Stainless steel composite plate for weathering steel bridge having low yield ratio and preparation method thereof

A stainless steel composite plate for a weathering steel bridge having a low yield ratio, comprising a base material and a covering material, wherein contents of chemical composition of the base material in percent by weight are:

• C:
• 0.04-0.09%, Si:
• 0.15%-0.40%, Mn:
• 1.20%-1.50%, P≥0.025%, S≥0.020%, Nb:
• 0.010%-0.060%, V≥0.080%, Ti:
• 0.008%-0.018%, Cr:
• 0.40%-0.60%, Ni:
• 0.31%-0.45%, Cu:
• 0.31%-0.50%, Mo≥0.25%, N≥0.0080%, and Als:
• 0.015%-0.045%, balances are Fe and a small amount of inevitable impurities, and the chemical composition of the base material meets requirements of an atmospheric corrosion resistance index (I)=26.01(% Cu)+3.88(% Ni)+1.20(% Cr)+1.49(% Si)+17.28(% P)−7.29(% Cu)(% Ni)−9.10(% Ni)(% P)−33.39(% Cu)2≤6.0;
• and contents of chemical composition of the covering material in percent by weight are:
• C≥0.03%, Si≥0.75%, Mn≥2.00%, P≥0.045%, S≥0.030%, Cr:
• 16.00%-20.00%, Ni:
• 8.00%-15.00%, Mo≥4.00%, Ti:
• 0.008%-0.300%, N≥0.10%, and balances are Fe and a small amount of inevitable impurities;
• and a preparation method of the stainless steel composite plate for the weathering steel bridge having the low yield ratio, comprising the following steps:
• a step (1) of preparing the base material and the covering material:
• a continuous casting blank whose chemical composition meets the requirements is selected as the base material and the covering material, and heated at a high temperature of 1230±20° C. and bloomed into a desired size;
• and a surface iron oxide scale on an upper surface and a lower surface of the base material after blooming is removed by polishing with an abrasive belt, attention is paid to checking that neither the upper surface nor the lower surface thereof has any defects including heavy skin, cracks, and pores;
• a step (2) of single-surface blank making:
• a right-angle hole is punched on a top of the base material at 60-70 mm from a side surface, with a hole diameter of 10±2 mm and a hole depth of ½ of the thickness of the base material;
• one covering material is placed onto the upper surface of the base material, wherein a width and a length of the covering material are processed to be smaller than those of the base material, the covering material is located in a middle of the upper surface of the base material, and areas with a width of about 40-50 mm around four edges of the base material not covered by the covering material are exposed;
• and gas shielded welding is employed to seal and weld surroundings of the covering material and the upper surface of the base material, welding width and height are controlled within a range of 5-20 mm, and a to-be-composited surface of a single-surface composite blank is vacuumized and sealed through the right-angle hole to form the single-surface composite blank;
• a step (3) of symmetrical blank making:
• a surface of the covering material of one single-surface composite blank is coated with an isolating agent and baked, and the covering material of another single-surface composite blank is symmetrically stacked downwards;
• surroundings of the covering material are surrounded by vertical strips, the vertical strips and the base material of the single-surface composite blank are welded and fixed by the gas shielded welding, and grooves around the vertical strips are fully welded and filled using submerged arc welding;
• and a hole is punched at a position of the submerged arc welding, vacuum pumping is performed between the covering material of the one single-surface composite blank and the covering material of the another single-surface composite blank, a vacuum degree is controlled to be below 50 Pa, and seal welding is then performed to obtain a symmetrical composite blank;
• a step (4) of controlled rolling and controlled cooling:
• the symmetrical composite blank is heated at 1180-1250° C., and a heating duration is controlled within 9-16 min/cm;
• the symmetrical composite blank is subjected to two-stage rolling, wherein a rough rolling is performed at a low speed and a heavy pressure, with a first rolling reduction rate of 10-15% and a final rolling reduction rate of ≥22%;
• a thickness of a two-stage warm-up blank is ≤1.8 h;
• an initial rolling temperature of finish rolling is ≥900° C., a sum of final three rolling reduction rates of the finish rolling is ≤30%, and a final rolling temperature falls within 800-850° C.;
• and after rolling, a composite plate is cooled, a water entry temperature is controlled within 720-800° C., a self-tempering temperature is controlled within 400-650° C., and air cooling is then performed;
• a step (5) of tempering heat treatment:
• a tempering temperature falls within 400-550° C., a tempering duration is controlled within a range of 2.0-3.5 min/mm according to a total thickness of the composite plate;
• and
• a step (6) of plate splitting and straightening:
• the composite plate is cut by plasma cutting on four sides, upper and lower single-surface composite blanks are automatically separated to obtain two single-surface stainless steel composite plates, and the two single-surface stainless steel composite plates are subjected to straightening and water blasting to obtain final products.