Impact and tensile properties of ferrite-martensite dual-phase steels

Fatigue & Fracture of Engineering Materials & Structures doi: 10.1111/j.1460-2695.2008.01318.x Impact and tensile properties of ferrite–martensite dual-phase steels T . D A L A L L I I S F A H A N I 1, A . S H A F Y E I 2 a n d H . S H A R I F I 1 1 Department 2 Department of Materials and Metallurgical Engineering, Iran University of Science and Technology, Narmak, Tehran 16846-13114, Iran of Materials Engineering, Isfahan University of Technology, Iran Received in final form 4 December 2008 A B S T R A C T The effect of martensite morphology on the impact and tensile properties of dual phase steels with a 0.25 volume fraction of martensite (V m ) under different heat treatments was investigated. These treatments are direct quenching (DQ) and step quenching (SQ) that result in different microstructures and mechanical properties. To process dual phase steels, a low carbon manganese steel was used. At first the banding present in the initial steel was eliminated, then the two different heat treatments were applied. To reach a 0.25 volume fraction of martensite a variation of intercritical annealing temperatures was adopted for both treatments that allowed the evolution of different volume fraction of martensite. Phase analysis showed that an intercritical temperature of 725 ◦ C (between A 3 , A 1 ) gives the desired 0.25 V m of martensite. A comparison of impact, tensile and ductile–brittle transition temperature (DBTT) indicates that the microstructure of the direct treatment has a better toughness. The DBTT for the DQ and SQ treatment is −49 and −6 ◦ C, respectively. Keywords dual phase steels; fixed volume fraction of martensite; impact properties; toughness. INTRODUCTION Dual phase steels are classed as high strength low alloy (HSLA) steels. These steels are developed from conventional low alloy steel, and their microstructure is composed of relatively hard martensite particles which imparts high strength dispersed in a relatively soft and ductile ferrite matrix1,2 which supplies good elongation that can produce a desirable combination of strength and ductility.3–6 They are generally produced by an intercritical annealing in the austenite and ferrite region followed by a rapid cooling to ensure the transformation of the austenite to the martensite.7,8 In addition to the two phases present in this composite microstructure, depending on cooling rate, it may also contain retained austenite, new ferrite (epitaxial ferrite) and bainite.9–11 In recent years, application of high strength steels, especially dual phase steels, have made a great improvement in the automotive industry. This improvement originates from the characteristics of dual phase steels. By this, we mean the combination of high strength, good ductility, good weldability, low yield stress to tensile strength ratio (YS/TS), continuous yielding and high initial workhardening rates (n values). Also, due to its high strength, Correspondence: T. Dalalli Isfahani. E-mail: tdi1359@iust.ac.ir  c 2009 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct 32, 141–147 a thinner plate can be used for a specific strength in contrast with usual steels, which results in weight saving of products.12,13 In the production of dual phase steels, different heat treatment procedures (or quenching paths) can be applied which result in different microstructures and mechanical properties due to the different morphology of martensite. Resulting microstructures consist of ferrite and martensite phases, but the volume fraction and morphology are largely varied by the quenching paths. Among the different procedures the direct (or intercritical) quenching and step quenching are mostly used in industry. The martensite in direct and step quenched specimens has the morphology of spherical and network martensite for the former, and aggregates of large, blocky shaped martensite islands for the latter.14–17 Using dual phase steels with a 0.25 volume fraction of martensite makes the best combination of the useful properties in the automotive industry.18–20 Some of the factors that influence the impact properties are morphology, distribution and carbon content of martensite phase as well as the grain size.21–24 Previous studies have mainly focused on high-martensite dual phase steels which are not appropriate for use in automotive industry because of their low ductility and toughness properties. The previous work, on low-martensite 141 142 T . D A L A L L I I S F A H A N I et al. Table 1 Composition of the steel used Steel C-Mn Composition (wt%) Fe C P S Si Mn Cr Mo Ni 97.84 0.137 0.028 0.022 0.30 1.57 0.02 0.01> <0.02 Al 0.03 Co <0.02 Cu 0.01 Nb 0.02 Ti <0.01 V <0.01 W <0.04 Zr <0.003 Pb <0.01 dual phase steels were mainly involved with fatigue properties, but in this research the impact properties are discussed, using the obtained Charpy curves. From these curves the ductile–brittle transition temperature (DBTT) are identified. As mentioned previously the purpose of the current study was to obtain the impact and tensile properties of dual phase steels with 0.25 V m martensite, different morphology and distribution of martensite with a fixed grain size. In this paper the DBTT for the different microstructures of dual phase steels with 0.25 V m is also presented. for 4 h (cooled in the furnace), then normalized at 910 ◦ C for 20 min. to make fine pearlite instead of the coarse ones which resulted in a smaller grain size. The austenitizing temperatures Ac 1 and Ac 3 , which define the ferrite and austenite region were calculated by using Eqs (1) and (2) with temperature degrees of 715 and 820 ◦ C, respectively. Ac1 = 723 − 10.7Mn − 16.9Ni + 29.1Si + 16.9Cr + 290As + 6.38W √ Ac3 = 910 − 203 C − 15.2Ni + 44.7Si + 104V + 31.5Mo + 13.1W EXPERIMENTAL PROCEDURE The composition of the steel used is shown in Table 1. It was supplied in the form of hot rolled plate with 10 mm thickness that had a ferrite-pearlite structure with slight banding (Fig. 1). To eliminate the banding present in the initial steel at the first stage, the specimens were homogenized at 1200 ◦ C (1) (2) The intercritical thermal treatments were performed at 715, 725,750,775 and 800 ◦ C for 1 h, followed by quenching in brine solution. After etching by nital 3% the microstructure was analysed with the help of an optical microscope and utilization of image analysis software for determination of volume fraction and grain sizes. In each sample, image analysis was carried out in at least ten Fig. 1 Banding present in the ferrite-pearlite microstructure of the initial steel.  c 2009 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct 32, 141–147 IMPACT AND TENSILE PROPERTIES OF FERRITE–MARTENSITE DUAL-PHASE STEELS 143 Fig. 2 Microstructure of dual phase steel achieved from direct quenching ×750 (a) and step quenching ×750 (b). different regions of the specimen. The analysing experiment showed that the 0.25 volume fraction of martensite was obtained with the use of intercritical temperature at 725 ◦ C .This temperature was the same for both, the direct quenching (DQ) and the step quenching (SQ) heat treatment schedules. The two different microstructures obtained are shown in Fig. 2. As mentioned before, in order to achieve different microstructures two different procedures were applied. The first procedure involved an intercritical treatment at 725 ◦ C for 60 min, followed by quenching in brine solution, and the second procedure involved austenitizing  c 2009 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct 32, 141–147 at 910 ◦ C for 20 min, followed by furnace cooling to the required intercritical temperature of 725 ◦ C for a 60 min duration before quenching in brine solution. The former has spherical and network morphology while the latter contains aggregates of large, blocky shaped martensite islands. The heat treatment procedures used to achieve different morphologies of martensite from the initial steel are schematically presented in Figs 3 and 4. To make sure that the two phases present in the microstructure are ferrite and martensite, microhardness was applied. At least six hardness values were taken in each sample. The soft phase had a hardness value around 144 T . D A L A L L I I S F A H A N I et al. 250 Initial Steel 200 Energy (Joule) Direct quenched 150 100 Step quenched 50 Fig. 3 Heat treatment used to achieve a dual phase structure from the initial steel using direct quenching. 0 -100 -50 0 50 100 -50 Temperature (°C) Fig. 6 Comparison of charpy curves for the two different dual phase steels and the initial steel (homogenized and normalized). Fig. 4 Heat treatment used to achieve a dual phase structure from the initial steel using step quenching. 1000 900 Step quenched Stress (Mpa) 800 700 Direct quenched phase steels. On the other hand the initial steel has a noncontinuous yielding which is characterized by the luder bands. Impact testing of the V notch specimens was carried out, after which the charpy curves were plotted for initial, direct quenched and step quenched specimens. The charpy curves for the two different microstructures of the dual phase steels and the homogenized and austenized initial steel are shown in Fig. 6. Fracture surfaces of impact specimens were analysed using a scanning electron microscope (Figs 7 and 8). 600 500 RESULTS AND DISCUSSION 400 300 Initial Steel 200 100 0 0 20 40 60 Strain Fig. 5 Comparison between the two different dual phase steels and the initial steel (homogenized and normalized). 200 Vickers while the harder phase had a value around 510 Vickers which proves that we have obtained a ferrite– martensite microstructure. Tensile testing of the specimens, in accordance with ASTM standard (A370-B), was conducted at room temperature in a computer controlled Housfield machine using a cross head velocity of 0.50 mm/min. The true stress–true strain curves of the dual phase steels obtained by step and direct quenching and also for the homogenized and austenitized initial steel are shown in Fig. 5. As shown the step quenched and direct quenched specimens have continuous yielding which is the unique characterization of ferrite–martensite dual The two morphologies obtained from different heat treatment procedures result in varying mechanical properties. From the true stress–true strain curves (Fig. 5) it can be seen that both of the dual phase steels have better tensile properties than the initial homogenized and normalized steel. The basic reason is that pearlite is substituted by martensite which is a harder phase. Martensite acts like a reinforcement in the dual phase steel. As well as this, the dual phase microstructures have continuous yielding which is a unique property of the dual phase steels due to their composite microstructure. This results in desirable formed surfaces but the initial steel has non-continuous yielding resulting in bad formed surfaces. The dual phase steel obtained by the step quenching procedure has a better tensile property than the direct quenching procedure. As mentioned by others25,26 due to the 3–4 volume percent expansion forced by the austenite to martensite transformation the dislocation concentration around the brittle martensite phase is high which causes the ferrite phase to be under stress. In the direct quenched specimens we have a uniform distribution of small spherical, network martensite resulting in a more uniform distribution of dislocations. This results in less concentration of dislocation  c 2009 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct 32, 141–147 IMPACT AND TENSILE PROPERTIES OF FERRITE–MARTENSITE DUAL-PHASE STEELS 145 Fig. 7 Fracture surfaces of the dual phase steel obtained from direct (a) and step quenched (b) specimens above the DBTT temperatures showing a ductile fracture. so the locking of dislocations is less. However, in the step quenched specimens we have large blocky martensite islands, which results in a non-uniform distribution of martensite and also the concentration of martensite in different parts of the specimen due to the less uniformity compared to the spherical and network morphology of the direct quenched specimen which originates from the larger size of the blocky martensite islands. This concentration causes more locked dislocations that result in a higher tensile property. The curves in Fig. 5 indicate that work hardening as a result of locked dislocations is higher in the step quenched specimen than the direct quenched specimen. From all the mentioned reasons and as shown  c 2009 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct 32, 141–147 by the curve we can mention that more elongation and a better formability are possible by direct quenched specimens. High work hardening rate and low yield stress to tensile strength ratio ( YTSS ) of steels are other important characteristics of dual phase steels making them suitable for many industries such as automobile industry. The ( YTSS ) ratio for the step quenched specimen, direct quenched specimen and the initial steel is 0.54, 0.55 and 0.676, respectively. The other tensile properties are given in Table 2. From the Charpy curves plotted with the data’s obtained by the impact test we see that the DBTT for the direct quenched, step quenched and initial steel are −49, −6 and 146 T . D A L A L L I I S F A H A N I et al. Fig. 8 Fracture surfaces of the dual phase steel obtained from direct (a) and step quenched (b) specimens under the DBTT temperatures showing a brittle fracture. Table 2 Tensile properties Initial Steel Direct quenched Step quenched YS (Mpa) UTS (Mpa) ( YTSS ) 359 406 523 531 751 950 0.676 0.540 0.550 −34 ◦ C, respectively. We can see that the impact property of the direct quenched specimen has improved from the initial steel but the step quenched specimen has become worse. The reason is that in the step quenched specimen we have a large number of locked dislocations which cause low ductility. Also in low temperatures the mobility of dislocation are low which causes more locked dislocations in different parts of high concentrated martensite. All these reasons cause lower impact energy and impact strength. In the direct quenched specimen we have more ductility resulting in better impact property. It can also be seen that the impact strength of the initial and direct quenched steel is greater while the step quenched specimen has a low value. The martensite phase in the step quenched specimen is located at some parts of the grain boundary in a noncontinuous way that results in poor impact property. SEM of fractured surfaces (Figs 7 and 8) above and beneath the  c 2009 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct 32, 141–147 IMPACT AND TENSILE PROPERTIES OF FERRITE–MARTENSITE DUAL-PHASE STEELS DBTT for both step quenched and direct quenched dual phase steels are shown (the micrographs were taken from the fracture surfaces of impact specimens). In the case of ductile fracture we see dimples and in the brittle case we have cleavage fracture. From the SEM of fractured surfaces above the DBTT it can be seen that in Fig. 7a we have only dimples but in Fig. 7b, some parts with cleavage fracture are seen along with dimples which indicates that the step quenched specimens have a more brittle property. Also, Fig. 7a has smaller dimples and a more uniform distribution compared to Fig. 7b. As shown in Fig. 8b we have only cleavage fracture while in the direct quenched specimen (Fig. 8a), both dimples and cleavage parts can be seen. Also, it can be seen from Fig. 8b that we have big cleavage parts and the cleavage parts are not uniformly distributed and are concentrated in different parts of the microstructure. This is probably due to the presence of large blocky martensite islands. Figures 7a and 8a are referred to direct quenched and Figs 7b and 8b are referred to step quenched specimens. CONCLUSION As Fig. 5 shows we can conclude that tensile properties for both of the dual phase steels are desirable, the dual phase steel obtained from step quenching has a better tensile strength while the direct quenched specimen has a better ductility. As Fig. 6 represents we understand that the specimen prepared with the use of step quenching has such a bad impact property that makes the use of this microstructure impossible. On the other hand using direct quenching improves the impact property while step quenching deteriorates it. 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