经济型铁素体/贝氏体高扩孔钢的组织与性能

 通过TMCP工艺实验,研究了Si、Mn含量对低碳Si Mn钢显微组织、力学及成形性能的影响,探讨了铁素体/贝氏体双相钢(FB钢)在扩孔过程中的裂纹形成及扩展行为。研究结果表明,增加Si含量,实验钢中等轴铁素体的体积分数增加,扩孔性能得到改善;而增加Mn含量,实验钢的强度和韧性显著提高,但塑性和扩孔性能有所下降。FB钢中的裂纹扩展主要是以微孔聚集机制进行,当遇到贝氏体时,裂纹通过铁素体 贝氏体相界面并剪断铁素体进行扩展。合理选择Si、Mn含量和TMCP工艺参数,可以获得690 MPa级的经济型热轧FB高扩孔钢,扩孔率达到了95%,综合性能较好。     

超细晶钒微合金化双相钢性能

研究了钒微合金化对高强双相钢微观组织及性能的影响。与Fe-0.186C-1.5Mn-0.3Si-0.008N参照钢相比,加入0.14%V带来如下效果:(1)在冷轧及退火状态铁素体晶粒高度细化;(2)严重推迟在连续退火过程中铁素体向奥氏体转变的初始动力学;(3)慢冷条件下铁素体开始转变温度稍微提高,但珠光体和贝氏体转变被抑制,导致淬透性提高;(4)在临界退火温度≤740℃时观察到未溶渗碳体;(5)750℃/180 s退火后铁素体相中发现大量V(C,N)析出(平均直径7.4 nm),而马氏体(奥氏体)中析出物稀少,尺寸更大(平均直径13.4 nm);(6)不含钒参照钢抗拉强度随马氏体体积分数增量为~16 MPa/%,而含钒钢由于晶粒细化和铁素体选择强化,强度随马氏体含量变化增量相当低(~4 MPa/%),在马氏体体积分数45%变软。     

贝氏体形态对细晶铁素体/贝氏体双相钢室温力学性能的影响

基于过冷奥氏体动态相变的思想,通过两道次压缩变形结合控制冷却的热模拟轧制工艺,获得不同贝氏体含量及形态的细晶铁素体贝氏体双相钢。通过显微组织观察及力学性能测试,考察了第二相贝氏体特征对双相钢室温拉伸变形行为的影响。研究结果表明,形变后快速冷却可获得无碳板条状贝氏体,较慢的冷速或在贝氏体转变区保温处理可获得粒状贝氏体。贝氏体体积分数大于20%左右的细晶铁素体/贝氏体双相钢具有低的屈服强度,高的抗拉强度,高的伸长率,低屈强比以及连续屈服特性。屈服强度既与铁素体晶粒尺寸相关,也与贝氏体形态和数量相关。板条贝氏体引起的屈服强度提高大于粒状贝氏体,粒状贝氏体具有比板条贝氏体更好的塑性。     

Micromechanical Behavior and Failure Mechanism of F/B Multi-phase High Performance Steel

The deformation and micro-voids formation mechanisms in ferrite / bainite( F / B) multi-phase steel with the volume fraction of bainite less than 50% were studied by numerical simulation and experimental observation. The results show that the micro-strain concentrates at the soft / hard phase( F / B) interface in the multi-phase steel,which should be correlated with the mechanism of incoordinate deformation. During the necking of the steel,the micro-voids initially form around the F / B interface,which also form in ferrite and bainite with the severe strain. The micro-voids in bainite are more dense and finer than those in ferrite. The failure mechanism of bainite is the coalescence of micro-voids,and the failure mechanism of ferrite is the growth and tearing of micro-voids. Due to the different failure mechanisms of ferrite and bainite,a suitable part of soft phase would be beneficial to the capability of anti-failure of F / B multi-phase steel during the ductile fracture.     

Effect of Morphology for Novel Bainite/Martensite Dual-Phase High Strength Steel on Its Hydrogen Embrittlement Susceptibility

It is very imperative to study the novel highstrength materials with high purity,high uniformityand refined grain,for improving the utilization rateand quality of materials.However,improving thestrength will reduce the resistance of materials tohydrogen embrittlement. For example,in lastcentury,some hydrogen embrittlements in thelanding gear of plane were reported[1] . When yieldstrength exceeds 1380 MPa,high strength steelsbeing used now exhibit strong susceptibility tohydrogen embrittlement…     

淬火温度对马氏体/铁素体双相钢组织性能的影响

双相钢具有优异的力学性能,而马氏体/铁素体的含量对其性能具有重要影响。通过实验调节淬火温度制备了不同马氏体体积分数的双相钢,采用金相显微镜对马氏体/铁素体组织形貌及分布进行了定性观察;其次利用电子背散射衍射技术(EBSD)并结合高斯拟合,发现马氏体/铁素体衍射花样衬度呈双峰分布,据此对钢中马氏体体积分数进行了定量统计。结果表明:淬火温度为730℃时,马氏体体积分数仅为19.09%。随淬火温度增加,双相钢中马氏体含量提高;于790℃淬火时,马氏体体积分数达到30.96%,提高了62%。此外,对试验双相钢力学性能进行对比分析发现:随淬火温度升高,双相钢的抗拉强度明显提高,屈服强度也呈上升趋势,这主要与马氏体含量增加有关;而双相钢延伸率显著降低,这主要是由于铁素体含量减少,且形貌由利于变形的针状转变为多边形所致。     

800～1200 MPa级超高强冷轧双相钢的组织与性能

在实验室试制了800~1200 MPa级超高强冷轧双相钢。DP800和DP1000的热轧组织为铁素体+珠光体,DP1200为铁素体+珠光体+贝氏体复相组织。热轧板经过冷轧和退火后呈现典型的双相钢组织特征,力学性能可以达到相应强度级别的要求。DP800和DP1000马氏体体积分数小于50%,铁素体相为基体;DP1200马氏体体积分数超过50%,马氏体转变为基体相。最后对退火板各力学性能之间的关系进行了对比分析。     

C Si Mn Cr Nb钢双相组织性能的柔性控制

 根据C Si Mn Cr Nb试验钢的双道次变形和分段冷却热模拟试验结果,进行了试验钢控轧控冷试验,分析了工艺参数对试验钢组织和性能的影响,获得了具有不同力学性能的铁素体+马氏体或铁素体+贝氏体双相组织。结果表明,试验钢两段轧制分段冷却后550 ℃卷取获得铁素体+马氏体双相组织,屈服强度415 MPa,抗拉强度710 MPa,伸长率23.0％,屈强比0.59。500 ℃卷取得到铁素体加粒状贝氏体双相组织,与550 ℃卷取相比,屈服强度升高35 MPa,抗拉强度降低45 MPa,伸长率略微降低。     

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。     

水淬工艺对冷轧微合金双相钢力学性能的影响

 研究了水淬工艺对微合金双相钢组织和力学性能的影响。结果表明,随着淬火温度的降低,双相钢的屈服和抗拉强度下降,同时总伸长率提高;不同的淬火温度使双相钢显微组织中不仅产生了不同体积分数的马氏体,而且也形成了不同数量的新生铁素体。TEM分析表明,新生铁素体中没有细小的NbCN粒子,因而避免了析出强化,这不仅可以改善铁素体的塑性从而有利于双相钢的塑性,而且也能够提高双相钢的强度。     

Study of Bake Hardening Effect on Laser Welded Hot Rolled Ferrite‐Bainite Dual Phase Steels

The influence of bake hardening (BH) effect on laser welded hot rolled ferrite‐bainite dual phase steel (FBS) with respect to the process conditions was investigated. The samples were firstly laser welded and then pre‐strained. Pre‐strained the samples with defined degrees of deformation and a subsequent aging treatment leads to enhanced hardness and strengthening for both conditions. The microstructure of the fusion zone (FZ) and heat affected zone (HAZ) was studied. A high volume fraction of martensite could be observed in the FZ as well as in the HAZ. Both steels exhibited a clear BH effect in both, the as‐received and the laser welded conditions. The BH effect is more pronounced in the pre‐strained laser welded condition.     

1000MPa级高强钢显微组织与析出相分析

利用金相显微镜、扫描电子显微镜及透射电子显微镜等分析手段,对1 000 MPa级高强钢的显微组织与析出相进行了研究。结果表明,试验钢的显微组织为板条状贝氏体和板条状马氏体,并存在少量残余奥氏体。大量析出相分布在基体上,平均尺寸30～60 nm,组织强化、析出强化、位错强化是高强钢主要的强化方式。     

980MPa级冷轧双相钢组织性能研究

为研究980 MPa级C-Si-Mn-Nb系冷轧双相钢组织性能,在试验室冶炼该钢并采用临界区保温+两段式冷却+过时效处理的工艺进行热处理。研究表明,试验钢的屈服强度为476 MPa,抗拉强度为1 021 MPa,伸长率为15%,n值为0.29;试验钢热轧组织为(F+P),铁素体晶粒尺寸约为3.3μm;退火组织为(F+M),马氏体体积分数约为63%。微合金元素Nb的添加,起到细晶强化和析出强化的作用。与热轧组织相比,连续退火板带状组织得到明显改善,试验钢表现出良好的强韧性匹配。     

CSP流程生产经济型热轧双相钢的工艺与组织性能

 为了在CSP产线上开发新一代经济型热轧双相钢,并确定生产的最佳成分和工艺,介绍了在武钢CSP生产线进行580MPa级热轧双相钢的工业化生产试制情况。分别采用C-Mn-Si系和C-Mn-Si-Cr系钢为原料,通过控制轧制和基于超强冷却设备的控制冷却工艺,成功开发出抗拉强度580MPa级热轧双相钢。通过比较分析2种成分钢的力学性能和微观组织,结果表明：经济型的C-Mn-Si系钢相对于C-Mn-Si-Cr系钢具有屈服强度低、屈强比小、伸长率大的特点,虽然马氏体量相对较少,但具有马氏体呈岛状更加均匀分布在铁素体晶界上等典型双相钢的特征,同时提出了生产过程中控制铁素体析出量和促进马氏体形成的具体措施。     

Void Nucleation and Growth in Dual-Phase Steel 600 during Uniaxial Tensile Testing

Commercial dual-phase (DP) steel in sheet form and comprised of ferrite, martensite, and bainite was subjected to uniaxial tension up to fracture. The damage characteristics were studied through extensive quantitative metallography and scanning electron microscope (SEM) observations of polished sections and fracture surfaces of failed specimens. The observed void nucleation mechanisms include nucleation at the martensite/ferrite interface or triple junction (most predominant), nucleation due to the cracking of martensite particles, and nucleation at the inclusions. The void characteristics in terms of area fraction, void density, void size ranges, and void orientations were analyzed as a function of thickness strain from various regions of the different uniaxial tensile test specimens taken to fracture. The damage analysis suggests that the void nucleation occurs during the entire deformation process with an almost constant rate and this rate reduces before fracture. A nucleation strain of 0.15 has been estimated for this material.     

钼微合金化低碳钢的显微组织与力学性能

借助物理模拟系统采用四种不同的多道次变形及控制冷却工艺,研究了成分为0.12C-0.78Si-1.42Mn-0.74Al-0.32Mo钢的显微组织和力学性能.结果显示:使用物理模拟系统进行高温区的多道次热连轧,并结合控制冷却处理,能够得到不同的复相组织（铁素体/贝氏体组织,贝氏体/马氏体组织）.依贝氏体含量和形态的不同,铁素体/贝氏体复相组织钢的屈服强度为388~558 MPa,抗拉强度为681~838 MPa,总延伸率为15%~27%;贝氏体/马氏体复相组织钢的屈服强度为746 MPa,抗拉强度为960 MPa,总延伸率为19%.      

Effect of Cooling Rate and Deformation on Microstructures and Critical Phase-Transformation Temperature of Boron-Nickel Added HSLA H-Beams

 Microstructures and critical phase-transformation temperature of boron-nickel added Nb-treated high strength low alloy (HSLA) H-beams cooled at different cooling rate, with different deformation were investigated. Continuous cooling transformation (CCT) diagram of this new type of steel was obtained by using Gleeble 1500 thermomechanical simulator. Microstructures and hardness, especially micro-hardness of the experimental steel were investigated by optical microscopy (OM), scanning electron microscope (SEM), Rockwell and Vickers hardness tests. Phase analysis was also studied by X-ray diffraction (XRD). The results indicated that with increase of cooling rate, microstructures of continuous cooled specimens gradually transformed from polygonal ferrite and pearlite, grain boundary ferrite and bainite, bainite and martensite to single martensite. The CCT diagram revealed that slow cooling was needed to avoid austenite-bainite transformation to ensure toughness of this steel. By plastic deformation of 40%, austenite-ferrite transformation temperature increased by 46 ℃, due to deformation induced ferrite transformation during continuous cooling, but Rockwell hardness has little change.     

Investigation of the Effect of Deformation on γ‐α Phase Transformation Kinetics in Hot‐Rolled Dual Phase Steel by Phase Field Approach

Dual phase steels, consisting of hard martensite particles in a ductile ferritic matrix, offer high strength and deformability at the same time. Additionally, they are cost effective by a dilute alloying concept. In industrial production, two manufacturing concepts have been implemented: intercritical annealing of cold rolled sheet, or hot rolling. The current work has investigated the effect of deformation on the γ‐α phase transformation kinetics in the dual phase steel production using the hot rolling scheme. The pancaked austenite grains containing denser nucleation sites have a strong influence on the ferrite transformation kinetics. In addition, the multiplication of dislocations which results in the increase in elastic strain energy and dislocation core energy contributes to some acceleration in ferrite growth kinetics. A modelling approach for the γ‐α phase transformation kinetics in dual phase steels has been developed employing the phase field theory. The nucleation behaviour, i.e. the number and size of nuclei developed after an elapsed time as well as their nucleation sites which were evaluated from microstructure analysis, and the increase in the driving force of grain growth were integrated into this model.     

双相热处理20MnSi钢的强化机理

分析了双相热处理工艺对20MnSi钢组织和力学性能的影响，探讨了其强韧化机理。研究结果表明：20MnSi钢经双相热处理后可得到马氏体与少量铁素体相间分布的细小均匀的双相组织，其屈服强度大幅度提高，同时塑性指标也明显改善，这是固溶强化、位错强化和细晶强化综合作用的结果。     

Forming Limit Curve (FLC) and Fracture Mechanism of Newly Developed Low‐Carbon Low‐Silicon TRIP Steel

The Forming‐Limited Diagram (FLD) of intercritically annealed 0.11C‐1.65Mn‐0.62Si TRIP‐assisted steel was investigated. The high FLD₀ value of this new low carbon TRIP steel was indicative of a superior formability. The micro‐structural changes during deformation and fracture were studied in detail. The polygonal ferrite phase was found to plastically deform first and deformed most at larger strains. Fracture was initiated by micro‐voids nucleated at ferrite grain boundaries, within ferrite grains or at the interface between ferrite and the harder phases. Cracks were formed after micro‐voids grew, coalesced, and expanded in one direction. When crack tips reached the bainite phase or the martensite/austenite constituent, the cracks propagated along the boundary of these phases. Cracks reaching retained austenite islands caused stress‐induced martensite transformation at the crack tip. The direction of motion of the cracks also changed in this case.