Refining slag for obtaining low-melting point inclusions of the CaO-SiO2-MgO system

Abstract

The present invention provides a refining slag for use in refining cord steel, which is a secondary refining technology in the steel metallurgy industry and can produce low-melting-point CaO-SiO2-MgO inclusions. Its unique feature is its rationally designed composition. The mass percentages of the refining slag are MgO = 15-25%, Al2O3 < 3%, and the remainder CaO and SiO2, with a CaO to SiO2 mass ratio of 0.7-1.0. Industrial applications have shown that the refining slag designed in this invention can produce low-melting-point CaO-SiO2-MgO inclusions. These inclusions undergo uniform and sufficient deformation during the hot rolling process, ultimately achieving a width of less than 2 microns in the wire rod. At the same time, the erosion of the refining slag on the ladle refractory material can be reduced, more than doubling its lifespan.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary refining in the iron and steel metallurgy industry, and particularly relates to a refining slag for obtaining CaO-SiO2-MgO-based low-melting-point inclusions.

Background Art

[0002] Cord steel is a steel wire with a diameter of 0.15 to 0.38 mm and is mainly used to make tire radial plies. Since the cord steel wire has a very thin wire diameter and a complex force-bearing situation in the manufacturing process, if there are inclusions in the steel that do not deform significantly in size, the steel wire will break during the drawing or stranding process, which will affect the production efficiency and may even lead to a price reduction or abandonment of the product. Research has shown that by controlling the inclusions in the steel to low-melting-point plastic inclusions, the wire breakage rate of cord steel can be reduced.

[0003] To obtain low-melting-point plastic inclusions, the industry currently employs a common control method that involves refining using a low-alkali acidic CaO-SiO2 slag system (CaO / )SiO2 = 0.8-1.2, Al2O3 < 10%, MgO < 10%), and controlling the reaction of the slag steel to control inclusions in the low-melting-point region of CaO-SiO2-Al2O3-MgO. While low-melting-point plastic inclusions can be obtained in the current refining process, low-alkali acidic CaO-SiO2 slag causes very serious erosion of the slag line in the ladle. The lifespan of ladles smelting cord steel using low-alkali slag is only one-third of the lifespan of ladles using normal refining slag, and even shorter. Therefore, refining with low-alkali acidic slag is currently a major cause of high ladle refractory costs in the cord steel production process. Furthermore, CaO-SiO2-Al2O3-MgO low-melting-point inclusions are prone to secondary crystallization during cooling and solidification or prolonged heating processes, generating hard Al2O3-MgO spinel inclusions. These inclusions are non-deformable and have a very high potential to cause wire breakage during the processing of cord steel. There are also several methods for applying CaO-SiO2-MgO slag systems. For example, "CN201610585085.3 Refining Method for Spring Steel" proposes a CaO-SiO2-MgO slag system, but the composition and morphology of the inclusions after refining have not been studied. Moreover, the melting point of the refined slag with this composition is high, and the refining structure does not easily melt into the slag, so it is necessary to add CaF2 separately as a flux. While the addition of CaF2 can lower the melting point of the refined slag, CaF2 causes very serious erosion of the ladle, which similarly leads to a reduction in the lifespan of the ladle. Furthermore, CaF2 is an environmentally unfriendly type of slag, and the element F it contains pollutes the environment. In many developed countries, the use of CaF2 in steelmaking slag is explicitly prohibited.

[0004] Therefore, the technical problem that this invention aims to solve is how to design a new type of refining slag that is environmentally friendly, does not contain CaF2 as a flux, improves the lifespan of the ladle, and simultaneously obtains low-melting-point inclusions with excellent deformation performance. [Overview of the Initiative]

[0005] According to the present invention, in order to solve the above technical problems, a refining slag is provided that yields CaO-SiO2-MgO-based low-melting-point inclusions.

[0006] To achieve the above objectives, the technical means employed in this invention are as follows.

[0007] In a refining slag for obtaining CaO-SiO2-MgO low-melting-point inclusions, the mass percentages of each component in the refining slag are MgO = 15-25%, Al2O3 < 3%, with the remainder being CaO and SiO2, and the mass ratio of CaO to SiO2 being 0.7-1.0.

[0008] In the LF refining process, electrodes are heated to raise the temperature, and in the tapping process, refining slag is added to create slag. Throughout the entire LF processing process, argon gas is blown into the ladle from the bottom to agitate it, and the molten steel composition is finely adjusted so that the molten steel composition meets the requirements of the finished product. The total LF processing time is ≥ 45 min, and the argon gas soft blow time is ≥ 25 min.

[0009] Furthermore, after the LF argon gas soft blowing is completed, the acid-fused aluminum (Als) in the molten steel is controlled to ≤8 ppm, and the dissolved oxygen (O) is controlled to 15-25 ppm.

[0010] Preferably, the total LF treatment time is 50 min, where the argon gas soft blow time is preferably 35 min.

[0011] According to the present invention, by jointly controlling the alkalinity (CaO / SiO2) to be in the range of 0.7 to 1.0, the MgO content to be 15 to 25%, and the refining time being ≥ 45 min at a refining temperature of 1565 to 1585°C for a limited amount of refined slag, it is ultimately possible to reduce ladle erosion while obtaining low-melting-point inclusions with sufficient deformation. Furthermore, there is no need to add other fluxes (such as CaF2) to melt the slag and lower its melting point, which reduces costs, is environmentally friendly, and avoids flux erosion of the ladle.

[0012] The Al2O3 content in the refined slag of the present invention is within 2%, and since cord steel generally uses low-titanium, low-aluminum silicon iron and metallic manganese smelting, and the Al content of these alloys is very low, the Al2O3 content in the inclusions is very low and can be ignored, thus avoiding the formation of hard MgO-Al2O3 spinel-type inclusions that do not crystallize and deform during continuous casting. The inclusions produced in the present invention are all low-melting-point inclusions that are sufficiently deformable and significantly reduce the wire breakage rate of the cord steel.

[0013] Compared to conventional technology, the beneficial effects of the present invention are that, after refining the slag to obtain the CaO-SiO2-MgO-based low-melting-point inclusions proposed in the present invention, the CaO-SiO2-MgO-based low-melting-point inclusions are obtained, while at the same time the erosion of the refining slag on the magnesium carbonaceous slag line is effectively reduced, improving the lifespan of the cord steel ladle by more than double compared to conventional processes. Furthermore, the oxide inclusions in the wire rod are sufficiently and uniformly deformed along the rolling direction and have a width of less than 2 microns, thus further reducing the rate of wire breakage in the cord steel, and significantly reducing the cost of the cord steel ladle. [Brief explanation of the drawing]

[0014] [Figure 1] This shows the morphology of the CaO-SiO2-MgO inclusions in the wire obtained in Example 1, along the rolling direction. [Figure 2]This shows the morphology of the CaO-SiO2-MgO inclusions in the wire obtained in Example 2, along the rolling direction. [Figure 3] This shows the morphology of the CaO-SiO2-MgO inclusions in the wire obtained in Example 3, along the rolling direction. [Figure 4] This shows the morphology of the CaO-SiO2-MgO inclusions in the wire obtained in Example 4, along the rolling direction. [Figure 5] This shows the lateral width dimension distribution of the inclusions in Examples 1 to 4. [Figure 6] This is a projection of the components of the inclusions in the wire obtained in Examples 1-4. [Figure 7] The components and morphology of the inclusions obtained in Comparative Example 1 are shown. [Figure 8] The components and morphology of the inclusions obtained in Comparative Example 2 are shown. [Modes for carrying out the invention]

[0015] The present invention will be described in more detail below with reference to the drawings and specific embodiments.

[0016] In the following examples, the test steel grade is LX82A, its chemical composition is shown in Table 1, and the test process can be the production process of cord steel commonly used in this field, namely "converter steelmaking" - LF refining - continuous casting - wire rod rolling". Specifically, the test process used in the following examples is as follows (other undisclosed conditions are all the normal smelting conditions for LX82A).

[0017] (1) In the converter process, the converter terminal employs a high-catch carbon process, and metallic manganese, low-titanium low-aluminum silicon iron, and a carburizing agent are added during the converter tapping process. After tapping is complete, metallic manganese, low-titanium low-aluminum silicon iron, and the refined slag designed in this invention are added. The amounts of metallic manganese and low-titanium low-aluminum silicon iron added are, in principle, such that the Mn and Si content in the steel reaches or approaches the requirements of the finished product, and the amount of refined slag added is 8-10 kg / ton of steel.

[0018] (2) In the LF refining process, the electrodes are heated to raise the temperature, and the heating temperature is controlled at 1565 - 1585 °C (preferably 1570 °C). Slag is made with respect to the refining slag (any one of 1# - 7#) added in the steel tapping process. During the entire LF treatment process, argon gas is bottom-blown and stirred against the ladle, and the molten steel components are finely adjusted so that the molten steel components meet the requirements of the finished product. The total LF treatment time ≥ 45 min (preferably 50 min), and the argon gas soft blowing time ≥ 25 min (preferably 35 min). After the LF argon gas soft blowing is completed, the acid-soluble aluminum Als in the molten steel is controlled at ≤ 8 ppm, and the dissolved oxygen [O] is controlled at 15 - 25 ppm.

[0019] (3) In the continuous casting and wire rod rolling process, a small billet of 160 mm × 160 mm is obtained by continuous casting. The small billet is heated to 1050 - 1100 °C, held for 2 hours, and rolled into a cord steel wire rod with a diameter of 5.5 mm. (Example 1)

[0020] The refining slag added to the ladle in this example is the 1# refining slag shown in Table 2. (Example 2)

[0021] The refining slag added to the ladle in this example is the 2# refining slag shown in Table 2. (Example 3)

[0022] The refining slag added to the ladle in this example is the 3# refining slag shown in Table 2. (Example 4)

[0023] The refining slag added to the ladle in this example is the 4# refining slag shown in Table 2. (Comparative Example 1)

[0024] The refining slag added to the ladle in this example is the 5# refining slag shown in Table 3.

[0025] The alkalinity of the smelting slag is higher than 1.0, and as can be seen from Figure 7, magnesium aluminum spinel precipitates as inclusions in the production wire. These inclusions are not sufficiently deformed, forming lumps, which increases the risk of wire breakage. (Comparative Example 2)

[0026] The slag added to the ladle in this embodiment is the 6# slag shown in Table 3.

[0027] The Al2O3 content in the inclusions is higher than 3%, and as can be seen from Figure 8, magnesium aluminum spinel precipitates in the inclusions within the wire. The inclusions are not sufficiently deformed, forming lumps, which increases the risk of wire breakage. (Comparative Example 3)

[0028] In this embodiment, the smelting slag added to the ladle is the 7# smelting slag shown in Table 3. Smelting is carried out until the end of the ladle's lifespan, and the lifespan of the ladle is statistically recorded.

[0029] The MgO content is less than 15%, meaning that a large amount of MgO in the refractory material dissolves into the refined slag, leading to severe erosion of the refractory material and a reduced lifespan.

[0030] Each of the wires obtained in the above examples was taken, and the morphology of the oxide inclusions along the rolling direction was examined using a scanning electron microscope. Figures 1 to 4 show the morphology of the oxide inclusions along the rolling direction in the wires obtained in Examples 1 to 4, respectively. The oxides in the wires obtained using the smelting slag of the present invention are sufficiently and uniformly deformed along the rolling direction, have a lateral width of 2 microns or less, and the dimensional width of most inclusions is 1.2 microns or less, as shown in Figure 5.

[0031] Figure 6 shows a projection of the inclusion components in Examples 1-4, where the inclusion components are CaO-SiO2-MgO and are located in the low melting point region of the CaO-SiO2-MgO system.

[0032] Figures 7 and 8 show the composition and morphology of the inclusions obtained in Comparative Example 1 and Comparative Example 2, respectively. Magnesium aluminum spinel precipitated in the inclusions, and the inclusions were insufficiently deformed and formed in a lumpy manner.

[0033] Furthermore, in long-term production practice, the lifespan of ladles using the smelting slag used in Comparative Example 3 (which currently represents the typical smelting slag for cord steel) varied from 23 to 29 furnaces (average 25 furnaces), and it has been shown that by adopting the smelting slag of the present invention, the lifespan can be improved to 71 to 83 furnaces (average 76 furnaces) after long-term production.

[0034] Table 1. Composition and mass percentage of test steel grade LX82A [Table 1]

[0035] In the above examples, the 1#, 2#, 3#, and 4# smelting slags were prepared according to the raw material ratios shown in Table 2 and then uniformly mixed.

[0036] Table 2. Composition and mass percentage of the smelting slag used in the examples. [Table 2]

[0037] Table 3. Components and mass percentages of the refined slag used in the comparative example. [Table 3]

[0038] Finally, it should be noted that the specific embodiments described above are merely illustrative of the technical means of the present invention and do not limit them. Although the present invention has been described in detail with reference to the examples, those skilled in the art should understand that the technical means of the present invention can be modified or replaced with equivalents without departing from the spirit and scope of the technical means of the present invention, and any such modifications should fall within the scope of the claims of the present invention.

Claims

1. A refining method for obtaining a CaO-SiO2-MgO-based low-melting-point inclusion using refining slag, In the LF refining process, the LF electrode is heated to 1565-1585°C, the refining slag is added in the tapping process to create slag, argon gas is blown into the ladle from the bottom and stirred throughout the LF processing process, the molten steel composition is finely adjusted so that the molten steel composition meets the requirements of the finished product, and CaO-SiO₂-MgO low melting point inclusions are obtained with a total LF processing time of ≥ 45 min and an argon gas soft blow time of ≥ 25 min. The aforementioned smelting slag contains CaO, SiO ２ , MgO, and Al ２ O ３ It consists only of, CaO, SiO ２ , MgO, and Al ２ O ３ The mass percentages of are, respectively, CaO ≥ 34.1%, SiO ２ ≥ 36.7%, MgO = 15 - 25%, and Al ２ O ３ < 3%, CaO and SiO ２ The mass ratio is 0.7 to 1.

0. A refining method characterized by the following features.

Images