Microalloyed Steel Research Articles

Cluster mediated high strength and large ductility in a strip casting micro-alloyed steel

Exhibiting exceptional mechanical properties and formability, high strength low alloy steels characterized by a single ferrite microstructure with finely dispersed nano-precipitates (ferritic HSLA steel) have garnered notable attention in the automotive industry. Nevertheless, to maximally utilize the precipitation hardening effect, these steels necessitate substantial additions of carbide-forming elements, unavoidably narrowing the process window and escalating the cost. Strip casting, featuring a streamlined process chain and high energy efficiency, has emerged as a promising technique for developing ferritic HSLA steels. In this work, leveraging the process characteristics of strip casting, we report that a novel single ferrite microstructure with multi-atomic layered clusters distributed in both interphase-precipitation and random fashions was engineered in a low Nb micro-alloyed ferritic HSLAs via raising the coiling temperature to 650 ℃. The multi-atomic layered clusters play a pivotal role in tailoring dislocation behaviors, facilitating local double cross-slips, contributing to dislocation multiplication and homogeneous distribution. These mechanisms collectively sustain mild work hardening to higher strains, leading to combined strength and ductility increments. In comparison to their cluster-free bainitic counterparts coiled at 480 ℃, the results demonstrate significant mechanical improvements with an increase in ultimate strength (630 MPa to 670 MPa) and a 90 % rise in plasticity (10.3 % to 19.1 %), signifying an alternative pathway for advancing the utilization of strip casting technology in designing and processing novel low-cost, high-performance HSLAs.

Carbonitride Precipitation Kinetics Model During Continuous Casting of Ti Microalloyed Steel

Carbonitride Precipitation Kinetics Model During Continuous Casting of Ti Microalloyed Steel

Correlative Electron Microscopy Analysis of Precipitates in a V–Nb–Mo Micro-alloyed Steel

Correlative Electron Microscopy Analysis of Precipitates in a V–Nb–Mo Micro-alloyed Steel

An artificial intelligence-based optimization framework for the optimal composition and thermomechanical processing schedule for specialized micro-alloyed multiphase steels

An artificial intelligence-based heuristic approach is presented to optimize the chemical composition and the thermomechanical processing schedule to obtain specialized micro-alloyed multiphase steels with desired mechanical properties, at minimal manufacturing cost. The optimization framework uses a modified form of genetic algorithm, called the micro-genetic algorithm (μGA), that uses a penalty-based cost function formulation operating on a multi-dimensional search space spanning 15 alloying elements, an average cooling temperature, an austenitizing temperature and eight time–temperature points from the cooling profiles of multiphase steels. With superior search speed and convergence rates to the traditional genetic algorithm, μGA uses a neural network-based reduced-order model to predict hardness. Additional correlation equations are used to determine the corresponding tensile strength and elongation. Microstructural analysis was performed using neurocomputing techniques to further validate the accuracy of the algorithm. The entire computational framework was validated using data from the literature, establishing its utility in steel design.

Exceptional cryogenic-to-ambient impact toughness of a low carbon micro-alloyed steel with a multi-heterogeneous structure

A low-carbon micro-alloyed (LCMA) steel with a body-centered cubic (bcc) crystal structure suitable for extremely low temperatures was developed by overcoming the intrinsic ductile-to-brittle transition in bcc alloys at cryogenic temperatures. The excellent cryogenic-to-ambient impact toughness in the LCMA rolled plate results from its heterogeneous microstructure, which gradually changes from bamboo-like ultrafine grains (∼ 1.1 μm) on the surface to relatively equiaxed coarse grains in the core (∼ 3.4 μm), accompanied by a distinct texture gradient variation. The heterostructured LCMA steel displays a cryogenic impact toughness of ∼200 J/cm2 at 77 K, which is 24 times higher than the coarse-grained LCMA steel. Such high impact toughness of heterostructured LCMA arises from the coordinated deformation mechanisms over different length-scales coupled with delamination toughening. At 77 K, the heterostructured steel plate deforms by forming cellular sub-structures at the core to the surface, which refines the microstructure and promotes hetero-deformation induced (HDI) hardening to improve intrinsic toughening. Moreover, the subsequent delamination process induces extrinsic toughening by shielding and blunting the cracks, with the local plane-stress conditions induced by delamination promoting ductile fracture of the coarse grains in the core regions. This low alloy steel with its heterogeneous microstructure exhibits extraordinary impact toughness at cryogenic temperatures highlights the possibility of materials design strategies for sustainable development.

Impact of cerium on microstructure of rust and corrosion resistance of Cu-Cr-Ni-V micro-alloyed steels in NaHSO3 acid accelerated test

Corrosion resistance, microstructure and corrosion products of rust and inclusions of Cu-Cr-Ni-V micro-alloyed steels containing 0.0015% cerium (Ce) were investigated by NaHSO3 acid-accelerated test. The results revealed the presence of three-layer microstructures in rust: bottom layer (Fe3O4), interface layer(α-FeOOH) and cracks layer(α-Fe2O3) from inner to outside. The introduction of cerium facilitated the formation of α-FeOOH and Fe3O4 in the rust, while reducing the prevalence of α-Fe2O3 in the rust. The type of inclusions transitioned from undissolved inclusions to dissolved Ce-based inclusions in the acid solution and corrosion rate was reduced by 27% by adding the Ce to the micro-alloyed steels.

Development of Neural Networks to Study Flow Behavior of Medium Carbon Microalloyed Steel during Hot Forming

In the present article, the application of an artificial neural network (ANN) model whose function is the development of plastic instability maps of a medium carbon microalloyed steel during the hot forming process is studied. Secondly, we proceed to create another ANN capable of providing the recrystallized grain size in the steady state resulting from forming deformation. We start from the experimental data of a medium carbon microalloyed steel obtained by hot compression tests with strain rates that vary between 10−4 s−1 and 3 s−1 and in a range of temperatures between 900 °C and 1150 °C. These experimental data are used to train the proposed ANN and obtain flow curves. Finally, the processing maps are developed by applying the dynamic materials model (DMM), according to which the safe hot forming domains and the plastic instability domains of the studied material are delineated. The comparison between the ANN and the experimental maps is carried out. It is ascertained that the optimal regions of forging in the ANN maps coincide with those obtained in the experimental maps. In addition, a study of the influence of the microstructure on the behavior of the studied steel during hot forming is carried out.

Microstructural evolution and carbides precipitation behavior and their effects on mechanical property of Nb–Ti microalloyed FB590 steel

Ferrite-bainite dual phase steels have been widely used in automobile manufacturing industry due to their high strength and excellent hole expansion performance. In this study, the effects of coiling temperature on mechanical properties and hole expansion performance of low carbon Nb–Ti micro-alloyed FB590 ferrite-bainite steel have been studied. Additionally, the microstructure evolution and carbide precipitation behavior during hot rolling and coiling processes have been investigated. The influence mechanism of microstructures and carbides on both mechanical properties and hole expansion performance has also been discussed. The results suggest that the fraction of the retained austenite and the martensite decreases gradually with increasing coiling temperatures from 450 °C to 500 °C. The main reason for the formation of martensite and retained austenite in steel CT450 coiled at 450 °C is that the coiling temperature is near the martensitic transformation temperature Ms. Increasing the coiling temperature to 500 °C which is above the Ms leads to ferritic and bainitic structure. The carbides precipitation behavior has been studied by TEM, thermodynamic and kinetics calculation. The results show that the size of the carbides increases with increasing coiling temperatures from 450 °C to 500 °C. Most of the large size carbides above 100 nm with high percent of Ti are the undissolved particles in austenite during slab heating. The medium size particles between 50 nm and 100 nm and enriched in Ti precipitate during rough rolling at higher temperature, while the particles smaller than 50 nm enriched in Nb precipitate during finish rolling and coiling processes. The strength of the steel is effectively increased by the fine carbides and martensite formed at low coiling temperature of 450 °C. With increasing coiling temperature from 450 °C to 500 °C, the steel's strength decreases from 630 MPa to 591 MPa, while the HER are improved from 76.8 % to 119.9 % due to the reduction of martensite and retained austenite. Although the strength is a little reduced due to the coarsening of carbides and reducing of the martensite, the plasticity and hole expansion performance is improved significantly with the coiling temperature of 500 °C.

Microstructure Evolution Behavior of Titanium Microalloyed Steel during Heat Treatment

Microstructure Evolution Behavior of Titanium Microalloyed Steel during Heat Treatment

Precipitate behaviour of large-sized Nb(C, N) in micro-alloyed steel

Precipitate behaviour of large-sized Nb(C, N) in micro-alloyed steel

Improving strength-toughness of low carbon bainitic microalloyed steel via tailoring isothermal quenching process and niobium microalloying

This study focuses on improving the strength-toughness properties of low carbon bainitic steel through the optimization of the isothermal quenching process and the adoption of a microalloying strategy. The impact of Nb microalloying on the microstructure and mechanical properties was thoroughly investigated. The optimal isothermal quenching process for achieving a balance between strength and toughness is as follows: the steels were austenitized at 900 °C for 30 min and then subjected to an isothermal treatment at 450 °C for 10 min. The resulting microstructure consisted mainly of granular bainite (GB) and lath bainite (LB), with GB accounting for 60 % of the structure. After the optimized isothermal quenching process, the ultimate tensile strength and yield strength of the Nb microalloyed steel increased to 1210 MPa and 693 MPa, respectively. The impact energy, KU2, was measured at 41.8 J. The study revealed that Nb microalloying refines the bainite structure by reducing activation energy and increasing the nucleation rate. This refinement was observed in the reduced length and thickness of the bainitic laths, as well as the increased dislocation density. Additionally, fine composite precipitated particles (Nb, V)C with an average diameter of 8.2 nm were dispersed throughout the microstructure. The addition of Nb significantly enhanced the effects of fine grain strengthening, precipitation strengthening, and dislocation strengthening in low carbon bainitic microalloyed steel. As a result, the strength was greatly improved without sacrificing toughness, thanks to the combined actions of these three strengthening mechanisms.

Effect of Rare Earth Elements on Microstructure and Tensile Behavior of Nb-Containing Microalloyed Steels.

The present investigation endeavors to explore the influence of rare earth elements on the strength and plasticity characteristics of low-carbon microalloyed steel under tensile loading conditions. The findings from the conducted tensile tests indicate that the incorporation of rare earths leads to a notable enhancement in the yield strength, ultimate tensile strength, and ductility properties of the steel. A comparative analysis of the microstructures reveals that the presence of rare earths significantly refines and optimizes the microstructure of the microalloyed steel. This optimization is manifested through a reduction in grain size, diminution of inclusion sizes, and a concomitant rise in their number density. Moreover, the addition of rare earths is observed to foster an increase in the volumetric fraction of carbides within the steel matrix. These multifaceted microstructural alterations collectively contribute to a substantial strengthening of the microalloyed steel. Furthermore, it is elucidated that the synergistic interaction between rare earth elements and both carbon (C) and niobium (Nb) in the steel matrix augments the extent of the Lüders strain region during the tensile deformation of specimens. This phenomenon is accompanied by the effective modification of inclusions by the rare earths, which serves to mitigate stress concentrations at the interfaces between the inclusions and the surrounding matrix. This article systematically evaluates the modification mechanism of rare earth microalloying, which provides a basis for broadening the application of rare earth microalloying in microalloyed steel.

Microstructure and Properties of Coarse Grain Zone in Single-Pass Welding of V-N Microalloyed Steel

Microstructure and Properties of Coarse Grain Zone in Single-Pass Welding of V-N Microalloyed Steel

A new resistance insert spot welding method for injection-molded FRP–steel component

For weight reduction, multi-material designs comprising metal and fiber-reinforced plastic (FRP) components in vehicle body structures have been increasingly used. However, the commonly used resistance spot welding (RSW) technology for car body assembly cannot be employed to join sheet metal and FRPs, limiting the use of FRPs. To solve this problem, a novel resistance insert spot welding (RISW) technique was developed in this work for RSW of steel parts and FRP structure parts made by injection molding. Small inserts were developed by using finite element method and experiments that may be welded to different micro-alloyed and dual-phase sheet steels using the projection welding method. The usual flange width of original equipment manufacturers could be kept unchanged. Using the developed insert and welding parameters, the maximum temperature in the FRPs surrounding the inserts was limited to 255 °C, minimizing the damage to polyamide 6 (PA6) material (with 40 wt% glass fiber). A weldability range between 2.5 and 7 kA could be achieved. The joining strength of RISW between a micro-alloyed HC340 steel in 0.75 mm and 1.5 mm thickness and a 2.5 mm/3.0 mm PA6-GF40 material is 20 to 80% higher than self-piercing riveting (SPR). For high-speed loading, RISW strength increases by 39 to 56% further. Finally, RISW was successfully applied to an FRP–steel roof-frame sub-assembly that consists of 19 simultaneously integrated inserts, achieving 10% weight reduction.

Determination of the onset of yielding and the Young’s modulus after a change in the loading direction

The onset of plastic deformation is an important parameter for an accurate description of the flow curve and the Young’s modulus. Determining the actual physical start of flow is already experimentally challenging for classic sheet metal materials. In addition to the experimental challenge, the onset of flow depends on numerous parameters such as strain rate, temperature and forming history. Non-proportional load paths in particular can significantly influence the onset of flow. Three different materials, a micro-alloyed steel HC340LA, a dual-phase steel CR330Y590-DP and an aluminium alloy AA6016-T4 are investigated in this publication. The physical onset of flow of the materials is determined at three different pre-strain levels as well as without and with a change in the load direction. Temperature-based approaches are used for this purpose. In-situ synchrotron diffraction is used to validate the results obtained. Those results can help to improve existing material models and springback prediction. Such models rely on material parameters that are as accurate as possible.

Characteristics of High-Strength Cast Steel Micro-Alloyed with Vanadium

This article presents the results of research into the characteristics of cast steel alloyed with chromium and vanadium, subjected to heat treatment for increased strength parameters. In the first part, it discusses the state-of-the-art knowledge regarding technological developments in the field of cast-steel alloys and the influence of individual alloying additives on the microstructure and the properties of the steel alloy. Further sections present the results of microstructure observations performed with light microscopy, scanning electron microscopy, and transmission electron microscopy. This research focuses on the material in the state directly after casting and after heat treatment, which involved quenching and tempering at 200 °C. The microstructural analysis performed as part of this research has informed the discussion of the results obtained from tensile and impact strength tests. The article also includes the results of a fractography analysis performed as the final part of the tests and offers a general summary and conclusions.

Effect of Magnesium on Sulfide and Microstructure in Microalloyed Steel

The effect of Mg on MnS inclusion and the formation mechanism of (Mn,Mg)S‐MgAl2O4 (MA) composite inclusion are analyzed by combining observation for the inclusions in medium microalloyed steel with (and without) 15 ppm Mg, thermodynamics, phase diagram, and crystallography. The addition of magnesium can improve the morphology of inclusions and promote microstructure uniformity. MA is formed by the reaction of dissolved Mg, Al, and O in the Mn‐Mg‐Al‐S‐O liquid melt, while residual Mg is dissolved in MnS, forming (Mn,Mg)S. As the Mg content in the Mn‐Mg‐Al‐S‐O liquid melt increases, the precipitation temperature of the melt increases. Excessive Mg content can lead to a decrease in the proportion of MA in the melt, forming MgO instead of MA. Based on the two‐dimensional slices of MnS and MA, the three‐dimensional morphology can be derived. The lowest mismatch value is 8.00% when the interface is {111} of MnS and {110} of MA.

Microscopic Investigation for Experimental Study on Transverse Cracking of Ti-Nb Containing Micro-Alloyed Steels.

The influence of Ti on the behavior of hot ductility was examined in four different Ti-containing micro-alloyed steels with a constant content of Nb. Thermomechanical investigations using a dilatometer were carried out to simulate the conditions during casting and cooling in the strand of a continuous caster with temperatures in the range of 650-1100 °C, strain rates of 0.01 s-1 and 0.001 s-1, and reheating rates between 60 and 180 Kmin-1. To understand the fracture mechanism, optical (LOM) and scanning electron microscopy (SEM), elemental analysis via energy dispersive X-ray spectroscopy (EDX), MatCalc "Scheil-Guilliver" calculations, and precipitation kinetics calculations were carried out for the critical conditions, showing low hot ductility between Ar3 and Ae3 temperatures and a brittle to ductile transition temperature at 900 °C. The existence of TiNb(CN), thin ferrite formation, and grain boundary sliding (GBs) due to limited dynamic recrystallization (DRX) has been documented and discussed. As a result, the reheating rate has no sufficient effect on the ductility. The existence of Nb-rich TiNb(CN) of sizes below ~1 μm triggers brittle fracture by increasing the frequency of micro-voids around grain boundaries. It can be stated that if the conditions in the hot ductility trough are avoided, the addition of Ti and high strain support minimize the risk of crack formation.

Influence of thermal loading parameters and microstructure on the formation of stratified surface layers on railway wheels

Due to an increasing trend towards environmentally friendly public transport, rail networks face higher speeds and wheel loads affecting the material degradation of the railway components. Within this study, influencing parameters on the formation of stratified surface layers, forming on railway wheels during service due to thermal loadings in the wheel-rail contact, are studied. These layers consist of white etching layer and underlying brown etching layer and are susceptible to fatigue crack initiation. By using laser surface treatments, its formation based on two consecutive thermal loadings is systematically demonstrated on ER7 wheel steel. Further, a decrease in layer thickness with decreasing laser power, and an increase in brown etching layer thickness with increasing laser power difference is shown. Moreover, the effect of finer grain size leading to an increased layer thickness, and the influence of the chemical composition by comparing the standard ER7 wheel steel to a micro-alloyed wheel steel are demonstrated.

Thermomechanical Control of Microstructure and Precipitation in Vanadium Microalloyed Steel: Influence of Finish Rolling and Coiling Temperatures

Six hot compression tests are conducted using the Gleeble3500 thermomechanical simulator to investigate the microstructural evolution and precipitation behavior in low‐C–Mn V‐microalloyed steel. The specimens are subjected to hot isothermal compression deformation of 87%. The optical microscopy and transmission electron microscopy using carbon extraction replica method are used to characterize the microstructures and precipitation after the simulated thermomechanical controlled process and coiling. The results indicate that increasing the finish rolling temperature benefits the refinement of ferrite grains but has little influence on the refinement of the precipitates. It is also observed that lower coiling temperatures (CTs) promote the formation of fine precipitates. When the CT is 500 °C, the average precipitate size is found to be 86 nm. Furthermore, it is found that the CT significantly influences the nucleation sites of the precipitates inter alia, the matrix, interphase, grain boundaries, and dislocations. As expected, at higher CTs, nucleation is predominantly on the defects rather than the matrix.