Materials Science Forum Vols. 500-501

Abstract: Six experimental low and ultra-low carbon C-Mn-Mo-Nb-B and one conventional TMCP steel heats have been prepared to study the effects of chemical composition and hot deformation on the microstructure and the strength-toughness properties. In physical simulation tests, it was found that the deformation of austenite below the non-recrystallization temperature enhances the formation of higher-temperature bainitic morphologies and polygonal ferrite. On the other hand, hardness exhibits relatively low sensitivity to the degree of deformation below Tnr, whereas the deformation results in a distinct refinement in the microstructures, as determined by SEM-EBSD measurements, suggesting an improvement in the impact toughness. Simultaneous alloying with Mo-Nb-B seemed to be most efficient to provide high hardness and strength. Hot rolling trials indicated that the yield strength in the range 500-700 MPa with the excellent toughness down to –80 °C can be achieved in low carbon (≈ 0.03%) bainitic grades.

Abstract: 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.

Abstract: In the present study HSLA steels of varying carbon concentrations, alloyed with Mn, Ni, Cr, Mo, Cu and micro-alloyed with Nb and Ti were subjected to different finish rolling temperatures from 850oC to 750oC in steps of 50oC. The microstructure of the steel predominantly shows martensite. Fine twins, strain induced precipitates in the martensite lath along with e-Cu precipitates are observed in the microstructure. With an increase in carbon content the strength value increases from 1200MPa UTS to 1700MPa UTS with a negligible reduction in elongation. Impact toughness values of 20-26 joules at room temperature and −40oC were obtained in sub-size samples.

Abstract: This report introduces a new deoxidation process that starts with a vacuum degassing followed by the addition of ferro-niobium in the same vacuum facility. The addition of strong deoxidation substance like aluminum and/or silicon is not adopted. Owing to the degassing in vacuum, which in reality reacts as deoxidation with C in molten steel, the solute oxygen in molten steel (O) is stabilized by niobium, which generates an ample number of fine oxide particles composed of (Nb, Mn)2O3-phase. Provided the carbon-degassing is conducted so as to reach the O prior to the FeNb addition would be around 100ppm and less, these oxide-inclusions do not agglomerate before and during solidification, because of the weak cohesivity of niobium-based oxides in molten steel, which would enable defect-free cast products as well as clogging-less casting operation. Also, the steel containing these fine (Nb, Mn)2O3-particles but no Al, exhibits so-called intra-granular acicular ferrite transformation and good toughness of weld heat affected zone (HAZ).

Abstract: This paper deals with the development of low carbon NbTiB micro-alloyed high strength low alloy steel for heavy plates with high wall thickness. In the production of heavy plate it is remarkably difficult to achieve a combination of high strength and good low-temperature toughness. Bainitic microstructures have shown the capability to attain such requirements. To achieve a bainitic microstructure even for heavy wall products the formation of bainite can be promoted and supported by the use of small amounts of boron as a micro-alloying element. This industrial research project is based on the addition of small amounts of boron to promote the desired bainitic structure. Mill rolling trials were carried out to determine the optimum process parameters. The results of experimental mill rolling trials on 35 mm plates will be presented in this paper.

Abstract: Steels with bainitic microstructures show the capacity to fulfil the requirements of high strength and low temperature toughness necessary for plate steels in specialised industrial constructions. The introduction of steels with higher strength allows for weight reductions of steel constructions. This paper investigates the development of hot rolled structural plate steels through laboratory hot rolling simulations of thermo-mechanically controlled processes (TMCP). Specific alloying and microalloying along with an optimised TMCP process has allowed high tensile properties to be achieved in combination with high levels of toughness. Tensile strengths of up to 900 MPa have been achieved with Charpy V-notch toughness greater than 200J at –40°C. Elements such as molybdenum, niobium and boron have been added to low carbon steels to promote the formation of fully bainitic microstructures with much lightened chemical compositions. The presented concepts allow the production of steel grades above S500 up to S690.

Abstract: Effect of the severe deformation by multi-pass rolling on microstructure and tensile properties was analyzed in terms of rolling temperature, plate thickness, and cooling rate for a modified API X65 steel containing B. The plates, 80 and 50 mm thickness, were rolled six times by 20%/pass (total 75%) to 20 and 12 mm, at 1023 K of unrecrystallized γ region or 973 K of intercritical (α+γ) region, and then quenched in water or oil. All specimens except one oil-quenched condition showed relatively high UTS 700-830 MPa and the continuous yielding(YR～0.6), typical mode of the (ferrite + martensite (bainite)) dual phase microstructure. In contrast, one oil-quenched specimen with the 973 K-20 mm condition, exhibited the discontinuous yielding (YR～0.8), indicating that the microstructure basically consists of ferrite plus pearlite, as well as a relatively low UTS 660 MPa. The degree of deformation really occurring within materials, i.e., strain hardening seems to be enhanced with a decrease in deformation temperature. As the degree of deformation increases, the remaining austenite, not dynamically transformed to fine ferrite, becomes increasingly unstable. A lower hardenability of this remaining austenite thus would lead to a higher possibility to transform into the (ferrite + pearlite) structure of lower strength rather than the (ferrite + martensite (bainite)) of higher strength.

Abstract: The effects of adding vanadium (up to 0.1%) and niobium (0.02%) to a steel containing 0.07%C – 1.5%Mn – 0.29%Si – 0.035%Al – 0.01%N have been studied in the laboratory. It was determined that a steel containing 0.05%V had a yield stress, after accelerated cooling, greater than 480 MPa and toughness, as measured by U-notch and Vee-notch impact specimens, improving as the finish rolling temperature was lowered from 950oC to 750oC. Consequently, a semi-industrial trial, involving the manufacture of a steel containing 0.07%V, rolled to 22 mm and 32 mm thick, narrow plates, was carried out. This demonstrated that after finish rolling at 750oC, followed by air cooling, the steel had a yield strength in excess of 470 MPa accompanied by excellent toughness. The work demonstrates the potential for achievement of high levels of strength and toughness in heavy plate steels, micro-alloyed with vanadium.

Abstract: One of the components required to successfully produce high strength pipeline steel is to optimize precipitation strengthening. Some high strength pipeline grades rely on increased levels of Nb; in these grades, it is important to ensure that all the Nb is effectively employed. It is generally accepted that choice of coiling temperature is critical in maximizing the Nb precipitation in ferrite. Additional control of this precipitation may be attained by deformation at these coiling temperatures, an approach termed ‘cool deformation’. In this work, steel specimens were heated to temperature of 1200°C and held at temperature for 20 minutes to ensure significant dissolution of Nb precipitates. Some specimens were aged at 400°C for times ranging from 10 minutes to 10 hours followed by air-cooling. Others were subjected to deformation at 400°C (‘cool deformation’) prior to aging. It was found that the cool deformation improves the mechanical properties by microstructure; both yield and tensile strengths are significantly higher than that of the aged only specimens. By using low voltage imaging on a field emission gun scanning electron microscopy (FE-SEM), precipitates were observed and identified. The effects of the thermal and cool deformation schedules on the precipitate characteristics are described in this paper.

Abstract: The effect of deformation temperature (800-1000°C) - strain accumulation (number of rolling passes: 3÷5) - cooling rate (8,6 – 25°C/s) parameters on the microstructure and mechanical properties of C-Ni-Nb and C-Ni-V steels after High Temperature Thermomechanical Processing (HTMP) are presented. The experiment planning method was used for the simulation and examination of the above mentioned parameters effect on the mechanical properties. The regression equations and diagrams describing the qualitative and quantitative effects of HTMP parameters on the mechanical properties of the steels are derived from the experimental results. The Ni-Nbbearing steel with the lower bainite structure that is a result of the HTMP with the maximum number of passes, the highest deformation temperature and cooling rate shows the highest strength (YS=987 MPa) in combination with a high impact strength (W=154 J). It is shown that the Ni- bbearing steel with the fine lower bainite structure that results from the transformation of the hot deformed fragmented substructure of austenite after water cooling has the best resistance to brittle fracture. The Nb-bearing steel has a higher strength compared to the V-bearing one, although it shows a lower impact strength in the negative testing temperature range.