Patent ID: 7381478

Claim:
A hot-rolled steel sheet for processing and having a superior bake hardenability after aging, comprising: at least one portion which comprises, in terms of percent by mass, C of approximately 0.01% to 0.2%, Si of approximately 0.01 to 0.3%, Mn of at most approximately (1.5-Si) %, P of at most approximately 0.1%, S of at most approximately 0.03%, Al of approximately 0.001% to 0.1%, N of at most approximately 0.006%, and a remainder of Fe and unavoidable impurities, wherein a microstructure consists essentially of a main phase having a form of a polygonal ferrite and a hard second phase of martensite, wherein a volume fraction of the hard second phase is approximately 3% to 20%, a hardness ratio of a hardness of the hard second phase to a hardness of the polygonal ferrite is about 1.5 to 6, and a grain size ratio of a grain size of the polygonal ferrite to a grain size of the hard second phase is at least approximately 1.5, wherein a BH amount after aging is at least approximately 60 MPa, and wherein the hot-rolled steel sheet is manufactured by: i. rough-rolling a slab to obtain a rough rolled bar, the slab including, in terms of percent by mass, C of approximately 0.01% to 0.2%, Si of approximately 0.01 to 0.3%, Mn of at most approximately (1.5-Si) %, P of at most approximately 0.1%, S of at most approximately 0.03% Al of approximately 0.001% to 0.1%, N of at most approximately 0.006%, and a remainder of Fe and unavoidable impurities, ii. finish rolling the rough rolled bar to obtain a rolled steel under conditions in which a sum of reduction rates of a final stage and a stage prior to the final stage is at least approximately 25%, a reduction rate of the final stage is about 1% to 15%, and a finishing temperature is in a temperature range from approximately an Ar 3 transformation point temperature to an Ar 3 transformation point temperature +100° C., iii. maintaining the rolled steel in a temperature range from approximately below the Ar 3 transformation point temperature to at least the Ar 1 transformation temperature for approximately 1 second to 15 seconds, iv. cooling the rolled steel to a temperature of approximately 350° C. at a cooling rate of approximately at least 100° C./sec to obtain the hot-rolled steel sheet; and v. coiling the hot-rolled steel sheet at a temperature of below approximately 350° C.