Effect of cooling rates after annealing on the microstructure and properties of 1000MPa grade automobile steel for cold forming

The effects of different cooling rates ( 0.05℃/s, 0.1℃/s, and 0.2℃/s ) on the microstructure and mechanical properties of 1000 MPa grade automobile steel for cold forming after two-phase annealing were studied. The microstructure of the experimental steel was observed by SEM and TEM, and its mechanical properties were tested by a universal tensile testing machine. The results showed that by increasing the cooling rate of two-phase annealing, more massive retained austenite, more uniform and fine ferrite, better elongation and higher ultimate tensile strength of steel can be obtained, so as to obtain better production of tensile strength and total elongation ( product of tensile strength and elongation, PSE ). The final result shows that after the test steel is quenched at 800℃ + 10 minutes and annealed in the two-phase region at 690℃ + 10 minutes, the faster the cooling rate, the better the mechanical properties. The mechanical properties of the steel plate are the best when the cooling rate reaches 0.2℃/s, and PSE can reach 27.44 GPa·%.

The effect of casting speed on slag-inclusion defects in 1050 × 200 mm ultra-low-carbon automobile steel slabs

The effect of casting speed on slag-inclusion defects in 1050 × 200 mm ultra-low-carbon automobile steel (UAS) slabs was studied by performing nail plate experiments and by monitoring the liquid level fluctuations during the continuous casting process. The number, location, length, and proportion of slag-inclusion defects in hot-rolled coils produced at different casting speeds were analyzed. The results showed that the defects in the hot-rolled coils were mainly owing to the mold protective slag. For the continuous casting speed of 1.6 m/min, the fraction of slag-inclusion defects was the lowest (at 5.3%), and the number of slag-inclusion defects was lower than for the other casting speeds. The length of slag inclusions was under 900 mm. Furthermore, the number of slag-inclusion defects on the upper and lower surfaces was smaller than those for the other casting speeds. At different casting speeds, slag-inclusion defects mainly existed within 100 mm of the edge on both surfaces of hot-rolled coil plates. The fluctuation within the 0–1 mm range was 98.1% for the casting speed of 1.6 m/min, indicating that the flow rate of molten steel on both sides of the nozzle was relatively stable, which helps to control slag-inclusion defects in hot-rolled coils.

Effect of the weir structure in the tundish on the cleanliness of IF steels and elimination of spot-like defects in deep drawing automobile steel sheets

The flow control devices (FCDs) in the tundish play an important role in steel cleanliness and removal of macro-inclusions. To eliminate the spot-like defects during deep-drawing process, the liquid steel samples were taken from tundish in three industrial IF steel heats, and the effect of weir structure in the tundish on the steel cleanliness were evaluated and compared with the aid of ASPEX with the total detection area of over 54 000 mm2. The results showed that non-metallic inclusions over 5 µm observed in the liquid tundish samples were categorized into two types: type 1: alumina based inclusions and type 2: calcium aluminates. Compared to the tundish without weir, the number density of macro-inclusions reduces by half when furnished with weir. In addition, several large sized calcium aluminates were occasionally found without weir whereas no calcium aluminates were detected with the weir. The mechanism of the inclusion distributions with the weir was proposed based on the experimental results, which shows that the weir act as a baffle that block the large sized inclusions from moving toward the mould. After using the weir in the actual casting practice, the occurrence ratio of spot-like defects was decreased by about 25%.

DYNAMIC STRESS IN AUTOMOBILE BRIDGEWORKS

The article considers and solves the problem of determining dynamic characteristics of automobile steel bridges. In the laboratory conditions, possible loads are simulated and stress diagrams are constructed. By measuring stresses in a real bridge, experimental loading diagrams are constructed for all possible loads. By comparison with the model diagrams, the stress effect in permanent joints is shown. The stress evolution is shown depending on the traffic flow.

Introduction to a Third-Generation Automobile Steel and Its Optimal Warm-Stamping Process

The medium-Mn steel is a promising third-generation automobile steel. Its chemical composition, microstructure, and thermal and mechanical properties are introduced and a warm-stamping process for the medium-Mn steel is proposed. The optimal process parameters are identified through the design of experiments (DOE) and range analysis. The evaluated experimental indexes include tensile strength, elongation, and hardness. The optimal forming process consists of an austenitization temperature of 840 °C, a soaking time of 4 min, and an initial stamping temperature of 500 °C. The proposed process was applied to the warm stamping of an automotive B-pillar. The microstructure of ultrafine, uniform, and complete martensite laths was obtained. The formed part exhibits approximately 1420 MPa tensile strength, over 11% elongation and 460 HV hardness. The optimal warm-stamping process has proved effective and applicable for forming medium-Mn steel parts. It will help promote the application of the third-generation automotive steels.