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.

ZHANG Qingdong, LIN Xiao, LIU Jiyang, HU Shushan. Modelling of Q&P Steel Heat Treatment Process Based on Finite Element Method. Acta Metall Sin, 2019, 55(12): 1569-1580.

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) (T_{q}—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 (M_{s}—martensite transformation start temperature)(a) the first quenching process (b) the second quenching process

Fig.7 The relationship between martensite volume fraction (${\xi}_{\mathrm{M}\mathrm{1}}$) 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

Time / s

Coefficient of primary temperature term

Coefficient of quadratic temperature term

Coefficient of cubic temperature term

R^{2}

5

2.98×10^{-4}

-6.26×10^{-4}

4.64×10^{-4}

0.9220

25

4.20×10^{-4}

-5.74×10^{-4}

4.73×10^{-4}

0.9622

45

3.56×10^{-4}

-1.87×10^{-4}

2.06×10^{-4}

0.9663

65

3.23×10^{-4}

-7.66×10^{-4}

9.35×10^{-5}

0.9900

85

1.85×10^{-4}

4.02×10^{-4}

-1.69×10^{-4}

0.9953

105

1.65×10^{-4}

5.59×10^{-4}

-3.01×10^{-4}

0.9964

125

6.53×10^{-4}

8.23×10^{-4}

-4.44×10^{-4}

0.9984

145

-4.74×10^{-4}

1.07×10^{-3}

-6.29×10^{-4}

0.9915

165

3.53×10^{-5}

9.34×10^{-4}

-5.21×10^{-4}

0.98288

185

-1.16×10^{-4}

1.39×10^{-3}

-8.35×10^{-4}

0.9769

195

-8.92×10^{-5}

1.28×10^{-3}

-7.49×10^{-4}

0.96565

Table 1 The results of cubic polynomial fitting of reduced temperature coefficient

Coefficient of equation

Coefficient value

Coefficient of equation

Coefficient value

Coefficient of equation

Coefficient value

p_{1}

2.65×10^{-5}

q_{1}

-4.16×10^{-5}

w_{1}

3.26×10^{-5}

p_{2}

-5.20×10^{-7}

q_{2}

1.03×10^{-6}

w_{2}

-7.69×10^{-7}

p_{3}

3.44×10^{-9}

q_{3}

-7.17×10^{-9}

w_{3}

5.31×10^{-9}

p_{4}

-7.65×10^{-12}

q_{4}

1.62×10^{-11}

w_{4}

-1.20×10^{-11}

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

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