U.S. patent application number 12/297091 was filed with the patent office on 2009-06-25 for brake disk having high temper softening resistance.
This patent application is currently assigned to JFE Steel Corp.. Invention is credited to Osamu Furukimi, Etsuo Hamada, Junichiro Hirasawa, Noriko Makiishi, Takumi Ujiro, Takako Yamashita.
Application Number | 20090162240 12/297091 |
Document ID | / |
Family ID | 38624678 |
Filed Date | 2009-06-25 |
United States Patent
Application |
20090162240 |
Kind Code |
A1 |
Hirasawa; Junichiro ; et
al. |
June 25, 2009 |
BRAKE DISK HAVING HIGH TEMPER SOFTENING RESISTANCE
Abstract
A brake disk including, by mass, 0.1% or less of C, 1.0% or less
of Si, 2.0% or less of Mn, 10.5% to 15.0% of Cr, 2.0% or less of
Ni, greater than 0.5% to 4.0% of Cu, 0.02% to 0.3% of Nb, and 0.1%
or less of N and further including 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)
0.03.ltoreq.{C+N-(13/93)Nb}.ltoreq.0.09 (2) and having a
martensitic structure having prior-austenite grains with an average
diameter of 8 .mu.m or more, a hardness of 32 to 38 HRC, and high
temper softening resistance.
Inventors: |
Hirasawa; Junichiro;
(Hiroshima, JP) ; Yamashita; Takako; (Kanagawa,
JP) ; Makiishi; Noriko; (Chiba, JP) ; Hamada;
Etsuo; (Kanagawa, JP) ; Ujiro; Takumi; (Chiba,
JP) ; Furukimi; Osamu; (Fukuoka, JP) |
Correspondence
Address: |
IP GROUP OF DLA PIPER US LLP
ONE LIBERTY PLACE, 1650 MARKET ST, SUITE 4900
PHILADELPHIA
PA
19103
US
|
Assignee: |
JFE Steel Corp.
Tokyo
JP
|
Family ID: |
38624678 |
Appl. No.: |
12/297091 |
Filed: |
October 5, 2006 |
PCT Filed: |
October 5, 2006 |
PCT NO: |
PCT/JP2006/320358 |
371 Date: |
October 30, 2008 |
Current U.S.
Class: |
420/61 ; 148/326;
420/41 |
Current CPC
Class: |
F16D 2200/0021 20130101;
C22C 38/58 20130101; C22C 38/02 20130101; C21D 9/46 20130101; C22C
38/04 20130101; C22C 38/42 20130101; C22C 38/48 20130101 |
Class at
Publication: |
420/61 ; 148/326;
420/41 |
International
Class: |
C22C 38/20 20060101
C22C038/20; C22C 38/22 20060101 C22C038/22 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 21, 2006 |
JP |
2006-117375 |
Claims
1. A brake disk comprising, by mass, 0.1% or less of C, 1.0% or
less of Si, 2.0% or less of Mn, 10.5% to 15.0% of Cr, 2.0% or less
of Ni, greater than 0.5% to 4.0% of Cu, 0.02% to 0.3% of Nb, and
0.1% or less of N and further comprising 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)
0.03.ltoreq.{C+N-(13/93)Nb}.ltoreq.0.09 (2) and having a
martensitic structure having prior-austenite grains with an average
diameter of 8 .mu.m or more, a hardness of 32 to 38 HRC and high
temper softening resistance.
2. The brake disk according to claim 1, wherein a precipitated
Nb-to-total Nb ratio that is defined as the ratio of the amount of
precipitated Nb to the amount of total Nb is less than 0.75 and the
following inequalities are satisfied:
5Cr+10Si+15Mo+30Nb-9Ni-5Mn-3Cu-225N-270C<45 (1)
0.03.ltoreq.{C+N-(13/93)Nb}.ltoreq.0.09 (2).
3. The brake disk according to claim 1, wherein the square root (
.rho.) of the density .rho. of dislocations present in the
martensitic structure is within a range from 0.8.times.10.sup.8 to
1.3.times.10.sup.8 m.sup.-1.
4. The brake disk according to claim 1, having a hardness of 30 HRC
or more after being tempered at 650.degree. C. for one hour.
5. The brake disk according to claim 1, having fine Cu precipitates
formed at dislocations present in the martensitic structure,
wherein the square root ( .rho.) of the density .rho. of the
dislocations present in the martensitic structure tempered at
650.degree. C. for one hour is within a range from
0.6.times.10.sup.8 to 1.3.times.10.sup.8 m.sup.-1.
6. The brake disk according to claim 1, further comprising one or
both of 0.01% to 2.0% of Mo and 0.01% to 1.0% of Co on a mass
basis.
7. The brake disk according to claim 1, further comprising one or
more selected from the group consisting of, by mass, 0.02% to 0.3%
of Ti, 0.02% to 0.3% of V, 0.02% to 0.3% of Zr, and 0.02% to 0.3%
of Ta.
8. The brake disk according to claim 1, further comprising, by
mass, one or both of 0.0005% to 0.0050% of B and 0.0005% to 0.0050%
of Ca.
9. The brake disk according to claim 2, wherein the square root (
.rho.) of the density .rho. of dislocations present in the
martensitic structure is within a range from 0.8.times.10.sup.8 to
1.3.times.10.sup.8 m.sup.-1.
10. The brake disk according to claim 2, having a hardness of 30
HRC or more after being tempered at 650.degree. C. for one
hour.
11. The brake disk according to claim 3, having a hardness of 30
HRC or more after being tempered at 650.degree. C. for one
hour.
12. The brake disk according to claim 9, having a hardness of 30
HRC or more after being tempered at 650.degree. C. for one
hour.
13. The brake disk according to claim 2, having fine Cu
precipitates formed at dislocations present in the martensitic
structure, wherein the square root ( .rho.) of the density .rho. of
the dislocations present in the martensitic structure tempered at
650.degree. C. for one hour is within a range from
0.6.times.10.sup.8 to 1.3.times.10.sup.8 m.sup.-1.
14. The brake disk according to claim 3, having fine Cu
precipitates formed at dislocations present in the martensitic
structure, wherein the square root ( .rho.) of the density .rho. of
the dislocations present in the martensitic structure tempered at
650.degree. C. for one hour is within a range from
0.6.times.10.sup.8 to 1.3.times.10.sup.8 m.sup.-1.
15. The brake disk according to claim 4, having fine Cu
precipitates formed at dislocations present in the martensitic
structure, wherein the square root ( .rho.) of the density .rho. of
the dislocations present in the martensitic structure tempered at
650.degree. C. for one hour is within a range from
0.6.times.10.sup.8 to 1.3.times.10.sup.8 m.sup.-1.
16. The brake disk according to claim 9, having fine Cu
precipitates formed at dislocations present in the martensitic
structure, wherein the square root ( .rho.) of the density .rho. of
the dislocations present in the martensitic structure tempered at
650.degree. C. for one hour is within a range from
0.6.times.10.sup.8 to 1.3.times.10.sup.8 m.sup.-1.
17. The brake disk according to claim 2, further comprising one or
both of 0.01% to 2.0% of Mo and 0.01% to 1.0% of Co on a mass
basis.
18. The brake disk according to claim 3, further comprising one or
both of 0.01% to 2.0% of Mo and 0.01% to 1.0% of Co on a mass
basis.
19. The brake disk according to claim 4, further comprising one or
both of 0.01% to 2.0% of Mo and 0.01% to 1.0% of Co on a mass
basis.
20. The brake disk according to claim 5, further comprising one or
both of 0.01% to 2.0% of Mo and 0.01% to 1.0% of Co on a mass
basis.
Description
RELATED APPLICATIONS
[0001] This is a .sctn.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.
TECHNICAL FIELD
[0002] 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.
BACKGROUND
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.degree. 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.
[0007] 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.
[0008] 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.degree. C. for a long time.
[0009] It would therefore be helpful to provide a brake disk having
proper hardness and high temper softening resistance.
SUMMARY
[0010] 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.degree. 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 .mu.m 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.
[0011] 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.
[0012] We thus provide a brake disk having a martensitic structure
having prior-austenite grains with an average diameter of 8 .mu.m
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)
0.03.ltoreq.{C+N-(13/93)Nb}.ltoreq.0.09 (2).
[0013] In the brake disk, a precipitated Nb-to-total Nb ratio that
is defined as the ratio of the amount of precipitated Nb to the
amount of total Nb is less than 0.75.
[0014] In the brake disk, the square root ( .rho.) of the density
.rho. of dislocations present in the martensitic structure is
within a range from 0.8 to 1.3.times.10.sup.8 m.sup.-1.
[0015] The brake disk has a hardness of 30 HRC or more after being
tempered at 650.degree. C. for one hour or fine Cu precipitates
formed at the dislocations. The square root ( .rho.) of the density
.rho. of the dislocations present in the martensitic structure
tempered at 650.degree. C. for one hour is within a range from 0.6
to 1.3.times.10.sup.8 m.sup.-1.
[0016] The brake disk further contains at least one component
selected from the following groups: one or two selected from the A
group consisting of, by mass, 0.01% to 2.0% of Mo and 0.01% to 1.0%
of Co; one or more selected from the B group consisting of, by
mass, 0.02% to 0.3% of Ti, 0.02% to 0.3% of V, 0.02% to 0.3% of Zr,
and 0.02% to 0.3% of Ta; and one or two selected from the C group
consisting of, by mass, 0.0005% to 0.0050% of B and 0.0005% to
0.0050% of Ca.
[0017] We provide a low-cost brake disk with high temper softening
resistance. The brake disk has a hardness of 32 to 38 HRC and also
has a hardness of 30 HRC or more after being tempered at
650.degree. C. for one hour.
BRIEF DESCRIPTION OF DRAWINGS
[0018] FIG. 1 is a graph showing the influence of the average
diameter of prior-austenite grains on the hardness (HRC) of
hardened or tempered steel sheets.
[0019] FIG. 2 is a graph showing the influence of the ratio of the
amount of precipitated Nb to the amount of total Nb on the hardness
(HRC) of a tempered steel sheet.
DETAILED DESCRIPTION
[0020] Experiments leading to the development of our brake disks
will now be described.
[0021] Low-carbon martensitic stainless steel sheets containing, by
mass, 0.04% of C, 12% of Cr, 0.1% of Si, 1.5% of Mn, 0.04% of N,
0.12% of Nb, 0.7% of Ni, and 1.0% of Cu, the remainder being
substantially Fe, were hardened in such a manner that the steel
sheets were heated at temperatures between 1000.degree. C. and
1150.degree. C. at four levels, held at the temperatures for one
minute, and then air-cooled to 200.degree. C. at an average cooling
rate of 110.degree. C./sec. A cross-section of the quenched steel
sheets were observed for metal microstracture, whereby
prior-austenite grains (hereinafter referred to as prior-.gamma.
grains) were measured for average diameter. The prior-.gamma.
grains of each steel sheet heated at 1000.degree. C., 1050.degree.
C., 1100.degree. C., or 1150.degree. C. had an average diameter of
5, 8, 10, or 15 .mu.m, respectively. The hardened steel sheets were
tempered in such a manner that the steel sheets were held at
650.degree. C. for one hour and then air-cooled. The untempered
steel sheets and the tempered steel sheets were measured for
surface hardness (HRC) with a Rockwell hardness meter after surface
oxide layers (scale) were removed therefrom.
[0022] FIG. 1 shows the influence of the average diameter of the
prior-austenite grains (prior-.gamma. grains), on the surface
hardness (HRC) of the steel sheets. As is clear from FIG. 1, the
quenched steel sheets having the prior-.gamma. grains with an
average diameter of 8 .mu.m or more have a hardness of 32 to 38 HRC
and the steel sheets tempered at 650.degree. C. for one hour have a
hardness of 30 HRC or more although these steel sheets do not
contain a large amount of alloying elements. These results suggest
that, to allow a quenched steel sheet to have a hardness of 32 to
38 HRC and to allow a steel sheet tempered at 650.degree. C. for
one hour to have a hardness of 30 HRC or more, prior-.gamma. grains
in a martensitic structure formed by quenching need to have an
average diameter of 8 .mu.m or more.
[0023] The reason why an increase in the diameter of prior-.gamma.
grains formed by quenching increases temper softening resistance is
not clear yet but is probably as described below. In usual,
alloying elements, such as Cr and Nb, present in crystal grains in
the form of solid solutions diffuse to form fine precipitates
(chromium carbides, niobium carbonitrides, and the like) in the
crystal grains during tempering subsequent to hardening. The
alloying elements reach grain boundaries to form coarse
precipitates at the grain boundaries. In a metal microstructure
containing fine prior-.gamma. grains, the diffusion length
necessary for the alloying elements present in the prior-.gamma.
grains to reach boundaries of the prior-.gamma. grains is small;
hence, coarse precipitates (chromium carbides) are readily formed
at the prior-.gamma. grains boundaries by tempering. This reduces
fine precipitates in crystal grains to cause insufficient
precipitation hardening. The coarse pre-cipitates formed at the
grain boundaries have a small contribution to precipitation
hardening. On the other hand, in a metal microstructure containing
course prior-.gamma. grains, the diffusion length that alloying
elements, such as Cr and Nb, present in crystal grains in the form
of solid solutions migrate to boundaries of prior-.gamma. grains is
large; hence, the alloying elements hardly reach the prior-.gamma.
grain boundaries and therefore form fine precipitates in the
prior-.gamma. grains. These precipitates prevent the movement of
dislocations. This probably leads to an increase in temper
softening resistance.
[0024] Low-carbon martensitic stainless steel sheets containing, by
mass, 0.04% of C, 12.1% of Cr, 0.2% of Si, 1.6% of Mn, 0.04% of N,
0.13% of Nb, 0.6% of Ni, and 1.0% of Cu, the remainder being
substantially Fe, were hardened in such a manner that the steel
sheets were heated at temperatures between 900.degree. C. and
1150.degree. C. at six levels, held at the temperatures for one
minute, and then air-cooled to 200.degree. C. at an average cooling
rate of 10.degree. C./sec. The amount of Nb precipitated in the
form of precipitates and the amount of total Nb contained in each
steel sheet were measured, whereby the ratio of the amount of
precipitated Nb to the amount of total Nb, that is, the
precipitated Nb-to-total Nb ratio was determined. The hardened
steel sheets were tempered in such a manner that the steel sheets
were held at 650.degree. C. for one hour and then air-cooled.
Surface oxide layers (scale) were removed from the steel sheets.
The resulting steel sheets were measured for surface hardness with
a Rockwell hardness meter. The amount of precipitated Nb was
determined by chemically analyzing the residue of electrolytic
extraction from the sample and the amount of total Nb was
determined by ordinary chemical analysis.
[0025] FIG. 2 is based on the analysis results and shows the
influence of the precipitated Nb-to-total Nb ratio of each quenched
steel sheet on the hardness of the tempered steel sheet. As is
clear from FIG. 2, to secure the temper softening resistance of the
steel sheets such that the steel sheets tempered at 650.degree. C.
for one hour have a hardness of 30 HRC or more, the precipitated
Nb-to-total Nb ratio of each steel sheet needs to be less than
0.75.
[0026] The reason why the quenched steel sheets with a low
precipitated Nb-to-total Nb ratio have high temper softening
resistance is probably as described below. Since most of Nb
contained in the unhardened steel sheets is present in the form of
precipitates, the precipitated Nb-to-total Nb ratio is usually 0.9
or more. A portion of precipitated Nb forms a solid solution
because of heating during hardening. Solute Nb forms fine
precipitates during tempering subsequent to hardening. This
contributes to precipitation hardening. If the steel sheets are not
sufficiently heated during quenching, precipitated Nb is
incompletely smelted and therefore the amount of solute Nb is
small. Hence, the amount of Nb precipitates formed by tempering is
small. This probably leads to a reduction in temper softening
resistance.
[0027] A martensitic structure obtained by quenching contains dense
dislocations. It is well known that an increase in the density of
the dislocations leads to an increase in hardness. We investigated
the relationship between the density of dislocations in martensitic
steels and the hardness of brake disks. As a result, we found that
there is a tight correlation between the brake disk hardness and
the dislocation density, that the brake disk hardness can be
therefore controlled within a proper range in such a manner that
the dislocation density is adjusted to a proper range, and that it
is effective in preventing a brake disk from being softened due to
tempering that the recovery of dislocations in a martensitic
structure is prevented by any means such that the density of the
dislocations therein is maintained within a proper range.
Furthermore, we investigated a technique for inhibiting the
recovery of such dislocations and then have found that the
dislocation recovery can be securely prevented in such a manner
that Cu is added to steel and fine Cu precipitates are formed at
the dislocations during tempering.
[0028] The reason why the content of each component in a brake disk
is established in the above range will now be described.
C: 0.1% by Mass or Less
[0029] C is an element determining the hardness of the brake disk.
To allow the quenched brake disk to have a proper hardness (32 to
38 HRC), the brake disk preferably contains 0.03% by mass or more
of C. When the content of C therein is greater than 0.1% by mass,
coarse grains of chromium carbide (Cr.sub.23C.sub.6) are formed,
thereby causing rust, a reduction in corrosion resistance, and a
reduction in toughness. Therefore, the C content needs to be 0.1%
by mass or less. In view of corrosion resistance, the C content is
preferably less than 0.05% by mass.
Si: 1.0% by Mass or Less
[0030] Si is an element used as a deoxidizer and therefore the
brake disk preferably contains 0.05% by mass or more of Si. Since
Si is stabilizes a ferrite phase, an excessive Si content exceeding
1.0% by mass causes a reduction in hardenability, a reduction in
quenching hardness, and a reduction in toughness. Therefore, the
content of Si therein is limited to 1.0% by mass or less. In view
of toughness, the Si content is preferably 0.5% by mass or
less.
Mn: 2.0% by Mass or Less
[0031] Mn is an element that is useful in securing constant
quenching hardness because Mn prevents a .delta.-ferrite phase from
being formed at high temperature to enhance hardenability. The
brake disk preferably contains 0.3% by mass or more of Mn. However,
an excessive Mn content exceeding 2.0% by mass causes a reduction
in corrosion resistance because Mn reacts with S to form MnS.
Therefore, the content of Mn therein is limited to 2.0% by mass or
less. In view of an increase in hardenability, the Mn content is
preferably greater than 1.0% and more preferably greater than 1.2%
on a mass basis.
Cr: 10.5% to 15.0% by Mass
[0032] Cr is an element essential to secure the corrosion
resistance of stainless steel. To secure sufficient corrosion
resistance, the basic material needs to contain 10.5% by mass or
more of Cr. However, a Cr content exceeding 15.0% by mass causes a
reduction in workability and a reduction in toughness. Therefore,
the content of Cr therein is limited to a range from 10.5% to 15.0%
by mass. To achieve sufficient corrosion resistance, the Cr content
is preferably greater than 11.5% by mass. To secure toughness, the
Cr content is preferably less than 13.0% by mass.
Ni: 2.0% by Mass or Less
[0033] Ni has an effect of improving corrosion resistance and an
effect of increasing temper softening resistance because Ni retards
the precipitation of chromium carbides at a temperature higher than
650.degree. C. to prevent the reduction in the hardness of a
martensitic structure containing an excessive amount of C.
Furthermore, Ni has an effect of improving the corrosion resistance
of stainless steel and an effect of improving the toughness
thereof. Such effects are achieved when the content of Ni in the
brake disk is 0.1% by mass or more. However, even if the Ni content
exceeds 2.0% by mass, an advantage appropriate to the Ni content
cannot be obtained because the increase in temper softening
resistance is saturated. Therefore, the Ni content is limited to
2.0% by mass or less. In order to achieve improved temper softening
resistance, the Ni content is preferably 0.5% by mass or more.
Cu: Greater than 0.5% to 4.0% by Mass
[0034] Cu is an element significantly improving temper softening
resistance because Cu forms fine E-Cu precipitates at dislocations
present in a martensitic structure during quenching. To achieve
such an effect, the content of Cu in the brake disk needs to be
greater than 0.5% by mass. However, a Cu content exceeding 4.0% by
mass causes a reduction in toughness. Hence, the Cu content is
within a range from greater than 0.5% to 4.0% by mass. In view of
toughness, the Cu content is preferably less than 1.5% by mass.
Nb: 0.02% to 0.3% by Mass
[0035] Nb is an element that improves temper softening resistance,
because Nb forms a carbonitride during heating at about 650.degree.
C. subsequently to hardening to cause precipitation hardening. To
achieve such an effect, the content of Nb is preferably 0.02% by
mass or more. However, an Nb content exceeding 0.3% by mass causes
a reduction in toughness. Therefore, the content of Nb in the brake
disk is preferably limited to a range from 0.02% to 0.3% by mass.
In view of an increase in temper softening resistance, the Nb
content is preferably greater than 0.08% and more preferably 0.11%
by mass or more. In view of toughness, the Nb content is preferably
0.2% by mass or less.
N: 0.1% by Mass or Less
[0036] N, as well as C, is an element determining the hardness of
quenched steel. N forms fine chromium nitride (Cr.sub.2N) grains at
a temperature of 500.degree. C. to 700.degree. C. and is effective
in increasing temper softening resistance because of the
precipitation hardening effect thereof. To achieve this effect, the
content of N is preferably greater than 0.03% by mass. However, an
N content exceeding 0.1% causes a reduction in toughness.
Therefore, the N content needs to be limited to 0.1% by mass or
less.
[0037] The brake disk needs to contain the above components within
the above ranges and also needs to satisfy the following
inequalities:
5Cr+10Si+15Mo+30Nb-9Ni-5Mn-3Cu-225N-270C<45 (1)
0.03.ltoreq.{C+N-(13/93)Nb}.ltoreq.0.09 (2)
wherein Cr, Si, Mo, Nb, Ni, Mn, Cu, N, and C each represent the
content (in mass percent) of the corresponding alloying elements.
The left-hand side value of Inequality (1) and the middle term
value of Inequality (2) are calculated on the basis that the
content of Cu, Nb, Mo, or Ni are assumed to be zero when the
content of Cu, Nb, Mo or Ni is, by mass, less than 0.01%, less than
0.02%, less than 0.01%, and less than 0.10%, respectively.
5Cr+10Si+15Mo+30Nb-9Ni-5Mn-3Cu-225N-270C<45 (1)
[0038] Inequality (1) defines a condition for securing excellent
hardening stability. The term "excellent hardening stability" used
herein means that the range of a quenching temperature achieved a
desired hardness after quenching is wide. The wide range is caused
when the amount of an austenite (.gamma.) phase formed during
quenching is 75 volume % or more and the austenite phase is
transformed into a martensite phase during cooling for quenching by
air-cooling or cooling at a rate faster than air-cooling. When the
left-hand side value of Inequality (1) is 45 or more, a constant
quenching hardness cannot be achieved because the amount of an
austenite phase formed during quenching does not exceed 75% by
volume or more or a temperature range forming such an amount of the
austenite phase is extremely narrow. Therefore, the left-hand side
value of Inequality (1) needs to be limited to less than 45.
0.03.ltoreq.{C+N-(13/93)Nb}.ltoreq.0.09 (2)
[0039] Inequality (2) defines a condition for controlling hardness
after quenching within a predetermined proper range. Hardness after
quenching strongly correlates with the content of C or N. However,
C or N has no contribution to hardness after quenching when C or N
is bonded with Nb to form Nb carbide or Nb nitride. Therefore,
hardness after quenching needs to be estimated using the amount of
C or N obtained by subtracting the amounts of C and N in
precipitates from the amounts of C and N, respectively, in steel,
that is, using the middle term value {C+N-(13/93)Nb} of Inequality
(2). When the middle term value of Inequality (2) is less than
0.03, the hardness after quenching of the brake disk is less than
32 HRC. When the middle term value is greater than 0.09, the
hardness is greater than 38 HRC. Therefore, to allow the quenched
brake disk to have a proper hardness (HRC 32 to 38), the middle
term value of Inequality (2) needs to be limited to a range from
0.03 to 0.09.
[0040] The brake disk may contain components below as required in
addition to the above fundamental components.
One or Both of 0.01% to 2.0% of Mo and 0.01% to 1.0% of Co by
Mass
[0041] Mo and Co are elements effective in improving corrosion
resistance and therefore the brake disk may contain 0.01% by mass
or more of Mo and/or Co, respectively. In particular, Mo retards
the precipitation of carbonitrides and has a significant
contribution to an increase in temper softening resistance. To
achieve such effects, the content of Mo is preferably 0.02% by mass
or more. The increase in temper softening resistance due to Mo can
be achieved even if the content of Mo is less than 0.05% by mass.
However, even if the content of Mo and/or the content of Co exceeds
2.0% and/or 1.0% by mass, respectively, the increase in corrosion
resistance due to Mo and/or Co and the increase in temper softening
resistance due to Mo are saturated. Therefore, the content of Mo is
preferably limited to up to 2.0% by mass and the content of Co is
preferably limited to up to 1.0% by mass.
One or More of 0.02% to 0.3% of Ti, 0.02% to 0.3% of V, 0.02% to
0.3% of Zr, and 0.02% to 0.3% of Ta by Mass
[0042] Ti, V, Zr, and Ta, as well as Nb, are elements which form
carbonitrides to increase temper softening resistance. One or more
of these elements may be contained in the brake disk as required.
Such an effect of increasing temper softening resistance is
obtained when the content of Ti, V, Zr, or Ta is 0.02% by mass or
more respectively. In particular, V is effective; hence, the
content of V is preferably 0.05% by mass or more and more
preferably 0.10% by mass or more. On the other hand, a Ti, V, Zr,
or Ta content exceeding 0.3% by mass causes a significant reduction
in toughness. Therefore, the content of Ti, at of V, Zr, or Ta is
preferably limited to a range from 0.02% to 0.3% by mass
respectively.
One or Both of 0.0005% to 0.0050% of B and 0.0005% to 0.0050% of Ca
on Mass Basis
[0043] B and Ca are elements that have an effect of increasing the
hardenability of steel even if their contents are slight.
Therefore, 0.0005% by mass or more of B and/or Ca may be contained
in the brake disk as required. However, a B or Ca content exceeding
0.0050% by mass causes such an effect to be saturated and also
causes a reduction in corrosion resistance. Therefore, the content
of B and that of Ca are preferably limited to up to 0.0050% by mass
respectively.
[0044] The brake disk further contains Fe and unavoidable
impurities such as P, S, and Al in addition to the above
components. The content of P, that of P, and that of Al in the
brake disk are preferably within ranges below.
P: 0.04% by Mass or Less
[0045] P is an element causing a reduction in hot workability;
hence, the content of P is preferably small. An excessive reduction
in the P content leads to a significant increase in production
cost. Therefore, the upper limit of the P content is preferably
0.04% by mass. In view of productivity, the P content is more
preferably 0.03% by mass or less.
S: 0.010% by Mass or Less
[0046] S, as well as P, is an element causing a reduction in hot
workability; hence, the content of S is preferably small. In
consideration of the cost of desulfurization during steel making,
the S content is preferably 0.010% by mass or less. In view of hot
workability, the S content is more preferably 0.005% by mass or
less.
Al: 0.2% by Mass or Less
[0047] Al is an element functioning as a deoxidizer. An excessive
amount of Al remaining in steel as an unavoidable impurity causes a
reduction in corrosion resistance, a reduction in toughness, and
deteriorations in surface properties. Therefore, the content of Al
is preferably limited to 0.2% by mass or less. To achieve
sufficient corrosion resistance, the Al content is more preferably
0.05% by mass or less.
[0048] The brake disk may further contain 0.05% by mass or less of
each of alkali metals such as Na and Li, alkaline-earth metals such
as Mg and Ba, rare-earth elements such as Y and La, and transition
metals such as Hf in addition to the above unavoidable impurities.
This does not negate advantages of our brake disks.
[0049] The metal structure of the brake disk will now be
described.
Average Diameter of Prior-.gamma. Grains: 8 .mu.m or More
[0050] The brake disk can be controlled to have a quenching
hardness of 32 to 38 HRC when a steel sheet (martensitic stainless
steel sheet) for producing the brake disk contains the components
within the above ranges. To allow the brake disk tempered at
650.degree. C. for one hour to have a hardness of 30 HRC or more,
the brake disk needs to have a martensitic structure having
prior-.gamma. grains with an average diameter of 8 .mu.m or more as
shown in FIG. 1. When the prior-.gamma. grains have an average
diameter of less than 8 .mu.m, the amount of fine precipitates in
the prior-.gamma. grains is small and therefore high temper
softening resistance cannot be achieved. To secure temper softening
resistance, the prior-.gamma. grains preferably have an average
diameter of 10 .mu.m or more and more preferably 15 .mu.m or more.
When the prior-.gamma. grains have an average diameter of greater
than 30 .mu.m, the facet sizes of brittle fracture surfaces are
large, which causes a reduction in toughness. Therefore, the
prior-.gamma. grains preferably have an average diameter of 30
.mu.m or less. Precipitated Nb-to-total Nb ratio of quenched brake
disk: less than 0.75
[0051] To allow the brake disk, tempered at 650.degree. for one
hour, to have a hardness of 30 HRC or more, the brake disk needs to
have a precipitated Nb-to-total Nb ratio of less than 0.75 as shown
in FIG. 2. This is because when the precipitated Nb-to-total Nb
ratio of the hardened brake disk is 0.75 or more, the amount of Nb
present in grains in the form of solid solutions is too small to
achieve sufficient temper softening resistance. To secure high
temper softening resistance, the precipitated Nb-to-total Nb ratio
of the brake disk is preferably 0.5 or less and more preferably 0.4
or less. When the precipitated Nb-to-total Nb ratio thereof is less
than 0.1, the brake disk has high temper softening resistance but
seriously low toughness, because a large amount of fine Nb
precipitates are formed during tempering because of a large amount
of solute Nb and the precipitates cause fracture. To secure
toughness, the precipitated Nb-to-total Nb ratio thereof is
preferably 0.1 or more and more preferably 0.2 or more. The amount
of precipitated Nb is determined by chemically analyzing the
residue of electrolytic extraction from the sample as described
below and the amount of total Nb is determined by ordinary chemical
analysis.
Density of Dislocations Present in Martensitic Structure
[0052] The brake disk needs to have a hardness of 32 to 38 HRC
after quenching. To secure such hardness, the square root ( .rho.)
of the density .rho. of dislocations present in the martensitic
structure of the hardened brake disk is preferably within a range
from 0.8.times.10.sup.8 to 1.3.times.10.sup.8 m.sup.-1. The brake
disk preferably has a hardness of 30 HRC or more after being
tempered at 650.degree. C. for one hour. To secure such hardness,
the square root ( .rho.) of the density .rho. of dislocations
present in the martensitic structure of the tempered brake disk is
preferably within a range from 0.6.times.10.sup.8 to
1.3.times.10.sup.8 m.sup.-1. The dislocation density of the
tempered brake disk is determined by fine Cu precipitates formed at
the dislocations during tempering.
[0053] A method for producing a martensitic stainless steel sheet
that is a basic material for producing the brake disk will now be
described. The stainless steel sheet may be hot-rolled or
cold-rolled one as long as conditions specified herein are
satisfied.
[0054] The martensitic stainless steel sheet, which is used to
produce the brake disk, is preferably produced from a steel
material (slab) that is obtained in such a manner that molten steel
having the above composition is melted in a steel converter, an
electric furnace, an electric furnace, or the like; subjected to
secondary refining such as VOD (vacuum oxygen decarburization) or
AOD (argon oxygen decarburization); and then cast into a steel
material (slab). The steel material is usually produced by an ingot
making-slabbing process or a continuous casting process. In view of
producibility and quality, the continuous casting process is
preferably used.
[0055] Hot-rolled steel sheets and cold-rolled steel sheets can
both be used to produce basic materials for brake disks as
described above. Hot-rolled steel sheets with a thickness of about
3 to 8 mm are usually used to produce brake disks for motorcycles
and motorcars. For these uses, the steel material is reheated to a
temperature of 1100.degree. C. to 1250.degree. C. and then
hot-rolled into a hot-rolled steel strip (steel sheet) with a
predetermined thickness. The hot-rolled steel strip is preferably
annealed at a temperature of higher than 750.degree. C. to
900.degree. C. for about ten hours in a batch-type box furnace as
required and then used to produce for such brake disk materials.
The resulting hot-rolled steel strip may be descaled by pickling,
shot blast, or the like as required.
[0056] Brake disks for bicycles and the like have a thickness of
about 2 mm and therefore are usually produced from cold-rolled
steel sheets. For these uses, the hot-rolled steel strip is
cold-rolled into a steel sheet, which is preferably annealed at a
temperature of 600.degree. C. to 800.degree. C., pickled as
required, and then used to produce basic materials for such brake
disk.
[0057] A method for producing the brake disk from a disk material
obtained from the martensitic stainless steel sheet will now be
described.
[0058] The disk material, which is obtained form the hot-rolled
martensitic stainless steel sheet or cold-rolled martensitic
stainless steel sheet, is machined a disk with a predetermined size
by punching or the like. The disk is machined so as to have cooling
holes having a function of dissipating the heat generated during
braking to enhance braking performance. A friction portion of the
disk that will meet brake pads is hardened so as to have a hardness
of 32 to 38 HRC in such a manner that the friction portion is
heated at a predetermined quenching temperature by high-frequency
induction heating or the like, held at the quenching temperature
for a predetermined time, and then cooled to room temperature.
Surface scales formed by hardening are removed from the disk by
shot blast or the like. Surfaces of the disk and sheared surfaces
thereof are subjected to coating as required. To increase
mechanical accuracy, the friction portion is mechanically ground,
whereby a product (the brake disk) is obtained.
[0059] In order to produce the brake disk, quenching conditions are
preferably as described below.
Quenching Temperature: Higher than 1000.degree. C.
[0060] The quenching temperature (heating temperature during
quenching) is preferably within a .gamma. region and is higher than
1000.degree. C. The term ".gamma. region" used herein means a
region of temperature in which the percentage of an austenite phase
is 75% by volume or more in the steel. When the quenching
temperature is higher than 1000.degree. C., the hardened friction
portion has proper hardness (32 to 38 HRC) after quenching and has
prior-.gamma. grains with an average diameter of 8 .mu.m or more
and/or a precipitated Nb-to-total Nb ratio of less than 0.75. Since
the friction portion is remarkably improved in temper softening
resistance, the friction portion can be prevented from being
reduced in hardness even if the friction portion is tempered at a
high temperature of 650.degree. C. for one hour. Even if the
quenching temperature is 1000.degree. C. or less, the friction
portion can be improved in temper softening resistance in such a
manner that the prior-.gamma. grains are enlarged and/or the amount
of precipitated Nb is reduced by increasing the holding time.
However, this is not preferable because of a reduction in
productivity. To enhance the temper softening resistance of the
friction portion, the quenching temperature is preferably
1050.degree. C. or more and more preferably 1100.degree. C. or
more. However, when the quenching temperature is higher than
1200.degree. C., a large amount of .delta.-ferrite is formed and
therefore 75% by volume or more of the austenite phase cannot be
achieved; hence, the quenching temperature is preferably
1200.degree. C. or less. In view of quenching stability, the
quenching temperature is preferably 1150.degree. C. or less. To
fully transform the ferrite phase into the austenite phase, the
holding time of the friction portion at the quenching temperature
is preferably 30 seconds or more. A technique for heating the
friction portion during quenching is not particularly limited. In
view of producibility, the friction portion is preferably heated by
high-frequency induction heating.
Cooling Rate: 1.degree. C./Sec or More
[0061] After being heated at the quenching temperature, the
friction portion is preferably cooled to an Ms point (martensitic
transformation starting temperature) or less and more preferably
200.degree. C. or less at a rate of 1.degree. C./sec or more. When
the cooling rate of the friction portion is less than 1.degree.
C./sec, the hardness of the quenched friction portion cannot be
adjusted to a proper range (32 to 38 HRC) because a portion of an
austenite phase produced at the quenching temperature is
transformed into a ferrite phase and therefore the amount of a
martensite phase produced during quenching is small. The cooling
rate thereof is preferably within a range from 5 to 500.degree.
C./sec. To achieve constant quenching hardness, the cooling rate is
preferably 100.degree. C./sec or more.
[0062] The brake disk, produced from the martensitic stainless
steel as described above has a hardness of 32 to 38 HRC and also
has prior-.gamma. grains with an average diameter of 8 .mu.m or
more and/or a precipitated Nb-to-total Nb ratio of less than 0.75.
Therefore, the hardness of the brake disk can be maintained at 30
HRC or more even after the brake disk is tempered at 650.degree. C.
for one hour, that is, the brake disk has excellent temper
softening resistance.
Example 1
[0063] Different 19 types of martensitic stainless steels A to S
having compositions shown in Table 1 were produced in a
high-frequency melting furnace and then cast into 50 kg ingots. The
ingots were hot-rolled into hot-rolled steel sheets with a
thickness of 5 mm under ordinary known conditions. The hot-rolled
steel sheets were annealed in such a manner that the hot-rolled
steel sheets were heated at 800.degree. C. for eight hours in a
reducing gas atmosphere, gradually cooled, and then descaled by
pickling. Specimens having dimensions of 30 mm.times.30 mm and a
thickness equal to that of the hot-rolled annealed steel sheets
were taken from the hot-rolled annealed steel sheets and then
hardened under conditions shown in Table 2 to 4. The hardened
specimens were observed for metal structure, measured for the
amount of precipitated Nb, and subjected to a hardening stability
test and a temper softening test. The specimens were measured for
dislocation density after being hardened and tempered. The maximum
temperature of .gamma.-regions shown in Tables 2 to 4 refers to a
maximum temperature at which an austenite (.gamma.) phase is formed
by 75 volume percent or more. At a temperature higher than or equal
to the maximum temperature, a .delta. phase (ferrite phase) is
increased and therefore the .gamma. phase cannot be formed by 75
volume percent or more.
Observation of Metal Structure
[0064] A sample for metal structure observation was taken from each
hardened specimen. A cross section of the sample was parallel to
the hot-rolling direction and the thickness direction was polished
and then etched with an Murakami reagent alkaline solution of red
prussiate (10 g of a red prussiate, 10 g of caustic potassium
(potassium hydrate), and 100 ml of water), whereby boundaries of
prior-.gamma. grains were exposed. Five or more fields (one field:
0.2 mm.times.0.2 mm) were observed with an optical microscope (a
magnification of 400 times). The grains contained in each field of
view were measured for area with an image analysis device, whereby
the equivalent circle diameters of the grains were determined. The
equivalent circle diameters of the grains were averaged, whereby
the average diameter of the prior-.gamma. grains of the sample was
determined.
Measurement of Amount of Precipitated Nb
[0065] A sample for electrolytic extraction was taken from each
quenched specimen. The sample was subjected to electrolysis using
an acetylacetone (10 volume percent)-tetramethyl-ammonium chloride
(1 g/100-ml)-methanol electrolyte. A residue was extracted from the
electrolyte using membrane filter (a pore size of 0.2 .mu.m) and
then cleaned, whereby a residual dross was extracted. The extracted
residue dross was measured for the amount of Nb by inductively
coupled plasma emission spectrometry, whereby the amount of
precipitated Nb was determined.
Hardening Stability Test
[0066] Each quenched sample was descaled by pickling and then
measured for surface hardness (HRC) at five points with a Rockwell
hardness meter according to JIS Z 2245. The obtained measurements
were averaged, whereby the hardness after quenching of the sample
was determined. The samples having a hardness of 32 to 38 HRC were
evaluated to have sufficient hardening stability.
Temper Softening Test
[0067] Each quenched sample was tempered in such a manner that the
sample was heated at 650.degree. C., held at this temperature for a
time shown in Table 2, and then air-cooled. The tempered sample was
descaled by pickling and then measured for surface hardness (HRC)
at five points with a Rockwell hardness meter according to JIS Z
2245. The obtained measurements were averaged, whereby the hardness
of the sample was determined. The samples having a hardness of 30
HRC or more were evaluated to have sufficient temper softening
resistance.
Measurement of Dislocation Density
[0068] The dislocation density .rho. of each sample was determined
in such a manner that the hardened sample and the tempered sample
were subjected to X-ray analysis. Each sample was subjected to
X-ray diffraction using an X-ray source including a Co bulb and an
integrated optical system under the following conditions: a step
scan mode with a step width of 0.01.degree., a divergence slit
angle of 1.degree., and a light-receiving slit width of 0.15 mm.
The measurement time of each peak was adjusted such that peaks in a
diffraction pattern had an intensity of several thousand counts.
Three peaks (in a component of steel, martensite was supposed to be
a cubic crystal) corresponding to plane indices {100}, {211}, and
{220} other than {200} were used to calculate the dislocation
density of the sample. After K.alpha.1 and K.alpha.2 in each peak
were separated from each other using the X-ray diffraction pattern
analysis program JADE 5.0 available from MDI, the half-value width
thereof was corrected with the spread of a half-value width
obtained by measuring an annealed Si powder used as an ideal sample
with no strain, whereby the true half-value width was obtained.
Nonuniformity strain (.epsilon.) was calculated from the true
half-value width by the Williamson-Hall method. The dislocation
density of the sample was calculated using the following
equation:
.rho.=14.4.epsilon..sup.2/b.sup.2
wherein b represents the magnitude of the Berger spectrum and is
0.25 nm.
TABLE-US-00001 TABLE 1 Left-hand middle term Steel side value value
of num- Chemical components (mass percent) of Inequality Inequality
ber C Si Mn P S Al Cr N Cu Nb Mo Ni Others (1) (2) Remarks A 0.041
0.33 1.55 0.03 0.005 0.003 12.25 0.016 1.03 0.11 -- 0.62 -- 36.8
0.042 Example B 0.044 0.25 1.44 0.02 0.003 0.005 12.33 0.038 0.88
0.15 -- 0.64 -- 32.6 0.061 Example C 0.043 0.21 1.65 0.03 0.002
0.033 12.18 0.035 0.61 0.10 -- 0.03 -- 36.4 0.064 Example D 0.055
0.15 1.28 0.02 0.003 0.005 12.68 0.037 2.05 0.25 -- 0.51 -- 32.1
0.057 Example E 0.042 0.23 1.55 0.03 0.002 0.004 12.88 0.042 1.22
0.11 -- 0.62 V: 0.13 32.2 0.069 Example F 0.044 0.08 1.85 0.02
0.005 0.003 11.63 0.042 1.44 0.12 1.15 0.72 -- 38.4 0.069 Example G
0.042 0.30 1.61 0.03 0.003 0.005 12.23 0.045 1.03 0.12 0.02 0.27
Co: 0.04 33.0 0.070 Example H 0.042 0.27 1.28 0.03 0.002 0.003
12.20 0.046 0.59 0.11 0.01 0.63 V: 0.12, 31.6 0.073 Example B:
0.0025 I 0.044 0.33 1.33 0.02 0.005 0.002 12.82 0.046 1.15 0.15
0.04 0.60 V: 0.08 34.8 0.069 Example J 0.042 0.28 1.42 0.03 0.003
0.003 12.40 0.039 1.22 0.10 -- 0.65 Ca: 0.0015 31.1 0.067 Example K
0.095 0.31 0.88 0.03 0.002 0.011 12.12 0.009 2.05 0.31 -- 0.51 V:
0.02 30.2 0.061 Example L 0.042 0.88 0.33 0.04 0.005 0.065 14.82
0.045 3.18 0.04 -- 1.05 V: 0.07, 42.0 0.081 Example Ta: 0.05 M
0.064 0.52 1.11 0.03 0.010 0.003 10.74 0.055 0.95 0.28 -- 0.15 Zn
0.08 27.9 0.080 Example N 0.034 0.26 1.84 0.02 0.004 0.005 12.55
0.048 1.21 0.13 -- 0.51 Ti: 0.06 31.9 0.064 Example O 0.044 0.22
1.51 0.03 0.004 0.003 12.11 0.036 0.35 0.11 -- 0.56 -- 32.4 0.065
Comparative Example P 0.034 0.25 1.61 0.03 0.005 0.005 12.22 0.035
0.68 0.00 -- 0.61 -- 31.0 0.069 Comparative Example Q 0.043 0.51
1.26 0.02 0.003 0.005 12.91 0.035 0.75 0.15 -- 0.13 V: 0.05 44.9
0.057 Comparative Example R 0.026 0.13 1.69 0.03 0.008 0.005 12.20
0.036 0.85 0.28 -- 0.56 -- 39.5 0.023 Comparative Example S 0.055
0.21 1.43 0.02 0.003 0.003 12.33 0.055 1.22 0.09 0.33 0.17 -- 31.8
0.097 Comparative Example
TABLE-US-00002 TABLE 2 Maximum Quenching conditions Hardness
Average temperature Quenching Holding Cooling after Evaluation of
diameter of Steel of .gamma. regions temperature time rate
quenching hardness after prior-.gamma. No. number (.degree. C.)
(.degree. C.) (min) (.degree. C./sec) (HRC) quenching grains
(.mu.m) 1 A 1170 950 1 10 31 x Inferior 4 2 A 1170 1000 1 10 32
.smallcircle. Superior 5 3 A 1170 1050 1 10 32 .smallcircle.
Superior 8 4 A 1170 1100 1 10 33 .smallcircle. Superior 10 5 A 1170
1150 1 10 32 .smallcircle. Superior 15 6 A 1170 1200 1 10 30 x
Inferior 20 7 B 1200 950 10 10 35 .smallcircle. Superior 5 8 B 1200
1000 10 10 35 .smallcircle. Superior 9 9 B 1200 1050 10 10 35
.smallcircle. Superior 15 10 B 1200 1100 10 10 35 .smallcircle.
Superior 20 11 B 1200 1150 10 10 34 .smallcircle. Superior 26 12 C
1170 1000 10 200 35 .smallcircle. Superior 5 13 C 1170 1050 10 200
35 .smallcircle. Superior 10 14 C 1170 1150 10 200 34 .smallcircle.
Superior 20 15 C 1170 1200 10 200 30 x Inferior 30 16 D 1200 1000 1
1 33 .smallcircle. Superior 4 17 D 1200 1030 1 1 33 .smallcircle.
Superior 8 18 D 1200 1100 1 1 34 .smallcircle. Superior 10 19 D
1200 1150 1 1 34 .smallcircle. Superior 15 20 E 1200 1000 1 10 35
.smallcircle. Superior 5 21 E 1200 1050 1 10 35 .smallcircle.
Superior 9 22 E 1200 1170 1 10 35 .smallcircle. Superior 17 23 E
1200 1200 1 10 34 .smallcircle. Superior 22 Amount of Holding
Hardness Evaluation precipitated time during after of hardness
{square root over (.rho.)} NB/amount tempering tempering after
(.times.10.sup.8 m.sup.-1) No. of total Nb (hr) (HRC) tempering
Quenched Tempered Remarks 1 0.87 1 23 x Inferior 0.7 0.1
Comparative Example 2 0.81 1 24 x Inferior 0.8 0.1 Comparative
Example 3 0.66 1 30 .smallcircle. Superior 0.8 0.6 Example 4 0.47 1
31 .smallcircle. Superior 0.8 0.7 Example 5 0.34 1 31 .smallcircle.
Superior 0.8 0.7 Example 6 0.30 1 28 x Inferior 0.6 0.3 Comparative
Example 7 0.82 1 25 x Inferior 1.0 0.2 Comparative Example 8 0.70 1
31 .smallcircle. Superior 1.0 0.7 Example 9 0.45 1 31 .smallcircle.
Superior 1.0 0.7 Example 10 0.35 1 32 .smallcircle. Superior 1.1
0.8 Example 11 0.20 1 32 .smallcircle. Superior 1.0 0.8 Example 12
0.79 1 24 x Inferior 1.0 0.2 Comparative Example 13 0.55 1 31
.smallcircle. Superior 1.0 0.7 Example 14 0.25 1 32 .smallcircle.
Superior 0.9 0.8 Example 15 0.19 1 29 x Inferior 0.7 0.6
Comparative Example 16 0.85 1 26 x Inferior 0.8 0.3 Comparative
Example 17 0.73 1 30 .smallcircle. Superior 0.8 0.6 Example 18 0.52
1 30 .smallcircle. Superior 0.9 0.7 Example 19 0.41 1 31
.smallcircle. Superior 0.9 0.7 Example 20 0.80 1 23 x Inferior 1.1
0.1 Comparative Example 21 0.38 1 32 .smallcircle. Superior 1.1 0.8
Example 22 0.32 1 33 .smallcircle. Superior 1.1 0.9 Example 23 0.28
1 34 .smallcircle. Superior 1.0 0.9 Example
TABLE-US-00003 TABLE 3 Maximum Quenching conditions Hardness
Average temperature Quenching Holding Cooling after Evaluation of
diameter of Steel of .gamma. regions temperature time rate
quenching hardness after prior-.gamma. No. number (.degree. C.)
(.degree. C.) (min) (.degree. C./sec) (HRC) quenching grains
(.mu.m) 24 F 1170 1000 1 1 35 .smallcircle. Superior 5 25 F 1170
1050 1 1 35 .smallcircle. Superior 8 26 F 1170 1150 1 1 35
.smallcircle. Superior 15 27 F 1170 1200 1 1 30 x Inferior 21 28 G
1200 1000 10 10 36 .smallcircle. Superior 8 29 G 1200 1050 10 10 36
.smallcircle. Superior 15 30 G 1200 1150 10 10 37 .smallcircle.
Superior 25 31 H 1200 1000 1 10 36 .smallcircle. Superior 4 32 H
1200 1030 1 10 36 .smallcircle. Superior 8 33 H 1200 1150 1 10 36
.smallcircle. Superior 15 34 H 1200 1230 1 10 31 x Inferior 25 35 I
1200 1000 1 10 35 .smallcircle. Superior 6 36 I 1200 1050 1 10 35
.smallcircle. Superior 9 37 I 1200 1100 1 10 35 .smallcircle.
Superior 11 38 I 1200 1150 1 0.1 31 x Inferior 17 39 I 1200 1150 1
10 35 .smallcircle. Superior 15 40 J 1230 1000 1 10 35
.smallcircle. Superior 5 41 J 1230 1050 1 10 35 .smallcircle.
Superior 9 42 J 1230 1170 1 10 35 .smallcircle. Superior 17 43 J
1230 1230 1 10 34 .smallcircle. Superior 26 44 K 1230 1000 1 10 35
.smallcircle. Superior 5 45 K 1230 1150 1 10 35 .smallcircle.
Superior 16 46 K 1230 1230 1 10 34 .smallcircle. Superior 26 Amount
of Holding Hardness Evaluation precipitated time during after of
hardness {square root over (.rho.)} NB/amount tempering tempering
after (.times.10.sup.8 m.sup.-1) No. of total Nb (hr) (HRC)
tempering Quenched Tempered Remarks 24 0.86 1 26 x Inferior 1.1 0.4
Comparative Example 25 0.69 1 33 .smallcircle. Superior 1.1 0.9
Example 26 0.37 1 34 .smallcircle. Superior 1.1 1.0 Example 27 0.32
1 29 x Inferior 0.6 0.6 Comparative Example 28 0.72 1 32
.smallcircle. Superior 1.2 0.9 Example 29 0.51 1 33 .smallcircle.
Superior 1.1 0.8 Example 30 0.22 1 34 .smallcircle. Superior 1.2
1.0 Example 31 0.82 1 24 x Inferior 1.0 0.2 Comparative Example 32
0.73 1 31 .smallcircle. Superior 1.2 0.7 Example 33 0.32 1 32
.smallcircle. Superior 1.2 0.8 Example 34 0.26 1 27 x Inferior 0.6
0.5 Comparative Example 35 0.85 1 26 x Inferior 1.0 0.5 Comparative
Example 36 0.69 1 33 .smallcircle. Superior 1.1 0.8 Example 37 0.51
1 33 .smallcircle. Superior 1.1 0.9 Example 38 0.77 1 29 x Inferior
0.7 0.6 Comparative Example 39 0.35 1 34 .smallcircle. Superior 1.2
1.1 Example 40 0.83 1 25 x Inferior 1.1 0.4 Comparative Example 41
0.69 1 31 .smallcircle. Superior 1.1 0.7 Example 42 0.31 1 32
.smallcircle. Superior 1.2 0.8 Example 43 0.24 1 32 .smallcircle.
Superior 1.1 0.8 Example 44 0.85 1 25 x Inferior 1.1 0.3
Comparative Example 45 0.39 1 31 .smallcircle. Superior 1.2 0.7
Example 46 0.28 1 31 .smallcircle. Superior 1.1 0.7 Example
TABLE-US-00004 TABLE 4 Maximum Quenching conditions Hardness
Average temperature Quenching Holding Cooling after Evaluation of
diameter of Steel of .gamma. regions temperature time rate
quenching hardness after prior-.gamma. No. number (.degree. C.)
(.degree. C.) (min) (.degree. C./sec) (HRC) quenching grains
(.mu.m) 47 L 1150 1000 1 10 37 .smallcircle. Superior 6 48 L 1150
1100 1 10 37 .smallcircle. Superior 10 49 L 1150 1150 1 10 37
.smallcircle. Superior 17 50 M 1250 1000 1 10 37 .smallcircle.
Superior 5 51 M 1250 1150 1 10 37 .smallcircle. Superior 15 52 M
1250 1250 1 10 36 .smallcircle. Superior 30 53 N 1200 1000 1 10 35
.smallcircle. Superior 5 54 N 1200 1150 1 10 35 .smallcircle.
Superior 15 55 O 1200 1000 1 10 35 .smallcircle. Superior 5 56 O
1200 1050 1 10 35 .smallcircle. Superior 8 57 O 1200 1100 1 10 35
.smallcircle. Superior 11 58 O 1200 1150 1 10 35 .smallcircle.
Superior 16 59 P 1200 1000 1 10 35 .smallcircle. Superior 5 60 P
1200 1100 1 10 35 .smallcircle. Superior 12 61 P 1200 1150 1 10 35
.smallcircle. Superior 18 62 Q 1100 1000 1 10 32 .smallcircle.
Superior 5 63 Q 1100 1050 1 10 32 .smallcircle. Superior 8 64 Q
1100 1150 1 10 31 x Inferior 16 65 R 1170 1000 1 10 29 x Inferior 5
66 R 1170 1100 1 10 30 x Inferior 11 67 R 1170 1150 1 10 30 x
Inferior 15 68 S 1200 1000 1 10 41 x Inferior 5 69 S 1200 1100 1 10
41 x Inferior 12 70 S 1200 1150 1 10 42 x Inferior 17 Amount of
Holding Hardness Evaluation precipitated time during after of
hardness {square root over (.rho.)} NB/amount tempering tempering
after (.times.10.sup.8 m.sup.-1) No. of total Nb (hr) (HRC)
tempering Quenched Tempered Remarks 47 0.76 1 25 x Inferior 1.2 0.3
Comparative Example 48 0.45 1 30 .smallcircle. Superior 1.2 0.7
Example 49 0.31 1 31 .smallcircle. Superior 1.2 0.7 Example 50 0.85
1 25 x Inferior 1.1 0.3 Comparative Example 51 0.44 1 31
.smallcircle. Superior 1.2 0.7 Example 52 0.30 1 31 .smallcircle.
Superior 1.1 0.7 Example 53 0.81 1 25 x Inferior 0.9 0.3
Comparative Example 54 0.35 1 31 .smallcircle. Superior 1.0 0.7
Example 55 0.83 1 24 x Inferior 1.0 0.3 Comparative Example 56 0.68
1 29 x Inferior 0.9 0.6 Comparative Example 57 0.51 1 29 x Inferior
0.9 0.6 Comparative Example 58 0.38 1 29 x Inferior 1.0 0.7
Comparative Example 59 -- 1 22 x Inferior 0.9 0.2 Comparative
Example 60 -- 1 26 x Inferior 0.9 0.5 Comparative Example 61 -- 1
27 x Inferior 0.9 0.6 Comparative Example 62 0.82 1 23 x Inferior
0.8 0.3 Comparative Example 63 0.69 1 29 x Inferior 0.9 0.5
Comparative Example 64 0.39 1 28 x Inferior 0.7 0.4 Comparative
Example 65 0.85 1 22 x Inferior 0.6 0.3 Comparative Example 66 0.51
1 28 x Inferior 0.6 0.5 Comparative Example 67 0.40 1 29 x Inferior
0.6 0.5 Comparative Example 68 0.78 1 32 .smallcircle. Superior 1.6
0.9 Comparative Example 69 0.52 1 35 .smallcircle. Superior 1.7 1.2
Comparative Example 70 0.33 1 36 .smallcircle. Superior 1.7 1.2
Comparative Example
[0069] The test results are summarized in Tables 2 to 4. The
Examples meet our requirements and have a quenching hardness of 32
to 38 HRC. That is, the Examples are excellent in hardening
stability. Furthermore, the Examples have a hardness of 30 HRC or
more after being tempered. That is, the Examples have sufficient
temper softening resistance. On the other hand, the Comparative
Examples have a quenching hardness outside a range from 32 to 38
HRC or a hardness of less than 30 HRC after being tempered. That
is, the Comparative Examples are inferior in temper softening
resistance.
* * * * *