Effect of coiling and annealing temperatures on yield point behavior of low-carbon steel

Low-carbon steel is widely used for household appliance and automotive panel steel because of its excellent plasticity. Unfortunately, yield point phenomena easily appear in the low-carbon steel produced by a continuous annealing process and cause degradation to the surface quality during processing. The effect of the coiling temperature (600–750 °C) and annealing temperature (740–820 °C) on the yield point behavior is studied. Tensile tests show that coiling temperature has a greater effect on yield point elongation (YPE) and aging index (AI) than the annealing temperature. Microstructure observations show that coiling temperature at 750 °C would make the micron-sized carbides appearing at the grain boundary disappear and a number of dispersed nanoscale carbides precipitate in grain interior, corresponding to the highest solid solution carbon content in the matrix of 750 °C coiled sample. The experimental results suggest that AI rather than YPE has a positive relationship with the solid solution carbon content of the low-carbon steel. And YPE has a positive relationship with the upper/lower yield strength.