Study on Continuous Annealing Process of Low Carbon Steel on a Continuous Annealing /Hot-dip Galvanizing Dual-purpose Line

The annealing cycle of the continuous annealing/hot-dip galvanizing dual-purpose line is different from that of the typical continuous annealing line which has no slow cooling section in addition to short annealing cycle and low cooling rate after annealing.The industrially-produced full-hard low carbon steel coils under high coiling temperature during hot rolling were used to investigate the effect of annealing temperature and time as well as over-aging treatment on the microstructures and mechanical properties of typical low carbon steels according to the characteristics of the dual purpose line.The results show that ferrite grains of both the low carbon steel and the extra-low carbon steel are fine with a grain size number of 10.5 even when the annealing temperature reaches 810℃.After over-aging treatment for 240 s at 400℃,the strength of the low carbon steel is lowered by about 10 MPa and its elongation is increased with a maximum of 4.5% while the strength of the extra-low carbon steel is lowered by about 10 MPa only at an annealing temperature of 810℃ or above,namely,the effect of over-aging treatment is limited.Steel sheets with excellent mechanical properties have been produced by optimizing the continuous annealing process and on-line skin-passing process as well as micro-alloying with baron on a continuous annealing/hot-dip galvanizing dual-purpose line.