High strength hot-rolled steel plate exhibiting excellent acid pickling property, chemical conversion processability, fatigue property, stretch flangeability, and resistance to surface deterioration during molding, and having isotropic strength and ductility

1. A high strength hot-rolled steel sheet that is superior in an acid pickling property, a chemical conversion processability, a fatigue property, a hole expandability, and a resistance to surface deterioration during forming, and that has isotropic strength and isotropic ductility, the steel sheet consisting of, in terms of percent by mass:

C: 0.05 to 0.12%;

Si: 1.0 to 1.2%;

Mn: 1.6 to 2.2%;

Al: 0.30 to 0.6%;

P: 0.05% or less;

S: 0.005% or less;

N: 0.01% or less; and

the remainder of Fe and unavoidable impurities,

wherein a microstructure comprises: 60 area % or more of ferrite phases; more than 10 area % of martensite phases; and 0 to less than 1 area % of residual austenite phases, or the microstructure comprises: 60 area % or more of ferrite phases; more than 10 area % of martensite phases; less than 5 area % of bainite phases; and 0 to less than 1% of residual austenite phases, and

a maximum concentration of Al detected by a glow discharge emission spectroscopic analysis is in a range of 0.75 mass % or less in a region from a surface of the steel sheet to a thickness of 500 nm after being acid-pickled.

2. A high strength hot-rolled steel sheet that is superior in an acid pickling property, a chemical conversion processability, a fatigue property, a hole expandability, and a resistance to surface deterioration during forming, and that has isotropic strength and isotropic ductility,

the steel sheet consisting of, in terms of percent by mass:

C: 0.05 to 0.12%;

Si: 1.0 to 1.2%;

Mn: 1.6 to 2.2%;

Al: 0.30 to 0.6%;

P: 0.05% or less;

S: 0.005% or less;

N: 0.01% or less;

one or more selected from a group consisting of

Ni: 0.002 to 1.0%,

Ti: 0.001 to 0.5%,

Nb: 0.001 to 0.5%,

Mo: 0.002 to 1.0%,

V: 0.002 to 0.2%,

Cr: 0.002 to 1.0%,

Zr: 0.002 to 0.2%,

Ca: 0.0005 to 0.0050%,

REM: 0.0005 to 0.0200%, and

B: 0.0002 to 0.0030%; and

the remainder of Fe and unavoidable impurities,

wherein a microstructure comprises: 60 area % or more of ferrite phases; more than 10 area % of martensite phases; and 0 to less than 1 area % of residual austenite phases, or the microstructure comprises: 60 area % or more of ferrite phases; more than 10 area % of martensite phases; less than 5 area % of bainite phases; and 0 to less than 1% of residual austenite phases, and

a maximum concentration of Al detected by a glow discharge emission spectroscopic analysis is in a range of 0.75 mass % or less in a region from a surface of the steel sheet to a thickness of 500 nm after being acid-pickled.

3. The high strength hot-rolled steel sheet that is superior in an acid pickling property, a chemical conversion processability, a fatigue property, a hole expandability, and a resistance to surface deterioration during forming, and that has isotropic strength and isotropic ductility according to claim 1 ,

wherein an average length of ferrite crystal grains in a rolling direction is in a range of 20 μm or less in a region from the surface of the steel sheet to a thickness of 20 μm.

4. The high strength hot-rolled steel sheet that is superior in an acid pickling property, a chemical conversion processability, a fatigue property, a hole expandability, and a resistance to surface deterioration during forming, and that has isotropic strength and isotropic ductility according to claim 2 ,

wherein an average length of ferrite crystal grains in a rolling direction is in a range of 20 μm or less in a region from the surface of the steel sheet to a thickness of 20 μm.

5. A high strength hot-rolled steel sheet that is superior in an acid pickling property, a chemical conversion processability, a fatigue property, a hole expandability, and a resistance to surface deterioration during forming, and that has isotropic strength and isotropic ductility, the steel sheet comprising, in terms of percent by mass:

C: 0.05 to 0.12%;

Si: 1.0 to 1.2%;

Mn: 1.6 to 2.2%;

Al: 0.30 to 0.6%;

P: 0.05% or less;

S: 0.005% or less; and

N: 0.01% or less,

with the remainder being Fe and unavoidable impurities,

wherein a microstructure comprises: 60 area % or more of ferrite phases; more than 10 area % of martensite phases; and 0 to less than 1 area % of residual austenite phases, or the microstructure comprises: 60 area % or more of ferrite phases; more than 10 area % of martensite phases; less than 5 area % of bainite phases; and 0 to less than 1% of residual austenite phases, and

a maximum concentration of Al detected by a glow discharge emission spectroscopic analysis is in a range of 0.75 mass % or less in a region from a surface of the steel sheet to a thickness of 500 nm after being acid-pickled.

6. The high strength hot-rolled steel sheet that is superior in an acid pickling property, a chemical conversion processability, a fatigue property, a hole expandability, and a resistance to surface deterioration during forming, and that has isotropic strength and isotropic ductility according to claim 5 ,

wherein the steel sheet further comprises, in terms of percent by mass:

one or more selected from a group consisting of

Cu: 0.002 to 2.0%,

Ni: 0.002 to 1.0%,

Ti: 0.001 to 0.5%,

Nb: 0.001 to 0.5%,

Mo: 0.002 to 1.0%,

V: 0.002 to 0.2%,

Cr: 0.002 to 1.0%,

Zr: 0.002 to 0.2%,

Ca: 0.0005 to 0.0050%,

REM: 0.0005 to 0.0200%, and

B: 0.0002 to 0.0030%.