Method for improving slag inclusion defect on surface of casting blank

Abstract

According to the method for improving the slag inclusion defect on the surface of the casting blank, aiming at the casting blank under a hot conveying and hot charging process, the oxidation burning loss amount is regulated and controlled in a heating furnace before rolling, so that the slag inclusion defect on the surface of the casting blank is improved and regulated and controlled. According to the method for improving the slag inclusion defect on the surface of the casting blank, the slag inclusion defect on the surface of the casting blank is improved by controlling oxidation burning loss of the surface of the casting blank aiming at the slag inclusion phenomenon on the surface of the casting blank under a hot delivery and hot charging process; and the specific regulation and control are carried out according to specific parameter values of two dimensions of the burning loss state of the casting blank and the surface quality of the casting blank. And in the casting blank surface quality, the two dimensions of the casting blank surface quality requirement and the casting blank actual surface quality state are considered.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgical production, and in particular relates to a method for improving slag inclusion defects on the surface of a casting blank. Background Art

[0002] The surface quality of the ingot directly affects the surface quality of the hot-rolled products and the efficiency of hot delivery and hot loading. In actual production, it is difficult to achieve defect-free surface of the ingot due to the fluctuation of process parameters and the occurrence of equipment abnormalities during the continuous casting process. Usually, for ingots with high requirements on product surface or ingots with abnormal continuous casting quality, flame cleaning will be used to clean the surface of the ingot. However, with the need for energy conservation, emission reduction and production efficiency improvement, more and more products are not flame cleaned, but directly sent to the hot rolling heating furnace for rapid heating, so as to achieve the purpose of energy conservation, emission reduction and production efficiency improvement.

[0003] However, if the ingot is directly heated without mechanical cleaning, the cleaning thickness of the ingot surface becomes smaller, and the probability of slag inclusion defects on the ingot surface increases. The changes in the heating process corresponding to the hot delivery and hot charging of the ingot also lead to changes in the oxidation and burning of the ingot surface compared with the previous ones, affecting the surface quality of the steel coil. In fact, the solidification process, mechanical cleaning process, and heating process of the ingot surface in the crystallizer all have an impact on the slag inclusion of the ingot. The source of the slag inclusion is the fluctuation of the process parameters of the crystallizer. The purpose of mechanical cleaning is to remove the slag inclusion. The combustion process of the heating furnace is a subsequent means to reduce the slag inclusion defects of the ingot. However, for the hot rolling production plan of hot delivery and hot charging, especially for the ingots with shallow slag inclusion depth, the heating furnace control is particularly important for slag inclusion control.

[0004] The invention application with application number: CN201010135605.3 discloses "a method for predicting slag inclusion on the surface of a continuous casting billet before rolling", which is specifically as follows: collecting the crystallizer liquid level value and the casting length; according to the casting length, calling the corresponding crystallizer liquid level value, calculating the difference between adjacent crystallizer liquid level values ​​and judging whether the crystallizer liquid level fluctuates and counting the number of fluctuations, calculating the fluctuation amplitude and fluctuation speed of each fluctuation; comparing the fluctuation amplitude and fluctuation speed with the set fluctuation amplitude standard value and fluctuation speed standard value respectively, and rating them to obtain the rating of each fluctuation; then determining the weight coefficient of each fluctuation rating according to the weight coefficient set for each rating; adding the weight coefficients of each fluctuation rating to obtain the slag inclusion index, comparing the slag inclusion index with the set slag inclusion index standard value, judging whether the continuous casting billet is qualified and outputting the forecast display.

[0005] The invention application with application number: CN201410173931.1 discloses "a method for dynamically setting the calorific value of coal gas in a steel rolling heating furnace". It first obtains the normal production or fault stop mark and related information of the slab in the furnace, distinguishes between normal production and fault stop. If it is normal production, the heat Q required for the hourly output of the slab within the specified range of the heating furnace is calculated, and the corresponding heat load level J is determined according to the heat demand Q, and then the corresponding calorific value Z is determined according to J; if it is a fault stop state, the heat load level J during the fault period is determined according to the fault time, and then the coal gas calorific value Z is determined according to the heat load level J; finally, the set value Z is sent to the PLC control system of the gas mixing station to realize dynamic setting control of the calorific value.

[0006] The invention application with application number: 202010602393.9 discloses "a method for predicting slag inclusion in a casting billet, a machine cleaning control method, a computing device and a storage medium". The prediction method is used in a production management system and includes the following steps: obtaining the steel grade information of the current casting billet; obtaining multiple sets of continuous casting parameters and corresponding hot rolling parameters of the current casting billet, and constructing a parameter set of the current casting billet containing multiple characteristic variables based on the continuous casting parameters and the hot rolling parameters; matching the steel grade information with the model database; if the match is successful, obtaining the matching pre-stored prediction model, and obtaining the slag inclusion probability of the current casting billet based on the parameter set and the pre-stored prediction model. Summary of the invention

[0007] In order to improve the problem of surface defects of continuous casting slabs, the present invention provides a method for improving slag inclusion defects on the surface of casting slabs, and the technical solution thereof is specifically as follows:

[0008] A method for improving slag inclusion defects on the surface of a casting billet,

[0009] For the ingots under the hot delivery and hot charging process, the slag inclusion defects on the surface of the ingots are improved and regulated by regulating the oxidation and burning loss in the heating furnace before rolling.

[0010] Further,

[0011] The regulation of the amount of oxidation burning of the ingot in the heating furnace is based on the regulation of the combustion medium.

[0012] Further,

[0013] The combustion medium contains hydrogen, and the regulation of the combustion medium is achieved by establishing a regulation of the hydrogen gas volume fraction in the combustion medium.

[0014] Further,

[0015] The combustion medium is a mixed gas, which is composed of coke oven gas produced by coking and blast furnace gas produced by ironmaking, or coke oven gas produced by coking and converter gas produced by steelmaking, or coke oven gas produced by coking, blast furnace gas produced by ironmaking and converter gas produced by steelmaking;

[0016] Based on this, the regulation of the hydrogen gas volume fraction in the combustion medium is achieved by adjusting the ratio of the mixed coal gas.

[0017] Further,

[0018] The oxidation and burning loss amount of the ingots in the heating furnace is regulated to establish the overall oxidation and burning loss amount regulation for all the ingots in the heating furnace;

[0019] Specifically:

[0020] Firstly, the target oxidation burning loss of each ingot is determined according to the burning loss state index and surface quality index of each ingot, and then the overall oxidation burning loss amount of the ingot in the furnace is regulated according to the target oxidation burning loss of each ingot.

[0021] Further,

[0022] The burnout state index is determined according to the burnout prediction value and the burnout reference value of the ingot;

[0023] The surface quality index is determined according to the actual surface quality state of the ingot, and the overall oxidation and burning loss amount of the ingot in the furnace is regulated according to the target oxidation and burning loss of each ingot, including the following steps:

[0024] S1: During the continuous casting production process, the actual surface quality state of the ingot is determined, a specific representation of the actual surface quality state of the ingot is formed, and the current surface quality index of the ingot is determined accordingly;

[0025] S2: When the ingot arrives at the furnace mouth of the heating furnace, the calculation and determination of the predicted value of the ingot burning loss is completed according to the charging temperature under the hot delivery and hot charging process and the predicted time in the furnace, and the charging temperature of the ingot with the same specification parameters under the continuous casting → flame cleaning → heating furnace heating → rolling process is retrieved from the historical data, and the calculation and determination of the reference value of the ingot burning loss is completed according to the actual time in the furnace; and the determination of the ingot burning loss state is completed according to the predicted value of the ingot burning loss and the reference value of the ingot burning loss, so as to form the characterization of the burning loss state index;

[0026] S3: determining the target oxidation burning of the billet according to the current billet surface quality index value in step S1 and the billet burning state in step S2;

[0027] S4: Repeat steps S1 to S3 until the target oxidation and burning losses of all the ingots entering the furnace are determined, and according to the target oxidation and burning losses of each ingot, the overall oxidation and burning losses of the ingots in the furnace are regulated.

[0028] Further,

[0029] The surface quality index is determined according to the surface quality requirements of the ingot and the actual surface quality status of the ingot; accordingly, step S1 is specifically as follows: during the continuous casting production process, the actual surface quality status of the ingot is determined to form a specific quality status characterization; and the current ingot surface quality index is determined in combination with the current ingot surface quality requirements.

[0030] Further,

[0031] The determination process of the burnout status index is as follows:

[0032] First, different judgment criteria are set, and different values ​​are assigned to the burnout status indicators according to the judgment criteria;

[0033] Then, based on the predicted value of the current ingot's burnout and the reference value of the ingot's burnout, calculation and judgment are performed to determine which judgment criterion the ingot falls into, and the corresponding burnout state index of the current ingot is determined according to the judgment result.

[0034] Further,

[0035] The “setting different determination criteria and assigning different values ​​to the burnout status indicators according to the determination criteria” is specifically:

[0036] Definition | S cal -S ref |≤0.1×,S ref For normal burning, a judgment standard for whether the burning is normal or not is formed, and a value is assigned to it to form a burning state index value corresponding to the normal burning;

[0037] Definition S cal <0.9×S ref For under-burning, a judgment criterion for under-burning is formed, and a value is assigned to it to form a burning state index value corresponding to the under-burning;

[0038] Definition S cal >1.1×S ref For over-burning, a judgment criterion for whether it is over-burning is formed, and a value is assigned to it to form a burn state index value corresponding to the over-burning;

[0039] In the above inequality,

[0040] S ref : Reference value of burning loss of ingot, unit: mm;

[0041] S cal : Prediction value of burning loss of ingot, unit: mm.

[0042] Further,

[0043] The actual surface quality state of the ingot is completed based on the ingot surface quality determination model in the production system or determined based on the crystallizer liquid level fluctuation value during the continuous casting process.

[0044] Further,

[0045] When the actual surface quality state of the ingot is determined based on the liquid level fluctuation value of the crystallizer during the continuous casting process, the specific steps are as follows: first, a liquid level fluctuation interval is established; then, each liquid level fluctuation interval is assigned a value to form a representation of the actual surface quality states of different ingots; finally, the interval into which the actual liquid level fluctuation falls is determined, and the actual surface quality state of the current ingot is determined according to the value of the actual surface quality state of the ingot in the interval.

[0046] Further,

[0047] The surface quality index is characterized by the actual surface quality state of the ingot.

[0048] Further,

[0049] The current process of determining the surface quality index of the ingot is:

[0050] Firstly, the surface quality requirements of the ingot are divided into different levels, and the surface quality requirements of the ingots with different levels are assigned values ​​respectively, so as to form a first array that can characterize the surface quality requirements of the ingots with different levels;

[0051] Secondly, the actual surface quality state of the ingot is divided into status levels, and the actual surface quality states of the ingots of different status levels are assigned values ​​respectively, so as to form a second array that can characterize the actual surface quality states of different ingots;

[0052] Thirdly, the first array and the second array are fully arranged and combined to form a third array, and the third array is assigned values ​​to form a set of surface quality indicators of the casting billet;

[0053] Finally, according to the surface quality requirement level of the current ingot and the actual surface quality status level of the ingot, the corresponding third array is searched in the third array, and the surface quality index of the ingot is determined according to the search result.

[0054] Further,

[0055] The determination of the reference value and the predicted value of the burning loss of the casting billet is carried out according to the set iterative calculation cycle, from the beginning of the casting billet entering the furnace to the end of the casting billet leaving the furnace, and the following steps are iterated:

[0056] SS1: When the billet arrives at the heating furnace entry position, the charging temperature is taken as the initial temperature, combined with the current furnace temperature, and the calculation of the absorbed heat is completed based on the heat radiation equation;

[0057] SS2: Based on the absorbed heat, combined with the one-dimensional heat conduction differential equation, the absolute temperature of the billet surface at the current moment is calculated;

[0058] SS3: The diffusion coefficient of oxygen atoms in iron is calculated based on the absolute temperature of the ingot surface at the current moment, and the oxidation loss at the current moment is calculated based on the calculation results.

[0059] Further,

[0060] The burnout amount in the current calculation cycle is determined according to the following formula:

[0061]

[0062] The cumulative burnout amount obtained after each iterative calculation up to the current calculation cycle is determined according to the following formula:

[0063]

[0064] In the above formula,

[0065] a n : The accumulated burn loss up to the current calculation cycle, unit: mm;

[0066] a n-1 : The cumulative burn loss as of the previous calculation cycle, unit: mm;

[0067] Δa n : The amount of burnout in the current calculation cycle, unit: mm;

[0068] Δt: iterative calculation time, unit: s;

[0069] D(T): The diffusion coefficient of oxygen atoms in the steel billet when the temperature is T.

[0070] The present invention provides a method for improving the slag inclusion defect on the surface of a billet, which aims at the slag inclusion phenomenon on the surface of the billet under the hot delivery and hot charging process, and improves the slag inclusion defect on the surface of the billet by controlling the oxidation and burning of the billet surface; the specific regulation is carried out based on the specific parameter values ​​of the two dimensions of the billet burning state and the billet surface quality. The billet surface quality takes into account the two dimensions of the billet surface quality requirements and the actual billet surface quality state. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Figure 1 It is a schematic diagram of the steps of iteratively calculating the burn-in loss reference value and the burn-in loss prediction value of the casting slab in the present invention;

[0072] Figure 2 It is a schematic diagram of the surface temperature and the burnt thickness curve when determining the burnt reference value in an embodiment of the present invention;

[0073] Figure 3 It is a schematic diagram of the surface temperature and the burn thickness curve when determining the burn prediction value in the embodiment of the present invention. DETAILED DESCRIPTION

[0074] Below, a method for improving slag inclusion defects on the surface of a casting billet according to the present invention is further described in detail based on the accompanying drawings and specific implementation methods.

[0075] In order to facilitate the formation of a basic understanding of this technical solution as a whole, the following is an overview of the setting ideas, theoretical basis and basic steps of this technical solution:

[0076] As described in the background section above, the process in the prior art adopts continuous casting → flame cleaning → heating in a heating furnace → rolling. The flame cleaning step under this process can clean the surface of the ingot. However, with the demand for energy conservation, emission reduction and production efficiency improvement, the existing process for certain products has been gradually replaced by hot delivery and hot charging. The so-called hot delivery and hot charging means removing the flame cleaning step above, and the ingot is directly heated in the heating furnace from continuous casting. Because there is no flame cleaning on the surface of the ingot, the probability of slag inclusion defects on the surface of the ingot increases, which brings problems to the quality of subsequent products. This technical solution has established an improvement plan for this problem and this process, namely: oxidize the surface slag inclusion by increasing the oxidation and burning loss of the ingot surface. In order to put this technical concept into practice, without changing the heating time and heating temperature, the following will point to the problem of the oxidizing medium and the oxidizing strength of each medium. For the atmosphere in the furnace, SO2 has the strongest oxidizing property, followed by O2, H2O, and CO2. In a high temperature environment, H2O has a stronger oxidizing and corrosive ability on the surface of the ingot than O2.

[0077] For example: see the table below, for a mixed gas with a given calorific value of 2200kcal / Nm3, the H20 volume fraction of the corresponding combustion flue gas from the lowest to the highest coke oven ratio increases from 8% to 17.1%. Although the corresponding volume fraction of CO2 is reduced, the high-temperature oxidizing property of the flue gas is enhanced. In theory, the burnt thickness of the ingot can be increased by more than 30% based on the original burnt thickness.

[0078] Calorific value Blast furnace ratio Coke oven ratio Converter ratio Excess air factor <![CDATA[O2]]> <![CDATA[H20]]> <![CDATA[CO2]]> 2200 0 0.09 0.91 1.1 0.014 0.08 0.253 2200 0.1 0.14 0.76 1.1 0.014 0.097 0.23 2200 0.2 0.19 0.61 1.1 0.014 0.113 0.209 2200 0.3 0.24 0.46 1.1 0.014 0.129 0.188 2200 0.4 0.29 0.31 1.1 0.014 0.143 0.169 2200 0.5 0.34 0.16 1.1 0.014 0.158 0.15 2200 0.6 0.39 0.01 1.1 0.014 0.171 0.132

[0079] Due to the constraints of production rhythm and product quality, it is impossible to establish control over combustion from the perspective of time and temperature factors, so the final method is to adjust the hydrogen gas volume fraction in the coal gas. Because the heating atmosphere in the actual heating furnace is a mixed gas composed of coke oven gas produced by coking, blast furnace gas produced by ironmaking, and converter gas produced by steelmaking, the adjustment of the hydrogen gas volume fraction in the coal gas is then transformed into adjusting the ratio of the mixed gas. Since the hydrogen content of coke oven gas in the mixed gas is relatively good, the final adjustment of the ratio of the mixed gas is focused on the proportion of coke oven gas in the mixed gas. The specific adjustment is based on the factors of the two dimensions of the set billet burnout state index and the billet surface quality index. The basic guiding principle is: on the basis of satisfying the predicted burnout amount to be greater than the burnout reference amount, combined with the billet surface quality, the final gas ratio is determined; the billet surface quality index here takes into account the two dimensions of the billet surface quality requirements and the billet actual surface quality state. The burnout state index of the ingot is determined by comparing the predicted burnout value of the ingot with the burnout reference value of the ingot.

[0080] Among them, the process of determining the burning state index is as follows: first, different judgment criteria are set, and different values ​​are assigned to the burning state index according to each judgment criteria; then, based on the burning prediction value of the current ingot and the burning reference value of the ingot, calculation and judgment are performed to determine which judgment criteria the ingot falls into, and the corresponding burning state index of the current ingot is determined according to the judgment result. The process of determining the surface quality index of the ingot is as follows: first, a division of requirement levels is established for the surface quality requirements of the ingot, and the surface quality requirements of the ingots of different requirement levels are assigned separately to form a first array that can characterize different surface quality requirements of the ingot; secondly, a division of state levels is established for the actual surface quality status of the ingot, and the actual surface quality status of the ingots of different state levels are assigned separately to form a second array that can characterize different actual surface quality status of the ingot; thirdly, the first array and the second array are fully arranged and combined to form a third array, and the third array is assigned separately to form a set of surface quality indexes of the ingot; finally, according to the surface quality requirement level of the ingot and the actual surface quality status level of the ingot of the current ingot, the corresponding third array is searched in the third array, and the determination of the surface quality index of the ingot is completed according to the search result.

[0081] The reference value of the burnout of the ingot here is determined by the burnout amount calculated based on the charging temperature of the ingot under the continuous casting → flame cleaning → heating in a heating furnace → rolling process; the predicted value of the burnout of the ingot here is determined by the burnout amount calculated based on the charging temperature of the ingot under the hot delivery and hot charging process.

[0082] Regardless of whether the burnout reference value or the burnout prediction value of the ingot is determined, the iterative calculation cycle is set, starting from the ingot entering the furnace to the end of the ingot leaving the furnace, and the iteration is performed as follows, see Figure 1 :

[0083] SS1: When the billet arrives at the heating furnace entry position, the charging temperature is taken as the initial temperature, combined with the current furnace temperature, and the calculation of the absorbed heat is completed based on the heat radiation equation;

[0084] SS2: Based on the absorbed heat, combined with the one-dimensional heat conduction differential equation, the absolute temperature of the billet surface at the current moment is calculated;

[0085] SS3: The diffusion coefficient of oxygen atoms in iron is calculated based on the absolute temperature of the ingot surface at the current moment, and the oxidation loss at the current moment is calculated based on the calculation results.

[0086] The method for improving slag inclusion on the surface of the ingot based on the above basic design ideas is basically implemented in the following steps:

[0087] 1. During the continuous casting process, the quality status of the ingot is determined and the specific quality status is stored in L2.

[0088] 2. When the ingot arrives at the furnace mouth of the heating furnace, the calculation and determination of the predicted value of ingot burning loss is completed according to the charging temperature under the hot delivery and hot charging process and the predicted time in the furnace. At the same time, the charging temperature of the ingot with the same specification parameters under the continuous casting → flame cleaning → heating in the heating furnace → rolling process is retrieved from the historical data, and the calculation and determination of the reference value of ingot burning loss is completed according to the actual time in the furnace.

[0089] 3. Determine the burning state of the billet according to the billet burning reference value and the billet predicted burning value;

[0090] 4. Obtain the surface quality requirement information of the ingot and the quality status information stored in L2, and determine the surface quality index of the ingot based on the two;

[0091] 5. Determine the mixed gas ratio according to the ingot burning state index and the ingot surface quality index (according to the above analysis, since the final regulation of the mixed gas ratio focuses on the coke oven gas ratio, the determination of the mixed gas ratio here can be specifically expressed as the determination of the proportion of coke oven gas, which will be explained in the following text based on the coke oven gas grade).

[0092] In order to form a clearer understanding of this technical solution, based on the above description, the specific process is described as follows:

[0093] Before the elaboration, it should be noted that: for the convenience of understanding, the above mentioned classification and assignment in the form of grades, and the selection scheme is formed according to the full permutation and combination method. In practice, other selection schemes can also be established, as long as they are constructed under the general control idea. The so-called general control idea is: first determine the surface quality requirements and quality status of the billet; then determine the surface quality index of the billet according to the surface quality requirements and quality status of the billet; then complete the determination of the coke oven gas ratio according to the billet surface quality index and the billet burning status index.

[0094] The specific process according to the technical solution described above is:

[0095] (1) According to the standard heating process of cold billet, the empirical model of oxidation burning is used to calculate the burning loss of the billet surface as the reference value of the burning loss;

[0096] For a given product, the heating process here specifically refers to the furnace temperature reference value of each control section corresponding to different heating times. The standard heating process is the furnace temperature reference value corresponding to the target heating time. Different production lines may have different heating processes for cold-charged and hot-charged ingots. In this method, the heating process is known.

[0097] In this scheme, an empirical formula for oxidation and burning loss is established to evaluate the differences in oxidation and burning loss of billets with different heating processes. Therefore, the empirical formula does not consider the influence of the furnace atmosphere. The empirical formula for oxidation and burning loss is used here:

[0098]

[0099]

[0100] a n : The accumulated burn loss up to the current calculation cycle, unit: mm;

[0101] a n-1 : The cumulative burn loss as of the previous calculation cycle, unit: mm;

[0102] Δa n : The amount of burnout in the current calculation cycle, unit: mm;

[0103] Δt: iterative calculation time, unit: s;

[0104] D (T): The diffusion coefficient of oxygen atoms in the steel billet when the temperature is T.

[0105] The heat conduction differential equations and corresponding boundary conditions involved in the heat diffusion process are as follows:

[0106]

[0107] Here, is the diffusion coefficient of oxygen atoms in the billet when the temperature is T, D0 is the frequency factor, Q is the diffusion activation energy, R is the molar gas constant, T is the absolute temperature of the billet surface during heating, and Δt is the iterative calculation period.

[0108] When calculating the temperature and burnout of the billet corresponding to the heating process, the billet temperature is calculated using a one-dimensional heat conduction partial differential equation. The billet moves at a uniform speed in the furnace, and the corresponding positions of the billet at different times are calculated periodically. The furnace temperature corresponding to the current position of the billet is calculated by linear interpolation along the length direction of the heating furnace according to the process furnace temperature. The comprehensive radiation coefficient is used to calculate the heat radiated from the billet surface, and then the one-dimensional heat conduction partial differential equation is solved to obtain the billet temperature distribution; the surface temperature is taken into the empirical formula for oxidation burnout to calculate the oxidation burnout at the current moment. The final oxidation burnout amount obtained after the iterative calculation of the entire heating time is used as the reference value S of the burnout amount corresponding to the current process. ref .

[0109] (2) Determine the surface quality index of the ingot according to the product surface quality requirements and the quality status of the ingot;

[0110] Obtain the product surface quality requirements and billet quality status in the production plan, and determine the billet surface quality index. The basic idea is that if the surface requirements are high and the quality judgment status is abnormal, a larger burnout amount is required to reduce the risk of slag inclusion on the billet surface. The judgment rules are defined as follows:

[0111] Surface quality requirements Quality determination status Burn target level 0 0 0 0 1 0 0 2 1 1 0 0 1 1 1 1 2 2 2 0 1 2 1 2 2 2 2

[0112] Among them, the surface quality requirements are 0, 1, and 2, which correspond to three levels: no requirements, conventional requirements, and high surface requirements. The actual surface quality status of the billet is output by the inherent billet quality judgment model: the billet quality status values ​​0, 1, and 2 correspond to normal, abnormal, and major abnormal crystallizer parameters, respectively; if the quality status information cannot be obtained from the existing production system, a simple judgment can be made by the corresponding liquid level fluctuation value during billet production, and 0 is taken when the liquid level fluctuation is <3mm; 1 is taken when the liquid level fluctuation is 3mm≤<5mm; 2 is taken when the liquid level fluctuation is ≥5mm. The billet surface quality index values ​​0, 1, and 2 correspond to no surface burnout requirements, normal burnout, and enhanced burnout, respectively.

[0113] (3) When the billet is charged with steel, the predicted value of the burnout is calculated based on the actual charging temperature of the billet and the predicted furnace time, and the burnout state is determined in combination with the reference value of the burnout;

[0114] The calculation method of the predicted value of the burnout is the same as step (1). The initial temperature of the ingot and the heating process are different. Here, the initial temperature of the ingot is T(x, t0) = T0(x) = T charge , T chargeThe temperature is measured for the current hot charging of the ingot. The heating process is the heating process corresponding to the predicted furnace time. The predicted heating time is based on the sum of the steel extraction rhythms of the ingots in the furnace when the current ingots are charged into the furnace. The steel extraction rhythms of different ingots are statistically averaged according to the historical steel extraction rhythms of the corresponding product specifications.

[0115] The predicted value S of the burning loss of the casting billet is calculated cal , compare the burnout reference value S ref ,satisfy:

[0116] 1. |S cal -S ref |≤0.1×,S ref , defined as normal burnout, state is 0;

[0117] 2. S cal <0.9×S ref , defined as underburn, the state is -1;

[0118] 3. S cal >1.1×S ref , defined as overburn, state is 1.

[0119] (4) Determine the coke oven gas grade required for the current casting according to the casting surface quality index and the casting burnout state index;

[0120] Surface quality index Burnout status indicator Coke oven gas grade 0 -1 0 0 0 0 0 1 0 1 -1 2 1 0 1 1 1 0 2 -1 2 2 0 2 2 1 1

[0121] Establish the coke oven gas grade search rules as shown in the table above, where the coke oven gas ratio grade values ​​0, 1, and 2 represent taking the small value, taking the average value, and taking the large value, respectively. The basic principle of gas ratio according to the burn loss is: the higher the burn loss target level, the smaller the current predicted burn loss, the higher the coke oven gas ratio, the higher the H20 volume fraction in the combustion flue gas, and the stronger the high-temperature oxidation capacity. The adjustment amount of the coke oven gas ratio needs to be designed according to the adjustment capacity of the gas mixing station.

[0122] When charging the ingot with steel, the coke oven gas grade is confirmed based on the ingot surface quality index and the burning state index.

[0123] (5) In actual production, the volume fractions of coke oven gas with different proportions are calculated according to the corresponding coke oven gas grades of all ingots in the furnace and the current calorific value of the mixed gas. The weighted average of the volume fractions of the coke oven gas proportions is then taken. Finally, the mixed gas proportion is calculated and sent to the PLC control system of the gas mixing station to adjust the calorific value of the gas.

[0124] In the combustion control process, the coke oven gas ratio required by different ingots in the furnace is obtained periodically, and then weighted average is performed to obtain the current coke oven gas ratio. The blast furnace and converter gas ratios are calculated in combination with the calorific value and the coke oven gas ratio, and sent to the PLC control system of the gas mixing station to adjust the gas calorific value. The weight of the coke oven gas ratio can be designed according to the actual performance of the slag inclusion defect in the on-site production of ingots. The higher the slag inclusion defect, the greater the corresponding weight of the maximum coke oven gas ratio; conversely, if the on-site slag inclusion defect rate is very low, the weight of the maximum coke oven gas ratio is smaller.

[0125] Generally speaking, the dynamic adjustment of the calorific value of coal gas mainly considers the heat load demand. When the heat load demand is high, using a high calorific value mixed coal gas will have a stronger heating and regulation capability. Therefore, the adjustment of the calorific value of the mixed coal gas is not within the scope of this method.

[0126] This technical solution mainly adjusts the gas ratio, and improves the slag inclusion defect on the surface of the billet by increasing the surface oxidation and burning loss during the heating process of the billet. However, in turn, when the amount of on-site oxidation and burning loss is too large, this method can also adjust the gas ratio, reduce the coke oven gas ratio in the mixed gas, and reduce the oxidizing atmosphere in the heating furnace, thereby achieving the purpose of reducing oxidation and burning loss.

[0127] Example

[0128] This technical solution is mainly aimed at the ingots that are not flame cleaned, and by increasing the surface burn loss of the ingot during the heating process, the purpose of improving the slag inclusion defects on the surface of the ingot is achieved. The specific implementation method is as follows:

[0129] Assuming there is a heating furnace, the proportion of the length of the five physical segments to the overall length is shown in the following table. Corresponding to the heating process, the heating time of each control segment is allocated according to the length ratio, and the heating process furnace temperature of each control segment corresponds to the middle position of each control segment:

[0130] Heat recovery section Preheating section OnePlus Two plus Heat evenly 0.250 0.205 0.182 0.182 0.182

[0131] Without loss of generality, assume that there is a 200mm thick steel billet with a surface quality grade of 2. In order to improve production efficiency, save energy and reduce emissions, hot delivery and hot loading are now adopted. The loading temperature is 700℃ and it is assumed that the temperature inside and outside the billet is consistent.

[0132] The design of the gas ratio weight in this embodiment is related to the pain points of the production site. For the slag inclusion quality problem of the ingot to be improved in this case, the coke oven gas will tend to be adjusted to a larger value, that is, the larger the coke oven gas ratio, the larger the corresponding weight; if the pain point on site is to reduce the oxidation and burning of the ingot or reduce the gas cost, because the high calorific value gas cost is high, and the oxidation capacity of the corresponding flue gas in the furnace will generally be enhanced, the larger the coke oven gas ratio required by the ingot at this time, the smaller the corresponding weight will be set. In this case, it is assumed that the corresponding weight design of the coke oven gas ratio is as follows:

[0133] Coke oven gas ratio Weight Small value 1 Median 2 Large value 10

[0134] (1) According to the standard heating process of cold billet, the empirical model of oxidation burning is used to calculate the surface burning amount of the billet as the reference value of the burning amount.

[0135] For a given product, the heating process here specifically refers to the furnace temperature reference value of each control section corresponding to different heating times. The standard heating process is the furnace temperature reference value corresponding to the target heating time. Different production lines may have different heating processes for cold-charged and hot-charged billets. In this method, the heating process is known. Heating processes for cold and hot billets at different times:

[0136] Cold billet heating process:

[0137]

[0138] Hot billet heating process: Corresponding to the cold billet heating process under the same heating time, the corresponding process furnace temperature decreases accordingly for every 100°C increase in the steel charging temperature. Please refer to the table below for details.

[0139] That is, X is rounded to multiples of 100 according to the steel charging temperature.

[0140]

[0141] Corresponding to the empirical formula for oxidation loss:

[0142]

[0143]

[0144]

[0145] Here, is the diffusion coefficient of oxygen atoms in iron, D0 is the frequency factor, and the value in this case is 5.71e-4m 2 ·s -1 , Q is the diffusion activation energy, which is 168.951e3 J·mol -1, R is the molar gas constant 8.31441 J·mol -1 ·K -1 , T is the absolute temperature of the billet surface during heating, K, Δt is the iterative calculation period, s.

[0146] When calculating the temperature and burnout of the billet corresponding to the heating process, the billet temperature is calculated using a one-dimensional heat conduction partial differential equation. The billet moves at a uniform speed in the furnace, and the corresponding positions of the billet at different times are calculated periodically. The furnace temperature corresponding to the current position of the billet is calculated by linear interpolation along the length direction of the heating furnace according to the process furnace temperature. The comprehensive radiation coefficient is used to calculate the heat radiated from the billet surface, and then the one-dimensional heat conduction partial differential equation is solved to obtain the billet temperature distribution; the surface temperature is taken into the empirical formula for oxidation burnout to calculate the oxidation burnout at the current moment. The final oxidation burnout amount obtained after the iterative calculation of the entire heating time is used as the reference value S of the burnout amount corresponding to the current process. ref .

[0147] Here, the 180-min cold billet heating process is taken as the standard cold billet heating process. The surface temperature and burnt thickness curves obtained by iterative calculation are as follows: Figure 2 The burn loss reference value is Sref = 0.782 mm.

[0148] (2) Determine the surface quality index of the ingot according to the product surface quality requirements and the ingot quality judgment status;

[0149] The existing rules for formulating billet surface quality indicators are as follows:

[0150]

[0151]

[0152] The surface quality and quality status information of the ingot is obtained from the hot rolling production plan. Here, the current steel product to be loaded is taken as an example. It is assumed that the surface quality grade requirement of the current steel product to be loaded is 2, and the continuous casting quality status is 1, that is, the surface quality grade requirement is high, and there are fluctuations in the process control process during the continuous casting production of the ingot. From the formulated ingot surface quality index, the corresponding surface quality index is 2, that is, it is necessary to strengthen the burn-in and require a larger burn-in amount, so as to reduce the risk of slag inclusion on the ingot surface.

[0153] (3) When the billet is charged with steel, the predicted value of the burnout is calculated based on the actual charging temperature of the billet and the predicted furnace time, and the burnout state is determined in combination with the reference value of the burnout;

[0154] The calculation method of the predicted value of the burnout is the same as step (1). The initial temperature of the ingot and the heating process are different. Here, the initial temperature of the ingot is T(x, t0) = T0(x) = T charge , T chargeThe temperature is measured for the current hot charging of the ingot. The heating process is the heating process corresponding to the predicted furnace time. The predicted heating time is based on the sum of the steel extraction rhythms of the ingots in the furnace when the current ingots are charged into the furnace. The steel extraction rhythms of different ingots are statistically averaged according to the historical steel extraction rhythms of the corresponding product specifications.

[0155] According to the assumption, the temperature of the steel product to be loaded into the furnace is 700℃, and the predicted time in the furnace is the sum of the steel extraction rhythms of the existing ingots in the furnace, which is calculated to be 158.2min.

[0156] According to the formula:

[0157]

[0158] Get the heating process corresponding to the current hot charging:

[0159]

[0160] According to the interpolation calculation of the heating time, the corresponding reference process, for example, the furnace temperature in the preheating section is:

[0161] Preheating section furnace temperature = 1010 + (980-1010) ÷ (180-150) × (158.2-150) = 1002°C.

[0162] When the existing process time interval is exceeded, the heating process corresponding to the existing process time critical value is taken. The heating process furnace temperature corresponding to the heating time of 158.2 is obtained, and after rounding:

[0163]

[0164] As in step (1), the oxidation loss under the corresponding heating process is iteratively calculated, see Figure 3 .

[0165] Get S cal =0.708mm.

[0166] |S cal -S ref |=|0.708-0.782|=0.074.

[0167] According to the given criteria:

[0168] 1. |S cal -S ref |≤0.1×,S ref , defined as normal burnout, state is 0;

[0169] 2. S cal <0.9×S ref , defined as underburn, the state is -1;

[0170] 3. S cal >1.1×S ref , defined as overburn, state is 1.

[0171] |S cal -S ref |=0.074≤0.1×,S ref =0.0782, it is considered as normal burning, and the burning state is set to 0. In other words, from the existing process, the hot charging temperature is increased, but the current heating time is shortened, so compared with the standard heating process of cold billet, the difference in the surface oxidation of the billet is not much.

[0172] (4) Determine the mixed gas ratio required for the current casting according to the casting surface quality index and the burning state index;

[0173] Surface quality index Burnout status indicator Coke oven gas ratio 0 -1 0 0 0 0 0 1 0 1 -1 2 1 0 1 1 1 0 2 -1 2 2 0 2 2 1 1

[0174] The surface quality index of the ingot is 2, the process burning status index is 0, and the corresponding coke oven gas ratio takes the larger value, marked as 2.

[0175] (5) In actual production, the volume fractions of coke oven gas with different proportions are calculated according to the corresponding coke oven gas grades of all ingots in the furnace and the current calorific value of the mixed gas. The weighted average of the volume fractions of the coke oven gas proportions is then taken. Finally, the mixed gas proportion is calculated and sent to the PLC control system of the gas mixing station to adjust the calorific value of the gas.

[0176] Here we assume that the calorific value ratio of the mixed gas at the gas station is [1800,2500] kcal / Nm 3 , the corresponding coke oven coal gas volume fraction ratio is [0,0.5]. Since this method does not change the calorific value, the gas ratio is calculated based on the calorific value of the current mixed gas. It is assumed that the current mixed gas calorific value is 2100kcal / Nm 3 , the mixed gas composition is as follows:

[0177]

[0178] Taking the increment of 0.05 of coke oven gas, we can get 2100kcal / Nm through iterative calculation. 3 The mixed gas ratios are as follows:

[0179] <![CDATA[Calorific value kcal / Nm 3 > Blast furnace ratio Coke oven ratio Converter ratio Theoretical air-fuel ratio 2100 0 0.05 0.95 1.73 2100 0.1 0.1 0.8 1.78 2100 0.2 0.15 0.65 1.82 2100 0.3 0.2 0.5 1.87 2100 0.4 0.25 0.35 1.92 2100 0.5 0.3 0.2 1.97 2100 0.6 0.35 0.05 2.01

[0180] Therefore, the calorific value of the current mixed gas is 2100 kcal / Nm 3 When the volume fraction of coke oven gas grade is:

[0181] Coke oven gas grade Weight Coke oven gas volume 0 1 0.05 1 2 0.2 2 10 0.35

[0182] Without loss of generality, it can be assumed that the coke oven gas grade, weight removal and volume fraction corresponding to all the ingots in the furnace are as follows:

[0183]

[0184]

[0185] The average volume fraction of coke oven gas is obtained by weighted average:

[0186]

[0187] This is how we get the final ratio information:

[0188] <![CDATA[Calorific value kcal / Nm 3 > Blast furnace ratio Coke oven ratio Converter ratio Theoretical air-fuel ratio 2100 0.3 0.2 0.5 1.87

[0189] The ratio information is transmitted to the gas distribution station to realize the ratio control of the mixed gas. It should be noted here that when the calorific value of the mixed gas remains unchanged or fluctuates slightly, if the current calculated coke oven ratio changes by less than 0.05, it is recommended that the gas distribution station retain the last ratio information unchanged to avoid frequent gas adjustments. In addition, for the heating furnace combustion control, when the ratio of the mixed gas changes, even if the calorific value of the mixed gas remains unchanged, the corresponding theoretical air-fuel ratio will also change. At this time, the basic combustion control needs to adjust the air-fuel ratio or excess air coefficient to ensure the best combustion effect.

Claims

1. A method for improving slag inclusion defects on the surface of a casting, characterized in that: For the ingots under the hot delivery and hot charging process, the slag inclusion defects on the surface of the ingots are improved and regulated by regulating the oxidation and burning loss in the heating furnace before rolling.

2. The method for improving slag inclusion defects on the surface of a casting according to claim 1, characterized in that: The regulation of the amount of oxidation burning of the ingot in the heating furnace is based on the regulation of the combustion medium.

3. The method for improving slag inclusion defects on the surface of a casting according to claim 2, characterized in that: The combustion medium contains hydrogen, and the regulation of the combustion medium is achieved by establishing a regulation of the hydrogen gas volume fraction in the combustion medium.

4. The method for improving slag inclusion defects on the surface of a casting according to claim 3, characterized in that: The combustion medium is a mixed gas, which is composed of coke oven gas produced by coking and blast furnace gas produced by ironmaking, or coke oven gas produced by coking and converter gas produced by steelmaking, or coke oven gas produced by coking, blast furnace gas produced by ironmaking and converter gas produced by steelmaking; Based on this, the regulation of the hydrogen gas volume fraction in the combustion medium is achieved by adjusting the ratio of the mixed coal gas.

5. The method for improving slag inclusion defects on the surface of a casting according to claim 1, characterized in that: The oxidation and burning loss amount of the ingots in the heating furnace is regulated to establish the overall oxidation and burning loss amount regulation for all the ingots in the heating furnace; Specifically: Firstly, the target oxidation burning loss of each ingot is determined according to the burning loss state index and surface quality index of each ingot, and then the overall oxidation burning loss amount of the ingot in the furnace is regulated according to the target oxidation burning loss of each ingot.

6. The method for improving slag inclusion defects on the surface of a casting according to claim 5, characterized in that: The burnout state index is determined according to the burnout prediction value and the burnout reference value of the ingot; The surface quality index is determined according to the actual surface quality state of the ingot, and the overall oxidation and burning loss amount of the ingot in the furnace is regulated according to the target oxidation and burning loss of each ingot, including the following steps: S1: During the continuous casting production process, the actual surface quality state of the ingot is determined, a specific representation of the actual surface quality state of the ingot is formed, and the current surface quality index of the ingot is determined accordingly; S2: When the ingot arrives at the furnace mouth of the heating furnace, the calculation and determination of the predicted value of the ingot burning loss is completed according to the charging temperature under the hot delivery and hot charging process and the predicted time in the furnace, and the charging temperature of the ingot with the same specification parameters under the continuous casting → flame cleaning → heating furnace heating → rolling process is retrieved from the historical data, and the calculation and determination of the reference value of the ingot burning loss is completed according to the actual time in the furnace; and the determination of the ingot burning loss state is completed according to the predicted value of the ingot burning loss and the reference value of the ingot burning loss, so as to form the characterization of the burning loss state index; S3: determining the target oxidation burning of the billet according to the current billet surface quality index value in step S1 and the billet burning state in step S2; S4: Repeat steps S1 to S3 until the target oxidation and burning losses of all the ingots entering the furnace are determined, and according to the target oxidation and burning losses of each ingot, the overall oxidation and burning losses of the ingots in the furnace are regulated.

7. The method for improving slag inclusion defects on the surface of a casting according to claim 6, characterized in that: The surface quality index is determined according to the surface quality requirements of the ingot and the actual surface quality status of the ingot; accordingly, step S1 is specifically as follows: during the continuous casting production process, the actual surface quality status of the ingot is determined to form a specific quality status characterization; and the current ingot surface quality index is determined in combination with the current ingot surface quality requirements.

8. The method for improving slag inclusion defects on the surface of a casting according to claim 6, characterized in that: The determination process of the burnout status index is as follows: First, different judgment criteria are set, and different values ​​are assigned to the burnout status indicators according to the judgment criteria; Then, based on the predicted value of the current ingot's burnout and the reference value of the ingot's burnout, calculation and judgment are performed to determine which judgment criterion the ingot falls into, and the corresponding burnout state index of the current ingot is determined according to the judgment result.

9. The method for improving slag inclusion defects on the surface of a casting according to claim 8, characterized in that: The "setting different determination criteria and assigning different values ​​to the burnout status indicators according to the determination criteria" is specifically: Definition | S cal -S ref |≤0.1×,S ref For normal burning, a judgment standard for whether the burning is normal or not is formed, and a value is assigned to it to form a burning state index value corresponding to the normal burning; Definition S cal <0.9×S ref For under-burning, a judgment criterion for under-burning is formed, and a value is assigned to it to form a burn state index value corresponding to the under-burning; Definition S cal >1.1×S ref For over-burning, a judgment criterion for whether it is over-burning is formed, and a value is assigned to it to form a burn state index value corresponding to the over-burning; In the above inequality, S ref : Reference value of burning loss of ingot, unit: mm; S cal : Prediction value of burning loss of ingot, unit: mm.

10. The method for improving slag inclusion defects on the surface of a casting according to claim 6, characterized in that: The actual surface quality state of the ingot is completed based on the ingot surface quality determination model in the production system or determined based on the crystallizer liquid level fluctuation value during the continuous casting process.

11. The method for improving slag inclusion defects on the surface of a casting according to claim 10, characterized in that: When the actual surface quality state of the ingot is determined based on the liquid level fluctuation value of the crystallizer during the continuous casting process, the specific steps are as follows: first, a liquid level fluctuation interval is established; then, each liquid level fluctuation interval is assigned a value to form a representation of the actual surface quality states of different ingots; finally, the interval into which the actual liquid level fluctuation falls is determined, and the actual surface quality state of the current ingot is determined according to the value of the actual surface quality state of the ingot in the interval.

12. The method for improving slag inclusion defects on the surface of a casting according to claim 6, characterized in that: The surface quality index is characterized by the actual surface quality state of the ingot.

13. The method for improving slag inclusion defects on the surface of a casting according to claim 7, characterized in that: The current process of determining the surface quality index of the ingot is: Firstly, the surface quality requirements of the ingot are divided into different levels, and the surface quality requirements of the ingots with different levels are assigned values ​​respectively, so as to form a first array that can characterize the surface quality requirements of the ingots with different levels; Secondly, the actual surface quality state of the ingot is divided into status levels, and the actual surface quality states of the ingots of different status levels are assigned values ​​respectively, so as to form a second array that can characterize the actual surface quality states of different ingots; Thirdly, the first array and the second array are fully arranged and combined to form a third array, and the third array is assigned values ​​to form a set of surface quality indicators of the casting billet; Finally, according to the surface quality requirement level of the current ingot and the actual surface quality status level of the ingot, the corresponding third array is searched in the third array, and the surface quality index of the ingot is determined according to the search result.

14. The method for improving slag inclusion defects on the surface of a casting according to claim 6, characterized in that: The determination of the reference value and the predicted value of the burning loss of the casting billet is carried out according to the set iterative calculation cycle, from the beginning of the casting billet entering the furnace to the end of the casting billet leaving the furnace, and the following steps are iterated: SS1: When the billet arrives at the heating furnace entry position, the charging temperature is taken as the initial temperature, combined with the current furnace temperature, and the calculation of the absorbed heat is completed based on the heat radiation equation; SS2: Based on the absorbed heat, combined with the one-dimensional heat conduction differential equation, the absolute temperature of the billet surface at the current moment is calculated; SS3: The diffusion coefficient of oxygen atoms in iron is calculated based on the absolute temperature of the ingot surface at the current moment, and the oxidation loss at the current moment is calculated based on the calculation results.

15. The method for improving slag inclusion defects on the surface of a casting according to claim 14, characterized in that: The burnout amount in the current calculation cycle is determined according to the following formula: The cumulative burnout amount obtained after each iterative calculation up to the current calculation cycle is determined according to the following formula: In the above formula, a n : The accumulated burn loss up to the current calculation cycle, unit: mm; a n-1 : The cumulative burn loss as of the previous calculation cycle, unit: mm; Δa n : The amount of burnout in the current calculation cycle, unit: mm; Δt: iterative calculation time, unit: s; D(T): The diffusion coefficient of oxygen atoms in the steel billet when the temperature is T.

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