JPH0211654B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical field) The technical content described in this specification regarding the processing method of welded steel pipes can be applied to curved material pipes used, for example, in river crossing sections of pipelines using welded steel pipes. In order to provide secondary workability that allows the steel pipe to undergo secondary workability, the composition range of the weld metal of the steel pipe was determined based on the results of investigating the processing conditions that would maintain the original performance of the original steel pipe even after the processing process. Therefore, the present invention proposes a method for processing a welded steel pipe in which processing conditions for using the welded steel pipe as a bent pipe, which are limited to the range of the weld metal composition, are specified. (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-temperature high-strength steel sheets containing Nb, but the rolling temperature and reduction rate are controlled to ensure strength and toughness. After forming by a method such as a method, pipes are usually manufactured by double-sided single-layer submerged arc welding. By the way, curved pipes with the same outer diameter as the main line are used for river crossing sections and curved piping parts around pump stations, but these curved pipes, which were previously manufactured separately by forging or welding, are also used. Recently, there has been a growing trend to bend welded steel pipes as described above due to delivery time and cost considerations. (Problem) Although the above-mentioned welded steel pipes are usually processed at high temperatures from the viewpoint of bending workability, they have high strength and high toughness when as welded. As a result, the toughness deteriorates, especially in the weld metal, so preventing this is a major problem. Methods for obtaining weld metal with high strength and high toughness by so-called quenching-tempering treatment or normalizing treatment after welding have already been disclosed, for example, in Japanese Patent Publication Nos. 1987-19297 and 1981-19381. The chemical composition of the metal and heat treatment conditions are indicated, but in the case of bent pipe manufacturing, considerable processing strain occurs in each part of the steel pipe during bending, and phenomena that are unfavorable for toughness such as precipitation and structural changes occur. This will accelerate the deterioration of toughness. 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 metal of welded steel pipes 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 60 Kgf/ ^mm2 and a low-temperature toughness of about 7 Kgfm at -46℃ after forming an as-welded raw pipe and a bent pipe at high temperatures, it is necessary to After specifying the chemical 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 the γ grains during hot bending. It was also found 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 includes C: 0.12wt% or less, Si: 0.10 to 0.50wt%, Mn: 0.80 to 2.30wt%, Al: 0.010 to 0.070wt%, Ni: 0.20 to 3.00wt%, Mo: 0.10 wt% or less, Ti: 0.015 to 0.050 wt%, and B: more than 0.0020 wt% to 0.0050 wt%, N: 0.010 wt% or less O: 0.025 to 0.050 wt%, and further contains 0.035 wt%.
% or less of Nb and 0.040wt% or less of V, and the remainder is iron and other contaminants that inevitably enter during welding. A welded steel pipe having a seam weld is heated. Processing of welded steel pipes characterized by performing hot secondary processing at a temperature of 850 to 1050°C for a holding time of 120 seconds or less, and then cooling to 300°C at an average cooling rate of 15 to 60°C/sec. It's a method. 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. That is, 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)
Since it has the effect of inhibiting grain growth, reducing the amount of oxygen excessively is not preferable from the viewpoint of toughness after bending heat treatment.
0.025~0.050% content and especially 850~1050℃
It is important to perform secondary processing within 120 seconds at a heating temperature in the range of . Note that bending at a temperature lower than 850°C has a large resistance to deformation, 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, but after When Nb carbonitrides are precipitated as fine Nb carbonitrides by reheating, the toughness deteriorates significantly. Therefore, when heat-treating weld metal containing Nb and using it, care must be taken not to produce fine carbonitrides, but the upper limit of the heating temperature should be set at 1050°C and the upper limit of the amount of Nb in the weld metal. By setting Nb to 0.035%, it is possible to reduce toughness deterioration 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, but since the transformation is almost completely completed by 300℃, the cooling rate after bending is 300°C. It is sufficient to consider temperatures up to ℃. If the average cooling rate from the heating temperature to 300°C is faster than 60°C/sec, the hardness of the weld metal will be too high, and even if tempering is performed as necessary, there will be little hardness loss, resulting in high strength. It becomes difficult to ensure sufficient toughness, and if the cooling rate is slower than 15°C/sec, coarse ferrite will form, making it difficult to ensure sufficient toughness and causing a significant decrease in strength. 300℃ even after completing processing quickly like this
It was discovered that it is necessary to control the cooling rate appropriately during the process, and that 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 the welded steel pipe that has been bent as described above, the processing heat treatment conditions described above are necessary.
In addition to this, it is difficult to ensure low-temperature toughness at the -46°C level unless the chemical composition of the weld metal is regulated. 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 0.12%, high carbon martensite, which is harmful to toughness, will be generated during quenching (cooling), and toughness will not improve even after tempering. The amount of C needs to be 0.12% or less. Si: Si is an essential component that enters this type of weld metal from the base metal, etc., and is also important from the viewpoint of toughness measures.
A lower limit of 0.10% or more is required. If it exceeds 0.50%, it will not only be difficult to secure toughness in the as-welded state, but also the polygonal ferrite grains will become large even after processing heat treatment, making it impossible to obtain good toughness, so the Si content should be 0.10% to 0.50%. did. 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 0.80%, deoxidation tends to be insufficient and it is difficult to maintain the strength of weld metal. It's difficult.
If the content 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%. Al: Al is used for deoxidizing and fixing nitrogen.
It is also a necessary element from the perspective of microstructural refinement, but if it is less than 0.010%, no effect can be expected, and if it exceeds 0.070%, the ferrite will become coarse and the toughness as welded will be significantly poor. Therefore, it is necessary to set it to 0.010 to 0.070%. Ni: Ni is an element that is effective in improving the strength and toughness of as-welded weld metal along with the above-mentioned Mn and later-mentioned Mo, 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 when added at the same time as Ti and B (described later), weld metal with extremely good toughness can be produced. can get. However, Mo tends to generate high carbon martensite during heat treatment, and the toughness does not improve even after tempering, so when considering the toughness after heat treatment, the upper limit of the amount added is 0.10%. Ti, B: Next, regarding Ti and B, the overall effect of these is not only when welding, but also when welding.
Even after processing heat treatment, fine grained ferrite is generated,
Since good low-temperature toughness can be obtained, we will discuss them together. The basic function of B is to suppress the precipitation of grain boundary ferrite generated at prior austenite grain boundaries, but this effect can no longer be expected if B becomes a nitride or oxide. 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 B at the same time. ,
If the amount added is limited, this effect will not be lost even after processing heat treatment. If the amount of B is less than 0.0020%, grain boundary ferrite is likely to be formed, and if it exceeds 0.0050%, concentrated B polarization occurs at the grain boundaries, resulting in low toughness. On the other hand, regarding Ti, the amount to effectively utilize B is
It is 0.015 to 0.050%, and if it is less than 0.015%, it is difficult to obtain fine ferrite, and
If it exceeds 0.050%, the amount of solid solution Ti increases, resulting in low toughness. N: In order to prevent nitridation of B and to prevent deterioration of toughness due to solid solute N, it is necessary to limit the content to 0.010% or less. As for O:O, as mentioned above, it is necessary to set it to 0.020% to 0.050%, considering both the as-welded toughness and the toughness after processing heat treatment. Nb, V: Normally, a controlled rolled steel plate containing Nb and V is used as the base material for welded steel pipes, and these elements are contained in the weld metal by diluting the base material during welding. As welded, these elements are in a solid solution state and do not have a decisive effect on toughness, but
When finely precipitated during processing heat treatment and tempering process,
Toughness deteriorates significantly. In the case of the above processing heat treatment conditions, the Nb amount is 0.035
% or less, and if the V amount is 0.040% or less, it is possible to secure toughness even if one or more of these is contained, so the upper limits are set at 0.035% and 0.040%, respectively.
%. 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 component range of this invention, up to 0.020% is allowed. Examples of the present invention will be described below. Example 1 A 25.4 mm thick steel plate having the chemical composition shown in Table 1 was machined with a V-groove at an angle of 60° and a depth of 11 mm, and the wire shown in Table 2 and the flux shown in Table 3 were combined to generate a heat input of 68 kJ/ cm V-groove single-layer submerged arc welding was performed. In order to adjust the composition of the weld metal, welding was performed by spraying the necessary alloy components in appropriate amounts into the groove before welding. The amount of oxygen in the weld metal is determined by the basicity of the flux and the amount of deoxidizing elements in the base metal, wire, and dispersed alloy, but it was mainly changed by changing the fluxes used in combination.

【table】

【table】

[Table] 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.

【table】

[Table] First, regarding the influence of heating temperature, weld metals in Table 4
No. 3 has an average cooling rate from each heating temperature to 300℃ with heating from 750℃ to 1150℃ and holding time of 60 seconds.
Figure 1 shows the change in absorbed energy at -46°C when heat treated at 30°C/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, austenitization occurs only partially, resulting in an uneven structure and poor toughness. Also 1050℃
At higher temperatures, the austenite grains become coarser and form a lath-like structure, resulting in brittleness, and the appropriate heating temperature is
It can be seen that the temperature is 850-1050℃. Next, regarding the effect of heating holding time, we also applied No. 3 weld metal in Table 4 by changing the holding time at 900°C and 1050°C from 20 to 180 seconds, and from each heating temperature to 300°C.
Figure 2 shows the change in absorbed energy at -46°C when heat treatment was performed at an average cooling rate of 30°C/sec, followed by tempering at 600°C. If the holding time at the above heating temperature is within 120 seconds, the structure of the weld metal will be fine ferrite.
It can be seen that if the time exceeds 120 seconds, the toughness deteriorates due to the formation of a lath-like structure, and therefore it is necessary to perform processing heat treatment within 120 seconds. Figure 3 is when the heating temperature is 950℃ and the holding time is 60 seconds.
The figure shows the absorbed energy at -46°C when the average cooling rate is changed from 950°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, in the range of 15 to 60°C/sec, the structure is good and exhibits high toughness, whereas outside this range, the toughness deteriorates. Next, regarding the influence of the oxygen content in the weld metal, we used the weld metal shown in Table 4 at a heating temperature of 950°C and a holding time of 60°C.
Average cooling rate 30℃/sec from 950℃ to 300℃
Figure 4 shows the results of heat treatment followed by tempering at 600°C. The same figure also shows the results for the as-welded condition. As welded, good low-temperature toughness can be obtained with an oxygen content in the range of 0.020 to 0.050%, but after heat treatment, good toughness can only be obtained with an oxygen content in the range of 0.025 to 0.050%. This is because when the amount of oxygen is small, it is easy to form a lath-like structure after heating and cooling. Considering the toughness of both as-welded and after heat treatment, the amount of oxygen is
It needs to be 0.025% or more. 0.050%
If it exceeds 20%, there will be too much oxide and high toughness will not be obtained. The relationship between the heat treatment conditions specified in this invention and the composition of the weld metal has been mainly described above with respect to No. 3 in Table 4, but it has been confirmed that other samples in the same table exhibit almost the same behavior. Example 2 A V-groove 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 V-groove submerged arc welding with a heat input of 68 kJ/cm. In order to adjust the composition of the weld metal, an appropriate amount of the necessary alloy was sprinkled into the groove before welding. Absorbed energy at -46°C of weld metal as welded and after heating to 950°C, holding for 60 seconds, cooling at an average cooling rate of 30°C/sec between 950°C and 300°C, and tempering at 600°C. Table 5 shows the chemical composition of the weld metal. In Table 5, weld metals A1 to A7 of the present invention were -46°C both as welded and after heat treatment.
The absorbed energy in all cases is 7Kgfm or more. On the other hand, in Comparative Examples B1 to B7, good toughness was not obtained in either state.

[Table] Note: Underlined items indicate components outside the scope of the present invention.
Example 3 Outer diameter 600 mm, wall thickness with weld metal shown in Table 6
A 25.4 mm API5LX-X65 test tube was subjected to secondary processing, and a curved tube with a radius of curvature of 3000 mm was heated at 950°C within 120 seconds, and cooled at an average rate of 30°C/sec between 950° and 300°C. In addition, a part of the curved pipe part was tempered at 600℃. Round bar tensile test pieces and impact test pieces were taken from the part that was left heated and cooled and the part that was tempered after cooling, and compared with the values before bending. The results are shown in Table 7. Weld metals that meet the conditions of the present invention have good absorbed energy at -46°C, while comparative weld metals do not have good toughness. Tensile strength is 60Kgf/ ^mm2 both as welded and after heat treatment.
We were able to secure more than that.

【table】

[Table] (Effects of the invention) As described above, it is possible to make welded steel pipes have sufficient performance as line pipes for pipelines for transporting oil, natural gas, etc. The welded steel pipe processed by the method of the present invention has a -46 Since the impact properties and sufficient tensile properties at °C can be maintained even after the processing, it is possible to advantageously construct pipelines without the need to adjust the delivery schedule between specially designed forged curved pipes.

[Brief explanation of drawings]

Figures 1 to 4 show the influence of thermal history conditions on the toughness of weld metal shown in Table 4. Figure 1 shows the heating temperature, Figure 2 shows the holding time at 900°C and 1050°C, and The figure is a graph showing the cooling rate 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.

Claims (1)

[Claims] 1 C: 0.12wt% or less, Si: 0.10 to 0.50wt%, Mn: 0.80 to 2.30wt%, Al: 0.010 to 0.070wt%, Ni: 0.20 to 3.00wt%, Mo: 0.10wt % or less, Ti: 0.015 to 0.050wt%, and B: more than 0.0020wt% to 0.0050wt%, N: 0.010wt% or less, O: 0.025 to 0.050wt%, and further 0.035wt.
% or less of Nb and 0.040wt% or less of V, and the remainder is iron and other contaminants that inevitably enter during welding. A welded steel pipe having a seam weld is heated. Processing of welded steel pipes characterized by performing hot secondary processing at a temperature of 850 to 1050°C for a holding time of 120 seconds or less, and then cooling to 300°C at an average cooling rate of 15 to 60°C/sec. Method.