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-%).

1 . A hot-rolled steel having a yield strength (Rp 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 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.