High strength steel sheet and hot dip galvanized steel sheet having high ductility and excellent delayed fracture resistance and method for manufacturing the same

1. A high strength cold rolled steel sheet comprising, by weight percent, 0.05 to 0.3% C, 0.3 to 1.6% Si, 4.0 to 7.0% Mn, 0.5 to 2.0% Al, 0.01 to 0.1% Cr, 0.02 to 0.1% Ni and 0.005 to 0.03% Ti, 5 to 30 ppm B, 0.01 to 0.03% Sb, 0.008% or less S, balance Fe and impurities, and comprising a microtexture including 40 to 50% by area fraction of annealed martensite as a matrix, 20 to 40% by volume fraction of retained austenite and balance ferrite.

2. The high strength cold rolled steel sheet of claim 1 , having a tensile strength of 980 MPa or more and an elongation of 28% or more.

3. A high strength galvanized steel sheet comprising:

a steel including, by weight percent, 0.05 to 0.3% C, 0.3 to 1.6% Si, 4.0 to 7.0% Mn, 0.5 to 2.0% Al, 0.01 to 0.1% Cr, 0.02 to 0.1% Ni and 0.005 to 0.03% Ti, 5 to 30 ppm B, 0.01 to 0.03% Sb, 0.008% or less S, balance Fe and impurities, and comprising a microtexture including 40 to 50% by area fraction of annealed martensite as a matrix, 20 to 40% by volume fraction of retained austenite and balance ferrite; and

a galvanized layer or a galvannealed layer.

4. A method of manufacturing a high strength cold rolled steel sheet, comprising:

heating a steel slab at a temperature range from 1150 to 1250° C., followed by hot finish rolling at a temperature range from 880 to 920° C., the steel slab including, by weight percent, 0.05 to 0.3% C, 0.3 to 1.6% Si, 4.0 to 7.0% Mn, 0.5 to 2.0% Al, 0.01 to 0.1% Cr, 0.02 to 0.1% Ni and 0.005 to 0.03% Ti, 5 to 30 ppm B, 0.01 to 0.03% Sb, 0.008% or less S, balance Fe and impurities, and comprising a microtexture including 40 to 50% by area fraction of annealed martensite as a matrix, 20 to 40% by volume fraction of retained austenite and balance ferrite;

coiling the resultant structure at a temperature range from 550 to 650° C.;

pickling the resultant structure using hydrochloric acid, followed by cold rolling at a cold reduction rate from 30 to 60%; and

performing continuous annealing on the resultant structure by holding a temperature range from 670 to 750° C. for 60 seconds or more, followed by cooling.

5. A method of manufacturing a high strength cold rolled steel sheet, comprising:

heating a steel slab at a temperature range from 1150 to 1250° C., followed by hot finish rolling at a temperature range from 880 to 920° C., the steel slab including, by weight percent, 0.05 to 0.3% C, 0.3 to 1.6% Si, 4.0 to 7.0% Mn, 0.5 to 2.0% Al, 0.01 to 0.1% Cr, 0.02 to 0.1% Ni and 0.005 to 0.03% Ti, 5 to 30 ppm B, 0.01 to 0.03% Sb, 0.008% or less S, balance Fe and impurities, and comprising a microtexture including 40 to 50% by area fraction of annealed martensite as a matrix, 20 to 40% by volume fraction of retained austenite and balance ferrite;

coiling the resultant structure at a temperature range from 550 to 650° C.;

pickling the resultant structure using hydrochloric acid, followed by cold rolling at a cold reduction rate from 30 to 60%;

performing reverse transformation by batch-annealing the resultant structure at a temperature range from 620 to 720° C. for 1 to 24 hours; and

cooling the resultant structure at a cooling rate from 10 to 200° C./s.

6. A method of manufacturing a high strength galvanized steel sheet, comprising:

heating a steel slab at a temperature range from 1150 to 1250° C., followed by hot finish rolling at a temperature range from 880 to 920° C., the steel slab including, by weight percent, 0.05 to 0.3% C, 0.3 to 1.6% Si, 4.0 to 7.0% Mn, 0.5 to 2.0% Al, 0.01 to 0.1% Cr, 0.02 to 0.1% Ni and 0.005 to 0.03% Ti, 5 to 30 ppm B, 0.01 to 0.03% Sb, 0.008% or less S, balance Fe and impurities, and comprising a microtexture including 40 to 50% by area fraction of annealed martensite as a matrix, 20 to 40% by volume fraction of retained austenite and balance ferrite;

coiling the resultant structure at a temperature range from 550 to 650° C.;

pickling the resultant structure using hydrochloric acid, followed by cold rolling at a cold reduction rate from 30 to 60%;

performing continuous annealing on the resultant structure by holding a temperature range from 670 to 750° C. for 60 seconds or more, followed by cooling; and

galvanizing the resultant structure at a temperature range from 450 to 500° C.

7. A method of manufacturing a high strength galvanized steel sheet of claim 6 , further comprising:

galvannealing the resultant structure at a temperature range from 500 to 600° C.

8. A method of manufacturing a high strength galvanized steel sheet, comprising:

heating a steel slab at a temperature range from 1150 to 1250° C., followed by hot finish rolling at a temperature range from 880 to 920° C., the steel slab including, by weight percent, 0.05 to 0.3% C, 0.3 to 1.6% Si, 4.0 to 7.0% Mn, 0.5 to 2.0% Al, 0.01 to 0.1% Cr, 0.02 to 0.1% Ni and 0.005 to 0.03% Ti, 5 to 30 ppm B, 0.01 to 0.03% Sb, 0.008% or less S, balance Fe and impurities, and comprising a microtexture including 40 to 50% by area fraction of annealed martensite as a matrix, 20 to 40% by volume fraction of retained austenite and balance ferrite;

coiling the resultant structure at a temperature range from 550 to 650° C.;

pickling the resultant structure using hydrochloric acid, followed by cold rolling at a cold reduction rate from 30 to 60%;

performing reverse transformation by batch-annealing the resultant structure at a temperature range from 620 to 720° C. for 1 to 24 hours;

cooling the resultant structure at a cooling rate from 10 to 200° C./s; and

galvanizing the resultant structure at a temperature range from 450 to 500° C.

9. A method of manufacturing a high strength galvanized steel sheet of claim 8 , further comprising:

galvannealing the resultant structure at a temperature range from 500 to 600° C.