HOT-ROLLED STEEL & METHOD FOR MANUFACTURING HOT-ROLLED STEEL

Abstract

A hot-rolled steel is described having a yield strength (Rp0.2) of at least 1100 MPa along and/or transverse to a rolling direction which has a chemical composition of (in mass-%): C: 0.10-0.2, Si: 0-0.7, Mn: 1.1-2.2, Nb: 0-0.06, Ti: 0-0.15, V: more than 0.03 and ≤0.25, Al: 0.01-0.15, B: 0.0005-0.010, Cr: 0.1-1.7, Mo: 0.15-0.8, Cu: 0-1.5, Ni: 0.3-2.5, P: 0-0.015, S: 0-0.008 Zr: 0-0.2, Ca: 0-0.004, the balance Fe and unavoidable impurities, whereby a) when 0.1<C<0.11 then Mn≥1.6 and V>0.14 and Mo≥0.5 (in mass-%), b) when 0.11≤C<0.125 then Mn≥1.45 and V≥0.13 and Mo≥0.35 (in mass-%), c) when 0.125≤C<0.15, then Mn≥1.35 and V≥0.12 and Mo≥0.20 (in mass-%), and d) when C≥0.15 and V>0.11, then Mn≥1.3 and Mo≥0.15 (in mass-%) or when C≥0.15 and V 0.03-0.11, then Mn>1.3 and Mo>0.15 and Nb>0.02 and Cr+Cu+Ni>1.4 (in mass-%).

Claims

1. A hot-rolled steel having a yield strength (Rp.sub.0.2) of at least 1100 MPa along and/or transverse to a rolling direction and a tensile strength of at least 1120 MPa along and/or transverse to a rolling direction, characterized in that: it has a chemical composition comprising (in mass-%): C 0.10-0.2 Si 0-0.7 Mn 1.1-2.2 Nb 0-0.06 Ti 0-0.15 V more than 0.03 and ≤0.25 Al 0.01-0.15 B 0.0005-0.010 Cr 0.1-1.7 Mo 0.15-0.8 Cu 0-1.5 Ni 0.3-2.5 P 0-0.015 S 0-0.008 Zr 0-0.2 Ca 0-0.004 balance Fe and unavoidable impurities, whereby: a) when 0.1<C<0.11 then Mn≥1.6 and V>0.14 and Mo≥0.5 (in mass-%) b) when 0.11≤C<0.125 then Mn≥1.45 and V≥0.13 and Mo≥0.35 (in mass-%) c) when 0.125≤C<0.15, then Mn≥1.35 and V≥0.12 and Mo≥0.20 (in mass-%), and d) when C≥0.15 and V>0.11, then Mn≥1.3 and Mo≥0.15 (in mass-%) or when C≥0.15 and V 0.03-0.11, then Mn>1.3 and Mo>0.15 and Nb>0.02 and Cr+Cu+Ni>1.4 (in mass-%).

2. The hot-rolled steel according to claim 1, characterized in that it comprises 0.4-1.7 mass-% Cr.

3. The hot-rolled steel according to claim 1, characterized in that said chemical composition comprises both Ni and Cu, in a total amount of at least 0.5 mass-%.

4. The hot-rolled steel according to claim 1, characterized in that it has an A %-elongation of at least 8% along and/or transverse to the rolling direction.

5. The hot-rolled steel according to claim 1, characterized in that it has an impact toughness of at least 34 J/cm.sup.2 when a Charpy V notched specimen having a thickness of 5-10 mm is measured at −40° C. longitudinally to the rolling direction.

6. The hot-rolled steel according to claim 1, characterized in that it has a minimum bending radius of 5.0× thickness longitudinally and/or transverse to the rolling direction.

7. The hot-rolled steel according to claim 1, characterized in that it is metal active gas (MAG) welded with reinforcement, using a V- or Y-groove welding method, whereby a first pass is welded from a bottom or top side and other passes from a top side, using welding consumables having a tensile strength of 1100 MPa and a t8/5 time of 8-12 seconds.

8. The hot-rolled steel according to claim 1, characterized in that it has an A %-elongation of at least 7% when a tensile test is carried out across a weld seam of a welded hot-rolled steel product where the weld is longitudinal to the rolling direction, whereby said hot-rolled steel is welded using welding consumables having a tensile strength of 890 MPa, and a t8/5 of 5-20 seconds.

9. A method for manufacturing hot-rolled steel having a chemical composition comprises (in mass-%): C 0.10-0.2 Si 0-0.7 Mn 1.1-2.2 Nb 0-0.06 Ti 0-0.15 V more than 0.03 and ≤0.25 Al 0.01-0.15 B 0.0005-0.010 Cr 0.1-1.7 Mo 0.15-0.8 Cu 0-1.5 Ni 0.3-2.5 P 0-0.015 S 0-0.008 Zr 0-0.2 Ca 0-0.004 balance Fe and unavoidable impurities. whereby: a) when 0.1<C<0.11 then Mn≥1.6 and V>0.14 and Mo≥0.5 (in mass-%) b) when 0.11≤C<0.125 then Mn≥1.45 and V≥0.13 and Mo≥0.35 (in mass-%) c) when 0.125≤C<0.15, then Mn≥1.35 and V≥0.12 and Mo≥0.20 (in mass-%), and d) when C≥0.15 and V>0.11, then Mn≥1.3 and Mo≥0.15 (in mass-%) or when C≥0.15 and V 0.03-0.11, then Mn>1.3 and Mo>0.15 and Nb>0.02 and Cr+Cu+Ni>1.4 (in mass-%). whereby the method comprises the following steps: heating to an austenitizing temperature of 1000-1350° C., hot-rolling such that a finishing rolling temperature is 760-1050° C., quenching to a temperature of 300° C. or less.

10. The method according to claim 9, characterized in that it comprises the step of temper annealing at a temperature of 500-650° C. if the tempering time is 1 hour or more, or temper annealing at a temperature of 500-750° C. if the tempering time is less than 1 hour after said quenching step.

11. The method according to claim 10, characterized in that the microstructure of the hot-rolled steel before said temper annealing step comprises at least 90% martensite when said microstructure is examined in ¼ thickness.

12. The method according to claim 9, characterized in that said quenching step is a direct quenching step.

13. The method according to claim 9, characterized in that it comprises the step of strip rolling said hot-rolled steel.

14. The method according to claim 13, characterized in that said hot-rolled steel comprises a maximum of 0.005 mass-% Nb and <0.15 mass-% C.

15. The method according to claim 9, characterized in that said hot-rolled steel comprises a minimum of 0.005 mass-% Nb when the hot-rolled steel is not strip rolled.

16. The hot-rolled steel according to claim 1, wherein the hot-rolled steel further comprises N 0-0.01.

17. The method according to claim 9, wherein the hot-rolled steel further comprises N 0-0.01.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0087] The present invention will hereinafter be further explained by means of non-limiting examples with reference to the appended figures where;

[0088] FIG. 1 shows a flow chart showing the steps of a method according to an embodiment of the invention,

[0089] FIG. 2 shows hardness profiles for material having a thickness of 8 mm over the weld tested from face side (i.e. the side at which welding took place) and the root side (i.e. the side opposite to the side at which welding took place), and

[0090] FIG. 3 shows hardness profiles for material having a thickness of 4 mm over the weld tested from face side and the root side.

[0091] It should be noted that all features disclosed with reference to a hot-rolled steel according to the present invention also apply to a method according to the present invention, and vice versa.

DETAILED DESCRIPTION OF EMBODIMENTS

[0092] FIG. 1 shows the steps of a method for manufacturing hot-rolled steel according to any of the embodiments of the invention having a chemical composition containing (in mass-%): [0093] C 0.10-0.2, preferably 0.10-0.18, more preferably 0.12-0.18, [0094] Si 0-0.7, preferably 0.03-0.50, more preferably 0.10-0.30, [0095] Mn 1.1-2.2, preferably 1.4-1.8, more preferably 1.4-1.7, [0096] Nb 0-0.06, preferably 0-0.04, more preferably 0-0.005, [0097] Ti 0-0.15, preferably 0-0.05 more preferably 0.005-0.02, [0098] V more than 0.03 and ≤0.25, preferably more than 0.10 and ≤0.20, [0099] Al 0.01-0.15, preferably 0.015-0.08, [0100] B 0.0005-0.010, preferably 0.0005-0.005, more preferably 0.001-0.003, [0101] Cr 0.1-1.7, preferably 0.4-1.7 or 0.6-1.5, or more than 1.0 mass-%, [0102] Mo 0.15-0.8, preferably 0.2-0.5, [0103] Cu 0-1.5, preferably 0.1-1.0, [0104] Ni 0.3-2.5, preferably 0.7-1.7, [0105] P 0-0.015, preferably 0-0.009, [0106] S 0-0.008, preferably 0-0.004, [0107] Zr 0-0.2, preferably 0-0.01 [0108] Ca 0-0.004, preferably 0.001-0.003, [0109] preferably N 0-0.01 mass-%, more preferably ≤0.006 mass-%. [0110] balance Fe and unavoidable impurities.

[0111] whereby: [0112] a) when 0.1<C<0.11 then Mn≥1.6 and V>0.14 and Mo≥0.5 (in mass-%), [0113] b) when 0.11<C<0.125 then Mn≥1.45 and V≥0.13 and Mo≥0.35 (in mass-%), [0114] c) when 0.125<C<0.15, then Mn≥1.35 and V≥0.12 and Mo≥0.20 (in mass-%), [0115] d) when C≥0.15 and V>0.11, then Mn≥1.3 and Mo≥0.15 (in mass-%) or [0116] when C≥0.15 and V 0.03-0.11, then Mn>1.3 and Mo>0.15 and Nb>0.02 and Cr+Cu+Ni>1.4 (in mass-%).

[0117] The method comprises the step of heating a steel slab having the chemical composition described above to an austenitizing temperature of 1000-1350° C.

[0118] The thickness of the steel slab is, for example, 210 mm and it is preferably heated to an austenitizing temperature of 1200-1350° C., where it is kept until it is of adequately even warmth and the alloying elements have adequately dissolved into the matrix. Typically, this takes several hours. If the austenitizing temperature is below 1200° C., there can be a danger that not all of the alloying elements will dissolve into the austenite, i.e. the austenite is not made homogenous and during the tempering, the precipitation hardening may remain at a low level. On the other hand, if the austenitizing temperature is higher than 1350° C., this will result in an exceptionally large grain size of the austenite and increased oxidation of the slab surface. Annealing time in reheating is typically varied in the range of 2-4 hours, but, depending on the selected furnace technology and the thickness of the slab, it can also be longer than 4 hours or shorter than 2 hours.

[0119] After the heating step, hot-rolling is conducted, which may typically comprise a roughing step and a subsequent finish rolling step. The temperature of hot-rolling at the last pass is 760-1050° C. Preferably, the finishing rolling temperature at the last pass of the hot-rolling is 760-960° C. The end temperature of hot-rolling is preferably above 830° C. or more preferably at least 850° C. so that rolling forces remain reasonable, and at the most 940° C., and more preferably 920° at the most, wherein i.a. excellent surface quality is assured.

[0120] After hot-rolling, or strip rolling, the steel is quenched, i.e. cooled at an accelerated cooling rate, typically of 30-150° C./s, using one step cooling for example, preferably at a maximum cooling rate of 120° C./s, in a suitable quenching medium, such as water or oil, to a temperature of 300° C. or less, or preferably 150° C. or less, i.e. any temperature between room/ambient temperature and 300° C. If it is a strip product it is coiled at that temperature, i.e. at a coiling temperature of 300° C. or less. Preferably, the quenching is direct quenching conducted a maximum of 15 seconds after the last hot-rolling pass.

[0121] This quenching gives the steel its exceptionally good mechanical properties including good impact toughness combined with good bendability. Preferably, the end temperature of quenching is a maximum of 150° C., because, in this case, after quenching, a steel product with good flatness is achieved.

[0122] The quenched steel is subsequently subjected to temper annealing at a temperature of 500-650° C. if the tempering time is 1 hour or more, or temper annealing at a temperature of 500-750° C. if the tempering time is less than 1 hour. If the tempering temperature is 400-750° C. then the temper annealing is typically carried out in a furnace other than a bell-type furnace so as to avoid the risk of adversely affecting the strength and toughness properties of the steel. However, if the tempering temperature is 150-250° C. then the tempering annealing may also be carried out in a bell-type furnace without adversely affecting the strength and toughness properties of the steel and to minimize production costs. Tempering at annealing temperatures of 250-400° C. is not recommended due to low temperature tempering embrittlement if good toughness properties are required. Typically, higher temperatures promote good total elongation values, and lower tempering temperatures promote higher strength properties.

[0123] A suitable tempering treatment is defined by the formula P=T*(20+log t), where the temperature T is in ° K and the time is in hours. The Larsen Miller parameter, P, is between 15-19.5, and preferably 16-18.

[0124] The temper annealing step may be carried out on quenched steel, such as steel sheet cut from a coil, or on a steel sheet that is continuously unwound from a coil, or a heavy plate. In the case of a strip product, the temper annealing step may alternatively be carried out on a whole coil, for example in a bell type furnace.

[0125] The microstructure of the hot-rolled steel before the temper annealing step contains at least 90% martensite, preferably at least 95% martensite and more preferably at least 99% martensite when the microstructure is examined in % thickness. The majority of the microstructure will be martensite although it may contain some bainite. The content of ferrite and pearlite before the temper annealing step must be in total less than 10%, preferably less than 5%.

[0126] Manganese content as a percentage by weight is 1.1-2.2 mass-% in order to assure good hardenability in the weld metal and HAZ of welded hot-rolled steel. Manganese also promotes hardenability of the base material during the quenching step. The expression “weld metal” is intended to mean the part of the weld seam that consists mainly of filler material.

[0127] The maximum Manganese content should be set according the equation so as to prevent excessive segregations and ensure good impact strength:

maximum Manganese content (in mass-%)=2.7−5*Carbon content (in mass-%).

[0128] Molybdenum precipitates in temper annealing, which decreases the lowering of strength caused by tempering treatment and thus helps in achieving high strength. Additionally, Molybdenum is used inter alia to prevent the brittleness of steel by slowing infiltration of inter alia Phosphorus into the grain boundaries during temper annealing. Molybdenum also efficiently increases the hardenability of the base material and ensures good strength properties of welded seams of welded hot-rolled steel.

[0129] It has been found that Niobium may decrease the bendability of a hot-rolled steel if it is present in a large amount. The use of Niobium as an alloying element is however of advantage in achieving adequate strength and impact toughness in hot-rolled steel. Niobium promotes smaller grain size in steel, which results in better properties of the steel. Niobium may be needed, especially in the case of heavy plate, to enable smaller amounts of other alloying elements that promote good strength and toughness properties to be used. In the case of a direct quenched strip product, the steel can be also made without using Niobium. Niobium is therefore an optional alloying element in the hot-rolled steel according to the present invention, the content of which should be limited to 0.06 mass-%, preferably to 0.04 mass-%, and more preferably to 0.005 mass-%, wherein the best possible bendability properties for the hot-rolled steel are assured.

[0130] Titanium is an optional alloying element in the hot-rolled steel according to the present invention, which may be required for binding Nitrogen in the steel, and so that Boron functions efficiently as an improver of hardenability and does not form Boron nitrides. Titanium is used, because it works more reliably with quenched steel than Aluminium. The Titanium content is 0-0.15 mass-%, preferably 0-0.05 mass-% and more preferably 0.005-0.02 mass-%. Titanium nitrides exhibit grain growth in the heat affected zone of a weld and improve the toughness properties of a welded seam. On the other hand, at contents higher than 0.02 mass-%, the amount of relatively large-sized Titanium nitrides, TiN may increase, which is detrimental in terms of the impact toughness and bending properties of the hot-rolled steel. The Ti/N ratio of the hot-rolled steel is preferably in the range 3-4. However, larger Titanium contents up to 0.15 mass-% may be used to increase strength in the as-tempered condition.

[0131] Vanadium content in the hot-rolled steel according to the present invention must be more than 0.1 mass-% and ≤0.25 mass-%, preferably more than 0.10 and ≤0.20 mass-%, or at least 0.11 mass-% Vanadium, or at least 0.12 mass-% Vanadium, or at least 0.13 mass-% Vanadium, or at least 0.14 mass-% Vanadium in order to assure high strength. It has however been found that a too high amount of Vanadium is detrimental to the impact toughness of the quenched and tempered steel. The amount of Vanadium should not therefore exceed 0.25 mass-%. Vanadium has a strong precipitation strengthening effect after tempering and is needed to achieve high strength both in base metal and in the HAZ.

[0132] Aluminium is used to condense steel, i.e. to bind oxygen from the steel. The Aluminium content is 0.01-0.15 mass-%, preferably 0.015-0.08 mass-% to prevent excessive formation of aluminium oxides.

[0133] Boron is an effective alloying element that promotes hardenability of the steel in quenching. It is an essential alloying element in this invention since it promotes strength and hardness properties in the weld metal and heat-affected zone (HAZ). During welding, Boron moves from the base material to the weld metal, thereby increasing the hardness of the weld metal. This ensures that fracture does not occur in the weld metal or fusion line. Fracture can be moved as far as possible away from the fusion line towards the base material in high static loads. The Boron content is 0.0005-0.010 mass-%, preferably 0.0005-0.005 mass-% and more preferably 0.001-0.003 mass-%. A Boron content of at least 0.0005 mass-% promotes hardenability of the base material and of the HAZ, ensuring good strength properties. On the other hand, a Boron content of more than 0.005 mass-% is worthless as regards the hardenability of the base material and the HAZ. When the Boron content is more than 0.001 mass-%, it ensures matching strength properties of the weld and fracture location as previously described. A Boron content of more than 0.010 mass-% can be detrimental for mechanical properties of the steel.

[0134] The Chromium content of hot-rolled steel according to the present invention is 0.1-1.7 mass-%, preferably 0.4-1.7 or 0.6-1.5 mass-%, or more than 1.0 mass-% in order to achieve high strength and good hardenability both in the hot-rolled steel as produced and in the HAZ of a welded hot-rolled steel product. Chromium also promotes tempering resistance.

[0135] According to an embodiment of the invention the chemical composition of hot-rolled steel according to the present invention contains both Ni and Cu in a total amount of at least 0.5 mass-%, or at least 1.0 mass-%, or at least 1.2 mass-%. Copper is an optional alloying element. It can be used in an amount up to 1.5 mass-%, preferably 0.1-1.0 mass-% in order to increase the strength or improve the weather resistance of the hot-rolled steel.

[0136] According to an embodiment of the invention the chemical composition contains both Ni and Cu, and the amount of Ni≥0.33×the amount of Cu, preferably the amount of Ni≥0.5×the amount of Cu. Cr+Cu+Ni is between 0.4-5.7, preferably between 1.4-3.5 and more preferably between 2-3.

[0137] Nickel is an essential alloying element in the hot-rolled steel according to the present invention and it improves the toughness of the heat-affected zones and the weld metal of welded seams and it also improves the surface quality of the hot-rolled steel containing Copper but may, under some circumstances, slightly decrease the impact toughness of the tempered steel.

[0138] Phosphorus weakens the impact toughness of quenched and tempered steel and the Phosphorus content should therefore be limited to a maximum of 0.015 mass-%, preferably to a maximum of 0-0.009 mass-%.

[0139] Sulphur content is limited to a maximum of 0.008 mass-%, preferably to a maximum of 0.004 mass-%, to assure good impact toughness and formability in the hot-rolled steel according to the present invention.

[0140] Zirconium is an optional alloying element that may replace Niobium if needed. The Zirconium content can be between 0-0.2 mass-%, preferably 0-0.01 mass-%.

[0141] Calcium is an optional alloying element that may be used to modify the morphology of inclusions in the steel. The Calcium content can be between 0-0.004 mass-%. If the amount of Calcium exceeds 0.004 mass-% the inclusions in the steel may be too large, which may adversely affect the physical properties of the steel.

[0142] According to an embodiment of the invention the hot-rolled steel has a tensile strength of at least 1120 MPa and up to 1450 MPa.

[0143] According to an embodiment of the invention the hot-rolled steel has an A %-elongation of at least 8% (i.e. permanent elongation of length, expressed in percent of length) or even of at least 10% or at least 12% along and/or transverse to the rolling direction. The hot-rolled steel has such an elongation in its as-produced condition. The hot-rolled steel also has an elongation of at least 7%, preferably at least 8%, more preferably at least 9% when a tensile test is carried out across a weld seam of a welded hot-rolled steel product where the weld is longitudinal to the rolling direction.

[0144] According to an embodiment of the invention the hot-rolled steel an impact toughness of at least 34 J/cm.sup.2 and more preferably an impact toughness of at least 50 J/cm.sup.2 when a Charpy V notched specimen having a thickness of 5-10 mm is measured at −20° C. and more preferably at −40° C. longitudinally and/or transverse to the rolling direction. The hot-rolled steel has such an impact strength in its as-produced condition.

[0145] The mechanical properties of the hot-rolled steel cited in this document were determined in accordance with the testing instructions of standard ISO 10025-6:2004.

[0146] According to an embodiment of the invention the hot-rolled steel is metal active gas (MAG) welded with our without reinforcement, using a V- or Y-groove welding method, whereby a first pass is welded from a bottom or top side, preferably from a bottom side, and other passes from a top side, using welding consumables having a tensile strength of 1100 MPa, preferably 960 MPa, more preferably 900 MPa, most preferably 890 MPa, and a t8/5 time of 8-12 seconds, preferably 6-18 seconds, more preferably 5-20 seconds, which may be determined by welding the hot-rolled steel and measuring the time it takes for the weld seam and the adjacent heat-affected zone (HAZ) to cool from 800° C. to 500° C.

[0147] According to an embodiment of the invention the hot-rolled steel has a minimum bending radius of 5.0× thickness or more preferably a minimum bending radius of 4.0× thickness or more preferably a minimum bending radius of 3.5× thickness longitudinally and/or transverse to the rolling direction. With a plate thickness of 7 mm or more the steel has a minimum bending radius of 5.0× thickness or preferably a minimum bending radius of 4.0× thickness or more preferably a minimum bending radius of 3.5× thickness longitudinally to the rolling direction, and a minimum bending radius of 5.0× thickness transverse to the rolling direction.

[0148] The hot-rolled steel according to the present invention is suitable for applications, such as wear or structural applications, in which the steel must exhibit high strength in combination with sufficient hardness, bendability and impact toughness both in the as-produced products and in the HAZ (heat affected zone) of welded hot-rolled steel products. For example, the hot-rolled steel according to the present invention may be used to produce any component for construction, mining, material-handling, earth-moving, pile driving, snow-plowing, landscaping or rock drilling equipment. The hot-rolled steel may for example be used to produce a lifting boom for an excavator or crane.

[0149] Test Results

[0150] Tests were conducted using the steels having the chemical compositions presented in Table 1 below. The amount of each element is given in mass-%, the balance being Fe and unavoidable impurities other than Nitrogen. It should be noted that Nitrogen may also be considered to be an unavoidable impurity. The amount of Nitrogen is however given in Table 1 along with the intentionally added alloying elements. The amount of Nitrogen is preferably in the range of 0-0.01 mass-%.

[0151] It should be noted that the compositions labelled “INV” in Table 1 are steels that have the chemical composition and the physical properties of steel according to the present invention and which have been manufactured using a method according to the present invention. Comparative examples that do not have the chemical composition or the physical properties of steel according to the present invention, or which have not been manufactured using a method according to the present invention are labelled “REF” in Table 1.

TABLE-US-00001 TABLE 1 Chemical compositions INV/ Comp. C Si Mn P S Al Nb V Cu Cr Ni N Mo Ti Ca B REF A1 0.13 0.20 1.5 0.01 0.001 0.05 0.00 0.15 0.4 1.3 1.0 0.004 0.4 0.01 0.002 0.001 INV B 0.14 0.18 1.5 0.01 0.001 0.05 0.00 0.15 0.5 1.3 2.0 0.004 0.1 0.01 0.002 0.001 REF C 0.17 0.19 1.5 0.01 0.001 0.05 0.04 0.05 0.5 0.7 1.0 0.004 0.4 0.01 0.002 0.001 INV A2 0.13 0.20 1.5 0.01 0.002 0.05 0.00 0.15 0.5 1.4 1.0 0.004 0.4 0.01 0.003 0.002 INV A3 0.13 0.18 1.5 0.01 0.002 0.06 0.00 0.15 0.4 1.3 1.0 0.004 0.4 0.01 0.003 0.001 INV D 0.17 0.30 1.5 0.01 0.001 0.05 0.00 0.04 0.4 0.7 1.0 0.003 0.4 0.02 0.001 0.000 REF E 0.13 0.19 1.5 0.01 0.001 0.05 0.00 0.04 0.4 0.7 1.5 0.003 0.4 0.02 0.001 0.000 REF F 0.17 0.19 1.5 0.01 0.001 0.06 0.00 0.04 0.4 0.7 1.0 0.003 0.4 0.02 0.001 0.001 REF G 0.18 0.03 1.5 0.01 0.001 0.05 0.00 0.04 0.4 1.0 1.0 0.003 0.4 0.02 0.001 0.001 REF

[0152] Steels having the chemical compositions presented in Table 1 were hot-rolled to an end thickness of 4 mm, 6 mm and 8 mm. Hot-rolling was performed in a hot strip rolling line and hot-rolled strips were directly quenched after rolling before coiling. Depending on the type of tempering furnace used, the tempering was carried out before or after a cut-to-length process. If the tempering was performed in bell type furnace (tempering code “C” in Table 2 below), then the cut-to-length processing for quenched strips was carried out after tempering. In the case of sheet tempering (tempering code “S” in Table 2), the cut-to-length processing was carried out before the tempering annealing. Depending on the tempering method, the holding time during tempering varied between 15-720 minutes.

[0153] More specific manufacturing parameters are presented in Table 2.

TABLE-US-00002 TABLE 2 Process parameters composition process tempering thickness furnT FRT CT T t INV/ code code code mm ° C. ° C. ° C. ° C. min REF A1 R02 S600 8.0 1280 856 50 600 30 INV B R04 S600 8.0 1280 862 50 600 30 REF C R06 S560 8.0 1280 878 50 560 30 INV A2 R12 S600 4.0 1278 924 50 600 15 INV A2 R13 S585 8.0 1267 871 50 585 15 INV A2 R13 S600 8.0 1267 871 50 600 15 INV A3 R14 S585 6.0 1279 897 50 585 15 INV A3 R14 S600 6.0 1279 897 50 600 15 INV A2 R16 S585 4.0 1279 917 50 585 15 INV A2 R16 S600 4.0 1279 917 50 600 15 INV D B05 C420 6.0 1250 919 100 420 720 REF A1 B09 C200 8.0 1250 891 100 200 720 REF E B10 C600 6.0 1250 876 100 600 240 REF F B12 C450 6.0 1250 897 100 450 540 REF G B14 C450 6.0 1250 918 100 450 540 REF D B15 C600 6.0 1250 917 100 600 240 REF where: furnT = reheat temperature before hot-rolling FRT = finishing rolling temperature CT = coiling temperature T = tempering temperature t = tempering time and the process code indicates the geographical location at which each process was carried out.

[0154] Test results from mechanical tests and bending tests are presented in Table 3. Steels according to invention have a yield strength of more than 1100 MPa, a tensile strength of more than 1120 MPa, good impact toughness and, surprisingly, extremely good elongation to fracture compared to known steels. Considering its high strength, steel according to the present invention also exhibits very good bending properties.

[0155] Bending tests were carried out using three-point bending with samples having an area of 300×300 mm.sup.2. Samples were bent to an angle of 905 with one press and all samples were bent into a Z-shape so that both the upper and lower surfaces of the samples were tested. Mechanical properties and bendability of the samples were tested both longitudinally with respect to the rolling direction, and transversely with respect to the rolling direction.

TABLE-US-00003 TABLE 3 Physical properties test T_ composition rolling tempering thickness direction Rp0.2 Rm A % test CVJ/ Bendability INV/ code code code mm T/L MPa MPa % ° C. CVJ cm2 Ri/t REF A1 R02 S600 8.0 T 1151 1188 13.6 −40 33 55.0 4.5 INV A1 R02 S600 8.0 L 1170 1180 15.0 −40 46 76.7 2.8 INV B R04 S600 8.0 T 1096 1128 14.7 −40 28 46.7 4.0 REF B R04 S600 S.0 L 1108 1119 15.4 −40 35 58.3 2.8 REF C R06 S560 8.0 T 1158 1196 13.3 −40 23 38.3 3.5 INV C R06 S560 8.0 L 1171 1185 14.2 −40 42 70.0 2.8 INV A2 R12 S600 4.0 T 1111 1166 13.9 −40 10 50.0 2.0 INV A2 R12 S600 4.0 L 1117 1170 15.1 −40 15 75.0 2.3 INV A2 R13 S585 8.0 T 1170 1204 14.0 −40 29 48.3 2.1 INV A2 R13 S585 8.0 L 1155 1177 15.1 −40 46 76.7 2.8 INV A2 R13 S600 8.0 T 1163 1196 14.0 −40 28 46.7 2.1 INV A2 R13 S600 8.0 L 1151 1171 15.4 −40 47 78.3 2.5 INV A3 R14 S585 6.0 T 1129 1173 14.5 −40 18 45.0 2.3 INV A3 R14 S585 6.0 L 1118 1152 15.6 −40 30 75.0 2.7 INV A3 R14 S600 6.0 T 1128 1171 14.2 −40 18 45.0 2.2 INV A3 R14 S600 6.0 L 1115 1147 15.5 −40 32 80.0 2.3 INV A2 R16 S585 4.0 T 1134 1178 14.6 −40 16 50.0 2.0 INV A2 R16 S585 4.0 L 1134 1163 15.8 −40 24 75.0 2.8 INV D B05 C420 6.0 T 1182 1277 7.3 −40 17 42.5 3.2 REF D B05 C420 6.0 L 1125 1251 9.0 −40 21 52.5 3.2 REF A1 B09 C200 8.0 T 1349 1551 6.7 −40 53 88.3 2.5 REF A1 B09 C200 8.0 L 1327 1519 9.0 −40 79 131.7 3.4 REF E B10 C600 6.0 T 1098 1137 8.9 −40 27 67.5 2.2 REF E B10 C600 6.0 L 1023 1108 11.8 −40 35 87.5 2.3 REF F B12 C450 6.0 T 1170 1257 7.9 −40 20 50.0 2.8 REF F B12 C450 6.0 L 1098 1229 8.9 −40 29 72.5 3.3 REF G B14 C450 6.0 T 1180 1265 7.0 −40 19 47.5 2.7 REF G B14 C450 6.0 L 1105 1234 9.9 −40 28 70.0 3.2 REF D B15 C600 6.0 T 1095 1143 8.7 −40 19 47.5 1.8 REF D B15 C600 6.0 L 1028 1132 11.3 −40 30 75.0 2.0 REF

[0156] Welding Tests

[0157] Welding tests were carried out using a metal active gas (MAG) welding method and V- and Y-grooves. The welding consumables used were according to standard ENG 89 5 M21 Mn4Ni2.5CrMo (Commercial grade X96). The first pass was welded from bottom or top side, preferably from the bottom side, and others passes were welded from the top side. Welding consumables having a tensile strength of 960 MPa were used and t8/5 was varied between 6-18 seconds. Tensile tests across the weld showed that the weld had a yield strength of 1100 MPa (Rp0.2) and the fracture was located at the base metal (BM).

[0158] The target was to achieve a combination of strength and toughness properties which are as good as possible in the weld so that matching tensile properties can be achieved without losing toughness. In addition, the aim was to obtain a fracture in a static tensile test over the weld that is located as far as possible from the weld metal (WM) and fusion line (FL), which enables extremely good elongation to fracture values for the welded structure. The behaviour of a welded structure is predictable and safe when the fracture in static loading takes place as far as possible away from the WM and FL and the elongation to fracture is high. The inventors have found that steels according to the present invention can fulfill these requirements even if the yield strength for the base material is more than 1100 MPa. Usually, known steels with such a high strength have a fracture located over the weld (WM or FL) when a tensile test is performed, especially when un-matching welding consumables are used (the yield strength of un-matching welding consumables is typically less than 1100 MPa).

[0159] Table 4 shows the welding parameters that were used in the tests and the test results obtained. Steels according the invention have a fracture that is located at a distance from the weld (WM and/or FL) when a static load in a tensile test is set over the weld. It is surprising compared to known steels that such behaviour can be achieved with or even without reinforcement. Without reinforcement, the achievement of such behaviour is very innovative. Fracture location is labelled “BM” in Table 4 when the fracture is in the base material, “HAZ” when it occurs in the Heat Affected Zone, and “WM” when the fracture occurs in the Weld Metal.

TABLE-US-00004 TABLE 4 Welding results Transverse tensile test on weld Processing variables Without reinforcement With reinforcement composition rolling tempering thickness t8/5 Rp0.2 RM A % fracture Rp0.2 Rm A % fracture INV/ code code code mm s MPa MPa % location MPa MPa % location REF A1 R02 S600 8 12 1125 1201 11.6 BM 1140 1194 10.8 BM INV B R04 S600 8 12 1078 1130 11.2 BM 1080 1123 11.2 BM REF C R05 S560 8 6 1154 1213 9.8 BM 1160 1204 9.6 BM INV C R05 S560 8 12 1140 1212 10.0 BM 1143 1201 10.3 BM INV C R05 S560 8 18 1115 1207 10.5 BM 1135 1198 10.4 BM INV D B05 C420 6 6 1128 1233 4.7 HAZ 1174 1275 8.4 BM REF D B05 C420 6 12 1081 1185 4.0 WM 1157 1252 6.2 HAZ REF D B05 C420 6 18 1007 1175 5.0 WM 1162 1248 5.5 HAZ REF F B12 C450 6 12 1101 1208 5.1 WM 1162 1244 7.3 HAZ REF G B14 C450 6 12 1106 1217 5.2 WM 1161 1247 5.7 HAZ REF A2 R12 S600 4 12 1107 1176 11.7 BM 1118 1190 11.6 BM INV

TABLE-US-00005 TABLE 4 Welding results Charpy V (5 × 10 × 55 mm) Processing variables J/−40° (mean of 3 tests) composition rolling tempering thickness t8/5 FL + FL + FL + INV/ code code code mm s WM FL 1 3 5 REF A1 R02 S600 8 12 18 * 25 37 20 INV B R04 S600 8 12 16 19 31 33 19 REF C R05 S560 8 6 * 18 31 18 17 INV C R05 S560 8 12 20 18 24 32 18 INV C R05 S560 8 18 22 19 26 34 15 INV D B05 C420 6 6 27 27 36 24 25 REF D B05 C420 6 12 31 33 49 32 28 REF D B05 C420 6 18 35 43 53 35 30 REF F B12 C450 6 12 31 30 33 53 28 REF G B14 C450 6 12 27 29 32 40 26 REF A2 R12 S600 4 12 * * * * * INV where t8/5 = cooling time from 800° C. to 500° C. in the welded seam Impact toughness was measured using specimens having a thickness of 5 mm.

[0160] FIGS. 2 and 3 shows typical hardness profiles over the welded seam tested near the face side and the root side of the welded samples. It is surprising that steels according to the present invention can have a very smooth hardness profile over the weld and that there are no soft zones that could start to neck during a tensile test and thereby influence the location of the fracture. Normally, steels with a yield strength of 1100 MPa welded with un-matching welding consumables (X90 and/or X96) exhibit some softening in the HAZ and especially in the WM. Steel according to the invention can maintain good hardness in the HAZ but also have good hardness in the WM due to the diffusion of alloying elements promoting hardening (i.e. Boron). Low carbon content in the steels according to the invention (i.e. 0.1-0.20 mass-% Carbon) ensures that a weld has high toughness as well as good hardness.

[0161] Further modifications of the invention within the scope of the claims would be apparent to a skilled person.