Steelmaker perfects hardening process for heavy plate applications

January 2016 - Sweden’s SSAB and its SSAB Americas division don’t employ half measures with anything. When the steelmaker wants to do something, such as developing new grades it hopes will become must-have products among its customers, it goes full bore.

The latest product line and associated process to emerge from research and development is Strenx, a series of extra-high-strength and ultra-high-strength performance grades that even at great thickness remains flat where needed but is also highly formable and bendable, and lasts a very long time in service.

Immediate applications are for structural needs: from truck frames to telescoping boom cranes, from shipbuilding and oil rigs to tow bars and plows, and to giant mobile equipment that lifts ocean-going cargo containers.

Customers can find in Strenx a lighter weight product that remains as durable and safe as previously proven specialty grades, while saving money via reduced fuel consumption and higher payload. SSAB guarantees the product for thickness accuracy, flatness and bendability.

Strenx is a lighter weight product that retains its mechanical properties and remains consistent through such processes as cutting, punching and bending.

A metallurgist and a plant manager—Ben Kowing, marketing and technical support director, based at SSAB Americas’ R&D center in Davenport, Iowa, and Mark Bush, general manager, Mobile Operations—sat down with Modern Metals at the Alabama mill to explain how the company developed Strenx and the proprietary process that makes that product exceed any current standard.

When steel plate has to match certain hardness and yield strength requirements, it must be heated and cooled. But to make sure the plate to be quenched and tempered remains flat across the entire width and length, “the trick is to quench it all uniformly so the material all transforms at the same time. If one little bit of the plate transforms before the other bits, it will be wavy, which makes it hard to apply water uniformly,” Kowing says.

“It really is an art form to make a quenched steel and the rate at which you do this has an affect on its physical properties,” he continues, but “quenching it uniformly is really an SSAB secret recipe.”

When SSAB wants to install a rolling mill or other processing lines, it typically has outside vendors build them. “When we wanted a quenching line, we built it ourselves with purchased equipment. Those vendors weren’t given the designs for the whole thing,” Kowing says. “It’s completely proprietary technology.”

Pieces were built “all over Europe,” adds Bush, “so no one had a complete set of drawings. Our engineers oversaw the manufacturing, assembly and startup. All the [machinery integration] control code was written in house, too.”

Part of the original startup team in 1999 for what was then Ipsco’s greenfield plate mill, Bush also supervised the installation of the previous generation quenching line (QL5) in 2005, which he says was built with off-the-shelf equipment. When SSAB acquired Ipsco two years later, it retrofitted QL5 with its own technology in order to evolve from producing commodity products to specialty products like Hardox. “Part of the reason SSAB bought Ipsco was to be able to make branded products in the United States. That was a major driving force,” Bush says.

The Strenx steel grades produced by Quench Line 6 in Mobile, Alabama, underwent rigorous tests and trials.

Defining the gap

SSAB assembled a team of 30 people including metallurgists, technicians, operators and engineers. “The first thing to do was define characteristics of branded products and where the gap lay between product coming off the existing quenching line versus what was being produced in Oxelösund, Sweden.”

What the company wanted, says Bush, is that when a customer receives a plate from either site, “they are not going to be able to tell the difference. The plate looks the same, acts the same and performs the same. If I get a Coca-Cola anywhere I want it to taste the same.”

Once the gap was defined, SSAB Americas brought in technical and engineering know-how from Sweden “and dug into the details to identify what had to be changed in how we operated the equipment and the physical characteristics,” he says. “We made a lot of changes to the quench portion of the line.” Although the unit remains in the original frame, “all the guts have been changed to make the next-generation products.”

Beta testing

Once the company redesigned the line, “we started producing product under a development brand. We sent Strenx grades out to specific customers partnering with us and they beta-tested the product,” Bush says. “We needed customers who would tell us what they were doing with the product,” and how it was working for them.

“At the same time, we did R&D and lab testing and more testing in Oxelösund to make sure both products screened alike. We waited for customer feedback, [for them] to say, OK, it machines the same, it bends the same, it cuts the same, it welds the same.”

Next, Mobile submitted all the technical data and evidence to SSAB’s in-house branding council to prove it was making product that matches that of Oxelösund, which finally approved the plant for production of branded products. “Every gauge and every product had to be individually tested and qualified,” Bush says.

SSAB Americas gathered additional customer feedback to determine if they found the product from Mobile as reliable as what they previously sourced from Sweden. “The whole process was years,” says Kowing.

Truck beds and frames, wind tower infrastructure and harvest equipment are among the applications for which SSAB developed its Strenx product line.

Bush says the QL6 line, as the newest quench line in Mobile is called, has “qualified everything in the original proposal that we said we would make.”

In 2014, SSAB marked the 30th anniversary of Hardox, a formable wear plate, according to Kowing.

Regarding Strenx, “The development cycle was shorter than with Hardox because we already had all the experience making Hardox,” Bush says. “But there are nuances to learn such as leveling that product, and the flatness criteria are more stringent. It reacts a little bit differently when leveling and works toward tighter tolerances.”

Demanding applications

The base of customers that now specify Strenx in their end product designs is widening, says Kowing. “The most demanding is the lifting industry, folks who make telescoping boom cranes, for example. Take a long plate of steel, fold it into a shape. Take another shape and weld it together. The tolerance has to be so good that you can take another long shape and slide it in and out. The tolerances are tremendous,” he says.

Many customers in that industry indicate they “prefer the material out of this plant,” he continues. “That’s why we offer tolerances tighter than industry standards for these grades—because of users making complex shapes. A crane manufacturer can sometimes use other steels but it’s a lot more rework in their shop to get it right. Whereas if they put a consistent steel in the front end, they can run their process normally and get what they expect without rework or adjustment.”

Bush says customers that have to bend heavy, thick plate know that with Strenx, “they can go to one setting consistently and get exactly what they are looking for with the bend. If they use another steelmaker’s product, they might get different results from bend to bend—which they don’t want.”

The three major flat product groups in the Strenx structural steel portfolio are quenched and tempered plate (as from Mobile), as-rolled strip produced via thermo-mechanically controlled processing (TCMP), and cold-rolled strip. From this, SSAB can make cold-formed tubes and profiles. The available yield strength classes for Strenx performance steel include 600, 650 and 700 MPa (extra-high-strength steels) as well as 900, 960, 1100 and 1300 MPa (ultra-high-strength steels). The available thickness range is 0.7 to 160 mm (0.028 to 6.3 inches). With Strenx, SSAB guarantees standard-exceeding minimum bend radii while maintaining internal purity, homogenous microstructure and uniform mechanical properties. MM