785 MPa LEVEL EXTRA-THICK QUENCHED AND TEMPERED RACK STEEL PLATE FOR OFFSHORE PLATFORMS AND PREPARATION METHOD THEREFOR

Abstract

The present invention discloses a 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms and a preparation method therefor, wherein chemical components of the steel plate by weight percentage are: 0.08%-0.15% of C, 0.60%-1.5% of Mn, 0.10%-0.50% of Si, 0.002%-0.005% of S, 0.005%-0.015% of P, 2.7%-4.0% of Ni, 0.40%-1.00% of Cr, 0.30%-0.70% of Mo, 0.10%-0.40% of Cu, 0%-0.080% of Nb, 0%-0.10% of V, 0%-0.08% of Ti, 0.02%-0.08% of Al, 0.0008%-0.0015% of B, and the balance of Fe and other unavoidable impurities. The present invention adopts a low C, high Ni—Cr—Mo alloying and Nb—V—Ti microalloying component system, has a yield strength of ≥795 MPa, a tensile strength of ≥865 MPa, a −60° C. Charpy V-notch impact energy of ≥140 J, a Z value of ≥35%, and a maximum thickness of 100 mm, and can be used to manufacture racks for offshore platforms.

1 . A 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms, wherein chemical components of the extra-thick quenched and tempered steel plate by weight percentage are: 0.08%-0.15% of C, 0.60%-1.5% of Mn, 0.10%-0.50% of Si, 0.002%-0.005% of S, 0.005%-0.015% of P, 2.7%-4.% of Ni, 0.40%-1.00% of Cr, 0.30%-0.70% of Mo, 0.10%-0.40% of Cu, 0%-0.080% of Nb, 0%-0.10% of V, 0%-0.08% of Ti, 0.02%-0.08% of Al, 0.0008%-0.0015% of B, and the balance of Fe and other unavoidable impurities. 
     
     
         2 . The 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms according to  claim 1 , wherein the extra-thick quenched and tempered steel plate has a thickness of 80-100 mm. 
     
     
         3 . The 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms according to  claim 1 , wherein the structure of the extra-thick quenched and tempered steel plate is a tempered martensite structure. 
     
     
         4 . The 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms according to  claim 1 , wherein the extra-thick quenched and tempered steel plate has a yield strength of 795-980 MPa, a tensile strength of 865-980 MPa, an elongation of 16%-19%, and a section shrinkage percentage of 56%-63%. 
     
     
         5 . The 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms according to  claim 1 , wherein the extra-thick quenched and tempered steel plate has a −60° C. impact energy of ≥140 J. 
     
     
         6 . Preparation method for the 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms according to  claim 1 , comprising the following process steps:
 {circle around (1)} forging a casting blank with a thickness of 180-210 mm into a billet with a thickness of 160-170 mm, heating the billet together with a furnace to 1100° C.-1200° C. and soaking for 2-4 hours, wherein chemical components of the billet by weight percentage are: 0.08%-0.15% of C, 0.60%-1.5% of Mn, 0.10%-0.50% of Si, 0.002%-0.005% of S, 0.005%-0.015% of P, 2.7%-4.0% of Ni, 0.40%-1.00% of Cr, 0.30%-0.70% of Mo, 0.10%-0.40% of Cu, 0%-0.080% of Nb, 0%-0.10% of V, 0%-0.08% of Ti, 0.02%-0.08% of Al, 0.0008%-0.0015% of B, and the balance of Fe and other unavoidable impurities;   {circle around (2)} rolling the billet into a hot-rolled steel plate with a thickness of 80-100 mm through a rough rolling stage and a finish rolling stage, wherein the rough rolling stage has an initial rolling temperature of 1050° C.-1150° C., and a final rolling temperature of 980° C.-1020° C.; the finish rolling stage has an initial rolling temperature of 850° C.-870° C., and a final rolling temperature of 840° C.-860° C.; and the steel plate is air cooled to room temperature after rolling;   {circle around (3)} placing the hot-rolled steel plate cooled to room temperature into a heating furnace for thermal refining including quenching and tempering, and air cooling to room temperature after tempering to obtain a product.   
     
     
         7 . The method according to  claim 6 , wherein in step {circle around (2)}, the number of rough rolling passes is 3-5, and the reduction in each pass is 9%-14%; the number of finish rolling passes is 2-4, and the reduction in each pass is 7%-12%. 
     
     
         8 . The method according to  claim 6 , wherein in step {circle around (3)}, the thermal refining process is that: quenching temperature is 900° C.-930° C., soaking time is 90-120 minutes, and quenching water temperature is 20° C.; tempering temperature is 600° C.-660° C., and tempering soaking time is 60-90 minutes.