Rapid cooling of high yield strength sheet steel

A method for reducing unevenness defects of a strip subjected to cooling by spraying a fluid along a cooling zone of a continuous heat treatment line, said cooling zone having means for adjusting a cooling intensity along the cooling zone, the method comprising:

• determining, before the strip is subjected to the cooling along the cooling zone, a thermal profile to be applied to the strip along the cooling zone to cause, at a first portion of the strip, substantial concomitance between (i) a predetermined critical strip temperature, called Leidenfrost temperature, and (ii) a first temperature at which a change in metallurgical structure of the strip is initiated;
• cooling, at a first cooling intensity, at a first point along the cooling zone, and according to the thermal profile, the first portion of the strip to the predetermined critical strip temperature;
• cooling, at a second cooling intensity, at a second point downstream of the first point along the cooling zone, and according to the thermal profile, the first portion of the strip to the first temperature;
• and
• adjusting the second cooling intensity at the second point, including adjusting one or more of a flow rate of the fluid, a flow pressure of the fluid, and an amount of time the fluid is sprayed onto the strip at the second point;
• wherein the thermal profile is determined by:
• a minimum cooling rate to achieve desired metallurgical structure changes;
• a metallurgical transformation temperature for achieving desired metallurgical structure changes;
• thermal critical points for achieving the Leidenfrost temperature;
• and
• conditions for initiating adjusting the thermal profile.