The Metallurgy of Zinc Coated Steels

By Arnold Marder and Frank Goodwin

The Metallurgy of Zinc Coated Steels provides a comprehensive overview of the science and engineering of zinc coatings. Beginning with a look at new innovations made in the hot-dip coating methods (CGL), the book goes on to discuss phase equilibria, Zn bath phenomena and overlay coating formations. Both processing methods and controls are covered, as well as corrosion resistance and coating product properties. The book concludes with a discussion of future opportunities for zinc coatings. This book is a vital resource for both individuals new to this area while also serving as a handbook for users and producers of zinc coatings.

• Presents a basic understanding of the science and engineering behind zinc coatings with a thorough and cutting-edge look at their processing methods, controls, properties, and applications
• Discusses corrosion resistance, overlay coating formation, heat treatment, interface reactions, deposition processes, and more
• Covers real-world applications of these coatings

• Language: English
• Publisher: Elsevier Science
• Release date: Feb 22, 2023
• ISBN: 9780323984898

Arnold Marder

Arnold R. Marder, PhD, FASM, is the R.D. Stout Distinguished Emeritus Professor of Materials Science and Engineering at Lehigh University. He received his BS and MS from the Polytechnic Institute of Brooklyn and his PhD from Lehigh University. After 20 years at Bethlehem Steel Research, he joined the Energy Research Center at Lehigh University and as Professor in the Department of Materials Science and Engineering. He is a Fellow of ASM International and has received awards for his research from ASM, ASTM, and AWS. His expertise is concentrated in physical metallurgy and structure/property relationships in hot-dip coatings and steel alloys.

Book preview

Introduction

As you walk around a galvanizing plant, you know there is whole lot going on. Have any one of these questions ever crossed your mind?

•What were the technical innovations that propelled this massive industry to produce these important steel-coated products that impact us daily?

•How is the steel being treated, and how is it reacting to make galvanized steel?

•Steels are becoming more advanced, with higher strengths and more complicated microstructures. How are these steels galvanized to meet both the mechanical property targets and achieve high-quality coatings?

•There are a lot of alloy coatings out there besides zinc, including zinc-aluminum, zinc-aluminum magnesium, and for general galvanizing, zinc-nickel. What is the metallurgy behind these coatings, why do they perform better in many situations?

•I would like to understand not only hot-dip galvanizing but also electrogalvanizing or vapor-deposited coatings—from a metallurgist’s point of view. How are these coatings applied and how do they behave? How about zinc alloy electrogalvanized and vapor coatings?

•There are lots of corrosion studies out there, written by corrosion engineers and electrochemists. How do zinc alloy contents and microstructures affect corrosion behavior, from a metallurgist’s point of view?

•I need to understand more about forming and welding of zinc-coated sheet steels, especially things that affect galvanneal formability. What is the metallurgical knowledge behind this?

•We know how important good coating weight/thickness control is, both to satisfy customer specifications and for the profitability of the line. What are the main sources of variability in coating weight control? What are the basic coating control relationships? What are triple-spot and single-spot coating weight specifications, and how do these relate to the coating weights we are measuring?

•Our galvanizing bath produces too much dross, or our pot rolls have too short of a life. What is the state of knowledge on these topics? Which operating conditions produce the least amount of dross?

•I need a good guide to coating defect identification and cure. Is there an atlas of typical defects available with this information?

•What is the future of galvanizing, and how is it being integrated into other plant operations as Industry 4.0 practices are developed? What is the digital twin of a galvanized coil, and how does this help me improve quality and reliability?

The answers to these and even more questions about the metallurgy of zinc coated steel can be found in this book. Separate chapters provide an overview of continuous galvanizing line operations and dive deeply into galvanizing bath phenomena and coating weight control. The galvannealing chapter provides a training tool for the operator and is at a high enough scientific level to be a useful reference to the engineering scientist seeking to understand the metallurgy behind many of the gas-metal, zinc-iron, and aluminum-iron reactions that occur during galvanizing. Chapters on phase diagrams and steel-coating interface reactions provide the road maps for understanding why zinc, iron, aluminum, and other elements react as they do during the galvanizing process. The knowledge described in these chapters is brought together and invoked in the chapter on identifying and analyzing coating defects.

For the general galvanizer, the latest advances in our knowledge on pretreatment steps including cleaning, pickling, and fluxing are provided, together with a detailed view on exactly what happens when structural steels and other long products are dipped in a zinc bath. For the first time, phase diagrams and bath management data are brought together to assist batch galvanizers refine their procedures for alloy additions and sampling procedures, all explained from a metallurgist’s view. Control of steel reactivity in the galvanizing bath is approached using basic metallurgical principles.

For the user or processor of galvanized sheet steel, the metallurgical influence on corrosion is provided both for uncoated and coil coated (continuously painted) sheet steel, in both conventional and alloy coatings, including galvanneal coatings. Details on corrosion of zinc-iron-coated hot press forming steels are included and both automotive and nonautomotive corrosion situations are addressed. Formability of both construction and automotive products is considered for conventional zinc, zinc-aluminum, and zinc-aluminum-magnesium alloy coatings. Recent knowledge on resistance spot welding and gas metal arc brazing includes an introduction to causes of liquid metal embrittlement in advanced high strength steels. A detailed description of coating weight specifications is included at the end of the coating control chapter.

The frontiers of zinc-coated steel are addressed in chapters on galvanizing high strength steels and the metallurgy of hot press-forming steels, and on the constant advances in process control and online characterization that, using increasingly fast computers together with artificial intelligence rather than predictive formulas, are revolutionizing metals processing. The scientific knowledge in this book provides a strong technical platform for the operating engineer, applications engineer, and research scientist to participate in these future advances.

The reader will learn from this book that although there are a great number of steel grades being coated with different zinc alloy coatings and processes, there is a unity of metallurgical knowledge behind it all. Understanding the knowledge presented in this book will enable the reader to see the big picture, increasing their professional worth together with satisfying their intellectual curiosity about what is really going on with zinc-coated steel.

As mentioned in the Preface, this book grew out of the 2000 review paper with the same title as this book, The Metallurgy of Zinc Coated Steel, written by A.R. Marder, with support from F.E. Goodwin and others. This 80-page paper became one of the most-cited papers in the metallurgical literature in the past 20 years and has been a valuable resource to several generations of students, their professors, and countless numbers of people involved, both producers and users, in the global galvanizing industry. After so many years, it became apparent that an update was required. Also, since 2000, there has been an enormous increase in global galvanizing capacity. As of the time of writing, over five hundred continuous hot-dip galvanizing lines in 73 countries have been identified, nearly doubling the numbers in 2000. There are an additional 90 electrogalvanizing lines [1].

Continuous vapor-coating processing has recently begun, for which capacity growth is also forecast. This has resulted in the involvement of many more people in this industry, all of whom require knowledge at some level of the metallurgy of zinc-coated steel. General galvanizing has kept pace with sheet galvanizing. The update of the 80-page paper quickly grew into a book of about five hundred pages together with more than eight hundred figures and tables, revising and unifying metallurgical knowledge on zinc-coated steels into one volume.

The authors combine over a century of experience working in zinc-coated steel research, production support, and applications support and have been involved in technical leadership in the galvanizing and steel industries for many of those years. Their balance of engineering and scientific experience make them ideal guides for readers who wish to gain a rich understanding of the metallurgy of zinc-coated steel.

Reference

[1] International Lead Zinc Study Group. World Directory: Continuous Galvanizing Lines, 1997 and 2003 editions, International Lead Zinc Study Group, Lisbon, Portugal.www.ilzsg.org.

Chapter 1: History of zinc-coated steel

Abstract

The history of zinc-coated steel has been well documented ever since Luigi Galvani conducted his experiments on frogs’ legs. In this chapter, the evolution of zinc-coated steel is followed through the patent literature, from the early works on electrochemistry by Volta, Davy, and Faraday, through the development of the electrodeposition and hot-dip processing, including the revolutionary patents by Sendzimir. The various alloy coatings such as GI, GALFAN, Galvalume, and the newer Al-Zn-Mg compositions are discussed. Finally, a chronological history of zinc-coated sheet steel development is presented in the Appendix.

Keywords

Cathodic protection; Corrosion; Electrogalvanize; Galvanize; Galvannealing; Galvalume; Hot-dip galvanizing; Patents; Processing; Sendzimir process; Zinc; Zinc-coated steel

The history of the development of zinc-coated steel has been well chronicled by Lamesch in The World History of Galvanizing [1]. The purpose of this chapter is to briefly review the highlights of those inventors and inventions that have brought us to this moment in time as summarized in Appendix 1, The Chronological History of Zinc-Coated Sheet Steel. For this review, there is a heavy emphasis on the patent literature that clearly shows the relationship between science and economics. Great scientific ideas are inspired but only enter the world of industry when they are paired with an economic driving force. Throughout the history of zinc coating development, these pairings can be seen. For example, Galvani and Faraday, Sorel and Daniell, Sendzimir and Cook-Norteman, Mayhew and Kohler, and Borzillo and Berke. Practical industrial needs influenced all of these inventions, from iron ships and metal buildings to lightweight energy-efficient automobiles. The history of zinc-coated steel is a testament to human genius and ingenuity.

1.1: Zinc

Zinc, atomic number 30, symbol Zn, has a silvery-grayish appearance and is brittle at room temperature as a result of its hexagonal close-packed crystal structure (Fig. 1.1).

Fig. 1.1

Fig. 1.1 HCP crystal structure.

Zinc ranks 24th in abundance in the Earth’s crust, and the most common zinc ore is sphalerite (zinc blende), a zinc sulfide mineral. Zinc is economical to produce because zinc deposits are located in concentrated areas in the Earth’s crust rather than being widely dispersed, and it can be refined at low cost, compared with rubidium (22nd) or titanium (9th). The production of zinc from recycled sources, mainly from recycling of zinc-coated steel, is expected to increase to 11% of total zinc metal supply by 2025, up from 6% in 2014 [2].

The elemental properties of zinc are given in Table 1.1:

Table 1.1

Large-scale production of zinc did not begin until the 12th century in India, although it was known to the ancient Romans and Greeks (300 BCE). Pure zinc production made by the distillation process was found as early as the 9th century in Zawar, in Rajasthan, India, where it continued into the 1830s when the Zawar complex was destroyed during a period of political instability. Zinc was known in China during the 16th century. Paracelsus (1493–1541), a Swiss-born German alchemist, named the metal zincum or zinken in the 16th century. Before zinc was produced in Europe, it was imported from India beginning about 1600. A Flemish metallurgist and alchemist, P.M. de Respour, extracted metallic zinc from zinc oxide in 1668.

In Bristol, England, William Champion (1709–89) rediscovered the Indian distillation process after a tour of European brass mills that imported Indian zinc and patented a process to extract zinc from calamine (Fe2O4Zn) by vapor condensation in a vertical retort furnace in 1738; the process was used for over 100 years through 1851. In 1746, a German Chemist, Andreas Marggraf (1709–82) heated a mixture of charcoal and calamine to obtain zinc, a procedure that became commercially practical by 1752. Marggraf usually gets credit for the discovery of pure metallic zinc, although zinc was distilled from calamine 4 years earlier by a Swedish Chemist, Anton von Swab (1702–68).

Pure metallic zinc oxidizes quickly on exposure to air and corrodes easily; however, in an atmosphere containing carbon dioxide, it can form a thin stable layer of insoluble zinc carbonate, giving the familiar gray color. Zinc has favorable casting properties. It also has good alloying characteristics and is an important alloying element in brass. Zinc oxide is used as an accelerant for vulcanization in the rubber-making industry, and because it is opaque, it is also used to protect polymers and plastics from ultraviolet radiation. Zinc oxide semiconductor properties make it useful in photocopying products and useful extensively as a white pigment in paints and ceramics. As a trace element, zinc is present in all parts of the body, e.g., organs, tissues, and bones, and zinc is essential for growth and reinforcement of the immune system. Zinc’s importance is reflected in the Recommended Daily Allowance for zinc by the USA Food and Drug Administration, 11 mg/d for adult men, 8 mg/d for adult women, and 12 mg/d for lactating women [3].

Zinc is nonaccumulative in the body, therefore is nontoxic as organisms ingest zinc with food and excrete excess quantities. Zinc supplements are used to fight the common cold, and zinc is thought to improve immune cell function that counters viral infections or by reducing the ability of viruses to multiply, as in the COVID-19 virus [4].

1.2: In the beginning

The history of galvanizing (see Lamesch for a complete review of zinc history, Ref. [1]) involves many scientists that goes back to the first use of zinc as a coating on steel. In 1742, Paul-Jacques Malouin (1701–78) the father of hot-dip zinc, dipped tin plate into a zinc bath and produced a new coating with rust-proof properties and a replacement for the standard at that time, tin plate [5]. Malouin’s discovery was overlooked by many including Antoine-Laurent Lavoisier (1743–94), the founder of modern chemistry, who continued to recommend that laboratory utensils be protected by tin rather than zinc. Little did scientists know that the tin plate in Malouin’s experiment acted as a shield between the iron and the oxygen in the air, preventing the iron from oxidizing, forming an oxide film on the surface of the iron, and preventing the zinc from adhering to the iron. However, in 1802, Karl Friedrich Buschendorf (1763–1811) proposed an improved method of zinc plating that instead of using tinplate involves pickling iron sheets in sal ammoniac, better known now as ammonium chloride [6]. Thus, the hot-dip zinc-coated steel process was born. It is interesting that almost no tin-plated items from this era or before can be found. Although tin provides excellent barrier protection, it cannot protect the steel from scratches and cut edges. At these locations, the unprotected iron corrodes, in part to sacrificially protect the tin coating until the article was destroyed. In contrast, one of the great values of zinc coatings are their ability to galvanically protect steel even when the coating is damaged.

True to form, theory followed practice, and it began with the discoveries of Luigi Galvani (1737–98) when in 1791 he published his experiments that caused spasmodic contraction of frog’s leg muscles by touching the spinal cord with wires of dissimilar metals [7]. Galvani labeled this animal electricity. Also, Alessandro Volta (1745–1827) reported his experiments in 1800 proving that electricity could be generated chemically by showing that any pair of dissimilar metals would give rise to an electrical force [8]. A result of his findings enabled Volta to arrange metals in a particular sequence, later to be called the Standard Potential of Elements, which showed the relative reactivity of the elements he studied. For these elements he found the sequence of reactivity to be:

Unlabelled Image

The relative position of iron between tin and zinc in this sequence shows the corrosion benefit of coating iron with zinc, the zinc being sacrificial to iron. This eventually led to the development of the battery or voltaic cell [9].

Around the same time in England, Sir Humphrey Davy (1778–1829), working on voltaic cells, was able to show that while producing electricity, one metal corroded protecting the other metal, suggesting that zinc could be used for the protection on iron even outside the voltaic cell [10]. Along with his laboratory assistant at that time, Michael Faraday (1791–1867), in 1829 he found an important property of zinc whereby it protected iron against rust even at a distance. He observed that iron exposed to moisture and air did not rust when placed in physical contact with zinc, leading to the concept of cathodic protection [11]. Faraday continued his studies producing the fundamental laws of electrochemistry, the cathode and anode, which was essential for electro galvanizing.

1.3: Zinc coating processing

Modern galvanizing was born when Stanislas Sorel received a French Patent in May 1837 (Fig. 1.2), followed by the USA Patent (US 510) in December 1837. Sorel built on the works of Davy and Faraday using electrochemical principles that zinc will protect iron from corrosion to produce a zinc-coated iron product. By pickling the surface of the iron prior to dipping into a zinc bath, he was able to show that the zinc was adhesive to the iron and that iron would not rust at the edges or even if filed. He called the process Galvanization of Iron in honor of Luigi Galvani and his frog experiments. The new terms galvanizing, to galvanize, and galvanic were quickly accepted in the French language and, soon afterward, in other languages.

Fig. 1.2

Fig. 1.2 Original Stanislas Sorel French Patent 93.

Actually, the term was first coined from the French galvanizer in 1801, to stimulate by galvanic electricity. The term became part of the local vernacular and was used in the general literature of that time, e.g., by Thomas Hood in his 1845 poem, Love has no Eyes [12] and in the novel Villete by Charlotte Bronte in 1853 [13]. During the American Civil War, 1861–1865, former Confederate prisoners of war who swore allegiance to the United States and joined the Union Army were called Galvanized Yankees. The term galvanized continues to be used metaphorically of any stimulus that results in any activity by a person or group of people, as in galvanize into action, meaning stimulating a complacent person or group to take action.

Timing is everything. Iron ships were being constructed from 1838, and corrosion in salt water was becoming a significant problem. In the past, copper plates were used to protect wooden hulls but, consistent with its place in the Standard Potential Series, would accelerate corrosion of the iron. Marine paints did not help, so the French naval command encouraged Sorel to galvanize hulls, anchor chains, bolts, and nails with great success. Industrial production of galvanized iron soon followed and so did patents. As an example, C.B. Miller received a US Patent, No. 10,976, in 1854, Fig. 1.3.

Fig. 1.3

Fig. 1.3 C.B. Miller, US Patent 10,976.

Concurrently, based on the experiments of Davy, Volta, and Faraday, electrochemistry became commercially very important. One result was to produce a zinc coating on iron, again for corrosion protection, called electrogalvanizing. Although originally a shortcoming of the process was the limited use of the voltaic cell battery, new battery research by Prof. J.F. Daniell (1790–1845), who invented the Daniell cell in 1840, utilized a semipermeable membrane between the anode and cathode to extend the life of the battery [14]. Sorel obtained an electrogalvanizing patent in 1840. Technological improvements on the electrogalvanizing process soon followed and as an example Charles Bresley in 1862, was awarded a US patent, No. 36,750, for an Improved Process of Electroplating Iron, steel, …, Fig. 1.4. Sherard O. Cowper-Coles (1866–1936), whose name is also connected with the sherardizing process used for coating of fasteners, obtained a GB Patent 189,419,797 in 1895, which proved impractical. Nevertheless in 1936, T. O. Tapp received a patent (GB445719A) for his continuous electrogalvanizing process. In 1938, Tapp was able to electrogalvanize wide thin sheets with remarkable quality. Development continued in electrogalvanized processing of sheet until it was largely replaced by hot-dip coatings whose quality significantly improved in the 1990s. Electrogalvanizing is still industrially important for the production of small items such as fasteners, where the coating must precisely follow the thread shape.

Fig. 1.4

Fig. 1.4 C. Beasley, US Patent 36,750.

In the early stages of electroplating of zinc on steel, both chloride baths (Fr patent 358,622) and cyanide baths were used (US Patents 1,451,543 and 2,080,483). It wasn’t until the 1930s when the first bright deposits of zinc were obtained by L.R. Westbrook, with an alkaline cyanide electrolyte (e.g., US patent 2,080,520 and US patent 2,233,500). Smooth electrogalvanized product was obtained in 1940 by J.P.Hibbell using an ammoniacal zinc solution (US Patent 2,200,987). Since that time, much work was carried out on alkaline electrolytes, acidic electrolytes (based on both chlorides and sulfates), and bath compositions in order to produce very bright deposits.

Thus, two zinc coating processes, hot-dip galvanizing and electrogalvanizing, became available to commercially produce zinc coatings. In the 1840s, an English company, Morewood & Rogers, commercially pioneered the use of hot-dip galvanized corrugated sheet for building construction. In particular, their 1843 patent GB 9720, mechanized the production of hot-dip galvanized steel, and is described as an iron frame placed on a zinc pot, which is driven by a steam engine, with supporting rolls designed to propel lengths of sheet through the zinc bath. Morewood & Rogers went on to construct many additional components to their semicontinuous process. In 1846, they proposed a mechanical system for controlling and balancing the thickness of the molten zinc with skimming rolls (GB 11476). In all, they obtained over 20 patents related to the production of zinc-coated steel. Many modifications to the process were made between the years of 1880 and 1920. Of significance was the design of Heathfield in 1879 (US Patent 211,905) and Poppleton at John Lysaght in 1888.

Preparation of the steel surface for galvanizing until this time was based on aqueous treatments. The breakthrough in the development of furnace treatments to prepare the steel surface for hot-dip galvanizing resulted from the inventions of Tadeus Sendzimir (1894–1989). After spending time in Shanghai as a galvanizer and returning to his country of origin, Poland, after WW1, Sendzimir developed the concept of a continuous galvanizing line, to eliminate the sheet-by-sheet processing that had up to this point characterized hot-dip galvanizing. He took his ideas to the United States in 1929, but because of economic circumstances at that time, had to return to Poland. Here, in conjunction with S. Inwald, he was able to build his CGL line and prove his invention.

Sendzimirs’ process, using a hydrogen atmosphere to reduce the iron oxides on the surface of the steel sheet, eliminated the need for the pickling-fluxing step and heating of the zinc bath. Eliminating the fluxing salts allowed for the efficient use of aluminum in the zinc bath, ranging between 0.1% and 0.2%. The resulting good-quality product had regular spangles and a very thin intermetallic inhibition layer. The steel strip vertically exited the zinc pot allowing for a uniform zinc layer on both sides of the steel sheet. In the United States, Sendzimir licensed Armco Steel, who acquired the rights to Sendzimir's patents (US Patents 2,110,893, 2,136,957 and 2,197,622), Fig. 1.5. As the Sendzimir concept continued to evolve, it was adapted to additional processes such as aluminizing, continuous annealing, and galvannealing.

Fig. 1.5

Fig. 1.5 T. Sendzimir, US Patent 2,110,893.

Although the Sendzimir process was meant to replace the need for the pickling-fluxing step, it gave rise to the development of a continuous fluxing process that used batch-annealed coils that required no further heat treatment as in Sendzimir, or could be processed full hard after cold rolling to give the highest possible strength. The new process, patented in 1958 (US Patents 2,824,020, 2,824,021, and 2,823,641), Fig. 1.6, was developed at Wheeling Steel by N.E. Cook (1900–64) and S.L. Norteman (1913–95). The furnace was therefore limited to drying the fluxing salts and ran at relatively low temperatures, 250°C. As a result, the Cook-Norteman lines led to high throughput, twice the current Sendzimir lines. However, a major problem was the management of flux fumes, despite this, the Cook-Norteman process, or its derivatives, is still used in many countries.

Fig. 1.6

Fig. 1.6 N.E. Cook and S. L. Norteman, US Patent 2,823,631.

Pretreatment furnace technology continued. In a series of patents (US Patent 2,462,202, 2,869,846, and 3,320,085), Fig. 1.7, Selas developed a direct fired furnace that by a mixture of gases produces a reducing atmosphere in the furnace chamber with radiant burners that will deoxidize and clean the wire or strip. This furnace enabled a more compact line and ran much lower percentage of hydrogen (5%) in contrast to the Sendzimir process (75% hydrogen). In contrast, the Heurty process, developed in France, provided the required heat by direct flames controlled to give a nonoxidizing atmosphere in a horizontal furnace. Further details on direct fired furnaces are given in Chapter 2.

Fig. 1.7

Fig. 1.7 C.A. Turner, US Patent 3,320,085.

Because of the requirements in the automotive industry for smooth, thin zinc coatings, jet wiping was invented. Prior to the 1970s, the control of coatings had been managed by coating rolls placed at the exit of the strip from the zinc bath. These had their origins in the mechanically driven wringer rolls used in the early semicontinuous galvanizing machines described earlier. Control of coating weight by coating rolls is still practiced by some lines producing roofing panels and other industrial products.

The concept of controlling zinc coating thickness by jet wiping was first developed in 1883 by H.A. Young using compressed air (US Patent 287,076). Because of the process being unsuccessful, it was abandoned, although work continued sporadically until the automotive need appeared. In 1970, after a series of failed patent applications dating back to the 1960s, J.T. Mayhew received a patent (US 3,499,418) for an apparatus that would control coating weight/thickness by using nozzles that would direct gas under pressure onto the surface of the molten zinc after emerging from the zinc bath, Fig. 1.8. This design was similar to those being used in the paper industry at that time. Further details on coating control are given in Chapter 7.

Fig. 1.8

Fig. 1.8 J.T. Mayhew, US Patent 3,499,418.

Galvannealing, a post pot heat treatment of the zinc coating in which the zinc coating is transformed to a series of Fe-Zn compounds, has been around since the 1920s. J.I. Herman received a patent (US 1,468,905) in 1923 for producing a heavy zinc coating that was flexible, malleable, and smooth. Commercial galvanneal coatings were also produced by removing as much liquid zinc as possible from a coating produced in a low Al(< 0.1235%Al at 460°C) bath giving a coating weight of 40 g/m². The development of heavy coatings proved difficult because the multiphase microstructure powdered easily. Major research in the 1990s on optimum ZnAlFe phase morphology, steel chemistry, and line control resulted in the successful application of Galvaneal coatings with coating thicknesses in excess of 8 μm [15]. Further information on galvanneal coatings is given in Chapter 8.

1.4: Zinc coating alloy development

Additions of alloying elements to the zinc bath for better control of the zinc coated product have long been of interest, particularly for hot-dip zinc coatings. One of the most important elements added to zinc coating is aluminum. J.W. Richards (1891–1918), Professor of Metallurgy at Lehigh University, was a strong proponent of the use of aluminum throughout his career. Yet, in 1891, he received a patent, US No. 456,204, Fig. 1.9, in which he proposed diffusing aluminum throughout a bath of metallic zinc to produce a much more uniform zinc galvanized coating, which is highly crystalline and permanently brilliant, while at the same time being exceedingly malleable and tenacious, as well as strongly adherent to the iron. Previously, several investigators experimented with zinc alloy additions such as lead, tin, copper, and antimony (1873, US Patent 144,403), or tin, antimony lead, and bismuth (1885, US Patent 328,239). Data gathered in 1911 and further reported by Cone [16], showed the relationship between the tightness (bend cracking resistance) of the coating and total aluminum, this favorable parameter increases rapidly in the Al range between 0.12 and 0.14 wt%, validating the discovery of Prof. Richards.

Fig. 1.9

Fig. 1.9 J.W. Richards, US Patent 456,204.

Zinc-coated iron or steel was coated with small amounts of lead in the zinc (1909, US Patent 933,612), or barium, strontium, and/or beryllium (1933, US Patent 1,939,667). In the 1950s, several investigators were able to show that additions of 0.1%–0.3% Al suppressed the formation of Fe-Zn compounds in the Zn coating [17–19]. This reaction was termed inhibition. In the 1960s and following years, systematic studies were conducted on Zn-Al alloy-coated steel. In 1967, Borzillo and Horton received patents for a zinc coating containing 25%–70% aluminum and silicon, not less than 0.5% (US Patents 3,343,930 and 3,393,089), Fig. 1.10. These patents were the basis for the Galvalume coatings. In 1970, Ghuman and Goldstein [20] studied the inhibition effect in 0%–10% Al-Zn baths, followed by research on the effect of silicon content on Zn-55% Al Galvalume coatings by Selverian, et al. [17,21,22]. Following promising work at Inland Steel (now Arcelor Mittal) by H.H. Lee (US Patents 4,029,478 and 4,051,366), Zn-5% Al coatings (Galfan) were developed by CRM in 1978 (US Patent 4,448,748), and in the late 1980s, a great amount of research was conducted on annealing Zn coatings [15]. In 1983, Berke and Townsend received a patent (US 4,401,727) for coatings of Al-Zn-Mg-Si based on the Galvalume composition, although Inland Steel had done research in the 1970s on small additions of 1.5%–5% Mg and at least 0.5% Si (US patents, 3,505,042 and 3,505,043) and further additions of antimony and lead (US Patent 4,056,366) at much lower aluminum contents.

Fig. 1.10

Fig. 1.10 A.R. Borzillo and J.B. Horton, US Patent 1,343,930.

A: Appendix

A.1: Chronological history of zinc-coated sheet steel

•200 BCE

Romans used zinc vapor to combine with copper to make brass

•9th Century

Zinc production by the distillation process in India

•1374

Large-scale production of a new metal zinc in India

•1526

Paracelsus names the element Zink

•1668

de Respour extracted metallic zinc from zinc oxide

•1738

Champion patented a process for extracting zinc from calamine

•1742

von Swab first distills zinc from calamine in Sweden

•1742

Malouin dipped tinplate into a zinc bath to produce a hot-dip coating

•1746

Marggraf heats calamine and charcoal to produce zinc metal

•1791

Galvani discovers the electrochemical process that occurs between metals

•1800

Volta proved electricity was generated chemically using dissimilar metals.

Davy discovers corrosion protection of iron by zinc

•1802

Buschendorf produces a zinc coating on a pickled iron substrate

•1829

Faraday proves zinc acts sacrificially in protecting iron from corrosion

•1837

Sorel obtained a French Patent to protect iron with a zinc layer coating

•1840

J.F. Daniell invents the Daniell cell

•!840

Sorel obtains an electro galvanizing patent

•1843

Morewood & Rodgers pioneer the commercial use of zinc hot-dip galvanized corrugated sheet for building construction

•1854

Industrial production of zinc coated French naval components

•1856

John Lysaght buys a small hardware galvanizing business to produce corrugated sheet

•1883

H.A. Young uses compressed air to control coating weight and thickness

•1891

J.W. Richards patents the use of precise aluminum additions to the zinc bath to produce more uniform coatings

•1936

T.O. Tapp receives a patent for a successful continuous electrogalvanizing process

•1938

T. Sendzimir obtains patents for the continuous processing of zinc-coated sheet

•1940

J.P. Hibbell using an ammoniacal zinc solution produces smooth electrogalvanized product

•1950

Bablik published a comprehensive review of Hot-Dip Galvanizing

•1958

Cook and Norteman patented a continuous fluxing process for the production of zinc coatings

•1967

Selas Corp. develops a vertical direct fired furnace that will deoxidize and clean wire and strip.

Heurty develops a horizontal direct fired furnace

•1967

Borzillo and Horton patent Galvalume high aluminum zinc coatings

•1970

J.T. Mayhew patents direct gas nozzles to control coating weight/thickness

•1978

Zn-5% Al (Galfan) is developed at CRM

•1979

Mackowiak & Short publish Metallurgy of Galvanized Coatings

•1983

Berke and Townsend receive a patent for Al-Zn-Mg-Si hot-dip coatings

•!983

Harvey and Richards publish Zinc-Based Coatings for Corrosion Protection Of Sheet Steel and Strip

•1990s

First application of hot-dip coatings and galvanneal coatings to stringent automotive requirements and advanced steel technology

•2000

Marder publishes a review of The Metallurgy Zinc-Coated Steel

•2004

Lamesch publishes monumental book on The World History of Galvannealing

•2016

Kuklik and Kudlacek publish Hot-Dip Galvanizing of Steel Structures.

References

[1] Lamesch J. The World history of Galvanizing. Arcelor Mittal; 2004.2-919969-25-0.

[2] Vandermissen M. (McKinsey & Co.) Zinc Market Outlook-What to Expect in Times of Uncertainty, Zinc Metal Roundtable Virtual Event, November 5–6. International Zinc Association; 2020.

[3] Institute of Medicine. Food and Nutrition Board, Dietary Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Washington, DC: Natinal Academy Press; 2001.

[4] Schmerling R.H. Do Vitamin D, Zinc and Other Supplements Help Prevent Covid-19 or Hasten Healing? Harvard Health Publishing; 5 April, 2021.

[5] Maloun P.J. Memore de L’Academie Royale des Science. 1745.70.

[6] Buschendorf K.F. Manufaktur, Hanlung, und Mode. J. Fabrik. 1802.

[7] Galvani L. De Viribus Electricitatus in Motu Musculari Commentarius. Annals of the University of Bologna; 1791.

[8] Volta A. Memorie Sull’electricita Animal. Brugnatelli, Pavin: Inserite nel GionarleFisico-Medico del Sig; 1792.

[9] Volta A. On the electricity excited by the mere contact of conducting substances of different kinds. Philos. Trans. R. Soc. Lond. (Fr.). 1800;90:403–431.

[10] Davy H. Philos. Trans. R. Soc. 1824 114, 151, 242, 328.

[11] Faraday M. Experimental Research in Electricity.. 1855;vol. 1 and 2.

[12] Hood T. Love has not Eyes. 1845.

[13] Bronte C. Villette. London: Smith, Elder & Co.; 1853.

[14] Daniell J.F. Daniell cell. Philos. Trans. 1836.

[15] Jordan C.E., Marder A.R. Morphology development in hot-dip galvanneal coatings. Metal. Mater. Trans. 1994;25A:937.

[16] Cone C. Iron and Steel Engineer. Association of Iron and Steel Engineers; March, 1962.80.

[17] Bablik H. Galvanizing (Hot Dip). third ed. London: Spon Ltd.; 1950.204–223.

[18] Hughes M.L. Proc. of the International Conference on Hot Dip Galvanizing. Copenhagen: Zinc Development Assoc.; . 1951;vol. 31.

[19] Houghton M.A. The effects of aluminum and iron on the structure of galvanized coatings. In: Proc. 2nd Int. Conf. on Hot Dip Galvanizing; Zinc Development Assoc.; . 1953;59.

[20] Ghuman A.R.P., Goldstein J.I. Reaction mechanisms for the coatings formed during hot dipping of iron in 0-10% Al-Zn Baths at 450° to 700°C. Metal. Trans. 1971;2:2903.

[21] Selverian J.H., Marder A.R., Notis M.N. The reaction between solid iron and liquid Al-Zn baths. Metal. Trans. A. 1988;19A:1193–1203.

[22] Selverian J.H., Marder A.R., Notis M.R. The effects of silicon on the reaction between solid iron and liquid 55 Wt pct Al − Zn baths. Metal. Trans. A. 1989;20A:543–555.

Chapter 2: Hot-dip coating methods: Continuous processing (CGL)

Abstract

The continuous galvanizing line is divided into several sections that prepare, coat, and provide additional processing involved in galvanizing coils of flat-rolled sheet. After an overview of line operations and processing objectives, descriptions are given of each section of the line, including cleaning, furnace pretreatments, coating with zinc and zinc alloys, coating weight regulation by gas wiping, galvannealing, and after-pot operations such as temper rolling and inspection. The types of steels typically coated are described. Metallurgical concepts such as internal versus external oxidation behavior of steel surfaces during processing, annealing, and overaging behavior including quench and partition treatments and the effects of surface oxides and roughness on pyrometry are described.

Keywords

Oxidation; Sheet galvanizing; Galvanized steel; Galvanizing furnace; Pyrometry; Zinc wettability; Solidification; Passivation; Austenite; Martensite

2.1: History and overview

As described in Chapter 1, the beginning of the modern era of continuous sheet galvanizing can be dated to Sendzimir’s 1938 USA patent 2,110,893 that for the first time describes that a sheet galvanizing process involves producing a thin and uniform film of oxides of the body (substrate) metal….by treating said…bodies in an oxidizing atmosphere, then under conditions of heat and a reducing atmosphere completely reducing said film so as to form a reduced metal layer in intimate contact and closely adhering to the body metal,……leading them beneath the surface of a bath of …molten coating metal. This process had many advantages over the Cook-Norteman lines that used wet chemicals to prepare the steel surface for galvanizing, as described in [1]. The genius of Sendzimir’s process was that both useful heat treatments of the steel sheet and preparation of a galvanizable surface could be accomplished simultaneously by the continuous galvanizing heat cycles and atmospheres. Most of the sheet steel delivered to modern continuous galvanizing lines (CGLs) is coming from cold rolling mills in full hard condition up to 2 mm in thickness; it is necessary to heat treat it to specified conditions for it to be useful to the customer. For the Cook-Norteman process, such heat treatments, including temper rolling, need to be performed before delivery to the galvanizing line. CGLs also process a wide variety of hot rolled strip up to 4 mm thickness, as described later in this chapter. CGLs have evolved since the time of Sendzimir’s patent, handling a thickness range of 0.15–6 mm and strip widths up to 2100 mm with production rates approaching 1 million metric tons of annual production, processing steels that could not be imagined in Sendzimir’s time.

Fig. 2.1 shows a modern CGL capable of applying Zn coatings to advanced high-strength steels (AHSS).

Fig. 2.1

Fig. 2.1 Schematic of a modern continuous galvanizing line [2].

As summarized in [1], the basic CGL process steps are precleaning (not shown in Fig. 2.1), heating and soaking in a reducing atmosphere, cooling, and immersion in the Zn coating bath, wiping of the coating after it exits the bath to obtain the desired coating weight, the optional use of a galvannealing furnace when production of galvannealed coatings is required, cooling, normally with the assistance of after-pot coolers before reaching the top roll where the strip temperature must not exceed 360°C, additional cooling, then (not shown in Fig. 2.1) postgalvanizing operations that can include tension leveling, temper rolling, then inspection before coiling or cutting to length of the coated sheet in the delivery section. This chapter explains the advances made in CGL processing technology since 2000 that relate to the metallurgy of Zn-coated steel.

2.2: Cleaning

Rolling mills typical deliver sheet steel to a galvanizing line with total surface residues present at a level of 100–200 mg/m², these must be removed so that they do not interfere with the galvanizing reaction, contaminate furnace rolls, or contribute to the dross levels in the galvanizing bath, as described further in Chapter 4. The original Sendzimir process relied on only an open-air direct-fired furnace to clean the steel sheet before entering the high-temperature soaking furnace. This was later changed to heating in a nonoxidizing furnace, because with an oxidizing flame, carbonaceous contaminants can burn on the steel surface, leaving a residue; if they are volatilized in a reducing flame, they can be removed. Iron fines, either in metallic or oxidic form, are commonly found on the surface of rolled sheet. The original Sendzimir process further oxidized these iron fines during the furnace cleaning step, resulting in higher iron levels in the zinc bath compared with a nonoxidizing cleaning step. To reach levels of cleanliness for automotive product, typically 20 g/m² per side of iron fines and oil [3], a wet cleaning step prior to furnace entry is almost always used, this involves cleaning with an alkali solution, applied by either spraying or immersion, combined with abrasion by brushes, and in many cases combined with electrolytic action that helps lift oil and iron residues from the surface. A schematic of a typical wet cleaning section is shown in Fig. 2.2.

Fig. 2.2

Fig. 2.2 Wet cleaning section of CGL (S. Hesling). (S. Hesling, Entry Section Processes Section 3 Chemical Cleaning of Strip Fig. 1, undated.)

The furnace cleaning step can include up to four furnace zones, with a typical furnace gas temperature of around 1200 °C. This gas temperature maximizes the potential for reducing contaminants and oxides on the strip. As the strip is heated in this section, the carbonaceous contaminants are removed at lower temperatures; surface oxides begin to be reduced in the final sections as the strip temperature reaches the range of 565–675°C. The furnace atmosphere is a mixture of CO from combustion and H2; the air-fuel ratio in the furnace burners is usually set to around 6% excess combustibles to be sure a reducing atmosphere is present, the sum of the CO and H2 compositions in the furnace atmosphere is typically 3%. If a neutral flame is used in the burners, soot deposition can result, contaminating the strip surface. The reduction of surface oxides continues in the soaking section that follows. Furnace cleaning that follows the wet cleaning stage has been enhanced in many cases by the use of direct flame impingement (DFI) burners, located a few cm from the strip. This has been found to enhance cleaning, while improving efficiency and productivity, in some cases making prior wet cleaning unnecessary and reducing the required duration of furnace treatment in this process step [4].

2.3: Steel types for continuous galvanizing

The wide variety of steel types processed in modern CGLs include the following major groups, for which Euronorm and ASTM standards are also shown:

•Commercial Steel (CS) ASTM A653, EN 10326

•C range 0.04%–0.10%, Mn range 0.2%–0.6%, depending on the product being made.

•steel is cold rolled with 50%–80% reduction.

•Forming Steel (FS) ASTM A653, EN 10142

•C range 0.04%–0.08%, Mn about 0.25%.

•to obtain better response to annealing, cold reduction is between 60% and 80%.

•Structural Steel (SS) ASTM A653, EN 10326

•C range between 0.04% and 0.20%, and Mn range 0.4%–1.6%, depending on the product being made.

•substrate is cold rolled anywhere from 50% to 70% reduction.

•Deep Drawing Steel (DDS) and Extra Deep Drawing Steel (EDDS) ASTM A653, EN 10292

•generally made from ultralow carbon stabilized steels: C range 0.0040%–0.0070%, also written as 40–70 ppm, although some DDS is made using extra low carbon (0.015%–0.020%) steel.

•EDDS and some DDS are produced to be fully stabilized (nonaging) after annealing and coating.

•To maximize annealing response, cold reduction is generally high at 75% minimum.

•High-Strength-Low Alloy (HSLAS) ASTM A653, EN 10292

•Typically made from microalloyed low carbon (C range 0.15%–0.25%) steel. The primary alloying element is Nb with a range of 0.005%–0.05%

•Cold reduction rarely exceeds 60% due to high cold rolling loads.

•Advanced High-Strength Steel (AHSS) ASTM A1088, EN HCT 490X through EN HCT 1180G2

•Produced using higher levels of alloying elements and carefully controlled annealing and cooling cycles. This category includes solution hardened steel (SHS), bake hardening steels (BHS), dual-phase (DP) steels, transformation-induced plasticity (TRIP) steels, complex-phase (CP) steels, and third-generation (3G) steels

•Cold reduction rarely exceeds 60% due to high cold rolling loads

Compared with drawing quality conventional C-Mn grades, AHSS alloy compositions include higher levels of elements that have higher oxidation potentials than Fe. These can include Mn, Si, Al, and Cr. For a discussion of the effects of these elements on microstructures and mechanical properties of these grades, the reader is referred to De Cooman and Speer’s book [5].

2.4: Galvanizing pretreatment steps for AHSS

The objective of pretreatment is to prepare the steel surface to be reacted with the zinc bath, which depends upon a surface consisting of clean, reduced iron. Simultaneously, the steel must be heat treated to achieve the desired microstructure. Alloying elements and their composition levels in AHSS strongly affect the choice of CGL process steps, if they are to be successfully galvanized. The main issue to be controlled is the surface-selective oxidation of several of these the alloying elements, mainly Mn, Si, Cr, and Al. In a CGL annealing furnace containing an atmosphere that is strongly reducing for Fe, say a partial pressure of oxygen PO2 = 10− 26 atm, which at a temperature of 675°C has a dew point of − 30°C, the concentration of each of these elements in solid solution at and near the steel surface is lowered by oxidation; equilibrium requires that replenishment by diffusion from the bulk (steel subsurface), which is accomplished by solid-state diffusion, particularly along steel grain boundaries. However, if a higher pO2 is used in the furnace, say 10− 24 atm, which at 675°C corresponds to a dew point of + 5°C, there is a surplus of oxygen on the surface. Because the diffusion coefficient of oxygen into the steel strip is on the order of 1000 times higher than that of Mn, Si, Al, and Cr, the oxidation of these elements occurs internally rather than on the steel surface when a sufficient oxygen supply is present, improving conditions for galvanizability [6–8]. This internal oxidation process is shown schematically in Fig. 2.3.

Fig. 2.3

Fig. 2.3 Schematic of diffusion of surface-selective oxidizing elements and oxygen in (Mn,Si) steels.

The transition point between internal and external oxidation for a binary steel composition is given by

si1_e

where

NB,Critical⁰ = critical mole fraction of alloying element B.

g∗= critical volume fraction of precipitated oxide (usually 0.3 is taken).

ν = stoichiometry of the oxide.

Valloy = molar volume of the alloy.

VBOν = molar volume of oxide BOν.

D = diffusion coefficient.

NOS = mole fraction of oxygen dissolved in A at surface.

Internal oxidation occurs when

si2_e

And external oxidation occurs when

si3_e

An example of the use of this equation, for a simple Fe-Mn composition, is shown in Fig. 2.4. The transformation boundary between Mn and MnO is shown by the Mn/MnO line on the left side. The furnace dew points required to reduce MnO to Mn are in the range from − 50°C to − 7°C even at a soaking temperature of 950°C. The other two lines show the transition boundary between external and internal oxidation for two different models, a simplified version based on the above equations (upper line) and a more generalized version that includes solute interactions and other factors (lower line). External MnO indicates the region where external MnO oxidation occurs on the steel surface (to the left side of these two lines), with internal oxidation occurring on the right side. At a constant dew point in the practical operating range for a CGL, i.e., − 30°C on the horizontal axis of the graph, solely internal oxidation occurs when Mn < 6 wt% according to the general model. This corresponds to the NB⁰,critical value given above; above this value, external oxides are present, according to the model.

Fig. 2.4

Fig. 2.4 Oxidation domains for binary Fe-Mn alloys oxidizing at 950°C in a N 2 -5%H 2 atmosphere. The simplified model is based on the above equations [9]. (V. Lashgari, Internal and External Oxidation of Manganese in Advanced High Strength Steels (Ph.D. thesis), University of Delft, September 2014.)

This equation has been broadened to ternary and higher alloyed steels. Interactions between alloying elements can be ignored in the case of low alloy (Mn,Cr) compositions because the solubility products of the oxidation products are very low [10]. A more complicated example is shown in Fig. 2.5, for a C < 0.25%-1.7%Mn-0.1%Si-1.5%Al TRIP composition. Here, as predicted by theory, the oxidation fronts for Al and Si are pushed deeper into the steel as dew point increases. However, Mn is not, because of several competing reactions with FeO (wüstite) and MnAl2O4. This illustrates some of the complications arising when modeling internal and external oxidation of these steels. However, the formed external oxides in this case are granular in morphology rather than film-like at high dew points, enabling a satisfactory quality Zn coating to be produced [11]. The internal oxidation layer grows as soaking time increases.

Fig. 2.5

Fig. 2.5 Effect of dew point on near-surface oxidation behavior of an Mn-Al TRIP steel [11] . (J. Staudte, J.-M. Mataigne, D. Loison, F. Del Frate, Galvanizability of high Mn grade versus mixed Mn-Al and Mn-Si grades, in: Proc. Galvatech 2011, Genoa, Associazione Italiana di Metallurgia.)

General predictions of the types of oxides to be found on Mn-Si steels are shown in Fig. 2.6A, for external oxides when a low dew point atmosphere is used, and for internal oxides in Fig. 2.6B, when a high dew point furnace is used.

Fig. 2.6A

Fig. 2.6A External surface oxides predicted for Mn-Si grades, using a low dew point furnace atmosphere [12]. (Y. Suzuki, T. Yamashita, Y. Sugimoto, S. Fujita, S. Yamaguchi, Thermodynamic analysis of selective oxidation behavior of Si and Mn-added steel during recrystallization annealing, ISIJ Int. 49 (4) (2009) 564–573.)

Fig. 2.6B

Fig. 2.6B Internal surface oxides predicted for Mn-Si grades, using a high dew point furnace atmosphere [12] . (Y. Suzuki, T. Yamashita, Y. Sugimoto, S. Fujita, S. Yamaguchi, Thermodynamic analysis of selective oxidation behavior of Si and Mn-added steel during recrystallization annealing, ISIJ Int. 49 (4) (2009) 564–573.)

Such steels can also be industrially processed using radiant tube furnaces (RTFs) in which the dew point is high in the first zones, resulting in internal oxidation of the Mn, Si, Al, and Cr with external oxidation of Fe to FeO (wüstite). Subsequent reducing furnace atmosphere zones then transform the surface FeO to metallic Fe, resulting in a surface suitable for galvanizing [13]. Alternatively a preoxidation section, containing an atmosphere of N2-O2, is contained in the first zones of an RTF, producing the desired internally oxidized condition, followed by a reducing section [14].

A similar technique uses DFF or DFI sections to produce the desired internally oxidized condition, the burners are set so that excess O2 is present. This is followed by reducing sections in either a DFF or RTF. Fig. 2.7 shows the effect of λ, the air/fuel ratio, on strip temperature as the strip is heated in the DFF, moving from right to left. The normal burner settings are for a slightly reducing flame, 0.85 < λ < 0.96. A λ = 1.02 setting increases oxidation of the strip and also its temperature; corresponding furnace temperatures are 500°C higher at any distance along these zones [15]. Surface oxides also increase surface emissivity, further increasing strip temperature at high λ values. External oxide thickness increases as the λ reaches the stoichiometric ratio for complete combustion because excess oxygen is then available to form these oxides, Fig. 2.8. The oxide composition for a Si-TRIP grade containing 1.48%Mn, 1.4wt%Si, and 0.69%Al formed under these oxidizing conditions is shown in Fig. 2.9. The surface has high proportions of both FeO and SiO2 so that a surface featuring these mixed oxides can be predicted; the lower value of Al suggests that mixed oxides of Al and Si may also be present.

Fig. 2.7

Fig. 2.7 Strip temperature (°K) evolution through the DFF with normal (0.85 <  λ  < 0.96) and increased ( λ  = 1.02) air/fuel ratio [15] . (L. Bordignon, G. Angeli, H. Bolt, R. Hekkens, W. Maschek, J. Paavalainen, X. vanden Eynde, Enhanced hot dip galvanising by controlled oxidation in the annealing furnace, in: Proc. 44th Mechanical Working and Steel Processing Conference, vol. XL, AIST, September 2002, pp. 833–844.)

Fig. 2.8

Fig. 2.8 External oxide thickness increases rapidly as λ reaches the stoichiometric ratio for combustion [15] . (L. Bordignon, G. Angeli, H. Bolt, R. Hekkens, W. Maschek, J. Paavalainen, X. vanden Eynde, Enhanced hot dip galvanising by controlled oxidation in the annealing furnace, in: Proc. 44th Mechanical Working and Steel Processing Conference, vol. XL, AIST, September 2002, pp. 833–844.)

Fig. 2.9

Fig. 2.9 Secondary ion mass spectroscopy surface depth profiles for a 1.48%Mn, 1.4wt%Si, and 0.69%Al TRIP steel preoxidized using the high λ DFF described in the previous figures [15] . (L. Bordignon, G. Angeli, H. Bolt, R. Hekkens, W. Maschek, J. Paavalainen, X. vanden Eynde, Enhanced hot dip galvanising by controlled oxidation in the annealing furnace, in: Proc. 44th Mechanical Working and Steel Processing Conference, vol. XL, AIST, September 2002, pp. 833–844.)

Process control, including control of the surface FeO layer and temperature measurement, has been found to be easier in the all-RTF process [14]. DFI preheating can also be used as the first step of oxidation/reduction treatments used to prepare the more difficult to galvanize AHSS grades for zinc coating. In this case, the surface must be essentially free of volatile contaminants because the DFI burner is run in oxidizing mode; the air/fuel ratio is controlled to give a specified external oxide thickness [16].

The cleanliness of the annealing furnace atmosphere has been enhanced by use of radiant tube heating elements. This also permits more precise control of furnace atmosphere, because burner combustion products are not released into the furnace. RTFs have high thermal inertia and are suitable for long production runs with the same product. Unfortunately, many CGLs are now required to produce short runs of a variety of steel grades (DP, TRIP, IF, etc.) all on the same line, and therefore, rapid changes in furnace temperature profile can be required. In this case, a direct-fired annealing furnace can be more suitable [17].

2.5: Overaging in the CGL

Absent from many CGLs in the years before 2000 was an overaging section after annealing and cooling that is now essential to obtain the desired microstructures and consequent mechanical properties for many steels used today. Aluminum-killed drawing quality steels, introduced in the 1960s, could not be processed on the older lines because of the need to produce a matrix of low-carbon ferrite that contained carbides, mainly cementite, which were initially nucleated during an initial slow cooling stage after high-temperature soaking. After overaging in an offline postannealing furnace, these carbides contained most of the carbon in these grades, enabling their excellent drawing capabilities, for example, transverse r values of 1.8–2 and n values of 0.19–0.2 [18]. Not until the development of ultralow carbon interstitial free (IF) steels in the 1980s was it possible to supply Zn-coated steels meeting automotive forming requirements on existing galvanizing lines, because IF steels need no overaging, in fact keeping some C in solution is required for bake-hardening grades [19]. High-strength low-alloy grades (HSLA) that provided higher strengths by carbide strengthening but less formability also were not necessary to overage. Typical heating, soaking, cooling, and overaging heat treatments used for steels on modern lines are shown in Fig. 2.10.

Fig. 2.10

Fig. 2.10 Heat treatment cycles used for various steel grades: DQ = drawing quality, IF = interstitial free, TRIP = transformation induced plasticity, Q&P = quench and partition, DP = dual phase, MS = martensitic [2] . (F. Brühl, C. Sasse, K. Watson, Annealing and galvanizing of third-generation AHSS grades, in: AISTech 2019 — Proceedings of the Iron & Steel Technology Conference, 6–9 May 2019, Pittsburgh, PA, USA, AIST, Warrendale, PA.)

Overaging is also not required for compatible dual-phase (DP) grades, where the final microstructure of ferrite and martensite can be produced by cooling at a controlled rate from an intercritical annealing temperature (where both ferrite and austenite are present), to below the martensite start (Ms) temperature. If a DP grade has its Ms temperatures below the temperature of strip entry into the galvanizing bath, which is typically 2–3°C above the zinc bath temperature of 460°C, a short heating step between cooling and strip entry into the zinc bath is required; alternatively it can be cooled to the zinc bath temperature, with transformations continuing as the steel is cooled further after coating with zinc. Overaging sections are essential for AHSS in which stable austenite is required in the final product, including TRIP, CP, and 3G steels. These grades have in common a process route that produces, after cooling from an annealing temperature either in or above the intercritical range, a mixture that can include old ferrite, martensite, bainite, and retained austenite. Overaging produces stabilization of the austenite by enabling its enrichment by carbon by reaction with surrounding phases or partitioning. A schematic of this carbon enrichment process for a quench and partition grade containing only austenite and martensite is shown in Fig. 2.11.

Fig. 2.11

Fig. 2.11 Microstructure and composition changes occurring in the quench and partition process for a 3G steel, where the partitioning temperature is greater than the quench temperature. Γ  = austenite (gamma) phases, m  = martensite phase [20]. (D.K. Matlock, J.G. Speer, Third generation of AHSS: microstructure design concepts, in: A. Haldar, S. Suwas, D. Bhattacharjee (eds.), Microstructure and Texture in Steels, Springer, Basel, 2009, ch. 11.)

It is not unusual to find 15%–50% retained austenite in TRIP and 3G steels, with austenite carbon contents up to 0.5%. Retained austenite is required in these grades to cause the TRIP (transformation-induced plasticity) effect: upon deformation, the retained austenite transforms to martensite, greatly increasing the strain hardening effect while permitting significant ductility. These enhanced stress-strain characteristics greatly increase energy absorption during deformation that is highly sought after by the auto industry to improve crashworthiness.

The process steps inducing internal oxidation in the steel can result in internally oxidized layer that is up to several microns thick. These can result in poor adhesion of the galvanized coating when the sheet is deformed. When the fracture surface is examined, the underside of the separated zinc coating is observed to have a surface composition similar to the internally oxidized layer [21]. Adhesion loss can be minimized if internal oxidation is limited to grain boundaries rather than also reacting in the bulk of the grain, this is controlled by sheet texture, composition, and oxidation treatment process variables [22].

2.6: Metallic coating pretreatments

Another approach to galvanizing steels containing relatively high levels of Mn, Cr, and Al is precoating the steel sheet with a very thin metallic layer. Precoating with Fe, Cu, Ni, and alloys of these metals has been the subject of research investigations [23–26]. Ni was found to affect the nucleation rate of the different intermetallic phases formed at the interface of the coating in steel, including NiAl3 and also NiZn delta and gamma phases, the optimal Ni preplated layer thickness is between 3 and 5 μm and depends upon dipping time in the Zn bath. Use of Ni as an addition to the bath with Al, to form an Ni-Fe intermetallic, has been explored. Fig. 2.12 shows the effect of Fe precoatings, in the range of 3–10 g/m² on the oxidation behavior of a 0.004%C-0.8%Si-0.29%Mn steel. An addition of sodium citrate was used for the precoating with 4 mass% oxygen, whereas the other precoating has less than 0.2 mass% oxygen. Fig. 2.11 shows that the surface-selective oxidation behavior of Si was the same in the oxygen-free coating as expected from a sample without precoating. By contrast, the Si combined with the oxygen in the oxygen-containing coating, resulting in internal oxidation that improved hot-dip galvanizability. Because of cost, these pretreatments are rare in industrial practice.

Fig. 2.12

Fig. 2.12 Effect of Fe preplating on oxidation behavior of Si during galvanizing furnace pretreatments [23]. (Y. Tobiyama, S. Fujita, T. Maruyama, Improvement of galvanizability of silicon-bearing steel by electrodeposited iron coating containing oxygen, ISIJ Int. 52 (1) (2012) 115–120.)

Beyond gas treatment of steel surfaces to influence oxidation behavior, use of a surface treatment technique involving lithium boron oxides has been described [27]. This liquid salt bath, which follows conventional annealing in an RTF, is operated between 650°C and 800°C and was successful in removing surface oxides so that a 1.5%Mn-1.5%Si Si TRIP steel could be successfully galvanized. A 23% Mn TWIP (twinning-induced plasticity) steel was also successfully galvanized. Samples showed good coating adhesion after