Hot-rolled steel strip and manufacturing method

Abstract

Disclosed is a hot-rolled steel strip having a tensile strength greater than 875 MPa and containing in mass-%: C 0.06-0.12, Si 0-0.5, Mn 0.70-2.20, Nb 0.005-0.100, Ti 0.01-0.10, V 0.11-0.40, whereby the total amount of V+Nb+Ti is 0.20-0.40 Al 0.005-0.150, B 0-0.0008, Cr 0-1.0, whereby the total amount of Mn+Cr is 0.9-2.5, Mo 0-0.5, Cu 0-0.5, Ni 0-1.0, P 0-0.05, S 0-0.01, Zr 0-0.1 Co 0-0.1 W 0-0.1 Ca 0-0.005, N 0-0.01, balance Fe and unavoidable impurities, and having a microstructure at ¼ thickness that is: at least 90% martensite and bainite with island-shaped martensite-austenite (MA) constituents, the remainder being: less than 5% polygonal ferrite and quasi-polygonal ferrite, less than 5% pearlite, less than 5% austenite, so that the total area percentage is 100%.

Claims

1. A hot-rolled steel strip having a tensile strength greater than 875 MPa and containing in mass-%: C 0.06-0.12, Si 0-0.5, Mn 0.70-2.20, Nb 0.005-0.100, Ti 0.01-0.10, V 0.11-0.40, whereby the total amount of V+Nb+Ti is 0.20-0.40, Al 0.005-0.150, B 0-0.0008, Cr 0-1.0, whereby the total amount of Mn+Cr is 0.9-2.5, Mo 0-0.5, Cu 0-0.5, Ni 0-1.0, P 0-0.05, S 0-0.01, Zr 0-0.1, Co 0-0.1, W 0-0.1, Ca 0-0.005, N 0-0.01, balance Fe and unavoidable impurities, and having a microstructure at ¼ thickness that is: an area percentage of at least 90% martensite and bainite with island-shaped martensite-austenite (MA) constituents, the remainder being: an area percentage of less than 5% polygonal ferrite and quasi-polygonal ferrite, an area percentage of less than 5% pearlite, an area percentage of less than 5% austenite, so that the total area percentage is 100%.

2. The hot-rolled steel strip according to claim 1, whereby the total amount of V+Nb+Ti is 0.22-0.40.

3. The hot-rolled steel strip according to claim 1, whereby the hot-rolled steel strip exhibits at least one of the following mechanical properties: a hardness of 260-350 HBW, a yield strength up to 1050 MPa, a tensile strength of 875-1100 MPa, a total elongation A5 of at least 8%, a Charpy V (−40° C.) impact toughness of 34 J/cm.sup.2, a minimum bend radius of ≤2.0×thickness of steel sample (t) when the bending axis is parallel to the rolling direction.

4. The hot-rolled steel strip according to claim 1, having a thickness of 12 mm or less.

5. The hot-rolled steel strip according to claim 1, whereby the niobium content is 0.01-0.05 mass-% when the thickness of steel sample (t), t≤6 mm and the niobium content is 0.01-0.10 mass-% when the thickness of steel sample (t), t>6 mm.

6. The hot-rolled steel strip according to claim 1, whereby the titanium content is 0.01-0.07 mass-% when the thickness of steel sample (t), t≤6 mm and the titanium content is 0.03-0.10 mass-% when the thickness of steel sample (t), t>6 mm.

7. The hot-rolled steel strip according to claim 1, whereby the carbon content is 0.07-0.10 mass-%.

8. The hot-rolled steel strip according to claim 1, whereby the manganese content is 1.20-2.20 mass-%.

9. The hot-rolled steel strip according to claim 1, whereby the niobium content is 0.005-0.080 mass-%.

10. The hot-rolled steel strip according to claim 1, whereby the vanadium content is 0.15-0.30 mass-%.

11. The hot-rolled steel strip according to claim 1, whereby the aluminium content is 0.015-0.090 mass-%.

12. The hot-rolled steel strip according to claim 1, whereby the total amount of Mn+Cr is 1.2-2.0 mass-%.

13. A method for producing a hot-rolled steel strip having a tensile strength greater than 875 MPa, whereby the method comprises the steps of providing a steel slab containing in mass-%: C 0.06-0.12, Si 0-0.5, Mn 0.70-2.2, Nb 0.005-0.100, Ti 0.01-0.10, V 0.11-0.40, whereby the total amount of V+Nb+Ti is 0.20-0.40, Al 0.005-0.150, B 0-0.0008, Cr 0-1.0, whereby the total amount of Mn+Cr is 0.9-2.5, Mo 0-0.5, Cu 0-0.5, Ni 0-1.0, P 0-0.05, S 0-0.01, Zr 0-0.1, Co 0-0.1, W 0-0.1, Ca 0-0.005, N 0-0.01, balance Fe and unavoidable impurities, heating the steel slab to a temperature of 900-1350° C., hot rolling said steel at a temperature of 750-1300° C., and direct quenching said steel after a final hot-rolling pass at a cooling rate of at least 30° C./s to a coiling temperature less than 400° C., whereby a hot-rolled steel strip having the following microstructure at % thickness is obtained: an area percentage of at least 90% martensite and bainite with island-shaped martensite-austenite (MA) constituents, the remainder being: an area percentage of less than 5% polygonal ferrite and quasi-polygonal ferrite, an area percentage of less than 5% pearlite, an area percentage of less than 5% austenite, so that the total area percentage is 100%.

14. The method according to claim 13, which further comprises the step of continuously annealing the quenched steel strip at an annealing temperature of 100-400° C. after the direct quenching step.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The present invention will hereinafter be further explained by means of non-limiting examples with reference to the appended figures where;

(2) FIG. 1 shows a flow chart of a method according to an embodiment of the invention,

(3) FIG. 2 shows the microstructure at the surface of a 6 mm thick hot-rolled steel strip according to an embodiment of the invention,

(4) FIG. 3 shows the microstructure 1.5 mm below the surface (i.e. at ¼ thickness) of a 6 mm thick hot-rolled steel strip according to an embodiment of the invention,

(5) FIG. 4 shows a feature of the microstructure of FIG. 3 at a greater magnification,

(6) FIG. 5 shows the microstructure 3.0 mm below the surface (i.e. at ¼ thickness) of a 6 mm thick hot-rolled steel strip according to an embodiment of the invention,

(7) FIG. 6 shows the weld groove geometry that was used in the weldability tests described herein, and

(8) FIG. 7 shows the weld pass arrangement that was used in the weldability tests described herein.

DETAILED DESCRIPTION OF EMBODIMENTS

(9) FIG. 1 shows the steps of a method according to an embodiment of the invention in which an optional step has been shown with dashed lines.

(10) The method comprises the step of providing a steel slab having the following chemical composition (in mass-%):

(11) C 0.06-0.12, preferably 0.07-0.10

(12) Si 0-0.5, preferably 0.03-0.5 more preferably 0.03-0.25%

(13) Mn 0.70-2.2, preferably 1.2-2.2, or more preferably 1.2-20

(14) Nb 0.005-0.100, preferably 0.005-0.08, more preferably 0.01-0.08

(15) Ti 0.01-0.10, preferably 0.01-0.08 more preferably 0.02-0.08

(16) V 0.11-0.40, preferably 0.15-0.30

(17) whereby the total amount of V+Nb+Ti is 0.20-0.40 or 0.22-0.40

(18) Al 0.005-0.150, preferably 0.015-0.090

(19) B 0-0.0008, preferably 0-0.0005

(20) Cr 0-1.0, preferably 0-0.3 or 0-0.25

(21) whereby the total amount of Mn+Cr is 0.9-2.5, preferably 1.2-2.0

(22) Mo 0-0.5, preferably 0-0.2 more preferably 0-0.1%

(23) Cu 0-0.5, preferably 0-0.15

(24) Ni 0-1.0, preferably 0-0.15

(25) P 0-0.05, preferably 0-0.02

(26) S 0-0.01, preferably 0-0.005

(27) Zr 0-0.1

(28) Co 0-0.1

(29) W 0-0.1

(30) Ca 0-0.005, preferably 0 0.001-0.004

(31) N 0-0.01, preferably 0.001-0.006

(32) balance Fe and unavoidable impurities.

(33) The steel for hot-rolling may be provided by casting or continuously casting such a micro-alloyed steel slab for example.

(34) According to an embodiment of the present invention the equivalent carbon content, Ceq, of the steel is 0.297-0.837.

(35) For example, the steel may have the following chemical composition (in mass-%): C: 0.09, Si: 0.175, Mn: 1.8, Cr: 0, (Mn+Cr=1.8), Nb: 0.027, V: 0.2, Ti: 0.045 (Nb+V+Ti=0.272), Al: 0.035, B: 0, Mo: 0, Cu: 0, Ni: 0, P: 0, W: 0, Co: 0, S: 0, Zr: 0, Ca: 0.003, Ceq: 0.430.

(36) Carbon is added to increase the strength of the steel by forming solid solution strengthening and precipitating as different kinds of carbides in the matrix. Carbon is also essential to get the desired hard microstructure, which is mainly martensite and bainite. To achieve a desired strength and to obtain the desired precipitation-related benefits, the steel contains carbon 0.06-0.12 mass-%, preferably 0.07-0.10 mass-%. The upper limits are set because if carbon is used excessively, it would weaken the weldability as well as the formability of the steel.

(37) Manganese is included in steel for reasons concerning smelt processing and it is also used to bind sulfur and form MnS. Manganese is also added to increase the strength of the steel. For those reasons, at least 0.70 mass-% is used. An upper limit of 2.20 mass-% is selected in order to avoid excessive strengthening and further to ensure weldability and suitability for optional coating processes. The manganese content is preferably 1.2-2.2 mass-%. Some of the manganese may be replaced by chromium as long as the total amount of Mn+Cr is 0.9-2.5 mass-%, preferably 1.2-2.0 mass-%.

(38) Titanium, niobium and vanadium are added to the steel to form precipitates providing beneficial effects, i.e. carbides, nitrides and carbonitrides and for refining the microstructure of the steel during hot rolling. Vanadium is important in the cooling step to obtain the desired microstructure. The titanium content of the steel is 0.01-0.10 mass-%, preferably 0.005-0.080 mass-%, more preferably 0.02-0.08 mass-%. The niobium content of the steel is 0.005-0.100 mass-%, preferably 0.005-0.08 mass-%, more preferably 0.01-0.08 mass-%. The vanadium content of the steel is 0.11-0.40 mass-%, preferably 0.15-0.30 mass-%. The total amount of V+Nb+Ti is 0.20-0.40 mass-% or 0.22-0.40 mass-%.

(39) Silicon may optionally be added since it, like aluminium, can function as a de-oxidation element, and it can also be also utilized in solid solution strengthening, especially if better surface quality is desired. The upper limit is selected in order to avoid excessive strengthening. The silicon content of the steel may be 0-0.5 mass-%, preferably 0.03-0.5 mass-%, more preferably 0.03-0.25 mass-%.

(40) Aluminium is utilized in an amount of 0.005-0.150 mass-%, preferably 0.015-0.090 mass-%, in order to affect the carbide formation during thermal processing of steel and in de-oxidation.

(41) Chromium can optionally be utilized in an amount of 0-1.0 mass-%, preferably 0-0.3 or 0-0.25 mass-% in order to increase strength. The upper limit is selected in order to avoid excessive strengthening. Furthermore, such a relatively low chromium content improves the weldability of the steel.

(42) Nickel can optionally be utilized in an amount of 0-1.0 mass-%, preferably 0-0.15 mass-%, in order to increase strength. The upper limit is selected in order to avoid excessive strengthening. Furthermore, such a relatively low nickel content improves the weldability of the steel.

(43) Copper can optionally be utilized in an amount of 0-0.5 mass-%, preferably 0-0.15 mass-%, in order to increase strength. The upper limit is selected in order to avoid excessive strengthening. Furthermore, such a relatively low copper content improves the weldability of the steel.

(44) If chromium, nickel and copper are added to the steel, this may impart weather-resistant properties to the steel.

(45) Molybdenum can optionally be utilized in an amount of 0-0.5 mass-%, preferably 0-0.2 mass-%, more preferably 0-0.1 mass-%, in order to increase strength. The upper limit is selected in order to avoid excessive strengthening. Furthermore such a relatively low molybdenum content can improves the weldability of the steel. However molybdenum is not normally needed in the present invention, which decreases the cost of alloying.

(46) Boron can optionally be utilized in an amount of 0-0.0008 mass-%, preferably 0-0.0005 mass-%, 30 in order to increase strength. However, due to the high hardenability factor of boron, it is preferred not to use boron. Boron is not intentionally added to the steel.

(47) Calcium can be included in the steel for reasons concerning smelt processing, in an amount up to 0.005 mass-%, preferably 0 0.001-0.004 mass-%.

(48) In addition to the intentionally and optionally added alloying elements and iron, the steel may comprise small amounts of other elements, such as impurities that originate from smelting. Those impurities are: nitrogen, which is an element that can bind micro-alloying elements existing in the steel to nitrides and carbonitrides. This is why a nitrogen content of up to 0.01%, preferably 0.001-0.006 mass-%, may be included in steel. However, a nitrogen content of more than 0.01 mass-% would allow the nitrides to coarsen. Nitrogen is not however intentionally added to the steel. phosphorus is usually unavoidably included in steel and should be restricted to 0-0.05 mass-%, preferably 0-0.02 mass-%, since a higher phosphorus content can be harmful for the elongation properties of the steel. sulphur is usually unavoidably included in steel and should be restricted to a maximum of 0.01 mass-%, preferably 0-0.005 mass-%. Sulphur decreases the bendability of the steel. oxygen may be present in the steel as an unavoidable element, but should be restricted to a maximum of 0.01 mass-%, preferably less than 0.005 mass-%. This is because it may exist as an inclusion that debilitates the formability of the steel. the steel may also contain 0-0.1 mass-% zirconium, 0-0.1 mass-% cobalt and/or 0-0.1 mass-% tungsten without adversely affecting the physical properties of the steel.

(49) The method according to the present invention comprises the step of heating the steel slab to a temperature of 900-1350° C. in order to dissolve the micro-alloying elements in the steel slab prior to hot-rolling, and then hot-rolling the steel at a temperature of 750-1300° C., whereby the final rolling temperature (FRT), i.e. a temperature of last hot-rolling pass in the hot-rolling step, that is for example between 850 and 950° C.

(50) The hot-rolling step can be performed at least partly in a strip rolling mill. The hot-rolling step can include hot-rolling at a temperature in the range 750-1350° C., but preferably in the range Ar3 to 1280° C. The hot-rolling step may be a thermomechanical rolling (TMCP) process consisting for example of two stages including rolling in a pre-rolling stage and a subsequent rolling stage in a strip rolling mill having a final rolling temperature (FRT) between 750 and 1000° C. It is however preferred that the final hot-rolling temperature (FRT) in the hot-rolling step is above the Ar3 temperature of the steel. This is because problems related to rolling-texture and strip flatness may otherwise arise. Thermomechanical rolling processes can help to achieve the desired mechanical properties by reducing the grain size of the phase hardened microstructure and increasing further phase substructures.

(51) After a final hot-rolling pass, the steel is direct quenched at a cooling rate of at least 30° C./s to a coiling temperature preferably in the range of 25-75° C. (i.e. residual heat from hot-rolling). A quenched steel strip includes a phase hardened microstructure, such as a microstructure consisting mainly of bainitic-ferrite and martensite, including phase sub-structures that are beneficial for the following process step(s). In addition, the quenching step results in at least part of, or preferably most of the micro-alloying elements being kept in the solution during the cooling from the hot-rolling heat.

(52) The steel strip is coiled after being direct quenched. The temperature of the steel strip can decrease continuously throughout the whole length of the steel strip from the end of direct quenching step to the start of coiling step. The coiling is carried out at low temperature, i.e. preferably at a temperature in the range of 25-75° C.

(53) According to an embodiment of the invention, after coiling, the hot-rolled steel strip may be subjected to one or more further method steps, such as continuous annealing.

(54) Continuous annealing may be carried out at a temperature between 100 and 400° C. The micro-alloying elements begin to precipitate or preliminary precipitates continue to grow when the quenched steel strip is continuously annealed after the direct quenching step if the annealing temperature is higher and the annealing time is long enough, which leads to softening. Such annealing may be performed in a continuous annealing line (CAL) or, in a hot-dip coating line (HDCL). Prior to the annealing step, the hot-rolled steel strip may be pickled.

(55) A hot-dip coating step may include immersing the hot-rolled steel strip into molten metal such as zinc, aluminum or zinc-aluminum, after the annealing step, whereby a hot-dip-coated steel strip having good formability and high strength is obtained.

(56) The continuous annealing temperature is not more than 400° C. Higher temperatures lead to softening. The annealing time in the annealing step can be 10 seconds to 1 week depending on the annealing temperature. Normally, annealing is not needed.

(57) The hot-rolled steel strip has a microstructure at % thickness that is: at least 90% martensite and bainite with island-shaped martensite-austenite (MA) constituents, preferably at least 95% and more preferably over 98%, the remainder being: less than 5% polygonal ferrite and quasi-polygonal ferrite, preferably less than 2%, more preferably less than 1%, less than 5% pearlite, preferably less than 2%, more preferably less than 1%, less than 5% austenite, preferably less than 2%, more preferably less than 1%, so that the total area percentage is 100%.

(58) The bainite may include granular bainite, upper and lower bainite and acicular ferrite, for example. According to an embodiment of the invention, the proportion of upper bainite is preferably less than 80%. According to an embodiment of the invention, the bainite content is preferably between 20-90%, and the martensite content preferably 10-80%. According to an embodiment of the invention, for a strip thickness under 3 mm, the bainite content is preferably 20-50% and the martensite content preferably 50%-80%. According to an embodiment of the invention, for a strip thickness greater than 5 mm the bainite content is preferably 50-90% and the martensite content is preferably 10-50%, whereby the total area percentage is 100% in all of the embodiments cited herein. The microstructure can be determined using a scanning electron microscope for example.

(59) According to an embodiment of the invention the hot-rolled steel strip manufactured using a method according to the present invention will also exhibit at least one of the following mechanical properties: a hardness of 260-350 HBW, preferably 270-325 HBW (whereby the Brinell hardness test is performed using a 2.5 mm diameter carbide ball up to 4.99 mm thickness, whereby the hardness is measured at least 0.3 mm from surface (and for thicknesses of 5-7.99 mm, the carbide ball diameter is 5 mm and the hardness is measured at least 0.5 mm from surface, and with a thickness of 8 mm and over, the carbide ball diameter is 10 mm and the hardness is measured at least 0.8 mm from surface, a tensile strength, Rm of 875-1100 MPa, preferably 900-1150 MPa, a total elongation of at least 8% or at least 10%, a Charpy V (−40° C.) impact toughness of 34 J/cm.sup.2 preferably 50 J/cm.sup.2, a minimum bend radius of ≤2.0× t or ≤1.9× t, or ≤1.8× t, or ≤1.7×t, preferably when the bending axis is parallel to the rolling direction and t is thickness (mm) of the steel sample.

(60) Table 1 shows the steel compositions that were studied in this work, whereby the balance is iron and unavoidable impurities. Steel compositions A1 and A2 are having a chemical composition as recited in the accompanying independent claims and are embodiments of the present invention (“INV”). Steel compositions B, C1, C2, D1, D2 and E1 comprise at least one element in an amount which lies outside the range given in the accompanying independent claims and are not embodiments of the invention, but comparative examples (“REF”).

(61) TABLE-US-00001 TABLE 1 C Si Mn P S Al Nb V Cu Cr Ni N Mo Ti Ca B A1 0.087 0.20 1.81 0.012 0.0017 0.033 0.031 0.200 0.02 0.06 0.04 0.0045 0.01 0.044 0.0026 0.0003 INV {open oversize brace} A2 0.092 0.18 1.81 0.010 0.0014 0.031 0.032 0.195 0.02 0.05 0.04 0.0045 0.01 0.045 0.0026 0.0004 B 0.059 0.16 1.30 0.009 0.0008 0.032 0.040 0.202 0.01 0.41 0.04 0.0060 0.15 0.002 0.0027 0.0003 C1 0.070 0.05 1.37 0.009 0.0006 0.034 0.040 0.010 0.02 0.69 0.04 0.0049 0.01 0.016 0.0024 0.0014 C2 0.070 0.04 1.37 0.009 0.0003 0.043 0.042 0.008 0.02 0.70 0.05 0.0043 0.01 0.017 0.0023 0.0013 REF {open oversize brace} D1 0.063 0.39 1.19 0.008 0.0038 0.032 0.083 0.012 0.38 0.51 0.25 0.0054 0.01 0.105 0.0023 0.0003 D2 0.056 0.40 1.19 0.008 0.0026 0.031 0.082 0.012 0.38 0.50 0.25 0.0049 0.01 0.106 0.0022 0.0002 E1 0.085 0.18 1.04 0.009 0.0045 0.026 0.003 0.045 0.03 1.33 0.06 0.0063 0.14 0.027 0.0030 0.0019

(62) Table 2 shows the process parameters that were used to manufacture the hot-rolled steel strips that were studied in this work

(63) TABLE-US-00002 TABLE 2 Inventive thickness furnace Rolling Coiling sample (I)/ t temp. t.sub.bar temp. FRT temp. Reference (mm) (° C.) mm) (° C.) (° C.) (° C.) sample (R) A1 6.0 1280 29.,5 1136 882 50 I A1 6.0 1280 29.4 1074 829 50 I A1 3.0 1280 28.4 1129 894 50 I A1 2.5 1280 27.4 1135 894 50 I A1 2.2 1280 27.4 1127 890 50 I A1 3.0 1280 28.4 1131 881 628 R A2 6.0 1280 30.5 1148 917 50 I B 3.0 1260 28.4 1139 859 50 R B 3.0 1260 27.4 1140 922 569 R C1 6.0 1280 30.4 1056 870 50 R C1 6.0 1280 30.4 1079 894 50 R C2 3.0 1280 28.5 1166 893 50 R D1 6.0 1276 30.6 1130 895 50 R D2 4.0 1271 30.6 1140 900 50 R E1 6.0 1279 30.5 1139 925 50 R

(64) Steel slabs of the steel compositions A1, A2 B, C1, C2, D1, D2 and E1 having a thickness t.sub.bar were namely heated in a furnace to the furnace temperature indicated in Table 2 and then subjected to hot-rolling to a final thickness, t, at the rolling temperature and final rolling temperature (FRT) shown in Table 2. After the final hot-rolling pass, the steel compositions were direct quenched at a cooling rate of at least 30 00/s to a coiling temperature of 5000 (apart from one of the steel compositions A1, (which was consequently not manufactured using a method according to the present invention which requires direct quenching to a coiling temperature in the range of 25-75° C.) and one of the comparative examples with steel composition B).

(65) Table 3 shows the mechanical properties of the steel compositions A1, A2 B, C1, C2, D1, D2 and E1.

(66) TABLE-US-00003 TABLE 3 Charpy Inventive thickness V (−40° C.) Bendability Hole sample (I)/ t Hardness Rp.sub.0.2 Rm Rp/Rm (J) R/t Expansion Reference (mm) HBW (MPa) (Mpa) ratio A % A80% (J/cm.sup.2) (L/T) (ISO) sample (R) A1 6.0 279 766 934 0.82 13.7 — 40  83 1.33/0.33 — I A1 6.0 271 746 923 0.81 15.1 — 53 110 1.33/0.33 — I A1 3.0 298 793 962 0.82 14.2 — — — 1.67/0.33 34 I A1 2.5 311 816 998 0.82 14.7 — — — 1.2/0.4 — I A1 2.2 302 854 994 0.86 13.9 — — — 0.91/0.45 — I A1 3.0 277 702 816 0.86 19.5 — 14  58 0.33/0.33 40 R A2 6.0 300 837 998 0.84 10.2 — 40 100 1.25/0.75 — I B 3.0 — 631 809 0.78 — 14.6 — — — 34 R B 3.0 — 641 777 0.82 — 17.6 — — — 63 R C1 6.0 328 886 1002 0.88 10.9 — 60 150 2.7/1.7 — R C1 6.0 327 942 1030 0.91 10.1 — 64 160 4.2/2.3 — R C2 3.0 330 987 1087 0.91 11.6 — — — 4.3/3.7 — R D1 6.0 — 735 865 0.85 15.1 — 60 125 1.0/0.2 — R D2 4.0 — 733 869 0.84 17.2 — 42 131 0.5/0.25 — R E1 6.0 — 1025 1124 0.91 11.9 — 42  88 — — R

(67) Conventional steel usually has a fully martensitic microstructure, a hardness of 400 HBW or more and a minimum bend radius, R/t of 2.5-5.0.

(68) Neither conventional steel nor the comparative examples exhibit such good bendability combined to high tensile strength as the hot-rolled steel strip according to the present invention. Furthermore, the hot-rolled steel strip according to the present invention exhibits good bendability both in its longitudinal direction, L, (i.e. rolling direction, RT) and its transverse direction, T.

(69) Additionally, the hot-rolled steel strip according to the present invention has a lower hardness than conventional steel and the comparative examples and is thereby more suitable for applications in which good bendability as well as good wear resistance and also high tensile strength are required together with high impact strength.

(70) FIGS. 2, 3 and 5 show the microstructure of a 6 mm thick hot-rolled steel strip according to an embodiment of the invention at the surface, 1.5 mm below the surface (i.e. at % thickness) and 3.0 mm below the surface (i.e. at %.sub.2 thickness) respectively.

(71) FIG. 4 shows a feature of the microstructure 1.5 mm below the surface (i.e. at % thickness) at a greater magnification than in FIG. 3.

(72) The microstructure at % thickness (shown in FIGS. 3 and 4) is at least 90% martensite and bainite with island-shaped martensite-austenite (MA) constituents. The remaining 10% of the microstructure may comprise polygonal ferrite and/or quasi-polygonal ferrite and/or pearlite and/or austenite.

(73) Tests

(74) Weldability testing was performed on 6 mm thick hot-rolled strips of the steel having the chemical composition Al in Table 1.

(75) Weldability testing was carried out by welding four butt joints using test pieces having the dimensions of 6×200×1050 mm. The test pieces were cut from the middle of the coil along the principal rolling direction so that 1050 mm-long butt welds were transversal to rolling direction.

(76) The joints were welded using a metal active gas (MAG) welding process and two different welding consumables were tested:

(77) a) unalloyed solid wire Lincoln Supramig (YS 420 MPa) which does not match (i.e. does not equal) the strength of the hot-rolled steel strip according to the present invention, but has a lower strength, and

(78) b) matching solid wire Böhler X70 IG (YS 690 MPa), which matches (i.e. equals) the strength of the hot-rolled steel strip according to the present invention.

(79) The butt joints were welded using single V-groove preparation with a 60° groove angle and without preheating. The calculated t.sub.8/5 time range during the welding tests was between 7-19 seconds, whereby the time t.sub.8/5 is the time in which a cooling of the welding layer from 800° C. to 500° C. occurs.

(80) FIG. 6 shows the weld groove geometry that was used in the weldability tests and FIG. 7 shows the weld pass arrangement.

(81) The results obtained from the above-mentioned tests are presented in Tables 4-6 below.

(82) In the tests labelled “Bohler Low”, the second weld pass t.sub.8/5 time was 6.7 s. Such a short cooling time from 800° C. to 500° C. (t.sub.8/5) means that a low heat input was used in the welding.

(83) In the tests labelled “Bohler High”, the second weld pass t.sub.8/5 time was 15.0 s. Such a long cooling time from 800° C. to 500° C. (t.sub.8/5) means that a high heat input was used in the welding.

(84) In the tests labelled “SupraMIG Low”, the second weld pass t.sub.8/5 time was 6.7 s.

(85) The mechanical testing of welded joints included the following tests two transverse tensile tests Charpy-V testing at −40° C. with three 5×10 mm specimen at the following locations: weld centerline, fusion line (FL)+1 mm, fusion line (FL)+3 mm and fusion line (FL)+5 mm.

(86) Both the yield strength and the tensile strength of the welds fulfilled the requirements set for S700 MC base material stated in standard EN 10149-2. When using a matching wire Bohler X70 IG and a higher heat input (t.sub.8/5=15 s), it was found that the strength requirements set for S700 MC base material in EN standard 10149-2 were also fulfilled.

(87) Typically, for high strength structural steels welding tests should be conducted in accordance with welding procedure test standard ISO 15614:2017. This standard requires that Charpy-V impact energy tests are conducted at two locations; from the middle of the weld metal and 1 mm from the weld's fusion line to base material. The impact toughness measured at the required locations fulfilled 34 J/cm.sup.2 at −40° C., or in other words 27 J with a full size test specimen when the t.sub.8/5 time was up to 15 seconds. However with higher heat input and when the t.sub.8/5 cooling time was 19 seconds, the impact toughness was less than 34 J/cm.sup.2 at −40° C. The achievement of 27 J with a full size test specimen is a minimum requirement for S700 MC.

(88) Usually, wear-resistant steels, such as the hot-rolled steel strip according to the present invention, are welded using lower strength welding consumables, i.e. undermatching welding consumables. Structural steels, are, on the contrary, welded using matching strength welding consumables.

(89) It is therefore surprising that the hot-rolled steel strip according to the present invention may be welded using matching strength welding consumables and achieve mechanical properties that fulfil standard requirements for structural steels.

(90) The inventors have namely found that the hot-rolled steel strip according to the present invention, which is a wear-resistant steel, may be welded like a structural steel and achieve mechanical properties fulfilling the requirements set for base steel S700 MC material.

(91) TABLE-US-00004 TABLE 4 Charpy impact test results at T: −40° C. Böhler Low Böhler High SupraMIG Low Dimension Area Impact Energy per Impact Energy per cm Impact Energy per cm Direction Notch position (10 .Math. x mm) mm2 (J/cm.sup.2) (J/cm.sup.2) (J/cm.sup.2) L WM 5 40 138 133 104 L FL 5 40 36 43 78 L FL + 1 mm 5 40 70 62 74 L FL + 3 mm 5 40 107 84 122 L FL + 5 mm 5 40 116 99 118

(92) TABLE-US-00005 TABLE 5 Böhler low Böhler high SupraMIG low Welding pass no.−> 1 2 1 2 1 2 Welding method: MAG 135 MAG 135 MAG 135 MAG 135 MAG 135 MAG 135 Welding direction: T T T T T T Weld position: PA PA PA PA PA PA Current (A) 249.0 259.0 250.0 260.0 249.0 259.0 Voltage (V) 25.1 27.5 24.9 28.0 24.5 27.0 Travel speed (cm/min) 63.0 71.5 62.4 48.3 61.7 69.7 Heat input (kJ/mm) 0.5 0.5 0.5 0.7 0.5 0.5 Cooling time, t8/5 (s) 6.7 6.7 6.7 15.0 6.4 6.7 Preheat temp (° C.) 25.0 25.0 25.0 25.0 25.0 25.0 Flux type: 0.0 0.0 0.0 0.0 0.0 0.0 Electrode diameter (mm) 1.0 1.0 1.0 1.0 1.0 1.0 Electrode: Böhler X70IG Böhler X70IG Böhler X70IG Böhler X70IG SupraMIG SupraMIG Shielding gas Mison 8 Mison 8 Mison 8 Mison 8 Mison 8 Mison 8

(93) TABLE-US-00006 TABLE 6 Tensile test results Böhler low Böhler high Rp0.2 Rm A80 Rp0.2 Rm A80 Number Direct. (N/mm2) (N/mm2) (%) Number Direct. (N/mm2) (N/mm2) (%) 1 L 775 881 8.5 1 L 723 837 7.0 2 L 774 883 6.5 2 L 717 830 7.2 Avg. 774.5 832 7.5 Avg. 720 834 7.1 SupraMIG low Rp0.2 Rm A80 Number Direct. (N/mm2) (N/mm2) (%) 1 L 710 810 4.9 2 L 706 810 4.9 Avg. 708 810 4.9

(94) Further modifications of the invention within the scope of the claims would be apparent to a skilled person.