Method for predicting shrinkage cavity segregation diffusion in continuous casting billet heating process

Abstract

The application discloses a kind of prediction methods of shrinkage cavity segregation diffusion in continuous casting billet heating process, comprising the following steps: 1) using field emission electron microscope to obtain the shrinkage cavity internal microsegregation ratio SR AH Before heating, formula (1) is used to calculate the time t' that satisfies the microsegregation ratio SR AH Before heating;2) according to formula (2), the microsegregation ratio SR BH After heating corresponding to different heating time t at heating temperature is calculated.This method determines the microsegregation degree of the internal continuous casting billet by analyzing the segregation ratio of the microsegregation of the internal shrinkage cavity of the continuous casting billet, so that the appropriate heating process parameters can be determined according to the analysis.The method gives the obtaining and calculation method of specific parameters, the analysis result is accurate and fast, and can effectively provide technical support for the heat treatment process of billet loading process.

Description

TECHNICAL FIELD

[0001] The present application relates to a continuous casting method, in particular to a method for predicting the diffusion of shrinkage segregation in the heating process of continuous casting billets. BACKGROUND

[0002] Due to the different solute distribution coefficients, macro and micro segregation will be formed in the solidification process of liquid steel, and center shrinkage and porosity will be produced under the action of dendrite bridging and solidification shrinkage. The two metallurgical defects of segregation and shrinkage often coexist. Macro segregation will reduce the yield, mechanical properties and corrosion resistance of the billet, and is easy to form cracks. Micro segregation will form banded structure in the subsequent rolling production process, affecting the mechanical properties and mechanical properties of the material. Center shrinkage and porosity will easily expand into cracks in the subsequent rolling production process, seriously affecting the product quality. Therefore, it is of great significance to improve the internal quality of the billet. The heating process of the delivery process of the continuous casting billet is the most effective method to improve the internal micro segregation of the billet, that is, the billet is heated and kept before rolling to make the micro segregation diffuse and homogenize, and reduce the influence of micro segregation of the billet on rolled materials.

[0003] In the currently disclosed patent applications, patent CN201810804789.4 "Casting billet microstructure simulation method" uses the micro segregation and secondary dendrite arm spacing in a certain area inside the casting billet to build an element concentration distribution diffusion model, and then predicts the required heating time and temperature; but this method can only obtain the diffusion time and temperature, and cannot predict the micro segregation ratio after diffusion. Patent CN202010394128.6 "Control method for promoting homogenization treatment of continuous casting billet" uses pulse current to improve the solute atom diffusion mobility in the heating process of continuous casting billet, effectively reducing the intergranular solute segregation; but it only considers the diffusion between dendrites, and does not consider the diffusion of micro segregation inside the shrinkage and the influence of shrinkage on the diffusion of micro segregation. Patent application CN202111134925.1 "High-temperature diffusion heating process for reducing micro zone carbon segregation of axle steel billet LZ50" provides a reasonable heating process to reduce the micro segregation of axle steel billet LZ50, which also does not predict the micro segregation after heating. SUMMARY

[0004] The technical problem to be solved by the present application is to provide a method for predicting the diffusion of shrinkage segregation in the heating process of continuous casting billets.

[0005] To solve the above technical problems, the steps taken by the present application are: including the steps of: 1) using a field emission electron microscope to scan the morphology of the shrinkage to obtain the micro segregation ratio SR AH before heating inside the shrinkage, and using formula (1) to calculate the time t' that satisfies the micro segregation ratio SR AH before heating;

[0006]

[0007] In formula (1), a is a parameter related to steel grade, and is 1+[C] / 2; SR AH is the microsegregation ratio before heating; R is the average diameter of shrinkage, μm; D is the thermal diffusion coefficient, cm 2 / s; t' is the time satisfying the microsegregation ratio SR AH before heating, s;

[0008] 2) calculating the microsegregation ratio SR BH after heating corresponding to different heating time t at the heating temperature according to formula (2);

[0009]

[0010] In formula (1), a is a parameter related to steel grade, and is 1+[C] / 2; SR BH is the microsegregation ratio after heating; R is the average diameter of shrinkage, μm; D is the thermal diffusion coefficient, cm 2 / s; t is the heating time, s; t' is the time satisfying the microsegregation ratio SR AH before heating, s.

[0011] Further, the sample of the continuous casting billet is subjected to layered scanning by using an ultrasonic scanning microscope, and the layered scanning picture is subjected to three-dimensional shrinkage reconstruction, the part with concentrated shrinkage in the sample is found and cut out to form a concentrated shrinkage sample; the concentrated shrinkage sample is subjected to shrinkage morphology scanning by using a field emission electron microscope, and the microsegregation ratio SR AH before heating is obtained.

[0012] Further, in the step 2), when the microsegregation ratio SR BH after heating is ≤1.5, it is considered that the continuous casting billet reaches homogenization.

[0013] The beneficial effects generated by the above technical scheme are that: by analyzing the segregation ratio of the internal microsegregation of the shrinkage of the continuous casting billet, the degree of the internal microsegregation of the continuous casting billet is determined, so that the appropriate heating process parameters can be determined according to the analysis. The present application gives the obtaining and calculation method of the specific parameters, the analysis result is accurate and fast, and can effectively provide technical support for the heat treatment process of the continuous casting billet conveying process. BRIEF DESCRIPTION OF DRAWINGS

[0014] The present application will be further described in detail below in combination with the drawings and specific embodiments.

[0015] Figure 1 is the ultrasonic scanning picture of the microshrinkage of the cross section of the continuous casting billet in the embodiment 1 of the present application;

[0016] Figure 2These are the shrinkage morphology and carbon element surface scan images before heating in Embodiment 1 of the present invention;

[0017] Figure 3 These are the shrinkage morphology and carbon element surface scan images after heating in Embodiment 1 of the present invention;

[0018] Figure 4 SR in Embodiment 1 of the present invention BH Graph showing the change with heating temperature;

[0019] Figure 5 SR in Embodiment 1 of the present invention BH Graph showing the change over heating time. Detailed Implementation

[0020] The method for predicting shrinkage porosity segregation diffusion during the heating process of this continuously cast billet adopts the following process steps: 1) Take a sample at the center of the continuously cast billet and grind the sample; the sample is a rectangular sample with a smooth and flat surface, and the dimensions are length ≤120mm, width ≤100mm, and thickness ≤20mm.

[0021] 2) Use an ultrasonic scanning microscope to perform layered scanning on the above samples, and reconstruct three-dimensional shrinkage cavities from the layered scanning images. Calculate the size range of shrinkage cavities inside the billet, and determine the average equivalent volumetric diameter of the shrinkage cavity, i.e., the average diameter R of the shrinkage cavity.

[0022] 3) Analyze the sample based on the average equivalent volume diameter R to determine the part of the sample where the shrinkage cavity is concentrated; cut out the part of the sample where the shrinkage cavity is concentrated into cubes with a length, width and height not exceeding 15mm, and grind and polish them so that there are no obvious scratches under the field of view of a 200x metallographic microscope, forming a shrinkage cavity concentrated sample.

[0023] 4) The shrinkage cavity samples were scanned using a field emission electron microscope (FET) to examine the shrinkage cavity morphology. Based on the statistically analyzed size range of shrinkage cavities inside the cast billet, representative shrinkage cavities were selected, and surface scanning was used to scan them. The microsegregation ratio (SR) inside the shrinkage cavity was calculated, which is the microsegregation ratio after heating. AH SR is the ratio of the maximum value of microsegregation inside the shrinkage cavity to the minimum value of microsegregation inside the shrinkage cavity. The representative shrinkage cavity is selected from shrinkage cavities within the following size range: located in the middle of the total size range, and the number of shrinkage cavities within this size range accounts for 30% to 50% of the total number of shrinkage cavities, and the average diameter R of the shrinkage cavity is within this size range.

[0024] 5) Calculate the microsegregation ratio SR before heating using formula (1). AH Time t′;

[0025]

[0026] In formula (1), α is a parameter related to the steel grade, which is 1+[C] / 2, where [C] is the carbon content in the continuously cast billet; SR AH R is the microsegregation ratio before heating; R is the average diameter of shrinkage cavities in μm; D is the thermal diffusivity in cm. 2 / s;t′ is to satisfy the microsegregation ratio SR before heating. AH Time, in seconds.

[0027] 6) Calculate the microsegregation ratio SR after heating at different heating times t under different heating temperatures according to formula (2). BH ;

[0028]

[0029] In equation (2), α is a parameter related to the steel grade, which is 1 + [C] / 2; SR BH R is the microsegregation ratio after heating; D is the average diameter of shrinkage cavities in μm; and D is the thermal diffusivity in cm. 2 / s; t is the heating time, in seconds; t′ is the value of the microsegregation ratio SR before heating. AH Time, in seconds.

[0030] 7) The microsegregation ratio SR calculated after heating in the above process. BH The smaller the value, the higher the homogeneity of the cast billet is considered; when heated, the microsegregation ratio SR BH When the ratio is ≤1.5, the billet is considered to have reached the required homogenization level. The heating temperature and holding time at this point are recorded as heating parameters for the subsequent continuous casting billet loading process. Several representative shrinkage cavities can also be selected for scanning calculations in steps 4), 5), and 6) above, respectively. Finally, the microsegregation ratio SR after heating is calculated. BH The average value, using this SR BH The average value is used for prediction.

[0031] Example 1: The method for predicting shrinkage cavity segregation diffusion during the heating process of this continuously cast billet is as follows.

[0032] Taking a certain type of gear steel billet from Baosteel as an example, the composition of this steel (wt) is as follows: C 0.18~0.22%, Si 0.2~0.3%, Mn 1.25~1.35%, P 0~0.15%, S 0.01~0.02%, with the balance being Fe and unavoidable impurities; the cross-sectional dimensions are 320×425mm.

[0033] 1) Take a sample from the center of the continuously cast billet and grind the sample; the sample size is 120mm in length, 100mm in width, and ≤20mm in thickness.

[0034] 2) Perform ultrasonic scanning on the entire cross-section to determine the locations where numerous micro-cavities exist, such as... Figure 1 As shown; the scanning area is reconstructed in three dimensions, and the size range of shrinkage cavities inside the billet is statistically analyzed. The size of the shrinkage cavity is equivalent to the diameter of a sphere with the same volume as the shrinkage cavity, and the average equivalent volume size diameter of the shrinkage cavity is determined to be R = 116 μm.

[0035] The ultrasonic scanning instrument used was a KSI 400E, with a 10Hz probe, a cross-sectional thickness of 20mm, and a scanning depth of 20mm.

[0036] 3) Cut and sample from areas with a large number of micro-cavities (central area), and grind and polish the concentrated cavity sample; the size of the obtained concentrated cavity sample is 15mm in length, 15mm in width and 15mm in height.

[0037] 4) Based on the statistical results from step 2, the shrinkage cavities were found to be 30.24% < 100 μm, 47.56% 100–300 μm, and 22.2% 300–1000 μm. The 100–300 μm range, with the highest percentage, was selected as the representative shrinkage cavity size range. An electron probe microanalysis was used to locate shrinkage cavities on the sample, finding nine cavities at different locations. All cavities were within the aforementioned size range. A surface scan was used to scan each shrinkage cavity, and the microsegregation ratios (SR) before high-temperature diffusion were calculated: 2.469, 2.655, 1.707, 2.024, 2.783, 2.287, 2.457, 2.174, and 2.455, respectively. The morphology of shrinkage cavity #7 and its carbon element surface scan image are shown below. Figure 2 As shown.

[0038] 5) The following calculation is performed using 7# shrinkage cavity as an example. Other shrinkage cavities are calculated using the same process. The preset heating process is: heating temperature 1250℃, heating time 90min. The micro segregation ratio inside the shrinkage cavity after heating is calculated using formula (1).

[0039]

[0040] In the formula, α is a parameter related to the steel grade. For a steel grade with a carbon content of 0.22 wt%, α = 1 + 0.22 / 2 = 1.11; SR AH The value represents the microsegregation ratio before heating; R is the average diameter of shrinkage cavities, 116 μm; D is the thermal diffusivity, which is related to temperature and elements, and the thermal diffusivity of this steel grade at 1250℃ is 291.04 μm. 2 / s;t′ is to satisfy the microsegregation ratio SR before heating. AH The time; according to formula (1), t′=252.8s is calculated.

[0041] 6) Calculate the microsegregation ratio SR after heating at different heating times t under different heating temperatures according to formula (2). BH ;

[0042]

[0043] In equation (2), α is 1.11; SR BH R is the microsegregation ratio after heating; R is the average diameter of the shrinkage cavity, 116 μm; D is the thermal diffusivity, 291.04 μm. 2 / s; t is the heating time, 5400s; t′ is the value of the microsegregation ratio SR before heating. AH The time was 252.8 s. The microsegregation ratio SR after heating was obtained from this example model. BH The value is 1.574. The microsegregation ratio (SR) before heating was measured for each shrinkage cavity. AH The calculated microsegregation ratio SR after heating BH See Table 1 below.

[0044] 7) Following the preset heating process: heating temperature 1250℃, heating time 90min; the concentrating sample with shrinkage cavities was heated, and then the actual microsegregation ratio SR after heating was obtained by electron probe microanalysis. 实际 The morphology and carbon elemental scanning image of shrinkage cavity #7 are shown below. Figure 3 As shown; the results are shown in Table 1;

[0045] Table 1: Microsegregation ratio of the sample in Example 1

[0046] Cavitation 1# 2# 3# 4# 5# 6# 7# 8# 9# SR AH ]] 2.469 2.655 1.707 2.024 2.783 2.287 2.457 2.174 2.455 SR 实际 ]]> 1.504 1.497 1.388 1.684 1.642 1.552 1.604 1.422 1.661 SR BH ]]> 1.502 1.500 1.500 1.506 1.505 1.516 1.538 1.542 1.606 Error % 0.133 -0.200 8.069 -11.620 -8.343 -2.320 -4.115 8.439 -3.311

[0047] Taking the #7 shrinkage cavity as an example, the SR calculated using this method is explained below. BH The value is 1.538, while the actual microsegregation ratio SR after heating, obtained by electron probe surface scanning, is... 实际 The value is 1.604, with an error of -4.115%.

[0048] 8) Several samples can be taken from the same continuously cast billet and analyzed and calculated under different heating processes using steps 1)-5) above. The calculated SR BH By plotting the values ​​against the corresponding heating temperature and heating time, the following can be obtained: Figure 4 SR shown BH graph showing the change with heating temperature Figure 5 SR shown BH Graph showing the change over heating time. Figure 3 This is an elemental distribution diagram of the cavity after heating in this embodiment; as shown. Figure 3 , Figure 4 , Figure 5As shown, when the microsegregation inside the shrinkage cavity is heated at different temperatures, the microsegregation inside the shrinkage cavity diffuses, and at the same time, the microsegregation inside the shrinkage cavity of the billet becomes more uniform (SR). BH The time required varies depending on the value of ≤1.5. Based on this prediction, different heating temperatures and matching times can be selected.

Claims

1. A method for predicting shrinkage porosity segregation diffusion during the heating process of continuously cast billets, characterized in that, The steps include: 1) Using field emission electron microscopy to scan the morphology of the shrinkage cavity and obtain the microsegregation ratio SR of the shrinkage cavity before heating. AH Formula (1) is used to calculate the microsegregation ratio SR before heating. AH Time t′; （1） In equation (1), α is 1 + [C] / 2, where [C] is the carbon content in the continuously cast billet; SR AH R is the microsegregation ratio before heating; R is the average diameter of shrinkage cavities, μm; D is the thermal diffusivity, cm. 2 / s;t′ is to satisfy the microsegregation ratio SR before heating. AH Time, s; 2) Calculate the microsegregation ratio SR after heating at different heating times t under different heating temperatures according to formula (2). BH ; （2） In equation (2), α is 1 + [C] / 2, and [C] is the carbon content in the continuously cast billet; SR BH R is the microsegregation ratio after heating; R is the average diameter of shrinkage cavities, μm; D is the thermal diffusivity, cm. 2 / s; t is the heating time, s; t′ is the value of the microsegregation ratio SR before heating. AH The time, s.

2. The method for predicting shrinkage cavity segregation diffusion during the heating process of continuously cast billets according to claim 1, characterized in that: The continuously cast billet samples were scanned layer by layer using an ultrasonic scanning microscope, and three-dimensional shrinkage cavities were reconstructed from the scanned images to locate and cut out the concentrated shrinkage cavities in the samples, forming shrinkage cavity concentration samples. The shrinkage cavity morphology of the shrinkage cavity concentration samples was scanned using a field emission electron microscope to obtain the microsegregation ratio SR before heating. AH .

3. The method for predicting shrinkage cavity segregation diffusion during the heating process of continuously cast billets according to claim 1 or 2, characterized in that: In step 2), the microsegregation ratio SR after heating BH If the value is ≤1.5, the billet is considered to have achieved homogenization.

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