A Precise Quantitative Analysis of Precipitates in Ti-bearing Interstitial-free Steel

Abstract

A more precise quantitative analysis of precipitates in Ti-bearing Interstitial-Free (IF) steel is made through the combination of both a Field-Emission Transmission Electron Microscope (FE-TEM) analysis and an analysis based upon chemical extraction and isolation. The non-stoichiometry of precipitates is precisely determined using either Energy Dispersive X-ray spectroscopy (EDX) or a new method by plasmon energy in Electron Energy-Loss Spectroscopy (EELS) attached to FE-TEM. The non-stoichiometric compositions of each precipitate are taken into consideration in the quantitative analysis.The precise quantitative method is applied to both the hot-rolled and the annealed steels in the process condition : the reheating temperature : 1220 deg . C , the finishing hot-rolling temperature : 890 deg . C , the hot-rolling coiling temperature : 680 deg . C and the annealing temperature : 770 deg . C . The precipitates are quantitatively analyzed from the viewpoint of the interstitial elements ; nitrogen , sulfur and carbon . All the nitrogen is precipitated as TiN when hot-rolling is completed. Most of the sulfur-related precipitate is TiS1.25 in the hot-rolled steel. The rest are Ti4C2S2 and (Mn , Fe) 51.25. After annealing , TiS1.25 decreases , while Ti4C2S2 increases significantly and (Mn , Fe) S1.25 slightly increases. Not all the carbon is combined with Ti in the hot-rolled steel and a lot of "free carbon" still remains even after the hot-rolling process is finished. After the annealing process, both TiC085 and Ti4C2S2 increase in quantity dramatically .

Sulfide Precipitation in Titanium-Added Steel with Residual Level of Copper (3) —Decrease of Copper Sulfide by Long-Time-Annealing in Ferrite Region—

“…Our prediction that Cu can diffuse very fast is inductively deduced. Interestingly, there have been some reports in the field of irradiation-induced embrittlement in material for atomic reactors, [32][33][34][35][36] which mention that Cu diffusion is about two orders of magnitude faster than Fe self-diffusion and is interpreted through Self-Interstitial-Atom (SIA) mechanism, although these experiments are carried out by using Fe-1%Cu or Fe/Cu/Fe diffusion pairs heat-treated at over 1000 C. In our opinion, these phenomena have the same origin to our reports [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] and each Cu-related or Cu-S-related phenomenon is viewed from a different angle.…”

“…Our standpoint [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] and reviews of past studies have already been mentioned in details in the first report. 1) In addition, the sulfide precipitation in ferrite region in Ti-added steel has been already reviewed again in the second paper.…”

“…Therefore, it has to be made clear whether solute carbon really exists in as-hot-rolled IF steel to prove that sulfide precipitation occurs along our proposed theory. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Please note that the terminology is very confusing in this scientific field. There have been some reports to mention the correlation between age-hardening and solute carbon in ''Tistabilized Ultra Low Carbon Bake-Hardenable steel (Ti-ULC-BH)'', 25,26) whose chemical compositions are designed to be lower sulfur (<20 ppm) and/or higher Mn (>1%) to obtain BH properties so that carbon is partially and/or weakly stabilized into TiC by inhibiting the precipitation of Ti 4 C 2 S 2 .…”

“…Also, the precipitation in Ti-IF steel has been traditionally interpreted to finish only in austenite region and not to occur in ferrite region, except for some limited papers. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] In other words, carbon in Ti-IF steel has been regarded as completely stabilized as carbide. Therefore, it has to be made clear whether solute carbon really exists in as-hot-rolled IF steel to prove that sulfide precipitation occurs along our proposed theory.…”

Sulfide Precipitation in Titanium-Added Steel with Residual Level of Copper (1) &mdash;Precipitation in Austenite Region&mdash;

“…[10][11][12][13][14][15][16][17][18][19][20] However, as in non-Ti-added steel does, (3) a residual level of Cu in Ti-added steel has been also believed to be solute in steel and there are no papers to mention the phenomenon of Cu-S precipitation induced by a tramp element of Cu.…”