Microalloyed Steel Research Articles

Effect of Initial Microstructure on the Toughness of Coarse-Grained Heat-Affected Zone in a Microalloyed Steel

Toughness of the coarse-grained-heat-affected-zone (CGHAZ) strongly depends on the prior austenite grain size. The prior austenite grain size is affected not only by chemical composition, thermal cycle, and dissolution of second-phase particles, but also by the initial microstructure. The effect of base metal microstructure (ferrite/pearlite obtained by air cooling and martensite obtained by water-quenching) on Charpy impact toughness of the CGHAZ has been investigated for different heat inputs for high-heat input welding of a microalloyed steel. A welding thermal cycle with a heat input of 100 kJ/cm and 400 kJ/cm were simulated on the MMS-300 system. Despite a similar microstructure in the CGHAZ of both the base metals, the average Charpy impact energy for the air-cooled base metal was found to be higher than the water-quenched base metal. Through thermo-kinetic simulations, it was found that a higher enrichment of Mn/C at the ferrite/austenite transformation interface of the CGHAZ of water-quenched base metal resulted in stabilizing austenite at a lower A1 temperature, which resulted in a coarser austenite grain size and eventually lowering the toughness of the CGHAZ.

Effect of strain-induced precipitation on microstructure and properties of titanium micro-alloyed steels

Effect of strain-induced precipitation on microstructure and properties of titanium micro-alloyed steels

Effect of Basicity and Boron Oxide in Slags of the AOD Process on the Equilibrium Interphase Distribution of Sulfur and Boron

The use of fluorspar in modern metallurgical slags, incl. slags of the argon-oxygen decarburization (AOD) process, as a fluxing agent, is associated with many disadvantages. Those disadvantages can be solved by using boron oxide as an alternative, which also provides conditions for direct microalloying of steel with boron. The paper presents the results of thermodynamic modeling of the effect of basicity and boron oxide content in slags of the CaO–SiO2–B2O3–Cr2O3–Al2O3–MgO system on the equilibrium interphase distribution of sulfur and boron, and their equilibrium content in the metal. Modeling was carried out using the HSC 8.03 Chemistry software package (Outokumpu). Slag from the desulfurization period of the AOD-process was used as the oxide phase. As a result, it was shown that, in the range of basicities 2.0-2.5 and a content of 2-4% B2O3, it is possible to carry out desulfurization of the metal, providing a sulfur content of 0.001-0.007%, and simultaneous microalloying of steel with boron in an amount of up to 0.0103%.

INFLUENCE OF THE CHEMICAL COMPOSITION AND METHOD OF SMELTING ON THE PROPERTIES OF THE METAL OF WELDED JOINTS OF ECONOMICALLY ALLOYED CHROMIUM-MANGANESE-NICKEL STEELS

The results of studies on the influence of the chemical composition on mechanical properties of the base metal and the metal of the near-seam zone of welded joints of ferrite-austenitic steels are presented. The positive effect of microalloying of steels with calcium and cerium on the resistance to embrittlement of the metal after exposure to the thermal welding cycle is shown. It is established that alloying ferrite-austenitic steels with molybdenum and vanadium reduces the tendency to grain growth during welding heating, and doping with nitrogen leads to the stabilization of the phase composition of steels under the influence of elevated welding and operating temperatures.

Effect of processing by high temperature Ex-ECAP on the microstructure, work hardening behavior and electrochemical properties of Ti-Nb microalloyed steel

Thanks to their good ductility and corrosion resistance, micro-alloyed steels have been widely used in many industries, such as manufacturing car's body. Ductility of metallic materials is influenced by their work-hardening behavior, which in turn varies with respect to grain refinement. In the present study, the effect of grain refinement using Ex-ECAP on work hardening behavior and electrochemical properties of Ti-Nb micro-alloyed ferritic steel was evaluated. For this purpose, tensile testing at room temperature and a series of electrochemical analyses (i.e. electrochemical impedance spectroscopy and potentiodynamic polarization test) were used. Two-stage strain hardening behavior was observed for both the coarse-grained and ultrafine-grained samples. It was concluded that uniform elongation of the coarse-grained steel is higher than that of the ultrafine-grained steel. Whereas, the elongation of Ex-ECAPed steel was related to non-uniform deformation occurring after the maximum stress in the engineering stress-strain curves. Results of electrochemical analysis indicated that the passive film formed on surface of iron in contact with acidic solutions is based on iron oxides, such as magnetite (Fe3O4), and hematite (Fe2O3). It was also concluded that the stability of the passive film formed on surface of micro-alloyed steel is increased along with its corrosion resistance through grain refinement achieved by high-temperature Ex-ECAP process.

Causes of Corner Cracks in Hypoperitectic Microalloyed Steel Billets

The cover image illustrates the causes of corner crack in hypoperitectic microalloyed steel billets. The hot ductility, solidification shrinkage, alloy composition, microstructure and segregation behavior of steel are analyzed in detail. In this study, article number 2100035, Yanping Bao and co-workers present a new mechanism for generating corner cracks in a billet.

Features in the formation of the structural state of low-carbon micro-alloyed steels after eletron beam welding

Welding thermomechanically-strengthened materials is accompanied with certain difficulties in terms of loss of strength characteristics in the zone of thermal influence. This issue can be resolved by using the technological welding schemes that include fusion of materials in a narrow contact area. One such technological scheme is electron beam welding, which is currently widely used to fabricate structures from refractory and chemically active materials. One of the main advantages of the electron beam welding process is a small quantity of heat input, which leads to the formation of narrow zones of melting and thermal influence and, as a result, minor deformities in the structure of the material. The welded joint can structurally be divided into several zones, which differ in the morphological characteristics of the structure. The most interesting, in terms of ensuring the quality of the joint, are the boundaries between the zones. It has been shown that the use of local heating sources, which is the case at electron beam welding, leads to the migration of the boundaries of grains. As a result, clear intersections, fusion lines, form at the boundaries between zones of the welded joint. The formation of the structural state of a welded joint is predetermined by the simultaneous course of several processes. First, a crystallization from the liquid state – the formation of a welded joint structure, as well as the boundary between a welded joint and the zone of thermal influence. Second, the phase-structural transformations in the solid state – a thermal impact zone, the boundary between a thermal impact zone and the main metal. Given this, one should note that the geometry and quality of joints at electron beam welding are more interrelated than in other welding techniques. Thus, one of the main parameters that ensure the quality of a welded joint is the structural state of the material that forms during welding

Phase Transformation Law of Nb Microalloyed Steel at Different Cooling Rates

Through the Gleeble3500 thermal simulation test machine, the phase transformation law of Nb microalloyed steel was studied and tested. After the compression deformation, it was cooled to room temperature at different speeds. Obtain the dynamic continuous cooling transformation diagram and the scanning structure diagram of the test steel, and then analyze the phase composition under different cooling speeds through JMatPro material performance simulation. The results show that: at a lower cooling speed (0.1°C/s), austenite decomposition is a diffusion-type phase change that takes place in a high-temperature region, and carbon atoms can diffuse sufficiently. At a moderate cooling rate (1°C/s), the bainite phase transition is a semi-diffusion phase transition in which carbon atoms are displaced in a non-cooperative thermally activated transition mode. When the cooling rate is high (15°C/s), the martensitic transformation is a non-diffusion-type transformation carried out in the low temperature region, and the atoms are directly transferred from the austenite lattice to the martensite lattice. With the increase of the cooling rate and the decrease of the transition temperature, from low-speed cooling→medium-speed cooling→high-speed cooling, respectively, the diffusion type phase transition→semi-diffusion type phase transition→the non-diffusion type phase transition. At different cooling rates, the continuous cooling transition diagram simulated by JMatPro is basically the same as the phase transition in the dynamic continuous cooling transition diagram of the test steel, which proves that the simulation prediction of the dynamic continuous cooling transition of the test steel by the JMatPro software has high accuracy and applicability.

Double-Twist Torsion Testing to Assess Partial Recrystallization in Microalloyed Steels

Double-Twist Torsion Testing to Assess Partial Recrystallization in Microalloyed Steels

КОЛИЧЕСТВЕННАЯ ОЦЕНКА ПРЕДЕЛА ТЕКУЧЕСТИ ФЕРРИТО-ПЕРЛИТНЫХ СТАЛЕЙ ПО ПАРАМЕТРАМ СТРУКТУРЫ

In various sectors of the economy, requirements are imposed on the quality of metallurgical products. The event that improves the quality of metallurgical products - thermomechanical treatment (TMT). TMT allows reducing the specific consumption of steel, increasing the service life, reliability and durability of products, which is tantamount to an increase in the volume of finished metal products. The problem of applied materials science is the establishment of a quantitative relationship between the structure and properties of steels and alloys, it underlies the development and creation of new effective ways to improve the operational characteristics of metal products. In the production of long products, (TMT) is increasingly used, which is a combination of two methods of strengthening steels: deformational by plastic deformation and thermal by phase transformations. Revealing the features of the properties of heat-treated steels makes it possible to approach the solution of this problem. The main mechanisms of hardening are solid solution hardening by alloying with relatively cheap alloying elements (Mn, Si) and dislocation and precipitation hardening using hardening heat treatment and microalloying of steel with carbide and nitride-forming elements V (C, N). The article quantifies the approximate contribution of various strengthening mechanisms to the yield stress of carbon and low-alloy steels. For St5ps steel (hot-rolled state), the yield stress is given by solid-solution and grain-boundary hardening (37.4.0% and 28.6%), in low-alloy steel 16G2AF (36.7% and 27.1%), the role of dispersion hardening (28.0%).Thermomechanical treatment of steel grade St.5ps leads to an increase in the value of dislocation hardening up to 27.6% due to an increase in the density of dislocations and the retention of most of the dislocations in the rolled stock during accelerated cooling of hot-deformed austenite.

Colloidal Analysis of Particles Extracted from Microalloyed Steels

AbstractDifferent colloidal particle characterization methods are examined for their suitability to determine the particle size distribution of particles extracted from steels. Microalloyed steels are dissolved to extract niobium and titanium carbonitride particles that are important for the mechanical properties of these steels. Such particles have sizes ranging from several nanometers to hundreds of nanometers depending on the precipitation stage during the thermomechanically controlled rolling process. The size distribution of the particles is analyzed by dynamic light scattering (DLS), analytical ultracentrifugation (AUC), and hollow fiber flow field‐flow fractionation (HF5) and compared to data obtained for reference particles as well as data from electron microscopy, the standard sizing technique used in metallurgy today. AUC and HF5 provide high‐quality size distributions, average over large particle numbers that enables statistical analysis, and yield useful insights for alloy design; however, DLS fails due to a lack of resolution. Important aspects in the conversion and comparison of size distributions obtained for broadly distributed particle systems with different measurement principles and the role of surfactants used in sample preparation are discussed.

Investigation of the influence of wire preheating current on dilution and angular distortion on thick plate of ASTM A709-Gr 36 steel

ABSTRACT In this work, the influence of preheat current of wire on the weld quality is investigated using a water-cooled ASAW process. The weld quality depends on direct process parameters and types of welding, which further influence the mechanical properties of weldments. To achieve a better weld quality, it is utter importance to understand the effect of process parameters on response like area of penetration, area of reinforcement, dilution, and angular distortion. Initially, the experiments were carried out using a bead-on-plate (BOP) technique on micro-alloyed ASTM A709 Gr-36 steel. Furthermore, the mathematical models were developed using the response surface methodology with a centre composite design technique. Then, ANOVA, F, and lack-of-fit tests were used to verify the statistical significance of the mathematical models. The individual and interaction effects of process parameters on response were investigated using perturbation and 3D surface graphs. Finally, the desirability and JAYA algorithm were used to optimize the process parameters to obtain optimum parameters for improving the welding performance. Preheat current has shown significant impact on the dilution and angular distortion.

Variant selection and the evolution of [formula omitted] and [formula omitted] texture in an 800 MPa grade hot rolled low carbon micro-alloyed complex steel

The ambient textures and their evolutions in an 800 MPa grade hot rolled low carbon micro-alloyed complex steel were analyzed. The prior austenite were firstly reconstructed and subsequently, determined the prior austenite texture unambiguously and calculated the corresponding variant fractions. It was found that the ambient texture was concurrently determined by the parent austenite texture and variant selection during the following phase transformation. The major prior austenite texture were B {011}〈211〉 and {011}〈533〉. They primarily give rise to the ambient transformation γ texture of {111}〈23-1〉, {111}〈25-3〉 and {111}〈1-2-3〉 by the selection of B1 or B2 group and α texture of {001}[110] by the selection of B3 group. The austenite textures of {210}〈361〉, {631}〈353〉, {618}〈121〉, also contribute to the overall transformation texture by strong variant selection. However, the austenite texture of Cu {112}〈111〉, Goss {110}〈001〉, S {123}〈634〉 orientations that were commonly transformed in the previous researches were not found in this steel.

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

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.

Study on Flow Behavior and Processing Maps of High‐Ti Low‐C Microalloyed Steel during Hot Compression

Deformation behavior of a high‐Ti low‐C microalloyed steel is studied by isothermal compression tests at temperatures of 800–1100 °C and strain rates of 0.1–10 s−1. The results indicate that the lower the strain rate and higher the temperature of steel, the more likely is the dynamic recrystallization. The stress–strain predicted by the constitutive model, which is established based on the experimental data, are in good agreement with the experimental results. It is noted from the processing maps of steel at various strains that there is instability zone which should be avoided during thermal processing at strain rate less than 1 s−1. The range of 850–1100 °C and 5–10 s−1 strain rate is more conducive to the processing of experimental steel. The movement of grain boundaries, subboundaries, and dislocations is hindered by precipitates in steel. In addition, the mean grain size of austenite at 0.1 s−1 is larger than 5 s−1. This is mainly because low strain rate provides more time for growth of grains.

Machine Learning Hot Deformation Behavior of Nb Micro-alloyed Steels and Its Extrapolation to Dynamic Recrystallization Kinetics

Machine Learning Hot Deformation Behavior of Nb Micro-alloyed Steels and Its Extrapolation to Dynamic Recrystallization Kinetics

Determination of Non-Recrystallization Temperature for Niobium Microalloyed Steel.

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.

Fatigue behaviour of material-adapted fibre-reinforced polymer/metal joints

By regarding the needs and requirements in modern multi-material joining, the Flow Drill Joining Concept (FDJ) was developed at the Chemnitz University of Technology. The technology allows an efficient and material-adapted joining of thin metal sheets with continuous fibre-reinforced thermoplastics, as required in modern lightweight engineering. For a better understanding of their fatigue behaviour, single-lap FDJ joints were examined in quasi-static and dynamic tests regarding shear loads, cross tension and superimposed shear/cross tension loads. By way of example, joints between micro-alloyed steel with high yield strength for cold forming and a continuous glass/carbon fibre-reinforced polyamide 6 were investigated. The fatigue curves show inclinations between k = 8.01 (shear loads) and k = 5.17 (cross tension loads), depending on the applied load angle. The results of the fatigue testings represent a basis for the enhancement of a failure criterion for FRP/metal joints in highly stressed multi-material designs.

Effect of ‘genetic’ rare earth on alloy dissolution behavior in micro-alloyed steel

The solid solubility of alloying elements and microstructures of insoluble second-phase particles of micro-alloyed steel containing genetic and additional rare earth were respectively investigated. Experimental results indicate that adding a rare earth can change the inclusion morphology and promote the formation of a core of composite inclusions with Al, Ca, Mg, S, and a small amount of Ti. The extremely small quantities of genetic rare earth in steel can promote the formation of a solid solution of Nb and Ti. Therefore, although the content of genetic rare earth in steel is negligible, the promotion effect on solid solubility is significant. The addition of a rare earth has no effect on the types of the insoluble second-phase particles; however, it has a distinct dissolution effect on the large particles in steel.

New Insight on Liquid Steel Microalloying by Pulse-Step Method in Two-Strand Slab Tundish by Numerical Simulations

Developing a technology for introducing alloy addition to liquid steel during the course of continuous casting process seems to be an interesting approach to enhancing the steelmaking process, especially as the effective introduction of micro-additives or non-metallic inclusion modifiers to the liquid steel is the key to the production of the highest-quality steel. This paper presents the results of investigation describing the process of liquid steel chemical homogenisation in the two-strand slab tundish. The alloy was fed to liquid steel by pulse-step method. Five tundish equipment variants with different flow control devices and alloy addition feeding positions were considered. The paper includes fields of liquid steel flow, alloy concentration vs. time curves, dimensionless mixing time, minimum time values and alloy concentration deviations at tundish outlets. The results pointed much more effectively with liquid steel mixing nickel than aluminium. For aluminium obtaining a 95% chemical homogenisation level requires three-fold more time. Moreover, it is definitely beneficial for chemical homogenisation to initiate the alloying process simultaneously in two sites. This procedure generates, among others, the least alloy deviation of concentration at tundish outlets.