高钢级管线钢中有效晶粒尺寸及与CVN关系研究

利用EBSD对系列高钢级管线钢(如X65,X70,X80)影响其韧性的晶体学参数进行了研究,发现高钢级管线钢中对材料韧性起到积极作用的是晶界取向差大于15°的晶粒,而且研究大于35°取向差的晶粒对管线钢的参数研究是没有意义的;有效晶粒尺寸、晶界取向差频度分布、大角度晶界对小角度晶界比率是高钢级管线钢研究的重要参数;晶粒尺寸越细、有效晶粒越小、高角度(大于50°)品粒频度越高,高钢级管线钢的韧性越好.     

X100级管线钢的组织和强韧性

用光学显微镜、扫描电镜、透射电镜和EBSD等方法研究了X100管线钢热连轧钢带的微观组织、析出物、晶粒尺寸等对X100管线钢强韧性的影响。结果表明,通过合理的成分设计和TMCP工艺得到的X100管线钢的平均有效晶粒尺寸约为2.38μm,晶内含有大量位错和亚结构;显微组织由粒状贝氏体、板条贝氏体和M/A岛组成,组织中粒状贝氏体含量较多,板条贝氏体含量较少,M/A岛尺寸较小,弥散分布;细小的第二相能有效钉扎位错的移动,产生沉淀强化效果;实验钢的抗拉强度高于970 MPa,屈服强度高于800 MPa,-40℃以上的Charpy冲击功大于250 J,韧脆转变温度在-40℃与-60℃之间。     

高钢级管线钢微观组织特征与强韧性能关系的研究及展望

随着油气管道建设的快速发展,高钢级管线钢强度水平有了显著提高,但是获得优良的强韧性匹配显得尤为重要.影响管线钢强韧性能的主要因素在于其内部微观组织特征.综述了国内外管线钢微观组织特征(晶粒尺寸、非金属夹杂物、带状组织)的研究现状,分析了以往工作在管线钢微观组织特征与强韧性能关系研究的局限性.在此基础上,提出了采用电子背散射衍射技术和原位观测技术,研究了有效晶粒尺寸的表征方法及夹杂物和带状组织的微观力学行为的解决方案.并讨论了该方案的越要性和可行性.     

X80管线钢焊接粗晶区韧化因素的研究

采用热模拟技术研究了不同热循环对X80管线钢焊接粗晶区低温冲击韧度的影响.实验结果表明,随着冷却时间t8/5的增加,第二相粒子的数量减少且出现聚集现象,晶粒尺寸增加,但是当t8/5小于6.8s时,粒状贝氏体含量较高,板条束贝氏体细小且方向性较弱,试样的冲击韧性较高;而当t8/5超过6.8s后,粒状贝氏体含量逐渐下降,板条贝氏体逐渐粗大、平行,试样韧性又逐渐降低.M-A组元由于其含量低,尺寸小,对韧性的影响不显著.因此为提高焊接粗晶区的韧性,应采用小线能量和合适的预热温度来控制晶粒尺寸和组织形态.     

抗大变形管线钢焊接粗晶区的组织和韧性

采用Gleeble-3800模拟研究了抗大变形管线钢中不同Nb含量和不同热输入时的焊接热循环过程,并通过光学显微镜(OM)、扫描电镜(SEM)、电子背散射衍射(EBSD)及冲击载荷试验等研究了抗大变形管线钢焊接粗晶区(CGHAZ)的微观组织特征和冲击韧性。结果表明:随着热输入量的增加显微组织逐渐从板条贝氏体到粒状贝氏体过渡,M/A尺寸增大,比例提高,同时显微组织的平均有效晶粒尺寸也增大;在热输入相同的条件下,高Nb钢中原始奥氏体细化明显,组织中M/A尺寸细小,分布更加弥散;随着焊接热输入量的增加实验钢的冲击韧性急剧降低,高Nb钢的韧性急剧降低的热输入临界值约为35 k J/cm,低Nb钢韧性降低的热输入临界值约为25 k J/cm,在整个实验参数范围内,高Nb钢的冲击韧性值明显比低Nb实验钢的高。     

热成型温度对连铸/模铸10CrNi3MoV钢组织与性能的影响

结合显微组织与晶粒度的观察,研究了热成型温度对连铸/模铸10CrNi3MoV钢组织与性能的影响以及相互关系.结果表明:当热成型温度高于960℃时,连铸钢晶粒粗化,粒状贝氏体含量增加,导致其强度上升,低温韧性显著下降;当热成型温度不高于1000℃时,模铸钢中晶粒并未产生粗化现象,粒状贝氏体含量较少,故其强韧性基本保持不变...     

TMCP工艺对高韧性X90管线钢组织性能的影响

对高韧性X90管线钢进行实验室模拟轧制,研究了TMCP的工艺参数加热温度、中间坯厚度和卷取温度对其综合力学性能和微观组织特征的影响。结果表明:随着加热温度的提高强度先呈现上升趋势,当加热温度达到1250℃左右时晶粒粗化严重,强度显著下降;中间坯厚度的增加有利于组织细化与均匀化,从而提高钢板强度与韧性;卷取温度对性能的影响最为显著,在320℃左右卷取时可得到粒状贝氏体+板条贝氏体组织,强韧性匹配达到最佳效果。结合实验室模拟参数进行了X90的工业试制,钢卷在得到足够强度的同时具有优异的低温韧性。本文研究的TMCP工艺具有良好的实践效果。     

控轧控冷工艺对高强度X100管线钢组织的影响

对一种X100管线钢进行热模拟试验,研究了过冷奥氏体的相变规律,提出了一种得到以粒状贝氏体+板条贝氏体为主的混合组织的控轧控冷工艺制度,分析了精轧变形量、冷却速度及终冷温度对实验钢微观组织的影响。结果表明,随着变形量的增大实验钢的微观组织逐渐细化,高强度的板条贝氏体含量减少而粒状贝氏体含量增多;随着冷却速度的增加和终冷温度的降低实验钢组织中的板条贝氏体含量明显提高,组织也逐渐细化;组织中板条贝氏体含量较高时实验钢具有较高的强度,但过多的板条贝氏体和针状M/A岛对材料的韧性造成不利的影响。     

690 MPa级以上高强钢焊接熔敷金属微观组织及其联合贝氏体的研究进展

690 MPa级以上高强钢的发展和工程应用受限于缺乏与其强韧性相匹配的焊接材料,其根本原因是以针状铁素体为主的690 MPa级以下高强钢熔敷金属的强韧化理论已经不能指导新一代高强钢焊接熔敷金属的设计。因此,解决该问题的关键是寻求最佳的熔敷金属微观组织结构。综述了690 MPa级以上高强钢焊接熔敷金属中出现的联合贝氏体和其它各类微观组织对其强韧性的影响。其中,由不同形貌贝氏体相和针状铁素体相组成的复相分割结构可使焊接熔敷金属实现良好的强韧性匹配;然而,联合贝氏体作为下贝氏体的一种,其大尺寸的特点会显著降低高强钢焊接熔敷金属的强韧性;此外,可以通过物理冶金改善高强钢焊接熔敷金属的性能。     

RE-Mg复合变质剂对准贝氏体铸钢的净化与变质作用

研究了RE—Mg复合变质对准贝氏体铸钢组织和性能的影响．结果表明：经复合变质处理，首先显著地细化了准贝氏体铸钢的晶粒组织，热处理后钢的晶粒度由变质前的4—5／级变为6—7级．其次，净化了熔体，减少夹杂和气体含量，改变了夹杂形态，使之由Ⅱ类变为I类球形夹杂．另外还纯化了钢的基体，提高了钢的致密度．大幅度地提高了钢的强韧性和耐磨性．     

抗大变形管线钢热影响区软化问题的研究

为了研究抗大变形管线钢热影响区出现软化区的具体原因,本文采用焊接热模拟实验研究了X80级抗大变形管线钢的焊接热循环过程,结合金相显微镜、扫描电镜、EBSD、透射电镜和冲击实验,分析了热影响区软化区的组织变化、晶体学特征和冲击韧性.结果表明:当母材组织为多边形铁素体+贝氏体时,焊接热影响区的软化区出现在峰值温度600~700℃的高温回火区,此时组织转变成硬度较低的粗大铁素体+回火贝氏体,并且回复过程加快,组织中亚结构的大幅度减少和位错密度的显著降低是产生软化区的主要原因;软化区的韧性较好,但是在800℃的临界区,M/A组元发生了聚集和粗化,并且大角度晶界比例降低,导致了韧性低谷的出现.     

Ultrahigh strength hot rolled microalloyed steels: microstructural aspects of development

Abstract

Ultrahigh strength hot rolled microalloyed steels of yield strength 690 and 760 MPa with ferrite–bainite microstructure have been developed. Impact toughness of ～135 J at -40°C and a ductile–brittle transition temperature of less than approximately -70°C have been obtained in steels of gauge ～3 mm. The attractive strength–toughness combination was achieved by applying microalloying concepts and controlled rolling in an interactive manner. Alloy design aspects are qualitatively described in relation to the contributions of solute, grain size, precipitates, and microstructural constituents.     

Effect of silicon content on microstructure and toughness of simulated heat affected zone in titanium killed steels

Abstract

hree steels having different silicon contents were prepared to study the microstructure and toughness of the thermally simulated heat affected zone (HAZ) in titanium killed steels. For a low silicon addition, the oxygen content in the molten steels decreased remarkably. This in turn caused a change in the inclusion phase from predominantly titanium oxide to titanium nitride (TiN), the change being accompanied by two major microstructural modifications. The austenite grain size became refined and the quantity of intragranularly nucleated acicular ferrite decreased. The microstructural change was found to cause coarsening of Charpy fracture surfaces and deterioration of HAZ toughness of the steels. The minor change of silicon content therefore has a profound influence on the properties of titanium killed steels.

MST/1503     

Fracture toughness of two experimental high-strength bainitic low-alloy steels containing silicon

Abstract

The fracture toughness of two experimental silicon-containing steels in the bainitic condition has been measured and related to the microstructural state of the steels. The optimum bainitic microstructure for high strength and high toughness combinations consists of bainitic ferrite and thin interwoven laths of retained austenite instead of cementite, this condition being achieved through the silicon addition to the steels. The thin films of retained austenite are thermally and mechanically stable and act to reduce the effective fracture grain size and also possibly help to blunt propagating microcracks; blockier volumes of retained austenite are unstable and hence not beneficial to toughness. The two experimental steels achieved strength and toughness values equal to, or better than, some commercial steels in the martensitic condition.

MST/528     

Microstructure and toughness of coarse grain heat-affected zone of domestic X70 pipeline steel during in-service welding

The microstructures and mechanical properties of coarse grain heat-affected zone (CGHAZ) of domestic X70 pipeline were investigated. The weld CGHAZ thermal cycles having different cooling time Δt _8/5 were simulated with the Gleeble-1500 thermal/mechanical simulator. The Charpy impact absorbed energy for toughness was measured, and the corresponding fractographs, optical micrographs, and electron micrographs were systematically investigated to study the effect of cooling time on microstructure, impact toughness, and fracture morphology in the CGHAZ of domestic X70 pipeline steel during in-service welding. The results of simulated experiment show that the microstructure of CGHAZ of domestic X70 pipeline steel during in-service welding mainly consists of granular bainite and lath bainite. Martensite–austenite (M–A) constituents are observed at the lath boundaries. With increase in cooling time, the M–A constituents change from elongated shape to massive shape. The reduction of toughness may be affected by not only the M–A constituents but also the coarse bainite sheaves. Accelerating cooling with cooling time Δt _8/5 of 8 s can be chosen in the field in-service welding X70 pipeline to control microstructures and improve toughness.     

Correlation of microstructures and low temperature toughness in low carbon Mn–Mo–Nb pipeline steel

Abstract

By adjusting thermomechanical controlled processing parameters, different microstructures were obtained in a low carbon Mn–Mo–Nb pipeline steel. The microstructural characteristic and its effect on low temperature toughness were investigated. The results show that under higher reduction in austenite non-recrystallisation region and faster cooling rate during accelerated cooling, the microstructure is dominated by acicular ferrite (AF) accompanied by a small amount of fine martensite/austenite (M/A) islands. In contrast, lower reduction and slower cooling rate lead to a predominantly quasi-polygonal ferrite microstructure with coarse M/A islands. The fine effective grain size (EGS) and the high fraction of high angle grain boundaries (HAGBs) make the cleavage crack propagation direction deflect frequently. The coarse M/A islands can lead to cleavage microcracks at the M–A/ferrite matrix interfaces. Compared with the microstructure mainly consisting of quasi-polygonal ferrite, the microstructure dominated by AF exhibits excellent low temperature toughness because of fine EGS, high fraction of HAGBs and fine M/A islands.     

The role of Cu and Al addition on the microstructure and fracture characteristics in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels with superior toughness

The effects of Cu and Al addition on the microstructure and fracture in the coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels with superior toughness were studied and compared with the X70 pipeline base steel counterpart. The microstructure in base steel was dominated by a small fraction of acicular ferrite and predominantly bainite. However, acicular ferrite microstructure was obtained in Cu-bearing steel, which nucleated on complex oxide with outer layer of MnS and CuS because of Cu addition. The microstructure in Al-bearing steel consisted of bainite with ultrafine martensite–austenite constituent, which was refined by Al addition. CGHAZ in Cu-bearing and Al-bearing steels had superior impact toughness and ductile fracture, which were attributed to acicular ferrite and ultrafine martensite–austenite constituent, respectively.     

氮含量对钒微合金钢粗晶热影响区组织和韧性的影响

采用焊接热模拟的方法,研究了氮含量对实验钢焊接粗晶热影响区(CGHAZ)显微组织和韧性的影响规律。结果表明:随着氮含量的增加,CGHAZ的组织从晶界铁素体、贝氏体和侧板条铁素体转变成针状铁素体、多边形铁素体和少量的贝氏体,且铁素体晶粒细化;CGHAZ韧脆转变温度(FATT₅₀)先降低后升高,屈服强度升高。氮含量从0.004 4%增加到0.009 4%时,有效晶粒尺寸减小,导致CGHAZ的FATT₅₀降低;氮含量从0.009 4%增加到0.019 0%时,CGHAZ中固溶氮、屈服强度增量对FATT₅₀的综合作用大于晶粒的细化作用,导致FATT₅₀升高。