Research of Strain Aging in Pipeline Steel with a Ferrite/Martensite Dual‐Phase Microstructure

Abstract

By applying ferrite-martensite microstructural control, high-deformability linepipes of grade X70 have been developed. The steel with an acicular ferrite microstructure was intercritically heat-treated at 8208C for 10 min, followed by rapidly water cooling, in order to get a ferrite/martensite dual-phase microstructure. After aging at 200-2508C for 5-15 min, the microstructure and mechanical properties of the ferrite-martensite microstructural steel were studied. The pipe with a ferrite/martensite dual-phase microstructure exhibited a superior strain aging resistance aging at 200-2508C for 5-15 min. The yield ratios were below 0.77 and the pipe still exhibited continuous yielding behavior, meanwhile tensile strength, uniform elongation and impact toughness did not experience significant change. By intercritical annealing, carbides and carbonitride containing Nb and Ti precipitated out of the ferrite phase further, at the same time, solute carbon atoms could adequately diffused from the ferrite to the uniformly distributed austenite derived from the acicular ferrite initial microstructures, leading to the reduction of the amount of free carbon and nitrogen to pin the mobile dislocations in the ferrite phase, therefore, strain aging effects were not pronounced in the temperature and time range investigated.

Refinement of Banded Structure via Thermal Cycling and Its Effects on Mechanical Properties of Dual Phase Steel

“…In fact, the volume change during quenching from the intercritical annealing range induces plastic deformation of adjacent ferrite grains, and therefore, creates a high density of unpinned dislocations in the vicinity of martensite. These martensite‐induced dislocations move at low stresses, creating low yield strengths, and interact to produce high rates of strain hardening . Among DP steels, DP3 shows higher initial n ‐values compared with DP1 while DP3* is even better than DP3.…”

Effect of Intercritical Annealing on Mechanical Properties and Work‐Hardening Response of High Formability Dual Phase Steel

“…In this regard, Cai et al, Das et al, Ahmad et al, Kalhor and Mirzadeh, Seyedrezai et al, and Schemmann et al showed that changing the initial microstructure can significantly affect the attributes of the DP microstructure. Moreover, the effect of tempering has also been noted . On another front, Calcagnotto et al and Papa Rao et al focused on warm deformation and intercritical annealing, where the importance of bimodal ferrite grain structure was also discussed .…”

“…Moreover, the effect of tempering has also been noted. [17][18][19] On another front, Calcagnotto et al [20] and Papa Rao et al [21] focused on warm deformation and intercritical annealing, where the importance of bimodal ferrite grain structure was also discussed. [11] Azizi-Alizamini et al [22] studied the effects of cold rolling of tempered martensite, subsequent tempering step, and heating rate to intercritical annealing temperature.…”

Effect of microstructure on the mechanical properties and corrosion resistance of a welded joint of 620-grade marine steel

“…When the WM mid containing WM 3 and WM 4 was subjected to the thermal effect of subsequent welding passes, the further diffusion of alloying elements of C, Mn, and Ni from ferrite into austenite occurred, resulting in the dense distribution of MA constituents with a slabby and blocky shape, as shown in Figures 4G-L. There were plenty of mobile dislocations in the ferrite (inset of Figure 9A), which were induced by the volume expansion from austenite to martensite transformation during cooling, leading to continuous yielding behavior or only an inflection point in the tensile curves (Figure 2C) (Zuo and Li, 2015). Table 2 shows that the dendrite spacing of the WM mid was 89.63 μm, which was much smaller than those of the WM in and WM out (142.34 μm and 116.51 μm, respectively).…”