challenges and advantages in usage of zinc-coated, press-hardened components with tailored...
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Berg- und Hüttenmännische Monatshefte
Originalarbeit
BHM (2012) Vol. 157(3): 97 – 101DOI 10.1007/ s00501-012-0061-4Printed in Austria© Springer-Verlag 2012
Tailored-properties for press-hardening parts Abstract: with zinc coating can be obtained by several ways. Tailor welded blanks is a well established technology and already in series application. Tailored-properties by partial heating is a new technology that makes different mechanical prop-erties available on one component made of one blank.
Zink-beschichtete, pressgehärteter Bauteile mit maßge-schneiderten Eigenschaften: Vorteile und Herausforderungen
Zur Herstellung pressgehärteter Bau-Zusammenfassung: teile mit anforderungsspezifischen Eigenschaften wurden in den letzten Jahren verschiedene Technologien und Meth-oden entwickelt. Während sich verzinkte Bauteile aus laser-geschweißten Platinen aus phs-ultraform bereits im Serie-neinsatz bewährt haben, ist die Methode, maßgeschnei-derte Eigenschaften durch partielle Erwärmung einzustel-len, vor der Umsetzung in Serienanwendungen.
Introduction
The demand for weight reduction in order to achieve the CO2 targets and achieve higher safety standards has led to increased application of press-hardened components over the past few years. Press-hardening steels are available with various coatings to meet the respective requirements. Uncoated press-hardening steel is used for parts in non-corrosive environments, aluminum-silicon coated steel for parts produced in the direct hot stamping process and zinc-coated steel for complex parts produced in the indirect process requiring cathodic corrosion protection. A com-mon property of press-hardened steel is rather limited duc-
tility in hardened condition. Press-hardened components with tailored properties have recently become more impor-tant to task-specific crash behavior.
The first approach to this task has been tailor-welded blanks of press-hardened steel and a partner material with reduced tensile strength and increased ductility. The prop-erties of these tailor-welded blanks are compared to those produced with more process-oriented technologies.
Various technologies for tailored properties based on the conventional press-hardening process have recently been developed. Voestalpines contribution is a new tech-nology to avoid austenitization in the softer zone of the component. This advanced technology has made it possi-ble to produce press-hardened parts with tailored proper-ties combined with a stable process-chain, thus fulfilling the demand of current safety requirements.
Tailor Welded Blanks
Components made from tailor-welded blanks have been in use in the automotive industry since the mid 1980s and have become an integral part of body-in-white develop-ment. Tailored blank manufacturing has achieved a high de-gree of maturity with respect to both economic feasibility and quality-assured production. Continual demand for light-weight construction combined with improved passive vehicle safety in the last decade have led to strong growth in the area of press-hardened steels. An additional benefit with respect to light weight and functionality in press-hard-ened components can be achieved through the application of tailor-welded blanks.
Typical already implemented tailored blanks in serial production are B-pillar reinforcements, longitudinal rear rails, tunnel reinforcements and inner roof rails. The blanks are made of press-hardened steel or combinations of con-ventional high-strength steel grades, especially micro-al-loyed steel grades.
Correspondence author: Dipl.-Ing. (FH) Thomas Manzenreiter Voestalpine Stahl GmbH, B3E / Forschung und Entwicklung Voestalpine-Straße 3 / BG04, 4020 Linz, Austria. e-mail: [email protected]
Challenges and Advantages in Usage of Zinc-Coated, Press-Hardened Components with Tailored Properties
Thomas Manzenreiter*, Martin Rosner*, Thomas Kurz*, Gerald Brugger**, Reiner Kelsch***, Dieter Hartmann***
and Andreas Sommer***
* voestalpine Stahl GmbH, Linz, Austria ** voestalpine Europlatine GmbH, Linz, Austria *** voestalpine Polynorm, Schwäbisch Gmünd, Germany
Received February 2, 2012; accepted February 8, 2012
Manzenreiter et al.BHM, 157. Jg. (2012), Heft 3 97© Springer-Verlag
Originalarbeit
Two coating systems for press-hardened steels are currently being used in addition to the uncoated 22MnB5 grade in large-scale serial production, namely phs-ultraform® (zinc-based coating) and aluminum-silicon coatings. More information on different component man-ufacturing processes is provided in1. The different process route used in the manufacturing of tailored blanks is de-scribed in the following. Stamped blanks made of phs-ul-traform can be welded as conventional hot-dip galvanized grades. However, the aluminum-silicon coating of the weld seam must be removed on both sides in hot-dip aluminized coating systems in order to achieve the required product characteristics. The weld seam coating is removed offline by means of laser ablation and incurs additional process costs.
Figures 3 and 4 show micrographs and hardness profile of a non-hardened and hardened hot-dip aluminized 22MnB5, where the coating was intentionally not removed in the area of the weld seam. The Al-Si coating is melted in the edge regions and reacts in the molten pool to become brittle intermetallic phases of Fe-Al-Si. The formation of bainite and ferrite is also promoted during the hardening process by the alloying of aluminum and silicon in the microstructure of the weld seam. Both of these negatively affect component properties such as static and dynamic strength2. The significant drop in hardness resulting from the intermetallic Fe-Al-Si phases is shown clearly in Fig. 1 and 2.
In contrast with hot-dip aluminized phs steels, phs-ultra-form® can be welded without an additional process step. Figure 3 shows a micrograph of a laser weld seam in a B-pillar reinforcement (Fig. 4) made of phs-ultraform® which is in serial production since June 2010. The sheet thickness combination is 1.6 mm and 2.0 mm. The hardening process yields a very homogeneous hardness profile throughout
the weld seam. The Zn-Fe layer occurs up to the beginning of the weld seam, as in conventional zinc coatings. This pro-vides optimum corrosion protection, and it can be assumed that the weld seam is also protected against corrosion to a certain extent by the remote cathodic effect of the zinc-iron coating. The zinc is directly subjected to the laser beam, va-porizes almost completely during the process and does not form any Zn-Fe phases in the weld seam.
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Fig. 1: 22MnB5 Al-Si coating, 1.0 mm/1.6 mm unhardened, hardness profile and micrograph
Fig. 2: 22MnB5 Al-Si coating, 1.0 mm/1.6 mm hardened, hardness profile and micrograph
Fig. 3: phs-ultraform®, 1.6 mm/2.0 mm hardened, hardness profile and micrograph
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Manzenreiter et al. BHM, 157. Jg. (2012), Heft 398 © Springer-Verlag
Originalarbeit
Analogous to conventional tailor-welded blanks a com-bination of other materials is possible with phs-ultraform®. The spectrum of properties required by the automotive in-dustry, including high levels of ductility, high levels of en-ergy absorption in the event of a crash and ultra-high strength, limits the possible types of suitable partner mate-rials. Furthermore, relatively low sensitivity to mechanical parameters dependent on the cooling rate has to be guar-anteed in order to both adjust homogeneous mechanical parameters in the component and guarantee a wide proc-ess window during the press-hardening process. These properties are most effectively achieved in a modified mi-cro-alloyed steel grade (HT490PS). The combination of phs-ultraform® and HT490PS is also used in the serial produc-tion of B-pillar reinforcement (Fig. 4).
Figure 5 shows a micrograph of a laser weld seam pro-duced with a combination of phs-ultraform® and HT490PS. The variation in hardness and thus the variation in strength
in the weld seam shows a relatively homogeneous transi-tion between the two materials.
The zinc coating of HT490PS is analogous to that of phs-ultraform® and offers cathodic corrosion protection after press-hardening.
Tailored Properties by Partial Heating
The objective is to keep the component below the critical heating temperature of 750 °C in the furnace by partially cooling it within defined areas during partial press-harden-ing with an absorption-mass. The absorption mass is in sur-face contact with the hot blank during the furnace heating process. This mass draws energy from the blank surface. The mass absorbs from the blank a portion of the energy introduced by the furnace. Figure 6 shows the heating curves with and without absorption mass. In this example, the critical temperature of 870 °C for complete hardening is achieved without absorption mass after approximately 250 seconds. The standard furnace temperature is achieved af-ter 330 seconds in this example. The temperature in the area of the absorption mass at this point in time is approx-imately 630 °C. The temperatures thus lie significantly be-low the critical temperature that marks the beginning of austenitization.
In contrast with methods in which the entire component is initially heated and portions of the soft components are subsequently slowly cooled, the selected technique featur-ing an absorption mass for partial hardening provides a
Fig. 4: Serially produced tailored-blanks made of phs-ultraform®/HT490PS for B-pillar reinforcement
Fig. 6: Area heating curves with and without absorption mass
Fig. 7: Hardness measurement of transition area between hard and soft zone
Fig. 5: phs-ultraform®/HT490PS, 2.0 mm/2.2 mm hardened, hardness profile and micrograph
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Manzenreiter et al.BHM, 157. Jg. (2012), Heft 3 99© Springer-Verlag
Originalarbeit
method without additional cycle time compared to uni-formly hardened components. The economic feasibility of this method is thus of high interest. It is only important that the critical component temperature of 750 °C is not ex-ceeded for the soft zones.
The transition from the hard to soft zone can be deter-mined relatively precisely with the absorption mass method. The variation in hardness from a hard to com-pletely soft zone is limited to a transition zone of approxi-mately 20 mm. Figure 7 shows the transition in the crash profile from the hard zone ranging between 450 and 470 HV to the soft zone ranging between 160 and 180 HV. The tran-sition zone here is exactly 20 mm.
The ability to select varying zones of hardness in press-hardened components is a big advantage. The soft zones can be defined at the component edge or middle according to engineering requirements. Several zones on one compo-nent are also possible.
Mechanical Properties
In order to determine the process limits for the manufac-ture of partially hardened components, samples (200 mm × 300 mm and sheet thickness of 1.5 mm) were annealed for six minutes in a radiation heating furnace at discrete tem-peratures ranging between 650 °C and 850 °C. The samples were subsequently cooled rapidly in a water-cooled press. Microhardness was determined from micrographs.
The results of the microhardness analysis shown in Fig. 7 indicate that the material below a temperature of 750 °C does not harden during quenching and remains soft at strengths ≤ 600 MPa. The microstructure remains ferritic / perlitic below this temperature. Sheet material heated above this critical temperature starts to austenitize and to harden non-specifically during quenching in a water-cooled press. Complete austenitization can be assumed at temper-atures of 850 °C and above. A typical strength of > 1 350 MPa is reached after quenching.
Layer Formation
An approximately 25 µm thick, typically single-phase layer of α-zinc-ferrite forms with a zinc concentration higher than 40 % from the originally 10-µm-thick Zn layer during com-plete hardening of phs-ultraform® by annealing in a radia-tion heating furnace at 910 °C. Depending on the reactivity of the substrate, small concentrations of Γ-ZnFe may re-main in the layer with a zinc concentration of roughly 80 %. In contrast, the component zones in the TPP process which remain soft after cooling in the press-hardening process should be heated only to slightly below the austenitization temperature. This has significant effects on the formation of the iron-zinc phases on the surface. The iron-zinc layer on the samples annealed in the laboratory furnace was characterized in order to obtain a deeper understanding of the formation of the iron-zinc phases as dependent on the component temperature.
Figure 8 summarizes the conditions of the iron-zinc layer after annealing. In accordance with the iron-zinc diagram, the δ-ZnFe phase (roughly 10 % Fe) is still stable at a tem-
perature of 650 °C. A thin seam of Γ-ZnFe (roughly 20 % Fe) begins to form at 700 °C at the interface between the steel substrates and the coating. This formation is complete at 750 °C. At this point a single-phase coating has formed. From a temperature of 780 °C, the Γ-ZnFe phase begins to decay, and α-FeZn (zinc ferrite) begins to grow from the steel substrate (roughly 60 % iron) and continues to grow as the temperature increases. At 850 °C the reaction has taken place throughout the entire layer.
Joining Tailored-Property Parts
One aim to ensure serial production of tailored-property parts (TPP), was to join parts by applying the same param-eters and utilities within the different areas. Therefore basic analyses and practical trials were carried out with several technologies. Major emphasis was placed on spot welding, bonding and bracing.
Development of Parameters for Resistance Spot Welding
At the very fist beginning of characterizing phs-ultraform® trials with resistant spot welding were made. As used, con-ventional parameters (AC-welding with one pulse) were applied. The unsatisfying result was the achievement of a welding range with less than 1,0 kA. Optimizing steps with DC 1-pulse welding and AC 2-pulse welding lead to the op-timum of DC 2-pulse welding.
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Fig. 8: Reaction of the zinc layer with the steel substrate as dependent on furnace temperature and resulting hardness of base material. Note: the bright nickel-layer on the top of the micrographs is for preparation purpose.
Fig. 9: Development of welding parameters (welding range defined by 2nd pulse)
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Manzenreiter et al. BHM, 157. Jg. (2012), Heft 3100 © Springer-Verlag
Originalarbeit
With this technique welding ranges of more than 2,5 kA were achieved. The first pulse posses a high of about 4,0 kA for 1,0 mm and 5,0 kA for 2,0 mm and is of the duration of 350 ms. It is used to evaporate the oxides. The second pulse of variable height is responsible for the formation of the welding spot and defines the with of the welding range. The duration is 400 ms (see Fig. 9). Double pulse welding is the procedure on which all results have been based to date.
Resistance Spot Welding of Samples with Tailored Properties
To ensure weldability of TPP by applying the same welding parameters in each zone, the welding range for the differ-ent zones of 1.0 mm material were determined and com-bined to a common one (see Fig. 10). With the current of 6.7 kA (200 A below the spatter limit), 400 spots were welded, 175 in the hardened zone, as well as in the soft zone and 50 in the transition zone. Afterward 50 spots in each zone were tested by chisel-test and measured after-wards. A statistical analysis of spot diameter within all three zones, showed that the spot diameter were keenly and the spread of the values differed in a tight measure (5,2 to 5,5 mm). At the end of the development, 250 parts of a serial sill (fully hardened for serial use), were supplied with a 400 mm long soft zone. These parts were welded to a complete side-inner component on the determined assem-bly line. Both, the welding-trial and also the following tests with chisel and super sonic equipment were highly satisfy-ing.
Bonding
Bonding-tests (according DIN EN 1465) done on CO2-condi-tioned 1.5 mm samples by the use of Betamate-1496 show,
that there are nearly no differences within the different zones regarding bonding-behavior (see Fig. 11), the shear tension is on an nearly equal level of 32–36 MPa after 10 weeks corrosion test the tension is reduced by about 30 % to 20–26 MPa. The fracture mode before corrosion is a 100 % cohesive fracture, after corrosion approx. 10 % adhesive fracture occur.
Conclusions
Press-hardened components made of tailor-welded blanks in combination with phs-ultraform®/HT490 PS represent a possibility of manufacturing tailored-property parts, which are already in serial production. Advantages over proc-esses that achieve material parameters through locally var-ying heat treatment processes include a precisely defined position of the strength transition and the possibility of thickness variation that leads to higher weight saving po-tential. Furthermore tailor welded blank solutions can have advantages in the engineering scrape rate, due to nesting possibilities which improves the cost effectiveness.
Tailored properties by partial heating can be made at press-hardened components in two strength levels, also re-sulting in hardened and soft areas. The transient area ex-hibits gradients in the strength and in the development of the Zn-Fe-layer. Regarding mechanical properties it can be explained by the temperature-dependent characteristics of the base material when austenitizing, while reactions of the Zn-coating with the steel can be described well by the zinc iron diagram.
The characteristics of demonstration components made in a press hardening line shows that the knowledge ob-tained in the laboratory in reference to formation of the Zn-Fe-phases and mechanical properties can be transferred for practical use.
Achieving a combination of tailored-properties by par-tial-heating with tailored-blanks combines the advantages of both methods. Concrete initial approaches have already been devised.
References1 Knezar K.; Manzenreiter T.; Faderl J.; Radlmayr K.M.: Form-
härten von feuerverzinktem 22MnB5: ein stabiler und re-produzierbarer Prozess, 2. Workshop Erlanger Workshop Warmblechumformung (2007)
2 Pic A.; Munera D.; Schmit F.; Pinard F.: Innovative press-har-dening solutions for tailored blanks, Stahl und Eisen 128 (2008) No. 8
3 Manzenreiter T.: Resistance spot welding of zinc-coated, press-hardened components with tailored properties, 6th International Seminar on Advances in Resistance Welding, Hamburg, (2010), pp. 19–27
4 Faderl J.; et al.: M.: phs-ultraform – Continuous galvanizing meets press-hardening. CHS2 Hot Sheet Metal Forming of High Performance Steel, Lulea Schweden, 15–17 June (2009), pp. 283–292
5 Radlmayr K.; et al.: The way to meet 1500 MPa and corro-sion protection in the BIW, 2nd International Conference on Steels in Cars and Trucks, Wiesbaden, 01–05 June (2008), pp. 222–229
Fig. 10: Welding range for different zones within a tailored component
Fig. 11: Shear tensile test on samples with tailored properties
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Manzenreiter et al.BHM, 157. Jg. (2012), Heft 3 101© Springer-Verlag