摘要
连铸生产中,常常由于不合理的浇注工艺和二冷配水制度导致铸坯内部产生质量缺陷,特别是以中间裂纹和中心裂纹为代表的铸坯裂纹缺陷占总质量缺陷的50%以上。为了消除铸坯内部裂纹缺陷,必须对内部裂纹产生的机理进行研究,并对其凝固过程进行控制。在实际生产过程中,由于受设备、流程和突发事情等因素的影响,过热度和拉速等工艺参数经常发生变化,使得基于稳定条件下建立的凝固传热模型在应用过程中往往满足不了生产高品质钢的要求;另一方面,为了消除铸坯内部裂纹等缺陷,必须对裂纹源和裂纹扩展的机理进行分析,从而为提出预防裂纹产生的措施,并为优化二冷水量改善铸坯质量提供理论依据。因此,热弹塑性应力场模型和实时凝固传热模型的研究对铸坯内部质量的提高具有实际意义。
本文以方坯凝固过程中出现的内部裂纹为研究对象,以减少乃至消除内部裂纹缺陷为主要目标。为了消除铸坯内部裂纹,就必须对产生内部裂纹的原因进行研究,这就要求对铸坯截面热应力分布及影响因素进行分析,为此必须建立铸坯热弹塑性应力场模型。由于传热模型是热应力场模型的基础,因此对传热模型也进行了深入的研究,并将应力场模型和传热模型在现场进行了应用研究。主要研究内容与创新工作如下:
(1)二维热弹塑性应力场模型的建立及内部裂纹产生的机理研究
针对以中间裂纹和中心裂纹为表征的内部裂纹是各现场存在的主要内部缺陷,建立二维热弹塑性应力场模型对裂纹产生的机理进行了研究。
通过应力场模型对铸坯应力分布和裂纹产生的原因进行了分析,提出了裂纹是由于铸坯表面温度回温过高导致铸坯凝固前沿过大的拉应力和凝固前沿凝固速率的不均造成的,并指出裂纹源是发生在补缩边界与固相边界之内,向着固相扩展。而过热度偏高和拉速波动频繁加剧了柱状晶生长速度不均从而加剧内部裂纹产生和扩展。通过钢种的性能和模型的分析,提出在二冷区最好使铸坯表面温度减少波动,减少各段回温的二冷优化目标。
(2)方坯实时凝固传热模型建立及动态特性分析
针对连铸二次冷却是影响铸坯内部质量的关键环节,因此必须对二次冷却凝固过程加以控制,也就是对铸坯温度场加以控制。由于连铸二冷区高温、水汽及铸坯表面的水膜、氧化铁皮等的影响,难以准确地测量铸坯表面温度,特别是铸坯内部温度难以测量,因此在分析连铸生产过程中浇注工艺条件频繁变化特性的基础上,建立了方坯实时凝固传热模型。
对建立的实时凝固传热模型,利用铸坯表面温度测量和射钉测厚对其进行了校正和验证。在此基础上,分析了网格划分对模型可靠性的影响,并在实际浇注过程中,通过变化浇注条件对模型的动态特性进行了分析,通过模型的在线运行和铸坯表面温度的测量对比,模型计算和实测值最大偏差在10℃范围内,为实时传热模型的在线应用和热应力场模型奠定了基础。
(3)连铸热应力场模型和实时传热模型的应用研究
对某钢厂Q235普碳钢和HRB400低合金钢进行了应用研究。从现场低倍数据和模型计算分析指出,铸坯内部裂纹主要是因为不合理的二冷配水导致二冷区的高回温在凝固前沿产生较大的拉应力;同时由于过热度偏高和拉速波动频繁加剧了柱状晶生长速度不均从而加剧内部裂纹产生和扩展。通过优化二冷各段水量,并对水量优化前后铸坯截面应力分布以及现场试验相结合,确定拉速和过热度的配水系数,同时把有效拉速和过热度引入到二冷动态配水控制系统中消除了因拉速突变引起的温度大幅波动。
该系统应用现场后,对六个月的铸坯低倍抽样检验,结果表明:Q235钢中间裂纹基本消除,中心裂纹的级别由原来大于1.0级占10.3%降低到4.7%,铸坯等轴晶区直径由优化前的φ28～32mm提高到优化后的φ35～42mm,其他缺陷的级别也在允许范围之内且比优化之前有所降低;HRB400钢中心裂纹基本消除,中间裂纹由原来大于1.0级占12.7%降低到7.4%,铸坯质量得到明显提高。
In the continuous casting process, the internal defects which can be formed in cast material are due to inappropriate casting operation and improper secondary cooling water distribution, especially the defects of cracks represented by the midway crack and centreline crack which cover over 50% of total quality problems. In order to avoid internal cracks and other defects, it is necessary to make studies on the formation mechanism of internal cracks and control the process of solidification. The casting conditions such as pouring temperature and casting speed fluctuate frequently caused by change of equipments, process and rhythm etc, so that the heat transfer model which is built based on the static conditions cannot meet the requirements of making high-quality billet. On the other hand, the formation mechanism of internal cracks should be studied in order to avoid internal cracks, which will offer theoretical grounds for the preventing of the occurrence of internal cracks and the optimization of secondary cooling water flow to improve the strand quality. Thus, it is of practical significance to study the real-time heat transfer model and thermal stress model to improve the billet quality.
With the internal crack of solidification process as the research object, and eliminating the internal crack as the final target, this dissertation was formed. In order to avoid the internal crack, the reason of internal crack generation was studied. The stress model was established to analyse the stress distribution and influence factors. The heat transfer model was systemically studied because it is the basis of the thermal stress model. The heat transfer model and thermal stress model were applied to the industrial caster. The main content and innovations are as follows:
(1) Establishing two-dimensional thermal elasto-plastic stress model and researching the formation mechanism of internal cracks
The internal cracks such as midway crack and centreline crack are the main internal defect. A two-dimensional thermal elasto-plastic stress model was established to analyze the formation mechanism of internal cracks.
The billet thermal stress distribution and the formation mechanism of internal cracks were analyzed by the model. It was presented that the internal cracks were caused by excessive surface reheating and solidification velocity disproportionation. The cracks occur between shrinkage boundary and solidoid boundary and spread to the solidoid boundary. Meanwhile, the high superheat and frequent casting speed fluctuation aggravated the expansion of cracks. By means of analysis of mechanical properties of steel and model, the surface temperature fluctuation should be reduced and the reheating should be decresased in each section of secondary cooling zones.
(2) Establishing the real-time heat transfer and solidification model and analysing the dynamic performance of billet temperature field
Secondary cooling control is one of the key techniques of influencing the quality of billet. The solidification process in the secondary cooling should be controlled, which is equal to control the temperature field of billet.Owing to the influence of high-temperature, water vapor, water film and scales on the strand surface, etc in the secondary cooling zone, it is difficult to measure the billet surface temperature accurately, let alone the internal temperature. According to the frequent changes of casting processes in the continuous casting production, a two-dimensional real-time heat transfer and solidification model was established.
In order to correct and test the real-time heat transfer and solidification model, the surface temperature and shell thicknesses were measured by the thermal imager and nail shooting respectively. The the influence of the grid division to the reliability of the model was analyzed. The dynamic performance and response to operation conditions were tested by changing casting conditions. The maximum deviation between the calculated surface temperature and measued ones were below 10°C at measured point by changing operation conditions, the foundation which the real-time model was on-line application and the thermal stress model was established.
(3) Application research of thermal stress model and real-time heat transfer model for the control system of billet continuous casting
The application research of steel grade Q235 and HRB400 were performed in a steel plant. On the basis of model calculation and the results of experimentation, the improper secondary cooling water distribution which leads to high reheating between sections in secondary cooling zone was the major cause of internal cracks. Meanwhile, the high superheat and frequent casting speed fluctuation aggravated the expansion of cracks. The secondary cooling water was optimized, and the stress distribution was calculated before and after the optimization of secondary cooling water. The water distribution coefficients of casting speed and superheat were obtained. The fluctuation of surface temperature was reduced greatly during casting speed abrupt changing beacuse the superheat and effective casting speed were imported in the dynamic secondary cooling control system
After the system was applied to the field, the analysis of macrostructure random specimens of six months showed that the Grade Q235 centreline cracks were basically eliminated and the midway cracks above 1.0 degree has been dropped from 10.3%o 4.7%. The equiaxed grains diameter has risen up from 28～32mm to 35～42mm, and other defect grades have been reduced; the Grade HRB400 centreline cracks were basically eliminated and the midway cracks above 1.0 degree has dropped from 12.7%to 7.4%. The quality of products has been improved obviously
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