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Modern Steel Products (2014) - Formable steels: lecture 20

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Modern Steel Products (2014) - Formable steels: lecture 20
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20 (2014)
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Professor de Cooman takes the topic of formable steels further. This is a part of a course of lectures given at the Graduate Institute of Ferrous Technology, POSTECH, Republic of Korea.
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SteelFood storageSheet metalRailroad carPhotographic processingVehicleGasnitrierenScrewSurface miningDeep drawingRolling (metalworking)TypesettingHot workingVolumetric flow rateVacuum pumpEngineRoll formingWorkshopShip breakingSpare partBlast furnaceHüttenindustrieWork hardeningMaterialAfterburnerCartridge (firearms)StagecoachRutschungSizingTauTexturizingPlane (tool)BookbindingHot isostatic pressingFrictionKickstandMatrix (printing)MechanicVertical stabilizerFord TransitInternational Space StationKopfstützeAufnäherAlcohol proofDistribution boardRoute of administrationSteelCoatingSheet metalPhotographic processingMicroformScreen printingRolling (metalworking)TypesettingRapid transitHot workingAutomobileHang glidingAfterburnerStagecoachTexturizingSemi-finished casting productsPlane (tool)Hot isostatic pressingCasting defectTin canFullingMechanicVertical stabilizerFord TransitSehr kohlenstoffarmer StahlHydrostatische BeanspruchungDistribution boardComputer animationDiagramEngineering drawing
Transcript: English(auto-generated)
And I'd mentioned the fact that the precipitation sequence can be rather complex. So just to repeat what we said at the end of lecture
on last Thursday, so titanium will not only stabilize nitrogen at high temperature and is very efficient in doing this, but it will also
bind sulfur as a sulfide or as a carbo-sulfide. And I stress the fact that this carbo-sulfide is a really interesting precipitate because it's the only precipitate that binds carbon at high temperatures.
But it requires, because of its solubility product, it would require, for instance, the use of low reheating temperatures. And if you do that, you can precipitate it, carbon at high temperatures, and you do get better properties also.
It's only after, at lower temperatures, and in particular in the ferrite phase, that you really start to form titanium carbide. Hmm? Titanium carbide. Now, I also mentioned the fact that in standard IF steels,
titanium IF steels, you always add a considerable amount of excess titanium. That gives rise to surface defects, oxides related to this titanium excess. And so when you galvanize the material, I should add to this, so galvanized materials
have these surface defects. The steel industries then develop what is called a titanium niobium IF steels, where the nitrogen is stabilized by the titanium, and the carbon is stabilized by the niobium.
And there is no need for these large excess titanium values. So important here, I want to stress this, that the nitrides of titanium
are much less soluble than the carbides. And the reason why you have to wait for the formation of titanium carbide that you are in ferrite
is the drop in solubility. When you go from gamma to alpha, there's a serious drop in solubility of the titanium carbide, and you form precipitates. So let's have a look here at a number of typical grades
that are produced. So we'll look at some standards, first a number of European standards. Low carbon steels, IF steels, are steels that develop for formability,
for their formability, a sheet material mainly. So deep drawing qualities, so you know that these, according to this standard, the first D refers to drawing qualities.
The second D here is not drawing, it refers to the fact that it's hot rolled. So we have about, well, the main grades are these four ones here. So D stands for, in this case, it's a table that reviews the main hot rolled low carbon grade.
So the first D stands for drawing, the secondary stands for hot rolled. And the numbers here, 11, 12, 13, and 14, are just numbers. They refer to increasing formability, going from 11 to 14. And this is achieved by, not surprisingly,
a reduction in the alloy content. So you see here, the maximum amount of carbon is reduced. The maximum of manganese, phosphorus, and sulfur all reduced for formability. OK, what are the properties?
Well, let's look at the properties that would be used in automotive applications for thicknesses 1.5 to 2, 170 to 300 yield strengths, and tensile strengths less than 400 megapascal.
And elongations, total elongations, are very large, more than 20%. And in the last one here, this DD 14, more than 30%. These are not typical values, right? These are the values set by the standards.
So the properties can be much better in practice, and we'll see a few numbers here. Same here, hot rolled grades according to GIST standards that you can compare.
So you remember here for GIST, S stands for steel, and P stands for plain carbon steels, and then these C here stands for commercial drawing, et cetera, extra drawing. And the H here is for hot rolled. So that way, this list allows
you to compare these two types of grade. US, some typical grades here. You have drawing steel, DS, type A, and type B. Again, low carbon contents, low values.
Again, these are low carbon steels. They're not IF steel. So you're looking at 200 to 400 ppm carbon, typically. Low manganese, less than 0.5. Low, where is silicon here? I don't have silicon, but that's low, too.
Less than 0.1. So very little alloying at all, yes? And these are typical values in terms of the tensile strength. If you want to know values, you have to multiply by about 7. If you want equivalent megapascal values, so that would be 200 to 350 megapascal.
OK, so comparable to what the European norms give. You have formable hot rolled grades. You have formable cold rolled grades.
Formable cold rolled grades. Again, if you look at the European standards, D for drawing, C for cold rolling. And then the numbers 1, 3, 4, 5, 6 here just refer to increasing formability. And you see here that only the DC06 contains titanium.
So that means that's the only one that requires the carbon and the nitrogen to be stabilized. OK.
These grades have to satisfy a requirement that they're not aging for six months. Why would that be a requirement? Well, many customers will buy coils, yes? And then store them for a while till they need them,
yes? So that storage period can be a few months. For instance, if it's automotive, a company doing buying, so they'll store them. And so they want to make sure that there is no strain aging when they start processing this material.
So that's this additional requirement. And again, titanium, usually indicated in these standards, can be substituted by niobium to stabilize carbon and nitrogen usually. You will actually see a combination
of titanium and niobium. So if we look at the properties, you see that these steels are softer than their hot rolled counterpart, yes? 140, 120 to maximum here of 280.
10 cell strengths, 270 to 350 typically. And the elongations must be in excess of 38 to 40. So that's a very large elongation. In the cold rolled grades, you have also requirements in terms of the strain
hardening, the N value, and the R value. Note that the R value is also indicated in what direction you have to take it because of anisotropy. So you have to take it in the direction 90. And remember, 90 degrees to the rolling direction,
that's usually the direction where you will measure the highest R value. And you see, of course, that as I go from DC01 to DC06, I get an increased R value so the materials become more formable as I go from low carbon steels to IF steels.
I want to remind you of the fact that this one here is a titanium IF steel or an equivalent grade containing both titanium and niobium.
So this is the equivalent ASTM. Here you have the drawing steels A and B, deep drawing steel and extra deep drawing steel. And the last one here is an IF steel.
You can see the requirement is less than 200 PPM of carbon. And you can see here the alloying of the addition of titanium and niobium is compatible with the stabilization
of carbon and nitrogen. Again, please note the fact that these contents of copper, nickel, moly, vanadium, et cetera, these are basically background values. There's no addition.
These steels are really not alloyed with manganese or silicon. You don't want to do this because these steels, that would make them harder. And you want soft, very formable steels. Again, here the American standards
for these cold roll grades. Again, very large elongations, high R values. In this case, the mean value of the normal anisotropy is specified.
But again, large values. In the case of the extra deep drawing steels, the mean R value has to be 1.7 to 2.1. So that's really high. And also, a large, high strain hardening exponent. And so here, if you would want
to compare these different grades, Europe, US, and Japan, you can see here how you can compare them. So very important here, the EDDS, DC06.
And the Japanese SBCG. These are all non-aging steels, absolutely non-aging. So they are IF steels, basically. They are required to be IF steels.
OK, so we have seen that we have these ferritic steels that are very formable, are high R values, very low yield strengths. And these steels are used because they're very nice
to work with in a press shop. You can make very complex shapes reliably, in terms of dimensionally. But they're soft. So what happens is that if you hit, for instance,
a car body with something hard, it will leave a dent, yes? Like this. Now in Korea, people seem to be very insensitive about dents, yes? But in North America and Europe, people can be upset about these things very much, yes?
And very, very much. So don't ever dent a car in America or Europe. You're going to be in trouble, yes? But in Korea, people seem to be much less sensitive, yes? But anyway, these steels are particularly sensitive to this
because they're so soft. And so big hardening steels have been developed to try to increase the yield strength of the material, but increase it after the press hardening.
You could make a stronger steel, but that's not what you want to do. So the big hardening is basically a steel where you actually use static strain aging, the aging effect, to create a slightly stronger steel after you have made
after you have made, for instance, the doors. And how does it work? Well, when you make panels, so you get your steel, it goes through presses, yes? For instance, this is the side of a car here, yes?
And then it gets assembled, welded to a body. And then you go and you paint the, you put the paint system on the car. And then you bake the paint, and the paint baking,
so you basically dry the paint. And the paint baking is a low temperature aging process, yes? So it will give you an annealing, basically
low temperature annealing, of your steel, typically 150 to slightly above 200, yes? And of the order of minutes, 20 minutes, yes? So 20 minutes. Yes.
And so that's actually a situation where we get aging in steels that can age. So this is the idea of this big hardening steel. So you start with a sheet, say that's, for instance,
200 megapascal strong, yes? You work harden it when you press form it, yes? So that's about 250, yeah? And then you do a paint baking. So after you, for instance, made a door, yes, a door panel, it's paint baked,
and you add an extra 50 megapascal. So your steel in the car body is now 200 megapascal. So that is the idea of the big hardening. And so what happens in practice, so in the sheet here,
you have some free carbon atoms, and the challenge now in these big hardening steels is to have a very well-defined concentration of carbon atoms so that the material doesn't age, yes,
before you do the press hardening, but ages rapidly, yes, after the press forming and during the paint baking. And what happens is, of course, the carbon atoms, when you do the work hardening, you have dislocations, yes?
And when you do the paint baking, the atoms will lock these dislocation into position, yes? And so if you would try to dent the car now, you will have the strength will be not here, but here,
200 megapascal stronger as a result of the work hardening and the paint baking, yes? All right, so we know how much you
need to dissolve carbon in, get carbon in solution, right? So we know we've got a maximum 200 ppm is possible, yes? These carbon atoms, yes, 200 ppm is way too much. These carbon atoms that we need, yes,
have to lock the dislocations that you form after a small amount of straining. Exterior panels are typically strained a few percents, yes? Not 40%, a few percent. So your dislocation density is relatively low, yes?
Right, and so these carbon atoms, what do they do? Well, say you have, you know, this is a unit cell here, yes, in BCC.
A dislocation that lies along the 111 direction is a screw dislocation, yes? If it has its Burgers factor, 111 Burgers factor, parallel to this same 111 direction. You can see the slip plane here of this dislocation is this 110 type glide plane.
This glide plane here, to this dislocation, good. And so this dislocation can now interact with carbon atoms that are not too far away from it. And these are in interstitial positions.
So you've got here, here, and here, three interstitial atoms, for instance, that can interact with the dislocations, OK? OK, and what they will do is you have a lattice distortion from the screw dislocation,
and you have a lattice distortion from the carbon atom, a tetragonal distortion, you remember. And that will cause the carbon atoms to be attracted to the screw dislocations and the edge dislocations, yes? And as a consequence, the carbon atoms
gather at the dislocation core. For instance, if it's an edge dislocation, they will tend to be in the tension part of the dislocation core. So you'll get here, where you have tension,
the carbon atoms can gather. Now, the carbon, it's the situation, yes, in practice is that what happens at room temperature when
I make a panel, yes, of carbon atoms, and I've introduced some dislocation. What happens at room temperature? Well, at room temperature, there is a phenomenon that we call snook ordering. Snook ordering.
Snook ordering. And what is that? That is the carbon atoms that are in the immediate vicinity of the dislocations will make a few diffusion hops, yes,
towards the dislocation. That happens very quickly, yes, because remember, carbon jumps about one times per second in the iron lattice, BCC lattice, yeah? So you introduce dislocations. If the carbon atom is close enough to the dislocation, it will hop to the dislocation and pin it, yes?
So that's what happens. But then, pretty much, that's about it, yes? That's about it. You would have to wait for many months and years to see anything in terms of serious strengthening, yes?
But if you heat up during the bake hardening process, you have two processes that happen. One process is carbon atoms moving to the dislocations,
So that is a diffusion process, yes? And what you can also have is carbon atoms moving together and forming carbides, for instance, cementite, yeah?
Now, the kinetics of these processes are not the same, yes? And this is shown here. So if you want to, if you say you plot the amount of carbon that's taken out of solution,
that's not interstitial anymore, it's proportional, it's equal to a fraction will go and form carbon atmosphere, and the rest will go and form carbide precipitates. So I get these are exponential laws.
You can derive them theoretically. The first one related to the formation of atmosphere is 1 minus exponential minus t divided by tau 1 to the power 2 thirds, yes?
And for the carbide precipitation, we have 1 minus exponential minus t divided by tau to the power 3 halves, yes? And the reason why you have these different exponents is due to the fact that this is purely diffusional,
and here you have a particle growth effect, yeah? Different processes, basically. So basically what it means is that if you look at,
say, this would be the carbon content, interstitial carbon content originally, and this is the time at a certain temperature, you will see there will be two steps in the removal of carbon
from the solution, yes? One will be usually the faster one, which is the atmosphere formation, and the second one is carbide formation.
Now when it comes to carbide formations, the question is what type of carbide do we form, yes? And so this is a diagram showing the kinetics,
the precipitation time temperature kinetics for carbon supersaturated ferrite at different temperature function of the time. And I want to remind you of the fact that the paint baking is done at 150 to 200 degrees C,
so it's around this temperature, yes? And the times are of the order of minutes, yes? So that would, yes, minutes. So if I have 20 minutes, if I'm right, that is,
what is it, 1,200 seconds? Something like this. Yes, 120, yeah? So about 1,000 seconds, yes? So 1,000 seconds is here, yeah? So 200, 1,000 seconds.
So we're not really forming cementite, but we're forming low temperature carbides, which we call transition carbides, yeah? And they're usually called etacarbides or epsilon carbides, you may have heard about them. We're not going to go into too much details
about these transition carbides, but they're not your regular cementite, yes? And so both these atmosphere formations and these precipitates give us strengthening, as you know from, right?
And if we look at these precipitates here, yes, the reason why they grow is because you have, if it were a cementite particle, is because around a little particle like this,
you create a diffusion profile, yes? Diffusion profile, which is here. So far away from the particle, you have your regular carbon content. The particle itself, say if it's cementite, it's got a very high carbon content, of course. And close to the interface, the ferrite cementite interface,
we assume we have equilibrium. And so the carbon content is dictated by the phase diagram. So as a consequence, because of this diffusion profile here,
there is a flow of carbon towards the particle. So you can calculate how much carbon needs to flow into the particle for growth.
It's been analyzed theoretically. There is a simple solution to this problem by assuming that this diffusion profile is linear, linearize this profile. And this allows you to determine the change of the radius of the particle as a function of time, yes?
And this is the very well-known relation. And once you know the radius of the particle, you can determine the volume of the carbide that you form as a function of time. And you see here, you find a function where the time
is to the power 3 halves. So this allows you to make simple calculations about how fast a particle will grow, or if you heat up, how fast particle will go into solution. OK, so good.
So anyway, we can, for instance, use this to study the over-aging, yes? Why is over-aging important in this for bake hardening steels?
Because a bake hardening steel, when you use a normal, low-carbon steel, normal, aluminum-killed, low-carbon steel, and you want to make a bake hardening steel out of this one, remember, the carbon content here
will be 200 to 400 ppm, very large. Yes? And so the way we work is by precipitating. Yes? You precipitate most of this carbon
during what's called the over-aging. Remember the over-aging? The over-aging. And here you can see, for instance, with the growth of these cementite particles
during the over-aging at 400 degrees C, yes? So you can really precipitate your carbon and keep a certain amount of, if you know the radius, you know the volume, and so you can calculate how much carbon you leave in solution
to give you the bake hardening effect. So you remember the over-aging and continuous annealing? So you have annealing like this. So here you precipitate your cementite,
but you leave a small amount of carbon in solution to give you the bake hardening. For instance, as shown here, you have your cementite particles and carbon in solution.
So if you have a bake hardening steel, you will have cementite particles and carbon atoms in solution. And you can make bake hardening grades
with batch annealing and with continuous annealing. And the most important one today is the one where you use the over-aging and continuous annealing.
And it's really important to control the precipitation of the cementite because you have these very large carbon contents in these low carbon steels. Yeah? Now, what about IF steels? So we're very happy about IF steels
because, well, you bind titanium, you bind carbon. So there's no problems in terms of properties along the strip. You have a very low yield strengths and tensile strengths. You have very high R values and work hardening rate. So it's a perfect steel, but it's fully stabilized.
Nitrogen is stabilized by titanium. Carbon is stabilized by titanium or by niobium. So you cannot make bake hardening steels because you stabilized it. OK, so that is where the idea of using niobium as a carbon
stabilizer also comes in. So let's look at the titanium or titanium niobium
IF steels. So when you have an IF steel, you have extremely low carbon contents. So this would be this 200 PPM, this is 100 PPM, this is 50 PPM.
So you have around this much carbon, 20 PPM. But all this carbon is stabilized. It's bound to carbon. So how do you work? Well, so you have the microstructure. Carbon is stabilized as niobium carbide.
So what do we do? We heat up, but we don't heat up to 700 degrees C. We heat up to really high temperatures, 800 degrees. Are we worried about making austenite? No, because we have so little carbon.
So the AE1 temperature becomes irrelevant. Now if you heat up, it's only alpha here. So there's no danger that you're going to get worse R values. So you heat up to very high temperatures.
And what happens here is that some of the niobium or some of the titanium carbide and niobium carbide will go back into solution and form carbon in solution again.
And now when we cool down, the solubility decreases very strongly. Some of the carbon may actually form carbides again. But if we cool down fast enough, about 50 degrees
per second, we can be left with a few PPM, less than 10 PPM, five or six PPM typically of carbon in solution. And that's enough to give you big hardening, good big hardening.
Alternatively, yes, some clever people have said, well, if we're going to make big hardening steels, let's not add so much niobium, or let's not add so much titanium.
Let's just leave carbon free at all times. And these are what are called ULC big hardening steels, ultra low carbon steels. So here we have extremely low carbon contents, and we don't stabilize.
We don't have to stabilize it. But the only way you can make these steels is by really being able to control your secondary metallurgy and have extremely low carbon contents. So in this case, you just reheat
to get your texture control and your grain growth of this ULC steel, and then you just do rapid cooling, keeping your carbon in solution. Again, you can perfectly control,
for instance, for these niobium, say these niobium carbide big hardening steels. We look, for instance, at the dissolution of niobium carbide during continuous annealing.
Say you have some niobium carbide. How long will it take for you to dissolve the niobium carbide? One minute, three hours? Well, you can calculate it. You can calculate it because we know how the kinetics of particle growth and particle dissolution.
So let's have a look, for instance. We have a big hardening steel, which has niobium as used to stabilize carbon. And say we have 150 ppm of niobium and 20 ppm of carbon. And we look at 800 degrees C. We look at the dissolution of the niobium carbide in ferrite.
So we need to have some data for the, so this should be niobium here. You could correct this. Diffusion constant for niobium in ferrite, yes.
And we need also data like the solubility product for niobium. Anyway, you don't have to look at the details. Just look at the results of a particle here that dissolves into the matrix.
And you can see here, it takes you about a little over two minutes to dissolve your niobium carbide. So you can do this in a controlled manner, yes.
And design your process around this data. And the kinetics, what controls the kinetics is the diffusion of niobium in the ferrite. Because when the niobium carbide goes into solution,
so you have carbon goes into solution and niobium goes into solution, yes. Now carbon is a really fast diffuser, yes. Really fast diffuser. So the dissolution of niobium carbon is not controlled by the carbon diffusion.
But it's controlled by niobium diffusing away from the niobium carbide particle. But you see, it's fast. Yes, it's fast. And you can do it in a continuous annealing furnace quickly.
Bake hardenable steels. If you look at the bake hardenable steel and a non-bake hardenable steel, the microstructure is actually the same. I mean, obviously, what makes a bake hardenable steel, bake hardenable are extreme, are PPM levels of carbon. So you're not going to see this in the microstructure.
So this is an example here of a bake hardenable steel. Typical ferrite grains, yes. So here, the kinetics, usually measuring
the kinetics of carbon in solution, it's not something you can do chemically. Because if you go to a lab and you say, OK,
this is a piece of steel. I think it's got 200 PPM of carbon. Please give me the carbon content. Then what usually happens is the chemist will basically destroy the sample, destroy the sample some way or another,
and then measure how much carbon is there in iron, et cetera, and magnesium, et cetera, manganese, et cetera. But if you go to the chemist and you say, this sample contains 200 PPM of carbon. Tell me how much carbon is in solid solution.
That is another. That's very difficult. Yes? And you need special techniques to analyze how much carbon is in solid solution. Typically, people will use a technique called internal friction. Because in internal friction, you can measure.
It's a technique that's sensitive to interstitial atoms like carbon or nitrogen. But it's not a simple technique. So what do we do? Well, we measure the mechanical properties, basically.
What we do is you will take your sample, your big, hardenable steel. This will be a big, hardenable steel. And you will pre-strain it. So here, I'm pre-straining my big, hardenable steel up
to this point. So in this case, I've strained it about how much? About 5%, or 0.05. And then I take this steel, and I'm aging it.
I'm aging it after pre-straining. That means I will take it. In this particular case, aging time, this particular thing, example here, was 170 degrees C. So 20 minutes at 170 degrees
C. So what you get then, you take the same material.
This is what you get. You get a material, of course, with a yield point and a yield point elongation because it's been aged. And we can now measure the difference here, this increase in strength from what's called the yield
plateau and the flow strength stress I had here. And we can measure this as a function of time, mechanically. And if we do this, for instance, for pre-stains of 5%, and we strain it 5%,
and then we heat it at 50 degrees, and we make 20 samples. And the first sample we take out after 10 seconds, after 30 seconds, after 60 seconds,
and we measure this delta sigma. So what we see is we see an increase in strength. And here there's a small plateau, and then it continues to increase, and then it seems to decrease. This is how bake hardening is evaluated.
Now, what is important for the industry is this. It's about 20 minutes. So this is 10 minutes, 20 minutes, 20 minutes at 50 degrees C. That's very, very low temperature.
Actually, I chose this temperature because it illustrated the fact that you have two levels of things happening here. First is the atmosphere formation, and then this stage here is the precipitation stage.
So firstly, you form atmospheres, then you form precipitates. And then the strength decrease here is because the precipitates continue to grow. They continue to grow, and they coarsen. And then you lose strengthening because they take away carbon,
and they become larger. And because they are larger, they provide less strength. Now, if you're ever involved in aging studies,
you will see that things are more complex in general. Why are they more complex? Because, for instance, we already talked about this. Very often, your steels are alloyed with elements that may interact with carbon.
So in addition to carbon in solid solution, carbon at dislocations, carbon at carbides, you also have carbon close to substitutional elements, which will behave differently.
And then what's really important, we also have boundaries, grain boundaries. And grain boundaries also act as sinks for carbon, yes? So when you ask your chemist, tell me how much solid solution carbon I have,
this is solid solution carbon, this is solid solution carbon, this is solid solution carbon, and this is solid solution carbon, yes? This is precipitated carbon, yes? So there are many different types of carbon in solid solution.
And they will all impact the mechanical properties differently. We know this, right? Carbon in grain boundaries probably causes the whole patch effect. Yeah? This one here, the aging effects.
This one here, precipitation hardening, et cetera. OK? So but let's have a look at grain boundary carbon in normal situations in a bake hardening steel. So if we have to look at what happens to bake hardening steel,
so here it's cold rolled, yes? We heat it up in continuous annealing, then we cool down quickly, yes? And then we over-aging, do the over-aging to precipitate carbide, and then we cool down to room temperature, OK?
And then it goes to the press shop, for instance, where it gets a small amount of deformation, a few percents, and then you go to the bake hardening. Bake hardening at typically, say, 170 degrees C,
and then it's quite long, 20 minutes, OK? So what happens here? Well, let's have a look at the carbon. We have carbon interstitial, carbon at dislocations, and carbon at grain boundaries, yes?
How does this, how can you think about the distribution? So at room temperature, carbon goes into grain boundaries,
yes? Because remember, carbon solubility in ferrite is nothing, yes? So and if a material is very well recrystallized, there are no dislocations to go to, yes? So it will go, carbon will go into grain boundaries.
So at the start here, at the start, we get of the annealing, we get lots of carbon in grain boundaries, and we get some of it in interstitial carbon.
When we heat up, however, yes, we heat this up, carbon in the grain boundaries leaves the grain boundaries pretty quickly, yes? And most of the carbon will be interstitial, yes? Most of it is interstitial. And then when I cool down, yes, it
increases the amount of carbon in the boundaries again. Remember, we don't have many dislocations, yes? Temperature constant, temperature constant. In this case, in this case, it's
assumed that we don't form carbides, OK? For instance, it's an ultra low carbon steel. OK, and then we continue cooling to room temperature, the carbon interstitial decreases, and the carbon goes back to grain boundaries, yes?
Deformation here creates a large amount of dislocations. The dislocations, amount of dislocation increase. So when I do the low temperature annealing, yes, a lot of the carbon goes to dislocations, yes?
And that's what causes this decrease in interstitial carbon. And with time, you also get an increase
in the amount of carbon that goes to grain boundaries, yes? So this is an important aspect. If you don't have many dislocations, yes, and you have a well-annealed structure that's going to be lots of carbon, the impact of the grain boundaries will be relatively important, yeah?
If you have small grain sizes also, impact of the grain boundaries are important. But once you strain the material, the impact of grain boundaries are smaller.
OK, so let's have a look at some typical bake hardening steels. What kind of compositions do we have? OK, this is a bake hardening steel here that's been vacuum treated, yes, that has niobium as a stabilizing element
and titanium to stabilize the nitrogen, OK? So there's not much in this material with the exception of carbon, niobium, and titanium. What are the properties here? It's still a pretty soft material, 300 MPa yield
strength. OK, what I want to focus on is how much bake hardening you get, typically 50 megapascal, OK? That's the amount of bake hardening you can get.
I also want to mention the fact that when people evaluate or quote the bake hardening values here, you will see either BH2 values, BH2 values,
or BH0 values. What's a BH0 value? Well, there is no pre-strain in this case. How does it work? You measure the yield strength of your material, yes?
Yes. And then you measure the, so you take your material and you strain it, yes, so you can measure the yield strength. And then you take the same material, don't strain it, you anneal it, typically 20 minutes at 170,
and you measure the tensile strength, stress strain curve, excuse me. And so this increase in the yield strength, yes, is called BH0. In the case of BH2, what we do is first,
you take your material, yes, you strain it 2%, yes? Excuse me, you strain it 2%, this much, OK? So that allows you to measure the yield strength and the flow stress, OK?
Then you unload the material and you anneal it, typically 170 degrees, 20 minutes. And then you retest the same material, you get an aged stress strain curve, yes? And so you can determine what is the amount of work hardening, the strength due to work hardening,
the strength due to bake hardening. And this is BH2, yes? So not this here, right? Not the difference in yield strengths, but the difference between the flow strength and the yield
strength. Excuse me, the flow strength and the yield strength after paint baking, OK? So some data here, standards data on bake hardenable steels, low carbon bake hardenable steels,
about 400 ppm of carbon. And what's interesting here is, of course, what are, according to the standards, the minimum required BH2 values, yes? So 35 megapascal is typical minimum standard requirements
for bake hardening. ASTM, you see the BHS, bake hardenable steels, compositional ranges here.
I'm trying to, these are the strengths. OK, so the bake hardening index here is giving us the upper yield and the lower yield. Let me see if I have values here.
No, I don't. OK, all right, just remember that standards usually minimally require 35 MPa, and that in normal cases, you can count on a 50 MPa, typically.
And for low carbon steels or IF steels or ultra low carbon steels, of course, you can do aging tests with other types of steels, like bake hardening steels or trip steels or other complex phase steels, and they will give you an other bake hardening.
response, which may be very much higher, actually. Now, another problem is that, related
to these IF steels and aluminum-killed low-carbon steels, is that they do have this inherently low strength, and that car makers and other users of steels
are always interested in lightweighting things and making things lighter. And so there is a need, also, for steels that are stronger,
so they can use thinner-gauged materials. And so this has led to IF steels and aluminum-killed low-carbon steels, which are alloyed with phosphorus, manganese, silicon, or other elements,
but usually phosphorus, manganese, or silicon, to increase the strength by solid solution hardening. And you see here that, of course, we already know that phosphorus, manganese, and silicon give us a lot of strength,
and that phosphorus gives us a lot of strengthening for very small additions. Yes? And this is one of the areas in which we will add phosphorus to steels in solid solution-strengthened steels.
For instance, this is an example here, taken from European standards, where we have so-called RE-phos, or phosphorus solid solution strengthened steels, where phosphorus is actually used as an alloying element to increase the strength.
And you can see here, strength levels, depending on the amount of phosphorus, can now increase to 350 yield strength, 400, 500
in tensile strength. And again, these are internationally recognized grades. You have RE-phos standards in Europe and in the US. And one of the things that happens, of course,
with phosphorus, and many people are always worried about the use of phosphorus as an alloying element, is embrittlement.
Now, when these IF steels first came, were used, they were used very widely. And when the RE-phos IF steels were developed,
there was also very much interest in them. But they looked like very safe materials, because the amount of phosphorus that you added was really low, around 500 ppm.
So below the danger level, where the phosphorus goes to grain boundaries and embrittles the steel. But it turned out that there was a phenomenon called secondary work embrittlement that was observed.
That means, take for instance, you make a cup, you form a cup made of a phosphorus alloyed IF steels. And the cup is perfect, it doesn't break or anything. But what happens is that, and this happens often
in press shops, is when you press a part, you don't press it at once. It can go to different press stages. So you, and during this processing, you deform already deformed material. And it turns out that these phosphorus alloyed steels
were kind of sensitive to this secondary extra work fracture. And you can see here, so if you, this is the Asteron cup, if you try to deform this cup again, like making it slightly wider, you
see that it will behave brittle, in a brittle fashion. And it will behave in a brittle fashion if you do the deformation at slightly lower temperatures. So that got people very worried, of course. Because as you can see, it's quite impressive,
the embrittlement. So let me go back here. So what did, it's not in the notes, I'm sorry about this. So what did they do? What did, what do you do to prevent this?
You add some boron. So that's why many of these steels, these phosphorus containing grades, nowadays contain some boron, very small amounts of boron, 10 ppm, yes. The boron goes to the grain boundaries.
And because it also likes to segregate to grain boundaries and enrich in the grain boundaries, it doesn't do any harm at the grain boundaries, the boron. But it prevents the phosphorus from going to grain boundaries because there are only so many positions that
are available for atoms on grain boundaries. So this grain boundary competition between boron and phosphorus is always in advantage of the boron. And the phosphorus stays in solid solution. And you don't get this secondary work embrittlement problem in practice.
And let's just finish today with this slide here. The, with the aluminum killed steels and the IF steels,
and structural steels we'll see, we'll start with on Thursday this week. We know we have a basic problem, yes. And the problem is that we have low strengths.
We, of course, we can increase them by doing, for instance, bake hardening. Or we can do solid solution strengthening with phosphorus, manganese, with silicon additions. But only so much so, yes.
So what people have been developing since in the last decade, yes, there's been lots of development, are steels with higher strengths.
So we can have higher strength engineering constructions that are also lighter weight, yes. And of course, one of the problems that needs to be addressed is that as we increase strength,
you usually see that plasticity measured as total elongation or uniformization and value will tend to decrease, yes. And that has been one of the major challenges in the development of higher strength steels
is how do you develop strength, yes, and keep very high formability in your steels, in these high strengths. And so we'll talk about a few of these solutions on Thursday.
Thank you.