Specification
[Title of the Itivention] COLD-ROLLED STEEL SHEET FOR VITREOUS
ENAMELNG, METHOD FOR PRODUCING THE SAME, AND ENAMELED
PRODUCT
[Technical Field of the Invention]
[OOOl]
The present invention relates to a high-strength steel sheet for vitreous
enameling having excellent workability, enameling characteristics (bubble-black point
resistance, adhesion, and fishscale resistance), and fatigue properties, and a method for
producing the same. In particular, the present invention relates to a high-strength
cold-rolled steel sheet for vitreous enameling having excellent fishscale resistance and
fatigue properties after vitreous enameling, and a method for producing the same. In
addition, the present invention relates to an enameled product which is obtained using
the steel sheet for vitreous enameling.
Priority is claimed on Japanese Patent Application No. 2013-187473, filed on
September 10,2013, the content of which is incorporated herein by reference.
[Related Art]
[0002]
In the related art, a steel sheet for vitreous enameling is used as an enameled
product after being imparted with functions of heat resisting properties, weather
resistance, chemical resistance, and water resistance tlxough vitreous enan~elingin
which glass is fiised to the steel sheet surface. In addition, by taking advantage of
these characteristics, the steel sheet for vitreous enameling is widely used as
kitchenware such as pans or sinks or materials such as building materials. Exatnples
of the characteristics required for the steel sheet for vitreous enameling include firing
strain resistance, fishscale resistance, adhesion, and bubble-black point resistance. In
addition, in the process of producing an enameled product frotn the steel sheet for
vitreous enameling, typically, pressing is performed in order to obtain a desired
product shape. To that end, in the steel sheet for vitreous enameling, not only the
above-described characteristics but also excellent formability (workability) is required.
[0003]
In addition, through vitreous enameling, corrosion resistance in a severely
corrosive environment containing sulfuric acid or the like is improved. Therefore,
recently, the steel sheet for vitreous enameling has been increasingly used in a wide
range of fields including the energy fields of power generation facilities and the like
(for example, a heat exchanger for a power generator). In these fields, the
improvement of reliability against fatigue and the like caused by a long period of use is
required. Moreover, in order to reduce the weight of conlponents, high-strengthening
of the steel sheet to be used is required.
[0004]
The high-strengthening of the steel sheet having eriameling characteristics is
described in, for example, Patent Document I. In the steel sheet disclosed in Patent
Document 1, Ti is added to steel, and Tic is finely precipitated in the steel sheet during
enamel firing (firing process in vitreous enameling), thereby realizing highstrengthening.
In addition, Patent Document 2 discloses a steel sheet in which not
only high-strengthening but also enameling characteristics are sin~ultaneouslys ecured
by controlling a ratio between the addition amounts orNi and P in components of the
steel sheet to be within a specific range.
[OOOS]
However, in the steel sheet obtained using the technique of Patent Doculnent
1, during vitreous enameling, surface defects such as bubbles or black point flaws are
likely to occur. In addition, in a short-term heat treatment during firing, Tic is not
likely to be sufficiently produced, and fishscale defects are likely to occur.
In the technique of Patent Document 2, the addition of expensive Ni is
essential in order to secure enameling characteristics. Therefore, the characteristics
can be secured, but there is a problem from the viewpoint of production cost.
[0006]
In a steel sheet for a vehicle or the like, in the related art, the in~provemenot f
fatigue properties is required, and various studies have been made. Howevel; a
technique of improving fatigue properties of the steel sheet for vitreous enameling after
vitreous enameling (that is, fatigue properties of an enameled product) has not been
reported. For example, Non-Patent Document 1 describes a technique of increasing
the P content to improve the fatigue properties of a steel sheet for a vehicle.
However, in the steel sheet for vitreous enameling, unlike the steel sheet for a
vehicle, it is necessary that a large amount of precipitates (in particular, oxides), which
cause a decrease in fatigue properties, are intentionally dispersed in the metallographic
structure in order to secure enameling characteristics, in particular, fishscale resistance.
In addition, unlike the steel sheet for a vehicle, in the steel sheet for vitreous enanleling,
vitreous enameling of performing heating at 80OoC or higher after processing is
performed, and thus the metallographic stlucture is changed by thermal history.
Therefore, as shown in FIG. 1 , in the steel sheet for vitreous enalneling, fatigue
properties deteriorate as compared to the steel sheet for a vehicle.
As a result, even when the tcclmiqne of improving fatigue properties \vhich is
performed in the steel sheet for a vehicle is simply applied to the steel sheet for
vitrcous enameling, the steel sheet for vitreous enameling cannot exhibit sufficient
fatigue properties.
[0007]
As described above, a high-strength steel sheet ivliich sufficiently satisfies
important characteristics of the steel sheet for vitreous enameling has not been
provided, the characteristics including: fishscale resistance; workability; and fatigue
properties of a product which is an index indicating the reliability of the steel sheet.
[Prior Ast Document]
[Patent Document]
[0008]
[Patent Document I] Japanese Uriexamined Patent Application, First
Publication No. S61-117246
[Patent Document 21 Japanese Patent No. 14561 99
[Non-Patent Document]
[0009]
[Nan-Patent Document 11 "Fatigue Strength of High-Strength Steel Sheet",
Nagae et al., Iron and Steel, Year 68 (1982), Vol. 9, pp. 1430-1436
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
An object of the present invention is to further improve the above-described
tecluliq~~eresg arding the steel sheet for vitreous enameling and thus to provide: an
inexpensive high-strength steel sheet for vitreous enameling having excellent
ivorkability, fishscale resistance, and fatigue properties, in particular, an inexpensive
high-strength cold-rolled steel sheet for vitreous enameling having excellent
workability, fishscale resistance, and fatigue properties even after vitreous enameling;
and a method of producing the samnc. In addition, another object of the present
invention is to provide an enameled product which is obtained using the inexpensive
high-strength cold-rolled steel sheet for vitreous enameling having excellent
workability, fishscale resistance, and fatigue properties.
[Means for Solving the Problem]
[OOll]
As a result of various investigations, the present invention has been made in
order to solve the problems of the steel sheet for vitreous enameling in the related art.
The present inventors have investigated effects of the component composition and
production conditions on the fishscale resistance, workability, and fatigue properties of
the cold-rolled steel sheet for vitreous enameling, thereby obtaining the following
findings (a) to (f).
[0012]
(a) Fishscale resistance is improved by adjusting the component composition
of steel to control precipitates in the steel sheet for entrapping hydrogen in the steel
sheet which causes fishscale defects. In particular, when oxides are present in the
steel sheet, fishscale resistance is improved.
[0013]
(b) When the-strength of the steel sheet increases, workability deteriorates.
However, even when the strength of the steel sheet increases, the deterioration for
workability can be reduced by adjusting the diameter and number of precipitates (in
the steel sheet for vitreous enameling, in particular, oxides) present in the steel sheet.
[OOI 41
(c) In the steel sheet for vitreous enameling, as described above, a large
amount of oxides are present in steel. In the steel sheet for vitreous enameling,
during processing such as cold rolling or press fonning, due to a difference in
deformatioti resistance between the oxides, which are present in the steel sheet, atld the
steel sheet, voids are formed between the oxides, which are present in steel, aud the
metallographic st~x~cture.D epending on its shape, the voids cause stress
concentration due to a notch effect, which may become startiug point of fatigue
fracture. Therefore, fatigue properties can be improved by appropriately controlling
the shape of the voids.
[0015]
(d) In the steel sheet for vitreous enameling, due to processing, strains are
likely to accumulate in the peripheries of the precipitates and the peripheries of the
voids. In pa~ticular\,v hen betiding deformation occurs during press forming, this
tendency is remarkable in a surface part (for example, at a distance of less than 20 pm
from the surface). Due to the accumulated strains, crystal grains grow during vitreous
enameling.
Fatigue properties after vitreous enameling are affected by the grain size of
the surface pai-t after vitreous enameling. Therefore, for the improvement of fatigue
properties, it is effective to reduce the average grain size. Howevel; even when the
average grain size is reduced, crystal grains which are paitially coarseued by grain
growth become starting point of fatigue fracture. Therefore, fatigue properties
decrease. In particular, when graitl growth occurs near the voids the grains likely
become starting point of fatigue fracture. Such graiu growth is not observed in the
steel sheet for a vehicle which does not undergo thermal history such as vitreous
enameling. Therefore, it is considered that the grain gro\vth is a phenotnenon unique
to the steel sheet for vitreous enameling.
1001 61
(e) The grain size of crystal grains can be controlled by appropriately
controlling hot rolling, pickling, and cold rolling conditions. In addition, the diatneter
of oxides can be controlled to be with it^ a preferable range, and the form of precipitates
in a final product can be controlled.
Further, during cold rolling, a friction coefficient between a roll and the steel
sheet can be controlled to be within an appropriate range through selection of cold
rolling oil or the like. As a result, strains acculnulating in a surface part can be
reduced.
[00 171
(f) Grain growth during vitreous enameling (enamel firing) can be prevented
by controlling the contents of components of the steel sheet, in particular, C, Mn, P,
and Nb to be within predetermined ranges. Therefore, by reducing the grain size
before processing and adjusting the contents of C, Mn, P, and Nb, the grain size of
crystal gains can be reduced after vitreous enameling, and fatigue properties can be
improved.
[0018]
The present invention has been made based on the above findings, and the gist
thereof is as follows.
[0019] .-
(1) According to an aspect of the present invention, there is provided a coldrolled
steel sheet for vitreous enameling, the steel sheet including, by mass%, C:
0.0005% to 0.0050%, Mn: 0.05% to 1.50%, Si: 0.001% to 0.015%,Al: 0.001% to
0.01%, N: 0.0010% to 0.0045%, 0: 0.0150% to 0.0550%, P: 0.04% to 0.10%, S:
0.0050% to 0.050%, Nb: 0.020% to 0.080%, Cu: 0.015% to 0.045%, and a remainder
including Fe and impurities, in which when a C content is represented by C (%), a Mn
content is represented by Mn (%), a P content is represented by P (%), and a Nb
content is represented by Nb (%), the following expression (i) is satisfied; a
nietallographic structure contains ferrite, and an average grain size of the ferrite at a
114 thickness position fiom a surface in a thickness direction is 12.0 pm or less; a
number density of Fe-Mn-Nb-based composite oxides containing Fe, Mn, and Nb and
having a diarneter of 0.2 pm to 10 pm is 2x 10' particle/nnn2 to 1 x lo4p al-ticlelmn~~a ;
fatigue limit ratio, which is a value obtained by dividing a fatigue strength by a tensile
strength, is higher than 0.42, the fatigue strength being a stress at lo7 cycles after
performing a heat treatment with an applied tensile strain of 10% at a heating
temperature of 830°C for a holding time of 5 minutes; voids are formed between the
metallographic structure and the Fe-Mn-Nb-based composite oxides, and an equivalent
circle diameter of the voids is 0.1 pm to 0.6 pm; and when each of the voids is
approximated as a triangle and a long side of the triangle is set as a base, a value
obtained by dividing a length of the base by a height of the triangle is 1 .O to 15.
2.2058xC (%)+1.3xMn (%)+lXxP (%)+S.lx(Nb (%))0.5<4.00 . (i)
[0020]
(2) According to another aspect of the present invention, tliere is provided a
cold-rolled steel sheet for vitreous enameling, the steel sheet conlprising, by mass%, C:
0.0005% to 0.0050%, Mn: 0.05% to 1.50%, Si: 0.001% to 0.015%,Al: 0.001% to
0.01%, N: 0.0010% to 0.0045%, 0: 0.0150% to 0.0550%, P: 0.04% to 0.10%, S:
0.0050% to 0.050%, Nb: 0.020% to 0.080%, Cu: 0.015% to 0.045%, B: 0.0005% to
0.0050%, and a remainder including Fe and impurities, in which when a C content is
represented by C (%), a Mn content is represented by Mn (%), a P content is
represented by P (%), and a Nb content is represented by Nb (%), the following
expression (ii) is satisfied; a nietallographic structurc contains ferrite, and an average
grain size of the ferrite at a 114 thickness position fioni a surface in a thickness
direction is 12.0 pm or less; a nunlber density of Fe-MI-Nb-B-based composite oxides
containing Fe, Mn, Nb, and B and having a diameter of 0.2 inn to 10 pm is 2x 10'
pa~-ticle/~nttno2 1 x lo4p article/mm2; a fatigue litnit ratio, which is a value obtained by
dividing a fatigue strength by a tensile strength, is higher than 0.42, the fatigue strength
being a stress at 10' cycles after performing a heat treatment with an applied tensile
strain of 10% at a heating temperature of 830°C for a holding time of 5 minutes; voids
are formed between the metallographic structure and the Fe-Mn-Nb-B-based
composite oxides, and an equivalent circle diameter of the voids is 0.1 ltm to 0.6 ptn;
and when each of the voids is approximated as a triangle and a long side of the triangle
is set as a base, a value obtained by dividing a length of the base by a height of the
triangle is 1 .O to 15.
2.5028xC (%)+1.3xMn (%)+18xP (%)+S.lx(Nb (%))0.5<4.00 . (ii)
[0021]
(3) The cold-rolled steel sheet for vitreous enameling according to (1) or (2)
may fill-ther contain, by mass%, one or more elements selected from the group
consisting of Cr, V, Zr, Ni, As, Ti, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, and Mg, in which
a total amount of the elements may be 0.1% or lower.
[0022]
(4) According to still another aspect of the present invention, there is
provided an enameled product which is produced using the cold-rolled steel sheet for
vitreous enameling according to (1) to (2).
[0023]
(5) According to still another aspect of tlie present invention, there is
provided an enameled product which is produced using the cold-rolled steel sheet for
vitreous enameling according to (3).
[Effects of the Invention]
[0024]
According to the present invention, it is possible to provide: a high-strength
steel sheet for vitreous enameling having excellent workability, fishscale resistance,
and fatigue properties after vitreous enameling; and an enameled product which is
produced using the cold-rolled steel sheet. When the high-strength cold-rolled steel
sheet for vitreous enameling according to the present invention is applied to the energy
fields in addition to kitchenware and building materials, the reliability against fatigue
and the like caused by a long period of use can be improved, and the weight of a
product can be reduced.
[Brief Description of the D~awings]
[0025]
FIG. 1 is a diagram showing a relationship between tensile strengths and
fatigue strengths of various steel sheets.
FIG 2 is a diagram showing a relationship between a value of 8xC
(%)+I .3 xMn (%)+I 8xP (%)+S.l x(Nb (%))'.' and a fatigue limit ratio.
FIG. 3 is a diagram showing an example in which voids are present in a steel
sheet for vitreous enameling according to an embodiment of the present invention.
[Embodiments of the Invention]
[0026]
Hereinafter, a higll-strength cold-rolled steel sheet for vitreous enameling
according to an enlbodiment of thc present invention (hereinaftel; also referred to as
"steel sheet for vitreous enameling according to the embodiment") having excellent
workability, fishscale resistance, and fatigue properties after vitreous enameling; a
metl~odfo r producing the salile (hereinaftel; also referred to as "method for producing
a steel sheet for vitreous enameling according to the embodiment); and an e~ia~lieled
product which is produced using the high-strength cold-rolled steel sheet for vitreous
enameling according to the embodiment (hereinafter, also referred to as "enameled
product according to the embodiment") having excellent workability and fishscale
resistance will be described.
[0027]
First, the reason for limiting the component conlposition (chemical
cotnposition) of a steel sheet for vitreous enameling according to the etnbodiment will
be described. Here, "%" regarding the colnponent composition represents "mass%.
The enameled product according to the embodiment is produced using the
steel sheet for vitreous enameling according to tlie embodiment. Therefore, the
component cotnposition of the enameled product according to the enlbodiment is the
same as that of the steel sheet for vitreous enameli~~acgc ording to the embodimet~t.
[0028]
C: 0.0005% to 0.0050%
C exhibits higher workability as the content thereof becomes lower.
Therefore, the upper limit of the C content is set as 0.0050%. In order to improve
elongation atid rvalue \vhich are indices indicating workability, it is preferable that the
upper limit of the C content is set as 0.0025%. It is more preferable that the upper
limit of the C content is set as 0.0015%. From the viewpoint of securing
characteristics of the steel sheet, the lower limit of the C content is not particularly
limited. However, \when the C content is reduced tnore than necessary, the
steelmaking cost increases. In addition, in order to secure the streagth as a product, it
is necessary to increase the contents of other alloy eleme~ltsw, l~iciln~c reases tlie
production cost. Therefore, it is preferable that the lower limit of the C content is set
as 0.0005%. It is more preferable that the lower limit of the C content is set as
0.0010%.
[0029]
MI]: 0.05% to 1.50%
Mn relates to the 0 content, theNb content, and the B content and affects the
co~npositiono f oxides which contribute to the improvement of fishscale resistance of
the steel sheet for vitreous enameling. In addition, Mn also affects the highstrengthening
of the steel sheet. Therefore, Mn is an important element in the steel
sheet for vitreous enameling. In addition, Mn is an element which prevents hot
brittleness caused by the presence of S during hot rolling. In order to obtain the
effects, in the steel sheet for vitreous enameling according to the embodiment
containing 0, the lower limit of the Mn content is set as 0.05%.
Typically, as the Mn content increases, enamel adhesion deteriorates, and
bubbles and black points are likely to occur. When Mn is present in steel as an oxide,
deterioration in enamel adhesion and bubble-black poiut resistance is small.
Accordingly, in the steel sheet for vitreous enameling according to the embodiment,
Mn is actively used to control oxides and to secure the strength of the steel sheet.
However, when the Mn content exceeds 1 SO%, solidifying segregation is likely to
occur, which may i~npaitro ughness and nlechanical properties. Therefore, the upper
limit of the Mn content is set as 1.50%. It is preferable that the tipper limit of the Mn
content is 1.20%.
[0030]
Si: 0.001% to 0.015%
Si is an elenlent having an effect of controlling the conlposition of oxides. In
order to obtain this effect, it is necessary that the lower litnit of the Si content is set as
0.001%. It is preferable that the lower limit of the Si content is set as 0.005%. On
the other hand, when the Si content is excessively high, enatneling characteristics
deteriorate. In particular, during hot rolling, a large amount of Si oxides are fortned,
and fishscale resistance may deteriorate. Therefore, the upper limit of the Si content
is set as 0.015%. 111 order to improve bubble-black point resistance and to obtain
excellent stirface propel-ties, it is preferable that the upper limit of the Si content is set
as 0.008%.
[003 11
Al: 0.001% to 0.010%
Al is an element which is effective for deoxidation of steel. However, since
A1 is a strong deoxidizing element, it is necessary to carefully control the A1 content.
When the A1 content exceeds 0.010%, it is difficult to maintain the 0 contetlt in steel to
be within a range which is required for the steel sheet for vitreous enanleling according
to the embodiment. In this case, it is difficult to form desired composite oxides, and
the number density of composite oxides which are effective for fishscale resistance
decreases. In addition, an A1 oxide having poor ductility during hot rolling is fonned,
which causes deterioration in fishscale resistance. In this case, it is difficult to control
oxides which are.effective for the imnprovement of fishscale resistance. Therefore, the
upper limit of the A1 content is set as 0.010%. On the other hand, when the A1
content is lower than 0.001%, a high load is applied during the steelmaking process.
Therefore, the lower limit of the A1 content is set as 0.001%. It is preferable that the
lower limit of the A1 content is set as 0.003%.
[0032]
N: 0.0010% to 0.0045%
N is an interstitial solid solution element. I11 a case wvl~erea large amount of
N is contained, even when a nitride-fonning element such as Nb or B is added,
workability tends to decrease, and it is difficult to produce a non-aging steel sheet.
Therefore, the upper limit of the N content is set as 0.0045%. The lower limit of the
N content is not particularly limited. However, in the existing techniques, a
significantly higll cost is required to reduce the N content to be 0.0010% or lower.
Therefore, the lower limit of the N content may be set as 0.0010%. It is more
preferable that the lower limit of the N content is set as 0.0020%.
[0033]
0: 0.0150% to 0.0550%
0 is an element which is required to form composite oxides and directly
affects fishscale resistance and workability. In addition, the 0 content relates to the
Mn content, the Nb content, and the B content and affects fishscale resistance, that is,
the number density of composite oxides and the size of voids present in steel.
Therefore, 0 is an essential element for the steel sleet for vitreous enameling
according to the embodiment. In order to obtain the effects, the lower limit of the 0
content is set as 0.0150%. It is preferable that the lower limit of the 0 content is set
as 0.0200%. When the 0 content is excessively reduced, the number density of
composite oxides present in the steel sheet is reduced, and concurrently, the size of
voids formed during the production process is also reduced. Therefore, fishscale
resistance deteriorates. On the other hand, when the 0 content is excessively high,
the number density of con~positeo xides formed and the size thereof increase. In this
case, the size of voids formed during a rolling process increases, which causes
deterioration in workability. Therefore, the upper limit of the 0 content is set as
0.0550%. It is preferable that the upper limit of the 0 contcnt is set as 0.0450%.
P is an element which is effective to refine the grain size of the steel sheet and
to strengthen the steel sheet. In order to obtain the effect, the lower limit of the P
content is set as 0.040%. It is preferable that the lower limit of the P content is set as
0.050%. On the other hand, when the P content is excessively high, during enamel
firing, a high concentration of P segregates in grain boundaries of the steel sheet,
which may cause bubbles, black points, and the like. Therefore, the upper limit of the
P content is 0.100%. It is preferable that the upper limit of the P content is set as
0.075%.
[0035]
S: 0.0050% to 0.0500%,
S is an element which forms a Mn sulfide with Mn. By precipitating the Mn
sulfide and oxides as complex precipitates, fishscale resistance can be significantly
improved. In order to obtain the effect, the lower limit of the S content is set as
0.0050%. The lower limit of the S content is preferably 0.0100% and more
preferably 0.0150%. However, when the S content is excessively high, the effect of
Mn which is required to control oxides may deteriorate. Therefore, the upper limit of
the S content is,set as 0.0500%. It is preferable that the upper limit of the S content is
set as 0.0300%.
[0036]
Nb: 0.020% to 0.080%
Nb is an essential elenlent for the steel sheet for vitreous enameling according
to the embodiment. Nb relates to the 0 content, the Mn content, and the B content
and affects the conlposition of oxides which contribute to the inlprovenlcnt of fishscale
resistatice of the steel sheet for vitreous enameling. In addition, Nb is an element
which also contributes to high-strengthening of the steel sheet through the refinement
of crystal grains. In order to obtain the effects, the lower limit of the Nb content is set
as 0.020%. It is preferable that the lower limit of the Nb content is set as 0.040%.
On the other hand, when the Nb content is excessively high, deoxidation occurs during
the addition of Nb, and it is difficult to form oxides in steel. 'Therefore, tlie upper
limit of the Nb content is set as 0.080%. The upper limit of the Nb content is
preferably 0.060% and more preferably 0.055%.
[0037]
Cu: 0.015% to 0.045%
Cu is an element having an effect of controlling a reaction between glass and
the steel sheet during enamel firing. In order to obtain the effect, the loww limit of
the Cu content is set as 0.015%. It is preferable that the lower limit of the Cu content
is set as 0.020%. On the other hand, when the Cu content is excessively high, a
reaction between glass and the steel sheet is inhibited, and the workability of the steel
sheet may deteriorate. Therefore, the upper limit of the Cu content is set as 0.045%.
The upper limit of the Cu content is preferably 0.040% and more preferably 0.030%.
[0038]
B: 0.0005% to@.0050%
When the steel sheet for vitreous enameling according to the embodiment
containing Mn, Nb, and 0 as essential eletnents contains B, the control range of oxides
increases, which is effective for tlie improvement of fishscale resistance. Even when
the steel sheet does not contain B, the steel sheet for vitreous enameling having
excellent fishscale resistance can be obtained. However, by the steel sheet containing
B, fishscale resistance can be easily improved. In order to obtain the effect, it is
necessary that tlie B content is set as 0.0005% or higher. In addition, B is an element
having an effect of improving enamel adhesion. From the viewpoint of adhesion, the
lower liti~iot f the B coritellt is preferably 0.0010% and more preferably 0.0015%.
On the other hand, when the B content is excessively high, castability in the
steelmaking process deteriorates. Therefore, tlie upper limit of the B content is set as
0.0050%. In additiotl, when the Nb content is relatively high, when the B content is
excessively high, the recrystallization temperature significantly increases, and
workability after cold rolling and annealing deteriorates. In addition, when the B
content is excessively high, in order to obtain sufficient workability, it is necessary that
annealing is performed at a significantly high temperature, which causes deterioration
in the productivity of annealing. Therefore, from this point of view, the upper limit of
the B content is set as 0.0050%. It is preferable that the upper limit of the B content
is set as 0.0035%.
[0039]
Fundamentally, the steel sheet for vitreous enameling according to the
embodiment contains the above-described elements and a remainder including Fe and
impurities. However, optionally, the steel sheet further cotltains one or more elements
selected from the group consisting of CI; V, Zr, Ni, As, Ti, Se, Ta, W, Ma, Sn, Sb, La,
Ce, Ca, and Mg, in which the total amount of the elements is 1 .O% or lower.
[0040]
One or more elements selected fiorn the group consisting of Cr, V, Zr, Ni, As,
Ti, Se, Ta, MI, Mo, Sn, Sb, La, Ce, Ca, and Mg: 1.0% or Lower in Total
C1; V, Zr, Ni, As, Ti, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, and Mg are
unavoidably i~~corporatefdio m steel raw materials such as ores or scrap. Therefore, it
is not necessary that the elenie~itsa re actively added. However, as in the case ofNb,
tlie elements fort11 oxides to exhibit an effect of preventing fishscale defects.
Therefore, the total amount of one or two or more of the elernents may be 1.0% or
lower. The total amount of the elenlents is preferably 0.5% or lower and nlore
preferably 0.1% or lower. When the total amount of the elements is excessively high,
a reaction with an oxide-forming element is intolerable, and it is difficult to control the
formation of desired oxides. As a result, fishscale resistance deteriorates. In
addition, when the total amount of the elements is excessively high, undesired oxides
are fornled in the steel sheet, and workability deteriorates.
[0041]
Further, \vl~enth e steel sheet for vitreous enameling according to the
embodiment does not contain B, it is necessary that the contents of C, Mn, P and Nb,
among the elements, which affect workability, fishscale resistance, and fatigue
properties and enamel adhesion after vitreous enameling satisfy the following
expression (1).
2.2018xC (%)+1.3xMn (%)+18xP (%)+S.lx(Nb (%))0.514.00... (1)
In the expression (I), C (%), Mn (%), P (%), and Nb (%) represent the
contents of C, Mn, P, and Nb, respectively.
In addition, when tlie steel sheet for vitreous enameling according to the
embodiment contains B, it is necessary that the contents of C, Mn, P, and Nb satisfy the
following expression (2).
2.5058xC (%)+1.3xMn (%)+I 8xP (%)+S.lx(Nb (%))0.5<4.00... (2)
[0042]
Typically, as the tensile strength of the steel sheet increases, the fatigue
properties ofthe steel sheet increase. However, in order for the steel sheet for
vitreous enanleling to be used as an enameled product, it is necessary that the steel
sheet undergoes a thermal history of performing heating (firing) for vitreous enameling
at a temperature of higher than 800°C after being processed into a desired shape. Due
to processing and vitreous enameling, the n~etallographics tl~lch~orfe t he steel sheet is
changed. Therefore, the tensile strength of the steel sheet after vitreous enameling is
different from that before vitreous enameling.
[0043]
Focusing on the change of the metallographic structure before and after
vitreous etlameliag, the present inventors fonnd that C, Mn, P, and Nb contained in the
steel sheet largely affected the change of the metallogr~phics truch~reb efore and after
vitreous enameling. In addition, it was found that, when the contents of C, Mn, P, and
Nb in the steel sheet satisfy a predetermined relational expression, the change of the
metallographic structure is suppressed, and the effects of the elements are added,
respectively.
[0044]
The present inventors prepared steel sheets having various cotnponent
compositions which contain Mn, Si, Al, N, 0, P, S, Nb, and Cu and optionally fi~rther
contain one element or two or Inore eletnents selected from the group consisting of Cr,
V, Zr, Ni, As, Ti, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, and Mg while changing the
contents of C, Mn, P, and Nb. After applying a tensile strain of 10% to the steel
sheets, a heat treatment of 830°Cx5 min corresponding to vitreous enameling was
perfornled. Nest, a fatigue test was performed using the steel sheets to investigate a
relationship between the fatigue limit ratio and the expression (1) of "8xC
('??)+I .3xMn (%)+I 8xP (%)+5.1 x(Nb (hereinafter, referred to as "expression
(I x)").
100451
As a result, the follo~vingw as found: when the value of the expression (1s) is
2.20 or higher, the fatigue strength corresponding to the tensile strength of the steel
sheet which undergoes processing and vitreous enameling is exhibited (that is, a
sufficient fatigue lirnit ratio is exhibited); and when the value of the expression (Ix) is
lower than 2.20, the fatigue strength relative to the tensile strength of the steel sheet is
reduced (that is, a fatigue limit ratio is reduced). It is preferable that the value of the
expression (Ix) is 2.40 or higher.
[0046]
In addition, the present inventors prepared steel sheets having various
component compositions which contain Mn, Si, Al, N, 0, P, S, Nb, Cu, and B and
optionally further contain one element or two or more elements selected from the
group consisting of Cr, V, Zr, Ni, As, Ti, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, and Mg
while changing the contents of C, Mn, P, and Nb. After applying a tensile strain of
10% to the steel sheets, a heat treatment of 83OoCx5 min corresponding to vitreous
enatneling was performed. Next, a fatigue test was performed using the steel sheets
to investigate a relationship between the fatigue limit ratio and the expression (2) of
"8xC (%)+I .3 xMn (%)+I 8 xP (%)+5. l x(Nb (%))0.5" (hereinafter, referred to as
"expression (2x)").
[0047] ,..
As a result, the following was found: when the value of the expression (2x) is
2.50 or higher, a fatigue strength corresponding to the tensile strength of the steel sheet
which undergoes processing and vitreous etlatneling is exhibited; and when the value
of tl~eex pression (2s) is lower than 2.50, a fatigue strength relative to the tensile
strengtll of the steel sheet is reduced. It is preferable that the value of the expression
(2x) is 2.70 or higher.
[0048]
When the structures of the steel sheets afier the fatigue test were observed, it
was found that the grain size of all the steel sheets increased. However, in the steel
sheets containing no B in which the value of the expression (Ix) was 2.20 or higher
and in the steel sheets containing B in which the value of the expression (2x) was 2.50
or higher, it was found that crystal grains were coarsened, but the coarsening degree
thereof was small.
[0049]
The reason why fatigue properties after vitreous enameling are changed
depending on the component composition of the steel sheet is not necessarily clear,
However, it is presumed that, by adding predetermined amounts of C, Mn, P, and Nb
within ranges which satisfy the expression (1) or (2), grain growth during vitreous
enameling is suppressed and a decrease in fatigue strength (fatigue limit ratio) relative
to the tensile strength of the steel sheet can be prevented.
[OOSO]
On the other hand, when the values of the expressions (lx) and (2x) exceed
4.00, adhesion between the steel sheet and glass during vitreous enameling deteriorates.
Therefore, the upper limits of the expressions (Ix) and (2x) are set as 4.00. The
preferable upper limits are 3.50.
[0051]
Next, colnposite oxides containing Fe, Mn, and Nb and composite oxides
containillg Fe, Mn, Nb, and B will be described.
[0052]
In the steel sheet for vitreous enameling according to thc e~nbodimentw, hen
the steel sheet does not contain B, composite oxides containing Fe, Mn, and Nb, in
particular, Fe-Mtl-Nb-based cotnposite oxides in which oxides formed of Fe, Mn, and
Nb are combined are present. In addition, when the steel sheet contains B, composite
oxides containing Fe, Mn, Nb, and B, in particular, Fe-Mn-Nb-B-based conlposite
oxides in which oxides formed of Fe, Mn, Nb, and B are combined are present.
Among the conlposite oxides, the number density of con~positeo xides having a
diameter of 0.2 ptn to 10 pm in the steel sheet is preferably 2x10~pa rticle/~nm2to
1 x104 pat-ticle/mm2. The Fe-Mn-Nb-based composite oxides and the Fe-Mn-Nb-Bbased
composite oxides have the same effect and thus will also be referred to as
"composite oxides according to the embodiment".
[0053]
The degree to which composite oxides having a diameter of lower than 0.2
pm contribute to the improvement of fishscale resistance is small. Therefore, the
diameter of the composite oxides according to the embodiment is set to be 0.2 pm or
more. The diameter of the composite oxides according to the embodiment is
preferably 0.5 pm or Illore and more preferably 1.0 pm or more. The definition of the
diameter of the composite oxides according to the enlboditnent and the method of
measuring the diameter will be described below.
[0054]
The uppcr limit of the diameter of the composite oxides according to the
embodiment is not particularly limited from the viewpoint of improving fishscale
resistance. However, when the amount of coarse coniposite oxides increases, the
number density of composite oxides decreases, and the effect of inhibiting hydrogen
penneation is reduced. Therefore, the effect of improving fishscale resistance is not
obtained. In addition, coarse composite oxides are likely to cause cracking during
processing. Therefore, when the atnot~not f coarse composite oxides increases,
workability decreases. Even if cracking does not occur, due to a diflerence between
the workability of the composite oxides and the workability of the metallographic
structure during processing, coarse voids are formed near boundaries between the
composite oxides and the metallographic structure. As a result, tlie fatigue properties
of an enameled product decrease, and the reliability deteriorates.
Therefore, the diameter of the composite oxides according to the embodiment
is set to be 10 pm or less. The diameter of the composite oxides according to the
e~nbodimenits preferably 5 pm or less.
[0055]
When the number density of the composite oxides according to the
embodiment in the steel sheet is less than 2x10~pa t-ticle/mm2,e xcellent fishscale
resistalice cannot be secured. Therefore, the number density of the composite oxides
according to the embodiment is necessarily 2x10~pa rticle/mm2 or tnore. The number
density of the composite oxides according to the embodiment is preferably 5x10~
particle/mm2 or more.
[0056]
On the other hand, when the number density of the co~npositeo xides
according to the enlbodiment in the steel sheet is more than 1x10~pa rticle/mm2, an
excess amount of voids arc formed in the boundaries between the co~npositeo xides
and the metallographic structure during processing, and the fatigue properties of an
enameled product decrease. Therefore, tlic number derlsity of the composite oxides
according to the e~nbodimenitn the steel sheet is set to be 1x 10>article/mm2 or less.
The number dcnsity of the composite oxides according to the embodiment is preferably
5x 1 o3 particle/nnn2 or less.
[0057]
A neth hod of identifying the conlposite oxides according to the etnboditnent is
not particularly limited. For exanlple, (a) oxides from which Fe, Mn, Nb, and 0 are
simultaneously detected or (b) oxides from which Fe, Mn, Nb, 0, and B are
simultaneously detected may be identified as the composite oxides according to the
embodiment. In order to identifj the oxides, for example, a field emission scanning
electron microscope (FE-SEM) or an energy dispersive X-ray analyzer (EDAX) may
be used.
I11 order to identify the composite oxides, a typical measurement method may
be used. Howevel; since it is necessary that the concentration of a micro region is
determined, it should be noted that the beam diameter of electron beams is s~~fficiently
reduced.
[0058]
The diameter and density of the composite oxides were defined using the
following method. That is, using a SEM, at an arbitrary position of the steel sheet,
the dimension and number of the composite oxides were measured in 10 or more view
fields at a magnification of 5000-fold, and the long diameter of the composite oxides
was measured as the diameter of the oxides. Among the oxides in the view fields, the
number of composite oxides having a long diameter of 0.2 pnl or more was calculated,
and the density (number density) per unit area (mm2) was calculated based on the
number of con~positeo xides.
[0059]
Next, the structure (metallographic structure) of the steel sheet for vitreous
enameling according to the embodiment will be described.
The structure of the steel sheet for vitreous enameling according to the
embodiment mainly including rerrite as a major component. Therefore, in order to
improve fatigue properties in addition to high-strengthening, it is effective to reduce
the grain size of crystal grains. In order to be used as an enameled product, as
described below, the steel sheet for vitreous enameling is processed into a desired
product shape by pressing or the like, is coated with an enamel, and then is heated to a
temperature of higher than about 800°C. Due to this heating, the adhesion between
glass of the enamel and the steel sheet is realized. Due to this heat treatment (vitreous
enameling), the grain size is changed by grain growth, and thus fatigue strength is also
changed. For the improvement of the fatigue strength of the steel sheet after vitreous
enameling, it is effective to reduce the grain size after vitreous enameling. In order to
reduce the grain size after vitreous enameling, it is important to reduce the grain size
before the heat treatment and to suppress the grain growth caused by vitreous
enameling.
It is necessaly that the average grain size of ferrite in the metallographic
structure before the heat treatment (vitreous enameling) is 12.0 pm or less at a 114
thickness (114t) position fiom the surface in the thickness direction. When the
average grain size exceeds 12.0 pm, it is difficult to realize the high-strengthening of
the steel sheet. In order to realize high-strengthening, the less the average grain size,
the better. However, as the average grain size decreases, workability deteriorates.
Therefore, it is necessa~yto determine the optimum grain size for the desired product
shape.
Furthet; typically, fatigue fiacture leads to breaking due to the initiation and
propagation of cracking. Cracking is likely to be initiated from the surface of the
steel sheet. Therefore, for the inlprovement of fatigue properties, it is preferable to
reduce the grain size of the surface of the steel sheet. The grain size of the steel sheet
for vitreous enameling is affected by the concentrations of eletneuts, in particular, P in
steel. As the P concentration increases, the grain size tends to decrease. The P
concentration distribution in the steel sheet is cl~angedin the hot rolling and pickling
processes.
In the steel sheet for vitreous enatneling according to the embodiment, the P
concentration at a position (surface part) at a distance of 20 pm from the surface in the
thickness direction is higher than that at the 1/4t position where the average grain size
is measured. As a result, the grain size of the surface part is less than that of the 1/4t
portion. In the steel sheet for vitreous enameling according to the embodiment, when
the P content (average concentration) in steel is about 0.04% or higher, the grain size of
the surface of the steel sheet is further reduced, which contributes to the itnprovement
of fatigue properties. The concentration distribution of the ele~nentsc an be measured
by, glow discharge optical etnission spectrometry and the like. The average grain size
of ferrite may be measured according to, an intercept method defined in JIS G 0552
and the like.
[0060]
Furthel; in order to suppress the grain growth caused by vitreous enameling, it
is important for the contents of C, Mn, P, and Nb among the components to satisfy the
following expression (1) when the steel sheet does not contain B and to satisfy the
following expression (2) when the steel sheet contains B.
2.2058xC (%)+1.3xMn (%)+18xP (%)+S.lxO\Tb (%))0.5~4.00... (1)
2.5058xC (%)+1.3xMn (%)+18xP (%)+5.1 x(Nb (%))0.5<4.~~... (2)
When the value of the expression (I) is less than 2.20 or wllen the value of the
expression (2) is less than 2.50, fatigue properties of an ena~neledp roduct which is
obtained after the steel sheet for vitreous enameling undergoes processing aud vitreous
enameling decrease.
In a laboratory, the present inventors prepared: steel sheets containing, as
components, C, Mn, Si, Al, N, 0, P, S, Nb, and Cu and optionally fi~rthecr ontaining
some of Cr, V, Zr, Ni, As, Ti, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, and Mg; and steel
sheets containing C, Mn, Si, Al, N, 0 , P, S, Nb, Cu, and B and optionally fi~rther
containing some of Cr, V, Zr, Ni, As, Ti, Se, Ta, W, Mo, Sn, Sb, La, Ce, Ca, and Mg.
By changing the contents of C, Mn, P, and Nb, steel sheets having various eompotlent
compositions were prepared. In addition, using these steel sheets, a fatigue test was
performed after a heat treatment of 830°Cx5 inin was performed with an applied
tensile strain of 10% to investigate a relationship between the fatigue limit ratio and
"SxC (??)+1.3xMn (%)+18xP (%)+5.1 x(Nb (%))'.'" of the expressions (1) and (2).
[0061]
The results are illustrated in FIG. 2. In the drawing, the horizontal axis
represents the value of "8xC (%)+1.3xMn (%)+18xP (%)+5.1 x(Nb (%))'.'" of the
expressions (1) and (2), and the vertical axis represents the fatigue limit ratio, that is, a
value ((TWITSw) hich is obtained by dividing a fatigue strength (ow) by a tensile
strength (TS) measured in a tension test, the fatigue strength being a stress at 10'
cycles.
The following results were found. When the value of "8xC (%)+I .3xMn
(%)+lSxP (%)+5.1 x(Nb of the expression (1) was 2.20 or higher, a
predetermined relationship was established between the fatigue limit ratio and the
value of "8xC (%)+1.3xMn (%)+18xP (%)+5.1 x(Nb (%))'.'" of the expression (1).
As the value of "8xC (%)+1,3xMn (%)+18xP (%)+S.lx(Nb (%))0.5" of the expression
(1) increased, the fatigue limit ratio was inlproved. On the other hand, when the
value of "8xC (%)+I .3xMn (%)+18xP (%)+5.1x(Nb of the expression (1) was
lower than 2.20, the above-described relationship was not satisfied, and a decrease in
fatigue litnit ratio increased. When the metallographic structure was observed after
the fatigue test, the following resnlts were found. In the steel sheets in which the
value of "8xC (%)+1.3xMn (%)+18xP (%)+5.1 x(Nb (%))'.'" was less than 2.20, the
grain size increased. In the steel sheets in which the value of "8xC (%)+I .3xMn
(%)+18xP (%)+5.1x(Nb (%))'-"'was 2.20 or higher, crystal grains of the steel sheet
were coarsened, but the coarsening degree thereof was small.
The following results were found. When the value of "8xC (%)+I .3xMn
(%)+18xP (%)+5.1 x(Nb of the expression (2) was 2.20 or higher, a
predetermined relationship was established between the fatigue limit ratio and the
value of "8xC (%)+1.3xMn (%)+18xP (%)+S.lx(Nb (%)lo-'" of the expression (I).
As the value of "8xC (%)+1.3xMn (%)+18xP (%)+5.lx@b (%))'.'" of the expression
(1) increased, the fatigue limit ratio was imnproved. When the metallographic
structure was observed after the fatigue test, the followitlg results were found. In the
steel sheets in which the value of "8xC (%)+I .3xMn (%)+l8xP (O/o)+S.lx(Nb
was less than 2.50, the grain size increased. In the steel sheets in which the value of
"8xC (%)+1.3xMn (%)+l8xP (O?)+S.lx(Nb (%))'.'"was 2.50 or highel; ctystal grains
of the steel sheet were coarsened, but the coarsening degree thereof was small.
[0062]
On the other hand, when the values of "8xC (%)+I .3xMn (%)+18xP
(%)+5.l x(Nb of the expressions (1) and (2) were higher than 4.00, adhesion
between the steel sheet and glass during vitreous enameling deteriorated. Therefore,
the upper limit of "8xC (%)+1.3xMll(%)+18xp (%)+5.1x(Nb (%))0.5" was set as 4.00.
[0063]
Next, the voids present in the steel sheet for vitreous enameling according to
the etnbodi~nenwt ill be described. The voids are formed in the boundaries between
the steel sheet and the co~npositeo xides duriug processing because there is a difference
in defor~nationre sistance between the steel shect and the composite oxides and it is
more difficult to defor~ltih e composite oxides than the steel sheet. The voids are
formed during hot rolling or cold rolling. Therefore, each of the voids exhibits a
pseudo-triangular shape (substantially triangular shape) in a direction in which the
steel sheet extends by rolling (in a cross-section in a rolling direction). FIG. 3 shows
an example of the voids. Such voids filnction as trap sites for hydrogen in steel, aud
it is preferable that the voids are present in order to suppress fishscale defects.
However, when the size of the voids increases, during processing such as press
forming for obtaining a product, the voids may become starting point of cracking due
to strains concentrated thereon. In addition, when vitl.eous enameling is performed
after processing, gain growth is likely to occur in the strain-concentrated portions.
Therefore, when a large void is present, crystal grains are coarsened after vitreous
enameling, and fatigue properties decrease. Furthel; when the steel sheet is used as
an enameled product, the concentration of strains on the voids causes a decrease in
fatigue properties.
In order to suppress a decrease in fatigue properties caused by the voids, it is
important to relax the strain concentration on the voids. The present inventors found
the following results. In the steel sheet for vitreous enameling according to the
embodinlent, by setting the equivalent circle diameter of the voids to be 0.6 ptn or less,
the stress concentration on tlie voids beconies relaxed and a decrease in fatigue
properties is suppressed even after processing arid vitreous enameling. However,
when the size of the voids is excessively small, the voids cannot fi~nctiona s the trap
sites for hydrogen in steel. Therefore, the lower limit of the equivalent circle
diameter ofthe voids is set as 0.1 pm.
[0064]
Further, the present inventors found that, even wvllen the equivalent circle
dianleter of the voids is 0.6 pm or less, fatigue properties may decrease. That is, the
present inve~ltorsfo und that, fatigue properties are affected not only by the size of the
voids but also by the shape thereof. As described above, each of the voids fonned in
the boundaries between the steel sheet and the conlposite oxides during hot rolling or
cold rolling exhibits a pseudo-triangular shape. The shape of the voids is changed by
conditions of hot rolling or cold rolling. When the angle of a tip end of the triangle
becomes more acute, strains are likely to be concentrated during the application of
stress, which may cause the coarsening of crystal grains after vitreous enameling. In
addition, when the steel sheet is used as a product, fatigue properties decrease due to
strain concentration.
As the tip angle of the triangle of the void becomes more acute, a decrease in
fatigue propetties beconles more severe. However, in a case where a long side of the
hiangle is set as a base, when a value obtained by dividing the length of the base by the
height of the triangle is higher than 15, a decrease in fatigue properties is remarkable.
Therefore, in the steel sheet for vitreous enameling according to the embodiment, when
each of the voids is approximated as a triangle and a long side of the triangle is set as a
base, a value obtained by dividing the length of the base by the height of the triangle is
15 or less. In addition, in a case where each of the voids is approximated as a triangle
and a long side of the triangle is set as a base, when a value obtained by dividing tl~e
length of the base by the height of the triangle is less than 1 .O, the vertical angle of the
triangle of the void is reduced, and strains are concetltrated. Therefore, the lower
limit of the value obtained by dividing the length of the base by the height of the
triangle is set as 1 .O.
The equivalent circle diameter and thc triangular shape of the voids \Irere
dcfincd using the following nlethod. That is, using a SEM, the long side and height
of the triangle of each of the voids were measured in 10 or more view fields at a
tnagnification of 5000-fold. In addition, the equivalent circle diameter was converted
from the area of the triangle.
100657
The method of producing the steel sheet for vitreous enameling according to
the embodiment and the nlethod of producing the enameled product accordi~~tog t he
embodinlent will be described.
[0066]
The steel sheet for vitreous enameling according to the embodiment can be
produced from molten steel having the above-described chemical composition through
refining, casting, hot rolling, pickling, cold rolling, continuous annealing, and temper
rolling and the like based on a typical method.
[0067]
During hot rolling, the heating temperature of a steel piece is preferably
11 50°C to 125OoC, the rolling reductio~(c~u mulative rolling reduction) is preferably
30% to 90%, and the finishing temperature is preferably 900°C or higher.
The composite oxides containing Fe, Mn, and Nb or the composite oxides
containing Fe, Mn, Nb, and B produced in the refining and casting processes are
stretched by hot rolling. During this hot rolling, the con~positeo xides are stretched
and crushed by rolling, and are changed into more preferable forms for securing the
desired properties. In order to uniformly disperse the conlpositc oxides in the steel
sheet, it is effective to perfor111 rolling at a given rolling reduction. That is, by setting
the hot-rolling reduction to be 30% or higher, the conlposite oxides in steel can be
sufficiently stretched, and the size and nunlber density of the conlposite oxides
obtained after cold rolling and continuous annealing can be easily controlled to be
within a desired range. I-Iowever, \when the hot-rolling reduction is higher than 90%,
the size of the conlposite oxides in steel is extremely small, arid excellent fishscale
resistance may not be obtained.
[0068]
In addition, in the pickling process after hot rolling, scales generated on the
surface are removed. In the pickling process, it is important to perform pickling
under conditions where the production in the cold rolling process, which is the next
process, is not inhibited by remaining scales and the like. For example, in the
pickling process using hydrochloric acid, basically, pickling may be performed at a
concentration of about 8% and a liquid temperature of about 90°C for a dipping time of
about 60 seconds. Pickling using sulfuric acid is not preferable. This is because, in
pickling using sulfuric acid, the surface having high concentrations of elements is
removed Inore than necessary by excessive pickling.
After pickling, in the cold rolling process, the steel sheet is fi~rthesr tretched at
a maximum temperature of about 150°C. Therefore, in the cold rolling, it is difficult
to stretch the hard composite oxides.
[0069] . ,,.
In the cold rolling, the cold-rolling reduction (cumulative rolling reduction) is
important to determine propel-ties of a product and is preferably 65% to 85%. The
hard conlposite oxides which filnctiori as hydrogen trap sitcs are crushed in the cold
rolling stcp. Thereforc, the size and number density of the conlposite oxides p~esent
in a final product change depending on the cold-rolling reduction. Likewise, the
voids which function as hydrogen trap sites are fortned by crushing the hard contposite
- 32 -
oxides in the cold rolling step. By crushing the hard composite oxides, the size and
number density of the composite oxides are optimized. Therefore, in order to form
the voids and to secure excellent formability after annealing, it is preferable that the
cold-rolling reduction is set to be 65% or higher. The voids act effectively on
fishscale resistance but act disadvantageously on workability. Accordingly, tlie
presence of an unnecessaly amount of voids causes a decrease in workability and
deterioration in the fatigue properties of a product after processing and vitreous
enameling. Therefore, it is preferable that the upper limit of the cold-rolling
reduction is set as 85%. When the cold-rolling reduction exceeds 85%, the composite
oxides are crushed more than necessary, and the size thereof is extremely small.
Therefore, the number density of the conlposite oxides vvhich are effective for fishscale
resistance is reduced. In addition, the metallographic structure is observed in which
the formed voids collapse and are eliminated. When the shape of the voids formed by
cold rolling, that is, each of the voids is approximated as a triangle and a long side of
the triangle is set as a base, a value obtained by dividing tlie length of the base by the
height of the triangle increases. Therefore, the effect of improving fishscale
resistance is reduced. Further, when the voids are not eliminated by sttxxctural
bonding, the voids cause cracking due to strains introduced by the processing, and thus
workability deteriorates.
[0070]
In general, in the cold rolling, a larger amount of strains are introduced into
the surface part of the steel sheet than the inside of the steel sheet. However, a
friction coefficient between a roll and the steel sheet can be reduced through selection
of cold rolling oil or the like. Accordingly, a difference in the introduced strains
between the surface part and the inside of tlie steel sheet can be reduced, and the
introduction of an excess amount of strains into the surface part can be suppressed.
As a result, the void shape can be preferably controlled.
I11 order to obtain a preferable void shape in the steel sheet for vitreous
enameling according to the embodiment, the friction coefficient between the rolling
mill roll and the steel sheet is preferably 0.015 to 0.060 and more preferably 0.015 to
0.040. However, a relationship between the friction coefficient and the void shape
varies depending on the settings of a rolling mill. The friction coefficient car1 be
calculated by repeatedly performing calculation according to a general rolling method,
that is, according to a rolling theory using a two-dimensional slab method such that
calculated values of a forward slip and a rolling force match measured values thereof.
In the related art, rolling in which the friction coefficient between a rollitlg
mill roll and a steel sheet is controlled was not performed.
[0071]
After cold rolling, the cold-rolled steel sheet is annealed. From the
viewpoint of productivity, it is preferable that the annealing is continuous annealing
using a continuous annealing line. The annealing temperature is preferably 700°C to
850°C. However, from the viewpoint of imparting distinctive mechanical properties,
the annealing temperature may be lower than 700°C or may be higher than 85OoC.
After continuous annealing, temper rolling may be performed to ~nainly
control the shape. In this temper rolling, a steel sheet for vitreous enameling having
desired characteristics call be obtained.
[0072]
The enanleled product according to thc cnlbodinlent can be obtained from the
stcel sheet for vitreous enameling according to the e~nbodimentth rough processing for
obtaining a desired shape, such as pressing or roll fornling, and vitreous enameling.
The processing such as pressing or roll fowling and vitreous enanleling majr be
performed according to a typical method. For esample, in the vitreous enameling, the
steel sheet coated with an enamel is heated to, for example, 800°C to 850°C and is left
to stand for 1 minute to 10 minutes such that glass of the enamel and the steel sheet
adhere to each other.
[Exanlples]
[0073]
Next, examples of the present inventiotl will be described. However,
conditions of the examples are merely exemplary to confirm the operability and the
effects of the present invention, and the present invention is not limited to these
condition examples. The present invention can adopt various conditions within a
range not departing from the scope of the present invention as long as the object of the
present invention can be achieved under the conditions.
[0074]
Steel having the component compositions shown in Table 1 was melted in a
converter, and slabs (steel pieces) were prepared through continuous casting based on a
typical method. These slabs were heated to 115OoC to 1250°C in a heating furnace
for hot rolling. Hot rolling was finished at a finishing temperature of 900°C or higher.
After hot rolling, the hot-rolled steel sheets were coiled at 700°C to 750°C.
[0075]
The hot-rolled steel sheets were pickled and cold-rolled at cold-rolliag
reductions shown in Table 2 to obtaul cold-rolled steel sheets. Nest, the cold-rolled
stccl sheets were conti~iuouslya nnealed at 78OoC. Next, through 1.2% of temper
rolling, stccl sheets for vitreous enameling haviug a thickness oC0.8 mu1 were prepared.
In order to make the thicknesses after temper rolling to be uniform, the thicknesses of
the hot-rolled steel sheets were changed relative to the rolling reductions of cold
rolling.
The fiiction coefficierlt between the rollillg lllill roll and the steel sheets was
0.025.
[0076]
The steel sheets for vitreous enameling were evaluated in various ways.
Regarding the mechanical properties, a tension test was performed according to JIS Z
2241 using JIS No. 5 specimen to measure the tensile strength (TS) and breaking
elongation. The average grain size of the steel sheets was measured near a 114
thickness position according to JIS G 0552.
[0077]
The diameter atid number densities of the oxides in each of the steel sheets
were measured using the above-described method by observing a cross-section of the
steel sheet parallel to a cold rolling direction with a SEM.
[0078]
Workability was evaluated in a 90" bending test using a V block method
according to JIS Z 2248. While changing the inner bending radius, each of the steel
sheets was bent by 90'. The outer surface of the bent portion was observed by visual
inspectioa to evaluate whether or not cracking occurred. The occurrence of cracking
was deterniined based on three stages: A: when the inner radius was 0.5 mm or less, no
cracking occurred; B: wheu the inner radius was more than 0.5 mnl and 2.5 mm or less,
no cracking occurred; and C: when the inner radius was Inore than 2.5 mnni, cracking
occurred. In this case, A and B were considered as "Pass".
[0079]
In the evaluation of fatigue properties, an alternating stress fatigue test was
perfor~lled on the steel sheets after performing a heat treatment with an applied tensile
strain of 10% at a heating temperature of 830°C corresponding to vitreous enameling
for a holding time of 5 minutes. In the evaluation of fatigue properties, a stress at lo7
cycles was obtained as a fatigue strength (cw), and this fatigue strength was divided by
a tensile strength (TS) obtained in the tension test which was performed on the steel
sheets after the heat treatment. The obtained value (OWITS) was set as a fatigue limit
ratio. When the value of the fatigue limit ratio exceeded 0.42, fatigue propelties were
considered as "Pass".
[OOSO]
In the evaluation of enameling characteristics, each of the steel sheets was
coated with an enamel at a thickness of 100 pm using a dry electrostatic powder
coating method and was fired in air at 830°C for 5 minutes. Using these steel sheets,
fishscale resistance and adhesion were evaluated. In the evaluation of fishscale
resistance, the steel sheets after vitreous enameling underwent a fishscale promoting
test of being put into a thermostatic chamber at 160°C for 10 hours. Next, using these
steel sheets, the occurrence of fishscale was determined by visual inspection based on
four steps: A: high; B: slightly high; C: normal; and D: problematic. In this case, A to
C were considered as "Pass".
In addition, in the evaluatioa of enamel adhesion, a 2 kg ball head weight is
caused to fall from a height of 1 m. Next, the enamel peeling state of a deformed part
was measured using 169 palpation needles, and the area rat?o of non-peeled portions
was obtained. The area ratio of non-peeled portions was evaluated based on four
stages: A: 95% or higher; B: higher than 85% and lower than 95%; C: higher than 70%
and lower than 85%; and D: 70% or lo\ver. In this case, A to C were considered as
"Pass".
[008l]
The evaluation results are shown in Table 2.
In Production Nos. 1 to 33 which are examples according to the present
invention, the Fe-Mn-Nb-based cotnposite oxides or the Fe-Mn-Nb-B-based composite
oxides having a diameter of tnore than 10 pm were not obsel-ved in the steel.
[0082]
In addition, it was found that, in the steel sheets in which the number of the
Fe-Mn-Nb-based con~positeo xides or the Fe-Mn-Nb-B-based composite oxides having
a diameter of 0.2 pm to 10 11m per unit area was within the range of the present
ir~veution(2 x lo2p a~ficle/mn12to 1X Io 4 particle/mn12), workability was satisfactory
while maintaining fishscale resistance.
[0083]
Furthel; it was found that, in the steel sheets in which the value of "8xC
(%)+I .3xMn (%)+I 8xP (%)+S.l x(Nb (%))0-5" (expression (lx)) of the expression (I)
was within the range of the present inventioti, fatigue properties and adhesion were
excellent. When the anlount of the cotnponents and the value of the expression (lx)
did not satisfy the range of the present invention, workability, enameling
characteristics, and fatigue properties were not able to be simultaneously satisfied.
[0084] .-
It can be seen from the results of Tables I and 2 that, in the high-strength steel
sheets for vitreous enameling of Production Nos. 1 to 33 which are examnples according
to thc pscsent invention, fatigue properties were higher while luaintaining workability
and fishscale resistance, as colnpared to a steel sheet for vitreous enameling of the
related ai-t. On the other hand, in Production Nos. 34 to 48 which are co~i~parative
exatnples, workability, fatigue propcl-ties, fishscale resistance, and adhesion were not

[OOSS]
[Table 11
[0086]
[Table 21
[Industrial Applicability]
[0087]
According to the present invelltion, it is possible to provide: a high-strength
steel sheet for vitreous enameling having excellent workability and fishscale
resistance; and an enameled product which is produced using the steel sheet for
vitreous enameling. When the high-strength steel sheet for vitreous enameli~lg
according to tlie present invention is applied to the energy fields in addition to
kitchenware arid building materials, the reliability against fatigue and the like caused
by a long period of use can be improved, and the weight of a product can be reduced.
Accordingly, the present invention is highly applicable to the industries in which the
steel sheet for vitreous enameling is produced and used.
[Brief Description of the Reference Symbols]
[OOSS]
1 : VOID
2: Fe-Mn-Nb-BASED COMPOSITE OXIDE

[Document Tl~pe] CLAIMS
1.A cold-rolled steel sheet for vitreous enameling, the steel sheet comprising, by
mass%,
C: 0.0005% to 0.0050%,
A h : 0.05% to 1.50%,
Si: 0.001% to 0.015%,
Al: 0.001% to 0.01%,
N: 0.0010% to 0.0045%,
0: 0.0150% to 0.0550%,
P: 0.04% to 0.10%,
S: 0.0050% to 0.050%,
Nb: 0.020% to 0.080%,
Cu: 0.015% to 0.045%, and
a remainder including Fe and impurities,
wherein when a C content is represented by C (%), a Mn content is
represented by Mn (%), a P content is represented by P (%), and a Nb content is
represented by Nb (%), the following expression (1) is satisfied;
a n~etallognphicstructurec ontains felxite, and an average grain size of the
ferrite at a 114 thickness position from a surface in a thickness direction is 12.0 pm or
less;
a number density of Fe-Mn-Nb-based composite oxides containing Fe, Mn,
and Nb and having a diameter of 0.2 prn to 10 pm is 2x lo2 i~a~-ti~le/ntnon 12 x 1 o4
particlelm~n~;
a fatigue linlit ratio, \vhich is a value obtained by dividing a fatigue strength
by a tensile strength, is higher than 0.42, the fatigue strength being a stress at lo7
cycles after performing a heat treatment with an applied tensile strain of 10% at a
heating temperature of 83OoC for a holding time of 5 minutes;
voids are formed between the metallographic structure and the Fe-Mn-Nbbased
co~npositeo xides, and an equivalent circle diameter of the voids is 0.1 pm to 0.6
pn; and
wlien each of the voids is approximated as a triangle and a long side of the
triangle is set as a base, a value obtained by dividing a length of the base by a height of
the triangle is 1.0 to 15.
2.2058xC (%)+1.3xMn (%)+18xP (%)+S.lx(Nb (%))0.5<4.~~... (1)
2.
A cold-rolled steel sheet for vitreous enameling, the steel sheet comprising, by
mass%,
C: 0.0005% to 0.0050%,
Mn: 0.05% to 1 SO%,
Si: 0.001% to 0.015%,
Al: 0.001% to 0.01%,
N: 0.0010% to 0.0045%,
0: 0.0150% to 0.0550%,
P: 0.04% to 0.10%,
S: 0.0050% to 0.050%,
Nb: 0.020% to 0.080%,
Cu: 0.015% to 0.045%,
B: 0.0005% to 0.0050%, and
a relnainder including Fe and impurities,
\vlierein ~vliena C content is represented by C (%), a Mn contcnt is
represented by MI](%), a P content is represented by P (%), arid a Nb content is
represented by Nb (%), the following expression (2) is satisfied;
a metallograpliic structure contains ferrite, and an average grain size of the
ferrite at a 114 thickness position from a surface in a thickness direction is 12.0 pm or
less;
a number density of Fe-Mn-Nb-B-based composite oxides containing Fe, Mn,
Nb, and B and having a diameter of 0.2 pm to 10 ptn is 2x10' particlelnnii2 to 1 x104
particle/mm2;
a fatigue limit ratio, which is a value obtained by dividing a fatigue strength
by a tensile strength, is higher than 0.42, the fatigue strength being a stress at lo7
cycles after perfaiming a heat treatment with an applied tensile strain of 10% at a
heating temperature of 830°C for a holding time of 5 minutes;
voids are formed between the metallographic structure and the Fe-Mn-Nb-Bbased
composite oxides, and an equivalent circle diameter of the voids is 0.1 pm to 0.6
pm; and
when each of the voids is approximated as a triangle and a long side of the
triangle is set as a base, a value obtained by dividing a length of the base by a height of
the triangle is 1.0 to 15.
2.5058xC (%)+1.3xMn (%)+18xP (%)+5.1 x(Nb (%))0.5<4.~~... (2)
3.
The cold-rolled steel sheet for vitreous enameling according to clai~1i ~or 2,
further comprising, by mass%,
one or more elements selected from the grot111 consisting of Cr, V, 21; Ni, As,
Ti, Se, Ta, W, Mo, Sn, Sb, La, Cc, Ca, and Mg,
wherein a total amount of the elements is 0.1% or lower.
4.An enanreled product ~vlrichis produced using the cold-rolled steel sheet for
vitreous enameling according to claim 1 or 2.
5.An enameled product which is produced using tlie cold-rolled steel sheet for
vitreous enalneling according to claim 3.