Causes of microcracks in tubes To make the low-temperature seamless steel tube Gr6 have good low-temperature impact properties of -45℃, its (C) is designed to be 0.08%~0.12%, and the other chemical components are to improve strength and refine grains. Usually, (C) is added to 0.09%. 0.12% steel is called peritectic steel, and an important feature of peritectic steel is solidification shrinkage, which includes volume shrinkage of liquid phase solidification and linear shrinkage of 8-Fe_1-Fe transformation at high temperatures. After the molten steel is poured into the crystallizer, the copper tube wall of the meniscus water-cooled crystallizer provides a large degree of supercooling. First, solid S-Fe is crystallized from the liquid and grows in the form of dendrites. In the high-temperature zone not far below 1495℃ (i.e., near the meniscus), 100% 8-Fe is converted into 100% y-Fe in the shell, and a y-Fe cladding is formed around the 8-Fe dendrites, forming an 8-Fe+L+3,-Fe three-phase interface. The dendrites and the liquid phase between the dendrites undergo a peritectic reaction.
When (C) is 0.10%, 6-Fe-leaf-Fe transformation occurs at 25℃ below the solidus temperature, and the linear contraction coefficient is 9.8×10-soC-1; when (C)>0.20%, the 8-Fe linear contraction coefficient is about 2.0×10-5oC~, that is, when 8-Fe-leaf-Fe transformation occurs, the linear contraction increases by 3.8%. The linear contraction of the shell is the largest, the gap formed is the largest, and the vibration mark is also the deepest. Accompanied by a large volume shrinkage, the billet shell separates from the copper tube wall to form an air gap, resulting in the smallest heat flow, the thinnest billet shell, and a depression on the surface. The cooling and solidification speed of the depressed part is slower than that of other parts; therefore, the structure of the depressed part is coarsened, and it is highly sensitive to cracks. Under the action of thermal stress and molten steel static pressure, stress concentration occurs at the weak part, resulting in crack induction. Peritectic steel continuous casting round billet is prone to depression defects. The deeper the shell depression, the more uneven the shell thickness, and the greater the probability of cracks. The place where cracks occur is often accompanied by depressions and the round billet depression morphology. In severe cases, cracks appear at the bottom of the depression, resulting in large external fold defects on the surface of the steel tube after rolling.
The primary solidification shell of peritectic steel is highly uneven, and the weak part of the shell is the “hot spot” position for cracks. The shell of the 350mm Gr6 continuous casting billet that leaked steel during casting was dissected. The thickness of the primary solidified shell of the cross section was measured. It was found that the inner wall of the primary shell was wavy in the longitudinal direction and the shell thickness was very uneven. Shell of the billet that leaked steel. The change curve of the shell thickness of the 350mm round billet in the early stage of solidification.
The actual measured value (e) of the solidified shell thickness in the early stage of solidification is consistent with the theoretically calculated value (e). The shell thickness gradually becomes thinner after 600mm from the crystallizer liquid surface. The shell thickness is the thinnest at 700mm from the crystallizer liquid surface, and then gradually thickens, which is consistent with the observed dissected shell. The microcracks generated in the weak part of the primary shell of the crystallizer continue to expand after leaving the crystallizer due to the influence of cooling water or straightening temperature in the secondary cooling zone. 2.2 The influence of microcracks on the tube surface on the rolled tube After the tube with microcracks is heated and rolled, cracks of different severity will form different degrees of external folding defects on the surface of the steel tube, some are in the shape of flakes, some are in the shape of “fingernails”, and the milder defects can be removed by grinding. However, once the external folding defect appears on the surface of the steel tube, it will spread throughout the entire tube body and cannot be ground, resulting in the steel tube being scrapped. After transverse metallographic analysis of the flake defect, it was found that the maximum depth of the defect into the matrix was about 0.95mm, and the tail was forked and pointed. After transverse metallographic analysis of the “fingernail” defect, it was found that the defect penetrated the matrix about 1.1mm, and the tail was pointed.
The crack extends from the surface of the tube to the inside. When the tube is rolled, the crack will not be welded, and can only be aggravated with the increase of rolling deformation. Therefore, to avoid the occurrence of external folding defects in steel tubes, only the microcracks on the surface of the tube can be eliminated.
Measures to prevent microcracks:
1. Improve the cleanliness of molten steel:
Strengthen the process control of “refining and refining” to reduce the P and S content of molten steel. When (S)>0.015% and (P)>0.020% in steel, the high-temperature strength and plasticity of steel are significantly reduced, and the probability of cracks increases. Therefore, control (P)≤0.015%, (S)≤0.010% and vacuum degassing improve the cleanliness of molten steel as much as possible, which is conducive to reducing the probability of cracks.
2. Control the superheat of molten steel and reduce the billet drawing speed:
The heat transfer of the crystallizer is the most important link in the cooling and solidification process of the continuous casting billet If the molten steel in the crystallizer is cooled too quickly, fine longitudinal cracks will be generated on the surface of the primary billet shell. When the heat flux density of the crystallizer is lower than 1.7MW/mz, no cracks will appear in peritectic steel. The drawing speed has an important influence on the heat flux density of the crystallizer. With the increase of the drawing speed, the heat flux density of the crystallizer increases, the non-uniformity of the lateral temperature distribution near the meniscus increases, and the index of crack occurrence increases. As the drawing speed increases, the thickness of the slag film decreases. When casting peritectic steel, special protective slag for peritectic steel is used to control the superheat of the molten steel at 20-30°C. The drawing speed is 10%-15% lower than that of ordinary carbon steel to ensure the drawing. During correction, the surface temperature of the ingot is far away from the brittle temperature zone.
3. Control the fluctuation of the crystallizer liquid level:
The fluctuation of the crystallizer liquid level affects the melting and uniform flow of the protective slag, causing the fluctuation of the meniscus heat flow and the uneven distribution of the lateral heat flow. The crystallizer liquid level fluctuation increases from ±5mm to ±20mm, and the crack index increases from 0 to 2.0. The liquid level control system can make the crystallizer liquid level fluctuate within ±3mm, effectively controlling the generation of cracks in the ingot.
4. Use a suitable crystallizer taper:
The crystallizer taper affects the contact state between the solidified ingot shell and the crystallizer copper tube. Its taper should be adapted to the shrinkage of the solidified ingot shell to prevent The solidified shell will being separated from the inner wall of the crystallizer too early to form an air gap, which will reduce the cooling effect of the shell. According to the growth law of the shell thickness of the crystallizer, e=flail (where k is the solidification coefficient, t is the solidification time, and n is the power index), the solidification thickness of the ingot is in a parabolic relationship with time. The solidified shell of the single-tapered crystallizer cannot maintain stable contact with the inner wall of the crystallizer. As the drawing time increases, the air gap increases. Under the action of the static pressure of the molten steel, the shell is deformed, causing uneven cooling. The use of a parabolic or multi-tapered crystallizer can make the solidified shell contact well with the inner wall of the crystallizer copper tube, preventing the shell from deforming and cracking.
Post time: Oct-10-2024