HIGH-RESISTANCE FREE SHELLED AND EXCELLENT HOT LAMINATED STEEL SHEET ON SURFACE PROPERTIES AND GRINDING PROPERTIES, AND PRODUCTION METHOD OF THE SAME

摘要

This hot rolled steel contains, in terms of mass %, C: 0.01 to 0.1%, Si: 0.01 to 0.1%, Mn: 0.1 to 3%, P: not more than 0.1%, S: not more than 0.03%, Al: 0.001 to 1%, N: not more than 0.01%, Nb: 0.005 to 0.08%, and Ti: 0.001 to 0.2%, with a remainder being iron and unavoidable impurities, wherein a formula: [Nb] × [C] ‰¤ 4.34 × 10 -3 is satisfied, a grain boundary density of solid solution C is not less than 1 atom/nm 2 and not more than 4.5 atoms/nm 2 , and a grain size of cementite grains precipitated at grain boundaries within the steel sheet is not more than 1 µm. This method for manufacturing a hot rolled steel sheet includes: heating a steel slab having the same composition as the above hot rolled steel sheet at a temperature that is not less than a temperature of SRTmin (°C) and not more than 1,170°C; performing rough rolling at a finishing temperature of not less than 1,080°C and not more than 1,150°C; subsequently starting finish rolling within not less than 30 seconds and not more than 150 seconds at a temperature of not less than 1,000°C but less than 1,080°C; completing the finish rolling within a temperature range from not less than an Ar 3 transformation point temperature to not more than 950°C so as to achieve a final pass reduction ratio of not less than 3% and not more than 15%; and conducting cooling at a cooling rate exceeding 15°C/sec from a cooling start temperature to a temperature within a range from not less than 450°C to not more than 550°C, and then coiling the steel sheet.