Effect of thermomechanical processing on microstructure, texture, and anisotropy in two Nb microalloyed steels

The process parameters that affect the anisotropy of mechanical properties of two Nb microalloyed linepipe steels (grades X-70 and X-80) were examined by controlled rolling and accelerated cooling on a pilot mill. The rolling schedules were first simulated by multi-pass torsion testing so as to determine the critical temperatures, such as T$ sb{ rm nr}$ and Ar$ sb3.$ Using the torsion test results, two finish rolling temperatures were chosen so as to be above and below the A$ sb3$ (in the $ gamma$ + $ alpha$ region). Two reheat temperatures were selected to study the effect of prior austenite grain size. The properties of air cooled samples are compared with those of specimens cooled at two different rates; in each case, cooling was interrupted at one of three different temperatures. The textures were measured by x-ray diffractometry and are presented in the form of ODF plots and skeleton lines. The yield strengths were measured by carrying out tensile tests along directions inclined at increasing angles to the rolling direction.The state of the pancaked austenite before transformation was characterized in terms of the effective interfacial area. It is shown that this parameter determines the sharpness of the transformation texture because it accounts for both the amount of pancaking strain applied to the austenite before transformation and the austenite grain size. Another important factor affecting texture development during transformation is the rate of cooling, as it determines the dislocation density present on each active slip system. The results of hardness testing, texture measurement, and mechanical testing showed that a moderate cooling rate and a medium cooling interruption temperature lead to the best combination of a fine microstructure and a desirable texture. It is shown that accelerated cooling increases both the yield strength and the planar anisotropy of the yield strength, the latter property rising to a maximum in the samples associated with the lowest cooling interruption temperature. Comparison between the experimental results and model predictions of the yield strength ratio indicates that the pancake and lath models are in the best agreement with the measured values