Annealing method

A method for the manufacture of a steel sheet, having a thickness t, in a device having a pre-heating section, a heating section, having a maximal heating rate for a steel sheet having a thickness t and a soaking section, the method comprising:

• A) a calibration step wherein
• i) a steel sheet having a thickness t and the following chemical composition in weight percent:
• 0.05≤C≤0.50%, 0.3≤Mn≤8.0%, 0.01≤Si≤5%, and optionally at least one of the following elements, in weight percent:
• 0.01≤Al≤1.5%, B≤0.004%, Co≤0.1%, 0.001≤Cr≤1.00%, Cu≤0.5%, 0.001≤Mo≤0.5%, Nb≤0.1%, Ni≤1.0%, Ti≤0.1%, N≤0.01%, P<0.1%, S≤0.01%, V<0.2%, a remainder of the composition being made of iron and inevitable impurities, is heated from room temperature to a temperature T1 lower than 600° C.,
• ii) the steel sheet is heated from Ti to a recrystallisation temperature T2 in the range of 720° C. to 1000° C. at said maximal heating rate, in an atmosphere A1 including 0.1 to 90% by volume of H2, the balance being an inert gas and unavoidable impurities and having a dew point DPCAL,
• iii) the steel sheet is then maintained at a temperature T2, in an atmosphere A2 including 0.1 to 90% by volume of H2, the balance being an inert gas and unavoidable impurities and having a dew point of at least −40° C.,
• iv) the steel sheet is then hot-dip coated in a coating bath and a quality of the coating is assessed,
• v) a) if the coating quality meets a predefined quality target, repeating the calibration steps i) to iv) with a lower dew point DPCAL, until the coating quality no longer meets the target, a penultimate dew point DPCAL being defined as DP1,
• b) if said coating quality is not satisfactory, repeating the calibration steps i) to v) with a higher dew point DPCAL, until the coating quality meets the target, the ultimate DPCAL being defined as DP1;
• and
• B) a production step wherein a steel sheet with the thickness t and the chemical composition undergoes:
• a recrystallization annealing including successively a pre-heating step, a heating step, a soaking step and a cooling step wherein:
• i) the pre-heating step includes a heating from room temperature to a temperature T1 lower than 600° C.,
• ii) the heating step includes a heating from T1 to a recrystallisation temperature T2 in the range from 720° C. to 1000° C. at a heating rate being lower or equal to the maximal heating rate in an atmosphere A1 comprising 0.1 to 90% by volume of H2, the balance being an inert gas and unavoidable impurities and having a dew point set at least at the DP1 value determined during the calibration step,
• iii) the soaking step including a holding at a temperature in the range from T2-30° C. to T2+30° C., in an atmosphere A2 comprising 0.1 to 90% by volume of H2, the balance being an inert gas and unavoidable impurities and having a dew point DP2 set at −40° C. or more, and
• a coating step wherein the steel sheet is hot dip coated in the coating bath.