This invention is a low carbon, carbide-free, nanostructured bainitic steel with a yield strength exceeding 1.2 GPa. This was achieved through a two-stage isothermal transformation process, resulting in 0.87 volume fraction of bainitic ferrite, with nearly 50% of bainitic laths measuring below 100 nm. The transformation time is under 8 hours, making it viable for industrial applications. The issue of coarse microstructure was addressed, specifically the coalescence of bainitic plates that typically degrade strength in low carbon steels transformed at low temperatures, without compromising the steel's strength. The material achieved a yield strength of over 1.2 GPa, an ultimate tensile strength exceeding 1.5 GPa, and ductility greater than 18%. Additionally, the plain strain fracture toughness reached 82 MPa√m, and the impact energy was recorded at 31 J, surpassing previous developments in low carbon bainitic steel. This low carbon, carbide-free, nanostructured bainitic steel also offers improved weldability compared to existing high carbon bainitic steels.
High-strength nanostructured bainitic steels can be produced in high carbon steels, but the process is time-consuming (without the addition of costly alloys) and results in poor weldability and significant loss of mechanical properties. Although these steels exhibit very high strength, they fail to meet many commercial and anticipated applications. A potential solution lies in using low carbon bainitic steels, but these steels often develop a coarse microstructure, which compromises their strength.
- This innovative technology produces low carbon, carbide-free nanostructured bainitic steel with a strength exceeding 1.2 GPa through a multistep transformation process.
- This method achieves reasonable transformation time (within 8 hours), making the process suitable for industrial applications.
- It addresses the issue of bainitic plate coalescence, which can degrade the strength of low carbon steels transformed at low temperatures.
- It demonstrates an exceptional combination of mechanical properties, including strength, ductility, impact toughness, and fracture toughness.
- It offers improved weldability compared to existing high carbon bainitic steels.
A nanostructured, low carbon bainitic steel composed of the following components by mass: carbon (0.24 - 0.28%), manganese (1.8 - 2%), silicon (2 - 2.5%), nickel (1.5 - 1.8%), molybdenum (0.2 - 0.25%), chromium (0.2 - 0.25%), aluminium (0.2 - 0.25%), and cobalt (0.45 - 0.5%), with the balance being Iron and unavoidable impurities.
The method for preparing this nanostructured bainitic steel involves the following steps:
- Austenitizing the steel at 945-955°C for 15-20 minutes
- Performing a first stage of isothermal transformation at 350-360°C for 20-25 minutes
- Conducting a second stage of isothermal transformation at 250-255°C for at least 6 hours
The invention is at the stage of early prototype development and/or validation in relevant environment.
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A composite microstructure of the steel with high strength, ductility and toughness makes it suitable for pipe line alloys, railway lines, railway wheels, bearings, automobile bodies, wind turbine gear box etc.
It has utility in many industries such as railways, automotive, aerospace, military, heavy machinery and equipment industry and so on, as a composite microstructure of the steel with high strength, ductility and toughness makes it suitable for pipe line alloys, railway lines, railway wheels, bearings, automobile bodies, wind turbine gear box etc.
202021002211
540248