Use Of Ultra High Strength Steels Research Articles

Ultra-high strength concrete filled steel tube members: Classification, experimental database, and design

Current design codes do not endorse the use of ultra-high strength steel and concrete materials (Fy ≥ 690 MPa and f’c ≥ 120 MPa) for concrete filled steel tube (CFST) members. This paper makes efforts to address this gap using a four-step method. In the first step, the classification of CFST members based on material strengths and tube slenderness ratio is introduced. In the second step, an experimental database comprised of 805 ultra-high strength concrete filled steel tube (UCFST) specimens under various loading conditions is compiled, and the research gaps in the database are identified. In the third step, the possibility of extending Eurocode 4 and AISC 360–22 to UCFST members are evaluated using the experimental database. Finally, a research program including experimental tests, finite element (FE) parametric studies and new design equations for UCFST rectangular members is introduced.

Development of design equations for ultra-high strength steel CHS columns under transient fire conditions considering thermal creep and axial restraint effects

Despite their potential for widespread structural applications, the use of ultra-high strength steel (UHSS) members in buildings is currently hindered by the lack of suitable design relationships for these components. In response to this technical gap, this paper presents design equations for Grade 1200 UHSS circular hollow section (CHS) columns under transient fire conditions. The design relationships are derived from an extensive parametric study on the in-fire behavior of UHSS tube columns using advanced finite element (FE) simulations in ABAQUS software. However, to verify the FE modeling framework, experiments are also conducted on axially-compressed UHSS tube specimens. The thermal creep of the UHSS tubes is explicitly included as a user-defined subroutine in the modeling by a robust creep model exclusively developed for the UHSS material under transient fire conditions. In addition to the utilization factor, the effects of non-dimensional slenderness, heating rate (or thermal creep), and axial restraint stiffness ratio are also studied and considered in the design equations obtained for the UHSS CHS columns. Finally, it is shown that the present design relationships can predict the columns' buckling and failure temperatures with a maximum absolute error (MAE) of 4%.

Column base joint made with ultrahigh-strength steel bars and steel tubular: An experimental study

The use of ultrahigh-strength steel in civil, bridge, and offshore engineering is increasing rapidly. In this study, the seismic behavior of a concrete-filled steel tubular (CFST) column base joint with implanted ultrahigh-strength reinforcing bars has been investigated. The steel tubular and the implanted reinforcing bars are made of ultrahigh-strength steel. Four column base joints with square and circular cross-sections and with 0 and 0.25 axial force ratios have been tested to discuss the hysteretic capacity, deformation, ductility, failure modes, and energy dissipation. The axial force is the main parameter to improve the flexural capacity of the column base joints. Owing to the slip between the infilled concrete and the steel tubular columns, the hysteresis loops show an obvious pinch phenomenon. The tested specimens demonstrate good ductility behavior. Finally, the flexural capacity of the CFST column base joint has been determined by analyzing the failure characteristics and stress of the CFST columns. The flexural capacity of the CFST column base joints determined by the calculation formulae matches well with the test results. The ratio of the calculated and tested flexural capacity is between 0.967 and 1.059, and the absolute relative error is between 0.131% and 5.595%.

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.

An Infrared Local-Heat-Assisted Cold Stamping Process for Martensitic Steel and Application to an Auto Part

The automotive industry has tried to employ ultra-high-strength steel (UHSS), which has a higher strength with a thinner thickness. However, because of its low formability, there is a limit to the use of UHSS in industrial applications. Even though the hot-press-forming method can resolve the formability problem, elevated-temperature conditions lead to side effects—heat transfer and productivity issues. This work presents the concept of an infrared local-heat-assisted cold stamping process. Before the forming process, local areas, where the formability problem occurs, are locally heated by the gathering of infrared rays and cooled to room temperature before delivery. Since the heat treatment is completed by the material supplier, the stamping companies can conduct cold stamping without new investments or the productivity issue. In this work, a heat-assisted cold V-bending test was conducted with a martensitic (MS) 1.5 GPa steel, the CR1470M steel provided by POSCO. The heating effects on the microstructure, hardness, and local ductility were also observed. Finally, a commercial door impact beam was successfully manufactured with the present method. In this application, only a targeted small area was heated. The results show that the present method can improve the formability and springback problems of MS steel in the stamping process.

Slenderness limits for fabricated S960 ultra-high-strength steel and composite columns

Ultra-high-strength steel (UHSS) has been gaining increasing attention globally due to its significant structural and economic benefits. Specifically, the use of UHSS in steel and composite column sections can reduce the dimensions and self-weight of the structural components. However, the utilisation of UHSS slender sections also renders the structural members more susceptible to local buckling. The reduced ductility as well as less strain hardening of UHSS can also affect the post-buckling load. In order to clarify the local buckling and post-buckling load of compressive members using UHSS plates, the authors herein present an extensive experimental program. A total of sixteen steel and composite specimens are tested with the slenderness limits (λey) and effective widths (be/b) being identified. Finite element models which account for the effects of residual stress and initial geometric imperfection are developed to capture the buckling behaviour of the box and I-section columns. The enhancement of local buckling and post-local buckling load of UHSS composite sections due to the presence of infilled concrete is demonstrated. In addition, the slenderness limits and effective widths of both bare steel sections and composite sections incorporating UHSS plates are further compared with existing codes of practice. Corresponding design recommendations are consequently provided.

Approaches for springback reduction when forming ultra high-strength sheet metals

Nowadays, the automotive industry is challenged constantly by increasing environmental regulations and the continuous enhancement of standards with regard to passenger's safety (NCAP, Part 1). In order to fulfil the aforementioned requirements, the use of ultra high-strength steels in research and industrial applications is of high interest. When forming such materials, the main problem results from the large amount of springback which occurs after the release of the part. This paper shows the applicability of several approaches for the reduction of springback amount by forming of one hat channel shaped component. A novel approach for springack reduction which is based on forming with an alternating blank draw-in is presented as well. In this investigation an ultra high-strength steel of the grade DP 980 was used. The part's measurements were taken at significant cross-sections in order to provide a qualitative comparison between the reference geometry and the part's released shape. The obtained results were analysed and used in order to quantify the success of particular approaches for springback reduction. When taking a curved hat channel shaped component as an example, the results achieved in the investigations showed that it is possible to reduce part shape deviations significantly when using DP 980 as workpiece material.

Influence of the base Material Strength and Edge Preparation on the Fatigue Strength of the Structures Made by High and Ultra-high Strength Steels

In recent years the high and ultra-high strength steels have been developed and applied for light-weight constructions such as the structural members of mobile equipment in order to reduce weight and fabrication costs as well as to contribute to the performance. In many applications, especially in the use of ultra-high strength steels, the welds are tended to locate outside of the highly stressed regions and use the post weld improvement for the critical welds. This means that the non-welded sections are in load-carrying positions and have to take into the consideration in the design of the structures.Novel test results have shown that the fatigue strength of the base material increases by increasing the yield strength of steel and the maximum fatigue class in the design codes FAT 160 and a slope, m=5, is conservative when the yield strength of steel is equal and higher than 355N/mm2. Only standard EN 13001-3-1:2012 allows applying for the higher fatigue classes by increasing the yield strength in cranes.In this paper the fatigue test results from high and ultra-high strength strip rolled steels are presented and compared with the fatigue classes in standard EN 13001-3-1:2012. Also a cross-member brace connection to main chord flange in vehicle chassis is presented as a design example.