Influence of Deformation Degree and Cooling Rate on Microstructure and Phase Transformation Temperature of B1500HS Steel

doi:10.1007/s40195-017-0594-3

Abstract

Abstract:

To study the effects of the deformation degree and cooling rate on the microstructure and phase transformation temperature for the B1500HS steel, the samples were heated at 900 °C for 5 min, compressed by 10, 20, 30 and 40% at the strain rate of 0.1 s^-1, and then cooled down at the rates of 50, 40, 25, 20 and 15 °C/s by the thermo-mechanical simulator, respectively. The start and finish temperatures of the phase transformation were determined by the tangent method, and the volume fraction of the phase transformation was ascertained by the level principle according to the dilatometric curves. The volume fraction of the retained austenite was determined by X-ray diffraction. The results show that the volume fraction of the bainite rises with an increase in the deformation degree as the cooling rate is lower than the critical rate. At the same cooling rate, the phase transformation temperature rises with an increase in the deformation degree, and the sizes of both the martensite and bainite phases reduce due to the austenite grain refinement induced by the deformation. The volume fraction of the retained austenite reduces as the deformation degree increases. The critical cooling rate of the un-deformed samples is approximately 25 °C/s and the critical cooling rate rises as the deformation degree increases.

Key words: Deformation, Cooling rate, Microstructure, Phase transformation temperature

Hui-Ping Li, Rui Jiang, Lian-Fang He, Hui Yang, Cheng Wang, Chun-Zhi Zhang. Influence of Deformation Degree and Cooling Rate on Microstructure and Phase Transformation Temperature of B1500HS Steel[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(1): 33-47.

Figures/Tables 19

Fig. 1 Schematic diagram of experiment

Fig. 2 Schematic diagram ascertaining the start and finish temperatures of phase transformation by the tangent method

Fig. 3 Dilatometric curves of B1500HS samples subjected to different deformation degree. a At the cooling rate of 50 °C/s; b at the cooling rate of 40 °C/s; c at the cooling rate of 25 °C/s; d at the cooling rate of 20 °C/s; e at the cooling rate of 15 °C/s

Fig. 4 Dilatometric curves of un-deformed samples at cooling rates of 25, 20 and 15 °C/s

Fig. 5 Transformation temperature of martensite at cooling rates of 50, 40 and 25 °C/s (Ms martensite start temperature, Mf martensite finish temperature): a Ms temperature; b Mf temperature

Fig. 6 Transformation temperatures of martensite and bainite at cooling rates of 20 and 15 °C/s (Ms martensite start temperature, Mfmartensite finish temperature, Bs bainite start temperature)

Fig. 7 Metallographic images taken by the Nikon ME600 metallographic microscope: a un-deformed sample; b sample subjected to 10% deformation degree; c sample subjected to 20% deformation degree; d sample subjected to 30% deformation degree; e sample subjected to 40% deformation degree

Fig. 8 Effects of deformation degree on the grain size of austenite

Fig. 9 Microstructures of B1500HS samples subjected to various deformation degrees at the cooling rate of 50 °C/s: a 0%; b 10%; c 20%; d 30%; e 40%

Fig. 10 Schematic illustration of lath martensite [19] The dilatometric curves of the B1500HS shown in Fig. 3 display the Mf temperature is higher than 200 °C under the cooling rate being 50 °C/s. Microstructure in Fig. 9a shows that the orientation relationship of laths is approximately 60° or 120°. Moreover, the B1500HS steel is a type of a low-carbon alloy steel with a carbon content of approximately 0.23% (<0.6%). Therefore, the microstructure of the B1500HS samples at the cooling rate of 50 °C/s displays the lath martensite structure as shown in Fig. 9a.

Fig. 11 Microstructures of samples at the 25 °C/s cooling rate: a un-deformed sample; b sample subjected to 10% deformation degree; csample subjected to 20% deformation degree; d sample subjected to 30% deformation degree; e sample subjected to 40% deformation degree

Fig. 12 Microstructures of bainitic ferrite (BF) in samples at the cooling rate of 25 °C/s: a sample subjected to 30% deformation degree; bsample subjected to 40% deformation degree (the polished samples were etched with a 4% nitric acid and alcohol solution, and the images were taken using the BSE mode of Nova Nano SEM 450)

Fig. 13 Microstructures of samples at the cooling rate of 20 °C/s: a un-deformed sample; b sample subjected to 10% deformation degree; c sample subjected to 20% deformation degree; d sample subjected to 30% deformation degree; e sample subjected to 40% deformation degree

Fig. 14 Microstructures of samples at the cooling rate of 15 °C/s: a un-deformed sample; b sample subjected to 10% deformation degree; c sample subjected to 20% deformation degree; d sample subjected to 30% deformation degree; e sample subjected to 40% deformation degree

Fig. 15 Determination of the volume fraction of phase transformation by the level principle

Fig. 16 Volume fraction of bainite for the samples at various cooling rates

Fig. 17 Micro-Vickers hardness of samples

Fig. 18 X-ray diffraction powder patterns of samples subjected to the deformation degree of 0, 10, 20, 30 and 40%: a At 50 °C/s cooling rate; b at 40 °C/s cooling rate; c at 25 °C/s cooling rate

Fig. 19 Volume fraction of the retained austenite in B1500HS samples

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