JP2005350724A - Ultrahigh strength pipe bend having excellent low temperature toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pipe bend having a tensile strength of ≥750 MPa and having excellent base material and weld zone toughness, and to provide its production method. <P>SOLUTION: The ultrahigh strength pipe bend has a composition comprising, by mass, 0.03 to 0.12% C, 0.01 to 0.2% Si, 0.8 to 2.0% Mn, ≤0.009% P, ≤0.003% S, 0.1 to 1.5% Ni, 0.003 to 0.05% Nb, 0.05 to 1.2% Mo, 0.005 to 0.03% Ti, ≤0.005% N and 0.002 to 0.07% Al, and the balance Fe with inevitable impurities, and has a microstructure where a tempered martensitic structure of ≥3% is incorporated into a tempered bainitic structure with a mean packet grain size of ≤20 μm. The pipe bend may further comprises 0.005 to 0.1% V and/or one or more kinds of metals selected from 0.1 to 1.4% Cu, 0.1 to 1.2% Cr, 0.0005 to 0.006% Ca and 0.0001 to 0.003% Mg. In its production, after heating at 820 to 1,050°C, bending is performed, thereafter, accelerated cooling is performed to ≤400°C at a cooling rate of ≥5°C/s, and subsequently, tempering treatment is performed in the temperature range of 350 to 750°C. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a bend pipe (bent pipe) having a high tensile strength of 750 MPa or more and excellent low-temperature toughness, and a method for producing the same.

  In pipelines that transport natural gas and crude oil over long distances, reducing transportation costs is a universal need, and for this purpose, transportation efficiency must be improved by increasing operating pressure. In order to increase the operating pressure, a method of increasing the wall thickness of a conventional strength grade pipe can be considered.

However, in such a method, there is a problem in that the welding efficiency in the field is lowered and the construction efficiency is lowered due to an increase in the weight of the structure. On the other hand, there is a growing need to increase the strength of pipe material itself and limit the increase in wall thickness.
80 grade steel has been standardized and put into practical use. In addition, steel pipes of X100 grade and higher strength grades are being developed.

  By the way, since it is impossible to construct a long pipeline or a pressure vessel with a complicated structure with only a straight straight pipe, a bent pipe capable of accommodating a steel pipe according to complicated topography and design is required.

Usually, a bent pipe, that is, a bend pipe, is manufactured by processing (bending) a straight pipe hot or warm. So far, various techniques have been proposed and put to practical use for bend pipes less than X100 grade up to X80 grade (bend pipes having a tensile strength of less than about 750 MPa). However, for high-strength grade bend pipes of X100 grade or higher, the role of controlled rolling technology is increasing for the purpose of refining the structure in order to achieve both weldability and low-temperature toughness in high-strength steel sheets. When steel pipes are hot / warm bent, the effect of controlled rolling disappears due to heating, and the mechanical properties that occur when manufacturing bend pipes tend to deteriorate drastically. It was considered difficult to obtain.

On the other hand, in Patent Document 1, for example, a steel pipe manufactured by adjusting the components is bent in a fine austenite state and immediately cooled with water to immediately transform bainite transformed from austenite grains having an average particle size of 10 μm or less in volume fraction. A high strength bend pipe of X100 grade or higher containing 70% or more has been proposed.

Further, in Patent Document 2, high strength and low temperature toughness are obtained by heating a very low C-Nb-trace amount Ti-based steel pipe containing 0.03% C or less containing an appropriate amount of alloying element and then quenching immediately after bending. X80 to X100 grade bend pipes that can be achieved simultaneously have been proposed.
JP 11-172374 A JP 2002-129288 A

  However, in a pipeline as a structure, a bend pipe portion is a strain concentration portion, and therefore, it is necessary to have extremely high brittle crack generation suppression characteristics in addition to crack propagation stop characteristics. In particular, it is necessary to pay particular attention to X100 grade or X100 grade super high strength steel because the material is more susceptible to brittle cracks.

  In general, the CTOD test is used to evaluate the cracking suppression characteristics of steel for line pipes. However, the conventional ultra-high-strength bend pipe manufacturing technology uses the crack generation suppression characteristics of steel pipes including welded HAZ parts. On the other hand, sufficient examination by CTOD test has not been made, and it cannot be said that safety has been secured. In particular, in high strength bend pipes manufactured with accelerated cooling as described in Patent Document 1 and Patent Document 2, since a harmful hard phase remains in the structure, sufficient crack generation suppression characteristics and safety are obtained. It could not be said that it was secured. Also, the tempering after molding of the bend pipe, which is often used in the production of conventional low-strength bend pipes, causes decomposition and softening of the hard phase just by diverting it to the ultra-high-strength bend pipes. It was not possible to provide sufficient cracking suppression properties due to precipitation of carbides on the top and on the packet interface.

That is, in the invention of Patent Document 1, since the as-quenched bainite structure is used, the austenite grains before cooling must be made into 10 μm fine grains. For this purpose, the heating temperature of the steel pipe is set to 780 to 950 ° C. It is necessary to heat the γ / α two-phase region. This not only results in variations in the processing accuracy and characteristics of the product and deteriorates productivity, but also has limitations in increasing strength and toughness over the X100 grade. Further, in the invention of Patent Document 2, because of extremely low C steel, it was difficult to produce a bend pipe having sufficient low temperature toughness at a high strength of 750 MPa or more.

  An object of the present invention is to provide a high-strength bend pipe having a tensile strength of 750 MPa or more, an excellent base metal and crack generation suppressing properties of welds and weldability, and a method for producing the same.

As a result of various studies to develop a high-strength bend pipe having excellent base material and weld toughness, crack generation suppressing characteristics and weldability, the present inventors have obtained the following knowledge.
In other words, the following points are important for producing a high-strength bend pipe having a tensile strength of 750 MPa or more by hot bending without impairing the toughness of the base material and the welded part.

 (i) The final structure of the bend tube is a tempered bainite (including lower bainite) structure containing tempered martensite, and the average packet particle size of bainite is 20 μm or less. At this time, in order to achieve both sufficient high strength and low temperature toughness, the production ratio of tempered martensite is set to 3% or more.

(ii) In order to obtain the above structure in the manufacturing process of the bend pipe, the value of the following formula (1) is controlled to 2.75 to 4.00 mass% for the component system of the steel pipe.
(iii) Obtaining the structure described in (i) in the manufacturing process of the bend pipe and further providing the material with sufficient crack generation suppressing properties as a bend pipe for a gas line pipe is based on the value of the following formula (2) − Control to 0.08 to 0.10% by mass.

 (iv) In order to efficiently obtain the structure of (i) and stably obtain a bend pipe having high cracking suppression characteristics, the component control of (ii) is performed simultaneously and hot bending is performed. When manufacturing a bend pipe, it is heated to a temperature range of 820 to 1050 ° C, bent and then accelerated to 400 ° C or less, and then tempered at a temperature range of 350 to 750 ° C. carry out.

M value = 5.5 C + 0.6Si + Mn + 0.5Cu + 0.7Cr + 0.4Ni + 1.5Mo + 2 V ... (1)
CP value = Nb−0.6Si −5S−5 (Ti−0.05) +0.2 V (2)
In order to improve the crack propagation characteristics of an ultra-high strength bend pipe, it is necessary to simultaneously perform 1) refinement of the structure and 2) removal of the hardened structure that can be the starting point of crack generation due to strain concentration.

  The M value defines the component values necessary for refining the structure in order to obtain high strength of 750 MPa or more and sufficiently high toughness. By this, baking with a sufficiently fine packet size is performed by bend processing and subsequent heat treatment. Returned bainite-martensite can be obtained. Although many of the hard island martensite (MA) phases, which are harmful to cracking suppression properties, tend to be decomposed by tempering treatment, some of them remain, or some carbides form a dotted line on grain boundaries or packets. Alternatively, sufficient characteristics may not be obtained due to deposition in the form of a film or a lump.

  In addition, the formation of coarse sulfides and the formation of precipitates that significantly weaken the brittle fracture resistance of the matrix such as TiC are also factors that reduce the cracking suppression resistance, but these inhibition points are eliminated by suppressing the CP value, It was found that the resistance to crack initiation was stabilized.

  FIG. 1 summarizes the relationship between the CTOD characteristics at −20 ° C. and the CP value of an ultrahigh strength bend pipe having a steel composition according to the present invention and having a tensile strength of 750 MPa or more, and is controlled within the scope of the present invention. Thus, it can be seen that a CTOD characteristic value of 0.2 mm or more, which is sufficiently safe as a gas line pipe, can be obtained.

  Therefore, according to the present invention, not only excellent low-temperature toughness and weldability can be obtained for high-strength bend pipes of x100 grade or higher, but there are few variations in processing accuracy (steel pipe dimensions) and steel pipe characteristics when manufacturing bend pipes. In addition, it is possible to obtain an effect of excellent productivity.

  The present invention also makes it possible to manufacture the performance bend pipe by a manufacturing method specifically limited to the claims of the invention.

  According to the present invention, a high-strength bend pipe having a tensile strength of 750 MPa or more and good low-temperature toughness can be obtained. As a result, it has become possible to dramatically improve the transportation efficiency and construction efficiency of the pipeline.

The reason why the present invention is limited as described above will be described in detail. In the following explanation, “%” of the alloy element content rate represents “mass%”.
C: 0.03-0.12%
C is an element effective for increasing the strength, and 0.03% or more is necessary to obtain a desired strength in the steel of the present invention. However, excessive addition exceeding 0.12% not only deteriorates the weldability of the steel, but also degrades the base metal after hot bending, the toughness of the weld heat affected zone, and the cracking suppression properties, so the upper limit is set to 0.12%. Restrict.

Si: 0.01 to 0.2% or less
Si is an element effective for deoxidation, but in order to obtain the effect, 0.01% or more is added. Adding more than 0.2% not only lowers the toughness of the weld heat affected zone, but also degrades the base metal after hot bending, the toughness of the weld heat affected zone, and cracking suppression properties, so the upper limit is set at 0.2%. Restrict. Preferably, it is 0.18% or less. More preferably, it is 0.15% or less.

Mn: 0.8 to 2.0%
Mn is an element effective in increasing the strength. For that purpose, 0.8% or more is added. However, adding over 2.0% not only deteriorates the toughness of the weld, but also degrades the base metal after hot bending, the toughness of the heat affected zone of the weld, and cracking suppression properties, so the upper limit is set at 2.0%. Restrict. Preferably, it is 1.8% or less, more preferably less than 1.7%.

P: 0.009% or less, S: 0.003% or less Since the content of P and S significantly affects the toughness of steel, it is necessary to reduce the content. Reducing the amount of P reduces the center segregation of the slab and reduces brittle fracture at the grain boundaries. S becomes MnS and precipitates in the steel, which is stretched by rolling and adversely affects the toughness and crack generation suppressing properties. In order to suppress these adverse effects, P is 0.009% or less and S is 0.003% or less.

  In particular, S is limited to 0.002% or less, more preferably 0.0015% or less, because it greatly affects the CP value described later. In order to stabilize the crack generation characteristics, the P content is preferably 0.006% or less.

Ni: 0.1 to 1.5%
Ni is an element effective for increasing the strength, and at the same time has an effect of improving the toughness and the propagation stop property of the brittle crack. Moreover, it has the effect | action which suppresses the toughness deterioration of the base material after a hot bending process, and a welding heat affected zone. To obtain these effects, 0.1% or more is added. However, an excessive amount exceeding 1.5% cannot increase strength and improve toughness to meet the cost increase, so the upper limit is set to 1.5%.

Nb: 0.003 to 0.05%
Nb is an element that suppresses the deterioration of the structure and properties due to hot bending and suppresses the deterioration of the toughness of the base material after welding and the heat affected zone. Add 0.003% or more for the effect. If added over 0.05%, the weldability deteriorates, and at the same time, the low temperature toughness of the base material after welding and the heat affected zone deteriorates, so the upper limit is made 0.05%. Considering the toughness of the welded portion, particularly the toughness of the circumferential welded portion, it is desirable to keep the upper limit to 0.03% or less, and further to less than 0.01%.

Mo: 0.05-1.2%
Mo is an element effective for increasing the strength, and at the same time has an effect of suppressing toughness deterioration of the base material after hot bending and the weld heat affected zone. To obtain these effects, 0.05% or more, preferably 0.1% or more is added. However, excessive addition exceeding 1.2% not only prevents an increase in strength and improvement in toughness commensurate with an increase in cost, but also causes a decrease in the toughness of the base metal and welded portion, so the upper limit is set to 1.2%.

Ti: 0.002 to 0.03%
Ti is an effective element for refining austenite grains during slab heating, and is added in an amount of 0.002% or more. In particular, in the case of Nb-added steel, it is effective for suppressing cracks on the surface of the continuously cast slab promoted by Nb. These effects are exhibited at 0.002% or more, but if added over 0.03%, TiN becomes coarse and the effect of refining austenite crystal grains disappears, and the toughness of the base metal heat-affected zone is reduced due to excess Ti. , Ti addition amount is 0.03% or less. Preferably 0.005% or more is added.

N: 0.005% or less N is a harmful element that not only causes a decrease in toughness of the base metal and the welded portion, but also makes the characteristic deterioration after hot bending remarkable. If the content exceeds 0.005%, good characteristics as a bend pipe cannot be obtained even if other conditions are adjusted, so the upper limit was made 0.005%. Desirably, it is 0.003% or less.

Al: 0.002 to 0.07%
Al is an element effective for deoxidation, but if it is not 0.002% or more, deoxidation will be insufficient, and the formation of lower oxides may cause a decrease in the toughness of the base metal, so the lower limit is set to 0.002% It was. In order to improve the toughness of the seam weld and the peripheral weld metal part, it is desirable to add more than 0.03%, desirably more than 0.05%. On the other hand, if added over 0.07%, not only the toughness of the weld heat affected zone is lowered, but also the base material after hot bending and the toughness of the weld heat affected zone are deteriorated, so the upper limit is made 0.07%.

In the present invention, preferably, the following elements may be further contained.
V: 0.005 to 0.1%
V is useful for strengthening the base material, but the effect is exhibited at 0.005% or more. However, if added over 0.1%, the toughness of the base metal and the welded part deteriorates, so the upper limit is made 0.1%.

In particular, in the case of V, it affects the CP value described later and greatly contributes to crystal refinement and toughness strengthening during heating of the steel pipe.
Furthermore, in the present invention, the following component elements can be added within the range, so that the base material, the low temperature toughness of the heat affected zone of the weld, and the weldability can be increased without increasing the strength and corrosion resistance. It can be used when producing thicker steel plates or high strength materials.

Cu: 0.1-1.4%
Cu is an element effective in increasing strength and improving corrosion resistance. However, if less than 0.1%, the effect cannot be expected, and if added over 1.4%, the toughness deteriorates, so the lower limit is made 0.1% and the upper limit is made 1.4%.

Cr: 0.1 to 1.2% or less
Cr is added to ensure strength and improve corrosion resistance, but the effect is demonstrated by addition of 0.1% or more, and if it exceeds 1.2%, the toughness of the base metal and weld zone deteriorates, so the upper limit is 1.2% And

Ca: 0.0005 to 0.006%
Ca is an element to control the morphology of MnS and improve the physical properties in the direction perpendicular to the rolling direction of steel. The fine oxide / sulfide also has the effect of improving the toughness of the base metal and the weld heat affected zone, but 0.0005% or more is added to expect these effects. If the content exceeds 0.006%, non-metallic inclusions in the steel increase and cause internal defects. Therefore, the Ca content is in the range of 0.0005 to 0.006%.

Mg: 0.0001 to 0.003%
Mg also has the effect of improving the toughness of the base metal and weld heat affected zone through the formation of fine oxides / sulfides, but 0.0001% or more is added to expect these effects. If the content exceeds 0.003%, non-metallic inclusions in the steel increase and cause internal defects. Therefore, the Mg content is in the range of 0.0001 to 0.003%. Preferably, it is 0.0002 to 0.002%.

In order to obtain the target base material and seam / circumferential weld toughness of the present invention, it is not sufficient to adjust the steel components within the above composition range, and the following component adjustments are necessary.
M value: 2.75 to 4.00%
By adjusting the M value shown in the following formula (1) to an appropriate range, the deterioration of the structure and properties due to hot bending is suppressed, and the toughness of the base metal after welding and the heat affected zone of the weld is suppressed. Is possible. In order to optimize the bainite and martensite structures and obtain the above effect, the M value is controlled in the range of 2.75 to 4.00% in mass%. When the M value is less than 2.75%, it becomes impossible to suppress the toughness deterioration of the base material after welding and the weld heat affected zone even if the individual elements shown above are within the specified values of the present invention. Also, if it exceeds 4.00%, not only the weldability and weld heat affected zone (HAZ) toughness will deteriorate, but also the steel pipe manufacturing cost will greatly deteriorate. From the viewpoint of optimization of the base metal, weld toughness balance, and cost, it is preferably 2.8 to 3.8%, and a more preferable component range is 2.9% to 3.5%.

M value = 5.5C + 0.6Si + Mn + 0.5Cu + 0.7Cr + 0.4Ni + 1.5Mo + 2 V ... (1)
CP value: -0.08 to 0.10%
The CP value shown in the following formula (2) is an index indicating the activation of Nb, and corresponds to the Nb equivalent.

  By adjusting the CP value to an appropriate range of -0.08 to 0.10%, the austenite grains during heating before bending are refined, and at the same time, the matrix toughness of the bainite and martensite structures formed after cooling is improved, thereby improving the matrix The toughness of the material part can be improved.

  In addition, the latter effect is also effective for the weld heat affected zone and suppresses toughness deterioration of the weld heat affected zone. These effects can be obtained by setting the CP value to −0.08% or more. If the CP value exceeds 0.10%, the island-like martensite in the welded thermal shadow zone increases rapidly, which may impair toughness. From the viewpoints of optimization of the base metal, weld toughness balance and cost, a more preferable component range is -0.05% to 0.05%.

CP value = Nb-0.6Si-5S-5 (Ti-0.05) +0.2 V ... (2)
Next, the manufacturing method of the bend pipe | tube concerning this invention is demonstrated.
In the present invention, the steel pipe having the above-defined component composition is heated to 820 to 1050 ° C. and then bent, accelerated and cooled to 400 ° C. or lower, and tempered, or after processing, a temperature of 450 to 250 ° C. A predetermined microstructure is obtained by performing accelerated cooling to room temperature and then air cooling to room temperature.

  Steel pipes used for bending use UOE steel pipes, roll-bend steel pipes, seamless steel pipes, etc. welded by various welding methods including SAW (submerged arc welding), MIG (metal inert gas welding), TIG (tungsten inert gas) welding, etc. can do.

Steel tube heating temperature:
When the heating temperature before bending is less than 820 ° C, the structure becomes non-uniform, and not only the processing accuracy (steel pipe dimensions) and characteristic variation are greatly deteriorated, but also the deformation resistance increases, so the bending productivity is significantly reduced. Resulting in. In addition, when the temperature exceeds 1050 ° C., the austenite grains are coarsened during heating, and the average packet size of bainite cannot be reduced to 20 μm or less even when the component adjustment or the control of the cooling conditions is performed. It becomes impossible to achieve both low temperature toughness.

Cooling rate after processing:
The cooling rate after processing is important for obtaining a bainite structure containing moderate martensite. If the cooling rate is less than 5 ° C./second, a desired microstructure cannot be obtained. The microstructure can be adjusted even if the cooling rate is high, but if it exceeds 50 ° C / second, it will be difficult to adjust the structure due to the components and cooling stop temperature, and problems such as excessive cooling equipment will occur. Therefore, it is desirable not to greatly exceed 50 ° C / second.

Cooling stop temperature / tempering temperature:
An appropriate amount of martensite cannot be obtained under conditions where the cooling stop temperature exceeds 400 ° C. Further, when tempering, if the tempering temperature is less than 350 ° C. or exceeds 750 ° C., both high strength and good low temperature toughness cannot be achieved.

  Next, the function and effect of the present invention will be described more specifically based on examples.

  The steel pipes to be used for bending are steel plates having the chemical components shown in Table 1 are melted and cast by hot rolling, and steel sheets are manufactured by hot rolling, and UOE pipes or roll bending (BR ) Obtained by pipe making. From these test steel pipes, bend pipes were produced under the heating and cooling conditions / heat treatment conditions shown in Table 2. The plate thickness of the steel pipe is 18-40mm.

  The test pieces thus manufactured and cut out from the bend pipe were subjected to a tensile test, a 2 mmV notch Charpy impact test, and a CTOD test, and the respective characteristics are summarized in Table 2.

  In the examples of the present invention, a tensile strength of 750 MPa or more and a good low temperature toughness of −80 ° C. or less in vTrs (shear rupture transition temperature) are obtained. Further, the CTOD characteristics at −20 ° C. all have good characteristics of 0.2 mm or more.

It is a graph which shows the relationship between CP value and CTOD value in -20 degreeC.

Claims (4)

By mass ratio C: 0.03-0.12% Si: 0.01-0.2%
Mn: 0.8% to 2.0% P: 0.009% or less,
S: 0.003% or less Ni: 0.1-1.5%
Nb: 0.003 to 0.05% Mo: 0.05 to 1.2%
Ti: 0.002 to 0.03% N: 0.005% or less
Al: 0.002 to 0.07%
It consists of the remainder Fe and inevitable impurities, and has a component composition in which the M value and CP value represented by the formulas (1) and (2) are in the range of 2.75 to 4.00% and −0.08 to 0.10%, respectively, and the microstructure is An ultra-high-strength bend pipe having a tensile strength of 750 MPa or more, wherein the tempered martensite structure is 3% or more in a tempered bainite structure having an average packet particle size of 20 μm or less.
M value = 5.5 C + 0.6Si + Mn + 0.5Cu + 0.7Cr + 0.4Ni + 1.5Mo + 2V (1)
CP value = Nb-0.6Si-5S-5 (Ti-0.05) +0.2 V ... (2)   The ultra-high-strength bend pipe according to claim 1, wherein the component composition further contains V: 0.005 to 0.1% by mass ratio. The component composition is a mass ratio, and
Cu: 0.1-1.4%, Cr: 0.1-1.2%
Ca: 0.0005 to 0.006% Mg: 0.0001 to 0.003%
The ultra high strength bend pipe according to claim 1 or 2, comprising one or more of the following.   The steel pipe having the component composition according to any one of claims 1 to 3 is heated to 820 to 1050 ° C, subjected to bending, and then cooled to a temperature of 400 ° C or lower at a cooling rate of 5 ° C / second or more. A method for producing an ultra-high strength bend pipe having a tensile strength of 750 MPa or more, characterized by performing accelerated cooling and then performing tempering treatment in a temperature range of 350 to 750 ° C.

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