EP3884077B1

EP3884077B1 - High strength steel product and method of manufacturing the same

Info

Publication number: EP3884077B1
Application number: EP19808728.0A
Authority: EP
Inventors: Petri JUSSILA; Olli Oja
Original assignee: SSAB Technology AB
Current assignee: SSAB Technology AB
Priority date: 2018-11-19
Filing date: 2019-11-19
Publication date: 2025-07-30
Anticipated expiration: 2039-11-19
Also published as: WO2020104437A1; EP3884077C0; EP3884077A1

Description

FIELD OF INVENTION

The present invention relates to a high strength steel sheet product and a method of manufacturing the same. In particular, the invention relates to a high strength steel sheet product with ultimate tensile strength of at least 950 MPa and having improved flatness and gauge uniformity.

BACKGROUND

In automotive industry, hot-dip galvanized (HDG) or galvannealed advanced high strength steel (AHSS) products are desired due to the combined advantages of light weight and high strength which allow for considerable reduction in automobile body weight without compromising safety requirements. AHSS grades can be used to make safety relevant body in white (BIW) components such as side impact protection beams.
In response to ever increasing demand for lighter yet stronger material solutions, AHSS grades with ultimate tensile strength levels of 950 MPa and above have been developed. Achieving such high strength requires quite high levels of alloying elements in the chemical composition, in particular when the steel is designed for processing in conventional continuous galvanizing lines (CGLs) with relatively low cooling capacity. The steel compositions also need to be fine-tuned for specific CGL heat treatment cycles in order to meet the desired mechanical property values e.g. strength, toughness, formability and weldability; and other practical customer requirements related to products exterior qualities e.g. dimensions, shape tolerance, flatness, gauge uniformity and surface quality.
Producing AHSS grades with ultimate tensile strength of 950 MPa and above requires complex alloying to increase hardenability of the steel, which severely suppresses transformation of austenite on hot rolling mill runout table such that the low-temperature microstructure is formed mainly during coil cooling. This leads to pronounced microstructural variations along the strip length, since the cooling rate in different parts of a hot rolled coil vary greatly because there is no viable means to control cooling of a coil due to unrepeatable handling and storage conditions, and seasonal variation in ambient temperature when coils are left to cool outdoors. The formed microstructures can range from ferritic-pearlitic to bainitic to even martensitic within a single coil, even when a constant coiling temperature has been achieved over the length of the strip.
The variability of microstructure and mechanical properties are not readily visible from an exterior view of the hot rolled strip. They only manifests in obvious defects, such as deteriorated flatness and gauge inconsistency, in the downstream operations that follow hot rolling. One widely recognized issue occurring in the downstream processing is the so-called cold-gauge hashing which is a periodic deviation in thickness. The periodicity of gauge spikes correlates with the microstructure and mechanical properties of the hot rolled strip that varies regularly once per revolution of the hot coil. This phenomenon causes unacceptable yield losses due to intolerably defective final products. In the worst case of uncontrolled coil cooling, the strength can become excessively high in one part of the hot rolled strip, which could cause disturbances or even strip breakages and damage to the equipment in the cold-rolling mill.
The problem of cold gauge hashing has been discussed in the following articles.

(1) "Thermomechanical Processing of Advanced High Strength Steels in Production Hot Strip Rolling", E.I. Poliak et al., La Metallurgia Italiana, February 2009.
(2) "Factors Affecting Gauge Uniformity of Flat Cold Rolled AHSS", A.V. Marmulev et al., Materials Science and Technology, October 16-20, 2011.
(3) "Control of AHSS Coil Cooling After Hot Rolling to Improve Downstream Processing", E.I. Poliak et al., Iron and Steel Technology, October 2016.

Two main set of factors were found affecting the amplitude of cold gauge hashing defects. They are steel chemistry and hot rolling mill process.
It was mentioned in (1) and (2) that the ferritic transformation on the run-out table can be accelerated by reducing the contents of austenite stabilizer (C, Mn) and transformation retarding elements (B, Cr, Mo). Hot strips with mainly ferritic microstructure are easy to cold roll with excellent results for both flatness and thickness accuracy. However, this approach is not realistic with the alloying levels of steels with ultimate tensile strength of 950 MPa class and above.
It was further mentioned in (2) that cold gauge hashing can be attenuated by lowering the coiling temperature, which favours phase transformation of austenite into bainite on the run-out table. However, this approach is not realistic which is dependent on the length of run-out table and the cooling system. For a relatively short run-out table, it is impossible to lower the coiling temperature without also lowering the finishing temperature since the hot rolled strip shape can be significantly distorted by intense rapid cooling. On the other hand, with high levels of alloying, the bainitic transformation could be too slow to occur on the run-out table before coiling.
It was mentioned in (3) that slow cooling of coils allows for improving longitudinal and transverse uniformity of microstructure and mechanical properties, and thereby enhancing cold rollability. However, slow coil cooling must be coupled with hot mill process timing, which is less flexible and causes decrease in productivity. Furthermore, there is no reliable means to control coil cooling rate even in a thermally insulated coil cooling box as the latent heat generated during austenite transformation affects the ambient temperature.
WO2017/108251A1 , concerning a high strength galvannealed steel sheet with an ultimate tensile strength of 1180 - 1300 MPa, recognizes that the hot rolled sheet needs to be subjected to batch annealing before cold rolling in order to obtain a microstructure which is necessary for cold-rolling with narrow thickness deviation. However, there is no evidence showing that the thickness deviation is diminished or the problem of cold gauge hashing can be solved by the extra step of batch annealing. An extra step of batch annealing would inevitably enhance production costs and reduce productivity. Further, EP3054025A1 discloses high-strength steel sheets having a similar composition and microstructure as claimed.
The present invention is intended to improve the production yield of high strength steel sheet products with ultimate tensile strength of at least 950 MPa, and to obtain a wide dimensional range for the respective final products. This is achieved by preventing the above-mentioned problem of cold gauge hashing and alleviating other issues related to strength variations or flatness defects in the hot rolled strip.

SUMMARY OF INVENTION

In view of the state of art, the object of the present invention is to solve the problem of providing a high strength steel sheet with ultimate tensile strength of at least 950 MPa and having improved flatness and gauge uniformity. The problem is solved by the combination of specific alloy designs and hot rolling mill process which attenuates the phenomenon of cold gauge hashing.
In a first aspect, the present invention provides a high strength steel sheet as defined in claim 1.
Specifically, Nb is not added in order for the steel product to have a wide dimensional range. The steel product is alloyed with essential alloying elements such as C, Si, Mn, Cr, Ti, B, Ca, and Al. Other elements such as Mo, V, Cu, and Ni may be present as residual contents that are not purposefully added.

In particular, the present invention provides a high strength steel sheet with an ultimate tensile strength (R_m) of at least 980 MPa and a thickness accuracy better than 1/5 of EN 10143:2006 normal thickness tolerances, which steel sheet has a composition consisting of, in terms weight percentages (wt. %):

C	0.07 - 0.1, e.g. 0.09
Si	0.2 - 0.29, e.g. 0.26
Mn	2.5 - 2.8, e.g. 2.65
Cr	0.2 - 0.4, e.g. 0.3
Ti	0.02 - 0.04, e.g. 0.03
B	0.002 - 0.005, e.g. 0.003
Ca	0.002 - 0.004, e.g. 0.003
Al	0.01 - 0.04, e.g. 0.025

remainder Fe and inevitable impurities.

Furthermore, the present invention provides a high strength steel sheet with an ultimate tensile strength (R_m) of at least 1180 MPa and a thickness accuracy better than 1/5 of EN 10143:2006 normal thickness tolerances, which steel sheet has a composition consisting of, in terms weight percentages (wt. %):

C	0.1 - 0.13, e.g. 0.12
Si	0.2 - 0.29, e.g. 0.26
Mn	2.5 - 2.8, e.g. 2.6
Cr	0.3 - 0.5, e.g. 0.45
Ti	0.02 - 0.04, e.g. 0.025
B	0.002 - 0.005, e.g. 0.003
Ca	0.002 - 0.004, e.g. 0.003
Al	0.01 - 0.04, e.g. 0.025

remainder Fe and inevitable impurities.

The steel sheet has a microstructure comprising a matrix consisting of, in terms of volume percentages (vol. %):

martensite 20 - 70

retained austenite ≤ 8, preferably ≤ 2, more preferably ≤ 1

pearlite and/or ferrite ≤ 5

remainder bainite and/or fine-grained ferrite.
The microstructure has an average grain size of less than 10 µm. Preferably, the bainite and/or fine-grained ferrite have an average grain size of less than 5 µm, preferably in the range of 1 µm to 5 µm.

Preferably, the steel sheet further has at least one of the following mechanical properties:

a minimum bending radius (Ri)	≤ 3.5 t, preferably ≤ 2.5 t
a total elongation (A₈₀)	≥ 5 %, preferably ≥ 8 %
a yield ratio (YR)	50 - 100, preferably 60 - 90
a hole expansion ratio (λ)	≥ 25 %

Preferably, the high strength steel sheet with an ultimate tensile strength (R_m) of at least 980 MPa has a minimum bending radius (Ri) of 2.5 t or less.
Preferably, the high strength steel sheet with an ultimate tensile strength (R_m) of at least 980 MPa has a total elongation (A₈₀) of at least 8 %.
Preferably, the high strength steel sheet with an ultimate tensile strength (R_m) of at least 1180 MPa has a total elongation (A₈₀) of at least 5 %.
Preferably, the steel sheet is a strip having a thickness in the range of 0.8 - 2.2 mm.
In a second aspect, the present invention provides a method for manufacturing the steel sheet as defined in claim 9.
The optional step of hot-dip coating is preferably hot-dip galvanizing or galvannealing.
Optionally, the method further comprises steps of temper rolling and/or levelling. Optionally, the method further comprises a step of extra batch annealing at a temperature in the range of 200 to 400 °C, e.g. 270 °C.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1: illustrates the microstructures.
Figure 2: illustrates the flatness maps.
Figure 3: illustrates the thickness deviations along the length of the tested steel sheets.

DETAILED DESCRIPTION OF INVENTION

The term "steel" is defined as an iron alloy containing carbon (C).
The term "flatness" is used to indicate deviations from a horizontal flat surface in a steel strip.
The term "thickness accuracy" is used to indicate deviations from a target thickness in a steel strip in length direction.
The term "gauge" refers generally to a measure of the thickness of a metal sheet.
The term "EN 10143:2006" refers to European Standard tolerances on dimensions and shape, which is applicable to continuously hot-dip coated steel sheet and strip.
The term "ultimate tensile strength (UTS, R_m)" refers to the limit, at which the steel fractures under tension, thus the maximum tensile stress.
The term "yield strength (YS, Rp_0.2)" refers to 0.2 % offset yield strength defined as the amount of stress that will result in a plastic strain of 0.2 %.
The term "yield ratio (YR)" refers to the ratio of Rp_0.2 and R_m, in terms of percentage, i.e. YR = (Rp_0.2/R_m) x 100.
The term "total elongation (TEL)" refers to the percentage by which the material can be stretched before it breaks; a rough indicator of formability, usually expressed as a percentage over a fixed gauge length of the measuring extensometer. Two common gauge lengths are 50 mm (A₅₀) and 80 mm (A₈₀).
The term "minimum bending radius (Ri)" is used to refer to the minimum radius of bending that can be applied to a test sheet without occurrence of cracks.
The term "bendability" refers to the ratio of Ri and the sheet thickness (t).
The term "hole expansion ratio (λ)" is a key indicator to evaluate stretch flanging performance of steel sheets, which is usually obtained by hole expanding test using cylindrical or conical punch.
Our approach to solve the problem of cold gauge hashing is beneficial to the formation of an essentially bainitic microstructure with minimal or at least consistent strength variation over the whole length and width of the cooled-down hot rolled strip. Concurrently, the strength of this bainitic microstructure is targeted as low as possible to increase hot and cold rollability for obtaining the widest possible dimensional range for cold-rolled products.
Through careful study of various hot strip microstructures and resulting properties, we have found a specific combination of alloying elements, which results in prevention of the formation of soft ferritic-pearlitic microstructures in the slow-cooling core of the hot rolled coil. The contents of austenite stabilizers C, Mn and Cr, and other transformation-retarding elements B and optionally Mo are adjusted to levels beneficial to bainitic transformation. As a consequence of the combined effects of suppressing the formation of soft ferritic-pearlitic microstructures and enhancing bainitic transformation, it is possible to utilize a high coiling temperature thereby obtaining generally slower cooling rate of the hot-rolled coil in storage, resulting in lower strength of the desired bainitic microstructure as it forms. The alloy design ensures that the ultimate tensile strength of at least 950 MPa can be obtained after hot-dip galvanizing or galvannealing. The alloy design also achieves a good balance of strength and elongation.
Next the chemical composition is described in more details.

Carbon C is used in the range of 0.07 wt. % to 0.13 wt. %.

C alloying increases strength of steel by solid solution strengthening, and hence C content determines the strength level. C content less than 0.07 wt. % may lead to insufficient tensile strength below 950 MPa. C also functions as an austenite stabilizer and delays transformation of austenite, which inhibits the formation of ferritic-pearlitic microstructures.
However, C content needs to be set to not more than 0.13 wt. % to prevent excessive strengthening within the hot rolled coil in parts that are locally cooled too fast to form the desired bainitic microstructures.

Silicon Si is used in the range of 0.1 wt. % to 0.3 wt. %.

Si is effective as a deoxidizing or killing agent that can remove oxygen from the melt during a steelmaking process. Si alloying enhances strength by solid solution strengthening, and enhances hardness by increasing austenite hardenability. The presence of Si favours good balance of strength and elongation since Si simultaneously increases formation and hardness of the ferrite and low carbide containing bainite which are the ductile components in the microstructure. Also, the presence of Si stabilizes ferrite and residual austenite.
However, zinc coating adhesion and surface quality may be deteriorated if Si is present in excessive amount more than 0.3 wt. %.
Preferably, the Si content is more than 0.1 wt. % and less than 0.3 wt. %.

Manganese Mn is used in the range of 2.3 wt. % to 3.0 wt. %.

Mn alloying enhances strength by solid solution strengthening, and enhances hardness by increasing austenite hardenability. As an austenite stabilizer, Mn stabilizes the remaining austenite at later stages of the bainitic transformation thereby inhibiting transformation of the carbon-rich constituent into different microstructures in different parts of the hot rolled coil. Mn is beneficial to the preferred carbon-rich second phase being martensite or autotempered martensite. Transformation to degenerate pearlite is inhibited in the presence of Mn.
However, Mn content needs to be set to not more than 3.0 wt. % to prevent excessive strengthening and hardenability.

Chromium Cr is used in the range of 0.2 wt. % to 0.5 wt. %.

Cr alloying enhances strength and hardness by increasing austenite hardenability. At least 0.2 wt. % of chromium is required for sufficient hardenability at the HDG line.
Cr also functions as an austenite stabilizer in the same manner as Mn. Cr alloying promotes the formation of the preferred carbon-rich second phase being martensite or autotempered martensite, while inhibiting the transformation to degenerate pearlite.
However, Cr in an amount above 0.5 wt. % would increase the strength of the desired bainitic structure to an unacceptable level.

Titanium Ti is used in the range of 0.02 wt. % to 0.05 wt. %.

Ti is added to bind free N that is harmful to toughness by forming stable TiN, which can efficiently prevent austenite grain growth in the reheating stage at high temperatures. TiN formation also suppresses BN precipitation, thereby leaving B free to make its contribution to hardenability. Thus, Ti is usually required to ensure the effectiveness of B in the case of B alloying.
However, if Ti content is too high, coarsening of TiN and precipitation hardening due to TiC develop and impact toughness may be deteriorated. Therefore, it is necessary to restrict titanium so that it is less than 0.05 wt. %.

Boron B is used in the range of 0.002 wt. % to 0.01 wt. %.

B is a transformation-retarding element that suppresses formation of diffusional transformation products such as polygonal ferrite, thereby promoting formation of low carbon bainitic structures. Thus, the presence of B prevents formation of soft ferritic-pearlitic microstructures in the slow-cooling core of the hot rolled coil.
Effective B alloying would require the presence of Ti to prevent formation of BN. Toughness is rapidly deteriorated when B content exceeds 0.01 wt. %.

Molybdenum Mo is used in a content of 0.2 wt. % or less.

Mo is a transformation-retarding element that has the effects of promoting the formation of low carbon bainitic structure. The presence of Mo also enhances strength and hardness by increasing austenite hardenability. In the case of B alloying, Mo is optionally required to ensure the effectiveness of B.
However, Mo is not an economically acceptable alloying element. Mo content should not exceed 0.2 wt. %. Excessive amount of Mo may impose limitations to the achievable dimensional range due to excessive strengthening of the bainitic microstructure beyond the desired level. If Mo is used in an amount above 0.2 wt. % toughness may be deteriorated thereby increasing risk of brittleness, and also the effect of B may be reduced.

Vanadium V is used in a content of 0.2 wt. % or less.

V is a strong carbide and nitride former, but V(C,N) can also form and its solubility in austenite is high. Thus, V alloying has potential for dispersion and precipitation strengthening, because large quantities of V are dissolved and available for precipitation in ferrite.
However, addition of V more than 0.2 wt. % has negative effects on hardenability.

Copper Cu is used in a content of 0.2 wt. % or less.

Cu promotes low carbon bainitic structures, causes solid solution strengthening and contributes to precipitation strengthening.
The upper limit of Cu content is set to 0.2 wt. % to prevent excessive strengthening. When added in excessive amount Cu also deteriorates toughness.

Nickel Ni is used in a content of 0.5 wt. % or less.

Ni is an alloying element that improves austenite hardenability and increases strength without any loss of toughness.
However nickel contents of above 0.5 wt. % would increase alloying costs too much without significant technical improvement. Excess amount of Ni may produce high viscosity iron oxide scales, which deteriorate surface quality of the steel product.

Calcium Ca is used in a content of 0.005 wt. % or less.

Ca is not used as alloying element due to its low solubility in steel and high vapor pressure. The optional Ca addition during a steelmaking process is for refining, deoxidation, desulphurization, and control of shape, size and distribution of oxide and sulphide inclusions.
Preferably, Ca is used in the range of 0.001 wt. % to 0.005 wt. %.

Aluminum Al is used in a content of 0.1 wt. % or less.

Al is effective as a deoxidizing or killing agent that can remove oxygen from the melt during a steelmaking process. Al also removes N by forming stable AIN particles and provides grain refinement, which has the effects of promoting high toughness. Also, Al stabilizes ferrite and residual austenite.
However, Al may increase non-metallic inclusions thereby deteriorating cleanliness if used in excessive amount above 0.1 wt. %.
Preferably, Al is used in the range of 0.01 wt. % to 0.1 wt. %.
Niobium Nb is considered as a major grain refining element. Nb contributes to the strengthening and toughening of steels. Specifically, Nb is not added in the composition according to the present invention in order to allow the steel product to have a wide dimensional range since excessive strengthening may decrease hot and cold rollability of the hot-rolled sheet.
The steel product is alloyed with essential alloying elements such as C, Si, Mn, Cr, Ti, B, Ca, and Al. Other elements such as Mo, V, Cu, and Ni may be present as residual contents that are not purposefully added. The difference between residual contents and unavoidable impurities is that residual contents are controlled quantities of alloying elements, which are not considered to be impurities. A residual content as normally controlled by an industrial process does not have an essential effect upon the alloy.
Unavoidable impurities can be phosphor P, sulfur S, nitrogen N. Their contents are preferably limited as follows:

Phosphor P ≤ 0.05 wt. %
Sulfur S ≤ 0.01 wt. %
Nitrogen N ≤ 0.008 wt. %

The method for producing the intermediate hot rolled product comprises the steps of:
- providing a steel slab with
a composition (I) consisting of, in terms weight percentages (wt. %):

C	0.07 - 0.13
Si	0.1 - 0.3, preferably more than 0.1 and less than 0.3
Mn	2.3 - 3.0
Cr	0.2 - 0.5
Ti	0.02 - 0.05
B	0.002 - 0.01
Mo	≤ 0.2
V	≤ 0.2
Cu	≤ 0.2
Ni	≤ 0.5
Ca	≤ 0.005, preferably 0.001 - 0.005
Al	≤ 0.1, preferably 0.01 - 0.1
P	≤ 0.05
S	≤ 0.01
N	≤ 0.008

remainder Fe and inevitable impurities, or
a composition (II) consisting of, in terms weight percentages (wt. %):

remainder Fe and inevitable impurities, or
a composition (III) consisting of, in terms weight percentages (wt. %):

C	0.1 - 0.13, e.g. 0.12
Si	0.2 - 0.29, e.g. 0.26
Mn	2.5 - 2.8, e.g. 2.60
Cr	0.3 - 0.5, e.g. 0.45
Ti	0.02 - 0.04, e.g. 0.025
B	0.002 - 0.005, e.g. 0.003
Ca	0.002 - 0.004, e.g. 0.003
Al	0.01 - 0.04, e.g. 0.025

remainder Fe and inevitable impurities;
- heating and hot-rolling to achieve a steel sheet with a finish rolling temperature of Ar₃ or above, typically 885 °C to 920 °C;
- accelerated cooling to a temperature of 720 °C or below;
- coiling at a temperature in the range of 600 °C to 720 °C, more preferably 640 °C to 670 °C; and
- cooling the coil to ambient temperature.

It is beneficial to apply a higher coiling temperature, which slows down the cooling rate of the hot-rolled coil in storage and therefore results in lower strength of the hot-rolled sheet. Consequently, cold rollability of the hot-rolled sheet is increased, which makes it possible to obtain a wide dimensional range for cold-rolled products. The microstructure of the intermediate hot-rolled steel sheet consists of less than 5% of pearlite and polygonal ferrite with a grain size above 10 µm, and the rest majority being bainite and/or fine-grained ferrite, and martensite, at all positions along the full length of the steel sheet. Fine-grained ferrite refers here to ferritic transformation products with a grain size in the order of typically 1 µm, which may nucleate above the bainitic temperature range, but are indistinguishable from low carbide containing bainitic ferrite by visual identification from a secondary electron microscope image. The martensite having variable carbon contents may be tempered and/or auto-tempered to variable degrees.
When the hot rolled coil has been cooled to ambient temperature, further process for producing the final product includes the steps of:

pickling the hot-rolled steel sheet;
cold rolling the pickled hot-rolled steel sheet, typically with a thickness reduction of 30 % to 70 %;
heating at an average rate in the range of typically 1 °C/s to 10 °C/s;
annealing at a temperature in the range of 780 °C to 860 °C;
optionally, (slow) cooling at an average rate of 10 °C/s or below to a temperature in the range of 720 °C to 780 °C;
(rapid) cooling at an average rate in the range of typically 5 °C/s to 50 °C/s to a holding temperature in the range of 440 °C to 525 °C;
holding at the holding temperature for 50 s to 200 s;
optionally, hot-dip coating; and
final cooling at an average rate in the range of typically 1 °C/s to 20 °C/s.

In some embodiments, the step of cooling from the annealing temperature in the range of 780 °C to 860 °C to the holding temperature in the range of 440 °C to 525 °C is continuous cooling at an average rate in the range of typically 5 °C/s to 50 °C/s.
In some embodiments, the step of cooling from the annealing temperature in the range of 780 °C to 860 °C to the holding temperature in the range of 440 °C to 525 °C is a two-step cooling comprising

a first slow cooling at an average rate of 10 °C/s or below to a temperature in the range of 720 °C to 780 °C; and
a second rapid cooling at an average rate in the range of typically 5 °C/s to 50 °C/s to the holding temperature in the range of 440 °C to 525 °C;

The first cooling step is not crucial. If omitted, the yield strength will increase slightly due to smaller amount of fine-grained ferrite in the microstructure.
The optional step of hot-dip coating is preferably hot-dip galvanizing or galvannealing.
Optionally, the method further comprises steps of temper rolling and/or levelling which are not crucial, but may improve yield strength of the final product.
Optionally, the method further comprises a step of extra batch annealing at a temperature in the range of 200 to 400 °C, e.g. 270 °C. The step of extra batch annealing results in bake hardening and tempering of the microstructure, which increases the yield strength of the steel while improving local formability parameters such as the hole expansion ratio.
The microstructure of the final steel sheet comprises a matrix consisting of, in terms of volume percentages (vol. %):

martensite 20 - 70

retained austenite ≤ 8, preferably ≤ 2, more preferably ≤ 1

pearlite and/or ferrite ≤ 5

remainder bainite and/or fine-grained ferrite.
The microstructure has an average grain size of less than 10 µm.
Preferably, the bainite and/or fine-grained ferrite have an average grain size of less than 5 µm, preferably in the range of 1 µm to 5 µm.
The bainite may comprise or consist of low carbide containing bainitic ferrite which is indistinguishable from fine-grained ferrite with a grain size of less than 2 µm. Together with martensite, bainite and/or fine-grained ferrite are the main microstructural components of the final steel sheet product. The martensite having variable carbon contents may be tempered and/or auto-tempered to variable degrees.
The final steel sheet has a thickness in the range of 0.8 - 2.2 mm, and a thickness accuracy better than 1/5 of EN 10143:2006 normal thickness tolerances at all positions along the full length of the steel sheet, excluding threading and tail-out sections in batch type cold rolling mills.
Tensile strength is determined mainly by the chemical composition of steel. The steel composition (I) is used for manufacturing a steel product with an ultimate tensile strength (Rm) of at least 950 MPa. The steel composition (II) is used for manufacturing a steel product with an ultimate tensile strength (Rm) of at least 980 MPa. The steel composition (III) is used for manufacturing a steel product with an ultimate tensile strength (Rm) of at least 1180 MPa.

It is preferable that the steel sheet further has at least one of the following mechanical properties:

Preferably, the high strength steel sheet with an ultimate tensile strength (Rm) of at least 980 MPa has a minimum bending radius (Ri) of 2.5 t or less.
Preferably, the high strength steel sheet with an ultimate tensile strength (Rm) of at least 980 MPa has a total elongation (A₈₀) of at least 8 %.
Preferably, the high strength steel sheet with an ultimate tensile strength (Rm) of at least 1180 MPa has a total elongation (A₈₀) of at least 5 %.
The following examples further describe and demonstrate embodiments within the scope of the present invention. The examples are given solely for the purpose of illustration and are not to be construed as limitations of the present invention, as many variations thereof are possible without departing from the scope of the invention.
The chemical compositions used for producing the tested steel sheets are presented in Table 1.
The production parameters of the inventive examples (Ex.) and the comparative example (Comp. Ex.) are summarized in Table 2.
The mechanical properties of the tested steel sheets are demonstrated in Table 3.

EXAMPLE 1

A steel slab having the composition grade A was prepared by conventional steel metallurgy and continuous casting. The slab was then hot rolled with a finish rolling temperature of 920 °C to produce a sheet of 2.5 mm thickness, then water cooled to a coiling temperature of 640 °C, and the coil was thereafter allowed to cool freely in coil storage. The hot-rolled steel sheet was then pickled and cold rolled with a thickness reduction of 40 % to a final thickness of 1.5 mm. Finally, the cold-rolled steel sheet was processed in a continuous galvanizing line, including steps of

heating at an average heating rate of 4 °C/s;
annealing at 830 °C for 81 s;
a first slow cooling to 745 °C, at an average cooling rate of 2 °C/s;
a second rapid cooling to 510 °C, at an average cooling rate of 11 °C/s;
holding at 510 °C for 100 s;
hot-dip galvanizing;
final cooling at an average rate of approximately 5 °C/s;
temper rolling and levelling.

The first cooling step is not crucial. If omitted, the yield strength will increase slightly due to smaller amount of fine-grained ferrite in the microstructure.
The steps of temper rolling and levelling are not crucial, but may improve yield strength of the final product.

Microstructure

Microstructures of the inventive steel Ex. 1 were investigated by scanning electron microscopy after preparation of cross-section samples by grinding, polishing, and etching with nital reagent. The constituent phases were visually identified and their relative fractions were determined based on surface proportions.
Figure 1(a) shows that the microstructure in the hot rolled condition comprises mainly bainite (bainitic ferrite - dark areas), and martensite with variable carbon contents or degrees of (auto)tempering (light areas).
Figure 1(b) shows that the microstructure in the cold-rolled and hot-dip galvanized condition comprises mainly bainite (bainitic ferrite - dark areas) and martensite (light areas).

Flatness

The flatness of cold rolled steel strips was measured by BFI flatness measurement roll with 54 piezoelectric encoders distributed across the barrel and expressed in I-units (IU) as 2D color map. The determination of the flatness distribution is realized by the measurement of the local deflection forces of the strip with the help of a measurement roll. This process is based on the fact that a strip which is under longitudinal tension, when deflected, exerts a radial force on the deflection roll.
From the flatness maps in Figure 2 it is obvious that the flatness of the cold rolled steel strip 2.5 → 1.51 x 1241 mm (a) according to the invention Ex. 1 has been improved compared to the comparative steel strip 2.8 → 1.51 x 1221 mm (b). Gray scales indicate the location and severity of buckles.

Thickness deviation

The center line thickness of cold rolled steel strips was measured by a non-contact X-ray thickness gauge.
Figure 3 shows that the cold rolled steel strip 2.5 → 1.51 x 1241 mm (a) according to the invention Ex. 1 has less thickness deviation along the body length than the comparative steel strip 2.8 → 1.51 x 1221 mm (b). High variations in the head and tail are due to threading and tail-out sections of the batch type cold rolling mill. Extra variation in the body of comparative strip (b) is due to the cold gauge hashing phenomenon, which is absent in the cold rolled steel strip (a) according to the invention Ex. 1. The dashed lines indicate 1/5 of EN 10143:2006 normal thickness tolerances.

Mechanical properties

The mechanical properties (Rm, Rp_0.2, and A₈₀ values) were determined by tensile testing of longitudinal test pieces according to EN ISO 6892-1:2009.
The minimum bending radius was determined by performing a 90° bending test with bend parallel to the longitudinal (rolling) direction, and measuring the minimum radii on approved bends.
The hole expansion test was carried out according to ISO 16630:2017.
The tested steel sheet has an ultimate tensile strength of 1032 MPa (Table 3).

EXAMPLE 2

A steel slab having the composition grade B was prepared by conventional steel metallurgy and continuous casting. The slab was then hot rolled with a finish rolling temperature of 920 °C to produce a sheet of 2.5 mm thickness, then water cooled to a coiling temperature of 670 °C, and the coil was thereafter allowed to cool freely in coil storage. The hot-rolled steel sheet was then pickled and cold rolled with a thickness reduction of 40 % to a final thickness of 1.5 mm. Finally, the cold-rolled steel sheet was processed in a continuous galvanizing line, including steps of

heating at an average heating rate of 4 °C/s;
annealing at 830 °C for 81 s;
a first slow cooling to 745 °C, at an average cooling rate of 2 °C/s;
a second rapid cooling to 510 °C, at an average cooling rate of 11 °C/s;
holding at 510 °C for 100 s;
hot-dip galvanizing;
final cooling at an average rate of approximately 5°C/s;
temper rolling and levelling.

The first cooling step is not crucial. If omitted, the yield strength will increase slightly due to smaller amount of fine-grained ferrite in the microstructure.
The steps of temper rolling and/or levelling are not crucial, but may improve yield strength of the final product.

Mechanical properties

The mechanical properties (Rm, Rp_0.2, and A₈₀ values) were determined by tensile testing of longitudinal test pieces according to EN ISO 6892-1:2009.
The minimum bending radius was determined by performing a 90° bending test with bend parallel to the longitudinal (rolling) direction, and measuring the minimum radii on approved bends.
The hole expansion test was carried out according to ISO 16630:2017.
The tested steel sheet has an ultimate tensile strength of 1230 MPa (Table 3).

EXAMPLE 3

heating at an average heating rate of 4 °C/s;
annealing at 839 °C for 62 s;
a first slow cooling to 755 °C, at an average cooling rate of 2 °C/s;
a second rapid cooling to 520 °C, at an average cooling rate of 14 °C/s;
holding at 520 °C for 75 s;
hot-dip galvanizing;
final cooling at an average rate of approximately 7°C/s;
temper rolling and levelling.

Mechanical properties

The mechanical properties (Rm, Rp_0.2, and A₈₀ values) were determined by tensile testing of longitudinal test pieces according to EN ISO 6892-1:2009.
The minimum bending radius was determined by performing a 90° bending test with bend parallel to the longitudinal (rolling) direction, and measuring the minimum radii on approved bends.
The hole expansion test was carried out according to ISO 16630:2017.
The tested steel sheet has an ultimate tensile strength of 1203 MPa (Table 3).

EXAMPLE 4

The hot-dip galvanized steel sheet of Example 3 was further batch annealed in a laboratory furnace at 270 °C with a holding time of 12 hours and tested after cooling to room temperature.

Mechanical properties

The mechanical properties (Rm, Rp_0.2, and A₈₀ values) were determined by tensile testing of longitudinal test pieces according to EN ISO 6892-1:2009.
The minimum bending radius was determined by performing a 90° bending test with bend parallel to the longitudinal (rolling) direction, and measuring the minimum radii on approved bends.
The hole expansion test was carried out according to ISO 16630:2017.

The tested steel sheet has an ultimate tensile strength of 1190 MPa (Table 3).

Table 1. Chemical composition (wt. %) of the tested steel sheets

Steel grade	C	Si	Mn	Al	Cr	Mo	Ca	Ti	B
A	0.09	0.26	2.65	0.025	0.3	-	0.003	0.03	0.003
B	0.12	0.26	2.60	0.025	0.45	-	0.003	0.025	0.003
C	0.165	0.24	1.85	0.025	0.5	0.15	0.003	-	-

Table 2. Production parameters of the tested steel sheets

Example	Steel grade	Hot rolling			Cold rolling		Hot-dip galvanizing							Extra batch annealing
Example	Steel grade	Finish rolling temp. (°C)	Hot rolled thickness (mm)	Coiling temp. (°C)	Cold rolling reduction (%)	Final thickness (mm)	Avg. heating rate (°C/s)	Annealing temp. (°C)	Annealing time (s)	Avg. rapid cooling rate (°C/s)	Holding start temp. (°C)	Holding time (s)	Avg final cooling rate (°C/s)	Extra batch annealing
Ex. 1	A	920	2.5	640	40	1.5	4	830	81	11	510	100	5	No
Ex. 2	B	920	2.5	670	40	1.5	4	830	81	11	510	100	5	No
Ex. 3	B	920	2.5	670	40	1.5	4	839	62	14	520	75	7	No
Ex. 4	B	920	2.5	670	40	1.5	4	839	62	14	520	75	7	Yes
Comp. Ex.	C	920	2.8	595	46	1.5	4	833	82	10	515	101	5	No

Table 3. Mechanical properties of the tested steel sheets

Example	Steel grade	Tensile strength Rm (MPa)	Yield strength Rp0.2 (MPa)	Yield Ratio	Total elongation A80 (%)	Minimum bending radius Ri	Hole expansion ratio (%)
Ex. 1	A	1032	814	79	9	1.3 t	32
Ex. 2	B	1230	950	77	7	2.5 t	25
Ex. 3	B	1203	892	74	7	-	19
Ex. 4	B	1190	1041	87	5	-	37
Comp. Ex.	C	1053	649	62	13	-	-

Claims

A high strength steel sheet having a composition consisting of, in terms weight percentages: C 0.07 - 0.13 Si 0.1 - 0.3, preferably more than 0.1 and less than 0.3 Mn 2.3 - 3.0 Cr 0.2 - 0.5 Ti 0.02 - 0.05 B 0.002 - 0.01 Mo ≤ 0.2 V ≤ 0.2 Cu ≤ 0.2 Ni ≤ 0.5 Ca ≤ 0.005, preferably 0.001 - 0.005 Al ≤ 0.1, preferably 0.01 - 0.1 P ≤ 0.05 S ≤ 0.01 N ≤ 0.008

remainder Fe and inevitable impurities, wherein the steel sheet has
an ultimate tensile strength R_m of at least 950 MPa, and a thickness accuracy better than 1/5 of EN 10143:2006 normal thickness tolerances,

wherein the steel sheet have a microstructure comprising a matrix consisting of, in terms of volume percentages: martensite 20 - 70 retained austenite ≤ 8, preferably ≤ 2, more preferably ≤ 1 pearlite and/or ferrite ≤ 5

remainder bainite and/or fine-grained ferrite, and

wherein the bainite and/or fine-grained ferrite have a microstructure with the average grain size of less than 10 µm.
The steel sheet according to claim 1, wherein the bainite and/or fine-grained ferrite have an average grain size of less than 5 µm, preferably in the range of 1 µm to 5 µm.
The steel sheet according to any one of the preceding claims, having at least one of the following mechanical properties: a minimum bending radius (Ri) ≤ 3.5 t, preferably ≤ 2.5 t a total elongation (A₈₀) ≥ 5 %, preferably ≥ 8 % a yield ratio (YR) 50 - 100, preferably 60 - 90 a hole expansion ratio (λ) ≥ 25 %, wherein these properties have been determined by the method mentioned in the description.
The steel sheet according to any one of the preceding claims, having a composition consisting of, in terms weight percentages: C 0.07 - 0.1 Si 0.2 - 0.29 Mn 2.5 - 2.8 Cr 0.2 - 0.4 Ti 0.02 - 0.04 B 0.002 - 0.005 Ca 0.002 - 0.004 Al 0.01 - 0.04

remainder Fe and inevitable impurities.
The steel sheet according to claim 4, wherein Ri ≤ 2.5 t and/or A₈₀ ≥ 8 %.
The steel sheet according to any one of the claims 1 to 3, having a composition consisting of, in terms weight percentages: C 0.1 - 0.13 Si 0.2 - 0.29 Mn 2.5 - 2.8 Cr 0.3 - 0.5 Ti 0.02 - 0.04 B 0.002 - 0.005 Ca 0.002 - 0.004 Al 0.01 - 0.04

remainder Fe and inevitable impurities, wherein $R_{m} \geq 1180 MPa .$
The steel sheet according to claim 6, wherein A₈₀ ≥ 5 %.
The steel sheet according to any one of the preceding claims is a strip having a thickness in the range of 0.8 - 2.2 mm.
A method for manufacturing the steel sheet according to any one of the preceding claims comprising the following steps of
- providing a steel slab with the composition according to any one of the claims 1, 4 and 6;

- heating and hot-rolling to achieve a steel sheet with a finish rolling temperature of Ar₃ or above;

- accelerated cooling to a temperature of 720 °C or below;

- coiling at a temperature in the range of 600 °C to 720 °C;

- cooling the coil to ambient temperature;

- pickling the hot-rolled steel sheet;

- cold rolling the pickled hot-rolled steel sheet;

- heating at an average rate in the range of 1 °C/s to 10 °C/s;

- annealing at a temperature in the range of 780 °C to 860 °C;

- optionally, cooling at an average rate of 10 °C/s or below to a temperature in the range of 720 °C to 780 °C;

- cooling at an average rate in the range of 5 °C/s to 50 °C/s to a holding temperature in the range of 440 °C to 525 °C;

- holding at the holding temperature for 50 s to 200 s;

- optionally, hot-dip coating; and

- final cooling at an average rate in the range of 1 °C/s to 20 °C/s.
The method according to claim 9, wherein the coiling temperature is in the range of 640 °C to 670 °C.
The method according to claim 9 or 10, further comprising steps of temper rolling and/or levelling.
The method according to any one of the claims 9 to 11, further comprising a step of extra batch annealing at a temperature in the range of 200 to 400 °C.
The method according to any one of the claims 9 to 12 wherein the optional step of hot-dip coating is hot-dip galvanizing or galvannealing.