【作者】 庄栋栋；

【导师】 王作成；

【作者基本信息】 山东大学 ， 材料加工工程， 2014， 博士

【摘要】 低碳铝镇静钢作为一种优质的碳素结构钢,具有优异的深冲性能和较低的强度,适于冲制4mm以下的各种形状复杂的冷冲压构件,如车身、驾驶室、各种仪表及机器外壳等。随着市场竞争日益激烈,为满足对不同厚度规格低碳铝镇静钢板的需求,需要为各厚度规格钢板制订最优的生产工艺,优化深冲性能,降低成本。近年来,关于连续冷轧及相应的退火等工艺参数对低碳铝镇静钢深冲性能影响的研究报道较多,而关于可逆冷轧过程中钢板内部组织结构的演变规律,压下率和相应的退火工艺参数等对钢板深冲性能影响的报道较少。开展可逆冷轧生产低碳铝镇静钢的研究工作,能够探讨钢板轧制过程中真实的受力状态及变形规律,为研究冶金生产过程中钢板组织性能和织构的演变规律提供了可能,更好的应用于不同厚度规格钢板生产工艺参数的制订,具有重要的理论意义和应用价值。本文以可逆冷轧生产的低碳铝镇静钢为研究对象,利用X射线衍射(XRD)、电子背散射衍射(EBSD)、扫描电镜(SEM)、透射电镜(TEM)、光学显微镜(OM)以及拉伸试验等测试方法,深入系统地研究了冶金工艺参数及酸溶铝含量对低碳铝镇静钢组织和深冲性能的影响。通过利用铜丝定点标记法对可逆冷轧生产过程中钢板变形规律的研究发现,可逆轧制时,钢板往复通过轧机,剪应力方向也随之发生相反的变化,剪应力没有因为轧制道次的增加而加剧,各道次可逆冷轧之后,都没有强烈的剪切变形发生,各压下率低碳铝镇静钢钢板的心部及表层的变形模式都为压缩变形,心部及表层的显微组织及织构情况相似。退火后,基体组织都为“饼形”的沿轧向伸长的再结晶晶粒。研究了不同冷轧压下率低碳铝镇静钢的组织、织构和性能后发现,冷轧压下率对深冲性能各项指标的影响都不大。由于可逆轧制时钢带在轧机后多次往复通过对织构的形成造成了一定的影响,不同于连续轧制后的织构情况。从宏观织构情况可以看出,各压下率钢板的有利织构{111}、不利织构{100}和高斯织构{110}所占的百分含量及其相应的α、γ线织构取向密度在冶金生产过程中的变化情况都较为一致。然而压下率对钢板的微观组织结构有一定的影响,随着压下率的增加,有利的显微取向织构密度升高,不利的显微取向织构密度降低,这是由于随着压下率的不同,冷轧过程中发生碎化及退火过程中发生再结晶转变的能力不同所造成的。研究了不同冷轧压下率低碳铝镇静钢板的退火再结晶规律发现,在4小时恒时退火过程中,再结晶温度随压下率的升高而降低,压下率为40%,52%,59%,64%,72%和81%的试验钢所对应的再结晶温度分别为602℃,600℃,596℃,590℃C,576℃和572℃。在610℃恒温退火过程中,再结晶保温时间随压下率的升高而减少,上述各压下率的钢板的再结晶保温时间分别为117分钟,80分钟,42分钟,18分钟,9分钟和5分钟。但在700℃恒温退火过程中,再结晶几乎不需要孕育期,并且压下率对再结晶的影响很小。通过理论推导可以计算不同冷轧压下率低碳铝镇静钢恒时及恒温条件下的再结晶温度及保温时间,计算结果与本文试验结果吻合度较高。采用EBSD技术对64%压下率的冷轧板在退火再结晶过程中的微观组织和织构演变规律进行了深入研究,结果表明在退火再结晶过程中,再结晶晶粒优先生长方向为轧向,晶粒长大为扁平的“饼形”。从取向图中看到,{111}取向晶粒在退火再结晶过程中优先形核及长大,其织构形成具有定向形核的特点。在取向密度图中还可以看到,再结晶组织中的晶粒取向织构沿α和γ取向线的特征分布,从再结晶初期到再结晶刚完成时,基本没有发生变化,只有强度发生了一些变化。说明低碳铝镇静钢最终的退火再结晶织构主要特征在再结晶初期就决定了,即再结晶初期的形核特点就决定了最终的再结晶织构特征。压下率为64%的冷轧试样中小角度晶界所占比例高达90.7%,因为其含有大量小角度的亚晶,随着退火再结晶过程的进行,小角度晶界从490℃退火时的88%降低到610℃退火时的5.1%,这是由于与变形基体呈大角度晶界的再结晶晶核消耗了基体中具有相近取向的变形亚晶而长大的缘故。再结晶晶粒长大阶段晶粒尺寸的增加是通过大晶粒吞并小晶粒来实现的,小尺寸晶粒由于曲率大,单位体积内晶粒的表面积大,能量高,不稳定,容易被大尺寸晶粒吞并,因此,不同于再结晶阶段的驱动力来自形变储能,晶粒长大阶段晶粒尺寸增加的驱动力来自晶界表面能量的降低。通过研究退火温度对68%和75%压下率低碳铝镇静钢板组织性能影响的研究发现,退火温度在660～720℃之间时对钢板的r值及屈服强度和抗拉强度都有一定的影响,但退火温度对强度的影响更加明显,强度随退火温度的升高而降低。通过深冲性能对比发现,68%-75%压下率低碳铝镇静冷轧板较适宜退火温度范围为680℃-700℃,较低压下率的冷轧板(68%)为提高其深冲性能应选择较高的退火温度(700℃),使退火再结晶过程充分进行,而较高压下率的冷轧板(75%)为节省资源,降低成本应选择该范围内较低的退火温度(680℃)。研究了酸溶铝含量对组织性能的影响。结果表明：酸溶铝含量为0.1%的低碳铝镇静钢,屈服强度和抗拉强度过高,分别为257MPa和336MPa,而塑性应变比为0.84,失去了深冲性能,而且各取向织构在冶金生产过程中没有明显的密度变化,退火后有利的{111}取向织构密度仍十分低,这主要是由于低熔点的铝粒子在冶金生产过程中的反复固溶析出造成的；而酸溶铝含量为0.021%的低碳铝镇静钢强度较低,塑性应变比为1.19,性能优异,且有利织构在退火后明显增加。因此,酸溶铝的质量分数应控制在0.021%左右是较为合适的。更多还原

【Abstract】 The aluminum killed low-carbon steel sheets, deoxidized by adding aluminum, are one of high quality carbon structural steel sheets. They are widely used for automobile manufactory, building industry and packaging industry because of their good formability and low strength. However, most scholars paid their attentions to recrystallization behavior of continually cold rolling steel sheets and their annealing processing parameters, but few are concerned with the influence of cold rolling reduction ratios on recrystallization behavior of reversely cold rolling steel sheets and their annealing processing parameters. With the development of modern industry, demands of the steel sheets owned different thickness specifications become increasingly pronounced. So making the most appropriate production parameters for different steel sheets, saving energy-consumption and reducing cost are the hotspots in research field at present.The research objects in this article are low-carbon aluminium killed steel sheets by reversely cold rolling. Influence of metallurgy process parameters and acid soluble aluminum content on microstructure and deep drawability of the steel sheets are studied systematically by using X-ray diffraction (XRD), electron back scattering diffraction (EBSD), scanning electron microscope (SEM), transmission electron microscopy (TEM), optical microscope (OM) and tensile testing machine.The research on deformation law of steel sheets during reversely cold rolling process by copper wire fixed-point notation shows that shear stress and shear deformation don’t increase obviously with increasing reduction ratios. Because steel sheets reciprocate through rolling mill, shear stress direction is also changed correspondingly. Deformation mechanism in surface and core of steel sheets are all compressed during reversely rooling process, so microstructure is homogeneous in thickness direction. The microstructure is composed of pie recrystallization grains along rolling elongation after annealing process. Volume fraction of{111} orientation textures is dominant and good for deep drawability of the steel sheets.The study on microstructure and deep drawability of steel sheets owned different cold rolling reduction ratios shows that reduction ratios have a little influence on deep drawability. Though the rolling plane isn’t changed, the rolling direction has been completely reversed after every reversely rolling pass. So the orientation texures are different from those by continually rolling. Seen from macro-textures measured using XRD, volume fractions of favorable{111}, unfavorable{100} and{110} orientation textures hasn’t yet been changed obviously with reduction ratios. However, reduction ratios have some influences on micro-textures measured using EBSD. The quantity of favorable micro textures increases and unfavorable micro textures decreases as reduction ratios increase. It mainly because steel sheets owned different reduction ratios have different fragmentation degrees of grains in cold rolling process and recrystallization driving forces in annealing process.Study on recrystallization law of steel sheets owned different cold rolling reduction ratios shows that as cold rolling reduction ratio increases from40%to81%, recrystallization temperature of steel sheets decreases from602℃to572℃during isochronal annealing for4h, and recrystallization holding time of steel sheets reduces from117min to5min during isothermal annealing at610℃. However, recrystallization process doesn’t need incubation period during isothermal annealing at700℃, and reduction ratios have less influence on recrystalization. Recrystallization temperatures and holding time of the steel sheets with different reduction ratios during different annealing processes all can be calculated using the experiment results in this paper.The Microstructure of steel sheets owned64%reduction ratios was examined by EBSD technology. The results show that preferentially growing direction of recrystallization grains is rolling direction and equiaxed grains grow into cake-type grains during recrystallization process.{111} orientation grains preferentially nucleate and grow up. Not only that, characteristic distributions of grain orientation in α and γ orientation line haven’t been changed obviously from early to just finishing recrystallization stage. So formation mechanism of texture can be explained by directional nucleation.Number fraction of low misorientation angle boundary (LMAB<15°) of cold rolling steel sheets owned64%reduction ratio reaches up to90.7%, because its microstructure contains a lot of subgrains with LMAB. As annealing tmeperatures increase from490℃to610℃, number fraction of LMAB decreases from88.0%to5.1%after annealing for4h. This is because recrystallization nucleuses owned great misorientation angle boundary (GMAB) grow up by consuming sub-grains owned similar orientation in recrystallization process. Increase of grain size is accomplished by annexing smaller grains in grain growth stage. Because smaller grains own larger curvature and surface area in per unit volume, they have higher energy and are annexed easily by larger grains. So the driving force coming from reducing of grain boundary surface energy is different from deformation storage energy in recrystallization stage.Study on the deep drawability of steel sheets owned68%and75%reduction ratios after annealing with different temperatures shows that annealing temperatures have some influences on r value, yield strength and tensile strength of the steel sheets. The optimal temperatures are680℃-700℃for the steel sheets owned68%-75%reduction ratios. Higher annealing temperatures (700℃) should be selected for the steel sheets owned lower reduction ratios (68%) in order to fully recrystallization. Lower annealing temperatures (680℃) should be selected for the steel sheets owned higher reduction ratios (75%) in order to save resources and reduce the cost.Effect of acid-soluble aluminium content on microstructure and deep drawability of the steel sheets have been researched. The results show that yield strength and tensile strength have already reached to257MPa and336MPa, respectively, for the steel sheets owned more acid-soluble (mass fraction0.1%). Moreover, the plastic strain ratio is0.84shows the steel sheets have almost lost theirs deep drawability. Textures haven’t been changed obviously during metallurgical process. Volume fraction of favorable{111} orientation texture is still very low after anneling. This is mainly because aluminum particles owned low melting point generate in the steel sheets. The aluminum particles are dissolved and precipitated over and over again during metallurgical production process. They appear in the steel sheets in forms of small droplets when temperatures are higher than its melting point. This has severe influences on deep drawability of the steel sheets. However, yield and tensile strength of the steel sheets owned less acid-soluble aluminium (mass fraction0.021%) is190MPa and282MPa, respectively. The plastic strain ratio is1.19. Favorable textures have been increased obviously after annealing process. The deep drawability of the steel sheets is very excellent. Therefore, the mass fraction of acid-soluble should be about0.021%in this paper.更多还原