HIGH TENSILE STEEL SHEET SUPERIOR IN FORMABILITY AND METHOD OF MANUFACTURING THE SAME

1 . A high tensile steel sheet superior in a formability, containing, in mass %:

C: 0.03% to 0.20%;

Si: 0.005% to 1.0%;

Mn: 1.0% to 3.1%; and

Al: 0.005% to 1.2%,

a P content being over 0% and equal to or less than 0.06%,

an S content being over 0% and equal to or less than 0.01%,

an N content being over 0% and equal to or less than 0.01%, and

a balance being composed of Fe and inevitable impurities,

wherein

a metal structure comprises a ferrite and a martensite,

a relation of a formula (A) is established about an Al content (%) and a Si content (%), and

an average value Y ave defined by a formula (B) regarding hardnesses measured at 100 points or more with a nanoindenter is equal to or more than 40.

0.3≦0.7×[Si]+[Al]≦1.5  (A)

Y ave =Σ(180×( X i −3) −2 /n )  (B)

wherein [Al] indicates the Al content (%), [Si] indicates the Si content (%), n indicates a total number of the measuring points of the hardnesses, and X i indicates the hardness (GPa) at the i-th measuring point, where i is a natural number equal to or less than n.

2 . The high tensile steel sheet superior in a formability according to claim 1 , further contains:

at least one selected from a group consisting of, in mass %,

B: 0.00005% to 0.005%,

Mo: 0.01% to 0.5%,

Cr: 0.01% to 1.0%,

V: 0.01% to 0.1%,

Ti: 0.01% to 0.1%,

Nb: 0.005% to 0.05%,

Ca: 0.0005% to 0.005%, and

REM: 0.0005% to 0.005%.

3 . The high tensile steel sheet superior in a formability according to claim 1 , wherein the high tensile steel sheet is a cold-rolled steel sheet.

4 . The high tensile steel sheet superior in a formability according to claim 1 , wherein the high tensile steel sheet is a galvanized steel sheet.

5 . The high tensile steel sheet superior in a formability according to claim 1 , wherein a martensite fraction in the steel structure is over 5%.

6 . A method of manufacturing a high tensile steel sheet superior in a formability, comprising:

obtaining a hot-rolled steel strip by performing hot rolling;

next, performing acid pickling of the hot-rolled steel strip;

next, obtaining a cold-rolled steel strip by performing cold rolling of the hot-rolled steel strip with a tandem rolling mill having a plurality of stands;

next, performing continuous annealing of the cold-rolled steel strip in a continuous annealing line; and

next, performing temper rolling of the cold-rolled steel strip,

wherein

the steel strip contains, in mass %:

C: 0.03% to 0.20%;

Si: 0.005% to 1.0%;

Mn: 1.0% to 3.1%; and

Al: 0.005% to 1.2%,

a P content being over 0% and equal to or less than 0.06%,

an S content being over 0% and equal to or less than 0.01%,

an N content being over 0% and equal to or less than 0.01%, and

a balance being composed Fe and an inevitable impurity,

a relation of a formula (A) is established about an Al content (%) and a Si content (%),

a relation of a formula (C) being established about a cold-rolling reduction in the first stand among the plurality of stands and a temperature increasing rate in a first heating zone in the continuous annealing line

0.3≦0.7×[Si]+[Al]≦1.5  (A)

50 ≦r 1 0.85 ×V≦ 300  (C)

wherein r1 indicates the cold-rolling reduction (%), and V indicates the temperature increasing rate (° C./s).

7 . The method of manufacturing a high tensile steel sheet superior in a formability according to claim 6 , further comprising, after said performing the continuous annealing:

performing a galvanizing treatment to the cold-rolled steel strip; and

next, performing a temper rolling of the cold-rolled steel strip.

8 . The method of manufacturing a high tensile steel sheet superior in a formability according to claim 7 , further comprising, after said performing the galvanizing treatment, holding the cold-rolled steel strip at a temperature of 400° C. to 650° C. for t seconds, wherein a relation of a formula (D) is established,

t≦ 60×[C]+20×[Mn]+24×[Cr]+40×[Mo]  (D)

wherein [C] indicates a C content (%), [Mn] indicates an Mn content (%), [Cr] indicates a Cr content (%), and [Mo] indicates an Mo content (%).

9 . The high tensile steel sheet superior in a formability according to claim 2 , wherein the high tensile steel sheet is a cold-rolled steel sheet.

10 . The high tensile steel sheet superior in a formability according to claim 2 , wherein the high tensile steel sheet is a galvanized steel sheet.

11 . The high tensile steel sheet superior in a formability according to claim 3 , wherein the high tensile steel sheet is a galvanized steel sheet.

12 . The high tensile steel sheet superior in a formability according to claim 9 , wherein the high tensile steel sheet is a galvanized steel sheet.

13 . The high tensile steel sheet superior in a formability according to claim 2 , wherein a martensite fraction in the steel structure is over 5%.

14 . The high tensile steel sheet superior in a formability according to claim 3 , wherein a martensite fraction in the steel structure is over 5%.

15 . The high tensile steel sheet superior in a formability according to claim 4 , wherein a martensite fraction in the steel structure is over 5%.

16 . The high tensile steel sheet superior in a formability according to claim 9 , wherein a martensite fraction in the steel structure is over 5%.

17 . The high tensile steel sheet superior in a formability according to claim 10 , wherein a martensite fraction in the steel structure is over 5%.

18 . The high tensile steel sheet superior in a formability according to claim 11 , wherein a martensite fraction in the steel structure is over 5%.

19 . The high tensile steel sheet superior in a formability according to claim 12 , wherein a martensite fraction in the steel structure is over 5%.