Formability of HSLA and EDDQ steels of tube products of India

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 12, April 2005, pp Formability of HSLA and EDDQ steels of tube products of India R Narayanasamy a & C Sathiya Narayanan b a Department of Production Engineering, National Institute of Technology,Tiruchirappalli , India b Department of Production Engineering, J.J.College of Engineering & Technology, Tiruchirappalli , India Received 14 June 2004; accepted 18 November 2004 In this work, the formability of HSLA (micro alloyed steel of 1.6 mm thickness) and EDDQ (1.2 mm thickness) steels have been studied and the suitability of HSLA and EDDQ steels for forming applications have been examined. The experimental work on microstructural aspects, tensile properties and formability parameters were carried out for both the steel sheet metals. The forming limit diagram for EDDQ and HSLA steels have been experimentally evaluated. A detailed analysis of the strain distribution profiles obtained from experiment has been attempted. The fracture surfaces of the formed samples have been observed using scanning electron microscopy with a view to obtain a correlation between fracture behaviour and formability of the sheet metals. The EDDQ steel has been found to possess good drawability and stretchability, whereas the HSLA steel has been found to possess limited drawability and stretchability. IPC Code: C22C 38/00 Forming limit diagram (FLD) is an effective tool to evaluate the formability of sheet metals in various strain conditions. The information derived from the FLDs is very much useful for the sheet metal manufacturers and users. Keeler 1 and Goodwin 2 introduced the concept of FLD in 1960s. Hecker 3 developed simplified techniques for evaluating FLD. Since then FLDs have been widely used for studying the formability of sheet metals and many investigations have been carried out, in the recent past, many theoretical studies have been carried out to improve the formability predictions 4-8. Since the formability and FLD depend on many different factors and conditions of sheet metals, many experimental investigations also have been carried out Although aluminium alloys are used automobile applications due to their less weight, steel sheets with high strength have been intensively applied to automobile bodies to improve crashworthiness without increasing the body weight under a strong pressure of the requirements for fuel conservation, energy saving, and crash-worthiness. Two important types of steels used in such applications are high strength low alloy (HSLA) steel and extra deep drawing quality (EDDQ) steel. In this study, the FLDs of these steels have been evaluated by following standard procedure and die and punch set up as explained elsewhere 15. The HSLA steel sheet of thickness 1.6 mm and EDDQ steel of thickness 1.2 mm have been used. Experiment Procedure Chemical composition and tensile test The chemical compositions of the sheet metals studied were found out by spectrometry and presented in Table 1. The microstructure of HSLA and EDDQ steels considered for the study were obtained by following standard metallography procedure and they are shown in Figs 1a and 1b. Tensile tests were carried out using Hounsfield tensometer. The samples were prepared as per the ASTM standard explained elsewhere 15 by cutting along three different directions namely 0, 45 and 90 to the rolling direction of the sheets. The load versus extension data were obtained from these tests. The important parameters namely the strain hardening exponent (n), the plastic strain ratio ( r ) and the strength coefficient (K) along the three directions mentioned above were found out from the Table 1 Chemical composition of the two steels (in weight %) Material Thickness C Mn Si S P Al Ti Nb HSLA Steel 1.60 mm EDDQ Steel 1.2 mm

2 142 INDIAN J. ENG. MATER. SCI., APRIL 2005 strain, viz., tension-tension, plane strain and tensioncompression by varying the width of the samples. During forming, circles became ellipses. The major diameter and minor diameter of the ellipses were measured using a traveling microscope with an accuracy of 0.01 mm. From the major diameters and minor diameters, major strains (ε 1 ) and minor strains (ε 2 ) were calculated. The major strains (ε 1 ) and minor strains (ε 2 ) were found out in three distinct regions, viz., safe region, necked region and fractured region. FLD was drawn by plotting the minor strain in abscissa and corresponding major strain in ordinate and by drawing a curve which separates the safe region from the unsafe region. Fig. 1a Microstructure of HSLA steel at 400 Fig. 1b Microstructure of EDDQ steel at 400 tensile tests. The normal anisotropy (r) and the planer anisotropy (Δr) were calculated from the r values determined along three directions, parallel (0 ), diagonal (45 ) and perpendicular (90 ) to the rolling direction. Forming limit diagram In this method, samples were cut by shearing. The sample sizes were of mm, mm, mm, mm, mm, mm and mm. In all the samples, grid patterns were printed by chemical etching method. In this experiment, the diameter of the grid circles is 3.5 mm. Forming up to fracture was carried out on a double action hydraulic press of capacity 2000 KN. The sheet samples were subjected to different state of Strain distribution profiles The major strain and the minor strain at different distances from the pole were measured and strain distribution profiles were drawn by plotting the distance from the pole in abscissa and the corresponding major strain and minor strain on the ordinate. The thicknesses of the blank at different distances from the pole in longitudinal (parallel to the length of the samples that is parallel to rolling direction), transverse (perpendicular to the length of the samples) and diagonal (45 to the length of the samples) directions were measured using a coordinate measuring machine (CMM) and thickness strain versus distance from the pole graphs were drawn. The above procedure was followed for testing both sheet metals namely HSLA steel and EDDQ steels. Fractography The specimens for SEM analysis were cut from the region closest to the origin of the fracture and they were examined in a scanning electron microscope (SEM) model JSM-5610LV. From the features of the fracture, the nature of fracture and the relation between the features and the formability were analyzed. From the SEM images obtained, the size of the microvoids that present in the images were measured and an attempt was made to relate the average size of the microvoids with the shear strain developed in sheet metals according to Mohr s circle as explained elsewhere 15. Results and Discussion Chemical compositions The chemical compositions of HSLA and EDDQ steels taken for study are provided in Table 1. When comparing both the steels, the HSLA steel consists of

3 NARAYANASAMY & NARAYANAN: FORMABILITY OF HSLA AND EDDQ STEELS OF TUBE PRODUCTS IN INDIA 143 slightly more amount of carbon, manganese, and silicon, which increase the strength of steel. In addition to these elements, the HSLA steel consists of small amount of Ti and Nb which are carbide formers. The EDDQ steel contains slightly more amount of S, P, and Al comparing with HSLA steel. The EDDQ steel possesses good formability. In HSLA steel, the high strength is derived from the precipitation of fine carbides and grain refinement by using microalloying elements like niobium. Tensile properties The mechanical properties of HSLA and EDDQ steels obtained from the tensile tests, using Hounsfield Tensometer as per the standard procedure explained elsewhere 15, are given in Tables 2 and 3. The n-value of HSLA steel is maximum along the rolling direction and the n-value is minimum along 45 orientation to rolling direction. The K-value of HSLA steel is minimum for 90 orientation to the rolling direction and maximum for 45 direction to the rolling direction. For EDDQ steel, the n-value is minimum for the direction 45 to the rolling direction and maximum along 0 direction to the rolling direction. The K-value is maximum along 0 direction to the rolling direction and minimum along 45 direction to the rolling direction. The strain hardening exponent (n) value indicates stretchability and formability as explained elsewhere 15. As the n value increases, the stretchability also increases. Formability parameters The formability parameters determined by Table 2 Tensile properties of HSLA steel experiment are shown in the Table 4. The r, average plastic strain ratio (or) normal anisotropy of the HSLA steel is low when compared with that of EDDQ steel. This means that the formability is poor for HSLA steel as reported elsewhere 15, because this steel shows poor resistance to thinning in thickness direction during deep drawing. This is also observed in the FLD of HSLA steel shown in Figs 2 and 4. In the FLD of HSLA, the maximum major strain is about 30% and maximum minor strain is about 13.5% in tension-compression region. For the same minor strain, the limiting major strain for EDDQ steel is about 40%. In tension-tension region, the maximum minor strain is 8.5% and corresponding major strain is about 24.5% for HSLA steel. For the same minor strain the limiting major strain is about 42.5% for EDDQ steel. The nr value of EDDQ steel is greater than that of HSLA steel. This means that the formability in tension-tension region is higher for EDDQ steel comparing with HSLA steel as explained elsewhere 15. This is also proved by the FLD. The planer anisotropy (Δr) is very low for both sheets. This shows the tendency to earing is very less for both the materials during drawing operations as explained elsewhere 15, because the earing during deep drawing is related to Δr value. Forming limit diagrams The forming limit diagrams of HSLA and EDDQ steels obtained from the experimental results are shown in Figs 2 and 3 respectively. From the FLDs of Table 3 Tensile properties of EDDQ steel Orientation relative to rolling direction n K MPa Orientation relative to rolling direction n K MPa Average* Average* * Average n = (n 0 + 2n 45 + n 90 )/4 *Average K = (K 0 + 2K 45 + K 90 )/4 *Average n = (n 0 + 2n 45 + n 90 )/4 *Average K = (K 0 + 2K 45 + K 90 )/4 Table 4 Formability parameters of two steels Orientation relative to HSLA Steel EDDQ Steel rolling direction r nr Δr r nr Δr Normal anisotropy r r = Average Average r =

4 144 INDIAN J. ENG. MATER. SCI., APRIL 2005 HSLA and EDDQ steels, the following observations were made. In tension-tension region (i.e. biaxial tension region), the maximum major strain is about 24% and the maximum minor strain is about 8.5% for HSLA steel. Whereas in tension-compression region, the maximum major strain is about 30%, but the maximum minor strain is only 13.5%. In plane strain condition, the limiting major strain for the HSLA steel is only 12%. In tension-tension region, the maximum major strain is 44% and the maximum minor strain is about 11% for EDDQ steel and whereas in tensioncompression region, the maximum major strain is Fig. 2 FLD for HSLA steel Fig. 3 FLD for EDDQ steel Fig. 4 Comparison of FLDs for HSLA and EDDQ steels Fig. 5 Longitudinal strain distribution profiles for HSLA steel

5 NARAYANASAMY & NARAYANAN: FORMABILITY OF HSLA AND EDDQ STEELS OF TUBE PRODUCTS IN INDIA 145 about 56% and maximum minor strain is about 29%. In the plane strain condition, the maximum major strain is 34%. The above results of HSLA and EDDQ steels are in good agreement with their tensile properties and formability parameters. When comparing both the steels, the EDDQ steel possseses good formability. In particular, the EDDQ steel possesses excellent drawability. Fig. 4 shows a comparison between the FLDs of HSLA and EDDQ steels. Strain distribution profile The strain distribution profiles for various blanks of HSLA and EDDQ steels are shown in Figs 5 and 6 respectively. From the strain distribution profiles, the following observations were made. The strain distribution profiles for both sheets are almost similar and symmetrical about the pole in all the blanks. For HSLA steel, there is minimum major strain (1-11%) at the pole for various blanks. The major strain value increases as the distance from the pole increases. This Fig. 6 Longitudinal strain distribution profiles for EDDQ steel Fig. 7 Thickness strain distribution profiles in longitudinal direction for HSLA steel

6 146 INDIAN J. ENG. MATER. SCI., APRIL 2005 reaches maximum value and then decreases. The same nature is also obtained for EDDQ steel. For this sheet, the major strain at the pole for various blanks is about 4-11%. The minor strain for HSLA steel for various blanks is about 1-6% at the pole. The minor strain for EDDQ steel at the pole for various blanks is also about 1-6%. For tension-compression condition, the difference between the magnitudes of the major strain peak and minor strain peak is high when comparing with tension-tension region, because in the case of tension-compression region the sheet accommodates more amount of plastic strain or deformation. The distance between major peaks for various blanks of EDDQ steel is high compared with HSLA steel because the depth of cup at fracture has less height in the case of HSLA steel. This means that formability of EDDQ steel is better than HSLA steel. The thickness strain distribution profiles for various blanks of both steels are shown in Figs For both steels, there is a minimum thickness strain at the pole in all the blanks. The thickness strain at the pole for various blanks of HSLA steel is about 3-6 % and that Fig. 8 Thickness strain distribution profiles in transverse direction for HSLA steel Fig. 9 Thickness strain distribution profiles in diagonal (45º to the rolling direction) direction for HSLA steel

7 NARAYANASAMY & NARAYANAN: FORMABILITY OF HSLA AND EDDQ STEELS OF TUBE PRODUCTS IN INDIA 147 Fig. 10 Thickness strain distribution profiles in longitudinal direction for EDDQ steel of EDDQ steel is about 6 16%. This shows that the EDDQ steel could accommodate high plastic deformation compared with HSLA steel which shows poor formability as explained earlier. The thickness strain increases, it reaches the peak value and then decreases. The strain distribution profiles are almost symmetrical about the pole and the nature of the variation is similar for all blanks. It resembles the major strain distribution profiles. The fracture point from the pole of the cup is close in the case of the HSLA steel and far in the case of EDDQ steel. In the case of longitudinal and diagonal directions, the peak value represents the fracture. Whereas along the transverse direction, no fracture region has been Fig. 11 Thickness strain distribution profiles in transverse direction for EDDQ steel encountered. The distance between two peaks of thickness strain (where fracture takes place) is high for tension-compression region when comparing with tension-tension region, for any given steel. The maximum percent thickness strain observed in the EDDQ steel is very high when comparing with the HSLA steel for any given blank width. Limiting minor strain The limiting minor strains at fracture and at necking were identified. A plot between the limiting minor strain at fracture and the blank width is shown in Fig. 13. For both the sheets, the limiting minor strain at fracture increases as the blank width

8 148 INDIAN J. ENG. MATER. SCI., APRIL 2005 Fig. 14 Variation of strain ratio (ε 2 / ε 1 ) with respect to blank width Fig. 15 Variation of depth of cup with respect to shear strain Fig. 12 Thickness strain distribution profiles in diagonal (45º to the rolling direction) direction for EDDQ steel Fig. 16 Variation of depth of cup with respect to blank width Fig. 13 Limiting minor strain at fracture versus blank width increases. This shows the variation from the tensioncompression condition to tension-tension condition. For the blank having width less than 140 mm, the limiting minor strain value is negative, which is in the case of tension-compression region (example of deep drawing). The limiting minor strain values obtained are high for the EDDQ steel compared with the HSLA steel. The range of the limiting minor strain is high for EDDQ steel. This means the EDDQ steel has better formability than HSLA steel.

9 NARAYANASAMY & NARAYANAN: FORMABILITY OF HSLA AND EDDQ STEELS OF TUBE PRODUCTS IN INDIA 149 Table 5 Average void sizes for various blanks Sheet metal Average void sizes (µm) for various blanks 80 mm 120 mm 140 mm 160 mm 180 mm 200 mm HSLA EDDQ The strain ratios were measured for the necked region and a plot has been drawn between the strain ratio (ratio between limiting minor strain at necked region and major strain at necked region (ε 2 / ε 1 )) and the blank width. This plot is linear one as shown in Fig. 14. This shows that the strain ratio at the necked region is negative in the tension-compression region comparing with tension-tension region. Depth of cup It is observed that as the blank width decreases, the depth of the cup increases for both the steels. An attempt is made to relate the depth of cup and the shear strain obtained from the radius of Mohr s circle radius. The Mohr s circles for various blanks of HSLA and EDDQ steels were drawn and their radii were measured. The Mohr s circle radius is the shear strain developed on the material. It is known that as the shear strain increases, the depth of the cup increases. As the width of the blank decreases, the depth of cup increases. The plots are shown in Figs 15 and 16. The depths of cup in EDDQ blanks are higher than that of HSLA blanks. Fractography Many authors as explained elsewhere 15 have worked on the fractography and it is possible to analyse the features of fractography. From these, the nature of failure and formability can be analyzed. fractography has been obtained using a scanning electron microscope (SEM). The SEM images were obtained for the fracture surface of all the blanks of HSLA and EDDQ steels. The various SEM images are shown in Figs 17 and 18. For the blanks subjected to tension-compression strain condition, the SEM images show many number of bigger size microvoids and shear type fracture consisting of deep dimples as shown in Figs 17a and 18a. For the blanks subjected to plane strain condition, the surface is smooth compared to the above images and number of voids is less as shown in Figs 17b and 18b. For the blanks subjected to tension-tension strain condition, the number of voids is less and it appears to be partly ductile and partly brittle as shown in Figs 17c and Fig. 17 SEM images taken at 1500 for the fracture surfaces of HSLA Steel 18c. The evidence for the brittle fracture is more. The average void sizes measured from the SEM images for various blanks of both steels are given in Table 5. The average void size decreases as the blank width increases. From the strain measurement and FLD, the shear strain (Mohr s circle radius) for various blanks were calculated and graph relating shear strain and average void sizes was drawn. The plot is shown in

10 150 INDIAN J. ENG. MATER. SCI., APRIL 2005 Fig. 18 SEM images taken for the fracture surfaces of EDDQ Steel Fig. 19 Average diameter of the voids versus shear strain obtained from radius Mohr s circle Fig. 19. The plot is a linear line. As the shear strain developed on the material increases (example: tension-compression region), the average void size also increases. For tension-compression region the average void size is bigger which indicates ductile fracture. For tension-tension condition, the average void sizes are small. The above observation is common for both steels. The average void sizes of HSLA steel are smaller than the average void sizes of EDDQ steel for various blanks. Conclusions From the tensile test, the n-value, the K-value, the r-value at different orientations (along three different directions namely 0, 45 and 90 to the rolling direction of the sheets) and nr-value of HSLA and EDDQ steels were found out and they were correlated with the formability of the sheets. The EDDQ steel is found to possess better formability compared to the other one. In particular, it possesses excellent drawability. These results are in good agreement with their formability parameters. Strain distribution profiles are symmetrical about the pole. The fractographic images show ductile-brittle dimples in tension-tension condition, deep voids and featureless areas in plane strain conditions and ductile shear type fracture in tension-compression condition. The average void sizes in SEM images increases with decreasing blank width (tension-compression region). This has a good agreement with the shear strain obtained from Mohr s circle radius. Owing to its excellent drawability, EDDQ steel of this quality can be used for automotive body applications. References 1 Keeler S P, Sheet Met Ind, 42 (1965) Goodwin G M, Metall Italiana, 60 (1968) Hecker S S, Sheet Met Ind, 53 (1975) Rao K P & Sing W M, Int J Mech Sci, 42 (2000) Takuda H, Mori K & Hutta H, J Mater Process Technol, 95 (1999) Banabic D, J Mater Process Technol, (1999) Bleck Wolfgang & Deng Zhi, J Mater Process Technol, 83 (1998) Sing W M & Rao K P, J Mater Process Technol, 63(1997) Takuda H, Kikuchi S & Hatta N, J Mater Process Technol, 84 (1998) Obermeyer E J & Majlessi S A, J Mater Process Technol, 75(1998) Doege E, Droder K & Griesbach B, J Mater Process Technol, 71(1997) Cada R, J Mater Process Technol, 60 (1996) Chow C L & Jie M, Int J Mech Sci, (in press). 14 Zhao L, Sowerby R & Sklad M P, Int J Mech Sci, 38 (1996) Narayanasamy R & Sathiyanarayanan, Report of FLD on HSLA steels, submitted to Tube Products of India, Avadi, Chennai, India, National Institute of Technology, Tiruchirappalli, India, 2004.