1. Technical Field
The present invention relates to a process for production of nitrided steel having high pitting corrosion resistance and superior fatigue strength.
2. Background Art
In recent years, CTV (Continuously Variable Transmission) is widely used as a stepless transmission for automobiles. The CTV is formed by annularly connecting plural pushing blocks with a metallic hoop. High fatigue strength is required for steel used in such hoops and springs because bending force is applied repeatedly. As a technique to improve fatigue strength of various kinds of steel, a nitriding method is known as is disclosed in, for example, Japanese Unexamined Patent Application Publications Nos. 1-142022, 2000-219956, or 2001-26857. However, since the surface is activated during the nitriding process, part of the surface is corroded and pitting corrosion is formed due to the presence of halogens, and corrosion resistance may be deteriorated. Such pitting corrosion often grows in a depth direction, and it is difficult to discover by visual inspection. In particular, in the case of a thin material such as the hoop mentioned above, pitting corrosion causes great deterioration in fatigue strength.
Many such halogens are the chlorine of NaCl. Fine NaCl particles which come from the sea or the human body are present in ordinary environments.
Conventionally, to prevent pitting corrosion, environmental measures or a method to improve pitting corrosion resistance of a steel itself has been performed. As the environmental measures, contact with water vapor or halogens is prevented. These measures can be performed when parts are produced and assembled in a clean room, although it is difficult to perform in all processes, and it is almost impossible to perform completely. Therefore, pitting corrosion resistance of steel itself is required to be improved.
To improve pitting corrosion resistance, it is effective to passivate the surface. Passivation of the surface by an alloy element control or other passivation treatment of the surface can be performed as the method. In particular, it is extremely efficient to add, for example, Cr as the alloy element control. However, in the case of a steel in which Cr cannot be added, for example, in the case of maraging steel, addition of Cr causes deterioration of strength characteristics. As a surface passivating treatment, a treatment in which steel is immersed in, for example, dichromate solution or nitrite solution, can be performed. However, immersing and drying processes are then required. Furthermore, it is difficult to perform uniform passivation unless the drying process is devised, and there is a case in which the steel becomes rusty.
It is known that thin passivated layer having a thickness of not more than 10 nm and containing FeOOH, Fe3O4, or Fe2O3 is formed on the surface of the iron (see “Corrosion and Corrosionproofing Handbook”, 2000, p. 23, published by MARUZEN). An important fact about pitting corrosion resistance is to form passivated layer as uniformly as possible. Partial passivated layer and partial oxide layer cause forming of a local battery and the pitting corrosion resistance may be deteriorated.
Therefore, an object of the present invention is to provide a process for production of nitrided steel in which a uniform passivated layer can be reliably formed by a simple method, and therefore, fatigue strength can be improved together with improvement of pitting corrosion resistance.
The inventors discovered that the surface of maraging steel is activated after nitriding, and that a uniform passivated layer can be formed on the surface by immersion in an oxidizing atmosphere, and thus the present invention was completed. That is, the present invention has a property that after steel is nitrided, passivation treatment in which the steel is heated in an atmosphere containing oxygen is performed.
In the present invention, only by a relatively simple process in which heating is performed in an atmosphere containing oxygen after nitriding, the surface of the steel is passivated to form a passivated layer which improves pitting corrosion resistance. Therefore, a conventional process which requires complicated control such as addition of an element to promote passivation or immersion in passivation treatment solution is no longer required, and the passivated layer can be easily formed.
In the present invention, the surface is oxidized by heating after the steel is nitrided. In the case in which extent of oxidizing by heating is insufficient, only a partial passivated layer is formed, and pitting corrosion occurs on an activated part which is not passivated. On the other hand, in the case in which the extent is too strong, an oxide layer containing mainly Fe2O3 is formed, local battery is formed between this oxide layer and the passivated layer, deteriorating pitting corrosion resistance. Therefore, optimum heating conditions (oxidizing conditions) as a passivating treatment after nitriding were researched, and it became clear that desirable passivated layer can be formed if the heating conditions are within a range of an area surrounded by coordinates of temperature and time (100° C., 120 min), (100° C., 10 min), (125° C., 5 min), (190° C., 5 min), (200° C., 10 min), (200° C., 20 min), (190° C., 30 min), (190° C., 40 min), (180° C., 60 min), and (180° C., 120 min). This range is the desirable aspect of the heating condition of the present invention.
Furthermore, more desirable heating conditions are in a range of area surrounded by coordinates (100° C., 120 min), (100° C., 30 min), (125° C., 20 min), (170° C., 20 min), (170° C., 40 min), (160° C., 60 min), and (160° C., 120 min).
It should be noted that if a nitriding treatment, in which the surface is activated by halogens or H2S before the nitriding, is performed, the passivating treatment of the present invention is extremely effective since corrosion resistance of the steel having high activity after the nitriding is deteriorated.
The passivating treatment is performed after the nitriding treatment in the present invention, and these series of treatments can be performed in respective heating furnaces, or continuously in the same furnace.
On the other hand, in the case in which nitriding treatment and passivating treatment are continuously performed in the same furnace, as is shown in
In addition to the passivating of the present invention, higher pitting corrosion resistance can be obtained by reducing humidity to prevent water from attaching, or by coating oil on the surface of steel during production thereof.
Desirable Examples of the present invention are explained as follows.
(1) Heating Conditions in Passivating Treatment
A number of test pieces were cut from maraging steel having a composition in which elements except Fe and inevitable elements shown in Table 1 are contained. These test pieces were nitrided, and passivating treatment was performed in the air with varying heating condition which is a combination of heating temperature and time to obtain nitrided steel of Examples. The heating condition of
(2) Measurement of Pitting Potential
The test pieces of the Examples and the Comparative Examples were immersed into solution of 0.1 N—NaCl+0.5 N—Na2SO4, an anodic polarization test was performed by a potential scanning method at 25° C. The testing device is shown in
As is clear from Table 2, a pitting potential similar to or greater than a pitting potential (360 mV vs. SCE) of a steel in which conventional passivating treatment is performed is shown within a range of the heating condition surrounded by the bold solid line, that is, the range surrounded by (100° C., 120 min), (100° C., 10 min), (125° C., 5 min), (190° C., 5 min), (200° C., 10 min), (200° C., 20 min), (190° C., 30 min), (190° C., 40 min), (180° C., 60 min), and (180° C., 120 min). This range (hereinafter referred to as a range A) of the heating condition is shown in
(3) Types of Passivated Layers
One test piece was selected from the test pieces of the Examples in which passivating treatment was performed in the range A, and the surface was analyzed by ESCA (electron spectroscopy for chemical analysis). A spectrum around O1s is shown in
(4) Thickness of Passivated Layers
Some test pieces which were treated in the heating condition shown in Table 3 were selected from the test pieces of the Examples which were passivated in the range A, and the thicknesses of these pieces and thickness of test piece of the Comparative Example which was not passivated were measured. The thicknesses of the passivated layer was measured by observing a distribution condition of oxygen along a depth direction by AES (auger electron spectroscopy) used together with sputtering, and then by calculating intersection of a sudden initial falling line of peak values which are reduced depending on the depth and a stable line in which the rate of reduction is gently sloping. The results are shown in Table 3. As is clear from Table 3, in the case in which the thickness of the passivated layer is not less than 7 nm, the pitting potential is not less than 360 mV vs. SCE.
(5) Hoop Fatigue Test
Hoops having dimensions of thickness 0.18 mm, width 9 mm, and circumference 600 mm were prepared by using maraging steel having compositions in which elements except Fe and inevitable elements shown in Table 1 are contained. These hoops were nitrided by the method shown in
The results of the fatigue tests are shown in Table 4, and the relationship of the results of the fatigue tests and the pitting potential is shown in
Number | Date | Country | Kind |
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2001-318602 | Oct 2001 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP02/07395 | 7/22/2002 | WO | 00 | 3/24/2004 |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO03/033757 | 4/24/2003 | WO | A |
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Number | Date | Country | |
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20040238073 A1 | Dec 2004 | US |