Fracture Morphologies of Advanced High Strength ... - Springer Link

Acta Metall. Sin. (Engl. Lett.), 2014, 27(1), 101–106 DOI 10.1007/s40195-014-0032-8

Fracture Morphologies of Advanced High Strength Steel During Deformation Ying Sun • Xifeng Li • Xiangyu Yu • Delong Ge • Jun Chen • Jieshi Chen

Received: 5 September 2013 / Revised: 21 October 2013 / Published online: 28 January 2014 Ó The Chinese Society for Metals and Springer-Verlag Berlin Heidelberg 2014

Abstract The fracture morphologies of several advanced high-strength steels (DP590, DP780, DP980, M1180, and M1300) formed in uniaxial tension and piercing were observed by scanning electron microscope, and then quantitatively analyzed by image processing technique. The tension-induced fractographs are dominated by obvious uniform or bimodal size dimples, while shearing-induced fractographs have smooth surfaces and few dimples. The fracture zone of higher grade DP steels is smoother. As for M1180 and M1300, the fracture zones consist of very small dimples and smooth brittle surfaces. The dimple size of M1300(*1.2 lm) is smaller than that of M1180(*1.6 lm). Moreover, in the tensile fracture, the quantitative correlation between average dimple diameter (d) and tensile strength (r) can be represented by d = 10,502.32r-1.21. However, the relation between dimple density and tensile strength is not monotonic due to the appearance of bimodal size dimples with increase of tensile strength. For shearing-induced fracture during piercing, the fitted empirical model between the percentage of burnish zone (f) and tensile strength can be described as f = 239.9r-0.36. KEY WORDS:

High strength steel; Fracture morphology; Dimple size; Burnish zone; Tensile strength

1 Introduction In the automotive industry, a lot of efforts are made to reduce vehicle weight while maintaining performance and cost competitiveness. One such effort is the use of advanced high strength steels (AHSS) as the primary body materials, including dual phase, transformation induced plasticity, complex phase, and martensitic steels [1, 2]. Fracture is a major defect occurred in sheet metal forming due to some factors, such as material property, stress state, strain rate, and temperature [3, 4]. Different kinds of fracture show different fracture surface morphologies.

Available online at http://link.springer.com/journal/40195. Y. Sun X. Li X. Yu D. Ge J. Chen (&) J. Chen Department of Plasticity Technology, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, China e-mail: [email protected]

Some researchers have focused on the study of fracture morphologies during tensile tests and other forming processes. Sun et al. [5] investigated the fracture surfaces of DP600, DP780, and DP980 under tensile loading conditions. They found that the amount of microvoids near fracture surface decreases with increasing strength level. Li et al. [6] studied the SEM fractographs of the in situ tensile test of Al-6061 samples under different stress states. They observed that in the in situ 0° tensile test, dimple-dominant fracture occurs; whereas in the in situ 90° test, shear fracture occurs and; and in the in situ 45° test, mixed fracture occurs. Similar findings were also reported by Agarwal et al. [7]. They found that the void is sensitive to stress triaxiality and the equivalent plastic strain for 6061-aluminum alloy. Das [8] investigated the quantitative relation between deformation-induced martensite and voids during the tensile deformation of metastable austenitic stainless steel at various strain rates under ambient temperature. Das et al. [9] also studied the surface morphologies and dimple geometries after tensile deformation of

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AISI 304LN stainless steel at various strain rates at room temperature by image processing technique. The results showed that dimple density was high, while dimple diameter was smaller at lower strain rate. Enami [10] investigated the fracture surfaces of the round notched tension specimens after compressive and tensile prestrain for TMCP steels and hot-rolled SM490B steels. They found that in the case of TMCP steel tested, completely ductile fracture without a local cleavage crack is observed throughout the test for 0, 10% and 30% compressive, and 10% tensile prestrain. In the case of SM490B steel tested, local cleavage cracks were observed in 10% and 30% compressively prestrain specimen. Yan et al. [11] carried out tensile tests of a 980 MPa high strength steel at various temperatures to compare the difference of fracture surfaces. He et al. [12] studied the fracture surfaces of DP1200 and M1200 under the uniaxial tensile test and the cup drawing test, respectively. They concluded that the fracture mode of the DP1200 tension sample is dimple-dominant ductile fracture, while quasi-cleavage fracture occurs in the tensile test of M1200. Both fracture surfaces of DP1200 and M1200 under the cup drawing tests are dominated by equiaxed dimples. Yu et al. [13] have simulated the fracture surface in sheet-metal blanking processes to obtain the effect of clearance on the fracture surface quality. Tasan et al. [14] observed the fracture surface of DP steel specimens along different strain paths. They indicated that the DP steel shows a through-thickness shear fracture in all strain paths, and the fracture surface is completely filled with dimples, average size of which is about 3 lm. Overall, fracture mode of AHSS and the quantitative correlation between fracture characteristics and tensile strength during deformation reported in the afore-mentioned literature are incomplete and scattered. Therefore, it is necessary to systematically study the fracture characteristics of typical AHSS. In this paper, a series of experiments including uniaxial tensile tests and piercing tests were carried out based on commercial DP590, DP780, DP980, M1180, and M1300 to study their fracture mode and the quantitative correlation between fracture morphologies and tensile strength during deformation.

Ying Sun et al.: Acta Metall. Sin. (Engl. Lett.), 2014, 27(1), 101–106

tester with the maximum load capacity of 20 kN. A constant tensile speed of 2 mm/min was utilized. Strains were measured by an extensometer with a gauge length of 50 mm. The geometry of the uniaxial tensile specimens is shown in Fig. 1. Piercing tests were carried out on a 200T H1F200 servo press. The steel sheets were cut into a series of circular sheets with a diameter of 100 mm. Figure 2 shows the schematic of experimental setup for the piercing test. The diameter of punch was 10 mm, and the unilateral clearance of punch-die was 0.125 mm. The edge radius of both punch and die was 0.05 mm. After uniaxial tensile tests and piercing tests, the fracture surfaces were sawed from the fracture specimens. Some contaminated fracture surfaces were cleaned with acetone ultrasonic cleaner. Then fracture surfaces were carefully examined under JEOL JSM-7600F scanning electron microscope (SEM) to record fractographic features. A set of fields were observed at an operating voltage of 20 kV throughout. After that, fracture surface morphologies were analyzed quantitatively by image processing of tensile and piercing fractographs. Image processing is a powerful tool for characterizing the void morphologies [3]. Extensive image processing technique was employed on the SEM fractographs to characterize the two-dimensional geometry of dimples (i.e., dimple diameter and dimple number density) on the fracture surfaces using the software DigitalMicrograph. After implementing several image processing operations such as scale calibration, image enhancement, and gray level adjustment, voids on a fracture surface were detected by the software DigitalMicrograph due to the high degree of contrast between the dark voids and the brighter peripheries. Tension-induced fractographs of selected materials (DP590 and DP780) and

Fig. 1 Geometry of the uniaxial tensile specimens (unit: mm)

2 Experimental Commercially available DP590, DP780, and DP980 dual phase steel sheets and M1180, M1300 martensitic steel sheets with a thickness of 1.4 mm supplied by Baosteel were used in this study. In order to obtain tensile and shearing fracture surfaces, uniaxial tensile tests and piercing tests were carried out at room temperature, respectively. Uniaxial tensile tests were performed on a Zwick/Roell Z020 tensile

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Fig. 2 Schematic of experimental setup for the piercing test (unit: mm)


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Fig. 3 SEM images showing the fracture surfaces and the corresponding void networks after image-processing of SEM fractographs: a, b DP590; c, d DP780

their corresponding dimple network processed through the image processing technique are given in Fig. 3. At the same time, the areas of all the voids were calculated. Finally, dimple diameter and dimple number density were obtained. As for piercing fractographs, the dimple characteristics were not obvious. As is well known, the cutting edge is made up of four typical bands: rollover, burnish zone, fracture zone, and burr. Therefore, the percentage of burnish zone, which is defined as f = h1/H, is used to identify the piercing fracture surface feature quantitatively, where f is the percentage of burnish zone, h1 is the height of burnish zone, and H is the total height of cutting edges. h1 and H were measured by the software SmileView on the SEM fractographs. All the fractographs were analyzed to obtain an average value.

3 Results and Discussion 3.1 Correlation Between Tensile Fracture Characteristics and Tensile Strength By uniaxial tensile tests, mechanical properties of five high strength steels at room temperature are obtained and listed

Table 1 Mechanical properties of five materials at room temperature Steel

Yield strength (MPa)

Tensile strength (MPa)

Elongation (%)

DP590

330

594

DP780

497

843

26 21

DP980

698

1,003

15

M1180

1,075

1,300

13

M1300

1,240

1,470

12

as shown in Table 1. Tensile strength of the five steel sheets increases from 594 to 1,470 MPa. Representative SEM images of the tension-induced fracture morphology at different strength levels are shown in Figs. 3a, c, and 4a, b, and c. A suitable magnification (91,000) was used in all cases so that representative fracture features were recorded. Comparing the five SEM fractographs, the tensile fracture surfaces of DP590 and DP780 (Fig. 3a, c) are dominated by a large number of round or equiaxed dimples, which is typical ductile fracture. The average dimple diameter of DP590 is higher than that of DP780. The calculated value is 4.6 and 2.8 lm, respectively. The dimple number density of DP590 and DP780 is estimated as 0.189

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Fig. 4 SEM images showing the fractographs of tensile fracture surface of DP980 a, M1180 b, M1300 c

and 0.457 lm-2, respectively. Figure 4a, b, and c show the fracture surfaces of DP980, M1180, and M1300, which reveal that a large number of smaller dimples mixed with several larger dimples distribute on the fracture surfaces. The formation of large-size dimples is related to brittle particles fracturing and particle–matrix decohesion. Similar findings were also reported by Sun et al. [5, 15]. They found that the failure in higher grade DP steels was driven by ferrite/martensite interface decohesion and fracture of martensite. Moreover, the dimples become shallower with increasing strength of AHSS. Based on SEM image processing, the dimple diameter can be obtained according to the equation as d ¼ 2ðS1 =pÞ1=2 ; where S1 is the area of each dimple. The dimple number density is estimated as q = n/S, where n and S are the number of dimples within a SEM fractograph and the area of a SEM fractograph, respectively. Figure 5 reveals the quantitative relationship between the average dimple diameter of tensile fracture and tensile strength. With increase of tensile strength from 594 to 1,470 MPa, the average dimple diameter (d) decreases from 4.6 to 1.6 lm. The trend curve follows the power function expression of d ¼ 10502:32r1:21 ;

Fig. 5 Variation of average dimple diameter with tensile strength

ð1Þ

where r is the tensile strength of high strength steel expressed as MPa. As shown in Fig. 6, when the tensile strength is below 1,000 MPa, the dimple number density increases with increase of tensile strength, but the trend reverses when the tensile strength is above 1,000 MPa. Such phenomenon is reasonable due to the appearance of bimodal size dimples with increase of tensile strength. Fig. 6 Variation of dimple number density with tensile strength

3.2 Correlation Between Piercing Fracture Characteristics and Tensile Strength To compare the piercing surfaces, the piercing tests were carried out under the same conditions for different materials. A specimen after the piercing process is shown in

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Fig. 7. One circular hole with a diameter of 10 mm appears in the middle of the specimen after piercing. There are three stages during the piercing process: elastic deformation stage, plastic deformation stage, and fracture stage. Different materials show different fracture


surface characteristics [16]. Figure 8a, c, e, g, and i show the low magnification fractographs of the cutting edges of DP590, DP780, DP980, MS1180, and MS1300, respectively. The results indicate that the percentage of burnish zone of different materials is different. By calculations of

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each fractographs (at least three parallel data under the same condition), the quantitative relation between the percentage of burnish zone and tensile strength is clearly shown in Fig. 9. It is observed that with increase of tensile strength, the percentage of burnish zone decreases. A fitted empirical model for describing the data trend is shown in Fig. 9. f ¼ 239:9r0:36 ;

Fig. 7 Photo of a specimen after the piercing test

ð2Þ

where f is the percentage of burnish zone, and the tensile strength of advanced high strength steel expressed as MPa. A similar method has been used to develop an empirical damage evolution equation [7]. Figure 8b, d, f, h, and j show the high magnification SEM images of areas A, B, C, D, and E for five materials as shown in Fig. 8a, c, e, g, and i, respectively. Compared with tensile fractographs, piercing fractographs are smoother and have fewer dimples, which results from the different stress states. Tensile fracture is mainly driven by tensile stress, while a large degree of shear stress is dominant during the fracture stage of piercing process. As for DP780 and DP980 (Fig. 8d, f), almost no microvoids appear on the smooth fracture surface, as other researchers have observed in the shearing-induced test for Al alloy 6061(T6) [6]. In addition, the fracture surface of higher grade DP steels is smoother. While for M1180 and M1300 (Fig. 8h, j), the fracture zone consists of very small dimples

Fig. 8 SEM images showing the fractographs of the fractured edge of the piercing sheets and the magnified images of the selected zone which marked by A, B, C, D, E: a, b DP590; c, d DP780; e, f DP980; g, h MS1180; i, j MS1300

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phase (DP) steels, tensile fracture surfaces are dominated by dimples, however, piercing fracture surfaces have smooth surface and few dimples. Moreover, the fracture zone of higher grade DP steels is smoother. As for M1180 and M1300, the fracture zones consist of very small dimples and smooth brittle surfaces. The dimple size of M1300 (*1.2 lm) is smaller than that of M1180 (*1.6 lm). Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 51105246).

References Fig. 9 Variation of burnish zone percentage with tensile strength

and smooth brittle surfaces. Obviously, the dimple diameter of M1300 (*1.2 lm) is smaller than that of M1180 (*1.6 lm). This trend is similar with that of tensioninduced fractographs.

4 Conclusions (1)

(2)

(3)

The fracture mode of tensile fracture for DP590 and DP780 is uniform size dimple-dominant ductile fracture, while bimodal size dimple-dominant fracture appears in tensile fracture for DP980, M1180, and M1300. The quantitative correlation between the average dimple diameter and tensile strength for tensile fracture is d = 10,502.32r-1.21. However, the relation between dimple density and tensile strength is not monotonic due to the appearance of bimodal size dimples with increase of tensile strength. For shearing-induced fracture during piercing, the quantitative relation between the percentage of burnish zone and tensile strength is f = 239.9r-0.36. Shearing-induced fracture surface during piercing is different from tensile fracture surface. As for dual

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