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Hydrogen-induced mechanical degradation was evaluated in different environmental conditions on two X70 pipeline steels having the same composition but different thermomechanical processing parameters, and a further correlation between microstructural features and hydrogen embrittlement (HE) susceptibility was established. HE susceptibility was determined by performing the Slow Strain Rate Tensile Test (SSRT) in air, ex-situ hydrogen-charged, in corrosive media, in-situ hydrogen-charged and Charpy impact tests before and after hydrogen charging in a wide range of temperatures. The SSRT results indicated that the synergistic action of stress and hydrogen in in-situ charged sample leads to maximum HE susceptibility in both X70 steels. Charpy results demonstrated that ductile-to-brittle transition temperature (DBTT) increases after hydrogen charging. The texture analysis revealed that X70-2 with a higher volume fraction ratio of Ƴ-fiber/{100}<011> demonstrated improved HE resistance, whereas X70-1 with higher fractions of {113}<110> and {100} || ND demonstrated a higher DBTT than before and after hydrogen charged X70-2. • Hydrogen embrittlement susceptibility was studied in various testing environments. • Synergy of stress and H enhances hydrogen embrittlement susceptibility significantly. • Ductile-to-brittle transition temperature increases after hydrogen charging. • Lower fraction of Ƴ fiber/{100}<011> enhanced sensitivity to hydrogen embrittlement. • Failure behavior depends on the H environmental conditions. [ABSTRACT FROM AUTHOR]

In this article, thermomechanical controlled processing (TMCP) plus tempering (T) is adopted to acquire a novel 690 MPa‐grade high‐strength low‐carbon bainitic construction structural steel, which has an optimum balance of strength‐ductility and lower yield ratio (YR). The results show that the as‐rolled steel consists of bainitic ferrite, lath‐like bainite, and granular bainite. With an increase in the tempering temperature, a great deal of carbonitrides precipitates and maintains a small nano‐scaled size without coarsening. Their interaction with dislocations induces an increase in strength. When the tempering temperature further increases, some grain boundaries between adjacent bainitic laths gradually blur or even disappear, and the average width of the bainitic laths increases to ≈860 nm. The recovery of bainitic laths is the main reason for the decreased strength. Due to the synergistic effect of the small dot‐shaped martensite/austenite (M/A) constituents and grain boundaries misorientation distribution, maximum impact absorbed energy is achieved after tempered at 500 °C. Furthermore, the lower YR is achieved by appropriate control of strength difference between the soft bainitic ferrite and hard M/A constituents via tempering. [ABSTRACT FROM AUTHOR]

This study is focused on investigating the microstructure and mechanical properties of high strength ferrite-martensite (F-M) dual phase steels manufactured through a simple hot rolling and step cooling. Additionally, it seeks to elucidate the strengthening mechanism of the 'unexpected microstructures', namely lower bainite and auto-tempered martensite, within F-M dual phase steels. During the step cooling process, the morphology of ferrite is determined by the air-cooling temperature, while the ferrite content is affected by the air-cooling temperature and time. Since the morphology of continuously cooled bainite is determined by the cooling rate, granular bainite is formed in the air-cooling stage and lower bainite is formed in the laminar water-cooling stage. Combined with the reasonable microstructure construction and the strengthening effect of 'unexpected microstructures', the sample air-cooled at 700 °C for 10s achieves the best comprehensive mechanical properties (yield strength: 1155 MPa, tensile strength: 1511 MPa, total elongation: 11.5%, impact toughness in −20 °C: 16.6 J). The strengthening effect of 'unexpected microstructure' is mainly to improve strain strengthening and impact toughness. The presence of lower bainite and auto-tempered martensite both promotes the incompatible strain during the deformation, resulting in higher strain hardening. This effect is further enhanced by the segmentation of the prior austenite grains by lower bainite and the plastic constraints imposed by surrounding martensite during deformation. Auto-tempering contributes to an increase in impact toughness by reducing areas with high dislocation density. The improvement of impact toughness by lower bainite is mainly attributed to the increase of high angle grain boundaries when partitioning the prior austenite. • Ary industry-friendly high-strength Ferrite-Martensite dual-phase steel with excellent comprehensive mechanical properties was obtained. • The microstructure evolution during the step cooling was analyzed. • The strengthening mechanism of "unexpected microstructures" (i.e., bainite and auto-tempered martensite) were proposed. [ABSTRACT FROM AUTHOR]

A novel strategy involving thermo-mechanical controlled processing (TMCP) and quenching to obtain martensite (Q&P) or bainite (B&P) followed by dynamic carbon partitioning was applied to two low carbon steels. Effect of cooling path on the microstructural evolution was studied with particular focus on dynamic partitioning, and the impact fracture behavior was observed and discussed based on the duplex structures, especially the retained austenite (RA). It was found that majority of the carbides were present in Al contained steel subjected to B&P process, leading to low fraction of RA ~ 5.1%, while ~ 9.2 to 20.1% RA was obtained in Si contained steel subjected to Q&P and B&P process. The impact energy at 20 °C of B&P steels exceeded 100 J, which was not related to RA. However, the impact energy at 20 °C of steels subjected to Q&P process indicated a large difference with impact energy of ~ 43 J to 117.1J, which resulted from carbon content in martensite and the RA fraction. The steel quenched to 380 °C followed by furnace cooling contained 14.7% RA and 5.5% RA experienced TRIP effect with transformation product of twinned martensite, which resulted in excellent impact toughness of ~ 106.6 J. During impact process, the micro-voids preferentially formed at ferrite/hard phases interface and in ferrite. In particular, the micro-voids also formed in martensite with high carbon concentration and thus led to low impact toughness of ~ 43 J. The bainite lath, M/A (martensite-austenite) island and RA acted as barriers and thus increased the resistance to crack propagation. During low temperature impact, the blocky RA has low thermal stability and is partially transformed leading to approximately 40 J decrease at 0 °C, while the microstructure of martensite lath and film-like RA exhibits high impact energy ~104.4 J at 0 °C. [ABSTRACT FROM AUTHOR]

The present study investigates the evolution of microstructure and mechanical properties of 304 stainless steel after thermo‐mechanical controlled processing (TMCP). Three different FRTs (finish rolling temperatures) have been adopted and the micro‐constituents are identified as austenite grains, stacking faults, annealing, and deformation twins. Fine austenite grains in the range of 1–30 μm are obtained at lower FRT (700 °C) whereas at higher FRT, coarse grains are formed. TEM and X‐ray analyses indicate the formation of M23C6 ((Cr, Fe)23C6) precipitates for higher FRT (900 °C). Specimen processed with 700 °C FRT results into 37% enhancement in UTS compared to the base metal which is attributed to fine partially recrystallized grain, extensive deformation twinning and high dislocation density. Maximum elongation (68%) is obtained due to the formation of strain‐free equiaxed grains (≈40 μm) at 900 °C FRT. [ABSTRACT FROM AUTHOR]

We elucidate here the impact of thermo-mechanical controlled processing (TMCP) in governing nucleation of second phase in Nb-Ti microalloyed dual-phase steels. A wide range of mechanical properties were obtained through change in the microstructure, depending on the TMCP cooling schedule. Polygonal ferrite matrix with martensite as the second phase exhibited highest initial strain hardening rate, lowest yield strength, and highest tensile strength as compared to the situation when bainite or bainite-martensite was the second phase. The Crussard–Jaoul (C-J) strain hardening behavior of granular bainite and granular bainite-martensite as the second phase exhibited one-stage characterized by linear and parabolic behavior, respectively. On the other hand, two stages were observed in steel containing martensite as the second phase, viz., linear stage I and parabolic stage II. [ABSTRACT FROM AUTHOR]

For the purpose of obtaining a niobium-microalloyed steel with a preferable ultrafine-grained ferrite microstructure, a thermomechanical controlled processing (TMCP) strategy that included a hot shear deformation conducted near the local phase transformation temperature and a subsequent cooling process is proposed. As a large plastic strain was enforced by the simple hot shear deformation, severe plastic deformation (SPD) of the niobium-microalloyed steel was realized. The proposed TMCP strategy was simulated physically on a Thermecmastor-Z compression machine combined with a multi-type cooling system. Various cooling rates of different cooling methods resulted in reconstructive and displacive phase transformations from γ-Fe to α-Fe and led to different microstructural morphologies. In addition to phase transformations, the precipitation of carbides, grain growth, plastic deformation, discontinuous dynamic recrystallization (dDRX) of retained austenite grains, and continuous dynamic recrystallization (cDRX) of ferrite grains occurred during hot shear and subsequent cooling. The effects of the strain rate and forming temperature on the microstructural and textural evolution of niobium-microalloyed steel during hot shear and subsequent cooling were investigated and discussed. A niobium-microalloyed steel with a homogenous ultrafine-grained ferrite microstructure and intense γ-fiber texture was fabricated when the forming temperature, strain rate of hot shear deformation, and cooling rate of subsequent mist cooling were 1073 K, 20 s−1, and 10 K·s-1, respectively. [ABSTRACT FROM AUTHOR]

The correlations existing among the hot mill processing, as-coiled microstructure and mechanical properties of a Mo–Ti–V microalloyed hot rolled high strength steel were investigated in this current study. Discrete processing parameters, i.e., finish rolling temperatures (FRT) and coiling temperatures (CT), were applied and the corresponding microstructures and mechanical properties were analyzed. It was found that the FRT had only a very minor influence on either microstructures or mechanical properties. However, the CTs strongly affected both the microstructures and mechanical properties. The microstructure in the matrix was observed to change from mainly polygonal ferrite to quasi-polygonal ferrite to granular bainite and upper bainite with falling CTs, accompanied by the formation of martensite/austenite (M/A) constituents at the lower CTs. The strength appeared to be increased by the dislocations originating by the shear component of the displacive phase transformation, and by fine (Ti, Mo)C precipitates, both formed during the coiling process. Strengths reaching values as high as 1166 MPa in yield strength and 1225 MPa in tensile strength were observed in specimens after coiling at 610 °C, and the steels still had a reasonable total elongation of around 20%. However, the hole expansion ratios of these steel conditions were rather low in this study, especially for those with higher strength. Several factors appeared to contribute to the poor hole expansion ratios found in these steel conditions: the presence of coarse TiN inclusions, a large amount of strengthening precipitates and a high dislocation density as well as the presence of M/A constituents. [ABSTRACT FROM AUTHOR]