Quality defects and prevention of seamless steel pipe blanks

The tube blanks used in the production of seamless steel pipes can be either continuous casting round tube blanks, rolling (forging) round tube blanks, centrifugal casting round hollow tube blanks, or directly using steel ingots. In the actual production process, the continuous casting round tube billet is mainly used because of the low cost and good surface quality of the continuous casting round tube billet.

Appearance and shape defects

For the round tube blank, the appearance and shape defects of the tube blank mainly include the diameter and ovality of the tube blank, and the inclination of the end face. For steel ingots, the appearance and shape defects of the tube blank mainly include the incorrect shape of the steel ingot due to the wear of the ingot mold.

The diameter and ovality of the round tube blank are out of tolerance:
In practice, it is generally believed that when the tube blank is perforated, the size of the reduction rate before the perforated plug is proportional to the inward fold of the perforated capillary. Internal surface cracks. In the normal production process, the hole type parameters of the punching machine are determined according to the nominal diameter of the tube blank and the outer diameter and wall thickness of the capillary. When the hole type is adjusted, if the outer diameter of the tube blank exceeds the positive tolerance, the reduction rate before the plug increases, and the perforated capillary tube produces inward bending defects; if the outer diameter of the tube blank is too negative, the reduction rate before the plug decreases, resulting in a tube blank. The first bite point moves to the pore throat, which will make the perforation process difficult to achieve. Out-of-tolerance ovality: When the ovality of the tube blank is uneven, the tube blank will rotate unstable after entering the perforation deformation zone, and the roller will scratch the surface of the tube blank, resulting in surface defects of the capillary. The inclination of the end face of the round tube blank is out of tolerance: the wall thickness of the front end of the perforated capillary of the tube blank is uneven. The main reason is that when the tube blank does not have centering holes, the plug meets the end face of the tube blank during the perforation process. Because the end face of the tube blank has a large slope, it is difficult for the nose of the plug to center the center of the tube blank, resulting in the wall thickness of the end face of the capillary tube. uneven.

Surface Quality Defects (Continuously Casting Round Tube Billets)

Cracks on the surface of the tube blank: longitudinal cracks, transverse cracks, mesh cracks Causes of longitudinal cracks:
A. The bias flow caused by the misalignment of the nozzle and the mold to the scouring of the solidified shell of the seamless carbon steel pipe blank;
B. The melting of the mold slag is poor, and the liquid slag layer is too thick or too thin, resulting in uneven thickness of the slag film, which makes the local solidification shell of the tube blank too thin.
C. Crystal liquid level fluctuation (when the liquid level fluctuation >±10mm, the crack occurrence rate is about 30%);
D. P and S content in steel. (P﹥0.017%, S﹥0.027%, the longitudinal crack increases);
E. When C in steel is 0.12%-0.17%, longitudinal cracks tend to increase.
Precaution:
A. Ensure that the nozzle and the crystallizer are aligned;
B. The fluctuation of crystal liquid level should be stable;
C. Adopt suitable crystal taper;
D. Choose mold powder with excellent performance;
E. Use a hot top crystallizer.

Causes of transverse cracks:

A. Too deep vibration marks are the main reason for transverse cracks;
B. The content of (niobium, aluminum) in steel increases, which is the cause.
C. The tube blank is straightened at a temperature of 900-700 °C.
D. The secondary cooling intensity is too large.
Precaution:
A. The mold adopts high frequency and small amplitude to reduce the depth of vibration marks on the inner arc surface of the slab;
B. The secondary cooling area adopts a stable weak cooling system to ensure that the surface temperature is greater than 900 degrees during straightening.
C. Keep the crystal liquid level stable;
D. Use mold slag with good lubricity and low viscosity.

Causes of surface mesh cracks:

A. The high temperature casting billet absorbs the copper of the mold, and after the copper becomes liquid, it seeps out along the austenite grain boundary;
B. Residual elements (such as copper, tin, etc.) in the steel remain on the surface of the tube blank and seep out along the grain boundary;
Precaution:
A. The surface of the mold is chrome-plated to increase the surface hardness;
B. Use a suitable amount of secondary cold water;
C. Controlling residual elements in steel.
D. Control Mn/S value to ensure Mn/S>40. It is generally believed that when the surface crack depth of the tube blank does not exceed 0.5mm, the cracks will be oxidized during the heating process and will not cause surface cracks on the steel tube. Because the surface cracks of the billet are severely oxidized during the heating process, the cracks are often accompanied by oxide particles and decarburization after rolling.

Tube blank scarring and heavy skin:

Causes: The temperature of molten steel is too low, the molten steel is too viscous, the nozzle is blocked, and the injection flow is deviated. Due to the scarring on the surface of the tube blank and the outward fold of the steel tube formed by the heavy skin, it is different from the scarring and outward fold defects of the barren tube produced during the rolling of the tube. Carbon phenomenon, ferrous oxide is present at the defect.
Tube blank pores: Generally, some small pores are formed on the surface of the tube blank due to the rupture of subcutaneous bubbles during the molten steel casting process. After the tube blank is rolled, small flying skins will be formed on the surface of the steel tube.

Tube blank pits and grooves:

The reasons for the pits and grooves in the tube blank: on the one hand, it may be caused by the crystallization process of the slab, which is related to the too large taper of the mold or the uneven cooling of the secondary cooling zone; on the other hand, it may be due to the fact that the slab is still When it is not completely cooled, the surface of the tube blank is mechanically bumped or scratched. After perforation, folds or scars (pits) and large outward folds (grooves) are formed on the surface of the capillary.
Tube billet “ears”: mainly due to the fact that the roll gap (the tension leveling roll of the continuous casting machine, the roll of the rolling mill) is not closed. The amount is too large or the roll gap is too small. Causes too much widening metal to enter the roll gap. After perforation, the surface of the capillary produces a spiral outward fold. No matter what kind of surface defects of the tube blank, there may be defects on the surface of the steel tube during the rolling process, and the rolled steel tube is scrapped in severe cases. Therefore, it is necessary to strengthen the control of the surface quality of the tube blank and the removal of surface defects. Only the tube blanks that meet the standard requirements can be put into tube rolling production.

Low magnification structure defects of tube blank:

Visually inspect the subcutaneous air bubbles of the tube blank:
The first reason is that the deoxidation of molten steel is insufficient, and the second is that the gas content (especially hydrogen) in the molten steel is also an important reason for the generation of subcutaneous bubbles in the tube blank. This defect forms a flying skin (irregular) on the outer surface of the steel pipe after perforation or rolling, and the shape is similar to the “nail” shape, and in severe cases, it will cover the outer surface of the steel pipe. Such defects are small and shallow and can be removed by grinding.

Subcutaneous crack of tube blank:
The main reason is that the temperature of the surface layer of the continuous casting round tube billet changes repeatedly and undergoes multiple phase transformations. Generally, there is no defect, if there is a slight outward fold.

Intermediate crack and center crack of tube blank:
The middle crack and center crack of the continuous casting round tube blank are the main reasons for the inward folding of the seamless steel tube. The causes of cracks are very complex, involving the effects of heat transfer, penetration and stress in the solidification of the billet, which are generally controlled by the solidification process of the billet in the secondary cooling zone.

Tube blank loose and shrinkage:
It is mainly due to the effect of advanced grains in the solidification process of the casting billet, and the liquid metal movement is formed based on the cooling to the solidification direction and the shrinkage is hindered. If the continuous casting round tube blank has loose and shrinkage holes, it does not have much influence on the quality of the capillary tube with cross-rolling piercing.

Microstructural defects of tube blanks: high magnification or electron microscope

When the composition and structure of the tube blank are uneven and serious segregation occurs, the rolled steel tube will show a serious band structure, which will affect the mechanical properties and corrosion properties of the steel tube, and make its performance unsatisfactory. When the inclusion content of the tube blank is too much, it will not only affect the performance of the steel tube, but also may cause the steel tube to crack during the production process.
Factors: Harmful elements in steel, composition and microstructure segregation of billet, and non-metallic inclusions in billet.