Studies On The Q&P Heat Treatment Processes Of Advanced High Strength Steel Used For Automobile And Elements Diffusion In Steels During Partitioning

With the requirement of safety and energy consuming increasing, lightweight become the main trend in automobile industry. The use of advanced high strength steel is the effective way to realize lightweight and ensure safety. Q&P steel, belonging to the third generation high strength steel, is a new type of martensite steels with high strength, high plasticity, high toughness and an excellent combination of strength and ductility. As an automotive structural steel, it can greatly reduce vehicle weight and enhance impact resistance. As a consequence, it has great development potential.This study focused on commercial C-Si-Mn and C-Mn-Al steel plate as experimental materials. Q&P steel heat treatment process was performed by the crucible type salt bath furnaces and DIL78 automatic dilatometer. Experimental methods such as optical microscopy(OM), scanning electron microscope(SEM), transmission electron microscope (TEM) and X-ray diffraction(XRD) to analysis the microstructure and the phase. Scanning transmission electron microscopy(STEM) were used to observe the distribution of elements. Electronic universal test machine to study the mechanical properties of Q&P steel. There are two research purposes, the first one is to study micro structural evolution, micro structure-mechanical properties of partial austenitizing Q&P steel. The second one is to study the element diffusion during partition.The main results are as follows:(1) In the austenitizing process of partial austenitizing Q&P steel, austenitization degree is lower with lower austenitizing temperature. As a consequence the content of martensite is less and the grain size of martensite is smaller. However,excessivelylow austenitizing temperature will lead to larger grain size of ferrite and affect the mechanical properties after quenching and partitioning.(2) When the quenching temperature is between 200℃ to 320℃, with the higher quenching temperature, the content of blocky martensite-austenite islands increases and the volume fraction of retained austenite decreases. When the quenching temperature is lower than 280℃, the content of carbon in retained austenite decreases as the quenching temperature increases. When the quenching temperature is higher than 280℃, the content of carbon in retained austenite remains the same as the quenching temperature increases.(3) It takes short period of time to reach the maximum volume fraction of retained austenite during partition. However above partition time is not enough for carbon to reach the maximum value in retained austenite. Later in the time of maximum volume fraction of retained austenite, when the partition temperature is low, the volume fraction of retained austenite remains almost the same as partition time increases, the content of carbon in retained austenite increases gradually as partition time increase; when the partition temperature is high, the volume fraction of retained austenite remains decreases as partition time increases, the content of carbon in retained austenite remains almost the same as partition time increase.(4) The elongation of A steel do not fully depend on the volume fraction of retained austenite, During partition the non-uniform elongation of A steel improves. In this experiment, the optimum heat treatment process of A steel is austenitizing at 800℃×5min+quenching at 240℃×10s+partitioning at 400℃×300s. With this process, about elongation of 23.4% and tensile strength of 1079MPa are obtained, and the strength-ductility product is up to 25000 MPa%.(5) By using Santofimia’s method, the calculated time for carbon to sufficiently diffusion into austenite is 6s(PT=400℃). And calculated time for carbon to sufficiently diffusion into austenite is 40s(PT=350℃)According to the image of STEM:the C atoms do enrich in the retained austenite after Q&P heat treatment; the diffusion behavior of Al and Mn not obvious when partition time is less than 80s.(6) When the partition time is less than 300s, The width of austenite don’t change obviously, the average width of austenite is about 60-70nm.