Grain boundary ferrite morphology and grain boundary crack in Nb/Ti microalloyed steel billets
YANG Zhong-min1，WANG Kai1，SUN Xiu2，DA Guang-jie2
（1. Institute of Structural Steels， Central Iron and Steel Research Institute， Beijing 100081， China 2 . College of Materials Science and Engineering， Hebei University of Science and Technology， Shijiazhuang 050018， Hebei， China）
Abstract：In order to study the causes of surface transverse cracks of Nb/Ti microalloyed steel billets，the metallographic observation，scanning electron microscopy and electron backscatter diffraction （EBSD） were used to comparative analysis of grain boundary ferrite microstructure of continuous casting billets and laboratory ingots of Nb/Ti microalloyed steel. Through microscopic observation and analysis，the grain boundary ferrite can be divided into two types according to the morphologies. One is that the side of interfaces of the grain boundary ferrite is linear and clear，and the other side is tortuous and unclear （called a clear grain interfaces on one side，GF-I type）. The second type is the tortuous and unclear types on both sides of grain boundaries （called it is non-clear on both sides of the grain interfaces，GF-II type）. In the two types of grain boundary ferrite，chain like micropores were observed. The chain linear micropores distributed along the clear interface side of grain boundary ferrite with a clear grain boundary on one side. The chain linear micropores distributed in a near the middle of grain boundary ferrite with non-clear interface on both sides. The existence of chain like micropores is due to the precipitation of Nb/Ti microalloying particles，which promoted the formation of grain boundary ferrite chain microporoes. As a result of Nb/Ti microalloying particles precipitated along the grain boundary of the prior austenite，the prior austenite grain boundary was pinned，meanwhile clear interface side of grain boundary ferrite are pinned，and changed the direction of the growth of the grain boundary ferrite. The growth direction of a clear grain boundary ferrite（GF-I type） grows toward austenite on the nucleation side and grows along grain boundaries of the prior austenite. Because of the pinning effect of precipitated particles，the austenite grain boundaries did not move，and the grain boundaries ferrite with non-clear on both sides interface（GF-II type） grew up to austenite on both sides. And micropores became the source of the transverse crack of continuous casting billet. The transverse crack of the continuous casting billet extended along a chain linear micropores.