JPH0592282A - Method for producing clad steel sheet with excellent sour resistance and low temperature toughness - Google Patents

Abstract

(57) [Summary] [Object] The present invention provides a rolling method capable of imparting both low temperature toughness of a base material and sour resistance of a liner material when rolling a clad steel sheet. [Structure] Low alloy steel-austenitic alloy-austenitic alloy-low alloy steel is assembled in a sandwich form, and when the clad slab is welded around the four upper and lower low alloy steels, the heating temperature of the slab, the low alloy steel Reduction ratio of
By optimizing the surface temperature of the steel sheet at the time of rolling finish, the cooling time after rolling, the water cooling rate after cooling, and the water cooling stop temperature, the austenitic alloy is solution-treated and has the same level of corrosion resistance as commercial materials, and a low alloy. The steel should have high toughness. [Effects] According to the present invention, it is possible to manufacture a clad steel plate for a large-diameter line pipe, which is excellent in sour resistance and low temperature toughness by metal bonding, without heat treatment.

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 clad steel sheet having excellent sour resistance and low temperature toughness, which is used as a raw sheet for a line pipe for transporting natural gas or crude oil containing hydrogen sulfide. It is a thing.

[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 causes an unnecessary increase in cost. Therefore, attention has been paid to the use of a so-called clad line pipe in which a high alloy is used on the inner surface side in contact with the corrosive gas and a low alloy is used on the outer surface side to reduce the amount of the high alloy used.

In the production of a clad line pipe, the material for the inner pipe is required to undergo solution treatment at 1000 ° C. or higher 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 UO 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 large diameter line pipes in the UO steel pipe class as market needs, establishment of a manufacturing technique for large diameter line pipes that does not rely on off-line steel pipe heat treatment is desired.

As a method of performing no off-line heat treatment of steel pipe, a method of obtaining a double pipe by fitting a solution-treated inner pipe and an outer pipe by a shrink fitting method (see, for example, JP-A-55-55).
No. 117514), and is attracting attention as a method for maximizing the characteristics of the respective materials of the inner pipe and the outer pipe.

[0005]

In the above shrink-fitting method, if the shrink-fitting temperature is set to the sensitization temperature or lower at which the corrosion resistance of the composite material decreases, the good corrosion resistance of the composite material that has been solution-treated in advance is impaired. Since it is possible to manufacture a clad pipe without using it, it is certainly not necessary to perform an off-line steel pipe heat treatment. However, the interface of the shrink-fitting type clad is not mechanically bonded but mechanically bonded, and therefore is not applicable when metallic bonding is desired.

Therefore, it is necessary to manufacture the clad material in which the base material and the composite material are metallically bonded to each other, and to impart the corrosion resistance of the composite material in the manufacturing process. Further, since the low temperature toughness is required for the base material in order to use it as a line pipe, it is necessary to pay attention to the manufacturing history of the base material.

The present invention advantageously solves the above-mentioned problems. In clad rolling, the rolling finish temperature, the cooling time after rolling and water cooling, and the water cooling rate are regulated to secure the corrosion resistance of the composite material. An object of the present invention is to provide a clad steel sheet excellent in sour resistance and low-temperature toughness in which the toughness of the base material is secured by regulating the rolling heating temperature, the reduction ratio, the rolling finishing temperature, the water cooling rate, and the water cooling stop temperature.

[0008]

In order to achieve the above object, the present invention has the following structures. That is, in the production of a clad steel plate using a low alloy steel containing Nb as a base material and an austenitic alloy excellent in sour resistance as a composite material, a low alloy steel (base material) -austenite alloy (composite material)- Austenitic alloy (composite material) -Stacked like a low alloy steel (base material), a sandwich-like slab in which the upper and lower low alloy steels are welded on four sides is assembled, and the slab is heated to 1100 ° C to 1150 ° C. Hot rolling with a reduction ratio of 5 or more for the low alloy steel is performed so that the steel sheet surface immediately before the final reduction is 950 ° C to 1050 ° C, and after the rolling is finished, the steel sheet is allowed to cool for 3 to 5 minutes, and then 550. ℃ ~ 350
A method for producing a clad steel sheet excellent in sour resistance and low temperature toughness, which comprises water cooling at a temperature of 10 ° C./s to 50 ° C./s and then cooling.

[0009]

The present invention will be described in detail below.

The present inventors have focused on utilizing sensible heat immediately after sandwich rolling of the clad in order to achieve solution treatment of the composite material online without performing solution heat treatment offline. API showing chemical composition in Table 1
Base material of low alloy line pipe steel for standard X65 grade,
Using Incoloy's Incoloy 825 as a composite material, a sandwich slab with a base material thickness of 150 mm and a composite material thickness of 20 mm was assembled and rolled in a plate mill until the total thickness was 40 mm, and the effect on the solution state after rolling was examined. did. Figure 1
The effects of the rolling finish temperature and the cooling time after rolling on the recrystallization of the laminated material are shown. According to this experiment, the rolling finishing temperature was set to 950 ° C or higher, and the cooling time after rolling was 3 mi.
It has been found that the composite material is completely recrystallized when n or more. Further, the rolled composite material was subjected to a corrosion test for 48 hours in a 10% ferric chloride solution to determine the temperature at which pitting corrosion did not occur. ) Is 32.5 ° C. or higher and excellent corrosion resistance is obtained, that is, it is clear that sufficient solution treatment is performed. On the other hand, it was also found that in the unrecrystallized or partially recrystallized state of the composite material, the critical pitting temperature was less than 32.5 ° C, and the sufficient solution treatment was not performed.

From the above experimental facts and from the viewpoint of ensuring the toughness of the base material described later, the surface of the steel sheet immediately before the final rolling is 9
It was concluded that if the temperature is set to 50 ° C. to 1050 ° C. and the steel sheet is left to cool for 3 to 5 minutes after the completion of rolling, the composite material is sufficiently solution-treated and excellent corrosion resistance is obtained. That is, the present invention uses a low alloy steel containing Nb as a base material, and an appropriate rolling finishing temperature in the production of a clad steel sheet using an austenitic alloy excellent in sour resistance as a composite material, and a cooling time until water cooling after rolling. Is a method for producing a clad steel sheet excellent in sour resistance and low temperature toughness, which is mainly to secure excellent corrosion resistance of the laminated material and low temperature toughness of the base material.

Next, the reasons for limiting the rolling conditions in the present invention will be described. The slab assembled into a sandwich is
1100 ° C in order to fully solutionize the bonding material during heating
Heat to above. 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 set to 1100 ° C to 1150 ° C.

The working ratio in rolling is 5 or more in terms of reduction ratio. This is because in the austenite recrystallization temperature range of the base material, the austenite grains are recrystallized by hot working, and the grain size before transformation is made fine to ensure good toughness after rolling, and in order to achieve this object. In addition, a reduction ratio of the base material of at least 5 is required.

Hot rolling is finished at 950 ° C. or higher. This is because, as shown in FIG. 1, the sensible heat of the clad steel sheet after rolling is utilized to completely recrystallize the composite material to obtain a solution state. For the purpose of solutionizing the composite material, the higher the rolling finish temperature is more desirable, but if it becomes too high,
Since the austenite grain size of the base material increases and the toughness decreases, the rolling finishing temperature is set to 1050 ° C or lower.

After the rolling is completed, the clad steel plate is allowed to cool for 3 to 5 minutes. This is because the sensible heat of the clad steel sheet that has been rolled at a high temperature of 950 to 1050 during cooling is utilized to solution-process the composite material, and as shown in FIG. It requires a cooling time. But,
When the cooling time is long, the composite material reaches the so-called sensitization temperature, and the corrosion resistance deteriorates. Also, since the base material also undergoes ferrite transformation before water cooling starts, the structure after water cooling is an air cooled structure +
It becomes a mixed state of a water-cooled structure, which is not preferable for securing toughness. Therefore, the cooling after rolling is within 5 minutes.

Thereafter, the clad steel plate is heated to 550 ° C.
Water cooling is performed at a temperature of 350 ° C. at a cooling rate of 10 ° C./s to 50 ° C./s. This is because the composite material is water-cooled in the sensitization temperature range at a cooling rate of 10 ° C./s or more after the on-line solution treatment to suppress precipitation of carbides and the like to prevent sensitization. Further, it is because the base material is water-cooled in the ferrite transformation temperature range 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. Therefore, the upper limit of the water cooling rate is 50 ° C./s. The water cooling stop temperature is set to 550 ° C. or lower where the ferrite transformation temperature range of the base material ends. If water-cooled to 550 ° C., it is possible to avoid sensitization of the liner material. When the water-cooling stop temperature is 350 ° C. or lower, a large residual strain accompanying cooling of the steel sheet occurs and the shape is deteriorated. Therefore, the lower limit of the water-cooling stop temperature is 350 ° C.

[0017]

EXAMPLES Table 1 shows chemical components. That is, a slab for API line X65 line pipe was used as a base material, and a commercially available thick plate of Incoloy 825 was used as a laminated material to assemble a sandwich slab as a hot-rolled material.
The slab thickness of the base material is 150 mm, the thickness of the laminated material is 20 mm,
The slab thickness after assembly was 340 mm. This assembled slab was rolled under the rolling conditions shown in Table 2 until the total thickness became 40 mm. However, in Comparative Example 2, in order to reduce the reduction ratio of the base material, the rolling was finished so that the total thickness was 72 mm. The thickness of one set of clad steel plates manufactured under each condition except Comparative Example 2 is about 20 mm, of which the thickness of the laminated material is about 1.5 mm on average between the upper and lower clad steel plates combined during rolling.
Became. In the case of Comparative Example 2, the base material was about 33.3 mm and the laminated material was 2.7 mm.

A 2 mm V-notch Charpy test piece was prepared perpendicularly to the rolling direction from the base material of the clad steel plate and subjected to an impact test. The low temperature toughness of the base metal is evaluated by the fracture surface transition temperature (vTrs), and considering the characteristics as a line pipe, vTrs
It was considered to have excellent toughness when ≦ −60 ° C. was achieved.

Further, a corrosion test piece was prepared from a clad steel sheet composite material, and a corrosion test was conducted in a 10% ferric chloride solution for 48 hours to determine the critical pitting temperature. According to the general evaluation standard of corrosion resistance, it was considered that the material had excellent corrosion resistance when the critical pitting temperature was 32.5 ° C. or higher, that is, the solution treatment was sufficiently performed. Incoloy 825 exhibits excellent corrosion resistance and stress corrosion cracking resistance in a sour environment if it is sufficiently solution-treated, and sour resistance can be evaluated by the level of the critical pitting temperature.

As shown in Table 2, when the clad steel sheet was manufactured under the conditions according to the present invention, a clad steel sheet having sour resistance and excellent low temperature toughness was obtained. However, the heating temperature in Comparative Example 1 and the reduction ratio of the base material in Comparative Example 2 were
In Comparative Examples 3 and 4, the steel plate surface temperature immediately before the final reduction, in Comparative Example 5 the cooling time after rolling was finished, in Comparative Examples 6 and 7, the water cooling rate was measured, and in Comparative Examples 8 and 9, the water cooling stop temperature was measured. Deviations from the conditions according to the invention reduce the low temperature toughness of the base metal or the corrosion resistance of the composite to below the desired level.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

According to the present invention, the sour resistance and low temperature toughness for large-diameter line pipes, which were difficult to manufacture without conventional off-line heat treatment, and the base material and the composite material are metallic Since the clad steel plate bonded to the can be manufactured by the online solution treatment, the industrial effect is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing influences of a steel plate surface temperature immediately before final rolling and a cooling time after completion of rolling on recrystallization behavior of a laminated material.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yoshinori Ogata 1 Kimitsu, Kimitsu-shi, Chiba Nippon Steel Corp. Kimitsu Steel Co., Ltd.

Claims (1)

[Claims] 1. A low alloy steel containing Nb as a base material and an austenitic alloy excellent in sour resistance as a composite material,
Low Alloy Steel (Base Material) -Austenitic Alloy (Composite Material)-
In the method for producing a clad steel sheet, which comprises stacking an austenitic alloy (composite material) and a low alloy steel (base material), and rolling a sandwich-shaped slab in which the upper and lower low alloy steels are welded on four sides, the slab is manufactured at 1100 ° C To 1150 ° C., hot rolling is performed so that the reduction ratio of the low alloy steel is 5 or more so that the steel sheet surface immediately before the final reduction is 950 ° C. to 1050 ° C., and the steel sheet is rolled for 3 to 5 minutes after rolling. Allow to cool, then cool to 550 ° C-350 ° C, 10 ° C / s-50 ° C / s
A method for producing a clad steel sheet having excellent sour resistance and low temperature toughness, which comprises cooling with water at a cooling rate of 1, and then allowing to cool.