Ferrite Grain Size Research Articles

Effect of Cr Element in Gas-Shielded Solid Wire for Oil and Gas Long-Distance Pipeline on Microstructure and Low Temperature Toughness of Weld.

In this paper, the influence of Cr element on the mechanical properties of welded joints of gas-shielded solid wire used in oil and gas long-distance pipelines was studied by means of tensile test, impact test, and hardness test, and the microstructure and crack propagation path of weld were characterized by means of an optical microscope, scanning electron microscope, and electron backscattering diffraction. The results show that with the addition of Cr, the strength and toughness of the weld are significantly improved, in which the tensile strength is increased from 607 MPa to 656 MPa, and the impact toughness is increased from 126.37 J to 223.79 J. The proportion of the ferrite side plate in the weld structure is reduced by about 20%, and the effective grain size of acicular ferrite is reduced by about 15%. The reason is that the addition of the Cr element improves the hardenability of the weld structure, inhibits the formation of the ferrite side plate, and promotes the effective refinement of acicular ferrite, which increases the proportion of high-angle grain boundaries in the weld, effectively hindering the crack propagation, improves the crack propagation work, and thus improves the strength and toughness of the weld.

Advanced Tuning of electric and dielectric properties in Ba0·5Ni0.5-xCdxFe2O4 spinel ferrites for high-frequency devices with barium dopants

Spinel ferrites are vital in developing next-generation technologies with potential applications covering the medical, electronic, and material industries. Nickel-cadmium spinel ferrites are soft magnetic materials having lots of applications ranging from electronics and magnetic to sensors industries. Herein, we developed cadmium-substituted spinel ferrites (Ba0·5Ni0.5-xCdxFe2O4) and their nanocomposite with polymer polyvinyl alcohol (PVA). The results evidently indicate that the successfully developed Ba0·5Ni0.5-xCdxFe2O4, which also confirmed the loading of PVA onto the Ba0·5Ni0.5-xCdxFe2O4 materials. XRD results confirmed orthorhombic single-phase structure. The optimal particle size was 27.22 nm, and the minimum was 21.25 nm, respectively, as shown by Ba0·5Ni0·1Cd0·4Fe2O4 (x = 0.4) and PVA composite of Ba0·5Ni0·2Cd0·3Fe2O4 (x = 0.3). It was also observed that the composites showed lower particle size than nano-ferrites. Spectral analysis (Fourier Transform Infrared Spectroscopy) confirmed the formation of spinel ferrites by detecting the M − O bonds in the 400-800 cm−1 region. υ1 and υ2 were appeared in the range 560–609 cm−1 and 439-466 cm−1, corresponding to M-O. The average grain size of spinel ferrites was 0.55, 0.50, 1.03, and 1.31 μm for Ba0·5Ni0.5-xCdxFe2O4 (x = 0.1, 0.2, 0.3, and 0.4) spinel ferrites. Dielectric parameters revealed that these materials are suitable for high-frequency devices due to their low dielectric loss, high quality factor, short relaxation time, and diverse responses.

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

Abstract 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.

The microstructure and properties of high-strength structural steel for fruit auxiliary picking equipment

The microstructure and mechanical properties of high-strength structural steel used in the fruit-picking equipment were studied by using a metallographic microscope, a tensile tester, and an impact toughness tester. The results revealed that the structure of the experimental steels was a mixed microstructure of ferrite and pearlite, with an average grain size of ferrite of 12 μm. The yield strength was between 515-540 MPa and the tensile strength was 635-645 MPa. The impact toughness at room temperature reached over 200 J. The physical properties of the studied steels completely fulfilled the requirements of the high-strength structural steel for fruit auxiliary picking equipment. The new method cut down the tempering heat treatment process and decreased production costs.

Structure and mechanical properties anisotropy of a steel product manufactured by layer-by-layer electric arc wire 3D printing

The work presents the study of structure and mechanical properties anisotropy of a metal wall obtained using electric arc wire 3D printing (WAAM) with ER70S-6 wire. The layers were deposited in the protective gases of carbon dioxide and argon. As a result of structural studies, it was found that the internal structure of the model product in form of a wall can be divided into three zones. Repeated heating, cooling cycles and degree of accumulated heat influence the formation of different wall zones. As a result of rapid heat removal to the substrate during deposition of the first layers, the wall base (zone 1) contains large elongated grains with acicular ferrite structure. The wall middle part (zone 2) consists of ferrite-pearlite structure, which was formed as a result of recrystallization under conditions of repeated heating and cooling during 3D printing. The size of ferrite grains in zone 2 varies from 11 to 16.3 µm with increasing the number of layers. The gradual accumulation of heat during 3D printing led to the formation of structures in zone 3 under conditions of overheating and a reduced cooling rate. As a result, the wall upper part (zone 3) consists of large ferrite grains (up to 29.8 μm), sorbite, and a small proportion of Widemanstatten ferrite and acicular ferrite. It is shown that the most uniform level of mechanical characteristics (σ0.2 = 340 MPa, σu = 470 MPa, ε = 28 %) correspond to the samples cut from zone 2 in a direction parallel to 3D prin­ting direction. The samples cut in the vertical direction relative to 3D printing and from zone 3 show the lowest level of microhardness and mechanical characteristics (σ0.2 = 260 MPa, σu = 425 MPa, ε = 20 %).

Anisotropic response in mechanical and corrosion performances of UNS S31803 duplex stainless steel fabricated by laser powder bed fusion

Mechanical and corrosion behaviors perpendicular to build direction (XOY) and along build direction (XOZ) of UNS S31803 duplex stainless steel (DSS) fabricated by laser powder bed fusion (LPBF) were investigated. With the post-annealing, the steel exhibited a ferrite-austenite microstructure. For XOY plane, austenite displayed a primarily <101> texture while ferrite kept <001> preferred orientation, and mainly held a Nishiyama-Wassermann orientation relationship. However, for XOZ plane, austenite showed a mixed texture. The average size of ferrite grains was larger for XOZ plane than XOY. The austenite in XOY plane was mostly island morphology grains, while in XOZ plane, there were a large number of coarse columnar grains. A higher geometrically necessary dislocation density and proportion of low-angle grain boundaries on XOY plane were observed. The yield strength and ultimate tensile strength values perpendicular to build direction were higher than those along build direction. While the total elongation perpendicular to build direction was lower. Moreover, the XOY plane shows slightly better corrosion resistance than the XOZ.

Structure and properties of fiber-reinforced LiZn ferrite ceramics prepared via spark plasma sintering

In order to improve the matrix cracking of LiZn ferrite (LZF) ceramics during spark plasma sintering, fiber-reinforced LZF materials were prepared using silicon carbide nanofibers (SCFs) and alumina whiskers (AWs) as reinforcement phases. The effect of fiber reinforcement on the microstructure and mechanical, electrical, and magnetic properties of LZF was systematically studied. The results reveal that SCFs lead to an increase in grain size of SCF-reinforced LiZn ferrites (LZF-S) and a deterioration of grain morphology due to the transgranular behavior of SCFs. The increase in complex dielectric constant and dielectric loss, as well as the decrease in resistance of LZF-S, is due to the increase in Fe2+ concentration in ceramics and low resistance of SCFs. AW-reinforced LiZn ferrites (LZF-A) show the opposite effect due to the high porosity caused by AWs and the high resistance of AWs. LZF-S exhibits high fiber/matrix bonding strength, which permits the transfer of the load from the matrix to the fiber to be more effective, leading to a high fracture toughness (KIC) of 5.1 MPa m1/2. The bonding strength between fiber and matrix of LZF-A is low, but a large number of pores passivate the crack tip. At an SCF content of less than 2.5 wt%, the saturation magnetization (Ms) of LZF does not decrease obviously, but a small amount of AWs leads to a significant decrease in Ms (pure LZF: 83 emu·g−1; 1.0 wt% SCFs: 80 emu·g−1; 1.0 wt% AWs: 62 emu·g−1), which can be ascribed to the partial substitution of Fe3+ by nonmagnetic Al3+ ions. When the content of reinforcement phases is 1 wt%, the ferroresonance linewidth increases significantly due to the high porosity caused by the secondary phases (pure LZF: 234 Oe; 1.0 wt% SCFs: 2161 Oe; 1.0 wt% AWs: 1020 Oe).

RELATION BETWEEN DISCONTINUOUS PLASTIC FLOW AND STRAIN HARDENING OF THE LOW-CARBON STEEL

Purpose. Determination of the conditions for the disappearance of a section of intermittent flow and its effect on the strain hardening of low-carbon steel.&#x0D; Actuality. The use of low carbon steels with intermittent flow for stamping has a significant limitation. Based on this, the issues of the effect of ferrite grain size on the occurrence of intermittent flow are relevant for determining the optimal structural state of steels intended for deep drawing.&#x0D; Research methods. The structure of the samples was studied under a light microscope, the grain size of the ferrite was determined by the methods of quantitative metallography. The mechanical properties of steel were determined in tension at room temperature and a strain rate of 10-3 s-1. The characteristics of the initiation of plastic flow and the parameters of strain hardening were determined from the analysis of tension curves in logarithmic coordinates. In the region of uniform strain hardening, the deformation of the violation of the directly proportional relation lgσ-lgε was determined as the moment of formation of dislocation structures with a certain periodicity.&#x0D; Results. An increase in the grain size of ferrite is accompanied by a decrease in the region of intermittent flow and a shift in the moment of formation of a dislocation cellular structure towards small plastic deformations. The strain hardening rate in the region of uniform work hardening and the Lüders strain are related by an inversely proportional relationship. With excessively large grain sizes of ferrite, the difficulty of maintaining conditions for a uniform distribution of slip lines is one of the reasons for the disappearance of the intermittent flow region on the deformation curves of low-carbon steel.&#x0D; Scientific novelty. With an increase in the grain size of ferrite, the deformation of the beginning of the decay of a uniform distribution of dislocations into periodic structures is shifted to a decrease. At the same time, an increase in the ability of the metal to work hardening during the formation of a deformation band contributes to a decrease in the length of the intermittent flow section.&#x0D; Practical value. Determining the nature of the influence of the grain size of low-carbon steel ferrite on the characteristics of work hardening and the formation of dislocation periodic structures can be useful in determining the optimal structural state of steel for deep drawing.

Research on the design of new high-strength and high-toughness steel and its manufacturing parameters

The lightweight of the steel structure can be achieved by the high strength and high toughness of steel. In this paper, the composition design and optimal manufacturing process of the experimental steel were investigated. The fine and uniform microstructure of tempered sorbite can be obtained under the conditions of TMCP (Thermo Mechanical Control Process), 920 °C quenching, and 650 °C tempering for the steel by micro-alloying with Nb, V, and Ti. The steel has properties as follows: yield strength of 1015 MPa, tensile strength of 1076 MPa, elongation of 14.5%, and -40 °C AKV 81 J. The parameters of quenching and tempering heat treatment have a decisive influence on the size and distribution of ferrite grains and precipitates of tempered sorbite.

STUDY OF THE WELDABILITY OF STEELS OF DIFFERENT ALLOYING COMPOSITIONS FOR THE LARGE-DIAMETER PIPES TRANSPORTING HYDROGEN SULPHIDE-CONTAINING GAS

Increased resistance to brittle fracture in any zone of the welded joint is one of the main requirements for quality of steel pipes for transportation of the hydrogen sulphide gas, which determines the relevance of identification of the reasons of the impact toughness decrease of the welded joint area. The purpose of the study was to investigate the microstructure formation features of the seam zone of pipes made of low-carbon steels of different chemical composition. The effect of microstructure of the coarse-grained area of the heat affected zone on toughness of welded joint has been estimated by the laboratory thermal cycling testing complex Gleeble 3180. The samples with cross-section of 10×10×110 mm made of commercially available steel grades 04ХНДБ, 06ГНФБ and 05ХГБ (Russian Standard) with different contents of niobium and manganese were used as material for the research. To eliminate the effect of heating rate and holding time in austenitic region on the microstructure, the simulation was carried out in a cyclic mode with the same heating rate up to 1350 °С and subsequent cooling to 800 °С. The cooling rate in the interval 800–500 °C varied from 2 to 64 °C/s. The microstructure formation of the seam zone was studied by scanning electron microscopy and reflected electron diffraction. Despite the differences in chemical composition, the studied steels showed similar microstructure after simulation of the coarse-grained area of the heat-affected zone. For all the steels an increase in the share of rack bainite and the density of high angle boundaries with increasing cooling rate was found. It was established that at close values of average grain size of ferrite in the samples from steel 04ХНДБ the grain homogeneity is higher due to decrease in size and quantity of the largest grains. In specimens made of 05ХГБ and 06ГНФБ steel grades, the impact toughness of simulated coarse-grained area of the heat affected zone stabilizes at high level due to formation of more favorable dispersed structure when cooling rate increases more than 8 °C/s.

A micro mechanical study on tensile behavior of steel sheets with spheroidal cementite using crystal plasticity finite element method

In this research, we investigate the effects of various microstructural features on the behavior of spherical steel sheets, using the crystal plasticity finite element method. The cementite phase ratio, the grain sizes of ferrite and cementite, and the percentage of residual pearlite in the steel structure due to incomplete annealing are the major microstructural parameters studied in this work. A grain generator software has been developed to generate hexagonal ferrite grains as well as circular cementite particles distributed in the ferrite matrix. We use a hard coating with special properties as an intermediate layer around the cementite grains to simulate the contact between ferrite grains and cementite particles. This software can also assist with the preprocessing including boundary condition and load producing and introducing constitutive modeling relations. We use representative volume elements (RVEs) to simulate the microstructure of the steel sheet. The hardening parameters in the simulations are calibrated according to the available experimental data in the literature. Several RVEs were prepared and solved to investigate the effect of the thickness of this layer on material behavior. We can replicate the experimental results by setting the ratio of this thickness to the cementite particles’ diameter to be 0.25. Moreover, by reducing the size of ferrite grains and cementite particles, the strength of steel increases. On the other hand, increasing the ratio of the cementite phase increases the strength of steel but the uniform plastic deformation of the material is reduced. We also model and discuss the remaining pearlite content and its effect on the flow behavior of the material. Finally, the effect of the cementite precipitation location and its bimodal size distribution were discussed in this study.

Microstructural Characteristics and Hardness Enhancement of Super Duplex Stainless Steel by Friction Stir Processing

In the present study, microstructural evolution and hardness of the friction stir processed (FSPed) SAF 2507 super duplex stainless steel fabricated at a rotational speed of 650 rpm and a traverse speed of 60 mm/min were investigated. A scanning electron microscope (SEM) equipped with an electron backscatter diffraction (EBSD) detector was used to study the microstructure of the stir zone. The grain sizes of austenite and ferrite in the FSPed 2507 were found to be smaller (0.75 and 0.96 μm) than those of the substrate (6.6 and 5.6 μm) attributed to the occurrence of continuous dynamic recrystallization (CDRX) in both phases. Higher degree of grain refinement and DRX were obtained at the advancing side of the FSPed specimens due to higher strain and temperature. A non-uniform hardness distribution was observed along the longitudinal direction of the SZ. The maximum hardness was obtained at the bottom (407 HV1).

Investigation into the evolution of tribological white etching layers

When wheels and rails are in operation, their complicated and changeable contact conditions contribute to the formation of white etching layers on their contact surface. Martensite white etching layers (M-WELs) can induce the initiation of rolling contact fatigue (RCF) cracks, accelerating wheel/rail failure; Tribological white etching layers (T-WELs) can improve wear resistance. Therefore, to investigate the formation of T-WELs is to provide theoretical bases for developing surface strengthening technology. In order to delve into the mechanism for the formation of T-WELs, this paper conducted dry friction sliding wear experiment on ingot iron (equiaxed ferrite), D1 wheel steel (lamellar eutectoid ferrite) and FCB wheel steel (lath-like bainite-ferrite), and generalized the impacts of ferrite with different morphology and the second phase on the evolution of T-WELs. The result showed that T-WELs formed on the worn surfaces of all of these materials under the condition of sliding wear, and the evolution of T-WELs displayed similarities. Under the impact of dislocation slip mechanism, size reduction and dimension reduction were realized and finally ultrafine ferrite grains formed. But the second phase would increase the critical shear stress value of the transformation of ultrafine ferrite grains, hindering the formation of T-WELs and eventually affecting the size of ultrafine ferrite grains inside T-WELs.

Magnetic and dielectric properties of low-temperature sintered NiCuZn/CaTiO3 composite dual-performance materials

The effect of CaTiO3 addition on the sintering behavior, microstructure, and magnetic and dielectric properties of NiCuZn/CaTiO3 composite dual-performance materials was investigated. The crystal and microstructure were characterized by X-ray diffraction and scanning electron microscopy. The magnetic and dielectric properties were obtained by vibrating sample magnetometry and impedance analysis. The CaTiO3 ceramic could coexist with the NiCuZn ferrite. The atom substitution occurred in the composite materials, and the increment in CaTiO3 content led to a gradual decline in grain size of NiCuZn ferrite. The saturation magnetization and permeability decreased gradually as CaTiO3 content increased, while the coercivity increased gradually. Meanwhile, the cut-off frequency and dielectric constant of the composite materials incremented, and all samples presented optimized performance with the increased sintering temperature. The NiCuZn/CaTiO3 composite duel-performance materials exhibited excellent dielectric and magnetic properties. All in all, the addition of CaTiO3 weakened the magnetic properties of the dual-performance material, while its dielectric properties were enhanced. These results would be useful for further study on NiCuZn/CaTiO3 composite dual-performance materials.

Post-fire mechanical behavior of Q345 structural steel after repeated cooling from elevated temperatures with fire-extinguishing foam

This study aims to investigate the post-fire mechanical behavior of Q345 structural steel after being repeatedly cooled from elevated temperatures using fire-extinguishing foam. The Q345 structural steel specimens were first repeatedly cooled 1–3 times from target temperatures of 200 °C, 500 °C, 800 °C and 900 °C to ambient temperatures using fire-extinguishing foam, respectively. After being repeatedly heated and cooled, the Q345 structural steel specimens were then subjected to the uniaxial tensile tests to obtain their post-fire mechanical behavior. A Scanning Electron Microscope (SEM) was used to observe the tensile fracture surfaces of the representative specimens to analyze the different tensile fracture characteristics induced by repeated heating and cooling. Meanwhile, the changes in stress-strain curves, yield strength, ultimate tensile strength and ductility were discussed by comparing with the experimental data of Q345 structural steel in the as-received state. Furthermore, the changes in the macro-mechanical properties of Q345 structural steel after repeated heating and fire-extinguishing foam cooling were explained in combination with the fracture morphology and phase transformation. It is observed that Q345 structural steel was able to remain its initial mechanical properties unchanged after being repeatedly cooled from temperatures of 200 °C and 500 °C using fire-extinguishing foam. In contrast, significant reduction can be observed in the mechanical properties when Q345 structural steel was repeated cooled from temperatures of 800 °C and 900 °C using fire-extinguishing foam. The yield strength, ultimate strength and ductility significantly reduced with the increase in the number of heating and cooling cycles. When the Q345 structural steel was repeatedly cooling from 500 °C to 900 °C, the shape and size of ferrite grains increased sharply, whereas those of the pearlites decreased accordingly with the increase in the number of heating and cooling cycles.

Superior strength-ductility combination in a Ni-modified medium manganese lightweight steel with the determining role of phase transformation and recrystallization behaviour

The present work elucidates the complex interplay between the changes in the microstructural features and tensile properties in a Ni-modified medium manganese steel under the combined influence of cold rolling (CR) and annealing treatment. The annealing temperature within 973 K–1273 K showed a wide range of microstructures, varying from single phase ferrite in CR-973 K to duplex microstructure (ferrite and austenite) in CR-1073 K and to multiphase structure (ferrite, austenite and martensite) in CR-1173 K and CR-1273 K annealed conditions. Additionally, B2 (NiAl) and intermetallic carbide precipitates were also found to be heterogeneously distributed in ferrite matrix of the CR-973 K annealed specimen. However, these precipitates dissolved and concurrently resulted in evolution of the austenite phase at an annealing temperature of ≥1073 K. Apart from the phase transformation, the recrystallization fraction increased and grain size became coarser at higher annealing temperature. The presence of duplex phase microstructure, optimum grain size of austenite and ferrite, and co-occurrence of recrystallized and deformed grains resulted in excellent strength-elongation synergy in CR-1073 K annealed specimen in comparison to the other annealing variants. More importantly, the austenite to martensite transformation during tensile test promoted the transformation induced plasticity (TRIP) effect, which improved the work hardening and ductility of the CR-1073 K annealed specimen. In contrast, the presence of pre-existing martensite in CR-1173 K and CR-1273 K annealed specimens adequately masked the TRIP effect leading to predominant brittle failure in these specimens without rendering any considerable ductility.

Microstructural Characteristics and Strengthening Mechanisms of Ferritic–Martensitic Dual-Phase Steels: A Review

Ferritic–martensitic dual-phase (DP) steels are prominent and advanced high-strength steels (AHSS) broadly employed in automotive industries. Hence, extensive study is conducted regarding the relationship between the microstructure and mechanical properties of DP steels due to the high importance of DP steels in these industries. In this respect, this paper was aimed at reviewing the microstructural characteristics and strengthening mechanisms of DP steels. This review article represents that the main microstructural characteristics of DP steels include the ferrite grain size (FGS), martensite volume fraction (MVF), and martensite morphology (MM), which play a key role in the strengthening mechanisms and mechanical properties. In other words, these can act as strengthening factors, which were separately considered in this paper. Thus, the properties of DP steels are intensely governed by focusing on these characteristics (i.e., FGS, MVF, and MM). This review article addressed the improvement techniques of strengthening mechanisms and the effects of hardening factors on mechanical properties. The relevant techniques were also made up of several processing routes, e.g., thermal cycling, cold rolling, hot rolling, etc., that could make a great strength–ductility balance. Lastly, this review paper could provide substantial assistance to researchers and automotive engineers for DP steel manufacturing with excellent properties. Hence, researchers and automotive engineers are also able to design automobiles using DP steels that possess the lowest fuel consumption and prevent accidents that result from premature mechanical failures.

Microstructure Evolution and Mechanical Properties of Medium Carbon Martensitic Steel during Warm Rolling and Annealing Process.

The microstructure evolution and mechanical properties of medium carbon martensitic steel during the warm rolling and annealing process were studied by scanning electron microscope (SEM), electron back scattering diffraction (EBSD), and electronic universal testing machine. The results showed that the microstructure of ferrite matrix with mass dispersive cementite particles was obtained by decomposition of martensitic in medium-carbon martensitic steel after warm rolling. The grain size of ferrite was ~0.53 μm, the yield strength and tensile strength were 951 MPa and 968 MPa, respectively, and the total elongation rate was 11.5% after warm rolling at 600 °C. Additionally, after the next 4 h of annealing, the grain size of ferrite and particle size of cementite increased to ~1.35 μm and ~360 nm and the yield strength and tensile strength decreased to 600 MPa and 645 MPa, respectively, with a total elongation increases of 20.9%. The strength of the material increased with increasing strain rate in tension, and the yield-to-tensile strength ratio increased from 0.92 to 0.94 and maintained good plasticity.

Micro-laminated and ultrafine-grained dual-phase steel plates generated via intercritical rolling followed by water quenching

In this study, a kind of heterogeneous structured dual-phase (DP) steels with micro-laminated and ultrafine-grained microstructures are obtained via intercritical rolling followed by water quenching. The microstructures are tailored by changing rolling temperature, i.e., reheating temperature. We find that with the decrease in reheating temperature from 810 °C to 750 °C, martensite aspect ratio increases from 4.5 ± 3.5 to 6.1 ± 2.9, and martensite long axis elongates from 1.72 ± 0.09 μm to 2.86 ± 0.98 μm. Meanwhile, the size of ferrite grain surrounded by high-angle grain boundaries refines from 1.98 ± 2.32 μm to 1.47 ± 2.14 μm. The yield strength improves 78 MPa from 432 ± 3 MPa to 510 ± 6 MPa, and the tensile strength increases 114 MPa from 745 ± 5 MPa to 859 ± 1 MPa. An important find is that the strain hardening rate increases abnormally with the refinement in ferrite grain due to enhanced hetero-deformation induced (HDI) hardening. So, the downtrend of tensile ductility varies with the improvement in strength is nearly stopped when reheating temperature reduces from 780 °C to 750 °C. Therefore, better comprehensive mechanical properties of DP steels are obtained compared with the approximately equiaxed and coarse-grained DP steel plate. The microstructure evolution mechanism is discussed from the perspective of deformation-induced ferrite transformation and ferrite dynamic recovery. The change of HDI hardening is related to the microstructure feature of the tested DP steels.

Quantitative analysis of microstructure and mechanical properties of Nb–V microalloyed high-strength seismic reinforcement with different Nb additions

Abstract HRB500E seismic steel bars are mainly used in high-rise buildings near the seismic zone. The influence of different niobium contents (0–0.023%) on the microstructure and mechanical properties of HRB500E seismic reinforcement was studied. Results showed that the grain size of ferrite was between 3.6 and 8.3 μm when only V was added. Meanwhile, as the niobium content increases, the ferrite particles are further refined. After adding niobium, the grain contribution increased by 9%. The addition of niobium significantly refined the grain size of HRB500E seismic reinforcement. The second-phase nano-elliptic precipitate is NbC. The precipitated phase is dispersed on the grain boundary and the matrix, and the dislocation density on the matrix promotes the precipitation of NbC particles along the dislocation line. The second-phase precipitation of niobium can form an effective pinning effect and then refine the pearlite spacing. The microhardness and the tensile strength also significantly improved. The yield strength increased from 509 to 570 MPa.