Sulfide Precipitation in Titanium-Added Steel with Residual Level of Copper (2) —Precipitation in Ferrite Region—

Abstract

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.

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

“…Please note that both ''0.04%Ti-IF'' and ''0.06%Ti-IF'' in this paper do not correspond to ''0.04%Ti-IF'' in the first and second papers 1,2) and ''0.07%Ti-IF'' in the first papers, 1) respectively. Each element slightly differs, but they are referred to as ''0.04%Ti-IF'' and ''0.06%Ti-IF'', because steel product has an allowable range of chemical compositions in real production and can be regarded as same steel sheets within an allowance.…”

“…In the third report of three consecutive papers, 1,2) the authors show the strange transitional change of copper sulfide precipitation in Ti-added steel in accordance with annealingtime in ferrite region. Through the analysis, our proposed theory of Cu-S precipitation is reinforced and also the whole phenomenon of sulfide precipitations in Ti-added steel is clearly shown in relation with Cu-S precipitation induced by a trace level of Cu, which has been regarded as solute in steel.…”

“…Our proposed mechanism has a common root through three consecutive papers, 1,2) which can apply to ferrite region and austenite region in both Ti-added and non-Ti-added steel. The concept is based on three things: (1) (re-)solute sulfur exists and it is re-precipitated along the estimated reaction ''4TiS + 2[C] !…”

“…1) In addition, the sulfide precipitation in ferrite region in Ti-added steel has been already reviewed again in the second paper. 2) In this third paper, the past studies on solute carbon in steel are mentioned below. It is related to the above mechanism ''4TiS + 2[C] !…”

“…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 .…”