Steel sheet for soft-nitriding treatment, method of manufacturing same, and soft-nitrided steel

1. A steel sheet for soft-nitriding treatment which

has a chemical composition consisting of, in mass %,

C: more than or equal to 0.02% and less than 0.07%,

Si: less than or equal to 0.10%,

Mn: 1.1 to 1.8%,

P: less than or equal to 0.05%,

S: less than or equal to 0.01%,

Al: 0.10 to 0.45%,

N: less than or equal to 0.01%,

Ti: 0.01 to 0.10%,

Nb: 0 to 0.1%,

Mo: 0 to 0.1%,

V: 0 to 0.1%,

Cr: 0 to 0.2%, and

the balance: Fe and impurities,

satisfies the following formula (i), and

has a total content of Ti, Nb, Mo, V, and Cr present as precipitates in the steel sheet of less than 0.03% in mass %,

wherein the steel sheet for soft-nitriding treatment has a metal structure in which a ferrite area ratio is more than or equal to 80%, and a ferrite dislocation density at a position of 50 μm from a surface of the steel sheet is 1×10 14 to 1×10 16 m −2 ,

Mn+Al≥1.5  (i)

where each chemical symbol included in the formula represents a content (mass %) of each element contained in the steel sheet.

2. The steel sheet for soft-nitriding treatment according to claim 1 ,

wherein the chemical composition comprises, in mass %, one or more selected from

Nb: 0.005 to 0.1%,

Mo: 0.005 to 0.1%,

V: 0.005 to 0.1%, and

Cr: 0.005 to 0.2%.

3. A method of manufacturing a steel sheet for soft-nitriding treatment, the method comprising:

starting rolling of a steel raw material having a chemical composition recited in claim 1 after the steel raw material is heated to higher than or equal to 1150° C., and ending the rolling at finishing temperature of higher than or equal to 900° C.;

performing coiling, after cooling, in a temperature region of 470 to 530° C. to cause ferrite area ratio to be more than or equal to 80%;

thereafter subjecting the steel raw material to pickling; and

subjecting the steel raw material to skin pass rolling after the pickling in conditions in which a rolling reduction ratio is 0.5 to 5.0%, and F/T (mm) is more than or equal to 8000, said F/T (mm) being a ratio of a line load F (kg/mm) determined by dividing a rolling mill load by a width of the steel sheet to a load T (kg/mm 2 ) per unit area applied in a longitudinal direction of the steel sheet to thereby obtain the steel sheet of claim 1 .

4. A soft-nitrided steel having a chemical composition consisting of, in mass %,

C: more than or equal to 0.02% and less than 0.07%,

Si: less than or equal to 0.10%,

Mn: 1.1 to 1.8%,

P: less than or equal to 0.05%,

S: less than or equal to 0.01%,

Al: 0.10 to 0.45%,

Ti: 0.01 to 0.10%,

Nb: 0 to 0.1%,

Mo: 0 to 0.1%,

V: 0 to 0.1%,

Cr: 0 to 0.2%, and

the balance: Fe and impurities,

wherein, at a depth position of 50 μm from an outermost surface, nitrides are precipitated on a {001} plane in a ferrite crystal,

an average value of maximum lengths of the respective nitrides is 5 to 10 nm, and

a number density of nitrides is more than or equal to 1×10 24 m −3 .

5. The soft-nitrided steel according to claim 4 ,

wherein the chemical composition contains, in mass %, one or more selected from

Nb: 0.01 to 0.1%,

Mo: 0.01 to 0.1%,

V: 0.01 to 0.1%, and

Cr: 0.01 to 0.2%.

6. The soft-nitrided steel according to claim 4 ,

wherein a Mn concentration in metal elements included in the nitrides is more than or equal to 80 at %.