METHOD FOR MANUFACTURING SAME
[Technical Field]
5 [0001]
The present invention relates to titanium cast products for hot rolling
composed of commercially pure titanium, and methods for manufacturing the same,
specifically to a titanium cast product for hot rolling, which bears a hot rolled sheet
excellent in a surface quality, and a method for manufacturing the same. This
10 application is based upon and claims the benefit of priority from the prior Japan
Patent Application No. 2013-075886, filed on April 1, 2013 with the Japan Patent
Office, the contents of which are incorporated herein by reference:
[Background Art]
[0002]
15 In general, commercially pure titanium is prepared usually in the form of a
large cast product by using titanium sponge obtained by a Kroll process and titanium
scraps as melting raw materials and melting them by vacuum arc remelting (VAR)
and electron beam remelting (EBR). In this connection, the form of the cast
product is limited to a cylindrical cast product in the case of VAR. On the other
20 hand, the materials can be casted into a rectangular cross-section cast product, that is,
a slab in the case of EBR.
[0003]
Further, when such the large cast product as described above is used as a
raw material to manufacture titanium materials such as titanium sheets and the like,
25 the large cast product is subjected, if necessary, to cutting treatment of a surface and
then to slab rolling or forging at a hot temperature to deform the large ingot into a
2
slab having a form and a size which are suited to subsequent hot rolling. A hot
working process carried out by the above slab rolling or forging is referred to as a
breakdown process in this application. Further, usually, the slab is subjected to
cutting treatment for removing a surface thereof by about several mm by cutting
5 work in order to remove an oxide layer and an oxygen-enriched layer which are
formed on the surface of the slab after the breakdown process, and then the slab is
subjected to hot rolling.
[0004]
However, the above conventional method requires a great deal of time and
10 costs for the breakdown process carried out by slab rolling or forging for deforming
the large cast product into a form and a size which are suited to hot rolling, and this
has largely hindered an improvement in a productivity and a reduction in a cost in
manufacturing titanium sheets.
[0005]
15 On the other hand, in recent years, a technique for manufacturing a
relatively thin slab-shaped cast product, that is, a titanium cast product having a form
and a size which make it possible to subject the cast product to hot rolling as it is, by
a DC slab casting method (direct casting method) is being established as a method
for casting a slab-shaped cast product instead of casting such the large ingot as
20 described above. According to the DC slab casting method, molten titanium
obtained by melting titanium in a hearth by an electron beam and the like is
continuously injected into a water-cooled copper mold maintained to be a vacuum
atmosphere, and a part of the molten titanium solidified in the water-cooled copper
mold is continuously pulled out from a lower end side of the mold to obtain a slab-
25 shaped cast product having a prescribed length.
[0006]
3
Applying the DC slab casting method carried out by the above EBR and the
like under vacuum makes it possible to omit the breakdown process which has
conventionally been required, which results in making it possible to improve a
productivity in manufacturing a titanium sheet and reduce a manufacturing cost
5 thereof.
[0007]
Further, there is the problem that even when a slab (omitting the breakdown
process) obtained by applying the DC slab casting method carried out by the EBR
and the like under vacuum as described above is subjected to hot rolling, the surface
10 property of a hot rolled sheet after hot rolling is not necessarily improved. That is,
there is the problem that many small and large overlapping flaws having a length of
several mm to about 10 mm are formed on the surface of the hot rolled sheet. Such
many overlapping flaws formed on the surface shall be referred to as surface flaws in
this application. Such the surface flaws formed on the hot rolled sheet are
15 considered to originate in coarse cast micro structure of a cast slab. That is, a slab
manufactured without passing through the breakdown process in which hot working
is carried out has cast microstructure composed of coarse crystal grains as cast, and
even if the surface thereof is subjected to cutting work to make undulations on the
surface smaller, the coarse microstructure is present in the surface layer after cutting.
20 It is considered that the surface flaws are formed on the hot rolled sheet due to the
cast micro structure of such the coarse cast microstructure in the surface layer.
[0008]
In this connection, a specific factor in which surface flaws are formed on a
hot rolled sheet due to coarse cast microstructure is considered to be attributable to
25 that relatively large dents are formed in a boundary part between a mother phase and
a twin crystal because of a large misorientation between the mother phase and the
4
twin crystal and a coarse hot twin crystal formed in the beginning of hot rolling and
metal is overlapped on the above dents to turn into surface flaws as subsequent hot
rolling proceeds.
[0009]
5 On the other hand, there has already been proposed some methods in which
a surface layer of a titanium slab for hot rolling which is obtained without passing
through the breakdown process is subjected to reforming treatment before hot rolling
in order to prevent surface flaws from being formed on a surface of a hot rolled sheet
after hot rolling.
10 [0010]
For example, in Patent Literature 1, it is proposed that a surface of a
titanium slab for hot rolling is struck (subjected to plastic deformation) at a room
temperature by a steel tool with a tip curvature radius of 3 to 30 mm or a steel ball
having a radius of 3 to 30 mm, which provides the slab with dimples having an
15 average height of 0.2 to 1.5 mm and an average length of 3 to 15 mm in a contour
curve element of a undulation. In the method proposed above, the surface layer of
the titanium slab is provided with prescribed plastic strain at the room temperature by
the steel-made tool or the steel ball each described above to thereby recrystallize the
surface layer in subsequent heating prior to hot rolling and form fine microstructure,
20 whereby dents can be prevented from being formed due to such the coarse
microstructure as described above. Accordingly, even when the breakdown process
is omitted, surface flaws of a hot rolled sheet can be reduced.
[0011]
In Patent Literature 2, there is proposed a method in which a surface of a
25 titanium slab for hot rolling, especially a surface of a side which is a surface to be
rolled in hot rolling is provided with high energy by high frequency induction
5
heating, arc heating, plasma heating, electron beam heating, laser heating and the like
to melt only the surface layer by a depth of 1 mm or more and in which the surface is
immediately quenched and solidified again. In the case of the method proposed
above, titanium has naturally a melting point which is higher a p transformation point,
5 and therefore as the surface is molten, a heat affected zone (HAZ) layer of a lower
side (parent metal side) than the molten layer on the surface is heated as well to the [3
transformation point or higher and subjected to [3 transformation. In the method
proposed above, the surface layer of the titanium slab for hot rolling is molten,
whereby the surface is smoothed; further, the molten layer is then quenched by
10 removing heat to the parent metal side and solidified; and at the same time, the HAZ
layer (p phase) at a lower side is quenched, whereby the molten layer and the HAZ
layer are turned into fine transformation microstructure (usually fine acicular
microstructure). Then, the surface layer which has been refined in the manner
described above is recrystallized in the subsequent reheating prior to hot rolling and
15 turned into granular microstructure (equiaxed grain microstructure) having a fine and
random orientation. Accordingly, dents attributable to the coarse microstructure
can be prevented from being formed, and the surface flaws on the hot rolled sheet
after hot rolling can be overcome as well.
[Prior Art Literature(s)]
20 [Patent Literature(s)]
[0012]
[Patent Literature 1] WO 2010/090352
[Patent Literature 2] JP 2007-332420A
[Summary of the Invention]
25 [Problem(s) to Be Solved by the Invention]
[0013]
6
It has been confirmed by experiments carried out by the present inventors
and the like that according to the surface layer reforming treatment method in which
a surface layer of a titanium slab for hot rolling is provided with plastic deformation
in a room temperature as shown in Patent Literature 1 and the surface layer
5 reforming treatment method in which the surface of a titanium slab for hot rolling is
provided with high energy to melt only the surface layer and in which the surface
layer is quenched and solidified again as shown in Patent Literature 2, even the
surface layer of the titanium slab for hot rolling which is manufactured without
passing through the breakdown process can effectively be reformed depending on the
10 surface situations thereof to prevent surface flaws from being formed on the hot
rolled sheet. That is, the surface layer of a cast product as cast by DC slab casting
under vacuum usually has marked undulations and is defective to a large extent as
already described above. It has been confirmed, however, that the surface layer of
the above slab is removed by a depth of several mm by cutting work and then
15 subjected to the surface layer reforming treatment as shown in Patent Literature 1 or
Patent Literature 2, whereby surface flaws on the hot rolled sheet after subsequent
hot rolling can be prevented from being formed.
[0014]
However, large amounts of labor and time are required for surface cutting
20 work before the surface reforming treatment described above, and the yield thereof is
reduced to a large extent. Accordingly, if it becomes possible to suppress formation
of surface flaws on the hot rolled sheet by the surface reforming treatment even when
omitting the above surface cutting work, a titanium sheet having an excellent surface
property can be manufactured at a high productivity and a low cost. However, it
25 has become clear that when an as cast product in which a black mill scale skin is
present on a surface is subjected to surface reforming treatment without subjecting
7
the surface to the cutting work described above before the surface reforming
treatment, it is difficult to surely and stably suppress formation of surface flaws on
the surface of the hot rolled sheet.
[0015]
5 Accordingly, the present invention focuses on providing a titanium cast
product for hot rolling and a method for manufacturing the same, the method not
only omits a breakdown process but also does not require cutting work before surface
reforming treatment and makes it possible to surely prevent surface flaws from being
formed on the surface of the hot rolled sheet after subsequent hot rolling, so that a
10 titanium hot rolled sheet can be improved in manufacturing and reduced in a cost.
[Means for Solving the Problem(s)]
[0016]
In order to solve the above problems, the present inventors have intensively
repeated experiments and investigations on the surface reforming technique shown in
15 Patent Literature 2 described above to result in obtaining the following knowledge.
[0017]
That is, a surface of a cast product is heated by heating means having a high
energy density such as an electron beam to melt only a surface layer, and then the
cast product is cooled usually by removing heat to a parent metal side. In this case,
20 the smaller the thickness of the molten layer is, the smaller the heat input per unit
area of a cast product surface (hereinafter, the unit area is 1 cm in terms of the heat
input) is, and therefore the cooling rate immediately after heating is increased, so that
the surface layer (molten and resolidified layer) solidified by cooling is turned into
finer microstructure. The microstructure of the surface layer heated for subsequent
25 hot rolling is more refined as well and results in making it possible to surely suppress
relatively large dents formed in the beginning of hot rolling and surface flaws formed
8
on the hot rolled sheet.
[0018]
However, when a melting depth is small, defects (originating in casting)
such as voids and wrinkles which are present in a position of a certain degree of a
5 depth from the surface do not disappear in some cases. That is, it has been
confirmed by experiments that the melting depth has to be controlled to several mm
or less in order to sufficiently refine the microstructure of the surface layer by
resolidification after remelting. However, in many cases, voids originating in
casting are present in a deeper position than the above level, that is, a position of a
10 depth of 5 to 8 mm that exceeds the several mm from the surface. Accordingly,
when the surface layer is molten only by a depth of several mm, voids present in a
relatively deeper position do not disappear and therefore it is acknowledged that
cracks are formed with the above voids as starting points in hot rolling and that
relatively large concave parts are produced on the surface to generate surface flaws.
15 [0019]
It is considered that the problem described above can be solved by
increasing a melting depth in heating the surface of the cast product by heating
means having a high energy density such as an electron beam to melt the surface
layer. In the above case, however, the heat input per unit area of a cast product
20 surface is increased contrary to the case described above, and the cooling rate in
removing heat to a parent metal side immediately after heating is decreased, so that
the microstructure of the surface layer (molten and resolidified layer) solidified by
cooling is not sufficiently refined. The microstructure of the surface layer heated
for subsequent hot rolling is not sufficiently refined as well and therefore relatively
25 large dents formed in the beginning of hot rolling and surface flaws formed on a hot
rolled sheet are not sufficiently reduced.
9
[0020]
Intensive experiments and investigations repeated by the present inventors
based on the above new knowledge have resulted in finding that relatively large
dents formed in the beginning of hot rolling and surface flaws formed on a hot rolled
5 sheet can surely be suppressed by further improving the surface reforming
technology shown in Patent Literature 2 and especially resulted in finding that
relatively large dents formed in the beginning of hot rolling and surface flaws formed
on a hot rolled sheet can surely be suppressed as well in a cast surface of the as cast
slab which is not subjected in advance to cutting work.
10 [0021]
That is, a surface layer of a cast product which is a material of a slab for hot
rolling is molten by irradiation with an electron beam or the like and resolidified, and
then the surface of the molten and resolidified layer is irradiated again with an
electron beam or the like to heat a surface region (region having a shallower depth
15 than a depth of the molten and resolidified layer) in the molten and resolidified layer
to a temperature of a p transformation point or higher to quench and solidify the
surface area. It has been found that since such heating is performed twice on the
surface layer by irradiation with an electron beam or the like, it is possible to surely
prevent relatively large dents formed in the beginning of hot rolling and surface
20 flaws formed on a hot rolled sheet and in addition to the above, formation of surface
flaws formed on a hot rolled sheet after subsequent hot rolling can surely be
suppressed as well in a cast surface of the as cast slab which is not subjected to
cutting work in advance. Thus, the present invention has been made.
[0022]
25 According to the present invention, there is provided a titanium cast product
for hot rolling composed of commercially pure litanium, the titanium cast product
10
including: a microstructural refinement layer having acicular microstructure on an
surface; and an inside microstructural refinement layer having acicular
microstructure provided in an inside of the microstructural refinement layer. Cast
solidification microstructure is present more inward than the inside microstructural
5 refinement layer. The microstructural refinement layer has finer microstructure
than the inside microstructural refinement layer. The microstructural refinement
layer is present in a range of a depth of 1 mm or more and less than 6 mm from the
surface. The inside microstructural refinement layer is present in an inside of the
microstructural refinement layer in a range of a depth of 3 mm or more and 20 mm or
10 less from the surface.
[0023]
In such the titanium cast product for hot rolling according to the present
invention as described above, a microstructural refinement layer present on an
outermost surface is turned, as explained later in the manufacturing method, into an
15 equiaxed fine granular microstructure in a random orientation in a state in which the
cast product is subjected to heat treatment prior to hot rolling or equivalent one and
recrystallized. In this connection, the heat treatment prior to hot rolling or
equivalent one shall mean heat treatment at 820°C for 240 minutes in the present
invention. That is, in general, a titanium slab is hot-rolled usually by heating at
20 approximately 720 to 920°C for approximately 60 to 420 minutes. Then, a hot
rolling heating condition which is in the middle of the above conditions is adopted in
the present invention, and a grain diameter at a time of subjecting the cast product to
heat treatment prior to hot rolling or equivalent one at 820°C for 240 minutes is
prescribed as an index of refinement of the microstructure refinement layer.
25 [0024]
According to the present invention, there is provided a method for
11
manufacturing a titanium cast product for hot rolling, the method including: a first
stage surface heat treatment process of heating a surface of a cast product material
composed of commercially pure titanium to be rolled in hot rolling to heat a region
of a depth of 6 mm or more and 20 mm or less from the surface to a p transformation
5 point or higher and to melt a range of a depth of 3 mm or more and 10 mm from the
surface, and a first stage cooling process of cooling the cast product material to
temperature lower than the p transformation point after the first stage surface heat
treatment process; and a second stage surface heat treatment process of reheating the
surface subjected to the first stage surface heat treatment process and the first stage
10 cooling process to heat a region of a depth of 1 mm or more and less than 6 mm from
the surface to the p transformation point or higher, and a second stage cooling
process of cooling the cast product material to temperature lower than the p
transformation point after the second stage surface heat treatment process.
In this connection, the p transformation point is temperature at which or
15 higher the p phase is a stable phase and at which or lower the a phase is substantially
a stable phase. The p transformation point is 880 to 920°C in commercially pure
titanium.
[0025]
According to the present invention, marked undulations present on a cast
20 surface after casting are removed and smoothed by melting, and at the same time,
defects such as internal voids originating in casting are eliminated. Further, coarse
cast microstructure disappears as well. In addition, the surface is turned into a
micro structural refinement layer by reheating and quenching. Accordingly, in
subjecting the titanium cast product for hot rolling according to the present invention
25 to hot rolling, surface flaws due to wrinkles and internal voids originating in casting
can be prevented in advance from being formed, and at the same time, relatively
12
large concave parts in the beginning of hot rolling originating in insufficient
microstructure refinement and surface flaws on the hot rolled sheet can surely be
prevented as well in advance from being formed.
That is, an inside micro structural refinement layer which is molten and
5 heated to a p transformation point or higher in melting and resolidifying at a first
stage has a sufficient thickness from 6 mm or more to 20 mm or less from the surface,
and the inside micro structural refinement layer which is molten and resolidified up to
a deeper position than a cutting stock (about several mm) in a conventional method.
Accordingly, voids (voids present in a position of a depth exceeding a usual cutting
10 stock) present in a deeper position than a position of several mm from the surface are
sufficiently removed, and at the same time, marked undulations on the surface are
eliminated as well.
On the other hand, a reheated and quenched micro structural refinement
layer at a surface side of a second stage is a thin layer present in a position of 1 mm
15 or more and less than 6 mm from the surface, and therefore the micro structural
refinement layer is turned into a layer having sufficiently fine microstructure by a
high-speed quenching effect provided by removing heat to the parent metal after
reheating. Accordingly, relatively large concave parts in the beginning of hot
rolling originating in insufficient microstructural refinement and surface flaws on the
20 hot rolled sheet can surely be prevented as well from being formed.
The respective actions described above can be obtained as well in a cast
product staying in a state in which the cast product does not pass through a
breakdown process carried out by slab rolling, forging or the like in hot working after
casting, and such actions can be obtained as well in a cast product with so-called
25 black mill scale skins as cast whose surface is not subjected in advance to cutting
work.
13
[0026]
The titanium cast product for hot rolling according to the present invention
may include at least one kind of a-phase stabilizing elements and neutral elements in
an amount of 0 % or more and less than 2.0 % in terms of total mass% in a range of a
5 depth of 4 mm or less from the surface. The titanium cast product for hot rolling
according to the present invention may include at least one kind of p-phase
stabilizing elements in an amount of 1.5 % or less in terms of total mass % in a range
of a depth of 4 mm or less from the surface. The titanium cast product for hot
rolling according to the present invention may include, in a range of a depth of 4 mm
10 or less from the surface, at least one kind of a-phase stabilizing elements and neutral
elements in an amount of 0 % or more and less than 2.0 % in terms of total mass%,
and at least one kind of p-phase stabilizing elements in an amount of 1.5 % or less in
terms of total mass %.
[0027]
15 With regard to the titanium cast product for hot rolling according to the
present invention, the number of crystal grains having a crystal grain diameter of 3
mm or more is preferably 5 or less per m of the surface in a state at room
temperature after heat treatment at 820°C for 240 minutes.
[0028]
20 With regard to the method for manufacturing a titanium cast product for hot
rolling according to the present invention, a heat input per unit area (1cm ) in the
second stage surface heat treatment process may be set to be lower than a heat input
per unit area in the first stage surface heat treatment process.
In this respect, the heat input in the second stage surface heating treatment
25 process described above is more reduced than the heat input in the first stage surface
heating treatment process since a thickness of the molten layer or the HAZ layer
14
formed in heating the second stage has to be smaller than a thickness of the layer
formed in the first stage.
[0029]
With regard to the method for manufacturing a titanium cast product for hot
5 rolling according to the present invention, an electron beam may be radiated while
continuously moving an electron beam radiation gun in a direction parallel to the
surface of the cast product material in the respective processes of the first stage
surface heat treatment process and the second stage surface heat treatment process.
[0030]
10 The first stage cooling process and the second stage cooling process may be
carried out by removing heat to a parent metal side of the cast product material. In
this case, the cast product material is allowed to pass through the J3 transformation
point at a cooling rate of 60°C/minute or more in the second stage cooling process.
In this regard, if the cooling rate at the second stage cooling process is less
15 than 60°C/minute, the crystal grains are likely to be insufficiently refined.
[0031]
The second stage surface heat treatment process and the second stage
cooling process can be carried out plural times.
[0032]
20 The surface may be molten together with a material containing at least one
kind of a-phase stabilizing elements and neutral elements in the second stage surface
heat treatment process. The surface may be molten together with a material
containing at least one kind of p-phase stabilizing elements in the second stage
surface heat treatment process. The surface may be molten together with a material
25 containing at least one kind of a-phase stabilizing elements and neutral elements and
a material containing at least one kind of p-phase stabilizing elements in the second
15
stage surface heat treatment process.
[0033]
In the method for manufacturing a titanium cast product for hot rolling
according to the present invention, the surface may be molten in the second stage
5 surface heat treatment process. In this case, the surface may be molten together
with a material containing at least one kind of a-phase stabilizing elements and
neutral elements in the second stage surface heat treatment process. The surface
may be molten together with a material containing at least one kind of p-phase
stabilizing elements in the second stage surface heat treatment process. The surface
10 may be molten together with a material containing at least one kind of a-phase
stabilizing elements and neutral elements and a material containing at least one kind
of p-phase stabilizing elements in the second stage surface heat treatment process.
[0034]
In the method for manufacturing the titanium cast product for hot rolling
15 according to the present invention, the material for the cast product described above
may be any of those prepared by casting a material by the DC slab casting method,
those prepared by casting a molten metal obtained by the melting method with an
electron beam and the like by the DC slab casting method, and those having a as cast
surface. The above the rectangular cross-section cast products are obtained without
20 passing through the breakdown process including slab rolling or forging. The
melting method for the same shall not specifically be restricted, and an EBR method,
a plasma arc melting method and the like can be applied. In the EBR method, since
melting is carried out in high vacuum, an inside of voids remaining in. the vicinity of
a slab surface after melting stays in vacuum, and therefore there is the advantage that
25 the voids are easy to be pressed in hot rolling and turned into harmlessness.
[Effect(s) of the Invention]
16
[0035]
The titanium cast product for hot rolling according to the present invention
has a flat and smooth surface and a few minute voids in an inside directly under the
surface and is provided with a markedly fine micro structure in an outermost surface
5 layer. Accordingly, when the titanium cast product is subjected to hot rolling, the
cast product can surely and stably be prevented from formation of relatively large
concave parts on the surface in the beginning of hot rolling and generation of surface
flaws on the hot roiled sheet. The above effects can be obtained as well by using a
cast product which does not pass through a breakdown process carried out by slab
10 rolling or forging and which is not subjected to surface finishing by cutting work as a
material for producing the titanium cast product for hot rolling. Accordingly, the
breakdown process and the surface finishing by cutting work can be omitted, and the
cost can be reduced more markedly than ever.
[Brief Description of the Drawing(s)]
15 [0036]
[FIG. 1] FIG. 1 is a schematic drawing showing a flow of an embodiment of
the method for manufacturing the titanium cast product for hot rolling according to
the present invention.
[FIG. 2] FIG. 2 is a schematic perspective drawing showing an outline of
20 one example of a material (rectangular cross-section titanium cast product) used in
an embodiment of the method for manufacturing the titanium cast product for hot
rolling according to the present invention, and a state of irradiating the titanium cast
product with an electron beam.
[FIG. 3] FIG. 3 is a schematic cross section showing, in stages, one example
25 of a transition in the surface layer of the rectangular cross-section titanium cast
product of the material in an embodiment of the method for manufacturing the
17
titanium cast product for hot rolling according to the present invention.
[FIG. 4] FIG. 4 is a schematic drawing showing one example of a crosssectional
structure in the vicinity of the surface of the titanium cast product for hot
rolling according to the present invention.
5 [FIG. 5] FIG. 5 is a schematic drawing showing one example of a crosssectional
structure in the vicinity of the surface of the titanium cast product staying in
a state in which the titanium cast product for hot rolling according to the present
invention is subjected to heat treatment prior to hot rolling or equivalent one.
[FIG. 6] FIG. 6 is a s cross-sectional observation photograph showing a
10 micro structural refinement layer, an inside micro structural refinement layer and a
casting solidification micro structure in a surface part of the titanium cast product for
hot rolling according to the present invention.
[Mode(s) for Carrying out the Invention]
[0037]
15 Hereinafter, referring to the appended drawings, embodiments of the present
invention will be described in detail.
[0038]
FIG. 1 schematically shows the respective processes PI to P4 of the overall
process in the method for manufacturing the titanium cast product for hot rolling
20 according to an embodiment of the present invention. In FIG. 1, an example of a
process for manufacturing the rectangular cross-section titanium cast product which
is the material is also shown as a pre-process P0. Also, FIG. 2 shows an outline of a
material (rectangular cross-section titanium cast product) used in the embodiment of
the method for manufacturing the titanium cast product for hot rolling according to
25 the present invention, and a state of irradiating the rectangular cross-section titanium
cast product with an electron beam. Further, FIG. 3 shows, in stages, a transition in
18
a cross-sectional state in the vicinity of the surface of the rectangular cross-section
titanium cast product in the respective processes in an embodiment of the
manufacturing method shown in FIG. 1.
[0039]
5 [Pre-process PO]
In manufacturing the titanium cast product for hot rolling according to the
present invention, only a prescribed amount of a melting raw material for
commercially pure titanium, for example, titanium sponge obtained by a Kroll
process and titanium scraps are molten in a hearth by EBR as shown in FIG. 1 as a
10 pre-process PO. The molten titanium thus obtained is teemed continuously into a
water-cooled copper mold for casting a DC slab, that is, a water-cooled copper mold
in which upper and lower parts are opened and in which a lateral cross section is
rectangular (including a case in which chamfers are formed in corners). Further, the
cast product solidified in the mold is continuously pulled out downward, whereby a
15 rectangular cross-section (slab-shaped) titanium cast product having a thickness, a
width and a length which are suited to hot rolling in a form and a dimension as cast is
obtained. In this regard, the cast product which is provided with chamfers in
corners shall also be referred to as a "rectangular cross-section cast product " in a
wide sense. An atmosphere in performing melting in the hearth by heating with an
20 electron beam and casting each described above is kept to vacuum.
[0040]
In this application, the commercially pure titanium includes commercially
pure titanium prescribed in JIS Class 1 to JIS Class 4, ASTM Grades 1 to 4, DIN
37025, DIN 37035, and DIN 37055 each corresponding to the JIS standards. That
25 is, the commercially pure titanium referred to in the present invention can be
composed of, in mass%, C: 0.1 % or less, H: 0.015 % or less, O: 0.4 % or less, N:
19
0.07 % or less, Fe: 0.5 % or less, and the balance: Ti. Further, high corrosion
resistant alloys (titanium materials prescribed in ASTM Grades 7, 11, 16, 26, 13, 30
and 33, or JIS standards corresponding to the ASTM Grades, or titanium materials
obtained by adding other kinds of elements thereto in small amounts) called modified
5 (improved) pure titanium which are obtained by adding slight amounts of platinum
group elements to the commercially industrial pure titanium are also referred to as
titanium included in the commercially pure titanium in the present invention.
[0041]
In manufacturing the titanium cast product for hot rolling according to the
10 present invention, a rectangular cross-section titanium cast product which is a
material for the titanium cast product can be obtained basically by an optional
melting method and an optional casting method A rectangular cross-section
titanium cast product obtained, as explained in the present embodiment, by melting a
raw material such as titanium sponge, titanium scraps and the like by EBR under
15 vacuum and casting the molten titanium under vacuum into a rectangular crosssection
form or rectangular parallelepiped form (slab-shaped) having a long
rectangular form in a cross section by the DC slab casting method can exert most
effectively the effects of the present invention. The rectangular cross-section
titanium cast product having a rectangular cross section of a form and a dimension
20 which are suited to hot rolling can readily be obtained according to the DC slab
casting method, and therefore the hot breakdown process including slab rolling and
forging at a hot temperature can be omitted.
[0042]
The dimension of the rectangular cross-section titanium cast product shall
25 not specifically be restricted as long as the titanium cast product has a dimension
which can be subjected to hot rolling as it is. When coil rolling is applied as the hot
20
rolling to manufacture a hot rolled coil thin and medium plates having a plate
thickness of about 3 mm to 8 mm, the dimension of the rectangular cross-section
titanium cast product can be set to a thickness of about 150 mm to 280mm, a length
of about 3 m to 10 m, and a width of about 600 mm to 1580 mm.
5 [0043]
Further, billets, blooms and the like which are subjected to hot rolling can
exert the same effects as well by subjecting the parts corresponding to surfaces to be
rolled to heat treatment and hot rolling in the manners of the present invention. The
titanium cast product which is the raw material includes not only rectangular cross-
10 section (slab-shaped) cast products but also billets and blooms.
[0044]
The rectangular cross-section titanium cast product obtained by DC slab
casting with EBR and the like in the manner described above is subjected as it is to,
as shown in FIG. 1, a first stage surface heat treatment process PI, a first stage
15 cooling process P2, a second stage surface heat treatment process P3 and a second
stage cooling process P4 in this order. In this connection, subjecting the rectangular
cross-section titanium cast product as it is to the respective PI to P4 processes means
subjecting the rectangular cross-section titanium cast product as a raw material as
cast to the respective PI to P4 processes without passing through a breakdown
20 process carried out by hot working such as slab rolling and forging and a cutting
process for surface finishing, as a material for producing a slab for manufacturing a
hot rolled titanium sheet. Accordingly, the rectangular cross-section titanium cast
product which is a material for the titanium cast product for hot rolling has not only
the surface property of coarse undulations originating in casting, but also coarse cast
25 micro structure, and many defects such as voids originating in casting are usually
present in the parts of up to a depth of about 8 mm to 10 mm from the surface.
21
[0045]
The respective PI to P4 processes described below are carried out to at least
two surfaces (that is, two wide surfaces) which are surfaces to be rolled in the hot
rolling process (surfaces brought into contact with the hot rolling rolls) out of four
5 surfaces excluding a front end surface (lower end surface corresponding to a cast
starting surface) and a rear end surface (upper end surface corresponding to a cast
finishing surface) in DC slab casting among the outer surfaces of the rectangular
cross-section titanium cast product. In the case of the rectangular cross-section cast
product having chamfers, the chamfer surfaces constitute a part of the two wide
10 surfaces described above.
[0046]
To be specific, in a rectangular cross-section titanium cast product 10 having
chamfers 11 as shown in FIG. 2 for example, two wide surfaces 10A and 10B
(surfaces containing chamfers 11) out of four surfaces 10A to 10D along a casting
15 direction D (a direction of pulling out the cast product in DC slab casting) are
surfaces to be rolled in hot rolling. Accordingly, at least the two wide surfaces 10A,
10B containing the chamfers 11 are subjected to the respective PI to P4 processes.
[0047]
When the two wide surfaces 10A and 10B described above are subjected to
20 the respective PI to P4 processes, the order of the respective surfaces and the
respective processes includes the following two cases of A and B. In the present
embodiment, explanations shall be given assuming that the case of B is applied for
the sake of simplifying the explanations. Also when the melting treatment of the
surface at the second stage is carried out plural times, the process of A or B may be
25 carried out, or both processes of A and B may be carried out in a mixture.
Case A: among the two surfaces 10A and 10B, one surface 10A is subjected to the
22
first stage surface heat treatment process PI to the first stage cooling process P2, and
then the other surface 10B is similarly subjected to the first stage surface heat
treatment process PI to the first stage cooling process P2. Thereafter, any one (for
example, 10A) of the above surfaces is subjected to the second stage surface heat
5 treatment process P3 to the second stage cooling process P4, and then the other
surface (for example, 10B) is subjected to the second stage surface heat treatment
process P3 to the second stage cooling process P4.
Case B: among the two surfaces 10A and 10B, one surface 10A is subjected to the
first stage surface heat treatment process PI to the first stage cooling process P2, and
10 then subsequently the same surface 10A is subjected to the second stage surface heat
treatment process P3 to the second stage cooling process P4. Thereafter, the other
surface 10B is subjected to the first stage surface heat treatment process PI to the
first stage cooling process P2, and then subsequently the same surface 10B is
subjected to the second stage surface heat treatment process P3 to the second stage
15 cooling process P4.
[0048]
Further, not only the two wide surfaces 10A and 10B (surfaces which are
surfaces to be rolled in hot rolling) out of the four surfaces 10A to 10D along the
casting direction D, but also two narrow surfaces IOC and 10D (surfaces which are
20 edge sides in hot rolling) may be subjected as well to the respective processes PI to
P4. In the above case, the two narrow surfaces IOC and 10D at the edge sides may
be subjected to the respective processes PI to P4 after subjecting the two wide
surfaces 10A and 10B which are surfaces to be hot rolled to the respective processes
PI to P4 is finished. Alternatively, in the case A described above, the two wide
25 surfaces 10A and 10B which are the surfaces to be hot rolled may be subjected to the
first stage surface heat treatment process PI to the first stage cooling process P2, and
23
then subsequently the two surfaces IOC and 10D at the edge sides may be similarly
subjected to the first stage surface heat treatment process PI to the first stage cooling
process P2. Thereafter, the two wide surfaces 10A and l'OB which are the surfaces
to be hot rolled and the two surfaces IOC and 10D at the edge sides may be subjected
5 to the second stage surface heat treatment process P3 to the second stage cooling
process P4 in order. In the present embodiment, however, the respective processes
PI to P4 for the two surfaces IOC and 10D at the edge sides are omitted for the sake
of simplifying the explanations.
[0049]
10 The respective processes PI to P4 are further explained below in detail.
[0050]
[First stage surface heat treatment process PI] to [first stage cooling process P2]
As described above, the rectangular cross-section titanium cast product
obtained by EBR and DC slab casting is subjected as it is to the first stage surface
15 heat treatment process PI. The first stage surface heat treatment process PI is, as
shown in FIG. 2, a process in which only the surface layers of the two wide surfaces
10A and 10B which are surfaces to be rolled (surfaces brought into contact with the
hot rolling rolls) at least in the hot rolling process out of the outer surfaces of the
rectangular cross-section titanium cast product 10 are molten by heating. In this
20 respect, one surface 10A out of the two surfaces 10A and 10B shall be first subjected
to the process. The surface layers are heated, for example, by being irradiated with
an electron beam. Hereinafter, electron beam irradiation shall be explained as one
example of a heating method.
[0051]
25 In this regard, an area of a region 14 irradiated with an electron beam by one
electron beam irradiation gun 12 on the surface 10A of the rectangular cross-section
24
titanium cast product 10 is, as shown in FIG. 2, usually very small as compared with
the whole area of the surface 10A to be irradiated. As a matter of fact, an electron
beam is usually radiated while continuously moving the electron beam irradiation
gun 12 or while continuously moving the rectangular cross-section titanium cast
5 product 10. A shape and an area of the above irradiated region can be adjusted by
regulating a focus of the electron beam or using an electromagnetic lens to oscillate a
small beam at a high frequency to form a beam bundle. In the present embodiment,
explanations are provided as follows, assuming that the electron beam irradiation gun
12 is continuously moved as shown by an arrow A in FIG. 2. A moving direction of
10 the electron beam irradiation gun 12 shall not specifically be restricted, and usually
the gun is continuously moved in a length direction (usually a casting direction D) or
a width direction (usually a direction vertical to the casting direction D) of the
rectangular cross-section titanium cast product 10 to continuously irradiate a width
W (diameter W in the case of a circular beam or a beam bundle) of the irradiated
15 region 14 described above in a belt form. Further, an unirradiated region adjacent
to the irradiated region 14 is irradiated with an electron beam in a belt form while
continuously moving the electron beam irradiation gun 12 to a reverse direction (or
the same direction). In a certain case, plural irradiation guns may be used to
irradiate plural regions with electron beams at the same time. In FIG. 2, a case in
20 which a rectangular cross-section beam is continuously moved along a length
direction (usually the casting direction D) of the rectangular cross-section titanium
cast product 10 is shown. Also, when a beam passes on a part adjacent to a part
once irradiated, 1/2 to 1/4 of the part once irradiated is allowed to be irradiated once
again, and the parts are treated so that a desired treatment depth can be achieved in
25 all regions, whereby the effects of the present invention can sufficiently be exerted.
[0052]
25
The surface (surface 10A) of the rectangular cross-section titanium cast
product 10 is irradiated with an electron beam in the above first stage surface heat
treatment process PI to heat the surface to temperature of a melting point (usually
about 1670°C) or more of commercial pure titanium, whereby the surface layer of
5 the surface 10A of the rectangular cross-section titanium cast product 10 is molten,
as shown in a central left side of FIG. 3 (A), by a depth dl corresponding to the heat
input. That is, a region from the surface to a position of the depth dl in a thickness
direction is a molten layer (first stage molten layer 16). Also, in a region inner than
the first stage molten layer 16 in the cast product, a part (heat affected layer = HAZ
10 layer) heated to a temperature of a p transformation point or higher of pure titanium
due to heat affection caused by irradiation with an electron beam is transformed into
a p phase. As shown above, the region transformed into the p phase due to heat
affection caused by irradiation with an electron beam in the first stage surface heat
treatment process PI is referred to as a first stage p transformation layer 18 in the
15 present specification. A thickness of the above first stage p transformation layer 18
is set to d2.
[0053]
In this regard, the depth dl + d2 of the first stage molten layer 16 and the p
transformation layer 18 falls in a range of 6 mm to 20 mm in the first stage surface
20 heat treatment process PI. The thickness dl of the first stage molten layer 16 shall
not specifically be restricted. The depth of dl + d2 can be controlled to be the
depth described above, and usually dl falls preferably in a range of 3 mm to 10mm.
[0054]
A heat input is related principally to a melting depth formed by irradiation
25 with an electron beam, and therefore electron beam irradiation conditions are
selected to control the heat input so that dl + d2 (6 mm to 20 mm) of the melting
26
depth + the p transformation layer each described above are obtained. In fact, since
the necessary heat input is varied depending on a thickness (heat capacity) of the cast
product, a parent metal temperature and cooling conditions of a parent metal side, the
heat input necessary for obtaining the melting thickness described above is not
5 simply determined, and usually the heat input per unit area (per 1 cm2) can be set to
80 to 300 J. In this regard, the electron beam irradiation conditions which affect the
heat input per unit area include an output of the irradiation gun and a beam diameter,
and a gun moving rate (irradiation position moving rate) when performing irradiation
while continuously moving the irradiation gun as described above. The above
10 conditions can suitably be set to secure the heat input described above.
[0055]
If an electron beam is radiated while continuously moving the irradiation
gun, the first stage molten layer 16 and the first stage p transformation layer 18 in a
part which has been finished to be irradiated with an electron beam are cooled, as
15 shown in the vicinity of the center in FIG. 3 (A), by removing heat to the parent
metal (inside of the cast product 10), and when the layers reach a solidifying
temperature or lower, they are solidified and turned into a resolidified layer
(hereinafter referred to as a first stage molten and resolidified layer) 20. Also, the
heat affected layer (first stage p transformation layer 18) at a lower side of the first
20 stage molten layer formed by irradiation with an electron beam is heated to a
temperature higher than the p transformation point and then cooed to a temperature
lower than the p transformation point, whereby the heat affected layer is reversely
transformed into an a phase. Coarse cast microstructure disappears and is turned
into fine acicular microstructure (hereinafter referred to as a first stage HAZ layer) in
25 the process in which the layer subjected to p transformation as described above is
reversely transformed into an a phase. Thus, the layer which is reversely
27
transformed into an a phase by cooling the first stage p transformation layer 18 is
shown as a first stage HAZ layer 22 in FIG. 3. The above cooling process
corresponds to the first stage cooling process P2. In the case of the present
embodiment in which the surface of the rectangular cross-section titanium cast
5 product 10 is irradiated with an electron beam while continuously moving the
irradiation gun 12, while the first stage surface heat treatment process PI proceeds by
irradiating some portion on the plate surface 10A of the rectangular cross-section
titanium cast product 10 with an electron beam, the first stage cooling process P2 for
cooling the layer to a temperature lower than the [3 transformation point proceeds in
10 an other portion (portion in which irradiation has already been finished).
[0056]
Though not specifically illustrated, in irradiating the surface of the
rectangular cross-section titanium cast product with an electron beam to perform the
first stage surface heat treatment process PI and then perform the first stage cooling
15 process P2, the rectangular cross-section titanium cast product can be placed on a
water cooled base composed of heat conductive material (metal) such as stainless
steel, copper, aluminum and the like so that the rectangular cross-section titanium
cast product is prevented from being wholly heated by irradiation with an electron
beam. Immediately after the first stage surface heat treatment process PI is
20 performed, removing heat to a parent metal side is allowed to rapidly proceed so that
the first stage cooling process P2 is performed. This makes it possible to further
enhance the effects of the present invention.
[0057]
In a process from the first stage surface heat treatment process PI to the first
25 stage cooling process P2, the surface (first stage molten layer 16) of the rectangular
cross-section titanium cast product molten by irradiation with an electron beam is
28
flattened by surface tension, and coarse undulations 10P on the cast surface are
eliminated. Further, voids 10Q originating in casting which are present in an inside
of the surface are eliminated as well by melting the surface (first stage molten layer
16). Accordingly, the first stage molten and resolidified layer 20 obtained by
5 cooling and solidifying the first stage molten layer 16 is a layer having less
undulations on a surface and less voids in an inside. Also, the coarse cast
microstructure disappears by melting, and the fine acicular microstructure is formed
by solidification in a subsequent cooling course and transformation from a p phase to
an a phase. The above cooling and solidification are carried out by removing heat
10 to a parent metal side, and a cooling rate by removing heat to the parent metal side is
considerably large, so that the acicular microstructure after solidification and
transformation is turned into fine microstructure.
[0058]
Also, the first stage p transformation layer 18 is heated to a temperature
15 higher than the p transformation point and then cooled at a large cooling rate by
removing heat to a parent metal side, and it is reversely transformed into an a phase
to be turned into the first stage HAZ layer 22. This allows the first stage HAZ layer
22 to be turned as well into a fine acicular microstructure.
[0059]
20 However, the thickness of the first stage molten and resolidified layer 20 +
the first stage HAZ layer 22 is as relatively large as 6 mm or more, and therefore it
should be noted that the cooling rate at the first stage cooling process P2 is smaller,
as explained later, than the cooling rate at the second stage cooling process P4.
[0060]
25 Melting to the melting depth (depth dl) at the first stage is a process carried
out in order to eliminate defects such as voids and wrinkles (originating in casting)
29
which are present in a position in a depth of some extent. Usually, levels of the
defects can be estimated to some extent by visually observing the surface of the cast
surface, and therefore a thickness of the first stage molten and resolidified layer 20
can be determined according to results obtained by visual observation.
5 [0061]
In this regard, if the depth dl of the molten layer (first stage molten layer
16) in the first stage surface heat treatment process PI is smaller than 3 mm, voids
originating in casting which are present in the vicinity of 3 mm to 10 mm from the
surface of the cast product (rectangular cross-section titanium cast product 10)
10 cannot be eliminated. As a result, the surface layer reforming effect is
unsatisfactorily exerted, and surface flaws originating in the voids described above
are likely to be formed on the hot rolled sheet. Also, defects such as voids and the
like which are present in an inside of the surface layer of the cast product are reduced
usually in a position of a depth exceeding 10 mm from the surface to such an extent
15 that can be almost ignored. If the defects are present, the defects can be made
harmless by pressing and being integrated in the hot rolling process. Accordingly,
even if the depth dl of the molten layer is increased to more than 10 mm, the
reforming effect cannot be expected to be enhanced further more. On the other
hand, for an increase in the melting depth exceeding 10 mm, it is necessary to delay
20 processing speeds (irradiation gun moving rate) and enhance an electron beam output
of the irradiation gun, and therefore a reduction in the processing efficiency and an
increase in the cost are likely to be brought about. Accordingly, the melting depth
(depth of the first stage molten layer) dl in the first stage surface heat treatment
process is set preferably to 3 mm to 10 mm. However, in the melting depth dl and
25 the depth d2 of the [3 transformation layer (the first stage p transformation layer 18)
which is present in a lower part of dl, the fine acicular microstructure is formed in
30
the first stage cooling process P2 by transformation from the p phase to the a phase,
and therefore it is difficult in certain cases to clearly distinguish dl from d2. On the
other hand, the parent metal part 28 in a lower part than the depth d2 is composed of
coarse micro structure (cast solidification microstructure) as cast, and therefore it can
5 be distinguished. Assuming that the total thickness of dl + d2 is 6 mm to 20 mm, it
has been found that a thickness of dl is approximately 3 to 10 mm, and therefore the
thickness of dl + d2 has been set to a range of 6 to 20 mm. A thickness of the first
stage molten and resolidified layer 20 obtained by allowing the first stage molten
layer 16 to be resolidified in the first stage cooling process P2 is substantially the
10 same as the melting depth dl of the first stage molten layer 16. Further, a thickness
of the first stage HAZ layer obtained by allowing the first stage p transformation
layer 18 to be cooled to the p transformation point or lower in the first stage cooling
process P2 is substantially the same as the dapth d2 of the first stage p transformation
layer 18. Accordingly, the thicknesses of the first stage molten and resolidified
15 layer 20 and the first stage HAZ layer 22 are set as well to dl and d2 in this
embodiment, and the total of dl and d2 has been set to a range of 6 mm to 20 mm.
Of course, in fact, the depths of the first stage molten layer 16 and the first stage p
transformation layer 18 are a little different in certain cases from the thicknesses of
the first stage molten and resolidified layer 20 and the first stage HAZ layer 22
20 depending on influences and solidification shrinkage of the undulations on the
surface of the raw material cast product (rectangular cross-section titanium cast
product 10) and influences brought about by elimination of the voids present in the
surface layer, but a difference between them is only small, and they can be regarded
as substantially the same. A lower limit of the first stage melting depth and the first
25 stage HAZ layer depth dl + d2 is particularly preferably set to 8 mm or more and an
upper limit is particularly preferably set to 16 mm or less, more preferably 13 mm or
31
less even in the range described above.
[0062]
[Second stage surface heat treatment process P3] to [second stage cooling process
P4]
5 The first stage molten and resolidified layer 20 and the first stage HAZ layer
22 are formed in a depth of 6 mm to 20 mm from the surface on the surface 10A out
of the two wide surfaces which are the surfaces to be rolled in the rectangular crosssection
titanium cast product 10 in the first stage surface heat treatment process PI
and the first stage cooling process P2 each described above. Then, the surface of
10 the first stage molten and resolidified layer 20 is irradiated again, as shown in a
central left side of FIG. 3 (B), with an electron beam in the second stage surface heat
treatment process P3 to rapidly heat the surface layer of the first stage molten and
resolidified layer 20. In the second stage surface heat treatment process P3, the
surface of the rectangular cross-section slab is irradiated with an electron beam while
15 continuously moving the irradiation gun relatively to the rectangular cross-section
slab in a way similar to the first stage surface heat treatment process PI, whereby
almost the whole surface of the surface 10A is reheated, and the reheated layer 24 is
quenched by removing heat to a parent metal side and turned into a micro structural
refinement layer 26.
20 [0063]
In this regard, the surface 10A of the rectangular cross-section titanium cast
product 10 is irradiated with an electron beam in the second stage surface heat
treatment process P3 to reheat the surface 10A (surface of the first stage molten and
resolidified layer 20) of the rectangular cross-section titanium cast product 10 so that
25 a region (region of a thickness d3) of up to a position of a depth of 1 mm or more and
less than 6 mm in a thickness direction from the outermost surface reaches the p
32
transformation point or higher, whereby the p transformation occurs. In this respect,
the region reheated to the p transformation point or higher is referred to as a reheated
layer 24 in this embodiment. The reheated layer 24 is turned into the
micro structural refinement layer 26 after being cooled.
5 [0064]
As described above, when the heating is performed to the p transformation
point or higher in a depth of 1 mm or more by irradiation with an electron beam, a
thin layer (about 0.5 to 2 mm or less: region 24A) in the outermost surface is heated
to a temperature of the melting point or higher, and the outermost surface layer is
10 molten again in many cases. Melting of the outermost surface layer shall not bring
about specific problems, and it is only necessary that the region up to the position of
a depth of 1 mm or more and less than 6 mm in a thickness direction from the
outermost surface is heated to the p transformation point or higher and turned into
the reheated layer 24. It may be also possible that the outermost surface is not
15 molten, the region of up to the position of a depth of 1 mm or more and less than 6
mm from the outermost surface is heated to the p transformation point or higher, and
the whole part of the reheated layer 24 is turned into a p transformation layer.
Accordingly, the reheated layer 24 formed in the second stage surface heat treatment
process P3 includes a case in which the reheated layer 24 is composed of a molten
20 layer (referred to as a second stage molten layer 24A in the present specification) and
a p transformation layer 24B at a lower side of the molten layer and a case in which
the reheated layer 24 is composed only of the p transformation layer 24B throughout
the whole part of the thickness direction. A case in which the outermost surface
layer of the reheated layer 24 is molten and turned into the second stage molten layer
25 24A is shown in the present embodiment.
[0065]
33
A heat input of irradiation with an electron beam in the second stage surface
heat treatment process P3 can be determined so that the region up to the position of a
depth of 1 mm or more and less than 6 mm is heated to the p transformation point or
higher. That is, the heat input can be controlled so that the thickness d3 of the
5 reheated layer 24 is 1 mm or more and less than 6 mm.
[0066]
In irradiation with an electron beam in the first stage surface heat treatment
process PI, the heat input is controlled so that the total of the melting depth
(accordingly a depth heated to the melting point or higher) dl and the depth d2 of the
10 HAZ layer d2 is set to 6 mm to 20 mm so as to control the melting depth dl to be 3
mm to 10 mm. On the other hand, in irradiation with an electron in the second
stage surface heat treatment process P3, the heat input is controlled so that the depth
d3 heated to the p transformation point or higher is 1 mm or more and less than 6
mm. The p transformation point is a markedly lower temperature than the melting
15 point, and the depth heated to the p transformation point or higher from the surface
which is prescribed in the second stage surface heat treatment process P3 is smaller
than the melting depth in the first stage surface heat treatment process PI.
Accordingly, the heat input is controlled so that the heat input (per unit time and unit
area) in irradiation with an electron beam in the second stage surface heat treatment
20 process P3 is decreased as compared with the heat input in irradiation with an
electron beam in the first stage surface heat treatment process PI. The specific
means for controlling the heat input includes, for example, controlling an output of
the irradiation gun to be a lower level than the first stage surface heat treatment
process PI, increasing a beam diameter of the irradiation gun more than a level in the
25 first stage surface heat treatment process PI, and raising a gun moving rate
(irradiation position moving rate) more than a rate in the first stage surface heat
34
treatment process PI. Any one of the above means can be applied, or two or more
means can be applied in combination. The specific heat input in irradiation with an
electron beam in the second stage surface heat treatment process P3 shall not
specifically be restricted, and the heat input can be usually about 15 to 80 J per unit
5 area (per 1 cm ).
[0067]
Also in the second stage surface heat treatment process P3, as is the case
with the first stage surface heat treatment process PI, an electron beam is radiated
while continuously moving the irradiation gun relatively to the cast product in order
10 to treat almost all area of the surface 10A of the cast product (rectangular crosssection
titanium cast product 10). In the above case, when a beam passes on a part
adjacent to a part once irradiated, 1/2 to 1/4 of the part once irradiated is allowed to
be irradiated once again, and the parts are treated so that the desired treatment depth
can be achieved in all regions, whereby the effects of the present invention can
15 sufficiently be exerted. In the above case, the reheated layer 24 in the part in which
irradiation is finished is quenched by removing heat to the parent metal (inside of the
cast product). In this regard, in a case where the outermost surface layer of the
reheated layer is molten and the second stage molten layer 24A is present, the second
stage molten layer 24A is solidified by quenching, is further quenched to the p
20 transformation point or lower, and turned into a second stage molten and resolidified
layer 26A having a phase micro structure. Also, a second stage p transformation
layer 24B is heated as well to a temperature of higher than a p transformation point
and then quenched to a temperature lower than the p transformation point to be
turned into a second stage HAZ layer 26B having a phase microstructure, and the
25 whole of the above layers 26A and 26B constitutes a micro structural refinement layer
26 described later. Such the cooling process corresponds to the second stage
35
cooling process P4.
[0068]
Also in the second stage surface heat treatment process P3 to the second
stage cooling process P4, the rectangular cross-section titanium cast product 10 can
5 be placed, as is the case with the first stage surface heat treatment process PI to the
first stage cooling process P2, on a water cooled base composed of a metal (metal)
having high heat conductivity so that the rectangular cross-section titanium cast
product 10 is prevented from being wholly heated by irradiation with an electron
beam, and heat removing to the parent metal side is allowed to quickly proceed in the
10 second stage cooling process P4, whereby the effects of the present invention can be
further enhanced.
[0069]
Also, in the present embodiment in which the surface of the rectangular
cross-section titanium cast product is irradiated with an electron beam while
15 continuously moving the irradiation gun relatively to the rectangular cross-section
titanium cast product in the second stage surface heat treatment process P3, while the
second stage surface heat treatment process P3 proceeds, as is the case with the first
stage surface heat treatment process PI to the first stage cooling process P2, by
irradiating a some portion on the surface of the rectangular cross-section titanium
20 cast product with an electron beam, the second stage cooling process P4 proceeds in
another portion (portion in which irradiation has already been finished).
[0070]
In this regard, the heat input per unit time and unit area in irradiation with
an electron beam in the second stage surface heat treatment process P3 is small as
25 compared with the heat input in irradiation with an electron beam in the first stage
surface heat treatment process PI, and therefore the cooling rate in the second stage
36
cooling process P4 by removing heat to the parent metal side after irradiation with an
electron beam is increased more than the cooling rate in the first stage cooling
process P2. That is, a solidifying rate of the second stage molten layer 24A in the
second stage cooling process P4 in a case where the surface of the reheated layer 24
5 is molten and turned into the second stage molten layer 24A is larger than a
solidifying rate of the first stage molten layer 16 in the first stage cooling process P2,
and the subsequent cooling rate in the second stage cooling process P4 is larger as
well than the cooling rate of the first stage cooling process P2. Further, a cooling
rate at which the second stage p transformation layer 24B is cooled to a temperature
10 lower than the p transformation point in the second stage cooling process P4 is larger
as well than a cooling rate of the first stage (3 transformation layer 24B in the first
stage cooling process P2. . Accordingly, the microstructure of the reheated layer 24
solidified and cooled in the second stage cooling process P4 is turned into
sufficiently finer microstructure (fine acicular microstructure) than the
15 micro structures (micro structures of the first stage molten and resolidified layer 20
and the first stage HAZ layer 22) cooled and solidified in the first stage cooling
process P2. Thus, the layer obtained by refining the microstructure of the reheated
layer 24 is referred to as the micro structural refinement layer 26.
[0071]
20 Also, the first stage molten and resolidified layer 20 and the first stage HAZ
layer 22 which are formed in the first stage surface heat treatment process PI and the
first stage cooling process P2 remain in an inside of the microstructural refinement
layer 26. In this respect, the first stage molten and resolidified layer 20 and the first
stage HAZ layer 22 remaining in an inside of the microstructural refinement layer 26
25 are turned into relatively coarse acicular microstructure as compared with the
microstructure of the microstructural refinement layer 26. In the present invention,
37
the first stage molten and resolidified layer 20 and the first stage HAZ layer 22
remaining in an inside of the micro structural refinement layer 26 are referred
generically to as "an inside microstructural refinement layer". The term "relatively
coarse" referred to herein means that " the first stage HAZ layer 22 is refined to less
5 extent as compared with the microstructural refinement layer 26", and according to
general standards, "the inside microstructural refinement layer" is composed as well
of a fine acicular micro structure.
[0072]
In this regard, if the depth d3 which is heated to the p transformation point
10 or higher by irradiation with an electron beam in the second stage surface heat
treatment process P3 is less than 3 mm, the microstructural refinement layer 26 is too
thin, and therefore an effect of surely preventing flaws from being formed on the
surface of the hot rolled sheet is not sufficiently obtained by microstructural
refinement. On the other hand, if the depth d3 is 6 mm or more, the cooling rate by
15 removing heat to the parent metal after irradiation with an electron beam is delayed,
and satisfactory microstructural refinement is not necessarily sufficiently obtained.
Accordingly, irradiation with an electron beam in the second stage surface heat
treatment process P3 is controlled so that the depth d3 which is heated to the p
transformation point or higher is 1 mm or more and less than 6 mm. That is, the
20 reheated layer 24 heated to the p transformation point or higher shall be regarded to
be in a position of 1 mm or more and less than 6 mm from the surface.
[0073]
A lower limit of the depth (thickness of the reheated layer 24) d3 which is
heated to the p transformation point or higher by irradiation with an electron beam in
25 the second stage surface heat treatment process P3 is particularly set to 2 mm or
more and an upper limit is set to 5 mm or less, preferably, even in the range of 1 mm
38
or more and less than 6 mm described above.
[0074]
Also, the second stage surface heat treatment may be earned out plural
times, and it is important that in any heat treatment, a depth is set to be smaller than a
5 depth in which the micro structure is reformed at least in the first stage surface heat
treatment.
[0075]
In this regard, an extent for quantitatively representing refinement of
microstructure (acicular micro structure) in the microstructural refinement layer 26
10 obtained by cooling the reheated layer 24 in the second stage cooling process
(including a case in which the process is carried out plural times) can be represented
by a state in which heat treatment prior to hot rolling or equivalent one is carried out
to recrystallize the microstructure instead of the state of the microstructure as it is.
That is, it is only necessary that the number of crystal grains having a grain diameter
15 of 3 mm or more is 5 or less per m of the surface of the slab in a state in which the
microstructure is turned into a fine recrystallized granular microstructure of a random
orientation. That is, it is difficult to determine an extent of refining the acicular
microstructure obtained by reheating and quenching, as it is. Accordingly, the grain
diameter staying in a state of heat treatment prior to hot rolling or equivalent one is
20 used in order to quantitatively represent the refinement of the microstructural
refinement layer 26 obtained by reheating and quenching. The treatment equivalent
to heat treatment prior to hot rolling means heat treatment at 820°C for 240 minutes.
[0076]
In a case where the number of crystal grains having a grain diameter of 3
25 mm or more exceeds 5 or more per m of the surface of the slab in a state in which a
microstructure (acicular microstructure) in the microstructural refinement layer 26 is
39
recrystallized by carrying out the treatment equivalent to heat treatment prior to hot
rolling, that is, a state in which the micro structure is turned into an equiaxed fine
granular micro structure of a random orientation, the refinement is not considered to
be achieved more notably than in a case where the second stage surface heat
5 treatment process to the second stage cooling process are not carried out (that is, a
case where a product of a slab for hot rolling is prepared in the first stage surface
heat treatment process to the first stage cooling process), and it becomes difficult to
surely and stably prevent relatively large dents and surface flaws on the hot rolled
sheet from being formed in the beginning of hot rolling. In the microstructural
10 refinement layer 26 after the heating prior to hot rolling or equivalent one, the
number of crystal grains having a grain diameter of 3 mm or more is particularly
preferably 1 or less even in the case of 5 or less per m of the surface of the slab.
The crystal grain diameters can surely be obtained by carrying out the second stage
surface heat treatment process in which the region having a depth of 1 mm or more
15 and less than 6 mm from the surface is heated to the p transformation point or higher.
[0077]
The crystal grain diameter means a crystal grain diameter in a corresponding
region of a cross section in a thickness direction of the slab. To be specific, the
crystal grain diameter means a crystal grain diameter obtained by measuring the
20 grain diameters of all crystal grains in a depth from the outer surfaces of the wide
surfaces 10A, 10B (surfaces to be rolled) up to a depth including the whole of the
corresponding region in a thickness direction of the slab, for example, in a cross
section (cross section in thickness direction) vertical to a length direction (rolling
direction D) of the slab and measuring the grain diameters throughout a prescribed
25 distance in a width direction of the slab. In this connection, the grain diameters are
measured preferably throughout a distance of about 1/2 of a width (half width) of
40
the slab in order to obtain the grain diameters with a high reliability.
[0078]
Further, in the second stage surface heat treatment process P3, at least one
kind of a-phase stabilizing elements and neutral elements may be allowed to be
5 present in the surface of the rectangular cross-section titanium cast product, and the
a-phase stabilizing elements and the neutral elements may be molten together in
melting the surface part of the rectangular cross-section titanium cast product to
allow the a-phase stabilizing elements and the neutral elements to be present densely
in the surface part. At least one kind of powders, chips, wires, thin films and swarfs
10 can be used in combination as a material for the a-phase stabilizing elements and the
neutral elements. The a-phase stabilizing elements and the neutral elements are
preferably Al, Sn and Zr. Addition of these elements to titanium makes it possible
to suppress the crystal grain growth in an a single phase region. Accordingly, the
crystal grains can be maintained fine even when the a phase is heated to be a high
15 temperature area in hot rolling. A concentration of more than a certain extent is
necessary for suppressing the crystal grain growth. At least one kind of the a-phase
stabilizing elements and the neutral elements is added preferably in an amount of
0 % or more and less than 2 % in terms of a total mass% in a range of a depth of 4
mm or less from the surface of the titanium cast product for hot rolling.
20 [0079]
Also, in the second stage surface heat treatment process P3, at least one kind
of [3-phase stabilizing elements may be allowed to be present in the surface of the
rectangular cross-section titanium cast product, and the p-phase stabilizing elements
may be molten together in melting the surface part of the rectangular cross-section
25 titanium cast product to allow the p-phase stabilizing elements to be present densely
in the surface part. At least one kind of powders, chips, wires, thin films and swarfs
41
can be used in combination as a material for the p-phase stabilizing elements. The
p-phase stabilizing element includes V, Mo, Fe, Cr, Mn, Ta, Nb, Ni, Cr, Co, Cu, W
and the like. However, in titanium, an element such as W having a high melting
point is causative of HDI (high density inclusion) and becomes a starting point of
5 fatigue failure when such element without being molten and sufficiently diffused
remains in the titanium material, and therefore such element has to be carefully used.
The p-phase stabilizing element can be classified into a complete solid solution type
such as V, Mo, Ta, Nb and the like and a eutectoid type such as Fe, Cr, Mn, Co, Ni,
Cu and the like. In the eutectoid type, each of the p-phase stabilizing elements has
10 a small solid solubility but has a large p stability, and therefore the p-phase
stabilizing elements of the eutectoid type are more effective even when being added
in a smaller amount. The p-phase stabilizing element is contained in the surface of
the rectangular cross-section titanium cast product by melting the p-phase stabilizing
element together in the second stage surface heat treatment process P3. As a result,
15 the hardenability is enhanced by adding the p-phase stabilizing elements, whereby
the finer microstructure can be obtained. "Enhancement in the hardenability"
referred to herein means that in the continuous-cooling transformation diagram
(CCT-curve), a nose of transformation in cooling is shifted to a long time side by
adding the p-phase stabilizing elements to the surface of the titanium cast product,
20 whereby the cast product is transformed at low temperature. The transformation at
low temperature makes it possible to increase the nucleation sites to increase and
refine the crystal grains. The microstructure stays in a state of a two phase of a + p
in heating in hot roiling, and a p phase is formed in a grain boundary of an a phase,
whereby grain growth in the a phase are suppressed. Accordingly, a hot rolled
25 titanium material having no surface flaws formed is produced due to that the crystal
grains in hot rolling are maintained in a state of fine crystal grains. At least one
42
kind of the {3-phase stabilizing elements is included preferably in an amount of 1.5 %
or less in terms of total mass% in a range of a depth of 4 mm or less from the surface
of the titanium cast product for hot rolling.
[0080]
5 Alternatively, in the second stage surface heat treatment process P3, at least
one kind of the a-phase stabilizing elements and the neutral elements and at least one
kind of the p-phase stabilizing elements may be allowed to be present in the surface
of the rectangular cross-section titanium cast product, and the a-phase stabilizing
elements, the neutral elements and the p-phase stabilizing elements, a-phase
10 stabilizing elements, and the neutral elements may be molten together in melting the
surface part of the rectangular cross-section titanium cast product to allow the aphase
stabilizing elements, the neutral elements, and the p-phase stabilizing elements
to be present densely in the surface part. In this case, at least one kind of the aphase
stabilizing elements and the neutral elements is included preferably in an
15 amount of 0 % or more and less than 2.0 % in terms of total mass%, and at least one
kind of the p-phase stabilizing elements is included preferably in an amount of 1.5 %
or less in terms of total mass% in a range of a depth of 4 mm or less from the surface
of the titanium cast product for hot rolling.
[0081]
20 When the second stage surface heat treatment is carried out plural times, the
operation of allowing the a-phase stabilizing elements, the neutral elements, and the
p-phase stabilizing elements to be present densely in the surface part is carried out
preferably in the final heat treatment.
[0082]
25 When the p-phase stabilizing element is added, recrystallization is not
brought about by heat treatment at 820°C for 240 minutes, and the micro structure
43
stays in a state of an acicular microstructure in a certain case. In such case, it is
difficult to measure accurately the crystal grain diameter. In general, however,
acicular microstructure is finer than recfystallized structure, and therefore formation
of surface flaws can be suppressed even after hot rolling.
5 [0083]
One surface 10A out of the two wide surfaces 10A and 10B ( surfaces to be
rolled in hot rolling) of the rectangular cross-section titanium cast product 10 is
subjected to the first stage surface heat treatment process, the first stage cooling
process, the second stage surface heat treatment process, and the second stage
10 cooling process in the manners described above, and then, for example, the
rectangular cross-section titanium cast product 10 is inverted to subject the other
surface 10B to the first stage surface heat treatment process, the first stage cooling
process, the second stage surface heat treatment process, and the second stage
cooling process in the same manners as described above. In some cases, after one
15 surface 10A is subjected to the first stage surface heat treatment process to the first
stage cooling process, the other surface 10B may be subjected to the first stage
surface heat treatment process to the first stage cooling process, and then the
respective surfaces 10A and 10B may be subjected to the second stage surface heat
treatment process to the second stage cooling process in order.
20 [0084]
In the embodiment described above, the two wide surfaces 10A and 10B
(surfaces to be rolled in hot rolling, and chamfers 11 are included if present; refer to
FIG. 2) are treated out of the four surfaces 10A to 10D in a casting direction D
(direction in which the cast product is pulled out in DC slab casting). However, the
25 narrow surfaces 10C and 10D (surfaces which are edge sides in hot rolling) (refer to
FIG. 2) out of the four wide surfaces may be subjected as well to the same treatment
44
as the treatment to which the two wide surfaces 10A and 10B are subjected.
[0085]
That is, a slab of a hot rolling material is subjected to reduction in hot
rolling, whereby at least a part of a surface at an edge side of the material goes
5 around usually toward a sheet surface side of the hot rolled sheet. Accordingly, if a
micro structure on the surface layer of the surface at the edge side of the rectangular
cross-section cast product is coarse, or many defects are present, surface flaws such
as dents are likely to be formed on the surface close to both ends in a width direction
of the hot rolled sheet. With regard to this matter, subjecting the surface at the edge
10 side of the rectangular cross-section cast product as well to the same reforming
treatment as described above makes it possible to effectively prevent such the matter
as described above from taking place.
[0086]
When the two surfaces 10C and 10D at the edge sides are subjected as well
15 to the first stage surface heat treatment process, the first stage cooling process, the
second stage surface heat treatment process, and the second stage cooling process in
the same manners as described above, the respective processes to which the two
surfaces 10C and 10O at the edge sides are subjected may be carried out after the
respective processes to which the two wide surfaces 10A and 10B are subjected are
20 finished. Alternatively, the respective processes for the two surfaces 10C and 10D
may be appropriately carried out between the respective processes for the two wide
surfaces lOAand 10B.
[0087]
Microstructure of a cross-section in the vicinity of a surface (for example,
25 the vicinity of the sheet surface 10A) of the titanium cast product for hot rolling
obtained by subjecting the titanium cast product for hot rolling obtained in the
45
manner described above, that is, the rectangular cross-section titanium cast product
to reforming treatment is shown schematically in FIG. 4. Further, microstructure in
a state in which the above titanium cast product for hot rolling is subjected to heat
treatment equivalent to the heating prior to hot rolling is shown schematically in FIG.
5 5. FIG. 6 is a cross-sectional observation photograph showing a refinement layer,
an inside refinement layer and a cast solidification microstructure in a surface part of
the titanium cast product for hot rolling corresponding to FIG. 4.
[0088]
The titanium cast product 30 for hot rolling shown in FIG. 4 corresponds to
10 a state (state shown at a right side of FIG 3 (B)) after finishing the second stage
cooling process. In the titanium cast product 30 for hot rolling, a parent metal part
28 (inner part of the slab than the first stage HAZ layer 22) is composed of a coarse
microstructure (cast solidification microstructure) as cast, and a part closer to the
surface side than the FtAZ layer 22 has a microstructural refinement layer 26
15 composed of acicular microstructure in the outermost surface and an inside
microstructural refinement layer 27 composed of an acicular microstructure in an
inside of the microstructural refinement layer 26. As described above, the inside
microstructural refinement layer 27 is composed of the first stage molten and
resolidified layer 20 and the first stage HAZ layer 22 each remaining in an inside of
20 the microstructural refinement layer 26 after carrying out the second stage surface
heat treatment process P3 and the second stage cooling process P4.
[0089]
FIG 6 (photograph) shows a surface part of the titanium cast product for hot
rolling which corresponds to a state (state shown at a right side of FIG. 3 (B)) after
25 finishing the second stage cooling process. In this titanium cast product 30 for hot
rolling, a parent metal 28 (part in an inner side of the slab than the inside
46
microstructural refinement layer 27 (the first stage HAZ layer 22) is composed of
coarse micro structure as cast. The surface of the titanium cast product 30 for hot
rolling is composed of double layer fine acicular micro structure of the
microstructural refinement layer 26 in the outermost surface and the inside
5 microstructural refinement layer 27 in an inner part of the slab than the
microstructural refinement layer 26. The inside microstructural refinement layer 27
can be observed in the form of two layers in a certain case depending on the
conditions of the first stage surface heat treatment process PI and the first stage
cooling process P2. Also, the microstructural refinement layer 26 can be observed
10 in the form of two layers in a certain case depending on the conditions of the second
stage surface heat treatment process P3 and the second stage cooling process P4.
Accordingly, the microstructural refinement layer 26 and the inside microstructural
refinement layer 27 can be observed in the form of three layers or four layers in a
certain case.
15 [0090]
As shown in FIG. 5, when the fine acicular micro structure of the
microstructural refinement layer 26 and the inside microstructural refinement layer
27 is recrystallized in a state in which the heat treatment equivalent to heating prior
to hot rolling (at 820°C for 240 minutes) has been carried out, particularly the
20 microstructural refinement layer 26 (a second stage molten and resolidified layer 26A
and a second stage HAZ layer 26B) at an outermost surface side of the slab is turned
into marked fine recrystallization equiaxed micro structure in which the number of
crystal grains having a grain diameter of 3 mm or more is 5 or less per m of the slab
surface. Also, the microstructure (inside microstructural refinement layer 27) of the
25 first stage molten and resolidified layer 20 and the first stage HAZ layer 22 each
present at an inner side of the slab than the microstructural refinement layer 26 is
47
refined to less extent than the microstructural refinement layer 26. In the first stage
molten and resolidified n layer 20, voids originating in casting are almost eliminated
by melting in the first stage surface heat treatment process. Voids 10Q remain
slightly in some parts, but an inside of the voids 10Q stays in vacuum, so that the
5 voids are pressed and eliminated in hot rolling and turned into harmlessness in a hot
rolled sheet product. Further, the outermost surface of the sheet surface 10A is
turned into a relatively smooth surface by melting in the first stage surface heat
treatment process.
[0091]
10 The recrystallization temperature is varied depending on the kind and the
concentration of impurities contained in the titanium slab, and the prior
microstructure. In general, if the heating temperature prior to hot rolling is 700°C
or higher, the microstructure can be recrystallized during heating prior to hot rolling,
but when the p-phase stabilizing element is added, a molten layer d4 at the second
15 stage remains in the form of a fine acicular microstructure in a certain case without
being recrystallized. However, the micro structures are very fine, and therefore
defects that turns into flaws formed in the subsequent hot rolling stay in a level
which makes no great difference as compared with a case in which the molten layer
d4 is recrystallized.
20 [0092]
In using actually the thus obtained titanium cast product for hot rolling, it is
hot-rolled into a hot rolled sheet having a prescribed sheet thickness. The method
of hot rolling shall not specifically be restricted, and when it is hot rolled into a thin
hot-rolled sheet product, coil rolling is usually applied. Also, the sheet thickness
25 after finishing hot rolling in the above case shall not specifically be restricted, and it
is usually 3 mm to 8 mm. The hot rolling conditions shall not specifically be
48
restricted, and the cast product is heated, as is the case with usual hot rolling, at 720
to 920°C for 60 to 420 minutes to initiate hot rolling at temperature falling in the
above range, and the hot rolling can be finished at a temperature of a room
temperature or higher according to the capacity of the rolling mill.
5 [0093]
The micro structural state of the cross section in the vicinity of the sheet
surface 10A in the hot rolled sheet after hot rolling is substantially equivalent to the
micro structure of a state in which the cast product is subjected to heat treatment
equivalent to the heating prior to hot rolling shown in FIG. 5 excluding extension of
10 the crystal grains in a rolling direction in the hot rolling. That is, in the
microstructural refinement layer 26 and the inside microstructural refinement layer
27 which are refined by melting treatment before the hot rolling, the microstructure
itself is worked and extended as well after the hot rolling, but the microstructure
maintains a sufficiently refined state as compared with the part 28.
15 [0094]
In the above embodiment, the rectangular cross-section titanium cast
product obtained by EBR - DC slab casting is subjected to the respective processes
as it is, that is, as a material for manufacturing a titanium cast product for hot rolling
in the form of a material as cast without passing through a breakdown process carried
20 out by hot working such as slab rolling and forging and passing through a cutting
process for finishing a surface. That is, a material having a cast surface as cast (cast
surface on which marked undulations originating in casting are present and which
has casting defects such as many voids and the like on a surface part and includes a
surface of a so-called black mill scale skin) is used. The effects of the present
25 invention can most effectively be exerted when the present invention is applied to
such the cast product as cast. However, the present invention is permitted as well to
49
be applied in certain cases to a cast product in which a layer up to several mm from
an outermost surface is subjected to cutting work and removed in order to remove
undulations on a cast surface and voids close to the surface, that is, a cast product of
a state in which a so-called descaled white skin appears. Further, the present
5 invention is permitted as well to be applied to a cast product with so-called partially
descaled white skin obtained by removing a part of an oxygen-enriched layer
(maximum about 1 mm) by a cutting work, the oxygen-enriched layer being formed
on a surface due to high temperature in taking out the cast product from a melting
furnace and a cooling furnace opened after casting and exposing the cast product to
10 air .
[Example(s)]
[0095]
The examples of the present invention shall be explained based on
experiments of Test No. 1 to 38 shown in Table 1, Tables 2 (Table 2A and Table 2B),
15 Tables 3 (Table 3 A and Table 3B), Tables 4 (Table 4A and Table 4B), Tables 5 (Table
5A and Table 5B), Tables 6 (Table 6A and Table 6B), and Tables 7 (Table 3A and
Table 7B) together with a reference example (- slab-rolled slab) according to the
conventional methods and comparative examples (comparative examples in which
the treatments of the present invention were not carried out at all and comparative
20 examples in which treatments deviating from the conditions of the present invention
were carried out).
[0096]
[Test No. 1 to 3 (Table 1)]
Test No. 1 shown in Table 1 is a reference example carried out by a
25 conventional method, wherein an electron beam molten cast product of pure titanium
of JTS Class 1 having a cross section of a width of about 1300 mm x a thickness of
50
about 400 mm and a length of about 7500 mm was hot rolled to be a cast product of a
width of about 1210 mm and a thickness of about 260 mm by slab rolling, a long slab
having a length of about 7000 mm was cut out, the whole surface of the slab was
subjected to cutting work by about 5 mm, and a slab-rolled slab obtained by
5 subjecting the slab to cutting work of chamfers having a width of 30 mm at an angle
of 45 degrees between upper and lower surfaces and side surfaces was used. The
dimensions of the slab are a width of about 1200 mm x a thickness of about 250 mm
x a length of about 7000 mm.
Test No. 2 shown in Table 1 is a comparative example, wherein a pure
10 titanium slab of JIS Class 1 having a cross section of a width of about 1220 mm x a
thickness of about 270 mm and a length of about 7000 mm was obtained by DC
casting by EBR, the whole surface of the slab was subjected to cutting work by about
10 mm, and a DC slab obtained by subjecting the above slab to cutting work of
chamfers having a width of 30 mm at an angle of 45 degrees between upper and
15 lower surfaces and side surfaces was used. The dimensions of the slab are a width
of about 1200 mm x a thickness of about 250 mm x a length of about 7000 mm.
Test No. 3 shown in Table 1 is a comparative example, wherein a pure
titanium slab of JIS Class 1 having a cross section of a width of about 1220 mm x a
thickness of about 270 mm and a length of about 7000 mm was obtained by DC
20 casting by EBR, the whole surface of the slab was not subjected to cutting work, and
a DC slab obtained by subjecting the above slab to cutting work of chamfers having a
width of 30 mm at an angle of 45 degrees between upper and lower surfaces and side
surfaces was used. The dimension of the slab is the same as those of the cast
product as DC cast.
25 The above slabs were inserted into a furnace at 820°C and then heated for
about 240 minutes to manufacture a hot rolled sheet coil having a thickness of 5 mm
51
by a continuous hot rolling strip mill. The sheet coil was allowed to pass through a
continuous pickling line containing nitric hydrofluoric acid to dissolve about 50 urn
per one surface. Then, both sheet surfaces of the sheet coil were visually observed
to measure the number of surface flaws. The number of the surface flaws generated
5 in a frame of 1 meter square was measured in 10 to 15 fields to determine the
average of the number of surface flaws. When the sheet length for observation does
not reach 1 m, a surface area of the hot-rolled sheet observed was converted to be 1
m to calculate the number of the surface flaws per m .
In this regard, in accordance with evaluation criteria for surface flaws on a
10 hot rolled sheet, 0.3 or less surface flaws per m were evaluated as passed, and 0.3 or
more surface flaws per m were evaluated as failure. The above evaluation criteria
shall apply to the respective Test Nos. 4 to 38 described later.
As shown in Table 1, in a slab-rolled material of Test No. 1, the density of
the flaws was lower than 0.3 per m that is the passing point, and the surface stayed
15 in a good condition. However, in the cases of both Test Nos. 2 and 3, many surface
flaws were generated on the surfaces of the hot rolled sheets, and the sheets were
evaluated as failure.
The good surface condition obtained in the slab-rolled material of Test No. 1
was obtained by passing through a process of slab rolling which takes labor, and it is
20 not an effect exerted by the present invention.
[0097]
[Test Nos. 4 to 15 (Table 2A and Table 2B)]
A DC slab of JIS Class 1 pure titanium having the same dimensions which
was manufactured by passing through the same manufacturing processes as Test No.
25 3 was irradiated with an electron beam in a longitudinal direction by moving the slab
and repeating a process for reciprocating the slab, whereby the whole surface to be
52
rolled was irradiated with an electron beam. The side surfaces of the slab were
irradiated as well with an electron beam.
Test No. 4 is a comparative example in which the slab was subjected only to
the first stage surface heat treatment and in which the slab was not subjected to the
5 second stage surface heat treatment. In Test Nos. 5 to 15, front surfaces of the slabs
were subjected to the first stage surface heat treatment; then, the slabs were inverted,
and rear surfaces were subjected to the first stage surface heat treatment.
Subsequently, the slabs were inverted again, and the front surfaces were subjected to
the second stage surface heat treatment. Thereafter, the slabs were inverted, and the
10 rear surfaces were subjected to the second stage surface heat treatment. Then, the
side surfaces of the slabs were irradiated as well with an electron beam in a similar
manner. In this case, the irradiation conditions were varied in various manners.
The electron beam was oscillated by using an electromagnetic lens to turn the
electron beam into a rectangular cross-section form. Also, when the adjacent part
15 was irradiated, the position of the electron beam was adjusted so that only 1/3 of the
part molten previously by irradiation was molten again. A change in the
temperature in cooling after irradiation with an electron beam was measured by an
infrared thermometer to calculate the cooling rate in passing through the p
transformation point.
20 The above slabs were inserted into a furnace to 820°C and then heated for
about 240 minutes to manufacture a hot rolled sheet coil having a thickness of 5 mm
by a continuous hot rolling strip mill. The sheet coil was allowed to pass through a
continuous pickling line containing nitric hydrofluoric acid to dissolve about 50 um
per one surface. Then, both sheet surfaces of the sheet coil were visually observed
25 to measure the number of surface flaws.
All of Test Nos. 5, 6, 7, 8, 10, 11, 12 and 14 are the examples of the present
53
invention and had, as shown in Table 2A and Table 2B, the form (at least double
layer acicular micro structure) of the surface part prescribed in the present invention,
and the examples presented the micro structure having the crystal grain diameter
prescribed in the present invention after subjected to the heat treatment equivalent to
5 heating prior to hot rolling; and the examples had less surface flaws after hot rolling
and exceeded the passing line.
On the other hand, Test Nos. 4, 9, 13 and 15 are comparative examples in
which the form of the surface part and the processing conditions each prescribed in
the present invention were not satisfied, and they had, as shown in Table 2A and
10 Table 2B, many surface flaws after hot rolling and the surface condition of the hot
rolled sheets were evaluated as failure.
[0098]
[Test Nos. 16 to 18 (Table 3 A and Table 3B)]
A DC slab of JIS Class 1 pure titanium having the same dimensions which
15 was manufactured by passing through the same manufacturing processes as Test No.
3 was irradiated with an electron beam by moving the slab and repeating a process
for reciprocating the slab, whereby the whole surface to be rolled was irradiated with
an electron beam. The side surfaces of the slab were irradiated as well with an
electron beam.
20 Test Nos. 16, 17 and 18 are examples in which the direction and the order of
irradiation were varied under the same processing conditions as in Test No. 5.
In Test No. 16, the slab was irradiated repeatedly in a width direction, and a
front surface of a slab was subjected to the first stage surface heat treatment. Then
the slab was inverted, and a rear surface was subjected to the first stage surface heat
25 treatment. Further, the slab was inverted again, and the front surface was subjected
to the second stage surface heat treatment. Thereafter, the slab was inverted, and
54
the rear surface was subjected to the second stage surface heat treatment. Then, the
side surfaces of the slab were irradiated as well with an electron beam in the same
manner.
In Test No. 17, the slab was irradiated repeatedly in a longitudinal direction,
5 and a front surface was subjected to the first stage surface heat treatment. Then, the
same surface was subjected to the second stage surface heat treatment. Further, the
slab was inverted, and a rear surface was subjected to the first stage surface heat
treatment. Thereafter, the rear surface was subjected to the second stage surface
heat treatment, and then the side surfaces of the slab were irradiated as well with an
10 electron beam in the same manner.
In Test No. 18, the slab was irradiated repeatedly in a width direction, and a
front surface was subjected to the first stage surface heat treatment. Then, the same
surface was subjected to the second stage surface heat treatment. Further, the slab
was inverted, and a rear surface was subjected to the first stage surface heat treatment.
15 Thereafter, the rear surface was subjected to the second stage surface heat treatment,
and then the side surfaces of the slab were irradiated as well with an electron beam in
the same manner.
In the above electron beam irradiations, the electron beam was oscillated by
using an electromagnetic lens to turn the electron beam into a rectangular cross-
20 section form, and when the adjacent part was irradiated, the position of the electron
beam was adjusted so that only 1/3 of the part molten previously by irradiation was
molten again.
The above slabs were inserted into a furnace to 820°C and then heated for
about 240 minutes to manufacture a hot rolled sheet coil having a thickness of 5 mm
25 by a continuous hot rolling strip mill. The sheet coil was allowed to pass through a
continuous pickling line containing nitric hydrofluoric acid to dissolve about 50 urn
55
per one surface. Then, both sheet surfaces of the sheet coil were visually observed
to measure the number of surface flaws.
All of the above Test Nos. 16, 17 and 18 are the examples of the present
invention and had, as shown in Table 3 A and Table 3B, the form of the surface part
5 prescribed in the present invention, and the examples presented the micro structure
having the crystal grain diameter prescribed in the present invention after subjected
to the heat treatment equivalent to heating prior to hot rolling. The examples had
less surface flaws after hot rolling and exceeded the passing line.
[0099]
10 [Test Nos. 19 to 23 (Table 4A and Table 4B)]
DC slabs of commercially pure titanium of various JIS Classes or ASTM
Grades or modified pure titanium (low-alloyed titanium) that have the same
dimensions which were manufactured by passing through the same manufacturing
processes as in Test No. 3 were irradiated with an electron beam in a longitudinal
15 direction by moving the slab and repeating a process for reciprocating the slab,
whereby the whole surfaces to be rolled were irradiated with an electron beam. The
side surfaces of the slabs were irradiated as well with an electron beam.
JIS Class 2 pure titanium was used in Test No. 19, JIS Class 3 pure titanium
was used in Test No. 20, JIS Class 4 pure titanium was used in Test No. 21, a
20 titanium alloy of ASTM Gr. 17 was used in Test No. 22, and a titanium alloy of
ASTM Gr. 13 was used in Test No. 23. The titanium alloys to which alloy element
was added were used in Test No. 22 and 23, but the addition amount of the alloy
element was small, and the titanium alloys were modified pure titanium regarded as
equivalent to pure titanium.
25 Front surfaces of the above slabs were subjected to the first stage surface
heat treatment. Then, the slabs were inverted, and rear surfaces were subjected to
56
the first stage surface heat treatment. Further, the slabs were inverted again, and the
front surfaces were subjected to the second stage surface heat treatment. Thereafter,
the slabs were inverted, and the rear surfaces were subjected to the second stage
.surface heat treatment. Then, side surfaces of the slabs were irradiated as well with
5 an electron beam in the same manner. In this case, the irradiation conditions were
varied in various manners. The electron beam was oscillated by using an
electromagnetic lens to turn the electron beam into a circular form. Also, when the
adjacent part was irradiated, the position of the electron beam was adjusted so that
only 1/2 of the part molten previously by irradiation was molten again in the first
10 stage surface heat treatment, and the position of the electron beam was adjusted so
that only 1/4 of the part molten previously by irradiation was molten again in the
second stage surface heat treatment.
The above slabs were inserted into a furnace to 820°C and then heated for
about 240 minutes to manufacture a hot rolled sheet coil having a thickness of 5 mm
15 by a continuous hot rolling strip mill. The sheet coil was allowed to pass through a
continuous pickling line containing nitric hydrofluoric acid to dissolve about 50 u.m
per one surface. Then, both sheet surfaces of the sheet coil were visually observed
to measure the number of surface flaws.
All of the above Test Nos. 19 to 23 are the examples of the present invention
20 and had, as shown in Table 4A and Table 4B, the form of the surface part prescribed
in the present invention, and the examples presented the micro structure having the
crystal grain diameter prescribed in the present invention after subjected to the heat
treatment equivalent to heating prior to hot rolling. The examples had less surface
flaws after hot rolling and exceeded the passing line.
25 [0100]
[Test Nos. 24 to 26 (Table 5Aand Table 5B)]
57
A cast product obtained by subjecting JIS Class 1 pure titanium slab having
a cross section of a width of 1000 mm x a thickness of 190 mm and a length of 5000
mm to DC casting by EBR was used in Test No. 24. A cast product obtained by
subjecting JIS Class 1 pure titanium slab having a cross section of a width of 950
5 mm x a thickness of 165 mm and a length of 4500 mm to DC casting by EBR was
used in Test No. 25. A cast product obtained by subjecting a slab having the same
dimensions as in Test No. 24 to DC slab casting by plasma arc-melting was used in
Test No. 26.
Front surfaces of the above slabs were subjected to the first stage surface
10 heat treatment. Then, the slabs were inverted, and rear surfaces were subjected to
the first stage surface heat treatment. Further, the slabs were inverted again, and the
front surfaces were subjected to the second stage surface heat treatment. Thereafter,
the slabs were inverted, and the rear surfaces were subjected to the second stage
surface heat treatment. Then, side surfaces of the slabs were irradiated as well with
15 an electron beam in the same manner. In this case, the irradiation conditions were
varied in various manners. The electron beam was oscillated by using an
electromagnetic lens to turn the electron beam into a rectangular cross-section form.
Also, when the adjacent part was irradiated, the position of the electron beam was
adjusted so that only 1/2 of the part molten previously by irradiation was molten
20 again in the first stage surface heat treatment, and the position of the electron beam
was adjusted so that only 1/3 of the part molten previously by irradiation was molten
again in the second stage surface heat treatment.
The above slabs were inserted into a furnace to 820°C and then heated for
about 240 minutes to manufacture a hot rolled sheet coil having a thickness of 5 mm
25 by a continuous hot rolling strip mill. The sheet coil was allowed to pass through a
continuous pickling line containing nitric hydrofluoric acid to dissolve about 50 um
58
per one surface. Then, both sheet surfaces of the sheet coil were visually observed
to measure the number of surface flaws.
The above slabs used in Test No. 24 to Test No. 26 have smaller dimensions
than that of the slab used in Test No. 5, and therefore have a small heat capacity, so
5 that cooling rates were decreased and grain diameters after the heat treatment
equivalent to heating prior to hot rolling were increased. However, the slabs
present microstructure having crystal grain diameters prescribed in the present
invention, have less surface flaws after hot rolling, and exceeded the passing line.
[0101]
10 [Test Nos. 27 to 34 (Table 6A and Table 6B)]
A DC slab of JIS Class 1 pure titanium having the same dimensions which
was manufactured by passing through the same manufacturing processes as Test No.
3 was irradiated with an electron beam by moving the slab and repeating a process
for reciprocating the slab, whereby the whole surface to be rolled was irradiated with
15 an electron beam. The side surfaces of the slab were irradiated as well with an
electron beam.
Front surfaces of the above slabs were subjected to the first stage surface
heat treatment. Then, the slabs were inverted, and rear surfaces were subjected to
the first stage surface heat treatment. Further, the slabs were inverted again, and Al
20 powders were dispersed on the front surface of the slab in Test No. 27, Sn powders
were dispersed on the front surface of the slab in Test No. 28, Fe powders were
dispersed on the front surface of the slab in Test No. 29, Cr chips were dispersed on
the front surface of the slab in Test No. 30, V chips were dispersed on the front
surface of the slab in Test No. 31, and swarfs of a titanium alloy were dispersed on
25 the front surfaces of the slabs in Test Nos. 32 to 34. Then, the front surfaces were
subjected to the second stage surface heat treatment. Thereafter the slabs were
59
inverted, and Fe powders were dispersed on the rear surfaces. Then, the rear
surfaces were subjected to the second stage surface heat treatment. Then, side
surfaces of the slabs were irradiated as well with an electron beam in the same
manner. In this case, the irradiation conditions were varied in various manners.
5 The electron beam was oscillated by using an electromagnetic lens to turn the
electron beam into a circular form. Also, when the adjacent part was irradiated, the
position of the electron beam was adjusted so that only 1/2 of the part molten
previously by irradiation was molten again in the first stage surface heat treatment,
and the position of the electron beam was adjusted so that only 1/4 of the part molten
10 previously by irradiation was molten again in the second stage surface heat treatment.
The above slabs were inserted into a furnace to 820°C and then heated for
about 240 minutes to manufacture a hot rolled sheet coil having a thickness of 5 mm
by a continuous hot rolling strip mill. The sheet coil was allowed to pass through a
continuous pickling line containing nitric hydrofluoric acid to dissolve about 50 urn
15 per one surface. Then, both sheet surfaces of the sheet coil were visually observed
to measure the number of surface flaws.
All of the above TestNos. 27 to 34 are the examples of the present invention
and had, as shown in Table 6A and Table 6B for the results for front surfaces, the
form of the front surface part prescribed in the present invention, and the examples
20 presented the microstructure having the crystal grain diameter prescribed in the
present invention after subjected to the heat treatment equivalent to heating prior to
hot rolling. The examples had less surface flaws after hot rolling and exceeded the
passing line. In addition, rear surfaces of Test Nos. 27 to 34, on which Fe powders
were dispersed, showed less surface flows around 0.02 per m and exceeded the
25 passing line.
[0102]
60
[Test Nos. 35 to 38 (Table 7A and Table 7B)]
A DC slab of JIS Class 1 pure titanium having the same dimensions which
was manufactured by passing through the same manufacturing processes as Test No.
3 was irradiated with an electron beam by moving the slab and repeating a process
5 for reciprocating the slab, whereby the whole surface to be rolled was irradiated with
an electron beam. The side surfaces of the slab were irradiated as well with an
electron beam.
In Test No. 35, front surface of the above slab was subjected to the first
stage surface heat treatment. Then, the slab was inverted, and rear surface was
10 subjected to the first stage surface heat treatment. Further, the slab was inverted
again, and the front surface was subjected to the second stage surface heat treatment.
Thereafter, the slab was inverted, and the second stage surface heat treatment was
performed. Further, the slab was inverted to disperse Fe powders on the front
surface, and then the front surface was subjected to the third stage surface heat
15 treatment. Thereafter, the slab was inverted to disperse Fe powders on the rear
surface, and then the third stage surface heat treatment was performed. In Test Nos.
37 and 38, Al powders and Fe powders were dispersed on surfaces of the slabs before
the third stage surface heat treatment, and the front and rear surfaces of the slabs
were subjected to the surface heat treatment. Also, in Test No. 36, the slab was
20 subjected to the surface heat treatment as was the case with Test No. 35. Then, the
slab was inverted, and front and rear surfaces of the slab were subjected to the fourth
stage surface heat treatment. Thereafter, side surfaces of the slab were irradiated as
well with an electron beam in the same manner. In this case, the irradiation
conditions were varied in various manners. The electron beam was oscillated by
25 using an electromagnetic lens to turn the electron beam into a circular form. Also,
when the adjacent part was irradiated, the position of the electron beam was adjusted
61
so that only 1/2 of the part molten previously by irradiation was molten again in the
first stage surface heat treatment, and the position of the electron beam was adjusted
so that only 1/4 of the part molten previously by irradiation was molten again in the
second stage surface heat treatment.
5 The above slabs were inserted into a furnace to 820°C and then heated for
about 240 minutes to manufacture a hot rolled sheet coil having a thickness of 5 mm
by a continuous hot rolling strip mill. The sheet coil was allowed to pass through a
continuous pickling line containing nitric hydrofluoric acid to dissolve about 50 urn
per one surface. Then, both sheet surfaces of the sheet coil were visually observed
10 to measure the number of surface flaws.
All of the above Test Nos. 35 to 38 are the examples of the present invention
and had, as shown in Table 7A and Table 7B, the form of the surface part prescribed
in the present invention, and the examples presented the microstructure having the
crystal grain diameter prescribed in the present invention after subjected to the heat
15 treatment equivalent to heating prior to hot rolling. The examples had less surface
flaws after hot rolling and exceeded the passing line.
[0103]
[Table 1]
20
75
[0116]
Heretofore, preferred embodiments of the present invention have been
described in detail with reference to the appended drawings, but the present invention
is not limited thereto. It should be understood by those skilled in the art that various
5 changes and alterations may be made without departing from the spirit and scope of
the appended claims.
[Reference Signs List]
[0117]
10 rectangular cross-section titanium cast product
10 lOAto 10D surfaces
12 electron beam irradiation gun
16 first stage molten layer
20 first stage molten and resolidified layer
24 reheated layer
15 26 micro structural refinement layer
30 cast product for manufacturing hot rolled titanium sheet
40 hot rolled sheet
PI first stage surface heat treatment process
P2 first stage cooling process
20 P3 second stage surface heat treatment process
P4 second stage cooling process

[Claim 1]
A titanium cast product for hot rolling composed of commercially pure
5 titanium, the titanium cast product comprising:
a microstructural refinement layer having acicular micro structure on an outermost
layer of a surface layer to be rolled; and
an inside microstructural refinement layer having acicular micro structure provided in
an inside of the microstructural refinement layer,
10 wherein cast solidification micro structure is present more inward than the inside
microstructural refinement layer,
wherein the microstructural refinement layer has finer micro structure than the inside
microstructural refinement layer,
wherein the microstructural refinement layer is present in a range of a depth of 1 mm
15 or more and less than 6 mm from the surface, and
wherein the inside microstructural refinement layer is present in an inside of the
microstructural refinement layer in a range of a depth of 3 mm or more and 20 mm or
less from the surface.
[Claim 2]
20 The titanium cast product for hot rolling according to claim 1, comprising
at least one kind of a-phase stabilizing elements and neutral elements in an amount
of 0 % or more and less than 2.0 % in terms of total mass% in a range of a depth of 4
mm or less from the surface.
[Claim 3]
25 The titanium cast product for hot rolling according to claim 1, comprising
at least one kind of p-phase stabilizing elements in an amount of 1.5 % or less in
77
terms of total mass % in a range of a depth of 4 mm or less from the surface.
[Claim 4]
The titanium cast product for hot rolling according to claim 1, comprising,
in a range of a depth of 4 mm or less from the surface,
5 at least one kind of a-phase stabilizing elements and neutral elements in an amount
of 0 % or more and less than 2.0 % in terms of total mass%, and
at least one kind of p-phase stabilizing elements in an amount of 1.5 % or less in
terms of total mass %.
[Claim 5]
10 The titanium cast product for hot rolling according to claim 1, wherein the
number of crystal grains having a crystal grain diameter of 3 mm or more is 5 or less
per m of the surface in a state of room temperature after heat treatment at 820°C for
240 minutes.
[Claim 6]
15 A method for manufacturing a titanium cast product for hot rolling, the
method comprising:
a first stage surface heat treatment process of heating a surface of a cast
product material composed of commercially pure titanium to be rolled in hot rolling
to heat a region of a depth of 6 mm or more and 20 mm or less from the surface to a
20 p transformation point or higher and to melt a range of a depth of 3 mm or more and
10 mm from the surface, and a first stage cooling process of cooling the cast product
material to temperature lower than the p transformation point after the first stage
surface heat treatment process; and
a second stage surface heat treatment process of reheating the surface
25 subjected to the first stage surface heat treatment process and the first stage cooling
process to heat a region of a depth of 1 mm or more and less than 6 mm from the
78
surface to the p transformation point or higher, and a second stage cooling process of
cooling the cast product material to temperature lower than the p transformation
point after the second stage surface heat treatment process.
[Claim 7]
5 The method for manufacturing a titanium cast product for hot rolling
according to claim 6, wherein a heat input per unit area in the second stage surface
heat treatment process is set to be lower than a heat input per unit area in the first
stage surface heat treatment process.
[Claim 8]
10 The method for manufacturing a titanium cast product for hot rolling
according to claim 6, wherein an electron beam is radiated while continuously
moving an electron beam radiation gun in a direction parallel to the surface of the
cast product material in the respective processes of the first stage surface heat
treatment process and the second stage surface heat treatment process.
15 [Claim 9]
The method for manufacturing a titanium cast product for hot rolling
according to claim 6, wherein the first stage cooling process and the second stage
cooling process are carried out by removing heat to a parent metal side of the cast
product material.
20 [Claim 10]
The method for manufacturing a titanium cast product for hot rolling
according to claim 6, wherein the cast product material is allowed to pass through the
p transformation point at a cooling rate of 60°C/minute or more in the second stage
cooling process.
25 [Claim 11]
The method for manufacturing a titanium cast product for hot rolling
79
according to claim 6, wherein the second stage surface heat treatment process and the
second stage cooling process are carried out plural times.
[Claim 12]
The method for manufacturing a titanium cast product for hot rolling
5 according to claim 6, wherein the surface is molten together with a material
containing at least one kind of a-phase stabilizing elements and neutral elements in
the first stage surface heat treatment process.
[Claim 13]
The method for manufacturing a titanium cast product for hot rolling
10 according to claim 6, wherein the surface is molten together with a material
containing at least one kind of p-phase stabilizing elements in the first stage surface
heat treatment process.
[Claim 14]
The method for manufacturing a titanium cast product for hot rolling
15 according to claim 6, wherein the surface is molten together with a material
containing at least one kind of a-phase stabilizing elements and neutral elements and
a material containing at least one kind of p-phase stabilizing elements in the first
stage surface heat treatment process.
[Claim 15]
20 The method for manufacturing a titanium cast product for hot rolling
according to claim 6, wherein the surface is molten in the second stage surface heat
treatment process.
[Claim 16]
The method for manufacturing a titanium cast product for hot rolling
25 according to claim 15, wherein the surface is molten together with a material
containing at least one kind of a-phase stabilizing elements and neutral elements in
the second stage surface heat treatment process.
[Claim. 17] •
The method for manufacturing a titanium cast product for hot rolling
according to claim 15, wherein the surface is molten together with a material
5 containing at least one kind of p-phase stabilizing elements in the second stage
surface heat treatment process.
[Claim 18]
The method for manufacturing a titanium cast product for hot rolling
according to claim 15, wherein the surface is molten together with a material
10 containing at least one kind of a-phase stabilizing elements and neutral elements and
a material containmg at least one kind of p-phase stabilizing elements in the second
stage surface heat treatment process.
[Claim 19]
' The method for manufacturing a titanium cast product for hot rolling
15 according to claim 6, wherein the cast product material is casted by a DC slab casting
method.
[Claim 20]
The method for manufacturing a titanium cast product for hot rolling
according to claim 6, wherein the cast product material is obtained by casting a
20 molten metal obtained by an electron beam remelting method by a DC slab castmg
method.
[Claim 21]
The method for manufacturing a titanium cast product for hot rolling
according to claim 6, wherein the cast product material has a cast surface as cast.