Page 22 - MetalForming Magazine April 2023

This deep-section crash part (a lower A- pillar reinforcement) stamped from Tata Steel’s HyperForm grade illustrates the extra formability that the steel exhibits, over and above that of other dual-phase steels, “developed by fine-tuning the steel recipe and optimizing the annealing curve,” says Basjan Berkhout, head of marketing, automotive at Tata Steel.
engine cradles and more; DP 700-1000 for roof rails, and DP 800-1180 for B- pillar uppers. DP steels have a microstructure consisting of a ferritic matrix with martensitic islands as a hard second phase. The soft ferrite phase is generally continuous, giving these steels excellent ductility. When these steels deform, strain is concen- trated in the lower-strength ferrite phase surrounding the islands of martensite, creating the unique high initial work-hardening rate (n-value) exhibited by these steels.
At Tata Steel, “we initially developed DP800-GI HyperForm as an alternative to TRIP (transformation induced plas- ticity) 800,” says Berkhout, “which was popular at the time but had some issues with weldability. We were looking for a new DP grade with good forma- bility compared to a normal DP, and because the TRIP 800 was popular at the time we focused the first Hyperform grade there.”
Recognizing the benefits of the con- cept, the firm has decided to expand to 600- and 1000-MPa Hyperform grades. “In fact,” Berkhout explains, “we now have started to upgrade one of our newest galvanizing lines to enable us to develop the HyperForm 1000-MPa grade. We see this grade fam- ily as falling between the DP steels and the extra-ductile (ED) quenched and partitioning (Q&P) grades in terms of formability. We’ll position it as a work- horse for future vehicles.”
How do these steels gain their extra formability and weldability? By fine- tuning the steel recipe and optimizing the annealing curve.
“The goal has been to make the grades lean in terms of composition— low silicon to ensure good weldability, and relatively low carbon content,”
Berkhout says. “Then the annealing curve is optimized to enable us to develop a microstructure capable of very high work-hardening rate. At 600 MPa, for example, the elongation is at least 6 percent higher than a conven- tional DP600 steel. So, for example, replacing a 300-MPa HSLA grade with DP600-GI HyperForm we can reduce part weight by 10 to 15 percent.”
Nucor on the Move
Speaking of new galvanizing lines boosting steelmakers’ capabilities, Nucor, which has a large portfolio of automotive grades, recently completed a new galvanizing line at its facility in Gallatin, KY. That plant now can pro- duce galvanized automotive products to 8.5 mm thick. The recent expansion of the mill nearly doubles its capacity, producing hot-rolled coated and uncoated steels to 1865 mm wide.
In addition, Nucor’s new specialty cold-rolling complex, opened late in 2019 in Hickman, AR, increased the mill’s cold-rolling capacity by 500,000 tons, with a robust reversing mill capa- ble of producing AHSS grades with far fewer passes and to lighter gauges, according to Nucor officials. That facil- ity’s Flex galvanizing line can produce 3rd Gen AHSS grades, coated and uncoated. With an added galvanizing capacity of 500,000 tons/yr., it also has an over-aging section capable of pro- ducing Q&P grades.
Lastly, Nucor is building a new 3- million-ton sheet mill in Mason Coun-
ty, WV, capable of producing 2133-mm- wide hot band and 1930-mm-wide cold-rolled sheet on its tandem mill. It will have two galvanizing lines, Met- alForming is told, one dedicated to automotive. This new mill is well-locat- ed, Nucor officials say, to serve the Midwest and Northeast regions, and will have one of the lowest carbon foot- prints of any steel facility in the world.
New DP Grade from Big River Steel (U.S. Steel) Earns Steelie Award Nomination
Earning a 2022 nomination for Inno- vation of the Year from worldsteel (the World Steel Association): the Big River Steel subsidiary of U.S. Steel, for its development of a single phase nano- precipitation strengthened batch- annealed SP590 sheet steel with report- edly outstanding formability and marketed as a low-cost, greener DP590 alternative. The high-strength highly formable sheet steel has been developed with very low-carbon lean alloying, the company reports, using an innovative fine-grained single-phase ferritic microstructure strengthened with TiC nano precipitates. It provides a hole- expansion ratio of 80 to 110 percent (a hole-expansion ratio of 90 percent and above has been registered for a sheet thickness of 1.2 mm) and tensile strength similar to DP 590, reportedly with formability properties far exceeding that of DP590 or any steels with similar strength. The steel has been commer- cially produced and successfully used
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