8.Plastic sheet, etc.
Steel is an alloy of iron and carbon. Carbon must be present to the extent of about 0.05 percent by weight in order for the material to be known as “steel” rather than commercial iron. The composition and processing of steels are controlled in a manner that makes them suitable for numerous applications. They are available in various basic product shapes: sheet, strip, sheet, and plate.
3.2.1 Hot-Rolled Steel
Hot-rolled sheets are formed easily. Low-carbon sheets are used for tanks, barrels, pails, farm implements, lockers, cabinets, truck bodies, and other applications where scale and discoloration are not objectionable because surfaces are painted after forming. Hot-rolled sheets are readily available in thicknesses ranging from #30 gage (0.012 in. or 0.3 mm) to #7 gage (0.1875 in. or 4.8 mm).
a) Pickled and Oiled Sheets
Pickling, or the immersing of hot-rolled sheets in acid solution, results in smooth, clean, scale-free surfaces having a uniform gray color. Oiling protects the surfaces against rust.
These sheets are readily stamped or welded. Long-lasting painting or enameling is possible because of the absence of scale. Pickled and oiled sheets are used for household appliances, automotive parts, toys, and the like.
b) Copper-Bearing Sheets
Copper-bearing sheets are hot-rolled sheets having 0.20 percent minimum copper content. They are used for parts designed for outdoor exposure, or for indoor use under corrosive conditions. These sheets have a service life from two to three times longer than can be expected from non-copper-bearing steels. They are used for roofing and siding, farm and industrial buildings, truck bodies, railroad cars, farm implements, signs, tanks, dryers, ventilators, washing machines, and other similar applications.
c) Medium-Carbon Sheets
Hot-rolled sheets having a 0.40 to 0.50 percent carbon content provide hardness, strength, and resistance to abrasion. They can be heat-treated to make the material even harder and stronger and are primarily used for scrapers, blades, hand tools, and the like.
3.2.2 Cold-Rolled Sheets
Cold-rolled sheets have a smooth, deoxidized satin finish, which provides an excellent base for paint, lacquer, and enamel coating. Thicknesses are held to a high degree of accuracy. Cold-rolled steel is produced by the cold rolling of hot-rolled sheets to improve size and finish. Refrigerators, ranges, panels, lockers, and electrical fixtures are among their many uses.
a) Possibility of Deformation
Six tempers of cold-rolled steel sheets and strips are available; it is important to know exactly what operations can be performed on each (Figure 3.1):
1. Hard. Hard sheets and strips will not bend in either direction of the grain without cracks or fracture. These tempers of steel are employed for flat blanks that require resistance to bending and wear. Direction of grain is shown along lines A in the illustration. Hardness is Rockwell B 90 to 100.
2. Three-quarter hard. This temper of steel will bend a total of 60 degrees from flat across the grain. This is shown as dimension B in the illustration. Hardness is Rockwell B 85 to 90.
3. One-half hard. This temper will bend to a sharp 90-degree angle across the grain, shown as dimension C. Hardness is between Rockwell B 70 and 85.
4. One-quarter hard. This commonly used temper of steel will bend over flat on itself across the grain and to a sharp right angle along the grain. Hardness is Rockwell B 60 to 70.
Figure 3.1 Various tempers of cold-rolled steel from hard (1) to dead soft (6) and kinds of deformation possible with each.
5. Soft. This temper will bend over flat upon itself both across the grain and along the grain. It is also used for moderate forming and drawing. Hardness is Rockwell B 50 to 60.
6. Dead soft. This temper of steel is used for deep drawing and for severe bending and forming operations. Hardness is Rockwell B 40 to 50.
b) Finish
Cold-rolled steel is available in three grades of finish:
1. Dull finish. This is a gray lusterless finish to which lacquer and paint bond well.
2. Regular bright finish. This is a moderately bright finish suitable for most work. It is not recommended for plating unless buffed first.
3. Best bright finish. This finish has a high lustre well suited for electroplating. It is the brightest finish obtainable.
c) Stretcher-Leveled Sheets
These are cold-rolled steel sheets that have been further processed by stretcher leveling and resquaring. They are used in the manufacture of metal furniture, table tops, truck body panels, partitions, and other equipment requiring perfectly flat material.
d) Deep-Drawing Sheets
Deep-drawing steel is prime quality cold-rolled steel having a low carbon content. Sheets are thoroughly annealed, highly finished to a deoxidized silver finish, and oiled. Deep-drawing sheets are used for difficult drawing, spinning, and stamping operations such as those which produce automobile bodies, fenders, electrical fixtures, and laboratory equipment.
e) Silicon Steel
Also called “electrical steel,” silicon steel is extensively used for motors and generators. Lighter gages are suitable for transformers, reactors, relays, and other magnetic circuits.
The shearing process involves the cutting of flat material forms, such as sheets and plates. The cutting may be done by different types of blades or cutters in special machines driven by mechanical, hydraulic, or pneumatic power.
Figure 3.2 shows the mechanics of shear in 8 steps:
1.This illustration shows the cutting edges of a die with clearance C applied. The amount of this clearance is important, as will be shown.
2.A material strip is introduced between the cutting edges and is represented by phantom lines. Cutting a material strip occurs when it is sheared between cutting edges until the material between the edges has been compressed beyond its ultimate strength and fracture takes place.
3.The upper die begins its downward travel and the cutting edge of the punch penetrates the material by the amount A. The following stresses occur: The material in the radii at B is in tension; that is, it is stretched. The material between cutting edges C is compressed, or squeezed together. Stretching continues beyond the elastic limit of the material, then plastic deformation occurs. Observe that the same penetration and stretching is applied to both sides of the strip.
4.Continued
