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Increasing magnetically favorable //ND texture components is a key challenge in the preparation of high-efficiency non-oriented electrical steels. In this study, an Fe-1.3 wt% Si steel with strong Cube ({100} ) and Goss ({110} ) texture was successfully produced by novel twin-roll strip casting, cold rolling and annealing process. The microstructure and texture of the material was characterized by optical microscopy, electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). The origin and formation mechanism of texture are described from the perspective of deformation and recrystallization behavior of specifically oriented grains. It was observed that initial Cube rotated toward {013} -{110} besides rotation toward {001} -{001} during cold rolling. In addition, new Cube deformation bands were developed in the deformed {115} -{115} grains. Cube components were partly retained as large block, small band structure and crystallite after heavy cold rolling. The Cube deformation structures served as nucleation sites of new Cube grains. The shear band within {114} , {112} and {111} matrix also provided some Cube nuclei. Morphology change from near bar-shaped to equiaxed occurred during the growth of Cube and Goss grains. The formation of recrystallization texture is attributed to the oriented nucleation mechanism, and the orientation pinning and size effects that impacted the intensity of texture component. The low thickness of strip and coarse solidification microstructure with strong {100} texture are the decisive factors to obtain strong Cube and Goss texture in strip-cast non-oriented electrical steel.

A novel processing strategy was proposed, first to promote Mn partitioning behavior during intercritical annealing, second to improve the fraction and stability of retained austenite (RA) and third to enhance mechanical properties of Mn-reduced transformation-induced plasticity (TRIP) steel. By deliberately preparing large-fractioned Mn-enriched cementite before final annealing, the intercritical austenite was encouraged to inherit relatively high Mn content from pre-existing cementite. Eventually, the formation of sufficiently stable RA together with the precipitation of fine VC particles in ferritic matrix contributes to much better yield strength-tensile strength-ductility combination of 763 MPa-1022 MPa-46.4%, which is superior to many steels containing higher Mn concentrations.

We applied two types of hot-rolling direct quenching and partitioning (HDQ&P) schemes to a low-C low-Si Al-added steel and obtained two ferrite-containing TRIP-assisted steels with different hard matrix structures, viz, martensite or bainite. Using quasi in-situ tensile tests combined with high-resolution electron back-scattered diffraction (EBSD) and microscopic digital image correlation (µ-DIC) analysis, we quantitatively investigated the TRIP effect and strain partitioning in the two steels and explored the influence of the strain partitioning between the soft and hard matrix structures on the TRIP effect. We also performed an atomic-scale analysis of the carbon partitioning among the different phases using atom probe tomography (APT). The results show that the strain mainly localizes in the ferrite in both types of materials. For the steel with a martensitic hard-matrix, a strong strain contrast exists between ferrite and martensite, with the local strain difference reaching up to about 75% at a global strain of 12.5%. Strain localization bands initiated in the ferrite rarely cross the ferrite/martensite interfaces. The low local strain (2%–10%) in the martensite regions leads to a slight TRIP effect with a transformation ratio of the retained austenite of about 7.5%. However, for the steel with bainitic matrix, the ferrite and bainite undergo more homogeneous strain partitioning, with an average local strain in ferrite and bainite of 15% and 8%, respectively, at a global strain of 12.5%. The strain localization bands originating in the ferrite can cross the ferrite/bainite (F/B) interfaces and increase the local strain in the bainite regions, resulting in an efficient TRIP effect. In that case the transformation ratio of the retained austenite is about 41%. The lower hardness difference between the ferrite and bainite of about 178 HV, compared with that between the ferrite and martensite of about 256 HV, leads to a lower strain contrast at the ferrite/bainite interfaces, thus retarding interfacial fracture. Further microstructure design for TRIP effect optimization should particularly focus on adjusting the strength contrast among the matrix structures and tuning strain partitioning to enhance the local strain partitioning into the retained austenite.

The low carbon micro-alloyed steel with start microstructure of ferrite-martensite was pre-tempered to be ferrite-tempered martensite microstructure before cold rolling. Then an identical quenching and partitioning process was applied to the cold rolled sheet with start microstructures of deformed ferrite-martensite (Q&P-UFC) and deformed ferrite-tempered martensite (Q&P-T). It revealed that the austenitization of Q&P-T was retarded and much more austenite with higher stability was obtained after annealing. In both cases, the austenite formed at recrystallized martensite boundaries was significantly fine, while that formed at recrystallized ferrite boundaries was much coarser. Although the retained austenite fraction (5–6%) and corresponding carbon content (1.1–1.2 wt.%) were identical in both cases, a significant Mn-enrichment of retained austenite was only observed in Q&P-T related to the pre-existed Mn-enriched cementite and hence the corresponding TRIP effect in Q&P-T occurred at relatively larger strain. Moreover, abundant martensite variant combination with twin boundary was observed and exhibited dispersive distribution in Q&P-T. The work hardening behaviors of both cases showed three-stage variation, successively ascribed to the dislocation glide of ferrite, TRIP effect of retained austenite and dislocation intersection/tangle of ferrite and martensite, respectively. Both samples exhibited excellent combination of strength and ductility with UTS larger than 1000 MPa and TEL larger than 20%, and Q&P-UFC was favorable to obtain high strength level, while Q&P-T possessed better ductility and excellent combination of strength and ductility.

A novel ultra-high strength transformation-induced plasticity (TRIP) steels with submicron-size martensite/austenite (M/A) constituents was processed by non-isothermal annealing process and compared with quenching and partitioning (Q&P) steels. The microstructure was characterized by means of SEM equipped with EPMA, XRD, EBSD and TEM and its effects on tensile strength and ductility were studied. The non-isothermal annealing significantly refined the parent austenite grains and enhanced inhomogeneous distribution of C atoms within the grains. Moreover, a large fraction of M/A constituents were separated and refined to hundreds of nanometers through bainite transformation in TRIP steels, while blocky retained austenite mainly existed near the carbon-rich prior grain boundaries in Q&P steel. TRIP steels had a higher content of less stable retained austenite by isothermal bainite transformation than Q&P steel, thereby favoring for improving the ductility. The contribution of fresh martensite to the ultimate tensile strength increased gradually from 49.7% to 67.3% with increase in the volume fraction of fresh martensite in TRIP steels. Optimizing the volume fraction and size of M/A constituents was important to obtain a high ultimate strength with desired ductility. The excellent mechanical properties were obtained in TRIP-350 (austempering at 350 °C) with ultimate tensile strength of 1011 MPa, total elongation of 28.80% and the product of strength and elongation (PSE) of 29.1 GPa%, exceeded the properties of Q&P steel.

We applied a hot rolling direct quenching and partitioning (HDQ&P) process to a low-C low-Si Al-added steel and obtained a Q&P steel containing 40 vol % of ferrite. Microstructure characterization was performed by means of SEM, EBSD, TEM and XRD. Atomic-scale characterization of carbon partitioning among the phases was carried out by atom probe tomography (APT). The carbon distribution in the retained austenite and near the martensite/retained austenite interfaces was quantitatively analyzed to study its partitioning behavior. The macroscopic strain distribution evolution across the tensile sample surface was investigated using macro digital image correlation (DIC) analysis. Combining these results with joint micro-DIC and EBSD analysis during quasi in-situ tensile testing, we investigated the strain partitioning among the different phases and the TRIP effect. Coupling of these results enabled us to reveal the relation among carbon partitioning, strain partitioning and the TRIP effect. The large blocky retained austenite with a side length of about 300–600 nm located near the ferrite/martensite (F/M) interfaces has low stability and transforms to martensite during the early deformation stages, i.e. at average strain below 21%. The retained austenite films in the centers of the martensite regions are more stable. The carbon distribution in both, the martensite and the retained austenite are inhomogeneous, with 0.5–2.0 at. % in the martensite and 4.0–7.5 at. % in the retained austenite. Strong carbon concentration gradients of up to 1.1 at. %/nm were observed near the martensite/retained austenite interfaces. The large blocky retained austenite (300–600 nm in side length) near the F/M interfaces has 1.5–2.0 at. % lower carbon content than that in the narrow retained austenite films (20–150 nm in thickness). The ferrite is soft and deforms prior to the martensite. The strain distribution in ferrite and martensite is inhomogeneous, varying by up to 20% within the same phase at an average strain of about 20%. Ferrite deformation is the main origin of ductility of the material. The balance between ferrite fraction and martensite morphology controls the TRIP effect and its efficiency in reaching a suited combination of strength and ductility. Reducing the ferrite volume fraction and softening the martensite by coarsening and polygonization can enhance the strain carried by the martensite, thus promoting more retained austenite in the martensite regions enabling a TRIP effect. The enhancement of the TRIP effect and the decrease of the strain contrast between ferrite and martensite jointly optimize the micromechanical deformation compatibility of the adjacent phases, thus improving the material's ductility.

In this study, two types of as-cast microstructure produced by strip casting were cold rolled and annealed to investigate the effect of initial microstructure on the textural evolution and magnetic properties of non-oriented silicon steel. The results indicated that the cold-rolled sheets of coarse-grained strip with pronounced {100} components exhibited stronger λ fiber (〈100〉//ND) and weaker γ fiber (〈111〉//ND) texture as composed to the fine-grained strip with strong Goss ({110}〈001〉) texture. After annealing, the former was dominated by η fiber (〈001〉//RD) texture with a peak at {110}〈001〉 orientation, while the latter consisted of strong {111}〈112〉 and relatively weak {110}〈001〉 texture. In addition, a number of precipitates of size ∼30–150 nm restricted the grain growth during annealing, resulting in recrystallization of grain size of ∼46 μm in the coarse-grained specimen and ∼41 μm in the fine-grained specimen. Ultimately, higher magnetic induction (∼1.72 T) and lower core loss (∼4.04 W/kg) were obtained in the final annealed sheets of coarse-grained strip with strong {100} texture.

The present study focuses on improving the magnetic properties and decreasing the anisotropy in non-oriented electrical steel by optimizing {1 0 0} recrystallization texture. As-cast Fe-1.3%Si strip with {1 0 0} texture produced by strip casting was subjected to hot rolling in the ferrite region, cold rolling, and recrystallization annealing. Magnetic properties and texture evolution after different stages of processing were studied. Annealed sample without hot rolling exhibited pronounced Cube and Goss texture, which led to high permeability but induced a large difference (∼0.15 T) in magnetic induction B50 between the maximum at 0° and minimum at 45° to the rolling direction. The introduction of hot rolling with 17–40% reduction weakened the intensity of recrystallization texture and had small influence on the nature of texture and magnetic induction. However, relatively complete {1 0 0} recrystallization texture was developed in the sample with hot rolling of 55% reduction. On the other hand, the average grain size of annealed sheets gradually increased with the increased hot rolling reduction. As a result, the magnetic induction and the core loss was optimized together with the improvement of anisotropy. The development of recrystallization texture is discussed on the basis of the deformed microstructure and nucleation mechanism, while the magnetic properties are correlated to the magnetic quality of the texture.

A high strength steel with Nb-V-Ti addition was heat treated by a series of isothermal bainite holding (IBH) processes and quenching and partitioning (Q&P) processes with different intercritical annealing temperatures. The microstructure observation showed that some martensite was formed during quenching in IBH condition with annealing temperature above 850 °C, mainly ascribed to heterogeneous carbon content distribution in parent austenite with relatively large size. Meanwhile, film-like RA in martensite, relatively coarser lath-like RA in bainite and blocky RA located in grain boundaries or phase boundaries were observed in both IBH and Q&P treatments. Moreover, Q&P process was proved to be more beneficial to retain austenite than IBH process, while the ability of austenite retention was similar in IBH conditions. An excellent combination of strength and ductility was obtained in Q&P process annealed at 880 °C with tensile strength of 1126 MPa and total elongation of ~ 18%, attributing to TRIP effect mainly occurred in the latter part of strain and fine microstructure with homogeneous distribution.

We describe here the aging characteristics and strengthening behavior of a low-carbon medium-Mn Cu precipitation-strengthened steel. Atom probe tomography (APT) was employed to characterize the evolution of Cu-rich precipitates in terms of mean radius, number density and volume fraction. Aging at 500 °C and 550 °C for 1 h resulted in substantial coherent body-centered cubic (bcc) Cu-rich precipitates with mean radius of 1.35 and 2.59 nm, respectively. The precipitation strengthening mechanism for these two aging conditions was shearing mechanism and the corresponding strengthening contribution was ~ 266 and ~ 312 MPa, respectively. Here, coherency strengthening and modulus strengthening played a major role, while the contribution of chemical strengthening was relatively small. With increased aging temperature to 600 °C, the precipitates grew and coarsened to elongated shape with incoherent face-centered cubic (fcc) structure, and the strengthening mechanism was Orowan mechanism with a contribution of ~ 232 MPa. Increasing the aging temperature also facilitated the formation of retained austenite, which was of great benefit to plasticity without pronounced deterioration on precipitation strengthening. Ultra-high yield strength of 1020 MPa with superior total elongation of 25.8% was obtained in the sample aged at 600 °C for 1 h. The excellent mechanical properties derived from the combination of precipitation strengthening by Cu-rich precipitates and plasticity effect of retained austenite can be considered as a design principle to simultaneously optimize strength and ductility.

Fe-6.5 wt% Si steel hot bands with different initial grain size and texture were obtained through different annealing treatment. These bands were then warm rolled and annealed. An analysis on the evolution of microstructure and texture, particularly the formation of recrystallization texture was studied. The results indicated that initial grain size and texture had a significant effect on texture evolution and magnetic properties. Large initial grains led to coarse deformed grains with dense and long shear bands after warm rolling. Such long shear bands resulted in growth advantage for {1 1 3} 〈3 6 1〉 oriented grains during recrystallization. On the other hand, sharp {11 h} 〈1, 2, 1/h〉 (α∗-fiber) texture in the coarse-grained sample led to dominant {1 1 2} 〈1 1 0〉 texture after warm rolling. Such {1 1 2} 〈1 1 0〉 deformed grains provided massive nucleation sites for {1 1 3} 〈3 6 1〉 oriented grains during subsequent recrystallization. These {1 1 3} 〈3 6 1〉 grains were confirmed to exhibit an advantage on grain growth compared to γ-fiber grains. As a result, significant {1 1 3} 〈3 6 1〉 texture was developed and unfavorable γ-fiber texture was inhibited in the final annealed sheet. Both these aspects led to superior magnetic properties in the sample with largest initial grain size. The magnetic induction B8 was 1.36 T and the high frequency core loss P10/400 was 17.07 W/kg.

A low carbon steel microalloyed with Nb-V-Ti were heat treated by means of two-step quenching and partitioning (Q&P) process after partial austenitization. Combined use of SEM equipped with EBSD, XRD and TEM showed that the microstructure was composed of intercritical polygonal ferrite, tempered lath martensite, secondary twin martensite and carbon-enriched retained austenite, as well as microalloyed carbides (mainly VC) precipitated inside ferrite and martensite. When increasing partitioning time from 0 s to 3600 s, the volume fraction of retained austenite increased up to 6.5% with mean carbon content in austenite keeping a constant level. The retained austenite kept K-S orientation relationship with adjacent tempered lath martensite and neighboring secondary twin martensite in different martensite packet. As partitioning time increased up to 3600 s, the initial lath martensite tempered gradually to ferrite without any cementite formation.

Bismuth vanadate (BiVO4) is one of the most promising metal oxide semiconductors for photoelectrochemical (PEC) water oxidation. Much efforts have been dedicated on accelerating the sluggish surface water oxidation kinetics. In this study, plasma enhanced atomic layer deposition and subsequent removal of Al2O3 ultrathin overlayers on bismuth vanadate were implemented to achieve the successful passivation of surface states and significant enhancement of PEC performance. Al2O3 ultrathin overlayers were first coated on BiVO4 surface via plasma enhanced atomic layer deposition with various deposition cycles, which resulted in the decrease of PEC water oxidation activity due to the poor conductivity. The subsequent removal of surface amorphous Al2O3 passivated the surface states of the photoanodes and significantly enhanced the photocurrent densities. The passivated BiVO4 exhibited a photocurrent density of 1.34 mA·cm−2 at 1.23 V vs. RHE, which is 73% higher than that of unmodified BiVO4. This work provides a novel strategy and deep insights on surface modification of semiconductor for photoelectrochemical energy conversion.

We elucidate the influence of intercritical rolling temperature on the microstructural evolution, mechanical properties and work-hardening behavior of a hot-rolled Fe-0.2C-6.5Mn-3Al-0.1V medium Mn transformation-induced-plasticity (TRIP) steel containing δ-ferrite. Tensile strength of 966 MPa, total elongation of 42.6% and yield strength of 705 MPa was obtained in the annealed steel subjected to a low intercritical rolling temperature. Rolling at a high intercritical rolling temperature promoted the partitioning of Mn from δ ferrite to prior austenite grains, and led to a martensitic matrix characterized by a fine lath structure. Subsequently, after intercritical annealing, the reversed austenite transformed from the martensitic matrix had high stability and small size. However, the reversed austenite with a high degree of Mn enrichment, fine lath structure and high stability provided a slow and less active TRIP effect. This was responsible for low work-hardening rate during deformation. In contrast, the high content of reversed austenite in the annealed steel subjected to low intercritical rolling temperature had relatively low stability and large lath width, exhibited serrated work-hardening behavior indicative of discontinuous TRIP effect. Additionally, the low intercritical rolling temperature led to a high density of dislocations in δ-ferrite, which effectively promoted VC precipitation after intercritical annealing, and enhanced yield strength. Furthermore, the formation of high angle boundary in δ-ferrite and the formation of pro-eutectoid ferrite at low intercritical rolling temperature also enhanced yield strength through grain boundary strengthening.

A low carbon C-Mn-Si-Al steel was hot rolled with coiling temperatures ranged from 510 oC to 710 oC. The microstructure of specimens consisted of deformed ferrite, lath martensite, twin martensite and bainite. Cementite was only observed in sample coiled at 570 oC. With increasing coiling temperature, deformed ferrite fraction decreased and recrystallized ferrite fraction increased. The ultimate tensile strength increased gradually from 1084 MPa to 1183 MPa due to the increasing fraction of martensite and bainite. The optimal mechanical properties were obtained when coiled at 570 oC with an ultimate tensile strength of 1085 MPa and ultimate elongation of 19.1 %.

Texture optimization has always been a challenge to fabricate non-oriented electrical steels (NOES). In the present study, thin-gauge NOES with high permeability was successfully processed using an innovative and convenient twin-roll strip casting process without hot rolling. The relation between the as-cast microstructure, processing route and texture evolution was studied. The results indicated that as-cast strip with coarse grains exhibited strong {100}〈0vw〉 texture and unique {110}〈110〉 component. Annealed sheets processed by one-stage rolling displayed pronounced {111}〈112〉, {223}〈110〉 components and weak Cube and Goss texture. Micro-texture characteristics revealed that {100}〈0vw〉 texture was partially retained from initial grains, and new Cube and Goss substructures were generated within {110}〈110〉 deformed grains during cold rolling. This was responsible for the development of Cube and Goss recrystallization texture. Furthermore, the application of two-stage rolling not only reinforced these two behavior, but also accelerated the nucleation of η grains because of the increased shear bands in cold-rolled sheets. In this manner, an improved texture consisting of dominant η-fiber, weak γ-fiber and optimized magnetic properties (B50 = 1.72 T, P10/400 = 14.91 W/kg) were obtained. Keywords: Thin-gauge non-oriented electrical steel, Twin-roll strip casting, Microstructure, Texture, Magnetic properties

Three different grain structures of low-carbon medium-manganese steel were prepared through appropriate controlled rolling process. The laminated microstructure with a strong //rolling direction (RD) fiber texture was characterized by ultra-fine elongated ferrite, retained austenite and martensite phase arranged alternately along the RD. The steel with equiaxed grain structure exhibited a relatively low tensile strength of 960 MPa and an extremely poor low-temperature toughness of ~ 8 J at −196 °C. An enhanced upper shelf energy (> 450 J) and low-temperature toughness (~ 105 J at −196 °C), as well as an improved tensile strength (1145 MPa) was obtained in the steel with laminated microstructure. The laminated microstructure enabled the steel to be significantly stronger and tougher along the RD, which contributed to the high tensile strength to some extent. It is concluded that the combined effect of the ultra-fine elongated laminated microstructure, the possible interface decohesion and the existence of numerous {001} cleavage planes resulted in the occurrence of delamination. The delamination fracture enhanced the upper shelf energy mainly by promoting crack branching along the RD and thus suppressing crack propagation along the v-notch direction, which finally resulted in greater plastic deformation and significant increase in absorbed energy. Besides delamination toughening, transformation-induced plasticity (TRIP) effect of metastable retained austenite is believed to be responsible for the high cryogenic toughness, which can release stress concentration of crack tips and thus blunting cracks propagation.

We elucidate here the impact of niobium in ultra-low carbon grain oriented electrical steel (GOES) in terms of microstructure, texture, precipitation and magnetic properties that was processed by twin roll strip casting. Coarse and complex MnS + NbN precipitates, and fine NbN were nucleated at the grain boundaries and in the interior of the grain in the as-cast strip, which contributed to a small degree of grain refinement together with relatively random texture, and AlN precipitation was suppressed during the strip casting process. NbN continuously precipitated during the entire process and exhibited high stability during the reheating cycle, which provided stronger inhibiting force in comparison to AlN precipitates. As a consequence, fine and homogeneous inhibitors were obtained in the primary annealed sheet in the presence of Nb under cold rolling and annealing parameters used in the present study. On considering the effect of NbN particles or Nb in solution on the deformation and recrystallization behavior, the primary annealed Nb-containing sheet exhibited significantly more homogeneous microstructure in relation to Nb-free GOES, with grain size in the range of ∼8–12 μm, and was characterized by relatively more pronounced γ-fiber and weak Goss texture, beneficial for the abnormal growth of Goss grains. Furthermore, Nb-containing GOES experienced complete abnormal growth during secondary recrystallization annealing, such that the enhanced magnetic induction (B 8 as high as 1.88 T) was obtained. In summary, the present study underscores that NbN can be used as an effective inhibitor in ultra-low carbon grain oriented electrical steel using strip casting technology.

The water-stable PAA/PVA/PW 12 nanofiber was firstly prepared by electrospinning and thermal treatments. The negatively charged phosphotungstic acid (PW 12 ), positively charged polyethylenimine (PEI) and silver nanoparticle (Ag NPs) were alternatively deposited onto the fibrous substrate by layer-by-layer (LBL) self-assembly technique. The photocatalytic activity of different coated LBL film was observed under visible light irradiation. The results indicated that the conversion rate of methylene blue (MB) was gradually decreased with increasing the number of bilayers pairs. The behavior might ascribe to the more polycation electrolyte PEI that prevented from permeating for the MB molecule. On the other hand, in terms of the effects of surface charges on the photocatalytic activity, negatively charged surface (PEI) n (PW 12 ) n indicated higher photocatalytic activity than that of positively charged surface (PEI) n (PW 12 ) n − 1 . For the Ag NPs deposited on the LBL film, Ag-(PEI) n (PW 12 ) n film were found to be a better photocatalytic activity than other multilayer films, specially, Ag-(PEI) 2 (PW 12 ) 2 exhibited the most excellent activity due to the influence of the synergy in the hybrid materials.

We investigate here the correlation between deformation behavior and retained austenite characteristics in a medium-Mn transformation-induced plasticity (TRIP) steel. The sample was characterized by a dual-phase microstructure consisting of ultra-fine grained ferrite and retained austenite with relatively high mechanical stability after annealing at 700 °C for 5 h. Both lath-like and blocky (granular) retained austenite with volume fraction of 38.7% and relatively inhomogeneous grain size was obtained. The tensile specimen exhibited outstanding mechanical properties with yield strength of 745 MPa, tensile strength of 1005 MPa and total elongation of 46%, as well as a distinctive work hardening behavior. The in-depth investigation on deformation behavior demonstrated that the transformation mechanism of retained austenite during deformation was strain-induced and the yielding behavior was controlled mainly by the deformation of soft ferrite phase. As to the multi-peak work hardening behavior, it is believed to be attributed to the inhomogeneous and discontinuous occurrence of TRIP effect, which resulted from the inhomogeneous stability of retained austenite. Moreover, the orientation of retained austenite (Schmid factors) was proved an important factor in determining the mechanical stability of retained austenite upon deformation, in addition to the heterogeneity of grain size. These two factors together resulted in the inhomogeneous stability of retained austenite.

Grain-oriented 6.5%Si steel was produced by strip-casting, followed hot rolling, cold rolling, primary annealing and secondary annealing. Microstructural and textural evolution was studied with particular focus on the effect of solidified microstructure on the ultimate microstructure and texture. The study indicated that solidified microstructure of the as-cast strip was determined by melt superheat. The relatively low melt superheat resulted in fine equiaxed grains with random texture, whereas large melt superheat led to coarse grains with moderate λ-fiber texture. After cold rolling, the fine-grained steel showed narrow pancake grain structure in comparison to the coarse-grained steel, which was similar to the microstructure of the initial as-cast strip. After primary annealing, fine equiaxed microstructure with favorable {111}⟨112⟩ texture was obtained in the fine-grained steel. While inhomogeneous microstructure with strong α-fiber and moderate γ-fiber texture was obtained in the coarse-grained steel. The desirable microstructure and texture in the fine-grained steel led to abnormal grain growth with B8 = 1.65 T after secondary annealing. By comparison, normal grain growth occurred in the coarse-grained sample with B8 = 1.37 T. Another observation was high frequency core losses of grain-oriented 6.5%Si steel sheet with adequate secondary grains was significantly lower than non-oriented 6.5%Si steel and grain-oriented 3.0%Si steel. The study suggests that the strip-casting is a suitable route to fabricate grain-oriented 6.5%Si steel, and the initial fine grain size with random texture is preferred to obtain adequate abnormal grain growth.

The prior carbide-free bainite (CFB) was introduced into a low carbon IQ&P steel via an isothermal bainitic holding (IBH) process, named as IBQ&P steel. It is essential to investigate the influence of prior CFB formation on microstructural evolution and mechanical properties, especially focused on austenite stability and mechanical behaviors. It reveals that the influence of prior CFB on austenite stability changed from deteriorated effect into mechanical stabilization with increasing IBH time, whilst the location of prior CFB expanded from the region adjacent to interface of constituent phases into the interior of parent austenite. Meanwhile, the amount of bainite with morphology of lath and blocky significantly increased accompanied by improvement of microstructural refinement, while the fraction of initial martensite with lath or twin structure decreased with existence of abundant tempered dislocation and e-carbide. As compared to IQ&P steel, the carbon content of retained austenite in IBQ&P steel was improved with elevated stability, ascribing to the retention of high carbon austenite in the exterior of parent austenite and refinement of microstructure. The practical carbon content of retained austenite was much larger than the predicted results merely considering carbon partitioning, while nearly close to that of NPLE/PLE transition line of γ to α transformation including synergistic partitioning of C, Mn and Si elements. Moreover, the high carbon retained austenite in IBQ&P steel gave rise to TRIP effect occurred at large strain with expanded range. Finally, the introduction of prior CFB significantly improved the combination of strength and ductility with optimal PSE value exceeding 30 GPa%, which was obtained by properly tailoring the decomposition and retention of metastable austenite.

Over the last decade, demand has increased for developing the hot-galvanized quenching and partitioning (Q&P) steel to overcome the disadvantages associated with vehicle safety, fuel consumption and corrosion resistant. The present work aims to elucidate the effects of quenching temperature on phase transformation kinetics, microstructure evolutions and mechanical properties of a hot-galvanized Q&P steel (0.225C-0.85Si-2.02Mn-0.91Al, in wt.%) by combining modeling and experimental research. Using dilatometry, SEM, EBSD, EPMA, TEM, PED, XRD and Image-Pro Plus (IPP) software, we quantitatively investigated the microstructure evolution at different quenching temperatures. Results indicated that a larger fraction of primary martensite at lower quenching temperature could strongly promote subsequent bainite transformation kinetics, which is attributed to more martensite-austenite interfaces and defect density. By fitting the dilatometer curves and establishing the equation of transformation rate vs. quenching temperature, a Kolmogorov-Johnson-Mehl-Avram (KJMA) equation was established to describe insufficient bainite formation kinetics during high-temperature short-time overaging. Furthermore, a modified CCE model taking into account intercritical ferrite and short-time bainite transformation was proposed and the predicted RA fractions are more consistent with the experimental values. As the quenching temperature decreases, small-sized blocky RA along martensite boundaries and filmy RA between martensite laths increase, while coarse lath/blocky RA inside bainite structures or at bainite boundaries decreases gradually. In addition, the YS decreases from 763 MPa to 431 MPa with the increase of quenching temperature, while the UTS varies in a narrow range between 967 MPa and 1036 MPa. A stable TEL of 22.6–25.7% can be obtained at a wide range quenching temperature (150–275 °C), which is attributed to the joint effects of TRIP effect and multiphase structure. This research would be of guiding significance for the industrial practice.

The relationship between crystallographic orientation, microstructure characteristic and mechanical properties in 3.5Mn transformation induced plasticity (TRIP) steel that subjected to different cold rolling reduction and annealing time was elucidated. Before the hot-rolled plates were cold rolled to different thicknesses, large-fractioned Mn-enriched cementite particles dispersed in ferrite matrix were deliberately prepared to enhance Mn-partitioning and improve austenite stability. The evolution of microstructure was performed by electron probe micro-analyzer (EPMA), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD), and the mechanical properties of the intercritical annealed samples were evaluated by uniaxial tensile test at room temperature. The results show that the cementite dissolution, ferrite recrystallization and austenite formation are strongly coupled and interdependent, resulting in a complex ultra-fine microstructure composed of retained austenite, ferrite and cementite in the intercritical annealed samples. Moreover, the Schmid factor (SF) distribution within the range of 0.25–0.5 was studied to understand its correlation with the crystallographic orientation and mechanical property of the test steel. It is concluded that the mechanical properties of 3.5Mn TRIP steel can be improved when the annealing period, orientation and austenite stability are optimized.

The nucleation and growth of Goss grains during primary and secondary recrystallization of a strip-cast grain-oriented Fe-6.5 wt% Si alloy were investigated using a quasi in-situ electron backscatter diffraction (EBSD) technique. During primary recrystallization at 700 °C, the Goss was found to nucleate from two locations in the deformed //ND (normal direction) grains: in the shear bands and at the grain boundaries. However, only those nucleated from the shear bands tended to survive during the subsequent grain growth. Even the survived Goss grains would not develop as abnormal Goss grains during secondary recrystallization. Abnormally growing Goss grains were formed at the surfaces when the primary recrystallization temperature was 850 °C, which grew into the center and consumed the entire thickness during secondary recrystallization. The abnormal growth of the Goss grains was attributed to the large fraction of 20–45° grain boundaries at the growing front, which essentially did not change during secondary recrystallization. Using a classical grain growth model, the migration velocities of Goss grains during abnormal growth were calculated, which were compared to the experimental migration velocities obtained from the quasi in-situ EBSD data. It was shown that the model could roughly predict the abnormal growth of the Goss grains during secondary recrystallization.

The microstructure-properties relationship, work-hardening behavior and retained austenite stability have been systematically investigated in a hot-rolled medium manganese transformation-induced-plasticity (TRIP) steel containing δ-ferrite subjected to one-step and two-step intercritical annealing. The steel exhibited tensile strength of 752 MPa and total elongation of 52.7% for one-step intercritical annealing at 740 °C, tensile strength of 954 MPa and total elongation of 39.2% in the case of intercritical quenching at 800 °C and annealing at 740 °C. The austenite obtained by two-step annealing mostly consists of refined lath structures and increased fraction of block-type particles existing at various kinds of sites, which is highly distinguished from those characterized by long lath morphology and small amounts of granular shape in one-step annealed samples. In spite of a higher C and Mn content in austenite and finer austenite laths, two-step annealing can lead to an active and continuous TRIP effect provided by a mixed blocky and lath-type austenitic structure with lower stability, contributing to a higher UTS. In contrast, one-step annealing gave rise to a less active but sustained TRIP effect given by the dominant lath-like austenite having higher stability, leading to a very high elongation. The further precipitation of vanadium carbides and the presence of both dislocation substructure and fine equiaxed grain in ferrite matrix facilitate the increase of yield strength after double annealing.

An unconventional cold rolling scheme (inclined rolling at 0°, 30°, 45°, 90° during second-stage cold rolling process) was adopted to process grain-oriented silicon steel based on strip casting process. The influences of inclination angles on microstructure, texture, inhibitor and magnetic properties were studied by a combination of EBSD, XRD and TEM. It was found that the α-fiber texture was weakened and γ-fiber was strengthened in cold rolled sheet with increase in inclination angle. The primary recrystallization sheet exhibited more homogeneous microstructure with relatively strong γ-fiber, medium α-fiber texture, weak λ-fiber texture and Goss component at high inclination angles. Fine and homogeneous inhibitors were obtained after primary annealing with increase in inclination angle from 0° to 90° because of more uniform deformation after inclined rolling. The grain-oriented silicon steel experienced completely secondary recrystallization at various inclination angles after final annealing process, with superior magnetic properties at 0° and 90°. Furthermore, Goss nuclei capable of final secondary recrystallization in strip casting process newly formed both in-grain shear bands and grain boundaries region during second-stage cold rolling and subsequent annealing process, which is different from the well-accepted results that Goss texture originated from the subsurface layer of the hot rolled sheet or during intermediate annealing process. In addition, the Goss texture that nucleated in-grain shear bands was weaker but more accurate as compared to that in grain boundaries region.

A Fe-3.0%Si non-oriented silicon steel was produced by a novel strip casting process. Cold-rolled sheets were annealed at temperatures varying from 900 to 1200 ° C with the aim to investigate the influence of recrystallization annealing process on the microstructure, texture and magnetic properties. The increase in annealing temperature significantly increased the recrystallized grain size and reduced the core loss. However, the oversized microstructure (~360 μm) in 1200 ° C annealed sheets resulted in an augment in core loss. In addition, with increasing annealing temperature, the magnetic induction diminished along the rolling direction while gradually increased along the transverse direction, even though the average value changed slightly. This interesting result could be attributed to the weakening in η -fiber and strengthening in λ -fiber and γ -fiber texture. The best combination of core loss (3.23 W/kg), magnetic induction (1.71 T) and anisotropy was obtained at recrystallization annealing of 1100 ° C. The specified and unspecified errors in this paper have been carefully corrected, and

A novel ultra-low carbon grain oriented silicon steel was successfully produced by strip casting and two-stage cold rolling method. The microstructure, texture and precipitate evolution under different first cold rolling reduction were investigated. It was shown that the as-cast strip was mainly composed of equiaxed grains and characterized by very weak Goss texture ({110} ) and λ-fiber ( //ND). The coarse sulfides of size ~100 nm were precipitated at grain boundaries during strip casting, while nitrides remained in solution in the as-cast strip and the fine AlN particles of size ~20–50 nm, which were used as grain growth inhibitors, were formed in intermediate annealed sheet after first cold rolling. In addition, the suitable Goss nuclei for secondary recrystallization were also formed during intermediate annealing, which is totally different from the conventional process that the Goss nuclei originated in the subsurface layer of the hot rolled sheet. Furthermore, the number of AlN inhibitors and the intensity of desirable Goss texture increased with increasing first cold rolling reduction. After secondary recrystallization annealing, very large grains of size ~10–40 mm were formed and the final magnetic induction, B 8 , was as high as 1.9 T.

A novel two-step intercritical annealing process was designed for an ultra-low carbon medium manganese steel plate. Excellent mechanical properties with yield strength of 590 MPa, tensile strength of 840 MPa, total elongation of 28.5% and high impact energy of 106 J at −80 °C were obtained. The microstructure comprised of ultra-fine grained ferrite and retained austenite together with a small amount of martensite after the two-step intercritical annealing. Both lath-like and blocky retained austenite with volume fraction of ~25% and relatively poor stability were obtained. The submicron-sized lath-like retained austenite exhibited Nishiyama-Wassermann (N-W) orientation relationship with the neighboring martensitic ferrite lath. The fine grain size played a crucial role in stabilizing austenite during phase transformation by significantly lowering Ms temperature and increasing the elastic strain energy. The overall stability of retained austenite during deformation was considered to be mainly governed by the chemical composition of the studied steel. The mechanism of toughening was elucidated. The superior low-temperature toughness was associated with TRIP effect of metastable retained austenite, which relieved the local stress concentration, enhanced the ability to plastic deformation and delayed the initiation and propagation of microcracks.

Quantitative modeling and experimental verification of carbide precipitation in a martesnsitic Fe-0,16 wt.%C-4.0 wt.%Cr alloy

A 0.23 mm grain-oriented 4.5 wt% Si steel sheet was prepared by strip casting, hot rolling, one-stage warm rolling, primary annealing and secondary annealing. A detailed study of microstructure, texture and precipitate was carried out by methods of OM, EBSD, XRD and TEM. It was found that the as-cast strip exhibited equaxied microstructure with random orientation, enabling the fine and homogeneous primary microstructure to be obtained after one-stage warm rolling. After hot rolling, weak Goss texture was observed, which could not act as Goss seed. In contrast, the Goss seed was found to originate within the shear bands in {111}〈112〉 grains during warm rolling. Another finding was that the precipitates was suppressed during strip casting due to the rapid solidification and subsequent cooling rate, and mainly precipitated during hot rolling. After secondary annealing, abnormal Goss grains evolved sufficiently, and showed prominent properties. The B 8 was 1.73 T, which was similar to 1.85 T for grain-oriented 3.0 wt% Si steel, and the core losses were low than the grain-oriented 3.0 wt% Si steel and non-oriented high silicon steel.

The present study on low-carbon medium-Mn steel demonstrates that the newly proposed “intercritical annealing-tempering” process led to good combination of strength, ductility and impact toughness, by simultaneously optimizing austenite reverted transformation and Cu precipitation. Intercritical annealing at high temperature accelerated the dissolution of cementite particles, promoted the formation of reverted austenite, and then increased the total elongation and impact energy. The additional tempering at 500 °C facilitated intensive precipitation of Cu-rich precipitates in secondary martensite. The precipitates were fine, uniform in size and high in density, which provided a significant strengthening effect. Strengthening by nano-scale Cu-rich precipitates compensated softening associated with high temperature annealing, and increased the yield strength of tempered medium-Mn steel by 75 MPa. In addition to facilitating Cu precipitation, the introduction of secondary martensite also promoted nucleation of reverted austenite during tempering with sufficient C and Mn concentration that stabilized austenite and further improved ductility. Excellent mechanical properties, 960 MPa yield strength, 1010 MPa tensile strength, 29.2% total elongation and 150 J impact energy at room temperature were obtained in the precipitation-strengthened low-carbon medium-Mn steel, on intercritical annealing at 645 °C, followed by tempering at 500 °C for 2 h.

A newly developed TRIP-assisted steel with ferrite/martensite initial structure was designed to investigate the microstructural evolution during overall annealing cycle. The results were compared with the traditional cold-rolled TRIP steel. The characteristic of austenite formation from different initial microstructure was clarified based on combining the experiments and modeling. Results indicated that the new austenite obtained during intercritical annealing was fine-grained and uniformly-distributed. It comprised of acicular structure formed in pre-existing martensite (Mpre) matrix and blocky structure occurred at phase interfaces or prior austenite grain boundaries. The kinetics of austenite formation in specimens involving Mpre are similar in the early heating stage, and then decreased with decreasing Mpre content at temperatures greater than 770 °C. Both experimental and DICTRA results indicated that the austenite fractions of specimens with 90% and 100% Mpre fraction attained almost the maximum at the end of heating, and decreased with isothermal holding time during the intercritical annealing. Moreover, the retained austenite at interfaces of ferrite and pre-existing phase followed the Kurdjumov-Sachs (K–S) relationship with adjacent tempered martensite, while only part of the retained austenite had identical relationship with neighboring ferrite. In addition, the amount of retained austenite increased with Mpre content, although their stability against martensite transformation during tensile testing was similar and greater than that of cold-rolled specimen. As a consequence, the yield strength was increased from 590 MPa to 753 MPa and the tensile strength increased slightly from 995 MPa to 1046 MPa with Mpre fraction increasing from 50% to 100%, and the total elongation was ~30%, similar to the third generation advanced high strength steels.

Here, we applied hot-rolling in conjunction with direct quenching and partitioning (HDQ&P) processes with different rolling schedules to a low-C low-Si Al-added steel. Ferrite was introduced into the steel by intercritical rolling and air cooling after hot-rolling. The effect of intercritcal deformation on the microstructure evolution and mechanical properties was investigated. The promotion of austenite stabilization and the optimization of the TRIP effect due to a moderate degree of intercritical deformation were systematically explored. The results show that the addition of 1.46 wt% of Al can effectively promote ferrite formation. An intercritical deformation above 800 °C can result in a pronounced bimodal grain size distribution of ferrite and some elongated ferrite grains containing sub-grains. The residual strain states of both austenite and ferrite and the occurrence of bainite transformation jointly increase the retained austenite fraction due to its mechanical stabilization and the enhanced carbon partitioning into austenite from its surrounding phases. An intercritical deformation below 800 °C can profoundly increase the ferrite fraction and promote the recrystallization of deformed ferrite. The formation of this large fraction of ferrite enhances the carbon enrichment in the untransformed austenite and retards the bainite transformation during the partitioning process and finally enhances martensite transformation and decreases the retained austenite fraction. The efficient TRIP effect of retained austenite and the possible strain partitioning of bainite jointly improve the work hardening and formability of the steel and lead to the excellent mechanical properties with relatively high tensile strength (905 MPa), low yield ratio (0.60) and high total elongation (25.2%).

The mechanism responsible for secondary recrystallization is still disputed in grain oriented silicon steel. This is mainly related to the complexly of itself. In this work, the occurrence of {2 1 0} texture through secondary recrystallization was observed in the grain oriented silicon steel with Cu 2 S as main inhibitor. In order to elucidate the abnormal growth of {2 1 0} grain, texture evolutions in Fe–3% Si–0.5% Cu steel were investigated using micro-texture analysis. In addition, the {2 1 0} texture was used to validate the current mechanisms of secondary recrystallization. The result indicates that the surface energy plays a significant role in the selection of secondary recrystallization besides the characteristics boundaries which are linked to high mobility.

An ultra-low carbon Fe–3%Si grain oriented electrical steel strip was produced by a twin roll strip casting process. Next, the strip with or without annealing was cold rolled and subsequently annealed. The effect of annealing after strip casting on the texture development was studied using macro-/micro texture analysis. Due to the rapid solidification, the precipitation behavior of second phase particles was suppressed and very few particles were observed in the as-cast strip. The direct cold rolled texture was characterized by strong α ( //RD) and weak γ ({111}//ND) fibers. While, in the annealed strip, a large number of particles were precipitated during the annealing of the cast strip and these particles hindered the crystal rotation during cold rolling such that part of the initial solidification texture remained in the cold rolled sheet. In the case of the direct cold rolling process, the precipitation occurred before recrystallization during primary annealing. These fine particles had a significant pinning effect on dislocations and further retarded the recrystallization behavior, leading to relatively strong α-fiber in the primary annealed sheet. On the other hand, compared with the fine and dispersed particles precipitated during the strip annealing process, the distribution of particles precipitated during primary annealing were inhomogeneous. Therefore, there was no secondary recrystallization during secondary annealing, which was distinct from the strip annealing process where abnormal growth of Goss grains were observed and the magnetic properties were significantly improved.

The crystallographic texture and Charpy impact toughness have been investigated in American Petroleum Institute (API) X100 pipeline steel, and the different rolling processes are applied to adjust the crystallographic texture for the sake of improving impact toughness. It has been found that the very weak textures exist in the steel only rolled in the recrystallization region of austenite, but the intensity of the unfavorable {001} 〈110〉 component is quite high. The {001} 〈110〉 texture is harmful for impact toughness and needed to be decreased in most cases. It is confirmed that the appropriate controlled rolling processes effectively reduce the intensity of the {001} 〈110〉 texture. By contrast, low temperature finish rolling in the non-recrystallization region produces the fine grain structures, beneficial textures and superior impact toughness at the low temperature. In addition, the occurrence of delamination in the ductile region generates the unfavorable cleavage crack on the fracture surface of Charpy specimen and inevitably reduces the impact energy in the upper shelf region. The delamination will occur continuously with decreasing temperature due to the large volume fraction of {001} planes parallel to the rolling plane.

Microstructures composed of lath martensite and retained austenite with volume fraction between 8.0 vol.% and 12.0 vol.% were obtained in a low-C low-Si Al-free steel through hot-rolling direct quenching and dynamical partitioning (HDQ&DP) processes. The austenite stabilization mechanism in the low-C low-Si Al-free steel under the special dynamical partitioning processes is investigated by analyzing the carbon partition behavior from martensite to austenite and the carbide precipitation-coarsening behavior in martensite laths combining with the possible hot rolling deformation inheritance. Results show that the satisfying retained austenite amount in currently studied low-Si Al-free HDQ&DP steel is caused by the high-efficiency carbon enrichment in the 30–80 nm thick regions of austenite near the interfaces in the hot-rolled ultra-fast cooled structure and the avoidance of serious carbides coarsening during the continuous cooling procedures. The excellent strength-elongation product reaching up to 26,000 MPa% shows that the involved HDQ&DP process is a promising method to develop a new generation of advanced high strength steel.

Non-oriented electrical steel as-cast strip was produced by twin roll strip casting process, and subsequently cold rolled and annealed at heating rates in the range of 3–450 °C/s with the aim to elucidate the effect of heating rate on the evolution of recrystallized microstructure and texture. The average grain size was rapidly increased when the heating rate was increased from 3 to 25 °C/s, and decreased when the heating rate was greater than 25 °C/s. The average grain size did not increase linearly with heating rate, which was related to different degree of nucleation and growth rate. The recrystallization texture exhibited pronounced improvement during rapid annealing. At high heating rate, the Goss and Cube had a higher probability of nucleation of shear bands with high stored energy, while the intensity of the γ-fiber texture was significantly reduced. The highest B50 value attained was 1.803 T at a heating rate of 300 °C/s. The study indicates that rapid heating has strong effect on the recrystallization behavior in non-oriented electrical steels, which facilitates optimization of microstructure and texture, especially in the coarse-grained structure.

Grain oriented silicon steel was produced by strip casting and two-stage cold rolling processes. The development of microstructure and texture was investigated by using optical microscopy, X-ray diffraction and electron backscattered diffraction. It is shown that the microstructure and texture evolutions of strip casting grain oriented silicon steel are significantly distinct from those in the conventional processing route. The as-cast strip is composed of coarse solidification grains and characterized by pronounced 〈001〉//ND texture together with very weak Goss texture. The initial coarse microstructure enhances {111} shear bands formation during the first cold rolling and then leads to the homogeneously distributed Goss grains through the thickness of intermediate annealed sheet. After the secondary cold rolling and primary annealing, strong γ fiber texture with a peak at {111}〈112〉 dominates the primary recrystallization texture, which is beneficial to the abnormal growth of Goss grain during the subsequent high temperature annealing. Therefore, the secondary recrystallization of Goss orientation evolves completely after the high temperature annealing and the grain oriented silicon steel with a good magnetic properties ( B 8 =1.94 T, P 1.7/50 =1.3 W/kg) can be prepared.