【作者】 韩福涛；

【导师】 王作成；

【作者基本信息】 山东大学 ， 材料加工工程， 2009， 博士

【摘要】 无间隙原子（IF）钢作为最新一代冲压用钢,具有十分优异的深冲性能和非时效性,被广泛用作汽车板等冲压件的原料。其传统生产工艺需经连铸—热轧—冷轧—退火等多道工序,工艺比较复杂,成本较高。随着市场竞争日益激烈,为简化深冲钢的生产工艺、降低成本,通过热轧生产深冲板引起冶金企业和钢板用户的普遍关注。近年来研究发现,IF钢在铁素体区热轧时的主要织构是{111}织构,这使通过热轧生产深冲板的设想在IF钢上成为可能。热轧IF钢生产周期短、成本低,具有明显的技术先进性和显著的经济效益,因此开展热轧IF钢的开发研制工作具有重要的理论意义和实用价值。本文以铁素体区热轧IF钢为对象,利用电子背散射衍射（EBSD）、X射线衍射（XRD）、光学显微镜（OM）、扫描电镜（SEM）、电子探针（EPMA）、透射电镜（TEM）以及拉伸试验等测试方法,深入系统地研究了冶金因素对热轧IF钢组织、织构和深冲性能的影响。研究了化学成分对热轧IF钢组织性能的影响。结果表明:固溶C、N严重损害热轧IF钢的深冲性能。未加Ti处理的ULC钢中由于固溶C、N的存在,热轧退火后不具有深冲性能;而Ti处理的IF钢中,Ti与C、N间隙原子化合形成TiN、TiS、Ti₄C₂S₂、TiC析出物将其从基体清除,热轧退火后获得良好深冲性能。两者深冲性能的显著差异主要归因于热轧组织中晶内剪切带含量的不同,而两者剪切带行为的差异则是热轧过程中固溶C、N与位错运动交互作用的结果。P可显著提高热轧IF钢的强度,但同时也导致其深冲性能降低。P的加入使Ti-IF钢的析出行为改变,在热轧和退火时沿晶界析出大量、细小FeTiP新相,热轧Ti+P-IF钢中P以置换固溶P原子和化合态FeTiP粒子两种形式存在。P原子的固溶强化作用是强度提高的主要因素;而退火时FeTiP粒子对有利织构发展的抑制则是深冲性能降低的主要原因。通过对不同铁素体区轧制温度和压下率下热轧Ti-IF钢组织、织构和性能的研究发现,铁素体区轧制温度和压下率对热轧Ti-IF钢的屈服强度、抗拉强度、总延伸率和（?）值影响较小,但对（?）值影响较大。随铁素体区轧制温度的降低和压下率的增加,（?）值显著增加。轧制温度的降低使{111}<112>及{554}<225>取向晶粒的形变储能增加,有利于其退火过程中优先形核长大,退火后形成了更强的{111}<112>及{554}<225>织构和更弱的{001}<110>～{223}<110>织构,是（?）值增加的内在原因。相同热轧温度下,铁素体区压下率的增加使热轧组织中有利织构组分强度增加,退火后有利织构进一步增强从而（?）值提高。研究了不同润滑条件下热轧IF钢的组织性能后发现,润滑条件对热轧IF钢的深冲性能有重要影响。未润滑热轧使Ti-IF钢的（?）值显著降低。未润滑热轧使板厚方向产生严重的不均匀变形,钢板表层由于剪切变形产生大量{110}等轴细晶粒并在退火后发生晶粒长大使{110}取向晶粒减少最终形成漫散织构状态,而中心层退火后形成少量{111}再结晶晶粒,表层和中心层晶粒取向差接近随机分布。这种板厚方向上织构的不均匀分布是（?）值大幅降低的本质原因。润滑热轧使板厚方向变形均匀,表层和中心层均形成长纤维状的{001}、{112}和{111}变形晶粒,退火后形成大量{111}再结晶晶粒,晶粒取向差偏离随机分布且小角度晶界含量较未润滑条件下高。因此,热轧IF钢为获得优异的深冲性能,良好的润滑条件是必须的。通过对热轧IF钢罩式退火再结晶规律的研究发现,在恒时3小时条件下,热轧Ti-IF钢在650℃退火时可发生完全再结晶;在恒温750℃条件下,保温1分钟后即可保证完全再结晶。退火温度对热轧Ti-IF钢的再结晶过程有显著影响,而保温时间的影响则取决于退火温度的高低。在较低温度退火时,保温时间的延长对热轧Ti-IF钢充分再结晶的进行至关重要。热轧Ti-IF钢经济合理的罩式退火工艺为:退火温度750℃,保温时间3小时。采用ODF分析和EBSD技术对热轧Ti-IF钢退火过程中的织构演变进行了深入研究,结果表明Ti-IF钢铁素体区热轧后形成较强的α-<110>//RD纤维织构和较弱的γ-<111>//ND纤维织构。变形组织中主要取向变形晶粒的形变储能按{001}<110>、{112}<110>、{111}<110>和{111}<112>的次序逐渐增加。再结晶初期,{111}变形晶粒的形变储能高,将优先形成{111}取向的晶核,织构转变发生在γ纤维织构内部,即{111}再结晶织构在形变的{111}晶粒中形成和发展;再结晶后期,变形组织中α取向晶粒被大量消耗,α纤维织构强度显著减弱,其中形变储能最低的{001}<110>组分最后被消耗,同时再结晶组织中的γ纤维织构增强。再结晶初期与后期及完全再结晶阶段再结晶晶粒的织构特征基本一致,再结晶初期的形核特点决定了再结晶织构,定向形核机制在热轧Ti-IF钢再结晶织构的形成中起主导作用。更多还原

【Abstract】 Interstitial-Free （IF） steels which represent a new generation of deep drawing steels have been widely used as raw materials of automobile sheet because of its excellent deep drawability and no-aging property. Their conventional production includes multi-step processes, such as continuous casting, hot-rolling, cold-rolling, annealing etc.. The process is complicated and costly. With the increase of market competition, in order to simplify the process and reduce costs, the idea that producing steel sheets for deep-drawing operation by hot-rolling has attracted the attentions of both steel company and steel users. In recent years, it has been founded that the dominant texture is {111} texture after IF steels are hot-rolled in ferrite region, this fact make it possible that deep-drawing steel can be produced by hot-rolling. Hot-rolled IF steel is characteristic of shorter production cycle and lower cost. It is an advanced technology product and can bring significant economic benefits. Therefore, to carry out research work of hot-rolled IF steel is not only of great theoretical significance but also has practical value.In this paper, the effects of metallurgical processing on mechanical properties, microstructure and texture of ferritic hot-rolled IF steel were investigated by means of electron back-scatter diffraction （EBSD）, X-ray diffractometer （XRD）, optical microscope （OM）, scanning electron microscope （SEM）, electro-probe microanalyzer （EPMA）, transmission electron microscope （TEM） and tensile test.By investigating the effects of chemical composition on microstructure and mechanical properties of hot-rolled IF steel, it has been shown that solute C, N cause detrimental effect on deep drawability of hot-rolled IF steel. In ultra-low-carbon （ULC） steel with no Ti addition, where solute C, N atoms exist, the deep drawability cannot be obtained even if hot-rolling is performed in ferrite region. But in Ti-stabilized IF steel, where Ti can combine C, N atoms to form TiN, TiS, Ti₄C₂S₂ and TiC precipitates and thus remove them from the matrix, good deep drawability is got after ferritic hot-rolling. The significant difference of deep drawability between the two experimental steels is mainly attributed to the different content of in-grain shear bands, and the difference of shear band behavior is in turn caused by the interaction of solute C, N and dislocation movement during ferritic hot-rolling. P enhances the strength of hot-rolled IF steel significantly, but it also reduces its deep drawability. The addition of P changes the precipitation behavior of Ti-IF steel, a large number of small FeTiP new phases precipitate along grain boundaries during ferritic hot rolling and annealing. That is to say, in hot-rolled Ti+P-IF steel, the phosphorus exists in the form of solid solution P atoms and compounds FeTiP particles. Solid solution strengthening of solute P atoms is the main factors in strength enhancing, and the inhibition of FeTiP particles to the development of favorable texture during annealing is the main reason for deep drawability reducing.Effects of ferritic hot-rolling temperature and reduction on microstructure, texture and mechanical properties were studied. Results show that ferritic rolling temperature and reduction have less influence on the yield strength, tensile strength, total elongation and n|--value of hot-rolled Ti-IF steel, but have a great influence on the r|--value. With the decrease of rolling temperature and the increase of reduction, the r|--value increases significantly. With the decrease of rolling temperature, stored energy of {111}<112> and nearby {554}<225> deformed grains increase which is beneficial for them to have priority to nucleation and grow up during recrystallization, so a stronger {111}<112> and {554}<225> recrystallization texture and a weaker {001}<110>～{223}<110> recrystallization texture are formed after annealing which is the reason for the increase of the r|--value. At the same hot-rolling temperature, the intensity of beneficial texture components is enhanced with the increase of reduction in ferrite region and their intensity would be further improved after annealing, so the r|--value increases.Lubrication condition has an important effect on deep drawability of hot-rolled IF steel, r|--value reduced significantly without lubricant condition. Inhomogeneous deformation is occurred when ferritic hot-rolled without using lubricant. Large amounts of {110} equiaxed fine grains are formed in the surface layer as a result of shear deformation during hot-rolling and the number of {110} grains reduces accompanied by grain growth during annealing, finally a weak texture is formed in the surface layer. In the center layer, a small amount of {111} recrystallization grain are formed after annealing. The misorientation of both surface and center layer are close to random distribution. This kind of texture inhomogeneity is the essence reason for r|--value decrease. Uniform deformed microstructure along the thickness direction is obtained when hot-rolling is done with good lubrication condition, long fibrous {001}, {112} and {111} deformed grains are formed in both surface and center layer. After annealing, large number of {111} recrystallized grains are developed in both layers, misorientation deviates from random distribution and the content of small-angle grain boundary is higher than that in the un-lubricant condition. Therefore, to obtain excellent deep drawability, good lubricant condition is necessary. The recrystallization behavior of hot-rolled IF steel sheet during batch annealing was investigated. The results show that it takes 3 hours for hot-rolled Ti-IF steel to be fully recrystallized at the annealing temperature of 650℃and when annealed at 750℃, the time that is needed is only 1 minute. Annealing temperature has significant effect on the recrystallization of hot-rolled Ti-IF steel, and the effect of holding time on recrystallization depends on the annealing temperature. Sufficient holding time is necessary for the complete recrystallization of hot-rolled Ti-IF steel when annealed at lower temperature. The economical and reasonable batch annealing process for hot-rolled Ti-IF steel is: annealing temperature 750℃and holding time 3 hours.The texture evolution of hot-rolled Ti-IF steel during annealing was investigated by using ODF analysis and EBSD techniques. It has been founded that a strongα-<110>//RD fiber texture and a weakγ-<111>//ND fiber texture are formed in ferritic hot-rolled Ti-IF steel and the stored energy of main deformed grains gradual increase in the sequence of {001}<110>, {112}<110>, {111} <110> and {111}<112> orientations. At the early stage of recrystallization, the {111} deformed grains which have higher stored energy will give priority to form {111}-oriented nuclei. Texture changes occur among the y fiber texture, that is, {111} recrystallization texture forms from {111} deformed grains. At the latter stage of recrystallization,α-oriented deformed grains are consumed gradually, and the intensity of a fiber texture decreased significantly and the {001}<110> component is consumed finally because of its smallest stored energy, while the intensity ofγfiber recrystallization texture is increased. The texture characteristics of recrystallized grains at the early and latter stage of recrystallization are basically the same as that at complete recrystallization. The initial recrystallization nucleation characteristics determines recrystallization texture and the oriented nucleation mechanism play a dominant role in the formation of recrystallization texture in hot-rolled Ti-IF steel.更多还原