Influence of thermal history on hot ductility of steel and its relationship to the problem of cracking in continuous casting

Abstract

A variety of heating and cooling programmes have been examined for plain C–Mn and high strength low alloy steels to examine their suitability in a hot tensile test for assessing the likelihood of transverse cracking occurring in the straightening operation. A tensile test temperature of 800°C was chosen for comparison, this being the temperature that generally results in poor ductility. For steels with 1·4–1·75%Mn, the simple procedure of heating to ∼1300°C to take all the microalloying additions into solution, followed by cooling to the test temperature, was found to be the easiest and most suitable. For high strength low alloy steels containing Ti and low Mn, high S steels, melting is required. For these steels, it is also advisable to have both primary rapid cooling followed by a slower secondary cooling stage, simulating more accurately the actual industrial operation. The addition of thermal oscillations to simulate slab roll contact makes the cycle even closer to the commercial process and generally led to a small decrease in reduction in area values. Because of its complexity, this latter method would not be generally recommended for steels showing wide trough behaviour (high Mn, peritectic C steels), and melting followed by primary and secondary cooling is sufficient. For narrow troughs (low Mn, low C steels), which require melting and where the minimum ductility will be at a temperature of >800°C, the more complex procedure will be required. It will be necessary to obtain the full hot ductility curve using a cycle that, as well as melting and having primary and secondary cooling, also incorporates commercial thermal oscillations or at least some limited thermal oscillations.