Cold-rolled steel sheet for vitreous enameling and its named enameled product thereof

1. A cold-rolled steel sheet for vitreous enameling, the steel sheet comprising, by mass %,

C: 0.0005% to 0.0050%,

Mn: 0.05% to 1.50%,

Si: 0.001% to 0.015%,

Al: 0.001% to 0.01%,

N: 0.0010% to 0.0045%,

O: 0.0150% to 0.0550%,

P: 0.04% to 0.10%,

S: 0.0050% to 0.050%,

Nb: 0.020% to 0.080%,

Cu: 0.015% to 0.045%, and

a remainder including Fe and impurities,

wherein when a C content is represented by C (%), a Mn content is represented by Mn (%), a P content is represented by P (%), and a Nb content is represented by Nb (%), the following expression (1) is satisfied;

a metallographic structure contains ferrite, and an average grain size of the ferrite at a ¼ thickness position from a surface in a thickness direction is 12.0 μm or less;

a number density of Fe—Mn—Nb-based composite oxides containing Fe, Mn, and Nb and having a diameter of 0.2 μm to 10 μm is 2×10 2 particle/mm 2 to 1×10 4 particle/mm 2 ;

a fatigue limit ratio, which is a value obtained by dividing a fatigue strength by a tensile strength, is higher than 0.42, the fatigue strength being a stress at 10 7 cycles after performing a heat treatment with an applied tensile strain of 10% at a heating temperature of 830° C. for a holding time of 5 minutes;

voids are formed between the metallographic structure and the Fe—Mn—Nb-based composite oxides, and an equivalent circle diameter of the voids is 0.1 μm to 0.6 μm; and

when each of the voids is approximated as a triangle and a long side of the triangle is set as a base, a value obtained by dividing a length of the base by a height of the triangle is 1.0 to 15;

2.20≤8×C (%)+1.3×Mn (%)+18×P (%)+5.1×(Nb (%)) 0.5 ≤4.00  (1).

2. A cold-rolled steel sheet for vitreous enameling, the steel sheet comprising, by mass %,

C: 0.0005% to 0.0050%,

Mn: 0.05% to 1.50%,

Si: 0.001% to 0.015%,

Al: 0.001% to 0.01%,

N: 0.0010% to 0.0045%,

O: 0.0150% to 0.0550%,

P: 0.04% to 0.10%,

S: 0.0050% to 0.050%,

Nb: 0.020% to 0.080%,

Cu: 0.015% to 0.045%,

B: 0.0005% to 0.0050%, and

a remainder including Fe and impurities,

wherein when a C content is represented by C (%), a Mn content is represented by Mn (%), a P content is represented by P (%), and a Nb content is represented by Nb (%), the following expression (2) is satisfied;

a metallographic structure contains ferrite, and an average grain size of the ferrite at a ¼ thickness position from a surface in a thickness direction is 12.0 μm or less;

a number density of Fe—Mn—Nb—B-based composite oxides containing Fe, Mn, Nb, and B and having a diameter of 0.2 μm to 10 μm is 2×10 2 particle/mm 2 to 1×10 4 particle/mm 2 ;

a fatigue limit ratio, which is a value obtained by dividing a fatigue strength by a tensile strength, is higher than 0.42, the fatigue strength being a stress at 10 7 cycles after performing a heat treatment with an applied tensile strain of 10% at a heating temperature of 830° C. for a holding time of 5 minutes;

voids are formed between the metallographic structure and the Fe—Mn—Nb—B-based composite oxides, and an equivalent circle diameter of the voids is 0.1 μm to 0.6 μm; and

when each of the voids is approximated as a triangle and a long side of the triangle is set as a base, a value obtained by dividing a length of the base by a height of the triangle is 1.0 to 15;

2.50≤8×C (%)+1.3×Mn (%)+18×P (%)+5.1×(Nb (%)) 0.5 ≤4.00  (2).

3. The cold-rolled steel sheet for vitreous enameling according to claim 1 , further comprising, by mass %,

one or more elements selected from the group consisting of Cr, V, Zr, Ni, As, Ti, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, and Mg,

wherein a total amount of the elements is 0.1% or lower.

4. An enameled product which is produced using the cold-rolled steel sheet for vitreous enameling according to claim 1 .

5. An enameled product which is produced using the cold-rolled steel sheet for vitreous enameling according to claim 3 .

6. The cold-rolled steel sheet for vitreous enameling according to claim 2 , further comprising, by mass %,

one or more elements selected from the group consisting of Cr, V, Zr, Ni, As, Ti, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, and Mg,

wherein a total amount of the elements is 0.1% or lower.

7. An enameled product which is produced using the cold-rolled steel sheet for vitreous enameling according to claim 2 .