| 摘要 |
This hot-rolled steel sheet has a thickness of 3-20 mm and contains specific amounts of C, Si, Mn, P, S, Al and N with the balance made up of iron and unavoidable impurities. The contents of solid-solved N, C and N are within specific ranges, and bainitic ferrite having a specific average crystal grain size and pearlite have specific area occupancies in the structure, with the balance occupied by polygonal ferrite. This hot-rolled steel sheet has a specific hardness distribution in the thickness direction. |
| 主权项 |
1. A hot-rolled steel sheet excellent in heavy cold workability and surface hardness after working, wherein:
a sheet thickness is from 3 to 20 mm; a component composition comprises, in mass %, C: more than 0% and 0.3% or less, Si: more than 0% and 0.5% or less, Mn: from 0.2 to 1%, P: more than 0% and 0.05% or less, S: more than 0% and 0.05% or less, Al: from 0.01 to 0.1%, and N: from 0.008 to 0.025%, with the remainder being iron and unavoidable impurities, wherein solute N: 0.007% or more and the contents of C and N satisfy the relationship 10C+N≦3.0; a microstructure comprises, in terms of area ratio relative to an entire microstructure, bainitic ferrite: 5% or more, pearlite: less than 20%, and remainder: polygonal ferrite; an average grain size of the bainitic ferrite is in a range of from 3 to 50 μm; and (Hvmax−Hvmin)/Hvmin≦0.3, wherein, in a hardness distribution in a thickness direction, Hvmax and Hvmin≦0.3, are respectively the maximal value and the minimal value of Vickers hardness values of three portions that are a surface portion, a t/4 portion where t is the sheet thickness, and a central portion. |
