Effect of duplex ferrite grain size distribution on local fracture stresses of Nb-microalloyed steels

Two commercial thermomechanically controlled rolled (TMCR) microalloyed steel plates have been used to investigate the relationship between the duplex ferrite grain size distribution and local fracture stresses. Statistical analyses of the grain size distributions were performed for the fine and coarse ferrite grains in the two steel plates. Microhardness values were measured for each grain size region and it was found that the fine grain areas have significantly higher microhardness values than the coarse grain areas. Tensile and blunt-notch slow bend tests were carried out over a range of temperatures on samples from the two commercial TMCR steel plates. The local fracture stress (σF) values were calculated and the results show that the σF values are almost independent of temperature. The presence of a mixed grain size distribution results in significant scatter in the local fracture stresses of the steels. The distribution of fracture stress values can be correlated to the coarse grain size distribution in the steels examined.