JP2005060838A - Steel pipe with low yield ratio, high strength, high toughness and superior strain age-hardening resistance, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel pipe with a low yield ratio, high strength, high toughness and superior strain age-hardening resistance, which is manufactured at high efficiency without adding a large quantity of alloy elements. <P>SOLUTION: The steel pipe with the low yield ratio, high strength, high toughness and superior strain age-hardening resistance comprises, by mass%, 0.03-0.1% C, 0.01-0.5% Si, 0.5-2.5% Mn, 0.08% or less Al, 0.05-0.5% Mo, 0.005-0.04% Ti, while controlling a ratio, by atom%, of a C content to the total content of Ti and Mo, namely C/(Mo+Ti) to 0.5-3, and the balance substantially Fe; and has a metallographic structure of a two-phase structure consisting substantially of ferrite and bainite, in the bainite phase of which fine precipitates containing Ti and Mo are dispersingly precipitated. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a large-diameter welded steel pipe (UOE steel pipe, spiral steel pipe) with small material deterioration after coating treatment, which is suitable mainly for line pipes for transporting crude oil and natural gas, and a method for producing the same.

  In line pipes that mainly transport crude oil and natural gas, in addition to high strength and toughness, a low yield ratio is also required from the viewpoint of earthquake resistance. In general, it is known that the yield ratio of steel can be reduced by making the metal structure of steel a structure in which hard phases such as bainite and martensite are moderately dispersed in a soft phase like ferrite. ing. As a production method for obtaining a structure in which a hard phase is appropriately dispersed in such a soft phase, quenching from a two-phase region of ferrite and austenite (Q ′) between quenching (Q) and tempering (T). There is known a heat treatment method (see, for example, Patent Document 1).

  Further, as a technique for achieving a low yield ratio without performing a complicated heat treatment as disclosed in Patent Document 1, the rolling of the steel material is completed at the Ar3 transformation point or higher, and the subsequent accelerated cooling rate and cooling stop temperature. Is known to achieve a low yield ratio with a two-phase structure of acicular ferrite and martensite (see, for example, Patent Document 2).

  However, welded steel pipes such as UOE steel pipes and ERW steel pipes used for line pipes are formed by cold forming the steel sheet into a tubular shape and welding the butt part, so that the outer surface of the steel pipe is usually coated from the standpoint of corrosion protection. Strain aging occurs due to processing strain during pipe making and heating during coating treatment, and yield stress increases. Therefore, even if the low yield ratio of the raw steel plate is achieved by the method described above, it is difficult to achieve a low yield ratio in the steel pipe.

Strain aging is suppressed by limiting the C and N contents that cause strain aging, and adding Nb and Ti to bond with C and N as a steel material with excellent strain aging characteristics and its manufacturing method. There is a known method (see, for example, Patent Document 3).
JP-A-55-97425 Japanese Patent Laid-Open No. 1-176027 JP 2002-220634 A

  However, in the technique described in Patent Document 3, in order to obtain high-strength steel, as shown in the examples, it is necessary to increase the carbon content of the steel material or to increase the addition amount of other alloy elements. Therefore, not only the cost of the material is increased, but also the deterioration of the toughness of the weld heat affected zone becomes a problem.

  As described above, in the conventional technique, it is difficult to manufacture a steel pipe having a low yield ratio even after the coating treatment without reducing productivity and without adding a large amount of alloying elements.

  Accordingly, an object of the present invention is to solve such problems of the prior art, and without adding a large amount of alloying elements, a high yield and high yield strength high toughness steel pipe with high production efficiency and excellent strain aging characteristics and its It is to provide a manufacturing method.

The features of the present invention for solving such problems are as follows.
(1), in mass%, C: 0.03-0.1%, Si: 0.01-0.5%, Mn: 0.5-2.5%, Al: 0.08% or less, Mo : 0.05 to 0.5%, Ti: 0.005 to 0.04% is contained, the balance is substantially made of Fe, and is the ratio of the amount of C in atomic% and the total amount of Mo and Ti C / (Mo + Ti) is 0.5 to 3, the metal structure is substantially a two-phase structure of ferrite and bainite, and fine precipitates containing Ti and Mo are dispersed and precipitated in the bainite phase. A low yield ratio, high strength, high toughness steel pipe with excellent strain aging characteristics.
(2) Further, by mass%, Nb: 0.005 to 0.07% and / or V: 0.005 to 0.1%, C amount in atomic% and Mo, Ti, Nb, C / (Mo + Ti + Nb + V), which is the ratio of the total amount of V, is 0.5 to 3, and fine precipitates containing Ti, Mo, Nb and / or V are dispersed and precipitated in the bainite phase. A low yield ratio high strength high toughness steel pipe excellent in strain aging resistance as described in (1).
(3) By mass%, C: 0.03 to 0.1%, Si: 0.01 to 0.5%, Mn: 0.5 to 2.5%, Al: 0.08% or less And at least two selected from Ti: 0.005-0.04%, Nb: 0.005-0.07%, V: 0.005-0.1%, with the balance being substantially Fe / Fe and C / (Ti + Nb + V), which is the ratio of the amount of C in atomic% to the total amount of Ti, Nb, and V, is 0.5 to 3, and the metal structure is substantially composed of ferrite and bainite. A low-yield ratio high with excellent strain aging resistance, characterized by a two-phase structure in which a precipitate containing two or more selected from Ti, Nb, and V is dispersed and precipitated in the bainite phase. High strength and toughness steel pipe.
(4) Furthermore, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, B: 0.005% or less, Ca: 0.0005-0. The low yield ratio, high strength, high toughness steel pipe according to any one of (1) to (3), comprising one or more selected from 003%.
(5) After heating the steel having the component composition according to any one of (1) to (4) to a temperature of 1000 to 1300 ° C. and hot rolling at a rolling end temperature below the Ar 3 temperature, 5 ° C. Accelerated cooling to 300-600 ° C. at a cooling rate of at least / s, and then immediately reheating to 550-750 ° C. at a rate of at least 0.5 ° C./s, and the metal structure is substantially ferrite and bainite As a steel sheet in which fine precipitates containing Ti and Mo or fine precipitates containing two or more selected from Ti, Nb, and V are dispersed and precipitated in the bainite phase. A method for producing a low-yield-ratio, high-strength, high-toughness steel pipe excellent in strain aging characteristics, characterized in that the steel sheet is cold-formed into a tubular shape, and a butted portion is welded to form a steel pipe.

  ADVANTAGE OF THE INVENTION According to this invention, the low yield ratio high strength high toughness steel pipe excellent in the strain aging characteristic can be manufactured with high manufacturing efficiency and low cost. For this reason, the steel pipe used as a line pipe can be stably manufactured in large quantities at a low cost.

  In order to solve the above-mentioned problems, the present inventors diligently studied a manufacturing method of a steel pipe original sheet, particularly a manufacturing process of accelerated cooling after controlled rolling and subsequent reheating. As a result, the following (a) and (b) Obtained knowledge.

  (A) In the manufacturing process of accelerated cooling after controlled rolling in the two-phase region of austenite and ferrite and subsequent reheating, by performing reheating after accelerated cooling, in addition to strengthening by bainite transformation during accelerated cooling, Due to precipitation strengthening by fine precipitates precipitated in the hard phase mainly composed of bainite at the time of reheating, high strength of the two-phase structure of ferrite and hard phase mainly composed of bainite even in low-component steels with few alloying elements. Can be realized. And by finely dispersing and precipitating fine composite carbide containing Mo and Ti (described below as the first embodiment), or fine containing at least two or more selected from Ti, Nb and V By dispersing and precipitating complex carbides (described below as a second embodiment), high strength of the hard phase in the two-phase structure can be achieved.

  (B) Because of the precipitation of fine precipitates in the hard phase mainly composed of bainite, the solid solution C and N that cause strain aging is reduced, so that the yield stress increases due to strain aging after steel pipe forming and coating treatment. It is possible to suppress.

  The present invention is a fine composition containing the ferrite formed during the two-phase rolling as described above, Mo and Ti generated by accelerated cooling after rolling and subsequent reheating, and optionally Nb and / or V. Excellent strain aging resistance having a two-phase structure of a composite precipitate and a hard phase mainly composed of bainite in which fine composite carbide containing two or more selected from Ti, Nb, and V is dispersed and precipitated. Low yield ratio, high strength, high toughness steel pipe and its manufacturing method. In order to make maximum use of precipitation strengthening in addition to transformation strengthening, it is not necessary to add a large amount of alloying elements, and the heat affected zone toughness is impaired. High strength can be achieved. Furthermore, since the duplex hardened steel produced by the present technology has a higher hardness of the hard phase than the conventional method, the hardness difference between the soft phase and the hard phase is large, and a further lower yield ratio can be achieved.

  Hereinafter, the high-strength steel pipe of the present invention will be described in detail. First, the structure of the high-strength steel pipe of the present invention will be described.

  The metal structure of the steel pipe of the present invention is substantially a two-phase structure of a soft phase and a hard phase, and is substantially a two-phase structure of ferrite and bainite. In the present invention, alloy elements are combined by utilizing transformation strengthening by bainite transformation during accelerated cooling and precipitation strengthening by fine precipitates that reheat after accelerated cooling and precipitate in a hard phase mainly composed of bainite. The strength can be increased without adding a large amount of. On the other hand, although ferrite is soft and rich in ductility, the steel pipe has sufficient strength by adopting a two-phase structure with a higher-strength hard phase. Furthermore, since the difference in hardness between the soft layer and the hard layer is increased, the yield ratio is further reduced. In the hard phase of the two-phase structure, bainite is mainly used, and different metal structures such as martensite and pearlite may be mixed in one or more kinds. However, since the strength is reduced, other than the ferrite phase and the bainite phase. The smaller the tissue fraction, the better. However, if the volume fraction of the structure other than the ferrite phase and the bainite phase is low, the effect can be ignored. Therefore, other metal structures of less than 5% in total volume fraction, that is, one type of martensite, pearlite, etc. Or you may contain 2 or more types. Further, it is desirable that the bainite fraction is 10% or more from the viewpoint of securing strength, and the ferrite fraction is 10% or more from the viewpoint of securing toughness of the base material.

  Furthermore, since solid solution C and N which are the cause of strain aging are fixed as fine precipitates, it is possible to suppress an increase in yield ratio due to strain aging after heating during steel pipe forming and coating.

  Next, regarding the precipitates dispersed and precipitated in the bainite phase (hard phase), the case where the steel pipe contains Mo will be described as a first embodiment, and the case where no steel is contained will be described as a second embodiment.

  In the steel pipe of the present invention in the first embodiment, precipitation strengthening by composite precipitates containing Mo and Ti in the hard phase as a basis is utilized. Mo and Ti are elements that form precipitates in steel, and strengthening of steel by precipitation of individual carbides has been performed in the past. It is a feature that a greater strength improvement effect can be obtained by finely dispersing and depositing the composite precipitate contained in the steel as compared with the case of precipitation strengthening of MoC or TiC. This unprecedented strength improvement effect is due to the fact that composite precipitates containing Mo and Ti as a basis are stable and have a slow growth rate, so that extremely fine precipitates having a particle size of less than 10 nm can be obtained. is there.

  When the composite precipitate containing Mo and Ti as a basis is composed of only Mo, Ti, and C, the total of Mo and Ti and C are combined in an atomic ratio of about 1: 1. It is very effective for increasing the strength. In the present invention, it has been found that by adding Nb and / or V in combination, the precipitate becomes a composite carbide containing Mo, Ti and Nb and / or V, and the same precipitation strengthening can be obtained.

  In the present invention, composite precipitates containing Mo and Ti, which are precipitates dispersed and precipitated in the steel pipe, are produced in the hard phase by producing steel sheets using the production method of the present invention on the steels having the components described below. It can be dispersed in

  Moreover, in the steel pipe of the present invention in the second embodiment, as other precipitates dispersed and precipitated in the bainite phase (hard phase), a composite carbide containing two or more selected from Ti, Nb, and V is used. Precipitation strengthening can also be used. Ti, Nb, and V are elements that form carbides in steel, and strengthening of steel by precipitation of individual carbides has been conventionally performed, but in the present invention, it is selected from Ti, Nb, and V. It is characterized in that a greater strength improvement effect can be obtained by finely depositing composite carbide containing two or more kinds. This unprecedented strength improvement effect is that a composite carbide containing two or more selected from Ti, Nb, and V is stable and has a slow growth rate, so that an extremely fine precipitate having a particle size of less than 10 nm is formed. It is because it is obtained.

  In the second embodiment of the present invention, a composite carbide containing two or more selected from Ti, Nb, and V, which is a precipitate that is dispersed and precipitated in a steel pipe, is applied to the steel of the components described below. By manufacturing a steel pipe using a manufacturing method, it can be obtained by being dispersed in a hard phase. In addition, the composite carbide containing two or more selected from Ti, Nb, and V is a composite carbide containing Mo and Ti as a basis even in the first embodiment in which the steel contains Mo. At the same time, dispersion and precipitation may occur.

  The number ratio of fine precipitates of these composite carbides is preferably 95% or more of the total precipitates excluding TiN. The average particle size of the fine composite carbide precipitates is obtained by subjecting a photograph taken with a transmission electron microscope (TEM) to image processing, and obtaining the diameter of a circle having the same area as each precipitate for each composite carbide. , And can be obtained as an average value of their diameters.

  As described above, the steel pipe of the present invention has a composite structure composed of two phases of a ferrite phase and a bainite phase in which precipitates are finely precipitated. Such a structure is obtained by using a steel having the following composition. It can obtain by manufacturing by the method like this.

  First, chemical components of the high-strength steel pipe of the present invention will be described. In the following description, all units represented by% are mass%.

  C: Set to 0.03 to 0.1%. C is an element that contributes to precipitation strengthening as a carbide, but if it is less than 0.03%, sufficient strength cannot be secured. Addition exceeding 0.1% not only deteriorates the HAZ toughness but also deteriorates the strain aging resistance, so the C content is specified to be 0.03% to less than 0.1%. More preferably, it is 0.03 to 0.08%.

  Si: 0.01 to 0.5%. Si is added for deoxidation, but if it is less than 0.01%, the deoxidation effect is not sufficient, and if it exceeds 0.5%, not only the toughness and weldability are deteriorated, but also the strain aging resistance is lowered. The Si content is specified to be 0.01 to 0.5%. More preferably, it is 0.01 to 0.3%.

  Mn: 0.5 to 2.5%. Mn is added for strength and toughness, but if it is less than 0.5%, the effect is not sufficient, and if it exceeds 2.5%, the toughness and weldability deteriorate, so the Mn content is 0.5 to 2. Specify 5%.

  Al: 0.08% or less. Al is added as a deoxidizer, but if it exceeds 0.08%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is specified to be 0.08% or less. Preferably, the content is 0.01 to 0.08%.

  The steel plate of the present invention in the first embodiment contains Mo and Ti.

  Mo: 0.05 to 0.5%. Mo is an important element when precipitating composite carbides containing Mo and Ti as a base, and when added, 0.05% or more is included to suppress pearlite transformation during cooling after hot rolling. However, a fine composite precipitate with Ti is formed, which greatly contributes to an increase in strength. However, if it exceeds 0.5%, the weld heat-affected zone toughness is deteriorated, so the Mo content is specified to be 0.05 to 0.5%. Furthermore, it is preferable to make Mo content into 0.1 to 0.3% from a viewpoint of weld heat affected zone toughness.

  Ti: 0.005 to 0.04%. Ti is an important element forming a composite carbide. Addition of 0.005% or more forms a composite precipitate with Mo, Nb and / or V, and greatly contributes to an increase in strength. However, since addition exceeding 0.04% causes deterioration of the weld heat affected zone toughness, the Ti content is specified to be 0.005 to 0.04%. Furthermore, when the Ti content is less than 0.02%, more excellent toughness is exhibited. For this reason, when adding Nb and / or V, it is preferable to make Ti content into 0.005% or more and less than 0.02%.

  The high-strength steel pipe of the present invention can obtain fine composite precipitates containing Ti and Mo by using the steel of the above components containing Mo and Ti, but in order to make maximum use of precipitation strengthening It is necessary to limit the content ratio of elements forming the composite precipitate as follows. That is, C / (Mo + Ti), which is a ratio of the amount of C in atomic% and the total amount of Mo and Ti, is set to 0.5 to 3. The increase in strength according to the present invention is due to precipitates containing Ti and Mo. In order to effectively use the precipitation strengthening by this composite precipitate, the relationship between the amount of C and the amounts of Mo and Ti which are carbide forming elements is important. By adding these elements in an appropriate balance, Thermally stable and very fine composite precipitates can be obtained. At this time, when the value of C / (Mo + Ti), which is the ratio between the amount of C in atomic% and the total amount of Mo and Ti, is less than 0.5, the precipitation of fine precipitates is insufficient, so the strength is insufficient. In addition, a higher strength hard phase cannot be obtained, and a low yield ratio cannot be achieved. Further, when the value of C / (Mo + Ti), which is the ratio between the amount of C in atomic% and the total amount of Mo and Ti, exceeds 3, C is excessive, strain aging resistance is lowered, and welding heat is reduced. Since a hardened structure such as island martensite is formed in the affected part and the weld heat affected part toughness is deteriorated, the value of C / (Mo + Ti) is set to 0.5 to 3. In addition, when using the content of mass%, the value of (C / 12.01) / (Mo / 95.9 + Ti / 47.9) is used as the content of each element in mass%. Is set to 1.2-3. More preferably, it is 1.4-3.

  Since Nb and / or V forms a fine composite precipitate with Ti and Mo, the steel pipe of the present invention may further contain Nb and / or V.

  Nb: 0.005 to 0.07%. Nb improves the toughness by refining the structure, but forms a composite precipitate with Ti and Mo, contributing to an increase in strength and anti-strain aging characteristics. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.07%, the toughness of the weld heat-affected zone deteriorates, so the Nb content is specified to be 0.005 to 0.07%.

  V: Set to 0.005 to 0.1%. V, like Nb, forms a composite precipitate with Ti and Mo and contributes to strength increase and strain aging resistance. However, if it is less than 0.005%, there is no effect, and if it exceeds 0.1%, the toughness of the weld heat affected zone deteriorates, so the V content is specified to be 0.005 to 0.1%.

  When Nb and / or V are contained, C / (Mo + Ti + Nb + V), which is a ratio of the amount of C in atomic% and the total amount of Mo, Ti, Nb, and V, is set to 0.5 to 3. Strengthening according to the present invention depends on precipitates containing Ti and Mo, but when Nb and / or V are contained, they become composite precipitates (mainly carbides) containing them. At this time, when the value of C / (Mo + Ti + Nb + V) represented by the content of atomic% of each element is less than 0.5, the precipitation of fine precipitates is insufficient, leading to insufficient strength, and higher strength. A hard phase cannot be obtained and a low yield ratio cannot be achieved. Further, when it exceeds 3, C is excessive, the strain aging resistance is lowered, and a hardened structure such as island martensite is formed in the weld heat affected zone, resulting in deterioration of the weld heat affected zone toughness. The value of / (Mo + Ti + Nb + V) is 0.5-3. In addition, when content of mass% is used, each element symbol is defined as the content of each element in mass% (C / 12.01) / (Mo / 95.9 + Ti / 47.9 + Nb / 92. 91 + V / 50.94) is set to 1.2-3. More preferably, it is 1.4-3.

  In the second embodiment that does not contain Mo, the steel pipe of the present invention contains two or more selected from Ti, Nb, and V. In this case, it is desirable that the chemical components of Ti, Nb, and V are also in the above range.

  The low yield ratio high strength high toughness steel pipe of the present invention is C: 0.03-0.1%, Si: 0.01-0.5%, Mn: 0.5-2.5%, Al: 0.08. %: Or less, Ti: 0.005-0.04%, Nb: 0.005-0.07%, V: contain at least two or more selected from 0.005-0.1% By using the component steel, fine precipitates containing Ti, Nb, and V can be obtained. In order to make maximum use of precipitation strengthening, the content ratio of elements that form precipitates is set as follows: It is necessary to restrict so that. That is, C / (Ti + Nb + V), which is a ratio of the amount of C in atomic% and the total amount of Ti, Nb, and V, is set to 0.5-3. The increase in strength in the second embodiment of the present invention is due to the precipitation of fine precipitates containing any two or more of Ti, Nb, and V. At this time, when the value of C / (Ti + Nb + V) represented by the content of atomic% of each element is less than 0.5, the precipitation of fine precipitates is insufficient, leading to insufficient strength, and higher strength. A hard phase cannot be obtained and a low yield ratio cannot be achieved. Further, when C exceeds 3, C is excessive, and a hardened structure such as island martensite is formed in the weld heat affected zone, resulting in deterioration of the weld heat affected zone toughness and deterioration of the strain aging resistance. The value of (Ti + Nb + V) is 0.5-3. In addition, when content of mass% is used, each element symbol is defined as content of each element in mass% (C / 12.01) / (Ti / 47.9 + Nb / 92.91 + V / 50.94). Is set to 1.2-3. More preferably, it is 1.4-3.

  In this invention, you may contain 1 type (s) or 2 or more types of Cu, Ni, Cr, B, Ca shown below in order to further improve the strength toughness of a steel pipe.

  Cu: 0.5% or less. Cu is an element effective for improving toughness and increasing strength. In order to acquire the effect, it is preferable to add 0.1% or more. However, if it is added in a large amount, weldability deteriorates.

  Ni: 0.5% or less. Ni is an element effective for improving toughness and increasing strength. In order to obtain the effect, it is preferable to add 0.1% or more, but adding a large amount is disadvantageous in terms of cost, and the weld heat affected zone toughness deteriorates. Is the upper limit.

  Cr: 0.5% or less. Cr, like Mn, is an element effective for obtaining sufficient strength even at low C. In order to acquire the effect, it is preferable to add 0.1% or more. However, when adding a large amount, the weldability is deteriorated, so when adding, the upper limit is 0.5%.

  B: Set to 0.005% or less. B is an element contributing to strength increase and HAZ toughness improvement. In order to obtain the effect, it is preferable to add 0.0005% or more, but if added over 0.005%, the weldability is deteriorated, so when added, the content is made 0.005% or less.

  Ca: 0.0005 to 0.003%. Ca improves the toughness by controlling the form of sulfide inclusions. The effect appears at 0.0005% or more, and when it exceeds 0.003%, the effect is saturated, and conversely, the cleanliness is lowered and the toughness is deteriorated. And

  N: Preferably it is 0.007% or less. N is treated as an unavoidable impurity, but if over 0.007%, the weld heat affected zone toughness deteriorates, so the content is preferably made 0.007% or less.

  Furthermore, by optimizing Ti / N, which is the ratio of Ti amount to N amount, the austenite coarsening of the weld heat affected zone can be suppressed by TiN particles, and good weld heat affected zone toughness can be obtained. Therefore, Ti / N is preferably 2 to 8, and more preferably 2 to 5.

  The remainder other than the above consists essentially of Fe, and it is possible to add a trace amount of elements that do not impair the effects of the present invention, including inevitable impurities. For example, each of Mg, REM, W, and Zr may be added by 0.02% or less.

  Next, the manufacturing method of the high intensity | strength steel pipe original plate of this invention is demonstrated.

  The base plate of the high-strength steel pipe of the present invention uses steel having the above-mentioned composition, and is hot-rolled at a heating temperature of 1000 to 1300 ° C. and a rolling end temperature of less than Ar 3 temperature, and then a cooling rate of 5 ° C./s or more. Is accelerated to 450 to 600 ° C., and then immediately reheated to a temperature of 550 to 750 ° C. at a temperature rising rate of 0.5 ° C./s or more, so that the metal structure is substantially made of ferrite and bainite. A fine precipitate containing Mo and Ti or a fine precipitate containing Ti, Nb and V can be dispersed and precipitated in the bainite phase (hard phase mainly composed of bainite). Here, the temperature is the average temperature of the steel sheet. The average temperature is obtained by calculation based on the surface temperature of the slab or steel plate, taking into account parameters such as plate thickness and thermal conductivity. Moreover, a cooling rate is an average cooling rate which divided the temperature difference required for cooling to the cooling end temperature (300-600 degreeC) after the completion | finish of hot rolling by the time required to perform the cooling. The temperature increase rate is an average temperature increase rate obtained by dividing the temperature difference required for reheating up to the reheating temperature (550 to 750 ° C.) by the time required for reheating after cooling. Hereinafter, each manufacturing condition will be described in detail.

  Heating temperature: 1000-1300 ° C. If the heating temperature is less than 1000 ° C, the solid solution of the carbide is insufficient and the required strength and yield ratio cannot be obtained, and if it exceeds 1300 ° C, the base metal toughness deteriorates, so the temperature is set to 1000 to 1300 ° C.

  Rolling end temperature: less than Ar3 temperature. This process is an important manufacturing condition in the present invention. In order to reduce the yield ratio, two-phase organization of the soft phase and the hard phase is effective, and the yield ratio decreases as the hardness difference between the soft phase and the hard phase increases. After precipitation of soft pro-eutectoid ferrite at the end of rolling at less than the Ar3 temperature, a high hardness bainite phase is obtained by accelerated cooling and precipitation strengthening in the reheating process described later, and the hardness difference between the soft phase and the hard phase is large. A phase structure is obtained. When the rolling end temperature exceeds the Ar3 temperature, the ferrite transformation does not proceed sufficiently and the yield ratio increases, so the rolling end temperature is set to be lower than the Ar3 temperature. If the rolling end temperature is too low, ferrite may be processed and soft pro-eutectoid ferrite may not be obtained. Therefore, the rolling end temperature is preferably 650 ° C. or higher.

  Immediately after completion of rolling, cooling is performed at a cooling rate of 5 ° C./s or more. When the cooling rate is less than 5 ° C./s, pearlite is generated during cooling, and transformation strengthening by bainite cannot be obtained, so that sufficient strength cannot be obtained. Therefore, the cooling rate after the end of rolling is specified to be 5 ° C./s or more. About the cooling method at this time, it is possible to use arbitrary cooling equipment by a manufacturing process.

  Cooling stop temperature: 300 to 600 ° C. If the cooling stop temperature is less than 300 ° C., island-shaped martensite (MA) is generated, so that the precipitation of fine carbides during reheating is insufficient and sufficient strength cannot be obtained, and the toughness of the base material is deteriorated, and further, strain resistance is increased. Aging characteristics are also reduced. When the temperature exceeds 600 ° C., pearlite is precipitated during cooling, so that the precipitation of fine carbides becomes insufficient and sufficient strength cannot be obtained, and further, the strain resistance is also lowered. Therefore, the accelerated cooling stop temperature is defined as 300 to 600 ° C.

  Immediately after the accelerated cooling, reheating is performed to a temperature of 550 to 750 ° C. at a heating rate of 0.5 ° C./s or more. This process is an important manufacturing condition in the present invention. Fine precipitates that contribute to the strengthening of the hard phase and the improvement of strain aging characteristics are precipitated during reheating. In order to obtain such fine precipitates, it is necessary to reheat to a temperature range of 550 to 750 ° C. immediately after accelerated cooling. In reheating, it is desirable to raise the temperature by at least 50 ° C. from the temperature after cooling. If the heating rate is less than 0.5 ° C./s, it takes a long time to reach the target reheating temperature, so that the production efficiency is deteriorated, and pearlite transformation occurs, so that no dispersion of fine precipitates can be obtained. A sufficient strength cannot be obtained. If the reheating temperature is less than 550 ° C, sufficient precipitation driving force cannot be obtained, and the amount of fine precipitates is small. Therefore, sufficient precipitation strengthening and strain aging resistance cannot be improved, and if it exceeds 750 ° C, the precipitates are coarse. Since sufficient strength cannot be obtained, the reheating temperature range is set to 550 to 750 ° C. There is no need to set the temperature holding time at the reheating temperature. Even if it cools immediately after reheating if the manufacturing method of this invention is used, since a sufficient fine precipitate will be obtained, high intensity | strength will be obtained. However, in order to ensure sufficient fine precipitates, the temperature can be maintained within 30 minutes. If the temperature is maintained for more than 30 minutes, precipitates may become coarse and the strength may decrease. Further, in the cooling process after reheating, the fine precipitates do not become coarse regardless of the cooling rate. Therefore, it is preferable that the cooling rate after reheating is basically air cooling.

  As equipment for performing reheating after accelerated cooling, a heating device can be installed downstream of the cooling equipment for performing accelerated cooling. As the heating device, it is preferable to use a gas combustion furnace or induction heating device capable of rapid heating of the steel sheet.

  An example of equipment for carrying out the production method of the present invention is shown in FIG. As shown in FIG. 1, a hot rolling mill 3, an acceleration cooling device 4, an induction heating device 5, and a hot leveler 6 are arranged in the rolling line 1 from the upstream side toward the downstream side. By installing the induction heating device 5 or other heat treatment device on the same line as the hot rolling mill 3 that is a rolling facility and the accelerated cooling device 4 that is a subsequent cooling facility, reheating is performed quickly after the end of rolling and cooling. Since it can process, it can heat, without reducing the steel plate temperature after rolling cooling too much.

  Furthermore, the manufacturing method of a welded steel pipe is demonstrated.

  In the welded steel pipe of the present invention, the metal structure of the steel having the above-described component composition is substantially a two-phase structure of ferrite and bainite according to the manufacturing conditions described above. In the first embodiment, Ti is contained in the bainite phase. As a steel plate in which fine precipitates containing Mo and Mo are dispersed and precipitated, in the second embodiment, as a steel plate in which fine precipitates containing two or more selected from Ti, Nb, and V are dispersed and precipitated. The steel plate is manufactured and formed into a tubular shape in the cold state, and the butt portion is welded to form a steel pipe. The method for forming into a tubular shape is not particularly specified. Further, the heating temperature of the steel pipe in the case of performing the coating treatment is not particularly defined, but it is desirable to set it at 300 ° C. or lower in order to prevent an increase in the stress ratio due to the softening of bainite.

  Steels of the chemical composition shown in Table 1 (steel types A to I) are made into slabs by a continuous casting method, and welded steel pipes (No. 1 to 15) having a plate thickness of 18, 26 mm and an outer diameter of 24, 48 inches are manufactured using this slab. did.

  After the heated slab is rolled by hot rolling, it is immediately cooled using a water-cooled accelerated cooling facility, reheated using an induction heating furnace or a gas combustion furnace, and a steel plate is produced. A welded steel pipe was produced by the process, and then the outer surface of the steel pipe was coated. The induction furnace was installed on the same line as the accelerated cooling equipment. Table 2 shows the production conditions of each steel pipe (No. 1 to 15). The heating temperature, rolling end temperature, cooling stop (end) temperature, reheating temperature, and other temperatures were the average temperature of the steel sheet. The average temperature was obtained by calculation from the surface temperature of the slab or steel plate, taking into account parameters such as plate thickness and thermal conductivity. The cooling rate is an average cooling rate obtained by dividing the temperature difference required for cooling to the cooling stop (end) temperature after the hot rolling is finished by the time required for the cooling. The reheating rate (temperature increase rate) is an average temperature increase rate divided by the time required to reheat the temperature difference necessary for reheating up to the reheating temperature after cooling.

  The tensile properties of the steel pipe manufactured as described above were measured. The measurement results are also shown in Table 2. Tensile properties were evaluated by taking the average thickness of two full thickness specimens in the rolling direction as tensile specimens, conducting tensile tests before and after coating, measuring tensile strength and yield ratio. The tensile strength of 580 MPa or more was determined as the strength required for the present invention, and the yield ratio of 85% or less was determined as the yield ratio required for the present invention.

  For base metal toughness, three full-size Charpy V-notch test pieces in the vertical direction of rolling were sampled and subjected to Charpy test. The absorbed energy at −10 ° C. was measured to obtain the average value. The absorption energy at −10 ° C. was 100 J or more.

  With respect to the weld heat affected zone (HAZ) toughness, a Charpy test was conducted by collecting test pieces as shown in FIG. FIG. 2 is a schematic view of a cross section of a seam welded portion of a steel pipe. From the center of the plate thickness of the seam welded portion, the notch 9 portion has a length ratio such that weld metal: HAZ = 1: 1. Three full-size Charpy V-notch test specimens 10 were collected, the Charpy absorbed energy at −10 ° C. was measured, and the average value was obtained. The absorption energy at −10 ° C. was 100 J or more.

  In Table 2, all of Nos. 1 to 7 which are examples of the present invention are within the scope of the present invention in terms of chemical composition and production method, have a high tensile strength of 580 MPa or higher, and a yield ratio before coating treatment of 80%. Below, even after the coating treatment, the yield ratio was a low yield ratio of 85% or less, the strain aging resistance was excellent, and the toughness of the base material and the weld heat affected zone was good at 100 J or more. Further, as a result of transmission electron microscope observation and analysis by energy dispersive X-ray spectroscopy, a fine composite having a grain size of less than 10 nm containing Ti and Mo in the ferrite phase and Nb and / or V in some steel plates. Dispersion precipitation of carbide was observed. In addition, the average particle diameter of the fine composite carbide is obtained by subjecting a photograph taken with a transmission electron microscope (TEM) to image processing, obtaining a diameter of a circle having the same area as that of each composite carbide, for each composite carbide. Obtained on average.

  In Nos. 8 to 11, the chemical components are within the scope of the present invention, but the manufacturing method is outside the scope of the present invention, so the strength or yield ratio is insufficient. Nos. 12 to 15 had chemical components outside the scope of the present invention, so that sufficient strength was not obtained, the yield ratio was high, or HAZ toughness was inferior.

  Steel of chemical composition shown in Table 3 (steel types A2 to I2) is made into a slab by a continuous casting method, and a welded steel pipe (No. 21 to 34) having a plate thickness of 18, 26 mm and an outer diameter of 24, 48 inches is manufactured using this. did.

  After the heated slab is rolled by hot rolling, it is immediately cooled using a water-cooled accelerated cooling facility, reheated using an induction heating furnace or a gas combustion furnace, and a steel plate is produced. A welded steel pipe was produced by the process, and then the outer surface of the steel pipe was coated. The induction furnace was installed on the same line as the accelerated cooling equipment. Table 4 shows the production conditions of each steel pipe (No. 21 to 34). Note that the heating temperature, rolling end temperature, cooling stop (end) temperature, reheating temperature, and other temperatures were average temperatures of the steel sheet and were determined in the same manner as in Example 1.

  The tensile properties, base metal toughness, and weld heat affected zone (HAZ) toughness of the steel pipe produced as described above were measured in the same manner as in Example 1. The measurement results are also shown in Table 4.

  In Table 4, all of Nos. 21 to 26, which are examples of the present invention, have chemical components and production methods within the scope of the present invention, have a high tensile strength of 580 MPa or more, and a yield ratio before coating treatment of 80%. Below, even after the coating treatment, the yield ratio was a low yield ratio of 85% or less, the strain aging resistance was excellent, and the toughness of the base material and the weld heat affected zone was good at 100 J or more. Further, as a result of analysis by transmission electron microscope observation and energy dispersive X-ray spectroscopy, a fine composite carbide having a particle diameter of less than 10 nm and containing two or more selected from Ti, Nb, and V in the ferrite phase. Dispersion precipitation was observed.

  In Nos. 27 to 30, the chemical components were within the scope of the present invention, but the manufacturing method was outside the scope of the present invention, so the strength and yield ratio were insufficient. Nos. 31 to 34 had chemical components outside the scope of the present invention, so that sufficient strength was not obtained, yield ratio was high, or HAZ toughness was inferior.

Schematic which shows an example of the manufacturing line for enforcing the manufacturing method of this invention. The schematic of the cross section of the seam weld part of a steel pipe which shows the sampling position of a test piece.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Rolling line 2 Steel plate 3 Hot rolling mill 4 Accelerated cooling device 5 Induction heating device 6 Hot leveler 7 Steel plate 8 Weld metal 9 Notch 10 Test piece 11 HAZ

Claims (5)

  In mass%, C: 0.03-0.1%, Si: 0.01-0.5%, Mn: 0.5-2.5%, Al: 0.08% or less, Mo: 0.05 -0.5%, Ti: 0.005-0.04%, the balance is substantially made of Fe, C / (Mo is the ratio of the amount of C in atomic% and the total amount of Mo and Ti + Ti) is 0.5 to 3, the metal structure is substantially a two-phase structure of ferrite and bainite, and fine precipitates containing Ti and Mo are dispersed and precipitated in the bainite phase. A low yield ratio, high strength, high toughness steel pipe with excellent strain aging characteristics.   Furthermore, it contains Nb: 0.005 to 0.07% and / or V: 0.005 to 0.1% by mass%, and the total amount of C, Mo, Ti, Nb, and V in atomic% The ratio of C / (Mo + Ti + Nb + V) is 0.5 to 3, and fine precipitates containing Ti, Mo, Nb and / or V are dispersed and precipitated in the bainite phase. The low yield ratio high strength high toughness steel pipe excellent in strain aging resistance according to claim 1.   In mass%, C: 0.03 to 0.1%, Si: 0.01 to 0.5%, Mn: 0.5 to 2.5%, Al: 0.08% or less, Ti: Contains at least two selected from 0.005 to 0.04%, Nb: 0.005 to 0.07%, V: 0.005 to 0.1%, with the balance being substantially Fe C / (Ti + Nb + V), which is a ratio of the amount of C in atomic% and the total amount of Ti, Nb, and V, is 0.5 to 3, and the metal structure is substantially a two-phase structure of ferrite and bainite. There is a low yield ratio high strength high toughness steel pipe excellent in strain aging resistance, characterized in that a precipitate containing two or more selected from Ti, Nb, and V is dispersed in the bainite phase. .   Furthermore, in mass%, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, B: 0.005% or less, Ca: 0.0005 to 0.003% The low yield ratio high strength high toughness steel pipe according to any one of claims 1 to 3, characterized in that it contains one or more selected from the group consisting of:   The steel having the component composition according to any one of claims 1 to 4 is heated to a temperature of 1000 to 1300 ° C and hot-rolled at a rolling end temperature lower than an Ar3 temperature, and then at least 5 ° C / s. Accelerated cooling to 300 to 600 ° C. is performed at a cooling rate, and then immediately reheated to 550 to 750 ° C. at a heating rate of 0.5 ° C./s or more, and the metal structure is substantially a two-phase structure of ferrite and bainite. The steel sheet is cooled as a steel sheet in which fine precipitates containing Ti and Mo or fine precipitates containing two or more selected from Ti, Nb, and V are dispersed and precipitated in the bainite phase. A method for producing a low yield ratio, high strength, high toughness steel pipe excellent in strain aging characteristics, characterized in that a steel pipe is formed by forming a tube in between and welding a butt portion.

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