LI Chuncheng, GAO Guoshuai, AN Junlin, JIANG Lepeng, LI Kelong
The large deviation of contract specifications in the production of Ti-containing deep drawing high strength steel with different yield strength grades led to the problems of more or less orders for individual products and too many heats of continuous casting in steelmaking. It was necessary to adopt the flexible production method of 'one steel multi-level' or 'one steel multi-function.The microstructure of Ti-containing deep drawing high strength steel was controlled to meet the performance requirements of products with different strength levels, and low-cost and efficient industrial production and management were realized. Cold rolling and continuous annealing process of HC220Y Ti-containing high strength steel were studied by cold rolling mill with hydraulic tension and continuous annealing simulator. Combined with industrial production test, the influence of cold rolling reduction rate, annealing temperature and temper rolling elongation on microstructure and mechanical properties of the tested steel was studied. The results indicate that the cold rolling reduction rate was increased from 70% to 85%, the yield strength of the tested steel increased by 39 MPa, the tensile strength increased by 42 MPa, the elongation after fracture increased by 4.5%, the plastic strain ratio (r value) increased by 0.25, and the strain hardening index (n value) is about 0.20. The recrystallized grains of the testd steel increased under the large cold rolling reduction rate, and the grain size became uniform and fine after annealing. When the annealing temperature was increased from 780 ℃ to 820 ℃, the yield strength ofthe tested steel was decreased by 32 MPa, the tensile strength was decreased by 17 MPa, the elongation after fracture was increased by 3.5%, the r value was increased by 0.25, and the n value was almost unchanged. On the one hand, the increase of annealing temperature during continuous annealing process increased the recrystallization temperature of the testd steel, increased the driving force of recrystallization, accelerated the nucleation rate of recrystallization, and greatly shortened the time of recrystallization process. On the other hand, Ti was easily combined with C/N during hot rolling production, and coarse titanium carbonitride was formed along the ferrite grain boundary. Higher continuous annealing temperature was easy to make titanium carbonitride aggregate and grow up, and it was difficult to hinder grain growth. Therefore, the grain size of the testd steel increased. When the temper rolling elongation was increased from 0.5% to 1.0%, the yield strength of the tested steel increased by 33 MPa, the tensile strength didn't change significantly, the elongation after fracture decreased by about 3%, the r value decreased by 0.25, and the n value decreased by 0.02. The industrial production of testd steel with different strength levels can adjust the temper rolling elongation to a small extent to meet the performance requirements of the finished product.
