3.2 Microstructure investigation
The microstructure of the experimental steel is shown in Fig.3(a, b). Due to a large number of substructures and dislocations generated in the deformed prior austenite grains during controlling and rolling process, fine final microstructure was promoted effectively. In addition, recrystallization of deformed austenite and the precipitates of Nb, Ti, V micro-alloying elements pining grain boundaries also promote the fine grains. According to Fig.3(a, b), X90 pipeline steel is a multiphase structure composed of quasi-polygonal ferrite (QPF), acicular ferrite (AF), lath bainite (LB) and a small amount of M/A constituent which is a mixture of martensite and retained austenite. Ferrite and bainite distributed evenly, M/A constituent dispersed on the grain boundaries and matrix in the form of granular or elongated, which is bright white under SEM, as shown in Fig.3 (b) by the arrows. The morphology and nucleation mechanism of QPF is different from that of AF. The coarse QPF is referred to as ‘proeutectoid ferrite’ formed at a lower temperature, and the latter is closed to Widmanstätten ferrite [24]. AF is slender and clutter than QPF, as shown in Fig. 3(a). M/A constituents in LB are in the form of long or short rods, and distributed in parallel. LB is the primary microstructure increasing the strength of X90 pipeline steel, and the consequent of LB enhancement is deeply depend on the total amount of M/A constituents. The strength increased with the number of M/A constituents increasing. The impact toughness is excellent when M/A constituent is finer and dispersed evenly [25]. In the TMCP process, substructures, dislocations and the increasing grain boundaries in the prior refinement austenite grains that could be used as nucleation sites of LB. As a consequence, the final microstructure composed of a number of fine LB. In general, M/A constituent as the brittle phase is possibly be the cracks initiation. The shape, size and distribution affecting the toughness effectively. Chunming Wang [25] proved that the yield strength of the material increasing with the increase of M/A constituents, but destroyed the continuity of the matrix. it will reduce the low temperature toughness when the M/A constituent is large in size and with angular.
After hot induction bending, the X90 bend has a relatively large size grain compared with the parent pipe. The same heat treatment process on the straight part and the bending zone, quenching and high temperature tempering. However, the outer arc side and inner arc side was subjected the maximum tensile and pressure tress according to the bending force during forming process. According to Fig. 3, the microstructure of the straight part is composed of polygonal ferrite (PF) and granular bainite (GB), as shown in Fig. 3(c, d). M/A constituents in GB are spherically distributed on the matrix or between the laths. The neutral axis has the same microstructure with the straight part because of only a little deformation in the positions, as shown in Fig. 3 (i, j). It can be found that there are some black clusters at the grain boundaries, which are the decomposition products of the retained austenite during the tempering process. There are many discontinuous carbides between bainite laths. In addition, some black carbide particles are found on the bainite and ferrite matrix. The microstructure of other positions in the bend zone is quite different, especially in the inner arc side and outer arc side. The bend zone was subjected the double effecting of deformation and heat treatment, the microstructure is consisted of PF and a large number of LB in the outer arc side. The microstructure is consisted of GB, LB and PF in the inner arc side, hardly observe AF in the bend zine. The prior austenite grain boundaries are clearly visible in the inner arc side and outer arc side, some point-like M/A constituents are distributed on the grain boundaries, as shown in Fig. 3(c-f). When the X90 parent pipe was reheated to austenitizing tempertaure, the fine PF was produced after quenching because of dynamic strain induced transformation (DSIT). When the temperature reached to the martensite start transformation temperature, a small portion of retained austenite transformed into martensite, forming M/A constituents [1]. The bainite laths are almost parallel, long and thin, which help improving toughness while increasing strength. Some of the carbides precipitation and the bainite grain boundaries became blurred after high temperature tempering.