CAUSE ANALYSIS AND CONTROL OF UNQUALIFIED 700 MPa HOT ROLLED PLATE
For 700 MPa high strength automobile beam steel,in the case of the same production process,the yield strength of the product is very different with two different composition schemes of Ti microalloying and Nb-Ti composite microalloying.The metallographic structure,morphology and composition of precipitates of automobile beam steel were analyzed by metallographic microscope,scanning electron microscope and transmission electron microscope.The reason of the difference in yield strength was discussed,and the corresponding process control measures were put forward.The results show that the main reasons for the low yield strength of Ti microalloyed automobile beam steel are the high Ti content,the coarse size of TiN and TiC precipitates,and the deterioration of steel properties.The unreasonable control of heating temperature and coiling temperature leads to the failure of Ti precipitation and ferrite grain refinement,and the effects of TiN and TiC precipitation enhancement,fine crystal refinement and phase transformation enhancement are not fully played.In the rolling process of NB-Ti composite microalloyed automobile large beam steel,the composite nanoscale precipitates(Nb,Ti)C are formed,which inhibits the growth of grain boundaries,prevents austenite grain growth and coarsening,plays the role of refining grains,and effectively improves the performance of steel.Therefore,in the actual production,the soaking temperature should be controlled at 1 210～1 230 ℃,the coiling temperature should be controlled at 600～620 ℃,and the Ti content should be controlled at 0.05％～0.10％,which effectively prevents the grain growth and coarsening and ensures the mechanical properties of the product.
precipitatesyield strengthautomobile beam steelmicroalloyingheating temperature
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