The history of the progress of technologies and theories of crown and shape control for sheet rolling have been reviewed. The western technologies had been introduced to Japan and assimilated skillfully. After that original technologies and theories of sheet rolling had been developed by industry-university cooperation. The many types of crown and shape control technology have been spreading worldwide from Japan.
This report discusses the development of promising system control technologies which have been mainly applied to thickness control after late 70 mainly. In the late 1970 s, modern control theory and optimization technique were introduced into the field of thickness control in earnest and they greatly contributed to the quantitative expansion of the domestic steel industry. As the examples, the AGC gain by optimum regulator theory and thickness and inter stands tension control system by multivariable control technique are shown. After the 90 s, the high added value and function improvements of the control system become more and more important for steel industry including the thickness control and they were indispensable for manufacturing of the high-strength steel. Here, the mill balance control for the tandem cold mill and the highly precise rolling load model by Genetic Programming are explained. As future problems, it is mentioned that practical use of the operation know-how is important and the development of the operation support system using them is significant.
In rolling processes, friction on the interface between the roll and workpiece is the most critical factor. It strongly affects not only deformation and load but also surface quality of the rolled material. Prediction and control of the friction are highly demanded. However, physical phenomenon on the interface has not been fully understood. Lubricant as well as surface oxide scale locates at the interface during rolling so that the two play important roles in frictional shear stress, surface deformation and heat transfer, especially in hot rolling. As flow stress of steels increases and better surface quality is demanded, tribological studies on rolling are required to develop new lubrication and scale technologies. In this article, past key studies on lubrication and oxide scale are reviewed and outlook for future tribology in rolling is given.
The theory of plasticity was first systematized in a textbook by R. Hill in 1950. This theory was further generalized in the framework of rational mechanics and is nowadays widely used in industrial as well as academic fields by means of the finite-element method. Because of the highly nonlinear process in metal forming, a finite-element simulation of metal forming processes suffers from various theoretical and technical problems. One of them is material modeling, i.e. a plastic constitutive model. Compared to the rapid progress in the analytical method of metal forming, a progress in the material modeling is not significant and a lot of problems still remain. Among them, a modeling of anisotropy is one of the problems of great importance. Recently a crystal plasticity model receives attention again as an alternative method to the conventional phenomenological constitutive model because this model can represent various phenomena including the anisotropy without giving specific assumptions. In this paper, progresses in the analytical method of metal forming and the material modeling during the period of the latter half of the twentieth century until now are comprehensively reviewed. Remaining problems to be solved and future perspectives are also discussed.
As a method for producing ultrahigh strength automotive steel components with high gauge accuracy the hot stamping technology draws attention and is practiced widely today. For supporting and developing this technology, numerous researches have been actively carried out all over the world. It is a good timing to review this technology on this special issue because further rapid development is to be expected in the next decade. The hot stamping technology consists of a variety of techniques. In this review, the authors focus two items suitable to this special issue, the first one is the formability of hot stamping and the second is the technology for improving its productivity. The review shows the importance of the temperature control at stamping parts to achieve excellent formability. Through the review on the technology for improving the productivity of hot stamping, the authors propose highly productive hot stamping processes consisting of rapid heating, short time stamping and rapid cooling.
Operational scheme and flatness control algorithm with related mathematical models are developed for OPL (Oita Plate Leveler) which is a large scale embodiment of the NSSMC Intelligent Mill (NIM); NIM is a new concept rolling mill which can estimate and control roll force distribution across the width acting between the work roll (WR) and the rolled material. For zero adjustment procedure of individual position control system of divided back-up rolls (BURs), force distribution of the BURs minimizing the WR horizontal deflection is calculated by means of Lagrange’s method of undetermined multipliers, and systematic way of realizing the target BUR force distribution is developed. Considering plan view inclination and temperature distribution of inlet material, flatness control algorithm is developed and practiced to obtain desired flatness as well as residual stress of the rolled material after cooling.
High accuracy online strip profile prediction model has been developed for realize mixed scheduled rolling of high tensile strength and mild steel in hot strip finishing mill. New method is based on matrix model. The force and tension distribution is considered by experiment and FEM analysis. First the force and tension distribution when there is no change in strip profile is calculated as the specified value, second the distribution when there is an actual change in strip profile is calculated as the variation from the specified value. Then the profile prediction accuracy of high tensile strength steel, whose rolling force is higher than that of mild steel, was improved. The model was applied to online crown, flatness, and walking control in hot strip finishing mill. As the result, mixed scheduled rolling of high tensile strength and mild steel was commercially promoted.
Cooling equipment of the pipe laminar type is utilized on run-out tables in hot strip mills. It is known that the stability of the water flow injected from the cooling equipment nozzles affects cooling performance. In this study, new criteria and an experimental method for quantifying water flow stability are proposed. The experimental apparatus consists of the water injection equipment, an electric circuit and a logger. The time variation of voltage represents the electric resistance of the injected water and is measured under various conditions of flow rate and nozzle diameter. We propose a new laminar stability index, σV/VAVE, in which σV and VAVE mean the standard deviation and the average value of voltage, respectively. The proposed index qualitatively shows good agreement with the appearance of the laminar flow in the experiment. Based on the experiment, an equation of continuous laminar length is suggested.
The influence of surface conditions such as scale thickness and surface roughness on water spray cooling and air jet cooling characteristics was investigated experimentally. SUS304 stainless steel with the thickness of 20 mm was used as the cooled sample. An artificial scale layer was formed on the sample surface by thermal-spraying using Al2O3 powder. The thickness of the Al2O3 layer was varied from 50 µm to 210 µm. A sample without an artificial scale layer was also studied; in this case, the surface was roughened by shot blasting up to 20 µmRa. As a result, the artificial scale layer showed a thermal resistance function in both water spray cooling and air jet cooling. In water spray cooling, the characteristics of which depend on surface temperature, the cooling rate during film boiling and the apparent quench point temperature at the interface increased with Al2O3 scale thickness. Surface roughness enhanced the cooling rate during film boiling and resulted in a higher quench point temperature in spray cooling. In air jet cooling, heat flux increases with surface roughness, but this tendency can be seen only with larger flow rates. Surface roughness has a much stronger influence on heat flux in water spray cooling, even though the average heat flux is not as large. In this research, the heat flux during impingement of water droplets was estimated to be much higher than that in air jet cooling. This is thought to explain the difference in the influence of surface roughness on cooling characteristics with the two cooling methods.
Possibility of applying new roll materials (FRM roll materials), which are consisted of high speed tool steel reinforced with alumina fiber, to work rolls for hot rolling process is investigated by several laboratory tests. The FRM roll material expected to have superior tribological and mechanical properties, have been manufactured using sintering method (Hot Isostatic Pressing process) with which ceramic content can be increased. Wear resistance and mechanical properties and hot rolling characteristics of the materials were investigated, and the FRM roll material was found to have three times or more wear resistance, a little lower rolling force and friction coefficient, and same or more thermal crack resistance and higher tensile strength in comparison with high speed tool cast steels. Therefore, FRM roll materials are expected to be used for hot rolling as high performance roll materials instead of high speed tool steel.
The effect of nitrogen(N) content on the hot-ductility of niobium(Nb) and boron(B) combined added extra low-carbon steel was investigated by using commercial steels. With containing N of 0.0048 mass%, Nb and B added extra low-carbon steel showed the bottom of ductility around 950ºC, which was slightly higher than the Ar3 transformation temperature. Reducing N content down to 0.0032 mass% or less improved the hot-ductility around the temperature of 950ºC. Both grain boundary ductile fracture and transgranular brittle fracture were observed in the fractured surface of the tested specimen. MnS,BN and AlN precipitates were observed at the grain boundaries, and the Nb(C, N) precipitates were observed in the ferrite matrix. These results suggest that the hot-ductility of Nb and B combined added extra low-carbon steel is deteriorated by both the grain boundary ductile fracture dominatied by the MnS, BN, AlN precipitates and the transgranular brittle fracture by the Nb(C, N) precipitates.
If surface defects on hot rolled wire rod subsequently remained, it might become the origin of cracks in the forging process. Therefore, the surface quality for hot rolled wire rod is strongly required. In order to avoid surface defects, it is important to establish methods for predicting the positions and the criteria of the defects as a function of rolling conditions. In the previous paper, we assumed a defect to be a kind of plastic buckling and we introduced a new parameter indicative thereof. We also indicated that a hard surface layer promoted the occurrence of surface defects. For this paper, we rolled a lead alloy with copper plating and a lead alloy without plating and compared their surface shapes after rolling. As a result, it was verified that “surface defects are caused by plastic buckling and defects are promoted by hardening the surface.” In addition, we quantified the effects of various rolling conditions on the occurrence of surface defects using the parameter above. The friction coefficient affects the depth and position of surface defects. The larger the friction coefficient, the greater the depth of the defect and the further to the side the position of the defect moves. Rolling tension also affects surface defects. Back tension has a greater affect than front tension. With compression tension, the depth of defects increases and the position moves to the free face.
The authors have reported that the machinability (lathe turning, drilling and sawing) of Type 304 austenitic stainless steel has been improved by the precipitation of hexagonal boron nitride (h-BN). In the present study, the influence of h-BN precipitation on surface roughness of Type 304 steel after lathe turning was investigated. Surface roughness was degraded with increasing the amount of boron addition, especially at lower cutting speed of 22 m/min. However, Type 304 steel with boron addition up to 0.05 mass % exhibited similar or better surface roughness compared to that of commercial Type 304 steel, and 0.1 mass % boron added Type 304 steel showed better surface roughness compared to that of Type 303 sulfur added free-cutting steel. Precipitation of h-BN enhanced the size of built up edge and degraded the surface roughness. On the other hand, increased Vickers hardness values due to solute nitrogen decreased the separation size of built up edge and improved the surfaced roughness.