New Multiphase CP and DP 1000 MPa strength level grades for improved performance after hot forming

Pure martensitic steels have after hot forming limited performance in terms of rest ductility which limits the application in crash relevant parts. New steel grades were designed in the EU project HOTFORM including the corresponding process routes. These steel grades have ferritic-martensitic dual phase (DP) and martensitic-bainitic complex phase (CP) microstructures after hot forming process. The laboratory tests show an improved formability after hot forming. The basic concepts of the new alloys are explained. Furthermore, for validation of upscaling purposes a semi-industrial test is carried out and the results are discussed. The main application is for vehicle safety. This is evaluated by comparing the crash performance of these hot formed grades with cold rolled DP1000 and CP1000 for crash cans in a drop tower test.The research leading to these results was carried out in the framework of HOTFORM project with a financial grant of the Research Programme RFCS (Research Funds for Coal and Steel) under grant agreement (RFSR-CT-2015-00017)

Influence of Ni and Process Parameters in Medium Mn Steels Heat Treated by High Partitioning Temperature Q&P Cycles

In this work, two medium Mn steels (5.8 and 5.7 wt pct Mn) were subjected to a quenching and partitioning (Q&P) treatment employing a partitioning temperature which corresponded to the start of austenite reverse transformation (ART). The influence of a 1.6 wt pct Ni addition in one of the steels and cycle parameters on austenite stability and mechanical properties was also studied. High contents of retained austenite were obtained in the lower quenching temperature (QT) condition, which at the same time resulted in a finer microstructure. The addition of Ni was effective in stabilizing higher contents of austenite. The partitioning of Mn and Ni from martensite into austenite was observed by TEM–EDS. The partitioning behaviour of Mn depended on the QT condition. The lower QT condition facilitated Mn enrichment of austenite laths during partitioning and stabilization of a higher content of austenite. The medium Mn steel containing Ni showed outstanding values of the product of tensile strength (TS) and total elongation (TEL) in the lower QT condition and a higher mechanical stability of the austenite.This research is partially funded by the European Commission in the HIGHQP Project, which has received funding from the Research Fund for Coal and Steel under Grant Agreement No 709855. This study reflects only the author's views and the European Commission is not responsible for any use that may be made of the information contained therein. The authors thank for technical and human support provided by SGIker (UPV/EHU/ERDF, EU). The authors also acknowledge the support of the sponsors of the Advanced Steel Processing and Products Research Center

Characterization of a Medium Mn-Ni Steel Q&P Treated by a High Partitioning Temperature Cycle

In this work, a medium Mn-Ni steel was treated through Quenching and Partitioning (Q&P) with a partitioning temperature (PT) of 650 °C, which corresponded to the start of the austenite reverse transformation (ART) phenomenon. The influence of the quenching temperature (QT) and partitioning time (Pt) on austenite stabilization and mechanical properties was investigated. A strong influence of the quenching temperature was observed. Results were compared with those obtained after a Q&P treatment with 400 °C partitioning temperature. The Q&P cycle with quenching to room temperature and a high partitioning temperature produced a steel with a high retained austenite (RA) volume fraction and exceptional strength–ductility balance. The analysis of the mechanical stability of the retained austenite revealed a significant stress-induced transformation. Nevertheless, the austenite, which was stable at stresses above the yield stress, provided significant TRIP-assisted ductility. Bending, hole expansion and post-stamping properties were also evaluated for the most promising conditions.This research was funded by the Research Fund for Coal and Steel, grant number 70985

Effects of Mn addittion, cooling rate and holding temperature on the modification and purification of iron-rich compounds in AlSi10MnMg(Fe) alloy

The use of secondary aluminum alloys in industry is still limited by the high Fe contents in recycled alloys. In general, the Fe-rich intermetallic compounds deteriorate the performance of secondary Al–Si alloys, specially the β-Fe phase. To mitigate the detrimental effects of iron, the influence of diferent cooling rates and holding temperatures on the modification and purification of iron-rich compounds in commercial AlSi10MnMg alloy with 1.1 wt % Fe was studied. According to the results obtained by CALPHAD calculations, the alloy was modified by adding a 0.7 wt%, 1.2 wt%. and 2.0 wt% of Mn. The phase formation and morphology of iron-rich compounds was systematically studied and correlated by different microstructural characterization techniques. The experimental results showed that the detrimental β-Fe phase can be avoided by adding at least 1.2 wt % of Mn at the studied cooling rates. Finally, the effect of different holding temperatures in the sedimentation of Fe-rich compounds also was studied. Hence, the gravitational sedimentation experiments at different holding times and temperatures were conducted to validate the feasibility of the methodology in different processing conditions. The experimental results showed a high Fe removal efficiency up to 64% and 61%, after a holding time of 30 min at 600 °C and 670 °C, respectively. The addition of Mn improved the Fe removal efficiency but not gradually, as the best results were obtained in the alloy containing 1.2 wt % Mn.This work was supported by the Basque Government through the projects Elkartek CIRCU-AL: KK-2020/00016 and SosIAMet KK-2022/00110

Influence of Mn and Ni on Austenite Stabilization during a High Temperature Q&P Treatment

The aim of this work was to study the influence of quenching and partitioning temperatures combined with various levels of Mn and Ni contents on the austenite stabilization along the quenching and partitioning (Q&P) cycle. Three steels with 2 wt.%, 4 wt.% and 6 wt.% manganese and one steel with 2 wt.% nickel content were investigated. Phase transformation temperatures and critical cooling rates were obtained experimentally using dilatometer for each alloy. Q&P cycles with different quenching and partitioning temperatures were also done in dilatometer, thus, allowing monitoring of the expansion/contraction during the whole Q&P cycle. Microstructure characterization was performed by means of a Scanning Electron Microscope and X-Ray Diffraction to measure retained austenite content. It was found that, strongly depending on the Q&P conditions, austenite stabilization or decomposition occurs during partitioning and final cooling. In case of high partitioning temperature cycles, austenite reverse transformation was observed. Certain cycles resulted in a very effective austenite stabilization and interesting microstructure.This research has been carried out in the framework of the HIGHQP project, which has received funding from the Research Fund for Coal and Steel under grant agreement No 709855

Impact of Si on Microstructure and Mechanical Properties of 22MnB5 Hot Stamping Steel Treated by Quenching & Partitioning (Q&P)

Based on 22MnB5 hot stamping steel, three model alloys containing 0.5, 0.8, and 1.5 wt pct Si were produced, heat treated by quenching and partitioning (Q&P), and characterized. Aided by DICTRA calculations, the thermal Q&P cycles were designed to fit into industrial hot stamping by keeping partitioning times £ 30 seconds. As expected, Si increased the amount of retained austenite (RA) stabilized after final cooling. However, for the intermediate Si alloy the heat treatment exerted a particularly pronounced influence with an RA content three times as high for the one-step process compared to the two-step process. It appeared that 0.8 wt pct Si sufficed to suppress direct cementite formation from within martensite laths but did not sufficiently stabilize carbon-soaked RA at higher temperatures. Tensile and bending tests showed strongly diverging effects of austenite on ductility. Total elongation improved consistently with increasing RA content independently from its carbon content. In contrast, the bending angle was not impacted by high-carbon RA but deteriorated almost linearly with the amount of low-carbon RA.The authors gratefully acknowledge the financial support by the Research Fund for Coal and Steel via ELONHOSTAMP project (Grant agreement No. RFSR-CT-2013-00010). A final project report with a focus towards industrialization is in process of being published

Phase Transformation and Microstructural Behavior of New Steel Grades for Hot Sheet Stamping

Since the last few years several researches have focused in proposing new approaches to decrease the energy impact of the hot sheet stamping process. Among them, one possibility is to use steel grades characterized by a lower austenitization temperature in order to reduce the heating energy required for the process, but still maintaining the steel quenchability under the investigated process conditions. In the paper, the behaviour of two new steel grades developed by ArcelorMittal and Tecnalia, and devoted to hot stamping is investigated in terms of transformation kinetics and microstructural behaviour. An extensive dilatometric analysis, performed by using a modified axial extensometer, permitted to evaluate the steel kinetics of phase transformation as a function of the applied cooling rate, amount of strain and strain rate. The micro-hardness of water and die quenched samples of the investigated steel grades was measured and their microstructure analyzed by means of optical and scanning electron microscopy to evaluate the characteristics of the formed martensite and the possible phenomenon of autotemperin