JPH05214499A - Production of high ni alloy-clad steel plate excellent in sour resistance and toughness at low temperature - Google Patents

Abstract

PURPOSE:To produce a clad steel plate for pipeline excellent in sour resistance by subjecting a clad material, where a high Ni alloy is used as a cladding material and a low alloy steel is used as a base material, to hot rolling and to cooling under respectively specified conditions. CONSTITUTION:A slab for a clad steel plate where a high Ni alloy steel containing, by weight, <0.03% C, 0.02-0.5% Si, 0.02-0.5% Mn, <0.02% P, <0.0050% S, 60-65% Ni, 20-25% Cr, 6-8% Mo, 0.05-0.2% Al, 0.0010-0.0060% Ca, and 0.01-0.2% of Ti and/or Nb so that 0.01%<=Ti+Nb<=0.2% is satisfied is used as a cladding material and a low alloy steel is used as a base material is heated to 1100-1150 deg.C and hot-rolled, and hot rolling is done so that the surface temp. of the steel plate just before final reduction becomes 875-925 deg.C. After the completion of rolling, the steel plate is air-cooled for 3-5min, further water-cooled down to 500-350 deg.C at (10 to 50) deg.C/sec cooling rate, and then air- cooled. By this method, the high Ni alloy-clad steel plate for large bore diameter linepipe excellent in sour resistance and toughness at low temp. can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high Ni alloy clad steel sheet having excellent sour resistance and low temperature toughness, which is used as an original plate for a line pipe for transporting natural gas or crude oil containing hydrogen sulfide. It is about.

[0002]

2. Description of the Related Art In sour gas wells and sour oil wells where high temperature, high pressure hydrogen sulfide and carbon dioxide are present, austenitic high alloy steel pipes excellent in corrosion resistance and stress corrosion cracking resistance (hereinafter referred to as sour resistance) are used as oil well pipes or It is now used as a line pipe. However, Ni, Cr, Mo
Using a large amount of an expensive high alloy steel pipe containing a large amount of as a line pipe brings about an unnecessary increase in cost. Therefore, attention has been paid to the use of so-called clad line pipes in which the amount of the high alloy used is reduced by using a high alloy on the inner surface side in contact with the corrosive gas and a low alloy on the outer surface side.

In the production of a clad line pipe, the clad material for the inner pipe is required to undergo solution treatment in order to sufficiently exhibit the corrosion resistance as a high alloy. However, when such heat treatment of a steel pipe is applied to a large-diameter line pipe such as a UOE steel pipe, there is a problem that a large-sized heat treatment equipment is required and the manufacturing process is complicated, resulting in an increase in manufacturing cost. Considering the strong demand for UOE steel pipe class large diameter line pipes as market needs, establishment of a large diameter line pipe manufacturing technique that does not rely on offline steel pipe heat treatment is desired. Further, since large-diameter clad line pipes have a great need in polar regions, excellent low-temperature toughness of the base material, which is the outer pipe, is required as in the case of conventional high-strength line pipes.

As a method for obtaining the low temperature toughness of the base material without performing the off-line steel pipe heat treatment, at a low temperature such as that used for a thick plate of a material for a large diameter steel pipe as long as the corrosion resistance of the clad material is not deteriorated. A method of performing controlled rolling is disclosed (JP-A-60-216984).

[0005]

According to the above-mentioned controlled rolling, since the metal structure of the base material becomes fine, it is easy to achieve the low temperature toughness required as a line pipe for the base material.
However, since the clad material is generally rolled at a temperature lower than the recrystallization temperature, the metal structure of the clad material after clad rolling has an unrecrystallized structure in which the processed state remains. The above-mentioned JP-A-60-216984 shows that the intergranular corrosion resistance of the clad material does not decrease even when low-temperature rolling is performed. However, if the clad material is in a non-recrystallized state, the pitting corrosion resistance is significantly reduced even if the intergranular corrosion resistance is not changed. If the pitting corrosion resistance decreases, the stress corrosion cracking resistance in the sour environment deteriorates. Therefore, it is not preferable that the clad material has a non-recrystallized structure in the line pipe premised on the use in the sour environment.

Therefore, in order to obtain excellent sour resistance, it is necessary to carry out rolling at a temperature range higher than the recrystallization temperature of the clad material. On the other hand, in order to secure the low temperature toughness of the base material, it is necessary to make the crystal grains fine, and it is preferable to roll in the unrecrystallized temperature range of austenite, and rolling in the low temperature range is desired. From this point of view, it is necessary to reduce the recrystallization temperature of the clad material as much as possible, to secure the rolling temperature range that obtains the corrosion resistance of the clad material and the low temperature toughness of the base material, and to set the optimal rolling conditions. Instead, it becomes an issue when manufacturing clad steel for large diameter steel pipes.

[0007]

Means for Solving the Problems The present invention advantageously solves the above-mentioned problems. In the clad rolling, the chemical composition of the clad material is limited, and the rolling and cooling conditions are regulated to improve the corrosion resistance of the clad material. The toughness of the base material is secured at the same time. That is, the gist of the present invention is that, by weight%, C: 0.03% or less, Si: 0.0
2 to 0.5%, Mn: 0.02 to 0.5%, P: 0.0
2% or less, S: 0.0050% or less, Ni: 60 to 65
%, Cr: 20 to 25%, Mo: 6 to 8%, Al: 0.
05-0.2%, Ca: 0.0010-0.0060
%, Ti: 0.01 to 0.2%, Nb: 0.0
1 to 0.2%, 0.01% ≦ Ti +
N-based high alloy containing Nb ≦ 0.2% and other unavoidable impurities as a clad material, low alloy steel as a base material, low alloy steel (base material) / high alloy (clad material) /
When manufacturing the two sets of clad steel plates by rolling the slabs assembled by superimposing the high alloy (clad material) / low alloy steel (base material), the slabs of 1100 to 115 are used.
The surface of the steel sheet immediately before the final rolling is 8
The temperature is set to 75 to 925 ° C., the steel sheet is left to cool for 3 to 5 minutes after the rolling is finished, and then water-cooled to a temperature of 550 to 350 ° C. at a cooling rate of 10 to 50 ° C., and then left to cool. A high Ni alloy clad steel sheet having excellent sour resistance and low temperature toughness, which is characterized by the above.

[0008]

The inventors of the present invention firstly investigated the low-temperature toughness required for line pipe steel, and the minimum rolling required to secure the brittle fracture propagation arresting property, which is the most difficult to achieve by increasing the rolling temperature. The finishing temperature was examined. Using a line pipe steel equivalent to API standard X65 shown in Table 1, a sandwich slab having a base material thickness of 162 mm and a clad material thickness of 28 mm was assembled, the slab was heated to 1150 ° C., and the steel sheet surface temperature immediately before final reduction was variously changed. Then, hot rolling with a total thickness of 40 mm is performed, and after the rolling is finished, the steel sheet is allowed to cool for 3 minutes and water-cooled to about 450 ° C. at a cooling rate of 30 ° C./sec to obtain the base material of the obtained clad steel sheet. A DWTT test was conducted. As a result, in order that the temperature at which the ductile fracture surface ratio becomes 85% is -20 ° C or lower, which is usually the criterion for judging the excellent brittle fracture propagation stopping property, the steel sheet immediately before the final rolling is shown in Fig. 1. We have found that it is necessary to set the surface temperature to 925 ° C or lower.

Next, the present inventors have found that the high Ni alloy having excellent sour resistance has a steel plate surface temperature of 9 before the final reduction.
Under the condition of 25 ° C. or less, various component systems in which recrystallization was completed by cooling after clad rolling were examined. When considering Ni, Cr, and Mo, which are basic components,
i: 60 to 65%, Cr: 20 to 25%, Mo: 6 to 8
The range of% was set as an examination target. When performing welding like a line pipe, the amount of C added is reduced in order to prevent sensitization associated with welding of a high Ni alloy, and addition of Ti and Nb which are carbide forming elements to prevent formation of Cr carbide. is necessary. The inventors of the present invention have found that, by repeating rolling experiments of clad steel, as shown in FIG. 2, the addition amounts of Ti and Nb have a great influence on the recrystallization temperature of the high Ni alloy after hot rolling. It has been found that if the Ti + Nb content is limited to 0.2% or less, the clad material will stably have a recrystallized structure even when the steel sheet surface temperature immediately before the final rolling is 925 ° C. or less. It was also found that even if the amounts of Ti and Nb were reduced, the recrystallization temperature of the clad material did not decrease so much that the steel sheet surface temperature immediately before the final reduction had to be 875 ° C or higher in order to obtain recrystallization.

Based on the above experimental facts, if the components of the clad material are limited and the rolling and cooling conditions are limited for the reasons described below, it is possible to produce a high Ni alloy clad steel sheet excellent in sour resistance and low temperature toughness. The conclusion is that it is possible. Next, the reasons for limiting the components of the cladding material in the present invention will be described.
When C is added in a large amount, it increases the susceptibility to stress corrosion cracking at grain boundaries. It also promotes sensitization during welding of line pipes. C is 0.03% or less as a range where these harmful effects do not occur.

Si and Mn are components necessary as deoxidizing components, and are added in 0.02% or more, respectively. But 0.5
If added in excess of%, non-metallic inclusions remain, leading to deterioration in corrosion resistance and deterioration in hot workability, so the upper limit is made 0.5%. P is an impurity element and deteriorates hot workability and stress corrosion cracking resistance in sour environment. But 0.02
% Or less, there is no practical problem, so 0.02% or less.

S is also an impurity element, and if it is present in a large amount, the hot workability deteriorates, so S is made 0.0050% or less.
Since Ni, Cr, and Mo have excellent stress corrosion cracking resistance in a sour environment, Ni: 60 to 65%, Cr:
20-25% and Mo: 6-8% are added.

Since Al is an effective deoxidizing element, 0.0
Add 5% or more. However, if added in excess of 0.2%, the stress corrosion cracking resistance deteriorates, so the content is made 0.2% or less. Since Ca is an effective element that is used as a deoxidizer and controls the form of sulfides to improve hot workability, 0.0010% or more is added. However, if added in a large amount, the amount of non-metallic inclusions in the steel increases, conversely the hot workability and corrosion resistance deteriorate, so it is made 0.0060% or less.

In addition to the above elements, in the present invention, Ti: 0.01-0.2%, Nb: 0.01-0.
1% or 2% of 2%, 0.01% ≦ Ti + Nb ≦
Add in the range of 0.2%. Ti and Nb are added in an amount of 0.01% or more in order to prevent sensitization during welding of line pipes. However, Ti and Nb increase the recrystallization temperature of the clad material after hot rolling and require an increase in the rolling finishing temperature. Therefore, the final condition that is necessary to secure the brittle fracture propagation stopping property of the base material is the final condition. In order to obtain a stable recrystallized structure of the clad material even when the surface temperature of the steel sheet immediately before rolling is 925 ° C. or lower, Ti + Nb ≦ 0.2% is set, and 0.01% ≦ Ti +
Nb ≦ 0.2%.

Next, the reasons for limiting the rolling conditions in the present invention will be described. The slab assembled into a sandwich is
In order to sufficiently dissolve the clad material during heating, 110
Heat to above 0 ° C. On the other hand, if the temperature is too high, the austenite grain size increases when the base material is heated, and the toughness of the base material after rolling decreases, so the heating temperature is set to 1150 ° C or lower. Therefore, the heating temperature of the slab is 1100 to 1150 ° C.
And

In hot rolling, the steel sheet surface immediately before the final rolling was 87
Finish to 5 to 925 ° C. This is because it is necessary to finish the hot rolling at a temperature of 925 ° C. or lower as shown in FIG. 1 in order to obtain the low temperature toughness of the base material, especially the brittle fracture propagation stopping property. On the other hand, in order to obtain the corrosion resistance of the clad material, it is necessary to obtain the recrystallized structure of the clad material, which requires a finishing temperature of 875 ° C. or higher as shown in FIG.

After the rolling is completed, the clad steel plate is allowed to cool for 3 to 5 minutes. This is because the clad material is recrystallized by utilizing the sensible heat of the clad steel sheet that has been rolled at 875 to 925 ° C. during cooling, and the cooling time of at least 3 minutes is required. However, if the cooling time is long, the clad material reaches the so-called sensitization temperature and the corrosion resistance deteriorates. Also, since the base material undergoes ferrite transformation before the start of water cooling,
The tissue after water cooling becomes a mixed state of air cooling tissue + water cooling tissue,
It is not preferable for securing toughness. Therefore, the cooling after rolling is within 5 minutes.

After that, the clad steel plate was 550 to 35
Water cooling is performed at a temperature of 0 ° C. at a cooling rate of 10 to 50 ° C./sec. This has a sensitization temperature range of 1 for clad materials.
This is because water cooling is performed at a cooling rate of 0 ° C./sec or more to suppress the precipitation of carbides and the like and prevent sensitization. Further, this is because the base material is water-cooled in the ferrite transformation temperature region to increase the cooling rate to make the ferrite grain size finer to increase the toughness and to accelerate the bainite transformation to increase the strength. If the water cooling rate becomes too high, the strength of the base material becomes unnecessarily high and the toughness deteriorates.
The upper limit is ° C / sec. The water cooling stop temperature is set to 550 ° C. or lower where the ferrite transformation temperature range of the base material ends. 55
If water-cooled to 0 ° C, sensitization of the clad material can be avoided. When the water cooling stop temperature is lower than 350 ° C, a low temperature transformation product that inhibits the low temperature toughness such as island martensite occurs in the base material, so the lower limit of the water cooling stop temperature is 350 ° C.
And

[0019]

[Example] API grade X65 whose chemical composition is shown in Table 1
As a base material, the slab for line pipe of No. 1 was used as a base material, and as a clad material, a high Ni alloy whose chemical composition is shown in Table 2, a sandwich slab as a hot-rolled material was assembled. The slab thickness of the base material is 162 mm, the thickness of the clad material is 28 mm,
The slab thickness after assembly was 380 mm. This assembled slab was rolled under the rolling conditions shown in Table 3 until the total thickness became 40 mm. The thickness of one set of clad steel plates manufactured under each condition is about 20 mm, and the thickness of the clad material is about 3.3 on average in the upper and lower clad steel plates combined during rolling.
It became 0 mm.

Corrosion test pieces were prepared from the clad material of the clad steel plate and subjected to a corrosion test for 48 hours in a 10% ferric chloride solution to determine the critical pitting temperature. According to the general evaluation standard of pitting corrosion resistance, when the critical pitting temperature was 32.5 ° C or higher, it was considered to have excellent corrosion resistance. When the critical pitting temperature of the clad material is 32.5 ° C. or higher, the clad material is sufficiently solution-treated, and therefore exhibits excellent corrosion resistance and stress corrosion cracking resistance even in a sour environment.

Further, a DWTT test piece was prepared from the base material of the clad steel plate in a direction perpendicular to the rolling direction, and an impact test was conducted. The low temperature toughness of the base material was evaluated at a temperature at which the ductile fracture surface ratio required by the DWTT test was 85%, and when the temperature was -20 ° C or less, it was considered to have excellent toughness. As shown in Table 4, when the clad steel sheet was produced under the conditions according to the present invention, a clad steel sheet having excellent corrosion resistance and low temperature toughness was obtained.

However, since the chemical compositions in Comparative Examples 1 to 3 and the rolling cooling conditions in Comparative Examples 4 to 13 deviate from the conditions according to the present invention, the corrosion resistance of the clad material or the low temperature toughness of the base material is desired. Falls below the level.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

[0026]

[Table 4]

[0027]

Industrial Applicability According to the present invention, a clad steel sheet having excellent sour resistance and low temperature toughness for a large diameter line pipe can be produced without performing an off-line heat treatment, and therefore the industrial effect is extremely large.

[Brief description of drawings]

FIG. 1 In order to obtain excellent low temperature toughness in which the temperature at which the ductile fracture surface ratio becomes 85% in the DWTT test of the base material becomes −20 ° C. or less, the steel plate surface temperature immediately before the final reduction is set to 925 ° C. or less. It is a figure which shows that it is necessary.

[Fig. 2] Even if the steel sheet surface temperature immediately before the final reduction is 925 ° C or lower, it is necessary to satisfy Ti + Nb ≤ 0.2% in order to stably secure the recrystallization structure of the clad material, that is, excellent corrosion resistance. , Nb amount is reduced, it is a diagram showing that the steel sheet surface temperature immediately before the final reduction needs to be 875 ° C. or higher.

Claims (1)

[Claims] 1. By weight%, C: 0.03% or less, Si: 0.02-0.5%, Mn: 0.02-0.5%, P: 0.02% or less, S: 0. 0.0050% or less, Ni: 60 to 65%, Cr: 20 to 25%, Mo: 6 to 8%, Al: 0.05 to 0.2%, Ca: 0.0010 to 0.0060%, , Ti: 0.01 to 0.2%, Nb: 0.01 to 0.2%, 0.01% ≦ Ti + Nb ≦ 0.2%
Ni-based high alloy containing inevitable impurities as a clad material, low alloy steel as a base material, low alloy steel (base material) / high alloy (clad material) / high alloy (clad material) / When rolling a slab assembled by stacking like low alloy steel (base material) to manufacture two sets of clad steel plates, the slab is heated to 1100 to 1150 ° C,
The steel sheet surface just before the final reduction in hot rolling is 875 to 925 ° C.
The steel sheet is allowed to cool for 3 to 5 minutes after the rolling is finished, and then the temperature is set to 550 to 350 ° C and 10 to 50 ° C.
A method for producing a high Ni alloy clad steel sheet excellent in sour resistance and low temperature toughness, which comprises cooling with water at a cooling rate of / sec and then cooling.