Ultra-high Strength Steel Grades Research Articles

Prediction of the Shear Tension Strength of Resistance Spot Welded Thin Steel Sheets from High- to Ultrahigh Strength Range

The tensile strength of newly developed ultra-high strength steel grades is now above 1800 MPa, and even new steel grades are currently in development. One typical welding process to join thin steels sheets is resistance spot welding (RSW). Some standardized and not standardized formulas predict the minimal shear tension strength (STS) of RSWed joints, but those formulas are less and less accurate with the higher base materials strength. Therefore, in our current research, we investigated a significant amount of STS data of the professional literature and our own experiments and recommended a new formula to predict the STS of RSWed high strength steel joints. The proposed correlation gives a better prediction than the other formulas, not only in the ultra-high strength steel range but also in the lower steel strength domain.

Flexural behavior of a novel high-strength RCFST column-to-column connection

Brittle fracture is one of the principal disincentives to limit the use of ultrahigh-strength steels in tall buildings. This paper proposes a novel column-to-column connection without full penetration welding in reinforced concrete-filled steel tubular composite columns. The paper describes an experimental investigation into the flexural behavior of the proposed RCFST column-to-column connection, in which the steel was manufactured using ultrahigh-strength steel grades H-SA700 and USD 685. Four specimens with varying configurations of reinforcing steel bars (separated type or gathered type) and column shapes (square or circular) were tested under four-point bending to evaluate the failure modes, flexural capacity, and deformation capacity. The results show that the gathered type of configuration of reinforcing steel bars can effectively improve the flexural capacities while having a negligible effect on the strain distribution of steel tubes or steel bars. Besides, the column type was found to significantly influence the strain distribution of the steel tube. The design formulae show accuracy and reliability and could be applied to assess the yield and ultimate strength of the proposed new connections.

The Effect of Ultra-Fast Heating on the Microstructure, Grain Size and Texture Evolution of a Commercial Low-C, Medium-Mn DP Steel

The effect of ultra-fast heating on the microstructures of steel has been thoroughly studied over the last year as it imposes a suitable alternative for the production of ultra high strength steel grades. Rapid reheating followed by quenching leads to fine-grained mixed microstructures. This way the desirable strength/ductility ratio can be achieved while the use of costly alloying elements is significantly reduced. The current work focuses on the effect of ultra-fast heating on commercial dual phase grades for use in the automotive industry. Here, a cold-rolled, low-carbon, medium-manganese steel was treated with a rapid heating rate of 780 °C/s to an intercritical peak temperature (760 °C), followed by subsequent quenching. For comparison, a conventionally heated sample was studied with a heating rate of 10 °C/s. The initial microstructure of both sets of samples consisted of ferrite, pearlite and martensite. It is found that the very short heating time impedes the dissolution of cementite and leads to an interface-controlled α → γ transformation. The undissolved cementite affects the grain size of the parent austenite grains and of the microstructural constituents after quenching. The final microstructure consists of ferrite and martensite in a 4/1 ratio, undissolved cementite and traces of austenite while the presence of bainite is possible. Finally, it is shown that the texture is not strongly affected during ultra-fast heating, and the recovery and recrystallization of ferrite are taking place simultaneously with the α → γ transformation.

Property Optimization in As-Quenched Martensitic Steel by Molybdenum and Niobium Alloying

Niobium microalloying is the backbone of modern low-carbon high strength low alloy (HSLA) steel metallurgy, providing a favorable combination of strength and toughness by pronounced microstructural refinement. Molybdenum alloying is established in medium-carbon quenching and tempering of steel by delivering high hardenability and good tempering resistance. Recent developments of ultra-high strength steel grades, such as fully martensitic steel, can be optimized by using beneficial metallurgical effects of niobium and molybdenum. The paper details the metallurgical principles of both elements in such steel and the achievable improvement of properties. Particularly, the underlying mechanisms of improving toughness and reducing the sensitivity towards hydrogen embrittlement by a suitable combination of molybdenum and niobium alloying will be discussed.

Microstructural Characterization of Ultra-High Strength Martensitic Steels

Abstract In the development of steels with increasing strength and toughness, ultra-high strength steel grades with a martensitic microstructure are gaining more importance. From the martensitic transformation product, however, strength-specific quantities derived from the former austenite grain cannot be accessed conveniently. A profound analysis of the prior austenite microstructure, its size distribution and aspect ratio is essential in order to allow conclusions on the mechanical properties. This paper presents an etchant to reveal the prior austenite grains of thermomechanical processed and press hardened martensitic steels. Furthermore, alternative detecting methods such as electron backscatter diffraction (EBSD) and high-temperature laser scanning confocal microscopy were evaluated. It was found that a picric-acid etchant in combination with a prior tempering treatment of the steel enables the visualization of prior austenite grains and their elongation in a micrometer-scale.

Analysis of hydrogen diffusion and trapping in ultra-high strength steel grades

The transport behavior of hydrogen in ultra-high strength steel grades (UHSS) has been analyzed by several test and evaluation methods. In particular, permeation and desorption measurements have been performed to evaluate material specific parameters such as the effective diffusion coefficient, the reversible trap density and the reversible trap activation energy. Subjects of this study were a dual phase steel grade (DP) with a ferritic-martensitic microstructure and a martensitic steel grade (MS). The results of the permeation measurements indicate that the influence of irreversible traps might be negligible for the investigated UHSS compared to other impact factors. The evaluated reversible trap densities were some orders of magnitude higher than those known for pure iron reflecting the more complex microstructure. The major influence on hydrogen trapping is attributed to reversible traps like grain boundaries and dislocations based on the results of desorption measurements.

Electroslag Refining of CrNiMoWMnV Ultrahigh-Strength Steel

Increasing demands for ultrahigh-strength steels in commercial as well as military applications have raised interest in finding alternatives to the high-cost high-alloyed steel and super-alloys currently used, e.g. the use of economic low-alloy compositions processed via low-cost air induction melting and electroslag refining (ESR). In this work the yield of alloying elements and the removal of the impurities nitrogen, sulphur and phosphorus as a result of electroslag refining (ESR) in a newly developed CrNiMoWMnV ultrahigh-strength steel (UHSS) have been studied in relation to their activities in the molten metal pool. Six experimental heats of CrNiMoWMnV UHSS with different chemical compositions were designed, melted in an induction furnace (IF) and refined using ESR. This was followed by hot forging of the ingots at 1100°C to 950°C. ESR using a CaF2-CaO-Al2O3 slag system led to a high yield in Cr, Ni, Mo, W, Mn and V, while the yield of Si is low. The desulphurization of all six UHSS grades was pronounced with most of the sulphur removed either to the slag or by gas reactions. The degree of dephosphorization was only 5% irrespective of the steel composition. On the other hand, denitrification (removal of nitrogen) was achieved. It ranged from 8% to 63% depending on the steel composition. The yield of the alloying elements and removal of impurities from the steel during ESR depends on the chemical and physical properties of the ESR slag and the activity of the elements in the molten state, taking into account elemental interactions.

Hydrogen embrittlement of high strength steel under in situ corrosive charging conditions and tensile load

It is known that advanced and ultra-high strength steel grades (AHSS and UHSS) are susceptible to hydrogen embrittlement under certain test conditions [1, 2]. In this study, punched hole specimens for constant load testing with in situ hydrogen charging were used to quantify the effect of different corrosive conditions. Flowing or stagnant electrolyte (5% NaCl) conditions were established and time to failure was recorded as a function of applied tensile stress. The pH of the solution was set at 2, 7, and 12. Bare and zinc-galvanized (EG) specimens were used. Hydrogen uptake as a function of immersion time was characterized.

Blank Hydroforming Using Granular Material as Medium-investigations on Leakage

For high and ultra-high strength steel grades a forming at elevated temperatures is advantageous due to higher forming limits and reduced flow stresses resulting in low process forces. For highly complex parts out of these materials hydroforming is an appropriate production method allowing undercuts and homogeneous stretching of the material. In case of forming temperatures above 350°C typical liquid forming media no longer can be used and gases are in the focus of interest. The big disadvantage of blank hydroforming at elevated temperatures results from the increase of friction coefficient by temperature and the need of sealing the cavity by high contact pressures in the flange area of the part. To overcome this challenge a third class of media, granular material like sand or small ceramic spheres can be used as forming media. For investigations of the process limits, especially the sealing limit of granular material, an experimental tool was built up.

Thermomechanically Hot Rolled High and Ultra High Strength Steel Grades - Processing, Properties and Application

In all application fields for hot rolled strip products for direct processing, e.g. construction and engineering but also crane and truck industry, there is a strong customers demand for grades with increased strength levels and well balanced formability. At voestalpine Stahl GmbH these requirements were met with the development of the high strength microalloyed steel grade ALFORM700M and the ultra high strength steel grade ALFORM900M with a minimum yield strength (YS) of 700 MPa and 900 MPa, respectively. In the present paper investigations on the steel grades ALFORM700M and ALFORM900M are introduced. To explain the evolution of the obtained complex microstructures consisting of bainitic ferrite, bainite and martensite alloy design and industrial production process is discussed on the base of dilatometric experiments and TEM investigations. The formation of precipitates is studied by using a numerical model, chemical methods and mechanical testing after heat treatment. Mechanical and mechanic-technological properties of the two steel grades are compared. Furthermore, some processing aspects as weldability an bending behaviour are highlighted. Finally, some typical applications for this high and ultra high strength steel grades are presented.