Effects of Nozzle and Roll on Surface Condition of Al-1%Si Strip Cast Using Vertical type High Speed Twin Roll Caster

When a strip of Al-Si alloy with an Si content of 1% was cast using a vertical-type high-speed twin-roll caster, cracks form in its surface. The effects of the pouring method, the shape and position of the nozzle, and the roll surface texture on surface crack formation were evaluated with a roll caster. The rolls were made of a copper alloy, and the roll speed was 30 m/min. The as-cast strips were bent to investigate the degree of crack formation, and the outer surface of the strips was observed without magnification and with a stereomicroscope to determine the influence of the pouring method, the shape and position of the nozzle, and the roll surface. A roll machined to form V-shaped grooves 0.4 mm deep on the surface of the strips was most useful for reducing surface cracking. Changing the shape of the nozzle tip was second-most effective. There was a clear correlation between the roll surface condition and surface cracking in the Al-Si strip.

Tempering effect on residual stress in bimetallic roll

In this study, tempering effect on the residual stress is studied after uniform heating–quenching and nonuniform heating–quenching for bimetallic work roll. Results for uniform heating treatment showed that the maximum stress at the center decreases by 68% from 396 MPa to 126 MPa after the first and second tempering. Results for nonuniform heating treatment showed that the maximum stress at the center decreases by 47% from 309 MPa to 165 MPa after the first and second tempering. It may be concluded that nonuniform heating–quenching and tempering are useful for reducing the central tensile stress preventing cracks at the roll surface.

Remote Cooling of Rolls in Hot Rolling; Applicability to Other Processes

A novel method of cooling rolls in hot rolling is proposed. This method uses a combination of solid jet nozzles and specially shaped deflecting vanes. The vanes transform the incoming cylindrical water jet into a flat fan. Thereby, the coolant can be directed into hardly accessible locations whilst maintaining the optimal angle of impingement. The vanes allow for effectively cooling the roll surface near the rolling gap, which is otherwise not possible with classical flat fan nozzles. The jet impact is tangential due to the limited room for nozzle mounts or a coolant supply. The developed method was laboratory tested. A similar cooling efficiency was found between the vane and the flat fan nozzle. The latter was however mounted in a position impossible in the plant. The potential of the proposed cooling is, therefore, eminent. Apart from hot rolling, it could be exploited in other technological processes such as high pressure die casting, machining, turning, hot stamping, etc.

Estimation of depth and degree of mill roll surface hardening during grit texturing

The paper presents shot blasting (SB) as the most common method of arrangement of temper mills rolls surface microgeometry, providing required roughness of the cold-rolled strip. The surface riffing studies were carried out at the Willibrator industrial unit; metallographic studies were performed using MEIJI 2700 optical microscope and JSM-6490LV scanning electron microscope. Refinement of the surface layer structure during interaction of the roll material with the grit was revealed. In the work the lower bound method, one of the methods of plasticity theory, is used for theoretical analysis of the SB process. To make computations easier, the roll microcave is approximated to spherical shape. Quantitative estimates of deformation degree and the depth of hardened layer were determined by speed of the grit and hardness of the roll surface. The depth of hardened layer is obtained by the size of used grit, and to a lesser extent, by the speed of grit impact on the roll surface and textured surface hardness. It has been searched out in literature that increase in hardness by one unit leads in average to 3 % increase in resistance. Thus, SB application allows reduction of time of rolls rehandling of temper mills by 6.0 – 10.5 %, depending on application modes of rolls riffing with grit. Analysis of microstructure have shown that increase in hardness of the surface layer is a consequence of refinement of its structure in process of interaction with the grit. It has been revealed that increase in hardness of the surface layer leads to an increase in its wear resistance and fatigue strength. The authors of the work have found that at speed of 60 m/s the hardness increases by 3.5 units.

On the Evolution of Temperature and Combined Stress in a Work Roll under Cyclic Thermo-Mechanical Loadings during Hot Strip Rolling and Idling

An accurate prediction of temperature and stress evolution in work rolls is crucial to assess the service life of the work roll. In this paper, a finite element method (FEM) model with a deformable work roll and a meshed, rigid body considering complex thermal boundary conditions over the roll surface is proposed to assess the temperature and the thermal stress in work rolls during hot rolling and subsequent idling. After that, work rolls affected by the combined action of temperature gradient and rolling pressure are investigated by taking account of the hot strip. The accuracy of the proposed model is verified through comparison with the calculation results obtained from the mathematical model. The results show that thermal stress is dominant in the bite region of work rolls during hot rolling. Afterwards, the heat treatment residual stresses which are related to thermal fatigue are simulated and introduced into the work roll as the initial stress to evaluate the redistribution under the thermal cyclic loads during the hot rolling process. Results show that the residual stress significantly changed near the roll surface.

Reduction of Surface Cracks at Al-Mg Alloy Strip Surface Cast by Unequal Diameter Twin Roll Caster

Al-Mg alloy strips were cast by an unequal-diameter twin-roll caster. It was found that cracks formed on the surface at grain boundaries. The grains near the surface were small in size, which likely contributed to crack formation. The use of a molten metal pouring method to increase the grain size near the surface is proposed to reduce cracks. In the previous method, molten metal is poured into a pool, which is on the lower roll surrounded by side-dam plates, a back-dam plate, and the upper roll. In this study, molten metal was directly poured onto the roll surface at a shallow angle using a launder. When the angle was smaller than 20°, cracks did not form. With the proposed method, the heat transfers between the molten metal and the roll surface decreased, as determined from the grain size and strip thickness. The cracks on the strip surface were color-checked and visually inspected.