【作者】 杨春政；

【导师】 翟启杰；

【作者基本信息】 上海大学 ， 钢铁冶金， 2011， 博士

【摘要】 以典型铌、钒、钛微合金化钢为研究对象,通过理论和试验研究分析了板坯表面角横裂的形成机理,建立了常规和带倒角结晶器内的流体流动、传热和宏观凝固以及矫直过程应力应变的数学模型,分析了常规和带倒角结晶器下板坯的凝固过程及其不同角部形状(包括倒角角度及倒角长度的变化)对铸坯角部钢水流动、温度变化、凝固过程和应力应变的相对影响。在此基础上,优化设计了倒角结晶器结构,采用带倒角的结晶器窄边铜板进行了工业试验,并考察了带倒角连铸坯对典型微合金化钢边部质量的影响。主要研究结果如下:(1)卧坯试验结果表明,结晶器内及垂直段铸坯角部无裂纹。在距弯月面3270mm处,即对应于弯曲开始后710mm铸坯即开始出现多处外弧横裂纹,因此外弧裂纹是弯曲过程产生的。(2)结晶器内的流体流动、传热和宏观凝固的数学模型分析表明,铸坯倒角形状的变化并不明显影响浸入式水口附近的总体流动模式。倒角形状的改变明显影响了弯月面位置处角部的流动分布,随着倒角角度的增加,弯月面角部的流动分离位置更靠近于铸坯的侧面,而且在铸坯宽面与窄面相交的角部附近的流动明显增强,流动对铸坯角部的冲击增加。随着倒角长度的增加,弯月面角部的流股对铸坯窄边的倒角部位冲击增加。在结晶器出口位置,随着倒角角度的增加,铸坯角部的表面温度近似呈线性增加,但是铸坯宽面与窄面的角部附近的流动增强。就不同倒角长度设计而言,较小的倒角长度如L=10mm就能将角部温度提高约102℃,随着倒角长度的增加,铸坯角部温度的提高幅值降低,当倒角长度从L=60mm增加到L=80mm时,铸坯角部的温度值提高幅度仅为26℃,而流动对铸坯倒角部位的冲击则明显增加,坯壳厚度变薄。因此,在优化结晶器倒角设计时,需要考虑铸坯倒角角度和倒角长度尺寸的改变对角部钢水流动、温度分布和凝固坯壳增长的综合影响。(3)矫直过程应力应变有限元模拟分析结果表明,当矫直(压下)速度一定时,铸坯温度的变化(700℃~1000℃)对铸坯截面切向等效应力应变影响比较小,但对等效应力影响比较大。最大的等效应力的位置发生在倒角斜面内,距角部约15mm~33mm。当矫直温度在900℃以上时,斜面内最大等效应力范围大幅下降。倒角角度对铸坯棱角部位切向应力应变影响很大,在等倒角长度条件下,30o和45o倒角铸坯棱角部位切向应力应变相对最小,只有同截面直角铸坯的40%~46%。不同倒角长度对铸坯棱角部位切向应力应变影响很大,在等铸坯角部倒角( 30o)一定条件下,当倒角长度控制在65mm~85mm之间时,铸坯棱角部位切向应力应变相对最小,当倒角长度为75mm时,铸坯棱角部位切向等效应变只有同截面常规铸坯的40%。(4)工业试验结果表明,倒角结晶器窄面铜板可用于首钢京唐公司板坯的规模化生产,其对液位波动、拉坯阻力没有明显的影响,在铸机矫直位置,大倒角的铸坯角部温度相比常规铸坯提高了100℃左右,提高了矫直段铸坯的高温延展性,有利于控制微合金钢板坯角横裂的发生;倒角结晶器在生产Q345B钢以及X65、L290等微合金钢板坯时,铸坯角横裂发生率得到了大幅度的降低,比现有技术降低了80%以上。更多还原

【Abstract】 The formation mechanism of slab transverse corner cracks in typical niobium, vanadium, titanium micro-alloyed steels was investigated by the theoretical models and industrial experimental trials. The chamfered mold, which is used to minimize the cracks, was developed. A numerical simulation of the fluid flow, heat transfer and macro-solidification in the conventional mold and the chamfered mold, together with a finite element stress-strain model in the straightening process of both molds, were performed to analyze the relative influence of the conventional model and chamfered mold with different chamfer shapes (including the chamfer angle and the chamfer length) on the fluid flow, temperature distribution and solidification, and the stress and strain in the slab corner. Then the mould copper plate with an optimum chamfer shape is designed on the basis of the numerical results and is applied in industrial tests. The effect of the chamfered mold on the slab corner quality of the typical micro-alloyed steels is analyzed. The following conclusion can be drawn:(1) The results from the“lying slab”experiment show that there is no crack in the mold and the vertical segment below the mold. However, at the location of 3270mm from the meniscus, where the slab bending starts after the 710mm from the vertical segment, the transverse corner cracks frequently occur in the outside curved surface of the slab. Therefore, the cracks are formed in the slab bending process.(2) The predicted results from numerical simulation of fluid flow, heat transfer and macro-solidification in the conventional mold and the chamfered mold show that the variations of the chamfer shapes don’t significantly affect the overall flow pattern near the SEN in both molds, but change the flow features near the meniscus corner. With the increase of the chamfer angle, the flow separation location near the meniscus corner is closer to the narrow face of the slab. The fluid flow near the intersection of the width face and narrow face and its impingement on the slab corner are also stronger. With the increase of the chamfer length, it is found that the flow near the corner becomes intensive. At the mold exit, the increased chamfer angle leads to an approximately linear increase of the slab surface temperature, but it also causes the strong flow near the corner. As far as the chamfer length is concerned, very small length (e.g. L=10mm) can lead to the significant increase of the temperature near the slab corner. As the chamfer length increases fromL=60mmto L=80mm, the temperature of the slab corner increased slightly while the flow near the chamfered corner of the slab obviously enhances and the thickness of solidified shell becomes thinner. Therefore, in order to optimize the design of the chamfer angle and length, it is necessary to comprehensively consider their effects on the flow, heat transfer and solidification on the slab corner.(3) The calculated results from the finite element stress-strain analysis in the straightening process show that when the straightening (pressing) velocity is constant, the slab temperature (700℃~1000℃) has little effect on the equivalent stress and strain on the cross sections in the slab. Equivalent stress are concentrated within the incline position in the chamfer, from the corner about 15mm~33mm; When the straightening temperature is above 900℃, the range of maximum equivalent stress within the sloped drops significantly, which reduces the possibility of occurrence of cracks. The chamfer angle has great impact on the tangential stress-strain near the slab edges and corners. At the same slope width,if the chamfer angle are chosen as 30o and 45o , the tangential stress-strain on the slab edges and the corners is least, only 40% to 46% of rectangular slabs with the same cross-sectional area. The chamfer angle of 30°is better than 45°. Chamfer length on the part of the slab edges and corners have great impact on the tangential stress-strain. If chamfer angle(30°) on the part of the slab corners were constant, when the chamfer length is controlled between 65mm~85mm, the tangential stress-strain on the part of the slab edges and corners were least. It is only 40% of conventional slabs with the same cross-sectional area if the slop width is 75mm.(4) Industrial test results show that the copper narrow plate with the chamfer shape can be used for Shougang Jingtang’s slab production. It hasn’t significant effect on the level fluctuations and mold withdrawal resistance. The service life of the mold plate is up to 236 furnaces. The slab corner temperature in the chamfered mold with the large chamfer increases by about 100℃, compared to the conventional molds. This increase corner temperature improves the high-temperature ductility of the slabs in the top bending section and the straightening section, and thus it is helpful for controlling of the transverse corner crack in the micro-alloy steel slabs. Using the chamfered mold for the production of X65, L290, SPA-H and other micro-alloy steel slabs, the slab transverse corner cracks have been reduced significantly, about 80% or more in comparison with those in the conventional mold.更多还原