Microstructural Aspects of Bainitic and Bainite-like Ferritic Structures of Continuously Cooled Low Carbon (<0.1%) HSLA Steels

Observation of .GAMMA..RAR..ALPHA. Transformation in Ultralow-carbon Steel under a High Temperature Optical Microscope.

“…Such growth behavior was more marked at lower cooling rates. Goodenow et al, 11) Wilson, 1) Maki 2) and Araki et al 3) reported on the growth behavior of a, but did not mention the above growth behavior. Figure 12 schematically shows two kinds of growth behavior of a in g→a transformation of (a) low-carbon and (b) ultralow-carbon steels.…”

“…Shibata and Asakura 4) observed g→a transformation in an ultralow-carbon steel in detail and found that the grain diameter of a did not decrease much even when the cooling rate was increased. Together with polygonal ferrite (a p ), 3,5) of which the grain boundaries are planar, and quasi-polygonal ferrite (a q ), 3,5) of which grain boundaries are curved or ragged, a variety of intermediate microstructures and martensite were observed and were found to be dependent of the cooling rate. In order to examine the relationship between the g grain boundary and the a grain of an ultralowcarbon steel, the present authors 4,6,7) performed several experiments.…”

“…Compared to the micrograph (a), it seems unreasonable to call microstructures in (c) and a q in the present research massive structures or massive ferrite, although microstructures in (c) and a q may transform from g by massive transformation. Araki and coworkers 3,5) have proposed calling ferrite surrounded by curved or ragged boundaries a q , quasi-polygonal ferrite, irrespective of the transformation mechanism. Following their nomenclature, all of the microstructures, revealed in Fig.…”

“…Moreover, relationship between transformation behavior and the microstructures, and the mechanism of the transformation remain unclear. 1,2) In addition, the terminology of the microstructures 2,3) is confusing. Shibata and Asakura 4) observed g→a transformation in an ultralow-carbon steel in detail and found that the grain diameter of a did not decrease much even when the cooling rate was increased.…”

A Bainite-Ferrite Multi-Phase Steel Strengthened by Ti-Microalloying

“…It emerged as required by society and the progress of metallurgical technologies over these 30 years and can fulfill multi-purpose application. [1][2][3][4][5][6] At present, most of pretty fine, high dislocation density bainite matrix structure is obtained by adding less carbon (carbon content 0:06 mass%) and alloying elements such as Ti, V, Nb, Cu, Ni, Mo, etc., and using secondary refining, TMCP, tempering technologies in manufacturing processes. Experts on steel materials developed low alloy steels of 600-800 MPa grades by means of different strengthening methods, however, relatively high amount of different alloy elements were used, which make the production cost high and requires more strict production conditions.…”