Low Carbon Microalloyed Research Articles

Mechanistic role of vanadium microalloying in improving corrosion resistance of low carbon bainitic steel

We here investigated the effect of vanadium microalloying on the microstructure, mechanical properties, especially corrosion performances of low carbon bainitic steels. The corrosion behaviors of the steels with different vanadium contents in 3.5 wt% NaCl solution were evaluated by electrochemical tests (including polarization curves and electrochemical impedance spectroscopic measurements) and alternating immersion test (including weight loss and rust layer observation). Results show that the mechanical properties and corrosion resistance of low carbon bainitic steels can be synergistically improved with vanadium microalloying. With help of the electron backscatter diffraction characterization and scanning Kelvin probe force microscopy, we first discovered that although the number of micro-galvanic couples increases because of grain refinement, the Volta potential gradient between the matrix and grain boundaries are decreased with vanadium microalloying, which can promote the formation of compact protective rust layers and improve the corrosion resistance of vanadium micro-alloyed low carbon bainitic steels.

A prediction model of brittle crack arrest toughness in TMCP heavy plates

Micro-alloyed low carbon MnCrNiCuMo steels were proposed to produce heavy plates with a maximum thickness of 100 mm. An integrated industrial route including a thermo-mechanical control process (TMCP) based on a study on continuous cooling transformation was applied in industrial production. Comprehensive mechanical properties tests including the measurement on resistance against crack initiation using crack tip opening displacement (CTOD) approach were conducted. Large scale wide-plate temperature gradient double tension (DT) technique was employed to evaluate the crack arrest toughness Kca in the heavy plates under different conditions. Nil-ductility transition temperature (NDTT) was also tested across the entire thickness section of the plates. The results showed that heavy steel plates were qualified being brittle crack arrest (BCA) products at EH47 grade with yield strength high than 460 MPa. High crack initiation toughness was exhibited by both Charpy-V notch impact property and CTOD values. Crack arrest toughness at −10 °C greater than 253 MPa √m was achieved in a plate even with thickness of 100 mm. The mean value of crack arrest toughness at different temperature measured by the DT approach can be indexed by the highest NDTT in the interior section of the heavy plates leading to an empirical prediction model. Further, fracture mechanics based master curve approach was used to statistically predict crack arrest toughness distribution over a wide range of temperatures showing improved predictability over the empirical model and an existing model in the literature.

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.

Effect of Zr Treatment on Inclusion and Solidification Microstructure in Ti Deoxidized Low Carbon Microalloyed Steel

Effect of Zr Treatment on Inclusion and Solidification Microstructure in Ti Deoxidized Low Carbon Microalloyed Steel

Effect of Cooling Rate on Phase Transformation Kinetics and Microstructure of Nb–Ti Microalloyed Low Carbon HSLA Steel

Nb–Ti microalloyed low carbon high strength low alloy (HSLA) steels are used to fabricate components, particularly for the automobile and piping applications requiring optimum combination of mechanical properties along with good weldability. These properties depend on the transformed ferrite microstructure and grain size obtained after cooling from hot rolling temperatures which are always well above the austenitizing temperature. Hence, an attempt was made to study the austenite decomposition (phase transformation) kinetics and the accompanying microstructural evolution in Nb–Ti microalloyed steel by subjecting it to austenitization at 1100 °C for 3 min followed by cooling at different rates ranging from 1 to 100 °C/sec in a B\(\ddot{a}\)hr DIL 805 A/D dilatometer. The first derivative method was employed to identify critical transformation temperatures from the dilation curves. A modified Johnson–Mehl–Avrami–Kolmogorov (JMAK) analysis (used for non-isothermal conditions) was carried out to find the time exponent (n’) values controlling the rate of transformation at different cooling rates. The nature of transformed ferrite was observed to change from polygonal to acicular type, and its grain size was found to decrease with an increase in cooling rate. EBSD analysis also revealed the cooling rate to have a profound effect on the microtexture evolution of the concerned alloy. The “\(\gamma\)” fibre and rotated cube components are replaced by the transformed copper (“\(\alpha\)” fibre) components owing to faster transformation and lack of recrystallization of transformed \(\alpha \) with increase in cooling rate. Finally, a power law and logarithmic relationship of grain size and microhardness with the applied cooling rate were established.

The Role of Splitting Phenomenon under Fracture of Low-Carbon Microalloyed X80 Pipeline Steels during Multiple Charpy Impact Tests

The ambiguity of the splitting effect on X80 low-carbon microalloyed pipeline steels’ tendency towards brittle fracture prompted an experimental study of impact toughness scattering based on multiple Charpy impact tests in a temperature range from 20 °C to −100 °C. A fractographic analysis of a large number of fractured samples was carried out. The relationships between impact toughness, deformability and splitting characteristics were studied. A number of common features of three X80 low-carbon microalloyed pipeline steel fractures were revealed. It was experimentally established that the reason for the scattering of the impact toughness values during completely ductile fracture of specimens, as well as during fracture accompanied by the splitting formation, is the local inhomogeneity of plastic properties. The higher the susceptibility to the formation of splits for a particular steel, the lower the impact toughness. Using the electron backscatter diffraction (EBSD) technique, an uneven distribution of local plasticity in the plastic zone of impact-fractured specimens was established. A comparative analysis of specimens with equal impact toughness values at different test temperatures makes it possible to identify the mechanism of negative splitting influence compensation by the increased plasticity of certain specimen.

Recrystallisation kinetics and yield-strength adjustment after annealing of cold-rolled microalloyed steel

After continuous annealing process (CAP) at 790°C, 85% of the coils of 50% cold-rolled low carbon microalloyed (LCM) steel did not exhibit yield-strength (YS) on the target range, while the 70% cold-reduced LCM coils did. In this context, the non-isothermal recrystallisation kinetics of ferrite for the above two full-hard LCM steel were investigated using differential scanning calorimetry and the Friedman differential isoconversional method. The recrystallisation kinetics of ferrite for the two deformed states showed different behaviour. Regarding a fixed degree of cold-rolling deformation, the soaking temperature was found as the manageable parameter to control YS during CAP. Consequently, a suitable YS of the 50% cold-rolled LCM steel was achieved by setting the soaking temperature at 773°C.

In-Depth Understanding of the Relationship between Dislocation Substructure and Tensile Properties in a Low-Carbon Microalloyed Steel

In-Depth Understanding of the Relationship between Dislocation Substructure and Tensile Properties in a Low-Carbon Microalloyed Steel

Formation Features of the Structure and Mechanical Properties in Rolled Products with Various Thicknesses from Low-Carbon Microalloyed Steel Produced at an Integrated Casting and Rolling Facility

Formation Features of the Structure and Mechanical Properties in Rolled Products with Various Thicknesses from Low-Carbon Microalloyed Steel Produced at an Integrated Casting and Rolling Facility

Study of Microstructural Evolution and Mechanical Properties of 1000 Mpa Low-Carbon Microalloyed Steel Prepared by Multiple Quenching Strategies Near Ms

Study of Microstructural Evolution and Mechanical Properties of 1000 Mpa Low-Carbon Microalloyed Steel Prepared by Multiple Quenching Strategies Near Ms

Design and Performance of 690 MPa Grade Low-Carbon Microalloyed Construction Structural Steel with High Strength and Toughness

Design and Performance of 690 MPa Grade Low-Carbon Microalloyed Construction Structural Steel with High Strength and Toughness

Heterogeneous Microstructure of Low-Carbon Microalloyed Steel and Mechanical Properties

The microstructure plays a major role in the performance of metallic materials, which can be tailored through the composition and/or processing technique. In this study, a heterogeneous microstructure was developed for low-carbon microalloyed API X65 steel, the most commonly used pipeline steel for oil and gas transportation, using a heat treatment process. The heat treatment process involved intercritical heating of the steel followed by high-temperature isotheral cooling, allowing for phase transformation, as well as alloying element partitioning. The heat treatment transformed the banded ferrite–pearlite microstructure of rolled steel to a quasi-polygonal ferrite microstructure, with the sporadic presence of austenite at the grain boundaries. The quasi-polygonal ferrite was distributed in a heterogeneous form with a fine-grain shell surrounding the coarse-grained core. The heterogeneity in the microstructure, despite being single phase, led to a significant improvement in the tensile yield strength, ultimate tensile strength, ductility and toughness of the steel, with a marginal reduction in microhardness values.

New Approach for Multiplying the Strength and the Formability of Cold Rolled BCC Based Structure Steel through Ultra Fine Structure Technique

Reduction in grain size of bcc based structure steel is still highly concerned in the cold rolled sheet to attain superior mechanical properties. As long as, the reduction of weight is much considered in the structure purposes, the strength/weight ratio of steel is highly demanded. In this study, an innovative technique was applied to attain ferrite grain size with hundreds of nanometer, in tandem with preserving the mechanical properties. In this approach, the micro-alloyed low carbon steel resulted from the thermomechanical process was followed by subcritical annealing regime prior to the first critical transformation temperature. To identify the effect of a micro-alloying element as vanadium, and the effect of subcritical annealing regime on the low carbon steel, two low carbon steel was subjected to studying in this research. The results refer that applying a subcritical annealing regime for the micro-alloyed low carbon steel after hot compression at intercritical annealing temperature can lead for attaining hundreds of nanometer ferrite grain size, which has a powerful effect on promoting the strength of the steel to exceed 1200 Mpa, in one hand with preserving the formability up to 20% as uniform elongation. Unexpectedly, the fine grain size obtained after the innovative technique promotes the impact toughness at room temperature, which is attributed to the fineness and the spheroid morphology of the secondary phase in conjugation with bcc ferrite structure.

Mechanical Properties and Strength Prediction of Ti Microalloyed Low Carbon Steel with Different Ti Content

The effects of Ti on microstructures, recrystallization and mechanical properties of microalloyed low carbon steels were studied. Hall-Petch relationship and Ashby-Orowan equation were applied to analyse the reason for the variation of strength. The results show that the recrystallization activation energy increases parabolically as the Ti content increases. Besides, the yield strength, tensile strength and the yield ratio increase in varying degrees with the increasing of Ti content, while the elongation decreases. The fine grain strengthening makes the greatest contribution to the strength, and the precipitation strengthening of TiC particles is the main reason why the strength of test steel increases significantly when the Ti content exceeds 0.042%. In addition, the strength prediction model was established for Ti microalloyed low carbon steel. The relative error of predicted yield strength and tensile strength of Q345B steel are within ± 8% and ± 7% respectively.

Study of combined effect of zirconium and titanium on microstructure and mechanical properties of micro-alloyed cast steels

Aim of the present work is to study the influence of zirconium as well as titanium in combinations on the microstructures and mechanical properties of low carbon micro-alloyed cast steels and to compare with non micro-alloyed low carbon steels. The quantum of research work on the zirconium bearing micro-alloyed steels with respect to grain refinement and mechanical properties is scanty. The micro-alloyed steels with 0.05 wt% to 0.10 wt% Zr and Ti were melted by a conventional air induction melting furnace and they were further subjected to normalizing at a temperature of 1000 °C for 1 h followed by cooling in atmospheric air. The metallurgical properties of the heat treated samples were observed by optical microscope and TEM. It is observed that steel with Zr and Ti shows the presence of acicular ferrite with a very fine grain size of around 5–10 microns as compared to the non micro-alloyed steel. In addition, the inter-lamellar spacing of pearlite as well as presence of intermetallic compounds were seen clearly in the heat treated samples by TEM. It is also observed that the presence of precipitates accounts for the improvement of strength and toughness of steels containing micro alloying content in the heat treated condition. The steel with 0.1 wt% Zr + 0.1 wt% Ti content showed tensile strength with nearly two times higher in comparison to steel without micro alloy addition in steel. However the elongation of both steels is almost equal (percentage of elongation is found to be around 36%). This can be attributed due to the presence of zirconium carbo-nitrides causing the formation of fine grains in steel with Zr and Ti contents.

2D automated SEM and 3D X-ray computed tomography study on inclusion analysis of steels

ABSTRACTIn this research work, inclusion analysis was done on titanium micro-alloyed low carbon steels using automated scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDS), and 3D X-ray computed tomography (XCT) techniques. Automated SEM/EDS analysis provided the information about the average size, number, shape, and the composition of the inclusions. The inclusions are categorized as calcium aluminates, titanium nitrides, and ‘other’ inclusions with an average size of 1.66, 1.52, and 1.31 µm, respectively. To estimate the actual amount of inclusions present and to understand their 3D morphologies, XCT analysis was done at two different resolutions of 1.8 µm and 590 nm. The employment of these resolutions enabled the technique to detect huge number of inclusions with a wide size range from 0.75 to 201.4 µm. Moreover, the XCT technique revealed the presence of complex irregular shaped inclusions. Effective combination of these two techniques for inclusion analysis gives complete quantified information about the inclusions present in steels.

Experimental determining of fracture behaviour of P460NL1 steel welded joint specimens

This paper is focused on the experimental measuring of impact energy as a part of the doctoral dissertation involving the fatigue behaviour of welded joints, made of fine grain normalised micro-alloyed low carbon high strength steel P460NL1 [1,2], typically used for pressure vessels working at subzero temperatures. VAC 65 [3] was used as filler material, and two plates were welded using the MAG welding procedure (with 82% Ar + 18% CO2 shielding gas).The specimens were taken from two opposite ends of the welded plate, taking into account the measured values of groove edge temperature in these locations, wherein the temperatures in Location 2 (near the end of the weld) were higher than those in Location 1 (near the weld’s beginning). Based on their specific position, in terms of temperature and notch position (weld root or face), the specimens were divided into four groups of three. This testing was performed using a SCHENCK-TREBEL instrumented Charpy pendulum. The total impact energy was determined, along with its components, crack initiation and crack propagation energy.The obtained results have confirmed that the test specimens were of high toughness, with total impact energy ranging from 165-200 J, at room temperature. The crack propagation energy was the dominant component, being several times greater than the crack initiation energy, as was expected.

New Technology to Produce 1 GPa Low Carbon Microalloyed Steels from Cast Strip

Global economy requires steel with further increasing mechanical properties and simultaneously decreasing price. In mass manufacturing three major methods can be used to increase strength: (i) increase microalloying element additions (increases cost), (ii) decrease deformation temperature and (iii) increase cooling rate after high temperature processing (both can be challenging for equipment). Thin strip casting is an effective way to reduce cost as it brings a reduction in number of deformation passes and shortens the production line. However, the mechanical properties can be missed due to insufficient microstructure development. In this article, we investigate a recently proposed technology based on Austenite Conditioning followed by Accelerated Cooling and Warm Deformation (AC2WD). Two low carbon steels microalloyed with either 0.012Ti or 0.1Mo-0.064Nb-0.021Ti (wt.%) were subjected to three processing modifications of the AC2WD-technology with two, one or no deformation of cast microstructure in the austenite temperature field. The Ti- and MoNbTi-steels exhibited 685–765 MPa and 880–950 MPa of the yield stress, 780–840 MPa and 1035–1120 MPa of tensile strength, and 20–30% and 22–24% of elongation to failure, respectively. The nature of strengthening mechanisms associated with the AC2WD-technology is discussed on the basis of detailed microstructure characterisation.

Precipitation Behavior of VN in High Nitrogen and Vanadium Micro-alloyed Low Carbon Weathering Steel

In this study, the high nitrogen and vanadium micro-alloyed low carbon weathering steel (0.0320%N–0.096V) with excellent mechanical properties was produced. TEM and EDS tests of the hot rolled specimens were conducted and it was found that a great number of VN precipitates were generated and the size of VN particles was in the range of 10–300 nm, which exceeded the critical size of 7.89 nm for pinning dislocation line. Thermodynamic calculation results showed that, at high temperature, the precipitation performance of VN remarkably increased due to high nitrogen content. For the experimental steel, the initial precipitation temperature of VN reached 1133 °C and the precipitation rate of V exceeded 90% at the finish rolling temperature of 850 °C. The precipitation temperature time (PTT) curves drawn from kinetic data showed that the fastest precipitation temperature was about 950 °C. The total precipitation time was shortened from 110 to 10 s at 850 °C due to the high dislocation density caused by deformation, and the VN particles could thus be completely precipitated with a brief relaxation process. High nitrogen and vanadium micro-alloying was beneficial for improving the grain refinement and precipitation strengthening effect of vanadium.

The Analysis of the Microstructure and Mechanical Properties of Low Carbon Microalloyed Steels after Ultra Fast Cooling

In this paper, two low carbon microalloyed steels, named as steel A and steel B, were fabricated by ultra fast cooling (UFC). In both steels, the microstructures containing quasi polygonal ferrite (QF), acicular ferrite (AF) and granular bainite (GB) can be obtained by UFC process. The amount of AF in steel B is more than that in steel A. The size and distribution of precipitates (Nb/Ti carbonitrides) in steel B are finer and more dispersed than those of in steel A due to relatively low finish cooling temperature. The mechanical properties of both steels are effectively enhanced by UFC process. UFC process produces low-temperature transformation microstructures containing a significant amount of AF. The mechanical properties of steel B were more satisfactory than those of steel A due to the finer average grain size, the greater amount of the volume fractions and smaller size of secondary phases.