JPH05144B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing welded steel pipes, and in particular, UOE is produced using high-strength steel sheets manufactured by a processing heat treatment method that has recently been widely adopted in the field of steel plate manufacturing. This invention relates to a method for manufacturing high-strength welded steel pipes using a pipe manufacturing method. (Conventional technology) In the UOE pipe manufacturing method, in which the joints are welded after UO forming a steel plate, it is customary to use a high heat input submerged arc welding method in which each layer is built up from the inside and outside in order to improve the efficiency of the welding process. It is. The steel plate used as the base material is one that has a low carbon equivalent or weld cracking susceptibility index (P _CM ), taking into account weldability during pipe manufacturing and weldability at the site where the product steel pipe is used. In order to manufacture high-strength steel pipes, it is necessary to use high-strength raw material steel sheets, but increasing the C content or reinforcing alloy components will increase the P _CM , and even if pipe manufacturing can be welded, , hardening of the weld heat affected zone during circumferential welding of steel pipes,
The risk of cracking increases. In order to deal with the above problems, recently, so-called water-cooled process heat treated steel plates (TMCP type steel plates), which are made from low carbon equivalent steel and made high in strength through process heat treatment during rolling, have come into use. However, TMCP
Shaped steel plates utilize transformation strengthening due to forced cooling after hot rolling, so in the pipe manufacturing method using high heat input welding, the above transformation strengthening effect is lost in the weld heat affected zone (HAZ), resulting in the weld zone being damaged. A problem arises in that the strength of the base metal is lower than that of the base material. This is because even if steel with a low carbon equivalent is transformed and strengthened by water cooling, it will be tempered by the temperature rise due to welding heat input and its strength will drop to near the standard state determined by the base metal components. For example, the Welding Society of Japan Welding Metallurgy Research Committee Symbosium "Welding Metallurgy of TMCP Steel" May 21, 1985,
P.140-150 of the handout on the 22nd shows the TMCP type HT50.
There is a detailed report on HAZ softening of steel sheets. Here, it is stated that the HAZ softening width of HT50 steel plate is about the same as the plate thickness, and the tensile strength is about 90% of the base material.
It is concluded that at this level, there is almost no decrease in strength due to HAZ softening of welded joints. As reported above, up to HT50 class
TMCP type steel plates have been studied to some extent. For example, considering welding conditions, the carbon equivalent of the base metal is increased to a level that does not impair weldability, and its strength is slightly higher than the standard value, thereby compensating for the decrease in strength due to HAZ softening. countermeasures have become possible. However, the problems of producing high-strength TMCP type steel sheets of HT60 class or higher, especially welded steel pipes using them, have remained completely unsolved. (Problems to be solved by the invention) For example, there is a strong demand for high strength large-diameter welded steel pipes used in oil delivery line pipes, etc.
Steel plates with a tensile strength of 60Kgf/mg or more are often used as the material. Increasing the carbon equivalent by increasing the alloy components to strengthen the base metal will lead to a fatal decline in weldability, so it is desirable to use TMCP type steel plates in this case as well. However, in such high-strength TMCP type steel sheets, the transformation strengthening by water cooling must be maximized, and therefore the problem of HAZ softening due to welding becomes even more serious. In manufacturing welded steel pipes from steel plates with a tensile strength of 60Kgf/mg or more, the present invention uses a low P _CM material steel plate that does not impair weldability, and a new welding method that does not cause a decrease in the strength of the welded part. The purpose is to provide a method for manufacturing steel pipes. (Means for Solving the Problems) The gist of the present invention is the following method for manufacturing a high-strength welded steel pipe. "In the method of manufacturing welded steel pipes using the UOE pipe manufacturing method using steel sheets that have a P _CM defined by the following formula of 0.175% or less and have been strengthened to a tensile strength of 60 Kgf/mg or more through processing heat treatment during rolling. , the ratio of the softening width (H ₀ ) of the base metal due to welding to the base metal wall thickness (t), , H ₀ /t is 0.30.
A method for producing a high-strength welded steel pipe, which is characterized by the following steps. _PCM =C+Si/30+Mn/20+Cu/20+
Ni/60+Cr/20+Mo/15+V/10+5B [%]" The present inventor joined steel plates that had been transformation-strengthened through water-cooled processing heat treatment under various welding conditions, conducted tensile tests and hardness distribution measurements of the joints, and By analyzing the data, we clarified the following. That is,
If the softening width H ₀ (area where the hardness is lower than the average hardness of the base metal) of the weld heat affected zone is 0.3 or less relative to the thickness of the steel plate, the joint tensile strength is equal to or higher than that of the base metal. That means the above. Based on this knowledge, it is possible to develop low-P _CM with few reinforcing components.
Welded steel pipes with sufficient joint strength can be manufactured even if TMCP type steel plates are used as the material. There are various specific ways to reduce H ₀ /t to 0.30 or less, one of which is to suppress the amount of heat input during welding. The experimental results will be shown later, but if the heat input per pass is 15 KJ/cm or less, H ₀ /t will be 0.30 or less, and the joint strength will be 60 Kgf/mg or more of the base metal. Furthermore, if the material steel plate contains 0.01 to 0.06% Nb (in this specification, unless otherwise specified, all "%" is "wt%"),
The heat input limit can be relaxed and the above heat input can be reduced.
It should be 40KJ/cm or less. The present invention has a tensile strength of 60Kgf/
It is assumed that the material for making pipes is a steel plate that has been strengthened to a strength of mg or higher. And this steel plate has a P _CM of 0.175
Must be less than %. P _CM is an index representing cold cracking susceptibility during welding, and the lower this value, the lower the cracking susceptibility. P _CM 0.175% corresponds to a value that does not cause cold cracking without preheating when product steel pipes are circumferentially welded using cellulose welding rods at the site of use. Regarding the chemical composition of the raw steel sheet, there are no special restrictions other than the above _PCM conditions. By processing heat treatment
All steel plates containing the minimum necessary alloy components sufficient to have a tensile strength of 60 Kgf/mg or more and having a P _CM of 0.175% or less are subject to the present invention. There are many possible combinations of alloy components for the composition of such steel sheets, but
Among the alloying elements used, the approximate contents of the elements involved in the above P _CM formula are as follows. C: 0.1% or less, Si: 0.5% or less, Mn: 2.0% or less, Ni: 0.5% or less, Cr: 0.5% or less, Cu: 0.35%
Below, Mn: 0.25% or less, V: 0.06% or less, B:
0.002% or less. Of course, it is not necessary to contain all of the above components, and components other than the above, such as Ti:
0.1% or less, Al: 0.07% or less, etc. may be contained as appropriate. Nb as an alloy component has a special effect. That is, when a steel plate containing 0.01 to 0.06% of Nb is used as a material, it is easy to reduce H ₀ /t to 0.30 or less even if the heat input during welding increases to some extent. The processing heat treatment process for the raw steel plate is, for example, rolling a slab heated to ₃ Ac points to 1280°C, finishing rolling at 800°C to ₃ Ar points, and then rolling at a temperature of at least 450°C from this rolling end temperature. area
Forced cooling is performed at a cooling rate of 15 to 50°C/sec. Of course, this process should be selected depending on the composition of the steel sheet and the required strength level, and in short, p _CM
The processing heat treatment conditions may be determined so that a high tensile strength of 60 Kgf/mg or more can be imparted even if the amount is 0.175%. (Function) Hereinafter, the function of the present invention will be explained based on experimental data. [Experiment 1] Steel with the ingredients shown in Table 1 was melted in a high-frequency melting furnace and made into a slab with a thickness of 100 mm and a width of 200 mm.
The steel plate was heated to 1250°C, rolled to a thickness of 30mm using normal rolling, and water-quenched at 850°C to produce a water-cooled heat-treated steel plate. A bevel as shown in the notes column of Table 5 was cut in this steel plate, and multilayer build-up welding was performed with the heat input per layer (one pass) varied to 9, 17, 25, 35, 50, and 75 KJ/cm. The hardness distribution of the cross section of the joint after welding was determined, and a tensile test piece (JIS 4
A tensile test piece) was taken from the center of the plate thickness. These test results are also shown in Table 1. In Table 1, TS _BM is the tensile strength of the base material (Kgf/mg),
TS _jpiot is the tensile strength of the joint (Kgf/mg). Also,
As shown in FIG. 1, H ₀ indicates the softening width of the weld heat-affected zone at the center of the plate thickness (the width of the part whose hardness is lower than the average hardness of the base material).

【table】

[Experiment 2]

In Experiment 1, it was found that if the material steel sheet contained Nb, the critical heat input that reduced the joint tensile strength increased, so here we investigated the effect of the Nb content. Steel with P _CM adjusted in the range of 0.16 to 0.175% and Nb content varied in five stages in the range of 0.005 to 0.073% was melted in a high-frequency melting furnace, and a steel plate with a thickness of 19 mm was prepared using the same method as in Experiment 1. The inner and outer surfaces were welded in a single layer with a heat input of 39.9KJ/cm. The chemical composition of the sample material and the results of the same tensile test and Charpey impact test as in Experiment 1 were summarized in the second experiment.
Shown in the table. Furthermore, FIG. 4 shows the relationship between Nb content, joint toughness (vE _-40 ), and TS _jpiot /TS _BM . As seen in these results, 0.01% Nb
In the above cases, the tensile strength of the joint exceeds that of the base metal. On the other hand, the joint toughness (vE _-440 ) initially increases with the Nb content, but when Nb is 0.06%
On the contrary, if it exceeds this value, it will be lower than that which does not contain Nb.

【table】

[Table] From the above experimental results, it is clear that by setting H ₀ /t to 0.30 or less, it is possible to weld without reducing the joint strength, and that when using a steel plate containing 0.01% or more Nb, the input per pass is By keeping the heat input below 40KJ/cm, and when using a steel plate that does not contain Nb, keeping the heat input below 15KJ/cm, H ₀ /t becomes 0.30 or below, which is equivalent to the strength of the base metal. It has become clear that a high-strength joint can be obtained. Furthermore, to ensure the toughness of the joint,
It is also clear from Figure 4 that the upper limit of the Nb content should be 0.06%. Example 1 Steel with the composition shown in Table 3 was made into a slab by continuous casting, heated to 1250°C, and then rolled by normal rolling as shown in Table 4.
The sheets were rolled to six different thicknesses from 12.7 mm to 38.1 mm, and the rolling was completed at 850°C and cooled from this temperature to 450°C at a cooling rate of approximately 30°C/sec. Select a steel plate with a tensile strength of 60Kgf/mg or more from the obtained steel plates, and
This method was used to form a blank pipe, and the joint was welded in one layer on the inner and outer surfaces using three-electrode submerged arc welding to create a welded steel pipe. Table 4 shows the welding conditions at this time.
The welding material used was a commercially available material applicable to the base metal strength specified by JIS, etc. Additionally, before submerged arc welding, tack welding is performed over the entire length of the joint using Ar-CO ₂ automatic welding. Normal conditions were used for welding current and welding speed. A tensile test piece and a hardness distribution test piece were taken from the welded joint of the obtained welded steel pipe based on API standards, and each was measured. The results are summarized in Table 4. Looking at the results in Table 4, joint tensile strength is higher than base metal strength in Nos. 1, 2, and 3.
No. where the plate thickness is thick and the welding heat input exceeds 40KJ/cm.
4, 5, and 6, the softening width (H ₀ ) is large, and H ₀ /t is
The joint tensile strength exceeds 0.30 and is lower than that of the base metal. Note that ΔHv in Table 4 indicates the difference between the average hardness of the base material and the minimum hardness of the softened part, as shown in FIG.

【table】

【table】

[Table] Example 2 Using steel plate No. 5 (plate thickness 31.7 mm) in Table 4, multilayer welding was performed under various conditions with single-sided welding, and measurements similar to those in Example 1 were performed. The conditions and measurement results are summarized in Table 5. From these results, regardless of the type of welding method,
If the heat input per pass is 30KJ/cm or less
It can be seen that when TS _jpiot /TS _BM is 1 or more, there is no decrease in the tensile strength of the joint. (Effect of the invention) According to the present invention, the weld cracking susceptibility index (P _CM )
A method for manufacturing high-strength welded steel pipes has been established, which uses steel plates with small sizes that have been made high-strength by transformation strengthening through processing heat treatment. According to the method of the present invention, a transformation-strengthened steel plate having a tensile strength of 60Kgf/mg or more is used.
With the UOE pipe manufacturing method, there is no concern that the strength of welded joints will decrease. Furthermore, the product steel pipe does not contain any unnecessary alloy components, and is therefore inexpensive and has excellent weldability.

【table】

【table】 [Brief explanation of the drawing]

Figure 1 is a cross-sectional view of the weld showing how to measure the softening zone width (H ₀ ) of the weld heat-affected zone, and Figure 2 shows the welding heat input and joint/base metal strength ratio (TS _jpiot /TS _BM ). Figure 3 is a diagram showing the relationship between the two, and Figure 4 is a diagram showing the influence of the ratio (H ₀ /t) of the softening zone width (H ₀ ) to the base material thickness (t) on TS _jpiot /TS _BM . is the Nb content of the base material and
3 is a diagram showing the relationship between TS _jpiot /TS _BM and joint toughness (vE _-40 ).

Claims (1)

[Scope of Claims] 1. Made by the UOE pipe manufacturing method using a steel plate whose P _CM defined by the following formula is 0.175% or less and which has been strengthened to a tensile strength of 60 kgf/mm ² or more by heat treatment during rolling. In the method of manufacturing welded steel pipes, the ratio (H ₀ /t) of the softening width of the base metal due to welding (H ₀ ) to the base metal wall thickness (t) is
A method for manufacturing a high-strength welded steel pipe, characterized in that the strength is 0.30 or less. _PCM =C+Si/30+Mn/20+Cu/20+
Ni/60+Cr/20+Mo/15+V/10+5B [%] 2 Claim 1: H ₀ /t is set to 0.30 or less by limiting the amount of welding heat input per pass.
A method for producing a high-strength welded steel pipe as described in Section 1. 3 The amount of welding heat input per pass is determined by the material steel plate.
Nb: 40KJ/ when it contains 0.01~0.06%
2. The method for manufacturing a high-strength welded steel pipe according to claim 2, wherein the manufacturing method is limited to 15 KJ/cm or less when the pipe does not substantially contain Nb.