High-strength hot rolled steel sheet and method for producing the same, and high-strength electric resistance welded steel pipe and method for producing the same

An electric resistance welded steel pipe comprising a base metal zone and an electric resistance welded zone, wherein:

• in a steel microstructure of the base metal zone at a center of the base metal zone in a wall-thickness direction of the electric resistance welded steel pipe,
• a volume fraction of bainite is 50% or more,
• a total volume fraction of ferrite and bainite is 95% or more,
• with the balance being one or more selected from pearlite, martensite, and austenite,
• an average grain size is 9.0 μm or less, and
• a dislocation density is 2.0×1014 m−2 or more and 1.0×1015 m−2 or less;
• in a steel microstructure of the base metal zone at a position 0.1 mm below an inner surface of the high strength-electric resistance welded steel pipe in a depth direction of the steel pipe,
• a volume fraction of bainite is 70% or more,
• a total volume fraction of ferrite and bainite is 95% or more,
• with the balance being one or more selected from pearlite, martensite, and austenite,
• an average grain size is 9.0 μm or less,
• a dislocation density is 6.0×1014 m−2 or more and 1.0×1015 m−2 or less, and
• a maximum low angle grain boundary density is 1.5×106 m−1 or less;
• and
• a wall thickness of the base metal zone is 15 mm or more.