THE HOT FORMING OF GALVANIZED STEELS - Voestalpine · 2021. 3. 13. · VDA 621-415 11 02.05. VDA 233-102 ... The classic base material 22MnB5 was used as the foundation for this process

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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

THE HOT FORMING OF GALVANIZED STEELS

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CONTENT

03 Abstract

04 01. The original: phs-ultraform®

05 02. Galvanized innovation in the field of direct hot forming:

phs-directform®

05 02.01. Precooling

06 02.02. Base material

09 02.03. Corrosion

10 02.04. VDA 621-415

11 02.05. VDA 233-102

12 02.06. Joining technology

13 02.07. Next step in development – phs-rollform®

14 02.08. Summary

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ABSTRACTThe 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.

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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).

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.

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Figure 1: phs-ultraform®, the indirect process for galvanized, press-hardening steel

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Figure 2: Possible tailored-property parts

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 1500 – 1800 MPa

phs-ultraform® 490: approx. 500 MPa

Patch inside/ Patch outside

Hardened and stress-relieved < 1500 MPa

Partially heated: approx. 500 MPa

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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.

02.01. PrecoolingThe 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).

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.

Figure 3: phs-directform®: Process featuring precooling method

Furnace exitPrecooling

Hot forming

Hardening process

Tem

per

atur

e

Time

Transfer 1 Transfer 2

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.

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Warmumformung verzinkter Stähle

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Bild 3: phs-directform-Prozess mit Vorkühlungsprinzip

Bild 4: Düsenfeldkonstruktion zur unterschiedlichen Kühlung von 4 Bereichen Figure 4: Nozzle arrangement for cooling of four zones


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Bild 5: Auswirkung der Vorkühleinheit auf die Mikrorissbildung

Bild 6: Vergleich der Prozessfenster der Analysen 22MnB5 und 20MnB8

Without precooling With precooling

Figure 5: Effects of precooling on microcracking

02.02. Base material20MnB8 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.

Microcracking of the first order (LME)

Microcracking of the second order

No Micro - cracking of

the first order

No Micro - cracking of the second

order

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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.

Figure 7 shows the comparison between the two zinc layers before and after the austenitization process.

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.

Figure 6: Process window comparisons between 22MnB5 and 20MnB8


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Bild 5: Auswirkung der Vorkühleinheit auf die Mikrorissbildung

Bild 6: Vergleich der Prozessfenster der Analysen 22MnB5 und 20MnB8

phs-ultraform®

22MnB5 + GI70/7022MnB5 + GI90/90

phs-directform®

20MnB8 + GA90/90

Figure 7: Microstructure and layer formation in unhardened and hardened condition

phs-ultraform® – unhardened phs-ultraform® – hardened phs-directform® – unhardened phs-directform® – hardened

Ste

el

Zin

c

Ste

el

Zin

c-

iron

Ste

el

Zin

c-

iron

Ste

el

Ga

lva

n-

ne

ale

d

Figure 8: Friction coefficient in hot-formed steels dependent on surface coatings


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Bild 7: Gefüge und Schichtausbildung im ungehärteten und gehärteten Zustand

Bild 8: Reibungskoeffizient der Warmumformstähle in Abhängigkeit ihrer Oberflächenbeschichtung

Different tool materials

Fric

tion

co

eff

icie

nt

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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.

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.

Figure 9: SEM image of the microstructure of Zn-Fe reaction layer on press-hardened hot-dip galvanized steel strip

Martensite

Oxide layerEmbedding agent

02.03. CorrosionCorrosion 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.

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.

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Figure 10: Galvanostatic resolution of press-hardened hot-dip galvanized steel strip (black) as compared to initial condition (red).


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Bild 9: REM-Aufnahme der Mikrostruktur der Zn-Fe-Reaktionsschicht von pressgehärtetem feuerverzinktem Stahlband.

Z140 unhardened

Z140 press-hardened

Po

tent

ial v

s. S

HE

/V

Time/Minutes

unhardened

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02.04. VDA 621-415The 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 50017-KFW and 28 hours of environmental testing pursuant to DIN 50014. 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.

The base material evidently does not begin corroding until eleven weeks have passed in VDA test 621-415. 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.

Figure 11: SEM images of transverse microsections of press-hardened hot-dip galvanized steel strips after exposure to VDA test 621-415

Electrolyte: 100 g l-1 ZnSO4 . 7 H2O + 200 g l 1 NaCl; T = 25 °C; ventilated; I = 11,76 mA cm-2

Initial strength

1 week

2 weeks

3 weeks

4 weeks

5 weeks

6 weeks

7 weeks

8 weeks

9 weeks

10 weeks

11 weeks

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Figure 12: SEM images of transverse microsections of press-hardened hot-dip galvanized steel strips after exposure to new VDA test

02.05. VDA 233-102The samples were also exposed to a newly developed VDA test (VDA 233-102) in order to verify the corrosion results of VDA test 621-415. The test is conducted in a fully automatic Weiss WK 1000 test chamber. VDA test 621-415 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 233-102. 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 233-102. 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

1 week

2 weeks

3 weeks

4 weeks

6 weeks

5 weeks

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02.06. Joining technologySpot 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.

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.

Figure 13: Spot weldability depending on material and dwell time


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Bild 13: Punktschweißbarkeit in Abhängigkeit von Werkstoffen und Verweilzeit

Bild 14: phs – rollform – Prozess und Möglichkeiten für Tailored Tubes

Wel

din

g ra

nge

(two-

pul

se) [

kA]

Material

Furnace dwell time (short*, long**)

Nug

get

dia

met

er [m

m]

Welding range

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02.07. 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.

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.


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Bild 13: Punktschweißbarkeit in Abhängigkeit von Werkstoffen und Verweilzeit

Bild 14: phs – rollform – Prozess und Möglichkeiten für Tailored Tubes

Figure 14: phs-rollform®: Process and possibilities of tailored tubes

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02.08. SummaryUse of phs-ultraform® to manufacture highly complex and large components with

extraordinary crash performanceManufacture of less complex components with lowest tool wear using phs-directform®

Intelligent furnace designs without any need of protective gas but with the same level of easily determined process limits and large processing windows

Cycle-time-neutral tailored-property parts with a high level of dimensional accuracy Any (alternative) heating method possibleManufacturing of tailored tubes and sections using phs-rollform®

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.


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_Bild 15: Härte und Festigkeitsverteilung eines partiell gehärteten Schwellers

Bild 16: Anwendungsmöglichkeiten für phs-rollform

Figure 15: Distribution of hardness and strength of a partially hardened rocker

Figure 16: Possible applications for phs-rollform®


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Hardened zonesSoft zones

Transition zones

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THE HOT FORMING OF GALVANIZED STEELS - Voestalpine · 2021. 3. 13. · VDA 621-415 11 02.05. VDA 233-102 ... The classic base material 22MnB5 was used as the foundation for this process