Influence of titanium on the static recrystallization of a medium carbon microalloyed steel

Static recrystallization kinetics of three medium carbon steels, microalloyed with vanadium and titanium (titanium varied from 0.003 to 0.039% in weight), were studied by hot torsion test simulation. A higher recrystallization time was observed in the steel with 0.019 wt% Ti. This difference in recrystallization time was checked by metallographic observations and mechanical softening. This time shift implies different activation energies, which were calculated by the time needed to obtain a 50% recrystallized structure and also by solving the Zener-Hollomon equation. Evolution of the kinetics of recrystallization as a function of temperature was also studied. In addition, the critical allowable temperature for a fully recrystallized structure was investigated.     

Influence of vanadium on the static recrystallization of austenite in microalloyed steels

Torsion tests were conducted to study the static recrystallization of three microalloyed steels manufactured by electroslag remelting (ESR) with different percentages of vanadium, 0.043%. 0.060% and 0.095%, respectively, and approximately equal percentages of the other alloyforming elements. It was seen that, in contrast to niobium, dissolved vanadium has no influence on the activation energy. The influence only becomes notable when the precipitates start to form and the activation energy increases rapidly, thus inhibiting recrystallization. The critical temperature at which inhibition commences was measured as a function of the vanadium content and the deformation performed, and in all cases it was lower than the dissolution temperature deduced from the solubility products for nitrides, mainly because the testing conditions lacked thermodynamic equilibrium. Finally, a comparison was made of the microstructures obtained in two commercial steels, namely a C-Mn type steel and a vanadium microalloyed steel. Both were subjected to the same cycle of successive deformations, whose temperatures were lower than the critical temperature. After the last deformation, a much harder austenite was obtained in the microalloyed steel than in the C-Mn steel.     

Nanoindentation response of interphase precipitation in a Nb microalloyed steel

ABSTRACT

In this work, dedicated isothermal transformation studies were performed in a Fe–C–Mn–Nb alloy to obtain two types of precipitate morphology embedded in a similar ferritic matrix. Nano-hardness of individual ferrite grain was measured by using an in situ nano-mechanical tester. The peak of nano-hardness distribution from interphase and dispersed precipitate strengthened ferrite corresponds to the nano-hardness range of 2.6–2.8 and 2.5–2.6?GPa, respectively. A wider spread of nano-hardness from interphase precipitate strengthened ferrite was observed and rationalised by considering the variation in inter-particle spacing on slip planes. The ‘pop-ins’ on the load-depth curve was not able to be correlated with the morphology of a precipitate.     

Precipitation in V bearing microalloyed steel containing low concentrations of Ti and Nb

Abstract

The paper describes the precipitation behaviour in a thermomechanically processed V bearing microalloyed steel containing small additions of Ti and Nb (0·007–0·008 wt-%) using analytical transmission electron microscopy. An intriguing aspect is the significant precipitation of titanium and niobium at these low concentrations, contributing to strength. A high density of multimicroalloyed precipitates of (V, Nb, Ti)(C, N) are observed instead of simple TiN, TiC, and NbC precipitates. They are characterised as cuboidal (45–70 nm), spherical (20–45 nm), irregular (20–45 nm), and fine (10–20 nm). Estimation of solubility products of carbides and nitrides of V, Nb, and Ti implies that the precipitation of titanium occurs primarily in austenite. Interphase precipitation of niobium occurs during austenite to ferrite transformation, while complete precipitation of vanadium takes place in the austenite–ferrite region close to completion of transformation. Substoichiometric concentrations of Ti and Nb, the presence of nitrogen, and the mutual extensive solubility of microalloying carbonitrides explains the formation of core shell (triplex/duplex) precipitates with highly stable nitrides ((Ti, Nb, V)N) in the core and carbides ((Ti, Nb, V)C) in the shell. The qualitative stochiometric ratios of triplex and duplex carbonitrides were Ti_0·53Nb_0·35V_0·12 and Ti_0·6V_0·4, Nb_0·51V_0·49 and Ti_0·64Nb_0·36. Extensive precipitation of fine carbides on dislocation substructures, and sub-boundaries occurred. They were generally characterised as vanadium carbide precipitates with ordered cubic L1₂ structure and exhibited a Baker–Nutting orientation relationship with the ferrite matrix. M₄C₃ types of carbides were also observed similar to the steel, having high concentrations of Ti and Nb.     

钒氮微合金钢动态再结晶动力学及影响因素

为研究钒氮微合金钢的动态再结晶动力学及影响因素,选取3种对比成分的钒氮微合金钢发生动态再结晶的流变应力曲线,利用硬化速率一应力(θ-σ)曲线获得了饱和流变应力σsat、峰值应力σp、动态再结晶临界应力σc及稳态应力σss的准确值及上述特征应力值与σp的依赖关系,回归得到应变速率敏感的中碳钒氮微合金钢动态再结晶临界应变ε...     

Phase-field model during static recrystallization based on crystal-plasticity theory

A numerical model and computational procedure for static recrystallization are developed using a phase-field method coupled with crystal-plasticity theory. In this model, first, the microstructure and dislocation density during the deformation process of a polycrystalline metal are simulated using a finite element method based on strain-gradient crystal-plasticity theory. Second, the calculated data are mapped onto the regular grids used in the phase-field simulation. The stored energy is calculated from the dislocation density and is smoothed to avoid computational difficulty. Furthermore, the misorientation required for nucleation criteria is calculated at all grid points. Finally, phase-field simulation of the nucleation and growth of recrystallization is performed using the mapped data. By performing a series of numerical simulations based on the proposed numerical procedure, it has been confirmed that the recrystallization microstructure can be reproduced from the deformation microstructure.     

Effects of V and Nb on static recrystallisation of austenite and precipitate size in microalloyed steels

Recrystallisation and precipitation in two microalloyed steels, one with Nb and the other with V, have been studied. The Nb-steel displayed two plateaus on the curves of recrystallised fraction against time. The difference between activation energies allow to predict the efficiency of different precipitates to strengthen the austenite during hot rolling. RPTT diagrams showed the interaction between both phenomena. It is found that NbCN particles nucleate and grow faster than VCN, but the latter are smaller.     

Study of microstructural evolution during static recrystallization in a low alloy steel

Hot compression tests of 42CrMo steel were carried out on Gleeble-1500 thermo-mechanical simulator. The effects of forming temperature, strain rate, deformation degree, and initial austenite grain size on the microstructural evolution during static recrystallization in hot deformed 42CrMo steel were discussed. Based on the experimental results, the grain size model for static recrystallization was established. It is found that the effects of the processing parameters on the microstructural evolution during static recrystallization are significant, while those of the initial austenitic grain size are not obvious. Additionally, a good agreement between the experimental and predicted grain sizes was also obtained.     

Model for static recrystallisation critical temperature in microalloyed steels

Abstract

By means of hot torsion tests, the static recrystallisation critical temperature (SRCT) has been determined for 18 microalloyed steels classified into two groups. In one group the metallic microalloying element is vanadium, and in the other it is niobium. In both groups the microalloying element, carbon, and nitrogen contents vary from one steel to another. Tests have been carried out at various strains and strain rates, and recrystallisation–precipitation–time–temperature (RPTT) diagrams have been drawn for each steel in each condition. The SRCT is the asymptote of strain induced precipitation start P _s and end P _f curves, and its determination has permitted the construction of a model that quantifies the effects of all the external variables implicit in hot working such as strain and strain rate, and the internal variables such as austenite grain size and chemical composition of the steel. Hence, the influence of each of these variables has been quantified, and the model's prediction, comparing experimental values with calculated values, gives a correlation index of ～0.9.     

Acicular ferritic microstructure of a low-carbon Mn–Mo–Nb microalloyed pipeline steel

The transformations during continuous cooling and isothermal processes, the effects of hot deformation and the morphology of the final microstructure of a low-carbon Mn–Mo–Nb microalloyed pipeline steel designed for acicular ferrite microstructure were investigated. The results show that there are three independent “C” curves for isothermal phase transformation, i.e., TTT diagram, of low-carbon microalloyed steel, namely, polygonal ferrite–pearlite transformation “C” curve, the massive ferrite transformation “C” curve and the bainitic transformation “C” curve, respectively. Hot deformation accelerates acicular ferrite transformation and refines the steel's matrix. The microstructure of acicular ferrite for pipeline steels was discussed.     

Austenite grain growth modelling in weld heat affected zone of Nb/Ti microalloyed linepipe steel

Abstract

A model to predict the austenite grain size in an Nb/Ti microalloyed steel weld heat affected zone (HAZ) was developed. The present work investigates grain growth behaviour under the influence of pinning carbonitrides. The steel has been subjected to austenitising heat treatments to selected peak temperatures at various heating rates that are typical for thermal cycles in the HAZ. The effect of temperature and heating rate on the grain size is studied. A model is proposed for the dissolution of NbCN precipitates. This model has been coupled to an austenite grain growth model in the presence of pinning particles. This coupling leads to accurate prediction of the austenite grain size along the heating path simulating selected thermal profiles of the HAZ.     

Effect of the welding mode and filler content on the structure of microalloyed Nb/Ti steel weldments

We describe the changes in the proportions of some microconstituents in the structure of the weld metal and the heat-affected zone and analyze the mechanical and technological properties of the welded joints of high-strength microalloyed Nb/Ti steel made by the E-process and MAG-process of welding with modification of the composition of the filler and welding parameters. The influence of different structures of the filler material and welding parameters on the microstructures of the HAZ and weld metal is studied with an aim to explain the changes in the mechanical and technological properties of the welded joints of microalloyed Nb/Ti steel.     

Determination of the critical conditions for the initiation of dynamic recrystallization in boron microalloyed steels

From the present research, the critical conditions associated with the onset of dynamic recrystallization (DRX) of hot deformed boron microalloyed steels were precisely determined based on changes in the strain hardening rate (θ) as a function of the flow stress. For this purpose, a low carbon steel microalloyed with four different amounts of boron (29, 49, 62 and 105 ppm) was deformed by uniaxial hot-compression tests at high temperature (950, 1000, 1050 and 1100 °C) and constant true strain rate (10⁻³, 10⁻² and 10⁻¹ s⁻¹). Results indicate that the critical conditions for the initiation of dynamic recrystallization depend on the temperature and strain rate. In addition, both critical stress σ_c, and critical strain ?_c, were noticed to decrease as boron content increased. Such a behavior is attributed to a solute drag effect by boron atoms on the austenitic grain boundaries and also to a solid solution softening effect. The critical ratios σ_c/σ_p and ?_c/?_p for all boron microalloyed steels remain fairly constant (≈0.82 and ≈0.53, respectively), such values are in agreement with those commonly reported for Al-killed, C-Mn, Nb, Nb-Ti, high carbon and stainless steels.     

Analysis of change in microstructure and properties during high temperature processing of ultralow C and high Nb microalloyed steel

Abstract

To further improve the strength and toughness, the advanced thermomechanical controlled processing has been applied in the development of an ultralow C and high Nb bearing steel. In the present investigation, the effects of processing parameters, consisting of the coiling and starting temperatures in non-recrystallisation region, on the final microstructure and mechanical properties of this steel have been studied by tensile, Charpy impact tests, optical microscopy and transmission electron microscopy. Results indicate that the acicular ferrite dominated microstructure can be greatly refined in grain size with decreasing the starting temperature of finishing rolling. However, for high Nb steels, the too low starting temperature would promote the formation of high temperature transformation products and consequently make against the improvement of mechanical properties. In addition, the optimum temperature window of finishing rolling is found to be also related to alloying levels of austenite stabilising elements. At the high starting temperature of finishing rolling, the precipitation strength contribution increases with increasing coiling temperature. However, the increase in strain accumulation associated with low temperature processing greatly reduces the sensitivity of the precipitation strength contribution to coiling temperature.     

Cellular automata modeling of static recrystallization based on the curvature driven subgrain growth mechanism

A two-dimensional cellular automata model was developed to describe the static recrystallization (SRX) arising from the subgrain growth, the driving force of which is dependent on boundary energy and local curvature. At the same time, the subgrain boundary energy and mobility rely on the boundary misorientation angle. On the basis, a deterministic switch rule was adopted to simulate the subgrain growth and kinetics of recrystallization quantitatively to provide an insight into the grain boundary bulging nucleation mechanism. Microstructure evolutions during SRX in different cases were simulated by the developed model. At the beginning of the simulation, the initial polycrystalline microstructure which contains large number of uniformly distributed subgrains in every pre-existing grain was prepared using simple assumption based on experimental observations. Then, both homogeneous and inhomogeneous subgrain growth phenomena were captured by the simulation with different inter-subgrain misorientation, which showed continuous and discontinuous recrystallization, respectively. The effects of initial mean subgrain radius, distribution of initial subgrains, distribution of inter-subgrain misorientations, and annealing temperature on the recrystallization kinetics were also investigated.     

基于本构分析的中碳钒微合金钢热变形行为

采用Gleeble-1500热模拟试验机对一种中碳钒微合金钢在变形温度900~1 100℃、应变速率0.01~10 s-1条件下的热变形行为进行研究.分别建立了实验钢的幂律、指数和双曲正弦本构方程,观察了实验钢在不同变形条件下的显微组织,得出了实验钢的动态再结晶稳态晶粒尺寸和峰值应变与Zener-Hollomon参数的关系.结果表明:双曲正弦本构方程具有最高的拟合精度;实验钢热变形激活能Q为273.225 kJ/mol,与奥氏体的自扩散激活能(270 kJ/mol)十分接近,说明实验钢在此变形条件下的速率控制机制是扩散控制的位错攀移;显微组织观察表明,实验钢的动态再结晶行为受变形温度和应变速率的影响;拟合得出实验钢的动态再结晶稳态晶粒尺寸(Ds)和峰值应变与Z参数的关系为ln Ds=-0.200 31ln Z+7.941 65和lnεp=0.184 56ln Z-5.373 83.     

Strengthening behavior analysis of weld metal of laser hybrid welding for microalloyed steel

The weld metal with high strength and good toughness was obtained for medium thickness microalloyed steel by using high power laser hybrid welding technology. Mechanical properties of weld metal were evaluated by using room temperature tensile test and low temperature three-side Charpy V-notch (CVN) impact test. The results show that the yield strength and ultimate tensile strength of weld metal are up to 713 MPa and 918 MPa, respectively. Both of them are almost 1.5 times higher than those of base metal. Under the strict three sides CVN condition, the −40 °C low temperature impact absorbed energy is up to 32 J and also higher than that of base metal. Weld metal predominately consists of granular bainite and carbon-free bainite. Both of them mainly contain lath morphology bainitic ferrite. The lath morphology bainitic ferrite with fine grain size plays an important role in higher strength. Dispersive carbide and high density dislocation are found in strengthening weld metal.     

A Fractal Study on Brittle-Ductile Transition forTi-24Al-11Nb Alloy

The mechanical behaviours of Ti-24Al-11Nb alloy in the brittle-ductile transition (BOT) have been investigated by using three-point bending tests. The temperature dependence of the fractal dimensions and fractal characterization of fracture surfaces are presented. The probable mechanism controlling BDT of intermetallic alloys are proposed according to fractal geometry.Additionally. it is found that there is a positive relationship between the fractal dimension and fracture toughness in BDT for Ti-24Al-11Nb alloy     

铁路车辆用V-N-Cr微合金化Q690高强耐候钢组织性能和腐蚀行为

为开发新一代铁路车辆用高强耐候钢,采用两阶段轧制制备V-N-Cr微合金化Q690耐候钢,并进行组织观察和力学性能检测。采用周期浸润腐蚀实验对V-N-Cr微合金化Q690耐候钢与Q345钢进行腐蚀行为研究。结果表明:V-N-Cr微合金化Q690耐候钢的显微组织为多边形铁素体、针状铁素体、板条贝氏体以及少量的M/A岛,屈服强度及抗拉强度分别为695 MPa和815 MPa,冲击性能优异,通过大小角度晶界共同作用,有效阻碍裂纹扩展。两种钢的表面均生成了明显的锈层,腐蚀产物主要包含α-FeOOH,β-FeOOH,γ-FeOOH和Fe₃O₄。腐蚀360 h后Q345钢的平均腐蚀失重速率为1.83 g/(h·cm²),V-N-Cr耐候钢的腐蚀失重速率为0.96 g/(h·cm²),显著低于Q345钢。