Why band structures appear and how to estimate it

1. Why band structures appear and how to estimate it
Banded structure in the low and medium carbon steel is formed along the direction of rolling,
which is mainly composed of proeutectoid ferrite, which is mainly composed of pearlite band and
stack with each other and form of organization form, metal material for the most part in smelting
casting after going through pressure processing to be a profile, but after hot rolling processing
material is easy to get along the direction of deformation of pearlite and ferrite in zonal
distribution.
1 The causes of the formation of band structure roughly classified into two kinds:
(1) the composition segregation.
Steel alloy elements other than the carbon and phosphorus harmful impurities in rolling, impurities
stretching along the rolling direction. When steel cooled below Ar3, these impurities will become
the core of the ferrite to make ferrite morphology in zonal distribution, then the pearlite also
assumes zonal distribution. The composition segregation band structure is difficult to eliminate by
the method of heat treatment.
(2) improper hot working temperature.
Namely hot working stopped forging or hot rolling temperature in two phase zone (Ar1 and Ar3),
ferrite along the direction of the metal flow strip from austenitic precipitation pearlite, the
organization can be avoided by normalizing or annealing. 20G, 15 CrMo, 12 Cr1Mo and SA - 210
series steel are low alloy heat-resistant steel, belonging to the steel grade easy to appear banded
structure.
As high pressure boiler tube and filter steel, band structure gets limited, the banded structure of
such kinds of steel pipe shall not exceed 3 level in the Soviet union TY14-3-460-75. The band
structure is not mentioned in the high pressure boiler pipe and filter tube standard GB5310-85 of
our country, but generally adopts the banded structure no more than 3 level in our country each big
boiler factory as a standard order.
2. High pressure boiler tube of management inspection institute is made with two kinds of process.
One is the tube billet by hot rolling - reheating furnace - tension reduction - finished product
reduction process, the other is the pipe billet PQF rolling - bypass - tension reduction - finished
product process.
Steel pipes of two kinds of process final specifications are Φ 76 mm x 9 mm, furnace no. 052480
and 053012 respectively.
(1) banded structure observation of sample taken from different rolling technology of steel pipe
(furnace no. 052480 and 05301 respectively) and different parts of the steel pipe. Make steel pipe
longitudinal section, metallographic specimen, along the rolling direction, polishing, polishing,
alcohol sulfate erosion, observe the banded structure, according to the level of national standard
GB13299-91 were assessed, then choose some samples for microstructure pictures. Visible, ferrite,
pearlite present zonal distribution, and steel pipe from the outside to the inside banded structure
increase in turn. The lining of the most serious, assessment level 4 level, the outer wall is the most
light, to evaluate the level of grade 1.
(2) band structure elimination process test is done for the normalizing treatment of the two kinds
of steel pipe with band structure.
Normalizing temperature is 910℃, thermal insulation for 30 minutes. After normalizing treatment

2. sampling do metallographic examination, after normalizing treatment, banded structure disappears,
homogeneously small cubic.
3. The results analysis
More than one through different rolling process of SA - 210 - c high pressure boiler tube banded
structure analysis shows that the production of steel pipe reheating furnace process banded
structure than the production of by-pass pipe banded structure, inside the steel tube of the former
higher than the average of the 2 ~ 3 grade. But according to the production of two kinds of steel
pipe of no difference between the conventional (vertical) performance.
(1) According to the research, due to the microstructure of banded structure adjacent zone need not,
their horizontal/vertical mechanical properties have obvious difference, under the action of
external force strength between belt can produce stress concentration, thus cause the overall
decrease of mechanical properties and have clear all the members of the opposite sex.
Two types of rolling technology preparation of steel pipe by mechanical and process performance
is meet the standard requirement, but due to the steel tube parts banded structure is serious, is
close to that caused the part of the standard floor, in the long-term use environment, banded
structure is easy to become a source of micro cracks, cause detonation tube.
(2) in order to explore the real cause of the formation of banded structure, to recover by reheating
furnace of steel tube heat treatment process test, normalizing temperature 910℃, heat preservation
of air cooling after 30 minutes. Banded structure completely eliminated, with even small F + P
group.
Through the trials show that normalizing can eliminate banded structure, show that the banded
structure is the result of a reheating furnace temperature setting is not reasonable, and not because
caused by chemical composition segregation.
Due to the reheating furnace temperature setting is unreasonable, that reduce steel tube in the
two-phase region (Ar1 ~ Ar3) rolling, ferrite along the metal flow direction from the strip of
precipitation in the austenite, has not yet been decomposition of austenite is divided into strip,
when cooled to Ar1, banded austenite into pearlite, the organization can be eliminated by the
method of normalizing or annealing.
4 Conclusion
(1) the production of two kinds of steel pipe has serious band structure, which is because of the
finishing temperature lower than Ar3, and there is no heat treatment after rolling, the banded
structure can be eliminated by 910℃ heat treatment process.
(2) serious band structure of steel pipe shows obvious anisotropy at room temperature mechanical
properties, for band structure of steel pipe routine inspection, results conforming to the
requirements of the product standard, still can run for safety, but still need to strengthen the
supervision and management.
(3) to save the production cost, optimize the production process, can take controlled rolling
technology, to suppress the formation of banded structure, but to strictly control the finishing
temperature and cooling rate instead of normalizing treatment.
(4) in order to prevent abnormal steel pipe, avoid unnecessary accident, all the ex-factory steel
pipe including seamless steel pipe and welded steel pipe should be strict inspected, in the product
manufacturing process file clear stated the microstructure inspection standard and related terms.