TiC carbides in TiNb and IF steels

Table 1: Chemical composition of the microalloyed steel heat [wt. %].

Table 2: Chemical composition of IF steel [wt. %].

Figure 1: Left: Cut outs from TiNb microalloyed steel slab. Right: Cut-out from IF steel slab.

Table 3: Precipitated particle size evaluation.

Figure 2: Fine and large globular particles, sample L3, Magnification 50 000x. Scale bar: 100 nm.

Figure 3: Globular particles, sample L3, Magnification 100 000x. Scale bar: 50 nm.

Figure 4: Mainly large globular and angled particles, sample L4, Magnification 68 000x. Scale bar: 100 nm.

Figure 5: Fine globular and large angular particles, sample L4, Magnification 85 000x. Scale bar: 100 nm.

Figure 6: Smaller angular particle EDX analysis, sample L3.

Figure 7: Angular particle EDX analysis, sample L4.

Figure 8: Small globular or oval particles, sample N9, Magnification 130 000x. Scale bar: 50 nm.

Figure 9: Small globular and single angular particle, sample N9, Magnification 85 000x. Scale bar: 100 nm.

Figure 10: TEM, globular particles, cut-out 1, Magnification 74 200x. Scale bar: 100 nm.

Figure 11: EDX spectrum.

Figure 12: TEM, dispersed globular particles, cut-out 3, Magnification 78 600x. Scale bar: 100 nm.

Figure 13: Stick shape particles, cut-out 3, Magnification 75 700x. Scale bar: 100 nm.

Carbide name: TiC
Record No.: 988
Carbide formula: TiC
Carbide type: MC
Carbide composition in weight %: No data
Image type: EDS, TEM
Steel name: TiNb and IF steels
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Microalloyed steels
Steel composition in weight %: See the table 1 and 2.
Heat treatment/condition: For the TiNb microalloyed steel, samples were produced from slab surface cut outs, from the small radius r side of the slab. In the secondary cooling zone the cooling rate of the slab was controlled by the program for microalloyed steel. Two casting rates were tested, specifically accomplished by two pulling rates. The cut outs were labelled in the following way:
Cut out K-1 (side cut out) and L-2 (central cut out): pulling rate 0.5m/min.
Cut out M-3 (side cut out) and N-4 (central): pulling rate 0.8m/min.
In this study central samples were used from L-2 and N-4, as shown in Fig. 1 (left). The continuouslycast slab specifications were as follows:
Slab dimensions 7360 x 1540 x 220 mm
Liquidus temperature 1527C
Cooling rate control curve for microalloyed steel, producing a moderate cooling rate in the secondary cooling zone
IF steel samples produced from 3 cut-outs across the slab width were used. The pulling rate was 0.8m/min. Steel produced in an oxygen-blowing converter was used. The chemical composition was designed for subsequent vacuum treatment, combined with inert argon blowing. The steel prepared this way was then continuously cast in a curved-type casting machine. The steel samples produced from 3 cut-outs across the slab width were cut from the small radius r side as shown in Fig. 1 (right). The slab specifications were as follows:
Slab dimensions: 220 x 1195 mm
Liquidus temperature 1532 C
Casting temperature 1561C
Cooling rate control in the secondary cooling zone - moderate.
Note: The aim of this paper was to evaluate the dependence of precipitation on casting rate in the surface zone of a TiNb microalloyed steel slab, and to compare the results with those in a TiNb IF steel slab. In the microalloyed steel TEM revealed fine and larger globular particles and larger angular particles of TiN and TiC. For higher casting rates TEM revealed more fine particles of similar shape, but NbC was combined with TiN and TiC. Precipitates in the slab surface zone of the TiNb IF steel were both globular and oval. They were based on Al and Ti in combination with MnS and stick shape particles based on Al. Comparison of precipitates in the surface zone of slabs cast at the same pulling rate 0.8m/min for both TiNb micro-alloyed steel and TiNb IF steel grades, showed fewer particles in the IF steel. This is assumed to be due to the very low carbon content in the IF steel compared to the microalloyed one.

TiNb micro-alloyed steel: Carbon replica observation of samples L3 and L4 produced from the central cut-out L2 of the slab pulled at the lower rate 0.5 m/min revealed a number of smaller and larger globular particles and large angular ones. More of the latter were seen in sample L4. The observed particles are documented in Figs. 2 and 3 for sample L3 and in Figs. 4 and 5 for sample L4. The types of particles with size ranges are listed in Tab. 3. In sample L3 the sizes of fine globular particles ranged from 3.8 to 11.5 nm, and the sizes of large globular particles ranged from 15.4 to 76.9 nm. The 2 angular particles were 76.9 107.7 nm in size. EDX analysis of sample L3 showed particles based on Ti and Nb (Fig.6). Diffraction plot analysis showed a TiN particle. Globular and angular particles were observed in sample L4 (Figs. 4 and 5). The sizes of fine globular and oval particles ranged from 4.8 to 9.5 nm. The sizes of the larger globular particles were from 12.5 to 18.7 nm and the angular particle size was from 14.3 to 75 nm. The number of angular particles in sample L4 was considerably higher compared to L3. This gives evidence of precipitation heterogeneity in the slab surface zone, though these two samples were close to one another. EDX analysis of the angular particle revealed a composition based on S, Ti and Nb (Fig.7). EDX analysis of one other particle from sample L4 also confirmed Ti and Nb. Diffraction plot results showed particles TiN and TiC. Sample N9 was cut out from the slab cast at the higher casting rate, actually pulled at a rate of 0.8 m/min. Carbon replicas extracted from sample N9 revealed small globular and oval particles, with sizes ranging from 6.3 to 9.5 nm (Fig.8). Some solitary large particles were there too, globular (12.5 28.6 nm) or angular (47.6 71.4 nm) (Fig.9). The results of the EDX analysis of the fine precipitated particles showed Ti and Nb. According to the diffraction results, the particles were TiC and NbC combined with TiN.

TiNb IF steel: Carbon replica observation of a sample produced from the left side cut-out 1 of the slab showed globular and oval particles in a ferrite matrix (Fig.10). Their sizes ranged from 28 to 71 nm. The pulling rate of the slab was 0.8 m/min. The EDX analysis results showed some complex particles with Al and Ti (Fig.11). In sample 3 from the central cut-out the size of the particles found ranged from 14 to 85.7 nm (Fig.12). The EELS analysis indicated nitride particles of Al and Ti. Fig.13 is the documentation of stick-like particles found deeper under the slab surface (20 mm). They were Al based and their sizes ranged from 71 nm to 114 nm. The obtained EDX spectrum confirmed in the sample from the right side cut out 2, particles based on Ti and Al. There was heterogeneity in the size of the particles and in places there were precipitates arranged in strings. There were very fine particles ranging from 4.1 nm to 41.6 nm in size, and larger particles from 57.1 to 100 nm. EDX analysis of the polished surface confirmed very fine cementite particles as well, and also large complex TiCN particles up to 160 nm in size, sometimes in combination with MnS deeper under the slab surface. Comparison of the precipitates in the tested steels showed differences in the distribution and size of particles. In micro-alloyed TiNb steel there were large numbers of particles, but in the TiNb IF steel the number of particles was smaller. This may have contributed to the very low carbon content in the IF steel compared to the micro-alloyed. This remains to be proven, however, requiring a large number of samples and statistics on them.
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