In Situ Observation of Phase Transformation in Low-Carbon, Boron-Treated Steels
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NTRODUCTION
IT is widely accepted that a chaotic arrangement of laths and a fine-grained interlocking microstructure of intragranular acicular ferrite (IAF) toward optimizing strength and toughness in both weld metals and heataffected zones (HAZs).[1,2] In the HAZs of high heatinput welds, the deterioration in toughness caused by providing preferential sites for crack nucleation and crack propagation paths becomes more significant because of the occurrence of grain coarsening.[3] This is mainly because in a coarse-grain heat-affected zone (CGHAZ), lower transformation products, such as Widmanstatten ferrite (WF), grain boundary allotriomorphic ferrite (GBAF), and intragranular bainitic ferrite (IBF), are likely to occur along the prior austenite grain boundary (AGB) and in the prior austenite grain. The better control of microstructural transition from WF, GBAF, and IBF to fine IAF in the prior austenite grain is a key technology for refining grains and, thus, improving the properties of a CGHAZ. This transition can be realized by eliminating the AGB areas as potential nucleation sites for WF and GBAF and by supplying potent inclusions for the formation of IAF, such as titanium-rich cores (TiO and Ti2O3), aluminarich cores (Al2O3), and crystalline galaxite spinel (MnOÆAl2O3).[4–10]
DI ZHANG, Research Fellow, and YU-ICHI KOMIZO, Professor, are with the Joining and Welding Research Institute, Osaka University, Ibaraki, Osaka 567-0047, Japan. Contact e-mail: zhangdi1211@ hotmail.com YOSHIAKI SHINTAKU, Research Engineer, is with the Plate & Bar/Wire Rod Research & Development Department, Sumitomo Metal Industries Ltd., Amagasaki, Hyogo 660-0891, Japan. SHUICHI SUZUKI, General Manager, is with Steel Sheet, Plate Titanium & Structural Steel Company, Sumitomo Metal Industries Ltd., Chuo-ku Tokyo 104-6111, Japan. Manuscript submitted April 18, 2011. Article published online September 27, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
The addition of a small amount of boron to steels improves their strength and toughness dramatically in the CGHAZs.[11–14] An effective approach for improving the strength and toughness of steels is to suppress the formation of WF and GBAF by the segregation of excess boron atoms at the AGB.[15] However, the best approach for improving strength and toughness lies in improving the nucleation potency of inclusions in the form of boron nitride (BN) and Fe-borocarbide (Fe23(C,B)6 and Fe2B) for c-a transformation in the prior austenite grain and in the acceleration of the completion of the transformation. To increase the volume fraction of the IAF content, we need a better control of the boron content in steels. Furthermore, variation of various factors such as microstructure, carbon and alloy composition, austenite grain size, and heat treatment must be taken into consideration. It is well known that microstructural evolutions are related to the size, spatial distribution, and formation of inclusions during heat treatment.[5] High-temperature laser scanning confocal microscopy (LSCM) has been used wide
