BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to steel products, such as hearth rolls and coiler
drums, for use in contact with steel materials such as slabs, blooms, and billets,
as well as steel sheets and plates.
2. Description of Related Art
[0002] Steel products, such as a hearth roll that conveys a steel sheet or strip in a coil
in an annealing furnace in which the steel sheet or strip in a coil is subjected to
continuous heat treatment and a coiler drum around which a steel sheet or plate is
wound during rolling within a high-temperature furnace, are employed in steelmaking
plants and the like.
[0003] Such steel products are used in direct contact with steel materials such as slabs,
blooms, and billets, as well as steel sheets and plates under high-temperature atmospheric
environments, and thus, metals in the base material are oxidized and a Cr oxide mainly
composed of Cr (and also containing Fe and Ni) is formed on the surface thereof. A
Cr oxide is likely to exfoliate as a result of coming into contact with the steel
material, and the exfoliation may result in damage to the steel material. Moreover,
due to the exfoliation of the Cr oxide, or polishing or the like that is performed
in order to remove the Cr oxide so as to suppress the exfoliation, reduction in the
thickness of the steel product itself may be accelerated.
[0004] To address these issues, a product has also been proposed in which a thermal sprayed
layer is formed on the outer circumference of a hearth roll by thermal spraying a
CoCrAlY alloy or the like (see Patent Document 1:
JP 2008-240072A, for example).
CITATION LIST
Patent Document
[0006] However, since a thermal sprayed layer also oxidizes, there is a risk that an oxide
film formed on the thermal sprayed layer will exfoliate, resulting in a deterioration
in oxidation resistance, and also the exfoliated oxide film will damage the steel
material. Furthermore, since a large amount of Al (about 10 mass%) is added in the
thermal sprayed layer, there is a risk that mechanical properties, such as tensile
ductility, of the thermal sprayed layer will deteriorate, and furthermore, the weldability
will deteriorate.
SUMMARY OF THE INVENTION
[0007] An object of the present invention is to provide a steel product that has excellent
oxidation resistance on a surface thereof that is to come into contact with a steel
material and that also has excellent weldability.
[0008] A steel product for use in contact with a steel material according to an aspect of
the present invention is a steel product for use in contact with a steel material,
the steel product including:
a centrifugally-cast portion formed on a surface of the steel product that is to come
into contact with the steel material, the centrifugally-cast portion being made through
centrifugal casting; and
a statically-cast portion to which the centrifugally-cast portion is welded, the statically-cast
portion being formed through static casting,
the centrifugally-cast portion containing, in terms of mass%:
C: 0.2% to 0.7%;
Si: more than 0% and 2.0% or less;
Mn: more than 0% and 3.0% or less;
Cr: 15.0% to 40.0%;
Ni: 18.0% to 55.0%;
Al: 1.0% to 5.5%; and
at least one type selected from the group consisting of Ti: 0.01% to 0.6% and/or Nb:
0.1% to 1.8%,
the balance being composed of Fe and inevitable impurities.
[0009] It is possible that the centrifugally-cast portion further contains, in terms of
mass%,
rare earth elements (REMs): more than 0% and 0.4% or less.
[0010] It is possible that the centrifugally-cast portion further contains, in terms of
mass%:
W: more than 0% and 5.0% or less; and/or
Mo: more than 0% and 2.0% or less.
[0011] It is desirable that the centrifugally-cast portion satisfies Pa<Ya, where
and
[0012] It is desirable that the statically-cast portion does not contain Al.
[0013] It is possible that the steel product further has a ceramic thermal sprayed layer
on the surface of the centrifugally-cast portion.
[0014] It is possible that the steel product is a hearth roll or a coiler drum.
[0015] According to the present invention, since the centrifugally-cast portion formed in
the steel product has the above-described composition, Al forms an Al oxide with priority
over Cr, and the formation of a Cr oxide can be suppressed and problems such as the
exfoliation of the Cr oxide can be suppressed. Moreover, since the amount of Al added
is as small as 1.0% to 5.5%, a deterioration in mechanical properties can be suppressed.
[0016] Moreover, since the amount of Al added to the centrifugally-cast portion is small,
weldability thereof can also be ensured, and thus, the centrifugally-cast portion
can be suitably welded to the statically-cast portion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017]
FIG. 1 is a graph showing the results of regression analysis of tested centrifugally-cast
portions based on weldability, where the vertical axis represents a Pa value and the
horizontal axis represents an Al content; and
FIG. 2 is criteria for judging the beads that were formed using the method A and the
method B.
DETAILED DESCRIPTION OF THE INVENTION
[0018] Hereinafter, an embodiment of the present invention will be described in detail.
Note that, in the following description, "%" means mass% unless otherwise specified.
[0019] A steel product according to the present invention is suitably applied to a product,
such as a hearth roll or a coiler drum, for use in direct contact with a steel material,
such as a slab, a bloom, or a billet, or a steel sheet or plate, under a high-temperature
atmospheric environment.
[0020] The steel product has a centrifugally-cast portion that contains:
C: 0.2% to 0.7%;
Si: more than 0% and 2.0% or less;
Mn: more than 0% and 3.0% or less;
Cr: 15.0% to 40.0%;
Ni: 18.0% to 55.0%;
Al: 1.0% to 5.5%; and
at least one type selected from the group consisting of Ti: 0.01% to 0.6% and/or Nb:
0.1% to 1.8%,
the balance being composed of Fe and inevitable impurities.
[0021] The centrifugally-cast portion can be made through centrifugal casting, and the reasons
for the limitation on components will be described below.
C: 0.2% to 0.7%
[0022] C has the effect of imparting favorable castability and increasing high-temperature
creep rupture strength. Also, C combines with Ti, Nb, Cr, or the like to form a carbide
and has the effect of increasing high-temperature strength. Therefore, C is contained
in an amount of at least 0.2%. However, if the C content is excessively high, a primary
carbide Cr
7C
3 is likely to be widely formed. Then, the transfer of Al to the surface of the centrifugally-cast
portion is inhibited, resulting in a shortage of Al, and thus, formation of an Al
oxide such as Al
2O
3 is suppressed. Moreover, excessive secondary carbide precipitation occurs, and this
leads to a deterioration in ductility and toughness. Therefore, the upper limit of
the C content is 0.7%. Note that it is more desirable that the lower limit of the
C content is 0.35% or more and the upper limit of the C content is 0.6% or less.
Si: more than 0% and 2.0% or less
[0023] Si is contained as a deoxidizer for a molten metal alloy and also for the purpose
of increasing the fluidity of molten metal alloy and improving oxidation resistance.
However, the addition of an excessively large amount of Si leads to a deterioration
in ductility, a deterioration in high-temperature creep rupture strength, a deterioration
in surface quality after casting, and a deterioration in weldability. Therefore, the
upper limit of the Si content is 2.0%. Note that the Si content is desirably 1.5%
or less and more desirably 1.0% or less.
Mn: more than 0% and 3.0% or less
[0024] Mn is contained so as to serve as a deoxidizer for a molten metal alloy and also
improve weldability by fixing S in the molten alloy and improve ductility. However,
the addition of an excessively large amount of Mn leads to a deterioration in high-temperature
creep rupture strength and a deterioration in oxidation resistance. Therefore, the
upper limit of the Mn content is 3.0%. Note that it is more desirable that the Mn
content is 1.0% or less.
Cr: 15.0% to 40.0%
[0025] Cr contributes to an improvement in high-temperature strength and cyclic oxidation
resistance. Also, Cr exhibits excellent heat resistance together with Ni and Fe in
a high-temperature range exceeding 1000°C, and generates a primary carbide with C
and N and improves high-temperature creep rupture strength. Moreover, Cr forms an
oxide layer together with Al, and imparts excellent characteristics in terms of oxidation
resistance and corrosion resistance to the centrifugally-cast portion. Therefore,
Cr is contained in an amount of at least 15.0% or more. On the other hand, excessive
generation of a Cr carbide and a Cr nitride leads to a deterioration in ductility,
and therefore the upper limit of the Cr content is 40.0%. Note that it is more desirable
that the lower limit of the Cr content is 22.0% or more and the upper limit of the
Cr content is 35.0% or less.
Ni: 18.0% to 55.0%
[0026] Ni is an element that is necessary to ensure cyclic oxidation resistance and metallographic
stability, ensure high-temperature creep strength, and stabilize the austenitization
of the centrifugally-cast portion. Moreover, Ni contributes to an improvement in high-temperature
strength and oxidation resistance together with Cr. Furthermore, if the Ni content
is low, the Fe content becomes relatively high, which inhibits the generation of an
Al oxide. Therefore, Ni is contained in an amount of at least 18.0% or more. On the
other hand, even if an excessively large amount of Ni is added, its effects become
saturated, and also this is economically disadvantageous. Therefore, the upper limit
of the Ni content is 55.0%. Note that it is more desirable that the lower limit of
the Ni content is 29.0% or more and the upper limit of the Ni content is 46.0% or
less.
Al: 1.0% to 5.5%
[0027] Al is an essential element for forming an Al oxide on the centrifugally-cast portion.
The formation of an Al oxide improves the carburization resistance of the centrifugally-cast
portion together with the Cr oxide. Also, Al forms a γ' phase together with Ni and
reinforces the austenitic phase of the centrifugally-cast portion. Therefore, Al is
contained in an amount of 1.5% or more. However, the addition of an excessively large
amount of Al leads to a deterioration in ductility, and makes the γ' phase unstable,
which results in the generation of an embrittled phase. Furthermore, the addition
of an excessively large amount of Al leads to a deterioration in castability and reduces
the cleanliness of the centrifugally-cast portion. Therefore, the upper limit of the
Al content is 5.5%. Note that it is more desirable that the lower limit of the Al
content is 2.0% or more and the upper limit of the Al content is 4.5% or less.
At least one type selected from the group consisting of Ti:0.01% to 0.6% and/or Nb:0.1%
to 1.8%
[0028] Ti and Nb are elements that are likely to form a carbide and contribute to an improvement
in creep rupture strength and high-temperature tensile strength. Moreover, Nb also
contributes to an improvement in aging ductility. Therefore, at least one type of
Ti in an amount of 0.01% or more and Nb in an amount of 0.1% or more is contained.
On the other hand, the addition of an excessively large amount of these elements leads
to a deterioration in ductility. Also, Nb causes a deterioration in the exfoliation
resistance of the Al oxide layer and also causes a deterioration in oxidation resistance.
Moreover, addition of an excessively large amount of Ti promotes the generation of
a Ti oxide and causes a deterioration in the cleanliness of the centrifugally-cast
portion. Therefore, the upper limit of the Ti content is 0.6%, and the upper limit
of the Nb content is 1.8%. Note that it is more desirable that the lower limit of
the Ti content is 0.05% or more and the upper limit of the Ti content is 0.30% or
less and the lower limit of the Nb content is 0.1% or more and the upper limit of
the Nb content is 1.3% or less.
[0029] In addition, the following elements may be contained in the centrifugally-cast portion.
Rare-earth elements (REMs): more than 0% and 0.4% or less
[0030] An REM means one of a set of eighteen elements in the periodic table including the
fifteen lanthanides from La to Lu as well as Y, Hf, and Sc. The REMs contained in
the centrifugally-cast portion are mainly Ce, La, and Nd, and the total amount of
these three elements accounts for preferably about 80% or more, and more preferably
about 90% or more, of the total amount of all the rare earth elements contained. The
REMs contribute to the stabilization of the Al oxide layer, and can improve the adhesion
of the oxide film since the REMs are reactive metals. Moreover, the REMs prevent spalling
breakdown of the oxide layer due to temperature changes in the furnace, and furthermore,
form a solid solution with the base material and contribute to an improvement in oxidation
resistance, and therefore, it is desirable that the REMs are contained. On the other
hand, the REMs preferentially form oxides and cause a deterioration in the cleanliness
of the base material and the ductility, and therefore, the upper limit of the REM
content is 0.4%. Note that it is more desirable that the lower limit of the REM content
is 0.01% or more and the upper limit of the REM content is 0.30% or less.
W: more than 0% and 5.0% or less and/or Mo: more than 0% and 2.0% or less
[0031] W and Mo form a solid solution with the base material, reinforce the austenitic phase
of the base material, and improve creep rupture strength, and therefore, it is desirable
that either one or both of them are contained. However, an excessively high W or Mo
content leads to a deterioration in ductility and carburization resistance, and also
inhibits the formation of an Al oxide in particular if the Al oxide is to be generated
at a temperature of 1050°C or lower. Moreover, an excessively high W or Mo content
leads to a deterioration in the oxidation resistance of the base material. Therefore,
the upper limit of the W content is 5.0% and the upper limit of the Mo content is
2.0%. Note that it is more desirable that the upper limit of the W content is 3.0%
and the upper limit of the Mo content is 1.0%.
[0032] Moreover, as shown in FIG. 1, it is desirable that the various elements contained
in the centrifugally-cast portion satisfy Pa<Ya, where Pa = -11.1+28.1×C+29.2×Si-0.25×Ni-45.6×Ti+18.0×REMs-16.6×Nb,
and Ya = -13.75×Al+63.75. Note that, with respect to the Pa value, if any of the elements
shown above is not contained, the value of that element is taken as 0.
[0033] If the Pa value and the Ya value satisfy the above-described expression, the weldability
and the oxidation resistance (formation of an Al oxide layer) of the centrifugally-cast
portion can be ensured.
[0034] The above-described Pa value is related to the amounts of the elements C, Si, Ni,
Ti, REMs, and Nb contained. Centrifugally-cast portions to be tested containing varying
amounts of these elements and Al were produced. Then, the centrifugally-cast portions
to be tested were subjected to a bead-on-plate test of Examples, which will be described
later, and data on the weldability of the tested centrifugally-cast portions were
acquired. The influence coefficients of elements that influence the weldability were
obtained from the acquired data through regression analysis.
[0035] With regard to the Pa value, referring to the influence coefficients, C, Si, and
REMs whose influence coefficients are positive numbers are the elements that each
adversely influence the weldability, and a greater numerical value (absolute value)
means a greater degree of the adverse influence. On the other hand, Ni, Ti, and Nb
whose influence coefficients are negative numbers are the elements that improve the
weldability, and a greater numerical value (absolute value) means a greater degree
of the favorable influence.
[0036] FIG. 1 is a plot of the Pa value on the vertical axis versus the Al content on the
horizontal axis with respect to the tested centrifugally-cast portions, and cases
with favorable weldability are plotted as rhombuses and cases with insufficient weldability
are plotted as squares.
[0037] In order for a tested centrifugally-cast portion to have a favorable Al oxide layer
formed thereon and have oxidation resistance, it is necessary to satisfy the above-described
range of the Al content (Al: 1.0% to 5.5%). The graph in FIG. 1 shows this range of
the Al content in an enlarged manner. Referring to FIG. 1, it can be found that the
boundary between a group with excellent weldability and a group with insufficient
weldability is distinctly delineated with respect to the Pa values and the Al contents
with which favorable Al oxide layers are formed. It can be seen from this graph that
the correlation with the Ya value, which includes the Al content, could be clearly
analyzed based on the weldability.
[0038] Then, the linear Ya value that separates these groups and that is based on the Al
content was determined to be Ya = -13.75×Al+63.75. That is to say, it can be seen
that a centrifugally-cast portion with excellent oxidation resistance and excellent
weldability can be obtained by satisfying Pa<Ya within the range of Al: 1.0% to 5.0%.
[0039] The centrifugally-cast portion is formed into a tubular shape, for example, through
centrifugal casting, and the steel product can be produced by welding the centrifugally-cast
portion to a base member such as an axle or a shaft portion that constitutes the steel
product. Since the centrifugally-cast portion has excellent weldability, welding to
the base member can be suitably performed, and a sufficient joining strength can be
ensured.
[0040] The base member can be made through static casting, for example, and this statically-cast
portion is suitably made from a material that does not contain Al, in order to suppress
a deterioration in mechanical properties and weldability.
[0041] In the steel product, the centrifugally-cast portion can be formed on a surface thereof
that is to come into contact with a steel material. In this case, it is necessary
to perform an Al oxide layer forming treatment for forming an Al oxide layer on the
surface of the centrifugally-cast portion. The Al oxide layer forming treatment can
be performed as an independent step by heat-treating the steel product in an oxidizing
atmosphere, or can also be performed in the same high-temperature atmosphere as that
in which the steel product is to be used, by placing the steel product in a heating
furnace.
[0042] Preferably, the Al oxide layer forming treatment is performed by heat-treating the
steel product at a temperature of 900°C, desirably 1000°C, and more desirably 1050°C
or more, in an oxidizing atmosphere in which an oxidizing gas that contains oxygen
in an amount of 1 vol% or more, steam, and CO2 are mixed. The heat treatment time
is preferably 1 hour or longer.
[0043] As a result of the steel product being subjected to the Al oxide layer forming treatment,
the centrifugally-cast portion comes into contact with oxygen, and Al, Cr, Ni, Si,
Fe, and the like that are diffused on the surface of the base material are oxidized
to form an oxide layer. At this time, due to the heat treatment being performed in
the above-described temperature range, Al forms an oxide with priority over Cr, Ni,
Si, and Fe. Moreover, Al in the base material also moves to the surface and constitutes
the oxide, and thus, an Al oxide layer mainly composed of Al
2O
3 is formed.
[0044] In the obtained steel product, since the centrifugally-cast portion has excellent
weldability, the centrifugally-cast portion is firmly joined to the base member without
causing weld cracking or the like, and thus the steel product has excellent mechanical
characteristics. Moreover, the centrifugally-cast portion has excellent mechanical
characteristics, and the Al oxide layer formed on the surface thereof allows the steel
product to exhibit excellent oxidation resistance when used in a high-temperature
atmosphere. Therefore, the steel product can be suitably used as a hearth roll or
a coiler drum that is to come into contact with a steel material in a high-temperature
atmosphere.
[0045] Note that a ceramic thermal sprayed layer may also be formed on the surface of the
centrifugally-cast portion, if necessary, by thermal spraying a ceramic onto the surface
of the centrifugally-cast portion.
Examples
[0046] Test pieces of centrifugally-cast portions having alloy compositions shown in Table
1 (unit: mass%, the balance being Fe and inevitable impurities) (test pieces for an
oxidation resistance test: thickness 25 mm × 3 pieces, and test pieces for a bead-on-plate
test for confirming the weldability (for only examples of the invention): thicknesses
25 mm or less and 25 mm or greater) were made through centrifugal casting. Examples
of the invention were sample Nos. 101 to 110, and comparative examples were sample
Nos. 201 to 206. In Table 1, "REMs" indicates the total amount of Ce, La, and Y. Note
that, although the examples of the invention satisfied the composition range of the
present invention, the sample Nos. 201 to 204 of the comparative examples contained
no Al and the sample Nos. 205 and 206 of the comparative examples each contained an
excessively small amount of Al, and the relevant portions are denoted by an asterisk
"*".
[0047] Moreover, with respect to each test piece in Table 1, Pa and Ya were calculated and
compared to each other to determine the relationship in magnitude therebetween. In
Table 1, for each test piece that satisfies Pa<Ya, a check mark is placed in the "Pa<Ya"
column. Referring to Table 1, it can be seen that all of the sample Nos. 103, 109,
110, 205, and 206 are test pieces that do not satisfy Pa<Ya.
Oxidation resistance test
[0048] The test pieces were weighed. After that, the test pieces were kept in a heating
furnace at 1000°C (in the atmosphere), and scales on the surfaces of the heated test
pieces were removed using an acid solution. Then, the test pieces were weighed again.
Based on the amount of change in weight before and after heating of each test piece,
the amount of weight reduction due to oxidation was calculated, and an average amount
of weight reduction due to oxidation per hour (mg/cm
2•h) was obtained. The results are shown in "Oxidation resistance" in Table 1. Moreover,
with respect to the oxidation resistance, as shown in "Evaluation" in Table 1, test
pieces with an amount of weight reduction due to oxidation of 0.1 mg/cm
2•h or less were evaluated as "A", those with 0.3 mg/cm
2•h or less were evaluated as "B", those with 1 mg/cm
2•h or less were evaluated as "C", and those with greater than 1 mg/cm
2•h were evaluated as "D".
[0049] Referring to Table 1, it can be seen that all of the examples of the invention were
evaluated as "A" or "B", and had smaller amounts of weight reduction due to oxidation
and had superior oxidation resistance, compared with the comparative examples, which
were evaluated as "C" or "D". The reason for this is that, in each of the test pieces
of the examples of the invention, an Al oxide was generated on the surface thereof
and suppressed further oxidation. On the other hand, in each of the test pieces of
the comparative examples, a Cr oxide and a Si oxide were generated on the surface
thereof, and these oxides were less dense than the Al oxide, did not have a sufficient
function of preventing the entry of oxygen, and was not able to suppress oxidation.
[0050] A comparison between the examples of the invention shows that the sample Nos. 102,
103, 106, and 108 to 110 were evaluated as "A" and had particularly excellent oxidation
resistance. It is assumed that this is because a particularly excellent Al oxide layer
was formed due to the high Al concentration, the relatively low concentration of Cr,
which is likely to form an oxide layer, and the like.
Bead-on-plate test
[0051] The test pieces (two types with a thickness of 25 mm or less and a thickness of 25
mm or more) of the examples of the invention shown in Table 1 were subjected to a
bead-on-plate test in the following manner, and resistance to cracking due to welding
was judged.
[0052] Prior to the bead-on-plate test, a test surface of each test piece was machined using
a grinder and smoothed. The test surface refers to a portion that was to constitute
a weld groove and a portion that was to be heat-affected by welding.
[0053] Moreover, the test surface of each test piece was subjected to a liquid penetrant
test, and it was confirmed that no crack was present in the test surface.
[0054] With respect to the test pieces whose test surfaces were confirmed to have passed
the test, the bead-on-plate test was performed through TIG welding under the conditions
shown in Table 2. The bead was a straight bead, and the bead length was 50 to 100
mm.
Table 2
Order |
Filler metal |
Thickness of material tested |
Current |
Speed |
Other conditions |
Method A |
Not used |
25 mm or less |
150 A |
150-200 mm/min. |
Straight bead 50-100 mm |
25 mm or more |
200 A |
150-200 mm/min. |
Method B |
Used |
25 mm or less |
150 A |
150-200 mm/min. |
Straight bead 50-100 mm |
25 mm or more |
200 A |
150-200 mm/min. |
[0055] Note that this test was conducted in the following order: a test was performed using
the method A, followed by a liquid penetrant test, and if a defect was found in the
liquid penetrant test, a test was performed using the method B.
[0056] FIG. 2 and Table 3 show criteria for judging the beads that were formed using the
method A (filler metal (welding rod) was not used) and the method B (filler metal
was used). Note that, in the method B, even a minute crack was judged as "OUT".
Table 3
Defect type |
Criterion for judgement |
Method A |
Method B |
Cracks |
Within Bead |
OK |
OUT |
Spanning between bead and base material |
OUT |
OUT |
Occurring in base material |
OUT |
OUT |
Within crater |
OK |
OUT |
Point defects at lateral side of bead |
OK |
OK |
[0057] As a result of the above-described test, the cracking resistance of test pieces with
respect to which no defects were found in all of the test pieces with a thickness
of 25 mm or less and a thickness of 25 mm or more using the method A was evaluated
as "A", the cracking resistance of test pieces with respect to which defects were
found using the method A, but no defects were found using the method B was evaluated
as "B", and the cracking resistance of test pieces with respect to which defects were
found using the method A and were also found using the method B was evaluated as "C".
The results are shown in "Cracking resistance" in Table 1.
[0058] Referring to Table 1, with regard to the test pieces of the examples of the invention,
the sample Nos. 101, 102, and 104 to 108 were evaluated as "A" and the sample Nos.
103, 109, and 110 were evaluated as "B".
[0059] If the examples of the invention are examined, it is found that all of the test pieces
whose cracking resistance was evaluated as "A" each had a Ya value greater than the
Pa value and satisfied Pa<Ya.
Overall judgment
[0060] Each test piece that was evaluated as "A" in both the oxidation resistance test and
the bead-on-plate test was judged to be "A" overall. On the other hand, each test
piece that was evaluated as "A" in one of the tests and "B" in the other test was
judged to be "B" overall, and each test piece whose evaluation results contained "C"
or "D" was judged to be "C" overall. The results are shown in "Overall judgment" in
Table 1. Referring to Table 1, all of the test pieces of the examples of the invention
were judged to be "A" or "B", and all of the test pieces of the comparative examples
were judged to be "C". That is to say, in the test pieces of the examples of the invention,
cracking is less likely to occur during welding, and it is thus found that the test
pieces of the examples of the invention have superior oxidation resistance to the
test pieces of the comparative examples. Therefore, the steel product of the present
invention is extremely suitable for application to a hearth roll, a coiler drum, and
the like that are used in contact with a steel material such as slabs, blooms, and
billets, as well as steel sheets and plates.
[0061] The foregoing description is solely intended to illustrate the invention and should
not be construed as limiting the present invention as set forth in the claims or restricting
the scope of the claims. It goes without saying that the configuration of each part
of the present invention is not limited to the foregoing examples, and various modifications
are possible within the technical scope indicated by the claims.
List of Reference Numerals
[0062]
10 bead
12 crater
14 crack
16 point defect