Analysis of influencing factors on internal cracks in billets of medium and low carbon steel in continuous casting

Abstract: An analysis of the influencing factors of internal cracks in the continuous casting of medium and low carbon steel billets produced by a company shows that controlling the carbon content according to the upper limit of process requirements to avoid the peritectic reaction zone is beneficial to reducing the occurrence of internal cracks in the billet. . Reducing the sulfur content of molten steel or increasing the manganese-sulfur ratio increases the critical strain value of steel and reduces the occurrence of internal cracks in the cast slab. Low superheat, high drawing speed and reduced secondary cooling strength inhibit the development of columnar crystals and are conducive to expanding the formation of equiaxed crystals, thereby reducing the probability of internal cracks in the slab.

Keywords: continuous casting; medium and low carbon steel; billet; internal cracks

Introduction

Cracks that appear from subcutaneous to the center of the cast slab are internal cracks. Because cracks occur during the solidification process, they are also called solidification cracks. The company produces billets using a production process of 120t top-bottom combined blowing converter → 120t LF refining furnace → full-arc continuous straightening billet caster. When producing a medium-low carbon steel billet with a cast section of 150 mm × 150 mm, internal cracks were found in part of the cast billet from the low-magnification sample. This article combines production practice to analyze the influencing factors of internal cracks in continuous casting billets, and obtains control measures to reduce the occurrence rate of internal cracks.

Analysis of factors affecting internal cracks in cast slabs

The incidence rate of internal cracks in the slab is defined as: the number of samples with internal cracks occurring within a certain range as a percentage of the total number of samples within that range.

Analysis of the influence of molten steel composition on internal cracks in cast slabs

Effect of carbon content on internal cracks in cast slabs

The process control requirements for carbon content are between 0.13% and 0.22%. The relationship between carbon content and crack occurrence rate is shown in Figure 1. It can be seen from the figure that as the carbon content increases, the crack occurrence rate decreases. The crack incidence rate is highest when the carbon content is between 0.13% and 0.16%. When the carbon content is greater than 0.16%, the crack incidence rate is significantly reduced.

Figure 1 Relationship between carbon content and crack occurrence rate

This is because the carbon content is exactly in the peritectic reaction zone when it is 0.13%~0.16%. At this time, as the temperature decreases, the newly solidified billet shell on the meniscus of the mold undergoes a δFe→yFe transformation. The volume shrinkage caused by the phase change causes uneven local heat transfer and solidification of the cast billets, thus forming depressions. The cooling rate and solidification rate of the depressed parts are slower than those of other parts, and the structure is coarsened and more sensitive to cracks. After the shell comes out of the copper mould tube, it shrinks after being sprayed by external secondary cold water. Coupled with the thermal expansion of the internal uncrystallized molten steel, stress concentration occurs in the weak areas of the depression, resulting in internal cracks. Therefore, avoiding the peritectic reaction zone can reduce the probability of internal cracks in the slab.

Effect of sulfur content and manganese-sulfur ratio on internal cracks in cast slabs

The relationship between the sulfur content in steel and the incidence of cracks is shown in Figure 2.

Figure 2 Relationship between sulfur content and crack occurrence rate

When the sulfur content is less than 0.01%, the crack incidence rate is 0. As the sulfur content in the steel increases, the crack incidence rate increases rapidly. When the sulfur content is greater than 0.02%, the crack incidence rate reaches 37.3%.

When a material is subjected to an external force, if the strength limit or strain limit of the material is insufficient to withstand the force, the material’s safety will be reduced or even fail. The material’s own characteristics determine the strength limit or strain limit of the material. In continuous casting production, if the critical strain value of steel is reduced, it is easy to cause the slab to crack. Many researchers have measured critical strain values for different steel types. Although the methods used are different, the rules obtained are consistent. This rule shows that an increase in the sulfur content in steel or a decrease in the manganese-sulfur ratio will cause the critical strain value of the steel to decrease sharply.

Increased sulfur content in molten steel will affect the properties of the steel. The sulfur in the molten steel easily combines with Fe to form low-melting type II sulfide FeS during the solidification process, and is distributed at the grain boundaries, causing grain boundary brittleness, greatly reducing the critical strain value of the steel, and reducing the strength and ductility of the steel. , eventually causing internal cracks in the cast slab. Therefore, the lower the sulfur content, the less chance of cracking.

However, although the lower the sulfur content, the smaller the chance of cracks, the pursuit of low sulfur content will inevitably increase production costs and reduce the price competitiveness of the product. Therefore, while controlling the sulfur content in steel to meet process requirements, consider increasing the manganese-sulfur ratio to further reduce the probability of cracks.

The relationship between the manganese-sulfur ratio in steel and the crack occurrence rate is shown in Figure 3, in which the sulfur content is distributed within the range of process requirements. It can be seen from the figure that when the manganese-sulfur ratio is less than 10, the crack incidence rate is as high as 66.67%. As the manganese-sulfur ratio increases, the crack rate incidence rate decreases rapidly. When the manganese-sulfur ratio is greater than 30, the crack rate incidence rate drops to 14.29%. .

Figure 3 Relationship between manganese-sulfur ratio and crack occurrence rate

When the manganese-sulfur ratio in steel increases, the melting point of MnS formed is as high as 1610°C. MnS in the form of rods is distributed on the grain boundaries, which helps to improve the degree of embrittlement of the grain boundaries and enhances the critical strain value of the steel, thereby effectively preventing the occurrence of cracks. This is also the reason for increasing the manganese-sulfur ratio. It can be seen that reducing the sulfur content or controlling the manganese content according to the upper limit of process requirements can enhance the critical strain value of steel and reduce the probability of cracks.

Analysis of the influence of the solidification structure of the cast slab on the internal cracks of the cast slab

The influence of the solidification structure of the cast slab on the formation of internal cracks in the cast slab

The solidification structure of the cast slab is composed of small equiaxed crystal bands, columnar crystal bands and central equiaxed crystal bands from the edge to the center. Not only are columnar crystals and central equiaxed crystals different in their own structures, but their influence on the surroundings is also different, which makes them significantly different in their impact on the comprehensive properties of steel.

When the columnar crystals are fully developed, they form a transgranular structure, which is prone to internal quality problems such as loose centers and reduces the density of steel. More importantly, due to the deposition of impurities in the columnar crystals, a weak surface is formed at the interface between the columnar crystals, which becomes a location where cracks can easily propagate. The equiaxed crystal structure is dense, has no obvious directional anisotropy, segregation and inclusions are easily dispersed, has high strength, plasticity, and toughness, has good processing performance and stability, and is not prone to concentrated damage. Therefore, suppressing columnar crystals and developing equiaxed crystals is a measure to reduce internal cracks.

Effect of superheat on internal cracks of cast slab

The relationship between superheat degree and crack occurrence rate is shown in Figure 4. It can be seen from the figure that the greater the degree of superheat, the higher the occurrence rate of cracks. When the superheat is below 30°C, the crack incidence rate is 5.56%, and when the superheat exceeds 30°C, the crack incidence rate increases significantly.

Figure 4 Relationship between superheat and crack occurrence rate

The solidification of continuous casting molten steel undergoes a crystallization process of nucleation and nucleation growth. The superheat and temperature gradient of the molten steel affect the nucleation rate and crystal growth rate in this process. They have the following relationship:

In the formula: A and B are constants; △T is the superheat of molten steel, ℃; Tc is the temperature of molten steel, ℃; Ts is the metal solidification temperature, ℃.

It can be seen from formula (1) that the nucleation rate 1 is inversely proportional to the superheat degree ΔT of molten steel. The smaller the superheat degree, the higher the nucleation rate of molten steel solidification, which is more conducive to the formation of equiaxed crystals. At the same time, it can be seen from formula (2) that the larger the Tc-Ts, the faster the nucleated unit cells will grow forward and the more developed the columnar crystals will be. Since the pouring temperature Tc of molten steel and the degree of superheat △T have a relationship of Tc=△T+T₁ (T₁ is the liquidus temperature of molten steel). Therefore, in the continuous casting production process, the main factor affecting crystal nucleation and growth is the superheat of molten steel. The higher the degree of superheat, the lower the nucleation rate at the solidification front of the molten steel, or the nucleation cannot occur, and it is difficult for equiaxed crystals to form. On the other hand, the higher the degree of superheat, the greater the temperature gradient and the greater the crystal growth rate, which is helpful for the growth and growth of columnar crystals. Therefore, the higher the degree of superheat, the more developed the columnar crystal area will be, and the higher the occurrence rate of cracks inside the cast slab. It is appropriate to control the degree of superheat at 20~30°C.

Effect of drawing speed on internal cracks in cast slab

The relationship between pulling speed and crack occurrence rate is shown in Figure 5. The process requirements for the drawing speed are 2.2~2.8 m/min. As shown in Figure 5, when the drawing speed is less than 2.4 m/min, the crack occurrence rate is around 28%. When the pulling speed is greater than 2.4 m/min, the crack occurrence rate decreases to 9.09%.

Figure 5 Relationship between pulling speed and crack occurrence rate

During the solidification process of the continuous casting shell, the growth of the shell obeys the square root law of solidification:

In the formula: δ is the thickness of the billet shell, mm; k is the solidification coefficient, mm/min1/2; t is the solidification time, min; l is the length of the billet, m; vc is the pulling speed, m/min.

It can be seen from the above formula that the higher the casting speed vc, the smaller the thickness δ of the billet shell. In other words, the slower the metal solidifies and grows, this will inhibit the development of columnar crystals and expand the formation of equiaxed crystals, so the incidence of cracks will be low. However, in the actual production process, in order to ensure the appropriate shell thickness, the drawing speed should not be too high and should be controlled at 2.4~2.8 m/min.

Effect of secondary cooling intensity on internal cracks of cast slab

In the secondary cooling zone, since the cooling water, air and rollers all directly act on the surface of the billet, the cooling intensity of the billet is relatively large, which makes the heat dissipation in this area account for about 70% of the total heat dissipation of the billet. Therefore, the internal quality of the slab depends to a large extent on the cooling intensity of the secondary cooling zone. When the secondary cooling intensity is high, the temperature gradient on the shell section of the cast slab is large, which promotes the development of columnar crystals. On the contrary, the secondary cooling intensity is small, the temperature gradient on the shell section of the cast slab is small, and the growth of columnar crystals is inhibited, thereby promoting the formation and expansion of the central equiaxed crystal. Therefore, appropriately reducing the cooling intensity can inhibit the growth of columnar crystals, facilitate the formation and expansion of central equiaxed crystals, and reduce the incidence of cracks. After reducing the secondary cooling intensity from the original 1.15 L/kg to 1.05 L/kg, the internal cracks were greatly reduced.

Conclusion

(1) Since the carbon content is exactly in the peritectic reaction zone when it is between 0.13% and 0.16%, the occurrence rate of cracks is the highest. Avoiding this peritectic reaction zone can reduce the occurrence of internal cracks in the cast slab.

(2) Reducing the sulfur content or controlling the manganese content according to the upper limit of process requirements will help enhance the critical strain value of steel, thereby reducing the occurrence of internal cracks in the cast slab.

(3) Superheat, drawing speed and cooling intensity affect the solidification structure of the slab. Low superheat, high drawing speed and weak cooling intensity all inhibit the development of columnar crystals, which is conducive to expanding the formation of equiaxed crystals and reducing the incidence of internal cracks in the slab.

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