Revisiting the Concept of Property Optimization of Low Temperature Phase Transformed Microstructures of GIGASTEEL Welds
Article information
Abstract
Abstract
Although the need for automobile lightening continues to increase in preparation for the era of electrical vehicles, the application of steel, which has strengths in price and recycling in the automotive industry, is expected to continue along with the developments of various driving energy sources. Recently, when lightening and stiffness reinforcement are required for the electric vehicles or medium and large SUVs, newly-designed parts using GIGASTEEL with a tensile strength of 980MPa or higher are being developed. However, it is difficult to secure sufficient weld strength and fatigue performances when applying welding consumables that have been applied normally and ultra-high strength welding materials that have been commercialized so far. It has also limitations that are not easy to apply due to excessive manufacturing costs in the automotive industry. In this study, the application of commercial welding consumables by strength grade to GIGASTEEL was reviewed and the properties of the weldments were compared to address all of the above issues. Here, we consider the effects of C, Mn, Si, Cr, Ni, Mo and microalloying to implement interlocked nature with dense and complex microstructures of lath-like bainitic and acicular ferrite that can effectively improve both strength and toughness of GIGASTEEL welds in a more economical way.
1. Introduction
The bainite phase transformation can be explained through a displacive mechanism using the Bain distortion model1). Since the bainite transformation and acicular ferrite transformation develop competitively depending on the grain size of the parent austenite, the cleanliness of the grain boundaries, and the distribution of effective oxide inclusions2), it is necessary to review the development of process technology that considers optimal alloy design and cooling rates to control these factors.
While research and development have been actively utilizing acicular ferrite transformation as a means to effectively enhance both the strength and toughness of high-strength weld metals, recently, in the case of GIGASTEEL with a tensile strength of 980 MPa or higher, which is increasingly being applied in the automotive industry, the steel thickness is typically less than 2.0 mm3). Compared to thicker plates, the cooling rate of the weld zone is relatively slower, and due to the influence of various alloying elements, it exhibits complex microstructural characteristics, including bainite transformation. Particularly, from the perspective of bainite transformation temperature and morphology, it is necessary to comprehensively consider the effects of major alloying elements such as C, Mn, Si, Ni, Mo, and Cr, as well as the grain size of austenite and the cooling rate4-8).
In this study, in line with the aforementioned perspective, the phase transformation behavior was examined for 1.0GPa-grade hot-rolled CP (Complex Phase) steel and 1.2GPa-grade cold-rolled TRIP (Transformation Induced Plasticity) steel, focusing on the alloying elements of the weld metal and the process variables of heat input during welding. The phase transformation microstructure was analyzed in detail using SEM, EBSD, and TEM. Through this study, we aim to explore ways to control the complex microstructure of low-temperature transformed bainite and acicular ferrite in GIGASTEEL weld metals into a more fine interlocked form to improve the mechanical properties of the weld zone, as well as the potential use of retained austenite to reduce hydrogen delayed cracking.
2. Experimental procedure
2.1 Microstructural Characterization and Mechanical Tests
In this work, we evaluated tensile/bending fatigue performances and high-speed tensile properties along with analysis of microstructures via EBSD and TEM for POSCO GIGASTEEL welds, and also measured the residual stresses of the weldments with XRD (XSTRESS 3000)5-8). In order to evaluate the impact characteristics of single pass welds in terms of practical application of thin plates, a low-temperature CVN (Charpy V-Notch) impact test of butt joint welds was also conducted from 0°C to -80°C8). Gas metal arc welding has been applied for the sample preparations8).
3. Results and discussion
3.1 Microstructural Characteristics and Mechanical Properties
In general, it is quite well known that more lath-like lower bainite and acicular ferrite (AF) are beneficial for good mechanical properties for the steel weld metal1-8). In particular, large amount of alloy pick-up occurs from base metal especially using modern pulsed-current arc welding. And cooling-rate for the thin steel sheet is considerably slower than that of thick steel plate according to the Rosenthal’s equations9).
As shown in Fig. 1, the overall microstructure of complex phase GIGASTEEL weld metal appeared to be mixtures of lath-like bainite and AF (welding heat input of 3.2kJ/cm for left figures and 5.2kJ/cm for right figures in Fig. 1). Representative features of AF formation with interwoven nature of GIGASTEEL weld metal are illustrated in Fig. 2. The transformation behavior of bainite (autocatalytic nucleation)1), AF (sympathetic nucleation)2) and retained austenite with a high fraction of high-angle grain boundaries4,8) was clearly revealed. Based on our concepts for alloying design of the welding material and steel, the bainite transformation start temperatures were estimated in the range from 312°C to 335°C for the case of 0.85[C]-1.77[Si+Mn]-4.68 [Ni+Cr+Mo](wt.%), which were quite in good agreement with the microstructural features4-6).
SEM and EBSD for 1.0GPa-grade steel weld (Representative features of intergranular AF formation)4,5)
(The readers would refer to https://youtube.com/watch?v=7WgwepmWCKU&si=EDkEslBFnBQqxlSl.)
Unlike the mechanical properties of conventional weldments, our newly-developed GIGASTEEL welds showed much greater tensile strength over 1GPa, and also revealed no failure even at a tensile speed of 15m/s (54km/h)5). In addition, the weld fatigue strength was greater than those for the conventional welds6). From the XRD measurements based on the change of lattice parameters (λ=2dsinθ), compressive residual stresses were quite considerably observed (Fig. 3), which in turn, most probably enhance the weld fatigue performances due to the effect of low temperature phase transformation4-8). For the detailed measurement methods, please refer to the supplementary document in Ref.8). These results have been elucidated based on the activity of carbon (tetragonality) via ThermoCalc8) and more detailed science behind the results will be addressed to prove our hypothesis further in the future studies.
In this study, we focused on predicting the phase transformation behavior of weld metals in 1.0 GPa- grade hot-rolled CP (Complex Phase) steel and 1.2 GPa-grade cold-rolled TRIP (Transformation Induced Plasticity) steel based on their alloy compositions. We conducted a detailed microstructural analysis using SEM, EBSD and TEM to explore new alloy designs that could potentially replace Ni (as summarized in Fig. 4). The detailed chemical compositions of the welds are presented in Ref.8).
By stabilizing the prior austenite grain boundaries in the weld metal and effectively lowering the transformation temperature, we were able to control the complex microstructure of low-temperature transformed bainite and acicular ferrite in GIGASTEEL weld metals into a finer and denser interlocked structure. Additionally, we controlled the thermodynamic stability of cementite (Fe3C) and increased the fraction of retained austenite distributed in a film form along the bainitic ferrite grain boundaries8,10).
In addition, tensile tests were conducted using micro-tensile specimens prepared from the weld metal. The results showed that both the 1.0 GPa-grade hot-rolled CP steel and the 1.2 GPa-grade cold-rolled TRIP steel exhibited tensile strength improvements of 14 MPa and 86 MPa, respectively, without any reduction in elongation. Consequently, we achieved excellent ultra-high strength and high ductility simultaneously in the GIGASTEEL weld metals8).
4. Summary
This new technology is expected to be highly applicable in the production of next-generation high-performance and low-cost GIGASTEEL welded components, aligning with the era of electric vehicles11). It also offers significant environmental benefits, such as reduced carbon emissions, making it highly promising for expanded application in the automotive industry.
Acknowledgements
This work was carried out based on POSCO’s PosZET® GIGA. The authors wishes to thank the POSCO technical research laboratory for technical and financial support.