2020 Volume 61 Issue 11 Pages 2121-2127
Automobile body parts produced by the hot stamping process exhibit excellent shape fixability with an ultra-high tensile strength of 1.5 GPa. We investigated the effect of flow stress during forming and phase transformation in the hot stamping process. Referring to both experimental and FEM coupled simulation results, we discussed the mechanism behind the excellent shape fixability in the hot stamping process. Steel of 0.2% C was used for hot stamping in this study. IF steel and SUS 304, which have different transformation behaviors, were used for comparison. The forming start temperature varied from 400°C to 800°C. After hot stamping, the springback of the parts was evaluated. The results showed that shape fixability in hot stamping is caused by low flow stress during stamping and martensitic transformation. When martensitic transformation occurs after stamping, excellent shape fixability is obtained regardless of the flow stress during forming. Accordingly, it was concluded that the stress introduced by hot stamping is relaxed and becomes uniform during martensitic transformation. The application of tensile stress due to thermal contraction also contributes to the decrease in springback.
This Paper was Originally Published in Japanese in J. JSTP 60 (2019) 45–50.
Influence of the strength of the material at the forming temperature on the shape fixability of hot-stamped part.
To reduce CO2 emissions as a global warming countermeasure, high automobile fuel efficiency has become indispensable, and reduced automobile weight is needed. In addition, safety regulations against collision are becoming increasingly strict. To meet these demands, the strength of steel sheets used for automobile body parts has increased markedly, and high-strength steel sheets exhibiting excellent characteristics have been developed.1) However, because increased steel sheet strength generally leads to a decrease in press formability, it is becoming difficult to manufacture high-strength parts by cold stamping.
Against this backdrop, the hot stamping process is finding increasingly widespread application as another approach to manufacture high-strength parts.2) In this forming process, a steel sheet is heated to a temperature in the austenitic region of approximately 900°C and then formed in a room-temperature die. The steel sheet is held and cooled while pressure is applied to the die. As a result, the sheet undergoes hardening and becomes a high-strength steel sheet. By adjusting the chemical components of the steel used for this process, parts with a tensile strength of approximately 1.5 GPa can be manufactured in appropriate shapes.
One distinctive characteristic of the hot stamping process is excellent shape fixability, that is, shape accuracy of parts after forming. The shape fixability of parts formed by cold stamping generally decreases due to springback, wall warp, torsion, and other shape changes that occur when stress applied mainly in the forming process is unloaded during die release.3) The residual stress that causes these shape changes is attributed to the elastic recovery of stress applied in the forming process; the amplitude of elastic recovery is proportional to the stress developed in the forming process. Accordingly, shape fixability decreases with higher material strength.4) In the hot stamping process, with forming performed at high temperatures, the deformation resistance of the material is low, and the stress applied in the forming process is small. It is presumed that low deformation resistance and low stress contribute to good shape fixability. According to results5) from actual operation, part shapes formed by the hot stamping process are nearly identical to those of a die used. Thus, the elastic recovery caused by this process is very small. However, stress is applied in the forming process, even at high temperatures. Thus, elastic recovery due to stress is thought to cause shape changes during die release, suggesting that in the hot stamping process, factors other than the small forming force at high temperatures act to produce good shape fixability.
Such non-stress factors include metallurgical effects, such as microstructure changes during hot stamping.6–11) In the hot stamping process, martensitic transformation occurs in materials that cool rapidly in a die after forming. As a result, it is expected that material volume may increase, leading to a change in the stress state produced during the forming process. Investigations conducted using a material with higher hardenability than materials normally used in a hot stamping process exhibited better shape fixability due to constraining materials during martensitic transformation.6,7,9–11) However, these results using a crank press are different from those obtained in conventional hot stamping processes using a hydraulic press. Recrystallization was reported to affect shape fixability in the hot stamping process.8) However, the study examined forming in the ferrite single phase region, which is not the target of our study. The current study focuses on the conventional hot stamping technique, which is accompanied by martensitic transformation from austenite.
In this study, hat-bending forming was performed using a material with typical chemical components for the hot stamping process. First, we compared hot stamping with cold stamping. Next, the enhanced shape fixability achieved in the hot stamping process was examined from two different viewpoints: the effect of hot deformation resistance, and the effect of martensitic transformation that occurs after forming. Furthermore, the development mechanism of good shape fixability was discussed. Hot deformation resistance was changed by adjusting the forming temperature. The effect of martensitic transformation was examined using a steel sheet for hot stamping that underwent martensitic transformation after forming, compared with steels that did not undergo martensitic transformation. The enhancement mechanism of shape fixability in the hot stamping process was determined through a forming analysis coupled with thermal and metallurgical analyses of temperature and phase transformation after forming.
An aluminized steel sheet with a thickness of 1.4 mm and dimensions of 241 mm × 297 mm was used for hot stamping.12,13) Its chemical components were 0.22% C, 1.2% Mn, and 0.002% B; hereafter, the material is referred to as 0.2% C steel. In this steel, the critical cooling rate producing martensitic transformation is 30°C/s, and the martensitic transformation start temperature, Ms, is approximately 400°C.13) Martensitic transformation is caused by rapid cooling in a die after forming. To compare springback in hot stamping and cold stamping, we used 270-MPa class, 440-MPa class, and 590-MPa class cold rolled steel sheets. Table 1 shows the tensile properties of these materials for comparison. To analyze the mechanism of shape fixability, we used 270-MPa class interstitial free steel (hereafter, IF steel) and austenitic SUS304 steel, which do not undergo transformation after hot forming, as materials for comparison.
Figure 1 shows the cross-sectional shape of the tools. The forming height was 50 mm, the clearance was 1.4 mm, and the part length was 297 mm. A steel sheet was heated to 950°C, transferred to a die, and formed by a hydraulic press. Subsequently, it was held at the bottom dead center under a pressure of 1,500 kN for 10 seconds. For comparison, cold stamping was performed under the same conditions with a steel sheet that was kept at room temperature.
Cross-sectional shape of the tools.
The forming start temperature was changed within the range of 400°C to 800°C, temperatures that occur in the air-cooling process after heating to 950°C. Figure 2 shows the temperature changes that occurred in the air-cooling of 0.2% C steel, IF steel, and SUS304 steel after heating to 950°C. During air cooling, the transformation exotherm due to bainitic transformation occurred at approximately 530°C in 0.2% C steel, suggesting that 0.2% C steel was in the austenitic phase at forming start temperatures of 550°C to 800°C and underwent martensitic transformation in the die cooling after forming. At forming start temperatures of 500°C and lower, the completion of bainitic transformation was confirmed before forming. IF steel was in the austenite phase upon heating to 950°C but transformed to ferrite at approximately 850°C; the ferrite transformation completed before forming. In SUS304 steel, the austenite phase was stable and the transformation exotherm did not occur. Thus, in the forming temperature range of 550°C to 800°C, IF steel was in the ferrite phase and SUS304 steel was in austenite phase, and neither underwent transformation after press forming.
Air-cooling curves for each type of steel.
To evaluate shape fixability, the clearances between the flange and the horizontal face and the wall warp in the middle of the side wall (curvature on the outside of the part) were measured, as shown in Fig. 3. The clearance between the end of the flange and the horizontal face was measured when springback occurred; the clearance between the end of the round portion of the die shoulder and the horizontal face was measured when spring-go occurred. Regarding wall warp, the curvature in the middle of the side wall was measured with a three-point gauge with a span of 30 mm.14)
Evaluation method for shape fixability.
Tensile strength at hot forming temperature was measured to evaluate the relationship between deformation resistance and shape fixability in hot forming. For IF steel and SUS304 steel, a tensile test was conducted at test temperatures of 400°C, 600°C, and 800°C at a strain rate of 0.1 s−1, in accordance with JIS G 0567.
The tensile strength of 0.2% C steel after bainitic transformation was determined by heating 0.2% C steel to 950°C and air cooling to room temperature to develop bainitic transformation. The tensile test was conducted at a temperature of 400°C and a strain rate of 0.1 s−1 in accordance with JIS G 0567. To obtain the tensile strength of 0.2% C steel in the austenite region, a strip specimen with a width of 30 mm was heated to 950°C, cooled to the test temperature at 30°C/s, and subjected to a tensile test at temperatures of 600°C, 700°C, and 800°C at a strain rate of 0.1 s−1 using a heat-cycle tensile-compressive tester manufactured by Fuji Electronic Industrial Co., Ltd.
2.5 Numerical analysis methodFEM analysis for the forming test was performed using DEFORM 3D Ver. 11.1.1, which can operate through phase transformation. A three-dimensional model expressing the blank and the die around the blank as elasto-plastic solid elements and elastic solid elements, respectively, was used as the analysis model. The work-hardening characteristic at high temperatures was evaluated using a stress-strain curve determined by temperature and strain rate dependence based on the compression test of cylindrical specimens. After heating to 950°C, a cylinder with a diameter of 8 mm and a length of 12 mm was cooled to the forming temperature at a rate of 50°C/s and compressed at strain rates of 0.01 s−1, 0.1 s−1, and 1 s−1 to obtain the stress-strain curve. A heat transfer coefficient, dependent on contact pressure, was used between the die and the blank.12) The friction coefficient was set to 0.55.16) The change in the coefficient of linear expansion due to martensitic transformation was determined by measuring the relationship between temperature and thermal expansion at martensitic transformation using a Formaster tester manufactured by Fuji Electronic Industrial Co., Ltd. Martensitic transformation behavior was defined as $\xi = 1 - \text{e}^{ - k(M_{\text{s}} - T)}$ from the experiment by determining the relationship between temperature and fraction of material transformed.17) In this expression, ξ is the fraction of transformed martensite and T is the temperature (K). Ms (K) and k (K−1) are the martensitic transformation start temperature and a constant, respectively. Transformation plasticity, which leads to plastic deformation, may occur when stress is applied during transformation.18) In this study, martensitic transformation that occurred during stress was introduced by forming. Accordingly, it was possible that transformation plasticity would occur, affecting the stress state. Therefore, we took transformation plasticity into consideration; when the increment of the fraction transformed and deviatoric stress are expressed as dξ and sij (MPa), respectively, the increment of transformation plasticity strain $d\varepsilon _{ij}^{\text{tp}}$ is expressed as $d\varepsilon _{ij}^{\text{tp}} = 3K(1 - \xi )d\xi s_{ij}$.15) In this expression, K (MPa−1) is the coefficient of transformation plasticity; the value reported in the literature18) was used. After cooling was completed, the blank was removed from the die and a springback evaluation was conducted. For comparison, the springback of a 590-MPa class cold rolled steel sheet was analyzed by cold-stamping at room temperature.
Figure 4 shows a comparison of shape fixability (wall warp) between hot-stamped parts with a forming start temperature of 800°C and cold-stamped parts. The tensile strength indicated on the horizontal axis is the strength obtained through hot stamping or the strength of the material before cold-stamping. Figure 5 shows the appearance of the stamped parts. The results indicate that neither a clearance nor wall warp developed during hot stamping, and good shape fixability was exhibited. However, in cold stamping, both the clearance and wall warp increased as the tensile strength of materials increased. The shapes after forming indicate that wall warp occurred in the side wall near the flange (Fig. 5). The wall warp presumably occurred when the bend formed in the round portion of the die shoulder became unbent as the forming of this portion progressed.
Shape fixability of hot-stamped and cold-stamped parts.
Appearance of stamped parts.
Figure 6 shows the effect of the forming start temperature on shape fixability after hot stamping.
Influence of forming start temperature on the shape fixability of hot-stamped part.
At forming start temperatures of 550°C to 800°C, 0.2% C steel developed only a small clearance and wall warp, exhibiting excellent shape fixability. However, at forming start temperatures of 500°C and lower, the shape fixability degraded rapidly. At forming start temperatures from 550°C to 800°C, forming started in the austenitic state. Therefore, martensitic transformation occurred due to retention at the bottom dead center after forming. At forming start temperatures of 500°C and lower, forming was conducted after the completion of bainitic transformation. Therefore, no martensitic transformation occurred after forming. When martensitic transformation occurs after forming, shape fixability is excellent, as reported in the previous research.10,11)
In IF steel and SUS304 steel, martensitic transformation did not occur during retention at the bottom dead center after forming, springback was present even at a forming start temperature of 800°C, and successive degradation was evident as the forming temperature decreased. This degradation is presumably caused by the reduced shape fixability that occurs when deformation resistance increases with a decrease in forming temperature. Accordingly, when martensitic transformation does not occur after forming, springback occurs even at high forming temperatures.
3.3 Effect of tensile strength at forming temperature on shape fixabilityFigure 7 shows the relationship between shape fixability after hot stamping and tensile strength at forming temperature, and the results obtained for cold stamping. For IF steel and SUS304 steel, which did not develop martensitic transformation after forming, and 0.2% C steel, which was formed at 400°C, shape fixability can be described in terms of tensile strength at forming temperature; shape fixability decreased as strength at forming temperature increased. However, even at high forming temperatures, the elastic deformation that occurs in response to the force applied for stamping causes springback. As Fig. 7 shows, even when deformation resistance is the same during forming, hot stamping is superior to cold stamping in terms of shape fixability. In hot stamping, the effect of tension stress introduced after forming presumably contributes to better shape accuracy as later discussed in 3.5.
Influence of the strength of the material at the forming temperature on the shape fixability of hot-stamped part.
When 0.2% C steel was formed at 600°C, 700°C, and 800°C in the austenite region and subjected to martensitic transformation after forming, springback became extremely low, regardless of the strength at the forming temperature.
3.4 Numerical analysis of changes in the stress state with time after formingFigure 8 shows the maximum principal stress distribution in a 590-MPa class cold rolled steel sheet formed by cold stamping, and Fig. 9 shows its shape after die release. In the round portion of the punch shoulder, a large stress difference was generated between the front and back of the steel sheet, causing springback after die release. In the lower portion of the side wall, a stress difference between the front and back was also generated due to bending and unbending, causing a wall warp after die release. In the numerical analysis, a clearance of 4.5 mm between the flange end and horizontal face was obtained, which is in generally good agreement with the value obtained in actual forming, 5.35 mm.
Maximum principal stress distribution of cold-stamped part calculated by finite element simulation (590-MPa class).
Shape of cold-stamped part after unloading calculated by finite element simulation (590-MPa class).
Figure 10 shows the results of analysis for the hot stamping process using 0.2% C steel when the forming start temperature was 800°C. Changes are observed in distributions of temperature and volume fraction of martensite with time in the formed part during the cooling process immediately after forming was completed. At 0.07 s after the start of cooling, martensitic transformation did not occur; at 0.47 s, martensitic transformation occurred in the web face and part of the side wall. At 0.87 s after the start of cooling, martensitic transformation was completed on the side wall; at 2.07 s, martensitic transformation was completed for the entire part.
Temperature distribution and change in volume fraction and distribution of martensite calculated by finite element simulation after forming and subsequent cooling in the hot-stamping process.
Figure 11 shows the changes with time in the maximum principal stress distributions at the round portion of the punch shoulder, at the round portion of the die shoulder, and at the lower portion of the side wall. Immediately after forming was completed, stress introduced by press forming was present, as is also the case with cold stamping. At this time, a large stress difference between the front and back of the steel sheet was generated in the round portion of the punch shoulder and the lower portion of the side wall. This stress state can lead to springback and wall warp after die release. At 0.07 s after the start of cooling, tensile stress was generated, and assumed to be caused by thermal contraction with the temperature decrease. At 0.47 s and 0.87 s after the start of cooling, the stress introduced by forming and thermal contraction decreased, and the stresses on the front and back of the steel sheet became nearly uniform. This uniformity is presumably caused by expansion accompanied by martensitic transformation and transformation plasticity. The tensile stress presumably decreased due to the expansion accompanied by martensitic transformation. Furthermore, the transformation plasticity generated plastic deformation in the direction of the applied stress. Plastic deformation reduces the stress difference between the front and back of the steel sheet; the amount of plastic deformation is determined by the amplitude of the transformation plasticity. At 0.87 s after the start of cooling, tensile stress occurred on the web face. Because martensitic transformation was completed earlier, thermal contraction due to the subsequent temperature decrease presumably caused this tensile stress. At 2.07 s after the start of cooling, tensile stress occurred again. This is also assumed to be caused by thermal contraction in an area in which martensitic transformation was completed earlier. At this point in time, although tensile stress was present, a stress difference between the front and back of the steel sheet, which causes poor shape accuracy, was not present. Accordingly, only the length is assumed to simply contract due to the tensile stress during die release. Figure 12 shows the part shape with good shape fixability after die release. It should be noted that the stress states during hot-stamping process are unknown in the previous report by Nakagawa et al.,11) or the final stress state is different from the simulation result (compressive stress) reported by Bok et al.10) Thus, we successfully reproduced excellent shape fixability when martensitic transformation occurs.
Maximum principal stress distribution calculated by finite element simulation after forming and subsequent cooling in the hot-stamping process.
Shape of the hot-stamped part calculated by finite element simulation after unloading.
Even at high forming temperatures, stress is introduced by deformation. Particularly in the round portion of the punch shoulder and the lower portion of the side wall, stress that produces shape changes after die release is often introduced. During cooling after the completion of forming, tensile stress is produced by thermal contraction. This tensile stress is presumed to reduce springback and wall warping, as with tension produced in cold stamping.19) With a stress difference between the front and back of a steel sheet, the stress state is presumably the same state as tensile stress by tension bending; as a result, springback and wall warping will be reduced. This effect is believed to occur in the hot stamping process and achieves better shapes than cold stamping with IF steel and SUS304 steel, in which martensitic transformation does not occur. Furthermore, it is presumed that in 0.2% C steel, in which martensitic transformation occurs after forming, the strain that was generated by the expansion accompanying transformation and transformation plasticity acted to reduce the stress difference introduced in the direction of the steel sheet thickness during forming. This effect can be used to achieve excellent shape fixability in hot stamping when martensitic transformation occurs after forming.
In future studies, we would like to examine the potential applications of forming simulation in different fields.
To examine the mechanism for achieving excellent shape fixability in the hot stamping process, we performed hat-bending forming, using materials that exhibit different transformation behaviors for comparison. We also performed FEM analysis with temperature and phase transformation. The main findings are as follows.
