Improving weldability of an advanced high strength steel by design of base metal microstructure

Abstract

Cold rolled samples of a low carbon Mo-Ti-Nb microalloyed steel were subjected to simulated continuous annealing cycles using a Gleeble 3500 thermomechanical simulator to suitably optimize the base metal micro-constituents in order to improve weldability. The cycles, CR-1 and CR-2 were designed to assess the cooling rate effect (i) CR-1: 10 degrees C/s-780 degrees C(120s)-2 degrees C/s-730 degrees C-100 degrees C/s, (ii) CR-2: 10 degrees C/s-780 degrees C(120s)-2 degrees C/s-730 degrees C-50 degrees C/s], CR-1-T to assess the tempering effect [CR-1-T: 10 degrees C/s-780 degrees C(120s)-2 degrees C/s-730 degrees C-100 degrees C/s-10 degrees C/s-200 degrees C/s (120s)-60 degrees C/s] and B-1 and B-2, to assess the effect of bainite [B-1: 10 degrees C/s-780 degrees C(60s)-2 degrees C/s-760 degrees C-40 C/s-470 degrees C/s (60s)-70 degrees C/s and B-2: 10 degrees C/s-780 degrees C(120 s)-2 degrees C/s-760 degrees C-20 degrees C/s-470 degrees C/s (605)-10 degrees C/s] on weldability of the steel. Dilatometric studies were conducted on a Gleeble 1500D to obtain the intercritical temperature regime (Ar-1-Ar-2) and the transformation temperatures for the low temperature transformation products. It was recorded that the cycle with 100 degrees C/s cooling rate (CR-1) resulted in a weldment full of cracks at the (Mn-Fe)S interfaces. A cooling rate of 50 degrees C/s (CR-2) caused cracks at the (Mn-Fe)S inclusions in the weld zone and the HAZ, and also softening of the heat affected zone (HAZ). The cracks generated by the CR-1 cycle could be completely avoided by introducing a short tempering cycle of 120s at 200 degrees C to the CR-1 cycle (CR-1-T). CR-1-T cycle resulted in tempering of martensite (31% by volume) and precipitation of Ti-C in the ferrite that helped avoid the weldment cracks and retain the tensile properties close to that of the CR-1 cycle. B-1 cycle with a nominal presence of bainite in the ferritie-martensite structure resulted in cracks in the weldment. However, in B-2 cycle with the introduction of an optimized amount of bainite (37%) in a ferritic-martensitic matrix in association with Mo-C precipitation in ferrite, a crack-free weldment with improved tensile properties could be obtained. Among the studied thermal cycles, CR-1-T and B-2 manifested the best combination of weldability and tensile properties. (C) 2015 Elsevier B.V. All rights reserved.