TECHNICAL FIELD
[0001] The present invention relates to a steel sheet used for a metal container such as
a beverage can and a method for manufacturing the same.
BACKGROUND ART
[0002] In general, a steel sheet for a container as typified by a beverage can, a food can
and the like has reached a stage where thinning of the steel sheet to reduce the cost
of the container is making a progress, and a raw material of 0.2 mm or less in thickness
has come to be adopted. One of the problems that become obvious when the container
is manufactured with such an ultrathin material is deformation of the container.
[0003] This refers to deformation of the container not only due to external forces applied
in the handling of the container in general after the manufacturing and filling of
the container, but also due to an increase or a decrease of the internal pressure
of the container, that is, a pressure increase at the time of heat treatment of the
contents or a decompression processing for conservation of the contents or a pressure
increase which becomes essential depending on the contents such as a carbonated drink,
and moreover, due to temperature changes during distribution and storage.
[0004] In order to improve resistance to deformation, it is necessary to use a much harder
raw material not only design-wise, but also as a raw material. However, in general,
hard material is low in ductility, and causes a problem of material rupture and the
like at the time of the can formation.
[0005] Furthermore, in the ultrathin raw material, since material rupture occurs by a relatively
lower distortion than in a thick material, a material having a better ductility than
that for a thick material is required for the ultrathin raw material. Further, in
the can forming, after welding the steel sheet, a welding unit is sometimes further
formed, and in such a case, concentration of deformation at a specific region is prone
to easily occur, and even from this point of view, good ductility is required.
[0007] However, these techniques lack viewpoints as to how to suitably control the hardness
of the surface and inner layers for the ultrathin raw material particularly by considering
steel sheet components and nitriding conditions, and when the can is manufactured
based on the ultathin raw material by these techniques, can moldability of the raw
material and the resistance to deformation of the can are not necessarily satisfied.
[0008] Further, a technique has been put to practical use, in which, to secure can strength
by using the design of the can, for example, small peak portions and valley portions
(beads) are formed on a can body portion so as to improve flexural rigidity. In this
case, since the shape of peak portions and valley portions affects the strength of
the can, when the raw material is soft or the sheet thickness is thin, a process of
providing the peak portions and valley portions is required to be severe, but by this
process, a surface-treated film is damaged, so that the deterioration of corrosion
resistance occurs, and this creates problems. If the hard material is used, even while
the raw material remains thin, a processing amount to provide the peak portions and
valley portions can be reduced, and the deterioration of corrosion resistance can
be avoided. Thus, the use of the hard material is desired, but the conventional hard
material is not sufficient in ductility, and a defect occurs in a flange forming process
to wind up a can lid and the like, and therefore, the use of the hard material is
limited.
DISCLOSURE OF THE INVENTION
[0009] An object of the present invention is to solve the above-described problems of the
conventional technique, and to provide a steel sheet and its manufacturing method,
in which, with respect to the problematic deformation of the container manufactured
by an ultrathin raw material, the qualities of materials of the surface layer and
the inner layer of a raw material are controlled by adopting nitriding, and are changed
to a large extent, and at the same time, good ductility is provided even in the case
of the hard material.
[0010] In order to solve the above-described problems, the present inventor and others,
during their research study of the components of the steel sheet of 0.4 mm or less
in thickness which is manufactured particularly by being subjected to a nitriding
process and the relationship between the nitriding condition and the material quality,
have found that the component, particularly, a N content is limited to a specific
range, and moreover, the nitriding conditions can be most suitably adjusted, whereby
it is possible to preferably control the nitride form of the surface layer portion
and the inner layer portion of the material, and as a result, the problematic deformation
in the container using the ultrathin steel sheet as a raw material can be controlled
to a large extent.
[0011] That is, the present inventors have found that the resistance to deformation of the
can does not improve substantially by simply providing a surface hardness by performing
the nitriding processing after cold rolling, and increasing the content of nitrogen
in the steel, and that there exist nitriding conditions required to improve the resistance
to deformation of the can made of the ultrathin raw material, and the control method
of the conditions, the outline of which will be shown as follows.
[0012] A first aspect of the steel sheet for an ultrathin container according to the present
invention contains, in terms of mass %, C of 0.0800% or less, N of 0.600% or less,
Si of 2.0% or less, Mn of 2.0% or less, P of 0.10% or less, S of 0.05% or less, and
Al of 2.0% or less, and has a region, in which nitrides of 1 µm or less and 0.02 µm
or more in diameter exist in a surface layer within 1/8 thickness of a steel sheet
at a number density of 0.2 pieces/µm
3 or more, and satisfying the following formula (A).
[0013] A second aspect of the steel sheet for an ultrathin container according to the present
invention contains , in terms of mass %, C of 0.0800% or less, N of 0.600% or less,
Si of 2.0% or less, Mn of 2.0% or less, P of 0.10% or less, S of 0.05% or less, and
Al of 2.0% or less, wherein nitrides of 1 µm or less and 0.02 µm or more in diameter
satisfy the following formula (B).
[0014] In accordance with the steel sheet for an ultrathin container of the present invention,
the resistance to deformation and the can formability of the container are compatible
with each other without sacrificing either of them, and can be remarkably improved.
Particularly, at the thickness of 0.400 mm or less, remarkably good can characteristics
can be realized.
[0015] In the steel sheet for an ultrathin container of the present invention, the number
density of nitrides of 1 µm or less and 0.02 µm or more in diameter at the 1/4 sheet
thickness position of the steel sheet may be 10 pieces/µm
3 or less.
[0016] As a. steel component, the steel sheet may further contain, in terms of mass %, one
or two or more selected from Ti of 0.08% or less, Nb of 0.08% or less, B of 0.015%
or less, and Cr of 2.0% or less.
[0017] As a steel component, the steel sheet may further contain , in terms of mass %, 0.1%
or less in total of Sn, Sb, Mo, Ta, V, and W.
[0018] The remainder of the steel component may be Fe and unavoidable impurities.
[0019] A first aspect of the method for manufacturing a steel sheet for an ultrathin container
according to the present invention includes subjecting a steel containing, in terms
of mass %, C of 0.0800% or less, N of 0.0300% or less, Si of 2.0% or less, Mn of 2.0%
or less, P of 0.10% or less, S of 0.05% or less, and Al of 2.0% or less to cold rolling
and then subjecting the steel to nitriding processing at the same time as recrystallization
annealing or after the recrystallization annealing so as to form a region in which
nitrides of 1 µm or less and 0.02 µm or more in diameter exist in a surface layer
within 1/8 thickness of a steel sheet at a number density of 0.2 pieces/µm
3 or more and to make N content in the steel sheet be 0.0600% or less by mass %.
[0020] A second aspect of the method for manufacturing a steel sheet for an ultrathin container
according to the present invention includes subjecting a steel containing, in terms
of mass %, C of 0.0800% or less, N of 0.0300% or less, Si of 2.0% or less, Mn of 2.0%
or less, P of 0.10% or less, S of 0.05% or less, and A1 of 2.0% or less to cold rolling
and then subjecting the steel to nitriding processing at the same time as recrystallization
annealing or after the recrystallization annealing so as to satisfy the following
formula (B) with respect to nitrides of 1 µm or less and 0.02 µm or more in diameter
a surface layer within 1/8 thickness of the steel sheet and to make N content in the
steel sheet be 0.600% or less by mass %.
[0021] A third aspect of the method for manufacturing a steel sheet for an ultrathin container
according to the present invention includes subjecting a steel containing, in terms
of mass %, C of 0.0800% or less, N of 0.0300% or less, Si of 2.0% or less, Mn of 2.0%
or less, P of 0.10% or less, S of 0.05% or less, and Al of 2,0% or less to cold rolling
and then subjecting the steel to nitriding processing at the same time as recrystallization
annealing or after the recrystallization annealing so as to satisfy the following
formula (C) with respect to nitrides of 1 µm or less and 0.02 µm or more in diameter
and to make N content in the steel sheet be 0.600% or less by mass %.
[0022] In accordance with the method for manufacturing a steel sheet for an ultrathin container
of the present invention, it is possible to obtain the ultrathin container steel sheet
which remarkably improves the resistance to deformation of the container and the can
formability without sacrificing either of them but making them compatible to each
other at high productivity. Particularly, at the thickness of 0.400 mm or less, the
steel sheet for an ultrathin container having remarkably good can characteristics
can be manufactured at high productivity.
[0023] In the method for manufacturing a steel sheet for an ultrathin container of the present
invention, the steel containing, in terms of mass %, C of 0.0800% or less, N of 0.0300%
or less, Si of 2.0% or less, Mn of 2.0% or less, P of 0.10% or less, S of 0.05% or
less, Al of 2.0% or less, and Fe and unavoidable impurities as reminder may be subjected
to cold rolling and then be subjected to nitriding processing at the same time as
the recrystallization annealing or after the recrystallization annealing, thereby,
the number density of nitrides of 1 µm or less and 0.02 µm or more in diameter at
the thickness 1/4 position of the steel sheet may made to be 10 pieces/µm
3 or less, and N content in the steel sheet may be made to be 0.600% or less by mass
%.
[0024] When the nitriding processing is performed at the same time as the recrystallization
annealing or after the recrystallization annealing, the steel may be held for 0.1
second or more and 360 seconds or less in the atmosphere containing ammonia gas of
0.02% or more in a state where the sheet temperature is 550°C to 800°C, and after
the nitriding processing, a product of the temperature and the time in the temperature
region of 550°C or more may be made to be 48000(°C.sec) or less, or the average cooling
rate from 550°C to 300°C may be set to 10°C/sec or more.
[0025] After the recrystallization annealing, re-cold rolling may be performed at a rolling
reduction of 20% or less before the nitriding processing or after the nitriding processing.
BRIEF DESCRIPTION OF THE DRAWING
[0026]
FIG. 1 is a view showing positions in the thickness direction of a steel sheet.
FIG. 2 is a view showing the relationship between a mold driving amount and a driving
load in a deformation test.
FIG. 3 is a view showing the relationship between a nitriding time and a can strength.
FIG.4 is a view showing the relationship between a ratio of the number of nitrides
before and after the nitriding and the can strength.
BEST MODE OF CARRYING OUT THE INVENTION
[0027] Hereinafter, the present invention will be described in detail.
[0028] First, steel material components in the present invention will be described. The
components are all shown in terms of mass percent. An upper limit of C content is
required in order to avoid the deterioration of workability, and the C content shall
be 0.0800% or less. Preferably it is 0.0600% or less, and more preferably 0.040% or
less.
[0029] In the steel of the present invention to allow N having the same property as C to
be increased in content by nitriding, the C content necessary in view of securing
strength may be low. Even if the C content is 0.0050% or less, necessary strength
can be secured, and the C content may be 0.0020% or less. Again, if the C content
is 0.0015% or less, though the nitriding amount has to be taken into consideration,
it is possible to manufacture an ultrasoft material. From the viewpoint that a value
r is improved and a drawing formability is maintained high, the C content is preferably
low.
[0030] An upper limit of N content before nitriding is also necessary in order to avoid
deterioration of the workability, and the N content shall be below 0.0300%. Preferably,
the N content is 0.0200% or less, and more preferably, the N content is 0.0150% or
less, and more preferably, the N content is 0.0100% or less, and more preferably,
the N content is 0.0100% or less, and more preferably, the N content is 0.0050% or
less, and more preferably, the N content is 0.0030% or less. From the viewpoint that
a value r is improved and a drawing formability is maintained high, the N content
before nitriding is preferably low. It is necessary to be careful that N which is
contained by nitriding to be described later exists in a different content depending
on the sheet thickness positions of the steel sheet in order to bestow resistance
to deformation of the can and the like, and it is slightly different in the effect
of N which exists before nitriding.
[0031] The upper limit of the N content after nitriding is required not only to avoid the
deterioration of the workability but also to avoid the deterioration of surface treatment
properties such as plating, and the N content shall be 0.600% or less. Preferably,
the N content is 0.300% or less, and more preferably, the N content is 0.0150% or
less, and more preferably, the N content is 0.100% or less, and more preferably, the
N content is 0.050% or less, and more preferably, the N content is 0.030% or less.
However, from the viewpoint that a hard portion by nitriding is made much harder,
it goes without saying that the N content is preferably higher.
[0032] Si content is 2.0% or less due to the fact that, though it is added for strength
adjustment, if the content to be added is too high, the workability is deteriorated.
In the steel of the present invention, N which is infiltrated into the steel by nitriding
in a grain boundary and nitrides are formed, but it is often the case that they not
only cause fragile breakage, but also harm the effect of the present invention, and
therefore, the necessity often arises where the Si content is 1.5% or less, and further
1.0% or less. Particularly, from the viewpoint that formability is maintained high,
the Si content is preferably low, and 0.5% or less, and further 0.1 % or less, thereby
improving formability.
[0033] Mn content is 2.0% or less due to the fact that, though it is added for strength
adjustment, if the content to be added is too high, the workability is deteriorated.
From the viewpoint that formability is maintained high, the Mn content is preferably
low, and 0.6% or less, and further 0.2% or less, thereby improving formability.
[0034] P content is 0.10% or less due to the fact that, though it is added for strength
adjustment, if the content to be added is too high, the workability is deteriorated.
From the viewpoint that formability is maintained high, the P content is preferably
low, and 0.05% or less, and further 0.01% or less, thereby improving formability.
[0035] S content is 0.05% or less due to the fact that it deteriorates hot rolling and is
a cause of failure of casting and hot rolling. From the viewpoint that formability
is maintained, the S content is preferably low, and 0.02% or less, and further 0.01%
or less, thereby improving formability.
[0036] Al is an element to be added for deoxidation, but if its content is high, casting
becomes difficult. Since it causes some harm such as an increase of scratches on the
surface, it is 2.0% or less. Further, when the Al content is higher than 0.2%, it
has an effect of combining with N which is infiltrated into the steel sheet by nitriding,
forming a large amount of AlN in the steel sheet, and hardening a nitriding portion.
From the viewpoint that the formability in the steel sheet thickness center portion
low in the extent of nitriding is maintained high, the Al content is preferably low,
and if the Al content is 0.2% or less, and further, 0.1 % or less, the formability
of the region low in the extent of nitriding is improved.
[0037] Other than the above-described basic elements, the effect of the elements considered
in the ordinary container steel sheet and its control will be described below.
[0038] Ti raises recrystallization temperatures of the steel sheet, and remarkably deteriorates
annealing stability of rolling of the target ultrathin steel sheet of the present
invention. Hence, Ti is 0.080% or less. Particularly, in the ordinary application,
which does not require a high value r, Ti is not required to be added, and it is 0.04%
or less, and preferably 0.01% or less. Further, Ti which is dissolved into the steel
before nitriding has a strong effect of combining with N which is infiltrated into
the steel sheet by nitriding, and forming a fine TiN in the steel, and hardening the
nitride portion. Hence, even in the steel sheet thickness center layer which is low
in the extent of nitriding, it is often the case that hardening of the material occurs
more than necessary, and therefore, when it is necessary to obtain a soft steel sheet,
the Ti content is preferably low and 0.005% or less, and more preferably 0.003% or
less, so that inadvertent hardening of the steel sheet can be inhibited.
[0039] Nb also has the same affect as Ti, and raises the recrystallization temperatures
of the steel sheet, and remarkably deteriorates stability of rolling of the target
ultrathin steel sheet of the present invention. Hence, Nb is 0.08% or less. Particularly,
in the ordinary application, which does not require a high value r, Nb is not required
to be added, and it is 0.04% or less, and preferably 0.01 % or less. Further, Nb which
is dissolved into the steel before nitriding has a strong effect of combining with
N which is infiltrated into the steel sheet by nitriding, and forming a fine NbN in
the steel, and hardening the nitriding portion. Hence, even in the steel sheet thickness
center layer also which is low in the extent of nitriding, it is often the case that
hardening of the material occurs more than necessary, and therefore, when it is necessary
to obtain a soft steel sheet, the Nb content is preferably low and 0.005% or less,
and more preferably 0.003% or less, so that inadvertent hardening of the steel sheet
can be inhibited.
[0040] B, when added to the steel sheet which contains Ti and Nb of approximately 0.01%
or more, raises the recrystallization temperatures of the steel sheet, and remarkably
deteriorates annealing stability of rolling of the target ultrathin steel sheet of
the present invention. However, when the content of Ti and Nb is low, an adverse effect
in this point is low, but rather causes a drop in recrystallization temperatures,
and therefore, the recrystallization annealing at low temperatures becomes possible,
and this has an effect of improving the annealing stability of rolling, so that B
can be positively added. However, an excessive amount of B allows the breakage of
a casting piece at the casting time to remarkably occur, and therefore, the upper
limit of B is 0.015% or less. With a view to reduce the recrystallization temperatures
and improve the annealing stability of rolling, if the relationship with the N content
before nitriding satisfies B/N = 0.6 to 1.5, it is sufficient. Further, B which is
dissolved into the steel before nitriding has a strong effect of combining with N
which is infiltrated into the steel sheet by nitriding, forming a fine BN in the steel,
and hardening a nitriding portion. When this surface layer hardening by BN is put
to practical use, a ratio of the content B and the content N before nitriding is preferably
set to B/N > 0.8. By making this ratio 1.5 or more, and further 2.5 or more, the hardening
by BN formation becomes apparent. On the other hand, when the hardening of the material
appears more than necessary due to the BN formation, the formability is often deteriorated,
and therefore, caution is required. In the steel of the present invention, if the
hardening by the BN formation is not particularly utilized, the ratio of the content
B and the content N before nitriding may be further set to B/N < 0.8, and more severely
to B/N < 0.1.
[0041] Cr which is dissolved into the steel before nitriding has an effect of combining
with N which is infiltrated into the steel sheet by nitriding, forming a fine Cr nitride
in the steel, and hardening the nitriding portion. Hence, it is often the case that
the hardening of the material occurs more than necessary, but on the contrary, by
utilizing this nitride, the hardness of the nitriding portion can be effectively increased.
With this view, it is preferable to add Cr of 0.01% or more. However, on the other
hand, Cr increases the recrystallization temperatures of the steel sheet, and if it
is execessively added, it often remarkably deteriorates annealing stability of rolling
of the target ultrathin steel sheet of the present invention. To avoid the deterioration
of annealing stability of rolling of the ultrathin steel sheet due to an increase
of recrystallization temperatures, the Cr content is preferably 2.0% or less, and
if it is 0.6% or less, the increase of recrystallization temperatures can be inhibited
to the extent that no problem practically occurs.
[0042] Further, while it is possible to add Cr, Ni, and Cu and the like in order to provide
a characteristic such as increasing the corrosion resistance, which is not defined
by the present invention, since the excessive addition of them often decreases nitriding
performance indispensable for the steel of the present invention, Cr is preferably
30% or less, Ni is 15% or less, and Cu is 5% or less, and more preferably, it should
be confined to the extent that Cr is 15% or less, Ni is 5% or less, and Cu is 2% or
less.
[0043] Further, while it is possible to contain Sn, Sb, Mo, Ta, V, and W at a total content
of 0.1% or less in order to provide the characteristics not defined by the present
invention, since the excessive addition of them often decreases nitriding performance
indispensable for the steel of the present invention, caution is required. Particularly,
since Sn and Sb inclusion often decreases nitriding efficiency, to perform a control
of nitride by applying nitriding, caution is required. In order not to hinder nitriding
efficiency remarkably with respect to Sn and Sb, each of them is in 0.06% or less,
and preferably 0.02% or less.
[0044] Here, the segments of a region of the steel sheet thickness direction used in the
present specification will be described with reference to FIG. 1.
[0045] The "surface layer within 1/8 thickness" represents a corresponding region in FIG.
1. Incidentally, the region corresponding to the "surface layer within 1/8 thickness"
exists in both surfaces of the steel sheet, and in the present invention, with respect
to either of the surfaces, the one falling into a limited range of the present invention
is taken as a target. Although it is relatively easy to change a nitride distribution
of the front face and the rear face by a method of nitriding and surface treatment
before nitriding and further some treatment after nitriding and the like, the present
invention includes the steel sheet having such front and rear different layers also
as a target. This is because it is possible to obtain the resistance to deformation,
which is the target of the present invention, only by a single surface.
[0046] Further, the "1/8 sheet thickness position" represents a corresponding region in
FIG. 1. Further, the region corresponding to the "1/4 sheet thickness position" represents
the corresponding position in FIG. 1. Incidentally, though the positions corresponding
to them exist in both surfaces of the steel sheet, the present invention includes
either surface of them as a target which falls into a limited range of the present
invention.
[0047] Although it is relatively easy to change a nitride distribution of the front face
and the rear face by a method of nitriding and surface treatment before nitriding
and further some treatment after nitriding, the present invention includes the steel
sheet having such front and rear different surface layers also as a target. This is
because it is possible to obtain the resistance to deformation, which is the target
of the present invention, only by a single surface.
[0048] Although not illustrated, the "1/20 sheet thickness position", similarly to the "1/8
sheet thickness position", designates a position of the 1/20 depth of the sheet thickness
from the steel sheet surface.
[0049] In the present invention, the size and number density of nitrides existing in a specific
position or a specific layer in the thickness direction of the steel sheet are defined,
while the existing precipitates can be identified by diffraction patterns of an electron
microscope and a suitable X-ray instrument. Of course, identification can be made
by a method other than the chemical analysis and the like. An average diameter of
the nitride taken as a target by the present invention is 0.1 µm or less.
[0050] If the diameter is more than this value, the efficiency of high intensification is
not only remarkably reduced, but also it becomes a starting point of the breakage
at the process time, and deteriorates ductility, and at the same time, when a coarse
nitride is exposed on the surface of the steel sheet, surface treatment such as plating
is adversely affected. In light of these characteristics, this average diameter is
preferably 0.40 µm or less, and more preferably 0.20 µm or less, and still more preferably
0.10 µm or less. These diameters and number densities to be described later can be
determined, for example, by electron microscopy.
[0051] This control of the size and number density of nitrides is very important in light
of the compatibility of high intensification and workability retention. The reason
is because they not only affect the intensity and workability, respectively, but also
the behaviors by which the intensity and the workability are changed. That is, they
are required to be controlled to a region where an intensity rising effect is high
and a degree of workability deterioration is low. To this end, it is effective to
suitably control the temperature and the time in the above-described temperature range
of 450 to 700°C and a cooling rate immediately before entering this temperature region,
and this effect is the same as the general precipitates formation if under ordinary
conditions.
[0052] That is, the higher the cooling rate, and the lower the temperature, the finer nitride
size and the higher the density become, and due to taking a long time, the size becomes
coarse.
[0053] Incidentally, rather than the precipitates of nitride only, the precipitates combined
with oxide, carbide, sulfide and the like are also included as the target. When the
combined precipitates are formed, while it is difficult to specify the type of one
precipitate and the size of each compound, except in the case where one precipitate
can be apparently separated into a nitride portion and other portions, the precipitates
are determined as one nitride.
[0054] Although in the present invention, an extraction replica obtained by a SPEED method
is observed by electron microscopy with EDX; thereby, nitrides are basically observed,
when it is considered that nitride is very fine and extraction is not good, the thin
film may be observed by transmission electron microscopy. The determination of the
composition is performed by analysis by EDX, and when a non-metallic element mainly
observed is N, it is taken as nitride. Further, because the size is small, even if
the characteristic spectrum of N is not distinct, Fe, Ti, Nb, B, Cr, and the like
are detected, and moreover, distinct spectra such as O and S are not observed, and
yet from the shape comparison with other precipitates specifiable as nitride, the
precipitates approximately determinable as nitride are also taken into consideration
as nitride in the present invention. Further, for the analysis of the precipitates,
electron diffraction patterns and the like may be used. The identification of nitride
is not only made by the EDX technique and electron diffraction patterns, and any type
of analytical instruments may be used, which are currently remarkably improved in
performance. That is, the identification may be determined by a method in which the
type, size, and number density of the precipitates are recognized as appropriate.
Although it is conceivable that discrimination between carbonate and nitride may be
difficult depending on the precipitates, those whose types are not suitably determined
by ordinary analytical instruments are excluded from the present invention. Nitrides
whose size is extremely fine and not definable by an EDX spectrum and ordinary analytical
instruments are excluded from nitrides to be considered by the present invention.
According to the analytical instruments ordinarily used by the inventor at the time
of filing the present application, the smallest size was approximately 0.02 µm, and
therefore, in the present invention, the size of 0.02 µm was taken as the lower limit.
It goes without saying that if an analytical instrument of a higher order is used,
and the determination of much finer nitrides is considered, the number density should
be increased.
[0055] Further, it is considered important to clarify the lower limit of the nitride size
which is taken as a target since a problem is involved as to how far an extremely
fine atomic coalescence of N and metal atoms can be determined as nitride when even
an individual atomic layout is clarified by an instrument not accustomed to the inventor.
[0056] A measurement is conducted on a visual field to the extent that the diameter and
number of nitrides are not biased. In the present invention, the magnifications are
set such that the number of nitrides whose diameter is taken as a target becomes approximately
500 pieces within one visual field, and ten visual fields are randomly selected, and
the number density is obtained such that the number of target nitrides is divided
by the field of view area and the electrolytic thickness by the SPEED method at that
time, and the average diameter is obtained such that a total of the diameter of individual
nitrides is divided by the number of pieces. Here, it goes without saying that nitrides
which become the target within the visual field are all required to be measured. Incidentally,
by using image analysis and the like, the number and the diameter of nitrides can
be also determined.
[0057] Further, though there are some nitrides whose shapes are elongated, with respect
to nitrides whose shapes are not isotropic, an average of the major axis and the minor
axis is taken as a diameter of the precipitate.
[0058] The number density of the precipitate is calculated on assumption that, in an electrolytic
process in a replica preparing process, all the charges energizing a test piece surface
are spent for electrolyzing the steel sheet as bivalent ions of Fe (Fe
2+), and the precipitates that remain as residuum at the electrolyzing time are all
added on the replica. For example, in the preparation of the replica, if electrolysis
is performed using a quantity of electricity of 50C (crone)/cm
2 in the test piece surface area, precipitates located within the thickness of 18 µm
from the test piece surface are observed on the replica. However, when the steel sheet
which is a target of the measurement is extremely thin, in the case where the precipitates
located within the thickness of 18 µm are collectively observed, it is not clear as
to which position of the sheet thickness the observing position corresponds, and the
meaning of the definition on the "1/8 thickness" or the "1/4 position", and the "1/8
position", the "1/20 position", and the like defined by the present invention becomes
obscure, and therefore, the electrolytic thickness in the SPEED method is not limited
to 18 µm. Ideally, the precipitates existing on the surface of 0 in thickness ought
to be observed, but this likely causes an apprehension that measurement errors may
become large. Although depending on the sheet thickness, the electrolytic thickness
should be approximately 5 to 20 µm, and a grinding is performed such that a target
sheet thickness position becomes the same as a thickness center of the electrolytic
portion.
[0059] Further, the electrolysis is performed not from the sheet surface to the direction
of the sheet thickness but from the sheet thickness section to the in-plane direction,
and a replica containing information on the direction to the sheet thickness is prepared,
and a distribution of the number density of nitride in the direction of the sheet
thickness on the replica is measured, and from this distribution, it is also possible
to decide the number density of nitrides at a specific sheet thickness position.
[0060] Hereinafter, the state of nitriding, which is an important factor of the present
invention, will be described. The target technique of the present invention is basically
adapted to the ultrathin steel sheet for a container excellent in can characteristics
adequately controlled in components and qualities of materials of the surface layer
and the center layer, which is filed by the present inventor in
Japanese Patent Application, First Publication No. 2002-337647, and displays an extremely excellent effect, but is not limited to this.
[0061] However, in the description of the present invention, the states of nitride within
the region of the "surface layer within 1/8 thickness" and at the "1/20 sheet thickness
position", the "1/8 sheet thickness position", and the "1/4 sheet thickness position"
are mainly used, and therefore, the main effect of the present invention is to control
the state of nitride so as to vary at the sheet thickness position, and by controlling
the state of nitride in this manner, it is possible more preferably to obtain the
effect of
Japanese Patent Application, First Publication No. 2000-337647.
[0062] This is in line with information that, in the raw material for an ultrathin container,
the state of the surface layer portion is important in terms of use characteristics
as a can, and for the purpose of expressing a distribution state of the precipitates
of the steel sheet having variations in characteristics in the direction of the sheet
thickness, the size and number density of nitrides at the sheet thickness position
are used.
[0063] The present invention mainly distributes nitrides of the surface layer portion in
larger quantities and more minutely as compared to the center portion, and is based
on the assumption that, in light of the general nitriding method assumed to be one
of the manufacturing methods of the present invention, basically the steel sheet surface
is preferentially nitrided, and accompanied with this nitriding, the amount of nitrides
to be generated is supposed to be increased as compared to the center layer. Further,
nitrides formed, at this time are rather not preferable if coarse in view of the object
of the present invention, and nitrides are preferably minutely distributed according
to heating history after nitriding, particularly according to cooling conditions and
the like, and therefore, in the present invention, a control on minute nitrides is
performed.
[0064] Thus, one of the features of the present invention is to allow the state of nitrides
at the steel sheet thickness position to be different. This difference has a region
existing by the number density of 0.2 pieces/µm
3 or more in (the surface layer within 1/8 thickness) of the steel sheet with respect
to nitride taken as a target of the present invention, and moreover, it is limited
by (the number density at (the 1/8 sheet thickness position) of the steel sheet) >
(the number density at (the 1/4 sheet thickness position) of the steel sheet)). The
number density of nitrides, though limited in the range obtainable by the relationship
between N content and the size of nitride, is preferably 0.2 pieces/µm
3 or more, and more preferably 2 pieces/µm
3 or more, and further, 20 pieces/µm
3 or more, and more preferably 200 pieces/µm
3 or more, and if preferably 1000 pieces/µm
3 or more, it is very effective in terms of hardness.
[0065] Further, this difference is also limited by (the number density at (the 1/20 sheet
thickness position) of the steel sheet) /(the number density at (the 1/4 sheet thickness
position) of the steel sheet)), and this ratio is made to be 1.5 or more, and preferably
3 or more, and more preferably 6 or more, and more preferably 10 or more, and more
preferably 30 or more, and more preferably 100 or more. When this ratio is small,
the effect of the present invention becomes small, and the target steel sheet cannot
be obtained. Further, when nitriding is applied as a method of increasing the number
density of nitrides of the surface layer portion in this manner, the ratio can be
defined by (the number density at (the 1/20 sheet thickness position) of the steel
sheet after the nitriding processing) / (the number density at (the 1/20 sheet thickness
position) of the steel sheet before the nitriding processing), and in this case also,
similarly to the above-described, this ratio is made to be 1.5 or more, and preferably
3 or more, and more preferably 6 or more, and more preferably 10 or more, and more
preferably 30 or more, and more preferably 100 or more. It goes without saying that
the larger this ratio, the larger the effect of the present invention becomes.
[0066] Further, as is evident from the fact that the main control object of the present
invention is to distribute fine nitrides in large quantities in the surface layer
of the steel sheet as compared to the center layer of the steel sheet, distribution
of the fine nitrides in large quantities in the steel sheet center layer is not preferable
from the viewpoint of the preferable effect of the present invention. To make the
effect of the present invention remarkable, the number density of the nitride of 1
µm or less and 0.02 µm or more in diameter at (the 1/4 sheet thickness position) of
the steel sheet is preferably 10 pieces/µm
3 or less.
[0067] Next, a nitriding condition will be described. It is convenient in view of the productivity
to perform the nitriding processing continuously with the recrystallization annealing
in parallel with or after the recrystallization annealing after cold rolling. However,
it is not limited. A method of annealing is applicable regardless of using a batch
method or continuous annealing.
[0068] However, in view of the productivity of the nitriding processing and uniformity of
the coil inner material of the nitriding member, the continuous annealing method is
by far the most advantageous. Further, as defined by the present invention, to control
the qualities of the materials of the surface and inner layers so as to obtain a large
effect, it is disadvantageous for the nitriding time and the subsequent heating history
to take a long time, and therefore, the nitriding processing is at least preferably
performed by a continuous annealing facility. Unless there is any specific reason,
the continuous annealing shall be applied. Particularly, in the continuous annealing
process, a process of partially controlling the atmosphere in a furnace and performing
recrystallization in a first half and nitriding in a last half has many merits such
as uniformity of productivity and materials and easy control of the state of nitriding.
[0069] Further, when the nitriding processing is performed before the recrystallization
is terminated, the recrystallization is remarkably inhibited, and non-recrystallized
areas remain, so that remarkable deterioration of the workability likely occurs, and
caution is required. Although this limit is intricately decided by the steel components,
nitriding conditions, recrystallization annealing conditions, and the like, if a person
has an ordinary skill in the art, it should be easy to determine under which conditions
the non-recrystallized areas remain after adequate trials.
[0070] The nitriding processing is decided by taking into consideration not only an increasing
content of N of the steel sheet by nitriding, but also the steel components and the
recrystallization annealing conditions, and moreover, a heating history and the like
after nitriding, the diffusion of N from the steel sheet surface into the interior
and the change of nitrides in the sheet thickness section. To simply take the material
quality only decided by Rockwell hardness, tension test, and the like as a reference
mark does not enable a favorable resistance to deformation to be obtained, which is
the target of the present invention.
[0071] Although this condition is required to be decided with reference to an adequate number
of trials in actual operations, a basic idea is as follows, and the present invention
will be defined based on this idea. That is, nitriding is required to be performed
in a state in which the sheet temperature is 550 to 800°C. This can be performed,
similarly to the ordinary annealing, by setting the nitriding atmosphere to this temperature
and allowing the steel sheet to pass through this atmosphere so as to make the sheet
temperature be within this range, and at the same time to perform nitriding. This
may be also performed by keeping the nitriding atmosphere at a lower temperature and
introducing the steel sheet heated to that temperature range into the nitriding atmosphere
so as to allow nitriding to advance.
[0072] When the nitriding atmosphere is raised to that temperature, it is often the case
that, due to alternation and decomposition of the atmosphere unrelated to nitriding
of the steel sheet, nitriding efficiency of the steel sheet is reduced. Therefore,
the sheet temperature is set to 550 to 750°C. Preferably, it is set to 600 to 700°C,
and more preferably 630 to 680°C.
[0073] The nitriding atmosphere contains a nitrogen gas of 10% or more in volume ratio,
and preferably 20% or more, and more preferably 40% or more, and most preferably 60%
or more, and according to needs, contains a hydrogen gas of 90% or less, and preferably
80% or less, and more preferably 60% or less, and most preferably 20% or less, and
further, according to needs, contains an ammonia gas of 0.02% or more, and the remainder
is taken as an oxygen gas, hydrogen gas, carbon dioxide gas, hydrocarbon gas or various
types of inactive gases. Particularly, the ammonia gas is high in efficiency at raising
the nitriding efficiency, and since it is possible to obtain a predetermined nitriding
amount within a short period of time, the diffusion into the steel sheet center of
N is inhibited, and a favorable effect can be obtained for the present invention.
This effect is sufficient even if the ammonia gas is 0.02% or less, but if preferably
0. 1% or more, and more preferably 0.2% or more, and more preferably 1.0% or more,
and more preferably 5% or more, and if the ammonia gas is 10% or more, even the nitriding
processing for five seconds or less can obtain a sufficient effect, and if the ammonia
gas is 20% or more, and further, if 40% or more, though depending on nitriding temperatures
and the sheet thickness, even a short nitriding processing for one second or less,
remarkable effect can be obtained.
[0074] Further, with respect to a ratio other than the ammonia gas, particularly in the
case in which nitrogen gas and hydrogen gas are main gas components, it is favorable
to make (the nitrogen gas) / (the hydrogen gas) into 1 or more in volume, from the
viewpoint of the nitriding efficiency, and making this ratio into 2 or more can further
provide an efficient nitriding.
[0075] Further, in the ordinary annealing, the annealing is performed under the conditions
not nitrided in the atmosphere mainly including nitrogen gas and hydrogen gas, but
if a person has an ordinary skill in the art, it is possible, after adequate trials,
to change the conditions to those in which the nitriding occurs not only by the above-described
mixture of the ammonia gas, but also by a change of dew point, the mixture of a minute
amount of gas, a change of the gas ratio, and the like. Those conditions that can
detect the nitriding realized at least by the heat treatment including annealing by
the current analytical capability are included as a target of the present invention.
[0076] Although a holding time in the nitriding atmosphere is not particularly limited,
when it is considered that the thickness of the steel sheet is 0.400 mm at the maximum
based on the temperature condition of the present invention, which is 550°C or more,
and N which has infiltrated from the steel sheet surface by nitriding reaches the
steel sheet center layer due to diffusion of N into the steel while being held so
that the N distribution or the nitride distribution which are the object of the present
invention end up being unobtainable, it is desirable that 360 seconds are taken as
the upper limit of the holding time. Further, even if the nitriding efficiency is
improved, in order to obtain the nitriding amount and the nitrogen and hardness distribution
in the direction of the steel sheet thickness, which are required by the present invention,
at least 0.1 seconds is required and preferably 1 to 60 seconds, and more preferably
2 to 20 seconds, and most preferably 3 to 10 seconds is required.
[0077] In order to control the nitride distribution in the direction of the steel sheet
thickness, the heating history of the steel sheet after nitriding is also very important.
When considering the sheet thickness of the target steel sheet, the diffusion of nitrogen
in the steel, and the formation and growth of nitrides, holding for a long time at
high temperatures is not preferable.
[0078] However, by making the nitrogen distribution adequately smooth by this heat processing,
it is possible to make the effect of the present invention even more remarkable. For
this purpose, the heating history at the temperature region of 550°C or more is important,
and a product of the temperature and time in this temperature region is preferably
48000 or less. Although this is equivalent to 80 seconds at 600°C, and 60 seconds
at 800°C, when the temperature continuously changes, it is split into a time region
of 5 seconds each and the temperature change is recorded, so that the effect can be
suitably estimated, and even by determining a product of the temperature and time
for each region, it is possible to make an estimation.
[0079] Of course, this may be estimated by splitting into temperature regions having certain
temperature widths, and it is preferably 24000 or less, more preferably 12000 or less,
and most preferably 6000 or less. Usually, nitriding conditions are set so that the
distribution of nitrogen into the steel is approximately decided at the end of nitriding,
and in the cooling process after that, the generation of nitrides is preferably controlled.
[0080] Nitriding, which is a target of the present invention, is performed in a state in
which large quantities ofN are dissolved, and large quantities of nitrides arise accompanied
with the temperature drop after that, and therefore, control of the cooling process
after nitriding is important. In association with the heating history in this cooling
process, the cooling rate after nitriding remarkably affects the effect of the invention.
[0081] That is, even in a short period of time at a low temperature in which the nitrogen
distribution hardly changes, a forming state of nitrides at the cooling process often
remarkably changes. By setting the average cooling rate from 550°C to 300°C to 10°C/s
or more, it is possible to generate fine nitrides in large quantities in the surface
layer portion in which N density is relatively high and the cooling rate is high particularly
as compared to the center layer. Preferably, the cooling rate is set to 20°C/s or
more, and more preferably 50°C/s or more. However, if the cooling rate is too fast,
the dissolved nitrides excessively remain, and depending on the applications, a problem
of aging property arises, and therefore, caution is required.
[0082] In the manufacturing of the thin container steel sheet, re-cold rolling is often
performed for hardening adjustment and sheet thickness adjustment after recrystallization
annealing. The rolling reduction of this re-cold rolling is put to practical use from
the extent of several percentages close to a skin pass performed for shape adjustment
to 50% or more similarly to cold rolling.
[0083] In the present invention also, the application of re-cold rolling similarly to the
convention steel is possible. Simply considering the case in which re-cold rolling
is applied in the steel of the present invention which is hard in the surface layer,
and soft in the inner layer, it seems that the soft inner layer alone is preferentially
vvork-hardened, and the change in the hardness distribution in the direction of the
sheet thickness, which is a feature of the present invention, ends up disappearing,
but this is not in fact the case. That is, in the steel of the present invention,
even if it is a re-cold rolling coefficient to the ordinary extent, by re-cold rolling,
the surface layer portion which is rather high in N content and hard is rather preferentially
hardened, the hardness difference between the surface layer and the inner layer, which
is the feature of the present invention, becomes even more apparent. This is because
the surface layer preferentially tends to work hardening due to large quantities of
solid solution N and nitrides, and on the other hand, since the inner layer is constrained
by the surface layer, it cannot be preferentially deformed, and cannot be selectively
hardened by that much exceeding the surface layer.
[0084] Granting that, if the re-cold rolling coefficient becomes remarkably high, the steel
sheet itself becomes hardened, and even without controlling the material, distribution
in the direction of the sheet thickness like the technique of the present invention,
it is possible to obtain sufficient can strength, and at the same time, since the
effect of the present invention tends to be reduced, the significance of increasing
the re-cold rolling coefficient even by exceeding the ordinary application range is
small. From this, when re-cold rolling is applied to the steel of the present invention,
its rolling reduction is preferably up to the extent of 70%.
[0085] Further, when a welding portion is considered, if it is hardened by the process distortion
of re-cold rolling, it is softened by the heat of welding, and the process distortion
is concentrated in the flange forming and the like, and a problem of the deterioration
of formability may occur. However, in the steel containing large quantities of N of
the present invention, softening due to this welding heat is inhibited, and therefore,
with respect to formability of the welding portion, it is also possible to obtain
an advantageous of re-cold rolling material.
[0086] The time for rc-cold rolling is after the nitriding processing in the process of
continuously performing the recrystallization annealing and the nitride processing
which is preferable in view of the productivity, but when the recrystallization annealing
and the nitriding processing are performed in a separate process, it is possible to
perform re-cold rolling before the nitriding processing.
[0087] The present invention is applied to the steel sheet of 0.400 mm or less in sheet
thickness. This is because, in the steel sheet thicker than this in sheet thickness,
deformation of the forming member rarely becomes a problem. Further, when the sheet
thickness is thicker, the thickness of the surface layer hardened layer by nitriding
becomes relatively smaller, thereby making it difficult for the effect of the invention
to manifest itself. Preferably, the steel sheet of 0.300 mm or less, and more preferably
0.240 mm or less is made as a target, and in the steel sheet of 0.190 mm or less,
and further, 0.160 mm or less, it is possible to obtain a very remarkable effect.
[0088] A mechanism is not clear which realizes a material quality unique to the steel of
the present invention by controlling a state of nitride after nitriding such that
a surface layer and a center layer are distinguished and its distribution in the direction
of the sheet thickness is taken into consideration, and the material quality does
not exist in a steel simply containing N and a steel nitrided for the only purpose
of varying the surface hardness. However, the reason seems to be that resistibility
to the bending deformation of the steel surface layer portion accompanied with deformation
of the can is effectively enhanced by nitrides.
[0089] Moreover, this effect is assumed to be due to very effective manifestation of the
resistance to deformation by an external force when deformation of the sheet thickness
of the target material occurs, a stress state involved with the conditions such as
an inner pressure and a shape of the container, and the size and the number density
of nitrides conscious of the difference between the surface layer and the center layer
combined with nitriding conditions defined by the present invention.
[0090] The effect of the present invention does not depend on the heating history and manufacturing
history subsequent to component adjustment and before annealing. Slabs when hot rolling
is performed are not limited to the manufacturing method such as an ingot method and
a continuous casting method, and also do not depend on the heating history until reaching
hot rolling. Therefore, the effect of the present invention can also be obtained by
a slab reheating method, a CC-DR method which directly hot rolls without reheating
a cast slab, and moreover, a thin slab casting which omits a coarse rolling and the
like.
[0091] Further, without depending on hot rolling conditions, the effect of the present invention
can also be obtained even by a two-phase region rolling in which finishing temperature
is set to in the two phase region of α + γ and the continuous hot rolling in which
rough rolled bars are bonded together and then rolled.
[0092] Further, when the steel of the present invention is used as a raw material for a
container having a welding portion, softening of a heat effect portion is inhibited,
and particularly, the surface layer portion having large quantities of nitrides is
quickly heated and quickly cooled, so that nitrides are dissolved, and are re-precipitated
as much finer nitrides, some of which remain as solid solution N and are hardened,
and this provides an effect of improving the strength of the welding portion. This
becomes more remarkable when elements ordinarily inhibiting the softening of the heat
effect portion such as B and Nb are added.
[0093] On the other hand, as for a so-called two piece can manufactured through drawing,
a drawing with ironing and the like, the sheet surface is hardened; thereby, a hardening
effect is obtained in which the coefficient of friction with a metal mold is reduced
and formability is improved since the sheet surface is hardened. Moreover, since the
surface layer is hardened and resistance to bending deformation is strengthened, flexural
buckling of the steel sheet during molding rarely occurs. That is, the effect of inhibiting
the generation of creases also manifests itself.
[0094] Usually, the steel sheet of the present invention is used as an original sheet for
the surface treatment, but the effect of the present invention is not harmed at all
due to the surface treatment. As the surface treatment for a can, nickel, tin, chrome
(Tin free), and the like are usually bestowed. Further, as an original sheet for a
laminate steel plate coated with an organic coating film which has come to be used
in recent years, it can be used without harming the effect of the present invention.
EXAMPLES
(Example 1)
[0095] For a three-piece can formed by welding a can body, a three-piece can body was manufactured
by a steel sheet in which nitriding conditions were changed and control of nitrides
was performed. A deformation resistance when the body portion of this can was compressed
by a cylindrical die of 10 mmφ and 40 mm in length was measured, and at the same time,
a can end portion was flange-shaped similarly to winding up a lid.
[0096] In a deformation test, a relationship between a compressing amount of a die and a
compressing load is shown FIG. 2, and a point of inflection is generated by a certain
load. This load which became the point of inflection was utilized as an index of a
resistance to deformation. The higher this value, the smaller deformation by an external
force is, and the resistance to deformation will become good.
[0097] Further, in a flange forming, a length of the flange until breakage occurred was
measured. The longer this length, the better the flange formability is, and a defect
at the winding up time of the lid will rarely occur.
[0098] With respect to the steel of each component shown in Table 1, after hot rolling,
cold rolling, and annealing accompanied with nitriding, skin pass or re-cold rolling
was performed so as to manufacture a steel sheet, and estimation was made on the resistance
to deformation and the flange formability. Hot rolling, cold rolling, annealing, nitriding
conditions and the like are shown in Table 1.
[0099] Nitriding was performed subsequent to the midst of annealing, and it is considered
as a condition that recrystallization had been terminated before nitriding occurred.
The N content in Table 1 is an average N content over a sheet thickness before nitriding.
Since the steel sheet was manufactured by an ordinary method, the change of components
in the direction of the sheet thickness and the change of a state of nitrides are
low before nitriding, which are changes to the extent of being negligible for the
effect of the present invention. That is, with respect to components of the steel
sheet and the size and number density of nitrides before nitriding, the values of
the surface layer within 1/20 thickness range, the surface layer within 1/8 thickness
range, and the center layer within 1/4 thickness range are all the same.
[0100] The materials qualities for these steel sheets are shown in Table 2. In the steel
sheet according to the manufacturing method of the present invention, it can be confirmed
that both good resistance to deformation and flange formability are compatible.
[0101] Here, in Table 2, symbol A shows the highest number density within (the 1/8 sheet
thickness), and symbol B shows (the number density in the surface layer within 1/20
thickness) / (the number density in the center layer within 1/4 thickness), and symbol
C shows (the number density in the surface layer within 1/20 thickness after the nitriding
processing) / (the number density in the surface layer within 1/20 thickness before
the nitriding processing).
(Example 2)
[0102] A steel slab of 250 mm in thickness containing, in terms of mass %, C of 0.02%, Si
of 0.02%, Mn of 0.2%, P of 0.01%, S of 0.01%, Al of 0.04%, and N of 0.002% was manufactured
by continuous casting, and was made into a hot rolled sheet of 2.0 mm at a slab heating
temperature of 1100°C, a finishing temperature of 880°C, and a winding up temperature
of 600°C. The hot rolled sheet was subjected to acid wash, cold rolled to be 0.17
mm, and recrystallization annealing at 650°C for 30 seconds in a continuous annealing
line.
[0103] Some of the steels were rolled through a nitriding processing furnace which was filled
up with an ammonia-containing atmosphere and was extended to the annealing furnace
of a continuous annealing line, thereby performing the nitriding processing. Inside
the nitriding processing furnace, a heating facility was not provided, and the sheet
heated in the recrystallization annealing furnace was passed into the nitriding processing
furnace at 650°C, so that the nitriding was performed. Since the atmosphere inside
the nitriding processing furnace was heated by the heat of the sheet, the drop in
the sheet temperature during the nitriding processing was not so large, and the temperature
of the sheet coming out from the nitriding processing furnace was approximately 600°C,
depending on the nitriding processing time.
[0104] The steel sheets thus manufactured were subjected to ordinary electric Sn plating
after the skin pass of 1.5%, and then, tinned-steel sheets were manufactured. By using
these, three-piece cans were manufactured by the same method as performed in an ordinary
can manufacture, and can strength was estimated by the same method as the first example.
Incidentally, in all the steels used for can manufacture, no problem of welding, winding
up of the lid, and the like occurred. The obtained can strength depending on ammonia
concentration in the nitriding processing atmosphere, the cooling rate after the nitriding
processing, and the nitriding processing time is shown in FIG. 3.
[0105] In FIG. 3, symbol A shows the cases in which the ammonia concentration is 4% and
the cooling rate is 20°C/sec after the nitriding, symbol B shows the cases in which
the ammonia concentration is 4% and the cooling rate is 120°C/sec after the nitriding,
symbol C shows the cases in which the ammonia concentration is 10% and the cooling
rate is 20°C/sec after the nitriding, symbol D shows the cases in which the ammonia
concentration is 20% and the cooling rate is 20°C/sec after the nitriding, and the
cooling rate after the nitriding is an average cooling rate from 550°C to 300°C.
INDUSTRIAL APPLICABILITY
[0107] The present invention can provide a steel sheet for an ultrathin container at high
productivity which can improve both of the resistance to deformation and the can formability
of the container remarkably without sacrificing either thereof. In accordance with
the present invention, as for a can in which beads are formed on the can body, for
example, a steel sheet is not only made thin, but also a processing amount of the
peak portions and valley portions processing can be reduced, and therefore, it is
possible not only to make the can light in weight but also to improve corrosion resistance.
Hence, it can be applied as a steel sheet for a container as typified by a beverage
can and food can.