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Transformation Induced Plasticity "TRIP" Steel
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=== Processing method === [[File:TRIPSteel Phasediagram.JPG|thumb|Fig 4: Iron carbon phase diagram showing the intercritical anealing process]] [[File:TRIPSteel Carbonconcentrationgraph.JPG|thumb|Fig 5: Diagram showing the carbon concentration in the bainitic ferrite phase and the austenite phase]] In order to produce a strong and ductile TRIP steel, an intercritical annealing process is used to obtain the correct phase distribution.<ref name="[5]">E. Emadoddin & Al. "Effect of cold rolling reduction and intercriticalannealing temperature on the bulk texture oftwo TRIP-aided steel sheets", Journal of Materials Processing Technology 203, 293-300, 2008.</ref> During intercritical annealing, the steel is brought to a temperature above the eutectoid{{W|eutectoid}}, where the material is composed of a solid austenite phase and a solid ferrite phase. The austenite phase is a high temperature solid phase which only exists in equilibrium at temperatures above 727 degrees Celsius.<ref name="[6]">William D. Callister, "Materials Science and Engineering An Introduction", 7th edition, Wiley, 2007. p.292</ref> The material is then isothermally cooled at a temperature of approximately 400 degrees Celsius,<ref name="[4]">S. Chatterjee & Al., "Delta TRIP steel", Materials Science and Technology, Vol 23 No 7,819-827, 2007.</ref> in order to allow the austenite to form a banitic ferrite phase. During the eutectoid transformation, excess carbon is produced by the formation of the low carbon ferrite phase. In a typical steel alloy, the excess carbon would form a high carbon cementite phase. However, the silicon and aluminium prevent the formation of cementite. In consequence, the excess carbon diffuses to the remaining austenite phase. In order to obtain the correct microstructure, it is important that the isothermal transformation be completed at a temperature where the formation of bainitic ferrite is slow enough to allow the carbon to diffuse to the austenite. The carbon enriched austenite phase eventually reaches a high enough carbon content that it is stable at room temperature.<ref name="[8]">Qiang Liu & Al., "Research and development of 780 MPa cold rolling TRIP-aided steel", International Journal of Minerals, Metallurgy and Materials, Vol 16, Num 4, 399-406, 2009.</ref> The result of the intercritical annealing process is a material composed primarily of ferrite, and bainite{{W|bainite}} formed from the austenite phase during intercritical annealing, as well as dispersed retained austenite, and martensite phases. The grain microstructure can be seen in Figure 2 which shows a schematic of the phases, and Figure 3 which shows a micrograph taken with a scanning electron microscope{{W|scanning electron microscope}}. Figure 4 depicts the intercritical annealing process on the iron phase diagram. Figure 5 demonstrates the carbon concentration of the ferrite and austenite phases during the intercritical annealing process.
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