Exploring New Strategies for Ultrahigh Strength Steel via Tailoring the Precipitates

Ultrahigh-strength (UHS) steels have shown great potential in the field of high-end equipment manufacturing in demand of lightweight engineering and performance upgrade. A significant research effort has been directed toward the development of advanced UHS steels with excellent combination of strength and toughness. In the course of development, tailoring precipitates by means of composition design and process optimization is absolutely a critical module. In this mini review, typical UHS steels strengthened by carbides and intermetallics are systematically summarized and discussed. With the increase of strength, the toughness losses of UHS steels strengthened by carbides and intermetallics have been compared in detail. In particular, the in-depth mechanisms leading to various strength/toughness variation trends have been discussed, extracting the bottleneck in developing new-generation UHS steels containing merely one type of precipitate. Meanwhile, prospects on designing advanced UHS steels strengthened by coexisting dispersive precipitates have been proposed to achieve better performance.

Multiobjective Optimization for Forming Process Parameters of Ultrahigh Strength Steel BR1500HS

This paper focuses on obtaining the optimum process parameters and improving the mechanical properties of ultrahigh strength steel BR1500S. Many experimental data are obtained, and then response surface methodology (RSM) is used to obtain the optimum parameters. Combining the experimental data with RSM, some conclusions are summarized. When the cooling rate reaches 30°C/s, martensite content in microstructure reaches up to 95%. The optimum regions of quenching hardness, tensile strength, and elongation are obtained when the temperature is about 900°C, and the holding time is about 0∼4 min. Results of multiobjective optimizations show that global optimal value is gained at 906.5°C, and the holding time is 0 min. Predicted optimum values of quenching hardness, tensile strength, and elongation are not less than 51.03 HRC, 1,671 MPa, and 8.75%, respectively. The application of RSM is notably successful in predicting the process parameters of hot forming.

Dynamic Recrystallization and Recovery Behaviors in Austenite of a Novel Fe-1.93Mn-0.07Ni-1.96Cr-0.35Mo Ultrahigh Strength Steel

Due to the complex composition and high proportion of alloys in traditional ultrahigh strength steel, the dilemma caused by ultrahigh strength and low toughness in casting and forging processes requiring subsequent heat treatment can be mitigated with an efficient and economical rolling process. In this work, the effect of deformation parameters on dynamic recrystallization (DRX) and dynamic recovery (DRV) is discussed through stress-strain analysis, the DRV mathematical model is obtained, and then the dynamic recrystallization activation energy, Zener–Hollomon equation, and hot working equation are obtained. The critical strain of DRX detected by the P-J method is ε c / ε p = 0.631 , which indicates that dynamic recrystallization of this novel steel is relatively easy to achieve by the rolling process. These models and conclusions have potential to be generalized for the formulation of process specification and process configuration without requiring extensive material testing.

Effect of the Welding Thermal Cycles Based on Simulated Heat Affected Zone of S1300 Ultrahigh Strength Steel

In the automotive industry there is an increasing demand for the wider application of high strength steels due to their favourable mechanical properties. The steel producers continuously developing new generations of high strength steels to insure higher strength and toughness properties. Since in most cases these steels are joined in welded structures, great attention must be taken to their weldability. The weldability of high strength steels has still challenges which are as follows: cold cracking sensitivity; reduction of strength and toughness of heat affected zone (HAZ); filler metal selection. Because the mechanical properties of ultrahigh strength steels are provided by using various alloying elements, micro alloys, and by different metallurgical methods, the steels may lose their outstanding properties during welding. In real welded joints the critical parts of the HAZ have small extent so their properties can be limitedly analysed by conventional material testing methods. With the help of physical simulators, the different parts of the heat affected zone can be produced in an adequate size for subsequent tests. In our research work the weldability, especially the HAZ properties of an ultrahigh strength structural steel (Rp0.2 = 1300 MPa) were investigated on thermal simulated samples with the help of Gleeble 3500 physical simulator. Three relevant technological variants for gas metal arc welding (GMAW), t8/5 = 5 s, 15 s and 30 s were applied during the HAZ simulations in the selected coarse-grained (CGHAZ), intercritical (ICHAZ) and intercritically reheated coarse-grained (ICCGHAZ) zones. Both the microstructure was studied by optical microscope and the mechanical properties were analysed by Vickers hardness tests and Charpy V-notch impact tests at -40 °C. According to the results the investigated ultrahigh strength steel was softened on account of the welding heat cycles, besides that the strength of the investigated ultrahigh strength steel can be better with the application of shorter t8/5 cooling time.