| As the main product in the modern iron and steel industry, micro-alloy steel is widely used in various industrial sectors of the high performance steel, but in the production of continuous casting slab process of micro alloyed steel, the corner transverse crack defect always has been a common problem in the major steel mills. It is known that the analysis of transverse corner cracking mechanism, and propose measures to eliminate transverse corner cracks is very important to improve the quality steel product and enhance the efficiency of steel enterprise. Therefore, the transverse corner crack sample from the continuous casting slab of micro alloyed steel produced by a domestic iron and steel enterprise was applied, the formation mechanism of the transverse corner crack was research and the measures to eliminate the transverse corner crack of continuous casting slab was proposed.The morphology characteristic of transverse corner crack, microstructure, grain size and precipitates distribution were examed by OM, SEM and EDS analyzing methods.The reasons for the transverse crack corner were concluded as: The slow cooling rate of second cooling zone of continuous casting slab, which resulted in dendrite ferrite and film-like primary ferrite and a large number of chain-like precipitate on the austenite grain boundaries in process of autenite to ferrite transformation. Ferritic phase is relatively soft and the strength of the film-like primary ferrite phase is only1/4 of the strength of austenite. While in the bending straightening, slab corner temperature will inevitably fall in the austenite ferrite two-phase coexistence temperature zone; Therefore, when subjected to bending and straightening stress, the stress mainly concentrated on the strength of the smaller film-like primary ferrite,when the stress is greater than the limit stress that the ferrtie can bear, it will produce micro hole. At the same time, the chain-like precipitates in the austenite grain boundaries will be separated from the grain boundaries with the stress of acting on the grain boundaries and produce micro hole, which will eventually form cracks and extend continuously.For the cause of the transverse corner cracks, slab surface microstructure control(SSC) process was put forward to prevent transverse corner cracks and accorded to the SSC process as the guiding ideology, researched on the SSC process with the actual production of micro alloy steel composition and proposed the optimum technological parameters.The second cooling of continuous casting slab process was simulated by Gleeble-3800 thermal simulation machine, and the test results were carried out under the condition of different cooling rate, cooling temperature, returnng rate and returnng temperature by using SSC cooling technology. When the temperature reaches the specified temperature, it will be cooled to the lowest hot ductility temperature point of the continuous casting slab at the cooling rate of 0.4℃/S, and then, use the strain rate of ε′=1×10-4s-1 to tensile. After the samples fractured, cooling it to the room temperature at the rate of 0.4℃/S, measuring the size of the fracture, calculating the reduction of area, analyzing the fracture of the sample by macro and micro analysis,observing the microstructure and micro-analysis by means of metallographic microscope(OM). At the same time, with the help of EDS to analyze the composition of the precipitates. Finally, obtained optimum process parameters: the cooling rate of10℃/S;the cooling temperature of 650℃; the reheating rate of 3℃/S; the reheating temperature of 930℃. Under the optimum second cooling process parameters of specimen section, the reduction of area in the whole was greater than 60% between700~900℃. The microstructure was fine and there were no film-like primary ferrite and precipitate generated in austenite boundary. It is shows that the continuous casting slab won’t be generated transverse corner cracks during bending and straightening, under the optimum second cooling process parameter. |
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