【作者】 于永梅；

【导师】 刘玠； 王国栋； 李长生；

【作者基本信息】 东北大学 ， 材料加工工程， 2008， 博士

【摘要】 近年来,针对传统工艺生产电工钢的流程长、成本高等问题,薄板坯连铸连轧(Thin Slab Cast Rolling,简称TSCR)短流程生产硅钢特别是取向硅钢的研究得到世界钢铁界的广泛关注,该技术的成功开发和工业化应用将会给电工钢生产带来一场深刻的变革。本文以Fe-3%Si电工钢为研究对象,利用热力模拟试验机,通过不同工艺参数的压缩、保温及淬火实验,分析了应力-应变曲线、显微组织形貌和再结晶动力学特征,研究了粗晶铁素体材料在高温变形过程的微观组织和晶粒取向的演变规律。在实验室条件下,模拟TSCR过程完成Fe-3%Si钢的冶炼、浇铸、高温热装加热、热轧和冷轧等工艺,其中包括铸模及成分设计、抑制剂的选取、铸坯保温输送与加热、热轧(保温、压下和冷却)、冷轧和热处理工艺的制定和优化等。在此基础上,对Fe-3%Si钢带的抑制剂的析出、显微组织和织构的形成及影响规律进行系统的研究,其成果将为短流程生产取向硅钢技术的研究和应用提供必要的参考与指导。本文的主要研究内容和结果如下：(1)采用不同应变、应变速率、变形温度的单道次压缩淬火等热模拟实验,通过对真应力-真应变曲线和淬火组织的分析,结果表明：Fe-3%Si钢真应力-真应变曲线是典型的动态回复型,其回复速率远高于奥氏体材料；初始奥氏体含量随着保温温度的提高而增加,大变形条件下材料内部出现连续动态再结晶特征；出现的少量奥氏体可以作为第二相钉轧晶界并阻止铁素体晶粒的长大。(2)采用高温压缩变形后保温不同时间喷水淬火方法,对Fe-3%Si钢再结晶行为及其显微特征进行了研究。其结果表明,随着保温(退火)时间的增加,再结晶体积分数增加,在达到峰值后形成软化率平台；显微组织主要由回复和再结晶晶粒组成,压缩后淬火的金相组织中位错线清晰、亚晶可见。(3)根据粗晶铁素体材料软化行为特点,采用30%再结晶的时间(t0.3)来确定静态再结晶激活能。晶界和三角地带(晶棱)是铁素体再结晶的优先形核点,奥氏体的出现促进了少量晶内再结晶的发生。静态回复发生迅速,再结晶晶粒长大速率以大于两个数量级的幅度降低,再结晶驱动力也随着退火时间的延长迅速降低。再结晶晶粒尺寸的变化依赖于初始组织(晶粒尺寸、奥氏体含量)和变形参数(应变、应变速率和温度)等因素。(4)在实验室模拟TSCR工艺条件下,在铸坯出模淬火的低倍组织中发现等轴晶和柱状晶的比例分别约为35%和65%,铸坯以大于1000℃入1200℃加热炉加热保温及热轧后高斯织构体积百分含量较高,且沿带钢厚度方向织构梯度比较理想。铸坯以1175℃和1150℃加热保温后热轧带钢的头部和中部显微组织差别显著。1200°C加热保温10-30min其热轧带钢各个位置的显微组织几乎没有差别,与传统工艺铸坯经1400℃高温加热生产取向硅钢热轧带钢有相似的组织形貌。(5)对铸坯出模、加热、保温、轧制3道次后的淬火试样和热轧及常化后带钢析出物进行TEM观察,发现在TSCR过程中热轧前保持铸坯在长、宽和厚三个方向均匀高温对抑制剂弥散、细小析出具有重要的意义。1200°C保温能够使铸坯中少量或不完全析出的抑制剂充分固溶,在随后的热轧过程通过应变诱导而析出并弥散分布,含Cu的硫化物析出尺寸相对较小。抑制剂的析出位置基本分布在晶内、位错、晶界和亚晶界上,所占的比例与热轧压下制度有重要关系。(6)对热轧次表层和冷轧各层宏观织构的ODF恒φ2=45。截面图分析表明,热轧次表层宏观织构主要分布在ε取向线上。而冷轧织构主要由α(<110>//RD)和γ(<111>//ND)纤维织构组成,ε取向线上高斯织构消失,高斯织构首先从H/8和H/4转变成{111}<112>织构；冷轧织构是热轧织构的连续性漫散,热轧工艺及成分对冷轧织构有一定的影响。含Al较高和添加Sn的硅钢材料更适合采用一次冷轧。工作辊辊径和冷轧压下制度对轧后{111}<112>取向密度有重大影响。更多还原

【Abstract】 In recent years, the research of silicon steel, especially oriented silicon steel, produced by a short process has attracted more attention in world steel industry, because of the questions of traditional process such as a long process and the high cost. The success of this technology development and industrialization will bring about a profound change in the electrical steel production.For the present Fe-3%Si steel, a series of laboratory tests were performed by a thermomechanical simulator including compression, soaking and quenching tests with different processing parameters. The evolution of microstructure and grain orientation during high-temperature processing in this coarse grained ferrite material was investigated by analyzing the characteristics of true strain-true stress curve, microstructure and recrystallization kinetics. Under laboratory conditions, smelting, casting, hot rolling and cold rolling were carried out to simulate thin slab cast rolling (TSCR) of Fe-3%Si steel, which mainly includes the designs of mould and chemical component, the selection of inhibiter, the determination and optimization of processing parameters in hot rolling (e.g. soaking temperature, time of ingot, reduction and cooling schedule), cold rolling and annealing etc.. The precipitation of inhibiter, formation of microstructure and texture and their influence factors in Fe-3%Si steel produced by TSCR were studied systematically. The results can provide the necessary guidance for the production and application of oriented-silicon steels using a short process. The main contents and results are as follows.(1) The true stress-strain curves with different stain, strain rate and deforming temperature and as-quenched microstructure for Fe-3%Si steel were analyzed. The true stress-strain curve shows that dynamic recovery is the dominant softening mechanism, and the recovery rate is far faster than austenite materials. Initial austenite content increases with heating temperature, the continuous-dynamic recrystallizing grains can be observed from in Fe-3%Si material by heavy deformation. The presence of minor austenite, acting as second phase, plays an important role in pinning the grain boundary and preventing grain growth.(2) The recrystallization behavior and microstructure feature of Fe-3%Si steel have been analyzed using samples which were deformed and subsequently quenched after annealing for different times. The volume fraction of recrystallization increase quickly with annealing time, and in coarse grained material, retardation of recrystallization resulted in the appearance of a plateau in the recrystallization curves. The microstructure mainly consists of recovery and recrystallization grain, and dislocation line and subgrain can be observed in optical micrographs for samples quenched immediately after deformation.(3) In coarse-grained ferrite materials, the activation energy for static recrystallization was determined using the time of 30% recrystallization. For the deformation conditions investigated, original grain boundaries, particularly triple points of boundaries are preferential nucleation sites for new recrystallized grains, boundaries, whereas some intragranular nucleation also occurs when austenite is present. As a result of rapid recovery in ferrite, growth rate falls by more than two orders of magnitude during recrystallization and driving pressure for growth of recrystallizing regions also falls with annealing time. The sizes of recrystallizing grains depend on initial microstructure, such as grain size and austenite content, and deforming parameters, such as strain, strain rate and temperature.(4) Under laboratory conditions, the proportions of equiaxed grains and columnar grains are observed to be close to 35% and 65% respectively in as-quenched macrostructure of ingot after it is pulled out from the mould, and the desired volume fraction of Goss texture and texture gradient in thickness are obtained for hot-rolled slab with reheating furnace entry temperature greater than 1000℃. There are significant differences between the microstructures of head and middle of hot-rolled sheets for ingots soaked at 1175 and 1150℃respectively. The microstructure has no clear differnce between head and middle of steel sheets when holding for 30min at 1200℃, which is very similar to that of GO steel produced by traditional process in metallography.(5) The TEM observation of as-quenched samples of ingot,3 pass and 7 pass hot rolling sheets, as-rolled and as-normalized samples, shows that it is very important that ingot maintains uniform and high temperature distribution in length, width and thickness direction before hot rolling for inhibitor precipitation with small-sized and homogenous-distributed particles in TSCR process. Minor and coarse precipitates formed in casting can be re-dissolved at 1200℃, and then precipitated once again with fine-grained and homogenous-distributed particles during hot rolling, and the particle size of copper sulfide is less than that of MnS particles. The precipitates are mainly distributed within grain, at dislocation, grain boundary and subgrain boundary, whose proportion is closely relate to rolling schedule.(6) Base onφ2=45°sections of the X-ray ODFs analysis, the subsurface texture of hot -rolled sheet consists of s TD//<110> fibers. But, the texture of cold-rolled sheet mainly consists ofα(<110>//RD) and y(<111>//ND) fibers, and Goss texture ({110}<001>) disappears. The Goss texture is translated to{111}<112> orientation from 1/8 and 1/4 thickness firstly. The hot rolling texture has a certain effect on the cold rolling texture. Al- or Sn-containing silicon steels are more suitable for one-stage cold rolling.{111}<112> orientation density mainly is dependent on diameter of working roll and reduction schedule of cold rolling.更多还原