The deep drawing process in metallic materials has several limitations, such as a stamped depth, the occurrence of wrinkles, the fracture of the sheet before total deformation and the generations of solids with variations in wall thickness. Considering these variables, this work studies the formability of AISI 441 stainless steel under different initial conditions, namely, as-received and rolled to investigate the effect of these variables on the formability of this ferritic stainless steel. The mechanical characterization of AISI 441 steel involved tensile, shearing and Vickers microhardness tests followed by cold stamping, using a deep stamping die. The structural investigation was accomplished by optical microscopy and electron backscatter diffraction techniques. The mechanical behaviour after the stamping process was analyzed by shearing tests on the bottom and wall of the cup and correlated to the deformation limit of the material, and the respective texture characteristics. The shearing test results indicated the effect of the initial condition on the mechanical behaviour after the stamping of AISI 441 steel, which exhibited hardening of the wall and softening at the bottom of the cups. Change in misorientation grain and strengthening of γ-fiber texture component for the rolled condition of AISI 441 steel.
Research Article
Study of the stampability of AISI 441 steel under different initial conditions
https://doi.org/10.21203/rs.3.rs-4177615/v1
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The constant evolution of material transformation methods to achieve increasingly demanding requirements for mechanical properties and better use of different types of products is a fundamental premise for the development and optimisation of forming processes. In this regard, new technologies and routes for materials processing, such as hybrid process chains connected with additive manufacturing are replacing traditional metal forming operations [1]. Considering the sheet forming processes, it is appropriate to mention the intense use of technologies, including the analysis of time-variant temperature distributions within a blank during forming using temperature sensors [2]. All these studies should consider the knowledge of the effect of material variables on product properties as a valuable method for increasing productivity in addition to avoiding rework and waste in metal forming industries [3, 4].
Texture, for example, is a common property used to select materials and conditions of processing for different applications, such as automotive components. Furthermore, inappropriate texture may contribute to the propagation of defects, such as ridging and cracking in ferritic stainless steels [5, 6]. Concerning this need, the initial condition of the materials associated with the structural arrangements and mainly, the routes used to process them, involving the amount and mode of plastic deformation affect the mechanical properties of stamped objects [7, 8].
This work investigates the effect of the initial state on the stampability of AISI 441 steel under two initial states: as received (as received by a steel plant) and rolled (the received steel followed by cold rolling up to 20% of effective deformation in rolling).
2.1 Material
AISI 441 ferritic stainless steel sheets 1.0 mm thick (original value) were used in this work. This material is commonly used in chimneys, condensing boilers, domestic boiler burners, civil construction profiles, doors and elevator cabins, and kitchen utensils [9]. The chemical composition exhibited 0.014% C, 18.01% Cr, 0.019% Mo and 0.56% Nb (weight percent), according to optical emission spectrometry (MIP OES).
Three initial conditions were used: as-received (grain size of 167 µm), and rolled (grain size of 164 µm with 0.20 effective strain, von Mises criterion) [10].
2.2 Cold Rolling and Mechanical Tests
Cold rolling was performed in a Frohling rolling mill with 200 mm diameter cylinders and rolling speed of 6.25 m/min. This rolling step followed the same initial rolling direction as that of the material, 0°RL. The effective (true) von Mises rolling strain, ee, was equal to 0.20 or 0.22 (engineering strain), calculated according to Eq. 1[10].
Vickers microhardness tests were performed using a load of 200 gf, and the penetration time was 15 s. Tensile tests were performed in an Instron 5982 universal testing machine with a Blue Hill 3 control system. Acquiring deformation values was accomplished by an automatic Instron 2630 − 100 extensometer. The cross-head speed was 4.5 mm/min, leading to an initial deformation rate of 10− 3s− 1. The yield strength, ultimate tensile strength and uniform elongation were calculated according to ASTM A370-19. Three tests for each loading condition were conducted according to ASTM E8/E8M-16a.
Tensile tests were also carried out with specimens at angles of 0° (RD), 45° and 90° (Transverse Direction, TD) relative to the rolling direction to investigate the anisotropy of the tensile properties. The Lankford values (r-value) at these angles were calculated using specimens deformed at a permanent engineering strain of 0.16. The weighted-average or normal anisotropy (rm) and the earing tendency or planar anisotropy (∆r) were also calculated according to ASTM E517-19.
The shearing tests required a special fixture mounted on the Instron machine, as exhibited in Fig. 1(b). This shear device was designed similarly to that developed by Bouvier et al [11]. All the tests were performed at the same strain rate as the tensile tests, 10− 3s− 1. The samples were extracted from as received and rolled cups using water jet cutting and then subjected to shearing tests, Fig. 1.
Equation (2) and Eq. (3) were used to convert the shearing strain (γ) into effective strain (ee) and the shearing stress (τ) into effective stress (σe), respectively. These equations employ a conversion factor of 1.84, according to the recommendations of Rauch [12].
2.3 Stamping: Swift Tests
The stamping of AISI 441 steel was carried out using circular blanks with diameters ranging from 80 to 100 mm in 5 mm increments, into a circular die with an internal diameter of 52.5 mm and a spherical punch with a diameter of 50 mm, Fig. 2. In this case, electrolytic surface etching was performed with 10 mm diameter circles, using etching solution C-1, composed of HNO3, and a 12 V power source to supply an alternating electric current. The process of etching the circular pattern concluded with acid neutralization using sodium bicarbonate to halt the corrosion. Stamping was conducted using a universal testing machine EMIC DL 200 kN, with a feed rate of 7 mm/min and a maximum feed of 70 mm. Images were taken of all the cups, including fractured samples, and deformations were measured using an image capture system and a specific software, Image ProPlus 6.0 based on varying the diameter of the circular mesh to measure the respective true deformation.
3.1 Characterization in initial state AISI 441 steel
The Vickers hardness values of the AISI 441 steel were 167 ± 2.0 for the as received and 260 ± 8.7 for the rolled condition, indicating hardening due to rolling processing. The pre-rolling deformation reduced the maximum ductility of AISI 441 steel, as shown in Table 1.
|
Rolling Direction |
Condition |
Yield strength, YS (MPa) |
Ultimate tensile strength, UTS (MPa) |
Uniform elongation, UE (%) |
|---|---|---|---|---|
|
0º RD |
As received |
282 ± 17.5 |
456 ± 16.5 |
22 ± 0.8 |
|
Rolled |
633 ± 9.8 |
636 ± 7.4 |
0.5 ± 0.1 |
|
|
45º RD |
As received |
315 ± 4.8 |
486 ± 4.3 |
21 ± 1.0 |
|
Rolled |
615 ± 29.2 |
621 ± 28.6 |
0.5 ± 0.1 |
|
|
90º RD |
As received |
318 ± 5.2 |
481 ± 5.8 |
20 ± 0.2 |
|
Rolled |
687 ± 9.6 |
704 ± 3.2 |
0.5 ± 0.1 |
The anisotropy of this material is evident in all the values of the mechanical properties presented in Table 1. The highest mechanical strength was observed at 45° to the rolling direction (RD) in the as received condition and at 90° for the rolled state, which indicates that the rolling process modified the properties of the AISI 441 steel. These results suggest the effect of the anisotropy of the material on its stampability, and it is also clear that the increase in mechanical resistance and the limited ductility in tension experienced by the rolled condition were the main parameters modified between the two states of AISI 441 steel [13].
3.2 Stamping AISI 441 steel
AISI 441 steel cups are shown in Fig. 3 for the as-received and rolled states from a blank diameter of 80 mm by the last blank diameter without fracture for both states, i.e., 105 and 100 mm, respectively. The earing phenomenon was more prominent in the rolled state than in the as received state, as also predicted by the higher value of planar anisotropy in the rolled condition as shown in Table 2.
|
Condition |
ΔR |
Rm |
R0º |
R45º |
R90º |
|
|---|---|---|---|---|---|---|
|
As received |
0.140 |
1.429 |
1.310 |
1.359 |
1.688 |
|
|
Rolled |
0.228 |
1.133 |
1.078 |
1.019 |
1.417 |
|
The LDR represents the ratio between the diameter of the blank just before the fracture of the sheet (as mentioned previously, 105 and 100 mm) and the diameter of the punch (d0 = 52.5 mm). As expected, the initial condition influenced the LDR value, with this parameter equal to 2.2 (as received) and 2.1 (rolled) for the samples of AISI 441 steel.
For the all the samples, the major deformation mode was stretching, as indicated by the arrows depicted in the circles shown in Figs. 4 and 5 corresponding to the occurrence of tensile stress along the cup height and compression in the orthogonal direction.
The stamping implemented in this work can be considered a 3 stages operation, as described by Barzegari and Khatir [14], based on the positioning of the punch with the sheet, the advancement of the punch to the die cavities and the subsequent removal of the punch. Under this condition, sheet tearing typically occurs in two preferential regions: at the bottom radius or at the cup wall. In this work, as observed in Figs. 4 and 5, tearing of the sheet occurred on the bottom radius of the AISI 441 steel cups, thus encouraging the need to measure the variation in the thickness of the cups from the flange to the region of the bottom of the cups.
Figure 6 exhibits the variation in the sheet thickness in the as received, Fig. 6 (a), and rolled states, Fig. 6 (b), considering that the initial thickness of the blank in the as received state was 1.0 mm and that in the rolled state was 0.84 mm, which corresponds to 0.20 effective deformation in rolling.
The variation in sheet thickness is associated with the state of stresses acting during stamping. In the cup flange region, compression stress resulting from the action of the blank holder, as well as radial stretching and circumferential compression, occurs, this last effort is associated with the increase in thickness experienced in this cup region [15].
On the wall of the cup, there is tensile stress responsible for stretching the cup, compression stresses that can cause thinning of the sheet, and frictional efforts between the sheet and the die wall/punch wall. Despite the existence of a gap between the die diameter (DD of 52.5 mm) and the punch diameter (Dp of 50 mm), a reduction in the sheet thickness was observed on the wall of the cup, indicating the possible action of friction between the punch, the sheet, and the stamping die. Finally, at the bottom of the cup, compression stresses result from the action of the punch, these stresses are subsequently transmitted to the rest of the cup through radial tensile stresses, causing no relevant change in the thickness of the sheet in this region of the stamped cup [16].
This distribution of the sheet thickness is an indication of the failure point of the sheet during the stamping operation, which needs to be associated with the prediction of the formability limit based on a method that evaluates the maximum deformation that a material can withstand until failure. Considering this, to measure the amount of effective deformation that the stamped cups experienced in the two conditions of AISI 441 steel, the deformation of the mesh of circles printed on the blanks after stamping was measured and exposed in Fig. 7.
As predicted by the anisotropy and LDR results, the deformation limit of AISI 441 steel in the rolled state was lower than that in the as-received condition, with the uniaxial stretching deformation mode being predominant (deep drawing, full circle) and some points, the presence of biaxial stretching (expansion, dashed circle).
The mechanical behaviour of AISI 441 steel after stamping was investigated using a shear test by removing test specimens from the bottom and wall of the stamped cups which defined the stampability limit, that is, 110 mm for the as-received state and 105 mm for the rolled condition. The results shown in Fig. 8 reveal the difference in mechanical behaviour between the positions of the embedded cup for the two states of AISI 441 steel. Commonly, hardening was noted on the wall of the cups, and minor softening at the bottom of the cups, when compared with the state before the stamping of AISI 441 steel.
The hardening in the region of the wall of the cups indicates that was the region that suffered the greatest amount of plastic deformation, with such hardening being associated with hardening resulting from stretching as revealed in Figs. 4 and 5. The softening detected at the bottom of the cup can be attributed to the cyclic loading cycle that the material experienced by the material from the flange to the bottom of the cup [17].
Comparing the response between the two states of AISI 441 steel, it is possible to observe that the hardening experienced in the as-received condition was greater than that detected in the rolled condition, despite the latter undergoing pre-deformation in cold rolling and consequently, a greater amount of accumulated plastic deformation. However, such an accumulation of plastic deformation limited the plasticity of the AISI 441 steel pre-deformed by cold rolling, as evidenced by the lower LDR value.
It is also noted that for both regions of the stamped cups, the stress-strain curves tend to coincide with the initial condition, i.e., before stamping, more prominently in the rolled condition. This finding suggested that the substructural change assumed by AISI 441 steel during stamping in the rolled state was smaller than that observed for the as-received condition [18].
Considering that in the as-received state, AISI 441 steel exhibited an LDR greater than that detected in the rolled state, 2.2 and 2.1, respectively, the maximum depth of the as-received cup (56.3 mm) was greater than that perceived for the rolled state (49.7 mm), Fig. 9, thus contributing to the increase in work hardening and, consequently, to the greater mechanical resistance of AISI 441 steel in the as-received state.
These results suggest that the intensity of the change in substructural arrangement, as well as the preferred crystallographic orientation assumed by AISI 441 steel after the stamping operation, was greater in the as-received state than in the rolled state.
3.3. Texture
The grain misorientation angles before and after the stamping of AISI 441 steel for the two states evaluated in this study are shown in Fig. 10, revealing a relevant modification. Before stamping, there was a predominance of low angle grain boundary (LAGB) misorientation in the range of 0° to 5° (approximately 95% of the total grains were analyzed for both states). After stamping, the concentration of grain misorientation was modified to high-angle grain boundaries (HAGBs) in the range of 15° to 62.5° (87% in the as-received state and 92% in the rolled condition) for both states.
The prevalence of low-angle grain boundaries in the range of 0° to 5° before stamping for both states of AISI 441 steel is typical of a dislocation substructure exhibiting a low density of dislocation lines compared to the predominance of high-angle grain boundaries in the range of 15° to 62.5°, as detected after stamping [14, 19].
Figure 11 displays the grain orientation map, before and after stamping, revealing a change from a homogeneous distribution around the [111] orientation to a more heterogeneous distribution to [001] orientation after stamping, for both states of AISI 441 steel. This heterogeneity confirmed the grain misorientation, as indicated in Fig. 10.
As shown in Fig. 12, the pole figures support the increase in misorientation, which is more pronounced in the [101] orientation and more intense in the rolled state than in the as-received state. In general, the formability of ferritic stainless steels is enhanced by increasing the average normal anisotropy parameter, rm, and by intensifying the texture in the [111] orientation, γ-fiber, with the consequent reduction of other textures [20]. Furthermore, it is known that a single cold rolling process generally induces a highly non-uniform γ-fiber recrystallization texture and nearly uniform recrystallization after two-step cold rolling. Additionally, choices such as annealing conditions, including the selection of temperature and time adopted in intermediate annealing, and the amount of subsequent plastic deformation through cold rolling also affect the texture and formability of ferritic stainless steels [21].
The stampability of AISI 441 stainless steel was investigated under different initial states using shearing tests to describe the mechanical behaviour and texture analysis. The main conclusions of this work can be summarized as follows:
(1) The intensity of the γ-fiber texture component (111) was slightly enhanced for rolling up to 20% effective strain.
(2) The stampability of the as received and rolled states are similar considering that the LDR values were equal to 2.2 and 2.1, respectively.
(3) For both states analyzed in this work, hardening was observed in the wall and softening was observed at the bottom of the cups from the shearing test, with a tendency for the sheet thickness to increase close to the flange and decrease close to the lower radius of the stamped cups.
(4) Occurrence of low-angle grain boundary (LAGB) misorientation in the range of 0° to 5° before stamping and high-angle grain boundaries (HAGB), in the range of 15° to 62.5°, after stamping.
Acknowledgments
The authors are grateful to CEFET-MG (Centro Federal de Educação Tecnológica de Minas Gerais), to CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), to CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) and to FAPEMIG (Fundação de Amparo à Pesquisa do Estado de Minas Gerais).
Competing interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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The authors declare no competing interests.