Effect of Niobium on Isothermal Transformation of Austenite to Ferrite in HSLA Low-Carbon Steel

Using thermomechanical simulation experiment,the kinetics of the isothermal transformation of austenite to ferrite in two HSLA low-carbon steels containing different amounts of niobium was investigated under the conditions of both deformation and undeformation.The results of optical microstructure observation and quantitative metallography analysis showed that the kinetics of the isothermal transformation of austenite to ferrite in lower niobium steel with and without deformation suggests a stage mechanism,wherein there exists a linear relationship between the logarithms of holding time and ferrite volume fraction according to Avrami equation,whereas the isothermal transformation of austenite to ferrite in high niobium steel proceeds via a two stage mechanism according to micrographs,wherein,the nucleation rate of ferrite in the initial stage of transformation is low,and in the second stage,the rate of transformation is high and the transformation of residual austenite to ferrite is rapidly complete.Using carbon extraction replica TEM,niobium carbide precipitation for different holding time was investigated and the results suggested that NbC precipitation and the presence of solute niobium would influence the transformation of austenite to ferrite.The mechanism of the effect of niobium on the isothermal transformation was discussed.     

钼、铬对低碳铌钛微合金钢连续冷却转变行为的影响

 为研究合金元素钼、铬对低碳铌钛微合金钢连续冷却转变行为的影响,采用Gleeble-3800热模拟试验机和热膨胀试验方法,测定了钼、铬含量不同的3种低碳铌钛成分微合金钢在不同冷却速度下的相变点,采用光学显微镜及扫描电子显微镜观察了其转变产物的微观组织,同时结合维氏硬度测试,绘制了动态CCT曲线。结果表明,钼和铬均降低奥氏体向针状铁素体转变的相变温度,并且在冷速大于1℃/s时,钼比铬的作用效果更加明显。钼、铬均能抑制先共析铁素体和珠光体的转变,扩大针状铁素体形成冷速范围,并能够显著细化组织。     

The isothermal decomposition of austenite in hot-rolled microalloyed steels

The isothermal decomposition of austenite has been examined in a set of 0.1 C, 1.4 Mn steels containing small amounts of Ti, V, or Nb. The volume fraction of ferrite was measured as a function of transformation temperature and holding time, after hot rolling. Precipitation of carbonitrides, in both the austenite and the ferrite, was examined by electron microscopy of extraction replicas. The decomposition is slowest in the Nb-alloyed steel, in which the start of transformation is delayed and ferrite growth rates are much lower than in the other steels. In the V-alloyed steels, ferrite growth rates are lower than in the plain carbon or Ti alloyed steels. These results are discussed in terms of the effects of carbonitride precipitation in the austenite during high temperature deformation and in the ferrite during transformation. The roles of V and Nb in solution are also considered.     

微合金钢连铸坯表面裂纹敏感性预测模型

为了从凝固及相变特性角度解决微合金钢连铸坯表面裂纹问题,建立了与合金化相关联的初始凝固包晶反应度模型、奥氏体晶粒长大模型、铁素体转变量模型以及碳氮化物的析出模型。结合铸坯实际冷却条件,进一步建立了包晶反应度预测、初生奥氏体晶粒长大、铁素体转变、析出相析出等对铸坯表面裂纹敏感性的预测模型。针对某J55钢连铸板坯,奥氏体晶粒尺寸超过1 mm、铁素体析出量为10%、二次相析出量增加时,横裂纹敏感性最大。表面裂纹敏感性预测模型有助于实现基于成分微调和组织调控的微合金钢连铸、热装等生产过程表面裂纹控制技术。     

Dynamic recovery in Nb-Ti IF steels during hot and warm working

Three ultra low carbon interstitial free steels, containing niobium and titanium, and a plain carbon steel were prepared to investigate their flow behaviour during hot and warm working. When the steels were subjected to compression at constant strain rates in the austenite region they exhibited typical work hardening and dynamic restoration behaviour. When they were worked in the ferrite region, they showed unusual and unexpected behaviour, featuring a rapid and quasi-linear increase in flow stress at a low strain value, followed by a decrease with further increasing strain. Such abnormal flow behaviour was attributed to the presence of dissolved Nb atoms. When the niobium precipitated or when the steel was free from Nb, the typical work hardening and dynamic restoration curves were obtained during deformation in the ferrite region as well.     

Effects of austenite grain size and cooling rate on Widmanstätten ferrite formation in low-alloy steels

Deformation dilatometry is used to simulate the hot rolling of 0.20 pct C-1.10 pct Mn steels over a product thickness range of 6 to 170 mm. In addition to a base steel, steels with additions of 0.02 pct Ti, 0.06 pct V, or 0.02 pct Nb are included in the study. The transformation behavior of each steel is explored for three different austenite grain sizes, nominally 30, 55, and 100 μm. In general, the volume fraction of Widmanst?tten ferrite increases in all four steels with increasing austenite grain size and cooling rate, with austenite grain size having the more significant effect. The Nb steel has the lowest transformation temperature range and the greatest propensity for Widmanst?tten ferrite formation, while the amount of Widmanst?tten ferrite is minimized in the Ti steel (as a result of intragranular nucleation of polygonal ferrite on coarse TiN particles). The data emphasize the importance of a refined austenite grain size in minimizing the formation of a coarse Widmanst?tten structure. With a sufficiently fine prior austenite grain size (e.g., ≤30 μm), significant amounts of Widmanst?tten structure can be avoided, even in a Nb-alloyed steel.     

低碳锰(铌)钢控轧控冷实验研究

通过TMCP研究了低碳锰钢和添加微量Nb的低碳锰铌钢的组织和力学性能的影响因素.研究了Nb对实验钢显微组织和力学性能的影响.Nb的添加能够细化铁素体晶粒,促进铁素体转变,对贝氏体形成有一定的抑制作用.相对低碳锰钢来说,铌的加入减少了贝氏体的体积分数,贝氏体强化效果减弱,钢的屈强比升高.通过增加冷速和降低卷取温度,可以使低碳锰铌钢获得一定量的贝氏体,综合性能较佳.低碳锰铌钢的主要强化机制有细晶强化、贝氏体相变强化和析出强化.     

铌微合金化抗震钢筋形变奥氏体连续冷却转变

 为了研究铌对高强抗震钢筋生产过程中组织转变的影响,通过热模拟试验对比研究了无铌碳素钢筋及铌微合金化钢筋(铌质量分数为0.03%)形变奥氏体在不同冷却速率下的组织和相变规律,获得动态CCT曲线。研究结果表明,添加0.03%铌使试验钢奥氏体连续冷却转变有明显变化。从连续冷却曲线(CCT)可看出,添加铌后,发生先共析铁素体、珠光体相变的冷却速度范围减小,铁素体、珠光体转变温度降低;贝氏体相变的冷却速度区间整体右移。添加铌能细化组织,各冷却速度下含铌钢的硬度均大于无铌钢。利用TEM对不同冷却速度下含铌钢中析出相进行观察,发现Nb(C,N)弥散分布于钢中,随着冷却速度的增加,析出的Nb(C,N)逐渐减少,析出相尺寸呈先减小后增大的规律,2 ℃/s冷却速度冷却得到的析出相尺寸细小且数量较多。     

Modelling of austenite decomposition of hot‐rolled plain carbon steels under complex cooling conditions

The purpose of the present work is to develop a mathematical model allowing the simultaneous prediction of both transformation product portions and mean ferrite grain size from the same common principles as a result of austenite decomposition during continuous cooling of plain carbon steels. The transformation products considered specifically are polygonal ferrite and pearlite. The model is based on the classical equations of nucleation‐growth theory and also contains some empirical parameters. The chemical driving forces for nucleation and composition of elements at the phase interfaces are derived from thermodynamic analysis. Three modes of ferrite nucleation are taken into account that correspond to the nucleation on the austenite grain corners, edges and faces. The model considers the reduction of the nucleation sites due to the occupation of austenite grain boundary surface by ferrite grains. Pearlite transformation starts at the γ/α interface and suppresses further ferrite grain growth. The parameters related to ferrite reaction were determined on the basis of a series of austenite transformation kinetic curves and grain size measurements for a steel with the composition 0.084%C‐0.58%Mn‐0.02%Si obtained by dilatometric technique for cooling rates from 0.032 to 2.5 K/s. The parameters related to pearlite reaction were determined on the basis of the data for a steel with 0.66%C. After determination of the model parameters the model was applied to complex cooling conditions of the run‐out table of the hot strip mill at Voest‐Alpine Stahl Linz GmbH. Predicted ferrite grain size appeared to be 1.2 ?1.3 times smaller than the observed one. With regard to experimental data on grain growth in iron, it was suggested that the underestimation of grain size is due to additional ferrite grain growth occurring after the coiling of the steel sheet. Taking that into account provided satisfactory agreement with observed values.     

Precipitation and fine structure in medium-carbon vanadium and vanadium/niobium microalloyed steels

Hot-rolled and continuously cooled, medium-carbon microalloyed steels containing 0.2 or 0.4 pct C with vanadium (0.15 pct) or vanadium (0.15 pct) plus niobium (0.04 pct) additions were investigated with light and transmission electron microscopy. Energy dispersive spectroscopy in a scanning transmission electron microscope was conducted on precipitates of the 0.4 pct C steel with vanadium and niobium additions. The vanadium steels contained fine interphase precipitates within ferrite, pearlite nodules devoid of interphase precipitates, and fine ferritic transformation twins. The vanadium plus niobium steels contained large Nb-rich precipitates, precipitates which formed in cellular arrays on deformed austenite substructure and contained about equal amounts of niobium and vanadium, and V-rich interphase precipitates. Transformation twins in the ferrite and interphase precipitates in the pearlitic ferrite were not observed in either of the steels containing both microalloying elements. Consistent with the effect of higher C concentrations on driving the microalloying precipitation reactions, substructure precipitation was much more frequently observed in the 0.4C-V-Nb steel than in the 0.2C-V-Nb steel, both in the ferritic and pearlitic regions of the microstructure. Also, superposition of interphase and substructure precipitation was more frequently observed in the high-C-V-Nb steel than in the similar low-C steel.     

Simulation of austenite decomposition in continuous cooling conditions: a cellular automata-finite element modelling

Transformation of austenite to ferrite under continuous cooling condition was investigated. The heat conduction problem was managed by finite element method while two-dimensional cellular automata modeling was simultaneously performed to predict the progress of austenite decomposition using a two-step algorithm to reduce surface-to-volume ratio. Continuous cooling experiments on low carbon steel were made and the ferrite structure was determined and compared with the simulation data. The predicted and the experimental results demonstrated an acceptable consistency and the activation energy for ferrite growth was determined as 171 kJ/mole. The rate of ferrite transformation increased under examined continuous cooling conditions owing to higher nucleation rate. Moreover, the initial austenite grain size has shown a significant impact on the rate of transformation e.g. in air-cooled samples as the austenite grain size decreased from 24 to 34 µm, the mean ferrite grain size decreased about 8 µm.     

Thermomechanical processing of microalloyed powder forged steels and a cast vanadium steel

The effects of controlled rolling on transformation behavior of two powder forged (P/F) microalloyed vanadium steels and a cast microalloyed vanadium steel were investigated. Rolling was carried out in the austenitic range below the recrystallization temperature. Equiaxed grain structures were produced in specimens subjected to different reductions and different cooling rates. The ferrite grain size decreased with increasing deformation and cooling rate. Ferrite nucleated on second phase particles, deformation bands, and on elongated prior austenite grain boundaries; consequently a high fractional ferrite refinement was achieved. Deformation raised the ferrite transformation start temperature while the time to transformation from the roll finish temperature decreased. Cooling rates in the cast steel were higher than in P/F steels for all four cooling media used, and the transformation start temperatures of cast steels were lower than that of P/F steel. Intragranular ferrite nucleation, which played a vital role in grain refinement, increased with cooling rate. Fully bainitic microstructures were formed at higher cooling rates in the cast steel. In the P/F steels inclusions and incompletely closed pores served as sites for ferrite nucleation, often forming a ‘secondary’ ferrite. The rolling schedule reduced the size of large pores and particle surface inclusions and removed interconnected porosity in the P/F steels. Formerly Postgraduate Researcher in the Department of Metallurgy and Materials Science, UMIST/University of Manchester, United Kingdom     

Effect of Austenite Deformation on the Microstructure Evolution and Grain Refinement Under Accelerated Cooling Conditions

Although there has been much research regarding the effect of austenite deformation on accelerated cooled microstructures in microalloyed steels, there is still a lack of accurate data on boundary densities and effective grain sizes. Previous results observed from optical micrographs are not accurate enough, because, for displacive transformation products, a substantial part of the boundaries have disorientation angles below 15 deg. Therefore, in this research, a niobium microalloyed steel was used and electron backscattering diffraction mappings were performed on all of the transformed microstructures to obtain accurate results on boundary densities and grain refinement. It was found that with strain rising from 0 to 0.5, a transition from bainitic ferrite to acicular ferrite occurs and the effective grain size reduces from 5.7 to 3.1 μm. When further increasing strain from 0.5 to 0.7, dynamic recrystallization was triggered and postdynamic softening occurred during the accelerated cooling, leading to an inhomogeneous and coarse transformed microstructure. In the entire strain range, the density changes of boundaries with different disorientation angles are distinct, due to different boundary formation mechanisms. Finally, the controversial influence of austenite deformation on effective grain size of low-temperature transformation products was argued to be related to the differences in transformation conditions and final microstructures.

     

Effect of magnesium addition in low-carbon steel part 1: behaviour of austenite grain growth

To reveal the effect of Mg addition on the austenite grain growth in low-carbon steel, the steels containing different Mg contents were refined with a vacuum induction furnace. First, the steels were subjected to the temperature range of 1000–1300°C for a holding time of 30?min. Moreover, using a confocal scanning laser microscope, the growth of austenite grains was investigated under isothermal holding conditions (1400°C), and the γ–α phase transformation was also identified after the samples were subjected to a cooling rate of 5°C?s^?1. It reveals that the grain growth is inhibited by Mg addition after increasing the temperature to 1300°C. The kinetic equations of austenite grain growth were further established by regression analysis based on the experimental results. Furthermore, a significant increase in the proportion of intra-granular ferrite takes place in 0.0026%Mg-added steel at the initial stage of γ?→?α with a cooling rate of 5°C?s^?1. This is mainly attributed to the plenty of Mg-containing inclusions, which are demonstrated to be effective nuclei for acicular ferrite, being in the Mg-added steel.     

Nb(C,N)溶解对高铌奥氏体晶粒长大行为的影响

为了定量研究铌对高铌钢加热过程奥氏体晶粒长大的影响,采用化学溶解过滤分离及电感耦合等离子光谱测定不同加热温度两种试验钢固溶铌质量分数,并对比研究了奥氏体晶粒长大行为。结果表明,在低温条件下,低铌钢固溶铌质量分数高于高铌钢;随加热温度升高,高铌钢固溶铌质量分数快速增加,但即使在1 300 ℃时,铌也不能完成固溶,少量铌存在于(Ti,Nb)(N,C)析出相中;奥氏体晶粒快速长大的温度与固溶铌质量分数快速增加的温度有关。随铌质量分数由0.082%增加到0.120%,奥氏体晶粒快速长大的临界温度由1 050升高到1 150 ℃。高铌钢在1 150～1 250 ℃加热温度范围内,奥氏体晶粒尺寸小于100 μm。     

Effect of hot‐rolling conditions on the tensile properties of multiphase steels exhibiting a TRIP effect

TRIP‐assisted multiphase steels have been thoroughly studied in the cold‐rolled and annealed state. The effects of hot‐rolling conditions on these steels are much less studied even though these are of major importance for industrial practice. This study was carried out in order to understand the effect of the hot deformation of austenite on the tensile properties of TRIP‐assisted multiphase steels. Two different compositions and microstructures are investigated. The first one is a low‐carbon steel (mass content of 0.15 %) with a microstructure consisting of an intercritical ferritic matrix, bainite and retained austenite. The second one is a medium‐carbon steel (mass content of 0.4 %) that consists of bainite and retained austenite. Both steels were deformed to various strain levels below the non‐recrystallisation temperature of austenite. The medium carbon steel was deformed in the fully austenitic temperature range whereas the low‐carbon steel was deformed in the intercritical temperature range. In both cases, the prior hot deformation of austenite brings about a large enhancement of the work‐hardening capabilities. In the case of the medium‐carbon steel, this effect can be attributed to a much larger TRIP effect taking place during straining. In the case of the low‐carbon steel, the improvement of the work‐hardening behaviour was attributed to an Interaction between the martensitic transformation and the dislocations already present within the surrounding ferrite matrix.     

Modeling of Austenite Decomposition in Low Si-Mn TRIP Steel During Cooling

Transformation behavior in low carbon Si Mn TRIP steel was investigated by means of microstructural ob servation and computer modelling. A transformation model in which transformation is controlled by carbon diffusion was suggested, which well described the volume fractional change of ferrite, pearlite, and bainite during continuous cooling. The influence of Si content and austenite grain size was thoroughly investigated. The calculated results indicated that Si retards pearlite transformation, accelerates polygonal ferrite transformation, refines the austenite grain, and increases the ferrite transformation rate.     

铬对超高强冷轧双相钢相变和组织性能的影响

实验室成功试制C-Si-Mn-Cr-Nb系和C-Si-Mn-Nb系超高强双相钢,利用热膨胀仪研究了铬对超高强双相钢相变规律的影响,利用光学显微镜、SEM以及拉伸试验对双相钢的微观组织和力学性能进行检测。实验结果表明:铬使实验用钢的CCT曲线整体右移,抑制铁素体和珠光体的生成,对铁素体开始转变温度影响不大,升高铁素体的终止转变温度,降低贝氏体转变温度,提高奥氏体的淬透性,在相同的冷速条件下,铬的加入更容易得到铁素体+马氏体的双相组织;合金元素铬显著改善双相钢的显微组织,细化晶粒,双相钢的屈服强度从510 MPa升高到535 MPa,抗拉强度从1 080 MPa升高到1 145 MPa,抗拉强度的增幅高于屈服强度,在抗拉强度提高的同时,伸长率升高。     

Sheet metal forming behaviour and mechanical properties of TRIP steels

Recently various kinds of high-strength sheet steels have been developed to meet the requirements of the automotive industry such as passive safety, weight reduction and saving energy. Usually the main problem of high-strength steels is their inferior ductility. Multiphase steels however show a very good combination of strength and formability so that the applicable region of high-strength steels has been widely enlarged. Multiphase steels have been developed for various purposes because of their ability to tailor properties by adjusting the type, the amount, and the distribution of different phases. Especially new developed triple-phase steels which make use of the TRIP effect (transformation induced plasticity) can further improve formability as well as strength due to the transformation of retained austenite to martensite during the deformation. In this work the transformation behaviour and the mechanical properties of low alloyed TRIP steels were investigated. The influence of the annealing parameters on transformation behaviour and on the amount of retained austenite were determined. In addition the temperature dependence of the mechanical properties and the effect of testing speed on the formability were studied. The investigation was carried out on seven different TRIP steels with different chemical compositions, especially the influence of the microalloying element niobium was considered. For reasons of comparison various mild and high-strength steels were tested parallel to the TRIP steels. It was found that the investigated TRIP steels offer very attractive combinations of elongation and strength values. An interesting temperature dependence of the mechanical properties can be observed, in such a way that the elongation values of the TRIP steels possess a maximum between +50 and +100°C. Due to its effect on grain size and on precipitation behaviour the addition of niobium leads to higher strength values without a strong decrease in ductility. In general, the mechanical properties are strongly affected by the type and the distribution of the different phases. The most important parameters, however, to influence the mechanical behaviour are the amount and the stability of the retained austenite, which are mainly controlled by the heat treatment and the chemical composition.     

Nb对2000 MPa桥索钢再加热过程中组织变化的影响

针对不同Nb含量的2种桥索钢,采用热膨胀仪、光学显微镜、扫描电子显微镜和硬度测试仪对其在箱式电阻炉连续加热过程中的组织演变和水冷淬火后的硬度进行了对比分析。结果表明:Nb元素可以细化桥索钢的原始组织,使其存在大量的铁素体和渗碳体的晶界,在连续加热过程中的开始阶段提供更多的奥氏体形核位置,使得奥氏体逆共析转变的起始温度降低,而终了温度升高,逆共析转变区域增大。同时,Nb元素形成的碳化物在加热阶段对奥氏体晶粒的长大具有拖拽作用,降低桥索钢在奥氏体形核后的长大速度,使得淬火后得到马氏体的硬度值降低,因此需要较高的温度来溶解合金碳化物使桥索钢充分奥氏体化。