High strength galvannealed steel sheet and method of producing such steel sheet

A method of producing a high strength galvannealed steel sheet, comprising:

• providing a hot rolled steel sheet,
• subjecting the hot rolled sheet to pickling,
• batch annealing the pickled sheet at a temperature of 450-620° C. for a time of 5-20 hours,
• cold rolling the annealed steel sheet to obtain a thickness reduction of at least 50%,
• annealing the cold rolled steel sheet in a continuous annealing line at a temperature of 780-870° C. for a time of 30-120 seconds,
• cooling the steel sheet to a temperature of 350-460° C.,
• optionally, reheating the steel sheet to 400-500° C.,
• hot dip galvanizing the steel sheet,
• galvannealing the sheet at a temperature of 500-600° C., and
• cooling the galvannealed steel sheet to ambient temperature,
• wherein the galvannealed steel sheet has:
• a) a steel composition consisting of the following elements (in wt. %):
• C
• 0.10-0.2,
• Mn
• 2.0-3.0,
• Si
• 0.2-0.5,
• Cr
• 0.1-0.7,
• Ti
• 0.01-0.07,
• Al
• ≤0.2,
• Nb
• <0.05,
• Mo
• <0.1,
• optionally,
• B
• 0.001-0.005,
• and
• balance Fe apart from impurities,
• b) a multiphase microstructure comprising (in vol. %):
• retained austenite
• 5-20,
• martensite
• 5-25,
• bainitic ferrite
• ≤10,
• polygonal ferrite
• ≤10,
• balance,
• bainite + tempered martensite
• 50-90,
• c) a tensile strength (Rm) 1180-1300 MPa, and
• optionally,
• at least one of the following mechanical properties:
• a yield strength (Rpo.2)
• 800-970 MPa,
• an elongation (A50)
• ≥8%, and
• an elongation (A80)
• ≥6%,
• and
• d) a bendability according to a 90° V Bend test value Ri/t of ≤4 for a sample having the size of 35 mm×100 mm, wherein Ri is the bending radius in mm and t is the thickness in mm of the steel sheet.