Microstructure and Properties of Seamless Steel Pipe Gr.6 after Rolling and Normalizing

Abstract

The microstructure morphology， low-temperature performance， and corrosion resistance of hot-rolled and normalized condition Gr.6 steel pipes were analyzed using optical microscopy， scanning electron microscopy， transmission electron microscopy， mechanical testing， and corrosion testing. The results show that the low-temperature mechanical properties and corrosion resistance of the developed low-temperature seamless steel pipe Gr.6 in the hot-rolled and normalized states meet the standard requirements. The microstructure in the hot-rolled state is pearlite and ferrite， with uniform and fine grains， but the banded structure is obvious.The precipitated phase is mainly composed of TiN particles， which undergo a ductile brittle transition at -60 ℃. After 900 ℃ normalizing heat treatment， the grains become finer， with a grain size of 10 grade or above. The proportion of large angle grain boundary differences in the normalized state sample is higher than that in the hot-rolled state， and the density of the large angle interface also increases. The precipitate phase is mainly TiN and a small amount of VN particles， and no ductile brittle transition has occurred at -80 ℃.The corrosion behavior of the material was studied through polarization tests with fast and slow polarization scanning rates. Corrosion tests have shown that the stress corrosion sensitivity of normalized specimens is relatively low in electrochemical simulation solutions， while the stress corrosion sensitivity of hot-rolled specimens is relatively high. Therefore， the electrochemical corrosion performance of the normalized state is better than that of the hot-rolled state， and it is suitable for more harsh corrosion environments.

Keywords

•  low temperature resistance； sour corrosion resistance； ductile-brittle transition； electrochemical corrosion； seamless steel pipe gr.6