Process method for controlling oxygen in converter tapping process of cold forging steel

Abstract

The invention discloses a technological method for controlling oxygen in the converter tapping process of cold forging steel. The technological method comprises the steps of molten iron pretreatment (desulfurization), converter smelting, LF furnace refining, VD vacuum degassing and square billet continuous casting production. In the converter smelting process, a slag washing process is adopted; in the LF refining process, lime, refining slag and fluorite are added, and meanwhile argon bottom blowing is carried out for whole-course stirring; molten steel obtained after ladle refining is subjected to vacuum degassing treatment through a VD furnace; whole-course protection casting is adopted in the continuous casting process; the LF refining needs to ensure enough refining time; ar bottom blowing stirring of the LF accelerates material transfer between steel and slag; and argon blowing can accelerate the upward floating speed of Al2O3 inclusions, and argon blowing is carried out in a sealed LF furnace for 12-18 min, so that the Al2O3 inclusions larger than 20 microns in the steel are basically removed. The invention aims to provide a process method for controlling oxygen in the tapping process of a cold forging steel converter so as to ensure that the subsequent LF refining process has higher cleanliness.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and in particular relates to a process method for controlling oxygen in a cold heading steel converter tapping process. Background Art

[0002] Cold heading steel is a type of steel that can be used to produce fasteners using the cold heading process at room temperature. It is generally low-carbon, medium-carbon high-quality carbon structural steel and alloy structural steel. Cold heading steel is widely used in the processing of fasteners, spare parts in the automotive, engineering construction, machinery, electronics and other industries due to its excellent machining properties.

[0003] In the process of steelmaking, oxygen content is an important factor affecting the quality of steelmaking, especially in oxygen converter smelting, the control of oxygen content in molten steel will affect the quality of molten steel. If the oxygen in the molten steel is not effectively treated during the steelmaking process, the oxygen content will be significantly higher than the control standard. During the solidification and subsequent cooling process of the steel, due to the sharp decrease in solubility, most of the oxygen originally dissolved in the steel is enriched in the form of fine inclusions such as iron oxides and oxysulfides at the austenite or ferrite grain boundaries; fine inclusions such as oxides and oxysulfides will cause grain boundary embrittlement, and easily become the starting point of grain boundary cracking during the processing and use of steel, resulting in brittle failure of the steel; the increase in oxygen content in steel will reduce the ductility, impact toughness and fatigue resistance of the steel, increase the toughness-brittleness transition temperature of the steel, and reduce the corrosion resistance of the steel.

[0004] In order to ensure the stability of product quality, cold heading steel must have a high degree of cleanliness during the smelting process. Usually, oxygen can be removed by adding composite deoxidizers such as Si, Al or composite SiAlBa to generate deoxidation products that are insoluble in molten steel. The deoxidation products then float up and enter the top slag, achieving a reduction in the oxygen content in the molten steel.

[0005] In order to reduce the difficulty of deoxidation before LF treatment after slag discharge from the converter, which leads to short LF treatment time and poor fluidity of molten steel, seriously affecting smooth production, it is very important to find a deoxidation system suitable for cold heading steel in industrial production. Summary of the invention

[0006] The object of the present invention is to provide a process for controlling oxygen in a cold heading steel converter tapping process, so as to ensure a higher cleanliness in the subsequent LF refining process.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] The present invention discloses a process for controlling oxygen in a cold heading steel converter tapping process, comprising: molten iron pretreatment (desulfurization) - converter smelting - LF furnace refining - VD vacuum degassing - billet continuous casting production; wherein:

[0009] Before smelting, the molten iron is first desulfurized to reduce the sulfur content in the molten steel;

[0010] In the converter smelting process, the slag washing process is adopted, lime and refining slag are added, and the good dynamic conditions of converter steel discharge are utilized to prepare the pre-slag for the refining furnace smelting;

[0011] During the LF refining process, lime, refining slag and fluorite are added, and argon is blown from the bottom for stirring throughout the process;

[0012] The molten steel after ladle refining is vacuum degassed in a VD furnace to further reduce the harmful gas content in the steel and improve the cleanliness of the steel;

[0013] The continuous casting process adopts full protection casting to prevent secondary oxidation of molten steel;

[0014] LF refining must ensure sufficient refining time to allow inclusions to fully float up; LF bottom blowing Ar stirring accelerates the material transfer between steel and slag, which is beneficial to the deoxidation reaction of molten steel; argon blowing can accelerate the floating speed of Al2O3 inclusions. Blowing argon in a sealed LF furnace for 12-18 minutes (preferably 15 minutes) can basically remove Al2O3 inclusions larger than 20μm in the steel.

[0015] Furthermore, the composition of the slag sample at the end of refining includes by mass percentage: CaO 50-60%, SiO2 3-5%, MgO 6-8%, and Al2O3 20-30%.

[0016] Furthermore, the refining soft blowing ensures that the molten steel surface is not exposed, the soft blowing time is ≥10min, and the upper steel temperature is between 1605 and 1615℃.

[0017] Furthermore, the composition of the top slag after steel is tapped from the converter is as follows by mass percentage: CaO 49.21%, SiO2 16.5%, MgO 8.99%, Al2O3 1.88%, FeO 12.89%, MnO 3.02%, and P2O 52.99%.

[0018] Furthermore, the composition of the top slag after steel is tapped from the converter is as follows by mass percentage: CaO 45.4%, SiO2 14.9%, MgO 9.1%, Al2O3 1.97%, FeO 11.57%, MnO 3.59%, and P2O 52.52%.

[0019] Furthermore, the composition of the top slag after steel is tapped from the converter is as follows by mass percentage: CaO 47.81%, SiO2 16.21%, MgO 8.71%, Al2O3 2.01%, FeO 11.94%, MnO 3.21%, and P2O 52.71%.

[0020] Furthermore, the one-step deoxidation method can not only improve the deoxidation efficiency and save deoxidizer, but also improve the cleanliness of molten steel.

[0021] Compared with the prior art, the beneficial technical effects of the present invention are:

[0022] The present invention adopts a one-step deoxidation method, which can increase the effect of the deoxidizer, improve the utilization rate of the deoxidizer, and improve the purity of molten steel. DETAILED DESCRIPTION

[0023] The present invention is further described in detail below in conjunction with embodiments:

[0024] This embodiment provides a method for controlling oxygen in a cold heading steel converter tapping process. This embodiment conducts experiments in three batches, which are marked as batch 1, batch 2 and batch 3 respectively.

[0025] The molten iron desulfurization process must ensure that the target sulfur content of the desulfurized iron is 0.010%. The molten iron conditions in this embodiment are shown in Table 1.

[0026] Table 1 Experimental heat molten iron composition (wt%)

[0027] Heat Si Mn P S Temperature(℃) 1 0.27 0.1 0.015 0.024 1321 2 0.28 0.09 0.014 0.028 1315 3 0.27 0.1 0.021 0.027 1320

[0028] The converter loading amount is strictly controlled at 160±2 tons to ensure that the steel output is controlled at more than 155 tons. The converter adopts high-pull supplementary blowing operation, and it is required that the carbon composition of the steel output meets the steel output requirements after one spot blowing.

[0029] Specifically, 200 kg of aluminum iron is added to one furnace at a time for deoxidation. The molten steel is taken to the argon station to take steel samples and slag samples, and then 500 meters of aluminum wire is fed into the argon station, and then steel samples and slag samples are taken again for data analysis and comparison of (FeO+MnO) in the slag.

[0030] Specifically, the deoxidizer of the second furnace is 200 kg of aluminum iron added to the steel, slag samples and steel samples are taken, and 80 kg of aluminum particles are added to the molten steel for deoxidation after 6 minutes of LF smelting process.

[0031] Specifically, 500 kg of aluminum iron was added to the three furnaces at one time for deoxidation.

[0032] The deoxidation cake samples of the converter are analyzed. At the same time, the temperature and oxygen measuring instrument is used to measure the oxygen content of the molten steel after deoxidation after the molten steel arrives at the station, so as to study and analyze the changes in oxygen content under different deoxidation conditions.

[0033] After tapping the converter, the composition of the molten steel in each test heat is shown in Table 2.

[0034] Table 2 Experimental furnace steelmaking conditions Molten steel composition (wt%)

[0035] Heat C Si Mn P S 1 0.09 0 0.1 0.009 0.014 2 0.07 0 0.09 0.011 0.016 3 0.095 0 0.1 0.01 0.017

[0036] From Table 2, we can see that after the converter process, the C content in the molten steel is 0.08-0.10%, and its content reflects the oxidation degree of the molten steel to a certain extent. The Mn content is 0.09-0.1%. Generally, the content of molten steel after the converter process is within this range. Mn at this content is difficult to be oxidized. The P content in the molten steel is 0.009-0.011%, and its content mainly depends on the composition requirements of the steel grade and the level of the operator; the Si content in the molten steel is 0, because Si is more oxidizing than Mn. Therefore, after the converter oxygen smelting, the original Si in the molten steel is oxidized to SiO2 and enters the slag. In general, the distribution of components in the molten steel after the converter is tapped is determined by the converter's oxygen smelting method.

[0037] Table 3 Composition of top slag in experimental furnaces (wt%)

[0038] Heat CaO <![CDATA[SiO2]]> <![CDATA[Al2O3]]> FeO MnO <![CDATA[P2O5]]> S MgO 1 49.21 16.5 1.88 12.89 3.02 2.99 0 8.99 2 45.4 14.9 1.97 11.57 3.59 2.52 0 9.1 3 47.81 16.21 2.01 11.94 3.21 2.71 0 8.71

[0039] The composition of the top slag after the converter is tapped is shown in Table 3. It can be seen from Table 3 that the SiO2 content in the slag is relatively high, which is because the Si in the steel enters the top slag after oxidation; the Al2O3 content in the slag is relatively low, which mainly comes from the addition of coolant or top slag modifier during the converter blowing process, so the content is relatively low. The FeO and MnO in the slag are 11-13% and 3-4% respectively, with relatively high contents; the (FeO+MnO) value is usually used to measure the oxidizability of the top slag. The value of the top slag used in this experiment is between 13-17%, indicating that the oxidizability of the top slag is relatively high.

[0040] The oxygen in the molten steel at the end of the converter was measured with an oxygen measuring gun. At the same time, the oxygen in the molten steel of the experimental heat was calculated based on the empirical formula of carbon-oxygen product at the end of the converter: [%C][%O]=0.0025. The results are shown in Table 4. The carbon in the table is the result of molten steel sampling analysis.

[0041] Table 4 Comparison of measured and calculated values ​​of [O]

[0042] Heat Carbon content (wt%) Determination of oxygen (%) Calculated oxygen (%) 1 0.09 0.0294 0.028 2 0.07 0.0349 0.0357 3 0.095 0.0271 0.0263

[0043] It can be seen from Table 4 that the measured value and the calculated value are very close, so the relevant calculation here uses the oxygen content value obtained according to the carbon-oxygen product formula for calculation.

[0044] Refining soft blowing ensures that the molten steel surface is not exposed, the soft blowing time is ≥10min, and the upper steel temperature is between 1605 and 1615℃.

[0045] Therefore, through the above comparison, it can be seen that the one-step deoxidation can not only improve the deoxidation efficiency and save deoxidizer, but also improve the cleanliness of molten steel. At the same time, it is also beneficial to shorten the inclusion removal time of the LF refining process. Under the same refining time, high-quality pure steel can be produced. However, considering the need for smooth production, the converter often slags and changes steel. If the third deoxidation method is used, it is easy to cause a waste of deoxidizer costs. The first method can be used without feeding aluminum wire after slag and steel, which is beneficial to production control.

[0046] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A process for controlling oxygen in a cold heading steel converter tapping process, characterized in that: include: Hot metal pretreatment - converter smelting - LF furnace refining - VD vacuum degassing - billet continuous casting production; including: Before smelting, the molten iron is first desulfurized to reduce the sulfur content in the molten steel; In the converter smelting process, the slag washing process is adopted, lime and refining slag are added, and the good dynamic conditions of converter steel discharge are utilized to prepare the pre-slag for the refining furnace smelting; During the LF refining process, lime, refining slag and fluorite are added, and argon is blown from the bottom for stirring throughout the process; The molten steel after ladle refining is vacuum degassed in a VD furnace to further reduce the harmful gas content in the steel and improve the cleanliness of the steel; The continuous casting process adopts full protection casting to prevent secondary oxidation of molten steel; LF refining must ensure sufficient refining time to allow inclusions to fully float up; LF bottom blowing Ar stirring accelerates the material transfer between steel and slag, which is beneficial to the deoxidation reaction of molten steel; argon blowing can accelerate the floating speed of Al2O3 inclusions. Blowing argon in a sealed LF furnace for 12-18 minutes can basically remove Al2O3 inclusions larger than 20μm in the steel.

2. The oxygen control process of cold heading steel converter tapping process according to claim 1 is characterized in that: The composition of the slag sample at the end of refining includes by mass percentage: CaO 50-60%, SiO2 3-5%, MgO 6-8%, and Al2O3 20-30%.

3. The oxygen control process of cold heading steel converter tapping process according to claim 1 is characterized in that: Refining soft blowing ensures that the molten steel surface is not exposed, the soft blowing time is ≥10min, and the upper steel temperature is between 1605 and 1615℃.

4. The oxygen control process of cold heading steel converter tapping process according to claim 1 is characterized in that: The composition of the top slag after steel is tapped from the converter is as follows by mass percentage: CaO 49.21%, SiO2 16.5%, MgO 8.99%, Al2O3 1.88%, FeO 12.89%, MnO 3.02%, and P2O 52.99%.

5. The oxygen control process of cold heading steel converter tapping process according to claim 1 is characterized in that: The composition of the top slag after steel is tapped from the converter is as follows: CaO 45.4%, SiO2 14.9%, MgO 9.1%, Al2O3 1.97%, FeO 11.57%, MnO 3.59%, and P2O 52.52% by mass.

6. The oxygen control process of cold heading steel converter tapping process according to claim 1, characterized in that: The composition of the top slag after steel is tapped from the converter is as follows by mass percentage: CaO 47.81%, SiO2 16.21%, MgO 8.71%, Al2O3 2.01%, FeO 11.94%, MnO 3.21%, and P2O 52.71%.

7. The oxygen control process of cold heading steel converter tapping process according to claim 1 is characterized in that: The one-step deoxidation method can not only improve the deoxidation efficiency and save deoxidizer, but also improve the cleanliness of molten steel.