Retained Austenite And Its Effect On The Mechanical Properties Of A Hot-stamped Steel

Forming-quenching integration is an advanced process for manufacturing parts of car body developed to meet the demand for automobile lightweighting. Major technical barriers faced by the process at the present are lack of suitable steel, which are quenchable, weldable, and with large product of strength and elongation that is achievable by corresponding treatment method. A new hot stamped steel grade has been developed in the present work that is a part of the key project "A study for an integrated process of forming and quenching for a quenchable and weldable automobile steel with high product of strength and elongation" supported by National Natual Science Fundation of China. Its chemical composition is C0.19%, Si1.55%, Mn1.53%, Ni0.95%, Cr1.01%, Cu1.01%, B0.0027%, A10.025%, Ti0.033%, Mo0.45%. The volume fractions and morphology of retained austenite obtained in samples processed by a series of heat treatment conditions, such as DQ (direct quenching), Q&P (quenching and partitioning) and Q&P-T (quenching, partitioning and tempering) were evaluated, and the effect the retained austenite on the mechanical properties was studied. The main contents of this article are as following:1. Based on the most recent published results in the area of ultra high strength steels and the alloy design principles, the chemical composition of the test steel was designed.2. The experimental steel was hot rolled and the specimens were made.3. Critical phase transformation points were measured by using an automatic phase change instrument, Formastor ?, and the CCT curve of the test steel was constructed. The heat treatment process of the experimental steel was designed based on the measured CCT curve.4. The tensile strength and elongation of the heat-treated samples were measured using a MTS universal tensile testing machine and its hardness was tested by a Digital Microhardness Testor. The impact toughness was measured by Instron 9250HV droping hammer impact testor at room temperature.5. The microstructure of the heat-treated samples were characterized by optical metallurgical microscope (OM)?scanning electron microscope (SEM)?electron probe microanalysis (EPMA)?X-ray diffraction (XRD) and transmission electron microscope (TEM). The volume fraction of retained austenite was calculated from the XRD dara and the effect of the retained austenite on mechanical properties was studied. The impact fracture surface and tensile fracture were viewed by SEM to determine the fracture mode.It was found that the tensile strength of the direct quenching samples is in the range from 1370 MPa to 1619 MPa and the impact toughness is from 50.2J to 38.65J. Partitioning processes have a strong effect on mechanical properties. When the partitioning time is 5 minutes, the samples partitioned at 280? have a higher impact energy than that at 250?. For samples partitioned at 250?, the impact energy may be improved when partitioning time is increased from 5 minutes to 10 minutes. All in all, as the partitioning condition alters, the tensile strength was changed only slightly but the impact energy was changed significantly. On the other hand, tempering has little effect on tensile strength, but considerably on the impact energy. Tempered at 200? for 60 minutes, the impact energy was increased to 71J, an up by 30%. By contrast, the tensile strength was reduced to 1320 MPa, a decrease only 6.4%. It was also found that the microstructure of the experimental steel is formed by lath martensite and retained austenite that were mainly located between the laths of martensite, film-like and of a volume fraction up to 15.The experimental results show that the tested steel has an excellent combined mechanical properties with a strength greater than 1320 MPa, an elongation upto 15%and an impact energy upto 71 J. Thus, it can be hot stamped to make aoto parts requiring high imact resistance, such as bumper, anticollision beam, etc.