The continuous hot-dip coating as galvanizing or aluminizing of advanced high strength steels (AHSS) is challenging for the producer regarding coating defects and degraded coating adhesion. Selective oxidation of less-noble alloying elements (Mn, Si, Cr, Al, …) was identified to be the main reason for degraded coating quality. This present paper aims to illustrate different approaches for the coating process of alloyed AHSS, which have been developed and discussed recently. Furthermore, the applicability of these approaches in industrial hot-dip coating lines will be evaluated.
Herausforderungen und Lösungen zur industriellen Feuerverzinkung von hochfesten Stählen – ein Überblick
Die kontinuierliche Schmelztauchveredelung als Feuerverzinkung oder Feueraluminierung von Bändern aus hoch-/ höchstfesten Mehrphasenstählen stellt besondere Herausforderungen an den Produzenten hinsichtlich der Vermeidung von unbenetzten Stellen und schlechter Überzugshaftung. Als Hauptursache solcher Beschichtungsfehler hat sich die selektive Oxidation der unedlen Legierungselemente dieser Stähle (Mn, Si, Cr, Al, ) herausgestellt. Der vorliegende Aufsatz möchte verschiedene Verfahrenskonzepte vorstellen und diskutieren, welche in der jüngeren Vergangenheit entwickelt wurden, um auch legierte hoch-/höchstfeste Mehrphasenstähle prozessstabil mit einem Schmelztauchüberzug zu beschichten. Des Weiteren soll die Anwendbarkeit dieser Verfahrenskonzepte an einer großindustriellen Feuerbeschichtungsanlage abgeschätzt und bewertet werden.
1 Introduction
The necessity of reducing fuel consumption and CO2 emission is the driving force for the increased use of cold- and hot-formed advanced high strength steels (AHSS) as structural parts in the automobile bode-in-white. Modern AHSS include steel classes with dual- or complex-phase microstructure (DP and CP steels) and such with induced plasticity (TRIP and austenitic TWIP steels) [1, 2]. Present and novel AHSS ask for high sophisticated production processes and lines in order to meet customer’s demands regarding mechanical properties, corrosion resistance and surface impression. Therefore, the application of hot-dipped zinc- or aluminum-based coatings, which are widely used in the automobile industry to protect the steel surface against corrosion, is facing production to high quality standards. However, modern AHSS are alloyed with considerable amounts of manganese, aluminum, chromium, silicon and boron aside other less-noble elements to obtain the required mechanical properties. As the hot-dip coating process includes continuous annealing prior to hot-dipping, interactions between the steel and the annealing atmosphere occur resulting, for instance, in selective oxidation of the alloying elements. The appearance of passive oxides on the external steel surface decreases reactive wetting between metallic iron at the steel surface and the galvanizing or aluminizing bath. Thus, the occurrence of coating defects (for example bare spots, uncoated areas, degraded coating adhesion) bases in many cases on selective oxidation on the external steel surface. Although the annealing atmosphere in modern continuous hot-dip galvanizing lines (CGL) consists of ~5 to 10 vol.% of hydrogen balanced in nitrogen, the oxygen partial pressure in the furnace is under typical industrial conditions not low enough to avoid selective oxidation completely [3-5].
Many scientific studies have been published dealing with gas/metal reactions and reactive wetting of alloyed and high alloyed AHSS during continuous annealing bringing new insights into thermodynamical equilibria and reaction kinetics on the steel surface [3-12, 18-21, 24, 25, 32]. Based on these new findings, a wide range of different processing approaches for the continuous hot-dip coating of AHSS have been developed in order to avoid coating defects caused by selective oxidation. This present paper aims to illustrate the most discussed and used approaches to support the operator, who familiarizes oneself with the interesting but challenging field of hot-dip coating of AHSS. Furthermore, the applicability of these approaches in industrial hot-dip coating lines will be evaluated. However, detailed discussions about mechanisms of selective oxidation or reactive wetting can be found in literature [6-10] and are not in scope of this paper. Approaches dealing with modified bath management and handling to avoid inadequate wetting of the steel surface (e.g. due to oxides from the coating bath surface [11]) keep unconsidered by this paper, too.
Generally, in this paper it is distinguished between processing approaches with in-line and off-line modifications of the steel surface in order to avoid external selective oxidation. An approach with in-line treatment includes a specifical modification of the steel surface during continuous annealing, what is typically carried out by modifying the gas/metal reaction. Otherwise, approaches with off-line modification characteristically reveal a pre-treatment of the steel surface before processing the steel strip in a CGL.
2 Approaches with off-line pre-treatment
2.1 Pre-coating (Flash coating)
Several processing approaches have been discussed describing a pre-treatment of the steel surface to avoid or hinder selective oxidation at the external surface during continuous annealing of AHSS in a CGL. One suggestion is pre-coating the steel surface with a thin electrolytically deposited layer of ≤1.0 µm in thickness – so-called flash coating [11]. For instance, a nickel flash is confirmed to improve reactive wetting and coating adhesion when hot-dip galvanizing of alloyed AHSS. Nickel having a low affinity towards oxygen do not oxidize during continuous annealing under conventional annealing conditions, but participates to reactive wetting with the hot-dip coating bath and forms Ni-Al or Ni-Zn(-Al) intermetallics (Fig. 1). At the same time, the pre-applied nickel flash on the steel surface extends diffusion paths for less-noble alloying elements in the steel substrate. By this, appearance of selectively formed external oxides can be inhibited effectively. Alternatively, more cost-effective iron can be used as flash pre-coating instead of nickel, too [13]. In the case of a iron flash, selective oxidation is only hindered by the elongation of the diffusion paths for oxidizing alloying elements and adsorbed oxygen by the additional distance of the iron flash thickness.
Fig. 1: Intermetallics in the coating after nickel flash (0.5 µm) and subsequent hot-dip zinc coating on laboratory scale (scanning electron microscopy)
Flash coating could actually be performed in-line in a CGL, if electrolytic cells are installed before the furnace. However, even modern CGLs are not equipped with an electrolytic cell. That means flash coating requires off-line pre-coating in an electrolytical galvanizing line before processing the steel strip in a CGL. Thus, industrial flash coating would have considerable cost as well as process disadvantages and is therefore not used in the industrial production of hot-dip coated AHSS (in Europe).
An innovative variation of pre-coating represents chemical or physical vapor deposition (CVD/ PVD). Hereby, thin metallic or even oxidic pre-coating of ≤0.1 µm in thickness can be applied. For instance, the hot-dip coating of high manganese alloyed AHSS was described, which was pre-coated with an aluminum layer applied by PVD [14]. Another research group revealed a method for pre-coating AHSS with an oxide layer system. In this case, an inner oxide layer is supposed to act as diffusion barrier and to protect the steel substrate against selective oxidation. The outer layer of iron oxide (FeO) can be reduced to metallic iron during continuous annealing, which participates to reactive wetting during hot-dipping [15]. However, innovative pre-coatings applied by CVD or PVD are currently limited at laboratory scale.
2.2 Pickling
Pickling represents a more conventional alternative to aforementioned pre-coating and is already used as process step in industrial hot-dip coating of hot-rolled AHSS. For hot-rolled strips do not need an annealing treatment for recrystallization in-line pickling in specially configured CGLs is state-of-the-art. Afterwards in-line pickling the steel strip is only heated to a temperature, which is close to the temperature of the hot-dip coating bath and which is not high enough to allow less-noble alloying elements to oxidize. Otherwise, if the steel strip to be produced is cold-rolled – as it is typical for strip dimensions used for automotive applications – annealing must be performed at temperatures between 700 °C to 850 °C. That means selectively formed oxides would appear at the external steel surface anyway whether in-line pickling before annealing was performed or not.
Therefore, pickling as processing approach for hot-dip coating of cold-rolled AHSS bases on the separation of the process steps high temperature annealing for recrystallization purpose and hot-dipping. In a first step, cold-rolled AHSS has to be heat-treated for recrystallization as off-line pre-treatment. During this first annealing selective surface oxidation occurs. Afterwards, the annealed strip can be subjected to a CGL, which is equipped with a pickling device before the furnace allowing hot-dip coating the strip in accordance to the method for hot-rolled strips described above. During pickling all selectively formed oxides on the external steel surface are removed. As the less-noble alloying elements will not form new oxides during the low-temperature heat-treatment prior to hot-dipping, reactive wetting can be clearly improved compared to conventional continuous hot-dip coating without pickling [16, 17]. However, if cold-rolled strips are considered, the pickling approach will have clear cost disadvantages, because recrystallization annealing as off-line pre-treatment needs an expanded production path via a batch-type annealing furnace or a continuous annealing line.
Other approaches with off-line conditioning of the steel surface, for instance, dealing with special heat-treatments of the pre-material [12, 31] can be found in literature, but are not considered by this paper.
3 Approaches with in-line conditioning of the steel surface
3.1 Internal oxidation
As illustrated in the previous section processing approaches with off-line pre-treatments are associated with higher production costs, because additional processing steps are necessary before subjecting the steel strip to a CGL. Therefore, processing approaches including an in-line conditioning of the surface of alloyed AHSS are more in focus of interest of modern flat steel producers. In this paper, in-line conditioning of the steel surface means exposing the strip to a gas/metal reaction, which is specifically modified to avoid external oxidation of less-noble alloying elements, during annealing. For this, it is crucial to control oxygen partial pressure by varying hydrogen and dewpoint of the annealing atmosphere, both dominating the gas/metal reaction during continuous annealing.
Alloying elements such as manganese, silicon, aluminum, chromium and boron tend to selectively form oxides at the external steel surface due to unavoidable impurities of oxygen and water vapor in the annealing atmosphere. However, the so-called Wagner model states that this external oxidation can be hampered kinetically, if the oxidation potential of the annealing atmosphere is high enough to promote internal oxidation of alloying elements [7, 8, 18]. Internal oxidation means that the flux of ad-/absorbed oxygen inwards the steel substrates dominates the flux of oxidizing alloying elements towards the steel surface. Thus, selective oxidation appears internally within the sub-surface of the steel. The sub-surface should be defined here as the steel region close beneath the steel surface with some micrometers in depth. According to the Wagner model, internal oxidation of an alloying element “Me” within an inert matrix will predominate, if the substance amount fraction of Me n(Me) is lying below the criterion given in equation (1):
Here, π ≈ 3.14 and g* ≈ 0.3. g* means critical volume fraction of formed oxides, which are blocking off all oxygen inwards diffusion. n(O)ad is the substance amount fraction of oxygen adsorbed on the steel surface, m is the metal/oxygen ratio of formed metal oxide, V and V(MexOy) (cm3/mol) are molar volume of the alloy and the oxide, respectively. D(O) and D(Me) (cm2/s) are the diffusion coefficients of oxygen and metal [19].
A feasible opportunity to meet the criterion given as equation (1) is increasing n(O)ad. In industrial CGLs this increase of n(O)ad can be put to practice by a controlled increase of the oxygen partial pressure or the dewpoint (→ H2O/H2 ratio) of the annealing atmosphere [3, 20]. For instance, that can be realized by a controlled and regulated injection of oxygen or water into the hydrogen-nitrogen mixture within the furnace and/or lowering the hydrogen fraction of the annealing atmosphere [3]. Laboratory investigations and industrial production confirm that selective oxidation of alloying elements will turn to internal accompanied by improved coating quality of hot-dipped AHSS, if the dewpoint during continuous annealing is increased (Fig. 2).
Fig. 2: Increased internal oxidation of AHSS (concept Fe-Mn-Cr) with increased dewpoint of the annealing atmosphere (scanning electron microscopy) [3]
Controlled internal oxidation distinguishes from well-known oxidation/reduction method (see below) by avoiding the generation of a iron oxide (FeO) layer on the steel surface. Thus, there is no danger of over-oxidation meaning the generation of an iron oxide layer too thick to be re-reduced to metallic iron by the reductive hydrogen-nitrogen annealing atmosphere [21]. That represents a clear advantage for this processing approach. However, the steel substrate tends to decarburization with increasing dewpoint of the annealing atmosphere limiting the process window of internal oxidation [32]. Additionally, a quick and homogeneous dewpoint regulation for the annealing furnace has to be realized technically.
The injection of controlled amounts of carbon oxide (CO) and carbon dioxide (CO2) into the annealing atmosphere was also discussed as method to promote internal oxidation of alloying elements. However, the presence of CO and CO2 in the annealing atmosphere can be accompanied by uncontrolled carburization of the steel bulk and is therefore supposed to be critical regarding mechanical properties of the strip [22].
3.2 Bright annealing
Promoting the internal oxidation of the less-noble alloying elements means hindering their external oxidation kinetically by increasing the oxidation potential of the annealing atmosphere according to the Wagner model. In contrast, the bright annealing process approach describes the increase of the reduction potential of the annealing conditions in order to avoid oxidation thermodynamically. Following the Richardson-Ellingham diagram, reduction potential of the annealing conditions will increase, if the oxygen partial pressure and the H2/H2O
ratio (→ dewpoint) of the annealing atmosphere decrease while keeping the annealing temperature constant. Additionally, increasing the annealing temperature while keeping the oxygen partial pressure and the
H2/H2O ratio constant increases the reduction potential of the annealing conditions, too. To illustrate this, bright annealing was successfully used at laboratory scale even to hot-dip galvanize high manganese alloyed TWIP steel alloyed with >20 wt.% of manganese [23, 24].
Unfortunately, annealing conditions necessary to effectively bright anneal (high) alloyed AHSS are far away from process parameters used in industrial CGLs. Realizing and maintaining a (super) low dewpoint is very difficult in a heavy-duty continuous annealing furnace. Furthermore, if the annealing temperature is increased, coarse grain microstructure could appear in the steel substrate degrading mechanical properties. The decrease of H2O/H2 ratio and dewpoint, respectively, by increasing the hydrogen amount in the annealing atmosphere to a value up to 100 vol.% could still be an option, but safety standards must be considered. Therefore, using bright annealing for the series production of hot-dip coated AHSS at industrial scales has to be evaluated as critical in terms of technical feasibility, safety and energy efficiency.
3.3 Nitrating
External surface oxidation of AHSS can also be hindered kinetically without regulating the oxidation/reduction potential of the annealing atmosphere. Nitrating of the sub-surface was suggested by exposing the steel to an annealing atmosphere, which includes up to ~5 vol.% of ammonia (NH3) aside hydrogen and nitrogen. On nitrate formation accompanied by improved hot-dip galvanizability of AHSS (concept Fe-Mn-Al, for instance) was reported by different working groups [3, 25]. The positive effect of this nitrating approach bases on diffusion of ad-/absorbed nitrogen inwards the sub-surface, where it forms intermetallic nitrates (AlN, BN, Fe3N) with the alloying elements and iron of the matrix (Fig. 3). As this nitrate formation preferably appears on crystal defects in the matrix such as grain and phase boundaries, diffusion paths for the less-noble alloying elements and inwards migrating oxygen are blocked off decreasing the amount of selectively formed oxides. Additionally, the nitrate formation during annealing increases the number of nucleation points for recrystallization resulting in a local grain refinement within the sub-surface. Thus, number of possible diffusion directions and paths is increased, decreasing the effective material flow towards the external steel surface.
Fig. 3: Sub-surface of AHSS (concept Fe-Mn-Al) after continuous annealing in nitrating atmosphere (light microscopy)
In comparison to other processing approaches nitrating could be realized easily in a CGL by injecting ammonia in addition to hydrogen and nitrogen into the annealing furnace. However, safety standards must be considered by handling with ammonia. Furthermore, effectiveness of nitrating decreases with decreasing amount of nitrogen-affine alloying elements – of aluminum in particular – in the steel composition. The danger of embrittlement of structural components of the annealing furnace limits the use of the nitrating approach additionally.
3.4 Oxidation/reduction method (Pre-oxidation)
The oxidation/reduction method – so-called pre-oxidation – is known to the operator of a CGL for decades and the industrial usage of pre-oxidation has been reported on since the 1960s. The pre-oxidation approach fundamentally bases on the initial generation of a covering iron oxide layer on the steel surface blocking off all diffusion of less-noble alloying elements and oxygen. The iron oxide layer formation (oxidation step) is typically carried out during heating the strip before the onset of selective oxidation. This iron oxide layer can be re-reduced to metallic iron during soaking the strip at annealing temperature in H2-N2 under normal industrial conditions (reduction step). Resulting from pre-oxidation, the external steel surface characteristically consists of almost pure metallic Fe with only few oxides prior to the immersion into the hot-dip coating bath. Thus, reactive wetting and coating quality can be considerably improved compared to a conventional annealing treatment [3, 26 - 28]. This established model of pre-oxidation is illustrated as Figure 4.
Fig. 4: Schematically illustration of the established model of oxidation/reduction method (pre-oxidation) used to hot-dip galvanize AHSS [3]
Performing pre-oxidation is challenging in terms of properly separating the specific oxidation and reduction atmospheres to avoid parasitic side-reaction and creating an even iron oxide layer on the steel surface. The iron oxide layer must be continuous and thick enough to hamper selective oxidation of less-noble alloying elements effectively. Otherwise, if iron oxide layer thickness is too high (~ > 200-300 nm), residual iron oxide could still exist after the reduction step and cause coating defects as well. Therefore, controlling and regulation the oxygen amount and the strip temperature during the oxidation step are of particular importance.
Generally, there are three different possibilities how a controlled oxidation step can be realized in an industrial CGL depending on the type of the CGL’s heating section:
- DFF-type heating section (direct fired furnace): Adjusting the air/fuel gas ratio (so-called λ value) of the open burners to >1.0 causing an excessive amount of oxygen within the DFF section [21]
- RTF-type heating section (radiant tube furnace): Implementation of a separated reaction chamber (so-called “oxidation chamber”), in which a controlled oxygen containing atmosphere exists [26 – 28]
- DFI-type heating section (direct flame impingement): Adjusting the λ value of the open DFI boosters to >1.0 in a similar manner as using a DFF-type heating section [29]
Today, pre-oxidation in CGLs equipped with DFF- and RTF-type heating sections represents a key technology to hot-dip galvanize or aluminize modern AHSS for both hot and cold sheet metal forming. The usage of an oxidation chamber in all-RTF CGLs additionally allows producing AHSS in best coating quality [5]. Beside λ value regulation and the usage of an oxidation chamber, the direct injection of air, oxygen, water vapor or mixtures of these gases has been discussed alternatively in order to perform an oxidation/reduction treatment of the steel surface during continuous annealing.
Recently, pre-oxidation could also be confirmed as an appropriate processing approach even to hot-dip galvanize TWIP steel alloyed with >20 wt.% of manganese. In the case of pre-oxidizing such high-manganese AHSS, a covering (Mn,Fe)O mix-oxide layer results from the oxidation step instead of the expected iron oxide layer. During reductive annealing this (Mn,Fe)O layer transforms to a manganese oxide layer with embedded metallic iron (→ MnO ·Femetall layer). When hot-dip galvanizing pre-oxidized high manganexe alloyed TWIP steel in laboratory and industrial trials in a CGL, good reactive wetting to a Fe2Al5 inhibitor layer was observed on top of this MnO ·Femetall layer accompanied clearly improved coating quality compared to conventionally annealed TWIP steel (Fig. 5) [4, 21, 30].
Fig. 5: Schematical illustration of the special model of oxidation/reduction method (pre-oxidation) used to hot-dip galvanize high manganese alloyed AHSS (TWIP steels) [21, 30]
4 Summary and conclusion
The aim of the present paper was to illustrate the main processing approaches for hot-dip coating alloyed and high alloyed AHSS such as DP, TRIP or TWIP steels in industrial CGLs. Several processing approaches with off-line pre-treatment or in-line conditioning of the steel surface were described and evaluated regarding their industrial applicability. By this, the present paper aims to support the operator of a CGL, who starts familiarizing oneself with the interesting field of continuous hot-dip coating of modern and future AHSS.
The main conclusions can be summarized as below:
- All processing approaches aim to protect the external steel surface against selective oxidation of less-noble alloying elements, which was identified as a main origin of degraded coating quality
- Processing approaches with off-line pre-treatment require an additional process step before subjecting the steel strip to a CGL, what is normally accompanied by higher over-all production costs
- Processing approaches with in-line conditioning of the steel surface focus on modifying the gas/metal reaction during continuous annealing, which can be challenging to control and regulate
- Well-known pre-oxidation in CGLs with DFF- or RTF-type heating sections is still a key technology to industrially hot-dip coat modern AHSS and even high manganese alloyed TWIP steel
Nevertheless, the individual decision, which processing approach is appropriate, depends on specific alloy composition and line specifications. Furthermore, recent results show that, in special cases, it is necessary to combine different approaches to obtain satisfying coating result [5].
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Contacts
(*corresponding author) ThyssenKrupp Steel Europe AG, Cold Rolling and Metallic Coating Dortmund/Finnentrop, Eberhardstraße 12, D-44145 Dortmund; marc.blumenau@thyssenkrupp.com,
ThyssenKrupp Steel USA, LLC, 1 ThyssenKrupp Drive, 36513 Calvert AL, United States; martin.norden2@thyssenkrupp.com
DOI: 10.7395/2012/Blumenau1
