Effect of Rare Earth on Hardness and Microstructure Evolution of Copper-Bearing Steel During Isothermal Aging
The copper-containing HSLA steels with different La contents were quenched at 900 ℃ and isother-mal aging heat treatment at 550 ℃.The microstructure and precipitation phase of the tested steel were observed by an optical microscope,a field emission scanning electron microscope and a transmission electron microscope.Combined with the hardness test results,the hardness change trend and microstructure evolution were explored.The influence of La on the hardness,microstructure and precipitation of the tested steel was discussed.The re-sults show that with the prolongation of aging time,the hardness of the tested steel increases firstly and then de-creases.The hardness of the tested steel increases firstly and then decreases with the increase of La content.Adding 0.005％La can significantly improve the hardness of the test steel.The quenched microstructure of the tested steel is a mixture of martensite and bainite,and the microstructure after isothermal aging is tempered mar-tensite;La can significantly refine the lath structure.The Cu-rich precipitates in the tested steel without rare earth are diffusely distributed on the matrix,and the Cu-rich precipitates in the tested steel with rare earth are mainly precipitated at the interface of dislocations and laths.The final hardness of the tested steel is determined by synergistic effects of microstructure and copper-rich precipitates.
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