FIELD OF THE INVENTION
[0001] The present invention relates to a hot work tool steel and a steel product using
the same. More specifically, the present invention relates to a hot work tool steel
capable of improving the thermal conductivity as compared with general-purpose die
steel (JIS SKD61) and having a higher impact value than that of general-purpose die
steel while maintaining machinability at a level equal to or higher than that of general-purpose
die steel, and a steel product using the same.
BACKGROUND OF THE INVENTION
[0002] As regards a die material used for die casting, hot forging and warm forging, JIS
SKD61 excellent in machinability is being used for general purposes. However, JIS
SKD61 has a low thermal conductivity and in turn carries a problem that the die temperature
tends to be high and the die life is decreased due to frequent occurrence of soldering
or heat checking. Furthermore, with an increase in size of the die, microstructure
refinement of JIS SKD61 becomes difficult because of its high transformation temperature
of bainitic transformation under the condition of small cooling rate (so called hardenability),
and this leads to a significant decrease in toughness. Therefore, JIS SKD61 is disadvantageous
in that heat checking is promoted and the die life is further shortened. Under these
circumstances, a hot work tool steel superior in the thermal conductivity and impact
value to JIS SKD61 is demanded in industry.
[0003] In this connection, various steels suitable for this kind of application have been
proposed.
For example, Patent Document 1 discloses a hot work tool steel excellent in transformation
behavior (hardenability) and creep property and substitutable for JIS SKD61, which
is a steel containing, as essential components, C: from 0.30 to 0.38 wt%, Si: from
0.10 to 0.40 wt%, Mn: from 0.60 to 0.80 wt%, Cr: from 5.40 to 5.70 wt%, Mo: from 1.50
to 1.70 wt%, and V: from 0.70 to 0.85 wt%, with the balance being Fe and unavoidable
impurities.
[0004] Patent Document 2 discloses a die for width sizing of a hot slab, improved in wear
resistance and heat crack resistance by introducing a thermal shock factor K, which
is a steel containing, in terms of wt%, C: from 0.1 to 0.5%, Si: from 0.1 to 1.5%,
Mn: from 0.2 to 1.5%, Ni: 5.0% or less, Cr: from 0.5 to 5.0%, Mo: 1.5% or less, V:
1.0% or less, and Cu: 0.2% or less, with the balance being Fe and impurities.
[0005] Patent Document 3 discloses a hot work tool steel with excellent low cycle fatigue
property obtained by electroslag remelting, which is a steel containing, in terms
of wt%, C: from 0.32 to 0.42%, Si: from 0.10 to 1.20%, Mn: from 0.10 to 0.50%, Cr:
from 4.50 to 5.50%, Mo: from 1.00 to 1.50%, V: from 0.30 to 0.80%, P: 0.010% or less,
S: 0.003% or less, Ni: 1.00% or less, Co: 1.00% or less, and W: 1.00% or less, with
the balance being Fe and impurities.
[0006] Patent Document 4 discloses a hot work tool steel succeeded in enhancing all of wear
resistance, cracking resistance and chipping resistance of a practical die, which
is a steel containing, in terms of wt%, C: from 0.15 to 0.80%, Si: less than 0.10%,
Mn: 3.0% or less, one member or two or members out ofNi: 4.0% or less, Cr: 10.0% or
less and Cu: 3.0% or less, one member or two or more members out of Mo: 5.0% or less,
W: 5.0% or less, V: 3.0% or less, Ti: 1.0% or less, Nb: 1.0% or less, Zr: 1.0% or
less and Co: 5.0% or less, S: 0.005% or less, P: 0. 015% or less, and O: 0.0030% or
less, with the balance being Fe and impurities.
[0007] Patent Document 5 discloses an alloy tool steel excellent in hot workability and
fatigue property, which is a steel containing, in terms of wt%, C: from 0.35 to 1.50%,
Si: from 0.1 to 2.0%, Mn: from 0.1 to 1.5%, Cr: from 2.0 to 10.0%, one member or two
or more members out of 2Mo+W: from 1.5 to 30.0% and V: from 0.5 to 5.0%, REM: from
0.001 to 0.60%, and one member or two or more members out of Co: from 1.0 to 20.0%,
Ni: from 0.01 to 2.0%, Cu: from 0.25 to 1.0% and B: from 0.001 to 0.050%, and being
bound by a restriction to S: 0.0020% or less, O: 0.0030% or less, N: 0.020% or less,
Al: 0.020% or less and P: 0.020% or less, with the balance substantially being Fe.
[0008] Patent Document 6 discloses a die steel succeeded in improving the thermal fatigue
property and softening resistance and thereby making it possible to suppress heat
checking and cracking of a cooling water hole and enhance the die life, which is a
steel containing, in terms of mass%, C: from 0.1 to 0.6%, Si: from 0.01 to 0.8%, Mn:
from 0.1 to 2.5%, Cu: from 0.01 to 2.0%, Ni: from 0.01 to 2.0%, Cr: from 0.1 to 2.0%,
Mo: from 0.01 to 2.0%, one member or two or more members out of V, W, Nb and Ta: from
0.01 to 2.0% in total, Al: from 0.002 to 0.04%, N: from 0.002 to 0.04%, and O: 0.005%
or less, with the balance being Fe and unavoidable impurities.
[0009] Patent Document 7 discloses an inexpensive die steel for plastic molding, satisfying
both machinability and thermal conductivity, which is a steel containing C: from 0.25
to 0.45%, Si: less than 0.3%, Mn: from 0.5 to 2%, S: from 0.01 to 0.05%, and sol.
Al: 0.02% or less, with the balance being Fe and impurities, wherein one or more of
Cr of up to 0.5% and V of less than 0.2% may be contained.
[0010] Patent Document 8 discloses a prehardened steel for a die-casting die, enabling extension
of the die-casting die life, which is a steel containing, in terms of the content
by mass, from 0.15 to 0.35% of C, from 0.05% to less than 0.20% of Si, from 0.05 to
1.50% of Mn, 0.020% or less of P, 0.013% or less of S, 0.10% or less of Cu, 0.20%
or less ofNi, from 0.20 to 2.50% of Cr, from 0.50 to 3.00% of Mo, from 0.05 to 0.30%
in total of V and Nb, from 0.020 to 0.040% of Al, 0.003% or less of O, and from 0.010
to 0.020% of N, with the balance substantially being Fe.
[0011] Patent Document 9 discloses a steel for press die, having high thermal fatigue property,
which is a steel containing C: from 0.10 to 0.45 wt%, Si: from 0.10 to 2.0 wt%, Mn:
from 0.10 to 2.0 wt%, Mo: from 0.50 to 3.0 wt% and V: from 0.50 to 0.80 wt%, and further
containing Cr: from 3.0 to 8.0 wt% and Ni: from 0.05 to 1.2 wt%, with the balance
being Fe and unavoidable impurities.
[0012] Patent Document 10 discloses a die steel with good spheroidization annealability
and good machinability, ensuring that satisfactory quenching and desired impact value
are obtained and the die life is enhanced, which is a steel having a composition containing,
in terms of mass%, C: from 0.2 to 0.6%, Si: from 0.01 to 1.5%, Mn: from 0.1 to 2.0%,
Cu: from 0.01 to 2.0%, Ni: from 0.01 to 2.0%, Cr: from 0.1 to 8.0%, Mo: from 0.01
to 5.0%, one member or two or more members out of V, W, Nb and Ta: from 0.01 to 2.0%
in total, Al: from 0.002 to 0.04%, N: from 0.002 to 0.04%, and the balance being Fe
and unavoidable impurities.
[0014] However, in general, when the heat check property is improved by enhancing the thermal
conductivity, machinability becomes poor and this may incur reduction of working efficiency
and rise in cost. Accordingly, the effects are totally cancelled out in many cases.
In addition, the steels disclosed in Patent Documents 1 to 10 are not a steel satisfying
both the thermal conductivity and the impact value that the present invention intends
to achieve.
For example, in Patent Document 1, the thermal conductivity is neither suggested nor
disclosed, and decrease of impact value due to excess V may be feared. Also, in Patent
Document 1, serious deterioration of machinability due to too little Si and difficulty
of working into a die shape may be feared.
In Patent Document 2, reduction of thermal conductivity due to excess Si and deterioration
of machinability due to too little Si may be feared. Furthermore, in Patent Document
2, decrease of impact value due to too little Mn or too little Cr may be feared.
Also in Patent Documents 3 to 5, thermal conductivity is neither suggested nor disclosed.
In Patent Document 3, insufficient transformation behavior (hardenability) and decrease
of impact value due to too little Mn may be feared. In Patent Document 4, deterioration
of machinability due to too little Si may be feared. Furthermore, in Patent Document
4, decrease of impact value due to too little Cr or too little or excess V may be
feared. In Patent Document 5, insufficient transformation behavior (hardenability)
or decrease of impact value due to too little Mn, reduction of high-temperature strength
due to too little Mo, or decrease of impact value due to too little or excess V may
be feared.
In Patent Documents 6 to 8, deterioration of transformation behavior (hardenability)
or decrease of hardness or impact value due to too little Cr may be feared.
In Patent Documents 9 and 10, deterioration of machinability due to too little Si,
reduction of thermal conductivity due to excess Si, or decrease of impact value due
to too little Cr may be feared.
SUMMARY OF THE INVENTION
[0015] The present invention has been made in consideration of these circumstances, and
an object of the present invention is to provide a hot work tool steel superior in
thermal conductivity to general-purpose die steel (JIS SKD61) and assured of a higher
impact value than that of general-purpose die steel while maintaining machinability
at a level equal to or higher than that of general-purpose die steel.
[0016] The general-purpose die steel (JIS SKD61) has excellent machinability but is low
in the thermal conductivity and impact value. Therefore, a steel enhanced in all of
machinability, thermal conductivity and impact value is being demanded in industry,
but in general, there is a relationship that when machinability is improved, thermal
conductivity is reduced and when the thermal conductivity is enhanced, machinability
becomes poor. Accordingly, a steel satisfying the properties that the present invention
requires in terms of all of machinability, thermal conductivity and impact value has
not been heretofore proposed.
The present inventors have made intensive studies to more improve the thermal conductivity
than in the general-purpose die steel and attain a higher impact value than that of
the general-purpose die steel while maintaining machinability at a level equal to
or higher than that of the general-purpose die steel. As a result, it has been found
that:
- (a) the thermal conductivity can be increased while preventing deterioration of machinability
by adjusting the Si amount, and at the same time,
- (b) the impact value can be raised while keeping higher thermal conductivity than
in the general-purpose die steel by adjusting the Mn amount, Cr amount, Mo amount
and V amount.
The present invention has been accomplished based on this finding.
[0017] In order to attain the above-described object, the present invention provides a hot
work tool steel containing:
0.20≤C≤0.50 mass%,
0.40<Si<0.75 mass%,
0.50<Mn≤1.50 mass%,
5.24≤Cr≤9.00 mass%,
1.08<Mo<2.99 mass%, and,
0.30<V<0.70 mass%,
with the balance being Fe and unavoidable impurities.
Herein, examples of the unavoidable impurities include W<0.30 mass%, Co<0.30 mass%,
Nb<0.004 mass%, Ta<0.004 mass%, Ti<0.004 mass%, Zr<0.004 mass%, Al<0.004 mass%, N<0.004
mass%, Cu<0.15 mass%, Ni<0.15 mass%, B<0.0010 mass%, S<0.010 mass%, Ca<0.0005 mass%,
Se<0.03 mass%, Te<0.005 mass%, Bi<0.01 mass%, Pb<0.03 mass%, Mg<0.005 mass%, and O<0.0080
mass%.
[0018] The hot work tool steel according to the present invention may further contains:
0.30≤W≤4.00 mass%.
[0019] The hot work tool steel according to the present invention may further contains:
0.30≤Co≤3.00 mass%.
[0020] The hot work tool steel according to the present invention may further contains at
least one element selected from the group consisting of:
0.004≤Nb:≤0.100 mass%,
0.004≤Ta≤0.100 mass%,
0.004≤Ti:≤0.100 mass%,
0.004≤Zr≤0.100 mass%,
0.004≤Al≤0.050 mass%, and
0.004≤N≤0.050 mass%.
[0021] The hot work tool steel according to the present invention may further contains at
least one element selected from the group consisting of:
0.15≤Cu≤1.50 mass%,
0.15≤Ni≤1.50 mass%, and
0.00 10≤B≤0.0100 mass%.
[0022] The hot work tool steel according to the present invention may further contains at
least one element selected from the group consisting of:
0.010≤S≤0.500 mass%,
0.0005≤Ca≤0.2000 mass%,
0.03≤Se≤0.50 mass%,
0.005≤Te≤0.100 mass%,
0.01≤Bi≤0.30 mass%, and
0.03≤Pb≤0.50 mass%.
[0023] A steel product according to the present invention uses the hot work tool steel according
to the present invention.
The term "steel product" as used herein indicates, for example, a die-casting die,
a hot forging die, or a warm forging die, but the present invention is not limited
thereto.
[0024] The hot work tool steel of the present invention and a steel product using the same
have the above-described component composition and therefore, produce an effect that
more excellent thermal conductivity and a higher impact value than in the general-purpose
die steel (JIS SKD61) are ensured while maintaining machinability equal to or higher
than that of the general-purpose die steel, that is, an unprecedented effect of ensuring
excellent balance among respective properties of machinability, thermal conductivity
and impact value.
In more detail, in the hot work tool steel of the present invention, the Si amount
is optimized and furthermore, the Mn amount, Cr amount, Mo amount and V amount are
optimized, so that more excellent thermal productivity than that of the general-purpose
die steel can be obtained and at the same time, machinability equal to or higher than
that of the general-purpose die steel can be ensured. Accordingly, there is an effect
that the hot work tool steel of the present invention has not only high thermal conductivity
but also low transformation temperature of bainitic transformation under the condition
of small cooling rate (so called hardenability) and a high impact value. Thanks to
this effect, in the case of the hot work tool steel of the present invention, the
cost required for die processing is kept from becoming higher than that for the general-purpose
die steel. Also, the hot work tool steel of the present invention hardly causes soldering
or heat check, as a result, a long die life can obtained and reduction of production
cost and enhancement of productivity in die casting or hot and/or warm forging can
be attained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025]
Fig. 1 is a graph showing the relationship between the machinability and the Si content.
Fig. 2 is a graph showing the relationship between the thermal conductivity and the
Si content.
Fig. 3 is a graph showing the relationship between the impact value and the Mn content.
Fig. 4 is a graph showing the relationship between the thermal conductivity and the
Mn content.
Fig. 5 is a graph showing the relationship between the impact value and the Cr content.
Fig. 6 is a graph showing the relationship between the thermal conductivity and the
Cr content.
Fig. 7 is a graph showing the relationship between the strength at 600°C (high-temperature
strength) and the Mo content.
Fig. 8 is a graph showing the relationship between the impact value and the V content.
BEST MODE FOR CARRYING OUT THE INVENTION
[0026] The hot work tool steel according to one embodiment of the present invention and
a steel product using the same are described below.
(Component Composition of Hot Work Tool Steel and Reasons for Limitations Therein)
[0027] The hot work tool steel according to this embodiment contains, as essential elements,
C, Si, Mn, Cr, Mo and V, with the balance being Fe and unavoidable impurities. The
hot work tool steel according to this embodiment contains, for example, W, Co, Nb,
Ta, Ti, Zr, Al, N, Cu, Ni, B, S, Ca, Se, Te, Bi, Pb, Mg and O as unavoidable impurities.
Herein, in the present specification, all the percentages defined by mass are the
same as those defined by weight, respectively.
(1) 0.20≤C≤0.50 mass%
[0028] C is an essential element necessary for adjusting the strength of the steel. If the
C amount is less than 0.20 mass%, the required hardness of 36 HRC or more can be hardly
obtained, whereas if the C amount exceeds 0.50 mass%, the hardness tends to be saturated
and at the same time, the carbide amount becomes excessive to deteriorate the fatigue
strength or impact value. For this reason, the C amount is set to 0.20≤C≤0.50 mass%.
Because of excellent balance of hardness, fatigue strength and impact value, the C
amount is preferably 0.24≤C≤0.46 mass%, more preferably 0.28≤C≤0.42 mass%.
(2) 0.40<Si<0.75 mass%
[0029] Si is an essential element necessary for adjusting the machinability of the steel.
If the Si amount is 0.40 mass% or less, it becomes difficult to ensure machinability
equal to or higher than that of the general-purpose die steel. If the Si amount is
0.75 mass% or more, significant reduction of the thermal conductivity is incurred.
For this reason, the Si amount is set to 0.40<Si<0.75 mass%. The Si amount is preferably
0.44≤Si≤0.70 mass%, more preferably 0.48≤Si≤0.65 mass%, where machinability and thermal
conductivity are well-balanced.
(3) 0.50≤Mn≤1.50 mass%
[0030] Mn is an essential element for improving the transformation behavior (hardenability).
If the Mn amount is 0.50 mass% or less, the effect of decreasing the transformation
temperature and refining microstructure is insufficient and therefore it is difficult
to ensure hardness or impact value. If the Mn amount exceeds 1.50 mass%, not only
the impact value is rather decreased but also a high thermal conductivity can be hardly
maintained. For this reason, the Mn amount is set to 0.50<Mn≤1.50 mass%. Also, the
Mn amount is preferably 0.55≤Mn≤1.35 mass%, more preferably 0.65≤Mn≤1.20 mass%, where
hardness and impact value can be ensured and at the same time, high thermal conductivity
is obtained.
(4) 5.24≤Cr≤9.00 mass%
[0031] Cr is an essential element for improving the transformation behavior (hardenability)
and at the same time, increasing the strength of the steel by forming a carbide. If
the Cr amount is less than 5.24 mass%, the effect of decreasing the transformation
temperature and refining microstructure is not sufficient and the hardness and impact
value cannot be sufficiently obtained. Furthermore, the corrosion resistance required
of a die-casting die that is exposed to a corrosion environment is higher as the Cr
amount is larger. On the other hand, if the Cr amount exceeds 9.00 mass%, it becomes
difficult to maintain a high thermal conductivity. For this reason, the Cr amount
is set to 5.24≤Cr≤9.00 mass%. Also, the Cr amount is preferably 5.40<Cr≤8.40 mass%,
more preferably 5.55≤Cr≤7.80 mass%, where the hardness, impact value and corrosion
resistance are ensured and at the same time, a high thermal conductivity is obtained.
(5) 1.08<Mo<2.99 mass%
[0032] Mo is an essential element not only for improving the transformation behavior (hardenability),
but also for increasing the strength of the steel by forming a carbide, particularly
for enhancing the high-temperature strength. If the Mo amount is 1.08 mass% or less,
satisfactory high-temperature strength is not obtained, whereas if the Mo amount is
2.99 mass% or more, the high-temperature strength tends to be saturated and at the
same time, a significant rise in the cost impairs the profitability. For this reason,
the Mo amount is set to 1.08<Mo<2.99 mass%. Also, the Mo amount is preferably 1.15<Mo≤2.80
mass%, more preferably 1.20≤Mo≤2.50 mass%.
(6) 0.30<V<0.70 mass%
[0033] V is an essential element not only for improving the transformation behavior (hardenability),
but also for increasing the strength of the steel by forming a carbide, particularly
for enhancing the high-temperature strength. If the V amount is 0.30 mass% or less,
austenitic grain is readily coarsened at the quenching to decrease the impact value,
whereas if the V amount is 0.70 mass% or more, the coarse carbide amount becomes excessive
and this deteriorates the impact value. For this reason, the V amount is set to 0.30<V<0.70
mass%. Also, the V amount is preferably 0.40≤V≤0.67 mass%, more preferably 0.50≤V≤0.64
mass%, where softening resistance can be ensured and at the same time, fatigue strength
and impact value can be satisfactorily obtained.
(7) Unavoidable impurities:
[0034] W<0.30 mass%, Co<0.30 mass%, Nb<0.004 mass%, Ta<0.004 mass%, Ti<0.004 mass%, Zr<0.004
mass%, Al<0.004 mass%, N<0.004 mass%, Cu<0.15 mass%, Ni<0.15 mass%, B<0.0010 mass%,
S<0.010 mass%, Ca<0.0005 mass%, Se<0.03 mass%, Te<0.005 mass%, Bi<0.01 mass%, Pb<0.03
mass%, Mg<0.005 mass%, O<0.0080 mass%, and so on.
In the case where the amounts of W, Co, Nb, Ta, Ti, Zr, Al, N, Cu, Ni, B, S, Ca, Se,
Te, Bi, Pb, Mg, O, etc. are in the above-described ranges, respectively, these elements
are included as the unavoidable impurities.
[0035] The hot work tool steel according to this embodiment may further contain, as selective
elements:
- (a) W,
- (b) Co,
- (c) at least one element selected from the group consisting of Nb, Ta, Ti, Zr, Al
and N,
- (d) at least one element selected from the group consisting of Cu, Ni and B, and/or
- (e) at least one element selected from the group consisting of S, Ca, Se, Te, Bi and
Pb.
(8) 0.30≤W≤4.00 mass%
[0036] W is a selective element that can be added so as to increase the strength by the
precipitation of a carbide (precipitation hardening). If the W amount is less than
0.30 mass%, the effect of increasing the strength is small, whereas if the W amount
exceeds 4.00 mass%, this incurs saturation of the effect and a significant rise in
cost. For this reason, the W amount is set to 0.30≤W≤4.00 mass%.
(9) 0.30≤Co≤3.00 mass%
[0037] Co is a selective element that can be added so as to increase the strength by the
solid solution in the matrix (solid solution hardening). If the Co amount is less
than 0.30 mass%, the effect of increasing the strength is small, whereas if the Co
amount exceeds 3.00 mass%, this incurs saturation of the effect and a significant
rise in cost. For this reason, the Co amount is set to 0.30≤Co≤3.00 mass%.
(10) At least one element selected from the group consisting of:
[0038]
0.004≤Nb≤0.100 mass%,
0.004≤Ta≤0.100 mass%,
0.004≤Ti≤0.100 mass%,
0.004≤Zr≤0.100 mass%,
0.004≤Al≤0.050 mass%, and
0.004≤N≤0.050 mass%.
Nb, Ta, Ti, Zr, Al and N are a selective element that can be added so as to increase
the strength and toughness by refining the austenitic grains at the quenching (grain
refining). As regards all of these elements, if the amount added is less than the
predetermined amount, the effect of improving the strength and toughness is small,
whereas if it exceeds the predetermined amount, a carbide, a nitride or an oxide is
excessively produced, and this rather incurs reduction of toughness.
(11) At least one element selected from the group consisting of:
[0039]
0.15≤Cu≤1.50 mass%,
0.15≤Ni≤1.50 mass%, and
0.0010≤B≤0.0100 mass%.
Cu, Ni and B are a selective element that can be added so as to improve the transformation
behavior (hardenability). As regards all of these elements, if the amount added is
less than the predetermined amount, the effect of improving the quenchability is small,
whereas if it exceeds the predetermined amount, the effect is saturated and the practical
benefit is poor. In particular, with respect to Cu and Ni, excessive addition gives
rise to reduction of the thermal conductivity.
(12) At least one element selected from the group consisting of:
[0040]
0.010≤S≤0.500 mass%,
0.0005≤Ca≤0.2000 mass%,
0.03≤Se≤0.50 mass%,
0.005≤Te≤0.100 mass%,
0.01≤Bi≤0.30 mass%, and
0.03≤Pb≤0.50 mass%.
S, Ca, Se, Te, Bi and Pb are a selective element that can be added so as to improve
the machinability (machinability enhancement). As regards all of these elements, if
the amount added is less than the predetermined amount, the effect of improving the
machinability is small, whereas if it exceeds the predetermined amount, hot workability
is significantly deteriorated to cause frequent occurrence of cracking in plastic
working and therefore the productivity and yield are reduced.
In this regard, with regard to each element contained in the steel of the invention,
according to an embodiment, the minimal amount thereof present in the steel is the
smallest non-zero amount used in the Examples of the developed steels as summarized
in Table 1 and 2. According to a further embodiment, the maximum amount thereof present
in the steel is the maximum amount used in the Examples of the developed steels as
summarized in Table 1 and 2.
(Production Method)
[0041] The steel according to this embodiment can be obtained, for example, by the following
procedure, but the present invention is not limited thereto.
(1) Casting
[0042] The raw materials blended to give the predetermined components above are melted,
and the melt is cast in a casting mold to obtain an ingot.
(2) Homogenization Heat Treatment/Hot Working
[0043] A homogenization heat treatment and hot working are performed to homogenize the components
of the obtained ingot and break the cast structure. As for the conditions of the homogenization
heat treatment and hot working, optimal conditions for each processing are preferably
selected according to the components.
The homogenization heat treatment is usually performed by holding the ingot at 1,100
to 1,500°C for approximately from 10 to 30 hours.
The hot working is usually performed at 1,000 to 1,300°C, and after the completion
of working, the ingot is air-cooled.
(3) Tempering/Spheroidization Annealing/Rough Machining
[0044] The steel according to this embodiment has relatively good transformation behavior
(hardenability) and therefore, is often hardened at the air cooling after hot working
due to occurrence of bainitic transformation or martensitic transformation. Therefore,
the material is preferably softened by performing tempering and spheroidization annealing
after the working and then subjected to rough machining.
[0045] As for the tempering conditions, optimal conditions are preferably selected according
to the components. The tempering is usually performed by holding the material at 600
to 750°C for approximately from 1 to 10 hours.
The spheroidization annealing is preferably performed to give a steel hardness of
approximately from 90 to 97 HRB. The spheroidization annealing is usually performed
by holding the material at 800 to 950°C for approximately from 1 to 10 hours and then
cooling it at a rate of 5 to 30°C per 1 hour.
The rough machining is performed by mechanically working the softened material into
a predetermined shape.
(4) Thermal Treatment (Quenching/Tempering)
[0046] The thermal treatment is performed for adjusting the roughly machined material to
a desired hardness. As for the quenching conditions and tempering conditions, optimal
conditions for each processing are preferably selected according to the components
and required properties.
The quenching is usually performed by holding the material at 1,000 to 1,050°C for
0.5 to 5 hours and then rapidly cooling it. The rapidly cooling method is not particularly
limited, and an optimal method is preferably selected according to the purpose. Examples
of the rapidly cooling method include water cooling, oil cooling and air blast cooling.
The tempering is usually performed by holding the material at 500 to 650°C for 1 to
10 hours.
By passing through these steps (1) to (4), a steel superior in the thermal conductivity
to the general-purpose die steel (JIS SKD61) and higher in the impact value than the
general-purpose die steel can be obtained while maintaining machinability at a level
equal to or higher than that of general-purpose die steel.
(5) Finish Machining
[0047] The material thermally treated to a desired hardness is subjected to finish machining.
By passing the step (5), a steel product using the hot work tool steel according to
this embodiment is obtained.
(Mode of Operation)
[0048] In the hot work tool steel of this embodiment, the Si amount is optimized, so that
machinability equal to or higher than that of the general-purpose die steel can be
ensured and a thermal conductivity more excellent than in the general purpose die
steel can be obtained. Also, in the hot work tool steel of this embodiment, the Mn
amount, Cr amount, Mo amount and V amount are optimized and therefore, an excellent
thermal conductivity and a high impact value as compared with the general-purpose
die steel are obtained while ensuring machinability equal to or higher than that of
the general-purpose die steel. Therefore, the hot work tool steel of the present invention
can keep the cost of die working from becoming higher than in using the general-purpose
steel. Furthermore, the hot work tool steel of this invention hardly causes soldering
or heat checking, as a result, a long die life can obtained and reduction of production
cost and enhancement of productivity in die casting or hot and/or warm forging can
be attained.
EXAMPLES
(Example A)
[0049] For producing each invention steel in Example B below, Examples 1 to 5 were performed
to examine the preferred Si amount, Mn amount, Cr amount, Mo amount and V amount.
(Example 1: Examination of Si Amount)
[0050] The preferred Si amount was examined and is described below by referring to Figs.
1 and 2.
Fig. 1 shows the distance machined by a cutting tool until the end of its life, with
respect to the Si amount when cutting a steel composed of 0.35 mass% of C, 0.82 mass%
of Mn, 5.73 mass% of Cr, 1.21 mass% of Mo, 0.62 mass% of V and x mass% of Si. In Fig.
1, the numerical values at each plotted point are such that the numerical value on
the upper side indicates the x value (mass%) and the numerical value on the lower
side indicates the distance (mm) machined. The specimen for evaluation of machinability
was a square bar of 55 mm x 55 mm x 200 mm (produced by the same procedure as in Example
B and softened to a hardness of 90 to 97 HRB by spheroidization annealing), and the
time when the maximum wear volume on the side clearance face of the cutting tool reached
300 µm was judged as the end of life. A larger machined distance is indicative of
better machining and is preferred.
According to Fig. 1, the machined distance increases with an increase of the Si amount
and therefore, in view of enhancing machinability, the Si amount is preferably larger.
According to Fig. 1, decrease in the machined distance is pronounced when the Si amount
is 0.40 mass% or less. Therefore, from the standpoint of ensuring machinability, the
Si amount is preferably more than 0.40 mass%, more preferably 0.44 mass% or more,
still more preferably 0.48 mass% or more. On the other hand, when the Si amount is
0.75 mass% or more, the improvement effect is not noticeable.
[0051] A round bar of φ 11 mm x 50 mm using the same material as in Fig. 1 was heated at
1,030°C and then treated to 49 HRC through rapid cooling and tempering. From this
round bar, a specimen of φ 10 mm x 2 mm for measurement of thermal conductivity was
produced. Fig. 2 shows the thermal conductivity measured at room temperature by a
laser flash method, with respect to the Si amount. In Fig. 2, the numerical values
at each plotted point are such that the numerical value on the upper side indicates
the x value (mass%) and the numerical value on the lower side indicates the thermal
conductivity (W/m/K). A larger thermal conductivity is indicative of higher cooling
ability of a die formed and is preferred.
According to Fig. 2, the thermal conductivity is reduced with an increase of the Si
amount and when the Si amount exceeds 0.80 mass%, the thermal conductivity is reduced
to such extent that there is almost no difference as compared with the general-purpose
die steel (JIS SKD61 (thermal conductivity: 24 W/m/K)). Therefore, from the standpoint
of obtaining a thermal conductivity higher than that of the general-purpose die steel
(JIS SKD61 (thermal conductivity: 24 W/m/K)), a value of less than 0.75 mass% is selected
as the upper limit of the Si amount.
Also, according to Fig. 2, a high thermal conductivity of 28.3 W/m/K or more is obtained
when the Si amount is from 0.10 to 0.40 mass%, and a good thermal conductivity of
26 W/m/K or more is obtained when the Si amount is from 0.10 to 0.70 mass%.
Concluding these, although the thermal conductivity is reduced with an increase of
the Si amount, in view of comparison with the general-purpose die steel; the upper
limit of the Si amount can be set to be less than 0.75 mass%. From the standpoint
of increasing the thermal conductivity, the Si amount is more preferably 0.70 mass%
or less, still more preferably 0.65 mass% or less.
(Example 2: Examination of Mn Amount)
[0052] The preferred Mn amount was examined and is described below by referring to Figs.
3 and 4.
Fig. 3 plots the impact value at room temperature of a steel composed of 0.32 mass%
of C, 0.42 mass% of Si, 5.03 mass% of Cr, 1.22 mass% of Mo, 0.60 mass% of V and x
mass% of Mn, with respect to the Mn amount. In Fig. 3, the numerical values at each
plotted point are such that the numerical value on the upper side indicates the x
value (mass%) and the numerical value on the lower side indicates the impact value
(J/cm
2). The specimen for evaluation of impact value was a square bar of 11 mm x 11 mm x
55 mm (produced by the same procedure as in Example B and softened to a hardness of
90 to 97 HRB by spheroidization annealing), which was heated at 1,030°C and then treated
to 49 HRC through rapid cooling and tempering. A JIS No. 3 impact test specimen of
10 mm x 10 mm x 55 mm was produced from the square bar above and measured for the
impact value. A larger impact value is indicative of higher cracking resistance of
a die formed and is preferred.
According to Fig. 3, it is understood that the impact value is relatively low when
the Mn amount is 0.50 mass% or less. Also, according to Fig. 3, the impact value is
enhanced with an increase of the Mn amount but is decreased when it exceeds 1.50 mass%.
According to Fig. 3, an impact value of 30 J/cm
2 or more is obtained when the Mn amount is 0.45 mass% and 0.55 mass%. Therefore, a
value of 0.50 mass% in-between the Mn amounts of 0.45 mass% and 0.55 mass% is taken
as the lower limit of the Mn amount. Also, according to Fig. 3, an impact value of
31 J/cm
2 or more is obtained when the Mn amount is 0.65 mass% or more. However, according
to Fig. 3, when the Mn amount exceeds 1.50 mass%, the impact value is decreased, though
the value is kept at a good level.
[0053] Fig. 4 plots the thermal conductivity at room temperature of the same material as
in Fig. 3, with respect to the Mn amount. In Fig. 4, the numerical values at each
plotted point are such that the numerical value on the upper side indicates the x
value (mass%) and the numerical value on the lower side indicates the thermal conductivity
(W/m/K). The measurement of the thermal conductivity was performed by a laser flash
method similarly to Example 1.
According to Fig. 4, the thermal conductivity is reduced with an increase of the Mn
amount. According to Fig. 4, the Mn amount can be 1.50 mass% or less for obtaining
a thermal conductivity of 26 W/m/K or more leading to an improvement of the cooling
ability as compared with JIS SKD61 (thermal conductivity: 24 W/m/K), can be 1.35 mass%
or less for obtaining 26.4 W/m/K or more leading to a further improvement of the cooling
ability, and can be 1.20 mass% or less for obtaining 26.8 W/m/K or more leading to
a still further improvement of the cooling ability.
(Example 3: Examination of Cr Amount)
[0054] The preferred Cr amount was examined and is described below by referring to Figs.
5 and 6.
Fig. 5 plots the impact value at room temperature of a steel composed of 0.35 mass%
of C, 0.51 mass% of Si, 0.84 mass% of Mn, 1.22 mass% of Mo, 0.61 mass% of V and x
mass% of Cr and treated to 49 HRC, with respect to the Cr amount. In Fig. 5, the numerical
values at each plotted point are such that the numerical value on the upper side indicates
the x value (mass%) and the numerical value on the lower side indicates the impact
value (J/cm
2). The production of specimen and the measurement of impact value were performed in
the same manner as in Example 2.
According to Fig. 5, the impact value increases with an increase of the Cr amount.
In particular, when the Cr amount exceeds 5 mass%, the effect of this element is prominent.
According to Fig. 5, it is understood that the Cr amount can be 5.24 mass% or more
for obtaining an impact value of 27.2 J/cm
2 or more. Therefore, in view of ensuring the impact value, the lower limit of the
Cr amount is set to be 5.24 mass% or more. Also, according to Fig. 5, when the Cr
amount is less than 5 mass%, the decrease of impact value is pronounced.
[0055] Fig. 6 plots the thermal conductivity at room temperature of a steel composed of
0.21 mass% of C, 0.41 mass% of Si, 0.52 mass% of Mn, 1.22 mass% of Mo, 0.61 mass%
of V and x mass% of Cr, with respect to the Cr amount. In Fig. 6, the numerical values
at each plotted point are such that the numerical value on the upper side indicates
the x value (mass%) and the numerical value on the lower side indicates the thermal
conductivity (W/m/K). The measurement of the thermal conductivity was performed by
a laser flash method similarly to Example 1.
According to Fig. 6, the thermal conductivity is reduced with an increase of the Cr
amount. According to Fig. 6, the Cr amount can be 9.00 mass% or less for obtaining
a thermal conductivity of 25 W/m/K or more leading to an improvement of the cooling
ability as compared with JIS SKD61 (thermal conductivity: 24 W/m/K), can be 8.40 mass%
or less for obtaining 25.6 W/m/K or more leading to an improvement of the cooling
ability, and can be 7.80 mass% or less for obtaining 26.3 W/m/K or more leading to
a further improvement of the cooling ability. Also, according to Fig. 6, the Cr can
be 6.70 mass% or less for obtaining a thermal conductivity of 28 W/m/K or more leading
to a remarkable improvement of the cooling ability as compared with JIS SKD61.
[0056] (Example 4: Examination of Mo Amount) The preferred Mo amount was examined and is
described below by referring to Fig. 7.
Fig. 7 shows the high-temperature strength (deformation resistance at 600°C) of a
steel composed of 0.35 mass% of C, 0.47 mass% of Si, 0.83 mass% of Mn, 5.74 mass%
of Cr, 0.59 mass% of V and x mass% of Mo, with respect to the Mo amount. In Fig. 7,
the numerical values at each plotted point are such that the numerical value on the
upper side indicates the x value (mass%) and the numerical value on the lower side
indicates the high-temperature strength (MPa). The specimen for measurement of deformation
resistance was a round bar of φ 15 mm x 50 mm (produced by the same procedure as in
Example B and softened to a hardness of 90 to 97 HRB by spheroidization annealing),
which was heated at 1,030°C and treated to 45 HRC through rapid cooling and tempering.
From this round bar, a specimen of φ 14 mm x 21 mm for measurement of deformation
resistance was produced. The specimen was heated to 600°C at 5°C/s and after holding
for 100 s, measured for the deformation resistance by working it at a strain speed
of 10 s
-1.
[0057] The term "deformation resistance" as used herein means a power per unit area necessary
for deforming a material. More specifically, the "deformation resistance" indicates
K
f determined as K
f=p
w/a
w from a power p
w during working at a stain rate 10 s
-1 and a contact area a
w perpendicular to the power (hereinafter, the "deformation resistance" is used in
the same meaning).
[0058] The deformation resistance measured in this way is defined as a strength at 600°C
(high-temperature strength) and plotted with respect to the Mo amount (see, Fig. 7).
A higher deformation resistance is indicative of higher strength and in turn less
wearing and is therefore preferred.
According to Fig. 7, the high-temperature strength increases with an increase of the
Mo amount. In particular, when the Mo amount is more than 1.08 mass% (corresponding
to the content in JIS SKD61), the increase of high-temperature strength enables obtaining
a relatively high degree of high-temperature strength (>930 MPa). According to Fig.
7, when the Mo amount is from 1.25 to 3 mass%, the increase of high-temperature strength
becomes gentle, and when the Mo amount is 3 mass% or more, the increase of high-temperature
strength is saturated. Therefore, in the range of the MO amount of 1.25 mass% or less
where the increasing tendency of high-temperature strength becomes gentle, the Mo
amount is, for example, preferably more than 1.15 mass%, more preferably 1.20 mass%
or more.
Also, according to Fig. 7, the Mo amount can be 1.23 mass% or more for obtaining a
high-temperature strength of 950 MPa or more, and can be 2.5 mass% or more for obtaining
970 MPa or more. However, an Mo amount of 3 mass% or more incurs a significant rise
in cost. Therefore, in view of cost reduction, the Mo amount is preferably less than
2.99 mass%, more preferably 2.80 mass% or less, still- more preferably 2.50 mass%
or less.
(Example 5: Examination of V Amount)
[0059] The preferred V amount was examined and is described below by referring to Fig. 8.
Fig. 8 shows the impact value of a steel composed of 0.34 mass% of C, 0.49 mass% of
Si, 0.82 mass% of Mn, 5.75 mass% of Cr, 1.23 mass% of Mo and x mass% of V and treated
to 48 HRC, with respect to the V amount. In Fig. 8, the numerical values at each plotted
point are such that the numerical value on the upper side indicates the x value (mass%)
and the numerical value on the lower side indicates the impact value (J/cm
2). The production of specimen and the measurement of impact value were performed in
the same manner as in Example 2.
According to Fig. 8, when the V amount is varied in the range of from 0.1 to 1 mass%,
a good impact value (20 J/cm
2 or more) is obtained irrespective of the amount. According to Fig. 8, the deflection
point exists in the vicinity of a V amount of 0.30 mass% and in the vicinity of a
V amount of 0.70 mass%. Therefore, when the V amount is set to be from more than 0.30
mass% to less than 0.70 mass%, this is considered to contribute to improvement of
transformation behavior (hardenability) and realization of high strength of the steel
by the formation of a carbide. On the other hand, according to Fig. 8, when the V
amount is 0.30 mass% or less, the decrease of impact value is pronounced and when
the V amount is 0.70 mass% or more, the rise in material cost becomes an industrially
large problem in addition to the decrease of the impact value. Therefore, the V amount
is preferably 0.30<V<0.70 mass%.
According to Fig. 8, it is understood that the V amount can be 0.40 mass% or more
for obtaining an impact value of 31 J/cm
2 or more, and can be 0.50 mass% or more for obtaining 34 J/cm
2 or more.
(Example B)
[0060] Based on the examination results of Example A, invention steels and comparison steels
were produced and evaluated, and this is described below.
(Production of Specimen and Die-Casting Die)
[0061] With respect to Examples and Comparative Examples (Comparative Steel A11 is JIS SKD61)
shown in Tables 1 and 2, each steel species was melted in vacuum, and the melt was
cast in a casting mold to obtain an ingot of 6 ton.
The obtained ingot was subjected to a homogenization treatment at 1,240°C. Thereafter,
a rectangular block having a cross-section of 310 mm x 660 mm was produced by hot
forging.
Subsequently, the rectangular block was tempered at 700°C, then heated to 900°C and
gradually cooled, whereby the rectangular block was softened to a hardness of 90 to
97 HRB. From the resulting rectangular block, a die-casting die of about 700 kg was
machined out.
This die-casting die was heated to 1,030°C in vacuum and after holding for 1 hour,
quenched by spraying a nitrogen gas. The die-casting die was then treated to about
42 HRC through tempering at 5 80 to 610°C.
After the thermal treatment, various specimens were cut out from the die-casting die.
Also, the die-casting die was subjected to finish machining, whereby a die-casting
die of about 650 kg was produced.
[0062]
[0063]
(Measurement and Examination of Basic Properties)
[0064] Using the specimens cut out from the die-casting die, basic properties (high-temperature
strength, thermal conductivity, impact value, corrosion resistance, cost) were measured
and examined.
The high-temperature strength was measured as follows. A specimen of φ 14 mm x 21
mm was cut out from the die-casting die. The obtained specimen was heated to 600°C
at 5°C/s and after holding for 100 s, then measured for the deformation resistance
by working it at a strain rate of 10 s
-1. The results are shown in Table 3.
The thermal conductivity was measured as follows. A specimen of φ 10 mm x 2 mm was
cut out from the die-casting die, and the thermal conductivity of the obtained specimen
was measured at room temperature by a laser flash method. The results are shown in
Table 3.
The impact value was measured as follows. A JIS No. 3 impact test specimen of 10 mm
x 10 mm x 55 mm was cut out from the die-casting die, and the impact value of the
specimen was measured at room temperature. The results are shown in Table 3.
The corrosion resistance was measured as follows. A specimen was cut out from the
die-casting die, a hole was provided in the specimen, and industrial water at 30°C
was passed into the inside of the hole at 5.0 liter/min for 24 Hr. The generation
status of rust on the hole inner surface after passing water was evaluated with an
eye. The results are shown in Table 3.
[0065]
Table 3
Basic Properties |
|
No. |
High Temperature |
Thermal Conductivity |
Impact Value |
Corrosion Resistance |
Cost |
|
A01 |
A |
A (26.1) |
A (35) |
A |
A |
|
A02 |
A |
A (26.2) |
A (32) |
A |
A |
|
A03 |
A |
A (26.3) |
A (30) |
A |
A |
|
A04 |
A |
A (26.1) |
A (27) |
A |
A |
|
A05 |
A |
A (26.4) |
A (36) |
B |
A |
|
A06 |
A |
A (26.5) |
A (31) |
B |
A |
|
A07 |
A |
A (26.2) |
A (34) |
A |
A |
|
A08 |
A |
A (26.3) |
A (35) |
A |
A |
|
A09 |
A |
A (26.5) |
A (36) |
A |
A |
|
A10 |
A |
A (27.9) |
A (36) |
A |
A |
|
A11 |
A |
A (27.8) |
A (33) |
A |
A |
|
A12 |
A |
A (27.7) |
A (31) |
A |
A |
|
B01 |
A |
A (27.9) |
A (33) |
A |
A |
|
B02 |
A |
A (27.8) |
A (34) |
A |
A |
Invention Steel |
B03 |
A |
A (27.8) |
A (34) |
A |
A |
B04 |
A |
A (27.2) |
A (35) |
A |
A |
B05 |
A |
A (26.7) |
A (33) |
A |
A |
B06 |
A |
A (26.2) |
A (32) |
A |
A |
|
C01 |
A |
A (27.8) |
A (36) |
A |
A |
|
C02 |
A |
A (27.8) |
A (35) |
A |
A |
|
C03 |
A |
A (27.8) |
A (35) |
A |
A |
|
C04 |
A |
A (28.0) |
A (34) |
A |
A |
|
D01 |
A |
A (26.4) |
A (34) |
A |
A |
|
D02 |
A |
A (26.3) |
A (35) |
A |
A |
|
D03 |
A |
A (26.2) |
A (33) |
A |
A |
|
D04 |
A |
A (26.3) |
A (34) |
A |
A |
|
E01 |
A |
A (27.9) |
A (26) |
A |
A |
|
E02 |
A |
A (27.8) |
A (25) |
A |
A |
|
E03 |
A |
A (27.8) |
A (21) |
A |
A |
|
E04 |
A |
A (27.8) |
A (21) |
A |
A |
|
E05 |
A |
A (27.9) |
A (25) |
A |
A |
|
E06 |
A |
A (27.8) |
A (26) |
A |
A |
|
A01 |
B |
A (27.1) |
B (19) |
C |
A |
|
A02 |
A |
B (25.2) |
B (14) |
C |
B |
|
A03 |
A |
B (25.7) |
A (32) |
B |
A |
Comparison Steel |
A04 |
A |
B (22.9) |
A (36) |
A |
A |
A05 |
A |
A (27.2) |
B (19) |
C |
A |
A06 |
A |
B (25.7) |
A (21) |
C |
A |
A07 |
A |
A (26.4) |
B (19) |
C |
A |
|
A08 |
A |
B (24.1) |
A (35) |
A |
A |
|
A09 |
B |
A (26.9) |
A (33) |
B |
A |
|
A10 |
A |
A (26.8) |
A (34) |
B |
B |
|
A11 |
A |
B (23.8) |
B (16) |
B |
A |
(Evaluation of Basic Properties)
[0066] The high-temperature strength was rated "good" (denoted by "A" in Table 3) when 920
MPa or more, and otherwise rated "bad" (denoted by "B" in Table 3). The thermal conductivity
was rated "good" (denoted by "A" in Table 3) when 26 W/m/K or more, and otherwise
rated "bad" (denoted by "B" in Table 3). The impact value was rated "good" (denoted
by "A" in Table 3) when more than 20 J/cm
2, and otherwise rated "bad" (denoted by "B" in Table 3): The corrosion resistance
was, based on the JIS SKD61 (Comparison Steel A11), rated "good" (denoted by "A" in
Table 3) when rust was less generated than that, rated "slightly bad" (denoted by
"B" in Table 3) when rust was generated equally, and rated "bad" (denoted by "C" in
Table 3) when rust was more generated than that.
The invention steels exhibited good properties in all items. Also, the machinability
of the invention steels was kept from becoming worse than the general-purpose die
steel (JIS SKD61). Incidentally, the machinability was evaluated by judging it from
the working efficiency and the wear damage of the cutting tool at the practical cutting
of the die-casting die. When a steel having poor machinability is cut, the cutting
tool is liable to cause locally abnormal wear or chipping, and this makes it unavoidable
to reduce the working efficiency due to frequent replacing of the cutting tool and
increase the cost due to use of a large number of cutting tools. The working efficiency
or wear damage of the cutting tool at the cutting of the invention steel was equal
to that when cutting the general-purpose steel, and it was confirmed in the practical
die working that the machinability of the invention steels is equal to that of the
general-purpose steel.
[0067] In the invention steels where the thermal conductivity exceeded 27 W/m/K, the Si
amount was 0.55 mass% or less (the Si amount was 0.52 mass% or less except for Invention
Steel A12), the Mn amount was from 0.81 to 1.04 mass% (from 0.81 to 0.95 mass% except
for Invention Steel A12, and from 0.81 to 0.84 mass% further except for Invention
Steel A11), and the Cr amount was from 5.55 to 5.74 mass% (from 5.63 to 5.74 mass%
except for Invention Steel A12, and from 5.71 to 5.74 mass% further except for Invention
Steel A11).
In the invention steels where the impact value was 34 J/cm
2 or more, the Mn amount was from 0.51 to 1.42 mass% (from 0.51 to 0.83 mass% except
for Invention Steel A05), the Cr amount was from 5.25 to 8.61 mass% (from 5.25 to
8.08 mass% except for Invention Steel A01), and the V amount was from 0.57 to 0.69
mass%.
[0068] On the other hand, in the case of Comparison Steel A11, rating was "C" in all items
except for high-temperature strength and cost. The specimen used was a specimen cut
out from a large die-casting die obliged to decrease in the quenching rate. Therefore,
particularly in Comparison Steel A11, a carbide of V was formed in a large amount
and the impact value was low.
Other comparison steels were better than Comparison Steel A11 (JIS SKD61) in some
evaluation items, but there was not a steel species where the rating was "A" in all
items.
[0069] For example, in Comparison Steel A01, the high-temperature strength was reduced due
to too little C. Also, the austenitic grain was coarsened at the quenching due to
too little V and the impact value was decreased. Furthermore, in Comparison Steel
A01, the corrosion resistance was bad due to relatively small contents of Cr and Mo.
In Comparison Steel A02, the amount of carbide became excessively large due to excess
C or excess V, and the impact value was decreased. Also, in Comparison Steel A02,
the thermal conductivity was reduced due to relatively large contents of Si and Mn.
Furthermore, in Comparison Steel A02, the corrosion resistance was bad due to relatively
small contents of Cr and Mo, and the cost was high due to excess V.
In Comparison Steel A03, the thermal conductivity was reduced due to excess Si.
In Comparison Steel A04, the thermal conductivity was reduced despite too little Si,
because the contents of Mn and Cr were relatively large.
[0070] In Comparison Steel A05, the effect of decreasing transformation temperature under
the condition of small cooling rate and refining microstructure was insufficient due
to too little Mn, and the impact value was decreased. Also, in Comparison Steel A05,
the corrosion resistance as bad because the Cr amount was relatively small.
In Comparison Steel A06, the thermal conductivity was reduced due to excess Mn. Also,
the impact value of Comparison Steel A06 was judged as good, but the value was barely
high enough to satisfy the evaluation standard. Furthermore, in Comparison Steel A06,
the C amount was large and in turn, a Cr carbide was formed in a large amount, as
a result, the amount of Cr as solid solution was decreased, giving rise to bad corrosion
resistance.
In Comparison Steel A07, the quenchability was insufficient due to too little Cr and
the impact value was decreased. Also, the corrosion resistance of Comparison Steel
A07 was bad due to too little Cr.
In Comparison Steel A08, the thermal conductivity was reduced due to excess Cr.
In Comparison Steel A09, the high-temperature strength was reduced due to too little
Mo.
In Comparison Steel A10, a significant rise in cost was incurred due to excess Mo.
In Comparison Steel A11, the thermal conductivity was reduced due to excess Si, and
the impact value was decreased due to too little Mn or excess V.
(Actual Machine Test Using Die-Casting Die)
[0071] An actual machine test using the die-casting die was performed as follows. The die-casting
die produced was mounted in a machine, and an aluminum alloy was cast. ADC12 was used
for the aluminum alloy, and the temperature of the melting and holding furnace was
set to 680°C. The weight of the die cast product was about 5 kg, and one cycle was
60 s. After casting 10,000 shots, the heat checking on the die surface and the corrosion
cracking of the internal cooling circuit were evaluated. Furthermore, evaluation was
made also on whether marked soldering or water leakage due to cracking of the internal
cooling circuit was generated until casting of 10,000 shorts was completed. The results
of the actual machine test are shown in Table 4. In Table 4, the thermal conductivity
and the impact value shown in Table 3 are directly inserted.
[0072]
Table 4
Result of Die Casting Test |
|
No. |
checking |
Heat Soldering |
Wear |
Cracking of Water Hole |
Cost |
Thermal Conductivity |
Impact Value |
Invention Steel |
A01 |
A |
A |
A |
A |
A |
A (26.1) |
A (35) |
A02 |
A |
A |
A |
A |
A |
A (26.2) |
A (32) |
A03 |
A |
A |
A |
A |
A |
A (26.3) |
A (30) |
A04 |
A |
A |
A |
A |
A |
A (26.1) |
A (27) |
A05 |
A |
A |
A |
B |
A |
A (26.4) |
A (36) |
A06 |
A |
A |
A |
B |
A |
A (26.5) |
A (31) |
A07 |
A |
A |
A |
A |
A |
A (26.2) |
A (34) |
A08 |
A |
A |
A |
A |
A |
A (26.3) |
A (35) |
A09 |
A |
A |
A |
A |
A |
A (26.5) |
A (36) |
A10 |
A |
A |
A |
A |
A |
A (27.9) |
A (36) |
A11 |
A |
A |
A |
A |
A |
A (27.8) |
A (33) |
A12 |
A |
A |
A |
A |
A |
A (27.7) |
A (31) |
B01 |
A |
A |
A |
A |
A |
A (27.9) |
A (33) |
B02 |
A |
A |
A |
A |
A |
A (27.8) |
A (34) |
B03 |
A |
A |
A |
A |
A |
A (27.8) |
A (34) |
B04 |
A |
A |
A |
A |
A |
A (27.2) |
A (35) |
B05 |
A |
A |
A |
A |
A |
A (26.7) |
A (33) |
B06 |
A |
A |
A |
A |
A |
A (26.2) |
A (32) |
C01 |
A |
A |
A |
A |
A |
A (27.8) |
A (36) |
C02 |
A |
A |
A |
A |
A |
A (27.8) |
A (35) |
C03 |
A |
A |
A |
A |
A |
A (27.8) |
A (35) |
C04 |
A |
A |
A |
A |
A |
A (28.0) |
A (34) |
D01 |
A |
A |
A |
A |
A |
A (26.4) |
A (34) |
D02 |
A |
A |
A |
A |
A |
A (26.3) |
A (35) |
D03 |
A |
A |
A |
A |
A |
A (26.2) |
A (33) |
D04 |
A |
A |
A |
A |
A |
A (26.3) |
A (34) |
E01 |
A |
A |
A |
A |
A |
A (27.9) |
A (26) |
E02 |
A |
A |
A |
A |
A |
A (27.8) |
A (25) |
E03 |
A |
A |
A |
A |
A |
A (27.8) |
A (21) |
E04 |
A |
A |
A |
A |
A |
A (27.8) |
A (21) |
E05 |
A |
A |
A |
A |
A |
A (27.9) |
A (25) |
E06 |
A |
A |
A |
A |
A |
A (27.8) |
A (26) |
Comparison Steel |
A01 |
B |
A |
C |
C |
A |
A (27.1) |
C (19) |
A02 |
C |
C |
A |
C |
C |
C (25.2) |
C (14) |
A03 |
B |
C |
A |
B |
A |
C (25.7) |
A (32) |
A04 |
B |
C |
A |
A |
A |
C (22.9) |
A (36) |
A05 |
C |
A |
A |
C |
A |
A (27.2) |
C (19) |
A06 |
B |
C |
A |
C |
A |
C (25.7) |
A (21) |
A07 |
B |
A |
A |
C |
A |
A (26.4) |
C (19) |
A08 |
B |
C |
A |
A |
A |
C(24.1) |
A (35) |
A09 |
A |
A |
C |
B |
A |
A (26.9) |
A (33) |
A10 |
A |
A |
A |
B |
C |
A (26.8) |
A (34) |
A11 |
C |
C |
A |
B |
A |
C (23.8) |
C (16) |
(Evaluation of Actual Machine Test)
[0073] The heat checking, soldering, wear and cracking of water hole were judged with an
eye and rated "good" when each was not generated (denoted by "A" in Table 4), rated
"slightly bad" when somewhat generated (indicated by "B" in Table 4), and rated "bad"
when generated (denoted by "C" in Table 4).
The invention steels exhibited good properties in all items, whereas the comparison
steels failed to satisfy the evaluation standard in any of items. This is because
the invention steels possessed the component composition above and were assured of
high thermal conductivity and high impact value, but the comparison steels did not
possess the above-described component composition and were low in the thermal conductivity
and/or impact value.
[0074] That is, in the invention steels, the thermal stress was small owing to high thermal
conductivity and heat checking hardly occurred. Also, in the case of the invention
steels, the high thermal conductivity suppressed overheating of the die, and soldering
between the aluminum alloy and the die was greatly reduced. Furthermore, wear by the
aluminum alloy injected at a high rate was negligible and responding to the highness
of high-temperature strength. In the case of the invention steels, corrosion of the
internal cooling circuit was not so much significant, and water leakage due to penetration
of a crack originated on the corroded part was not generated.
[0075] On the other hand, it is seen that Comparison Steels A01 to A10 (excluding Comparison
Steel A02) were on an improved trend as compared with JIS SKD61 (Comparison Steel
A11) but were inferior to the invention steels and Comparison Steel A02 was worse
than JIS SKD61 (Comparison Steel A11).
In the steel species where both the thermal conductivity and the impact value were
low (Comparison Steels A02 and A11), heat checking was readily generated. Also, in
the steel species having a low thermal conductivity (Comparison Steels A02, A03, A04,
A06, A08 and A11), soldering was frequently generated. In the steel species having
low corrosion resistance (Comparison Steels A01, A02, A05, A06 and A07), the corrosion
of the internal cooling circuit was quite serious, and cracks originated on the corroded
part were scattered in diffusely. In the steel species low in the internal high-temperature
strength (Comparison Steels A01 and A09), wear was conspicuous. Comparison Steel A10
had a high Mo content and was not a recommendable material in view of cost or resource
saving.
[0076] In particular, Comparison Steel A11 (JIS SKD61) was rated "bad" or "slightly bad"
in all items except for wear and cost, similarly to the evaluation of basic properties.
Comparison Steel A11 caused overheating of the die due to its low thermal conductivity
and allowed for frequent occurrence of soldering between the aluminum alloy and the
die. Also, many heat checks were generated, because the thermal conductivity was low
and in turn, the thermal stress was large.
[0077] The die used in this actual machine test is a die in a large size. The results of
this test revealed that despite a large size, the die using the invention steels can
have a high impact value and can be high in the thermal conductivity and the high-temperature
strength.
[0078] While the present invention is described in the foregoing pages, it should be understood
that the present invention is not limited to these embodiments by any means.
[0079] The hot work tool steel of the present invention and the steel product using the
same are superior in thermal conductivity to general-purpose die steel (JIS SKD61)
and assured of a higher impact value than that of general-purpose die steel while
maintaining machinability at a level equal to or higher than that of general-purpose
die steel and therefore, are industrially very valuable for die manufacturers and
die users.