Effect of Sub-rapid Solidification and Secondary Cooling on Microstructure and Properties of Strip Cast Low-Carbon Bainitic–Martensitic Steel

The low-carbon bainitic–martensitic steel added with microalloying elements was designed, and samples with different cooling rates were produced through ingot casting and simulated strip casting. The ingot cast presented a multiphase microstructure constituted of allotriomorphic ferrite, polygonal ferrite, acicular ferrite and pearlite. The air-cooled strip consisted of polygonal ferrite, acicular ferrite and bainite ferrite, and the gas-cooled strip was composed of acicular ferrite, bainite ferrite and martensite. The results indicated that the duplex sulfide-oxide particles were firstly formed during solidification and refined by the high solidification rate during strip casting; the embedded structure of copper sulfide in duplex particles suggested that the copper sulfide precipitated from the liquid oxide. Independent copper sulfides with an average diameter of ~ 80 nm were formed in the ingot cast and a large number of nanoscale carbides or nitrides less than 25 nm was also observed. However, independent copper sulfides and nanoscale precipitates were hardly observed in the gas-cooled strip cast. For the air-cooled strip cast, only a small number of the nanoscale precipitates were observed. Vickers hardness, yield strength (YS) and ultimate tensile strength (UTS) increased with the fraction of bainite ferrite and martensite, in which the gas-cooled strip showed the highest YS and UTS, and the ingot cast presented the highest total elongation (TE). Nevertheless, the gas-cooled strip showed the best toughness due to the highest product of UTS and TE.