| Electrical steels are important metallic functional materials. Strip profile and flatness are key factors which determine the quality of electrical steels. Hot rolling is a key and necessary process in the manufacturing of electrical steel. The hot rolling process for electrical steels is subjected to working conditions with schedule free rolling of a large variety of the same width steels, while the profile and flatness of the steels are difficult to meet the quality requirements. Furthermore, severe roll wear and spallings are caused by the service conditions of electrical steel rolling, which constraint the improvement of strip profile quality. The research work is based on a hot strip mill with the largest production of electrical steels in China. In order to improve the strip profile quality of hot rolling electrical steels, fundamental experiments, finite element (FE) simulations, theoretical analysis, industrial tests and applications were conducted through this work. The main research works and achievements are as follows:(1) The flow behaviors of electrical steel during hot compressive process were investigated by thermal simulation experiments. The steel showed strain hardening and thermal softening effects in single-phase region, while it appeared strain hardening and low temperature softening in dual-phase region. A constitutive model with high prediction accuracy was developed to describe the flow behaviors in single and dual phase region respectively.(2) The FE models for electrical steel rolling process were built and the constitutive model was applied to the FE simulations. The strip profile characteristics with multi-factors were investigated by the FE simulation. The influences of temperature differences across the transverse direction of the strip steel in actual hot rolling process were researched. FE results showed that temperature differences especially rolling in dual phase region would cause huge changes in strip crown, wedge and wave, making it difficult to achieve the desired profile in normal conditions.(3) The serious roll wear and spalling in electrical steel hot rolling process were investaged. The roll wear characteristics were summarized and analyzed. FE calculations showed that roll wear would increase the strip crown and decrease the effect of work roll bending force. A multi-parameters descriptions for work roll wear were put forward so that the effect of work roll wear on strip profile could be well evaluate. In addition, roll spalling causes were revealed by crack detection, spalled fracture analysis and contact stresses calculations between rolls during the whole service periods of work and backup rolls. Severe stress concentration originated from roll wear was supposed to be the main reason of roll spalling.(4) In order to improve the strip profile quality of wide electrical steel in hot rolling process, specific control measures were proposed. Roll wear improvement effects of high-speed steel roll, sinusoidal roll shifting strategy and rolling lubrication were investigated by theoretical analysis and industrial experiments. The UVC work roll technology with good strip profile control performance for wide-range steels was developed; a design method of VCR backup roll to meet the same width steel rolling schedule was put forward. New measures have been applied to industry production and over 5.9 million tonnage steel has been rolled steadily so far. Industry application showed that the new measures manifested remarkable performances in preventing roll spalling, improving roll wear and improving strip profile quality. |
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