STEEL SHEET WITH EXCELLENT COLD WORKABILITY DURING FORMING AND METHOD FOR MANUFACTURING THE SAME

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.