PROCESS FOR MANUFACTURING COLD-FORMED PRECISION STEEL PIPES

PROCEDE DE FABRICATION DE TUBES D'ACIER DE PRECISION FABRIQUES A FROID

English Abstract

The invention relates to a process for manufacturing cold-formed, in
particular
cold-drawn, precision steel pipes for application in particular as pressure-
operated cylinder
pipes with optimum addition of one or several alloying elements as well as
impurities
caused by melting. A seamless, hot-formed pipe blank or welded pipe blank made
from a hot strip with defined starting condition is hereby drawn in one pass
or in
several passes into a finished pipe, and the pipe undergoes a heat treatment
before
the finishing pass.

French Abstract

L'invention concerne un procédé de fabrication de tubes d'acier de précision fabriqués à froid, notamment étirés à froid, notamment destinés à être employés en tant que tubes de cylindres sous pression, avec addition optimale d'un ou plusieurs éléments d'alliage et d'impuretés dues à la fusion. Une ébauche de tube continue fabriquée à chaud ou fabriquée par soudage à partir d'une bande à laminer est étirée à partir d'un état initial défini, dans une ou plusieurs phases d'étirage, de manière à obtenir un tube fini, et le tube est soumis à un traitement de finition avant la phase d'étirage finale.

Claims

CLAIMS:
1. A
process for manufacturing a cold-formed, precision steel pipe with the
following chemical composition in weight %:
C = 0.05 - 0.25,
Si = 0.15 - 1.0,
Mn = 1.0 - 3.5,
Al = 0.020 - 0.060,
V max. 0.20,
N max 0.15,
S max 0.03,
Cr max. 0.80,
Mo max. 0.65,
Ni max. 0.90,
W max. 0.90,
Ti max. 0.20,
Nb max. 0.20, and
the remainder iron and inevitable impurities, comprising:
melting said elements; and
7

preparing a seamless, hot-formed pipe blank or welded pipe blank
made from a hot strip drawn in one pass or in several passes into a finished
pipe, and
heat treating the pipe before the finishing pass, wherein the heat treatment
comprises
heating the pipe to a temperature of 910-940°C, cooling the pipe, and
subsequently
exposing the pipe to a tempering treatment.
2. The process according to claim 1, wherein the cold-formed pipe is cold-
drawn.
3. The process according to claim 1 or 2, wherein the cooling involves an
accelerated cooling.
4. The process according to claim 3, wherein the accelerated cooling
involves quenching.
5. The process according to claim 4, wherein the quenching is
implemented by means of a water shower.
6. The process according to claim 1, wherein the cooling is carried out
through exposure to static air.
7. The process according to any one of claims 1 to 6, wherein the
tempering treatment is carried out at a temperature range of 540 to
720°C.
8. The process according to any one of claims 1 to 7, wherein the finish-
drawn pipe is subjected to a final annealing.
9. The process according to claim 7, wherein the final annealing is carried
out at a temperature range of 500 to 700°C.
10. Use of the pipe produced according to the process defined in any one
of claims 1 to 9, for a cylinder pipe under pressure.
8

Description

CA 02622410 2014-03-04
20337-627
Process for Manufacturing Cold-Formed Precision Steel Pipes
The invention relates to a process for manufacturing cold-formed, in
particular
cold-drawn, precision steel pipes.
Involved in particular in this context are precision steel pipes according to
DIN EN
10305, part 1 and 2, which are under high internal pressure during operation
and
find application for example as cylindrical pipes in the hydraulic or
pneumatic
fields.
The basic manufacturing process of seamless or welded, cold-drawn precision
steel pipes is described for example in the Stahlrohr Handbuch [Steel Pipe
Handbook], 12. ed. 1995, Vulkan Verlag Essen.
Pipes manufactured in this way are characterized in particular by narrow wall
thicknesses and diameter tolerances..
Starting product may either be a seamlessly produced hot-rolled pipe blank or
a
pipe blank made from a hot strip by means of high frequency induction welding
(HFI welding).
This pipe blank, labeled also as hollow, is drawn in the following cold
drawing
process, which includes one or more passes, to the required final size
(diameter,
wall thickness) for the finished pipe.
Cold forming causes the material to solidify, i.e. yield point and strength
thereof
increase while elongation and toughness thereof become smaller at the same
time.
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This is a desired effect for many applications. As a consequence of the
reduced
deformation capability, it is, however, necessary to execute in some instances
a
recrystallizing heat treatment before carrying out further forming processes,
so that
the material can be cold formed again for the next drawing process.
The properties of precision steel pipes made in this way are described in DIN
EN
10305 part 1 and 2.
Unalloyed quality steels up to E 355 as well as higher strength grades up to
StE 690 are used as steel grades.
In order to use such pipes under high pressure, e.g. as hydraulic cylinder
pipes,
they have to meet high standards as far as their toughness is concerned.
Hydraulic cylinders control movement patterns of many devices and machineries
which are used, i.a., also outdoors at great temperature fluctuations.
When exposed to temperature conditions of up to -20 C, risk of harm to persons
and material cannot be absolutely ruled out in view of the brittle fracture
tendency
of materials used to date for cylinders or pipes under pressure.
Tests have shown that a notch impact energy of 27 J at -20 C, as typically
demanded heretofore, is not sufficient for standardized specimens to
absolutely
rule out structural failure as a result of brittle fracture at this
temperature.
Comparative systematic tests of ready-for-use cylinder pipes, that include
notch
impact bending tests, drop weight tear tests, and structure tests have shown
that a
substantially ductile structural failure can be expected only when the notch
impact
energy is at a minimum value commensurate with a shear fracture area of 50% in
the DWT test.
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20337-627
This means for example for a St52 that the values to be attained in the notch
impact
bending test need to have a minimum value of about 80 J at operating
temperature to
provide the structural part with enough plastic deformation reserves to
prevent the risk of
a brittle, multipart disintegration of the structural part.
The notch impact energy determined in the notch impact bending test for the
finished
pipe cannot be raised to the necessary level by the currently employed
manufacturing
process.
The invention relates to a process for manufacturing cold-formed, in
particular cold-
drawn precision steel pipes for application in particular as pressure-operated
cylinder
pipes, to positively ensure a substantially ductile failure of the pipe in a
simple and cost-
efficient manner, even at operating temperatures of up to -20 C.
In one process aspect, the invention relates to a process for manufacturing a
cold-
formed, precision steel pipe with the following chemical composition in weight
%:
C = 0.05 - 0.25,
Si = 0.15 - 1.0,
Mn = 1.0 - 3.5,
Al = 0.020 - 0.060,
V max. 0.20,
N max 0.15,
S max 0.03,
Cr max. 0.80,
Mo max. 0.65,
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20337-627
Ni max. 0.90,
W max. 0.90,
Ti max. 0.20,
Nb max. 0.20, and
the remainder iron and inevitable impurities, comprising:
melting said elements; and
preparing a seamless, hot-formed pipe blank or welded pipe blank made
from a hot strip drawn in one pass or in several passes into a finished pipe,
and heat
treating the pipe before the finishing pass, wherein the heat treatment
comprises heating
the pipe to a temperature of 910-940 C, cooling the pipe, and subsequently
exposing the
pipe to a tempering treatment. Suitably, the cold-formed pipe is cold-drawn.
Suitably,
the cooling involves an accelerated cooling. Suitably, the accelerated cooling
involves
quenching. Suitably, the quenching is implemented by means of a water shower.
The
cooling may also be carried out through exposure to static air. Suitably, the
tempering
treatment is carried out at a temperature range of 540 to 720 C. Suitably, the
finish-
drawn pipe is subjected to a final annealing. Suitably, the final annealing is
carried out at
a temperature range of 500 to 700 C.
In one use aspect, the invention relates to use of the pipe produced
according to the process defined above, for a cylinder pipe under pressure.
Fig. 1 is a graph of test results for notch impact energy values on cylinder
pipes
produced by the process of the invention; and
Fig. 2 is a graph comparing structural parts produced by the process of the
present
invention and a conventional process for ductile fracture behaviour.
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' 20337-627
According to the teaching of the invention, a process is applied in which the
pipe blank is
finish-drawn in one or more passes, wherein the pipe undergoes a heat
treatment before
finish-drawing, and the steel pipe has the following chemical composition (in
%):
C = 0.05 - 0.25
Si = 0.15 - 1.0
Mn = 1.0 - 3.5
Al = 0.020 - 0.060
V max. 0.20
N max 0.150
S max 0.030,
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with optional addition of one or more alloying elements such as Cr, Mo, Ni, W,
Ti,
or Nb as well as impurities caused by melting.
The optional addition of the alloying elements is dependent on the required
property profile, i.e. according to the desired mechanical properties, and
have
advantageously the following contents (in weight-%):
Cr max. 0.80
Mo max. 0.65
Ni max. 0.90
W max. 0.90
Ti max. 0.20
Nb max. 0.20.
The heat treatment itself includes a classical hardening with subsequent
tempering
of the pipes. Austenitizing is carried out at temperatures of about 910-940 C
depending on the respective material, followed by a quenching process to form
a
hardening structure. Quenching may be executed using various quenching media,
typically quenching is implemented by means of water using a water shower.
When using air-hardening materials, cooling may be realized through exposure
to
static air.
Tempering treatment follows hardening and is carried out at temperatures of
about
540 - 720 C depending on the material.
The advantage of the proposed process is the realization of a very even
homogenous microstructure with superior toughness by providing a heat treating
step before the finishing pass, which microstructure is substantially
maintained
even after the finishing pass of the pipe. Tests have shown that the values
for the
notch impact energy at -20 C and 50% shear fracture area in the DWT test lie
for
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transverse test specimen at a superior 80 J and for longitudinal test specimen
at
100 J.
A possible demand by customers for a final annealing in the form of a stress-
free
annealing after the finishing pass leads to an additional improvement of the
notch
impact energy values and thus toughness of the structural part.
The final annealing is carried out advantageously at a temperature range of
600 -
700 C in dependence on the material, whereby care should be taken that the
temperature should be precisely set in dependence on the material properties
to
be attained, like e.g. strength, elongation at fracture, and notch impact
energy.
Test of pipes made in accordance with the process according to the invention
have shown the elimination of the otherwise typically encountered ferritic-
pearlitic
microstructure of the construction steels with pronounced variations in the
notch
impact energy level in transverse as well as longitudinal test specimens in
materials produced by the process according to the invention.
This is clearly shown by the test results for notch impact energy values on
cylinder
pipes of StE 460 mod., as illustrated in Figure 1. An almost identical notch
impact
energy level of up to about 180 J is reached in longitudinal as well as
transverse
direction.
As illustrated in Figure 2, the structural parts made from the steel pipe StE
460
mod. in accordance with the invention have, compared to the steel pipe
produced
in a conventional manner, a sufficiently high proportion of ductile fracture
behavior
at temperatures of up to -20 C, and thus have sufficient plastic deformation
reserves to positively prevent the risk of a disintegration of the structural
part into
several parts.

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The material concept according to the invention thus allows the operation of
hydraulic cylinders even at the temperature range of up to -20 C.
In certain steel grades, there is the positive side effect of a significant
increase of
the strength values. This allows advantageously a reduction in wall thickness
of
the cylinder pipes by about up to 30% and thus a reduction in weight,
satisfying
the demands of lightweight construction.
In summary, it should be noted that the process in accordance with the
invention
for manufacturing cylinder pipes subject to pressure positively prevents a
multipart
structural failure even at operating temperatures of up to -20 C and moreover
permits a reduction in wall thickness of the cylinder wall of up to 30%.
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Representative Drawing