Why The Banded Structure Defects Appear In The Low Carbon Steel
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Why the Banded Structure Defects Appear In the Low Carbon Steel

Metallographic examination for the hot rolled or hot expanded seamless steel pipe, in 20, 20G, A106B, A106C deformation of low carbon steel, often find banded structure defects, seriously even reach level 4 ~ 5.

Sampling different parts of steel tube, inspection results show that the banded structure is lighter in the surface of steel pipe circumference, inner hole surface obvious banded structure, wall thickness in half banded structure are more serious. Under the condition of the same process and material, thick wall steel pipe is more serious than thin wall steel tube of banded structure.

Banded structure is one of the steel structure defects, when banded structure is serious, the mechanical properties of steel tube appear obvious anisotropy, and reduce steel pipe transverse reduction of area is more, the longitudinal impact energy and the transverse impact energy by one times, about steel pipe plastic or toughness can not meet the requirements of technical standard, even if the mechanical properties of steel pipe has reached the standard of the lowest, but the level of banded structure in the steel pipe in the technical agreement also limit, banded structure overweight doesn't pay the goods. In order to eliminate the banded structure in the steel tube, we made normalizing and normalizing treatment for many times, but the effect is not ideal. Therefore, to explore the heating process to eliminate or reduce the banded structure in the low carbon steel is very pressing task. Low carbon deformation at room temperature is ferrite + pearlite, if there is a banded structure ferrite + pearlite and alternate the banded structure.

Have banded structure of low carbon steel seamless steel pipe, heated from room temperature to normalizing temperature, when over than the Ac temperature, the eutectoid structure change, pearlite transformation of austenite, the composition of the austenite was not uniform, with the increase of heating temperature, solid dissolved in austenite, ferrite constantly when temperature more than Ac, ferrite after solid solution, the ShiGang organization into a single phase austenite, with the constant improvement of the temperature and the normalized temperature insulation, to homogenization austenitic thoroughly.

After normalizing temperature thermal insulation on cooling, the cooling speed is greater than the critical cooling rate of steel to eliminate banded structure (that is, the inhibition of ferrite from precipitation in the austenite cooling speed), the proeutectoid ferrite in the steel tube is inhibited, the ferrite without desolventizing precipitation remains in austenite, with the increase of temperature reduction and supercooling degree, will not happen again, the precipitation of proeutectoid ferrite and directly by supercooled austenite decomposed into eutectoid products. Ferrite and cementite, but the composition is different from that of eutectoid steel, known as the "false eutectoid structure".

Because there are no "false eutectoid structure" proeutectoid ferrite, the microstructure of steel tubes of banded structure doesn't exist also.

Seamless steel pipe out from the furnace under the normalizing temperature insulation, cooling speed is less than the critical cooling rate of steel to eliminate banded structure, when the temperature drops to the cooling temperature, first of all in austenitic steel low carbon zone leading exhalation cable (this poor carbon area is related to the parent phase of banded structure), with the loss of the slow cooling and temperature, proeutectoid ferrite also increase gradually, until the cold austenite turns into pearlite, then at room temperature, the microstructure of steel tube, the ferrite + pearlite banded structure appears.


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