High-strength steel sheet exhibiting excellent stretch-flange formability and bending workability, and method of producing molten steel for the high-strength steel sheet

1. A high-strength steel sheet comprising:

C: 0.03 to 0.25 mass %,

Si: 0.1 to 2.0 mass %,

Mn: 0.5 to 3.0 mass %,

P: not more than 0.05 mass %,

T.O: not more than 0.0050 mass %,

S: 0.0001 to 0.01 mass %,

N: 0.0005 to 0.01 mass %,

acid-soluble Al: more than 0.01 mass %,

Ca: 0.0005 to 0.0050 mass %,

a total of at least one element of Ce, La, Nd, and Pr: 0.001 to 0.01 mass %, and

optionally one or more selected from the group consisting of:

acid-soluble Ti: 0.008 to 0.20 mass %,

Nb: 0.01 to 0.10 mass %,

V: 0.01 to 0.10 mass %,

Cu: 0.1 to 2 mass %,

Ni: 0.05 to 1 mass %,

Cr: 0.01 to 1 mass %,

Mo: 0.01 to 0.4 mass %,

B: 0.0003 to 0.005 mass %, and

Zr: 0.001 to 0.01 mass %

with a balance including iron and inevitable impurities, wherein:

the steel sheet contains a chemical component on a basis of mass that satisfies

0.7<100×([Ce]+[La]+[Nd]+[Pr])/[acid-soluble Al]≦70, and

0.2≦([Ce]+[La]+[Nd]+[Pr])/[S]≦10,

where [Ce] is an amount of Ce contained, [La] is an amount of La contained, [Nd] is an amount of Nd contained, [Pr] is an amount of Pr contained, [acid-soluble Al] is an amount of acid-soluble Al contained, and [S] is an amount of S contained;

the steel sheet has a compound inclusion including a first inclusion phase containing at least one element of Ce, La, Nd, and Pr, containing Ca, and containing at least one element of O and S, and a second inclusion phase having a component different from that of the first inclusion phase and containing at least one element of Mn, Si, Ti and Al;

the compound inclusion forms a spherical compound inclusion having an equivalent circle diameter in a range of 0.5 μm to 5 μm; and

a ratio of a number of the spherical compound inclusions relative to a number of all inclusions having the equivalent circle diameter in the range of 0.5 μm to 5 μm is 50% or more.

2. The high-strength steel sheet according to claim 1 , wherein

the spherical inclusion is an inclusion having an equivalent circle diameter of 1 μm or more, and

a ratio of a number of elongated inclusions having a major axis/minor axis of 3 or less relative to a number of all inclusions having the equivalent circle diameter of 1 μm or more is 50% or more.

3. The high-strength steel sheet according to claim 1 , wherein

the spherical inclusion contains at least one element of Ce, La, Nd, and Pr, a total of which is in a range of 0.5 mass % to 95 mass % in an average composition.

4. The high-strength steel sheet according to claim 1 , wherein

an average grain diameter of a crystal in a structure of the steel sheet is 10 μm or less.

5. A method of producing molten steel for the high-strength steel sheet according to any one of claims 1 to 4 , the method having a refinement process for producing a steel, the refinement process including:

a first process of obtaining a first molten steel including

applying processing so as to obtain P of not more than 0.05 mass % and S of not less than 0.0001 mass %, and

performing addition or adjustment such that C is not less than 0.03 mass % and not more than 0.25 mass %, Si is not less than 0.1 mass % and not more than 2.0 mass %, Mn is not less than 0.5 mass % and not more than 3.0 mass %, and N is not less than 0.0005 mass % and not more than 0.01 mass %;

a second process of obtaining a second molten steel including

performing addition to the first molten steel such that Al is more than 0.01 mass % in acid-soluble Al, and T.O is not more than 0.0050 mass %;

a third process of obtaining a third molten steel including

adding at least one element of Ce, La, Nd, and Pr to the second molten steel so as to satisfy on a basis of mass

0.7<100×([Ce]+[La]+[Nd]+[Pr])/[acid-soluble Al]≦70,

0.2≦([Ce]+[La]+[Nd]+[Pr])/[S]≦10, and

0.001≦[Ce]+[La]+[Nd]+[Pr]≦0.01,

where [Ce] is an amount of Ce contained, [La] is an amount of La contained, [Nd] is an amount of Nd contained, [Pr] is an amount of Pr contained, [acid-soluble Al] is an amount of acid-soluble Al contained, and [S] is an amount of S contained; and

a fourth process of obtaining a fourth molten steel including

adding Ca to or performing adjustment to the third molten steel such that Ca is not less than 0.0005 mass % and not more than 0.0050 mass %.

6. The method of producing molten steel for a high-strength steel sheet according to claim 5 , wherein

the third process includes, before the at least one element of Ce, La, Nd, and Pr is added to the second molten steel, adding at least one element of Nb and V to the second molten steel such that the second molten steel further contains at least one element of Nb of not less than 0.01 mass % and not more than 0.10 mass % and V of not less than 0.01 mass % and not more than 0.10 mass %.

7. The method of producing molten steel for a high-strength steel sheet according to claim 5 , wherein

the third process includes, before the at least one element of Ce, La, Nd, and Pr is added to the second molten steel, adding at least one element of Cu, Ni, Cr, Mo, and B to the second molten steel such that the second molten steel further contains at least one element of Cu of not less than 0.1 mass % and not more than 2 mass %, Ni of not less than 0.05 mass % and not more than 1 mass %, Cr of not less than 0.01 mass % and not more than 1 mass %, Mo of not less than 0.01 mass % and not more than 0.4 mass %, and B of not less than 0.0003 mass % and not more than 0.005 mass %.

8. The method of producing molten steel for a high-strength steel sheet according to claim 5 , wherein

the third process includes, before the at least one element of Ce, La, Nd, and Pr is added to the second molten steel, adding Zr to the second molten steel such that the second molten steel further contains Zr of not less than 0.001 mass % to 0.01 mass %.

9. A high-strength steel sheet comprising:

C: 0.03 to 0.25 mass %,

Si: 0.03 to 2.0 mass %,

Mn: 0.5 to 3.0 mass %,

P: not more than 0.05 mass %,

T.O: not more than 0.0050 mass %,

S: 0.0001 to 0.01 mass %,

acid-soluble Ti: 0.008 to 0.20 mass %,

N: 0.0005 to 0.01 mass %,

acid-soluble Al: more than 0.01 mass %,

Ca: 0.0005 to 0.005 mass %, and

a total of at least one element of Ce, La, Nd, and Pr: 0.001 to 0.01 mass %,

with a balance including iron and inevitable impurities, wherein:

the steel sheet contains a chemical component on a basis of mass that satisfies

0.7<100×([Ce]+[La]+[Nd]+[Pr])/[acid-soluble Al]≦70, and

0.2≦([Ce]+[La]+[Nd]+[Pr])/[S]≦10,

where [Ce] is an amount of Ce contained, [La] is an amount of La contained, [Nd] is an amount of Nd contained, [Pr] is an amount of Pr contained, [acid-soluble Al] is an amount of acid-soluble Al contained, and [S] is an amount of S contained;

the steel sheet has a compound inclusion including a first inclusion phase containing at least one element of Ce, La, Nd, and Pr, containing Ca, and containing at least one element of O and S, and a second inclusion phase having a component different from that of the first inclusion phase and containing at least one element of Mn, Si, Ti, and Al;

the compound inclusion forms a spherical compound inclusion having an equivalent circle diameter in a range of 0.5 μm to 5 μm;

a ratio of number of the spherical compound inclusion relative to number of all inclusions having the equivalent circle diameter in the range of 0.5 μm to 5 μm is 50% or more; and

number density of an inclusion with more than 5 μm is less than 10 pieces/mm 2 .

10. The high-strength steel sheet according to claim 9 , wherein

the spherical inclusion is an inclusion having an equivalent circle diameter of 1 μm or more, and

a ratio of number of elongated inclusions having a major axis/minor axis of 3 or less relative to number of all inclusions having the equivalent circle diameter of 1 μm or more is 50% or more.

11. The high-strength steel sheet according to claim 9 , wherein

the spherical inclusion contains at least one element of Ce, La, Nd, and Pr, a total of which is in a range of 0.5 mass % to 95 mass % in an average composition.

12. The high-strength steel sheet according to claim 9 , wherein

an average grain diameter of a crystal in a structure of the steel sheet is 10 μm or less.

13. The high-strength steel sheet according to any one of claims 9 to 12 , further containing at least one element of

Nb: 0.005 to 0.10 mass %, and

V: 0.01 to 0.10 mass %.

14. The high-strength steel sheet according to any one of claims 9 to 12 , further containing at least one element of:

Cu: 0.1 to 2 mass %,

Ni: 0.05 to 1 mass %,

Cr: 0.01 to 1.0 mass %,

Mo: 0.01 to 0.4 mass %, and

B: 0.0003 to 0.005 mass %.

15. The high-strength steel sheet according to any one of claims 9 to 12 , further containing Zr: 0.001 to 0.01 mass %.

16. The high-strength steel sheet according to any one of claims 9 to 12 , further containing at least one element of:

Nb: 0.005 to 0.10 mass %,

V: 0.01 to 0.10 mass %,

Cu: 0.1 to 2 mass %,

Ni: 0.05 to 1 mass %,

Cr: 0.01 to 1.0 mass %,

Mo: 0.01 to 0.4 mass %,

B: 0.0003 to 0.005 mass %, and

Zr: 0.001 to 0.01 mass %.

17. A method of producing molten steel for the high-strength steel sheet according to any one of claims 9 to 12 , having a refinement process for producing a steel, the refinement process including:

a first process of obtaining a first molten steel including:

applying processing so as to obtain P of not more than 0.05 mass % and S of not less than 0.0001 mass % and not more than 0.01 mass %, and

performing addition or adjustment such that C is not less than 0.03 mass % and not more than 0.25 mass %, Si is not less than 0.03 mass % and not more than 2.0 mass %, Mn is not less than 0.5 mass % and not more than 3.0 mass %, and N is not less than 0.0005 mass % and not more than 0.01 mass %;

a second process of obtaining a second molten steel including

performing addition to the first molten steel such that Al is more than 0.01 mass % in acid-soluble Al, and T.O is not more than 0.0050 mass %;

a third process of obtaining a third molten steel including

adding Ti of not less than 0.008 mass % and not more than 0.20 mass % in acid-soluble Ti to the second molten steel;

a fourth process of obtaining a fourth molten steel including

adding at least one element of Ce, La, Nd, and Pr to the third molten steel so as to satisfy on a basis of mass

0.7<100×([Ce]+[La]+[Nd]+[Pr])/[acid-soluble Al]≦70,

0.2≦([Ce]+[La]+[Nd]+[Pr])/[S]≦10, and

0.001≦[Ce]+[La]+[Nd]+[Pr]≦0.01,

where [Ce] is an amount of Ce contained, [La] is an amount of La contained, [Nd] is an amount of Nd contained, [Pr] is an amount of Pr contained, [acid-soluble Al] is an amount of acid-soluble Al contained, and [S] is an amount of S contained; and

a fifth process of obtaining a fifth molten steel including

adding Ca to or performing adjustment to the fourth molten steel such that Ca is not less than 0.0005 mass % and not more than 0.0050 mass %.

18. The method of producing molten steel for a high-strength steel sheet according to claim 17 , wherein

the third process further includes, before the at least one element of Ce, La, Nd, and Pr is added to the second molten steel, adding at least one element of Nb and V to the second molten steel such that the second molten steel further contains at least one element of Nb of not less than 0.005 mass % and not more than 0.10 mass %, and V of not less than 0.01 and not more than 0.10 mass %.

19. The method of producing molten steel for a high-strength steel sheet according to claim 17 , wherein

the third process further includes, before the at least one element of Ce, La, Nd, and Pr is added to the second molten steel, adding at least one element of Cu, Ni, Cr, Mo, and B to the second molten steel such that the second molten steel further contains at least one element of Cu of not less than 0.1 mass % and not more than 2 mass %, Ni of not less than 0.05 mass % and not more than 1 mass %, Cr of not less than 0.01 mass % and not more than 1 mass %, Mo of not less than 0.01 mass % and not more than 0.4 mass %, and B of not less than 0.0003 mass % and not more than 0.005 mass %.

20. The method of producing molten steel for a high-strength steel sheet according to claim 17 , wherein

the third process further includes, before the at least one element of Ce, La, Nd, and Pr is added to the second molten steel, adding Zr to the second molten steel such that the second molten steel further contains Zr of not less than 0.001 mass % and not more than 0.01 mass %.

21. The high-strength steel sheet according to claim 1 , comprising acid-soluble Ti: 0.008 to 0.20 mass %, wherein

a number density of inclusions with more than 5 μm is less than 10 pieces/mm 2 .

22. A high-strength steel sheet consisting essentially of:

C: 0.03 to 0.25 mass %,

Si: 0.1 to 2.0 mass %,

Mn: 0.5 to 3.0 mass %,

P: not more than 0.05 mass %,

T.O: not more than 0.0050 mass %,

S: 0.0001 to 0.01 mass %,

N: 0.0005 to 0.01 mass %,

acid-soluble Al: more than 0.01 mass %,

Ca: 0.0005 to 0.0050 mass %,

a total of at least one element of Ce, La, Nd, and Pr: 0.001 to 0.01 mass %, and

optionally one or more selected from the group consisting of:

acid-soluble Ti: 0.008 to 0.20 mass %,

Nb: 0.01 to 0.10 mass %,

V: 0.01 to 0.10 mass %,

Cu: 0.1 to 2 mass %,

Ni: 0.05 to 1 mass %,

Cr: 0.01 to 1 mass %,

Mo: 0.01 to 0.4 mass %,

B: 0.0003 to 0.005 mass %, and

Zr: 0.001 to 0.01 mass %

with a balance of iron and inevitable impurities, wherein:

the steel sheet contains a chemical component on a basis of mass that satisfies

0.7<100×([Ce]+[La]+[Nd]+[Pr])/[acid-soluble Al]≦70, and

0.2≦([Ce]+[La]+[Nd]+[Pr])/[S]≦10,

where [Ce] is an amount of Ce contained, [La] is an amount of La contained, [Nd] is an amount of Nd contained, [Pr] is an amount of Pr contained, [acid-soluble Al] is an amount of acid-soluble Al contained, and [S] is an amount of S contained;

the steel sheet has a compound inclusion including a first inclusion phase containing at least one element of Ce, La, Nd, and Pr, containing Ca, and containing at least one element of O and S, and a second inclusion phase having a component different from that of the first inclusion phase and containing at least one element of Mn, Si, Ti, and Al;

the compound inclusion forms a spherical compound inclusion having an equivalent circle diameter in a range of 0.5 μm to 5 μm; and

a ratio of a number of spherical compound inclusions relative to a number of all inclusions having an equivalent circle diameter in the range of 0.5 μm to 5 μm is 50% or more.

23. The high-strength steel sheet according to claim 22 , wherein

the spherical compound inclusion is an inclusion having an equivalent circle diameter of 1 μm or more, and

a ratio of a number of elongated inclusions having a major axis/minor axis of 3 or less relative to a number of all inclusions having the equivalent circle diameter of 1 μm or more is 50% or more.

24. The high-strength steel sheet according to claim 22 , wherein

the spherical compound inclusion contains at least one element of Ce, La, Nd, and Pr, a total of which is in a range of 0.5 mass % to 95 mass % in an average composition.

25. The high-strength steel sheet according to claim 22 , wherein

an average grain diameter of a crystal in a structure of the steel sheet is 10 μm or less.

26. The high-strength steel sheet according to claim 22 , comprising acid-soluble Ti: 0.008 to 0.20 mass %, wherein

a number density of inclusions with more than 5 μm is less than 10 pieces/mm 2 .