WO2011042936A1

WO2011042936A1 - High-strength steel pipe, steel plate for high-strength steel pipe, and processes for producing these

Info

Publication number: WO2011042936A1
Application number: PCT/JP2009/005263
Authority: WO
Inventors: 朝日均; 原卓也; 土井直己
Original assignee: 新日本製鐵株式会社
Priority date: 2009-10-08
Filing date: 2009-10-08
Publication date: 2011-04-14
Also published as: BR112012007753B1; CN102549186B; BR112012007753A2; CN102549186A; JPWO2011042936A1; JP4824143B2

Abstract

A steel pipe produced by cold-forming a steel plate and then subjecting the formed plate to seam-welding, wherein (i) the steel plate contains carbon, manganese, niobium, and titanium, has a weld crack susceptibility composition (Pcm) of 0.23 or less, and has a metallographic structure including bainite and ferrite and (ii) the ratio of the thickness of the steel plate (t) to the outer diameter (D) of the steel pipe, t/D, is 0.030 or lower and the steel pipe has a circumferential yield ratio, measured using a round-bar test piece, of 0.90 or lower.

Description

High strength steel pipe, steel plate for high strength steel pipe, and manufacturing method thereof

The present invention particularly relates to a high-strength steel pipe suitable for a transportation line pipe for crude oil, natural gas, and the like, a steel sheet for a high-strength steel pipe that is a material thereof, and a method for producing the same.

In recent years, pipelines have become increasingly important as long-distance transportation methods for crude oil and natural gas. In particular, in order to reduce pipeline installation costs and operation costs, it is required to use steel pipes with smaller diameters and operate at higher pressure. As a means for realizing this, it is desired to use a higher-strength line pipe.

Conventionally, as shown in FIG. 2A, the test piece for measuring the strength of the line pipe has been collected from the surface of the steel pipe 1 so that the longitudinal direction of the test piece 2 coincides with the circumferential direction of the steel pipe 1. Since the collected test piece 2 has an arc shape, the yield stress in the circumferential direction is measured by a flat test piece flattened by a press or the like. However, when the test piece is flattened, a compressive strain is generated, and the yield stress of the test piece is reduced by this compressive strain (Bauschinger effect), so that the correct circumferential yield stress of the test piece cannot be measured. In particular, this effect is large at X80 or more (yield stress of 555 MPa or more). Therefore, recently, as shown in FIG. 2B, the test piece is often collected from the cross section of the steel pipe 1 so that the longitudinal direction of the test piece coincides with the circumferential direction of the steel pipe 1. Since the collected test piece (round bar test piece) 3 has a round bar shape, it is not necessary to make it flat, and a correct yield stress in the circumferential direction can be measured.

As shown in FIG. 3, when the yield stress in the circumferential direction of a high strength steel pipe of X80 or higher is measured, the yield stress of the round bar test piece is higher than the yield stress of the flat test piece. Therefore, the yield ratio of the round bar test piece is higher than the yield ratio of the flat test piece, and the yield ratio specified in ISO 3183 may not be satisfied. For example, in ISO 3183, the yield ratio of X80 is defined as 0.93 or less.

Moreover, the steel pipe is manufactured by cold forming of a thick steel plate as a material. Therefore, generally the yield ratio of a steel pipe becomes higher than the yield ratio of a thick steel plate. In order to solve such a problem, a method of suppressing an increase in yield ratio has been proposed by using a thick steel plate having a bainite-based structure and a yield point elongation when manufacturing a steel pipe (for example, Patent Document 1). reference).

JP 2002-363686 A

However, in the method described in Patent Document 1 that suppresses an increase in yield ratio by using a thick steel plate having a bainite-based structure and a yield point elongation during the production of a steel pipe, the amount of increase in yield stress due to cold forming Is larger than the amount of increase in tensile stress, the yield ratio Y / T, which is the ratio between the yield stress Y in the circumferential direction and the tensile stress T, inevitably increases. Therefore, even when the thick steel plate as the material satisfies the yield ratio specified by ISO 3183, the steel pipe may not satisfy this yield ratio. In particular, high strength steel pipes of grades X80 to 100 have a problem in that an increase in yield ratio due to cold forming and pipe expansion must be considered so as to satisfy the yield ratio specified by ISO 3183.

The present invention has been made in view of the above circumstances, and is particularly suitable for line pipes for transporting crude oil, natural gas, and the like, and has a low yield ratio Y / T in the circumferential direction and high strength of X80 to 100 grade. It aims at provision of the steel pipe, the steel plate for steel pipes which is the material, and these manufacturing methods.

In addition, the present invention aims to reliably reduce the yield ratio of a high-strength steel pipe measured using a round bar test piece, in particular, more than the yield ratio (steel plate yield ratio) of a steel plate thick steel plate. Another object of the present invention is to provide a method for producing a high-strength steel pipe that lowers the yield ratio (steel pipe yield ratio) of a cold-formed high-strength steel pipe.

The present invention employs the following means in order to solve the above problems and achieve the object.

(1) A steel pipe manufactured by cold forming a steel plate and then seam welding. (I) The steel plate is in mass%, C: 0.04 to 0.10%, Mn: 1. 20 to 2.50%, Nb: 0.01 to 0.10%, Ti: 0.005 to 0.03%, Si: 0.50% or less, P: 0.03% or less, S: 0.01% or less, Al: 0.10% or less, N: 0.008% or less,% C,% Si,% Mn,% Cu,% Ni,% Cr,% Mo, and% V, respectively Pcm =% C +% Si / 30 +% Mn / 20 +% Cu / 20 +% Ni / 60 +% Cr / 20 +% Mo / when the content of C, Si, Mn, Cu, Ni, Cr, Mo, V is used The weld cracking susceptibility composition Pcm calculated by 15 +% V / 10 is 0.23 or less, and the balance is made of iron and inevitable impurities. The metal structure is composed of bainite and ferrite, and (ii) the ratio t / D of the plate thickness t of the steel sheet to the outer diameter D of the steel pipe is 0.030 or less, and a round bar test piece is used. A high-strength steel pipe having a measured yield ratio in the circumferential direction of the steel pipe of 0.90 or less.

(2) In the high-strength steel pipe described in (1) above, by mass, Ni: 1.00% or less, Mo: 0.50% or less, Cr: 1.00% or less, Cu: 1.00% or less V: 0.10% or less, Ca: 0.01% or less, REM: 0.02% or less, Mg: 0.006% or less.

(3) In the high-strength steel pipe described in (1) above, a structure in which the area ratio of the ferrite is more than 10% to 30% may be adopted.

(4) The high-strength steel sheet used for the high-strength steel pipe described in (1) above may have a yield point elongation of 0.5% or more.

(5) In the method for producing a high-strength steel sheet described in (4) above, the steel slab is reheated to the austenite region, rough rolling is performed in the recrystallization region, and then unrecrystallized at an Ar ₃ point or higher and 900 ° C. or lower. Finish rolling with a cumulative reduction of 50% or more in the temperature range, air cooling, and accelerated cooling at a cooling rate of 5-50 ° C / s from a temperature in the range of Ar ₃ -50 ° C to Ar ₃ -5 ° C And a step of stopping the accelerated cooling at 400 ° C. or higher may be employed.

(6) The method for producing a high-strength steel pipe according to (1) above, wherein the steel sheet is cold-formed into a cylindrical shape, the seam portion is arc-welded, and thereafter 0.5% to 1.5% Tube expansion is performed at a tube expansion rate of less than or less, and the ratio t / D between the plate thickness t of the steel plate and the outer diameter D of the steel tube is made 0.030 or less.

According to the present invention, it is possible to provide a high-strength steel pipe having a low yield ratio in the circumferential direction. Moreover, the yield ratio of a high-strength steel pipe can be reduced rather than the yield ratio of the thick steel plate which is a raw material. Therefore, it is not necessary to consider the increase in yield ratio due to cold forming and pipe expansion when manufacturing a high-strength steel pipe, and the industrial contribution is extremely remarkable.

It is a figure which shows the relationship between the yield ratio of the steel plate before cold forming, and the yield ratio of the high strength steel pipe obtained by cold forming and seam welding. It is a figure which shows the collection method of a flat test piece. It is a figure which shows the sampling method of a round bar test piece. It is a figure which shows the relationship between the yield stress of a flat test piece, and the yield stress of a round bar test piece. It is a figure which shows the SS curve of the steel plate for steel pipes of this invention, and the SS curve of a steel pipe. It is a figure which shows the SS curve of the conventional thick steel plate for steel pipes, and the SS curve of a steel pipe.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In each drawing used for the following description, each member and element are appropriately changed in order to make each member and element recognizable.

The inventors of the present invention prototyped thick steel plates having various strengths ranging from X80 (yield stress of 555 MPa or more) to X100 (yield stress of 690 MPa or more). From the resulting thick steel plate, a plate-shaped test piece having a longitudinal direction (width direction) perpendicular to the rolling direction as a longitudinal direction was collected and subjected to a tensile test. Furthermore, these thick steel plates were cold-formed and seam welded to produce steel pipes. From the obtained steel pipe, as shown in FIG. 2B, a round bar test piece having a circumferential direction as a longitudinal direction was collected and subjected to a tensile test. The tensile test was performed according to ISO3183. Thus, it is not necessary to flatten a test piece by cutting a round bar test piece from a steel pipe. Therefore, a decrease in yield stress due to the Bauschinger effect can be prevented, and the yield stress in the circumferential direction of the steel pipe can be accurately measured.
Moreover, as a steel pipe satisfying the requirements of ISO 3183, the yield ratio in the circumferential direction of the steel pipe measured using a round bar test piece needs to be 0.90 or less from the viewpoint of product quality control.

First, the relationship between the stress-strain curve (SS curve) of the thick steel plate, which is the material of the steel pipe, the yield ratio of the thick steel plate (steel plate yield ratio), and the yield ratio of the steel pipe (steel pipe yield ratio) was investigated in detail.

As shown in FIG. 4A, it was found that when the thick steel plate has a yield point elongation of 0.5% or more, the yield stress of the steel pipe is lower than the yield stress of the steel plate. Therefore, the steel pipe yield ratio is lower than the steel plate yield ratio. Specifically, even if the steel sheet yield ratio is 0.90 or more, the steel pipe yield ratio is about 0.80 to 0.90.

On the other hand, as shown in FIG. 4B, in the case of a work hardening type (round type) in which the SS curve of the thick steel plate does not show elongation at yield point, the steel pipe yield stress is always higher than the steel plate yield stress. Therefore, the steel pipe yield ratio is higher than the steel plate yield ratio. Specifically, even if the steel sheet yield ratio is less than 0.90, the steel pipe yield ratio may be 0.98 at the maximum.

The relationship between the steel pipe yield ratio and the SS curve of the thick steel plate is considered as follows.

The inner surface of the steel pipe has a strain history due to compressive deformation due to bending and subsequent tensile deformation due to pipe expansion. Therefore, the yield stress of the steel pipe before pipe expansion is reduced by the Bauschinger effect due to compression deformation. In particular, when the thick steel plate has a yield point elongation, the amount of decrease in yield stress due to the Bauschinger effect appears more prominently. As a result, the steel pipe yield ratio after pipe expansion is estimated to be lower than the steel plate yield ratio.

On the other hand, when the SS curve of the thick steel plate is round, the amount of decrease in yield stress due to the Bauschinger effect is small. Therefore, the influence of the increase amount of the yield stress accompanying work hardening by molding and pipe expansion appears more greatly. As a result, it is considered that the steel pipe yield ratio increases more than the steel plate yield ratio.

However, even if the yield point elongation of the thick steel plate is 0.5% or more, if the ratio t / D between the thickness t of the base material of the steel pipe and the outer diameter D of the steel pipe is large, tensile deformation of the outer surface of the steel pipe will occur. The influence of the work hardening which arises becomes large. Therefore, depending on the shape of the steel pipe, it is impossible to achieve a decrease in yield ratio due to the Bauschinger effect.

The present inventors have conducted a detailed study and found that if the t / D is 0.030 or less and the yield point elongation of the steel sheet is 0.5% or more, the yield ratio of the steel pipe decreases due to the Bauschinger effect. did.

FIG. 1 is an example of the test results, the horizontal axis is the yield ratio (steel yield ratio) of the steel sheet before cold forming, and the vertical axis is the yield of the steel pipe obtained by cold forming and seam welding. Ratio (steel pipe yield ratio). The broken line in FIG. 1 indicates that the steel sheet yield ratio and the steel pipe yield ratio are equal. Therefore, the test results below the broken line in FIG. 1 indicate that the steel pipe yield ratio is lower than the steel plate yield ratio.

The t / D of the steel pipe was 0.020 to 0.030. The test result that the yield point elongation of the thick steel plate is 0.5% or more is indicated by “” in the figure.

As shown in FIG. 1, when the SS curve of the thick steel plate as a material is a round type, the steel pipe yield ratio is higher than the steel plate yield ratio. On the other hand, when the yield point elongation of the thick steel plate is 0.5% or more, the steel pipe yield ratio is lower than the steel plate yield ratio. Furthermore, even though the yield ratio of the steel sheet satisfies 0.90 or less, the yield ratio of the steel pipe becomes 0.90 or more, so ISO 3183 may not be satisfied.

Next, the metallographic structure of the thick steel plate having the yield point elongation and the hot rolling conditions for obtaining such a thick steel plate were investigated in detail. As a result, when the metal structure is a bainite single-phase structure or when the stop temperature of accelerated cooling after hot rolling is 400 ° C. or less, the SS curve of the thick steel plate is found to be a round shape. It was.

On the other hand, when the metal structure is composed of bainite and ferrite and the stop temperature of accelerated cooling is 400 ° C. or higher, it was found that the thick steel plate has a yield point elongation. In particular, when the area ratio of ferrite is more than 10% to 30%, a structure is obtained in which fine ferrite is dispersed in bainite, so that a steel pipe having a high strength and a low yield ratio is obtained.

Hereinafter, the present invention will be described in detail. % Means mass%.

C is an extremely effective element for improving the strength of steel. Therefore, in order to obtain the target strength of the steel sheet, it is necessary to add C by 0.04% or more. Further, in order to increase the ductility of the steel sheet, in particular, in order to increase the uniform elongation, the C content is preferably 0.05% or more. On the other hand, when the amount of C is too large, the amount of C is made 0.10% or less in order to deteriorate the low temperature toughness and field weldability of the base metal and the weld heat affected zone (HAZ).

Si is a deoxidizing element. However, if the amount of Si is too large, the HA amount is reduced to 0.50% or less in order to significantly deteriorate the HAZ toughness and field weldability. Since deoxidation of steel is sufficiently possible using Al or Ti, it is not always necessary to add Si. Further, since Si is an effective element for improving the strength, it is preferable to add 0.05% or more.

Mn is an indispensable element for ensuring the balance between strength and toughness by making the microstructure of the matrix of the steel sheet of the present invention a microstructure mainly composed of bainite. Therefore, the amount of Mn is made 1.20% or more. However, when the amount of Mn is too large, it becomes difficult to form dispersed ferrite, so the amount of Mn is made 2.50% or less.

Nb is added to suppress the recrystallization of austenite and to refine the structure when hot rolling the steel sheet. Nb also contributes to an increase in hardenability. Therefore, in order to strengthen steel, it is necessary to add 0.01% or more of Nb. On the other hand, when the amount of Nb is too large, the HAb toughness and on-site weldability are adversely affected, so the amount of Nb is set to 0.10% or less.

Ti is added to suppress coarsening of austenite grains by forming fine TiN during reheating or welding of a steel plate (slab). In order to improve the toughness of the base material and HAZ by refining the microstructure, it is necessary to add 0.005% or more of Ti.

However, when the amount of Ti is too large, TiN becomes coarse or precipitation hardening due to TiC occurs. Therefore, since the low temperature toughness of the steel sheet is deteriorated, the Ti amount is limited to 0.03% or less. Further, in order to fix the solid solution N as TiN, it is preferable to add Ti amount 3.4 times or more of N amount.

Al is a deoxidizing element. However, in order to suppress a decrease in toughness due to an increase in Al-based nonmetallic inclusions, the Al content is made 0.10% or less. Since deoxidation of steel is sufficiently possible using Ti or Si, it is not always necessary to add Al. Further, when Al is reduced to 0.005% or less, Ti forms an oxide. Since this oxide acts as an intragranular ferrite formation nucleus in HAZ, the structure of HAZ is refined. For this reason, the Al content is preferably 0.005% or less.

N is an impurity. In order to suppress generation of surface flaws and toughness deterioration due to the formation of nitride, the N content is set to 0.008% or less. In order to make the base material and HAZ finer by forming TiN, the N content is preferably 0.001% or more.

P and S are impurities. By reducing the amount of P, center segregation in the continuously cast slab is reduced, and grain boundary fracture is prevented, so that the toughness of the steel sheet is improved. By reducing the amount of S, MnS stretched by hot rolling is reduced, so that ductility and toughness are improved. Therefore, the P amount and the S amount are 0.03% or less and 0.01% or less, respectively. The P amount and the S amount are preferably as small as possible, but may be appropriately determined based on a balance between characteristics and cost.

In order to make the steel structure a multiphase structure composed of bainite and ferrite, the Pcm (weld cracking susceptibility composition) of the following formula (1) needs to be 0.23 or less.

Pcm =% C +% Si / 30 +% Mn / 20 +% Cu / 20 +% Ni / 60 +% Cr / 20 +% Mo / 15 +% V / 10 (1)
Here,% C,% Si,% Mn,% Cu,% Ni,% Cr,% Mo, and% V are the contents of C, Si, Mn, Cu, Ni, Cr, Mo, and V, respectively. (Mass%). When the selective elements described below, Cu, Ni, Cr, Mo, and / or V are not added intentionally, the variable corresponding to the element not added is calculated as 0.

Furthermore, one or more of Ni, Mo, Cr, Cu, V, Ca, REM, and Mg may be added. These elements are added mainly for the purpose of further improving the strength and toughness of the steel of the present invention and increasing the size of the steel material that can be produced.

Ni is an element that improves the strength. However, when there is too much addition amount of Ni, not only economical efficiency but HAZ toughness and field weldability will be degraded. Therefore, the amount of Ni is made 1.00% or less. Further, the addition of Ni is effective in preventing Cu cracking during continuous casting and hot rolling. In this case, it is preferable to add 1/3 or more of the amount of Ni.

Mo is an element that improves the hardenability of steel. Further, when coexisting with Nb, since recrystallization of austenite is suppressed during controlled rolling, it is also effective in refining the structure. Therefore, the addition of Mo is effective for increasing the strength. However, if the amount of Mo added is too large, the HAZ toughness and on-site weldability may deteriorate and a dispersed ferrite phase may not be generated. Therefore, the Mo amount is 0.50% or less.

Cr and Cu are elements that increase the strength of the base steel plate and HAZ. However, when these elements are added excessively, the HAZ toughness and on-site weldability may deteriorate. Therefore, the Cr amount and the Cu amount are both set to 1.00% or less.

V has almost the same effect as Nb. However, the effect of refining and strengthening the structure of the steel sheet by adding V is smaller than that of Nb. When V is added excessively, the HAZ toughness and field weldability deteriorate. Therefore, the V amount is set to 0.10% or less. Preferably, the V amount is 0.08% or less. On the other hand, V is an element that suppresses softening of the weld. Therefore, the V amount is preferably 0.03% or more.

Ca and REM are elements that improve toughness by controlling the form of sulfide, particularly MnS. However, when the Ca content exceeds 0.01% or the REM exceeds 0.02%, the CaO—CaS or REM—CaS becomes a large cluster or inclusion, so that the steel is cleaned. Degree and weldability on site may deteriorate.

Accordingly, the Ca content is preferably 0.01% or less, and the REM content is preferably 0.02% or less. In particular, in a high-strength steel pipe, the S content and the O content are reduced to 0.001% and 0.002% or less, respectively, and the ESSP represented by the following formula (2) is 0.5 or more and 10.0. It is preferable to satisfy the following.
ESSP = (Ca) [1-124 (O)] / 1.25S (2)

Mg is an element that contributes to improvement of toughness by suppressing the coarsening of the particle size of HAZ by forming dispersed fine oxides. On the other hand, when the amount of Mg added is excessive, a coarse oxide is generated, which may deteriorate toughness. Therefore, the Mg amount is preferably 0.006% or less.

Steel plates and steel pipes containing the elements as described above, the balance being Fe and unavoidable impurities are preferable basic compositions of steel used in the present invention.

The metal structure of the steel plate and steel pipe of the present invention is a bainite structure in which fine ferrite is dispersed. The metal structure of the steel of the present invention is preferably a multiphase structure in which bainite is the phase with the highest area ratio. Furthermore, in order to improve the strength of the steel sheet and to surely provide a yield point elongation of 0.5% or more, the area ratio of ferrite is preferably more than 10% to 30% or less.

In order to obtain a steel sheet having such a metal structure, after rough rolling, the steel sheet is processed in an unrecrystallized temperature range, the crystal grains are flattened in the thickness direction, and austenite grains stretched in the rolling direction. Next, the steel sheet is cooled at a cooling rate at which ferrite is finely formed, and then rapidly cooled to transform the remaining structure at a low temperature. The structure generated by this low-temperature transformation is generally called by a name such as bainite or bainitic ferrite. However, in the present invention, these low temperature transformed structures are collectively referred to as bainite.

Next, a method for producing a thick steel plate that is a material of the high-strength steel pipe of the present invention will be described.

The thick steel plate for high-strength steel pipe of the present invention is manufactured by the following method. Steel is melt | dissolved by a conventional method, a component composition is adjusted, and a steel piece is manufactured by continuous casting or a lump of molten steel. This steel slab is reheated and hot-rolled to produce a thick steel plate for high strength steel pipe.

The reheating temperature needs to be a temperature at which the steel structure becomes an austenite phase (austenite region), that is, a temperature of Ac ₃ points or more in the case of heating.

Ac ₃ points vary depending on the chemical composition and the heating rate. Therefore, the Ac ₃ points may be measured in advance using a sample collected from a steel piece or a sample having almost the same composition as the sample. Ac ₃ points may be measured by performing transformation expansion measurement while performing heat treatment simulating reheating of hot rolling in a laboratory.

In order to sufficiently dissolve the additive element, it is preferable to set the reheating temperature to 1050 ° C. or higher. On the other hand, if the reheating temperature exceeds 1250 ° C, the crystal grains may be coarsened, so the upper limit is preferably set to 1250 ° C or less.

鋼 First, the reheated steel slab is roughly rolled in the recrystallization temperature range. The lower limit temperature of the recrystallization temperature range is generally over 900 ° C. and varies depending on the component composition. The rolling reduction of rough rolling may be appropriately determined from the thickness of the steel slab and the thickness of the product. In order to make the crystal grain size as fine as possible by rough rolling before the non-recrystallization zone rolling, it is preferable to lower the rolling temperature and increase the rolling reduction.

After rough rolling, finish rolling is performed in an unrecrystallization temperature range of 900 ° C. or lower. The cumulative rolling reduction of finish rolling was 50% or more. By this finish rolling, the crystal grains become flat and fine, and the strength and toughness are improved.

Cumulative rolling reduction is calculated by dividing the difference between the thickness of the steel sheet before rolling in the non-recrystallization zone and the thickness of the steel plate after rolling in the non-recrystallization zone by the thickness of the steel plate before rolling in the non-recrystallization zone. It is the expressed value. Here, the temperature of finish rolling shall be ₃ points or more of Ar which is an austenite temperature range at the time of cooling.

After finish rolling, air cooling is performed to generate ferrite, followed by accelerated cooling. The stop temperature of air cooling, that is, the start temperature of accelerated cooling is set in the range of Ar ₃ -50 ° C to Ar ₃ -5 ° C. When the steel sheet is air-cooled to less than Ar ₃ -50 ° C., high strength cannot be obtained because the amount of ferrite increases. On the other hand, when accelerated cooling is performed from a temperature higher than Ar ₃ −5 ° C., ferrite is not sufficiently generated.

The Ar ₃ point varies depending on the component composition and the air cooling rate. Therefore, Ar ₃ points may be measured in advance using a sample collected from a steel piece or a sample having almost the same component as the sample. For the measurement of Ar ₃ points, transformation expansion measurement may be performed while performing a heat treatment that simulates hot rolling and air cooling in a laboratory.

After air cooling, when accelerated cooling at an average cooling rate of about 5 to 50 ° C./s at the center thickness of the steel sheet is performed from a temperature of Ar ₃ −50 ° C. to Ar ₃ −5 ° C., two ferrite and bainite layers are obtained. A phase structure is obtained. When the cooling rate is less than 5 ° C./s, granular bainite is generated at the center of the plate thickness of the steel sheet, so that the strength and toughness are lowered. On the other hand, when the cooling rate is higher than 50 ° C./s, martensite is generated, so that the strength increases and the toughness decreases.

The stop temperature for accelerated cooling of steel sheets must be 400 ° C or higher. When accelerated cooling is performed to below 400 ° C., the yield point elongation of the steel sheet does not occur. This is presumably because part of austenite remaining at a temperature of 400 ° C. or higher is transformed into martensite, and strain is introduced in the vicinity thereof.

When accelerated cooling is stopped at 400 ° C. or higher, austenite transforms into bainite or ferrite and cementite, so that the yield point elongation of the steel sheet occurs.

The cooling rate at the center of the plate thickness when cooling the steel plate may be obtained by dividing the temperature difference at the center of the plate thickness before and after cooling by the cooling time. The temperature at the center of the plate thickness before and after cooling is obtained by heat conduction calculation after measuring the temperature of the steel plate surface before and after cooling with a radiation thermometer or the like.

Moreover, if the plate thickness and cooling conditions, for example, water cooling conditions are changed, and the time change of the temperature at the center of the plate thickness of the steel plate is measured with a thermocouple, the cooling rate can be controlled according to the cooling conditions. is there.

In order to obtain the calibration value of the radiation thermometer and the parameters of the heat conduction calculation, cooling was performed under various conditions simulating actual operation, and the time change of the temperature of the surface of the steel plate and the center of the plate thickness was measured with a thermocouple. It is preferable to keep it.

The steel plate manufactured by the above method is formed into a tubular shape by cold forming into a tubular shape and seam welding the butt portion. As a forming method, a UOE method generally used for manufacturing a steel pipe can be applied. The joining method is arc welding. In order to make the steel pipe yield ratio lower than the steel plate yield ratio, the ratio t / D between the thickness t of the base steel sheet and the outer diameter D of the steel pipe needs to be 0.030 or less.

It is also preferable to expand the steel pipe in order to improve the roundness of the steel pipe. However, when processing strain is introduced into the steel pipe by pipe expansion, the yield ratio increases. Therefore, the tube expansion rate is 0.5 to less than 1.5%.

Steels having chemical components shown in Table 1 were cast and then cast. A steel plate was manufactured by cooling the obtained steel piece after hot rolling under the conditions shown in Table 2. Under any manufacturing conditions, finish rolling of the steel sheet was performed at a temperature of Ar ₃ or higher, and the steel sheet was air-cooled from the end of finish rolling to the start of accelerated cooling. From the obtained steel plate, a test piece having a rectangular section having a full thickness with the direction perpendicular to the rolling direction (width direction) as the longitudinal direction was collected and subjected to a tensile test. Further, the metal structure was observed with an optical microscope, and the area ratio of ferrite was measured from the metal structure.

Ar ₃ shown in Table 1 is a ferrite transformation start temperature obtained by measuring transformation expansion at a cooling rate of 1 ° C./s. A sample for transformation expansion measurement was prepared by collecting a cylindrical test piece from a steel piece, heating the test piece to 1100 ° C. in a laboratory, and then compressing it at 810 ° C. for 30%.

Furthermore, a steel pipe having a t / D of less than 0.03 was manufactured by cold forming the steel sheet by UOE process and arc welding the seam part. Further, as shown in Table 2, the steel pipe was expanded so that the expansion ratio was 0.8 to 1.2. From the obtained steel pipe, a round bar test piece having a circumferential direction as a longitudinal direction was collected and subjected to a tensile test. Therefore, the test piece is not flattened by a press. The results are shown in Table 3.

Manufacturing No. Examples 1 to 9 are examples in which the chemical composition of the steel sheet and the manufacturing conditions of the steel pipe are within the scope of the present invention. In the examples, the metal structure is composed of ferrite and bainite, and the yield ratio of the steel pipe is lower than the yield ratio of the steel sheet. Therefore, the steel pipe yield ratio in the circumferential direction satisfies 0.90 or less. Furthermore, production No. As shown in FIG. 3, even when the steel sheet does not satisfy 0.90 or less, the steel pipe yield ratio satisfies 0.90 or less, and thus the flexibility with respect to the steel sheet yield ratio is high.

On the other hand, manufacturing No. In Comparative Examples 10, 11, and 13, the accelerated cooling start temperature was high, and ferrite was not generated. In addition, production No. In the comparative example No. 12, since the stop temperature of accelerated cooling is low and martensite is generated, the yield point elongation of the steel sheet does not occur. Therefore, the SS curve of the steel plate is a round type, and the steel pipe yield ratio in the circumferential direction does not satisfy 0.90. In addition, production No. Since No. 13 has a small amount of C, the tensile strength of the steel pipe is low. Furthermore, when a flat test piece, which is a conventional yield strength measurement method, is used, the steel pipe yield ratio is reduced by the Bauschinger effect, so that the steel pipe yield ratio satisfies 0.90. Therefore, in the prior art which presupposed the flat test piece, the objective which lowers the yield ratio of a steel pipe rather than the yield ratio of the thick steel plate which is a raw material is not assumed. Furthermore, the prior art may not accurately determine the steel pipe yield ratio.

A high-strength steel pipe having a low yield ratio in the circumferential direction and a method for producing a high-strength steel pipe capable of lowering the yield ratio of a steel pipe than the yield ratio of a thick steel plate as a material can be provided.

1 Steel pipe 2 Test piece 3 Test piece (Round bar test piece)

Claims

A steel pipe manufactured by cold forming a steel plate and then seam welding,
(I) The steel sheet is mass%,
C: 0.04 to 0.10%,
Mn: 1.20 to 2.50%,
Nb: 0.01 to 0.10%,
Ti: 0.005 to 0.03%
Containing
Si: 0.50% or less,
P: 0.03% or less,
S: 0.01% or less,
Al: 0.10% or less,
N: limited to 0.008% or less,
When% C,% Si,% Mn,% Cu,% Ni,% Cr,% Mo and% V are the contents of C, Si, Mn, Cu, Ni, Cr, Mo and V, respectively,
Pcm =% C +% Si / 30 +% Mn / 20 +% Cu / 20 +% Ni / 60 +% Cr / 20 +% Mo / 15 +% V / 10
The weld cracking susceptibility composition Pcm calculated by the following is 0.23 or less, the balance consists of iron and inevitable impurities, the metal structure consists of bainite and ferrite, and
(Ii) The ratio t / D between the plate thickness t of the steel sheet and the outer diameter D of the steel pipe is 0.030 or less, and the yield ratio in the circumferential direction of the steel pipe measured using a round bar test piece is 0. 90 or less,
High strength steel pipe characterized by that.
% By mass
Ni: 1.00% or less,
Mo: 0.50% or less,
Cr: 1.00% or less,
Cu: 1.00% or less,
V: 0.10% or less,
Ca: 0.01% or less,
REM: 0.02% or less,
The high-strength steel pipe according to claim 1, containing one or more of Mg: 0.006% or less.
The high-strength steel pipe according to claim 1, wherein an area ratio of the ferrite is more than 10% to 30%.
It is used for manufacturing the steel pipe according to claim 1,
A steel plate for high-strength steel pipes having a yield point elongation of 0.5% or more.
It is a manufacturing method of the steel plate for high strength steel pipes according to claim 4,
The steel slab is reheated to the austenite region, roughly rolled in the recrystallization region, and then finish-rolled with a cumulative reduction ratio of 50% or more in the non-recrystallization temperature region of Ar 3 to 900 ° C, and air-cooled. , Accelerated cooling is performed at a cooling rate of 5 to 50 ° C./s from a temperature within the range of Ar 3 −50 ° C. to Ar 3 −5 ° C., and the accelerated cooling is stopped at 400 ° C. or higher. Manufacturing method of steel sheet for high strength steel pipe.
It is a manufacturing method of the high strength steel pipe according to claim 1,
The steel plate is formed into a cylindrical shape in the cold, the seam is arc welded, and then the tube is expanded at a tube expansion rate of 0.5% to less than 1.5%. The thickness t of the steel plate and the outer diameter of the steel tube A method for producing a high-strength steel pipe, wherein a ratio t / D to D is 0.030 or less.