Processing of phosphorus-alloyed lovv-carbon steel sheet by continuous annealing

Abstract

Low-carbon steel containing 0·07 wt-% phosphorus has been heat treated in the laboratory in a manner designed to simulate an industrial continuous-annealing process. The resulting structures have been examined by optical metallography and X-ray diffraction, while mechanical properties were determined from tensile tests. Process variables investigated were: (i) hot-band ‘coiling’ temperature, (ii) cold-rolling reduction, (iii) time and temperature of annealing, and (iv) the quenching temperature of the sheet after annealing. A standard overaging treatment was given in all cases. Strength levels of the steel sheets were rather insensitive to processing treatments, but increased somewhat with lower coiling and annealing temperatures as a result of grain refinement. Both ductility and plastic anisotropy were favoured by a higher coiling temperature, and an optimum combination of properties was foundfor the 67% cold-rolled sheet annealed for 80 s at 750°C. With heavier cold reductions, the optimum annealing temperature was ∼800°C. The quenching temperature has a significant effect on ductility, because it controls the dispersion of carbide particles and the amount of carbon remaining in solid solution in the matrix. A good combination of properties could be achieved by appropriate control of these processing variables.