The welding continuous cooling transformation (CCT) diagram concerning microstructure evolution in a fine zone of the weld joint provides essential data for microstructure control in practice. In this study, a welding CCT diagram was improved to be elaborate by using the expanded lever rule. Processes for obtaining pieces of phase information are explained in detail. Improvements are: (i) phase areas are clearly presented, (ii) phase volume percentages are given, and (iii) overlapped phase areas are revealed. To meet high-strength requirement, a heat input of 21 kJ cm−1 was referenced from the welding CCT diagram, for achieving acicular ferrite and lath-like martensite. Transformed phase volume fractions are estimated and high strength and good toughness are obtained due to the elaborate phase information.
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