Manufacturing method of carbon steel sheet superior in formability
A carbon steel sheet having high formability due to a microscopic and uniform carbide distribution and having a good characteristic of final heat treatment, and a manufacturing method thereof. The carbon steel sheet having excellent formability, includes, in wt %, C at 0.2-0.5%, Mn at 0.1-1.2%, Si at less than or equal to 0.4%, Cr at less than or equal to 0.5%, Al at 0.01-0.1%, S at less than or equal to 0.012%, Ti at less than or equal to 0.5×48/14×[N]% when the condition of B(atomic %)/N(atomic %)>1 is satisfied or by 0.5×48/14×[N]% to 0.03% when the condition of B and N is not satisfied, B at 0.0005-0.0080%, N at less than or equal to 0.006%, Fe, and extra inevitable elements; an average size of carbide is less than or equal to 1 μm; and an average grain size of ferrite is less than or equal to 5 μm.
1. A manufacturing method of carbon steel sheet having excellent formability, the method comprising:
manufacturing a steel slab that comprises, in the unit of wt %, C at 0.2-0.5%, Mn at 0.1-1.2%, Si at less than or equal to 0.4%, Cr at less than or equal to 0.5%, Al at 0.01-0.1%, S at less than or equal to 0.012%, Ti at 0.5×48/14×[N] to 0.03%, B at 0.0005-0.0080%, N at less than or equal to 0.006%, Fe, and extra inevitable impurities;
reheating and hot finish rolling the slab at a temperature above an Ar 3 transformation temperature;
cooling a hot-rolled steel sheet manufactured by the hot finish rolling at a cooling speed in a range of 20° C./sec-100° C./sec;
coiling the cooled hot-rolled steel sheet at a temperature in a range of Ms (martensite transformation temperature) to 530° C.; and
annealing the hot-rolled steel sheet at a temperature range of 600° C. to Ac 1 transformation temperature;
wherein the carbon steel sheet microstructure in the as-hot-rolled and coiled condition comprises free ferrite and pearlite having a lamellar carbide structure at respectively greater than 0 vol % and less than or equal to 5 vol %, and bainite greater than or equal to 90 vol %; and
the carbon steel sheet in the as-annealed condition has an average particle size of carbide less than or equal to 1 μm and an average grain size of ferrite in the carbon steel sheet is greater than 0 μm and less than or equal to 5 μm.