微量硼对低碳贝氏体钢过冷奥氏体转变的影响

采用Gleeble-3800热模拟实验机对加入微量硼的低碳钢进行了动态CCT曲线的测定和组织分析,研究了钢中微量硼在奥氏体晶界上的偏聚作用。结果表明,微量硼的加入降低了碳原子在晶界上的吸附趋势,导致了晶界处珠光体形核率的下降,改变了动态CCT曲线的形状,抑制了珠光体的转变,能明显提高钢的淬透性和强韧性。     

Unified Kinetics Modeling of Isothermal Bainite Transformation in 60Si2CrA Steel

In this study, isothermal bainite transformation kinetics in hypo-eutectoid steel 60Si2CrA was investigated. Isothermal transformation at temperature ranging from [528 K (255 °C)] to [653 K (380 °C)] after austenitization was performed on a quenching dilatometer. Based on the dilation data, a shear bainite kinetics model was established using the internal state variable (ISV) approach. The model is coupling with bainite phase nucleation and growth, and the incubation time is characterized by the proposed normalized bainite nucleus radius and normalized radius growth rate. Material constants within the unified kinetics model were calibrated with genetic algorithm (GA)-based optimization methodology using GA toolbox in MATLAB. Furthermore, fairly close agreement between model predicted and tested data was achieved, and predicted evolution of ISVs in the model was illustrated. The proposed shear-type bainite kinetics model may be applied to other hypo-eutectoid steels.     

铬对C-Si-Mn-Nb铁素体贝氏体双相钢相变规律的影响

通过比较C-Si-Mn-Nb和C-Si-Mn-Cr-Nb两种实验钢热变形后的CCT曲线,分析了合金元素Cr对铁素体贝氏体双相钢连续冷却过程相变规律和组织演变的影响.结果表明,Cr可轻微抑制实验钢的铁索体转变,细化铁素体晶粒,但对铁素体显微硬度没有影响.在铁素体转变量很少或未转变的情况下,Cr可增强亚稳奥氏体的稳定性和淬透性,细化贝氏体产物中的铁素体板条和MA岛,提高贝氏体的显微硬度.     

硼对780 MPa低碳贝氏体钢组织和力学性能的影响

试验低碳贝氏体钢（/%:0.08C,0.11~0.13Si,1.10~1.20Mn,0.008~0.009P,0.002S,0.21~0.23Ni,0.020~0.021Ti,0.003~0.004Nb,0~0.0010B,0.000 7~0.0008O,0.0031~0.0033N）由50kg真空感应炉熔炼,轧成45mm钢板,并经930℃淬火,610℃回火。研究了0.0010%硼对780 MPa低碳贝氏体钢45mm板组织和力学性能的影响。结果表明,硼可显著提高试验钢的淬透性,不含硼试验钢淬火后得到粒状贝氏体,0.0010%硼试验钢淬火后得到板条贝氏体。硼明显改善试验低碳贝氏体钢的力学性能,含0.0010%硼试验钢淬、回火后的抗拉强度834MPa和屈服强度771MPa远高于不含硼试验钢的抗拉强度702MPa和屈服强度591MPa,实际生产中应加入适量硼可使低碳贝氏体钢得到板条贝氏体。     

V含量对V-Mo-N微合金钢贝氏体区析出行为的影响

通过热模拟和透射电镜(TEM)等研究方法对三种不同V含量的V-Mo-N微合金钢在贝氏体铁素体基体中析出粒子分布、形貌和尺寸进行了观察和分析。结果表明:在V-Mo-N微合金钢中,当Mo的添加量较低时,析出粒子为V(C,N),Mo元素全部固溶于基体中起到固溶强化作用,并不会形成Mo的碳氮化物析出。随着V含量的提高,组织中的贝氏体板条逐渐变细并向粒状贝氏体转变。V含量在0.05%~0.16%时,试验钢析出量随V含量增加而增加,同时,钢的硬度也增加。     

Bainite Transformation in Deformed Austenite

This study documents the characteristics of bainite transformations in deformed austenite during continuous cooling of a low-carbon microalloyed steel. In particular, it describes the distinguishing features of the two types of bainite observed: conventional bainite (CB) and acicular ferrite (AF). CB nucleates at prior austenite grain boundaries and grows as packets of parallel ferrite laths. Growth of CB packets is limited by either the deformation substructure (“mechanical stabilization”) or austenite grain boundaries. AF nucleates at intragranular sites and grows as individual ferrite laths or groups of parallel laths. In an AF group of parallel laths, some neighboring laths have the same orientation, but a significant number have dissimilar orientations. Neighboring laths having dissimilar orientation are two variants of either Kurdjumov–Sachs (KS) or Nishiyama–Wasserman (NW). It is proposed that the AF nucleation sites are dislocation boundaries having a relatively high misorientation (i.e., microbands (MBs) and minimized by the variant selection mechanism).     

Effect of Isothermal Bainite Treatment on Microstructure and Mechanical Properties of Low‐Carbon TRIP Seamless Steel Tube

In this work, a low‐carbon transformation‐induced‐plasticity (TRIP) seamless steel tube (Fe–0.15C–1.34Si–1.45Mn–0.029Nb–0.024Ti, in wt%), having potential in application of hydroforming process, has been successfully manufactured by using piercing, cold‐drawing, and two‐stage heat‐treatment process. The optimal heat‐treatment conditions, inter‐critical annealing (IA), and isothermal bainite treatment (IBT) were firstly obtained to maximize the volume fraction and stability of the retained austenite (RA). The effects of temperature and holding time IBT on the microstructures of the TRIP steel tube were studied via optical microscopy (OM), scanning microscopy (SEM), transmission electron microscopy (TEM), and X‐ray diffractometer (XRD). The mechanical properties in the axial direction and hydroformability were also evaluated by conventional tensile test and flaring test, respectively. Two‐stage heat‐treatment was finally performed to achieve the required mechanical properties for the hydroformed tube. The results shows that the RA volume fraction increased at first and then decreased with the increase of IBT holding time and IBT temperature for a particular set of IA temperature and IA holding time. It was also demonstrated that high tensile strength of 618 MPa, total elongation of 35.5%, n‐value of 0.23, and better hydroformability could be successfully produced in this TRIP steel tube at IA temperature of 800°C, holding for 10 min, and IBT of 410°C for 4 min holding time.     

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The continuous cooling transformation process and the organization performance of transformation product was investigated by means of dilatation test during single pass deformation on Gleeble-3500. The result shows that bainite transition can be happened when the cooling speed about 3??/s on 460B steel with boron, and the grain size is coarse and hardness is higher when the cooling speed (0. 1, 0. 3, 0. 5??/s)is slowly, it is more tiny than without boron. Boron can make the CCT curves shift to right, the transition from austenite to ferrite and pearlite is restrained, the hardenability of the steel are obviously improved, the cooling speed from austenite to martensite is lower and Ac3, Ac1, Ar3, Ar1, Ms is dropped.     

Effect of Mo Addition on the Transformation Stasis Phenomenon During the Isothermal Formation of Bainitic Ferrite

The effect of Mo addition on the transformation stasis (TS) phenomenon during bainitic transformation has been investigated in Fe-0.2C-1.5Mn-1.5Si-(0.2, 1.5, 3)Mo (wt pct) alloys. Both diffusionless and diffusional models failed in predicting the TS that is strongly affected by Mo concentration in experiment. The Gibbs energy balance approach could capture TS in alloys with 0.2 and 1.5 wt pct Mo, but significantly underestimate bainite fraction at stasis for alloy with 3 wt pct Mo, which is deduced to arise from the so-called anti-coupled solute drag effect.     

硼对Q690D钢CCT曲线的影响

利用Formastor-F全自动相变仪测定了不同硼含量Q690D钢的临界点Ac1、Ac3,并绘制Q690D钢的连续冷却转变曲线。结果表明:硼的加入,使Q690D钢的Ac3、Ac1略有提高,使Ms点及Bs点有所降低,使Q690D钢的临界冷速降低,淬透性明显提高。硼对Q690D钢先共析铁素体转变及贝氏体转变均有不同程度的推迟作用,抑制了先共析铁素体的析出,在很宽的冷速范围可以得到完全的贝氏体组织。     

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.     

Bainite Formation Kinetics During Isothermal Holding in Modified High Cr Ferritic Steel

Microstructural observation and high-resolution dilatometry has been employed to investigate the course of isothermal holding at various temperatures during cooling after austenization in a modified high Cr ferritic steel. The formation of bainite during isothermal holding was identified. The amount of bainite increases as transformation temperature decreases. The phenomenon of thermal stabilization of austenite disappears after isothermal holding due to the formation of bainite. The kinetics of incomplete bainite transformation was described by a displacive model in view of autocatalytic nucleation. Kinetics analysis suggests that reduction of holding temperature promotes to bainite transformation by means of an increase in the number of embryos for autocatalytic nucleation and a decrease in activation energy.     

稀土对16Mn钢等温相变及显微组织的影响

采用金相观察与硬度测量相结合的方法测定了不同稀土含量16Mn钢的TTT曲线,并研究了稀土含量对其等温相变及显微组织的影响.研究表明,随稀土含量的增加,先共析铁素体的晶粒大小及其析出的孕育期减小,完成珠光体相变所需时间和组织中铁素体含量增大;贝氏体形态逐渐由羽毛状向粒状转变.含稀土钢组织中的碳化物具有明显的偏聚现象.本文对这些结果产生的原因进行了讨论.     

Effect of Rare Earth Elements on Isothermal Transformation and Microstructures in 20Mn Steel

The effect of rare earth elements on the isothermal transformation and microstructures in 20Mn steel is in-vestigated by means of metallography and dilatometry.Rare earth elements decrease both the incubation periodof pro-eutectoid ferrite and the rate of pearlitic transformation.In addition,rare earth elements play a role ofreducing needle-like ferrite and the amount of pearlite,densifing the lamellar space of pearlite and enhcingsegregation of carbide in granular bainite.It is suggested that rare earth elements may decrease the interfacialenergy of grain boundary and interphase,hinder the diffusion of carbon atoms and form rare earth carbides withhigh melting point which reduce the carbon content in austenite.     

Dilatometric Investigation on Isothermal Transformation after Hot Deformation

Itiswellknownthattheferritetransformationisacceleratedandenhancedbyaustenitedeforma tion ,especiallyinnon recrystallizationregion ,andbysubsequentcoolingwithrelativehighrate[1] .Suchprocessobviouslyrefinestheferritegrainandotherphasetransformationproductsandbecomesaneffectivewaytoimprovethepropertiesofsteel ,basedonwhichthecontrolledrollingandcontrolledcoolingtechniquegoesforwardfurther .　　However ,someyearsago ,itwasfoundthatthedeformationofunder cooledaustenitecaninducefer ritetransformati…