MANUFACTURING METHOD OF STEEL COMPONENT AND STEEL COMPONENT

1 . A manufacturing method of a steel component, comprising:

heating a steel sheet in a carburizing atmosphere to form a carburized layer on a surface of the steel sheet, the steel sheet having:

a chemical composition represented by:

in mass %,

C: 0.0005 to 0.1%;

Si: 0.01 to 2.0%;

Mn: 0.05 to 3.0%;

Al: 0.9% or less;

P: 0.05% or less;

S: 0.01% or less;

Ti: 0.0 to 0.2%;

Nb: 0.0 to 0.1%;

Cr: 0 to 2%;

Mo: 0.0 to 0.2%;

B: 0.000 to 0.005%; and

the balance: Fe and impurities; and

a steel structure represented by

ferrite with an area fraction of 70% or more; and

forming the steel sheet by using metal dies, and performing quenching on the steel sheet in a state of housing the steel sheet in the metal dies to transform the carburized layer into martensite and make a part of the steel sheet on the further inside than the carburized layer to be a steel structure represented by ferrite with an area fraction of 50% or more.

2 . The manufacturing method of the steel component according to claim 1 , wherein

in the formation of the carburized layer on the surface of the steel sheet, the steel sheet is subjected to soaking at a temperature of 820° C. or more.

3 . The manufacturing method of the steel component according to claim 2 , wherein

in the formation of the carburized layer on the surface of the steel sheet, the steel sheet is subjected to soaking for one hour or more.

4 . The manufacturing method of the steel component according to claim 3 , further comprising

performing working on the steel sheet at an equivalent strain of less than 5% or 20% or more, before the formation of the carburized layer on the surface of the steel sheet.

5 - 7 . (canceled)

8 . The manufacturing method of the steel component according to claim 2 , further comprising

performing working on the steel sheet at an equivalent strain of less than 5% or 20% or more, before the formation of the carburized layer on the surface of the steel sheet.

9 . The manufacturing method of the steel component according to claim 1 , wherein

in the formation of the carburized layer on the surface of the steel sheet, the steel sheet is subjected to soaking for one hour or more.

10 . The manufacturing method of the steel component according to claim 9 , further comprising

performing working on the steel sheet at an equivalent strain of less than 5% or 20% or more, before the formation of the carburized layer on the surface of the steel sheet.

11 . The manufacturing method of the steel component according to claim 1 , further comprising

performing working on the steel sheet at an equivalent strain of less than 5% or 20% or more, before the formation of the carburized layer on the surface of the steel sheet.

12 . A steel component, comprising:

a base material having:

a chemical composition represented by:

in mass %,

C: 0.0005 to 0.1%;

Si: 0.01 to 2.0%;

Mn: 0.05 to 3.0%;

Al: 0.9% or less;

P: 0.05% or less;

S: 0.01% or less;

Ti: 0.0 to 0.2%;

Nb: 0.0 to 0.1%;

Cr: 0 to 2%;

Mo: 0.0 to 0.2%;

B: 0.000 to 0.005%; and

the balance: Fe and impurities; and

a steel structure represented by in an area fraction,

ferrite: 50% or more; and

a carburized layer made of martensite and existing on a surface of the base material.

13 . The steel component according to claim 12 , wherein:

the carburized layer made of the martensite contains C of 0.5 to 0.9% in mass %; and

in a range of t/4 to 3t/4 from a surface of the steel component when a thickness of the steel component is set to t, a total area fraction of ferrite grains each having a grain diameter of 200 μm or more in terms of a circle-equivalent diameter is 5% or less.

14 . The steel component according to claim 13 , wherein

an effective case depth is 0.05 to 0.5 mm.

15 . The steel component according to claim 12 , wherein

an effective case depth is 0.05 to 0.5 mm.