Abstract: 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.

Abstract: 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.

Abstract: A steel for high-strength spring has an Ac3 transformation temperature as an indicator of the decarburization performance, which is calculated by Equation (1) below, is from 859 to 885° C., a maximum hardened diameter DI as an indicator of the hardening performance, which is calculated by Equation (2) below, is from 70 to 238 mm, and a temper hardness HRC as an indicator of the spring performance, which is calculated by Equation (3) below, is from 50 to 55. Ac3=910?203×?{square root over (C)}?15.2Ni+44.7Si+104V+31.5Mo+13.1W??(1) DI=DO×fSi×fMn×fP×fS×fCu×fNi×fCr??(2) HRC=38.99+17.48C+2.55Si?2.28Ni+2.37Cr+8.04Ti??(3) wherein, D0=8.65×?{square root over (C)}, fSi=1+0.64×% Si, fMn=1+4.10×% Mn, fP=1+2.83×% P, fS=1?0.62×% S, fCu=1+0.27×% Cu, fNi=1+0.52×% Ni, and fCr=1+2.33×% Cr.

Abstract: A steel for high-strength spring has an Ac3 transformation temperature as an indicator of the decarburization performance, which is calculated by Equation (1) below, is from 859 to 885° C., a maximum hardened diameter DI as an indicator of the hardening performance, which is calculated by Equation (2) below, is from 70 to 238 mm, and a temper hardness HRC as an indicator of the spring performance, which is calculated by Equation (3) below, is from 50 to 55. Ac3=910?203×?{square root over (C)}?15.2Ni+44.7Si+104V+31.5Mo+13.1W??(1) DI=DO×fSi×fMn×fP×fS×fCu×fNi×fCr??(2) HRC=38.99+17.48C+2.55Si?2.28Ni+2.37Cr+8.04Ti??(3) wherein, D0=8.65×?{square root over (C)}, fSi=1+0.64×% Si, fMn=1+4.10×% Mn, fP=1+2.83×% P, fS=1?0.62×% S, fCu=1+0.27×% Cu, fNi=1+0.52×% Ni, and fCr=1+2.33×% Cr.