TECHNICAL FIELD

The present invention relates to strip material of a lead alloy, and to a method of making such strip material for use in the manufacture of electrode grids for lead acid storage batteries. 5. BACKGROUND ART

.The lead acid battery industry is currently beginning to make greater use of expanded lead electrodes and this technology permits the use of a continuous line in which lead strip is expanded, punched, pasted

10. and then cut up into individual plates. A number of methods of producing lead alloy strip is known, such as casting followed by rolling, however none of these is ideally suited to continuous production of strip in a line producing battery electrodes.

15. , DISCLOSURE OP THE INVENTION ^'

According to one aspect of the present invention a strip of lead or "lead alloy has a thickness of ^" at least 0.25mm and a grain structure which is substantially homogeneous across its width and which is elongate

20. in a direction through the thickness of the strip. It has been found that such strip material is particularly suitable for making lead acid battery electrode grids in ^' that it may be easily handled and expanded, and its tensile strength is such that growth

25. of the positive plates in service is not excessive.

The invention also embraces a method of producing such a strip and thus according to a further aspect of the present invention a method of producing a strip of lead or lead alloy comprises the steps of casting,

30. e.g. centrifugally casting, a substantially circular

section billet of lead alloy such that the grain structur of the resultant billet is substantially homogeneous across its width at any particular radius and peeling a strip of at least 0.25mm thickness from the cir- 5. cumference of the Billet.

If the billet is formed by centrifugal casting it may be ensured that the grain structure is substantial homogeneous across its width, by introducing molten metal into the mould at an appropriate rate and rotating the

10. mould at an appropriate speed.

In the preferred embodiment th.e lead alloy contains not more than 0.13%, e.g. 0.06 - 0.1% e.g. 0.08 - 0.09% calcium by weight, and preferably also contains not more than 0.99%, e.g. 0.5 - 0.9%, e.g. 0.6 - 0.8% ^" tin by

15. weight and optionally also up to 0.015%, e.g. 0.007 to 0.01% aluminium by weight.- Preferably th-e cast cast billet has a thickness substantially less than its diameter. The strip material desirably has a tensile strength of more than 35 Newtons/mm 2.

20. As cast the billet may have a diameter of between 50 and 100 cms, e.g. 75 cms and a thickness of between 8 and 25 cms e.g. 15 cms. It will however be appreciated that if desired the billet may have a thickness approaching or even exceeding its diameter. The billet

25. may have an uneven surface coated with dross when removed from th.e mould thus the method preferably includes the step of removing a layer of material for instance by machining off 1 to 5mm or even ^' more e.g. 3mm from each planar face so as to expose clean

30. metal.

As discussed in more detail below it is essential that the billet be cast such that its grain structure is substantially homogeneous across its ^' width at any particular radius otherwise even 5. satisfactory peeling of the strip is not possible. In the preferred embodiment in which, the billet is formed by centrif gal casting,molten lead alloy is introduced into the mould in a number of separate charges each of which is added before the

10. previous charge has fully solidified and the total casting operation may last between 10 and 30 minutes. If the mould is rotated too slowly voids may occur and molten metal may "run back" and the grain structure may not be suf iciently homogeneous,

15. The mould is, therefore preferably rotated at at least 100 r.p.m.. e.g. at between 150 and 250 r.p.m. In addition it may be advantageous to increase the rotational speed of the mould as the casting proceeds, because the solid metal

20. boundary gradually advances radially inwards which leads to a reduction in the centrifugal force acting on the solidifying metal if the mould is rotated at constant speed.

We have gravity cast a billet by simply

25. pouring molten metal into an open mould and attempted to peel it but the results were most unsatisfactory for a number of reasons. The primary reason was that it was found that the strip had a number of holes in it due to the

30. voids inevitably formed as the metal cooled in

the mould, and that the amount of dross and the extent of* surface _, shrinkage cracking on the billet resulted in an unnacceptable amount of -material having to be removed before peeling. In addition there was found 5. to be a variation in grain structure across the width of the peeled strip. It has been found essential that the grain structure of the billet must be substantially homogenous across its width at any point if even reliabl peeling is to be achieved.

10. The reason for these defects is that after the molten metal is _. poured into the mould, the metal at the top, bottom and side of the mould cools rapidly and thus has a relatively fine grain structure. How¬ ever, as the cooling rate drops, the rate of nucleation

15. of grains decreases and the growth of course columnar grains is accelerated. Large grains grow in the direction of heat extraction, and the net effect is a variation in grain structure across the billet. As the metal cools, shrinkage inevitably occurs and

20. voids appear throughout the cast billet.

However the method in accordance with the invention - hich includes centrifugally casting the billet is found to substantially eliminate., these problems. Centrifugal casting basically involves

25. gradually pouring molten metal into the centre of a mould rotating at high speed about a vertical axis. The metal is forced against the outer wall of the mould by the centrifugal force and it is from this wall that the metal solidifies. Any voids

30. that occur during solidification. ill do so at the

solid/liquid interface and are immediately forcibly filled by fresh liquid. Thus all the shrinkage appears at the inner face of the annular billet. In addition the rapid rate of solidification of the metal 5. coupled with the high, degree of turbulence ensures that the solid metal has a fine substantially homogeneous grain structure. The forces generated also accelerate the separation of non-metallic inclusions and precipitated gases.

10. When the strip is peeled from the billet it is the peeling operation itself which distorts the grain stuctύ e and leads to the grains being elongated in a direction having a component through, the thick¬ ness of the strip.

15. The reason for this is that the strip decreases in length, as it is being peeled. Thus it was found that, with the cutting tool inclined at -+0 to the tangental direction of the billet, two marks that were placed 60cms apart on the peripheral surface

20. of the billet were ^' only 37 cms apart on the peeled strip. The shrinkage or compression of the strip at the point of peeling leads to similar shrinkage or compression of the grain structure in the plane of the plate. Thus the net effect in the peeled

25. strip is that the grain structure appears elongate in a direction perpendicular to the plane of the strip, though in truth it is compressed in the plane of the strip. It will be appreciated that for this reason if the cutting tool is set to cut about 1mm into

30. the billet the peeled strip actually obtained will

be thicker e.g. of the order of 2mm thick.

The peeled strip has a .smooth surface on the side which was contacted by the cutting tool, but has ripples or corrugations extending in a direction 5. transverse to its length on the side remote from the cutting tool caused by the contraction of the strip which occurs at the point of peeling. A strip having one rough side does not lend itself to easy mechanical handling, and it is also more prone to failure during

10. expansion into a grid. Furthermore the rough surface leads to increased corrosion of the battery plates, and a greater likelihood of grain Boundary cracking. For this reason the method preferably includes the step of rolling the peeled strip from a thickness

15. of say 2 to Hmm down to 0.5 to 1.5mm in one or more passes. This distorts but does not radically alter the grain structure and renders the strip moresatisfactory for use in batteries. In addition it is found that the rolling operations lead to an

20. increase of between 10 and 30% typically 20% of the tensile strength, of the strip, e.g. to in excess

2 of 45 Newtons7mm . The reason for this is not fully understood, however it is believed that the rolling induces either work hardening or an accelerated

25. age hardening, process.

BEST MODE OF CARRYING OUT THE INVENTION

Further features and details of the invention will be apparent from the following description of one specific embodiment of the invention which is

30. given by way of example with reference to the accompanying drawings in which: -

"$13kr.

FIGURE 1 is a plan view at X 80 magnification of the grain structure of a strip peeled from a centrifugally cast billet in accordance with the present .invention; and

5. FIGURE 2 is a cross-sectional elevation at X

63 magnification of the strip shown in ^' Figure 1 showing the elongate grain structure.

Between 650 and 850 Kg. of lead alloy containing 0.08% calcium, 0.6% tin and 0.007% aluminium was

10. heated in a crucible furnace to a temperature of 500°c, and an initial charge of 300 kg. was introduced through an axial hole into a mould rotated about avertical axis at a speed of 200 r.p.m. The mould was- made of steel having a wall thickness of 100mm and having

15. an internal diameter of 75cms and an internal height of 19cms. The remainder of the molten metal was then introduced into the mould in further charges of between about 50 to 100kg. over a time of between 5 and 10 minutes. After all- the metal had been introduced the

20. mould was rotated for a further ten minutes until all the metal had solidified. When the moμld was cooled it was disassembled and the annular billet, having a central hole whose diameter was between 15 and SOcms, was removed. It will be appreciated that the hole has

25. no particular .importance and it is therefore made as small as possible. As previously described the billet was void-free and had a substantially homogeneous grain structure across its width.. A centre-piece for securing the billet to a chuck of a lathe was then secured in

30. the central hole by any convenient means, such as

forcing a square section rod into the round hole or bolting on a face plate or inserting an expanding man¬ drel into the hole. Dross and surface irregularities were then removed from each of the end faces of the 5. billet until clean metal was reached. Typically about 2mm of material was removed from each face and the amount should not exceed 3.5mm.

After about two weeks the billet was secured to the chuck of a lathe by means of the central bar and

10. rotated at such a speed that its linear peripheral speed was between 0.1 and 0.5m/s, typically 0.25m/s. A cutting tool having a width, of about 19 cms was then presented to the peripheral edge of the rotating billet and set to move radially inwards at a constant speed such.

15. that the thickness of the peeled strip remains constant, for instance between 1 and 4mm. The angular speed of the lathe was steadily increased as its diameter decreases so as to maintain its linear peripheral speed substantially constant.

20. As explained above the peeled strip had one smooth side and one rough side, and was heated up to a temperature of about 80°c by the shearing action of the cutting tool. The peeled strip was withdrawn at a speed substantially equal to the linear peripheral

25. speed of the billet and wound onto a take-up reel.

The take-up tension used was 11 kg force, and it was found that this tension can slightly change the grain structure of the strip. However the change produced was not significant and the take-up tension

30. was found not to be critical.

At this stage the strip had a grain structure which was substantially homogeneous across its width as seen in Figure 1 and which was elongate through the thickness of the strip as seen in Figure 2, and

5. had a tensile strength of between 40 and 45 N/mm 2.

The tensile strength (.which largely correlates with the growth of the positive battery plates in service) at this stage would be satisfactory for battery plates since "it has been found that a strength in excess 2 10. of 35 N/mm is adequate. However as mentioned above such strip. cannot be handled very easily in the preferred subsequent expanding process and is subject to an increased rate of corrosion in the battery environment..

15. The strip.was therefore subsequently rolled in one or more operations to reduce its thickness to between 10 and 80% of its original thickness, typically to 0.9mm. The rolling smooths the roughened side of the strip rendering it easier to

20. handle and less subject to corrosion. In addition the tensile strength was increased by about 20%,

2 typically to between about 50 and 55 N/mm .

The strip was now expanded into a grid, punched pasted with active electrode material and cut into

25. individual battery plates.

In the production process it may be convenient to omit the take-up roll after peeling and to feed the peeled strip straight into the flattening rollers. In this case the peeling will be at a lower speed than

30. the remainder of the line, determined by the percentage by which the strip's length is increased by the rolling operation.

The lead alloy of a Battery electrode desirably

2 has a tensile strength of 35 NJmm or more, and the method described above produces a strip having such a tensile strength, even Before the rolling step.

Thus, in production, it may Be possiBle to omit the rolling step and the strip may nevertheless have an adequate tensile strength and corrosion resistance in the Battery environment to Be immediately processed into Battery electrodes.

INDUSTRIAL APPLICABILITY

The strip according to the invention finds applicaBility as a material from which lead acid battery electrodes may be made by punching to perforate for or slitting and expanding to mesh. form.