Recently, with the high speed rotation and miniaturization of the demand is higher and higher, therefore puts forward higher requirements on bearing performance. Especially the automotive industry to decrease the size of cars and reduce fuel consumption have very high demand. On the other hand, the electronics industry also requires very little bearing on energy saving and no vibration when electronic instruments more hours.
Despite the use of the Parameter Of Bearings demand is higher and higher, but also requires its service life is longer than before. However, due to the bearing ball occasionally happen spalling, therefore extending the service life of the bearing is very difficult. In order to meet the requirements of users is becoming more and more strict, must by reducing oxide and nitride inclusion in steel to improve steel cleanliness.
For more clean steel, using the traditional ASTM methods to analyze the fatigue life and steel clean sex is very difficult, so they need a new inclusion method. This article introduces Japanese kobe steel company, which is used in the development of bearing super clean steel production technology and a new determination of inclusions.
A, bearing with super clean steel production technology
1, ultra clean steel production
Kobe steel in kobe steel smelting, continuous casting and the expedition factory for high carbon chromium steel for bearings. This kind of steel in addition to adopt more stringent refining process in order to make mixed oxide and nitride minimizing, its production process is the same as ordinary steel.
By the molten iron pretreatment of dephosphorization and desulphurization of hot metal in converter decarburization process. Slag is a kind of oxidizing slag. In order to reduce oxide inclusions, slag is cut off, so that the oxide inclusions will not flow into the ladle. (heating, degassing) in refining and continuous casting molten steel were more severe than usual of inert gas shielded to prevent secondary oxidation in contact with air. In the process of refining, due to the basicity of slag is very high, molten steel by fully mixing, the oxide inclusions (mainly 3 oxidation 2 aluminium) to rise into the slag, as a result, steel become more clean, only a small amount of inclusions. During continuous casting, kobe factory using steel tundish heater acceleration of mixed up, plus the expedition factory is to use a larger tundish.
To reduce the nitride inclusion, it is necessary to minimize the titanium content in steel. Titanium mainly comes from iron alloy, especially used to adjust the ferrochrome toughened learn composition. Usually, ferrochrome are added to the molten steel in ladle refining process. Because the oxygen content in molten steel is very low, so in the form of titanium oxide (tio2) is very difficult to remove titanium. In kobe steel, chromium iron is added and melting in converter, while the oxygen content is very high, so the titanium react with oxygen to form titanium dioxide, then be absorbed into the converter slag.
2, the performance of the ultra clean steel
In 52100 super clean steel, the oxygen content can be controlled to an average of 4.1 PPM, titanium content with an average of 6.0 PPM. The fatigue life of 52100 super clean steel compared with the contrast test of steel, L10 is higher. In addition, it adopted many detection methods of super clean steel and ordinary steel clean sex was compared. Found by comparison and clean sex of super clean steel with ordinary bearing steel is not very big difference, however, the fatigue life of super clean steel is much longer than ordinary steel.
Second, the new determination method for inclusion in steel
Due to ultra clean steel has a very low oxygen content, so the traditional method of ASTM test was carried out on the oxygen content and inclusion measurement and fatigue life of a good corresponding relation cannot be given. For this purpose, the experiment of electron beam melting to determine inclusion in clean steel. In kobe steel, the researchers try to find out a from inclusions size and number of the new determination method for evaluating clean sex. Therefore developed acid dissolving method to detecting small inclusions and inclusions.
Acid dissolution methods including melt tested materials to isolate inclusion. Rich in 3 oxidation 2 aluminium inclusions can be separated, but calcium oxide inclusions can not be detected, because it is dissolved in acid.
First of all, 30 ~ 100 g samples dissolved in nitric acid and heated to 90 ℃. After cooling solution for filtering, filter the residue is separated as inclusions. Filter mesh size to 10 mesh, size is more than 10 purpose of inclusions in the filter. Inclusions were observed with scanning electron microscopy (sem), electron probe microanalysis method of inclusions were analyzed.
By adopting these methods, can detect a lot of inclusions, but also can avoid the influence of carbide. By using the method of acid dissolution, researchers can detect oxygen content is lower than 9 PPM, used for fatigue life test sample. Test results show that the number of inclusions in the ultra clean steel and inclusion size less than and less than ordinary steel respectively. When oxygen levels are lower than 9 PPM, acid dissolving method also can be used to measure the super clean steel and ordinary steel clean sex differences.