Hot-rolled steel strip and method for producing same

Abstract

The invention relates to a hot-rolled steel strip or sheet according to claim 1, and which is in particular suited for structural members of construction machines and industrial machines requiring a good abrasive wear resistance and bendability. The invention also relates to a method of manufacturing such a hot-rolled steel strip.

Description

[0001] HOT-ROLLED STEEL STRIP AND METHOD FOR PRODUCING SAME

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a hot-rolled steel strip or sheet, and which is in particular suited for structural members of construction machines and industrial machines requiring a high strength, good abrasive wear resistance, impact toughness and bendability. The invention also relates to a method of manufacturing such a hot-rolled steel strip.

[0004] BACKGROUND TO THE INVENTION

[0005] Hot-rolled abrasive wear resistant steel products are typically used in harsh abrasive environments, such as in lifting and excavating applications. Typically the aim of the end users is to extend the service life of these abrasive wear resistant products as much as possible in order to reduce maintenance / downtime and therewith the costs.

[0006] There is a very strong correlation between the abrasion resistance and the surface hardness of steel, thus to further improve the durability of these abrasive wear resistant steel products, high strength, high hardness as well as high wear resistance properties are required. Therefore, hot-rolled martensitic steels with high hardness and desired impact toughness are extensively used in the lifting and excavating industry.

[0007] With the continuing development of hot-rolled abrasive wear resistant steel strip over the years and a demand for longer service time, the Brinell hardness has been steadily increased resulting in a Brinell hardness of 400 HBW and higher. The general notation to identify the abrasive-resistant steel grades is to classify them according to their surface hardness in terms of Brinell hardness (HBW), and the most common grades are 400 HBW, 450 HBW and 500 HBW. The high hardness and good hardenability are realized commonly by adding more carbon and / or alloying elements like Ni and Cu to the hot-rolled abrasive wear resistant steel strips. An important disadvantage is that as more alloying elements are added to increase the hardness and the yield strength of the steel strip, the bendability of the steel strip deteriorates. This problem limits the application potential of the steel strip since a high bendability is necessary for these applications while maintaining the other properties at a sufficient high level.This problem becomes more prominent as industrial machines tend to be lighter or upsized gradually. Such trends require that the self-weight of these machines be reduced. Thus, there is a demand for steel strips with a high strength. However, with increasing the strength they have difficulties in attaining the desired shapes as well as in realizing stable and facilitated manufacturing of hot-rolled steel shapes. That is, bendability is often not sufficient for producing complex parts.

[0008] Patent document EP2692890-B1 discloses an abrasion resistant hot-rolled steel plate suitable for use in construction machines, industrial machines, and the like, and a method for manufacturing the same. The steel plate has a composition, in wt.%, as its main alloying elements C: 0.20 to 0.27%, Si 0.05 to 1.0%, Mn: 0.30 to 0.90%, Nb: 0.005 to 0.025%, Ti: 0.008 to 0.020%, Al: 0.1% or less, and N: 0.0010 to 0.0060%, the composition further contains one or more of Cr, Mo, W, and B, one or more of Cu, Ni, V, an REM, Ca, and Mg as required, the remainder being Fe and inevitable impurities. After being heated, a semi-finished product having the steel composition is hot-rolled and is subjected to reheat-quenching or direct quenching. The presence of substantial amounts of Ni, Mo, Nb, and V are present which significantly increase the costs of the steel. The abrasion resistant hot-rolled steel plate has a good resistance to stress corrosion cracking. However, no bending related properties are disclosed.

[0009] Patent document EP3719148-B1 discloses a hot-rolled steel strip having a composition consisting of, in wt.%, 0.17-0.38% C, 0.01-0.5% Si, preferably 0.03-0.25% Si, 0.1-0.4% Mn, 0.015-0.15% Al, 0.1-0.6% Cu, 0.2-0.8% Ni, 0.1-1% Cr, 0.01-0.3% Mo, 0-0.005% Nb, 0-0.05% Ti, 0-0.06% V, 0.0005-0.005% B, 0-0.025% P, 0-0.008% S, 0-0.1% N, 0.0008-0.003% Ca, remainder Fe and inevitable impurities, wherein the steel product has a Brinell hardness in the range of 420 - 580 HBW, and a corrosion index (ASTM G101-04) of at least 5, and wherein the steel product has a microstructure consisting of, in terms of vol.%, martensite greater than or equal to 90, and 0-1 residual austenite, remainder bainite, ferrite and / or pearlite, and wherein, the steel strip product has a thickness of 10 mm or less, and the amount of Ti is in the range of 0-0.005% when the amount of N is in the range of 0-0.003%, the amount of Ti is more than 0.005% and not more than 0.05% when the amount of N is more than 0.003% and not more than O.01%, and wherein the steel product has a Charpy-V impact toughness of at least 34 J / cm2at a temperature of minus 20°C or minus 40°C in transversal and / or longitudinal direction. The steel strip requires the presence of substantial amounts of the alloying elements Cu, Ni, Cr, and Mo which significantly increase the costs of the steel. The hot-rolled steel strip has a good corrosion resistance, however, the bending behaviour in terms of r / t is not very good.

[0010] Patent document EP3631031-B1 discloses a high strength, hot rolled abrasive wear resistant steel strip, wherein the strip has a thickness in the range of 3-20 mm and has a microstructure comprising martensite, auto-tempered martensite with iron carbides and NbC, Nb(C,N) and NbV(C,N) particles and trace amounts of retained austenite in martensite-austenite islands, with low carbon equivalent values CEV, CET and Pern, and wherein the steel contains in wt.%: C: 0.13 - 0.29, Si: 0.01 - 0.05, Mn: 0.6 - 0.9, Cr: 0.05 - 0.8, Mo: 0.05 - 0.4, Ni: at most 0.1, Cu: at most 0.1, Al: 0.01 - 0.08, Ti: at most 0.02, B: at most 0.004, Nb: 0.005 - 0.035, V: 0.03 - 0.15, P: at most 0.020, S: at most 0.010, N: at most 0.006, H: at most 0.0004, Ca additions for sulphide shape control: 0.0005 - 0.005, wherein the total content of Nb + V is in a range of 0.035 - 0.16, other elements in amounts of impurity level, balance iron, and wherein CEV is at most 0.46, CET at most 0.34 and Pern at most 0.32, and wherein the strip has a Brinell hardness of at least 400 HBW and a tensile strength of at least 1316 MPa, CEV, CET and Pern being defined as follows:

[0011] CEV = C + Mn / 6 + Cr / 5 + Mo / 5 + V / 5 + Cu / 15 + Ni / 15;

[0012] CET = C + Mn / 10 + Mo / 10 + Cr / 20 + Cu / 20 + Ni / 40;

[0013] Pern = C + Si / 30 + Mn / 20 + Cu / 20 + Cr / 20 + Mo / 15 + V / 10 + Ni / 60 + 5B.

[0014] The steel composition requires substantial amounts of Cr, V, Nb, and Mo. Furthermore, the manufacturing process is complex as it requires the hot rolling slab to be contained in a hot box for a period of 2-6 days at a temperature in the range of 400-500°C, prior to reheating to at least 1150°C and keeping the reheated slab for a period of up to 3 hours, preferably of 0.3-3 hours, at the temperature of at least 1150°C, and subsequently hot rolling to the required thickness.

[0015] There is a demand for hot-rolled steel strips having high strength, good ductility, high wear resistance as expressed in hardness and having improved bendability.

[0016] DESCRIPTION OF THE INVENTION

[0017] As will be appreciated herein, for any description of steel compositions or preferred steel compositions, all references to percentages are by weight percent unless otherwise indicated.

[0018] As used herein, the term “steel strip” is defined to include steel strips and steel sheets.As used herein, the term "about" when used to describe a compositional range or amount of an alloying addition means that the actual amount of the alloying addition may vary from the nominal intended amount due to factors such as standard processing variations as understood by those skilled in the art.

[0019] The terms “up to” and “up to about”, as employed herein, explicitly include, but are not limited to, the possibility of zero weight-percent of the particular alloying component to which it refers. For example, up to 0.030% Nb may include a steel having no Nb.

[0020] As used herein, the term “at ! thickness” when used to describe microstructural features refers to an area between 1 / 8 and 3 / 8 of the steel strip thickness below the surface of the steel strip.

[0021] It is an object of the invention to provide a hot-rolled steel strip with a high tensile yield strength, improved bendability and high hardness.

[0022] It is another object to provide a method of producing a hot-rolled steel strip with a high tensile yield strength, improved bendability and high hardness.

[0023] These and other objects and further advantages are met or exceeded by the present invention providing a hot-rolled high-strength steel strip having:

[0024] i) a composition, in wt.%,

[0025] C: 0.15 to 0.30%,

[0026] Mn: 1.30 to 2.50%,

[0027] Si: 0.10 to 1.0%,

[0028] Al: 0.10 to 1.0%,

[0029] Si + Al: 0.50 to 1.50%,

[0030] Cr: 0.05 to 1.50%,

[0031] Ti: 0.001 to 0.10%,

[0032] Nb: up to 0.030%,

[0033] Mo: up to 0.50%,

[0034] Ni up to 0.50%,

[0035] Cu: up to 0.30%,

[0036] V: up to 0.10%,

[0037] Sn: up to 0.05%,

[0038] P: up to 0.02%,

[0039] S: up to 0.01%,N: up to 0.02%,

[0040] B: up to 0.007%,

[0041] Ca: up to 0.005%;

[0042] the remainder being residual and tramp impurities and Fe;

[0043] ii) and a microstructure having at !4-thickness as its main phrase a tempered martensite phase or a mixture of a low temperature bainite phase and a tempered martensite phase, not less than 95% in volume fraction;

[0044] iii) and having the following properties:

[0045] a minimum bending radius of not more than 2.5 x strip thickness (t), a Brinell hardness of more than 370 HBW,

[0046] a low-temperature Charpy impact toughness at -40°C of not less than 27 J in both transversal and longitudinal direction, and

[0047] a yield strength of at least 960 MPa.

[0048] The microstructure, the bending radius, the Brinell hardness, the Charpy impact toughness, the yield strength, the ultimate tensile strength, and the tensile elongation are measured as herein described.

[0049] In accordance with the invention and based on extensive research, the inventors have found that by careful control of the steel composition having these narrow compositional ranges (notably the purposive addition of Si, the control of the combined additions of Si and Al, and limiting the amounts of Nb and Ti to adjust the nonrecrystallization temperature of the steel), the hot rolling process, and the coiling temperature (CT) in particular, to allow for the completion of recrystallization after hot rolling, a hot-rolled steel strip is obtained having microstructural features (notably a uniformly distributed almost equiaxed austenite microstructure with fine grain size after hot rolling and prior to quenching) that provides for an improved bendability in combination with the favourable balance of other properties, in particular strength, hardness, weldability and corrosion resistance. The hot-rolled steel strip is in particular an ideal candidate for use in structural members of construction machines and industrial machines requiring a high abrasive wear resistant and bendability. The hot-rolled steel strip, in particular for a thickness up to about 6 mm, can also be used for automotive applications in the chassis and suspension (C&S) domain and areas including frame rails of trucks, bumper beams or battery boxes for electrical vehicles.

[0050] The inventors have realized that the presence of some elongated, pancake-shaped prior austenite grain structures, which have a higher aspect ratio, is to a large extentresponsible for the reduced bendability of the high strength hot-rolled steel strips. This highly non-homogeneous grain size distribution is generated by partial recrystallization which cannot be precisely controlled under most industrial conditions.

[0051] The hot-rolled steel sheet has an improved minimum bending radius of not more than 2.5 x strip thickness, preferably of not more than 2.0 x strip thickness, and more preferably of not more than 1.5 x strip thickness. Bending tests are performed using test pieces of size 60 mm x 30 mm sampled from the hot-rolled steel strips. The test pieces were subjected to a 180° bending test, and the minimum bending radius was determined by measuring the minimum inner bending radius (mm) which did not cause any cracks. The minimum bending radius / sheet thickness ratio was then calculated. Those steel strips with a minimum bending radius / sheet thickness ratio of not more than 2.5 were evaluated to be "excellent in bendability". The lower this ratio the better qualification of the bendability. The bending test pieces were sampled in two directions, viz. in L specimens, the length of the specimen is along the rolling direction, while the bending axle is perpendicular to the rolling direction, and in T specimens, the length of the specimen is along the transverse direction, while the bending axle is parallel to the rolling direction.

[0052] The Brinell hardness of the hot-rolled steel strip is at least 370 HBW, preferable of at least 385 HBW, and more preferably of at least 425 HBW, to ensure excellent abrasion resistance. In an embodiment the hardness does not exceed 480 HBW, and preferably it does not exceed 475 HBW. Too high a hardness has an adverse effect on bendability. The Brinell hardness (HBW) is measured according to test standards ASTM E10-14 and ISO 6506-1:2005.

[0053] The hot-rolled steel strip has parallel to the rolling direction (L-direction) a yield strength (Rp0.2) of at least 960 MPa, preferably of at least 1020 MPa, and more preferably of 1080 MPa.

[0054] The hot-rolled steel strip has an ultimate tensile strength (Rm) of at least 1050 MPa, preferably of at least 1100 MPa, and more preferably of 1200 MPa in the rolling direction. The tensile properties have been determined using JIS5 test pieces (gauge length is 50 mm and width is 25 mm) machined from the obtained hot-rolled sheets such that the tensile direction was parallel to the rolling direction. Room temperature tensile tests are performed in a ZwickRoell testing machine following NEN-EN10002-1:2001 standard to determine tensile properties, viz. the yield strength Rp0.2 in MPa, theultimate tensile strength UTS in MPa, and the tensile elongation A50 in %. For each condition, three tensile tests were performed and the average values of mechanical properties are reported.

[0055] The hot-rolled steel strip has parallel to the rolling direction a tensile elongation (A50) of at least 8.0%, and preferably of at least 8.6%.

[0056] The toughness at low-temperatures is determined by Charpy impact toughness tests in accordance with ASTM A370 (JIS Z 2242) to determine the absorption energy at a test temperature of -40°C. Charpy V-notched test pieces were sampled in two directions, viz. in L specimens, the length of the specimen is parallel to the rolling direction, while the notch is oriented along the transverse direction, and in T specimens, the length of the specimen is parallel to the transverse direction, while the notch is oriented along the rolling direction. The hot-rolled steel strip has a low-temperature Charpy impact toughness of not less than 27 Joule, and preferably of not less than 30 Joule in both the transversal and longitudinal direction.

[0057] The hot-rolled steel strip of this invention has as key feature at !4-thickness a microstructure consisting of as its main phase a tempered martensite phase or a mixture of a low temperature bainite phase and a tempered martensite phase, not less than 95% in volume fraction, and preferably not less than 97 vol.%. The term “low temperature bainite” indicates bainite transformed at lower temperatures below Ms. The second phase other than the main phase is a ferrite phase and / or a pearlite phase and / or a retained austenite. Thus, the volume fraction of the second phase is preferably not more than 5 vol.%, and preferably not more than 3 vol.%. With an increasing fraction of the second phase in the microstructure the desired high strength and wear resistance cannot be ensured and the bendability is lowered. This microstructure feature is characterized on a cross section of RD-ND plane (RD is the rolling direction and ND is the normal direction) !4-thickness of a hot-rolled strip.

[0058] In a preferred embodiment, the average grain diameter of prior austenite (y) grains measured at !4-thickness with respect to a cross section parallel to the rolling direction is not more than 30 pm, and preferably not more than 25 pm. In an embodiment the average grain diameter of prior austenite (y) grains measured at !4-thickness with respect to a cross section parallel to the rolling direction is at least 10 pm.

[0059] In an embodiment, the aspect ratio of the prior austenite (y) grains (defined as the average length of prior y grains in RD divided by the average length of prior y grains in ND) is not more than 3.0, and preferably is not more than 2.5. With this configurationindicating a more equiaxed grain structure, the hot-rolled steel strips achieves high strength and excellent bendability. The aspect ratio of the prior austenite (y) grains is at least 1.0.

[0060] Another key feature of the invention is related to the composition (in wt.% or weight percent) of the hot-rolled steel strip.

[0061] C: 0.15 to 0.30%. Carbon (C) is a necessary element for increasing the hardenability, the strength and room temperature surface Brinell hardness of the steel strip. Carbon content below 0.15% is not sufficient to obtain the high strength guaranteed by this invention. Also, lower carbon contents reduce the hardenability so that a higher cooling rate is needed after hot rolling to avoid the formation of non-martensitic microstructures, which can reduce the strength and the bendability due to the introduction of weak interfaces with the martensite. On the other hands, if C is above 0.30%, weldability and bendability of the steel product will be poor. In an embodiment the C content is at least 0.18%, and preferably at least 0.21%. In an embodiment the C content does not exceed 0.26%.

[0062] Mn: 1.30 to 2.50%. Manganese (Mn) is a necessary element for increasing the hardenability, yield strength, ultimate tensile strength and hardness of the steel strip. Mn levels below 1.30% will not give the required strengthening and the hardenability. If Mn level is above 2.50%, Mn will increase the formation of segregation bands after casting which will remain in the final product. This will contribute to poor bendability due to heterogeneous microstructure. In an embodiment the Mn content is at least 1.50%. In an embodiment the Mn-content does not exceed 2.20%, and preferably does not exceed 1.90%.

[0063] Si: 0.10 to 1.0%. Silicon (Si) increases the strength of steel by effecting solid solution hardening and by improving hardenability. These effects are obtained by adding 0.10% or more Si. If Si is added in an amount exceeding 1.0%, carbon is concentrated in the y phase and the y phase stabilization is promoted to lower strength, and too high a Si content deteriorates the quality of the welds. To suppress the formation of retained austenite, the Si content is not more than 1.0%. Due to the presence of Si there is a more homogeneous austenite formation during hot rolling resulting in a more homogeneous martensite microchemistry. In an embodiment the Si-content is at least 0.30%, and preferably at least 0.50%. In an embodiment the Si content does not exceed 0.80%.Al: 0.10 to 1.0%. Aluminium (Al) is a necessary element for deoxidising during steelmaking and together with Si is an important element for the present invention. Al suppresses the formation of coarse cementite and increases the low temperature toughness. However, excessive Al of more than 1.0% will increase the number of Al-based coarse inclusions, resulting in the degradation of bendability. Al also increase the non-recrystallization temperature of a steel, which leads to the formation of partially recrystallization austenite. The Al content is at least 0.10%, and preferably at least 0.20%. In an embodiment the Al content does not exceed 0.50%.

[0064] The combined addition of Si+AI should be at least 0.50%, and preferably least 0.70%, and more preferably at least 0.80%. Si and Al are added in the present steels to promote homogeneous austenite formation during hot rolling, so that a homogeneous martensite phase is formed after rapid cooling and coiling to room temperature and also carbide formations during cooling and coiling from austenite are suppressed. The minimum amount of Si+AI is 0.50% to have optimum desirable effects. The combined addition of Si+AI does not exceed 1.50%, and preferably does not exceed 1.30%, and more preferably does not exceed 1.10%. Si and / or Al contents above the upper limits defined here will increase the rolling force during hot rolling and also will cause excessive oxidation of the steel surface.

[0065] Cr: 0.05 to 1.50%. Chromium (Cr) is a necessary element for increasing the hardenability of the steel strip and reduces the critical cooling rate for the martensite formation. Below 0.05% it is taken as a tramp element having a limited or no effect. In an embodiment the Cr content is at least 0.10%, and preferably at least 0.15%. In an embodiment the Cr content does not exceed 0.80%, and preferably does not exceed 0.40%, as too high a Cr content result in poor performance in weldability.

[0066] Ti: 0.001-0.10%. Titanium (Ti) is present in the steel in a range from 0.001% to up to 0.10% to form fine carbide precipitates preventing austenite grain growth during reheating, and also to prevent the occurrence of cracks in the steel slab by fixing nitrogen in the form of nitride. These effects are markedly obtained when the Ti content is 0.001% or above, and better results are achieved when at least 0.005%. If the Ti content exceeds 0.10%, however, the amount of coarse TiN particles formed from liquid during solidification is increased and toughness of the steel strip is lowered. Further, heating at a high temperature of above 1250°C is entailed for the melting of titanium carbonitride to invite the coarsening of prior austenite grains, thus making it difficult to adjust the aspect ratio of prior austenite grains to the desired range. Thus, preferably thecontent of Ti is limited to the range of 0.001% to 0.10%, and more preferably of 0.005% to 0.080%.

[0067] In a preferred embodiment the hot-rolled steel strip has a composition comprising:

[0068]

[0069] Niobium (Nb) can be present in the steel up to 0.030%. Although Nb is finely precipitated as carbonitride and increases the strength of hot rolled steel strip, it can lead to a significant increase in the non-recrystallization temperature of the steel and thus promotes the formation of partially recrystallized microstructure, which is thought to be a main cause for low bendability of a hot-rolled steel strip. Preferably, Nb is present only up to 0.020%, and more preferably up to 0.010%.

[0070] Molybdenum (Mo) up to 0.50% is an optional alloying element in the steel. In particular it may be added when producing a steel strip having a high thickness of more than 6 mm. Mo increases the strength of steel strips by improving hardenability as well as by forming carbonitride. In order to obtain these effects, the Mo content is desirably not less than 0.020%. Mo less than 0.020% is taken as residual element. If molybdenum is present in a high content exceeding 0.50% weldability is lowered. Thus, the content of Mo when purposively added as alloying element is limited to the range of 0.020% to 0.50%, and preferably of 0.05% to 0.30%. In an embodiment Mo is not added as a purposive alloying element and is present as an impurity to a level of up to 0.020%, and preferably up to 0.010%.Vanadium (V) up to 0.10% is an optional alloying element in the steel strip. It can be added when the Ti addition is in the lower end of the described range. V contributes to increasing the strength of steel strip by solid solution hardening and by precipitation hardening. In order to obtain these effects, the V content is preferably not less than 0.005%. If V is present in excess of 0.10%, however, toughness of the steel strip is lowered. Thus, the content of vanadium, when purposively added, is limited to the range of 0.005% to 0.10%, and preferably of 0.02% to 0.05%. V less than 0.005% is taken as a residual element. In an embodiment V is not added as a purposive alloying element and is present as an impurity to a level of up to 0.005%.

[0071] Copper (Cu) is not required in the steel of the present invention but it can come from scraps to a level of up to 0.30%. Cu may contribute to increasing strength by being dissolved in the steel, and also improves corrosion resistance. In order to obtain these effects, the Cu content is desirably not less than 0.05%. However, any Cu content exceeding 0.30% results in deteriorations in surface properties of steel strip by the segregation of Cu in the surface layer, which may also reduce the bendability. Thus, the content of Cu, when present, is limited to not more than 0.30%, preferably not more than 0.20%. In an embodiment Cu is not added as a purposive alloying element and is present as an impurity to a level of up to 0.05%.

[0072] Nickel (Ni) is not required in the steel but it can come from scraps to a level of up to 0.50%. Ni may contribute to strength increasing by being dissolved in steel, and improves toughness of the steel strip. Ni can also reduce the Cu segregation at the surface. In order to obtain these effects, the Ni content is desirably not less than 0.05%. However, adding nickel to a high content exceeding 0.50% results in an increase in material costs. Thus, the content of nickel, when purposively present, is limited to 0.05% to 0.50%, and preferably of 0.05% to 0.30%. In an embodiment Ni is not added as a purposive alloying element and is present as an impurity to a level of up to 0.05%.

[0073] In an embodiment one or more, or all, of V, Cu, Ni, Mo, and Nb, are not purposively added and only present as an inevitable impurity resulting from the ironmaking and steelmaking process.

[0074] Tin (Sn) comes from steelmaking raw materials such as scraps. Tin is an element that is easily segregated in grain boundaries and the like. When Sn is present in a large amount exceeding 0.05%, the grain boundary strength is lowered and the toughness isdecreased. Thus, it is desirable that the Sn content is not exceeding 0.05%, preferably it is not exceeding 0.02%, and more preferably not exceeding 0.01%.

[0075] Nitrogen (N), sulphur (S) and phosphorus (P) are residual elements present in the steel as a result of steelmaking and refining process. Their amounts are limited to up to about 0.01% S, up to about 0.02% P, and up to about 0.02% N. Amounts higher than these are detrimental for mechanical properties, formability, toughness, fatigue, and weldability. In an embodiment P is present only up to 0.015%. In an embodiment S is present only up to 0.005% (50 ppm) and more preferably only up to 0.0025% (25 ppm), and most preferably up to 0.0012% (12 ppm). In an embodiment N is present only up to 0.006%, and preferably up to 0.003% as an excessively high N content increases the frequency of cracks during the casting of steel slab and decrease that toughness of the steel strip.

[0076] Boron (B) is an element that can be added to the steel up to 0.007% to increase the hardenability, and is in particular useful for the thicker gauge hot-rolled steel strips. B is segregated in austenite (y) grain boundaries and markedly improves hardenability when added in a low content. To obtain the effect, the B content is desirably not less than 0.0001%. On the other hand, the effects are saturated after 0.007% and thus any further addition cause the formation of TiB2 or NB coarse particles, which are deleterious to toughness. Thus, the content of boron, when purposively added, is limited to the range of 0.0001% to 0.007%, and preferably of 0.0005% to 0.0050%.

[0077] Calcium is added for the Ca treatment of the steel to control sulfide shape and composition; this results in a modification to the MnS inclusions, resulting in an improved Charpy toughness and bendability but also improving processability. Other potential improvements associated with Ca additions (and low S) would be a reduction of welding defects such as lamellar tearing. In an embodiment the amount of Ca in the invention is 0.0005 to 0.005%. However, when Ca is added excessively, the effect is saturated and the economic efficiency is reduced. Therefore, it is better to maintain Ca levels below 0.005%.

[0078] The remainder being iron and residual and tramp impurities from the ironmaking and steelmaking process.

[0079] In an embodiment the hot-rolled steel strip has a composition consisting of, in wt.%, C: 0.15 to 0.30%, Mn: 1.30 to 2.50%, Si: 0.10 to 1.0%, Al: 0.10 to 1.0%, Si+AI: 0.50 to 1.50%, Cr: 0.05 to 1.50%, Ti: 0.001 to 0.10%, Nb: up to 0.030%, Mo: up to0.50%, Ni up to 0.50%, Cu: up to 0.30%, V: up to 0.10%, Sn: up to 0.05%, P: up to 0.02%, S: up to 0.01%, N: up to 0.02%, B: up to 0.007%, Ca: up to 0.005%, the remainder being residual and tramp impurities and iron; and with more preferred ranges as herein described and claimed.

[0080] In an embodiment the hot-rolled steel strip has a composition consisting of, in wt.%, C: 0.15 to 0.30%, Mn: 1.30 to 2.50%, Si: 0.10 to 1.0%, Al: 0.10 to 1.0%, Si+AI: 0.50 to 1.50%, Cr: 0.05 to 1.50%, Ti: 0.001 to 0.10%, Nb: up to 0.030%, Mo: up to 0.020%, Ni up to 0.05%, Cu: up to 0.05%, V: up to 0.005%, Sn: up to 0.05%, P: up to 0.02%, S: up to 0.01%, N: up to 0.02%, B: up to 0.007%, Ca: up to 0.005%, the remainder being residual and tramp impurities and iron; and with more preferred ranges as herein described and claimed.

[0081] In an embodiment the thickness of the hot-rolled steel strip is in a range of 3 to 25 mm, and preferably of 4 to 12 mm.

[0082] In another aspect of the invention it relates to a method of manufacturing a hot-rolled steel strip as herein described and claimed, the method comprising the steps of, in that order,

[0083] casting a slab having a composition as set out in this description and any of the claims;

[0084] reheating of the material for homogenization of the chemistry to a temperature above 1200°C;

[0085] rough rolling followed by hot rolling to desired thickness and followed by finish rolling with a finish rolling temperature above the non-recrystallization temperature Tnr in the austenitic phase range;

[0086] cooling from above the Ar3 temperature to a temperature below the martensitic transformation temperature (Ms) on the run-out table (ROT) with a fast enough cooling rate with an average cooling rate of 40°C / s or more, preferably of 50°C / s or more, that can mostly avoid austenite decomposition to ferrite or pearlite or bainite;

[0087] coiling of the hot-rolled steel strip at a temperature below the Ms temperature and cooling to room temperature to obtain a microstructure having as its main phrase a tempered martensite phase or a mixture of a low temperature bainite phase and a tempered martensite phase, not less than 95% in volume fraction;optionally temper annealing at a temperature in a range of 150°C to 380°C for a time span of 1 min to 24 hours, and preferably in a range of 250°C to 350°C for 1 to 10 hours.

[0088] The method of manufacturing as herein described and claimed results in a hot-rolled steel strip having the desired microstructure providing for the aimed improved balance of properties. The invention is also embodied in a hot-rolled steel strip manufactured by the method described herein and claimed having said microstructure and improved balance of properties.

[0089] In a first step, a slab having the above-described compositions is cast. There is no particular limitation on the production method before the hot rolling process. That is, subsequently to melting in a blast furnace, electric arc furnace, or the like, secondary refining is performed in a manner so that the steel composition is adjusted to be within the range according to the invention. Scraps may be used as a raw material. The molten steel is cast into slabs using continuous cast lines.

[0090] Next the cast slab is reheated to a temperature of 1200°C or more, and subjected to a hot rolling step in which the steel is rough rolled into a plate, which plate is then subjected to finish rolling into a hot-rolled strip in such a manner that hot-rolling is completed at a temperature of Tnr or more, Tnr is given by the equation hereinafter. Preferably, after the completion of the finish rolling, the steel strips are hold on the line for 1 to 20s before the steel sheet is discharged from the finish rolling stand. Subsequently, the steel strip is cooled at a temperature not lower than the Ar3 temperature of the steel, where Ar3 is the temperature at which temperature transformation of austenite to ferrite starts during cooling and can be calculated by equation given hereinafter, at a cooling speed of 40°C / s or more to a temperature below the respective Ms point of the steel and is then coiled at a temperature of less than the Ms point, preferably of less than Ms-30°C. Ms is the temperature at which the transformation from austinite to martensite starts and can be calculated by the equation given hereinafter. The cooling rate during coil cooling is not critical. The coil can be left in the air for natural cooling or can be also soaked into water for a faster cooling.

[0091] Tnr(°C) = 887 + 464C + 36341 - 3575i + 890 + (?32 - 230-^ ) + (6445JW - 644y®5) + 6 3e-ft3,6£-3 (°C) = 910 - 3 IOC - 80Mn - 15( u + +Mi + Cr) + 335i + 4W1 + 0.35(11 - 8)

[0092]

[0093] 540 - 420C - 3SMn - 12Cr - 2QMi - 21Af o - USi + 20^1

[0094] In these equations Nb, Ti, V, Al, C, Si, Mn, Ni, and Mo are the elements in wt.% of the steel, e is the rolling pass reduction, h is the final thickness of the hot-rolled steel strip. The elements which are absent in the steel are regarded as zero in the calculation. It can be seen that Nb and Ti play a significant role in influencing the temperature Tnr and being the reason for the strict control of their presence in the steel as herein described.

[0095] The slab reheating temperature before hot rolling needs to be 1200°C or more. In the steel according to the invention, austenite grains are prevented from being coarse by using dissolved Ti, and accordingly, it is necessary to dissolve coarser TiC particles that have been precipitated during casting. If the reheating temperature is less than 1200°C, TiC will take a long time to be melted, and thus the austenite grain size will not be refined. On the other hand, the invention aims to obtain the fine grain structure by stimulating recrystallization in the finish stage. Considering the amount of temperature decrease in the finish rolling mill, a higher reheating temperature can ensure that the finish rolling temperature is above the Tnr. Therefore, the temperature for heating the slab needs to be 1200°C or more. However, excessively high temperature for heating can lead to austenite grain growth, which may decrease strength and toughness. Therefore, the upper limit on the temperature for heating the stab is desirably less than about 1300°C.

[0096] Next, a hot rolling step is performed to convert the slab into strip, which includes a rough rolling step and a finish rolling step.

[0097] The rough rolling conditions are not particularly limited as long as the steel may be rolled into a plate with desired size and shape. However, it is preferable that the plate thickness after rough rolling be designed in consideration of the amount of temperature drop in the finish rolling mills. For example for producing hot-rolled steel strips having a thickness of about 3 mm to 12 mm, the plate thickness after rough rolling is preferably controlled to about 30 to 45 mm.

[0098] The finish rolling in the hot rolling step is usually tandem rolling in which the time intervals between passes are short. During this time recrystallization remains partial, orlimited as recrystallization of any kind requires certain time for completion. This step is important for the invention to control the microstructure of a product to improve its bendability by uniformly and finely recrystallizing the austenite in the finish rolling. Because of very high sensitivity of recrystallization kinetics to slight variations in workpiece temperature and speed, amount of strain applied, etc., and chemical composition, the finish hot rolling should be precisely controlled.

[0099] To obtain a fully recrystallized austenite microstructure, the finish rolling end temperature should be higher than the Tnr temperature. The temperatures in finish rolling are surface temperatures. If the final rolling is performed in a temperature range of less than Tnr, austenite will be unlikely to be recrystallized and grains extending in the rolling direction will be generated, easily causing the degradation of toughness and bendability. Furthermore, when uncrystallized austenite is transformed into martensite or bainite, dislocations accumulated in austenite are inherited to martensite. Moreover, with the decreasing of finish rolling temperature, the microstructure of the hot rolled steel was changed from lath martensite to complex microstructures composed of lath martensite, ferrite, granular bainite and lath bainite, uncrystallized prior austenite grains are the main reason for the formation of complex microstructures. Therefore, the finish rolling temperature is Tnr or more. Preferably the finish rolling temperature does not exceed 960°C such that a smaller grain size is obtained. Moreover, the reduction schedule of each pass during finish rolling should be optimized. It is preferred to set the reduction ratio in the last pass to 30% or more to reduce the Tnr and to accelerate the recrystallization of austenite.

[0100] In a preferred embodiment, after the completion of the finish rolling and prior to cooling on a run-out table, the steel strip is not immediately discharged from the finish rolling stand but is retained on the line for about 1 to 20s, and preferably of 1 to 15s, before applying the rapid cooling. The holding time or retention time should be selected such that the starting cooling temperature should be not lower that the Ar3 temperature. The purpose of this retention is to allow the recrystallization to be complete. If the retention time before the start of cooling is too short, recrystallization may not be complete so partially recrystallized austenite grain may form adversely affecting final engineering properties, in particular bendability and toughness, of the steel strip. On the other hand, if the retention time is prolonged, the temperature drop is too much which leads to the formation of non-martensite microstructures during subsequent rapidcooling, which leads to a decrease in strength and bendability. In addition, too long a retention time slows down the manufacturing process.

[0101] In the case for producing thinner hot-rolled strips, in particular when less than 6 mm in thickness, where the temperature drop is too fast during the retention, the holding treatment may involve methods such as induction heating to keep the temperature of the strip above the Ar3 temperature. If additional heating is applied, the temperature of the steel strip should not exceed Tnr+50°C to ensure the complete recrystallization of the austenite but to avoid significant grain growth of the austenite.

[0102] In the rapid cooling step, the steel sheet is subjected to a cooling treatment in which the strip is cooled to a cooling termination temperature in the range of the Ms point of the steel to room temperature. If the cooling termination temperature is higher than the Ms temperature, it is not possible to ensure the required microstructure having a lower temperature-transformed bainite phase or a tempered martensite phase as the main phase or containing a mixture of these phases. In an embodiment the cooling termination temperature is below Ms minus 30°C (Ms-30°C).

[0103] It is necessary to perform rapid cooling following finish rolling at an average cooling rate of 40°C / s or more, preferably of 50°C / s or more, from the final rolling temperature to the Ms point. If the average cooling speed is less than 40°C / s, ferrite or pearlite or upper bainite might be formed halfway on the cooling, and it will become difficult to make the volume ratio of the main phase, one or both of tempered martensite and lower bainite, be 95 vol.% or more.

[0104] After the completion of the rapid cooling step, the steel strip is coiled into a coil at a coiling temperature below the Ms temperature, and preferably below Ms-30°C, to room temperature. During coil cooling, the remaining austenite transforms to martensite or bainite, tempering proceeds in the formed martensite, forming fine carbides, reducing dislocation density and relieving stress. The coiling step can be realized by transferring the coil to a furnace with a pre-set temperature or putting the coil in a hot water pool or to leave the coil in air for natural cooling. For hot-rolled steel plate engineering applications in parts requiring a high resistance and where the high hardness is required for abrasion in addition to toughness and bendability, a lower cooling termination temperature and a lower coiling temperature are recommended. A preferred range for the cooling termination temperature for these application is in a range of 25°C to 200°C.For hot-rolled steel plate engineering applications in lifting and machinery parts, where a high ductility is required in addition to high toughness and bendability, a higher cooling termination temperature and a higher coiling temperature are recommended. A preferred range for cooling termination temperature for these application is 200°C to 375°C.

[0105] In embodiment, the hot-rolled strip following coil cooling can be further heat-treated by temper annealing with a peak temperature from 150°C to 380°C to in an inline or off-line manner in order to obtain a slightly different set of steel plate properties. The annealing time at peak temperatures should be in a range of 1 min to 24 hours, and preferably in a range of 1 to 10 hours. For industrial-scale steel coils, in a tempering annealing treatment the hot-rolled coil is heated up at a slow heating rate of about 20-50°C / h to the peak temperature, soaked at the peak temperature and subsequently cooled at a cooling rate of about 30-60°C / h to room temperature. As the toughness and the bendabillity increase while the strength and hardness decrease with increasing the tempering temperature, the strength of the strip can be adjusted by selecting different tempering temperatures to meet different set of requirements of different parts in use for components requiring wear resistance and high bendability. The tempering temperature should be not higher than 380°C to ensure that the minimum yield strength is at least 980 MPa and the ultimate tensile strength is at least 1050 MPa.

[0106] After the coiled steel strip has cooled to room temperature, the oxides (scale) on the hot-rolled steel strips can be removed either by pickling in an acid solution (e.g., HCI) at warm temperatures (about 80-120°C) or by a combination of pickling and mechanical brushing of the strip surface as is known in the art. This step is necessary for rendering the steel strip surface suitable for direct use as uncoated hot-rolled steel or making it amenable to the coating process, when optionally needed for improved corrosion resistance.

[0107] Furthermore, after pickling, the resulting hot-rolled steel sheet may be subjected to skin-pass or cold rolling at a reduction of 3% or less in an in-line or off-line manner to improve strip flatness.

[0108] The hot-rolled steel strip product can be a bare product or uncoated product or it can be provided on one or both of its main surfaces with a thin metallic coating layer, typically up to about 100 g / m2per side of the steel strip, and preferably up to about 50 g / m2per side. The metallic coating is preferably selected from the group comprising an aluminium alloy coating (e.g., an Al-Si alloy, or Al-Zn alloy), a magnesium alloy coating,a zinc coating, and a zinc alloy coating (e.g., a Zn-AI alloy, Zn-Mg alloy, Zn-Fe alloy, Zn-Al-Mg alloy, or Zn-Mg-AI alloy).

[0109] The invention will now be illustrated with reference to non-limiting comparative and examples according to the invention.

[0110] EXAMPLES

[0111] Four steels A to D having compositions as listed in Table 1 were cast by melting charges in a vacuum induction furnace into 25 kg ingots of 200x110x110 mm in dimensions. Then, the following process schedule was used to manufacture hot rolled strips of 6 mm thickness to mimic the manufacturing process in accordance with the invention:

[0112] • Reheating of the ingots at 1230°C for 1 hour;

[0113] • Rough rolling of the ingots from 200 mm to 35 mm;

[0114] • Reheating of the rough-rolled ingots at 1230°C for 30 min;

[0115] • Hot rolling from 35 mm to 6 mm (25 - 16- 9- 6 mm);

[0116] The reduction per pass in the finish rolling is 36%, 44% and 33% successively. The strips were reheated to 1230°C for 10 min before the last pass. The finish rolling temperature (FRT) is as set out in Table 2 and retained for a short period of time prior to rapid cooling;

[0117] • Run out table cooling according to the process parameters as given in Table 2;

[0118] • T ransfer the strip to a preheated furnace at coiling temperature;

[0119] • Cooling the strip with controlled cooling rate to room temperature to simulate the coil cooling process;

[0120] • Pickling the hot rolled sheets in HCI at 85°C to remove the oxide layers;

[0121] • Test pieces for microstructure observations, tensile tests, bending tests and toughness tests were sampled from the hot-rolled strips.Table 1. The compositions (in wt.%) of tested steel.

[0122] Balance is Fe and unavoidab e impurities.

[0123]

[0124]

[0125] The microstructure was determined by optical microscopy (OM) using a commercially available image-processing program. The microstructure was observed on the cross section with plane normal oriented to the transverse direction of the hot rolled strips. The observation took place at !4-sheet thickness. The polished cross sections were etched by using a 3%-nital solution. Under optical microscopy, ferrite is revealed as white, pearlite and carbide are revealed as black, a mixture of bainitic ferrite, martensite and tempered martensite or partitioned martensite is revealed as grey. A very small amount of nano scale carbides might be present inside temperate martensite. The prior austenite grains were revealed by etching using picric acid. The average austenite grain size and the aspect ratio (average length of prior austenite grains in rolling direction) / (average length in direction perpendicular to rolling direction) were measured. The retained austenite was measured using x-ray diffraction method. The minority second phases such as MnS, TiN, and AIN were characterized using optical microscopy on the polished but unetched samples.

[0126] The Charpy impact test was performed in accordance with ASTM A370 (JIS Z 2242) to determine the absorption energy at a test temperature of -40°C. Since the steel strips of the examples have a thickness of 6 mm, half sub-sized Charpy V-notched test pieces were sampled with two directions: in L specimens, the length of the specimen is parallel to the rolling direction, while the notch is oriented along the transverse direction, in T specimens, the length of the specimen is parallel to the transverse direction, while the notch is oriented along the rolling direction. The obtained absorption energy values were arithmetically averaged of 3 tests. Data measured with respect to subsize test pieces areconverted to full size equivalent. The hot-rolled steel strip has a low-temperature Charpy impact toughness of not less than 27 Joule, and preferably of not less than 30 Joule in both testing directions.

[0127] The Brinell hardness (HBW) was measured according to test standards (ASTM E10-14 and ISO 6506-1:2005).

[0128] Bending test pieces of size 60 mm x 30 mm were sampled from a prescribed position of the hot-rolled strips. The test pieces were subjected to a 180° bending test, and the minimum bending radius was determined by measuring the minimum inner bending radius (mm) which did not cause any cracks. The minimum bending radius / sheet thickness ratio was then calculated. The bending test pieces were sampled with two directions, namely in L specimens, the length of the specimen is along the rolling direction, while the bending axle is perpendicular to the rolling direction, and in T specimens, the length of the specimen is along the transverse direction, while the bending axle is parallel to the rolling direction. Those steel strips with a minimum bending radius / sheet thickness ratio of not more than 2.5 were evaluated to be “excellent in bendability”.

[0129] Table 2. The finish hot-rolling process parameters and the relevant temperatures of the steels.

[0130]

[0131] And with:

[0132] FRT is the finish rolling temperature;

[0133] Tnr is the non-recrystallization temperature calculated from the equation herein above given;

[0134] Ar3 is the starting temperature of austenite to ferrite during cooling calculated from the equation herein above given;

[0135] Ms is the martensite starting transformation point calculated from the equation herein above given.

[0136] The microstructures of the final hot rolled strips are shown in Table 3. The matrix comprises of a mixture of tempered martensite, martensite and bainite. The fraction of the retained austenite and other non-martensite microstructures is less than 5 vol.%. The sum of the minority second phase particles such as MnS, AIN and TiN is less than 0.1 vol.%. The average prior austenite grain size in all the steels is 18 to 30 pm with an aspect ratio of less than 2.5. Examples B-1, B-2 and B3 are out of the invention because the steels contain too many coarse TiN particles due to the high Ti content, which can exacerbate the low temperature impact toughness. Example D-4 shows that if the retention time after hot rolling is too long, the prior austenite grain will coarsen and non-martensite microstructure will form, which will deteriorate all the mechanical properties as can be seen from Tables 4, 5 and 6.

[0137] The properties of the steels in the as-coiled conditions after coiling at 25°C (room temperature), 275°C and 375°C are listed in Table 4, Table 5 and Table 6, respectively.

[0138] Depending on the coiling temperature the yield strength of the steels varies from 980-1150 MPa, and the tensile strength varies from 1192-1440 MPa. The Charpy impact toughness at cryogenic temperatures up to -40°C is >27 Joule, which meets the requirement for high strength abrasion resistance steels and engineering construction steels. The bendability of the tempered steels is at least r = 2.5t, which is much higher than that of the most commercialised high strength abrasion resistance hot-rolled steels and engineering construction steels.

[0139] The effect of the temper annealing treatment on the mechanical properties are shown in Table 7 for the hot-rolled strip coiled at 25°C and Table 8 for the hot-rolled strip coiled at 375°C. To simulate the tempering treatment for industrial-scale steelcoils, the hot-rolled coil is heated up at a slow heating rate of 30°C / h to the peak temperature and then is cooled at 40°C / h to room temperature. The results show that the strength of the hot-rolled steels remain relatively high when a tempering treatment at a temperature below 350°C for 10 hours while the toughness and the endability increase with increasing the tempering temperature. The strength of the hot rolled strips can be adjusted by selecting different tempering temperatures to meet different requirements of different parts. That is , one steel composition for multi-purposes. The tempering temperature should be not higher than 400°C to ensure that the minimum yield strength is higher than 960 MPa.

[0140] Table 3. Microstructures of the as-hot rolled steel strips (% = vol.%). RA = retained austenite, F= ferrite, P = perlite.

[0141]

[0142] Table 4. The properties of the steels in the as-coiled conditions after coiling at 25°C (RT).

[0143]

[0144] R / t-T and CVN-T are bending properties and Charpy toughness measured from the T specimens.

[0145] R / t-L and CVN-L are bending properties and Charpy toughness measured from the L specimens.

[0146] R is the minimum bending radius and t is the thickness of the hot-rolled strips.

[0147] Table 5. The properties of the steels in the as-coiled conditions after coiling at 275°C (RT).

[0148]

[0149] Table 6. The properties of the steels in the as-coiled conditions after coiling at 375°C (RT).

[0150]

[0151] Table 7. The properties of the steels after coiling at 25°C and subsequently temper annealed at different temperatures for 10 hours.

[0152]

[0153] Table 8. The properties of the steels after coiling at 375°C, cooling to room temperature and subsequently temper annealed at different temperatures for 10 hours.

[0154]

Claims

CLAIMS1. A hot-rolled steel strip having a composition, in wt.%,C: 0.15 to 0.30%,Mn: 1.30 to 2.50%,Si: 0.10 to 1.0%,Al: 0.10 to 1.0%,Si+AI: 0.50 to 1.50%,Cr: 0.05 to 1.50%,Ti: 0.001 to 0.10%,Nb: up to 0.030%,Mo: up to 0.50%,Ni: up to 0.50%,Cu: up to 0.30%,V: up to 0.10%,Sn: up to 0.05%,P: up to 0.02%,S: up to 0.01%,N: up to 0.02%,B: up to 0.007%,Ca: up to 0.005%,the remainder being residual and tramp impurities and Fe;and a microstructure having at !4-thickness as its main phrase a tempered martensite phase or a mixture of a low temperature bainite phase and a tempered martensite phase, not less than 95% in volume fraction;and having the following properties:a bending radius of not more than 2.5 x strip thickness,a Brinell hardness of at least 370 HBW,a low-temperature Charpy impact toughness at -40°C of not less than 27 J in both the longitudinal and longitudinal direction,and a yield strength of at least 960 MPa.

2. Hot-rolled steel strip according to claim 1, wherein the steel composition has a C content of at least 0.18%, and preferably of at least 0.21%.

3. Hot-rolled steel strip according to claim 1 or 2, wherein the steel composition has a Si content of at least 0.30%, and preferably of at least 0.50%, and preferably does not exceed 0.80%.

4. Hot-rolled steel strip according to any one of claims 1 to 3, wherein the steel composition has Si+AI > 0.70%, and preferably Si+AI > 0.80%.

5. Hot-rolled steel strip according to any one of claims 1 to 4, wherein the steel composition has a Cr-content of 0.10 to 0.80%, and preferably of 0.10 to 0.40%.

6. Hot-rolled steel strip according to any one of claims 1 to 5, wherein the steel composition has:Nb: up to 0.030%, preferably up to 0.020%;Mo: up to 0.020%, preferably up to 0.010%;Ni: up to 0.05%;Cu: up to 0.05%; andV: up to 0.005%.

7. Hot-rolled steel strip according to any one of claims 1 to 6, wherein the prior austenite grains have a ratio of the average length in the rolling direction relative to the average length in a direction perpendicular to the rolling direction, of not more than 3.0, and preferably not more than 2.5.

8. Hot-rolled steel strip according to any one of claims 1 to 7, wherein the average grain diameter of prior austenite grains is not more than 30 pm, and preferably not more than 25 pm, as measured at !4-thickness with respect to a cross section parallel to the rolling direction.

9. Hot-rolled steel strip according to any one of claims 1 to 8, wherein the hot- rolled steel strip has:a bending radius of < 2.0 x strip thickness, preferably < 1.5 x strip thickness; a Brinell hardness of > 385 HBW, preferably > 415 HBW;a low-temperature Charpy impact toughness of not less than 30 J;and a yield strength of > 1020 MPa, preferably > 1080 MPa.

10. Hot-rolled steel strip according to any one of claims 1 to 9, wherein the hot- rolled steel strip has:an ultimate tensile strength (Rm) > 1050 MPa, preferably > 1100 MPa; and a tensile elongation (A50) > 8.0%, preferably > 8.6%.

11. Hot-rolled steel strip according to any one of claims 1 to 10, wherein the thickness of the hot-rolled steel strip is in a range of 3 to 25 mm, and more preferably of 4 to 12 mm.

12. Method of manufacturing a hot-rolled steel strip according to any one of claims 1 to 11 , the method comprising the steps of:casting a slab having a composition according to any one of claims 1 to 6; reheating of the material for homogenization of the chemistry to a temperature above 1200°C;rough rolling followed by finish hot rolling with a finish rolling temperature above the non-recrystallization temperature Tnr;cooling of the hot-rolled strip from above the Ac3 temperature with an average cooling rate of 40°C / s or more, preferably of 50°C / s or more, to a temperature below the martensitic transformation temperature Ms; coiling of the cooled hot-rolled strip at a temperature below the Ms temperature;cooling of the coiled hot-rolled strip from below the Ms temperature to room temperature.

13. Method according to claim 12, wherein coiling of the hot-rolled strip is at a temperature below Ms minus 30°C (Ms-30°C).

14. Method according to claim 12 or 13, wherein cooling of the hot-rolled strip is with an average cooling rate of 50°C / s or more.

15. Method according to any one of claims 12 to 14, wherein the reduction ratio is at least 30% in the last pass of the finish hot rolling step.

16. Method according to any one of claims 12 to 15, wherein the hot rolled strip after cooling to room temperature is temper annealed at a temperature in a range of 150°C to 380°C, preferably for a time in a range of 1 min to 24 hours, and more preferably for 1 to 10 hours.

17. Method according to any one of claims 12 to 16, wherein following finish rolling at a temperature above the non-recrystallization temperature Tnr the hot rolled strip is retained for 1 to 20s, preferably for 1 to 15s, at a temperature above the Ac3 temperature prior to the subsequent rapid cooling step.

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