development of advanced high strength steels by ...

XX Congress of Chemists and Technologists of Macedonia V Congress of the Metallurgists of Macedonia _______________________________________________________________________________________

DEVELOPMENT OF ADVANCED HIGH STRENGTH STEELS BY PHYSICAL SIMULATION AND LABORATORY TESTING Mahesh Somani1, Pasi Suikkanen2, Atef Hamada3, Pentti Karjalainen4 e-mail: [email protected] 1-2-3-4 University of Oulu, Oulu, Finland

As a response to the demand of high-performance steels, processing of advanced steel types such as DP, TRIP, CP and TWIP, and high-strength low-carbon bainitic and martensitic DQ-T steels, has been developed based on physical simulation and modelling studies. Physical simulation has been used by employing a Gleeble thermo-mechanical simulator to reveal the phenomena occurring in the hot rolling stage (strain hardening, flow resistance, recrystallization kinetics and microstructure evolution), and in the cooling stage (CCT diagrams). Connecting these data with microstructures examined in optical and electron microscopes and resultant mechanical properties have improved the understanding of those phenomena and the role of numerous process variables in the optimization of enhanced mechanical properties. Based on a well-designed experimental plan, new regression models have been developed for the start of bainite/martensite transformation temperature and also for the hardenability for boron-containing steels for the direct quenching route. Processing and properties of Ti-microalloyed TRIP steels were studied. For multi-phase steels, realtime phase fractions as a function of heating rate and holding in the intercritical annealing for optimized processing have been estimated, and for TWIP steels, high-temperature deformation and recrystallization have been investigated. Keywords: advanced steels, strength, toughness, physical simulation, modelling INTRODUCTION An ever-increasing demand for developing advanced high strength steels (AHSS) with excellent combinations of mechanical properties such as high strength, ductility, impact toughness and formability has led to extensive research worldwide for optimal design of new compositions and/or thermo-mechanical controlled processing (TMCP) routes. As an example, the European Commission has outlined in the Steel Technology Platform (ESTEP) its vision for the year 2030 aiming at highly enhanced combinations of high tensile strength with excellent ductility for steel sheets used in automotive applications [1]. A twin-pronged approach directed at developing newer compositions and/or improving the processing routes for both the bcc-lattice-based high strength steels (such as DP, TRIP, CP, bainitic and martensitic steels) as well as fccaustenite-based second generation steels (e.g., cold-strengthened austenitic stainless and TWIP steels) has been adopted at the University of Oulu in collaboration with its industrial supporters and/or under the framework of the research programmes of the European Commission (such as those funded by the Research Fund for Coal and Steel). A Gleeble thermo-mechanical simulator has been suitably employed to simulate and comprehend the microstructural phenomena occurring during different thermo-mechanical processing stages for optimization of processing parameters and data acquisition for modelling, e.g., the constitutive flow behaviour, restoration characteristics and kinetics, microstructural evolution and influence of alloying elements and/or controlled deformation on phase transformation characteristics and hardenability. Some examples of the recent approaches to the optimal design and processing of these high strength steels using physical simulation and modelling are briefly described in this paper.

XX Congress of Chemists and Technologists of Macedonia V Congress of the Metallurgists of Macedonia _______________________________________________________________________________________ Development of direct-quenched martensitic steels Direct quenching (DQ) of low carbon steels following TMCP is an effective way of producing high strength steel plate products, e.g., [2,3]. The higher strength of DQ plates compared to reheat-quenched (RQ) plates has been attributed to the refinement of the martensite structure and an increased dislocation density of the martensite [4]. Precipitation of finer carbides in heavily dislocated laths during tempering further enhances the strength. Improved toughness in DQ steels has been attributed to the shortening and randomization of martensitic laths [5]. In direct quenching, the critical cooling rate for obtaining 100% martensite is of interest and phase constituents other than martensite can also be facilitated thus opening up the possibilities of multiple-constituent steel [6]. For the formation of bainitic-martensitic microstructures, Ar3 and Ar1 temperatures together with the resultant hardness levels are useful. In order to produce quenched martensitic or bainitic-martensitic plate mill products (the yield strength level of 690 MPa at the minimum), a new direct quenching equipment has been installed in Finland. Meanwhile, research investigations have been intensively pursued at the University of Oulu with the company to design newer compositions for developing high strength steel plates with enhanced mechanical and technological properties through controlled rolling and direct quenching and tempering route (DQ-T). The aim is to reduce the cost by controlling the alloying additions and designing cost-effective processing routes through physical simulation and modelling. For the investigations of the effects of a wide range of alloying elements on phase transformation temperatures and hardenability under direct quenching conditions, an orthogonal Taguchi L8 experimental matrix was adopted [7]. The elements chosen were C, Mn, Cr, Ni, Mo, Nb and V, which were alloyed at two levels. In wt %, these levels were C (0.1, 0.2), Mn (0.75, 1.5), Cr (0.2, 1.0), Ni (0, 1.0), Mo (0, 0.5), Nb (0, 0.03) and V (0, 0.05). All steels contained constant additions of B (∼30 ppm), Ti (∼0.033%) and Si (∼033%). Only the main linear effects of the alloying elements were considered, thus ignoring the interactions between them. Alloy factor (AF) is a measure of overall level of alloying that is used to calculate Jominy curves in the ASTM standard A 255. For the low carbon matrix steels, AF ranged from 6 to 65, and for the high carbon steels from 15 to 45. Four other steels were also included in the study taken from industrial plates or sheets, having compositions within the ranges covered by the L8 matrix. CCT diagrams covering cooling rates 1.5 – 48°C/s were determined with the aid of dilatation measurements using Gleeble simulations with or without controlled deformation below Tnr. In the case of straining, samples were processed through a complex schedule, aimed at simulating the controlled rolling before continuous cooling at various linear rates, as given in Fig. 1. The cooling rates of greatest interest for direct quenching are in the range 12 – 48°C/s. A detailed summary of the CCT diagrams obtained for all the steels is presented elsewhere [8]. As an example, the CCT diagram plotted from the dilatation data for the matrix steel 8 is shown in Fig. 2. Furthermore, the start of the bainite transformation temperature was suitably modelled as a function of chemical composition and cooling rate, as also seen in Fig. 2.

Fig. 1. A typical test plan for dilatation experiments – tests with the prior deformation.


Fig. 2. CCT diagram of Steel 8 plotted by employing the data estimated from the dilatation tests and also predicted curves for the start of bainite formation. (Abbreviations: F = ferrite, B = bainite, M = martensite). Hardness values are also given. In the quenching of the plate, it is important to be able to predict the composition required to achieve the theoretical maximum hardness at the centre-line of the plate. Hardenability models can be developed from the Jominy curves or ideal critical diameters (DIB), which can be estimated by the procedure given in the ASTM standard A 255–89 (re-approved 1994) [9]. However, in the case of direct quenching, Jominy models are relatively inappropriate for boron steels, as the efficacy of boron on hardenability is determined by the concentration and distribution of boron at grain boundaries, which is further influenced by the TMP history and state of the austenite (grain size, shape and dislocation structure) [10,11]. Results also revealed that the mean values of DIB predicted using the ASTM formulae are generally larger than those obtained from the CCT data. It was inferred that present steels with low-temperature straining of the austenite leads to higher hardenability than simple quenching from a higher temperature. A thorough analysis indicated that chromium and especially molybdenum may be less effective at increasing hardenability than given in the ASTM standard. In modelling, non-linear regression of DIB data estimated for different matrix and industrial steels was carried out by dynamically varying both the alloy factor (AF) and the factor of boron (BF) as using MatLab software. It also seems that the straining prior to quenching is affecting both the AF as well as BF, if considering the factor for carbon to remain unchanged. Processing of low carbon bainitic steels The quest for higher strength structural steels with excellent toughness, weldability and formability have led to the development of modern bainitic steels for pipeline and offshore structures, and suitable for mobile vehicles, too. A brief account of the preliminary successful trials to develop low and ultra-low C bainitic steels (ULCB; in wt.%; 0.01-0.03C, 0-0.1Cr, 0.2-0.4Mo, 0.03-0.1Nb, 0-0.002B) at the University of Oulu has been presented in detail elsewhere [12, 13]. Based on the stress relaxation and dilatometric testing conducted on the Gleeble simulator, parametric optimisation was carried out to achieve grain refinement in the hot rolling regime, followed by controlled rolling below the non-recrystallization temperature and cooling at desired rates in order to achieve optimum strength-impact toughness combinations. Fig. 3a shows the recrystallization kinetics for a large number of bainitic steels (compared to conventional TMCP steels) and Fig. 3b a typical CTT diagram for a bainitic steel that is quite insensitive to the cooling rate. For example, it has been shown that deformation below Tnr results in refined bainitic microstructures and consequently improved impact toughness. Microstructures have been extensively examined by SEM and SEM-EBSD. At the moment a project has been initiated with the University of Pittsburgh, where bainitic microstrures will be examined by SEM-EBSD and analysed by the Image Quality technique to distinguish between various bainitic structures on the basis of their dislocation density, as described in a recent paper [14]. Quantitative characterization of complex microstructures is a target.


a)

b) Fig. 3. Recrystallization rate (t50 time) for bainitic steels (a) and a typical CTT diagram (b).

Based on extensive physical simulation and laboratory rolling exercises during recent years, product development has led to new commercial high-strength structural steels for both the plate and strip mills. In the case of plate mill, using the very low carbon approach, it has been possible to develop new bainitic steel PC F500W with a yield strength of 500 MPa. The steel has found application in off-shore structures in the Arctic Sea. The increased demand for structural steel with a minimum specified strength of 690 MPa has resulted in the development of another low-carbon bainitic steel S700ML as an alternative to the quenched and tempered steels, such as S690Q/QL, with good toughness and weldability properties [15]. Development of TRIP steels The potential of steel as light-weight material was further demonstrated in the development of low alloyed TRIP steels. The phase fractions of about 50% ferrite, 35-38% bainite and 12-15% austenite are desired. In a research program for TRIP steels, both with and without microalloying, extensive simulation experiments were carried out on the Gleeble simulator according to test schedules commensurate with possible industrial processing routes. The processing parameters can only be controlled and optimised by appropriately simulating the industrial process. Fig. 4 shows an example of the variation of incremental strain hardening exponent with true strain for a Timicroalloyed TRIP steel processed through different test schedules, revealing greater austenite stability for the specimen processed through the schedule Cycle-7, which involved reheating to a relatively high intercritical annealing temperature (820°C), compared to those of Cycle-1 (780°C) and Cycle-3 (800°C), prior to holding at a lower temperature in the bainitic regime (460°C). This is in accordance with the results in the literature [16]. Besides, the size and distribution of the austenite grains are considered extremely important for the strain hardening capability, particularly for microalloyed TRIP steels. The corresponding microstructure shown in Fig. 5 illustrates more evenly distributed, extremely fine (≤1 µm), C-enriched austenite constituents (shining white) in the matrix.


Fig. 4. Variation of the incremental strain hardening exponent (n) with true strain for the 0.12% Timicroalloyed TRIP steel.

Fig. 5. Typical microstructure of the 0.12% Ti-microalloyed TRIP steel following one specific simulated cycle. Development of CP steels In response to the recent requirements for improving the forming properties of an industrial TS800 CP steel (C=0.17%, Si+Mn+Cr=2.2%, Mo=0.2%) elaborate simulation experiments were planned to establish proper annealing conditions in the intercritical regime. Fig. 6 shows a typical example of the complex test schedules employed for the simulation studies. For the prediction of kinetics of austenite formation during heating and subsequent soaking, the specimens were heated at different rates in the range 4-25°C/s to a temperature in the intercritical range (720-880°C), followed by holding for 10 min to reveal the effect of holding times at different temperatures (740-880°C).

Fig. 6. A typical simulated processing route for TS800 CP steel.

XX Congress of Chemists and Technologists of Macedonia V Congress of the Metallurgists of Macedonia _______________________________________________________________________________________ As expected, the elongation decreased slightly with the increase in the bainite fraction, even though the data are quite scattered (Fig. 7). A detailed summary of the development work concerning the CP steels, including the dilatometric investigations, and mechanical and forming properties, is presented elsewhere [17].

Fig. 7. Variation of the yield ratio (Rp0.2/Rm) and uniform elongation (Ag in %) with the bainite phase fraction. Processing of TWIP steels There is an interest in high-Mn steels with austenitic microstructures due to their potential for applications in automotive industry owing to their very attractive strength-ductility combinations, derived from the twinning induced plasticity (TWIP) effect [18]. For instance, the tensile elongation up to 90% can be achieved at the tensile strength level of 800-1000 MPa [19]. Extensive investigations have been carried out at the University of Oulu dealing with the manufacturing route of these steels [19-22]. Special emphasis has been put on the high-temperature constitutive flow behaviour affected by the Al alloying (0-8 wt.%), recrystallization characteristics, and the evolution of microstructure in tests conducted on the Gleeble simulator. Typical flow curves are displayed in Fig. 8 revealing the influence of Al alloying and that the flow resistance is quite high compared to that of carbon steels and the Cr-Ni austenitic stainless steel (AISI 304) [20].

Fig. 8. Flow stress curves for various TWIP steels compared to those of some conventional steels. Acknowledgement The financial support from Research Fund for Coal and Steel, the Finnish Funding Agency for Technology and Innovation, TES, as well as from Rautaruukki Oyj steel company is acknowledged with gratitude.

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