JPS5834133A - Production of api standard class x80 steel pipe having excellent low temperature toughness - Google Patents

Abstract

PURPOSE:To obtain a high tensile large diameter steel pipe having excellent low temp. toughness by subjecting steel contg. prescribed contents of C, Si Mn, P, S, Nb, Al, etc. to machining and heat treatment under specific conditions. CONSTITUTION:The steel contg., by weight %, <=0.15% C, <=0.7% Si, <=0.5-2.5% Mn, <=0.025% P, <=0.005% S, 0.001-0.15% Nb, <=0.07% Al, etc. is melted. A slab of thickness from 300mm. up to 3 times the thickness of the final product is cast continuously. At the point of the time when the surface temp. attains 1,000- 800 deg.C, rough rolling is started. Upon ending of the rough rolling, finish rolling is started at <=60sec time and 950-750 deg.C temp., and the finish rolling is finished at >=60% draft and Ar3 transformation point -650 deg.C temp. The hot-rolled steel strip is coiled at 750-450 deg.C, and is formed into a steel pipe. The steel pipe is heated to a temp. range from Ac3 transformation point -50 deg.C up to 1,100 deg.C and is cooled between 800-500 deg.C at the average cooling rate CR ( deg.C/sec) satisfying the equation. Finally, the steel pipe is cooled down to ordinary temp.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing an API standard XIO class steel pipe with excellent low-temperature toughness, and particularly to a method for manufacturing a high-tensile, large-diameter steel pipe for pipelines in cold regions.

BACKGROUND ART In recent years, the development of energy resources such as oil and natural gas has been progressing, and the development of energy resources such as oil and natural gas has become widespread, especially in cold regions, and the construction of transportation pipelines is rapidly increasing. Moreover, the steel pipes used in these pipelines are becoming increasingly larger in diameter and require higher tensile strength. Since steel pipes for pipelines in these cold regions are installed at low temperatures, there are naturally strict requirements for low-temperature toughness. Therefore, these materials for steel pipes must have properties that combine excellent low-temperature toughness and high tensile strength for use in large-diameter steel pipes.

Large-diameter high-tensile copper pipes currently used for pipelines are mainly manufactured by the UOE method, the spiral pipe manufacturing method, or the method of electrical resistance welding after a 7-ohm rail. Among these, UO-crowned copper pipes are made from steel plates produced by thick plates, while Su-Keiral steel pipes and ERW steel pipes are made from hot-rolled steel strips. Materials having strength are mainly manufactured by controlled rolling.

Control-rolled materials generally have relatively high strength and excellent low-temperature toughness, and can have material properties suitable for high-strength, high-value copper pipe materials for pipelines. The controlled rolling material has an austenitic temperature region where recrystallization starts immediately after rolling (hereinafter referred to as the recrystallized T region), and an austenitic temperature region where recrystallization does not occur between rolling passes (hereinafter referred to as the non-recrystallized T region).
), and austenite, 7 erite 2
The required material quality can be achieved by regulating the amount of rolling reduction and finishing temperature of t5 final rolling for each region of the phase region (hereinafter referred to as T+i), and adjusting the distribution of rolling reduction and finishing temperature that correspond to each radius region. A method is adopted that satisfies the characteristics. Customers' requirements for low-temperature toughness have become more severe as pipeline sites have become colder in recent years, and there is a general trend toward increasing transportation pressure in order to increase transportation capacity. For this reason, some plans are being made to use steel pipes that have high strength of the X80 class and good low-temperature toughness, and the demand for such pipes is likely to increase in the future. This X80 class steel pipe material not only has good low temperature toughness.

In order to facilitate on-site welding, a low carbon equivalent is required, which is extremely difficult to ensure strength with as-rolled materials.

In order to obtain X80 class strength using a material with a composition with a limited carbon equivalent, it is necessary to increase the amount of reduction in the low temperature region during rolling of the material and lower the hot rolling finishing temperature.

However, if the rolling temperature is significantly lowered compared to Ar, and the rolling reduction amount is increased too much in the low temperature range, the strength will increase significantly, but the low temperature toughness will deteriorate. In order to perform effective controlled rolling, the amount of reduction in this temperature range must be as large as possible, and the rolling process must be repeated continuously before the processed structure recovers to prevent the unrecrystallized material from forming. It is necessary to enlarge the ink pressure lower part,
It is difficult to perform this with the current hot rolling method, and it is particularly difficult to roll a hot rolled steel strip that is rolled in a hot strip mill to be used as a material for spiral steel pipes or electric resistance welded steel pipes.

Hot rolling by a hot strip mill usually consists of two stages of rolling called rough rolling and finishing rolling. Some reversible rolling mills are also used for rough rolling, but finishing rolling consists of two steps called rough rolling and finishing rolling. Since the sheet bar is continuously rolled in one direction by one roll, if the shape of the sheet bar is bent during rolling, rolling becomes impossible.

Therefore, the shape of the sheet bar before finishing rolling becomes a problem, and the leading end portion, which is not uniform in shape, is cut off with a shearing machine to give it an appropriate shape before rolling. However, the thickness of the sheet bar is regulated by the capacity of the shearing machine, and therefore it is not possible to take a large amount of shearing in the unrecrystallized T region as in the case of a plate mill.

Therefore, the strength and toughness of hot-rolled steel strip produced at hot stops is quite limited.

With the current rolling mill and rolling method, sheet bar II
In the case of a slab where it is not necessary to cut off one end of 11r or II, or when finish rolling is performed after very light rough rolling, after the slab is extracted from the heating furnace or from the end of rough rolling to the start of finish rolling. During this period, it is usually necessary to wait for a long time to cool down to the specified temperature. As a result, there is a drawback that in addition to a severe decrease in rolling efficiency, low-temperature toughness deteriorates due to coarsening of γ grains after rolling. This long temperature waiting period causes other problems as follows. That is,
In this case, since the slab is heated in a low temperature region for a long period of time, all of the solid solute Nb, which is essential for controlled rolling, will precipitate as carbon and nitrides, so even if the rolling reduction in the low temperature region is increased. There is a problem that desired high strength and high toughness cannot be obtained.

As-rolled hot-rolled steel strip produced by conventional methods has the above-mentioned constraints on strength and low-temperature toughness, and when this is applied to x80 class steel pipe material, from the strength perspective Mo, Nl It is necessary to add large amounts of expensive special elements such as , Nb, and v, and it is unlikely that sufficient low-temperature toughness can be obtained while satisfying the strength of the XSO class. Although high strength and high toughness can be obtained by adding a large amount of the above-mentioned alloying elements, the prices of these special alloying elements have risen sharply in recent years, so it is difficult to use materials produced by conventional methods. X8G class steel pipes are significantly more expensive.

In order to manufacture χ80 class steel pipes from hot-rolled copper strips, which have a low carbon equivalent composition required from the viewpoint of on-site weldability, without adding large amounts of expensive alloying elements, conventional methods and new methods are required. In addition to obtaining a high-strength, high-toughness material by rolling, it is necessary to take steps to further increase the strength of steel pipes formed from that material. Strain aging treatment may be considered, but if the carbon equivalent of the material is low, it is difficult to ensure the strength of the X80 class even if the steel pipe is aged, so this is a known method to increase the strength of materials with low carbon equivalents. , quenching treatment, quenching, and tempering treatment, but steel pipes that have undergone quenching treatment, quenching, and tempering treatment, so-called tempered steel pipes, have a lower overall thickness than steel pipes made from controlled rolled materials. Tempered copper pipe was not applied to a pipeline with a low degree of pressing at a depth of 5 dew, which is said to evaluate brittle fracture resistance and positive characteristics.

An object of the present invention is to provide a method for manufacturing an API standard XSO class copper tube that improves the DWTT characteristics of a tempered steel tube and has excellent low-temperature toughness.

In order to improve the DWTT characteristics of heat-treated steel pipes, the present inventors performed quenching or quenching and tempering on steel pipes manufactured from many materials with different chemical compositions and rolling methods, thereby improving their strength. As a result of investigating the DWTT characteristics, we found that steel pipes with a specific chemical composition, manufactured by appropriately controlled rolling, and formed from materials with good low-temperature toughness should be subjected to vA treatment or quenching and tempering treatment. It was found that the steel pipe had sufficient strength and good WTT characteristics as an X80 class steel pipe.

The present inventors have completed the following four inventions based on the above knowledge.

The main points for the first flight are as follows. That is, CIo in weight ratio, 15% or less, 5ft0.70X
Below, ) & to, 5o ~2.56%, PI3.0
25% or less, sto, oos% or less, Nbto, 010
~0.1SON, ,u! Contains 0.070% or less and must also contain 11 d J9 V l 0.010~@, 15
0X, TI! 0.005~0.111ON, Z
r t O, 005-0.150%, Mo to, os
~o, so%, Cu g O, 10-1.00%,
N1t6.10 ~4.00X, Cr t
0.1 0 ~ 1.0 0X, rare earth element to, 02
0% or less, CaI 2.010% or less, and has a carbon equivalent Ceq of o, 45 or less, and the remainder is substantially Fe. In the method for manufacturing API standard
A step of starting rough rolling when the surface temperature of the slab reaches 1000 to 100°C after keeping or heating the slab as it is or within 20 minutes, and 950 to 750°C within 60 seconds after the completion of the rough rolling. Finish rolling is started at a temperature range of 60% or more, and Ar, l
a step of finishing finish rolling in a temperature range of 750 to 450° C.; a step of forming the hot rolled copper strip into a steel pipe; A step of heating the steel pipe to a temperature range from a transformation point of -50°C to 1100°C, and heating the heated copper pipe to a temperature range of 5oo-so.
API standard χ80 with excellent low temperature toughness, characterized by comprising a step of cooling to room temperature such that the average cooling rate CR (t/5ee) during the temperature range of 0°C satisfies the following formula:
Q(III digitogenic index −vlr (x+o, 5
4st)(t+4.1ow)(1+2.23Cr)(
1+O,! 52Ni)(1+3.14Mo)(1+
0.27Cu) The main point of the second invention is the above-mentioned first invention.
In a method for producing an API standard X80 class steel pipe of steel having the same composition as the invention, after the same continuous casting, hot-fight rolling and pipe-making steps as in the first invention, the steel pipe is heated from a knitting transformation point of -50°C to 1°C.
heating the heated steel pipe to a temperature range of up to 100°C and cooling the heated steel pipe at an average cooling rate CR (
'C/HIC) is cooled to below 500°C so that it satisfies the following formula, and the cooled steel pipe is heated to 500°C again.
~Temperature II of Ae1 transformation point! A with excellent low-temperature toughness, characterized by comprising a step of heating and then cooling.
This is a method for manufacturing PI standard XSO class steel pipes.

CR≧30.8/Q The gist of the third invention is that after the step of manufacturing the same continuous casting slab of the first invention in the method for manufacturing an API standard X80 class steel pipe of steel having the same composition as the first invention, , After the slab is kept warm or heated for 20 minutes or less, finish rolling is started when the surface temperature of the slab reaches 1000 to 750°C, and finish rolling is performed with a reduction rate of 60% or more at 95C1°C or less. After finishing the finish rolling in a temperature range of Ar, f-state to 650°C, winding the hot rolled copper strip in a temperature range of 750 to 450°C, and forming the hot rolled copper strip into a steel pipe. heating the steel pipe to a temperature range of Ac, transformation point -50°C to 1100°C; heating the heated steel pipe to 800-500°C;
and a stage of cooling to room temperature such that the average cooling rate CR (℃/5ee) during the period satisfies the following formula. 4. The main point of Takamei is that in the method for manufacturing an API standard Steel pipe A% l' state from -50℃ to 110
heating the heated steel pipe to a temperature range of up to 0°C, and heating the heated steel pipe to an average cooling rate CR (t/5
ec) satisfies the following formula (including the step of cooling to 500°C or less, and the step of heating the cooled steel pipe again to a temperature range of 500°C to A self-transformation point and then cooling it. API standard X80 with excellent low temperature toughness characterized by
This is a method for manufacturing grade steel pipes.

CR≧30. s/Q That is, 1 The present invention is a low carbon continuous casting slab containing Nb that is heated to 1000 to 800°C after being kept warm or heated for a short time.
Rough rolling is started when the temperature is 950 to 75, assuming that the elapsed time from the end of rough rolling to the start of finish rolling is within 60 seconds.
Start finish rolling at 0°C, or start finish rolling at 1000-750°C without rough rolling, and reduce the total rolling reduction in the unrecrystallized r region and a+1 region during rolling to 6
0% or more, and the finishing positive arc end temperature is set to Ar, ~650°C. During rolling in the low-temperature T region and 7+a region where the amount of solid solution of Nb decreases, Nb is added to the dislocation positions caused by rolling. Carbonitrides precipitate, which prevents rearrangement due to movement and annihilation of dislocations, and recrystallization does not occur on short-time positive arc passes, resulting in continuous rolling with short interpass times as in the above process. If this process is carried out over time, the amount of reduction will be accumulated without recrystallization occurring, and a large amount of chicken carbonitride will be precipitated in a finely dispersed manner.

These Nb carbonitrides, which are precipitated in large amounts and finely dispersed, make the austenite grains much finer than in conventional control-rolled materials during quenching and reheating after rolling, and significantly improve the DWTT characteristics after tempering. .

If the characteristics required for a steel pipe are X80 class strength and a low brittle fracture appearance temperature required for D-WTT, the material manufactured by the method of the present invention can be heated to a temperature range of (Ac, -50℃) to 1100℃. It is sufficient to perform forced cooling after heating, and if a high impact absorption energy is required in addition to the above requirements, a further tempering treatment is performed.

The reasons for limiting the ingredients of the slab used in the present invention are as follows.

I C is necessary to increase strength, but if it exceeds 0.15%, weldability and low temperature toughness will deteriorate significantly.
The content was limited to 0.15% or less.

8 parts are added to deoxidize steel and increase its strength, but 0.
If it exceeds 70%, low temperature toughness deteriorates, so 0.70
% or less.

Hoso-wo-an has the property of increasing strength without deteriorating low-temperature toughness, so it is an essential element for high-strength, high-toughness steel such as the present invention, and requires at least 0.50%.) ,50%
If it is less than 2,50%, the effect on increasing strength will be small.
If it exceeds 0.1So, cracks will occur frequently in the slab.
~2.2$0% range.

s An element contained in steel as an unavoidable impurity, especially 0.02! If it exceeds S%, the low temperature toughness will be significantly deteriorated, so the upper limit was set at 0.021%.

S! Like P, it is an element that is included in steel as an unavoidable impurity, but if it exceeds 0.005%, the impact absorption energy in the direction perpendicular to the rolling direction will be seriously reduced, so the upper limit was set at 0.005%. .

Nb t Nb is an element that has a recrystallization retarding effect and a precipitation hardening effect, and is an essential element for controlled rolling materials. But 0.01
If it is less than 0%, the effect will be extremely small.

On the other hand, addition of a large amount exceeding 0.150% significantly deteriorates the low-temperature toughness of weld metal during steel pipe manufacturing, so 0.010%
It was limited to a range of 0.150%.

11 Red is an extremely effective element for deoxidizing steel and refining crystal grains, but if it exceeds o or oyo%, it deteriorates the surface quality of the steel plate and causes internal defects, and it also Since it causes many defects, it is limited to 0.070% or less.

The above limited composition is the basic composition of the steel of the present invention, but if necessary, the following limited amounts Nov, 'rt, Zr%Mo, Cu
.

The object of the present invention can be more effectively achieved by adding one or more selected from among Nl5Cr, rare earth elements (hereinafter referred to as REV), and REV.

The reasons for limiting these selective addition elements are as follows.

■Snow■ is sometimes added as an effective element for improving strength due to its precipitation hardening effect, but o, ot.

If it is less than 0.1%, the effect is small, and if it exceeds 0.150X, the low-temperature toughness deteriorates, so it is limited to a range of 0.010 to 0.150%.

i11 〒1 is sometimes added for the purpose of refining grains and increasing strength, but if it is less than 0,00% it has little effect (if it exceeds 0,150%, surface defects of the steel sheet will occur frequently). Therefore, it was limited to a range of 0.005 to 0.150%.

Zr-Zr is sometimes added to control the morphology of sulfides and refine grains, but if it is less than 0.005%, its effect is extremely small, and if it exceeds 0.15otX, surface defects in steel materials occur frequently. Therefore, it was limited to a range of 0.005% to 0.150%.

At-dynamics is sometimes added as an element to increase strength without deteriorating low-temperature properties, but if it is less than 0.05%, its effect is small, and if it exceeds 0.50%, it will increase the welding heat when welding steel pipes to TII. 0.0 because it significantly deteriorates the low-temperature toughness of the affected zone.
The range was limited to 0.05 to 0.50X.

1 Like Mo, Ca is sometimes added as an element that increases strength without deteriorating low-temperature toughness, but if it is less than 0.10%, the effect is small, and if it exceeds 100%, red-hot brittle defects will occur. It was limited to a range of 0.10 to 1.00%.

Nl Snow N1 is sometimes added as an element to improve low-temperature toughness and strength, but if it is less than 0.10%, its effect is small, and it does not improve the low-temperature toughness required for large-diameter steel pipe materials for pipelines. In the range, it is necessary to add a large amount exceeding 4.0% (and it is also a bird value, so it is 0.10~
limited to the range of toox.

rtCr is sometimes added to increase strength, but
If it is less than 0.10%, there is almost no effect.

If it exceeds 1.00%, low-temperature toughness will be significantly deteriorated, so it is limited to a range of 0.10 to 1.00%.

REM I REM is sometimes added to control the shape of sulfides and to increase the impact absorption energy in the direction perpendicular to the rolling direction, but if it exceeds 0.020%, it will damage the surface and interior of the steel sheet. Since it causes many defects, it is limited to 0.020% or less.

m I CI has almost the same effect as REV, but if it exceeds 0.010%, it will cause many surface and internal defects of the steel plate, so it should not be
．． It was limited to 0IOX or less.

If the carbon equivalent C@q exceeds 0.45%, it will be necessary to preheat to prevent cracking during on-site welding, making the work difficult, so we limited it to 0.45X or less.
It is preferably 0.43% or less.

In addition to the above-mentioned essential limited ingredients, the slab used in the present invention has:
Other than elements that are selectively added as necessary.

The remainder consists essentially of Fe.

Next, the reason for limiting the controlled rolling in the present invention will be explained.

First, the thickness of the slab was regulated from 300 mm to 3 times the thickness of the final product.If the thickness of the slab exceeds 300 mm, it will take a long time to cool down to the starting temperature of the control.・Nitrides precipitate, making it impossible to improve strength and toughness through controlled rolling. Further, if the slab thickness is less than three times the final product thickness, effective controlled rolling cannot be performed.

Furthermore, the reason why the slabs used in the present invention are limited to continuous casting slabs is because when ingot making or blooming rolling methods are used, if a steel ingot with a thickness of 300 mm or less is to be obtained, the steel ingot is This is because the dimensions are extremely small9, which not only decreases the yield but also decreases heating and forward arc efficiency, resulting in a severe strike height.If continuous casting is used, it is easier to obtain slabs with the above dimensions. It is from.

Next, if necessary, the slab before rolling is stretched within 20 minutes.If the temperature in the part where the cooling rate is faster than the inside of the slab at the end drops excessively, it will be difficult to roll uniformly, so keep warm or heat the part that is easy to cool. The aim is to However, the treatment time was regulated to within 20 minutes because if it exceeded 20 minutes, Nb carbon/nitride would precipitate, and even if the reduction amount in the low temperature region was increased, the desired strength and high strength could not be achieved. This is because toughness cannot be obtained.

When performing rough rolling, the starting temperature is 1000 to 800.
The reason why it is limited to 0.degree. C. is because the low-temperature toughness deteriorates significantly at rough rolling start temperatures below 800.degree. C. or above 1000'C.

When rough rolling is performed, the elapsed time from the end of rough rolling to the start of finish rolling is regulated to within 60 seconds because if it exceeds 60 seconds, coarsening of one grain occurs and low temperature toughness deteriorates.

The rolling start temperature when performing rough rolling and finish rolling is -10
~750℃, and the rolling start temperature when performing finish rolling without rough rolling was set to 1000~7$Otl: in both cases, when deviating from this range by 4 degrees, the deterioration of low temperature toughness is severe by 1 degrees. be.

The reason why the rolling reduction ratio in the temperature range of 950°C or lower is set to 60% or more is that the elapsed time from the rough rolling end force 1 to the start of finish rolling is within 60 seconds, and ζ is that finish rolling is performed without rough rolling. In combination with the process, recrystallization is not caused between each pass during each rolling process, the cumulative amount of unrecrystallized particles is increased, the dislocation density is significantly increased, and Nb carbonitrides are precipitated in a fine 6-bidisperse manner. This is because, as shown in the attached drawings, the DWT properties of steel pipes that have been subjected to rolling, heating after forming, controlled cooling, quenching, and tempering are good only when this is done.

The attached drawing is C10,09 Kai SM0.24%, isl, 6
2%, Plo, 013%, 8sO,O (1%, Nb10
．． 036%, A/, 10.031%, Vgo, 071t
Part XIII consists essentially of F@, carbon equivalent force (0,
Using a slab with a roughness of 37, the rough rolling start temperature force τ980
~880℃, finishing pressure resistance end temperature force (730~700℃,
A hot-rolled steel strip manufactured at a winding degree of 590 to 550°C is formed into a tube with an outer diameter of 1016 mm and a wall thickness of 143 m. After forming, it is heated to 920° C. After heating, it is heated to 5oo-soo during cooling.
The cooling rate between ℃ and 15℃/se@(>7.611/Q
-2,9) always! Spiral steel pipe cooled at lt, heated to 950℃, SOO~500℃ during cooling
Cooling rate between ℃ 40″17secC>80.tj/Q
-7,IJ) tL "t" Cooled to 250℃,
For steel pipes that have been tempered again by heating to 600°C and holding for 1 minute, the rolling reduction ratio in the temperature range below 950°C, the elapsed time from the end of rough rolling to the start of finish rolling, and the direction perpendicular to the pipe axis are shown. It shows the relationship with the temperature (DWT knife 85jlFATT) at which the abrasive surface fracture rate of DWT knife is 85%.

The display marks in the figure are as shown in Table 1.
As is clear from the figure, when the elapsed time from the end of rough rolling to the start of finishing is 4 seconds, the DWTT characteristics are excellent when the total rolling reduction in the temperature range of 950°C or less is 60% or more. The total rolling reduction rate was limited to 60% or less.

The finishing positive arc finishing temperature f Ar is set at ~650°C because when the finishing temperature exceeds Arm,
This is because the surface crystal cumulative reduction amount is not sufficient and fine austenite grains cannot be obtained during heating after forming the steel pipe, and when the temperature is lower than 6150°C, the strength of the material increases and forming becomes difficult.

The reason why the coiling temperature range is 750 to 450℃ is that when the coiling temperature exceeds 750℃, Nb carbonitride and Fe
, C agglomeration and coarsening occur, resulting in deterioration of low-temperature toughness, resulting in 4sO℃
If it is less than 1, the strength of the material becomes extremely high and it becomes difficult to mold it.

Next, the reason for limitations on the heat treatment conditions after steel pipe forming in the present invention will be explained.

Heating temperature after steel pipe forming (Ar, -30℃) ~ 1100
The range of °C was set as mAr lower than 550 °C,
This is because it causes a decrease in strength and deterioration of low-temperature toughness, and when the temperature exceeds 1100°C, austenite grains become coarser, resulting in deterioration of low am property.

After heating, the product is cooled to room temperature.

The reason for setting the value to 79 or higher is that if a steel pipe with a carbon equivalent of 045 or less, particularly 0.43 or less, which is within the range of the present invention, is cooled at a slower rate, the strength of the X80 class cannot be obtained. The range 1 was limited to 800-500℃.
This is because if the cooling rate in this range is managed as described above, a sufficiently satisfactory material can be obtained.

Heating 611 In the case of the second and fourth inventions in which cooling to a temperature of 500°C or less is performed, the cooling rate during cooling production after the first heating is 1 m & 8/
The reason for setting it above Q is that if a steel pipe of the present invention having a carbon equivalent of 0.45 or less, particularly preferably 0.43 or less, is cooled at a slower rate than this, the strength of the X80 class cannot be obtained after tempering. . In addition, the reason why the cooling after the first heating was set to a temperature of 500°C or less is because if cooling is stopped at a temperature exceeding SOO°C and tempered and reheated, the steel pipe with the carbon equivalent within the range of the present invention will have a strength of X80 class. This is because it is not possible to obtain
If it is cooled to a temperature of 500°C or less, and the subsequent tempering and reheating is carried out under appropriate conditions within the scope of the present invention, the X
This is because strength of grade 80 and excellent low-temperature toughness can be obtained.

Further, in the second and fourth inventions, the temperature range during tempering heating is set to 500°C to Ac.
This is because if the temperature is less than 0°C, the desired high absorbed energy cannot be obtained, and if the temperature exceeds the Ac transformation point, the low temperature toughness deteriorates significantly. How to choose temperature and holding time during tempering? (1 total IAgt)xlO” (T+absolute temperature, it
The tempering parameter rewarded by holding time is 115.
-19.5, taking into account the chemical composition and efficiency in actual work.

As mentioned above, the present invention uses a continuously cast slab with a limited steel composition, implements the controlled positive arc unique to the present invention, and produces a pipe from the hot rolled steel strip and heats it at a low temperature. API standard X80 class steel pipe with excellent toughness could be obtained.

Examples A continuously cast slab whose chemical composition satisfies all the limiting compositions of the present invention is used, heat is kept or heated to meet the requirements of the present invention, and then controlled forward arcing is performed, followed by pipe making and tempering treatment. Invention steel pipe, invention 815% FATT and 2■
Comparative tests such as Charpy absorbed energy at -80°C were conducted using V-notch full-size test pieces, and the results are shown in Table 2.

That is, slabs with the thickness shown in Table 2 were manufactured using the continuous casting method from the test material A-L steel having the chemical composition and carbon equivalent shown in Table 2, and the front, back, and end portions of these slabs were After heating for 3 minutes to keep warm, rough rolling start temperature 98
0~880'C, finish rolling start temperature 930~830'
c, the elapsed time from the rough rolling end temperature to the start of finish rolling shown in Table 2 (however, rough rolling was not performed on the sample material), and the total rolling reduction in the temperature range of 950°C or less shown in Table 2.

A copper strip was obtained by rolling at a finish rolling temperature of 730 to 700° C. and a winding temperature of s of 90 to 550° C. A spiral copper tube having an outer diameter of 111011 cm and a wall thickness of 14.3 cm was formed from this copper strip. These spiral steel pipes are heated to 920'C by induction heating, and some of them are heated to 1
It is cooled to room temperature under the cooling conditions shown in Table 12, and the residue is
After cooling to 250° C. under the conditions shown in the table, the samples were tempered as shown in Table 2.

On the other hand, in order to compare with the steel pipe made by this JIV4 method, we decided to use the original!
A steel pipe M-X having the same chemical composition, carbon equivalent button, wall thickness and outer diameter as the steel pipe of II theory was manufactured from a continuous cast slab under manufacturing conditions that do not satisfy any of the requirements of the present invention. Note that processing conditions that do not satisfy the requirements of the present invention in the comparative steel pipes are underlined in Table 2.

Ar of all steel pipes shown in Table 2 is 800-750℃
, 880 to 830°C, and Ac, 670 to 720°C.

As is clear from Table 2, the steel pipe according to the present invention has superior DWTT characteristics and Charpy absorbed energy at -80°C, and is glossy compared to the comparative steel pipe.

It can be seen that it has X80 class strength and excellent low temperature toughness.

As is clear from the above examples, the present invention uses a continuously cast slab of steel with a specific composition, performs controlled rolling with a hot strip mill, and further heat-treats the steel pipe after pipe making to achieve excellent low-temperature toughness. API standard X8
We were able to manufacture grade 0 steel pipes.

[Brief explanation of the drawing]

The attached drawing shows the total rolling reduction in the temperature range below 950°C during rolling of the copper strip material, the elapsed time from the end of rough rolling to the start of finish rolling, and the direction perpendicular to the tube axis for a spiral steel pipe that has been heat-treated after forming. DWTT85%FAT
It is a correlation diagram showing the relationship with T. Agent Takeo Nakaji

Claims (4)

[Claims] (1) C10.15% or less by weight, Si t
0.70% or less, Mn 50.50-2.50%, ? ! 0
．． 025% or less, s+o, oos% or less, Nb! 0
．． 010 to 0.150%, AAto, 070X or less, and further as necessary eV to, o to to o, i
so%, TiI4.005-0.150%, Zr
+0.005-0.150%, MotO, 05-0.
50%, Cut 0.10-1.00%, Nl+0.
10-4.00%, Cr: 0.10-1.00%,
Rare earth elements: 0.020% or less, Cm + 0.010
Contains one or more types selected from below %,
In the method for manufacturing API grade XIO class steel pipes of steel with a carbon equivalent Ceq of 0.45 or less and the remainder substantially consisting of Fe, manufacturing a continuous cast slab having a thickness, and after the slab is kept warm or heated as it is or within 20 minutes, the surface Wa degree of the slab is 1000 to 800.
℃, and within 60 seconds after the completion of the rough rolling, 9! A step of starting finish rolling in a temperature range of sO to 750°C, setting a rolling reduction rate of 60911 or more during the rolling, and finishing finish rolling in a temperature range of transformation point to 6sO°C; a step of winding the hot-rolled copper strip in a temperature range of 450°C; a step of forming the hot-rolled copper strip into a steel pipe; a step of heating the steel pipe to a temperature range of Ac, a transformation point of -50°C to 1100°C; 80 steel pipes
Average cooling rate OR (C/sec
) satisfies the following formula (API standard X80Q (castability index) = v'6 (1
+0.6451)(1+4.10Mn)(1+2.
23Cr)(1+0.52Ni)(1+3.14M
o) (1+0.27Cu) However, C, Si, Mn, Cr
, Ni%Mo, Cu are weight% of steel. (2) C: 0.15% or less in weight ratio, St: 0.
70% or less, Mn: 0.50-2.50%, P: 0
．． 025% or less, S: 0.005% or less, Nb: o, o
io ~ 0.150%, l'J-: 0.070% or less, and if necessary, V: 0.010-0.150%
, Ti: 0.005-0.150%, Zr: 0
．． 005-0.150%, Mo: 0.05-0.5
0%, Cu: 0.10-1.00%, Ni: 0.1
0-4.00%, Cr: 0.10-1. 0%,
Contains one or more selected from rare earth elements 40.020% or less and Ca: 0.010% or less, and has a carbon equivalent Ceq represented by 6 5 15 of 0.45 or less. API standard X80 for steels with the remainder essentially consisting of Fe
In the manufacturing method of grade steel pipe, the step of manufacturing a continuous casting slab having a thickness of 300 mm to 3 times the final product thickness, and the step of manufacturing the slab as it is or after keeping it warm or heating it for within 20 minutes, Temperature is 1000-80
A step of starting rough rolling when the temperature reaches 0°C, and a step of starting finish rolling within 60 seconds after the completion of the rough rolling in a temperature range of 950 to 750°C, and setting a rolling reduction rate of 60% or more at Ar
, a step of finishing finish rolling in a temperature range of transformation point to 650° C., a step of winding the hot rolled steel strip in a temperature range of 750 to 450° C., and a step of forming the hot rolled steel strip into a steel pipe. heating the steel pipe to a temperature range from Ac3 transformation point -50°C to 1100°C; heating the heated steel pipe to a temperature range of 800°C to 1100°C;
Average cooling rate during 500 °C (1; R ( °C/sec
) to satisfy the following formula to below 500°C, and a step of heating the cooled steel pipe again to the temperature range of 500°C to Ac, the attack point, and then cooling it. A manufacturing method for API standard X80 class steel pipe with excellent low-temperature toughness. CR≧30.8/Q Q (hardenability index)=l -(1+0.64Si)(1
+4.10 pigeon) (1+2.23Cr) (1+0.52N
i) (1+3.14Mo)(1+0.27Cu) only
, C%Si, Mn, Cr, Ni%Mo, Cu
is the weight of steel %0 (3) C: o, is% or less in weight ratio, Si: 0.
70% or less, Mn: 0.50-2.50%, P: 0
．． 025% or less, S: o, oos% or less, Nb: 0
．． 010 to 0.150%, A-t: o, o'yo% or less, and if necessary V: o, o1o to 0.150%.
, Ti: 0.005-0.150%, Zr: 0
．． 005-0.150%, Mo: 0.05-0.5
0%, Cu: 0.10-1.00%, Ni: 0
．． Contains one or more selected from 10 to 4.00%, Cr: 010 to 1.00%, rare earth elements: 0.020% or less, Ca: 0.010% or less, and 6 5 In the manufacturing method of API standard xso'' class steel pipe of steel in which the carbon equivalent Ceq shown in No. 15 is 0.45 or less and the remainder is substantially Fe, the thickness of the final product is from 300 mm to 3 times the length. A step of manufacturing a continuous cast slab having a thickness, and after keeping the slab as it is or keeping it warm for 20 minutes or heating it, finish rolling is started when the surface temperature of the slab reaches 1000 to 750°C and 950°C or less. After finish rolling with a rolling reduction of 260% or more, Ar, transformation point ~
a step of finishing finish rolling in a temperature range of 650°C; a step of winding the hot rolled steel strip in a temperature range of 750 to 450°C; a step of forming the hot rolled steel strip into a steel pipe;
c. Heating the heated steel pipe to a temperature range from -50°C to 1100°C, and heating the heated steel pipe to room temperature such that the average cooling rate OR (°C/5ee) between 800 and 500°C satisfies the following formula: Q (hardenability index) - VC (1 + 0.64 Si
)'(1-1-4,10Mn)(1+2.23Cr
)(1+0.52Ni)(1+3.14Mo)(1+
0.27Cu) However, C%Si, Mn, Cr%Ni,
Mo1Cu is the weight percent of steel. (4) C10.15% or less by weight, 812 Q
, 7Q% or less, Mn f 0.50-L50%, Pl
o, 025% or less, Sgo, 005% or less, Nb f
0.010-0.150%% ktto, 07 (JI6
Contains the following, and if necessary, V l 0.010 to 0.
150X, 71 I O, 005~0. IJ$0%,
Zr t O, 005-0.150X% Net 0.
05-0.50% i', Cut 0.10-1.0
ON%NitO, 10-400. ! %, Or r
0.10~1.00X, rare earth elements to, ozo
API of steel that contains one or more types selected from pg or less, Caro, 0.10% or less, has a carbon equivalent Ceq of 0.45 or less, and the remainder substantially consists of F@. In the method for manufacturing standard When the surface ms degree of the slab reaches 1000~7JSO1l:, finish rolling is started and the reduction rate at 950℃ or less is 60℃.
After finish rolling to X or higher, Ar, transformation point ~aS
A step of finishing finish rolling in a temperature range of 0°C, a step of winding the hot rolled copper strip in a temperature range of 750 to 450°C, a step of forming the hot rolled steel strip into a steel pipe, and a step of forming the steel pipe into an Acs
The step of heating from the transformation point -50C to 1100 °C t"c a degree*rm, and the average cooling rate CR (t: / sec) of the heated steel pipe between SOO and 500 °C satisfies the following formula: The step of cooling the steel pipe to below 500°C, and the step of cooling the cooled steel pipe again to the temperature I of 500C-Act transformation point.
! A method for producing an API standard X80 class steel pipe with excellent low temperature toughness, comprising the steps of heating and then cooling. CR≧30.8/Q Q (castability index) = ≠Σ(1+0.6481)
(1+4.10Mw□(1+2.23Cr)(1+0
．． 52N1) (1+3.14M0)C1+0.27Cu
)t=lic%ss, earth, Cr, Nj%Mo
, wt% of Cu1it14.