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Herein, several treatments, including modified quenching and tempering (QT) treatment, ultrafast cooling and tempering (UFC‐T) treatment, and ultrafast cooling, intercritical quenching, and tempering (UFC‐LT) treatment, are used to prepare low‐Ni steels and compared with conventional QT treatment. All the three treatments enable low‐Ni steel to achieve equivalent or better cryogenic toughness than 9% Ni steel. The toughening effect of modified QT treatment is better than that of UFC‐T treatment, which is attributed to the increase in density of high‐angle grain boundaries (HAGBs). For modified QT treatment, fine equiaxed prior austenite grains and weak variant selection of martensite transformation lead to higher density HAGBs. Although the coarse elongated prior austenite grains in UFC‐T treatment are beneficial to Bain grouping and not conducive to matrix refinement, the intercritical quenching step in UFC‐LT treatment further increases the density of HAGBs due to the jagged prior austenite grain boundaries (PAGBs) and fresh martensite formed along PAGBs. Therefore, UFC‐LT treatment achieves high‐density HAGBs and excellent cryogenic toughness similar to modified QT treatment. Due to the wider tempering temperature process window, UFC‐LT treatment is the optimal process route for low‐Ni liquefied natural gas tank steel. [ABSTRACT FROM AUTHOR]

A novel treatment comprised of ultra-fast cooling, intercritical quenching and tempering is proposed to modify the microstructure and improve the mechanical properties of low-concentration Ni-containing steel (6.5 pct Ni steel). The influence of intercritical temperature on the microstructure and mechanical properties has been systemically investigated. The results reveal that a mixed microstructure of alloy-depleted intercritical ferrite and alloy-enriched fresh martensite and retained austenite is obtained after intercritical quenching. Moreover, reversed austenite is formed at the grain boundaries of fresh martensite and retained austenite during subsequent tempering. Furthermore, the increase in intercritical temperature promoted the formation of lath-like fresh martensite, leading to the refinement of intercritical ferrite and the formation of secondary acicular reversed austenite. When the intercritical temperature was increased to 670 °C and 700 °C, reversed austenite exhibited sufficient stability at − 196 °C because of the fine-grained structure and acicular morphology. Finally, an optimal combination of strength and cryogenic toughness has been achieved by intercritical quenching from 700 °C. The obtained high yield strength can be ascribed to the decrease in the amount and size of intercritical ferrite, whereas excellent cryogenic toughness can be attributed to the increase in fine-grained stable acicular reversed austenite and decrease in effective grain size. [ABSTRACT FROM AUTHOR]

In this study, the correlation between reversed austenite formed at different tempering temperatures and mechanical properties in a low Ni steel treated by ultra-fast cooling, intercritical quenching and tempering was investigated in detail. It was found that reversed austenite was formed at fresh martensite and retained austenite during tempering. As tempering temperature increased, volume fraction and size of reversed austenite increased, alloy concentration in reversed austenite decreased, and reversed austenite stability was thus deteriorated. When tempering temperature is not higher than 590 °C, reversed austenite that remains completely stable at − 196 °C greatly improves cryogenic toughness. When tempering temperature reaches 610 °C and above, partial reversed austenite transforms into hard secondary fresh martensite at − 196 °C or even room temperature, which is harmful to cryogenic toughness. By tempering at 590 °C, an optimum combination of mechanical properties was achieved, which is comparable to that of 9% Ni steels. [ABSTRACT FROM AUTHOR]

A 0.23 mm thick grain-oriented silicon steel sheet was successfully produced based on strip casting and two-stage cold rolling route. The evolutions of microstructure and texture along the complete processing route were briefly investigated. It was shown that the as-cast strip was composed of coarse ferrite grains and martensite, and the texture was mainly characterized by strong {001} $\langle 0vw\rangle $ texture. Goss texture was absent in the hot-rolled strip, which was distinct from the well accepted results that Goss texture had its origin during hot rolling. After the cycle of the first-stage cold rolling, an intermediate annealing, the second-stage cold rolling, and a primary annealing, the strip exhibited a fully recrystallized microstructure with a weak \alpha $ -fiber texture and a relatively strong \gamma $ -fiber texture. Goss texture was not observed through the thickness of the primary annealed strip. This was significantly different from the previous results that Goss texture was the most populous component in the primary recrystallization texture of two-stage route in conventional process. After secondary recrystalization annealing, the magnetic induction of the sheet was as high as 1.84 T. [ABSTRACT FROM AUTHOR]

Based on the mechanisms of plasticity and toughness, a low carbon medium manganese alloyed steel plate with the outstanding combination of ductility and toughness was designed. The microstructure characteristics, strain hardening behavior and toughening mechanisms were investigated in detail. The microstructure is composed of ferrite/tempered martensite and austenite after heat treating at 650 ° C for 2 h. The retained austenite shows multi-type morphology, multi-scale size and relatively poor mechanical stability, resulting in multi-peak strain hardening behavior and sufficient TRIP effect. The excellent strain hardening capacity and ductility are hence achieved. The excellent low temperature toughness is also achieved due to the relatively lower concentrations of Mn and C, the formation of fine ferrite grains and a small volume fraction of fine retained austenite. The steel hence possesses the excellent ductility (the total elongation is ~37.3%) and low temperature toughness (the Charpy V-notch impact energy is ~158 J at −80 °C). [ABSTRACT FROM AUTHOR]

Abstract: Based on ultra fast cooling (UFC), the microstructural characteristics and mechanical properties with various cooling paths and the mechanism of enbrittlement and toughening for different microstructural characteristics in low-carbon high performance bridge steel were investigated in details using optical microscope (OM), scanning electron microscope (SEM), electron back-scattered diffraction (EBSD), electron probe micro-analyzer (EPMA) and transmission electron microscope (TEM). The results show that using UFC can effectively refine the size of M/A constituent, promote the formation of lath bainite with high misorientation between laths, suppress the re-partition of carbon, and enhance the relative frequency of high-angle grain boundaries during bainite transformation. However, at the higher UFC cooling finish temperature of 560°C, the bainite transformation mainly takes place during air cooling. The larger block-form M/A constituent is almost twin martensite with zone axis of B=[113] and twin plan of (pqr)=(21−1) due to sufficient re-partition of carbon and carbon concentration of approx. 0.22wt% not making residual austenite so stable that they become twin martensite below the martensite transformation start temperature (M s ). The balance of high strength with yield strength of 876MPa and better toughness with ductile–brittle transition temperature (DBTT) of lower than −60°C was realized using the cooling path of UFC→400°C→air cooling. In addition, based on observation and analysis of cracks initiation and cracks propagation, the mechanism of embrittlement and toughening for the cooling paths of UFC→560°C→air cooling and UFC→400°C→air cooling, respectively, was discussed in details. For the cooling path of UFC→560°C→air cooling, the microcracks can easily nucleate at larger block-form brittle twin martensite or twin martensite–matrix interface and easily propagate through twin martensite or along twin-martensite–matrix interface; furthermore, low-angle grain boundaries, even high-angle grain boundaries, cannot effectively arrest cracks propagation, resulting in the higher DBTT. However, for the cooling path of UFC→400°C→air cooling, the microvoids can hardly nucleate at fine M/A constituent or carbides and their growth is not along lath boundaries, but through bainite laths with high misorientation between laths. Moreover, the larger plastic deformation is observed at turning sites or coalescence sites, resulting in the lower DBTT. [Copyright &y& Elsevier]

The microstructural heredity of as-rolled lath martensite with different prior austenite grain morphologies during inter-critical quenching has been studied by using a quasi in-situ EBSD technique in a 6.5%Ni-containing cryogenic steel, to reveal the effect of as-rolled microstructure on final microstructure and toughness at 77 K. It is found that the refinement of final microstructure can be achieved by refining the as-rolled lath martensite through both refinement and elongation of prior austenite grains, which is attributed to two aspects of heredity effect. On the one hand, the fine as-rolled martensitic structure in the interiors of prior austenite grains is inherited during inter-critical quenching. On the other hand, the refinement of as-rolled lath martensite promotes the formation of newly oriented grains along the prior austenite grain boundaries to further refine the microstructure. The cryogenic toughness of Ni-containing cryogenic steels is significantly improved through the heredity effect of as-rolled lath martensitic refinement. [ABSTRACT FROM AUTHOR]