Seamless steel pipe quality objection analysis and preventive measures
We conduct statistical analysis on the product quality of seamless steel pipes. From the statistical results, we can understand that each manufacturer has processing defects (processing cracks, black leather buckles, internal screws, close pitch, etc.), geometric dimensions, and performance in terms of product quality. (mechanical properties, chemical composition, fastening), steel pipe bending, flattening, dents, steel pipe corrosion, pitting, missed defects, mixed regulations, mixed steel, and other defects.

Production standards for seamless steel pipes: quality requirements for seamless steel pipes
1. The chemical composition of steel; the chemical composition of steel is the most important factor affecting the performance of seamless steel pipes. It is also the main basis for formulating pipe rolling process parameters and steel pipe heat treatment process parameters. In the seamless steel pipe standard, according to the different uses of the steel pipe, corresponding requirements are put forward for the smelting of steel and the manufacturing method of pipe blanks, and strict regulations are made on the chemical composition. In particular, requirements are put forward for the content of certain harmful chemical elements (arsenic, tin, antimony, lead, bismuth) and gases (nitrogen, hydrogen, oxygen, etc.). To improve the uniformity of the chemical composition of the steel and the purity of the steel, reduce non-metallic inclusions in the tube blanks, and improve their distribution, external refining equipment is often used to refine the molten steel, and even electro slag furnaces are used to refining the tube blanks. Melting and refining.

2. Steel pipe geometric dimension accuracy and outer diameter; steel pipe outer diameter accuracy, wall thickness, ovality, length, steel pipe curvature, steel pipe end cut slope, steel pipe end bevel angle and blunt edge, cross-sectional dimensions of special-shaped steel pipes

1. 2. 1 Steel pipe outer diameter accuracy The outer diameter accuracy of seamless steel pipes depends on the method of determining (reducing) diameter (including tension reduction), equipment operation conditions, process system, etc. The outer diameter accuracy is also related to the hole processing accuracy of the fixed (reducing) diameter machine and the distribution and adjustment of the deformation of each frame. The outer diameter accuracy of cold-rolled (抜) formed seamless steel pipes is related to the accuracy of the mold or rolling pass.

1. 2. 2 Wall thickness The wall thickness accuracy of seamless steel pipes is related to the heating quality of the tube blank, the process design parameters and adjustment parameters of each deformation process, the quality of the tools, and their lubrication quality. The uneven wall thickness of steel pipes is distributed as uneven transverse wall thickness and uneven longitudinal wall thickness.

3. Surface quality of steel pipes; the standard stipulates the “smooth surface” requirements of steel pipes. However, there are as many as 10 types of surface defects in steel pipes caused by various reasons during the production process. Including surface cracks (cracks), hair lines, inward folds, outward folds, punctures, inner straights, outer straights, separation layers, scars, pits, convex bumps, pits (pits), scratches ( Scratches), inner spiral path, outer spiral path, green line, concave correction, roller printing, etc. The main causes of these defects are surface defects or internal defects of the tube blank. On the other hand, it occurs during the production process, that is, if the rolling process parameter design is unreasonable, the tool (mold) surface is not smooth, the lubrication conditions are not good, the pass design and adjustment are unreasonable, etc., it may cause the steel pipe to appear. Surface quality problems; or during the heating, rolling, heat treatment, and straightening process of the tube blank (steel pipe), if it occurs due to improper heating temperature control, uneven deformation, unreasonable heating and cooling speed, or excessive straightening deformation Excessive residual stress may also cause surface cracks in the steel pipe.

4. Physical and chemical properties of steel pipes; the physical and chemical properties of steel pipes include the mechanical properties of steel pipes at room temperature, mechanical properties at a certain temperature (thermal strength properties or low-temperature properties), and corrosion resistance (anti-oxidation, water corrosion resistance, acid and alkali resistance, etc.). Generally speaking, the physical and chemical properties of steel pipes mainly depend on the chemical composition, organizational structure, and purity of the steel, as well as the heat treatment method of the steel pipe. Of course, in some cases, the rolling temperature and deformation system of the steel pipe also have an impact on the performance of the steel pipe.

5. Steel pipe process performance; the process performance of steel pipe includes the properties of flattening, flaring, curling, bending, ring-drawing, and welding of steel pipes.

6. Steel pipe metallographic structure; the metallographic structure of steel pipe includes low-magnification structure and high-magnification structure of steel pipe.

7 Special requirements for steel pipes; special conditions required by customers.

Quality issues in the production process of seamless steel pipes – Quality defects of tube blanks and their prevention
1. Tube blank quality defects and prevention The tube blanks used in the production of seamless steel pipes can be continuous cast round tube blanks, rolled (forged) round tube blanks, centrifugally cast round hollow tube blanks, or steel ingots can be used directly. In the actual production process, continuous cast round tube blanks are mainly used because of their low cost and good surface quality.

1.1 Appearance, shape, and surface quality defects of the tube blank

1. 1. 1 Appearance and shape defects For round tube blanks, the appearance and shape defects of the tube blank mainly include the diameter and ovality of the tube blank, and the end face cutting slope. For steel ingots, the appearance and shape defects of the tube blanks mainly include the incorrect shape of the steel ingot due to 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 reduction rate before the perforated plug is proportional to the amount of inward folding of the perforated capillary tube. The greater the reduction rate of the plug, the better the pipe blank will be. The pores are formed prematurely, and the capillaries are prone to inner surface cracks. During the normal production process, the hole shape parameters of the punching machine are determined based on the nominal diameter of the tube blank and the outer diameter and wall thickness of the capillary tube. When the hole pattern 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 will produce inward folding defects; if the outer diameter of the tube blank exceeds the negative tolerance, the reduction rate before the plug decreases, resulting in the tube blank The first bite point moves towards the pore throat, which will make the perforation process difficult to achieve. Excessive ovality: When the ovality of the tube blank is uneven, the tube blank will rotate unstable after entering the perforation deformation zone, and the rollers will scratch the surface of the tube blank, causing surface defects in the capillary tube. The end-cut slope of the round tube blank is out of tolerance: The wall thickness of the front end of the perforated capillary tube of the tube blank is uneven. The main reason is that when the tube blank does not have a centering hole, the plug meets the end face of the tube blank during the perforation process. Since there is a large slope on the end face of the tube blank, 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.

1. 1. 2 Surface quality defects (continuous cast round tube blank) Surface cracks on the tube blank: vertical cracks, transverse cracks, network cracks. Causes of vertical cracks:
A. The deflection flow caused by the misalignment of the nozzle and the crystallizer washes the solidified shell of the tube blank;
B. The reliability of the mold slag is poor, and the liquid slag layer is too thick or too thin, resulting in uneven slag film thickness and making the local solidification shell of the tube blank too thin.
C. Crystal liquid level fluctuation (when the liquid level fluctuation is >± 10mm, the crack occurrence rate is about 30%);
D. P and S content in steel. (P >0. 017%, S > 0. 027%, longitudinal cracks increasing trend);
E. When C in steel is between 0. 12% and 0. 17%, longitudinal cracks tend to increase.

Precaution:
A. Ensure that the nozzle and the crystallizer are aligned;
B. The crystal liquid level fluctuation must be stable;
C. Use appropriate crystallization taper;
D. Select protective powder with excellent performance;
E. Use a hot top crystallizer.

Causes of transverse cracks:
A. Too deep vibration marks are the main cause of transverse cracks;
B. The content of (niobium, and aluminum) in steel increases, which is the cause.
C. The tube blank is straightened when the temperature is 900-700℃.
D. The intensity of secondary cooling is too great.

Precaution:
A. The crystallizer 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 zone 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 powder with good lubrication performance and low viscosity.

Causes of surface network cracks:
A. The high-temperature cast slab absorbs the copper from the mold, and the copper becomes liquid and then oozes out along the austenite grain boundaries;
B. Residual elements in the steel (such as copper, tin, etc.) remain on the surface of the tube blank and seep out along the grain boundaries;

Precaution:
A. The surface of the crystallizer is chromium-plated to increase surface hardness;
B. Use an appropriate amount of secondary cooling water;
C. Control residual elements in steel.
D. Control the 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 in the steel pipe. Since the cracks on the surface of the tube blank will be severely oxidized during the heating process, the cracks are often accompanied by oxidation particles and decarburization phenomena after rolling.