Iron and Steel Sheets

SHEETS, IRON AND STEEL. The production of rolled sheet iron dates back before 1620 in Bohemia and was introduced in Wales in 1720. (See TIN-PLATE AND TERNE-PLATE.) Most Of the rolling was done by hand, and great skill was required. The sheets were usually made of puddled or wrought iron. Upon the development of the Bessemer (q.v.) process and the open hearth (q.v.) process, steel was produced more rapidly and cheaply and practically replaced iron. However, steel sheets showed shorter service life under corrosive conditions, and interest in sheet iron reawakened. Metallurgists found methods of manufacturing it in larger quantities and less laboriously. To-day iron sheets are gen erally available and marketed at prices not greatly different from those of steel sheets.

Production.

In the production of sheets and strip, the first part of the process is much the same as that employed for plates, sections, and other products. The metal is refined, teemed into ingot moulds, heated again in the soaking pits to insure uniform heat before rolling.

Old Rolling Practice.

Under the old hand-operating practice —now largely outmoded by the continuous mill—ingots were re duced to slabs in the blooming mill, further reduced in the bar mill, cut into specified bar lengths, which became the width of the sheets to be rolled from the bars. Bar thickness governed the ultimate gauge of the sheet.

Continuous Mill.

In modern mills the ingot is rolled to a slab in a mill having horizontal rolls and a second mill with verti cal rolls, known as an edger, arranged in tandem with a turning mechanism between and spaced approximately thirty feet apart. With this mill arrangement it is possible to roll a slab having any desired width (within the capacity of the mill) without regard to the ingot width. The width of the finished strip is not limited to the width of the ingot from which it is rolled. The usual practice is to roll the slab from 4" to 6" thick and equal to the strip width.

Slab is trimmed at ends and reheated before rolling on the con tinuous bar and strip mill. The reheated slab is discharged upon the mill table, which carries the slab to the bar mill. The bar mill may consist of four to six sets of rolls arranged in tandem, each succeeding roll reducing the thickness, and turning slightly faster.

These rolling mills reduce the slab to a plate ranging from -g-" to in thickness. Plates can be removed beyond the last bar mill stand when plates of such thickness are desired.

Proceeding from the bar mill the steel passes to the strip mill, consisting usually of five to six stands of rolls, also arranged in tandem, which roll the plates (a" to i" thick) to thinner gauges such as .0625" to .100". These are delivered on a mill table beyond the last mill at surface speeds ranging from 1,000 to 2,00oft. per minute. The finished hot strip travels approximately 350ft. to a coiling machine that winds the strip into coils to facilitate handling for storage and further processing.

The coils may be cut up into units for rolling into lighter sheets or tin plate on the conventional hand-operated hot mill, or may further be reduced in gauge on the cold-reduction tandem mill. The wide tandem cold mill has now come into general use

for producing tin plate and sheets and strip in the range of 14 to 32 gauge. Before the hot strip is delivered to the cold mill for further reduction, hot mill scale is removed in a strip pickier. The strip is passed through a sulphuric acid solution, washed, dried, oiled, and recoiled.

The strip is now ready for the cold-reduction mill, which con sists of three or more roll stands arranged in tandem and a spool type coiling machine beyond the last stand. The operation here is much the same as described for the hot mill, except that the strip is under considerable tension between the mills and between the last mill and the coiling machine. Rolling under tension im proves the gauge and flatness, and also gives a high finish texture to the sheets. The strip is either finished and marketed in coils or cut into sheet lengths as ordered ; yet in either case the finishing process is the same. The sheets or strip are annealed in a controlled atmosphere to relieve the strains caused by rolling and to provide a dead soft sheet, which is then temper rolled on a mill to produce such hardness or temper as is specified. On orders requiring a dead flat surface, such as for metal furniture, the sheets are stretched flat in a hydraulic machine. The product is inspected, resquared when necessary, and shipped.

Galvanizing.

If the sheets are to be galvanized they are coated with zinc in galvanizing pots. (See GALVANIZED IRON AND STEEL.) The speed of the exit rolls and the temperature of the zinc control the weight of coating. Zinc makes a very good coat ing for iron and steel because it forms an alloy bond with the iron.

Painting.

Paint is often used to cover galvanized iron. Once the common practice was to permit the zinc coating to weather several months. A quicker alternative was to treat the zinc with acetic acid, which etched the smooth surface. Both methods had obvious drawbacks. Now many galvanized sheets are rendered paintgripping by a special phosphate treatment at the mill. No zinc coating is lost ; the galvanized sheets take paint readily ; and no zinc compounds are left on the surface prematurely to dry out the paint. Bitumastic or asphalt coatings are sometimes applied to galvanized sheets and plates. They are especially valuable where erosion is encountered.

Uses.

In general, galvanized sheets are used in products and construction where corrosion is a factor. Among the important uses are culverts, roofing, air conditioning and general sheet metal work. In the uncoated classification are the cold-rolled grades for products that require a fine finish, such as automobiles and metal furniture ; enamelling iron for products to be finished with porce lain enamel; electrical sheet steel for motors and transformers, as well as hot-rolled sheets for a wide variety of consumer products and industrial applications.

(See TIN-PLATE AND TERNE-PLATE; GALVANIZED IRON AND