Method for producing a steel sheet having excellent processability before hot forming

A process to manufacture a steel sheet having the following chemical composition comprising, by weight percent:

• C:
• 0.26-0.40%
• Mn:
• 0.5-1.8%
• Si:
• 0.1-1.25%
• Cr 0.1-1.0%
• Al 0.001-0.1%
• Ti 0.01-0.1%
• B 0.001-0.004%
• P≤0.020%
• S≤0.010%
• N≤0.010%
• and one or more of the following optional elements:
• Ni≤0.5%
• Mo≤0.40%
• Nb≤0.08%
• Ca≤0.1%
• a remainder of the composition being iron and unavoidable impurities resulting from processing, the impurities reaching up to the following levels, according to the process employed:
• Sb≤0.03%
• As≤0.03%
• Pb≤0.03%
• Sn≤0.010%
• wherein Q is less than 20, Q being defined as:
• Q=114−68*C−18*Mn+20*Si−56*Cr−61*Ni−37*Al+39*Mo+79*Nb−17691*B, wherein all elements are expressed in weight percent,
• the process comprising the steps of:
• reheating a cast slab having the composition to a temperature Treheat from 1100° C. to 1300° C. before being hot rolled at a finishing hot rolling temperature between 800° C. and 950° C. to obtain a hot rolled steel sheet;
• cooling and coiling the hot rolled steel sheet at a temperature Tcoil lower than 670° C. and optionally pickling said hot rolled steel sheet;
• cold rolling the hot rolled steel sheet to obtain a cold rolled steel sheet, applying a reduction ratio ranging from 20% to 80%;
• annealing the cold rolled steel sheet using the following annealing line process parameters:
• −K<0.50 wherein K is defined by
• If the steel composition is such that Mn−Si in weight %>1.5 weight %:
• K
• =
• Tsoaking
• -
• Ae
• ⁢
• 1
• Ae
• ⁢
• 3
• -
• Ae
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• 1
• *
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• (
• trex
• )
• 5
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• (
• 1
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• 0
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• (
• heating
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• rate
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• Mn
• 0
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• 6
• -
• Si
• 1
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• 2
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• *
• 0
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• 0
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• 3
• If the steel composition is such that Mn−Si≤1.5 weight %:
• K
• =
• Tsoaking
• -
• Ae
• ⁢
• 1
• Ae
• ⁢
• 3
• -
• Ae
• ⁢
• 1
• *
• ln
• ⁢
• (
• trex
• )
• 5
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• (
• 1
• +
• 0
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• 1
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• ln
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• (
• heating
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• rate
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• And wherein:
• Tsoaking is the soaking temperature expressed in ° C., and a maximum temperature reached by the steel sheet during the annealing process;
• trex is the recrystallisation time expressed in seconds, defined as being the time spent above 700° C. during the annealing process;
• heating rate is the speed at which the steel sheet reaches the soaking temperature expressed in ° C./s, so that heating rate=(Tsoaking−30)/(time spent between 30° C. and T soaking
• Ae1=734+77*C−29*Mn+14*Si+7*Cr−38*Ni−22*Al−25*Mo+123*Nb−8621*B, wherein all elements are expressed in wt %
• Ae3=935−257*C−25*Mn+32*Si−17*Cr−83*Ni+17*Al+129*Mo+156*Nb−19473*B, wherein all elements are expressed in wt %.