【作者】 李琦；

【导师】 刘相华； 刘伟杰；

【作者基本信息】 东北大学 ， 材料加工工程， 2021， 博士

【摘要】 连铸板坯角部横裂纹是钢厂在生产过程中最常见的铸坯缺陷,特别是角部横裂纹高频次的出现会严重影响铸坯质量,降低铸坯合格率。本文以某钢厂的Nb-Ti微合金钢连铸坯为研究对象,对铸坯角部横裂纹的宏观和微观形貌进行分析研究,确定铸坯角部横裂纹产生温度区间和位置,分析铸坯角部横裂纹的形成机理,并优化连铸工艺以改善铸坯表面质量。本文主要研究工作如下:(1)通过对连铸坯角部横裂纹宏观和微观形貌观察,发现裂纹周围存在大量的铁素体膜,厚度达到40μm以上,并且裂纹周围没有脱碳现象。这表明连铸坯角部裂纹发生在较低温度范围,即裂纹发生在第Ⅲ脆性温度区间,该区间内发生了奥氏体向铁素体转变,因此说明裂纹与铁素体的析出有关。(2)通过热模拟实验研究四种实验钢的高温塑性,确定不同成分实验钢的脆性温度区间。通过观察高温拉伸试样断口附近的金相组织,发现在高温塑性低谷区试样裂纹边缘存在大量沿晶铁素体,其断口形貌呈现出晶间脆性断裂特征。通过添加0.002%的B元素,能够有效降低奥氏体向铁素体转变温度,将高温塑性低谷区向低温区方向移动。实验发现含少量B的钢断面收缩率大于40%,优于不含B的同类钢种。这说明添加少量B能够降低钢高温塑性低谷区的宽度和深度,减小铸坯角部出现裂纹的倾向。(3)通过对实验钢连续冷却过程热膨胀曲线的分析表明:随着冷却速率增大,奥氏体向铁素体转变温度Ar3逐渐降低。在铸坯二冷阶段,提高冷却速率会导致奥氏体晶界处的铁素体膜减薄,沿晶铁素体的体积分数降低。钢中添加少量B降低了奥氏体向铁素体转变温度,在相同的冷却速率下,含B钢中晶界处先共析铁素的厚度和体积分数均低于无B钢。(4)采用ABAQUS有限元软件对现有连铸工艺进行数值模拟,研究在冷却凝固过程中铸坯温度场的变化规律。计算结果发现:铸坯角部降温速率较快,形成角部低温区;在弯曲和矫直阶段,铸坯角部温度处于裂纹敏感温度区间,为铸坯角部横裂纹的形成和扩展提供了条件。(5)对连铸过程中的二冷工艺进行优化,在弯曲矫直时使连铸坯角部温度控制在900℃以上,以避开钢种的裂纹敏感区间。采用优化后的二冷配水制度,铸坯芯部的液芯厚度只增加了4mm,并改善了铸坯温度场整体分布的均匀性。(6)对实验钢的合金成分进行优化,降低Ti含量,不添加Mo和V,并添加少量B,改善了铸坯的高温塑性。在保证钢材力学性能满足标准要求的前提下,延缓先共析铁素体的析出,避开铸坯脆性敏感温度区间,从而降低了弯曲矫直阶段铸坯的裂纹敏感性。同时添加B元素还提高钢材的淬透性,增加钢中贝氏体含量,有利于保证常温下钢材的力学性能。更多还原

【Abstract】 The transverse corner cracks of the continuous casting slab are one of the most common slab defects and have seriously affected the quality of the slabs,reducing the pass rate of the casting slabs.In this paper,the continuous asting slabs for Nb-Ti micro-alloyed steel were used as the research object.The macroscopic and microscopic morphologies of the slab corner cracks were observed and analyzed,and the forming temperature range and location of transverse corner cracks were ascertained.The forming mechanism of transverse corner cracks of the casting slabs was analyzed,and the continuous casting process is optimized to improve the surface quality of the casting slab.The main research work of this paper is as follows:(1)By observing the macroscopic and microscopic morphologies of the transverse corner cracks,it is found that there is a large amount of ferrite film with a thickness of more than 40μm,and there is no decarburization around the cracks.It indicates that the transverse corner cracks of the continuous casting slab occurred in the lower temperature range.It means that the crack occurred in the third brittle temperature range,in which the austenite to ferrite transformation occurred.Therefore,the cracks are related to the precipitation of ferrite.(2)The hot ductility of steels was studied through thermal simulation tests to obtain the brittle temperature range of the steels.By observing the metallographic structure near the fracture of the hot tensile specimens,it is found that there is a large amount of intergranular ferrite at the crack edge of the specimens in the hot ductility trough zone,and the fracture morphology also shows intergranular brittle fracture.By adding 0.002%B element,the temperature of austenite to ferrite transformation can be effectively lowered,and the hot ductility trough can be moved to the low-temperature zone.Also,it is found that the reduction of area of B-containing steels is more than 40%,which is better than that of B-free steels.It shows that adding a small amount of B can reduce the width and depth of the hot ductility trough zone and reduce the tendency of corner cracks.(3)The thermal expansion curves of the steels during the continuous cooling process show that as the cooling rate increases,the temperature of the austenite-to-ferrite transformation,Ar3,gradually decreases.In the secondary cooling stage,increasing the cooling rate will result in the thinning of the ferrite film along the austenite grain boundaries and the decrease in the volume fraction of intergranular ferrite.Adding a small amount of boron in steel reduces the temperature of austenite-to-ferrite transformation.At the same cooling rate,the thickness and volume fraction of proeutectoid ferrite at the grain boundaries of the boron-containing steels are lower than those of the boron-free steels.(4)The ABAQUS finite element software was used to simulate the continuous casting process,and the change law of the temperature field of the slab during the solidification and cooling process was studied.The calculation results show that the temperature of the slab corners has a faster cooling rate,forming a low-temperature area at the corner.In the bending and straightening stages,the temperature of the slab corner is in the crack sensitive temperature range,which provides conditions for the formation and propagation of transverse corner cracks of the slab.(5)The secondary cooling process in the continuous casting process has been optimized,so that the corner temperature of the continuous casting slabs is controlled above 900℃ in the bending and straightening section to avoid the crack sensitive zone.By optimized secondary cooling water distribution system,the thickness of the liquid core of the casting slab increases only by 4 mm,and the uniformity of the overall temperature field distribution is improved.(6)The alloy composition of the steel is optimized,reducing Ti content,no adding Mo and V,and adding a small amount of B element,improving the hot ductility of slab.On the premise of ensuring that the mechanical properties of the steel meet the requirements,the precipitation of proeutectoid ferrite is delayed,and the brittleness sensitive temperature range of the slab is avoided,thereby reducing the crack sensitivity of the casting slab during the bending and straightening stage.The B element can also improve the hardenability of the steels,increase the bainite content in the steel,and ensure the mechanical properties of the steel at room temperature.更多还原