A test method for improving the surface quality of microalloyed steel continuous casting slab

Abstract

The present application relates to the field of continuous casting and rolling production process, and relates to a test method for improving surface quality of microalloy steel continuous casting slab. The test method for improving surface quality of microalloy steel continuous casting slab directly simulates actual industrial continuous casting and rolling process by using a Gleeble-3800 thermal simulation testing machine, and includes: obtaining a test continuous casting slab; performing overall continuous casting and rolling process simulation experiment on the sample: crystallizer internal heating and holding process, cooling process after the molten steel leaves the crystallizer and cooling process of the steel slab entering the secondary cooling zone, steel slab bending and straightening process; measuring the section shrinkage of the sample after the simulation of continuous casting and rolling, obtaining the section shrinkage-temperature relationship curve of the sample, and determining the easy-cracking critical interval; determining the optimized continuous casting slab straightening temperature interval according to the easy-cracking critical interval; determining the optimized cooling rate V 冷 according to the width of the easy-cracking critical interval 拉 , and taking the optimized cooling rate as the actual industrial continuous casting process parameter.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of continuous casting and rolling production process, and relates to a test method for improving the surface quality of micro-alloy steel continuous casting slab. BACKGROUND

[0002] Thin slab continuous casting and rolling is a revolutionary technology of steel industry developed in the late 1980s and widely used, which has significant advantages in energy saving, cost, benefit, efficiency and investment. In recent years, with the implementation of the steel enterprise's variety strategy, the proportion of micro-alloy steel with typical alloying elements such as Nb, V, Ti and Al has been increasing year by year, and the product structure of thin slab continuous casting and rolling is also expanding. However, micro-alloy steel has high crack sensitivity in the continuous casting process, and often has high incidence of edge transverse cracks, resulting in serious edge curling and edge defects of thin slab hot-rolled coil. How to improve the crack problem in the process of thin slab continuous casting and rolling is one of the key work of the current enterprise production.

[0003] In the smelting of micro-alloy steel, the temperature range of the bending and straightening process of the continuous casting slab produces more cracks due to the influence of deformation, and more carbonitride is precipitated, and the generated ferrite has weak strength, so the process improvement of the straightening part of the casting slab is extremely important for improving the surface performance of the product.

[0004] The prior art directly adjusts the continuous casting parameters and the secondary cooling zone process in actual production to change the cooling speed of the blank to control the thermal plasticity, to prevent the casting blank from cracking and improve the surface quality of the casting blank, which is prone to cause waste of materials, low efficiency and high cost. Therefore, an improved micro-alloy steel continuous casting slab surface quality test method is needed, which does not need to explore the process parameters by directly adjusting the continuous casting parameters and the secondary cooling zone process in actual production, improves the efficiency and reduces the cost. SUMMARY

[0005] In view of the above analysis, the embodiments of the present application aim to provide a test method for improving the surface quality of micro-alloy steel continuous casting slab, to solve the problem that the prior art directly adjusts the continuous casting parameters and the secondary cooling zone process in actual production to prevent the casting blank from cracking and improve the surface quality of the casting blank, which is prone to cause waste of materials, low efficiency and high cost.

[0006] The purpose of the present application is mainly realized by the following technical solutions:

[0007] The present application provides a test method for improving the surface quality of micro-alloy steel continuous casting slab, which directly simulates the actual industrial continuous casting and rolling process by using a Gleeble-3800 thermal simulation testing machine, including the following steps:

[0008] Step 1: obtaining a test continuous casting slab;

[0009] Step 2: using a thermal simulation testing machine to perform a whole continuous casting and rolling process simulation experiment on the sample obtained in step 1, wherein the continuous casting and rolling process simulation experiment comprises: a heating and holding process in a crystallizer in a continuous casting and rolling process, a cooling process after the molten steel leaves the crystallizer, a cooling process of the billet entering a secondary cooling zone, and a bending and straightening process of the billet;

[0010] In the cooling process of the billet entering the secondary cooling zone, the cooling rate V 冷 satisfies:

[0011] V 冷 = (t1-t2) / (L / V 拉 );

[0012] t1 is a second fixed point temperature, in units of ℃, which is determined according to the actual measured temperature of the continuous casting billet when leaving the crystallizer in the actual industrial continuous casting machine production;

[0013] t2 is a straightening temperature of the continuous casting billet, in units of ℃, which is determined according to the actual industrial continuous casting machine parameters;

[0014] L is the length of the secondary cooling zone of the continuous casting machine, in units of m, which is determined according to the actual industrial continuous casting machine parameters;

[0015] V 拉 is the casting speed of the continuous casting billet, in units of m / min, which is determined according to the parameters of the actual industrial continuous casting machine;

[0016] Step 3: measuring the area reduction of the sample after the simulation of continuous casting and rolling to obtain a relationship curve between the area reduction of the sample and the temperature, and determining a critical cracking interval according to the curve, wherein the critical cracking interval is a temperature interval with an area reduction RA≤40%;

[0017] Step 4: determining an optimized straightening temperature interval of the continuous casting billet according to the determined critical cracking interval, wherein the optimized straightening temperature interval of the continuous casting billet is a temperature interval avoiding the critical cracking interval;

[0018] Step 5: determining an optimized cooling rate V 冷 according to the width of the critical cracking interval, wherein the width of the critical cracking interval increases with the increase of the cooling rate V 冷 ;

[0019] Step 6: substituting t1, L, and the optimized t2 and the optimized V 冷 into V 冷 = (t1-t2) / (L / V 拉 ) to determine an optimized casting speed V 拉 , which is used as an actual industrial continuous casting process parameter.

[0020] Further, the step 1 obtains the test continuous casting billet, comprising the following steps:

[0021] S11: Selecting the alloy system and composition required for continuous casting and rolling, carrying out small batch smelting, and obtaining the ingot through charging, melting, refining and pouring;

[0022] S12: After smelting, the ingot is forged into a square billet, and hot rolling is carried out to obtain a test hot-rolled plate.

[0023] Further, the step 1 obtains the test continuous casting billet, comprising: directly sampling the continuous casting billet, sampling in the 1 / 4 thickness direction of the continuous casting plate along the drawing direction, processing into a sample of φ10mm×110mm, and M10mm×10mm at both ends.

[0024] Further, the step 2 comprises:

[0025] S21: Simulating the heating and holding process in the mold in the continuous casting and rolling process: heating the sample at a certain heating rate to a first fixed point temperature at a constant speed, and holding;

[0026] S22: Simulating the cooling process of the molten steel after leaving the mold in the continuous casting and rolling process: cooling the sample at a certain cooling rate to a second fixed point temperature;

[0027] S23: Simulating the cooling process of the billet entering the secondary cooling zone in the continuous casting and rolling process: cooling the sample to a third fixed point temperature at different cooling rates, and taking points every interval Δt during the cooling process;

[0028] S24: Simulating the bending and straightening process of the billet in the continuous casting and rolling process: after the sample reaches the third fixed point temperature, holding, then carrying out a tensile test on the sample at a constant strain rate, and then water cooling or air cooling after the tensile test is completed.

[0029] Further, in step S21, the first fixed point temperature is 1300℃-1350℃, and the holding time is >3min.

[0030] Further, in step S22, the cooling rate is 8℃ / s-15℃ / s.

[0031] Further, in step S22, the second fixed point temperature is 1050-1200℃.

[0032] Further, in step S23, the third fixed point temperature is 650-1100℃, and the Δt is 25℃-50℃.

[0033] Further, in step S24, the holding time is 15-20s.

[0034] Further, in step S24, the constant strain rate is 1×10 -3s -1 .

[0035] Compared with the prior art, the present application can achieve at least one of the following beneficial effects:

[0036] 1. The present application directly simulates the whole continuous casting and rolling process through a thermal simulation testing machine, not only including the bending and straightening process of the billet, but also including the heating and holding process in the crystallizer, the cooling process after the molten steel out of the crystallizer and the cooling process of the billet into the secondary cooling zone, so that the cooling rate V 冷 control and / or the continuous casting billet casting speed control can realize the control of the surface quality of the micro-alloy steel continuous casting slab.

[0037] 2. In the process of simulating the whole continuous casting and rolling through the thermal simulation testing machine, the present application fully considers the parameters in the actual industrial continuous casting machine production, including the temperature when the continuous casting billet out of the crystallizer, the length of the secondary cooling zone, the straightening temperature parameter of the continuous casting billet and the continuous casting billet casting speed of the actual industrial continuous casting machine, designs the relational expression among the multiple parameters, and determines the cooling rate V 冷 of the cooling process of the billet into the secondary cooling zone based on the parameters of the actual industrial continuous casting machine and the parameters in the actual industrial production, solves the problem that the cooling rate V 冷 of the secondary cooling zone cannot be measured in the actual production, and the V 冷 controlled in the thermal simulation experiment can be converted into the value in the actual industry through the formula V 拉 =(t1-t2) / (L / V 冷 ), which is of great significance for guiding the actual production.

[0038] 3. The present application firstly determines the initial straightening temperature of the continuous casting billet according to the parameters of the actual industrial continuous casting machine, and then determines the critical cracking interval, i.e. the straightening temperature interval of the continuous casting billet prone to cracking, according to the relationship curve between the sample section shrinkage rate and the temperature, so as to avoid the straightening temperature interval in the critical cracking interval and obtain the optimized straightening temperature interval of the continuous casting billet, thereby avoiding the surface quality problem caused by the unsuitable straightening temperature.

[0039] 4. The present application determines the cooling rate V 冷 of the cooling process of the billet into the secondary cooling zone based on the parameters of the actual industrial continuous casting machine and the parameters in the actual industrial production, and clearly defines the relationship between the cooling rate V 冷 and the critical cracking interval, i.e. the straightening temperature interval of the continuous casting billet prone to cracking, so as to provide a basis for optimizing the cooling rate V 冷 .

[0040] 5. The present application can determine the optimized casting speed V 拉 based on the optimized straightening temperature and the cooling rate.The drawing speed is taken as an actual industrial continuous casting process parameter, and the surface quality control target can be directly realized through the control of the drawing speed.

[0041] 6、The present application realizes the direct simulation of the continuous casting process, directly guides the process parameters during the continuous casting production, guides the designation of the straightening temperature, so as to reduce the crack occurrence rate and improve the surface quality of the continuous casting blank; the optimal drawing speed of the continuous casting plate when the surface quality required by the industry is met can be predicted, the actual industrial production can be guided according to the simulated optimal drawing speed, so as to improve the production efficiency;

[0042] 7、The present application directly simulates the continuous casting and rolling process through the thermal simulation testing machine, determines the cooling rate in the simulation experiment according to the actual continuous casting machine parameters, adjusts and controls the V 冷 The conversion of the parameters in the thermal simulation experiment into the parameters in the actual industrial production realizes the accurate simulation of the actual industrial production, can be directly used to guide the actual production, and has great significance.

[0043] 8、The present application can be used for the exploration of the finished product composition in the product development through the thermal simulation experiment on the small-size sample by the thermal simulation testing machine, because the thermal simulation experiment is only carried out by using the small-size sample, the expensive and high-yield continuous casting and rolling machine is not needed for production, the cost of the composition exploration stage in the product development can be greatly reduced, and the composition can be adjusted more freely and flexibly.

[0044] The above technical solutions in the present application can be combined with each other to realize more preferred combination solutions. Other features and advantages of the present application will be described in the subsequent specification, and some advantages will become apparent from the specification, or will be understood by implementing the present application. The purposes and other advantages of the present application can be realized and obtained from the contents particularly pointed out in the specification and the drawings. BRIEF DESCRIPTION OF DRAWINGS

[0045] The accompanying drawings are included to provide a further understanding of the application, and are incorporated herein and constitute a part of the detailed description. The drawings illustrate embodiments of the application and, together with the description, serve to explain the principles of the application. In the drawings:

[0046] Figure 1 The thermal simulation process flowchart of the present application method;

[0047] Figure 2 The thermal simulation process flowchart of the present application method;

[0048] Figure 3 The cross-section shrinkage rate curve diagram of the experimental steel under different cooling rates in the embodiment;

[0049] Figure 4The schematic diagram of sampling position of the experimental steel in the examples. DETAILED DESCRIPTION

[0050] The preferred embodiments of the present application will be described in detail below with reference to the drawings, which form a part of this application, and together with the implementation examples of the present application, illustrate the principles of the present application, but not limit the scope of the present application.

[0051] The present application provides a test method for improving the surface quality of micro-alloyed steel continuous casting slab, which directly simulates the actual industrial continuous casting and rolling process by using Gleeble-3800 thermal simulation testing machine, including the following steps:

[0052] Step 1: obtaining a test continuous casting slab;

[0053] Step 2: using a thermal simulation testing machine to perform a whole continuous casting and rolling process simulation experiment on the sample obtained in step 1, wherein the continuous casting and rolling process of the simulation experiment includes: a heating and holding process in the crystallizer in the continuous casting and rolling process, a cooling process after the molten steel leaves the crystallizer, and a cooling process of the slab in the secondary cooling zone, and a bending and straightening process of the slab;

[0054] In the cooling process of the slab in the secondary cooling zone, the cooling rate V 冷 satisfies:

[0055] V 冷 = (t1-t2) / (L / V 拉 );

[0056] t1 is the second fixed point temperature, in ℃, which is determined according to the measured temperature of the continuous casting slab when it leaves the crystallizer in the actual industrial continuous casting machine production;

[0057] t2 is the straightening temperature of the continuous casting slab, in ℃, which is determined according to the parameters of the actual industrial continuous casting machine;

[0058] L is the length of the secondary cooling zone of the continuous casting machine, in m, which is determined according to the parameters of the actual industrial continuous casting machine;

[0059] V 拉 is the casting speed of the continuous casting slab, in m / min, which is determined according to the parameters of the actual industrial continuous casting machine;

[0060] Step 3: measuring the reduction of area of the sample after the simulation of continuous casting and rolling to obtain the relationship curve between the reduction of area and the temperature, and determining the critical cracking interval according to the curve, wherein the critical cracking interval is the temperature interval with the reduction of area RA≤40%;

[0061] Step 4: determining the optimized straightening temperature interval of the continuous casting slab according to the determined critical cracking interval, wherein the optimized straightening temperature interval of the continuous casting slab is a temperature interval avoiding the critical cracking interval.

[0062] Step 5: determining the optimized cooling rate V according to the width of the easy-to-crack critical interval 冷 , wherein the width of the easy-to-crack critical interval increases with the increase of the cooling rate V 冷 ;

[0063] Step 6: substituting the above t1, L, and the optimized t2 and the optimized V 冷 into V 冷 = (t1-t2) / (L / V 拉 ), to determine the optimized drawing speed V 拉 as the actual industrial continuous casting process parameter.

[0064] The existing method directly adjusts the continuous casting parameters and the secondary cooling zone process in actual production to prevent the cracking of the casting blank and improve the surface quality of the casting blank, and the process parameter exploration is extremely easy to cause material waste, low efficiency and high cost; the method of the present application directly simulates the continuous casting and rolling cooling process on small-size samples by a thermal simulation testing machine, realizes the intuitive simulation of the continuous casting process, directly guides the process parameters during the continuous casting production through the simulation results, guides the specification of the straightening temperature, can predict the optimal drawing speed of the continuous casting plate when meeting the required surface quality of the industry, reduces the crack occurrence rate, improves the surface quality of the continuous casting blank, greatly reduces the cost in the composition exploration stage of product development, and improves the production efficiency.

[0065] Specifically, step 1 of obtaining a test continuous casting blank comprises the following steps:

[0066] S11: selecting an alloy system and composition required for continuous casting and rolling, carrying out small-batch smelting, and obtaining a steel ingot through charging, melting, refining and pouring;

[0067] S12: forging the steel ingot into a square billet after smelting, and hot rolling to obtain a hot-rolled plate for testing.

[0068] It should be noted that the test sample required by the present application can also be obtained by sampling the continuous casting blank. If it is obtained by sampling the continuous casting blank, steps S11 and S12 are replaced by:

[0069] Sampling the continuous casting blank according to the requirements of the national standard "GB_T 2975-2018 Steel and Steel Products Sampling Position and Sample Preparation for Mechanical Property Testing". Sampling along 1 / 4 thickness in the slab, sampling along the drawing direction, processing into a sample of φ10mm*110mm, and M10mm*10mm at both ends.

[0070] Specifically, the method of the present application simulates the whole continuous casting process, such as Figure 1As shown, A-B section is to simulate the heat preservation in the crystallizer, B-C section is to simulate the cooling after the molten steel out of the crystallizer, and C-D section is to simulate the cooling in the secondary cooling zone. The sample obtained in step 1 is subjected to the whole continuous casting process simulation experiment by using Gleeble-3800 thermal simulation testing machine, and the section shrinkage of the sample under different experimental conditions is measured, including:

[0071] S21: simulating the heating and heat preservation process in the crystallizer in the continuous casting and rolling process: the sample is heated at a certain heating rate to a first fixed point temperature, and is preserved;

[0072] It should be noted that the heating and heat preservation process in the crystallizer in the continuous casting and rolling process is as shown in A-B section of Figure 1 . The first fixed point temperature is as shown in A point of Figure 1 , which is 1300-1350℃, and should be as high as possible to simulate the heating process in the crystallizer in the continuous casting and rolling process under the condition that the steel grade is allowed and does not affect the subsequent thermal simulation experiment. The heat preservation time is > 3 min, so as to determine that the sample is fully heat preserved and completely solid-solved, thereby better simulating the state of the continuous casting billet out of the crystallizer in the actual industrial production.

[0073] S22: simulating the cooling process after the molten steel out of the crystallizer in the continuous casting and rolling process: the sample is cooled at a certain cooling rate to a second fixed point temperature;

[0074] It should be noted that Figure 1 B-C section is to simulate the cooling process after the molten steel out of the crystallizer in the continuous casting and rolling process. The cooling rate of 8-15℃ / s is determined according to the cooling rate of the billet out of the crystallizer in the production example, and the value can be adjusted according to the actual cooling rate. Figure 1 The temperature of C point is the second fixed point temperature, which is 1050-1200℃, i.e. the temperature t1 of the billet after being cooled out of the crystallizer and entering the secondary cooling zone, and the temperature can be adjusted according to the actually measured temperature entering the secondary cooling zone. When the temperature is reduced to C point, no heat preservation is performed, and the thermal simulation process in C-D section is directly performed to ensure that the whole process simulates the actual continuous casting process as much as possible. Figure 1

[0075] S23: simulating the cooling of the billet entering the secondary cooling zone in the continuous casting and rolling process: the sample is cooled at different cooling rates to a third fixed point temperature; the third fixed point temperature is determined according to the actually measured temperature of the continuous casting billet entering the secondary cooling zone in the actual industrial continuous casting and rolling production.

[0076] It should be noted that Figure 1 C-D section is to simulate the cooling of the billet entering the secondary cooling zone in the continuous casting and rolling process. First, the approximate billet drawing speed range V of the steel grade to be studied is determined according to the continuous casting and rolling production process of the steel grade, and the billet drawing speed range V is determined according to the actual production process of the steel grade.​拉 , determine the temperature range t2 of the steel type under study into the straightening area, determine the length L of the secondary cooling zone of the continuous casting machine. According to the above parameters, the cooling rate V 冷 range is calculated in the hot simulation experiment 冷 The calculation formula of V

[0077] Formula 1: V 冷 = (t1-t2) / (L / V 拉 )

[0078] Wherein, t1 is the second fixed point temperature, ℃;

[0079] t2 is the straightening temperature of the continuous casting billet, ℃;

[0080] L is the length of the secondary cooling zone of the continuous casting machine, m;

[0081] V 拉 is the casting speed of the continuous casting billet, m / min.

[0082] t1, L in formula 1 are actual measured values, so V 冷 is determined by t2, V 拉 According to the type of sample, determine the appropriate t2, V 拉 Parameters, so as to calculate a plurality of V 冷 value as the cooling rate of C-D section, respectively under different cooling rate, the sample is cooled to the third fixed point temperature. Figure 1 The temperature of point D in the formula is the third fixed point temperature, the temperature is any value in the range of 650-1100℃, for example, every interval of 25-50℃ is taken, a series of third fixed point temperatures are determined, such as 650℃, 675℃, 700℃, 725℃, 750℃, 775℃, 800℃, 825℃, 850℃, 875℃, 900℃, 925℃, 950℃, 975℃, 1000℃, 1025℃, 1050℃, 1075℃, 1100℃. The temperature range and the range of taking points can be increased or decreased according to actual needs.

[0083] Because the cooling rate V 冷 of the secondary cooling zone cannot be measured in actual production, the V 冷 controlled in the hot simulation experiment can be converted into the value in actual industry through formula 1, which is of great significance for guiding actual production.

[0084] S24: simulate the bending and straightening process of the billet in the continuous casting and rolling process: after the sample reaches the third fixed point temperature, it is kept warm, and then the sample is stretched at a constant strain rate. After the stretching experiment is finished, it is water cooled or air cooled.

[0085] It should be noted that the holding time is 5-20 s to ensure that the temperature inside and outside the sample is stable, and the holding time should not be too long, otherwise the performance and organization will change. The constant strain rate is generally 1x10 -3 s -1 The constant strain rate does not represent a limitation on the method of the present application, and other constant strain rates can also be applicable to the method of the present application. After the tensile test is completed, if the sample morphology needs to be analyzed, the sample is water-cooled to ensure that the organization is stable, otherwise it can be air-cooled.

[0086] Step 3: Measure the reduction of area of the sample after simulating continuous casting and rolling to obtain a curve of the reduction of area of the sample versus temperature, and determine the critical cracking interval according to the curve, wherein the critical cracking interval is a temperature interval with a reduction of area RA≤40%;

[0087] Measure the reduction of area of the series of temperature tensile samples under different cooling rates to draw a curve of temperature and reduction of area. It should be noted that the hot ductility of the billet is quantified by measuring the reduction of area (RA) of the sample fracture, the lower the reduction of area, the greater the crack tendency of the experimental steel, and the higher the probability of crack defects. RA can be expressed as: RA=(S0-S1) / S0x100%; wherein S0 and S1 are the original cross-sectional area and the cross-sectional area after fracture of the sample, respectively.

[0088] Step 4: Determine the optimized continuous casting billet straightening temperature interval according to the determined critical cracking interval, wherein the optimized continuous casting billet straightening temperature interval is a temperature interval that avoids the critical cracking interval;

[0089] The formation of surface cracks of H-shaped steel or other shaped steel and plate material and the straightening temperature in the continuous casting and rolling process are in the third brittle zone, and there is a clear relationship between the poor plasticity and toughness at this time. Therefore, improving the plasticity and toughness of the third brittle zone of the steel continuous casting plate, controlling the straightening temperature, and avoiding the area with low plasticity and toughness in the third brittle zone as much as possible will help to improve the surface quality of the steel continuous casting plate. In the present application, through the hot simulation tensile test, the range of the third brittle zone of the steel continuous casting plate under different cooling rates and the corresponding plasticity and toughness performance can be directly obtained, which is helpful for selecting a reasonable straightening temperature in the subsequent actual production to avoid the area with low plasticity and toughness in the third brittle zone. The present application method obtains the relationship between the sample temperature and the reduction of area under different pulling speeds by analyzing the reduction of area curve, so that the straightening temperature can be set according to the curve to avoid the third brittle zone. Generally, the reduction of area less than 40% (RA≤40%) is set as the brittle zone. According to the type of continuous casting billet, a suitable straightening temperature is selected to avoid the temperature interval with a reduction of area ≤40%.

[0090] Step 5: Determine the optimized cooling rate V according to the width of the critical cracking interval冷 wherein the width of the critical easy-to-crack temperature interval increases with the increase of the cooling rate V 冷 ;

[0091] The temperature interval where the section shrinkage rate in the observation curve is less than or equal to 40% is the critical easy-to-crack temperature interval, and the straightening temperature is selected according to the determined critical easy-to-crack temperature interval and in combination with the actual production of the continuous casting machine and the product type, and the straightening temperature needs to avoid the critical easy-to-crack temperature interval; the optimized cooling rate V 冷 wherein the width of the critical easy-to-crack temperature interval increases with the increase of the cooling rate V 冷 , and the appropriate cooling rate is selected.

[0092] Step 6: substituting t1, L, the optimized t2 and the optimized V 冷 into V 冷 = (t1-t2) / (L / V 拉 ), to determine the optimized withdrawal speed V 拉 as the actual industrial continuous casting process parameter.

[0093] The present application directly simulates the whole continuous rolling and casting process by the thermal simulation testing machine, which not only includes the bending and straightening process of the billet, but also includes the heating and holding process in the crystallizer, the cooling process after the molten steel is out of the crystallizer and the cooling process of the billet in the secondary cooling zone in the continuous rolling and casting process, so that the surface quality of the micro-alloy steel continuous casting slab can be controlled from the cooling rate V

[0094] Embodiment

[0095] The present application is further described below by taking the hot-rolled thick-wall H-shaped steel produced by the continuous casting process in Ma Steel as an example. The chemical composition of the hot-rolled thick-wall H-shaped steel is shown in Table 1.

[0096] Table 1 Chemical composition of the embodiment (mass fraction, %)

[0097]

[0098] The specific steps include:

[0099] Step 1: obtaining the test continuous casting billet, including:

[0100] S1: Select the hot-rolled thick-walled H-shaped steel produced by Ma Steel. Since the hot-rolled thick-walled H-shaped steel is prone to corner crack during production, the sampling position is required to be at 1 / 4 of the thickness direction according to the requirements of GB_T 2975-2018 Steel and Steel Product Mechanical Property Test Sampling Position and Sample Preparation, as shown in the position shown in Figure 4

[0101] S2: Process the hot simulation tensile sample from the specified sampling position along the direction of the blank, with a size of φ10mmx110mm, and M10mmx10mm threads at both ends, a total of 3 groups of samples, 12 each, a total of 36.

[0102] Step 2: Use Gleeble-3800 thermal simulation testing machine to simulate the whole continuous casting process of the 3 groups of samples obtained in step 1, and measure the reduction of area of the samples under different experimental conditions, as shown in Figure 2 , the specific process is as follows:

[0103] S21: Simulate the heating and holding process in the mold in the continuous casting and rolling process: according to the characteristics of the H-shaped steel used, heat the 3 groups of samples to the first set point temperature of 1350℃ at a rate of 10℃ / s in a vacuum environment and hold for 3min to coarsen the grains and dissolve the micro-alloy carbonitride;

[0104] S22: Simulate the cooling process of the molten steel after leaving the mold in the continuous casting and rolling process: cool the 3 groups of samples to the second set point temperature of 1150℃ at a cooling rate of 10℃ / s to ensure that all vanadium carbonitride in the steel is completely dissolved at point C;

[0105] S23: Simulate the cooling condition of the billet entering the secondary cooling zone in the continuous casting and rolling process:

[0106] According to the data of the steel plant, it is known that the H-shaped steel bloom continuous casting machine parameters of the steel plant are that the length L of the secondary cooling zone of the continuous casting machine is 7.1m, the straightening point temperature t2 is 850℃, and the range of the drawing speed V 拉 is generally 0.3-1.5m / min. Substituting the formula: V 冷 =(t1-t2) / (L / V 拉 ), the approximate range of the cooling rate is calculated to be 0.20℃ / s-1.06℃ / s, which is used as a reference in the thermal simulation test of the present embodiment. Three cooling rates are set, which are 0.2℃ / s, 0.5℃ / s and 1.0℃ / s.

[0107] Cool the 12 samples of the first group to the third set point temperature D point at a cooling rate of 0.2℃ / s; take points every 25℃-50℃, and the third set point temperature is set to 690℃, 725℃, 750℃, 775℃, 800℃, 825℃, 875℃, 900℃, 925℃, 950℃, 925℃ and 1000℃, respectively;​

[0108] The 12 samples in the second group were cooled to a third set temperature D point at 0.5°C / s; every 25-50°C was taken as a point, and the third set temperature was set to 690°C, 725°C, 750°C, 775°C, 800°C, 825°C, 875°C, 900°C, 925°C, 950°C, 925°C, and 1000°C, respectively;

[0109] The 12 samples in the third group were cooled to a third set temperature D point at 1.0°C / s; every 25-50°C was taken as a point, and the third set temperature was set to 690°C, 725°C, 750°C, 775°C, 800°C, 825°C, 875°C, 900°C, 925°C, 950°C, 925°C, and 1000°C, respectively;

[0110] S24: simulate the bending and straightening process of the billet in the continuous casting and rolling process: after the three groups of samples were cooled to the third set temperature, they were kept for 5s to ensure temperature stability. The samples were stretched at a constant strain rate of 1×10 -3 s -1 The samples were stretched until they were broken, and the broken samples were taken out and naturally cooled to room temperature in air; the reduction of area (RA) of the fracture was measured, RA=(S0-S1) / S0×100%; wherein S0 and S1 are the original cross-sectional area and the cross-sectional area after breaking of the sample, respectively, and the unit is mm 2 .

[0111] Step 3: measure the reduction of area of the sample to obtain a curve of the reduction of area of the sample versus temperature, as shown in Figure 3 , wherein the horizontal axis temperature is the different third set temperatures;

[0112] According to the curve, the critical cracking interval is determined, and the critical cracking interval is the temperature interval in which the reduction of area RA is less than or equal to 40%; it can be seen from Figure 3 that the critical cracking interval at 1°C / s is 734°C-828°C, at 0.5°C / s is 745°C-793°C, and at 0.2°C / s is 744°C-782°C.

[0113] Step 4: determine the optimized straightening temperature interval of the continuous casting billet according to the determined critical cracking interval, wherein the optimized straightening temperature interval of the continuous casting billet is a temperature interval that avoids the critical cracking interval;

[0114] Step 5: determine the optimized cooling rate V 冷 according to the width of the critical cracking interval, wherein the width of the critical cracking interval increases with the increase of the cooling rate V 冷 ;

[0115] FromFigure 3 As can be seen from the figure, the critical temperature interval of easy cracking is 734℃-828℃ at 1℃ / s, 745℃-793℃ at 0.5℃ / s, and 744℃-782℃ at 0.2℃ / s. The width of the critical temperature interval of easy cracking increases by 56℃ as the cooling rate increases from 0.2℃ / s to 1.0℃ / s. It can be seen that the higher the cooling rate, the wider the critical temperature interval of easy cracking. When determining the straightening temperature in combination with the type of continuous casting billet, the temperature interval of easy cracking must be avoided.

[0116] Step 6: substituting t1, L, and the optimized t2 and the optimized V 冷 into V 冷 = (t1-t2) / (L / V 拉 ), to determine the optimized casting speed V 拉 as an actual industrial continuous casting process parameter.

[0117] In this embodiment, by Figure 3 It can be seen that the thermal plasticity decreases significantly when the temperature decreases from 900℃ to 850℃. According to the graph, the part with RA above 40% can ensure that the product has good surface quality. After selecting the appropriate straightening temperature according to the type of continuous casting billet and the actual production conditions of the factory, selecting the appropriate secondary cooling rate, and determining the optimized casting speed V 冷 = (t1-t2) / (L / V 拉 ) in reverse, the optimal casting speed is determined and applied to the actual continuous casting process to guide actual production.

[0118] For example, as can be seen from the figure, the temperature interval of easy cracking of the billet is 744℃-782℃ at a cooling rate of 0.2℃ / s, so the setting of the straightening temperature must avoid this temperature interval. If the cooling rate is 0.2℃ / s, the length L of the secondary cooling zone of the continuous casting machine is 7.1m, and the straightening temperature is 850℃, the appropriate casting speed can be calculated as 0.28m / min. If the straightening temperature can be maintained at 900℃ in actual production, the appropriate casting speed is 0.34m / min. At this cooling rate, the casting speed is small, which can easily lead to low production efficiency in actual production, and is not suitable.

[0119] For example, as can be seen from the figure, the temperature interval of easy cracking of the billet is 744℃-782℃ at a cooling rate of 0.2℃ / s, so the setting of the straightening temperature must avoid this temperature interval. If the cooling rate is 0.2℃ / s, the length L of the secondary cooling zone of the continuous casting machine is 7.1m, and the straightening temperature is 850℃, the appropriate casting speed can be calculated as 0.28m / min. If the straightening temperature can be maintained at 900℃ in actual production, the appropriate casting speed is 0.34m / min. At this cooling rate, the casting speed is small, which can easily lead to low production efficiency in actual production, and is not suitable.

[0120] For example, from the figure, it can be seen that when the cooling rate is 1.0 ℃ / s, the temperature range in which the billet is easy to crack is 734 ℃-828 ℃, and therefore the setting of the straightening temperature must avoid this temperature range. If the cooling rate of 1.0 ℃ / s is selected, the length L of the secondary cooling zone of the continuous casting machine is 7.1 m, and the straightening temperature is 850 ℃, the appropriate casting speed can be calculated to be 1.42 m / min, but at this time the section shrinkage at this temperature is 43%, which is too close to the requirement of RA≤40%, and therefore this casting speed is not recommended for production, and surface quality problems are likely to occur. If the straightening temperature can be maintained at 900 ℃ in actual production, the appropriate casting speed is 1.70 m / min, which can be used for actual production.

[0121] The present application directly simulates the cooling process of continuous casting and rolling by a thermal simulation testing machine, realizes direct simulation of the continuous casting process, and can directly guide the process parameters in the continuous casting production and the designation of the straightening temperature according to the simulation results, which is of great significance for improving the surface quality of the billet.

[0122] The above merely describes the preferred embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can easily think of changes or replacements within the technical scope disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A test method for improving the surface quality of microalloyed steel continuously cast slabs, characterized in that, The actual industrial continuous casting and rolling process was directly simulated using the Gleeble-3800 thermal simulation testing machine, including the following steps: Step 1: Obtain the test continuous casting billet; Step 2: Use a thermal simulation testing machine to conduct an overall continuous casting and rolling process simulation experiment on the sample obtained in Step 1. The continuous casting and rolling process of the simulation experiment includes: the heating and heat preservation process in the crystallizer in the continuous casting and rolling process, the cooling process after the molten steel exits the crystallizer, the cooling process of the billet entering the secondary cooling zone, and the bending and straightening process of the billet. During the cooling process of the steel billet entering the secondary cooling zone, the cooling rate V 冷 satisfy: V 冷 =(t1-t2) / (L / V 拉 ); t1 is the second fixed point temperature, in °C, which is determined based on the actual measured temperature of the continuously cast billet exiting the crystallizer during actual industrial continuous casting machine production. t2 is the straightening temperature of the continuously cast billet, in °C, which is determined based on the actual parameters of the industrial continuous casting machine; L is the length of the secondary cooling zone of the continuous casting machine, in meters, and is determined based on the actual parameters of the industrial continuous casting machine. V 拉 This refers to the billet drawing speed in continuous casting, expressed in m / min, and is determined based on the parameters of the actual industrial continuous casting machine. Step 3: Measure the reduction of area of ​​the sample after simulated continuous casting and rolling to obtain the relationship curve between the reduction of area and temperature. Determine the critical range for easy cracking based on the curve, wherein the critical range for easy cracking is the temperature range where the reduction of area RA ≤ 40%. Step 4: Determine the optimized straightening temperature range for the continuously cast billet based on the determined critical cracking range, wherein the optimized straightening temperature range for the continuously cast billet is a temperature range that avoids the critical cracking range. Step 5: Determine the optimal cooling rate V based on the width of the critical cracking range. 冷 The width of the critical cracking region varies with the cooling rate V. 冷 It increases with the increase of; Step 6: Combine the above t1, L, optimized t2, and optimized V. 冷 Substitute V 冷 =(t1-t2) / (L / V) 拉 Determine the optimal throwing speed V. 拉 , as the actual industrial continuous casting process parameters.

2. The method according to claim 1, characterized in that, Step 1, obtaining the test continuous casting billet, includes the following steps: S11: Select the alloy system and composition required for continuous casting and rolling, carry out small-batch smelting, and obtain steel ingots through charging, melting, refining and casting; S12: After smelting, the steel ingot is forged into a square billet, hot-rolled, and used to obtain a hot-rolled plate for testing.

3. The method according to claim 1, characterized in that, Step 1, obtaining the test continuous casting billet, includes: directly taking samples from the continuous casting billet, taking samples along the casting direction in the 1 / 4 thickness direction of the continuous casting plate, and processing them into φ10mm×110mm samples with M10mm×10mm at both ends.

4. The method according to claim 1, characterized in that, Step 2 includes: S21: Simulates the heating and heat preservation process in the crystallizer of continuous casting and rolling process: The sample is heated to the first fixed point temperature at a certain heating rate and then held at that temperature. S22: Simulates the cooling process of molten steel after exiting the crystallizer in continuous casting and rolling process: the sample is cooled to the second fixed point temperature at a certain cooling rate. S23: Simulates the cooling process of steel billet entering the secondary cooling zone in continuous casting and rolling process: The sample is cooled to the third fixed point temperature at different cooling rates, and points are taken at intervals of Δt during the cooling process. S24: Simulate the billet bending and straightening process in continuous casting and rolling: After the sample reaches the third fixed point temperature, it is kept at the temperature, and then a tensile test is performed on the sample at a constant strain rate. After the tensile test is completed, it is water-cooled or air-cooled.

5. The method according to claim 4, characterized in that, In step S21, the first fixed-point temperature is 1300℃~1350℃, and the heat preservation time is >3min.

6. The method according to claim 5, characterized in that, In step S22, the cooling rate is 8°C / s to 15°C / s.

7. The method according to claim 6, characterized in that, In step S22, the second fixed-point temperature is 1050-1200℃.

8. The method according to claim 7, characterized in that, In step S23, the third fixed-point temperature is 650-1100℃, and the Δt is 25℃-50℃.

9. The method according to claim 8, characterized in that, In step S24, the heat preservation time is 15-20 seconds.

10. The method according to claim 9, characterized in that, In step S24, the constant strain rate is 1×10⁻⁶. - 3 s -1 .

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