CCT diagram Research Papers - Academia.edu (original) (raw)

Continuous cooling transformation (CCT) diagrams and properties of four kinds of low-silicon C-Mn-Si-Al transformation-induced plasticity (TRIP) steels with different carbon contents, with or without microalloy element Ti/V, as well as a reference TRIP steel containing 1.19 wt.% Si were studied. The CCT diagrams exhibited that as the carbon equivalent (CE) increased, it caused a shift of the ferrite forming and pearlite forming temperatures to the right side and the bainite forming and martensite forming to the lower temperatures of the diagram. The microstructural evolution obtained from the dilatometry samples revealed that the highest cooling rates produced fully martensitic microstructure in all cases except the reference TRIP steel. As the cooling rate decreased, more ferrite and bainite were formed. The increase of CE caused the increase of the amount of martesite in the microstructure. Tensile test and Erichsen test of the investigated steels showed an excellent mechanical strength and ductility combination, with tensile strength between 800 and 1000 MPa, total elongation of around 20%, and a quite good formability with a dome height of about 10 mm in all cases.

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- Materials Engineering, Mechanical Engineering, Microstructure, Ductility

The phase transformation kinetics under continuous cooling conditions for intercritical austenite in a cold rolled low carbon steel were investigated over a wide range of cooling rates (0.1–200 °C/s). The start and finish temperatures of the intercritical austenite transformation were determined by quenching dilatometry and a continuous cooling transformation (CCT) diagram was constructed. The resulting experimental CCT diagram was compared with that calculated via JMatPro software, and verified using electron microscopy and hardness tests. In general, the results reveal that the experimental CCT diagram can be helpful in the design of thermal cycles for the production of different grades of dual-phase–advanced high-strengh steels (DP-AHSS) in continuous processing lines. The results suggest that C enrichment of intercritical austenite as a result of heating in the two phases (ferrite–austenite) region and C partitioning during the formation of pro-eutectoid ferrite on cooling significantly alters the character of subsequent austenite phase transformations.

This paper considers the issues related to plotting of continuous cooling transformation (CCT) phase diagram based on the dilatometric test results. The numerical data processing algorithm for uniformly analysis of the dilatometric curves is developed and implemented. Computing of the phase transformation ranges and coefficient of thermal expansion (CTE) were made in Microsoft Excel. Numerical algorithm contains determination of the critical points as the deviation dots from linear approximation function of the monotonous segment of experimental curve which is extrapolated to the start (or finish) of the phase transformation. Method of CCT diagram plotting based on the quantitative analysis of CTE during austenite continuous cooling is proposed. CTE quantitative evaluation of austenite and its decomposition products were accomplished by the analysis of linear approximation function of monotonous segment of dilatometric curve. Dependence of the CTE values on the range of cooling rates were used to define the phase transformation ranges on the CCT diagram. Obtained CTE dependence clearly defines critical cooling rate (ССR) as an intersection point of functions, describing austenite CTE changes with cooling rates and austenite-ferrite mixture. Point of intersection was found by solving the system of functions equations. Confirmation and clarification of results, based on the numerical analysis of the dilatometric curves, could be done as usual, using metallographic analysis and microhardness testing. The application of the developed numerical algorithm for dilatometric curves provides opportunity to unify dilatometric test analysis and to improve the accuracy of the CCT diagram plotting.

- by Anton Tsvetkov
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- CCT diagram