【作者】 张鹏；

【导师】 汪凌云；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2008， 博士

【摘要】 IF钢又称为无间隙原子钢,属于第三代深冲钢,在汽车、机械等领域得到了广泛的应用。本文为探索其在铁素体区热变形工艺与组织性能关系,用Gleeble-1500热模拟试验机,针对Nb+Ti-IF钢在铁素体区进行了拉伸变形,在试验数据的基础上拟合了该钢在铁素体高温区流变应力峰值σ的解析表达式,建立了铁素体高温区压缩变形的功率耗散图和加工失稳图,并形成了铁素体高温区间的热加工图。为探索IF钢冷轧工艺与组织性能关系,论文采用一次冷轧制度和增加了中间退火工艺的二次冷轧制度进行了Nb+Ti-IF钢冷轧工艺试验,完成了酸洗、一次冷轧、中间退火、精整、二次冷轧、再结晶退火等工序,制备出尺寸规格为200×1000×(0.325～2.16)mm的IF钢板带。通过拉伸实验对板料的基本深冲力学性能进行了测试和计算。结果表明:二次冷轧工艺能获得相对一次冷轧工艺更优的力学性能,在同等变形程度下,二次冷轧退火后获得的塑性应变比值最高达到3.08,比一次冷轧提高了18%,在此基础上综合基本深冲力学性能参数对模拟深冲力学性能之一的极限拉深系数进行了预测。非线性参数拟合了三种硬化曲线数学模型,并得到了本试验用Nb+Ti-IF钢最优的加工硬化模型。用该模型揭示了不同冷轧工艺下板材的硬化能力:本试验中硬化能力相对较好的是首次冷轧60%,二次冷轧30%工艺对应的板料。通过金相和透射电子显微镜对IF钢的冷变形和退火组织进行了观察,随变形量的增加,变形愈趋均匀,再结晶后晶粒愈来愈细,二次冷轧工艺对应的退火组织总体更加均匀。在变形量大而且层错能较高的IF钢冷变形组织中,以刃位错为主的混合位错呈曲线分布在滑移带周围,并由于局域剪应力的存在,形成大量的排列很密的长条状的“剪切带”。通过X-射线衍射方法,测定了不同轧制工艺下板材主要织构分布。二次冷轧的形变织构的显著特征是<111>//ND织构相对中间退火得到了明显增强,增长最多的是{111} < 110>组分;α织构和γ织构的演变与第二次冷轧压下率之间没有简单的线性关系,但{001}织构几乎随着二次压下率的增加而增加。与一次冷轧后退火织构相比,二次冷轧退火织构有强度相对较低的{001}织构和较高的<111>//ND织构;在退火状态下,首次冷轧70%、二次冷轧20%的工艺对应的{111}与{001}面织构含量的比值最高,说明该工艺下退火态薄板测得的最高r值是准确的。本文研究获得的IF钢铁素体区热加工图、本构关系和二次冷轧优化工艺及该钢种在冷轧过程中的织构演变分析,丰富了超深冲钢理论,对生产具有指导意义。更多还原

【Abstract】 Interstitial free (IF) steel is one of the third generation deep-drawing steels. It is widely used in automobile and machinery and so on. In this paper, in order to obtain the relationship between the hot working process in ferrite state and the microstructure and properties, the compressive deformation data of Nb+Ti-IF steel at high temperature of ferrite area have been fit into an express of peak stressσ. Through the power dissipation map and the instability map, the process map for hot working was eventually obtained.In order to obtain the relationship between the cold working process and the microstructure and properties of Nb+Ti-IF steel, single cold rolling and double cold rolling with an intermediate annealing were performed. IF steel sheets with the dimension of 200mm×1000mm×(0.325 ~ 2.16 mm) were attained via the following manufacturing procedure: acid pickling, primary cold rolling, intermediate annealing, finishing, secondary cold rolling and recrystallization annealing,.The basic mechanical properties of deep-drawing sheet have been evaluated by tensile test and computation. The results indicated that double cold rolling process resulted in better mechanical properties than the single cold rolling process with the same total reduction did. After annealing, the plastic strain ratio resulting from double cold rolling techniques was 3.08, seeing an increment of 18% in comparison to that resulting from single cold rolling. The ultimate stretch coefficient, a simulated mechanical property for deep stamping, was predicted using the basic mechanical properties data. Three kinds of hardened curves were ploted via the non-linear simulation, in which the optimum work hardening model applicable to the Nb+Ti-IF steel was also obtained.The work hardening properties of sheets under different cold rolling processes were studied using this model. The result indicated that the sheet with the cold reduction of 60% in the first time and 30% in the second time had the best work hardening properties. The microstructures after cold rolling and annealing were observed using a transmission electron microscope (TEM) and an opticle metallographic microscope. As the cold reduction increased, the deformed microstructures became more uniform, and the recrystallized grains became more symmetrical. Moreover, the microstructure resulting from the double cold rolling was more homogeneous than that resulting from the single cold rolling. In the IF steel sample heavily cold deformed, a lot of mixing dislocations, most of which were edge-dislocations, distributed around the slip-bands. As a result of the shearing stress concentration, a lot of very dense“shear bands”with the shape of parallel sliver were formed in some areas.The textures resulting from different cold rolling processes were measured using XRD. The deformation textures resulting from double cold rolling were remarkably charactered by the enhenced <111>//ND texture after intermediate annealing, among which the {111} < 110> component was the most enhanced. The evolution ofαandγtextures were not linearly related to the second cold reduction, whereas the {001} texture incerased with the increase of the reduction. After double cold rolling and annealing, the recrystallization textures included lower intensity of {001} texture and higher intensity of <111> //ND texture in comparison to the textures resulting from the single cold rolling; The highest recrystallization texture content of {111} and {001} was measured after double cold rolling when the reduction assignment was 70% in the first time and 20% in the second time, suggesting that the highest r value of the sheet after this cold rolling condition was correct.The process map for hot working in ferrite region, stress-strain relationship and double cold rolling process were established for IF steel. The evolution of the textures during cold rolling was systemically analyzed. These results not only added new ideals to the theoretic system of deep stamping steels, but provided guideance to the production of such steels also.更多还原