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Rolled stock quality assurance improvement of mechanical properties The article shows that the existing system of controlling the mechanical properties on the basis of on-line tests does not ensure high quality of products due to insufficient data representation and the locality of control. The authors propose a new system of controlling the mechanical properties of rolled stock based on extensive application of non-destructive magnetic method providing for 100% product control at the mill. The said system ensures the significant improvement of products quality. At present the industrial world shows a considerable interest to manufacturing of products with the guaranteed quality. Such manufacturing does not imply that the products are to be made of the best materials with the application of the most sophisticated equipment and technologies. To produce the product of guaranteed quality means to manufacture the products accurately meeting the custom requirements, i.e. to assure quality of high level. Quality assurance provides for the system of quality indices control and technological parameters control. The important characteristic of the system of quality indices control are data representation and repetition factor. To enhance the said characteristics means to improve the quality of products. The mechanical properties of steel products are the main factor affecting the customer’s interest. Mechanical properties are tested on-line by sampling. This method cannot be considered accurate due to its destructive character. In practice only 1% of steel is tested (per lot) with explicit locality factor. It explains low level of quality assurance. At present there is no way to improve the efficiency of the said method. Along with on-line tests, non-destructive methods are widely used: magnetic methods according to GOST 304150-96 and statistic method according to OST 14-1-43-90. Magnetic method is based upon measurement of structural and sensitive magnetic characteristics of steel, statistic method – upon predicted analysis of the on-line test indices. The above mentioned methods are used to replace the on-line tests. The said methods reduce considerably the time of obtaining the information on the mechanical properties indices. They also reduce the expenses for sampling and sample testing. However, data representation and repetition factor remain the same, thus the quality is similar to the one of the on-line tests. Magnetic methods have a potential for the quality improvement. First of all, this method is the alternative for on-line tests in respect to physical nature. Like on-line tests it evaluates the steel structure determining its mechanical properties. Being nondestructive, it has several principal advantages in comparison with on-line tests: 1.Efficiency of control. The time for determining the properties is significantly reduced. 2.Higher accuracy and reliability of control due to the lack of errors connected with sampling, sample testing providing for multiple measurements and calculations. 3.100% control of the length and area of each cogged ingot which is similar to ultrasound control of rolling defects. 4.Maintaining control directly on moving cogged ingots; it enables to control efficiently the technological parameters in the process of feedback. In spite of the said advantages, the magnetic method of control at present is not widely used in steel practice, as compared to the use and development of non-destructive methods of control of rolling defects. At the same time the magnetic control is supported by a developed technical base. It ensures contact and non-contact measurements of different magnetic characteristics of steel. However, arranging control on the definite production site is sometimes problematic. Quality assurance (as to the accuracy of properties’ reproducibility within the given limits) is supported significantly by the system of technological parameters control. Analysis of production process shows that at present it is impossible to execute on-line control of technological processes of steel structure formation while rolling and subsequent heat treatment. It is conditioned by the fact of receiving the results of the process only after on-line tests, i.e. out of technological process. That is why; control of the said processes should be based on predicted analysis. The magnetic method enables to implement the efficient control of the processes and to produce with mechanical properties within the limited range. The advantages of nondestructive magnetic method enable to propose a new system for controlling the mechanical properties of the rolled stock. It ensures the perfect quality of steel products meeting all the modern requirements. The system is based on 100% complete control of the rolled stock by magnetic methods. In this case the magnetic method can be used optionally or for final acceptance. Preference is given to arrangement of control in the process of mill’s operation. In the new system the method of on-line tests is used for final acceptance, and as checking and arbitration one. The new system provides for improvement of data representation and repetition factor. The implementation of a new system of control ensures the perfect quality, improvement of accuracy of quality indices reproducibility and the extension of the technological process. The new system is especially applicable for the rolled stock, the mechanical properties of which are considered to be operational in as supplied condition, i.e. it ensures operability and reliability of product. It is planned to implement a new system of control in the Blooming Mill of CJSC “MMW “ISTIL (Ukraine)”. The assortment of steel for the billets of the Blooming Mill includes carbon, alloyed and low alloyed steel grades. The Mill produces wide range of billets of different grades and size. The rolled stock is supplied in hot-rolled condition after slow cooling or low-temperature annealing, as well as with anti-flake treatment. The hardness of steel is guaranteed in as supplied condition. The required properties are controlled on the heat-treated samples. At present the system of control of mechanical properties provides for on-line tests as the major control, magnetic and statistic methods are used for special types of rolled stock according to STP-70-98 and STP-99-98. The magnetic control is carried out on the samples selected from the billets after hot rolling. In the nearest future it is planned to proceed to the system of controlling the quality indices of the rolled stock and to ensure the realization of the conception accepted at CJSC “MMW “ISTIL (Ukraine)”: “ ZERO DEFECTS”. To effect it, the Mill implements the effective non-destructive methods of 100% control of the external and internal defects of the rolled products. It also provides for effective modes of billet heat treatment: normalization and temper quenching to ensure the required properties of billets, not samples. The objective of the Company is to apply the magnetic control of mechanical properties for wider assortment of steel grades of the rolled stock and to ensure 100% magnetic control for billets in the process of operation of the Mill and efficient control of the technological parameters. It will promote the quality improvement of the supplied products and their competitiveness. |
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