Showing papers on "Microalloyed steel published in 2013"

Development of Ti and Mo micro-alloyed hot-rolled high strength sheet steel by controlling thermomechanical controlled processing schedule

Abstract: A new Ti and Mo micro-alloyed hot-rolled high strength sheet steel was developed by controlling thermomechanical controlled processing (TMCP) schedule, in particular focusing on rolling and coiling temperatures. The results revealed that the steels developed were strengthened mostly by a combined effect of ferrite (α) grain refinement and precipitation hardening. The rolling at austenite (γ) non-recrystallization region provided desirable features by introducing an accumulation of local strains (i.e., dislocations), which served as nucleation sites for γ→α transformation during coiling process, making ferrite grain more fine and homogeneous, although the precipitation hardening was somewhat weakened due to the precipitation of large (Ti, Mo)C carbides (i.e., γ region precipitation). Moreover, these fine and homogeneous ferrite grain, and well developed dislocation structure improved impact properties. The TMCP schedule of the rolling at γ non-recrystallization region (final rolling temperature of 880 °C) and coiling at 620 °C was found to provide an attractive combination of tensile and impact properties.

Microstructural evolution and mechanical properties of high strength microalloyed steels: Ultra Fast Cooling (UFC) versus Accelerated Cooling (ACC)

Abstract: We describe here the microstructural evolution and mechanical properties of high strength microalloyed steels processed using different cooling trajectory. Pilot-scale studies demonstrated that high strength of ∼700 MPa can be obtained in a microalloyed steel using ultra fast cooling (UFC) positioned at the exit of hot rolling mill, while the yield strength obtained via the conventional thermo-mechanical controlled processing (TMCP) with accelerated cooling (ACC) is ∼100 MPa less. The underlying reason is that ultra fast cooling positioned immediately after hot rolling enhances strengthening associated with precipitation and grain refinement. Theoretical calculations and experiments indicated that grain refinement and precipitation in TMCP with in-front UFC led to strength increment of ∼49 and 54 MPa, respectively over the conventional TMCP with ACC process. Furthermore, the microstructural characterization indicated that the density of high angle grain boundaries was increased and the average size of precipitates was reduced from ∼34 nm to ∼10 nm, when the cooling pattern is changed from ACC to UFC. The theoretical estimate also indicated that when the cooling profile is changed from the conventional ACC to UFC+ACC, and to UFC, a higher degree of precipitation is responsible for increase in strength in UFC processed hot rolled microalloyed steels.

Dynamic recrystallization behavior of a medium carbon vanadium microalloyed steel

Abstract: The dynamic recrystallization behavior of a medium carbon vanadium microalloyed steel was systematically investigated at the temperatures from 900 °C to 1100 °C and strain rates from 001 s −1 to 10 s −1 on a Gleeble-1500 thermo-simulation machine The flow stress constitutive equation of hot deformation for this steel was developed with the activation energy Q being about 273 kJ/mol, which is in reasonable agreement with those reported before Activation energy analysis showed that vanadium addition in microalloyed steels seemed not to affect the activation energy much The effect of Zener–Hollomon parameter on the characteristic points of flow curves was studied using the power law relation, and the dependence of critical strain (stress) on peak strain (stress) obeyed a linear equation Dynamic recrystallization is the most important softening mechanism for the experimental steel during hot compression The dynamic recrystallization kinetics model of this steel was established based on flow stress and a frequently-used dynamic recrystallization kinetics equation Dynamic recrystallization microstructure under different deformation conditions was also observed and the dependence of steady-state grain size on the Zener–Hollomon parameter was plotted

Structure–mechanical property relationship in low carbon microalloyed steel plate processed using controlled rolling and two-stage continuous cooling

Abstract: Controlled rolling followed by two-stage continuous cooling was carried out in-house to study the microstructure and mechanical properties of a low carbon microalloyed steel plate of medium thickness gauge. The objective of the study was to develop a process of obtaining excellent mechanical properties (strength, toughness, and ductility) in a microalloyed steel. The process is industrially viable because it does not require high degree of cooling and large reduction during thermo-mechanical processing. The study demonstrates that it is possible to obtain yield strength, tensile strength, elongation, and percentage reduction in area 585 MPa, 680 MPa, 29.5%, and 55%, respectively, with total deformation reduction of 50% and small first stage cooling rate. The total impact energy at −20 °C was 140 J. The microstructure consists of polygonal ferrite and acicular ferrite. The strain-induced 20–30 nm precipitates act as nucleation sites for acicular ferrite. The acicular ferrite contributes to high strength and good toughness, while polygonal ferrite provides excellent elongation.

Effect of microstructure on the impact toughness of Nb-microalloyed steel: Generalisation of existing relations from ferrite–pearlite to high strength microstructures

Abstract: The present work relies on the production of selected microstructures through the application of thermal and thermomechanical laboratory tests, followed by mechanical testing and microstructural characterisation. The relations that link the microstructure parameters and the tensile properties have already been discussed and extended from ferrite–pearlite to high strength microstructures in a previous work. Using these results as a starting point, the present work goes a step forward and develops a methodology to consistently incorporate the effect of mesotexture (EBSD) into the existing relations that link the Charpy impact toughness to the microstructure. The result is an extension of the existing equation for the FATT from ferrite–pearlite to high strength microstructures (bainite, tempered martensite). The upper shelf energy for the Nb-microalloyed steel under consideration correlates linearly with the sum of the different terms of the FATT equation (solutes, grain boundary carbides, pearlite and precipitation/dislocations strengthening) excepting the grain size.

Microstructure, texture, property relationship in thermo-mechanically processed ultra-low carbon microalloyed steel for pipeline application

Abstract: An ultra-low carbon Ti-containing microalloyed steel has been thermo-mechanically controlled rolled and water-quenched. Microstructural characterization of the samples finish rolled in the temperature range of 750–850 °C revealed the presence of polygonal ferrite, quasi-polygonal ferrite, acicular ferrite and granular bainite. Micro-texture study showed the dominance of random texture and cube orientation at higher deformation temperatures, whilst, α-fiber and γ-fiber components intensified with the decrease in finish rolling temperature. Characteristic ferrite-bainite microstructure with fine ferrite grain size (

Development of high strength and ductile ultra fine grained dual phase steel with nano sized carbide precipitates in a V–Nb microalloyed steel

Abstract: Ultrafine grained dual phase steel with yield strength of 865 MPa and tensile strength of 1640 MPa with a high work hardening rate and uniform elongation of 7% was produced by cold rolling and intercritical annealing. The fine scale Nb-V based carbides contributed to improving the strength and work hardening.

Artificial neural networks application to predict the ultimate tensile strength of X70 pipeline steels

Abstract: The paper presents some results of the research connected with the development of new approach based on the artificial neural network (ANN) of predicting the ultimate tensile strength of the API X70 steels after thermomechanical treatment. The independent variables in the model are chemical compositions (carbon equivalent), based upon the International Institute of Welding equation (CEIIW), the carbon equivalent, based upon the chemical portion of the Ito-Bessyo carbon equivalent equation (CEPcm), the sum of the niobium, vanadium and titanium concentrations (VTiNb), the sum of the niobium and vanadium concentrations (NbV), the sum of the chromium, molybdenum, nickel and copper concentrations (CrMoNiCu), Charpy impact energy at −10 °C (CVN) and yield strength at 0.005 offset (YS). For purpose of constructing these models, 104 different data were gathered from the experimental results. The data used in the ANN model is arranged in a format of seven input parameters that cover the chemical compositions, yield stress and Charpy impact energy, and output parameter which is ultimate tensile strength. In this model, the training, validation and testing results in the ANN have shown strong potential for prediction of relations between chemical compositions and mechanical properties of API X70 steels.

Determination of the Critical Strains for the Initiation of Dynamic Transformation and Dynamic Recrystallization in Four Steels of Increasing Carbon Contents

Abstract: Hot torsion tests are carried on three plain carbon steels and a Nb microalloyed steel of increasing C concentrations. The tests are performed at strain rates up to 4 s−1 and over the temperature range 743–917°C. The onsets of dynamic transformation (DT) and dynamic recrystallization (DRX) are detected using the double-differentiation method. Both mechanisms are initiated under all testing conditions but one. The critical strain for DT increases with temperature while the reverse dependency is exhibited by the critical strain for DRX.

Prediction of martensite fraction of microalloyed steel by artificial neural networks

Abstract: The final microstructure and resulting mechanical properties in the linepipe steels are predominantly determined by austenite decomposition during cooling after thermomechanical and welding processes. The paper presents some results of the research connected with the development of a new approach based on the artificial neural network to predicting the martensite fraction of the phase constituents occurring in five microalloyed steels after continuous cooling. The in- dependent variables in the model are chemical compositions, niobium condition, austenitizing temperature, initial austenite grain size and cooling rate over the temperature range of the occurrence of phase transformations. For the purpose of constructing these models, 104 different experimental data were gathered from the literature. According to the input parameters in feedforward backpropagation algo- rithm, the constructed networks were trained, validated and tested. In this model, the training and testing results in the artificial neural network have shown a strong potential for prediction of effects of chemical compositions and heat treatments on phase transformation of microalloyed steels.

Strengthening mechanisms in a pipeline microalloyed steel with a complex microstructure

Abstract: The microstructure of a commercial pipeline microalloyed steel has been characterized by optical and electron microscopy considering the particularity of the thermomechanical processing without accelerated cooling. The microstructure was a mixture of polygonal ferrite (PF) and granular bainite (GB). The well-known structure–property relationship for PF microalloyed steels is used in structures where high misorientation boundaries in the acicular ferrite are significant. In order to quantify the contributions of the precipitation strengthening as well as the dislocation hardening, representative carbonitride particles and dislocation densities were determined in sample areas by transmission electron microscopy.

A Novel Approach to Model Static Recrystallization of Austenite During Hot Rolling of Nb Microalloyed Steel. Part I: Precipitate-Free Case

Abstract: A physically based model is developed to describe static recrystallization during the hot rolling of Nb microalloyed austenite. A key feature of the model is a detailed description of the recrystallization nucleation process; the model predicts the recrystallization incubation time as well as the time evolution of the recrystallization nucleation rate. In addition, the effects of static recovery and solute drag on the growth of the recrystallized grains are captured. The predicted recrystallization kinetics and recrystallized grain size are shown to be in good agreement with published data.

Effect of reheat temperature on continuous cooling bainite transformation behavior in low carbon microalloyed steel

Abstract: The effect of reheat temperature on continuous cooling bainite transformation in a low carbon microalloyed steel was investigated using a dilatometer based on welding thermal simulation process. The variation of microstructure was analyzed in detail by means of optical microscope and transmission electron microscope (TEM). The results showed that the morphology of the main microstructure changes from polygonal ferrite to granular bainite with increasing reheat temperature at a given lower cooling rate. For the higher cooling rate, the microstructure is predominantly lath bainite irrespective of the reheat temperature. The specimens with the relatively fine austenite grain size have the lowest bainite start and finish temperatures among the simulated sub-zones of heat affected zone, which is consistent with the result of the bainite lath width size observed using the TEM. Meanwhile, although the prevailing type of impingement mode of transformation is anisotropic growth impingement for all heat treatment processes, the reheat temperature has some influence on the maximum transformation rate and effective activation energy of bainite transformation.

Development of Ultrafine-Grained Dual-Phase Steels: Mechanism of Grain Refinement During Intercritical Deformation

Abstract: Heavy deformation of metastable austenite (below Ae3) or both austenite and ferrite in the two-phase region (between Ar3 and Ar1) is known to develop an ultrafine ferrite grain structure with an average grain size of less than 3 μm. Different dynamic softening mechanisms, such as dynamic recovery, dynamic recrystallization, and dynamic strain-induced austenite→ferrite transformation (DSIT), are responsible for such grain refinement. However, the sequence of those metallurgical events and the temperature range over which any particular mechanism dominates is not yet well understood. The current study throws some light on this aspect by applying heavy, single-pass compressive deformation (with true strain of 1.0) on the microalloyed steel samples over a temperature range of 1173 K to 873 K (900 °C to 600 °C) using a Gleeble simulator (Dynamic Systems Inc., Poestenkill, NY) and water quenching the samples immediately after deformation. The current study showed the dominating effect of the following mechanisms with respect to the deformation temperature: (1) DSIT followed by conventional dynamic recrystallization (Conv-DRX) of ferrite at higher deformation temperatures (≥1073 K [800 °C]), (2) extended recovery and continuous dynamic recrystallization (Cont-DRX) of ferrite at intermediate deformation temperatures (~1023 K [750 °C]), and (3) simple dynamic recovery of ferrite at lower deformation temperatures (≤923 K [650 °C]).

Effect of deformation temperature on niobium clustering, precipitation and austenite recrystallisation in a Nb-Ti microalloyed steel

Abstract: The effect of deformation temperature on Nb solute clustering, precipitation and the kinetics of austenite recrystallisation were studied in a steel containing 0.081C–0.021Ti–0.064 Nb (wt%). Thermo-mechanical processing was carried out using a Gleeble 3500 simulator. The austenite microstructure was studied using a combination of optical microscopy, transmission electron microscopy, and atom probe microscopy, enabling a careful characterisation of grain size, as well as Nb-rich clustering and precipitation processes. A correlation between the austenite recrystallisation kinetics and the chemistry, size and number density of Nb-rich solute atom clusters, and NbTi(C,N) precipitates was established via the austenite deformation temperature. Specifically, we have determined thresholds for the onset of recrystallisation: for deformation levels above 75% and temperatures above 825 °C, Nb atom clusters

Tribological properties of magnetite precipitates from oxide scale on hot-rolled microalloyed steel

Abstract: Abstract Nano-magnetite (Fe 3 O 4 ) particles have a potential to lead to the formation of lubrication tribofilm that reduces the friction and wear in hot steel strip rolling. In this paper, an attempt to fabricate the oxide film with magnetite precipitates from thermally-grown wustite (Fe 1− x O) layer during isothermal cooling of low carbon microalloyed steel, was obtained. The precipitation behaviors were investigated on Gleeble 3500 thermo-mechanical simulator under the humid air with water vapour content of 19.5 vol%. Several types of magnetite precipitates were examined using scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) analysis. The tribological properties of magnetite precipitates were investigated in pin-on-disc configuration. It was found that the dispersed magnetite particles originate from either the pro-eutectoid precipitation above 570 °C or the partial decomposition of wustite below 570 °C. The oxide film on the presence of free particles during eutectoid precipitation could be a lubricant and consequently resist wear, particularly for the oxide scale with a typical thickness in the range of 8 to 11 μm in dry air and moisture atmosphere. Furthermore, characterisation and precipitation process of the oxide scale are discussed, with respect to a probable mechanism to explain the lubricated properties has been proposed.

Effect of Cooling Rates on the Second‐Phase Precipitation and Proeutectoid Phase Transformation of a Nb–Ti Microalloyed Steel Slab

Abstract: During the continuous casting of low-carbon Nb–Ti microalloyed steel, control of the slab surface microstructure and the behavior of the second-phase precipitation are significantly influenced by the cooling rate. Through confocal laser scanning microscopy, the effect of the cooling rate on the behavior of ferrite precipitation both at the grain boundary and within the austenite was observed in situ and analyzed. The relationship between the cooling rate and precipitation of the microalloying elements on the slab surface microstructure was further analyzed by transmission electron microscopy. The results showed that the effect of microalloying element precipitation on proeutectoid ferrite phase transformation is mainly manifested in two aspects: (i) the carbonitrides of microalloying elements act as inoculant particles to promote nucleation of the proeutectoid ferrite and (ii) the carbon near the grain boundary is depleted when the microalloying elements precipitate into carbonitrides, inducing a decrease in the local carbon concentration and promoting ferrite precipitation.

Effect of Dynamic Transformation on the Mean Flow Stress

Abstract: Flow curves were determined in torsion at a series of temperatures on four plain C steels and a Nb microalloyed steel of increasing C concentration. The mean flow stresses (MFSs) pertaining to each experimental condition were calculated from the flow curves by integration. These are plotted against inverse absolute temperature in the form of Boratto diagrams. The stress drop temperatures, normally defined as the upper critical temperature , were determined from these diagrams. These are shown to be about 40°C above the paraequilibrium and about 20°C above the orthoequilibrium temperatures. This type of behavior is ascribed to the occurrence of dynamic transformation (DT) during deformation. The general characteristics of the DT of austenite to ferrite are reviewed. It is suggested that some of the unexpected load drops that have been reported to take place above the Ae3 temperature in strip mills may be attributable to this phenomenon.

Gradient ultrafine ferrite and martensite structure and its tensile properties by asymmetric rolling in low carbon microalloyed steel

Abstract: Asymmetric rolling (AsR) was carried out to achieve a gradient ultrafine ferrite and martensite duplex structure in a commercial low-carbon microalloyed steel and compared with symmetric rolling. Two specific fiber textures 〈110〉//RD and 〈111〉//ND were developed through the thickness when imposing shear strains by AsR. In hibition of plastic instability in the ultrafine ferrite was closely associated with the gradient grain-size distribution, beneficial shear fiber textures and the dispersed martensite, resulting in high strength (~905 MPa) with good ductility (~17%).

Improvement in Mechanical Properties of Microalloyed Steel 30MSV6 by a Precipitation Hardening Process

Abstract: The microstructural evolutions and mechanical properties of vanadium microalloyed steel (30MSV6) during precipitation hardening were studied. The effects of aging temperature and cooling rate on mechanical strength (yield strength and ultimate tensile strength) were similar. Increasing aging temperature or cooling rate firstly increased the mechanical strength of specimens up to their maximum values, which then decreased with further increase in aging temperature or cooling rate. Microstructural evolutions revealed that cooling rate had significant effects on the pearlite interlamellar spacing and size of pre-eutectoid ferrite. Unlike the effect of austenitizing temperature, the pearlite interlamellar spacing and pre-eutectoid ferrite size were decreased by increasing the cooling rate from austenitizing temperature. According to the microstructural evolutions and mechanical properties, the optimal heat treatment process of microalloyed steel 30MSV6 was austenitizing at 950 °C for 1 h, air cooling (3. 8 °C/s) and aging at 600 °C for 1. 5 h. This optimal heat treatment process resulted in a good combination of elongation and yield strength.

Medium-carbon microalloyed steel for engineering machinery caterpillar chain piece and production process thereof

Abstract: The invention discloses a medium-carbon microalloyed steel for an engineering machinery caterpillar chain piece. The medium-carbon microalloyed steel comprises the following chemical components by mass percent: 0.30-0.37% of C, 0.15-0.35% of Si, 0.80-1.50% of Mn, less than or equal to 0.025% of P, 0.005-0.030% of S, less than or equal to 0.055% of Al, 0.0005-0.0035% of B, 0.008-0.15% of V, less than or equal to 0.080% of Ti, less than or equal to 0.30% of Cr, 0.05- 0.30% of Ni, 0.08- 0.35% of Cu, 0.04- 0.15% of Mo, 0.005- 0.020% of Pb, 0.003- 0.030% of Sn, and the balance of Fe and inevitable impurities. The invention also discloses a preparation method for producing the medium-carbon microalloyed steel for the engineering machinery caterpillar chain piece. The metallurgical quality, for example, surface quality, macrostructure, non-metallic inclusion, grain size, metallographic structure and the like, of the steel obtained by the method provided by the invention all meets the technical conditions.