Cold-rolled Precision Stainless Steel Strip: Features & Uses

With the rapid development and transformation of communication, electronic, and office automation equipment, user requirements for stainless steel strips are becoming increasingly stringent. Unlike conventional stainless steel sheets, precision stainless steel strips typically refer to strips with a thickness of less than 0.3mm. As a high-end product in the field of stainless steel strips, precision stainless steel strips have excellent strength, precision, surface roughness, and other properties, and are widely used in electronic stamping parts, optical cables, diamond blades, light-blocking materials, and the like. In this post, we’ll take a close look at the manufacturing process and features of the cold-rolled precision stainless steel strip.

Currently, the main production processes for precision stainless steel strips include cold rolling, bright annealing, and finishing.

Precision Stainless Steel Strip Rolling Process

The performance requirements of precision stainless steel strips for extreme thinness, high strength, and high precision place high demands on the rolling process. Conventional rolling processes cannot meet the production of precision stainless steel strips.

Currently, the 20-roll rolling machine is mainly used to produce precision stainless steel strips, and its complete production process includes cold rolling, annealing, and finishing. In the rolling process, attention should be paid to selecting an appropriate reduction system, tension system, speed system, and roll shape. To improve the production capacity of the rolling mill, while fully utilizing the main drive power of the rolling mill and the front and rear coilers, it is necessary to maximize the reduction rate in each pass to reduce the number of rolling passes.

Tension can reduce rolling pressure, improve plate shape, and stabilize the rolling process. The rolling process of a 20-high mill generally uses high tension and should be adjusted according to the plate shape at any time. When there are waves in the middle of the material, the tension should be reduced to prevent pulling the edge or breaking the strip. When the strip produces side waves, the tension can be appropriately increased.

Within the speed range allowed by the mill, the highest possible rolling speed should be adopted to improve the production capacity of the mill. At the same time, when the rolling speed increases, the rolling pressure decreases accordingly. During the first pass of rolling, a lower rolling speed is generally used.

Cold Rolling Equipment

Based on the rolling principle, the maximum pressure condition should use the work rolls with the smallest possible diameter, and to achieve this goal, multi-roll rolling mills such as 6-roll, 12-roll, and 20-roll rolling mills have been developed. Among them, the 20-roll rolling mill is the preferred equipment for producing precision stainless steel strips due to its high rigidity and small elastic flexural deformation of the work rolls.

Thickness and Shape Control

To maintain good consistency in the thickness of the rolled strip and eliminate the influence of the incoming material thickness, the most reliable method is to add feedforward control to the traditional control mode. Based on the measured change in strip thickness, the delay controller adjusts the roll gap to ensure that the final rolled strip thickness remains constant.

In the AGC control system of traditional roller mills, product thickness accuracy is achieved by repeated calculations by the feedback system. In modern AGC control systems, based on trigonometric principles and automatic roller management systems, geometric calculations of the roller system are solved, as well as position setting.

In modern roller mills, AGC control calculates the set position of the support roller eccentric wheel based on the actual size of the working roller to keep the working roller at zero position. This method avoids accidents caused by reverse adjustment during manual adjustment.

In terms of shape control, a series of advanced shape-control technologies have been developed internationally in the past 20 years. By using certain roll profiles with bending, shifting, and other technologies, the increasingly stringent requirements of users for shape quality accuracy have been met. Two shape measurement rolls are installed at the inlet and outlet of the rolling mill, and numerous pressure sensors are installed along the axial direction of the shape measurement rolls.

The strip is pressed on the shape measurement rolls during the rolling process, so the output signal of the pressure sensors in the shape measurement rolls changes with the strip shape. These signals are comprehensively processed by the shape control system and then act on the hydraulic valve, and the movement of the hydraulic valve adjusts the support roller eccentric slightly. At the same time, these signals also control the transverse movement of the first intermediate roll, so that the strip shape is effectively controlled.

Tension Control

The tension control system of the 20-roll rolling mill adopts a mixed control method of indirect tension and direct tension. When the steel strip is engaged by the coiler tongs and enters the coiler, the indirect tension control is carried out by the speed control system, which is a three-loop speed/current/voltage control system.

As the low-speed creeping speed of the coiler is generally 5%-10% higher than the creeping speed of the main rolling mill, the steel strip is quickly tensioned. When the tension value reaches 70%-80% of the set value, the system automatically switches to direct tension control, and the tension regulator is put into operation to achieve three-loop control of tension, current, and voltage.

Roll Shape Adjustment Mechanism

The roll shape adjustment mechanism of the 20-roll rolling mill includes AS-U convexity adjustment for the support roll, a movable intermediate roll with a conical end. The AS-U convexity adjustment passes through two intermediate rolls, and the first intermediate roll is transmitted to the working roll. Therefore, this adjustment method has a small influence on the roll shape of the working roll, and can only make a small adjustment to the roll gap, increasing convexity and causing the central part of the strip to extend more, and reducing convexity and causing the edge part of the strip to extend more.

However, for more complex shape defects such as quarter-scale camber, the AS-U convexity adjustment effect is small. The first intermediate roll has a conical end and can be moved axially, effectively eliminating the influence of the working roll end deflection on the strip edge wave. Axial movement of the first intermediate roll not only has a large effect on the edge part of the strip deformation but also has some effect on the central part of the strip extension.

Heat Treatment and Finishing Process

The heat treatment process of precision stainless steel strips is the same as the heat treatment process of ordinary stainless steel plates. The typical heat treatment process for 300 series austenitic stainless steel is solution treatment. In the heating process, the carbides dissolve into the austenite, heated to 1050-1150°C, properly held at a short time, so that the carbides are completely dissolved in the austenite, and then quickly cooled to below 350°C, resulting in a supersaturated solid solution, i.e. uniform one-way austenitic structure. The key to this heat treatment process is rapid cooling, requiring cooling rates of up to 55°C/s, quickly passing through the temperature range of post-solidification of carbides (550-850°C).

The holding time should be as short as possible, otherwise, the grains will become coarse and affect the surface finish. 400 series ferritic stainless steel has a lower heating temperature (around 900°C) and is often slow-cooled to obtain an annealed softened structure. Martensitic stainless steel can also be treated by segmented quenching and tempering.

Another key issue in the heat treatment of cold-rolled stainless steel strips is the requirement for uniform microstructure in both the width and length of the entire strip. The Muffle-type bright annealing furnace uses large-size muffle tubes, and the heating air flows uniformly in a spiral manner from the outside of the muffle tube, ensuring uniform heating of the strip.

To ensure uniform microstructure along the length of the strip, it is necessary to maintain a constant linear velocity of the strip in the heating furnace. Therefore, modern vertical bright heat treatment furnaces are equipped with precision adjustable roller tension adjusting devices at both the inlet and outlet. These devices not only meet the heat treatment speed requirements by adjusting the strip speed at the inlet and outlet, regardless of whether the looping quantity is empty or full but also establish and precisely adjust the small tension of the strip according to its shape condition to meet the requirements for shape control.

The finishing of precision stainless steel strips mainly includes tension leveling and longitudinal cutting. After annealing, to obtain good shape, machining performance, and deep drawing performance, the strip usually undergoes a stretching and leveling unit, which operates by combining stretching and continuous alternating bending to produce plastic elongation and correction of the strip.

The strip can eliminate defects such as center wave, quarter wave, edge wave, longitudinal bending, and transverse bending through the stretching and leveling process, but the brightness is reduced by 1%. The flatness accuracy of the finished strip after stretching and leveling can reach 1% of the incoming flatness. To trim and slit wide strips into finished narrow strips, longitudinal cutting machines are commonly used.

Nowadays, many new technologies have been developed for longitudinal cutting machines, such as automatic adjustment of the cutting tool insertion and axial clamping, quick change systems for scissors that reduce changeover time by 50% compared to conventional methods, and vacuum technology for safe transportation, automatic cutting of strips, and prevention of any damage to the strip surface.

Cold-rolled Precision Stainless Steel Strip Supplier

Thank you for reading our article and we hope it can help you to have a better understanding of the cold-rolled precision stainless steel strip. If you want to find more information about cold-rolled precision stainless steel strips, we advise you to visit Sino Stainless Steel for more information. As a leading supplier of precision stainless steel strips across the world, Sino Stainless Steel offers high-quality cold-rolled precision stainless steel strips, stainless steel wire, sheets, and embossed stainless steel plates at an extremely competitive price.