Papers by Keyword: Dual-Phase Steel

Abstract: Gaining a better understanding of the structure-property relationship in materials is a vital step in optimizing forming processes in order to minimize the induced damage and thereby maximizing the materials’ performance.Dual phase (DP) steels are comprised out of hard martensite surrounded by a soft and ductile ferrite matrix. Due to the complex microstructure of DP-steels, different mechanisms of damage initiation can occur, such as martensite cracking or ferrite-martensite phase boundary decohesion. A key problem with computational microstructure optimization focusing on one specific damage mechanism is, that this can lead to virtual microstructures, which are good against one mechanism, but vice-versa problematic for another mechanism. This is why all optimization strategies have to consider more than one mechanism. In this study, a multi-objective Bayesian optimization (moBo) approach is developed for the design of damage-tolerant DP-microstructures. It combines full-field crystal plasticity simulations on 3D representative volume elements with computational optimization. By employing the moBo, the sets of microstructure parameters are determined, where the combined minimum of both damage indicators is located. The proposed algorithm was applied to identify pareto-optimal microstructure configuration for DP800, considering both prevalent damage mechanisms It also provides an estimate of the variance associated with each parameter, which defines how critical the correct regulation of that aspect is. The results are in line with prevailing knowledge about DP steel, thus showing that the proposed approach is a promising tool for computational microstructure design

Abstract: This study examines electric vehicle (EV) crashworthiness with a focus on side impact scenarios affecting high-voltage (around 400V) battery packs. Using a 2001 Ford Taurus model, the research compares the performance of side door beams constructed from HSLA steel, boron steel, and Dual-Phase (DP-590) steel in crash simulations. The results indicate that boron steel significantly enhances impact resistance, minimizing battery pack damage and improving occupant safety over HSLA and DP-590 steel. The findings recommend boron steel for critical areas in EV design, with DP-590 steel emerging as an alternative option that still maintains safety standards. Future research is suggested to confirm these results through empirical testing and to investigate advanced materials for further safety improvements in EVs.

Abstract: Corrosion rate behavior of laser welded dual-phase galvanized steel, DP 600, has been assessed in comparison with the material without the laser weld, in 3.5% NaCl solution. Three combinations of both scanning speed and laser power parameters were selected, maintaining the thermal input of 30 J mm^-1, calculated as the ratio between the laser beam power [W] and the scanning speed [mm s^-1]. The corrosion studies included measurements of open circuit potential, micro and macro polarization, showing higher corrosion rates as scanning speed decreased. Optical microscopy showed the formation of a grain size refined morphology in the heat affected zone and fusion zone. A mechanism has been proposed to explain the corrosion behavior as a function of the laser parameters, which dictated the galvanized coating vaporization.

Abstract: Arc welding processes are widely used in the automotive industry among other welding processes. Consequently, laser welding technology is being used instead of arc welding due to the rapid heating and cooling characteristics of the laser. In this study, empirical investigations and comparative study are held out on the arc and laser beam welded joints of DP780 dual-phase steel. Accordingly, weld joint microstructures, hardness distribution, and fatigue properties cross the butt-welded joints were investigated. The results showed that laser beam welding produces narrow fusion and heat-affected zones while gas metal arc welding produced wide welds with incomplete penetration. It was observed that the microstructure of the laser joint weld metal has mainly lath martensite in the ferritic matrix, while microstructure of gas metal arc weld metal relies upon filler type. Heat-affected zone in DP780 steel exhibit hardness softening in both laser beam welding and gas metal arc welding due to martensite tempering, a wider softening region was clearly observed in heat-affected zone welded by gas metal arc welding than laser beam welding. Generally, fatigue ratio, fatigue limit and fatigue life of the welded joints were improved by using laser welding.

Abstract: The effect of the characteristics of austenite interface with ferrite on the pearlite transformation behaviour after intercritical annealing was investigated. Most austenite grains were situated mainly on ferrite grain boundaries and had the Kurdjumv-Sachs (K-S) or near K-S relationship to one of the neighbor ferrite grains before pearlite transformation. The pearlite transformation started mainly from the austenite grain boundary faced to ferrite. The pearlite transformation showed stasis. This indicates that some austenite is stabilized thermally against the pearlite transformation. The fraction of austenite having only the K-S or near K-S interface to neighbor ferrite grains was correspond to the fraction of austenite grains which does not include pearlite. The pearlite transformation was difficult to start from austenite interface having the K-S relationship to ferrite since the interface between austenite grains and ferrite grains was stabilized energetically in the case of their interface having the K-S relationship.

Abstract: One industrially C-Mn steel was studied in the present work. The Mn pre-partitioning process during continuous annealing was used. The results showed that the stability of retained austenite is increased because of the Mn pre-partitioning. The amount of retained austenite was about 5%, when increase the partition time to 100s. With the increase of the partition time, the amount of retained austenite increases, the tensile strength decreases, the yield strength and elongation both increase. Compared with quenching and tempering (Q&T) process, the tested steel after pre-partitioning quenching and partitioning (PQ&P) process has higher uniform elongation and the product of strength and elongation (UTS×TEL).

Abstract: Dual-phase (DP) steel sheets composed of both soft ferritic and hard martensitic phases are typical advanced high strength steel sheets applicable to a variety of automobile parts. The crystallite texture of the steel sheet is one of the important factors that influence press formability. However, the texture of the martensite itself in DP steels has not been discussed since the texture was generally measured by the X-ray diffraction method, which does not distinguish the texture of martensite from that of ferrite. The objective of this study is to investigate the effects of intercritical and γ single-phase annealing on the texture evolution in DP steels by a newly-developed analysis method using Electron Back-Scatter Diffraction (EBSD) to obtain the texture of each phase separately. The chemical composition of the steel used was 0.1%C-1.2%Si-2.3%Mn-0.1%Ti (mass%). The 1st-annealing was carried out at 948K, which is below the Ac₁ temperature, in order to finish recrystallization after hot and cold rolling so as to focus on the transformation texture evolution itself. The steels were subsequently annealed both at 1123K in the intercritical region and at 1223K in the γ single-phase region to obtain DP microstructures with approximately 40% volume fraction of martensite. The overall texture including martensite in the case of intercritical annealing was similar to the initial texture before annealing, while the texture became randomized in the case of γ single-phase annealing. Moreover, our unique EBSD analysis method clearly showed that the textures of the martensite themselves were close to those of ferrite under the two annealing conditions.

Abstract: In the present study, the effect of cold-rolling for the amount of reduction in thickness ranging from 25% to 75% on microstructure and mechanical properties of plain low carbon steel processed from dual-phase ferrite-martensite starting microstructure was studied. As the cold-rolling, the microstructure elongated to rolling direction and more compressed with increasing the rolling reduction and strength also increased. After annealing at warm temperature 500°C, the ultrafine grained was obtained in the 75% rolling reduction. Moreover, it was exhibited excellent strength of 82% and hardness of 66.1% higher than as-received condition with adequate uniform elongation 9.6%.

Abstract: Backward Ultra-fast cooling (UFC) plays a key role in low-cost dual-phase steels production. Cooling process control system for backward UFC was developed to meet the process requirements. Basing on basic theory of heat transfer, cooling control model was established to accomplish temperature calculation. To deal with the influence of technological conditions fluctuations an adaption system, including self-learning function and feedback function was proposed to intelligently realize temperature correction. The developed cooling strategy can achieve diversified cooling path control. The controlling of significant technological parameters, including cooling rate and air cooling time for dual-phase steels, was also accomplished. Furthermore, flexible and diversified UFC strategies were developed aiming at high temperature precision and low temperature deviation control in thickness direction. The system has been applied successfully in dual-phase steel production with high stability and reliability. The precision of medium temperature (MT) can be controlled within ±10°C, and the UFC delivery temperature (UFC-T) can be limited within ±30°C of the target values.

Abstract: In this article, the authors have analysed the influence of quenching temperature (T_Q) on the microstructure of a dual-phase steel with a low carbon and manganese content (0,094 % C and 0,53 % Mn). The ferrite-martensite structures, typical of the dual-phase steels, has been obtained by intercritical quenching that consisted of heating at temperatures (T_Q) ranging between 750 ^°C and 830 ^°C, maintaining for 30 minutes and cooling in water. After carrying out intercritical heat treatments, samples have been subjected to metallographic analysis through which the volume fraction of martensite (V_M), the volume fraction of ferrite (V_F), the carbon content of the martensite (C_M), the morphology and distribution of these phases have been determined, and then, the influence of quenching temperature (T_Q) has been established.