An 80mm-thick 690MPa-grade ultra-high strength and toughness marine-engineering steel plate and the preparation method thereof

Abstract

The present invention relates to an 80 mm-thick 690 MPa-grade ultra-high strength and toughness marine-engineering steel plate and the preparation method thereof, wherein the chemical composition of the steel plate, by mass percentage, is shown as follows: C: 0.08%˜0.10%, Si: 0.20%˜0.30%, Mn: 1.10%˜1.25%, P≤0.007%, S≤0.002%, Nb: 0.020%˜0.030%, Ti: 0.010%˜0.020%, V: 0.030%˜0.045%, Cr: 0.40%˜0.60%, Ni: 1.40%˜1.50%, Cu: 0.15%˜0.25%, Mo: 0.25%˜0.35%, Als: 0.015%˜0.045%, and Pcm≤0.33%, Ceq≤0.64%, the rest are Fe and inevitable impurity elements. The 80 mm-thick 690 MPa-grade ultra-high strength and toughness marine-engineering steel plate according to the present invention has the performance indicators that meets all the certification requirements of China Classification Society for EH690 steel. Also, the saturated corrosion current density at −300 mV (relative to Ag/AgCl reference electrode) is ≤1.90 mA/cm 2 ; the corrosion-active inclusion has a density of ≤9/mm 2 . The product has excellent characteristics such as high toughness, low temperature resistance, and corrosion resistance, etc.

1 . An 80 mm-thick 690 MPa-grade ultra-high strength and toughness marine-engineering steel plate, wherein, the chemical composition of the steel plate, by mass percentage, comprises: C: 0.08%˜0.10%, Si: 0.20%˜0.30%, Mn: 1.10%˜1.25%, P≤0.007%, S≤0.002%, Nb: 0.020%˜0.030%, Ti: 0.010%˜0.020%, V: 0.030%˜0.045%, Cr: 0.40%˜0.60%, Ni: 1.40%˜1.50%, Cu: 0.15%˜0.25%, Mo: 0.25%˜0.35%, Als: 0.015%˜0.045%, and Pcm≤0.33%, Ceq≤0.64%; and the rest are Fe and inevitable impurity elements. 
     
     
         2 . The 80 mm-thick 690 MPa-grade ultra-high strength and toughness marine-engineering steel plate according to  claim 1 , wherein, the content of each component in the inevitable impurity elements is shown as follow: H is ≤0.0002%, O is ≤0.003%, N is ≤0.004%, B is ≤0.0005%, As is ≤0.006%, Sb is ≤0.010%, Sn is ≤0.020%, Pb is ≤0.010%, Bi is ≤0.010%, by mass percentage. 
     
     
         3 . The 80 mm-thick 690 MPa-grade ultra-high strength and toughness marine-engineering steel plate according to  claim 1 , wherein, the major performance indicators of the ultra-high strength and toughness marine-engineering steel plate include that: yield strength is ≥690 MPa; tensile strength is 770-940 MPa; elongation at break is ≥16%; lateral impact energy at the core and at −40° C. is ≥100J; CTOD at −40° C. is ≥0.15 mm; saturated corrosion current density at −300 mV, relative to Ag/AgCl reference electrode, is ≤1.90 mA/cm2; corrosion-active inclusion has a density of ≤9/mm2. 
     
     
         4 . A method for preparing the 80 mm-thick 690 MPa-grade ultra-high strength and toughness marine-engineering steel plate according to  claim 1 , comprising the steps of converter smelting, LF+RH double refining, continuous casting, casting billet heating, rolling, and heat treatment. 
     
     
         5 . The method according to  claim 4 , wherein, in the step of converter smelting, KR-treated molten iron is used; in the molten iron, S is ≤0.008%; nickel plates, copper plates, and ferromolybdenum are added along with scrap steel; a double-slag deep dephosphorization process is used for smelting; metal manganese, ferroniobium, ferrovanadium, low-carbon ferrochromium, and ferrosilicon are used for alloying; and aluminum ferromanganese is added, according to 3-3.5 kg/t steel, for deoxidation. 
     
     
         6 . The method according to  claim 4 , wherein, in the step of LF+RH double refining, argon is blown at the bottom throughout the LF refining process; the refining period is ≥50 min, and the soft blowing period is 10-15 min; in the RH refining process, the vacuum degree is ensured within 30 Pa, the pure degassing period is ≥5 min; after RH treatment, the calcium-aluminum wire is fed according to 1-1.5 m/t, the soft blowing period is 15-20 min, and the RH smelting period is ≥50 min. 
     
     
         7 . The method according to  claim 4 , wherein, in the step of continuous casting, whole-process protective casting is adopted; the superheat level is controlled within 20° C.; 300 mm-thick continuous casting billet is used and pulled at a speed of 0.70-0.90 m/min; and the obtained casting billet is cooled down slowly for 72 hours and longer. 
     
     
         8 . The method according to  claim 4 , wherein, in the step of casting billet heating, a multi-stage heating process is adopted; the temperature is 1190-1240° C. in the soaking section; the soaking period is ≥60 min; the tapping temperature is 1200-1230° C.; and the total heating period is 290-310 min. 
     
     
         9 . The method according to  claim 4 , wherein, the step of rolling comprises two stages of rolling, i.e. rough rolling and precision rolling; the rough rolling is a recrystallization rolling process with no more than 5 rough rolling passes, the reduction rates of at least 2 passes are ensured to be ≥19%, and the intermediate billet obtained after rough rolling has a thickness of 120-130 mm; the precision rolling is a un-recrystallization rolling process with an initial rolling temperature of 835-865° C. and no more than 7 precision rolling passes; after rolling, the initial cooling temperature of the rapid cooling process is 800˜820° C., the final cooling temperature is 550˜590° C., and the cooling rate is 6˜10° C./s. 
     
     
         10 . The method according to  claim 4 , wherein, in the step of heat treatment, a quenching and high-temperature tempering process is used; the quenching temperature is 920±5° C., heating period is 1.3˜1.6 min/mm×plate thickness, the holding time is 30±3 min; the water pressure in the high-pressure section and the low-pressure section of the quenching machine is 0.7˜0.9 bar and 0.3˜0.4 bar, respectively; the roller speed is 1.6-1.8 m/min, the water volume in the high-pressure section is 5376-6067 m3/h, and the water volume in the low-pressure section is 3499-3888 m3/h; the ratio of the water volume from the upper nozzle to the water volume from the lower nozzle is about 1:1.4, the tempering heating temperature is 600±5° C., the heating period is 2˜2.5 min/mm*plate thickness, the holding time is 30±3 min, and the product is air-cooled after being discharged from the furnace.