STD11강의 상변화 및 탄화물 석출에 미치는 열처리 조건의 영향

Abstract

STD11 steel widely used as cold press mold steels has been studied to investigate microstructural change with polishing conditions and phase change and the precipitates of the carbide with heat treatment conditions. The characteristics of microstructure and hardness was also examined with austenitizing temperature, cryogenic treatment conditions. The results obtained are as follows;

(1) The different microstructures were shown according to the polishing conditions and the etchant conditions. by electrolytic-polishing and etching in the Super picral solution for 12 sec, it could be confirmed that the microstructure was clearly distinguished with martensite and retained austenite structures. (2) Martensitic transformation that appears to determine (111)γ//(011)α crystal orientation as untransformed austenite and second phases was confirmed by using the above polishing conditions and the etchant conditions.

(3) As the austenitizing temperature increased, the carbides dissolved in the matrix increased but the hardness decreased because of the increased volume fraction of the retained austenite by stabilizing the austenite phase.

(4) The volume fraction of the retained austenite in STD11 steel containing carbides was determined by using the image analysis and X-ray diffraction methods. It was confirmed that both methods for measuring retained austenite small test error, we can use the image analysis method for measuring the volume fraction of retained austenite in the filed and the exact analysis of the mechanical properties is to be possible.

(5) The cryogenic treatment, which is immersed the specimen in the liquid nitrogen at -196 ℃ for 6 hours, removed the retained austenite by austenite → martensite transformation, but even if liquid nitrigen immersed time was increased from 12 to 24 hours, the volume fraction of the retained austenite was no longer a significant change. Also after quenching at 1,080 ℃ and austenitizing, the austenite remained since a large amount of the C atoms were employed in the matrix austenite. if we had the cryogenic treatment for 24 hours, the martensite transformation of retained austenite to martensite at this time was obtained and had a high hardness due to a large content of C atoms.

(6) When the tempering temperature was rised from 1,080 ℃, the hardness decreased, but when tempering at 500 ℃, the hardness was increased by the precipitates of carbides, I.e. the 2nd hardening.