ROLLING BEARING STEEL HAVING A SURFACE WITH A LOWER BAINITIC STRUCTURE AND A METHOD FOR THE PRODUCTION THEREOF

Abstract: Method for producing a rolling bearing component, such as a rolling bearing ring from the 1C-1.5Cr type series steel as well as a rolling bearing component produced by said method. At least the surface of the rolling bearing component has a bainitic structure. It has been found that the service life of said structure is considerably increased over a component not subjected to cold rolling. Preferably a lower bainitic structure is used which can be obtained starting form usual 1C-1.5Cr type steel in ferritic condition. Before austenizing the steel is subjected to cold deformation. Quenching is such that the bainitic structure is obtained.

Description: Rolling bearing steel having a surface with a lower bainitic structure and a method for the production thereof. The invention relates to a rolling bearing steel from the 1 C - 1,5 Cr type series. Such steel comprises generally the following composition: Carbon: 0.85 - 1.10 by weight % Silicon: 0.005 - 0.6 by weight % Manganese: 0.005 - 0.80 by weight % Chromium 1.25 - 2.05 by weight % Nickel: 0.35 max by weight % Molybdenum 0.36 max by weight % balance Fe and usual impurities. Steels within this composition are is widely used in the production of rolling bearing components. Starting from a ferritic structure the steel is subjected to an austenizing heat treatment after which a quenching treatment results in a martensitic surface structure of the final component. This martensitic structure is relatively hard and has good basic properties. For applications wherein the rolling contact fatigue life and toughness are of interest carburised steels are used. The carburising steels and heat treatments are more costly and the related heat treatments are generally much more complicated. EP 0896068 Al discloses a method of bainite hardening of a bearing steel. To that end the starting material is in ferritic condition, is austenized followed by quenching such that a bainitic final structure results. The invention aims to obtain a steel with improved properties and more particular having improved rolling contact fatigue and good toughness properties. According to the invention this is realised in that the steel in ferritic condition is subjected to a deformation. This deformation can either be warm or cold. If warm deformation is used, preferably a deformation in the ferrite phase, i.e. of a temperature below 700 °C takes place. During warm forming the dislocation cells ob- tained during deformation recover to form fine sub grains during heating to the hardening temperature and therefore a finer structure as a result of the applied lower bainitic hardening process is obtained. More particular the steel is subjected to shaping by rolling. More preferable, if a ring has to be produced as a rolling bearing component starting from a tube, cold deformation is effected during which also the ring itself is produced from the tube with less metal cutting operations. This means that there is less material loss. It has been found that if cold rolling is used the austenite start and the austenite finish temperature will decrease, i.e. the transformation from ferrite to austenite will be at a lower temperature level and will be more complete at the same temperature level. The bainitic transformation time is preferably at least 180 minutes. Except from lowering the austenizing temperatures by rolling and more particular cold rolling the martensite start temperature is also lowered by about 30°C and well below 250°C. Generally the microstructure shows a much-refined grain. Preferably the bainite comprises lower bainite which re- suits in an extra extension of the service life of rolling bearing components made from such steel. It is of course possible to start from an other article as a tube at deformation. For example parts or rings are mentioned possible followed by a pri shaping process (turning, milling). Cold forming can comprise rolling, forging, shaping and so on. The ferrite subgrain boundaries are probably austenite nuclination sites at the intersections with spheroidal carbides, which result in refinement of the austenite grain size compared to undeformed 1C - 1.5 Cr austenized under the same conditions . The steel used is preferably relatively pure, i.e. comprises 9 ppm oxygen max, 0.004 wt % sulphur max. 15 ppm titanium max and 0.015 wt % phosphorus max.. To show the beneficial effect of cold rolling relative to hot rolling when producing a rolling bearing ring comparative tests have been conducted. Apart from either the hot rolling or cold rolling, the heat treatment in both samples has been exactly the same. It has been shown that in a spherical roller bearing the relative L10 life of the hot rolled variant is 106 with 95 upper and lower confidence interval of 52-157 mil- lion revolutions. Under the same test condition a cold rolled ring had an L10 life of more than 294 million revolutions. Although not essential for the scope of protection for the invention it is meant that because of cold rolling of the ferritic matrix dislocations recover to cells resulting in sub grain formation. This sub grain formation will lead to finer austenite. Quenching will be starting from the temperature above martensite start. The treatment described above is an alternative for a rolling bearing steel having a generally lower carbon content to increase the rolling contact fatigue life . Such steel will generally be carburised or carbonitrided to increase the surface hardness to a sufficient level. The % deformation will have an effect on the size.of the austenite grains obtained during the austenitizing treatment. A relatively low deformation will result in a coarse material having a grain size of several μm. However, if considerable deformation is used, for example more than 30% and more particular more than 60% the grain size will decrease considerably to below 2 μm. It is noted that the scope of protection is not limited to the embodiments given in the description but is determined by the appended claims. $*) ! --**

Claims: CL A IM S 1. Method for producing a rolling bearing steel comprising the provision of an 1 C-l .5 Cr type steel in ferritic condition, austenizing said steel followed by quenching the- reof such that a bainitic final structure results, characterised in that, the steel is deformed in ferritic condition. 2. Method according to claim 1, wherein said deformation comprises a cold deformation. 3. Method according to claim 1 or 2, wherein said deformation comprises at least 30%. 4. Method according to one the proceeding claims wherein said austenizing temperature is between 800 and 900° C. 5. Method according to one of the proceeding claims, wherein said quenching temperature is below 250° C. 6. Method according to one of the proceeding claims, wherein said quenching treatment is such that lower bainite results. 7. Method for producing a 1C - 1,5 Cr steel roller bearing ring, comprising cold rolling of a tube blank having a ferritic matrix structure, separating said tube into rings, austenizing of said steel followed by quenching such that a bainitic structure results. 8. Rolling bearing steel produced with the method according to one of the proceeding claims wherein at least the surface comprises a bainitic structure and does not comprise martensite. 9. Rolling bearing component procuded as a rolling bearing steel according to claim 8. 10. Rolling bearing component according to claim 9 comprising a spherical roller bearing component.

Inventors: BESWICK JOHN MICHAEL, FOGELSTROM JOACIM, KERRIGAN AIDAN MICHAEL, STRANDELL HAN INGEMAR, BESWICK, JOHN, MICHAEL, FOGELSTROM, JOACIM, KERRIGAN, AIDAN, MICHAEL, STRANDELL, HAN, INGEMAR
Applicants: BESWICK JOHN MICHAEL, FOGELSTROM JOACIM, KERRIGAN AIDAN MICHAEL, SKF ENG & RES CENTRE BV, STRANDELL HAN INGEMAR, BESWICK, JOHN, MICHAEL, FOGELSTROM, JOACIM, KERRIGAN, AIDAN, MICHAEL, SKF ENGINEERING & RESEARCH CENTRE B.V., STRANDELL, HAN, INGEMAR
Application year: 2000
Main classification: C21D9
IPC codes: C21D1, C21D8, C21D9, C22C38