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Modern Steel Products (2014) - Corrosion of coated steel: lecture 15

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don't let some starts to weaken finish on time or earlier perhaps so what terms which we've been talking about was saying go tanks on steel and there so what the use was basically to protect steel surfaces against corrosion extended test to look at corrosion protection areas To do souls protests we have and so for instance that you can look at with so much time it takes to have 5 per cent of red rust on a panel during a souls protests and so on and if you do this for the coded panels with different coating weights yes and you see that well Elektra galvanized coatings tend to be thinner so that's why they are the only go from 20 to 40 grams per square meter serious you see that as you have thicker coatings you have no longer time to of rust red rust Formation B. that's 1 thing the other thing that's interesting is I don't a loss of saying so we have coatings are not only cinco cinco pure saying but also saying alloys and the 2 main ones always think irony and zinc nickel and sink Byron is usually made by a process called Galvin knee-length which will discuss and zinc nickel is gone by electrode deposition and you can see here that although zinc nickel you have really seen a coatings that they're very corrosion resistance another thing that is important too realize is that the Mattel's Power will corrode depending on the situation you whether you have strongly oxidizing situation or not or an or in particular whether it's very acidic the a environment or not and and so the user to corrosion rate of Zank as stated in centimeter per year we as a function of the pH of the environment and you see that and it's pretty low at neutral pH of 7 2 2 was somewhat more alkaline environments but in very alkaline environments the very high futures or very acidic pH is very acidic environment surreal pH is that is not the corrosion rate goes up that's important to to realize that using coatings are very good but in normal conditions would not too acidic and also told not to alkaline either right so let's
hope it limited technologies 1st so that there so basically full galvanizing steel steel the they're basically 2 ways you cold-rolled material yes and then you do a process that is called a hot dipped galvanized hot dipped galvanized and in the process of hub that galvanized In involves many things if it means but Re crystallization annealing last coatings zinc of applying the coating plus skim past I was skimpy or temporal that's all being done in 1 production unit and what comes out is at Cornell of code material there is no alternative way of doing things now it is you take the cold rolled sheet and you anyway and you can India led as you read remember bye backs annealing or by continues annually then once it's In the old you applied the skin pass and then you can electoral galvanized the electoral that gives you the product so it evolves many were much more manipulation so the steel let's have a look here at such an Electra galvanizing plant so UN role distract yes this and continuous process so you you can weld the start of this trip to the end of the previous trip you go through 8 accumulated that's where you accumulate distance of stripped in the line this is also an exit accumulated and as I said these accumulators are used so that you can have a constant struck the velocity in light would hide you accumulate struck in the underlying well you have be a set of roles yes this year under wraps the strike over these roles that you that's the only thing they did this and districts are these roles are attached to each other and they can move upward the stick and move up to to here for instance about 20 meters high In the strip your most and at the facility that here I have to make a Weld for instance yes but I have to make a well so I mean I have to stop this little piece of still has to stop so you can well right so in the meanwhile these roles go down yes they provide straight to the line that's when you can keep the velocity of distress calls we don't give develops the constant due process is time-dependent and get variations In the process of that's that's where you have this accumulator L right you clean the air district and then you pass through a number of of cells you use this little bit of pickling before you start to make to have a nice clean surface and then you go through all these electrolytic cells very quickly that's why you have so many all right and then afterward there is you can you can apply some of post surface treatments address phosphate in the eye of the surface spoiling it etc. many but you go through the accumulate exit accumulator and and you are you make history at Dundee egg in the exterior of the line is not much to see yes these electrolytic cells look like this Of course there felt with acid electrolyte but usually the sulfate electrolytes and you have to this this would be to strip it goes down around this role and then back up and then sued the strip is B and a cavernous cathode kept polarizes a cathode yes and it moves between 2 insoluble and roads and and that is that you can use a stainless steel you can use leaded as insoluble Arnold but modern and those are the region bauxite coated tight Tania I'm and and basically the insoluble and stated that just very neutral the electrodes basically and there is a sums which which you get here is the unnoticed positive saying the Saudis steel is negative and you deposit a thin layer of sitting on the Strip as it passes by means of very high velocities and 200 meters per minute it's so it's this quick give high current densities and these cells the ovary Bay their 8 spared few meters it's not a small a small unit and you have how many haven't nailed 15 to 20 years of the cells so far saleable typically and new
depending on what you coach this is what the surface looks like generally used CDs these very fast surfaces this is for purists think think nickel doesn't have these facets and that they would encode level here is about 10 per cent and this is sirens Inc and the siren level is also about 10 per cent so we were talking
about them alloys Of the zinc now typically would so we have galvanized or Galvin yield Calvin contains about 10 per cent the irony and if you look at the face I diagram that corresponds to an IRA and zinc intermetallic called the Delta the delta phase but also have zinc aluminum coatings yes In there to industrial I types 1 is called golf fans it has 5 per cent of aluminum and then there is another 1 which has about 50 per cent of aluminum and that's called Galva looms this is a very corrosion resistant coating has and it can be used for over 20 years and uh without the need for painterly layer or anything In a certain countries like in Australia where it's extremely popular building materials for instance for roofs and and there are also coatings that the hot dip coatings in particular that are applied to I steals and 1 of them is alumina aluminum coating their it's a coating that contains about 10 per cent of the aluminum with 10 per cent of silica and the most famous application for this coating is automotive exhaust systems it's a it's a high-temperature coated
let's look at how the technology now for the these hot galvanizing has we can make in this house the galvanizing line we can make piercing coatings using coatings for zinc iron coatings and this is usually referred to Galva Neil console so what do we get here there you see In this cross section of the line that there are sections this the big section here there is a section here and there is a section here 1 here here then and then there is a lot here this section here selected so when you say you come in you go 1st to the which called direct fired furnace In this the furnace that this trip is exposed to flames Of the burgers that's when gold direct fire for instance and that is usually a using its In lines where you want to oxidize slightly oxidized to surface In order to remove all a carbon residues at the surface there are some lines that do not contain this section the direct fire furnace instead they have a radiant June furnace To do that heating yes if you have a region to furnace to do that he thinks you need to have decreasing and removal of fines before there's any good to the radiant tube furnace where you get to stoke anger and then you go through the big did slow heating a slow cooling section Excuse me man of fast heating section in the vast seating section is characterized by a very large the diameter tubing yes here because you need to applied a very high cooling rates and then after this this is stripped goes through and over aging section you remember that overreaching is needed when you look process deals that I contain carbon in solution and you want to precipitate 7 tied to you and overreaching and then you go into VI and this what's called snout here into the zinc the strip goes from the furnace exit into the zinc box literally if you don't don't get into liquid sank at the time typically for 64 70 degree C zinc bath and where gets coated with zinc you control the thickness here will look into this and you cool district school district and then you do final call with water quench for instance and then you go to the drying and the and this can pass and the oil application of 2 years the on the surface if that's necessary and it's a typical lines will process In this line here could process 1 . 6 meter 1 . 8 meters is quite common is stripped thickness is also limited in this particular case 2 . 5 mm production is typically needs 350 350 thousand tons per year this that means that you can do process about 80 Thomas Brower of steel that how is this determined all that's the deed do the heating capacity of your the 1st is how much tons of steel you can and yield per hour and the temperatures here that you can reach would to region you furnaces typically h 100 and 20
seriously it again the section of this line but then with a few more details so how does this radiant tube furnace look alike while you've got the students here CDs S shaped tubes and S & M basically in these juices picture here you you have birds that will seek to the interior all of these troops and that and they'll be red hot and they will heed the district that moves In between these this tubes here the debate the cooling and in particular the fast cooling section as a set consists of the areas where you blow large amounts of the gas atmosphere nitrogen hydrogen gas atmosphere on to destruct and in order to avoid a build-up of pressure indeed indifferent as you also need to remove all theirs extra gas because that's when you have lot of these just as big tubing associated with the fast and you see going from the snout into the the missing about them out right so this is
a fuel here off the the coating area with his victory you this is the snout here you can't see the strip disappearing under the under the sink in so let's look at the schematic district comes under the zinc indeed think pot there is a role yes rolled around which the strip turns and it passes gas wipers that's how you control the thickness is basically by having gas fibers or air knives as we call them which is below very sharp on the stream of gas before air or nitrogen on this on the same liquid sink and Canton and thereby control the thickness of the SEC it's actually quite a quite accurate and there are hot gages here which will measure the tickets that you obtain and controlled the these gas wipers but if you want to make Galvin yields sink IRA the way it works is very simple you and Neil this new alloy basically the same and the buyer and you do this in a Galvin kneeling furnace which is followed by holdings of the Gulf annealing furnace the strip resides about 20 seconds In this Calvin kneeling furnace in the governing link furnaces typically at around 500 deg C so you don't have to do a long heating all are the very air X in the very high temperature heat and because the zinc and the IRA readily react very quickly the react to form these solid interim intermetallics so that brings us to the question when you Duncan the stretch into but when you dunks to steal into liquid saying just pure liquid saying what happens almost instantly is because it because irony reacts very strongly wait as saying you instantly for intermetallic compound very quickly a lot of 2 years so stress resides about 5 seconds 5 seconds in the back that's enough 2 but to have a lot of intermetallics for years and if you form intermetallics yes you cannot control the thickness yes because it's a very um burst like reaction so why does it work why does that work quite as this process work this process works because this is not a pure saying this the same contains about between 0 . 1 0 . 1 the sink and a 0 . 1 to 0 . 2 per cent of aluminum ends when you introduced these strip in the zinc that contains a little bit aluminum has delivered of aluminum the reaction between saying this series between aluminum and iron is the first one to happen I knew for In a solid layer of irony aluminum intermetallic 1st very thin layers the solid thin layer of
irony of 2 the irony aluminum intermetallic at the interface so let me just show you what I mean when you but U.S. Steel In 2 designing that contains a little bit of aluminum you for a very thin layer of irony aluminum intermetallic layer which we call the inhibition later and why do we call it the inhibition later we call individually because it's a process the reaction between irony and saying very effective effective and that's why the whole holiday galvanizing process works yes is because of this this and this and this this fraction of a per cent of and so it seems and that's why although the process from the outside looks very simple the it's actually very important to manage this zinc back to manage but what happens in this saying that very precisely so let's have a look at some more details so and then we talk about this and that management is what does it involve a default did control of the effective aluminum in the back effective aluminum beating the aluminum In solution in zinc yes so that you can time the aluminum additions to isn't bad but you also want to know what is the solutes Irish content they knew about this and you also need to control do what how things are going with the bad hardware because the number of their pieces of equipment here that are submerged in zinc continuously and and it's important that they get replaced them to write more so of course when you put Byron in as saying but there will not be there will always some dissolution of the irony in music said so this is Inc composition will be saturated Irish it's it's it's not only do what do we have alumina and aluminum that is added to that saying we also have elements that come from the From the steel that's sold 1 of the elements the debt that is when you when you make a new sink baths yes France's no magnate manganese in using That yes but over time the the the bats will pick up Irish and manganese and chrome Newman whatever allying elements that you had in the state so so but the most important things we we see forming in the using bad air which we call drops dross particles and so in years in using bad you have iron and aluminum well iron aluminum and zinc and 4 compounds which we call Top yes In case and at the top draws there has if it is stock draws because it floats it has a lower density than zinc so it floats them which you can also form bottom dross and these are Byron zinc compounds are in sync 13 iron zinc 7 years and therefore so did solutes irony with the saying that forms this Bottom dross so we need to control these beasts that Top Trost has to be removed manually from this from the surface of the of the back him endit so you you can control this uh new of course don't want to be producing too much of this draws the of UC France's a particle so that you take a sample of this you quench its yes and anew you do the Madelon review of this zinc has and you see very often these particles here you have a big particle of bottom drawers which smaller particles of top dross In In your saying I think so
and the deep keep part to be a losing battle management is to know what are they the conditions in which you run do vast so and usually it's done with this particular diagram yes it's a diagram that's that's basically these you know if you have to really you know from your Of course is stepped and thermodynamics when you have the binary alloy right you'll have for instance this is dire saying yes as a function of temperature you'll have lines like this if you I have a Ternary alloy IRA and zinc aluminum has then you will have for each temperature you will make isothermal sections the Thurner we phase diagram while this is such a directory of phase diagram and it's basically the zinc Raich corridor Of this space diagram at a temperature of 465 which is a very common bath temperature and instead of having here and uncle of 60 degrees you make this a straight on goal yes it's more practical an and and in this particular case you plot the iron content and the aluminum content in the except that it's basically a tertiary and distinct rich corner of eternally phase diagram and of course if you add very little Ireland over a little of aluminum they'll be in solution so there is in the very corner of In this very corner here right you'll have a pure saying aluminum and iron in solid support beams in to the notice something In another in solution liquids with zinc in solution that is it right and as it in there In depending on your bath situation yes you can have a liquid plus but this is the interface or a liquid persist delta phase or liquid plus this at the two-wheeled 5 faces so this is what we see here right and when we took a sample we saw Delta faces we saw yield 205 and we sell same right so that means Lakewood saying Delta phase if he too so that means are Byron of an aluminum content of bad must have been somewhere in this area yes 2 because I see all these faces as they right so that what we do I Bloomberg In early galvanizing we have will actually control the aluminum content and will control at between 2 main values the values in this range and a value in this range is and no value arrange and a high value in the range here of the about 0 . 13 0 . 12 0 . 14 and arrange curable .period to this range is used when we want to make gals aluminum excision of Calvin yield Excuse me cover against the law aluminum content the high aluminum company is used when we want to make galvanized steel we intend to what happens when we win we increase the aluminum bat content from from this value to this value well well 1st of all you can see we lowered the solubility of fire right the conceded but the other thing with the compounds that we 4 yes and instead of being Delta will be as the 2 will fight in the In the zing back as before we take a sample of my zinc here and I will find dealt effects which is bottom drops if I'm in here I will find the effort to whale 5 this top drops look at and and and this is very important there is a change in the alloy being kind this and in fact that is the main reason why we use these lower aluminum content is because we can't have faster reaction between zinc and the Irish you can easily How do you know of the sink and tyrants and make a Galvin yield later public interest right so so did
so this is the aluminum control the idea again low for Galvin healing so that you don't inhibit the iron zinc reaction to much that's because you need to half the the reaction and high 0 . 2 % when we want strong inhibition of the iron zinc reaction now how can we how is this but how is this is done this aluminum situation you know because when you adapt this trip into do saying back the 1st thing that happens is a reaction between iron and aluminum you basically taking out aluminum continuously so you need to track the aluminum level in your back yes adds aluminum so How do you can you add aluminum so you track the composition of your aluminum you can do you can pretty belts the summer aluminum rich I sink and then added to using bad or not and that happens most of the time is you add borrowers little bars of zinc aluminum alloy to do the molten saying yes and that and you call them brightening bars brightening bars and so that's also tightened to very nicely again you need to do do you control the process right now these I don't have a number here but I know
don't if my memory serves me right but the same bats are huge right this moment not just a little bit of saying it's here you talking about 200 50 tons of liquid say right at that time so you know what's when we say the temperature is 465 the temperature is actually very whole genius of distributed them so these are the sum the modeling results and experiments show that they're quite right you have no if so for a temperature of about 450 guests you that there there were destroyed planters you're slightly higher hotter but about 1 degree Close to the back wall you a little bit cooler but most of this track experiences very small temperature a variations but what about when you add a little aluminum to the saying that so you will add that usually you added 2 words the back here of you will add blocks yes OK and you can see you typically 1 . 13 per cent of aluminum there where you add the the aluminum Richard zinc iron blocks you have but put 14 per cent but very quickly the whole thing it's homogenized do you need to add a lot of heat to this molten saying No you don't because it's basically heated up by the steel strip that is the war years and its 1 . and of course to start to sink uh melting yes you do have stuff to sink too well but that there so there are induction heat heaters so you know it's it's basically an induction heater heated up in back think about it so in terms of temperature and composition leasing Bassett are actually quite Homewood genius here as I
said to there you have very efficient mixing because he had the strip that move so dizzying that and also because you have in doctors for the Heat and control here for instance some
some typical things you would do it during zinc bath measurement and management you out and use you can sample the bath at different heights in the India Anderson bad would indeed aluminum and zinc content is the aluminum and iron content yes and what you should find which is you always find the specific back years is that the data should line up lineup and the intersection of uh the intersection where the solubility line that will give you the effective aluminum in the sing them so directive alone in this case is . 4 2 and so so what's important again and that uses a problem that you that you have very often is that the composition of a phase of of should be the composition of a sample yes for instance here if you take a about sample there was some of the aluminum will be present as a precipitate yes and some of the aluminum will be present in solution b what is important for the galvanizing process is the aluminum bats in solution so that's given by the yellow .period "quotation mark because just before the thickness control units but when you come out of the zinc bath using thickness is actually quite quite large can be found you know a few hundred my and after the zinc 94 yes you can reduce it to less than 10 microns quite accurately until now the
there are many things happening in and that are of importance In there in this process at the surface and at the interface and I just want to point out to you that when you enter III continuous annealing furnace yes it's very important that In order to make the the zinc hot dipping process to work you must reduce all the oxides at the surface there cannot be any oxide said so but in the furnace when you have any of the area atmosphere that contains about 5 per cent hydrogen you will reduce all the iron oxide but you will not reduce manganese oxide you will not reduce silicon oxides chromium oxide etc. so do you have selective oxidation so you your IRA is reduced to you have metallic iron at the surface but your manganese Silicon etc. following elements are usually present as surface oxide In OK so you want to minimize this the very fact In this now to the strip is exposed to zinc metal vapor in this snout some of the surface Of the Steelers dissolved and you form and in addition later yes the aluminum reacts with district then you come out you get wet final wetting and solidification and then you get if you do the Galvin healing you 1st reaction is to break down the In addition layer and then he it's followed by the very rapid fire sink reaction can a typical the cycle shown here knew he'd up to 800 you get lover rearing crystallization of the texture of development and bring growth in you cool down if needed and that depends on the the type of material that is being processed in your line this and this a particular case that would be a low-carbon steels you do over you hot dipped in the zinc about and then you hear reheat To do the IRA and sink reaction for Galvin if you don't do any Galvin yelling you you cool down after the hot dipping in this and but so is dipping here is typically a few seconds 5 seconds this Galvin dealing is also very sure typically 20 seconds so the whole process but of annealing and we crystallization the and coating this is not much longer than any the then what you see in a continuous annealing alike having said this it's important for you to realize that if you're ever I'm involved in working in there the galvanizing line or in continuous annealing lines is the most lines are different tests and most lines will have different slightly different I thermal cycles that they can apply this was the continuous this is an example of continuous annealing line with overeating and so there is no the coating going on this is a hotbed galvanizing line without all 4 aging processes only by have still so very short you the process this is a hard deadline with over answer and this is an advanced hold that line where you can do cooling and heating yes In the in-line years 2 obtained different Mike rastructure 2 you can't not only hot dipped cold-rolled strip but you can also hot dip hot-rolled strip why would you do this well because there are many applications were it's interesting too In the data constructional applications were it's interesting to note provide this type of material it's various interesting type of material well because it's very cost-effective why when you hot rolled a low-carbon steel in the hot ruled that state the material is we crystallized so you don't need to do a week crystallization and knee-length it's enough to reheat the district I 2 temperature that's high enough so you can galvanize yes that is called heat to coat he took cold that's 1 thing the other thing is In terms of the pretreatment all of this trend you have to do a pickling that's because the hot strip is oxidized so these lines where you do the at the galvanizing of hot stretch still well you'll start by pickling after that you do drying and rinsing and then you go to a furnace yet where you heat the new stable homogenize temperature and then cool to coach the district in the Moulton said yes and the temperature where you go up to it is not the 800 degrees with a 6 and agrees so much lower temperatures in this an example here and because the temperature is so much lower you don't need huge furnaces there's a small induction furnace is enough so this is a very In comparison to standard I hope galvanizing line for cold stress is very very small in relatively small investment Clinton OK let's have a look at how this Gulf annealing goes yes In Delta Lake yes so what what's been done here is the the reaction has been stopped at the very beginning of the year at the end and what you see is that when you reheat you're Galvin New galvanized sheet has it's very quickly the In addition layered disappears yes thank you get very very burst strong burst like reaction between design and the IRA and after 20 minutes at 20 minutes just 20 seconds to very quickly this coating which was originally a pure saying is replaced by an intermetallic called the Delta layer and between the Delta layer and the Iron there is the gamma later and usually this is the time the coating that you want to have to put you help with not too much gamut later because it's a very brittle layer so if if
you look at the micro structure of a Galvin yield the galvanizing the Gulf yield coating so galvanized you have steeled New have on top of you still have a very thin inhibition layer I love this should be my what mm I know correct this distributing Mike wrong here this mixture the 2
this year is wrong should the microloans and then on top of this you have a zinc yes which contains of course aluminum which we have added as a lawyer in element to form visionary and it's saturated in Irish also about 0 . 0 5 mass persons and on top of it yes we form a very thin aluminum oxide layer which later was when we do we'll wind up there material Galvin yields yes this structure is replaced by the thin gamma phase later the interface yes then most of the coating consists of the delta phase In about 8 to 11 per cent of our and then on top there may be some residues of this Zita ferries but usually offered presence at this inhibition layer if you want to know how it looks like and I just to prove to you that it's a very home watching nears yes it's really a layer that covers the entire surface very effectively and prevents reaction between zinc and Irish you can see it here you can actually be extracted and put it under a microscope and you can see it's very very I very very thin but also it covers the entire surface to there is no contact between zinc and the industry so bad
this uh Gulf annealing reaction yeah that's a lot of things happen in these 20 seconds and so is also very important to control this reaction because you you don't want you really want to have a layer yes which contains the maximum amount of this Delta ferries yes that's so you so what happens during the the reaction well this is the pure saying disappearance quickly then the Zita layer Zita intermetallic is formed and disappears the Delta layer is forums this form to me then reaches a maximum and then there you have to stop the reaction here and there you have to exit the furnace that's that's that's the right moment to stop why because Gamal layer increases and it's flat during this is formed in that it doesn't increase for awhile that's because you formed Delta and then as soon as you the Delta decreases then you have a very quick increase in the gamma phase and this is not a good face to have in your coating because it it's and brittle the coating very much and you can see
that if you make for instance little cups Out of Calvin yield yes and you measured the amount of wait Of the cup June 2 powdery you too the powdering of the of the coating you see that once you reach about 10 I weight per cent in the Code of irony in this area in the in the coating if you go beyond there you just powdering increases very much and that's the reason for that is because you have increasing amounts of gamma phase in Yokota you have to make sure you stay below 10 or at about 10 per cent in coating the and and that corresponds basically to this situation where you have not too much gamma here and in Europe in the highest possible amount of Delta
directions just a few words about other coatings the IRA in addition to the zinc aluminum coatings 5 per cent aluminum coatings there are also hot dip coatings they use the same type of lines to do decoding I know this is a U tactic so you can see the mike structure Of the a of the new tactic is a new season very nice Lamela Mike rastructure them it has a primary Zinc Matrix and the darker areas are aluminum rich and when we have this very nice Lamela rolled like Mike rastructure these coatings on in situations where we I want to have to cook public protection from designing but we also want need a coating that does not crack as easily as pure as saying Pierce is very good to in terms of protecting the steel against Galvin the Indian dense corrosion however it's not very good in applications where you where you have to band In Bender the formation which because it cracks it basically is the cracks easily but that is not the case with when you add aluminum to distinct and in particular in these zinc 5 cent aluminum coatings to cracking is very small amount of crack so there are uh you know applications where that's very much needed another hot dip
coating is the um alumina ice coating with 10 cent silica yes I where you see here aluminum of primary aluminum dendrites are formed during solidification and you have also your aluminum silicon new tactics now let's go back these coatings are also made in public life basically and we don't call them called the galvanizing logical and what the alumina rising life now remember I told you that when you dead the irony in saying yes zinc and iron will react very quickly yes to form an intermetallic crystals as an and we need to add a little bit of alumina to form the inhibition later this but we can't add very much aluminum because again we would get a lot of reactions yes so what do we do in this case because here we did pure irony that a fire into the IRA steel into an aluminum bat yes again the reactivity is extreme as very high right reactivity and so the aluminum will react wit irony very quickly instantly to 4 intermetallics yes so how do we avoid that is the reason why we use this silica in this case this silica here as well together with the aluminum and form this intermetallic layer it's a thicker layer here but it's also and it also works as an inhibition later has to when a new strip comes out of the aluminum raising that yes it's steel then you have this inhibition layer and then you have the quit aluminum rising liquid on top of it you pass the the error nights so you can control the thickness nicely as if it was full of intermetallics You'll never you would never be able to do this us they would never be able to achieve the nice and visual appearance students of this this this intermetallic here this is an innovation layer and it basically that's the reason why we had to survive colluded
products there in a multitude of coated products has available From the steel industry parliament and it doesn't stop with zinc and zinc alloy coatings on aluminum aluminum alloy coatings but it also goes beyond that there is a 2 steals are covered with organic coatings with paint layers and so this is an example here yes you will have steel sheet and it will have a metallic coating for instance the zinc is on top of that it will have be phosphate layer or ChromaID made free pretreatment some kind of an organic later here yes then you'll have an organic primer yes filler or whatever or base coat and then you'll have your topcoat and this topcoat is usually only applied on 1 side yes on the exterior but this decided will be visible on the site it's not visible you have an organic back over this been great for instance can these and the materials are also produced in continuous wines yes which we where we apply these paint layers using special coaches in this this is you can see here this the coating unit paint coating I said this and the type of line is called a coil coating facility and usually you need what you bring in it is in the zinc coated with the material In and this is coaches this is used is called can Cota here again the type of coatings that are applied can be very diverse yes for instance is an example here where the organic coating includes zinc particles yes would think particles you have a very corrosion resistance the system later coating layer on top of the just in
relation to the paint there's but you cannot put paint directly on safe because whenever you see places where the the I paint layers just falls off From the steel it's usually because people have put painterly rights on 2 this thing coated steel yes that's very bad it's very very bad it's extremely important that the coating layers but I can but attached themselves to the sector has and for that you always need generally nowadays prosperity and phosphate that makes very tender yes a layer of phosphate crystals yes between the zinc and the painterly yes and that is used as this is used to anchor ankara b on the paint layer and there are different types of love phosphate crystals and usually they will contain zinc manganese phosphate they don't contain chromium these days so there's the environmental impact of these letters is very very small and you can see Chrysler graphically musical force Filide and hoping yes phosphate layers very portends that play a key role there very tiny layers and in in certain cases there actually applied on top of that can be applied on steals or on zinc to improve the friction property properties of the Of to steel the phosphate layers are known to drastically reduce friction and that's why they're sometimes used by a In particular in automotive construction that some of the car makers like to use preheat a cold pretty phosphate and stress because it reduces the friction and when it reduces the friction it also reduces the powdering during the died so tickets so we've
we've talked about it aspects of corrosion is on stilts and it is it is a problem that is basically solved years I don't think there so there many technical and engineering solutions for that and so there is a lot a lot of very large technology there in in that area that's that's available and that can be applied but 1 of the the key elements there is is that the use of I'm very cost-effective metallic gold thanks in particular zinc and zinc alloys and aluminum and aluminum alloys is to we've discussed mainly the structure of galvanized and Galvin coatings and also the technologies about this look at so it's a just a few
minutes past 4 so we about midway have just finished his parts of I will learn let's take a five-minute break
and then reconvene here 10 past 4 years this
go head to head the but
it so
what I was hoping to doing that university of news talk a lot about mechanical behavior of steel by I want to let you know that there there is a chorus you I have to I think it's 6 6 9 yes but I will teach In this this fall which is specifically about mechanical behavior of steel With wasn't very much in much the details and it's over To the southerners really not the aim to talk about everything just a bit the essentials and also make sure that you have some background material In your notes for your own information should use it later so it's mainly for the people who will not were not really interested in taking on 6 6 9 in the fall so that they have been a little bit more about how chemical composition can impact or Mike restrictions will impact the the mechanical properties can still go rather quickly over the material and so let's
start Wilson a few things about the elastic and plastic behavior role of crystal plasticity again I will so make sure that it's you know it is is always important certainly at your level to know there the history sales is very much made possible by this locations related mechanisms yes and that these dislocation mechanisms are very specific in low carbon steel in very specific in in Austin that takes steals and so a and then you should be aware of this unique is that many things that apply 2 far-right that do not apply to also let's say we take for instance the very simple example manganese is a very common alloy in element to fatigue steals when you add manganese yes you will do many things to steal 1 of the things you lose you make it more hardened both that but also you will make this deal was stronger and stronger by solid solution hardened and efficient element in that's says however you cannot say that manganese is a solid solutions for ignorant all Semitic steals actually it's not at all you can add as much manganese as you want to a specific stainless-steel it's not going to change the structure OK so very dense the just just 1 of the many examples where to illustrate the difference between Austinite and fairer and and of course there is also the crystallographic texture that that impacts on the the material behavior can top
general you know we're all familiar with elasticity and would long the EDI actual elastic the actual Nova in general books law for force for irony or for the Al-Faran demand would look like this stress is matrix times id and so matrix for with elastic constants times In your strengths you can so these the constants that appear here yes this time the reason why we we only have 1 2 3 constants is because of cubic symmetry and uh so once you know these constants which determined constant experimentally you can actually use this generalized if you're interested for instance and the properties of start date and plastic properties of far-right for Austinite and you need to to have the the talks Lofgren and so we we we have data to these these parameters C 1 1 C 1 2 and C 4 4 are known to have been determined but there may be some variability in the constants that it's pretty pretty small but of course what can you do with these constants well 1 of the things you can do is you can determine elastic properties of single crystals as of a of far-right were also nite or alloys for to galleries or Austin Alice a single crystals the difference if you want to know the and on the the modular of a single crystals along the HK direction yes but this is the formula that will allow you to do this see the 1st parameter is just a combination of these 3 single crystals the elastic constants this parameter also and then you have here a strange factor where you have these elves and al-Sadr on owners direction co-sign contempt and for different directions here we 1 direction "quotation mark together so it's possible to determine the elastic a modulus in any directions you are interested what it what is the meaning of these 1 1 C 1 to C 4 4 well for instance and C 1 1 well if if this is a single crystal menu you stretch it in this direction elastic while it well this case Sigmar y y in musical to C 1 1 times epsilon why 1 the strain in the wider action was basically along 1 of the few directions but if I had no incentive I was doing extending the crystal in this direction I shared a crystal With force the sheer force tell why zinc has been the relation between tell wisely and the the sheer Downer y the ISS given here ways is equal to the C 4 4 times I Doumar why's so it's it's certain the C 4 4 is some kind is is a single crystal modulus disability that that day they have the simple the properties the C 1 2 Is this a bit more complicated yes and there is is related to the fact that if you were heaven tension in this direction and compression in this direction the fire the deformation of the the of of the a single crystal unit cell will be related to this parameter in some way that I don't want to discuss because then when they have physical properties physical meaning I said you can't do this and using this formula just showed you can calculate what is the modulus along the 1 0 or direction 1 1 0 direction 1 wonder and you see that was not well Our irony in this case the big differences you know along 1 0 0 the modulus is less than half the modulus along 1 1 1 so that means if you take it 1 1 1 Crystal 1 1 1 Cristo thank you you do inelastic
deformation there will be a very big very large difference between 1 1 1 oriented crystal and 1 oriented pressed 2 it also means that because the modulus is different yes not that you are steel as a crystal of is is not an isotropic material has exactly very I'm isotropic so inherently the from at Chrysler graphic point of view the Byron is not isotropic not 1 of the things that we are also interested in is saying well I certainly when you would deal with steals this well that's very nice to know the elastic modulus of a single crystal but might I never deal with single crystals make unless maybe you working with a grain oriented so steals but usually we work with pretty small grains just isn't possible To determine the of the modulus of the Holy crystallizing material if I know c 1 1 you want to and support for yes you can guess for instance these are the modulus for a single crystals what do you do for the modulus if you have a Pollack slang material of the 1st thing you have to Su is that your crystals a randomly oriented yes and then you have to assume something else you have to assume that the response of the material to an applied stress there are 2 ways to extreme cases that you can imagine is that when you do elastic deformation either all the grains will be subjected to the same strain or all grains will be subject to the same stressed the same strain is kind and nice because then you don't have compatibility problems the grains state because the older form the same way they can also stay in contact if there if they are subject to the same stressed yes that's a little bit conceptually a little bit more difficult because if there subject to the same stressed they are not subject to the same strain yes and so on basically there should be holes in your structure OK but let's assume that that doesn't happen and you can compute these 2 extremes there known as the Royce and avoid the case has a new conceived in 1 case it's 190 about 195 and in the other case it's 227 we don't know what's which 1 is really exact no but we know that the exact value is somewhere in between yes so we can take for instance the geometric average which is the square root of the product of 2 2 yes and we find 210 Giga Pascal yes and so that's very good so far so this approach is is excellent we and because we know the 200 And if you ever have to do a quick calculation and you don't know or you don't know you you you wonder what value should I use for a hopeful by modulus 210 is excellent value but it doesn't mean that it is 210 at the foot of the particular case you you wanted others with incidents is definitely a good starting the value and here you you see some summer values of the main elastic properties so the Young's modulus again usually use the values between 110 thousand 192 20 if if if the value that somebody uses out of that range you know you should check the shear modulus 75 to 85 typical range and the point source modulus is also given them so that when you do this of course you assume that your material steel is elastically isotropic consumers who use these parameters we that the
modulus of steel is not a you know a natural concept it's it varies it varies with composition and it and it's certainly varies with temperature you see that I saw you see here of some modulus they'd this as a function of temperature and uh so you can see here that you have about the a 10th of a giggle Pascal reduction Of the modulus 4 degrees as the material doesn't stay stiff as there as a measurement of the stiffest as it is at room temperature and and you can also see that there is a variability the related to the type of composition I also want it to remark that aluminum In comparison aluminum is very low much of and that's in general what we see With the materials and the light weight to my metals and alloys is that in comparison to steal the modulus as it is very much lower this and and that's it that has a big impact on the lighter-weight potential of these materials because very often but it's not only enough 2 change over from steel to light weight solution you also still have to have the same stiffness on the same strength yes and no just on the basis of stiffness already has it's impossible 2 I To use aluminum as such you need to use much thicker along with them to the lower modulus usually means that you need to use a lot more Of that lightweight materials and the light with potential is therefore much reduced I'm the is even lower and then polymer is a very very come and there is also compositional affected the modulus here for is of steel will decrease with 4 the amount of carbon that you but please note this is not a very much a large decrease of could discuss from 210 to 210 so that is not a big impact tickets
went and we moved to do the question of plasticity and we stand around of the crystallographic thing of you probably know that the uh NBC see Materials BCC metals and puerile for foreigners saying the case when you are when you do when you have plastic deformation you get to shear deformation and sharing of lattice planes over each other and in specific lactose directions and this this specific letters direction is 4 1 1 1 directions has in the specific lattice planes are 1 1 0 planes and so have indicated 1 playing here and I've indicated such a sheer direction from the price and so at this you can look at this uh this slept yes as a homogeneous left where you are as you as you select Over and 1 1 1 plane in 1 or to meet 1 1 0 planes in a 1 1 1 direction Europe you go through a maximum and then the minimum again when you're back in the registry is and this is a change in the sheer force you can have a sine curve go through this yes which which shown here this factor here they all 4 2 1 1 1 takes into account a period this city of the latter's look at and so if you do this it is as if you do
this you find very very large volumes 4 but the sheer force that is needed to classically deformed the crystals that Alpha crystals but if
you measure you know if you measure on single crystals for instance here you this is a single crystal of RUC here around this area like and a little bit and study here but there is a change in slope in curves this is where this alpha 1 1 1 In single crystal starts to deform plastic yes I measure the stress of about 6 thousand mega Pascal In the case of Alpha Arab my stress is that I think it's about here in the is about 3 thousand make a Pascal yes and that so this is for single crystals I can also do measurements where we and we look at 1 0 0 0 whiskers and whispers are very very fine crystals as for very tiny crystals single crystals and so here is is a function of whisker size is plotted then the yield strength because we that the strength at that which a fracture that's the and you can see that there is a huge variety a of data points here it's all over the place but in general the whiskers give me a much lower strength when they're very when they have a very high section yes and so that means something so 1st of all so when you compute the sheer strength and you assume it's a homogeneous cheered as you get very high values unrealistically high values which are much higher than the values that you measure and then but there appears to be at least for these West the dimensional effect on the on the strength of the single crystals of some and so that this is an example here they yield strength of theoretical yield strength it is about g divided by 2 so 2 pie so you remember I had about what sees 80 AT dig a pastel 14 and to pipe is about 6 Sewell .period gives me what is it wants and 2 1 2 years about 3 something right to get about 3 13 at 12 bigger Pascal that strength now it's clear that my the measurements here my single crystals yes were much lower than in the best
case and if you look here at the very very fine whiskers they will go on we hear that the highest values about 8 thousand 500 minutes so the maximum values that you measure on single crystals will be about 10 bigger Pascal's so but if you actually measure on the eve of the public crystallize piece of pure irony you'll you the values of much much lower than are typically so opened all four-digit passcodes so we're talking about 40 48 the make Pascoe the tiny yes so on this on this graph yes it's about here and and so the reason is that well there's something that happens in the crystal that makes plastic deformation very varies easy and and and so and and that those of the dislocations this locations and and you probably know this from you of course you can think of dislocation as and localized amount of defamation that you can propagate through the crystals that's and it's this this and this packet of information that you move through the crystal that uh today to call this dislocation and the way we like to presenters formerly is by saying it's as if you had an extra half playing in them in the crystal structure so so if you want to share this the block of this single crystals of iron for instance you and you want to share it on this playing tennis in this direction with the AU you squeeze on you for a a plane of of volume of material that used to be here you squeeze it into yes this you squeeze it into the crystal so that you have an extra half playing in this in in the structure and this extra have blamed them travels through the crowd to the crystal and comes out on this that and on this point and when it does that I have shared the crystal I have plastic deformed it permanently White and and and I it's was very simple From the point of view of Chris lot crystallography to determine what to share is just basically be over each beaches this week is the amount of sheer carried by this the extra have played basically the winner of this extra have play so what what is important now this is how does this happen and it appears that in most of the crystal wine systems is a very small number of claims on which the sharing happens and is a very small number of directions in which the crystal will share and that's determined by the which would we called the Pyros stress Powell stressed we're going to go into this to the great detail but this is the piles stressed this is the equation for the pile stressed it's also called the lactose resistance the lattice resistance and so we get Shearer and in direction where the lattice resistance is small this and so that means slip is favored for large the spacing and small Burgers vectors and Westerberg respectable that's the small slipped associated with the the weight of the extra half played Francis Sosa when at what is happening for instance if I if I pull in a 1 0 0 0 grew grain yes but a certain forest units what actually happens are these would recall edge dislocation or extra have claims he has moved on 1 1 0 0 planes in 1 1 1 direction answers this this extra planes will will move out will move on this plane and then when it arrives here it pops out yes and you can see now that's when it pops out my original Crystal length has become larger yes by an amount determined by the burgers factor of this dislocation and so I can make a very simple relations between this strain rate yes the density of these dislocations so how many do I have per unit volume their velocity and the factor and this is the equation this is an example here of steel where you can see His dislocations In the far-right grains and they're responsible for the defamation and and it's important to want to stress here is that most of the steel all our students actually you the dislocations are responsible 4 almost all of history yes at room temperature at high temperature increase conditions things can be different but at room temperature whether you are talking about the trip steel or at which the or dual face steal or whatever you know moderate steals did we talk about you may have heard about and sometimes I'm not very surprised that this is not emphasized
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Metadaten

Formale Metadaten

Titel Modern Steel Products (2014) - Corrosion of coated steel: lecture 15
Serientitel Modern Steel Products
Teil 15 (2014)
Anzahl der Teile 31
Autor Cooman, Bruno C. de
Lizenz CC-Namensnennung 3.0 Unported:
Sie dürfen das Werk bzw. den Inhalt zu jedem legalen Zweck nutzen, verändern und in unveränderter oder veränderter Form vervielfältigen, verbreiten und öffentlich zugänglich machen, sofern Sie den Namen des Autors/Rechteinhabers in der von ihm festgelegten Weise nennen.
DOI 10.5446/18330
Herausgeber University of Cambridge
Erscheinungsjahr 2014
Sprache Englisch

Inhaltliche Metadaten

Fachgebiet Technik
Abstract Professor de Cooman builds on the previous lectures in the sequence, this time dealing with the hot-strip mill. The lecture then breaks into a new topic, on the mechanical behaviour of steel, elasticity in particular. This is a part of a course of lectures given at the Graduate Institute of Ferrous Technology, POSTECH, Republic of Korea.
Schlagwörter The Graduate Institute of Ferrous Technology (GIFT)

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