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Modern Steel Products (2015) - lecture 6

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Modern Steel Products (2015) - lecture 6
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6 (2015)
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31
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CC Attribution 3.0 Unported:
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A series of lectures on steels, given by Professor Bruno de Cooman, Graduate Institute of Ferrous Technology (GIFT), POSTECH, Republic of Korea
SteelSheet metalMachineSurface miningDeep drawingRolling (metalworking)EngineRoll formingRemotely operated underwater vehicleWork hardeningMaterialCartridge (firearms)Roots-type superchargerRutschungSizingFinishing (textiles)TexturizingWiderstandsschweißenShip of the lineTotholz <Schiffbau>NegationMechanicProgressive lensAlcohol proofSteelRolling (metalworking)AfterburnerSizingFinishing (textiles)MechanicComputer animation
Transcript: English(auto-generated)
All right. So we have started some in, so we were talking about composition of steels and relation to how you can make different microstructures.
And now we're going through the discussion of compositions and how they affect the strength properties. I told you that strengthening could be achieved by adding some elements that give you
solid solution strengthening. You can deform the material, so then you have dislocations that give you strain hardening. You can reduce the grain size, yes? And you can introduce precipitates, and you'd like to have a high density of very
small precipitates. And obviously, this mechanism here will require you to change the composition to add these precipitates. This mechanism here, you will need to add phosphorus or silicon and manganese, so that will affect the composition.
And we'll also see that in order to get very small grain sizes, we can also change the composition to achieve some special effects which will reduce the grain size. And we'll be discussing that in a moment.
All right. OK, so what is mechanical properties? Things like yield strength, ultimate tensile strength. So usually a low carbon steel will have stress strain curve like this. You may get some looters elongation, as you know.
And then the applied engineering stress will reach a maximum. The material will then start to neck, so you'll first form a diffuse neck, yes? At this time, there's only deformation in this zone here. In the zone that doesn't neck, the deformation stops.
So after the material starts to neck, you get all the deformation is localized in this neck. And eventually, you form a local neck before fracture. This is the behavior you'll see for a sheet material, sheet
steel, for instance, in tension. As what is important here to realize is that if you, so if I may go back to the picture here, you see this is your sample here.
It's in a tensile machine. So you pull on it in the length direction. So it has length strain, yes? But it also changes in thickness and in width. And the reason why is, of course, because when you do plastic deformation, you have to have constant volume, yes? So if you make something longer, you
make it longer in this direction, you get width strain and thickness strain. And that's important. So if this is the length strain, you have the width strain and the thickness strain
are negative, because they will be reduced. OK? And so this red curve here is the thickness strain. So it's negative. The more the, so if I have, so the volume
is constant during plastic deformation, that means that the strain in the length direction plus the strain in the thickness direction plus the strain in the width direction is zero, right? It means there's no change in volume.
So when you, this is positive, because I pulled, yes? And these two, when we deform sheet material,
that usually is a constant ratio between these two, which we call R, which is the width strain over the thickness strain. This ratio R is the width strain over the thickness strain. And we'd like to have this value, we'd like to have it high.
The reason is, so if I have a volume here, a material, and a steel, and I pull on this in this direction, so there will be a width strain and also thickness strain.
So you want the width strain to be larger than the thickness strain. Why? Because when something gets thinner, yes, there is a higher risk that it will fracture, that the deformation will localize and you will get fracture.
That's the reason why we like to have materials with this R value, width strain over thickness strain, large, positive and large, of course, yes? So this, because when, so if we have less width strain,
the thickness strain becomes larger. And for instance, when you make a difficult deep drawing part, deep drawn parts, just as this oil pan here, there is a higher risk for fracture. So how do we control this?
Well, we'll see in the lecture progress that this is done by texture control, texture control. But, and it's a very important parameter
for sheet steel, in particular. All right. So the yield strength, tensile strength, elongation, and R value, as we call it, R, we also call it the,
this factor is also called the normal anisotropy, normal anisotropy, yes, have to be high.
And we control, we can control the strength, for instance, by means of solid solution, hardening. Another thing we like to control is the grain size, yes? And you know that that's the reason is because this equation,
which is an empirical equation, yes, appears to hold in many cases for steels. And it basically says that if you reduce the grain size, so the grain size is smaller, if you plot here in the x-axis
one over square root of D, you get a higher strength. So that's a very nice way to get a large strength, yeah? In technology, steel industry, we usually do not define grain size the same way you do in research.
First of all, we tend to talk about A, grain size. And obviously, if you look at this here, well, you can see that there is a distribution of grain sizes. That's one of the things. And second, well, what do you define as the grain size?
You have a distribution. So what you usually do in technology, you just assume that these grains are spherical, yes? Of course, which is a very great simplification, yes? And then you do not really define a grain size. In technology, you define a parameter
which is called the number of grains per surface area, yes? And that is the basis for what we call the ASTM number, yes, ASTM number.
And that is often used in technology to describe grain size of steels. And it basically is the description of the density of grains in a metallographic sample observed at a certain magnification
in an optical microscope. So that's something. And these ASTM numbers, they go from, typically, the scale is larger, but typically will go from one to about 10 or more. And so the way you have to look at it, yes,
is the ASTM is an organization, professional organization, that publishes standards for materials and also for methods of measuring things.
So what you do, you observe a 250 micron by 250 micron surface of a metallographic sample with a metallographic microscope. So that corresponds to 62,500 square microns here.
And say you have one grain in here, yes, it will have a diameter of 250 microns. We assume all the grains are circular.
All the grains are spherical. And so if you make a cut, you'll see a grain diameter. So I have one grain with 250. And if I have a grain that's only one micron in size,
I will have, I can put, if I would fill this surface, 512 grains. So you see here, grain diameter, 250, corresponds with ASTM number of one, yes? So that means this is the size of this grain,
the surface covered by this grain. So the number of grains per surface is one, yes? And then on the other end of the scale, I have a diameter of 11 in this particular scale. 11 microns corresponds to, I said one micron,
I take this back as 11 microns, 512. And that's in that particular scale corresponds to 10, yes? And for those who are familiar with steels, you know that most of the grain sizes for steels
will be within that range of grains. So that's how grain size is defined from a technical point of view. So using this formula here, you can go from the ASTM number to the grain diameter, yes?
And you find a single number, OK? But you have to be aware of the fact that it's not defined the way you would expect. In research, what we will do is
we will measure, for instance, intercept lengths and average that. And we have other techniques to measure grain size, average grain size, or the distribution of grain sizes.
So we won't be talking about this. So we'll just assume that the grain size we get from, for instance, the ASTM technique that measures the surface density of grains, that that's the good way to obtain a grain size.
What happens to material when we go reducing the grain size? Is everything positive? Usually, we'd like to think that it is positive. So definitely, we know we get an increase in yield strength and an increase in UTS.
We do get also an improvement in toughness. Now, toughness doesn't mean that the elongation increases. On the contrary, you get less elongation when you reduce the grain size. You get less elongation, OK?
So the material has less plasticity. So what's the meaning of toughness? Toughness refers to the resistance that the material gives to fracture propagation. It's different from plasticity. Why is it that we get less uniform elongation when
we reduce the grain size? It's because, so if this is the, you may have to write this down because it's not in the slide. When we look at the Hall-Petch equation, for the yield
strength, we find a curve like this. And when we do, you can measure the Hall-Petch equation also for the tensile strength. And if you do that, the tensile strength,
you find something, a line which has a slope that is smaller. So what does this mean? Is that there is a grain size here where the yield strength and the tensile strength are the same, yes?
And when you have yield strength is the same as tensile strength, you have no elongation anymore, yes? It's like if I have something like this. So it's like the yield strength becomes the same as this, right? So that means I have no uniform elongation anymore, yes?
So that's what you generally see, is that grain size has negative impact on plasticity. So it's something you have to be
aware of when you improve matters through grain size reduction. Now, how do we control the grain size in steel in technological circumstances?
Well, we usually do that during hot deformation of the material. And so let's have a look at what happens when we do hot deformation of steel in a hot strip mill. In a hot strip mill, we roll the material at different temperatures, yes?
So what do we get? Well, first of all, this graph shows the temperature as a function of time, position in the mill. And these wiggles here are deformation steps. You can think of them as being in a rolling mill.
These would be rolling passes. So you start with the material at high temperatures. High temperature, so we know it's austenite. And you roll it. You roll it. And then you pass it through another mill, roll it again.
There can be many steps of this time, four or five steps of what we call roughing mill at relatively high temperatures. What is high temperature? 1,100 degrees C or more, yes? And what you get is the austenite deforms
and recrystallizes constantly. As you deform it, it recrystallizes and it becomes soft again. As it recrystallizes, I get some grain refinement, yes? But the grains refine, and then they grow. And because we are at high temperatures,
the kinetics of this grain growth is actually rather fast. So even though we do a lot of deformation, there's a lot of deformation plus recrystallization, yes? We end up with a relatively coarse austenite grain, yes?
And then we go through a very thin region, which is called a non-recrystallization region. It's a very small region where the austenite, if we were able to deform in this particular region, the austenite would not recrystallize. And I would be able to transform.
So as we cool down, yes, we stop the rolling, finish the rolling.