TPT July 2011

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Article
2. Researching and developing of new
pipe straightening roll, HFW/ERW
welding pipe roll and cold roll forming
steel roll
2.1 Design
Any material is not universal, the key is whether the material
corresponds to use in the conditions of a working environment. Any
development of an advice with new material must correspond to
the requirement of working environment and to reach as possible
as ideal condition. At the same time, the ability of the user to afford
the cost must be considered. If the manufacturing cost is too high to
reach the best material property, maybe this product will be difficult
to make popular.
First, the company systematically analyses the working features and
service condition of steel pipe straightening roll, HFW/ERW welding
pipe roll and cold roll forming steel roll, they then make up the
design plan of new roll which is different from the design of forging
steel roll and plate, strip roll at home and abroad.
2.1.1 Material design of roll
In the past ten years the roll production technology has developed
at a pace, the working layer of roll is developed in a direction of high
alloying and high resistance to wear, the technology of high speed
steel cast roll is constantly being perfected, and new production
methods are continuously appearing. Because of the limit of
manufacture and equipment some of these technologies are not
suitable for our research.
If using D2, X155CrVMo121, SKD11 and Cr12MoV material to cast
shapes, the microstructure is poorer than forging microstructure and
defects of segregation and uneven carbide can be present and the
utilisation property is certainly poorer than that of forging steel roll,
so it cannot satisfy all the requirements.
Before the 1990s high chromium alloy cast iron roll was extensively
used in the hot mills for plate and strip steel. Because of certain
limits in the resistance to heat, cracking and resistance to wear, the
used quantity of high chromium alloy cast iron roll is continuously
reduced[2] , so it cannot also be used to satisfy the requirement to
copy high chromium cast iron roll.
Considering the above mentioned factors the only way to optimise
material design and to invent new high chromium alloy material is
by raising quality and extending utilisation potential.
At the present, the boundary line between high chromium steel
and high chromium iron is not clear in the international market. A
number of countries in the world are in habit of dividing steel and
iron by 2% carbon content, but the carbon content of high carbon
high chromium steel is limited to 2.20-2.30%. For example China’s
Cr12, Germany’s X210Cr12, America’s D3, Britain’s BD3 and
Japan’s SKD1[3], and the carbon content of high chromium iron
чX22C, чX28 are separately 0.6-1.0% and 0.5-1.6% in Russia. [4]
The Association of Iron and Steel Engineers state that the content
of 0.3-2.5% carbon is called a steel matrix roll and the content of
2.5-3.5% carbon is called as iron matrix roll.[5]
The carbon content limit of high chromium material researched by the
project consists of high chromium alloy steel and high chromium alloy
iron as a traditional idea, so we call this material high chromium alloy.
2.1.2 Design of manufacturing process
The cast process is a shaping process and it is optimised and
innovative. At the same time the heat treatment process and
machining process are also optimised and innovative.
2.2 Researching and developing
2.2.1 Material researching
Through researching the difference of working environment of rolls,
the company has researched and developed a new series of high
chromium alloy material with different chemical compositions and
different heat treatment processes. Because of the strengthening
and use of fine crystal and multi-alloy deterioration, changing the
growth condition of eutectic structure, improving the kind, shape
and distribution of carbide, the utilisation quality is raised, the cast
property is improved and the defect of the cast microstructure is
also improved.
The carbide is mainly M7C3 and the micro-hardness up to 1800Hm,
microstructure appeared as no continuous strip state, block state,
particle state and chrysanthemum state. The matrix of alloy is
austenite or martensite, the hardness and toughness have fine
combination, macro-hardness is up to HRc50-65, so the excellent
wear resistance of the product is apparent during utilisation.
Tempered chromium austenite eliminates casting stress and has a
special strong mounting ability and good support ability. It can be
used to prevent and passivate the development of brittle cracks
from a carbide crystal to another carbide crystal and can be used
to produce work hardening on the face layer, and the hardness can
be up to 1000HV[6] after work hardening. High chromium alloy with
austenite is suitable for working condition of high wear resistance,
high stress and low energy impact. The matrix of high chromium
alloy quenching microstructure is martensite – adding a little bit of
residual austenite, first carbide is still isolated. Second, carbide is
precipitated from the original austenite and appeared small grain,
the uneven degree of carbide is reduced and appears even in
distribution. The macrohardness is raised and further homogeneity
is possible.
At present, high chromium alloy for pipe straightening roll
manufactured by Shandong Province Sifang Technical Development
Co Ltd has been first selected material and this material has also got
good approval in the way of selected material for rolls of big HFW/
ERW welding pipe and cold roll forming steel at home and abroad.
This material system has achieved proprietary intellectual property
rights and two patents in China.
2.2.2 Researching and developing of casting process
Smelting is used in the degassing process and purifying process
for raising purity and avoiding the production of cast defects, at the
same time reducing influence of harmful trace elements. Through
setting the controlling temperature device and machinery power
plant in cast mould system during the casting process and achieving
effective control to solidification process, the ideal casting can be
completed.
Controlling the solidification process can be fine dendritic crystal and
eutectic structure, effectively controlling and reducing segregation,
lowering unevenness of carbide. Adjusting and controlling cooling
speed can be used to get the required matrix structure. Achieving
“controlling order solidification” can effectively avoid some cast
defects such as cracks and shrinkage.