COMPOSITE ADDITIVE FOR FORMING INCLUSIONS WITH CORE-SHELL STRUCTURE, PREPARATION METHOD AND SMELTING METHOD

Abstract

A composite additive for forming inclusions with a core-shell structure, a preparation method and a smelting method are provided. The composite additive includes the following chemical components by mass percentage: Fe: 41-59%, Zr: 5-11%, Ti: 14-26%, Mg: 11-19%, and RE: 4-10%, in which the mass percentage content of Zr element, Ti element, Mg element and RE element satisfies a formula: (Ti+Mg+RE)/Zr=4-8. The composite additive for forming inclusions with a core-shell structure has the characteristics of fineness, spheroidization and obvious dispersion effect. This type of inclusions has a bulk modulus similar to that of the iron matrix, which can significantly improve the plasticity, toughness, fatigue resistance, local corrosion resistance, welding performance, cold bending performance, etc. of steel materials, suitable for steel types with strict requirements on the shape of inclusions, such as marine steel, pipeline steel, container steel, cold heading steel, tool and die steel, etc.

Claims

1. A composite additive for forming inclusions with a core-shell structure, comprising the following chemical components by a mass percentage: Fe: 41-59%, Zr: 5-11%, Ti: 14-26%, Mg: 11-19%, and rare earth (RE): 4-10%, wherein the mass percentage of the Zr, the Ti, the Mg and the RE satisfies a formula: (Ti+Mg+RE)/Zr=4-8.

2. The composite additive for forming the inclusions with the core-shell structure according to claim 1, comprising the following chemical components by the mass percentage: the Fe: 46-57%, the Zr: 6-11%, the Ti: 15-16%, the Mg: 12-19%, and the RE: 9-10%.

3. The composite additive for forming the inclusions with the core-shell structure according to claim 1, comprising the following chemical components by the mass percentage: the Fe: 50%, the Zr: 8%, the Ti: 20%, the Mg: 15%, and the RE: 7%.

4. The composite additive for forming the inclusions with the core-shell structure according to claim 1, wherein the RE comprises an La element and a Ce element, and a mass ratio of the La element to the Ce element is (70-90):(10-30).

5. The composite additive for forming the inclusions with the core-shell structure according to claim 1, wherein the Zr is sponge metal zirconium and/or metal zirconium; and the Mg is one or a combination of at least two of a metal magnesium block, magnesium grains, magnesium-zirconium alloy blocks, and magnesium-zirconium alloy grains.

6. A method for preparing the composite additive for forming the inclusions with the core-shell structure according to claim 1, comprising the following steps: first adding the Zr, the Ti, and the RE to an FeTi alloy to obtain a resulting product, and then smelting the resulting product in a vacuum induction furnace to obtain a smelted product, and then casting the smelted product under vacuum conditions to obtain the composite additive.

7. The method for preparing the composite additive for forming the inclusions with the core-shell structure according to claim 6, wherein a time for smelting the resulting product in the vacuum induction furnace is 4-8 hours.

8. A smelting method, comprising steps of: (1) after steelmaking a molten iron and/or a scrap steel using a converter or an electric arc furnace to obtain a resulting steel, adjusting a temperature and a composition of the resulting steel to obtain a molten steel; (2) making the molten steel enter a ladle, performing a pre-deoxidation on the molten steel first to obtain a pre-deoxidated molten steel, and then performing a final deoxidation on the pre-deoxidated molten steel using the composite additive according to claim 1 to obtain a final deoxidized molten steel; and (3) refining and continuously casting the final deoxidized molten steel in sequence.

9. The smelting method according to claim 8, wherein a temperature of the molten steel in step (1) is 1551-1690 C., and a free oxygen content in the molten steel is 101-399 ppm; step (2) comprises: making the molten steel enter the ladle, firstly pre-deoxidizing the molten steel in the ladle using an FeSi alloy or an FeSiMn alloy under micro-subbubble stirring, adjusting a free oxygen content in the pre-deoxidated molten steel to 11-99 ppm, and then under the micro-subbubble stirring, performing the final deoxidation on the pre-deoxidated molten steel using the composite additive to obtain the final deoxidized molten steel; and in step (3), the final deoxidized molten steel is firstly subjected to a ladle furnace (LF) refining, a vacuum degassing (VD) refining, or an Ruhrstahl-Heraeus (RH) refining, and then continuously cast.

10. The smelting method according to claim 8, wherein in step (2), an amount of the composite additive added per ton of the pre-deoxidated molten steel is 0.51-4.9 kg.

11. The composite additive for forming the inclusions with the core-shell structure according to claim 2, wherein the RE comprises an La element and a Ce element, and a mass ratio of the La element to the Ce element is (70-90):(10-30).

12. The composite additive for forming the inclusions with the core-shell structure according to claim 3, wherein the RE comprises an La element and a Ce element, and a mass ratio of the La element to the Ce element is (70-90):(10-30).

13. The composite additive for forming the inclusions with the core-shell structure according to claim 2, wherein the Zr is sponge metal zirconium and/or metal zirconium; and the Mg is one or a combination of at least two of a metal magnesium block, magnesium grains, magnesium-zirconium alloy blocks, and magnesium-zirconium alloy grains.

14. The composite additive for forming the inclusions with the core-shell structure according to claim 3, wherein the Zr is sponge metal zirconium and/or metal zirconium; and the Mg is one or a combination of at least two of a metal magnesium block, magnesium grains, magnesium-zirconium alloy blocks, and magnesium-zirconium alloy grains.

15. The method for preparing the composite additive for forming the inclusions with the core-shell structure according to claim 6, wherein the composite additive comprises the following chemical components by the mass percentage: the Fe: 46-57%, the Zr: 6-11%, the Ti: 15-16%, the Mg: 12-19%, and the RE: 9-10%.

16. The method for preparing the composite additive for forming the inclusions with the core-shell structure according to claim 6, wherein the composite additive comprises the following chemical components by the mass percentage: the Fe: 50%, the Zr: 8%, the Ti: 20%, the Mg: 15%, and the RE: 7%.

17. The method for preparing the composite additive for forming the inclusions with the core-shell structure according to claim 6, wherein in the composite additive, the RE comprises an La element and a Ce element, and a mass ratio of the La element to the Ce element is (70-90):(10-30).

18. The method for preparing the composite additive for forming the inclusions with the core-shell structure according to claim 6, wherein in the composite additive, the Zr is sponge metal zirconium and/or metal zirconium; and the Mg is one or a combination of at least two of a metal magnesium block, magnesium grains, magnesium-zirconium alloy blocks, and magnesium-zirconium alloy grains.

19. The smelting method according to claim 8, wherein the composite additive comprises the following chemical components by the mass percentage: the Fe: 46-57%, the Zr: 6-11%, the Ti: 15-16%, the Mg: 12-19%, and the RE: 9-10%.

20. The smelting method according to claim 8, wherein the composite additive comprises the following chemical components by the mass percentage: the Fe: 50%, the Zr: 8%, the Ti: 20%, the Mg: 15%, and the RE: 7%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIG. 1 shows morphology of inclusions in low-alloy high-strength steel obtained after non-aqueous solution electrolytic extraction according to the present invention;

[0040] FIG. 2 shows element distribution of the inclusions in the low-alloy high-strength steel obtained after non-aqueous solution electrolytic extraction according to the present invention;

[0041] FIGS. 3A-3B show SEM morphology and EDS analysis of inclusions in conventional Al deoxidized steel low-alloy high-strength steel after electrolytic extraction according to the present invention;

[0042] FIG. 4 shows precipitation curves of particles such as sulfide and nitride in a solidification process according to the present invention;

[0043] FIGS. 5A-5C show the density of states of different oxides, in which FIG. 5A represents the density of state of Ti.sub.2O.sub.3, FIG. 5B represents the density of state of ZrO.sub.2, and FIG. 5C represents the density of state of Al.sub.2O.sub.3, and the dotted line is Fermi surface, according to the present invention;

[0044] FIGS. 6A-6B show KAM test results after complex deoxidation of rare earth La, in which FIG. 6A represents an IQ map, and FIG. 6B represents a KAM map, according to the present invention; and

[0045] FIGS. 7A-7C show Young's moduli of Fe matrix, Al.sub.2O.sub.3, and AlLaO.sub.3 inclusions, in which FIG. 7A represents Fe matrix, FIG. 7B represents Al.sub.2O.sub.3, and FIG. 7C represents AlLaO.sub.3, according to the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0046] The principles and features of the present invention are described below, and the examples given are only used to explain the present invention, and are not intended to limit the scope of the present invention. If no specific technique or condition is indicated in the examples, it shall be carried out according to the technique or condition described in the literature in the art, or according to the product specification. The reagents or instruments used were not indicated by the manufacturer, and they were all conventional products that can be purchased through formal channels.

Example 1

[0047] This example relates to a composite additive for forming inclusions with a core-shell structure, which includes the following chemical components by mass percentage: Fe of 50 wt %. Zr of 8 wt %, Ti of 20 wt %, Mg of 15 wt %, and RE of 7 wt %, with the rest being unavoidable impurities. The mass percent content of Zr element, Ti element, Mg element and RE element satisfies a formula: (Mg+Ti+RE)/Zr=5.2. The RE includes La element and Ce element. The mass ratio of the La element to the Ce element is 80:20.

[0048] This example relates to a method for preparing a composite additive for forming inclusions with a core-shell structure, which includes the following steps: based on FeTi alloy, adding Zr, Ti and RE, wherein the RE includes lanthanum La and cerium Ce, and La accounts for 80%, and Ce accounts for 20%, smelting in a vacuum induction furnace for 6 hours, and casting under vacuum conditions. The Zr used in the composite additive is sponge metal Zr or metal Zr. The RE used in the composite additive is mixed rare earth, including lanthanum La and cerium Ce. La accounts for 80%, and Ce accounts for 20%. The Mg used in the composite additive is metal magnesium block, magnesium grain, MgZr alloy block, or MgZr alloy grain. This example relates to a smelting method, which includes the following steps: [0049] (1) after steelmaking molten iron, or scrap steel, or molten iron and scrap steel using a converter (BOF) or electric are furnace (EAF), adjusting the temperature and composition of the molten steel, in which the tapping temperature is adjusted to 1620 C., and the free oxygen content in the molten steel is 250 ppm; [0050] (2) stirring the molten steel for 6 minutes with fine agron bubbling after entering a ladle, then performing pre-deoxidation using FeSi alloy or FeSiMn alloy in the ladle, adjusting the free oxygen content in the molten steel to 55 ppm, and after micro-subbubble stirring for 4 minutes, performing final deoxidation with a composite additive, in which the composite additive is added into the molten steel in the form of bulk alloys or cored wires, the particle size of the composite additive is 11 mm, and the amount of the composite additive added is 2.7 kg per ton of molten steel; and obtaining molten steel after final deoxidation; and [0051] (3) then performing LF refining, VD refining, or RH refining on the obtained molten steel after final deoxidation according to a conventional process; and finally, continuously casting the refined molten steel according to a conventional process.

Example 2

[0052] This example relates to a composite additive for forming inclusions with a core-shell structure, which includes the following chemical components by mass percentage: Fe of 46 wt %. Zr of 11 wt %, Ti of 15 wt %, Mg of 19 wt %, and RE of 10 wt %, with the rest being unavoidable impurities. The mass percent content of Zr element, Ti element, Mg element and RE element satisfies a formula: (Mg+Ti+RE)/Zr=4. The RE includes La element and Ce element. The mass ratio of the La element to the Ce element is 90:10.

[0053] This example relates to a method for preparing a composite additive for forming inclusions with a core-shell structure, which includes the following steps: based on FeTi alloy, adding Zr, Ti and RE, in which the RE includes lanthanum La and cerium Ce, La accounts for 90%, and Ce accounts for 10%, smelting in a vacuum induction furnace for 4 hours, and casting under vacuum conditions. The Zr used in the composite additive is sponge metal Zr or metal Zr. The RE used in the composite additive is mixed rare earth, including lanthanum La and cerium Ce. La accounts for 90%, and Ce accounts for 10%. The Mg used in the composite additive is metal magnesium block, magnesium grain, MgZr alloy block, or MgZr alloy grain.

[0054] This example relates to a smelting method, which includes the following steps: [0055] (1) after steelmaking molten iron, or scrap steel, or molten iron and scrap steel using a converter or electric arc furnace, adjusting the temperature and composition of the molten steel, in which the tapping temperature is adjusted to 1560 C., and the free oxygen content in the molten steel is 150 ppm; [0056] (2) stirring the molten steel for 5 minutes with fine agron bubbling after entering a ladle, then performing pre-deoxidation using FeSi alloy or FeSiMn alloy in the ladle, adjusting the free oxygen content in the molten steel to 20 ppm, and after micro-subbubble stirring for 3 minutes, performing final deoxidation with a composite additive, in which the composite additive is added into the molten steel in the form of bulk alloys or cored wires, the particle size of the composite additive is 3-19 mm, and the amount of the composite additive added is 0.70 kg per ton of molten steel; and obtaining molten steel after final deoxidation; and [0057] (3) then performing LF refining. VD refining, or RH refining on the obtained molten steel after final deoxidation according to a conventional process; and finally, continuously casting the refined molten steel according to a conventional process.

Example 3

[0058] This example relates to a composite additive for forming inclusions with a core-shell structure, which includes the following chemical components by mass percentage: Fe of 57 wt %, Zr of 6 wt %, Ti of 16 wt %, Mg of 12 wt %, and RE of 9 wt %, with the rest being unavoidable impurities. The mass percent content of Zr element, Ti element, Mg element and RE element satisfies a formula: (Mg+Ti+RE)/Zr=6.2. The RE includes La element and Ce element. The mass ratio of the La element to the Ce element is 70:30.

[0059] This example relates to a method for preparing a composite additive for forming inclusions with a core-shell structure, which includes the following steps: based on FeTi alloy, adding Zr, Ti and RE, in which the RE includes lanthanum La and cerium Ce, La accounts for 70%, and Ce accounts for 30%, smelting in a vacuum induction furnace for 7 hours, and casting under vacuum conditions. The Zr used in the composite additive is sponge metal Zr or metal Zr. The RE used in the composite additive is mixed rare earth, including lanthanum La and cerium Ce. La accounts for 70%, and Ce accounts for 30%. The Mg used in the composite additive is metal magnesium block, magnesium grain, MgZr alloy block, or MgZr alloy grain.

[0060] This example relates to a smelting method, which includes the following steps: [0061] (1) after steelmaking molten iron, or scrap steel, or molten iron and scrap steel using a converter or electric arc furnace, adjusting the temperature and composition of the molten steel, in which the tapping temperature is adjusted to 1670 C., and the free oxygen content in the molten steel is 350 ppm; [0062] (2) stirring the molten steel for 8 minutes with fine agron bubbling after entering a ladle, then performing pre-deoxidation using FeSi alloy or FeSiMn alloy in the ladle, adjusting the free oxygen content in the molten steel to 80 ppm, and after micro-subbubble stirring for 5 minutes, performing final deoxidation with a composite additive, in which the composite additive is added into the molten steel in the form of bulk alloys or cored wires, the particle size of the composite additive is 16 mm, and the amount of the composite additive added is 3.9 kg per ton of molten steel; and obtaining molten steel after final deoxidation; and [0063] (3) then performing LF refining, VD refining, or RH refining on the obtained molten steel after final deoxidation according to a conventional process; and finally, continuously casting the refined molten steel according to a conventional process.

Experimental Example

[0064] Below in conjunction with the low-alloy high-strength steel prepared in Example 1, the main features of its inclusions are observed, analyzed and tested, and the results are as follows:

(1) Features of Core-Shell Structure of Inclusions

[0065] FIGS. 1 and 2 show a morphology diagram and an element distribution diagram of inclusions in low-alloy high-strength steel obtained after non-aqueous solution electrolytic extraction. It can be seen from FIGS. 1 and 2 that the morphology of the inclusions is spherical. The type of inclusions extracted by non-aqueous solution electrolysis is complex oxysulfide, in which the core of the inclusions is Zr, Ti, Mg, RE complex oxide, and the surface of the spherical particles is sulfide (MnS) and carbide (TIN).

[0066] The types and sizes of inclusions in low-alloy high-strength steel added with different amounts of composite additives were compared by scanning electron microscopy. In conventional Al-deoxidized steel, the oxide inclusions are clustered (reference: Deng Z, Zhu M. Evolution Mechanism of Non-metallic Inclusions in Al-Killed Alloyed Steel during Secondary Refining Process [J].Isij International, 2013, 53(3):450-458.), the sizes of inclusions are mainly concentrated in 2-5 m, and the sulfide is in the form of strips, usually 5-20 m in length; and after complex deoxidation treatment, the sizes of inclusions are obviously refined, mainly concentrated in 1-3 m (as shown in FIGS. 3A-3B).

(2) Complex Deoxidation Thermodynamics

[0067] In the above experimental example (1), the inclusions were observed and analyzed. The inclusions are core-shell structures with sulfur oxides as the core and nitrides and sulfides attached to the periphery. This part is about the formation of complex inclusions and evolution for thermodynamic analysis. Considering the diversity of alloying elements in steel, a series of physical and chemical reactions will occur in the process of steelmaking and molten steel solidification to form non-metallic inclusions such as oxides, sulfides and nitrides. The Gibbs free energy change is the criteria to determine whether the reaction can proceed spontaneously under constant temperature and pressure, and G<0 indicates that the reaction can occur spontaneously. The deoxidation reaction equation and thermodynamic formula involved in adding alloying elements into molten steel are shown in formulas (1) and (2).

[00001]$\begin{array}{cc}\frac{X}{y}\left[M\right]+\left[O\right]=\frac{1}{y}\left(M_x{O}_y\right)& \left(1\right)\end{array}$$\begin{array}{cc}G=G^{}+\mathrm{RT}\ln K=G^{}+\mathrm{RT}\ln \frac{a_{M_x{O}_y}^{1/y}}{a_{\left[O\right]}{a}_{\left[M\right]}^{x/y}}& \left(2\right)\end{array}$

[0068] In the formula, G and G.sup. represent the Gibbs free energy and standard Gibbs free energy of the reaction (J/mol), respectively. R is the gas constant, (J/(mol.Math.K)), T is the temperature, (K), and a.sub.i represents the activity of the element. Table 1 gives the corresponding Gibbs free energy data of inclusion formation that may be involved in low-alloy high-strength steel, and the molten steel temperature is set to 1873 K. As shown in Table 1, according to the Gibbs free energy value, the formation order of pure metal oxides is Al.sub.2O.sub.3 (734.33 KJ/mol). La.sub.2O.sub.3 (610.85 KJ/mol), Ti.sub.2O.sub.3 (273.72 KJ/mol), ZrO.sub.2 (156.09 KJ/mol), and CaO (96.36 KJ/mol). In addition, Zr and RE can completely or partially replace Al elements in Al.sub.2O.sub.3 in molten steel to form ZrO.sub.2 (7298.12 KJ/mol) and LaAlO.sub.3 (646.15 KJ/mol). Ca element can react with molten aluminum and oxygen to form calcium aluminate (9458.40 KJ/mol). In traditional aluminum deoxidation techniques used in steelmaking, Al.sub.2O.sub.3 dominates the inclusions. At the same time, refractory bricks are easy to react with Al.sub.2O.sub.3 to form Al.sub.2O.sub.3.Math.MgO (similar to spinel).

TABLE-US-00001 TABLE 1 Gibbs free energy of inclusion formation reaction at 1873 K No. Chemical reaction G.sup. (J/mol) G (KJ/mol) 1 2[Al] + 3[O] = (Al.sub.2O.sub.3) 1682927 + 323.240T 734.33 2 [Mg] + 2[Al] + 4[O] = (MgOAl.sub.2O.sub.3) 1848696 + 574.144T G.sup. = 773.32 3 [Zr] + 2[O] = (ZrO.sub.2) 845532 + 266.100T 156.09 4 2[Ti] + 3[O] = (Ti.sub.2O.sub.3) 1072872 + 346.000T 273.72 5 [Ca] + [O] = (CaO) 138227 63.000T 96.36 6 [Ca] + 6[Al] + 4[O] = [CaOAl.sub.2O.sub.3] 1023637 + 142.120T 9458.40 7 2[La] + 3[O] = (La.sub.2O.sub.3) 542531 + 124.150T 610.85 8 3[Zr] + 2(Al.sub.2O.sub.3) = 4[Al] + 3(ZrO.sub.2) 8233279 + 464.510T 7298.12 9 3[Zr] + 2(Ti.sub.2O.sub.3) = 4[Ti] + 3(ZrO.sub.2) 1073389 + 538.830T 11.99 10 [La] + 3[O] + [Al] = (LaAlO.sub.3) 801616 + 129.000T 646.15 11 [Ce] + (Al.sub.2O.sub.3) = (CeAlO.sub.3) + [Al] 423900 247.300T 833.13 12 [Ca] + [S] = (CaS) 542531 + 124.150T 126.03 13 [Ti] + [N] = (TiN) 307620 + 113.400T 65.09

[0069] Zr, Ti, Mg, RE (rare earth) elements are used for complex deoxidation, and complex sulfur oxides are easily formed in molten steel. Taking Zr, La, Ce alloy elements as examples, thermodynamic calculations were carried out (Formula 3-6). The results show that Zr can directly react with Ti.sub.2O.sub.3 and Al.sub.2O.sub.3 (Formula 3 and 4), and the complex addition of Zr and Ti deoxidizing elements will lead to a uniform distribution of Zr and Ti complex oxides in molten steel. This provides theoretical support for the addition of Zr to modify common inclusions in steel (such as Al.sub.2O.sub.3). Formula 5 and 6 show that under low oxygen concentration, RE elements can directly react with Al in molten steel to form (La, Ce)AlO.sub.3.

[00002]$\begin{array}{cc}\begin{array}{c}3\left[\mathrm{Zr}\right]+2\left(A{1}_2{O}_3\right)=4\left[A1\right]+3\left({\mathrm{ZrO}}_2\right)\\ G_1=G_1^{}+\mathrm{RT}\ln J_1=-8233279+464.51T+\mathrm{RT}\ln \frac{a_{\left({\mathrm{ZrO}}_2\right)}^3.Math.a_{\left[A1\right]}^4}{a_{\left(A{1}_2{O}_3\right)}^2.Math.a_{\left[\mathrm{Zr}\right]}^3}\end{array}& \left(3\right)\end{array}$$\begin{array}{cc}\begin{array}{c}3\left[\mathrm{Zr}\right]+2\left({\mathrm{Ti}}_2{O}_3\right)=4\left[\mathrm{Ti}\right]+3\left({\mathrm{ZrO}}_2\right)\\ G_2=G_2^{}+\mathrm{RT}\ln J_2=-1073389+538.83T+\mathrm{RT}\ln \frac{a_{\left({\mathrm{ZrO}}_2\right)}^3.Math.a_{\left[\mathrm{Ti}\right]}^4}{a_{\left({\mathrm{Ti}}_2{O}_3\right)}^2.Math.a_{\left[\mathrm{Zr}\right]}^3}\end{array}& \left(4\right)\end{array}$$\begin{array}{cc}\begin{array}{c}\left[\mathrm{La}\right]+3\left[O_3\right]+\left[A1\right]=\mathrm{LaA}1O_3\left(s\right)\\ G_3=G_3^{}+\mathrm{RT}\ln J_3=-801616+129T+\mathrm{RT}\ln \frac{a_{\left(\mathrm{LaA}1O_3\right)}}{a_{\left(\mathrm{La}\right)}.Math.a_{\left[O\right]}^3.Math.a_{\left[A1\right]}}\end{array}& \left(5\right)\end{array}$$\begin{array}{cc}\begin{array}{c}\left[\mathrm{Ce}\right]+\left(A{1}_2{O}_3\right)=\mathrm{CeA}1O_3\left(s\right)+\left[A1\right]\\ G_4=G_4^{}+\mathrm{RT}\ln J_3=-423900-247.3T+\mathrm{RT}\ln \frac{a_{\left(\mathrm{CeA}1O_3\right)}.Math.a_{\left[A1\right]}}{a_{\left(A{1}_2{O}_3\right)}.Math.a_{\left[\mathrm{Ce}\right]}}\end{array}& \left(6\right)\end{array}$

[0070] G and G.sup., (J/mol) represent Gibbs free energy and standard Gibbs free energy, R is a constant, (J/(mol.Math.k)), T represents temperature, (K), a.sub.i represents the activity of element I, and J.sub.1, J.sub.2, J.sub.3, and J.sub.4 represent

[00003]$\frac{a_{\left({\mathrm{ZrO}}_2\right)}^3.Math.a_{\left[A1\right]}^4}{a_{\left(A{1}_2{O}_3\right)}^2.Math.a_{\left[\mathrm{Zr}\right]}^3},\frac{a_{\left({\mathrm{ZrO}}_2\right)}^3.Math.a_{\left[\mathrm{Ti}\right]}^4}{a_{\left({\mathrm{Ti}}_2{O}_3\right)}^2.Math.a_{\left[\mathrm{Zr}\right]}^3},\frac{a_{\left(\mathrm{LaA}1O_3\right)}}{a_{\left(\mathrm{La}\right)}.Math.a_{\left[O\right]}^3.Math.a_{\left[A1\right]}},\mathrm{and}\frac{a_{\left(\mathrm{CeA}1O_3\right)}.Math.a_{\left[A1\right]}}{a_{\left(A{1}_2{O}_3\right)}.Math.a_{\left[\mathrm{Ce}\right]}},$

respectively. In the example low-alloy high-strength steel, the melting temperature is set to 1873 K, and the corresponding Gibbs free energies are 7298.12 kJ/(mol.Math.K), 11.99 KJ/(mol.Math.K), 646.152 kJ/(mol.Math.K), and 833.13 KJ/(mol.Math.K), in which all Gibbs free energies are negative, indicating that all reactions can proceed spontaneously.

(3) Formation Mechanism of Complex Sulfur Oxides

[0071] Table 2 shows the solid solubility product formulas of carbides, nitrides, and sulfides. FIG. 4 shows precipitation curves of molten steel during solidification and cooling calculated using JMatPro thermodynamic software. It can be seen from FIG. 4 that nitrides start to precipitate and grow at around 1400 C., and sulfides start to precipitate and grow at around 1300 C. Carbonitrides start to precipitate at a low temperature of about 1100 C. In the later stage of the solidification process, CaS and TiN mainly precipitate on the surface of the pre-formed oxide inclusions. Therefore, after complex deoxidation of Zr, Ti, Mg, and RE (rare earth) elements, the molten steel is continuously cast or die-cast, and then rolled to obtain the complex inclusions with a core-shell structure as shown in FIG. 1.

TABLE-US-00002 TABLE 2 Solid solubility product formulas of carbides, nitrides, and sulfides No. Precipitation phase Solid solubility product formula 1 NbC lg{[Nb] .Math. [C]} = 2.26-6770/T 2 NbN lg{[Nb] .Math. [N]} = 2.80-8500/T 3 TiC lg{[Ti] .Math. [C]} = 2.75-7000/T 4 TiN lg{[Ti] .Math. [N]} = 0.32-8000/T 5 MnS lg{[Mn] .Math. [S]} = 5.02-11625/T

[0072] MnS and ZrO.sub.2 have very similar lattice constants, see Table 3 for specific data. Since MnS and ZrO.sub.2 have a good lattice matching relationship, this will reduce the interfacial energy between the two. Lower interfacial energy leads to better adhesion between grains at different interfaces. This further proves the reason why strips and strings of sulfides were not formed in the test samples. This is because MnS tends to precipitate on the pre-formed ZrO.sub.2 particles, and the sulfides are thus refined, spheroidized and dispersed. Thus, it is beneficial to improve the plasticity and toughness of Zr and Mg complex deoxidized ferritic stainless steel.

TABLE-US-00003 TABLE 3 Lattice constants of MnS and ZrO.sub.2 Crystal Planes Plane Category structure (hkl) distance int h k l a b c ZrO.sub.2 Monoclinic 002 2.621 99.23 20 0 0 2 5.145 5.207 5.311 022 1.847 14 0 2 2 113 1.509 4 1 1 3 MnS Fcc 111 2.612 90 100 1 1 1 5.224 5.224 5.224 220 1.847 50 2 2 0 222 1.509 20 2 2 2

(4) Fine Dispersion Mechanism of Complex Deoxidized Inclusions

[0073] Density of states (DOS) is a useful tool for analyzing the electronic structure of solids. The DOS of the oxide is shown in FIGS. 5A-5C. For Ti.sub.2O.sub.3, there is no gap at the Fermi surface, indicating that it is metallic. For ZrO.sub.2 and Al.sub.2O.sub.3, the interstitial energies at the Fermi surface are 3.5 eV and 6.3 eV, respectively. The larger the interstitial energy, the stronger the insulation of the material. At 1273 K, the electrical conductivities of Ti.sub.2O.sub.3, ZrO.sub.2 and Al.sub.2O.sub.3 are 10.sup.2 .sup.1m.sup.1, 10.sup.1 .sup.1m.sup.1 and 10.sup.2 .sup.1m.sup.1, respectively. The DOS data is consistent with the conductivity data in the literature (Zhang X, Qin R. Controlled motion of electrically neutral microparticles by pulsed direct current[J]. 2015, 5:10162.).

[0074] The conductivity of oxides is a key factor for their movement in molten steel. According to the existing research results, the driving force of Al.sub.2O.sub.3 movement in molten steel is greater than the driving force of ZrO.sub.2 movement. During refining and electrification, ZrO.sub.2 particles tend to repel each other and are difficult to agglomerate, while Al.sub.2O.sub.3 tends to collide with each other to form large particles, which float over the surface of molten steel and are absorbed by the surface covering agent of molten steel. Therefore, compared with conventional Al deoxidation, fine and dispersed complex oxides can be formed by Zr, Ti, Mg, RE complex deoxidation, which is proved by the experimental results in FIGS. 1 and 2.

[0075] According to the basic principles of metallurgical thermodynamics, Zr, Ti, Mg, and RE are all strong oxide-forming elements, and the use of Zr, Ti, Mg, and RE for complex deoxidation is beneficial to the removal of free oxygen content in molten steel. Density of main oxides in steel is shown in Table 4. It can be seen from Table 4 that the density of ZrO.sub.2 is 5.68 g/cm.sup.3, which is greater than that of Al.sub.2O.sub.3 (3.97 g/cm.sup.3), especially the density of ZrO.sub.2 is closer to that of molten steel (7.15 g/cm.sup.3). Rare earth oxides have a density greater than that of ZrO.sub.2 (6.87 g/cm.sup.3), so the addition of rare earths makes the density of the complex oxides closer to that of molten steel. Therefore, once stable oxides are formed at high temperatures, the complex oxides mainly composed of ZrO.sub.2 and rare earth oxides can float evenly in molten steel, while Al.sub.2O.sub.3 will collide and gather on the surface of molten steel to become a component of steel slag. The unfloating part of Al.sub.2O.sub.3 will remain in the steel as clusters of large inclusions.

TABLE-US-00004 TABLE 4 Densities of molten steel and various inclusions Inclusions Ce.sub.2O.sub.3 Al.sub.2O.sub.3 ZrO.sub.2 Ti.sub.2O.sub.3 TiO.sub.2 CaO FeO MnO MnO.sub.2 Density 6.87 3.97 5.68 4.48 4.23 3.34 5.75 5.37 5.03 (g/cm.sup.3) Inclusions MgO SiO.sub.2 Fe.sub.2O.sub.3 CaS MnS MgS FeS TiN AlN Density 3.58 2.65 5.24 2.59 4.89 2.80 4.85 5.22 3.26
(5) The Physical Mechanism that the Bulk Modulus of Inclusions is Close to that of Iron Matrix

[0076] Table 5 shows the crystal structure of the selected iron matrix and inclusions, and Table 6 shows the physical properties of the relevant inclusions determined in combination with first-principle calculations. According to the calculation results, the bulk moduli of LaAlO.sub.3 (192.61) and La.sub.2O.sub.7Zr.sub.2 (165.83) are lower than those of Al.sub.2O.sub.3 (249.54) and ZrO.sub.2 (271.06), and the bulk modulus of LaAlO.sub.3 (192.61) is closer to that of the iron matrix (194.76). Other inclusions, such as CaO (114.11), MgO (165.84), TiN (175.02) and CaS (57.05), exhibit a lower bulk modulus than the matrix.

TABLE-US-00005 TABLE 5 Crystal structure parameters of inclusions and BCC matrix Lattice parameters Space group Atomic position (/) Fe (BCC) Im-3m (229) Fe (0.00000, 0.00000, 0.00000) a = b = c = 2.8336 = = = 90 Al.sub.2O.sub.3 R-3C (167) Al (0.00000, 0.00000, 0.35216) a = b = 4.7650, O (0.30668, 0.00000, 0.25000) c = 13.0024 = = 90, = 120 ZrO.sub.2 Fm-3m Zr (0.00000, 0.00000, 0.00000) a = b = c = 5.09 (225) O (0.25000, 0.25000, 0.25000) = = = 90 LaAlO.sub.3 Pm-3m O (0.50000, 0.00000, 0.00000) a = b = c = 3.81160 (221) Al (0.00000, 0.00000, 0.00000) = = = 90 La (0.50000, 0.50000, 0.50000) CaO Fm-3m Ca (0.00000, 0.00000, 0.00000) a = b = c = 4.79496 (225) O (0.50000, 0.50000, 0.50000) = = = 90 MgO Fm-3m Mg (0.00000, 0.00000, 0.00000) a = b = c = 4.21214 (225) O (0.50000, 0.50000, 0.50000) = = = 90 MnS Fd-3m Mn (0.00000, 0.00000, 0.00000) a = b = c = 5.24000 (225) O (0.50000, 0.50000, 0.50000) = = = 90 CaS Fm-3m Ca (0.00000, 0.00000, 0.00000) a = b = c = 4.8365 (225) S (0.50000, 0.50000, 0.50000) = = = 90 TiN Fm-3m Ti (0.00000, 0.00000, 0.00000) a = b = c = 4.24400 (225) N (0.50000, 0.50000, 0.50000) = = = 90 Al.sub.2MgO.sub.4 Fd-3m Mg (0.00000, 0.00000, 0.00000) a = b = c = 4.79496 (227) Al (0.62500, 0.62500, 0.62500) = = = 90 O (0.37500, 0.37500, 0.37500) La.sub.2O.sub.7Zr.sub.2 Fd-3m La (0.50000, 0.50000, 0.50000) a = b = c = 10.80760 (227) O1 (0.41890, 0.12500, 0.12500) = = = 90 O2 (0.12500, 0.12500, 0.12500) Zr (0.00000, 0.00000, 0.00000)

TABLE-US-00006 TABLE 6 Calculation results of physical properties of inclusions and matrix Bulk Shear Young's modulus modulus modulus Poisson's (GPa) (GPa) (GPa) ratio Fe (BCC) 194.76 81.52 214.61 0.32 Al.sub.2O.sub.3 249.54 152.76 380.61 0.25 ZrO.sub.2 271.06 109.14 288.68 0.32 La.sub.2O.sub.7Zr.sub.2 165.83 60.56 161.95 0.34 LaAlO.sub.3 192.61 121.10 300.35 0.24 CaO 114.11 78.81 192.19 0.22 MgO 165.83 127.64 304.73 0.19 CaS 57.05 39.07 95.42 0.22 TiN 175.02 79.89 208.01 0.30

(6) Local Stress Characterization of Complex Inclusions

[0077] Electron back-scattering diffraction (EBSD) technology can provide information on crystal orientation, phase distribution and strain of the material microstructure. In this test work, a voltage of 20 kV and a current of 13 nA are selected for the EBSD test. In order to determine the lattice distortion between the inclusions and the matrix, it is necessary to eliminate the influence of external stress on the inclusions as much as possible. Therefore, it is necessary to use an Argon ion polisher (GATAN 685) for further grinding the polished sample. The Image Quality (IQ) map tested by EBSD is mainly used to describe the pattern quality of EBSD. Specifically, the strain distribution in the microstructure is represented by the variation of the pattern quality. For example, when the lattice is distorted, the IQ map will produce diffuse, lower-quality diffraction patterns, and the gray level in the IQ map will increase accordingly. The kernel average misorientation (KAM) map can be used to characterize the homogenization degree of local stress concentration or lattice distortion. Generally, a higher KAM value indicates a higher degree of deformation/dislocation density in the region, and the larger the KAM value, the higher the stress concentration.

[0078] FIGS. 6A-6B show EBSD characterization results of a region containing complex inclusions in a developed steel sample. The results show that the image quality of the IQ map in the sample is clear; and the KAM map shows that the blue (low strain) is mainly around the complex inclusions, that is, there is only a relatively small strain concentration around the complex inclusions. It can be seen that the stress generated in the matrix by the complex inclusions formed after complex deoxidation of rare earth La is very small. The addition of rare earth elements is beneficial to the formation of rare earth complex inclusions in steel, such as LaAlO.sub.3 and La.sub.2O.sub.7Zr.sub.2, which is beneficial to reduce the stress caused by the presence of inclusions.

(7) Mechanism of Influence of Rare Earth Addition on Physical Properties of Complex Inclusions

[0079] The addition of rare earth elements not only affects the grain size and microstructure, but also affects the type and physical properties of inclusions. Rare earth (RE) elements have a stronger affinity for oxygen and sulfur than Zr. Ti and other deoxidizers, and can form rare earth oxides and rare earth sulfides, as well as rare earth complex inclusions with other deoxidizers, such as LaAlO.sub.3, La.sub.2O.sub.7Zr.sub.2, etc. Compared with Al.sub.2O.sub.3, the bulk modulus of LaAlO.sub.3 is closer to that of Fe matrix, indicating that LaAlO.sub.3 inclusions have similar incompressibility to the matrix. In addition, the Young's modulus of LaAlO.sub.3 is also closer to the Fe matrix (FIGS. 7A-7C), and the Young's modulus is proportional to the hardness of the material, that is, the hardness of the rare earth complex inclusions is closer to that of the matrix.

[0080] In summary, compared with the traditional Al deoxidation process, the rare earth complex inclusions formed by the rare earth deoxidation process can reduce the micro gaps between the inclusions and the matrix, resulting in uniform deformation between the inclusions and the iron matrix, which is conducive to improving the mechanical properties, fatigue resistance, localized corrosion resistance, etc. of steel.

[0081] It can be seen from the above results that the composite additive and smelting method are especially suitable for steel for marine engineering, steel for pipeline containers, steel for cryogenic containers, steel for bridges, steel for iron towers, steel for tracks, steel for bearings, steel for gears, steel for curtain wires, spring steel, cold heading steel, automobile steel, electrical steel, bridge steel, bridge cable steel, stainless steel, H-shaped steel and other steel types that have strict requirements on the shape of inclusions. The obtained inclusions have fine, spherical, and dispersed complex inclusions with a core-shell structure having a bulk modulus similar to that of the iron matrix, which can significantly improve the corrosion resistance of neutral aqueous media, seawater corrosion resistance, fatigue resistance, plasticity and toughness, and reduce local stress concentration.

[0082] In the description of this specification, descriptions referring to the terms an embodiment, some embodiments, example, specific examples, or some examples mean that specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms are not necessarily directed to the same embodiment or example. Furthermore, the described specific features, structures, materials or characteristics may be combined in any suitable manner in any one or more embodiments or examples. In addition, those skilled in the art can corporate and combine different embodiments or examples and features of different embodiments or examples described in this specification without conflicting with each other.

[0083] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention, those skilled in the art can make changes, modifications, substitutions and variations to the above-mentioned embodiments.