WO2006103894A1

WO2006103894A1 - Thick seamless steel pipe for line pipe and method for production thereof

Info

Publication number: WO2006103894A1
Application number: PCT/JP2006/304613
Authority: WO
Inventors: Kunio Kondo; Yuji Arai; Nobuyuki Hisamune
Original assignee: Sumitomo Metal Industries, Ltd.
Priority date: 2005-03-29
Filing date: 2006-03-09
Publication date: 2006-10-05
Also published as: AU2006229079C1; AU2006229079B2; BRPI0608953A2; BRPI0608953B1; JP4792778B2; EP1876254A4; CA2602526C; EP1876254B1; US20080047635A1; CN101151387A; NO20074257L; EP1876254A1; NO340772B1; US20100236670A1; AU2006229079A1; BRPI0608953B8; CN100543167C; JP2006274350A; AR052706A1; CA2602526A1

Abstract

[PROBLEMS] To provide a seamless steel pipe for a line pipe which is a thick steel pipe, has high strength and is excellent in toughness; and a method for producing the seamless steel pipe. [MEANS FOR SOLVING PROBLEMS] A thick seamless steel pipe for a line pipe having high strength and good toughness, characterized in that it has a chemical composition that c: 0.03 to 0.08 %, Si: 0.25 % or less, Mn: 0.3 to 2.5 %, Al: 0.001 to 0.10 %, Cr: 0.02 to 1.0 %, Ni: 0.02 to 1.0 %, Mo: 0.02 to1.2 %, Ti: 0.004 to 0.010%, N: 0.002 to 0.008 %, the total of one or more of Ca, Mg and REM: 0.0002 to 0.005 %, V: 0 to 0.08%, Nb: 0 to 0.05 %, Cu: 0 to 1.0 %, and the balance: Fe and impurities, with the proviso that P in the impurities is 0.05 % or less and S in the impurities is 0.005 % or less. The steel pipe may further comprise 0.0003 to 0.01 % of B in addition to the above components. A method for producing the steel pipe is characterized by the cooling rate of a cast steel product, the heating conditions for piercing and the heat treatment conditions after the formation of a pipe.

Description

Specification

Thick-walled seamless steel pipe for line pipe and manufacturing method thereof

Technical field

The present invention relates to a thick-walled seamless steel pipe for line pipes excellent in strength, toughness, and weldability, and a method for producing the same. Thick-walled seamless steel pipe means a seamless steel pipe with a wall thickness of 25 mm or more. The seamless steel pipe of the present invention has a strength of X70 or higher as specified in API (American Petroleum Institute) standards, specifically X70 (yield strength 482 MPa or higher), X80 (yield strength 551 MPa or higher), X90 (yield strength). 620MPa or higher), X100 (yield strength 689MPa or higher), X120 (yield strength 827MPa or higher), combined with good toughness. Especially suitable for submarine flow lines

Background art

[0002] Oil and gas resources in oil fields located on land and in shallow water have been depleted in recent years, and development of deep-sea submarine oil fields has become active. In the deep-sea oil field, it is necessary to transport crude oil and gas from oil and gas well wells installed on the seabed to offshore platforms using flow lines and risers.

[0003] The Neuve that forms the flow line laid in the deep sea is subjected to high internal fluid pressure with deep formation pressure, and the pipe is repeatedly distorted by waves, and when the operation is stopped, Influenced by water pressure. Therefore, a thick steel pipe having high strength and high toughness is desired as a pipe used for such applications.

[0004] Conventionally, seamless steel pipes with high strength and toughness are produced by punching a billet heated to a high temperature with a piercing and rolling machine, rolling and stretching it, and then forming it into a product pipe shape, followed by heat treatment. It was manufactured. However, in recent years, from the viewpoint of energy saving and process saving, application of in-line heat treatment to simplify the manufacturing process has been studied. In particular, focusing on the effective use of the heat retained by the material after hot working, a process of quenching has been introduced without cooling it to room temperature. This method achieves significant energy savings and manufacturing process efficiencies, which can significantly reduce manufacturing costs. [0005] Steel pipes manufactured by an in-line heat treatment process, which is directly quenched after finish rolling, cannot be reheated after rolling and after cooling to room temperature, as in the past! Not going through the process. Accordingly, it is not easy to secure toughness and corrosion resistance as soon as the crystal grains become coarse. In order to deal with such problems, several techniques have been proposed for making the crystal grain of the finish-rolled steel pipe finer and for ensuring toughness and corrosion resistance even if the crystal grain is not so small.

[0006] For example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2001-240913) discloses that a crystal grain can be obtained by adjusting the time until charging of a finishing rolling force reheating furnace using a reheating furnace after finish rolling. A technique for miniaturizing the size is disclosed. Patent Document 2 (Japanese Patent Application Laid-Open No. 2000-104117) discloses a technique that has good performance even when crystal grains are relatively large by adjusting the component composition, particularly the Ti and S contents. Has been.

[0007] However, in order to manufacture high strength and thick steel pipes for deep seabed oil fields where demand has been increasing in recent years, the technology disclosed in Patent Document 1 described above is applicable. I can't do it. For example, in the case of a thick-walled steel pipe, the finish rolling temperature becomes high, and it takes a long time to reach the target reheating furnace charging temperature, and the production efficiency is greatly reduced. In addition, the above patent document

The method described in 2 is also difficult to apply to thick materials. With thick materials, the cooling rate during in-line heat treatment is small, so there is a problem that the toughness decreases even when steel having the composition disclosed in Patent Document 2 is applied.

Patent Document 1: Japanese Patent Application Laid-Open No. 2001-240913

Patent Document 2: JP 2000-104117 A

Disclosure of the invention

Problems to be solved by the invention

An object of the present invention is to solve the above-described problems, and in particular, to provide a seamless steel pipe for a line pipe having high strength and stable toughness with a thick steel pipe, and a method for producing the same. And

Means for solving the problem

[0010] 1. Basic examination and knowledge First, the factors governing the toughness of thick-walled seamless steel pipes were analyzed. As a result, the following was found.

[1] (1) Cooling conditions during and after solidification of molten steel greatly affect toughness. If the cooling rate is low, the toughness decreases, so it is necessary to cool at a certain cooling rate or higher.

[0012] (2) Furthermore, the ingot rolling process in which the steel ingot is heated to a high temperature range for hot working does not affect the toughness.

[0013] (3) The cause of the above-mentioned decrease in toughness is that the precipitation form of Ti carbonitride is affected by the cooling rate during solidification and after solidification. In order to prevent this decrease in toughness, it is important to precipitate Ti carbonitride finely.

(4) Precipitation strengthening deteriorates the balance between strength and toughness in the in-line heat treated material. Although it is disadvantageous to obtain high strength, it is desirable to obtain high toughness by using transformation strengthening and solid solution strengthening without using precipitation strengthening as much as possible.

[0015] (5) In order to obtain a uniform metal structure, it is necessary to prevent the formation of retained austenite and low-temperature transformation martensite.

[0016] (6) As an alloy composition, Si is reduced, P and S are further reduced, Nb and V are controlled so as not to exceed a certain upper limit, and appropriate amounts of Ti and Ca, Mg and RE

A composition containing an appropriate amount of one or more of M is desirable. This greatly improves the toughness of the thick-walled material.

(7) The above findings (1) to (6) are obtained on the premise of in-line heat treatment. However, even better toughness can be obtained when applied to steel pipes that are heat treated off-line. Therefore, the above knowledge can also be used when manufacturing a high-strength material by offline heat treatment.

[0018] 2. Basic test and results

In-line heat treatment does not have a crystal grain refinement process due to “transformation and reverse transformation” in off-line heat treatment, so it is necessary to refine the crystal grains themselves at the end of rolling to ensure toughness.

[0019] Normally, the solidified crystal grains are coarse. It is said that the crystal grains become finer by reheating and performing block rolling. Therefore, a laboratory experiment was conducted to investigate the optimization of the ingot rolling process for in-line heat treated materials. As a result, with in-line heat treatment material Found that there is a tendency for crystal grains to become finer and toughness to be improved if the rolling is not carried out in the first place, even if the rolling conditions are not specified. In other words, it has been found that the above-mentioned conventional common sense is not necessarily correct.

[0020] In order to understand this unexpected result, further simulation tests were conducted in laboratory experiments. First, as a process that has undergone a lump process, the inserted ingot is heated to 1250 ° C and hot-worked to produce a block, and further heated to 1250 ° C and hot-rolled and water-cooled to in-line with the drilling process. The heat treatment process was simulated.

[0021] As a process that does not go through the lump process, a block of the same size as the block made by the above hot working is cut out from the inserted ingot by machining, and the block is heated to 1250 ° C and heated. The drilling process and in-line heat treatment process were simulated by hot rolling and water cooling.

[0022] As a result of the above-described two tests, the crystal grains on which the partial rolling was not performed became overwhelmingly fine and the toughness was improved.

[0023] However, when the same prototype was made with an actual machine, it was found that the effect as expected was not obtained. Therefore, the cause of the large difference in crystal grain size was investigated in the above simulation. As a result, almost all of the added Ti precipitates as Ti carbonitride in the split rolling simulation material, and the precipitated particles are formed by Ti carbon nitride grain growth by heating and hot working during the split rolling simulation. I was convinced that the number was decreasing. When the number of precipitated particles decreases, the ability to pin the crystal growth of the parent phase decreases, and the coarsening of the crystals cannot be suppressed during the subsequent block heating for drilling simulation.

[0024] On the other hand, in a process that does not carry out the block rolling simulation, first, Ti carbonitride precipitates finely during heating for simulation of the drilling process without carbonitride precipitation in the ingot. It has been clarified that the crystal grains are remarkably reduced by carbonitride pinning the grain growth of the parent phase.

[0025] Even if the split rolling process was omitted when the prototype was manufactured on an actual machine, the cause of the force that made the crystal grains almost fine was investigated, and the cooling rate at the time of squeezing was not sufficiently high. It was found that Ti carbonitride was already deposited at the stage of penetration and that it was caused by the absence of solid solution Ti. [0026] Since Ti carbonitrides that precipitate at the time of pouring precipitate at a high temperature, they coarsen and the number of precipitates immediately decreases. Therefore, the ability to pin the parent phase crystal grains is reduced. On the other hand, if a sufficient amount of solute Ti is ensured with little precipitation of Ti carbonitride during pouring, Ti carbonitride precipitates at a low temperature when the billet is heated in the subsequent pipe making process. Precipitates and the number of precipitated particles increases. When the number of precipitated particles is large, coarsening of the parent phase crystal grains, which has a large effect of pinning the parent phase crystal grains, is suppressed. Therefore, it is very important to properly control the cooling rate in the pouring process.

[0027] In particular, when cooling after solidification is slow, Ti carbonitride precipitates in the high temperature region during cooling. This is precipitation in the austenite region where there is relatively little dislocation, so there are fewer nucleation sites. The precipitate becomes coarse and becomes a coarse dispersion state. Once coarsely precipitated, Ti carbonitride is difficult to dissolve in the solid phase, making fine dispersion impossible.

[0028] On the other hand, when the cooling rate after solidification is set to a rate at which Ti carbonitride does not precipitate, Ti carbonitride does not exist in the inserted billet, and Ti exists in a solid solution state. . Ti carbonitride precipitates at a relatively low temperature during subsequent heating for heating. In the case of precipitation during heating, because of low-temperature precipitation in a bainite structure with many dislocations, a large number of nucleation sites are dispersed and deposited. It has also been clarified that if the heating rate is too high, the precipitation becomes a high temperature region, and fine precipitation is difficult.

[0029] In order to sufficiently precipitate 1 carbonitride, it is also effective to carry out a soaking process in an appropriate temperature range during heating. When Ti carbonitrides are finely precipitated, the effect of suppressing the coarsening of crystal grains is exhibited even in the case of partial rolling that is difficult to coarsen. However, since Ti carbonitrides are slightly coarsened during batch rolling, it is better to have a large amount of solid solution Ti during solidification when not performing batch rolling.

[0030] Precipitation strengthening with V or Nb makes it easy to obtain high strength. Therefore, precipitation strengthening has been widely applied to steel materials that require high strength and weldability. However, in the thick in-line heat treated material, the above precipitation strengthening greatly reduces the toughness, so it is better not to use it as much as possible. In particular, Nb significantly reduces the toughness of the in-line heat-treated material, so when it is included, it is necessary to set an upper limit strictly. Regarding V, an upper limit of force not set to Nb is set, and an alloy design that secures strength based on transformation strengthening and solid solution strengthening is performed. It is necessary.

[0031] Furthermore, when the material is thick, the toughness, which makes it difficult to obtain a uniform metal structure in the first stage of the heat treatment, tends to decrease. Since the cooling rate decreases with thick materials, it is difficult to obtain a uniform transformation structure. That is, it transforms into martensite and bainite sequentially during cooling, but if the cooling rate is low and diffusion of C is possible to some extent, C is concentrated in untransformed austenite, and that portion has a high C content after the final transformation. It changes to martensite and bainite, and it becomes residual austenite with high C content. Therefore, it is desirable to set the cooling rate as large as possible and perform forced cooling to as low a temperature as possible.

However, in the case of a thick steel pipe, there is a limit to increasing the cooling rate. Therefore, a study was conducted to develop a technique for producing a uniform structure even at a cooling rate that can be achieved even with thick materials. As a result, it was found that the concentration of C to the second phase can be reduced by reducing the content of the element to be concentrated, that is, the C content and suppressing the Si content.

[0033] Based on the above knowledge, the basic idea of alloy design and manufacturing process was clarified as follows, and the present invention was completed. Hereinafter, “%” regarding the component content means “mass%”.

[0034] First, the C content is limited to 0.08% or less. Furthermore, the upper limit of Si is set to 0.25% or less, more preferably 0.15% or less, and still more preferably 0.10% or less. Ti should be controlled within a narrow range of 0.004-0.010% as a suitable content for Ti not to precipitate during solidification and to precipitate as fine Ti carbonitride during subsequent billet heating. Furthermore, in the case of in-line heat treatment, the Nb additive strength decreases toughness and causes variation in strength. Therefore, Nb is not added, and the upper limit as an impurity is preferably 0.005% or less. V also lowers toughness, so it is necessary to keep it at 0.08% or less even if it is not added or contained.

[0035] The other elements are adjusted from the viewpoint of Norrance between high strength and good toughness. For P and S, which have an adverse effect on toughness, an upper limit is set. Mn, Cr, Ni, Mo, and Cu must be selected and adjusted according to the target strength in consideration of toughness and weldability. Add A1 required for deoxidation. One or more of Ca, Mg and REM It is also effective to select and add to secure the pitting characteristics and improve toughness. Furthermore, in order to precipitate stable Ti carbonitride, it is necessary to control the N content in a narrow range.

Next, as a manufacturing process, it is important to first obtain a solidified steel ingot in which precipitation of Ti carbonitride is suppressed and solid solution Ti is secured. The present inventor has found that if the contents of C, Ti and N are the above, Ti carbonitride is not precipitated immediately after solidification, and coarse Ti carbonitride is precipitated when the subsequent cooling rate is low. Therefore, it is necessary to cool at a specific speed or more after solidification.

[0037] Forging is a process of forming a continuous billet into a round billet (a billet with a round cross-section), which is ideally shaped into an ingot and then into a round billet. Is also possible. However, in that case, it is important to control the cooling rate after fabrication more strictly to secure a sufficient amount of solute Ti by suppressing the precipitation of coarse TiN.

[0038] The round billet is reheated to a temperature at which hot working is possible, and is subjected to piercing, stretching, and regular rolling. If there is enough Ti in solid solution, Ti carbonitride precipitates during reheating, and the precipitation temperature is relatively low, so that much finer Ti carbonitride is formed than when precipitated during cooling after solidification. Precipitate. The finely precipitated Ti carbonitride suppresses grain boundary migration during heating and holding of billets that are large in number and prevents coarsening of the grains. When rapid heating is performed, fine precipitation at low temperatures becomes impossible, and the effect of preventing grain coarsening cannot be obtained. Therefore, if heating is performed slowly or kept in the middle temperature range, fine Ti carbonitride The precipitation of objects is promoted.

[0039] In heat treatment after pipe formation, it is necessary to secure toughness to obtain a uniform structure. To that end, it is important to use steel with a controlled chemical composition and to make the forced cooling end temperature as low as possible and to cool it down sufficiently. As a result, it is possible to prevent the formation of transformation strengthening structure and residual austenite that are partially enriched in c, and toughness is improved.

[0040] The present invention made in accordance with the basic idea described above includes the following (1) and (2) seamless pipes for line pipes and (3) production of seamless pipes for line pipes up to (6). The method is as follows.

[0041] (1) C: 0.03 to 0.08%, Si: 0.25% or less, Mn: 0.3 to 2.5%, A1: 0.001 to 0.00. 10%, Cr: 0.02 to: L 0%, Ni: 0.02 to: L 0%, Mo: 0.02 to: L 2%, Ti: 0.004 to 0.010%, N: 0 002 to 0.008%, and a total of one or more of Ca, Mg and REM 0.0002 to 0.005%, V: 0 to 0.08%, Nb: 0 to 0.05%, Cu: 0 ~ l. 0%, the balance is Fe and impurity power, P in impurities is 0.05% or less and S is 0.005% or less A thick-walled seamless steel pipe for line pipes with high strength and good toughness.

[0042] (2) A thick-walled seamless steel pipe for line pipes having high strength and good toughness, containing 0.0003 to 0.01% B in place of a part of Fe, the force of the above components.

[0043] (3) A method for producing a thick-walled seamless steel pipe for a line pipe having high strength and good toughness characterized by the following steps (a) and (e).

[0044] (a) A step of solidifying molten steel having the chemical composition described in (1) or (2) above into a billet having a round cross section by continuous forging.

[0045] (b) A step of cooling the billet to room temperature with an average cooling rate between 1400 ° C and 1000 ° C being 6 ° CZ min or more.

[C] (c) 1150 to 1280 with an average heating rate between 550 ° C and 900 ° C of 15 ° CZ or less

The process of producing seamless steel pipes by piercing and rolling after heating to ° C.

[0047] (d) Immediately after pipe making, at 850 to 1000 ° C, or after cooling and after cooling, continue heating to 850 to 1000 ° C, or immediately after pipe making, 800 ° A process in which the average cooling rate between C and 500 ° C is forced to cool continuously to 100 ° C or less with an average cooling rate of 8 ° CZ seconds or more.

[0048] (e) A step of tempering at a temperature in the range of 500 to 690 ° C.

[0049] (4) The following (a) force is also provided (a method for producing a thick-walled seamless steel pipe for line pipes having high strength and good toughness characterized by the steps up to D.

[0050] (a) A step of solidifying molten steel having the chemical composition described in (1) or (2) above into a bloom or slab having a square cross section by continuous forging.

[0051] (b) Average cooling rate between 1400 ° C and 1000 ° C for the above bloom or slab.

The process of cooling to room temperature over 8 ° CZ minutes.

[0052] (c) After heating to 1150 to 1280 ° C with an average heating rate from 550 ° C to 900 ° C of 15 ° CZ or less, billets with a round cross section were produced by forging or Z and rolling. , Room The process of cooling to temperature.

[0053] (d) A step in which the billet is heated to 1150 to 1280 ° C, and a seamless steel pipe is produced by drilling and rolling.

[0054] (e) Immediately after pipe-making, after soaking at 850 to 1000 ° C, or after cooling once after pipe making, and subsequently heated to 850 to 1000 ° C, or immediately after pipe making A process in which the average cooling rate between 800 ° C and 500 ° C is continuously forcibly cooled to 100 ° C or less with an average cooling rate of 8 ° CZ seconds or more.

[0055] (1) A step of tempering at a temperature in the range of 500 to 690 ° C.

[0056] (5) A method for producing a high-strength, thick tough seamless steel pipe for line pipe characterized by the following steps (a) and (e):

[0057] (a) A step of solidifying molten steel having the chemical composition described in (1) or (2) above into a billet having a round cross section by continuous forging.

[0058] (b) A step of cooling the billet to room temperature with an average cooling rate between 1400 ° C and 1000 ° C being 6 ° CZ min or more.

[0059] (c) A step of producing a seamless steel pipe by drilling and rolling after soaking in a temperature range from 550 ° C to 1000 ° C for 15 minutes or more and heating to 1150 to 1280 ° C.

[0060] (d) Immediately after pipe making, at 850 to 1000 ° C, or after cooling and after cooling, continue heating to 850 to 1000 ° C, or immediately after pipe making, 800 ° A process in which the average cooling rate between C and 500 ° C is forced to cool continuously to 100 ° C or less with an average cooling rate of 8 ° CZ seconds or more.

[0061] (e) A step of tempering at a temperature in the range of 500 to 690 ° C.

[0062] (6) The following (a) force is also provided (a method for producing a thick-walled seamless steel pipe for line pipes characterized by the steps up to D and having high strength and good toughness.

[0063] (a) A step of solidifying molten steel having the chemical composition described in (1) or (2) above into a bloom or slab having a square cross section by continuous forging.

[0064] (b) Average cooling rate between 1400 ° C and 1000 ° C for the above bloom or slab.

The process of cooling to room temperature over 8 ° CZ minutes.

[0065] (c) Soaking for 15 minutes or more in the temperature range from 550 ° C to 1000 ° C, 1150-1280

A process in which billets with a round cross section are produced by forging or Z and rolling after heating to ° C, and then cooled to room temperature. [0066] (d) A step of heating the billet to 1150 to 1280 ° C and producing a seamless steel pipe by drilling and rolling.

[0067] (e) Immediately after pipe making, after soaking to 850-1000 ° C, or after cooling once after pipe making, continue to heat to 850-1000 ° C, or immediately after pipe making A process in which the average cooling rate between 800 ° C and 500 ° C is continuously forcibly cooled to 100 ° C or less with an average cooling rate of 8 ° CZ seconds or more.

[0068] (1) A step of tempering at a temperature in the range of 500 to 690 ° C.

BEST MODE FOR CARRYING OUT THE INVENTION

[0069] 1. Chemical composition of steel pipe of the present invention

First, the reason why the chemical composition of the steel pipe is limited as described above in the present invention will be described below. In addition, as above-mentioned,% showing chemical component content (concentration) means the mass%.

[0070] C: 0.03 to 0.08%

C is an important element for ensuring the strength of steel. 0.03% or more is required to increase the hardenability and obtain sufficient strength with thick materials. On the other hand, if it exceeds 0.08%, the toughness S decreases, so it was set to 0.03-0.08%.

[0071] Si: 0.25% or less

Si has an effect as a deoxidizer in steelmaking, but it is better not to add it as much as possible. The reason is that the toughness of the thick-walled material is greatly reduced. When the Si content exceeds 0.25%, the toughness of the thick-walled material is remarkably lowered. Therefore, when it is added as a deoxidizer, the content should be 0.25% or less. If the content is less than 15%, the toughness can be further improved. The most desirable is to keep it below 0.10%. Extremely good toughness can be obtained by limiting Si, which is an impurity, to an extremely low force of less than 0.05%, which is difficult in the steelmaking process.

[0072] Mn: 0.3 to 2.5%

Mn needs to be contained in a relatively large amount in order to enhance hardenability and strengthen even thick-walled materials to the center, while at the same time increasing toughness. If the content is less than 0.3%, these effects cannot be obtained. If the content exceeds 2.5%, the HIC resistance decreases, so the content should be 0.3 to 2.5%.

[0073] A1: 0.001 ~ 0.10%

A1 is added as a deoxidizer in steelmaking. In order to obtain this effect, its content is It is necessary to add so that it may become 0.001% or more. On the other hand, if the Al content exceeds 0.10%, inclusions will form clusters and deteriorate toughness, and surface defects will occur frequently during the beveled surface of the pipe. Therefore, A1 is set to 0.001-0.10%. Regarding the viewpoint power for preventing surface defects, it is desirable to limit the upper limit, and the preferable upper limit is 0.03%, and the more preferable upper limit is 0.02%. Since the steel pipe of the present invention cannot be expected to have a large deoxidation effect due to Si-added iron, the lower limit of the preferable A1 content is 0.001% for sufficient deoxidation.

[0074] Cr: 0.02 ~: L 0%

Cr is an element that improves the hardenability and improves the strength of the steel with a thick material. The effect becomes remarkable when the content is 0.02% or more. However, if its content is excessive, the toughness will be reduced, so it was made 1.0% or less.

[0075] Ni: 0.02: L. 0%

Ni is an element that improves the hardenability and improves the strength of the steel with a thick material. The effect becomes remarkable when the content is 0.02% or more. However, Ni is an expensive element, and even if contained excessively, the effect is saturated, so the upper limit was set to 1.0%.

[0076] Mo: 0.02 ~: L 2%

Mo is an element that improves the strength of steel by transformation strengthening and solid solution strengthening. The effect becomes remarkable when the content is 0.02% or more. However, if added in excess, the toughness decreases, so the upper limit was made 1.2%.

[0077] Ti: 0.004-0.001%

The Ti content does not precipitate during cooling during solidification, but is controlled to a narrow range of 0.004% to 0.001% as a content suitable for depositing Ti carbonitride during subsequent billet heating. There is a need to. When the content is less than 0.004%, the number of Ti carbonitrides to precipitate cannot be secured, and when it exceeds 0.000%, it precipitates coarsely during cooling after solidification.従Tsu Te, the content of Ti ί or, from 0.004 to 0.010 is ^_0/0 force proper.

[0078] Trap: 0.002-0.008%

In order to secure finely dispersed Ti carbonitrides, soot must contain 0.002% or more. On the other hand, if it exceeds 0.008%, coarse Ti carbonitride will precipitate during solidification. Therefore, it is necessary to control in the narrow range of 0.002 to 0.008 ⁰ / o.

[0079] V: 0 ~ 0.08%

V is an element that determines the content based on a balance between strength and toughness. When sufficient strength is obtained with other alloy elements, the toughness is better when the additive is not added. When added as a strength-enhancing element, the content is preferably 0.02% or more. On the other hand, if it exceeds 0.08%, the toughness is greatly reduced, so when it is added, the upper limit of the content is set to 0.08%.

[0080] Nb: 0 ~ 0.05%

In the case of off-line heat treatment, Nb has a remarkable effect of suppressing grain coarsening during heating for quenching. In order to obtain the effect, the content is preferably 0.005% or more. However, if the Nb content exceeds 0.05%, coarse carbonitrides precipitate and the toughness decreases, so the upper limit was made 0.05%.

[0081] In the case of in-line heat treatment, Nb carbonitride precipitates non-uniformly, lowering the toughness and increasing the strength variation. Therefore, it is basically better not to add Nb. The strength knockout becomes noticeable and the manufacturing problem is when the content exceeds 0.005%. Therefore, when applying in-line heat treatment, the allowable upper limit should be 0.005%. It is.

[0082] Cu: 0 to l. 0%

Cu does not need to be added, but has the effect of improving the HIC resistance (hydrogen-induced cracking resistance), so it may be added to improve the HIC resistance. The minimum content at which the effect of improving HIC resistance is manifested is 0.02%. On the other hand, even if it exceeds 1.0%, the effect is saturated, so when added, the content is preferably 0.02 to: L 0%.

[0083] Ca, Mg, REM: One kind, or a total of two or more kinds 0.00002 to 0.005%

These elements are added for the purpose of improving toughness and corrosion resistance by controlling the form of inclusions, and for the purpose of improving clogging characteristics by suppressing nozzle clogging during clogging. In order to obtain these effects, it is necessary to contain 0.0002% or more of one kind or a total of two or more kinds. On the other hand, if the force exceeds 0.005% for one type or exceeds 0.005% for a total content of two or more types, the above effect will be saturated, and if no further effect is exhibited, The inclusions are easily clustered, and the toughness and HIC resistance are reduced. Therefore, the original When adding element alone, the content is 0.0002 to 0.005% in all cases, and when adding two or more elements, the total content is 0.0002 to 0.005%. REM is a lanthanoid element, 17 elements of Y and Sc.

[0084] B: 0.0003-0.01%

B does not need to be added, but if added, it improves the hardenability even in a trace amount, so it is effective to add it when higher strength is required. In order to obtain the above effect, the content is desirably 0.003% or more. However, excessive addition reduces toughness, so when B is added, its content should be 0.01% or less.

[0085] The steel pipe for a line pipe of the present invention is composed of Fe and impurities in the balance in addition to the above components. However, the upper limit of the content of P and S in impurities must be kept as follows.

[0086] P: 0.05% or less

P is an impurity element that lowers toughness, and its content is preferably as low as possible. If the content exceeds 0.05%, the toughness deteriorates remarkably, so the upper limit is made 0.05%. 0.02% or less is preferable. 0.01% or less is more preferable.

[0087] S: 0.005% or less

S is also an impurity element that lowers toughness, and is preferably as small as possible. If the content exceeds 0.005%, the toughness is remarkably lowered, so the upper limit is made 0.005%. 0.003% or less is preferable, and 0.001% or less is more preferable.

[0088] 2. Manufacturing method

Next, preferred production conditions for the production method of the present invention will be described.

[0089] (1) Cooling after fabrication and solidification

First, the steel is smelted in a converter or the like so as to have the above composition, forged and solidified to obtain a flake. At this time, it is important to obtain a solidified steel ingot that suppresses the precipitation of 1 carbonitride. If the C, Ti, and N contents specified above are used, Ti carbonitrides are not basically analyzed during solidification. However, if the subsequent cooling rate is low, coarse Ti carbonitride precipitates, so it is necessary to cool at a specific rate or higher.

[0090] As a manufacturing process, it is ideal to continuously produce a round billet shape. However, a process that slabs into a square-shaped bowl as a continuous ingot and then into round billets. You can also take In that case, it is important to control the cooling rate after fabrication more strictly to suppress coarse TiN precipitation.

[0091] The cooling rate is an average cooling rate in the temperature range of 1400 to 1000 ° C where Ti carbonitride is likely to form after solidification, and when cooling into a round billet, a cooling rate of 6 ° CZ or more is When bulk rolling is performed, a cooling rate of 8 ° CZ or more is required. More preferably, an average cooling rate of 8 ° CZ or more is used when rolling into a round billet, and an average cooling rate of 10 ° CZ or more is used when performing block rolling. In either case, the higher the average cooling rate, the better, so there is no restriction on the upper limit.

[0092] The cooling rate of the piece varies depending on the part of the piece, but in the case of continuous production in a circular bowl shape, the cooling rate is controlled by the cooling rate at a place away from the center by a distance of 1Z2 of the radius. To speak. In the case of continuous forging in a rectangular shape, the cooling rate is controlled at a location between the center of gravity and the surface on a line passing through the center of gravity of the rectangle and parallel to the long side. Temperature measurements can also be made with a numerical simulation corrected with a temperature history of the force surface that can be attached with a thermocouple.

[0093] (2) Processing of billet or lump

Round billets are reheated to a temperature where hot working is possible and drilled, stretched, and shaped. In addition, when forging a bloom or slab with a square cross section, after reheating, round billets are formed by forging or Z and rolling, and drilling, stretching, and regular rolling are performed.

[0094] When the reheating temperature is less than 1150 ° C, the hot deformation resistance increases and the generation of soot increases, so 1150 ° C or more is necessary. On the other hand, if the temperature exceeds 1280 ° C, the heating fuel intensity will become too large, the scale loss will increase and the yield will decrease, and the heating furnace life will be shortened and it will become uneconomical. The temperature was 1280 ° C. The lower the heating temperature, the finer the crystal grains and the better the toughness. Therefore, the preferred heating temperature is 1200 ° C or lower.

[0095] When sufficient solid solution Ti exists, Ti carbonitride precipitates during reheating. However, unlike precipitation during cooling after solidification, the precipitation temperature is relatively low. Therefore, much finer Ti carbonitride precipitates than when it precipitates during cooling after solidification. Finely precipitated Ti carbonitrides move grain boundaries during heating and holding of large numbers of billets. Suppresses and prevents grain coarsening. However, if rapid heating is performed, fine precipitation at low temperatures becomes impossible, and the effect of preventing crystal grain coarsening cannot be obtained. In order to promote fine precipitation at low temperature, the average heating rate between 550 ° C and 900 ° C should be 15 ° CZ or less during reheating, or between 550 ° C and 1000 ° C. It is effective to perform soaking for 15 minutes or more.

[0096] The piercing, drawing, and regular rolling should be carried out under the usual conditions for producing seamless steel pipes.

[0097] 3. Heat treatment after pipe making

In heat treatment after pipe making, it is necessary to obtain a uniform structure to ensure toughness. The quenching process is basically an in-line heat treatment in which quenching is performed to room temperature after hot rolling and without cooling, but once cooled, re-heating and quenching further refines the crystal grains. Toughness is improved. As an in-line heat treatment method, steel pipes with small strength variation can be obtained by performing quenching after hot working and soaking in a soaking furnace.

[0098] The higher the cooling rate during quenching, the easier it is to obtain high strength and high toughness with a thick material, and the closer to the theoretical limit cooling rate, the higher the strength and high toughness. The required cooling rate is an average cooling rate of 800 ° C to 500 ° C and 8 ° CZ seconds or more. More preferred is 10 ° CZ seconds or more, and most preferred is 15 ° CZ seconds or more.

[0099] For securing excellent toughness, the cooling end temperature is also important in addition to the cooling rate.

It is important to cool down the forced cooling end temperature to as low a temperature as possible below 100 ° C by using steel with adjusted chemical composition. Preferably, forced cooling is continuously performed up to 80 ° C or less, more preferably 50 ° C or less, and most preferably 30 ° C or less. As a result, it is possible to prevent the formation of retained austenite if the transformation strengthened yarn and weave are partially concentrated in C, and the toughness is greatly improved.

[0100] After quenching, tempering is performed at a temperature within a range of 500 ° C to 700 ° C. The purpose of tempering is to adjust strength and improve toughness. The holding time at the tempering temperature should be appropriately determined according to the thickness of the steel pipe, etc. Usually, it should be set to about 10 to 120 minutes.

Example

[0101] Steels having the chemical composition shown in Table 1 were melted in a converter, and round billets were produced by a method in which the steel was poured into a continuous forging mold having a round cross section, and into a square mold. After And a method of manufacturing a round billet by split rolling. Tables 2 and 3 show the manufacturing conditions when inserted into a round continuous mold. The solidification process is denoted as “RCC”. The process of pouring into a square mold is denoted as “BLCC” and the manufacturing conditions are shown in Table 4 and Table 5.

[0102] A round billet was heated under the tube-forming heating conditions shown in Tables 2 to 5, and a hollow shell was obtained using an inclined roll punch. The hollow shell was finish-rolled using a mandrel mill and a sizer to obtain a steel pipe with a thickness of 30 to 50 mm. Then, it cooled on the hardening conditions of Table 2-Table 5. That is, a method of cooling immediately after pipe making, a method of immediately cooling after pipe making and immediately soaking in a reheating furnace and soaking, and cooling to room temperature and then re-heating to cool. And implemented the method. Thereafter, tempering was performed under the conditions described in Table 2 to Table 5 to obtain a product.

[0103] As a tensile test, a JIS No. 12 tensile test piece was sampled from the obtained steel pipe, and the tensile strength (TS) and yield strength (YS) were measured. Tensile test WIS

Performed according to Z 2241. The impact test piece was tested in accordance with JIS Z 2202 No. 4 test piece by collecting a 10 mm X 10 mm, 2 mm V notch test piece in the longitudinal direction at the center of the wall thickness.

[0104] In trial number 1 of Table 2, two examples with branch numbers 1 and 2 are described. 1-1 and 1-2 use invention steel A, and the manufacturing conditions of 11 are within the range specified in the invention, and good toughness is obtained. On the other hand, sample No. 12 does not have good toughness because the heating rate for pipe making is too high and deviates from the manufacturing process defined in the present invention. Below, there are branch numbers 1 and 2 for trial numbers 2 to 24, and the same steel grade is used for the same trial number. The production conditions with branch number 1 are within the range specified in the invention, and good toughness is obtained. On the other hand, branch number 2 deviated from the manufacturing process defined in the present invention, and good toughness was not obtained.

[0105] Similarly, in Table 4 and Table 5, the same steel type is used in one trial number, and branch number 1 is a manufacturing process within the range specified in the present invention, and good toughness is obtained. Yes. On the other hand, branch No. 2 does not have good toughness because it deviates from the manufacturing process defined in the present invention. [0106] Test numbers 25 to 30 are examples of steels (comparative steels) that deviated from the alloy composition range defined in the present invention. Both of them are insufficient in performance as line pipes that require high toughness with a thick wall that does not have sufficient toughness.

[0107] [Table 1]

[] 0108

table

Chemical composition [mass%, Fe: baIJ

^] [] [30109

Table 2

[]

Table 3

]

Table 4

Table 5

Industrial applicability

According to the present invention, by specifying the chemical composition of seamless steel pipe and its manufacturing method, the yield stress is X70 class (yield strength 482 MPa or more), X80 class (yield strength 551 MPa or more), especially for thick steel pipes. , X90 class (yield strength 620MPa or higher), X100 class (yield strength 689MPa or higher), X120 class (yield strength 827MPa or higher), high strength and toughness can be produced for line pipe seamless steel pipe Become. The seamless steel pipe of the present invention is a steel pipe that can be laid in more severe deep seas, particularly for submarine flow lines. Therefore, the present invention greatly contributes to the stable supply of energy.

Claims

The scope of the claims

[1] in a weight ^{_{0/0, C: 0.03~0.08%}} , Si: 0.25% or less, Mn: 0.3 ~ 2.5%, A1: 0.00 1~0.10%, Cr: 0.02~: L 0%, Ni: 0.02~ : L 0%, Mo: 0.02 to: L 2%, Ti: 0.004 to 0.010%, N: 0.002 to 0.008%, and a total of one or more of Ca, Mg and REM 0.0002 to 0.005%, V: 0 to 0.08%, Nb: 0 to 0.05%, Cu: 0 to 1.0%, balance is Fe and impurities, P in impurities is 0.05% or less, S is 0.005 A thick-walled seamless steel pipe for line pipes with high strength and good toughness.

[2] Mass ^{_{0/0, C: 0.03~0.08%}} , Si: 0.25% or less, Mn: 0.3~2.5%, A1: 0.00 1~0.10%, Cr: 0.02~: L 0%, Ni: 0.02~ : L 0%, Mo: 0.02 to: L 2%, Ti: 0.04 to 0.010%, N: 0.002 to 0.008%, B: 0.0003 to 0.01%, and one or two of Ca, Mg and REM Contains 0.0002 to 0.005% in total for species and more, V: 0 to 0.08%, Nb: 0 to 0.05%, Cu: 0 to 1.0%, the balance is Fe and impurities, and P in impurities is 0.05 A thick-walled seamless steel pipe for line pipes with high strength and good toughness, characterized by% or less and S of 0.005% or less.

[3] A method of manufacturing a thick-walled seamless steel pipe for high-strength, high-toughness line pipes characterized by the following steps (a) and (e).

(a) A step of solidifying the molten steel having the chemical composition according to claim 1 or 2 into a billet having a round cross section by continuous forging.

(b) A step of cooling the billet to room temperature with an average cooling rate between 1400 ° C and 1000 ° C being 6 ° CZ min or more.

(c) A step of producing a seamless steel pipe by piercing and rolling after heating to 1150-1280 ° C with an average heating rate between 550 ° C and 900 ° C of 15 ° CZ or less.

(d) Immediately after pipe making, soak at 850 to 1000 ° C, or cool down after pipe making and continue heating to 850 to 1000 ° C, or immediately after pipe making, from 800 ° C to 500 The process of forced cooling continuously to 100 ° C or lower with an average cooling rate between 8 ° C and 8 ° CZ seconds.

(e) A step of tempering at a temperature in the range of 500 to 690 ° C.

[4] The following (a) force is also available (thickness for line pipe with high strength and good toughness characterized by the process up to D Manufacturing method for seamless steel pipes.

(a) A step of solidifying molten steel having the chemical composition according to claim 1 or 2 into a bloom or slab having a square cross section by continuous forging.

(b) A step of cooling the bloom or slab to room temperature at an average cooling rate between 1400 ° C and 1000 ° C of 8 ° CZ or more.

(c) After heating to 1150-1280 ° C with an average heating rate from 550 ° C to 900 ° C of 15 ° CZ or less, billets with a round cross-section were produced by forging or Z and rolling, and the room temperature The process of cooling to.

(d) A process in which the above billet is heated to 1150 to 1280 ° C, and a seamless steel pipe is produced by drilling and rolling.

(e) Immediately after pipe making at 850 to 1000 ° C, or after cooling down after pipe making, and after heating to 850 to 1000 ° C, or immediately after pipe making, 800 ° C A process of forced cooling continuously to 100 ° C or lower with an average cooling rate from 1 to 500 ° C of 8 ° CZ seconds or longer.

(1) A step of tempering at a temperature in the range of 500 to 690 ° C.

[5] A method for producing a thick-walled seamless steel pipe for line pipes, characterized by the following steps (a) and (e):

(c) 550. C force is also 1000. A process of producing seamless steel pipes by drilling and rolling after soaking for 15 minutes or more in the temperature range to C and heating to 1150-1280 ° C.

(e) A step of tempering at a temperature in the range of 500 to 690 ° C.

[6] The following (a) force is also provided (a method of manufacturing a high-strength, high-toughness thick-walled seamless steel pipe characterized by processes up to D). (a) A step of solidifying molten steel having the chemical composition according to claim 1 or 2 into a bloom or slab having a square cross section by continuous forging.

(c) 550. C force is also 1000. A step of soaking in the temperature range up to C for 15 minutes or more, heating to 1150 to 1280 ° C, producing a billet with a round cross section by forging or Z and rolling, and cooling to room temperature.

(e) Immediately after pipe making, soak to 850-1000 ° C, or cool down after pipe making and continue heating to 850-1000 ° C, or immediately after pipe making, 800 ° C A process of forced cooling continuously to 100 ° C or lower with an average cooling rate from 1 to 500 ° C of 8 ° CZ seconds or longer.

(1) A step of tempering at a temperature in the range of 500 to 690 ° C.