Advances in the Optimization of Thin Strip Cast Austenitic 304 Stainless Steel

This study is about the latest advances in the optimization of the microstructure and properties of thin strip cast austenitic stainless steel (AISI 304, 1.4301). Concerning the processing steps the relevance of different thin strip casting parameters, in‐line forming operations, and heat treatments for optimizing microstructure and properties have been studied. The microstructures obtained from the different processing strategies were analysed with respect to phase and grain structures including the grain boundary character distributions via EBSD microtexture measurements, the evolution of deformation‐induced martensite, the relationship between delta ferrite and martensite formation in austenite, and the texture evolution during in‐line deformation. It is observed that different process parameters lead to markedly different microstructures and profound differences in strip homogeneity. It is demonstrated that the properties of strip cast and in‐line hot rolled austenitic stainless steels are competitive to those obtained by conventional continuous casting and hot rolling. This means that the thin strip casting technique is not only competitive to conventional routes with respect to the properties of the material but also represents the most environmentally friendly, flexible, energy‐saving, and modern industrial technique to produce stainless steel strips.     

Flow,Thermal and Microstructural Analysis during Cast Rolling of Steel in In‐line Strip Production Process

A mathematical model was developed to describe the in‐line hot rolling deformation and recrystallization behaviour of austenite after the solidification on a thin slab casting plant. The most significant features of the cast rolling process were taken into consideration: through‐thickness thermal gradients, inhomogeneous stress and strain, temperature discontinuity between the strip and the rolls. A HSLA (High Strength Low Alloy) steel has been chosen to perform the experiments of cast rolling. The characteristic constants ruling the microstructural evolution of that steel were computed and integrated into the computational module which manages the structural stress‐strain and strain rate computation. The developed approach is based on the Navier‐Stokes’ equations which were used to compute the speed field in the strip during the deformation. Then a model providing a proper constitutive equation was structured on the basis of the Yada's model based on evolution of the dislocation populations. The use of the Navier‐Stokes’ formalism allows to reach the resolution of the structural problem from the data measured easily during the industrial practice (i.e. speed of the rolled product at the entry and at the exit of a stand, the temperature of the rolled material). The validation of this computational approach was obtained by a comparison between the prior austenite grain size of the strip in different positions of the hot rolling process, as well as by a comparison between the computed deformation power and the measured one provided by the engines moving the rolls.     

CSP线高强度细晶热轧板的混晶和变形拉长晶粒的成因

对CSP线生产的高强度细晶热轧板的混晶和拉长晶粒的成因进行了分析,用有限元分析法模拟了热轧带钢的变形区的剪切应变场和温度场,用Gleeble实际模拟轧制工艺和组织变化。结果表明,CSP线高强度细晶热轧板的混晶和拉长晶粒的形成与钢板轧制过程中的钢板表层的变形场及温度场有关,也与先析出铁素体的形成后再进行轧制变形的过程有关;采用奥氏体深过冷轧制,既保证得到细晶粒又避免产生混晶和被变形拉长的晶粒。新的CSP轧制工艺,成功地生产了高强度高成形性细晶粒C-Mn热轧板。     

Modelling the Temperature Distribution along the Length of Strip during Hot Rolling Process

Hot strip rolling process includes four main stages, which are reheating process, roughing and finishing process, laminar‐cooling process, and coiling process respectively. Temperature is the most sensitive parameter and has direct effect on the microstructural evolution and further the mechanical properties, and the accurate control of temperature guarantees the quality of products and homogeneity of properties along the strip length. However, for the conventional hot strip rolling process, thermal history along the strip length is very complex, the related temperature variation concerns air cooling, water cooling, heat transmission by roll contact, heat generation by deformation and friction. Based on the actual hot strip mill, the thermal models are established in this paper to simulate the temperature distribution along the whole strip length from the reheating furnace exit to the down coiler. Different interface heat transmission coefficients are selected for the scale breaking and spray water‐cooling process, and a self‐learning algorithm is thus employed to improve the calculation accuracy. This model is characterized as simple and fast, and convenient for on‐line/off‐line prediction of temperature. Finally the simulated results are verified by the on‐line temperature detection at typical points such as roughing exit (RT2), finishing exit (FT7) and coiling position (CT).     

CSP产品范围扩展及其工艺技术前景

在过去的１０年里，ＣＳＰ工艺生产的产品范围不断扩展。现在它已能生产高强度结构钢、弹簧钢、工具钢、可热处理钢、耐磨钢、硅钢和不锈钢。ＣＳＰ工艺浇涛区域特有的冶金条件及其直接装炉、轧制、冷却的工艺方法导致热轧带钢晶粒细、强度高、塑性好。ＣＳＰ生产流程中尚可汇集形变热处理工艺，因此ＣＳＰ工艺技术将技术将拓展出更广阔的产品范围。     

薄板坯连铸连轧生产线工艺技术及应用

简单介绍了薄板坯连铸连轧技术特点、产品范围及工艺优劣势;还阐述了与传统热轧工艺相比,该薄板坯连铸连轧生产线生产(超)薄带钢、(超)低碳钢的优势以及相关产品质量保证的特点.     

Prediction of Rolling Load, Recrystallization Kinetics, and Microstructure During Hot Strip Rolling

 The recrystallization kinetics and grain size models were developed for the C Mn and niobium containing steels to describe the metallurgical phenomenon such as softening, grain growth, and strain accumulation. Based on the recrystallization kinetics equations, the mean flow stress and the rolling load of each pass were predicted and the optimum rolling schedule was proposed for hot strip rolling. The austenite grain refinement is associated with the addition of niobium, the decrease of starting temperature of finish rolling, and the reduction of finished thickness. The mean flow stress curve with a continuous rising characteristic can be usually observed in the finish rolling of niobium containing steel, which is formed as a result of the heavy incomplete softening and strain accumulation. The predicted rolling loads are in good agreement with the measured ones.     

热连轧过程中组织性能预报系统的应用

简要回顾和总结了热连轧过程中组织性能预报系统研究和开发的历史。重点论述了组织性能预报系统对可控热连轧生产的重要意义 ,指明了组织性能预报系统的开发和应用是新世纪钢铁工业发展的主流趋势之一。在热连轧的过程模拟中 ,层流冷却是决定产品最终力学性能的最关键过程。也指出当前对热连轧组织性能预报的研究已从离线预报向离线预报和在线预测相结合方向发展     

传统热连轧生产技术的发展

介绍了传统热连轧新技术,包括无头轧制技术、连铸板坯热送热装和直接轧制技术、铁素体轧制技术、热轧工艺润滑技术、自动化控制技术的特点.传统热连轧机分为粗轧和精轧两部分,使用的板坯厚度一般大于180 mm,最小产品厚度为1.2mm.近年来传统热连轧新技术、新装备的出现推动了炼钢一连铸一轧钢生产的一体化,加速了钢铁生产向连续化、低成本和高质量方向发展,扩大了传统热带轧机的轧制范围,可批量生产0.8mm的超薄带钢.先进的传统热连轧生产技术,是传统热连轧机组改造和发展关键.     

Rolling and Coiling Technology for the Production of Thin Strip on a Casting Rolling Line

One aim of the casting rolling plant is the production of hot strip with low thicknesses [1]. Results form an ECSC project [2], dealing with the introduction and optimisation of the new rolling and coiling technology for thin strip production at ThyssenKrupp Stahl are presented. Several series of hot rolling with different mild steels were performed. The strips were produced by two‐step (ferritic) rolling, as well as by austenitic rolling. In both cases direct application and hot‐dip galvanising of the strips were tested. The thinnest strip thickness achievable was 0.8 mm. Industrial trials were also carried out successfully with HSLA and dual phase steels.     

Prediction of Microstructure and Mechanical Properties of Hot Rolled Steel Strip: Part I ‐ Description of Models

A metallurgical through‐process model is presented which describes the microstructural evolution and predicts the final mechanical properties of low carbon steel during hot strip rolling. Process models concern the thermal and deformation phenomena, which take into account the strain, strain rate and temperature distribution along the length of the strip. And the metallurgical models cover five modules, which are (i) austenitization of cast slab in reheating furnace, (ii) recrystallization of austenite in hot rolling, (iii) phase transformation of austenite‐ferrite in laminar cooling on the run‐out‐table, (iv) grain growth after coiling, and (v) final structure‐mechanical properties of products. Temperature is the main parameter and has dominant influence on the microstrutural evolution and the mechanical properties. The related temperature variation in hot strip rolling concerns air cooling, scaling, water cooling, heat transmission by roll contact, heat generation by deformation and friction. These complex factors are incorporated into the thermal models to simulate the temperature distribution along the length of the strip from the reheating furnace exit to the down‐coiler. A self‐learning algorithm is employed to improve the calculation accuracy and the computational temperatures are compared with the measured ones at typical locations. In the structure‐property relationships, two key process parameters (e.g., finishing exit temperature (FT7) and coiling temperature (CT)) are introduced in the model to consider the influence of morphology of microstructure on mechanical properties.     

620mm热连轧窄带钢生产线快节奏轧制的全线跟踪

热连轧全线跟踪对其生产过程中的全自动轧钢具有重要的意义。结合国丰620mm热连轧生产线现场实际设备和仪表配置,针对窄带钢热连轧生产线的快节奏特点,通过对轧区进行细致划分,提出轧区内多队列跟踪概念,进而建立了新的全线跟踪方法。该方法结合钢坯出炉系统和过程自动化模型设定系统,实现了全线跟踪,并已成功应用于620mm热连轧生产线中。现场实际生产表明,这里的全线跟踪方法保证了数据传递的准确性,有效地提高了轧制节奏和生产效率,具有良好的实际应用价值。     

板带热连轧过程力能参数、温度和组织变化的预报

采用0rowan公式计算轧制过程中轧件应力应变、有限差分法计算温度变化，建立了板带热连轧生产过程温度变化、微观组织演变综合预报模型。用此模型对鞍钢现场轧制工艺进行了温度变化、奥氏体晶粒大小以及相变过程进行了解析计算，其预报值与现场实测结果吻合较好。     

Physical Simulation of the Hot Direct Rolling of Steel

Thin slab casting and direct rolling technologies became important for hot strip production to increase the productivity, to lower the investment costs and to increase the energy efficiency. The main objective of our investigation is to gain a better understanding of the microstructural evolution starting from solidification through to the final hot rolled sheet. Small ingots were cast and direct rolled and reheated and rolled, using a thermomechanical testing machine. The differences in the recrystallization kinetics during both production routes were measured using double hit hot compression tests. In addition, multipass deformation trials were performed to reveal differences in the resulting microstructure and the grain size homogeneity. It was found, that the coarse initial microstructure before rolling become negligible after three rolling passes. This means that despite the simple and economical direct rolling similar mechanical strip properties can be obtained in comparison to conventional strip production routes.     

Theoretical Analysis of “Slow Hot Rolling” in Direct Linkage with Thin Slab and Strip Continuous Casting

The present investigation was undertaken in context with the development of the single‐belt strip casting process. In this new process the casting is directly connected to hot rolling which must be carried out at lower speed than in conventional hot rolling to match the casting rate. We have used an integrated thermal/mechanical finite element model to theoretically study the effect of rolling speed on rolling parameters. Since the model is general and the question of “slow hot rolling” is of general interest, the computations were carried out for a wide range of volumetric flow rate extending from that in thin slab casting to that in conventional finishing rolling. The results are given on the influence of rolling speed on roll force, torque, power, temperature change of stock and roll, and roll life. The most important effect of slow hot rolling seems to be the increased heat up of the rolls. This causes an increase of plastic strain in the near surface region of the rolls that will intensify the formation of fatigue cracks.     

奥钢联的热带钢连铸连轧的技术思想

VAI公司的热带钢连铸连轧基于不同考虑分别主张采用中厚板坯或者薄板坯，文中介绍了VAI的技术思想的特点和热带钢连铸连轧生产线配置方案。     

Effects of TMCP Schedule on Precipitation,Microstructure and Properties of Ti-microalloyed High Strength Steel

Using the similar compositions of the Ti-microalloyed high-strength steels produced by the thin-slab casting process of compact strip production(CSP),four thermo-mechanical control processes(TMCP)after the simulated thickslab casting,i.e.the two hot rolling routes and the two cooling processes,were designed,aiming at achieving the same mechanical properties as the thin strip products.The final microstructures after the four TMCP processes were examined by optical microscope(OM),scanning electron microscope(SEM)and transmission electron microscope(TEM).The tensile properties and Charpy impact energy were measured correspondingly.Strain-induced TiC precipitation was found in the two-stage rolling route with the finish rolling temperature at low levels,leading to grain refinement due to the pinning effect during austenite recrystallization.Precipitation hardening in ferrite was observed when a period of isothermal holding was applied after hot rolling.It could be concluded that both finish rolling temperature and the subsequent isothermal holding temperature were crucial for the achieved strength level due to the combined effect of grain refinement and precipitation hardening.At the same time,it was found that the isothermal holding led to poor impact toughness because of remarkable precipitation hardening.Therefore,it was suggested that the precipitation kinetics of titanium carbides in both austenite and ferrite should be investigated in future.     

薄板坯连铸连轧改造为无头轧制产线的分析及展望

热轧带钢无头轧制技术诞生于常规热轧宽带钢轧机,之后有一批薄板坯连铸连轧产线采用半无头轧制技术,但由于轧制机理及其他方面的局限性,不能认为是一个理想的工艺产线.近几年国外两套以原有ISP(在线带钢工艺)型式的薄板坯连铸连轧产线为原型进行了无头轧制改造,特别是意大利阿维迪无头轧制产线的构成及特点,揭示了无头轧制技术发展的某些特征.国内薄板坯连铸连轧最多的是CSP产线,以国内某套一代半的CSP机组迁建项目为例,对改造成无头轧制产线进行技术分析,提出改造的设想方案及改造的效果预期.未来热轧带钢无头轧制技术的发展前景取决于在技术性、经济性及适用性方面的竞争力.     

热轧带钢尾部“伪板形不良”问题研究

 热轧带钢头部与尾部处于非稳态轧制区,容易出现各种尺寸形状质量缺陷。在轧制软薄料时,带钢尾部经常出现一种外观与中间浪类似的“伪板形不良”,但其产生机理与板形不良完全不同,因此常规的板形控制手段无法解决该问题。为此,首先对该问题的产生机理进行了分析,发现这一现象的原因是抛钢一瞬间张力消失引起的突然收缩。根据上述机理,对抛钢前F8机架与卷取机的速度匹配关系进行了优化设定,使带钢尾部的“伪板形不良”问题得到极大改善,还避免了带钢表面与辊道之间的擦伤与划伤。