Brake disk having high temper softening resistance

This is a §371 of International Application No. PCT/JP2006/320358, with an international filing date of Oct. 5, 2006 (WO 2007/122754 A1, published Nov. 1, 2007), which is based on Japanese Patent Application No. 2006-117375, filed Apr. 21, 2006.

This disclosure relates to disks used for disc brakes for motorcycles, motorcars, bicycles, and the like. The disclosure particularly relates to a brake disk having proper hardness and high temper softening resistance. The term “high temper softening resistance” used herein means such a feature that a reduction in hardness due to high temperature is small and initial proper hardness can be substantially maintained.

Disc brakes used for motorcycles, motorcars, and the like slow the rotation of wheels in such a manner that kinetic energy is converted into heat energy by the friction between brake disks and brake pads.

Therefore, the brake disks need to have excellent abrasion resistance and toughness in addition to proper hardness. In particular, a low hardness of the brake disks accelerates abrasion of disks because of the friction with brake pads and reduces the braking performance. An extreme hardness thereof causes brake squeal. Therefore, the hardness of the brake disks is controlled to be about 32 to 38 HRC in Rockwell C hardness (HRC) as specified in JIS Z 2245.

A material conventionally used for the brake disks is martensitic stainless steel in view of hardness and corrosion resistance. In the past, martensitic stainless steel, such as SUS 420J2 (JIS Z 4304), having a high carbon content was used for the disks after quenching and tempering treatment. Since the workload of tempering treatment is large, low-carbon martensitic stainless steel has been recently used for the brake disks as disclosed in Japanese Unexamined Patent Application Publication No. 57-198249 or 60-106951 because this steel can be used directly after quenching treatment.

In view of global environmental conservation, recent motorcycles and motorcars need to have high fuel efficiency. A reduction in vehicle weight is effective in achieving high fuel efficiency; hence, lightweight vehicles are demanded. Even disc brakes, which are a part of brake mechanism (or brake system), are no exception. Compact or low-thickness (thin) brake disks are being experimentally produced. Compact or thin brake disks have low heat capacity. Hence, the temperature of the disks is increased to 650° C. or more by friction heat during braking. Therefore, there is a problem in that conventional brake disks made of martensitic stainless steel are reduced in durability because the conventional brake disks are tempered by the friction heat and therefore is softened.

To cope with such a demand, the following sheet has been proposed as disclosed in Japanese Unexamined Patent Application Publication No. 2002-146489: a low-carbon martensitic stainless steel sheet which contains N and one or more of Ti, Nb, V, and Zr and which can be effectively prevented from being softened by heating during the use of a disc brake. Japanese Patent 3315974 (Japanese Unexamined Patent Application Publication No. 2001-220654) discloses a stainless steel for disc brakes. The stainless steel contains Nb or Nb and one or more of Ti, V, and B and therefore can be prevented from being temper-softened. Japanese Unexamined Patent Application Publication No. 2002-121656 discloses steel for disc brake rotors. The GP value (the percentage of austenite at high temperature) of this steel is adjusted to 50% or more and this steel contains one or both of Nb and V, the GP value being determined by a function of the content of C, N, Ni, Cu, Mn, Cr, Si, Mo, V, Ti, and Al in this steel. This steel is prevented from being temper-softened by heating during braking.

The stainless steel, for brake disks, disclosed in Japanese Unexamined Patent Application Publication No. 2002-146489, Japanese Patent 3315974, or Japanese Unexamined Patent Application Publication No. 2002-121656 has a problem in that a relatively large amount of high-cost alloying elements need to be used and therefore the production cost thereof is high and also has a problem in that the stainless steel is significantly reduced in hardness after being held at 650° C. for a long time.

It would therefore be helpful to provide a brake disk having proper hardness and high temper softening resistance.

We intensively investigated factors affecting the temper softening resistance of brake disks made from martensitic stainless steel sheets. As a result, we found that the following disk has proper hardness after quenching (32 to 38 HRC) and excellent temper softening resistance (a hardness of 30 HRC or more after being tempered at 650° C. for one hour): a brake disk that is produced from low-carbon martensitic stainless steel with a specific composition and then tempered so as to have prior-austenite grains with an average grain diameter of 8 μm or more and/or tempered such that the ratio of the amount of precipitated Nb to the amount of total Nb is adjusted to a predetermined value or less.

Furthermore, we found that the following operation is effective in enhancing the temper softening resistance of low-carbon martensitic stainless steel: the density of dislocations present in a martensitic structure formed by quenching is controlled within a proper range and the dislocation density of such a martensitic structure is controlled within a proper range in such a manner that an element, such as Cu, primarily forming fine precipitates on the dislocations is used to prevent the recovery of the dislocations.

We thus provide a brake disk having a martensitic structure having prior-austenite grains with an average diameter of 8 μm or more, a hardness of 32 to 38 HRC, and high temper softening resistance. The brake disk contains 0.1% or less C, 1.0% or less Si, 2.0% or less Mn, 10.5% to 15.0% Cr, 2.0% or less Ni, greater than 0.5% to 4.0% Cu, 0.02% to 0.3% Nb, and 0.1% or less N on a mass basis and further contains C, N, Nb, Cr, Si, Ni, Mn, Mo, and Cu, the remainder being Fe and unavoidable impurities, such that the following inequalities are satisfied:

5Cr+10Si+15Mo+30Nb−9Ni−5Mn−3Cu−225N−270C<45  (1)

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