Deformation of Non-metallic Inclusions in Steel during Rolling Process: A Review

Abstract

Non-metallic inclusions were deformed and broken during hot and cold rolling processes. The deformation of inclusions during the rolling process had an influence on the property of steel products. Investigations on the deformation of inclusions in steel was reviewed in the current study. Physical properties of inclusions were summarized and compared, such as the melting temperature, viscosity, Young’s modulus, hardness, thermal expansion coefficient, crystallization, and Poisson’s ratio. The deformability index of inclusions showed a decreasing tendency with a higher rolling reduction. During the hot rolling process, the deformability index of oxide inclusions was mainly related to the melting temperature and viscosity of inclusions. With the increase of the rolling temperature, the deformability index of oxide inclusions showed an increasing tendency due to the higher liquid fraction and the lower viscosity of inclusions. During the hot rolling process, the deformability index of MnS inclusions increased with a decrease of the rolling temperature until reaching the austenite to ferrite matrix transformation temperature. During the cold rolling process, deformability index of oxide inclusions was mainly influenced by Young’s modulus and hardness of inclusions. Inclusions with a lower Young’s modulus exhibited a higher deformability index during the cold rolling process. Moreover, the focus on the inclusion deformation in the future was proposed in this paper.

1. Introduction

Non-metallic inclusions in steel were inevitable during the steelmaking process. The composition, size, shape, number, and distribution of inclusions had a significant influence on the steel performance, such as strength, toughness, fatigue, corrosion resistance, etc.^1,2) A lot of technologies were developed to achieve the precise control of inclusions in the molten steel. Inclusions were mainly generated during the deoxidation process of steel.^3,4,5) The cleanliness of the ferroalloy had an obvious influence on the composition of inclusions due to impurity elements of Al and Ca.^6,7,8,9) The calcium treatment was widely used for the Al-killed steel to modify solid Al₂O₃ inclusions to liquid ones and avoid the nozzle clogging.^10,11,12,13) The slag refining can effectively modify the inclusion composition^14,15,16) and absorb inclusions in the slag.^17,18,19) The argon bubbling^20,21,22) and vacuum treatment^23,24,25) during the refining process promoted the removal of inclusions from steel to slag. During the continuous casting process, the protective casting was conducted to avoid the formation of inclusions caused by the reoxidation of the molten steel.^26,27,28) Recently, the evolution of inclusions during continuous casting^29,30) and reheating^31,32) processes attracted more attentions, which was mainly caused by the change of the equilibrium between inclusions and steel at various temperatures. Thus, it was concluded that the control of inclusions in the molten steel was relatively mature.

Inclusions were deformed and broken during hot and cold rolling processes. The deformation of inclusions during the rolling process had a direct influence on the property of steel products. For the non-oriented silicon steel, the grain pinning by deformed inclusions on the steel grain was responsible for the magnetic induction performance of steel.^33,34) For the tire cord steel, the steel bar was cold-drawn into very fine wires, which required not only high cleanliness, but also good deformability of inclusions to avoid the fracture of the tire cord steel.³⁵⁾ Large inclusions in heavy rail steels caused in cracks in rails, which was harmful to the railway safety.^36,37,38) The fatigue life of the bearing steel was significantly influenced by the number and size of inclusions. The fragile calcium aluminate inclusions were suggested to be avoided due to their large size after the rolling process. The string calcium aluminate inclusions with the low melting temperature after the rolling was the source of the hydrogen induced crack in the linepipe steel.³⁹⁾ It was reported that micro-voids generated around inclusions after rolling were one of the main reasons of the initiation of pitting corrosion of the stainless steel.^40,41,42) It was of great importance to investigate the deformation of inclusions to determine the control target of inclusions and improve the steel performance.

2. Overview of The Investigation of Deformation of Inclusions in Steel

The deformation of inclusions in steel rolling process was first investigated by Echeil and Schell⁴³⁾ as early as in the 1950s. Figure 1⁴³⁾ shows the morphology of Al₂O₃–SiO₂–MnO inclusions after the hot rolling process. Spherical inclusions changed to long strip ones after the hot rolling. The deformation of inclusions during rolling was compared using the ratio of the major axis and minor axis of the inclusions, as Eq. (1). Afterwards, the issue of inclusion deformation attracted more and more attentions. As summarized in Table 1, during the period from 1960s to 1990s, a large number of experimental works were conducted to investigated the deformation of inclusions during the rolling process. The influence of chemical composition of inclusion, rolling temperature, and rolling reduction on the deformation of inclusions was investigated. Most of investigations focused on the deformation of MnS inclusions during hot rolling. Meanwhile, several works focused on the deformation of Al₂O₃–SiO₂–CaO and Al₂O₃–SiO₂–MnO inclusions during hot rolling process. However, reported studies on the deformation of inclusions during cold rolling were relatively less than that of the hot rolling. Since the inclusion deformation in Eq. (1) ignored the steel deformation, which can hardly be used for the measurement of the inclusion deformation with varied rolling reductions. In 1963, Malkiewicz and Rudnlk proposed a deformability index of inclusions to quantitatively compare the deformability of various types of inclusions during the rolling process of plates and bars, as Eqs. (2) and (3),⁴⁴⁾ respectively. The deformability index considered not only the inclusion deformation but also the steel deformation.   

$$\lambda =\frac{b}{a}$$(1)

  

$$\upsilon =\frac{{\epsilon }_i}{{\epsilon }_m}=\frac{ln\lambda }{2lnh}=\frac{lnb/a}{2ln{F}_0/F_1}$$(2)

  

$$\upsilon =\frac{{\epsilon }_i}{{\epsilon }_m}=\frac{2ln\lambda }{3lnh}=\frac{2lnb/a}{3ln{F}_0/F_1}$$(3)

where v is the deformability index of inclusions, ε_i and ε_m are the true elongation of the inclusion and steel; b is the major axis of inclusions, and a is the minor axis of inclusion measured on the longitudinal section of steel, m; λ is the inclusion deformation of b/a; F₀ is the initial cross-section, and F₁ is the cross-section after rolling, m²; h reflects the working amount of F₀/F₁.

Fig. 1.

Morphology of Al₂O₃–SiO₂–MnO inclusions after the hot rolling process with various rolling reductions reported in 1953.⁴³⁾

Table 1. Summarized investigations on the inclusion deformation during the rolling process.

Authors	Year	Steel matrix	Inclusion property		Rolling parameters		Method	Details
			Compositions	Size (μm)	Temperature (°C)	Reduction (%)
Malkiewicz44)	1963	Low C steel	Si–Al–Mn	–	850–1150	40–80	Experiment	Rolling
Uciyama et al.45)	1965	Iron	Si–Mn	2–25	1000–1250	88, 97	Experiment	Rolling
Baker et al.46)	1976	NA	MnS	1–40	800–1200	30–90	Experiment	Rolling
Segal et al.47)	1977	NA	MnS	1–5	400–1000	50–75	Experiment	Rolling
Maiti et al.48)	1985	Linepipe steel	Ca–Al–Si–Mn	–	900	38–97	Experiment	Rolling
Luo et al.49)	2002	Carbon steel	MnS	–	900–1200	30–85	Experiment	Rolling
Jin et al.50)	2009	Tire cord steel	Al–Si–Ca	20	Room temperature	<83	Experiment	Tensile
Manshadi51)	2010	Low C steel	MnS	1–4	900–1100	–	Experiment	Compressive
Yamamoto37)	2011	Carbon steel	MnS, Al2O3	5–20	1000–1250	82	Experiment	Compressive
Ji et al.52)	2015	304 stainless steel	Ca–Si–Mn–Al	5–10	Room temperature	83	Experiment	Rolling
Moroz et al.53)	2017	Bearing	–	5	800–1200	60	Experiment	Rolling
Wang et al.54)	2020	304 stainless steel	MnS	2–8	900–1200	–	Experiment	In situ tensile
Wang55)	2021	Linepipe steel	Ca(O·S)–Al–Mg		1200–1500	20	Experiment	Thermal simulator
Huang3)	2018	1215MS steel	MnS	8–11	1050	10–70	Experiment	Compressive
Niu4)	2021	Tire cord steel	Ca–Si–Mn–Al	1–7	950–1050	–	Experiment	Rolling
Luo et al.49)	2001	Low alloy steel	Mn–Ca–Al–Si	–	800–1200	5–42	Experiment	Rolling
Yang et al.56)	2020	Tire cord steel	Ca–Si–Mn–Al	1–7	940–1020	80–90	Experiment	Thermal simulator
Bandstra et al.57)	2004	HY-100	MnS	1–6	Room temperature	–	Modeling	ABAQUS 2D
Ervasti et al.36)	2005	Si-Mn-killed steel	Hard inclusions	8000	1200	13.6	Modeling	LS-DYNA 3D
Yu et al.59,60,61)	2007	304 stainless steel	Al2O3	48	Room temperature	66	Modeling	LS-DYNA 3D
	2008		Hard inclusions	10–40	Room temperature	33	Modeling	LS-DYNA 2D
	2009		Al2O3	10–50	Room temperature	33–66	Modeling	LS-DYNA 3D
Matsuika et al.62)	2014	Al-killed steel	Al2O3, MnS	5	1100	0–80	Modeling	DEFORM 2D
Gupta et al.63)	2015	–	Hard inclusions	5.5, 25	800–1400	–	Modeling	ANSYS 2D
Zhang et al.64,65)	2017	Si-Mn-killed steel	Mn–Al–Si	500	800–1000	10, 20	Modeling	ABAQUS 3D
	2018		Ca–Al–Si	500	800–1100	10, 20	Modeling	–
Wang et al.66)	2019	304 stainless steel	MnS	–	Room temperature	–	Modeling	ABAQUS 2D

After 2000s, the deformation of inclusions during the rolling process was widely simulated using modeling software with the development of the numerical simulation computing techniques. The effect of the shape, size, position, and orientation of inclusions in steel on inclusion deformation was widely studied, which can hardly be achieved through experimental methods. Meanwhile, the stress concentration between inclusions and steel matrix after rolling was revealed by numerical simulations, as shown in Fig. 2. However, the deformation of complex inclusions was hardly simulated accurately due to the data lack of physical properties of inclusions at various rolling temperatures. Nowadays, the experiment and simulation were combined to better reveal the deformation mechanism of inclusions during the hot rolling and cold rolling process.

Fig. 2.

The effective stress distribution for inclusion and its surrounding material:⁶⁷⁾ (a) A soft inclusion at 1200°C; (b) A hard inclusion at 700°C.

3. Physical Property of Inclusions

Inclusions were deformed or broken during the rolling process. The deformation mechanism of various inclusions is proposed as the schematic shown in Fig. 3.⁶⁸⁾ The hard inclusion was undeformed during the rolling process, leading to the cavity around the inclusion in the rolling direction. The hard crystalline inclusion was broken into a long strip one during the rolling process. The hard cluster inclusion was rolled to a chain of small particles. The soft inclusion was rolled to a linear one, which exhibited a good deformability during the rolling process. Crystals precipitated in a soft inclusion lowered the deformability of inclusions due to their high hardness. The deformability of inclusions was obviously related to their composition, which was mainly determined by inclusion properties, such as melting temperature, viscosity, Young’s modulus, hardness, thermal expansion coefficient, crystallization, poisson’s ratio, etc.

Fig. 3.

Schematic of deformation mechanism of various inclusions.⁶⁸⁾

The deformability of inclusions during the rolling process was significantly influenced by the melting temperature of inclusions. The lower the melting temperature of inclusions, the better deformability of inclusions during rolling. When the liquidus of inclusions was lower than the hot rolling temperature, liquid inclusions exhibited a good deformability of fluid during rolling. Inclusions with a solidus higher than the rolling temperature were deformed a little during the hot rolling process. The melting temperature of typical Al₂O₃–SiO₂–CaO, Al₂O₃–SiO₂–MnO, and Al₂O₃–MgO–CaO inclusions are calculated using FactSage 7.0, as results shown in Fig. 4. Liquid inclusions with a low melting temperature were generally larger than the solid ones, which exhibited a lower floating rate due to their smaller contact angle with the molten steel.⁶⁹⁾ Thus, the full liquid inclusion was rarely suggested to avoid the formation of the long strip Al₂O₃–SiO₂–CaO and Al₂O₃–MgO–CaO inclusions for Al-killed steels,⁷⁰⁾ while it was the target inclusion for the Al₂O₃–SiO₂–MnO in Si-Mn-killed steel to improve the deformability of inclusions during the hot rolling process.⁷¹⁾

Fig. 4.

Calculated melting temperature of typical inclusions: (a) Al₂O₃–SiO₂–CaO; (b) Al₂O₃–SiO₂–MnO; (c) Al₂O₃–MgO–CaO. (Online version in color.)

The viscosity was the flow resistance of a fluid, which had an obvious influence on the deformability of inclusions in steel. During the hot rolling process, voids around inclusions were closed due to the high rolling stress, leading to the change of voids from spherical to ellipsoid or line. The inclusion with a lower viscosity easily flowed and filled the deformed voids, showing a better deformability during the hot rolling process. The inclusion of a higher viscosity deformed little during the hot rolling process, leading the formation of cavity and stress concentration around the inclusion. It was reported that the viscosity of Al₂O₃–SiO₂–CaO and Al₂O₃–SiO₂–MnO inclusions can be calculated as Eq. (4).⁶⁸⁾ The viscosity parameter of typical Al₂O₃–SiO₂–CaO and Al₂O₃–SiO₂–MnO inclusions are shown in Fig. 5. The viscosity of Al₂O₃–SiO₂–CaO and Al₂O₃–SiO₂–MnO inclusions increased with a higher SiO₂ content and a lower CaO content in inclusions. A higher rolling temperature was beneficial to lower the viscosity of inclusions, improving the deformability of inclusions during the hot rolling process.   

$$\mu =\frac{T}{1.17\times {10}^6}exp\left[\left(\frac{1\mathrm{\hspace{0.17em} }000}{T}-0.27\right)\times B\right]$$(4)

where μ is the viscosity of Al₂O₃–SiO₂–CaO and Al₂O₃–SiO₂–MnO inclusions, Pa·S; T is the temperature, K; B is the viscosity parameter of inclusions.

Fig. 5.

Viscosity parameters of Al₂O₃–SiO₂–CaO and Al₂O₃–SiO₂–MnO inclusions:⁶⁸⁾ (a) Al₂O₃–SiO₂–CaO; (b) Al₂O₃–SiO₂–MnO.

During the heat treatment before hot rolling, inclusions and steel matrix expanded with the change of temperature. Thermal expansion coefficient was an important parameter of materials, reflecting the volume change of materials at different temperatures. When the thermal expansion coefficient of inclusions was higher than that of the steel matrix, there was a stress concentration around inclusions. A thermal expansion coefficient of inclusions lower than steel matrix resulted in the formation of voids between the inclusions and steel matrix. The difference between the steel inclusion had an obvious influence on the performance of steel. Figure 6 shows the thermal expansion coefficient of typical inclusions reported by Nicholson,⁷²⁾ Bernard,⁶⁸⁾ and Brooksbank.⁷³⁾ Thermal expansion coefficient of simple oxides was higher, while it was lower for the complex oxides. It was noted that thermal expansion coefficient of sulfides was generally higher than that of oxides. Meanwhile, the thermal expansion coefficient of inclusions was a function of the temperature. There was linear relationship between thermal expansion coefficient of inclusions and the temperature,^73,74) as shown in Fig. 7.

Fig. 6.

Thermal expansions coefficient of typical inclusions. (Online version in color.)

Fig. 7.

Thermal expansion equation for inclusion materials. (Online version in color.)

Young’s modulus was the stress/strain ratio during the elastic deformation process, which reflected deformation resistance of materials. The lower Young’s modulus of inclusions, the higher deformation ability of inclusions during the rolling process. The Young’s modulus of inclusions at the room temperature reported by Kimura,⁷⁵⁾ Ashizuka,⁷⁶⁾ Makishima,⁷⁷⁾ Inaba,⁷⁸⁾ Bernard,⁶⁸⁾ Brooksbank⁷⁹⁾ was summarized in Fig. 8. The mean atomic volume of oxides can be calculated by Eq. (5). Then, it was found that there was an empirical correlation between the mean atomic volume and Young’s modulus of inclusions at the room temperature, as Eq. (6).⁸⁰⁾ Young’s modulus of oxides was a function of temperature. The relationship between Young’s modulus of oxides and temperature was summarized in Fig. 9, as reported by Kimura et al.,⁷⁵⁾ Kang et al.,⁸¹⁾ Wachtman et al.⁸²⁾ A higher temperature was beneficial to improve the deformability of inclusions due to the lower Young’s modulus.   

$$V=\frac{M}{\rho \cdot n}$$(5)

  

$$E=39\mathrm{\hspace{0.17em} }811V^{-2.9}$$(6)

where V is the mean atomic volume of oxides, 10⁻⁶·m³/mol; M is the relative molecular mass of oxides, kg/mol; ρ is the density of oxides, kg/m³; and n is the number of atoms in oxides; E is the Young’s modulus of oxides, GPa.

Fig. 8.

The relationship between Young’s modulus and mean atomic volume of oxides. (Online version in color.)

Fig. 9.

Relationship between Young’s modulus of oxides and temperature. (Online version in color.)

Young’s modulus of inclusions at the room temperature was responsible for the deformability of inclusions during the cold rolling process, while it hardly reflected the inclusion deformability during the hot rolling process. However, the data of Young’s modulus of complex oxides at a high temperature were little reported. To evaluate the deformability of inclusions during the cold rolling process, Young’s modulus of typical complex Al₂O₃–SiO₂–CaO, Al₂O₃–SiO₂–MnO, and Al₂O₃–MgO–CaO inclusions at the room temperature were calculated using Eq. (3), as results shown in Fig. 10. For Al₂O₃–SiO₂–CaO and Al₂O₃–SiO₂–MnO inclusions, inclusions with a lower Al₂O₃ content had a smaller Young’s modulus, which exhibited a better deformability during the cold rolling process. For Al₂O₃–MgO–CaO inclusions, the deformability of 3CaO·Al₂O₃ and 12CaO·7Al₂O₃ inclusions was better during the cold rolling process due to their lower Young’s modulus.

Fig. 10.

Young’s modulus of typical inclusions: (a) Al₂O₃–SiO₂–CaO;⁸⁰⁾ (b) Al₂O₃–SiO₂–MnO;⁸⁰⁾ (c) Al₂O₃–MgO–CaO. (Online version in color.)

The hardness was also an important parameter of materials reflecting the deformation resistance during plastic deformation.⁸³⁾ The inclusion with a high hardness deformed little during the rolling process. However, the hardness of complex inclusions can hardly be calculated due to complicated hardness models.^{84,85,86,87,88,89)} The reported Vickers hardness⁹¹⁾ and the liquidus of inclusions are summarized in Fig. 11, as reported by Ladutkin,⁹⁰⁾ Tian,⁸⁴⁾ Hillig,⁹²⁾ Gao,⁸⁵⁾ Dai.⁸⁹⁾ The Vickers hardness increased with a higher liquidus of inclusions. The Vickers hardness of complex inclusions was lower than that of simple inclusions due to their lower melting temperature.

Fig. 11.

Relationship between Vickers hardness and melting temperature of inclusions. (Online version in color.)

The viscosity and Young’s modulus of inclusions were important parameters of the inclusion deformability, which were obviously influenced by the crystallization of inclusions during cooling and heating processes. Generally, the hardness of an oxide crystal was higher than its glass.⁶⁸⁾ Thus, it was suggested to lower the crystallization ability of inclusions to improve the deformability of inclusions during the cold rolling process. The activation energy of crystallization reflected the required energy for the crystallization of slag. Inclusions with a lower activation energy of crystallization showed a better crystallization ability. Crystallization activation energies of typical Al₂O₃–MgO–CaO⁹²⁾ and Al₂O₃–SiO₂–CaO inclusions⁹³⁾ were reported by Li et al. and Sugimura et al., respectively. The liquidus of corresponding inclusions was calculated using FactSage 7.0 software. The relationship between the crystallization activation energy and the liquidus of inclusions was shown in Fig. 12. The crystallization activation energy of inclusions reduced with a higher liquidus of inclusions, which indicated that inclusions with a higher melting temperature exhibited the crystallization ability of inclusions. It was reported that the addition of Li₂O or Na₂O in CaO–Al₂O₃–MgO slag can significantly lower the crystallization ability of inclusions.

Fig. 12.

Relationship between the crystallization activation energy and liquidus of inclusions. (Online version in color.)

Inclusions in steel become narrower in cross section during rolling process. Poisson’s ratio was the ratio of transverse contraction strain to longitudinal extension strain in the direction of stretching force, which was related to the change of inclusion shape during rolling. Inclusions with a larger Poisson’s ratio exhibited a better deformability during rolling. Poisson’s ratio was a necessary parameter in the mathematical simulation of inclusion deformation during the rolling process. The reported poisson’s ratio⁹⁴⁾ of typical inclusions was summarized in Fig. 13.

Fig. 13.

Poisson’s ratio of typical inclusions. (Online version in color.)

4. Deformation of Inclusions in Steel during Hot Rolling Process

MnS inclusion was considered as a highly deformable inclusion during rolling process. Figure 14⁹⁵⁾ shows the morphology of typical MnS inclusions during hot rolling process. The spherical Type I MnS inclusions in the free-cutting steel deformed less than the matrix during hot rolling process, while the irregular type III MnS were highly deformable.

Fig. 14.

Morphology of MnS inclusions during the hot rolling process: (a) Type I MnS; (b) Type III MnS.⁹⁵⁾

Li et al. observed the three-dimensional morphology of MnS inclusions in high sulfur steel using Micro-CT, as shown in Fig. 15.⁹⁶⁾ The morphology, size, number, and distribution of MnS inclusions were characterized, illustrating the deformation behavior of MnS inclusions during rolling of the steel samples. MnS inclusions in steel before rolling were spherical and spindle, while they were elongated with a rolling reduction of 80% at 1150°C.

Fig. 15.

Three-dimensional morphology and distribution of MnS inclusions: (a) As cast sample; (b) Rolled sample.⁹⁶⁾ (Online version in color.)

The deformation of MnS in a high sulfur steel during rolling process at 1050°C was simulated using Gleeble thermomechanical simulator, as shown in Fig. 16.⁹⁷⁾ With the increase of the reduction ratio from 10% to 70%, the average aspect ratio of inclusions increased from 1.20 to 2.39. In addition, the deformability of MnS inclusions was lower than that of the matrix. The stress concentration around inclusions were characterized by the kernel average misorientation using electron backscattering diffraction. The stress mainly concentrated at the interface of the inclusion and steel matrix.

Fig. 16.

Image quality (a, c, e) and kernel average misorientation (b, d, f) maps of MnS at 1050°C for 180 s with various reduction ratio.⁹⁷⁾ (Online version in color.)

Figure 17 shows the deformability of MnS inclusions during hot rolling process. The data of MnS deformation reported by Baker⁹⁵⁾ and Luo⁴⁹⁾ was compared, which showed a similar tendency. With the increase of rolling reduction of steel, the deformability index of MnS inclusions shows a decreasing tendency. The deformability index of MnS inclusions increased with a decrease of the rolling temperature until reaching the austenite to ferrite matrix transformation temperature. Segal⁴⁷⁾ et al investigated the effect of inclusion size on the deformability of MnS inclusions during hot rolling process in Fig. 18. Smaller MnS inclusions in steel deformed less than larger ones under the same deformation conditions. It was proposed that the inclusion/matrix interfacial energy played an important role in determining the deformation index of inclusions with various sizes.

Fig. 17.

The deformability of MnS inclusions during the hot rolling process. (Online version in color.)

Fig. 18.

The effect of inclusion size on the deformability of MnS inclusions during the hot rolling process (Segal⁴⁷⁾). (Online version in color.)

Figure 19 shows the deformability index of Al₂O₃–SiO₂–MnO inclusions during hot rolling process. The rolling temperature was 1000°C–1250°C and the rolling reduction was roughly 80%. It was found that the deformability index of inclusions increased from 0.25 to 1.07 with the inclusion evolution in the order of MnO·Al₂O₃→3Al₂O₃·2SiO₂→MnO·SiO₂→2MnO·SiO₂→2MnO·SiO₂→3MnO·Al₂O₃·3SiO₂. The 3MnO·Al₂O₃·3SiO₂ inclusions with the lowest melting temperature exhibited the highest deformability index of Al₂O₃–SiO₂–MnO inclusions during hot rolling process. As shown in Fig. 20, 3MnO·Al₂O₃·3SiO₂ inclusions with the lowest melting temperature highly deformed during the hot rolling process, while inclusions with high melting temperature deformed less.

Fig. 19.

The relationship between the deformability index and the solidus of Al₂O₃–SiO₂–MnO inclusions during the hot rolling process: Iwai.⁹⁸⁾

Fig. 20.

The morphology of Al₂O₃–SiO₂–MnO inclusions after the hot rolling process.⁹⁸⁾

Figure 21 shows the deformability index of Al₂O₃–SiO₂–CaO inclusions during hot rolling process. With the increase of the rolling temperature, the deformability index of inclusions showed an increasing tendency due to higher liquid fraction of inclusions. The deformability index of inclusions slightly decreased at 1100°C. Figure 22 shows the deformability of simple oxide inclusions during the hot rolling process, including SiO₂, TiO₂, Al₂O₃, SiO₂, SiO₂–Al₂O₃. The rolling temperature was 1100°C–1200°C, respectively. The rolling reduction was roughly 95%.

Fig. 21.

The deformability index of Al₂O₃–SiO₂–CaO-riched inclusions during the hot rolling process: Al₂O₃–CaO–SiO₂₋Hitosugi.⁸⁾ (Online version in color.)

Fig. 22.

The deformability of simple oxide inclusions during the hot rolling process at 800–1000°C: Malkiewicz,⁴⁴⁾ Iwai.⁹⁹⁾

During hot rolling process, the deformation of inclusions was significantly influenced by the viscosity of inclusions and the steel matrix. Ren³³⁾ and Kurosaki³⁴⁾ investigated the influence of the viscosity of inclusions on the deformability of inclusions in the non-oriented electrical steel. The reduction of the hot rolling process was 98%. Figure 23 shows the relationship between the deformability index and the viscosity of Al₂O₃–SiO₂–CaO–MnO inclusions. During hot rolling process, the steel matrix viscosity was 1.2 × 10⁷ Pa·s and the viscosity of inclusions at 1000°C was calculated using FactSage7.0. It was found that the deformability index of inclusions obviously increased when the viscosity of inclusions was larger than the steel matrix.

Fig. 23.

The relationship between the deformability index with the reduction of 98% and the viscosity of Al₂O₃–SiO₂–CaO–MnO inclusions at 1000°C. (Online version in color.)

During rolling process, there were voids and self-welding lines formed around hard inclusions since the deformability of the steel matrix was higher than hard inclusions, while it was hardly detected around soft inclusions. Figure 24 shows the effect of the rolling reduction on voids and self-welding lines around hard Al₂O₃–SiO₂–MnO inclusions.¹⁰⁰⁾ With the increase of the rolling reduction from 0% to 40%, the horizontal width of voids between inclusions and the matrix extended. Meanwhile, it was noted that the formed volume of the voids first increased due to the higher rolling reduction, then it decreased due to the formation of self-welding lines. The effect of the rolling reduction on the rotation of elliptic MnS inclusions is shown in Fig. 25.⁴⁹⁾ The elliptical 45° inclusion was rolled to an elliptical 15° one with the increase of the rolling reduction from 0% to 40%. Meanwhile, the elongation of inclusions increased with the higher rolling reduction.

Fig. 24.

The effect of the rolling reduction on voids and self-welding lines around hard Al₂O₃–SiO₂–MnO inclusions.¹⁰⁰⁾

Fig. 25.

The effect of the rolling reduction on rotation of elliptic 45° MnS inclusions.⁴⁹⁾

5. Deformation of Inclusions in Steel during Cold Rolling Process

Gove⁴⁾ investigated the deformability of MnS inclusions during cold rolling process with various reductions and temperatures. Figure 26 shows the deformability of MnS inclusions during cold rolling process. With the increase of the rolling reduction of steel, the deformability index of MnS inclusions shows a decreasing tendency. A higher rolling temperature was beneficial to improve the deformability of inclusions.

Fig. 26.

The deformability of MnS inclusions during the hot rolling process. (Online version in color.)

Figure 27 shows the deformability index of Al₂O₃–SiO₂–MnO inclusions⁹⁸⁾ during cold rolling process. The deformability index of Al₂O₃–SiO₂–MnO inclusions after hot rolling is given in Fig. 19. The rolling reduction was roughly 50%. From hot rolling to cold rolling, the deformability index of MnO·SiO₂ changed from 0.71 to 2.40, the deformability index of 2MnO·SiO₂ changed from 0.79 to 2.14, the deformability index of 3MnO·Al₂O₃·3SiO₂ changed from 1.07 to 1.81, the deformability index of 3Al₂O₃·2SiO₂ changed from 0.51 to 1.56, the deformability index of MnO·Al₂O₃ changed from 0.26 to1.51. It was found that the deformability index of inclusions decreased from 2.41 to 1.51 with the inclusion evolution in the order of MnO·SiO₂→2MnO·SiO₂→3MnO·Al₂O₃·3SiO₂→3Al₂O₃·2SiO₂→MnO·Al₂O₃. The MnO·SiO₂ inclusions with the lowest Young’s modulus exhibited the highest deformability index of Al₂O₃–SiO₂–MnO inclusions during cold rolling process. The morphology of Al₂O₃–SiO₂–MnO inclusions after hot rolling and cold rolling process are compared in Figs. 20 and 28.

Fig. 27.

The deformability index of Al₂O₃–SiO₂–MnO inclusions during the cold rolling process.

Fig. 28.

The morphology of Al₂O₃–SiO₂–MnO inclusions after the cold rolling process.⁹⁸⁾

Yu et al.⁶⁰⁾ simulated the deformation of inclusions during the rolling process of strip with the thickness from 3.0 mm to 2.0 mm. The size of spherical inclusions was set as 10, 20, 30, and 40 μm separately. The initial position of inclusions was 1/2, 3/8, 1/4, and 1/8 of strip thickness. Figure 29 shows the effect of the size and position of inclusions on the deformation of inclusions during cold rolling. The void formed around hard inclusions along the rolling direction, while it was rarely detected for soft inclusions. In the actual production process, inclusions in steel have been already aligned during hot rolling. In the simulation, the initial shape of the inclusions was set as spherical to reveal the deformation behavior during the cold rolling process. Inclusions with various sizes rotated with the same angle after the rolling, indicating that the rotation of inclusion was little influenced by the inclusion size. The rotation of inclusions was obviously influenced by the position in the steel strip. The void size decreased with increase of the distance between the inclusion and the strip surface. The inclusion at the center of the strip thickness rotated less.

Fig. 29.

Effect of the size and the position of inclusions on the deformation of inclusions during the cold rolling:⁶⁰⁾ (a). Size; (b). Position.

6. Opportunities and Challenges in Deformation of Inclusions in Steel

The deformation of inclusions in steel during the rolling process had a significantly influence on the steel performance. It was meaningful to reveal behaviors of inclusions during the rolling process, optimizing the control target of inclusions in steel. Although many works had been conducted to investigate the evolution of inclusions during the rolling process, following directions would be the research focus in the area of inclusion deformation in the future. (1) The deformation of the complex inclusions in steel in various types of steels. Inclusions formed in various types of steel were different, exhibiting various variations of inclusions after the rolling process. More experimental works are still necessary to better design and control of inclusion behaviors in various types of steels. (2) Numerical modeling of the deformation and broken of inclusions in steel. With the development of the numerical simulation computing techniques, the simulation of the inclusion deformation during the rolling process will be a more effective and accurate method in the future. Inclusions in steel are deformed or broken during the rolling process. However, current developed models mainly focused on the deformation of plastic inclusions, while the broken of hard inclusions in steel during the rolling process was rarely considered. Besides, complex inclusions were generated in steel during deoxidation, refining, and continuous casting processes, while the evolution of complex inclusions was rarely simulated accurately using available models. (3) The database of physical properties of complex inclusions at various temperatures. The deformation ability of inclusions was mainly related to physical properties of inclusions, including the melting temperature, viscosity, Young’s modulus, hardness, thermal expansion coefficient, crystallization, and Poisson’s ratio. However, physical properties of inclusions at the various temperatures were insufficient, especially for the complex inclusions. More empirical formula, theoretical calculation, and even machine learning models are needed to evaluate deformation and broken behaviors of inclusions during rolling in the future works.

(4) The quantitative relationship between the deformability of inclusions and rolling parameters. Evolution behaviors were not only influenced by physical properties of inclusions, but also the rolling parameters. Previous works were mainly conducted through the industrial trials, which can only adjust limited rolling parameters. Thus, more systematic laboratorial experiments are necessary to be carried out to investigate the effect of temperature, rolling reduction, and rolling passes on the characters of inclusions during rolling.

7. Conclusions

(1) Physical properties of inclusions, such as melting temperature, viscosity, Young’s modulus, hardness, thermal expansion coefficient, crystallization, and Poisson’s ratio were summarized and calculated.

(2) During hot rolling process, the deformability index of inclusions showed a decreasing tendency with a higher rolling reduction. The deformability index of MnS inclusions increased with a decrease of the rolling temperature until reaching the austenite to ferrite matrix transformation temperature. The deformability index of oxide inclusions during hot rolling process was mainly related to the melting temperature and the viscosity of inclusions. With the increase of the rolling temperature, the deformability index of oxide inclusions showed an increasing tendency due to higher liquid fraction of inclusions.

(3) During the cold rolling process, inclusions with the lowest Young’s modulus and hardness exhibited the highest deformability index of oxide inclusions during the cold rolling process. With the increase of the rolling reduction of steel, the deformability index of inclusions showed a decreasing tendency.

(4) In the future, the quantitative relationship between the deformability of inclusions and rolling parameters will be a research focus. Meanwhile, the deformation of complex inclusions still needs further study. Moreover, it is necessary to develop better mathematical models to reveal the deformation mechanism of inclusions during the rolling process.

Acknowledgements

The authors are grateful for support from the National Nature Science Foundation of China (Grant No. U1860206, No. 51725402, No. 51874032), S&T Program of Hebei (Grant 20311006D), the High Steel Center (HSC) at North China University of Technology, Yanshan University and University of Science and Technology Beijing, China.

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