【作者】 王洋；

【导师】 王国栋； 许云波；

【作者基本信息】 东北大学 ， 材料加工工程， 2017， 博士

【摘要】 薄带连铸短流程和亚快速凝固的特点决定了其在简化取向硅钢生产流程和抑制剂调控方面具有独特的优势。本文基于双辊薄带连铸工艺,调整与优化了取向硅钢的成分、后续轧制及退火工艺。系统研究了单道次热轧工艺、一阶段冷轧工艺及两阶段冷轧工艺路线下,取向硅钢组织和织构的演变规律以及抑制剂固溶和析出行为的相关机理,并在实验室条件下成功制备了高磁感取向硅钢的原型钢。对比分析了 Nb的析出粒子作为取向硅钢抑制剂,对常规流程与薄带连铸流程制备超低碳取向硅钢组织、织构和磁性能的影响,阐明了常规流程不用Nb作为抑制剂的原因。最后,讨论了抑制剂强度和织构对取向硅钢二次再结晶动力学的影响,提出了两次利用二次再结晶现象快速高温退火制备取向硅钢的工艺路线。论文的主要创新性工作如下:(1)基于薄带连铸工艺亚快速凝固的特点提出了取向硅钢超低碳的成分设计,取消了后续脱碳退火工艺,进一步简化了生产流程。常规流程取向硅钢成分体系中碳元素起到调整组织和抑制剂的关键作用,但是脱碳过程也会带来工艺复杂和成本高的不利影响,这一矛盾一直无法解决。薄带连铸条件下,取向硅钢成分中加碳引入的奥氏体相多分布在晶界处,对凝固组织的细化作用不大,而通过改变熔池与铸辊之间的导热效率可有效控制铸带的初始凝固组织。另一方面,薄带连铸亚快速凝固的优势可实现铸带中的第二相粒子过饱和固溶或者细小弥散析出,并可通过后续冷轧退火工艺精确调控其数量与尺寸分布,无需加碳引入奥氏体及利用γ/α相变控制抑制剂的固溶与析出行为。(2)明确了薄带连铸条件下单道次热轧工艺对超低碳取向硅钢组织、织构、抑制剂析出和磁性能的影响。研究结果表明,AIN在超低碳取向硅钢成分体系下的析出PTT鼻点温度为1100℃。随着热轧压下率的增加,热轧板组织不断细化且Goss织构逐渐增加,但是由于热轧压下率的限制,热轧Goss织构显著低于常规流程热轧板的水平。初次再结晶后Goss织构与常规流程基本相当。铸带最佳的热轧工艺为1100℃压下35%,其相应的磁感应强度B8为1.85 T。(3)研究了一阶段冷轧工艺条件下常化工艺对薄带连铸取向硅钢组织、析出物演变和磁性能的影响。结果表明,铸带不常化直接冷轧时,MnS主要在冷轧后续初次退火过程中析出,且不同变形晶粒内部MnS析出粒子分布不均。这种析出的分布差异导致了初次再结晶组织不均匀。AIN主要在高温退火过程中析出,初次再结晶过程中析出的MnS在AIN大量析出之前已经开始粗化失效,导致Goss晶粒无法异常长大。冷轧前引入常化工序时,MnS及AIN在常化过程中开始析出,且分布较为均匀,初次再结晶组织均匀。高温退火过程中MnS粗化之前已存在部分AIN粒子,可持续抑制初次再结晶晶粒长大,促进了 Goss晶粒发生二次再结晶。最优磁感应强度B8为1.75 T。(4)阐明了两阶段冷轧工艺条件下薄带连铸取向硅钢Goss织构起源与发展的基本规律、抑制剂固溶与析出的调控原理。结果表明,两阶段冷轧工艺可显著改善由粗大初始凝固组织引起的冷轧及后续退火组织不均匀的问题,可控制初次再结晶晶粒尺寸在5～15 μm范围内。有效的Goss种子主要起源于γ取向冷变形晶粒的内部剪切带上,且沿中间退火板厚度方向分布均匀。中间退火板Goss织构的强度随一阶段冷轧压下量的升高而升高。抑制剂的固溶与析出行为与常规流程利用热轧及常化工艺的控制策略完全不同。利用亚快速凝固及后续水冷工艺可实现抑制剂元素过饱和固溶并在后续中间退火过程中大量析出。抑制剂尺寸分布可由一阶段冷轧压下量及中间退火工艺精确调控。该工艺克服了常规含碳取向硅钢AIN析出分布不均的固有问题,显著提高了抑制剂的利用效率。可成功制备0.18 mm厚的薄规格高磁感取向硅钢,磁感应强度B8为1.9 T以上。(5)对比研究了 Nb的析出粒子作为抑制剂对常规流程与薄带连铸流程制备超低碳取向硅钢组织、织构和磁性能的影响。结果表明,常规流程下Nb的析出抑制了热轧Goss织构的形成,后续退火过程中Goss种子在数量与尺寸上均不占优势,高温退火后取向硅钢不发生二次再结晶或{113}<110>取向晶粒异常长大。这种非Goss的{113}<110>取向晶粒异常长大现象,可以通过CSL晶界理论解释。薄带连铸条件下,含Nb取向硅钢Goss种子起源于冷轧及后续退火过程中,Nb的引入不会导致Goss种子的减少。因此,Goss晶粒可以发生二次再结晶并获得良好的磁性能,磁感B8为1.85～1.89T。(6)分析了抑制剂强度及中间退火织构对取向硅钢Goss晶粒二次再结晶动力学的影响。提出了两次利用二次再结晶现象快速高温退火制备取向硅钢的工艺路线。结果表明,二次Goss晶粒的晶界迁移速率呈非线性变化,二次再结晶初期晶界迁移速率较快而后随着二次再结晶的进行逐渐减慢。通过高温原位观察测量了二次Goss晶粒的迁移速率,为40～1300 μm/min。在A1N为主抑制剂的基础上添加Nb作为辅助抑制剂,并提高中间退火温度,可促进中间退火板发生不完善的二次再结晶,获得强烈的Goss织构。高温AIN抑制剂粗化失效,低温含Nb抑制剂在中间退火的冷却过程中细小弥散析出。经第二阶段冷轧和最终退火后,Goss晶粒可快速完成第二次异常长大过程。磁性能B8为 1.75 T。更多还原

【Abstract】 Twin-roll strip casting with the characteristics of short process and rapid solidification has a distinctive advantage on simplification grain oriented silicon steel production process and inhibitors control.Based on the rapid solidification characteristic of strip casting process,the composition,subsequent rolling and annealing process of grain oriented silicon steel were adjusted and optimized.The microstructure and texture evolution and the dissolve and precipitation behaviors of inhibitors were systematic investigated during single-stage hot rolling,one-stage cold rolling and two-stage cold rolling processes.The effect of Nb-particles as grain growth inhibitors on the microstructure,texture and magnetic properties of conventional grain oriented silicon steels and strip casting grain oriented silicon steels was.comparative studied.The reason of Nb-particels cannot be acted as inhibitors of silicon steels produced by conventional process were explained.Finally,the effect of inhibitor strength and texture distribution on the secondary recrystallization kinetics of grain oriented silicon steel was discussed,and the rapid annealing process of grain oriented silicon steels by using twice secondary recrystallization phenomenon was proposed.The main innovative results are as follows:(1)Based on the rapid solidification characteristic of strip casting process,the composition system of ultra-low carbon oriented silicon steel was proposed and the subsequent decarburizing annealing process was cancelled,leading to the further simplify the production process.The conventional process through a tedious technology that addition carbon during smelting and then decarburization’ before subsequent high temperature annealing successfully solved the contradiction between the controls of precipitation distribution by introduced the austenite phase during the hot rolling and normalization processes,and the requirements of high temperature annealing process in the single ferrite area.In the strip casting process,the refinement of solidified structure by carbon addition was ineffective.The austenite phase was distributed at the grain boundary.However,the solidified structure and texture can be effectively controlled by changing the thermal conductivity between molten pool and cast rollers.On the other hand,Due to the rapid solidification advantages of strip casting,the second phase particles were supersaturated solid solution or fine and dispersion precipitated,and then the main precipitation can be precisely controlled in the subsequent annealing process.It is not necessary to add carbon and introduce of the austenite phase and use the y/a transformation controlling the solid solution and precipitation behaviors of inhibitors.(2)The effect of single hot rolling process on the microstructure,texture,inhibitors and magnetic properties of ultra-low carbon grain oriented silicon steels was clearly explained.The results shown that the nose temperature of the PTT curve of AIN is 1090 ℃ and with the increase of hot rolling reductions,the hot rolled microstructure was refined and the Goss texture gradually increased,but because of the reduction limitation of the single hot rolling process,the hot rolled Goss texture was significantly lower than that of the conventional hot rolled sheets.After cold rolling and primary annealing,the amount of Goss texture was almost the same with that of the conventional annealed sheets.The optimal hot rolling process was 1100℃ and 35%reduction,the magnetic induction of B8 was 1.85 T.(3)The effect of normalization after strip cast on the microstructure,texture and inhibitors was studied based on one-stage cold rolling process.It was shown that without normalization process,the MnS was precipitated during primary annealing,and the precipitation behavior was started before the primary recrystallization.The distributions of MnS particles were inhomogeneous in different deformed grains,leading to the inhomgenetiy of the microstructure of primary annealed sheets.AIN particles were precipitated during high temperature annealing.The per-precipitates MnS was coarsening and failure the inhibited effect of grain growth before the beginning of AIN particles was precipitated,leading to no secondary recrystallization.When the normalization was introduced before cold rolling process,the MnS and AIN particles was precipitated homogeneous during normalization,and the primary annealed microstructure was also homogeneous.The.AlN particles have been existed before MnS particles became coarsening,which can continually inhibit the grain growth,promoting the secondary recrystallization during high temperature annealing.The optimal magnetic induction of B8 was 1.75T(4)The origin of Goss texture and its development in strip casting grain oriented silicon steel and the inhibitors solid solution and precipitation behaviors were studied based on the two-stage cold rolling process The results showed that the two stage cold rolling process could significantly improved the inhomogeneity of cold rolling and subsequent annealing microstructure caused by the coarse initial solidification structure.The primary recrystallization grain size can be controlled in the range of 5～15 μm.The effective Goss seed was originated at the shear bands formed in the cold deformed y-grains,and which distributed homogeneous along the thickness direction of intermediate annealed sheets.The Goss texture intensity of intermediate sheets was increases with the first cold rolling reduction increased.The control strategy of inhibitor solid solution and precipitation behaviors was completely different from that of the conventional process by hot rolling and normalizing.Due to the sub-rapid solidification and subsequent cooling process,the inhibitor elements can be supersaturated solid solution in the as-cast strip and then a large number of inhibitors can be precipitated fine and dispersed in the subsequent intermediate annealing process.The size and density of inhibitors can be precisely controlled by changing the first stage cold rolled reduction and annealing temperature.This control strategy can overcome the inherent problems of AIN precipitated inhomogeneous in the conventional grain oriented silicon steel bearing carbon.The utilization efficiency of the inhibitor elements was significantly improved,and a 0.18 mm thickness grain oriented silicon steel can be successfully prepared.The magnetic induction of B8 was more than 1.9 T.(5)The microstructure and texture evolutions in strip casting grain oriented silicon steels and conventional process grain oriented silicon steels with Nb-particles as grain growth inhibitors was comparatively studied.In the conventional process,the formation of hot rolled Goss texture was inhibited by Nb-particles.The Goss seeds were less in quantity and smaller in size during the stage of high temperature annealing,there was no secondary recrystallization or non-Goss {113}<110>texture was abnormal growth.The phenomenon of{113}<110>abnormal growth can be explained by CSL grain boundary theory.In the strip casting process,the Goss seed was originated in the cold rolling and subsequent annealing process,the introduction of the Nb-particles could not lead to a drop in Goss seeds.Therefore,Goss could occur secondary recrystallization,and the magnetic induction of B8 was 1.85～1.89 T.(6)The effects of inhibitor strength and texture distribution on the secondary recrystallization kinetics of grain oriented silicon steel were analyzed.The rapid high temperature annealing process of grain oriented silicon steel using twice secondary recrystallization phenomenon was proposed.It was shown that the growth rate of abnormal Goss grain was changed.At the early stage of secondary recrystallization,the grain boundary migration was higher,while the growth rates were gradually slowed as the proceeding of grain growth.The in situ observation experiments by high temperature scanning confocal microscope shown that abnormal grain growth rate is about 40～1300 m/min.The addition of Nb as grain growth inhibitor and using twice secondary recrystallization phenomenon could significantly improve the growth rate of abnormal grains.The first abnormal grain growth was obtained during high temperature intermediate annealing.Due to the strong Goss texture inherited form first abnormal grain growth,and the low temperature Nb inhibitor was obtained during the cooling process of intermediate annealing,the subsequent second abnormal grain growth accelerated,and the final magnetic induction of B8 was 1.75 T.更多还原