Temperature and thermal stresses in work rolls during the hot rolling of strip

In this paper the solution of the unsteady, three-dimensional heat transfer in work rolls was derived by using Bessel functions and the δ-function and expressed by an infinite series. In this manner other problems of heat transfer in solid cylinders and hollow cylinders can also be calculated. The temperature profile of the work roll due to the distribution of the temperature in the axial direction can cause unregular pressure distribution between work roll and backup roll and can influence the quality of strip. Therefore, the thermal crowns must be considered at various moments during rolling. The thermal crown can be determined by using the FE-Method or others. The reason for the fire-cracking in the roll surface can be determined as the thermal shock load in the contact zone, in which the sharp compressive stresses lead to local plastic deformation. The local plastic deformations are followed by residual deformations. Therefore, residual tensile stresses occur in the cooling zone. On each revolution, the surface undergoes plastic strain in compression and in tension. The result is thermal fatigue. The fine network of cracks in the roll surface can result in a sharp stress concentration which is dangerous for the rolls loaded with a bending moment. The maximum tensile stresses due to the temperature distribution occur in the roll core, which usually do not lead to damages of the rolls.     

Finite element modelling of strip curvature during hot rolling

Abstract

Finite element techniques have been used to decouple some of the major causes of strip curvature in the roughing stages of a hot strip mill. A plane strain elastic-plastic model is used to predict the direction and severity of strip curvature caused by asymmetrical factors at each pass of the rougher. Asymmetrical factors considered include work roll speed mismatch, temperature differentials across the thickness of the workpiece, pass height, and frictional differences between upper and lower work rolls. A full factorial experiment was then designed and performed, using finite element predictions, to identify which asymmetrical factors are most influential to strip curvature, and to determine the interactions, if any, between factors. The results show that three of the asymmetrical factors are significant to strip curvature, but their influence depends on the rolling pass. This study will allow the rolling operator to identify which asymmetrical factor may be causing strip curvature and, thus, provide a suitable course of action.     

Thermal profile of work roll during strip hot rolling

The thermal expansion of work rolls during hot strip rolling was modeled by a two-dimensional difference method.The calculation model was validated by comparison between the predictions to the measured results.With that model,the effects of rolling process parameters such as the rolling plan,mill pacing,coolant flow rate and stepped cooling on the roll thermal profile were investigated.The result shows that stepped cooling of work rolls can be used to control complicated flatness defects such as quarter buckles during hot strip rolling.     

Modelling recalescence after stock reduction during hot strip rolling

Abstract

The relationships between stock recalescence time/distance and process variables, such as exit thickness, reduction, rolling speed, work roll diameter and slab–roll heat transfer coefficient during hot strip rolling, have been established. The behaviour of the temperature gradient was analysed and used to estimate the slab and transfer bar mean temperatures from measured surface temperature in three hot strip mills located in northeast México. It was found that the recalescence critical variables, in order of importance in minimising temperature measurement variation, are slab–roll heat transfer coefficient, reduction, exit thickness and roll speed.     

Modelling water cooling of steel strip during hot rolling

Several tests of water cooling of steel samples are presented in the paper. The water spray and quenching conditions are considered. The temperature variations during the tests are monitored using the thermocouples embedded in the side of the samples. The experimental time – temperature profiles are compared with the results of calculations based on the finite element model. Comparison of the theoretical and experimental results allowed to suggest heat transfer coefficients of 10000 W/m²K for water in quenching, 6000 W/m²K for the top surface during water spray cooling and 4000 W/m²K for the bottom surface during water spray cooling.     

Defect prediction in hot strip rolling

Abstract

In steel manufacturing, enormous amounts of data are gathered from many kinds of processes and it is difficult to distinguish useful knowledge from the resulting extensive databases. The recorded databases in the rolling process contain hundreds of features, and new methods are needed to reveal the novel and useful information. In the present study, the data from a hot strip rolling process were analysed in order to identify the rolling conditions in which some common defects, such as too thin or too narrow strip, do and do not occur. At the beginning, the dataset was reduced from its original size of over 200 features to 17% by using basic statistical analysis and linear correlation. After this, self-organising maps, parallel coordinates display, and k means clustering were used to find out the conditional probabilities of the common defect types. As a result, the method presented here revealed the rolling conditions for the four common defect types, i.e. coiling temperature close to the limits, too thin strip, too narrow strip, and torn tail end. Most important, the knowledge gained can be used to reduce the number of these defects.     

热轧带钢层流冷却控制策略研究

在带钢层流冷却过程中,为实现高品质的冷却效果,针对不同的参数需要采取不同的控制策略。控制的参数主要有:带钢运行速度、冷却区开启阀门数、冷却阀门喷水量、喷水模式等。在实际的生产过程中往往采用设定固定的带钢运行速度值,然后通过控制喷水量的大小来进行冷却控制。针对国内某热轧厂层流冷却实时生产数据进行分析,分析了"速厚积"对冷却效果的影响规律,制定出合适冷却策略,并验证其有效性。     

Influence of process variables on development of camber during hot rolling of strip steel

Abstract

Experimental design techniques have been used to study the effects of variations in horizontal scale breaker process parameters in relation to camber generation during the early reduction stages in a hot strip mill. A fractional factorial experiment was designed and performed to identify which process variables are most influential to camber generation. The parameters which emerge from this analysis as being significant are: slab wedge, temperature differentials across the strip width, the deviation of the slab from the mill centreline, and mill tilt. A full factorial experiment was then designed and performed to further investigate these significant process parameters to determine where to set influential and controllable variables so that camber is minimised. The results show that none of the process parameters is significant to camber generation when acting alone; however, they are all significant when combined with one or two other process parameters. This study has enabled a predictive equation to be developed which is able to predict camber to within ±4 mm. This equation may be used in conjunction with mill tilting strategies to reduce camber in the early reduction stages of a hot strip mill.     

Improved hot and cold rolling of steel strip

The potential for decreasing the end thickness of hot-rolled strip in traditional broad-strip rolling mills is demonstrated. The change in thickness depends on the temperature difference over the length of the hot-rolled intermediate strip. Additional reduction of the rear end of hot-rolled intermediate strip is worthwhile for other reasons. The rolling of thinner rear ends of hot-rolled strip in continuous cold-rolling mills decreases the loss of metal in end trimming.     

提高热连轧轧制节奏生产实践

介绍新钢热连轧厂生产产能的制约瓶颈,通过对生产节奏、运输节奏的合理控制,降低粗轧区温降,最大化地满足生产,发挥产能潜力,顺利完成生产目标.     

热宽带钢无头轧制技术进展及趋势

 首先回顾分析了热宽带钢无头轧制技术的发展历程及趋势,针对不同工艺流程生产热宽带钢的冶金工艺特征进行了比较;其次,从无头轧制板带产品的尺寸精度、组织性能均一性、稳定批量生产超薄规格板带、成材率及工序能耗以及钢材品种和应用等方面,总结分析了中国热宽带钢无头轧制技术开发及应用进展情况;同时,结合实例具体讨论了无头连铸连轧（ESP）生产低碳/微碳钢工艺及组织性能分析与控制方法;最后,结合中国热轧板带的产线、产能、产品等实际情况提出了发展热宽带钢无头轧制技术的建议。     

热轧带钢的热卷箱控制

介绍了热卷箱的原理,对于卷径计算数学模型很先进,速度匹配处理技术简单可行,总体控制安全可靠,对于轧制控制的设计应用有着很大参考价值。     

热轧带钢组织性能预报模型及应用

 基于物理冶金理论,研究了热轧带钢过程中的奥氏体晶粒长大模型、奥氏体再结晶模型、奥氏体相变模型以及力学性能模型。奥氏体再结晶模型中,通过研究位错密度的变化来描述由于再结晶不完全造成的变形抗力的变化。奥氏体相变模型中,通过碳扩散理论描述了奥氏体-铁素体相界面随冷却过程的变化规律。基于热轧带钢过程中的冶金物理模型,开发热轧带钢组织性能预报系统。系统包括4个模块,分别用于计算板坯在加热炉、粗轧精轧、层流冷却和卷取完成各阶段的组织和力学性能参数,生产工艺是该系统的重要输入参数。利用该系统对某钢厂实际生产过程的组织性能进行预报,预报的力学性能和现场实测值有较好的一致性。     

基于PCA-DBN的热连轧数据特征提取

在热连轧生产过程中会产生大量的数据,使用这些海量数据进行诊断和预报非常困难,运算时间较长,甚至会导致"维度灾难"。因此,需要对特征进行提取,去除冗余和不相关的特征。提出基于主成分分析-深度置信网络(principal component analysis-deep belief network,简称PCA-DBN)的热连轧数据特征提取,采用主成分分析(principal component analysis,简称PCA)对数据集进行特征提取,并将选择出的特征按照贡献率可视化,直观显示提取的特征。二次特征提取采用深度置信网络(deep belief network,简称DBN)对已提取数据集进行二次特征提取。将最终得到的数据集作为粒子群优化的最小二乘支持向量机(particle swarm optimization-least squares support vector machine,简称PSO-LSSVM)对精轧轧制力进行预报的数据集,以检验特征提取的合理性。试验显示,预报相对误差要小于机理模型预报相对误差,说明提取特征比较合理。     

基于行处理算法的带钢热连轧负荷分配

负荷分配是带钢热连轧精轧模型设定的基本问题,传统 Newton-Raphson算法存在初始值要求严格、易发散等计算性能较差的缺点。针对其在求解非线性方程组方面的不足,基于行处理法提出了一套适用于热连轧机组快速厚度分配的计算模型。该方法以中间机架出口厚度为变量,在采用高精度、非简化轧制模型的基础上,计算速度快,弥补了传统方法的不足。同时为了加快收敛速度,根据经验统计提出了一种新的初始值的确定方法。并利用C＋＋根据得出的算法编制了计算程序进行分析计算,在采用实测数据基础上的验证结果表明该算法具有运算速度快、收敛性好等优点。     

Effect of temperature sequence during hot strip rolling on structure and properties of mild steels

Reduced temperatures can be applied for rolling of mild steels in wide hot strip mills. This has been investigated by laboratory and production trials with unalloyed and microalloyed deep drawing grades. A reduced slab reheating temperature leads to a grain refinement prior to rolling and to incomplete dissolution of precipitates. Decreased rolling temperature results in partial deformation in the ferrite region. The flow curves of mild steels show a relative minimum below γ/α transformation temperature, thus only low deformation forces are necessary. Ferrite rolling can develop coarse or partly recrystallized microstructures, depending on amount of strain and on coiling temperature. The yield strength, as a measure of cold formability, can be lowered compared to conventional rolling in the austenite region.     

Precision online model for prediction of strip temperature in hot strip rolling

Abstract

Investigated via a series of finite element (FE) process simulation is the effect of diverse process variables on some selected non-dimensional parameters characterising the thermomechanical behaviour of the roll and strip in hot strip rolling. Then, on the basis of these parameters, online models are derived for precise prediction of the temperature changes occurring in the bite zone as well as in the interstand zone in a finishing mill. The prediction accuracy of the proposed models is examined through comparison with predictions from a FE process model.     

热轧带钢轧制过程的稳定性

在分析热轧带钢时产生带钢运行故障原因的基础上，对带钢粗轧和精轧等过程提出了相应的控制措施．提高了带钢轧制过程的稳定性，保证了带钢产品的质量．