Influence of partitioning on mechanical behavior of Q&P steels

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Abstract

The effect of partitioning conditions during Quenching and Partitioning (Q&P) annealing on mechanical behavior of cold rolled 0.2C-2.22Mn-1.44Si-0.21Cr steels was studied. Q&P heat treatments followed by tensile tests were performed. The evolution of mechanical properties as a function of partitioning temperature and time was analyzed. Microstructure and retained austenite fraction in the initial state and after interrupted tensile tests were characterized by SEM and magnetic measurements. Tensile tests were also performed on tempered martensite to assess the mechanical properties of partitioned martensite. Finally, efforts were made to bring results together and explain the microstructure and mechanical properties of Q&P steels.

Introduction

Current requirements on automotive safety and on reduction of environmental impact are the driving forces for the research of advanced high-strength steels (AHSS). New metallurgical solutions are investigated to answer these ambitious targets and propose innovative steels with enhanced strength and ductility. Quenching and Partitioning process appears to be a relevant way to produce such type of steels. Proposed by Speer et al. [1], [2], [3], Q&P process allows producing a new kind of TRIP steels (Transformation-Induced Plasticity) where conventional ferrite matrix is replaced by stronger martensite phase, thus resulting in an attractive combination of strength and ductility. In fact, Q&P concept is an original heat treatment consisting of several steps. Heating and soaking is followed by rapid cooling in between Ms and Mf temperatures, thus assuring formation of required martensite amount. Next, reheating and holding at the temperature inferior to Ac1, called as partitioning step, is operating to promote carbon diffusion from supersaturated martensite to austenite [4], [5]. Ideal partitioning results in the carbon depleted martensite (partitioned martensite) and carbon enriched austenite which is retained during the final cooling to the room temperature. In the real conditions, austenite stabilization can be partial, thus producing some fresh martensite in the final microstructure. Characterization and modeling of carbon partitioning from martensite to austenite [4], [5], [6], [7], [8] and of the resulted Q&P microstructures [9], [10], [11], [12] have been done recently. On the other hand, several studies dealt with the mechanical properties of Q&P steels [12], [13], [14], [15], [16], [17] and their relation with a certain microstructure constituents like retained austenite [18] and fresh martensite [19]. Finally, even though, there are some works considering different partitioning conditions [15], [17], [19], [20], the understanding of the partitioning temperature and time influence on the evolution of microstructure and mechanical behavior is not complete.

In the present work, characterization of the Q&P steels obtained with different partitioning conditions was done using Field Emission Gun Scanning Electron Microscope (FEG-SEM), saturation magnetization measurements and tensile tests. Evolution of microstructure and mechanical properties was then analyzed as a function of partitioning temperature and time. Complementary tempering treatments of fully martensitic structure were performed to assess the mechanical behavior of partitioned martensite (matrix phase). In addition, interrupted tensile tests were realized and stability of retained austenite against strain increase was characterized using saturation magnetization measurements and FEG-SEM. Finally, explanations of the observed tendencies between partitioning conditions and tensile properties are proposed.

Section snippets

Experimental procedure

The steel containing 0.2C-2.22Mn-1.44Si-0.21Cr (wt%) and with the thickness of 1.2mm was supplied by ArcelorMittal Ghent in the cold rolled state. Q&P heat treatments were realized using the combination of Nabertherm furnace and the salt baths in the following manner. First, austenitization was done in the Nabertherm furnace, then the sheet was moved to the first salt bath which was set up at the quench temperature (Tq). When the quenching temperature was reached, sample was immersed in another

Mechanical behavior of Q&P steels

The true stress-true strain curves and the corresponding evolution of work hardening (WH) rate with the true strain of the steels obtained with different partitioning conditions are shown in Fig. 2, Fig. 3. As well, Table 1 summarizes all the obtained mechanical properties. These results demonstrate that a very high strength steels with reasonable ductility can be obtained using Q&P annealing. The best combinations of UTS and TE were obtained with the partitioning at 400 °C for 100 s and at 460 °C

Mechanical properties of Q&P steels

Mechanical behavior of Q&P and tempered martensite steels with, respectively, different partitioning and tempering conditions are presented in Fig. 2, Fig. 7. Comparison between these two different thermal treatments is given in Fig. 9. Both 400 °C and 460 °C holding temperatures but with only one holding time (10 s) are considered. It can be seen that both Q&P and tempered martensite steels have similar tendencies with increasing holding temperature: strength, in particular YS, is decreasing and

Conclusions

Q&P treatments with different partitioning temperatures and times were performed on cold rolled 0.2C-2.22Mn-1.44Si-0.21Cr steel. Evolution of mechanical behavior with partitioning conditions was investigated. Partitioning temperature has an important influence on the strength and ductility of Q&P products. On the other hand effect of partitioning time was different for 400 °C and 460 °C partitioning temperatures. In the first case (400 °C) this influence was almost negligible. But in the case of

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