Steel member and steel sheet

A steel member comprising, as a chemical composition, by mass %:

• C:
• 0.26% to 0.65%;
• Si:
• 0% to 2.00%;
• Mn:
• 0.05% to 3.00%;
• P:
• 0.050% or less;
• S:
• 0.0100% or less;
• N:
• 0.010% or less;
• O:
• 0.010% or less;
• Nb:
• 0.01% to 0.10%;
• Ti:
• 0.005% to 0.100%;
• Cu:
• 0.10% to 2.50%;
• Ni:
• 0% to 0.30%;
• Cr:
• 0% to 0.50%;
• B:
• 0% to 0.0100%;
• Mo:
• 0% to 1.00%;
• V:
• 0% to 1.00%;
• Ca:
• 0% to 0.010%;
• Mg:
• 0% to 0.010%;
• Al:
• 0% to 1.00%;
• Sn:
• 0% to 1.00%;
• W:
• 0% to 1.00%;
• Sb:
• 0% to 1.00%;
• Zr:
• 0% to 1.00%;
• Se:
• 0% to 1.00%;
• Bi:
• 0% to 1.00%;
• As:
• 0% to 1.00%;
• Ta:
• 0% to 1.00%;
• Re:
• 0% to 1.00%;
• Os:
• 0% to 1.00%;
• Ir:
• 0% to 1.00%;
• Tc:
• 0% to 1.00%;
• Co:
• 0% to 1.00%;
• REM:
• 0% to 0.30%;
• and
• a remainder:
• Fe and impurities,
• wherein, when a ¼ position of a thickness in a thickness direction from a surface is defined as a ¼ depth position,
• a metallographic structure at the ¼ depth position includes 90% or more of martensite by volume fraction,
• in the metallographic structure at the ¼ depth position, L49-56°/L4-12°, which is a ratio of a length L49-56° of grain boundaries having an angle of rotation of 49° to 560 to a length L4-12° of grain boundaries having an angle of rotation of 4° to 12° about a <011> direction as a rotation axis, among grain boundaries of grains having a body-centered structure, is 1.10 or more, and
• a tensile strength of the steel member is more than 1,500 MPa.