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

Abstract

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...

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

“…They are consecutive numbers from the first paper. 1) As shown in Fig. 1, samples of 0.04%Ti-IF and 0.06%Ti-IF are prepared (10) as-hot and then annealed at 850 C for (11) 90 s !…”

“…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.…”

“…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] !…”

“…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 .…”