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JIS SCM440M (SAE4140H), heat treated to the strength level of 120 to 140 kgf/mm2(171 to 199 ksi) -ISO 12.9 class-, is currently used for cylinder head bolts of Japanese passenger cars. Lower alloy steels, such as SAE 1541 for example, have not been substituted for JIS SCM440H so far because of their high susceptibility to delayed fracture. Daido Steel has tackled this problem and succeeded in applying the lower alloy SAE 1541 steel to 12.9 class cylinder head bolts by enhancing the resistance to delayed fracture by reducing impurities, especially sulphur. In this paper mechanical properties and delayed fracture characteristics of SAE 1541-ULS (Ultra Low Sulphur) steel are reported. 1541-ULS (S<0.005%, S+P< 0.020%) shows outstanding resistance to delayed fracture compared to conventional steel. Furthermore, the amount of MnS inclusions decreases remarkably in ULS steel, which results in high toughness.

In order to save molybdenum (Mo) in chromium - molybdenum steels for automobile components, medium carbon - boron steels were investigated. Boron is not a new alloying element for structural steels, however, to date boron steels have not been widely used because of their unstable hardenability and poor machinability. Therefore, in this paper, the optimum content of boron was reexamined, and also the appropriate addition of titanium as a stabilizer of boron was investigated from the view point of hardenability. Furthermore the upper limit of manganese (Mn) content was studied to keep good machinability. The new steel grades, established on the basis of the above fundamental research, have been used on vital components of passenger cars.