High strength steel sheet and method for manufacturing the same

Abstract

The present invention relates to a high strength steel sheet consisting essentially of 0.04 to 0.1% C, 0.5% or less Si, 0.5 to 2% Mn, 0.05% or less P, 0.005% or less O, 0.005% or less S, by weight, having 10 μm or less of average ferritic grain size, and 20 mm/mm 2 or less of generation frequency A, which generation frequency A is defined as the total length of a banded secondary phase structure observed per 1 mm 2 of steel sheet cross section along the rolling direction thereof. The steel sheet is manufactured by, for example, a method comprising the steps of: hot-rolling a continuously cast slab having the composition described above at temperatures of Ar 3 transformation point or above directly or after reheating thereof; and cooling the hot-rolled steel sheet within 2 seconds down to the temperatures of from 600 to 750° C. at cooling speeds of from 100 to 2,000° C./sec, followed by coiling the cooled steel sheet at temperatures of from 450 to 650° C. The present invention provides a high strength steel sheet having strengths of 340 MPa or more and having excellent stretch flanging performance, ductility, and shock resistance, providing a sufficient coil shape with good surface properties, even when hot dip zinc-coating is applied.

What is claimed is:  
     
         1 . A high strength steel sheet consisting essentially of 0.04 to 0.1% C, 0.5% or less Si, 0.5 to 2% Mn, 0.05% or less P, 0.005% or less O, 0.005% or less S, by weight, having 10 μm or less of average ferritic grain size, and 20 mm/mm 2  or less of generation frequency A, which generation frequency A is defined as the total length of a banded secondary phase structure observed per 1 mm 2  of steel sheet cross section along the rolling direction thereof.  
     
     
         2 . The high strength steel sheet of  claim 1  further containing 0.01 to 0.3% as the sum of at least one element selected from the group consisting of Ti, Nb, V, Mo, and Cr.  
     
     
         3 . The high strength steel sheet of  claim 1 , wherein the variations of tensile strength in the width direction and in the longitudinal direction of the steel sheet is within ±8% to the average value thereof.  
     
     
         4 . The high strength steel sheet of  claim 2 , wherein the variations of tensile strength in the width direction and in the longitudinal direction of the steel sheet is within ±8% to the average value thereof.  
     
     
         5 . A method for manufacturing high strength steel sheet comprising the steps of: hot-rolling a continuously cast slab having the composition described in  claim 1  or  claim 2  at temperatures of Ar 3  transformation point or above directly or after reheating thereof; and cooling the hot-rolled steel sheet within 2 seconds down to the temperatures of from 600 to 750° C. at cooling speeds of from 100 to 2,000° C./sec, followed by coiling the cooled steel sheet at temperatures of from 450 to 650° C.  
     
     
         6 . The method for manufacturing high strength steel sheet of  claim 5  further comprising the step of either applying pickling and annealing to the coiled steel sheet or applying pickling and cold-rolling, followed by annealing thereto.  
     
     
         7 . The method for manufacturing high strength steel sheet of  claim 5 , wherein a treatment for reducing segregation is applied during the continuous casting.  
     
     
         8 . The method for manufacturing high strength steel sheet of  claim 6 , wherein a treatment for reducing segregation is applied during the continuous casting.  
     
     
         9 . The method for manufacturing high strength steel sheet of  claim 5 , wherein, after cooled the steel sheet at cooling speeds of from 100 to 2,000° C./sec, the variations of temperature in the width direction and in the longitudinal direction of the steel sheet are controlled within 60° C.  
     
     
         10 . The method for manufacturing high strength steel sheet of  claim 6 , wherein, after cooled the steel sheet at cooling speeds of from 100 to 2,000° C./sec, the variations of temperature in the width direction and in the longitudinal direction of the steel sheet are controlled within 60° C.  
     
     
         11 . The method for manufacturing high strength steel sheet of  claim 7 , wherein, after cooled the steel sheet at cooling speeds of from 100 to 2,000° C./sec, the variations of temperature in the width direction and in the longitudinal direction of the steel sheet are controlled within 60° C.  
     
     
         12 . The method for manufacturing high strength steel sheet of  claim 8 , wherein, after cooled the steel sheet at cooling speeds of from 100 to 2,000° C./sec, the variations of temperature in the width direction and in the longitudinal direction of the steel sheet are controlled within 60° C.  
     
     
         13 . The method for manufacturing high strength steel sheet of  claim 9 , wherein the cooling is conducted at heat transfer coefficients of 2,000 kcal/m 2 h° C. or more.  
     
     
         14 . The method for manufacturing high strength steel sheet of  claim 10 , wherein the cooling is conducted at heat transfer coefficients of 2,000 kcal/m 2 h° C. or more.  
     
     
         15 . The method for manufacturing high strength steel sheet of  claim 11 , wherein the cooling is conducted at heat transfer coefficients of 2,000 kcal/m 2 h° C. or more.  
     
     
         16 . The method for manufacturing high strength steel sheet of  claim 12 , wherein the cooling is conducted at heat transfer coefficients of 2,000 kcal/m 2 h° C. or more.  
     
     
         17 . A method for manufacturing high strength hot dip zinc-coated steel sheet comprising the steps of: hot-rolling a steel slab consisting essentially of 0.01 to 0.3% C, 0.7% or less Si, 1 to 3% Mn, 0.08% or less P, 0.01% or less S, 0.08% or less sol.Al, and 0.007% or less N, by weight, at temperatures of Ar 3  transformation point or above; cooling the hot-rolled steel sheet within 2.5 seconds down to the temperatures of from above 500° C. to 700° C. at average cooling speeds of 100° C./sec or more, followed by coiling the cooled steel sheet; and picking or pickling and cold-rolling the coiled steel sheet, then annealing thereto in a continuous hot dip zinc-coating line at temperatures of 720° C. or above to perform zinc coating.  
     
     
         18 . The method for manufacturing high strength hot dip zinc-coated steel sheet of  claim 17 , wherein the steel slab further contains at least one element selected from the group consisting of 0.005 to 0.5% Nb, 0.005 to 0.5% Ti, and 0.0002 to 0.005% B.  
     
     
         19 . The method for manufacturing high strength hot dip zinc-coated steel sheet of  claim 17 , wherein the steel slab further contains at least one element selected from the group consisting of 0.01 to 1% V, 0.01 to 1% Cr, and 0.01 to 1% Mo.  
     
     
         20 . The method for manufacturing high strength hot dip zinc-coated steel sheet of  claim 18 , wherein the steel slab further contains at least one element selected from the group consisting of 0.01 to 1% V, 0.01 to 1% Cr, and 0.001 to 1% Mo.  
     
     
         21 . The method for manufacturing high strength hot dip zinc-coated steel sheet of  claim 17 , wherein the steel sheet after completed the hot-rolling is cooled in a period of from more than 0.5 second to 2.5 seconds at average cooling speeds of 100° C./sec or more.  
     
     
         22 . The method for manufacturing high strength hot dip zinc-coated steel sheet of  claim 18 , wherein the steel sheet after completed the hot-rolling is cooled in a period of from more than 0.5 second to 2.5 seconds at average cooling speeds of 100° C./sec or more.  
     
     
         23 . The method for manufacturing high strength hot dip zinc-coated steel sheet of  claim 19 , wherein the steel sheet after completed the hot-rolling is cooled in a period of from more than 0.5 second to 2.5 seconds at average cooling speeds of 100° C./sec or more.  
     
     
         24 . The method for manufacturing high strength hot dip zinc-coated steel sheet of  claim 20 , wherein the steel sheet after completed the hot-rolling is cooled in a period of from more than 0.5 second to 2.5 seconds at average cooling speeds of 100° C./sec or more.  
     
     
         25 . A method for manufacturing high strength steel sheet comprising the steps of: hot-rolling a continuously cast slab consisting essentially of 0.05 to 0.2% C, 0.15% or less Si, 0.4 to 2.0% Mn, 0.025% or less P, 0.005% or less O, 0.01% or less S, 0.006% or less N, and 0.004% or less Sn, by weight, and having Mn/S≧50 at temperatures of Ar 3  transformation point or above directly or after reheating the slab; and cooling the hot-rolled steel sheet down to the temperatures of from 400° C. to 700° C. at cooling speeds of from 20 to 2,000° C./sec, followed by coiling the cooled steel sheet.  
     
     
         26 . The method for manufacturing high strength steel sheet of  claim 25 , wherein the continuously cast slab further contains 0.005% or less Ca.  
     
     
         27 . The method for manufacturing high strength steel sheet of  claim 25 , wherein the reduction in thickness at the final stand during hot-rolling is in a range of from 8 to 30%.  
     
     
         28 . The method for manufacturing high strength steel sheet of  claim 26 , wherein the reduction in thickness at the final stand during hot-rolling is in a range of from 8 to 30%.  
     
     
         29 . The method for manufacturing high strength steel sheet of  claim 25 , wherein the cooling starts in a period of from more than 0.1 second to less than 1.0 second after completed the hot-rolling.  
     
     
         30 . The method for manufacturing high strength steel sheet of  claim 26 , wherein the cooling starts in a period of from more than 0.1 second to less than 1.0 second after completed the hot-rolling.  
     
     
         31 . The method for manufacturing high strength steel sheet of  claim 27 , wherein the cooling starts in a period of from more than 0.1 second to less than 1.0 second after completed the hot-rolling.  
     
     
         32 . The method for manufacturing high strength steel sheet of  claim 28 , wherein the cooling starts in a period of from more than 0.1 second to less than 1.0 second after completed the hot-rolling.  
     
     
         33 . The method for manufacturing high strength steel sheet of  claim 25  further comprising the steps of cold-rolling then annealing the coiled steel sheet.  
     
     
         34 . The method for manufacturing high strength steel sheet of  claim 26  further comprising the steps of cold-rolling then annealing the coiled steel sheet.  
     
     
         35 . The method for manufacturing high strength steel sheet of  claim 27  further comprising the steps of cold-rolling then annealing the coiled steel sheet.  
     
     
         36 . The method for manufacturing high strength steel sheet of  claim 28  further comprising the steps of cold-rolling then annealing the coiled steel sheet.  
     
     
         37 . The method for manufacturing high strength steel sheet of  claim 29  further comprising the steps of cold-rolling then annealing the coiled steel sheet.  
     
     
         38 . The method for manufacturing high strength steel sheet of  claim 30  further comprising the steps of cold-rolling then annealing the coiled steel sheet.  
     
     
         39 . The method for manufacturing high strength steel sheet of  claim 31  further comprising the steps of cold-rolling then annealing the coiled steel sheet.  
     
     
         40 . The method for manufacturing high strength steel sheet of  claim 32  further comprising the steps of cold-rolling then annealing the coiled steel sheet.