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.
Keywords The Graduate Institute of Ferrous Technology (GIFT)
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and I mentioned the fact
that the precipitation sequence itself can be rather complex so this is just a repeat what we said at the end of it lecture on last Thursday so taking you will not only stabilized nitrogen at high temperature and is very efficient in doing this but it will also blamed sulfur as a sulfide or as a car sulfide and I stress the fact that this Cobb was sulfides and really interesting precipitated because it's the only precipitated by mines carbon at high temperatures but it requires because of its solubility of product it requires it would require France's use of low reheating temperature and if you do that you can't precipitated carbon in the at high temperatures and you do get a better properties also it's only after but at lower temperatures and in particular in the far-right phase that you that you really start to but forum the titanium carbide at Union Carbide know I also mentioned fact in standard ISO steals taking you might have after you always add a considerable amount of excess Tania that gives rise to the surface defects oxides related to this Titaniums excess and so on when you galvanized material should add to this galvanized materials have resurfaced affects the other L steel industries then develop would this golden Titaniums Niobe BMI of steals where debates the and the nitrogen is stabilized by the Tania and the carbon is stabilized by day and I and and and there is no need for these large excess Titaniums values so
important here I want to stress this year that's
the L so we did nitrites of taking over much less soluble then and in the car its nose and the the reason but why you are what you have to wait for the the the formation of tightening and carbide that you are and is this the drop in solubility that's what you get when you go from gamma Alpha is a serious drop in solubility of detecting and Carbide in new you form precipitates let's have a look here at the number of a typical grades that are produced so we love and we'll look at some of the standards of the 1st number of European standards but low-carbon steels IS seals are steals the day developed for form ability for their former ability right as a sheet material mainly it's so bad deep drawing qualities and uh so you know that these according to the standards of the 1st the is refers to drawing qualities yes the 2nd Didier is not drawing it refers to the fact that it's hopes world could we have about them while the main grades are reduced for once here so the stands in this case it said it's a at the table and refused to mean hopped rolled low-carbon grades of the 1st decent for drawing the 2nd transfers the cultural and the numbers here 11 12 13 and 14 are just I just know they referred to increasing of form ability going from 11 to 14 and and this is achieved by not surprisingly a reduction In the alloy content you see here the maximum amount of carbon is reduced a maximum of manganese phosphorus and sulfur all reduced for form ability look at this whatever the properties yes well let's look at the properties that are that would be used in automotive applications for thickness is 1 . 5 to 2 170 to 300 yield strengths and tensile strength less than 400 make Pasco and elongate shuns totally longer agents are a ovary large more then 20 per cent and in the last 1 year his DDE 14 more than 30 per cent these the these are not typical values deserted the value set by the standard yet so there the properties can be much better in practice and will
see fewer numbers here I'm saying here hot rolled grades according to just standards that you can compare so you remember here for just that stands for steel and so on the stands for playing carbon steels and then the use see here stands for commercial drawing etc. extra drawing and H here is for hopeful conceded that where you can this list allows you to compare these 2 types of
grapes in U.S. and some of the typical great series have drawing steel DS Type a and Type B again carbon contents low values against the Sept low-carbon steel or not I have still listed so you looking at 200 to 400 ppm of carbon typically I know of no manganese less than a point 5 lower Silicon here and enhance image candidates to most important 1 we intend to at the very end very little outlawing 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 mega Pascal values so that would be 200 to we 100 and 15 the maker Pascoe this a comparable to what the European norms together but
you have formidable hot great you have formidable cold-rolled rates for mobile cold-rolled parades again if you look at the European standards d for drawing C for cold rolling and then that the numbers 1 3 4 5 6 here I just referred to In increasing form ability and you see here that only the these CEOs 6 yes contains Thai Tania so I that means that's the only 1 that meets the uh that requires so the carbon and nitrogen to be stabilized under these grades and have to satisfy the requirement that they're not not Beijing for 50 the forest of 6 months White why would die the requirement while many customers will buy coils yes and then store them for a while until they need them sold that storage period can can be a few months from for instance if his automotive and accompanied the doing of buying still storm and and so they want to make sure that there is no strain aging when they start processing this material itself that's this this additional requirements and again a Titaniums usually indicated in the standards can be substituted by Niobe to stabilize carbon and nitrogen usefully you will will actually see a combination of titanium and end ital if you look at the
properties is seeded the the steals a softer than their hot-rolled counterpart 100 and 40 maximum heroes of 2 18 the tensile strength 272 about 350 typically Inc and the elongate since must be in excess of 38 to 40 students this Avery large envisioned in the cold-rolled grades you have also requirements in terms of the the strain hardening of the end value and the R value note that T R value is In the you is also indicate in what direction you have to take it because of an eye socket you have to take it in there the direction 90 and remember 90 so it 90 degrees to the rolling direction that's usually the direction where you have you will measure the highest our values and you see of course set as a goal from the sea will want to deceive 6 5 in Magadan increased our value to the material become more formal ball as I go from low-carbon steels to IST standards we want to remind you of the fact that this 1 here is titanium the have steel or an equivalent of grade containing both fighting and my own view OK In that so this is the equivalent of here you have the drawing steals a and B. the drawing stealing extra gene drawing steel and the last 1 here is the idea of steel In you could see the requirement is less than 200 ppm of carbon and as you can see here the following Of the addition of tightening Newman there is is compatible with the the stabilization of carbon and nitrogen and again at the please note affected you know these contents of call poorer nickel Molly vanadium etc. These are these and basically background values that this this is no addition these these deals are really not alloyed with manganese ore silicon you don't want to do this because of the steals and that that would make them harder and you want soft very formidable steals again in the period to the American uh those standards for these cold greater again very large elongates sense as high art values In this case the mean value Of the normal anisotropy is specified began large rather than in the case of the extra the drawing steals yes you did mean are value has to be .period war with 1 which 2 2 .period once it has really high and also a large high strain hardening export but in end and so here if you would want to compare these these different grades of Europe the U.S. and Japan you you can see here too How can compare to so very important here the EDD yes the sex and the Japanese sp CGE yes these are all normal aging steals absolutely normal aging so that they are I have steals basically that's the way it there required to be I steals but that so we
have been seeing that of we have the sporadic steals at a very formidable yes far values very little I yield stress and how these these steels are are used because they're very nice to work with In the press shop you can you can make very complex shapes as a reliably In terms of dimension only but they're soft so so with what happens the answer is that I if you are if you had a forensic car-body with 88 something hot it will leave a dent yes like this now in Korea people seem to be very insensitive about dance yes right but in North America and Europe people can be upset about these things very much yes and I very very much so don't ever donned a car in America or in Europe you know it you're going to be in trouble yeah but in Korea people seem to be much less sensitive so but anyway city sales are particularly sensitive to this because they're so soft and so big hardening steals have been developed to try to increase the yield strength of the material but increase said after the press hardening what you could make a stronger steel but you know what that's not what you want to do you what so the debate hardening is basically a steel where you actually use static strain aging the aging effects to create a slightly stronger steel after you have made panels after you have made
France's dollars and how does it work well but when you make panels years get stealing goes through process has for instance this this is the site side of the the car here and then it gets assembled to welded to the body and then you go a you paint the input the paint system on the cart set up and then you bake the pies and the pain breaking through tried painter and that the paying baking is 80 a low-temperature aging process yes do it well give you and annealing basically low-temperature annealing of your steel typically 152 slightly above 200 years end of the Order of minutes 20 minutes has so 20 minutes and so that's actually a the situation where we get aging in steels that can change so this is the
idea of this big Corning sales you start with a a sheet so that's for instance 200 mega Pascal strong against you work hard and it when you press Format Nos 0 that's about 250 and then you do a paint baking so after the for intimated door yes the door panel it's paying and you add an extra 50 make pasta so that your steel in the car bodies out 200 makeup Oscar so that is the the idea of the cotton and so what happens in practice and in the sheet here you have some free carbon atoms and that the the challenge now in these big conning steel sister have a very well defined concentration of carbon atoms so that the material doesn't age yes 4 the deal they press hardening but ages rapidly yes after the press forming enduring the pain Baker and what happens is of course carbon when you do the work hardening you have this locations and when you do the paint banking the the atoms will walk these dislocation into position has an and so but if you would try to dent the now yes you will have the strength will be not here but here to hundreds of 100 Mega Pascal stronger as a result of the work hardening and debate the paint Baker
rights so we we have and we know um how much so you need to be dissolved carbon in get carbon and solution right so we know we've got a maximum 200 ppm is possible these carbon atoms yes I was 200 ppm is way too much these carbon atoms and it is and that we need cast we have to lock the dislocations that you form after a small amount of strain exterior panels that typically strained a few cent that's not not 40 per cent of few cent dislocation density is relatively low right and so these carbon atoms what do they do well let's say you have it the you know this is necessary the unit-cell here as in BCC at this location that lies along the 1 1 1 the direction is a screw dislocation has what if it has is Burgers factor 1 1 1 Burgos factor parallel true this same one-on-one direction you can see the slip playing here of business location is this 1 1 0 0 tie the light plane His quietly this is looking good and so this is location can now interact with carbon atoms in the not too far away from it and these are in interstitial positions said so you've got here here and here 3 interstitial atoms for instance that can interact with his dislocations and
what they will do this you have as lattice distortion from the school dislocation and you have all the lattice distortion from the the carbon atoms at a Reginald distortion you really remember and that will cause the carbon atoms to be attractive 2 the screw dislocations and the edge dislocations and as a consequence the carbon atoms down there at the dislocation conferences if it's an edge dislocation they will tend to be in the tension part of the after dislocation call so you can hear a lot where you have tension due to carbon atoms can gather now the carbon it's
look at 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 called Psuik ordering smoke drink the recorded at and what is that that is the carbon atoms that are in the immediate vicinity Of the dislocations Wales make a few diffusion hops yes 2 wars the dislocation it happens very quickly and because remember carbon jumps about 1 times per 2nd in the iron lattice BCC left them so you introduce dislocations the carbon atom is close enough to the dislocation it will hop to the dislocation and pain this so that's what happened but then pretty much that's about it that's about it you would have to wait for many months and years "quotation mark to see anything in terms of a serious strengthening but if you he got during the big hardening process yes you have to processes that happen 1 process is carbon atoms moving To the dislocations that so that is diffusion process the and what you can also have its carbon atoms moving together and forming Carbide's for instance similar now the kind that takes of these processes are not the same yes these L and this is shown here so if you want to know what to if if you say you you plot the amount of carbon yeah it's taken out of solution that's that's that's not interstitial anymore is proportional it's is equal to the fraction will go and form carbon atmosphere and aunt and the rest will go form carbide precipitates so I get to be the it's these are exponential lost you can derived and theoretically the first one related to the bid formation of atmosphere is 1 minus exponential -minus T divided by 2 now 1 the power two-thirds and for the carbide transportation we have one-liners exponential minus the goodbye to the power the rehab and and and 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 the effect different different the process basically so the so basically what it means is that if you look at said this would be the the carbon content of interstitial carbon content originally and this is the temperature at the time at a certain temperature you will see no B 2 steps indeed the removal of carbon From the solution has 1 will be usually the faster 1 which is all that the atmosphere formation yes and the 2nd 1 is ,comma formation not when it comes to Carbide formations on the question is types of Carbide Dewey for and so this is a diagram showing the kind for the presentation time temperature kind for super set carbon supersaturated far-right at different times the function of the time and we met I want to remind you of the fact that the paint baking some 150 to 200 degrees C that surround this temperature yes and at times are of the order of minutes this so old that would just minutes so so if I have a 20 minutes from right that is what is 1200 seconds like this it's 120 about thousands seconds yes thousands seconds is here so 204 thousand 2nd so we're not really forming statement but we form a low temperature Carbide's which would which transition carbine and and the EU's recalled he took Carbide's or epsilon Carbide's you may have heard about them with nothing go into and too much details about Tuesday's transition Carbide's but that there had been a knock your regular Simonton yes and and they will and so they are both these atmosphere formations and use precipitation give give us strength and as you know From right and if we if we will look
at these precipitates here yes you the reason why the growers because you have if it worse pursuant I particle is because you around a little particle like theirs you create aid diffusion profiles and diffusion profiles which is here far away from the particle you have your regular carbon content the particle itself safe at some it's called a very high carbon content of course and close to the interface the fair rights verite Seaman tight interface we assume we have equilibrium so uh went into the carbon content is dictated by the phase diagram so soon as a consequence because of this the diffusion profile here there is a flow of carbon toward the particles so you can calculate but you know how much carbon needs to the flow into the particle 4 growth it's been analyzed theoretically of the resistance simple solution to this problem by assuming that the this diffusion profile is linear linear arises from profile and this
allows you to determine the change of the radius of the particle asks the function of time nears and and and this is due to the very well known relations and once you know the radius of the particle you can determine the volume of the the carbide that you for as a function of time and you see here you find it functions it where the time is to the power of 3 compared to this allows you to make simple calculations about how fast a particle will grow or if you heat up how fast particles will go into solution you can't good so anyway we can't for instance use this 2 to study the over aging units why over aging important in this big hardening steals because a big hardening steel when you when you use a normal low-carbon steel yes look even normal aluminum killed low-carbon steel and you want to make a big hardening steel out of this 1 yes I remember the carbon content here will be 200 to 400 ppm was very large yes and so on that's the way we we we we we work is by precipitating as you precipitate most of this carbon now during the what's called the over aging remember the overreaching the over and here you can see for instance I wouldn't have do the growth of the Simpson type particles during the overeating at 400 degrees C people because you can really precipitate precipitated carbon and keep a certain amount of With you know the Rangers you know the volume and so you can calculate how much carbon you leave in solution to give you the a break hardening effect that the suit remember the overreaching in continuous annealing so you have you 1 like so here you precipitate the same entire but you you you you leave a small amount of carbon in solution to give him the the heart the for which is shown here you have your the semen tight particles and carbon in the uh solution so if you have a big hardening steel you will have seen type particles and carbon atoms in solution and you can make big hardening grades With batch annealing and which continues in 10 and it would be the most important 1 2 days is this the 1 where you see the overreaching in continuous annealing and it's really important to control the precipitation of the same attack because you have these very large carbon contents In these low-carbon steel no what
about St right so we're very happy about have steals because while you know you buying Titaniums buying carbon so there's no problems in terms of properties along the Strip you have a very low strength and tensile strength in very high our values and work hardening rate that's a perfect steel but it's fully stabilized nitrogen is stabilized by tightening carbon is stabilized by dieting or by so you cannot make bake hardening steals yes because you stabilized OK so that is where we a the the the idea of using albeit as a carbon stabilizer also comes because let's look at the the citing you or tightening in Iulian skills so when you have an eye S. steel you have an extremely low carbon contents so this would be his 200 ppm does is 100 ppm this is 50 ppm so you have around this much carbon 20 ppm content but all this is stabilized this is about to come so how do you work well so you have the Microsoft touches carbon is stabilizes niobium carbide so what do we do we heat up but we don't heat up to 700 deg C we had we heat up to really high temperatures 800 degrees Wheat are we worried about making Austinite no because we have so little carbon so that the the 81 temperature becomes irrelevant yes now if you he up it's only Alpha here OK so there's no danger that you're going to get worse our values because it so so he had to very high temperatures and what happens here yes is that some of the 90 or some other type thing you will will be entering of some of the Titaniums Carbide and niobium carbide will go back into solution yes and for denial BIA inform carbon insulting them in solution again and now when we cool down yes Of course the solubility decreases very strongly some of the carbon may actually form Carbide's again but if we cool down fast enough about 450 degrees so we can be left to wait a few ppm less than 10 ppm 5 or 6 ppm typically of carbon in solution and that's enough to give you the cotton but they can't "quotation mark alternatively yes some clever people have said well if we're going to make it an ideal way sorry freedom to make big hardening seals yes let's not so much niobium or let's not add so much tighter yes let's just leave carbon free at all times and these are what are called you LCD they Coddington ultra-low carbon steels and so here we have extremely low carbon contest and we don't stabilize we don't stay but we don't have to stabilize them but the only way you can make these seals is by really being able to control your secondary metal achieved and have extremely low carbon called so in this case you just reheat you you received to get your texture control in the grain growth of this you will see steel and then you just do rapid cooling and keeping your your carbon in solution again you can
note perfectly control for for instance for these and niobium Sadie's niobium carbide Bay coddling steals and we look for instance at their dissolution both the Union Carbide in but during continuous annealing and say you have some Niobe in How long will it take you to dissolve the nite William Cartwright 1 minute 3 hours while you can calculate you calculated because we know how the kinetics of particle the growth and particle dissolution so let's have a look friends we have a big hardening steel yes which has nailed as used to stabilize carbon and say we have 150 ppm of niobium and 20 ppm of carbon and we look at 800 deg C will look at the dissolution of the Carbide in Feira so we we need to have some data for the so this would be an ideal here could correct this but diffusion constant for niobium and far-right and then we we need also data like the solubility product for myopia anyway is that you don't have to worry noted that look at the deals just look at the results of the particle here that dissolves into the the matrix and you can see here it takes you about a little over 2 minutes yes To dissolved but of you know you'll be Carbide so you can do this in a controlled manner yes and so on and get your and and design your process around this data and the kind that takes what is and what is the will and what controls the kind that Texas is the diffusion of 90 opium in the far-right because when the niobium carbide goes into solution yes you have carbon goes into solutions and niobium goes into solution yes no carbon is a really fast diffuser yes really fast diffuse yet so the dissolution of Malian ,comma is not controlled by the carbon diffusion but it's controlled by diffusing away from the niobium carbide particles but you see it's fast yes it's fast and then you can do it in a continuous annealing furnace quickly but
Baker the steals if you look at the big Conable steel an unknown carnival steel the microfracture exactly the same thing obviously what makes a Bay Conable steel they Conable optics to read are ppm levels of carbon so you're not going to see this on in the Microsoft rupture was this an example here of the big-hearted of all steel a typical for grains yes so here the
well yes the kind that takes usually measuring the kind that takes of carbon and solution it's not something you can do chemically because when when and if you go to a lab and you say on it is a piece of steel I think it's got to 100 ppm of carbon please give me the carbon content than what usually happens is the chemist Will basically destroyed the sample yes destroyed a sample some way or another yes and then measure how much carbon is there in an hour and etc. like magnesium it's manganese etc. but if you want to the chemist in side it has this sample contains 200 ppm of carbon tell me how much carbon isn't solid solution that is another that's very difficult task you go on and you need special techniques to analyze how much carbon is in solid solution was technically people will use a technical internal friction because an internal friction you can measure of you it's a technique that sensitive to interstitial atoms wide carbon or nitrogen but it's a it's not an a simple technique that's so so what do we do well we measure then the mechanical properties basically what we what we do yes because of that you will take a sample of of your big Conable steel and this will be a big choruses and you will priest the severe I'm amp restraining might be Connell steal up to this point In this case I stranded about much about 5 per cent OK 4 . 0 4 OK and that's that's going to get in and then I take this steel and I'm aging it yes I'm aging it after priests trained and that means I will take that in this particular case Beijing time this 1 this particular thing example here was 170 degrees as for 20 minutes minutes that's 170 degrees C so the so what you get when you take the same material this is what you get you get a material of course with the yield .period and the yield .period elongation yes because it's been aged Nos and we can now measure this is the difference here this increase in strength From so which called yield plateau the the flow strength stress ahead here and we can measure this as a function of time mechanically and if we do this for instance for priest stains of 5 per cent and we we strayed 5 said and then we put it in far enough In the heated at 50 degrees and we make 20 samples in the 1st sample we take out of the 5 of 10 seconds after 30 seconds after 60 seconds and we measure this Delta Sigma so what we see is we do we see an increase in stress and here there is a small plateau and then it continues to increase and analysis of decrease OK this is this is how they Cotton Inc it is evaluated now the what is it important for the the industry is is this is that about 20 minutes or so this is 10 minutes 20 minutes 20 minutes at 50 degrees C that's down it's very young very low temperature I actually I chose this temperature because it illustrated the fact that you see you have 2 levels of things happening here versus the atmosphere formation and then this stage here is the precipitation staged the 1st week form atmospheres venue for precipitates and then the strength decrease here is because the precipitates continue to grow if continued to grow and you the cost and then you lose strengthening because they pick the away carbon and then they become larger and because their larger than the the provide less strength 10 no as if you were
ever involved in aging studies and you will see that things are more complex In general why either more complex because but for instance we already talked about this very often your steals are alloyed with elements that may interact with carbon so in addition to carbon and solid solution carbon and dislocations carbon at Carbide's you also have carbon Close to substitution all elements which will behave differently and then what's really important we also have boundaries grain boundaries and grain boundaries also act as sinks 4 carbon and so when you ask your chemists tell me how much solid solution Carbon I have this is solid solution carbon this is solid solution ,comma this is solid solution ,comma and this is solid solution ,comma yes this is precipitated cars so there are many different types of carbon in solid solution and they will all in fact the mechanical properties differently we know what we know that carbon and grain boundaries is probably the calls the whole patch affect him on this 1 here the aging effects that this 1 here precipitation hardening etc is so but let's have a look at grain boundary carbon in the normal situations In a brake on steel so if we have look what happens to bake hardening steel this is so here it's cold rolled this week heated up foot in continuous annealing and we cool down quickly yes and then we over a change to the overrating to precipitate car behind and then we cool down to room temperature OK and then it goes to the price shop for instance where he gets small amount of defamation few % His and then you go to the bank hardening they Cotton Inc that typically it's a 170 degrees C and then that's quite long and 20 minutes OK so what happens well let's have a look at the Cobb we have carbon interstitial carbon and dislocations and carbon and grain boundaries how does this you have to know how can you think about the distribution of so the at at room temperature carbon goes into grain boundaries because remember carbon still ability in verite is nothing yes so and if the material is very well read crystallized their no this locations to go to yes so it will go carbon will go into grain so at the start here in the car at the start we get Of The year annealing we get lots of carbon in grain boundaries and we gets some of it in interstitial of interstitial when when we heat up however as we keep this up In the common in the grain boundaries that leaves the during batteries pretty quickly and and we do most of the carbon will be intersection most of it is when I cool down yes it's increases the amount of carbon in boundaries again remember we don't have much to so many dislocations the temperature constant temperature constant in this case in this case it's assumed that we don't for Carbide OK for instance it's an ultra-low carbon steel look at and then we continue cooling to room temperature of the the carbon interstitial decreases and the carbon goes back to grain boundaries defamation year creates a large amount of dislocations the dislocations the amount of dislocation increased so when I do the low-temperature annealing yes a lot of the carbon goes to dislocations yes and and that's it that's what causes a decrease in the interstitial carbon and With tide you also get an increase in the amount of carbon the ghost grain boundaries this is an important aspect and if you don't have many dislocations yes and you have a well structure is going to be lots of carbon the impact of the grain boundaries be relatively import if you have small grain sizes .period the impact of the grain boundaries are imported but once you strain material but the impact of grain boundaries are world
smaller OK so let's have a look at some of typical big hardening steals what kind of compositions do we have OK this is the big hardening steel here there's been vacuumed treated announced that it has the money will be used as a stabilising element and by Tania to stabilize the nitrogen track so there's not much in this material With the exception of carbon 90 William and tightening I went on the properties here it's it's still a pretty soft material 300 and the yield strength I would look at what I want to focus on is how much they conning you get typically 50 megabytes "quotation mark Is it that's that's the amount of the card meaning you can get I also want to mention the fact that when people evaluate "quotation mark the big hardening values here you will see either the H 2 values the itch to values or B H 0 values H 0 value as as well there is no priest trained in this case In How does it work you measure the yield strength of new material yes and then you have measure the so you take your material I knew strain Antonis so you can marry the yield strength and then you take the same material don't strain it you emulated the typically 20 minutes at 170 and and you make sure deeds cancel stress stress on stress-strain curve and this increases In the the yield strength yes it's called the it's 0 In the case of BH true what we do is 1st you take your material yes you strain it 2 per cent yes the strain at 2 % this much OK so that allows you to measure the yield strength and the flow stress then you unload the material I knew immediately typically 170 degrees 20 minutes and then you retest the same material you get aged stress-strain curve and and so you can determine what is the amount of work hardening the strength you to work hardening strength due to bank and this is the yes so not not not this year right not the difference in yield strengths but the difference between the flow strength and the yield stress takes to the flow strength and the yield strength after thank you rights a summer day
to hear standards data on Conable steals low-carbon they Conable steals about 400 ppm of carbon then what's interesting here is of course what are according to the standards the minimum required beach to values yes so 35 major Pascal is is typical minimum standard requirements for cotton the STM you see on the beach carnival steals a compositional ranges here now I'm trying to do the strains the focus of the big hardening index here is that given as upper yields and lower yields he should have values here and I don't
think the dead just remember and
that standards submitted usually minimally required 35 MPA and that in the in normal cases you can count on it 50 and years difficult and so forth low-carbon steels or ideas steals or ultra low-carbon steels but of course you can do Beijing tests with other types of steel led Corning steals trips to other complex face field and they will give you all the bay Congress spots but which may be very much higher actually book now another the problem is that In a related to the design of steals and aluminum killed low-carbon steels Is that I they did do have is inherently lows drank yes and that carmakers and other users of steel we are always interested in a light waiting facts in making things like that and I'm so there is a need also 4 steals the are stronger so that we can use that I can use their engaged 10 gage materials and so on indeed this has led to I steals and aluminum killed low-carbon steel which are alloyed with phosphorus manganese silicon all the elements which usually phosphors manganese ore silicon to increase the strength by solid solution hard and you see here that of course we already know that phosphorus magnesium silicon give give us all a lot of strength and that phosphorus this is a lot of strengthening for very small In addition to and that is best this isn't 1 of the areas in which we
will act phosphorus to stick this in solid solution strengthen steal some news inches as an example here I've taken from European centers where we have the so-called really false or phosphorus solid solution strength steals where phosphorus it actually used as an AL-leading element to increase the strength and in you can see here strength levels depending on the amount of time phosphorus can now increase 2 300 350 yield strength 400 to 500 in tensile strength and again these are recognized grades you have we false standards for in Europe and in the in the West 1 of the things
that happens of course with phosphorus and and many people are always worried about the use of phosphorus as an alloy heating element is unbridled there's no when the sign steals of 1st came there were used I they were around used very widely and when the reformist ideas deals that were developed and there was also a very much interest in but the in the end they looked like very safe materials because the amount of phosphorus that you added was really love around 500 ppm so below the danger level where TO phosphorus was to grain boundaries and and brittle statistic yes but it turned out that and there is a phenomenon called secondary work and brittle meant that was observed that means that take for instance you make a cup yes you former cop made of phosphorus alloyed steals and 2 compass perfect it doesn't break or anything but what happens is that when this happens often in um in pressure ulcers when you press apart you don't process but once you it can go to different press stages so you in during this processing you deformed already deformed material used and it turns out that these phosphorus alloyed steels were kind is sensitive to this secondary in extra work of fracture vacancy here so if you want this is the ouster on cup if you try to do deform this Cup again like making it slightly wider now you see that it will be behaved brittle in a brittle fashion and it'll be haven a brittle fashion and if you do the deformation at slightly lower temperatures so that got people very worried of course and because as you can see it's it's quite impressive the unbridled this is so
yet so let me go back if so what
what it's not in the notes for about this so what it what did they do what to do or what do you do to prevent us if you add some boar so that's why many of these deals this phosphorus containing grades on nowadays contains some more on very small amounts of boron 10 ppm has had to Boren goes to the grain boundaries and because it's also likes to segregate to grain boundaries and rich in the grain boundaries it doesn't do any harm at a grain boundaries the boron blocked it prevents the phosphorus from going to bring back because there are only so many of the positions that are available 4 atoms on grain boundaries so far this grain boundary competition between boron and phosphorus is always an advantage of the boron and the phosphorus stays in solid solution and you don't get this secondary work and brittle problems in practice
right and let's let's just start and finish it today with this slide here VII I With the aluminum killed steals and be IDS steals and and structural steels will see I will start with a on Thursday this week about with you we know we we have a basic problem and there the the problem is that we have a low strengths we courts we can increase them by doing for instance the continent or we can Joe a solid solutions wrangling with phosphorus magnesium silicon editions but only so much so and so on what people has been developing since In the last decade has a colossal of development are and steals with higher stress it's so 0 so we can have a higher strength the engineering and construction under are also weight and and of course 1 of the problems that needs to be addressed that as we increase strength tests we usually see that plasticity measured as total along Asian are uniformly and value will tend to decrease so and and that has been to 1 of the major challenges in the development of highest-ranked steels is how do you developed a strength test and keep very high for mobility in your Europe in university in this house and so will talk about a few of these solutions the that's the thing