JPS60258411A - Method for working welded steel tube - Google Patents

Abstract

PURPOSE:To prevent the deterioration of strength and toughness after the working by using a welding metal having a specified compsn. and applying the secondary hot working under a specified temp. condition in manufacturing a welded steel tube. CONSTITUTION:As the welded steel tube to be used as pipe line in cold district, etc., a seamed steel tube having the seamed welded part by the welding metal made of a steel contg. by weight, <0.12% C, 0.10-0.50% Si, 0.80-2.30% Mn, 0.010-0.070% Al, 0.20-3.00% Ni, <0.10% Mo, 0.015-0.050% Ti, 0.0020- 0.0050% B, <0.010% N, 0.025-0.050% O and one or two kinds of <0.035% Nb, <0.040% V is subjected to bending working. In this case said tube is heated to 850-1,050 deg.C, then hot worked with >=120sec holding time. In this way the secondarily worked product of the welded steel tube such as bent tube, etc. without the deterioration of strength and toughness by bending working is manufactured by rapid cooling to 300 deg.C at 15-60 deg.C/sec rate.

Description

[Detailed Description of the Invention] (Technical Field) The technical content described in this specification regarding the processing method of welded steel pipes is applied to the material of curved pipes used, for example, in river crossing sections of pipelines using welded steel pipes. In order to have secondary workability that can be applied to pipes, the composition range of the weld metal of the steel pipe has been changed so that the original performance of the original steel pipe is maintained even after the processing process. Based on the results and conditions, we propose a method for processing welded steel pipes that specifies processing conditions for using the welded steel pipe as a curved pipe, which is limited to the range of the weld metal composition.

(Background technology) Pipelines are an efficient method for transporting large volumes of oil, natural gas, etc., and many long-distance pipelines have been constructed around the world. There is a tendency to increase.

The higher the pressure inside the pipe, the higher the strength required of the pipe, but in particular, when used in cold regions, not only strength but also high toughness at low temperatures is required, and steel plates require adjustment of chemical composition and special controlled rolling. By applying this method, we have obtained products that can almost satisfy the required performance.

This type of steel pipe generally uses non-tempered high-strength steel sheets containing Nb, but the rolling temperature and reduction rate are controlled to ensure strength and toughness, and the UOE method, bending roll method, etc.
After forming by spiral method etc., pipes are usually manufactured by double-sided single-layer submerged arc welding method.

By the way, curved pipes with the same outer diameter as the line main pipe are used for river crossing sections and curved piping parts around pump stations, but this curved pipe was previously manufactured separately by forging or welding. However, due to delivery time and cost considerations, there has recently been a growing trend to bend welded steel pipes as described above.

(Problem) Although the welded steel pipes mentioned above are usually processed at high temperatures from the viewpoint of bendability, but have high strength and high toughness when as welded, after being bent into pipes by high-temperature heating processing, depending on the reheating conditions. Since the toughness deteriorates, and the toughness of weld metal in particular deteriorates significantly, preventing this is a major issue.

Methods of obtaining weld metal with high strength and high toughness by so-called quenching and tempering treatment or normalizing treatment after welding have already been disclosed, for example, in Japanese Patent Publication No. 55-1.
9297 and 56-19381, the chemical composition of weld metal and heat treatment conditions are shown, but in the case of bent pipe manufacturing, considerable processing strain occurs in each part of the steel pipe during bending, causing precipitation and microstructure. Phenomena that are unfavorable for toughness, such as changes, occur and the deterioration of toughness is accelerated.

Therefore, it is difficult to solve this problem simply by applying the heat treatment conditions for straight pipes, and the heat treatment methods shown in the above publications are completely useless. It is necessary.

(Motivation for the Invention) In view of the current situation, the inventors have developed a method that not only ensures the toughness of welded steel pipe weld metal before processing, but also avoids the deterioration of toughness after forming bent pipes at high temperatures. A detailed study was conducted on the weld metal composition and processing conditions.

As a result, in order to obtain a weld metal that has a yield strength of about 40 to 604 f/mm2 and a low-temperature toughness of about 7 kgfm at -46°C after forming an as-welded raw pipe and a bent pipe at high temperatures, it is necessary to After specifying the composition, it was found that it was necessary to limit the heating temperature range and the elapsed time until the completion of bending in order to prevent coarsening of γ grains during hot bending. It was also discovered that in order to simultaneously ensure strength and toughness, it is important to appropriately control the average cooling rate during the cooling process after processing.

(Object of the Invention) The present invention not only can be used as a line pipe, but also can be easily processed into a curved pipe of the same diameter as a part of a pipeline without deterioration in strength and toughness. The purpose is to provide a method for processing welded steel pipes.

(Structure of the Invention) This invention contains C: 0.12 wt% or less, Si: 0.
10-0.50wt%, Mn: 0.80-2.30
wt%, AJ2: 0,010-0.070wt%, N
i: 0.20-3,00wt%, Mo20.10
wt% or less, Ti: 0.015 to 0.050 wt%, and B
: Contains more than 0.0020 wt% and up to 0,0050 wt%, N: o, o1wt% or less 0: 0.025 to 0,050 wt% and further contains 0.
A welded steel pipe having a seam welded part containing 0.035wt% or less of Nb and 0.040wt% or less of one or more of , heating temperature 850-1050℃
This is a method for processing welded steel pipes, in which hot secondary processing is performed for a holding time of 120 seconds or less, and then the average cooling rate to 300° C. is 15 to 15°C.

In this invention, the processing conditions after heating the welded steel pipe have the following important meaning in relation to the chemical composition of the weld metal.

In other words, regardless of whether or not it is subjected to hot secondary processing, the weld metal must have sufficient strength and low-temperature toughness as welded, and for this purpose, the lower the oxygen content, the better. .

However, in high-temperature heating, oxygen (oxide) has the effect of inhibiting the growth of γ grains, so reducing the amount of oxygen excessively is not preferable from the viewpoint of the stiffness after bending heat treatment. 1 between a content of .025 and 0.050% and a heating temperature in the range of 850 to 1050°C.
It is important to perform secondary processing within 20 seconds. In addition, when bending at a temperature lower than 850° C., the resistance to deformation is large, making it difficult to perform the bending process in a short time.

In addition, controlled rolled steel plates containing Nb are generally used as the base material for welded steel pipes, but Nb is dissolved in the weld metal as it is being welded and does not have a decisive effect on the toughness of the weld metal.
When fine Nb carbonitrides are precipitated by subsequent reheating treatment, the toughness deteriorates significantly. Therefore, when using heat-treated weld metal containing Nb, care must be taken not to produce fine carbonitrides, but the upper limit of the heating temperature should be set to 1050°C and the upper limit of the amount of Nb in the weld metal should be set to 0. By setting it to .035%, it becomes possible to reduce the deterioration of toughness due to fine Nb precipitation during tempering.

Next, in continuous cooling after heating, it is important to control the cooling rate until the transformation is completely completed.
Since the transformation is almost completely completed by 300°C, it is sufficient to consider cooling up to 300°C after bending. Average cooling rate from heating temperature to 300℃ is 60℃/s
If the cooling rate is faster than ec, the hardness of the weld metal will become too large, and even if tempering is applied as necessary, the hardness will not decrease much, so the strength will become too high and it will be difficult to ensure toughness. If it is slower than sea, coarse ferrite will be formed, making it difficult to ensure toughness and also causing a large decrease in strength.

It was discovered that even after such rapid processing is completed, it is necessary to control the cooling rate appropriately until the temperature reaches 300°C, and it is also necessary to regulate the composition of the weld metal accordingly.

Of course, the tempering treatment after cooling may be carried out as necessary.

In order to ensure the weld metal strength and low-temperature toughness of welded steel pipes that have been bent as described above, the processing heat treatment conditions described above are necessary. Without regulations, it will be difficult to ensure low-temperature toughness at the -46°C level. Only by specifying the chemical composition of the weld metal and applying appropriate processing heat treatment conditions can weld metals with sufficient strength and low-temperature toughness both in the as-welded and post-process heat treatment states be obtained.

Next, we will discuss the reasons for limiting the chemical composition of the weld metal.

C: Under the above heat treatment conditions, if the C content exceeds 12%, high carbon martensite that is harmful to toughness will be generated during quenching (cooling), and toughness will not improve by tempering. Therefore, Ct needs to be 0.12% or less.

3i: 3i is an essential component that enters this type of weld metal from the base metal, etc., and from the viewpoint of toughness, it is 0.10
% or more is required as the lower limit. On the other hand, 0.50%
If the Si content exceeds 0.1, it will not only be difficult to ensure toughness in the as-welded state, but also the polygonal ferrant grains will become large even after processing heat treatment, making it impossible to obtain good toughness.
The content was set at 0% to 0.50%.

Mn: Mn is an essential element for deoxidizing weld metal, and is also important from the viewpoint of strength and toughness.
If it is less than %, deoxidation tends to be insufficient and it is difficult to maintain the strength of the weld metal. On the other hand, if it exceeds 2.30%, the hardenability becomes too large, resulting in a lath-like structure and the toughness deteriorates, so the upper limit should be 2.30%.

Aβ: A°C is an element necessary for deoxidation, nitrogen fixation, and microstructural refinement, but o, oio
If it is less than 0.0, the effect cannot be expected.
If it exceeds 70%, the ferrite will become coarse and the toughness as welded will be extremely poor, so 0.010 to 0.07
It needs to be set to 0%.

Ni: Ni is an element that is effective in improving the strength and toughness of as-welded weld metal together with Mn described above and MO described below, but if it is less than 0.20%, no such effect can be expected. The above-mentioned effects of Ni remain even after processing and heat treatment, and it does not cause deterioration of toughness even when added in a wide range of amounts, making it an extremely effective element. However, if the amount added is too large, there is a risk of hot cracking occurring during welding, so the upper limit was set at 3.00%.

Mo: Mo is also an effective element for increasing hardenability and improving the toughness of as-welded weld metal, especially Ti and B described later.
When added at the same time, a weld metal with extremely good toughness can be obtained. However, MO tends to generate high carbon martensite during heat treatment and tempering does not improve toughness, so when considering the toughness after heat treatment, the upper limit of the addition amount is 0.10%.

Ti, 3: Next, Ti and 8 will be discussed together because their overall effect is that fine-grained ferrite is generated not only as welded but also after processing heat treatment and good low-temperature toughness can be obtained. B(7) The basic function is
The purpose is to suppress the precipitation of grain boundary ferrite generated at prior austenite grain boundaries, but if B becomes a nitride or oxide, the effect cannot be expected. By adding Ti, it is possible to suppress the nitridation and oxidation of B, and since Ti has the function of making ferrite grains finer, it is easy to ensure low-temperature toughness by adding Ti and 8 at the same time. If the amount added is limited, this effect will not be lost even after processing heat treatment.

If the B content is less than 0.0020%, grain boundary ferrite is likely to be formed, and if it exceeds 0.0050%, dense segregation of B occurs at the grain boundaries, resulting in low toughness.

On the other hand, for Ti, effective use of B is 0.015
~0.050%, and if it is less than 0.015%, it is difficult to obtain fine grain ferrite, and if it exceeds o or oso%, solid solution T1 increases, resulting in low toughness.

N: Regarding N, to prevent nitridation of B, and to prevent solid solution N.
In order to prevent the deterioration of toughness due to
It is necessary to limit it to 0.010% or less.

O: Regarding O, as mentioned above, considering both the as-welded toughness and the toughness after processing heat treatment, it is 0.02.
It is necessary to set it to 0% to 0.050%.

Nb, V: Normally the base material for welded steel pipes contains Nb and V.
Jill rolled steel plates are used, and these elements are contained in the weld metal by dilution of the base metal during welding.

As welded, these elements are in a solid solution state and do not have a decisive effect on toughness, but if they are finely precipitated during processing heat treatment and tempering processes, toughness will be significantly degraded.

In the case of the above processing heat treatment conditions, Nb1i is 0.035%
Hereinafter, as long as the V content is 0.040 or less, toughness can be ensured even if one or more of these is contained, so the respective upper limits are set as 0.035% and 0.040%.

Note that P and S enter the weld metal as impurity elements, but since they are elements that deteriorate toughness, they are not a big deal. Within the range of ingredients of this invention, all are 0.
．． Examples of the present invention will be described below, in which up to 0.020% is allowed.

Example 1 A V-groove with an angle of 606 and a depth of 11 mm+ was machined on a steel plate with a thickness of 25.4 mm and the chemical composition shown in Table 1.The wire shown in Table 2 and the flux shown in Table 3 were combined to create a heat input of 6.
11 V-groove single-layer submerged arc welding was performed at 8kJ/. In order to adjust the composition of the weld metal, welding was performed by spraying an appropriate amount of the necessary alloy components into the groove before welding.

The amount of oxygen in the weld metal is determined by the basicity of the flux and the presence of deoxidizing elements in the base metal, wire, and scattering alloy, but it was mainly changed by changing the combined fluxes.

Table 4 shows the chemical composition of weld metals, absorbed energy and hardness in as-welded state. Using these weld metals, we investigated the influence of processing heat treatment conditions.

First, regarding the influence of heating temperature, when weld metal No. 3 in Table 4 was heated from 750°C to 1150°C and held for 60 seconds, the average cooling rate from each heating temperature to 300°C was 30°C/
FIG. 1 shows the change in absorbed energy at -46° C. when heat treated to become SeC and then tempered at 600° C.

As is clear from FIG. 1, good toughness can be obtained in the range of 850 to 1050°C. At temperatures lower than 850°C, the austenitic structure is only partially transformed, resulting in an uneven structure and poor toughness. At temperatures higher than 1050°C, the austenite grains become coarse and lath-like, resulting in brittleness. It can be seen that the appropriate heating temperature is 850 to 1050°C.

Next, regarding the influence of heating holding time, No.
3 Holding time at 900℃ and 1050℃ for weld metal for 20
- 180 seconds, the average cooling rate from each heating temperature to 300°C is 30°C/Sec, and then tempering at 600°C.
Figure 2 shows the change in absorbed energy at 6°C.

If the holding time at the above heating temperature is within 120 seconds, the structure of the weld metal will be fine ferrite, but if it exceeds 120 seconds, a lath structure will occur and the toughness will deteriorate.
7Jl processing heat treatment within seconds! It can be seen that i1' is necessary.

Figure 3 shows 95% when the heating temperature is 950℃ and the holding time is 60 seconds.
It shows the absorbed energy at -46°C when the average cooling rate is changed from 0°C to 300°C.

The weld metal, tempering conditions, etc. used were exactly the same as in the previous example.

As shown in FIG. 3, 15-b has a good structure and exhibits high toughness, while outside this range the toughness deteriorates.

Next, regarding the influence of the amount of oxygen in the weld metal, using the weld metal shown in Table 4, the heating temperature was 950°C and the holding time was 950°C for 60 seconds.
Figure 4 shows the results of heat treatment at an average cooling rfJ of 30°C/sec from 300°C to 300°C, followed by tempering at 600°C. The same figure also shows the results for as-welded products.

As welded, good low-temperature toughness can be obtained with an oxygen content in the range of 0.020-0.050%, but after heat treatment, good toughness can only be obtained with an oxygen content in the range of 0.025-0.050%. I won't be able to do it. This is because when the amount of oxygen is small, a lath-like structure tends to form after heating and cooling, and considering the toughness after heat treatment while welding, the amount of oxygen needs to be 0.025% or more. On the other hand, if it exceeds 0.050%, there will be too much oxide, making it impossible to obtain high/υ properties.

The relationship between the heat treatment conditions specified in this invention and the composition of the weld metal has been mainly described for N013 in Table 4 above, but it has been confirmed that the sample No. 1 in the same table also exhibits almost the same behavior.

Example 2 A groove was added to the steel plate shown in Table 1, and the welding materials shown in Tables 2 and 3 were combined to perform single-layer submerged arc welding in a V-groove with a heat input of 68 kJ/(, m. In order to adjust the composition, an appropriate amount of the necessary alloy was sprayed into the groove before welding.Welding was performed by heating at 950°C as welded and after welding.
Average cooling rate between 950℃ and 300℃ after holding for seconds 30℃/
Table 5 shows the absorbed energy at -46° C. and the chemical composition of the weld metal after cooling with SeC and tempering at 600° C.

In Table 5, weld metals Δ1 to A7 of the present invention have an absorbed energy of 7 kgfm or more at -46° C. both as welded and after heat treatment.

On the other hand, none of Comparative Examples B1 to B7 had good toughness in both conditions.

Example 3 Outer diameter 600Il 1111 Wall thickness 25.4ml 1 (7) AP I 5LX-X65 test steel pipe having weld metal shown in Table 6 was subjected to secondary processing at a curvature of half 93000mm within 120 seconds by heating at 950°C 950°～
It was cooled at an average rate of 30°C/sec between 300°C. In addition, a part of the curved pipe part was tempered at 600°C. Round bar tensile test pieces and impact test pieces were taken from the part that had been heated and cooled and the part that had been tempered after cooling, and their values before bending were compared.

The results are shown in Table 7. Weld metals that do not meet the conditions of the present invention have good absorbed energy at -46°C, whereas comparative weld metals do not have good toughness.

Tensile strength is 60 kgr/mm2 both as welded and after heat treatment.
We were able to secure more than that.

(Effects of the Invention) As described above, it is possible to make welded steel pipes sufficiently provide the required performance in line pipes for transporting oil and natural gas, etc. The secondary workability required for the material used for curved pipes is achieved by specifying the composition of the weld metal and limiting the processing conditions.
Welded steel pipes processed into curved pipes using this method can maintain their impact properties at 6°C and sufficient tensile properties even after processing, so it is possible to adjust the delivery date between special specification 1 forged bent pipes. Pline construction is advantageously possible without the need for.

[Brief explanation of the drawings] Figures 1 to 4 show the influence of the thermal history conditions on the weld metal shown in Table 4. Figure 1 shows the thermal temperature, and Figure 2 shows the thermal temperature at 900°C and 1050°C. FIG. 3 is a graph showing the relationship between time, cooling between the heating temperature and 300° C., and FIG. 4 is a graph showing the relationship with the amount of oxygen in the weld metal. Figure 1 J degree (r:) Figure 2 Figure 3 Figure 4 Figure 4

Claims (1)

[Claims] 1, (:, : 0, 12 wt% or less, S i : 0
．． 10~0.50wt%, Mn: 0.80~2.3
0wt%, AA: 0.010-0.070wt%,
Ni: 0.20-3.00wt%, Mo: 0.
10 wt% or less, Ti: 0.015 to 0.050 wt%, and B
: More than 0.0020wt% and 0,0050wt%
N: 0,010 wt%, N: 0.025 to 0.050 wt%, further containing at least 0.035 wt% of Nb and 0.040 wt% or less of V, A welded steel pipe having a seam weld is heated to a temperature of 850 to 105°C, with the remainder being a weld metal composition of iron and contaminants that inevitably enter during welding.
Hot secondary processing is performed at 0℃ for a holding time of 120 seconds or less, and then the average cooling speed to 300℃ is 15~bs
A method for processing a welded steel pipe, characterized by cooling within the EC range.