Effect of the Cr Content and Coiling Temperature on the Properties of Hot Rolled High Strength Lower Bainitic Steel

摘要

The effect of the Cr content and the coiling temperature on the tensile properties and the micostructure of an ultra high strength hot rolled lower bainitic steel has been evaluated by lab simulations. It was shown that an increase in Cr reduces the yield strength, but increases the tensile strength and the elongation. This observation was made for both coiling temperatures (425°C and 500°C) tested. For coiling at 500°C, the effect was directly attributed to the presence of an increasing fraction of secondary phases, mainly martensite-austenite islands. At low coiling temperature (425°C), a banded structure of granular and lower bainite occured. The granular bainite bands contained more martensite-austenite islands for the high Cr level, explaining the low yield strength, high tensile strength and high elongation. It was shown that the lower bainite forms prior to coiling, whereas the granular bainite forms during slow coil cooling. The yield and the tensile strength is higher for coiling at 425°C compared to 500°C as a consequence of a higher lower bainite fraction.