The influence of residual copper (;0:01 mass%) on sulfide precipitation in ultra low carbon steel was studied with microanalysis using Field Emission-Transmission Electron Microscopy (FE-TEM), X-Ray Diffraction (XRD) of the extracted precipitates and quantitative chemical analysis of extraction residue.A small amount of copper (;0:01 mass%) plays an important role in sulfide formation in steel, as does manganese. Two types of sulfides were found in ultra low carbon steel with residual copper: (1) MnS covered with copper sulfide (Cu-S) and (2) ''free-standing'' Cu-S, with sizes of 100$300 nm and less than about 40 nm, respectively. The atomic ratio of Cu/S was determined to be 1:8 AE 0:3 by Energy Dispersive Xray spectroscopy (EDX), and the phase was determined to be Cu 8 S 5 from XRD of the extracted precipitates.Quantitative chemical analysis revealed that almost all sulfur (71 mass ppm) was precipitated as sulfides in steel containing 73 ppm of sulfur. The concentration of sulfur as Cu 8 S 5 was 24 ppm, which accounts for about 35% of the total quantity of sulfur. This indicates that the formation of copper sulfide has to be taken into account in precise analysis of sulfides in low Mn and S systems (Mn ; 0:20%, S ; 0:007%). The precipitation of Cu-S in water-quenched (WQ) samples and the morphologies of MnS-cored BN & Cu-S-covered MnS in hot-rolled and WQ samples suggest that Cu-S is formed at temperatures between ''below 750 C'' and 620 C and is also precipitated even during a short time in water-quenching.

The authors previously reported that even a trace level of copper (0.01% Cu) acts as a sulfide-former to form copper sulfide (Cu-S) in nonTi-added steel in which Mn has been thought to be the only sulfide-former so far. A new concept of copper-related sulfide precipitation is now extended to Ti-added steel. In addition, the total sulfide precipitation in Ti-added steel is reconfigured with the new knowledge of Cu-S. The new concept is shown in three consecutive papers. This first paper focuses on the sulfide precipitation in austenite-heat-treated Ti-added steel.Through TEM observation and quantitative chemical analysis, it is reported that the sulfide precipitation in austenite-heat-treated Ti-added steel reflects not only the precipitation in the austenite region but also the inevitable precipitation behavior during the cooling process from austenite to ferrite region, especially trace-level-Cu-induced Cu-S precipitation in ferrite region during rapid cooling. The mechanism of the inevitable sulfide precipitation during the cooling process is mainly based upon TiS-decomposition-induced morphological change and the reprecipitation of sulfur which remains solute over precipitatable limit during the heat-treatment in austenite region, and is precisely discussed in relation with Cu-S precipitation. Purpose of Our Studies and Reviews of Past ReportsThrough three consecutive papers, 1,2) the authors demonstrate the comprehensive concepts of sulfide precipitation in Ti-added steel in austenite and ferrite regions with the addition of a new concept of copper sulfide (Cu-S) precipitation induced by a small amount of 0.01%Cu, as well as the rarely-mentioned concept that Ti 4 C 2 S 2 and MnS are formed in ferrite region. The first paper focuses on sulfide precipitation in austenite region, including the phenomena in austenite-heat-treated Ti-added steel with the subsequent various cooling pattern.The following review provides explanations and comments on past studies of Cu-related precipitation, together with our purposes. These items are mentioned only in this paper, the first part of three consecutive papers. Therefore, the following description includes phenomena in both austenite and ferrite regions, because these phenomena are related to each other and cannot be completely separated. Table 1 shows the overviews of past studies on both Curelated precipitation in steel and sulfide precipitation in steel, together with the purpose of the three consecutive studies. Each item below is referred to as a ''parenthetic No.'' in Table 1. First of all, it has been believed as an established theory that (1) a small amount of Cu in non-Ti-added steel exists as solid solution, and many people have not paid any attentions to it. However, as our group has already pointed out, (2) such a trace level of Cu acts as a sulfide-former to form copper sulfide (Cu-S) in non-Ti-added steel whose sulfide-former is only Mn (more precisely, 0.01%Cu). [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] The first purpose is (#1) to...

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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 .

This report is the second paper in three consecutive papers to know the whole sulfide precipitation in Ti-added steel by adding a new knowledge that even a trace level of copper (0.01%Cu) acts as a sulfide-former to form copper sulfide (Cu-S). The second paper focuses on the sulfide precipitation in ferrite region in Ti-added steel, especially in real-produced hot-rolled and annealed steel sheets.Although it has been believed that the sulfide precipitation is completed only in austenite region except for a few limited papers, Ti 4 C 2 S 2 , MnS and Cu-S are really formed in ferrite region. When as-hot-rolled Ti-added steel is annealed, the sulfide precipitation transforms morphologically and compositionally. The phenomenon can be explained through the same mechanism in the first paper that TiSdecomposition-induced re-solute sulfur is re-precipitated. When annealed in ferrite region, already-precipitated TiS is changed into Ti 4 C 2 S 2 and re-solute sulfur comes out along the estimated reaction ''4TiS þ 2[C] ! Ti 4 C 2 S 2 þ 2[S]'', and it is re-precipitated in ferrite region as other precipitates (Ti 4 C 2 S 2 , MnS and Cu-S). The precipitation of Cu-S is estimated to occur at the lowest temperature among re-precipitated sulfides, and therefore is affected by the precedent sulfide precipitation phenomena.

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