미세조직 제어에 의한 고강도강의 강도 개선에 관한 연구

Abstract

Effects of coiling temperature, alloying elements i.e., C and Cr, and annealing conditions on the spheroidization of cementite have been examined in high strength Fe-(0∼1.45%)Cr-(0.38∼0.43%)C steels. In addition, resultant microstructure and mechanical property of those steels after austenitizing and quenching were investigated in connection with the spheroidization of cementite. The results obtained are as follows: (1) Spheroidization of cementite of the hot-rolled sheet depended upon coiling temperature; lamellar cementite in coarser pearlite colony was observed by raising coiling temperature but globular cementite was shown by lowering coiling temperature. (2) Lamellar cementite was sperodized by annealing above 640℃×10hr but the rate of spheroidization became lower with increasing Cr contents, though same annealing condition. (3) Banded structure was revealed owing to dendrite segregation in the sheet annealed above 640℃×5hr and thed quenched at 900℃×5min. The unfavorable structure leaded to the mixture of lath martensite and lower bainite due to inhomogeneity of solute atoms even at same cooling rate. (4) Comparing Vickers hardness values of the cold-rolled sheet after annealing and quenching with alloying elements(C or Cr), Vickers hardness values largely increased with increasing C content but those values were not altered even though Cr content increased. (5) When cold-rolled Fe-0.43%C-1.45%Cr alloy sheet was annealed at 640℃×5hr and then quenched at 900℃×5min, the maximum hardness of HV=740 was obtained while the microstructure revealed lath martensite and insoluble carbide.