EFFECT OF INTERMEDIATE COOLING ON GRAIN-REFINEMENT AND PRECIPITATION DURING ROLLING OF SEAMLESS TUBES

The effect of intermediate reheating during tube piercing and rolling was studied by means of laboratory simulation. This was done by allowing the samples to cool below the Ar3 and Ar1 temperatures during torsion testing before reheating and applying the final passes. In the simulation, the process was divided into high and low temperature stages, separated by the reheating stage. The tests were performed on a plain C-Mn and five microalloyed steels (four Ti-V and one Nb-V grade). It is shown that the most effective grain refinement is produced when the sample is cooled below the Ar1 temperature before reheating, i.e. when an on-line normalising concept is used. The largest grain size decrease was produced in the C-Mn steel and the smallest in the Ti-V grades. To retain such grain refinement, static recrystallisation must be inhibited during subsequent processing. A disadvantage of on-line normalising is that it reduces the precipitation strengthening produced in the ferrite in microalloyed steels.