In the present invention it has been observed that a low carbon boron treated steel having a
coarse grain ferritic microstructure does not work harden during cold forming.
A highly soft galvanized electric resistance welded (ERW) tube can be produced from a hot-
rolled strip of such steel without the expensive normalizing treatment in a Fretz Moon mill.
The ERW tube can be made continuously by forming the accurately rolled strip over a
mandrel and welding the edges electrically.
Thus the present invention provides a method for producing electric resistance welded (ERW)
tubes for plumbing applications comprising the steps of: preparing a hot rolled steel strip; processing
the strip by rolling in a conventional tube rolling line for producing ERW tubes; and galvanizing the
ERW tubes; characterized in that said hot rolled strips are prepared from low carbon boron treated
steel with coarse grain ferritic microstructure, and in that said hot rolling of strip is carried out
independently of any further normalizing treatment for producing a highly soft ERW tube suitable for
plumbing application.
The microstructure of low carbon steel for ERW tubes shows fine ferritic structure only while
steel for FM tubes reveal comparatively coarser ferritic and some amount of pearlite.
The input material for both the processes is the high precision hot-rolled (HR) strips in coils,
the ends of which is welded together to facilitate a steady supply to the flash welder. The
welding temperature is reached passing through a 48 meter long tunnel-type continuous
furnace. As soon as strips leave the furnace, a blast of air (mixed with oxygen) is blown on
both the edges. This is done to remove all the oxides and clean the edges; as well as to
raise the edge temperature which is 30 degree higher than the body temperature of
approximately 1280 to 1300° C. The strips, now ready for welding and sizing operation,
proceed to the 8 - stand welding mill. The edges of the skelp formed are blown with
compressed air after which they are butt - welded in an oval roll pass and air cooled
subsequently.
The ERW process begins with a steady flow of HR coils from a super coil accumulator to the
welding head, where, right after welding, an Eddy current non destructive testing machines
screen out improperly welded tubes. The mill proper consists of a series of forming rolls
which blend the strip progressively into a tubular form by centre forming with abutting
edges. These abutting edges are then heated to the required welding temperature by
means of high frequency induction welding. A three - roll squeeze assembly operated under
controlled pressure affects the welding, thus forming the tubes. The continuous tube is then
cooled to ambient temperature by coolant sprays, passed through a series of sizing rolls,
straightened and automatically cut off to the required length without stopping the mill. The
tubes are then galvanized, packed and finally dispatched to the end users.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention will now be described in detail with reference to figures of the accompanying
drawings where
Figure 1 shows the spring back properties of the electric resistance
welded tubes of the present invention.
Figure 2 shows hardness profile across the weld zone of the
electric resistance welded tube of the present invention.
Figure 3 shows the microstructures at different locations of weld zone of the
electric resistance welded tubes.
DETAILED DESCRIPTION
Two steel chemistries were designed for the present invention as shown in Table 1 and
rolled into hot strip coils. Two steels, namely IF and a low carbon steel with boron, were
taken for experimental electric resistance welded (ERW) tube rolling into 15 mm NBC
(Nominal Bore dia) tubes and the tubes were then galvanized. The mechanical properties of
the galvanized tubes were evaluated and compared with the Fretz Moon tubes.
Microstructures and micro hardness profile across the welded zone were also studied.
The mechanical properties of the galvanized tubes are shown in Table 2. The low
carbon steel after tube rolling and galvanizing shows the yield stress of 271 MPa as
against the yield stress of 270 MPa of Fretz Moon tube. This indicates that the steel
used for electric resistance welded tube does not work harden during cold forming of
tube. This can be attributed to the addition of boron and to the coarse grain ferritic
microstructure. On the countrary, the IP steel having leaner chemistry and fine grain
microstructure, work hardens more and the yield strength went up as high as 308 MPa.
The tensile strength remained same for both the materials as expected after tube
Foiling and galvanizing. The spring back properties, which is an important parameter
for plumbing application and which depends on the yield strength of the material,
reveals that the low carbon steel could give 0 mm spring back with respect to Fretz
Moon tube (as shown in Figure 1).
The hardness profile across the weld - HAZ is shown in Fig 2. The higher hardness at
the weld and HAZ was due to the formation of Fine grain ackuiar ferrite structure (as
shown in Figure 3). The Fretz Moon tube does not have any such weld zone and such a
variation in hardness across the welded zone. However, the weld - HAZ was found to
be about 1.5 mm and as it is coated with zinc through galvanizing treatment, this will
not give any problem in terms of the performance of the tube, which is evident from
the Table 2. This difference between the Fretz Moon tube and the electric resistance
welded (ERW) tube of the present invention can be eliminated by providing a short
seam annealing treatment.
Table 3 shows the performance of the tubes at the actual sites. This indicates that the
newly developed edlectric resistance welded tube of the present invention is as good as
Fretz Moon tube and very much superior to the other known products for the same
application.
Table 3: Comparative study by local plumbers
Scale of impact by hammer 1 to 5
1- difficult, 5-very easy (in sees)
A trial of an HFIW product for plumbing application with suitable steel was carried out
to look into the viability of this route and to establish the new steel which will possess
the similar characteristics of Fretz Moon tubes without any normalizing treatment.
Thus the present invention provides a low carbon boron treated steel as shown in Table
1 processed through electric resistance welded route can match the properties of Fretz
Moon tube. The above-mentioned steel does not work harden during cold forming of
tubes due to boron and coarse grain ferritic structure. A highly soft tube can be
produced without normalizing by using the steel as described above. The electric
resistance welded tube produced from the said steel is less expensive than Fretz Moon
tubes. The electric resistance welded galvanized tube using the chemistry as described
is far superior to the available material in the market for plumbing application. The
weld -HAZ was found to be about 1.5 mm and as it is coated with zinc through
galvanizing treatment, this will not give any problem in terms of the performance of the
tube.
WE CLAIM
1. A method for producing electric resistance welded (ERW) tubes for plumbing applications
comprising the steps of:
preparing a hot rolled steel strip;
processing the strip by rolling in a conventional tube rolling line for producing ERW
tubes; and
galvanizing the ERW tubes;
characterized in that
said hot rolled strips are prepared from low carbon boron treated steel with coarse grain
ferritic microstructure, and in that said hot rolling of strip is carried out independently of
any further normalizing treatment for producing a highly soft ERW tube suitable for
plumbing application.
2. The method as claimed in claim 1, wherein said low carbon steel contains at least 0.0020 %
boron.
3. The method as claimed in claim 1, wherein said low carbon steel comprises 0.02 - 0.06 %
C, 0.10 - 0.25 % Mn, 0.015 % (max) S, 0.02 % (max) P, 0.03 % (max) Si, 0.02 - 0.06 %
Al and 0.002 % (min) B.
4. An electric resistance welded (ERW) galvanized tube produced by the method as claimed in
preceding claims having mechanical properties comparable to Fretz Moon tubes free from
work hardening during cold forming of the tube.
5. The ERW tube as claimed in claim 1, wherein said mechanical properties are:
YS = 271 MPa, UTS = 333 MPa, Elongation = 35 % and Spring Back = Nil.
6. A method for producing electric resistance welded (ERW) tubes for plumbing applications
substantially as herein described and illustrated in the accompanying drawings.

A method for producing electric resistance welded (ERW) tubes for plumbing application
comprises preparing hot rolled steel strip and processing the strip by rolling in a conventional
tube rolling line for producing ERW tubes. The ERW tubes produced are galvanized. The
method is characterized by preparing the hot rolled strip from low carbon boron carbide steel
treated with coarse grain ferritic microstructure and the hot rolling of strip being carried out
independently of any further normalizing treatment.