A kind of protective slag for continuous casting of high carbon cutting tool steel and its preparation method

Abstract

The present invention discloses a protective slag for continuous casting of high-carbon cutting tool steel and a preparation method thereof, belonging to the technical field of steel smelting. The components of the protective slag for continuous casting of high-carbon cutting tool steel described in the present invention are, by mass fraction, CaO: 25-35%, SiO2: 35-45%, Al2O3: 1.0-2.0%, Fe2O3: 2.0%-5.0%, MnO: 7-15%, Na2O: 2-5%, K2O: 1-3%, MgO: 3-5%, and C: 0-5%. The chemical composition and mass percentage of the protective slag for continuous casting of high-carbon cutting tool steel used in the present invention are as follows: C: 0.72-0.85%; Si: 0.50-0.70%; Mn: 1.60-2.0%; P≤0.015%; S≤0.010%; Cr: 1.50-1.80%; Cu: 0.50-0.80%; the balance being iron and unavoidable impurities. The protective slag for continuous casting of high carbon cutting tool steel of the present invention has the characteristics of suitable properties, uniform thickness, short transmission time, etc., and can eliminate defects such as central segregation, longitudinal cracks, transverse cracks and surface slag inclusions of high carbon cutting tool steel.

Description

Technical Field

[0001] The invention belongs to the technical field of steel smelting, and in particular relates to a novel protective slag for continuous casting of high-carbon cutting tool steel and a preparation method thereof, which is suitable for protective pouring of high-carbon cutting tool steel. Background Art

[0002] With the rapid development of national infrastructure construction, mineral mining and other industries, the demand for cutting tool steel has increased year by year. In order to improve cutting efficiency and yield rate, saw blades are developing towards large diameter, high speed and thin slices. Due to the harsh working environment and the increase in saw blade speed, the impact, friction and vibration that the saw blade needs to withstand during operation are intensified, resulting in increased stress on the saw blade and reduced stability. A series of problems such as severe center segregation, severe vertical and horizontal cracks on the surface, and low fatigue resistance have occurred, which seriously affect the service life.

[0003] The continuous casting process for producing high-carbon cutting tool steel billets requires continuous addition of mold slag to the mold's molten steel surface and slag replacement during the casting process. This creates a slag film between the mold and the billet shell, reducing drawing resistance and improving lubrication. This film also improves heat transfer, filling the air gap between the billet shell and the mold. It also purifies the molten steel and absorbs inclusions that rise to the slag interface. Furthermore, it increases fluidity, preventing secondary oxidation and maintaining heat preservation. Poor mold slag performance can lead to uneven heat transfer and inconsistent air gap flow. Frictional forces on the mold surface and interior can easily cause central porosity and segregation within the billet, leading to defects such as longitudinal and transverse cracks on the billet surface.

[0004] Patent publication number CN1047724441A, "Continuous Casting Mold Flux for High-Carbon High-Alloy Steel," uses a continuous casting mold slag with a basicity of 1.0 to 1.3 and contains 3 to 10% B2O3. However, this invention is primarily used to treat surface slag inclusions, longitudinal cracks, and other quality defects that occur during the casting process of high-carbon high-alloy steels with a carbon content exceeding 1%. Patent publication number CN106111928A, "A Novel Mold Flux Containing Mn and Al for Steel and Its Application," is primarily used for casting high-manganese, high-aluminum steels with carbon contents of 0.14 to 0.20%, Al contents of 1.5 to 2.5%, and Mn contents of 21.5 to 25%. The mold slag contains relatively high levels of oxides such as K2O, BaO, and SrO. Patent Publication No. CN102039386A, "A Continuous Casting Mold Powder and a Method for Continuous Casting of Low-Alloy Steel Slabs," provides a continuous casting mold powder with a basicity of 0.95-1.05, a viscosity of 0.22-0.32 Pa·s, and Li₂O. It aims to reduce and eliminate microcracks on the surface of hot-rolled coils and microcracks and longitudinal cracks in continuously cast slabs. Patent Publication No. CN101406939A, "A High-Sodium, Low-Fluorine Continuous Casting Mold Powder and Its Preparation Method," describes a mold powder with a basicity of 0.8-1.15 and a Na₂O content of 12%-25%. It addresses the casting of crack-sensitive steel grades. However, the addition of a relatively high Na₂O content and a certain amount of lithium carbonate results in a relatively high product cost.

[0005] In summary, the problems of uneven shell, transverse and longitudinal cracks, central segregation and shrinkage cavity of high carbon cutting tool steel are more serious than those of ordinary steel. The existing continuous casting protection slag has a series of problems such as high cost and performance that cannot fully meet the use requirements of this series of steel grades. Therefore, it is urgent to develop a special continuous casting protection slag suitable for new high carbon cutting tool steel. Summary of the Invention

[0006] The harsh working environment places higher demands on the surface quality and internal quality of the steel used for the saw blade base, especially the creep resistance, cutting resistance, impact resistance and fatigue performance of the saw blade. In order to solve the above problems, the present invention provides a special protective slag for high-carbon cutting tool steel with good surface quality, high fatigue life and creep resistance, and a preparation method thereof, which successfully solves the problems of uneven shell of continuous casting billet and large billet drawing resistance during casting of high-carbon cutting tool steel; vertical and horizontal cracks on the surface; poor cutting performance and low fatigue life; and central segregation and shrinkage holes inside the continuous casting billet.

[0007] The object of the present invention is to achieve the following goals:

[0008] The present invention provides a protective slag for continuous casting of high-carbon cutting tool steel. The components of the protective slag for continuous casting of high-carbon cutting tool steel are, by mass fraction, CaO: 25-35%, SiO2: 35-45%, Al2O3: 1.0-2.0%, Fe2O3: 2.0%-5.0%, MnO: 7-15%, Na2O: 2-5%, K2O: 1-3%, MgO: 3-5%, and C: 0-5%.

[0009] Based on the above technical solution, further, the physical properties of the protective slag for continuous casting of high carbon cutting tool steel are: basicity (R) 0.70±0.15, viscosity at 1300℃ (η1300℃) = 0.52±0.05Pa.S, hemisphere point temperature (Thalf) = 1142±5℃, transition temperature 1190±5℃, spreading angle 35±2°, specific gravity 0.62±0.1g / cm 3 , crystallization rate 0.

[0010] Based on the above technical solution, the chemical composition and mass percentage of the high carbon cutting tool steel used in the protective slag for continuous casting of high carbon cutting tool steel protected by the present invention are as follows: C: 0.72-0.85%; Si: 0.50-0.70%; Mn:

[0011] 1.60-2.0%; P≤0.015%; S≤0.010%; Cr: 1.50~1.80%; Cu: 0.50~0.80%; the balance is iron and unavoidable impurities.

[0012] Because this composition steel contains relatively high levels of alloying elements such as Si, C, Cu and Cr, this type of steel has special cooling characteristics. When cooling in the crystallizer, the shell shrinks significantly in the initial stage, and the cooling shrinkage is inconsistent, which can easily cause more defects such as longitudinal and transverse cracks, copper cracks, central porosity or shrinkage cavities on the surface of the shell of the continuous casting insulate.

[0013] In the high-carbon cutting tool steel of this composition, the Cu element improves the corrosion resistance and cutting performance of the steel plate, and the C and Cr elements effectively improve the strength of the steel plate and the hardenability of the steel. The precipitation at the grain boundary increases the tendency of "copper brittleness", increases stress concentration, and increases the brittle low valley area of ​​the steel. This requires that the protective slag has good fluidity, low adhesion, low basicity, and the turning point temperature and melting temperature are appropriately low, so that a sufficient liquid slag layer can be formed to ensure good heat conduction and lubricity between the ingot and the crystallizer.

[0014] At the same time, in order to control the uniformity of heat conduction of the continuous casting billet shell, the basicity, transition temperature and melting point temperature of the protective slag must not be too low, so as to form a sufficient solid slag layer so that the protective slag has the characteristic of uniform heat conduction. In addition, the protective slag of the present invention promotes the full floating of inclusions such as MnS, CuO, Cr2O3, Cr2O5 and Al2O3, which enter the slag and are oxidized and absorbed, so the basicity cannot be too low.

[0015] In response to the above problems, the mold slag for continuous casting of high carbon cutting tool steel of the present invention is based on CaO-SiO2-Al2O3. The continuous casting mold slag of the present invention needs to focus on the thermal conductivity (including heat transfer capacity and uniformity of heat transfer), and secondly, the lubricity of the mold slag needs to be considered. To this end, the basicity R, melting point temperature, transition temperature, viscosity and other properties of the mold slag must be strictly controlled within a reasonable range, as follows:

[0016] A protective slag with a basicity of R = 0.65 to 0.80 is used to ensure good heat conduction and lubrication between the continuous casting billet and the crystallizer. This is achieved by adjusting the mass fractions of CaO and SiO2. At the same time, solvents such as Na2O and K2O are used to further adjust the viscosity and melting temperature of the protective slag. At the same time, in order to reduce pollution to the environment and water, Na2O: 2 to 5% is added, and elements such as CaF2, NaF, and KF are not added.

[0017] In order to improve the heat conduction effect of the protective slag and adjust the melting point and viscosity of the protective slag, 7-15% of the neutral oxide MnO is added. At the same time, a certain amount of components with the ability to absorb inclusions need to be added to the protective slag. For this purpose, the basic oxide MgO is used with a mass fraction of 3-5%.

[0018] By reducing basicity and adding a certain amount of MnO, MgO, and other ingredients, this invention ensures good heat conduction and lubricity between the ingot and the mold. This also improves heat transfer uniformity, reducing the occurrence of central porosity and longitudinal and transverse cracks on the ingot surface. The incidence of surface crack defects can be reduced from 18% before use to less than 1% after use.

[0019] Typically, carbon particles are added to mold slag as a melt rate regulator. However, in the present invention, coal lumps are added instead of carbon particles. When the amount and particle size of the added coal lumps are appropriate, some of the coal powder particles are burned out in the sintering layer, while the remaining slag is effectively controlled by the coal lumps. This results in a sintering layer and liquid slag layer of suitable thickness. The coal lumps of the present invention have a particle size of 5 to 20 μm, exhibiting significant separation and retardation effects, a reasonable starting oxidation temperature (520°C), and a moderate oxidation rate. Therefore, the coal lumps have a strong ability to control the melt rate in the lower temperature areas of the slag layer and high control efficiency in the higher temperature areas. This not only compensates for the shortcomings of graphite and carbon black, but also effectively reduces costs.

[0020] The present invention also provides a method for preparing the above-mentioned mold slag for continuous casting of high-carbon cutting tool steel, which mainly comprises the following steps:

[0021] The protective slag is produced by a pre-melting method. Industrial raw materials such as quartz sand, soda, fluorite, wollastonite, blast furnace slag, borax and coal blocks used to make the protective slag are weighed according to the mass percentage of the designed target composition; the weighed raw materials are mixed and mechanically stirred to ensure that the components are evenly mixed; the mixed sample is made into balls or blocks, dried, and poured into a crucible, and then placed in a medium frequency induction furnace for heating and melting at a heating temperature of 1300-1600°C. The temperature is kept for a period of 2.0h-4.0h to effectively remove volatiles and uniform slag components to form molten slag; the molten slag is poured into cold water for rapid cooling to obtain a uniform glassy amorphous substance; the glassy amorphous substance is dried and crushed into powder to obtain the required protective slag powder.

[0022] The present invention has the following beneficial effects compared to the prior art:

[0023] 1. The high-carbon cutting tool steel targeted by the present invention has high carbon and copper content, and requires rapid heat conduction and the formation of a certain shell thickness in the crystallizer. The heat conduction rate of the protective slag with high basicity (CaO / Si2>1.2) cannot meet the requirements. In order to solve a series of quality problems on the surface of the continuous casting billet of this steel type, a protective slag with fast, uniform air gap flow and good lubricity has been developed to strictly control the uniformity of heat conduction of the billet shell in the crystallizer, and solve the problems of surface quality cracks, internal center segregation and looseness of the billet in the production of high-carbon cutting tool steel.

[0024] 2. The protective slag prepared by the present invention not only meets the lubrication and heat transfer properties of continuous casting protective slag for crack-sensitive steel grades, but also does not contain elements such as CaF2, NaF, and KF, thereby reducing pollution to the environment and water and reducing corrosion to equipment.

[0025] 3. The protective slag for continuous casting of high-carbon cutting tool steel of the present invention has the characteristics of suitable properties, uniform thickness, short transmission time, etc. It can eliminate the central segregation, longitudinal cracks, transverse cracks and surface slag inclusion defects of high-carbon cutting tool steel. In addition, adding coal blocks instead of carbon particles as a melting rate regulator is beneficial to reducing manufacturing costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention, the drawings related to the embodiments are briefly introduced below.

[0027] Figure 1 This is a picture of the surface of the continuous casting slab obtained when continuous casting is carried out using the mold slag of Comparative Example 1;

[0028] Figure 2 This is a picture of the surface of the continuous casting billet produced when the protective slag in Example 1 is used for continuous casting. DETAILED DESCRIPTION

[0029] The present invention is described in detail below with reference to the embodiments, but the embodiments of the present invention are not limited thereto. Obviously, the embodiments described below are only some embodiments of the present invention. For those skilled in the art, other similar embodiments obtained without creative work all fall within the scope of protection of the present invention.

[0030] The method for preparing protective slag for continuous casting of high-carbon cutting tool steel in the embodiment includes the following processes: weighing industrial raw materials such as quartz sand, soda, fluorite, wollastonite, blast furnace slag, borax and coal blocks used to make protective slag according to the mass percentage of the designed target components; mixing the weighed raw materials and mechanically stirring them to ensure that the components are evenly mixed; making the mixed sample into balls or blocks, drying them and pouring them into a crucible, and then placing them in a medium frequency induction furnace for heating and melting, the heating temperature is 1300~1600℃, and keeping warm for a period of time (2.0h~4.0h) to effectively remove volatiles and uniform slag components to form molten slag; pouring the molten slag into cold water for rapid cooling to obtain a uniform glassy amorphous substance; drying the glassy amorphous substance and crushing it into powder to obtain the required protective slag powder.

[0031] During the steel casting process, the selection of protective slag is strictly carried out in accordance with the process system of the steel grade; during the casting process, the inspection and monitoring of the liquid level in the crystallizer are strengthened, and the slag strips on the steel liquid surface are cleaned in time; during the casting process, the monitoring of the performance of the protective slag is strengthened, and the thickness of the liquid slag layer is measured regularly for each furnace. Under normal circumstances, the thickness of one-quarter of the crystallizer should be controlled at 10-16mm, and the slag consumption per casting should be kept stable, and the slag consumption per furnace should be kept between 50-80kg.

[0032] The protective slag is used for continuous casting of high carbon cutting tool steel with the following chemical composition: C: 0.72-0.85%; Si: 0.50-0.70%; Mn: 1.60-2.0%; P≤0.015%; S≤0.010%; Cr: 1.50-1.80%; Cu: 0.50-0.80%; the balance is iron and unavoidable impurities.

[0033] Embodiment 1:

[0034] The chemical composition of the high-carbon cutting tool steel protective slag prepared in this embodiment is: CaO: 29.15%; SiO2: 44.31%; Al2O3: 1.2%; Fe2O3: 2.8%; MnO: 13.14%; Na2O: 2.5%; K2O: 1.1%; MgO: 3%; C: 2.8%.

[0035] The prepared protective slag has a basicity (CaO / SiO2) of 0.66, a hemisphere point temperature of 1145°C, and a viscosity of 0.56 Pa.S at 1300°C.

[0036] The protective slag is used for continuous casting of high carbon cutting tool steel with the following composition, and the steel composition is: C: 0.73%; Si: 0.51%; Mn: 1.65%; P: 0.009%; S: 0.002%; Cr: 1.51%; Cu: 0.55%; the balance is iron and inevitable impurities.

[0037] Example 2:

[0038] The chemical composition of the high-carbon cutting tool steel protective slag prepared in this embodiment is: CaO: 31.25%; SiO2: 41.45%; Al2O3: 1.5%; Fe2O3: 2.3%; MnO: 11.8%; Na2O: 3.5%; K2O: 1.52%; MgO: 3.58%; C: 3.1%.

[0039] The prepared protective slag has a basicity (CaO / SiO2) of 0.75, a hemisphere point temperature of 1139°C, and a viscosity of 0.52 Pa.S at 1300°C.

[0040] The protective slag is used for continuous casting of high carbon cutting tool steel with the following composition, and the steel composition is: C: 0.76%; Si: 0.57%; Mn: 1.69%; P: 0.010%; S: 0.004%; Cr: 1.62%; Cu: 0.62%; the balance is iron and inevitable impurities.

[0041] Example 3:

[0042] The chemical composition of the high-carbon cutting tool steel protective slag prepared in this embodiment is: CaO: 33.25%; SiO2: 41.48%; Al2O3: 1.5%; Fe2O3: 3.2%; MnO: 9.52%; Na2O: 3.5%; K2O: 1.6%; MgO: 4.15%; C: 1.8%.

[0043] The prepared protective slag has a basicity (CaO / SiO2) of 0.80, a hemisphere point temperature of 1135°C, and a viscosity of 0.51 Pa.S at 1300°C.

[0044] The protective slag is used for continuous casting of high carbon cutting tool steel with the following composition, and the steel composition is: C: 0.79%; Si: 0.62%; Mn: 1.72%; P: 0.012%; S: 0.005%; Cr: 1.71%; Cu: 0.72%; the balance is iron and inevitable impurities.

[0045] Example 4:

[0046] The chemical composition of the high-carbon cutting tool steel protective slag prepared in this embodiment is as follows: CaO: 27.85%; SiO2: 39.22%; Al2O3: 1.8%; Fe2O3: 3.2%; MnO: 12.53%; Na2O: 4.2%; K2O: 2%; MgO: 5%; C: 4.2%

[0047] The prepared protective slag has a basicity (CaO / SiO2) of 0.71, a hemisphere point temperature of 1137°C, and a viscosity of 0.51 Pa.S at 1300°C.

[0048] The composition of the high carbon cutting tool steel used in the protective slag is the same as that in Example 2.

[0049] Comparative Example 1

[0050] In this comparative example, a high basicity protective slag is prepared, and its chemical composition is: CaO: 35.19%; SiO2: 25.96%;

[0051] Al2O3: 4.1%; Fe2O3: 1.9%; MnO: 6.69%; Na2O: 8.2%; K2O: 0.19%; CaF: 8.96%; MgO: 2.21%; C: 6.6%.

[0052] The prepared protective slag has a basicity (CaO / SiO2) of 1.35, a hemisphere point temperature of 1105°C, and a viscosity of 1.08 Pa.S at 1300°C.

[0053] The composition of the high carbon cutting tool steel used in the protective slag is the same as that in Example 1.

[0054] Comparative Example 2

[0055] In this comparative example, a high basicity protective slag is prepared, and its chemical composition is: CaO: 34.59%; SiO2: 26.15%;

[0056] Al2O3: 3.96%; Fe2O3: 1.8%; MnO: 4.98%; Na2O: 8.7%; K2O: 0.36%; CaF: 8.21%;

[0057] MgO: 3.25%; C: 8.1%

[0058] The prepared protective slag has a basicity (CaO / SiO2) of 1.32, a hemisphere point temperature of 1108°C, and a viscosity of 1.05 Pa.S at 1300°C.

[0059] The composition of the high carbon cutting tool steel used in the protective slag is the same as that in Example 2.

[0060] Comparative Example 3

[0061] In this comparative example, a high basicity protective slag is prepared, and its chemical composition is: CaO: 34.19%; SiO2: 26.95%;

[0062] Al2O3: 3.56%; Fe2O3: 1.7%; MnO: 5.53%; Na2O: 8.3%; K2O: 0.33%; CaF: 7.79%;

[0063] MgO:3.75%; C:7.9%.

[0064] The prepared protective slag has a basicity (CaO / SiO2) of 1.27, a hemisphere point temperature of 1102°C, and a viscosity of 1.03 Pa.S at 1300°C.

[0065] The composition of the high carbon cutting tool steel used in the protective slag is the same as that in Example 3.

[0066] Comparative Examples 1 to 3 are high basicity protective slags commonly used to reduce cracks. Figure 1 The continuous casting slab obtained by continuous casting using the mold slag of Comparative Example 1 is: Figure 2 The continuous casting slabs were produced using the mold flux in Example 1. By comparison, it can be seen that when the high-basicity mold flux in Comparative Example 1, which is generally known to reduce cracks, was used, the continuous casting slabs exhibited more cracks on their surfaces. However, the continuous casting slabs produced using the mold flux in Example 1 of the present invention had good surface quality, with no surface vertical or horizontal crack defects.

[0067] According to statistics, when high carbon cutting tool steel is made of protective slag in comparative examples 1 to 3, the quality of the continuous casting billets is poor, and the incidence rate of surface vertical and horizontal cracks reaches 18%; when the protective slag in embodiments 1 to 4 is used, the quality of the continuous casting billets is good, and the incidence rate of surface defect cracks is reduced to below 1%.

[0068] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A mold slag for continuous casting of high carbon cutting tool steel, characterized in that: The components of the mold slag for continuous casting of high-carbon cutting tool steel are, by mass fraction, CaO: 25-35%, SiO2: 35-45%, Al2O3: 1.0-2.0%, Fe2O3: 2.0%-5.0%, MnO: 7-15%, Na2O: 2-5%, K2O: 1-3%, MgO: 3-5%, and C: 0-5%. Alkalinity (R) 0.70 ± 0.15; The chemical composition and mass percentage of the high-carbon cutting tool steel used in the protective slag for continuous casting of high-carbon cutting tool steel are as follows: C: 0.72-0.85%; Si: 0.50-0.70%; Mn: 1.60-2.0%; P≤0.015%; S≤0.010%; Cr: 1.50~1.80%; Cu: 0.50~0.80%; the remainder is iron and unavoidable impurities.

2. The mold slag for continuous casting of high carbon cutting tool steel according to claim 1, characterized in that: The physical properties of the mold slag for continuous casting of high carbon cutting tool steel are as follows: viscosity at 1300°C (η1300°C) = 0.52±0.05 Pa.S, hemisphere point temperature (Thalf) = 1142±5°C, transition temperature 1190±5°C, spreading angle 35±2°, specific gravity 0.62±0.1g / cm 3 , crystallization rate 0.

3. The method for preparing mold slag for continuous casting of high carbon cutting tool steel according to any one of claims 1 to 2, characterized in that: The following steps are involved: (1) Weigh the raw materials for making protective slag according to the designed target composition mass percentage, and mechanically stir the weighed raw materials to make the components evenly mixed; (2) The mixed sample is made into balls or blocks, dried and poured into a crucible, and then placed in a medium frequency induction furnace for heating and melting to remove volatiles and uniform slag components to form molten slag; (3) The molten slag is poured into cold water for rapid cooling to obtain a uniform glassy amorphous material, and the glassy amorphous material is dried and crushed into powder to obtain protective slag for continuous casting of high carbon cutting tool steel.

4. The preparation method according to claim 3, characterized in that The raw materials for preparing the protective slag in step (1) include quartz sand, soda, fluorite, wollastonite, blast furnace slag, borax and coal blocks.

5. The preparation method according to claim 3, characterized in that The heating temperature in step (2) is 1300~1600℃.

6. The preparation method according to claim 3, characterized in that The holding time in step (2) is 2.0h~4.0h.

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