Keywords: Dual phase steel, martensite, ferrite, intercritical annealing, quenching, K, n

Transcription

1 Applied Mechanics and Materials Online: ISSN: , Vol. 493, pp doi: / Trans Tech Publications, Switzerland Effect of intercritical annealing temperature and holding time on microstructure and mechanical properties of dual phase low carbon steel ALFIRANO 1, a, SAMDAN Wibawa 1, b, MAULUD Hidayat 2, c 1 Department of Metallurgy Engineering, University of Sultan Ageng Tirtayasa, Cilegon, Indonesia 2 Division of Applied Research, PT. Krakatau Steel, Cilegon, Indonesia a alfirano@ft-untirta.ac.id, b samdanwibawa@rocketmail.com, c maulud.hidayat@krakatausteel.com Keywords: Dual phase steel, martensite, ferrite, intercritical annealing, quenching, K, n Abstract. Dual phase steels are an important advanced high strength steel, which have been widely used in the automotive industry for vehicle components requiring light weight and safety. In this study, the formation of dual phase structure with various volume fraction of martensite in a low carbon steel SS400 during intercritical annealing were investigated. It was found that intercritical annealing temperature and holding time affected the microstructure and mechanical properties of dual phase low carbon steel. The specimens were heated at intercritical annealing temperature of 750 o C, 775 o C, 800 o C and 825 o C, for holding periods of 6-18 minutes, followed by water quenching in order to get a dual phase ferrite and martensite. After quenching, it was obtained the optimal annealing conditions at 800 o C with a holding periods of 10 minutes. In this condition, the tensile strength was increased up to 621 N/mm 2 or 39.24% higher than the initial condition, while the elongation decreased up to 13.8%. The hardness of specimens increased from to HVN or up to 84.67% higher than the initial condition. Meanwhile the volume fraction of martensite was 24.08%. The higher the temperature of the heating value of grain growth rate constant (K) increases. In addition, at the optimal poin, the value of K (grain growth rate constant) and n (Avrami s exponent) were and 0.318, respectively, with activation energy (Q) of 3.98 J/mol. Introduction In order to decrease the energy consumption in automotive industry, optimization of well-known materials and developing new materials with a high ratio of strength to density and a good suitability for metal forming operation are still progressed [1, 2]. Dual phase steels are one of the important steel products developed for the automotive industry, in which steel grade exhibiting high strength and good formability is required so that weight of vehicle can be reduced for fuel saving purpose. The pure dual phase microstructure consists of a matrix of soft a -ferrite grains, strengthened by a finely dispersed, hard MA (Martensite Austenite) constituent [3]. Thus, dual phase steels have characteristic mechanical properties which include low yield strength and high ultimate tensile strength as compared with conventional low-carbon steel [3]. The steel obtains its specific properties from the low temperature transformation of intercritical austenite to lath-type martensite. This transformation results in a non-uniform dislocation distribution and internal stresses. The austenite, enriched in carbon during the processing, may not transform entirely into martensite, resulting in the presence of small amounts of retained austenite. The amount of retained austenite depends among others, on the hardenability of the transforming austenitic phase and the size of the austenitic particles [4]. However, the combination of a good strength and ductility can be obtained by developing a dual phase or multiphase steels through the heat treatment process. In this study, the microstructural change in low carbon steel during intercritical annealing has been investigated with a focus on the formation of dual phase structure with various volume fraction of martensite. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-19/09/16,11:25:19)

2 722 Advances in Applied Mechanics and Materials Experimental procedure The chemical composition of the low carbon steel used in this study are shown in Table 1. Table 1. Chemical composition of steel used in this study, wt% Intercritical annealing was tested on tensile test sample with standard JIS Z 2201 test piece no 5. The intercritical annealing temperatur behind A 1 and A 3. A 1 = ,9 Ni + 29,1 Si 20,7 Mn - 16,9 Cr ± 11,5 o C (1) A 3 = ,7 Si 15,2 Ni + 31,5 Mo V ± 16,7 o C (2) Then, it was found that A 1 =710,32 o C ± 11,5 o C and A 3 =872 ± 16,7 o C. The specimens were heated at intercritical annealing temperature of 750 o C, 775 o C, 800 o C and 825 o C, for holding periods of 6-18 minutes, followed by water quenching. For mechanical properties, the samples were cut in to two parts. Parts one for tensile test and part two for hardness test. For tensile test sample with standard JIS Z 2201 test piece no 5. A cross head speed of 10 mm/min was maintained upto the yield point and, thereafter speed of 50 mm/min was maintained upto fracture. The strain to necking in each specimen was recorded using an ektensometer. From the tensile test the yield strength, ultimate tensile strength and % elongation were measured. For hardness test used macro vickers hardness.the microstructures of samples were studied by optical microscopy. Martensite volume fraction was measured by point counting method based on ASTM E standard. Result and discussion C Si Mn P S Cu Ni 0,1602 0,0078 0,6229 0,0086 0,0089 0,0153 0,0133 Al V As N Mo Ti Cr 0,0377 0,0009 0,0017 0,0044 0,0023 0,0007 0,0123 W Ca Pb Sb Fe Co Sn 0,0041 0,0010 0,0003 0,0041 <99,09 0,0023 0,0025 The microstructure of specimens before and after heat treatment is shown in Fig. 1. The specimens before heat treatment were consisted of pearlite and ferrite phases (Fig. 1(a)). Microstructures formed after the quenching were formed ferrite and martensite phase, which can be seen in Fig. 1(b). Martensite was formed when the steel material is cooled rapidly from the austenitic region. Figure 1. Microstructure of specimens (a) before heat treatment and (b) after heat treatment at 775 o C for 15 minutes, water quenched.

3 Applied Mechanics and Materials Vol Figure 2. Effect of heating time on (a) volume fraction of martensite and (b) hardness of specimens heated at various temperature. The effect of heating time on volume fraction of martensite at various temperature are shown in Fig. 2(a). Volume fraction of martensite increases with increasing heating time. It shows that the higher in heating temperature and the longer in holding time, it will produce more martensite. In the A 1 temperature austenite began to transform by eutectoid reaction into ferrite, cementite and pearlite. If the temperature is increased above the A1 then carbon can dissolve into austenite, so that the carbon was contained on ferrite gradually dissolved into the austenite. Because not only the austenite formed from pearlite, but also the dissolution of ferrite [5]. Holding timeduring the heating process aims to provide an opportunity for atoms to diffuse in austenite [6]. Austenite phase is unstable if the steel is cooled rapidly from the austenitic region which has a fcc structure, therefore the bcc structure will not formed because of the time which needed to complete the transformation into bcc is not enough. Austenite has a higher carbon content than ferrite should remove carbon from the solution, but due to the rapid cooling, carbon was trapped, so that bct structure or so-called martensite was formed [7]. The hardness value of specimens can be seen on Fig. 2(b). The hardness value of the specimen before heat treatment was HVN. Heating was performed on the ferrite and austenite region. Increased in hardness occurs caused by increasing in the volume fraction of martensite. The difference of cooling rate between specimens before and after heat treatment can cause the difference in phases; ferrite-pearlite and ferrite-martensite, respectively. In low carbon steel, ferrite and pearlite usually were formed together and its hardness is determined by the carbon content, so the hardness of low carbon steel will be affected by ferrite and pearlite content. While the specimens were cooled in the quenched medium, generated the final structure of ferrite and martensite, where the carbon content in the martensite is saturated. In can be cause increasing in hardness value. Increasing heating temperature will increase the cooling rate so that martensite easily formed. Increase in martensite volume led to the increasing of hardness value [4]. The effect of heating time on elongation at various temperatures is shown in Figure 3. The volume fraction of martensite increases with increasing heating temperature and holding time, while the volume fraction of ferrite decreases with increasing heating temperature and holding time. Decrease in the volume fraction of ferrite and increase in volume fraction of martensite cause the decrease of elongation, because the nature of the hard martensite and ferrite gertas increased while that is resilient reduced [8,9]. Despite the decrease in elongation, the dual-phase ferrite-martensite steels with a microstructure which is easily formed because it consists of a fixed and martensite phase in a soft ferrite matrix [10]. The increase in temperature will increase cooling rate resulting a decrease in strain. Decrease in strain can occur due to two factors, such as the residual stress during the transformation of austenite to martensite during cooling and dislocations increases with increasing volume of martensite [4].

4 724 Advances in Applied Mechanics and Materials Figure 3. Effect of heating time on elongation at various temperatures. Figure 4. The effect of martensite volume fraction on tensile strength. The effect of martensite volume fraction on tensile strength is shown in Fig. 4. At 750 o C, tensile strength shows the fluctuation value. This fluctuation of tensile strength can be caused by a difference in the dimensions of the sample when cooled rapidly [11]. The presence of pearlite phase is likely a lot compared to other heating temperature and the amount of pearlite volume fraction at a temperature of 750 o C heating at various times of fluctuating resistance values indicate fluctuating tensile strength. In addition, the pearlite phase formed at temperatures of 750 o C has a large grain size. It is generally known steel with a large grain size will have low power [7]. At 775 o C, increase in the volume fraction of martensite, it will improve the value of tensile strength [9]. When compared to the initial material before intercritical annealing and quenching process, where tensile strength was 446 N/mm 2. Surely there is a very significant increase in tensile strength including the volume fraction of martensite. This could be due to the structure of the soft ferrite and hard martensite dual phase steel resulting in a strong and resilient. The main source of strength dual-phase steel is located in the martensite microstructure acts as a strong and able to load carrying constituent in the soft ferrite matrix. If the steel is supplied load voltages then there will be propagated to the strong particle with the interfacial shear matrix [8]. Dual phase steel also has a high rate of work hardening due to dislocation density resulting in high tensile strength [12]. Presence of dislocation density may impede the movement of other dislocations, due to the constraints that the energy stored in the material to be increased so that the material becomes harder and stronger. At 825 o C, the higher of volume fraction of martensite will lower the tensile strength. Tensile strength values decrease because of the increase in the volume fraction of martensite beyond the optimum value, where the nature of the hard martensite phase brittle causing atomic defects when martensite formation that is the source of the rift. Because of the reduced ferrite will reduce bonding between phases, then tensile strength values also decrease. According Smallman martensitic structure optimum proportion of 20% [13]. Dual phase steel ferrite-martensite has martensite volume ranged from 20% to 30% in the ferrite matrix including the advanced high strength stainless steel (AHSS) making it suitable for applications in the automotive sector [14].

5 Applied Mechanics and Materials Vol Kinetics of formation of austenite during the heating process at the point of intercritical annealing has long been studied. This shows the kinetics of formation of austenite formation of austenite in dual phase steel is diffusion control growth. Johnson-Mehl-Avrami equation (JMA) with kolomogrov (JMAK) can be used for transformation of austenite during intercritical annealing. The equation is as follows [7]: f = 1 exp (-Kt n ) (3) By using the modified JMA equation, where a large volume of austenite is limited, then the following equation: ln [ln (1/(1- )] = n ln t + ln K (4) the austenite volume, fɤ, can be considered to the volume of martensite. fe is an austenite volume at equilibrium, the value can be found using the level rule Fe 3 C diagram. t is the holding time on heating at the point of intercritical annealing. K is a constant growth rate of austenite, n is the Avrami exponent. Through a modified JMA equation then can be searched value grain growth rate constant (K) and the value of Avrami exponent (n) is obtained from the slope and intercept resulting from linear equation relationship between ln [ln (1 / (1 - fɤ / fe) ] versus ln t (Fig. 5). Using the data from experiment (Fig. 5), the n and K value can be determined, as listed in Table 2. Rate constant (K) is a parameter that depends on the temperature and velocity associated with grain growth, nucleation frequency. So the higher the heating temperature intercritical annealing point it will increase the value of K vice versa. While the Avrami exponent (n) depends on the nucleation and growth processes and the grain nucleation rate has a significant effect on the value of n [7]. Figure 5. The relationship between ln [ln (1 / (1 - fɤ / fe) ] and ln t from experiment. Table 2. n and K value from experimental data in various temperature. Temperature ( o C) Equilibrium volume fraction of austenit,fe Avrami s exponent (n) 750 0,2373 0,409 0, ,3494 0,266 0, ,5357 0,279 0, ,7635 0,318 0,318 Rate constant (K) From data grain growth rate constant (K) can be searched through the magnitude of the activation energy Arrhenius equation: ln K = ln A ( ) (5)

6 726 Advances in Applied Mechanics and Materials Through the Arrhenius equation can be determined the value of activation energy (Q) and constant (A) is obtained from the slope and intercept resulting from linear equation between ln K and 1/T. From this relationship, activation energy (Q) and constant (A) will be 3,98 J/mol and 0,606, respectively. Summary 1. Dual phase steel shows an excellent combination between strength and ductility due to the coexistence of harder (martensite) and softer (ferrite) phase in their microstructure. 2. The hardness, tensile strength and martensite volume increase with increasing heating temperature and holding time. At the optimum point, the value of hardness, tensile strength and martensite volume were HVN (higher 84.67% than the initial value), 621 N/mm 2, and 24,08 %, respectively. In the other hand, elongation decrease until 13.8% at the optimum poin. 3. At the optimum point, K and n value was and 0.318, respectively with activation energy (Q) of 3.98 J/mol. References [1] M. J. Lee, Advanced High Strength Steel Technology in the Ford500 and Freestyle, Great designs in steels, Semina, [2] M.S. Rashid, Dual Phase Steels, Ann. Rev. Mater. Sci. (1981), [3] S. Sodjit and V. Uthaisangsuk, High Strength Dual Phase Steels and Flow Curve Modeling Approach, TSME-ICOME, [4] G.R. Speech and R.L. Miler, Mechanical properties of ferit martensit steels, New Orleans, LA, USA, [5] ASM Metal handbook, Heat Treating, Vol 4. [6] M.F. Ashaby and K.E. Easterling, Acta metallurgical. 30 (1982), [7] A. Agusto, Steel forming and heat treating handbook, [8] Saefudin, Analisa sifat mekanik baja fasa ganda pada proses perlakuan panas intercritical annealing dengan quenching untk baja karbon rendah, LIPI, Tangerang. [9] Y. Okitsu, Fabrication of ultrafine grained steels without severe plastic deformation and their application to outomotive body structure, [10] A. Sadabad et al., Kinetics of austenite formation in dual phase steels, ISIJ International. 48 (2008), [11] S.H. Avner, Introduction to physical metallurgy, Mc.Graw Hill Book Company, 2 nd ed, [12] A. Kustiawan, Studi analisa sifat mekanik dan struktur mikro baja fasa ganda (dual phase steel) pada proses perlakuan panas intercritical annealing dengan pendinginan cepat (quenching) untk baja karbon rendah spec SPHC grade OA 0804, skripsi program sarjana teknik Untirta, Cilegon, [13] A. Murugaiyan et al., Phase Transformations in Two C Mn Si Cr Dual Phase Steels, ISIJ International. 46 (2006), [14] T. Waterschoot et al., Tempering Kinetics of the Martensitic Phase in DP Steel, ISIJ International. 46 (2006),