U.S. patent application number 11/790714 was filed with the patent office on 2008-10-30 for hot working die steel for die-casting.
This patent application is currently assigned to DAIDO TOKUSHUKO KABUSHIKI KAISHA. Invention is credited to Koichiro Inoue.
Application Number | 20080264526 11/790714 |
Document ID | / |
Family ID | 39885578 |
Filed Date | 2008-10-30 |
United States Patent
Application |
20080264526 |
Kind Code |
A1 |
Inoue; Koichiro |
October 30, 2008 |
Hot working die steel for die-casting
Abstract
The invention provides a hot-working die steel for die-casting
obtainable by quenching a steel comprising, in terms of % by mass,
C: 0.1 to 0.3%, Si: 0.1 to 1.5%, Mn: 0.3 to 2%, Cr: 6 to 12%, P:
0.05% or less, S: 0.01% or less, Mo: 1 to 3%, V: 0.5 to 1.5%, s-Al:
0.005 to 0.025%, N: 0.005 to 0.025%, and O: 0.005% or less, with
the remainder being Fe and inevitable impurities, followed by
tempering the steel at a temperature of 500.degree. C. or
lower.
Inventors: |
Inoue; Koichiro;
(Nagoya-shi, JP) |
Correspondence
Address: |
BACON & THOMAS, PLLC
625 SLATERS LANE, FOURTH FLOOR
ALEXANDRIA
VA
22314-1176
US
|
Assignee: |
DAIDO TOKUSHUKO KABUSHIKI
KAISHA
Nagoya-shi
JP
|
Family ID: |
39885578 |
Appl. No.: |
11/790714 |
Filed: |
April 27, 2007 |
Current U.S.
Class: |
148/334 ;
148/325; 148/332 |
Current CPC
Class: |
C22C 38/02 20130101;
C22C 38/04 20130101; C22C 38/22 20130101; C22C 38/24 20130101; C22C
38/06 20130101 |
Class at
Publication: |
148/334 ;
148/325; 148/332 |
International
Class: |
C22C 38/42 20060101
C22C038/42; C22C 38/18 20060101 C22C038/18; C22C 38/22 20060101
C22C038/22 |
Claims
1. A hot-working die steel for die-casting obtainable by quenching
a steel comprising, in terms of % by mass, C: 0.1 to 0.3%, Si: 0.1
to 1.5%, Mn: 0.3 to 2%, Cr: 6 to 12%, P: 0.05% or less, S: 0.01% or
less, Mo: 1 to 3%, V: 0.5 to 1.5%, s-Al: 0.005 to 0.025%, N: 0.005
to 0.025%, and O: 0.005% or less, with the remainder being Fe and
inevitable impurities, followed by tempering the steel at a
temperature of 500.degree. C. or lower.
2. The hot-working die steel for die-casting according to claim 1,
which further comprises at least one member selected from the group
consisting of, in terms of % by mass, Ni: 2% or less, and Cu: 1% or
less.
3. The hot-working die steel for die-casting according to claim 1,
which further comprises, in terms of % by mass, Co: 5% or less.
4. The hot-working die steel for die-casting according to claim 2,
which further comprises, in terms of % by mass, Co: 5% or less.
5. The hot-working die steel for die-casting according to claim 1,
which further comprises at least one member selected from the group
consisting of, in terms of % by mass, Ti: 0.2% or less, Zr: 0.2% or
less, and Nb: 0.2% or less.
6. The hot-working die steel for die-casting according to claim 2,
which further comprises at least one member selected from the group
consisting of, in terms of % by mass, Ti: 0.2% or less, Zr: 0.2% or
less, and Nb: 0.2% or less.
7. The hot-working die steel for die-casting according to claim 3,
which further comprises at least one member selected from the group
consisting of, in terms of % by mass, Ti: 0.2% or less, Zr: 0.2% or
less, and Nb: 0.2% or less.
8. The hot-working die steel for die-casting according to claim 4,
which further comprises at least one member selected from the group
consisting of, in terms of % by mass, Ti: 0.2% or less, Zr: 0.2% or
less, and Nb: 0.2% or less.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a hot-working die steel for
use as die-casting molds. More particularly, the invention relates
to a hot-working die steel for die-casting which inhibits the
cracking from a water-cooling hole, which is a major cause of
serious cracks in die-casting molds, and is capable of coping with
a higher cycle speed in the production of die-casting products. The
hot-working die steel for die-casting of the invention can be
advantageously used as a material for aluminum die-casting
molds.
BACKGROUND OF THE INVENTION
[0002] Aluminum die-casting molds have hitherto had a problem that
cracks generate at the cavity surface due to thermal fatigue (i.e.,
heat check). This heat check is a phenomenon in which the cavity
surface, when sprinkled with cooling water after mold opening,
comes to have an tensile stress due to a temperature difference
between the rapidly cooled cavity surface and inner parts in a
heated state, and the thermal fatigue resulting from repetitions of
this stress generation causes cracks at the cavity surface.
[0003] It is said that it is advantageous to heighten the hardness
of the mold for diminishing the heat check.
[0004] On the other hand, there recently has been a desire for a
reduction in cycle time (higher cycle speed) in the production of
aluminum die-casting products. For the purpose of reducing a mold
closing time in order to realize that desire, the water cooling of
an aluminum cast in a mold tends to be enhanced. Specifically, this
enhancement of water cooling is accomplished by disposing a
water-cooling hole in a position closer to the cavity surface. In
this case, the thermal stress generating at the surface of the
water-cooling hole during the casting of an aluminum product is
increased and the phenomenon in which a crack generates from the
water-cooling hole becomes problematic.
[0005] Such a crack generating from a water-cooling hole is not
attributable only to the thermal stress repeatedly imposed during
casting but is thought to be a delayed-fracture phenomenon
including a combination of cracking caused by thermal stress and
stress corrosion cracking caused by rust generating on the surface
of the water-cooling hole.
[0006] The higher the hardness of a mold, the more the cracking
from the water-cooling hole is apt to occur. Consequently, it is
advantageous to reduce the hardness of a mold for inhibiting such
cracking from the water-cooling hole.
[0007] Namely, to increase mold hardness is advantageous for
diminishing the heat check but is disadvantageous for diminishing
cracking from a water-cooling hole, whereas to reduce mold hardness
is advantageous for diminishing cracking from a water-cooling hole
but is disadvantageous for diminishing the heat check, resulting in
impaired heat check resistance.
[0008] From the standpoint of inhibiting the cracking from a
water-cooling hole, it is desirable to regulate the mold hardness
to HRC 45 to 40.
[0009] Hot-working die steels of the 5Cr type represented by
JIS-SKD61 have been mainly used for current aluminum die-casting
molds. In recent years, the use hardness thereof has been
increasing so as to inhibit the heat check generating at the cavity
surface, and the risk of cracking from the water-cooling hole in
the mold has been increasing with the trend toward a higher cycle
speed in the production of aluminum die-casting products.
[0010] In the case of the JIS-SKD61, this steel contains about 0.4%
of C and the hardness of the steel in a quenched state is, for
example, about HRC 53.
[0011] For reducing the hardness thereof to HRC 45 or lower for the
purpose of inhibiting cracking from a water-cooling hole, it is
necessary to conduct annealing at a high temperature of 600.degree.
C. or above. However, when annealing at such a high temperature is
conducted, the corrosion resistance of the steel decreases
considerably.
[0012] This material, which contains Cr in an amount of about 5%,
in itself is a material having excellent corrosion resistance.
However, when this steel is annealed at a temperature as high as
600.degree. C. or above, most of the Cr contained therein separates
out as a Cr carbide due to this high-temperature annealing.
Accordingly, the Cr contained in the steel thus comes not to
contribute to an improvement in corrosion resistance.
[0013] In any event, the hot-working die steels presently in main
use as aluminum die-casting molds, which are represented by
JIS-SKD61, are ineffective in satisfactorily overcoming the problem
concerning cracking from a water-cooling hole.
[0014] It is thought that an effective measure in satisfactorily
overcoming each of the problem concerning cracking from a
water-cooling hole and the problem concerning heat check at the
cavity surface is to prevent rusting in the water-cooling hole and
to reduce the hardness of that inner part of the mold in which the
water-cooling hole is present, as well as to increase the hardness
of the mold cavity surface where a heat check may generate.
However, no material satisfying such properties has been provided
yet.
[0015] Incidentally, reference document 1 shown below discloses an
invention concerning a technique in which the inner circumferential
surface of the water-cooling hole of a die-casting mold is
regulated so as to have a lower hardness than the mold surface to
thereby reconcile the prevention of water-cooling hole cracking and
the heat check resistance of the mold surface.
[0016] The steel disclosed in this reference document 1 is produced
by regulating JIS-SKD61, which has been used hitherto, so as to
have a high hardness by quenching and tempering and then regulating
the surface of the water-cooling hole so as to have a low hardness
by local tempering with induction heating, burner heating, laser
heating, or the like.
[0017] All the methods disclosed in this reference document 1
necessitate local heating, and have a problem that the shape of the
water-cooling hole is limited, for example, that the diameter of
the water-cooling hole should be a size which enables burner
insertion.
[0018] Reference Document 1: JP-A-6-315753
SUMMARY OF THE INVENTION
[0019] The present invention has been achieved under the
circumstances described above. An object of the invention is to
provide a hot-working die steel for die-casting which has excellent
heat check resistance and can satisfactorily inhibit cracking from
a water-cooling hole.
[0020] The present inventors have made eager investigation to
examine the problem. As a result, it has been found that the
foregoing objects can be achieved by the following hot-working die
steels for die-casting. With this finding, the present invention is
accomplished.
[0021] The present invention is mainly directed to the following
items.
[0022] 1. A hot-working die steel for die-casting obtainable by
quenching a steel comprising, in terms of % by mass,
[0023] C: 0.1 to 0.3%,
[0024] Si: 0.1 to 1.5%,
[0025] Mn: 0.3 to 2%,
[0026] Cr: 6 to 12%,
[0027] P: 0.05% or less,
[0028] S: 0.01% or less,
[0029] Mo: 1 to 3%,
[0030] V: 0.5 to 1.5%,
[0031] s-Al: 0.005 to 0.025%,
[0032] N: 0.005 to 0.025%, and
[0033] O: 0.005% or less,
[0034] with the remainder being Fe and
[0035] inevitable impurities,
followed by tempering the steel at a temperature of 500.degree. C.
or lower.
[0036] 2. The hot-working die steel for die-casting according to
item 1, which further comprises at least one member selected from
the group consisting of, in terms of % by mass,
[0037] Ni: 2% or less, and
[0038] Cu: 1% or less.
[0039] 3. The hot-working die steel for die-casting according to
item 1 or 2, which further comprises, in terms of % by mass,
[0040] C: 5% or less.
[0041] 4. The hot-working die steel for die-casting according to
any one of items 1 to 3, which further comprises at least one
member selected from the group consisting of, in terms of % by
mass,
[0042] Ti: 0.2% or less,
[0043] Zr: 0.2% or less, and
[0044] Nb: 0.2% or less.
[0045] The hot-working die steel for die-casting of the invention
has a reduced C content and, on the other hand, has high and
optimized Cr and Mo contents. Accordingly, the steel of the
invention, when used as a die-casting mold, can effectively inhibit
cracking from the water-cooling hole and can impart excellent heat
check resistance to the die-casting mold. The hot-working die steel
for die-casting of the invention can be advantageously used
especially as a material for aluminum die-casting molds.
[0046] Cr is known as an element which improves corrosion
resistance. In ordinary JIS-SKD61, however, the Cr for improving
corrosion resistance separates out disadvantageously as a carbide
during the heat treatment for obtaining a use hardness because this
steel is tempered at a temperature as high as 600.degree. C. or
above as described hereinabove. Accordingly, the effect of the Cr
is almost lost. On the other hand, when the tempering temperature
is lowered to such a degree that Cr carbide separation does not
occur, the steel comes to have an exceedingly high hardness of 50
HRC or above. When such a steel is used as a die-casting mold,
cracking from the water-cooling hole is apt to occur.
[0047] A target hardness may be obtained through tempering at a low
temperature of 500.degree. C. or below by reducing the C content.
In this case, however, the hardness of the cavity surface also
decreases to cause a problem that heat check resistance becomes
poor.
[0048] Herein, in the hot-working die steel for die-casting of the
invention, the C content is reduced and Mo is added in an
appropriate amount.
[0049] By reducing the C content, a hardness of HRC 45 or below,
which is less apt to result in cracking from a water-cooling hole,
can be obtained through tempering at a low temperature of
500.degree. C. or below.
[0050] Furthermore, by the addition of an appropriate amount of Mo,
the mold cavity surface can be partly increased in hardness by
utilizing the heat transferred from the melt (e.g., aluminum melt)
during die-casting when this steel is used as a die-casting
mold.
[0051] Specifically, the Mo added separates out as a carbide when
the mold is used for the casing of a die-casting product and the
cavity surface is heated by the heat transferred from the melt
(about 600-650.degree. C. in the case of aluminum melt) to thereby
serve to partly heighten the hardness of the cavity surface.
[0052] Namely, the hot-working die steel for die-casting of the
invention has an effect that the hardness of the cavity surface
increases by means of age hardening during the use of the mold. Due
to this effect, heat check in the cavity surface can be
satisfactorily inhibited.
[0053] Namely, in the hot-working die steel for die-casting of the
invention, the phenomenon in which, when the steel is used as a
die-casting mold, the cavity surface thereof undergoes age
hardening due to the heat transferred from the melt can be
ingeniously utilized. As a result, it is possible to obtain a mold
which retains a low hardness in inner parts thereof but has a
partly increased hardness in the cavity surface. In this respect,
the hot-working die steel for die-casting of the invention has an
excellent effect over conventional ones.
[0054] Moreover, Cr as a corrosion-resistant element has been added
in a larger amount in the invention than in JIS-SKD61. In the
invention, annealing is conducted at a temperature as low as
500.degree. C. or below after a quenching treatment. Accordingly,
the Cr added does not separate out as a carbide but is in the state
of being a solid solution in the matrix to effectively serve to
improve the corrosion resistance of the steel. Namely, due to this
corrosion-resistance-improving function of the Cr, when the
hot-working die steel for die-casting of the invention is used as a
die-casting mold, rusting in the water-cooling hole is inhibited
and the cracking from the water-cooling hole, which is caused by
the rusting, is satisfactorily inhibited.
[0055] Furthermore, when the hot-working die steel for die-casting
of the invention is used as a die-casting mold, the cavity surface
of the mold undergoes secondary hardening (age hardening) due to
the separation of a Mo carbide, whereby it hardens to come to have
a hardness of HRC 45 or higher, at which heat check resistance can
be secured.
[0056] Next, reasons for the limitation of each chemical component
in the invention will be described below in detail. Hereinafter,
"%" means "% by mass".
[0057] C: 0.1 to 0.3%
[0058] C is an element necessary for securing hardness and wearing
resistance, which are important mold performances.
[0059] Ordinary hot-working die steels contain C in an amount of
about 0.4%. In the invention, however, the C content is lower than
in the ordinary hot-working die steels so that a hardness of HRC 45
or lower can be obtained through low-temperature tempering at
500.degree. C. or lower. The range thereof is 0.1 to 0.3%,
preferably 0.15 to 0.25%.
[0060] Si: 0.1 to 1.5%
[0061] Si is an element necessary as a deoxidizing element in
steelmaking.
[0062] Furthermore, by increasing the content thereof,
machinability and resistivity to temper softening can be
improved.
[0063] However, excessively large addition amount thereof results
in reduced impact value toughness. Consequently, the range of the
addition amount thereof is 0.1 to 1.5%, preferably 0.1 to 0.5%.
[0064] Mn: 0.3 to 2%
[0065] Mn is a component necessary for securing hardenability and
hardness. The addition amount thereof id set at 0.3% or larger.
[0066] On the other hand, when Mn is added excessively,
hardenability becomes too high and there are some cases where
quenching yields a large amount of residual .gamma. to reduce the
impact value or where annealing does not result in a reduction in
hardness. Consequently, the upper limit thereof is set at 2%. The
upper limit of the addition amount of Mn is preferably set at
1%.
[0067] Cr: 6 to 12%
[0068] Cr is an element which improves hardenability and also
improves the corrosion resistance of a water-cooling hole.
[0069] For obtaining the effect of improving corrosion resistance,
it is necessary to add Cr in an amount of 6% or larger. It is
preferred to add Cr in an amount of 8% or larger.
[0070] However, addition in an excessively large amount reduces
resistivity to temper softening and also reduces mold performances.
Therefore, the upper limit thereof is set at 12%. Further, it is
preferred that the upper limit of the content of Cr be set at
10%.
[0071] P: .ltoreq.0.05%
[0072] P is an element which is preferably diminished because it
reduces impact value. When the steel contains it inevitably, it is
preferred to diminish the content thereof to 0.05% or below.
[0073] S: .ltoreq.0.01%
[0074] S is an element which is preferably diminished because it
forms MnS to reduce impact value.
[0075] When the steel contains it inevitably, it is preferred to
diminish the content thereof to 0.01% or below.
[0076] Mo: 1 to 3%
[0077] Mo is necessary for strengthening the matrix and improving
the wearing resistance through carbide formation and also for
securing hardenability.
[0078] Furthermore, when the hot-working die steel for die-casting
of the invention is used as a die-casting mold, this Mo carbide
separates out due to the heat transferred from the melt (around
600.degree. C. in the case of aluminum melt) to thereby heighten
the hardness of the mold.
[0079] Although the mold hardness after quenching and subsequent
tempering has been set at HRC 45 or lower in the invention in order
to prevent cracking from the water-cooling hole, the temperature of
the cavity surface rises during die-casting (around 600.degree. C.
in the case of aluminum die-casting) and a hardness of HRC 45 or
higher can be obtained. Thus, heat check resistance can be
improved.
[0080] For obtaining such an effect, it is necessary to add Mo in
an amount of 1% or larger and it is preferred to add Mo in an
amount of 1.5% or larger.
[0081] However, even when it is added excessively, the effect is
saturated and such an excessive addition is economically
disadvantageous. The upper limit of the addition is therefore set
at 3%. It is preferred that the upper limit of the addition amount
of Mo be set at 2.5%.
[0082] V: 0.5 to 1.5%
[0083] V is an element which forms a carbide and separates out
during tempering to thereby strengthen the matrix and improve
wearing resistance.
[0084] Furthermore, during heating for quenching, it forms a fine
carbide and this has the effect of inhibiting crystal grain
enlargement to thereby inhibit impact value decrease.
[0085] For obtaining such an effect, it is necessary to add V in an
amount of 0.5% or larger.
[0086] On the other hand, in a case where V is added excessively,
it yields coarse carbonitride crystals during solidification to
reduce toughness. Consequently, the upper limit of the addition
amount of V is set at 1.5%. It is preferred that the upper limit of
the addition amount of V be set at 1%.
[0087] s-Al: 0.005 to 0.025%
[0088] Al not only functions as a deoxidizing element during
steelmaking, but is an element which combines with the N in the
steel and finely disperses as a nitride to inhibit crystal grain
enlargement during heating for quenching.
[0089] For obtaining such effects, it is necessary to add Al in an
amount of 0.005% or larger.
[0090] However, even when it is added in a large amount, the effect
is saturated.
[0091] Consequently, the upper limit of the addition amount thereof
is set at 0.025%.
[0092] N: 0.005 to 0.025%
[0093] N is an element which combines with the Al and V in the
steel to form nitrides. The nitrides finely disperse to thereby
inhibit crystal grain enlargement during heating for quenching. N
is hence an element effective for preventing impact value
decrease.
[0094] For obtaining such an effect, it is necessary to add N in an
amount of 0.005% or larger.
[0095] However, even when it is added in a large amount, the effect
is saturated.
[0096] Consequently, the upper limit of the addition amount thereof
is set at 0.025%.
[0097] O: .ltoreq.0.005%
[0098] O forms oxide inclusions to decrease impact value. For
inhibiting impact value decrease, it is necessary to reduce the
content of O to 0.005% or lower.
[0099] Ni: .ltoreq.2%, Cu: .ltoreq.1%
[0100] Since Ni enhances hardenability and are thus effective in
toughening the matrix, it can be added according to need.
[0101] However, even when these elements are added excessively, the
effects are saturated and the excessive addition thereof is
economically disadvantageous. The upper limits of the addition
amount thereof are hence set at 2% and 1%, respectively.
[0102] Co: .ltoreq.5%
[0103] Co is an element which improves strength through
solid-solution strengthening. It can be added according to
need.
[0104] However, even when it is added excessively, the effect is
saturated and the excessive addition thereof is economically
disadvantageous. Consequently, the upper limit of the addition
amount thereof is set at 5%.
[0105] Ti: .ltoreq.0.2%, Zr: .ltoreq.0.2%, Nb: .ltoreq.0.2%
[0106] These are elements which form Ti(CN), Zr(CN), Nb(CN), and
composite carbonitrides thereof and finely separate out to inhibit
crystal grain enlargement during heating for quenching. When it is
desired to form fine crystal grains to secure toughness, these
elements can be added according to need.
[0107] However, in case where those elements are added excessively,
they separate out as coarse carbonitride crystals during
solidification to reduce rather than increase impact value.
Consequently, the upper limits of the addition amount thereof are
set at 0.2%, respectively.
[0108] Furthermore, in the case where those elements are added in
combination, it is preferred that the total amount thereof be 0.5%
or smaller.
DETAILED DESCRIPTION OF THE INVENTION
[0109] Embodiments of the present invention will be described below
in detail.
[0110] The present invention is now illustrated in greater detail
with reference to Steels of the invention and Comparative Steels,
but it should be understood that the present invention is not to be
construed as being limited thereto.
[0111] Steels respectively having the compositions shown in Table 1
each were melted in a 150-kg vacuum high-frequency induction
furnace. Each ingot thus obtained was forged at 1,200.degree. C.
into a square bar having a section of 60 mm.times.60 mm.
[0112] This square bar was cut into a length of 500 mm,
subsequently heated to 1,030.degree. C., and then subjected to oil
quenching.
[0113] Thereafter, tempering was conducted twice under the
conditions with a temperature of 450.degree. C. and a period of 1
h. Each square bar which had been tempered was subjected to each of
a measurement of the hardness of a 1/4 H part (a part located
midway between the surface and the central part), a Charpy impact
test in the T direction (width direction for the square bar) using
a 2-mm U-notch test piece, and a corrosion test in which a block of
10 mm.times.10 mm.times.10 mm was cut out of the 1/4 H part, the
surface thereof was polished with an emery paper, and this block
was then wholly immersed in 20.degree. C. industrial water for 24 h
and examined for rusting.
[0114] In the evaluation of corrosion resistance, ones which
suffered no rusting are rated as A and ones which suffered rusting
are rated as B.
[0115] Furthermore, for the purpose of simulating a heat history in
repetitions of the casting of an aluminum die-casting product, each
of the square bars which had been tempered at 450.degree. C. was
subjected to repeated 1,000 cycles each including heating from room
temperature to 650.degree. C. by high-frequency heating, holding at
this temperature for 4 seconds, and subsequent water cooling.
Thereafter, the surface hardness thereof was measured.
[0116] The results of these evaluations are shown in Table 2.
TABLE-US-00001 TABLE 1 Composition (mass %) No. C Si Mn P S Cr Mo V
s-Al N O Others Invention 1 0.12 0.1 0.42 0.008 0.002 8.2 2.8 0.6
0.015 0.01 0.003 Steel 2 0.2 0.3 0.45 0.012 0.002 9 2.5 0.8 0.02
0.012 0.002 3 0.18 0.5 0.8 0.01 0.005 11.3 2 1.1 0.005 0.022 0.004
4 0.23 0.3 1.2 0.024 0.008 6.5 2.8 1.4 0.008 0.008 0.002 5 0.28 0.8
0.6 0.003 0.009 10.1 1.6 0.9 0.013 0.005 0.003 6 0.2 1.3 1.4 0.018
0.001 8.8 1.8 0.8 0.022 0.008 0.001 Ni: 1% 7 0.2 0.3 0.5 0.009
0.002 9.2 2.3 0.6 0.021 0.011 0.002 Ni: 0.5%, Cu: 0.5% 8 0.21 0.25
0.45 0.003 0.002 9 2.5 0.7 0.018 0.009 0.003 Ni: 0.7%, Co: 2% 9
0.18 0.3 0.6 0.007 0.002 9.5 2.2 0.6 0.011 0.021 0.002 Co: 4%, Ti:
0.05% 10 0.22 0.22 0.65 0.031 0.001 10.1 2.3 0.55 0.016 0.023 0.002
Zr: 0.1%, Nb: 0.1% 11 0.18 0.25 0.67 0.021 0.001 8.9 2.5 0.61 0.02
0.018 0.003 Co: 1%, Zr: 0.2%, Nb: 0.05% Comparative a 0.05 0.2 0.52
0.015 0.002 8.3 2.3 0.62 0.021 0.011 0.002 Steel b 0.38 0.21 0.48
0.011 0.002 9.1 2.3 0.65 0.022 0.015 0.002 c 0.2 2 0.45 0.012 0.002
9.2 2.5 0.6 0.018 0.012 0.002 d 0.2 0.2 2.5 0.011 0.002 8.9 2.4
0.61 0.019 0.011 0.003 e 0.2 0.2 0.5 0.08 0.001 9.1 2.5 0.6 0.021
0.011 0.002 f 0.2 0.21 0.5 0.012 0.05 9 2.5 0.61 0.02 0.01 0.002 g
0.21 0.22 0.5 0.011 0.001 5.1 2.3 0.6 0.021 0.01 0.002 h 0.19 0.21
0.45 0.012 0.002 13.5 2.4 0.61 0.019 0.009 0.003 i 0.21 0.25 0.44
0.009 0.002 9 0.6 0.6 0.02 0.009 0.002 j 0.2 0.21 0.48 0.011 0.002
9.1 2.1 0.3 0.015 0.008 0.001 k 0.19 0.22 0.51 0.011 0.002 9.3 2.4
0.6 0.003 0.007 0.002 l 0.21 0.3 0.52 0.012 0.001 9 2.3 0.7 0.012
0.002 0.002 m 0.19 0.29 0.46 0.011 0.001 9 2.3 0.6 0.012 0.009
0.008 Conventional A 0.38 1 0.45 0.011 0.001 5.5 1.2 0.85 0.02
0.012 0.002 Steel
TABLE-US-00002 TABLE 2 450.degree. Tempering Hardness after Impact
repetitions Hardness value Corrosion of 650.degree. No. (HRC)
(J/cm.sup.2) resistance heating (HRC) Invention 1 40 52 A 46 Steel
2 42 48 A 48 3 41 50 A 46 4 43 46 A 49 5 44 45 A 48 6 42 48 A 47 7
42 48 A 48 8 42 49 A 47 9 41 50 A 46 10 43 48 A 48 11 41 48 A 47
Com- a 36 58 A 42 parative b 53 21 A 48 Steel c 42 25 A 48 d 42 23
A 48 e 42 18 A 47 f 42 15 A 48 g 42 49 B 48 h 42 21 A 48 i 42 48 A
44 j 42 32 A 47 k 41 33 A 47 l 42 28 A 48 m 40 30 A 48 Conventional
A 53 18 B 47 Steel * Corrosion resistance: A . . . no rusting, B .
. . rusting occurred
[0117] Furthermore, a steel obtained by heating Invention Steel No.
2 shown in Table 1 to 1,030.degree. C. and subsequently subjecting
it to oil quenching and then to tempering twice under the
conditions with a temperature of 450.degree. C. and a period of 1
h, one obtained by heating Conventional Steel A to 1,030.degree. C.
and subsequently subjecting it to oil quenching and then to
tempering twice under the conditions with a temperature of
450.degree. C. and a period of 1 h, and one obtained by subjecting
Conventional Steel A to tempering twice under the conditions with a
temperature of 630.degree. C. and a period of 1 h were respectively
evaluated for delayed-fracture resistance as an index to
receptivity to cracking from a water-cooling hole.
[0118] Here, the evaluation of delayed-fracture resistance was
conducted in the following manner.
[0119] Namely, industrial water was dropped (in order to cause
rusting) onto the notched part of a test piece having a 0.1-R
annular notch, and the relationship between flexural stress and
fracture time was examined.
[0120] The delayed-fracture resistance was evaluated by comparing
in the ratio of static flexural stress (0-h rupture stress) to the
stress causing rupture at 200 h.
[0121] Furthermore, 10,000 cycles each including heating from room
temperature to 650.degree. C., holding at this temperature for 4
seconds, and subsequent water cooling were repeatedly conducted.
Thereafter, the length of the heat crack generated at the surface
was measured and evaluated as an index to heat check
resistance.
[0122] The results of these evaluations are shown in Table 3.
[0123] In Table 3, the desired value of delayed-fracture resistance
was set at 0.7 or higher.
TABLE-US-00003 TABLE 3 Heat check Delayed-fracture Tempering
resistance resistance temperature Hardness Length of largest
Proportion of 200-h No. (.degree. C.) (HRC) heat crack (.mu.m)
rupture stress 2 450 42 120 0.98 A 450 53 123 0.65 630 42 253
0.91
[0124] As the results given in Table 2 show, Invention Steels No. 1
to No. 11 have hardnesses of HRC 40 to 44 after the tempering at
450.degree. C. and have hardnesses after the repetitions of heating
at 650.degree. C. of HRC 46 to 49. The hardnesses thereof have
increased.
[0125] Furthermore, since the tempering is low-temperature
tempering at 450.degree. C., almost no Cr carbide has separated
out. Each steel shows satisfactory corrosion resistance.
[0126] In contrast, Comparative Steel a has a C content of 0.05%,
which is lower than the lower limit of 0.1% in the invention, and
hence has a hardness after the 450.degree. C. tempering as low as
HRC 36. The hardness thereof after the repetitions of heating at
650.degree. C. also is as low as HRC 42. It has poor heat check
resistance.
[0127] Comparative Steel b conversely has a C content of 0.38%,
which is higher than the upper limit of 0.3% in the invention, and
hence has a hardness after the 450.degree. C. tempering as high as
HRC 53. It has a low impact value.
[0128] Comparative Steel c has a Si content of 2%, which is higher
than the upper limit of 1.5% in the invention. It has a low impact
value.
[0129] Comparative Steel d has a Mn content of 2.5%, which is
higher than the upper limit of 2% in the invention. It has a low
impact value.
[0130] Comparative Steel e has a content of P as an impurity of
0.08%, which is higher than the upper limit of 0.05% in the
invention. This steel also has a low impact value.
[0131] Furthermore, Comparative Steel f has a content of S also as
an impurity of 0.05%, which is higher than the upper limit of 0.01%
in the invention, and hence has a low impact value.
[0132] Comparative Steel g has a Cr content of 5.1%, which is lower
than the lower limit of 6% in the invention, and hence has low
corrosion resistance.
[0133] Comparative Steel h conversely has a Cr content of 13.5%,
which is higher than the upper limit of 12% in the invention, and
hence has a low impact value.
[0134] Comparative Steel i has a Mo content of 0.6%, which is lower
than the lower limit of 1% in the invention. Because of this, even
through the repetitions of heating at 650.degree. C., the hardness
has not increased sufficiently. This means that heat check
resistance is insufficient.
[0135] Comparative Steel j has a V content of 0.3%, which is lower
than the lower limit of 0.5% in the invention. Because of this,
crystal grain enlargement has occurred and the steel has a low
impact value.
[0136] Comparative Steel k has an s-Al content of 0.003%, which is
lower than the lower limit of 0.005% in the invention. Because of
this, crystal grain enlargement has occurred and the steel has a
low impact value.
[0137] Comparative Steel l has an N content of 0.002%, which is
lower than the lower limit of 0.005% in the invention. Because of
this, crystal grain enlargement has occurred in this case also and
the steel has a low impact value.
[0138] Comparative Steel m has an O content of 0.008%, which is
higher than the upper limit of 0.005% in the invention. Because of
this, the steel contains a larger amount of inclusions and has a
low impact value.
[0139] Next, Conventional Steel A is JIS-SKD61 and has a hardness
after the 450.degree. C. tempering of HRC 53. The hardness thereof
after the repetitions of heating at 650.degree. C. has decreased to
HRC 47. It is poor also in corrosion resistance.
[0140] Next, in Table 3, Invention Steel No. 2 has a low hardness
after the low-temperature tempering at 450.degree. C. However, this
steel is equal in heat check resistance and superior in
delayed-fracture resistance to the high-hardness material obtained
by tempering Conventional Steel A at 450.degree. C.
[0141] Furthermore, as compared with the steel having the same
hardness obtained by the 630.degree. C. high-temperature tempering
of Conventional Steel A, Invention Steel No. 2 has higher corrosion
resistance and better heat check resistance because of the
low-temperature tempering.
[0142] It can be seen as demonstrated above that the steels of the
invention have both of the property of inhibiting cracking from a
water-cooling hole and heat check resistance; these two properties
have hitherto being inconsistent with each other.
[0143] While the present invention has been described in detail and
with reference to specific embodiments thereof, it will be apparent
to one skilled in the art that various changes and modifications
can be made therein without departing from the spirit and scope
thereof.
[0144] The present application is based on Japanese Patent
Application No. 2005-346156 filed on Nov. 30, 2005, and the
contents thereof are incorporated herein by reference.
* * * * *