Steel sheet

A steel sheet comprising:

• a chemical composition including, by mass %,
• C:
• 0.07% or more and 0.15% or less,
• Si+Al:
• 0.20% or more and 2.50% or less,
• Mn+Cr:
• 1.20% or more and 4.00% or less,
• P:
• 0% or more and 0.040% or less,
• S:
• 0% or more and 0.010% or less,
• N:
• 0% or more and 0.010% or less,
• O:
• 0% or more and 0.006% or less,
• Mo:
• 0% or more and 0.50% or less,
• Ti:
• 0% or more and 0.20% or less,
• Nb:
• 0% or more and 0.20% or less,
• B:
• 0% or more and 0.010% or less,
• V:
• 0% or more and 0.50% or less,
• Cu:
• 0% or more and 1.00% or less,
• W:
• 0% or more and 0.10% or less,
• Ta:
• 0% or more and 0.10% or less,
• Ni:
• 0% or more and 1.00% or less,
• Sn:
• 0% or more and 0.050% or less,
• Co:
• 0% or more and 0.50% or less,
• Sb:
• 0% or more and 0.050% or less,
• As:
• 0% or more and 0.050% or less,
• Mg:
• 0% or more and 0.050% or less,
• Ca:
• 0% or more and 0.040% or less,
• Y:
• 0% or more and 0.050% or less,
• Zr:
• 0% or more and 0.050% or less,
• La:
• 0% or more and 0.050% or less, and
• Ce:
• 0% or more and 0.050% or less,
• wherein a remainder includes Fe and impurities,
• a metallographic structure in a thickness ¼ portion includes, by volume percentage,
• ferrite:
• 10% or more and less than 50%,
• granular bainite:
• 5% or more and less than 40%,
• martensite:
• 30% or more and 55% or less,
• upper bainite and lower bainite:
• less than 30% in total,
• pearlite:
• less than 10%, and
• residual austenite:
• less than 5%,
• a proportion of the number of the martensites adjacent to the ferrite to the number of the metallographic structures adjacent to the ferrite is 30% or less in the thickness ¼ portion, and
• a difference between an absolute maximum value and an absolute minimum value of Vickers hardness at a load of 50 gf obtained at an interval of 30 μm from a position of a depth of 100 μm in a sheet thickness direction from a surface of the steel sheet as a starting point to a thickness middle portion position is 60 HV or less,
• wherein the metallographic structure is identified by observing a 100 μm×100 μm region of a first cross section of the steel sheet parallel to a rolling direction and perpendicular to the sheet surface at a magnification of 1000 to 50000 times by electron back scattering diffraction (EBSD), X-ray measurement, corrosion using a nital reagent or a LePera solution, and a scanning electron microscope, and
• wherein the proportion of the number of the martensites adjacent to the ferrite to the number of the metallographic structures adjacent to the ferrite is calculated by observing a 100 μm×100 μm region of a second cross section of the steel sheet parallel to the rolling direction and perpendicular to the sheet surface in the thickness ¼ portion using EBSD at a step interval of 0.2 μm.