【作者】 郭卫民；

【导师】 王作成；

【作者基本信息】 山东大学 ， 材料加工工程， 2011， 博士

【摘要】 高强度无间隙原子钢(简称HIF)由于其具有优异的深冲性能和较高的强度,它在轿车覆盖件上的使用,可以显著降低汽车重量,降低生产成本,节能环保,近二十年来,受到汽车工业的青睐。高强IF钢在普通IF钢成分的基础上添加了P、Si和Mn等固溶强化元素。有成熟的研究表明,P的固溶强化作用最强,对深冲性能的损害最小。P元素以置换固溶的形式固溶在Fe基体中,由于P原子半径比Fe原子要大,P原子置换固溶时会引起基体晶格畸变,其应力场与位错应力场发生交互作用,阻碍钢板变形时的位错运动,从而提高强度。有研究发现,在铁素体区轧制IF钢可以获得较强的可以与冷轧态相媲美的{111}//RD织构类型。铁素体区轧制工艺简化了传统的冷轧深冲钢板的工艺流程,降低了生产成本,从而使热轧板具有深冲性能。因此,铁素体区轧制生产高强IF钢具有明显的技术先进性和显著的经济效益,开展铁素体区轧制高强IF钢的开发研制工作具有重要的理论意义和实用价值。本文所用实验钢由宝钢CONSARC500公斤真空感应炉冶炼,并在钢铁研究总院进行铁素体区轧制实验。利用光学显微镜(OM)、扫描电镜(SEM)、透射电镜(TEM)、电子背散射衍射(EBSD)、硬度测定以及单向拉伸实验等测试方法,系统深入了研究了冶金因素以及铁素体区轧制工艺参数对含P高强IF钢微观组织和力学性能的影响。研究了P元素对铁素体区轧制含P高强IF钢组织性能的影响。结果表明：P元素的加入使钢板深冲性能略有下降,并显著提高了钢板的强度。P元素在钢中以两种形式存在：置换固溶的P原子和形成FeTiP化合物。置换固溶的P原子使Fe基体晶格发生畸变,并阻碍位错运动,这是钢板强度提高的主要因素。退火时FeTiP粒子对有利织构的发展起到抑制作用,同时,FeTiP的形成会消耗钢中用于消除C、N间隙原子的Ti原子,使得钢中的二相粒子除TiN、TiC、TiS和Ti4C2S2外,还有少量的Fe4N、Fe3C、AlN,钢中C、N间隙原子的存在是高强IF钢深冲性能降低的另一个原因。铁素体区轧制时设计了不同的开轧温度(ST)、终轧温度(FT)和轧制压下率(RED),通过对不同轧制参数的含P高强IF钢组织、织构和力学性能的对比研究发现,铁素体区轧制温度和压下率对含P高强IF钢的屈服强度(YS)、抗拉强度(TS)、总伸长率(E1)和n值的影响较小。开轧温度为840℃时,钢板深冲性能较差,了<1,开轧温度为700℃和750℃时,钢板深冲性能较好,r>1；终轧温度和轧制压下率对了值影响较大,随终轧温度逐渐上升,r值呈上升趋势；随轧制压下率逐渐增大,r值呈下降趋势；同时,交叉比较发现,当终轧温度较低时,如540℃,可在较小的轧制压下率下获得较高的了值,较好的深冲性能；当终轧温度较高时,如660℃,则可在较大的轧制压下率下获得较高的r值,较好的深冲性能。通过对比润滑轧制和不润滑轧制的含P高强IF钢的组织、织构和力学性能发现,良好的润滑条件是使钢板轧后获得较好的深冲性能的必要条件。未润滑轧制使含P高强IF钢沿板厚方向产生严重的不均匀变形,钢板与轧辊的摩擦使钢板表层发生剪切变形产生大量取向为<110>//ND的等轴细晶粒,这些细小的等轴晶在退火后发生晶粒长大,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r值大幅降低的本质原因。润滑轧制使钢板沿厚方向变形均匀,均为压缩变形。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,具有优异的深冲性能。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。更多还原

【Abstract】 As a result of excellent deep drawing properties and high strength of high strength interstitial-free steels (referred to as HIF steels), they can dramatically reduce the weight of vehicles, decrease the product cost, save energy and environment friendly, therefore, the application of HIF steels in automobile industry has attracted much attention in recent twenty years. HIF steels are achieved by adding P, Si, Mn or other solution strengthening elements to traditional IF steels. Among these solution strengthening elements, P has the strongest solid solution strengthening function and the smallest damage to formability. P element exists in Fe matrix in the form of replacement solid solution. As a result of larger radius of P atoms than that of Fe atoms, the replacement of P atoms leads to lattice distortion of the matrix. The interaction of its stress field and the dislocation stress field blocks the movement of dislocations during the deformation of steels, and then results to the improvement of steel strength.Studies showed that Ferritic-Rolled-IF steels possessed a strong{111}//RD texture type comparable with cold-rolled-IF steels. The technology of ferritic-rolling simplifies the traditional process of cold-rolled deep drawing steel sheet and reduces production cost, leading to the hot-rolled plate with a good deep-drawing performance. Therefore, producing high-strength IF steel with ferritic-rolling technology is advanced and has significant economic benefits, furthermore, the development of ferritic-rolling technology has importantly theoretical meaning and practical value.The steel used in this experiment was smelt in CONSARC500 kg vacuum induction furnace in Baoshan Iron and Steel Company, and the ferritic-rolling test was carried out in Iron and Steel Research Institute. Optical microscope (OM), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), Electron Backscattered Diffraction (EBSD), Rockwell Hardness Tester and Uniaxial Tensile Tester were used to investigate the effect of metallurgical factors and ferrite-rolling process parameters on microstructure and mechanical properties of P-added high-strength IF steels.The effect of P element on the microstructures and mechanical properties of ferritic-rolled P-added high-strength IF steel were studied. Results showed that after adding P element, the strength of steel was significantly improved and deep drawing performance was decreased slightly. P element exists in the steel in two kinds of forms:P atoms that replace Fe atoms in matrix lattice and P atoms that form FeTiP compounds. The replacement of P atoms to Fe atoms causes lattice distortion of the matrix and impedes dislocation movement, which is a major factor on improving strength steel. FeTiP particles restrain the growth of favorable texture during annealing; meanwhile, the formation of FeTiP in the steel consumes Ti atoms that are supposed to eliminate C and N interstitial atoms, which leads to the existence of Fe4N, Fe3C, AlN, besides TiN, TiC, TiS and Ti4C2S2. The existence of C and N interstitial atoms in the steel is the other reason that the formability of high strength P added IF steel decreases.Different rolling parameters such as start-rolling temperatures (ST), finish-rolling temperatures (FT) and the rolling reduction (RED) were designed when ferrite regions were rolled. Comparing the microstructures, textures and mechanical properties between P-added high-strength IF steel sheets rolled with different rolling parameters, it is found that ferritic-rolling temperature and reduction have slight effect on yield strength (Rp), tensile strength (Rm), elongation (e) and n-value, but have significant effect on r-value. When the start-rolling temperature was at 840℃, the steel showed poor of formability,r<1; when the temperatures were at 700℃and 750℃, the steel showed good formability,r<1. Finish-rolling temperature and rolling reduction have great influence on r-value.r-value increases with increasing finish-rolling temperature, in contrast,r-value decreases with increasing rolling reduction. Meanwhile, from cross-comparison, it is found that, when finish-rolling happens at lower temperature, such as 540℃, higher r-value and good formability are obtained at lower rolling reduction; when finish-rolling happens at higher temperature, such as 660℃, higher r-value and good formability are obtained at higher rolling reduction.Comparing the microstructures, textures and mechanical properties of the steel sheets that are rolled in lubricate conditions and nonlubricate conditions, it is found that good lubricate condition plays an important role in obtaining good formability. Serious inhomogeneous deformation occured when the steels were rolled in ferrite region without lubricate and large amount of<110>//ND fine grains formed in the surface layer of the steel sheet, which finally leaded to weak texture condition. However, a small amount of{111} recrystallization grains form after annealing in the center layer, and the dis-orientation between surface and center layers is very close to random distribution. This texture in-homogeneity is the essential reason for the decrease of r-value. Lubrication condition leads to the uniformly pressed deformation along thickness direction. Long fibrous{001},{112} and{111} deformed grains were formed in both surface and center layer, and large amount of {111} recrystallized grains were formed after annealing treatment, which have excellent formability. Therefore, good lubricant condition is necessary to obtain excellent formability.更多还原