The yield ratios (ultimate tensile strength/yield strength) of our as received steel sheet are varying in a wide range ( from 1.0 t0 1.5) while the ultimate tensile strength and the elongation of steel are quite stable. What is the reason behind this?
The reason might be found in the state of the "as received" steel Sheets, meaning the stress state (residual stress Level). Since the ultimate tensile strength and the elongation of steel are quite stable, the variation arises most likely because of a Variation in the deformation state (from sheet rolling) or Level of annealing. If you carry out a stress annealing heat Treatment, the differences should become smaller in this case.
The difference in yield strength between baches really become smaller after heating on 800C to 850 C for 20 minutes , our enamelling process temperature, higher than the temperature of stress annealing heat Treatment, )
i don not know how the coil temperature varying between batch to batch or the difference of cooling rate between edge to the centre of the sheet after coiling
Hence you have already a good hint to look at. Maybe you can further check for the effect of temperature Variation by taking samples from the Center, the surfaces and or the edges. If they show repeatedly similar behavior which corresponds to the one you would expect due to temperature variations your assumption become more realistic and you could decide on maybe more expensive analyses to make your point.
Variations in the yield level among a set of specimens indicates that the on set of plastic deformation is changing from specimen to specimen. Since you are dealing with presumably the same sheet thickness, the state of stress is supposed to be the same and should not be a contributor to the observed variation.
Assuming it is a standard industrial product, I do not think that rolling/coiling temperature should be an issue as well (you may check it out by taking specimens from various locations of the coil/sheet as pointed out by others or by simply checking the variations in microstructures, if any, at different locations).
The most probable reason that appears to me is the manner in which the specimens were cut and the cold working if any imparted to the material during the specimen making process. You have not mentioned the sheet thickness. However, it is very easy to bend sheet specimen during specimen making process thereby inadvertently cold working the material. If you are fabricating specimens through a machinist who does not understand the metallurgy, it is quite possible that the piece of the coil material is straightened to remove curvature prior to specimen fabrication. Check out how the specimens were made. If necessary, use EDM or water jet to fabricate specimens.
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