Optimization of flow stress in cool deformed Nb-microalloyed steel by combining strain induced transformation of retained austenite, cooling rate and heat treatment

In this study cool deformation was incorporated in the overall thermo-mechanical processing of a Nb-microalloyed steel. Included in this was the effect of cooling rate subsequent to hot rolling on precipitate formation in the ferrite phase. The results show that increasing the cooling rate prevents precipitate formation in the ferrite phase at the cool deformation temperature. As well, the amount of retained austenite under the low cooling condition in the temperature range of cool deformation, 700-300 °C, was measured by neutron diffraction. It is then shown that strain-induced transformation of retained austenite to martensite is the main factor in increasing the strength of cool deformed Nb microalloyed steel. Combining accelerated cooling, strain-induced transformation of austenite to martensite during cool deformation and a subsequent heat treatment stage to increase precipitation maximizes the flow stress of the steel. Finally, it is shown that this process also lowers the yield strength/ultimate strength ratio.     

控轧工艺对厚壁X70管线钢低温止裂性能的影响

针对壁厚33 mm、管径φ1 422 mm管线用X70钢板在-30 ℃下DWTT性能异常问题,利用扫描电镜和背散射电子衍射技术,分析了两种不同控轧工艺下X70管线钢板的微观组织、晶粒大小及取向等因素对其低温止裂性能的影响。结果表明:粗轧采用7道次,道次压下量21~35 mm,轧后冷却速率19.4 ℃/s的条件下,钢板可获得细小的多边形铁素体、针状铁素体、以及弥散分布的M/A岛组织;同时,由于采用大压下率,提高了钢板心部变形渗透能力,可以细化心部奥氏体晶粒,增加了心部大角度晶界比例;还有,由于钢板厚度方向晶界取向差接近且心部存在38.2%的大角晶界,均有利于提高管线钢板的止裂性能。     

Role of tandem submerged arc welding thermal cycles on properties of the heat affected zone in X80 microalloyed pipe line steel

The influence of thermal cycles on the properties of the coarse grained heat affected zone in X80 microalloyed steel has been investigated. The thermal simulated involved heating the X80 steel specimens to the peak temperature of 1400 °C, with different cooling rates. The four-wire tandem submerged arc welding process, with different heat input values, was used to generate a welded microstructure. The martensite/austenite constituent appeared in the microstructure of the heat affected zone region for all the specimens along the prior-austenite grain boundaries and between the bainitic ferrite laths. The blocky-like and stringer martensite/austenite morphology were observed in the heat affected zone region. The fractional area of M/A particles due to different cooling rate was the main factor in increasing of the hardness values in the coarse grained heat affected zone. The Charpy absorbed energy of specimens was assessed using Charpy impact testing at −50 °C. The martensite/austenite constituent's size such as mean diameter and length are important factors influencing Charpy impact properties of coarse grained heat affected zone. The micro crack nucleation may occur from M/A particles at the intersection of prior-austenite grain boundaries. Similar crack initiation sites and growth mechanism were investigated for specimens welded with different heat input values.     

GRAIN REFINEMENT OF MICROALLYED STEEL THROUGH HEAVY HOT DEFORMATION AND CONTROLLED COOLING

1.IntroductionAsfinegrainedsteelsshowmanyfaVowhlemechanicalpropertiessuchassbength,ductility,impactproperty,etc,grainrefinementhasbeenanimportantitemamongresearchstudies.Niohitunisveryoftenaddedtoimprovestrengthandlow--temperaturetoughnessOfTMCPsteelsbyinhibitingausteniterecrystallizationandgrain~dabingthermomechAncaltreatment.ItisgenerallybelievedthatthehanimtunferritegrainsizeofcommercialTMCPsteelsrangesfrom3to5Urn['--3),butithasbeenrePOrtedthatthefenitegrainsizecanbereducedto1--ZUmusi…     

变形条件对Mn-Cr齿轮钢连续冷却相变的影响

使用Gleeble 1500热模拟实验机研究了Mn—Cr齿轮钢在奥氏体再结晶区不同温度变形后的连续冷却相变行为及相变组织．实验结果表明,变形温度降低,促进了多边形铁索体及珠光体相变,获得多边形铁索体加珠光体混合组织的临界冷速增大．贝氏体与针状铁索体之间存在相互竞争机制,随着变形温度及冷速的降低,大量的晶界仿晶型铁索体占据了奥氏体晶界,中温相变产物由贝氏体向针状铁索体转变．降低变形温度,奥氏体在中温相变区稳定性增加,相变结束温度下降,室温组织中马氏体／奥氏体岛的数量增多．     

23MnNiMoCr54钢的热变形行为

利用DIL 805A型热膨胀仪测定了23MnNiMoCr54钢的热膨胀曲线,结合硬度检验绘制出试验钢的CCT曲线,并对其动态相变及动态再结晶规律进行了研究分析。结果表明,23MnNiMoCr54钢的临界转变点Ac₃=806 ℃,Ac₁=713 ℃,CCT曲线中无珠光体转变区,当冷速≥0.5 ℃/s时,开始发生马氏体相变。变形量为10%时,变形温度在850~1150 ℃范围内时,试验钢的奥氏体晶粒边界稳定,晶粒大小没有发生明显变化,没有发生动态再结晶,软化机制以动态回复为主。变形量为40%时,变形温度在850 ℃时试验钢没有发生动态再结晶,软化机制以回复为主;温度为900~950 ℃时出现了不稳定的奥氏体晶界和细小晶粒,动态再结晶开始发生;温度为1000 ℃时,发生了完全动态再结晶。变形量为50%时,变形温度在850~950 ℃时试验钢出现了不稳定的奥氏体晶界和细小晶粒,发生了部分再结晶;温度为1000 ℃时,发生了完全动态再结晶。变形量为60%时,变形温度在850~950 ℃时试验钢出现了不稳定的奥氏体晶界和细小晶粒,发生了部分再结晶;温度为1000 ℃时,发生了完全动态再结晶。     

Semisolid structure for M2 high speed steel prepared by cooling slope

Effects of cooling slope angle and the temperature of molten metal on the globular structure of M2 high speed steel after holding at the semisolid state have been investigated. The globular structure was achieved by pouring the molten metal at 1595 °C on the ceramic cooling slope with the length of 200 mm and the angle of 25°. The globular structure of M2 high speed steel in the form of rolled–annealed and as cast condition after holding at semisolid state has been achieved. The size of globular grains of cooling slope sample was smaller than that of the rolled–annealed and as cast samples. Solid particles of rolled–annealed sample after holding at semisolid state had better roundness compared with cooling slope sample. Dissolution of carbides in the austenite phase at grain boundaries leads to formation of globular particles in the semisolid state. MC-type and M₆C-type eutectic carbides reprecipitate during cooling cycle along grain boundaries.     

低碳钢形变强化相变的特征

介绍了低碳钢形变强化相变的基本概念及主要特征．系统的研究工作证实了变形显著地加速了低碳钢过冷奥氏体向铁素体的相变过程．形变强化相变是一个以形核为主导的过程直到相变完成以前，形核始终存在于新相与原奥氏体相界面的高应变区．由于几何空间与成分条件上受到一定的限制，长大及各向异性都不太明显，铁素体晶粒超细化．实验工作还证实了转变动力学呈现明显的3个阶段，它们分别与相变铁素体在原奥氏体晶界上的形核，在铁素体／奥氏体相界前沿高畸变区的形核，及被铁素体晶粒所包围的残存奥氏体上的相变形核等过程相对应。     

The role of crystal misorientations during solid-state nucleation of ferrite in austenite

The role of grain and phase boundary misorientations during nucleation of ferrite in austenite has been investigated. Electron back-scatter diffraction (EBSD) was performed on a high-purity iron alloy with 20 wt.% Cr and 12 wt.% Ni with austenite and ferrite stable at room temperature in order to identify the crystallographic misorientation between austenite grains and between ferrite and austenite grains. It is observed that the specific orientation relationships between ferrite and austenite play a dominant role during solid-state nucleation of ferrite. Ferrite grains nucleate on grain faces independently of the misorientation between austenite grains, although random high-angle grain boundaries have a slightly higher efficiency. Different types of nucleation mechanisms are found to be active during ferrite formation at grain faces. A slight deformation of the austenite matrix was found to triple the number of ferrite nuclei during isothermal annealing.     

Understanding the effect of nitrogen in austenitic stainless steel on the intergranular stress corrosion crack growth rate in high temperature pure water

Understanding the effect of nitrogen content on the crack growth rate (CGR) due to intergranular stress corrosion cracking (IGSCC) in high temperature (288 °C) pure water, in non-sensitised and strain-hardened stainless steel (SS) type 304 LN was the focus of this study. Non-sensitised SS containing two different levels of nitrogen (0.08 and 0.16 wt.%) in the solution annealed condition was strain-hardened by cross-rolling at 200 °C (warm rolling). It has earlier been reported that SS with a higher nitrogen level in the warm rolled condition has a higher CGR in high temperature pure water. Tensile testing was carried out using both the SS in the warm rolled as well as in the solution annealed condition at 288 °C. Samples were prepared for transmission electron microscopy (TEM) from the warm rolled SS and from the tensile tested (at 288 °C) specimens. TEM studies indicated that twinning and shear band formation were the major modes of deformation due to rolling at 200 °C and these feature were observed to terminate at grain boundaries, leading to regions of higher strain and stresses at grain boundaries. Higher nitrogen SS has higher grain boundary strain and stresses making the grain boundary regions more susceptible to IGSCC, resulting in higher CGR values. At 288 °C dislocation entanglement and cross-slip were the predominant modes of deformation.     

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     

Twinning, grain orientation and texture variation of AZ31 Mg alloy during compression by EBSD tracing

In order to investigate the micro-mechanism of warm forming of Mg alloys, three specimens cut from a rolled AZ31 sheet were chosen to be compressed along the Rolling Direction (RD) at 100 °C, 170 °C and 230 °C, separately. During compression, an in situ measurement of grain orientation in the plane of RD × TD (Transverse Direction) was carried out with EBSD method. Experimental and analytical results show that temperature has remarkable impact on activation of twinning and variation of texture. As the temperature was raised from 100 °C to 230 °C, the number of grains with twins activated decreased substantially during deformation, and rolling texture varied from quick vanishing at 100 °C to always existing at 230 °C. Tracing for orientation of individual grains during deformation shows that there are obvious different orientation changes between grains with twins activated and those without twins activated. Twinning plays a significant effect on texture variation during compression. The extension twin variant really activated during deformation is the one with maximal Schmid factor.     

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.     

变形温度对含Nb双相钢显微组织的影响

通过单道次压缩及连续冷却实验,研究了变形温度(810-720℃)对具有超细原始奥氏体晶粒的含Nb双相钢显微组织的影响.实验结果表明:实验钢最终组织为铁索体加马氏体的双相组织.压缩过程中,实验钢应力-应变曲线上出现峰值,且峰值应力随变形温度的降低先增大后减小;随着变形温度的降低,铁索体的含量先增大再减小,但增减幅度不大,在最低变形温度(720℃)时,铁素体品粒尺寸降低到2.8 μm,弥散分布于铁素体晶界上的马氏体含量达到22.7%;随着变形温度的增加,铁索体晶粒硬度减小,最低可降至230 GPa;EBSD取向分析显示,随着变形温度的降低,组织中小角度晶界增多.     

Development and characterization of steel containing very fine ferrite grains

Ferrite grain sizes of the order of 1 to 2 μm were obtained by optimizing the strain, strain rate, the stage of cooling, as well as the cooling rate during hot rolling of 0.15C-0.92Mn-0.01Si-0.036S-0.04P-0.013Nb steel. It was found that in single-pass rolling of a 10 mm plate to a thickness of 3.5 mm with an entry temperature of 800 °C, and early-stage water cooling, very fine grains of ferrite (1–2 μm) were formed at the surface and in subsurface regions. It was also found that the threshold level of reduction during rolling, which is required for the refinement of ferrite grains, is >50%. The 3.5 mm thermomechanically processed plate was found to possess very attractive mechanical properties in terms of the yield strength (485 MPa), the ultimate tensile strength (UTS) (763 MPa), and particularly the yield strength to ultimate tensile strength (YS/UTS) ratio (0.63). This combination of properties can be explained on the basis of the composite microstructure consisting of ferrite and bainite that was obtained as a result of the thermomechanical processing.     

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.     

Microstructural evolution of twin-roll cast Mg–3Al–0.5Mn–0.2Mm alloys during warm rolling and subsequent annealing

Microstructure of twin-roll casted (TRC) Mg–3Al–0.5Mn–0.2Mm (or AM30 + 0.2Mm) alloy strips consisted of columnar dendrites in the surface and equiaxed dendrites in the center regions, as well as widely dispersed fine primary intermetallic compounds located in the interdendritic region. Warm rolling of the TRC strips developed both deformation or shear bands and homogeneously dispersed fine particles. No evident dynamic recrystallization (DRX) was found in the TRC sheets during warm rolling. The dispersed fine particles seemed to retard DRX. The warm-rolled TRC sheets showed equiaxed fine grains with an average size of around 8 μm after annealing at 350 °C for 60 min. The TRC sheets had superior yield and tensile strengths to ingot cast (IC) samples. Elongation was similar to both TRC and IC samples.     

Thixoforming of EN AW-2014 alloy at high solid fraction

EN AW-2014 extruded alloy slugs were thixoformed at 615 °C where the solid fraction is estimated to be 80%. The recrystallization process occurred during heating to the thixoforming temperature, between 550 °C and 600 °C, well above the solidus temperature owing to the pinning of grain boundaries by Al₂Cu precipitates. The equiaxed polygonal grains thus obtained have become increasingly globular upon soaking. Si was enriched in the grain boundaries during soaking while the solid solution matrix was gradually depleted off Cu. The grain boundary composition has moved closer to that of the Al-Cu-Si ternary eutectic with a lower melting point than the binary Al-Cu eutectic, facilitating grain boundary melting. The liquid phase has then penetrated between the grains, forming a more or less continous intergranular network. Microstructural features essential for forming in the semi-solid state were obtained after about 10 min at 615 °C. The subsequent forming process has occurred in the semi-solid state with no evidence of grain deformation. The thixoformed EN AW-2014 part was solutionized at 500 °C for 2 h and was subsequently quenched in water. Artificial ageing at 160 °C has produced hardness values as high as 160 HV after only 8 h. It is concluded that the high strength wrought EN AW-2014 alloy feedstock processed by the RAP route respond to a thixoforming operation in a very favorable fashion.     

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.     

INFLUENCE OF HOT DEFORMATION ON CONTINUOUS COOLING BAINITIC TRANSFORMATION IN A LOW CARBON STEEL

1.IntroductionRecently,greatprogresshasbeenmadeintheresearchofthebainiticmicrostructuralcharacteristicsandtransformationbehaviorfor(ultra)lowcarbonsteel,especiallyoncontinuouslycoolingtransformatio.[ll.Itisknownthatthemorphologyofbainiticstructurefromcontinuouslycoolingtransformationisverydifferentfromthatfromisothermalheatt...t...t[2--v].Fromtheviewpointofindustrialapplicationthattheultralowcarbonbainiticsteelareproduc,gdbythermomechanicalcontrolledprocess(TMCP),thestudyofcontinuouscoolingt…