US3729973A

US3729973A - Roll passes for rolling a bar of continuously cast non-ferrous metal and the method improving the metal structure

Info

Publication number: US3729973A
Application number: US00130702A
Authority: US
Inventors: R Wykes
Original assignee: Morgan Construction Co
Current assignee: Siemens Industry Inc
Priority date: 1971-04-02
Filing date: 1971-04-02
Publication date: 1973-05-01
Anticipated expiration: 1990-05-01
Also published as: FR2132055B1; DE2213173C2; DE2213173A1; ZA72816B; JPS5222944B1; CA963701A; FR2132055A1; GB1375317A

Abstract

A first pair of rolls preferably of unequal diameter and grooved in a manner to change the cross-sectional configuration of a bar of non-ferrous metal delivered from a continuous casting machine, from a generally trapezoidal section of substantial depth to a wider flattened less asymmetrical section, flat on both sides or flat on one side and concave on the other, and a second pair of rolls having concave grooves which will reduce the flattened bar delivered by the first pair of rolls to a generally symmetrical bar. The successive passes effectively carry out a method of operation which works the loose structure of the continuously cast bar into a more homogeneous fibrous structure.

Description

United States Patent 11 1 1111 3,729,973 Wykes I [4 1 May 1, 1973 [5 ROLL PASSES FOR ROLLING A BAR 2,139,872 12/1938 Worthington ..72/203 0F CONTINUOUSLY CAST NON- 3.561105 2/1971 Cofer FERR U METAL AND THE METHOD 3,606,785 9/1971 Olsson ..72/199 IMPROVING THE METAL STRUCTURE [58] Field Inventor:

Assignee:

Filed:

App].

Robert D. Wykes, Worcester, Mass.

Morgan Construction Worcester, Mass.

Apr. 2, 1971 Company,

US. Cl ..72/234, 164/76 Int. Cl.

..B21b 13/12 of Search ..72/234, 199, 225, 72/366, 235; 164/76, 270; 29/5277 References Cited UNITED STATES PATENTS Fox 72/234 Peters ..72/235 Wean ..72/205 X Primary Examiner-Milton S. Mehr Attorney-C'hittick, Pfund, Birch, Samuels & Gauthier [5 7 1 ABSTRACT A first pair of rolls preferably of unequal diameter and grooved in a manner to change the cross-sectional configuration of a bar of non-ferrous metal delivered from a continuous casting machine, from a generally trapezoidal section of substantial depth to a wider flat tened less asymmetrical section, flat on both sides or flat on one side and concave on the other, and a second pair of rolls having concave grooves which will reduce the flattened bar delivered by the first pair of rolls to a generally symmetrical bar. The successive passes effectively carry out a method of operation which works the loose structure of the continuously cast bar into a more homogeneous fibrous structure.

18 Claims, 8 Drawing Figures Patented May 1, 1973 3,729,973

2 Sheets-Sheet -1- ROLL PASSES FOR ROLLING A BAR OF CONTINUOUSLY CAST NON-FERROUS METAL AND THE METHOD IMPROVING THE METAL STRUCTURE BACKGROUND OF THE INVENTION In the rolling of hot non-ferrous bar being delivered by a continuous casting machine to a series of roll stands, one of the objectives is to rapidly work the relatively loose structure of the cast section into a more homogenous fibrous structure. In current practice, it has been found that the normal pass sequence in the first two stands tends to induce surface defects in the form of cracks. These cracks persist through to the finished product with adverse effect on the quality.

SUMMARY OF THE INVENTION The present invention provides non-identical grooves of novel configuration in the first stand and substantially identical grooves in the second stand which while reducing and changing the cross-sectional area will work the metal in a manner to provide the desired homogenous fibrous structure without creating any cracks. The design of the grooves in the first pair of rolls is contrary to current practice in that the initial and maximum working of the bar occurs at its midsection, whereas, according to the prior art, the initial and maximum working occurred at the corners and sides. The new design using flat grooves in both rolls or a flat groove in the upper roll and a convex groove in the lower roll results in the performance of a method in which the greatest deformation occurs at the midsection rather than at the sides. This method eliminates the development of the above referred to cracks.

A further important feature of this new design is to have relatively deep sides for each of the grooves and a small parting between the rolls. By proper spacing of the sides of the grooves in relation to the width of the cast bar, the present design results in complete filling of both grooves with the sides of the newly formed section being strongly supported by the deep sides of the grooves. The generally elongated quadrilateral crosssection resulting from the working at the first stand. presents to the second stand a bar of favorable configuration for reduction to a generally symmetrical bar and preferably to a substantially cylindrical bar at the second stand which will not overfill the passes. The working at the second stand furthers the development of the desired homogenous fibrous structure.

The refining of the non'ferrous continuously cast metal in the first two stands is so improved over prior practices that conventional rolling can then proceed from the third stand to the end of the rolling train with the final product being of the expected quality.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows the general arrangement of first and second pairs of rolls with their axes at right angles to each other according to conventional practice with the bar stock passing therethrough without twist.

FIG. 2 is a side elevation showing the continuously cast bar passing between the first pair of equal diameter rolls.

FIG. 3 is a section taken on the line 3-3 of FIG. 2 showing the original shape of the bar and the novel design of the grooves in the rolls of the first stand in which the groove of the top roll is flat and the groove of the lower roll is convex.

FIG. 4 is a section taken on the line 4-4 of FIG. 2 showing the final cross-section of the bar produced by the rolls of the first stand.

FIG. 5 is a side elevation showing the rolls at the second stand receiving the bar produced at the first stand and reducing the bar to a second configuration.

FIG. 6 is a section taken on the line 66 of FIG. 5.

FIG. 7 is a section taken on the line 7-7 of FIG. 6.

FIG. 8 is a schematic side elevation similar to FIG. 2 in which the rolls are of unequal diameter.

DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1 shows a general view of the invention as it applies to the first two stands of the rolling mill. The continuously cast bar 2 which ordinarily will have a crosssection substantially as shown in FIG. 3, is delivered to the first pair of

rolls

4 and 6 where it is reduced to a flattened condition with a section of less cross-sectional area as shown at 8 in FIG. 4. The bar in the form of section 8 then moves on to the rolls l0 and 12 at the second stand, as illustrated in FIGS. 5 and 6, and is then reduced to a substantially cylindrical section 14 shown in FIG. 7. Thereafter, the bar travels on to successive stands in which the rolls are grooved and spaced according to conventional practice.

It will be noted in FIG. 1 that the axes of

rolls

4 and 6 of the first stand are at right angles to the axes of

rolls

10 and 12 ofthe second stand as is conventional in a socalled no-twist mill. The non-ferrous bar 2 being delivered from the continuous casting machine is customarily substantially trapezoidal in cross-section as shown in FIG. 3. It has a flat upper major surface 16, a flat lower minor surface 18 substantially parallel to surface 16, and drafted

sides

20 and 22. Usually the corners are rounded or beveled slightly.

The circular groove in roll 4 is illustrated at 24 in FIG. 3. It is substantially flat in cross-section. When roll 4 engages bar 2, which occurs approximately at the sec tion line 33 of FIG. 2, the flat cylindrical wall of the groove 24 will make instantaneous engagement across the entire width of upper major surface 16 of bar 2. Groove 24 has relatively

deep sides

26 and 28 whose spacing is somewhat greater than the initial upper width of bar 2.

The circular groove 30 in roll 6 in the preferred form is not flat like groove 24, but rather in cross-section is convex as shown at 32. Groove 30 has relatively

deep sides

34 and 36, the distance therebetween being greater than the width across the minor surface 18 of the bottom of bar 2, and proportionately greater than' the width of sides 26 and28 to the width of the upper major surface 16 to accommodate the greater spreading of the narrower metal ofthe lower part of the trapezoidal bar. The degree of convexity of groove 30 may be varied, it being understood that the design shown in the drawings is illustrative and not limiting.

Due to the convex form of groove 30 as at 32, initial engagement of roll 6 with the minor surface 18 of the bottom of bar 2 is at the transverse middle portion. Then, as the bar progresses between :

rolls

4 and 6, the engagement of groove 30 with the minor surface quickly widens to cover the entire width. While the vertical dimension of bar 2 is being reduced to the minimum which occurs at the position of section line 4--4 of FIG. 2, the sides of bar 2 are spread laterally to engage the

deep sides

26 and 28 of upper roll 4 and

sides

34 and 36 of lower roll 6. The new reduced crosssection of the bar 2 is shown at 8 in FIG. 4. This section has a cross-sectional area less than the original crosssectional area so the velocity of section 8 leaving

rolls

4 and 6 is greater than that of the speed of bar 2 arriving at

rolls

4 and 6. For comparison purposes, the crosssection of original bar 2 is shown in dotted line in FIG. 4.

From theforegoing, it will be understood that the preferred convex groove 30 of the lower roll helps to spread the narrower minor bottom surface of bar 2 proportionately more than the groove 24 spreads the upper major surface 16 to reduce some of the asymmetry and as will be pointed out hereinafter the shape of section 8 substantially eliminates the tendency of the concave side of the section to overfill in the next stand as illustrated in FIG. 7.

The downward force of the flat surface of groove 24 of roll 4 and the equal cooperating upward force of the slightly arched surface 30 of roll 6 acts necessarily on equal areas of the major and minor surfaces. Thus, since the minor surface is narrower than the major surface, the length of engagement with the lower roll groove is greater than the length of engagement with the upper roll groove. This causes relatively uniform vertical compression over the width of the bar and desired lateral spreading as the bar moves from the position of section 3-3 to section 44 of FIG. 2. There is no tendency for the central upper portion of bar 2 to be pulled apart to cause cracks therein as often happens when the grooves of the upper and lower rolls are concave as in current practice whereby the comers of the bar are the areas initially engaged creating transverse tension at the midsection.

The bar in the form of section 8, shown in FIG. 4, is completely free of any surface cracks. The nature of the movement of the metal at the first stand has been effective to improve the metal structure by creating a more homogenous fibrous condition.

The bar 2 now in the cross-section configuration of section 8 with a concave bottom surface, passes to the

rolls

10 and 12 at the second stand as shown in FIG. 1. The grooves in

rolls

10 and 12 as illustrated at 38 and 40 in FIG. 6 are concave and in the preferred form are substantially semicircular in section. Section 8 is only slightly asymmetrical and hence when received on edge between rolls l and 12 as at section 6-6, it is converted readily to the cylindrical form 14 by the time it has reached the discharge position of section 77 as in FIGS. 5 and 7. The metal of the bar as it moves from section 6-6 to section 77 is progressively supported with increasing lateral resistance as the metal widens and comes into contact with the steepening sides of the groove walls as at 42, 44, 46 and 48. This lateral support aids in producing a desirable effect in the texture of the bar at this position. Since the parting between

rolls

10 and 12 is small, the extent of the opposite walls of the bar that are unsupported in the vicinity of the parting is likewise small, being in the order of not more than of the bar circumference.

After leaving the rolls l0 and 12 of the second stand in substantially cylindrical form, the bar, having now been refined to a desirable metal structure, may then move on to subsequent stands whose grooves are of conventional design for reduction to final size.

The foregoing explanation of the shape of the upper and lower roll passes has been made with respect to rolls of equal or unequal diameter. The rod on leaving the rolls in which the upper groove conforms to the major upper surface of the bar and the lower groove is narrower and preferably slightly convex and with each groove having deep restraining sides, will be greatly improved as to the metal structure in comparison with the product produced through the use of conventional concave roll passes.

The product will be further improved through the use of rolls having the same shape of roll passes but with the diameter of the lower roll somewhat less than the diameter of the upper roll.

In further explanation of the principles of operation of this aspect of the invention, reference is now made to FIG. 8 in which the rolls are of unequal diameter. The cast section 50 is asymmetrical, the width WT of its upper surface 52 being greater than the width WB of its lower surface 54. Because of this, the area of contact between the continuously cast bar and the upper roll in the first stand is shorter and wider than the area of contact between the continuously cast bar and the smaller lower roll.

It is axiomatic that the rolling force exerted against the upper surface must equal the rolling force exerted against the lower surface. Hence, the contact areas schematically shown at AT and AB in FIG. 8 must be substantially the same. This equalization of contact areas is automatically accomplished by the cast bar passing through the pass with its centerline 56 on the entering side of the rolls below the midpoint 58 between

rolls

60 and 62, thus lengthening the contact area LB at the lower roll which is acting on the minor width WB, and shortening the contact area LT at the upper roll which is acting on the wider major width WB. This means that the increment of height reduction HB performed by the lower roll is greater than the corresponding value I-IT for the upper roll.

In the rolling process, the bar issues with reduced section as at 64 from the pair of

rolls

60, 62 with a greater linear velocity than the peripheral velocity of the rolls. This difference is commonly referred to as forward slip."

The magnitude of forward slip has a relationship to the amount of height reduction (HB plus HT) performed in the pass. Thus, because of the inherent cross sectional shape of the continuously cast non-ferrous bar the surface of the stock leaving the bottom roll has a natural tendency to travel faster than the wider upper surface of the bar leaving the top roll. Because of this unequal forward slip, the trapezoidal bar when rolled by rolls of equal diameter will if unrestrained issue from the rolls in a slight curving upward are. In a continuous mill however, the bar on leaving the rolls of one stand is forced to travel to the next stand in a straight line at uniform speed regardless of its issuing condition from the previous stand.

The result of this inherent unequal forward slip when the rolls are of equal diameter is a build up of stress within the bar 50 as the lower roll fights the upper roll. The. situaTion is finally relieved when the stress reaches a value which exceeds the tangential forces between bar and roll, and the bar slips over one of the rolls, generally the upper roll.

The present invention offers a solution to this problem of unequal forward slip by reducing the diameter of the lower roll 62 by an amount proportional to the forward slip difference between upper and

lower rolls

60, 62, thereby equalizing the natural delivery speeds of both upper and lower surfaces and removing this source of stress from the bar. The result is that the bar issues from the rolls in substantially straight condition.

The amount of diameter difference between

rolls

60 and 62 depends on the difference in width between upper and

lower surfaces

52 and 54 of the cast bar 50. These widths differ according to the angle of the sides of the groove in the casting wheel.

The difference in forward slip between the wider upper surface and narrower lower surface of the continuously cast trapezoidal non-ferrous bar when rolled between equal diameter rolls can be determined experimentally. With this difference known, the diameter of the lower roll will then be correspondingly reduced with the result that the forward slip of the wider upper surface and the narrower lower surface will be substantially equal.

For simplification in explanation, the

rolls

60 and 62 in FIG. 8 are shown without the sides such as were referred to and illustrated in FIGS. 1, 2, 3 and 4 at 26, 28, 34 and 36.

The line 66 of roll 62 represents the flat circular groove of that roll 60 and the line 68 of roll 62 represents the groove, either flat or convex, of that roll.

When it is said in the claims that the groove in the roll that engages the lower minor surface of the bar substantially conforms to the said minor surface, it is intended to mean that the groove could be either flat to conform substantially exactly or convex to conform relatively closely.

Modifications and further applications of the invention will now be apparent to those skilled in the art without departing from the spirit of the invention.

Iclaim:

1. In a rolling mill, a pair of rolls for performing a rolling operation on a bar of continuously cast'non-ferrous metal having a cross-section generally in the form of a trapezoid with major and minor parallel surfaces, one roll having a first circular groove which in crosssection substantially conforms to the major surface of said bar, said first groove including relatively deep sides, the other roll having a second circular groove narrower than the said first groove and which'in crosssection substantially conforms to the minor surface of said bar, said second groove also including relatively deep sides, the spacing of the sides of the said first and second grooves being such that in the rolling operation the displaced metal of said bar will spread laterally thereagainst and the bar delivered from the rolls will have substantially parallel major and minor surfaces, the diameter of the said first and second grooves being such that the forward slip of the major surface of said bar with respect to said first groove is substantially equal to the forward slip. of the minor surface of said bar with respect to said second groove.

2. The construction set forth in claim 1 and a second pair of rolls adjacent said first pair of rolls and whose axes are at right angles to the axes of said first rolls, both grooves in said second rolls being generally semicircular whereby said bar produced by said first pair of rolls will be engaged in its long dimension and then reduced to a generally cylindrical. bar and the parting between said second pair of rolls being small whereby the unsupported sides of said delivered section total not more than 25 percent of the bar circumference.

3. The construction set forth in claim 1, the diameter of the said first circular groove being greater than the diameter of the said second circular groove.

4. In a rolling mill, a pair of rolls for performing a rolling operation on a bar of continuously cast non-ferrous metal having a cross-section generally in the form of a trapezoid with major and minor parallel surfaces, one roll having a first groove which in cross-section substantially conforms to the major surface of said bar, said first groove including relatively deep sides, the other roll having a second groove narrower than the said first groove and which in cross-section substantially conforms to the minor surface of said bar, said second groove also including relatively deep sides, the spacing of the sides of the said first and second grooves being such that in the rolling operation the displaced metal of said bar will spread laterally thereagainst and the bar delivered from the rolls will have substantially parallel major and minor surfaces, the spacing of the sides of the second groove being less than the spacing of the sides of the first groove but proportionally more with respect to the initial width of the minor surface than the spacing of the sides of the first groove is to the initial width of the major surface.

5. In a rolling mill, a pair of rolls for performing a rolling operation on a bar of continuously cast non-fen rous metal having a cross-section generally in the form of a trapezoid with major and minor parallel surfaces, one roll having a first circular groove which in crosssection substantially conforms to the major surface of said bar, said first groove including relative deep sides, the other roll having a second circular groove narrower than the said first groove and which in cross-section substantially conforms to the minor surface of said bar, said second groove also including relatively deep sides, the spacing of the sides of the said first and second grooves being such that in the rolling operation the displaced metal of said bar will spread laterally thereagainst and the bar delivered from the rolls will have substantially parallel major and minor surfaces, the diameter of the said first and second grooves being such that the forward slip of the major surface of said bar with respect to said first groove is substantially equal to the forward slip of the minor surface of said bar with respect to said second groove, the spacing of the sides of the second groove being less than the spacing of the sides of the first groove but proportionally more with respect to the initial width of the minor surface than the spacing of the sides of the first groove is to the initial width of the major surface.

6. The construction set forth in claim 4, said second groove being transversely convex.

7. The method of improving the grain structure in a bar of non-ferrous continuously cast metal in which the bar in cross-section is delivered from the casting process substantially in the form of a trapezoid having major and minor parallel surfaces, said method comprising the steps of delivering the bar to a pair of rolls of unequal diameter and then through the operation of said rolls acting simultaneously on said major and minor surfaces over substantially their full widths from the time of initial engagement by the rolls, compressing and widening said bar to a new generally trapezoidal form in which the widened major and minor surfaces remain substantially parallel and the cross-sectional area of said new form is less than the cross-sectional area of said original bar, the larger of said rolls acting on the said major surface over an area which is shorter and wider than the area being acted on by the smaller of said rolls, said larger roll producing less height reduction than said smaller roll and both rolls producing substantially equal forward slip of the respective surface.

8. The method set forth in claim 7, the minimum thickness dimension of the bar being along the longitudinal center line.

9. The method set forth in claim 7, the sides of said bar during said compressing step being restrained over a major portion of their reduced side dimensions from further widening.

10. The method set forth in claim 7, the percentage widening of the minor surface being greater than the percentage widening of the major surface.

11. The method set forth in claim 7 and delivering the bar in its new form to a second pair of rolls and then, through the action of the second pair of rolls, compressing the bar in the direction of its long crosssection dimension, and simultaneously applying lateral restraints of increasing resistance to widening of said bar as the long cross-section dimension is diminished whereby a substantially cylindrical bar is delivered from said second pair of rolls.

12. The method set forth in claim 7, said new form being less asymmetrical than said original cast bar.

13. The method set forth in claim 7, the initial compressing occurring along the center line of said bar and spreading immediately thereafter over the full width of said major and minor surfaces as the bar advances between said rolls.

14. The method of improving the grain structure in a bar of non-ferrous continuously cast metal in which the bar in cross-section is delivered from the casting process substantially in the form of a trapezoid having major and minor parallel surfaces comprising compressing the bar on both sides according to opposed cylindrical patterns in which the length of the pressure are against the wider major surface is less than the length of the pressure are against the narrower minor surface whereby the decrease in thickness of said bar on the major surface side will be less than the decrease in thickness on the minor surface side and the forward slip of the major and minor surfaces will be substantially equal.

15. The method set forth in claim 14 and in which the cylindrical pressure pattern against said minor surface side will be of less diameter than the cylindrical pressure pattern against said major surface side.

16. The method set forth in claim 14 and in which lateral spreading is limited and the extent of the lateral spreading of the major surface side is less than the extent of the lateral spreading of the minor surface side.

7. The method of improving the gram structure in a bar of non-ferrous continuously cast metal in which the bar in cross-section is delivered from the casting process substantially in the form of a trapezoid having major and minor parallel surfaces, said method comprising the steps of delivering the bar to a pair of rolls and then through the operation of said rolls acting on said major and minor surfaces compressing and widening said bar to a new generally trapezoidal form in which the major and minor surfaces are substantially parallel and the cross-sectional area of said new form is less than the cross-sectional area of said original bar, the percentage widening of the minor surface being greater than the percentage widening of the major surface.

18. The method of improving the grain structure in a bar of non-ferrous continuously cast metal in which the bar in cross-section is delivered from the casting process substantially in the form of a trapezoid having major and minor parallel surfaces, said method comprising the steps of delivering the bar to a pair of rolls and then through the operation of said rolls acting on said major and minor surfaces compressing and widening said bar to a new generally trapezoidal form in which the major and minor surfaces are substantially parallel and the cross-sectional area of said new form is less than the cross-sectional area of said original bar, said new form being less asymmetrical than said original cast bar.

Claims

1. In a rolling mill, a pair of rolls for performing a rolling operation on a bar of continuously cast non-ferrous metal having a cross-section generally in the form of a trapezoid with major and minor parallel surfaces, one roll having a first circular groove which in cross-section substantially conforms to the major surface of said bar, said first groove including relatively deep sides, the other roll having a second circular groove narrower than the said first groove and which in cross-section substantially conforms to the minor surface of said bar, said second groove also including relatively deep sides, the spacing of the sides of the said first and second grooves being such that in the rolling operation the displaced metal of said bar will spread laterally thereagainst and the bar delivered from the rolls will have substantially parallel major and minor surfaces, the diameter of the said first and second grooves being such that the forward slip of the major surface of said bar with respect to said first groove is substantially equal to the forward slip of the minor surface of said bar with respect to said second groove.

2. The construction set forth in claim 1 and a second pair of rolls adjacent said first pair of rolls and whose axes are at right angles to the axes of said first rolls, both grooves in said second rolls being generally semi-circular whereby said bar produced by said first pair of rolls will be engaged in its long dimension and then reduced to a generally cylindrical bar and the parting between said second pair of rolls being small whereby the unsupported sides of said delivered section total not more than 25 percent of the bar circumference.

5. In a rolling mill, a pair of rolls for performing a rolling operation on a bar of continuously cast non-ferrous metal having a cross-section generally in the form of a trapezoid with major and minor parallel surfaces, one roll hAving a first circular groove which in cross-section substantially conforms to the major surface of said bar, said first groove including relative deep sides, the other roll having a second circular groove narrower than the said first groove and which in cross-section substantially conforms to the minor surface of said bar, said second groove also including relatively deep sides, the spacing of the sides of the said first and second grooves being such that in the rolling operation the displaced metal of said bar will spread laterally thereagainst and the bar delivered from the rolls will have substantially parallel major and minor surfaces, the diameter of the said first and second grooves being such that the forward slip of the major surface of said bar with respect to said first groove is substantially equal to the forward slip of the minor surface of said bar with respect to said second groove, the spacing of the sides of the second groove being less than the spacing of the sides of the first groove but proportionally more with respect to the initial width of the minor surface than the spacing of the sides of the first groove is to the initial width of the major surface.

11. The method set forth in claim 7 and delivering the bar in its new form to a second pair of rolls and then, through the action of the second pair of rolls, compressing the bar in the direction of its long cross-section dimension, and simultaneously applying lateral restraints of increasing resistance to widening of said bar as the long cross-section dimension is diminished whereby a substantially cylindrical bar is delivered from said second pair of rolls.

14. The method of improving the grain structure in a bar of non-ferrous continuously cast metal in which the bar in cross-section is delivered from the casting process substantially in the form of a trapezoid having major and minor parallel surfaces comprising compressing the bar on both sides according to opposed cylindrical patterns in which the length of the pressure arc against the wider majoR surface is less than the length of the pressure arc against the narrower minor surface whereby the decrease in thickness of said bar on the major surface side will be less than the decrease in thickness on the minor surface side and the forward slip of the major and minor surfaces will be substantially equal.

17. The method of improving the grain structure in a bar of non-ferrous continuously cast metal in which the bar in cross-section is delivered from the casting process substantially in the form of a trapezoid having major and minor parallel surfaces, said method comprising the steps of delivering the bar to a pair of rolls and then through the operation of said rolls acting on said major and minor surfaces compressing and widening said bar to a new generally trapezoidal form in which the major and minor surfaces are substantially parallel and the cross-sectional area of said new form is less than the cross-sectional area of said original bar, the percentage widening of the minor surface being greater than the percentage widening of the major surface.