Evaluation of nitrogen remaining in solution in an Al-killed steel by measurement of Thermo-Electric Power and Internal Friction

The evaluation of nitrogen and carbon remaining in solid solution in deep-d rawing steels is very important with respect to their ageing susceptibility This evaluation is usually carried out by indirect chemical analysis or mo re directly by Internal Friction (IF) measurements associated to C and/or N Snoek relaxation peaks under cyclic loading. Unfortunately, this East tech nique is not easy to exploit as soon as the steel is textured or C and N co exist in solid solution with substitutional atoms like manganese in particu lar. Application of TEP measurements to cold rolled steel sheets In consequence, we have attempted to develop a new method which is giving a ccess quantitatively and very easily to the N, and eventually C, solid solu tion concentrations. For that, we used the Thermo-Electric Power (TEP) meas urements based on the Seebeck effect and took advantage of the well establi shed sensitivity to the microstructural evolutions in alloys, and steels in particular, of this technique. We developed an original experimental proto col allowing to determine the quantity of nitrogen and/or carbon in solutio n while following their trapping by dislocations resulting from an ageing t reatment (similar to 1/2 h at 120 degrees C) after a significant degree of cold-rolling (similar to 80 % reduction). In these conditions, the measurem ent of TEP variations between the cold-worked state and the aged state perm its to quantify with a good accuracy (+/- 2 ppm) the concentration of inter stitials in solid solution. Internal friction measurements On the other hand, the experimental measurement of Internal Friction was im proved so that it becomes possible to separate the C and N relaxation peaks . This allowed to show a very good agreement in the relative evolutions of nitrogen concentration in solid solution determined by TEP and IF measureme nts during an isothermal treatment at 600 degrees C (1 min to 230 h) realis ed in a low-Al steel (15.10(-3) %) with increased nitrogen content (similar to 12.10(-3) %). Comparison with the results of chemical analysis Chemical analysis also showed the validity of the result obtained for the a mount of nitrogen in solution determined by TEP In particular the possibili ty of completion of nitrogen precipitation in this steel found by TEP is co nfirmed by chemical analysis. This last result is very surprising because t here is an excess of N with respect to Al in the steel composition and not all the nitrogen present is required to form AlN. So, a more specific conti nuation of the metallurgical studies should be undertaken in order to deter mine the nature of other compounds suitable to be formed Other possible applications of TEP measurements This new method, which is able to quantify N and/or C in solid solution fro m variations of TEP after cold-working and ageing, can be applied to all st eels which contain less than about 20.10(-3) % interstitials in solid solut ion (low-carbon steel in the annealed condition, ULC steel, IF steel, etc.) .