Effect of Intercritical Annealing Time on the Microstructures and Tensile Properties of a High Strength TRIP Steel

doi:10.1007/s40195-014-0061-3

Abstract

Abstract:

A new Mn–Si–Al–Mo–Nb transformation-induced plasticity steel was annealed by intercritical annealing for different durations to investigate the partitioning of C element and the volume fraction change of the microstructural constituents. Direct experimental evidence confirms the partitioning of C elements in different phases during heat treatment by Electron probe microanalysis and X ray diffraction. The distribution of the precipitates was investigated as well. It was revealed that the microstructures and mechanical properties of the investigated steels were affected by the intercritical annealing time. According to the present experiment, the volume fraction of retained austenite and the product of tensile strength and total elongation of investigated steel decrease with increasing intercritical annealing time. It was observed that high tensile strength of 1,103 MPa, total elongation of 21.3%, and strength-ductility product of 23,493.9 MPa % could be successfully produced in this experimental steel at intercritical annealing temperature of 830 °C, holding for 1 min, and isothermal bainite treatment of 440 °C for 5 min holding time.

Key words: TRIP steel, Intercritical annealing, Carbon partitioning, Retained austenite, Stability

Chao Wang, Hua Ding, Jun Zhang, Hongyan Wu. Effect of Intercritical Annealing Time on the Microstructures and Tensile Properties of a High Strength TRIP Steel[J]. Acta Metallurgica Sinica (English Letters), 2014, 27(3): 457-463.

Figures/Tables 7

Fig. 1 SEM images of the cold-rolled sample and the heat-treated sample with different annealing times (F ferrite, B bainite, P pearlite, RA retained austenite, M/A M/A island): a cold-rolled; b 830 °C/1 min + 440 °C/5 min; c 830 °C/3 min + 440 °C/5 min; d 830 °C/15 min + 440 °C/5 min; e 830 °C/30 min + 440 °C/5 min

Fig. 2 XRD patterns of the samples after intercritically annealed at 830 °C for different holding times and then isothermally heat treated at 440 °C for 5 min prior to tensile deformation

Table 1 Volume fraction of RA and the average carbon content in RA for samples after annealing at 830 °C for different times and isothermally heat treated at 440 °C for 5 min

Annealing time (min)	Fraction of RA (vol%)	Carbon content in RA (wt%)
1	13.8	1.21
3	10.3	1.01
15	8.5	0.89
30	<5.0	0.63

Fig. 3 EPMA images of cold-rolled sample and heat-treated sample with different annealing times (F ferrite, B bainite, P pearlite, RA retained austenite, M/A M/A island): a cold-rolled sample, b 830 °C/1 min + 440 °C/5 min, c 830 °C/30 min + 440 °C/5 min

Fig. 4 EDX analysis results of precipitates in the heat-treated sample with different annealing time: a 830 °C/1 min + 440 °C/5 min; b 830 °C/30 min + 440 °C/5 min

Fig. 5 a Engineering stress–strain curves of the samples after intercritically annealed at 830 °C for different holding times and then isothermally heat treated at 440 °C for 5 min; b the instantaneous strain hardening exponent exponent (n) versus true strain

Table 2 Mechanical properties of the samples after annealing at 830 °C for different times and isothermally heat treated at 440 °C for 5 min

Annealing time (min)	YS (MPa)	TS (MPa)	TEL (%)	TS × TEL (MPa %)
1	591	1,103	21.3	23,493.9
3	548	1,034	19.9	20,576.6
15	530	965	19.1	18,431.5
30	494	871	18.5	16,113.5

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