Influence of finish cooling temperature on microstructure and properties of Nb containing high strength weathering steel
High strength weathering steel plays an important role in structural steel with its advantages of high strength,weather resistance and light weight.At present,high strength weathering steel usually adopted composition design of high Ti content can inevitably produce Ti inclusion,which deteriorates the properties.Therefore,the high strength steel with high Ti content needs to make up for the lack of performance by appropriately reducing Ti and increasing Nb,and optimizing cooling control meanwhile.Aiming at the Nb containing high strength weathering steel under different finish cooling temperatures,the microstructure was characterized by means of metallographic micros-copy,scanning electron microscopy and electron backscattered diffraction.The tensile and impact properties were determined.The effect of finish cooling temperature on the microstructure and mechanical properties of Nb contain-ing high strength weathering steel was analyzed.The results show that a mixed microstructure of ferrite and granular bainite formed in each sample with the finish cooling temperature range from 575 to 650℃.With the decrease of the finish cooling temperature,the morphology of ferrite changed from massive to acicular,the content of M/A compo-nents gradually decreased,and the average grain size was refined.Moreover,the yield strength,tensile strength,yield to strength ratio and impact energy increased,and the elongation decreased gradually with the decrease of the finish cooling temperature.With the decrease of finish cooling temperature,the grain refinement effect was obvious,the proportion of grain boundary strengthening increased gradually,resulting in the increased yield strength.The fraction of high angle grain boundaries increased,the energy required for impact crack propagation increased,and the impact energy was improved.On the one hand,the decrease of finish cooling temperature reduces the fraction of M/A con-stituents,weakens the strain hardening ability,and leads to the increase of yield ratio.On the other hand,with the increase of supercooling,dislocation density increases and grain size decreases,and the resistance during dislocation slip increases,resulting in the decrease of elongation after fracture.For this experimental steel,the optimal tempera-ture range is 600-625℃.
Nb microalloyinghigh-strength weathering steelfinish cooling temperaturemicrostructuremechani-cal property
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