The invention relates to an engineering steel comprising a steel of the 1 C-1.5 Cr type rolling bearing steels, having: -0.9-1.0 % by wt C; -0.15-. 0.40% by wt Si, -0.25-0.80 % by wt Mn, -1.30-1.95% by wt Cr, -0.25 % by wt Ni max and -0.35% by wt Mo max.

Such a steel is generally known and, for example, used to produce rolling bearing components. However, it should be noted that the invention is not restricted to the production of a rolling bearing component. Steel, according to the invention can be used as well for other applications such as parts of a continuously variable transmission wherein rollers move relative to discs.

Generally through a heat treatment the surface of a component made from the above engineering steel will be martensite. This is relatively hard and has good contact fatigue and wear resistance properties. However, the toughness of martensite is relatively low. To obviate this problem it is suggested in the prior art to start from steel having a lower carbon percentage and use a carburising/carbonitriding treatment to enrich the surface in order to obtain a sufficiently hard surface martensite. Because of the lower carbon content of the core, the core of the related component will be much tougher.

This method is relatively costly and time consuming because of the carburising/carbonitriding treatment.

The invention aims to provide an engineering steel from which rolling bearing components could be produced having a much tougher structure without sacrifying too much of its hardness properties.

According to the invention this is realised in that at least 0.05 wt % Mo is present, the structure of the steel comprising ultra fine bainite and having the structure according to fig. 5. It has been found that ultra fine bainite results in much improved toughness whilst the hardness of the surface is sufficient for most rolling bearing applications. The length of the fine lenticular-shaped carbide is preferably about 250

nm and a width of about 80 nm. Preferably it is orientated at 50 to 65 degrees and more particular 55 to 60 degrees relative to the ferritic spine.

The engineering steel according to the invention can be produced with any method known in the art. Preferably the starting steel should be relatively pure, i. e. be of a high internal cleanliness standard. For example such steel being in the ferritic, matrix condition should comprise maximum 9 ppm oxygen, 0.04 wt % sulphur, 15 ppm titanium and 0.015 wt % phosphorus. Starting from such steel it has been found that through cold deformation, e. g. during the production of rings from a tube blank, the austenite start and austenite finish temperature will be lowered. This also relates to the martensite start temperature. In order to obtain the ultra fine bainite, quenching is effected at a temperature somewhat above the martensite start temperature. For example quenching to 250°C and holding at this temperature.

The lower bainite transformation in relatively high carbon through hardening steels is a diffusion controlled transformation within the ferrite after transformation of the austenite phase. The lower the temperature of the transformation, the longer the time for the transformation and the finer the product of the transformation and the higher the surface hardness. Through the use of cold deformation in the ferritic phase the transformation temperature to lower bainite can be decreased. The typical temperature to start transformation is about 250°C and the typical duration is about 180 minutes. This is further elucidated in fig. 1 which shows a typical relationship of time- temperature for transformation to lower bainite in a 1C-1.5 Cr type steel.

Lower bainite transformation takes place in the ferrite, but before this can take place, the ferrite has to form from the austenite, this being a shear induced transformation. The partitioning of the alloying elements, originally in solid solution in the austenite, to the remaining non-transformed residual austenite will result in an increased shear resistance. This increased shear resistance will require a greater degree of undercooling to initiate the shear transformation from austenite to ferrite.

The literature teaches us that the the stages in the lower bainite transformation can be described referring to fig. 2a-d as a four stage process, namely: a. The formation a ferrite'spine'across the austenite grain, the length therefore being austenite grain size dependent. b. Secondary nucleation of ferrite platelets, generally oriented on one side of the

sspine,'at an angle of 55 to 60 degrees to the ferrite'spine'formed in stage a. c. The precipitation of carbides at the ferrite to austenite interface resulting in gaps in the adjacent secondary ferrite platelets. The higher the temperature the coarser the carbides. d. During the transformation the gaps are filled by growth of the ferrite and additiottal carbide precipitation resulting in the lower bainite microstructure.

Fig. 3 compares the relative toughness against the impact test speed both for martensite (dotted line) and bainite (solid line). Fig. 5 is a photo micrograph of the final ultra fine bainite structure obtained. This is a transmission electron microscope photograph is made in the following conditions: The microstructure is described by electrolytic thinning of the heat treated steel to make a so called thin foil which is transparant to the electron beam in the transmission eletron microscope. By suitable imaging the ultra fine structure of the lower bainitic structure can be resolved. This photo micrograph shows ultra fine bainite in a 52100 steel. Finally fig. 4 shows relative impact toughness for martensite (solid line) and bainite as well as bainite according to the invention i. e. being molybdenum alloyed.

As indicated above the steel according to the invention is particularly useful in the production of spherical rolling bearing rings, but has other applications. Depending on the application the composition of the steel can be adapted within the range given in the independent claim.