Upgrades help meet special challenges of this material
December 12, 2006
Forming lubricants always have been a critical component in the metal forming process, but the performance characteristics of these products are even more essential when working with HSS.
The features of high-strength steels (HSS) can help improve the fuel efficiency and safety of vehicle structures. However, this developing steel technology has created new challenges for the metal forming industry and, as a result, has spawned a frontier of techniques and supporting technologies designed to address these challenges.
Forming lubricants always have been a critical component in the metal forming process, but the performance characteristics of these products are even more essential when working with HSS. As HSS technology has developed into high-strength, low-alloy (HSLA) steel and further to the advanced high-strength steels (AHSS), metal stampers have come to understand the importance of forming lubricant properties.
Many metal stampers have experienced the pains associated with forming problems—;such as part galling and splits and tool coating wear—so lubricant companies have responded with new products. Specialized lubricants have been and are being developed as a supporting technology for forming AHSS. These developments provide specialized relief for the immediate forming pains that AHSS creates, but many lubricants do not address the peripheral issues associated with stamped parts.
A forming lubricant serves in one of the most basic roles of metal forming production, yet it offers the most far-reaching effects. Its most obvious job is to facilitate the metal forming process: protect tools and parts from abrasive friction, cool parts and tooling, and promote controlled metal flow into the working sections. These functions are not unique to the needs of HSS, but these steels, especially AHSS, do a lot to test this area of lubricant performance characteristics.
Lubricants also affect postprocessing procedures such as welding, compatibility with various substrates, antirust protection, die condition, parts cleaning, paint quality, housekeeping, environmental compliance, health and safety, waste treatment, and disposal costs.
Automotive original equipment manufacturers (OEMs) place great emphasis on each of these areas and formally approve lubricants based on independent laboratory tests established by groups such as the Auto/Steel Partnership (A/SP), weld engineering consortiums, and sealer/adhesive companies. Because AHSS use has grown primarily within the automotive industry, it becomes important that stampers and their lubricant suppliers are familiar with and accommodating of these OEM requirements.
The first synthetic, water-based lubricant to pass OEM test requirements and receive formal body-in-white approval as a heavy-duty stamping lubricant was introduced in 2000. It provided benefits to postprocessing procedures, was compatible with various automotive substrates, and at a use dilution of 5 percent by volume, demonstrated coefficient of friction values from 0.07 to 0.13.
The new generation of this lubricant incorporates multiphase performance additives that endure the additional duress associated with forming higher-yield-strength AHSS, such as dual-phase steel. Many OEMs have begun to use materials such as dual-phase steel (780 MPa) and hot-dip-galvanized HSLA steel (350 MPa) in vehicles. The higher-strength steels undoubtedly require exceptional lubricity, but the introduction of new HSLA galvanized steel material means that unique substrate-compatibility features also will be required of the newer lubricants.
For instance, the higher yield strength of these steels can require unique welding techniques such as laser welding, so the lubricant residue becomes a critical factor. If the residue is not compatible with all types of welding, postprocess parts washing will be required. Lubricants specialized to meet the forming requirements of ferrous AHSS might fall short of the compatibility requirements for newer zinc-coated HSLA steel and could cause other postprocess issues.
Studies related to antifriction characteristics represent only one-sixth of the testing parameters that are meaningful to the OEMs. Lubricants need to satisfy both the lubricity properties and many other related parameters. Success in these areas can provide significant cost savings.
How is it that the unique features of HSLA steel and AHSS create such differences in stamping operations? The Society of Automotive Engineers (SAE) distinguishes steel grades such that low-carbon materials are specified when formability is the prime consideration (SAE J2329). Typically, mills control these steels to meet certain chemical properties. The SAE distinguishes higher-strength steels such that the yield and tensile strength levels are the prime considerations (SAE J2340).
Typically, these steel processes are controlled to meet certain mechanical properties. The focus of this steel technology prioritizes the strength of the end product, or component part, rather than the ease with which such parts might be shaped or formed. This change in technology priorities can present significant challenges to the company forming high-strength steel. The metal former can best meet these challenges when armed with a basic understanding of the characteristics of this steel and of the services, strategies, and technologies available to ease these challenges.
The features of HSS provide superior benefits to the end product. As a rule, AHSS provides greater work-hardening rates and higher tensile strengths than conventional low-carbon steel. These features are accomplished through chemical compositions or alloying with elements such as magnesium, chromium, vanadium, and niobium.
Special mill processing, such as rolling techniques and time and temperature controls, also can increase steel strength. The goal usually is to replace a heavier-gauge, weightier steel with a thinner-gauge, lighter-weight steel that provides the same or improved features. A heavier-gauge floor pan rail, for instance, can be fabricated with a lighter-gauge AHSS.
A body structure made of AHSS can reduce vehicle weight to improve fuel efficiency. The work-hardening effect improves crash energy management. A seat support made of AHSS provides more secure harnessing and so improves passenger safety.
Metal forming operations with conventional steels and HSS differ primarily because of the latter's unique characteristics. With both types of steel, an initial stress experienced at the point of first deformation requires the full complement of a lubricant's properties.
The boundary protection (hydrodynamic and elastodynamic fluid-pressure and film characteristics) must remain present throughout the part-to-tooling contact. If friction degrades the boundary film, the associated heat must activate extreme-pressure (EP) additives that create molecular metallic coatings and provide additional residual protection. Ideally, the fluid itself will delay the need for this EP effect by dissipating the heat and protecting the boundary film and fluid viscosity.
As conventional steel forms, it generally softens or at least does not harden, and the duress on the lubricant properties generally subsides. In effect, the lubricant must survive the initial stress of each deformation to perform effectively. However, high-strength steel requires multiphase performance from the lubricant.
HSS requires latter-stage lubricant performance for two reasons. First, materials such as dual-phase HSS contain alloys that improve the strength and formability of lighter-weight steel. These alloys increase yield strength as strain increases; for example, the yield strength of dual-phase steel typically increases 50 MPa when work-hardened.
This trait in HSS means that as the steel is shaped, it hardens and gains strength compared with its original composition. Not only must the lubricant provide traditional property values, but its performance must increase throughout the forming process to counteract the increasing friction that occurs. The work-hardening effect also makes secondary forming operations more difficult.
As a result, die designers tend to form as much of the part as possible in the first forming station. Usually these types of designs lead to intricate and severe forms, which require robust lubricant properties.
Second, the lubricant must accommodate the unique stamping techniques that are recommended to successfully produce a quality AHSS part. According to the A/SP "High Strength Steel Stamping Design Manual," the wrong die process is the greatest contributor to poor AHSS part quality. It notes the residual stress and elastic recovery experienced in stamping AHSS, which refers to the springback and part distortions that can occur.
Die processes sometimes require a shape set that induces a 2 percent poststretch in the latter stage of the forming process to prevent side-wall curl or springback. This procedure requires that the lubricant boundary film and lubricity properties be as effective at the latter stages of the form as they were at the initial stages of the deformation. Under these conditions, the lubricant residue must provide an effective and tenacious boundary film, usually some form of EP reaction, and exceptional heat-transfer effect.
Chlorinated oils can provide significant chemical EP, but because petroleum retains heat, it provides minimal cooling as friction develops. These products also create the greatest problems in welding operations (often requiring preweld washing) and other postprocess procedures.
Synthetic lubricants are well-suited for postprocess wash operations and environmental requirements, yet very few have satisfied the automotive postprocess compatibility concerns. The OEM approved synthetic lubricants perform well in many HSLA and HSS operations. Multiphase performance additives have been incorporated into a new generation of synthetic lubricants to address AHSS issues while maintaining postprocess advantages.
Paul Bosler Jr. is CEO and Michael Brown is technical director with Mid-State Chemical & Supply Corp., 2100 Greenbrier Lane, Indianapolis, IN 46218, 800-486-1407 or 317- 925-1407, firstname.lastname@example.org email@example.com, www.midstatechem.com. Debra Reining is manager for TOMASCO mulciber Inc., 2001 Courtright Road, Columbus, OH 43232, 614-236-5436, firstname.lastname@example.org.