2005 年 2005 巻 50 号 p. 1-9
In order to improve the durability of the coil springs used in the automotive suspensions against the corrosion fatigue fracture and the delayed fracture, the effects of the alloying and the chemical passivation treatment with HNO3 solution of 63 mass% concentration on that durability were investigated through the following procedures. Firstly, the anordic polarization curves were measured for 4 commercial spring steels and 22 laboratory steels. Secondly, the quasi-pitting potential of V'cq100 was analyzed from these polarization curves to discuss those effects. Then, the commercial spring steels whose pitting resistance was enhanced by the measures noted above were put to the following tests: the slow strain rate tensile test using the test pieces corroded by wet-dry cyclic corrosion in NaCl solution of 5 mass% concentration; and the rotating bending fatigue test under wet condition in dropping NaCl solution of 5 mass% cocentration. The major results are as follows.
(1) The effective alloy elements to improve the pitting corrosion resistance are Si, Ni, Cr, Cu, Co, Sb, and B.
(2) The chemical passivation treatment for 1 hour is effective against the pitting corrosion for the spring steels with less alloy elements.
(3) The guide to the alloying design is proposed from the regression of V'cq100 on the content of alloy elements.
(4) The spring steel (HDS13) with the alloy elements effective against pitting corrosion showed superior hydrogen embrittlement resistance and corrosion fatigue strength.
(5) It was found that the addition of alloy elements noted in (1) and the chemical passivation treatment were effective to reduce the sensitivity to the corrosion fatigue and the hydrogen embrittlement.