Microstructures and Mechanical Properties of High Strength Low Carbon Bainitic Steel

Abstract

Based on TMCP and UFC technology, the microstructures and mechanical properties of 0.05% C bainitic steel were studied in this paper. The bainite morphology and precipitation within bainite lath were observed by SEM and TEM, and the mechanical properties of bainitic steel were measured by tensile and impact test. The results showed that the yield and tensile strengths of steel were 713 MPa and 891 MPa respectively, and the elongation was 15.8% with impact energy of 95J at the temperature of-20°C as the final cooling temperature in hot rolling of 550°C. For comparison, the steel obtained the yield strength of 725 MPa, tensile strength of 930 MPa and elongation of 18% as the final cooling temperature of 450°C. However, the impact energy of steel was 195J at the temperature of-20°C. While at the same final cooling temperature of 450°C, the fast cooling-holding temperature-fast cooling was applied to experimental steel with a faster cooling rate of 50°C/s, hence the steel acquired the yield strength of 845 MPa, tensile strength of 1037 MPa, and elongation of 15.5% with impact energy of 168J at the temperature of-20°C. The strength and toughness of 0.05%C bainitic steel is related to the bainite morphology and precipitation distribution. Hence, the strength and toughness can be improved by control the different cooling processes for adjusting the content of lath bainite, distribution of granular bainite and precipitation.

Influence of Chromium Content on the Microstructure and Mechanical Properties of Thermomechanically Hot-Rolled Low-Carbon Bainitic Steels Containing Niobium

“…Different alloying elements like Cr, Nb, Mo, and B are used to improve the strength and toughness through promoting the formation of bainite and refining the microstructure [3]. Bainitic ferrite and granular bainite represent the two main microstructure constituents in low-carbon bainitic steels and they are associated with distinct mechanical properties [4][5][6]. Cr is widely used to improve the hardenability and thereby promote the formation of bainite by suppressing the formation of high temperature transformation products [7][8][9][10][11][12][13][14].…”

Influence of Chromium Content and Prior Deformation on the Continuous Cooling Transformation Diagram of Low-Carbon Bainitic Steels

“…Typical LSCM and SEM micrographs of the transformed products, without and with deformation, for the cooling rates 2, 10 and 60 °C/s are shown in Fig. (3)(4)(5)(6)(7)(8).…”

“…There are three main types of bainite that differ in respect of the morphology of the ferritic component and the type and distribution of the second phase: I) granular bainite with irregular ferrite; II) upper bainite with lath-like ferrite and the second phase on lath boundaries (BF); and III) lath-like or plate-like lower bainite with cementite within the ferrite plates or laths [4]. Granular bainite and lath-like bainitic ferrite represent the two main types of microstructure encountered in low-carbon bainitic steels and they are associated with distinct mechanical properties [5][6][7].…”