JP2010046681A - Method for manufacturing high-strength thick-walled electric resistance welded steel pipe for line pipe - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a high-strength thick-walled electric resistance welded steel pipe with a seam part having high toughness. <P>SOLUTION: A steel strip having a specific composition is formed into an open pipe by a continuous pipe making process, and the ends in the circumferential direction of the open pipe are joined to each other by electric resistance welding to make a pipe having a seam part. When subjecting the seam part of the pipe to seam heat treatment by heating it from the outer periphery of the pipe, Q treatment is first carried out in such a manner that the peripheral part of the pipe in which the width of the weld zone from the weld center is substantially the same width of the seam part is heated to an Ac<SB>3</SB>point or above and is then cooled with water, followed by T treatment in such a manner that the peripheral part of the pipe in which the width of the weld zone from the weld center is 1.5 to 3 times the heating width in the Q treatment is heated to 400 to 650°C and then is cooled. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for producing a high-strength thick-walled electric-welded steel pipe for line pipes. Here, high strength means the strength of API standard grade X65 or higher, and thick means a thickness of 14.3 mm or more.

  The ERW welded portion of the ERW steel pipe and the heat-affected zone in the vicinity (hereinafter referred to as the seam portion) are extremely harder than the base metal due to the rapid heating experienced during welding and the subsequent rapid cooling due to heat removal. It is well known that it becomes high and consequently brittle. Therefore, in order to reduce the hardness of the seam portion to the same level as the base metal portion and increase fracture toughness, it is common to perform a seam heat treatment including a process of heating only the seam portion to the austenite temperature range after ERW welding ( Patent Document 1 [0002]).

Also, as commentary in Non-Patent Document 1, the process to heat only the seam portion to the austenite temperature region (or A ₃ points below the temperature) after electric resistance welding is referred to as post-annealing, the post annealing, It is carried out using a special local heat treatment facility called post-snealer with an induction heating method. The heating width (post-circumferential width of the region to be heated) of post annealing is about 20 to 30 mm.

Regarding seam heat treatment of ERW steel pipe, as in Patent Document 2, after ERW welding, the ERW weld and the heat affected zone in the vicinity thereof are heated at 750 to 1050 ° C. for 5 seconds or more, and then forcedly cooled. Is known (Patent Document 1 [0003]).
JP-A-6-145794 JP 59-35629 A Issued by Maruzen on November 20th, 1980, pp. 1084-1086, 3rd edition Steel Handbook, Volume III (2) Common Steel, Steel Pipe, and Rolling Equipment, edited by Japan Iron and Steel Association

In recent years, with the increase in strength and thickness of ERW steel pipes, the amount of alloy components added has increased, and there has been a concern that the two-phase zone heating part during seam heat treatment will become the starting point of brittle fracture. However, since the seam heat treatment relied on experience, no special measures have been taken to improve the toughness of the two-phase zone heating section.
In a relatively small diameter ERW pipe mill in Europe, a method of heat treating the entire circumference of the pipe after pipe making and welding is adopted. In this case, the problem of insufficient toughness of the two-phase zone heating part due to the seam heat treatment does not occur, but since the entire circumference of the steel pipe is heated, the heat treatment cost during production increases significantly. In addition, when manufacturing steel strips used as materials for ERW steel pipes, high strength and high toughness are achieved by refining the structure imparted to the steel strips by applying heat treatment technology. Since it will be canceled by the circumferential heat treatment, it is necessary to increase the strength by adding the alloy components, leading to a significant increase in the cost of the alloy components.

  That is, in the conventional technique, it is difficult to sufficiently improve the seam toughness of the high-strength thick-walled electric resistance welded steel pipe by the seam heat treatment, and this is a problem.

In view of the above-mentioned situation, the present inventors do not rely on the heat treatment of the entire circumference of the tube, and in the seam heat treatment, the structure effective for improving the toughness with respect to the two-phase zone heating part that has caused the toughness deterioration. The present invention was made by intensively studying a simple method for promoting generation. That is, the present invention is as follows.
(Claim 1)
In mass%, C: 0.02-0.12%, Si: 0.01-0.5%, Mn: 0.4-2.0%, P: 0.01% or less, S: 0.01 %, Al: 0.1% or less, and a steel strip having the balance consisting of Fe and inevitable impurities is made into an open pipe by continuous pipe making, and the circumferential ends of the open pipe are electrically connected to each other. A pipe having a seam portion formed by sewing and welding is formed, and when the seam heat treatment is performed on the seam portion of the pipe by heating from the outer periphery side of the pipe, first, the width around the welded joint is set to be almost the width of the seam portion. The pipe circumference portion is heated to Ac ₃ points or more and then subjected to water treatment Q treatment, and then the pipe circumference portion whose width around the welded joint is 1.5 to 3 times the heating width at the time of the Q treatment. High-strength thickness for line pipes, characterized in that T treatment is performed by heating to 400 to 650 ° C. and then allowing to cool. A method for manufacturing a meat electric welded steel pipe.
(Claim 2)
The composition according to claim 1, further comprising one or two kinds selected from Cu: 0.5% or less and Ni: 0.5% or less by mass% in addition to the composition. A manufacturing method for high strength thick ERW steel pipes for line pipes.
(Claim 3)
3. In addition to the composition, the composition further comprises one or two selected from Cr: 1.5% or less and Mo: 2.0% or less by mass%. The manufacturing method of the high intensity | strength thick-walled ERW steel pipe for description of a line pipe.
(Claim 4)
In addition to the above composition, the composition further contains one or more selected from the group consisting of Nb: 0.1% or less, V: 0.1% or less, and Ti: 0.1% or less in terms of mass%. The manufacturing method of the high strength thick-walled electric-welded steel pipe for line pipes of any one of Claims 1-3 characterized by the above-mentioned.
(Claim 5)
The high-strength thick-walled electric-welded steel pipe for line pipes according to any one of claims 1 to 4, further comprising Ca: 0.005% or less by mass% in addition to the composition. Production method.

  According to the present invention, a steel sheet having a specific composition is made into a pipe by an electric seam welding process following the pipe making process, and subjected to a QT process as a seam heat treatment. In the QT process, the heating width at the T process is By restricting to 1.5 to 3 times the heating width, it is possible to manufacture a high-strength thick-walled electric-welded steel pipe for line pipes with excellent seam toughness.

  From the viewpoint of suppressing brittle fracture in cold districts of high-strength thick-walled electric-welded steel pipes for line pipes having an API standard X65 grade or higher in strength, the present inventors applied seam portions after seam heat treatment (referred to as seam heat treatment portions for short). We examined the required toughness and the component systems that satisfy it. As a result, the required toughness is high toughness in which the fracture surface transition temperature is −46 ° C. or lower and the absorbed energy at −46 ° C. is 100 J or higher in the Charpy impact test in which a seam heat treatment portion is notched. In addition, the QT (quenching-tempering) treatment is performed as the seam heat treatment, and at that time, the heating width at the Q treatment is substantially the seam width (1.0 to 1.1 times the seam width). It has been found that the heating width at the T treatment can be realized by setting the heating width at 1.5 times to 3 times the heating width at the Q treatment.

Hereinafter, the chemical composition optimized in the present invention will be described. This optimized chemical composition considers the overall cost reduction when laying ERW steel pipes as line pipes, and in order to meet the demands of customers who place emphasis on reducing the transportation costs of steel pipes. The chemical composition is based on the premise of high strength. The unit of component content is mass% and is abbreviated as%.
C: 0.02 to 0.12%. C is an element that contributes to precipitation strengthening as a carbide, but if it exceeds 0.1%, the structural fraction of the second phase of pearlite, bainite, martensite, etc. increases, and the excellent material toughness required as a line pipe Cannot be secured. For this reason, in this invention, it limited to 0.1% or less. However, if the C content is less than 0.02%, sufficient strength as a line pipe cannot be secured. For this reason, it is desirable to contain C 0.02% or more. More preferably, the C content is 0.02 to 0.07%.
Si: 0.01 to 0.5%. Si is added for deoxidation, but if it is less than 0.01%, the deoxidation effect is not sufficient, and if it exceeds 0.5%, the weldability is deteriorated, so the Si content is 0.01 to 0. Stipulated to be 5%.
Mn: 0.4 to 2.0%. Mn is added to ensure strength and toughness, but if it is less than 0.4%, the effect is not sufficient, and if it exceeds 2.0%, the second phase fraction increases, and it is an excellent material necessary as a line pipe. Since toughness cannot be ensured, the Mn content is specified to be 0.4 to 2.0%. In addition, Preferably, it is Mn: 0.06-1.8%.
-P: 0.01% or less. Since P is an impurity element that degrades electroweldability, the upper limit of the P content is specified to be 0.01%.
-S: 0.01% or less. In general, S becomes MnS inclusions in steel, and not only deteriorates toughness, but also serves as a starting point for hydrogen-induced cracking (HIC). However, since there is no problem if it is 0.01% or less, the upper limit of the S content is defined as 0.01%.
-Al: 0.1% or less. Al is added as a deoxidizer, but if it exceeds 0.1%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is specified to be 0.1% or less.

In the present invention, in order to further improve the strength, yield ratio, and toughness of the ERW steel pipe for line pipe, in addition to the above components, Cu: 0.5% or less, Ni: 0.5% or less is selected. 1 or 2 selected from the group consisting of Cr: 1.5% or less, Mo: 2.0% or less, Nb: 0.1% or less, V: 0.1% or less , Ti: 0.1% or less, and one or more selected from Ca and 0.005% or less can be selected and contained.
Cu: 0.5% or less. Cu is an element effective for improving toughness and increasing strength, but if added in a large amount, weldability deteriorates, so when added, the upper limit is 0.5%.
Ni: 0.5% or less Ni is an element effective for improving toughness and increasing strength, but if added in a large amount, a hardened second phase is likely to be formed, leading to a decrease in material toughness. Therefore, when added, the upper limit is 0.5%. .
-Cr: 1.5% or less. Like Mn, Cr is an element effective for obtaining sufficient strength even at low C. However, when added in a large amount, the second phase tends to be formed and the material toughness is lowered. The upper limit.
-Mo: 2.0% or less. Mo is an element effective for obtaining sufficient strength even at low C as in Mn and Cr. However, when added in a large amount, the second phase is easily formed and the material toughness is lowered. % Is the upper limit. More preferably, Mo: 0.5% or less.
-Nb: 0.1% or less. Nb improves strength and toughness by fine precipitation of carbonitride and fine graining of the structure. However, if it exceeds 0.1%, the cured second phase tends to increase, and conversely, the material toughness deteriorates remarkably, so the Nb content is specified to be 0.1% or less.
-V: 0.1% or less. V, like Nb, contributes to strength increase by fine precipitation of carbonitride. However, if it exceeds 0.1%, the fraction of the cured second phase increases in the same manner as Nb, and the material toughness deteriorates remarkably, so the V content is specified to be 0.1% or less.
-Ti: 0.1% or less. Ti, like Nb and V, contributes to strength increase by fine precipitation of carbonitride. However, if it exceeds 0.1%, the fraction of the cured second phase increases in the same manner as Nb and V, and the toughness of the material deteriorates remarkably, so the Ti content is specified to be 0.1% or less.
-Ca: 0.005% or less. Ca is an element necessary for controlling the morphology of elongated MnS that tends to be the starting point of HIC. However, if added over 0.005%, excessive Ca oxides and sulfides are generated and lead to toughness deterioration, so the Ca content is specified to be 0.005% or less.

The balance other than the above consists of Fe and inevitable impurities. However, as long as the effects of the present invention are not lost, a part of Fe may be substituted with a trace element other than the above.
In the present invention, pipe-forming and electric seam welding may be performed by a normal technique, but seam heat treatment after pipe-making and electric seam welding is different from usual. That is, in the seam heat treatment in the present invention, first, the pipe peripheral portion whose width around the welded joint is substantially the width of the seam is heated to Ac ₃ points or more and then water-cooled, and then the welded joint is processed. It is assumed that T treatment is performed in which a tube peripheral portion whose center width is 1.5 to 3 times the heating width in the Q treatment is cooled to 400 to 650 ° C. and then allowed to cool. The heating width here is defined in the pipe circumferential direction within the pipe outer peripheral surface.

In the T process, as a method of heating a region having a wide heating width of 1.5 to 3 times compared to the Q process, the T process induction coil is enlarged, the output during the T process is increased, and the T process is increased. Various methods such as lowering the frequency of the high-frequency heating apparatus for use are available, but any method may be adopted as long as a heating width 1.5 to 3 times that in the Q treatment can be secured.
It is the same as usual that the heating width at the time of the Q treatment is approximately the seam width (for example, 1.0 to 1.1 times the width of the seam width) with the welded joint as the center. The heating width at the time of the T treatment is 1.5 times or more that at the time of the Q treatment because the two-phase region heating-quenching region in the Q treatment is surely subjected to the T treatment, so that martensite and residual austenite are deteriorated in toughness. In order to make tempered martensite or ferrite + carbide.

Conventionally, as shown in FIG. 1, the heating width is the same during Q treatment and T treatment. In FIG. 1, 1 is an ERW steel pipe, 2 is a weld joint, 3 is an austenite region (Ac ₃ points or more) heating unit, 4 is a two-phase region (Ac ₁ point or more and less than Ac ₃ points) heating unit, 5 is a temper It is a temperature range (400-650 degreeC) heating part. The Q-treated two-phase zone heating unit 4 has a mixed structure of ferrite + bainite + martensite + residual austenite, but conventionally, the heating width at the T treatment is the same as the heating width at the Q treatment. In addition, the tempering temperature zone heating section 5 does not spread to the inner surface of the pipe, that is, the pipe inner peripheral side of the two-phase zone heating section 4 is not sufficiently T-treated, and therefore martensite and residual austenite that deteriorates toughness deteriorates toughness. It cannot be made tempered martensite or ferrite + carbide, and the toughness of the seam heat treatment part cannot be ensured.

In changing the heating width of the T treatment, when the heating width is less than 1.5 times that of the Q treatment, the entire two-phase zone heating unit 4 cannot be made into the temper temperature zone heating unit 5, that is, the two-phase zone. The zone heating part 4 is not sufficiently T-processed and the high toughness of the seam heat-treatment part cannot be ensured.
On the other hand, when the heating width at the time of T treatment exceeds three times that at the time of Q treatment, the toughness of the seam heat treatment part does not change significantly, but the base material part that does not require the T treatment is heated. Has no merit.

  From these things, in this invention, the heating width at the time of the T treatment is specified to be 1.5 to 3 times that at the time of the Q treatment.

  Test steel (steel types A to J) slabs having the chemical composition shown in Table 1 are hot-rolled to the plate thickness shown in Table 1, and wound hot coil is used as a raw material for pipe making, electric seam welding, seam heat treatment Through these sequential steps, thick, high-strength ERW steel pipes of X65, X70, and X80 grades (corresponding to steel types are shown in Table 1) having an outer diameter of 20 inches were manufactured. Any of the seam heat treatment conditions shown in Table 2 was adopted.

  In order to investigate the seam toughness of the product pipe after seam heat treatment (seam heat treatment part toughness), 10 V-notch test pieces defined in JIS Z 2202 were measured from the position where the pipe circumferential distance from the weld joint position was 10 mm. Samples were taken so that the tube circumferential direction was the test piece length direction and the tube longitudinal direction was the notch depth direction, and a Charpy impact test was conducted at -46 ° C. to measure the absorbed energy and the brittle fracture surface ratio. In consideration of manufacturing variations, the minimum value and average value of the measurement data were taken as the acceptable range of the seam heat treatment part toughness for the absorbed energy of 125 J or more and the brittle fracture surface ratio of 35% or less. The results are shown in Table 3.

  From Table 3, in the present invention examples where both the composition and the seam heat treatment conditions fall within the scope of the present invention, the seam heat treatment part toughness passed in any of the examples, whereas either one or both of the composition and seam heat treatment conditions However, in the comparative examples that deviate from the scope of the present invention, the seam heat-treated part toughness in each example failed.

It is a schematic diagram which shows the heating area | region in the conventional QT process for seam heat processing.

Explanation of symbols

1 ERW pipe 2 welded portion 3 the austenite region (Ac ₃ points or more) heating unit 4 two-phase region (Ac less than ₁ point or more Ac ₃ point) heating unit 5 tempering temperature range (400 to 650 ° C.) heating unit

Claims (5)

In mass%, C: 0.02-0.12%, Si: 0.01-0.5%, Mn: 0.4-2.0%, P: 0.01% or less, S: 0.01 %, Al: 0.1% or less, and a steel strip having the balance consisting of Fe and inevitable impurities is made into an open pipe by continuous pipe making, and the circumferential ends of the open pipe are electrically connected to each other. A pipe having a seam portion formed by sewing and welding is formed, and when the seam heat treatment is performed on the seam portion of the pipe by heating from the outer periphery side of the pipe, first, the width around the welded joint is set to be almost the width of the seam portion. The pipe circumference portion is heated to Ac ₃ points or more and then subjected to water treatment Q treatment, and then the pipe circumference portion whose width around the welded joint is 1.5 to 3 times the heating width at the time of the Q treatment. High-strength thickness for line pipes, characterized in that T treatment is performed by heating to 400 to 650 ° C. and then allowing to cool. A method for manufacturing a meat electric welded steel pipe.   The composition according to claim 1, further comprising one or two kinds selected from Cu: 0.5% or less and Ni: 0.5% or less by mass% in addition to the composition. A manufacturing method for high strength thick ERW steel pipes for line pipes.   3. In addition to the composition, the composition further comprises one or two selected from Cr: 1.5% or less and Mo: 2.0% or less by mass%. The manufacturing method of the high intensity | strength thick-walled ERW steel pipe for description of a line pipe.   In addition to the above composition, the composition further contains one or more selected from the group consisting of Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less in terms of mass% The manufacturing method of the high strength thick-walled electric-welded steel pipe for line pipes of any one of Claims 1-3 characterized by the above-mentioned.   The high-strength thick-walled electric-welded steel pipe for line pipes according to any one of claims 1 to 4, further comprising Ca: 0.005% or less by mass% in addition to the composition. Production method.

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