GB1575204A - Drawing hollow articles from sheet - Google Patents

Description

(54) DRAWING HOLLOW ARTICLES FROM SHEET (71) We, METAL BOX LIMITED, of Queens House, Forbury Road, Reading RG1 3JH, Berkshire, a British Company, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to the selection of mild steel sheet for drawing into hollow articles and more particularly, but not exclusively, to the drawing of cans from tinplate.

During the drawing of cylindrical containers from tinplate it has often been observed that, in spite of feeding blanks of equal diameter to the press tools, the useful height of the drawn cans varies from batch to batch of steel. This variation in height is usually attributed to varying stretching of the tinplate, but hitherto this factor in the can height has been an erratic quantity which could not be relied upon in mass production. A further variation arises due to earing. The usual remedy has been to deliberately make the blank somewhat larger than that theoretically required and trim the excess metal off the can after pressing. However, this produces considerable quantities of scrap.

The suitability of sheet metals for drawing into hollow articles can be graded in terms of the coefficient of normal plastic anisotropy denoted r. However, as this coefficient is essentially the ratio of width strain to thickness strain arising in a specimen subjected to tension beyond the yield stress, it can vary with the orientation to the strip rolling direction of the test piece. Therefore, in this specification, a mean coefficient of normal plastic anisotropy denoted r is used. The coefficient r is defined as the average of such strain ratios from tests on three specimens whose longitudinal axes were respectively 00, 450 and 90" to the rolling direction of the sheet metal from which they were cut. In general terms, sheet metals having a high r value are more suitable for deep drawing. However, we have found that allowance for the earing behaviour restricts this general rule.

In a first aspect this invention provides mild steel sheet suitable for drawing into hollow articles, said sheets having a mean coefficient of normal plastic anisotropy within the range 1.3 to 1.6. The sheet may be blackplate or steel coated with tin or other protective coatings such as composite chromium-chromium oxide coatings which are universally known as "Tin-Free-Steel" (TFS) materials. It is advantageous that the sheet has a steel grain siie finer than 3500 grains per square millimetre, particularly when the sheet is used in prelacquered form.

In a second aspect the invention provides hollow articles drawn, in one or more press tools, from mild steel sheets having a mean coefficient of normal plastic anisotropy within the range 1.3 to 1.6.

In a third aspect the invention provides a method of drawing hollow articles from mild steel sheets, said method comprising the steps of selecting sheets having a mean coefficient of normal plastic anisotropy within the range 1.3 to 1.6 and subjecting the sheet to at least one drawing tool. In a preferred method the sheets are in the form of blackplate, tinplate or chromiumchromium oxide coated plate (TFS) and may be prelacquered before drawing into hollow articles.

One embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 is a sectioned side elevation of a press tool for re-drawing; Fig. 2 is a perspective view of a can after drawing and before trimming; Fig. 3 is a graph of minimum flange width plotted against the mean coefficient of normal plastic anisotropy (r), to show the principle; Fig. 4 is a graph of degree of earing plotted against the mean coefficient of normal plastic anisotropy r; Fig. 5 is a graph, in which the observed minimum flange is plotted against the flange width calculated, to show the correlation; Fig. 6 is a graph in which the calculated flange width is plotted against rgo; and Fig. 7 is a graph of r90 plotted against h (mm), the earing on first operation cups.

In Fig. 1 the redrawing tool comprises a punch 1, an annular die 2 and a blankholder 3. The punch 1 is reciprocable into and out of the die 2. The blankholder is movable towards and away from the upper face 4 of the die 2 so that coaction between the blankholder and the die face 4 restrains the passage of metal between them.

In use of the tool, a shallow flatbottomed cup 5 (shown dashed), drawn in a previous operation, is placed over the die 2 and the blankholder 3 is moved to hold the cup bottom against the die face 4. The punch 1 is then moved towards and through the die 2 to the end of its forward stroke to form the can 6 shown in section in Fig. 1. The can so produced has a flange which has not been drawn into the die 2. The overall height of the can is denoted H and the outside diameter denoted D, and in this example the ratio of H to D is 1:1.

In Fig. 2 the can 6 is depicted in perspective to show the flange 7 in detail. In this example the flange has four ears, one of which is denoted 8. These ears are trimmed off to make the can rim 9 suitable for receiving a can end and the maximum rim diameter available is denoted by the circle (shown in dashes) of radius F min. The outer dashed circle of radius F max. defines the extent of earing and the degree of earing is given by subtracting F min. from F max.

The minimum flange width is given by subtracting the can radius from the F min.

Using a tool of the kind shown in Fig. I cups, of the kind shown in Fig. 2, have been redrawn, to 202x212 cans (54 mm diameter x 70 mm deep) to permit comparison of the r value of various tinplates and the flanges produced.

The following table shows the results obtained from tests on aluminium stabilised steel sheets coated with a D 11.2/5.6 bright finish tin coating and lacquered. The tin coating was passivated and oiled.

Degree of Minimum Sample - Earing Flange No. r (mm) Width (mm) 1.15 4.50 4.55 2 1.27 2.95 5.92 3 1.33 2.44 7.06 4 1.44 2.51 6.60 5 1.49 2.24 7.75 6 1.50 2.08 7.67 7 1.64 2.92 7.92 We have confirmed the principle that higher r value materials give deeper cans as depicted in Fig. 3 by the straight line marked P, in which figure any material of the blank, not drawn into the die, will be evident in the flange. However, the curve denoted E, based on the results tabulated above, is not a straight line and indicates that relatively greater flange widths are available from the sheet samples having r values in the range 1.3 to 1.6. This greater flange width can be partly explained by reference to Fig. 4, in which the degree of earing is plotted against r value. In this graph the degree of earing is greater on each side of a range of r values between 1.3 to 1.6.

Metal in the ears may be considered lost and therefore subtraction of the degree of earing predicted in Fig. 4 from the flange generally predicted by the line P in Fig. 3, confirms the observed results depicted as the curve E.

In order to substantiate this interpretation of the data of Figures 3 and 4 a more comprehensive investigation has been made into the influences of various material properties upon the minimum untrimmed flange width of cans drawn in two stages to a final size of 202x212 (54 mm diameter x 70 mm deep).

Analysis of the data from this investigation has shown that the minimum untrimmed flange width can be estimated reasonably accurately from four basic material parameters, namely gauge, temper, earing susceptibility and the degree of normal plastic anisotropy. The regression equation is: Minimum untrimmed flange width (mm)= 61.74 A+l.19 B-0.45 C-0.09 D.

where A=sheet thickness in millimetres (gauge) B=r90 which experience has already shown can be used to estimate r value (the mean coefficient of normal plastic anisotropy).

C=the degree of earing (Ah) in millimeters on first stage cups.

D=hardness of the sheet (R30T scale).

(Temper).

Fig. 5 is a graph plotting the observed minimum flange width in millimeters against the flange width calculated by the equation and shows a satisfactory correlation.

The various influences of these factors upon flange width are that increases in gauge and r,0 and or r value lead to an increase in flange width whereas increases in temper and the degree of earing give a reduction. If, as is thought, there is no increase in earing upon redrawing, a coefficient of -0.5 would be expected. The computed value of -0.45 is therefore sensible.

By standardising gauge and hardness to the nominal specified values of 0.20 millimeters and 53 HR30T (Rockwell Superficial Hardness) the regression equation reduces to: Minimum untrimmed flange width (mm)= 7.53+1.19 (r90)-0.45 (ash) Since the individual coefficients of normal plastic anisotropy rO, r45 and r90 appear to be interrelated, a linear relationship must exist between r and Ar.

This is maintained from the parabolic relationship which exists between r90 and Ash., as shown in Fig. 7. By substituting paired values of r90 and Ah into the above equation, the curve A shown in Fig. 6 is obtained. This is equivalent to Fig. 3 but theoretical in origin.

We have observed that the steels obtained from different suppliers have slightly different relationships between their coefficients of normal anisotropy r90 and their degree of earing. Ah). Whilst these differences change the absolute values of r90 (and hence r) and Ah the differences do not significantly alter the relationship expressed in Fig. 6 and the equation upon which it is based.

In Fig. 6 three inclined graphs (X,Y,Z) shown as dashed straight lines indicate the relationship between the calculated minimum flange width and r90 when the degree of earing Ah is maintained at constant values of 3 millimeteres, 3.5 millimetres and 4 millimetres respectively.

These graphs enable the drawing behaviour of a material to be partly predicted from a knowledge of degree of earing only, but more particularly demonstrate that the effect of r90 and or r is real and additional.

Whilst it would appear preferable to use tinplates having an r value near 1.6 in order to achieve the maximum flange width, or achieve economy by using a smaller blank, such high r value tinplates tend to have an excessively large steel base grain size. Large grain size materials tend to develop particularly high surface roughness during drawing and this roughening may disturb a preapplied lacquer coating. Therefore, when prelacquered or otherwise precoated mild steel sheet is to be deep drawn it is advisable to select steels having a grain size finer than 3,500 grains per square millimetre, but preferably steels having a grain size finer than 4,000 grains per square millimetre are used. Thicker organic coatings such as lacquers are better able to survive some roughening arising during drawing but in principle they will be prone to a similar disturbance by the roughening as thinner coatings.

WHAT WE CLAIM IS: 1. Mild steel sheet suitable for drawing into hollow articles, said sheet having a mean coefficient of normal plastic anisotropy (as hereinbefore defined) within the range 1.3 to 1.6.

2. A mild steel sheet according to claim 1 having a grain size finer than 3,500 grains per square millimetre.

3. Mild steel sheet according to claim 1 or claim 2 having a surface coating applied to the opposed planar surfaces thereof.

4. Mild steel sheet according to claim 3, wherein the coating is tin.

5. Mild steel sheet according to claim 3, wherein the coating is a composite chromium/chromium oxide layer.

6. Mild steel sheet according to any preceding claim having a lacquer or other organic coating material applied to at least one of the planar surfaces.

7. A hollow article drawn from material according to any of the preceding claims.

8. A method of drawing a hollow article from mild steel sheet, said method comprising the steps of selecting mild steel sheet having a mean coefficient of normal plastic anisotropy (as hereinbefore defined) within the range 1.3 to 1.6 and a grain size finer than 4000 grains per square millimetre, and drawing said sheet in at least one press tool operation into a hollow article.

9. A method according to claim 8 wherein the mild steel sheet is tinplate.

10. A method according to claim 8 wherein the mild steel sheet has a composite coating of chromium/chromium oxide.

11. A method according to any of claims 8, 9 or 10 wherein a lacquer or other or organic coating material is applied to at least one of the surfaces of the sheet before drawing.

12. A drawn can according to claim 7 substantially as hereinbefore described with reference to Figs. 1 and 2 of the accompanying drawings.

13. Mild steel sheet according to claim 1 for drawing, substantially as herein before described with reference to the accompanying drawings and graphs.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. the nominal specified values of 0.20 millimeters and 53 HR30T (Rockwell Superficial Hardness) the regression equation reduces to: Minimum untrimmed flange width (mm)= 7.53+1.19 (r90)-0.45 (ash) Since the individual coefficients of normal plastic anisotropy rO, r45 and r90 appear to be interrelated, a linear relationship must exist between r and Ar. This is maintained from the parabolic relationship which exists between r90 and Ash., as shown in Fig. 7. By substituting paired values of r90 and Ah into the above equation, the curve A shown in Fig. 6 is obtained. This is equivalent to Fig. 3 but theoretical in origin. We have observed that the steels obtained from different suppliers have slightly different relationships between their coefficients of normal anisotropy r90 and their degree of earing. Ah). Whilst these differences change the absolute values of r90 (and hence r) and Ah the differences do not significantly alter the relationship expressed in Fig. 6 and the equation upon which it is based. In Fig. 6 three inclined graphs (X,Y,Z) shown as dashed straight lines indicate the relationship between the calculated minimum flange width and r90 when the degree of earing Ah is maintained at constant values of 3 millimeteres, 3.5 millimetres and 4 millimetres respectively. These graphs enable the drawing behaviour of a material to be partly predicted from a knowledge of degree of earing only, but more particularly demonstrate that the effect of r90 and or r is real and additional. Whilst it would appear preferable to use tinplates having an r value near 1.6 in order to achieve the maximum flange width, or achieve economy by using a smaller blank, such high r value tinplates tend to have an excessively large steel base grain size. Large grain size materials tend to develop particularly high surface roughness during drawing and this roughening may disturb a preapplied lacquer coating. Therefore, when prelacquered or otherwise precoated mild steel sheet is to be deep drawn it is advisable to select steels having a grain size finer than 3,500 grains per square millimetre, but preferably steels having a grain size finer than 4,000 grains per square millimetre are used. Thicker organic coatings such as lacquers are better able to survive some roughening arising during drawing but in principle they will be prone to a similar disturbance by the roughening as thinner coatings. WHAT WE CLAIM IS:

1. Mild steel sheet suitable for drawing into hollow articles, said sheet having a mean coefficient of normal plastic anisotropy (as hereinbefore defined) within the range 1.3 to 1.6.

2. A mild steel sheet according to claim 1 having a grain size finer than 3,500 grains per square millimetre.

3. Mild steel sheet according to claim 1 or claim 2 having a surface coating applied to the opposed planar surfaces thereof.

4. Mild steel sheet according to claim 3, wherein the coating is tin.

5. Mild steel sheet according to claim 3, wherein the coating is a composite chromium/chromium oxide layer.

6. Mild steel sheet according to any preceding claim having a lacquer or other organic coating material applied to at least one of the planar surfaces.

7. A hollow article drawn from material according to any of the preceding claims.

8. A method of drawing a hollow article from mild steel sheet, said method comprising the steps of selecting mild steel sheet having a mean coefficient of normal plastic anisotropy (as hereinbefore defined) within the range 1.3 to 1.6 and a grain size finer than 4000 grains per square millimetre, and drawing said sheet in at least one press tool operation into a hollow article.

9. A method according to claim 8 wherein the mild steel sheet is tinplate.

10. A method according to claim 8 wherein the mild steel sheet has a composite coating of chromium/chromium oxide.

11. A method according to any of claims 8, 9 or 10 wherein a lacquer or other or organic coating material is applied to at least one of the surfaces of the sheet before drawing.

12. A drawn can according to claim 7 substantially as hereinbefore described with reference to Figs. 1 and 2 of the accompanying drawings.

13. Mild steel sheet according to claim 1 for drawing, substantially as herein before described with reference to the accompanying drawings and graphs.