JPH0747226B2 - Heat treatment method for welded joints with excellent strength and toughness - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a welded joint having excellent strength and toughness, which improves the strength and toughness of a welded joint such as steel pipes butt resistance welded by flash butt welding or the like. The heat treatment method of

[Prior Art] When laying a pipeline for oil transportation or gas transportation, a steel pipe of a predetermined length manufactured in a factory is transported to the site and the end faces of the steel pipe are butt-welded together at the site. Unlike the in-factory welding under favorable conditions, this on-site welding operation is a heavy labor due to the poor working environment. Further, in the circumferential welding of the fixed pipe, since the welding is performed in all the postures such as upward, vertical, and downward, usually, a high technique is required and the groove precision needs to be relatively high. Conventionally, for butt circumferential welding of pipelines, hand welding (coated arc welding) and flux cored wire semi-automatic welding (Flux cored arc weldi
FCAW), MAG welding (Metal active gas welding), etc. are used. However, since these welding processes are low in productivity and cannot accelerate the progress rate of pipeline laying work, mechanization of the welding process is being considered.

Recently, in laying a long-distance pipeline, there is a growing demand for a highly efficient welding process due to demands for shortening delivery times and cost reductions, and flash butt welding is drawing attention as a suitable welding process. Since flash butt welding has the advantages of higher efficiency than the above-mentioned conventional welding processes and of not requiring high groove precision, practical application is being considered for butt welding of steel pipes for pipelines.

On the other hand, steel pipe materials for pipelines tend to have higher toughness in order to meet the demand for improved economic efficiency, and also have higher toughness in consideration of the severer usage environment in cold regions and the like.
In addition, in order to improve the on-site weldability, the composition is designed so that the carbon equivalent value Ceq of the steel material is made as low as possible. Therefore, when manufacturing steel pipe materials for pipelines, the amount of addition of the strength compensating alloying element is limited, and therefore, the Thermo-Mechanical which accelerates and cools down the material in the low temperature region
-Control-Process (hereinafter referred to as TMCP) is adopted. Such TMCP steel has high strength because internal strain is imparted to the steel material in the manufacturing process.

By the way, when flash butt welding a TMCP steel pipe, the central region of the joint is kept at a high temperature for a relatively long time during the flushing process, so that the crystal grains are significantly coarsened and the toughness of the joint is significantly reduced.

Therefore, in the conventional heat treatment method, in order to improve the toughness of the flash butt welded joint, a so-called normalizing treatment is performed in which the joint is heated and held at about 950 ° C. for about 1 minute and then air-cooled.

[Problems to be solved by the invention] However, in the conventional heat treatment method, the internal strain imparted to the TMCP steel material is released by normalization, and the joint strength is about 10 kgf higher than the as-welded strength. / m
Since m ^{2 is} also reduced, there is a problem that the high tension effect of TMCP steel is impaired.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a heat treatment method for a welded joint having excellent strength and toughness that can improve the strength and toughness of the welded joint.

[Means for Solving Problems] A heat treatment method for a welded joint portion excellent in strength and toughness according to the present invention is C: 0.08% or less by weight%, Si: 0.50% or less, Mn:
0.05-2.50%, Nb: 0.005-0.100%, Ti: 0.100% or less,
Solid solution Al: 0.005 to 0.100%, nitrogen 0.0015 to 0.0100%, the balance substantially consisting of Fe and unavoidable impurities, butt flash butt welding of steel products manufactured by water cooling or air cooling after controlled rolling, and then, The weld joint is made of steel with Ac
It is characterized in that it is heated to ₃ transformation points or higher, held for 1 minute or less, and then the welded joint is rapidly cooled at a cooling rate of 5 ° C. or more per second. In this case, the steel material further comprises, by weight, Cu: 1.0% or less, Ni: 1.0% or less, Cr 1.0% or less, M
One or two or more of o: 0.5% or less and V: 0.15% or less may be contained.

[Operation] In the heat treatment method for a welded joint portion excellent in strength and toughness according to the present invention, when the steel material produced by water cooling or air cooling after controlled rolling is subjected to butt resistance welding, the base metal side of the weld heat affected zone appears. The structure becomes coarse, a softened region is generated, and the joint strength is slightly reduced. On the other hand, in the central region of the welded joint, the austenite crystal grains are remarkably coarsened because they are kept at a high temperature for a relatively long time in the melting process before pressurization, and after cooling, coarse austenite grains are formed on the former austenite grain boundaries. With precipitation of ferrite, an upper bainite structure containing a large amount of martensite and austenite is formed in the grains, and the toughness in this region is significantly deteriorated as compared with other regions. However, after welding, the joint is heated and held at the Ac ₃ transformation point or higher of the steel material, so that the heating region becomes an austenite single phase,
The coarse structure in the central region of the joint and the coarse structure in the heat affected zone disappear. And after heating and holding, continue to 5 per second
Since the joint part is rapidly cooled at a cooling rate of ℃ or more, the upper bainite structure containing a large amount of martensite and austenite is improved to a structure mainly composed of fine ferrite, and the toughness of the center part of the joint is improved. Further, since the softened region disappears and the entire heat-affected zone has a fine structure, the joint strength is improved. In particular, in the case of low carbon steel with a carbon content of 0.08% by weight or less, the welded joint is heated and maintained in the temperature range of the Ac ₃ transformation point or higher, and then rapidly cooled, so that the as-welded structure (old The martensite in which coarse pro-eutectoid ferrite is precipitated at the austenite grain boundaries and the upper bainite structure containing a large amount of retained austenite) are improved to a fine ferrite structure. That is, the carbon content in the steel is
When the content is 0.08% by weight or less, first, the structure is made into an austenite single phase by heating and holding it at the Ac ₃ transformation point or higher, and then this is quenched at a cooling rate of 5 ° C. or more per second, from the austenite phase to the pearlite phase and bainite phase. Only the ferrite phase can be precipitated without precipitating the phase. The ferrite structure thus obtained is uniform and fine, and has a toughness equal to or higher than that of the base material. Further, the heat-affected zone of welding is softened by the coarsening of crystal grains, but such a softened structure disappears by the above-mentioned heat treatment, and the strength level of the joint portion is improved more than that of the base metal.

[Examples] Recently, the demands of consumers for pipeline steels have become more sophisticated, and many steel materials having high strength and high toughness while satisfying the requirements for properties such as weldability and corrosion resistance. It came to be used. In order to meet such demands, a so-called Th in which steel sheet is water-cooled or air-cooled after being controlled-rolled
The ermo-mechanical-contorol-process (hereinafter referred to as the TMCP method) has been developed, and high-strength and high-toughness steel pipe pipes have been manufactured and put into practical use. In the TMCP method, the steel material to be rolled is heated to 950 ° C or higher, and hot finish rolling is performed in the non-recrystallized region or the two-phase mixed region of austenite / ferrite so that the total reduction rate is 20% or more, Immediately after rolling is finished, water cooling or air cooling is performed. Therefore, the structure of TMCP steel becomes finer, the internal strain increases, and high strength and high toughness are imparted to TMCP steel.

As an example of TMCP steel, the composition shown in Table 1 below is used.
There is API standard grade X80 (hereinafter referred to as X80). X80 has excellent mechanical properties such that the base material has a yield strength of about 55.3 kgf / mm ² and a tensile strength of about 63.4 kgf / mm ² .

By the way, the tensile strength of the welded joint of X80 as welded is about 60.7 kgf / mm, which is slightly lower than the base metal strength.
Become ² . This is because a softened region having a coarse grain structure is generated in the heat affected zone of welding. Further, the toughness value of the as-welded X80 weld joint is, for example, when the test temperature is 0 ° C. in the 2 mmV notch Charpy impact test.
1.0kgf ・ m, only 0.7kgf ・ when the test temperature is -20 ℃
It is only m, which is extremely low. Therefore, in the past, after welding, the welded part was heated to about 950 ° C. and then air-cooled for normalization (Normalizad) treatment to recover the toughness. When the test temperature was 0 ° C., 15.0 kgf · m, the test temperature was At -20 ° C
Although the toughness has been restored to the level of 7.4 kgf ・ m, the tensile strength of the welded joint is approximately 60.7 kgf / mm after normalizing.
² (as welded) to about 53.0kgf / mm ² (after normalizing). This is about 10 kgf / mm ² lower than the tensile strength of the base metal.

As a result of various studies on the strength and toughness of the X80 flash butt welded joint, the inventors have obtained the following findings regarding two points: (1) elimination of the softened region and (2) improvement of toughness.

FIG. 6 is a schematic diagram showing a flash butt welding apparatus. Steel pipes 10 and 12 are mounted on a support frame (not shown) so that their end surfaces face each other. The steel pipe 10 is fixed, but the steel pipe 12 is supported so as to be movable in the axial direction. The end faces of the steel pipes 10 and 12 are machined to be flat. Electrodes 14 and 16 are attached to steel pipes 10 and 12, respectively, and transformer 18
Also, an electric current is supplied from the power source to the steel pipes 10 and 12 via the control device 20.

When flash butt welding the steel pipes 10 and 12 with such an apparatus, the end faces of the steel pipe 10 and the steel pipe 12 are made to face each other with a slight gap, and a predetermined flushing current is applied to both to heat the end faces to each other. To do. And steel pipe 12
Press on 10 and lightly contact both end faces, and continue to energize. Electric sparks are generated on the contact surface to heat and melt it. Eventually, when the welding surface is covered with the metal vapor and the molten metal, the steel pipe 12 is upset to the steel pipe 10 and a large current is passed for only a few seconds to stop energization. After that, the welded joint is heated to a temperature above the transformation point of the steel material Ac ₃ by a gas flame, a resistance heating element, or the like, held for a predetermined time, and then cooled with water. In addition,
After welding, the joint may be rapidly cooled before the temperature of the joint falls below the Ac ₃ transformation point. In any case, the temperature of the joint shall be grasped quickly and accurately, and the temperature control of the heat treatment after welding shall be strictly performed.

FIG. 7 is a view showing a cross-sectional macrostructure of a flash butt welded joint portion of an X80 steel pipe. Normally, in the case of flash butt welding, the joint portion rises as shown in the drawing due to the pressure applied during the upsetting process, resulting in burrs. Burr is removed by grinder grinding or the like.

FIG. 8 is a view showing an as-welded microstructure of a flash butt welded joint portion of an X80 steel pipe. In the flushing process, the joint is kept at a high temperature for a relatively long time, so that the austenite crystal grains are remarkably coarsened and a structure in which coarse proeutectoid ferrite is precipitated at the former austenite grain boundaries is exhibited. In addition, the intragranular structure exhibits an upper bainite structure containing a large amount of martensite and retained austenite.

Next, the erasure of the softened region generated in the joint portion of the steel pipe thus welded will be described. TM with composition shown in Table 1
Butt welding of CP steel pipes by flash butt welding causes a softening zone on the base metal side of the heat-affected zone, resulting in a decrease in joint strength. The hardness in the softened region is about 50 lower than that of the base metal in Vickers hardness conversion value. There is almost a correlation between hardness and joint strength, and as hardness increases, joint strength also increases. Therefore, eliminating the softened region of the welded joint increases the strength of the joint.

Table 2 shows the composition of each test material (steel types A to F) used for the examination of the strength of the flash butt welded joint. Each test material is a steel material manufactured by TMCP treatment.

3 (a) to 3 (d), the horizontal axis indicates the distance from the welding center and the vertical axis indicates the Vickers hardness when the measured load is 10 kg. 3 is a graph diagram in which the hardness distribution of the cross section of the welded joint portion when flash butt welding B and B (pipe thickness 17.5 mm) is respectively investigated. 3 (a) and 3 (c) show the as-welded and heat-treated steel types A in Table 2 respectively. 3B and 3D show the as-welded and heat-treated steel types B in Table 2 respectively. In addition,
Regarding the heat treatment conditions for the steel types A and B, the heating temperature is about 950 ° C., the holding time is about 1 minute, and the cooling rate after heating is about 15 ° C. per second.

As shown in Fig. 3 (a) and (b) respectively, the as-welded type has a softened zone about 10 mm from the center of the weld, but if the welded joint is quenched under steam heat treatment conditions, As shown in FIGS. 3 (c) and 3 (d),
Hardness of base material, heat affected zone and weld metal is almost averaged,
The softened area disappears.

In addition, each numerical value in the table indicates% by weight and carbon equivalent value
For Ceq, the recommended formula of the Japan Welding Society (WES) shown in the following formula (1) was used.

Ceq = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 (1) In Fig. 1, the horizontal axis represents the cooling rate and the vertical axis represents the joint strength. Fig. 2 is a graph showing the joint strengths when the welded joints were rapidly cooled at various cooling rates after being heated to about 950 ° C. Fig. 2 shows the heating temperature on the horizontal axis and the joint strength on the vertical axis. Steel types A and B above
FIG. 5 is a graph chart in which the respective joint strengths of the respective welded joints are investigated when variously heated to a predetermined temperature in the range of 700 to 1100 ° C. and then rapidly cooled at a cooling rate of about 20 ° C. per minute. In the figure, the black circles show the results for steel type A, and the white circles show the results for steel type B.

As is clear from FIG. 1, when the cooling rate is 5 ° C. or more per second, the joint strength sharply increases. Further, as is clear from FIG. 2, when the heating temperature is set to the Ac ₃ transformation point or higher,
The joint strength sharply increases, and the joint strength exceeds the base metal strength at high temperature heating. The Ac ₃ transformation point was calculated by the following equation (2).

Next, the improvement of the toughness of the joint by improving the structure will be described. When flash butt welding a steel pipe, the central part of the joint is kept at high temperature for a long time in the flushing process before pressurization, so the austenite grains become significantly coarse, and coarse proeutectoid ferrite precipitates at these grain boundaries, and An upper bainite structure containing a large amount of martensite and austenite is formed in the steel, and the toughness of the central part of the joint is significantly reduced (see the microstructure photograph of FIG. 8). Such an as-welded metallographic structure depends on the chemical composition of the base metal and the thermal history of the welding. Therefore, it is possible to improve the toughness of the flash butt welded joint portion by setting the chemical composition of the base material in an appropriate range and performing an appropriate post-welding heat treatment.

The effect of each component element in the steel on the toughness of the welded joint will be described below, and the preferable component range of the TMCP steel to be joined by flash butt welding will be described.

In Fig. 4, the horizontal axis represents the amount of carbon in the steel, and the vertical axis represents the impact test value. After welding the steel types shown in Table 2, the as-welded joints (indicated by black circles in the figure) 3 is a graph showing the results of an impact test of a joint portion (indicated by a white circle in the figure) that is heat-treated at 950 ° C. for about 1 minute and then quenched at a cooling rate of about 30 ° C. per second. As is clear from the figure, the toughness of the as-welded joint is extremely low, but in the quenched joint, the toughness is improved when the carbon content of the low-carbon steel is about 0.08 wt% or less.

Furthermore, Fig. 9 (structure of steel B (carbon content 0.038% by weight) welded joint heated to 950 ° C and then rapidly cooled at a cooling rate of 30 ° C / sec) and Fig. 10 (steel C (carbon content 0.120%) (% By weight) welded joints were heated to 950 ° C and then rapidly cooled at a cooling rate of 30 ° C / sec), as is clear from the comparison of the metallographic structures of steel types with different carbon contents, In the case of (carbon content 0.038% by weight), the structure mainly composed of fine ferrite is exhibited, whereas the carbon content is 0.120%.
When the content is increased to wt%, the structure mainly composed of martensite and lower bainite is exhibited, so that the toughness is remarkably lowered.

Therefore, in order to improve the toughness of the flash butt welded joint by quenching, the upper limit of the carbon content of the steel is set to 0.08
Must be wt%.

As shown in Fig. 5, the joint strength decreases as the carbon content in the steel decreases, but the joint part is heated and held at about 950 ° C for about 1 minute and then quenched at a cooling rate of about 30 ° C per second (Fig. The white circle in the middle) has higher strength than the as-welded joint (indicated by the black circle in the figure), so a sufficient strength level of the base metal is secured even when the carbon content is low. be able to.

In the chemical composition of the steel material, the constituent elements other than carbon are preferably within the ranges shown below.

Si is necessary for deoxidation, but if added in excess, the toughness decreases, so the upper limit is made 0.50% by weight.

Mn needs to be added in an amount of 0.05% by weight or more for deoxidation, but if it exceeds 2.5% by weight, weldability deteriorates, so the upper limit is made 2.5% by weight.

Nb is effective in preventing coarsening of austenite grains during heating and softening of the welded joint when heating and quenching the welded portion, so the lower limit is made 0.005% by weight. However, if Nb is excessively added, the toughness of the welded portion is lowered, so the upper limit is made 0.10% by weight.

Ti improves the toughness of the weld by fixing nitrogen and also improves the toughness of the base material by refining the crystal grains, but when adding a large amount, it decreases the toughness conversely, so the upper limit The value is 0.10% by weight.

Solid solution Al needs to be added for deoxidation, but if added in excess, non-metallic inclusions increase and the absorbed energy value of the Charpy test decreases, so the lower limit is set to 0.005 wt% and the upper limit is set. Is 0.10% by weight.

Nitrogen has the effect of refining the crystal grains as a nitride and improving the toughness of the material. 0.00 for this effect
It is necessary to add more than 15% by weight of nitrogen,
If the content exceeds 0.010% by weight, the toughness of the heat-affected zone of the weld decreases, so the upper limit is made 0.010% by weight.

Further, in addition to the above elements, at least one of Cu, Ni, Cr, Mo and V can be selected and added.

Cu is effective as a strength compensation element for improving strength,
If added excessively, the toughness of the base material and the heat-affected zone deteriorates, so the upper limit is made 1.0 wt%.

Ni contributes to the improvement of the strength and toughness of the base metal, but if the addition amount increases, the cost increases, so the upper limit is 3.0% by weight.
And

Cr and Mo both contribute to the improvement of the base metal strength,
Since excessive addition impairs weldability, the upper limit of Cr is 1.0% by weight and the upper limit of Mo is 0.5% by weight.

V also contributes to the improvement of the base metal strength, but if the addition amount exceeds 0.15% by weight, the effect is saturated and the toughness decreases, so the upper limit is made 0.15% by weight.

Example 1 Using steel type G having the composition shown in Table 3, after flash butt welding, quenching was performed under each heat treatment condition shown in the same table. Regarding the TMCP conditions for steel type G, the heating temperature is 1150 ° C., the cumulative reduction rate in the unrecrystallized region and the two-phase region is 70%, and the cooling method is air cooling.

Under the conditions 1 and 4 satisfying the heat treatment conditions of the present invention, the joint strength is 58 kgf / mm ² or more, which is equal to or higher than the strength of the base metal. Also, in the impact test (test temperature: -20 ° C), the value was 8.4 kgf · m or more, which was a good result. On the other hand, in the case of condition 2 given as a comparative example, the cooling rate is slow, so the joint strength is 50 kgf / mm ²
Less than Further, in the case of the condition 3 of the comparative example, the heating temperature is below the Ac ₃ transformation point, so the joint strength becomes lower than that of the base metal.

Example 2 Using steel types H to T produced under the TMCP conditions as shown in Table 4, a steel pipe was flash-butt welded. Steel grade HT
The composition of is as shown in Table 5. After flash butt welding, the welded joint portion was quenched under the heat treatment conditions shown in Table 6 (heating and holding at 950 ° C., followed by rapid cooling at a cooling rate of 20 ° C. per second).

Steel grades satisfying the preferred composition of the present invention H, I, L, M, N, O, P
Also, at T, as shown in Table 6, the strength and toughness of the welded joint have good values. On the other hand, the steel type J of the comparative example has a high carbon content of 0.09% by weight and thus has a low impact test value of 3.2 kgf · m. Further, the steel type K of the comparative example has a higher carbon content of 0.1% by weight, so that the toughness value decreases to 2.1 kgf · m. Further, in the steel types Q, R, and S of the comparative examples, since the contents of Nb, Ti, and N are not within the proper ranges, the toughness values are all extremely low.

According to the above example, the flash butt welded joint is quenched to improve the microstructure, so that the high strength and high toughness characteristics of the TMCP steel pipe can be fully utilized. In particular, a remarkable effect is recognized in steel materials having a carbon content of 0.08% by weight or less, and since only one heat treatment operation can improve both the toughness and strength of the welded joint to a sufficient level, flash butt welding We were able to increase the chances of adopting to the welding process of pipeline laying work.

In addition, although the welded joint portion of the steel pipe has been described in the above embodiment, the present invention is not limited to this, and the present invention may be applied to a welded joint portion such as a round bar.

Further, in the above embodiment, the joint portion welded by butt welding by flash butt welding has been described, but the present invention is not limited to this, and the present invention may be used for other joint portions for butt resistance welding.

EFFECTS OF THE INVENTION According to the present invention, the strength and toughness of the joint can be improved by heat-treating the joint after the butt resistance welding. Therefore, laying a long-distance pipeline that requires high strength and high toughness. A highly efficient rush butt welding method can be applied to the construction. In particular, according to the present invention, the heat treatment operation of heating and quenching can be completed in a short time. Therefore, TMCP steel pipes can be joined with high efficiency, and the cost can be significantly reduced as compared with the conventional arc welding method.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the cooling rate of the welded joint and the joint strength, FIG. 2 is a graph showing the relationship between the heating temperature of the welded joint and the joint strength, and FIGS. 3 (a) to 3 (d). Is a graph showing the hardness distribution of each welded joint as-welded and after heat treatment, FIG. 4 is a graph showing the relationship between the carbon content in steel and the impact test value of the joint, and FIG. 5 is steel FIG. 6 is a graph showing the relationship between the carbon content in the steel and the joint strength, FIG. 6 is a schematic diagram showing the flash butt welding apparatus, FIG. 7 is a photograph showing the metal structure of the flash butt welding joint, and FIG. 8 is the flash butt. A micrograph showing the as-welded metallographic structure of the welded joint, FIG. 9 is a micrograph showing the metallographic structure of the flash butt welded joint with a carbon content of 0.038% by weight after heat treatment, and FIG. 10 shows the carbon content. Heat treatment of 0.120 wt% flash butt weld joint Is a photomicrograph showing the metal structure after. 10,12; Steel pipe, 14, 16; Electrode, 18; Transformer, 20; Control device

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Eiji Morishige Eiji Morishige, 1-2, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (56) Reference JP-A-59-43826 (JP, A) JP-A-SHO 58-19438 (JP, A) JP-B-51-43985 (JP, B2)

Claims (2)

[Claims] 1. C: 0.08% or less by weight%, Si: 0.50% or less, M
n0.05-2.50%, Nb: 0.005-0.100%, Ti: 0.100% or less,
Solid solution Al: 0.005 to 0.100%, nitrogen 0.0015 to 0.0100%, the balance substantially consisting of Fe and unavoidable impurities, butt flash butt welding of steel products manufactured by water cooling or air cooling after controlled rolling, and then, The weld joint is made of steel with Ac
_A method for heat treatment of a welded joint having excellent strength and toughness, characterized by heating to ₃ transformation points or higher, maintaining this for 1 minute or less, and then rapidly cooling the welded joint at a cooling rate of 5 ° C. or more per second. . 2. Further, the steel material is Cu: 1.0% or less in weight%,
Ni: 1.0% or less, Cr 1.0% or less, Mo: 0.5% or less, V: 0.15%
The heat treatment method for a welded joint having excellent strength and toughness according to claim 1, characterized in that the heat treatment method comprises one or more of the following.