Abstract
In this study the strain capacity and work-hardening behavior of bainite (B), bainite + polygonal ferrite (B + PF), and bainite + polygonal ferrite + pearlite (B + PF + P) microstructures are compared. The work hardening exponent (n), instantaneous work hardening value (ni), and differential Crussard-Jaoul (DC-J) analysis were used to analyze the deformation behavior. The best comprehensive mechanical properties were obtained by the introduction of the pearlite phase in B + PF dualphase with the tensile strength of 586 MPa and total elongation of 31.0%. The additional pearlite phase adjusted the strain distribution, which increased the initial work hardening exponent and then maintained the entire plastic deformation at a high level, thus delayed necking. The introduction of pearlite reduced the risk of micro-void initiation combined with the high frequency of high angle grain boundaries (HAGBs) in triple-phase steel, which led to a low crack propagation rate.
Highlights
Pipeline transport is considered the most economic, safe, uninterrupted, and largescale method to transport oil and gas, geologically unstable regions become common in oil and gas transportation [1]
When the initial cooling temperature decreased to 790 ◦ C, a dual-phase microstructure was obtained (Figure 2b,e)
The kernel average misorientation (KAM) maps (Figure 3c,f,i) show that the strain is mainly distributed in the bainite and pearlite phase with large lattice distortion; the average KAM value decreased with the introduction of polygonal ferrite and pearlite phases
Summary
The equilibrium transformation temperatures Ar1 and Ar3 were determined using a DIL805A/D as 660 ◦ C and 790 ◦ C, respectively; the parameters of TMCP (Figure 1b) are formulated using deformation continuous cooling transformations (DCCT) diagram (Figure 1a). Rough rolling was performed within the austenite recrystallization region to reduce the thickness from 80 to 35 mm. The finishing rolling temperature ended at 820 ◦ C, and the thickness of the plates was ~15 mm. Steel with a bainite microstructure was obtained by direct super-fast cooling after rolling. B + PF and B + PF + P steels were air-cooled to temperatures below. The relaxation process in the ferrite-austenite dual-phase region can induce a certain amount of polygonal ferrite formation; the cooling rate was controlled by a super-fast cooling process. Coil cooling for all steel blocks was simulated by air cooling to room temperature.
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