Surface defects of billet continuous casting of medium carbon steel and selection of protective slag properties

Abstract: Shougang No. 2 Steelmaking Co., Ltd. uses high-carbon steel type FRK-45 mold flux to produce 160 mm × 160 mm billet continuous casting with a carbon mass fraction of 0. 35% ~ 0. 50%. A large number of longitudinal cracks and transverse pit defects appear on the surface of the continuous cast slab. Technical measures include reducing the casting speed, increasing the alkalinity of the mold slag, slowing down the melting speed of the mold slag, and improving the heat transfer of the slag film between the billet shell and the copper mould tube wall. The surface defects of the cast slab are effectively controlled.

Key words: medium carbon steel; continuous casting; mold slag; surface defects

The 160 mm × 160 mm section billet casting machine of Shougang No. 2 Steelmaking is pouring medium carbon steel with a carbon mass fraction of 0. 35% ~ 0. 50%. Due to the inappropriate performance of the mold powder, a large number of surface defects such as longitudinal cracks and transverse pits appeared on the surface of the slab during the continuous casting process, accounting for 25% of the total. It has a negative impact on subsequent processes and product quality, and also brings great economic losses to the company. In order to reduce the occurrence of longitudinal cracks and transverse pits on the surface of the cast slab, the alkalinity of the continuous casting mold slag is increased, the melting speed of the mold slag is slowed down, and the heat transfer of the slag film between the billet shell and the wall of the copper mould tube is improved, thereby reducing the surface defects of the cast slab. obtain effective control. This article analyzes the mechanisms and characteristics of different surface defects in cast slabs of two steel types, and discusses the influence of steel composition and physical and chemical properties of protective slag on the surface quality of cast slabs.

FRK-45 type and improvement Effect of FRK-45 type mold powder performance on the surface quality of medium carbon steel

Surface defects of typical steel types using FRK-45 mold flux

The composition (mass fraction) of steel grades A and B with longitudinal cracks and pits are shown in Tables 1 to 2.

Table 1 Chemical composition of steel A

CSiMnPSV
0. 41~ 0. 490. 40~ 0. 601. 20~ 1. 50<0. 025>0. 0400. 10~0. 20

Table 2 Chemical composition of B steel

CSiMnPSCr
0. 35~ 0. 451. 70~ 1. 901. 20~ 1. 50<0. 025<0. 0200. 40~ 0. 60

Steel A and B adopt the same continuous casting process: the secondary cooling uses 0.98 L/kg specific water volume, the electromagnetic stirring of the copper mould tube: current 350 A, frequency 5 Hz, and the pulling speed is controlled at a constant pulling speed of 1.65 m/min. .

The statistical results are shown in Table 3: defective cast slabs in steel A accounted for 24.8% of the total, and defective cast slabs in steel B accounted for 25.1% of the total. A typical defect of steel A is that the cast billet has a serious longitudinal depression in the middle of the inner arc side, and the length runs through the entire cast billet. At the same time, there is a longitudinal crack in the middle with a depth of 10~12mm. Its morphology is shown in Figure 1.

Table 3 Statistical results of slab surface inspection

Heat number Steel typeNumber of defects Defect typeHeat number Steel typeNumber of defects Defect type
90C0462A28pits plus longitudinal cracks90C0498B21indentation
90C0461A38pits plus longitudinal cracks90A0434B25Indentation plus transverse cracks
90B0422A17pits plus longitudinal cracks90C0499B191 piece reattached and 18 pieces indented
90A0438A13pits plus longitudinal cracks90B0497B14indentation
90A0437A8pits plus longitudinal cracks90B0496B16indentation
90B0428A16pits plus longitudinal cracks90B0499B24indentation
90C0460A27pits plus longitudinal cracks90C0501B31indentation
total147total150

(a) 4 times; (b) 4 times; (c) 1 times.

Figure 1 Longitudinal cracks and longitudinal depressions on the surface of A steel continuous casting billet

Transverse pits appeared on the surface of steel B billet, and there was a shallow transverse groove in the middle of the pits. When the shallow groove develops deeper, it becomes a transverse crack, and the bottom of the vibration mark is often accompanied by slag entrainment. Its morphology is shown in Figure 2: The vibration mark is the deepest in the middle of the pit. According to on-site measurement, the deepest vibration mark of the casting billet reaches 5~ 6mm. After pickling, transverse cracks appeared at the bottom of the transverse shallow grooves.

(a) 12 times; (b) 4 times.

Figure 2 Transverse depressions on the surface of B steel continuous casting billet and vibration marks on the surface of the cast billet

The effect of improving FRK-45 mold powder on improving the surface quality of medium carbon steel

The 160 mm×160 mm section billet casting machine of No. 2 Steelmaking produces FRK-45 mold flux for 35% of the steels with m([C])>0.35, and its performance has always been relatively stable. The surface quality of the cast slabs produced by high carbon steel grades such as SWRH 82B, GCr15 and 60Si2Mn is good. When pouring steel with a carbon mass fraction in the range of 0. 35% to 0. 50%, the above defects occur.

To this end, the physical and chemical properties of the original mold slag were improved, the alkalinity of the mold slag was increased, the melting speed of the mold slag was adjusted, and the heat transfer uniformity of the slag film was improved. In order to test the performance of two types of mold powder, the original mold powder was used in streams 1 to 4 of the casting machine, and the improved mold powder was used in streams 5 to 8 to compare the surface quality of the slab on site. The test results show that when the original mold powder is used, pits still occur in some cast slabs; when the improved mold powder is used, the slag film of the cast slab is uniform, the surface quality of the slab is good, and there are no pits or longitudinal cracks. Figure 3 shows the morphology of the cast slab after pickling, magnified 4 times. The surface quality of the cast slab is good.

Figure 3 Photos of the surface of the cast slab after pickling after using improved mold powder

Analysis and Discussion

There were different numbers of defective slabs in all heats of the entire pouring of steel A and steel B, and the flow number of the casting machine was not fixed. It can be concluded that when the continuous casting machine equipment and process operation are normal, the surface and subcutaneous quality of the slab depend on the performance of the mold powder. In other words, various defects on the surface and subsurface of the cast slab are closely related to the performance of the mold powder. If you choose a mold powder with suitable performance, you can get a cast slab with no defects on the surface; if you choose it improperly, it will easily cause a lot of defects on the surface of the slab. In general, the mold powder must have good lubrication properties to reduce the friction between the shell and the mold in the mold, thereby reducing the occurrence of longitudinal cracks. The mold slag should have the effect of uniform heat transfer, especially in the upper cross-sectional direction of the mold. To achieve this, the slag film between the upper copper plate of the mold and the shell must be kept uniform.

Mold powder performance and changes in mold heat flow and mold powder consumption

When the casting speed of the continuous casting machine is constant, there is a significant difference in the heat flow derived from the copper mould tube when using the original mold powder and the mold powder with improved performance. It can be seen from Figure 4 that at a pulling speed of 1. 40 m/min, the difference in heat exported by the two types of mold slag copper mould tubes is not too big. At the pulling speeds of 1.60 m/min and 1.80 m/min, after using the improved FRK-45 mold flux, the heat exported from the copper mould tube was 29 MJ/min and 27 MJ/min less than the original heat exported. Figure 5 shows the consumption of mold powder per unit surface area of the slab under the conditions of different casting speeds for the two types of mold powder: at a pulling speed of 1. 4m/min, the consumption of improved FR K-45 mold powder is 0. 32 kg/m2 , the consumption of FRK-45 mold powder is 0. 28 kg/m2; as the drawing speed increases to 1. 65m/min, the consumption of the two types of mold powder decreases to 0. 24 kg/ m2 and 0. 21kg/ m2 respectively; When the pulling speed increases to 1. 85m/min, the consumption of the two types of mold fluxes has almost no change.

Figure 4 Relationship between pulling speed and copper mould tube export heat flow

Figure 5 Relationship between casting speed and mold powder consumption

When the improved mold powder is used at the same pulling speed, the heat exported from the copper mould tube is reduced accordingly. Using the improved mold powder, the consumption per unit area of the casting slab increases, and the thickness of the liquid slag film in the copper mould tube becomes correspondingly thicker, which makes the heat conduction of the copper mould tube slow, thus improving the problem of uneven heat conduction of the copper mould tube.

Molding powder composition and physical and chemical indicators

Judging from the changes in the composition and physical and chemical indexes of the mold powder in Tables 4 and 5, it can be seen that the FRK-45 mold powder is a low melting point, low viscosity, and fast-melting mold powder. When pouring Steel A and Steel B, the pouring temperature is relatively high, the melting speed of the mold slag is too fast, the slag is exposed to the atmosphere prematurely, and the heat loss is large. This causes local molten steel to be supercooled on the upper cross-section of the mold, and the shell at the liquid level shrinks away from the copper mould tube wall to form lateral pits. After increasing the CaO and MgO content in the slag, as m (CaO) / m (SiO2) increases from 0.62 to 0.87, the hemispheric melting point of the mold slag increases by 90 to 140°C. The m (Na2O) in the slag was reduced from 11. 05% to 1. 72%, which delayed the melting speed of the mold slag, extending from the original 29 ~ 34 s to 59 ~ 70 s. At the same time, the m(Al2O3) in the slag was increased from the original 3.80% to 11.98%, thereby increasing the viscosity of the slag. Horst A bat is research believes that when m (Al2 O3) in the slag is above 2%, the viscosity of the slag hardly changes, but when m (Al2 O3) is greater than 10%, the viscosity increases sharply. The viscosity of the improved mold powder changed significantly, from the original 0.47Pa.s to 0.89Pa.s. By controlling m (Al2O3) in the mold flux to more than 10%, higher viscosity mold flux can be obtained.

Table 4 Comparison of mold powder composition (mass fraction)

TrademarkCaOSiO2Al2 O3K 2 ONa2OMgO
FRK-4519. 5231. 183. 800. 1811. 050. 34
Improved FRK-4523. 5126. 9211. 980. 781. 722. 85

Table 5 Comparison of physical and chemical indicators of protective powder

Sample nameHemispheric melting point/℃Melting rate (1350℃)/s Viscosity (1300℃)/(Pa. s) Alkalinity (binary)
FRK-451 050~ 109029~ 340. 470. 62
Improved FRK-451 180~ 125059~ 700. 890. 87

Analysis of solidification mechanism of steel

Judging from the solidification characteristics of steel A and steel B, a peritectic reaction occurs when carbon steel with a carbon mass fraction of 0. 09% ~ 0. 17% is cooled from the liquid phase to 1495°C. δFe(solid) +L(liquid)% YFe(solid). When the δFe+L % YFe transformation occurs, the linear shrinkage coefficient is 9. 8 x10-5/℃. When the mass fraction of carbon is greater than 0.53%, no peritectic reaction δFe % YF occurs when the molten steel solidifies, and the linear shrinkage coefficient is 2×10-5/℃. Steel types with a carbon mass fraction between 0. 17% and 0. 53% are shown in Figure 6 before peritectic reaction occurs. Taking steel with a carbon mass fraction of 0.45% as an example, during the solidification process of molten steel 1 to 2, the content of δFe phase is less than the content of δFe at point J (79.5%), but (0.53- 0. 45) / (0. 53-0. 09) x 100% =20. 9%.

Figure 6 Schematic diagram of solidification of liquid steel in peritectic steel

It is set that the linear shrinkage coefficient of the molten steel in the solidification interval has a linear relationship with the δFe content, then the linear shrinkage coefficient at this time is 4. 6 x 10-5 /℃ (Figure 7). Various alloying elements in the molten steel interact with each other. The interaction coefficients of V and Cr elements in steel A and steel B for C at 1600°C are -13 and -34. These two elements are elements that strongly reduce the carbon activity in steel, causing the amount of δFe precipitated during the solidification process of molten steel to be greater than the relative carbon mass fraction. The actual linear shrinkage falls within the circle shown in Figure 7, which is one of the main reasons for the high incidence of pits and longitudinal crack defects in the cast slab.

Figure 7 Correspondence between the mass fraction of δFe precipitated during solidification of molten steel and the amount of linear shrinkage

The use of weak cooling in the copper mould tube can greatly improve the surface quality of peritectic steel. The mass fraction of carbon is between 0. 17% and 0. 53%, and the shrinkage of molten steel during the solidification process is relatively large. By increasing the alkalinity, melting point, and viscosity of the original mold powder, the melting time of the mold powder becomes longer, and the melting temperature of the mold powder is increased to between 1100 and 1250°C. In this way, the melting speed of the mold slag slows down, and a slag layer and slag layer of appropriate thickness are formed on the molten steel surface to prevent oxidation of the molten steel and heat loss, which has the same effect as weak cooling of the copper mould tube. Similarly, as the viscosity of the mold slag increases, the consumption of the mold slag becomes relatively larger, the thermal resistance between the billet shell and the slag layer becomes relatively larger, and the heat exported is relatively less. This means that the use of improved mold slag will reduce the heat dissipated in the copper mould tube, and the quality of steel billets with severe solidification shrinkage will be greatly improved.

Conclusion

1) When the cast billets with medium carbon steel containing high carbon activity weakening elements (Cr and V), peritectic reaction occurs when the molten steel solidifies, and its linear shrinkage is greater than that of the corresponding carbon content. The use of mold powder with low melting point, low viscosity and fast melting speed is a combination of the uneven thermal conductivity of the four walls of the mold and the large linear shrinkage of the molten steel during solidification. It is the main cause of longitudinal cracks and transverse pits on the surface of the cast slab.

2) Using mold powder with high melting point, high viscosity and low melting speed will increase the consumption of mold powder per unit surface area of the casting slab and increase the thermal conductivity and thermal resistance in the mold. The uniformity of the weak cooling heat conduction of the mold is improved, reducing the occurrence of pits and longitudinal cracks in the cast slab.

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