【作者】 郭皓；

【导师】 张延玲；

【作者基本信息】 北京科技大学 ， 冶金工程， 2021， 博士

【摘要】 钢中残留的大型夹杂物会导致材料裂纹萌生而损害其机械性能,同时由于晶粒粗大而导致大幅度降低材料的强韧性。上世纪有学者提出了“氧化物冶金”技术用于解决以上问题,即控制材料中细小弥散的夹杂物作为异质形核点,诱导晶内铁素体形核。随着外加技术和设备的不断成熟,通过喷吹等方式向钢液中加入合适成分的第二相粒子,可以起到促进晶粒细化、细化夹杂物等作用。之前有研究在钢中外加纳米级第二相粒子,一定程度细化了钢中的夹杂物和微观组织。然而,由于纳米粒子比表面积大、表面能高的特性,加入钢液后粒子容易聚集并上浮到钢液表面,造成纳米粒子在钢液中的使用效率大幅度降低。纳米粒子之间的团聚现象是软团聚,传统物理手段不能从本质上消除粒子间的作用力,因此有必要改变炼钢用纳米粒子的表面特性。首先,采用化学手段对MgO纳米粒子表面改造,根据表征结果,制备出一种新型的具有核壳结构的炼钢用纳米粒子,碳化后的粒子表面有一层厚度为10nm的碳层,在溶液中具有良好的单分散性。在氦气气氛下,原始MgO纳米粒子在高温钢液的润湿角达到了 130°,而表面改造的MgO@C纳米粒子的润湿角只有50°,具备更小的润湿角意味着改性后的纳米粒子具有更良好的润湿性。通过高温预实验分段取样,测定合金元素的含量计算得知,试验钢中改性纳米粒子的收得率达到了 65%,远高于原始纳米粒子的收得率。利用化学表面改性的方法,提高了炼钢用纳米粒子的收得率,解决外加纳米粒子技术的关键技术问题。其次,应用改性的炼钢用纳米粒子设计高温冶炼实验,研究发现纳米粒子对钢中非金属夹杂物的特性有很大影响。根据Factsage热力学模拟软件和SEM-EDS测试结果得知,纳米试验钢中生成大量不规则形状的TiN夹杂物,而且MgAl2O4尖晶石也逐渐取代了原始钢中单相Al2O3夹杂物。添加同质量的纳米粒子时,含改性纳米粒子的试验钢中的细小夹杂物的数量也高于含原始纳米粒子的试验钢。特别地,在含0.03%改性MgO@C纳米粒子的试验钢中,亚微米级别的夹杂物数量比例达到了所有夹杂物数量的77.2%。细小的夹杂物可以阻碍原奥氏体晶粒迁移并诱导针状铁素体形核。根据夹杂物异质形核诱导铁素体的理论,热力学计算得出TiN夹杂物的等效临界形核直径为0.346μm。改性纳米粒子在不同冷却条件下,对试验钢中微观组织的演变也有很大影响。在低碳高合金钢中,冷却速率的增加会减少试验钢中多边形铁素体的比例,并且会生成贝氏体相。更大的冷却速度为铁素体相变提供了更高的过冷度。同时,纳米试验钢中细小弥散的夹杂物会对原奥氏体晶界起到钉扎的作用,试验钢中细小的晶粒也会促进针状铁素体形核。在原位观察实验中,板条铁素体总是沿着晶界形成,而且总是先于针状铁素体形核,这些铁素体大多是在夹杂物表面被诱导。当冷却速率上升到-15℃/s时,板条铁素体和针状铁素体的开始转变温度都会降低,并且针状铁素体的比例会增大。同时,一定温度范围内,针状铁素体的长度与时间呈线性比例关系,说明在相变过程中针状铁素体形核的驱动力随时间基本不变。当冷速相同时,纳米试验钢中针状铁素体的开始形核温度高于原始钢,而且形核速率更大。最后,将外加纳米粒子技术应用于试验钢形变诱导相变强化工艺中,通过控制热压缩形变参数,研究双强化技术下实验钢中微观组织的演变及力学性能的变化。通过热压缩形变实验得知,更大的形变量导致钢中铁素体与马氏体相的平均尺寸均降低。钢中出现了大量细小链状的形变诱导铁素体和交错的针状铁素体,极大地提升了钢中微观组织的交错度,提高了材料的强韧性。在同一形变温度下,纳米钢中的应力峰值始终高于原始钢中的应力峰值。当形变温度为750℃时,纳米试验钢对应的最大应力峰值为516MPa,比原始钢的最大应力峰值高出28.4%。更多还原

【Abstract】 The residual large inclusions in steel material can lead to the development of cracks and impair its mechanical properties,while the coarse grains also lead to a significant reduction in the toughness of the material.In the last century,the technique of "oxide metallurgy" was proposed to solve the above problems,which is to form the fine dispersed inclusions in the material as heterogeneous nucleation points to induce intragranular ferrite nucleation.With the continuous development of external addition technique and equipment,the addition of second-phase particles with suitable composition into the molten steel by spraying method can promote grain refinement and fine inclusions.Previously,the addition of nanoscale second-phase particles to steel has been investigated,and the inclusions and microstructure of the steel material have been refined to some extent.However,due to the characteristics of large specific surface area and high surface energy of nanoparticles,the particles tend to agglomerate and float to the steel surface after entering into the molten steel,resulting in a significant decrease in the efficiency of using nanoparticles in the steel.The agglomeration phenomenon between nanoparticles is soft agglomeration,and the traditional physical means cannot essentially eliminate the interparticle forces,so it is necessary to change the surface properties of nanoparticles for steelmaking.Firstly,a new type of nanoparticles with core-shell structure for steelmaking was prepared by chemically modifying the surface of MgO nanoparticles,and based on the characterization results,the carbonized particles had a carbon layer of 10 nm thickness on the surface with excellent monodispersity in solution.Under helium atmosphere,the wetting angle of the original MgO nanoparticles reached 130° in the high temperature molten steel,while the wetting angle of the surface modified MgO@C nanoparticles was only 50°.The surface-modified nanoparticles exhibited better wettability with a smaller wetting angle.The high temperature pre-experimental segmental sampling and the determination of the alloying element content were calculated to show that the yield of modified nanoparticles in the test steel reached 65%,which was much higher than that of the original nanoparticles.By using the chemical surface treatment method,the yield of nanoparticles for steelmaking was improved,and the key technical problem of additive nanoparticle technology was solved.Then,the modified nanoparticles for steelmaking were applied to design high-temperature smelting experiments,and it was found that nanoparticles had a great influence on the characteristics of non-metallic inclusions in steel.According to the results of Factsage thermodynamic simulation software and SEM-EDS analysis,it was known that a large number of irregularly shaped TiN inclusions were formed in the tested steel with nanoparticle addition,and the MgAl2O4 spinel also gradually replaced the single-phase Al2O3 inclusions in the original steel.The number density of fine inclusions in the test steel containing modified nanoparticles was also higher than that in the test steel containing original nanoparticles.In particular,the proportion of submicron-scale inclusions in the test steel containing 0.03%modified MgO@C nanoparticles reached 77.2%of the total number of inclusions.The fine and stable inclusions could hinder the migration of prior austenite grains and induce acicular ferrite nucleation.According to the theory of heterogeneous nucleation of ferrite induced by inclusions,the equivalent critical nucleation diameter of TiN inclusions was thermodynamically calculated to be 0.346 μm.The modified nanoparticles also had a significant effect on the evolution of microstructure in the tested steels under different cooling conditions.In low-carbon high-alloy steels,an increase in cooling rate reduced the proportion of polygonal ferrite in the test steels and generated bainite phases.The greater cooling rate provided higher subcooling for the ferrite phase transformation.Meanwhile,the fine dispersed inclusions in the nano-test steels had a pinning effect on the prior austenite grain boundaries,and the fine grains also promoted acicular ferrite nucleation.In the in-situ observation experiments,ferrite side plate always formed on the boundary prior to acicular ferrite formation on the intragranular inclusions.When the cooling rate increased up to-15℃/s,the starting transformation temperature of both ferrite side plate and acicular ferrite decreased,and the proportion of acicular ferrite increased.Meanwhile,the length of acicular ferrite was linearly proportional to time within a certain temperature range,indicating that the driving force of acicular ferrite nucleation during the phase transition process was basically constant with time.When the cooling rate was the same,the starting nucleation temperature of the acicular ferrite in the nano-test steel was higher than that of the original steel,and the nucleation rate was greater.Finally,the external nanoparticle addition technology was combined with deformation strengthing technology to investigate the evolution of microstructure and mechanical properties in the experimental steel under the double strengthening by controlling the hot compression deformation parameters.The compression deformation experiments showed that the larger compression led to a decrease in the average size of both ferrite and martensite phases in the steel.A large amount of chain-like deformation induced ferrite and interlaced acicular ferrite were formed in the test steel,which greatly enhanced the interlacing of microstructures in the steel and improved the strength and toughness of the material.At the same deformation temperature,the stress peaks in the test steel were always higher than those in the original steel.When the deformation temperature was 750℃,the maximum stress peak of the nano-test steel was 516 MPa,which was 28.4%higher than that of the original steel.更多还原