Nanostructured Steel Industrialization - A plausible reality
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Transkript: Englisch(automatisch erzeugt)
00:28
Our next speaker is Dr. Carlos Garcia Mateo from the National Center for Metallurgic Research in Spain, in Madrid, and he's going to talk to us today about nanostructured
00:40
steel industrialization. Over to you, Carlos. Good afternoon to everyone. As I've been introduced, my name is Carlos Garcia Mateo. I belong to the National Center of Metallurgical Research, and what I'm going to present here today is a story. It's a story of the results we obtained within this consortium with all these research groups,
01:07
companies, steel makers, and final users under the auspices of a European project. Nanostructured steel industrialization, a plausible reality.
01:22
So it was in this same house at Cambridge University almost 15 years ago where Caballero and Vadesia, using Vadesia's binite phase transformation theory, developed a set of alloyings, a set of steels that allow for the transformation of austenite into binite at very low temperatures.
01:43
The transformation temperatures ranged from 120 to 350. As a result, they did obtain this beautiful and elegant microstructure composed of bainitic ferrite plates, the white feature, with just between 20 and 40 nanometers of plate thickness,
02:02
and some retained austenite also in the range of the nanoscale. The plate thickness increases their size as the transformation temperature increases, obviously, and the hardness also decreases as a result of that increase in the size.
02:20
Some issues or some other peculiarities of the transformation is, of course, a part of the high density of a strong interface is given the nanosize of the phases we are dealing with. As it is a displacive transformation, the plastic deformation that accompanies the transformation
02:41
is accommodated and through the generation of this location of nanotwins as those you that you can see here in the high resolution GM micrographs. As it is a diffusion-less transformation, the carbon, once a bainitic ferrite plate has finished its growth, tries to escape from the bainitic ferrite and goes into the parenaustenite.
03:04
On its way, it might find this location and twins and it might get trapped, it might get just clustered in carbon clusters, those measured by atom-proof tomography, or trapped in these locations that has been observed also as cultural atmospheres in atom-proof tomography.
03:27
So the carbon is not the, I mean, it's not homogeneously distributed, it's not, we have two types of morphologies of retinostenite. We have these blocks of retinostenite trapped between the sieves of bainitic ferrite.
03:44
In the case of nanobain, instead of being the typical blocks with a micron size, as you see, it's a micron. And then we have the thin films trapped between the plates of bainitic ferrite. Both of them have very distinctive carbon content.
04:01
As you see in these results, the smaller the austenite feature is, the higher the carbon content is and we'd rather have that type of morphology than even the micron because the mechanical response and the stability is much better. So no wonder the mechanical properties we did obtain with such a fine microstructure were good.
04:21
We obtained UTS over two gigapascals accompanied by very reasonable toughness. And most of the strength, as I said, it came from the size of the microstructure of the matrices, the bainitic ferrite plates and the amount of heat that we have in the microstructure, but also from the dislocation density.
04:42
But there are other ways to produce a nanostructure steel. We can do it by martensitic transformation, but we need very expensive alloying elements. We need rapid cooling rates so we have problems with recalescence during transformation, or we may introduce some residual stresses. We can use ECAP or other severe plastic deformation techniques.
05:05
We can use warm deformation, friction steel, processing or advanced thermal mechanical processing. And this is the range of mechanical properties in terms of geostand and uniform elongation that you can obtain. But as I said, we have problems of expensive alloying elements,
05:24
recalescence. We have a problem also with the amount of material that we can treat and the shape that we can achieve. But if you think about the range of mechanical properties we can obtain with nanobain, with such a simple heat treatment, there is a big potential for this type of alloys.
05:46
So it is not strange that a group of researchers, steel makers and final users gather all together to form what was known as the nanobain consortium, just with one aim, take a step forward to the industrialization of these nanobain alloys.
06:06
For the design of the alloys we use fundamental and industrial design considerations. And those considerations can be summarized as follows. We wanted, obviously, a carbide-free microstructure because cementite is a hard and brittle phase that we wanted to avoid at all instances.
06:24
So we know that an addition of 1.5 silicon in weight percent is more than enough for that purpose. If we want to have a competitive alloy, and we want to put it into the market, it has to be a simple alloy system. So we keep it in this system.
06:42
Carbon, silicon, manganese, chromium. Fairly cheap and fairly common alloying elements. We want the transformation of austenitin-2 by night to happen in reasonable times, as the two first generation took like, let's say, a 215 days.
07:01
And then the second generation took like a week or a bit more. We, that industrial is not, it will never put an alloy into the market. It is too long. So for that, to reduce the transformation time, we took two approaches. We reduced the pre-austenite grain size, and we tailored the chemical composition
07:21
so the free energy change of the transformation would suit our purpose. Final user had in mind two components. One, which is pretty big, and the other one is smaller. For the big one, the isothermal heat treatment was going to be applied by salt bath.
07:41
The other one by dry-bain technology. And in a case there are, again, there are some models, thermodynamic base, that again playing with the chemical composition, we can change the free energy of the transformation to suit the hardenability that we need. As we want a hard material, a hard microstructure,
08:03
we want to keep the fraction of anitiferite as high as possible, and we can do that playing with the T0 line and keeping the transformation temperature low. But keeping the transformation temperature low is also a way to have a smaller plate thickness,
08:21
to have more thin morphology as compared to blocky austenite. So it suits our purpose in terms of mechanical properties. Again, we can do that just knowing how to calculate the Vs and Ms, the free energy changes in plate in both transformations. So again, just by means of chemical composition tailoring,
08:43
we can keep the transformation temperatures very low. So we end up with a set of nine alloys. One group with nearly one percent of carbon, the other one with 0.6, 0.7. The high carbon content is to ensure the highest strength
09:02
and low Bs and Ms temperature. Silicon in surface quantities to avoid the cementite precipitation. Manganese and chromium for hardenability purposes. Niobium, it was added to control the pure austenite grain size through the carbon nitride precipitation. And the extra, the molybdenum and the extra silicon,
09:23
it was added to get an extra strengthening on the austenite prior to the bainite transformation. So we expected to have even thinner plates of anitiferite. So as you see, we managed to have a simple alloy system. It's silicon, manganese, chromium, and in some cases,
09:42
some moly and some niobium, but in really small quantities. We keep the transformation temperatures very low because the transformation, it happened between 220 and 350. It is a nanomicrostructure. As you see, for the higher carbon, the worst scenario was 40 nanometers
10:01
at the higher temperature we tested. For the 0.6 alloys, it was below 70. And it contains a higher fraction, a very high fraction of anitiferite that in all the cases, it was almost above 70% in all the cases. It was faster as compared to the benchmark alloys
10:23
we used during the design process. This is the time taken for the benchmark alloys at 200, and this is at 250 and 300. As you see in general terms, we managed to obtain an increase in the transformation kinetic, and we managed to get sufficient hardenability
10:41
for the bigger component, which was the most critical in terms of hardenability. This is the microstructure. We end up, OK, blocks, some micron blocks. Look at the scale of the micro. Those are some micron blocks of retained austenite.
11:00
The thin films at these little whiskers, the black features are the bainitic ferrite. This is the one carbon silicon treated at 215 degrees during 16 hours. So no wonder the mechanical properties, the strength, it was so good. I mean, we have a nanoscale microstructure
11:21
with a high fraction of the harder phase, which is bainitic ferrite. So UTS in all cases is above two gigapascals. We have GL strengths almost over 1.7 for the higher carbon and 1.5 for the lower carbon, but the utility level is quite reasonable given the strength levels.
11:44
I just want to point out these two cases, which are just extraordinary combination of strength and ductility. For the one carbon silicon treated at 250, the total elongation is more than 21 percent. The uniform elongation is over 10 percent, well above.
12:03
And for the 0.6 carbon niobium treated at the same temperature, 250, the level of total elongation is almost 20 percent. So we were quite impressed with those results. We measured also where properties as rolling and sliding were test, and what we did is compare it against some reference material,
12:24
as for example the 100 chromium-6, and some others use it for those application with different type of microstructure. B stands for bainite, C for carbides, B for P for pearlite, sorry, and M for martensite. And as you see, those are the results for all those microstructure.
12:42
And when we compare it with a nanobain treated at different temperatures, we obtain a completely different behavior. Even for the same level of hardness, the behavior of the nanobain, it's 50 percent better than for the reference alloy.
13:01
One of the outcomes of this work, it was the importance of the hardening of the surface of the nanobain alloys. As the experiment went on, the transformation of austenite into martensite, which is a hard phase, kind of controlled the amount of material that was removed from the surface.
13:24
There were another type of wear properties. This is the high pressure abrasion, the final user usually does that type of experiment, where we press over a chunk of our material, of our nanobain material, we press it against an abrasive material at this interval during this time,
13:44
with this force. And what we did is compare it against the reference material, which is the Hardox, which is a very expensive material with high nickel, high moly and chromium content, treated to half of this hardness.
14:02
As you see, the behavior, which is in white, is pretty good compared to the reference material. But the final user doesn't use only this high pressure abrasion, he also uses the Charpy to make a selection of the material.
14:21
So if we compare all together, the toughness multiplied by this HPA, what we can see is naturally a selection of three states of this nanobain that perform much better than the reference material. Fatigue properties, rotation bending on notch specimens,
14:42
there are several reasons to use the notch specimens, as the final product will have a stress concentration, fatigue is sensitive to the austenite decomposition and to the cleanliness and inclusion level. What we see is that for the three different states that we tested, the 0.6 carbon behave far much better than the higher carbon.
15:04
Okay, and this is in pair with the reference material, which was a 100 chromium-6. So based on the results that we had so far, we did a material selection to fabricate this material at industrial level. For the small component, we looked for a strength and ductility balance,
15:24
so we went for the high carbon silicon, the one carbon silicon. For the small component, sorry, for the big component, keeping in mind that we needed hardenability and we wanted a good balance of HPA and Charpy performance,
15:40
we went for the 0.6 carbon niobium. On the way during the selection process, we realized that this alloy was giving, we performed some theoretical calculations and we realized that we anticipate some problems with the primary niobium carbides in the liquid state.
16:02
So we changed the approach, and we decided to go for carbon-banadium system to control the pre or austenite grain size. We did a battery of tests on lab scale casts, and we found out that the microstructure is the same,
16:21
the hardness, the BS and MS temperature were exactly the same, but even the kinetic was slightly faster. So finally, we fabricate that at industrial scale. These two alloys, the one carbon silicon, at the 0.6 carbon silicon, and before fabricating components, proper components or demonstrator,
16:43
we run a battery of tests to check if things were changing or were exactly the same. Very fast, this is a comparison of the result of the lab scale and industrial scale in terms of strength and ductility, which are roughly the same, not big differences.
17:01
If we compare the result in terms of where rolling sliding of the nano vinyl lab scale compared to the industrial scale, we improve even the wear resistance of the material. For the same level of hardness, which is this 100 chromium-6, I mean the level of wear is almost just 1%.
17:24
Just take a look to this one that correspond to the one carbon silicon with almost no wear at all. So we wear, and the final user was pretty impressed with those results. In terms of the other type of wear, the HPA, the high pressure abrasion,
17:42
we use the 0.6 carbon vanadium. We did two heat treatment at 220, which was the lowest temperature, and 280, which was slightly higher, and the performance of the 280 compared to the Hardox material is just impressive. We tested the fatigue properties.
18:02
You see the industrial material and the different conditions. What you see here is that, let's say, for the same material, same cleanliness level, the higher the UTS, the higher the fatigue resistance is. If we compare again the high carbon
18:22
and the lower carbon, the results of the one carbon silicon are worse than were expected somehow. There were some problems during casting, and the cleanliness level, it wasn't as good as expected. If we compare the industrial scale
18:41
and the lab scale of the 0.6, we see that the performance is very similar. Bosch, that was one of the final user, tested for notch tension, tension specimens, which is even more representative of the injectors they were aiming for heavy loaded DSL engines.
19:03
We have here the results of the reference material. And as you see, again, the 0.6 carbon vanadium alloy is performing much, much better. Well, it's performing like the 100 chromium-6, but with much lower UTS,
19:22
which is also a very good result. The one carbon silicon, it wasn't performing as good as expected, and it's still under research. Bosch went a step forward and fabricated what they call a demonstrator for the injector. It has this shape,
19:41
and the results, they tested both alloys. Again, the carbon vanadium and the one carbon silicon, and I think they are pretty happy with the results of the 0.6 carbon vanadium, not that happy with the one carbon silicon. They think there is still room for improvement, as most of the failures in the 0.6 carbon vanadium
20:02
happen on titanium carbides that they found on the surface on the cracks. This is the big component fabricated by Mezzo Mineral, this is the size of the component, it's fabricated of the 0.6 carbon vanadium treated at 280.
20:23
I have to point out that the microstructure after the heat treatment was completely homogeneous through the whole section of the piece, which is pretty big. This piece is used as a scrap-share blade, and Mezzo Mineral estimated a gain of almost 20%
20:43
due to the use of a significantly cheaper material. Before leaving into the conclusions, I would like to highlight what we think it was the biggest achievement in this project. In just over three years, the nanobank project took the concept of nanostructure magnetic steel
21:02
from a laboratory experiment to a full-scale industrial production and testing, and we did that just solely by using the phase transformation theory. Thank you very much.
21:20
Thank you very much for your presentation, Carlos. Are there any questions? Hi, so this is really interesting. As you know, obviously, your work has previously kind of produced steels with values that are now famously quoted by Harry as a K1C toughness of about 45 megapascal root meters
21:43
for strength levels of around two gigapascals or so. I think that was your work. And this one, the K1C value has dropped quite considerably. It's not K1C, what we measure here. It was a Charpy test, the results I saw.
22:01
Was it Charpy in the original? No, no, there was a slide very early on. Yeah, 20 something, yeah. Yeah. That is reported in, I can't remember which publication it is. It's all data. So that's not this alloy?
22:21
No, no, no, no. In this alloy, the only fracture we tested it was Charpy and it was tested by Mezzo-Minerale to find out which material suits the purpose. And so what Charpy values were you getting from this alloy? Say for two gigapascals strength? Let me see.
22:46
I think I missed it. Yes, I missed it. Yes, HPA should be, should be here. Yeah, those are. Oh wow, so that's actually quite impressive then compared to previous work.
23:05
I mean the material is impressive, would you expect? I know I just got confused because I thought you've halved the toughness, what's going on, but that was all data, okay. I mean considering the two gigapascals, just as a standard of UTS, I think it's pretty good. Okay, so you haven't done fracture toughness on this alloy? Not yet, I mean we are running a second European project
23:24
focusing now on fatigue properties and trying to understand what lies beneath. And at the same time we are performing a lot of research in terms of what is controlling ductility, which is a pretty tough issue. Okay, thank you.
23:42
The questions? What is the time of upstream pairing in your steels? Um, for the 250 is lower than 15 hours, a 250.
24:00
That is for the one carbon silicon. I cannot give you, I mean the Europe, the final report is on the web, is in the European, so you have the exact numbers there, but I'm sorry, it's in logarithmic scale. But I'm talking about the 220 something like 30 hours
24:21
and a 250 between 12 and 16. It is acceptable for industry for 40 hours. Yeah, I mean what you are obtaining is something you cannot obtain in such a cheap manner otherwise. I mean we did it shoulder to shoulder
24:40
with final users and with steel makers and heat treatment companies, so yeah. Okay, did you study the transition temperature from ductate to brittle? No, we didn't know. Maybe you know that we made a combinatorial thermodynamics
25:01
computation to estimate the most important elements contributing to shortening the treatment and we concluded that manganese reduction was very, very manganese reduction was strongly contributing to reducing treatment. So if you chart all your alloys heat treatment temperature
25:22
versus manganese content, do you see that trend? I mean of course it's not. Yeah, it's not represented here. To be honest, right now I cannot answer you. But we always try, I mean we decrease too much the manganese level, you know. I mean there is a moment where you have to balance
25:43
between this, how fast the transformation happened at which temperature you want it to happen. So manganese helps you to deplete the BS and MS. It's true, it makes the transformation very sluggish. But yeah, I mean manganese, it has a strong effect.
26:01
Thank you. Thank you. Are there any internet questions, Steve? Hello? Yep. You choose for the 2.5 silicon, reasonably high value. Yep. Why? And does it better help?
26:22
Yeah, I did not choose the consortium, did it? I mean because there were many interested in plate. But as you see, I mean take a look to this result here. I mean the combination of strength and ductility is pretty impressive.
26:41
Can you give a methodological explanation for that? Not yet, not yet. I mean we are thinking, in terms of ductility, we don't know yet. We put this extra silicon because we knew silicon is a solid solution strengthener of the austenite.
27:02
And we expected to have even thinner bainite ferrite plates, but it was of the same order as the other alloys. So it's like we reached a lower limit or we cannot resolve. But I don't have an explanation right now. I mean we wrote a paper and we speculated
27:21
of some of the reasons. Maybe it's the distribution of the retinostenite, but we don't know if silicon is directly linked to that part. Maybe it's the stocking fault energy, I don't know.
27:42
The vanadium was used in a later stage just to substitute the problems with the niobium. It was to control the prior austenite grain size during the austenitization prior to the isothermal heat treatment. A small prior austenite grain size creates more nucleation sites,
28:02
therefore accelerates the transformation. It was another approach to accelerate the transformation. Hi Carlos. So what do you think the difference in terms of mechanical properties between the nanobane and the superbainite commercially produced by tartar still at the moment is?
28:26
I don't think there is any. I think this consortium aimed to look for a set of alloys that suit their purpose. We have final users, they had pretty clear what they wanted, where they wanted, and what was the mechanical properties range
28:44
in which they can move. So we were looking for a new family. Just the composition and the properties look identical.
29:03
What was the industrial production route for your industrial heat? I'm sorry, I don't have the details. It was Sydenor, Gerdau now, and it was Ascometal, the ones that produced them. So it would have been an electric arc heat probably by an Inga route?
29:29
Matias is also from the consortium. I can probably try to answer it. It was not an abnormal route for the industrialization, so they just put in an industrial heat as far as I know for Ascometal,
29:42
they just put in an industrial heat in their normal sequences and fabricated 100 tons and scrapped 90 tons and kept 10 for the experiment. It was a bloom Comcast route then, was it? Yeah, it was bloom, yeah.
30:05
Any further questions? Okay, in that case, thank you very much, Carlos. Thank you very much.