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Based on the strain-based concept of linepipe products, materials with high deformability are desirable and a dual-phase microstructure consisting of harder and softer phases is essential to obtain higher deformability. Martensite-austenite constituents (MA) are very important in the hard phase for achieving good mechanical properties such as deformability and toughness. For understanding the formation process and its effect on mechanical properties, microstructural analysis of the MA formed through two different heat treatments (on-line heat process and off-line heat process) is conducted.Based on SEM and TEM observation, it is found that the MA consisted mainly of martensite and its distributions and shapes are different between on-line and off-line heat process samples. Because of the different formation processes, grain boundary character is also different. The interface of MA and matrix in the on-line heat process sample is divided by short length Kurdjumov-Sacks (K-S) orientation grain boundaries. On the other hand, long length random grain boundaries are formed at the interface of the off-line heat process material. This characteristic morphology results in the difference in the toughness of the materials by causing a different strain accumulation and crack propagations mechanism at the vicinity of MA.KEY WORDS: martensite-austenite constituent; high deformable steel; on-line heat process; grain boundary character.

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The history of construction of thick plate mills in Japan and trends in the development of rolling technology (gauge control, plan view control, and crown control) during the 100 year history of plate technology in this country are discussed in outline, and the Thermo Mechanical Control Process (TMCP) is reviewed.In 1901, the blast furnace at the state-owned Yawata Steel Works was blown-in and the medium gauge plate mill was started up, followed by startup of a 3-high rolling mill in 1905. Today, Japan has an annual thick plate production capacity of more than 10 million tons. Various important technical advances have also been achieved over the years, including gauge control, plan view pattern control, and crown control. The Thermo Mechanical Control Process was applied to controlled rolling for the first time in the 1960s, and accelerated cooling was applied in the 1980s. These technologies first reached full maturity in Japan and are now global technologies. The heat-treatment on-line process was also developed and continues to be a world-leading technology.