Study On Hardenability Prediction Of Gear Steel And Quality Control

Benxi Steel has built up the large rectangular billet production line for special steel by "Converter→Refining→Continuous Casting→Continuous Rolling" process, according to its actual situation and development plan. Its products orientate high-grade, including gear steel. This paper summarizes various gear steels around world, their development state and steel-making qualities of gear steel. It points out hardenability band and dispersion is too broad, which is an important measurement of the grade of gear steel. This paper compares the superiority-inferiority in various calculations of hardenability, uses the hardenability data of 20CrMnTiH gear steel produced by the large rectangular billet production line to modify the mathematical model of calculating hardenability with the method of nonlinear regression equation. In considering the relationship of variables, the method determines the key index of end quenching curve shape is half martensite(Es), transits to the nonlinear relationship of end quenching hardness and Jominy distance from the nonlinear relationship of quenching end distance(Jo), Es and chemical composition, and establishes the nonlinear regression mathematical model. The method has high prediction precision and its result is closer to the measured value. Programmed by VB6.0, the hardenability prediction software can calculate random Jominy distance hardness according to chemical composition and grain size. The forecast accuracy is better than that of unmodified nonlinear regression equation and multiple linear regression equation, but equal to that of artificial neural network. The hardenability prediction software not only provides the basis for design and selection of steel and metallurgical controls, but also lays a foundation for on-line detection. Meanwhile, the equation of chemical composition and hardenability and grain size is deduced on the base of the mathematical model. The composition prediction software, which has empirical model and nonlinear model, is established for the purpose of assistant to refining.The influential factors of hardenability and error reason of prediction model are analyzed in this paper. Measures are put forward to control the narrow band of hardenability, suggesting that the precise and homogenization of chemical composition be paid more attention to. For the accurate chemical composition control, firstly based on the influences of different alloying elements to hardenability, the chemical target has been decided, and then adjusted by alloying at converter and at LF refining slightly; For the homogenization of the chemical composition, it is mainly to control billet segregation, melt superheat, proper electromagnetic stirring, tapping velocity, reasonable secondary cooling and soft reduction at final stage of solidification.In addition, the actual product level of gear steel is described in this article. A research of the formation, prevention and elimination of the main quality problem flake is made. Then the production process of gear steel is modified to improve the quality, and to win benefits for the enterprise.