2018 Volume 58 Issue 8 Pages 1538-1544
Local formability is one of the most critical properties of Advanced High Strength Steels for successful implementation in automotive applications. However, attaining satisfactory local formability is a challenge for many Advanced High Strength Steels. In the present study, a hot dip galvannealed dual-phase steel with 980 MPa minimum tensile strength was selected as the AHSS and its local formability was evaluated by conducting 90-degree V-bend tests. The bend samples, particularly those that experienced failure during testing, were subjected to intense characterization using Scanning Electron Microscope, Cathodoluminescence Microscope and Thermal Desorption Analyzer. Fractography study indicated that the bend failure is associated with the interaction between inclusions near the steel surface and the diffusible hydrogen in the steel. The same underlying failure mode was also observed in two additional zinc coated AHSS grades. Improved steel cleanliness resulted in reduction of inclusions and diffusible hydrogen amounts, and facilitated enhanced steel bendability. Diffusible hydrogen content in the steel could be reduced by removing the surface zinc coating or by heat treatment; such treatments also led to a significant improvement in bendability.
Automotive manufacturers are using Advanced High Strength Steels to enhance vehicles’ crash resistance and for weight reduction. Steels with tensile strength of 980 MPa and above are the targeted structural materials for such applications. However, such steels are not as formable as mild strength steels. This meant that automakers could not simply replace the existing parts made of mild steel with a higher strength steel. This limits the application of the high strength steels and adds extra cost for automakers to redesign parts. In the current study, mill-produced galvannealed dual-phase (DP) steel with minimum tensile strength of 980 MPa was evaluated by conducting 90-degree V-bend test. The root causes of bending failure were identified by conducting fractographic investigations with Scanning Electron Microscope (SEM) and Cathodoluminescence Microscope (CLM) and by measuring diffusible hydrogen content in the steel with Thermal Desorption Analyzer (TDA). The impacts of inclusion reduction and hydrogen removal on bendability were investigated. Two additional zinc coated AHSS grades with bend failure were also discussed.
The factors impacting AHSS’s bendability had been studied in the past. In general, these factors could be categorized into three types: steel microstructure, inclusions and hydrogen. Kaijalainen et al. investigated the hot rolled-quenched steel with yield strength of 960–1100 MPa. It was found that the steel with upper bainite and auto tempered martensite microstructure exhibited poor bendability when the subsurface texture was {112}<111>α. Such textured microstructure tended to promote shear localization and crack formation during bend test. It was also found that if the subsurface microstructure was replaced by a soft layer of polygonal ferrite and granular bainite, the bendability could be significantly improved.1,2) The steels in the current study were hot rolled, cold rolled and annealed with same processing parameters, the microstructure was consistent among all samples. There was no soft layer at the steel subsurface. Based on authors’ study, the bend failure and bendability variation were not related to the steel microstructure in the present study. Kaijalainen also found that the MnS inclusion had negative impact on the bendability. It was reported that when the S content was above 40 ppm, the bendability started to be deteriorated.1) Chang reported that ductile MnS and hard oxides inclusions had adverse effect on the bendability. These inclusions would cause voids formation, coalesce and lead to cracking eventually.3) In the current study, the bend failure was found to be related to oxide inclusions, but not MnS. Gao et al. investigated the impact of hydrogen on bendability of DP1180 steel. Two conditions were tested, electrogalvanized vs. hydrogen pre-charged and electrogalvanized. It was found that the latter condition always showed inferior bendability under various strain conditions.4) It should be noticed that electrogalvanized sample might also contained certain amount of hydrogen, which was not addressed by the authors. In the current study the bend failure mechanism associated with hydrogen was intensively studied.
The material in the present study was galvannealed dual-phase steel, with tensile strength of 980 MPa–1100 MPa in the transverse direction. The chemistry of steel is listed in Table 1. The microstructure was composed of approximately 45 volume% of martensite and remnants of ferrite and bainite.
| C | Mn | Al | Ca | Mg | Others |
|---|---|---|---|---|---|
| 0.09% | 2.1% | 0.05% | ≤0.001% | ≤0.0005% | Si, Cr, Mo, Ti, B |
The 35 × 100 mm rectangle coupons were cut from the steel coil. A coupon was placed in the bend tester and the punch pushed the sample into the die at constant speed. The bent sample was then held under 80 kN load for 5 sec before the punch traveled back to its initial position. The bend axis was along the rolling direction. The bent coupon was taken out of the fixture and examined for cracking at the bending surface.
The steel coupons were tested using a series of punches with varied tip radii. When three or more coupons could be bent consecutively at one die radius without showing any surface crack, they were labeled as “pass” for that specific die radius. Additional coupons were then tested at smaller radii, until it reached a minimum die radius beyond which samples started to crack. The minimum bending die radius (r) divided by the sample thickness (t) is defined as the bendability value (r/t). The smaller is the ratio of r/t, the better is the bending performance of the material.
2.3. FractographyFractography investigation was critical in identifying the root cause of the bend failures. After observing bend cracks, the cracked areas were marked and the samples were manually bent to complete fracture. Then the cracked areas were exposed for further examination.
The fractured samples were observed in a SEM operated at 20 kV. Secondary electron and backscattered electron signals were used for imaging. Energy Disperse Spectrum (EDS) was used to identify the chemical composition of the investigated features.
The CLM was used to visualize the inclusions on the fracture surface of samples.5) The schematic of CLM and its working mechanism are shown in Fig. 1. Fractured samples were placed inside the vacuum chamber. After evacuating the chamber, samples were charged with electrons, which caused the non-metallic inclusions to glow. The samples were observed and photographed using an optical microscope above the chamber in a darkroom.
Schematic drawing of Cathodoluminescence Microscope.
The majority of inclusions in the steel were oxides. The inclusion amount and the measure of steel cleanliness are indicated by the total oxygen content in the steel.6) Samples were taken from selected heats and were tested in a LECO tester to obtain total oxygen amount.
2.5. Hydrogen MeasurementThe TDA was used to measure the diffusible hydrogen content in the steel. The samples were heated to 250°C at 1.66°C/min by a radiation furnace in the TDA. Hydrogen released from the steel samples was carried out by nitrogen gas and measured using mass spectroscopy. Hydrogen desorbed below 250°C was characterized as diffusible hydrogen, because it has relatively low binding energy with the crystal defects in the steel, such as dislocations and grain boundaries.7)
2.6. Hydrogen RemovalIn the current study, it was found that the diffusible hydrogen had great impact on bendability. To experimentally remove diffusible hydrogen, the coated steel samples were subjected to two processes, surface coating removal and heat treatment. The galvannealed coating was removed from the steel surface by dipping samples for 20 min in a HCl solution (72 g/l) with inhibitor hexamethylenetetramine (3.5 g/l). Then the steel was left in ambient atmosphere for 24 hr prior to further testing. In the other process, the coated steel samples were heat treated in a box furnace at 190°C for 18 hours with air atmosphere. Then samples were subjected to further testing.
2.7. Tensile TestTo understand the impact of heat treatment on the final mechanical properties, tensile tests were performed on samples before and after heat treatment. Steel sheets were cut for tensile tests according to JIS standard,8) with a gauge length of 50 mm and a strain rate of 0.003/sec.
The 90-degree V-bend test is one of the standard tests to evaluate the local formability of AHSS. As shown in Fig. 2, the failed sample had large surface cracks, indicative of a potential failure when the same steel is stamped to make automotive parts. Prior to improvement, the bendability of the galvannealed DP980 steel was unsatisfactory; it exhibited a high r/t ratio and large variability in performance. The challenge was to improve the bendability, i.e. decrease the r/t ratio, and reduce the scatter.
Examples of (a) passed and (b) failed bend coupons of DP980 steel.
To identify the root cause of the bend failures, the cracked areas were marked and opened up further to completely expose the substrate steel underneath the cracks. Then the fractured samples were subjected to microanalysis using SEM.
Figure 3 shows two examples of the areas underneath the bend crack. In each micrograph, there was an inclusion particle located right underneath the surface crack. These particles were identified as calcium aluminate using EDS. In Fig. 3(a) the particle was spherical with the other half broken during the bend test. The diameter was approximately 45 μm. Its distance from the steel surface was 210 μm. In Fig. 3(b), the particle had a perfect spherical shape with diameter of 18 μm. At the two horizontal ends of these particles, there were two conical shaped voids; they were elongated along the rolling direction with curvature towards the inclusion. These voids could act as crack initiation sites during bend testing. Around the particle, there was an elliptical area, which exhibited more brittle-like features in comparison to the rest of the fractured steel surfaces. The major axis of the elliptical area was along the rolling direction. The most commonly observed inclusion particle was calcium aluminate. In some cases, a spinel or alumina particle was observed.
The micrographs of area underneath bend cracks (secondary electron images).
This presence of an inclusion particle and a surrounding quasi-cleavage area is very similar to the morphology described as “fish eye” in literatures. “Fish eye” was reportedly observed when hydrogen-containing steel welds fractured under tensile stress. The pupil of the eye usually was a pore or an inclusion particle. The surrounding area usually had a quasi-cleavage fracture morphology.9,10,11) In those studies, hydrogen was entrapped at steel crystal defects when steel solidified from molten state during welding. In the manufacturing of advanced high strength steels, there are multiple processes that can introduce hydrogen into the steel: steelmaking, acid pickling, and final-step annealing in a hydrogen-nitrogen protective atmosphere. In the current work, the formation of voids, the “fish eye” and the surface cracking could be explained as follows.
3.2. Voids FormationThe voids at two ends of an inclusion were observed after sample cracking. Literature reported that voids were formed during heavy reduction at the hot rolling stage; it had been validated using FEM simulations.12,13,14) It is believed that there was no void at the steel/inclusion interface after the steel was cast; the steel/inclusion interface started to de-bond at hot rolling stage due to the tensile stress along the rolling direction caused by the deformation of the surrounding steel. The voids were formed symmetrically at the two ends of the inclusion. As the void expanded in the rolling direction, steel was pushed into the voids from the vertical direction due to the compressive stress, leading to formation of the characteristic curvature on the conical voids surface.12,15) The formation of the void was largely dependent on the relative strength of the inclusion to the steel and the strain-stress status. In general, when the inclusion was much harder in comparison to the steel at the deformation temperature, the void formation was more pronounced.12) The growth of the void was dictated by the balance of the tensile stress at the void tip and the compressive stress surrounding the void. At the beginning, the voids expanded as the strain increased, then the voids kept their shape unchanged due to the closing caused by the compressive stress, even as the strain further increased.13,14) It has been reported that the distance the voids extended into the steel usually equaled to the half-height of the inclusion particle, which was observed in Figs. 3(a) and 3(b) in the current study.
3.3. Fish Eye FormationThere were three processes during which hydrogen could be introduced into steel; steelmaking, acid pickling, and annealing in hydrogen atmosphere. After steelmaking and casting, steel was heated up in reheat furnace for rolling and diffusible hydrogen could escape. After pickling, the steel surface was bare; the diffusible hydrogen could escape as well. In the final annealing, steel was heated in hydrogen-nitrogen atmosphere and subsequently coated with zinc. It was reported that zinc could act as a hydrogen diffusion barrier,16) thus the diffusible hydrogen in the final product should be mainly from the final annealing step.
During the annealing process, hydrogen diffused into the steel and built up at the inclusion/steel interface and at the voids. So, it is possible that hydrogen was kept at these sites in the final product. When the final product went through bend testing, the upper part of the bent area was under tension. If the inclusions were present in the tension area, the two voids at the ends of the inclusion would grow under stress. Due to trapped hydrogen, the steel surrounding the inclusion became brittle such that no new ductile void could form. The existing void tips continued to extend and could not be blunted by coalescing with new voids. The lack of blunting caused the tip to remain sharp and extend in a brittle fashion into the steel, leading to the formation of the quasi-cleavage zone.10) It was believed that the voids would propagate along prior austenite grain boundaries in ferritic or martensitic steels and exhibit an intergranular fracture surface.10) However, in the current study, the prior austenite grain boundaries were barely seen. The fracture surface was much flatter and many small facets were noted on the surface, which were much smaller than prior austenite grain. As the void grew, the concentration of hydrogen dropped due to increased void volume such that the void front became less brittle. A brittle to ductile transition was formed, which became the boundary of the “fish eye”.
3.4. Surface Crack FormationThe big voids coalesced with newly formed new voids and continued to grow. When it reached the steel surface, a surface crack was formed. The crack would grow further until the stress was released, or when the stress was not sufficient to drive further growth.
In some cases, the inclusion was so close to the steel surface that the quasi-cleavage area directly reached the steel surface and caused the crack, before it reached the brittle to ductile transition. Occasionally, even when the inclusion was far away from the surface, due to its bigger size the surrounding quasi-cleavage area was also large, which intercepted the steel surface and caused a crack.
Besides individual inclusion particles, the other frequently observed morphology was linear stringers of inclusions, as shown in Fig. 4(a). These two types of inclusion morphologies were described in the work of Bernard and Luo. In their work, inclusion morphologies were classified into five types. In the current study, the “hard” inclusion under rolling condition and the “hard” inclusion cluster “strung out” during rolling were two scenarios similar to the previously reported.12,17)
Micrograph of an inclusion stringer underneath the bend crack, (a) at lower magnification and (b) at higher magnification.
In the following example (Fig. 4), the length of the inclusion stringer was 445 μm and the distance to the surface was 60 μm. The quasi-cleavage zone was around the stringer and was much longer (580 μm). As shown in Fig. 4(a), the upper portion of quasi-cleavage zone reached the steel surface. This inclusion stringer was composed of many small individual inclusion particles. Most of them were smaller than 10 μm, as seen in the Fig. 4(b) at higher magnification. These particles were identified as alumina, spinel, and mixture of spinel and calcium aluminate. The formation of such inclusion stringers was likely due to the breakage of a large inclusion or a conglomerate of inclusions during hot rolling, causing the broken pieces to spread out along the rolling direction. As the broken particles were spread out, gaps were formed among the particles. Though each individual inclusion was small, the impact zone of such inclusion stringer was very large.
3.5. Bend Failure in Other AHSSThe bend failure was also frequently observed in other AHSS grades with tensile strength greater than 1000 MPa. The “fish eye” type fractography was always observed inside the cracks. In Fig. 5(a), the steel was dual-phase steel with minimum tensile strength of 1180 MPa and with hot dip galvanized zinc coating on surface. Diffusible hydrogen was introduced into steel during high temperature annealing similar to DP980 steel. In Fig. 5(b), the steel was a fully martensitic grade with minimum tensile strength of 1700 MPa. When the zinc was electrochemically plated after the steel was annealed, hydrogen was introduced into the steel. In both cases, zinc coating could act as a diffusion barrier to seal the hydrogen inside the steel. Additionally, in both products, the presence of calcium aluminate inclusions and diffusible hydrogen caused the formation of “fish eye” and bend failures. The investigations on these additional products indicated that both inclusions and diffusible hydrogen affect the formation of cracks during bend testing.
The fish-eye morphology inside bend crack in (a) hot dip galvanized dual-phase 1180 MPa steel and (b) electro-galvanized martensitic 1700 MPa steel.
The other issue with the DP980 steel was its inconsistent bending performance. The bendability exhibited a large variation between different heats, at times even within the same coil. CLM is an efficient methodology to generate an overview of inclusions at the fracture surface. Figure 6 shows examples of a pass sample and a failed sample, which were obtained from the same coil. In the CLM analysis, the steel surface remained as dark-red background, the inclusions fluoresced and their images were recorded with an optical microscope equipped with a digital camera. In these two examples, there were more inclusions in the failed sample. Their proximity to the tension surface seemed to be the cause of bend failure. These observations suggest that the distribution of inclusions was heterogeneous inside the steel, which likely caused the large variation of the bendability. It was hypothesized that overall reduction in inclusion amount might be able to mitigate such variation.
CLM image of fracture surface for (a) a passed sample and (b) a failed sample. (Online version in color.)
The above analysis suggested that to improve bendability of zinc coated DP980 steel, the steel cleanliness is one of the variables needed to be addressed. To reduce inclusions in the steel, particularly near the steel surface, several steel refining and casting practices were developed. As a result, as shown in Fig. 7(a), the total oxygen level was reduced from 18 ppm to 5 ppm, which indicated significant reduction of oxide inclusion density and improvement of steel cleanliness. Corresponding improvement in product bendability was noted and its variation was also reduced. Figure 7(b) shows that the diffusible hydrogen was also reduced in the steel with improved cleanliness. This could be due to a reduction in the amount of trapping sites, viz. the inclusions.
(a) the bendability and (b) diffusible hydrogen amount respecting to total oxygen level.
Therefore, by reducing the inclusion amount, the “fish eye” formation sites were reduced and so did the diffusible hydrogen trapping sites and the diffusible hydrogen amounts. Both factors were beneficial for superior bendability.
3.8. Effect of Hydrogen RemovalTo investigate the impact of diffusible hydrogen on the bending performance, the galvannealed coated DP980 steel was subjected to two processes. The as produced steel was dipped into HCl solution for 20 min to completely remove the surface zinc coating. Then the steel was left in ambient atmosphere for 24 hr to diffuse out the hydrogen. The Table 2 and Fig. 8 reveal that after this process the diffusible hydrogen content dropped from 0.42 ppm to 0.04 ppm. This also confirmed that the bare steel could not hold any diffusible hydrogen. The diffusible hydrogen was introduced into the steel at the last stage of processing while the steel was coated with zinc, such as hot dip galvanizing or electrochemical plating. After the coating was removed, the bendability was significantly improved from 3.9 r/t to 1.6 r/t. The tensile properties remained the same. However, without the zinc coating, it could not be commercially used.
| steel | t, mm | condition | YS, MPa | TS, MPa | YS/TS | TE, % | r/t | Diff H, ppm |
|---|---|---|---|---|---|---|---|---|
| DP980 | 1.8 | As produced | 707 | 1070 | 0.66 | 14.0 | 3.9 | 0.42 |
| Coating removed | 1.6 | 0.04 | ||||||
| Heat treated | 915 | 1073 | 0.85 | 12.7 | 1.7 | 0.06 |
The hydrogen thermal desorption profile of DP 980 steel with varied conditions.
The other process to remove the diffusible hydrogen was via heat treatment. The as-produced steel samples were heat treated in a box furnace at 190°C for 18 hours with air atmosphere. After heat treatment, the diffusible hydrogen content in the steel was also greatly reduced to 0.06 ppm. Bendability was improved to 1.7 r/t accordingly, similar to the improvement after the coating removal process. However, the final tensile properties were changed by this process. The yield strength drastically increased from 707 MPa to 915 MPa. The tensile strength remained the same. The total elongation decreased from 14.0% to 12.7%. Increased yield strength and decreased elongation occur when dual-phase steel is tempered. After heat-treatment, the carbides act as obstacles to dislocation gliding, which increases the yield strength and decreases the elongation. The changes in yield strength and elongation should be taken into consideration when the automakers stamp the parts.
The association of improved bendability with reduced diffusible hydrogen amount confirmed that hydrogen was one of the key factors impacting the bendability. Dislocations and grain boundaries are well-known trapping sites for diffusible hydrogen in ferritic steels.7) In the current study, the drop of diffusible hydrogen from 0.42 ppm to 0.04–0.06 ppm was partly contributed by the removal of hydrogen trapped at these sites.
In the “fish eye” structure, there were two types of hydrogen trapping sites, the inclusion/steel interface and the void. Based on the present study, the improvement in bendability after hydrogen removal implied that some of the amount of hydrogen trapped at these sites might be reduced. In the previous section of inclusion control, it was noticed that the reduced inclusion amount also resulted in the reduction in diffusible hydrogen content. Both facts indicated that there was some diffusible hydrogen trapped at these sites, i.e. these trapping sites exhibited some low-energy binding behavior.
However, whether the hydrogen trapped at these sites were completely removed by heat treatment remained unconfirmed, i.e. whether these sites only had low binding energy remained unknown. It was reported that AlN and Alumina inclusion/steel interfaces had high binding energy.18,19) The binding energy of steel interfaces with calcium aluminate or spinel has not been reported in literature. It was reported that the micro voids had low binding energy,7) however, the voids in the current study were much larger in size. The interfaces between steel and large particles typically were incoherent. Incoherent interfaces were known to have high binding energy; the trapped hydrogen was non-diffusible.20)
In the present study, the hydrogen trapped at the inclusion/steel interfaces might be partially diffusible, which could be removed by experiments resulting in improved bendability to a certain extent. The rest of the trapped hydrogen might be non-diffusible, which could only be removed by higher temperature treatment to affect a further enhancement in bendability. But this could not be simply achieved as the steel microstructure would be totally changed and strength would be greatly reduced.
The root causes of bend failure of galvannealed DP980 steel were identified as follows. The presence of subsurface inclusions caused the formation of voids at both ends during hot rolling. Hydrogen was introduced and trapped at the inclusion/steel interface and at the voids during the steel manufacturing. At the time of bend test, the voids extended under stress in a brittle fashion due to the presence of hydrogen, which formed a “fish eye” region around the inclusion. This brittle region propagated and grew, intercepted steel surface and caused the surface crack. This same bend failure mechanism was also observed in other zinc coated AHSS products. The large variation of bendability was likely due to the heterogeneous distribution of inclusions.
The inclusions causing the bend failures were found to be calcium aluminate, alumina, spinel and their mixtures. The inclusions could be a single large inclusion or a stringer composed with many small inclusions.
The bendability was improved by changing the steelmaking and casting practices to enhance the steel cleanliness. The improved cleanliness also resulted in reduced diffusible hydrogen content.
The bendability was also improved by removing diffusible hydrogen. Two artificial processes were conducted: coating removal and heat treatment. In both cases, the diffusible hydrogen dropped to similar values and the bendability was improved to similar levels. However, the zinc coating was removed in the first process and the tensile properties were changed in the second process. Therefore, improving steel cleanliness was the only way that could be industrialized to improve bendability of commercial AHSS products.
It was estimated that the hydrogen trapped at inclusion/steel interface and voids were possibly partially diffusible and partially non-diffusible. This needs to be confirmed by further investigations.
The technical support from ArcelorMittal Cleveland is deeply acknowledged. The support from David Reilley, Dan Druyor, Mark Milliron and Mark Yahraus is specially appreciated. Laboratory support from Jeff Tai, Jerome Cap, Eric Kekeis, Eric Ellis, Luisito Laus, and Jeff Thacker is greatly acknowledged. Naveen Ramisetti is acknowledged for explaining TDA data. Rongjie Song is acknowledged for providing the data of electrogalvanized martensitic steel.
