New generation thermo-mechanical control process（TMCP） based on ultra-fast cooling is being widely adopted in plate mill to product high-performance steel material at low cost. Ultra-fast cooling system is complex because of optimizing the temperature control error generated by heat transfer mathematical model and process parameters. In order to simplify the system and improve the temperature control precision in ultra-fast cooling process, several existing models of case-based reasoning（CBR） model are reviewed. Combining with ultra-fast cooling process, a developed R5 CBR model is proposed, which mainly improves the case representation, similarity relation and retrieval module. Certainty factor is defined in semantics memory unit of plate case which provides not only internal data reliability but also product performance reliability. Similarity relation is improved by defined power index similarity membership function. Retrieval process is simplified and retrieval efficiency is improved apparently by windmill retrieval algorithm. The proposed CBR model is used for predicting the case of cooling strategy and its capability is superior to traditional process model. In order to perform comprehensive investigations on ultra-fast cooling process, different steel plates are considered for the experiment. The validation experiment and industrial production of proposed CBR model are carried out, which demonstrated that finish cooling temperature（FCT） error is controlled within±25℃ and quality rate of product is more than 97%. The proposed CBR model can simplify ultra-fast cooling system and give quality performance for steel product.
Bainite is metastable due to its high dislocation density, and consequently bainitic steel structures have the problem of thermal stability. Plastic deformation of bainite can further increase dislocation density and change dislo- cation configuration at the same time. The influence of plastic deformation on thermal stability of low carbon bainitic steel during isothermal holding at 650 ℃ was investigated with hardness analysis, in-situ tracing metallographic analysis and transmission electron microscopy, Bainite in the low carbon steel evolves into polygonal ferrite via recov- ery and reerystallization during isothermal holding at 650 ℃. There is a considerably long period （about 20 h） be tween end of recovery and commencement of recrystallization of undeformed bainite, in which the hardness of the sample maintains a constant value slightly lower than that before reheating. Slight plastic deformation of bainite in- duces rearrangement of pre existing dislocations and forming of low-energy dislocation cells inside bainite laths, which has little influence on thermal stability of bainite, whereas heavy plastic deformation results in obvious dislocation multiplication and accelerates recrystallization of bainite. Recrystallization of heavily-deformed bainite occurs preferen tially at prior austenite grains boundaries. The samples subjected to heavy torsion exhibit obviously higher thermal stability than the samples subjected to heavy compression despite their same initial hardness, which can be attributed to different influences of torsion and compression on dislocations and boundaries of bainite.
