Architectured Steel
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Transkript: Englisch(automatisch erzeugt)
00:33
Welcome to the meeting, and we are now ready for the first lecture, which will be given by Professor Toshi Koseki from Tokyo
00:41
University of Technology. Good morning, ladies and gentlemen. My name is Toshi Koseki from the University of Tokyo, Japan. And first of all, I would like to thank Professor Haribade for organizing
01:02
this wonderful conference and for including me here. And also, I'd like to thank my colleagues here. Actually, Nambu is over there, who have contributed to the research that I'm going to talk about today. The title of my talk is Architecture of Steel.
01:23
This name was given by Hari when I gave a talk somewhere else on our multi-layer steel. Because I like the name, so I used the name again here. So all of us here know that many different kinds
01:40
of steel were developed in the 20th century. And the performance and the properties of the steel significantly improved over the century. Those development and advancement, certainly thanks to theoretical basis, that was also developed in the 20th century
02:02
and also achieved by a low design with a different combination of rare metals and also achieved by microstructure control by some mechanical process and also by high purification. Also, we have used fully the strengthening mechanism
02:21
to develop different steels. So looks like to improve the steel, we have done almost everything. But still, the demand for high performance steels is never ending. And with increasing the environment consciousness,
02:41
the demand becomes stronger and stronger. For example, if we look at the automobile steels, higher strength and the higher ductility demanded. And so we need to meet the demand. We have to go this direction. How can we do that?
03:02
We may need a new alloy design and we may need a new micro and nano microstructure control that we haven't tried in the 20th century. And also we may need the ultimate refinement of microstructure and grained structures.
03:21
Alternatively, we may need the external architecture of steel or externally designed steel where we can get away from monolithic steels and we can part from some dynamic restriction in the design of materials, in the design of steels.
03:40
So this is my proposal here. Today, I'm talking about externally designed multilayer steels. This is an example of a multilayer steels where we combine high strength, as pitched mountainside and the high ductility steel
04:01
and to achieve a combination of high strength and high ductility. And although this is a 25 layer steel, the number of layers could be fewer depending on the combination of steels as I will mention later. To fabricate multilayer steels,
04:22
we stuck the steel of our interest and then hot rolled or warm rolled or even cold rolled for bonding. And finally, we heat treated to achieve desired microstructure and to increase the interfacial toughness.
04:42
And for the high strength layers, we use as pitched mountainside and for high ductility layers, we use austenitic steels or ferritic steels or trip steel or dual phase steel, whatever steel that has the ductility. In other words, you can combine any steels of your interest.
05:02
And sometimes we insert thin nickel layers to prevent carbon diffusion between the layers. Why the layer structure? To achieve the high elongation, we need to elongate the mountain side, as pitched mountainside.
05:20
In a dual phase steels, mountainside is not deformed because the stress is not partitioned to the mountainside. And there is a stress concentration in a ferrite matrix, particularly between mountainside island, which result in a voiding and eventually a fracture. In case of layered structure,
05:42
stress is partitioned both ductile layer and high strength layers. So with a plastic constraint, the mountainside should be elongated, as long as the local fracture is suppressed, which is not easy.
06:02
These are the data of a strength and the ductility of a laminated metals, different laminated metals, which was summarized by Liget and Charby. And the strength always follow the rule of mixture, rule of average,
06:20
along this line. But the ductility does not follow the rule of mixture. This vertical axis is elongation of laminated metals consisting of 50% ductile and 50% brittle component. And this elongation come down here,
06:43
far below the rule of average as the elongation of brittle component becomes low. Why do we have such low elongation? Because we have a brittle fracture during elongation
07:02
in the brittle layer, as shown here, which is caused by delamination and resulting H-shaped cracks and also so-called Pundit cracks. We have to suppress those local fracture to obtain larger elongation.
07:21
In terms of delamination and H-shaped cracks, naturally increasing interfacial toughness, increased elongation of multi-layer steel. And when there is a delamination, brittle layer behaves like a single component and without plastic constraint,
07:42
it breaks, it fractures with low elongation like here. And if the interfacial toughness is not enough, the elongation is still below the uniform elongation predicted by rule of mixture.
08:01
And with increasing interfacial toughness, the multi-layer steels are fractured with diffuse necking in ductile manner. This boundary can be predicted by this equation which was also developed in the research of composite.
08:21
And this criterion is given by this line. Looks like this criterion works well. For the prevention of a tunnel crack, there is also work about the tunnel crack in the research of semiconductors, which is given here, where the thickness of a brittle layer is limited,
08:45
which is a function of a fracture toughness of brittle layers. So you need to reduce the thickness of brittle layer to avoid the tunnel crack. But this criterion was derived in an elastic situation.
09:02
In case of metals, you have a plastic zone in ductile layers in the vicinity of the tunnel crack. We have to consider that. So considering the elastic plastic situation, we derived this criterion. Again, the thickness of brittle layer should be reduced to increase the elongation,
09:23
which is a function of fracture toughness of a brittle layer and yield strength of ductile layers. Certainly, the decrease of the layer thickness increases the elongation of as quenched mountainside. By decreasing the thickness of mountainside,
09:43
this type 420 high carbon mountain silica stainless steel can be elongated up to near 20%. So without multi-layer structure, we can't elongate the as quenched mountainside in this way.
10:03
Also the fracture surface changed from brittle to ductile dimpling as the thickness of mountainside layer is decreased. This is the effect of thickness of brittle layer on the elongation.
10:22
And these two lines are from the elastic model and the elastic plastic models. And many use alternate type 304 stainless steel for ductile layers, because the type 304 stainless steel has a good work hardening. The transition from low elongation to the high elongation
10:42
is close to the elastic model. And when you use ferrite, interstitial free ferrite as a ductile layer, because this steel does not show much work hardening, so the transition behavior from low elongation to the high elongation is close to the elastic plastic model.
11:02
So the transition in a multi-layer steel, the transition from low elongation to the high elongation is somewhere in between. And the elastic plastic model we developed gives a lower boundary in the design of multi-layer steel. And also the limitation of the brittle layer
11:24
is a function of the fracture toughness. So these are confirmed here. We provided the mountain silic steel having a different fracture toughness and measure the transition. And certainly increasing a fracture toughness gives a thicker brittle layers.
11:42
In other words, you can increase the thickness of brittle layer if you have mountain silic steel with a better toughness, or you can reduce the number of layers. So by controlling interfacial toughness and controlling the thickness of brittle layers,
12:02
we can now elongate the unquenched mountain side. And if you apply a neutron diffraction, this is the result, you will measure the fully partitioning of stress. And because of partitioning of stress, mountain side is being elongated here.
12:21
And as a result, we obtain the steels which have a high strength and high ductility. Those are plotted here. Those steels have a strength more than 1200 megapascal, and the steel have a elongation of more than 20%. And the product of strength and elongation
12:42
is more than the double of a conventional monolithic steels. This mountain layer steels keep the elongation even under the high strain rate deformation. Here's the stress strain curves and the different strain rate,
13:03
and the maximum is 800 per second. And if the strength increases with increasing strain rate, but the elongation does not change much. And those photos are the test result of high speed buckering, which simulate the collision
13:22
of front side member of automobiles. And the 1200 megapascal multi-layer steel deforms perfectly in the same way as the 590 dual-phase steels. And the steel there is a space for additional deformation because of the high strength.
13:41
And I'd like to note, there is no delamination or a local crack during this high strain rate deformation. Here's another high strain rate deformation. This is a impact bending test which simulate pillars of automobiles.
14:01
And the bending strength is increased in the multi-layer steel. So here's a DP 590 for comparison, and significantly the bending strength is increased here. And in the application of multi-layer steel, we need a welding.
14:21
So we are trying to weld the multi-layer steels using a friction stir welding. And this is a cross section. And the welding is successful, which have a joint efficiency more than 90%. And it is interesting to note here,
14:41
this layer structure is still remain not only in a heat affected zone, but also in a steel zone. By using a multi-layer steel, now we can look at the deformation behavior of a sprenched mountainside, which was difficult before, because of the load activity of a sprenched mountainside.
15:02
And we are now conducting many in situ observation on the deformation of a sprenched mountainside, and using an EBSP. Here's a multi-layer steels, and this is the monolithic mountain steels. And we found the slip is always along the in-plane.
15:25
In that plane, and up to the certain strain levels, and beyond that, the slips across the, across the last direction start to appear.
15:41
And this slip is concentrated in the region where the Schmidt factor along the in that plane is high, and no slips in those regions where the Schmidt factor is low. This is a similar result
16:01
using a digital image correlation using a silver nanoparticle during a tensile test. Again, the stress concentration is in a block, mountainside block, where the Schmidt factor along the in, last plane, is high, and the other part is not deformed significantly,
16:23
even though the Schmidt factor is high, which is out of last plane, and the Schmidt factor along the in last plane is low, because this is low. So further improvement of the multi-layer steels,
16:42
we need to improve the process. We are now looking at the lower temperature, lower pressure bonding, which makes the fabrication much easier and more efficient. And in terms of a component, not only use the high carbon mountain acidic steels, we are now using a HTP metals,
17:01
such as a magnesium and the titanium to achieve a light multi-layer steels. And we are now using a steel with a high impurity, like a scrap steel, so that we can use a scrap to fabricate a high performance steel. Here's an example of a magnesium steel multi-layer.
17:22
We have developed a good bonding process to join the magnesium steel, and we fabricate three layers, magnesium steel multi-layer. And magnesium is commercially available, lightest metal. But the problem is the ductility,
17:41
because of HTP structures, the elongation is up to 20% or even less. But by employing a multi-layer structure, we can increase the ductility of magnesium without any break, and up to 35 to 40%.
18:01
And the strength is also increased because of steels. So, this is the summary of my talk, and this is all my talk. Thank you very much for your attention.
18:23
Hi. So, I was wondering how you deal with differential volume contractions and expansions that you get when you quench the multi-layers, because obviously you're going to get different volume expansions with the martensite transformation. And does that set up like residual stresses between the layers that might result
18:40
in like crack propagation being easier and delamination becoming easier? How do you deal with that? Yeah, you're right. There must be some residual stress. And we are now measuring how much residual stress and what the effect on the mechanical properties. And yeah, certainly there is some residual stress.
19:01
You're saying that decrease in martensite layer thickness is increasing the toughness. Is it not due to locally plain strain loading condition is starting in those only the martensitic layer only? Well, you're right. As the thickness decreases, situation come to that.
19:22
But the thickness can be thicker if the fracture toughness of martensite is maybe medium or not high. Today I showed the martensite,
19:41
which is a type 420 high carbon martensitic stainless steel. This is really brittle. The fracture toughness is really low. But the normal carbon steel, the martensite of carbon steel is not so such low.
20:00
So we can increase more. So the situation is not simply a plain. So far you've made these materials by starting with the original layers and then rolling them together. Is that right? Right. Have you looked at other methods of fabricating this kind of structure?
20:21
Additive layer manufacturing looks like it would be well suited to a layered structure like that with electron beam or laser depositing powder and then just obviously got a lot of flexibility to pick whatever material or thickness you want there. Well, multi-layer structure is employed everywhere, like in a semiconductor. In that case, as you mentioned,
20:40
they deposit layer by layer. But this is a structure steel. We need a body. So the easier and the simple fabrication is better. So at the moment, we thought of many possibilities, but at the moment, I think this is the simplest way. Okay, what sort of thickness are you aiming to get?
21:01
Excuse me? You say it's easy to get good volume material. What sort of thickness and volumes are you interested in fabricating? In our case, the thickness, anyway, this is the sheet material. So the thickness is about 0.5 to two millimeter
21:20
or something like that, the final thickness. Okay, but is sheet quite wide, I guess? Yeah, yeah, yeah. And in our research, we already made a 60-meter steels, coil to coil. So when you do the hot rolion,
21:41
warm rolion of martensite and multi-layer, so the martensite phase actually, during rolling, it changes back to austenite or it's a martensite, hot rolion and a warm rolion. Could you repeat again? So when you do hot rolion or warm rolion of your multi-layer steels,
22:01
one layer, suppose, is a martensite. So the martensite in that temperature has actually changed back to austenite or... I understand. During fabrication, martensitic steel does not have the microstructure of martensite. It's like a mixture of ferrite and the cementite
22:23
or something during a hot rolion. And after that, the heat treatment is made and after the austenite region and the quenched. So where the martensite is formed.
22:44
Thanks for a very interesting presentation. Reminds me of the fiber metal laminate work was done a few years ago. Introducing polymers and metal layers together. I just wondered if you'd done any corrosion studies, introducing dissimilar metals, often problematic.
23:01
That's also a good question. We haven't done the corrosion test. There is a possibility that the corrosion resistance is decreased if we combined like magnesium. Did you measure the mass of the machinist materials?
23:21
Yes, but at the moment we combine steel to steel. Young's modulus is the same. Yes, it means just follow the mixture. Yes, yes, yes. This is well known. I was wondering if you did fatigue testing
23:40
and stress localization testing for formability. We are conducting a fatigue test too. At the moment there is nothing I can say about that. Certainly the metal layer still affects
24:01
the fatigue behavior. Did you make stress localization experiments to understand the visco-plastic behavior under forming or foreforming? At the moment we haven't done much of that. Thank you.
24:21
3D deformation. Thank you for a very nice presentation. But could you explain me in a few words what is the difference between your idea of composite material on the base of steel with, for example, the Mahakana steel or Damascus steel? It seems to me that it is the same idea.
24:43
Yes, maybe the origin is the same. In the past, many people studied a lot of metals, even the Damascus too. But in the past, it looks like this attention
25:01
was paid to the ductility. And the people tried to increase the strength, but the research on the ductility is limited as far as we investigated it. Yes, during heat treatment, do you study how important is the diffusion,
25:22
for example, of carbon between the same layers of high to low carbon steels? Yes, it is very important to suppress the diffusion of carbon. And when we combine the different steels, we're always thinking about activity of carbon
25:41
during a heat treatment so that maybe alloy design is needed to suppress the diffusion of carbon for brittle layer and ductile layers. Toshi, how do you measure the interfacial strength? We use a peel test,
26:04
but we can't measure if the interfacial strength is really high. Just only a brittle interface can be measured. And how do you control it? Nambu, can you explain that? In the case of interfacial toughness,
26:23
the majority of the test is just 180 degrees. So the majority and the storage direction is almost constant. So we can evaluate the difference.
26:41
Supposing that the interfacial strength is too low, how can I change it? Of course, in the case of very, very weak interface toughness, it is very difficult to evaluate. But in the case of, for example, when 500 degrees C or 600 degrees C,
27:03
we can evaluate the interfacial toughness. So the interfacial toughness is increased by a bonding process and heat treatment. So you can increase the interfacial toughness. And I've got to note,
27:20
the interfacial toughness does not be really high unless you give a peel test. So based on, I guess, that, would it be possible if you did have quite a high stress at the interface
27:41
to design the ductile phase so that it underwent slight plastic relaxation at the interface, which would provide a bit of work hardening and then you wouldn't have the, would that be beneficial in a way to reduce any stresses that you might get generated in the alloy? Would that be feasible or would the work hardening limit your layer thickness
28:01
and the further work hardening capability of it? Would that then have an adverse effect on ductility, do you think? Could you repeat that? Oh, so with the residual stresses that are set up in the interface, if they were reasonably high, would it be feasible to design your ductile layer
28:23
to have a yield strength such that it undergoes slight plastic deformation to relieve the residual stresses? And would that work hardening then have an adverse effect on the ductility as a result? Well, up to now, we haven't found
28:42
any adverse effect on that. Yes, let me be a bit adventurous here and about a little bit speculative as well. Perhaps you could decrease the carbon diffusion between layers and perhaps improve the cohesion effect
29:05
if you dealt with, let's say, nano structure still. So you would get a strength out of a nano structure still layer and keep your soft layer as well.
29:20
This is highly speculative, you see. So it seems adequate an audience to do that. What are your ideas about that? You mean the diffusion across the interface? We are now researching that and we want to minimize the diffusion layer to increase the strength and the ductility
29:43
for brittle layer and the ductile layer. The diffusion of carbon decrease the strength and the ductility both. So that's why I'm saying low temperature bonding is necessary to improve the performance of multi-layer steel.
30:05
And we have a feeling that the bonding of interface is possible at medium or lower temperatures. And we don't need big diffusion to increase the interfacial strength.
30:24
Did you look at your interfaces at higher magnification and in general, how important is the quality of interfaces for you? So do you have rough interfaces or more kind of intermixing? We are looking at the interface using a transmission electron microscope.
30:41
And of course, there is some small void and some discontinuity and some part continues. And the details of the development of interface need to be researched more. I have a more macroscopic question.
31:02
You're reducing the thickness of the martensitic layer to increase elongation. This is traditional bi-phase philosophy. You're also saying that strength is a rule of mixtures. Would you not then expect strength to drop?
31:22
So you sacrifice strength to increase elongation, which is exactly what happens in any bi-phase microstructure. Well, this is a combination. So we can't go beyond the rule of a mixture. So there is some compensation, but we can use a higher strength of steels,
31:42
like a very high carbon steels so that we can increase the strength. And the elongation is always, if we make all the effort, we can achieve the rule of mixture even for the elongation.
32:01
So you can increase. Let me explain more. So anyway, this is a mixture of a high strength layer and a low strength layer and a mixture of high ductility and low ductility. And we can go this way more
32:21
by using a higher strength steel. Thank you very much indeed, Tashif. Excellent talk and excellent discussion. Thank you.
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