Precipitation Behaviour of Titanium Carbo-sulfide in Extra-low-carbon Titanium-added Steels

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...

Section: Purpose Of Our Studies and Reviews Of Past Reportsmentioning

confidence: 99%

Section: Purpose Of Our Studies and Reviews Of Past Reportsmentioning

confidence: 99%

Section: Purpose Of Our Studies and Reviews Of Past Reportsmentioning

confidence: 99%

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

Ishiguro¹,

Murayama²,

Fujita³

et al. 2009

“…As compared to the past studies, [21][22][23][24] our viewpoint can be regarded as innovative and comprehensive. As mentioned in ''Purpose of our studies and reviews of past reports'' in the first paper 1) and in ''Introduction'' in the second paper, 2) the sulfide precipitation in ferrite region has been exceptionally reported only by K. Yamada et al 21,23) and H. Murakami et al, 22,24) but their mechanisms are mutually contradictory and can be restrictedly used only for their reporting phenomena. In contrast, our mechanism has a wide application to explain the whole sulfide precipitation phenomena in ferrite region in Tiadded steel and their reported phenomena can be also interpreted logically.…”

Section: Samples and Experimental Proceduresmentioning

confidence: 99%

“…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.…”

Section: Introductionmentioning

confidence: 99%

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

Ishiguro¹,

Murayama²,

Fujita³

et al. 2009

This report corresponds to the third 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. The authors show the strange phenomenon of copper sulfide in Ti-added steel in long-time-annealing in ferrite region. When annealed for short time (ex. 90 s), much of Cu-S is precipitated, but the amount of Cu-S is small at longer-time-annealing, while TiS reduces and Ti 4 C 2 S 2 increases monotonically. This phenomenon does not mean that Cu-S is initially formed and then dissolved away, but the Cu-S precipitation should be interpreted as causal evidence of ''(re-)solute sulfur and its reprecipitation'' along the same concept in the first and the second papers. Long-time-annealing means that our proposed reaction ''4TiS + 2[C] ! Ti 4 C 2 S 2 + 2[S]'' proceeds rightward and the re-solute sulfur is re-precipitated as Ti 4 C 2 S 2 (and MnS). Consequently, the amount of Cu-S is small. In contrast, short-time-annealing is not long enough to stabilize re-solute sulfur as Ti 4 C 2 S 2 (and MnS) and much of sulfur still remains solute. Therefore the amount of Cu-S is increased.Furthermore, the phenomena in three consecutive papers is organized into an integrated concept to understand the whole sulfide precipitation in Ti-added steel in both austenite and ferrite region.

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

Ishiguro¹,

Murayama²,

Fujita³

et al. 2009

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.