Study And Application Of Ultra-fast Cooling Technique For H Beam

The final rolling temperature difference between flange and web for H-beam is very large, resulting in nonuniform microstructure at H-beam cross-section. The high strength H-beam can be manufactured with micro-alloying technology, but the cost of per ton is high and the mechanical properties is unstable. Mechanical properties of H-beam can be improved by ultra-fast cooling (UFC). The UFC has been applied to hot strip mills, but for H-beam, the uneven cooling phenomenon such as out of square, web-wave and crack can easily occur due to complex shape at H-beam cross-section, resulting in nonuniform mechanical properties. In this paper, the mathematic model of UFC control system for H-beam was established, the temperature fields for H-beam were researched, the uneven distorition and residual stress were analysed, and the effect of UFC on microstructure and mechanical properties of H-beam is systematically investigated. The main work are as follows:(1) UFC equipment for H-beam is outlined. The UFC system can be adapted for the cooling requirements of H-beam with different specifications through the signal transfer in basic automation and the co-operation of hydraulic cylinders. Based on the development of H-beam and the requirements of UFC process, the temperature-drop models of water/air cooling were established, and the temperature control was achieved by modular design. The UFC process became reasonable gradually through the self-learning of the models. Operation results showed that the temperature difference between web and flange can be reduced to 30 ℃, and the readings of magnetic feet on the two hydraulic cylinders are broadly in line, thus the UFC system is controllable, precise and reliable.(2) Based on finite element method (FEM), an analytical model was established for H-beam during controlled cooling. The temperature fields during controlled cooling and air cooling were analyzed. After the H-beam was controlled cooled for 4.5 s, its mean surface temperature decreased from 850 to 460℃, and the lowest and the highest temperatures were measured at edge of flange and at R corner, respectively. On the whole, the cooling rate on the surface of H-beam was higher up to 100℃/s, which coincided with the actual experiment. Whereas, for the H-beam air-cooled for 30 s, the mean temperature at R corner and web was 700 and 540℃, respectively.(3) Based on finite volume method (FVM), coupled simulation for UFC of H-beam was researched. Firstly, the coupled interface of flow-solid for H-beam was cooled. With the passage of time, heat quantity of region which corresponded to the coupled interface was absorbed by cooling water at last. The average cooling rate on the surface of H-beam was higher up to 100℃/s, which coincided with the actual experiment.(4) The cause of out of square is large residual stress due to the uneven cooling rate between upper and lower groove of the H-beam. The out of square can be solved by improving the structure of UFC equipment layout and process control mode. An experiment and FEM study of cooling distortion showed that the main reason of out of square was uneven cooling rate between upper and lower web, and the secondary cause was uneven cooling rate between upper and lower R corner. The temperature of air cooling for H-beam after rolling were tested by means of thermal imager, nonuniform temperature of H-beam resulted in residual stress in initial air cooling stage. The residual stress of H-beam under the two cooling processes were tested using X-ray stress analyzer. After UFC, the mean residual stress of H-beam can be reduced by 30MPa.(5) The microstructure and mechanical properties of Q235, Q345 and BS55C were studied by means of optical microscope (OM), electron probe micro-analyzer (EPMA) and transmission electron microscope (TEM). The results showed that the microstructures for different parts of Q235 and Q345 H-beam consisted of ferrite and pearlite, and the grain size at R corner was coarser than those at flange and web. The difference of yield and tensile strengths of web, flange and R corner was within 30 MPa, and the elongation was similar. The BS55C H-beam consisted of a little bainite, and the precipitation strengthening at web was greater than those at flange and R corner, but the uniformity of cross-section mechanical properties is acceptable.