EFFECT OF COILING TEMPERATURE ON MICROSTRUCTURE AND MECHANICALPROPERTIES OF QStE550TM STEEL
With the same finishing parameters,QStE550TM steel was studied by using optical microscope(OM),scanning electron microscope(SEM),and tensile test machine at different coiling temperatures.The results showed that the change of the coiling temperature could change the grain size of microstructure,morphology,amount and size of precipitates in QStE550TM steel,and furtherly affect the mechanical properties of QStE550TM steel.When the coiling temperature is 600 ℃,the matrix structure of the test steel is mainly ferrite and pearlite.When the coiling temperature is 560 ℃ and 580 ℃,the matrix structure of the test steel is mainly ferrite and bainite.The result is slightly different from the calculated TTT curve.In addition,as the coiling temperature increases from 560 ℃ to 600 ℃,the grain size of the matrix decreases gradually,and the effect of fine grain strengthening decreases.However,as the number of precipitated phase increases gradually,the precipitation strengthening effect will be enhanced,and the size of precipitated phase will gradually grow,which will weaken the precipitation strengthening effect.Accordingly,under the game of precipitation strengthening and fine grain strengthening,the effect of fine grain strengthening is greater than that of precipitation strengthening at 560 ℃,and the comprehensive strengthening effect is poor.When the coiling temperature is 600 ℃,the effect of precipitation strengthening is greater than that of fine crystal strengthening,and the comprehensive strengthening effect is also poor.When the coiling temperature is 580 ℃,the two strengthening effects reach a balance,and the yield strength,tensile strength and elongation of the test steel QStE550TM meet the target requirements.
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