Acta Metall Sin  2019, Vol. 55 Issue (12): 1569-1580    DOI: 10.11900/0412.1961.2019.00082
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Modelling of Q&P Steel Heat Treatment Process Based on Finite Element Method
ZHANG Qingdong,LIN Xiao,LIU Jiyang(),HU Shushan
School of Mechanical and Engineering, University of Science and Technology Beijing, Beijing 100083, China
Abstract

Quenching and partitioning (Q&P) steel is a kind of high strength and toughness steels which has a majority of martensite at room temperature and a certain amount of retained austenite through the quenching and carbon distribution heat treatment process of cold-rolled carbon-silicon-manganese steel. In this work, the typical Q&P high-strength steel, QP980 steel, is taken as an example to carry out the physical simulation study of the whole process of heat treatment. A creep-like strain equation coupled with temperature and time is proposed to describe the volume change of materials during Q&P heat treatment. The phase transformation kinetics equation, phase transformation strain and phase transformation plasticity equation of Q&P heat treatment with the influence of quenching temperature were established, and the thermal expansion coefficient of each phase of Q&P steel was obtained. According to the coupling principles of temperature, microstructure and stress-strain field, a numerical simulation model for the whole process of Q&P heat treatment was developed. In this model, the physical simulation of QP980 steel thermal-elastoplastic incremental constitutive equations are implemented to commercial finite element software ABAQUS as the user subroutines. The model was validated by Q&P heat treatment experiment on Gleeble thermal-mechanical simulator. The calculated values of the models are both in good agreement with the experimental values.

 ZTFLH: TG156.34
Fund: National Science and Technology Support Project(No.2011BAE13B05);National Natural Science Foundation of China(No.51075031)
Corresponding Authors:  Jiyang LIU     E-mail:  b20170270@xs.ustb.edu.cn
 Fig.1  Experimental scheme (a) and continuous cooling transformation (CCT) curves (b) of continuous cooling of quenching and partitioning (Q&P) QP980 steel (A—austenite, B—bainite, F—ferrite, M—martensite) Fig.2  Normal temperature structures after quenching at the cooling rates of 1 ℃/s (a), 2 ℃/s (b), 5 ℃/s (c), 10 ℃/s (d), 30 ℃/s (e) and 50 ℃/s (f) Fig.3  Schemes of thermal expansion experiment (a) and phase change kinetics experiment (b) (Tq—quenching temperature) Fig.4  Experimental scheme of phase change plasticity Fig.5  Experimental heating and cooling system of Q&P heat treatment with different distribution temperatures Fig.6  Relationships between axial strain and temperature during stress-free quenching and tempering (Ms—martensite transformation start temperature)(a) the first quenching process (b) the second quenching process Fig.7  The relationship between martensite volume fraction ($ξM1$) and quenching temperature in the first quenching process Fig.8  Microstructure changes before and after the second quenching process with different quenching temperatures Fig.9  Phase change kinetics curves of the second quenching process with different quenching temperature Q&P heat treatment Fig.10  Wide-direction strain of martensite transformation process under external loading Fig.11  The measured value of phase change plasticity coefficient (k) and linear fitting (σ—stress in the same direction as phase transformation plastic strain) Fig.12  Axial strain during Q&P heat treatment with different distribution temperatures Table 1  The results of cubic polynomial fitting of reduced temperature coefficient Table 2  Creep strain equation coefficients of QP980 high strength steel Fig.13  Comparison of the calculated and measured values of creep-like strain Fig.14  Loading system of heating and cooling for Q&P heat treatment experiments on gleeble Fig.15  Model validation finite element model of heat distribution of test samples Fig.16  Comparisons between simulated and measured deformations of samples in different stages of Q&P heat treatment process(a) first quenching process (b) partitioned heating process(c) partitioned holding process (d) second quenching process Fig.17  Variations of phase volume fraction with time in Q&P heat treatment stages(a) first quenching process (b) distribution process (c) second quenching process