Cold Rolled Steels

Abstract:

Cold rolled steels provide excellent press formability, surface finish, and thickness and flatness tolerances. Steel companies manufacture three groups of low- or ultra-low-carbon grades to meet a variety of customer formability requirements: CS Type B, DS Type B, EDDS, and EDDS+. They also produce HSLA steels and structural steel grades for those applications that require specified strength levels.

Cold rolled steels provide excellent press formability, surface finish, and thickness and flatness tolerances. Steel companies manufacture three groups of low- or ultra-low-carbon grades to meet a variety of customer formability requirements: CS Type B, DS Type B, EDDS, and EDDS+. They also produce HSLA steels and structural steel grades for those applications that require specified strength levels.

Cold rolled steels can also be specified as dent resistant or bake hardenable for applications that require dent resistance after forming and painting. Each grade can be processed with several surface finishes depending on customer requirements. Lubricants can be applied to enhance formability and to avoid at-press lubrication.

Cold rolled steels have the following features:

  • Excellent Surface Appearance. Cold Rolled Steels have manufacturing controls in place assuring consistent surface quality to satisfy customer requirements.
  • Formability. Cold Rolled Steels can be used to produce parts containing simple bends to parts with extreme deep drawing requirements.
  • Paintability. Due to stringent surface roughness controls, Cold Rolled Steels are readily paintable using essentially any paint system.
  • Weldability. Cold Rolled Steels can be joined using virtually any accepted welding practice.
Standard grades for cold rolled steels are:
  • Commercial Steel (CS Type B). May be moderately formed; a specimen cut in any direction can be bent flat on itself without cracking.
  • Drawing Steel (DS Type B). DS Type B is made by adding aluminum to the mol steel and may be used in drawing applications.
  • Extra Deep Drawing Steel (EDDS). Interstitial Free (I-F) steels are made Drawing Steel by adding titanium and/or niobium to the molten steel after vacuum degassing and offer excellent drawability.
  • Extra Deep Drawing Steel Plus (EDDS+). Interstitial Free (I-F) steels are made by adding titanium and/or niobium to the molten steel after vacuum degassing and offer excellent drawability.

Surface Finish

Cold rolled steels are manufactured with a matte finish obtained by rolling with specially roughened rolls on the cold mill and the temper mill. This finish helps to maintain effective lubrication during metal forming and improves the appearance of painted surfaces. Non-standard matte finishes can be provided that optimize the opposing effects of surface roughness on painted part appearance and lubrication during press forming.

Surface Protection and Lubrication

To prevent rusting in transit and storage, cold rolled steels can be supplied with a rust protective oil film or press forming lubricants. A pre-applied press forming lubricant provides uniform lubrication and eliminates the housekeeping problems.

A dry film (acrylic/polymer) lubricant can also be supplied by further processing the cold rolled product through a coil coating facility. These specialty organic coatings are easily removed with a mild alkaline cleaner.

Formability and Mechanical Properties

The formability of all steel products is a result of the interaction of many variables, the main ones being the mechanical properties of the steel, the forming system (tooling) used to manufacture parts, and the lubrication used during forming.

Tight control over chemical composition, hot rolling parameters, amount of cold reduction, annealing time and temperature, and the amount of temper rolling allow the production of high-quality cold rolled steel products to meet customers requirements. Commercial Steel (CS Type B) should be used for moderate forming or bending applications. CS Type B products are produced from aluminum-killed continuously cast slabs and, unless otherwise specified, have a carbon content of less than 0.15%.

To prevent the occurrence of fluting or stretcher strains during forming, CS products are tempered as a normal step in the mill processing.

For more severe forming applications, Drawing Steel Type B (DS Type B) should be used. DS Type B has a controlled carbon content (<0.06%) and is produced in such a manner that parts formed from DS Type B Steel should not exhibit stretcher strain.

Extra Deep Drawing Steel (EDDS) or Extra Deep Drawing Steel plus (EDDS+) should be used for the most demanding forming applications. These steels (also known as Interstitial Free or I-F steels) are produced from a vacuum degassed, titanium stabilized grade. EDDS and EDDS+ have the lowest carbon content available (<0,010%) and have been specially formulated to be most ductile products.

For high strength or structural applications, cold rolled steels are also available in yield strengths up to 50 ksi.

Paintability

Cold rolled steels can be easily painted using a variety of paint systems provided proper care is taken in preparing the material. Prior to painting, the surface should be carefully cleaned with either a solvent or alkaline cleaner.

Cleaning should be followed by a pre-treatment prior to painting. Zinc or iron phosphates give good results on cold rolled steels. Mild abrasion prior to pre-treating may also be used to enhance mechanical bonding of the paint.

Cold rolled steels can be in general supplied as pre-painted or pre-primed.

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