Modern Steel Products (2015) - lecture 6

right so we had started some of it in so we were visiting and talking about composition of steels and in relation to how you can make different Mike restaurateurs and now we're going to the discussion of compositions and how they affect the the strength properties told you that strengthening could be achieved by by young adding some elements that if you solid solution strengthening you can do you can deform the material the you have dislocations that give you strain hardening you can reduce the grain size has and you can I 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 I need to add false resource silicon manganese so that will affect the the composition and will also see that I 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 will be discussed and that at the moment right OK so I'm so

what is mechanical properties things like real strength ultimate tensile strength usually a low-carbon steel will have stress-strain curve like this you may get some university long-duration as you know and then be applied engineering stress will reach a maximum the material will then start to next the 1st former diffuse yes at this time there's only deformation in this zone here than the disowned doesn't matter the defamation stops so after the material starts tonight you get all the deformations the localized in this neck and eventually you form a local neck before fracture this is for this is the behavior you will see but for the cheap material should steal for instance in intentions when as what is important here to realize is that but if

you so if I may go back to

this the picture here you see this is your sample here it's in the tensile machines so you pull on it in the length direction to his length strains but it also changes in thickness and in West the reason why is of course because when you do plastic deformation you have to have a constant volume so if you make something longer you make it longer in this direction you can you get with strange and thickness straight and that's

important I want to know if this is the length strain you have the wits strain and the thickness training Our negative because they will be reduced and so this is this the red curve here is the the thickness strength negative them more the so if I have Cindy volume is constant during plastic deformation that means that the strain in the length direction was the strain in the thickness the the strain in the west direction is 0 president misses no change In fall so when you were and but this is positive the pool and I these 2 when would the form of sheet material that usually is a constant ratio between these 2 which we call are which is the wet strain Over the thickness string racial artist which strength over the thickness and we'd like to to have this the value we like to have a high the reason is With the help of volume here the the material and steel and I pull on this in this direction so there will be a wet strains and also the thickness trade so you want to win strain to be larger than the thickness string wide because when something gets thinner yes there is a higher risk that it will fracture but it will but that it will allow that to defer their information will localize and you will get fracture and that's the reason why we like to have the some materials with this ah value strains over thickness strain also large largely positive and lots of cancel this because when so this is if we have less weird strange that the thickness strain becomes larger differences when you make it difficult for the drawing Pardee drum parts as such as this oil and here I you there is a risk a higher risk of fracture so how do we know how do we can all this while we will see I think you do this was lecture progress that this is done by texture control texture control but and and it's a very important parameter for hours should still be close Florida that said it needs the power and the source of the yield strength tensile strength along variations and of Our value as we call it are musicology this factor is also called the normal anisotropy normal anisotropy best won't have to be Hi there and we control but we can control the strength for instance by means of the solid solution hardening and another thing we like to control is to the grain size and and you know that that's the reason is because this equation of his empirical equations as appears to hold in many cases steals and basically says that if you reduce the grain size of grain size is smaller lot here in the XX won over square root of the the you get higher strength that's a very nice way to get a large

stressed the aim of the this technology the steel industry we will usually do not define a grain size the same way you doing research 1st of all we tend to still talk about 80 grain size and obviously if you look at this year I what you can see that there is a distribution of grain sizes and that's 1 of the things and 2nd well how would you define as the grave sites you have distribution so what usually do in technology you just assume that these grains are spherical Of course which is a a very great simplification has and then you just wrote you do not

really define a grain size In Technology Inc in technology you define a parameter which is called the the number of grains poor surface area yes and that is the basis for what we call the AST M number yes the STM number and that is often used in technology and foot to describe grain size of steel and it and basically is this description of the density of grains aid metal graphics sample observed at the certain magnification in an optical microscope so that's something I'm so and AST numbers and they go from typically of the skill is larger but typically will go from 1 to about 10 or more room and so so the way you have to look at it it is this is the AST is ATI it is an organization professional organization that publishes the standards for materials and also for methods of measuring things I'm so what would you do you observe that 80 250 micron by 250 microns on the surface of a middle of graphic sample that went in the middle of graphic microscope so that corresponds to 26 at 62 thousand 500 square microns here and as I say you have 1 grain yet 1 grain in the year yes it will have a diameter of 250 might do we assume all the grains of circular but on the of grains spherical and so if you make a got you use the agreement hammered out so this is so I had 1 grain with a 250 L so on and so if I had come to me a grain that's only 1 micron in size I will have complete if I would fill the surface 512 practice from care so this so you see here grain diameter 250 corresponds with the esteemed and number of 1 that means and this is the size of this grain as to the surface covered by this disgrace to the number of grains for 1st surfaces 1 and then on the other end of the scale I have a diameter of 11 in this Dakhla scaling 11 microns corresponds to said 1 micro 9 the because inspectors of Michael's at 512 and at the UN in that particular scale corresponds to tap yes and for those who familiar with steals you know that most of the grain sizes of 4 steals will be within that range of very great here and out so the Sitser that's how would the grain sizes is defined from a technical point of view but so of using this formula here you can go from the BSE and number 2 the grain diameter listen you can you find a single number good but you have to be aware of the fact that I was it's it's it's it's not defined this the way you would expect me and in research we would we will do it we will measure for instance intercept lengths and averaged out and we have all the techniques to to measure grain size average grain size for the distribution of grain size OK but but so we won't be talking about this so we will just assume that there is a risk the grain size we get from it for instance STM technique that measures that the surface area of of the surface density of cranes that that's that's the good way 2 1 2 obtained a grains of 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 but we do get also improvement in toughness now toughness but that doesn't mean that the elongation increases and the contrary you get last elongation when you reduce the grain the last longer so the material has less plasticity what so what's what's the meaning of toughness that toughness refers to the resistance that material the gives too fracture propagation and that's different from last dissidents why is it that we get less uniform elongate when we reduced the grain size it's because 2 this is the you may have to write this down because it's not in the slide when 9 when we have look at the whole batch equation for the yield strength we find a curve like this as and when we have to do you can measure did hold pedigrees and also for the tensile strength and if you do that the tensile strength you find something a line which has a lot slope that is smaller so what does this mean is that there is a grain size here yes where the yield strength and the tensile strength are the same yes and when you have a yield strength is the same as tensile strength you have no a long vacation anymore yes it's like the fact my so it's like the yield strength becomes the same as that it's so that means I have no no we'll nation no uniform illumination anymore yes but so so that's what you generally see is that the grain sizes negative impact on the plasticity so it's something you you have to be aware of when you want

improve matters through grain size reduction now how do we control the grain size being in steel in the technological circumstances well we usually do that during the hot deformation Of the material so let's have a look at what happens when we do hot deformation of a steel in hot strip mill in the hot strip mill we roll the material at different temperatures as so what do we get but will 1st of all this crashes the temperature as a function of time position in the mail and you and these windows here are defamation steps you can think of them as being I was in a rolling mill these would be rolling passes so in there and so you start with the material is at high temperatures high temperatures so we know it's Austinite and as you roll let you roll in and then you pass it to another male Poland it again but there can be many steps of this time 4 or 5 steps of what we call roughing mail at at relatively high temperatures with this high temperature the 1100 deg C or more yes and which you get is bit of Austinite the forests and we crystallizes constantly as he it 3 crystallizes and becomes soft again as a tree crystallizes I get some grain refinement yes but the grains refined and ending growth and it's good because we are at high temperatures of the kinetics of this grain growth this is expected rather fast and so even tho we you will want a defamation is a lot of defamation plus 3 crystallization because of we end up with a relatively chorus also grain and then we go through a very thin region which is called the nonpareil crystallization region various small region where the Austinite if we were able to deform in this particular region of the Austinite would not trick crystallized and I would be able to transform when she was as weak as we cool down we we stop this year finished rolling