U.S. patent application number 10/615193 was filed with the patent office on 2004-06-03 for thixocast fe-based alloy material and process for heating the same.
This patent application is currently assigned to Honda Giken Kogyo Kabushiki Kaisha. Invention is credited to Tsuchiya, Masayuki, Ueno, Hiroaki.
Application Number | 20040105776 10/615193 |
Document ID | / |
Family ID | 27329678 |
Filed Date | 2004-06-03 |
United States Patent
Application |
20040105776 |
Kind Code |
A1 |
Tsuchiya, Masayuki ; et
al. |
June 3, 2004 |
Thixocast Fe-based alloy material and process for heating the
same
Abstract
A thixocast Fe-based alloy material is provided, from which a
cast product having mechanical properties uniform over the whole
thereof can be produced. The Fe-based alloy material comprises 1.8%
by weight.ltoreq.C.ltoreq.2.5% by weight, 1.0% by
weight.ltoreq.Si.ltoreq.3.0% by weight, 0.1% by
weight.ltoreq.Mn.ltoreq.1.5% by weight, 0.5% by
weight.ltoreq.Ni.ltoreq.3.0% by weight, and as the balance, iron
(Fe) including inevitable impurities. The eutectic crystal amount
Ec is in a range of 10% by weight<Ec<50% by weight.
Inventors: |
Tsuchiya, Masayuki;
(Wako-shi, JP) ; Ueno, Hiroaki; (Wako-shi,
JP) |
Correspondence
Address: |
WESTERMAN, HATTORI, DANIELS & ADRIAN, LLP
1250 CONNECTICUT AVENUE, NW
SUITE 700
WASHINGTON
DC
20036
US
|
Assignee: |
Honda Giken Kogyo Kabushiki
Kaisha
Tokyo
JP
|
Family ID: |
27329678 |
Appl. No.: |
10/615193 |
Filed: |
July 9, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10615193 |
Jul 9, 2003 |
|
|
|
09508458 |
Mar 10, 2000 |
|
|
|
6616777 |
|
|
|
|
09508458 |
Mar 10, 2000 |
|
|
|
PCT/JP99/03794 |
Jul 14, 1999 |
|
|
|
Current U.S.
Class: |
420/9 |
Current CPC
Class: |
C22C 38/08 20130101;
B22D 17/007 20130101; C21D 6/008 20130101; C21D 6/00 20130101; C22C
38/02 20130101; C22C 38/04 20130101; C22C 37/10 20130101 |
Class at
Publication: |
420/009 |
International
Class: |
C22C 038/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 14, 1998 |
JP |
10-214828 |
Sep 8, 1998 |
JP |
10-253750 |
Nov 12, 1998 |
JP |
10-322565 |
Claims
What is claimed is
1. A thixocast Fe-based alloy material comprising 1.8% by
weight.ltoreq.C.ltoreq.2.5% by weight, 1.0% by
weight.ltoreq.Si.ltoreq.3.- 0% by weight, 0.1% by
weight.ltoreq.Mn.ltoreq.1.5% by weight, 0.5% by
weight.ltoreq.Ni.ltoreq.3.0% by weight, and as the balance, iron
(Fe) including inevitable impurities, wherein a eutectic crystal
amount Ec is in a range of 10% by weight<Ec<50% by
weight.
2. A thixocast Fe-based alloy material comprising 1.8% by
weight.ltoreq.C.ltoreq.2.5% by weight 1.0% by
weight.ltoreq.Si.ltoreq.3.0- % by weight 0.8% by
weight.ltoreq.Mn.ltoreq.1.5% by weight, and as the balance, iron
(Fe) including inevitable impurities, wherein a eutectic crystal
amount Ec is in a range 10% by weight<Ec<50% by weight.
3. A thixocast Fe-based alloy material, comprising carbon (C) of a
content in a range of 1.8% by weight.ltoreq.C.ltoreq.2.5% by
weight, silicon (Si) of a content in a range of 1.0% by
weight.ltoreq.Si.ltoreq.3.0% by weight, manganese (Mn) of a content
in a range of 0.6% by weight.ltoreq.Mn.ltoreq.1.5% by weight, at
least one of nickel (Ni) of a content in a range of 0.2% by
weight.ltoreq.Ni.ltoreq.3.0% by weight and titanium (Ti) of a
content in a range of 0.05% by weight.ltoreq.Ti.ltoreq- .0.6% by
weight, the total sum of the Mn content, the Ni content and the Ti
content being equal to or larger than 0.8% by weight
(Mn+Ni+Ti.gtoreq.0.8% by weight), and the balance being iron (Fe)
including inevitable impurities, wherein a eutectic crystal amount
is in a range of 10% by weight<Ec<50% by weight.
4. A thixocast Fe-based alloy material according to claim 1, 2 or
3, wherein a solid phase rate R in a semi-molten state is set at
R>50%.
5. A process for heating a thixocast Fe-based alloy material having
a chilled structure into a semi-molten state in which solid and
liquid phases coexist, the process comprising setting an average
rate H.sub.R of heating to a point A.sub.1 in an Fe--C based
equilibrium diagram to be in a range of 0.5.degree.
C./sec.ltoreq.H.sub.R.ltoreq.6.0.degree. C./sec, and setting a
maximum temperature gradient T.sub.G of the inside of the Fe-based
alloy material per unit distance to be at T.sub.G.ltoreq.7.degree.
C./mm.
6. A process for heating a thixocast Fe-based alloy material
according to claim 5, further setting a sonic velocity Sv of said
Fe-based alloy material determined by an ultrasonic velocity
measurement to be at Sv.gtoreq.5,600 m/sec.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a thixocast Fe-based alloy
material, and a process for heating the same.
BACKGROUND ART
[0002] In carrying out a thixocasting process, a procedure is
employed which comprises heating an Fe-based alloy material into a
semi-molten state in which a solid phase (a substantially solid
phase and this term will also be applied hereinafter) and a liquid
phase coexist, pouring the semi-molten Fe-based alloy material
under a pressure into a cavity in a casting mold, and solidifying
the semi-molten Fe-based alloy material under a pressure.
[0003] There is such a known Fe-based alloy material having a
eutectic crystal amount Ec set in a range of 50% by
weight.ltoreq.Ec.ltoreq.70% by weight (see Japanese Patent
Application Laid-open No. 5-43978).
[0004] However, if the eutectic crystal amount Ec is set to be
equal to or larger than 50% by weight, the amount of graphite
precipitated is increased in such an Fe-based alloy material, and
hence, the mechanical properties of a cast product are
substantially equivalent to those of a cast product made by
casting. Therefore, with the conventional material, it is
impossible to achieve an intrinsic purpose of enhancing the
mechanical properties of the cast product made by the thixocasting
process.
[0005] In a quenched area such as a thinner portion in the cast
structure of the cast product, a portion which has been a spherical
solid phase is transformed into a mixed structure of austenite and
martensite. On the other hand, in a slowly cooled area such as a
thicker portion, a portion which has been a spherical solid phase
is transformed into a pearlite structure. Portions which have been
liquid phases in both the areas are transformed into a ledeburite
structure (a chilled structure).
[0006] If such a cast product is subjected to a thermal treatment,
the following problem also arises: Graphite is finely precipitated
in the quenched area, while it is precipitated in a coalesced
manner in the slowly cooled area. As a result, the mechanical
properties of both the areas are different from each other. For
this reason, it is impossible to produce a cast product having
mechanical properties uniform over the whole thereof.
[0007] Further, in the thixocasting process, the temperature of the
semi-molten Fe-based alloy material, namely, the casting
temperature is low as compared with the temperature of a molten
metal. Therefore, when a cast product having a smaller thickness or
having a complicated shape is produced by casting, the semi-molten
Fe-based alloy material is cooled rapidly by the casting mold, and
as a result, a portion which has been a liquid phase has a chilled
structure having a low toughness. The chilled structure is liable
to become a starting point for cracking on the solidification and
shrinkage of the material, which is undesirable. Therefore, a
measure to form an inner wall of a casting mold from a carbon
material such as graphite is employed to moderate the quenching of
the material. However, the following problem is encountered by
utilizing the thixocasting process: The carbon material is worn
violently and for this reason, the replacement of the casting mold
must be performed frequently, which is uneconomic, and moreover,
which results in a reduced productivity.
[0008] On the other hand, if the stability and productivity of
components and metallographic structure and the like of the
Fe-based alloy material are taken into consideration, it is optimal
to produce such material by a continuous casting process. In the
continuous casting process, however, the cooling rate of the
Fe-based alloy material is high, and for this reason, a chilled
structure may be produced in the material in some cases. When such
an Fe-based alloy material is heated, the following problem arises:
The temperature gradient of the inside of the material is increased
depending on heating conditions, whereby cracks are produced in the
material, and the material cannot be heated to a target temperature
during induction-heating.
DISCLOSURE OF THE INVENTION
[0009] Accordingly, it is an object of the present invention to
provide a thixocast Fe-based alloy material of the above-described
type, from which a cast product having mechanical properties which
are more excellent than those of a cast product made by casting,
and which are uniform over the whole of the cast product, can be
produced.
[0010] To achieve the above object, according to the present
invention, there is provided a thixocast Fe-based alloy material
comprising
[0011] 1.8% by weight.ltoreq.C.ltoreq.2.5% by weight,
[0012] 1.0% by weight.ltoreq.Si.ltoreq.3.0% by weight,
[0013] 0.1% by weight.ltoreq.Mn.ltoreq.1.5% by weight,
[0014] 0.5% by weight.ltoreq.Ni.ltoreq.3.0% by weight, and
[0015] as the balance, iron (Fe) including inevitable impurities,
and wherein a eutectic crystal amount Ec is in a range of 10% by
weight<Ec<50% by weight.
[0016] A semi-molten Fe-based alloy material having liquid and
solid phases coexisting therein is prepared by subjecting the
Fe-based alloy material having the above composition to a heating
treatment. In this semi-molten Fe-based alloy material, the liquid
phase produced by a eutectic melting has a large latent heat. As a
result, in the course of solidification of the semi-molten Fe-based
alloy material, the liquid phase is supplied in a sufficient amount
around the solid phase in response to the solidification and
shrinkage of the solid phase, and is then solidified. Therefore,
the generation of voids of a micron order in the cast product is
prevented. In addition, the amount of graphite precipitated can be
reduced by setting the eutectic crystal amount Ec in the
above-described range. Thus, it is possible to enhance the
mechanical properties, i.e., the tensile strength, the Young's
modulus, the fatigue strength and the like of the cast product. In
the Fe-based alloy material with the eutectic crystal amount Ec in
the above-described range, it is possible to lower the casting
temperature of the Fe-based alloy material, thereby providing the
prolongation of the life of a casting mold.
[0017] However, if the eutectic crystal amount Ec is equal to or
smaller than 10% by weight, the casting temperature of the Fe-based
alloy material approximates to a liquidus temperature due to the
small eutectic crystal amount Ec. Therefore, a heat load of a
material transporting equipment to a pressure casting apparatus is
high, thereby making it impossible to carry out the thixocasting.
On the other hand, a disadvantage raised when Ec.gtoreq.50% by
weight is as described above.
[0018] In the above-described composition, manganese (Mn) is a
cementite and austenite producing element, and nickel (Ni) is an
austenite producing element. Therefore, Mn and Ni inhibit the
slowly cooled area from being transformed into a pearlite
structure. Thus, the cast structure of the entire cast product is
such that a portion which has been a solid phase is transformed
into a mixed structure of austenite and martensite, and a portion
which has been a liquid phase is transformed into a ledeburite
structure.
[0019] By subjecting such a cast product into a predetermined
thermal treatment, a cast product having a uniformly thermally
treated structure with fine graphite dispersed in a mixed structure
of ferrite and pearlite is produced. This cast product has
mechanical properties uniform over the whole thereof.
[0020] In the above-described composition, carbon (C) and silicon
(Si) participate in the eutectic crystal amount, and the C content
and the Si content are set in the above-described ranges to control
the eutectic crystal amount in the above-described range. However,
if the C content is smaller than 1.8% by weight, the casting
temperature must be high, even if the Si content is increased to
increase the eutectic crystal amount. Therefore, the advantage of
the thixocasting is degraded. On the other hand, if C>2.5% by
weight, the amount of graphite is increased. For this reason, the
mechanical properties of the cast product is degraded, and the
eutectic crystal amount is increased and hence, the handlability of
the semi-molten Fe-based alloy material is deteriorated. If the Si
content is smaller than 1.0% by weight, the casting temperature is
raised as the case where the C content is smaller than 1.8% by
weight. On the other hand, if Si>3.0% by weight, silico-ferrite
is produced and for this reason, the mechanical properties of the
cast product cannot be enhanced.
[0021] Manganese (Mn) functions as a deoxidizing agent and is
required for producing cementite. However, if the Mn content is
smaller than 0.1% by weight, the deoxidizing effect is smaller and
for this reason, defects due to inclusion of an oxide caused by the
oxidation of the molten metal and due to bubbles are liable to be
produced. On the other hand, if Mn>1.5% by weight, the amount of
cementite [(FeMn).sub.3C] crystallized is increased. For this
reason, it is difficult to finely divide the large amount of
cementite by a thermal treatment, resulting in a reduced toughness
and a reduced cutting property of a cast product.
[0022] Nickel (Ni) is an austenite producing element, as described
above, and has an effect which allows austenite to exist in a very
small amount at normal temperature to enclose impurities in the
austenite, thereby enhancing the toughness. To provide such effect,
it is necessary to set the Ni content at about 1% by weight.
However, if the Ni content is smaller than 0.5% by weight, the
addition of nickel is meaningless. On the other hand, if Ni>3.0%
by weight, a matrix is transformed into a martensite structure with
an increased hardness in the course of cooling following a
cementite-eliminating thermal treatment.
[0023] It is another object of the present invention to provide a
thixocast Fe-based alloy material of the above-described type,
wherein the generation of cracks in a thin cast product and the
like can be avoided.
[0024] To achieve the above object, according to the present
invention, there is provided a thixocast Fe-based alloy material
comprising
[0025] 1.8% by weight.ltoreq.C.ltoreq.2.5% by weight
[0026] 1.0% by weight.ltoreq.Si.ltoreq.3.0% by weight
[0027] 0.8% by weight.ltoreq.Mn.ltoreq.1.5% by weight, and
[0028] as the balance, iron (Fe) including inevitable
impurities,
[0029] and wherein a eutectic crystal amount Ec being in a range of
10% by weight<Ec<50% by weight.
[0030] When a thixocasting is carried out using the Fe-based alloy
material having the above composition and using a conventional
casting mold, a portion which has been a solid phase is transformed
into a mixed structure of austenite and martensite in the entire
thin cast product due to the presence of Mn which is an austenite
producing element, and a portion which has been a liquid phase is
transformed into a ledeburite structure. In this way, the toughness
of the entire structure is enhanced by the austenite remaining in
the portion which has been the solid phase. Therefore, in the thin
cast product and the like, the generation of cracks due to the
solidification and shrinkage is avoided. In addition, it has been
made clear that if the above Fe-based alloy material is used, the
pearlite transformation of a thick portion cooled at a low speed in
the cast product can be inhibited to ensure that austenite remains
in the portion which has been the solid phase.
[0031] In the alloy composition of this material, manganese (Mn) is
an austenite producing element and has an effect of permitting
austenite to remain in the portion which has been the solid phase,
as described above. If the Mn content is smaller than 0.8% by
weight, the amount of austenite remaining in the portion which has
been the solid phase is insufficient, and the amount of austenite
crystallized in ledeburite presenting a chilled structure is also
insufficient. On the other hand, if Mn>1.5% by weight, the
amount of cementite [(FeMn).sub.3C] precipitated in ledeburite is
increased, resulting in reduced toughness and cutting property of a
product. Mn also has a function as a deoxidizing agent.
[0032] The reason why the eutectic crystal amount Ec, the C content
and the Si content are limited in the Fe-based alloy material is
the same as described above.
[0033] In addition, according to the present invention, there is
provided a thixocast Fe-based alloy material, comprising carbon (C)
of a content in a range of 1.8% by weight.ltoreq.C.ltoreq.2.5% by
weight, silicon (Si) of a content in a range of 1.0% by
weight.ltoreq.Si.ltoreq.3.0% by weight, manganese (Mn) of a content
in a range of 0.6% by weight.ltoreq.Mn.ltoreq.1.5% by weight, at
least one of nickel (Ni) of a content in a range of 0.2% by
weight.ltoreq.Ni.ltoreq.3.0% by weight and titanium (Ti) of a
content in a range of 0.05% by weight.ltoreq.Ti.ltoreq- .0.6% by
weight, the total sum of the Mn content, the Ni content and the Ti
content being equal to or larger than 0.8% by weight
(Mn+Ni+Ti.gtoreq.0.8% by weight), and the balance of iron (Fe)
including inevitable impurities, a eutectic crystal amount Ec being
in a range of 10% by weight<Ec<50% by weight.
[0034] If the Fe-based alloy material having the above composition
is used, the generation of cracks due to the solidification and
shrinkage can be further reliably avoided in a thin cast
product.
[0035] The reason why the eutectic crystal amount, the C content
and the Si content are limited in the Fe-based alloy material is
the same as described above.
[0036] Nickel (Ni), which is an austenite producing element, acts
to further promote the remaining of austenite and to enclose
impurities in the remaining austenite for harmlessness. Namely,
nickel (Ni) has an effect of dispersing the impurities reducing the
toughness into the austenite rich in toughness, thereby preventing
the impurities from influencing the mechanical properties. In
addition, nickel (Ni) also has an effect of preventing the pearlite
transformation of a portion cooled slowly such as a thick portion.
However, If the Ni content is smaller than 0.2% by weight, the
addition of nickel is meaningless. On the other hand, if the Ni
content is larger than 3.0% by weight, when the cast product is
subjected to a thermal treatment in order to ensure that cementite
disappears, thereby forming spherical fine graphite grains, the
precipitated graphite grains are agglomerated at points at points
to bring about a reduction in toughness. In addition, the matrix is
transformed into martensite by the cooling carried out after the
thermal treatment, resulting in an increased hardness. Further, the
addition of an excessive amount of nickel brings about an increase
in material cost.
[0037] Titanium (Ti) has an effect of finely dividing the crystal
grains in the solid phase to further enhance the toughness of the
cast product. However, if the Ti content is smaller than 0.05% by
weight, the addition of titanium is meaningless. On the other hand,
if Ti>0.6% by weight, TiC is precipitated and for this reason,
the cutting property is reduced and the flowability of the molten
metal is reduced, resulting in the generation of casting
defects.
[0038] The lower limit value of the Mn content may be decreased
down to 0.6% by weight, lower than that of the Fe-based alloy
material, because of the containment of titanium (Ti) and/or nickel
(Ni). The reason why the upper limit value of the Mn content is
limited is the same as described above.
[0039] Even in a casting process by casting, it is possible to
allow austenite to remain, but for this purpose, the cooling rate
must be managed extremely severely. According to the present
invention, the remaining of austenite in a portion which has been a
solid phase has been realized in the thixocasting process by
specifying the total amount of the Mn content and the Ni and Ti
contents (or the Ni or Ti content). A lower limit value of the
total amount of the Mn content and the Ni and Ti contents (or the
Ni or Ti content), 0.8% by weight, is a condition for providing the
above-described effect without being influenced by the cooling
rate.
[0040] It is desirable that the solid phase rate R in the
semi-molten Fe-based alloy material in the thixocasting process is
larger than 50%. This makes it possible to shift the casting
temperature to a lower level to prolong the life of a pressure
casting apparatus. If the solid phase rate R is equal to or smaller
than 50%, the amount of the liquid phase is increased. For this
reason, when a short columnar semi-molten Fe-based alloy material
is transported in a standing state, the self-standing property
thereof is degraded, and the handlability thereof is also
degraded.
[0041] Further, it is an object of the present invention to provide
a heating process, by which a thixocast Fe-based alloy material
having a chilled structure can be heated into a semi-molten state
without generation of cracks in the material.
[0042] To achieve the above object, according to the present
invention, there is provided a process for heating a thixocast
Fe-based alloy material having a chilled structure into a
semi-molten state in which solid and liquid phases coexist, wherein
the average rate H.sub.R of heating to a point A.sub.1 in an Fe--C
based equilibrium diagram is set in a range of 0.5.degree.
C./sec.ltoreq.H.sub.R.ltoreq.6.0.degree. C./sec, and the maximum
temperature gradient T.sub.G of the inside of the Fe-based alloy
material per unit distance is set at T.sub.G.ltoreq.7.degree.
C./mm.
[0043] The average rate H.sub.R of heating to a point A.sub.1 and
the maximum temperature gradient T.sub.G are specified as described
above, the cracking due to the heating of the Fe-based alloy
material having the chilled structure can be prevented, and the
oxidation of the material and the coalescence of crystal grains
cannot occur. After the temperature exceeds the point A.sub.1, the
heating rate is increased to effect the decomposition of dendrite
and the spheroidization of the solid phase. At this time, a
.gamma.-phase appears in the Fe-based alloy material, resulting in
an enhanced toughness of the material. Therefore, even if the
heating rate is increased, cracks cannot be produced in the
Fe-based alloy material.
[0044] Both of 6.0.degree. C./sec which is an upper limit value for
the average heating rate H.sub.R and 7.degree. C./mm which is an
upper limit value for the maximum temperature gradient T.sub.G are
limit values for preventing the generation of cracks due to the
heating. If the average heating temperature H.sub.R is lower than
0.5.degree. C./sec, problems of a reduction in producibility of a
cast product, the coalescence of the solid phases and the oxidation
of the material surface arise.
[0045] The Fe-based alloy material which is the subject of the
present invention is not limited to a material produced by a
continuous casting process, and may be a material produced by
casting and having a chilled structure.
[0046] To determine whether the Fe-based alloy material has a
chilled structure, it is a common practice to observe the material
by a metal microscope, but it is convenient to use an ultrasonic
velocity measuring process which is one of non-destructive
inspecting processes for a metal. The sonic velocity Sv measured by
the ultrasonic velocity measuring process is in a range of 5,800
m/sec.ltoreq.Sv.ltoreq.6,000 m/sec in a case of a steel. On the
other hand, the reviews by the present inventors have made it clear
that in a thixocast graphite-crystallized Fe-based alloy material,
a flake-formed graphite phase is reflected as a defect to a
measured value of sonic velocity and hence, the sonic velocity Sv
assumes a low value in a range of 5,100
m/sec.ltoreq.Sv.ltoreq.5,450 m/sec, but in an Fe-based alloy
material having a chilled structure, the sonic velocity assumes a
value near that of a steel due to non-precipitation of graphite.
Therefore, it can be determined from such a difference between the
sonic velocities that if the sonic velocity Sv measured for the
Fe-based alloy material by the ultrasonic velocity measuring
process is a value.gtoreq.5,600 m/sec, this material is an Fe-based
alloy material having a chilled structure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] FIG. 1 is a sectional view of a pressure casting
apparatus;
[0048] FIG. 2 is a graph showing the relationship between C and Si
contents and a eutectic crystal amount Ec;
[0049] FIG. 3 is a graph showing the relationship between a heating
temperature and a solid phase rate in correspondence to the C and
Si contents;
[0050] FIG. 4 is a diagram for explaining a cast product;
[0051] FIG. 5A is a photomicrograph of the texture showing a cast
structure of a tip end portion A of example (1) of a cast
product;
[0052] FIG. 5B is a photomicrograph of the texture showing a cast
structure of an intermediate portion B of example (1) of the cast
product;
[0053] FIG. 5C is a photomicrograph of the texture showing a cast
structure of a base end portion C of example (1) of the cast
product;
[0054] FIG. 6A is a photomicrograph of the texture showing a cast
structure of a tip end portion A of example (1a) of cast
product;
[0055] FIG. 6B is a photomicrograph of the texture showing a cast
structure of an intermediate portion B of example (1a) of the cast
product;
[0056] FIG. 6C is a photomicrograph of the texture showing a cast
structure of a base end portion C of example (1a) of the cast
product;
[0057] FIG. 7A is a photomicrograph of the texture showing a first
example of a thermally treated structure in the base end portion C
of example (1) of the cast product;
[0058] FIG. 7B is a photomicrograph of the texture showing a first
example of a thermally treated structure in the base end portion C
of example (1a) of the cast product;
[0059] FIG. 8A is a photomicrograph of the texture showing a second
example of a thermally treated structure in the base end portion C
of example (1) of the cast product;
[0060] FIG. 8B is a photomicrograph of the texture showing a second
example of a thermally treated structure in the base end portion C
of example (1a) of the cast product;
[0061] FIG. 9A is a photomicrograph of the texture showing a third
example of a thermally treated structure in the base end portion C
of example (1) of the cast product;
[0062] FIG. 9B is a photomicrograph of the texture showing a third
example of a thermally treated structure in the base end portion C
of example (1a) of the cast product;
[0063] FIG. 10 is a sectional view of a pressure casting
apparatus;
[0064] FIG. 11 is a plan view of an oil pump cover;
[0065] FIG. 12 is a view of a first example of the oil pump
cover;
[0066] FIG. 13 is a view of a second example of the oil pump
cover;
[0067] FIG. 14 is a view of a third example of the oil pump
cover;
[0068] FIG. 15 is a photomicrograph of the texture showing the
first example of a metallographic structure of the oil pump cover;
and
[0069] FIG. 16 is a photomicrograph of the texture showing the
second example of a metallographic structure of the oil pump
cover;
[0070] FIG. 17 is an Fe--C based equilibrium diagram;
[0071] FIG. 18 is a photomicrograph of the texture showing the
metallographic structure of an Fe-based alloy material having a
chilled structure;
[0072] FIG. 19 is a photomicrograph of the texture showing the
metallographic structure of an Fe-based alloy material having no
chilled structure;
[0073] FIG. 20 is a sectional view of the Fe-based alloy
material;
[0074] FIG. 21 is a graph showing the relationship between the
heating time and the temperature of the Fe-based alloy
material;
[0075] FIG. 22 is a graph showing the relationship between the
average temperature of the Fe-based alloy material having the
chilled structure and the temperature difference;
[0076] FIG. 23 is a graph showing the relationship between the
average heating rate and the maximum temperature gradient;
[0077] FIG. 24 is a graph showing the relationship between the
average temperature of the Fe-based alloy material having no
chilled structure and the temperature difference; and
[0078] FIG. 25 is a graph showing the relationship between the
ultrasonic velocity and the maximum temperature gradient.
BEST MODE FOR CARRYING OUT THE INVENTION
[0079] Embodiment I
[0080] A pressure casting apparatus 1 shown in FIG. 1 is used to
produce a cast product by casting by using an Fe-based alloy
material and utilizing a thixocasting process. The pressure casting
apparatus 1 includes a stationary die 2 and a movable die 3 which
have vertical mating surfaces 2a and 3a, respectively, so that a
cast product forming cavity 4 is defined between both the mating
surfaces 2a and 3a. A chamber 6 is defined in the stationary die 2,
so that a columnar semi-molten Fe-based alloy material 5 is placed
horizontally in the chamber 6. The chamber 6 communicates with a
base end of the cavity 4 through a truncated bore 7 and a gate 8. A
sleeve 9 is horizontally mounted to the stationary die 2 to
communicate with the chamber 6. A pressing plunger 10 is slidably
received in the sleeve 9, so that it is inserted into and removed
out of the chamber 6. The sleeve 9 has a material inlet 11 in an
upper portion of a peripheral wall thereof. Each of the stationary
and movable dies 2 and 3 is formed of a Cu--Be based alloy as a
copper-based alloy. The copper-based alloy which may be used is a
Cu--Cr based alloy, Cu--Ni based alloy and the like. Pure copper
may be utilized as a die forming material.
[0081] FIG. 2 shows the relationship between the C and Si contents
and a eutectic crystal amount Ec in an Fe-based alloy material. In
FIG. 2, a 10% by weight eutectic crystal line with a eutectic
crystal amount Ec of 10% by weight exists adjacent a high
C-concentration side of a solidus, and a 50% by weight eutectic
crystal line with a eutectic crystal amount Ec of 50% by weight
exists adjacent a low C-concentration side of a 100% by weight
eutectic crystal line with a eutectic crystal amount Ec of 100% by
weight. Three lines between the 10% by weight eutectic crystal line
and the 50% by weight eutectic crystal line are 20, 30 and 40% by
weight eutectic crystal lines in order from the 10% by weight
eutectic crystal line, respectively.
[0082] For the composition range of the Fe-based alloy material,
the eutectic crystal amount Ec is in a range of 10% by
weight<Ec<50% by weight and therefore, in a range between the
10% by weight eutectic crystal line and the 50% by weight eutectic
crystal line. However, compositions on the 10% by weight eutectic
crystal line and the 50% by weight eutectic crystal line are
excluded. In addition, the C content is in a range of 1.8% by
weight.ltoreq.C.ltoreq.2.5% by weight and the Si content is in a
range of 1.0% by weight.ltoreq.Si.ltoreq.3.0% by weight. Hence,
when the C content is taken on an X axis, and the Si content is
taken on a Y axis in FIG. 2, the composition range of the Fe-based
alloy material is in a range represented by a substantially
hexagonal figure formed by connecting a coordinate (2.08, 1.0)
point a.sub.1, a coordinate (2.5, 1.0) point a.sub.2, a coordinate
(2.5, 2.6) point a.sub.3, a coordinate (2.42, 3.0) point a.sub.4, a
coordinate (1.8, 3.0) point a.sub.5 and a coordinate (1.8, 2.26)
point a.sub.6 to one another. However, compositions at the points
a.sub.3and a.sub.4lying on the 50% by weight eutectic crystal line
and on a line segment b.sub.1 connecting the points a.sub.3 and
a.sub.4, and compositions at the points a.sub.1 and a.sub.6 lying
on the 10% by weight eutectic crystal line and on a line segment
b.sub.2 connecting the points a.sub.1 and a.sub.6 are excluded from
the compositions on a profile b of the figure indicating the limit
of the composition range.
[0083] It is desirable that the solid phase rate R of the
semi-molten Fe-based alloy material is larger than 50%. FIG. 3 is a
graph showing the relationship between a heating temperature and a
solid phase rate R for an Fe--C--Si based alloy. A line L1
corresponds to the case where the C and Si contents are 1.8% by
weight and 1.0% by weight which are lower limit values,
respectively, and a line L2 corresponds to the case where the C and
Si contents are 2.5% by weight and 3.0% by weight which are upper
limit values, respectively. It can be seen that if the C and Si
contents are smaller than the lower limit values, the casting
temperature must be considerably high in order to provide a solid
phase rate R higher than 50% by weight. Namely, at a casting
temperature set from the viewpoint of the durability of the
pressure casting apparatus and the like, the solid rate R of the
material is high and for this reason, casting defects due to a
filling failure or a cold shut are produced. On the other hand, if
the C and Si contents are higher than the upper limit values, the
solid phase rate R of the material is lower and for this reason,
the chilled structure is increased and cracks are liable to be
produced.
[0084] Table 1 shows the composition and the eutectic crystal
amount Ec for example (1) and comparative example (1a) of Fe-based
alloy material.
1 TABLE 1 Eutectic Fe-based Chemical constituent crystal alloy (%
by weight) amount Ec material C Si Mn Ni P S Fe (% by weight)
Example (1) 2.3 2.0 1.2 1.1 <0.04 <0.04 Balance 33
Comparative 2.3 2.0 0.2 -- <0.04 <0.04 Balance 33 Example
(1a)
[0085] Example (1) and comparative example (1a) are also-shown as
points (1) and (1a) in FIG. 2.
[0086] To produce a cast product by casting, example (1) was
subjected to an induction heating up to 1180.degree. C. which is a
casting temperature, thereby preparing a semi-molten Fe-based alloy
material having solid and liquid phases coexisting therein. The
solid phase rate R of this material was equal to 58%.
[0087] Then, the temperature of the stationary and movable dies 2
and 3 in the pressure casting apparatus 1 shown in FIG. 1 was
controlled, and the semi-molten Fe-based alloy material 5 was
placed into the chamber 6. Thereafter, the pressing plunger 10 was
operated to pour the Fe-based alloy material 5 into the cavity 4.
In this case, the pouring pressure for the semi-molten Fe-based
alloy material 5 was 36 MPa. Then, a pressing force was applied to
the semi-molten Fe-based alloy material 5 filled in the cavity 4 by
retaining the pressing plunger 10 at a terminal end of its stroke,
thereby solidifying the semi-molten Fe-based alloy material 5 under
such pressure to produce example (1) of a cast product 12 shown in
FIG. 4. Using comparative example (1a), example (1a) of the cast
product 12 was produced in a similar manner. However, the casting
temperature was set at 1180.degree. C.
[0088] In a cavity-correspondence portion 12a of the cast product
12, an area from a site B4 in the vicinity of agate-correspondence
portion 12b and nearer to a tip end of the cavity than the
gate-correspondence portion 12b to a base end c of the
cavity-correspondence portion 12a is a scrap S and hence, an area
from the site B4 to a tip end e of the cavity-correspondence 12a is
a product P.
[0089] Central portions of a tip end portion B1, an intermediate
portion B2 and a base end portion B3 in each of the products P of
both the cast products 12 were microscopically examined, whereby
their cast structures were examined to provide results in FIGS. 5A
to 5C for example (1) of the cast product 12 and in FIGS. 6A to 6C
for example (1a) of the cast product 12.
[0090] In example (1) of the cast product 12 shown in FIGS. 5A to
5C, those areas of the tip end portion B1, the intermediate portion
B2 and the base end portion B3, which had been a spherical solid
phase, were of a mixed structure of austenite and martensite, and
the areas which had been a liquid phase were of a ledeburite
structure.
[0091] In example (1a) of the cast product 12 shown in FIGS. 6A to
6C, those areas of the tip end portion B1 and the intermediate
portion B2 which had been a spherical solid phase were of a mixed
structure of austenite and martensite; those areas of the base end
portion B3 which had been spherical solid phase were of a pearlite
structure; and the areas which had been a liquid phase were of a
ledeburite structure.
[0092] In example (1) of the cast product 12 made using example
(1), the cast structure of the base end portion B3 was the same as
those of the tip end portion B1 and the intermediate portion B2,
notwithstanding that the base end portion B3 was slowly cooled by
the heat insulating effect of the scrap S. On the contrary, in
example (1a) of the cast product 12 made using comparative example
(1a), the base end portion B3 had a cast structure different from
those of the tip end portion B1 and the intermediate portion B2,
because the base end portion B3 was slowly cooled by the heat
insulating effect of the scrap S and no means for avoiding the slow
cooling effect was taken.
[0093] A plurality of test pieces including the base end portions
B3 were made from examples (1) and (1a) of the cast product 12.
Then, the test pieces were subjected to a thermal treatment.
Thereafter, the test pieces were microscopically examined for
examination of their thermally-treated structures to provide
results shown in FIGS. 7A, 7B to 9A and 9B.
[0094] FIGS. 7A and 7B show thermally treated structures provided
by subjecting the test pieces to a ledeburite eliminating thermal
treatment for 30 minutes at 900.degree. C. and for 60 minutes at
750.degree. C. FIG. 7A corresponds to the base end portion B3 of
example (1) of the cast product 12, and FIG. 7B corresponds to the
base end portion B3 of example (1a) of the cast product 12.
[0095] FIGS. 8A and 8B show thermally treated structures provided
by subjecting the test pieces to a ledeburite eliminating thermal
treatment for 30 minutes at 900.degree. C. FIG. 8A corresponds to
the base end portion B3 of example (1) of the cast product 12, and
FIG. 8B corresponds to the base end portion B3 of example (1a) of
the cast product 12.
[0096] FIGS. 9A and 9B show thermally treated structures provided
by subjecting the test pieces to a cementite spheroidizing thermal
treatment for 60 minutes at 800.degree. C. FIG. 9A corresponds to
the base end portion B3 of example (1) of the cast product 12, and
FIG. 9B corresponds to the base end portion B3 of example (1a) of
the cast product 12.
[0097] As is apparent from FIGS. 7A, 7B to 9A and 9B, fine graphite
grains having a grain size d equal to or smaller than 10 .mu.m were
precipitated in the base end portion B3 of example (1) of the cast
product 12. This applies to the tip end portion B1 and the
intermediate portion B2. As a result, example (1) of the cast
product 12 has mechanical properties uniform over the whole
thereof. On the other hand, coalesced graphite grains having a
grain size d larger than 10 .mu.m were precipitated in the base end
portion B3 of example (1a) of the cast product 12, but each of the
tip end portion B1 and the intermediate portion B2 was of a
thermally treated structure having fine graphite grains, as was the
base end portion B3 of example (1). As a result, the mechanical
properties of the tip end portion B1 and the intermediate portion
B2 in example (1a) of the cast product 12 are different from those
of the base end portion B3.
[0098] The graphite area rate, the hardness, the Charpy impact
value (toughness) and the Young's modulus in the base end portions
B3 of examples (1) and (1a) of the cast product 12 are as given in
Table 2. In this case, the graphite area rate was determined using
an image analysis device (IP-1000 PC made by Asahi Kasei, Co.) by
polishing the test pieces without etching thereof.
2TABLE 2 Base end portion of Graphite cast area rate Hardness
Charpy impact Young's product (%) HB value (J/cm.sup.2) modulus
(GPa) 4.3 153 9.0 180 4.3 162 7.0 180 4.1 260 7.8 183 4.1 285 5.5
183 3.0 192 8.0 188 2.5 298 2.1 193
[0099] As is apparent from Table 2, the base end portion B3 of
example (1) of the cast product 12 shown in each of FIGS. 7A, 8A
and 9A has excellent mechanical properties, as compared with the
base end B3 of example (1a) of the cast product 12 shown in FIGS.
7B, 8B and 9B.
[0100] Table 3 shows the composition and the eutectic crystal
amount Ec for examples (2) to (4) and comparative examples (2a) to
(4a)
3 TABLE 3 Eutectic Fe-based crystal alloy Chemical constituent (%
by weight) amount material C Si Mn Ni P S Fe Ec Example (2) 2.3 2.0
0.6 1.1 <0.04 <0.04 Balance 33 Example (3) 2.0 2.0 1.2 1.1
<0.04 <0.04 Balance 17 Example (4) 2.0 2.0 0.6 2.0 <0.04
<0.04 Balance 17 Comparative 2.0 2.0 0.6 -- <0.04 <0.04
Balance 17 Example (2a) Comparative 2.0 2.0 0.2 -- <0.04
<0.04 Balance 17 Example (3a) Comparative 2.0 2.0 0.6 3.1
<0.04 <0.04 Balance 17 Example (4a)
[0101] Examples (2) to (4) and comparative examples (2a) to (4a)
are given as points (2) to (4) and points (2a) to (4a) in FIG. 2,
respectively.
[0102] Examples (1) to (4) and comparative examples (1a) to (4a) of
the cast products 12 were produced using the above-described
examples (1) to (4) and comparative examples (1a) to (4a) in a
manner similar to the above-described manner. Each of example (1)
and other examples of the cast product 12 was subjected to an
annealing treatment for 30 minutes at 900.degree. C. and then
microscopically examined for examination of their thermally treated
structures.
[0103] Table 4 shows results of the above-described experiment. In
Table 4, "Cu" in the column of material of die means the
above-described Cu--Be based alloy, and "Fe" means an alloy tool
steel for a high-temperature die. Further, "O" in the column of
thermally treated structure means that the grain size d of graphite
grains is equal to or smaller than 10 .mu.m, and "X" means that the
grain size d of graphite grains is larger than 10 .mu.m.
4 TABLE 4 Casting Example temper- Thermally treated structure of
cast ature Material Tip end Intermediate Base end product (.degree.
C.) of die portion portion portion Scrap (1) 1180 Cu .largecircle.
.largecircle. .largecircle. .largecircle. Fe .largecircle.
.largecircle. .largecircle. X (2) 1180 Cu .largecircle.
.largecircle. .largecircle. .largecircle. Fe .largecircle.
.largecircle. .largecircle. X (3) 1200 Cu .largecircle.
.largecircle. .largecircle. .largecircle. Fe .largecircle.
.largecircle. .largecircle. X (4) 1200 Fe .largecircle.
.largecircle. .largecircle. X (1a) 1180 Cu .largecircle.
.largecircle. X X (2a) 1220 Fe .largecircle. X X X (3a) 1220 Fe X X
X X (4a) 1200 Fe graphite grains were agglomerated at points at
crystal grain boundary
[0104] In Table 4, in examples (1) to (3) of the cast product 12
produced using the stationary and movable dies 2 and 3 made of the
Cu--Be based alloy, the thermally treated structures of the
products P thereof are uniform, and moreover, the thermally treated
structures of the scraps S thereof are equivalent to those of the
products P due to the cooling promoting effect of the stationary
and movable dies 2 and 3. However, if the stationary and movable
dies 2 and 3 made of the above-described steel, including example
(4) of the cast product 12, are used, the cooling promoting effect
thereof is inferior to that of the dies made of the Cu--Be based
alloy and hence, graphite grains are precipitated in coalesced
forms in the scrap S.
[0105] In examples (1a) to (3a) of the cast product 12, an effect
of nickel (Ni) is not obtained, because the Fe-based alloy
materials (1a) to (3a) do not contain nickel (Ni). As a result, in
the cases of examples (1) and (2a) of the cast product 12, the
thermally treated structures of the products P are non-uniform over
the whole thereof. In the case of the example (3a) of the cast
product 12, coalesced graphite grains were dispersed over the whole
thereof. In the case of example (4a) of the cast product 12,
graphite grains were agglomerated at points at a crystal grain
boundary due to the Ni content of the Fe-based alloy material (4a)
larger than 3.0% by weight.
[0106] Embodiment II
[0107] FIG. 10 shows a pressure casting apparatus 1 used to produce
an oil pump cover by casting. In FIG. 10, the same components or
portions as those in the apparatus 1 shown in FIG. 1 are designated
by the same reference characters as in FIG. 1, and the detailed
description of them is omitted. A scrap portion 21 is connected to
an oil pump cover 20 shown in FIG. 11. In a cavity 4, a scrap
portion forming area 4b exists between an oil pump cover forming
area 4a and a gate 8. A movable die 3 is provided with a core 22
for forming a central bore 23 in the oil pump cover 20, and a
plurality of cores 25 for forming a plurality of bolt bores 24
around the central bore 23. Each of the stationary and movable dies
2 and 3 is formed of a steel such as JIS SKD61 and the like, but
may be formed of a copper-based alloy such as a Cu--Be based alloy,
a Cu--Cr based alloy, a Cu--Ni based alloy and the like, when it is
desired to enhance the cooling rate.
[0108] The relationship between the C and Si contents and the
eutectic crystal amount Ec in the Fe-based alloy material is in
accordance with FIG. 2.
[0109] Table 5 shows the composition and the eutectic crystal
amount Ec for examples (5) to (13) and comparative examples (5a) to
(10a).
5TABLE 5 Fe-based Eutectic crystal alloy Chemical constituent (% by
weight) amount Ec (% by material C Si Mn Ti Ni P S Fe Mn + Ni + Ti
weight) Example (5) 2.37 2.02 0.8 -- -- 0.008 0.006 Balance 0.8 37
Example (6) 2.28 1.96 0.6 -- 0.2 0.008 0.007 Balance 0.8 32 Example
(7) 2.37 1.97 0.65 0.16 -- 0.009 0.005 Balance 0.81 37 Example (8)
2.28 1.96 1.19 -- -- 0.008 0.005 Balance 1.19 32 Example (9) 2.24
2.02 0.72 -- 1.01 0.008 0.007 Balance 1.73 29 Example (10) 2.27
1.98 1.19 -- 1.1 0.01 0.005 Balance 2.29 31 Example (11) 2.3 1.96
0.67 0.52 1.11 0.009 0.005 Balance 2.3 33 Example (12) 2.38 1.99
0.6 -- 1.95 0.009 0.006 Balance 2.55 37 Example (13) 2.35 2.08 0.6
-- 2.02 0.008 0.005 Balance 2.62 36 Comparative 2.36 2.02 0.21 --
-- 0.01 0.005 Balance 0.21 36 Example (5a) Comparative 2.35 2.01
0.57 -- -- 0.014 0.005 Balance 0.57 36 Example (6a) Comparative 2.4
1.9 0.5 0.05 0.21 0.011 0.005 Balance 0.76 39 Example (7a)
Comparative 2.39 2.03 0.78 -- -- 0.012 0.005 Balance 0.78 38
Example (8a) Comparative 2.3 2 0.1 0.51 0.2 0.008 0.007 Balance
0.81 33 Example (9a) Comparative 2.33 2.01 0.11 0.31 0.39 0.012
0.005 Balance 0.81 36 Example (10a)
[0110] In FIG. 3, a line L3 indicates the relationship between the
heating temperature and the solid phase rate R in example 2.
[0111] Each of example (5) of the columnar Fe-based alloy material
having a diameter of 50 mm and a length of 65 mm and other examples
was heated into a semi-molten state to produce the oil pump cover
20 having cored holes 23 and 24 at nine points and having the
thinnest portion having a thickness of 2.5 mm using the pressure
casting apparatus 1 shown in FIG. 10. In this case, the preheating
temperature for the dies was set at 250.degree. C., and the
pressure maintaining time was set at 5 seconds.
[0112] Then, the presence or absence of cracks generated in each of
the oil pump covers 20 was examined by a red mark check.
[0113] Table 6 shows the casting temperature, the solid phase rate
R and the presence or absence of cracks for examples (5) to (13)
and examples (5a) to (10a) of the oil pump covers 20. Examples (5)
to (13) and examples (5a) to (10a) correspond to examples (5) to
(13) and comparative examples (5a) to (10a) given in Table 5,
respectively.
6TABLE 6 Presence or Casting Solid phase rate absence of Oil pump
cover temperature (.degree. C.) R (%) cracks (5) 1180 59 absence
(6) 1190 60 absence (7) 1190 55 absence (8) 1190 60 absence (9)
1190 60 absence (10) 1200 53 absence (11) 1190 66 absence (12) 1190
60 absence (13) 1180 59 absence (5a) 1190 56 presence (6a) 1190 56
presence (7a) 1180 58 presence (8a) 1180 57 presence (9a) 1200 55
presence (10a) 1200 56 presence
[0114] As is apparent from Table 6, no crack was generated in each
of examples (5) to (13), whereas cracks were generated in all of
examples (5a) to (10a). FIG. 12 is a view of the oil pump cover
free of cracks, and FIG. 13 is a view of the oil pump cover having
large fractures and hair cracks generated around the bolt bores.
FIG. 14 is a view of the oil pump cover having cracks generated by
being restrained by the two cores. As compared with the case where
manganese (Mn) is contained alone, the cracks were generated in
comparative examples (5a), (6a) and (8a), when the Mn content was
equal to or smaller than 0.78% by weight, whereas no crack was
generated in examples (5) and (8), because the Mn content was equal
to or larger than 0.8% by weight. Therefore, it can be seen that
when manganese (Mn) is contained alone, it is necessary to set the
Mn content at Mn.gtoreq.0.8% by weight.
[0115] In examples (6), (7), (9) and (11) to (13) in which the Mn
content was smaller than 0.8% by weight, but Mn.gtoreq.0.6% by
weight and Mn+Ni+Ti.gtoreq.0.8% by weight, no crack was generated,
whereas in comparative examples (9a) and (10a) in which
Mn+Ni+Ti.gtoreq.0.8% by weight and yet, the Mn content is smaller
than 0.6% by weight, cracks were generated. In this way, it can be
seen that when nickel (Ni) and/or titanium (Ti) were contained in
addition to manganese (Mn), the Mn content must be equal to or
larger than 0.6% by weight and the Mn+Ni+Ti content must be equal
to or larger than 0.8% by weight.
[0116] FIG. 15 is a photomicrograph of the texture showing the
metallographic structure of example (10) of the oil pump cover. In
FIG. 15, a black needle-shaped portion is martensite, a light gray
portion adjacent the black needle-shaped portion is austenite. The
portion of the mixed structure comprising martensite and austenite
is a portion which was a solid phase in the casting. A dark gray
portion around the portion which was the solid phase is ledeburite
comprising a eutectic crystal of austenite and cementite, and is a
portion which was a liquid phase in the casting.
[0117] FIG. 16 is a photomicrograph of the texture showing the
metallographic structure of example (6a) of the oil pump cover. In
FIG. 16, a black portion is a portion which was a solid phase in
the above-described casting, and such black portion has a pearlite
structure. A dark gray portion around the portion which was the
solid phase is ledeburite comprising a eutectic crystal of
austenite and cementite, and is a portion which was a liquid phase
in the casting. As can be seen from comparison of FIGS. 15 and 16
with each other, in example (10), austenite exists in the portion
which was the solid phase and hence, the entire example (10)
includes a large amount of austenite and has an excellent
toughness.
[0118] Embodiment III
[0119] An Fe--C (2% by weight) alloy material was selected as the
Fe-based alloy material. FIG. 17 is an Fe--C based equilibrium
diagram, wherein a point A.sub.1 of the Fe--C (2% by weight) alloy
material is 740.degree. C.
[0120] FIG. 18 shows the photomicrographic structure of a material
having such composition and produced by a continuous casting
process, namely, a continuously-cast material, wherein it can be
seen that this metallographic structure is a mixed structure
comprising dendrite and a chilled structure (a white portion). FIG.
19 shows the photomicrographic structure of a material having such
composition and produced by casting using a die, namely, a die-cast
material, wherein it can be seen that this metallographic structure
is a structure having a graphite phase precipitated in
dendrite.
[0121] Then, a columnar Fe-based alloy material 5.sub.0 having a
diameter D of 50 mm and a length L of 65 mm as shown in FIG. 20 was
fabricated from the continuously-cast material, and thermocouples
were embedded into one 5b of end surfaces and an outer peripheral
surface 5c of the material 5.sub.0, respectively. The position of
the thermocouple in the end surface 5a is a point E at a depth of 5
mm from the center O of the end surface, while the position of the
thermocouple in the outer peripheral surface 5b is a point F at a
depth of 5 mm from a bisected position in the direction of a
generating line. During heating of the material 5.sub.0, the
temperature of the point E is lowest, and this temperature is a
criterion in the casting process. Therefore, the point E is defined
as a casting reference-temperature point. The point F is a site
which is heated to the highest temperature in the induction heating
and hence, the point F is defined as the highest-temperature
point.
[0122] FIG. 21 shows one example of a temperature rise curve
provided when the Fe-based alloy material 5.sub.0 was subject to an
induction heating. In the induction heating, the heating rate is
controlled by an on-off control and hence, in the
highest-temperature point F intensively influenced by the
turning-on/off, the temperature is lowered slightly at the
off-time, but in the casting reference-temperature point E, the
temperature is raised substantially rectilinearly, because the
point E is less influenced by the turning-on/off. However, the
heating rate at the highest-temperature point F is larger than that
at the casting reference-temperature point E.
[0123] Therefore, the average value (H.sub.RE+H.sub.RF)/2 of the
heating rates H.sub.RE and H.sub.RF at the points E and F is
defined as the average heating rate H.sub.R, and the maximum
temperature gradient T.sub.G is defined as being equal to
.DELTA.Tmax/d (.degree. C./mm) from the maximum value .DELTA.Tmax
of the difference .DELTA.T between the temperatures at the points E
and F and the distance d between both the points E and F. The
relationship between the average value (H.sub.RE+H.sub.RF) as well
as the maximum temperature gradient T.sub.G and the cracking due to
the heating of the Fe-based alloy material 5.sub.0 was
examined.
[0124] The Fe-based alloy material 5.sub.0 was heated to
740.degree. C. (the point A.sub.1) at the average heating rate
H.sub.R set at 2.9.degree. C./sec, 4.7.degree. C./sec, 6.4.degree.
C./sec and 7.2.degree. C./sec. The relationship between the average
temperature of the material 5.sub.0 and the difference .DELTA.T
between the temperatures at the casting reference-temperature point
P and the highest-temperature point Q was examined, thereby
providing a result shown in FIG. 22. The term "average temperature"
as used herein means an average value (T.sub.E+T.sub.F)/2 of
temperatures T.sub.E and T.sub.F at the points E and F. The maximum
temperature gradient T.sub.G was calculated from a maximum value of
the temperature differences .DELTA.T and the distance d.apprxeq.34
mm between both the points E and F. The relationship between the
maximum temperature gradient T.sub.G and the average heating
temperature H.sub.R was examined, thereby providing a result shown
in FIG. 23. When the average heating temperature H.sub.R was set at
4.7.degree. C./sec in this heating test, cracks were not generated
in the Fe-based alloy material, but when the average heating rate
H.sub.R was set at 6.4.degree. C./sec, cracks were generated in the
Fe-based alloy material.
[0125] From such results, in the present invention, the average
heating rate H.sub.R to the point A.sub.1 is set at
H.sub.R.ltoreq.6.0.degree. C./sec, and the maximum temperature
gradient T.sub.G of the inside of the material per unit distance is
set at T.sub.G.ltoreq.7.degree. C./mm.
[0126] Then, for comparison, an Fe-based alloy material fabricated
from the die-cast material was heated to 740.degree. C. (the point
A.sub.1) at an average heating rate set at 11.74.degree. C./sec,
and the relationship between the average temperature of the
material and the difference .DELTA.T between the temperatures at
the casting reference-temperature point E and the
highest-temperature point F was examined, thereby providing a
result shown in FIG. 24. In this case, the maximum value .DELTA.T
max of the temperature differences .DELTA.T was 463.4.degree. C.
and hence, the maximum temperature gradient T.sub.G was 13.6, but
cracks were not generated in the material. This is attributable to
the absence of a chilled structure in the material.
[0127] B. Ultrasonic Velocity Measuring Test
[0128] Examples 1 to 4 of test pieces as shown in Table 7 were
fabricated from the continuously-cast material and the die-cast
material made of an Fe--C (2% by weight) alloy. Each of examples 1
to 4 was of a disk shape having a diameter of 50 mm and a thickness
of 30 mm. Examples 1 to 4 were subjected to the ultrasonic velocity
measurement. EGT1K made by Kusaka Rare Metal Co., was used as an
ultrasonic measuring apparatus, and the measurement of the sonic
velocity was carried out two times for each of examples 1 to 4 in a
state in which a probe of the ultrasonic measuring apparatus was
placed against the outer peripheral surface, the center of an end
surface and a point of the end surface corresponding to one half of
its radius. Results are shown in Table 7.
7 TABLE 7 Sonic velocity Sv (mm/sec) Outer periph- Center One Test
Measuring eral of end half of piece Material position surface
surface radius Example 1 Continuously-cast Measured 5887 5891 5872
material value 5872 5888 5880 Chilled structure: Average 5880 5890
5876 presence value Total 5882 average value Example 2 Mold-cast
material Measured 5861 5869 5862 Chilled structure: value 5856 5820
5850 presence Average 5859 5845 5856 value Total 5853 average value
Example 3 Mold-cast material Measured 5267 5132 5197 Chilled
structure: value 5269 5123 5198 absence Average 5268 5128 5198 Long
flake-formed value graphite: presence Total 5198 average value
Example 4 Mold-cast material Measured 5457 5280 5396 Chilled
structure: value 5458 5314 5401 absence Average 5458 5297 5399
Short flake-formed value graphite: presence Total 5384 average
value
[0129] Then, each of examples 1 to 4 was subjected to a heating
test at various maximum temperature gradients T.sub.G, whereby it
was observed whether cracks were generated, thereby providing a
result shown in FIG. 25. The sonic velocities for a spherical
graphite cast iron and a steel are also shown in FIG. 25. As is
apparent from FIG. 25, it can be seen that the ultrasonic velocity
measurement is an effective means for determining whether the
material has a chilled structure, because the sonic velocity for
examples 1 and 2 having the chilled structure is remarkably high,
as compared with examples 3 and 4 having no chilled structure and
an FCD material. It was confirmed that cracks was generated due to
the heating at the temperature gradient T.sub.G equal to or higher
than 7.degree. C./mm in examples 1 and 2 having the sonic velocity
Sv equal to or higher than 5,600 m/sec.
[0130] C. Casting Test
[0131] Example 1 shown in Table 7 was heated to the point A.sub.1
at an average heating rate H.sub.R equal to 2.9.degree. C./sec and
a maximum temperature gradient T.sub.G equal to 4.5.degree. C./mm,
and example 2 was heated to the point A.sub.1 at an average heating
rate H.sub.R equal to 4.7.degree. C./sec and a maximum temperature
gradient T.sub.G equal to 6.1.degree. C./mm. Subsequently, they
were heated to about 1,200.degree. C. into their semi-molten
states. Then, examples 1 and 2 in the semi-molten states were
placed into a pressure casting apparatus 1 shown in FIG. 1, where
they were subjected to a casting process. The resulting cast
products were examined and as a result, it was made clear that they
were free of defects such as the coalescence of crystal grains and
had a good quality.
[0132] This embodiment is not limited to the Fe--C based alloy
material, and is also applicable to the other Fe-based alloy
materials such as an Fe--C--Si (1% by weight) alloy material (point
A.sub.1: 758.degree. C.), an Fe--C--Si (2% by weight) alloy
material (point A.sub.1: 780.degree. C.), an Fe--C--Si (3% by
weight) alloy material (point A.sub.1: 820.degree. C.), and the
like.
* * * * *