THE ROLE OF A COOLING RATE IN THERMO-MECHANICAL TREATMENT WITH INCORPORATED Q-P PROCESS

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1 VÝZNAM RYCHLOSTI OCHLAZOVÁNÍ V TEPELNĚ-MECHANICKÉM ZPRACOVÁNÍ ZAHRNUJÍCÍM Q-P PROCES THE ROLE O A COOLING RATE IN THERMO-MECHANICAL TREATMENT WITH INCORPORATED Q-P PROCESS Bohuslav MAŠEK, Ludmila KUČEROVÁ, Hana JIRKOVÁ, Danuše KLAUBEROVÁ ORTECH, University of West Bohemia in Pilsen, Univerzitní 8, Plzeň, Česká republika, skalova.lida@seznam.cz Abstrakt Inovativní strategie zpracování vysoce-pevných ocelí umožňují dosáhnout lepší kombinace pevnosti a tažnosti, než klasickými metodami. Jednou z těchto nových strategií tepelného zpracování je rovněž Q-P (quenching and partitioning) proces, který využívá pozitivní vliv zbytkového austenitu ve struktuře na mechanické vlastnosti. Použitím Q-P procesu je výsledná mikrostruktura tvořena martenzitickou matricí s 5-15% zbytkového austenitu. To umožňuje dosáhnout pevnosti přesahujících 1800MPa při tažnostech kolem 10%. V této práci je Q-P proces začleněn do komplexnějšího tepelně mechanického zpracování s inkrementální deformací, která zjemní výslednou mikrostrukturu a tím dále zlepší její mechanické vlastnosti. Aby bylo možné sledovat vliv dvou hlavních legujících prvků, manganu a křemíku na vlastnosti oceli, bylo provedeno termomechanické zpracování na třech ocelích s 0,43%C a s různým obsahem manganu (0,6-1,2%) a křemíku (2-2,6%). Zpracování začínalo 100s austenitizací na 900 C nebo 950 C. P ři ochlazování z této teploty byla vždy aplikována 20-ti násobná inkrementální deformace. Od teploty 820 C následovalo rychlé ochlazením na teplotu 200 C. Pro každou ocel bylo provedeno zprac ování třemi různými průměrnými rychlostmi chlazení 20, 7 a 4,7 C/s. Přerušením ochlazování na teplotě ležící mezi M s a M f vznikla martenziticko-austenitická mikrostruktura. Bezprostředně poté byly oceli ohřáty na teplotu 250 C, kde během 600s prodlevy proběhla difůze uhlíku z přesyceného martenzitu do austenitu. Změna rychlosti ochlazování měla výrazný vliv na vývoj mechanických vlastností. Byly dosaženy tažnosti A 5mm v rozmezí 6-15% při pevnostech mezi 1570 až 2060MPa. Klíčová slova: Q-P proces, tepelně mechanické zpracování, zbytkový austenit, inkrementální deformace Abstract Innovative processing strategies of high-strength steels enable better combinations of strength and ductility than the classical ones. One of these new heat treatment methods is also quenching and partitioning (Q-P) process, which benefits from a positive effect of retained austenite on mechanical properties. Resulting microstructure after Q-P process consist of martensite with 5-15% of retained austenite, which allows it to achieve strengths above 1800MPa with ductility around 10%. In this work was Q-P process incorporated into the more complex thermo-mechanical treatment with incremental deformation, which refines final microstructure and thus further improve its mechanical properties. Three steels with 0.43%C and varying manganese (0,6-1,2%) and silicon (2-2,6%) content were used to investigate the influence of these main alloying elements. The processing began with 100s austenization at 900 C or 950 C. Twenty deformation steps were always applied during the cooling from austenization temperature to 820 C. urther cooling to 200 C was carri ed out by three different cooling rates 20, 7 and 4.7 C/s. Martensitic-austenitic microstructures were created because this temperature lies between Ms and Mf. The steels were subsequently heated to 250 C, where

2 600s hold was performed to allow carbon diffusion from supersaturated martensite to austenite. Variations in cooling rate had pronounced effect on mechanical properties development. The steels reached ductility A5mm of 6-15% with the strengths among 1570 MPa and 2060 MPa. Key words: Q-P process, thermo-mechanical treatment, incremental deformation 1. INTRODUCTION The need of high strength low alloyed steels has been increasing in recent years. Quenching has been the most conventional method of strength enhancement for steels, however new applications of high strength steels demand higher ductility, formability and toughness than conventionally quenched steels provide. This requires new innovative strategies of heat and thermo-mechanical treatment to be developed so that high ultimate strengths can be obtained without much loss of ductility. Several methods of heat and thermomechanical treatment have been proposed in last two decades which have used the positive effect of retained austenite on strength to ductility balance of low alloyed steels. One of the new methods of heat treatment is also quenching and partitioning (Q-P) process [1]. It consist of austenization of the steel, rapid cooling to the quenching temperature which lies between Ms and Mf temperature and subsequent heating to partitioning hold temperature. Rapid cooling creates martensitic microstructure with a certain volume fraction of metastable retained austenite. Partitioning hold then enables carbon to diffuse from the supersaturated martensitic lattice into the remaining austenite. The austenite is thus chemically stabilized and does not transform to martensite during final cooling. Resulting microstructure is made of martensitic matrix with around 10% of retained austenite. This structure combines the advantage of high strength of martensite and improved ductility caused by gradual transformation of retained austenite to martensite during cold deformation. In the case of low alloyed 42SiCr steel the strength after Q-P process can reach 2000MPa with ductility of around 10% [2]. Even further enhancement of mechanical properties can be achieved when the Q-P process is combined with intensive incremental deformation carried out in the austenite region to obtain finer martensitic matrix. The amount of applied deformation might also influence morphology and stability of the retained austenite [3]. This combination of heat treatment and controlled deformation has an important practical impact, as incremental deformation is a fundamental principle of several processing methods used in industrial practice. To obtain good combination of mechanical properties it is however necessary to optimize several parameters of Q-P process. Among the most important ones belong austenization temperature, cooling rate, quenching temperature and partitioning hold. 2. EXPERIMENTAL PROGRAMM The aim of this work was to investigate the influence of cooling rate from austenization temperature to the quenching temperature. A thermo-mechanical simulator was used for sample processing, as it applies precisely controlled and monitored thermal and deformation regimes, which might include rapid incremental deformations. Due to these abilities, precise temperature and deformation parameters can be set up, similar to the real process of technology or material development. This equipment also enables rapid changes in process parameters and precise simulation of processing conditions. Particularly for steels, temperature gradients of over 250 C per second for heating and cooling can be achieved. A speed of 3 m/s can be reached by deformation component. These parameters of the thermo-mechanical simulator have enabled the combination of heat treatment processing according to the Q-P method with incremental deformation steps.

3 2.1. Characterization of used materials Three high strength steels with slightly different chemical compositions were used in this work. While carbon content was 0.43% for all of them, various contents of manganese ( %) and silicon (2-2.6%) were chosen to investigate the influence of these main alloying elements (Tab.1). Table1. Chemical composition in weight % Tab. 1. Chemické složení ocelí v hm.% 2.2. Thermo-mechanical processing Steel C Si Mn Cr 2Si- 0.6Mn Si- 0.6Mn Si- 1.2Mn Three thermo-mechanical processing strategies with different cooling rates were designed for each steel (Tab.2, ig. 1). On the basis of previous optimization experiments performed on 2Si-0.6Mn steel, thermo-mechanical processing with 20 incremental deformation steps with incorporated Q-P process and cooling rate of 20 C/s was chosen as the most c onvenient one. This strategy was applied to all three steels, however due to the shift of transformation temperatures caused by higher silicon content, austenization temperature had to be increased for 2.6Si-0.6Mn and 2.6.Si-1.2Mn steels from 900 C to 950 C. Austenization hold of 100s was carried out i n all the strategies. Incremental deformation was always applied in 14.7s during cooling from austenization temperature in the form of subsequent tension and compression deformation steps where the tensile step was always slightly larger, with the total logarithmic deformation equal to 5. As the cooling rate is one of the most important parameters of each thermomechanical processing, three different average cooling rates of 20, 7 and 4.7 C/s were used to cool t he steels after incremental deformation to quenching temperature of 200 C. Tab.2. Parameters of thermo-mechanical processing Tab. 2. Podmínky tepelně-mechanického zpracování Steel Heating 2Si- 0.6Mn 900/ Si- 0.6Mn 2.6Si- 1.2Mn 950/100 Average cooling rate Quenching hold Partitioning hold 200/10 250/600 Different cooling rates were applied always in temperature interval of 820 C 200 C. Quenching temperature of 200 C and tempering hold conditions 250 C/600s were the same for all proposed processin g strategies. As the main aim of this experiment was to investigate the influence of cooling rate on final microstructure and properties of steels, microstructure analysis was carried out at processed samples and their mechanical properties were also determined.

4 Temperature [ C] C /100s 4.7 C/s 7 C/s 20 C/s 20x incremental deformation steps to 820 C 200 C/10s C/600s Time [s] ig. 1. Scheme of Thermo-mechanical processing Obr. 1. Schéma tepelně-mechanického zpracování 3. RESULTS AND DISCUSSION Si-0.6Mn Steel The processing with a cooling rate of 20 C/s resulted in a typical Q-P microstructure consisting of the mixture of matensitic matrix with lower bainite and 17% of retained austenite (ig. 2). The austenite was mainly in the form of thin films lining the martesitic laths and the ultimate strength of this microstructure reached 2096MPa (Tab.3). Slower cooling with a rate of 7 C/s already touched pearlite nose of CCT curves and therefore small islands of pearlite and free ferrite were found in final microstructure (ig. 3). This change in the microstructure was also accompanied by a decrease of strength. Even slower cooling by 4.7 C/s only increased the amount of pearlite in t he final microstructure and further deteriorated the properties enabling the steel to reach an ultimate strength of only 1662MPa. Ductility values followed the same trend as tensile strength. The growth of pearlite also means that carbon is used to crate cementite laths and therefore less carbon is available to stabilize the austenite. However, pearlite formed only relatively small fraction in the final microstructure and cementite laths were rather fine, so 8% of retained austenite was measured in the microstructure obtained after the cooling at a rate of 4.7 C/s. Tab. 3. Influence of cooling rate on mechanical properties of experimental steels Tab. 3. Vliv rychlosti chlazení na mechanické vlastnosti experimentálních ocelí Steel 2Si-0.6Mn 2.6Si-0.6Mn 2.6Si-1.2Mn Average cooling rate HV 10 [-] R p0,2 [MPa] R m [MPa] A 5mm [%] , , , Si-0.6Mn Steel A similar microstructure development was also observed for this steel with higher silicon content. Cooling by 20 C/s produced mostly martensitic microstructure ( ig. 4) with 13% of retained austenite and ultimate strength of 1965MPa. Decrease of cooling rate to 7 C/s caused the occurrence of free ferrite predominantly at the original austenite grain boundaries and lower amount of pearlitic areas was also found in the final microstructure (ig. 5). urther lowering of cooling rate to 4.7 C/s did not lead to any further signi ficant

5 , Brno, Czech Republic, EU change in final microstructure (ig. 6). The occurrence of pearlite in the final microstructure was reflected by the drop of ultimate strength by 400MPa to 1572MPa (Tab.3). B P 1µ ig. 2. 2Si-0.6Mn steel, cooling rate 20 C/s, martensite with lower bainite (B) Obr. 2. Ocel 2Si-0.6Mn, rychlost chlazení 20 C/s, martenzit a spodní bainit (B) 1µ ig. 3. 2Si-0.6Mn steel, cooling rate 4.7 C/s, large pearlitic area (P) Obr. 3. Ocel 2Si-0.6Mn, rychlost chlazení 4.7 C/s, perlitické oblasti (P) P 5µ ig Si-0.6Mn steel, cooling rate 20 C/s, martensite Obr. 4. Ocel 2.6Si-0.6Mn, rychlost chlazení 20 C/s, martenzit 15µ ig Si-0.6Mn steel, cooling rate 7 C/s, large ferrite areas (), pearlite (P) Obr. 5. Ocel 2.6Si-0.6Mn, rychlost chlazení 7 C/s, rozsáhlé oblasti feritu (), perlit (P) P ig Si-0.6Mn steel, cooling rate 4.7 C/s, large ferrite areas (), pearlite (P) Obr. 6. Ocel 2.6Si-0.6Mn, rychlost chlazení 4.7 C/s, rozsáhlé oblasti feritu (), perlit (P) ig Si-1.2Mn steel, cooling rate 4.7 C/s, martensite, bainite, ferrite () Obr. 7. Ocel 2.6Si-1.2Mn, rychlost chlazení 4.7 C/s, martenzit, bainit, ferit ()

6 , Brno, Czech Republic, EU Si-1.2Mn Steel The steel alloyed with higher contents of silicon and manganese showed different behavior from the previous two steels. The main difference was in the absence of grown pearlite colonies in the final microstructure even after the slowest cooling by 4.7 C/s (ig. 7). All three processing strategies managed to produce a microstructure of tempered martensite and lower bainite with some amounts of free ferrite (ig. 7-ig. 9). The volume fraction of retained austenite after the quickest cooling by 20 C/s was 14%. Another interestin g feature is that the cooling rate of 7 C/s did not r esult in any significant drop of mechanical properties, as the ultimate strength was still nearly 2000MPa. This can be explained by very little changes in the final microstructure in comparison with strategy with 20 C/s cooling. Only the slowest cooling by 4.7 C/s le d a distinguished decrease by approximately 450 MPa. 4. CONSLUSIONS Three thermo-mechanical processing strategies with different cooling rates were applied to three steels with 0.43% of carbon and varying silicon and manganese content. After austenization at C incrementa l deformation steps with a total logarithmic deformation equal to 5 were applied. Deformation finished at 820 C and three cooling rates of 20, 7 and 4.7 C/s were used to quench the steels from 820 C to 200 C. M 5µ ig Si-1.2Mn steel, cooling rate 20 C/s,martensite (M) and ferrite () Obr. 8. Ocel 2.6Si-1.2Mn, rychlost chlazení 20 C/s, martenzit (M) a ferit () 15µ ig Si-1.2Mn steel, cooling rate 7 C/s, martensite, bainite, free ferrite () Obr. 9. Ocel 2.6Si-1.2Mn, rychlost chlazení 7 C/s, martenzit, bainit, volný ferit () Subsequent heating to 250 C for 600s was carried ou t for all tested strategies. The ultimate strengths of MPa were obtained with ductility A5mm of 6-17%. Quicker cooling always resulted in higher ultimate tensile strength. The highest values of ultimate strengths were for all cooling rates achieved for 2.6Si-1.2Mn steel with the highest silicon and manganese content. It can be contributed to the fact that this steel had martesitic microstructures with bainite, a small amount of ferrite and retained austenite even after the slowest cooling, while the other two steels possessed fully grown pearlite in microstructures after the cooling by 7 and 4.7 C/s. The ductility followed th e same trend as ultimate strengths and decreased with decreasing cooling rate. Ductility of 2.6Si-0.6Mn and 2.6Si-1.2Mn steel was slightly higher for all cooling rates than corresponding ductility of 2Si-0.6Mn steel. 5. ACKNOWLEDGEMENT This paper includes results created within the project 1M06032 Research Centre of orming Technology and within the project GAČR 106/09/1968 Development of New Grades of High-Strength Low-Alloyed Steels with

7 Improved Elongation Values. The projects are subsidized from specific resources of the state budget for research and development. 6. LITERATURE [1] EDMONDS, D.V. et al. Quenching and Partitioning Martensite - A Novel Steel Heat Treatment. Materials Science and Engineering A, 2006, Vol , p [2] KUČEROVÁ, L. et al. Optimization of Q-P Process Parameters with Regard to inal Microstructures and Properties. In Conference proceedings of Annals of DAAAM for Vienna: DAAAM International Vienna, 2009, p [3] KLAUBEROVÁ, D. et al. Influence of Deformation Intensity and Cooling Parameters on Microstructure Development in QP Process. In Conference proceedings of Annals of DAAAM for Vienna: DAAAM International Vienna, 2010, p