JPH04314826A - Production of clad steel tube excellent in toughness at low temperature - Google Patents

Abstract

PURPOSE:To produce a steel tube excellent in toughness at low temp. and corrosion resistance by using, as a stock, a slab consisting of a base material composed of low alloy steel and a cladding material composed of stainless steel or Ni alloy, forming the slab into a clad steel plate by means of heat treatment and rolling under respectively specified conditions, and then forming and welding into tubular state by positioning the cladding material inside. CONSTITUTION:A slab of clad material is formed by using stainless steel or Ni alloy as a cladding material and also using a low carbon low Nb-Ti steel which has a composition containing, by weight, 0.02-0.07% C, <0.5% Si, 1.0-1.8% Mn, <0.03% P, <0.005% S, 0.02-0.15& Nb, 0.005-0.03% Ti, <0.05% Al, and 0.002-0.006% N or further containing specific trace amounts of one or >=2 elements among V, Ni, Cu, Cr, Mo, and Ca as a base material, performing cladding, and then welding the four corners. The slab is heated to 1100-1250 deg.C, hot-rolled at >5% reduction ratio and 850-1000 deg.C rolling finishing temp., air- cooled for 100-200sec, and cooled from >=750 deg.C down to <=550 deg.C at 5-40 deg.C/sec cooling rate so as to be formed into a clad plate. Then, the clad plate is formed and welded into a clad steel tube by positioning the cladding material inside.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is a large-diameter clad steel pipe (UOE steel pipe,
This relates to a high-quality, highly efficient manufacturing method for bending roll steel pipes, etc.).

0002

[Prior art] Corrosive substances (
Crude oil containing a lot of H2S, CO2, Cl-)
High-alloy clad steel pipes made of stainless steel and nickel alloy are increasingly being used as large-diameter line pipes for natural gas transportation. Conventionally, such steel pipes were manufactured by forming rolled clad steel sheets (UOE forming), seam welding, and then reheating and cooling the entire steel pipe (solution treatment), but this method was extremely unproductive. . Therefore, recently, technologies have been developed that can omit solution treatment and achieve target properties as rolled (for example, JP-A No. 60-216984, 62
-16892, 63-130283) were developed, and the manufacturing technology of clad steel sheets has made great progress. However, the corrosion resistance of the composite material and the low-temperature toughness of the base material that can be achieved using these techniques are not necessarily satisfactory.

On the other hand, in seam welding, the TIG welding method (Japanese Patent Application Laid-Open No. 1983-1998) is used as a welding method for the joining material inside the steel pipe.
-154875) is often applied. But T.I.G.
The welding method has an extremely slow welding speed, which has been a major obstacle to mass production of clad steel pipes. On the other hand, in high-speed welding (for example, MIG welding), the welding energy is large, and the weld metal is exposed to high temperatures for a long time even after welding is completed, forming a strong oxide film of Cr, Ti, etc. on the bead surface. do. As a result, it took a lot of effort to clean the bead surface, and it was difficult to increase the speed.

0004

[Problems to be Solved by the Invention] The present invention provides a manufacturing technology for high-alloy clad steel pipes that can simultaneously achieve excellent corrosion resistance of the laminated material, strength and low-temperature toughness of the base material without solution treatment of the steel pipes. . Furthermore, the present invention applies seam welding by high-speed MIG welding, and is characterized by high productivity as well as high quality.

[0005]

[Means for Solving the Problems] The gist of the present invention is to provide a composite material of stainless steel or nickel alloy and carbon
: 0.02 to 0.07, Si: 0.5 or less, Mn: 1.0 to 1.8, P
: 0.03 or less, S: 0.005 or less, Nb:
0.02-0.15, Ti: 0.005-0.03, Al: 0.05
Hereinafter, N: 0.002 to 0.006, and if necessary, V: 0.01 to 0.1, Ni: 0
．． 05-1.0, Cu: 0.05-1.0, Cr: 0.
05-0.5, Mo: 0.05-0.3, Ca: 0.05-0.5, Mo: 0.05-0.3, Ca: 0.
001 to 0.005, with the remainder consisting of iron and unavoidable impurities, to assemble a slab by welding the four circumferences. After heating to a temperature range, reduction ratio of 5 or more, rolling end temperature of 850 to 10
After rolling at 00℃ and air cooling for 60 to 200 seconds, 750℃
From a temperature above ℃ to 550℃ at a cooling rate of 5 to 40℃/sec
Cool to the desired temperature below, then air cool to produce a clad steel plate, then form it into a steel pipe with the laminated material inside,
In seam welding, three-electrode MIG welding is performed from the inside with a low-alloy wire for the leading electrode, a flux-cored high-alloy wire for the intermediate electrode, and a high-alloy solid wire for the trailing electrode, and then a low-alloy wire is attached from the outside. It is used for multi-electrode submerged arc welding.

[0006] The stainless steel of the present invention refers to austenitic, ferritic, martensitic, and two-phase stainless steel, and the nickel alloy is a nickel alloy such as Incoloy 825 or Inconel 625, which is a material with excellent corrosion resistance. . In addition, the properties of the base material (value in the direction perpendicular to the rolling direction) are strength X52 or more (API standard), low temperature toughness 2v
It is a low alloy steel with high strength and high toughness such that E-30≧10 kgf-m and vTrs≦-40°C.

In the present invention, slabs are assembled in two ways. The first method is to assemble a slab by overlapping the laminating material 2 on the surface of the base material 1 and welding the four circumferences. At this time, it is preferable that the bonding surfaces of the base material and the bonding material be smoothed in advance by planing, polishing, etc., and that they be cleaned by degreasing or degassed by a vacuum pump. The second method is to make a slab (sandwich slab) by bringing the two slabs assembled in the first method into close contact with each other through a separating material, and welding the four circumferences.
This is a method of assembling.

[0008] The method for manufacturing a clad steel plate according to the present invention will be explained below. The feature of the steel of the present invention is that the base material composition is low in C-Nb.
- The use of a trace amount of Ti makes it possible to simultaneously achieve excellent corrosion resistance of the laminated material and excellent strength and toughness of the base material even if rolling is completed at high temperatures. In order to obtain excellent corrosion resistance in a composite material, the alloying elements are dissolved during reheating, rolled at a high temperature, and cooled for an appropriate period of time to recrystallize the γ structure. Precipitation of phases (Cr carbide) and the like must be suppressed. However, the γ of the laminated material
If rolling is carried out at such a high temperature that the structure recrystallizes, the crystal grains of the base material will not be sufficiently refined, making it impossible to obtain sufficient low-temperature toughness as a line pipe. For this reason, we investigated a composition system that provides a good balance of strength and toughness even after rolling is finished at high temperatures. As a result, we found that a low C-Nb-trace Ti system (preferably adding Ni, Cu, and Mo) is effective as a base material component, and by applying this to rolled clad steel, we can manufacture a completely new clad steel sheet. invented a method.

The reheating and rolling cooling conditions of the present invention will be explained. In the present invention, the above assembled slab is 1100 to 1
Reheat to a range of 250°C. This is necessary to simultaneously ensure the corrosion resistance of the laminated material and the strength and toughness of the base material. The lower limit temperature of 1100° C. is the minimum heating temperature necessary to sufficiently solutionize the laminated material to obtain excellent corrosion resistance and to recrystallize the γ structure after rolling at a rolling end temperature of 850° C. or higher. However, when the reheating temperature is 1250° C. or higher, the austenite (γ) grains become coarser, the crystal grains after rolling also become larger, and the low-temperature toughness deteriorates. Therefore, a suitable reheating temperature is 1100-1250°C.

[0010] The reheated slab must be rolled at a reduction ratio of 5 or more, and the rolling end temperature must be 850 to 1000°C. The reason why the reduction ratio is set to 5 or more is to (1) completely metallurgically bond the laminated material and the base material, and (2) refine the crystal grains of the base material. For the usability of line pipes, it is necessary that the laminated material and the base material are in complete metallurgical contact, and for this purpose, it is preferable that the rolling reduction ratio be as large as possible. The minimum rolling ratio depends on the reheating temperature and the rolling temperature, but when the rolling temperature is high as in the present invention, it is 5 or more. In the present invention, rolling is completed at a temperature in the range of 850 to 1000°C. If the rolling end temperature is 850° C. or lower, the γ structure of the composite material does not recrystallize and the corrosion resistance (for example, pitting corrosion resistance, test conditions: 48 hrs immersion in 10% FeCl3.6H2 O solution) deteriorates significantly. From the viewpoint of corrosion resistance of the laminated material, the higher the rolling end temperature is, the more preferable. However, if the rolling end temperature is too high, the crystal grains of the base material will not become finer, leading to deterioration of low-temperature toughness. For this reason, the rolling end temperature was set at 10
The temperature was limited to 00°C or below.

Furthermore, in the present invention, after the rolling is completed, 60 to 20
Air cooling is performed for 0 seconds, and cooling is performed from a temperature of 750° C. or higher to an arbitrary temperature of 550° C. or lower at a cooling rate of 5 to 40° C./second, and then air cooling is performed. The reason for allowing air cooling time after rolling is to promote recrystallization of the γ structure of the laminated material and improve corrosion resistance. If the steel is rapidly cooled immediately after rolling, good corrosion resistance cannot be obtained. When the rolling end temperature is 850°C or higher, an air cooling time of at least 60 seconds is required (preferably 100 seconds or more). However, extending the air-cooling time causes a decrease in the temperature of the clad steel sheet, causing precipitation of σ, α' phases (Cr carbides). It also interferes with the toughening of the base metal by accelerated cooling. Therefore, although it depends on the thickness of the steel plate, the air cooling time must be 200 seconds or less, and water cooling must be performed from a temperature of 750° C. or higher. At this time, in order to suppress the precipitation of the σ and α' phases (Cr carbides) and to strengthen the base material by accelerated cooling, the cooling conditions are 550°C at a cooling rate of 5 to 40°C/sec.
It is necessary to cool down to below. Note that reheating (tempering treatment) of the rolled clad steel plate to a temperature of Ac1 or lower for the purpose of improving low-temperature toughness, dehydrogenation, etc. does not impair the features of the present invention in any way.

[0012] The reasons for limiting the base material components of the present invention will be explained below. In order to ensure the strength and low-temperature toughness of the base material and the corrosion resistance of the composite material, the amounts of C, Mn, Nb, and Ti are set to 0.02 to 0.07%, 1.0 to 1.8%, and 0.01%, respectively.
02-0.15%, 0.005-0.03%. The lower limit of the amount of C and Mn is the minimum amount in order to achieve the desired base metal, weld strength and toughness, precipitation hardening due to Nb addition, and crystal grain refinement effects. Furthermore, the upper limit is the limit value for obtaining excellent low-temperature toughness and on-site weldability of the base metal. Base material C
If the amount is too high, C in the base material will diffuse into the laminated material when the assembled slab is reheated, resulting in poor corrosion resistance. Therefore, from the viewpoint of corrosion resistance of the laminated material, it is necessary to limit the amount of C in the base material to 0.07% or less. In the steel of the present invention, Nb: 0.02-0.15%, Ti: 0.005-0.0% as essential elements.
03%. Nb contributes to grain refinement and precipitation hardening in controlled rolling, and has the effect of toughening steel. For the steel of the present invention, rolling must be completed at a high temperature of 850°C or higher in order to improve the corrosion resistance of the laminated material.
Nb needs to be added at least 0.02% or more. As a result, grain refinement and precipitation hardening progress even under special manufacturing conditions based on high-temperature rolling as in the present invention, resulting in superior strength and toughness compared to conventional clad steel sheets. However, if more than 0.15% of Nb is added, the on-site weldability and the toughness of the weld will deteriorate, so the upper limit should be set at 0.15% or more.
%. Furthermore, addition of Ti forms fine TiN, suppresses coarsening of γ grains during slab reheating and welding, and is effective in improving base metal toughness and weld heat affected zone (HAZ) toughness. This effect is particularly important in the steel of the present invention, which is finished rolling at a high temperature. In order to fully demonstrate the effect of TiN,
A minimum of 0.005% Ti addition is required. However, Ti
If the amount is too large, coarsening of TiN and precipitation hardening due to TiC will occur, resulting in deterioration of low temperature toughness, so the upper limit is 0.
It is necessary to limit it to 0.3%.

[0013] The reasons for limiting other elements will be explained. Si is an element that toughens steel, but if added in large quantities it deteriorates weldability and HAZ toughness, so the upper limit is set at 0.5%.
And so. Ti alone is sufficient for deoxidizing steel, and Si does not necessarily need to be added.

The reason why the impurities P and S in the steel of the present invention are set to 0.03% and 0.005% or less, respectively, is to further improve the low-temperature toughness of the base metal and the welded part. Reducing the amount of P prevents grain boundary fracture, and reducing the amount of S prevents deterioration of toughness due to MnS. The preferable amounts of P and S are 0.01% and 0.003% or less, respectively.

Al is an element normally contained in steel as a deoxidizing agent, but deoxidizing can also be done with Ti or Si, and it is not necessary to add it. If the Al amount exceeds 0.05%, Al-based nonmetallic inclusions will increase and impair the cleanliness of the steel, so the upper limit was set at 0.05%.

[0016]N forms TiN and improves base material toughness and HAZ toughness through the effect of suppressing coarsening of γ grains. The minimum amount for this is 0.002%. However, if it is too large, it may cause cracks on the slab surface and deterioration of HAZ toughness due to solid solution N, so the upper limit needs to be suppressed to 0.006% or less.

The reason for adding V, Ni, Cu, Cr, Mo, and Ca will be explained. The main purpose of adding these elements to the basic components is to improve properties such as strength and toughness of the base metal without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount added is self-limited.

Although V has almost the same effect as Nb, its effect is weaker than Nb. The upper limit is weldability, HA
From the point of view of Z toughness, it is 0.1%.

[0019] Ni improves both strength and toughness without adversely affecting weldability and HAZ toughness.
It is also effective in preventing hot cracking when u is added. But 1.0
If it exceeds 1.0%, it is unfavorable from an economic point of view, so the upper limit was set at 1.0%.

[0020] Cu is effective in corrosion resistance and hydrogen-induced cracking resistance, but if it exceeds 1.0%, Cu-cracks occur during hot rolling, making manufacturing difficult. Therefore, the upper limit is set to 1.0
%. Both Cr and Mo are effective in improving strength. However, if added in large amounts, weldability and HAZ toughness will be impaired, so the upper limits were set at 0.5% and 0.3%, respectively. Note that the lower limit of V amount is 0.01% and Ni, Cu, Cr, M
The lower limit of 0.05% of the amount of o is the minimum amount for obtaining the effect on the material quality by adding the element.

[0021]Ca controls the morphology of sulfide (MnS),
In addition to improving low-temperature toughness (increasing Charpy absorbed energy, etc.), it also has a remarkable effect on improving hydrogen-induced cracking resistance. However, if the amount of Ca is less than 0.001%, it has no practical effect, and if it exceeds 0.005%, Ca
A large amount of O and CaS are generated and become large inclusions, which not only impair the cleanliness of the steel but also adversely affect the toughness and on-site weldability. For this reason, the amount of Ca added is 0.001 to 0.005.
%. In addition, to improve hydrogen-induced cracking resistance, S
, O amount to 0.001% and 0.002% or less, respectively, and ESSP≧(Ca)[1-124(O)]/1.2
5(S) is particularly effective. In the present invention, a steel pipe is manufactured using the clad steel plate manufactured as described above.

FIG. 1 is a schematic view of the side surface of a welded part when internal seam welding of a high alloy clad steel pipe according to the present invention is performed in three passes and one run. Also, Figure 2 shows A and A' in Figure 1.
FIG. Here, 1, 3, and 5 are the first, second, and third electrodes, respectively, 2, 4, and 6 are the first, second, and third electrode wires, respectively, 7 is the high alloy layer, 8 is the low alloy steel base material, and 9, 10
, 11 are the first, second and third electrode arcs, 12 and 13 respectively.
, 14 are each low alloy first pass (hereinafter referred to as the first layer)
, a high alloy second pass (hereinafter referred to as a buffer layer), a high alloy third pass (hereinafter referred to as a final layer), 15 is a buffer layer slag, 16 is a final slag, and 17 is a temporary bead on the outer surface.

In FIG. 1, first, using the low alloy solid wire 2 as the first electrode, the groove of the low alloy steel base material 8 is subjected to MIG.
After forming a first layer bead by welding, MIG welding is performed using a high alloy flux-cored wire 4 as a second electrode to form a buffer layer 13 and a slag layer 15. In the third electrode of the final layer, a high alloy solid wire 6 is used, and MIG
Welding is performed to form the final layer bead. At this time, the slag 15 created by buffer layer welding is transferred to the third electrode arc 11.
The slag is remelted and becomes the final layer slag 16 that covers the bead surface, resulting in a high-alloy weld bead with no surface oxide film and excellent corrosion resistance.

[0024] Here, the first electrode wire (initial layer) must be a low alloy wire. When high-alloy wire is used, the high-alloy component of the first layer weld is diluted during the final submerged arc welding from the outside surface, causing cracks in the weld metal. The second electrode wire (buffer layer) uses a high-alloy flux-cored wire. With low alloy wires, the dilution during final layer welding becomes large and excellent corrosion resistance cannot be obtained. At this time, if a high alloy solid wire is used, an oxide film on the bead surface cannot be prevented. Naturally, high-alloy wire must be used in the final layer from the viewpoint of corrosion resistance, but if flux-cored wire is used, the arc becomes unstable, spatter occurs, and a good bead cannot be obtained, so high-alloy wire must be used. Use solid wire.

After internal welding, high-efficiency multi-electrode latent arc welding is finally performed from the external surface using low-alloy wire. Since the inner first layer has a low alloy content, no weld cracks will occur even if it is diluted. At this time, the low-alloy wire may be either a solid wire or a flux-cored wire, and is not limited.

[0026]

[Example] Next, an example of the present invention will be described. Converter - Base material slabs of various steel compositions (thickness 240 m) are produced by continuous casting.
m) was produced. After rolling this slab to a predetermined thickness, one surface is mechanically planed and SUS316 of a predetermined thickness is
L or Incoloy 825 laminated material (adjusted so that the thickness of the laminated material of the steel plate after rolling was 3 mm) was overlapped and welded on all four sides. Furthermore, the two slabs produced in this manner were stacked together with a separating material in between, and the four circumferences were welded to assemble a sandwich slab. All adhesive surfaces were smoothed, dirt was removed, degreased, and air was removed using a vacuum pump. Sandwich slabs are reheated, rolled, and cooled under various conditions to produce clad steel plates, and from this, UOE steel pipes with an outer diameter of 762 mm are produced to evaluate the strength of the base material, low-temperature toughness (Charpy impact test), and corrosion resistance of the laminated material. (Evaluated by the presence or absence of pitting corrosion, test conditions: 10%FeCl3 ・6H
2 O solution for 48 hours at 15°C for SUS316L and 48 hours at 30°C for Incoloy 825), and the adhesion between the base material and the composite material (by ultrasonic flaw detection) was investigated.

Examples are shown in Table 1. The clad steel plates (Steels 1 to 10) manufactured according to the method of the present invention have good properties in both the base material and the laminated material. On the other hand, the comparative steels (steels 11 to 26) that are not according to the present invention have inferior properties of the base material or the laminated material.

Steels 11 and 12 have a high C content and a low Mn content, and steel 12 has a low Nb content, resulting in poor low-temperature toughness. Further, since Ti is not added to Steel 13, Steel 14 has a low amount of N, and thus has poor low-temperature toughness. Steel 15 also has poor low-temperature toughness because the amount of N is too large. Steel 16 has high Si and Mn contents, so it has good strength but poor low-temperature toughness. steel 1
Sample No. 7 has poor strength, corrosion resistance, and adhesion between the laminated material and the base material because the reheating temperature is too low. Steel 18 has poor low-temperature toughness because the reheating temperature is too high. Steel 19 has poor corrosion resistance because the rolling end temperature is too low. Steel 20 has poor low-temperature toughness because the rolling end temperature is too high. Steel 21 requires a short air cooling time and has poor corrosion resistance. Since steel 22 requires a long air cooling time and a low water cooling start temperature, its strength and corrosion resistance are poor. Since the cooling rate of steel 23 is too low, its strength and corrosion resistance are poor. Since the cooling rate of Steel 24 is too high, its low-temperature toughness is poor. Steel 25 has a small reduction ratio, so the laminated material and the base material do not adhere sufficiently. Since steel 26 has a high water-cooling stop temperature, it has poor strength and corrosion resistance.

Table 2 shows examples of MIG welding. Outer diameter 7
UO of 62mm, thickness 20mm (laminated material thickness 3mm)
Using E steel pipes (invention steels 5, 6, and 5 in Table 1 are SUS316L clad steel pipes, and 6 is an Incoloy 825 clad steel pipe), welding was performed with the groove shape shown in FIG. 3. Inconel 625 series wire was used as the high alloy wire.

The weld metals of steels 1 and 2 welded according to the present invention have excellent corrosion resistance, have good bead appearance, and do not cause weld cracks. On the other hand, in Comparative Steels 3 and 4, the combination of the inner weld buffer layer and the final layer of high-alloy wire was inappropriate, so a good bead could not be formed. Also, in steel 5,
Low-alloy wire is used for the inner weld buffer layer, which has poor corrosion resistance and weld metal cracking occurs in the final layer. steel 6
In this case, all internal welding is done with high-alloy wire, so not only does the internal bead look bad, but cracks occur in the external weld metal.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

[Table 4]

[0035]

[Effects of the invention] The present invention has made it possible to manufacture high-quality clad steel plates with excellent strength and low-temperature toughness of the base material and corrosion resistance of the laminated material with high efficiency and without solution treatment of the entire steel pipe. . As a result, energy and process savings have become possible. Additionally, improvements in various properties have significantly improved the safety of pipelines.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a side surface of a welded part when internal seam welding of a high-alloy clad steel pipe according to the present invention is performed in three passes and one run.

FIG. 2 is a cross-sectional view of part A' in FIG. 1A.

FIG. 3 shows the groove shape of a UOE clad steel pipe with an outer diameter of 762 mm and a thickness of 20 mm (laminated material thickness: 3 mm).

[Explanation of symbols] 1 First electrode 2 First electrode wire 3 Second electrode 4 Second electrode wire 5 Third electrode 6 Third electrode wire 7 High alloy layer 8 Low alloy steel base material 9 First electrode arc 10 Second electrode arc 11 Third electrode arc 12 Low alloy first layer 13 High alloy buffer layer 14 High alloy final layer 15 Buffer layer slag 16 Final layer slag 17 Temporary bead on outer surface

Claims (1)

[Claims] Claim 1: C: 0.02 to 0.07, Si: 0.02 to 0.07 by weight with stainless steel or nickel alloy composite material.
5 or less, Mn: 1.0 to 1.8,
P: 0.03 or less, S: 0.005 or less, Nb:
0.02-0.15, Ti: 0.005-0.03, Al: 0.05
Hereinafter, N: 0.002 to 0.006, and if necessary, V: 0.01 to 0.1, Ni: 0
．． 05-1.0, Cu: 0.05-1.0, Cr: 0.
05-0.5, Mo: 0.05-0.3, Ca: 0.05-0.5, Mo: 0.05-0.3, Ca: 0.
001 to 0.005, with the remainder consisting of iron and unavoidable impurities, to assemble a slab by welding the four circumferences. After heating to a temperature range, reduction ratio of 5 or more, rolling end temperature of 850 to 10
After rolling at 00℃ and air cooling for 60 to 200 seconds, 750℃
From a temperature above ℃ to 550℃ at a cooling rate of 5 to 40℃/sec
Cool to the desired temperature below, then air cool to produce a clad steel plate, then form it into a steel pipe with the laminated material inside,
In seam welding, three-electrode MIG welding is performed from the inside with a low-alloy wire for the leading electrode, a flux-cored high-alloy wire for the intermediate electrode, and a high-alloy solid wire for the trailing electrode, and then a low-alloy wire is attached from the outside. A method for manufacturing clad steel pipes with excellent low-temperature toughness, characterized by using multi-electrode latent arc welding.