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A surface treated steel sheet for battery container, a battery container and a battery using thereof
(Field of the invention)
The present invention concerns a surface treated steel sheet for
battery container, a battery container and a battery using thereof.
(Prior art)
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A method of post-plating, that is barrel plating, after press forming cold-rolled steel strip into a battery container or a method of pre-plating, that is press forming nickel-plated steel strip into a battery container, have been adopted for manufacturing a battery container for primary battery such as alkali-manganesie batteries and secondary battery such as nickel cadmium batteries and nickel-hydrogen battery of which extending demand is expected in which strong alkaline solution is packed. Prom these points of view, the inventors of the present application previously proposed a surface treated steel sheet excellent for a battery container having small
internal resistance (WO 95/1152?) .
Recently, DI (drawing and ironing) forming method is increasingly used as a method of thinning wall to increase the capacity cf the battery replacing the past multi step deep drawing method (see Published Japanese Patent Hel 7-99686) . This DI forming method and DTK (drawing thin and redraw) forming method is capable of increasing the battery capac-
tive materials to be contained. Moreover, the thick bottom has an advantage to improve the pressure resistance of the battery. (Objective of the present invention )
By the way, although the DI forming method and the DTR forming method are effective for increasing battery capacity as mentioned above, there is a disadvantage when they are used for continuous forming because the deformation resistance of the material in those methods is greater than in the conventional multi step deep-drawing forming method. Concretely, when the powdering quality (powdery dropout of the plating layer) in the cupping process of the DI forming method and the DTR forming method is inferior, the powder adheres to the die and the punch in the ironing process causing a defect in the container side wall. Although the similar phenomenon happens in the deep-drawing forming, the above-mentioned defect is more remarkable in the DI form-Ing method and the DTR forming method because the container wall has small surface roughness, which produced more lustrous appearance. Moreover, powdering quality is more critical in the DI forming method and the DTR forming method. Also, because the contact pressure of the material and the tool is greater in the DI forming m«thod and the DTR forming method than that in the drawing method, favoiable lubrication is requested for tool life. Therefore, materials which have a favorable powdering quality and retention of the press lubricant are requested.
For the surface to be the outside of a battery container, as-plated steel sheet by the gloss nickel plating has been adopted. However, as it produces a hard and brittle gloss plating layer, it has inferior powdering quality at the press-forming. In addition, since gloss plating involves organic additives containing sulfir (for example, sulfomic acid havlng=C-SO 2 -group) to make electrolytically deposited crystal grains fine, sulfur is absorbed in the plating layer, which causes the em-brittlement with sulfur promoted by the temperature rise Of the material in the ironing and the stretching process of the Dl forming and the DTR forming resulting in a deteriorated powdering quality.
The present Invention is based on such findings and it is aimed at a battery container having a surface treated layer with the small internal resistance at the inner surface of it and high quality and excellent continuous formability at the outer surface of it. And it is further aimed at a surface treated steel sheet which is suitable for producing the said battery container. Another objective of the present invention is to improve the removability of container (strippability) after the DI forming and the DTR forming. This is taken into consideration since the difficulty of stripping the container from the punch (strippability) in the final pressing process is critical in the container manufacturing in addition to the above-mentioned powdering quality. At stripping where the container is pulled out from the punch hitching fingernails or the edge of the container, there was a problem that an inferior stripping caused
breaking and split at the open edge portion of the container more frequently, which deteriorated the productivity. (The best manner realizing the present invention )
A surface treated steel sheet of the present invention is explained below.
A surface treated steel sheet of the present invention has the different component of the surface treated layer at the side to be the inner surface of a battery container and that to be the outer surface of it. At first, the composition of the surface treated layer at the
x
surface to be the inner surface of a battery container is explained in detail. A nickel-tin diffusion layer or a iron-nlckal-tin diffusion layer is formed on the surface to be the inner surface. The formation of these diffusion layer on the inner surface of a battery container can reduce the internal resistance and improve the battery performance. The thickness of the above-mentioned nickel-tin diffusion layer or iron-nickel-tin diffusion layer is preferably 0.15 to 3.0 LL m. more preferably 0.2 to 2.0 U, m. When it is less than 0.15 IL m, the internal resistance of a battery can not be reduced. On the other hand, when it is more than 3.0 µm, the improvement of the adhesion to the positive electrode mix is saturated, which is not economical.
In order to form a nickel-tin diffusion layer, nickel tin alloy is plated or nickel is plated and subsequently tin is plated, then followed by heat treatment. In addition, it is favorable to form a nickel layer or a iron-nickel diffusion layer unde:r the nickel-tin diffusion layer in order to improve the corrosion resistance of the
Whole surface treated steel sheet. The thickness of these layers are
not particularly defined, but they are preferably 3 µm or less taking
into the economical consideration.
Next, a nickel-cobalt layer is formed on the outermost surface of the
surface treated layer at the side to be the outer surface of a battery
container for the following reason. Namely, it is because that a
nickel-cobalt diffusion layer has superior powdering quality and
formability by DI forming and DTK forming to the usual nickel layer
alone.
Next, the manufacturing process of a surface treated steel sheet of the
present invention is explained referring to Figure 1.
(Steel sheet)
In general, a low carbon aluminum killed steel sheet is favorably used
as a substrate for plating. A non-aging hyper low carbon steel sheet
added by niobium, boron or titanium is also available. A steel strip
electrolytically degreased, annealed and skin-parsed after cold
rolling is usually used as a substrate.
(Nickel plating)
After the pre-treatment consisting of electrolytecal degreasing,
rinsing, pickling in sulfuric acid or hydrochloric acid
( electrolytieal or dipping) and rinsing is practiced on the
above-mentioned steel substrate, nickel is plated on both sides of the
steel sheet. Since the nickel plating is diffused by the post heat
treatment, the usual problem can be solved. In the present invention.
any of known Watt bath, sulfamic bath or chloride bath is available. In
addition, any of non-gloss plating, semi-gloss plating or gloss
plating is available. The sulfur content involved in :he brightener is
rejected during the post heat treatment.
The plating thickness can be different on either side. The thickness of
the nickel plating is 0.5 to 5 Mm, preferably 1 to 4 u m.
When the above-mentioned thickness of the nickel placing is less than
0.5 At m, a lot of pin-holes are formed in the nickel plating layer,
which causes the increasing dissolving of iron (steel) into alkali
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solution that is packed in the battery container and the formation of iron oxide, that is unfavorable. Furthermore, i: increases the deterioration of the corrosion resistance at the out«r surface of the battery container, that is unfavorable.
Next, the formation of a nickel-cobalt alloy plating in the above-mentioned battery container and the surface treated steel sheet is explained. When cobalt sulfate is added into Watt bath or sulfamic bath, a surface treated layer in which cobalt co-deposiites with nickel is formed.
By the way, the suitable cobalt content of the nickel-cobalt alloy plating is from 0.5% to 10%. When the cobalt content is less than 0.5%, it is not effective for hardening the cobalt plating layer. On the other hand, when the cobalt content is exceeding 10%, it is uneconomical because the hardening effect on the surface treated steel sheet is saturated and also because cobalt is expensive precious metal. (Tin plating) After the afore-mentioned plating, tin is plated on the side of the
steel sheet to be the inner surface of a. battery container. The bath
composition can be any of usual acid bath or alkali bath, however,
stannous sulfate bath or phenol-sulfonic acid bath are preferably used
in the present invention.
In this formation of the tinplating layer, tin plating weight is
determined on the following view point. Namely, all the tin plating
layer should be changed into a nickel-tin diffusion :.ayer by the heat
treatment which forms a nickel-tin diffusion layer in the present
invention, since the layer of tin remains, tin dissolves into alkali
solution which i« an electrolyte of an alkali battery, which generates
hydrogen and possibly deteriorates the battery performance.
When heated at 700 C or less in the heat treatment process, nickel-tin
alloy is mainly composed of Ni 3 Sn, Ni 3 Sn 2 and Ni a Sn 1
The above-mentioned method is the first one which forms nickel-tin
diffusion layer comprising forming tin plating layer after forming
nickel plating layer followed by heat treatment.
There is the second method comprising directly plating nickel-tin
alloy plating on the steel sheet followed by heat treatment.
Next, the steel substrate on which nickel-tin alloy plating is formed
is selected from the following two of them.
(D cold rolled steel sheet
(2) steel sheet previously plated with nickel
There are 2 methods in which nickel-tin diffusion layer is formed as
mentioned above. In the present invention, disregarding the adoption
of the first or the second method, the heat treatment should be
practiced after plating.
It is because that the formation of the diffusion layer involving
nickel and tin can reduce the internal resistance at the inner surface
of the battery container.
Next, the latter method of nickel-tin alloy plating (the second one in
which the diffusion layer is formed after alloy plating) is explained
in detail.
Chloride-fluoride bath or pyro-phosphate bath are adopted as the
plating bath for nickel-tin alloy plating. The thickness of the
nickel-tin alloy plating is preferably 0.15 to 3,0 µ m.
(Heat treatment)
The post heat treatment is practiced in order to form a diffusion layer.
The heat treatment is preferably practiced under the non oxidizing or
reducing protective gas atmosphere in order to prevent the formation
of the oxide film on the surface. The diffusion layer is formed at
300 C or more. The heating duration is ranged from 30 seconds to 15
hours.
Batch annealing or continuous annealing are available for the heat
treatment of the steel in the present invention. The heating
conditions are preferably 30 seconds to 5 minutes at (500 to 850 * C for
continuous annealing and 5 to 15 hours at 300 to 650 C for batch
annealing. Furthermore, a diffusion layer of iror-niclcel- tin (3
components) can also be formed between the steel substrate and nickel
and tin plating layer in the present invention. In this case, tin
plating layer is formed on the nickel plating layer after nickel
plating followed by heating at a higher temperature for a longer duration. In which 3 components diffuse each other. (Skin-pass)
Skin-pass is practiced for the purpose of preventing the stretcher-strain caused by the heat treatment after nickel plating. Another purpose of the practice of skin-pass Is th
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selecting the work roil having various surface roughness in the
skin-pass.
(Explanation of a battery container)
Next, the manufacturing of the battery container using the
above-mentioned surface treated steel sheet is explained.
The battery container of the present invention is manufactured
press-forming the surface treated steel sheet manufactured as
mentioned above into cylindrical shape.
The inventors of the present invention founds out that when the
above-mentioned surface treated steel sheet was applied to a battery
container for an alkali battery, the more excellent battery
performance could be obtained rather than that using the usual battery
container.
(Composition of the outer surface of a battery container)
Tn the present invention, a nickel-cobalt diffusion liyer is formed on
the outermost surface of the surface treated layer of l:he outer surface
of a battery container. The formation of the nickel-cobalt diffusion
layer on the outermost surface can suppress the powdering when it is press-formed in comparison with the case where the surface treated steel sheet covered with a single layer of niokel plating is press-formed. Furthermore, nickel-cobalt diffusion layer has excellent corrosion resistance.
In addition, a nickel layer is favorably formed under the nickel-cobalt diffusion layer in the present invention. It is favorable to form the nickel layer because it further can improve the corrosion resistance of the nickel-cobalt diffusion layer and also improve the whole outer surface of the battery container. The thickness of the nickel-cobalt diffusion layer formed on the outer surface of the battery container is preferably 0.1 to I u. m, more preferably 0 . 2 to 0.6 li m.
Accordingly, the present invention relates to a method for producing a surface treated steel sheet for a battery case comprising the steps of:
plating the steel sheet with metal , wherein the inner side of the battery case is coated with Ni plating Ni-Sn alloy plating and the outer side of battery case is coated with Ni plating Ni-Co alloy plating,
heating the said plated steel sheet at a temperature of from 300 to 850°C to obtain surface treated steel sheet.
(Embodiment)
The present invention is explained in more detail showing embodiments
below,
[Example l]
A low carbon stael sheet after cold rolling and sinneallng having
thickness of 0.25 mm was used as a substrate for plating. The chemical
composition of the substrate is as follows :
C : 0.04 % (% means weight %, the same hereinafter) , Mn • 0.19 %, Si :
0.01 %, P • 0.012 %, S : 0.009 *, Al : 0.064 % and N : 0.0028 %.
The above-mentioned steel sheet wets electrolytic ally degreased in the
alkali solution on the following conditions.
(Alkali degreasing)
Condition of the electrolysis :
Bath composition s Caustic soda 30 g/1
Current density : 5 A/dm2 (anodic treatment) X 10 -seconds
5 A/dm 2 (cathodic treatment) X 10 seconds Bath temperature : 70 ' C
After that, pickling in the sulfuric acid (sulfuric acid 50 g/l, bath temperature 30 " C, dipping for 20 seconds) was practiced, and then nickel plating wsis carried out on the conditions described below.
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(Nickel plating)
Bath composition : nickel sulfate 320 g/l
boric acid 30 g/l
nickel chloride 40 g/l
Sodium laury 1 - sulfate 0.5 g/l
Bath temperature : 55 ± 2 " C
pH : 4 .1 to 4 . 6
Stirring : air bubbling Current density : 10 A/dm '
Anode : nickel pellets (nickel pellets were packed in a
titanium basket and the titanium basket was covered
with a polypropylene bag)
Non-gloss plating was formed on both sides of the steel substrate on the above conditions. The thickness of the nickel plating was varied by changing electrolysis duration on the conditions mentioned above. The samples having differential thickness of plating on either side were also prepared.
(Nickel-cobalt alloy plating)
Cobalt was involved in the nickel plating layer using a sulfamic bath
added by cobalt sulf ate in several quantity.
Bath composition : nickel sulf amide Ni(NH 2 SO a) • 4H 2 0 600 g/l
nickel chloride NiCl 2 10 g/l
cobalt sulfate CoSO 4 • 6H 2 0 5 to 20 g/l
boric acid H 3 BO 3 40 g/l
citric acid 0.6 g/l
saccharin 0.5 g/l
pH : 4 (controlled by addition of sulfamic acid)
Stirring : air bubbling
Bath temperature : 60 ' C
Cathodic current density : 10 A/dm '
Anode ! S pellets (brand name of INCO co., ltd, granular)
were packed in a titanium basket, and the titanium
basket was covered with a polypropylene bag.
The cobalt content in the plating and the thickness of the plating were
varied by changing the quantity of added cobalt sulfate and the
electrolysis duration on the conditions mentioned above.
(Tin plating)
After nickel-cobalt plating, tin plating layer was formed on the
opposite side using gtannous sulfate bath on the following conditions.
Bath composition : stannous aulfate 30 g/l
phenol-sulf onic acid 60 g/l ethoxyrated α naphthol 5 g/l
Bath temperature t 55 ± 2 ' C
Current density : 10 A/dm '
Anode : tin board
The several kinds of samples having varied thickness of tin plating
were prepared changing electrolysis duration on the conditions
mentioned above.
(Heat treatment)
Next, the heat treatment was practiced to form a diffusion layer on the
following conditions.
Protective gas : comprising 6.5 % of hydrogen and the reminder of
nitrogen
dew point — 55 ° C
The several kinds of the surface treated steel sheiit were prepared varying the soaking temperature and the soaking period of time. The thickness of the plating layer and the diffusion layer were measured with CDS (glow discharge atomic emission spectrochemical analysis) . The surface analysis of the samples heat which were treated after tin was plated on nickel plating layer by X-ray diffraction analysis and CDS (glow discharge atomic emission spectrochemical analysis) revealed the formation of the nickel-tin diffusion layer. [Example 2]
The following nickel plating was formed on both sides af the same steel substrate as that of Example 1. After that, nickel-cobalt alloy was plated on the side to be the outer surface of a battery container on the same conditions as those of Example 1, and tin was plated on the side to
be the inner surface of a battery container on the s?ime conditions as
those of Example 1. Subsequently, the surface treated steel sheet was
completed practicing heat treatment and skin-pass rolling.
Nickel plating
Bath composition : nickel sulfate 300 g/1
boric acid 30 g/1
nickel chloride 45 g/1
Sodium lauryl-sulfate 0. 5 g/1
Bath temperature : 50 ± 2 * C
pH : 4.0 to 4.5
Stirring ; air bubbling Current density : 15 A/dm 2
The samples having several thickness of the nickel plating were prepared varying electrolysis duration and subsequent heat treatment was practiced as in Example 1. [Example 3]
(Nickel-tin alloy plated example)
Nickel was plated on both sides of the same steel subutrate as that of Example 1 on the same conditions as those of Example 1, and then nickel-cobalt alloy was plated on the one side of the steel substrate on the same conditions as those of Example 1. After that, nickel-tin alloy was plated on the one side of the steel substrate using chloride-fluoride bath. The conditions of nickel-t:Ln alloy plating are as follows. Bath composition : stannous chloride 50 g/1
niclcel chloride 300 g/1
sodium fluoride 30 g/1
acid ammonia fluoride 35 g/1
Bath temperature \ 65 ' C
pH : 4.5
Stirring : air bubbling Current density : 4 A/dm "' Nickel-tin alloy involving 28 % of tin was used as the anode,
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The samples having several thickness of the nickel-tin alloy plating were prepared varying electrolysis duration and subsequent heat treatment was practiced as in Example 1. These results are shown in Table l.
Table I
(Table Removed) £
Remarks * IN : Inner surface OUT : Outer surface
(Manufacturing of battery container)
As for the battery container formed by the DI forming method, the above-mentioned plated steel sheet having the thickness of 0.38 mm was punched out into a blank 41 mm in diameter, drawn into a cup 20. 5 mm in diameter, and then formed into 13.8 mm in outside diameter, 0.20 mm in container wall thickness and 56 mm in height by redrawing and two-step ironing using DI forming machine. Finally, the upper part was trimmed
off to shape a LR-6 type battery container 49.3 mm in height. On the other hand, as for the battery container formed by the DTR forming method, the plated steel sheet 0.25 mm in sheet thickness was punched out into a blank 58 mm in diameter, and then shaped into a LR-6 type battery container 13.8 mm in outside diameter, 0.20 mm in container wall thickness and 49.3 mm in height by several times of drawing and redrawing.
(Production of battery)
After manufacturing battery container as described above, LR-6 type alkali-manganese battery was produced as follows.
At first, positive electrode mix was prepared mixing manganese dioxide and graphite at the 10 : 1 by weight ratio and etdding potassium hydroxide (8 mole) to them. After that, the positive electrode mix wag pressed in the did and shaped into a doughnut shaped positive electrode mix pellet having prescribed dimensions, and then it was pressed and fitted into the battery container. Then, the batte:ry container was formed by necked-in forming at just below the open edge portion of it in
order to fit a negative electrode board on which a negative electrode collector rod into the battery container.
After that, a separator made of unwoven vinylon cloth was inserted along the inner circuit of the pellet pressed and fitted into the battery container, and then negative electrode gel composed of zinc granules and potassium hydroxide saturated with zinc dioxide was inserted into the battery container. After that, the negative electrode board fitted with an insulating gasket was .inserted into the battery container, and then it was calked around the open edge portion of the battery container.
In case where graphite is coated on the inner surface of the battery container, graphite 80 part (by weight) and thermoset.tlng epoxy resin 20 part (by weight) are mixed and diluted with methyl-ethyl fcetone, and then it is air-sprayed onto the inner surface of the battery container followed by drying at 150 ° C for 15 minutes. (Evaluation of powdering quality)
Furthermore, the powdering quality was evaluated in the 3 forming method, which are drawing. DI forming and DTR forming, in order to evaluate the continuous productivity of the battery ccntainer. The powdering quality was evaluated by the decrease in weight at the following weight measurements of (l) and (2) after forming in the manufacturing process of the above-mentioned battery container. The process was comprising blanking → cupping → degraasing → weight measurement (l) → forming → degreasing → weight measurement (2) . Furthermore, the whole evaluation of powdering quality was evaluated
observing the mass of powder fallen off from the plating, that was glued to a cellophane tape after the lubricant on the inner and the outer surfaces of the battery container was rejected by organic solvent, using a microscope of 25 magnification.
Degreasing was conducted by alkali dipping degreasing followed by ultrasonic cleaning in acetone.
Since the error might be large if the weight decrease was measured every one container, 30 of them as one measurement unit were repeated three times. Table 2 shows the result.
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Table2
(Table Removed)
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As la apparent in Table 2, while in the comparative example of 1 and 2, which were formed using a surface treated steel sheet of which one side to be the outer surface of a battery container was covered with mono layer of nickel plating, a large number of powders fell off from the containers (from 74 to 160 mg/30 containers) , in the examples of the present invention, a small number of powders fell oi:f (from 23 to 33 mg/30 containers) . This shows that the battery container of the present Invention is excellent in powdering quality.
By the way, in the present invention, the powdering quality and the anti-scratch quality can be improved not only using VI forming or DTK forming, which are the methods to reduce the wall thickness of a battery container, but also using the conventional multistage drawing. Therefore, it can preferably be used, also. ( Effect of the invention )
In the battery container as claimed in Claim 1 to 6, the low internal resistance is revealed on the inner surface, and powdering can remarkably be reduced during the forming by DI or DTR, which can suppress the scratch occurrence by metal contact, prolong the life span of the die, and improve the continuous productivity of the battery container.
In the surface treated steel sheet as claimed in Claim 7 to 11, they can be used as the material for the battery container as claimed In Claim 1 to 6.
Moreover, in the battery of the present invention using them, excellent: battery performance such as low internal resistance and
large short circuit current can be obtained.
(Brief explanation of figures)
Figure 1 shows the manufacturing process of the surface treated steel
sheet of the present invention.
(Summary)
(Objective)
The present invention aims to produce the material for a battery
container having the improved battery performance^ wherein the
internal resistance caused by contacting with the positive electrode
mix is remarkably reduced and the corrosion resistance is excellent.
It also aims to produce the material, wherein powdering Is reduced at
the inner surface of the battery container during forming. Furthermore,
it aims to produce a battery container manufactured u*!ing the material
and a battery manufactured using the battery container.
(Solution)
The surface treated steel sheet of the present invention for a battery
container is comprising a steel substrate on which one side to be the
inner surface of the battery container nickel- tin-iron diffusion
layer is formed on the uppermost surface. On the sid«» to be the outer
surface of the battery container nickel-cobalt-iron diffusion layer
is formed on the uppermost surface. Further, on the inner surface of
the battery container manufactured by drawing, the nickel-tin-iron
diffusion layer is formed, while on the outer surface of it
nickel-cobalt-iron diffusion layer is formed. In addition, a battery
is manufactured packing the active material into this battery
contalner. (Selected Figure) Figure 1
WE CLAIM:
1. A method for producing a surface treated steel sheet for a battery case
comprising the steps of :
plating the steel sheet with metal , wherein the inner side of the battery case is coated with Ni plating Ni-Sn alloy plating and the outer side of battery case is coated with Ni plating Ni-Co alloy plating,
heating the said plated steel sheet at a temperature of from 300 to 850°C to obtain surface treated steel sheet.
2. A method as claimed in claim 1, wherein the said inner side of the
battery case is coated with Ni plating -Sn plating and the outer side of
battery case is coated with Ni plating Ni-Co alloy plating.
3. A method for producing a surface treated steel sheet for a battery case
substantially as herein described with reference to and as illustrated
in the accompanying drawings.
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