Comparison of Two Different Rolling Processes on Microstructure and Properties of Ferrite-Bainite Dual-Phase Pipeline Steels

Abstract

The microstructure and mechanical properties of ferrite-bainite dual-phase pipeline steels respectively produced by thermomechanical control process (TMCP) and intercritical annealing were studied using optical microscope, scanning electron microscope (SEM), Vickers hardness tester and MTS 810 servo-hydraulic machine. The results show that these two processes obtain different morphology and distribution of microstructure, but the mechanical properties of them are similar and superior, especially the yield ratios are low. It is found that for TMCP steel, the grains of ferrite are large and the average ferrite grain size is approximately 6.1µm. The bainite structures are lump-shaped with concentrated distribution in the ferrite matrix and the band structure is obvious. For steels produced by intercritical annealing process, the grains are finer and the distribution of bainite is uniform on the ferrite grain boundaries. From the rolling plane to the centre, the grain sizes of ferrite in these two microstructures increase obviously, and the volume fraction of bainite first increases significantly, then decreases slightly in the TMCP steel and the average volume fraction is approximately 60.1%. However, the bainite volume fraction in the intercritical annealing steel decreases gradually and the average volume fraction is approximately 35.4%. Moreover, hardness values in through-thickness direction are uniform in the TMCP steel and these values are also similar in the intercritical annealing steel except the values in the region near the rolling plane are much larger.

Effect of Bainite to Ferrite Yield Strength Ratio on the Deformability of Mesostructures for Ferrite/Bainite Dual-Phase Steels

“…Usually, the F/B dual-phaseX80 pipeline steels are produced by an advanced thermo-mechanical controlled process (TMCP) [ 9 , 10 ]. The TMCP parameters, including reheating temperature, beginning rolling temperature, rolling reduction, finishing rolling temperature, beginning quick cooling temperature, cooling rate, and finishing cooling temperature, strongly affect the microstructure of F/B dual-phase X80 pipeline steels, such as the volume fraction, distribution, morphology and mechanical properties of each phase [ 9 , 10 , 11 , 12 , 13 ]. Thus, the F/B steels’ microstructure effects on mechanical properties, strain hardening behavior, microscopic deformation, and failure mechanism have been studied [ 14 , 15 ].…”

“…During the F/B pipeline steel TMCP processes, the ferrite transformation occurs during the cooling process, inthetime between the finishing of rolling and beginning of quick cooling, i.e., relaxation time. Then, the bainite is obtained during the subsequent rapid cooling [ 9 ]. Previous research shows that the finishing rolling temperature and beginning quick cooling temperature, i.e., relaxation time, are very important in determining the volume fractions of ferrite and bainite [ 12 , 13 ].…”

Effect of Bainite Volume Fraction on Deformability of Mesostructures for Ferrite/Bainite Dual‐Phase Steel

“…e distributed bainite in the ferrite matrix for the DP pipeline steel is usually in elongated form. Meanwhile, the amount, size, distribution, and hardness of the bainite strongly depend on the parameters of TMCP including rolling temperature, finishing rolling temperature, beginning cooling temperature, cooling rate, and finishing cooling temperature [26][27][28]. In this case, the volume fraction of bainite with elongated form will become an important influencing parameter of the deformation behavior and mechanical properties.…”

Effect of initial microstructures on the properties of Ferrite-Martensite Dual-Phase pipeline steels with Strain-Based design

“…Therefore, steel makers turn their attention to the strain-based design pipeline steels to obtain an optimal combination of high strength, high toughness and good deformability. Due to the characteristic properties of the dual-phase steels such as better deformability than other HSLA steels of similar strengths and the successful using in the automobile industries, this type of dual-phase steel inspires steel makers and provides a good candidate for the strain-based design pipeline steels 15,16 . In the past, some researches discussed about the effect of many factors on the properties of ferrite-martensite dual-phase (FMDP) steels, such as martensite morphology 17,18 , martensite amounts 19 , the tempering and carbon content of ferrite and martensite 20,21 .…”