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Strong steel

Roll forming high-strength steel offers advantages for automobile manufacturing

Research and development in automobile manufacturing is an ongoing process, particularly in identifying new materials and ways of processing them. Engineers are focused on finding cost efficiencies while simultaneously improving safety and lightweighting vehicles for better energy consumption.

A recent material trend that follows these initiatives involves a type of steel that’s actually been around for quite a while – martensite, a high-strength and lightweight material that has long been thought to be a substandard material for forming parts for automobiles. SSAB, a Swedish-Finnish metal manufacturer, has found that those historical opinions of the material are no longer relevant. Upon closer examination and testing of martensite – along with processing the material the right way – the company says it is an incredibly viable option for automakers.

These images show how materials are stretched to determine their elasticity. The graph on the bottom shows the critical area on high-strength steel where deformation is measured to ensure the material’s usability in manufacturing.

SSAB, which formed in 1878, specializes in processing raw materials into steel. One of its product lines, which the company calls Docol, is aimed at the auto industry. With seven variations in tensile strength, from 900 to 1,700 MPa, SSAB’s Docol martensitic line is a high-strength steel that makes lightweighting, safety enhancements and cost savings possible.

Martensitic advantage

As evidence of these possibilities with Docol, Shape Corp., a Michigan-based Tier 1 automotive and industrial component supplier, recently won the Swedish Steel Prize for its groundbreaking use of SSAB’s martensitic steel in 3-D formed tubes for automotive roof rail applications.

Specifically, Shape used Docol 1700M material in tubular parts that are being implemented in the 2020 Ford Explorer and Escape models. Docol 1700M is SSAB’s strongest martensitic steel and is considered an ultra-high-strength steel that is used as an alternative to aluminum for lightweighting vehicles. The material is also being considered in other areas of automobiles, including side impact beams, bumpers and structural components for improving high energy absorption.

This graph shows the span of tensile strength and elongation values of several types of steel. Martensitic steel is a great match for applications requiring high strength and light weight.

The elongation factor

One of SSAB’s missions is to test materials and improve their use in various applications, including automotive manufacturing. When testing steel, a number of attributes are considered, but one that gets a lot of attention is the elongation value. The elongation test involves pulling apart a piece of steel until it breaks. The deformation of the steel, which occurs prior to it breaking, is measured as a percentage, thereby determining its elongation value. Ultimately, the elongation determines the formability of steel into high-functioning parts.

It’s generally thought that steels with a higher percentage of elongation, or higher A80 value (A80 is a test that stretches materials to 80 mm and beyond to measure the elongation value), are more desirable. While mild steel grades show global deformation behavior, high-strength and ultra-high-strength steels that undergo the A80 test show a low elongation value, because they are being measured over the entire 80 mm, which is considered “global.” But because deformation occurs locally on higher strength steels rather than globally, the A80 test alone is not an accurate indication of how the material will behave when formed.

Roll forming martensitic steel parts, such as this rocker panel, is a cost-effective and efficient process, which is now being used by automobile manufacturers using the Docol line of martensitic steel.

In a recent webinar, Peter Alm, a forming specialist at SSAB, makes the argument that because a new high-strength material has a low percentage of global elongation (as opposed to “local” elongation, which measures deformation closer to the area where the actual fracture occurs) doesn’t mean that it’s not an option for specific applications.

Alm says that a material’s global elongation value is not equal to formability where advanced high-strength or ultra-high-strength steels are concerned. The concern with using materials with low elongation values is based on testing performed on mild steels long before the emergence of high-strength steels. In mild steels, elongation is definitely a factor in determining the formability of a part. However, high-strength steels, such as those represented in the Docol martensite line, have more complex microstructures and don’t behave the same way as mild steels behave.

To get a better idea of how high-strength steels rate, SSAB uses a test that shows results in a forming limit diagram, or forming limit curve, which more accurately predicts forming behavior of this type of steel. The team at SSAB developed seven standardized tests to measure the local strain. Rather than looking at the deformation over the 80 mm of material, as is normally done in the A80 test, they measure the forming limit by creating a 2-mm-by-2-mm grid on a tensile test specimen and analyze that small, local area after stretching the material.

With this type of testing, SSAB has found that the Docol 1000DP material, for example, elongates by 20 percent. Looking at the A80 test, the global deformation value is only 10 percent, which is far less desirable and also misleading. This is important because when forming a part, nearly all the deformation occurs locally, not globally.

This image illustrates springback, which is the term given to the occurrence of steel going partially back to its original formation after being roll formed. Roll forming while using the right tooling can eliminate springback.

Rolling metal

After establishing that high-strength and ultra-high-strength steels are appropriate materials for structural parts in automobiles, the next step is to determine how to manufacture the parts, which comes down to tooling and forming.

Hot stamping of parts has been a popular method of forming parts for automobiles. However, SSAB found that a preferred method of making automobile parts out of high-strength and ultra-high-strength steels is through roll forming, which is done in cold conditions. When sheets of the Docol line are rolled incrementally while under constant pressure from the roll forming tooling, the desired cross section of parts is achieved.

Gustav Olsson, senior forming specialist at SSAB, says roll forming is popular, in part, due to cost savings compared to other forming processes, such as hot stamping.

Each piece of raw material is fed through rollers, which progressively shape the sheet into its desired form.

“The cost savings come in different forms,” he says. “One is that material utilization is close to 95 percent for roll forming compared to about 75 percent for (hot) stamped parts. Another is lower tool costs. Also, roll forming can free up space on the workshop floor. Compared to hot stamping, it has significant energy savings, as well, because it’s done in cold conditions and no furnace is required.”

Cold forming the high-strength steel also controls springback, which is a reaction in formed metals that leads them to spring back to a percentage of the original form after being rolled.

There are also performance issues to take into account with roll forming, such as reducing edge strain. Edge strain is deformation that occurs in roll forming parts made from steel. Conventional steels are more prone to edge strain than ultra-high-strength steels because they have lower yield strength.

“It is of huge importance in roll forming to avoid shape defects,” Olsson says, adding that edge strain often occurs as the rolling mechanism bends and twists the material, stretching it by varying degrees. “We need the edge to stretch without any plastic deformation happening. That is why increased yield strength is so beneficial. A high yield strength means you have a higher elastic limit, meaning a bigger safe zone for edge stretching.”

What does all of this mean to the consumer – the millions of drivers who buy new cars every year? The answer is that using lighter and stronger materials can reduce the size of parts while improving on safety, which means overall vehicle designs are changing. According to Olsson, this is definitely the case at Ford.

“Not only do they save weight and cost and increase strength,” he says, “but they have the added benefit of increased headroom and visibility.”