Influence of heating conditions on (D)CCT diagram of microalloyed steel for seamless tubes

Abstract

With the aim of improving processing of seamless tubes made of a HSLA steel containing Cr, V, Nb and N. Three continuous cooling transformation diagrams were constructed for the HSLA steel containing Cr, V, Nb and N – standard CCT diagram as well as DCCT diagram including the effect of previous deformation for austenitizing temperature 900 °C, and another DCCT diagram for austenitizing temperature 1280 °C. Combining the location in continuously cast bloom with the material’s thermomechanical treatment, initial austenite grain of size approx. 10 µm was obtained at low-temperature heating and approx. <200 µm at high-temperature heating of the dilatometry samples. Deformation at temperature 900 °C influenced the phase transformations only little, but the effect of coarse grain was considerable, by dropping down the Ferrite-start temperature up to 200 °C in comparison with the fine-grain structure. The initial coarse-grain microstructure accelerated the acicular ferrite formation. The developed diagrams can be used in a seamless tube rolling mill for the stage of cooling from finish-rolling temperature or for the subsequent thermal processing.