THE HOT FORMING OF GALVANIZED STEELS

Transcription

1 THE HOT FORMING OF GALVANIZED STEELS Karl M. Radlmayr, voestalpine Metal Forming GmbH Reiner Kelsch, Andreas Sommer, voestalpine Automotive Components Schwäbisch Gmünd GmbH & Co. KG Christian Rouet, voestalpine Krems GmbH Thomas Kurz, Josef Faderl, voestalpine Stahl GmbH

2 CONTENT 03 Abstract The original: phs-ultraform Galvanized innovation in the field of direct hot forming: phs-directform Precooling Base material Corrosion VDA VDA Joining technology Next step in development phs-rollform Summary

3 3 ABSTRACT The demands on safety-related structural components include high strength in combination with good energy absorption in order to meet the requirements of light-weight design and crash performance. The materials themselves must also be easy to process (forming, joining, coating) and must feature a high level of corrosion resistance. The direct hot forming of bright-finished and hot-dip aluminized MnB steels has made it possible to meet the requirements of strength and crash performance. While the first galvanized serially produced body in white went to market in 1985, it took twenty years to put the first hot-formed galvanized component produced using the indirect process into serial operation. In order to achieve this feat, it was necessary to raise the physical limits between zinc and steel (Zn melting point: 420 C, boiling point: 907 C, austenitizing temperature of Fe: 900 C). Knowledge of the layer reactions and microstructural transformations during the heat treatment of boron steel have apparently made the impossible possible. The possible occurrence of liquid metal embrittlement (LME) was avoided in the indirect process by means of the upstream cold forming process. The phs-directform process now also solves the problem of LME in the direct process as well and thus allows the comprehensive application of hot-formed components in the carbody. Precooling and alloy adaptations in the base material were necessary in order to achieve this.

4 4 01 The original: phs-ultraform The classic base material 22MnB5 was used as the foundation for this process because the optimized production parameters lead to excellent cold formability in the hot-rolling, cold-rolling and the hot-dip galvanizing routes. Analysis of the 1500 MPa version is comparable to HSLA340 with respect cold formability. Forming and particularly the cutting and punching operations prior to austenitizing result in significantly improved crash performance when compared to hard- or laser-cut components. Finalizing the cold-formed geometry with the patented form-hardening process also allows the highest degree of dimensional accuracy and reproducibility without having to cut in hardened condition (Figure 1). Figure 1: phs-ultraform, the indirect process for galvanized, press-hardening steel Undercutting can be achieved using sliding elements, and the furnace design allows any technically feasible SiC roll lengths, which means that complete side panels are possible. The unique furnace design and technology combined with cycle-time-neutral tailored-property parts (TPP) has permitted the development of tailored heating (partial austenitization in the furnace) of regions that prevent plastically formed areas from being hardened while all other regions achieve full strengths of up to 1500 MPa. Figure 2 shows an overview of currently available TPP versions that can be produced with phs-ultraform technology. The Zn layer exhibits excellent cold-forming properties and minimized friction values, which guarantees the lowest tool wear.

5 5 The afore mentioned austenitization, which at first sight seems to defy the physical limits of zinc and iron, is achieved through oxidation of Zn as well as with the aluminum contained in the Zn layer, both of which prevent vaporization of the Zn layer during the reaction between Zn and Fe. The oxide skin that forms during heat treatment of the cold-formed component in combination with the zinc-iron crystals that also grow out of the steel surface prevent the melted zinc itself from flowing into the vertically standing component regions. Hardened MPa phs-ultraform 490: approx. 500 MPa Patch inside/ Patch outside Hardened and stress-relieved < 1500 MPa Partially heated: approx. 500 MPa Figure 2: Possible tailored-property parts

6 6 02 Galvanized innovation in the field of direct hot forming: phs-directform It was long thought that the problem of grain-boundary attack caused by liquid zinc on steel with simultaneous tension overlay would never be solved. In-house research using a variety of zinc layers, quantities and process parameters did not yield any positive results Precooling The breakthrough was finally achieved when contact-free cooling was applied to avoid liquid phases during hot forming (see Figure 3). The cooling units implemented in the first two lines are based on separately regulated nozzle fields that blow carefully adjusted volumes of air onto the hot sheet surfaces from both sides after the material has exited the austenitization zone (furnace). Temperature Furnace exit Precooling Hot forming Hardening process Transfer 1 Transfer 2 Time Figure 3: phs-directform : Process featuring precooling method The wide control range not only achieves comparable cycle times with various sheet thicknesses, but the process also forms the foundation for processing laser-welded blanks with varying sheet thicknesses that require different cooling volumes.

7 7 A possible nozzle field arrangement is shown in Figure 4. Figure 5 shows the positive effect of intermediate cooling. All types of microcracks can be eliminated using this cooling method. Varying sheet thicknesses, any differences in transfer times between the furnace and the press and the time needed for (cycle-time-neutral) contact-free precooling require a stable and sufficiently large process window. These benefits were not guaranteed by the 22MnB5 base material. Without precooling With precooling No Microcracking of the first order Microcracking of the first order (LME) Microcracking of the second order No Microcracking of the second order Figure 5: Effects of precooling on microcracking Figure 4: Nozzle arrangement for cooling of four zones Base material 20MnB8 was developed to meet these requirements. The ferrite, perlite and bainite regions were shifted to achieve longer times as well as a more stable and broad cooling corridor for this production process. Figure 6 shows the effect of the change in analysis type. A ZF180 zinc layer (Galvannealed) GA (90/90) is used instead of the classical Z140 or Z180 (GI70/70 or GI90/90) zinc layers as a further modification of the indirect phs-ultraform process.

8 8 As a result of its higher emission level, this new zinc layer heats more quickly by 30% to 40% and yields a robust furnace process window. The layer hardness, which is lower than that of Al-Si coatings, and the halved coefficient of friction substantially reduce tool wear and permit more complex geometries. phs-ultraform 22MnB5 + GI70/70 22MnB5 + GI90/90 phs-directform 20MnB8 + GA90/90 Figure 7 shows the comparison between the two zinc layers before and after the austenitization process. Figure 6: Process window comparisons between 22MnB5 and 20MnB8 Steel Zinc Steel Steel Zinciron Galvannealed Steel Zinciron phs-ultraform unhardened phs-ultraform hardened phs-directform unhardened phs-directform hardened Figure 7: Microstructure and layer formation in unhardened and hardened condition Figure 8 shows the friction coefficients as they depend on tool materials and coatings. The previously manufactured prototypes show the large process window available with this technology, which will lead, especially in combination with current further developments, to a substantial increase in the share of hot-formed components. Friction coefficient Different tool materials Figure 8: Friction coefficient in hot-formed steels dependent on surface coatings

9 9 The first industrial-scale production line equipped with this technology was put into operation at the beginning of July 2016 and is located at voestalpine Automotive Components Schwäbisch Gmünd GmbH & Co KG. As in the phs-ultraform production lines, the phs-directform line also operates in normal ambient environments because hydrogen embrittlement does not occur in zinc coatings during furnace processing. It is not necessary for this reason to control dew points or protective-gas atmospheres. A further advantage of the Zn layer lies in the process window. Only a few seconds are required to reach austenitizing temperature in order to achieve the full strength potential. This is because no diffusion times are required for the formation of layers, which is not the case in Al-Si layers. A variety of different heating technologies can be used because no special heating curves are necessary and only the target temperature is of relevance. The layer composition determined by the heating process is relevant to the corrosion behavior. Figure 9 shows a SEM image of the Fe-Zn reaction layer on hot-dip galvanized steel strip following the press-hardening process. Intermetallic phases of varying compositions are characterized by different shades of gray. Zinc-rich phases appear in lighter shades than iron-rich phases. Embedding agent Oxide layer Martensite Figure 9: SEM image of the microstructure of Zn-Fe reaction layer on press-hardened hot-dip galvanized steel strip It is easy to see the change in the zinc layer caused by the press-hardening process. The initially 10-micrometer-thick Zn layer grows to a thickness of between 20 and 30 micrometers as a result of the reaction and consists of two different zinc-iron phases, each with a very different composition. The lighter-shaded regions in the layer have a zinc content of roughly 70% by mass. The darker regions have a zinc content of roughly 40% by mass.

10 Corrosion Corrosion performance is one of the essential benefits, which is why galvanostatic testing is conducted regularly during production. The testing procedure is based on standardized VDA test. Figure 10 shows a galvanostatic curve depicting press-hardened hot-dip galvanized steel strip in comparison with material in unhardened condition. Potential vs. SHE/V Z140 press-hardened Z140 unhardened unhardened Figure 10: Galvanostatic resolution of press-hardened hot-dip galvanized steel strip (black) as compared to initial condition (red). Time/Minutes The potential at the beginning of galvanostatic resolution applied to press-hardened hot-dip galvanized steel strip is 0.54 V SHE, which is shown by the black line in Figure 10. This value corresponds to a -ZnFe phase, which goes into solution first. Following a period of roughly twenty minutes at a current density of approximately 12 ma per square centimeter, the potential rises to -0.4 V SHE. This means that the -ZnFe phase is completely usurped and that the zinc ferrite goes into solution continuously. The steel potential of 0.22 V SHE is reached after roughly sixty minutes. Both potentials in the zinc-iron phase are significantly smaller than the potential of the steel substrate and thus offer excellent cathodic corrosion protection potential in the steel substrate. These assumptions have been verified by two corrosion tests recognized in the automotive industry.

11 VDA The VDA alternating test is an accelerated, cyclical corrosion test in which test samples are successively subjected to a 24-hour salt spray test pursuant to EN ISO 9227, a 4-day condensation climate-change test pursuant to DIN KFW and 28 hours of environmental testing pursuant to DIN The VDA alternating test is used primarily on automotive components. The test is conducted in a fully automatic Weiss SC 1000 test chamber. Transverse microsections of the corroded samples were created and examined using SEM in order to determine corrosion progress as well as the point at which the base material is attacked. Figure 11 shows the transverse microsections with varying exposure periods. Initial strength 6 weeks Electrolyte: 100 g l-1 ZnSO4. 7 H2O g l 1 NaCl; T = 25 C; ventilated; I = 11,76 ma cm- 2 1 week 7 weeks 2 weeks 8 weeks 3 weeks 9 weeks 4 weeks 5 weeks 10 weeks 11 weeks Figure 11: SEM images of transverse microsections of press-hardened hot-dip galvanized steel strips after exposure to VDA test The base material evidently does not begin corroding until eleven weeks have passed in VDA test The base material of press-hardened hot-dip galvanized steel strip is protected five times longer than unhardened material. Corrosion in unhardened material begins as early as two weeks in the form of red rust. The main reasons for this are the thicker layer created in the press-hardening process and the slower corrosion rate. Inspection of the corroded transverse microsections and comparing them to their initial condition shows that the -ZnFe phase (light-gray areas in the image) corrodes most readily. This behavior is expected as a result of the significant difference in potential when compared with zinc ferrite.

12 VDA The samples were also exposed to a newly developed VDA test (VDA ) in order to verify the corrosion results of VDA test The test is conducted in a fully automatic Weiss WK 1000 test chamber. VDA test and the new VDA test differ primarily in factors such as salt concentration, methodology, temperature, humidity and testing duration. Figure 12 shows the transverse microsections after varying periods of exposure in VDA test Base material corrosion is still not observed after six cycles. Press-hardened hot-dip galvanized steel strip exhibits substantially longer corrosion protection when compared to the unhardened base material, which corrodes after only four weeks. The time point of base material attack was not determined. The main reason for the higher level of protection is the thicker layer created in the press-hardening process. The corrosion rates in the new VDA test can be compared. The -ZnFe phase also corrodes first in VDA test Corrosive attack on the zinc-ferrite does not occur until the -ZnFe phase has been usurped. Both Zn-Fe phases provide the base material with cathodic protection. Both tests confirm the excellent cathodic corrosion protection potential of galvanized hot-formed components. 0 weeks Figure 12: SEM images of transverse microsections of press-hardened hot-dip galvanized steel strips after exposure to new VDA test 1 week 2 weeks 3 weeks 4 weeks 5 weeks 6 weeks

13 Joining technology Spot welding is still the most frequently applied joining technique in the automotive industry. This welding process is influenced by surface resistance and the base material. Different Zn-Fe layer compositions, which depend on the furnace dwell time, can also have an influence. Figure 13 shows the welding ranges of 22MnB5 and 20MnB8 with various coatings as well as a long and short furnace dwell time. The respective weld nugget diameters are also indicated. As can be seen, the weld nugget diameters are affected very little by the furnace dwell time and changes in alloys. In these welding ranges, longer furnace dwell times lead to a lower spatter limit of around 1 ka. Welding range Welding range (two-pulse) [ka] Nugget diameter [mm] Material Furnace dwell time (short*, long**) Figure 13: Spot weldability depending on material and dwell time The welding range itself becomes somewhat narrower. The process and the results are stable at any rate. At more than 1500 spot welds, the electrode life is especially long, which far exceeds the values of alternative coating systems. Further joining techniques such as MIG and laser welding are not detailed here, even though they have proven effective millions of times in serial production. Composite joints such as those involving aluminum and galvanized hot-formed steels are achieved using adhesive and rivet bonding. Both adhesive bonding and fusion welding processes can be used without a problem. Mechanical joining techniques circumvent the problem of high strengths by using softer joining regions of laser-welded blanks or partially unhardened regions (see Figure 2). Methods used to join fully martensitic regions are currently being developed.

14 Next step in development phs-rollform The methods described are generally capable of producing any desired geometry but will always require the implementation of furnaces with lengths of up to at least 20 meters in order to account for industrial cycle times. Variable component lengths and closed cross sections are not achievable because of the tool geometry. Rollform technologies used in the production of tubes and sections have solved this problem. The phs-rollform hardening process is based on this experience. The production of tubes and sections up to a strength of 1800 MPa is made possible by integrating an inductively heated and/or gas heated austenitization zone with subsequent hardening in the roll-forming process. As shown in Figure 14, tubes and sections can be partially hardened by means of selective heating across both the length and cross section of the component. Figure 14: phs-rollform : Process and possibilities of tailored tubes The process automatically includes any desired length in the same geometry, which means that different vehicle lengths can be achieved in a modular fashion. This means that tailored tubes are as viable as tailored-property parts. Figure 15 shows the variation in hardness across the entire cross section and the strength parameters of the hardened roof region as compared to the soft frames. The homogeneous transitions are easy to recognize. Figure 16 shows possible areas of application of components manufactured this way.

15 15 Hardened zones Soft zones Transition zones Figure 15: Distribution of hardness and strength of a partially hardened rocker Figure 16: Possible applications for phs-rollform Summary n Use of phs-ultraform to manufacture highly complex and large components with extraordinary crash performance n Manufacture of less complex components with lowest tool wear using phs-directform n Intelligent furnace designs without any need of protective gas but with the same level of easily determined process limits and large processing windows n Cycle-time-neutral tailored-property parts with a high level of dimensional accuracy n Any (alternative) heating method possible n Manufacturing of tailored tubes and sections using phs-rollform n Further performance-enhancing developments underway These factors show that the galvanized ultra-high-strength components of today have no limit to their scope of application.