High strength thick steel plate superior in crack arrestability
The present invention provides high strength thick steel plate superior in crack arrestability high in strength, free of deterioration of HAZ toughness, and free of anisotropy, that steel plate containing, by mass %, C: 0.03 to 0.15%, Si: 0.1 to 0.5%, Mn: 0.5 to 2.0%, P: ≦0.02%, S: ≦0.01%, Al: 0.001 to 0.1%, Ti: 0.005 to 0.02%, Ni: 0.15 to 2%, and N: 0.001 to 0.008% and having a balance of iron and unavoidable impurities as chemical components, having a microstructure of a ferrite and/or pearlite structure with bainite as a matrix phase, and having an average circle equivalent diameter of crystal grains with a crystal misorientation angle of 15° or more of 15 μm or less in the regions of 10% of plate thickness from the front and rear surfaces and of 40 μm or less in the other region including the center part of plate thickness.
1. High strength thick steel plate containing, by mass %, C: 0.03 to 0.15%, Si: 0.1 to 0.5%, Mn: 0.5 to 2.0%, P: ≦0.02%, S: ≦0.01%, Al: 0.001 to 0.1%, Ti: 0.005 to 0.02%, Ni: 0.15 to 2%, and N: 0.001 to 0.008% and having a balance of iron and unavoidable impurities as chemical components, having a microstructure of a ferrite and/or pearlite structure with bainite as a matrix phase, and having an average circle equivalent diameter of crystal grains with a crystal misorientation angle of 15° or more of 15 μm or less in the regions of 10% of plate thickness from the front and rear surfaces and of 40 μm or less in the other region including the center part of plate thickness.
2. High strength thick steel plate superior in crack arrestability as set forth in claim 1 characterized by further containing, by mass %, one or more of Cu: 0.1 to 1%, Cr: 0.1 to 1%, Mo: 0.05 to 0.5%, Nb: 0.005 to 0.05%, V: 0.02 to 0.15%, and B: 0.0003 to 0.003% as chemical components.
3. High strength thick steel plate superior in crack arrestability as set forth in claim 1 characterized by further containing, by mass %, one or more of Ca: 0.0003 to 0.005%, Mg; 0.0003 to 0.005%, and REM: 0.0003 to 0.005% as chemical components.
4. High strength thick steel plate superior in crack arrestability as set forth in any one of claims 1 to 3 wherein {100} planes forming an angle of ±15° with respect to a plane vertical to loading direction have an area ratio of 30% or less in said regions of 10% of plate thickness from the front and rear surfaces.
5. High strength thick steel plate superior in crack arrestability as set forth in any one of claims 1 to 3 wherein said {100} planes forming an angle of 15° with respect to the plane vertical to loading direction have an area ratio of 15% or less in said regions including the center part of plate thickness other than said regions of 10% of plate thickness from the front and rear surfaces.
6. High strength thick steel plate superior in crack arrestability as set forth in any one of claims 1 to 3 wherein the plate thickness is 40 mm or more.
7. High strength thick steel plate superior in crack arrestability as set forth in any one of claims 1 to 3 wherein the yield stress is 390 MPa or more.