Influence of Processing Parameters on Recrystallised Microstructure of Extra-Low Carbon Steels

1. Influence of Processing Parameters on Recrystallised Microstructure of Extra-Low Carbon Steels C. Capdevila, J.P. Ferrer, F.G. Caballero and C. García de Andrés Solid-Solid Phase Transformations Group (MATERALIA) Department of Physical Metallurgy Centro Nacional de Investigaciones Metalúrgicas (CENIM) Consejo Superior de Investigaciones Científicas (CSIC) Avda. Gregorio del Amo, 8 E-28040 Madrid, Spain www.cenim.csic.es

2. Focus the problem The recrystallisation in the as-coiled during production of thin hot strips of extra low carbon steel (ELC) for direct application, is obtained if the finishing rolling temperature (FRT) is low enough (FRT ranging from 750 to 820 ºC) to strain the ferrite and the coiling temperature high enough (above 600 ºC) to recrystallise. Bearing in mind that that the inner and outer wraps, as well as the edges, cool faster than the centre, the coiling temperature must generally be kept between 700°C and 650°C in an industrial line. As a consequence, it is very difficult to obtain a fully recrystallised material after coiling in the thinnest sheets due to thermal looses. In a joint project with ARCELOR and CRM it has been proposed a new strategy which consists on transferring a hot coil (2 – 4 mm) from the hot strip mill (HSM) directly to an additional rolling stand. After recoiling, a heavy warm deformation (HWD) is performed below the  -  transformation temperature to reduce the thickness down to 1.2 mm or lower, followed by recoiling. The conditions of this last rolling process and the coiling temperature are of vital importance to obtain a fully recrystallised material. In this sense, the goal of the present paper is to study the influence of those parameters on microstructural evolution of ELC steels.

3. Simulation of the process Soaking 1250°C - 60min hot rolling FRT 900°C, 850°C 20-12-6.5-4 ROT cooling 4-5°C/s final coiling coil transfer HWR CT = THWR + 50°C entry thickness 20mm Study of rex-kinetics at CENIM

4. Simulation of the process 0.03C-0.17Mn-0.009P-0.005Si-0.006S-0.041Al-0.003N (in wt.%) Specimen processing parameters After HWD material is annealed at temperatures ranging from 525 to 700 ºC for 1.5 h to simulate coiling

5. Evolution of the microstructure Thermo-Electric Power (TEP) measurements have been used to track microstructure evolution DS = DV / DT S depends on the microstructure: -Decrease of elements in solid solution: S  -Increase of dislocations S

6. Evolution of the microstructure Influence of annealing temperature after HWD (simulation of coiling) final coiling Annealing temperatures ranging from 525 to 700 ºC. Holding time of 1.5 h. Heating rate up to desire temperature of 50 ºC/s.

7. Evolution of the microstructure A detailed analysis of TEP results presented in this figure revealed several common patterns that should be highlighted: 1- Higher TEP values are recorded in samples with high CT temperature (ELC-F, ELC-H, and ELC-I) as compare with those with low CT temperature (ELC-C and ELC-L). 1- Higher TEP values are recorded in samples with high CT temperature (ELC-F, ELC-H, and ELC-I) as compare with those with low CT temperature (ELC-C and ELC-L). 2- The same TEP value is reached at annealing temperature of 700 ºC for all the studied samples. 2- The same TEP value is reached at annealing temperature of 700 ºC for all the studied samples. 3- There is a drop in TEP first, followed by a sharp increase.

8. Evolution of the microstructure Role of cementite Simulation of TEP evolution assuming cementite dissolution as the only phenomenon occurring ELC-F at 575ºC ELC-F at 625ºC

9. Evolution of the microstructure Role of AlN Cooling from hot rolling is fast enough to avoid AlN precipitation: ELC-C and ELC-L samples where transfer was undergone at 650 and 600 ºC, respectively, present the lowest TEP values. This is consistent with the assumption that most of Al and N are still in solid solution in this material. By contrast, steels ELC-F, ELC-H and ELC-I where transfer was undergone at temperatures around 700 ºC present a higher TEP values which indicate that some AlN precipitation events have occurred during the transfer and HWD The rise in TEP at temperatures above 625 – 650 ºC during the reheating cycles could be also explained in base of AlN precipitation. Moreover, the similar results achieved in all the materials studied after annealing at 700 ºC suggest that most of the AlN precipitation events have taken place

10. Evolution of the microstructure Role of AlN

11. Influence of AlN precipitation on recrystallised grain morphology Evolution of hardness and ReX volume fraction with annealing temperature

12. Influence of AlN precipitation on recrystallised grain morphology For the same deformation level: The higher the CT is, the bigger the delay in the beginning of ReX is (ELC-H vs ELC-C, and ELC-I vs ELC-L) Those samples with lowest CT temperature (ELC-C and ELC-L), and the highest HWR (ELC-F) present the earliest recrystallization start temperature.

13. Influence of AlN precipitation on recrystallised grain morphology Evolution of grain size and grain density Two different tendencies can be distinguished: 1- ELC-C, ELC-F and ELC-L samples (low CT with exception of ELC-F) present smaller grain size and higher values of n as annealing temperature is increased up to 650ºC. 2- ELC-H and ELC-I (high CT) present increasing grain size and hence decreasing n values as annealing temperature is increased.

14. Influence of AlN precipitation on recrystallised grain morphology ELC- C HWD=46% CT=590 ºC 525 ºC 550 ºC 575 ºC 600 ºC 625 ºC 650 ºC ELC- H HWR=48% CT=634 ºC 575 ºC 600 ºC 625 ºC 650 ºC 700 ºC CT   ReX starts at lower temperatures CT   Coarser ReX grain size. Nucleation is difficulted

15. Conclusions The results obtained after different warm rolling conditions and reheating temperatures have been analyzed to conclude that slight modification on HWD conditions could induce big differences on the subsequent recrystallisation processes which take place on reheating. The absence of AlN precipitation, and a higher stored energy, produce finer and more equiaxed recrystallised grains in materials with high HWR and/or low CT temperatures. High transfer temperature promotes the AlN precipitation during the transfer. This precipitation, together with the low stored energy for recrystallisation because of low HWR, leads to coarser and irregular in shape recrystallised grains after subsequent reheating. The lower the transfer temperature, the small amount of AlN precipitation during the transfer is. The low store energy in samples with high CT temperatures and low HWR induce the overlapping of recrystallisation and AlN precipitation, promoting the block of recrystallization.