Steel wire for cold-formed spring excellent in corrosion resistance and method for producing the same

A steel wire for a spring containing C:

• 0.45-0.54%, Si:
• 1.8-2.5%, Mn:
• 0.05-0.9%, Cr:
• 0.05-2.0%, and at least one kind selected from among the group of Nb:
• 0.01-0.10%, V:
• 0.07-0.40% and Mo:
• 0.10-1.0%, comprising a fraction of martensite and bainite of 60% or less in area percentage, wherein
• P and S are controlled to 0.020% or less (including 0%), respectively;
• a martensitic transformation start temperature Ms2, shown by the following expression (2), is in the range from 280° C. to 380° C., where
• MS2=550−361[C]−39[Mn]−20[Cr]−35[V]−5[Mo] (2), and
• [C], [Mn], [Cr], [V] and [Mo] represent the contents (mass %) of C, Mn, Cr, V and Mo, respectively;
• and
• the steel wire has been austenized and tempered such that
• the prior austenite grain size number N of austenite grains is No. 12 or more,
• the grain boundary share of carbide precipitated along the austenite grain boundaries is 50% or less,
• the amount of retained austenite is 20 vol. % or less, and
• the tensile strength is 2,000 MPa or more.

A steel wire for a spring containing C:

• 0.45-0.54%, Si:
• 1.8-2.5%, Mn:
• 0.05-0.9%, Cr:
• 0.05-2.0%, and at least one kind selected from among the group of Ni:
• 0.05-1.0%, Cu:
• 0.05-1.0% and W:
• 0.10-1.0%, comprising a fraction of martensite and bainite of 60% or less in area percentage, wherein
• P and S are controlled to 0.020% or less (including 0%), respectively;
• a martensitic transformation start temperature MS3, shown by the following expression (3), is in the range from 280° C. to 380° C., where
• MS3=550−361[C]−39[Mn]−20[Cr]−17[Ni]−10[Cu]−5[W] (3), and
• [C], [Mn], [Cr], [Ni], [Cu] and [W] represent the contents (mass %) of C, Mn, Cr, Ni, Cu and, respectively;
• and
• the steel wire has been austenized and tempered such that
• the prior austenite grain size number N of austenite grains is No. 12 or more,
• the grain boundary share of carbide precipitated along the austenite grain boundaries is 50% or less,
• the amount of retained austenite is 20 vol. % or less, and
• the tensile strength is 2,000 MPa or more.

A steel wire for a spring containing C:

• 0.45-0.54%, Si:
• 1.8-2.5%, Mn:
• 0.05-0.9%, Cr:
• 0.05-2.0%, at least one kind selected from among the group of Nb:
• 0.01-0.10%, V:
• 0.07-0.40% and Mo:
• 0.10-1.0%, and at least one kind selected from among the group of Ni:
• 0.05-1.0%, Cu:
• 0.05-1.0% and W:
• 0.10- 1.0%, comprising a fraction of martensite and bainite of 60% or less in area percentage, wherein
• P and S are controlled to 0.020% or less (including 0%), respectively;
• a martensitic transformation start temperature MS4, shown by the following expression (4), is in the range from 280° C. to 380° C., where
• MS4=550−361[C]−39[Mn]−20[Cr]−35[V]−5[Mo]−17[Ni]−10[Cu]−5[W] (4),
• and
• [C], [Mn], [Cr], [V], [Mo], [Ni], [Cu] and [W] represent the contents (mass %) of C, Mn, Cr, V, Mo, Ni, Cu and, respectively;
• and
• the steel wire has been austenized and tempered such that
• the prior austenite grain size number N of austenite grains is No. 12 or more,
• the grain boundary share of carbide precipitated along the austenite grain boundaries is 50% or less,
• the amount of retained austenite is 20 vol. % or less, and
• the tensile strength is 2,000 MPa or more.