TRIP assisted press hardened steel by the anisothermal bainitic ferrite transformation

https://doi.org/10.1016/j.jmatprotec.2020.116950Get rights and content

Highlights

  • New manufacturer friendly TRIP assisted steel chemistry.

  • New economical press hardening process.

  • New TRIP assisted Press Hardened Steel parts exhibit uniform austenite distribution.

  • Stress induced transformation of austenite in cold formed regions of parts is avoided.

  • 22 % increase to energy absorption under axial crushing compared to cold formed parts.

Abstract

A new steel chemical composition is combined with a new press hardening process, in which die-quenching is interrupted by opening the forming tool to permit slow cooling of the hot formed part through the anisothermal bainitic ferrite transformation. This promotes carbon partitioning to austenite before the forming tool is re-closed and die-quenching is resumed to near-ambient temperature. The final microstructure is predominantly bainitic ferrite with dispersions of martensite and up to 11 % retained austenite. Retained austenite can undergo stress induced transformation to martensite in an automobile crash event. The steel exhibits up to 25 % elongation and 930 MPa tensile strength. In contrast to traditional cold formable Transformation Induced Plasticity assisted steels, where retained austenite is consumed during work hardening of cold forming, here, the desired microstructure is achieved after hot forming meaning the retained austenite is more uniformly distributed within the formed part, which enhances energy absorption. The new steel chemical composition is carefully designed to provide optimal microstructural evolution within the constraints of the new press hardening process, yet relatively lean and manufacturer friendly. The new press hardening process is energy efficient as secondary heating is not required since retarded cooling through the bainitic ferrite transformation is provided by residual heat accumulation of the newly developed titanium alloy forming tool. Development of the new technology is demonstrated by press hardening experiments, tensile testing, microstructural analysis, transversal & axial crush testing of formed parts and numerical simulation of crush testing, including a new modelling technique that more accurately simulates deformation of hot versus cold formed parts. Results show a 22 % increase to energy absorption under axial crushing compared to traditional cold formed Transformation Induced Plasticity assisted steels owing to greater work hardening capacity in formed radii of the part, which are shown to be exposed to the highest stresses during crushing.

Introduction

Zackay et al. (1967) first reported the Transformation Induced Plasticity (TRIP)’ effect in 1967 during investigation of austenitic stainless steels. The authors discovered unusually high uniform elongation values, which they attributed to stress or strain induced transformation of austenite to martensite, giving rise to dilatation and internal plastic strain, hence the term TRIP. Mild carbon low alloy TRIP steels, exploiting the same phenomenon discovered by Zackay et al. (1967) were developed by the steel industry in the early 1990s and continue to gain much attention within academic research owing to the impressive combinations of strength and ductility. Tan et al. (2020) applied different hot rolling, quenching and partitioning sequences to a controlled steel chemical composition to obtain eight different TRIP steel products with different microstructures and then evaluated the effect on mechanical properties. Compared to Dual Phase (DP) steels characterised by microstructures of ferrite and martensite, exhibiting an ultimate tensile strength-total elongation product of approximately 15 GPa.%, TRIP steels exhibit much higher values of up to 25 GPa.%. The characteristically impressive strength-ductility combination of TRIP steels provides superior cold formability and potentially, superior application performance, such as automobile crash performance. Oliver et al., 2007a conducted impact crash box testing on DP and TRIP steels with equivalent tensile strength. For a given strength category, the TRIP steels with enhanced ductility consistently exhibited superior energy absorption by preventing folded regions of the deformed crash box from splitting.

Sakuma et al. (1995) documented development of first generation TRIP steels by the steel industry in the early 1990s. Chemical compositions are typically of 0.15−0.25 % C, 1–2 % Mn and 1.5 % Si and are processed during hot rolling or strip annealing to exhibit a multi-phase microstructure of predominantly (proeutectoid) ferrite, with dispersions of bainitic ferrite, martensite and 10–15 % retained austenite. The silicon content retards iron carbide precipitation during bainite formation giving rise to carbide-free bainitic ferrite with excess carbon over and above the ferrite saturation limit partitioning to austenite and remaining in solid solution. The carbon enriched austenite is then stabilised at ambient temperature. The retained austenite is designed to be stress induced transformed to martensite during plastic deformation, as originally cited by Zackay et al. (1967), but in this manner, can be achieved with the inexpensive addition of just 1.5 % Si rather than 20 % (Cr + Ni) in austenitic stainless steel. While TRIP steels inherit their name from the transformation induced plasticity effect originally cited by Zackay et al. (1967), Bhadeshia (2002) has mathematically demonstrated that only 2 % of the uniform elongation value may be attributed to the TRIP effect, with for the most part, the impressive strength-ductility combination attributed to ‘composite deformation’ behaviour, where ductility of ferrite is exploited early during deformation and then hardness of martensite resulting from stress induced transformation exploited late during deformation.

While economical, the typical silicon content of up to 1.5 % has significantly restricted application of first generation TRIP steels. Slab cracking, high rolling loads, poor weldability and poor metallic coatability are the major drawbacks. To overcome these problems, second generation TRIP steels (sometimes called TRIP assisted DP steels) with silicon content partially substituted by aluminium have been developed. Mein et al. (2012) investigated a commercial C-Mn-Cr DP steel and an equivalent experimental steel with the addition of 0.52 % Al. The aluminium addition not only allows for higher percentages of retained austenite, but also broadens the processing window of DP steels by raising the A3 temperature to reduce distribution of mechanical properties.

The next problem identified with first and second generation TRIP steels has been relatively low proof to ultimate tensile strength ratio, typically of the order of 0.5, compared to 0.6 of DP steels and 0.8 of martensitic steels. While the low proof to ultimate ratio is indicative of high work hardenability and energy absorption, many automotive structural parts require a minimum proof strength. Therefore, parts may require over forming (work hardening) in order to meet the minimum requirement. To overcome this, third generation TRIP assisted Bainitic Ferrite (TBF) steels have been developed. Bachmaier et al. (2013) illustrated the design principles behind TBF steels, consisting predominantly of bainitic ferrite with retained austenite dispersions produced by isothermal holding in the bainitic phase field following rapid cooling from the austenite phase conducted either on the Run Out Table following hot rolling, or during the continuous annealing cycle following cold rolling.

Ridderstrale (1977) developed the original Press Hardened Steel (PHS) technology to overcome poor formability and high springback of ultrahigh strength martensitic steels. The steel blank (typical chemical composition of 0.20−0.25 % C, 1.2 % Mn and 30−50 ppmB) is furnace heated to the austenite phase, transferred to the water cooled forming tool, formed in the austenite phase and then die-quenched to martensite, giving rise to tensile strength and total elongation values of 1400−1600 MPa and 3–6 % respectively. The automotive industry desires lightweighting (primarily to reduce exhaust emissions and fuel consumption). Down-gauging reduces weight, but compromises formability and springback. These compromises are more significant when strength is increased, but they still exist for lower strength steels. Press hardening offers exceptional formability and almost eliminates springback, thus press hardening has gained attention for lower strength parts. Naderi et al. (2011) developed three C-Mn-Cr steels with chemical compositions of 0.14−0.19 % C, 1.45–1.71 % Mn and 0.01−0.55 % Cr. Following press hardening, the three steels gave rise to multiphase microstructures of ferrite, bainite and martensite exhibiting tensile strength of up to 910 MPa and total elongation of up to 9.3 %.

The design principles behind TBF steels are compatible with the press hardening process, consisting of rapid cooling from the austenite phase to bainite and / or martensite. However, in order to achieve the isothermal holding step in the bainitic phase field following rapid cooling from austenite, the original press hardening process requires modification. Liu et al. (2011) made an attempt towards this by interrupting die-quenching in the temperature range of 280−320 °C with a secondary isothermal heating step for up to 60 s before die-quenching is resumed to near-ambient temperature. Using a steel with chemical composition of 0.22 % C, 1.58 % Mn and 0.81 % Si, the result was reported to be inter-lath retained austenite within the martensitic matrix, giving rise to a retained austenite volume fraction of 18 %, ultimate tensile strength of 1510 MPa and total elongation of 15 %. While the result is impressive, modification to the press hardening line with the secondary heating step is costly in terms of both infrastructure requirements and energy consumption. Xu et al. (2020) introduced a similar idea as above, but instead used a first generation TRIP steel of 0.2 % C, 1.9 % Mn and 1.4 % Si; and instead of isothermal holding, used retarded cooling by pre-heating the forming tool to 200 °C and opening the forming tool in the region of 300−200 °C for up to 60 s to conduct the anisothermal bainitic ferrite transformation. The optimum result was a retained austenite volume fraction of 11 %, tensile strength of 1449 MPa and total elongation of 14.5 %. These impressive results, which represent properties of the formed part are very similar to the traditional cold formable strip TRIP and TBF steels. However, the retained austenite in the latter is consumed during the plastic strain of forming. Oliver et al., 2007b conducted pre-strain tensile tests on a commercial TRIP steel, showing that pre-strain of 10 % reduced the retained austenite volume fraction from 10 to 5 %. Proof strength was raised from 550 to 825 MPa owing to work hardening, while total elongation was reduced from 30 to 15 % owing to degradation of the stress induced transformation effect. Thus, to compare like for like, the measured mechanical properties of a traditional cold formed TRIP or TBF steel part (in formed regions) will be inferior to the mechanical properties of the part produced by Xu et al. (2020), since in the latter the optimal retained austenite volume fraction and stress induced transformation capability is determined after forming and thus, uniformly distributed throughout the part. However, as previously mentioned, the first generation TRIP steel used by Xu et al. (2020), rich in silicon, is unfavourable because of manufacturing drawbacks and thus, the technology is unlikely to reach commercialisation. Pre-heating the forming tool also necessitates modification to the press hardening line, which increases the infrastructure requirements and energy consumption and reduces the production efficiency. In addition, pre-heating the forming tool decreases the die-quenching rate, limiting capability of the process to produce a variety of microstructures (particularly with limited ferrite volume fractions) across parts with different dimensions and gauges.

In this paper, we introduce a new steel chemical composition combined with a new press hardening process in which die-quenching is interrupted by opening the forming tool at a temperature in the bainitic phase field. The resulting slow cooling of the hot formed part through the anisothermal bainitic ferrite transformation allows carbon partitioning to austenite to occur. The forming tool is then re-closed and die-quenching is resumed to near-ambient temperature. The new steel chemical composition is carefully designed to provide optimal microstructural evolution within the constraints of the new press hardening process, yet relatively lean, manufacturer friendly and compatible with conventional steelmaking, strip rolling, metallic coating and welding practices. The new press hardening process is economical and energy efficient as secondary heating is not required, since retarded cooling through the bainitic ferrite transformation is provided by residual heat accumulation of the newly developed titanium alloy forming tool following hot forming. Fig. 1 illustrates the principal advantage of the proposed TRIP assisted PHS technology compared to traditional cold formable TRIP (and TBF) steels.

Section snippets

New press hardening process

The new press hardening process is schematically illustrated by Fig. 2a. The steel blank is furnace heated above the A3 temperature, transferred to the water cooled forming tool and hot formed into the part geometry while in the highly formable and isotropic austenite phase, as per the original press hardening process by Ridderstrale (1977). Die-quenching from austenite is interrupted by opening the forming tool at a temperature in the bainitic phase field between the A1 and Ms temperatures, in

Press hardening with flat dies for tensile testing and optimisation of process parameters

Fig. 8 presents representative time-temperature plots from the start of cooling. Upon interrupting die-quenching and opening the tool, temperature increased due to substantial residual heat accumulation in the dies characterised by low thermal conductivity, before slow cooling commenced by natural air circulation. Even with the lowest tool-open temperature of 350 °C and longest tool-open time of 300 s, minimum temperature did not drop below 255 °C during the tool-open time, demonstrating

Conclusion

The optimised press hardened microstructure of 20MnSiAlPB5 consisted of 48.3 % bainitic ferrite, 42.6 % martensite and 9.1 % retained austenite.

The retained austenite can undergo stress induced transformation to martensite during application of a formed part such as an automobile crash event, giving rise to 867 MPa tensile strength and 20.5 % total elongation.

Traditional cold forming of TRIP assisted steel reduced the retained austenite volume fraction from 9.1 to 3.3 % in the formed radii,

Author credit statement

T. Taylor – Principal Investigator and lead author.

K. Kim – EBSD analysis and assistant author.

J. Zhang – FEA simulation and assistant author.

D. Penney – lab cast processing and assistant author.

J. Yanagimoto – project supervisor and assistant author.

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgments

This research was funded by the Japan Society for the Promotion of Science (JSPS).

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