JPS61190019A - Electric welded steel pipe having excellent sour resistance in electric weld zone - Google Patents

Abstract

PURPOSE:To improve the sour resistance of an electric weld zone by subjecting the specific range on both sides of the electric welded butt surfaces as a center to a heating treatment under specific conditions after electric welding. CONSTITUTION:The parts in the range within 100mu on both sides of the electric welded butt surfaces as a center are heated for >=1min to a temp. range of 450-720 deg.C, more preferably 450-620 deg.C after production of an electric welded steel pipe. The basic component compsn. of the electric welded steel pipe is composed, by wt%, of about <=0.30% C, about 0.02-1.0% Si, about 0.2-1.8% Mn, about <=0.03% P and about <=0.003% S and is added with a rare earth element, Ca, Mg, Al, etc., at an adequate ratio. The electric welded steel pipe which obviates the generation of a hydrogen blister crack even in the severe environment of low pH and has excellent sour resistance of the electric weld zone is thus obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an ERW steel pipe having an ERW welded part with excellent sour resistance. This invention relates to electric resistance welded steel pipes that have high cracking resistance even in environments containing hydrogen sulfide.

(Conventional technology and its problems) Oil and natural gas produced in recent years often contain hydrogen sulfide, and when water such as seawater and fresh water coexists, corrosion occurs on steel surfaces. In addition to thinning caused by corrosion, hydrogen generated on the steel surface may penetrate into the steel, causing fracture.This fracture is caused by sulfides, which have long been recognized in high-tensile steel. This is different from stress cracking, and can occur even when there is no externally applied stress.

This fracture occurs because hydrogen that has entered from the environment φ accumulates at the boundary between the steel base and A-based sulfide inclusions such as MnS that are present in the base metal and extend in the rolling direction, and is gasified by the gas pressure. The above-mentioned A-based sulfide inclusions such as MnS form sharp notches, which serve as crack nuclei to grow into parallel cracks on the plate surface, and these parallel cracks are connected in the thickness direction. It is something. This type of cracking is hereinafter referred to as "parallel hydrogen bulge cracking."

Various studies have been conducted on steels that have high resistance to parallel-to-plate hydrogen blister cracking, and various steels have been proposed. For example, they are
Typical examples can be found in Japanese Patent Publication No. 57-16184 and Japanese Patent Publication No. 57-16184.

Prevention of cracking by addition, reduction of MnS by extremely low S, C
These techniques utilize the fixation of S by adding a or rare earth elements, and to date, steels that can withstand even quite severe environments have been developed using these techniques.

By the way, ERW steel pipes are manufactured by forming steel plates such as hot coils and ERW welding them, and needless to say, the decisive difference from steel plates is the presence of welded parts and weld heat-affected zones. However, there have been few examples in the past in which the sour resistance of the periphery of the electric resistance welded part has been studied. This is because in normal manufacturing processes, A-based sulfide inclusions such as MnS are present in large numbers in the inverted V segregation area and V segregation area in large steel ingots, and in the central segregation area in continuous slabs. A such as MnS is extremely rare at the edges of steel sheets, and the micro-segregation of Mn and P, which promote parallel cracking of the sheet surface, is severe.
In so-called single-weight materials, which are produced by electric resistance welding the edge parts of steel plates, because these are the same parts where many sulfide-based inclusions exist, and there are almost no inclusions in the edge parts. This is because it has been understood that the sour resistance of the area around the electric resistance welding part is good.

In addition, in the so-called multi-stripe electric resistance welded steel pipe, which is manufactured by dividing one hot coil into two or more parts in the width direction, an inverted V
Since areas that are highly susceptible to parallel-to-plate hydrogen blistering, such as segregated areas and central segregation, are located on one or both of the ERW welds, parallel-to-plate hydrogen blistering was recognized, but this In this case, measures similar to those for the base material have been taken, mainly to reduce A-based sulfide inclusions such as MnS and to reduce microsegregation.

On the other hand, the present inventors conducted a detailed study on the sour resistance of ERW welds of ERW steel pipes, and found that hydrogen blisters occur in ERW welds even when sulfide-based inclusions such as MnS are not present. In some cases, it was found that in the case of electric resistance welded parts, hydrogen blistering cracks occur in the form of vertical cracks on the plate surface, which is different from the base metal part. Furthermore, it was found that hydrogen blister cracking occurs even in single-weight steel sheets with little micro-segregation at the edges of the steel sheet. This cracking has not been known in the past, and is as serious a problem as, or even more serious than, parallel-to-plate hydrogen bulging cracks in the base metal. Furthermore, it was found that this cracking occurs even in electric resistance welded steel pipes that use conventional parallel-plate type steel against hydrogen bulging cracks, and cannot be prevented using conventional techniques.

The present inventors have continued their research to develop a completely new type of steel pipe that is perpendicular to the plate surface, which has high resistance to hydrogen blistering cracking. The cause of hydrogen bulging cracks perpendicular to the plate surface of the welded part is that the electric resistance sewing abutment part 2 and its both sides z1 and z2 are 10
It was determined that these were plate-shaped oxide-based inclusions existing in the heat-affected zone 8 within 0 μm. Furthermore, among these plate-shaped oxide-based inclusions, the shape of the inclusions as seen in a cross section within 100 μm on both sides of the electric resistance stitching abutting portion 2 shown in FIG. Inclusions with a length ratio of 2 or more in the circumferential direction and a major diameter of 10 μm or more become the core of hydrogen blister cracking perpendicular to the plate surface, and that the ratio of the length in the thickness direction to the length in the circumferential direction is 2. When the density of oxide inclusions exceeds 5 inclusions with a major diameter of 10 μm or more in the cross section of the IW2 hole, nucleated hydrogen bulges perpendicular to the sheet surface occur. It was found that they bond with each other and grow into macroscopic cracks. Furthermore, according to the research of the present inventors, 1. These plate-shaped oxide inclusions are formed when nearly spherical oxide inclusions that previously existed in the base metal are heated to near the melting point of the steel by the heat effect during electric resistance welding, and then squeeze rolls are applied to both sides. It has become clear that the material deforms into a plate shape due to the pressure applied to it.

Based on the above knowledge, the present inventors have already applied for a patent application in 1982-1970.
No. 546, 100μm on both sides centering on the sutured surface
Among the oxide-based inclusions contained within, inclusions whose shape as seen in cross section has a ratio of the length in the plate thickness direction to the length in the circumferential direction of 2 or more and a major axis of 10 μm or more are 1w12
We have proposed an electric resistance welded steel pipe with excellent sour resistance in which the number of cuts in the cross section of the hole is 5 or less. As a result, it was found that even in the presence of plate-like oxide inclusions as described above, a nine-density welded steel pipe with excellent sour resistance can be obtained. The sour resistance of the electric resistance welded part is improved by heat treatment.

(Means for Solving the Problems) The present invention has been made based on these findings, and its gist is that after manufacturing an ERW steel pipe, a portion within 100 μm on both sides of the ERW abutting surface is 450~ including
The present invention provides an electric resistance welded steel pipe with excellent sour resistance of the electric resistance welded part, which is heated to a temperature range of 720° C. for 1 minute or more.

The present invention will be explained in detail below.

First of all, the present invention is aimed at general electric resistance welded steel pipes with excellent sour resistance, and the basic components are 0.80% or less by weight, 0.02 to 1.0% Si,
Mn 0.2-1.8%, po, oa% or less, so,
It is preferable that it is less than or equal to ooas. The range of these components was limited for the following reasons.

First, C is an element that most stably increases the strength of steel, but if it exceeds 0.30%, it impairs toughness and weldability, making it difficult to use.

Next, since Si is an element necessary for deoxidation, it is necessary to contain it in an amount of 0.02% or more, but in order to ensure toughness, the upper limit content should be kept at 1.0%.

In addition, Mn is a necessary element from the viewpoint of deoxidation and maintenance of strength and toughness, but the upper limit content is set at 1.8 to ensure weldability.
It should be kept in check. In addition, from the perspective of ensuring strength and toughness, the content is required to be 0.21 or more. On the other hand, P is an element that facilitates the propagation of hydrogen blistering cracks in the base material and should be 0.03 or less. .

Furthermore, since S combines with Mn to form MnS, which becomes the starting point for hydrogen blistering cracks in the base material, it must be suppressed to less than o, ooa% in order to ensure the sour resistance of the base material.

In order to ensure the sour resistance of the base material, it is preferable to add an appropriate amount of rare earth elements, Ca, Mg, etc. to control the shape of sulfides. In this case, it is preferable to add an appropriate amount of At to strongly deoxidize the steel.

Therefore, the main point of the present invention is to heat the electric resistance suture abutting surface, including the parts within 100 μm on both sides thereof, to a temperature range of 450 to 720° C. for 1 min or more, This is based on the findings obtained through the following experiments.

That is, the present inventor investigated the effects of various heat treatments using steel A whose components are shown in Table 1 as a test material.

First, steel having the composition shown in Table 1 was hot-rolled into a steel plate with a thickness of 11 mm, and then made into an electric resistance welded steel pipe using a normal process. Note that the electric resistance welded portion was subjected to seam normalization heat treatment, and the heating temperature was 1020°C. After cutting the electric resistance welded steel pipes manufactured by welding under the same conditions, heat treatment was applied to a portion within 100 μm around the electric resistance welded abutment surface under the conditions shown in Table 2.

In Table 2, AQ is the seam norm (weld normalization) heat treated, A1 to A9 are heat treated only in the vicinity including the part within 100 μm on both sides centering on the electric resistance welding abutment surface, For Al0-Al2, the entire steel pipe was heat treated. The heating temperatures shown in Table 2 are those obtained by accurately measuring the temperature within 100 μm on both sides of the ERW abutment surface using a thermocouple attached to the ERW abutment, and are based on the heating time. is the holding time after the temperature measurement point reaches the heating temperature. After holding, the steel tube was air cooled.

Next, the wall thickness tl is measured from these ERW steel pipes as shown in Figure 2.
``Thickness t2 = 9 including the electric resistance sewing abutment part 2 of the 11 mm steel pipe.
A test piece 5 with a width W of 20 mm and a length t of 100 mm was taken and subjected to a sour resistance evaluation test. Note that 4 in the figure is the welding direction.

As an evaluation test for sour resistance, the above test piece was subjected to HzS
Saturated, 0.5% CH3C0O in 5% NaCl aqueous solution
96hr in a solution (pH 2,8-3.8) with addition of H.
It was immersed and cracking was measured. The presence or absence of cracking was determined by ultrasonic flaw detection on two cross sections of the test piece as shown in FIG. 3, and then by microscopic observation of the cross sections. In the same figure, P is the UST detection direction for surface-parallel hydrogen bulging cracks, and R
is the UST detection direction for hydrogen bulging cracks perpendicular to the plate surface.

As shown in Table 2, the sour resistance evaluation test results show that the heat treatment after ERW welding is only 7-mm norm (weld zone standard).
Alternatively, heat treatment at a heating temperature of less than 450°C causes hydrogen blistering cracks perpendicular to the plate surface, whereas 450°C
It is clear that steel pipes heated to a temperature of .degree. C. or higher and 720.degree. C. or lower do not develop hydrogen blister cracks perpendicular to the surface of the plate and have significantly improved sour resistance, making it possible to completely prevent this type of cracking. On the other hand, steel pipes heated to temperatures exceeding 720°C suffer from vertical hydrogen blister cracking and have poor sour resistance.

As a result of observing the cross section of the test piece after the test, AO to A
2 and A8 to A9, the hydrogen blister crack perpendicular to the sheet surface propagates within 100 μm on both sides centering on the ERW abutment surface, and the plate-like oxide-based inclusion is the starting point. On the other hand, in A3 to A7 and AIO to AI2, C
Microscopic observation of the cross section confirmed that despite the presence of plate-like oxide inclusions, no hydrogen blister cracks perpendicular to the plate surface had occurred. As mentioned above, A3 to A7 are heat-treated only in the vicinity of the electric resistance welded abutting surface, including within 100 μm on both sides, and Al0-Al2 are heat-treated to the entire steel pipe. However, the sour resistance of the ERW welded part is significantly improved.When heat-treating, it is necessary to apply heat treatment to the entire steel pipe, as long as it includes at least a portion within 100 μm on both sides of the ERW abutment surface. does not affect the sour resistance of the ERW weld. That is, it is important to heat-treat the portions within 100 μm on both sides of the electric resistance welding abutment surface, which is the area where hydrogen blister cracks perpendicular to the sheet surface occur and propagate.

Therefore, either heat-treat a part of the steel pipe in the circumferential direction, including the area within 100 μm on both sides of the ERW abutment surface, or
Whether or not to heat treat the entire circumferential direction of the steel pipe can be selected after considering factors such as cost.

Table 1 (% by weight) The reason why the heating temperature was set at 450 to 720°C when heat-treating the area within 100 μm on both sides of the electric resistance sewing abutment surface in the present invention is based on the above experimental results. This is because the effect is not sufficient if the heating temperature is lower than 450°C, and the sour resistance of the electric resistance welded part is poor even if the heating temperature exceeds 720°C. On the other hand, if the heating time is less than 1 min, the effect is insufficient, so the heating time should be 1 m1n or more.

The mechanism by which the sour resistance of the ERW weld is improved by the heat treatment described above is not clear, but it is probably due to the
Residual strain and residual stress around the plate-shaped oxide-based inclusions, which are caused by the difference in thermal expansion coefficient between the plate-shaped oxide-based inclusions and the surrounding steel, disappear when heated to 450-720°C. Therefore, the occurrence of hydrogen blistering cracks perpendicular to the plate surface from plate-shaped oxide inclusions is thought to be suppressed, and the hardened area around the electric resistance welding part that occurs during the cooling process is Point 02 is considered to be that the heat treatment at 720° C. softens the material and lowers the hardness to the same level as the base material, which increases the resistance to the propagation of hydrogen blister cracks perpendicular to the sheet surface.

In this case, if only the sour resistance of the ERW weld is considered, the effect is the same regardless of the heating temperature within the temperature range of 450 to 700°C; On the other hand, especially when it is desired to ensure the same strength as the base material, it is preferable to set the heating temperature to a temperature range of 450 to 620°C.

As for the manufacturing process of the steel that is the raw material for the steel pipe of the present invention, the steel may be hot-rolled as it is, or a process of standardizing, tempering, or quenching and tempering the rolled material may be applied. The pipe-making process uses electric resistance welding.

In the production of the material, which process is to be applied may be determined depending on the need to ensure properties such as strength and toughness of the ERW steel pipe base material.

Hereinafter, the effects of the present invention will be described in more detail with reference to Examples.

(Example) Steel with the composition shown in Table 3 was hot-rolled into a 12.7 mm thick steel plate, and then made into an ERW steel pipe with an outer diameter of 400 mm through a normal process. Heat treatment was applied. In Table 3, 1 to 3 are steel pipes of the present invention, and 4 is a comparative material.

Test pieces were taken from these electric resistance welded steel pipes as shown in FIG. 2, and sour resistance evaluation tests were conducted in the same manner as in Table 2. The results are shown in Table 4. As is clear from Table 4, no cracks occurred in the steel pipe of the present invention, whereas cracks perpendicular to the plate surface did occur in the comparative material, although no cracks occurred parallel to the plate surface.

(Effects of the Invention) As can be seen from the above test results, the present invention makes it possible to provide an ERW steel pipe with excellent sour resistance at the ERW welded part without hydrogen blistering cracking even in a harsh environment with low pH. However, there are some things that make a huge contribution to the development of industry.

[Brief explanation of drawings]

Figure 1 is a schematic diagram showing the abutting part of an ERW steel pipe and the area to be heat-treated on both sides thereof, Figure 2 is a diagram showing the procedure for collecting a test piece, and Figure 3 is a diagram showing the UST flaw detection direction. . 1... ERW steel pipe 2... Abutment zone 3... Heat affected zone 4... Welding direction 5... Test piece applicant Nippon Steel Corporation @IIIJ

Claims (1)

[Claims] After manufacturing the ERW steel pipe, 10 parts on both sides centering on the ERW mating surface.
An electric resistance welded steel pipe having excellent sour resistance at an electric resistance welded part, which is heated to a temperature range of 450 to 720° C. for 1 minute or more including a portion within 0 μm.