Cold Rolled Sheet 


Cold Rolled Sheet
Application Considerations
As noted in the previous section regarding typical applications, U. S. Steel Cold Rolled Steel Sheet can be fabricated into many different parts to meet applications requiring high strength, high formability, a high quality surface finish, an embossed finish, rigid dimensional tolerances, excellent flatness, including many applications that require the combination of these attributes.

Forming

Cold rolled steel sheet can be formed by bending, hemming, flanging, drawing, stretching, and complex draw/stretch forming. In simple press-bending and hemming, processes that are often employed for forming steel sheet products, the minimum bend radii are limited by the material properties (thickness, strength etc.), the angle of the bend, orientation of the bend with respect to the rolling direction of the steel, and other more subtle conditions such as the condition of the sheared edge.

Examples of conditions that affect bending are:

1)	thicker steels are not as readily bent to small radii as thinner steels,
2)	high-strength steels cannot be bent to as small a bend diameter as low strength steels,
3)	work-hardened edges (from shearing, for example) may adversely affect the minimum acceptable bend diameter, and
4)	bends made transverse to the rolling direction typically can be made to smaller bend radii than bends made parallel to the rolling direction.

In press-forming, an operation where the workpiece (steel sheet) takes the shape imposed by the punch and die, issues such as the steel characteristics (thickness, formability, strength), the die clearances, the type of lubrication, the surface finish of the steel sheet, and the die material itself can influence the success of forming without breakage or galling. To assure good performance during press-forming, it is important to use a steel that is manufactured to exhibit excellent drawing characteristics, and minimal to no tendency to develop stretcher strains.

Deep drawing is another forming process applied to steel sheet. In deep drawing, the sheet metal is drawn between a set of dies in which the sheet edges are restrained by opposing force applied to the sheet edges while the steel is being drawn into an open cavity. As the sheet is drawn into the die cavity, the outer portions of the steel blank move radially toward the center of the blank until they flow over the die radius as the blank is drawn into the die cavity by the punch. This process requires that the steel exhibit very excellent deep-drawing characteristics, and that the drawing practices be defined rigorously. For example, the die clearances, the type of lubricant, the hold-down pressure, the design of the hold-down ring, the rate of drawing, and various other practice variables must be well-defined and controlled.

Roll forming is another common method of fabrication used for cold rolled steel sheet products. In this process, the steel is fed through a series of roll stations during which the steel is gradually bent to take a final profiled shape. This type of forming is suited to the fabrication of long lengths to close tolerances without much handling. Most steels can be roll formed although special practices need to be used for the roll forming of high-strength steel grades. In order to assure matchup of parts, it is important to control the final dimensions. One of the biggest problems is springback that occurs after the formed part exits the roll forming stations. The yield strength of the steel sheet has a very significant effect on the amount of springback. Thus, it is important to assure that the steel sheet has a consistent yield strength throughout the coil as it is being processed.


Welding
Cold rolled steel sheet can be readily welded using either arc or resistance welding techniques. Typical resistance welding techniques include spot-welding and seam welding. Both methods can be readily applied to join cold rolled steel sheet parts during manufacture. There are no special requirements for resistance welding although the welding setup conditions, i.e., voltage, current, weld time, and electrode force, need to be developed for each specific application (steel chemistry, steel thickness). Similarly, no extraordinary special procedures are needed for arc welding. 

The one area in which special attention and special welding practices are needed is during the arc-welding of high strength steels since it is equally important that the welds in fabricated structures have the strength and ductility to match that of the steel sheet itself. In some instances when the steel strength is less than about 50 ksi, low-carbon electrodes are generally acceptable. When the steel strength exceeds about 50 ksi, low-alloy steel electrodes are generally required. Low-hydrogen type electrodes may be required.


Painting
Most cold rolled steel sheet is painted after production of a finished part. There are a number of paint treatments, primers and topcoats (color coat) that work suitably for good performance during use in the final application. Perhaps, the most important feature of the steel with respect to obtaining good performance for the final application is that the surface be well cleaned prior to application of the treatment. There are a number of cleaning processes used for steel sheet products to remove the surface oils, grease, dirt and other surface contaminants that interfere with good performance. These include: alkaline cleaning, solvent degreasing (including vapor degreasing), ultrasonic cleaning, and acid cleaning. The two most common cleaning practices are alkaline cleaning and solvent cleaning.

Alkaline cleaning is a process that includes the use of heated alkaline, water-based solutions to remove soils. Soils removed by alkaline cleaning include oils, grease, waxes, metallic fines and other dirt. Two methods of alkaline cleaning are used: immersion and spray cleaning. The makeup of alkaline cleaner solutions is a science unto itself, but the principle involves cleaning by saponification of oils and grease – a mechanism that involves the breakup of the oils and grease by a chemical reaction with the chemicals in the cleaner, displacement which is the lifting of oily soils from the steel surface by the action of surfactants, and dispersion and emulsification – mechanisms that allow the removed oils and dirt to be retained in the water-based cleaning solution.

It is very important to assure that the parts to be painted be thoroughly rinsed after alkaline cleaning. Alkaline residues may interfere with good paint adhesion after painting.

Solvent cleaning involves the removal of oils and grease by solubilizing them in organic solvents. Common organic solvents are chlorinated hydrocarbons, chlorofluorocarbons, and hydrofluorocarbons. Parts are cleaned by immersing and soaking in the solvent. Solvent cleaning is used typically when the parts to be cleaned are small.

	Typical Applications
	Application Considerations
	Tolerances