Brakes – Elements – Brake wheels
Reexamination Certificate
2001-10-09
2002-05-14
Schwartz, Christopher P. (Department: 3613)
Brakes
Elements
Brake wheels
C148S325000
Reexamination Certificate
active
06386342
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a martensitic stainless steel suitable for use in manufacturing a disc brake rotor. A disc brake rotor formed from the steel can stably function under the severe conditions of a surface temperature which may reach 500-550° C. due to the heat generated during braking while maintaining an excellent hardness, toughness, and corrosion resistance. The steel can also be manufactured using a wide range of soaking temperatures and cooling speeds.
In particular, the present invention provides a martensitic stainless steel which can be used to form a disc brake rotor which has excellent properties even when the steel is hardened with a cooling speed slower than air cooling.
2. Description of the Related Art
In the past, a martensitic stainless steel such as SUS 403, 410, or a 420 type referred to as a “13% Cr steel” having a hardness adjusted to H
R
C 30-45 was used as a disc brake rotor for a motorcycle (herebelow, percent when used to express proportions of components refers to mass percent).
The reason why the hardness of a disc brake rotor is adjusted to the above-described range is that if the hardness of a disc brake rotor is too low, the wear resistance necessary for a disc brake rotor is not obtained, while if the hardness is made too high due to consideration only of wear resistance, the hardness has an adverse effect on braking stability.
Thus, a prescribed hardness after hardening is necessary for a material for a disc brake rotor, but at the same time, it is necessary for the material to have excellent properties, such as corrosion resistance, toughness, and stability with respect to its hardness after hardening, and the like. Stability here refers to the ability of the steel to obtain a certain level of hardness without regarding a change in cooling speed.
Stability is important because the surface of a disc brake rotor may rise to a high temperature of 500° C. or above due to the heat generated during braking. At such temperatures, a reaction between the surface of the disc brake rotor and the brake pads or an oxidation reaction between the surface of the disc brake rotor and the atmosphere becomes a problem, and in addition, there is a tendency for changes in material properties such as softening of the rotor or a decrease in corrosion resistance to occur.
A disc brake rotor requires a high degree of flatness. In recent years, therefore, it has become desired to perform hardening with a cooling temperature which is slower than air cooling in order to minimize the occurrence of strains during hardening, which could affect flatness.
In order to increase productivity, it is common to carry out hardening treatment by stacking a large number of disc brake rotors on top of each other. Performing hardening in this manner also leads to a decrease in cooling speed.
However, with a conventional material for a disc brake rotor, corrosion resistance and toughness deteriorated when hardening was performed at a low cooling speed.
Japanese Published Unexamined Patent Application No. Hei 10-152760 discloses a material having improved properties required of a disc brake rotor. This material is a martensitic stainless steel containing 0.5-2.5% of Cu in order to expand the possible temperature range for hardening and to suppress softening caused by heat generated during braking. That steel has an excellent resistance to softening such that a material adjusted to have a hardness of approximately H
R
C 35 undergoes a decrease in hardness of less than 10 when subjected to heat treatment at 600° C.×10 minutes, which simulates tempering occurring during braking. Therefore, the material is said to be able to prevent a decrease in braking force.
However, that publication does not take into consideration a decrease in corrosion resistance due to a slow cooling speed during hardening at the time of rotor manufacture or a decrease in corrosion resistance caused by temper softening, which occurs to a certain extent during use (due to increased temperatures during braking), so it is thought that the above-described steel leaves room for improvements with respect to these characteristics.
Japanese Published Examined Patent Application Hei 3-79426 discloses a technique in which corrosion resistance is guaranteed by adding 0.05-0.5% of Mo while reducing the Cr content to 10.0-11.5%, and if necessary adding 0.04-0.1% of Nb as a proposal for improving corrosion resistance of a martensitic stainless steel for a disc brake rotor.
Mo increases the corrosion resistance of stainless steel in an additive manner. It has a smaller tendency to form carbides than does Cr, so it is added in order to suppress a deterioration in corrosion resistance due to precipitation of carbides. However, Mo is expensive, so the martensitic stainless steel described in the above-described Japanese Published Examined Patent Application Hei 3-79426 is not completely satisfactory with respect to costs.
Thus, there is still a need for further improvements in the stability of the properties of a steel for a disc brake rotor in order to provide a disc brake rotor of greater reliability.
As described above, in a conventional material for a disc brake rotor, if hardening at a slow cooling speed is carried out in order to decrease strains or to permit a large number of discs to be stacked on top of each other during hardening to improve productivity, the corrosion resistance and toughness of the steel decrease.
In addition, a deterioration in material properties (a decrease in hardness, a deterioration in corrosion resistance) due to the high temperatures to which the steel is exposed during use (at the time of braking) easily occur, so conventional materials have problems with respect to the reliability of the resulting disc brake rotor.
SUMMARY OF THE INVENTION
Accordingly, an object of this invention is to provide an inexpensive steel for a disc brake rotor which has a satisfactory hardness after hardening, toughness, and corrosion resistance and which undergoes almost no changes in material properties (decrease in hardness, deterioration in corrosion resistance) due to high temperatures during use (at the time of braking), and which can be manufactured using a wide range of soaking temperatures and cooling speeds at the time of hardening.
Another object of the present invention is to provide a disc brake rotor made from such a steel, and a vehicle equipped with such a disc brake rotor.
The present inventors evaluated the hardness, the corrosion resistance, and the toughness of a cold rolled steel plate made from 13% Cr martensitic stainless steel which contained various amounts of C and which was subjected to hardening, i.e., cooling from a temperature in the range of 900-1050° C. and tempering in the range of 200-600° C., and they made the following discoveries.
(a) When the cooling speed during hardening falls below a speed of 5° C. per second corresponding to air cooling, the as-hardened corrosion resistance of the steel decreases. This decrease in corrosion resistance becomes more marked as the C content of the steel increases.
(b) When the cooling speed is reduced to 0.5° C. per second, if the C content of the steel is reduced to 0.1% or less, a fairly good corrosion resistance is obtained compared to a material having a comparatively high C content. Even so, the corrosion resistance of the steel is not improved to the level of an air cooled material.
(c) If the Cr content of the steel is made to exceed 11.5%, minute amounts of Nb or V are added, and the overall content in the steel of C, N, Ni, Cu, Mn, Cr, Si, Mo, V, Ti, and Al is adjusted, the corrosion resistance resulting from gradual cooling at a speed of 0.5° C. per second at the time of hardening is markedly improved.
In order to clarify the effects of guaranteeing the stability of an excellent corrosion resistance under the conditions of a slow cooling speed at the time of hardening, it is necessary to perform control such that the structure at a high temperature has a high austenite ratio.
Matsuda Takaaki
Nago Daisuke
Okada Yasutaka
Sakashita Tsuyoshi
Tsuge Shinji
Burns Doane , Swecker, Mathis LLP
Kramer Devon
Schwartz Christopher P.
Sumitomo Metal Industries Ltd.
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