低成本热轧双相钢焊接性能研究

研究了DP590低成本热轧双相钢采用合适的焊丝、焊剂及合理的焊接工艺后,钢板焊接接头的拉伸性能、冲击韧性及硬度的变化情况。证明了实验钢具有优良的抗软化能力和冷弯成型性能且淬硬倾向不明显。实验表明,开发的DP590低成本热轧双相钢焊接性能优良,适于焊接制造汽车车轮、横梁、纵梁等构件。     

冷却模式对热轧双相钢组织及断裂机制的影响

为研究冷却模式对热轧双相钢显微组织及断裂机制的影响,采用两段式(空冷+水冷)、连续式两种冷却方式,得到不同相比例和力学性能的热轧双相钢,轧后取样并在扫描电镜上进行原位拉伸实验.结果表明,两段式冷却模式得到的马氏体呈小岛状,而连续式冷却模式得到的马氏体呈块状,马氏体含量和形貌的不同导致两种冷却方式得到的双相钢力学性能存在差异.原位拉伸过程中,裂纹首先萌生于铁素体与夹杂物界面处,随着变形继续进行,在铁素体与马氏体界面处开始出现裂纹,当变形量进一步增大时,细小岛状马氏体始终不发生断裂,而块状马氏体在颈缩阶段发生断裂.     

低碳双相钢冲击韧性及断裂特征

本文研究了低碳双相钢中不同体积百分比及不同强度比的铁素体、马氏体双相组织对钢的冲击韧性及断裂过程的影响。结果表明:马氏体数量增加使钢的韧性降低,回火温度升高使钢的韧性提高。冲击断口形貌由室温的韧窝型转变为-78℃时以准解理为主及-196℃时全部解理型。     

800MPa冷轧热镀锌双相钢组织性能及其织构演变

对800MPa级热镀锌双相钢热轧、冷轧及退火后的显微组织进行了观察,分析比较了热轧和退火后的力学性能,并考察了其织构演变过程.结果表明:实验用钢经820℃保温140s热镀锌退火后,可获得抗拉强度819MPa,伸长率为17%的铁素体+马氏体双相钢,铁素体晶粒尺寸在1.5~4μm之间,马氏体体积分数为34%左右;热轧织构密度较弱,但已呈现出γ织构的雏形;冷轧后α织构和γ织构密度显著增长;热镀锌退火后α织构变化不大,不利织构{001}〈110〉织构密度有较大程度地攀升,γ织构取向密度值波动很大,最大织构组分为{112}〈110〉织构;快冷过程中形成的马氏体阻碍了有利织构{111}的发展,使得不利织构{001}〈110〉得到一定程度的发展.     

热轧双相钢回火后的显微组织研究

研究了热轧以相钢Ｆｅ－０．０７８Ｃ－１．４４Ｍｎ－１．１１Ｓｉ－０．３８Ｃｒ经不同温度回火后的显微组织变化。透射电镜观察表明：热轧双相钢的组织中不可避免地混有少量珠光林（＜５％），它们对钢的基本力学性能并没有多大影响。本钢种２００℃回火组织中，马氏体基本不变化，为板条态，双相钢仍保持屈服行为；３００℃回火组织中，大量马氏体发生分解，形成碳化物，与少量珠光体交界处的马氏体首先发生分解。该温度回火完全     

马氏体含量对双相钢力学性能的影响

在Ｅｓｈｅｌｂｙ等效夹杂的基础上，建立了双相钢在单向应力作用下的自洽模型，通过该模型对双相钢的弹塑性变形进行了数值模拟，拟合的结果与试验数据十分吻合。模型分析表明，对于马氏体，铁素体双相钢，随着马氏体含量的增加，存在着三种不同的破坏形式，其强度受三种不同的因素影响：在马氏含量奶小时，双相钢的强度由铁素体控制；在马氏体含量很高时，双相钢的强度由马氏体控制；马氏体含量中等时，双相钢的强度由马氏体，铁素     

高强热轧双相钢中组织对性能的影响

采用低成本成分设计,应用超快冷技术为核心的新一代TMCP技术可以得到强韧性较好的高强热轧双相钢,本文研究了该试验钢组织对性能的影响。研究表明:铁素体晶粒尺寸在5μm处波动;条状马氏体比块状马氏体的n值要高;抗拉强度随马氏体体积分数的增加而增加;组织中小尺寸的铁素体和马氏体提高了裂纹弛豫能力,有利于试样的韧性和n值的提高。     

600MPa级冷轧双相钢的试制与研究

在实验室试制了600MPa级低成本C-Mn系冷轧双相钢,比较了热轧态普通组织和退火后双相组织与力学性能的关系,分析了双相组织的强化机理,讨论了热轧卷取温度对最终力学性能的影响,分析并且优化了连续退火工艺参数.采用X衍射和电子背散射EBSD观察了试制双相钢组织的宏观与微观取向.     

C-Si—Mn冷轧双相钢的应变硬化特性

试制了C-Si-Mn冷轧双相钢.采用力学测试、显微组织观察与修正的C-J分析方法研究了双相钢的应变硬化特性.研究结果表明,双相钢的应变硬化具有两阶段.第一阶段应变硬化能力较强,第二阶段硬化能力减弱.两阶段硬化之间存在一个转折应变.当马氏体体积分数小于16%,随马氏体体积分数的增加,两阶段硬化能力均增强.当马氏体体积分数大于16%,随马氏体体积分数的增加,两阶段硬化能力均减弱.硬化转折应变则随马氏体体积分数增加单调递减.铁素体与马氏体的弹塑性行为差异是导致双相钢两阶段硬化的主要原因.马氏体体积分数增加,其强化效果增加,但是由于马氏体中的碳含量降低,其塑性抗力降低.只有当马氏体量增加带来的强化效应大于碳含量减少的弱化效应时,双相钢的应变硬化能力才随之增加.     

Nb微合金化对Q500MPa级热轧H型钢组织和性能的影响

采用纳米压痕试验、电子背散射衍射、透射电镜等研究Nb元素对热轧H型钢组织和性能的影响。结果表明,H型钢中添加0.025%(质量分数,下同)的Nb,在轧制和冷却工艺相同的情况下,屈服强度和-20℃低温冲击功分别为552 MPa、102 J,相比不含Nb的试样分别提升了17.7%和20.0%,且晶粒尺寸从5μm细化至4μm。通过热力学计算发现铁素体基体内形成的复合析出物(Nb_(1-x)V_x)C与基体的界面能比NbC与基体的界面能更小且形核驱动力更大,因此更易促进Nb以复合析出物形式析出,且纳米析出物平均尺寸从16.3 nm减小至11.46 nm,体积分数从0.053%提高至0.108%,析出强化由55 MPa增加至102 MPa,铁素体硬度由247HV提升至279HV,添加Nb元素后析出强化贡献增量较细晶强化增量高18 MPa。     

Transit damage and hot rolled steel

The investigation of alleged corrosion damage to hot rolled steel during transit requires metallurgical, chemical, and corrosion knowledge. Familiarity with non-destructive techniques and sampling procedures are of assistance. A complete record of shipment history is also required, including the purchasing specifications and observations and photographs taken during surveys enroute. A frequent conclusion of such investigations is that the alleged corrosion is of no significance or did not occur during the voyage.     

热轧TRIP钢组织性能的研究

采用实验室热轧机对高硅和低硅TRIP钢(A钢和B钢)进行控制轧制试验,研究了热轧后等温淬火对热轧TRIP钢组织性能的影响.通过显微组织观察,力学性能分析,探讨了两种钢的应变诱导相变和相变诱发塑性行为.研究表明:A、B钢均能够获得铁素体、贝氏体和大量稳定残余奥氏体的混合组织,具有较高的力学性能;残余奥氏体稳定性是提供TRIP的重要因素,B钢中贝氏体和残余奥氏体较多,相变诱发塑性效果更好,其性能优于A钢;等温时间影响热轧TRIP钢的力学性能,随等温时间的延长,A、B钢的伸长率增加,等温时间超过120 min,导致碳化物析出,残余奥氏体的稳定性降低;B钢经热轧后在400 ℃等温25 min,抗拉强度和伸长率分别达到了784 MPa和36%的最高值.     

屈服准则用于高强度热轧钢板的适用性分析

为确定适合描述高强度热轧钢板变形行为的屈服准则,采用Hollomon流动应力方程和三种屈服准则对几类高强度热轧钢板在不同应变路径下达到成形极限的成形过程进行了模拟.比较了Barlat(1989)、Hill(1948)、Barlat六参数3种屈服准则,对热轧酸洗板QStE340TM、SAPH370和热轧镀锌板ZStE260P在单向拉伸、平面应变和双向等拉3种应变路径下的变形过程进行了比较.结果表明,Barlat(1989)屈服准则能较好地描述单元的变形行为,且在平面应变路径下的模拟结果最符合实验结果.     

Silicon as grain refiner in niobium microalloyed hot rolled steel

The effect of silicon on grain refinement has been investigated in industrially produced Nb microalloyed steel. The synergistic effect of silicon in the presence of Nb is found to favour grain refinement in hot rolled strip, which has resulted in very fine ferrite grains of <3; μm. Evolution of such a fine grain has been related to the enhanced precipitation of Nb(CN) in the presence of silicon. The concept of effective niobium (Nb + Si) in such steels has been introduced.     

Ultrahigh strength hot rolled microalloyed steel: microstructure and properties

Abstract

A low carbon steel alloyed with Ni, Mn, Mo, Cu and microalloyed with Nb and Ti was prepared. Continuous cooling transformation behaviour of the steel was evaluated. Formation of polygonal or Widmanstätten ferrite is suppressed at high temperature and the 'C' curve is shifted to an extreme right. At lower temperatures a flat top 'C' curve with a mixed structure of bainite and martensite was obtained and the transformation temperatures do not vary much with a wide range of cooling rates. The steel was thermomechanically processed at different finishing temperatures and ultrahigh strength values were obtained as a result of austenite grain refinement, highly dislocated fine lath martensite structure along with tiny precipitates of microalloying carbide and carbonitride at all finish rolling temperatures. The stable and large TiN/TiCN particles formed during casting have impaired the impact toughness values at ambient and at ?40°C temperatures.     

Transformation textures in new ultrahigh strength hot rolled microalloyed steel

Abstract

Texture development during thermomechanical processing of a newly developed ultrahigh strength microalloyed steel was investigated, with particular attention to through thickness texture gradient. A considerable texture gradient was evident, particularly in the 1/2 depth compared to the other three positions (surface, 1/8 depth, 1/4 depth). The most intense deformation texture at all depths was {113}〈110〉, which was also more prevalent at the centre than elsewhere, implying an overall dominance of {112}〈111〉 in the deformed austenite during hot rolling. The recrystallisation texture of austenite, {100}〈001〉, transformed into {100}〈011〉 component in the ferrite indicated an increase in intensity with increase in depth. The {100}〈011〉 type of texture has an undesirable effect on the delamination behaviour of steels. However, the centre and 1/4 depth contained considerable intensity of the desired texture, {332}〈113〉, which offsets the undesired {100}〈011〉 texture.     

Ferrite/pearlite band formation in hot rolled medium carbon steel

Abstract

The influence of the microsegregation of Mn, Si, and Cr on the austenite decomposition during isothermal transformations in hot rolled medium carbon steel has been studied by neutron depolarisation, electron probe microanalysis (EPMA), and optical microscopy. Eight specimens of the same alloy were held at 1173 K for 30 min and were rapidly cooled to different isothermal transformation temperatures. Two-dimensional EPMA maps of the specimen annealed at 1013 K showed that microsegregation of alloying elements in hot rolled steel is strongly related to the ferrite/pearlite band formation. The local variations in alloying element concentration lead to variations in local transition temperatures, which were calculated with the thermodynamic database MTDATA. Similar EPMA maps for the specimen transformed at 953 K demonstrate the presence of microchemical bands, while optical microscopy reveals the absence of microstructural bands. It is shown that the formation of microchemical bands is a prerequisite for band formation, but that the kinetics of the phase transformation determines the actual formation of microstructural bands. A quantitative model has been developed, which describes the observations in terms of the relative difference between ferrite nucleation rates in regions with a high and low local undercooling and the subsequent growth of the ferrite. The isothermal transformation experiments have led to generalised nucleation and growth criteria for the formation of microstructural bands.     

Development of high strength hot rolled steel sheet for wheel disc application

Abstract

A hot rolled steel with unique combination of high strength and excellent ductility coupled with high hole expansion ratio has been developed for automotive applications using a new metallurgical approach. The steel chemistry and processing parameters were designed in such a way that austenite–ferrite phase transformation takes place at lower temperature with interphase precipitation of very fine carbides. A fine ferritic microstructure was produced with an average grain size of 3·4 μm resulting into attractive mechanical properties. Interphase precipitation was used as one of the major strengthening mechanisms apart from grain refinement. This led to an achievement of tensile strength as high as 800 MPa (min.) with an excellent stretch formability and low impact transition temperature. This is a potential grade with unique features which can have wider applications.     

Predicting microstructural evolution and yield strength of microalloyed hot rolled steel plate

Abstract

A mathematical model has been developed to optimise process parameters for production of API grade steel plates by thermomechanical controlled processing at a plate mill in Bhilai Steel Plant, India. The model comprises the prediction of the microstructural evolution during hot rolling, the subsequent phase transformation, and, finally, the mechanical properties of microalloyed steels. Effects of chemistry and mill parameters on recrystallisation, grain growth, and precipitation kinetics were taken into consideration to describe the metallurgical processes. . The model has been validated through laboratory experiments as well as full-scale rolling at the plate mill.