KR101327643B1 - STEEL SHEET FOR LOW YIELD RATIO THICK STEEL PIPE WITH 780 MPa OR ABOVE TENSILE STRENGTH, METHOD FOR PRODUCING THE SAME, AND LOW YIELD RATIO THICK STEEL PIPE WITH 780 MPa OR ABOVE TENSILE STRENGTH - Google Patents

Abstract

The present invention achieves both high strength of at least 780 MPa and resistance of 90% or less at the time of strong bending such as D / t = 10 to 20 at thick plates having a maximum plate thickness of 80 mm, and good toughness even after steel pipe processing. We propose a steel sheet for a round steel pipe that can be achieved by.
The steel sheet for round steel pipe of the present invention satisfies a predetermined chemical composition, has a hardenability index DI defined by a given relational expression of 8 inches or more, and satisfies the following requirements (A), (B) and (C):
(A) in the microstructure at the quarter thickness of the plate, the bainite is 90 area% or more,
(B) In the microstructure in the plate thickness 1/4 site | part, the average circle equivalent diameter of the area | region enclosed by the diagonal grain boundary whose orientation difference is 15 degrees or more is 4 micrometers or less,
(C) Microstructure in the plate | board thickness 1/4 site | part WHEREIN: 3-10 area% of island-like martensite whose average circular equivalent diameter is 0.5-3 micrometers, and Vickers hardness Hv are 700 or more.

Description

SHEET FOR LOW YIELD RATIO THICK STEEL PIPE WITH 780 MPa OR ABOVE TENSILE STRENGTH, METHOD FOR PRODUCING THE SAME, AND LOW YIELD RATIO THICK STEEL PIPE WITH 780 MPa OR ABOVE TENSILE STRENGTH}

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a steel sheet for thick circular steel pipes, which are mainly used in building structures and the like, and have high strength and high toughness even in strong bending, and a resistance to high strength thick circular steel pipe using such steel sheets. The yield ratio is 90% even in the strong bending process where D / t (D: steel pipe diameter, t: steel plate thickness) is 10 to 20, especially in the thickness of 80mm or more with a tensile strength of 780 MPa or more. Hereinafter, the impact resistance (waveform transition temperature) in a round steel pipe, a tensile strength thickened circular steel pipe with a tensile strength of 780 MPa or more with a vTrs of -20 ° C. or less, and a steel sheet for obtaining such a circular steel pipe, and a useful method for manufacturing such steel sheet Etc.

In recent years, with the increase in size and the large span of building structures, thickening and high strength of steel materials used therein have been advanced. In addition, the steel used has a ratio of yield stress YS to tensile strength TS (YS / TS) under the idea of absorbing seismic energy by plastic deformation after elastic deformation from the viewpoint of securing safety against earthquakes of building structures. It is required to lower the displayed yield ratio YR, and the upper limit is prescribed.

On the other hand, in recent years, the use of the steel pipe column which has a circular cross section is expanded from the viewpoint of improving the design freedom of a building structure, the freedom of structural design, such as beam installation.

The circular steel pipes as described above are manufactured by cold forming a centrifugal casting method or a thick steel sheet, and the circular steel pipes used in a building structure are often manufactured in the latter. In the method of cold forming a thick steel sheet by a press bending method or the like, in general, the sheet thickness of a raw material (steel sheet) is t and the outer diameter of the steel pipe is D as an index of the amount of deformation received by the steel sheet during molding. The light weight ratio, expressed as D / t, is used. As the value of this D / t decreases, the amount of work hardening increases, the yield ratio YR increases, and the toughness also deteriorates, resulting in a drop in vibration resistance. In particular, the increase in yield ratio YR becomes more remarkable on the outer surface side of the steel pipe than in the portion of the plate thickness 1/2 at which the tensile stress acts. Therefore, in order to obtain a steel pipe with a resistive ratio, it is necessary to considerably reduce the yield ratio YR of the raw material steel sheet before molding in consideration of the amount of work hardening by cold forming.

As a technique regarding the steel pipe as described above, various techniques have been proposed until now. For example, Patent Documents 1 and 2 disclose a steel sheet for a resistive high strength steel pipe, a method for producing the same, and a technique relating to a resistive high strength steel pipe. In these techniques, the steel sheet base material has a yield stress YS of 650 MPa or more and a yield ratio YR of 90% or less by making the raw material steel sheet a mixed structure of bainite and hard island martensite. However, there is a problem that a special facility (for example, an induction heating facility, etc.) for heating after the accelerated cooling is required online.

In addition, Patent Literature 3 proposes a technique for a 780 MPa class low-wear ratio circular steel pipe for a building structure excellent in shock resistance and a manufacturing method thereof. In this technique, the microstructure of the steel pipe is defined as bainitic ferrite phase: 80 area% or more, and martensite phase: 5 area% or less, and the hardness of the central portion is defined from the front and back surfaces of the steel pipe.

This technique is to reheat after online cooling or to perform a two-phase reverse quenching treatment to produce a soft phase and a hard phase, and to provide a resistive ratio characteristic even after steel pipe processing, but the hardness of the hard phase is not large with respect to the mother phase. Therefore, it was not possible to stably satisfy the resistivity ratio property (YR ≦ 90%) on the outer surface side of the steel pipe where tensile stress acted upon, in the case of strong bending such as D / t = 10 at a thickness such as 80 mm.

On the other hand, the following technique is also proposed about the resistance ratio reduction of the steel plate which is effective in obtaining a steel tube of resistance ratio after cold forming. For example, Patent Literature 4 discloses a technique relating to a resistive ratio high tensile strength steel sheet having a low acoustic anisotropy and excellent weldability and a method of manufacturing the same. This technique obtains a resistance ratio by utilizing MA having a predetermined hardness ratio with respect to bainitic ferrite, but at the level of the obtained base material toughness of vE- ₅₀ , after deterioration of toughness due to steel pipe processing, It is difficult to stably satisfy good toughness.

In addition, Patent Documents 5 and 6 propose a 780 N / mm ² or more class of high-resistance high tensile strength tensile steel having excellent uniform elongation and toughness and a method of manufacturing the same. These techniques produce a hard phase by air-cooling (bi-phase annealing) after two-phase induction heating, and exhibit a resistance ratio by the composite structure of a soft phase and a hard phase. However, in these techniques, precipitation strengthening elements such as Nb and V are essential, and since the soft phase is strengthened by the precipitation strengthening element, the difference in hardness between the soft phase and the hard phase decreases, and at a thick thickness such as a maximum plate thickness: 80 mm. In the strong bending process such as D / t = 10 to 20, it is difficult to stably obtain the resistance ratio in the steel pipe.

Japanese Patent Publication No. 2009-161811 Japanese Patent Publication No. 2009-161812 Japanese Patent Publication No. 2009-256780 Japanese Patent Publication No. 2006-89789 Japanese Patent Application Laid-Open No. 05-112843 Japanese Patent Application Laid-Open No. 05-112824

The present invention has been made under such circumstances, and its object is to provide a strong bending such as D / t = 10-20 for thick steel plates with a maximum plate thickness of 80 mm or more, especially for round steel pipes with a tensile strength of 780 MPa or more, which is the highest strength in building steel applications. In processing, it is possible to achieve a high strength of 780 MPa or more and a resistance ratio of 90% or less, and to achieve a good toughness even after steel pipe processing. The present invention proposes a cold-molded thick-walled round steel pipe.

The steel sheet for resistance-to-heap thick circular steel pipes according to the present invention, which was able to solve the above problems, C: 0.02 to 0.15% (the meaning of "mass%", the same as for the chemical composition), Si: 0.10 to 0.40%, Mn : 1.5 to 2.5%, P: 0.012% or less (0% not included), S: 0.005% or less (0% not included), Ti: 0.005 to 0.02%, N: 0.002 to 0.006%, and Al : Ni: 2.5% or less (not including 0%), Cr: 2.0% or less (not including 0%), and Mo: 0.5% or less (not including 0%) in addition to satisfying 0.02 to 0.08% One or two or more selected from the group consisting of: a balance consisting of iron and unavoidable impurities, and the hardenability index DI defined by the following formula (1) is not less than 8 inches (inch), It satisfies the requirements of (A), (B) and (C):

DI (inch) = {1.16 × ([C] / 10) ^1/2 } × (0.7 × [Si] +1) × {5.1 × ([Mn] −1.2) +5} × (0.35 × [Cu] +1) × (0.36 × [Ni] +1) × (2.16 × [Cr] +1) × (3 × [Mo] +1) × (1.75 × [V] +1) × (200 × [B] +1)… (One)

However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are C, Si, Mn, Cu, Ni, Cr, respectively. The content (mass%) of Mo, V, and B is shown.

(A) in the microstructure at the quarter thickness of the plate, the bainite is 90 area% or more,

(B) In the microstructure at the plate thickness 1/4 site | part, the average circle equivalent diameter d of the area | region enclosed by the diagonal grain boundary whose orientation difference is 15 degrees or more is 4 micrometers or less,

(C) Microstructure in the plate | board thickness 1/4 site | part WHEREIN: 3-10 area% of island-like martensite whose average circular equivalent diameter is 0.5-3 micrometers, and Vickers hardness Hv are 700 or more.

According to the present invention, (a) Cu: 1.0% or less (0%) is not included in the steel sheet for thick-walled circular steel pipes with a resistivity ratio of 780 MPa or more (hereinafter sometimes referred to simply as "steel plate for circular steel pipes") of the present invention. And / or B: 0.0025% or less (does not contain 0%), (b) Ca: 0.0050% or less (does not contain 0%), etc., and is also effective. The properties of the steel sheet for round steel pipes are further improved.

In manufacturing the steel sheet for a circular steel pipe as described above, the steel piece composed of the chemical composition is heated to 950-1200 ° C., and the plate thickness is 1/4 at the thickness position of t / 4 when the plate thickness is t. 40% or more of hot rolling was carried out at a cumulative reduction ratio in the temperature range where the temperature becomes the austenite microrecrystallization temperature, and cooled to 350 ° C or less at an average cooling rate of 3 to 25 ° C / sec from a temperature of Ar ₃ transformation point or more. Thereafter, the substrate may be reheated and calcined to a temperature range of (Ac ₁ transformation point + 30 ° C) to (Ac ₃ transformation point -20 ° C), followed by tempering at 450 to 550 ° C.

By forming the circular steel pipe by the press bending method using the steel sheet for circular steel pipes as described above, it is possible to realize a resistance-to-heap thick circular steel pipe with a tensile strength of 780 MPa or more that exhibits good characteristics. In addition, the round steel pipe of this invention includes what was further subjected to the stress relief annealing at 400-500 degreeC after shape | molding into a round steel pipe.

According to the present invention, the chemical composition of the steel sheet (steel sheet constituting the steel pipe) is appropriately adjusted, and the fraction of bainite and island martensite (MA) in the microstructure is appropriately controlled and is also diagonal. By appropriately controlling the size of the area enclosed by the grain boundary and the size of the MA, it is possible to achieve both high strength and resistance ratio of 780 MPa or more, and to suppress excellent toughness due to bending during steel pipe forming, thereby ensuring excellent toughness. A steel sheet for circular steel pipe can be realized, and by using the steel sheet for circular steel pipe, it can be formed into a round steel pipe by a press-bending method, thereby realizing a low-strength-weight thick round steel pipe with a tensile strength of 780 MPa or more that exhibits good characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS It is a graph which shows the relationship between the average circle equivalent diameter d of a region enclosed by diagonal grain boundaries, and steel pipe toughness vTrs.
2 is a graph showing the relationship between MA hardness and yield ratio YR of steel pipes.
3 is a graph showing the relationship between the bainite fraction and the tensile strength TS of the steel pipe.
4 is a graph showing the relationship between the hardenability index DI and the bainite fraction.
5 is a graph showing the relationship between the MA fraction and the steel pipe toughness vTrs.
6 is a graph showing the relationship between the MA fraction and the yield ratio YR of steel pipes.
7 is a graph showing the relationship between the MA mean circle equivalent diameter and steel pipe toughness vTrs.
8 is a graph showing the relationship between the MA average circle equivalent diameter and the yield ratio YR of steel pipes.

MEANS TO SOLVE THE PROBLEM In order to solve the said subject, the strength (tensile strength) and yield ratio in the steel pipe manufactured by cold-forming of strong bending processing, such as D / t = 10-20, especially in the thickness of 80 mm maximum plate | board thickness, And various factors affecting toughness. In the prior art, in order to obtain a resistance ratio, the soft phase and the hard phase have a double phase structure, but there is a limit in the resistance ratio increase during the strong bending process. Therefore, in order to maintain the high strength and maintain the resistance ratio, the present inventor has focused on making the hard phase further harder, and has carried out further studies.

As a result, in steel pipes produced by cold forming with strong bending, such as D / t = 10-20, in a thick plate having a thickness of 30 to 80 mm, high strength and high toughness, and furthermore, a resistance ratio (YR ≦ 90%). In order to achieve stably), after the chemical composition of the steel sheet is appropriately adjusted, the structure of the steel sheet is mainly composed of bainite tissue, and the structure of the steel sheet is fined to secure toughness and strength. We have found that it is important to properly control the MA fraction.

In addition, in order to obtain the steel plate which has the said structure, when hot-rolling the steel material which controlled the chemical composition of the composition in the appropriate range, after performing predetermined | prescribed pressure reduction in a suitable temperature range, the cooling process which controlled the cooling rate and cooling stop temperature is performed. Then, it was further discovered that it is important to further heat to a predetermined two-phase temperature range and to perform a tempering treatment after air cooling. The present invention has been made by further examining the above findings.

Although the steel sheet for round steel pipes of this invention needs to adjust suitably the chemical composition, the reason for the range limitation of a basic component (C, Si, Mn, Ti, N, Al, Ni, Cr, Mo) is as follows.

[C: 0.02-0.15%]

Although C has the effect of increasing the strength of the steel sheet, it is also an element that degrades weldability such as crack resistance. When the C content is less than 0.02%, it becomes difficult to secure the necessary base metal (steel pipe) strength, and the amount of MA and the hardness that are generated at the time of two-phase reversal are reduced, thereby reducing the yield ratio reduction effect. However, when the C content exceeds 0.15%, island martensite MA is excessively generated in the welded portion, the weld heat affected zone HAZ becomes excessively hard, and cracks are likely to occur, resulting in a breakdown point during an earthquake. On the other hand, the minimum with preferable C content is 0.03% or more (more preferably 0.05% or more), and a preferable upper limit is 0.12% or less (more preferably 0.10% or less).

[Si: 0.10 to 0.40%]

Si is an effective element for improving the strength. In order to exhibit such a strengthening mechanism, it is necessary to contain Si 0.10% or more. However, when Si content becomes excess, since base material toughness, HAZ toughness (toughness of a weld heat influence part), and weldability will deteriorate, you may be 0.40% or less. On the other hand, the minimum with preferable Si content is 0.15% or more (more preferably 0.20% or more), and a preferable upper limit is 0.35% or less (more preferably 0.30% or less).

[Mn: 1.5 to 2.5%]

Mn is an effective element in improving hardenability and securing the strength and toughness of a steel plate (that is, steel pipe). In order to exert such an effect, it is necessary to contain Mn 1.5% or more. However, when Mn is excessively contained, the toughness deteriorates, so the upper limit is made 2.5%. On the other hand, the minimum with preferable Mn content is 1.6% or more (more preferably 1.8% or more), and a preferable upper limit is 2.4% or less (more preferably 2.2% or less).

[Ti: 0.005% to 0.02%]

Ti is an element which forms N and nitride (TiN), prevents coarsening of austenite grains (γ grains) during heating before hot rolling, and is effective in improving toughness. It is also effective to secure the hardenability of B by fixing N. In order to exhibit these effects, it is necessary to contain Ti 0.005% or more. However, when Ti content becomes excess, since TiN coarsens and a base material toughness deteriorates, it is necessary to be 0.02% or less. On the other hand, the minimum with preferable Ti content is 0.008% or more (more preferably 0.010% or more), and a preferable upper limit is 0.018% or less (more preferably 0.015% or less).

[N: 0.002-0.006%]

N is an element effective in producing TiN, preventing coarsening of γ grains during heating and welding before hot rolling, and improving base metal toughness and HAZ toughness. If the content of N is less than 0.002%, TiN is insufficient, the heating? Grains are coarsened, and the toughness deteriorates. Therefore, it is necessary to contain 0.002% or more. Moreover, when N content becomes excess and exceeds 0.006%, the toughness of a base material (steel pipe) will deteriorate by the brittleness by bending work. On the other hand, the minimum with preferable N content is 0.0025% or more (more preferably 0.003% or more), and a preferable upper limit is 0.0055% or less (more preferably 0.005% or less).

[Al: 0.02-0.08%]

Al is an element necessary for securing the hardenability of B by deoxidation and free nitrogen fixation. In order to exhibit these effects, it is necessary to contain 0.02% or more. However, when excessively contained, a coarse inclusion of alumina is formed and the toughness of the base metal (steel pipe) is lowered. Therefore, it is necessary to make it 0.08% or less. On the other hand, the minimum with preferable Al content is 0.03% or more (more preferably 0.04% or more), and a preferable upper limit is 0.07% or less (more preferably 0.06% or less).

[Ni: 2.5% or less (0% not included), Cr: 2.0% or less (0% not included) and Mo: 0.5% or less (0% not included) Or 2 or more types]

In order to make the hardenability index DI defined by said Formula (1) into 8 inches or more, it is necessary to contain 1 type (s) or 2 or more types chosen from the group which consists of Ni, Cr, and Mo in addition to C, Si, and Mn. . The reason for each limitation is as follows.

[Ni: 2.5% or less (0% not included)]

Ni is an element that is effective in preventing Cu cracks and weld cracks, as well as improving the strength of the base metal and the HAZ toughness and the hardenability. However, when the Ni content is excessive, the solder crack resistance deteriorates and scale flaws tend to occur at the time of rolling. Therefore, the Ni content needs to be 2.5% or less. On the other hand, the upper limit with preferable Ni content is 2.35% or less (more preferably 2.3% or less). Moreover, the minimum with preferable Ni content for exhibiting the said effect is 0.20% or more (more preferably 0.50% or more).

[Cr: 2.0% or less (0% not included)]

Cr is an effective element for improving hardenability and improving strength. However, when Cr content is excessive, the content of crack resistance deteriorates, and therefore it is necessary to make it 2.0% or less. On the other hand, the upper limit with preferable Cr content is 1.85% or less (more preferably 1.5% or less). Moreover, the minimum with preferable Cr content for exhibiting the said effect is 0.5% or more (more preferably 0.8% or more).

[Mo: 0.5% or less (does not include 0%)]

Mo is an element that improves strength by increasing hardenability and is easy to produce carbide. However, when Mo content is excessive, the hardenability becomes excessive and the content of crack resistance deteriorates, so it is necessary to make it 0.5% or less. have. On the other hand, the upper limit with preferable Mo content is 0.45% or less (more preferably 0.4% or less). In addition, the minimum with preferable Mo content for exhibiting the said effect is 0.1% or more (more preferably 0.2% or more).

In the steel sheet for circular steel pipe of the present invention, in addition to the above basic components, it is made of iron and unavoidable impurities (for example, P, S, etc.), but also includes trace components (allowable components) inevitably mixed in a solvent. It is possible (for example, Zr, H, etc.), such a round steel pipe is also included in the scope of the present invention. However, about P, S, etc. as an unavoidable impurity, it is necessary to suppress in the following ranges respectively from the following viewpoint.

[P: 0.012% or less (does not include 0%)]

P, which is an unavoidable impurity, adversely affects the toughness of the base material (steel pipe) and the welded part, and in order not to cause such a problem, the content thereof needs to be suppressed to 0.012% or less, and preferably 0.010% or less.

[S: 0.005% or less (not including 0%)]

Since S forms MnS and degrades weld cracking, it is preferable to use as few as possible. From this viewpoint, S content needs to be suppressed to 0.005% or less, Preferably it is good to set it as 0.003% or less.

In the steel sheet for circular steel pipe of the present invention, if necessary, (a) Cu: 1.0% or less (does not contain 0%) and / or B: 0.0025% or less (does not contain 0%), (b) Ca: It is also effective to contain 0.0050% or less (not containing 0%) and the like, and the characteristics of the steel sheet for circular steel pipes (and circular steel pipes) are further improved depending on the kind of elements to be contained. The reason for limiting the range of these components is as follows.

[Cu: 1.0% or less (not including 0%)]

Cu is an element useful for improving the strength of the base metal (steel pipe) by solid solution strengthening. However, when the Cu content is excessive, Cu cracks may occur during gas cutting, so it is preferably 1.0% or less. On the other hand, the minimum with preferable Cu content for demonstrating the said effect is 0.1% or more (more preferably 0.2% or more), and a more preferable upper limit is 0.80% or less.

[B: 0.0025% or less (not including 0%)]

Free B exists in a gamma grain boundary, and is an effective element in improving hardenability and improving base material strength. However, when the B content is excessive, inclusions are generated and the base metal toughness deteriorates. Therefore, the content is preferably 0.0025% or less. On the other hand, the minimum with preferable B content for exhibiting the said effect is 0.0003% or more (more preferably 0.0008% or more), and a preferable upper limit is 0.002% or less.

[Ca: 0.0050% or less (not including 0%)]

Ca is an effective element for detoxifying the crack cracking resistance by spheroidization of MnS. However, when Ca content becomes excess exceeding 0.0050%, coarsening will be coarsened and the toughness of a base material (steel pipe) will be degraded. On the other hand, the minimum with preferable Ca content for exhibiting the said effect is 0.0005% or more (more preferably 0.0015% or more), and a preferable upper limit is 0.0040% or less (more preferably 0.0030% or less).

[DI: 8 (inch) or more]

The hardenability index DI prescribed | regulated by said Formula (1) is an index | index (corresponding to an ideal critical diameter) which shows hardenability of steel, and needs to be 8 inch or more in this invention. If it is less than 8 inches, polygonal ferrite will be produced at the time of air cooling (sintering) after the two-phase heating in the steel plate for circular steel pipe of this invention, and it will become a bainite main structure, and it will become difficult to ensure strength. The minimum with preferable DI is 9 inches or more (more preferably 9.5 inches or more). In addition, in said Formula (1), besides what is prescribed | regulated as a basic component of the steel plate of this invention (C, Si, Mn, Ni, Cr, Mo), the element (for example, Cu, B) contained as needed, Although the element (for example, V) which does not contain fundamentally in the steel plate of this invention is included, when it does not contain these elements, DI is computed as there is no item, and when it contains these elements, it is represented by said formula (1). Calculate DI based on this.

In the steel plate for circular steel pipe of this invention, it is necessary to make the structure in the plate | board thickness 1/4 site | part into a bainite principal structure. The bainite main body structure here means that the fraction of bainite is 90 area% or more. The term "bainite" in the present invention includes bainitic ferrite and granular bainitic ferrite, and does not include polygonal ferrite. When the fraction of bainite is less than 90 area% and the number of structures other than bainite (for example, polygonal ferrite) increases, strength of 780 MPa or more even if other requirements (tissue size, size of MA, MA hardness, and MA fraction) are satisfied. It becomes difficult to secure. On the other hand, the fraction of bainite is preferably 95 area% or more, and more preferably 97 area% or more.

In the steel sheet for circular steel pipe of the present invention, it is also important that the average circle equivalent diameter d of the region surrounded by the diagonal grain boundary whose orientation difference is 15 ° or more is 4 µm or less. Here, the average circle equivalent diameter d means the average value of the diameter (circle equivalent diameter) when it converts to the circle of the same area in the area | region enclosed by the diagonal grain boundary of two adjacent crystal grains of 15 degrees or more. In addition, said "azimuth difference" is also called a "deviation angle" or an "inclined angle", and can measure by employing the EBSP method (Electron Backscattering Pattern method). When the average circle equivalent diameter d of the area enclosed by the diagonal grain boundary with the azimuth difference prescribed | regulated as mentioned above becomes larger than 4 micrometers, ie, the base bainite structure becomes coarse, the base material in a steel pipe is deteriorated by the toughness at the time of steel pipe processing. It is difficult to secure toughness. The average circle equivalent diameter d is preferably 3.5 µm or less, and more preferably 3 µm or less.

In the steel sheet for circular steel pipe of the present invention, in order to achieve high strength, high toughness, and a resistance ratio after steel pipe processing, the bainite main body structure has an average circular equivalent diameter of 0.5 to 3 µm and Vickers hardness Hv of 700 or more island-like martens It is necessary to include 3% to 10 area% of the site MA. When the average circle equivalent diameter of MA is less than 0.5 µm, the effect of lowering the yield ratio of hard MA is small, and the resistance yield ratio after steel pipe processing cannot be achieved. Moreover, when the average round equivalent diameter of MA exceeds 3 micrometers, the effect of reducing yield ratio becomes large, but it becomes easy to become a starting point of a crack, and the base metal toughness after steel pipe processing deteriorates. On the other hand, the minimum with preferable average equivalent circle diameter of MA is 1 micrometer or more, and a preferable upper limit is 2 micrometers or less.

When Vickers hardness Hv of MA is less than 700, the yield ratio reduction effect after steel pipe processing will become small, and the resistance yield ratio after steel pipe processing cannot be achieved. Preferably it is 800 or more (more preferably 850 or more).

Moreover, when the fraction of MA is less than 3 area%, the yield-reduction ratio reduction effect by hard MA is small, and it becomes impossible to achieve the resistance yield ratio (YR <= 90%) after steel pipe processing. However, when the fraction of MA exceeds 10 area%, the effect of reducing the yield ratio is increased, but it is likely to be a starting point of cracking, and the base metal toughness after steel pipe processing is deteriorated. On the other hand, the minimum with preferable fraction of MA is 5 area% or more, and a preferable upper limit is 8 area% or less.

In order to manufacture the steel sheet for circular steel pipe of this invention, after heating the slab which consists of said chemical composition to 950-1200 degreeC, the temperature which becomes an austenite uncrystallized temperature in a 1/4 plate | board thickness Inversely, hot rolling is performed at a cumulative reduction ratio of 40% or more, and after cooling to an average cooling rate of 3 to 25 ° C / sec (plate thickness direction average cooling rate) from a temperature of Ar ₃ transformation point or more, to 350 ° C or less, (Ac ₁ transformation point) It may be reheated to an temperature of + 30 ° C) to (Ac ₃ transformation point-20 ° C), followed by annealing, and then tempering at 450 to 550 ° C. Then, the obtained steel sheet may be molded into a round steel pipe by a press bending method. The reasons for defining the conditions of each process are as follows.

[The casting is heated to 950-1200 degreeC]

The heating temperature of the cast steel greatly influences the structure control before hot rolling. If heating temperature is less than 950 degreeC, rolling final pass (finishing rolling) temperature will be less than 750 degreeC, ferrite will precipitate from the surface before water cooling, and it becomes difficult to ensure the base material strength of 780 Mpa or more. On the other hand, when heating temperature exceeds 1200 degreeC, base material toughness will deteriorate by coarsening of (gamma) grain diameter.

[40% or more hot rolling is carried out at a cumulative reduction ratio in a temperature range where the temperature at a quarter thickness of the plate becomes an austenite uncrystallized temperature.

In anticipation of toughness deterioration at the time of steel pipe processing, it is necessary to make the steel plate for steel pipes high toughness. For that purpose, it is necessary to apply 40% or more reduction in cumulative reduction ratio in the austenite unrecrystallized temperature range. Thereby, by the deformation | accumulation accumulated during hot rolling in an austenite uncrystallized temperature range, and the lower bainitization by cooling after rolling mentioned later, the structure unit of the bainite in a steel plate for steel pipes is made fine, and an orientation difference The average circle equivalent diameter d of the area | region enclosed by the diagonal grain boundary of 15 degrees or more can be 4 micrometers or less, and high toughness can be obtained even after steel pipe processing. If the cumulative reduction ratio is less than 40%, the above effect is less, and the toughness in the steel pipe is deteriorated due to the deterioration in toughness during steel pipe processing. The cumulative reduction ratio is preferably 45% or more. On the other hand, it is necessary to control the structure at that position because the tensile test and the impact test are often performed at this position because the temperature at the plate thickness 1/4 portion is controlled. The rolling temperature can be controlled by managing at the calculated temperature. In addition, the plate of the cumulative rolling reduction is, austenite slide the plate thickness before rolling h ₀ in the recrystallization temperature range, after the final rolling in the austenite non-recrystallized temperature range of where (in the case of performing the two-step or more reduction in the final step) When the thickness is h ₁ , it means a reduction ratio expressed by (h ₀ -h ₁ ) / h ₀ .

[Cooling at an average cooling rate of 3 to 25 ° C / sec from a temperature above the Ar ₃ transformation point]

The cooling process (acceleration cooling process) after rolling is an important process for structure control. If the cooling rate is less than 3 ° C / sec, the structure becomes rough upper bainite, and the average circle equivalent diameter d of the region surrounded by the diagonal grain boundary having an orientation difference of 15 ° or more cannot satisfy 4 µm or less, resulting in deterioration of toughness. . The faster the cooling rate at this time, the finer the bainitic ferrite structure, the higher the toughness.

[Cooling stop temperature: surface temperature of steel sheet is 350 ° C. or less]

By the cooling stop temperature, the presence form of the lower bainite changes and the tissue size of the bainite changes. When the cooling stop temperature exceeds 350 ° C, the low temperature transformation structure decreases, coarse upper bainite is mixed, and the toughness deteriorates. In order to transform uniformly, cooling stop temperature needs to be 350 degrees C or less.

[Re-heated to a temperature of (Ac ₁ transformation point + 30 ° C) to (Ac ₃ transformation point -20 ° C)

In order to obtain a soft phase and a hard phase composite structure that realizes a resistance-ratio ratio (YR ≦ 90%), heating to a two-phase inverse temperature between the Ac ₁ transformation point and the Ac ₃ transformation point is an effective means. By heating to the temperature of a biphase, a part becomes a soft structure by tempering, a part becomes reverse transformation into an austenite phase, and becomes a hard structure by subsequent cooling. By controlling this two-phase temperature, the yield stress YS, tensile strength TS, and yield ratio YR can be controlled by changing the fraction and hardness of the hard phase. In addition, by slowing down the cooling rate after the two-phase heating (air cooling) (sintering), C can be diffused during cooling, local concentration can be promoted, and extremely hard MA can be produced in the reverse transformation part. When the reheating temperature is lower than (Ac ₁ transformation point + 30 ° C), the reverse transformation fraction is low, so that the reverse transformation portion is concentrated and the component becomes easy to become a very hard MA. In addition to decreasing the yield ratio reducing effect, strength over 780 MPa cannot be secured. On the other hand, when the reheating temperature exceeds (Ac ₃ transformation point -20 ° C), the reverse transformation fraction increases and the strength is high, but the fraction of MA generated from the reverse transformation portion also rises and the toughness deteriorates. In addition, as the reverse transformation fraction increases, the thickening of the reverse transformation portion decreases, the hardness of MA also decreases, and the yield ratio reduction effect decreases.

[Tempering in the temperature range of 450 ~ 550 ℃]

The tempering treatment lowers the strength, but is effective for improving the toughness by adjusting the amount of the MA produced by the two-phase banding. In that case, if tempering heat processing is a temperature range of 450-550 degreeC, appropriate yield ratio YR and toughness can be obtained. If the tempering temperature is less than 450 ° C., MA is present in large amounts, and the toughness improvement is not sufficient. On the other hand, when the tempering temperature exceeds 550 ° C, the hardness of MA decreases, so that a desired resistance ratio cannot be obtained. On the other hand, both the heat treatment and the tempering treatment in the two-phase region can be controlled by controlling the temperature of the plate thickness 1/4 portion calculated by the process computer by the method described later.

[Formed into round steel pipe by press bending method]

The steel sheet is cold bent by the press bending method to form a round steel pipe. Plate thickness as applied to line pipe: If the steel plate is less than 30mm, round steel pipe can be manufactured by UOE forming method (Uing-Oing press-expander method), but in circular steel pipe for building structure, the plate thickness is thick and strong Since is high, it is necessary to shape | mold to round steel pipe by the press bending method (namely, press-bending process). In the application of this method, since the strong processing of D / t: 10-20 is performed, the yield ratio YR increases and the toughness deteriorates large at the time of bending. Therefore, by carrying out press bending by using the steel sheet manufactured as described above, a circular steel pipe having a low yield ratio YR and excellent toughness can be produced.

[Heat treatment of round steel pipe]

After forming into a round steel pipe, stress relieving (sometimes referred to as "SR heat treatment") may or may not be performed. By performing SR heat treatment, the strength (tensile strength TS, yield ratio YR, toughness vTrs) in the steel pipe can be adjusted. According to the method of the present invention, since the high strength, the yield ratio YR is low, and the toughness is also good, the SR heat treatment may not be performed basically. However, when the heat treatment is performed, the heat treatment temperature is preferably in the temperature range of 400 to 500 ° C. Do. If the heat treatment temperature is less than 400 ° C, there is little influence on the strength, yield ratio and toughness. On the other hand, when it exceeds 500 degreeC, the fall of intensity will become large and it will become impossible to ensure the intensity | strength of 780 Mpa or more.

Example

Hereinafter, the configuration and effect of the present invention will be described in more detail with reference to Examples, but the present invention is not limited by the following Examples, of course. It is also possible to add and implement, and they are all included in the technical scope of this invention.

Example 1

With the solvent, the steel materials of each chemical component composition shown in following Tables 1 and 2 were dissolved, and after hot-rolling to the slab obtained by continuous casting after completion of the solvent, direct quenching (DQ) was performed (some air cooling). .

In addition, the "-" column of Table 1, 2 shows that an element is not added. In Tables 1 and 2, the upper limits of Ac ₁ transformation point (Ac ₁ ), Ac ₃ transformation point (Ac ₃ ), Ar ₃ transformation point (Ar ₃ ), and austenite unrecrystallized temperature are given together. These Ac ₁ transformation points and Ac _{3 are described together.} The upper limit of the transformation point, the Ar ₃ transformation point, and the austenite uncrystallized temperature was measured by the following method, respectively.

<Measurement method of Ac ₁ transformation point (austenite transformation start temperature at heating) and Ac ₃ transformation point (austenite transformation end temperature at heating)>

In the process of heating the processed formaster test piece from room temperature to 1000 ° C. at a heating rate of 10 ° C./sec, the temperature at which the volume begins to shrink is Ac ₁ transformation point, and the temperature at which the volume continues to expand to Ac ₃ transformation point. It was.

<Measurement method of Ar ₃ transformation point (ferrite transformation start temperature at the time of cooling after rolling)>

After processing the formate test piece at 1100 ° C. and holding it for 10 seconds, the cumulative reduction ratio 25% is processed at 1000 ° C., and the cumulative reduction rate 25% is processed at 900 ° C., and then cooled at 800 ° C. Cooling at a rate of 1 ° C./sec, the temperature at which the volume starts to expand during cooling was determined as the Ar ₃ transformation point.

<Measurement Method of Upper Limit of Austenitic Uncrystallized Temperature>

After processing the formate test piece at 1100 ° C. for 10 seconds, processing 25% of the cumulative reduction ratio at 1000 ° C., and then performing multipass machining at 10% reduction rate per pass. The temperature at which the strain resistance at the second pass starts to increase with respect to the strain resistance of was taken as the upper limit of the austenite uncrystallized temperature.

Next, after heating to the austenite-ferrite two-phase region or the vicinity of this two-phase region, and air-cooling, tempering was performed and the steel plate of each plate thickness shown in following Tables 3 and 4 was obtained. The temperature, the two-phase heating temperature, and the tempering temperature in the hot rolling are the temperatures calculated at the 1/4 part thickness. These conditions are shown in Tables 3 and 4 below.

In addition, the temperature of the plate | board thickness 1/4 site | part at the time of completion | finish of rolling of Table 3, 4 is calculated | required by the method of following (a)-(f).

(a) In the process computer, the heating temperature of the arbitrary positions from the surface of the steel piece to the back surface is calculated based on the atmosphere temperature from the start of heating to extraction and the furnace stay time.

(b) A method suitable for calculating a rolling temperature at an arbitrary position in the plate thickness direction, such as a differential method, based on data of the rolling pass schedule during rolling and the cooling method (water cooling or air cooling) between the passes, using the heating temperature. Rolling is carried out while calculating using.

(c) Steel plate surface temperature is measured using the radial thermometer provided on the rolling line (however, calculation is also performed on the process computer).

(d) The steel plate surface temperatures actually measured at the start of rough rolling, at the end of finish rolling and at the start of finish rolling are compared with the calculated temperatures on the process computer.

(e) When the difference between the calculated temperature and the measured temperature is ± 30 ° C or more, the measured surface temperature is replaced with the calculated surface temperature to be the calculated temperature on the process computer.

(f) The rolling temperature of the area | region made into control object was managed using the corrected calculation temperature.

The heat treatment temperature (temperature of 1/4 plate | board thickness) in two phases (nearby) is calculated | required by the method of following (i) and (ii). In addition, the quenching temperature (temperature of 1/4 part of plate thickness at the time of quenching start) and tempering temperature (temperature of 1/4 part of plate thickness) were similarly calculated | required.

(i) In a process computer, the heating temperature of the arbitrary position of the plate thickness direction from the surface of a steel piece to the back surface is computed based on the atmospheric temperature and furnace stay time from a heating start to a heating completion.

(ii) From the calculated calculation temperature, the temperature of 1/4 sheet thickness is obtained.

For the steel sheet obtained as described above, observation of metal structure (bainite fraction), average circle equivalent diameter d, MA fraction, average circle equivalent diameter (MA size) of the area enclosed by the diagonal grain boundary having an orientation difference of 15 ° or more, The hardness of MA was measured by the following method.

<Observation of metal structure>

The bainite fraction of the steel plate for round steel pipes was measured as follows.

(a) A sample is taken from the steel sheet so that the sheet thickness cross section including the steel plate front and back surfaces parallel to the rolling direction and perpendicular to the steel sheet surface can be observed.

(b) A mirror surface finish of the observation surface is performed by polishing with wet emery abrasive papers (# 150 to # 1000) or by a polishing method (such as polishing using an abrasive such as diamond slurry) having a function equivalent thereto.

(c) The polished sample is corroded using a 3% nital solution to reveal grain boundaries of bainite structure.

(d) Photograph the extruded tissue at 400 times magnification (here, as a 6 cm x 8 cm photograph) at 1/4 plate | board thickness, and color the ferrite structure black.

(e) Next, the said photograph is input into an image analysis device (the area | region of the said photograph corresponds to 150 micrometer x 200 micrometers in 400 times). The input to an image analysis device is input so that the sum total of an area may be 1 mm x 1 mm or more in any magnification (namely, in the case of 400 times, at least 35 said photographs are input).

(f) In the image analysis device, the area ratio of black is calculated for each photograph, and the average value of all photographs is used as the ferrite fraction (polygonal ferrite fraction), and the fraction obtained by subtracting the fraction of MA to be described later is defined as the bainite fraction. .

<Measurement method of average circle equivalent diameter d by EBSP>

(a) The sample containing the front and back surface part of the plate thickness cut | disconnected in the direction parallel to a rolling direction is prepared.

(b) Mirror surface finishing of an observation surface is performed by the wet emery polishing paper of # 150- # 1000, or the polishing method (polishing etc. using abrasive | polishing agent, such as a diamond slurry) which has a function equivalent to it.

(c) Measurement area: 200 × 200 (μm), measurement pitch: 0.5 μm intervals were measured at a quarter of the plate thickness direction using an EBSP apparatus manufactured by Tex SEM Laboratories, and the boundary having a crystal orientation difference of 15 ° or more was measured. The grain size was measured as a grain boundary. At this time, the measurement point whose confidence index indicating the reliability of the measurement orientation is smaller than 0.1 was excluded from the analysis object.

(d) The average value of the diagonal grain boundary diameter calculated | required in this way was computed, and it was set as "diagonal grain boundary diameter (average circle equivalent diameter d)" in this invention. On the other hand, the diagonal grain boundary diameter of 1.0 µm or less was determined to be measurement noise and excluded from the average value calculation target of the crystal grain diameter.

<Method of measuring the fraction of MA and the average equivalent circle diameter (MA size) of MA>

The fraction of MA and the average equivalent circle diameter were measured as follows.

(a) A sample is taken from the steel sheet so that the sheet thickness cross section including the steel plate front and back surfaces parallel to the rolling direction and perpendicular to the steel sheet surface can be observed.

(b) A mirror surface finish of the observation surface is performed by polishing with wet emery abrasive papers (# 150 to # 1000) or by a polishing method (such as polishing using an abrasive such as diamond slurry) having a function equivalent thereto.

(c) The polished sample is corroded using the repera solution to reveal the MA. At this time, it is colored white on an optical microscope photograph.

(d) Photographs of the tissues exhibited at the plate thickness quarter portion at a magnification of 1000 times (in this embodiment, photographed as 6 cm x 8 cm photographs). Next, the photograph is input to an image analysis device (the area of the photograph corresponds to 60 µm x 80 µm at 1000 times). Input to an image analysis device is input so that the sum total of an area may be 0.4 mm x 0.4 mm or more (that is, in the case of 1000 times, input at least 35 said photographs).

(e) In the image analysis device, the area ratio of the MA and the average circle equivalent diameter are calculated for each picture, and the average value of all the pictures is taken as the area ratio of the MA and the average circle equivalent diameter.

<Measuring hardness of MA>

Since the MA is very fine, the hardness of the MA was measured as follows using the repera corroded sample and using the nano indentation method. As a hardness measuring device of MA, using a Nano Indenter XP / DCM manufactured by Agilent Technologies, at least 10 particles of MA were measured at a sheet thickness of 1/4 at a press-in depth of 100 nm. It converted into Vickers hardness Hv by the formula, and made the average value into the hardness of MA.

Hv = 76.2 × (nano indentation hardness) +6.3

These measurement results are shown in Tables 5 and 6 below. On the other hand, the remainder of not meeting 100 area% by the sum of the bainite fraction and MA fraction is polygonal ferrite.

Circular steel pipe was manufactured by the press bending method using the steel plate for circular steel pipes obtained as mentioned above. About the obtained round steel pipe, SR heat treatment was performed as needed, the tension test and the impact test were performed, and the structure observation was performed with the following method.

Tensile Test

No. 4 specimen of JISZ 2201 was taken in the tube axis direction from the site of the plate thickness 1/4 on the outer surface side of the steel pipe of each steel pipe, and subjected to a tensile test according to the method of JISZ 2241, yield stress (YS: 0.2% yield strength), tensile strength TS The yield ratio YR (yield stress YS / tensile strength TS) was obtained. As the acceptance criteria, the tensile strength TS was 780 MPa or more and the yield ratio YR was 90% or less.

<Charpy impact test>

The V notched test piece of JISZ 2242 was taken in the tube axis direction from the plate thickness 1/4 site of the steel pipe outer surface side of each steel pipe, and the Charpy impact test was carried out according to the JIS Z2242 method, and the brittle fracture rate (or the soft wavefront) was measured in accordance with JIS. Rate), and the brittle wavefront transition temperature vTrs at which the brittle fracture rate is 50% was calculated from the curve of (test temperature vs brittle fracture rate). And it evaluated that the brittle wavefront transition temperature vTrs is -20 degrees C or less that the impact characteristic (steel pipe toughness) was excellent.

These measurement results are shown in Tables 7 and 8 below together with the outer diameter D of the circular steel pipe, the thickness ratio (D / t), and the temperature (“-” is no SR heat treatment) when the SR heat treatment was performed on the steel pipe.

From these results, it can be considered as follows. First, 1 to 3, 6 to 10, 17, 22, 23, and 27 to 30 are steels of the present invention that satisfy all of the requirements specified in the present invention, and are compatible with high strength of 780 MPa or more and yield ratio YR of 90% or less. In addition, it can be seen that good toughness (below -20 ° C in vTrs) can be stably achieved even after steel pipe processing.

On the contrary, test No. 4, 5, 11 to 16, 18 to 21, 24 to 26, and 31 to 46 lack any of the requirements specified in the present invention, and any of the characteristics deteriorate and the object of the invention can be achieved. There was no. That is, 4 is an example in which the heating temperature in the cast stage is too high. Due to the coarse grain size, the coarsening of the average circle equivalent diameter d in the region surrounded by the diagonal grain boundary with an orientation difference of 15 ° or more results in deterioration of the toughness of the steel pipe. No. 5 is an example in which the SR heat treatment temperature is increased, and the strength of the steel pipe is decreased.

Test No. 11 to 16 are examples which deviate from the requirements of any of the steel sheet manufacturing conditions (cumulative reduction ratio, cooling start temperature, plate thickness direction average cooling rate, and cooling stop temperature) specified in the present invention. The coarsening of the average circle equivalent diameter d of the area | region enclosed by the system was brought about, and the toughness of the steel pipe deteriorated.

Test No. 18 is an example in which the two-phase heating temperature is too low. Since the inverse transformation fraction is low, the MA fraction is low, and the inverse transformation is not performed, and the fine bainite produced by the controlled rolling and controlled cooling is mainly composed of a tempered structure at a high temperature. Yield lowered and yield ratio increased. Test No. 19 is an example in which the two-phase heating temperature is too high. Since there are many reverse transformation fractions, the thickening of the components does not proceed, the MA hardness is lowered, the yield ratio of the steel pipe is increased, and the reverse transformation fraction is excessively controlled. The fine bainite produced by the control cooling disappeared, and coarse bainite became coarse, resulting in coarsening of the average circle equivalent diameter d in the region surrounded by the diagonal grain boundary having an orientation difference of 15 ° or more.

Test No. 20 is an example in which the cooling rate in the two-phase temperature range is too high. The MA fraction is low, the MA hardness is low, and the yield ratio of the steel pipe is high. Test No. 21 is an example in which the heating temperature at the time of tempering of the steel sheet is low, the MA fraction is high, and the toughness of the steel pipe is deteriorated.

Test No. 24 is an example in which the heating temperature at the time of steel plate tempering became high, MA hardness fell, and the yield ratio of the steel pipe became high. Test No. 25 is an example in which the two-phase reverse heat treatment was not performed. MA did not satisfy predetermined requirements (fraction, size, and hardness), and the yield ratio of the steel pipe was increased. Test No. 26 is an example in which the average cooling rate in the plate thickness direction is slow and the two-phase inverse heat treatment is not performed. The MA size increases, the MA hardness decreases, the yield ratio of the steel pipe increases, and the toughness deteriorates.

Test No. Although the manufacturing conditions are suitable for the thing of 31-46, the characteristic of which was deteriorated as the example which the chemical composition of the steel plate lacked the requirement prescribed | regulated by this invention. That is, 31 is an example of using a steel grade (steel grade B1) lacking the hardenability index DI, and the hardenability is lowered and the bainite fraction is lowered due to the increase in polygonal ferrite, the strength of the steel pipe is low, and the yield ratio is high.

Test No. 32 is an example of using a low-C steel grade (steel grade B2). The MA fraction was low, the strength of the steel pipe was low, and the yield ratio was high. Test No. 33 is an example of using a C-rich steel grade (steel grade B3). The MA fraction was high, and the yield ratio of the steel pipe was low, but the toughness was deteriorated.

Test No. 34 is an example using the steel grade (steel grade B4) with low Si content, hardenability fell, the bainite fraction fell by the increase of polygonal ferrite, and the strength of the steel pipe fell. Test No. 35 is an example of using a high Si content (steel grade B5), the MA fraction is high, the yield ratio of the steel pipe is low, but the toughness is deteriorated.

Test No. 36 is an example using the steel grade (steel grade B6) with a low Mn content, hardenability fell, the bainite fraction became low by the increase of polygonal ferrite, and the strength of the steel pipe fell. Test No. 37 is an example using steel grades with high Mn content (steel grade B7). MA fraction was high and the yield ratio of steel pipe was low, but toughness was deteriorated.

Test No. 38 uses the high P content steel grade (steel grade B8), and the toughness of the steel pipe deteriorated. Test No. 39 is an example using steel grades with high S content (steel grade B9), and toughness of steel pipes was deteriorated by MnS production. Test No. 40 is an example using the high Al content (steel type B10), and the toughness of steel pipe deteriorated by formation of coarse alumina inclusions.

Test No. 41 is an example of using a low Ti content (steel grade B11), which is insufficient in TiN production, resulting in coarsening of an average circle equivalent diameter d in an area surrounded by diagonal grain boundaries having an orientation difference of 15 ° or more due to coarse grain size. As a result, the toughness of the steel pipe deteriorated. Test No. 42 is an example of using a Ti-rich steel grade (steel grade B12), in which TiN is coarsened, resulting in coarsening of an average circle equivalent diameter d in an area surrounded by diagonal grain boundaries having an orientation difference of 15 ° or more due to coarse grain size. The toughness of steel pipes has deteriorated.

Test No. 43 is an example of using a high B steel grade (steel grade B13), the toughness of the steel pipe deteriorated. No. 44 is an example of using a low-N steel grade (steel grade B14), which lacks TiN production, resulting in coarsening of an average circle equivalent diameter d in an area surrounded by diagonal grain boundaries having an orientation difference of 15 ° or more due to coarse grain size. As a result, the toughness of the steel pipe deteriorated.

Test No. In the case of using 45 steel grade (steel grade B15) with 45 N, the amount of free N increased, the toughness fell at the time of bending, and the toughness of the steel pipe deteriorated. Test No. 46 is an example using the high Ca content (steel grade B16), and coarse inclusions (Ca type | system | group inclusions) were produced and the toughness of steel pipes deteriorated.

Based on these data, the relationship between the average circle equivalent diameter d of the area | region enclosed by the diagonal grain boundary whose orientation difference is 15 degrees or more, and steel pipe toughness vTrs is shown in FIG. 1 (However, the average circle equivalent of the area | region enclosed by the diagonal grain boundary whose orientation difference is 15 degrees or more. Other than diameter d and steel pipe toughness vTrs, those satisfying the requirements specified in the present invention are selected). As is apparent from this result, it can be seen that good steel pipe toughness is achieved by setting the average circle equivalent diameter d of the region surrounded by the diagonal grain boundary having an orientation difference of 15 ° or more to 4 m or less.

The relationship between MA hardness and the yield ratio YR of steel pipes is shown in FIG. 2 (except that MA hardness and yield ratio YR of steel pipes satisfy the requirements specified in the present invention). As is clear from this result, it can be seen that the resistivity ratio of the steel pipe is achieved by setting the MA hardness to 700 (Hv) or more.

The relationship between the bainite fraction and the tensile strength TS is shown in FIG. 3 (except that the bainite fraction and the tensile strength TS are selected to satisfy the requirements specified in the present invention). As is clear from this result, it can be seen that good steel pipe toughness is achieved by setting the bainite fraction to 90 area% or more.

The relationship between DI and bainite fraction is shown in FIG. 4 (except that DI and bainite fraction satisfy the requirements defined in the present invention). As is clear from this result, it can be seen that the bainite fraction can be secured to 90 area% or more by setting the DI to 8 (inch) or more.

The relationship between the MA fraction and the steel pipe toughness vTrs is shown in FIG. 5 (except for the MA fraction and the steel pipe toughness vTrs, those satisfying the requirements specified in the present invention are selected). As is clear from this result, it can be seen that good steel pipe toughness (vTrs is -20 ° C or less) is achieved by setting the MA fraction to 10 area% or less.

The relationship between the MA fraction and the yield ratio YR of the steel pipe is shown in FIG. 6 (except for the MA fraction and the yield ratio YR of the steel pipe, those satisfying the requirements specified in the present invention are selected). As is apparent from this result, it can be seen that the yield ratio YR of the steel pipe can be 90% or less by setting the MA fraction to 3 area% or more.

The relationship between MA average circle equivalent diameter and steel pipe toughness vTrs is shown in FIG. 7 (except that MA average circle equivalent diameter and steel pipe toughness vTrs satisfy | fill the requirements prescribed | regulated by this invention). As is clear from this result, it can be seen that good steel pipe toughness (vTrs is -20 ° C or less) is achieved by setting the MA average circle equivalent diameter to 3 µm or less.

The relationship between MA average round equivalent diameter and the yield ratio YR of steel pipes is shown in FIG. 8 (except that MA average round equivalent diameter and the yield ratio YR of steel pipes satisfy | fill the requirements prescribed | regulated by this invention). As is apparent from this result, it can be seen that the yield ratio YR of the steel pipe can be 90% or less by setting the MA average circle equivalent diameter to 0.5 µm or more.

Claims (6)

C: 0.02 to 0.15% (the meaning of "mass%", the same as below for the chemical component composition),
Si: 0.10 to 0.40%,
Mn: 1.5-2.5%,
P: 0.012% or less (not including 0%),
S: 0.005% or less (not including 0%),
Ti: 0.005-0.02%,
N: 0.002-0.006%, and
Al: in addition to satisfying 0.02 to 0.08%
Ni: 2.5% or less (not including 0%),
Cr: 2.0% or less (not including 0%), and
Mo: contains one or two or more selected from the group consisting of 0.5% or less (not containing 0%), and the balance consists of iron and inevitable impurities, and the hardenability index defined by the following formula (1) A steel plate for resistance-to-heap thick round steel pipes having a tensile strength of 780 MPa or more, characterized by satisfying the following requirements (A), (B) and (C) while having a DI of 8 inches or more:
DI (inch) = {1.16 × ([C] / 10) ^1/2 } × (0.7 × [Si] +1) × {5.1 × ([Mn] −1.2) +5} × (0.35 × [Cu] +1) × (0.36 × [Ni] +1) × (2.16 × [Cr] +1) × (3 × [Mo] +1) × (1.75 × [V] +1) × (200 × [B] +1)… (One)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are C, Si, Mn, Cu, Ni, Cr, respectively. The content (mass%) of Mo, V, and B is shown.
(A) in the microstructure at the quarter thickness of the plate, the bainite is 90 area% or more,
(B) In the microstructure at the plate thickness 1/4 site | part, the average circle equivalent diameter d of the area | region enclosed by the diagonal grain boundary whose orientation difference is 15 degrees or more is 4 micrometers or less,
(C) Microstructure in the plate | board thickness 1/4 site | part WHEREIN: 3-10 area% of island-like martensite whose average circular equivalent diameter is 0.5-3 micrometers, and Vickers hardness Hv are 700 or more. The method of claim 1,
And further comprising Cu: 1.0% or less (not including 0%) and / or B: 0.0025% or less (not including 0%). 3. The method according to claim 1 or 2,
In addition, Ca: a steel plate for a resistivity-ratio thick round steel pipe having a tensile strength of 780 MPa or more including 0.0050% or less (not including 0%). In manufacturing the steel sheet for resistance-to-heap thick circular steel pipes having a tensile strength of 780 MPa or more according to claim 1 or 2, the steel pieces composed of the above chemical composition are heated to 950-1200 ° C., and the temperature at a quarter thickness area. after the austenite US (未) is performed more than 40% in the cumulative rolling reduction in the temperature range where the recrystallization temperature of the hot rolling, cooling to below 350 ℃ by an average cooling rate of 3~25 ℃ / sec from the above Ar ₃ transformation point temperature, Re-heated thick steel pipe with a tensile strength of 780 MPa or more, characterized in that it is reheated to a temperature range of (Ac ₁ transformation point + 30 ° C) to (Ac ₃ transformation point -20 ° C) and then tempered at 450 to 550 ° C. Method of manufacturing steel plate for use. A resistance-compartment thick circular steel pipe having a tensile strength of 780 MPa or more formed into a circular steel pipe by a press bending method using a steel sheet for resistance-strength thick circular steel pipe having a tensile strength of 780 MPa or more according to claim 1. The method of claim 5, wherein
After forming into a round steel pipe, the stress ratio annealing thick steel pipe with a tensile strength of 780MPa or more that is further subjected to stress relief annealing at 400 ~ 500 ℃.

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