| Continuous casting slabs provided from caster without cooling then charge directlyto heating furnace usually named direct charge, which can save energy. But for Nb, Vand Ti containing steel, crack usually can be found on final product surface after directcharge-rolling. Those cracks decreased direct charge percentage of microalloyed steelsignificantly. To solve this problem, we conducted below researches to reveal specificmechanism of microalloyed steel direct charge:1. When and how the crack formed in microalloyed steel direct charge2. Effect of microstructure on related crack formation.3. How the crack can be avoided through process optimization.Research method: Choose E36steel as research object and Q235as comparison.First, we performed heat treatment on samples in tube furnace. The whole process wasas far as possible to simulate direct charge. Then we analyzed its microstructure withoptical microscope, SEM and TEM to reveal the information of microstructure, cracksand precipitates.Through lab experiments, some concludes can be formed:The cracks formed from charging in heat furnace around700℃till heating to800℃（Ac3）, in this100℃section, experimental samples were in ferrite-austenitecoexist zone. α'γ change cause huge tensile stress, Nb、V、Ti containing participatesincreased to its maximum quantity at800℃.Reheating samples to1200℃with different charging mircostructure:No cracks were found in pearlite and lower bainite microstructure samples. But forgranular bainite, upper bainite and martensite samples, cracks were formed because ofprecipitates and hardness.Feature of precipitates under different microstructures:1. In pearlite: large quantity of precipitates but single one is small and withhomogeneous distribution.2. In granular bainite and upper bainite: with little quantity of precipitates buteasy to gather on grain boundary.3. In lower bainite and martensite: extremely low quantity of precipitates.Carbonitride of microalloy element precipitates:1. Solubility of carbonitride of microalloy element in different microstructure is not the same. This phenomenon affects the distribution of precipitates.2. High cooling speed will decrease the size of precipitates. This will affect thequantity of precipitates.Austenite, pearlite and lower bainite are microstructures which not easy to causecracks, and lower bainite is the best. Pearlite and lower bainite can be formed aftercontrolled water spraying before charging. The key point is how to find the sprayingbegin/end temperature and cooling speed of slabs. Then we found the best process flowto form pearlite: after slabs come out from caster, cooling its surface with2℃/sspraying to530℃to promote surface γ'α transformation. And the best process flowto form lower bainite: after slabs come out from caster, cooling its surface with60℃/sspraying to360℃to promote surface microstructure transform to lower bainite. |
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