Advanced lubricants improve high-strength steel forming

Advanced or ultrahigh-strength steel (AHSS or UHSS) use in more than 60 percent of structural automotive stampings has changed the rules when it comes to tooling surface, heat and friction control, robotic automation, and paint pretreatment. With yield strengths now reaching production levels of 80-plus kilopounds per square inch (KSI), elevated temperatures, abrasiveness, and springback, AHSS has created a real market need for new lubricant technology.

In the article "Advanced lubricant technology for high-strength steel" on thefabricator.com, extreme-temperature (ET) high-solids polymers (HSPs) were presented as a viable lubricant technology for AHSS grades. Today additional field results document the success of HSP for structural component production at Tier 1 and 2 auto and truck suppliers. Field insight from tool coating, automation, and paint pretreatment suppliers also has provided data indicating that advances in lubricant technology are playing a significant role in helping automotive stamping suppliers regain much-needed productivity and profit.

Field Testing Results

Recent production studies at OEM suppliers show measured process and quality improvements through a change in lubricant technology alone. According to these companies, switching to HSP has shown measurable improvement in tool performance, welding, parts cleaning, and painting. Figure 1shows results from four studies by major automotive and truck suppliers.

Figure 1 Source: Three leading Tier 1 & 2 automotive component suppliers

Tool Coating Performance Linked to Lubricant Quality

Many tool coating suppliers believe AHSS is not the place to compromise on forming lubricants. Steve Chamberlain of the Arvin TD Center said, "The thermal diffusion [TD] process can address problems of galling and abrasive wear. The vanadium carbide resulting from TD provides a durable and hard surface that can significantly increase the tool life of critical forming and draw die surfaces. If the lubricant can't hold up to the elevated temperature, abrasiveness, and work-hardening characteristics of advanced high-strength steel, this value is put in jeopardy. With the trend to use less lubricants and dilute further, using a lubricant that can handle the demands of high-strength steel is critical to getting the longest life out of the coating."

Traditional physical vapor deposition (PVD) and chemical vapor deposition (CVD) coatings require external lubrication for maximum performance. Because of their increased yield strength, work-hardening behavior, and springback, UHSS and AHSS require significantly higher forming pressure than low-carbon steel. Higher pressure means higher frictional forces, which places more demands on lubricants. TD and CVD coatings do well in the 60- to 120-KSI ranges of UHSS and AHSS, but the lubricant also needs to provide consistent protection.

Some newer coatings have low coefficients of friction, so they are effective at reducing frictional forces present in the die and extending die life. These products do not eliminate the heat generated from the forces required to form the material; therefore, external lubricants are recommended to aid in heat reduction.

Implications for Automation and Paint

Lab data shows that some lubricants can significantly influence the ability of a suction cup to adhere to a steel surface (see Figure 2).

Figure 2 Cup adhesion simulation test performed by Greenleaf Technologies.

Parts with excessive amounts of lubricant can be handled in several ways. Systems can be designed with larger or an increased number of ribbed or patterned vacuum cups, increased vacuum pressure, or magnets (see Figure 3). Locator pins or gauges can be added to control potential location slipping, although all this adds to system cost and weight, which in turn slows down the process. Ideally, a robotic process using vacuum cups works best when the lubricant film allows the system to perform without additional controls.

Figure 3

Using heavy chlorinated paraffin oils to increase AHSS performance may create problems for paint quality. Chlorinated paraffins can be difficult to remove in the pretreatment cleaning process. Welding operations, before the pretreatment and painting process, tend to bake the paraffin residue to a shellaclike consistency, which is even more difficult to remove. The chemistry of a HSP is oil free and 100 percent water dissolvable, which helps eliminate potential paint rejects caused by poor cleaning.

Lab Screening Can Help Save Production Time

Although extreme-temperature HSP field test results are encouraging, you should prequalify potential lubricants in a lab environment before using it in a production environment. Most process variables can be tested without using production tools and time. New lab tests have been shown to help predict a lubricant's reaction to friction (see "Gaining from friction and formability data" on thefabricator.com) and heat, vacuum cup adhesion, welding quality, pretreatment and paint. Although lab tests cannot replicate a complete production condition, they can provide valuable data to help you narrow your choices significantly. With production time at an all-time premium, reducing your risk before you get to the press is essential.