| 摘要 |
The invention provides a high tensile strength steel material having a tensile strength of 600 MPa, which is excellent in delayed fracture resistance property, and a method of manufacturing the steel material. As means for this, a steel material contains, in mass percent, C of 0.02 to 0.25%, Si of 0.01 to 0.8%, Mn of 0.5 to 2.0%, Al of 0.005 to 0.1%, N of 0.0005 to 0.008%, P of 0.03% or less, and S of 0.03% or less. In addition, the steel material contains at least one element selected from Mo, Nb, V, and Ti, and contains at least one of Cu, Ni, Cr, W, B, Ca, REM and Mg, as needed. The remainder includes Fe and inevitable impurities. In addition, in the steel material, precipitates having an average grain size of 20 nm or less, which contains at least one of Mo, Nb, V and Ti, are contained in steel in the number of at least 5 per 250000 nm2, and a microstructure includes residual austenite in a volume fraction of 0.5 to 5%. When Ca to be added is specified to be 0.0010% to 0.0020%, it is specified that S is 0.0005% to 0.0020% and O is 0.0008% to 0.0025%. ACR is specified to be 0.2≰ACR(=(Ca−(0.18+130*Ca)*O)/1.25/S)≰1.0. |
