Hot-rolled steel sheet and weld joint, and methods for producing same

A hot-rolled steel sheet having a chemical composition comprising, in mass %:

• C:
• 0.02 to 0.20%,
• Si:
• 0.01 to 1.50%,
• Mn:
• 0.10 to 3.00%,
• P:
• 0.10% or less,
• S:
• 0.010% or less,
• Al:
• 0.005 to 0.100%,
• Ti:
• 0.02 to 0.20%,
• N:
• 0.001 to 0.010%,
• Cu:
• 0 to 0.50%,
• Ni:
• 0 to 0.50%,
• Cr:
• 0 to 1.00%,
• Mo:
• 0 to 0.40%,
• Nb:
• 0 to 0.060%,
• V:
• 0 to 1.00%,
• B:
• 0 to 0.0100%,
• Ca:
• 0 to 0.0050%,
• O:
• 0.0100% or less, and
• the balance:
• Fe and impurities,
• wherein:
• a steel micro-structure includes, in area %:
• ferrite:
• 60 to 80%, and
• a total of ferrite and bainite:
• 90% or more;
• an average of a crystal grain size of ferrite and bainite is 7.0 μm or less, and a standard deviation of the crystal grain size is 2.0 μm or less;
• a standard deviation of a diameter of Ti carbo-nitrides is 10 nm or less,
• a tensile strength is 780 MPa or more;
• a relation TS×λ0.5 between the tensile strength TS in MPa and a hole expansion ratio λ in % is 6000 MPa·%0.5 or more;
• and
• a Charpy impact absorbed energy at −40° C. is 150 kJ/cm2 or more.