Method for discriminating crystallization rate of casting powder with strong crystallization property

Abstract

The invention discloses a method for discriminating the crystallization rate of casting powder with a strong crystallization rate, which comprises the following treatment steps: (1) melting the casting powder subjected to carbon removal at high temperature, and quickly pouring the molten casting powder into a thin cake forming mold to form thin cake-shaped slag sheets by compression molding; the thickness of the cooled slag sheet is equivalent to the thickness of a real slag film in a field crystallizer; (2) slowly cooling the slag pieces together with part or all of the mold; and (3) taking down the slag piece after the slag piece is fully cooled, observing the condition of the crystallization rate of the cross section of the slag piece, and taking the condition as data of the crystallization rate of the casting powder. According to the method, the slag is pressed into the slag diaphragm through the pressing tool to be rapidly solidified, the crystallization rate of the casting powder with different strong crystallization properties can be well distinguished, and a good technical guidance effect is provided for field application of the casting powder of crack sensitive steel types such as peritectic steel.

Description

Technical Field

[0001] The present invention relates to auxiliary materials for steelmaking continuous casting, and particularly relates to a method for discriminating the crystallization rate of a strongly crystallizable mold powder. Background Art

[0002] Continuous casting mold powder is a powdery or small granular auxiliary material for steelmaking, which is used to cover the surface of molten steel in the continuous casting machine mold. Under the action of the high temperature of the molten steel, the mold powder forms a solid-liquid two-layer structure. The molten layer is adjacent to the molten steel, and above the molten layer, the mold powder still remains in its original granular or powdery state, thus playing a good heat insulation and heat preservation role to prevent the surface of the molten steel from solidifying. Under the action of the periodic vibration of the mold, the molten layer will continuously flow into the gap between the mold copper plate and the primary shell of the molten steel, lubricating the relative movement between the shell and the copper plate, thereby ensuring good surface quality of the cast slab. In addition, the molten layer also has the function of absorbing non-metallic inclusions floating in the molten steel and purifying the molten steel. The mold powder film flowing into the gap between the mold copper plate and the shell is usually only 1-2 mm. The side close to the copper plate is solid, and the side close to the shell is still liquid. The liquid phase plays a lubricating role, and the solid phase can well control the cooling capacity of the mold copper plate on the shell, thereby adjusting the cooling rate of the molten steel and achieving the effect of controlling heat transfer. Therefore, the mold powder is the last process technology for controlling the surface quality of the cast slab in the steelmaking process. An inappropriate mold powder will cause surface defects such as slag inclusion and cracks in the cast slab, and in severe cases, even cause the shell to tear and trigger a breakout accident. Therefore, the mold powder is an important means to ensure the smooth progress of the continuous casting process and the surface quality of the cast slab.

[0003] Generally, the mold powder is mainly based on the binary system of CaO and SiO2, with fluxes such as CaF2, Na2O, and Li2O added externally. Its main function is to lower the melting point and viscosity of the CaO-SiO2 binary system. In addition, a small amount of components such as Al2O3, MgO, MnO, and Fe2O3 are also added to achieve suitable metallurgical properties. Since the melting point of the mold powder is about 400°C lower than the temperature of the molten steel, in order to control the slow melting of the relatively low-melting mold powder on the surface of the molten steel, a certain amount of carbonaceous material must be added. The carbonaceous material has a very high melting point and can effectively prevent the aggregation of mold powder droplets, thus delaying the melting of the mold powder. Among these mold powder components, by adjusting the ratio of CaO to SiO2 (i.e., CaO / SiO2, hereinafter referred to as the basicity) and the addition amount of F, the precipitation amount of cuspidine (3CaO·2SiO2·CaF2) can be effectively controlled to achieve the purpose of reasonably adjusting the crystallization property of the mold powder. The strength of crystallization is the most effective means for the mold powder to control heat transfer. The stronger the crystallization property, the greater the thermal resistance of the molten slag and the lower the heat conduction intensity; the molten slag with complete vitrification has the smallest thermal resistance and the largest heat conduction intensity. For low-carbon, ultra-low-carbon steel and steel grades with poor thermal conductivity (such as silicon steel, etc.), in order to strengthen the cooling of the continuous casting billet, the mold powder is not expected to crystallize, and the addition amount of F is generally relatively low, about 3-5%. However, for peritectic steel and steel grades containing crack-sensitive elements, once the molten steel cools unevenly and too quickly in the mold, the primary shell is easily torn at the weak points under the action of various stresses, thus triggering longitudinal cracks. For these steel grades, the mold powder must have strong crystallization properties to achieve the purpose of slow cooling and suppressing crack generation. At this time, the F content added to the mold powder is often as high as 8-10%. It can be seen that F in the mold powder not only plays a role in lowering the melting point and viscosity, but also acts as an important role in crystallization, so it is an essential component of the mold powder.

[0004] For the mold fluxes used for peritectic steel and steel grades containing crack-sensitive elements, the basicity is usually greater than 1.35, and in some cases even higher than 1.5, and the fluorine content is above 8%. Such mold fluxes have very strong crystallization properties. The existing methods for detecting the crystallization rate of laboratory mold fluxes mainly involve decarburizing and melting the corresponding mold flux and then pouring it into a steel or copper mold for cooling. After the molten slag is completely solidified, the slag body is taken out, and the proportion of crystals at the cross-section of the slag body is measured. This value is used as the crystallization rate of the mold flux to characterize the crystallization strength of the mold flux, such as the crystallization rate detection methods described in Chinese patent documents with application numbers 201210078394.3, 201410387703.4, and 201611081050.2. However, due to the extremely strong crystallization properties of the mold fluxes for peritectic steel and steel grades containing crack-sensitive elements, the results obtained by the existing crystallization rate detection methods often show complete crystallization (crystallization rate of 100%). For mold fluxes with the same crystallization rate of 100%, when casting peritectic steel and other crack-sensitive steel grades on-site, the monitored heat flux and the results of longitudinal surface cracks on the continuous casting billet are quite different, indicating that the existing detection methods cannot distinguish the strength of the crystallization rate of such mold fluxes for peritectic steel crack-sensitive steel grades. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the above deficiencies of the prior art. For the detection of the crystallization rate of strongly crystallizing mold fluxes, a method of rapidly cooling a thin slag sheet is provided to rapidly cool the strongly crystallizing mold flux. By the method of the present invention, the cross-section of the slag sheet after cooling can also exhibit a structure in which a glass layer and a crystallized layer coexist.

[0006] The technical problem to be solved can be implemented by the following technical solutions.

[0007] A method for discriminating the crystallization rate of a strongly crystallizing mold flux, characterized by comprising the following treatment steps:

[0008] (1) High-temperature melting of the decarburized mold flux, and rapidly pouring the melted mold flux into a thin round cake forming mold to press and form a thin cake-shaped slag sheet; and ensuring that the thickness of the slag sheet after cooling is equivalent to the actual slag film thickness in the on-site mold.

[0009] (2) Slowly cooling the slag sheet together with part or all of the mold.

[0010] (3) After sufficient cooling, remove the slag sheet and observe the crystallization rate condition of the cross-section of the slag sheet, and use this as the data of the crystallization rate of the mold flux.

[0011] Preferably, the forming mold includes a base made of high-purity graphite that is non-wetting to high-temperature molten slag, a high-temperature resistant thin ring placed on the base, and a pressing tool with a thermal conductivity greater than 300 w / m·K.

[0012] As a further improvement of the technical solution, it further includes the step of placing the high-temperature resistant thin ring on the base before forming and heating them together in a silicon carbide rod heating furnace with unidirectional heating, and controlling the temperature in the furnace at 700°C ± 50°C.

[0013] Also as a further improvement of the technical solution, the thickness of the formed pancake-shaped slag flakes is 1.4 - 2.0 mm, and the diameter is 30 - 50 mm.

[0014] Furthermore, the diameter-to-thickness ratio of the formed pancake-shaped slag flakes is 8 or more (preferably 8 - 10).

[0015] Preferably, the high-temperature resistant thin ring is an alloy ring resistant to high temperatures of 1100 - 1200°C.

[0016] Preferably, the pressing tool is a copper cylindrical structure, and the base is a frustum-shaped structure with a larger upper part and a smaller lower part.

[0017] Furthermore, when the decarburized mold powder is melted at high temperature in step (1), the decarburized mold powder is put into a high-purity graphite crucible and placed in a high-temperature furnace with a controlled temperature of 1350°C for heating and melting.

[0018] Still further, after the mold powder is heated and melted for 15 - 30 minutes, first take out the base carrying the high-temperature resistant thin ring from the unidirectional heating furnace and place it on the furnace mouth brick, and then quickly pour the melted mold powder into the high-temperature resistant thin ring.

[0019] Furthermore, in the mold powder composition, CaO / SiO2 is 1.4 - 1.6.

[0020] The method for discriminating the crystallization rate of the high crystallization rate mold powder adopting the above technical solution has the following characteristics and

[0021] beneficial effects:

[0022] The traditional crystallization rate detection method cannot effectively distinguish the crystallization rates of high crystallization rate peritectic steel mold powders (most of the crystallization rates are 100%). Applying the method of the strong cold thin slag flakes proposed by the present invention can rapidly cool the high crystallization rate mold powder. Through the strong cooling ability of the pure copper detection pressing tool with strong heat conduction ability for a small amount of molten slag of the slag flakes, the slag flakes present a coexistence structure of glass and crystallization. At the same time, the pressed slag flakes are slowly cooled in a unidirectional furnace, which is beneficial to the stress release during the solidification process of the slag flakes and prevents the slag flakes from cracking and pulverizing. Through this invention, the strengths of the crystallization rates of different high crystallization rate mold powders can be well distinguished, providing good technical guidance for on-site applications. Description of the Drawings

[0023] Figure 1 It is the metal ring in the method of the present invention;

[0024] Figure 2For Figure 1 is a sectional view;

[0025] Figure 3 is the pure copper detection press tool in the method of the present invention;

[0026] Figure 4 is the frustum-shaped high-purity graphite detection base in the method of the present invention. Specific embodiments

[0027] During the solidification process of crack-sensitive steel types such as peritectic steel, there is a peritectic reaction during the transformation of the δ-phase of ferrite to the face-centered cubic γ-phase of austenite. This phase transformation reaction generates a large shrinkage. Therefore, during the solidification process, longitudinal cracks are easily generated due to the concentration of various stresses. For this reason, special requirements are put forward for the heat transfer control function of the corresponding mold powder, that is, a certain amount of crystals should be formed in the slag film flowing into the gap between the copper plate of the mold and the billet shell, so as to generate a large thermal resistance, and the primary billet shell with as uniform thickness as possible is ensured through slow cooling, preventing stress concentration and achieving the purpose of suppressing longitudinal cracks. Since the existing detection methods for the crystallization rate of mold powder in the laboratory cannot distinguish and differentiate the strength of the crystallization rate of mold powder for such crack-sensitive peritectic steel types. For this reason, the present invention has developed a discrimination method for the crystallization rate of isostructural crystallization mold powder for peritectic steel, providing reliable technical support for the research and development of mold powder with appropriate crystallization performance to meet the actual production needs of such steel types.

[0028] The technical solution of the present invention is as follows:

[0029] 1) Use a high-temperature alloy tube resistant to 1100 - 1200 °C to process a thin ring. The high-temperature alloy has the characteristics of resisting the thermal deformation of high-temperature liquid slag, not rusting, and not reacting chemically with the liquid slag.

[0030] 2) Control the thickness of the processed ring within 1.4 - 2.0 mm to ensure that the thickness of the slag piece after cooling is equivalent to the actual slag film thickness in the on-site mold.

[0031] 3) Control the inner diameter of the high-temperature alloy ring within 30 - 50 mm, and the ratio of the inner diameter to the thickness of the ring needs to reach more than 8 (preferably 8 - 10) to ensure one-way heat transfer during cooling.

[0032] 4) Use a metal such as pure copper with a thermal conductivity greater than 300 w / m·K to process a crystallization rate detection press tool to achieve rapid cooling of the liquid slag. The detection press tool refers to the Figure 3 cylindrical structure.

[0033] 5) Use high-purity graphite that is not wetted by high-temperature liquid slag to process a crystallization rate detection base. For the convenience of clamping operation, the detection base is processed into a frustum shape with a larger top and a smaller bottom. Specifically, refer to Figure 4 .

[0034] The specific operation steps are as follows:

[0035] (1) Place the metal rings shown in Figure 1 and Figure 2 on the detection base of Figure 4 , and place them together in a silicon carbide rod heating furnace with unidirectional heating using tongs. Control the temperature in the furnace at 700°C ± 50°C.

[0036] (2) Put the de-carbonized flux with a certain weight (3 - 5 g) into a high-purity graphite crucible, place it in a high-temperature furnace with a controlled temperature of 1350°C for heating and melting. After 15 - 30 minutes, first take out the detection base with the metal ring from the unidirectional heating furnace and place it on the furnace mouth brick, and then quickly pour the melted flux into the metal ring.

[0037] (3) Use the detection press tool in Figure 3 to quickly press the mold above the metal ring, and then put the metal ring with the slag piece together with the detection base into the unidirectional heating furnace for slow cooling.

[0038] (4) After 1 - 1.5 hours, take out the detection base and the metal ring with the slag piece. After sufficient cooling, remove the slag piece from the metal ring, and observe the crystallization rate of the cross-section of the slag piece, which is used as the data of the crystallization rate of this flux.

[0039] The following further elaborates on the specific implementation manners of the present invention in combination with specific embodiments.

[0040] Examples 1 - 4:

[0041] Weigh 5 grams of flux A, B, C, and D after de-carbonization respectively, add them into a high-purity graphite crucible, and place them in a high-temperature furnace at 1350°C for melting for 15 minutes; place the superalloy compression ring on the detection base and put them together in a unidirectional heating furnace at 700°C for heat preservation and standby. Take out the detection base with the compression ring and place it on the furnace mouth brick, quickly pour the melted slag into the compression ring, use the detection press tool to quickly press the slag until it is completely solidified, then place it in a heating furnace at 700°C for heat preservation for 1 - 1.5 hours and then take it out. After complete cooling, measure the proportion of crystals and glass in the cross-section of the slag body. Table 1 below shows the main components and test results of several typical high-alkali fluxes. Among them, the influence of a small amount of impurity elements such as Fe2O3, K2O, and MnO on the crystallization performance of the flux is very small, so they are not listed in the table)

[0042] Table 1:

[0043]

[0044]

[0045] The method of the present invention for providing strongly cooled slag flakes for strongly crystallizing mold fluxes, which presses the molten slag into a slag film with a thickness of 1.4 to 2.0 mm by a pure copper press tool for rapid solidification, can well distinguish the strength of the crystallization rate of different strongly crystallizing mold fluxes, and provides good technical guidance for the on-site application of mold fluxes for crack-sensitive steel grades such as peritectic steel.

Claims

1. A method for discriminating the crystallization rate of a mold powder with a high crystallization rate, characterized in that, It includes the following processing steps: (1) High-temperature melting of the de-carbonized mold powder, and quickly pouring the melted mold powder into a thin round cake forming mold for molding into a thin cake-shaped slag piece; and ensuring that the thickness of the slag piece after cooling is equivalent to the true slag film thickness in the on-site mold; (2) Slowly cooling the slag piece together with part or all of the mold; (3) After sufficient cooling, remove the slag piece and observe the crystallization rate condition of the cross-section of the slag piece, and use this as the crystallization rate data of the mold powder.

2. The method for discriminating the crystallization rate of the mold powder with a high crystallization rate according to claim 1, wherein The forming mold includes a base processed from high-purity graphite that is non-wetting with high-temperature liquid slag, a high-temperature resistant thin ring placed on the base, and a pressing tool with a thermal conductivity greater than 300 w / m·K.

3. The method for discriminating the crystallization rate of the flux with high crystallization rate according to claim 1 or 2, characterized in that It also includes the step of placing the high-temperature resistant thin ring on the base before forming and heating them together in a silicon carbide rod heating furnace with unidirectional heating, and controlling the temperature in the furnace at 700°C ± 50°C.

4. The method for discriminating the crystallization rate of the strong crystallization rate mold powder according to claim 1 or 2, characterized in that, The thickness of the formed thin cake-shaped slag piece is 1.4 - 2.0 mm, and the diameter is 30 - 50 mm.

5. The method for discriminating the crystallization rate of the flux with a high crystallization rate according to claim 4, characterized in that, The diameter-to-thickness ratio of the formed thin cake-shaped slag piece is 8 or more.

6. The method for discriminating the crystallization rate of the flux with a high crystallization rate according to claim 1 or 2, characterized in that, The high-temperature resistant thin ring is an alloy ring resistant to high temperatures of 1100 - 1200°C.

7. The method for discriminating the crystallization rate of the mold powder with strong crystallization rate according to claim 1 or 2, characterized in that, The pressing tool is a copper cylinder structure, and the base is a frustum-shaped structure with a larger top and a smaller bottom.

8. The method for discriminating the crystallization rate of the flux with strong crystallization rate according to claim 1 or 2, characterized in that, In step (1), when high-temperature melting the de-carbonized mold powder, the de-carbonized mold powder is placed in a high-purity graphite crucible and heated and melted in a high-temperature furnace with a controlled temperature of 1350°C.

9. The method for discriminating the crystallization rate of the flux with strong crystallization rate according to claim 3, characterized in that After the mold powder is heated and melted for 15 - 30 minutes, first remove the base carrying the high-temperature resistant thin ring from the unidirectional heating furnace and place it on the furnace mouth brick, and then quickly pour the melted mold powder into the high-temperature resistant thin ring.

10. The method for discriminating the crystallization rate of the strong crystallization rate mold powder according to claim 1, characterized in that, The CaO / SiO2 in the mold powder composition is 1.4 - 1.6.

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