【作者】 李娇；

【导师】 文光华；

【作者基本信息】 重庆大学 ， 冶金工程， 2011， 博士

【摘要】 大型钢锭一般指其重量大于400吨的钢锭。百万千瓦级核电常规岛整锻低压转子的重量约为170~180吨,是目前世界上所需钢锭体积最大、锻件毛坯重量最大、截面尺寸最大、技术要求最高的实心锻件,需浇注约580~650吨大型钢锭。为解决低压转子在工作温度条件下的回火脆性问题,要求该类钢需进行超纯净控制。目前,国内外转子钢的生产多采用EAF—LF—VD—VCD多炉连浇工艺,通过上注法真空浇注而成。大型钢锭的浇注受冶炼炉容量的限制,须采用多炉连浇技术,但由于浇注速度大(通钢量是连铸的1.5-2.5倍),故既同于常规连铸又有别于常规连铸。在多炉连浇大型钢锭过程中,中间包是连接钢包与钢锭模之间的中间容器,其主要功能为贮存一定量的钢水,以保证在更换钢包时实现多炉连续浇注。随着中间包冶金技术的发展,常规连铸中间包的去夹杂功能成为了冶金工业洁净钢生产的一个关键技术。即,采用大容量、深熔池、设置挡墙挡坝中间包来提高钢水在中间包内的平均停留时间,从而促使夹杂物在中间包内聚合上浮、排除。目前国内重型机械行业大型钢锭生产为满足浇注速度的要求,使用的圆形中间包虽然具有大容量、深熔池的特点,但由于其大包注点到注流出口的距离短,浇注流量大,导致钢液在包内的停留时间较短,夹杂物来不及上浮就直接进入钢锭模,没能发挥中间包去夹杂的冶金功能,降低了大型铸锻件的质量。因此,正确设计满足模铸过程多炉连浇、保证大通钢量和高去除夹杂物能力的中间包几何尺寸,对提高低压转子锻件及大型钢锭洁净度具有重要意义。本文以第一重型机械厂多炉连浇大钢锭中间包为研究对象,通过物理模拟和数理模拟实验,对不同熔池深度(H)的圆形中间包和不同中间包长度(L)的槽型中间包内钢水流动模式和夹杂物上浮状况进行了研究。针对大流量、深熔池的特点,结合现场的安装、砌墙和吊浇等因素,首次提出了有效提高多炉连浇大钢锭质量的中间包的合理几何尺寸(即L/H),设计了新型椭圆形中间包及控流装置。现场应用证明,采用本研究结果设计的多炉连浇大钢锭新型椭圆形中间包能有效地去除钢水中非金属夹杂物,较大提高钢水洁净度,确保了低压转子锻件的成品质量。本文还通过不同湍流模型对多炉连浇大钢锭中间包内钢液流动行为和状况的预测能力的讨论和研究,建立了大钢锭圆形和椭圆形中间包的三维稳态计算模型,提出了最优湍流模型流场数值模拟方案,提高了预测能力和效果。主要的研究内容和成果如下:1、现用圆形多炉连浇大钢锭中间包的评估采用数理模拟对某重型机械厂多炉连浇大钢锭圆形中间包内钢液流动状况和夹杂物去除情况进行了评估比较,结果表明:①圆形中间包无任何控流装置时,中间包内钢液的平均停留时间较短,死区大,夹杂物上浮率较低。②安装控流装置后,中间包内钢液的平均停留时间虽然有所增加,但是由于大包注点至中间包出口距离过近,夹杂物来不及上浮,仍然不能达到提高钢水洁净度的要求。因此,需要对圆形中间包进行优化。2、多炉连浇大钢锭中间包结构尺寸研究首次进行了多炉连浇大钢锭中间包不同熔池深度和长高比对中间包内钢液流动行为和夹杂物上浮率的影响研究,并提出了针对大流量多炉连浇大钢锭的中间包合理的几何尺寸(论文的第一个创新点)。①增大熔池深度和延长中间包长度都有利于夹杂物的上浮,但中间包体积相同时,延长中间包长度更有利于夹杂物上浮。②结合钢液在中间包内的流动模式和夹杂物上浮率的结构分析认为,多炉连浇大钢锭中间包液位熔池深度应在1500mm～2000mm,长度与熔池深度之比应控制在2.0～2.7之间。3、多炉连浇大钢锭椭圆形中间包内控流装置研究结合现场实际生产情况和中间包吊运、砌砖、安放等因素设了一种新型的深熔池、大容量的椭圆形中间包,并对其控流装置进行了优化,提出了最优挡墙挡坝的安装距离和挡墙的开口位置(论文的第二个创新点)。①通过对18组控流装置设计方案的研究,从钢液流动停留时间、流动模式和夹杂物上浮率分析研究,挡墙距离中间包底部为300mm、挡坝高度为400mm,大包注点与挡墙距离为1500mm、挡墙与挡坝距离为490mm为最优控流装置安装位置方案。②根据前文研究结果和现场实际情况,在设计的中间包长度与熔池深度比为2.0的椭圆形中间包中对挡墙开孔位置进行了优化分析,研究结果表明,开孔位于中间包对称轴上的方案为最优方案。③相同容量的椭圆形中间包与圆形中间包模拟结果相比可知,安装控流装置后,椭圆形中间包的开始响应时间是圆形中间包的2.5倍,钢水平均停留时间和活塞流停留时间分别延长了6%和24%,其混合流体积增大了30%;死区体积减小了28%, Vp/Vd之比增大了68%,并且没有短路流存在,夹杂物上浮率增加了17%,有效提高了钢水洁净度。4、湍流模型在大钢锭中间包内流场数值模拟中的适应性研究本文采用标准k-ε模型和Realizable k-ε模型模拟的圆形多炉连浇中间包内空况和加挡墙挡坝后的钢液流动状况,并结合物理模拟结果进行了验证,提出Realizable k-ε模型更适合深熔池、大流量的多炉连浇大钢锭中间包流场的模拟(论文的第三个创新点):①两种湍流模型都能用于圆形和椭圆形中间包内钢液流动状况模拟。但标准k-ε模型中粘度系数为常数,对中间包内漩涡模拟效果差;模拟圆形中间包时与物理模拟结果最大误差达50%,模拟椭圆形中间包时最大误差为56.3%。而在Realizable k-ε模型中粘度系数是一个变化量,能更准确地模拟中间包内钢液流动行为,模拟圆形中间包时与物理模拟结果最大误差减小到了9.6%,模拟椭圆形中间包时最大误差减小为16.5%。②对比Realizable k-ε模型与物理模拟结果的比较可知,与空况相比Realizable k-ε模型对加控流装置的模拟结果与物理模拟结果更接近,其中模拟圆形中间包空况时最大误差为50%,安装控流装置后的最大误差为9.6%;模拟椭圆形中间包空况时最大误差为56.3%,安装控流装置后的最大误差为16.5%。5、椭圆形中间包现场应用根据前文研究结果,将椭圆形中间包应用到实际生产过程中。现场试验结果表明,与圆形中间包生产的钢锭相比,采用椭圆形中间包后相同材质和体积的钢锭内氧含量得到了明显减少;还对两种形状中间包生产的钢锭所制造的百万千低压转子锻件进行了无损检测和氧含量分析,结果表明:①使用椭圆形中间包浇注后,中间包内全氧量减小了25%～33%。②锻件单个缺陷的当量直径从2.5mm减小到了1.6mm,并不存在密集性缺。③百万千瓦低压转子全氧量减小了30%。这说明采用椭圆形中间包大大提高了锻件的质量,更适合用于浇注超纯低压转子锻件。更多还原

【Abstract】 Generally, the weight of a heavy steel ingot is bigger than 400 tonne. Due to the limitation on the volume of smelting furnace, the multi-heat teeming is usually used as that in traditional continuous casting by flow rate 1.5 to 2.5 times. In the present study, the steel ingot is made for the 1000MW nuclear conventional island integral solid rotor forgings. It is one of the most important parts with the biggest weight of forging blank, which is the biggest of section size and the highest technical requirements in the world. Since the weight of rotor is 170 to 180 tonne, it requires the weight of ingot as large as 580 to 650 tonne. This kind of steel demands a super pure control in order to solve the problem of the temper brittleness. So, the multi-heat teeming of EAF—LF—VD—VCD, up-pour technology and vacuum casting are commonly used both at home and abroad. In the process of multi-heat teeming in heavy ingot, the tundish acts as an intermediate vessel which connecting the ladle and ingot mold, storing liquid steel to ensure the multi-heat teeming when ladle is changing. With the development of tundish metallurgical technology, the function of inclusions removal in a tundish of conventional continuous casting has turned into a key technology for the clean steel production of metallurgy industry. Large capacity tundish, high bath depth and flow control devices were used to enhance the residence time of liquid steel in a tundish, and to promote inclusions aggregation and floatation. In China, round tundish with large-capacity and deep bath is mainly used in the production of heavy ingot to meet the requirement of a high casting flow rate. However, the short residence time of liquid steel and the low inclusions floatation rate in the round tundish caused by the short distance from the zone of ladle injection to the outlet and large flow rate, weakened the inclusions removal. Therefore, it is quite significant to design a proper geometry size of tundish for the multi-heat teeming and the function of inclusions removal in heavy ingot casting processes, which will largely improve the cleanliness of low pressure rotor forging.In this paper, focused on the heavy steel ingot, physical modeling was performed on the research of the flow field pattern and inclusions removal efficiency in the different types of multi-heat teeming tundishs, the round tundishs with different bath depths (H) and the groove tundishs with different lengths (L). And a reasonable geometry size (L/H) of the multi-heat teeming with a large flow rate was obtained. A new ellipse tundish was designed by a comprehensive consideration of industrial field installation, walling.The trial results showed that the modified multi-heat teeming tundish of heavy ingot can effectively remove the nonmetal inclusions in liquid steel, improve the cleanliness of liquid steel and ensure the product quality of low pressure rotor. Study on the fluid flow in a multi-heat teeming tundish for the heavy steel ingot by using different turbulence models in numerical simulation.There are four main contents and results of the dissertation as follows:1、The evaluation of fluid flow in the round for multi-heat teeming tundishsThe flow condition of the molten steel and the inclusion floatation efficiency were evaluated in the round shape tundish for heavy steel ingot. The results showed that:①Without flow control devices, the residence time of molten steel was short, the dead volume was big and the inclusion floatation efficiency was low.②After installing the flow control devices, the mean residence time slightly increased,the inclusions has no time to float due to the short distance from the pouring point to the outlet of the tundish.Thus, the purpose of improving the molten steel cleanliness cannot be achieved.2、Study on the geometric dimension design in a multi-heat teeming tundish for heavy steel ingotEffect of fluid flow and inclusion motion on different bath depth and the ratio length and bath depth in a multi-heat teeming tundish for heavy steel ingot were firstly studied. The geometric dimension was afforded for a multi-heat teeming tundish of large flow rate. This is the first innovation in the dissertation.①Increasing the bath depth and tundish length was beneficial to the inclusions floatation. When the tundish volume was the same, prolonging the tundish length was more advantageous to inclusions floatation.②Considering the flow field patterns of liquid steel in the tundish and the inclusions floatation efficiency, the bath depth of the multi-heat teeming tundish for heavy steel ingot should be between 1500 mm and 2000 mm, and the L/H should be controlled between 2.0 to 2.7.3、Study on the flow control device in a multi-heat teeming tundish for heavy steel ingotConsidering the tundish hoisting, brickwork and placing, a new elliptical tundish of deep bath and large flow rate was designed. And the flow control device was optimized. This is the second innovation in the dissertation. ①According to the study of 18 design schemes of flow control devices and the residence time of molten steel, flow pattern and inclusion floatation efficiency, the optimal installation position of the flow control device is that, the distance from the weir to the bottom of tundish is 300mm, the height of the dam is 400mm,,the distance from pouring point to the weir is 1500 mm, and the distance from the weir to the dam is 490 mm.②Based on the research above and the site actual conditions, a new ellipse tundish with the ratio of length and bath depth is 2.0 was designed, and an optimization analysis was performed on the position of the weir exit, the results indicate that the scheme that the outlet locates on the axis of symmetry is the optimal one.③The comparison of simulation results between the two shapes of tundishs with the same capacity showed that, in the elliptical shape tundish with the flow control devices, the response time was 2.5 times longer than that in the round shape tundish, the mean residence time and the residence time of the plug increased by 6% and 24% respectively, the mixed volume increased by 30%, the dead volume decreased by 28%, the ratio of the plug volume to the dead volume increased by 68%, and no short -circuiting flow, the inclusion flotation efficiency increased 17%.4、Applicability of turbulence models in numerical simulation of flow field in a tundish for heavy steel ingotThe flow field of molten steel in a multi-heat teeming tundish for heavy steel ingot was simulated by using the standard k-εmodel and Realizable k-εmodel. And the accuracy of simulation was validated by the comparison between the results of physical modeling and numerical simulation. For the flow field simulation of the tundish with deep molten pool, large flow rate and vortex, the adoption of the Realizable k-εModel was more suitable than the Standard k-εModel. This is the three innovation in the dissertation The results indicate that:①Both two turbulence models can be applied in the simulation of flow conditions of molten steel inside the round and elliptical shape tundishs. In the Standard k-εModel, the viscosity coefficient was a constant, and it has a poor effect of vortex simulation inside the tundish, the maximum deviation of the round tundish reached 50% while the maximum deviation of the elliptical tundish reached 56.3% compared to the results of physical modeling. In the Realizable k-εModel, the viscosity coefficient was a variable, the flowing behavior of molten steel inside the tundish can be simulated more accurately, the maximum deviation of the round tundish was reduced to 9.6%, and the maximum deviation of the elliptical tundish was reduced to 16.5%.②It can be known from the comparison between the Realizable k-εModel and physical modeling that, as for the Realizable k-εModel, the simulation results of molten steel flowing condition inside the two tundishs with flow control device was even closer to that of physical modeling than that without flow control device. The maximum deviation was 50% in the round tundish without flow control device, and 9.6% in the round tundish with flow control device. The maximum deviation was 56.3% in the elliptical tundish without flow control device, and 16.5% in the elliptical tundishs with flow control device.5、The trial application of the elliptical tundishAccording to the research results mentioned above, the elliptical tundish was used in trial application. The trial results indicated that after using the elliptical tundish,①The content of the total oxygen in the elliptical was decreased by 25%～33%.②The equivalent diameter of single defect decreased from 2.5 mm to 1.6 mm, and no intensity defect observed.③The content of the total oxygen in the rotor was 21 ppm, which was decreased by 30%.Therefore, the quality of rotor forgings was improved greatly by using the elliptical shape tundishs.更多还原