High-strength Microalloyed Steel Research Articles

Micro-Alloying Effect on the Inter-Critical Grain Coarsened Heat Affected Zone of a S355 Steel Welded Joint

The inter-critical heat affected zone (ICHAZ) appears to be one of the most brittle sections in the welding of high-strength micro-alloyed steels (HSLA). Following repeated heating cycles in in with temperature ranging Ac1 /Ac3, the ICHAZ will face with an evident toughness and fatigue behavior reduction especially due to martensite-austenite constituent (MA) formation. Microalloying in high strength steels causes the generation of some phases in the matrix able to increase the mechanical properties of the joint. In this paper we report an investigation related to 1000 ppm vanadium addition in the welded joint of a structural S355 steel. The inter-critical zone of ta double pass welded joint is here reproduced by dilatometer, with second peak temperature ranging 720°C-790°C. The residual austenite dependence on inter-critical temperature is analyzed and related to the hardness behavior.

Microstructure evolution and precipitation behavior of hot-rolled high-strength Ti–Mo–V micro-alloyed steel

The microstructure evolution and precipitation behavior of hot-rolled Ti–Mo–V micro-alloyed high-strength steel processed at different final rolling temperatures (FRT) and coiling temperatures (CT) were investigated. The results indicated that the decrease of FRT (880 °C–820 °C) promoted the ferrite refinement and strain-induced precipitation in austenite, but it also led to the decrease of pearlite content, which finally led to a slight difference in properties, and CT obviously changed the phase component, grain size, and precipitation behavior. Appropriate reduction of the CT (650 °C–600 °C) could promote the precipitation of (Ti, Mo, V)C and refine ferrite grains, thus improving strength. As the CT further lowered to 550 °C, the precipitation hardening effect would be seriously lessened owing to the significant reduction of (Ti, Mo, V)C volume fraction, finally resulting in the loss of strength. The optimum mechanical properties were attained for the FRT850-CT600 sample, and the ultimate tensile strength (UTS), yield strength (YS), and total elongation (TE) were 908 MPa, 848 MPa, and 21.5 %, respectively, with precipitation hardening up to 383.51 MPa.

Study on microstructure evolution in welded lap joint configuration of dissimilar advanced high-strength steels using low-frequency laser wobble welding

Laser wobble welding (oscillating laser beam) experiments were performed on high-strength micro-alloyed steels (HSLA) in both a low-carbon state (top plate) and carburized state (lower plate) using various process parameters. The welds were characterized regarding microstructure and microhardness. The cross sections of the welds had a predominantly symmetric geometry and were free of defects. Repeatable heating along an oscillating path resulted in a gradual transformation along heat-affected (HAZ) region of the top HC380LA plate from partial recrystallization through acicular and Widmanstätten ferrite formation to bainite. The microstructure of the lower 16MnCr5 plate experienced self-tempering of HAZ martensite along with bainite formation. The microhardness distribution across the entire fusion zone (FZ) of the dissimilar joint exhibited relatively consistent value, with the most significant variation of 80 HV attributed to the carburized layer of the lower plate.

Optimising Two-Stage Vacuum Heat Treatment for a High-Strength Micro-Alloyed Steel in Railway Spring Clip Application: Impact on Microstructure and Mechanical Performance.

The heat treatment process is a vital step for manufacturing high-speed railway spring fasteners. In this study, orthogonal experiments were carried out to obtain reliable optimised heat treatment parameters through a streamlined number of experiments. Results revealed that a better comprehensive mechanical performance could be obtained under the following combination of heat treatment parameters: quenching temperature of 850 °C, holding time of 35 min, medium of 12% polyalkylene glycol (PAG) aqueous solution, tempering temperature of 460 °C, and holding time of 60 min. As one of the most important testing criteria, fatigue performance would be improved with increasing strength. Additionally, a high ratio of martensite to ferrite is proven to improve the fatigue limit more significantly. After this heat treatment process, the metallographic microstructure and mechanical properties satisfy the technical requirements for the high-speed railway practical operation. These findings provide a valuable reference for the practical forming process of spring fasteners.

Hot Ductility, Homogeneity of the Composition, Structure, and Properties of High-Strength Microalloyed Steels: A Critical Review

High-strength microalloyed steels are widely used in various branches of technology and industry due to the simultaneous combination of high indicators of strength, ductility, fatigue, corrosion resistance, and other service properties. This is achieved due to the reasonable choice of the optimal chemical composition and parameters of temperature-deformation treatment of steel that provide a synergistic effect on the dispersed microstructure and characteristics of excess phase precipitates, which control the achievement of these difficult-to-combine properties of rolled products. Additionally, the improvement of the level and stability of these properties, as well as the prevention of the occurrence of defects, is largely determined by the indicators of the homogeneity of the composition, structure by volume and manufacturability of the metal, and primarily hot ductility, which are controlled by the presence of precipitation of excess phases, including microalloying elements. In accordance with the circumstances noted, in the present review, a generalization, systematization, and analysis of the results of the studies are conducted on the effect of phase precipitates on the hot ductility and homogeneity of composition and structure, depending on the chemical composition and parameters of the temperature-deformation treatment of steel.

Favorable Impact Toughness of High Heat Input Coarse-Grained HAZ in an Ultra-Low Carbon High-Strength Microalloyed Steel

Favorable Impact Toughness of High Heat Input Coarse-Grained HAZ in an Ultra-Low Carbon High-Strength Microalloyed Steel

Microstructure and Impact Toughness of the Coarse-Grain HAZ Physically Simulated with Different Heat Inputs in a 690 MPa Ultra-low Carbon High-Strength Microalloyed Steel

Microstructure and Impact Toughness of the Coarse-Grain HAZ Physically Simulated with Different Heat Inputs in a 690 MPa Ultra-low Carbon High-Strength Microalloyed Steel

Design of a novel austenitising bending process in forming characteristics of high-strength quenched and micro-alloyed steel: Experiment and simulation

A systematic study was carried out on a novel designed austenitising bending process to explore the effects of quenching temperatures on the evolution of microstructure and mechanical properties of high-strength micro-alloyed steel. Results reveal that the qualified parts could be bent into a satisfactory shape without spring-back and cracks in bending temperatures of 850–1000 °C. The simulation was applied to predict the equivalent stress, stress neutral layer (SNL) and plate thickness. Strength presents a decreasing trend with the increase of bending temperature owing to integrated influences of high-temperature dynamic recovery (DRV) and dynamic recrystallisation (DRX). Beyond that, SNL nearby the geometrical centre line throughout the plate thickness and dislocation pile-ups on the tension/compression zones are responsible for the smaller hardness on middle regions while higher hardness on the surface areas. Through austenite reconstruction, a smaller martensite packet size would directly improve the strength of the plates bent at a lower temperature. The prior austenite size may be related to the appearance of martensite variants but not necessary. Furthermore, the high accuracy by comparing the experimental results with simulation would provide a high-value reference for the practical manufacturing process of railway spring clips.

Evolution of Carbides during Prestrain and Tempering

The effect of thermo-mechanical treatment on the microstructural evolution of low carbon micro-alloyed high strength steel was studied by combining prestrain with tempering (PST) in this paper. It was found that the prestrain causes the dislocation to plug up around the grain boundary and carbide, resulting in carbide boundary fragmentation. Moreover, it breaks the thermo-dynamic equilibrium between the matrix and carbide, induces the dissolution of carbon in the high energy state, and then changes the distribution of carbon in the matrix. In the subsequent tempering process, the precipitation regularity of carbide was changed, which promoted the precipitation carbide at low temperature. The influence of carbide precipitation on dislocation can be divided into two stages: the first stage was precipitation induced creep, which promoted stress relaxation; the second stage was precipitation pinning dislocation, which improved material strength and inhibited stress relaxation.

The Selection of Materials for the Auto-Body Deformation Zones

Nowadays, it is important to select the proper material for parts of the car-body’s deformation zones in terms of passenger safety. Due to wide range of high-strength steels on the market, the article presents the way of selecting the material by determining the deformation work and the stiffness constant, both measured by the stretch-bending test. The procedure was verified on a high-strength micro-alloyed steel HSLA, high-strength multi-phase steels DP 600 and TRIP, and an austenitic stainless steel AISI 304. The results indicate the austenitic steel is better to use in the deformation zones for the frontal impact, while multi-phase steels are more suitable for deformation zones at the lateral impact. The regression model describing the dependence of the deformation work measured at the stretch-bending test and the deformation work measured at uniaxial tensile test was defined as well. The model allows predicting the deformation work at stretch-bending test based on the mechanical properties measured by the tensile test.

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

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.

Flow Behavior and Microstructure of a Mo–V–Ti Micro-Alloyed High-Strength Steel

Flow behavior and microstructure characteristics of a Mo–V–Ti micro-alloyed high-strength steel were investigated through hot compression tests with two procedures. In group 1, the specimens were quenched after deformation at temperature range of 800–1100 °C, with strain rate of 10/s. Initiation of dynamic transformation (DT) and dynamic recrystallization (DRX) were confirmed by the evolution characteristics of work hardening rate (θ) during deformation. Critical stresses of both DT and DRX decreased with the increase in deformation temperature. The critical strain for DRX also decreased with the increase in temperature, while the critical strain of DT was less temperature dependent. In group 2, with different holding times at 630 °C after 3-pass deformation, complex precipitates were acquired from all the specimens. For the specimen that was water quenched after deformation, the particles were as fine as 20 nm. For the specimens that were held at 630 °C for 30–120 min, the particles were around 100–200 nm. Mo content in the precipitates decreased with holding time at 630 °C. V content in the particles is much less than Ti content because the Gibbs free energy of TiC is less than that of VC at the same temperature.

Stress relaxation behaviour of hot deformed austenite in high strength micro-alloyed pipeline steel

The objective of the present study is to investigate the static restoration of CMn(V-Nb-Ti) pipeline steel utilising comparative study of the stress relaxation and micro-alloying precipitates behaviour. The hot compression tests in temperature range of 850°C to 1,000°C, strains of 0.04, 0.1, 0.25, 0.4 and strain rates of 0.1 and 0.5 s−1 have been carried out and the hot flow stress of austenite has been evaluated by stress relaxation technique at annealing time intervals of 0.05 to 1000 s. The results indicate that at the specific deformation temperature of 850°C, the strain induced precipitation starts with the beginning of annealing time which retards recrystallisation. At higher temperatures than 900°C and strains more than 0.25 and 0.4, first recrystallisation occurs and then the plateau appears. The electron backscattering diffraction (EBSD) image also confirms the partial recrystallisation in a sample quenched at the end of the plateau.

Stress relaxation behaviour of hot deformed austenite in high strength micro-alloyed pipeline steel

The objective of the present study is to investigate the static restoration of CMn(V-Nb-Ti) pipeline steel utilising comparative study of the stress relaxation and micro-alloying precipitates behaviour. The hot compression tests in temperature range of 850°C to 1,000°C, strains of 0.04, 0.1, 0.25, 0.4 and strain rates of 0.1 and 0.5 s−1 have been carried out and the hot flow stress of austenite has been evaluated by stress relaxation technique at annealing time intervals of 0.05 to 1000 s. The results indicate that at the specific deformation temperature of 850°C, the strain induced precipitation starts with the beginning of annealing time which retards recrystallisation. At higher temperatures than 900°C and strains more than 0.25 and 0.4, first recrystallisation occurs and then the plateau appears. The electron backscattering diffraction (EBSD) image also confirms the partial recrystallisation in a sample quenched at the end of the plateau.

Experience of Mastering in NLMK Dansteel Production of Arctic Usage High-Strength Microalloyed Steels F240, F460, and F500 for Shipbuilding and Marine Structures

Shipbuilding remains one of the most metal-consuming segments of the transport industry. For various types of vessels there is a trend of a successive increase in specifications for the specific volume of freight transport that predetermines an increase in the use of materials for construction, including thick steel plate. Considering the general trends in development of the thick plate market for shipbuilding, NLMK DanSteel has mastered production technologies for rolled product up to 35 mm thick produced from low-carbon microalloyed extra high-strength steels F420, F460, F500 intended for vessels of various types for Arctic application. Pilot plant, certified and industrial batches of heavy plates have shown a high level of strength, low-temperature impact strength at – 60°C, and a high level of welded joint mechanical properties employing various welding methods with heating input in the range from 15 ± 1 to 50 ± 2 kJ/cm. The results obtained provide NLMK DanSteel with favorable certification test results for production of extra high-strength grades F420, F460, F500 and their analogs in thickness up to 35 mm according to the IACS rules and confirm the quality of heavy plates in accordance with the requirements of DNV-GL and other marine certification association classification programs and standards.

Microstructural Characteristics and Mechanical Properties in the Laser Beam Welded Joints of High-Strength Microalloyed Steel

The laser beam welded joints of the high-strength microalloyed steel consist of the base metal (BM), the partially transformed heat-affected zone I (PTHAZ-I), the partially transformed heat-affected zone II (PTHAZ-II), the fully transformed heat-affected zone (FTHAZ) and the fusion zone (FZ). The martensite/austenite (M/A) islands of the bainites in the PTHAZ-II are dense versus those in the PTHAZ-I, and the martensites exist in the FTHAZ and the FZ together with the bainites under the slow welding speed. The M/A islands of the bainites and/or the martensites in the PTHAZ become denser under the high welding speed, and the martensites in the FTHAZ and the FZ predominate. The low-angle boundary density and the high local misorientation proportion are used to identify the constituents roughly, consistent with the microstructural characteristics in the welded joints. The misorientation angle distributions are employed to evaluate the degree of the phase transformation, reflecting the change law of the degree of the phase transformation in the welded joints. The hardness and the tensile tests indicate that the HAZ and the FZ are stronger than the BM, dependent on the microstructures in the welded joints. The strengthening in the HAZ and the FZ is mainly attributed to the products of non-equilibrium phase transformation with the high dislocation density.

Effect of Cooling and Isothermal Holding on the Amount of Martensite/Austenite (M/A) Constituents, Microstructure and Mechanical Properties of Microalloyed Pipeline Steel

Constant increase of energy consumption in modern industry requires construction of heavily loaded pipelines with high throughput capacity. Therefore, high-strength steels should be used for the cost reasons. Additionally, the pipelines are also often used in the areas with cold climate and high seismicity. Therefore, strength and plasticity reduction is unacceptable. Bainitic steels with retained austenite (RA) or martensite/austenite (M/A) constituents meet these requirements. The purpose of this investigation is to determine thermo-mechanical treatment parameters with further accelerated cooling and additional isothermal holding for M/A-phase and mechanical properties formation. Experimental modeling of the production process was carried out using Gleeble HDS-V40 thermo-mechanical simulator. All investigations were realized with two high-strength micro-alloyed steels with different molybdenum and carbon content. Results showed that decrease of temperature and duration of isothermal holding as well as addition of molybdenum promote bainitic microstructure nucleation and reduce grain size and M/A-constituents. All these factors lead to a slight improvement in mechanical properties.

Simulating toughness properties under varying temperatures with micromechanical and phenomenological damage models

The sustainable and efficient use of high-strength micro-alloyed steels is of particular interest in today’s industry. Using modern steels with high strength and excellent toughness enables thinner constructions, for example in infrastructure applications, for pressure vessels or pipelines. Sufficient toughness is of special importance to avoid catastrophic brittle failure. Yet, the simplified, conventional design rules prevent a full exploitation of the properties of modern high strength steels. Modern simulation models are able to represent the actual failure characteristics of high strength steels and thereby help to fully exploit the potential of these materials. To include toughness properties, it is essential that the models are able to describe the transition behavior of these steels. The materials’ behavior in the transition region is characterized by a competition of ductile and cleavage failure mechanisms. To simulate the corresponding behavior, it is necessary to adapt damage mechanics models for considering both effects. While numerous studies on the simulation of either ductile or cleavage failure exist, only a limited selection is available on investigations in the transition range. Therefore, the presented study investigates two representatives of the most popular groups of ductile failure models. The Gurson-Tvergaard-Needleman model (GTN) is investigated as a micromechanical model while the Modified-Bai-Wierzbicki model (MBW) is selected as a phenomenological model. Both are combined with the Orowan cleavage fracture model. Investigations are performed on a thermomechanical rolled S355, for which the cleavage fracture stress and the parameters for the ductile failure models were experimentally determined. The study compares simulations of Charpy impact toughness tests to experimental results to determine which model class delivers results that are more accurate. The focus of the investigation is hereby set on the simulation of the lower shelf and the lower transition area. In addition, the efficiency of the simulations performed is also evaluated.

Effect of Hot Rolling and Thermal Treatment on the Microstructure Evolution of Microalloyed Bainitic Steels for Pipeline

Heavy plate for pipelines, which are used in earthquake-prone areas, must have among other a good ductility. The ductility is needed to prevent cracking in the case of local plastic deformation. The bainitic steels with retained austenite or martensite meet these requirements. The aim of this investigation is the determination of the thermo-mechanical treatment parameters with which such microstructures can be generated during the heavy plate production or an additional heat treatment. Experimental modelling of the production process for heavy plate was realized on a Gleeble HDS-V40 thermo-mechanical simulator. Moreover, the microstructure evolution processes during cooling on the round out table were investigated by dilatometry. The investigations on the formation of the microstructure during following accelerated cooling and heat treatment combination were carried out. All investigations were realized with two high-strength micro-alloyed steels, one of which was additionally alloyed with molybdenum. Results revealed that the decreasing of the temperature and duration of the heat treatment as well as addition of molybdenum reduce the size of grains and promotes the nucleation of the bainitic microstructure.

Influence of Ti on the toughness of the FGHAZ and the CGHAZ of high-strength microalloyed S700MC steels

The influences of titanium on the microstructure and the mechanical properties of three microalloyed steels were investigated. Light optical microscopy as well as scanning electron microscopy were used to determine any changes in microstructure and precipitations. Charpy V-notch impact and tensile tests were performed to examine the mechanical properties. Only the low titanium alloyed steel showed the formation of granular bainite/ferrite in the fine-grained heat-affected zone at 15-s t 8/5 time. All other steels showed a microstructure consisting of tempered and untempered martensite, leading to very similar strength values in all samples. Particle analysis showed the formation of coarse titanium containing precipitations in the two steels with an overstoichiometric titanium-to-nitrogen ratio. These lead to a decrease of the maximum impact energy in the coarse-grained heat-affected zone as well as to an increase of the T 27 transition temperature when compared to the fine-grained heat-affected zone. The drop in impact energy was less in the low titanium alloyed steel, showing the harmful impact of these coarse titanium-containing precipitations. Finally, the comparison between 2- and 15-s t 8/5 times showed that with the right (softer) matrix structure, the detrimental influences of the coarse titanium-containing precipitations can be lessened.