U.S. patent application number 15/511905 was filed with the patent office on 2017-09-07 for martensitic stainless steel for brake disk and method for producing said steel.
This patent application is currently assigned to NIPPON STEEL & SUMIKIN STAINLESS STEEL CORPORATION. The applicant listed for this patent is NIPPON STEEL & SUMIKIN STAINLESS STEEL CORPORATION. Invention is credited to Junichi HAMADA, Yoshiharu INOUE, Yuji KOYAMA, Toshio TANOUE, Shinichi TERAOKA.
Application Number | 20170253945 15/511905 |
Document ID | / |
Family ID | 55803980 |
Filed Date | 2017-09-07 |
United States Patent
Application |
20170253945 |
Kind Code |
A1 |
TERAOKA; Shinichi ; et
al. |
September 7, 2017 |
MARTENSITIC STAINLESS STEEL FOR BRAKE DISK AND METHOD FOR PRODUCING
SAID STEEL
Abstract
A martensitic stainless steel used for a brake disk of a
two-wheeled vehicle includes: in % by mass, C of 0.025% to 0.080%,
Si of 0.05% to 0.8%, Mn of 0.5% to 1.5%, P of 0.035% or less, S of
0.015% or less, Cr of 11.0% to 13.5%, Ni of 0.01% to 0.50%, Cu of
0.01% to 0.08%, Mo of 0.01% to 0.30%, V of 0.01% to 0.10%, Al of
0.05% or less, and N of 0.015% to 0.060%; a DFE value defined by a
formula (1) ranging from 5 to 30; and a .delta. ferrite fraction
observed in a cross section structure ranging from 5% to 30% by an
area ratio. Ti, B, Nb, Sn and Bi may be added.
DFE=12(Cr+Si)-430C-460N-20Ni-7Mn-89 (1)
Inventors: |
TERAOKA; Shinichi; (Tokyo,
JP) ; INOUE; Yoshiharu; (Tokyo, JP) ; KOYAMA;
Yuji; (Tokyo, JP) ; HAMADA; Junichi; (Tokyo,
JP) ; TANOUE; Toshio; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMIKIN STAINLESS STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
NIPPON STEEL & SUMIKIN
STAINLESS STEEL CORPORATION
Tokyo
JP
|
Family ID: |
55803980 |
Appl. No.: |
15/511905 |
Filed: |
September 2, 2015 |
PCT Filed: |
September 2, 2015 |
PCT NO: |
PCT/JP2015/074912 |
371 Date: |
March 16, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/48 20130101;
C21D 8/0226 20130101; C21D 8/0263 20130101; C21D 2211/008 20130101;
C22C 38/42 20130101; C22C 38/44 20130101; C22C 38/50 20130101; C22C
38/002 20130101; C22C 38/06 20130101; C21D 9/46 20130101; C22C
38/008 20130101; C22C 38/02 20130101; C22C 38/04 20130101; C22C
38/001 20130101; C22C 38/46 20130101; C22C 38/54 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/50 20060101 C22C038/50; C22C 38/48 20060101
C22C038/48; C22C 38/46 20060101 C22C038/46; C21D 8/02 20060101
C21D008/02; C22C 38/42 20060101 C22C038/42; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; C22C 38/54 20060101
C22C038/54; C22C 38/44 20060101 C22C038/44 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 17, 2014 |
JP |
2014-188401 |
Mar 25, 2015 |
JP |
2015-062216 |
Claims
1. A martensitic stainless steel used for a brake disk of a
two-wheeled vehicle, the martensitic stainless steel comprising: in
% by mass, C of 0.025% to 0.080%, Si of 0.05% to 0.8%, Mn of 0.5%
to 1.5%, P of 0.035% or less, S of 0.015% or less, Cr of 11.0% to
13.5%, Ni of 0.01% to 0.50%, Cu of 0.01% to 0.08%, Mo of 0.01% to
0.30%, V of 0.01% to 0.10%, Al of 0.05% or less, and N of 0.015% to
0.060%; the balance being Fe and inevitable impurities; a DFE value
defined by a formula (1) ranging from 5 to 30; and a .delta.
ferrite fraction observed in a cross section structure ranging from
5% to 30% by an area ratio, DFE=12(Cr+Si)-430C-460N-20Ni-7Mn-89
Formula (1) where: Cr, Si, C, N, Ni and Mn in the formula (1)
respectively indicate contents.
2. The martensitic stainless steel according to claim 1, further
comprising: in % by mass, one or two of Ti of 0.03% or less and B
of 0.0050% or less.
3. The martensitic stainless steel according to claim 1, further
comprising: in % by mass, Nb 0.30% or less.
4. The martensitic stainless steel according to claim 1, further
comprising: in % by mass, one or two of Sn of 0.1% or less and Bi
of 0.2% or less.
5. A manufacturing method of the martensitic stainless steel
according to claim 1, comprising: heating a rough bar at a
temperature ranging from 10 degrees C. to 50 degrees C. between a
rough hot rolling and a finish hot rolling.
6. The martensitic stainless steel according to claim 1, wherein
the martensitic stainless steel is a hot-rolled steel plate that is
not subjected to hot-rolling-and-annealing.
7. The martensitic stainless steel according to claim 1, wherein
the martensitic stainless steel is a hot-rolled and annealed steel
plate.
8. The martensitic stainless steel according to claim 2, further
comprising: in % by mass, Nb 0.30% or less.
9. The martensitic stainless steel according to claim 2, further
comprising: in % by mass, one or two of Sn of 0.1% or less and Bi
of 0.2% or less.
10. The martensitic stainless steel according to claim 3, further
comprising: in % by mass, one or two of Sn of 0.1% or less and Bi
of 0.2% or less.
11. The martensitic stainless steel according to claim 8, further
comprising: in % by mass, one or two of Sn of 0.1% or less and Bi
of 0.2% or less.
12. A manufacturing method of the martensitic stainless steel
according to claim 2, comprising: heating a rough bar at a
temperature ranging from 10 degrees C. to 50 degrees C. between a
rough hot rolling and a finish hot rolling.
13. A manufacturing method of the martensitic stainless steel
according to claim 3, comprising: heating a rough bar at a
temperature ranging from 10 degrees C. to 50 degrees C. between a
rough hot rolling and a finish hot rolling.
14. A manufacturing method of the martensitic stainless steel
according to claim 4, comprising: heating a rough bar at a
temperature ranging from 10 degrees C. to 50 degrees C. between a
rough hot rolling and a finish hot rolling.
15. A manufacturing method of the martensitic stainless steel
according to claim 8, comprising: heating a rough bar at a
temperature ranging from 10 degrees C. to 50 degrees C. between a
rough hot rolling and a finish hot rolling.
16. A manufacturing method of the martensitic stainless steel
according to claim 9, comprising: heating a rough bar at a
temperature ranging from 10 degrees C. to 50 degrees C. between a
rough hot rolling and a finish hot rolling.
17. A manufacturing method of the martensitic stainless steel
according to claim 10, comprising: heating a rough bar at a
temperature ranging from 10 degrees C. to 50 degrees C. between a
rough hot rolling and a finish hot rolling.
18. A manufacturing method of the martensitic stainless steel
according to claim 11, comprising: heating a rough bar at a
temperature ranging from 10 degrees C. to 50 degrees C. between a
rough hot rolling and a finish hot rolling.
19. The martensitic stainless steel according to claim 2, wherein
the martensitic stainless steel is a hot-rolled steel plate that is
not subjected to hot-rolling-and-annealing.
20. The martensitic stainless steel according to claim 3, wherein
the martensitic stainless steel is a hot-rolled steel plate that is
not subjected to hot-rolling-and-annealing.
21. The martensitic stainless steel according to claim 4, wherein
the martensitic stainless steel is a hot-rolled steel plate that is
not subjected to hot-rolling-and-annealing.
22. The martensitic stainless steel according to claim 8, wherein
the martensitic stainless steel is a hot-rolled steel plate that is
not subjected to hot-rolling-and-annealing.
23. The martensitic stainless steel according to claim 9, wherein
the martensitic stainless steel is a hot-rolled steel plate that is
not subjected to hot-rolling-and-annealing.
24. The martensitic stainless steel according to claim 10, wherein
the martensitic stainless steel is a hot-rolled steel plate that is
not subjected to hot-rolling-and-annealing.
25. The martensitic stainless steel according to claim 11, wherein
the martensitic stainless steel is a hot-rolled steel plate that is
not subjected to hot-rolling-and-annealing.
26. The martensitic stainless steel according to claim 2, wherein
the martensitic stainless steel is a hot-rolled and annealed steel
plate.
27. The martensitic stainless steel according to claim 3, wherein
the martensitic stainless steel is a hot-rolled and annealed steel
plate.
28. The martensitic stainless steel according to claim 4, wherein
the martensitic stainless steel is a hot-rolled and annealed steel
plate.
29. The martensitic stainless steel according to claim 8, wherein
the martensitic stainless steel is a hot-rolled and annealed steel
plate.
30. The martensitic stainless steel according to claim 9, wherein
the martensitic stainless steel is a hot-rolled and annealed steel
plate.
31. The martensitic stainless steel according to claim 10, wherein
the martensitic stainless steel is a hot-rolled and annealed steel
plate.
32. The martensitic stainless steel according to claim 11, wherein
the martensitic stainless steel is a hot-rolled and annealed steel
plate.
Description
TECHNICAL FIELD
[0001] The present invention relates to a stainless steel plate
used for a brake disk of a two-wheeled vehicle and a producing
method of the stainless steel plate, more specifically, to a
martensitic stainless steel plate used for a brake disk of a
two-wheeled vehicle, the martensitic stainless steel plate having
excellent properties of a surface and an end surface.
BACKGROUND ART
[0002] The brake disk of a two-wheeled vehicle is required to have
properties such as wear resistance, corrosion resistance, and
toughness. In general, wear resistance is increased as hardness is
increased. However, since an excessively high hardness causes a
so-called brake squeak between a brake and a pad, the hardness of
the brake is required to range from 32 to 38 HRC (Rockwell hardness
C-scale). Because of these demanded properties, a martensitic
stainless steel plate is used for the brake disk of the two-wheeled
vehicle.
[0003] Typically, SUS420J2 has been quenched and tempered to be
adjusted to have a desired hardness, thereby providing a brake
disk. In this case, two heat treatment processes of quenching and
tempering have been adversely required. In order to solve this
problem, Patent Literature 1 discloses an invention relating to a
steel composition capable of stably obtaining a desired hardness in
a wider quenching temperature range than that of a typical steel of
a SUS420J2 steel, the steel composition being usable in the
as-quenched state. Similarly to SUS410, SUS403 and SUS410S steels,
the invention of Patent Literature 1 is provided by reducing a C
content and compensating narrowing of the austenitic single phase
temperature range caused by the reduction of the C content, in
other words, narrowing of the quenching temperature range, by
adding Mn that is an austenite stabilizing element.
[0004] Moreover, Patent Literature 2 discloses an invention
relating to a steel sheet for a motorbike disk brake, the steel
sheet being a low Mn steel and is used in the as-quenched state.
This steel sheet is obtained by reducing Mn and simultaneously
adding Ni and Cu having the same effect as an austenite forming
element.
[0005] Recently, also in the two-wheeled vehicle, a reduction in a
weight of a vehicle body has been demanded and a reduction in a
weight of the two-wheeled-vehicle brake disk has been studied. In
this case, a problem that is disk deformation due to softening of a
disk material caused by heat generation at the time of braking is
caused. In order to solve the problem, it is necessary to improve
heat resistance of the disk material. One of solutions of the
problem is to improve temper softening resistance. Patent
Literature 3 discloses an invention relating to a method for
improving heat resistance by adding Nb and Mo. Patent Literature 4
discloses an invention relating to a disk material having an
excellent heat resistance obtained by subjecting the disk material
to a quenching treatment at a temperature higher than 1000 degrees
C.
[0006] As a brake disk having an excellent temper softening
resistance, Patent Literature 5 discloses a brake disk having a
martensitic structure in which a prior austenite grain has an
average grain size of 8 .mu.m or more, and Patent Literature 6
discloses an invention in which martensite accounts for 75% or more
and Nb accounts for from 0.10% to 0.60% at an area ratio of a
quenched structure.
[0007] Patent Literature 7 discloses control of components to a
limited range in which cracks are unlikely to be generated, since
the above low-C martensitic stainless steel has a low hot
workability and easily causes a so-called cracked edge at a
widthwise end at the time of hot rolling.
[0008] Patent Literature 8 relates to a manufacturing method of a
ferritic stainless steel strip. Particularly, Patent Literature 8
discloses optimum conditions for sheet bar heating in relation to a
manufacturing method with a high productivity of a ferritic
stainless hot-rolled steel strip having excellent moldability and
material uniformity.
CITATION LIST
Patent Literature(S)
[0009] Patent Literature 1: JP-A-57-198249
[0010] Patent Literature 2: JP-A-8-60309
[0011] Patent Literature 3: JP-A-2001-220654
[0012] Patent Literature 4: JP-A-2005-133204
[0013] Patent Literature 5: JP-A-2006-322071
[0014] Patent Literature 6: JP-A-2011-12343
[0015] Patent Literature 7: JP-A-2008-285692
[0016] Patent Literature 8: JP-A-2000-61524
SUMMARY OF THE INVENTION
Problem(s) to be Solved by the Invention
[0017] Because of these techniques, a low-C martensitic stainless
steel has been widely used for a disk brake of a two-wheeled
vehicle. On the other hand, an improvement in productivity in
manufacturing the disk brake has been demanded in recent years. For
instance, a reduction in a heating time for heating and quenching
and a reduction in a polishing time after heating and quenching
have been demanded. Moreover, an improvement in a yield by even
using a widthwise end of a steel strip also has been demanded.
[0018] When a polishing amount per a unit time is increased in
order to reduce the polishing time, unfavorably, a jig is
increasingly worn and temper softening of a material occurs by heat
generated by friction caused during a processing. Accordingly, in
order to reduce the polishing time without heat generation by
friction caused during the processing, a polishing thickness is
generally decreased. Here, an edge seam defect at the widthwise end
of the steel strip has been problematic.
[0019] FIG. 1A shows an appearance of an edge seam defect in an
actual product. FIG. 1B shows a microscope photograph of a cross
section of the edge seam defect in the actual product. A typical
manufacturing process of a hot-rolled steel strip includes: heating
a slab having a thickness from 150 mm to 250 mm to a temperature
ranging from 1100 degrees C. to 1300 degrees C.; rolling the slab
to form a rough bar having a thickness from 20 mm to 40 mm using a
rough hot rolling mill; subsequently, rolling the rough bar to form
a plate having a thickness from 3 mm to 6 mm using a finish hot
rolling mill; and coiling the obtained plate. Since tension is not
applied during the rough hot rolling, the slab expands widthwise,
so that a part of an end surface of the slab becomes a surface of
the rough bar. Since the end surface of the slab does not contact
with the rolling roller at the beginning of the rough hot rolling,
roughness of the end surface of the slab is large, which causes
defects when the end surface of the slab is brought into contact
with the rolling roller.
[0020] The edge seam defect is often observed in a hot-rolled steel
strip of a steel material. FIG. 2 is a photograph showing an end
surface of a 20-mm-thick steel ingot obtained by hot-rolling an
original 80-mm-thick steel ingot of each of various stainless
steels in a laboratory. It is understood that the stainless steels
are considerably different from each other in a level of roughness
of the end surface thereof. Moreover, it is understood that
roughness of the end surface of SUS410 steel is considerably
changed depending on a hot-rolling heating temperature. Since
roughness of the end surface of the slab in the rough hot rolling
is caused by a difference in a deformation pattern caused by a
difference in a crystal orientation between crystal grains of the
slab, the roughness becomes noticeable when the crystals grains are
large-sized. For instance, in the course of cooling to the room
temperature after solidification, a common steel is transformed
twice of .delta./.gamma. and .gamma./.alpha. to have a fine
structure. Herein, .delta. refers to .delta. ferrite, .gamma.
refers to austenite, and a refers to .alpha. ferrite. It should be
noted that the expression of "ferrite" usually means a ferrite.
.delta. ferrite is ferrite precipitated at A4 transformation point
or higher. .alpha. ferrite is ferrite precipitated at A3
transformation point or lower.
[0021] Since the structure of the common steel is micronized by
another transformation of .alpha./.gamma. in the hot-rolling
heating and the rough hot rolling is performed in a .gamma. single
phase in which recrystallization easily occurs, the structure of
the common steel becomes finer also with the effect of micronizing
crystal grains by recrystallization, so that edge seam defect is
unlikely to occur. On the other hand, as in the ferritic stainless
steel, when ferrite grains are kept in a state as at the
solidification until the hot-rolling heating without a single
transformation, the edge seam defect is likely to occur due to a
large grain size. In general, .delta. ferrite is not differentiated
from a ferrite in a steel not forming a .gamma. single phase after
solidification as in the ferritic stainless steel.
[0022] As long as components are 13% Cr-0.2% C as those in SUS420J1
even in a martensitic stainless steel, an austenitic single phase
is formed in hot-rolling heating, so that an edge seam defect is
unlikely to occur due to a microstructure obtained by
transformation and a microstructure obtained by recrystallization
of austenite.
[0023] However, since a temperature range in which the low-C
martensitic stainless steel exhibits the austenitic single phase is
narrow, the low-C martensitic stainless steel has a duplex
structure of .delta. ferrite and austenite in hot-rolling heating.
The edge seam defect is likely to occur because of the .delta.
ferrite at this time. Accordingly, the polishing thickness
exceeding a depth of the edge seam defect is required in the
polishing process after quenching of the disk brake, thereby
hampering productivity.
[0024] When the hot-rolling heating temperature is decreased and an
austenite ratio is increased, deformation resistance is increased
to deteriorate hot workability, resulting in generation of a
cracked edge in hot rolling. When the C amount is increased to
raise the .gamma. phase fraction, a quenching hardness becomes
excessively high. When the austenite stabilizing elements such as
Mn, Ni and Cu are further added, the material cost is increased and
an annealing cooling time is prolonged in an annealing process of a
hot-rolled plate, thereby hampering productivity. When the Cr
amount is decreased to raise the austenite fraction, corrosion
resistance is hampered.
[0025] Although it is required to know a change in the .delta.
ferrite fraction in the course of the hot rolling in order to
control the .delta. ferrite fraction, it is impossible to measure
the .delta. ferrite fraction of the hot-rolled plate. In case of
the slab in the hot-rolling heating, the .delta. ferrite fraction
is measurable according to a phase diagram calculation method or a
heat treatment test in a laboratory. When the duplex structure of
austenite and martensite is rapidly cooled, since the austenitic
phase forms a martensitic structure, the .delta. ferrite phase is
easily distinguished from the martensitic structure as a less
deformed .delta. ferrite phase. However, in the actual hot rolling
process, it has been impossible to know a change in the .delta.
ferrite amount in the course of the hot rolling after the slab is
taken out of a hot-rolling heating furnace. The hot-rolled steel
strip coiled after the finish hot-rolling exhibits a low toughness
because of including the martensitic structure obtained by
transformation of austenite. Accordingly, it is difficult to uncoil
the hot-rolled steel strip without any treatment. The hot-rolled
steel strip can be uncoiled by being subjected to a hot-rolling and
annealing in a box annealing furnace to temper martensite to
ferrite and carbide. However, it is impossible to examine a
structure of the hot-rolled plate before annealing. The structure
of the plate after hot-rolling and annealing is a structure of
ferrite and carbide as shown in FIG. 3. It is impossible to measure
the .delta. ferrite fraction.
Means for Solving the Problem(s)
[0026] The inventors studied an examination method of the .delta.
ferrite fraction in a ferrite mother phase of the hot-rolled and
annealed steel plate of the low-C martensitic stainless steel. As a
result of testing various etching liquids for structure analysis
according to Electron Backscatter Diffraction (EBSD) and
observation using an optical microscope, it was found that the
.delta. ferrite can be colored using a Murakami reagent. The
Murakami reagent is an aqueous solution of potassium ferricyanide.
The solution is heated and a sample is immersed in the solution to
be etched. The Murakami reagent is usually used for distinguishing
austenite from the .delta. ferrite phase by coloring the .delta.
ferrite mixed in an austenite mother phase as seen in a
solidification structure of the austenitic stainless steel.
Although it was not expected at first that it became possible to
distinguish the .delta. ferrite in the hot-rolled and annealed
steel plate of the martensitic stainless steel in which the .delta.
ferrite and ferrite were mixed, the .delta. ferrite was able to be
clearly distinguished as shown in FIG. 4. The .delta. ferrite is
shown by a gray contrast part in FIG. 4. Although a mechanism of
distinguishing the .delta. ferrite by coloring the .delta. ferrite
using the Murakami reagent is not clarified, as a result of the
examination by the inventors, it is inferred that a high-Cr .delta.
ferrite phase is colored by the Murakami reagent to be
distinguished since the .delta. ferrite phase and the austenite
phase (the martensite phase at the room temperature) are different
from each other in a Cr concentration by about 1.5% in the
hot-rolling heating. No example with use of the Murakami reagent
for appearance of the .delta. ferrite of such a low-Cr martensite
stainless steel has been found. It is a new finding that a just
about 1.5% difference in the Cr amount is distinguished. Use of
this method reveals a behavior of the .delta. ferrite which has not
been known. For instance, it was found that the ferrite amount of
the hot-rolled plate is considerably decreased as compared with the
.delta. ferrite amount at the hot-rolling heating temperature.
Moreover, the ferrite amount at a widthwise end of the steel strip
is larger than that at a widthwise center thereof. There seems to
be a possibility that a temperature difference is generated between
a widthwise end and a widthwise center of the slab and a
possibility that the .delta. ferrite amount is increased by
decarburization in a surface layer of the slab. With respect to the
hot-rolled steel plate that is not subjected to
hot-rolling-and-annealing, the .delta. ferrite can be distinguished
by evaluating the steel plate sample as described above.
[0027] A relationship between the edge seam defect and the .delta.
ferrite amount is shown in FIG. 5. No edge seam defect is observed
in the austenitic stainless steel having 0% .delta. ferrite
fraction. As the .delta. ferrite fraction is increased, a depth of
the seam defect is increased. However, an increase amount of the
depth of the seam defect is small until the .delta. ferrite
fraction reaches 30%. However, it is found that, when the .delta.
ferrite fraction exceeds 30%, the depth of the seam defect
drastically becomes large.
[0028] On the other hand, a cracked edge at the end of the steel
plate is likely to occur when the .delta. ferrite fraction falls
below 5%. FIG. 6 shows forms of ends of (11% Cr, 12% Cr)-0.04%
C-1.4% Mn-0.03% N steels respectively with the .delta. ferrite
fractions of 4% and 20% after subjected to hot-rolling in a
laboratory. When the .delta. ferrite fraction is low, an apparent
cracked edge occurs.
[0029] As described above, in both of the hot-rolled and annealed
steel plate and the hot-rolled steel plate, the .delta. ferrite
fraction is strongly related to the depth of the edge seam defect
and the cracked edge at the widthwise end of the steel plate. In
case of the martensitic stainless steel in which the .delta.
ferrite fraction is controlled, since the cracked edge is not
observed and the depth of the edge seam defect is shallow, a
grinding depth in a manufacturing process of the brake disk can be
shallow, thereby improving productivity of the brake disk. Further,
since the very end of the steel plate can be used, a yield is also
improvable.
[0030] Thus, it is considered that, in a method of controlling the
.delta. ferrite fraction considerably affecting a surface quality,
controlling of (1) a chemical composition and (2) a hot-rolling
heating temperature is effective. However, in the chemical
composition (1), it is not preferable to control the .delta.
ferrite fraction by C and N amounts since a quenching hardness
necessary for the disk brake is not obtained. Moreover, since Si,
Mn, Cr, Ni, Cu and the like affect a thickness of a hot-rolled
scale, temper softening resistance and corrosion resistance and
addition of a large amount of Si, Mn, Cr, Ni, Cu and the like
increases alloy costs and the like, a range of the .delta. ferrite
fraction controllable by the chemical composition (1) is limited.
In controlling of the .delta. ferrite amount by the hot-rolling
heating temperature (2), when the heating temperature is decreased
to 1150 degrees C. or less in order to decrease the .delta. ferrite
amount, a cracked edge is likely to occur because of a difference
in strength between the austenite phase and the ferrite phase
slightly left. Accordingly, an improvement in the surface quality
by controlling the .delta. ferrite amount is not easy.
[0031] The inventors scrutinized the .delta. ferrite amount of the
hot-rolled and annealed steel plate, hot rolling operational
conditions and the chemical compositions, and found a method
effective for satisfying the surface quality, prevention of the
cracked edge, and hardness and corrosion resistance required as the
disk brake. Specifically, it is necessary to heat the rough bar by
induction heating or the like to increase a temperature of the
rough bar, the rough bar provided between the rough hot-rolling and
the finish hot-rolling, in order to control the components to
satisfy (1) the .delta. ferrite amount in the hot-rolling heating
and (2) various properties and in order to prevent (3) the .delta.
ferrite amount from being decreased due to lowering of the
temperature during the rough rolling after being taken out of the
hot-rolling heating furnace.
[0032] Based on these findings, the hot-rolled steel plate and the
hot-rolled and annealed steel plate of martensitic stainless steel
used for a brake disk, in which the edge seam defect is reduced and
the widthwise end of the hot-rolled steel strip is prevented from
forming a cracked edge, and a controlling method of a structure of
the steel plates can be provided.
[0033] The invention has been achieved based on the findings. A
solution of the problem of the invention, specifically, a
martensitic stainless steel (including a hot-rolled steel plate
(which is not subjected to hot-rolling-and-annealing) and a
hot-rolled and annealed steel plate)) used for a brake disk of a
two-wheeled vehicle, and a manufacturing method of the martensitic
stainless steel of the invention are described as follows.
(1) According to an aspect of the invention, a martensitic
stainless steel used for a brake disk of a two-wheeled vehicle
includes: in % by mass, C of 0.025% to 0.080%, Si of 0.05% to 0.8%,
Mn of 0.5% to 1.5%, P of 0.035% or less, S of 0.015% or less, Cr of
11.0% to 13.5%, Ni of 0.01% to 0.50%, Cu of 0.01% to 0.08%, Mo of
0.01% to 0.30%, V of 0.01% to 0.10%, Al of 0.05% or less, and N of
0.015% to 0.060%; the balance being Fe and inevitable impurities; a
DFE value defined by a formula (1) ranging from 5 to 30; and a
.delta. ferrite fraction observed in a cross section structure
ranging from 5% to 30% by an area ratio,
DFE=12(Cr+Si)-430C-460N-20Ni-7Mn-89 Formula (1).
In the formula (1), Cr, Si, C, N, Ni and Mn in the formula (1)
respectively indicate contents. (2) With the above arrangement, the
martensitic stainless steel further includes: in % by mass, one or
two of Ti of 0.03% or less and B of 0.0050% or less. (3) With the
above arrangement, the martensitic stainless steel further
includes: in % by mass, Nb 0.30% or less. (4) With the above
arrangement, the martensitic stainless steel further includes: in %
by mass, one or two of Sn of 0.1% or less and Bi of 0.2% or less.
(5) According to another aspect of the invention, a manufacturing
method of the martensitic stainless steel includes: heating a rough
bar at a temperature ranging from 10 degrees C. to 50 degrees C.
between a rough hot rolling and a finish hot rolling. (6) With the
above arrangement, the martensitic stainless steel is a hot-rolled
steel plate that is not subjected to hot-rolling-and-annealing. (7)
With the above arrangement, the martensitic stainless steel is a
hot-rolled and annealed steel plate.
[0034] According to the control technology of the structure and the
compositions of the invention, the hot-rolled steel plate and the
hot-rolled and annealed steel plate used for the brake disk of a
two-wheeled vehicle, in which the edge seam defect is reduced on
the widthwise end of the hot-rolled steel strip and the widthwise
end thereof is prevented from forming a cracked edge, can be
obtained. The quality of each of the obtained steel plates is
favorable in terms of an improvement in productivity and a yield of
the brake disk.
BRIEF DESCRIPTION OF DRAWING(S)
[0035] FIG. 1A shows an appearance of an edge seam defect at a
widthwise end of a hot-rolled and annealed steel strip of a
martensitic steel used for a brake disk.
[0036] FIG. 1B shows a microscope image obtained by observing a
cross section of the edge seam defect at the widthwise end of the
hot-rolled and annealed steel strip of the martensitic steel used
for the brake disk.
[0037] FIG. 2 is a photograph showing a surface of a widthwise end
of a 20-mm-thick steel ingot obtained using a laboratory
hot-rolling mill by rolling an original steel ingot cast in
dimensions of 300 L.times.180w.times.80t (mm) in a laboratory, in
order to demonstrate a generation process of the edge seam
defect.
[0038] FIG. 3 is a photograph showing a structure (in which ferrite
grains and carbides mainly appear) after being hot-rolled and
annealed, the structure being a general cross-sectional structure
of a hot-rolled and annealed plate of a 11% Cr-1% Mn-0.04% C-0.04%
N steel. The structure has been subjected to etching by aqua regia
for a short period of time.
[0039] FIG. 4 is a photograph showing a distribution of .delta.
ferrite in a TD cross section of a hot-rolled and annealed steel
strip of a 11% Cr-1% Mn-0.04% C-0.04% N martensitic stainless
steel, in which an edge seam defect and a cracked edge are not
observed, the photograph showing a .delta. ferritic structure of
the hot-rolled and annealed plate having a favorable quality in
terms of the edge seam defect and cracked edge.
[0040] FIG. 5 illustrates a relationship between the depth of the
edge seam defect and the .delta. ferrite amount of a sample taken
from each of several types of martensitic stainless steels used for
the disk brake of a two-wheeled vehicle, in which each of the
martensitic stainless steels was subjected to a change in the
hot-rolling heating temperatures from 1100 degrees C. to 1280
degrees C. and hot-rolled to form a plate having a 3.8-mm
thickness, the hot-rolled plate was annealed, subsequently, the
hot-rolled coil was uncoiled and the sample was taken.
[0041] FIG. 6 is a photograph showing a relationship between a
cracked edge on an end surface of a 3-mm-thick plate and a .delta.
ferrite amount affecting the cracked edge, in which the plate is
obtained by heating a 50-mm steel ingot of 11-to-12% Cr-0.04%
C-0.5-to-1.4% Mn-0.03% N steel to 1250 degrees C. at a laboratory
and subsequently hot-rolling the steel ingot into the 3-mm-thick
plate.
DESCRIPTION OF EMBODIMENT(S)
[0042] Exemplary embodiment(s) of the invention will be described
below. Firstly, a reason why a steel composition of a stainless
steel plate in an exemplary embodiment is limited will be
described. A mark % with respect to the composition means a mass %
unless otherwise particularly indicated.
[0043] C: From 0.025% to 0.080%
[0044] C is an essential element for obtaining a predefined
hardness of the stainless steel plate after quenching and is added
in combination with N so as to attain a predefined hardness level.
In the exemplary embodiment, an upper limit of C is defined to be
0.080% to maximize the effect of N without an excessive addition of
C When C is added beyond the upper limit, the hardness becomes
excessive to cause disadvantages such as brake squeak and reduction
of toughness of the stainless steel plate. The upper limit of the C
content is desirably 0.060% in terms of hardness control and an
improvement in corrosion resistance. On the other hand, when the C
content is less than 0.025%, N needs to be excessively added in
order to obtain the hardness. Accordingly, a lower limit of the C
content is defined to be 0.025%. The C content is desirably 0.040%
or more in terms of stability of quenching hardness.
[0045] Si: From 0.05%% to 0.8%
[0046] Si is required for deoxidation at melting and refining
process and is also useful for inhibiting generation of oxidized
scale at a quenching heat process. Since an effect of Si is exerted
at 0.05% or more, a Si content is set at 0.05% or more. However,
since Si is mixed in a raw material such as molten pig iron and an
excessive decrease of the Si content increases cost, the Si content
is desirably 0.20% or more. Moreover, since Si narrows an austenite
single phase temperature region to impair a quenching stability,
the Si content is defined to be 0.8% or less. However, the Si
content is desirably 0.6% or less in order to decrease an additive
amount of the austenite stabilizing element and reduce the
cost.
[0047] Mn: From 0.5% to 1.5%
[0048] Mn is an element to be added as a deoxidation agent and
contributes to expansion of the austenite single phase region and
an improvement in hardenability. Since an effect of Mn is clearly
exerted at 0.5% or more, a Mn content is set at 0.5% or more. The
Mn content is desirably 1.1% or more in order to stably obtain the
hardenability. However, since Mn promotes generation of oxidized
scale at quenching heating, an upper limit of the Mn content is
defined to be 1.5% or less in order to increase abrasion load. Also
in consideration of a decreased in corrosion resistance caused by
grains (e.g., MnS), the Mn content is desirably 1.3% or less.
[0049] P: 0.035% or Less
[0050] P is an element contained as impurities in a raw material
such as molten pig iron and a main raw material such as
ferrochromium. Since P is a harmful element to toughness of a
hot-rolled and annealed plate after quenching, a P content is
defined to be 0.035% or less. P is preferably 0.030% or less. Since
excessive reduction of P essentially requires use of a high purity
raw material, leading to cost increase, a lower limit of P is
preferably 0.010%.
[0051] S: 0.015% or Less
[0052] Since S forms sulfide inclusions and causes deterioration of
a general corrosion resistance (whole surface corrosion and pitting
corrosion) of a steel material and S decreases hot workability and
increases cracked-edge sensitivity of the hot-rolled steel plate,
the upper limit of the S content is preferably small and set at
0.015%. The upper limit is more preferably 0.008%. As the S content
becomes smaller, corrosion resistance becomes more favorable.
However, since the reduction of the S content causes an increase in
desulfurization burden and an increase in production cost, the
lower limit of the S content is preferably set at 0.001%.
[0053] Cr: From 11.0% to 13.5%
[0054] Cr is an essential element for ensuring oxidation resistance
and corrosion resistance in the exemplary embodiment. The Cr
content of less than 11.0% does not exert these effects, while the
Cr content of more than 13.5% narrows the austenite single phase
region to impair hardenability. Accordingly, the Cr content is set
in a range from 11.0% to 13.5%. In consideration of stability of
corrosion resistance, the Cr content is desirably 12.0% or more.
Moreover, in consideration of press formability, the Cr content is
desirably 13.0% or less.
[0055] Ni: From 0.01% to 0.50%
[0056] Ni is mixed as inevitable impurities in an alloy raw
material of a ferritic stainless steel and is generally contained
in a range from 0.01% to 0.10%. Moreover, Ni is an element
effective for suppression of progress of pitting corrosion and the
effect of Ni is stably exerted by the addition of 0.03% or more of
Ni. Accordingly, a lower limit of a Ni content is preferably set at
0.03%. On the other hand, since a large added amount of Ni may
deteriorate press formability due to solid-solution hardening in a
hot-rolled and annealed steel plate, an upper limit of the Ni
content is set at 0.50%. In consideration of alloy cost, the Ni
content is desirably 0.15% or less.
[0057] Cu: From 0.01% to 0.08%
[0058] Cu is effective for improving corrosion resistance of the
martensitic structure including .delta. ferrite and the effect of
Cu is exerted at 0.01% or more. Moreover, a positive addition of Cu
is occasionally performed in order to improve the hardenability as
the austenite stabilizing element. However, since an excessive
addition of Cu causes a decrease in the hot workability and an
increase in the raw material cost, an upper limit of a Cu content
is set at 0.08% or less. In consideration of generation of
corrosion due to acid rain, a lower limit of the Cu content is
desirably set at 0.02% or more. Moreover, in consideration of press
formability of the hot-rolled and annealed steel plate, the Cu
content is preferably 0.08% or less.
[0059] Mo: From 0.01% to 0.30%
[0060] Mo is effective for improving corrosion resistance of the
martensitic structure including .delta. ferrite and the effect of
Mo is exerted at 0.01% or more. Accordingly, a lower limit of a Mo
content is set at 0.01%. Since Mo is effective for improving the
hardenability and improving heat resistance after quenching, the Mo
content is preferably 0.02% or more. The steel is occasionally
tempered by heating after quenching to cause a decrease in the
hardness. Herein, the improvement in heat resistance after
quenching means a small decrease in the hardness, which is also
referred to temper softening resistance. Although a disk brake for
use is subjected to quenching, a disk material is heated by
resistance heat generated at the time of braking in use.
Accordingly, this property is important.
[0061] Since Mo is an element for stabilizing a ferrite phase and
an excessive addition of Mo narrows the austenite single phase
temperature region to impair the hardenability, an upper limit of
the Mo content is set at 0.30% or less.
[0062] In order to improve heat resistance after quenching, a
composite addition of Mo and Nb is desirable. When both of Mo and
Nb are simultaneously added, the Mo content in a range from 0.05%
to 0.20% and the Nb content in a range from 0.05% to 0.20% are
particularly preferable.
[0063] V: From 0.01% to 0.10%
[0064] V is mixed as inevitable impurities in an alloy raw material
of a ferritic stainless steel and is not easy to remove in the
refining process. Accordingly, V is generally contained in a range
from 0.01% to 0.10%. Moreover, V is an intentionally added element
as needed since V forms a fine carbonitride to improve wear
resistance of the brake disk and exerts an effect of improving
corrosion resistance. Since an effect of V is stably exerted by the
addition of 0.02% or more, a lower limit of a V content is
preferably set at 0.02%, more preferably 0.03% or more. On the
other hand, since an excessive addition of V may form large-sized
precipitates, leading to deterioration of toughness after
quenching, an upper limit of the V content is set at 0.10%. In
consideration of the production cost and productivity, the V
content is desirably set at 0.08% or less.
[0065] Al: 0.05% or Less
[0066] Al is an element to be added as a deoxidizing element and
improve oxidation resistance. Since an effect of Al is exerted at
0.001% or more, a lower limit of an Al content is preferably set at
0.001% or more. On the other hand, since solid-solution hardening
and formation of large-sized oxide inclusions may cause
deterioration of toughness of the brake disk, an upper limit of the
Al content is set at 0.05%. The Al content is preferably 0.03% or
less. It is not a requisite to contain Al.
[0067] N: From 0.015% to 0.060%
[0068] N is one of very important elements in the exemplary
embodiment. Similarly to C, N is an essential element for obtaining
a predetermined hardness after quenching and is added in
combination with N so as to attain a predetermined hardness level.
In the case of quenching as the duplex structure of austenite and
ferrite at the time of quenching heating, precipitation of Cr
carbide, in other words, a sensitization phenomenon is likely to
occur, leading to deterioration of corrosion resistance. However,
addition of nitrogen enables suppression of precipitation of Cr
carbide, which may exert the effect of improving corrosion
resistance. Since the effect is exerted at 0.015% or more, a N
content is set at 0.015% or more. On the other hand, since the
effect becomes saturated at 0.060% and formation of defects such as
air bubbles is likely to decrease a yield, an upper limit of the N
content is set at 0.060%. In consideration of the improvement in
corrosion resistance by reinforcing a passivation film, the N
content is desirably 0.030% or more. Moreover, the N content is
desirably 0.050% or less.
[0069] An amount of .delta. ferrite (.delta. ferrite fraction),
which is represented by an area ratio, observed in a hot-rolled
steel plate or a hot-rolled and annealed steel plate is defined in
a range from 5% to 30%.
[0070] The amount of .delta. ferrite in the steel affects
generation of the edge seam defect and hot-rolled cracked edge at
the time of hot rolling. When the .delta. ferrite fraction is less
than 5%, hot workability is reduced to easily cause the cracked
edge. Accordingly, the .delta. ferrite fraction is set at 5% or
more. On the other hand, when the .delta. ferrite fraction is more
than 30%, a grain size is increased to easily generate the edge
seam defect and a large polishing thickness of the brake disk is
required to remove the edge seam defect by polishing in a polishing
process after quenching of the brake disk. Accordingly, the .delta.
ferrite fraction is set at 30% or less. It should be noted that the
.delta. ferrite is observed in a cross section of the hot-rolled
and annealed steel plate and the hot-rolled steel plate at the time
of hot rolling, and evaluated through observation using a typical
microscope. A structure etching of the .delta. ferrite is desirably
performed by a method of immersing a sample in a solution provided
by heating the Murakami reagent (an aqueous solution of potassium
ferricyanide).
[0071] A DFE value defined by the formula (1)
(DFE=12(Cr+Si)-430C-460N-20Ni-7Mn-89) is in a range from 5 to
20.
[0072] When the DFE value is low, the .delta. ferrite amount is
decreased to increase a generating frequency of the cracked edge at
hot rolling. Accordingly, the DFE value is set at 5 or more. When
the DFE value is high, the .delta. ferrite amount is increased to
easily cause the edge seam defect. Accordingly, the DFE value is
set at 20 or less. It should be noted that Cr, Si, C, N, Ni and Mn
in the formula (1) indicate the respective contents (% by
mass).
[0073] Moreover, in the exemplary embodiment, in addition to the
above elements, the following elements may be added in order to
improve rust resistance, heat resistance, hot workability and the
like.
[0074] Ti: 0.03% or Less
[0075] Ti, which forms carbonitride, is an element for suppressing
a sensitization phenomenon by precipitation of chrome carbonitride
and a decrease in corrosion resistance in a stainless steel. A Ti
content is preferably 0.001% or more. However, since formation of a
large-sized TiN causes deterioration of toughness and squeaking of
the brake disk, an upper limit of the Ti content is set at 0.03% or
less. In consideration of toughness in winter, the Ti content is
desirably 0.01% or less. It is not a requisite to contain Ti.
[0076] B: 0.0050% or Less
[0077] B is an element effective for improving hot workability.
Since the effect of B is exerted when a B content is 0.0002% or
more, B may be added at 0.0002% or more. In order to improve hot
workability in a wider temperature range, the B content is
desirably set at 0.0010% or more. On the other hand, since
excessive addition of B causes deterioration of hardenability due
to composite precipitation of boride and carbide, an upper limit of
the B content is set at 0.0050%. In consideration of corrosion
resistance, the B content is desirably 0.0025% or less.
[0078] Nb: 0.3% or Less
[0079] Nb, which forms carbonitride, is an element for suppressing
a sensitization phenomenon by precipitation of chrome carbonitride
and a decrease in corrosion resistance in a stainless steel. A Nb
content is preferably 0.001% or more. Further, Nb is an element for
largely improving heat resistance after quenching. Herein, heat
resistance means how the stainless steel is unlikely to be softened
when receiving heat after quenching. In other words, heat
resistance is also referred to as temper softening resistance.
[0080] However, since excessive addition of Nb forms NbN in the
brake disk to adversely cause a decrease in toughness and squeaking
of the brake disk, an upper limit of the Nb content is set at
0.3%.
[0081] In order to improve heat resistance after quenching, a
composite addition of Mo and Nb is desirable. When both of Mo and
Nb are simultaneously added, Mo in a range from 0.05% to 0.20% and
Nb in a range from 0.05% to 0.20% are particularly preferable.
[0082] Sn: 0.1% or Less
[0083] Sn is an element effective for improving corrosion
resistance after quenching. A Sn content is preferably 0.001% or
more, more preferably 0.02% or more as needed. However, since
excessive addition of Sn promotes edge cracking at hot rolling, the
Sn content is preferably set at 0.10% or less.
[0084] Bi: 0.2% or Less
[0085] Bi is an element for improving corrosion resistance.
Although the mechanism is not clarified, it is deduced that
addition of Bi decreases probability that MnS becomes a starting
point of corrosion generation because Bi micronizes MnS that is
likely to be the starting point. The effect is exerted when Bi is
added at 0.01% or more. Since the effect is only saturated when Bi
is added at more than 0.2%, an upper limit of the Bi content is set
at 0.2%.
[0086] In addition to the above-described elements, impurity
elements are contained as long as the effect of the invention is
not hampered. Not only the above-described P and S which are
general impurity elements but also Zn, Pb, Se, Sb, H, Ga, Ta, Ca,
Mg, Zr and the like are preferably reduced as much as possible. On
the other hand, to the extent of the range to solve the problem of
the invention, content rates of these elements are controlled. The
respective contents are Zn.ltoreq.100 ppm, Pb.ltoreq.100 ppm,
Se.ltoreq.100 ppm, Sb.ltoreq.500 ppm, H.ltoreq.100 ppm,
Ga.ltoreq.500 ppm, Ta.ltoreq.500 ppm, Ca.ltoreq.120 ppm,
Mg.ltoreq.120 ppm, and Zr.ltoreq.120 ppm.
[0087] In the hot rolling process, using an induction heater (bar
heater) between a rough rolling and a finish rolling, a rough bar
having a plate thickness from 20 mm to 40 mm is preferably heated
at a temperature from 10 degrees C. to 50 degrees C. When the
temperature for heating the rough bar is less than 10 degrees C.,
the .delta. ferrite amount is small to decrease hot workability, so
that the cracked edge is likely to be generated. On the other hand,
when the temperature for heating the rough bar is more than 50
degrees C., the .delta. ferrite amount is excessively large to
increase a grain size, thereby increasing roughness of an end
surface of the rough bar, so that deep edge seam defect is likely
to be generated. The temperature of the rough bar is increased also
by increasing a slab heating temperature prior to the hot rolling
process instead of heating by a rough bar heater. However, since
the grain size is increased when the heating temperature exceeds
1250 degrees C., the roughness of the end surface of the rough bar
is increased during the rough rolling to deepen the edge seam
defect. Accordingly, the heating temperature in the hot rolling is
desirably 1250 degrees C. or less. When the heating temperature in
the hot rolling is less than 1150 degrees C., deformation
resistance of the austenite mother phase is increased and the
.delta. ferrite amount is decreased, so that a .delta. ferrite
phase in a small amount is concentrically deformed to decrease
hot-rolling deformability, whereby a cracked edge is generated to
decrease a yield. Accordingly, the heating temperature in the hot
rolling is desirably 1150 degrees C. or more.
[0088] With the components and the .delta. ferrite fraction recited
in the claims, a quality defined in the claims is attainable. The
martensitic stainless steel for a brake disk for a two-wheeled
vehicle can exert effects in both of a hot-rolled steel plate
without being subjected to hot-rolling-annealing, and a hot-rolled
and annealed steel plate.
Modification(s)
[0089] The effects of the invention will be described below with
reference to Examples. However, the invention is by no means
limited to conditions used in the following Examples.
[0090] In each of Examples, firstly, a steel having a component
composition shown in Tables 1-1 and 1-2 was melted and cast to
obtain a 200-mm-thick slab. This slab was heated to a temperature
in a range from 1150 degrees C. to 1250 degrees C. and subsequently
subjected to a rough hot-rolling and a finish hot-rolling to obtain
a 4-mm-thick hot-rolled steel plate, which was coiled in a
temperature range of 750 degrees C. to 900 degrees C. The coiled
hot-rolled steel plate was heated in a range of an increasing
temperature from 10 degrees C. to 50 degrees C. using a rough bar
heater with use of induction heating between the rough hot-rolling
and the finish hot-rolling. Subsequently, the coiled hot-rolled
steel plate was annealed in a box annealing furnace. The box
annealing furnace was heated up to a temperature range of 800
degrees C. to 900 degrees C. After scales of a surface of the
hot-rolled and annealed steel plate were removed by shot blasting
and pickling, the hot-rolled and annealed steel plate was evaluated
in terms of the edge seam defect and the cracked edge. The edge
seam defect was judged as "Pass" when a depth of the edge seam
defect was less than 150 .mu.m, in which a judgment S was given
when the edge seam defect was not visually observed; and a judgment
A was given when the edge seam defect was visually observed. When
there was an edge seam defect having a depth of 150 .mu.m or more,
the edge seam defect was judged as "Fail" (a judgment C).
[0091] The cracked edge was judged as: "Pass" (a judgment A) when
no cracked edge having a depth of 10 mm or more was generated; and
"Fail" (a judgment B) when a cracked edge having a depth of 10 mm
or more was generated. When the cracked edge was continuously
generated, the cracked edge was judged as "Fail" (a judgment
C).
[0092] A cross-sectional structure was observed using an optical
microscope and a .delta. ferrite amount was measured by image
analysis. .delta. ferrite appeared using the Murakami reagent.
[0093] Subsequently, the hot-rolled, annealed and pickled plate was
quenched and a surface thereof was subjected to a polish finish
(#80). A JIS surface hardness (quenching hardness) was evaluated by
a Rockwell C-scale hardness tester. The plate having the surface
hardness from 32 to 38 was judged as "Pass" and the plate not
having the surface hardness from 32 to 38 was judged as "Fail." A
disk brake was quenched under conditions of heating at an average
heating rate of about 50.degree. C./s to reach 1000 degrees C.,
keeping the temperature for one second after reaching 1000 degrees
C., and cooling at an average cooling rate of 70.degree. C./s to
reach the ambient temperature.
[0094] In order to evaluate heat resistance after quenching, after
tempering at 500 degrees C. for one hour and the polish finish
(#80) of the surface, a JIS surface hardness (quenching hardness)
was evaluated by a Rockwell C-scale hardness tester. The disk
having the surface hardness of less than 32 was judged as "Fail
(B)" and the disk having the surface hardness of 32 or more was
judged as "Pass (A)." Further, the disk was subjected to a test in
which the tempering temperature was 530 degrees C. in the same
manner as the above. The surface hardness of 32 or more was judged
as "Pass (S)" and put in a column of "Temper softening resistance"
of Tables 2-1, 2-2 and 2-3.
[0095] In order to evaluate corrosion resistance, after a polish
finish (#600), the hot-rolled, annealed and pickled plate was
subjected to a salt spray test for four hours (JIS Z 2371: "Salt
Spray Test Method") and a rust area ratio was measured and judged
as "Fail (B)" at 10% or more and "Pass (A)" at less than 10%.
Particularly, the rust area ratio at zero was judged as "Pass
(S)."
[0096] With respect to polishing performance, the depth of the edge
seam defect of 150 .mu.m or less was judged as "Pass (A)" and the
depth of the edge seam defect of more than 150 .mu.m was judged as
"Fail (B)."
[0097] For Comparative Examples, the same evaluation was performed
on samples with compositions, heating conditions of quenching, and
a .delta. ferrite area ratio of the hot-rolled and annealed plate,
which were beyond the scope of the invention.
TABLE-US-00001 TABLE 1-1 Sample Content (mass %) Steel Others DFE
No. C Si Mn P S Cr Ni Cu Mo V Al N (Ti, B, Nb, Sn, Bi) Value
Examples A A1 0.025 0.28 1.05 0.025 0.004 11.5 0.04 0.01 0.02 0.03
0.003 0.040 15.1 A2 0.080 0.30 1.10 0.026 0.004 13.5 0.02 0.03 0.01
0.02 0.004 0.016 26.7 A3 0.045 0.05 1.07 0.024 0.005 12.3 0.05 0.02
0.02 0.02 0.002 0.035 15.3 A4 0.060 0.80 0.55 0.027 0.003 12.5 0.04
0.02 0.01 0.05 0.004 0.028 27.3 A5 0.040 0.30 0.50 0.026 0.006 12.2
0.05 0.01 0.02 0.04 0.003 0.033 0.1% Sn 24.1 A6 0.050 0.38 1.50
0.025 0.004 12.1 0.02 0.02 0.02 0.03 0.004 0.036 11.8 A7 0.065 0.57
0.60 0.035 0.002 12.2 0.06 0.01 0.01 0.02 0.005 0.040 0.03% Ti 12.5
A8 0.045 0.60 1.01 0.028 0.015 12.0 0.05 0.02 0.02 0.01 0.004 0.036
18.2 A9 0.042 0.35 1.40 0.024 0.003 11.0 0.05 0.03 0.02 0.01 0.001
0.015 11.4 A10 0.041 0.30 1.50 0.035 0.003 12.1 0.05 0.02 0.02 0.02
0.004 0.035 14.6 A11 0.043 0.41 1.30 0.025 0.015 12.2 0.50 0.02
0.01 0.03 0.003 0.035 0.10% Nb 8.6 A12 0.038 0.40 1.20 0.021 0.004
11.5 0.03 0.08 0.02 0.02 0.001 0.041 9.6 A13 0.035 0.35 1.50 0.025
0.003 12.1 0.05 0.05 0.30 0.02 0.003 0.060 6.2 A14 0.050 0.40 1.21
0.025 0.002 11.5 0.02 0.03 0.01 0.10 0.004 0.040 0.005% B 5.0 A15
0.035 0.40 0.80 0.026 0.005 11.3 0.03 0.03 0.02 0.02 0.05 0.052
0.02% Ti 6.2 A16 0.052 0.60 1.10 0.028 0.005 12.5 0.03 0.05 0.03
0.04 0.005 0.050 0.05% Sn, 0.02% Ti 14.5 A17 0.041 0.50 1.05 0.026
0.005 11.4 0.03 0.02 0.03 0.03 0.014 0.050 5.2 A18 0.040 0.58 1.20
0.024 0.005 11.5 0.05 0.03 0.02 0.04 0.003 0.040 0.01% Nb 11.0 A19
0.041 0.41 1.20 0.025 0.001 12.4 0.05 0.01 0.02 0.02 0.003 0.035
0.005% B 21.6 A20 0.041 0.41 1.20 0.025 0.001 12.4 0.05 0.01 0.02
0.02 0.003 0.035 0.25% Nb 21.6 A21 0.037 0.42 1.20 0.025 0.001 11.7
0.05 0.01 0.20 0.02 0.003 0.035 0.15% Nb 15.0 A22 0.034 0.38 1.20
0.025 0.001 11.8 0.05 0.01 0.07 0.02 0.003 0.035 0.1% Bi 17.0 A23
0.038 0.41 1.20 0.025 0.001 12.2 0.05 0.01 0.05 0.02 0.003 0.035
0.05% Sn, 0.05% Bi 20.5 A24 0.042 0.41 1.20 0.025 0.001 12.4 0.05
0.01 0.15 0.02 0.003 0.035 0.005% B, 0.15% Nb, 21.2 0.05% Sn A25
0.041 0.38 1.20 0.025 0.001 11.7 0.05 0.01 0.10 0.02 0.003 0.035
0.005% B, 0.15% Nb, 12.8 0.05% Bi A26 0.039 0.41 1.20 0.025 0.001
11.5 0.05 0.01 0.20 0.02 0.003 0.035 0.005% B, 0.15% Nb, 11.7 0.05%
Sn, 0.05% Bi
TABLE-US-00002 TABLE 1-2 Sample Content (mass %) Steel Others DFE
No. C Si Mn P S Cr Ni Cu Mo V Al N (Ti, B, Nb, Sn, Bi) Value
Comparatives B B1 0.085 0.44 0.80 0.025 0.003 12.1 0.04 0.02 0.01
0.02 0.002 0.025 7.0 B2 0.042 0.85 1.05 0.025 0.003 12.0 0.06 0.01
0.02 0.02 0.002 0.050 15.6 B3 0.043 0.04 1.40 0.024 0.005 12.2 0.05
0.02 0.02 0.02 0.005 0.021 18.9 B4 0.039 0.33 0.40 0.026 0.010 12.2
0.02 0.01 0.01 0.02 0.002 0.020 32.2 B5 0.020 0.31 1.60 0.025 0.005
12.4 0.01 0.02 0.02 0.02 0.005 0.020 34.3 B6 0.040 0.30 1.05 0.036
0.004 12.5 0.02 0.02 0.03 0.01 0.002 0.040 21.3 B7 0.048 0.31 0.80
0.020 0.020 12.6 0.04 0.01 0.02 0.02 0.10 0.025 27.4 B8 0.042 0.28
1.20 0.025 0.003 10.5 0.06 0.02 0.02 0.02 0.003 0.030 -1.1 B9 0.040
0.32 1.18 0.021 0.003 13.6 0.05 0.03 0.02 0.02 0.006 0.047 30.0 B10
0.041 0.30 1.30 0.027 0.005 12.4 0.00 0.02 0.02 0.02 0.005 0.035
20.6 B11 0.043 0.32 1.28 0.026 0.003 12.3 0.60 0.01 0.02 0.01 0.005
0.037 6.0 B12 0.044 0.29 1.20 0.025 0.003 12.5 0.06 0.00 0.02 0.02
0.002 0.040 17.6 B13 0.045 0.30 1.20 0.025 0.006 12.3 0.05 0.10
0.02 0.02 0.004 0.025 22.0 B14 0.044 0.31 1.20 0.027 0.003 12.3
0.07 0.01 0.00 0.02 0.008 0.032 18.9 B15 0.040 0.32 1.20 0.024
0.002 12.2 0.04 0.02 0.40 0.02 0.005 0.044 14.6 B16 0.038 0.33 1.20
0.026 0.003 12.4 0.08 0.01 0.02 0.00 0.004 0.040 19.0 B17 0.040
0.35 1.08 0.025 0.004 12.6 0.01 0.02 0.02 0.20 0.003 0.036 24.9 B18
0.048 0.32 1.30 0.025 0.004 12.3 0.05 0.02 0.02 0.02 0.004 0.010
27.1 B19 0.046 0.30 1.30 0.026 0.003 13.5 0.05 0.01 0.01 0.02 0.003
0.065 16.8 B20 0.068 0.36 1.50 0.027 0.004 12.7 0.05 0.02 0.02 0.05
0.004 0.050 4.0
TABLE-US-00003 TABLE 2-1 Rough bar Sample heater .delta. ferrite
Edge seam Cracked Quenching Temper Salt steel heating temp. area
ratio defect edge Polishing hardness softening spray Other NO. No.
(.degree. C.) (%) evaluation evaluation performance (HRC)
resistance test properties Examples 1 A1 10 13 A A A 32 A A 2 A2 10
30 A A A 37 A A 3 A3 10 15 A A A 34 A A 4 A4 10 30 S A A 36 A A 5
A5 10 24 A A A 33 A S 6 A6 10 12 A A A 35 A A 7 A7 10 15 A A A 37 A
A 8 A8 10 18 A A A 34 A A 9 A9 50 11 A A A 32 A A 10 A10 10 15 A A
A 33 A A 11 A11 10 9 A A A 34 S A 12 A12 10 10 A A A 33 A A 13 A13
10 6 A A A 34 A A 14 A14 10 5 A A A 35 A A 15 A15 10 6 A A A 34 A A
16 A16 10 15 A A A 36 A S 17 A17 10 5 A A A 34 A A 18 A18 10 11 A A
A 34 A A 19 A19 10 22 A A A 33 A A 20 A20 10 22 A A A 35 S A
TABLE-US-00004 TABLE 2-2 Rough bar Sample heater .delta. ferrite
Edge seam Cracked Quenching Temper Salt steel heating temp. area
ratio defect edge Polishing hardness softening spray Other NO. No.
(.degree. C.) (%) evaluation evaluation performance (HRC)
resistance test properties Examples 21 A21 10 15 A A A 34 S A 22
A22 10 17 A A A 33 A S 23 A23 10 20 A A A 34 A S 24 A24 10 21 A A A
33 S S 25 A25 10 13 A A A 34 S S 26 A26 10 12 A A A 34 S S 27 A17
50 30 S A A 34 A A 28 A17 30 25 A A A 34 A A 29 A17 20 20 A A A 35
A A 30 A6 50 30 S A A 35 A A 31 A6 40 25 A A A 35 A A 32 A17 10 7 A
A A 34 A A 33 A10 40 25 A A A 33 A A 34 A1 30 20 A A A 32 A A 35
A12 50 28 S A A 33 A A 36 A13 50 26 A A A 34 A A 37 A11 50 30 S A A
34 A A
TABLE-US-00005 TABLE 2-3 Rough bar Sample heater .delta. ferrite
Edge seam Cracked Quenching Temper Salt steel heating temp. area
ratio defect edge Polishing hardness softening spray NO. No.
(.degree. C.) (%) evaluation evaluation performance (HRC)
resistance test Other properties Comparatives 38 B1 10 7.0 A A A 39
A A 39 B2 0 16.0 A A A 31 B A 40 B3 0 19.0 A A B 33 A A 41 B4 20
32.0 B A A 32 A A 42 B5 0 34.0 B A B 29 B A 43 B6 0 21.0 A B A 34 A
A poor toughness 44 B7 10 27.0 A C A 34 A A 45 B8 0 0.0 A C A 33 A
B 46 B9 0 32.0 B A A 31 B A 47 B10 10 21.0 A A A 33 A B 48 B11 20
4.0 A A A 34 A S poor press formability 49 B12 0 18.0 A A A 34 A B
50 B13 20 22.0 A B A 33 A A poor press formability 51 B14 0 19.0 A
A A 34 A B 52 B15 0 15.0 A A A 30 B S 53 B16 10 19.0 A A A 33 A B
54 B17 10 25.0 A A B 33 A A poor toughness 55 B18 10 27.0 A A A 31
B A 56 B19 0 17.0 A A A 39 B A 57 B20 0 4.0 A C A 38 A A 58 A17 0
3.0 A C A 34 A A 59 A17 60 40.0 C B A 34 A A 60 B9 40 35.0 C B A 30
B A 61 B9 5 4.0 A C A 31 B A 62 B9 60 35.0 C B A 30 B A
[0098] As is apparent from Tables 1-1, 1-2, 2-1, 2-2 and 2-3, in
Examples of the invention, which had the component composition to
which the invention was applied, and the .delta. ferrite area ratio
was in a range from 5% to 30%, the quality of the edge seam defect
was judged as "Pass" and the quality of the cracked edge was also
judged as "Pass." The quenching hardness, heat resistance, and
corrosion resistance were also favorable. Further, when the heating
temperature applied to the rough bar using the rough bar heater
falls within the range of the invention, the depth of the edge seam
defect was further reduced, so that a polishing time of the disk
after quenching was reducible. Moreover, the quality of the cracked
edge was further improved to be unrecognizable. On the other hand,
in the component composition falling out of the scope of the
invention, it was difficult to control the .delta. ferrite amount
in the hot-rolled and annealed plate. One or more of the quality of
the edge seam defect, the quality of the cracked edge, the
quenching hardness, and the corrosion resistance after quenching
were judged as "Fail." From the above, it is understood that the
properties of the brake disks in Comparatives are inferior.
[0099] Specifically, since the values of C and N were high in the
test Nos. 38 and 56 and the values of C and N were low in the test
Nos. 42 and 55, the quenching hardness was beyond the target range.
Since the value of Si was low in the test No. 40 and the value of V
was high in the test No. 54, the polishing performance in the
polishing process after quenching was inferior. Since the .delta.
ferrite amount of the hot-rolled and annealed plate each in the
test Nos. 41, 42, 45, 46, 57, 58, 59, 60, 61 and 62 exceeds 30% or
falls less than 5%, the quality of the edge seam defect or the
cracked edge was evaluated as inferior. Since the value of P was
high in the test No. 43, the value of S was high in the test No.
44, and the value of Cu was high in the test No. 50, the cracked
edge was evaluated as inferior.
[0100] Since the values of Cr, Ni, Cu, Mo and V were low in the
test Nos. 45, 47, 49, 51 and 53, corrosion resistance was inferior.
Since the large amounts of Ni and Cu were added in the test Nos. 48
and 50, press formability was inferior. Since the value of Cr was
high in the test Nos. 46 and 60 to 62 and the values of Si and Mo
were high in the test Nos. 39 and 52, hardenability was decreased
to decrease the quenching hardness. Moreover, since the material
cost was high in the test Nos. 48, 50, 52 and 54, the produced
samples were judged economically inferior. Moreover, since the DFE
value was low in the test No. 57, the .delta. ferrite fraction was
low and the cracked edge was inferior.
[0101] From the above results, the above findings can be confirmed
and the grounds for limiting the above steel compositions and
structures can be supported.
[0102] As is apparent from the above description, the martensitic
stainless steel plate of the invention used for the brake disk is a
high-quality brake disk having favorable qualities of the edge seam
defect and the cracked edge and being free from deterioration in
hardness and corrosion resistance after quenching, which attains
optimization of the .delta. ferrite amount observed in the
hot-rolled and annealed steel plate and the hot-rolled steel plate
by controlling the component design and the hot rolling conditions.
Moreover, the qualities of the edge seam defect and the cracked
edge were further improved by heating the rough bar between the
rough hot rolling and the finish hot rolling under the optimum
conditions depending on the composition of each of the steel
plates. A material to which the invention is applied is used for
the two-wheeled brake disk, whereby a yield is improvable and an
examination burden is reducible, and further, productivity is
improvable due to a shortening of the polishing time, so that the
invention can increasingly contribute to the society. In other
words, the invention has a sufficient industrial applicability.
* * * * *