Improved properties of Mn series ultralow carbon steel with ultrafine bainitic structure

Abstract

In order to reduce the cost of alloying elements, Mn series ultralow carbon bainitic (ULCB) steels have been developed. The different cooling conditions have been used to improve the mechanical properties of Mn series ULCB steels in the present study. The hot rolled ULCB steel plates with 26 mm thickness were water quenched to different finishing quenching temperatures (550, 450 and 350°C) followed by air cooling. Results show that the as rolled ULCB steel plates exhibit high yield strength of 775 MPa and low energy transition temperature of −55°C when the finishing quenching temperature is chosen at 450°C. Low temperature impact toughness was improved through control of the finishing quenching temperature. Microstructure observations show that a fine granular bainite structure is obtained, and the granular bainite structure exhibited shorter crack propagation path than both granular ferrite and lath martensite structures, which contributed to the improved combination of strength and toughness. Furthermore, the formation process of fine granular bainite structure was investigated.

Keywords:

• Bainite steels
• Cooling condition
• Mechanical properties
• Microstructure control
• Granular bainite
• Brittle cleavage fracture

The present research was supported by The Science and Technology Project of Beijing, China (grant no. D08050303450804).