**1. Introduction**

With the development of large-scale engineering structures, high heat input welding has been widely used in shipbuilding, marine engineering, and oil container fields due to its advantages, such as high efficiency and high stability. However, the increment in the heat input will result in the coarsening of grains in the heat affect zone (HAZ), the origin of a tiny cleavage crack, and a heterogeneous microstructure, thereby significantly deteriorating the toughness [1].

To address these problems, researchers have started to focus on inclusions that are inevitable in metal products. Some non-metallic inclusions can have positive impacts on the steel microstructure and mechanical properties [2–4]. Initially, in the 1970s, nanosized TiN particles were found to be effective at inhibiting the growth of austenite grains and beneficial for the mechanical property of steel [5]. They were formed during the cooling

**Citation:** Rong, Z.; Liu, H.; Zhang, P.; Wang, F.; Wang, G.; Zhao, B.; Tang, F.; Ma, X. The Formation Mechanisms and Evolution of Multi-Phase Inclusions in Ti-Ca Deoxidized Offshore Structural Steel. *Metals* **2022**, *12*, 511. https://doi.org/10.3390/ met12030511

Academic Editor: Mark E. Schlesinger

Received: 17 February 2022 Accepted: 14 March 2022 Published: 17 March 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

process after solidification due to their relatively low solubility. In the 1990s, titanium oxide-containing inclusions, formed in the molten steel and solidification process, were reported to act effectively as intragranular nucleation sites for acicular ferrite, contributing to a smaller grain size and consequently improving the weldability of steel. These practical inclusions were formed during the second refining process and had a relatively high liquidus, so they had excellent thermostability. The technology of titanium oxides and their positive effect on the steel microstructure and phase transformation behaviour was proposed and termed "oxide metallurgy" [6–9]. Since then, the understanding of inclusions in the steelmaking industry has been rejuvenated. Harmful inclusions, such as large-sized and Al2O<sup>3</sup> inclusions that may cause clogging, should be removed as much as possible while those with a small size and particular chemical composition can be maintained and utilized to achieve outstanding mechanical properties [10,11]. Oxide metallurgy has been one of the most effective methods used to address the toughness problem in HAZ.

In recent decades, the use of strong deoxidizers, such as Ca, Mg, Zr, and rare Earth metals (REMs) [10,12–15], has attracted considerable research interest due to their stronger affinity with O and S and higher thermal stability of inclusions. Traditionally, Ca treatment is mostly used for modifying Al2O<sup>3</sup> inclusions with a high melting point into CaO-Al2O<sup>3</sup> with a lower melting point, thereby solving the nozzle clogging problem [16–18]. Kobe Steel also reported that modified inclusion of CaO-Al2O<sup>3</sup> could help prohibit the coarse TiN precipitating on oxides and facilitate the formation and dispersive distribution of fine TiN particles [19]. So far, research has mainly focused on Ca treatment in Ti-bearing Al-killed steel. Wang et al. [20] compared Al-Ca deoxidized steel and Ti-Ca deoxidized steel in the lab, pointing out that CaO-Al2O3-TiO2-MnS-related inclusions were dominant and active in facilitating fine IAF formation [21,22]. CaO-Al2O3-TiO<sup>2</sup> will act as a nucleation core while MnS will precipitate around the core during the cooling process and form a manganese depletion zone (MDZ) to induce IAF formation [23–25].

Although several investigations regarding Ti-Ca deoxidized steel have been reported, they mainly focused on the effect of different deoxidizers on inclusions in final products or the effect of inclusions on the microstructure in HAZ. However, few studies harnessing the integrated understanding of inclusion evolution in each step of actual industry practice are available. In this paper, the inclusion characteristics regarding categories, number density, and evolution were systematically investigated to understand and clarify the formation mechanisms and evolution of complex inclusions in Ti-Ca-treated offshore structural steel, which will benefit the translation Ti-Ca oxide metallurgy of offshore steels.

#### **2. Materials and Methods**

Steel samples were obtained from an industrial process for offshore structural steel, which followed the sequential steps through a basic oxygen furnace (BOF)→ladle furnace (LF)→vacuum degassing (VD)→continuous casting (CC)→thermomechanical control process (TMCP) in Wusteel, HBIS. Ferrosilicon (Si ≥ 75.0%, Al ≤ 1.5%, Fe ≥ 21.0%) and ferromanganese (65.0–72.0% Mn, Si ≤ 2.5%, C ≤ 7.0%) alloys together with lime were added to the molten steel during BOF tapping. Then, the molten steel was transferred into LF for refining and bottom argon blowing was adopted for homogenizing the steel composition and accelerating the removal of large-sized inclusions. LF refractories were composed of MgO-C bricks. After refining in LF at 1650 ◦C, the molten steel was transferred to VD at 1600 ◦C and was eventually transported into tundish for continuous casting. In particular, samples were collected at the beginning of LF 1 min after Ti addition (Ti-Fe: 25.0–35.0%Ti, Al ≤ 8.0%, Si ≤ 4.5%), 1 min after Ca addition, VD soft blowing, tundish, and hot-rolled plate (final products). The chemical compositions of the steel samples from different stages are shown in Table 1. The product sample underwent heat treatment as welding thermal simulation at a peak temperature of 1400 ◦C. The measured temperature curve of the welding thermal simulation is shown in Figure 1. The welding samples were held at 1400 ◦C for 180 s, and the cooling rates of the 1400–800 ◦C and 800–500 ◦C range were approximately 3 and 0.75 ◦C/s (t8/5 = 400 s), respectively.


approximately 3 and 0.75 °C/s (t8/5 = 400 s), respectively.

**Table 1.** Chemical composition of samples taken from different stages (wt%).

1400 °C for 180 s, and the cooling rates of the 1400–800 °C and 800–500 °C range were

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**Figure 1.** The measured temperature curve of welding thermal simulation. **Figure 1.** The measured temperature curve of welding thermal simulation.

Steel samples were mounted in resin and ground, polished, and etched for inclusion composition analysis and microstructure observation. Composition analysis of inclusions was conducted by using a JXA 8200 electron probe microanalyzer (EPMA, JEOL, Akishima, Japan) with wavelength-dispersive spectrometers (WDSs). Steel samples were mounted in resin and ground, polished, and etched for inclusion composition analysis and microstructure observation. Composition analysis of inclusions was conducted by using a JXA 8200 electron probe microanalyzer (EPMA, JEOL, Akishima, Japan) with wavelength-dispersive spectrometers (WDSs).

The EPMA operating details are as follows: the standard samples are CaSiO3 for Ca and Si, pure MgO for Mg, pure Al2O3 for Al, pure TiO2 for Ti, Spessartine for Mn, Fe2O3 for Fe, and FeS for S. An acceleration voltage of 15 kV, probe current of 15 nA, probe diameter of "zero" (the smallest operating probe diameter achieved by the focused electron beams), peak measuring time of 30 s, and background measuring time of 10 s were used. Backscattered electron imaging and EDS analysis were utilized to recognize the nonmetallic inclusions, especially those complex inclusions with multiple phases. Then, WDS was conducted on the inclusions to analyze the inclusion composition accurately. EPMA can accurately determine the composition of inclusions exceeding 1.5 µm. Field emission scanning electron microscopy (FE-SEM) was used to analyze inclusions sized 1.5 µm or less by element spot analysis, line scanning, and map scanning to demonstrate the ele-The EPMA operating details are as follows: the standard samples are CaSiO<sup>3</sup> for Ca and Si, pure MgO for Mg, pure Al2O<sup>3</sup> for Al, pure TiO<sup>2</sup> for Ti, Spessartine for Mn, Fe2O<sup>3</sup> for Fe, and FeS for S. An acceleration voltage of 15 kV, probe current of 15 nA, probe diameter of "zero" (the smallest operating probe diameter achieved by the focused electron beams), peak measuring time of 30 s, and background measuring time of 10 s were used. Backscattered electron imaging and EDS analysis were utilized to recognize the non-metallic inclusions, especially those complex inclusions with multiple phases. Then, WDS was conducted on the inclusions to analyze the inclusion composition accurately. EPMA can accurately determine the composition of inclusions exceeding 1.5 µm. Field emission scanning electron microscopy (FE-SEM) was used to analyze inclusions sized 1.5 µm or less by element spot analysis, line scanning, and map scanning to demonstrate the elementary distribution of multiphase inclusions.

mentary distribution of multiphase inclusions. To describe the number densities of inclusions, 20 photos were taken randomly under 500 magnifications for each sample to ensure reliable statistics. The area of each photo was 0.246 mm × 0.196 mm = 0.0482 mm2, so the total observation area of each sample was 0.964 mm2. To describe the number densities of inclusions, 20 photos were taken randomly under 500 magnifications for each sample to ensure reliable statistics. The area of each photo was 0.246 mm <sup>×</sup> 0.196 mm = 0.0482 mm<sup>2</sup> , so the total observation area of each sample was 0.964 mm<sup>2</sup> .

#### **3. Results and Discussions**

#### *3.1. Characteristics of the Inclusions at Different Stages*

Due to complex alloy systems including Si, Mn, Al, Ti, and Ca related to the deoxidization process, the inclusions can be classified into nine groups: Al2O3-MnO, Al2O3SiO2-MnO, CaO-Al2O3-SiO2-MnO, CaO-SiO2, CaO-Al2O3-SiO2, CaO-Al2O3-SiO2-TiOx, Al2O3-MnO-TiOx, SiO2-MnO, and CaO-TiOx, as shown in Table 2. Generally, several valences of Ti, such as TiO, TiO2, Ti2O3, and Ti3O5, exist in steel, and it is assumed that Ti-oxide was mainly TiO<sup>2</sup> when analyzing the inclusion composition, so a TiO<sup>2</sup> standard sample was used during the EPMA analysis. Among these inclusions, Al2O3-MnO, Al2O3-SiO2-MnO, and CaO-Al2O3-SiO2-MnO are either primary deoxidization products or those that contacted with lime, which was added during the BOF tapping process. CaO-SiO<sup>2</sup> and CaO-Al2O3-SiO<sup>2</sup> are grouped into slag entrapment products (large-sized inclusions) and reduction products (small-sized inclusions). CaO-Al2O3-SiO2-TiO<sup>x</sup> and CaO-TiO<sup>x</sup> belong to the refining products. Al2O3-MnO-TiO<sup>x</sup> and SiO2-MnO inclusions are considered to be secondary oxidization products during the vacuum break of VD and Tundish processes.


**Table 2.** Classification of inclusions based on the chemical compositions (wt%).

The initial inclusions in the LF-entry sample include Al2O3-MnO, Al2O3-SiO2-MnO, CaO-Al2O3-SiO2-MnO, and CaO-SiO2. Among these inclusions, Al2O3-MnO and Al2O3- SiO2-MnO are primary deoxidization products with small sizes generally less than 3 µm. Although Fe-Si and Fe-Mn ferroalloys are mainly used as pre-deoxidizer in industrial practice, inevitably a small amount of Al impurities exist in alloys. As Al has a stronger ability to combine with oxygen than Si and Mn, almost all the primary deoxidization products contained Al2O3. CaO-Al2O3-SiO2-MnO can be considered as the coalescence between primary deoxidization products and CaO from lime and CaO-SiO<sup>2</sup> from refining slag that was added to molten steel during BOF tapping. CaO-SiO<sup>2</sup> inclusions in the LF entry stage were generally larger than 10 µm, but the proportion of CaO-SiO<sup>2</sup> inclusions in this stage was very low. They originated from slag entrapment. After Ti was added to LF, the main inclusion types were CaO-Al2O3-SiO<sup>2</sup> and CaO-Al2O3-SiO2-TiO<sup>x</sup> 1–10 µm in size. The dissolved aluminium coming from continuous reaction of ferroalloys with CaO-SiO<sup>2</sup> from refining slag then formed CaO-Al2O3-SiO<sup>2</sup> inclusions. The additive Ti entered molten steel, reacting with CaO-Al2O3-SiO<sup>2</sup> inclusions and forming CaO-Al2O3-SiO2-TiO<sup>2</sup> inclusions. After Ca was added to the molten steel, a considerable proportion of CaO-SiO<sup>2</sup> inclusions was observed apart from CaO-Al2O3-SiO<sup>2</sup> and CaO-Al2O3-SiO2-TiO<sup>x</sup> inclusions due to the severe slag entrapment caused by the splashing of the Ca addition, and the reduction of Al2O<sup>3</sup> by additive calcium. In the following VD and Tundish, until the final products, CaO-SiO2, CaO-Al2O3-SiO2, and CaO-Al2O3-SiO2-TiO<sup>x</sup> inclusions remained the main types. The most significant difference was that Al2O3-MnO-TiO<sup>x</sup> and SiO2-MnO inclusions were observed after the vacuum break of VD and Tundish. In general, these were typical re-oxidization products due to the excessive surface turbulence during molten steel teeming into the Tundish ladle. However, these kinds of re-oxidization products were seldom found in the final products, which indicated that most of them floated upwards into the mold flux and were removed effectively. In the final products, CaO-TiO<sup>x</sup> inclusions

were found apart from CaO-SiO2, CaO-Al2O3-SiO2, and CaO-Al2O3-SiO2-TiO<sup>x</sup> inclusions. They may be generated from either the enrichment of the CaO component and the reduction of Al2O<sup>3</sup> and SiO2, or the precipitation of CaO-TiO<sup>x</sup> from CaO-Al2O3-SiO2-TiO<sup>x</sup> inclusions during the cooling process. Al2O3 and SiO2, or the precipitation of CaO-TiOx from CaO-Al2O3-SiO2-TiOx inclusions during the cooling process. *3.2. Number Density and Size Distribution of Inclusions* 

into the Tundish ladle. However, these kinds of re-oxidization products were seldom found in the final products, which indicated that most of them floated upwards into the mold flux and were removed effectively. In the final products, CaO-TiOx inclusions were found apart from CaO-SiO2, CaO-Al2O3-SiO2, and CaO-Al2O3-SiO2-TiOx inclusions. They may be generated from either the enrichment of the CaO component and the reduction of

#### *3.2. Number Density and Size Distribution of Inclusions* The number density of all inclusions in industrial steel is shown in Figure 2. The primary inclusions in the LF entry sample were generally primary deoxidization products,

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The number density of all inclusions in industrial steel is shown in Figure 2. The primary inclusions in the LF entry sample were generally primary deoxidization products, such as Al2O3-MnO and Al2O3-SiO2-MnO, featuring a high proportion of fine inclusions that were less than 1 µm and had a number density of 1560/mm<sup>2</sup> . With ladle furnace refining processing, when Fe-Ti alloy was added to the molten steel, the total amount of inclusions decreased dramatically to about 300/mm<sup>2</sup> , especially those with a small size (<1 µm). This is due to the coalescence of small inclusions, floating upward into the refining slag. When Si-Ca wire was added to the molten steel and the end of LF refining, the total amount of inclusions increased slightly to higher than 400/mm<sup>2</sup> owing to the severe splashing during Si-Ca wire feeding. It also featured a higher proportion of 1–2 µm and 2–5 µm inclusions, and this can be explained by inclusion coalescence and growth to be larger ones. In the VD section, the total amount of inclusions dropped gradually to around 200 mm<sup>2</sup> , small-sizd inclusions of less than 1 µm. This is because small-sized inclusions kept coalescing and gathering during the vacuum break and soft blowing process of VD. From VD to Tundish and products, the total amount of inclusions decreased steadily, and the main factor was the decrease in the inclusions with small sizes less than 1 µm. such as Al2O3-MnO and Al2O3-SiO2-MnO, featuring a high proportion of fine inclusions that were less than 1 µm and had a number density of 1560/mm2. With ladle furnace refining processing, when Fe-Ti alloy was added to the molten steel, the total amount of inclusions decreased dramatically to about 300/mm2, especially those with a small size (<1 µm). This is due to the coalescence of small inclusions,floating upward into the refining slag. When Si-Ca wire was added to the molten steel and the end of LF refining, the total amount of inclusions increased slightly to higher than 400/mm2 owing to the severe splashing during Si-Ca wire feeding. It also featured a higher proportion of 1–2 µm and 2–5 µm inclusions, and this can be explained by inclusion coalescence and growth to be larger ones. In the VD section, the total amount of inclusions dropped gradually to around 200 mm2, small-sizd inclusions of less than 1 µm. This is because small-sized inclusions kept coalescing and gathering during the vacuum break and soft blowing process of VD. From VD to Tundish and products, the total amount of inclusions decreased steadily, and the main factor was the decrease in the inclusions with small sizes less than 1 µm.

**Figure 2.** The number density of inclusions at different stages.

**Figure 2.** The number density of inclusions at different stages. The oxide components in inclusions at different stages are shown in Figure 3. As can be seen, Al2O3 and MnO, as primary deoxidization products, accounted for the majority initially when LF began. With the LF refining processing, when Fe-Ti and Si-Ca alloys were added to the molten steel, CaO and SiO2 were substituted for Al2O3 and MnO, accounting for more than 50% and 30%, respectively. Therefore, the content of Al2O3 and The oxide components in inclusions at different stages are shown in Figure 3. As can be seen, Al2O<sup>3</sup> and MnO, as primary deoxidization products, accounted for the majority initially when LF began. With the LF refining processing, when Fe-Ti and Si-Ca alloys were added to the molten steel, CaO and SiO<sup>2</sup> were substituted for Al2O<sup>3</sup> and MnO, accounting for more than 50% and 30%, respectively. Therefore, the content of Al2O<sup>3</sup> and MnO dramatically decreased to less than 10%. From the Ca addition to vacuum degassing, the CaO content dropped a little to 40% due to the evaporation effect of Ca, and accordingly, the Al2O<sup>3</sup> content increased a little to about 20%. However, the CaO content increased gradually to about 50% after the vacuum break of VD, which was due to extra Si-Ca alloy being added to the molten steel. The TiO<sup>2</sup> concentration fluctuated from 10% to 15% after Ti-Fe addition.

to 15% after Ti-Fe addition.

80

to 15% after Ti-Fe addition.

**Figure 3.** Oxide component in the inclusions at different stages. CaO-SiO2, CaO-Al2O3-SiO2, and CaO-Al2O3-SiO2-(MgO)-TiOx inclusions remained the main types among all the processes after Ti addition. Particularly, CaO-SiO2 and CaO-Al2O3-SiO2 can be described in a CaO-Al2O3-SiO2 system, thus CaO-Al2O3-SiO2-(MgO)- TiOx and CaO-Al2O3-SiO2 inclusions were focused on and their formation mechanisms CaO-SiO2, CaO-Al2O3-SiO2, and CaO-Al2O3-SiO2-(MgO)-TiO<sup>x</sup> inclusions remained the main types among all the processes after Ti addition. Particularly, CaO-SiO<sup>2</sup> and CaO-Al2O3-SiO<sup>2</sup> can be described in a CaO-Al2O3-SiO<sup>2</sup> system, thus CaO-Al2O3-SiO2-(MgO)- TiO<sup>x</sup> and CaO-Al2O3-SiO<sup>2</sup> inclusions were focused on and their formation mechanisms and evolutions were investigated and are discussed here. the main types among all the processes after Ti addition. Particularly, CaO-SiO2 and CaO-Al2O3-SiO2 can be described in a CaO-Al2O3-SiO2 system, thus CaO-Al2O3-SiO2-(MgO)- TiOx and CaO-Al2O3-SiO2 inclusions were focused on and their formation mechanisms and evolutions were investigated and are discussed here.

CaO-SiO2, CaO-Al2O3-SiO2, and CaO-Al2O3-SiO2-(MgO)-TiOx inclusions remained

MnO dramatically decreased to less than 10%. From the Ca addition to vacuum degassing,the CaO content dropped a little to 40% due to the evaporation effect of Ca, and accordingly, the Al2O3 content increased a little to about 20%. However, the CaO content increased gradually to about 50% after the vacuum break of VD, which was due to extra Si-Ca alloy being added to the molten steel. The TiO2 concentration fluctuated from 10%

MnO dramatically decreased to less than 10%. From the Ca addition to vacuum degassing,the CaO content dropped a little to 40% due to the evaporation effect of Ca, and accordingly, the Al2O3 content increased a little to about 20%. However, the CaO content increased gradually to about 50% after the vacuum break of VD, which was due to extra Si-Ca alloy being added to the molten steel. The TiO2 concentration fluctuated from 10%

> CaO Al2O3

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#### and evolutions were investigated and are discussed here. *3.3. Formation Mechanism and Evolution of CaO-Al2O3-SiO2-(MgO)-TiO<sup>x</sup> Inclusion 3.3. Formation Mechanism and Evolution of CaO-Al2O3-SiO2-(MgO)-TiOx Inclusion*  The initial CaO-Al2O3-SiO2-(MgO)-TiOx (MgO was marked here for discussing the

*3.3. Formation Mechanism and Evolution of CaO-Al2O3-SiO2-(MgO)-TiOx Inclusion*  The initial CaO-Al2O3-SiO2-(MgO)-TiOx (MgO was marked here for discussing the formation mechanism) inclusion was found after Ti addition. Three typical inclusions (size ranged from 1 to above 10 µm) with different sizes are shown in Figure 4. The size of the three inclusions were 10.8, 5.1, and 2.5 µm, respectively. All of them belonged to CaO-Al2O3-SiO2-(MgO)-TiOx inclusions, among which the MgO content was relatively low at about 5%. The detailed chemical composition is shown in Figure 5 to demonstrate The initial CaO-Al2O3-SiO2-(MgO)-TiO<sup>x</sup> (MgO was marked here for discussing the formation mechanism) inclusion was found after Ti addition. Three typical inclusions (size ranged from 1 to above 10 µm) with different sizes are shown in Figure 4. The size of the three inclusions were 10.8, 5.1, and 2.5 µm, respectively. All of them belonged to CaO-Al2O3-SiO2-(MgO)-TiO<sup>x</sup> inclusions, among which the MgO content was relatively low at about 5%. The detailed chemical composition is shown in Figure 5 to demonstrate the varying trend of each oxide component with different sizes. formation mechanism) inclusion was found after Ti addition. Three typical inclusions (size ranged from 1 to above 10 µm) with different sizes are shown in Figure 4. The size of the three inclusions were 10.8, 5.1, and 2.5 µm, respectively. All of them belonged to CaO-Al2O3-SiO2-(MgO)-TiOx inclusions, among which the MgO content was relatively low at about 5%. The detailed chemical composition is shown in Figure 5 to demonstrate the varying trend of each oxide component with different sizes.

**Figure 4.** Morphologies of typical inclusions with different sizes after Ti addition.

It is obvious from Figure 5 that the mass proportion of CaO and SiO<sup>2</sup> steadily dropped with the decrease in the inclusion size. The mass proportion of MgO remained stable at a relatively low level. Adversely, the mass proportion of Al2O<sup>3</sup> and TiO<sup>2</sup> showed a gradually increasing trend. CaO and SiO<sup>2</sup> were generally the main contents of the refining slag, and MgO caneither come from refining slag or the corrosion of MgO-C refractory and Al2O3, and TiO<sup>x</sup> was a typical deoxidization product of Ti-Ca killed steel. Generally, deoxidization products are small in size while the size of particles from slag and refractory can range widely. When small-sized deoxidization products are combined with large-sized

CaO-SiO2(-MgO) particles, the CaO-SiO2(-MgO) content in the average composition will be high, and Al2O3-TiO<sup>2</sup> will be low. On the other hand, when a small-sized deoxidization product is combined with small- or medium-sized CaO-SiO2(-MgO) particles, the CaO-SiO2(-MgO) content in the average composition will be relatively lower, and Al2O3-TiO<sup>2</sup> will be moderately higher. As a result, it can be concluded from Figure 5 that the CaO-Al2O3-SiO2-(MgO)-TiO<sup>x</sup> inclusion in LF originated from the reaction between CaO-SiO<sup>2</sup> from slag, MgO from refractory, and the deoxidization products Al2O<sup>3</sup> and TiOx. *Metals* **2022**, *11*, x FOR PEER REVIEW 7 of 20 **Figure 4.** Morphologies of typical inclusions with different sizes after Ti addition.

**Figure 5.** Oxide component proportion in different sized CaO-Al2O<sup>3</sup> -SiO<sup>2</sup> -(MgO)-TiOx inclusions.

**Figure 5.** Oxide component proportion in different sized CaO-Al2O3-SiO2-(MgO)-TiOx inclusions. It is obvious from Figure 5 that the mass proportion of CaO and SiO2 steadily dropped with the decrease in the inclusion size. The mass proportion of MgO remained stable at a relatively low level. Adversely, the mass proportion of Al2O3 and TiO2 showed a gradually increasing trend. CaO and SiO2 were generally the main contents of the refining slag, and MgO caneither come from refining slag or the corrosion of MgO-C refractory and Al2O3, and TiOx was a typical deoxidization product of Ti-Ca killed steel. Generally, deoxidization products are small in size while the size of particles from slag and refractory The CaO-Al2O3-SiO2-MgO-TiO<sup>x</sup> inclusions' composition at different stages was projected into pseudo-ternary phase diagrams of CaO-Al2O3-TiO<sup>x</sup> at fixed 20%SiO<sup>2</sup> (14.8–23.2% SiO2) and 5%MgO (2.7–7.8%MgO), as shown in Figure 6. In Figure 6a, all the inclusions' composition is projected, and relatively dispersive. Accordingly, the average composition of each stage was calculated and is shown in Figure 6b. After the addition of Ti in the LF refining, the main inclusions are generally located in the regions of CaO-Al2O3-MgO and melilite, and with Ca addition in LF and following VD, the main inclusions are basically located in melilite and Ca2SiO<sup>4</sup> regions, and the inclusion composition in final product is eventually located in the CaTiO<sup>3</sup> region.

can range widely. When small-sized deoxidization products are combined with largesized CaO-SiO2(-MgO) particles, the CaO-SiO2(-MgO) content in the average composition will be high, and Al2O3-TiO2 will be low. On the other hand, when a small-sized deoxidization product is combined with small- or medium-sized CaO-SiO2(-MgO) particles, the CaO-SiO2(-MgO) content in the average composition will be relatively lower, and Al2O3- TiO2 will be moderately higher. As a result, it can be concluded from Figure 5 that the CaO-Al2O3-SiO2-(MgO)-TiOx inclusion in LF originated from the reaction between CaO-SiO2 from slag, MgO from refractory, and the deoxidization products Al2O3 and TiOx. The CaO-Al2O3-SiO2-MgO-TiOx inclusions' composition at different stages was projected into pseudo-ternary phase diagrams of CaO-Al2O3-TiOx at fixed 20%SiO2 (14.8– 23.2% SiO2) and 5%MgO (2.7–7.8%MgO), as shown in Figure 6. In Figure 6a, all the inclusions' composition is projected, and relatively dispersive. Accordingly, the average composition of each stage was calculated and is shown in Figure 6b. After the addition of Ti The morphologies of CaO-Al2O3-SiO2-(MgO)-TiO<sup>x</sup> at different stages are shown in Figure 7. Generally, they were spherical. From the liquidus temperature in the pseudoternary phase diagram, the compositions of inclusions at different stages are located in the 1400–1500 ◦C liquidus region, which means they were in a liquid state in molten steel and thereby presented a sphere shape. The morphologies shown in Figure 7 indicate relatively fast cooling and the precipitation of different components; thus, the morphologies reflect the formation mechanism of inclusion. The component with a light grey colour is TiO2, and the area proportion of TiO<sup>x</sup> continuously increased with the refining process proceeding, which was in accordance with the TiO<sup>x</sup> component trend shown in the phase diagram. During the early stage of LF refining after Ti addition, large amounts of Al-Tioxide particles started to gather around and stick to the CaO-SiO2-(MgO) inclusion on its surface. Particularly, Al2O<sup>3</sup> was from the oxidization of residual Al in the ferroalloys, and TiO<sup>x</sup> was from the oxidization of Ti in the. Ti-Fe alloy. These small deoxidization

> in the LF refining, the main inclusions are generally located in the regions of CaO-Al2O3- MgO and melilite, and with Ca addition in LF and following VD, the main inclusions are

products did not have enough time to dissolve into the inclusion from the refining slag, so they stayed on the surface. With the refining process proceeding, Ti-oxide continuously increased and had sufficient time to gradually dissolve into the inclusion from the refining slag and formed new inclusions as a whole. As can be seen in the phase diagrams shown in Figure 6, the liquidus of the average composition of the inclusions was in the range from 1400 to 1450 ◦C. So, these new inclusions were mainly liquid inclusions in molten steel. During the cooling of the sample-taking process, different phases precipitated and formed multiphase inclusions, as shown in Figure 7. As a result, the morphology after cooling presented, such as the Ti-oxide component, "entered" the inclusion and formed the core of the multiphase inclusions. Meanwhile, the Al2O<sup>3</sup> and SiO<sup>2</sup> component generally decreased due to the reduction of Ca, so the average content of CaO showed an increasing trend when compared with that of Al2O<sup>3</sup> and SiO2, as shown in Figure 6. The formation mechanism of CaO-Al2O3-SiO2-(MgO)-TiO<sup>x</sup> is in accordance with that of the CaO-Al2O3-SiO2-(MgO) inclusions in Si-Mn killed steel with a limited aluminum content [21]. *Metals* **2022**, *11*, x FOR PEER REVIEW 8 of 20

**Figure 6.** Pseudo-ternary phase diagrams CaO-Al2O3-TiO2 at fixed 20%SiO2 (14.8–23.2% SiO2) and 5%MgO (2.7–7.8%MgO); (**a**) experimental data; (**b**) average data. **Figure 6.** Pseudo-ternary phase diagrams CaO-Al2O<sup>3</sup> -TiO<sup>2</sup> at fixed 20%SiO<sup>2</sup> (14.8–23.2% SiO<sup>2</sup> ) and 5%MgO (2.7–7.8%MgO); (**a**) experimental data; (**b**) average data. *Metals* **2022**, *11*, x FOR PEER REVIEW 9 of 20

from 1400 to 1450 °C. So, these new inclusions were mainly liquid inclusions in molten steel. During the cooling of the sample-taking process, different phases precipitated and formed multiphase inclusions, as shown in Figure 7. As a result, the morphology after **Figure 7.** The morphologies of CaO-Al2O3-SiO2-(MgO)-TiOx at different stages: (**a**) Ti addition; **(b**) Ca additon; (**c**) LF end; (**d**) VD; (**e**) Tundish; (**f**) product. **Figure 7.** The morphologies of CaO-Al2O<sup>3</sup> -SiO<sup>2</sup> -(MgO)-TiO<sup>x</sup> at different stages: (**a**) Ti addition; **(b**) Ca additon; (**c**) LF end; (**d**) VD; (**e**) Tundish; (**f**) product.

cooling presented, such as the Ti-oxide component, "entered" the inclusion and formed

In summary, the formation and evolution mechanism of CaO-Al2O3-SiO2-(MgO)- TiOx inclusion during the whole refining process can be described in a schematic diagram,

**Figure 8.** The formation and evolution mechanism of CaO-Al2O3-SiO2-(MgO)-TiO2 inclusion during

There are generally two morphology types of CaO-SiO2-Al2O3 inclusions as shown in Figure 9. One is a single-phase inclusion with a smooth surface, and the other has a rough surface. These two types of CaO-SiO2-Al2O3 inclusions have different chemical compositions as shown in Table 3. The basicity CaO/SiO2 of them is similar at about 1.9–2.0. The difference is mainly the component of Al2O3: Al2O3 of the inclusion in Figure 9a is 24.5% while that of the inclusion in Figure 9b is only 2.8%, so the inclusion in Figure 9b can be defined as CaO-SiO2. The chemical composition distribution of CaO-SiO2-Al2O3 inclusions in steel samples taken from different stages of industrial manufacturing was projected

trend when compared with that of Al2O3 and SiO2, as shown in Figure 6. The formation mechanism of CaO-Al2O3-SiO2-(MgO)-TiOx is in accordance with that of the CaO-Al2O3- SiO2-(MgO) inclusions in Si-Mn killed steel with a limited aluminum content [21].

*3.4. Formation Mechanism and Evolution of CaO-Al2O3-SiO2 Inclusion* 

into CaO-Al2O3-SiO2 ternary phase diagrams, as shown in Figure 10.

the whole refining process.

*Metals* **2022**, *11*, x FOR PEER REVIEW 9 of 20

(**a**) (**b**) (**c**)

(**d**) (**e**) (**f**)

Ca additon; (**c**) LF end; (**d**) VD; (**e**) Tundish; (**f**) product.

**Figure 8.** The formation and evolution mechanism of CaO-Al2O3-SiO2-(MgO)-TiO2 inclusion during the whole refining process. **Figure 8.** The formation and evolution mechanism of CaO-Al2O<sup>3</sup> -SiO<sup>2</sup> -(MgO)-TiO<sup>2</sup> inclusion during the whole refining process.

**Figure 7.** The morphologies of CaO-Al2O3-SiO2-(MgO)-TiOx at different stages: (**a**) Ti addition; **(b**)

#### *3.4. Formation Mechanism and Evolution of CaO-Al2O3-SiO2 Inclusion 3.4. Formation Mechanism and Evolution of CaO-Al2O3-SiO<sup>2</sup> Inclusion*

There are generally two morphology types of CaO-SiO2-Al2O3 inclusions as shown in Figure 9. One is a single-phase inclusion with a smooth surface, and the other has a rough surface. These two types of CaO-SiO2-Al2O3 inclusions have different chemical compositions as shown in Table 3. The basicity CaO/SiO2 of them is similar at about 1.9–2.0. The difference is mainly the component of Al2O3: Al2O3 of the inclusion in Figure 9a is 24.5% while that of the inclusion in Figure 9b is only 2.8%, so the inclusion in Figure 9b can be defined as CaO-SiO2. The chemical composition distribution of CaO-SiO2-Al2O3 inclusions in steel samples taken from different stages of industrial manufacturing was projected into CaO-Al2O3-SiO2 ternary phase diagrams, as shown in Figure 10. There are generally two morphology types of CaO-SiO2-Al2O<sup>3</sup> inclusions as shown in Figure 9. One is a single-phase inclusion with a smooth surface, and the other has a rough surface. These two types of CaO-SiO2-Al2O<sup>3</sup> inclusions have different chemical compositions as shown in Table 3. The basicity CaO/SiO<sup>2</sup> of them is similar at about 1.9–2.0. The difference is mainly the component of Al2O3: Al2O<sup>3</sup> of the inclusion in Figure 9a is 24.5% while that of the inclusion in Figure 9b is only 2.8%, so the inclusion in Figure 9b can be defined as CaO-SiO2. The chemical composition distribution of CaO-SiO2-Al2O<sup>3</sup> inclusions in steel samples taken from different stages of industrial manufacturing was projected into CaO-Al2O3-SiO<sup>2</sup> ternary phase diagrams, as shown in Figure 10. *Metals* **2022**, *11*, x FOR PEER REVIEW 10 of 20

**Figure 9.** Two morphology types of CaO-Al2O3-SiO2 inclusions. (**a**) CaO-Al2O3-SiO2, (**b**) CaO-SiO2 **Table 3.** Chemical compositions of CaO-Al2O3-SiO2 inclusions. **Figure 9.** Two morphology types of CaO-Al2O<sup>3</sup> -SiO<sup>2</sup> inclusions. (**a**) CaO-Al2O<sup>3</sup> -SiO<sup>2</sup> , (**b**) CaO-SiO<sup>2</sup> . **Table 3.** Chemical compositions of CaO-Al2O<sup>3</sup> -SiO<sup>2</sup> inclusions.


(**a**) (**b**)

**Figure 10.** CaO-Al2O3-SiO2 ternary phase diagrams.(**a**) experimental data, (**b**) average data

The CaO-Al2O3-SiO2 and CaO-SiO2 inclusions' compositions at different stages were projected into ternary phase diagrams of CaO-Al2O3-SiO2, as shown in Figure 10. Since the inclusion composition of the industrial sample is distributed dispersedly (Figure 10a), the average composition of inclusions during different stages is shown in Figure 10b. The initial composition of the LF entry is located in the 2CaO∙ SiO2 region, and with the refining processing from LF→VD→TD→final products, the average composition moved from the 2CaO∙ SiO2 region to the 2CaO∙ Al2O3∙ SiO2 region (Gehlenite) and eventually returned to the 2CaO∙ SiO2 region (Ca2SiO4). First, additive lime during the BOF tapping process brought CaO into molten steel, and it met the primary deoxidization product SiO2 and formed 2CaO∙ SiO2. When Ti-Fe alloy was added, residual Al enterted the molten steel and formed the deoxidization product Al2O3. The combination of Al2O3 and Ca2SiO4 formed 2CaO∙ Al2O3∙ SiO2, and significantly decreased the liquidus of inclusion. Then, Ca treatment led to the reduction of Al2O3 and SiO2 in 2CaO∙ Al2O3∙ SiO2, so the Al2O3 and SiO2 content kept dropping while the CaO content gradually increased. When LF began, the average inclusion composition located in the 2CaO∙ SiO2 region and the CaO/SiO2 ratio was about 1.7–1.8, and its liquidus was around 2000 °C. After the Ti addition, the average

**Table 3.** Chemical compositions of CaO-Al2O3-SiO2 inclusions.

**Figure 10.** CaO-Al2O3-SiO2 ternary phase diagrams.(**a**) experimental data, (**b**) average data **Figure 10.** CaO-Al2O<sup>3</sup> -SiO<sup>2</sup> ternary phase diagrams.(**a**) experimental data, (**b**) average data.

(**a**) (**b**) **Figure 9.** Two morphology types of CaO-Al2O3-SiO2 inclusions. (**a**) CaO-Al2O3-SiO2, (**b**) CaO-SiO2

**No. CaO Al2O3 SiO2 MgO TiO2 MnO FeO S**  a 40.6 25.5 20.5 2.3 2.9 0.2 7.0 0.8 b 59.2 2.8 31.3 0.3 0.9 0.2 5.1 0.1

The CaO-Al2O3-SiO2 and CaO-SiO2 inclusions' compositions at different stages were projected into ternary phase diagrams of CaO-Al2O3-SiO2, as shown in Figure 10. Since the inclusion composition of the industrial sample is distributed dispersedly (Figure 10a), the average composition of inclusions during different stages is shown in Figure 10b. The initial composition of the LF entry is located in the 2CaO∙ SiO2 region, and with the refining processing from LF→VD→TD→final products, the average composition moved from the 2CaO∙ SiO2 region to the 2CaO∙ Al2O3∙ SiO2 region (Gehlenite) and eventually returned to the 2CaO∙ SiO2 region (Ca2SiO4). First, additive lime during the BOF tapping process brought CaO into molten steel, and it met the primary deoxidization product SiO2 and formed 2CaO∙ SiO2. When Ti-Fe alloy was added, residual Al enterted the molten steel and formed the deoxidization product Al2O3. The combination of Al2O3 and Ca2SiO4 formed 2CaO∙ Al2O3∙ SiO2, and significantly decreased the liquidus of inclusion. Then, Ca treatment led to the reduction of Al2O3 and SiO2 in 2CaO∙ Al2O3∙ SiO2, so the Al2O3 and SiO2 content kept dropping while the CaO content gradually increased. When LF began, the average inclusion composition located in the 2CaO∙ SiO2 region and the CaO/SiO2 ratio was about 1.7–1.8, and its liquidus was around 2000 °C. After the Ti addition, the average The CaO-Al2O3-SiO<sup>2</sup> and CaO-SiO<sup>2</sup> inclusions' compositions at different stages were projected into ternary phase diagrams of CaO-Al2O3-SiO2, as shown in Figure 10. Since the inclusion composition of the industrial sample is distributed dispersedly (Figure 10a), the average composition of inclusions during different stages is shown in Figure 10b. The initial composition of the LF entry is located in the 2CaO· SiO<sup>2</sup> region, and with the refining processing from LF→VD→TD→final products, the average composition moved from the 2CaO· SiO<sup>2</sup> region to the 2CaO· Al2O3· SiO<sup>2</sup> region (Gehlenite) and eventually returned to the 2CaO· SiO<sup>2</sup> region (Ca2SiO4). First, additive lime during the BOF tapping process brought CaO into molten steel, and it met the primary deoxidization product SiO<sup>2</sup> and formed 2CaO· SiO2. When Ti-Fe alloy was added, residual Al enterted the molten steel and formed the deoxidization product Al2O3. The combination of Al2O<sup>3</sup> and Ca2SiO<sup>4</sup> formed 2CaO· Al2O3· SiO2, and significantly decreased the liquidus of inclusion. Then, Ca treatment led to the reduction of Al2O<sup>3</sup> and SiO<sup>2</sup> in 2CaO· Al2O3· SiO2, so the Al2O<sup>3</sup> and SiO<sup>2</sup> content kept dropping while the CaO content gradually increased. When LF began, the average inclusion composition located in the 2CaO· SiO<sup>2</sup> region and the CaO/SiO<sup>2</sup> ratio was about 1.7–1.8, and its liquidus was around 2000 ◦C. After the Ti addition, the average inclusion composition was located in the Gehlenite region, and the liquidus significantly decreased to about 1500–1600 ◦C due to the increment of Al2O3. The inclusions after the Ti addition had a spherical and smooth shape because they remained liquid in molten steel. After Ca treatment, the CaO/SiO<sup>2</sup> ratio tended to be slightly higher than the initial CaO/SiO<sup>2</sup> ratio, and the average composition entered the Ca2SiO<sup>4</sup> region, the liquidus of which was about 1700–1800 ◦C. As a result, the CaO-SiO<sup>2</sup> inclusions from VD, tundish, and the final product presented rough surfaces, which indicated that they remained in a solid state in molten steel as shown in Figure 11. The reactions during these processes can be described as Formula (1)–(3):

$$\text{2CaO} + \text{SiO}\_2 \rightarrow \text{2CaO} \cdot \text{SiO}\_2 \tag{1}$$

$$\text{2CaO} \cdot \text{SiO}\_2 + [\text{Al}] + [\text{O}] \to \text{2CaO} \cdot \text{Al}\_2\text{O}\_3 \cdot \text{SiO}\_2 \tag{2}$$

$$2\text{CaO} \cdot \text{Al}\_2\text{O}\_3 \cdot \text{SiO}\_2 + [\text{Ca}] \rightarrow \text{CaO} - \text{Al}\_2\text{O}\_3 - \text{SiO}\_2 + [\text{Al}] + [\text{Si}] \tag{3}$$

**Figure 11.** The morphologies of CaO-Al2O3-SiO2 inclusions at different stages. **Figure 11.** The morphologies of CaO-Al2O<sup>3</sup> -SiO<sup>2</sup> inclusions at different stages.

#### *3.5. Effect of Inclusions on IAF Formation 3.5. Effect of Inclusions on IAF Formation*

be described as Formula (1), (2), and (3):

As many researchers have reported, titanium oxide is the key part of "oxide metallurgy". Generally, they perform as an effective and stable nucleation site for the induction of intragranular acicular ferrite (IAF). As discussed above, the main categories of inclusion in Ti-Ca industrial steel are CaO-Al2O3-SiO2-(MgO)-TiOx and CaO-Al2O3-SiO2 (CaO-SiO2 can be defined as one of CaO-Al2O3-SiO2). However, CaO-Al2O3-SiO2 may not be an effective inclusion for IAF promotion while CaO-Al2O3-SiO2-(MgO)-TiOx can be a potential nucleation site for IAF. As a result, the chemical composition of some typical inclusions was investigated using EDS mapping scanning of FE-SEM to clarify the element distribution in multiphase inclusions. Figure 12 shows the morphologies of typical Ti-oxide-containing inclusions in final products, and their chemical composition is shown in Table 4. They were generally CaO-Al2O3-(SiO2-MgO)-TiOx inclusions, with MnS precipitating around As many researchers have reported, titanium oxide is the key part of "oxide metallurgy". Generally, they perform as an effective and stable nucleation site for the inductionof intragranular acicular ferrite (IAF). As discussed above, the main categories of inclusion in Ti-Ca industrial steel are CaO-Al2O3-SiO2-(MgO)-TiO<sup>x</sup> and CaO-Al2O3-SiO<sup>2</sup> (CaO-SiO<sup>2</sup> can be defined as one of CaO-Al2O3-SiO2). However, CaO-Al2O3-SiO<sup>2</sup> may not be an effective inclusion for IAF promotion while CaO-Al2O3-SiO2-(MgO)-TiO<sup>x</sup> can be a potential nucleation site for IAF. As a result, the chemical composition of some typical inclusions was investigated using EDS mapping scanning of FE-SEM to clarify the element distribution in multiphase inclusions. Figure 12 shows the morphologies of typical Ti-oxide-containing inclusions in final products, and their chemical composition is shown in Table 4. They were generally CaO-Al2O3-(SiO2-MgO)-TiO<sup>x</sup> inclusions, with MnS precipitating around them. *Metals* **2022**, *11*, x FOR PEER REVIEW 12 of 20

inclusion composition was located in the Gehlenite region, and the liquidus significantly decreased to about 1500–1600 °C due to the increment of Al2O3. The inclusions after the Ti addition had a spherical and smooth shape because they remained liquid in molten steel. After Ca treatment, the CaO/SiO2 ratio tended to be slightly higher than the initial CaO/SiO2 ratio, and the average composition entered the Ca2SiO4 region, the liquidus of which was about 1700–1800 °C. As a result, the CaO-SiO2 inclusions from VD, tundish, and the final product presented rough surfaces, which indicated that they remained in a solid state in molten steel as shown in Figure 11. The reactions during these processes can

2CaO + SiOଶ → 2CaO · SiOଶ (1)

2CaO · SiOଶ + [Al] + [O] → 2CaO · AlଶOଷ · SiOଶ (2)

2CaO · AlଶOଷ · SiOଶ + [Ca] → CaO − AlଶOଷ − SiOଶ + [Al] + [Si] (3)

**Figure 12.** The morphologies of typical Ti-oxide-containing inclusions in the final products. **Figure 12.** The morphologies of typical Ti-oxide-containing inclusions in the final products.

**Table 4.** The chemical composition of typical Ti-oxide-containing inclusions in the final products. **No. CaO Al2O3 SiO2 MgO TiO2 MnO FeO S**  1 22.7 15.7 0.7 6.6 15.3 16.1 12.1 11.0 2 27.7 19.2 6.9 9.2 22.8 2.8 6.6 4.7

multiphase inclusions is shown in Figures 13 and 14. The size of the inclusion in Figure 13 is about 2 µm. The shape of the inclusion shows different parts combined, and it mainly consists of two parts: MgO-Al2O3 spinel formed the core with two small MgO + MgS particles attached to it. The core was half surrounded by the CaO-TiO2 layer, and the interface between the core and surrounding layer was relatively clear. The size of the inclusion in Figure 14 is about 1.5 µm. The shape looks like a whole, and it contains two phases: the left part is MgO-Al2O3 spinel, and the right part is CaO-TiOx and (Ca, Mn) S. Generally, MgO-Al2O3 spinel is formed by the combination of [Mg], [Al], and [O] in molten steel and has a very high melting point. However, different mechanisms of the formation of MgO-Al2O3 spinel in Ti-Ca-treated industrial steel may exist. As for the multiphase inclusion in Figure 13, it is indicated that MgO-Al2O3 spinel was first formed when dissolved [Al] from ferroalloy combined with [Mg] from the refractory. Then, spinel acted as a heterogeneous nucleation site, and MgS and CaO-TiOx started to precipitate around the core with the decreasing temperature of molten steel and the solidification process. As a result, clear interfaces between each phase existed. As for the multiphase inclusion in Figure 14, the explanation of its formation may be illustrated, as shown in Figure 8: liquid inclusion CaO-Al2O3-SiO2-(MgO)-TiOx was formed in the steel refining process, but with the decreasing temperature of molten steel and the solidification process, MgO-Al2O3 spinel firstly crystallized and precipitated due to its highest melting point, and then other

phases, such as CaO-TiOx and (Ca, Mn) S, precipitated in succession.


**Table 4.** The chemical composition of typical Ti-oxide-containing inclusions in the final products.

Additionally, element mapping scanning was used to identify the chemical composition of different phases in multiphase inclusions. The element distribution of 2 typical multiphase inclusions is shown in Figures 13 and 14. The size of the inclusion in Figure 13 is about 2 µm. The shape of the inclusion shows different parts combined, and it mainly consists of two parts: MgO-Al2O<sup>3</sup> spinel formed the core with two small MgO + MgS particles attached to it. The core was half surrounded by the CaO-TiO<sup>2</sup> layer, and the interface between the core and surrounding layer was relatively clear. The size of the inclusion in Figure 14 is about 1.5 µm. The shape looks like a whole, and it contains two phases: the left part is MgO-Al2O<sup>3</sup> spinel, and the right part is CaO-TiO<sup>x</sup> and (Ca, Mn) S. Generally, MgO-Al2O<sup>3</sup> spinel is formed by the combination of [Mg], [Al], and [O] in molten steel and has a very high melting point. However, different mechanisms of the formation of MgO-Al2O<sup>3</sup> spinel in Ti-Ca-treated industrial steel may exist. As for the multiphase inclusion in Figure 13, it is indicated that MgO-Al2O<sup>3</sup> spinel was first formed when dissolved [Al] from ferroalloy combined with [Mg] from the refractory. Then, spinel acted as a heterogeneous nucleation site, and MgS and CaO-TiO<sup>x</sup> started to precipitate around the core with the decreasing temperature of molten steel and the solidification process. As a result, clear interfaces between each phase existed. As for the multiphase inclusion in Figure 14, the explanation of its formation may be illustrated, as shown in Figure 8: liquid inclusion CaO-Al2O3-SiO2-(MgO)-TiO<sup>x</sup> was formed in the steel refining process, but with the decreasing temperature of molten steel and the solidification process, MgO-Al2O<sup>3</sup> spinel firstly crystallized and precipitated due to its highest melting point, and then other phases, such as CaO-TiO<sup>x</sup> and (Ca, Mn) S, precipitated in succession. *Metals* **2022**, *11*, x FOR PEER REVIEW 13 of 20

**Figure 13.** The element distribution of typical multiphase inclusion. **Figure 13.** The element distribution of typical multiphase inclusion.

Mn S Ca Si

Ti N Fe O

**Figure 14.** The element distribution of typical multiphase inclusion.

ing the rolling process of TMCP, so it spread along the rolling direction.

A large-sized multiphase inclusion is shown in Figure 15 and the results of EPMA analysis for each phase are listed in Table 5. The main body of this large-sized inclusion consisted of CaO-Al2O3-SiO2 (red number 1) and CaO-TiOx (red number 2). It seemed that this large-sized inclusion was crushed and separated into several parts composed of (Ca, Mn)S (red number 3), CaO-Al2O3-MgO (red number 4) and CaO-SiO2 (red number 5) dur-

Mg Al

**Figure 13.** The element distribution of typical multiphase inclusion.

Mn S Ca Si

Ti N Fe O

Mg Al

**Figure 14.** The element distribution of typical multiphase inclusion. **Figure 14.** The element distribution of typical multiphase inclusion.

A large-sized multiphase inclusion is shown in Figure 15 and the results of EPMA analysis for each phase are listed in Table 5. The main body of this large-sized inclusion consisted of CaO-Al2O3-SiO2 (red number 1) and CaO-TiOx (red number 2). It seemed that this large-sized inclusion was crushed and separated into several parts composed of (Ca, Mn)S (red number 3), CaO-Al2O3-MgO (red number 4) and CaO-SiO2 (red number 5) during the rolling process of TMCP, so it spread along the rolling direction. A large-sized multiphase inclusion is shown in Figure 15 and the results of EPMA analysis for each phase are listed in Table 5. The main body of this large-sized inclusion consisted of CaO-Al2O3-SiO<sup>2</sup> (red number 1) and CaO-TiO<sup>x</sup> (red number 2). It seemed that this large-sized inclusion was crushed and separated into several parts composed of (Ca, Mn)S (red number 3), CaO-Al2O3-MgO (red number 4) and CaO-SiO<sup>2</sup> (red number 5) during the rolling process of TMCP, so it spread along the rolling direction. *Metals* **2022**, *11*, x FOR PEER REVIEW 14 of 20

**Figure 15.** The morphology of a large-sized multiphase inclusion. **Figure 15.** The morphology of a large-sized multiphase inclusion.

**Table 5.** The chemical composition of each part of the large-sized multiphase inclusion.

After welding simulation, the inclusions and microstructure were investigated, and some typical inclusions that can effectively induce IAFs were observed and detected using EPMA. The morphologies and chemical composition of typical inclusions are shown in Figure 16 and Table 6, respectively. The induced IAFs are marked in red (AF1-AF6, AF1- AF3) in Figure 16. It can be confirmed that only TiO2 containing inclusions were effective for IAF promotion while CaO-Al2O3-SiO2 inclusions were not found to be the nucleation site of IAF formation. These effective IAF inclusions were generally multiphase, and their size ranged from 2 to 5 µm and consisted of CaO-Al2O3-(SiO2-MgO)-TiOx. This indicates that CaO-Al2O3-(SiO2-MgO)-TiOx was essential for inducing IAFs in Ti-Ca-treated offshore structural steel. Particularly, it can be found that the concentration of Ti-oxide in these effective IAF inclusions (20–30%wt) was significantly higher than the average concentration of Ti-oxide in all inclusions, as shown in Figure 13, which was about 15%wt. The higher concentration of Ti-oxide is generally considered to increase the possibility of IAF formation, thereby enhancing the mechanical property of steel. As a result, how to increase the proportion of CaO-Al2O3-(SiO2-MgO)-TiOx and how to increase the concentration of Ti-oxide can be a potential and meaningful research direction, and more work

4 15.1 46.2 3.2 19.6 0.9 2.8 8.1 3.6 CaO-Al2O3-MgO 5 53.4 0.2 27.6 1.9 0.1 0.5 16.4 0.1 CaO-SiO2

is yet to be done in the future.

**No. CaO Al2O3 SiO2 MgO TiO2 MnO FeO S Composition** 


**Table 5.** The chemical composition of each part of the large-sized multiphase inclusion.

After welding simulation, the inclusions and microstructure were investigated, and some typical inclusions that can effectively induce IAFs were observed and detected using EPMA. The morphologies and chemical composition of typical inclusions are shown in Figure 16 and Table 6, respectively. The induced IAFs are marked in red (AF1–AF6, AF1–AF3) in Figure 16. It can be confirmed that only TiO<sup>2</sup> containing inclusions were effective for IAF promotion while CaO-Al2O3-SiO<sup>2</sup> inclusions were not found to be the nucleation site of IAF formation. These effective IAF inclusions were generally multiphase, and their size ranged from 2 to 5 µm and consisted of CaO-Al2O3-(SiO2-MgO)-TiOx. This indicates that CaO-Al2O3-(SiO2-MgO)-TiO<sup>x</sup> was essential for inducing IAFs in Ti-Ca-treated offshore structural steel. Particularly, it can be found that the concentration of Ti-oxide in these effective IAF inclusions (20–30%wt) was significantly higher than the average concentration of Ti-oxide in all inclusions, as shown in Figure 13, which was about 15%wt. The higher concentration of Ti-oxide is generally considered to increase the possibility of IAF formation, thereby enhancing the mechanical property of steel. As a result, how to increase the proportion of CaO-Al2O3-(SiO2-MgO)-TiO<sup>x</sup> and how to increase the concentration of Ti-oxide can be a potential and meaningful research direction, and more work is yet to be done in the future. *Metals* **2022**, *11*, x FOR PEER REVIEW 15 of 20

**Figure 16.** *Cont*.

C). The results are shown in Figures 17–19.

**Figure 16.** The morphologies of three typical inclusions after welding simulation.

**Table 6.** The chemical composition of typical inclusions after welding simulation.

**No. CaO Al2O3 SiO2 MgO TiO2 MnO FeO S**  1 13.4 29.3 8.0 24.8 19.1 1.6 10.6 0.1 2 11.7 16.5 16.3 20.8 30.1 1.7 3.3 0.2 3 3.7 18.2 0.3 3.8 36.1 3.2 37.3 1.0

To clearly identify different phases in these multi-phased inclusions and thoroughly clarify the formation mechanisms, mapping scanning of FE-SEM was used for detecting the elementary distribution of effective inclusions that induced IAFs (inclusion A, B, and

**Figure 16.** The morphologies of three typical inclusions after welding simulation. **Figure 16.** The morphologies of three typical inclusions after welding simulation.



To clearly identify different phases in these multi-phased inclusions and thoroughly clarify the formation mechanisms, mapping scanning of FE-SEM was used for detecting the elementary distribution of effective inclusions that induced IAFs (inclusion A, B, and C). The results are shown in Figures 17–19. To clearly identify different phases in these multi-phased inclusions and thoroughly clarify the formation mechanisms, mapping scanning of FE-SEM was used for detecting the elementary distribution of effective inclusions that induced IAFs (inclusion A, B, and C). The results are shown in Figures 17–19. *Metals* **2022**, *11*, x FOR PEER REVIEW 16 of 20

**Figure 17.** EDS mapping analysis of one typical inclusion A effective for IAF nucleation in Ti-Ca deoxidized steel. **Figure 17.** EDS mapping analysis of one typical inclusion A effective for IAF nucleation in Ti-Ca deoxidized steel.

respectively. Particularly, CaO-Al2O3-SiO2-MgO is the main content of secondary refining slag and has a low liquidus, thereby presenting a liquid state in molten steel. An Al2O3- MgO spinel particle 1.2 µm in size is embedded into the CaO-Al2O3-SiO2-MgO liquid inclusion, and it has a very high liquidus temperature. CaO-TiO2 also has a higher liquidus temperature than that of molten steel except for the condition when TiO2 = 80%. As a result, it can be concluded that the CaO-Al2O3-SiO2-MgO-TiOx liquid inclusion was formed in molten steel as described in Figure 8. Then, Al2O3-MgO spinel and CaO-TiOx perovskite

As can be seen in Figure 17, the equivalent diameter of inclusion A is about 5.1 µm and one piece of acicular ferrite lath is induced by this inclusion. The morphology of the

As shown in Figure 18, inclusion B also shows the spherical shape, and two pieces of acicular ferrite laths are induced by this inclusion. It consists of CaO-Al2O3-SiO2-MgO, (Ca, Mn) S, and CaO-TiOx. Particularly, the left part of the main body is CaO-Al2O3-SiO2-MgO, and the right part of the main body is (Ca, Mn) S while CaO-TiOx perovskite precipitates

precipitated in order during the cooling process.

along the edge of the above two phases.

**Figure 18.** EDS mapping analysis of one typical inclusion B effective for IAF nucleation in Ti-Ca deoxidized steel. **Figure 18.** EDS mapping analysis of one typical inclusion B effective for IAF nucleation in Ti-Ca deoxidized steel.

In Figure 19, five pieces of acicular ferrite laths are induced by inclusion C. Different from the above two inclusions, the morphology reveals that it is formed due to the coalescence of several particles: CaO-Al2O3-SiO2-MgO, (Ca, Mn) S, and CaO-TiOx. During the secondary refining process, phases with a higher liquidus temperature, such as (Ca, Mn) S and CaO-TiOx, are "captured" by liquid-phase CaO-Al2O3-SiO2-MgO. These particles do not melt and form a new liquid inclusion but mechanically coalesce together. Almost all the detected inclusions effective at inducing IAFs are found to be CaO-Al2O3-SiO2-MgO-TiOx-based inclusions. It can be confirmed that CaO-Al2O3-SiO2-MgO-TiOx-based inclusions are effective nucleation sites for IAF promotion while CaO-Al2O3- SiO2-based inclusions are ineffective at inducing IAFs. The above three elementary distribution analyses also prove the formation mechanism of CaO-Al2O3-SiO2-MgO-TiOx-based inclusions due to the combination of particles from refining slag and deoxidization prod-As can be seen in Figure 17, the equivalent diameter of inclusion A is about 5.1 µm and one piece of acicular ferrite lath is induced by this inclusion. The morphology of the multi-phased inclusion has a spherical shape. This is a typical CaO-Al2O3-SiO2-MgO-TiO<sup>x</sup> inclusion, and its main body consists of CaO-Al2O3-SiO2-MgO, Al2O3-MgO, and CaO-TiOx, respectively. Particularly, CaO-Al2O3-SiO2-MgO is the main content of secondary refining slag and has a low liquidus, thereby presenting a liquid state in molten steel. An Al2O3-MgO spinel particle 1.2 µm in size is embedded into the CaO-Al2O3-SiO2-MgO liquid inclusion, and it has a very high liquidus temperature. CaO-TiO<sup>2</sup> also has a higher liquidus temperature than that of molten steel except for the condition when TiO<sup>2</sup> = 80%. As a result, it can be concluded that the CaO-Al2O3-SiO2-MgO-TiO<sup>x</sup> liquid inclusion was formed in molten steel as described in Figure 8. Then, Al2O3-MgO spinel and CaO-TiO<sup>x</sup> perovskite precipitated in order during the cooling process.

ucts. Then, different phases precipitate in order during the cooling process after secondary refining. As shown in Figure 18, inclusion B also shows the spherical shape, and two pieces of acicular ferrite laths are induced by this inclusion. It consists of CaO-Al2O3-SiO2-MgO, (Ca, Mn) S, and CaO-TiOx. Particularly, the left part of the main body is CaO-Al2O3-SiO2-MgO, and the right part of the main body is (Ca, Mn) S while CaO-TiO<sup>x</sup> perovskite precipitates along the edge of the above two phases.

**Figure 19.** EDS mapping analysis of one typical inclusion C effective for IAF nucleation in Ti-Ca deoxidized steel**. Figure 19.** EDS mapping analysis of one typical inclusion C effective for IAF nucleation in Ti-Ca deoxidized steel.

**4. Conclusions**  To clarify the formation mechanism and evolution of oxide inclusions in Ti-Catreated offshore structural steel, industrial sampling was conducted from LF, VD, and TD to final products. Continuous changes and correlations of various inclusions were explained by analysing the number density, morphology, and chemical composition using EPMA and FE-SEM. The primary findings are concluded as follows: In Figure 19, five pieces of acicular ferrite laths are induced by inclusion C. Different from the above two inclusions, the morphology reveals that it is formed due to the coalescence of several particles: CaO-Al2O3-SiO2-MgO, (Ca, Mn) S, and CaO-TiOx. During the secondary refining process, phases with a higher liquidus temperature, such as (Ca, Mn) S and CaO-TiOx, are "captured" by liquid-phase CaO-Al2O3-SiO2-MgO. These particles do not melt and form a new liquid inclusion but mechanically coalesce together.

1. The evolution of inclusions during different stages in Ti-Ca-treated offshore structural steel is from primary deoxidization products (Al2O3-MnO, Al2O3-SiO2-MnO) and their combination with lime (CaO-Al2O3-SiO2-MnO and CaO-SiO2)→CaO-Al2O3- SiO2-(MgO)-TiOx and CaO-Al2O3-SiO2→ CaO-Al2O3-SiO2-(MgO)-TiOx, CaO-Al2O3- SiO2, and secondary deoxidization products (Al2O3-MnO-TiOx and SiO2-MnO)→ CaO-Al2O3-SiO2-(MgO)-TiOx and CaO-Al2O3-SiO2. The number density of the inclusions in Ti-Ca-treated industrial steel generally dropped from LF, VD, and Tundish to the final product, except for the Ti-Fe and Si-Ca addition in the LF, the number Almost all the detected inclusions effective at inducing IAFs are found to be CaO-Al2O3-SiO2-MgO-TiOx-based inclusions. It can be confirmed that CaO-Al2O3-SiO2-MgO-TiOx-based inclusions are effective nucleation sites for IAF promotion while CaO-Al2O3- SiO2-based inclusions are ineffective at inducing IAFs. The above three elementary distribution analyses also prove the formation mechanism of CaO-Al2O3-SiO2-MgO-TiOxbased inclusions due to the combination of particles from refining slag and deoxidization products. Then, different phases precipitate in order during the cooling process after secondary refining.

density slightly increased. The total decrease in the inclusion number density was mainly due to the significantly decreasing number density of small inclusions (<1

µm) during the refining process.
