high tensile strength steels that have both favorable delayed fracture resistance and a tensile strength of 600 mpa or higher and are suitably used in construction machinery , tanks , penstocks , and pipelines , as well as methods for manufacturing such steels are provided . the safety index of delayed fracture resistance is 100 ×, where x 0 : reduction of area of a specimen substantially free from diffusible hydrogen , and x 1 : reduction of area of a specimen containing diffusible hydrogen .