Search

An ultra-low carbon and low nickel medium manganese heavy steel plates was fabricated by quenching and intercritical annealing process. The effect of annealing time on the tensile properties and impact toughness were studied in relation to the formation of reversed transformation. The results show that excellent mechanical properties with yield strength of 680 MPa, tensile strength of 871 MPa, total elongation of 38.2% and high impact energy of 187 J at −60 °C were obtained. The microstructure comprised of ultra-fine grained ferrite and retained austenite (RA) together with a small amount of martensite after intercritical annealing. Increasing the intercritical annealing time resulted in coarser matrix microstructures, larger and more voluminous retained austenite, but with notably reduced stability. Highly stable RA enhances the work hardening capacity of the steel via sustained deformation-induced transformation (TRIP effect), significantly contributing to the toughening of the multiphase system. In addition, Austenite stability, rather than large-angle grain boundaries and the amount of austenite transformation, is the primary factor determining the low-temperature toughness of medium manganese steel.

The study describes large strain high-speed asynchronous rolling process to process AZ91 magnesium alloy sheets with an overall thickness reduction of 84 % via two-pass rolling at 350 °C. Rolling speed ratios of 1.0, 1.1, 1.2, and 1.3 were used to obtain bimodal grain structure with different organizational states. Microstructural analysis unraveled that the formation of the bimodal structure was a consequence of combined effect of precipitation-strengthening and the Zener pinning effect. The R3 exhibiting an asynchronous ratio of 1.2 exhibited an optimized bimodal grain structure characterized by smaller recrystallized grains and a more homogeneous distribution. This phenomenon underscored the beneficial impact on enhancing strength and alleviating the anisotropic behavior.

Carbide-free nano-bainite steel holds significant potential for applications in mining as an ultra-high strength wear-resistant steel, but the wear mechanisms are not yet fully understood. This study aims to explore the effect of austempering temperature on microstructure, mechanical properties and wear resistance of carbide-free nano-bainitic steel. The relationship between the content and mechanical stability of retained austenite (RA) and mechanical properties was investigated to elucidate the mechanisms of pin-on-disc wear and impact wear. The results show that in the range of Ms∼300 °C, the phase transformation incubation period and completion time significantly increase with decreasing austempering temperature, which is mainly related to higher phase transformation driving force resulting from a larger subcooling degree and slow growth of carbon content in RA. Optimal strength and hardness of 2099 MPa and 615 HBW, respectively, are achieved after austempering at 210 °C for 83 h. Austempering at 270 °C for 11 h yields the best balance of strength and plasticity, with a strength-plasticity product of 28.8 GPa%. The micro-cutting mechanism of pin-on-disc wear leads to serious weight loss. The RA with poor mechanical stability accelerates the stress-induced martensite transformation, while internal stress support due to volume expansion reduces the penetration depth of abrasive particles. The weight loss in impact wear is primarily caused by shedding due to fatigue failure. With the increase of RA content and mechanical stability, the persistent stress-induced martensite transformation effectively hinders micro-crack propagation, thereby enhancing impact wear resistance.

We elucidate here profound effects of quenching and partitioning process after partial (IQP) or full austenitization (FQP) on the microstructure and mechanical properties of high strength hot-dip galvanized steel with high Al and low Si content. The results showed that the FQP steel contains 3.38% vol.% retained austenite with mainly film-like morphology, while the IQP steel mainly consists of 6.44–8.13 vol.% retained austenite with block shape. Additionally, retained austenite in the IQP sample transforms into martensite more easily duo to higher nucleation rate, which means lower austenite stability. A combination of various austenite grain size and morphology make the TRIP effect throughout the whole deformation process and increased the elongation for IQP sample. When the steel was treated by annealing temperature of 967 °C, partitioning temperature and time of 460 °C and 100 s respectively, the steel exhibited excellent mechanical properties with tensile strength of 754 MPa, total elongation of 27% and the product of strength and ductility exceeds 20 GPa%. The proposed content in the present study demonstrates compatibility with the continuous hot-dip galvanizing process.

The present work aims to study the effect of quenching temperature on phase transformation, microstructural characteristics and mechanical properties of a novel Cu–Ni bearing Q&P steel based on combining the experiments and modeling. In this study, a new constrained carbon equilibrium (CCE) model was proposed by considering the effect of intercritical ferrite and multi-element synergistic partitioning. The optimum quenching temperature and corresponding retained austenite (RA) fraction can be reflected in the new CCE model, which is in good agreement with XRD results. Furthermore, the significant role of fresh martensite in the tensile strength and fracture behavior were elucidated. As the volume fraction of fresh martensite increases from 9.8% to 57.8%, the strengthening contribution to the tensile strength increases from 13.7% to 68.8%. Meanwhile, the fracture mechanism transforms from ductile into quasi-cleavage fracture due to an increased amount of fresh martensite and the absence of tempered martensite. The superior mechanical properties, ultimate tensile strength of 1152 MPa, yield strength of 797 MPa, total elongation of 25%, and the product of strength and elongation (PSE) up to 28.8 GPa% were obtained in QP-200, which is largely due to the combined effect of the better deformation ability of a considerable amount of tempered martensite and the continuous TRIP effect provided by multiscale RA with different morphologies.

The objective of this study is to establish a quantitative relationship between the strengthening effects of different phases and yield strength, and also to gain a new insight into the relationship between crystallographic variants and impact toughness in the simulated inter-critical heat affected zone (ICHAZ) of 690 MPa grade thick-plate medium Mn steel. The results indicated that the heterogeneous microstructure consisted of completely tempered martensite (CTM), partly tempered martensite (PTM), fresh martensite (FM) and retained austenite (RA) was formed in ICHAZ. As the peak temperature increased from 700 to 800 °C, the relative percentage of FM significantly increased from 5.5 to 79.2 vol %, while that of RA, PTM and CTM decreased from 20.3 to 11.3 vol %, from 65.5 to 9.3 vol %, and from 8.7 to 0.2 vol %, respectively. The yield strength correspondingly increased from 800 to 1200 MPa as the peak temperature rose due to the increasing contribution of various strengthening mechanisms of distinct phases. Furthermore, impact toughness decreased paradoxically from 184.9 to 47.8 J with increasing density of high angle boundary (HAB) from 0.806 to 1.47 μm−1 and decreasing size of prior austenite grains (PAGs) from 12.5 to 4.1 μm because of the variable crack propagation path induced by different variant selection mechanisms. In concluding, this research proposed a feasible strategy to improve the welding performance of medium Mn steel.

The evolution of shear band (SB) and texture in strip-cast electrical steel was studied in order to investigate the crystallographic character of SB and its influence on recrystallization texture. Shear deformation in the form of SB played an effective role in tailoring the local lattice rotation to accommodate heavy deformation in the case of inhomogeneous deformation under conditions of initial coarse grain in strip-cast electrical steel. During rolling process, η-oriented SBs appeared at different stages of deformation as a result of geometric softening. Cube-oriented SB was mainly retained from initial exact Cube grain at early stage of shear deformation and Goss-oriented SB was newly developed during the formation of γ-fiber matrix after heavy deformation. {210} SB formed as a transition form among different SBs. Crystal rotation inside SB, SB dimension and SB boundary misorientation progressed in proportion to cold rolling reduction. It is demonstrated that crystallographic direction of SBs tended to be parallel to under the driving force of reduction in stored energy to maintain relatively quasi-stable state. Low orientation gradient as well as relatively low density of dislocations was developed within SB region, while high orientation gradient formed between SB region and surrounding matrix. Interestingly, the η-oriented cell blocks inside SBs provided preferential nucleation sites for Goss and Cube texture at early stage of recrystallization, which significantly contributed to beneficial recrystallization texture in a manner of texture inheritance and superior magnetic properties (B50 was as high as 1.81 T, and P1.5/50 was as low as 4.0 W/kg).

Different scale microstructures at different scales were obtained by one and two-step isothermal transformation. The deformation behavior and stability of retained austenite under the action of nanoindentation stress was investigated in these different microstructures. The results demonstrate that the boundary fraction and density of the K–S relationship increase significantly after two-step isothermal transformation, promoting the nucleation of bainite and resulting in a refined microstructure. Changes in the nano-hardness as a function of indentation depth and morphology confirmed the quantity of RA transformation and the composite deformation/transformation behavior of blocky martensite/retained austenite (M/RA). A higher dislocation density is enriched in RA with higher KAM with the two-step isothermal process. This resulted in higher stability to delay the occurrence of the pop-in phenomenon. The stability of RA is highly dependent on the crystal orientation, with (001) oriented RA having an increased propensity to undergo mechanical transformation to martensite, with additional pop-ins generated during loading.

Titanium (Ti)/stainless steel weld joints with copper (Cu) transition layer thicknesses of 0.2, 0.4, and 0.6 mm were studied through combined experimental and finite element methods. Then, the formation mechanism and distribution of the Ti–Cu compounds (intermetallic compounds, IMCs) were analyzed. The results showed that the thicknesses of the Ti–Cu IMCs decreased with increasing thickness of the Cu layer, and the tensile strength and elongation increased when welding was conducted under a ring oscillation laser with a spot diameter of 0.3 mm, an oscillating diameter of 0.1 mm, a welding speed of 220 cm/min, and a laser power of 2000 W. In the 0.2 and 0.4 mm Cu layers, Ti–Fe IMCs appeared in the welds, and the thicknesses of the Cu–Ti IMC layers were 60 and 30 μm, respectively. When the 0.6 mm Cu transition layer was used, a pure Cu layer appeared in the weld, which effectively inhibited the formation of Ti–Fe IMCs. The thickness of the Cu–Ti IMC layer was reduced owing to the faster cooling rate at the Cu–Ti interface. Furthermore, the Cu4Ti phase was distributed in a thin area of 15 μm, which improved the mechanical properties of the welded joint and allowed the tensile strength to reach 330 MPa. Compared to the welding joints with Cu layer thickness less than or equal to laser irradiation width, the joints with Cu layer thickness larger than laser irradiation width have a significantly improved tensile strength.

The sub-micron bimodal grain structure of 301 austenitic stainless steel stripe obtained by phase reversion process, and the relationship between grain size distribution and mechanical properties was described in this study. With the change of annealing heating rate under the industrial condition 10 °C/min, the bimodal grain size of submicron scale was obtained at 750 °C. By analyzing the micrograph of bimodal structure, it was found that the deformed martensite transformed to austenite through diffusion transformation, and the recovery austenite grain had obvious size distribution. It was found that the bimodal structure was easier to form under the condition of slow heating rate, and the ultra-fine grain materials with high performance were obtained by controlling the annealing parameter. The high-density grain boundary of ultrafine grain presents microvoid in the tensile fracture, and the streaks formed by the aggregation of holes can be observed, which is considered to be caused by deformation twinning. The large dimples in the coarse grain are observed, as the strain-induced martensite was formed in the large grain of the bimodal structure.

Regeneration of long-bone segmental defects remains a challenge for orthopedic surgery. Current treatment options often require several revision procedures to maintain acceptable alignment and achieve osseous healing. A novel hollow tubular system utilizing magnesium-strontium (Mg–Sr) alloy with autogenous morselized bone filled inside to repair segmental defects was developed. To improve the corrosion and biocompatible properties, two coatings, Ca–P and Sr–P coatings, were prepared on surface of the implants. Feasibility of applying these coated implants was systematically evaluated in vitro and in vivo, and simultaneously to have a better understanding on the relationship of degradation and bone regeneration on the healing process. According to the in vitro corrosion study by electrochemical measurements, greater corrosion resistance was obtained for Ca–P coated sample, and attributed to the double-layer protective structure. The cytotoxicity and alkaline phosphatase (ALP) assays demonstrated enhanced bioactivity for Sr–P coated group because of the long-lasting release of beneficial Sr2+. At 12 weeks post-implantation with Mg–Sr alloy porous device, the segmental defects were effectively repaired with respect to both integrity and continuity. In addition, compared with the Ca–P coated implant, the Sr–P coated implant was more proficient at promoting bone formation and mineralization. In summary, the Sr–P coated implants have bioactive properties and exceptional durability, and promote bone healing that is close to the natural rate, implying their potential application for the regeneration of segmental defects.

We elucidate here profound effects of quenching and partitioning process on the microstructure and mechanical properties of high strength hot-dip galvanized steel with high Al and low Si content. The results indicated that partitioning time significantly affected the morphology and volume fraction of retained austenite (RA). A change in austenite from film-like to blocky structure with the increase of partitioning time was observed. Increasing partitioning time also increased the volume percentage of RA up to 8.95 vol.%. Using annealing temperature of 967 °C, quenching temperature of 267 °C, partitioning temperature and time of 460 °C and 90 s respectively, the experimental steel exhibited excellent mechanical properties with tensile strength of 728 MPa, total elongation of 30% and the product of strength and ductility was 21,840 MPa%. The variation in strength, elongation, strain-hardening behavior and phase transformation mechanism with different partitioning time is discussed in relation to the RA stability.

In the present study, a zinc phosphate conversion coating was successfully deposited onto steel fiber surface to improve the corrosion resistance of steel fiber reinforced concrete. An understanding of phosphating temperature–coating properties was established through potentiodynamic polarization curves and electrochemical impedance spectroscopy analysis, in conjunction with scanning electron microscope (SEM) and X-ray diffraction (XRD). The results showed that increasing the phosphating temperature could promote the formation of coating, the best bath efficiency and corrosion behavior was observed at 75 °C. Also, the potential-time curves during phosphate treatment were used to elucidate the mechanism of film formation, and indicated that higher temperatures minimized the time of film formation. But at higher phosphating temperature (85 °C) was detrimental in causing matrix corrosion and grain growth. This lead to increase of porosity and decrease of corrosion resistance. Furthermore, the corrosion behavior, interface properties and bending toughness of zinc phosphate coated steel fiber-reinforced concrete in 3.5% NaCl solution was investigated and compared with general steel fiber concrete.

The static recrystallization behavior of a low-density δ-ferritic steel containing 4 wt% Al was investigated by means of hot compression tests. The fractional softening obtained from the interrupted hot compression tests was used as a measure of the combined effect of static recovery and recrystallization. The single-pass hot compression tests followed by holding were performed to reveal the degree of static recrystallization. The experimental results indicated that the onset of static recrystallization occurred at a fractional softening of 0.6, which was lower than that of other ferritic steels. Moreover, the nucleation of recrystallization was dominated by strain induced grain boundary migration. First-principles calculations were performed to verify the effect of Al on recovery, which was influenced by stacking fault energy. It was found that the stacking fault energy of the steel is 105 mJ/m2, much lower than that of pure iron, which explained the suppressed recovery and correspondingly the promoted recrystallization. Based on the experimental results, a new multi-pass hot rolling process with on-line annealing was developed. The grain size of the low-density δ-ferritic steel was refined to 85 μm by static recrystallization and the mechanical properties can meet the requirement for hull steels. Keywords: δ-Ferrite steel, Recovery, Recrystallization, Stacking fault energy

Isothermal treatment affects the microstructural evolution and the precipitation behavior of high-strength low alloy (HSLA) steels. In this regard, thermal simulation of different isothermal treatment temperatures was adopted by using a thermomechanical simulator. The results showed that hardness reached the maximum value at 600 °C holding temperature, which was related to a finer grain structure and granular bainite. The strengthening effect of precipitates was remarkable due to the combination of small particle size and small interparticle spacing. It is presumed that the precipitation started after 600 s at 600 °C. Precipitation strengthening continued to exist, even though coarsening of ferrite grains led to softening phenomena when the specimen was isothermally held at 750 °C, which led to relatively high hardness. The precipitates were fcc (Ti, Nb) (N, C) particles, and belonged to MX-type precipitates. Average size of precipitates increased from 3.14 to 4.83 nm when the specimens were isothermally held between 600 °C and 800 °C. Interparticle spacing of precipitates also increased with increasing isothermal treatment temperatures. These led to a reduction in precipitation strengthening. At the same time the polygonal ferrite content increased and ferrite grain size got larger, such that the hardness decreased continuously.

In this study, a 304/20MnSi stainless-steel clad rebar was prepared by single-pass compression process using the MMS-200 Thermal Mechanical Simulator. The impact of different degrees of deformation and deformation temperature on microstructure evolution and the mechanical properties of stainless steel clad rebars were investigated. The study indicated that with the increase of the degree of deformation, the content of pearlite in a carbon steel matrix was increased, and the grains refined. The metallurgical bonding of the bonded interface was formed under high temperature and high extrusion force. With the increase of the deformation temperature, more bainite was obtained on the side of carbon steel, and the grain size increased. The obvious diffusion of Fe, Cr and Ni elements near the bonding interface resulted in higher microhardness of the stainless steel side and smaller microhardness of the carbon steel side. Moreover, the engineering stress-strain curves obtained by the tensile test showed that the plastic deformation of stainless steel and carbon steel was more coordinated. With the increase of deformation temperature and the degree of deformation, the tensile strength of the stainless steel clad rebar was as high as 690 MPa and the elongation was 26%, which was superior to the properties of the clad rebar prepared by other process parameters.

Low variability in mechanical properties is required for pipeline project designs to meet a strain-based design, which is used in regions of large ground movements. The objective of this study is to elucidate the influence of centerline segregation in continuously cast slab on variability in the mechanical property of pipeline steel, and controlling centerline segregation can meet the requirements of a strain-based design. Mannesmann rating method was used to evaluate the degree of segregation of two slabs and its effect on variability in mechanical properties of corresponding plates. Microstructural characterization indicated that bainite/martensite was formed in a segregated area where the content of C and Mn enriched. The mechanical property results indicated that controlling the degree of centerline segregation can reduce tensile strength variability and improve ductile-brittle transition temperature (DBTT).