Steel sheet for can having high strength and high formability, and method for manufacturing the same

Abstract

A steel sheet is provided containing C: 0.070% or more and less than 0.080%, Si: 0.003% or more and 0.10% or less, Mn: 0.51% or more and 0.60% or less, wherein the average grain size is 5 μm or more, the grain elongation rate is 2.0 or less, the difference in hardness is 10 points or more and/or 20 points or more depending on how it is determined, the tensile strength is 500 MPa or more, and the elongation after fracture is 10% or more.

1 . A high-strength high-formability steel sheet for a can, the steel sheet comprising C: 0.070% or more and less than 0.080%, Si: 0.003% or more and 0.10% or less, Mn: 0.51% or more and 0.60% or less, P: 0.001% or more and 0.100% or less, S: 0.001% or more and 0.020% or less, Al: 0.005% or more and 0.100% or less, N: 0.010% or less, and the remainder composed of Fe and incidental impurities, on a percent by mass basis, wherein in a cross-section in the rolling direction, the average grain size is 5μm or more, the grain elongation rate is 2.0 or less, the difference in hardness determined by subtracting an average Vickers hardness of a cross-section between the surface and a depth of one-eighth of the sheet thickness from an average Vickers hardness of a cross-section between a depth of three-eighths of the sheet thickness and a depth of four-eighths of the sheet thickness is 10 points or more and/or the difference in hardness determined by subtracting a maximum Vickers hardness of a cross-section between the surface and a depth of one-eighth of the sheet thickness from a maximum Vickers hardness of a cross-section between a depth of three-eighths of the sheet thickness and a depth of four-eighths of the sheet thickness is 20 points or more, the tensile strength is 500 MPa or more, and the elongation after fracture is 10% or more. 
     
     
         2 . The high-strength high-formability steel sheet for a can, according to  claim 1 , wherein the difference in average grain size determined by subtracting an average grain size between a depth of three-eighths of the sheet thickness and a depth of four-eighths of the sheet thickness from an average grain size between the surface and a depth of one-eighth of the sheet thickness is 1 μm or more. 
     
     
         3 . The high-strength high-formability steel sheet for a can, according to  claim 1  wherein the difference in average amount of N determined by subtracting an average amount of N between the surface and a depth of one-eighth of the sheet thickness from an average amount of N between a depth of three-eighths of the sheet thickness and a depth of four-eighths of the sheet thickness is 10 ppm or more. 
     
     
         4 . The high-strength high-formability steel sheet for a can, according to  claim 1 , wherein for nitrides having a diameter of 1 μm or less and 0.02 μm or more, the average nitride number density between the surface and a depth of one-quarter of the sheet thickness is larger than the average nitride number density between the surface and a depth of one-eighth of the sheet thickness. 
     
     
         5 . The high-strength high-formability steel sheet for a can, according to  claim 1 , wherein for nitrides having a diameter of 1 μm or less and 0.02 μm or more, the value obtained by dividing the average nitride number density between the surface and a depth of one-twentieth of the sheet thickness by the average nitride number density between the surface and a depth of one-quarter of the sheet thickness is less than 1.5. 
     
     
         6 . The high-strength high-formability steel sheet for a can, according to  claim 1 , wherein the amount of solid solution C in the steel is 51 ppm or more. 
     
     
         7 . A method for manufacturing a high-strength high-formability steel sheet for a can comprising the steps of slabbing a steel containing C: 0.070% or more and less than 0.080%, Si: 0.003% or more and 0.10% or less, Mn: 0.51% or more and 0.60% or less, P: 0.001% or more and 0.100% or less, S: 0.001% or more and 0.020% or less, Al: 0.005% or more and 0.100% or less, N: 0.010% or less, and the remainder composed of Fe and incidental impurities, on a percent by mass basis, through continuous casting, performing hot rolling so as to coil at a temperature lower than 620° C., performing rolling with a primary cold reduction ratio of 86% or more in total and a cold reduction ratio of a final stand in primary cold rolling of 30% or more, performing annealing in an atmosphere containing less than 0.020 percent by volume of ammonia gas, and performing secondary cold rolling with a reduction ratio of 20% or less.