Experimental study of in-line heat treatment of 1.0577 structural steel
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In-line heat treatment is frequently used in rolling mills because it offers a significant improvement of rolled product mechanical properties with costs benefits. This method allows achieving required mechanical properties without necessity of additional alloying and rolled product reheating. Disadvantage of in-line heat treatment is fixed rolling velocity which is typically strong parameter in controlling of final cooling regime. Water flow rate, pressure, type, size and position of nozzles, water temperature are examples of parameters influencing cooling intensity and the Leidenfrost temperature. Laboratory experimental study is needed to design well controllable cooling system which allows keeping required cooling regimes for various product steel grades and dimensions. This paper describes experimental stages of cooling system designing procedure for improving structural steel 1.0577 mechanical properties. First experimental part began with building of cooling intensities (heat transfer coefficients - HTC) database for tested several nozzles configurations. Then required cooling regime was selected according to the continuous cooling transformation diagram. The target was obtaining harder (quenched) material with good ratio between elongation and strength. The final equalization temperature was set to 600 °C in the whole body. Numerical simulations of cooling followed based on the knowledge of heat transfer coefficients from database. Appropriate nozzle configuration was chosen and numerical results were experimentally validated using modified Jomminy test. A hardness was improved significantly up to thickness of 12 mm (275 HV under sprayed surface decreasing to 180 HV in 12 mm). When the required material structure and hardness verified appropriateness of cooling regime by previous tests, the first design of cooling section was done. Full scale sample was heat treated on a new experimental stand (Karusel) which was developed by HeatLab. It enabled to simulate real cooling process in laboratory conditions. The sample was heated to rolling temperature and moved through the intensive spray (surface temperature drop to 300 °C) and then through the soft spray because of water savings. The cooling stopped in required time and the sample tempered to the target equalization temperature of 600 °C in the whole body. Finally, the hardness was measured, tensile and Charpy pendulum tests were done to confirm design of cooling unit. The hardness of the original material was constant along depth – 160 HV. It was improved by heat treatment – decreasing from the sprayed surface (265 HV) to the depth of 12 mm (175 HV). The minimal yield strength of the heat treated material increased from 355 MPa 400 MPa (maximal up to 750 MPa – closed to the sprayed surface). The sample was cooled to -20 °C for Charpy pendulum test. The pendulum energy of heat treated material rapidly increased from 50 J to 150 J.
KeywordsHeat treatmentWater quenchingSelf temperingTensile testsS355J21.0577Structural Steel
Document typePeer reviewed
SourceProcedia Manufacturing. 2018, vol. 15, p. 1696-1603.