Effect of continuous casting process on carbon segregation in cast slabs

Research shows that the uneven distribution of carbon during the solidification process of the slab is the main reason for the low-magnification segregation of the slab. The degree of uneven carbon distribution is basically proportional to the carbon content of the steel type. Therefore, the standard deviation and range of the carbon distribution of different steel types are corrected into the carbon distribution segregation of 20CrMoH steel and then unified studied. Under normal production conditions, the pouring superheat is controlled within the range of 15 to 25°C, the secondary cooling ratio is 0.35 L/kg, and the crystallizer and terminal electromagnetic stirring torques are 18 cN·cm and 15 cN·cm respectively. It is beneficial to improve carbon segregation. Using combined parameters for production practice testing, the standard deviation of corrected carbon segregation and the corrected carbon segregation range of the continuous casting billet dropped to below 0. 015% and 0. 025%.

Keywords: continuous casting process; casting billet; carbon segregation

Rolled products usually have varying degrees of low-magnification segregation defects in the cross-section, that is, uneven distribution of chemical components. This defect will have many adverse effects on machining, heat treatment, dimensional stability, safety of use and performance of various applications. The level of segregation defects in rolled materials is mainly determined by the severity of uneven crystallization components on continuous casting billets or steel ingots. Subsequent processing and heat treatment can only alleviate segregation defects but cannot effectively solve the problem. The influencing factors of low-magnification segregation of the cast slab are mainly reflected in the molten steel composition, pouring temperature, pouring speed, cooling rate of the crystallizer and secondary cooling, electromagnetic stirring intensity, etc. This paper mainly studies the influence of continuous casting process on carbon segregation of cast slab.

Research conditions and methods

The production process route of Xingcheng Branch No. 1 is: 100 t ultra-high power electric furnace (U HP) – 100 t secondary refining furnace (LF) – 100 t vacuum degassing furnace (VD) – argon soft blowing – R12 five machines Flowing bloom continuous casting machine – billet hot feeding or cold rolling, in which the continuous casting machine is equipped with slag detection, full protection pouring of molten steel, automatic mold level control, automatic addition of mold slag, and electromagnetic stirring at the mold and end , secondary cooling vaporization cooling and other technologies.

In order to compare the effects of various factors affecting segregation, various parameter adjustment tests can be carried out in a certain stream of the continuous casting machine without significantly deteriorating product quality. The cast slab is managed separately, and low-magnification samples of the most representative positions are cut. The samples are first pickled to check the segregation level of the slab, and then spectral analysis or metal shavings are drilled at different positions on the cross section of the slab. Analyze the segregation of chemical components (Figure 1)

1 – Edge chilled positive segregation zone 2 – Edge white quantity negative segregation zone 3 – Middle columnar positive segregation zone 4 – Middle frame negative segregation zone 5 – Central equiaxed negative segregation zone 6 – Experimental research and analysis sample points.

Figure 1 Schematic diagram of segregation in the cross section of the cast slab

Points 1 to 6 in the figure are respectively 7, 20, 50, 70, 110, and 150 mm away from the edge of the slab. The standard deviation and range are calculated based on the analyzed data to evaluate the degree of segregation.

The steel types involved in this study mainly include Cr, CrMo and CrMnTi series, and the carbon mass fraction is distributed between 0. 15% and 0. 45%. The target melting components representing the requirements of steel types are shown in Table 1. In the actual production process, the deviation of carbon in the melting composition is generally ±0.01%, the deviation of manganese and chromium is generally ±0.02%, and the deviation of other trace elements is ±0.005 %.

Table 1 Target composition (mass fraction) of gear steel smelting

Steel typeCMnCrPSAlTi
20CrMo H0 . 200 . 781 . 15≤0 . 020≤0 . 0080 . 0300 . 010
40Cr0 . 400 . 701 . 00≤0 . 020≤0 . 0080 . 0300 . 015

Research results and discussion

Effect of chemical composition on segregation

In order to analyze the composition segregation of different steel types and different chemical compositions during the continuous casting and solidification process, and for the convenience of using mathematical methods to compare the segregation degrees of different steel types with a unified conversion index. The degree of component segregation of various steel types was compared under similar pouring superheat and drawing speed, similar cooling intensity and electromagnetic stirring conditions. The representative data are shown in Table 2. The data in the table are calculated after selecting representative low-magnification samples of continuous casting billets and measuring the spectral components.

Table 2 Segregation of gear steel components under similar conditions

Steel typeCMnCrSAl
σ/  %δ/  %σ/  %δ/  %σ/  %δ/  %σ/%            δ/  %σ/  %              δ/  %
20CrMo H0 . 0170 . 0440 . 0090 . 0230 . 0110 . 0310 . 00080 . 00130 . 00040 . 00130 . 00350 . 0015
40Cr0 . 0290 . 0850 . 0080 . 0240 . 0100 . 0240 . 00080 . 0033

In Table 2, σ and δ refer respectively to the standard deviation and range values of the spectral chemical composition at the six sampling points on the cross-section of the cast slab as shown in Figure 1. The standard deviation reflects the uniformity of the component distribution, and the range reflects the seriousness of the uneven distribution of the components. It can be seen from the data in the table that the relative deviation of the carbon composition is much larger than that of manganese and chromium. The uneven distribution of carbon solidification components has the greatest impact on the segregation of the cast slab. Sulfur and aluminum are trace elements, and the composition deviation has little effect on low magnification. Test data shows that where the carbon component is positively segregated, other components also basically show positive segregation, and vice versa. Under the same conditions, the size of the carbon component distribution deviation is basically proportional to the melting carbon content of the steel type and has little relationship with other characteristics of the steel type. Therefore, this paper uses carbon composition as a representative to analyze the influencing factors of slab segregation, and corrects the standard deviation and range of carbon segregation according to the carbon content. After correction, the segregation of all steel types is converted into segregation equivalent to 20CrMoH steel for study. The correction formula is:

Corrected carbon segregation standard deviation = carbon segregation standard deviation detection data / (steel type melting carbon composition / 0. 20 %)

Corrected carbon segregation range = carbon segregation range detection data / (steel type melting carbon content / 0. 20 %)

Effects of superheat and drawing speed on carbon segregation in cast slabs

It is generally believed that the temperature of the molten steel poured into the continuous casting tundish and the casting speed have a great influence on the component segregation during the steel crystallization process. The superheat of molten steel pouring increases, the solidification heat dissipation of the slab increases and the shell thickness decreases, resulting in a large temperature gradient across the slab section. Elements that are prone to segregation, such as carbon and sulfur, are conditionally diffused and solidified through selective crystallization, thus forming obvious segregation defects. Under the same pouring superheat, increasing the pulling speed will also increase the solidification heat dissipation and thin the billet shell. The effect is equivalent to increasing the superheat, resulting in an increase in the degree of segregation. When the superheat degree is high during the production process, the general drawing speed will be reduced accordingly. Therefore, under the same drawing speed and superheat degree matching process, the influence of the drawing speed on segregation is not considered.

Under similar cooling intensity and electromagnetic stirring conditions, the effect of superheat on carbon segregation in the slab was analyzed, as shown in Figure 2.

Figure 2 Relationship between pouring superheat and carbon segregation of the slab

从图2可以看出, 中间包浇注钢水过热度在13~32℃内, 修正碳偏析标准偏差波动范围0. 013 %~0. 024 %、修正碳偏析极差波动范围0. 026 %~0. 051 % ,对碳偏析影响不太明显。考虑 到正常生产要求和过热度控制水平,将过热度控制在15~25℃的范围内对控制碳偏析可行。

As can be seen from Figure 2, the superheat of the molten steel poured in the tundish is within the range of 13 to 32°C, the fluctuation range of the corrected carbon segregation standard deviation is 0. 013% to 0. 024%, and the fluctuation range of the corrected carbon segregation range is 0. 026% to 0 . 051 %, the effect on carbon segregation is not obvious. Considering normal production requirements and superheat control level, controlling the superheat within the range of 15 to 25°C is feasible to control carbon segregation.

Effect of continuous casting cooling intensity on carbon segregation of cast slab

Continuous casting cooling intensity mainly includes two factors: the crystallizer cooling water volume and the secondary cooling specific water volume. Generally speaking, increasing the crystallizer cooling water volume does not necessarily improve the cooling effect of the casting shell. Because the initial billet shell is subjected to strong cold shrinkage, the air gap increases, and the billet shell warms up and expands, then approaches the copper wall of the mold, shrinks again, and so on. Therefore, the amount of crystallizer cooling water under safe conditions should not have a great impact on segregation. The data in Table 3 also illustrates this point.

Table 3 Effect of crystallizer cooling water volume on carbon segregation under similar conditions

projectdata
Crystallizer cooling water volume/(m3·h – 1)160180200220
Corrected carbon segregation standard deviation0 . 0160 . 0180 . 0170 . 020
Corrected carbon segregation range0 . 0340 . 0390 . 0370 . 043

The function of the secondary cooling vaporization cooling is to uniformly cool the slab and steadily reduce the temperature of the slab. If the surface temperature of the slab rises significantly, the structure will become coarse and component segregation will intensify. Figure 3 shows the impact of the secondary cooling specific water volume on the carbon segregation of the cast slab. It can be seen that the specific water volume has the best effect at 0.35 L/kg. Below 0.35 L/kg, the degree of carbon segregation increases significantly. Above 0 . 35 L/kg The carbon segregation change is not obvious and will have a negative impact on the surface quality.

Figure 3 The relationship between the secondary cooling water content and the carbon segregation of the slab

Effect of electromagnetic stirring intensity on carbon segregation in cast slab

Electromagnetic stirring is divided into crystallizer electromagnetic stirring and terminal electromagnetic stirring. Crystallizer electromagnetic stirring breaks the dendrites at the solidification front and suspends them in the molten steel. The temperature of most of the broken dendrite fragments is lower than the temperature of the molten steel, so they cannot be dissolved in the steel again. They settle and accumulate at the crystallization front of the cast slab to form fine equiaxed crystals. However, the growth thickness of the billet shell within the electromagnetic stirring distance of the mold is limited, and only a thin layer of chilled fine equiaxed crystals is formed on the surface of the cast billet. After the slab leaves the stirring zone, due to the secondary cooling effect and the high sensible heat of the liquid core, the growth of the slab still produces obvious relatively coarse columnar crystals and segregation areas. Therefore, it is necessary to combine the terminal electromagnetic stirrer to control the crystal structure and segregation defects in the middle part of the billet. If the mixing intensity of the two is not properly matched, the positive and negative carbon segregation areas shown in Figure 1 will appear on the cross section of the slab. In order to study the influence of the two on the carbon segregation of the cast slab, the main effect analysis method was used, and the crystallizer and terminal electromagnetic stirring torque were divided into five levels. They are 10, 12, 18, 23, 32 and 8, 15, 36, 60 and 100 cN·cm respectively. Each factor is paired with each level once for a total of 25 experiments. The main effect data after analysis are shown in Figures 4 and 5.

Figure 4 Effect of crystallizer electromagnetic stirring intensity on carbon segregation under similar conditions

Figure 5 Effect of terminal electromagnetic stirring intensity on carbon segregation under similar conditions

It can be seen from Figures 4 and 5 that electromagnetic stirring has a significant impact on carbon segregation. The crystallizer and terminal electromagnetic stirring torques of 18 and 15 cN·cm respectively are beneficial to improving segregation. Among them, the effect of crystallizer electromagnetic stirring is obviously stronger than that of terminal electromagnetic stirring, rising from 10cN·cm to 32cN·cm, and the correction range of carbon segregation range is 0.018% ~ 0.052%.

Practical results of optimal continuous casting parameters

The combined parameters of superheat degree of 15-25℃, secondary cooling specific water volume of 0.35 L/kg, crystallizer and terminal electromagnetic stirring torque of 18 and 15 cN·cm respectively were selected for production practice testing, and the segregation defects of the continuous casting billet were significantly improved. The standard deviation of corrected carbon segregation and the range of corrected carbon segregation in the continuous casting billet dropped to less than 0.015% and 0.025%, and the contrast of positive and negative segregation chromaticity at low magnification of the billet dropped significantly.

Conclusion

1) The relative deviation of carbon distribution in the cast slab is larger than that of manganese, chromium, sulfur and aluminum. The carbon composition deviation has the greatest impact on low-magnification segregation; the size of the carbon composition distribution deviation is basically proportional to the melting carbon content of the steel type.

2) Under similar cooling intensity and electromagnetic stirring conditions, the degree of carbon segregation change is not obvious within the normal range (13~32℃) of the superheat of molten steel poured in the tundish. Controlling the superheat degree within the range of 15 to 25°C is feasible to control carbon segregation.

3) The amount of cooling water in the crystallizer has little effect on segregation. The amount of secondary cooling water at 0. 35 L/kg is beneficial to reducing carbon segregation. Electromagnetic stirring has a significant impact on carbon segregation. The crystallizer and terminal electromagnetic stirring torques are 18 and 15 cN·cm respectively, which is beneficial to improving carbon segregation.

4) Using combined parameters for production practice testing, the corrected carbon segregation standard deviation and corrected carbon segregation range of the continuous casting billet dropped to below 0.015% and 0.025%, and the segregation defects of the casting billet were significantly improved.

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