‘Gen 3 steels’ bring increased strength, formability and weldability to the automotive industry
John Sutter, of Abicor Binzel, discusses how Generation 3 steels perfectly fit the needs of vehicle manufacturers
Over the years, automotive manufacturers have continuously sought out new means to make their vehicles more desirable, more consistent, more profitable and more fuel efficient. There are many ways of achieving these targets. One way, in particular, that has enabled manufacturers to hit a number of them in one go, is using advanced materials such as high-strength steels. These steels can be made to exacting standards with application-specific qualities.
For example, a manufacturer might need an extremely strong steel in a difficult shape to stamp, like a bumper. This presents a need for a steel that can be stretched a great deal, but once assembled has the strength needed to prevent injury to the occupants in the event of a collision. Steels in today’s vehicles are often tailored per specific application.
Some steels get incredibly strong after they are heated up or stamped (or a combination of both). These steels offer some incredible benefits to weight savings, as well as an ability to style the shape of the vehicle, while increasing safety – all of which translate to desirability for the buyers of these vehicles. But there are also some incredible challenges, along with these benefits.
Challenges in welding
One of the most glaring challenges is that, in many cases, the traditional methods of welding need to be re-thought when using high-strength steels in vehicles. Some of these steels have extremely high carbon content, which makes them sensitive to phase changes when the heat from a welding process is applied. This, in turn, often leads to welds that break in minutes or even seconds after the process is complete.
This isn’t the first time the automotive industry has faced such challenges. Over the last 50 years, the change from frame construction to unibody; the addition of rust-inhibiting coatings; the change from mild steel to high-strength steel; and the change to advanced high-strength steels, were all met with answers robust enough to change the way that vehicles are assembled forever.
What are Gen 3 steels?
While Generation 3 advanced high-strength steels (Gen 3 steels) were developed in the early 2010s, it has taken several years for manufacturers to understand how to implement them in vehicles.
So what are Gen 3 steels and why do they break the traditional rules for joining? The answer begins in understanding the needs of the automotive industry: formability and high strength.
It would seem these characteristics are diametrically opposed. However, as manufacturers were looking to meet industry regulations for fuel efficiency and safety, while also addressing competition in vehicle styling, the raw material suppliers answered with high-strength steels. Steels that, through chemistry and an understanding of mechanical properties and phase changes, under certain conditions would be able to meet both the needs of formability and high strength at certain times throughout the manufacturing cycle.
Gen 3 steels were designed to fill the gap in formability-to-strength left by pre-existing steels (Image: Abicor Binzel)
From mild steels and interstitial free (IF) steels, to martensitic steels and transformation induced plasticity (TRIP) steels, the menu of steels that carmakers have to pick from is growing. This menu can be charted out by plotting elongation percentages for a given material against its tensile strength. As more and more recipes are tried, there have been some serious advancements in what is possible in the world of steel.
Some of these advancements led to twinning induced plasticity (TWIP) steels, which have extremely high percentages of manganese in their recipe, and austenitic stainless steels, which left a very large gap in the formability-to-strength chart from traditional steels and high-strength steels. Gen 3 steels were designed to fill that gap. Some examples of current Gen 3 steels are dual phase (DP) 980, SHF 1180 and TBF980 (both trip aided bainitic ferrite steels), and PHS 1300 and 1500 – a conversion delayed boron steel designed to be hot stamped at around 900°C.
Why are they used?
Much attention is given to these steels that exist in the range of 1,200MPa ultimate tensile strength and 30 per cent elongation ductility.
A primary goal for Gen 3 steels was to achieve the tensile strength of martensitic steels with a ductility closer to mild steels. This translates to being able to stamp parts that are deeper, without splitting or cracking occuring.
Some of the automotive components appropriate for using these steels are bumpers, frames, door impact bars, b-pillar reinforcements, roof reinforcements, load beam reinforcements, and floor cross members. The payoff for manufacturers to use these steels instead of older high-strength steels is the continued weight reduction.
The increased weldability, formability and strength of Gen 3 steels make them suitable for use in door impact bars. (Image: Abicor Binzel)
While conventional steels have simple structures and generally have a yield strength less than 550MPa, advanced high-strength steels are characterised by complex or dual-phase structures and specific microstructural features.
A lot goes into the chemistry or recipe for these materials in order to manipulate the physical properties to a specific need or application. Many of these steels have been developed with a specific part in mind from the start, and many of these steels have multi-phase microstructures in order to improve ductility, while meeting specific strength requirements.
Education in welding required
One of the challenges alluded to earlier is the weldability of these new steels. With high carbon content and susceptibility to experiencing phase changes with heat, limiting heat input in welding processes becomes far more important than it ever has been.
This is pushing the envelope of the automotive industry’s welding knowledge. While some research facilities and labs might understand how to get good welds with these materials, not every production line or repair welder in the shop necessarily has a handle on the importance of these factors.
A re-education is therefore happening on both the shop floor and in the process engineering world for the automotive industry. While welding has always been a skilled trade, there is a marked deficit of welders with the skills required to use these new materials. Processing engineers are also looking to systems they may not have used before, such as riveting and laser welding. Both of these processes have some significant costs associated with them, but they offer serious advantages to the manufacturer when they can successfully join the metals that give them the weight reductions they are seeking.
The industry is continuously learning. The steel companies tried with their second generation of advanced high-strength steels to achieve some of the same things they’ve done with the Gen 3 steels, but most of these attempts were rejected because manufacturers were not able to find efficient ways to join these steels. Now, with the Gen 3 steels, steel producers have modified their chemistries to improve weldability, while providing the same formability and high-strength properties that they were orginally seeking. And in this industry, success is measured by acceptance. As these materials are being implemented right now on almost every vehicle platform hitting the market, the Gen 3 steels are here to stay.
In summary, with formidability, weldability and high strength, Generation 3 advanced high-strength steels perfectly fit the needs of the automotive industry.
John Sutter has served eight years on the executive committee for the American Welding Society, Detroit Section and is part of the Abicor Binzel team, as a key accounts manager and laser welding specialist