Effect of intercritical temperature on quenching and partitioning steels originated from martensitic pre-microstructure
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Dong, Junye Cheng, and Xiang Meng Engineering Research Institute, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China (Received 19 April 2014; accepted 4 September 2014)
The effect of the intercritical temperature on the microstructure and mechanical properties of a newly developed quenching and partitioning steel using martensitic microstructure prior to the heat treatment process was studied. Such a quenching and partitioning process possessed a unique microstructure evolution, especially during intercritical annealing after prequenching. Excellent mechanical properties were obtained due to this unique multiphase microstructure. Significant amount of interlath-retained austenite was acquired and the relationship between the microstructure and work-hardening behaviors was proposed. The martensite/austenite islands increased at elevated annealing temperature, which deteriorated the total elongation and increased the tensile strength as hard constituents when it was excessive. The result indicated that the present full martensitic microstructure before the intercritical annealing is probably more suitable to an industrial application and is a better way to produce high strength steels with suitable ductility.
I. INTRODUCTION
In the past few years, the automobile industry has an increasing demand to the development of new advanced high strength steels (AHSS) to manufacture weightreduced, environment friendly and fuel efficient vehicles. Multiphase steels were put forward to be ideal solutions that cater to these demands, such as dual-phase (DP)1 and transformation-induced plasticity (TRIP) steels.2 A new heat treatment process known as “quenching and partitioning” (Q&P) has been shown to be a superb processing option for the production of high strength steels with significant amounts of retained austenite,3–5 and has been assessed as a potential solution to provide excellent strength/ductility combinations in high strength steels. Although autobody application represented the first implementation of Q&P on an industrial scale, this heat treatment is also applicable to a range of other potential applications.6 In general, a Q&P process includes quenching from full austenization or intercritical region and partitioning at the quenching temperature between the martensite start temperature (Ms) and the martensite finish temperature
(Mf) or above Ms temperature.3–7 Generally, a ferrite plus pearlite initial microstructure is used prior to the Q&P process. However, mechanical properties after the Q&P process significantly depend on the original microstructure prior to the Q&P process regardless of the heat treatment conditions. Therefore, the resulting microstructure and mechanical properties after using different original microstructures prior to the Q&P process are very appealing. On the research by Santofimia et al.,8 on increasing the Mn content to 3.5 wt%, the initial microstructures after hot rolling and air cooling consisted of martensite and polygonal ferrite. The application of Q&P
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