A carburization device includes a heating furnace which heats a material, a transfer mechanism, an alcohol vapor generator, an alcohol vapor spraying portion, a quenching tank, and an exhaust heat intake tube. The transfer mechanism moves a plurality of materials. The alcohol vapor generator uses part of heat generated by the heating furnace as a heat source. As the alcohol vapor spraying portion repeats a vapor spraying step and a diffusion step a plurality of times in the heating furnace. In the vapor spraying step, by spraying alcohol vapor on the material which moves inside the heating furnace, carbon in the alcohol is adsorbed to the material. In the diffusion step, an interval for diffusing the carbon adsorbed to the material is taken.

An aspect of the present invention relates to a wire rod for springs with high strength and excellent corrosion fatigue resistance, in which a combination of Cr, Cu, and Ni content is controlled to an appropriate level, the maximum depth of corrosion pits is set to be below a certain level, and fine carbides containing Mo are set to be at a certain level or greater.

An aspect of the present invention relates to a wire rod for springs with high strength and excellent corrosion fatigue resistance, in which a combination of Cr, Cu, and Ni content is controlled to an appropriate level, the maximum depth of corrosion pits is set to be below a certain level, and fine carbides containing Mo are set to be at a certain level or greater.

A leaf spring device includes a main leaf made of a steel plate including an elastic section configured to generate elastic force when bent; and an eye section formed in an end portion of the elastic section, the elastic section and the eye section being tempered. There is also provided a method for manufacturing the leaf spring device. The eye section is formed by rolling the end of the elastic section into a circular form. The eye section is tempered at a higher temperature than the elastic section.

A leaf spring device includes a main leaf made of a steel plate including an elastic section configured to generate elastic force when bent; and an eye section formed in an end portion of the elastic section, the elastic section and the eye section being tempered. There is also provided a method for manufacturing the leaf spring device. The eye section is formed by rolling the end of the elastic section into a circular form. The eye section is tempered at a higher temperature than the elastic section.

A method for manufacturing steel, by quenching and tempering a seamless pipe for use as a material of a hollow spring, where the seamless pipe including predetermined components is subjected to a heat treatment which is performed to satisfy quenching conditions (1) and tempering conditions (2), (1) quenching conditions:
26,000(T1+273)(log(t1)+20)29,000
900 C.T11,050 C.,
10 secondst11,800 seconds,formula (1) where T1 is a quenching temperature ( C.), and t1 is a holding time (seconds) in a temperature range of 900 C. or higher, and (2) tempering conditions:
13,000(T2+273)(log(t2)+20)15,500
T2550 C., and
t23,600 seconds,formula (2) where T2 is a tempering temperature ( C.), and t2 is a total time (seconds) from start of heating to completion of cooling.

A method for manufacturing steel, by quenching and tempering a seamless pipe for use as a material of a hollow spring, where the seamless pipe including predetermined components is subjected to a heat treatment which is performed to satisfy quenching conditions (1) and tempering conditions (2), (1) quenching conditions:
26,000(T1+273)(log(t1)+20)29,000
900 C.T11,050 C.,
10 secondst11,800 seconds,formula (1) where T1 is a quenching temperature ( C.), and t1 is a holding time (seconds) in a temperature range of 900 C. or higher, and (2) tempering conditions:
13,000(T2+273)(log(t2)+20)15,500
T2550 C., and
t23,600 seconds,formula (2) where T2 is a tempering temperature ( C.), and t2 is a total time (seconds) from start of heating to completion of cooling.

A spring consists of, by mass %, 0.5 to 0.7% of C, 1.0 to 2.0% of Si, 0.1 to 1.0% of Mn, 0.1 to 1.0% of Cr, not more than 0.035% of P, not more than 0.035% of S, and the balance of Fe and inevitable impurities. The spring has a structure including not less than 65% of bainite and 4 to 13% of residual austenite by area ratio in a cross section. The spring has a compressive residual stress layer in a cross section from a surface to a depth of 0.35 mm to D/4, in which D (mm) is a circle-equivalent diameter of the cross section. The spring has a high hardness layer with greater hardness than a center portion by 50 to 500 HV from a surface to a depth of 0.05 to 0.3 mm.

A spring consists of, by mass %, 0.5 to 0.7% of C, 1.0 to 2.0% of Si, 0.1 to 1.0% of Mn, 0.1 to 1.0% of Cr, not more than 0.035% of P, not more than 0.035% of S, and the balance of Fe and inevitable impurities. The spring has a structure including not less than 65% of bainite and 4 to 13% of residual austenite by area ratio in a cross section. The spring has a compressive residual stress layer in a cross section from a surface to a depth of 0.35 mm to D/4, in which D (mm) is a circle-equivalent diameter of the cross section. The spring has a high hardness layer with greater hardness than a center portion by 50 to 500 HV from a surface to a depth of 0.05 to 0.3 mm.

An arcuate spring having a plurality of coils which are configured and dimensioned to provide an arcuate shape to the spring and being substantially free of internal stresses which would tend to urge the coils into linear alignment. The spring is designed to function under load conditions while maintaining its natural arcuate shape. The spring is can be heated by use of an induction heating process.