Development of Mn and Si based 3rd generation advanced high strength steels for automobile applications

Abstract

Automotive industries require materials with higher strength, plasticity and crashworthiness, aiming for 3rd generation of advanced high strength steels (AHSS) with medium Mn, Si and minor alloying elements. In this investigation, 3rd generation AHSS were developed using a vacuum arc melting furnace with manganese (Mn) and silicon (Si) as major alloying elements as well as minor additions such as Cr, Al, Ni, etc. Time Temperature Transformation (TTT) diagrams were simulated using JMatPro. AHSS homogenized treatment was performed at 1200 °C for 4 hours. Ferrite peaks were identified in homogenized steels. Field emission scanning electron microscope revealed ferrite and pearlite in the homogenized steels. The alloy steels after casting and homogenization were subjected to hot rolling at 900 °C and 1100 °C. The steels thus developed were characterized using FE-SEM, XRD, microhardness tester, universal testing machine (UTM) and 3-dimensional Atom Probe Tomography (APT). The homogenized AHSS were hot rolled at 900 C for multiple passes followed by air cooling. The microstructure revealed 1-2% martensite, 20-40% bainite, 10-12% retained austenite, and 46-69% ferrite. The presence of retained austenite was also verified by XRD analysis. The ultimate tensile strengths (UTS) of Alloy 1 (Fe-4Mn-1.5Si), Alloy 2 (Fe- 6Mn-1.5Si), and Alloy 3 (Fe-8Mn-1.5Si) hot rolled at 900 C were found to increase from 1418 MPa to 1625 MPa with elongation of 17% to 15%. The addition of manganese increased UTS and hardness while decreased ductility of alloys. The ultimate tensile strengths (UTS) of Alloy 4 (Fe-6Mn-1Si), Alloy 2 (Fe-6Mn-1.5Si), and Alloy 5 (Fe-6Mn- 2Si) were not changed significantly and elongation was observed to decrease from 18% to 12%. Effect of silicon variation from 1 to 2 wt.% keeping Mn constant at 6 wt.% was also analyzed. The addition of silicon increased hardness while decreased the ductility of alloys. Further, the alloy steels after casting and homogenization were subjected to hot rolling at 1100 °C. The FE-SEM micrographs revealed a complex phase microstructure with martensite, ferrite, bainitic ferrite and retained austenite in the specimens obtained after rolling and air cooling. The microhardness of the developed alloys was found to be in the range of 395 to 502 VHN in the hot-rolled and air-cooled condition of the specimen. The tensile strength of alloys was measured to be in the range of 1412 to 1614 MPa with elongation of 12% to 19%. The analysis of fracture surfaces after tensile tests for developed alloys revealed that Alloy 1 (Fe-4Mn-1.5Si) had dimples indicating ductile fracture while Alloy 2 (Fe-6Mn-1.5Si) has a mixture of dimples and facets, and Alloy 3 (Fe-8Mn-1.5Si) has lower dimples but larger facets, confirming quasi-ductile fracture with a lower ductility limit of 12%. Atom Probe Tomography was performed to study the complex phase structure at nanoscale through re-distribution of carbon and other alloying elements. 3D APT revealed the presence of very fine retained austenite film of thickness ~4-5 nm and carbon content of 6-8 at.%. This complex phase microstructure obtained after hot rolling and normalizing is made of very fine bainitic ferrite with film type retained austenite also providing TRIP effect, in addition martensite and ferrite resulting in high strength and toughness.