Low-carbon Microalloyed Steel Research Articles

An automated morphological classification of ferrite-martensite dual-phase microstructures

In this study, a simple classification scheme based on image processing is proposed to categorize optical microstructures of dual-phase steels with varying morphology (i.e., shape, size and distribution) of ferrite-martensite phases. Dual-phase microstructures with varying morphologies have been developed by employing different heat treatment schedules; namely, intercritical annealing (IA), intermediate quenching (IQ) and step quenching (SQ) using a low carbon micro-alloyed steel. The images of optical microstructures of the developed steels have been first segmented using Otsu thresholding technique and then, the ratio of the black (martensite) and white (ferrite) run-lengths for the phases have been calculated. The mean and median values of these ratios have been considered further, and it has been demonstrated that the mean values are more appropriate than the median values for the classification purpose. The mean and standard deviation of the former parameter for IA, IQ and SQ microstructures are 0.679 ± 0.040, 1.540 ± 0.090 and 0.989 ± 0.119, respectively. The simple classification technique is found to be advantageous from different aspects.

Processing of Ultrafine-Grained Steels by Warm Rolling and Annealing

Low-carbon microalloyed steel was subjected to warm rolling followed by rapid transformation annealing (RTA) at 800-850 °C and subcritical annealing (SCA) at 600 °C to develop ultrafine ferrite grain structures (UFFG) with grain size less than 3 μm. The present study investigated the influence of light (40%) and heavy (80%) warm rolling deformation (LWR and HWR) applied during the finishing pass of two-pass rolling schedules on the microstructural evolution after rolling and subsequent annealing treatments. RTA treatment of HWR sample at a lower intercritical temperature for an optimum duration (800 °C, 30 s) developed UFFG-martensite dual-phase structure that offered the best combination of strength (YS ~ 900 MPa and UTS ~ 1200 MPa) and ductility (25% elongation). The SCA treatment provided sufficient time to achieve a uniform distribution of carbide particles throughout the ferrite matrix. SCA treatment of HWR at 600 °C for 4 h developed UFFG-carbide structure achieving YS of 800 MPa with 20% ductility. The SCA of LWR resulted in coarser ferrite grain structures (grain size > 5 μm) having higher ductility (more than 30%) but lower strength (UTS of 400-550 MPa) as compared to RTA.

Coupling a Cellular Automaton Model with a Finite Element Model for Simulating Deformation and Recrystallization of a Low-Carbon Micro-alloyed Steel During Hot Compression

This study simulated the dynamic recrystallization (DRX) of a 2.25Cr-1Mo-0.25V steel by coupling a cellular automaton (CA) method with a finite element (FE) model. A secondary development program (SDP) on DEFORM-2D was carried out and the stress strain response, dislocation density evolution and DRX were considered. In this way, an interaction between the microstructural evolution and the deformation behavior can be taken into account. The values of temperature, strain and strain rate from the FE analysis were passed to the CA model. In turn, the recrystallized grain size and volume fraction of DRX predicted by the CA model were passed back to the FE model, which influenced the flow stress. To validate the SDP, the predicted loads and flow stress were compared with the experimental values, which show a good agreement. Then, the SDP program was used to simulate the microstructural evolution. Morphological characteristic, recrystallized grain size and volume fraction of DRX were predicted and compared with experimental data. The results show that the comparison is in a good agreement, which indicates the SDP is reliable.

Features of Technology for Pipe Steel Modification with Calcium and Cerium with Specification for Resistance to H2S-MEDIA

Among many factors influencing steel resistance to hydrogen induced cracking, the sulfur content in the metal and steel modification technology with calcium are of particular importance. In assessing the impact of process parameters on HIC resistance many authors do not consider their effect on production process efficiency and other quality indicators. The combined effect of calcium and rare earth metals on hot-rolled steel quality is described (with respect to steel HIC resistance, and contamination of metal by nonmetallic inclusions) in this paper. Formation of nonmetallic inclusions containing calcium and rare earth metals is described, and nonmetallic inclusions in the metal with different versions of modification are analyzed. The optimum amount of calcium and cerium required for obtaining stable results for HIC resistance in low-carbon micro-alloyed steels while retaining good rolled steel surface quality is determined.

Correlation of Structure and Fractographic Characteristics of High-Strength Pipe Steel Welded Joint Metal Failure Micro-Mechanism

The structure and microstructure of impact fracture specimens of the heat-affected zone (HAZ) of low carbon micro-alloyed high-strength pipe steel welded joints are investigated. Correlation of structure parameters and fractographic characteristics of the failure micro-mechanism channel is established, providing information about elementary acts of failure, i.e., the dimensions of ductile and brittle microcracks.

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.

Corrosion behavior of low-carbon Cr micro-alloyed steel for grounding grids in simulated acidic soil

To improve the corrosion resistance of steels for grounding grids, a low-carbon Cr micro-alloyed steel was developed (C1 steel), and corrosion behavior of Q235 steel and newly developed C1 steel in simulated acidic soil was investigated. The corrosion rate was evaluated with the mass loss measurements, while the corrosion morphology of surface and cross section of rust layer was observed by scanning electron microscopy. The corrosion products were analyzed by energy-dispersive X-ray spectrometry, X-ray diffraction and X-ray photoelectron spectroscopy, and the polarization curve was measured using potentiodynamic polarization method. Results indicated that C1 steel displayed good corrosion resistance in the simulated acidic soil, of which the corrosion rate was only 30% of that of Q235 steel after corrosion for 360 h. The analysis of rust layer showed that lower carbon content in steel could reduce the tendency of micro cell corrosion and appropriate amount of chromium could improve the corrosion potential of metal matrix. Moreover, the analysis of X-ray photoelectron spectroscopy revealed that the chromium enriched in inner rust layer of C1 steel existed mainly in the form of Fe2CrO4, which facilitated the formation of Cr-goethite and improved the protection of corrosion products.

Effect of Pre-deformation on Grain Ultrafining by Intercritical Deformation in Low-Carbon Microalloyed Steels

In this study, the effect of pre-deformation at recrystallization and non-recrystallization zone on the grain ultrafining by the subsequent intercritical deformation (ID) was investigated on low-carbon microalloyed steel. The results showed that ultrafine grain microstructure with an average size of ~ 1.0 μm was fabricated through pre-deformation in the recrystallization zone followed by ID. When pre-deformed at the non-recrystallization zone prior to ID, the grain size increased to 1.6 μm with a heterogeneous distribution along with the well-developed dynamic recovery of ferrite. The grain ultrafining mechanism was attributed to the combined action of the deformation-induced ferrite transformation and the continuous dynamic recrystallization. In particular, the continuous dynamic recrystallization process during ID occurred on the pro-eutectoid ferrite as a result of the subgrain rotation mechanism and the absorbing dislocations mechanism.

Fabricating ultrafine grain by advanced thermomechanical processing on low-carbon microalloyed steel

The ultrafine grain with the effective grain size of 1.10 μm was fabricated by intercritical deformation on low-carbon microalloyed steel, which was finer than the deformation induced ferrite transformation (DIFT) at 1.55 μm. During intercritical deformation, the DIFT process was enhanced, showing a colony distribution of the refined grains. Meanwhile, the continuous dynamic recrystallization of pro-eutectoid ferrite was developed by both the subgrains progressively trapping dislocations and the serrated boundary pinching off the elongated grains. These processes were directly observed through the grain average misorientation maps of electron back-scattered diffraction and confirmed by transmission electron microscopy micrographs.

Effect of Continuous Casting Technology on Low-Alloy Steel Structure in Different Production Stages

The effect of “soft reduction” technology on chemical inhomogeneity of a continuously-cast slab, and also on the structure and properties of finished rolled product made of low-carbon microalloyed steel is studied. In slabs prepared without soft reduction, a defect is often observed in the form of white bands leading to a reduction in the proportion of ductile component in low-standard rolled product. In a slab transverse section, from the surface into the depth there are coarse grains with ferrite over boundaries and bainite within grains. A white band is formed by fine polyhedral ferrite grains with a small amount of finely acicular bainite. In this case, the microhardness of ferrite and bainite in the axial zone, and in a white band is considerably higher than in other slab zones. Also within the limits of a white band there is formation of an increased amount of sulfides and carbonitrides. It is demonstrated that as a result of liquation processes there is an increase in carbon, manganese, sulfur, and phosphorus, and also microalloying element content within a white band.

Effects of Thermo-mechanical Process Parameters on Microstructure and Crystallographic Texture of High Ni–Mo Ultrahigh Strength Steel

A novel low-carbon micro-alloyed steel has been developed with ultrahigh strength (UTS ~ 1700 MPa), satisfactory ductility (total elongation ~ 13%) and impact toughness (25 J/cm2 at − 40 °C) for light-weight applications in automobile, aerospace and defence sectors. The effect of finish rolling temperatures (850–750 °C) and cooling rate (air cooling versus water quenching) on the evolution of microstructure and crystallographic texture and finally on the mechanical properties of thermo-mechanically controlled processed steel has been studied. A refinement in mixed microstructure comprised of granular/lower bainite and lath/plate martensite and an intensification of Goss and rotated Goss texture components were found with the decrease in finish rolling temperature and increase in cooling rate. Interaction of fine-scale carbide/carbonitride precipitates of Nb and Ti with the dislocation substructure present within bainite and martensite contributed significant precipitation strengthening to the steel.

Effect of 0.1 wt.% Co on the Hot Deformation and Toughness of Fine-Grained Low-Carbon Steel at Sub-zero Temperatures

The effect of 0.1 wt.% Co on the hot deformation behavior of fine-grained low-carbon microalloyed steel was investigated at temperatures of 850-1200 °C and a strain rate of 5 s−1. Furthermore, the toughness of the steel with and without Co at sub-zero temperatures was evaluated. The results suggest that the addition of 0.1 wt.% Co increases the flow stress and delays the occurrence of dynamic recrystallization (DRX) at the same deformation temperature and strain. The DRX fraction of steel specimens without and with 0.1 wt.% Co was about 67.4 and 43.9% at 850 °C, respectively. Then, it increased to 100% at 1100 °C. Compared with steel without Co, cementite particles in the tempered sorbite of steel with 0.1 wt.% Co decreased in size but increased in quantity, yield strength increased from 756 to 787 MPa, and Charpy V-notch energy at − 20 and − 50 °C improved from 69 and 41 to 102 and 65 J, respectively. The fracture morphology and crack propagation characteristics were consistent with the variation in impact energy.

Production of thick steel sheet for high-pressure equipment operating at temperatures from 450°C to–50°C

The industrial production of low-carbon microalloyed steel plates (thickness 150–200 mm) on the NLMK Dansteel 420 reversible rolling mill is considered. Such plates are intended for operation at temperatures between–50 and +450°C in high-pressure equipment. The technologies used for steel casting and thick-sheet production ensure stable strength over the whole sheet thickness at 20–450°C, with satisfactory impact strength between +40 and–50°C. The research results are used in certification on the basis of TUV Pressure Equipment Directive (PED) DGR 2014/68/EU and the AD2000 directive regarding the production of rolled P355N, P355NH, P355NL1, and P355NL2 steel sheet of thickness up to 200 mm.

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.

Micro- and Nanostructural Nonuniformity Through the Thickness of 100 mm Structural Steel Plate After TMT and HT

Traditionally high strength (σy ≥ 400 N/mm2) rolled sheet thicker than 50–60 mm of low-carbon structural steel is produced by hot rolling technology followed by normalizing or hardening and tempering. Contemporary equipment makes it possible to produce rolled product 50–140 mm thick of low carbon steel by thermomechanical rolling and to obtain a required set of properties without expenditure on production operations connected with additional metal heating such as normalizing and tempering. Results are given in this article for studying one of the thick sheet property indices, i.e., structure nonuniformity through plate thickness. The microstructure is compared for rolled sheet 100 mm thick of low-carbon microalloyed steel structural steel produced in the 4200 NLMK Dansteel mill by thermomechanical rolling technology (TMT) with the microstructure obtained as a result of hot rolling followed by normalizing (HT). Grain sizes are determined at different points of rolled product layer thickness, and histograms are plotted for their distribution, making it possible to compare the degree of inhomogeneity for the microstructure with respect to grain size. The type, size, and density of carbonitride particles are determined at distances of 1/2 and 1/4 through the thickness of 100 mm plate after TMT and HT.

Conditions for Obtaining a Good Set of Properties for Low-Carbon Microalloyed High-Strength Steels Clad with Corrosion-Resistant Austenitic Steels

Conditions are studied for preparation with temperature and deformation treatment of a good set of mechanical properties and high quality rolled product characteristics for low-carbon Ti–Nb–V- and Mo–Timicroalloyed high-strength steels, clad with austenitic class corrosion-resistant chromium-nickel steels. It is established that an increase in uniformity of deformation for layers of a two-layer workpiece for steels of these types is facilitated by an increase in heating temperature and for the end of rolling to 1250 and 900°C, respectively. In order to provide good process ductility of corrosion-resistant austenitic steels in this temperature range, their chemical composition should satisfy a condition Crequ/Niequ not more than 1.5–1.6. On the basis of modeling results of hot rolling in a Gleeble 3500 unit and a laboratory duo 300 rolling mill, it is shown that with the use of a high temperature for workpiece heating and for the end of rolling it is possible to achieve good strength properties (ultimate strength more than 850 MPa) for lowcarbon Ti–Nb–V- and Mo–Ti-microalloyed steels. Test results for clad rolled product prepared from four two-layer workpieces with a different combination of steels in layers points to the achievement not only of high strength for low-carbon microalloyed steels of one layer, but also other mechanical properties, and also high quality layer joint properties.

A new low-carbon microalloyed steel wire in drilling rope

SAE 1065 steel wire ropes are prone to individual wire fracture due to the martensite structure with high carbon and high microhardness formed on the wire surface because of friction. A new low-carbon microalloyed steel wire composed of lath bainite with fine carbides dispersed in bainitic ferrite and lath martensite is developed in this paper. For the developed steel wire with a diameter of 1.0 mm, the tensile strength and low-cycle fatigue resistance are higher than those of commercial SAE 1065 wire. It is shown that the individual wire fracture caused by as-quenched martensite formation and severe plastic deformation can be avoided in the developed steel wire. Also, the design process and associated manufacturing process are discussed in detail.

Effect of vanadium addition on the microstructure and mechanical properties of low carbon micro-alloyed powder metallurgy steels

Abstract In the present experimental work, the effects of vanadium additions on the microstructures and mechanical properties of powder metallurgy (PM) steel and microalloyed powder metallurgy (PM) steels were investigated. The microstructures of the PM steel and microalloyed PM steels were characterized by optic microscope, SEM and EDS. Experimental results showed that vanadium microalloyed steels can be produced by PM technology. The addition of vanadium limits grain growth during austenitization prior to air cooling and increases the strength in the sintered conditions. By limiting austenite grain growth, the precipitates result in significant improvement in strength.

Influence of cooling rate on secondary phase precipitation and proeutectoid phase transformation of micro-alloyed steel containing vanadium

During continuous casting process of low carbon micro-alloyed steel containing vanadium, the evolution of strand surface microstructure and the precipitation of secondary phase particles (mainly V(C, N)) are significantly influenced by cooling rate. In this paper, influence of cooling rate on the precipitation behavior of proeutectoid α-ferrite at the γ-austenite grain boundary and in the steel matrix are in situ observed and analyzed through high temperature confocal laser scanning microscopy. The relationship between cooling rate and precipitation of V(C, N) from steel continuous casting bloom surface microstructure is further studied by scanning electron microscopy and electron dispersive spectrometer. Relative results have shown the effect of V(C, N) precipitation on α-ferrite phase transformation is mainly revealed in two aspects: (i) Precipitated V(C, N) particles act as inoculant particles to promote proeutectoid ferrite nucleation. (ii) Local carbon concentration along the γ-austenite grain boundaries is decreased with the precipitation of V(C, N), which in turn promotes α-ferrite precipitation.

Thermo-mechanically controlled processed ultrahigh strength steel: Microstructure, texture and mechanical properties

A low-carbon microalloyed steel containing high Ni and Cu content has been developed and subjected to thermo-mechanical processing by varying the finish rolling temperature (FRT∼850–750°C) and cooling rates (air cooling and water quenching). Microstructures of air cooled samples consist of granular bainite and lath or plate-like bainite, whereas, water quenched samples exhibit a mixture of lower bainite and lath martensite. A refinement in microstructure has been noticed with the decrease in FRT and increase in cooling rate. Transmission electron microscopy demonstrates the presence of coarse (Ti, Nb)C precipitates (~90–160nm) and fine Cu precipitates (<20nm). Macro-texture and micro-texture results reveal the dominance of Goss and rotated Goss texture components, which strengthened with the decrease in FRT and increase in cooling rate. The proposed steel composition and TMCP schedule have offered YS ∼ 1000MPa, UTS ∼ 1400MPa, total elongation greater than 10% maintaining a low YS: UTS ratio (0.68–0.80). Such a satisfactory combination of tensile properties achieved in as-cooled or as-quenched conditions (without the need of any tempering treatment) makes the steel suitable for automotive application.