JP4193757B2 - Steel sheet for ultra-high-strength line pipe, manufacturing method thereof and welded steel pipe - Google Patents

Description

  The present invention relates to a steel sheet for ultra-high-strength line pipe excellent in unstable ductile fracture resistance having a tensile strength of 750 MPa or more, a manufacturing method thereof, and a welded steel pipe manufactured by molding and welding using the steel sheet as a raw material, These steel pipes and steel plates are particularly suitable as steel pipes for high-pressure line pipes having a thickness of 10 to 40 mm for transporting crude oil and natural gas, or materials thereof. In the present invention, the ultra-high strength steel plate refers to a steel plate of 750 MPa or more with respect to a conventional steel plate of about 600 MPa.

  Line pipes that transport crude oil and natural gas are more efficient to transport crude oil and gas at higher pressures, so there is a tendency for higher strength steel materials to be used to withstand high pressures. Ultra-high strength steel has been proposed.

For example, Patent Document 1 discloses a high-strength steel pipe suitable for an American Petroleum Institute standard API X100 grade (tensile strength of 760 MPa or more) line pipe excellent in weld toughness.
Patent Document 2 discloses a steel material for line pipe having a tensile strength exceeding 950 MPa and excellent in low-temperature toughness.

  In the inventions disclosed in these documents, the structure is optimized by controlling the chemical composition, rolling, and water-cooling conditions, and the toughness of the base material and the welded portion of high strength steels of APIX100 grade and X100 grade is improved.

  In line pipes that transport high-pressure gas, not only material characteristics such as strength and toughness required as characteristics of ordinary structural steel, but also material characteristics related to fracture resistance unique to gas line pipes, so-called unstable ductile fracture resistance It is necessary to ensure sex.

  Fracture toughness values in ordinary structural steels show resistance to brittle fracture and are used to design so that brittle fracture does not occur in the environment of use. On the other hand, in high-pressure gas line pipes, it is not sufficient to suppress brittle fracture for avoiding large-scale fracture, and it is also necessary to suppress ductile fracture called unstable ductile fracture.

  This unstable ductile fracture is a phenomenon in which ductile fracture propagates in the direction of the pipe axis at a speed of 100 m / s or more in a high-pressure gas line pipe, which may cause a large-scale fracture of several kilometers. Therefore, for ultra-high strength steel pipes, as a measure to suppress unstable ductile fracture, the characteristic value of Charpy impact test (absorbed energy) required for unstable ductile fracture suppression obtained based on many actual pipe gas burst test results. Value, fracture surface transition temperature) and DWTT test characteristic value (85% fracture surface transition temperature) are specified, and by these measures, the suppression of unstable ductile fracture of ultra-high-strength steel pipes is maintained. There wasn't.

  In conventional ultra-high strength steels of X100 class and X100 superclass as shown in Patent Documents 1 and 2, the ductility and brittle fracture surface transition temperature characteristics required by the strength and Charpy test are sufficiently shown. However, the resistance characteristic against the unstable ductile fracture has not been sufficiently considered.

  The steel sheets for ultra-high-strength steel pipes shown in Patent Documents 1 and 2 are steel sheets that have been accelerated and cooled after hot rolling. In general, in the production of steel plates for line pipes, in order to produce large quantities and as cheaply as possible, offline processes such as normalizing and quenching-tempering processes (processes in batches outside the rolling line) are mostly used. Instead, the product is a steel plate as rolled (after rolling, air cooling) or accelerated cooled (after rolling, subjected to accelerated cooling treatment and then air cooled).

  In conventional steel pipe manufacturing processes up to API X70 grade, accelerated cooling has been applied to increase the strength and thickness, but there has been no particular decrease in unstable ductile fracture resistance. However, in some cases, X100 grade or X100 grade super-strength steel pipes made of a steel plate that has been accelerated and cooled cannot be sufficiently secured with unstable ductile fracture resistance. Therefore, it is necessary to consider the use of a crack arrester or the like in its use, and there is a problem that the use of such a crack arrester causes an increase in pipeline construction cost.

Moreover, in the steel plate used for the conventional steel pipe for ultra-high-strength line pipes, a steel plate becomes hard and it becomes difficult to bend into a steel pipe, and especially when the surface is hardened, bending work becomes difficult. Specifically, the probability of causing cracks during processing increases. Slow bending to prevent cracking will reduce machining efficiency. In addition, there is a problem that the accuracy of the bending process is deteriorated, the groove accuracy of the seam welding cannot be secured, and the efficiency of the pipe making welding is lowered.
JP 2000-313935 A Japanese Patent Laid-Open No. 11-041074

  The present invention has been made in view of the above-mentioned problems, and the object thereof is to provide a steel sheet and a steel pipe for an ultrahigh strength line pipe having an excellent tensile ductility of 760 MPa or more, and a method for producing the steel sheet. There is.

In order to solve the above problems, the present inventors have conducted experiments on the chemical composition and manufacturing method of steel, and as a result, have obtained the following knowledge.
1) Ultra-high-strength steel sheets of 760 MPa or higher are used with accelerated cooling treatment as the cause of lack of unstable ductile fracture resistance, even if Charpy absorbed energy values and DWTT ductile fracture surface ratio are good. This is because the structure of the surface layer portion of the steel sheet is not appropriate and the spread of the ductile region near the crack is limited.

  2) In the conventional Charpy test, the effect of the steel sheet surface layer cannot be seen. Also in the DWTT test, the ratio of the surface area of the steel sheet to the fracture surface is small. The adverse effects on ductile fracture resistance were underestimated.

3) Therefore, it is necessary to evaluate using the absorbed energy of DWTT, and the unstable ductile fracture resistance can be improved by securing the absorbed energy of DWTT of 300 J / cm ² or more.

  4) To improve bendability in the pipe making process without reducing high strength, to improve ductility characteristics to prevent work cracking, and to improve unstable ductile fracture resistance After the accelerated cooling in the TMCP process, a special heat treatment is performed to temper only the surface layer portions on the front and back surfaces of the steel sheet, thereby improving the structure of the surface layer portion and reducing the hardness.

5) Unstable ductile fracture resistance is improved by making the average prior austenite grain size a fine grain structure of 18 μm or less.
The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) By mass%, C: 0.02 to 0.1%, Si: 0.5% or less, Mn: 1 to 2.5%, P: 0.01% or less, S: 0.005% or less Mo: 0.1-0.8%, Nb: 0.005-0.06%, Ti: 0.004-0.015%, sol. Al: 0.05% or less, N: 0.001 to 0.005% included, the balance has a chemical composition consisting of Fe and impurities, and the Pcm1 value represented by the following formula (1) is 0.16 to A steel sheet having an average prior austenite grain size in the center of the plate thickness of 18 μm or less and a bainite, martensite, or mixed structure of both, and a tempered martensite structure Is a steel sheet for an ultra-high-strength linepipe having a tensile strength of 750 MPa or more, characterized in that the structure containing 80% or more occupies at least 5% or more and 30% or less of the sheet thickness ratio in the steel sheet front and back surfaces . .

Pcm1 ＝ C ＋ Si / 30 + Mn / 20 + Ni / 60 + Mo / 15 (1)
(2) By mass%, C: 0.02 to 0.1%, Si: 0.5% or less, Mn: 1 to 2.5%, P: 0.01% or less, P: 0.01% or less , S: 0.005% or less, Mo: 0.1-0.8%, Nb: 0.005-0.06%, Ti: 0.004-0.015%, sol. Al: 0.05% or less, N: 0.001-0.005% included, Ni: 0.1-2.5%, Cu: 1.5% or less, Cr: 0.1-1%, V: 0.005 to 0.1%, B: contain one or more of 0.0003 to 0.002%, the balance has a chemical composition consisting of Fe and impurities, A steel sheet having a Pcm2 value in the range of 0.16 to 0.30%, an average prior austenite grain size at the center of the sheet thickness of 18 μm or less, and a bainite, martensite, or mixed structure of both. And the structure containing 80% or more of the tempered martensite structure occupies at least 5% or more and 30% or less of the sheet thickness ratio in total on the front and back surfaces of the steel sheet, and the tensile strength is 750 MPa or more It is a steel plate for ultra high strength line pipe.

Pcm2 = C + Si / 30 + Mn / 20 + Ni / 60 + Mo / 15 + Cu / 20 + Cr / 20 + V / 10 + 5B (2)
(3) In place of a part of Fe, the mass as described in (1) or (2) above containing one or two of Zr: 0.03% or less and Ca: 0.003% or less Steel plate for high-strength line pipe.

(4) It is a manufacturing method of the steel plate for ultra-high-strength line pipes in any one of said (1)-(3) , Comprising: The steel piece which has a chemical composition as described in said (1)-(3) , After heating to a temperature range of 950 to 1200 ° C. and hot rolling, an accelerated cooling treatment is performed from a temperature in the range of 850 to 650 ° C. to a temperature of at least 450 ° C., and then the tempered martensite structure is 80 % At a rate of temperature increase of 3 ° C./s or more so that only the surface layer portion of the steel plate is 400 to 700 ° C. The manufacturing method of the steel plate for ultra-high-strength line pipes characterized in that the temperature is raised to the temperature up to the temperature, and the temperature raising and holding treatment for holding at that temperature for 60 seconds or less is performed once or more.

(5) The method for producing a steel sheet for ultra-high strength line pipe excellent in unstable ductile fracture resistance according to (4) , wherein the temperature rise after the accelerated cooling treatment is performed by an induction heating device .
(6) A welded steel pipe for an ultrahigh strength line pipe having a tensile strength of 750 MPa or more, comprising the steel plate according to any one of (1) to (3 ) above.

  According to the present invention, it is possible to economically supply a steel pipe for a line pipe excellent in unstable ductile fracture resistance having an ultra-high strength with a tensile strength of 760 MPa or more, and a steel sheet having a good formability of the material. it can.

Hereinafter, the best mode for carrying out the present invention will be described in detail.
(1) Metal structure Average old austenite grain size:
The reason why the average prior austenite particle size (D) is set to 18 μm or less is that when the particle size exceeds 18 μm, a steel sheet excellent in ultra-high strength unstable ductile fracture resistance cannot be obtained.
The smaller the particle size, the finer the structure after transformation, and the finer structure after transformation is indispensable for obtaining a steel sheet for ultra-high strength line pipes excellent in unstable ductile fracture resistance. Desirably, it is 15 μm or less.

  The prior austenite grain boundaries can be distinguished relatively clearly in the structure of a steel sheet composed of an ultra-high-strength bainite structure, a martensite structure, or a mixed structure of both, as in the present invention. If it is unclear, it may be devised to use a corrosive solution with a surfactant added.

  As a method for measuring the grain size, there are a method applying JIS G0501 (1998) “Austenite grain size test method for steel” and a method based on image analysis. In the present invention, the old austenite grains are not recrystallized austenite. Since the hardened flat grains are also included, the average interval between the austenite grain boundaries in the thickness direction, which becomes the resistance of the through-thickness crack, was regarded as the average grain size of the prior austenite grain size. That is, for the steel surface parallel to the rolling direction and perpendicular to the plate surface, the vicinity of the center of the plate thickness is observed at a magnification of 500 times, and five straight lines of 100 mm (actual size: 0.2 mm) are drawn in the plate thickness direction, Count the number of intersections of the straight lines and determine the average intercept length. The same operation was repeated 5 times or more, and the obtained average section length was defined as the average particle diameter.

Tempered martensite organization:
It is possible to increase the unstable ductile fracture resistance by appropriately heat-treating the surface layer part of the front and back surfaces of the steel sheet to obtain a tempered martensite structure, and for forming ultra-high-strength steel sheets when forming steel sheets into pipes. It is possible to alleviate the difficulty of cracking and bending, and to improve the pipe manufacturing efficiency.

  The reason why the tempered martensite structure is 80% or more is that it is necessary to make the structure mainly composed of the tempered martensite structure in order to realize ultra-high strength with high unstable ductile fracture resistance, and the ratio is 80%. It is because unstable ductile fracture resistance will run short if it is less than. That is, a steel sheet containing a large amount of martensite structure and other bainite, ferrite structure, etc. that are not tempered on the steel sheet surface cannot achieve both ultrahigh strength and high ductile fracture resistance.

Next, the reason why the part where the tempered martensite structure is generated is limited to the respective surface layer portions corresponding to 5 to 30% of the plate thickness from the front and back surfaces of the steel plate is as follows.
If a martensite structure is formed over 30% of the plate thickness, strength will be reduced, heat treatment costs and heat treatment time will be prolonged, and economic efficiency will be impaired. At the same time, strength reduction, weldability and weld HAZ toughness will be reduced. This is because it causes a decrease. On the other hand, if it is less than 5%, the proportion of the entire thickness is too small, and a sufficient effect of the tempered martensite structure cannot be obtained, so the lower limit was made 5%. In order to obtain an ultra-high strength with a tempered martensite structure, a method of making a high alloy is also conceivable. However, if a high alloy is used, there is a possibility that deterioration of weldability and weld HAZ toughness may occur. Cannot be adopted.

  Here, “martensite” does not relate to the structure of the structure, but rather is a general term that refers to a transformation product generated by the martensitic transformation, which is a solid phase transformation caused by the cooperative movement of atoms. [Refer to "Leslie Steel Materialology" (S60.5.31. Maruzen) p67].

  That is, when the steel is cooled at a sufficient speed from the austenite state, it undergoes martensitic transformation and becomes a so-called baked state. The structure formed at this time is martensite.

In the steel sheet for ultra-high strength line pipe according to the present invention, the martensite structure in the quenched state is tempered to improve the characteristics of the martensite phase.
For the tempering of the present invention, a tempering treatment mainly by heating is preferable, but “self-tempering or automatic tempering” using a process from the Ms point to room temperature is also possible [see “Leslie Steel Materialology” (p. 242)]. . In this case, for “self-tempering or automatic tempering”, it is necessary to control the temperature after the martensitic transformation and the subsequent cooling process. In the case of tempering by heating, there is an advantage that quality control can be easily performed without such control difficulty.

Bainite organization:
“Bainite” is a structure that occurs between pearlite and martensite, and has been studied as a structure generated by isothermal transformation [Refer to “Third Edition Steel Handbook I Basics” (S56.6.20 Maruzen) P461]. Subsequently, the transformation structure from the processed austenite of controlled rolling and accelerated cooling related to the present invention was studied, and from the similarity of the structure form, the name of bainite was also used for the transformed structure from the processed austenite [“ "Transformation Behavior of Steels-Transformation and Properties of Practical Materials" (H1.10.2 Japan Iron and Steel Institute, see P11, P58, etc.)].

Here, “bainite” is generally classified into upper bainite and lower bainite.
The upper bainite is a bainite produced at a relatively high temperature and is characterized by rod-like or needle-like carbides at the boundary between lath-like ferrite and lath-like ferrite.

  Lower bainite is a bainite produced at a relatively low temperature and is composed of a lens-like or plate-like ferrite phase. When observed with an electron microscope, the lower bainite is characterized by a structure in which carbides are precipitated in the ferrite phase ["Third Edition". See “Handbook of Iron and Steel I Fundamentals” (ibid.) P.

  In the present invention, it is not necessary to strictly distinguish between lower bainite and upper bainite. However, the bainite according to the present invention is bainite that is produced under production conditions in which 80% or more of martensite is produced. are categorized.

The bainite structure can be identified by observation with an optical microscope or, if necessary, with an electron microscope or a CCT diagram.
Machining CCT diagrams are based on the development of measuring equipment, as described in Chapter 3 “Explanation of CCT Diagrams from Machining Austenite” in “Chapter 3 of“ Transformation Behavior of Iron and Steel—Transformation and Properties of Practical Materials ””. Since it is easy to obtain, and “CCT charts from processed austenite” are contained in Chapter 4, it can be used for steels of similar composition. Moreover, the metal data book (S49.7.20. Maruzen) etc. can be used for normal CCT collections.

  The martensite and bainite according to the present invention may be either a transformed structure from processed austenite or a transformed structure from unprocessed austenite (by ordinary heat treatment).

However, the transformation structure from the processed austenite is finer and preferable in terms of the refinement of the structure, but can be selected depending on the owned equipment.
(2) Chemical composition The reason for limiting the chemical composition of the steel sheet will be described. In addition,% which shows content of each element is "mass%".

C: 0.02-0.1%
C is an element necessary for ensuring the strength of the steel sheet. If it is less than 0.02%, sufficient strength cannot be ensured. If it exceeds 0.1%, the toughness and unstable ductile fracture resistance deteriorate. Let Therefore, the C content is set to 0.02 to 0.1%.

Si: 0.5% or less Si is added for deoxidation, but if it exceeds 0.5%, toughness and weldability are deteriorated. Therefore, the Si content is set to 0.5% or less.

Mn: 1 to 2.5%
Mn has the effect of improving the strength and toughness of the steel, and if it is less than 1%, the effect is not sufficient, while if it exceeds 2.5%, the weldability deteriorates. Therefore, the Mn content is set to 1 to 2.5%.

P: 0.01% or less P is an inevitable impurity element and deteriorates weldability. Since this tendency becomes remarkable when it exceeds 0.01%, the P content is set to 0.01% or less.

S: 0.005% or less S is generally an MnS-based inclusion in steel, and deteriorates unstable ductile fracture resistance. Further, although the form is controlled from the MnS type to the CaS type inclusion by addition of Ca, if the content of S is large, the amount of CaS type inclusion also increases and the unstable ductile fracture resistance is deteriorated. This tendency becomes remarkable when the S content exceeds 0.005%. Therefore, the S content is set to 0.005% or less.

Mo: 0.1 to 0.8%
Mo is an element effective for improving toughness and increasing strength. However, if contained over 0.8%, weldability and unstable ductile fracture resistance are deteriorated. Therefore, the upper limit of the Mo content is set to 0.8%. On the other hand, since the effect of improving toughness and increasing the strength is small unless the content is 0.1% or more, the lower limit is set to 0.10.

Nb: 0.005 to 0.06%
Nb has the effect of suppressing grain growth during rolling and quenching and improving toughness by making fine grains. However, if the Nb content is less than 0.005%, the effect is not obtained, and if it exceeds 0.06%, the toughness of the heat affected zone is deteriorated. Therefore, the Nb content is set to 0.005 to 0.06%.

Ti: 0.004 to 0.015%
Ti has the effect of forming TiN and improving the toughness of the welded HAZ part. However, when the Ti content is less than 0.004%, there is no effect, and when it exceeds 0.015%, the toughness deteriorates. Therefore, the Ti content is set to 0.004 to 0.015%, preferably 0.005 to 0.015%.

sol. Al: 0.05% or less Al is added as a deoxidizing agent, but if it exceeds 0.05%, unstable ductile fracture resistance is deteriorated due to a decrease in cleanliness. Therefore, the Al content is set to 0.05% or less.

N: 0.001 to 0.005%
N has the effect of forming TiN and improving the toughness of the welded HAZ part. However, if the N content is less than 0.001%, the effect is not obtained, and if it exceeds 0.0050%, the toughness deteriorates. Therefore, the N content is set to 0.001 to 0.005%.
The following elements can be contained if necessary.

One or more of Ni, Cu, Cr, V and B:
Since these elements have an effect of increasing the strength, one or more of these elements may be contained as necessary.
Ni has an effect of improving the toughness in addition to the effect of increasing the strength. However, even if it is an expensive element and exceeds 2.5%, the effect is small for the increase in cost. Therefore, when Ni is contained, the content is set to 2.5% or less. On the other hand, unless the content is 0.1% or more, the toughness improvement and strength improvement effects are small, so the lower limit was made 0.1%.

  Cu is an element that improves toughness in addition to increasing the strength, but if it exceeds 1.5%, weldability deteriorates. Therefore, the upper limit when Cu is contained is set to 1.5%. In addition, since the said effect is acquired even if it contains a trace amount of Cu, a minimum is not specifically limited.

  Cr, like Mn, is an element effective for obtaining sufficient strength even at low C, but if it exceeds 1%, weldability deteriorates. Therefore, the upper limit when Cr is contained is set to 1%. On the other hand, if the content is not more than 0.1%, the effect of improving toughness and increasing the strength are small, so the lower limit was made 0.1%.

  V has the effect of increasing the strength without deteriorating toughness and weldability, but if it exceeds 0.1%, the weldability is significantly impaired. Therefore, the upper limit when V is contained is set to 0.1% or less. On the other hand, if 0.005% or more is not contained, the effect of improving toughness and increasing strength is small, so the lower limit was made 0.1%.

  B is useful as an element that improves the hardenability of the steel and improves the strength in a small amount. If the B content is less than 0.0003%, the effect is not obtained, and if it exceeds 0.002%, the hardenability deteriorates, so the upper limit was made 0.002%. Therefore, the content when the B content is included is set to 0.0003 to 0.002%.

One or two of Zr and Ca:
Zr and Ca are elements that exert an effect on improving the toughness of the welded HAZ part, and may be contained as required.

  Zr is useful for improving the toughness of the welded HAZ part by forming nitrides, oxides, sulfides and the like. However, if the Zr content exceeds 0.03%, the toughness is deteriorated, so the upper limit of the Zr content is set to 0.03%. Zr has the above-mentioned effects even in a very small amount, so the lower limit is not limited, but 0.003% or more is desirable to make the effect sufficient.

  Ca is an element effective for controlling the form of sulfide inclusions and is effective even in a trace amount, so the lower limit is not particularly limited. On the other hand, even if the content exceeds 0.003%, the effect is saturated, and rather the HIC resistance is deteriorated due to a decrease in cleanliness. Therefore, the upper limit of the Ca content is set to 0.003%. Moreover, although there exists an improvement effect of HAZ part toughness by oxide control, when it is desired to improve toughness positively, it is desirable to make it contain 0.0004% or more.

Weld cracking susceptibility composition: Pcm 0.16-0.3%
The weld cracking susceptibility composition Pcm needs to be 0.16% or more in order to ensure the strength of X65 grade or higher, so the lower limit was made 0.16%. On the other hand, if it exceeds 0.3%, the weldability deteriorates, so the upper limit was made 0.3%. However, when emphasizing weldability, the upper limit is preferably set to 0.25%. Pcm needs to be properly used according to the following two formulas depending on the chemical composition.

Pcm1 = C + Si / 30 + Mn / 20 + Ni / 60 + Mo / 15
Pcm2 = C + Si / 30 + Mn / 20 + Ni / 60 + Mo / 15 + Cu / 20 + Cr / 20 + V / 10 + 5B
The balance of the steel of the present invention is substantially iron, and elements other than the above and inevitable impurities can be contained unless the effects of the present invention are impaired.

(3) Steel plate manufacturing method Heating temperature:
If the heating temperature of the slab is less than 950 ° C., the deformation resistance of the steel is large, and not only the predetermined rolling finishing temperature cannot be secured but also the target strength cannot be obtained, so the lower limit is set to 950 ° C. . On the other hand, when the temperature exceeds 1200 ° C., not only does the austenite grain size of the steel become coarse and the toughness of the steel after rolling deteriorates, but also the energy efficiency becomes worse and the rolling scale due to surface oxidation of the slab is remarkable, so the upper limit is set. It was 1200 degreeC.

  In addition, about finishing temperature, although it does not prescribe | regulate in particular, 880 to 680 degreeC is desirable. The reason is that at a finishing temperature exceeding 880 ° C., a sufficient microstructure cannot be obtained and sufficient unstable ductile fracture resistance may not be obtained, so the upper limit is desirably 880 ° C. Further, at a temperature lower than 680 ° C., the influence of rolling becomes so strong that the rolling texture remains and sufficient unstable ductile fracture resistance may not be obtained, so the lower limit is desirably 680 ° C.

Accelerated cooling:
After rolling, an accelerated cooling treatment is performed in order to obtain a quenched structure. Accelerated cooling refers to cooling a steel sheet at a faster speed than air cooling using a cooling medium such as water.

  When the accelerated cooling start temperature exceeds 850 ° C., the difference in structure between the center of the plate thickness after cooling and the surface is enlarged, and sufficient unstable ductile fracture resistance cannot be obtained even if the surface layer structure is changed after water cooling. In some cases, the upper limit of the start temperature of accelerated cooling was set to 850 ° C. In addition, if accelerated cooling is performed from a temperature lower than 650 ° C., not only sufficient hardenability can be secured and the desired strength cannot be obtained, but also the toughness and unstable ductile fracture resistance are impaired. The lower limit of the temperature was 50 ° C.

  When the accelerated cooling stop temperature exceeds 450 ° C., the martensite transformation or the lower bainite transformation cannot be sufficiently completed, and a predetermined structure ratio cannot be obtained. Therefore, the upper limit is set to 450 ° C. The lower limit is not particularly limited, but is preferably 200 ° C. or higher from the viewpoint of energy efficiency.

Temperature rise after accelerated cooling:
If the rate of temperature increase is less than 3 ° C./s, it takes too much time to increase the temperature, and the characteristics of the steel material are affected during the temperature increase, making it impossible to secure the predetermined characteristics stably. The lower limit of 3 ° C./s or more. The upper limit is not particularly limited, but the upper limit is necessarily limited by the capability of the heating means.

  The temperature of the steel plate surface layer means the temperature in the vicinity of the steel plate surface portion measured by a radiation thermometer or a thermocouple welded to the steel plate surface portion. It is preferable to measure both the front and back surfaces of the steel plate, but when it is estimated that the front and back surfaces of the steel plate have a substantially symmetrical temperature distribution, measurement on only one side may be performed.

  Regarding the temperature elevation temperature, if the temperature is lower than 400 ° C., the tempering is insufficient and sufficient unstable ductile fracture resistance cannot be secured, so the lower limit is set to 400 ° C. On the other hand, when the temperature exceeds 700 ° C., the strength decreases greatly, and not only the target tensile strength of 760 MPa or more cannot be secured, but also the heating energy increases, so the upper limit is set to 700 ° C.

  If the time of holding at the raised temperature exceeds 60 s, not only will it be impossible to ensure a tensile strength of 760 MPa or more, but also the productivity will be lowered due to the increased temperature raising time, so the upper limit is set to 60 s. did. Further, the lower limit is not particularly limited, but an effect can be obtained if the steel plate reaches the target temperature rise even instantaneously, and may be 1 second or less.

  The reason why the temperature raising and holding treatment is performed once or more is that if it is less than once, the steel sheet surface structure cannot be a predetermined tempered structure, and a desired unstable ductile fracture resistance cannot be obtained. Further, the unstable ductile fracture resistance tends to increase as the number of treatments increases, but from the viewpoint of cost, the temperature raising / holding treatment is desirably performed 1 to 3 times.

As for the temperature raising method after rolling, there is a method such as using a heating furnace using an induction heating device or a direct or reflective heating burner, but this induction heating is a method of the steel sheet surface texture hardened by the accelerated cooling of the present invention. Suitable for tempering treatment.

(3) Welded steel pipe for line pipes Welded steel pipes for ultra-high-strength line pipes join the ends of steel sheets formed into steel pipe shapes by welding by cold working or warm working of steel sheets specified in the present invention. It is a steel pipe manufactured by this. Usually, cold forming at room temperature is performed. However, in winter and the like, cracks may occur in the steel sheet during cold forming, and the steel sheet may be heated to a temperature that is not so high above room temperature. This is called warm processing.

  The welding for joining the end portions of the steel plates may be a normal method. In many cases, the groove portions of the cylindrical steel plates are first tack welded by carbon dioxide gas welding or the like, and then main welding is performed by submerged arc welding.

  If the bend formability of the steel sheet is poor, the groove accuracy deteriorates and weld defects are often generated. Therefore, the formability of the steel sheet is an important characteristic for a steel pipe for an ultra high strength line pipe. According to the present invention, sufficient bending accuracy can be ensured.

  After melting steels having respective chemical compositions of symbols A to L shown in Table 1, the slab is formed by forging, hot rolling is performed under the manufacturing conditions shown in Table 2, and then the conditions are changed variously as shown in Table 2. After performing the accelerated cooling process by water cooling, the reheating process was performed using the induction heating apparatus, and the hot rolled steel plate with a plate thickness of 20 mm was manufactured.

  From these steel sheets, a sample for microscopic observation, a plate-shaped tensile test piece specified in API (American Petroleum Institute) 5L, a Charpy impact test piece of 2 mm V notch specified in JIS (Japanese Industrial Standard) Z2202, and API RP5L3 A specified DWTT specimen was taken.

As for the metal structure, the ratio of the tempered structure in the surface layer part and the depth from the steel plate surface occupied by the structure were examined with an optical microscope, and the state of the structure other than the surface layer part was examined.
The average prior austenite grain size was measured by observing the vicinity of the center of the thickness of the steel material surface parallel to the rolling direction and perpendicular to the plate surface at a magnification of 500 times, and measuring 100 mm in the plate thickness direction (actual size: 0.2 mm). 5 were drawn, the number of intersections of the straight lines was counted, the average intercept length was obtained, the same operation was repeated 5 times or more, and the obtained mean intercept length was defined as the average prior austenite grain size. The results of the metal structure investigation are shown in Table 2.

In the DWTT test, an impact load was applied to a test piece having a V-shaped press notch with a depth of 0.2 inch at the center of the test piece having a thickness of 3 inches and a width of 12 inches. In this test, the energy value and the ductility / brittle fracture surface ratio of the specimen fracture surface are measured, and the unstable ductile fracture resistance of the steel material is evaluated. In conventional strength grade steel, if the ductile fracture surface ratio at the test temperature is 85%, it was required to have sufficient unstable ductile fracture resistance at that temperature, but in ultra-high strength steel with a tensile strength of 750 MPa or more. In addition to securing the ductile fracture surface ratio, it is necessary to secure an absorbed energy value of 300 J / cm ² .

  The test results are shown in Table 3.

  As is apparent from Table 3, each steel plate shown in the present invention example has an extremely large absorbed energy in the DWTT test and a tensile strength exceeding 750 MPa or more, compared with a comparative example not having a metal structure defined in the present invention. It turns out that it is excellent in unstable ductile fracture resistance though it is high intensity | strength.

Claims (6)

In mass%, C: 0.02 to 0.1%, Si: 0.5% or less, Mn: 1 to 2.5%, P: 0.01% or less, S: 0.005% or less, Mo: 0.1-0.8%, Nb: 0.005-0.06%, Ti: 0.004-0.015%, sol. Al: 0.05% or less, N: 0.001 to 0.005% included, the balance has a chemical composition consisting of Fe and impurities, and the Pcm1 value represented by the following formula (1) is 0.16 to In the range of 0.3%,
A steel sheet having an average prior austenite grain size of 18 μm or less at the center of the plate thickness and comprising a bainite, martensite, or a mixed structure of both, and a structure containing 80% or more of a tempered martensite structure. A steel sheet for ultra-high-strength line pipes having a tensile strength of 750 MPa or more, characterized by occupying at least 5% to 30% of the total thickness of the front and back surfaces .
Pcm1 ＝ C ＋ Si / 30 + Mn / 20 + Ni / 60 + Mo / 15 (1) In mass%, C: 0.02 to 0.1%, Si: 0.5% or less, Mn: 1 to 2.5%, P: 0.01% or less, P: 0.01% or less, S: 0.005% or less, Mo: 0.1-0.8%, Nb: 0.005-0.06%, Ti: 0.004-0.015%, sol. Al: 0.05% or less, N: 0.001-0.005% included, Ni: 0.1-2.5%, Cu: 1.5% or less, Cr: 0.1-1%, V: 0.005 to 0.1%, B: contain one or more of 0.0003 to 0.002%, the balance has a chemical composition consisting of Fe and impurities, The represented Pcm2 value is in the range of 0.16 to 0.30%,
A steel sheet having an average prior austenite grain size of 18 μm or less at the center of the plate thickness and comprising a bainite, martensite, or a mixed structure of both, and a structure containing 80% or more of a tempered martensite structure. A steel sheet for ultra-high-strength line pipes having a tensile strength of 750 MPa or more, characterized by occupying at least 5% to 30% of the total thickness of the front and back surfaces .
Pcm2 = C + Si / 30 + Mn / 20 + Ni / 60 + Mo / 15 + Cu / 20 + Cr / 20 + V / 10 + 5B (2) The super-high content according to claim 1 or 2 , wherein one or two of Zr: 0.03% or less and Ca: 0.003% or less are contained in mass% instead of a part of Fe. Steel plate for strength line pipe. It is a manufacturing method of the steel plate for ultra-high-strength line pipe according to any one of claims 1 to 3,
The steel slab having the chemical composition according to any one of claims 1 to 3 is heated to a temperature range of 950 to 1200 ° C and subjected to hot rolling, and then at least 450 from a temperature in the range of 850 to 650 ° C. Accelerated cooling treatment is performed to a temperature of 0 ° C., and then the structure containing 80% or more of the tempered martensite structure occupies at least 5% or more and 30% or less of the sheet thickness ratio in the steel sheet front and back surfaces. It is characterized in that only the surface layer of the steel sheet is heated to a temperature of 400 to 700 ° C. at a temperature rising rate of at least ° C./s, and the temperature raising and holding treatment for holding for 60 s or less at that temperature is performed once or more. The manufacturing method of the steel plate for super high-strength line pipes. The method for producing a steel sheet for ultra-high-strength line pipe according to claim 4 , wherein the temperature rise after the accelerated cooling treatment is performed by an induction heating device . Ultra-high-strength line welded steel pipe for tensile strength consisting of steel plate is not less than 750MPa according to any one of claims 1-3.