CN102203302B - Method for manufacturing steel plate and steel pipe for ultrahigh-strength line pipe - Google Patents

Abstract

Disclosed is a method for manufacturing a steel plate for an ultrahigh-strength line pipe, whereby: steel comprised of 0.03-0.08% C, 0.01-0.50% Si, 1.5-2.5% Mn, 0.01% or less P, 0.0030% or less S, 0.0001-0.20% Nb, 0.0001-0.03% Al, 0.003-0.030% Ti, 0.0010-0.0050% N, 0.0050% or less O, in terms of mass%, with the remainder being iron and unavoidable impurities, is shaped into ingots; this steel is cast into billets; these billets are hot-rolled to form a steel plate; the surface of the steel plate is cooled until the surface temperature of the aforementioned steel plate is a prescribed temperature that is higher than 540 DEG C, with a water quantity concentration of 0.6 m3/(m2 DEG min) or less; and then the surface of the aforementioned steel plate is cooled with a water quantity concentration of 1.3 m3/(m2 DEG min) or more.

Description

Manufacturing method of steel plate and steel pipe for ultra-high strength line pipe

technical field

The present invention relates to a method for manufacturing an ultra-high-strength line pipe steel plate having a tensile strength (TS) in the circumferential direction of 625 MPa or more and excellent low-temperature toughness, and a method for manufacturing an ultra-high-strength line pipe steel pipe manufactured using the steel plate. In particular, the steel pipe obtained by the production method of the present invention can be widely used as a line pipe for natural gas and crude oil transportation. the

This application is based on Japanese Patent Application Japanese Patent Application No. 2008-285837 for which it applied on November 6, 2008, and the priority is claimed, The content is used here. the

Background technique

In recent years, line pipes have become more and more important as a method of long-distance transportation of crude oil and natural gas. Now, as the trunk line pipe for long-distance transportation, the American Petroleum Institute (API) standard X65 has become the basis of design, and the actual usage of X65 line pipe is overwhelmingly large. However, higher strength line pipes are desired for (1) improvement of high-pressure transportation efficiency and (2) improvement of on-site construction efficiency by reducing the outer diameter and weight of the line pipe. So far, line pipes up to X120 (with a tensile strength of 915 MPa or more) have been put into practical use. the

On the other hand, in recent years, the way of thinking of designing line pipes has been changing. In the past, it was the design of the linepipe that made the stress uniform ("stress based design"). However, recently, a design ("strain based design") has been adopted that does not damage the circumferential welded portion of the steel pipe or buckles the steel pipe itself even if deformation is applied to the line pipe. So far, for high-strength linepipes of X80 or higher, chemical components and manufacturing conditions for ensuring the low-temperature toughness of the base metal and the toughness of the welded heat-affected zone have been studied. However, in the case of "strain based design", the deformability of the base material or the deformability of the steel pipe after painting is also required. It is impossible to manufacture X80 or higher line pipe steel pipes for "strain based design" unless these problems related to toughness and deformability are solved. In the ultra-high-strength line pipe, it is necessary to ensure the balance between the strength of the base metal and the low-temperature toughness, the toughness of the weld metal and the toughness of the weld heat-affected zone (HAZ), field weldability, resistance to joint softening, and rupture tests. Excellent resistance to cracking of the pipe body, etc., while manufacturing steel pipes with excellent deformability of the base material. Therefore, it has been desired to develop ultra-high-strength thick-walled line pipes of X80 or higher satisfying these steel pipe characteristics. the

With regard to the production method of steel pipes for linepipes, for example, the following methods have been proposed so far in order to improve the above-mentioned steel pipe characteristics. In Patent Document 1 and Patent Document 2, in order to improve the deformability of the steel pipe, it is proposed that the steel plate is slowly cooled in the front stage of 500-600°C, and then cooled at a faster cooling rate than the front stage in the subsequent stage. method. In this method, the microstructure of the steel sheet and the steel pipe can be controlled. Furthermore, in Patent Document 3 and Patent Document 4, in order to improve the buckling resistance of the steel pipe, the steel pipe is cooled at a constant cooling rate of 15° C./s or higher to manufacture a 16 mm thick steel plate. the

Prior technical literature

Patent Documents

Patent Document 1: Japanese Patent Laid-Open No. 2004-131799

Patent Document 2: Japanese Patent Laid-Open No. 2003-293089

Patent Document 3: Japanese Patent Application Laid-Open No. 11-279700

Patent Document 4: Japanese Patent Laid-Open No. 2000-178689

Contents of the invention

The problem to be solved by the invention

However, in the methods disclosed in Patent Document 1 and Patent Document 2, since the water cooling stop temperature of the steel plate varies greatly, there is a problem that the material of the steel plate varies greatly. Also, in the methods disclosed in Patent Document 3 and Patent Document 4, since the water-cooling stop temperature of the steel sheet varies greatly, in addition to the large variation in the strength of the steel sheet, there is also a big problem in securing the deformability of the steel sheet. the

The invention provides a method for manufacturing ultra-high-strength linepipe steel plates and steel pipes with excellent base material strength, low-temperature toughness, and deformability, and a tensile strength of 625 MPa or more (API standard X80 or more) that is easy to weld on site. the

The means used to solve the problem

The inventors of the present invention have intensively studied the production conditions of steel sheets and steel pipes for obtaining ultra-high-strength steel sheets and steel pipes having a tensile strength of 625 MPa or more and excellent low-temperature toughness. As a result, new manufacturing methods of steel sheets for ultra-high-strength linepipes and steel pipes for ultra-high-strength linepipes have been invented. The gist of the present invention is as follows. the

(1) A method for producing a steel plate for ultra-high strength linepipe, wherein the melting contains C: 0.03 to 0.08%, Si: 0.01 to 0.50%, Mn: 1.5 to 2.5%, P: 0.01% or less, S: 0.0030% or less, Nb: 0.0001 to 0.20%, Al: 0.0001 to 0.03%, Ti: 0.003 to 0.030%, N: 0.0010 to 0.0050%, O: 0.0050% or less, and the remainder includes iron and unavoidable impurities steel; casting the steel to form a steel slab; subjecting the steel slab to hot rolling to form a steel plate; cooling the surface of the steel plate to the surface temperature of the steel plate at a water quantity concentration of 0.6 m ³ /(m ² ·min) or less A specified temperature greater than 540°C; and then cooling the steel plate surface with a water density of 1.3m ³ /(m ² ·min) or greater.

(2) In the method for producing an ultra-high-strength linepipe steel sheet described in the above (1), the steel may further contain Mo: 0.01 to 1.0%, Cu: 0.01 to 1.5%, and Ni: 0.01% by mass %. At least one of ～5.0%, Cr: 0.01～1.5%, V: 0.01～0.10%, B: 0.0001～0.0003%, W: 0.01～1.0%, Zr: 0.0001～0.050%, Ta: 0.0001～0.050% . the

(3) In the method for producing an ultra-high-strength linepipe steel sheet described in the above (1), the steel may further contain Mg: 0.0001 to 0.010%, Ca: 0.0001 to 0.005%, REM: 0.0001% by mass % At least one of -0.005%, Y: 0.0001-0.005%, Hf: 0.0001-0.005%, and Re: 0.0001-0.005%. the

(4) In the method for manufacturing an ultra-high-strength linepipe steel sheet as described in the above (1), when the surface temperature of the steel sheet is a predetermined temperature higher than 540° C., the surface cooling rate of the steel sheet may be 10°C or higher. °C/s or less; when the surface temperature of the steel plate is lower than the specified temperature, the surface cooling rate of the steel plate is 40 °C/s or more. the

(5) In the method for manufacturing an ultra-high-strength linepipe steel sheet described in the above (1), in the hot rolling, the reheating temperature of the steel slab may be 950°C or higher, and the unrecrystallized steel slab may be The reduction ratio in the temperature zone may be 3 or more. the

(6) In the method of manufacturing the steel plate for ultra-high strength linepipe described in the above (1), cooling may be started from a cooling start temperature of 800° C. or lower. the

(7) In the method for manufacturing ultra-high-strength line pipe steel pipe, the steel plate manufactured by the method for manufacturing ultra-high-strength line pipe steel plate described in the above (1) is formed into a tubular shape by UO pipemaking; using welding wire and Sintering type or melting type flux, carry out submerged arc welding to the butt joint of described steel plate; Then, carry out pipe expansion. the

(8) In the method for manufacturing an ultra-high-strength linepipe steel pipe described in the above (7), after the submerged arc welding is performed and before the pipe expansion is performed, a welded portion is heat-treated. the

(9) In the method of manufacturing an ultra-high-strength linepipe steel pipe described in the above (7), the welded portion is heat-treated at a temperature of 200°C to 500°C. the

Invention effect

According to the present invention, after hot-rolling a steel plate with a defined chemical composition, slow cooling is performed in the front stage before the surface temperature of the steel plate reaches the transition boiling temperature range, and rapid cooling is performed in the latter stage, thereby reducing the size of the steel plate and The strength deviation of the steel pipe improves the deformation performance of the steel plate and steel pipe before and after deformation aging. As a result, safety for line pipe is greatly improved. the

Description of drawings

FIG. 1 is a schematic diagram showing an example of the relationship between the cooling pattern on the surface of a steel sheet and the transformation curve of the steel. the

Detailed ways

Hereinafter, the content of the present invention will be described in detail. the

The present invention relates to an ultra-high-strength linepipe having a tensile strength (TS) of 625 MPa or more and excellent low-temperature toughness. The ultra-high-strength line pipe of this level of strength can withstand about 1.2 to 1.8 times the pressure compared with the conventional mainstream X65, so it can transport more gas with the same size as before. When using X65 at higher pressures, the wall thickness of the line pipe needs to be thickened. Therefore, material costs, transportation costs, and on-site welding construction costs have increased, and line pipe laying costs have increased significantly. Therefore, in order to reduce the cost of laying line pipes, ultra-high-strength line pipes having a tensile strength (TS) of 625 MPa or more and excellent low-temperature toughness are required. On the other hand, while the required strength of the steel pipe increases, the manufacture of the steel pipe becomes very difficult. In particular, when "strain based design" is required, it is necessary to obtain not only the balance between the strength of the base metal and the low temperature toughness and the toughness of the seam weld, but also the target characteristics including deformation energy after deformation aging. However, satisfying all these properties is very difficult. the

In the line pipe requiring "strain based design", the strength of the weld metal connecting the line pipes (the strength of the circumferential weld) must be higher than the length direction of the base metal (the part of the steel plate that is a steel plate or steel pipe) (the strength of the line pipe) strength along the tube axis). In environments where line pipes are used, permafrost sometimes melts in summer or regenerates in winter. In this case, the line pipe is deformed and broken from the circumferential weld. In particular, in the case where the strength of the circumferential weld is low-matched to the strength of the base metal, fracture occurs with smaller deformation. Therefore, the strength in the longitudinal direction of the base material needs to be lower than the strength of the circumferential weld, and an upper limit is set for the strength of the base material in the longitudinal direction depending on the strength of the circumferential weld. In particular, since each grade of linepipe has a strength range, the strength of the base material used to manufacture the linepipe is limited within a narrow range depending on the upper limit. Therefore, it is required to stably manufacture a linepipe and its base material in which variations in strength are suppressed. the

The inventors conducted intensive studies in order to limit the tensile strength of the base material of the linepipe to 625 MPa or more and within a narrow range. As a result, it was found that it is very important to optimize the cooling conditions at the time of hot rolling of the steel sheet by using low-carbon steel as the steel sheet. For example, if the amount of C exceeds 0.08%, the hardenability is too high, so that the strength varies greatly between the center and the surface of the steel sheet. Therefore, low carbon steel is used as the steel plate. Also, for example, even if the amount of C is 0.08% or less, martensite may or may not be formed depending on the cooling method of the steel plate surface if cooling is performed without limiting the cooling conditions of the steel plate surface. In this case, variation occurs in the strength of the surface of the steel plate within one steel plate or among manufactured steel plates, and therefore, it is impossible to manufacture a line pipe having a strength within a narrow range. the

The inventors did not cool the steel plate surface at one time, but by properly adjusting the amount of cooling water in the front section before the surface temperature of the steel plate reaches the transitional boiling temperature zone and the cooling water amount in the rear section thereafter, successfully suppressed the internal and manufacturing of a steel plate. The strength deviation between the steel plates. The inventors of the present invention think that the reason why the variation in strength of the steel sheet can be significantly suppressed by appropriately adjusting the water density and the cooling rate at the front and rear stages is as follows. the

If the steel plate is cooled from a high temperature, the cooling mechanism of the steel plate changes in the order of film boiling, transition boiling, and nucleate boiling from a high temperature. It is known that in a temperature region where transition boiling occurs (transition boiling temperature region), the cooling mechanism transitions from film boiling to nucleate boiling, and thus is unsteady (unstable) cooling. Therefore, if the steel plate is cooled for a long time in the transition boiling temperature region, the temperature variation within the steel plate increases. As a result of the investigation, it was found that in this transition boiling temperature range, the surface temperature of the steel sheet is in the range of 450 to 560°C, and it is necessary to rapidly cool the steel sheet. In addition, in the present invention, in order to make the steel sheet have good deformability, the structure of the steel sheet is not formed into a martensite structure, but a bainite/ferrite mixed structure is formed. Therefore, when the surface temperature of the steel plate exceeds 540°C, cooling is performed with a small water density or a cooling rate that causes ferrite transformation. However, as described above, it is necessary to shorten the cooling time of the steel plate in the transition boiling temperature region. Therefore, when the surface temperature of the steel sheet is 540° C. or lower, cooling is performed at a large water density or a cooling rate such that variations in the surface temperature of the steel sheet due to transition boiling are reduced. According to this method, the cooling stop temperature of the steel sheet can be made uniform, and thus the strength in the width direction and the longitudinal direction of the steel sheet can be made substantially uniform. Therefore, the timing of switching the water density or the cooling rate of the steel plate, that is, the timing of switching the front and rear stages of cooling needs to make the surface temperature of the steel plate a predetermined temperature of 540° C. or higher. Regarding the timing of switching between the front stage and the rear stage of cooling, the surface temperature of the steel sheet is preferably 560°C or higher, more preferably 580°C or higher. the

Hereinafter, the reasons for limiting the composition of the steel sheet (base material) in the present invention will be described. In addition, % represents mass % about the chemical component of this invention. the

C is indispensable as a basic element for increasing the strength of the base material. Therefore, it is necessary to add 0.03% or more of C. If C is added excessively exceeding 0.08%, the weldability and toughness of a steel material will fall. Therefore, the upper limit of the amount of C added is made 0.08%. the

Si is necessary as a deoxidizing element in steelmaking. For deoxidation, 0.01% or more of Si needs to be added to the steel. However, if Si is added in excess of 0.50%, the HAZ toughness of the steel material will decrease. Therefore, the upper limit of the amount of Si added is made 0.50%. the

Mn is an element necessary for securing the strength and toughness of the base material. However, if the amount of Mn exceeds 2.5%, the HAZ toughness of the base material will remarkably decrease. When the amount of Mn is less than 1.5%, it becomes difficult to secure the strength of the base material, so the range of the amount of Mn is made 1.5 to 2.5%. the

P is an element that affects the toughness of steel. If the amount of P exceeds 0.01%, not only the toughness of the base material but also the toughness of the HAZ will decrease significantly. Therefore, the upper limit of the amount of P is made 0.01%. the

When S is added in excess of more than 0.0030%, coarse sulfides are formed. The coarse sulfides lower the toughness, so the upper limit of the amount of S is made 0.0030%. the

Nb is an element having an effect of increasing strength by forming carbides and nitrides. However, when Nb is added in an amount of 0.0001% or less, there is no effect. In addition, adding more than 0.20% of Nb leads to a reduction in toughness. Therefore, the range of the amount of Nb is defined as 0.0001 to 0.20%. the

Al is usually added as a deoxidizing material. In the present invention, if Al is added in excess of 0.03%, oxides mainly of Ti will not be formed. Therefore, the upper limit of the amount of Al is made 0.03%. In addition, in order to reduce the amount of oxygen in molten steel, it is necessary to add 0.0001% or more of Al. Therefore, the lower limit of the amount of Al is made 0.0001%. the

Ti is an element that acts as a deoxidizing material and as a nitride-forming element to refine crystal grains. However, the addition of a large amount of Ti brings about a remarkable decrease in toughness due to carbide formation, so the upper limit of the amount of Ti needs to be made 0.030%. However, in order to obtain a predetermined effect, it is necessary to add 0.003% or more of Ti. Therefore, the range of the amount of Ti is made 0.003 to 0.030%. the

N is necessary for finely precipitating TiN and making the austenite grain size finer. When the amount of N is 0.0010%, miniaturization is insufficient, so the lower limit of the amount of N is made 0.0010%. In addition, if the amount of N exceeds 0.0050%, the amount of solid solution N increases and the low-temperature toughness of the base material deteriorates, so the upper limit of the amount of N is made 0.0050%. the

When O is added excessively exceeding 0.0050, coarse oxides are formed and the toughness of the base material is lowered. Therefore, the upper limit of the amount of O is made 0.0050%. the

A steel containing the above elements, with the remainder including iron (Fe) and unavoidable impurities is a preferable basic composition employed as the steel sheet and steel pipe of the present invention. the

In addition, in the present invention, at least one element selected from Mo, Cu, Ni, Cr, V, B, Zr, and Ta can be added as an element for improving strength and toughness as required. the

Mo is an element that improves hardenability and forms carbides and nitrides to improve strength. In order to obtain this effect, it is necessary to add 0.01% or more of Mo. However, the addition of a large amount of Mo exceeding 1.0% increases the strength of the base material more than necessary while significantly reducing the toughness. Therefore, the range of the amount of Mo is made 0.01 to 1.0%. the

Cu is an effective element for increasing strength without reducing toughness. However, when the amount of Cu is less than 0.01%, there is no effect, and when the amount of Cu exceeds 1.5%, cracks are likely to occur when the steel slab is heated or welded. Therefore, the content of the amount of Cu is made 0.01 to 1.5%. the

Ni is an effective element for improving toughness and strength. In order to obtain this effect, it is necessary to add 0.01% or more of Ni. However, when Ni is added in excess of 5.0%, weldability decreases. Therefore, the upper limit of the amount of Ni is made 5.0%. the

Cr is an element that increases the strength of steel by precipitation strengthening. Therefore, it is necessary to add 0.01% or more of Cr. However, if a large amount of Cr is added, a martensitic structure is formed due to an increase in hardenability, and the toughness is lowered. Therefore, the upper limit of the amount of Cr is made 1.5%. the

V is an element having an effect of increasing strength by forming carbides and nitrides. However, when V is added in an amount of 0.01% or less, there is no effect. In addition, when V is added in excess of 0.10%, the toughness is lowered. Therefore, the range of the amount of V is defined as 0.01 to 0.10%. the

B is generally an element that increases hardenability by solid solution in steel and significantly suppresses the formation of ferrite. Therefore, the amount of B is made less than 0.0003%. However, in order to secure a certain degree of hardenability of the steel, 0.0001% or more of B may be added. Therefore, the range of the amount of B is defined as 0.0001 to 0.0003%. the

W is an element that improves hardenability and forms carbides and nitrides to improve strength. In order to obtain this effect, it is necessary to add 0.01% or more of W. However, the addition of a large amount of W exceeding 1.0% increases the strength of the base material more than necessary and significantly reduces the toughness. Therefore, the range of the amount of W is defined as 0.01 to 1.0%. the

Like Nb, Zr and Ta are also elements that have an effect of increasing strength by forming carbides and nitrides. However, when added at 0.0001% or less, there is no effect. In addition, adding more than 0.050% of Zr or Ta leads to a reduction in toughness. Therefore, the range of the amount of Zr or Ta is defined as 0.0001 to 0.050%. the

In addition, in the present invention, at least one element of Mg, Ca, REM, Y, Hf, and Re can be added in order to improve the pinning effect of the oxide or lamellar tear resistance as needed. the

Mg is mainly added as a deoxidizing material. However, if Mg is added in excess of 0.010%, coarse oxides are likely to be formed, and the toughness of the base material and HAZ decreases. In addition, when Mg is added in an amount of less than 0.0001%, the generation of oxides necessary for intragranular transformation and pinning of particles cannot be expected sufficiently. Therefore, the addition range of Mg is specified as 0.0001 to 0.010%. the

Ca, REM, Y, Hf, and Re suppress the formation of MnS, which tends to elongate in the rolling direction, by forming sulfides, and improve the properties of the steel material in the thickness direction, especially the lamellar tear resistance. When Ca and REM, Y, Hf, and Re are all less than 0.0001%, this effect cannot be obtained. Therefore, the lower limit of the amount of Ca, REM, Y, Hf, and Re is set to 0.0001%. Conversely, if Ca and REM, Y, Hf, and Re all exceed 0.0050%, the number of oxides of Ca, REM, Y, Hf, and Re increases, and the number of ultrafine Mg-containing oxides decreases. Therefore, the upper limit of the amount of Ca, REM, Y, Hf, and Re is set to 0.0050%. the

After melting the steel containing the above components in the steelmaking process, it is cast by continuous casting or the like to form a billet (slab). This steel slab is subjected to hot rolling (rolling after heating the slab) to form a steel sheet. At this time, the slab is heated to a temperature above the A _C3 point (reheating temperature), and rolled so that the reduction ratio in the recrystallization temperature range is 2 or higher and the reduction ratio in the non-recrystallization temperature range is 3 or higher. As a result, the average prior-austenite grain size of the obtained steel sheet was 20 μm or less.

The reheating temperature of the steel slab (cast slab) is preferably 950° C. or higher. In addition, if the reheating temperature is too high, the γ grains will be coarsened during heating, so the reheating temperature is preferably made 1250° C. or lower. the

With regard to the reduction ratio in the recrystallization temperature range, if the reduction ratio is less than 2, recrystallization cannot sufficiently occur, so it is preferable to set the reduction ratio to 2 or more. the

If the reduction ratio in the non-recrystallization temperature range is set to 3 or more, the average prior-austenite grain size of the steel sheet can be 20 μm or less. Therefore, it is preferable to set the reduction ratio in the non-recrystallization temperature range to 3 or more. More preferably, the reduction ratio in the non-recrystallization temperature range is 4 or more. In this case, the average prior-austenite grain size of the steel sheet can be 10 μm or less. the

Regarding the temperature at which water cooling is started (water cooling start temperature), it is preferable to start cooling the steel plate from a water cooling start temperature of 800° C. or lower. That is, the cooling of the steel plate is started from A _e3 point or less. In this case, since the yield ratio of the steel sheet is lowered by ferrite transformation, the deformability of the steel sheet is good.

Regarding the cooling method, the surface of the steel plate is cooled at a water density of 0.6 m ³ /(m ² ·min) or less until the surface temperature of the steel plate exceeds a predetermined temperature of 540°C (in the preceding stage). When the water density exceeds 0.6 m ³ /(m ² ·min), ferrite does not form in the steel sheet. Then (at the latter stage), the surface of the steel plate is cooled at a water density of 1.3 m ³ /(m ² ·min) or more. When the water volume density is lower than 1.3 m ³ /(m ² ·min), the time for the steel plate to stay in the transition boiling temperature region is prolonged, and the temperature variation in the steel plate increases to a non-negligible level.

In addition, regarding the temperature on the surface of the steel plate, the central portion in the width direction of the steel plate was measured. the

Furthermore, when the surface temperature of the steel sheet is above a predetermined temperature (at the front stage) of 540°C or higher, the surface cooling rate of the steel sheet is preferably 10°C/s or less. When the surface cooling rate of the steel sheet exceeds 10°C/s, ferrite does not form in the steel sheet. On the other hand, when the surface cooling rate of the steel sheet is lower than the predetermined temperature (in the latter stage), the surface cooling rate of the steel sheet is preferably 40° C./s or higher. When the surface cooling rate of the steel plate is lower than 40°C/s, the stagnation time of the steel plate in the transition boiling temperature region is prolonged, and the temperature deviation in the steel plate increases to a level that cannot be ignored. In the cooling device used in the present invention, there are several places (called regions) where nozzles that can be controlled so that the water density becomes the same are concentrated. In the present invention, for example, these regions are divided into the aforementioned front stage (predetermined temperature region of 540° C. or higher) and rear stage. After the water density is set in the front and back sections, the actual surface temperature of the steel plate before and after water cooling, the steel plate furnace penetration speed and the cooling distance of the steel plate can be used to calculate the cooling rate of the steel plate surface. In addition, the position (area) for switching the front stage and the rear stage can be determined arbitrarily, and can be determined by considering the cooling condition of the steel plate and the like. the

The reason for performing cooling under the above cooling conditions will be described in more detail below with reference to FIG. 1 . Fig. 1 is an example of the relationship between the cooling pattern on the surface of the steel plate and the phase transformation curve of the steel. As shown by the dotted line (i) in Fig. 1, when the water density in the previous stage or the surface cooling rate of the steel plate does not satisfy the conditions of the present invention, the surface of the steel plate does not have a ferrite/bainite mixed structure, but has a substantially martensitic structure. Body organization. Therefore, even when the water density at the front and back sections or the surface cooling rate of the steel plate satisfy the conditions of the present invention, the toughness of the steel plate surface is often significantly reduced, and surface defects such as surface cracks may occur on the steel plate during steel pipe production. In addition, since the rapid cooling is performed before ferrite transformation or bainite transformation starts, variations in strength often occur in the steel sheet. In addition, as shown by the dotted line (ii) in Figure 1, when the water density in the latter stage or the surface cooling rate of the steel plate does not meet the conditions of the present invention, the time for the steel plate to stay in the transition boiling temperature region is prolonged, and the temperature in the steel plate The deviation increases to a level that cannot be ignored. Therefore, even if the water density at the previous stage or the surface cooling rate of the steel plate is the condition for forming ferrite in the steel plate, there will be a variation in strength within one steel plate or between produced steel plates. On the other hand, as shown by the solid lines (iii) and (iv) in FIG. 1 , when the water density at the front and rear stages or the surface cooling rate of the steel plate satisfies the conditions of the present invention, the steel plate is the Bainian steel plate of the present invention. Ferritic/ferritic mixed structure. the

With regard to the cooling stop temperature, if the final water cooling (final water cooling) is stopped at 200° C. or lower, defects thought to be caused by hydrogen are formed in the thickness center of the steel sheet. Therefore, it is preferable to set the lower limit of the cooling stop temperature to 200°C. the

Next, a method for producing a linepipe by the UO process (UO pipemaking) using the steel plate for ultra-high strength linepipe produced by the above-mentioned production method will be described. After manufacturing a steel plate with a thickness of 12 to 25mm, it is formed into a tubular shape by UO pipemaking (C extrusion, U extrusion, O extrusion). Then, the ends of the steel plates formed into a tubular shape are butt-butted and tack welded. In this tack welding, MAG welding or MIG welding is used. After tack welding, submerged arc welding is performed on the butt joints of steel plates formed into a tubular shape from inside and outside. In this submerged arc welding, welding wire and fired or melted flux are used. Finally, pipe expansion is performed to manufacture a steel pipe. the

In the method of manufacturing an ultra-high-strength linepipe steel pipe according to the present invention, it is preferable to heat-treat the welded portion (seam welded portion) after performing the above-mentioned submerged arc welding of the inner and outer surfaces and before performing pipe expansion. In addition, as the heat treatment conditions of the steel pipe, it is preferable to heat-treat the welded portion at a temperature of 200°C to 500°C. This heat treatment can reduce the MA (mixed structure of austenite and martensite) generated in the welded part (weld metal) which is detrimental to toughness. When the welded part is heated at a temperature of 200°C to 500°C, the coarse MA generated along the prior-austenite grain boundaries decomposes into fine cementite. However, when the welded part is heat-treated at a temperature lower than 200°C, the coarse MA does not decompose into cementite. Therefore, the lower limit of the heat treatment temperature of the welded portion is 200°C. In addition, if the welded portion is heat-treated at a temperature exceeding 500° C., the toughness of the welded portion deteriorates. Therefore, the upper limit of the heat treatment temperature of the welded portion is 500°C. the

Example

Next, examples of the present invention will be described. the

After heating a steel billet with a thickness of 240mm having the chemical composition in Table 1 to 1000-1210°C, hot rolling was carried out in a recrystallization temperature range above 950°C until the thickness (transfer thickness) of the billet reached 70-100mm. Furthermore, hot rolling is performed in a non-recrystallization temperature range in the range of 880 to 750° C. until the thickness (sheet thickness) of the slab becomes 12 to 25 mm. Then, the cooling of the steel sheet is started from a temperature of 650°C to 795°C (water cooling in the first stage), and rapid cooling is performed from a predetermined temperature higher than 540°C. Then, the cooling is stopped at a temperature of 200 to 500° C. (water cooling in the latter stage). In addition, in Table 1, as a reference, the carbon equivalent _Ceq , weld crack susceptibility index P _cm , martensitic transformation start temperature M _S , and the critical cooling rate at which 90% martensitic structure can be obtained are also shown. MC90.

In order to evaluate the yield strength and tensile strength of each steel plate produced, a total thickness test piece according to the API5L standard was collected from each steel plate, and a tensile test was performed at normal temperature. Regarding the taking direction, test pieces of their total thickness are taken. In addition, the collection position of the overall thickness test piece is such that the longitudinal direction of the overall thickness test piece coincides with the width direction of the steel plate, at a position 1m away from the top end and the end portion of the steel plate in the longitudinal direction of the steel plate. Two total thickness test pieces were taken from both sides of the center portion of the plate width of the steel plate at these positions. the

Next, after forming this steel plate by UO pipemaking, tack welding is performed on the butt joint of the steel plate by carbon dioxide gas shielded welding. Then, using welding wire and molten flux, seam welding by submerged arc welding is performed from the inner and outer surfaces of the butted portions of the steel plates to form steel pipes. The average linear energy of seam welding is 2.0-5.0kJ/mm. In addition, for some steel pipes, heat treatment at 250 to 450°C is performed on the seam weld. The manufacturing conditions of steel plates and steel pipes are shown in Table 2. the

In order to evaluate the yield strength and tensile strength of each steel pipe produced, an API test piece was taken from each steel pipe and subjected to a tensile test. Regarding the collection direction, these API test pieces were taken such that the longitudinal direction of the API test pieces coincided with the pipe axis direction of the steel pipe. In addition, with respect to the collection position, these API test pieces were collected on both sides of the center of the position 1/4 of the seam welded part of each steel pipe in the cross section perpendicular to the pipe axis. In addition, as a reference, in order to evaluate the deformability after deformation aging, these steel pipes were heat-treated at 210°C (holding for 5 minutes and then air-cooled), and two API test pieces were taken from the same positions as above to perform a tensile test. Tensile tests were performed in accordance with API Standard 2000. In addition, in order to evaluate the toughness of the steel pipe, a Charpy test and a DWT test at -30°C were implemented. Charpy test and DWT test are also carried out in accordance with API Standard 2000. The Charpy test piece and the DWT test piece were taken at a position 1/2 circle away from the seam welded part of the steel pipe in a section perpendicular to the pipe axis so that the longitudinal direction of the test piece coincided with the circumferential direction of the steel pipe. Two DWT test pieces were taken from each steel pipe, and three Charpy test pieces were taken from each steel pipe at the center of the wall thickness. the

Furthermore, the HAZ toughness of each steel pipe produced was evaluated. The test piece for evaluating HAZ toughness is taken from the welding heat-affected zone (HAZ) near the seam welded part of the steel pipe, and formed at FL+1mm (1mm away from the boundary between the HAZ part and the seam welded part on the side of the HAZ part) gap. As these test pieces, three pieces were collected from each steel pipe. All of these test pieces were evaluated by the Charpy test at -30°C. the

The results of these tests are shown in Table 3. In addition, in Table 3, not only the tensile strength but also the yield strength and yield ratio are shown for reference. the

Steels 1 to 22 represent examples of the present invention. Table 3 shows that these steel sheets and steel pipes have a tensile strength of X80 or higher, and the variation in strength in the steel sheets and steel pipes is also suppressed to 60 MPa or less. In addition, the Charpy energy of the steel pipe is more than 200J, the DWTT plastic fracture rate is more than 85%, and the Charpy absorbed energy (HAZ toughness) of the welding heat-affected zone exceeds 50J. Thus, the steel pipes in the examples of the present invention have high toughness. Steels 23 to 35 represent comparative examples that do not satisfy the production conditions of the present invention. That is, in steel 23, the amount of C in the steel is less than the range of the present invention, so the tensile strength is not sufficient. In steels 24 to 29 and 31, at least one element among the basic components and optional elements was added to the steel beyond the scope of the present invention, so the HAZ toughness was not sufficient. On the other hand, in Steels 30 and 32 to 35, the cooling conditions of the steel sheets did not satisfy the present invention. That is, in Steels 30 and 33, the steel plate was quenched in the front stage. In steels 32 and 35, the steel plate is slowly cooled in the latter stage. In steel 34, the quenching start temperature of the steel plate is low, and after the surface temperature of the steel plate enters the transition boiling temperature range, the subsequent quenching is performed. Therefore, in the steels 30 and 32 to 35, the variation in the strength of the steel plate and the steel pipe is as large as 100 MPa or more.

Industrial availability

It can provide the manufacturing method of ultra-high-strength line pipe steel plate and steel pipe with a tensile strength of 625MPa or more (API standard X80 or more) with excellent base material strength, low-temperature toughness and deformability, and easy on-site welding. the

Claims (8)

1. the manufacture method of a pipe for ultrahigh-strength line steel plate is characterized in that:

Melting in quality % contain C:0.03～0.08%, Si:0.01～0.50%, Mn:1.5～2.5%, below P:0.01%, below S:0.0030%, Nb:0.0001～0.20%, Al:0.0001～0.03%, Ti:0.003～0.030%, N:0.0010～0.0050%, below O:0.0050%, remainder comprises the steel of iron and inevitable impurity;

Cast this steel to form steel billet;

This steel billet is implemented hot rolling to form steel plate;

With 0.6m ³/ (m ²Min) the following water yield density surface temperature that surface of steel plate is cooled to described steel plate is greater than the specified temperature of 540 ℃;

Then with 1.3m ³/ (m ²Min) above water yield density is carried out cooling to described surface of steel plate;

Wherein, in described hot rolling, the temperature that reheats of described steel billet is more than 950 ℃, the depressing than being more than 3 of the non-recrystallization humidity province of described steel billet.

2. the manufacture method of pipe for ultrahigh-strength line steel plate according to claim 1, is characterized in that, described steel further contains in quality %:

Mo：0.01～1.0%、

Cu：0.01～1.5%、

Ni：0.01～5.0%、

Cr：0.01～1.5%、

V：0.01～0.10%、

B：0.0001～0.0003%、

W：0.01～1.0%、

Zr：0.0001～0.050%、

In Ta:0.0001～0.050% more than a kind.

3. the manufacture method of pipe for ultrahigh-strength line steel plate according to claim 1, is characterized in that, described steel further contains in quality %:

Mg：0.0001～0.010%、

Ca：0.0001～0.005%、

REM：0.0001～0.005%、

Y：0.0001～0.005%、

Hf：0.0001～0.005%、

In Re:0.0001～0.005% more than a kind.

4. the manufacture method of pipe for ultrahigh-strength line steel plate according to claim 1 is characterized in that: in the described surface temperature of described steel plate when above greater than the specified temperature of 540 ℃, the surface cool speed of described steel plate be 10 ℃/below s; In the described surface temperature of described steel plate during lower than described specified temperature, the surface cool speed of described steel plate be 40 ℃/more than s.

5. the manufacture method of pipe for ultrahigh-strength line steel plate according to claim 1, is characterized in that: begin to cool down since the cooling temperature below 800 ℃.

6. the manufacture method of a pipe for ultrahigh-strength line steel pipe is characterized in that:

To be shaped as tubulose by the UO tubing with the steel plate of the manufacture method manufacturing of pipe for ultrahigh-strength line steel plate claimed in claim 1;

Use welding wire and burn till type or fusion solder flux from interior outside, union-melt weld is carried out in the docking section of described steel plate;

Then, carry out expander.

7. the manufacture method of pipe for ultrahigh-strength line steel pipe according to claim 6, is characterized in that: after having carried out described union-melt weld and before carrying out described expander, weld part is heat-treated.

8. the manufacture method of pipe for ultrahigh-strength line steel pipe according to claim 7, is characterized in that: at the temperature of 200 ℃～500 ℃, described weld part is heat-treated.

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