Characterization of Centreline Segregation of Continuously Cast Microalloyed Pipeline Steel

• Author / Creator

  Wang,Pusong

• Quantitative electron microprobe analysis (EMPA) was used in this study to quantify the segregation of alloying elements (e.g., Mn, Si, etc.) at the centreline for continuous cast slabs and pipes of microalloyed steel exhibiting different Mannesmann Ratings. Elemental segregation levels at the centreline region were compared to the microhardness readings. Mn levels at the centreline were studied at various homogenization temperatures for a variety of times. An EMPA elemental mapping technique was developed to quantify elemental distributions over a relatively large region of the centreline segregation band for continuous cast slab samples. Line scans for slab samples were conducted based on the EMPA maps. Segregation ratio (S.R.) was used to evaluate the level of elemental segregation in the centreline region. Peak Mn levels of 2.6 wt% (S.R. = 1.65) and 2.67 wt% (S.R. = 1.53) were found for Mannesmann 3 samples in the as-cast and pipe conditions, respectively. The peak Mn levels for Mannesmann 2 samples were 2.32 wt% (S.R. = 1.47) and 2.30 wt% (S.R. = 1.32). Image analysis showed that both slab and pipe samples have wider centreline segregation bands in Mannesmann 3 steels than in Mannesmann 2 steels. Microhardness testing across the segregation band showed higher hardness values for the Mannesmann 3 samples relative to the Mannesmann 2 samples. In addition, the Mannesmann 3 samples had a higher hardness peak to average hardness ratio (P/A) than the Mannesmann 2 samples. The maximum microhardness values for slabs were 263 HV and 289 HV, with average values of 241 HV and 251 HV for Mannesmann 2 and Mannesmann 3 steels, respectively. This gives a P/A ratio as 1.09 and 1.15 for Mannesmann 2 slab and Mannesmann 3 slab respectively. For pipes, the Mannesmann 2 samples had a maximum microhardness value of 250 HV (P/A was 1.16), while the Mannesmann 3 samples had a maximum microhardness value of 303 HV (P/A was 1.55). Homogenizations (1100 ˚C, 1200 ˚C, and 1300 ˚C) reduced Mn segregation at the centreline for both Mannesmann 2 and Mannesmann 3 slab samples. For the Mannesmann 2 samples, the Mn S.R. decreases most (from 1.54 to 1.08) at 1300˚C for 2 h, with average microhardness decrease from 241 HV to 208 HV. For the Mannesmann 3 samples, the Mn S.R. decreases most (from 1.69 to 1.36) at 1100˚C for 16 h, with average microhardness decrease from 251 HV to 205 HV. S.R. decreased dramatically for single segregation band samples, while there was only a slight reduction for multiple segregation bands. A macro analysis was used to distinguish the difference of segregation behaviour between Mannesmann 2 and Mannesmann 3 samples. The sum seg (SS) number was applied to microhardness, ferrite grain size, prior austenite grain size (PAGS), image analysis and Mn composition for slab samples, pipe samples and as-homogenized samples. The SS number indicates an increase level of segregation from Mannesmann 2 to Mannesmann 3 sample. In the future, a SS number database could be developed with more samples analyzed. This database provides a fast and easy way (using image analysis) to diagnose the casting process parameters for both research and industrial use.

• Subjects / Keywords

  • Centreline segregation
  • Continuous casting
  • Microalloyed steel

• Graduation date

  Spring 2017

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3SQ8QW13

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.