INFLUENCE OF PARAMETERS OF Q-P PROCESS ON PROPERTIES AND MICROSTRUCTURE OF CMnSiMo STEEL

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1 Abstract INFLUENCE OF PARAMETERS OF Q-P PROCESS ON PROPERTIES AND MICROSTRUCTURE OF CMnSiMo STEEL Daniela HAUSEROVÁ a, Jaromír DLOUHÝ b Zbyšek NOVÝ c a,b,c COMTES FHT a.s., Průmyslová 995, Dobřany, Czech Republic, daniela.hauserova@comtesfht.cz, jaromir.dlouhy@comtesfht.cz, zbysek.novy@comtesfht.cz Current efforts in production of low-alloyed steels are aimed at achieving high ultimate and yield strengths while maintaining sufficient elongation and good weldability in these materials. Among advanced heat treatment processes capable of reaching this goal there is also the Q-P process (Quenching and Partitioning). The process consists in rapid quenching of the material between the M s and M f temperatures in order to prevent full martensitic transformation. The immediately following heating leads to tempering of the martensite and to diffusion of excess carbon from martensite to retained austenite. This increases the stability of the latter. The aim of the Q-P process is to produce very fine martensite microstructure with retained austenite between martensite plates. The experimental programme was carried out on a highstrength low-alloyed steel containing 0.2% carbon and a higher amount of silicon about 1.5%. Higher silicon content in the microstructure contributes to stabilization of retained austenite by suppressing formation of carbides. This grade of steel is an advantageous material thanks to its low amount of alloying elements. This group of low-alloyed steels, if heat treated or thermomechanically treated in a suitable manner, offers a favourable combination of strength, elongation and toughness. The paper is aimed at testing the influence of the Q-P process on evolution of final microstructure and mechanical properties of the steel CMnSiMo. 1 INTRODUCTION The requirements on mechanical properties of steels are constantly increasing and new heat and thermomechanical treatment processes for such steels are being developed. High-strength low-alloyed steels, as a group of steels, offer favourable proportion of strength, elongation and toughness. The level of ultimate tensile strength required in these materials, upon suitable heat treatment, reaches 1,500 MPa. Their elongation should be around 15%, while the maximum content of alloying and residual elements should not exceed five weight per-cent. Another important requirement consists in good weldability. One of modern heat treatment techniques capable of meeting these requirements is the Q-P process which consists in rapid quenching of a material between the M s and M f temperatures in order to prevent full martensitic transformation.

2 Subsequent heating to a temperature below M s initiates martensite tempering and a diffusion flow of excess carbon from martensite to retained austenite. Cooling down to room temperature stabilizes the retained austenite thanks to prior diffusion of carbon from supersaturated martensite to the still untransformed austenite. The purpose of the Q-P process is to produce very fine martensite Fig. 2. Schematic Q-P heat treatment [1] with retained austenite between martensite plates. (Fig. 2.) After austenitizing, the steel should be quenched (Q-P) to a specific temperature calculated in such a way as to produce a pre-defined ratio of martensite and non-transformed austenite. Subsequently, the temperature of the material should be raised to the partitioning level (PT). The carbon will diffuse to the existing austenite and increase its stability to the level where it does not transform upon cooling to ambient temperature. As the austenite becomes enriched in carbon Fig. 1 Experimental heat treatment schedule: Q-P process during the partitioning stage, its actual M s - M f temperatures decrease. Full stabilisation requires that the M s temperature is depressed to or below room temperature to prevent martensitic or bainitic transformation of insufficiently stabilised austenite during final cooling [2-3]. 2 EXPERIMENTAL The material used in the experiments was low-alloyed steel of CMnSiMo type with good weldability (Table 1). It had an initial bainite microstructure with hardness of 285 HV30, proof stress of 550 MPa and a strength just below 1,000 MPa (Table 3). Table 1. Chemical composition of the experimental steel [weight%] C Si Mn P S Cr Mo Ni Al V Nb Ti N B <0.007 <0.001 <0.002 < Heat Treatment Data obtained from literature [4] was used as a basis for designing a Q-P schedule with an austenitising temperature of 850 C, water quenching to 250 C and tempering at 350 and 450 C. Experimental results indicated that the soaking temperature of 850 C was not sufficient for full austenitisation of the material. The austenitising temperature was therefore increased to 900 C (Fig. 1). The soaking time was set at 25 min.

3 Partial water quenching was performed in a special fixture. Tempering in a furnace at various temperatures was carried out for 30 minutes. First two Q-P schedules were proposed to examine the influence of the tempering temperature on the evolution of microstructure and mechanical properties of the experimental steel. The remaining schedules were intended to examine the intensity of influence of the lower limit of partial quenching temperature. Reference specimens for comparison with the Q-P-treated ones were prepared by conventional quenching and tempering at 400 C for 150 minutes (HT 900/25min /150min) (Table 2). Table 2 Parameters of experimental heat treatment and their influence on stabilization of retained austenite Treatment Partial quenching 900/25min-250-air /30min /30min /30min /30 HT 900/25min /150min Austenitising[ C] 2.2 Metallographic Analysis Partial quenching temperature [ C] Tempering temperature [ C] Tempering time [min] Retained austenite in microstructure [%] Metallographic analysis was carried out in light and scanning electron microscopes. The resulting microstructure upon all Q-P schedules consisted of bainite, tempered martensite and a small amount of residual austenite. The majority of carbides can be found between the plates, while small proportion precipitated within the plates. Considering the size of martensite laths or bainite plates, the microstructure is relatively fine and uniform. Retained austenite was detected by means of quantitative X-ray diffraction phase analysis. Its amount in tested specimens was near the detection limit of the method. In Q-P-treated specimens, up to 4% retained austenite was found. No retained austenite was found in specimens upon conventional heat treatment (Table 2). The resulting microstructure in reference specimens quenched in water to 250 C and subsequently air-cooled (Fig. 3) contains majority of martensite and some bainite. Carbides are also present both in and between the bainite plates.

4 Fig. 3 Martensite-bainite microstructure upon water quenching 900 C/25min-250 C-air. Fig. 4 Bainite microstructure with tempered martensite, HT 900 C/25min-20 C-400 C/150min. Fig. 5 Bainite microstructure with tempered martensite, Q-P 900 C/25min-290 C-300 C/30min.

5 2.3 Mechanical Testing Tension tests have been performed on specimens with the diameter of 8 mm and length of 50 mm. Specimens used for impact testing were of the miniature type, with the size of mm and a 1 mm deep V-notch. Table 3 Results of mechanical testing R p0,2 R m A g A 5 Z KV KCV Schedule [MPa] [MPa] [%] [%] [%] [J] [J/cm 2 ] /30min /30min /30min / HT 900/25min /150min Initial state Results of the tension test (Table 3) clearly show that specimens treated with the first two Q-P schedules, where only the tempering temperature was varied from 350 to 300 C, do not exhibit significant differences in yield and ultimate strength. Elongation values are almost identical as well. Changing the partial quenching temperature leads to a slight decline in yield and ultimate strength with increasing temperature. Specimens upon reference conventional heat treatment (denoted as HT 900/25min /150min) showed a 130 MPa higher ultimate tensile strength and a 70 MPa lower yield strength. 3 DISCUSSION OF RESULTS The Q-P process did not lead to formation of the typical fine martensite-residual austenite microstructure without carbides in the present CMnSiMo steel. Metallographic and X-ray analyses indicated that the process led to stabilising only a minimum amount of retained austenite: up to 4%. At the same time, carbide precipitation was extensive in the microstructure. These carbides were probably formed in the course of the Q-P process, as the microstructure upon conventional water quenching from 900 C was free from carbides. With the relatively low total carbon content the presence of carbides impedes redistribution of carbon by diffusion. The carbon concentration is then too low to provide the required stabilisation of retained austenite. The aim of further investigation is to find process parameters leading to formation of martensitic microstructure with considerable amount of retained austenite upon partial quenching. The temperatures of subsequent tempering should not enable decomposition of retained austenite but should suffice for diffusion of carbon from martensite to austenite.

6 4 CONCLUSION Experiments using the Q-P process carried out on the CMnSiMo steel with the carbon content of 0.21% did not lead to formation of microstructure consisting of tempered martensite and retained austenite without carbides. The resulting microstructure consisted of martensite and bainite and of up to 4% of retained austenite. However, the Q-P process led to certain improvement in some parameters of mechanical properties. The ultimate tensile strength reached 1,400 MPa, which means about 100 MPa increase over that achieved by conventional heat treatment. Elongation measured upon all alternative schedules was about 15%. Similar trends were observed in impact toughness (KCV) where all variants showed the value of about 70 J/cm 2 without significant variance. These properties impart good potential for practical application to the experimental material. ACKNOWLEDGEMENT This paper includes results achieved within the project GACR 106/09/1968: Development of New Grades of High-Strength Low-Alloyed Steels with Improved Elongation Values. REFERENCES [1] EDMONDS, D.V.,etc. Quenching and partitioning martensite A novel steel heat treatment. Materiale Science and Engineering A (2006) [2] GERDEMANN, F.L.H. Microstructure and hardness of 9260 steel heat-treated by the quenching and partitioning process, Aachen University of Technology, Germany, [3] BHADESHIA, H.K.D.H. High performance bainitic steels Materiále Science Forum A (2005) [4] DE MOOR, E.,etc. Quench and partitioning response of a Mo-alloyed CMnSi steel, Proceedings of New developments on metallurgy and applications of high strength steels, Vol. 1 and 2, 2008, pp