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Here we present a novel approach of intercritical heat treatment for microstructure tailoring, in which intercritical annealing is introduced between conventional quenching and tempering. This induced a heterogeneous microstructure consisting of soft intercritical ferrite and hard tempered martensite, resulting in a low yield ratio (YR) and high impact toughness in a high-strength low-alloy steel. The initial yielding and subsequent work hardening behavior of the steel during tensile deformation were modified by the presence of soft intercritical ferrite after intercritical annealing, in comparison to the steel with full martensitic microstructure. The increase in YR was related to the reduction in hardness difference between the soft and hard phases due to the precipitation of nano-carbides and the recovery of dislocations during tempering. The excellent low-temperature toughness was ascribed not only to the decrease in probability of microcrack initiation for the reduction of hardness difference between two phases, but also to the increase in resistance of microcrack propagation caused by the high density of high angle grain boundaries.

During the past thirty years, two generations of low alloy steels (ferrite/pearlite followed by bainite/martensite) have been developed and widely used in structural applications. The third-generation of low alloy steels is expected to achieve high strength and improved ductility and toughness, while satisfying the new demands for weight reduction, greenness, and safety. This paper reviews recent progress in the development of third-generation low alloy steels with an M3 microstructure, namely, microstructures with multi-phase, meta-stable austenite, and multi-scale precipitates. The review summarizes the alloy designs and processing routes of microstructure control, and the mechanical properties of the alloys. The stabilization of retained austenite in low alloy steels is especially emphasized. Multi-scale nano-precipitates, including carbides of microal-loying elements and Cu-rich precipitates obtained in third-generation low alloy steels, are then introduced. The structure–property relationships of third-generation alloys are also discussed. Finally, the promises and challenges to future applications are explored.

The influence of inclusion size and number density on high-temperature urea corrosion (HTUC) behavior of ferritic stainless steels was investigated in a simulated working environment of selective catalytic reduction (SCR) system in commercial vehicles. There is a positive correlation between the control level of inclusions and the resistance of HTUC. By slightly increasing the content of Nb in ferritic stainless steels, the inclusions, especially TiN, were significantly refined, and thus displayed an improvement in HTUC resistance. The interface between inclusions and the matrix becomes a fast channel for chromium precipitation during high-temperature nitriding induced by the decomposition of urea. Chromium nitrides will precipitate around the inclusions and wrap the inclusions, which will decrease the chromium equivalent of the matrix and reduce the resistance of ferritic stainless steels to HTUC. In addition, the high-temperature oxidation accompanied with thermal fatigue also makes the inclusions more likely to become the crack nucleation source, which can accelerate the material thinning and reduce its service life.

The evolution behavior of Cu precipitates under intercritical tempering in a low carbon low alloy steel is presented in the study here. A complete crystal structure transformation process of Cu precipitate is presented together with newly discovered steps. The complete transformation sequence of crystal structure of Cu precipitates in the alloy system was (I) Cu-rich nano-order clusters→9R Cu→detwinned 9R Cu→transition phase (detwinned 9R + FCC)→FCC Cu and (II) Cu-rich nano-order cluster→non twin 9R Cu→FCC Cu. The entire evolution process of Cu precipitates was caused by the continuous enrichment of Cu atoms in Cu precipitates and the size of Cu precipitates. In the present study, we have discovered the increase of Cu content in Cu-rich nano-ordered clusters, the movement rule of twin planes and the combination of twin variants in the multiple twin 9R Cu, the transition rule from 9R structure to FCC structure.

The impact of intercritical annealing (IA) temperature and time on retained austenite and mechanical properties are elucidated in a low carbon steel containing 2.85 wt% Mn. Experimental studies indicated that the multi-phase microstructure consisted of intercritical ferrite, martensite/bainite, retained austenite and fine dispersion of precipitations. The volume fraction of retained austenite attained a maximum value of 14% on IA at 680 °C for 30 min. The partition behavior of alloying elements during intercritical annealing was studied by scanning transmission electron microscopy (STEM) and analyzed kinetically by DICTRA simulation. Appropriate fraction of retained austenite obtained at 680 °C for 30 min was attributed to the extent of partitioning of carbon and manganese into the reverted austenite that contributed to best balance of mechanical properties.

The carbon is one of the basic alloying solutes in steels. In the present study, the newly carbon microalloyed X80 steel was designed for the next generation of pipeline and the carbon microalloying effect of base material on the crystallographic structure of coarse grained heat affected zone (CGHAZ) of X80 girth welded joints was systematically investigated using electron back-scattering diffraction (EBSD), considering the fact that crystallography is the most intrinsic aspect in evaluating the microstructure. The results indicated a remarkable variation in the density of high angle grain boundaries and their dispersion by increasing the carbon content of 0.03wt%. It was attributed to the differential shearing mechanism, which was primarily sheared by variant pairing of V1/V2 at 0.06 wt% carbon, whereas at lower 0.03wt% carbon, V1/V4 pairing mechanism dominated the transformation. The preferential variant pairing transformation mechanism at differential carbon content therefore generated dissimilarly selected variants. Thus, as the carbon content increased, the variant selection effect upon phase transformation was attenuated by generating more variants from different Bain groups. The variant pair V1/V2 belonging to different Bain groups neighboring in CGHAZ of high carbon steel increased the density of high angle boundary (Σ3 boundary). Moreover, the variation in CTOD (crack-tip opening displacement) property of HAZ suggested that it should be primarily correlated to the block size (or density of block boundaries) of CGHAZ.

Stress&ndash, strain behavior of a low carbon low alloy multiphase steel with ferrite, tempered bainite, and retained austenite was studied at different cryogenic temperatures. Results indicated that both strength and ductility were enhanced with decreasing tensile testing temperature. The enhancement of both strength and ductility was attributed to the decreased mechanical stability of retained austenite with decreasing temperature, resulting in sufficient transformation induced plasticity (TRIP) effect for increasing work hardening rate.

Two wear-resistant steel plates with different Ni content were studied to reveal the significant role of Ni on hardenability from the perspective of crystallography. From the surface to the quarter, and then to the center of the plates, the 0.44Ni steel exhibited higher hardness compared with the 0.02Ni steel, indicating that the 0.44Ni steel was of higher hardenability. The microstructure at the center changed from granular bainite to lath bainite with increased Ni content, together with an enhanced density of block boundaries. Ni improves the hardenability by weakening the variant selection through inducing larger transformation driving force and more self-accommodation of transformation strain to obtain a high density of block boundaries.

Carbide precipitation and hydrogen embrittlement susceptibility of a 960 MPa grade ultra-high strength low carbon low alloy steel processed by quenching and tempering (Q-T) via different reheating ...

A significant volume fraction of 26% retained austenite was obtained via a two-step intercritical treatment in a low carbon low alloy steel with ultra-high tensile strength of ~1200 MPa, ultra-low yield ratio of 0.32, excellent uniform elongation of ~16.4% and total elongation of ~23%. The ultra-low yield ratio and excellent uniform elongation implied high work hardening and excellent formability. The high work hardening and ultra-high tensile strength are attributed to deformation-induced twin-martensite.

Nano B2 FeCu ordered clusters and multiphase microstructure consisting of intercritical ferrite, tempered martensite and nano-γ phase (reverted austenite) were obtained by two-step heat treatment involving intercritical annealing and intercritical tempering. The experimental steel with nano Cu precipitates and nano-γ phase exhibited high strength and high ductility combination. The yield strength and total elongation of the experimental steel increased from 758 MPa and 16.8% to 984 MPa and 29.5% after the second step intercritical tempering for 5 min. High resolution transmission electron microscopy (HRTEM) and three-dimensional atom probe (3DAP) studies provided evidence to support that high density of nano B2 FeCu ordered clusters contributed to high strength. First principle calculations suggested that the feasibility of B2 FeCu nano-ordered clusters, and the stability of B2 structure is related to the coherent stress field at the interface between clusters and the BCC-Fe matrix. The average size of B2 FeCu nano-ordered clusters was 4 nm with a lattice constant of 0.2893 nm and an orientation relationship of (1 1 0)B2//(1 1 0)α and [0 0 1]B2//[0 0 1]α. 9R Cu without twinned structure was discovered at different tempering times. The proportion of Cu in Cu precipitates varied from 24.4 at% to 61.2 at% with change in crystal structure and increase in the size of precipitates. The significant increase in yield strength is discussed in terms of ordered domain strengthening, modulus strengthening and lattice misfit strengthening mechanisms. Bright field TEM image and selected area electron diffraction (SAED) pattern discovered and proved that spindle nano-γ phase was present at phase boundary between tempered martensite and intercritical ferrite. The size of them were among 242–375 nm in length and 52–80 nm in width. The two needle tip ends of nano-γ phase were just at phase boundaries, indicating the preferred growth direction. (1 −1 1)γ plane of nano-γ phase was parallel to (1 −1 0)α plane of the adjacent tempered martensite. The effect of nano-γ phase on enhancing the plasticity was revealed by instantaneous work hardening index curve.

This study aims at providing a new insights into the impact toughness scatter from the aspect of crystallographic structure. It demonstrated that the large impact toughness scatter associated much to the microstructure diversity. The crystallographic structure with evident scatter will display obvious discrepancy. In this work, three groups of samples simulated coarse-grained heat affected zone (CGHAZ) of an offshore engineering steel were obtained at different cooling rates. The Charpy test results showed that the toughness decreases dramatically with the decrease of cooling rate. However, the largest scatter in impact toughness occurred in the sample with medium cooling rate (15 °C/s), which was attributed to the heterogeneity in crystallographic structures. The visualization of crystallographic features showed that the prior austenite grain size has a significant effect on bainitic variant selection, which governed the effective grain size and crack propagation mechanism. CP (close-packed plane) grouping of variants is more likely to take place in large austenite grain, indicating that the size of CP region is larger than Bain zone, and the crack is short and flexural. On the contrary, in smaller austenite grain, Bain grouping of variants that always forms low angle grain boundary and favors crack propagation dominates the transformation, and it will promote the crack to propagate through the entire Bain zone and then yield large long crack. However, these two cases can co-exist in the same sample at medium cooling rate, indicating that the cleavage fracture is controlled by the effective grain size (Bain-zone size) and the scatter in impact toughness is associated much to the proportion and relative location between fine and coarse Bain zones.

This work describes here the synergistic effect of niobium micro-alloying in combination with boron addition on the development of a 900 MPa grade ultra-high strength low carbon cost-effective bainitic steel processed by direct-quenching and induction tempering. A mixed microstructure consisting of acicular ferrite and lath bainite associated with high density of high angle grain boundaries was obtained by controlled rolling and direct-quenching. Nano-sized precipitation behavior during controlled rolling and induction tempering was studied by transmission electron microscopy (TEM). A number of nano-sized precipitates were observed in the matrix after controlled rolling and direct-quenching. These precipitates were identified by electron energy loss spectrometry (EELS) in scanning TEM (STEM) to be TiN-Nb(C,N) or TiN-NbC composite precipitates, and were associated with mean austenite grain size of ~ 34 ± 6 µm in steel before finishing rolling. A finish rolling reduction of 67% below non-austenite recrystallization temperature pancaked the austenite grains to 10–15 µm in thickness. Nano-sized NbC formed during induction tempering at 670 °C had an average diameter of ~ 4.3 nm and 9.5 nm for tempering duration of 5 min and 30 min, respectively. It is noted that while nano-sized NbC precipitates smaller than 5 nm provide significant precipitation hardening effect to increase the mechanical strength, acicular ferrite in the mixed microstructure helps in retaining high elongation of low carbon direct-quenched and tempered steel. Ultra-high yield strength of 944 MPa with high ductility (uniform elongation of 6.3% and total elongation of 20%) was obtained after induction tempering at 670 °C for 5 min.

Intercritical tempering at 680 °C for different times was carried out in a low carbon copper-bearing steel to study the evolution of the crystal structure of Cu precipitates by high resolution transmission electron microscopy. With increased tempering time, four different types of crystal structure of copper precipitates with different sizes were observed, namely, (a) nano-ordered clusters comprised of B2 FeCu nano-ordered clusters (2–3 nm) and weak ordered BCC Cu nanoclusters (2–3 nm), (b) 9R Cu (5–12 nm) - multiple twinned structure consisted of two, six or seven 9R twins with an orientation relationship of (1 1–4)9R∥(0 1 1)α, [−1 1 0]9R∥[1 −1 1]α, (c) detwinned 9R Cu (24–26 nm) consisted of two 9R parts and a removable interface (1 1–4)9R, and (d) FCC Cu (~37 nm) precipitates consisted of two parts of FCC Cu and a micro-twinned region. The evolution sequence of crystal structure of Cu precipitates was: nano-ordered clusters → 9R Cu → detwinned 9R Cu → FCC Cu. The maximum contribution to precipitation hardening is attributed to nano-ordered clusters. Keywords: Nano-ordered cluster, 9R Cu, Intercritical tempering, Evolution sequence, FCC Cu

A two-step intercritical heat treatment was designed to obtain a multi-phase microstructure consisting of intercritical ferrite, tempered martensite/bainite and stable retained austenite in a low carbon and copper alloyed steel, characterized by high strength and high ductility combination. The evolution of copper precipitation during intercritical tempering was studied by transmission electron microscopy (TEM). Electron microscopy studies indicated that the precipitation of copper during tempering followed the sequence (as a function of time): twinned 9R-Cu (0.5 h) → de-twinned 9R-Cu (1 h) → e-Cu (greater than 3 h), which was accompanied by increase in the size of precipitates from ~ 11 nm to ~ 30 nm. Considering the cutting mechanism of precipitation strengthening, e-Cu precipitation contributed to ~ 248 MPa and ~ 207 MPa toward yield strength for 3 h and 5 h tempering, respectively. The average size of niobium-containing carbides varied marginally from ~ 11–16 nm and had a Baker–Nutting (B-N) orientation relationship with the ferrite matrix. The combination of transformation induced plasticity (TRIP) effect and nano-sized precipitation strengthening contributed to excellent mechanical properties (yield strength > 700 MPa, tensile strength > 800 MPa, the uniform elongation > 16% and the total elongation > 30%).

The objective of this paper is to study the influence of thermal cycles produced by the later welding passes on the properties of the sub-regions in reheated zone of multi-pass weld metal for a 550 MPa grade offshore engineering steel. A Gleeble-3500 simulator was applied to simulate microstructural evolution in sub-regions of the reheated zone and its effect on the properties. The results indicated that the reheating process changed the prior austenitic morphology from columnar structure to equiaxed structure and similar columnar structure with quasi-polygonal ferrite (QPF) or blocky M-A (martensite/austenite) constituent distributed on the grain boundaries while the matrix microstructure (acicular ferrite) changed slightly. Charpy impact results indicated that WM region (as-deposited) had the highest impact energy. However, the actual impact sample showed lower impact energy because the machined notch contained one or more brittle reheated zones. In these brittle reheated zones, the necklace-type M-A constituent was hard phase decorating prior columnar or equiaxed austenite grain boundaries, which yielded stress concentration significantly and was mainly responsible for lower toughness of the entire weld metal. Fortunately, this deterioration in toughness could be reduced as decomposition of necklace-type M-A constituent occurred due to later welding passes.

Film-like retained austenite with a volume fraction of ~ 10% was obtained in a Fe-0.08C-0.5Si-2.4Mn-0.5Ni (wt%) steel via a two-step intercritical treatment. The retained austenite exhibited ultra-high stability even on cooling to −100 °C. Atom probe tomography indicated notable enrichment of C (0.43 wt%) and Mn (6.6 wt%) in retained austenite. The ultra-high stability of austenite in low carbon low alloy steel is attributed to the combination of partitioning of C and Mn to austenite and the size effect of ultrafine austenite based on thermodynamic analysis. It is underscored that controlled growth of reverted austenite is important for austenite stabilization.

We elucidate here the microstructure-property relationship in high strength and high toughness 60 mm thick ultra-heavy gauge plate steels characterized by ferrite-bainite multi-phase microstructure and compare with the conventional bainitic microstructure. Pilot-scale results indicated that high yield strength of ~486–508 MPa with high ductility of ~10% in uniform elongation, low yield to tensile (Y/T) ratio of ~0.74–0.77, excellent low temperature toughness of ~186–228 J at −60 °C and good weldability are obtained in a multi-phase steel through combination of controlled rolling and accelerated cooling. The multi-phase microstructure steel consisted of quasi-polygonal ferrite and lath bainite with the corresponding phase proportion of 3:2 and 2:3 at 1/4 thickness (1/4 t) and 1/2 thickness (1/2 t) locations, respectively. The positive impact of multi-phase microstructure on ductility, Y/T ratio, toughness and weldability were attributed to the following aspects: (a) results from Crussard-Jaoul (C-J) analysis indicated that the soft ferrite phase in the multi-phase microstructure favorably modifies the work hardening behavior and results in high ductility and low Y/T ratio; (b) multi-phase microstructure is beneficial in enhancing toughness by enhancing initiation energy for nucleation of crack; (c) multi-phase microstructure improves the toughness of heat affected zone by altering the size and distribution of martensite/austenite (M/A) constituent.

The study provides new sights on thermal and mechanical stability of reverted austenite, which interestingly were modulated by intercritical heat treatment in a low alloy low carbon steel. Thermal and mechanical stability of reverted austenite was strongly governed by the intercritical tempering temperature during the second step of heat treatment and decreased with the increase of temperature. Hollomon analysis indicated that reverted austenite grains experienced martensitic transformation and the tensile strength was significantly enhanced. Furthermore, thermal stability studied by low temperature magnetic experiments suggested that Ms and Mf of reverted austenite generated by intercritical tempering at 720℃ were −97.5℃ and −120.5℃ respectively, and the transformation product was twinned martensite.

The combination of partially sintered titanium tool electrode and graphite powder mixed dielectric is proposed to prepare titanium carbide coatings by electrical discharge coating technique with the major objective to improve their tribological properties. The phase composition and microstructure of the obtained coatings are characterized by X-ray diffraction, Raman spectrum, transmission electron microscopy and scanning electron microscopy. Their tribological performances are investigated by ball-on-disk tribometer and the corresponding mechanism of friction and wear is explored. Compared with the coatings obtained in dielectric without graphite powder, they show more uniform thickness and higher microhardness. They also show lower and steadier friction coefficient as well as better wear resistance especially in high load condition. These enhanced properties can be attributed to the suspended graphite powders in dielectric. They, as the additional carbon source, increase the transition of titanium tool electrode to titanium carbide and introduce amorphous carbon into the coatings. It is this amorphous carbon that continuously supplies the lubricant at the contact, helping to accommodate sliding motion and preventing the surfaces from mechanical wear and oxidation.

Microstructure evolution and properties were studied in a low carbon low alloyed hot-rolled bainitic steel by annealing and annealing plus tempering. Microstructure of the hot-rolled steel consists of lath bainite and martensite. By annealing at 720 °C for 30 min and water quenching, multi-phase microstructure consisting of intercritical ferrite, tempered bainite/martensite, retained austenite and fresh martensite was obtained. With increasing annealing temperature to 760 °C, microstructure of the steel consisted of intercritical ferrite, fresh martensite without retained austenite. After the second step of tempering at 680 °C for samples annealed both at 720 °C and 760 °C, ~ 8–9% volume fraction of retained austenite was obtained in the multi-phase microstructure. Moreover, fine precipitates of VC with size smaller than 10 nm and copper precipitates with size of ~ 10–50 nm were obtained after tempering. Results from scanning transmission electron microscopy (STEM) give evidence to support that the partitioning of Mn, Ni and Cu is of significance for retained austenite stabilization. Due to the combined contribution of multiphase microstructure, the transformation-induced-plasticity effect of retained austenite and strengthening effect of nanometer-sized precipitates, yield strength greater than 800 MPa, yield to tensile ratio of 0.9, uniform elongation of ~ 9% and good low temperature impact toughness of 147 J at − 40 °C were achieved.

This paper elucidates the determining role of grain boundary types/misorientations on formation of intragranular globular austenite, which has random orientation or large deviation from the parent austenite, in a high-strength low-alloy bainitic steel during reaustenitization. Results revealed that within the parent austenite with lath bainite, reverted austenite with random orientation or large deviation from the parent austenite can nucleate at the packet and block boundaries with high misorientations, and rapidly evolve to become globular austenite with increase of temperature, making a significant contribution in final austenite grain refinement. While in granular bainite, the low misoriented boundaries belonging to the same Bain group are the preferred nucleation sites for the reverted austenite, which is detrimental to the formation of intragranular globular austenite. In addition, the precipitation mode of cementite is different in the reverse process, that is, cementite is precipitated from the matrix of lath bainite, but it is precipitated from retained austenite or martensite/austenite (M/A) particles in granular bainite, which will also lead to the different or consistent variant orientation of reverted austenite and matrix. Therefore, the final austenite grain refinement in initial granular bainite was mainly due to the formation of intergranular globular reverted austenite, and the refining effect is much lower than that of lath bainite.

The present study was conducted to evaluate the additivity of apparent (AID) and standardized (SID) ileal digestibility of AA in diets at two feed intake (FI) levels. A total of 28 growing barrows were used in this study with 7 treatments and two 10-d experimental periods (7 d adaptation and 3 d ileal digesta collection). The seven treatments contained 3 experimental diets (Corn, SBM, and Corn + SBM mixed diet) at 2 feeding levels (low and high, represented 3% of BW per day and ad libitum, respectively) in a 3 × 2 factorial arrangement and a nitrogen-free diet (NFD) fed at low feeding level. There were 8 replicates for each of the 7 treatments. The AID of AA determined in low FI was greater than the AID determined in high FI levels for nitrogen and all AA except for Leu, Met, Cys, and Tyr in corn, SBM, and the mixed diet (P

The significant roles of Nb and Mo on enhancing high temperature urea corrosion (HTUC) resistance of ferritic stainless steels applied in selective catalytic reduction (SCR) system of commercial vehicle were investigated. By slightly increasing of Nb or Mo, the reactivation rate (Ra) of ferritic stainless steels was significantly reduced, indicating an improvement in HTUC resistance. High temperature nitriding induced by the decomposition of urea resulted in precipitation of chromium nitride particles, which caused chromium depletion in the matrix and resultant deterioration of corrosion resistance. Moreover, the high temperature oxidation accompanied by thermal fatigue further accelerates the formation and exfoliation of oxide layer, showing detrimental consequences on service life of materials for commercial vehicle.

Two regions with different degrees of centerline segregation in a bainitic steel were studied to explore the influence of segregation on hardenability and mechanical properties. With increased severity of segregation, the microstructure at centerline regions changed from lath bainite to martensite, associated with a deterioration of toughness, the mechanism of which was investigated from the perspective of crystallographic features. The high-level local enrichment of alloying elements led to a large packet and Bain zone size and strong variant selection, which were unfavorable for crack deflection and transformation strain accommodation. As a result, the toughness of severely segregated regions was decreased.

In this work, we elucidate the effects of tempering on the microstructure and properties in a low carbon low alloy steel, with particular emphasis on the thermal stability of retained austenite during high-temperature tempering at 500&ndash, 700 &deg, C for 1 h. Volume fraction of ~14% of retained austenite was obtained in the studied steel by two-step intercritical heat treatment. Results from transmission electron microscopy (TEM) and X-ray diffraction (XRD) indicated that retained austenite had high thermal stability when tempering at 500 and 600 &deg, C for 1 h. The volume fraction was ~11&ndash, 12%, the length and width remained ~0.77 and 0.21 &mu, m, and concentration of Mn and Ni in retained austenite remained ~6.2&ndash, 6.6 and ~1.6 wt %, respectively. However, when tempering at 700 &deg, C for 1 h, the volume fraction of retained austenite was decreased largely to ~8%. The underlying reason could be attributed to the growth of austenite during high-temperature holding, leading to a depletion of alloy contents and a decrease in stability. Moreover, for samples tempered at 700 &deg, C for 1 h, retained austenite rapidly transformed into martensite at a strain of 2&ndash, 10%, and a dramatic increase in work hardening was observed. This indicated that the mechanical stability of retained austenite decreased.

A novel and feasible method of enhancing the low temperature (−40 °C) impact toughness of coarse grained heat affected zone (CGHAZ) is put forward. By slightly increasing the holding time at peak temperature, the completeness of austenite-bainite transformation was enhanced and the toughness of CGHAZ was improved significantly. The product bainite was refined with increased holding time and more crystallographic variants from different Bain groups, namely, more Bain group boundaries were formed, which proved to be effective in preventing crack propagation. Moreover, when the inter-spacing of these boundaries is controlled below the critical Griffith crack length by self-accommodation during transformation, micro-cracks can be arrested by these high angle boundaries, thereby suppressing brittle fracture initiation and increasing fracture properties.

The present work focuses on microstructure evolution and precipitation strengthening during tempering at region of 550–680 °C to elucidate the structure–property relationship in the steel. The effect of tempering on the development of a 700 MPa grade high strength hot rolled cost-effective bainitic steel was studied for infrastructure applications. Granular bainite with dispersed martenisit–austenite (M–A) constituents in the bainitic ferrite matrix was obtained after hot rolling and air cooling to room temperature. The decomposition of M–A constituents to cementite carbides and the precipitation of nano-sized NbC carbides in bainitic matrix on tempering were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Nano-sized precipitates of NbC precipitated during tempering were in average diameter of ~4.1–6.1 nm. There were ~86–173 MPa increases in yield strength after tempering at region of 550–680 °C. It is noticeable that those nano-sized NbC precipitates provide an effective way to significantly increase the strength of the low carbon bainitic steel. High yield strength of 716 MPa with high ductility (uniform elongation of 9.3% and total elongation of 22.4%), low yield to tensile ratio of 0.9 and good low temperature toughness of 47 J (half thickness) at –40 °C was obtained after tempering at 680 °C for 30 min.

The present study describes the microstructure and precipitation behavior in an ultra-high strength low carbon niobium–vanadium microalloyed steel that was processed by quenching and high frequency induction tempering. Ultrahigh yield strength of ~1000 MPa with high elongation of ~15% and high low temperature toughness of 55 J (half thickness) at −40 °C was obtained after quenching from austenitization at 900 °C for 30 min, and tempering at 600 °C for 15 min by induction reheating with a reheating rate of ~50 °C/s. While the yield strength increase on tempering was similar for both induction reheating and conventional reheating (electrical resistance reheating), there was ~100% increase in low temperature toughness in induction reheated steel compared to the conventional reheating process. The underlying reason for the increase in toughness was attributed to the transformation of cementite film observed in conventional reheating and tempering to nanoscale cementite in induction reheating and tempering. The precipitation of nanoscale carbides is believed to significantly contribute to ultra-high strength, good ductility, and high toughness in the high frequency induction reheating and tempering process.

The structure–mechanical property relationship, with particular focus on effect of tempering process on the microstructural evolution and mechanical properties was investigated in a low carbon Cu-bearing steel that was processed in three-steps, namely, intercritical annealing, intercritical tempering, and tempering heat treatment. The objective of adopting three steps was to elucidate the nature and evolution of microstructural constituents that contributed to high strength–ductility combination in the studied steel. The three-step processing led to a microstructure primarily comprising of ferrite, retained austenite, and small amount of bainite/martensite. The mechanical properties obtained were: yield strength > 720 MPa, tensile strength > 920 MPa, uniform elongation > 20%, total elongation > 30%, and low yield ratio of 0.78. The tempering step led to a significant increase in both yield and tensile strength and decrease in yield ratio, without reducing ductility, a behavior attributed to the precipitation of copper in retained austenite and ferrite. The precipitation of copper enhanced the stability of retained austenite and work hardening rate, leading to a high volume fraction of retained austenite (∼29%), with consequent increase in elongation and significant increase in yield and tensile strength during tempering.

Fine film-like stable retained austenite was obtained in a Fe–0.08C–0.5Si–2.4Mn–0.5Ni in weight percent (wt.%) steel by the two-step intercritical heat treatment. The first step of intercritical annealing creates a mixed microstructure of preliminary alloy-enriched martensite and lean alloyed intercritical ferrite, which is called as “reverted structure” and “un-reverted structure”, respectively. The second step of intercritical tempering is beneficial for producing film-like stable reverted austenite along the reverted structure. The stabilization of retained austenite was studied by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), dilatometry and X-ray diffraction (XRD) analysis. The two-step austenite reverted transformation associated with intercritical partition of C, Mn and Ni is believed to be the underlying basis for stabilization of retained austenite during the two-step intercritical heat treatment. Stable retained austenite is not only beneficial for high ductility, but also for low temperature toughness by restricting brittle fracture. With 10% (volume fraction) of retained austenite in the steel, high low temperature toughness with average Charpy impact energy of 65 J at −80 °C was obtained.

The contribution of multi-phase microstructure and retained austenite on mechanical properties of austempered and intercritical annealed Fe–0.23C–1.8Mn–1.35Si (wt%) steel was studied. The multi-phase microstructure comprised of intercritical ferrite (IF), bainite/martensite, and retained austenite. During austempering, the retained austenite was stabilized, which was studied using a combination of experimental (XRD, TEM) and thermodynamic analysis. The termination of bainitic transformation combined with carbon rejection into residual austenite during the second step austempering treatment is believed to be the underlying basis for stabilization of retained austenite. This led to significant increase in uniform and total elongation (25% and 36%, respectively) and the product of tensile strength and % elongation was 33 GPa%. The work hardening behavior of retained austenite exhibited a three-stage process such that necking was delayed. The increased work hardening rate is attributed to the multi-phase microstructure and TRIP effect.

The present work focuses on structure-property correlation in a new low alloy multi-phase microstructure steel engineered with retained austenite via Cu alloying plus a three-step heat treatment, namely, intercritical annealing, intercritical tempering and sub-critical tempering of an as hot rolled structure. Particular attention was paid to characterizing nano-sized precipitates and dislocations after the three-step heat treatment by atom probe tomography (APT) and transmission electron microscopy (TEM), in order to establish their respective contribution to the yield strength. Cu-rich atomic clusters and ultra-fine precipitates were obtained in intercritical ferrite after the three-step heat treatment. The average radius of clusters and precipitates was 1.3±0.4 nm and 5.6±0.7 nm, respectively. The number density of clusters and precipitates was estimated as 2.55 × 1023 m-3 and 1.68 × 1021 m-3, respectively. The strength contribution from clusters and precipitates was ~72 MPa and ~101 MPa, respectively, assuming the number density of the APT dataset to represent the whole material. In addition, a large number of dislocations were observed in the intercritical ferrite by TEM study. The average density of dislocations was measured to be ~1.66 × 1014 ± 7.12 × 1013 m-2, which contributed ~250 MPa to yield strength according to the Bailey–Hirsch relationship.

A Cu-bearing Nb-microalloyed bainitic steel with high yield strength (>700 MPa), good elongation (>35%) and low temperature toughness was developed for heavy plates. An intercritical annealing plus tempering (L–T) heat treatment was applied for combining reversed transformation with the precipitation during the tempering stage. The enrichment of alloying elements during the annealing process leads to the drop of A c1 temperature, which makes the reversed transformation and precipitation occur simultaneously in the subsequent tempering process. The secondary enrichment of Mn and Ni makes the reversed austenite stable enough to be retained at room temperature. About 29% retained austenite introduced by the L–T process contributes to excellent elongation more than 10% by the TRIP effect. The formation of Nb(C,N) and (Nb, Mo)C precipitates during the annealing and cooling stages inhibits the recovery of dislocations, while more Nb precipitates smaller than 10 nm and Cu precipitates with sizes of 20 nm play significant strengthening role on the matrix. The combination effects of retained austenite and nano-sized dual-precipitate increased yield strength, ductility and toughness remarkably.

This paper presents a smart energy management strategy (SEMS) to optimise the economic operation of a micro-grid. As an important component of smart grids, the micro-grid acts as a bridge to bundle renewable generation, energy storage systems (ESSs) and electric vehicles (EVs) together. It can be operated either by connecting to upstream distribution systems or isolated with the help of ESSs. In the paper, an economic model of a micro-grid is established based on the influence of energy prices in a day-ahead market. The SEMS is used to schedule power supply/demand of diversified sources to obtain optimal economic operation. Smart management of ESSs, EVs and power exchange with the upstream grid, if possible, are simplified into a single-objective optimisation. Artificial Fish Swarm Algorithm (AFSA) has been used to achieve a practical method for energy management based on three different operation policies. The paper presents results which exemplify the proposed model.