Seamless steel pipe cracking is as follows:.
1. The morphology of hot-rolled and cracked seamless steel tube was observed under scanning electron microscope, and obvious cracks were observed in the middle of the fracture.
2. Further observation of hot-rolled and cracked seamless steel tube cracks and their ends revealed that there were obvious oxidized particles near the cracks, and there were intra-crystalline cracks at the crack tip, and a complete polygonal ferrite morphology was present near the crack.
3. The characteristics of oxidized particles in the vicinity of 30 um of the crack are closely related to the original defects of the continuous casting billet. Continuous heating of the billet in the heating furnace causes the elements near the cracks to be oxidized to form a thicker oxidized point layer. This defect is eventually found. Inherited into the final hot roll.
4. It is found that there are obvious cracks and oxidation bands in the center of the thickness and 1/4 of the thickness, and there are also oxidized particles near the crack.
There are different internal cracks in the whole volume of the broken coils. The cracks mainly exist near the middle of the coils, showing continuous topography and higher cracking risk.
1, Billet heating
The heating should be averaged to stop the rapid rise and fall temperature. Every time the temperature rises, it must maintain steady and slow, and the maximum lifting temperature does not exceed 30°C.
2, Mill mandrel
The thick-walled tubes and solid billets with the average wall thickness have greatly reduced the probability of bending and deformation of the mandrel, which can effectively improve the accuracy of the wall thickness of the steel pipe.
3, The accuracy of mandrel
The mandrel's external machining accuracy is controlled to ±0.1mm, and the mandrel's straightness does not exceed 5mm. During welding, a two-piece mandrel is punctured into a finished pin for positioning to avoid excessive total straightness of the weld.
4, The perfect process
Perfect technology to avoid the situation that the central thinning and thickening of exceed the limitations of restraint and improve the precision of wall thickness.
5, Centering roller
Determine whether the centering roller device can be in place, adjust the middle and opening angles of the relevant core roller, and the opening and closing of all measures is consistent. The center of the core roller must be on the rolling line.
6, Perforated top rod
The perforated ejector rod is generally a thick-walled tube with an outer diameter of Φ108 mm-Φ114 mm and a wall thickness requirement of ≥25 mm and an average wall thickness.
7, Rolling middle line
Ensure that the middle line of the piercing mill is consistent with the middle line of the perforated trolley to prevent "up-rolling" or "down-rolling", so that the tube will maintain its average force during perforation.
8, Rolling things
Rolling on worn heads, guide plates, rolls, etc. must be changed in real time.
9, Rolling equipment
The middle of the roller distance and the lead must be on the rolling line. The middle line of the wrapping pitch and the roller pitch is on the middle line of the piercing and rolling, that is, the upper and lower roller gaps are equal, and the left and right guiding distances are equal. In the production process of straight seam steel pipes, since the primary processing is performed in a hot state, the heating operation is a very important process for determining the quality of products. The key to this heating operation is to uniformly heat the billet to a temperature suitable for processing.
Inappropriate heating will cause cracks, folding, and partial pain on the outer surface of the tube blank. Since the perforation has a great influence on the quality, that is, the temperature during perforation processing is an important condition that affects the quality, the temperature of the blank during perforation processing is generally controlled. The second key to heating operation is to minimize the amount of scale. In particular, when we knead the elbows hot, we start from the number of things and the surface quality of the kneading pipe, and pray for more severe.
The concept of hot rolling: hot rolling is relative to cold rolling, cold rolling is rolling performed below the recrystallization temperature, and hot rolling is rolling performed at a temperature above the recrystallization temperature.
Advantages of hot rolling:
a. Hot rolling can significantly reduce energy consumption and reduce costs. During hot rolling, the metal plasticity is high, deformation resistance is low, and the energy consumption of metal deformation is greatly reduced.
b. Hot rolling can improve the processing performance of metals and alloys, that is, the coarse grains in the as-cast state are fractured, significant cracks are healed, casting defects are reduced or eliminated, the as-cast microstructure is transformed into deformed structure, and the processability of the alloy is improved.
c. Hot rolling usually adopts large ingots, and rolling at large reductions not only improves the production efficiency, but also creates the conditions for increasing the rolling speed and achieving the continuous and automated rolling process.
First, the prerequisite must be polished
However, the galvanized layer at the weld must be polished away, otherwise bubbles, trachoma, false welds, etc. will be generated. It also makes the welds brittle and their rigidity decreases.
Second, the galvanized steel welding characteristics
Galvanized steel is generally coated with a layer of zinc outside the low carbon steel. The galvanized layer is generally 20 um thick. Zinc has a melting point of 419°C and a boiling point of about 908°C. In welding, zinc melts into a liquid that floats on the surface of the bath or at the root of the weld. Zinc has a greater solid solubility in iron, zinc liquids will penetrate the weld metal deeper along the grain boundaries, and low-melting-point zinc will form "liquid metal embrittlement."
At the same time, zinc and iron can form intermetallic brittle compounds. These fragile phases reduce the plasticity of the weld metal and cause cracks under tensile stress.
If the welding fillet weld, especially the T-joint fillet weld, is most likely to produce penetration cracks. When the galvanized steel is welded, the zinc layer on the surface and edge of the groove will be oxidized, melted, evaporated, and even emit white fumes and steam under the action of the arc heat, and the pores of the weld can easily be caused.
ZnO formed due to oxidation, its melting point is higher, about 1800 °C above, if the parameters in the welding process is too small, will cause ZnO slag, at the same time. As Zn becomes a deoxidizer. FeO-MnO or FeO-MnO-SiO2 low-melting oxide slags are generated. Secondly, due to the evaporation of zinc, a large amount of white fumes are volatilized, which has a stimulatory and harmful effect on the human body. Therefore, the galvanized layer at the welding place must be ground off.
Third, the welding process control
The pre-welding preparation of galvanized steel is the same as that of ordinary low-carbon steels. It should be noted that the size of the groove and the nearby galvanized layer must be carefully handled. For penetration, the groove size should be appropriate, generally 60 ~ 65 °, to leave a certain gap, usually 1.5 ~ 2.5mm; In order to reduce the penetration of zinc on the weld, before welding, the galvanized within the groove The layer is removed and then soldered.
In the actual supervision work, the use of a concentrated beating, no blunt edge process for centralized control, two welding processes, reducing the possibility of incomplete penetration.
The electrode should be selected according to the material of the base material of the galvanized pipe. Generally, low-carbon steel is widely used due to the ease of operation.
Welding methods: When welding the first layer of multilayer welding, as much as possible to make the zinc layer to melt and vaporize, evaporation and escape the weld, can greatly reduce the liquid zinc left in the weld.
In order to obtain a seamless steel pipe with excellent surface quality, the surface defects of the pipe must be cleaned. Although there are many ways to clear up defects on the tube blanks, the main methods used are flame spade cleaning, manual grinding wheels, automatic grinding wheels and belt cleaning, and turning and cleaning.
(1) Fire cleanup. The equipment is simple, efficient and economical. It is mainly suitable for the surface defect removal of seamless steel tube blanks with a carbon content less than Q.4%. This is because when the tube is cleaned with flame, the tube blank is locally heated, so thermal stress occurs. For billets with high carbon content, hot cracks may occur after the billet is cleaned, even after it has been placed for a period of time, and the quality of the seamless steel tube billet is affected. In severe cases, the billet may be scrapped.
(2) Air shovel cleaning. The equipment is simple, but it is labor-intensive and inefficient. It is not limited by steel. It is suitable for the cleaning of local defects on the surface of bone blanks.
(3)) Manual grinding wheel and automatic wheel cleaning. Mainly used for seamless steel tube blanks that are not suitable for flame cleaning. The manual grinding wheel equipment is simple, the investment is saved, and the operation is flexible, but the labor intensity is large; the automatic grinding wheel cleaning efficiency is high, and currently it is used more.
(4) Using turning or abrasive belts, wheel cleaning. This method of overall grinding and cleaning the surface defects of seamless steel pipe blanks is suitable for the cleaning of surface defects of all kinds of steel pipes, but has low efficiency, high metal consumption, high cost, and is mainly used for high added value and high quality requirements. The surface defects of the first billet (such as stainless steel blanks) are cleaned. It should be pointed out that the surface defects of continuously cast round blanks are generally not removed by the turning method because the fine grained areas of the outer surface layer of the continuously cast round blanks may be turned away, leaving the large columnar crystals exposed. Seamless steel pipe blanks are likely to cause perforated hairs to produce external fold defects when they are pierced.
In short, the inspection and inspection of the quality of the tube blanks and the elimination of the surface defects of the tube blanks are the basic prerequisites for ensuring that the blanks used in the production of seamless steel tubes meet the quality requirements.