工艺参数对过共析钢过冷奥氏体动态转变的影响

利用Gleeble 1500热模拟试验机进行单轴热压缩实验,研究了含Al过共析钢和不含Al过共析钢过冷奥氏体形变过程中,形变温度和应变速率对组织演变的影响。结果表明:过共析钢过冷奥氏体形变过程中的动态转变经历动态相变和相变所得珠光体的动态球化、超细化。过冷奥氏体动态相变抑制晶界网状渗碳体的形成。动态转变所经历的两个过程均为热激活过程,受形变温度和应变速率的影响。降低形变温度,使过冷度增加;降低应变速率,使完成应变的时间延长,两者均减小了过冷奥氏体完成动态相变所需的应变量,使相变所得的珠光体经历的变形程度增加,有利于珠光体实现球化、超细化。合金元素Al的添加阻碍铁、碳原子的扩散,推迟过冷奥氏体动态相变的发生,细化所得的球化、超细化复相组织。     

中碳钢过冷奥氏体形变过程中碳的分布与扩散

利用热模拟压缩变形实验以及SEM、XRD和热磁法,研究了中碳钢过冷奥氏体变形时组织演变过程中碳原子的分布与扩散.结果表明,动态相变过程中碳的有效扩散系数与等温过程相比明显增大,相变完成时间显著缩短.在随后的片层状珠光体的球化过程中,相界以及形变过程中产生的高密度位错和空位等缺陷促进了间隙碳原子的扩散,使得球化动力学过程与等温退火相比显著缩短.渗碳体的溶解和铁素体中碳的过饱和现象得到证实,其中过饱和碳原子高度聚集在铁素体晶界和位错核心处,而不是均匀地分布在铁素体点阵的间隙位置.     

Microstructure evolution during continuous cooling in niobium microalloyed high carbon steels

Effects of microalloyed niobium (Nb) on the austenite decomposition behaviors and microstructure evolution during continuous cooling in the near eutectoid steels were investigated. Compared to the Nb free steel, the Nb microalloyed steel was refined with regard to polygonal ferrite grain, pearlite block and colony sizes. This was because its austenite grain size was smaller. The volume fraction of polygonal ferrite transformed was more in the Nb microalloyed steels, which indicated the eutectoid carbon content exceeded that of pure carbon steel. The spheroidization of pearlite during continuous cooling was enhanced by Nb microalloying, mainly due to a higher critical transformation temperature and the finer pearlite structure with smaller colony size and narrower interlamellar spacing. Hot deformation right above the equilibrium eutectoid temperature accelerated the spheroidization kinetics of pearlite, especially in the Nb microalloyed steel.     

TEM STUDY OF AUSTENITE FORMATION IN A LOW CARBON STEEL

<正> 低碳铁素体加马氏体双相钢的强度和塑性取决于马氏体的数量和分布,这与亚温区加热过程中奥氏体形成的特点有关。由于奥氏体的形成是在高温下进行,速度快,难于直接获得有关形核的信息。本文研究低碳钢中奥氏体形成的部位,以及预先冷轧的影响。 试验用低碳1.5Mn钢,其化学成分为(wt-％):C 0.08,Mn 1.45,si 0.21,Al 0.045,N 0.005。经高频感应电炉熔炼成45kg的钢锭后,热轧成2.5mm厚的板材。在真空炉中经过1200℃均匀化退火24h。然后,在盐炉中900℃加热15min后空冷,获得铁素体加珠光体型的正火原始组织。另将一部分正火的坯料进行厚度压下量     

马氏体温变形超细化过共析钢

利用Gleeble 1500热模拟试验机进行单轴热压缩实验, 研究了含Al和不含Al两种过共析钢马氏体温变形和等温回火过程中的组织超细球化演变及超细球化组织的力学性能. 结果表明: 与马氏体等温回火相比, 马氏体温变形加快马氏体的分解动力学, 在较短的时间 内即获得超细化 (α+θ)复相组织. 温变形过程中的组织超细化演变主要经历渗碳体粒子的析出与粗化及铁素体基体的动态回复和动态再结晶; 而在等温回火过程中, 铁素体主要发生静态回复和晶粒长大, 并没有再结晶现象发生. 合金元素Al的加入在等温回火和温变形过程中均抑制马氏体的分解, 阻碍渗碳体粒子的粗化和铁素体晶粒的长大, 导致复相组织的细化. 同时, Al的加入使马氏体温变形和等温回火后所得超细化 (α+θ) 复相组织在不降低总延伸率的前提下, 强度得以明显提高.     

MICROSTRUCTURE EVOLUTION OF HYPEREUTECTOID STEELS DURING WARM DEFORMATION I. Formation of Equiaxial Ferrite and Effects of Al

利用Gleeble 1500热模拟试验机进行单轴热压缩实验, 结合SEM, TEM和EBSD等方法研究了过共析钢温变形过程中的组织演变规律, 重点讨论了铁素体的等轴化演变过程, 同时考察了合金元素Al的影响. 结果表明: 过共析钢温变形经历片层渗碳体的熔断球化、铁素体的等轴化以及渗碳体的溶解再析出等过程. 温变形初期, 片层状渗碳体缺陷处局部熔断的同时其近邻铁素体内产生大量位错, 并通过动态回复过程形成亚晶; 继续变形过程中, 由于渗碳体粒子钉扎, 亚晶发生转动, 导致大角度晶界的形成, 即铁素体通过连续动态再结晶过程实现等轴化.Al的加入细化了铁素体晶粒尺寸, 提高了等轴状铁素体晶粒大角度晶界的比例.     

Development of microstructure and texture of medium carbon steel during heavy warm deformation

The microstructure and texture development of a medium-carbon steel (0.36% C) during heavy warm deformation (HWD) was studied using scanning electron microscopy and electron back scattering diffraction. The spheroidization of pearlite is accelerated due to the HWD, which leads to the formation of completely spheroidized cementite already after the deformation and coiling at 873 K (600 °C). The homogeneity of the cementite distribution depends on the cooling rate and the coiling temperature. The cooling rate of about 10 K/s (ferrite–pearlite prior to HWD) and deformation/coiling at 943–973 K (670–700 °C) lead to a homogeneous cementite distribution with a cementite particle size of less than 1 μm. The ferrite softening can be attributed to continuous recrystallization. Even up to fairly high deformation/coiling temperatures of 983 K (710 °C) the texture consists of typical deformation components. During the continuous recrystallization the amount of high angle grain boundaries can increase up to 70% with a ferrite grain size of 1–3 μm. An increase of the cooling rate up to 20 K/s (ferrite–pearlite–bainite prior to HWD) deteriorates the homogeneity of the cementite distribution and the softening of ferrite in the final microstructure.     

亚共析钢搅拌摩擦加工组织与力学性能

采用K40钨钴硬质合金搅拌头对3 mm厚热轧退火态亚共析钢板进行了搅拌摩擦加工,对加工区域的宏观形貌、微观组织及力学性能进行了分析．结果表明,搅拌区和热力影响区为先共析块状铁素体、“针状”铁素体及珠光体,组织转变受动态再结晶和相变共同作用,热影响区组织为等轴状铁素体和片层状珠光体．搅拌摩擦加工对各区域中珠光体及析出渗碳体的分布形态影响显著．搅拌摩擦加工后试样显微硬度明显增加,抗拉强度相比母材提高8.2%,断裂位置位于母材处,加工前后试样断裂形式均为微孔聚合韧性断裂．固溶强化与相变强化对硬度和抗拉强度的提高起主要作用．     

形变条件下过冷奥氏体组织转变特征

结合过冷奥氏体形变的真应力-真应变变化特征和显微组织观察,对Q235钢冷奥氏体的形变过程进行了深入分析,讨论了不同阶段的形变特点,并初步讨论了形变强化铁素体相变和铁素体动态再结晶在获得均匀、超细、等轴的铁素体组织方面所起的作用.     

A computational study of austenite formation kinetics in rapidly heated steels

Surface hardening of steels involves rapid austenitization and subsequent quenching of the surface. The resulting extent of hardening largely depends on the rate of austenitization of the surface under the applied high heating rates. In the present work the kinetics of austenite formation in Fe-C alloys during rapid, non-isothermal heating conditions, characterized by high heating rates and short austenitization periods, were studied by means of computational simulation. Austenitization of lamellar pearlite/proeutectoid ferrite microstructures was simulated by assuming two kinetically distinct stages: i) dissolution of lamellar pearlite followed by ii) dissolution of proeutectoid ferrite. The two stages were simulated by two corresponding 1-D diffusion models employed in series. Numerical solution of the resultant moving-boundary diffusion problems provide calculated results regarding the dependency of vol. fraction austenite on thermal cycle parameters and on initial microstructural features of the steel. Analysis of calculated results showed that the vol. fraction of pearlite transforming to austenite during pearlite dissolution depended on maximum temperature, dwell time and pearlite interlamellar spacing. A functional relationship between these variables, consisting of a thermodynamic and a kinetic term, was established. On the other hand, the total vol. fraction of austenite forming in the steel, after both stages of austenitization, was found to follow a typical sigmoidal kinetic behaviour.     

不同形变条件下非调质钢45MnSiVSQ的连续冷却转变

在Gleeble-3500热模拟机及热膨胀试验仪上测定了45MnSiVSQ钢动态及静态膨胀曲线,并采用切线法结合组织及硬度,测定了试验钢的静态和动态连续冷却转变(CCT)曲线,研究分析了形变温度和冷却速度对非调质钢45MnSiVSQ相变及珠光体片层间距的影响。结果表明:在0.1~3 ℃/s冷却速度范围内,珠光体片层间距随着冷却速度的增大而减小;对比950 ℃的动、静态CCT曲线可知,形变使试验钢相变起始温度有所升高,即相变孕育期缩短,其中对铁素体和珠光体相变区间影响尤为明显,而对贝氏体和马氏体相变区间孕育期的影响较小,表现为动态CCT曲线相比静态CCT曲线向左上方移动;对比不同形变温度下的动态CCT曲线可知,形变温度950 ℃时,贝氏体相变冷速区间为0.5~20 ℃/s,850 ℃形变时的贝氏体相变冷速区间为0.8~10 ℃/s。低温形变更利于铁素体和珠光体相变发生,减少了贝氏体、马氏体等非理想组织出现的机率。     

奥氏体形成机制

以T8等钢为例研究并综合分析了珠光体逆共析转变为奥氏体的过程和机制,指出,奥氏体是碳或各种化学元素溶入γ-Fe中所形成的固溶体。奥氏体在铁素体／渗碳体相界面形核,也可在珠光体领域交界处和原奥氏体晶界上形核。奥氏体以体扩散形式长大。奥氏体形核及长大过程中可同时吞噬铁素体和渗碳体,或只吞噬铁素体片长大,或吞噬铁素体较快,而吞噬渗碳体较慢,完成逆共析转变,最后剩下部分渗碳体。为减少相变积累的畸变和缓和应变能,而调整生长方向,形成了孪晶。奥氏体中存在未溶碳化物,需继续溶解和均匀化。     

Mn-Si-Cr系中碳钢在过冷奥氏体状态下变形时的显微组织演变

将Mn-Si-Cr系中碳钢在过冷奥氏体状态下进行低速率变形, 变形促进先共析铁素体转变, 但未见层状珠光体形成. 铁素体在奥氏体晶界和晶内形核, 以近似等轴状长大、交联, 并分割奥氏体, 形成富碳奥氏体区. 随着变形量的增大, 铁素体可在富碳奥氏体区内部继续形核长大并交联, 导致富碳奥氏体区不断被分割且碳浓度升高, 当碳浓度足够高时, 一次析出球状碳化物可在富碳奥氏体区边界处形成, 尺寸为0.5-1 μm; 变形过程中铁素体的动态回复和再结晶导致碳原子从Cottrell气团中逸出, 在铁素体内部形成几十纳米的二次析出球状碳化物.     

双相不锈钢2205热变形行为的实验

利用扫描电镜的EBSD技术,通过对单道次压缩热模拟实验后淬火试样组织的深入系统分析,研究了2205双相不锈钢热变形过程中的软化行为。研究结果表明,2205双相不锈钢的主要软化机制为铁素体的连续动态再结晶和奥氏体与铁素体之间的相转变,变形速率为主要影响参数,并通过影响应变在两相之间的分配控制组织在变形过程中的软化进程。变形速率很小时,铁素体的动态再结晶和铁素体向奥氏体的相转变是互为竞争的两个软化机制;随着变形速率的增加,主导软化机制转变为铁素体的动态再结晶和奥氏体相向铁素体相的转变。     

变形温度对形变强化相变完成时临界应变量的影响

利用“形变强化相变”机制研究了低碳钢过冷奥氏体在740℃和780℃,10s-1变形时的变形温度对相变完成时临界应变量εc的影响。结果表明,变形温度对εc和组织演变的影响很大。在740℃和780℃变形时,εc分别为0.96和1.39,变形温度降低明显促进了相变。变形温度对εc的影响在组织演变上主要表现为铁素体形核地点的不同。740℃变形时,铁素体由奥氏体晶界形核过渡到以形变带形核为主,形核速率极高;780℃变形时,铁素体由奥氏体晶界形核过渡到在铁素体/奥氏体相界面前沿高畸变区快速形核。     

EFFECT OF SILICON ADDITION ON RECRYSTALLIZATION AND PHASE TRANSFORMATION BEHAVIOR OF HIGH-STRENGTH HOT-ROLLED TRIP STEEL

Because Si is a noncarbide forming element, a multiphase microstructure consisting of ferrite, bainite, and retained austenite, at room temperature, can be formed by controlling the thermomechanical process strictly. The cooling schedules must be restricted by the formation of pearlite and cementite. In the present article, a new integrated mathematical model for prediction of microstructure evolution during controlled rolling and controlled cooling is developed for a typical kind of low carbon Si-Mn TRIP steel, which consists of temperature simulation, recrystallization, and transformation models. The influence of Si contents has been thoroughly investigated. The calculated results indicate that Si retards recrystallization, restrains austenite grain growth as well as accelerates polygonal ferrite transformation.     

电工钢热压缩时动、静态再结晶组织及取向分析

采用热模拟方法及EBSD技术,研究Fe-3 wt%Si电工钢在不同温度下组织的动、静态再结晶及晶粒取向特征,特别是少量奥氏体对铁素体动、静态再结晶组织及取向的影响.结果表明,不同温度形变的组织主要分三类:形变长条铁素体、珠光体和等轴细小铁素体.长条形变铁素体内发生动态回复或连续式的动态再结晶,奥氏体周同的铁素体动态再结晶加速,部分以传统的不连续方式动态再结晶.铁素体、奥氏体都可发生静态再结晶.奥氏体的静态再结晶在1050℃以上明显,铁素体的静态再结晶随温度的升高逐渐进行,最显著的再结晶发生在1050℃.不同温度形变的样品,其形变晶粒取向主要以<111>和<100>为主,小等轴铁素体晶粒除与大形变铁素体取向相近外,出现了<110>取向及其它取向.     

低碳钢压缩变形时的形变不均匀性及其对铁素体转变的影响

宏观的形变不均匀性会导致相变和再结晶的差异 ,从而造成最终组织和性能的变化。本文观察并测量了单向压缩的低碳钢形变强化相变、铁素体热、冷形变时样品中心与边缘的铁素体形貌、珠光体或渗碳体分布和织构的差异。分析了不同区域相变及再结晶时的组织演变特点。讨论了形变储存能、渗碳体粒子及形变温度对铁素体相变及再结晶的影响。     

原奥氏体晶粒度对45V非调质钢连续冷却转变的影响

采用ＦｏｒｍａｓｔｏｒＦ热膨胀分析仪及金相显微镜研究了原奥氏体晶粒度对４５Ｖ非调质钢相变点及显微组织的影响。结果表明,在相同的冷速下,相变前奥氏体晶粒尺寸越大,先共析铁素体量越少,先共析铁素体和珠光体转变温度越低     

Pearlite to austenite transformation in an Fe–2.6Cr–1C alloy

The mechanism of austenite formation, the kinetics of cementite lamellae dissolution and the crystallography of the austenitization from pearlite have been studied in an Fe–2.6 wt% Cr–0.96 wt% C alloy. Austenite grains nucleate after rather long incubation time both at pearlite colony boundaries and at the ferrite/cementite interfaces within a pearlite. Characteristic morphologies of transformation products were observed at various stages of transformation. Particular attention was paid to the structural evolution close to the α/γ interfaces. No indications of the diffusion ahead of the α/γ interface were found. The kinetics of austenite growth is controlled initially by carbon diffusion but in later stages by chromium diffusion. The results are discussed by assuming local equilibrium at the moving interfaces with the software and database ThermoCalc. The method involves the driving force determination for the diffusion of carbon and substitutional element during austenitization. The orientation relationships between ferrite, martensite and cementite were also determined. The possible austenite orientations were evaluated assuming the α/θ, α/γ and γ/θ orientation relationships so far obtained.