NO965425L - Process for the production of welded steel pipes using duplex stainless steel - Google Patents

Description

The background of the invention

1. The scope of the invention

The present invention relates to a method for the production of welded pipes using duplex stainless steel, in particular by the laser welding method.

2. Description of prior art

Duplex stainless steel, which consists of a ferritic phase and an austenitic phase, is an excellent

corrosion-resistant steel used in chemical factories, pipelines, oil wells, etc. Control of the content of Cr, Ni, Mo and N in this type of steel allows the improvement of corrosion resistance in an environment containing chlorine ions or carbon dioxide. The yield strength and tensile strength of this type of steel are higher than those of austenitic stainless steels and ferritic stainless steels.

In a method for continuous pipe manufacturing that is in use, welded steel pipes are produced by roll forming a strip of steel into a pipe using a group of forming rolls and welding together the opposite edges of the rolled steel. When using duplex stainless steel, it is advantageous to use this method for the production of welded pipes

The welding method used in this method is either fusion welding, such as TIG welding, plasma welding or powder welding, or pressure welding such as electric resistance welding (ERW).

In general, it can be said that fusion welding rarely leads to the formation of welding defects and provides excellent weldability. However, the fusion welding results in poor productivity due to the low welding speed.

Powder welding gives a relatively high welding speed because the welding method allows welding with a large heat input. However, powder welding inevitably causes gases such as 0 or N to enter the steel because the welding method uses powdered welding flux in air. As a result, oxides and nitrides are precipitated in the welded part so that toughness deteriorates. In addition, powder welding increases the frequency of high-temperature cracking due to its large heat input.

The deterioration of toughness is improved by using a strong basic flux to reduce the amount of oxygen at the welding part. The use of a strong basic flux also improves corrosion resistance because the flux improves the irregular concentration distribution of elements that become starting points for pitting.

However, strongly basic flux is unsuitable for increasing the welding speed because the flux has a high melting point. Furthermore, a strong basic flux tends to introduce welding defects such as slag inclusions and edge wounds (concave at the molten part).

On the other hand, a method of pressure welding, by electric resistance heating or induction heating, (or electroseam welding), gives higher productivity than the fusion welding. However, with electroseam welding, there is a tendency for oxides to form on the joining rafts due to the heating during welding, and the formation of welding defects. Especially for duplex stainless steel, oxides such as those of Cr, Si or Mn are released. These oxides are probably left in the welded part as defects called penetrators, because they have a higher melting point than the base mass. Thus, the toughness deteriorates and

the corrosion resistance of the welded part.

As for electroseam welding is pressed

the joining section together for welding, so that a thicker part emerges from the plastic deformation (increased metal flow) at the welded part. Although usually machined away, the machining exposes non-metallic inclusions on the pipe surface, and the toughness and corrosion resistance deteriorate.

The mechanical properties in general, and particularly the toughness, are worse in the direction vertical to the steel plate surface than in the direction parallel to the steel plate surface. Accordingly, the elevation of metal flow at the joining section causes shearing of the direction vertical to the steel plate surface at right angles to the circumference of the pipe, which causes deterioration of toughness.

Summary of the invention

The aim of the present invention is to provide a method for the production, with high productivity, of welded steel pipe, with excellent toughness and corrosion resistance, of duplex stainless steel.

In order to achieve this objective, a method of manufacturing a welded steel pipe using duplex stainless steel is employed, which method comprises the steps of: (a) producing a hot-rolled duplex stainless steel plate comprising 0.03% by weight or less of C, 1% by weight or less Si, 0.8 to 2.0 wt% Mn, 0.03 wt% or less P, 0.01 wt% or less S, 20 to 30 wt% Cr, 2.5 to 4 wt% Mo, 4 to 7 wt% Ni, 0.08 to 0.2 wt% N and the rest essentially Fe; (b) continuously forming the hot-rolled steel sheet into an open tube using multi-stage forming rolls; (c) welding both of the opposite edges of the open tube with a laser welding method while simultaneously butting them together; (d) grinding the portion increasing in plate thickness of the welded portion; and (e) solid solution treating the welded part at a temperature in the range from 950 to 1100 °C for 30 to 300 s. The following step is advantageously introduced between step (b) and step (c); (f) heating both opposite ends of the open tube by electrical resistance heating.

The object is also achieved by a process for producing a welded steel tube of duplex stainless steel, which process comprises the steps: (a) producing a hot-rolled duplex stainless steel sheet comprising 0.03 wt% or less C, 1 wt% or less Si, 0 .8 to 2.0 wt% Mn, 0.03 wt% or less P, 0.01 wt% or less S, 20 to 30 wt% Cr, 2.5 to 4 wt% Mo, 4 to 7 wt% Ni , 0.08 to 0.2 wt% N and the rest essentially Fe; (b) continuously forming the hot-rolled steel sheet into an open tube using multi-stage forming rolls; (g) heating both opposite edges of the open tube by electrical resistance heating, pressure joining the edges and splicing them together; (h) grinding the portion increasing in plate thickness of the joined portion; (i) remelting the joined part with a laser beam to form a fused part; and (e) solid solution treating the welded part at a temperature in the range of 950 to 1100 °C for 30 to 300 s.

Brief description of the drawing

Fig. 1 shows a schematic drawing of test equipment to simulate the production of a welded steel pipe in an embodiment of the present invention.

Description of the preferred embodiment

In order to produce a hot-rolled duplex stainless steel sheet as a base material for welded pipe according to the present invention, the chemical composition (expressed in weight%) of the steel is as follows.

C: Carbon forms carbides and leads to the formation of Cr-depleted layers. Since C content above 0.03% increases the amount of Cr-depleted layers so that the corrosion resistance deteriorates, the C content is specified to be less than 0.03%, preferably less than 0.02%.

Si: Since a Si content of more than 1% impairs hot workability, the Si content is specified at 1% or lower, preferably lower than 0.5%.

Mn: To ensure sufficient solid solution of N, which is an element effective for pitting corrosion resistance in a chloride environment, it is necessary to add Mn of 0.8% or more. On the other hand, excessive addition of Mn, higher than 2.0%, deteriorates the pitting corrosion resistance in a hydrogen sulphide environment. Accordingly, the Mn content is specified to range from 0.8 to 2.0%, preferably from 1.0 to 1.7%.

P: As over 0.03% content of P entails

stress corrosion cracking in a chloride environment or hydrogen sulphide environment, the content of P is specified at 0.03%

or lower, preferably lower than 0.02%.

S: Since higher S content than 0.01% impairs hot workability, the S content is specified as 0.01% or lower, preferably 0.001% or lower.

Cr: Chromium is an element that is effective for

corrosion resistance. Less than 20% Cr content cannot provide sufficient resistance to pitting corrosion, and more than 30% Cr content impairs hot workability. Therefore, the Cr content is specified in the range from 20 to 30%, preferably from 22 to 25%.

Mo: Molybdenum is an element that is effective for chloride corrosion resistance. Less than 2.5% Mo content cannot provide sufficient resistance to pitting corrosion, and more than 4% Mo content impairs hot workability. Accordingly, the optimum range of Mo content is specified to range from 2.5 to 4%, preferably from 2.9 to 3.3%.

Ni: Nickel is effective in ensuring toughness at an additive amount of 4% or more. Above 7% Ni content deteriorates pitting corrosion resistance. As a result, the appropriate range of Ni content is specified from 4 to 7%, preferably from 5.0 to 6.2%.

N: Nitrogen is effective in adjusting the fraction of ferrite phase. Addition of N in amounts higher than 0.08% improves chloride corrosion resistance. Amounts of N greater than 0.2%, however, impair corrosion resistance due to precipitation of Cr2N. Accordingly, the appropriate range of N content is specified from 0.08 to 0.2%, preferably from 0.10 to 0.15%.

Elements other than those given above may be added or included as long as the aim of the present invention is not adversely affected.

After the hot-rolled steel plate with composition as described above is formed, a pipe forming step for the production of welded pipe starts.

In the tube forming step, a hot-rolled sheet is continuously formed into tubes using multi-stage forming rollers, as is the case for the usual production of electro-seamed steel tubes. During the step, the hot-rolled steel sheet is formed into a shape that becomes a tube when the seam is welded, or into an open tube. The equipment includes a number of forming rollers corresponding to the production equipment for conventional electroseam-welded pipes.

Both edges of the steel plate to be joined are then butted against each other using an indentation roller or the like to produce a pucker, and laser welding is performed using an apparatus that emits a laser beam.

As laser welding isolates the welding part from air by using gas shielding, the shielding effect at the welding part is higher than that of powder welding which isolates the welding part by using powder flux. Consequently, laser welding reduces the amount of oxygen that penetrates to the weld. Laser welding does not involve slag inclusions as the method does not use flux.

As laser welding uses an energy beam with a high density, a much lower heat input is required than with powder welding. As a result, the width of the heat-affected zone, where properties are likely to deteriorate, becomes significantly narrower. In addition, laser welding is performed at a higher welding speed, so that productivity is improved.

Laser welding is a fusion welding that melts the entire joining surface, so that the method hardly introduces defects such as the penetrator defects that often occur in electroseam welding.

When the edge part is bent during laser welding, edge wounds and separation of the melted part are prevented.

Grinding the part that increased the plate thickness during the splicing ensures uniform plate thickness, which allows the weld to be heated evenly in the next step with solid solution treatment.

Welding a duplex stainless steel increases the proportion of ferritic phase at the weld and degrades

the corrosion resistance and the low temperature toughness. To prevent these disadvantages, homogenization of the structure is effective by solid solution treatment of the welded part at a temperature in the range from 950 to 1100 °C for 30 to 300 s. Below 950 °C heating temperature results in the formation of sigma phase which significantly impairs the low-temperature toughness, and

heating temperature higher than 1100 °C results in more than 60% ferritic phase fraction, which degrades

the corrosion resistance. A holding time shorter than 30 s cannot provide a sufficiently firm solution, and a holding time longer than 300 s leads to coarsening of the crystal grains, which impairs the low-temperature toughness. The heating temperature is preferably from 1020 to 1070 °C, and the holding time is from 30 to 200 s.

As described above, the method according to the present invention will produce, with high productivity, welded pipes of a duplex stainless steel with excellent toughness and corrosion resistance at the welded part.

In the laser welding method described above, it is preferred, after the pipe forming step, to heat both opposite edges of the open pipe by an electrical resistance method to preheat the part to be welded. This preheating shortens the melting time of the subsequent laser welding step and improves productivity.

The present invention also provides a method where, after the step of pipe forming, both the opposite edges of the formed pipe are preheated by an electric resistance method to weld them together so as to provide a joint, after which the increased part of the plate thickness is ground at the joining part, i.e. . the splicing, to facilitate the focusing of the laser beam on the joint part in the subsequent remelting process, followed by remelting the joint part from the inside of the pipe to the outside of the pipe by laser beam heating, for example by a carbon dioxide laser beam heating with 10 kw laser power, so that a welded pipes are manufactured, with high productivity, from a duplex stainless steel with excellent toughness and corrosion resistance.

As the method described in the previous paragraph includes pipe forming and welding steps identical to the method for making electro-seam steel pipe, further increases in speed are possible. Such a joined part still contains oxide inclusions which cause penetrator or other defects. However, a steel pipe with excellent toughness and corrosion resistance is produced if a laser beam is directed at the joint, whereby oxide inclusions are broken up and the broken-up inclusions are distributed or emitted to the periphery by the convection action during melting. The methods of manufacture described above are applicable in an existing production facility for the production of electro-seamed steel pipes by introducing a radiation unit for laser beam, although they can be used in a newly constructed production facility.

Example 1

The main aim of the present invention is to improve the mechanical properties and corrosion resistance of the welded part. To this end, the inventors have carried out simulation tests in the laboratory to simulate the welded part on real, welded steel pipes.

Table 1 shows the chemical analysis of the steels used in the experiment. Each of the steels was vacuum melted in the laboratory to produce a 50 kg block. The block was hot-rolled to a sample thickness of 12 mm.

Fig. 1 shows a test apparatus for simulating the fabrication of welded pipe. The reference number 1 indicates the steel plate, 2 indicates the electrode (contact tip) for electric resistance heating, 3 indicates the impression roller, 4 indicates the laser beam and 5 indicates the forming roller.

The form roller 5 receives two test plates which simulate both edges of the steel plate 1 which are facing each other. These samples are heated by the resistance heating method which uses high frequency current supplied from the contact tip 2. They are then pressed together with the impression rollers 3. A carbon dioxide laser beam 4 is directed at both joining edges. For commercial impression rollers, a sprain is formed from the outer surface of the open tube. The indentation rollers in the simulator provide the buckling in a direction across the sample.

Each of the samples, with chemical composition given in Table 1, was welded using the apparatus shown in

Figure 1 to prepare the welded sample.

The welding conditions were 10 m/min welding speed, 10 kW laser power and 0.5 mm beam diameter at the focal point. The laser beam was directed from vertically above the steel plate so that it focused on the joint edge.

For each of the thus produced welded samples, solid solution treatment was carried out at 1050 °C for 180 s., followed by water cooling. The samples were subjected to a joint tensile strength test, a Charpy impact test, and a pitting corrosion test. The presence/absence of welding defects was determined by observing the fracture surface of the impact-tested Charpy specimen.

The corrosion test of the welded part was carried out by a pitting corrosion test at varying test temperatures with the sample immersed in 10% FeCl36H20 solution for 72 hours. The evaluation was given at the critical temperature, CPT, which caused pitting corrosion. The toughness of the welded part was evaluated by absorbed energy, vE.40, at -40 °C.

The test results are summarized in Table 2.

Samples Nos. 1 to 8 in the example, which are according to the present invention, gave a vE.40 value greater than 100 J, ranging from 119 to 225 J, and a CPT value greater than or equal to 35 °C, ranging from 35 to 50 °C, which gives favorable toughness and corrosion resistance. The tensile strength of the joint varied from 753 to 785 N/mm<2>(MPa), which values are in a sufficient strength range. No welding defects were found in these steels in the example.

In contrast, the comparative steels No. 9 and No. 10 in the example had poor toughness, a vE_40 value of 75 J and 63 J, respectively, and a CPT value of 20°C, giving poor corrosion resistance. In the comparative steel No. 10 in the example, weld failure appeared on the fracture surface of the impact tested Charpy specimen, and a low tensile strength of the joint, 507 N/mm<2>(MPa), was obtained.

Example 2

When using the hot-rolled samples from steel no.

5 and 7 described in Example 1, laser-welded samples and electroseam-welded samples were produced with the test apparatus shown in

Fig. 1. The production of the electroseam welded samples was carried out without laser beam heating by heating the samples using high frequency current supplied from the contact tip, then compressing the samples using the impression rollers, followed by the same solid solution treatment as stated in Example 1.

The toughness of the welded part for all these samples was evaluated by the same method described above.

The test results are summarized in Table 3.

Far lower values for vE _40 were obtained for the electroseam welded samples compared to the laser welded samples. Consequently, the laser welding is advantageous for achieving good toughness for the welded part.

Example 3

The samples with solid solution treatment at 1050 "C for 180 s or 30 s, and the samples without solid solution treatment, were produced after laser welding of the hot-rolled steel samples No. 5 and 7 described in example 1 with the test apparatus shown in

Fig. 1.

The Charpy impact test and the pitting corrosion test described above were carried out for all these samples.

The test results are summarized in Table 4.

The values for CPT and vE _40 for the samples without solid solution treatment are much lower than those for the solid solution treated samples. The solid solution treatment according to the present invention is necessary for good toughness and good corrosion resistance in the welded part.

LW: Laser welding, ERW: Electric resistance welding

Claims (6)

1. Method for the production of welded steel pipes using duplex stainless steel, characterized in that it comprises the steps: forming a hot-rolled duplex stainless steel sheet continuously into an open tube with opposite edges using multi-stage forming rolls, wherein the duplex stainless steel sheet comprises 0.03 wt% or less C, 1 wt% or less Si, 0.8 to 2, 0 wt% Mn, 0.03 wt% or less P, 0.01 wt% or less S, 20 to 30 wt% Cr, 2.5 to 4 wt% Mo, 4 to 7 wt% Ni, and 0.08 to 0.2 wt% N and the rest essentially Fe; welding the opposite edges of the open tube to form a welded tube, while butturing the edges together, by heating the edges to welding temperature by applying a laser beam thereto; to grind down the part that increased in plate thickness during the splicing of the welded part; to solid solution treat the welded part at a temperature in the range from 950 to 1100 °C for 30 to 300 s. 2. Method according to claim 1, characterized in that the laser beam is a carbon dioxide gas laser beam. 3. Method according to claim 1, characterized in that after the open pipe has been shaped, and before the welding, the edges of the open pipe are preheated by electric resistance heating. 4. Method according to claim 2, characterized in that after the open pipe has been shaped, and before the welding, the edges of the open pipe are preheated by electric resistance heating. 5. Procedure for the production of welded steel pipes using duplex stainless steel, characterized in that it includes the steps: forming a hot-rolled duplex stainless steel sheet continuously into an open tube with opposite edges using multi-stage forming rolls, wherein the duplex stainless steel sheet comprises 0.03 wt% or less C, 1 wt% or less Si, 0.8 to 2, 0 wt% Mn, 0.03 wt% or less P, 0.01 wt% or less S, 20 to 30 wt% Cr, 2.5 to 4 wt% Mo, 4 to 7 wt% Ni, and 0.08 to 0.2 wt% N and the rest essentially Fe; heating the opposite edges of the open pipe by electric resistance heating and compression joining the edges by splicing them to form a welded pipe; to grind the part that increased in plate thickness during the splicing of the welded part; remelting the welded portion by applying a laser beam thereto to form a fused portion; and to solid solution treat the welded part at a temperature in the range from 950 to 1100 °C for 30 to 300 s. 6. Method according to claim 5, characterized in that the laser beam is a carbon dioxide gas laser beam.