MXPA99008950A - Steel and process for the manufacture of a part for ball bearing - Google Patents

Abstract

La presente invención describe un acero usado para manufacturar una parte para un cojinete de bolas. La composición química del acero en peso es como sigue:0.6%1 ( C ( 1.5%;0.4%( Mn ( 1.5%;0.75%( Si ( 2.5%;0.2%( Cr ( 2%;0%( Ni ( 0.5%;0%( Mo ( 0.2%;0%( Al ( 0.05%;S ( 0.04%;lo restante de la composición es hierro e impurezas que resultan del proceso. La composición adicionalmente satisface las siguientes relaciones:Mn ( 0.75 + 0.55 x Si y Mn ( 2.5 - 0.8 x Si. También se describe un procedimiento para manufacturar un cojinete de bolas.

Description

STEEL AND PROCEDURE FOR THE MANUFACTURING OF A ROUGH PIECE FOR A BALL BEARING DESCRIPTION OF THE INVENTION The present invention relates to the manufacture of a ball bearing part, and in particular of a ring for a ball, needle or roller bearing. Parts for the ball bearing, such as rings, balls, needles or rollers, are generally made of steel type 100Cr6 or 100CrMn6, containing 0.6 to 1.5% carbon, from 1.3 to 1.6% of chromium, from 0.3 to 1% manganese and less than 0.4% silicon, which has a very good inclusion property. The steel is used in the form of rolled bars, seamless tubes or wires, in which pieces are cut, which are shaped by cold or hot plastic deformation and then subjected to annealing and annealing before of being mechanized. The parts thus obtained have a high hardness and the necessary tenacity to enable them to withstand bearing fatigue well, at least under the usual conditions of use, particularly for temperatures in service below 150 ° C. However, the parts thus constituted have an insufficient bearing fatigue resistance for the most severe service conditions, which tend to REF .: 31021 generalize These more severe service conditions are characterized, in particular, by a service temperature higher than 150 ° C, and can reach 350 ° C, and / or by the presence of a phenomenon of deterioration of the bearing surfaces by indentation. This phenomenon consists of the initiation of cracks in the surface caused by the indentations, that is, the deformations generated by hard particles present in the lubricant. To limit the effect of indentation, it has been proposed to use very hard materials such as ceramics or hard material deposits. But this technique has the disadvantage of being unreliable due to the too great fragility of these materials, fragility that makes them very sensitive to the minor defect. It has also been proposed, for example in US Pat. No. 5,030,017, to use a steel containing in particular from 0.3% to 0.6% carbon, from 3% to 14% chromium, from 0.4% to 2% of .molybdenum, of 0.3% to 1% vanadium and less than 2% manganese. The pieces are cemented or carbonitrided in the vicinity of the rolling surface, to obtain a sum of the carbon and nitrogen contents between 0.03% and 1%, and then tempered so that their micrographic structure comprises from 20% to 50% (in% by volume) of residual austenite in a surface layer that represents 10% to 25% of the volume of the piece. This This technique has the double disadvantage of requiring the use of a steel heavily loaded with alloying elements, and thus costly, and the performance of a cementing or carbonitriding treatment, a prolonged and expensive treatment. It has also been proposed in the German patent application DE 195 24 957 to use a steel containing 0.9% to 1.3% carbon, 0.6% to 1.2% silicon, 1.1% to 1.6% manganese and 1.3% a 1.7% chromium, the rest is iron or impurities that result from processing, the structure of this steel contains 7% to 25% residual austenite. On the other hand, due to its chemical composition, this steel does not provide any guarantee of its ability to be castable, of its capacity for cold deformation, as well as the content and stability of the residual austenite. The content of residual austenite specified and necessary to improve the fatigue resistance under indentation needs, on the other hand, for this steel, the application, on the bearings, of an uncomfortable heat treatment comprising a maintenance stage at approximately 100 ° C for more than 10 hours between annealing and annealing without return to the environment after tempering or before annealing. On the other hand, in the presence of multidirectional stresses below the cyclic elastic limit, its austenite is not stable more than 2000 hours longer than for thermal stresses lower than 120 ° C, which is Very little for certain applications. To improve the fatigue resistance of bearing steels, it has been proposed, particularly in US Pat. No. 1 439 072, to carry out a heat treatment consisting of austenization followed by rapid cooling to a temperature between 480 ° C and 725 ° C, followed by prolonged maintenance at this temperature to generate a perlific transformation, after rapid reheating to an austenitization temperature and finally a tempering. This treatment applies, in particular, to a steel containing 0.8 to 1% carbon, 0.5 to 0.8% silicon, 1.4 to 1.7% manganese and 1.4 to 1.8% chromium. But, this treatment and its application to the steel considered does not solve the problem of resistance to indentation. The object of the present invention is to remedy these drawbacks by proposing a means for manufacturing, economically and in particular with the aid of a relatively standard heat treatment, an indentation-resistant bearing part, particularly when there are occasional or occasional uses above. 300 ° C, and little fragile. For this, the invention relates to a steel for the manufacture of a bearing part whose chemical composition comprises, by weight: 0.6% < C < 1.5% 0. 4% < Mn < 1.5% 0.75% < Yes < 2.5% 0.2% < Cr < 2% 0% < Ni < 0.5% 0% < Mo < 0.2% 0% < To < 0.05% 0% < You < 0.04% S < 0.04% the rest is iron and impurities that result from the elaboration, the composition satisfies, in addition, the relations: Mn < 0.75 + 0.55 x Yes Mn < 2.5 - 0.8 x Si Preferably the chemical composition of the steel is such that, separately or better yet simultaneously, on the one hand: 0.8% < Mn < 1.2% 0.8% < Yes < 1.7% and on the other hand: 0.9% < C < 1.1% 1.3% < Cr < 1.6% Preferably, also, the silicon content is higher than 1.2%; the inventors have indeed observed unexpectedly that, when simultaneously, the silicon content is higher than 1.2% and the manganese content is less than 1.5%, and preferably less than 1.2. % but higher than 0.8%, the stability of the austenite is greatly improved. The invention also relates to a process for the manufacture of a bearing part, according to which: a semi-steel product according to the invention is supplied, - the semi-product is put into shape by hot plastic deformation in order to to get a piece in the rough, and, more particularly, a blank of seamless pipe, a globulization treatment consisting of a heating at a temperature between 750 ° C and 850 ° C is carried out on the blank, followed by a cooling whose maximum speed is 10 ° C / hour until 650 ° C, in order to obtain a hardness structure of less than 270 HV, and comprising a fine dispersion of carbides and, optionally, a shaping by cold plastic deformation, for example by cold rolling or by stretching in cold, in order to obtain a product, 20 - the product is cut to obtain a piece that is put into shape by cold or hot plastic deformation, or by machining, in order to obtain a bearing blank, - and a heat treatment by cooling, for example, is carried out on the bearing blank. an oil, then austenitization between 800 ° C and 950 ° C, and a annealing heat treatment between 100 ° C and 400 ° C and preferably below 250 ° C, to obtain a bearing part having a structure whose hardness is between 58 HRC and 67 HRC, and which is made up of residual carbides , from martensite and from 5% to 30 or from residual austenite. The invention relates, finally, on the one hand, to a seamless steel tube according to the invention, and on the other hand, to a steel bearing part according to the invention., which has a structure composed of residual carbides, of martensite and from 5% to 30% residual austenite thermally stable up to at least 400 ° C. The invention will now be described more precisely, but not limiting, and is illustrated by the examples. To make a bearing part such as a ring or a rolling body having a good resistance to indentation, for example from a seamless pipe, a steel whose chemical composition comprises, by weight: - more than 0.6% is used and preferably more than 0.9% carbon to obtain a sufficient hardness and a sufficient percentage of residual austenite, but less than 1.5% and, preferably, less than 1.1% to avoid the formation of very important segregations, and to limit the formation of primary carbides, more than 0.75%, and preferably more than 0.8%, and better still, more than 1.2% silicon, to increase the heat stability of the residual austenite (between 170 ° C and 450 ° C, approximately, and preferably by above 300 ° C) and hardness, but less than 2.5%, and preferably less than 1.7, because when the silicon content is very high, the steel becomes very brittle particularly to be able to be put into shape by plastic deformation , more than 0.4% manganese, and preferably, more than 0.8% in order to obtain a tempered structure having a residual austenite content greater than 5% and preferably greater than 15%; the content of manganese must be such that: Mn < 0.75 + 0.55 x Yes to obtain a good castability, without which it becomes difficult to obtain a steel sufficiently appropriate for it to have a good resistance to rolling fatigue, and such that: Mn < 2.5 - 0.8 x Yes, to allow the operations of finishing and putting form by cold plastic deformation; it results from these relationships that the manganese content must be less than 1.5%, and it is preferable that it be less than 1.2%, of 0.2. to 2% chromium, and preferably from 1.3% to 1.6%, in order, on the one hand, to obtain sufficient hardenability, and on the other hand, to form globular carbide seeds smaller than 2 μm, uniformly distributed and in sufficient quantity, less than 0.5% nickel, a residual element that is not essential other than for a favorable effect on the hardenability, less than 0.2% of molybdenum, an element that decreases the 5% sweetened in the annealing, - between 0% and 0.05% aluminum and less than 0.04. of sulfur, the rest is iron and impurities that result from processing. This steel is cast and, optionally laminated to manufacture a semi-product that when it is desired to manufacture a ring from a seamless tube, is a round one for tube. The semi-product is then put into shape by hot plastic deformation to obtain a rough piece for example, by hot rolling to obtain a seamless pipe. The blank is then subjected to a thermal treatment of globulization which consists of heating at a temperature between 750 ° C and 20 850 ° C, followed by a cooling whose maximum speed is 10 ° C / hour up to 650 ° C , in order to obtain a structure of hardness lower than 270 HV and comprising a fine dispersion of carbides. This heat treatment is necessary in order to give the steel a good aptitude for putting into shape by cold plastic deformation and good machinability, process used to manufacture, for example, by cold rolling or by cold stretching, a product. This product, which can be a seamless tube, is characterized in that it is well calibrated. It is used to manufacture 5 blanks, for example, the blanks of bearing rings. The manufacture of the blanks, which are made by cold or hot forming or by machining of the pieces cut into the product, is terminated by a heat treatment consisting of tempering and annealing. A bearing part is thus obtained. The austenitization temperature before tempering, above 800 ° C, is adjusted to obtain after tempering a structure constituted of martensite, from 5% to 30% of residual austenite and a network of residual carbides. The percentage of residual austenite, whose presence is essential to obtain a good resistance to indentation depends on the value of the Ms point (martensitic transformation initiation temperature) that depends on it. same, at the same time of the composition of the steel and the conditions of austenización. Those skilled in the art know how to determine these parameters, for example with the aid of dilatatric tests. Annealing, which is more exactly - a relaxation, is done by heating above 100 ° C in order to stabilize the structure, but less than 400 ° C, and preferably less than 250 ° C in order not to destabilize the residual austenite. As a first example, 10 laboratory strains were made, 2 according to the invention (identified A and B) and 8 as comparison (identified C, D, E, F, G, H, I and J). These castings, to which was added the standard 100Cr6, intended essentially to highlight the effects of the alloying elements on the various properties of a bearing steel, had the following chemical compositions (in% by weight, only the main elements). are indicated, the rest is iron and impurities): These steels were cast in the form of 65 kg ingots which were forged to form square bars of 20 mm long, and after globulized by a maintenance for 1 hour at 30 ° C above the end of transformation temperature of the perlite in austenite, followed by cooling to 650 ° C at a speed between 8 and 10 ° C per hour, completed by air cooling to room temperature. The aptitude for cold deformation was then evaluated by a KCU impact resistance measurement at 60 ° C expressed in daJ / cm2; When this resistance to impact is greater than 4.2 daJ / cm2, the fitness for cold forming is good, and it is bad in the opposite case. The pieces were then tempered in cold oil after austenitization at 895 ° C, and the proportion t of residual austenite, and the temperature, were measured on the one hand. of destabilization of residual austenite. Collability was also evaluated. The results were the following : These results show that only the castings A and B according to the invention have all the desired properties, namely a good castability, a good capacity for cold deformation, a high proportion of residual resins, and a stable structure up to temperatures elevated these last two characteristics are significantly higher than the corresponding characteristics of the standard lOOCrβ. Furthermore, stability tests of residual austenite under monotonic tension and in cyclic compression showed that: for castings whose silicon content is higher than 1%, residual austenite remains stable when subjected to compression at an equivalent shear stress of 1400 MPa, while under the same conditions, 50% of the residual austenite of a 100Cr6 steel (containing less than 0.5% Si) is destabilized, for austenite destabilization tests residual in cyclic compression (equivalent shear stress that evolves between 25 MPa and 1025 MPa at the frequency of 200 Hz) no destabilization appears after 1 million cycles, for a cast that contains around 1% manganese and 1.5% silicon (cast A). As a second example, an industrial casting of a steel according to the invention was carried out with which a bearing ring was manufactured. The chemical composition of steel comprises, by weight: C = 0.9% Si = 1.25% Mn = 1% Cr = 1.4% Ni = 0.25% Mo = 0.015% the rest is iron and impurities that result from processing. This steel was cast and rolled in the form of a tube ring 100 mm in diameter. The tube ring was hot-drilled between two cylinders and then hot rolled to obtain a blank of seamless tubes with outside diameters 67. 5 mm and inside 36.5 mm. The tube blank was subjected to a globulization treatment consisting of a 2 hour maintenance at 800 ° C followed by a cooling up to 650 ° C at the speed of 10 ° C per hour, to obtain a Brinell hardness of 240 HB. The tube blank was then cold rolled to obtain a seamless tube of 42.9 mm outside diameter and 22.7 mm inside diameter. The bearing rings were cut and machined in the tube, then subjected to an oil quenching treatment after austenitization at 900 ° C and annealing at 200 ° C to obtain a structure containing 18% residual austenite. The indentation resistance of the fatigue-type stop test under strong pressure in Hertz was tested using rings previously indented at the level of the tracks for two symmetrically arranged Vickers impressions, and measuring the times of flaking of the rings. The rings according to the invention were thus compared to the 100C6 rings according to the prior art, for impressions whose diagonal dimensions were 267 μm and 304 μm. In addition for the rings according to the invention as for the rings of the prior art, the hardness was 63 HRC. The results were the following: These results show that the rings according to the invention have a useful life more than doubled for important indentations. The steel according to the invention is particularly adapted for the manufacture of rings for bearings from seamless tubes, but is also adapted for the manufacture of rings, balls, needles, and rollers from rolled bars or wires. These pieces can be put in shape by plastic deformation in hot or cold, or by machining. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (11)

CLAIMS Having described the invention as above, the content of the following claims is claimed as property:

1. Steel for the manufacture of a bearing part, characterized in that its chemical composition comprises, by weight: 0.6% < C < 1.5% 0.4% < Mn < 1.5% 0.75% < Yes < 2.5% _ 0.2% < Cr < 2% 0% < Ni < 0.5% 0% < Mo < 0.2% 0% < To < 0.05% S < 0.04% the rest is iron and impurities that result from the elaboration, the composition satisfies, in addition, the relations: Mn < 0.75 + 0.55 x Yes Mn < 2.5 - 0.8 x If with the exception, on the one hand, steels whose chemical composition comprises, by weight: 0.4% ^ C 0.8% Cr < . 0.8% Mo < 1% 0.01% < Yes < 2% 0.2% < Mn < 2%, on the other hand, steels whose chemical composition they comprise, by weight: 0.9% < . C < 1.3% 0.6% < Yes < 1.2% l.l% < Mn < 1.6% 1.3% < Cr < 1.7%

2. Steel according to claim 1, characterized in that its chemical composition is such that: 0.8% < Mn < 1.2% 0.8% < Yes < 1.7%

3. Steel according to claim 1 or 2, characterized in that its chemical composition is such that: Si > 1.2%

4. Steel according to claim 1, characterized in that its chemical composition is such that. 0.9% < C < 1.1% 1.3% < Cr < 1.6%

Steel according to claim 4, characterized in that its chemical composition is such that: 0.8% < . Mn < 1.2% 0.8% < . Yes < 1.7%

6. Steel according to claim 4 or 5, characterized in that its chemical composition is such that: Si > 1.2%

7. Process for the manufacture of a ball bearing part, according to which: a semi-steel product for bearing is formed by hot plastic deformation in order to obtain a blank, and, more particularly, a part in Gross seamless tube, a globulization treatment is carried out on the blank in order to obtain a structure of hardness lower than 270 HV, and comprising a fine dispersion of carbides, and optionally, a shaping is carried out by cold plastic deformation, for example, cold rolling or cold stretching, for a product, - a piece is cut into the product which is put into shape by cold or hot plastic deformation, or by machining, in order to obtain a blank for bearing, and a final heat treatment to obtain a ball bearing part is made on the blank. having a structure whose hardness is between 58 HRC and 67 HRC and which is made up of residual carbides, martensite and from 5% to 30% of residual austenite, the final heat treatment consists of isothermal tempering or cooling, for example in oil after austenitization between 800 ° C and 950 ° C, and annealing between 100 ° C and 400 ° C and preferably below 250 ° C, characterized in that: the bearing steel is according to any of claims 1 to 6, the globulization treatment consists of a heating at a temperature comprised between 750 ° C and 850 ° C, followed by a cooling whose maximum speed is 10 ° C / hour up to 650 ° C.

8. Conformity method - with claim 7, characterized in that the product is a tube without solder.

9. Seamless steel tube according to any of claims 1 to 6.

10. Part for steel ball bearing according to any of claims 1 to 6, characterized in that it has a structure constituted by a network "of carbides, of martensite and of 5% to 30% of residual austenite.

11. Piece according to claim 10, characterized in that it is a ring.