GB2308385A

GB2308385A - Fabrication method of welded steel pipe using dual-phase stainless steel

Info

Publication number: GB2308385A
Application number: GB9621984A
Authority: GB
Inventors: Tsuyoshi Shiozaki; Moriaki Ono; Masanori Ohmura; Yutaka Nagahama; Akio Sato
Original assignee: NKK Corp; Nippon Kokan Ltd
Current assignee: JFE Engineering Corp
Priority date: 1995-12-18
Filing date: 1996-10-22
Publication date: 1997-06-25
Anticipated expiration: 2016-10-22
Also published as: JPH09170050A; NO965425L; NO965425D0; GB9621984D0; CA2188632A1; GB2308385B; CN1157768A

Abstract

A method of making welded steel pipe using dual-phase stainless steel comprises: (a) forming hot-rolled dual-phase stainless steel plate continuously into open pipe having opposed edges using multi-stage forming rolls, wherein the steel has composition: 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 balance essentially Fe; (b) welding said opposed edges of open pipe to form a welded pipe, while upsetting, by healing said edges to welding temperature by application of a laser beam thereto; (c) grinding the portion that increases in thickness during upsetting on the welding part, and (d) solid solution treating the welded part at a temperature ranging from 950 to 1100 {C for 30 to 300 seconds. Alternatively, the opposed edges of open pipe may be compression-joined by electric resistance heating and upsetting, and re-fusing the welded part with a laser beam after the grinding step.

Description

FABRICATION METHOD OF WELDED STEEL PIPE USING DUAL-PHASE STAINLESS STEEL BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fabrication method of welded pipe using dual-phase stainless steel, and particularly to the method by laser welding.

2. Description of the Related Arts Dual-phase stainless steel consisting of a ferritic phase and an austenitic phase is an excellent corrosion resistant steel used in chemical plants, linepipes, oil well pipes and so on. The control of the content of Cr, Ni, Mo, and N of this type of steel allows to improve corrosion resistance in an environment containing chlorine ion or carbon dioxide. Proof stress and tensile strength of this type of steel are higher than those in austenitic stainless steel and in ferritic stainless steel.

In a current continuous pipe-forming process, welded steel pipes are fabricated by roll-forming a strip of steel into a pipe using a group of forming rolls and welding the facing edges of the roll-formed steel together. It is preferable to apply this process to the fabrication of welded pipe usinng dual-phase stainless steel.

The welding method used in this process is either fusion welding such as TIG welding, plasma welding, or submerged arc welding, or compression welding such as electric resistance welding (ERW).

Generally speaking, the fusion welding hardly generates weld defects and provides excellent weldability. The fusion welding is, however, poor in productivity owing to the low welding speed.

Submerged arc welding gives relatively high welding speed because the welding method allows to conduct large heat input welding. Submerged arc welding, however, unavoidably induces invasion of gases such as O or N into the steel because the welding method uses powder welding flux in air. As a result, oxides or nitrides precipitate at the welded part to degrade the toughness of the welded part. In addition, submerged arc welding increases the frequency of high temperature crack generation because of its large heat input.

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

Nevertheless, strong basic flux is not suitable for increasing the welding speed because the flux has a high melting point.

Furthermore, strong basic flux likely induces weld defects such as slag inclusion or undercut (concave at fused portion).

On the other hand, a method of compression welding by electric resistance heating or induction heating, (or the electroseam welding), gives higher productivity than the fusion welding. The electroseam welding, however, tends to form oxides on the joining face by heating during welding, and to generate weld defects.

Particularly for a dual-phase stainless steel, oxides such as those of Cr, Si, or Mn precipitate. These oxides are likely left in the welded part as defects called penetrator because they have a higher melting point than the matrix. Thus the toughness and the corrosion resistance of the welded part degrade.

As for the electroseam welding, the joining section is compressed for welding so that there appears a thicker portion resulted from the plastic deformation (rise of metal flow) at the welded part. Although the portion is usually machined, the machining exposes non-metallic inclusions on the pipe surface, and the toughness and the corrosion resistance degrade.

Generally, mechanical characteristics, particularly of toughness, are inferior in the direction vertical to the steel plate surface to the direction parallel to the steel plate surface.

Accordingly, the rise of metal flow at the joining section causes the intersection of the direction vertical to the steel plate surface at right angle against the periphery of the pipe, which causes toughness degradation.

SUMMARY OF THE INVENTION The object of the present invention is to provide a method of fabrication of welded steel pipe having excellent toughness and corrosion resistance from dual-phase stainless steel at high productivity.

To achieve the object, a fabrication method of welded steel pipe using dual-phase stainless steel is applied, which method comprises the steps of: (a) producing a hot-rolled dual-phase stainless steel plate 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 balance essentially Fe; (b) forming the hot-rolled steel plate continuously into an open pipe using multistage forming rolls; (c) welding both edges of the open pipe facing each other while upsetting by a laser welding method (d) grinding the portion that increased in sheet thickness on the welded part; and (e) applying solid solution treatment to the welded part at a temperature range of from 950 to 1100 "C for 30 to 300 sec of holding time.

The following step is preferably inserted between the step (b) and the step (c); (f) heating both edges of the open pipe facing each other by electric resistance method.

The object is also achieved by a fabrication method of welded steel pipe from dual-phase stainless steel, which method comprises the steps of: (a) producing a hot-rolled dual-phase stainless steel plate 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.S Mo, 4 to 7 wt.% Ni, 0.08 to 0.2 wt.% N and the balance essentially Fe; (b) forming the hot-rolled steel plate continuously into an open pipe using multistage forming rolls; (g) heating both edges of the open pipe facing each other by electric resistance method, compression-joining the edges and providing an upset thereto; (h) grinding the portion that increased in plate thickness on the joined part; (i) re-fusing the joined part by a laser beam to form a melted part; and (e) applying solid solution treatment to the welded part at a temperature range of from 950 to 1100 "C for 30 to 300 sec of holding time.

BRIEF DESCRIPTION OF THE DRAWING Fig. 1 shows a schematic drawing of test apparatus to simulate the fabrication of a welded steel pipe in an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT To fabricate a hot-rolled steel plate of the dual-phase stainless steel as a base material of welded pipe of the present invention, the chemical composition (expressed in wt.% unit) of the steel as follows.

C: Carbon forms carbide and generates Cr-deficient layer. Since above 0.03 % of C content increases the amount of Cr-deficient layer to degrade the corrosion resistance, the C content is specified to 0.03 % or less and preferably 0.02 % or less.

Si: Since above 1 % of Si content degrades the hot working performance, the Si content is specified to 1 X or less and preferably 0.5 X or less.

Mn: To assure a sufficient solid solution of N which is an element effective in pitting resistance under a chloride environment, Mn addition is necessary at 0.8 % or more. On the other hand, excessive addition of Mn at above 2.0 % degrades the pitting resistance under a hydrogen sulfide environment. Accordingly, the Mn content is specified to a range of from 0.8 to 2.0 % and preferably from 1.0 to 1.7 X.

P: Since above 0.03 % of P content induces stress corrosion cracking under a chloride environment or a hydrogen sulfide environment, the P content is specified to 0.03 % or less and preferably 0.02 % or less.

S: Since above 0.01 X of S content degrades the hot workability, the S content is specified to 0.01 % or less and preferably 0.001 % or less.

Cr: Chromium is an element effective for corrosion resistance.

Less than 20 % of Cr content cannot give sufficient pitting resistance, and above 30 % of Cr content degrades the hot workability. Therefore, the Cr content is specified to a range of from 20 to 30 % and preferably from 22 to 25 %.

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

Ni: Nickel is effective to assure toughness at an additive rate of 4 % or more. Above 7 % of Ni content degrades the pitting resistance. Consequently, the suitable range of Ni content is specified as from 4 to 7 % and preferably from 5.0 to 6.2 %.

N: Nitrogen is effective for adjusting the fraction of ferritic phase. Addition of N at above 0.08 % improves the chloride corrosion resistance. Above 0.2 X of N content, however, degrades corrosion resistance because of the precipitation of Cr2 N.

Accordingly, the suitable range of N content is specified as from 0.08 to 0.2 % and preferably from 0.10 to 0.15 X.

Other elements than given above may be added or included as far as the object of the present invention is not adversely effected.

After completing the formation of hot-rolled steel plate having the composition described above, a pipe-forming step for fabricating welded pipe begins.

In the pipe-forming step, a hot-rolled steel plate is continuously formed into pipe using the multistage forming rolls as in the case of usual electroseam steel pipe production. During the step, the hot-rolled steel plate is formed into a shape that becomes a pipe when the seam is welded, or into an open pipe. The equipment comprises a series of forming rolls similar to the production equipment for conventional electroseam welded pipes.

Both edges of the steel plate to be joined together are then butted each other using a squeeze roll or the like to give a upset, and laser welding is performed using an apparatus to radiate a laser beam.

Since laser welding isolates the welding part from air using gas shielding, the shield effect at the welding part is higher than that of submerged arc welding which isolates the welding part using powder flux. Accordingly, laser welding decreases the amount of oxygen entering the welded part. Laser welding does not induce slag inclusion because the method does not use flux.

Since laser welding uses a high density energy beam, it requires much lower heat input than submerged arc welding. As a result, the width of the heat affecting part where the characteristics are likely degraded becomes significantly more narrow. In addition, laser welding is carried out at a high welding speed, so productivity is improved.

Laser welding is a fusion welding that fuses the whole joining face, so the method hardly generates defects such as the penetrator defects which likely appear in electroseam welding.

When upset is given to the edge portion during laser welding, separation of the fused portion and undercut are prevented.

Grinding the portion that increased the plate thickness caused by upsetting assures the uniformity of plate thickness, which allows to heat the welded part uniformly in the next step of solid solution treatment.

Welding of a dual-phase stainless steel increases the ferritic phase fraction at the welded part and degrades the corrosion resistance and low temperature toughness. To prevent these disadvantages, uniformalization of structure is effective through solid solution treatment on the welded part at a temperature range of from 950 to 1100 "C for 30 to 300 sec of holding time. Below 950 "C heating temperature induces the formation of sigma phase to significantly degrade the low temperature toughness, and above 1100 "C heating temperature results in 60 X or more of ferritic phase fraction to degrade the corrosion resistance. Shorter than 30 sec.

of holding time cannot give sufficient solid solution, and longer than 300 sec of holding time leads to the coarsening of crystal grains to degrade the low temperature toughness. Preferably, the heating temperature is from 1020 to 1070 "C, and the holding time is from30 to 200 sec.

As described above, the method of the present invention fabricates welded pipes from a dual-phase stainless steel having excellent toughness and corrosion resistance at the welded part at high productivity.

In the laser welding method as described above, it is preferred, after the pipe-forming step, to heat both edges of the open pipe facing each other by an electric resistance method to pre-heat the portion to be welded. This pre-heating shortens the time for fusion in the succeeding laser welding step, and improves the productivity.

The present invention also provide a method wherein, after completing the pipe-forming step, both edges of the formed open pipe facing each other are heated by electric resistance method to weld them together to provide an upset, then grinding the portion increased in the sheet thickness at the joined part, i.e., the upset to facilitate the laser beam focusing on the joined part in the next re-fusing process, followed by re-fusing the joined part from the inside of the pipe to the outside of the pipe by laser beam heating, for example by carbon dioxide laser beam heating at 10 kW of laser output to produce welded pipe from a dual-phase stainless steel which has excellent toughness and corrosion resistance at a high productivity.

Since the process described in the preceding paragraph includes the pipe-forming step and the welding step whichi are similar to the process for producing electroseam steel pipe, further increase in speed is possible. Such a joined part still contains oxide inclusion which causes penetrator or other defects. However a welded steel pipe having excellent toughness and corrosion resistance is produced if a laser beam is directed on the joined part whereby oxide inclusions are broken and the broken inclusions are dispersed or discharged to the priphery by the convection effect during fusing. The above-described fabrication methods are applicable to an existing electroseam steel pipe production facility by adding a laser beam radiation unit, though they may be applied in a newly constructed production facility.

EXAMPLE 1 The main object of the present invention is to improve the mechanical characteristics and the corrosion resistance of the welded part. In this respect, the inventors carried out laboratory simulation tests to simulate the welded part on actual welded steel pipes.

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

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

The forming roll 5 receives two sheets of specimens which simulate both edges of steel plate 1 facing each other. These specimens are heated by resistance heating method using high frequency current supplied from the contact tip 2, then they are compressed by the squeeze rolls 3. A carbon dioxide laser beam 4 is directed onto both edge joining part. For commercial squeeze rolls, an upset is formed from external face of the open pipe. The squeeze rolls of the simulator provide the upset in a transverse direction of the specimen.

Each of the specimens having the chemical composition given in Table 1 was welded using the apparatus shown in Fig. 1 to prepare the specimen welded part. The welding conditions were 10 m/min. of welding speed, 10 kW of laser output, and 0.5 mm of beam diameter at the focal point. The laser beam was directed from vertically above the steel plate focusing on the edge joining part.

For each of thus prepared specimen of welded part, solid solution treatment was applied at 1050 "C for 180 sec of holding time followed by water cooling. The specimens were subjected to a joint tensile test, a charpy impact test, and a pitting test. The presence/absence of weld defect was confirmed through the observation of the fracture face on the charpy impact tested specimen.

The corrosion test at the welded part was given by a pitting test at various test temperature levels while immersing the specimen into 10 % FeCl3 6H20 solution for 72 hrs. The evaluation was given by the critical temperature CPT generating pitting. The toughness at the welded part was evaluated by the absorption energy vE -40 at -40 0C.

The test results are summarized in Table 2.

Example steels No. 1 through No. 8 of the present invention gave higher than 100 J of vE -40 ranging from 119 to 225 J, and at or above 35 "C of CPT ranging from 35 to 50 C providing favorable toughness and corrosion resistance. Their joint tensile strength ranged from 753 to 785 N/mm2(MPa), which figures are in a sufficient strength range. No weld defect was found in these example steels.

To the contrary, comparative example steels No. 9 and No. 10 gave poor toughness, 75 J and 63 J of vE ~rio, respective, and gave poor corrosion resistance with 20 "C of CPT. In comparative example steel No. 10, weld defects appeared on the fracture face of the charpy impact tested specimen, and a low joint tensile strength of 507 N/mm2(MPa) was given.

EXAMPLE 2 Using the hot-rolled specimens of steel No.5 and 7 described in example 1, laser welded sammples and electroseam welded sammples were fabricated by the test apparatus shown in Fig. 1. The fabrication of the electroseam welded sammples was conducted without laser beam heating by heating samples using high frequency current supplied from the contact tip, then compressing them using the squeeze rolls, followed by the same solid solution treatment recited in example 1.

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

The test results are summarized in Table 3.

Much lower vE -40 values are obtained in the electroseam welded sammples than in the laser welded sammples. Accordingly, the laser welding is preferable to achieve good toughness of the welded part.

EXAMPLE 3 The samples with solid solution treatment at 1050 "C for 180 sec or 30 sec and the samples without solid solution treatment were fabricated after laser welding the hot-rolled specimens of steel No.5 and 7 described in example 1 by the test apparatus shown in Fig. 1.

The charpy impact test and the pitting test described above were conducted on all these samples.

The test results are summarized in Table 4.

The vE 4O and CPT values of the sammples without solid solution treatment is much lower than that of the solid solution treated sammples. The solid solution treatment of the present invention is necessary for good toughness and good corrosion resistance of the welded part.

Steel Chemical Composition(wt%) Note No. C Si Mn P S Cr Mo Ni N 1 0.010 0.49 1.01 0.016 0.0010 23.02 2.97 4.96 0.124 invention 2 0.012 0.48 0.94 0.019 0.0006 22.12 2.93 5.90 0.130 invention 3 0.019 0.41 1.48 0.018 0.0008 21.30 2.92 5.80 0.150 invention 4 0.024 0.45 1.11 0.017 0.0010 22.80 2.90 4.90 0.100 invention 5 0.018 0.48 1.01 0.015 0.0009 24.30 3.00 6.20 0.110 invention 6 0.008 0.00 0.82 0.010 0.0006 23.60 3.10 6.90 0.140 invention 7 0.018 0.49 1.77 0.019 0.0010 22.40 3.05 5.85 0.143 invention 8 0.025 0.48 1.68 0.016 0.0007 22.56 3.28 6.18 0.088 invention 9 0.020 0.70 0.70 0.020 0.0110 22.58 5.00 9.50 0.210 comparison 10 0.038 0.60 0.61 0.015 0.0010 24.50 2.93 9.94 0.160 comparison

SLeel Joint tensile vE-ao CPT Presence/absence Note No. strength (N/mm2) (J) ( 'C) of weld defect 1 785 203 50 None invention 2 774 173 35 None invention 3 781 131 30 None invention 4 753 120 35 None invention 5 762 231 55 None invention 6 768 225 50 None invention 7 783 121 40 None invention 8 756 119 40 None invention 9 752 75 35 - None comparison 10 507 63 20 Exist comparison

Steel Welding vE-40 Note No. method (J) 5 LW 231 invention 5 ERW 25 comparison 7 LW 121 invention 7 ERW 10 comparison LW: Laser welding. ERW: Electric resistance welding Table 3

Steel Solid solution vE-40 CPT Note No. treatment (J) ( C) 5 1050 C x 180sec 231 55 invention 5 no treatment 65 20 comparison 7 1050 C x 180sec 131 40 invention 7 1050 C x 30sec 120 35 invention 7 no treatment 30 20 comparison Table 4

Claims

CLAIMS 1. A method of making welded steel pipe using dual-phase stainless steel, comprising the steps of: forming a hot-rolled dual-phase stainless steel plate continuously into an open pipe having opposed edges using multi-stage forming rolls, said dual-phase stainless steel plate 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, and 0.08 to 0.2 wt. % N and the balance essentially Fe; welding said opposed edges of the open pipe to form a welded pipe, while upsetting, by heating said edges to welding temperature by application of a laser beam thereto; grinding the portion that increases in thickness during upsetting on the welded part, and solid solution treating the welded part at a temperature ranging from 950 to 1100 "C for 30 to 300 seconds of holding time.
2. A method as claimed in claim 1 wherein, after forming said open pipe and before said welding, said edges of the open pipe are pre-heated by electric resistance heating.
3. A method of making welded steel pipe using dual-phase stainless steel, comprising the steps of: forming a hot-rolled dual-phase stainless steel plate continuously into an open pipe having opposed edges using multi-stage forming rolls, said dual-phase stainless steel plate 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, and 0.08 to 0.2 wt. % N and the balance essentially Fe; heating said edges of the open pipe facing each other by electric resistance heating and compression-joining said edges by giving upset thereto to form a welded pipe; grinding the portion that increases in thickness during upsetting on the welded part; re-fusing the welded part by application of a laser beam thereto to form a melted part, and solid solution treating the welded part at a temperature ranging from 950 to 1100 "C for 30 to 300 seconds of holding time.
4. A method as claimed in any preceding claim wherein said laser beam is a carbon dioxide gas laser beam.
5. A method of making welded steel pipe using dual-phase stainless steel substantially as described herein with reference to the Figure.
6. A welded steel pipe made by a method substantially as described herein with reference to the Figure.