Research Progress of Corrosion Induced by Second-Phase Particles in Microalloyed High-Strength Rebars—Review

The research progress surrounding second-phase particle-induced corrosion has been expounded through extensive work, including local corrosion (pitting corrosion, crevice corrosion, stress corrosion) of Al2O3, (RE)-AlO3, CaS, MnS, NbC, and other particles in microalloyed high-strength rebars. By summarizing the local corrosion mechanism of these particle-induced rebars, this review further explores the fact that these particles play an inducing role in the local corrosion of microalloyed high-strength rebars, which has guiding significance for research on the induced corrosion of microalloyed high-strength rebars

Effect of final cooling temperature on the microstructure and mechanical properties of high-strength anti-seismic rebar

Rebar is an important material in the major structural engineering, and its fine structure has a very important effect on the performance of the rebar. In this work, the Gleeble-3800 thermal simulator was used to simulate and control the final cooling temperature process to explore the effect of the precipitation behavior of the microalloying elements on the microstructure and mechanical properties of the rebar. The electron backscatter diffraction (EBSD), high-resolution transmission electron microscope (TEM), and universal tensile testing machine were used to characterize the microstructural transformation and mechanical properties of high-strength anti-seismic rebar. The results shows that under the conditions of different final cooling temperatures, the microstructure of the rebar were mainly composed of ferrite and pearlite. When the final cooling temperature decreased from 750 °C to 650 °C, the ferrite grain size decreased from 0.01237 mm to 0.00678 mm and the pearlite lamellar spacing decreased from 0.226 μ m to 0.114 μ m. The EBSD results found that the most of ferrite grains with larger misorientation angle (20° ∼ 60°) formed by the different austenite grains. The TEM results found that the main precipitates were (Nb, Ti, V) C, which precipitated on the ferrite matrix, and the shapes were oval, and the average particle sizes were about 20 ∼ 30 nm. When the final cooling temperature was 650 °C, the tensile strength and yield strength of the rebar reached 712.94 MPa and 562.97 MPa, respectively, and strength yield ratio was 1.27. With the decreases in the final cooling temperature, the tensile strength and yield strength of the rebar gradually increased

Effect of Thermal Simulation Process on Microstructure of Seismic Steel Bars

Thermal deformation has a significant influence on the microstructure of high-strength antiseismic steel. The effect of hot deformation on the microstructure of experimental steel was studied by the Gleeble-3800 thermal simulator. The microstructure of the steel was characterized by the metallographic microscope, microhardness, tensile test, field emission scanning electron microscope, electron backscatter diffraction, and high-resolution transmission electron microscope. The results show that the core microstructure of the test steel is composed of polygonal ferrite and lamellar pearlite. The test steel is mainly ductile fracture. Tensile strength and hardness increase with the decrease of temperature. At 650 °C isothermal temperature, the ferrite distribution was uniform, the average grain size was 7.78 μm, the grain size grade reached 11, the pearlite lamellar spacing was 0.208 μm, and the tensile fracture was distributed with uniform equiaxed dimples. Polygonal ferrite grain boundaries have high density dislocations that can effectively block the initiation and propagation of cracks. However, there are some low dislocation boundaries and subgrain boundaries in ferrite grains. Precipitation strengthening is mainly provided by fine precipitates of V-rich carbonitride in experimental steel. The precipitates are round or narrow strips, about 70–100 nm in size, distributed along ferrite grain boundaries and matrix

Effect of Complex Strengthening on the Continuous Cooling Transformation Behavior of High-Strength Rebar

The effects of niobium and composite strengthening on the phase transformation characteristics and precipitation behavior of continuous cooling transformation of high-strength rebar during thermal deformation and subsequent cooling were investigated. The results show that when the cooling rate was within 0.3–5 °C/s, ferrite transformation and pearlite transformation occurred in the experimental steels. The Nb content increased to 0.062 wt.%, and the starting temperature of the ferrite transformation decreased. Meanwhile, the ferrite phase transformation zone gradually expanded, and the pearlite phase transformation zone gradually narrowed with the increase in the cooling rate. When the cooling rate was 1 °C/s, bainite transformation began to occur, and the amount of transformation increased with the increase in the cooling rate. It was found that the main precipitates in the experimental steels were (Nb, Ti, V)C, with an average particle size of about 10–50 nm. When the Nb content was increased to 0.062 wt.% and the cooling rate was increased to 5 °C/s, the ferrite grain size was reduced from 19.5 to 7.5 μm, and the particle size of the precipitate (Nb, Ti, V)C could be effectively reduced. The strength of the steel was significantly improved, but the elongation of the steel was reduced. However, the comprehensive mechanical properties of 0.062 wt.% Nb experimental steel was significantly improved at a cooling rate of 5 °C/s

Influence of Nb/V on the corrosion behavior of high-strength anti-seismic rebar in marine environments

Abstract In this study, the immersion test, surface analysis, cross-section analysis, quantitative analysis and electrochemical analysis were used to study the influence of Nb/V on the corrosion behavior of high-strength anti-seismic rebar in marine environments. The corrosion results clarified that the addition of Nb/V improved the corrosion resistance of the rebar, thereby reducing the corrosion rate of the rebar and improving the stability of corrosion layers. Firstly, the addition of Nb/V promoted the transformation of unstable Fe oxyhydroxides to stable Fe oxyhydroxides in the surface corrosion layers of the rebar, thus increasing the α/(β + γ) ratio, corrosion potential and total impedance value. Secondly, the addition of Nb/V induced the formation of Nb oxides and V oxides in the surface corrosion layers of the rebar, and the existence of these oxides repaired the surface defects of corrosion layers, thus enhancing the corrosion resistance performance of surface corrosion layers of the rebar

Effect of Controlling Nb Content and Cooling Rate on the Microstructure, Precipitation Phases, and Mechanical Properties of Rebar

Seismic anti-seismic rebar, as materials for supporting structures in large buildings, need to have excellent mechanical properties. By increasing the Nb content and controlling the cooling rate, the microstructure and precipitation behavior of the steel are adjusted to develop seismic anti-seismic rebar with excellent mechanical properties. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and a universal tensile testing machine were used to characterize the microstructure, precipitation phases, and mechanical properties of the experimental steels. The results show that the ferrite grain size, pearlite lamellae layer (ILS), and small-angle grain boundaries (LAGB) content of the high-Nb steels decreased to 6.39 μm, 0.12 μm, and 48.7%, respectively, as the Nb content was increased from 0.017 to 0.023 wt.% and the cooling rate was increased from 1 to 3 °C·s−1. The strength of the {332}α texture is the highest in the high-Nb steels. The precipitated phase is (Nb, Ti, V)C with a diameter of ~50 nm, distributed on ferrite, and the matrix/precipitated phase mismatch is 8.16%, forming a semicommon-lattice interface between the two. The carbon diffusion coefficient model shows that increasing the Nb content can inhibit the diffusion of carbon atoms and reduce the ILS. The yield strength of the high-Nb steel is 556 MPa, and the tensile strength is 764 MPa

Effect of isothermal transformation temperature on the microstructure, precipitation behavior, and mechanical properties of anti-seismic rebar

The synergy between Nb/Ti strengthening and precise isothermal transformation temperatures has resulted in the optimal microstructure and mechanical properties in Nb/Ti anti-seismic rebars. The microstructure, precipitates, and mechanical properties of experimental steels at different isothermal transformation temperatures were characterized using scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction, and universal tensile testing machine. As the isothermal transformation temperature decreased from 700 to 600°C, the ferrite grain size, pearlite interlamellar spacing, and carbon diffusion coefficient of the experimental steel decreased to 6.69 μm, 0.17 μm, and 0.14 cm2·s−1, respectively, while the yield strength and tensile strength increased to 584 and 714 MPa, respectively. At 600°C, the pearlite transformation rate in the experimental steel was the fastest, accompanied by the most rapid precipitation kinetics. The precipitates were (Nb, Ti)C of approximately 50 nm in size, with a mismatch of 14.24% at the matrix/precipitate interface and a screw dislocation angle of 2.07°. The presence of screw dislocation steps may facilitate nucleation of Nb/Ti precipitates, forming semi-coherent interfaces

Influence of V on the Microstructure and Precipitation Behavior of High-Carbon Hardline Steel during Continuous Cooling

High-carbon hardline steels are primarily used for the manufacture of tire beads for both automobiles and aircraft, and vanadium (V) microalloying is an important means of adjusting the microstructure of high-carbon hardline steels. Using scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM), the microstructure and precipitation phases of continuous cooled high-carbon steels were characterized, and the vanadium content, carbon diffusion coefficient, and critical precipitation temperature were calculated. The results showed that as the V content increased to 0.06 wt.%, the interlamellar spacing (ILS) of the pearlite in the experimental steel decreased to 0.110 μm, and the carbon diffusion coefficient in the experimental steel decreased to 0.98 × 10−3 cm2·s−1. The pearlite content in the experimental steel with 0.02 wt.% V reached its maximum at a cooling rate of 5 °C·s−1, and a small amount of bainite was observed in the experimental steel at a cooling rate of 10 °C·s−1. The precipitated phase was VC with a diameter of ~24.73 nm, and the misfit between ferrite and VC was 5.02%, forming a semi-coherent interface between the two. Atoms gradually adjust their positions to allow the growth of VC along the ferrite direction. As the V content increased to 0.06 wt.%, the precipitation-temperature-time curve (PTT) shifted to the left, and the critical nucleation temperature for homogeneous nucleation, grain boundary nucleation, and dislocation line nucleation increased from 570.6, 676.9, and 692.4 °C to 634.6, 748.5, and 755.5 °C, respectively

Effect of cooling rate and Nb synergistic strengthening on microstructure and mechanical properties of high-strength rebar

Rebar is an extremely important building material. The cooling rate and the presence of niobium (Nb) element are key factors influencing the overall performance of rebars. In this work, the high-strength rebar’s microstructure, precipitated phase, and mechanical properties were characterized using scanning electron microscopy, transmission electron microscopy, HVS-1000 hardness tester, and MTS810 universal tensile testing machine. The results showed that a shift in cooling rate from 0.3 to 3°C·s−1 resulted in noticeable changes in the microstructures of rebars, particularly between Nb-free and Nb-containing variants. In the case of Nb-containing rebars, there was an increase of 8.26% in the proportion of pearlite, along with a decrease of 10.63 μm in the average grain size of ferrite. Furthermore, the lamellar spacing of pearlite experienced a decrease of 0.0495 μm, the proportion of low-angle grain boundaries saw an increment of 4.13%, and the size of the precipitated phase (Nb, Ti, V) C reduced by 18.9 nm. These changes collectively led to a significant increase in hardness (98.56 HV), yield strength (179.02 MPa), and ultimate strength (199.43 MPa). The resultant fracture morphology manifested as a dimple pattern

Effect of Nb content and cooling rate on the microstructure transformation and mechanical properties of hot-rolled rebar

In this study, the synergistic effect of the Nb content and cooling rate on the microstructural transformation, precipitation behavior, and mechanical properties of hot-rolled rebar was studied using a Gleeble-3800 thermal simulation machine, scanning electron microscopy, transmission electron microscopy, and a universal tensile testing machine. The results showed that the synergistic effect of Nb content and cooling rate could optimize the microstructure and precipitation behavior of rebar, promote ferrite and pearlite transformation, and reduce the ferrite grain size, pearlite lamellar spacing, and precipitated phase size, thereby improving the strength and plasticity of Nb-containing rebar. When the cooling rate was 3 °C/s and the Nb content was 0.035 wt%, the ferrite grain size decreased to 21.45 μm, the pearlite lamellar spacing refined to 0.222 μm, and the particle size of the (Nb, Ti, V) C precipitates reduced to 5 nm. Accordingly, the yield and tensile strength increased to 779 ± 6 MPa and 973 ± 8 MPa, respectively, the elongation at fracture increased to 16.35%, and the microhardness increased to 311.39 ± 8.0 HV