METHOD FOR PRODUCING A ROLLING BEARING STEEL

Abstract: Method for producing a steel and more particularly a rolling bearing steel as well as a rolling bearing component produced with such a method. In order to be able to directly roll the steel after continuous casting or to roll the steel immediately after heating without a soaking heat treatment, it is proposed to use a steel comprising nominally 0.96 wt% C, 0.25 wt% Si, 0.85 wt% Mn and 0.8 wt% Cr. Such a steel can favourably be used in rolling bearing components and more particularly bearing rings.

Description: Method for producing a rolling bearing steel The present invention relates to a method for producing a steel, comprising the provision of a steel containing : - 0.9 -1.03 wt% C; 0.15-0.35 wt% Si; 0.75-1.00 wt% Mn; 0.70-0.90 wt% Cr; and balance Fe and impurities, wherein said steel is produced by continuous billet casting followed by rolling. The use of blooms continuous casting is well known and the definitions of blooms casting is that the cast size requires two heating stages prior to rolling the strand to the final bar or the round. See Hengerer et al. 'Evaluation of the Continuous Casting ASTM STP 1195, 1993,' pages 237-251. In order to reduce the costs of producing engineering steels it is suggested in the prior art to produce such steel by continuous billet casting, see J.Lichtenstein, Brunei's Steel World Quarterly Vol. Issue 4, pages 25-29. The most generally used rolling bearing steel ASTM A295 52100, comprising approximately 1 wt% C, 0.25 wt% Si, 0.35 wt% Mn and 1.45 wt % Cr can be used for this purpose. However it is necessary to introduce a soaking treatment after continuous casting and before rolling. This is necessary because during solidification, segregation of the alloying elements C and Cr produces primary carbides which will persist in the steel if a reheating process and a soaking heat treatment is not used. The segregation and primary carbides result in voids after hot working and other defects in a final roll- ing bearing component. Such a soaking treatment will necessarily be at relatively high temperatures and during relatively long times. This means that the advantage of continuous casting of ASTM A295 52100 steel is at least partially reduced, because of the increasing energy costs. Generally such soaking heat treatment is not effected in a special atmosphere and steel losses result because of scaling. In view of the above drawbacks continuous casting of bearing steels has not fulfilled its full economic potential as suggested. GB 2294270A discloses a bearing steal comprising 0.70-.093 wt% C, 0.15-.0.50 wt% Si 0.50-1.10 wt% Mn and 0.30-0.65 wt% Cr. The Cr/C ratio is within the range of .4-.7. EP 0825270A1 discloses a bearing metal comprising 0.95-1.10 wt% C, 0.15-0.70 wt% Si, 1.15 wt% max Mn, 0.90-1.60 wt% Cr. In view of the cost reduction being presently possibly through the use of continuous casting the invention aims to provide a method for producing a steel wherein such a soaking treatment to remove segregation can be avoided and thereby achieve the full cost saving potential without a major sacrifice in quality. According to the invention this is realised with a method as described above in that said steel comprises nominally 0.96 wt% C; 0.25 wt% Si; 0.85 wt% Mn and 0.8 wt% Cr, and in that either immediately after casting said steel is rolled or is rolled after casting and reheating without a soaking heat treatment, wherein the Cr/C ratio is between 0.7 and 1.20. Surprisingly it has been found that a steel having a lower carbon and chromium content and a somewhat higher manganese content does not show the segregation and primary carbide formation as is found in ASTM A295 52100 steels. This means that it is now possible to immediately roll steel after continuous casting or to effect rolling in a later stage wherein immediately after heating the billet the steel can be rolled without a soaking treatment. It is assumed that because of a somewhat lower carbon content and a considerably decreased chromium content, segregation and primary carbide problems are (gener- ally) avoided. However it must be understood that this is only a theory from which the validity of the appended claims is not dependent. According the a preferred embodiment of the invention the Cr/C ratio is between .75 and .95. According to a further preferred embodiment the Cr/C ratio is between 0.78 and 0.87. Preferably the steel described above comprises nominally 0.96 wt% by wt. C, 0.25 wt% Si, 0.85 wt% Mn and 0.80 wt% Cr. Such a steel is also known as ASTM A295 5195. Hardening of this steel can either be obtained by through martensitic hardening or surface (induction) hardening. The suitability of the ASTM A295 5195 steel as a substitute for ASTM A295 52100 is addressed in figures 1 and 2 which show the martensitic through harde- nability and tempering response. Although the steel as described above can be used for all kinds of rolling bear- ings, preferably bearings which are not subjected to severe loading are particularly suited. A particular application area is wherein the service life of the bearing is not determining the service life of the product in which it is mounted i.e. wherein the service live of the bearing is much longer. Typical examples are electrical motor rolling bearings. However it should be noticed that the subject invention is not restricted to these application and a person skilled in the art will be able to find further applications being within the range of the appended claims.

Claims: C l a i m s Method for producing a steel, comprising the provision of a steel containing 0.9 - 1.03 wt% C; 0.15 - 0.35 wt% Si; 0.75 - 1.00 wt% Mn; 0.70 - 0.90 wt% Cr, balance Fe and impurities, wherein said steel is produced by continuous casting followed by rolling, characterised in that, said steel comprises 0.25 wt% Si; 0.85 wt% Mn and 0.80 wt% Cr, wherein the Cr/C ratio is between 0.7 and 1.20 and in that either immediately after casting said steel is rolled or is rolled after casting and reheating without a soaking heat treatment. 2. Method according to claim 1, wherein said steel comprises about 0.96 wt% C. 3. Method according to one of the proceeding claims wherein the Cr/C ratio is between 0.75 and 0.95. 4. Method according to one of the preceding claims, wherein said steel comprises a rolling bearing steel. 5. Method according to claim 4, wherein a component of said steel is hardened by through martensitic hardening or surface induction hardening. 6. Method according to claim 4, wherein the component of said steel has a through hardened martensitic structure. 7. Rolling bearing element component produced with the method according to one of the preceding claims. 8. Rolling bearing element component according to claim 6, comprising a bearing ring.

Inventors: BESWICK JOHN MICHAEL, WICKS GARRY, BESWICK, JOHN, MICHAEL, WICKS, GARRY
Applicants: BESWICK JOHN MICHAEL, SKF ENG & RES CENTRE BV, WICKS GARRY, BESWICK, JOHN, MICHAEL, SKF ENGINEERING & RESEARCH CENTRE B.V., WICKS, GARRY
Application year: 2000
Main classification: C21D9
IPC codes: C21D8, C21D9, C22C38, F16C33