Critical Cooling Rate - The minimum rate of continuous cooling just sufficient to prevent undesired transformations. For steel, the slowest rate at which it can be cooled form above the upper critical temperature to prevent the decomposition of austenite at any temperature above the Ms. The limiting rate at which austenite must be cooled to ensure that a particular type of transformation product is formed.
Critical cooling rate is slowest cooling rate that converts 100% austenite to 100% martensite