Showing papers on "Microalloyed steel published in 2021"

Determination of Non-Recrystallization Temperature for Niobium Microalloyed Steel.

Abstract: In the present investigation, the non-recrystallization temperature (TNR) of niobium-microalloyed steel is determined to plan rolling schedules for obtaining the desired properties of steel. The value of TNR is based on both alloying elements and deformation parameters. In the literature, TNR equations have been developed and utilized. However, each equation has certain limitations which constrain its applicability. This study was completed using laboratory-grade low-carbon Nb-microalloyed steels designed to meet the API X-70 specification. Nb- microalloyed steel is processed by the melting and casting process, and the composition is found by optical emission spectroscopy (OES). Multiple-hit deformation tests were carried out on a Gleeble® 3500 system in the standard pocket-jaw configuration to determine TNR. Cuboidal specimens (10 (L) × 20 (W) × 20 (T) mm3) were taken for compression test (multiple-hit deformation tests) in gleeble. Microstructure evolutions were carried out by using OM (optical microscopy) and SEM (scanning electron microscopy). The value of TNR determined for 0.1 wt.% niobium bearing microalloyed steel is ~ 951 °C. Nb- microalloyed steel rolled at TNR produce partially recrystallized grain with ferrite nucleation. Hence, to verify the TNR value, a rolling process is applied with the finishing rolling temperature near TNR (~951 °C). The microstructure is also revealed in the pancake shape, which confirms TNR.

Effects of chromium on the microstructure and hot ductility of Nb-microalloyed steel

Abstract: It is well-known that the surface quality of the niobium microalloy profiled billet directly affects the comprehensive mechanical properties of the H-beam. The effects of chromium on the γ/α phase transformation and high-temperature mechanical properties of Nb-microalloyed steel were studied by Gleeble tensile and high-temperature in-situ observation experiments. Results indicated that the starting temperature of the γ→α phase transformation decreases with increasing Cr content. The hot ductility of Nb-microalloyed steel is improved by adding 0.12wt% Cr. Chromium atoms inhibit the diffusion of carbon atoms, which reduces the thickness of grain boundary ferrite. The number fractions of high-angle grain boundaries increase with increasing chromium content. In particular, the proportion is up to 48.7% when the Cr content is 0.12wt%. The high-angle grain boundaries hinder the crack propagation and improve the ductility of Nb-microalloyed steel.

On the correlation among continuous cooling transformations, interphase precipitation and strengthening mechanism in Ti-microalloyed steel

Abstract: The correlation among continuous cooling transformations, interphase precipitation and their strengthening effect was investigated quantitatively in a Ti- microalloyed steel subjected to a two-stage controlled rolling and cooling process. Interphase precipitates, strain-induced and random precipitates were observed, of which the latter two mainly nucleated on dislocations. Interphase precipitation occurred only at low cooling rates of 0.1–0.5 °C/s and the highest density of nanoscale interphase precipitates with the smallest inter-sheet spacing was obtained at 0.5 °C/s. Subsequently increasing the cooling rate resulted in inhibited precipitation, but the dislocation density gradually increased and the grains were refined. Based on microhardness analysis, the highest peak consisting of 290 HV was located at 0.5 °C/s, where the best strengthening effect was obtained mainly due to interphase precipitation. When the continuous cooling rate exceeded 3 °C/s, dislocation and grain refinement strengthening gradually became more significant, transitioning from interphase precipitation to dislocation and grain refinement strengthening with increasing cooling rate.

Fatigue Properties of Resistance Spot Welded Dissimilar Interstitial-Free and High Strength Micro-Alloyed Steel Sheets

Abstract: This study aims to investigate the influence of nugget diameter on tensile-shear and fatigue properties of the dissimilar resistance spot-welded joints of interstitial free and high strength niobium microalloyed steel sheets. The spot-weldings are done at currents of 7 kA and 9 kA at welding time of 300 ms with electrode force of 2.6 kN. Fatigue tests are done at different load amplitudes, considering the maximum tensile-shear load bearing capacity of the joints. The tests are interrupted before the final failure to understand the crack initiation and propagation paths. The study is supplemented by the microstructural and detailed fractographic analyses. The results indicated that the fatigue strength of the spot-welds increased with a decrease in nugget diameter. This could be attributed to the higher microhardness of heat affected zone (HAZ), presence of compressive residual stress and lower variation in thickness reduction on the IF steel side, welded at 7 kA. However, the tensile-shear load bearing capacity of the spot-welds increased with increase in the nugget diameter. The failure of the spot-welded joint under static loading initiates from the HAZ/base metal interface of the IF steel side. Nevertheless, under cyclic loading crack initiates from the notch root at the interface of two sheets lying in the HAZ of IF steel side. Fractographic investigations indicate intergranular fracture features in combination with striations, secondary cracks on the IF steel side under cyclic loading. Fracture surfaces of the specimens failed under static loading shows however, shear dimples on the IF steel side.

High cycle fatigue performance, crack growth and failure mechanisms of an ultrafine-grained Nb+Ti stabilized, low-C microalloyed steel processed by multiphase controlled rolling and forging

Abstract: An effort has been made to examine the high cycle fatigue (HCF) properties including crack propagation characteristics and related fracture mechanisms of submicron-grained (SG) Nb + Ti stabilized low C steel processed through advanced multiphase-controlled rolling (MCR) and multiaxial forging (MAF). The HCF and other mechanical properties have been correlated with microstructural features characterized by light optical (LOM), transmission electron (TEM) and scanning electron microscopy (SEM), aided with electron backscatter diffraction (EBSD). TEM analysis near the fracture zones of the fatigue tested samples and corresponding fractographic analysis corroborated well in explaining the improved fatigue life of the SG steel. The fatigue strength was found to have a linear relationship with the tensile strength in both types of processed samples. The fatigue strength of the forged specimens was estimated to be nearly twice than that of the untreated annealed steel, demonstrating significantly different fracture characteristics. Intergranular fracture is found to be dominant in the rolled/forged specimens, in comparison to the transgranular fracture observed in the as-received steel. Such variances in fatigue strength and fracture characteristics have been endorsed to their microstructural constituents. Superior combinations of yield strength (YS), tensile strength (UTS), elongation (% El.) and high cycle fatigue strength (σf) (YS = 1027 MPa, %El. = 8.3%, σf = 355 MPa) were obtained in multiphase-controlled 15-cycle multiaxially forged (MAFed) specimens (processed in intercritical α+γ phase regime). An enhancement of the fatigue strength can be ascribed to the formation of evenly dispersed nano-sized fragmented cementite (Fe3C) particles (~35 nm size) present in the SG ferritic matrix (average ~280 nm size). The fine dislocation substructures/cells together with the nano-sized Fe3C particles could efficiently block the initiation and propagation of cracks thereby enhancing the fatigue endurance limit of the steel. Superior mechanical properties together with high fatigue resistance in the SG material render the present steels highly beneficial for high-strength structural applications.

Punching above its weight: life cycle energy accounting and environmental assessment of vanadium microalloying in reinforcement bar steel.

Abstract: Steel-reinforced concrete is ubiquitously used in construction across the world. The United Nations estimates that the worldwide energy consumption of buildings accounts for 30–40% of global energy production, underlining the importance of the judicious selection of construction materials. Much effort has focused on the use of high-strength low-alloy steels in reinforcement bars whose economy of materials use is predicated upon improved yield strengths in comparison to low-carbon steels. While microalloying is known to allow for reduced steel consumption, a sustainability analysis in terms of embodied energy and CO2 has not thus far been performed. Here we calculate the impact of supplanting lower grade reinforcement bars with higher strength vanadium microalloyed steels on embodied energy and carbon footprint. We find that the increased strength of vanadium microalloyed steel translates into substantial material savings over mild steel, thereby reducing the total global fossil carbon footprint by as much as 0.385%. A more granular analysis pegs savings for China and the European Union at 1.01 and 0.19%, respectively, of their respective emissions. Our cradle-to-gate analysis provides an accounting of the role of microalloying in reducing the carbon footprint of the steel and construction industries and highlights the underappreciated role of alloying elements.

Isothermal transformation and precipitation behaviors of titanium microalloyed steels

Abstract: The microstructure transformation and precipitation behavior of nano-carbides in Ti microalloyed steel during isothermal process were studied by a compression test on a Gleeble 3800 thermomechanical simulator and analyzed by optical microscopy, transmission electron microscopy and other methods. The results show that γ → α phase transformation and TiC precipitation take place in Ti microalloyed steel during the isothermal process, and time–temperature–transformation curve and precipitation–time–temperature (PTT) curve are all of “C”-type. During the isothermal process, the interphase precipitation of TiC mostly occurs at the period of the phase transformation, and the random precipitation of TiC mostly occurs on the ferrite after the phase transformation. The increment in yield strength at the initial stage of isothermal transformation mainly comes from phase transformation strengthening. With the increase in isothermal time, the precipitation hardening effect becomes more important for nucleation and growth of titanium carbides and eventually reaches the maximum value at the precipitation finished point of the PTT curve.

Interphase precipitation hardening of a TiMo microalloyed dual-phase steel produced by continuous cooling

Abstract: Precipitation hardening is a promising way to strengthen the ferrite phase in dual-phase steels. This work studied the microstructure and precipitation behaviour of a TiMo microalloyed steel processed by continuous cooling. A dual-phase steel with interphase precipitation-strengthened ferrite has been successfully produced by continuous cooling. Transmission electron microscopy and atom probe tomography proved that precipitates formed during continuous cooling were nano-sized (Ti, Mo)C interphase precipitates. More importantly, the interphase precipitation-strengthened ferrite phase became harder than the bainite phase, leading to a new strategy for producing dual-phase steels. The continuous cooling process for interphase precipitation can be used for a wide range of conventional steel production lines, since there is no need for coiling operation of hot strip mills.

Effect of Mold Cavity Design on the Thermomechanical Behavior of Solidifying Shell During Microalloyed Steel Slab Continuous Casting

Abstract: Corner transverse cracks are frequently observed on microalloyed steel slabs during continuous casting. As a solution to this problem, double phase-transformation technology could improve the ductility of the shell surface and avoid corner cracks. However, this technology requires a high cooling rate, which is difficult to reach in traditional flat plate molds (TFMs). Therefore, a novel convex structure mold (NCM) was designed to intensify corner cooling. To investigate the effects of mold design on interfacial heat transfer between the solidifying shell and mold, a thermomechanical model was developed considering the dynamic distributions of the mold slag layers and air gaps. Afterward, the interfacial heat fluxes between mold and solidifying shell obtained from the thermomechanical model were loaded on the flow, heat transfer, and solidification model to study the comprehensive influence of mold cavity design and steel flow on the shell temperature. Based on the models, the contact conditions, distributions of interfacial heat transfer media, interfacial heat fluxes, and temperatures and thicknesses of the solidifying shells were thoroughly compared between the TFM and NCM. The results show that the NCM provides a more appropriate compensation for the shell shrinkage; as a result, the thick slag layers concentrating in the corners of the TFM are flattened and homogenized in the NCM. Thicker slag layers in the TFM weaken the corner heat transfer and lead to uneven shell growth in the off-corner area. Meanwhile, the NCM could homogenize the off-corner heat transfer and increase the cooling rate of the shell corner to help implement double phase-transformation technology in the high-temperature zone of casters.

Examining the machinability of 38MnVS6 microalloyed steel, cooled in different mediums after hot forging with the coated carbide and ceramic tool:

Abstract: In this study, the effect of the microstructure, hardness, and cutting speed on main cutting force and surface roughness in medium carbon microalloyed steel cooled in different mediums after hot fo...

Study on Microstructure Characterization and Impact Toughness in the Reheated Coarse-Grained Heat Affected Zone of V-N Microalloyed Steel

Abstract: Ultra-fine grained ferrite and V(C, N) precipitates formed in the welding process are important factors affecting the toughness of welded joints. In this paper, the microstructure evolution and mechanical properties of heat affected zone were investigated by welding thermal cycle simulation experiment. The effect of V(C, N) precipitates on microstructure and mechanical properties of reheating coarse grain heat affected zone was analyzed. The results showed that the dispersed V(C, N) precipitates can effectively motivate ferrite nucleation. The distribution of large angle grain boundary is more dense at the fine ferrite grain boundaries. The fine polygonal ferrite increases the percentage content of large angle grain boundaries, provides greater resistance to cleavage fracture and increases impact toughness. Because V(C, N) precipitates refine the M–A component, so the size is less than 1 μm. In addition, V(C, N) precipitates enrich carbon, therefore the content of carbon in M–A component is reduced, the hardness is decreased and the toughness is improved. It has a good inhibition effect on the formation and propagation of cracks.

Modification for prediction model of austenite grain size at surface of microalloyed steel slabs based on in situ observation

Abstract: The initial solidification process of microalloyed steels was simulated using a confocal scanning laser microscope, and the growth behavior of austenite grain was observed in situ. The method for measuring the initial austenite grain size was studied, and the M 0 * (the parameter to describe the grain boundary migration) values at different cooling rates were then calculated using the initial austenite grain size and the final grain size. Next, a newly modified model for predicting the austenite grain size was established by introducing the relationship between M 0 * and the cooling rate, and the value calculated from the modified model closely corresponds to the measured value, with average relative error being less than 5%. Further, the relationship between Tγ (the starting temperature for austenite grain growth) and equivalent carbon content CP (CP > 0.18%) was obtained by in situ observation, and it was introduced into the modified model, which expanded the application scope of the model. Taking the continuous casting slab produced by a steel plant as the experimental object, the modified austenite grain size prediction model was used to predict the austenite grain size at different depths of oscillation mark on the surface of slab, and the predicted value was in good agreement with the actual measured value.

Structure and Strength of Isothermally Heat-Treated Medium Carbon Ti-V Microalloyed Steel

Abstract: Isothermal transformation characteristics of a medium carbon Ti-V microalloyed steel were investigated using light microscopy, scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), and by uniaxial compressive testing. Samples austenitized on 1100 °C were isothermally treated in the range from 350 to 600 °C and subsequently water quenched. The final microstructure of the samples held at 350 °C consisted of bainitic sheaves and had compressive yield strength, approximately from 1000 MPa, which is attributed to high dislocation density of low bainite. At 400 and 450 °C, acicular ferrite became prevalent in the microstructure. It was also formed by a displacive mechanism, but the dislocation density was lower, leading to a decrease of compressive yield strength to approximately 700 MPa. The microstructure after the heat treatment at 500 °C consisted of coarse non-polygonal ferrite grains separated by pearlite colonies, principally dislocation free grains, so that the compressive YS reached a minimum value of about 700 MPa. The microstructure of the samples heat-treated at 550 and 600 °C consisted of pearlite and both grain boundary and intragranular ferrite, alongside with some martensite. After 600 s, austenite became stable and transformed to martensite after water quenching. Therefore, the presence of martensite increased the compressive YS to approx. 800 MPa.

A Thermodynamic Analysis of Strengthening Mechanisms and Process-Structure-Property Relationships in Ti-Nb-Mo High-Strength Ferritic Alloy

Abstract: We elucidate here the significant impact of processing and coiling temperature on microstructure and mechanical properties of Ti-Nb-Mo high-strength ferritic steel through thermodynamic modeling and quantitative analysis of strengthening mechanisms. The study clearly demonstrated that the moderate coiling temperature (570 °C) exhibited superior mechanical properties (yield strength of 773 MPa and elongation of 13%). At this coiling temperature, high percentage (58.64%) of nano-size (4-10 nm) precipitates were obtained with the finest average ferrite grain size of 3.34 ± 0.28 µm. The quantitative analysis of strengthening effects suggested that contributions of precipitation hardening from (Ti, Mo, Nb)C particles and grain refinement were 36.7 and 37.7%, respectively, and were remarkable. A precipitation model was utilized to predict the average size and volume fraction of precipitates and mass fraction of solute elements in the steel matrix. The thermodynamic model predicted precipitate size of 6.81 nm, consistent with the experimentally observed size of 7.01 ± 0.51 nm, when the steel was coiled at 570 °C for 2 h.

Physical Simulation Based on Dynamic Transformation Under Hot Plate Rolling of a Nb-Microalloyed Steel

Abstract: The deformation of austenite in the single austenite phase field leads to the formation of Widmanstatten ferrite plates, which eventually merge into polygonal ferrite grains at higher strains. This unusual metallurgical process, known as dynamic transformation (DT), mainly occurs via a displacive mechanism. The metastable austenite phase undergoes reverse transformation when the temperature is held above the Ae3 via a diffusion process. These phenomena affect the rolling load during high-temperature plate rolling, which was investigated in this work. Therefore, a linepipe X70 steel was studied under plate rolling with two-pass roughing and seven-pass finishing strains of 0.4 and 0.2, respectively, applied at strain rate of 1 s-1 and interpasses of 10, 20, and 30 seconds. The samples were cooling down during deformation, which mimics the actual industrial hot rolling. It was observed that the alloy softens as the hot rolling progresses, as depicted by flow curves and mean flow stress plots, which are linked to the combined effects of dynamic transformation and recrystallization. The former initially occurs at lower strains, followed by the latter at higher strains. The critical strain to DT was affected by the number of passes and temperature of deformation. Shorter interpass time allows higher amounts of ferrite to form due to higher retained work hardening. Similarly, the closer the deformation temperature to the Ae3 permits a higher DT ferrite fraction. The information from this work can be used to predict the formation of phases immediately after hot rolling and optimize models applied to the accelerated cooling.

Effect of helical rolling on the bainitic microstructure and impact toughness of the low-carbon microalloyed steel

Abstract: Ferrite-bainite microstructures and impact toughness of the X65 low-carbon microalloyed steel were investigated after helical rolling at 1000, 920, 850, and 810 °C followed by continuous cooling in air. After helical rolling at 1000 °C, granular bainite with large areas of the massive-shape martensite-austenite constituent (d = 1.5 μm) and a high fraction of twinned martensite (d > 2.0 μm) were observed in the steel. This caused a decrease in impact energy at low test temperatures (for example, 70 J at –70°С). Lowering the helical rolling temperature contributed to a reduction of dimensions of both ferrite-bainite and martensite-austenite constituent areas, as well as the replacement of the latter by a slender type one and an improvement in fracture toughness at the low temperatures. The highest impact energy level (210 J at –70 °C) was achieved after helical rolling at 850 °C due to the formation of a homogeneous microstructure, which included dispersed ferrite grains, granular bainite and small fractions of the slender type martensite-austenite constituent (d = 0.1–0.7 μm). In this case, areas of twinned martensite were absent.