EP4663803A1

EP4663803A1 - Micro-alloyed hot rolled strip with improved formability, above 700 mpa

Info

Publication number: EP4663803A1
Application number: EP24182344.2A
Authority: EP
Inventors: Martin SCHICKINGER; Helmut Spindler; Gottfried HRIBERNIG; Markus Sonnleitner
Original assignee: Voestalpine Stahl GmbH
Current assignee: Voestalpine Stahl GmbH
Priority date: 2024-06-14
Filing date: 2024-06-14
Publication date: 2025-12-17
Also published as: WO2025257439A1

Abstract

The invention relates to a hot rolled steel strip or sheet having a composition comprising of: C: 0.045 - 0.09, Mn: 1.8 - 2.4, Al: 0.01 - 0.1, Ti : 0.020 - 0.2, Nb:0.01 - 0.08, Si: 0.01 - 0.20, Cr: ≤ 0.2, Ni: ≤ 0.25, Mo: ≤ 0.2, Cu ≤ 0.4, V: ≤ 0.02, and balance Fe apart from impurities, and having microstructure comprising (in vol. %): polygonal ferrite (≤ 35), bainite and quasi polygonal ferrite (≥ 65), pearlite (< 3), cementite (< 3), and having a Yield strength of 700 - 900 MPa and a Hole expansion ratio (λ) ≥ 50 %, and a thickness of 2.0 - 6.5 mm. The invention also relates to method for producing the strip or sheet.

Description

TECHNICAL FIELD

The invention relates to a hot rolled steel strip or sheet and manufacturing method thereof.

BACKGROUND ART

Conventional micro-alloyed hot-rolled strips having S700MC according to EN10149-2, are typically produced on the hot wide strip mill (HWSM) at moderate cooling rates of ~20-30°C/s, from the hot rolling finish temperature to cooling temperatures of ~550-600°C.The microstructure of such steels typically contains ferrite, coarse cementite and perlite. The global ductility is good, e.g. elongation. Hower, the local ductility is worse, e.g. formability of punched edges: quantifiable by means of Hole Expansion Ratio HER.
With conventional micro-alloyed hot-rolled strip, complex forming on stamped edges often cannot be realised. There is a need for a micro-alloyed hot-rolled strips having a minimum yield strength of 700 MPa which have an improvement in the formability of punched edges (HER).
EP3715491 A1 disclose a hot rolled sheet and a manufacturing method thereof. The tensile strength is 440 MPa or higher. The microstructure comprises 30 - 70% of a first ferrite and another structure of at least one among bainite and a second ferrite such that the total sum is 95 % or more. The average grain size of the first ferrite is less than 5 µm and the average gran size of the other structure is less than 10 µm. The hot rolling finishing temperature is below Ae3.
EP3516085 B1 disclose a method of manufacturing a high-strength hot-rolled steel strip. The tensile strength is at least 570 MPa, preferably 780 MPa or higher. The method includes cooling the hot-rolled steel strip with a primary cooling rate between 50 to 150°C/s to an intermediate temperature between 600 and 720°C and coiling between 580 and 660°C. The steel has a substantially single-phase ferritic microstructure.
EP3926064 B1 disclose a high strength steel product and method of manufacturing the same. The steel is among other elements alloyed with Ti and V. The yield strength is ≥ 700 MPa and the tensile strength is ≥ 760 MPa. The hole expansion ratio is ≥ 50 %. The microstructure comprises more ≥90 % ferrite of which 10 - 50 % is quasi polygonal ferrite and the remainder of the ferrite being polygonal ferrite and/or bainitic ferrite. The average grain size of the ferrite is < 10 µm. The most preferred finish rolling temperature is 930 - 970 °C. The accelerated cooling is preferably done to 620-660 °C at a rate of 35 - 350 °C/s. Coiling is done at 560 - 670 °C.
WO2023084926 A1 disclose a hot rolled sheet alloyed with Ti and B.
EP2546377 B1 disclose a hot rolled sheet and a manufacturing method thereof. The tensile strength is above 780 MPa. The finish rolling temperature of the inventive examples are from 940 °C or higher and the cooling rate ranges between 26 and 35 °C/s.
EP2689045 B1 discloses a hot-rolled steel sheet with a yield strength of more than 690 MPa and less than or equal to 840 MPa.

DISCLOSURE OF THE INVENTION

The invention is described in the claims.
In a preferred embodiment the strip or sheet has a composition consisting of the following alloying elements (in wt. %):

C 0.045 - 0.09

Mn 1.8 - 2.4

Al 0.01 - 0.1

Nb 0.01 - 0.08

Ti 0.020 - 0.2

Si 0.01 - 0.20

Cr ≤ 0.2

Ni ≤ 0.25

Mo ≤ 0.2

Cu ≤ 0.4

V ≤ 0.02

balance Fe apart from impurities
The composition is excluding any coatings applied to the strip or sheet.
The steel balance strength and formability and is suitable for applications requiring improved edge stretch capability.
The importance of the separate elements and their interaction with each other as well as the limitations of the chemical ingredients of the claimed alloy are briefly explained in the following. All percentages for the chemical composition of the steel are given in weight % (wt. %) throughout the description. Upper and lower limits of the individual elements can be freely combined within the limits set out in the claims. The arithmetic precision of the numerical values can be increased by one or two digits for all values given in the present application. Hence, a value of given as e.g., 0.1 % can also be expressed as 0.10 or 0.100 %. The amounts of the microstructural constituents are given in volume % (vol. %).

C: 0.045 - 0.09 %

C (carbon) is an element having an action of improving the strength of the steel sheet. A certain amount is therefore required. Increasing the C content too much can lead to insufficient ductility, and that the strength becomes too high. Furthermore, iron-based carbides such as cementite (Fe₃C) may form. These can be the starting point of cracking in punching operations and can cause deterioration of the stretch flangeability. A preferred range is 0.05 - 0.085 %.

Mn: 1.8 - 2.4 %

Mn (manganese) improves the strength of the steel sheet through solid-solution strengthening and quench-hardening. Further, increasing Mn can expand the austenite temperature region at the lower temperature side. This improves hardenability and facilitates the formation of a low temperature transformation structure having an excellent burring property. The excellent burring property can be seen by improved Hole Expansion Ratio's. Mn assists to retard the austenite to ferrite transformation such that the desired amount of bainite in the cooling step d) can be reached. The lower limit may be set to, 1.8, 1.85 or 1.9 %. The amount of Mn may be > 1.9%. The upper limit may be set to 2.4, 2.3, 2.2, 2.1, or 2.0 %. A preferred range is 1.9 - 2.2 %.

Si: 0.01 - 0.20%

Si (silicon) is an element having an action of improving the strength of the steel sheet, but also functions as a deoxidizer in the steel melt. Si supresses formation of cementite. However, too high amounts of Si can affect surface quality negatively. The upper limit is therefore limited to 0.20. The upper limit may be 0.20,0.15,0.10, 0.09, or 0.08 %. A lower limit may be 0.01 %. A preferred range is 0.02 -0.15.

Al: 0.01 - 0.1 %

Al (aluminium) has an action of deoxidizing molten steel in a refining process of the steel to sound the steel. Al further has, similarly to Si, an action of suppressing the precipitation of the iron-based carbides such as cementite and thereby improving the strength and Hole expansion ratio. An upper limit of 0.1 % is set to suppress the generation of a non-metallic inclusion in the steel thereby improving the ductility and the low-temperature toughness. The lower limit may be set to 0.01 or 0.02 %. The upper limit may be set to 0.1, 0.08, or 0.06 %. A preferred range is 0.02 - 0.08 %.

Ti: 0.02- 0.2%

Ti (titanium) can improve the strength of the steel sheet by precipitation strengthening or solid-solution strengthening. The addition of Ti can lead to a finer initial structure (the prior austenite grain size). A finer prior austenite grain size can increase the elongation at fracture and the Hole expansion ratio. However, too much Ti can cause coarse titanium nitrides, which are bad for the Hole expansion ratio. The upper limit of Ti may be set to 0.2, 0.19, 0.18, 0.17, 0.16 or 0.15. The lowest amount of Ti may be set to 0.02, 0.03, 0.04,0.05, 0.06, 0.07 or 0.08 %. A preferred range is 0.05 - 0.16 %.

Nb: 0.01 - 0.08 %

Nb(niobium) improves the strength of the steel sheet by precipitation strengthening or solid-solution strengthening. Furthermore, the addition of Nb can lead to a finer initial structure (the prior austenite grain size). A finer prior austenite grain size can increase the elongation at fracture and the Hole expansion ratio. The upper limit may be set to 0.08, 0.07, or 0.06 %. The lower limit may be set to 0.01 or 0.02 %. A preferred range is 0.02 - 0.065 %.

Optional elements

Cr, Mo, Ni

Cr(chromium), Mo (molybdenum) and Ni (nickel) can improve the strength of the steel sheet by solid-solution strengthening or quench-hardening strengthening. Cr, Mo, and Ni affect the Ac3 temperature, and they also promote transformation behaviour. Too much of Cr, Mo, and Ni can affect the Hole expansion ratio and the forming properties negatively.

Cr ≤ 0.2%

Cr can assist to retard the austenite to ferrite transformation, to set the desired amount of bainite in the cooling step d). The upper limit of Cr may be set to 0.2, 0.15, 0.1, 0.05, 0.01 %. The lowest amount of Cr may be set to 0.001, 0.005, 0.01, 0.02, 0.03, 0.04 or 0.05 %. A deliberate addition of Cr is not necessary according to the present invention.

Ni ≤ 0.25%

The upper limit of Ni may be set to 0.25, 0.2, 0.15, 0.1, 0.05, or 0.01 %. The lowest amount of Ni may be set to 0.001, 0.005, 0.01, 0.02, 0.03, 0.04 or 0.05 %. A deliberate addition of Ni is not necessary according to the present invention.

Mo ≤ 0.2%

The upper limit of Mo may be set to 0.2, 0.15, 0.1, 0.05, 0.01 %. The lowest amount of Mo may be set to 0.001, 0.005, 0.01, 0.02, 0.03, 0.04 or 0.05 %. A deliberate addition of Mo is not necessary according to the present invention.

Cu ≤ 0.4%

Cu (copper) improves scale removal and thus has an influence on a better surface quality. If the steel is made from scrap, as is common when using an electric arc furnace, Cu impurities can be up to 0.4 %. The upper limit of Cu may be set to 0.4, 0.3, 0.2, 0.15, 0.1, 0.05, 0.01 %. The lowest amount of Cu may be set to 0.001, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.1, or 0.15 %. A deliberate addition of Cu is not necessary according to the present invention.

Further preferred restrictions

Nb + Ti: ≤ 0.2 %

The combined content of Ti and Nb is preferably restricted to ≤ 0.2%, more preferably ≤ 0.15%. Higher amounts of these elements are not necessary to achieve the desired properties of the steel.

Mn + Si + Cr ≤ 2.3 %

The combined content of Mn, Si and Cr is preferably restricted to ≤ 2.3%, preferably ≤ 2.2%. Higher amounts of these elements are not necessary to achieve the desired properties of the steel.

Impurities

The following impurities may optionally be limited as disclosed below.

V: ≤ 0.02 %

The steel does not contain deliberately addition of V (vanadium). The upper limit may be set to 0.02 or 0.01 %.

N: ≤ 0.015 %

N (nitrogen) is an element generally contained as an impurity. When the N content is too high, N causes cracking in the hot rolling, and deteriorates the aging resistance. Therefore, the N content is set to 0.015 % or less, preferably 0.10 % or less. A nominal amount may be in the range of 0.001 - 0.008 %.

P: ≤ 0.10 %

P (phosphorus) is an element generally contained as an impurity. When the P content is more than 0.10 %, P causes cracking in the hot rolling, and is segregated at a grain boundary to decrease the low-temperature toughness and also decrease the workability and the weldability. Therefore, the P content is set to 0.10 % or less. The upper limit may be set to 0.10, 0.05, 0.04, 0.03 or 0.02 %

S: ≤ 0.010 %

S (sulfur) is an element generally contained as an impurity. When the S content is more than 0.010%, S causes cracking in the hot rolling, and can generate MnS inclusions in the steel which deteriorates the Hole expansion ratio. Therefore, the S content is set to 0.010 or less. The upper limit may be set to 0.010, 0.05, 0.03, 0.01, 0.005 or 0.001 %
Other impurity elements may be comprised in the steel in normal occurring amounts. However, it is preferred to limit the amounts of As, Zr, Sn, Sb to the following optional maximum contents:

As: ≤ 0.020, or ≤ 0.015%
Zr: ≤ 0.010, or ≤ 0.006%
Sn: ≤ 0.050, or ≤ 0.040%
Sb: ≤ 0.020, or ≤ 0.010%

Oxygen and hydrogen can further be limited to

O: ≤ 0.001, or ≤ 0.0003%
H: ≤ 0.0050, or ≤ 0.0020%

Mechanical properties

The steel should fulfil the following conditions:

YS, Yield strength (R_p0.2) HER, Hole expansion 700 - 900 MPa, preferably 700 - 850 MPa

ratio (λ) ≥ 50 %, preferably 60 - 100 %
And optionally one or more of the following mechanical properties:

TS, Tensile strength (R_m) 700 - 1000 MPa, preferably 750 - 950 MPa,

TE, Total elongation ≥ 10 %, preferably 13 - 30 %, more preferably 15 - 25 %
TS, YS, TE are examples of properties related global ductility. HER is a property related to local ductility.
The R_m, R_p0.2 values are derived in accordance with the Industrial Standard DIN EN ISO 6892-1:2019, wherein the samples are taken in the longitudinal direction of the strip.
The Total Elongation is determined according to EN10149-2 2013 (D). For sheet thickness t <3mm, A_80mm used, and from t ≥3mm As (proportional measuring length related to the sample cross-section) is used.
The Hole expansion ratio (λ) is determined by the Hole expansion test according to ISO 16630:2017. In this test a conical punch having an apex of 60 ° is forced into a 10 mm diameter punched hole made in a steel sheet having the size of 100 x 100 mm². The test is stopped as soon as the first crack is determined, and the hole diameter is measured in two directions orthogonal to each other. The arithmetic mean value is used for the calculation.
The Hole expansion ratio (λ) in % is calculated as follows: $λ = (Dh - Do) / Do \times 100$ wherein Do is the diameter of the hole at the beginning (10 mm) and Dh is the diameter of the hole after the test.
The lower limit of the Hole expansion ratio (λ) can be 50, 60, 70 or 80 %. The upper limit may be 90, 100, 120, 150 or 200 %.
The strip or sheet thickness of the final product is 2.0 - 6.5 mm, preferably 2.5 - 5 mm, more preferably 3-4 mm. The strip or sheet width may be 500 - 2000 mm, preferably 700 - 1750 mm in non-slit condition.

Microstructure

The microstructure of the steel contains the main phases ferrite and bainite. High elongation is associated with ferrite, and bainite is associated with good edge stretchability expressed as HER. The steel balance strength and formability and is suitable for applications requiring improved edge stretch capability.
The microstructural constituents are in the following expressed in volume % (vol. %).
a)

polygonal ferrite ≤ 35, preferably ≤ 30, most preferably ≤ 25

b)

bainite and quasi polygonal ferrite ≥ 65, preferably ≥ 70, most preferably ≥ 75

c)

pearlite < 3, preferably < 2, most preferably <1

cementite < 3, preferably < 2, most preferably <1

retained austenite < 1
The phases a) or b) in the list above may balance the microstructure.
Bainite represents a carbon-rich second phase of the present steel. Bainite can be upper bainite and/or lower bainite and/or granular bainite. Quasi polygonal ferrite looks similar to granular bainite (irregular, undulating grain boundaries and a dislocation substructure) and is therefore included in the range.
Pearlite and/or cementite may be present in lesser amounts. The content of pearlite and/or cementite can be 0%.
Retained austenite should preferably not be present but may be present at smaller amount. The content may be 0%.
The microstructure, including the amount of each phase, can be identified in scanning electron microscope (SEM) using 20000 times magnification. Preferably by cutting out a sample from a steel plate and polishing a cross section of a plate parallel to the rolling direction. The microstructure should be taken from ¼ of the thickness. The surface can be etched to make the phases easier to identify.
The amount of retained austenite can e.g. be determined by means of the saturation magnetization method such as described in detail in Proc. Int. Conf. on TRIP-aided high strength ferrous alloys (2002), Ghent, Belgium, p. 61 - 64.

Production of the coiled strip.

The coiled strip can be produced by the following steps:

a) making steel slabs with the composition defined above. The steel slabs may e.g. be produced by converter melting or in an electric steel plant and secondary metallurgy to finalise the composition
b) Reheating the slabs to a reheating temperature TRH between 1150 °C and 1300 °C at a heating rate of 0.05 - 10 °C/s.
c) Hot rolling the slabs in austenitic range including finishing rolling to a final thickness t (2 - 6.5 mm), wherein the hot rolling finishing temperature FRT is within the range of 910 to 940 °C, preferably 920 to 940 °C. A high hot rolling finishing temperature FRT is beneficial for the Hole Expansion Ratio (λ). An increase in hot rolling finishing temperature FRT leads to more elongated austenite grains which affect the final microstructure positively expressed by an increased Hole Expansion Ratio (λ). The rolling should be accomplish rolling in the austenite region and the hot rolling finishing temperature FRT is preferably above Ae3 as defined by the formula: $Ae3 (° C) = 919 - 266 \times [C] + 38 \times [Si] - 28 \times [Mn] - 27 \times [Ni] + 12 \times [Mo] .$ The amount of the elements in brackets are in weight %.
d) Cooling the hot rolled strip wherein the cooling rate (CR) is 40 - 60 °C/s in the temperature range from 850 to 650 °C. The cooling of the strip is preferably done via water cooling. Cooling from 650 °C to the coiling temperature may be done at any rate, typically at a lower rate, e.g. in the range of 10 - 40 °C/s. Cooling from 650 °C may e.g. be done by water cooling.
e) Coiling the cooled strip at a coiling temperature between Bs -150 °C and Bs -50 °C, where Bs (°C) = 637-58 x [C] - 10 x [Mn] - 15 x [Ni] - 34 x [Cr] - 41 × [Mo] + 67 x [V + Nb +Ti]. The amount of the elements in brackets are in weight %.
f) optionally batch annealing of the coiled strip at 400 - 600 °C for 1 - 48 h, or sheet annealing at 400 - 650 °C for 1-60 minutes, or hot dip galvanising of the strip.

The cooling rate of 40 - 60 °C/s in the temperature range from 850 to 650 °C and the coiling temperature between Bs -150 °C and Bs -50 °C facilitates a ferritic matrix without coarse cementite and without or at most small amounts of pearlite. If the cooling rate is too high and/or the coiling temperature is too low, martensite may form. Preferably the coiling temperature is between Bs - 125 °C and B_S - 75 °C.

EXAMPLE

Steel slabs were produced by conventional metallurgy by converter melting and secondary metallurgy. The compositions of the produced steels 1- 5 are shown in Table. 1.

Table 1

steel	C	Si	Mn	P	S	Al	Cr	Ni	Mo	Cu	V	Nb	Ti	N
1	0,059	0.249	1.860	0.008	0.001	0.054	0.029	0.012	0.004	0.016	0.004	0.027	0.106	0.0048
2	0.058	0.250	1.890	0.008	0.001	0.053	0.030	0.010	0.002	0.022	0.003	0.028	0.098	0.0039
3	0.055	0.119	1.960	0.007	0.001	0.049	0.045	0.027	0.012	0.013	0.006	0.027	0.089	0.0056
4	0.057	0.091	1.940	0.005	0.001	0.049	0.023	0.014	0.003	0.012	0.005	0.028	0.095	0.0047
5	0.055	0.106	1.980	0.007	0.001	0.047	0.031	0.014	0.009	0.015	0.007	0.029	0.100	0.0059

Steel strips were produced from the steels. Process values and mechanical parameters are shown in Table 2.

Table 2

steel	t [mm]	FRT [°C]	CR 850-650°C [°C/s]	CT [°C]	YS [Mpa]	UTS [Mpa]	A80mm [%]	HER [%]	remark
1	2.55	920	21	B_S- 8	725	842	15.7	44	comparative
2	2.55	918	21	B_S- 5	715	815	17.0	47	comparative
3	2.53	928	56	B_S - 98	736	799	14.7	73	inventive
4	2.54	929	52	B_S - 93	743	808	15.4	85	inventive
5	2.54	929	57	B_S - 96	780	835	14.6	84	inventive

Underlined values are outside the scope of the invention.

The steels #1-5 were reheated to a reheating temperature TRH at a heating rate of about 0.1 °C/s. The reheated samples where thereafter hot rolled in austenitic range to a hot rolled strip. The hot rolling finishing temperatures FRT were between 918 °C (steel #1) and 929 °C (steel #4 and #5). The inventive steel had hot rolling finishing temperatures FRT above 920 °C.
The comparative steels #1 and #2 were cooled at a cooling rate CR of 21 °C/s from 850 to 650 °C, whereas the inventive steel was cooled between 50-60 °C/s from 850 and 650 °C. By cooling the strip faster in the range 650 to 850 °C, the formation of coarse cementite and perlite can be avoided or be supressed.
The inventive steels were all coiled at a coiling temperature within Bs - 150 °C and Bs - 50 °C, whereas the comparative examples were coiled at temperatures closer to Bs. The coiling temperatures CT for the inventive steels were chosen to avoid or supress formation of coarse cementite and perlite and to achieve a high fraction of bainite in the microstructure. After coiling phase transformation is finished for these steels.
The mechanical properties of the produced steel strips #1-5 were determined using the measurement methods defined in the description. Mechanical properties are shown in Table 2. As can be seen, the inventive steels #3-5 met the mechanical criterions, particular in having a Tensile strength YS in the range of 700 - 850 MPa while having a Hole expansion ratio HER ≥ 50 %, while the comparative example failed to meet the requirement of the Hole expansion ratio HER.
The microstructures of the inventive steel fulfilled in vol. %:

polygonal ferrite ≤ 25

bainite and quasi

polygonal ferrite ≥ 75

pearlite < 1

cementite < 3

Claims

A hot rolled steel strip or sheet having a composition consisting of the following alloying elements (in wt. %): C 0.045 - 0.09 Mn 1.8 - 2.4 Al 0.01 - 0.1 Ti 0.020 - 0.2 Nb 0.01 - 0.08 Si 0.01 - 0.20 Cr ≤ 0.2 Ni ≤ 0.25 Mo ≤ 0.2 Cu ≤ 0.4 V ≤ 0.02

balance Fe apart from impurities, and having microstructure comprising (in vol. %): polygonal ferrite ≤ 35 bainite and quasi polygonal ferrite ≥ 65 pearlite < 3 cementite < 3

determined by use of scanning electron microscope (SEM) at 20000 times magnification, and having the following mechanical properties: YS, Yield strength 700 - 900 MPa Hole expansion ratio (λ) ≥ 50 %

the yield strength is derived in accordance with the Industrial Standard DIN EN ISO 6892-1:2019 in which samples are taken in the longitudinal direction of the strip or sheet and the Hole expansion ratio (λ) determined by the Hole expansion test according to ISO 16630:2017, and the strip or sheet having a thickness of 2.0 - 6.5 mm.
The hot rolled steel strip or sheet according to claim 1, having a composition consisting of the following alloying elements (in wt. %): C 0.05 - 0.085 Mn 1.8 - 2.2 Al 0.02 - 0.08 Ti 0.02 - 0.16 Nb 0.02 - 0.065 Si ≤ 0.2 Cr ≤ 0.15 Ni ≤ 0.2 Mo ≤ 0.15 Cu ≤ 0.15 V ≤ 0.01

balance Fe apart from impurities.
The hot rolled steel strip or sheet according to claim 1 or 2, wherein the Hole expansion ratio (λ) is at least 60 %.
The hot rolled steel strip or sheet according to any one of claims 1 - 3, wherein the microstructure comprising (in vol. %): polygonal ferrite ≤ 30 bainite and quasi polygonal ferrite ≥ 70 pearlite < 2 cementite < 3
The hot rolled steel strip or sheet according to claim 4, wherein the microstructure comprising (in vol. %): polygonal ferrite ≤ 25 bainite and quasi polygonal ferrite ≥ 75 pearlite < 1 cementite < 3
The hot rolled steel strip or sheet according to any one of claims 1 - 5, wherein the composition fulfils at least one of the following conditions in weight %: Mn + Si + Cr ≤ 2.3 Nb + Ti: ≤ 0.2
A method for producing the hot rolled steel strip or sheet according to any one of claim 1 - 5: the method comprising the steps of:
a) making steel slabs according to the composition;

b) reheating the slabs to a reheating temperature (TRH) between 1150 °C and 1300 °C at a heating rate of 0.05 - 10 °C/s;

c) hot rolling the slabs in austenitic range to a hot rolled strip having a final thickness (t) of 2 - 6.5 mm, wherein the hot rolling finishing temperature (FRT) is within the range of 910 to 940 °C;

d) cooling the hot rolled strip wherein the cooling rate (CR) is 40 - 60 °C/s in the temperature range from 850 to 650 °C,

e) coiling the cooled strip at a coiling temperature between Bs -150 °C and Bs -50 °C, where B_S = 637-58 x [C] - 10 x [Mn] - 15 x [Ni] - 34 x [Cr] - 41 x [Mo] + 67 x [V + Nb +Ti], and the content of the element is in weight %,

f) optionally batch annealing the coiled strip at 400 - 600 °C for 1- 48 h, or sheet annealing at 400 - 650 °C for 1-60 minutes, or hot dip galvanising of the strip
The method according to claim 6, wherein the coiling temperature is between Bs - 125 °C and B_S - 75 °C.
The method according to claim 6 or 7, wherein the hot rolling finishing temperature (FRT) is within the range of 920 to 940 °C.