The battle of the metals
HSS sheet for chassis parts
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Mitsubishi uses 590-MPa (86-ksi) ultra HSS for 80 chassis parts in the third-generation Pajero.
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Mitsubishi Motors Corp. developed a new 590-MPa (86-ksi) ultra HSS sheet containing retained austenite for 80 chassis parts on the third-generation Pajero released last fall. Mitsubishi worked with Nippon Steel Corp., Sumitomo Metal Industries, Ltd., and Kobe Steel Ltd. to develop the new steel. The sulfur and retained austenite content is controlled in the new HSS sheet to realize improved stretchability, and to enable it to undergo stretch flanging, in which high degrees of localized flexing and stretching are generated. This has resulted in an increase in the hole expansion ratio to more than 80%-an indicator of stretch flange formability-and enabled for the first time the use of retained austenite sheet steel in chassis parts, many of which are difficult to form.
Generally the greater the tensile strength of the steel, the more possible it is to use the thinner gauges, necessary to attain automobile mass reductions. However, steel sheet formability deteriorates with increasing tensile strength, as evidenced by cracking and splitting during pressing, and also softens by the heat effect during welding, which results in the fatigue of welded joints. This puts a limit on the strength of the steel that can be used in forming parts; to date most chassis sheet metal parts have been made using hot-rolled steel sheet between 370 to 400 MPa (54 to 58 ksi).
In the new 590-MPa (86-ksi) ultra HSS sheet, the sulfur content has been lowered from 0.006 to 0.002%, while the retained austenite content has been reduced from between 8-12% to 4-8%. This has made it possible to provide the stretchability needed for the manufacture of chassis parts requiring stretch flanging. In addition, the steel's high silicon and manganese content prevents softening during welding and enables increased welded-joint fatigue strength. According to Mitsubishi, the use of the new steel in the Pajero series reduced the mass of the parts by 13% for a total mass reduction of 14 kg (31 lb).
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