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The effect of extended intercritical annealing time on the changes of the microstructure and tensile properties is studied in a medium Mn steel. Nanosized lamellar structures are formed after intercritical annealing at 550 °C for 8 and 16 h; they consist of mainly tempered‐α′ martensite with retained austenite. Many cementite precipitates are observed in the tempered‐α′ martensite matrix and at the high‐angle boundaries. The orientation relationship between θ‐cementite and tempered‐α′ martensite is identified to be ()θ//()tempered‐α′, [011]θ//[011]tempered‐α′, and []θ//[]tempered‐α′. Fresh α′ martensite is detected between the tempered‐α′ martensite and austenite lamellae in the quenched specimens. The fresh α′ martensite is formed due to partial martensitic transformation of austenite upon cooling. The fractions of cementite and fresh α′ martensite increase with extended annealing time, leading to reduced austenite fraction after quenching. Additional baking treatment at 200 °C increases yield stress and elongation due to the stabilization of the austenite. This study provides a detailed understanding of these complex nanosized microstructures and discusses the key factors governing their yield stresses and work hardening behaviors.

An electron probe X-ray microanalyzer (EPMA) is an essential tool for studying chemical composition distribution in the microstructure. Quantifying chemical composition using standard specimens is commonly used to determine the composition of individual phases. However, the local difference in chemical composition in the standard specimens brings the deviation of the quantified composition from the actual one. This study introduces how to overcome the error of quantification in EPMA in the practical aspect. The obtained results are applied to evaluate the chemical composition of retained austenite in multi-phase steel. Film-type austenite shows higher carbon content than blocky-type one. The measured carbon contents of the retained austenite show good coherency with the calculated value from the X-ray diffraction.

An electron probe X-ray microanalyzer (EPMA) is an essential tool for studying chemical composition distribution in the microstructure. Quantifying chemical composition using standard specimens is commonly used to determine the composition of individual phases. However, the local difference in chemical composition in the standard specimens brings the deviation of the quantified composition from the actual one. This study introduces how to overcome the error of quantification in EPMA in the practical aspect. The obtained results are applied to evaluate the chemical composition of retained austenite in multi-phase steel. Film-type austenite shows higher carbon content than blocky-type one. The measured carbon contents of the retained austenite show good coherency with the calculated value from the X-ray diffraction.