Abstract

This paper investigated a novel technique that uses a slow-cooling heat treatment to reduce the strength of existing steels to produce a regional soft area for energy dissipation. This research investigated the performance and affecting mechanism of the slow-cooling heat-treatment on the mechanical properties of high strength steels. A total of 18 heat-treating cases were applied to Q420 and Q690 steels. The relation of critical strengths and ductility of the materials versus the heat-treating parameters, such as peak temperature, cooling rate, holding time and terminate temperature, were investigated and discussed. The strength reduction from the slow-cooling heat treatment process in high strength steels and normal strength steels was discussed. The results indicated that the slow-cooling heat treatment can effectively reduce the yield and ultimate strengths of high strength steels. The Q690 steel displayed a more prominent strength reduction than the Q420 steel, especially for the yield strength. The heat treatment also improved the material ductility, especially for the Q690 steels. The peak temperature and cooling rate were two determining factors for the strength reduction performance. A heat treatment process with a 900–1000 °C peak temperature and a 1–2 °C/min cooling rate was suggested for the strength reduction of Q420 steel.

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