Over the past decade extensive development of advanced high strength steel (AHSS) was driven by the demand from the automotive industry for stronger materials that can enable lightweighting to meet increasing fuel efficiency requirements. However, achievement of higher strength in many AHSS grades comes with reductions in ductility, leading to geometric constraints on formability and limiting their application. In this paper, a 3 rd Generation AHSS with a compelling property combination of high tensile strength of ~1200 MPa and total elongation > 40% was used for laboratory and stamping studies. Various auto related laboratory tests were done including tensile testing, 180 degree bending, bulge testing, and cup drawing to estimate the steel’s formability under different applied conditions. Additionally, since laboratory testing provides only an estimation of the potential stamping response, the 3 rd Generation AHSS sheet was stamped into B-pillars under industrial stamping conditions. Non-destructive and destructive analysis of the resulting stampings were done to evaluate the microstructural and property changes occurring during stamping. Significant strengthening of material in the stamped part is attributed to the structural changes through the complex Nanophase Refinement and Strengthening mechanism.
With current market trends and governmental regulations pushing towards higher efficiency in vehicles, AHSS are increasingly being pursued for their ability to provide reductions in gauge thickness compensated by higher strength [
Formability may be broken into two distinct forms: local formability and bulk formability [
The World Auto Steel “banana plot” represents the trends of new steel development and the resulting paradox of strength and ductility [
In
follow closely the calculated true stress?true strain curve with slight deviation at high strain levels affected by localized strengthening in deformation bands. After each test, the gauge section of the sample was analyzed with a Feritscope and the total magnetic phases volume percent (Fe%) is additionally shown. Note that the Feritscope uses a magnetic induction technique whereby the applied field in the probe interacts with the volume magnetization in the sample. Thus the data is reported as total magnetic phases volume percent since this technique does not distinguish between different magnetic phases such as alpha-ferrite and alpha-martensite, for example. The maximum value recorded during the testing was 49 Fe%.
In this paper, the focus will be on structure and property changes occurring in NanoSteel 3rd Generation AHSS when cold deformed. Results will include studies on laboratory tensile specimens, incremental tensile testing for property evaluation as a function of strain and data from tests commonly used to estimate metal sheet formability including 180 degree bending, bulge testing, and cup drawing. Additionally, results will be included on non-destructive and destructive testing of stampings from industrial produced blanks of NXG™ 1200 and the results will be compared to those generated from laboratory samples.
As shown in the previous section, NXG™ 1200 demonstrates a combination of high strength and ductility reaching 69,400 MPa% in the case of the example tensile curve in
During tensile testing to failure, the initial structure (3GNS-BD) undergoes Nanophase Refinement and Strengthening (NR&S) leading to formation of the final structure (3GNS-AD). The NR&S mechanism leading to structural evolution during cold deformation described above involves complex interaction of dislocation dominated deformation mechanisms along with phase transformation, nanoscale phase formation, nanoprecipitation, and dynamic strain aging effects. In-depth information related to the details of this mechanism has been presented recently [
The microstructure of the 3rd Generation NanoSteel sheet after deformation (i.e. 3GNS-AD), is demonstrated by the SEM and TEM micrographs in
Further details of the 3GNS-AD structure highlighting microstructural features are shown for Microconstituent 1 in
In
Microconstituent 2 is characterized by the formation of dislocation cells in the austenite grains and concurrent formation of nanoprecipitates. In
micron-sized austenite structure can be seen. In these areas, the austenite does not transform during deformation but accumulates a high density of dislocations forming dislocation block boundaries (i.e. black regions outlining the grains) and nanoscale cell structures with cell size typically from 10 to 20 nm (i.e. the internal structures observed inside the grains).
Various laboratory tests can be used to gain an understanding of a material’s potential performance upon stamping. A summary of various test results for commercially produced NXG™ 1200 sheet with thickness of 1.4 mm is presented below.
Tensile testing is one of the tests sometimes utilized for cold formability evaluation. Uniaxial tensile testing was performed in accordance with the ASTM E8 standard [
Bendability was evaluated by ISO 7438 guided bend test method with a range of nose diameters down to approximately 0.5 mm. In this method, the specimen is set across two supports and lubricated. The forming nose is pushed downward in the middle of the supports and into the specimen until 180˚ bending occurs or until the sample fails. No cracking was observed in any bend samples down to R/t ratio of 0.34 at a 180° bend and in many samples as low R/t ratio as 0.23 (
Drawability was evaluated by drawing full cups from circular blanks with a range of diameters into a die using a cylindrical ram having a 1.9” diameter. During the testing, a blank was centered in the hole of the die and the cylindrical ram was advanced into the die, thereby drawing in the blank. Drawing of the blank progressed until the material had either fully drawn into the die to form a cup, or ruptured. Using this method, the ability of the 1.4 mm gauge sheet to be drawn with draw ratio up to 1.9 without rupture or cracking was demonstrated (
Another test to demonstrate formability of the material was mechanical bulge testing utilizing 7” square blanks that were formed using rounded cylindrical ram of 3” in diameter. Samples were bulged until fracture to determine the maximum dome height before rupture.
Hole expansion testing was used to evaluate edge stretchability of the material. Using samples with punched holes of 10 mm in diameter with punching speed of 228 mm/sec, hole expansion was performed at quasi static ram speed of 50 mm/min. An average hole expansion ratio of 45% was demonstrated (
As shown in the background section, based on the laboratory results, including tensile testing, 180 degree bending, bulge testing, and cup drawing indicated that NanoSteel 3rd Generation AHSS should exhibit good cold formability during cold stamping. To test this assumption, samples of sheet from commercially produced coil with tensile properties provided in
Feritscope measurements provide an indication of the structural changes occurring during deformation from stamping. As shown previously, in the NXG™ 1200 sheet the initial 3GNS-BD structure changes from paramagnetic austenite to the ferromagnetic ferrite in the 3GNS-AD structure during cold deformation through the NR&S mechanism. Increase in the volume fraction of Microconstituent 1 after deformation results in higher Fe% measured due to the formation of alpha-Fe. Measurements were taken by using a Fisher Feritscope from the stamped B-pillar surface with an ~20 mm grid pattern shown in
to allow a correlation of non-destructive measurements with the destructive measurements in the next section. Note that the Feritscope uses a small 1.5 mm probe that allows data to be measured even in areas where destructive tensile testing is not possible due to a non-planar geometry. From the Feritscope measurements, Fe% varied in a range from 0.1% to 0.4% Fe% in most flat areas of the B-pillar (same as in the initial sheet before stamping) and up to 31 Fe% in the most deformed areas.
Rockwell C (HRc) hardness measurements were also used as a non-destructive method to evaluate the deformation effect during stamping operation on material structure and properties. However, in contrast to Feritscope measurements, this method is limited to flat surfaces for measurements and cannot provide information on the most deformed curved areas of the B-pillar. The hardness of the sheet before stamping was ~20 HRc. Hardness in a range from 20 to 47 HRc was recorded for the stamped part confirming the strengthening effect from the NR&S mechanism activation during stamping.
For destructive analysis, tensile specimens were cut along the entire length of the B-pillar as shown in
Examples of the stress ? strain curves for specimens cut from the B-pillar with various levels of magnetic phases volume percent (Fe%) are presented in
Detailed TEM analysis was done on the samples cut from different locations of the stamped part to demonstrate the structural response to the deformation during stamping.
Selected samples containing 4.6 Fe%, 13.9 Fe%, and 24.5 Fe% after stamping as provided in
Fe% | 0.2% Yield Strength (MPa) | 0.5% Yield Strength (MPa) | Ultimate Tensile Strength (MPa) | Tensile Elongation (%) |
---|---|---|---|---|
Unstamped (<1%) | 463 | 496 | 1188 | 56.6 |
1.3 | 591 | 664 | 1215 | 59.0 |
4.4 | 588 | 743 | 1253 | 51.2 |
4.6 | 503 | 652 | 1212 | 54.2 |
9.0 | 621 | 774 | 1231 | 46.7 |
13.9 | 716 | 896 | 1326 | 39.1 |
20.2 | 787 | 1007 | 1320 | 37.0 |
24.5 | 954 | 1229 | 1410 | 27.0 |
studying multiple locations of the stamped part, a clear correlation is found with increases of Fe% in the samples and the amount of activated NR&S occurring during stamping.
In
measurements showing 13.9 and 24.5 Fe%, respectively. Higher dislocation density can be noted within the grains with increasing Fe% values. Twins are present in all three microstructures.
Additional TEM analysis of the microstructure was also done in the gauge section of the corresponding samples tested in tension. After deformation, similar Fe% was measured in all three sample gauges that ranged from 38 to 43 Fe%. Bright-field TEM images are provided in
The structural analysis of the samples cut from the stamped part confirms that the NR&S mechanism observed in laboratory samples during tensile testing as described in the Background section is also occurring during stamping operations. The NR&S mechanism is complex in nature [
Using the dataset of tensile properties measured from the destructive testing of the B-pillar, correlations of the measured tensile data can be made with both Rockwell C hardness and Feritscope measurements. With increasing hardness and magnetic phases volume percent (Fe%), both the yield and ultimate tensile strength are found to increase commensurate with reductions in elongationwhich is related to phase changes in the material during stamping. These phase changes depend on the level of straining leading to variable values of measured properties. The Pearson correlation coefficient (ρ) was calculated for yield strength (0.2% and 0.5% offset), ultimate tensile strength, and elongation, respectively, as a function of both hardness and magnetic phases volume percent (Fe%). Plots are provided in
For hardness the absolute value of the Pearson correlation coefficient (|ρ|) varies from 0.72 to 0.87, indicating moderate to strong linear correlations between these measurements. The absolute value of the Pearson correlation coefficient (|ρ|) for magnetic phases volume percent (Fe%) ranges from 0.78 to 0.91, indicating a moderate to strong linear correlation and one that is slightly stronger than for the hardness measurements. Based on the strength of these correlations, a linear model for predicting yield strength, ultimate tensile strength, and elongation can be created using hardness or magnetic phases volume percent.
Through non-destructive evaluation, a maximum value of 31 Fe% was measured in highly deformed areas of the B-pillar. Due to the physical limitations in cutting of tensile specimens in these areas, the maximum Fe% observed in the cut tensile specimens was 24.5%. Using established correlations based on ~200 data points and linear model described above, yield and tensile strength as well as total elongation were strength the tensile properties were estimated by extrapolation of the linear relationships to 31 Fe% as shown in
ductility in the most deformed areas of the B-pillar after stamping is similar to other grades of AHSS in their respective undeformed, pre-stamped conditions. In addition to showing the level of strengthening achieved in the stamping, these results indicate that the material has a potential for applications requiring stamping of even more complex geometries and the resulting stamped parts retain capability for high energy absorption.
As shown above, a detailed analysis of property changes in the NanoSteel alloy sheet after stamping was performed on the B-pillar. The correlations of the tensile properties with the Feritscope measurements were found to be good with coefficient of determination or R2 from 60.2% to 83.7% as shown in
Bulk and local formability was demonstrated in the 3rd Generation NanoSteel NXG™ 1200 material by a number of laboratory tests including hole expansion, biaxial bulge, cup drawing, and guided bend testing. The extension of these test predictions for cold formability in an industrial process was confirmed by commercial stamping of the material into B-pillars at ambient conditions. A detailed analysis of the stamped parts revealed variations in structural changes and strengthening of the material after stamping depending on the localized deformation applied in relation to the part geometry. The effect of the NR&S mechanism occurring in the areas of the B-pillar during forming has been quantified by using magnetic phases volume percent measurement coupled with tensile testing of the specimens cut from the stamped part.
The measured magnetic phases volume percent, yield strength, and ultimate tensile strength are shown to increase with increasing deformation level during forming especially in the areas of extensive bending and stretching with correlation to microstructural changes from the NR&S mechanism. Yield strengths (0.2% offset) up to 1085 MPa and tensile strengths up to 1490 MPa were estimated to be achieved in the stamped B-pillar while retaining a minimum of 15% estimated elongation. Initial correlation studies have demonstrated a capability to relate non-destructive evaluation methods with actual measured properties to develop a holistic picture of the structure and properties produced during stamping allowing for subsequent more detailed modeling and analysis. The developed correlations allowed estimation of properties in the areas where direct measurement are not available showing, in some locations, even higher strengthening effect from the NR&S mechanism. In the future, work will continue to understand the details of this 3rd Generation AHSS performance during industrial stamping operations as well as to correlate resulting properties in the stamped part with specific localized deformation conditions.
Branagan, D., Parsons, C., Machrowicz, T., Cischke, J., Frerichs, A., Meacham, B., Cheng, S., Justice, G. and Sergueeva, A. (2018) Effect of Deformation during Stamping on Structure and Property Evolution in 3rd Generation AHSS. Open Journal of Metal, 8, 15-33. https://doi.org/10.4236/ojmetal.2018.82002