STEEL WIRE FOR SPRING AND MANUFACTURING METHOD THEREFOR

PROBLEM TO BE SOLVED: To provide a steel wire for a spring capable of enhancing settling resistance and fatigue properties by a manufacturing process without depending on relying on addition of alloy elements.SOLUTION: A steel wire for a spring has a structure obtained by hardening and tempering, comprises a first layer 1 on a surface, a second layer 2 more inside than the first layer 1 and a third layer 3 more inside than the second layer 2 and reaching a center, where hardness of the second layer 2 is lower than the first layer 1 and the third layer 3, the first layer 1 has lower average crystal particle diameter than the second layer 2, hardness of the first layer 1 and the third layer 3 are 500 to 700 HV and hardness of the second layer 2 is 400 to 650 HV. A manufacturing method of the steel wire for a spring includes a hardening process of heating the whole of the steel wire to a temperature higher than an austenite transformation point and then hardening it, a border hardening process of heating only a surface layer of the steel wire to a temperature higher than the austenite transformation point and cooling a center part from a state at a temperature lower than a tempering temperature in a next process and a tempering process of heating the whole of the steel wire.SELECTED DRAWING: Figure 1 【課題】合金元素の添加に頼らず、製造プロセスによって耐へたり性と疲労特性を向上させることができるばね用鋼線を提供する。【解決手段】焼入れ焼戻しにより得られる組織を有するばねであって、表面の第１層１と、該第１層１よりも中側の第２層２と、第２層２よりも中側で中心に至る第３層３とからなり、第２層２は第１層１および第３層３よりも硬さが低く、第１層１は第２層２と比べて平均結晶粒径が小さく、第１層１および第３層３の硬さは５００〜７００ＨＶであり、第２層２の硬さが４００〜６５０ＨＶであるばね用鋼線。鋼線の全体をオーステナイト変態点よりも高い温度に加熱してから焼入れする焼入れ工程と、鋼線の表層のみオーステナイト変態点よりも高い温度に加熱し、中心部は次工程での焼戻し温度よりも低い温度の状態から冷却する輪郭焼入れ工程と、鋼線の全体を加熱する焼戻し工程と、を備えた、ばね用鋼線の製造方法。【選択図】図１