Method for producing a TWIP steel sheet having an austenitic microstructure

A method for manufacturing a recovered steel sheet having an austenitic matrix presenting at least one mechanical property (M) equal or above a target value Mtarget whose composition comprises, in weight:

• 0.1<C<1.2%,
• 13.0≤Mn<25.0%,
• S≤0.030%,
• P≤0.080%,
• N≤0.1%,
• Si≤3.0%,
• the remainder of the composition making up of iron and inevitable impurities resulting from development,
• the method comprising:
• A. calibrating, wherein the calibrating includes:
• I. performing heat treatments between 400 and 900° C. during 40 seconds to 60 minutes on at least 2 samples of said steel, the heat treatments having Pareq values P, Pareq being defined by an equation:
• Pareq=−0.67*log(∫−ΔH/RT)*dt),
• with ΔH:
• energy of diffusion of iron in iron, T=temperature of the heat treatment, the integration being over the heat treatment time, the heat treatments being different heat treatments carried out at different temperatures for different times,
• II. submitting said samples to X-ray diffraction so as to obtain spectrums including a main peak whose width at mid height FWHM is being measured,
• III. measuring a mechanical property (M) of said samples,
• IV. identifying a recovery or recrystallization state of each sample,
• V. drawing a curve of M as a function of FWHM in a domain where the samples are recovered from 0 to 100%, but not recrystallized,
• B. calculating, wherein the calculating includes:
• I. determining, from the curve, a value FWHMtarget corresponding to a target mechanical property Mtarget,
• II. determining a pareq value Ptarget of the heat treatment to perform to reach Mtarget based on the M and Pareq values P determined during the calibrating, and
• III selecting a time ttarget and a temperature T°target for the Ptarget value using the equation,
• C. feeding a recrystallized steel sheet having a Mrecrystallization,
• D. cold-rolling the recrystallized steel sheet in order to obtain a steel sheet having a Mcold-roll and
• E. annealing the cold rolled steel sheet at the selected temperature T°target for the selected time ttarget to achieve at least one mechanical property M equal or above the target value Mtarget.