Papers by Keyword: Ultra High-Strength Steel

Abstract: This paper focuses on the development of a new type of roll bending machine. Our primary aim was to build a machine that could form ultra-high-strength steels (UHS) with smaller inner radii than those achieved by traditional bending methods. One of the main planning principles was modular construction, so a length of a bending line could be easily selected or changed later by the user without major changes to the basic construction of the machine. In contrast, in traditional roll forming, the blank does not move during the forming process, so the accuracy of the profile can be better controlled. Different kinds of cut to size-open profiles can be produced by this machine, which utilizes and combines bending and rolling techniques. In the initial stages of the project, the needs of smaller companies that do short-run productions are taken into account. First, the prototype is designed mainly for research use; moreover, it is important that the properties of the machine are multifunctional. In addition, forming can be done in several ways by this machine. In this paper, there is shown creation of a machine, designing of construction and manufacturing steps of the whole machine including assembling. Also detailed description of the various functional components and the operating principle is presented. The results of the forming tests are also presented.

Abstract: nnovative ultra high-strength steels have excellent mechanical properties which commonly relate to the materials martensitic microstructure. As thermal heat treatments are state-of-the-art for obtaining the desired microstructure, innovative thermo-mechanical treatments are likely to give rise to even better material qualities. This article highlights various aspects of innovative thermo-mechanical hardening strategies for the processing of ultra high-strength steels, involving both press hardening and friction spinning operations.

Abstract: According to the properties request of energy storage flywheels running in high speed, chemical composition of a new ultra-high-strength steel has been designed. The designed steel specimen was prepared using intermediate frequency induction furnace and its transformation point, which had been simulated and calculated through J-Mat software in advance together with cooling curves, was investigated using by thermal dilatometer. Then the microstructure and mechanical properties of the designed steel have been evaluated by means of OM, SEM, durometer and universal material tensile tester. Simulation results showed that the pearlitic transformation of designed steel occurred at 687-453°C and bainite transformation at 453-340°C. Martensitic transformation started at 340°C and terminated at 220°C. The experimental results indicated that the casting microstructure of the designed steel was a duplex structure consisting of martensite and acicular bainite with a small amount of retained austenite. The austenization temperature ranged from 698°C to 790°C. The superior comprehensive mechanical properties of tensile strength of 1900MPa and elongation of 6.65% as well as the microstructure of tempered martensite was obtained after heat treatment.

Abstract: Ultra-high Strength Steel heat process mechanical property energy spectrometer Abstract: Microstructure and mechanical property variation vs. heat process was investigated by means of metallography and electron microscopy in Ultra-high Strength Steel with tensile strength 1000Mpa. The results show that microstructure variation in the steel with tempering temperature increasing is as follows:tempered martensite→main tempered sorbite and a small amount of M/A, at the meantime, growing in quantities and volume of second-phase, always decreasing in strength, firstly increasing and then decreasing in impact energy and elongation. Study on precipitated mechanism of second-phases, the crystal structure and volume of precipitation was characterized by TEM observation and energy spectrometer.

Abstract: The superior mechanical and physical properties of ultra-high strength steel 30Cr3SiNiMoVA have awarded the material such desirable and increasing demands in aerospace and aviation industries. Due to its excellent properties, 30Cr3SiNiMoVA is usually classified as a difficult-to-cut material. The machining process of ultra-high strength steel is often characterized by high cutting forces, low surface finish and severe tool wear. In actual production, hard cutting which is a profitable alternative to finish grinding is usually employed in order to improve machining efficiency and reduce processing cost. Therefore, this research concerns the machinability evaluation in hard milling of 30Cr3SiNiMoVA by using TiAlN coated carbide tools. The machinability of the steel was investigated with respect to cutting forces, surface roughness, chip morphology and tool wear, respectively. Finally, a comparative test was also conducted between hard milling and grinding process. The results pointed out that hard milling and coated tools were suitable, beneficial and effective for manufacturing ultra-high strength steel 30Cr3SiNiMoVA.

Abstract: Overall elastic-plastic stability behavior of cable-stayed ultra-high strength steel (660MPa) column with welded I-section is studied by geometric and material nonlinear FEM of software ANSYS. Longitudinal residual stresses and initial imperfections are taken into consideration. The results indicate that cable-stayed type can significantly improve overall stability bearing capacity and effectively control mid-span deflection of ultra-high strength steel column.

Abstract: The use of ultra-high-strength steels (UHS) has become more and more popular within last decade. Higher strength levels provide lighter and more robust steel structures, but UHS-steels are also more sensitive to surface defects (e.g. scratches). Practically this means that the critical crack size decreases when the strength increases. The aim of the study was to study if the formula of critical crack size is valid on forming processes of UHS-steels. Surface cracks with different depths were created by scratching the surface of the sheet by machining center. Effect of the scratch depth was determined by bending the specimens to 90 degrees. Bents were then visually compared and classified by the minimum achieved bending radius. Test materials used were direct quenched (DQ) bainitic-martensitic UHS steels (YS/TS 960/1000 and 1100/1250). Results from the bending tests were compared to the calculated values given by the formula of critical crack size.

Abstract: Extremely high strength of the ultra-high-strength steels leads to increased load factors on the tooling machines and punching tools. This experimental study examines how much convex punch geometry affects cutting forces when punching ultra-high-strength steels. Tools used in punching tests were four different convex sheared rooftop punches and one conventional flat end punch, to which rooftop punches were compared to. The material in punching tests was ultra-high-strength steel Ruukki Optim 960 QC, with a thickness of 4 mm. The test material in punching tests was sheared with rooftop punches and a flat end punch and occurred cutting forces were measured. The qualities of punched holes were evaluated visually and the roundness measurements were also performed. The results show that the cutting forces of Optim 960 QC can be reduced radically with optimal convex punch geometry. With using 14-degree shear angle of the punch end, the cutting forces reduced up to 57 % compared to forces of the conventional flat end tool. However, largest tested shear angles caused several negative effects on the cutting quality of the holes and therefore they are not suitable in all applications. Punching tests proved that the cutting clearance had no appreciable effect on cutting forces when punching ultra-high-strength steel. Instead there was a noticeable effect on the quality of the punched hole, especially when large shear angles were used.

Abstract: Bainitic steels, which are transformed at very low temperatures, offer an excellent combination of strength and ductility where the strength comes from the nano-structured bainitic plates and thin-film of austenite sandwiched between two bainite sheaves offers the ductility. The main drawback of this structure is the long transformation time which is not ideal for industrial application. Through the microstructural engineering, the extent and kinetics of transformation can be manipulated by judicious selection of alloy composition and process variables. The main challenge is to delay the transformation till the coiling stage and allow the formation of bainite only during the cooling of the coil. In the current work, an approach will be shown, starting from the alloy design based on thermodynamics till the cooling after coiling, which can satisfy the requirements to develop such steel with 1300 MPa UTS combined with 20% elongation (min).

Abstract: Recently developed ultra high-strength low alloy transformation-induced plasticity (TRIP)-aided steel with martensitic lath structure matrix or "TRIP-aided Martensitic steel; TM steel" possesses a high impact toughness. In this study, to apply the TM steel to some hot-forging parts, the effects of hot-forging on microstructure, retained austenite characteristics, tensile properties and toughness in the TM steels with chemical composition of 0.3-0.4%C, 1.5%Si, 1.5%Mn, 0.002%B, 0.02Ti, 0.05Nb (mass%) were investigated. The hot forging brought on an excellent combinations of tensile strength of 1500-2000 MPa or 0.2% offset proof stress of 1200-1560 MPa and Charpy impact absorbed value of 35-80 J/cm² when partitioned at 250-350°C after quenching in oil. The combinations exceeded so much those of the conventional quench and tempering structural steels. From examinations of microstructure and retained austenite characteristics, it was found that the excellent combinations are mainly caused by (i) refined and uniform martensitic lath structure matrix with a small amount of carbide, (ii) increasing narrow martensite with high dislocation density and (iii) the increased stability of retained austenite, resulting from the FQP process.