against the background of the prior art explained above, it was an object of the invention to provide a cold-rolled flat steel product which can be produced in a simple and operationally reliable manner and has an optimized combination of a further increased strength and good deformability. In addition, the intention was to provide a method for producing such a cold-rolled flat steel product.
In relation to the cold-rolled flat steel product, this object has been achieved according to the invention by the flat steel product indicated in claim 1.
In relation to the method, the object mentioned above is achieved according to the invention in that at least the working steps indicated in claim 12 are performed to produce a cold-rolled flat steel product according to the invention.
Advantageous configurations of the invention are indicated in the dependent claims and will be explained in detail hereinbelow as the general concept of the invention.
The cold-rolled flat steel product according to the invention is distinguished by the fact that it comprises, in addition to iron and unavoidable impurities (in % by weight):

- C: 0.10-0.60%,
- Si: 0.4-2.5%,
- Al: up to 3.0%,
- Mn: 0.4-3.0%,
- Ni: up to 1.0%,
- Cu: up to 2.0%,
- Mo: up to 0.4%,
- Cr: up to 2%,
- Co: up to 1.5%,
- Ti: up to 0.2%,
- Nb: up to 0.2%,
- V: up to 0.5%.

Here, in the cold-rolled state, the microstructure of the flat steel product according to the invention consists of bainite to an extent of at least 20% by volume, of residual austenite to an extent of 10-35% by volume and of martensite as remainder, it being self-evident that technically unavoidable traces of other microstructure constituents may be present in the microstructure of the flat steel product. A cold-rolled flat steel product according to the invention provided in this way regularly achieves tensile strengths Rm of at least 1400 MPa and an elongation A80 of at least 5%. The C content of the residual austenite is typically more than 1.0% by weight.
The method according to the invention for producing a flat steel product provided or being composed according to the invention comprises the following working steps:

- providing a preliminary product in the form of a slab, thin slab or a cast strip, which, in addition to iron and unavoidable impurities, comprises (in % by weight) C: 0.10-0.60%, Si: 0.4-2.5%, Al: up to 3.0%, Mn: 0.4-3.0%, Ni: up to 1.0%, Cu: up to 2.0%, Mo: up to 0.4%, Cr: up to 2%, Co: up to 1.5%, Ti: up to 0.2%, Nb: up to 0.2%, V: up to 0.5%;
- hot-rolling the preliminary product to form a hot strip in one or more rolling passes, wherein the hot strip obtained has a hot-rolling end temperature of at least 830° C. when it leaves the last rolling pass;
- coiling the hot strip obtained at a coiling temperature which lies between the hot-rolling end temperature and 560° C.;
- cold-rolling the hot strip to form a cold strip with a degree of cold-rolling of at least 30%;
- heat-treating the cold strip obtained, wherein, during the course of the heat treatment, the cold strip
- is heated to an annealing temperature amounting to at least 800° C.,
- is optionally held at the annealing temperature for an annealing duration of 50-150 s,
- is cooled proceeding from the annealing temperature at a cooling rate amounting to at least 8° C./s to a holding temperature which lies in a holding temperature range having an upper limit of 470° C. and having a lower limit which is higher than the martensite start temperature MS, from which martensite forms in the microstructure of the cold strip, and
- is held in the holding temperature range for a period of time which is sufficient to form at least 20% by volume bainite in the microstructure of the cold strip.

A steel strip according to the invention has a three-phase microstructure, the dominant constituent of which is bainite and which moreover consists of residual austenite and also of martensite as remainder. It is optimal here that the bainite proportion is at least 50% by volume, in particular at least 60% by volume, and that the residual austenite proportion lies in the range of 10-25% by volume, with the remainder of the microstructure being made up here too in each case by martensite. The optimum martensite proportion is at least 10% by volume. A microstructure having such a composition brings about the best combination of Rm*A80 with the required tensile strength.
In addition to the main components “bainite”, “residual austenite” and “martensite”, it is possible for contents of other microstructure constituents to be present, but the proportions of these are too low to have an influence on the properties of the cold strip according to the invention. The residual austenite is present in a cold strip according to the invention predominantly in film form with small, globular islands of block residual austenite having a grain size of <5 μm, such that the residual austenite has a high stability in the initial state and, associated therewith, a low tendency towards undesirable transformation into martensite. At higher degrees of deformation, martensite is formed from this residual austenite (TRIP effect), and this increases the elongation at break.
Cold strip produced according to the invention regularly achieves tensile strengths Rm of more than 1400 MPa, with elongations A80 which similarly regularly lie above 5%. Accordingly, the quality Rm*A80 of flat steel products according to the invention is regularly above 7000 MPa*%, with qualities Rm*A80 of at least 13 500 MPa*% typically being achieved. A cold strip according to the invention as such has an optimum combination of extreme strength and sufficient deformability.
The martensite start temperature, i.e. the temperature from which martensite forms in steel processed according to the invention, can be calculated on the basis of the procedure explained in the article entitled “Thermodynamic Extrapolation and Martensite-Start-Temperature of Substitutionally Alloyed Steels” by H. Bhadeshia, published in Metal Science 15 (1981), pages 178-180.
In the steel according to the invention, carbon delays the transformation to ferrite/pearlite, reduces the martensite start temperature MS and contributes to an increase in the hardness. In order to utilize these positive effects, the C content of the flat steel product according to the invention can be set to at least 0.25% by weight, in particular at least 0.27% by weight or at least 0.28% by weight, it being possible for the effects achieved by the comparatively high carbon content to be utilized particularly reliably when the C content lies in the range of >0.25-0.5% by weight, in particular 0.27-0.4% by weight or 0.28-0.4% by weight.
The strength-increasing action of copper can also be utilized in a cold-rolled flat steel product according to the invention. In this respect, a minimum Cu content of 0.15% by weight, in particular at least 0.2% by weight Cu, can be present in the flat steel product according to the invention. Cu makes a particularly effective contribution to the strength if it is present in the flat steel product according to the invention in contents of at least 0.55% by weight, it being possible for negative effects of the presence of Cu to be limited by virtue of the fact that the Cu content is limited to at most 1.5% by weight.
In the steel processed according to the invention, Mn in contents of at least 0.4% by weight and up to 3% by weight, in particular up to 2.5% by weight, promotes the bainite formation, the Cu, Cr and Ni contents which are optionally additionally present likewise contributing to the formation of bainite. Depending on the respective other constituents of the steel processed according to the invention, it can be expedient here to limit the Mn content to at most 2% by weight or to increase the minimum Mn content to 1.5% by weight.
The optional addition of Cr can also lower the martensite start temperature and suppress the tendency of the bainite to transform into pearlite or cementite. Moreover, in contents up to the upper limit of at most 2% by weight as predefined according to the invention, Cr promotes the ferritic transformation, with optional effects of the presence of Cr in the cold-rolled flat steel product according to the invention arising when the Cr content is limited to 1.5% by weight. The positive influence of Cr can be utilized particularly effectively if at least 0.3% by weight Cr is present in the flat steel product according to the invention.
The addition of Ti, V or Nb, which is likewise optional, can support the formation of a finer-grained microstructure and promote the bainitic transformation. In addition, these microalloying elements contribute to an increase in the hardness through the formation of precipitations. The positive effects of Ti, V and Nb can be utilized in a particularly effective manner in the cold-rolled flat steel product according to the invention when the content of each of these elements lies in the range of 0.002-0.15% by weight, in particular does not exceed 0.1% by weight.
Si is present in a flat steel product according to the invention in contents of 0.4-2.5% by weight and brings about a considerable solid solution solidification. In order to utilize this effect in a particularly reliable manner, the Si content can be set to at least 1.0% by weight. Similarly, to avoid negative influences, it may be expedient to limit the Si content to at most 2% by weight.
In the steel processed according to the invention, Al can partly replace the Si content. At the same time, Al, like Si, has a deoxidizing action during the steel production. For this purpose, a minimum Al content of 0.01% by weight can be provided. Higher contents of Al prove to be expedient, for example, when the addition of Al is intended to set the hardness or tensile strength of the steel to a relatively low value in favour of improved deformability.
A further function of Si and Al consists in suppressing the carbide formation in the bainite and therefore stabilizing the residual austenite by dissolved C down to low temperatures.
The positive influences of the simultaneous presence of Al and Si can thereby be utilized particularly effectively when the Si and Al contents within the limits predefined according to the invention satisfy the following condition: % Si+0.8% Al>1.2% by weight (where % Si: respective Si content in % by weight, % Al: respective Al content in % by weight).
The formation of the microstructure predefined according to the invention can be ensured in particular by virtue of the fact that the Mn, Cr, Ni, Cu and C contents of the steel processed according to the invention and accordingly the Mn, Cr, Ni, Cu and C contents of the flat steel product according to the invention satisfy the following condition
1<0.5% Mn+0.167% Cr+0.125% Ni+0.125% Cu+1.334% C<2
where % Mn denotes the respective Mn content in % by weight, % Cr denotes the respective Cr content in % by weight, % Ni denotes the respective Ni content in % by weight, % Cu denotes the respective Cu content in % by weight and % C denotes the respective C content in % by weight.
To produce a flat steel product according to the invention, the primary or preliminary product cast from a steel having a composition according to the invention is firstly brought to a temperature or held at a temperature which is sufficient to end the hot-rolling carried out proceeding from this temperature at a hot-rolling end temperature lying in the range of 830-1000° C. After it leaves the last rolling stand used for the hot-rolling, the hot strip cools down on the roller table adjoining the rolling stand in question. Subsequent to the roller table, the hot strip passes into a coiling device, in which it is wound to form a coil.
The coiling temperature has to be at least 560° C., so that a relatively soft hot strip microstructure consisting of ferrite and pearlite is formed. A temperature profile which is optimal for this purpose arises if the hot-rolling end temperature lies in the range of 850-950° C., in particular in the range of 880-950° C. To this end, it is typically the case that the preliminary product is heated to a temperature lying in the range of 1100-1300° C. or is held at this temperature before the hot-rolling. The microstructure of the hot strip thus obtained consists primarily of ferrite and pearlite. The risk of grain boundary oxidation arising can be minimized by virtue of the fact that the coiling temperature is limited to at most 750° C.
After the coiling, the hot strip is cold-rolled, it going without saying that the hot strip can be conventionally descaled by chemical or mechanical means before the cold-rolling.
The cold-rolling is effected with a degree of cold-rolling of at least 30%, in particular at least 45%, in order to accelerate the recrystallization and transformation during the subsequent annealing. It is generally the case that a better surface quality is also obtained by observing a correspondingly high degree of cold-rolling. Degrees of cold-rolling of at least 50% have proved to be particularly favourable for this purpose.
After the cold-rolling, the cold strip obtained according to the invention completes an annealing cycle in a continuous pass, during which it is heated in a first annealing phase to a temperature of at least 800° C., preferably at least 830° C. This first annealing phase lasts at least for such a period of time that the cold strip is completely austenitized. 50-150 s are typically required for this.
At the end of the first annealing phase, the product is quenched, the cooling rate being at least 8° C./s, in particular 10° C./s. The target temperature for this quenching is a holding temperature which is at most 470° C. and is higher than the martensite start temperature MS, from which martensite forms in the microstructure of the cold strip. In practice, the range of 300-420° C., in particular 330-420° C., can be used as an indication of the range in which the holding temperature is to lie.
Proceeding from the respective holding temperature, the cold strip is held in the holding temperature range in the second annealing phase, to be precise until at least 20% by volume of the microstructure of the cold strip has transformed into bainite. The hold can be carried out here as an isothermal hold at the holding temperature reached during the cooling or as a slow decrease in temperature within the holding temperature range.
The flat steel product produced according to the invention can be coated in a conventional manner with a metallic protective layer. This can be effected by hot-dip coating, for example. If annealing is required before the application of the metallic coating, the heat treatment provided according to the invention can be carried out in the course of this annealing.
The invention will be explained in more detail hereinbelow on the basis of exemplary embodiments.
Five steels S1-S5 were melted, the composition thereof being shown in Table 1.
The steel melts of corresponding composition were cast in a conventional manner to form a strand, from which slabs were separated. The slabs were then heated in a similarly conventional manner to a reheating temperature.
The heated slabs were hot-rolled in a similarly conventional group of hot-rolling stands to form hot strips having a thickness of 2 mm.
The hot-rolling end temperature was in the range of 830-900° C. in each case. The hot strips were cooled proceeding from this temperature to a coiling temperature lying above 560° C. and then coiled to form coils.
The hot strips thus obtained were descaled after the coiling and cold-rolled to form cold strip with degrees of cold-rolling of 50% after the descaling.
A relatively large number of specimens of these cold strips were then subjected to a heat treatment, in which they were heated in a first annealing step at a heating rate of at least 1.9° C./s to a first annealing temperature in the range of 830-850° C. The cold strips were held at this temperature for a period of time of 120 s, until they had been completely heated through.
This was followed by quenching, during which cold strips were quenched at a cooling rate amounting to at least 8° C./s to a holding temperature T2 in the range of 350-420° C. Specifically, the holding temperatures T2 in a first batch of tests were 300° C., 310° C., 330° C., 340° C., 375° C., 390° C. and 410° C. The cold strip specimens were held at the respective holding temperature T2 for an annealing duration t2.
In FIG. 1, the tensile strengths Rm achieved are plotted against the respective annealing temperature T2. It can be seen that the cold strip specimens produced from the steel S5 each achieve the required minimum tensile strength of 1400 MPa only under certain annealing conditions, whereas the tensile strengths of the cold strip specimens produced from the other steels were always reliably above the minimum limit of 1400 MPa. The comparatively low carbon content of the steel S5, lying at the lower limit of the content range predefined according to the invention, has been identified as the reason for this.
In FIG. 2, the tensile strengths of the cold strip specimens produced from the steel S4 are plotted against the annealing duration t2 of the second annealing stage. It can be seen that the cold strip specimens held at a holding temperature of 310° C., 330° C. and 350° C., i.e. in the holding temperature range of 310-350° C., achieved the required tensile strength Rm of 1400 MPa, irrespective of the respective annealing duration t2.
In FIG. 3, the tensile strengths of the cold strip specimens produced from the steel S5 are similarly plotted against the annealing duration t2 of the second annealing stage. It can be seen here that the cold strip specimens held at a holding temperature of 350° C. and 390° C., i.e. in the holding temperature range of 350-390° C., achieve the required tensile strength Rm of 1400 MPa if the annealing duration t2 is shorter than 145 s.
In FIG. 4, the elongation A80 of the cold strip specimens produced from the steel S4 is plotted against the annealing duration t2 of the second annealing stage. The cold strip specimens held at a holding temperature of 310° C., 330° C. and 350° C., i.e. in the holding temperature range of 310-350° C., achieved the required minimum elongation A80, irrespective of the respective annealing duration t2.
In FIG. 5, the elongation A80 of the cold strip specimens produced from the steel S5 is plotted against the annealing duration t2 of the second annealing stage. Here, too, it can be seen that the cold strip specimens achieve the required elongation A80 of at least 5% irrespective of the respective holding temperature T2 thereof and irrespective of the respective annealing duration t2. Accordingly, if a short annealing duration and suitably low holding temperatures T2 are observed, it is also possible for a cold-rolled flat steel product according to the invention in which a high tensile strength Rm is combined with a sufficient elongation A80 to be produced from the steel S5, in spite of the comparatively low C content thereof.
FIG. 6 shows, in a section, a magnified view of a cross section of a cold strip according to the invention. In this figure, by way of example, residual austenite blocks RA-b are marked and a point at which film-like residual austenite RA-f is present in a lamellar stratification is emphasized by being circled.

TABLE 1

Steel	C	Mn	Si	Cu	Cr	Ti	Nb	V	Al	N	Other

S1	0.52	1.48	0.40	1.51	0.88	0.009	—	0.093	1.400	—	—
S2	0.301	1.41	1.46	1.47	0.87	0.014	0.005	0.09	0.021	0.0015	Ni: 0.021
											Mo: <0.002
S3	0.505	1.50	0.40	0.6	1.30	0.011	—	0.098	0.012	0.002	Ni: 0.63
											Mo: 0.30
S4	0.384	1.97	0.41	0.57	1.37	0.0016	—	<0.0005	0.018	0.0014	Ni: 0.59
											Mo: 0.30
S5	0.252	1.47	2.15	0.32	0.41	0.020	—	0.11	0.009	—	Ni: 0.02
											Mo: <0.002

Amounts, in % by weight
Remainder iron and unavoidable impurities

Claims

1. A cold-rolled flat steel product, having a tensile strength Rm of at least 1400 MPa and an elongation A80 of at least 5% and comprising, in addition to iron and unavoidable impurities (in % by weight):

C: 0.10-0.60%,

Si: 0.4-2.5%,

Al: up to 3.0%,

Mn: 0.4-3.0%,

Ni: up to 1.0%,

Cu: up to 2.0%,

Mo: up to 0.4%,

Cr: up to 2%,

Co: up to 1.5%,

Ti: up to 0.2%,

Nb: up to 0.2%, and

V: up to 0.5%,

wherein the microstructure of the flat steel product consists of bainite to an extent of at least 20% by volume, of residual austenite to an extent of 10-35% by volume and of martensite as the remainder.

2. The flat steel product according to claim 1, wherein the C content thereof is at least 0.25% by weight.

3. The flat steel product according to either of claim 1, wherein the C content thereof is at least 0.27% by weight.

4. The flat steel product according to claim 1, wherein the Si content thereof is at least 1.0% by weight.

5. The flat steel product according to claim 1, wherein the Al content thereof is at least 0.01% by weight.

6. The flat steel product according to claim 1, wherein the Cu content thereof is at least 0.2% by weight.

7. The flat steel product according to claim 5, wherein the Cu content thereof is at least 0.55% by weight.

8. The flat steel product according to claim 1, wherein the Cr content thereof is at least 0.3% by weight.

9. The flat steel product according to claim 1, wherein the Mn, Cr, Ni, Cu and C contents thereof satisfy the following condition:

1<0.5% Mn+0.167% Cr+0.125% Ni+0.125% Cu+1.334% C<2, where

% Mn: respective Mn content in % by weight,

% Cr: respective Cr content in % by weight,

% Ni: respective Ni content in % by weight,

% Cu: respective Cu content in % by weight,

% C: respective C content in % by weight.

10. The flat steel product according to claim 1, wherein the microstructure thereof comprises at least 50% by volume bainite.

11. The flat steel product according to claim 1, wherein the microstructure thereof comprises 10-25% by volume residual austenite.

12. A method for producing a flat steel product, said method comprising the following work steps:

providing a preliminary product in the form of a slab, thin slab or a cast strip, which, in addition to iron and unavoidable impurities, comprises (in % by weight) C: 0.10-0.60%, Si: 0.4-2.5%, Al: up to 3.0%, Mn: 0.4-3.0%, Ni: up to 1.0%, Cu: up to 2.0%, Mo: up to 0.4%, Cr: up to 2%, Co: up to 1.5%, Ti: up to 0.2%, Nb: up to 0.2%, V: up to 0.5%;

hot-rolling the preliminary product to form a hot strip in one or more rolling passes, wherein the hot strip obtained has a hot-rolling end temperature of at least 830° C. when it leaves the last rolling pass;

coiling the hot strip obtained at a coiling temperature which lies between the hot-rolling end temperature and 560° C.;

cold-rolling the hot strip to form a cold strip with a degree of cold-rolling of at least 30%;

heat-treating the cold strip obtained, wherein, during the course of the heat treatment, the cold strip

is heated to an annealing temperature amounting to at least 800° C.,

is cooled proceeding from the annealing temperature at a cooling rate amounting to at least 8° C./s to a holding temperature which lies in a holding temperature range having an upper limit of 470° C. and having a lower limit which is higher than the martensite start temperature (MS), from which martensite forms in the microstructure of the cold strip, and

is held at the holding temperature for a period of time which is sufficient to form at least 20% by volume bainite in the microstructure of the cold strip.

13. The method according to claim 12, wherein the hot-rolling end temperature is 850-950° C.

14. The method according to claim 12, wherein the holding temperature is 300-420° C.

15. The method according to claim 12, wherein the cold strip is coated with a metallic protective layer after the heat treatment.