Steel sheet with excellent cold workability during forming and method for manufacturing the same

Abstract

The present invention provides a steel sheet having an excellent cold workability during forming and a method for producing the same. The steel sheet of the present invention is characterized in that: (a) the ratio of the number of carbides at the ferrite grain boundary to the number of carbides in the ferrite grain exceeds 1, (b) the ferrite grain diameter is 5 μm or more and 50 μm or less, (c) the in-plane anisotropy |Δr| of the r value is 0.2 or less, (d) the Vickers hardness is 100 HV or more and 150 HV or less, (e) the random intensity ratio of the {311} <011> orientation at the ½-thickness portion of the steel sheet is 3.0 or less.

1 . A steel sheet having an excellent cold workability during forming, comprising, in terms of % by mass:
 C: 0.10 to 0.40%,   Si: 0.01 to 0.30%,   Mn: 0.30 to 1.00%,   P: 0.0001 to 0.020%,   S: 0.0001 to 0.010%,   Al: 0.001 to 0.10%, and   a balance of Fe and inevitable impurities,   wherein (a) a ratio of the number of carbides at a ferrite grain boundary relative to the number of carbides in the ferrite grain is more than 1,   wherein (b) a diameter of the ferrite grain is 5 μm or more and 50 μm or less,   wherein (c) an in-plane anisotropy |Δr| of the r value standardized according to JIS Z 2254 is 0.2 or less,   wherein (d) a Vickers hardness of the steel sheet is 100 HV or more and 150 HV or less, and   wherein (e) a ratio of X-ray diffraction intensity of the {311} <011> orientation at the ½-thickness portion of the steel sheet relative to the X-ray diffraction intensity obtained when a sample with a random orientation distribution of crystal grains in the steel sheet is subjected to X-ray diffraction is 3.0 or less.   
     
     
         2 . The steel sheet with excellent cold workability during forming according to  claim 1  further comprising, in terms of % by mass, one or a plurality of:
 N: 0.0001 to 0.010%, 
 O: 0.0001 to 0.020%, 
 Cr: 0.001 to 0.50%, 
 Mo: 0.001 to 0.10%, 
 Nb: 0.001 to 0.10%, 
 V: 0.001 to 0.10%, 
 Cu: 0.001 to 0.10%, 
 W: 0.001 to 0.10%, 
 Ta: 0.001 to 0.10%, 
 Ni: 0.001 to 0.10%, 
 Sn: 0.001 to 0.050%, 
 Sb: 0.001 to 0.050%, 
 As: 0.001 to 0.050%, 
 Mg: 0.0001 to 0.050%, 
 Ca: 0.001 to 0.050%, 
 Y: 0.001 to 0.050%, 
 Zr: 0.001 to 0.050%, 
 La: 0.001 to 0.050%, and 
 Ce: 0.001 to 0.050%. 
 
     
     
         3 . A method for producing a steel sheet with excellent cold workability during forming according to  claim 1 , said method comprising:
 subjecting a steel strip having an ingredient composition according to  claim 1  to hot rolling by heating, followed by completing the finish hot rolling at a temperature range of 800° C. or higher and 900° C. or lower;   coiling said hot-rolled steel sheet at a temperature of 400° C. or higher and 550° C. or lower;   pickling said hot-rolled steel sheet, and then subjecting said hot-rolled steel sheet to a two-step type annealing in which said hot-rolled steel sheet is retained in two temperature ranges,   wherein the two-step type annealing comprises   (i) subjecting said hot-rolled steel sheet to a first step annealing performed by retaining said hot-rolled steel at a temperature range of 650° C. or higher and 720° C. or lower for 3 hours or longer and 60 hours or shorter, and then a second step annealing performed by retaining the hot-rolled steel at a temperature range of 725° C. or higher and 790° C. or lower for 3 hours or longer and 50 hours or shorter, and thereafter   (ii) cooling said hot-rolled steel sheet to 650° C. or lower at a cooling rate of 1° C./hour or more and 30° C./hour or less.   
     
     
         4 . The method for producing a steel sheet according to  claim 3 , wherein the steel sheet has a cross-sectional shrinkage percentage of 40% or more. 
     
     
         5 . A method for producing a steel sheet with excellent cold workability during forming according to  claim 2 , said method comprising:
 subjecting a steel strip having an ingredient composition according to  claim 2  to hot rolling by heating, followed by completing the finish hot rolling at a temperature range of 800° C. or higher and 900° C. or lower;   coiling said hot-rolled steel sheet at a temperature of 400° C. or higher and 550° C. or lower;   pickling said hot-rolled steel sheet, and then subjecting said hot-rolled steel sheet to a two-step type annealing in which said hot-rolled steel sheet is retained in two temperature ranges,   wherein the two-step type annealing comprises   (i) subjecting said hot-rolled steel sheet to a first step annealing performed by retaining said hot-rolled steel at a temperature range of 650° C. or higher and 720° C. or lower for 3 hours or longer and 60 hours or shorter, and then a second step annealing performed by retaining the hot-rolled steel at a temperature range of 725° C. or higher and 790° C. or lower for 3 hours or longer and 50 hours or shorter, and thereafter   (ii) cooling said hot-rolled steel sheet to 650° C. or lower at a cooling rate of 1° C./hour or more and 30° C./hour or less.   
     
     
         6 . The method for producing a steel sheet according to  claim 5 , wherein the steel sheet has a cross-sectional shrinkage percentage of 40% or more.