Effect of Annealing Time on Microstructural Evolution and Deformation Characteristics in 10Mn1.5Al TRIP Steel
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INTRODUCTION
THE third generation advanced high-strength steel (AHSS) has drawn considerable interest in the automobile industry, due to its attractive combination of strength and ductility. Compared with the 1st generation, the 3rd generation automotive steels have more austenite with moderate stability, and therefore the effect of transformation-induced plasticity (TRIP) can be fully utilized.[1,2] Because of the formation of ultrafine-grained ferrite and austenite with controllable TRIP effects, the medium Mn (4 to 8 pct) steels processed by intercritical annealing are distinguished by their outstanding mechanical properties, and thus considered as potential candidates for the 3rd generation automotive steels.[3–5] The strength–ductility combination (tensile strength 9 elongation) of the medium Mn steels usually varies from 20 to 35 GPa pct[6–8] and could be further improved by enhancing the TRIP effect. It was found that a strength– ductility balance of >40 GPa pct could be achieved using>9 pct Mn (intermediate Mn steels) because of the increased fraction and stability of the austenite.[9] In this case, the intercritical annealing time does not only influence the Mn content in the austenite,[10] which is already very high before diffusion, but also considerably affects the microstructural characteristics.[2] Thus, the interaction of grain size distribution, Mn/Al QIHANG HAN and YULONG ZHANG, Research Fellows, and LI WANG, Chief Researcher, are with the Research Institute, Baoshan Iron & Steel Co., Ltd., Shanghai 201900, P.R. China, and also with State Key Laboratory of Development and Application Technology of Automotive Steels (BaoSteel), Shanghai 201900, P.R. China. Contact e-mail: [email protected] Manuscript submitted August 18, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
concentration, and slip systems activation has been studied to fully understand the excellent mechanical balance of intermediate Mn steel.
II.
EXPERIMENTAL METHODS
The composition of the steel investigated was 0.14C-10.2Mn-1.5Al (wt pct). Thirty-six percent coldrolled steel sheets were intercritically annealed at 913 K (640 C) for 3, 5, 8 minutes, 1, 6, 12, 24, and 48 hours, respectively. Tensile specimens were prepared from these materials with a gage length of 50 mm, gage width of 25 mm, and a thickness of 1.4 mm. The tensile tests were carried out on an Instron tensile frame at an initial strain rate of 3 mm/min until failure. The strain was measured with a non-contact video extensometer. Specimens were polished with diamond paste and then with colloidal silica for 1 hour. The colloidal silica would slightly etch the sample surface and therefore remove the deformation layer. These specimens were used for EBSD/EDS, XRD, and EPMA analysis. The EBSD combined with EDS mapping was carried out by the ZEISS ULTRA 55 (EBSD, NORDLYS) with a scanning step size of 30 nm. Data acquisition was performed using AZtec-EBSD. The mean Mn and Al contents of ferrite and austenite were measured via the EDS–EBSD maps. The EPMA test (JXA-8230) was c
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