Low-yield-ratio ultra-high-strength high-toughness steel for pressure hulls and preparation method therefor

A preparation method for a steel for pressure hulls, comprising the following steps:

• step 1, melting
• melting to obtain a casting blank using the following chemical components by weight percentage:
• 0.05%-0.10% of C, 0.15%-0.35% of Si, 0.60%-1.00% of Mn, 0.10%-0.50% of Cu, 0.10%-1.00% of Mo, 0.40%-0.70% of Cr, 0.05%-0.15% of V, 5.00%-10.00% of Ni, and the balance of Fe and unavoidable impurities,
• step 2, hot rolling
• keeping the casting blank obtained in step 1 at 1150° C.-1220° C. for soaking, and then performing hot rolling to a final thickness of 5-80 mm to obtain a hot-rolled steel plate;
• wherein the hot rolling comprises a two-stage rolling process, wherein a first stage of the two-stage rolling process is carried out at a rolling temperature of 1000° C.-1150° C. and a reduction of >50%, and wherein a second stage of the two-stage rolling process is carried out at a rolling temperature of 750° C.-900° C. and a reduction of >50%;
• and air cooling the hot-rolled steel plate after the hot rolling to room temperature;
• step 3, heat treatment
• heating a sample of the hot-rolled steel plate in step 2 to 790° C.-810° C., soaking for 20-40 minutes, and water quenching to room temperature;
• then heating again to 650° C.-690° C., soaking for 20-40 minutes, and water quenching to room temperature;
• and finally, tempering at 590° C.-610° C. for 50-70 minutes.