High-strength steel sheet, strength member for vehicles using the same, and method for producing strength member for vehicles

A high-strength steel sheet comprising:

• a metallic structure consisting of a ferrite phase and a hard second phase dispersed in the ferrite phase;
• the hard second phase in the metallic structure having an area ratio of 30 to 70%;
• and
• the ferrite, of which grain sizes are not more than 1.2 μm, having an area ratio of 57 to 90% in the ferrite phase,
• wherein ds is an average grain size of the ferrite of which grain sizes are not more than 1.2 μm, dL is an average grain size of ferrite of which grain sizes are more than 1.2 μm, and ds and dL satisfy the following equation (1):
• dL/ds>3 (1), and
• in a stress-strain diagram obtained by a tensile test of the steel, gradient of stress on the stress-strain diagram, dσ/dε in 3 to 7% of true strain is 5000 MPa or more.