JP6435122B2 - Thick steel plate for cold-pressed square steel pipe, cold-pressed square steel pipe, and welding method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a thick steel plate for a cold press-formed square steel pipe used as a steel structure column or the like, a cold press-formed square steel pipe using the same, and a welding method .

  A square steel pipe is generally used as a column of steel structure, and in particular, a thick-walled square steel pipe by cold forming is formed into a cross-section or a U-shaped section by cold pressing (bending), and a submerged arc Cold press-formed square steel pipes joined by welding are used. A cold-formed square steel pipe used for such a steel structure column is generally called a column.

FIG. 1 shows an example of a steel-structure column-beam joint structure using a square steel pipe as a pillar, for example, a joint structure when a through diaphragm type is applied.
In FIG. 1, a dice (also referred to as Tyco) 13 made of a square steel pipe is disposed between a pair of upper and lower vertical pillars 11 made of a square steel pipe, and the end of each pillar 11 and the dice 13 are arranged. Between the two, a diaphragm 12 made of a steel plate is horizontally inserted and welded between the two. Further, for example, the end of the beam 14 made of H-shaped steel is welded to the peripheral edge of the diaphragm 12.

Furthermore, FIG. 2 schematically shows an example of a welded portion structure (joint structure) between the square steel pipe 20 as the column 11 (or the dice 13) and the diaphragm 12. In FIG. 2, unlike FIG. 1, the pillars 11 are shown arranged in the horizontal direction.
In FIG. 2, a rectangular groove 21 is formed at the end of a square steel pipe 20 that becomes a column 11, and the groove 21 is laminated and welded (multilayer welding) to the plate surface of the diaphragm 12 made of a steel plate by MAG welding. A welded joint portion 22 is formed. Reference numeral 15 denotes a backing metal, reference numeral 23 denotes a weld metal (molten metal) of the welded joint portion 22, and reference numeral 24 denotes a welding heat affected zone (hereinafter referred to as "HAZ portion") on the square steel pipe 20 side. Furthermore, the code | symbol 25 shows the part (base material part) of the base material side of the square steel pipe 20 rather than the HAZ part 24. FIG.

  In such a steel structure, stress concentration is likely to occur in the joint between the column and the beam, particularly in the vicinity of the welded portion (welded joint portion 22) between the square steel pipe 20 as the column 11 and the diaphragm 12, and therefore in the vicinity of the welded portion. It is strongly desired that the strength and fracture toughness are excellent.

  By the way, in the welded joint, a region having a certain width from the weld line to the base material side (a region having a certain width from the groove surface to the base material side) is a part affected by heat during welding, that is, a HAZ part. In many cases, the HAZ portion is coarsened due to thermal history during welding. For this reason, the area of the HAZ part often has lower toughness than the base metal (the part of the area away from the welded joint in the square steel pipe as the column) and the weld metal.

Here, the fracture of the joint between the column member (rectangular steel pipe as a column) and the diaphragm assumed at the time of the earthquake causes a ductile crack in the vicinity of the boundary position between the weld metal and the HAZ part, and the base metal part from the HAZ part. Triggered by progressing toward the territory. This is because damage due to plastic deformation is concentrated in a region where the cross-sectional area from the HAZ portion to the base metal portion is smaller than that on the weld metal side where the cross-sectional area is large and difficult to plastically deform.
However, since the fracture toughness of the HAZ portion is generally lowered due to thermal effects, brittle fracture may occur. Due to the initial ductile crack generated near the boundary position between the weld metal and the HAZ portion, a driving force for generating a brittle crack caused by stress concentration acts on the crack tip. Here, if the fracture toughness of the HAZ part is low, the brittle crack generation resistance of the HAZ part is insufficient, and after the occurrence of the ductile crack, it immediately transforms into a brittle crack and propagates along the HAZ part to reach the final fracture. There is a concern that the base metal part does not exhibit sufficient plastic deformation performance.

  Further, if the fracture toughness of the base metal part is low even if the HAZ part has a high fracture toughness, brittle fracture may occur in the base metal part. When the fracture toughness of the HAZ part is high, the ductile crack generated near the boundary position between the weld metal and the HAZ part progresses from the HAZ part to the base material part side, and the crack tip reaches the base material part. Here, if the fracture toughness of the base metal part is low, the brittle crack generation resistance of the base metal part is insufficient with respect to the driving force to generate the brittle crack due to stress concentration at the tip of the ductile crack, and the ductile crack immediately becomes a brittle crack. There is a fear that the base material part does not exhibit sufficient plastic deformation performance due to conversion and progress in the thickness direction of the base material part leading to final fracture.

  Here, FIG. 3 schematically shows the process from the occurrence of a crack to the final breakage at the joint between the conventional column member (a square steel pipe as a column) and the diaphragm as described above.

  In FIG. 3, when a large bending moment is generated at the beam-column joint during an earthquake, first, as shown in FIG. 3A, the boundary position between the weld metal 23 and the HAZ portion 24 of the square steel pipe 20 ( Usually, a crack 27 is generated at the weld toe) 26. Then, when the HAZ toughness is low, the crack 27 is converted from a ductile crack to a brittle crack in the HAZ part 24 as shown in FIG. 3B, and the inside of the HAZ part 24 in the thickness direction (square shape). If it progresses in the direction toward the back surface of the steel pipe 20 and reaches the back surface, it will eventually break. On the other hand, if the base metal toughness is low even if the HAZ toughness is high, the crack 27 propagates from the HAZ part 24 to the base material part 25 as shown in FIG. It progresses in the thickness direction as a brittle fracture, reaches the back surface and reaches the final fracture.

  Conventionally, for example, in Patent Document 1 and Patent Document 2, as a technique for reducing the problem of crack generation and fracture of the welded portion as described above, the position of the front end of the groove of the base material (general A technique for adjusting the lamination method of the final layer is proposed so that the weld metal covers the region closer to the base metal than the region on the front side of the weld line in welding (region exceeding the HAZ portion in general welding). Yes. According to these proposed techniques, cracks that occur at the front boundary position (final layer weld toe) between the weld metal and the base metal due to stress concentration propagate to the base metal, thus reducing the toughness of the base metal. If it is increased, the crack progresses as ductile fracture on the base metal side, so it is considered possible to delay the progress of the crack and delay the final welded joint fracture.

Japanese Patent No. 3937389 Japanese Patent No. 3711495 Japanese Patent No. 4611250 Japanese Patent No. 5385760

"Steel Structure Papers Vol.8 No.32" P17-P31 (Japan Steel Structure Association, issued in December 2001) "Square Steel Pipe Design Research Report" P144-P145 (Construction Research Institute, Ministry of Construction, Steel Club, published in November 1993)

  As shown in Patent Documents 1 and 2, in the technique for adjusting the method of laminating the final layer, management in the process of face-welding the groove shoulder in multi-layer welding between the base material groove and the plate surface of the diaphragm In addition to requiring strict condition control in the welding equipment, in order to perform face-to-face welding with appropriate conditions and appropriate width and height, measure the final layer dimensions. For example, it is necessary to make a crease near the groove shoulder before construction, and to measure the dimension of the final layer after construction and record the dimension measurement. There is a problem of not getting.

On the other hand, in Patent Document 3, the weld heat affected zone toughness index f _HAZ , carbon equivalent Ceq, weld cracking susceptibility composition P _CM , and cleanliness up to 2 mm below the surface of the steel pipe are regulated. Although it has been proposed to make a cold-worked formed steel pipe with excellent deformability, these regulations alone will cause sufficient crack propagation from the weld toe depending on the microstructure of the weld heat-affected zone. There is a problem that it may not be able to be fully controlled.

  Furthermore, Patent Document 4 proposes that the bainite fraction in the microstructure of the steel material be set to 40% or more and the number of inclusions having a circle equivalent diameter of 100 μm or more to be regulated. There is a problem that the progress of a crack generated from the end portion may not be sufficiently suppressed.

  As described above, in the conventionally proposed methods, as a thick steel plate for cold press-formed square steel pipes (columns) used for steel structure columns, etc., it is not accompanied by a large construction load and a large earthquake or the like. It must be said that it is still insufficient to reliably and stably prevent breakage in the vicinity of the weld when an impact is applied.

  The present invention has been made against the background described above. As a thick steel plate for a cold press-formed square steel pipe (column) used for a steel structure column or the like, the end of the square steel pipe is used as a mating material (usually through threaded steel). When welding with a diaphragm), without applying final layer welding as in the above-mentioned proposal, while applying conventional multi-layer welding, maintaining the toughness of the welded joint, it is possible to make a large earthquake The objective is to provide a thick steel plate for rectangular steel pipes that can reduce the risk of brittle fracture from welded joints due to stress concentration when a bending moment occurs and can improve the safety and reliability of steel structures. .

As a result of various experiments and examinations by the present inventors to solve the above-mentioned problems, a thick steel plate for cold press-formed square steel pipe, preferably a thickness produced by applying a thermomechanical processing method as described later. When the steel component for the square steel pipe is selected so that the carbon equivalent Ceq, the weld crack susceptibility composition P _CM , and the weld heat affected zone toughness index f _HAZ calculated from the steel component are within appropriate ranges as the steel plate At the same time, by appropriately setting the yield strength, tensile strength, yield ratio, and uniform elongation, and further the 2 mm V notch Charpy value, particularly the Charpy value at −40 ° C. and the Charpy value at −10 ° C. It has been found that the above problem can be solved, and the present invention has been made.

In particular, it is important to adjust the steel composition so that the weld heat-affected zone toughness index f _HAZ becomes a low value among the above-mentioned conditions while simultaneously increasing the base metal toughness, and at the same time, setting the uniform elongation to be large. Furthermore, it has been found that it is important to regulate the Charpy value by two-stage values of a value at −40 ° C. and a value at −10 ° C.

Specifically, by increasing the base metal toughness and adjusting the steel components so that the weld heat affected zone toughness index f _HAZ is 0.30% or more and 0.47% or less, It has been found that when cracks occur, the progress of cracks due to brittle fracture in the region of the HAZ part is suppressed, and the cracks can progress to the base metal side as ductile fracture.

  In addition, when the steel sheet manufactured by applying the thermomechanical method is cold-formed into a square shape, by ensuring that the uniform elongation of the steel sheet is 13% or more, good press formability is ensured, for example, Manufacturing a square steel pipe with a thickness t (mm) in the range of 9 to 60 mm and an outer bending radius (mm) in the range of 2.0 to 4.0 t without causing cracks. I found out that it is possible. Further, at the stage of the square steel pipe obtained by cold press forming and seam welding of the steel sheet, if the square steel pipe has sufficiently high toughness and sufficiently large uniform elongation, it will increase from the HAZ part when cracks occur. It propagates to the base material side of toughness and progresses as ductile fracture in the base material part. At that time, necking occurs in the base material part, but by setting the uniform elongation sufficiently large, it is possible to suppress an early local thickness reduction due to early necking, and as described above. It has been found that a crack due to ductile fracture that has progressed to the base metal side can be surely delayed from reaching the back surface and reaching the final fracture.

FIG. 4 schematically shows such a crack occurrence state and a process from the progress of the crack to the final fracture.
When a large bending moment occurs at the beam-column joint during an earthquake, as shown in FIG. 4A, first, the boundary position between the weld metal 23 and the HAZ portion 24 of the square steel pipe 20 (weld toe) A crack 27 occurs at the position 26. When the HAZ toughness is high, the crack 27 propagates from the HAZ part 24 to the base metal part 25 as a ductile fracture as shown in FIG. 4B, and when the base metal toughness is high, the ductile crack is It progresses in the thickness direction in the base material part toward the back surface. At this time, necking occurs in the base material portion, and as shown in FIG. 4 (c), the ductile crack 27 in the base material portion 25 reaches the necking portion (thinned portion) 28, thereby finally breaking. However, if the uniform elongation is set large as described above, the occurrence of early necking can be prevented, and as a result, the final breakage can be prevented early. Here, when a large horizontal force is generated by an earthquake, depending on the direction of the horizontal force, a large bending moment may be generated at the corner subjected to plastic working during cold press forming. It has been found that if the uniform elongation in the length direction is 4% or more, it is possible to reliably and stably suppress the early breakage at an early stage.

Furthermore, the 2 mm V notch Charpy value representing the impact absorption energy as an index of the base metal toughness is not a conventional value at 0 ° C., but a value at two steps of −40 ° C. and −10 ° C. It was found that the fracture toughness of the base metal part can be sufficiently ensured by setting the above in order to prevent brittle fracture in the base metal part.
Therefore, each aspect of the present invention is as follows.

The thick steel plate for cold press-formed square steel pipe of the basic aspect (first aspect) of the present invention is:
In mass%, C: 0.06 to 0.16%, Si: 0.05 to 0.50%, Mn: 0.90 to 2.00%, P: 0.015% or less, S: 0.006 %: Al: 0.005 to 0.060%, Ti: 0.005 to 0.025%, N: 0.001 to 0.006%, with the balance being Fe and inevitable impurities, 0.43% carbon equivalent Ceq is 0.33% or more, which is calculated from the steel component (mass%) or less, the weld crack susceptibility composition _{P CM} 0.24% 0.15% or less, the weld heat-affected zone toughness index f _HAZ has a composition of 0.30% or more and 0.47% or less, and the metal structure observed by an optical microscope at a 1/4 thickness position of the cross section in the thickness direction is composed of ferrite and the remaining second phase, and ferrite And the second phase is bainite and / or pearlite. Rannahli, yet made of steel circle-equivalent mean diameter of the ferrite is 12μm or less,
Yield strength in the tensile test at the steel plate original thickness is 325 to 505 MPa, tensile strength is 490 to 670 MPa, yield ratio is 80% or less, uniform elongation with the steel plate original thickness is 13% or more,
2mmV notch Charpy value taken from the steel plate front and rear surfaces is characterized in that only at -40 ℃ 90J / cm ² or more is also -10 ° C. at 180 J / cm ² or more.
However, the carbon equivalent Ceq, the weld crack susceptibility composition P _CM , and the weld heat affected zone toughness index f _HAZ are values defined as follows.
Ceq (%) = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
P _CM (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
f _HAZ (%) = C + Mn / 8 + 6 × (P + S) + 12 × N−4 × Ti
(However, N is total nitrogen, and Ti = 0 when Ti ≦ 0.005%.)

Further, the thick steel plate for cold press-formed square steel pipe of the second aspect of the present invention is the thick steel plate for cold press-formed square steel pipe of the first aspect,
The said steel is the mass%, contains Ca: 0.0005-0.005% further, and the mass ratio of Ca / S is 0.15 or more, It is characterized by the above-mentioned.

Furthermore, the thick steel plate for cold-press-formed square steel pipe of the third aspect of the present invention is the thick steel plate for cold-press-formed square steel pipe of any of the first and second aspects,
The steel is characterized by containing% by mass and further Nb: 0.005 to 0.030%.

Moreover, the thick steel plate for cold-press-formed square steel pipes of the fourth aspect of the present invention is the thick steel plate for cold-press-formed square steel pipes of any one of the first to third aspects,
The steel contains one or more of Cu, Ni, Cr, and Mo by mass%, each 0.04% or less, and a total of 0.10% or less.

Furthermore, the thick steel plate for cold press-formed square steel pipe of the fifth aspect of the present invention is any one of the first to fourth aspects,
The plate thickness t is in the range of 9 to 60 mm.

In yet a sixth aspect of the present invention, as the original plate of cold press forming RHS for steel plate as described above, it defines the produced cold press forming RHS.
That is, the thick steel plate for cold press-formed square steel pipe according to the sixth aspect of the present invention is a cold press-formed square steel pipe using the thick steel plate for the cold press-formed square steel pipe according to the fifth aspect as a base plate ,
Outer corner bend radius (mm) is in the range of 2.0T～4.0T,
Furthermore, the Charpy value by a 2 mmV notch Charpy test piece taken along the length direction of the square steel pipe from directly below the bent outer surface of the corner is 90 J / cm ² or more at −10 ° C. .

In the seventh aspect of the present invention, a welding method for creating a welded joint formed by welding the above-mentioned cold press-formed square steel pipe as a steel structure column and welding the end of the square steel pipe to the plate surface of the diaphragm. Is stipulated.
That is, the welding method according to the seventh aspect of the present invention is a welding method for creating a welded joint obtained by welding the end of the cold press-formed square steel pipe according to the sixth aspect to a plate surface of a diaphragm made of a steel plate. A method ,
A reed groove is formed at the end of the square steel pipe, and the reamed groove of the square steel pipe and the plate surface of the diaphragm are welded by multilayer welding with an average heat input of 7 to 40 kJ / cm.
2mmV notch Charpy value of the outer surface side weld heat affected zone of the corners of the square tube is characterized in that to create a 90 J / cm ² or more der Ru welded joint at 0 ° C..

  According to the present invention, as a thick steel plate for a cold press-formed square steel pipe (column) used for a steel structure column or the like, when the end of the square steel pipe is welded to a mating member (for example, a diaphragm), Without applying the final layer welding as proposed in the past, that is, only by conventional general multi-layer welding, the strength and fracture toughness of the welded joint are improved, and the beam-to-column joint is applied to the steel structure during an earthquake. Therefore, the risk of breakage of the welded joint when a large bending moment is applied is reduced, and the safety and reliability of the steel structure can be improved. And since the final layer welding as proposed in the past is not required, the process management becomes easier and the risk of quality deterioration due to inadequate process management is eliminated as compared with the case where the welding is performed. Time can be shortened and work efficiency can be improved.

It is a perspective view which shows roughly an example of the structure of the column-beam junction part in a steel structure, for example, a joint structure at the time of applying a through diaphragm form. It is a schematic diagram which shows an example of the welded part structure (joint structure) of the square steel pipe as a column and a diaphragm about the column-beam junction part in a steel frame structure. It is typical explanatory drawing which shows the process from the crack generation in the welding part of the conventional general square steel pipe and a diaphragm to final destruction in steps. It is typical explanatory drawing which shows the process from the crack generation in the welding part of a square steel pipe and a diaphragm at the time of applying this invention to final destruction in steps. It is a graph showing the relationship between 2mmV notch Charpy value at 0 ℃ in the weld heat affected zone toughness index f _HAZ values and Wanbido welding of the steel sheet. It is a graph which shows the relationship between the uniform elongation of the corner | angular part of a square steel pipe, and a cumulative plastic deformation magnification. It is a schematic diagram which shows the collection position of the Charpy impact test piece about the corner | angular part of a square steel pipe.

  The present invention is described in detail below.

<Basic aspect of steel sheet>
The thick steel plate for cold press-formed square steel pipe of the present invention is basically a thick steel plate obtained by applying a work heat treatment method, that is, a so-called TMCP (Thermo Mechanical Control Process) to manufacture the plate. It is desirable. Here, the thermomechanical processing method (TMCP) is a heating process for heating the continuously cast slab to the hot rolling temperature, a multi-pass hot rolling process comprising rough rolling and finish rolling, a flattening process by a hot leveler, In a series of processes leading to a cooling process such as water cooling applied as necessary, the conditions in each process, in particular, the controlled rolling to appropriately control each pass temperature and rolling reduction in the hot rolling process, and the cooling process In this technique, controlled rolling is performed to appropriately control the cooling start temperature, the cooling rate, the cooling stop temperature, etc., and thereby the metal structure of the steel sheet and the grain size of the crystal grains are appropriately controlled. In the case of the present invention, as will be described later, by appropriately controlling the various conditions of the thermomechanical processing method (TMCP), it was desirably observed with an optical microscope at a ¼ plate thickness position in the cross section in the plate thickness direction. A steel sheet in which the metal structure is composed of ferrite and the remaining second phase, the fraction of ferrite is 30% or more, the second phase is composed of bainite and / or pearlite, and the average equivalent circle diameter of ferrite is 12 μm or less. Can be obtained.

And as mentioned above, the thick steel plate for cold-press-formed square steel pipes of the present invention is preferably a steel plate obtained by applying the thermomechanical processing method (TMCP), basically from steel components (mass%). and the carbon equivalent Ceq is calculated, and the weld cracking susceptibility composition P _CM, with appropriately defining a weld heat-affected zone toughness index f _HAZ, as the characteristic value, a yield strength in a tensile test at steel original thickness, tensile strength And, the uniform elongation with the original thickness of the steel sheet and the Charpy value at −40 ° C. and the Charpy value at −10 ° C. are defined as 2 mmV notch Charpy values.

Specifically, the carbon equivalent Ceq 0.43% to 0.33% or less, the weld crack susceptibility composition _{P CM} 0.24% to 0.15% or less, the weld heat-affected zone toughness index _{f HAZ} 0.30 The component composition of the steel is adjusted so as to satisfy each condition of not less than 0.4% and not more than 0.47%. And while satisfying these component composition conditions, the yield strength in the tensile test at the steel sheet original thickness is in the range of 325 to 505 MPa, the tensile strength is in the range of 490 to 670 MPa, and the yield ratio is 80% or less. , defines that more uniform elongation while the steel sheet original thickness is not less than 13%, yet 2mmV notch Charpy value is -40 ℃ at 90 J / cm ² or more and -10 ° C. at 180 J / cm ² or more doing. Regarding the uniform elongation, as will be described later, it is also important that the uniform elongation in the length direction of the steel pipe is 4% or more at the corner after the steel plate is formed into a square steel pipe by cold pressing. is there.

In the thick plate for cold-press-formed square steel pipe of the basic aspect of the present invention as described above, the weld heat-affected zone toughness index f _HAZ is 0.47% or less among the above-mentioned conditions for regulating the steel components. By adjusting the steel components so as to have a low value, the fracture toughness of the HAZ part is secured sufficiently high, the occurrence of brittle cracks in the HAZ part is suppressed, and the cracks propagate in a ductile manner to the base metal part. Can do. Furthermore, by setting the 2 mmV notch Charpy value to a value in two stages, a value at −40 ° C. and a value at −10 ° C., the fracture toughness of the base material portion is sufficiently high to ensure the base material portion. The occurrence of brittle cracks can be suppressed, and the base metal part can be propagated in a ductile manner until final fracture.

  In addition, by making the uniform elongation at the steel plate stage 13% or more, it is possible to sufficiently ensure the corner workability in cold press forming into a square steel pipe. Furthermore, the final fracture | rupture in the necking part of a preform | base_material can be delayed by making the uniform elongation of the corner | angular part in the state shape | molded in the square steel pipe into 4% or more. As a result, it is possible to sufficiently enhance the safety and reliability of the welded joint, while eliminating the need for decorative welding as described above.

Therefore, first, the carbon equivalent Ceq, the weld crack susceptibility composition P _CM , and the weld heat affected zone toughness index f _HAZ defined in the present invention will be described.

[Carbon equivalent Ceq: 0.33% to 0.43%]
The carbon equivalent Ceq is a value defined by the following formula (1).
Ceq (%) = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
... (1)
Here, the carbon equivalent Ceq can be increased in strength as its value increases, but with a low carbon equivalent of less than 0.33%, even if the thermomechanical processing method (TMCP) is applied to the production of the steel sheet, It is difficult to stably secure the yield strength YS and the tensile strength TS required as a base plate of a square steel pipe having a maximum thickness of 60 mm used for a steel structure column. On the other hand, at a high carbon equivalent of more than 0.43%, the risk of hardening of the base material and the HAZ part and the formation of martensite increases, so it is difficult to stably secure the base material toughness and the HAZ toughness at a high level. Become. Therefore, the carbon equivalent Ceq is limited to a range of 0.33 to 0.43%.

[Weld cracking susceptibility composition P _CM : 0.15% or more and 0.24% or less]
Weld crack susceptibility composition P _CM is a value defined by the following equation (2).
P _CM (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (2)
The steel sheet of a low _{P CM} components is less than 0.15% weld crack susceptibility composition _{P CM,} as in the case the carbon equivalent Ceq is less than 0.33%, by applying the thermo-mechanical treatment method (TMCP) to the production of steel plate However, it is difficult to stably secure the yield strength YS and the tensile strength TS that are required as an original plate of a square steel pipe having a maximum thickness of 60 mm and used for a steel structure column. On the other hand, 0.24 percent higher P _CM components of the steel, the base material and HAZ of curing, increased risk of martensite, is that the order to ensure a stable high levels of base material toughness and HAZ toughness It becomes difficult. For this reason, the content is limited to 0.15 to 0.24%. Note _{P CM} is preferably 0.23% or less.

[Welding heat-affected zone toughness index f _HAZ : 0.30% to 0.47%]
The weld heat affected zone toughness index f _HAZ is a value defined by the following equation (3).
f _HAZ (%) = C + Mn / 8 + 6 × (P + S) 12 × N−4 × Ti (3)
However, in (3), N is total nitrogen, and Ti is Ti = 0 when Ti ≦ 0.005%.
HAZ toughness index f _HAZ is an index which gives an indication of the multi-layer welding joint toughness which are well known (HAZ toughness) those skilled in the art, bake enhances C, Mn, etc., Ceq, the P _CM The value of f _HAZ increases as the number of elements that increase the permeability increases and the amount of impurity elements P and S described later increases. Therefore, generally speaking, the lower the value of f _HAZ , the more advantageous for HAZ toughness.
In addition, the value of f _HAZ needs to be set appropriately from the viewpoint of securing the strength of the base metal and the economy as a rectangular steel pipe used as a column of the steel structure. From the enormous HAZ toughness evaluation test results accumulated by the present inventors, taking into account the economic rationality due to the increased load of reducing impurities, ensuring the strength required for the rectangular steel pipes targeted by the present invention. In addition to the required strength, as described in the seventh embodiment, as a range that can satisfy 90 J / cm ² or more at a heat input of a square steel pipe (column) and a diaphragm of 7 to 40 kJ / cm, 0.30 to 0.00. The value of f _HAZ was limited within the range of 47%.

Specifically, if the value of f _HAZ exceeds 0.47%, it becomes difficult to ensure that the ² mmV notch Charpy value in the HAZ part of the final layer in multilayer welding is 90 J / cm ² or more at 0 ° C. There is a risk of brittle fracture along the part. The value of f _HAZ is an index capable of predicting one-bead HAZ toughness, for example, as proposed in Non-Patent Document 1, and the HAZ toughness in the case of multilayer welding is usually higher than the one-bead HAZ toughness. . Therefore, if it is possible to ensure 90 J / cm ² or more at 0 ° C. as the one-bead HAZ toughness, it is considered that the HAZ toughness in multilayer welding can also ensure 90 J / cm ² or more at 0 ° C. According to the inventors' experiment on one-bead HAZ toughness, the approximate curve of the relationship between the value of f _HAZ and the 2 mmV notch Charpy value at 0 ° C. is a curve C ₀ as shown by the solid line in FIG. It has been confirmed. If the error is assumed to be a normal distribution in consideration of the variation, the standard deviation σ is 50 J / cm ^{2. If} the standard deviation (σ) is taken into consideration, the deviation value curve on the −1σ side is shown in FIG. As shown by the broken line C- _σ . From this deviation value curve C- _σ , if the value of f _HAZ is 0.47 or less, it is considered that 90 J / cm ² or more can be secured as the ² mmV notch Charpy value at 0 ° C. Although this experimental result is about one bead HAZ toughness, as described above, the HAZ toughness in the case of multilayer welding usually exceeds the one bead HAZ toughness, and therefore the value of f _HAZ is 0.47 or less. If it exists, 90 J / cm ² or more can be secured as a ² mmV notch Charpy value at 0 ° C. even in multi-layer welding. Therefore, in the present invention, the upper limit of the value of f _HAZ is defined as 0.47.
In addition, in the thick steel plate for rectangular steel pipes of the present invention, the plate thickness is 9 to 60 mm, and considering that the yield strength is 325 to 505 MPa and the tensile strength is 490 to 670 MPa, it is stable at all plate thicknesses. In order to secure the strength, the value of f _HAZ is preferably 0.30% or more. Therefore, the value of f _HAZ is defined to be within a range of 0.30 to 0.47%.

[Yield strength / Tensile strength / Yield ratio]
The yield strength in the tensile test at the original thickness of the steel sheet is in the range of 325 to 505 MPa, the tensile strength is in the range of 490 to 670 MPa, and the yield ratio is 80% or less. Here, these strengths (yield strength and tensile strength) and yield ratio are as follows. A square steel pipe obtained by cold press forming and seam welding the thick steel plate of the present invention is used as a column of a steel structure. The required strength and yield ratio range as a column when used are defined. When the yield strength is less than 325 MPa and the tensile strength is less than 490 MPa, the square steel pipe for a column cannot satisfy the target strength in the present invention, while the yield strength is over 505 MPa and the tensile strength is over 670 MPa. In this case, the condition described as the first aspect may not be satisfied.
Here, the above yield strength, tensile strength, and yield ratio are defined as values when a tensile test is performed using a JIS No. 5 test piece.

[Uniform elongation of steel sheet]
The uniform elongation of the steel plate has a great influence on the cold workability of the plate. When the uniform elongation is less than 13%, for example, as described in the sixth aspect, the thickness t (9 to 60 mm) There is a possibility that it is difficult to cold-press corners having an outer bending radius (mm) of 2.0 t to 4.0 t. Therefore, the uniform elongation of the steel plate is limited to 13% or more. In addition, uniform elongation is prescribed | regulated here as a value at the time of performing a tensile test by a JIS5 test piece like the above. The uniform elongation of the steel plate is preferably 15% or more, and more preferably 17% or more, even within a range satisfying the condition of 13% or more. In addition, although the upper limit of the uniform elongation of the steel sheet is not particularly defined, it is generally about 30% or less. Here, the value of the uniform elongation is defined as a value when a tensile test is performed using a JIS No. 5 test piece as described above.

  Regarding uniform elongation, in addition to uniform elongation at the steel plate stage, the uniform elongation at the corner of the square steel tube at the stage of the square steel pipe obtained by cold press forming using the steel plate is also specified. This will be explained later.

[2mmV notch Charpy value]
In a conventional general steel plate, the 2 mmV notch Charpy value is normally defined as a value obtained by testing at 0 ° C. as an index of impact absorption energy, but in the present invention, the 2 mmV notch Charpy value is at a lower temperature. Moreover, it is defined as a value in two stages (−40 ° C. and −10 ° C.). Thus, by controlling at a temperature lower than the conventional general 0 ° C. and in two steps, excellent base material toughness can be secured at a higher level and more stably than in the past.
Here, when the 2mmV notch Charpy value at -40 ℃ and the 90 J / cm ² or more conditions, 2mmV notch Charpy value at -10 ° C. not met even either one of 180 J / cm ² or more conditions, The target high level of toughness in the present invention cannot be secured stably.

<Structural conditions of steel sheet>
In still cold press forming RHS for steel plate of the present invention, the metal structure was observed with an optical microscope at a quarter thickness location of the plate-thickness direction cross-section, it consists of ferrite and the balance a second phase, and minute ferrite It is preferable to have a microstructure in which the main phase is ferrite, in which the ratio is 30% or more, the second phase is composed of bainite and / or pearlite, and the average equivalent circle diameter of ferrite is 12 μm or less.
Such a structure can be obtained by appropriately controlling the conditions of the thermomechanical processing method (TMCP) as described above, particularly the controlled rolling and controlled cooling conditions. For example, by terminating hot rolling at a relatively low temperature, recrystallization occurs from the austenite structure during rolling to produce ferrite crystals, and ferrite growth and coarsening are further achieved by controlling the cooling process after rolling. By suppressing the above, it is possible to obtain a tissue that satisfies the above conditions.
The reasons for setting the organizational conditions as described above are as follows.

[Metal structure: Ferrite fraction of 30% or more, balance bainite and / or pearlite]
In the cold press forming RHS for steel plate of the present invention, as a condition of the metal structure, the metal structure was observed with an optical microscope at a quarter thickness location of the plate-thickness direction cross-section, of ferrite and the balance second phase And the ferrite fraction is preferably 30% or more, and the second phase is preferably composed of bainite and / or pearlite. That is, the metal structure of the steel plate has a great influence on the strength, yield ratio, toughness and elongation (cold workability) of the base material. In particular, ferrite, which is a soft phase, contributes to a low yield ratio. If the ferrite fraction is less than 30%, it is difficult to make the yield ratio of the base material 80% or less. The remaining second phase with respect to ferrite (30% or more) may be bainite and / or pearlite.
In addition, although the upper limit of a ferrite fraction is not prescribed | regulated, in order to make the yield ratio of a steel plate 80% or less as mentioned above, it is preferable that a ferrite fraction shall be 80% or less.
Such a metal structure can be obtained by appropriately controlling the conditions of the thermomechanical processing method (TMCP) at the time of manufacturing the thick steel sheet, together with the ferrite grain size conditions described below.

[Average equivalent circle diameter of ferrite: 12 μm or less]
Furthermore, in the thick steel plate for cold press-formed square steel pipe of the present invention, the average crystal equivalent diameter is required to be 12 μm or less as the crystal grain size condition of ferrite in the metal structure. That is, the crystal grain size of the steel sheet metal structure affects the base material toughness. The smaller the average crystal grain size, the better the base material toughness. On the other hand, the crystal grain size also affects the strength, particularly the yield strength. Therefore, in the present invention, in order to make the relationship between strength (yield strength) and toughness, which are generally in a trade-off relationship, compatible at a high level, the average equivalent circle diameter of ferrite is defined as 12 μm or less on average. The lower limit of the circle equivalent average diameter of the ferrite is not particularly specified, but is generally about 5 μm or more.

<Specific component composition>
The specific component composition of the cold press-formed square steel pipe thick steel plate of the present invention is that the carbon equivalent Ceq, the weld crack susceptibility composition P _CM , and the weld heat affected zone toughness index f _HAZ are within the above-mentioned ranges. rather it is determined as, in mass%, C: 0.06~0.16%, Si : 0.05~0.50%, Mn: 0.90~2.00%, P: 0. 015% or less, S: 0.006% or less, Al: 0.005-0.060%, Ti: 0.005-0.025%, N: 0.001-0.006%, the balance being The component composition of Fe and inevitable impurities is preferable. The reason of the preferable range of each of these elements is demonstrated below.

[C: 0.06-0.16%]
C is an important element for improving the strength. In a thick steel plate manufactured by applying a thermomechanical processing method (TMCP) that is thoroughly subjected to low temperature heating and low temperature rolling, it is necessary to contain 0.06% or more of C in order to ensure a predetermined strength stably. Preferably, the strength can be increased more stably by containing C by 0.07% or more, or 0.08% or more. However, in order to ensure stable good HAZ toughness, it is necessary to suppress C to 0.16% or less. The C content is more preferably limited to 0.15% or less, or 0.14% or less.

[Si: 0.05 to 0.50%]
Si has a deoxidizing action, but Si is essentially unnecessary when Al, which is a strong deoxidizing element, is sufficiently contained. However, lowering the amount of Si more than necessary would rather increase the load of the steelmaking process, so the lower limit of the amount of Si was set to 0.05%. On the other hand, since Si promotes martensite formation that adversely affects toughness, it is preferable to reduce the amount of Si as much as possible from the viewpoint of HAZ toughness. From this viewpoint, Si needs to be suppressed to 0.50% or less. Note that the Si content is preferably limited to 0.20% or less, or 0.16% or less, and more preferably 0.13% or less.

[Mn: 0.90 to 2.00%]
Mn is an element effective for improving the strength, and Mn is added to ensure the strength economically. Here, in order to obtain 0.33% or more as the value of the carbon equivalent Ceq, an Mn content of 0.90% or more is necessary from the relationship between the C amount and the Si amount. However, if Mn is contained in excess of 2.00%, the harmfulness of center segregation of the slab becomes remarkable, and the toughness is deteriorated through HAZ hardening and martensite formation promotion. % Was the upper limit. In order to secure the strength, it is preferable to set Mn to 1.0% or more. On the other hand, from the viewpoint of suppressing HAZ curing and martensite formation, Mn is preferably limited to 1.8% or less, or 1.6% or less, and more preferably 1.5% or less.

[P: 0.015% or less]
It is known that P is an impurity element and is liable to segregate at the grain boundary, thereby degrading ductility. For this reason, in order to ensure the base material toughness and the HAZ toughness stably at a high level, the content is limited to 0.015% or less.

[S: 0.006% or less]
S is also an impurity element like P, and it is necessary to suppress it to 0.006% or less in order to suppress generation of MnS which is stretched by rolling and adversely affects toughness and lamellar resistance. In order to further improve toughness, it is preferable to limit the amount of S to 0.004% or less, and further to 0.003% or less.

[Al: 0.005 to 0.060%]
Al is an indispensable element for deoxidation and reducing O (oxygen) to increase the cleanliness of steel. In order to stably and reliably exert its effect, it is necessary to make it 0.005% or more. is there. However, if Al exceeds 0.060%, the tendency of the alumina-based coarse oxide to cluster is strengthened, clogging of the steelmaking nozzle occurs, and the harmfulness as a failure starting point becomes obvious, so 0.060% Was the upper limit. In addition, it is more preferable to limit the upper limit of the Al amount to 0.05% or less and 0.04% or less.

[Ti: 0.005 to 0.025%]
Ti is useful for improving the base material and HAZ toughness. That is, Ti combines with N and finely precipitates in the slab as TiN, suppresses the coarsening of the austenite grains during heating, functions effectively to refine the rolling structure, and exists in the steel sheet. Fine TiN has the effect of refining the HAZ structure during welding. In order to obtain these effects, Ti needs to be 0.005% or more. However, if the amount of Ti is too large, TiC is formed and the low-temperature toughness and weldability are deteriorated. Ti is more preferably limited to 0.015% or less.

[N: 0.001 to 0.006%]
N is contained in the steel as an unavoidable impurity, but combines with Ti to form TiN, thereby improving the properties of the steel as described above. In addition, when Nb is added as described in the third aspect, N combines with Nb to form carbonitride and contributes to increasing the strength. However, the increase in the amount of N is extremely harmful to the HAZ toughness and weldability. Therefore, in the present invention, the amount of N is limited to 0.006% or less.

Basically, the balance of each of the above components may be Fe and inevitable impurities other than those described above. However, in addition to the above component elements, as described in the second embodiment, Ca may be 0.0005. ~ 0.005% (Ca / S ratio when Ca is added is 0.15 or more) may be added, and Nb is added 0.005-0.030% as described in the third aspect May be. Furthermore, Cu, Ni, Cr, and Mo, which are often added as steel strengthening elements, are described in the fourth aspect. Is preferably 0.10% or less. The reason for defining these components will be described next.

[Ca: 0.0005 to 0.005%, Ca / S: 0.15 or more]
Depending on the amount of S, it is preferable to add Ca in order to control the form of MnS described above. In order to exert the form control effect of Ca, it is preferable to add at least 0.0005% of Ca. However, excessive addition of Ca worsens the cleanliness of the steel, so it is preferable to limit the upper limit to 0.005%. In addition, in order to control the form of MnS, it is necessary to appropriately control the stoichiometric ratio of Ca and S together with the upper and lower limits of these Ca contents. The S ratio is preferably 0.15 or more.

[Nb: 0.005 to 0.030%]
The addition of Nb raises the non-recrystallization temperature of austenite and maximizes the effect of controlled rolling during hot rolling, and also prevents softening of the heat affected zone during welding and cutting. It is an effective element, and in order to exert these effects, 0.005% or more of Nb needs to be added. Nb also contributes to strength improvement as precipitation hardening. However, excessive addition of Nb causes deterioration of HAZ toughness, so the upper limit of Nb was made 0.030%.

[One or more of Cu, Ni, Cr, and Mo are 0.04% or less in total by mass%, and the total is 0.10% or less]
Proper addition of Cu, Ni, Cr, and Mo often enhances the properties of steel, but these are also rare metals, and it is desirable not to add them intentionally in consideration of economy. In a square steel pipe as a column of a building structure, its economic efficiency is emphasized. Therefore, in the present invention, these elements are 0.04% or less, which is a contamination level inevitably mixed from scrap or the like. Also, it is desirable to keep the total amount to 0.10% or less.

[Thickness of steel sheet]
As described in the fifth aspect, the thickness of the thick steel plate for cold press-formed square steel pipe of the present invention is preferably in the range of 9 to 60 mm. If the plate thickness is less than 9 mm, it will be difficult to ensure sufficient strength as a structural member of the steel structure, while if it exceeds 60 mm, the outer bending radius (mm) will be 4 mm against the plate thickness t (mm) by cold press molding. It may be difficult to ensure 0 t or less.

[Steel plate manufacturing method]
The method for manufacturing the cold press-formed square steel pipe thick steel plate as described above is not particularly limited, but it is desirable to apply the thermomechanical processing method (TMCP) as described above, and it is appropriately controlled by appropriately setting conditions. By the controlled heat treatment method, it is possible to reliably and stably produce a thick steel plate having the above-described metal structure and performance.
Here, the specific manufacturing conditions to which TMCP is applied are not particularly limited. Usually, the slab obtained by continuous casting is chamfered as necessary and then heated before hot rolling. Heating to about 1250 ° C., hot rough rolling is performed at a rising temperature of about 850 to 1000 ° C., and finish rolling is further performed, and the rising temperature is about 700 to 850 ° C., each pass reduction ratio is about 3 to 15%, and the number of passes is 10 to 10. After performing at about 30 and then flattening with a leveler as necessary, as control cooling, the cooling start temperature is about 600 to 800 ° C., the cooling rate is 0.1 to 30 ° C./sec, and the cooling stop temperature is 200 to 600 ° C. It is preferable to perform cooling by water cooling or non-water cooling.

<Square steel pipe (column)>
Furthermore, in this invention, the aspect of the cold press forming square steel pipe using the thick steel plate for cold press forming square steel pipes as described above is also defined in the sixth aspect. Attached to is this will be described below.

[Outer bending radius and Charpy value of corner]
The outer bending radius of the corner portion of the square steel pipe is within the range of 2.0t to 4.0t with respect to the plate thickness t (9 to 60mm) of the base plate, and as an index of the toughness (impact absorption energy) of the corner portion. In the sixth aspect, the ² mm V notch Charpy value collected in the length direction of the steel pipe from directly below the outer surface layer of the corner is 90 J / cm ² or more at −10 ° C.
This is the same as the square steel pipe as a conventional general column in the above-mentioned corner bend radius.
In addition, when a large horizontal force is generated by an earthquake, depending on the direction of the horizontal force, a large bending moment may be generated at the corner, but the corner is a place where large work hardening occurs during cold pressing. By setting the −10 ° C. Charpy value of the corner to 90 J / cm ² or more, the toughness of the corner can be secured at a high level.
As described above, the corner portion is a portion that has undergone work hardening by cold press forming, and the toughness is usually lower than other portions (side portions). Therefore, if the −10 ° C. Charpy value at the corner is set to 90 J / cm ² or more, the side portion can secure a Charpy value higher than that of course.

[Uniform elongation at corners]
In the sixth aspect, the uniform elongation at the corners of the square steel pipe is 4% or more as a value in the length direction (base plate rolling direction) of the square steel pipe.
Here, if the uniform elongation at the corner of the square steel pipe is 4% or more, the local thickness reduction due to necking at the corner when a sudden impact due to an earthquake or the like is applied is suppressed. It is possible to delay the crack due to ductile fracture that has progressed to the base material side to reach the back surface and reach the final fracture, and thus the reliability and safety of the steel structure can be sufficiently improved, etc. It has been confirmed by the experiment.

That is, as shown in the Example mentioned later, a full-scale square steel pipe-diaphragm test body was manufactured, and a three-point bending experiment of the member was performed to confirm the bending performance. On the other hand, a tensile test piece was taken from the longitudinal direction of the corner on the square steel pipe side of the test body, and the uniform elongation was calculated. FIG. 6 shows the relationship between the uniform elongation of the corner and the cumulative plastic deformation magnification. The figure also shows the test results of a conventional square steel pipe for comparison.
From this test result, the square steel pipe having a uniform elongation of 4% or more in the corner is deformable (cumulative) required for the column member in the design that allows the plasticization of the column proposed in Non-Patent Document 2. It was confirmed that the material had sufficient performance exceeding the plastic deformation magnification 28). Moreover, in the test body of the present invention in which the final fracture type was fracture, a ductile crack was generated in the vicinity of the HAZ portion, and then propagated to the base material side, and progressed as ductile fracture in the base material. At that time, necking occurred in the base material, but by having a sufficiently uniform elongation, early local thickness reduction due to the occurrence of necking was suppressed, cracks due to ductile fracture reached the back surface, and finally It was confirmed that it would lead to destruction.
As described above, the corner portion is a portion that has undergone work hardening by cold press forming, and the uniform elongation is usually smaller and lower than other portions (side portions). Therefore, if the uniform elongation of the corner is set to 4% or more, it is natural that the uniform elongation of the side is secured.

[Method of manufacturing square steel pipe]
The manufacturing method of the square steel pipe is, in short, a thick steel plate as described above, formed by cold press forming into a cross-section or a U-shaped section, and a square closed section by submerged arc welding or the like. Apply methods and conditions.

<Welding method >
Furthermore, in the present invention, as described in the seventh aspect, in order to create a welded joint in which the cold-pressed square steel pipe is welded to a diaphragm (generally a through diaphragm) as a column of a steel structure. The welding method is also prescribed.
This welding method is a welding method for creating a welded joint formed by welding the end of the cold-pressed square steel pipe to the plate surface of a diaphragm made of a steel plate, and is attached to the end of the square steel pipe. forming a shape groove, it is welded by multi-layered welding between les shape groove and the diaphragm plate surface of square tube by an average heat input 7~40kJ / cm, corner outer surface welding heat of the RHS It has prescribed | regulated that the ² mmV notch Charpy value of an influence part (HAZ part) obtains the welded joint which is 90 J / cm < ² > or more at 0 degreeC.

Here, the corners of a square steel pipe are prone to stress concentration when a large bending moment is applied to the steel structure due to an earthquake or the like when the column is welded to a mating material (generally through diaphragm) as a column. Therefore, the HAZ portion of the corner on the square steel pipe side in the welded joint is also a place where breakage easily occurs. By setting the 0 ° C. Charpy value to 90 J / cm ² or more for the HAZ portion, the square steel pipe side of the welded joint The HAZ part toughness of the corner can be secured at a high level. In addition, the HAZ toughness of the corner portion subjected to work hardening is usually inferior to the HAZ toughness of the side portion. Therefore, the 0 ° C. Charpy value as the HAZ toughness index at the corner portion is 90 J / cm ² or more. In general, a 0 ° C. Charpy value equal to or higher than that of the side portion is generally secured.

[Diaphragm as welding partner]
The material of the diaphragm is not particularly limited, and may be a steel plate similar to that conventionally used for steel structures. For example, the conventional BCP 325 and 385 N / mm ^second grade column standards may be used. In general, JIS SN490C, Minister's accredited standard 385N / mm grade ² thick plate with a drawing direction restriction, etc. are the targets.

[Welding conditions]
The welding conditions for forming the welded joint are not particularly limited, and may basically be the same as those of the conventional BCP325 or the like. That is, in general, the welding current is 200 to 400 A when the welding position is either a side or a corner, the welding voltage is 22 to 40 V at both the side and the corner, and the welding speed is between the side and the corner. All are 15 to 60 cm / min, the heat input is 40 kJ / cm or less at the side, 30 kJ / cm or less at the corner, the temperature between passes is 350 ° C. or less at the side in the case of semi-automatic welding, and the corner in the case of semi-automatic welding. In the case of full automatic welding, the sides and corners may be 250 ° C. or lower. Moreover, the lamination conditions in the multi-layer welding as the complete penetration welding between the through diaphragm and the column may be welded by a laminating method based on JASS6 used for general architectural steel frame production management.

  Examples of the present invention will be described below together with comparative examples.

No. in Table 1 and Table 2 1-No. 40, each of the continuous casting slabs having a thickness of 240 mm having the composition indicated by R1 to R11 in Table 3 was hot-rolled to finish the thick steel plates for square steel pipes having the thicknesses shown in Tables 4 to 6. In the hot rolling-cooling, controlled rolling and controlled cooling were performed as so-called thermomechanical processing (TMCP). Various conditions for hot rolling and cooling are shown in Tables 4 to 6. In addition, each steel No. 1-No. Tables 1 to 3 show composition index values (carbon equivalent Ceq, weld crack susceptibility composition P _CM , weld heat affected zone toughness index f _HAZ ) calculated from the component compositions of 40 and R1 to R11.

  Each steel plate obtained was observed with a metallographic microscope, and as its microstructure, the ferrite fraction, ferrite grain size (equivalent circle average diameter), and second phase structure (bainite and / or pearlite) were examined. The results are shown in Tables 7-9. In Tables 7 to 9, B in the second phase column indicates bainite, and P indicates pearlite.

  Further, a tensile test was performed on each steel plate, and the yield strength (YP), tensile strength (TS), yield ratio (YR), and uniform elongation (uEL) were examined. Here, the tensile test was carried out in accordance with JIS Z 2241 by collecting JIS No. 5 tensile test pieces along the rolling direction.

Moreover, about each steel plate, the 2mmV notch Charpy impact test piece was cut out along the rolling direction, and the Charpy impact test was done based on JISZ2242 at the test temperature of -40 degreeC and -10 degreeC. Tables 7 to 9 show the Charpy value (vE _-40 ) at -40 ° C and the Charpy value (vE _-10 ) at _-10 ° C.

  Furthermore, a cold press-formed square steel pipe was produced according to a conventional method using each steel plate. Specifically, each steel plate is formed into a U-shaped cross-section by cold press forming, and two of the U-shaped cross-section members are butted so that the cross-section is rectangular, and a seam by carbon dioxide arc welding or submerged arc welding is used. Internal welding and external welding were performed by welding to form a square steel pipe. In addition, the cross section of the obtained square steel pipe was made into the square whose length of one side is 500 mm. Further, the outer bending radius r (mm) of the corner portion was set to be r = 3.5 t according to the plate thickness t (mm).

JIS No. 5 tensile test specimens were collected along the axial direction from the corners of the respective square steel pipes obtained, and subjected to a tensile test in accordance with JIS Z 2241 to examine uniform elongation (uEL). Further, as shown in FIG. 7, a 2 mm V notch Charpy impact test piece 30 is cut out along the tube axis direction around the position of the depth of 6 mm from the bent outer surface of the corner portion 20A in the square steel pipe 20, and the test is performed. A Charpy impact test was performed at a temperature of −10 ° C. in accordance with JIS Z 2242. The uniform elongation (uEL) at the corners and the Charpy value (vE- ₁₀ ) at -10 ° C are shown in Tables 10-12.

In addition, a rectangular groove was formed at the end of each square steel pipe, and the diaphragm was welded to the diaphragm by a multi-layer welding method to produce a full-scale square steel pipe-diaphragm welded joint. The welding conditions at this time were a groove angle of 35 °, a root gap of 7 mm, a heat input of 30 kJ / cm or less, a number of passes of 3 to 21, a temperature between passes of 250 ° C. or less, and a welding material equivalent to JIS Z 3312 was used as the welding material. Downward robot welding was applied.
The 2 mmV notch Charpy value (HAZ vE ₀ ) at 0 ° C. of the outer surface side heat-affected zone at the corner on the square steel pipe side of the obtained welded joint was examined. The test method and specimen collection position of the Charpy impact test are the same as in the Charpy impact test on the corners of a square steel pipe.
The test results (HAZ vE ₀ ) are shown in Tables 10-12.

As shown in the tables above 0.43% carbon equivalent Ceq is 0.33% or more, which is calculated from the steel component (mass%) or less, the weld crack susceptibility composition _{P CM} 0.15% or more 0.24% Hereinafter, Example No. of the present invention No. 1 in which the heat-affected zone toughness index f _HAZ satisfies each condition of 0.30% to 0.47%. 1-No. In the case of 40, the yield strength, tensile strength, yield ratio, uniform elongation of the original steel sheet, 2 mmV notch Charpy value at −40 ° C., 2 mmV notch Charpy value at −10 ° C. It has been confirmed that both can ensure the level specified in the present invention.

Furthermore, these No. 1-No. In steel 40, the metal structure condition, i.e. ferrite fraction, also the conditions of the ferrite grain size (circle-equivalent mean diameter), and the second phase structure (bainite and / or pearlite), condition defined in claim 1 Was met.

These Nos. 1-No. The RHS using steel sheet 40, 2 mm V-notch Charpy value at -10 ° C. for corners defined in claim 6, Oyo that each satisfy the condition of uniform elongation for beauty corner was confirmed .

Furthermore, no. 1-No. It was confirmed that the HAZ portion of a square steel pipe-diaphragm welded joint using a square steel pipe made of 40 steel plates satisfies the 2 mmV notch Charpy value condition at 0 ° C. defined in claim 7 .

  On the other hand, in Comparative Example R1 in which the carbon equivalent Ceq of the steel plate was too low, the yield strength and tensile strength of the steel plate were below the provisions of the present invention.

In addition comparative examples carbon equivalent Ceq and weld cracking susceptibility composition P _CM is too high, and the fraction of ferrite of the steel sheet was too low R2, although higher yield strength and tensile strength of the steel sheet, one remained steel original thickness The uniform elongation, the 2 mmV notch Charpy value at −40 ° C., and the 2 mmV notch Charpy value at −10 ° C. were all below the regulation of the present invention. Moreover, the HAZ part toughness (2 mmV notch Charpy value at 0 degreeC) of the square steel pipe-diaphragm welded joint using the steel plate of this comparative example R2 became extremely low.

  Comparative Example R3 is an example in which the ferrite grain size of the steel sheet is too large. In this case, the HAZ part toughness (2 mmV notch Charpy value at 0 ° C.) of the square steel pipe-diaphragm welded joint using the steel sheet is low. It is had.

Further, Comparative Example R4 is an example in which the carbon equivalent Ceq of the steel sheet, the weld cracking susceptibility composition P _CM , and the weld heat affected zone toughness index f _HAZ are too low, and the ferrite grain size is too large. Strength and tensile strength have been lowered.

  Comparative example R5 is an example where the carbon equivalent Ceq of the steel sheet is too high and the ferrite fraction is too low. In this example, the yield strength and tensile strength of the steel sheet are high, but the original thickness of the steel sheet remains unchanged. Elongation, 2 mmV notch Charpy value at −40 ° C., and 2 mmV notch Charpy value at −10 ° C. were below the regulation of the present invention. In addition, in the square steel pipe using the steel plate of this comparative example R5, the 2 mmV notch Charpy value at −10 ° C. of the corner part becomes low, and the HAZ part toughness of the square steel pipe-diaphragm welded joint (2 mmV notch Charpy value at 0 ° C.) ) Has become extremely low.

Comparative Example R6 is an example in which the carbon equivalent Ceq of the steel sheet and the weld heat affected zone toughness index f _HAZ were too high. In this case, the yield strength and tensile strength of the steel sheet were high, but 2 mmV at −40 ° C. The notch Charpy value, 2 mmV notch Charpy value at −10 ° C., was below the regulation of the present invention. Moreover, in the square steel pipe using the steel plate of this comparative example R6, the 2 mmV notch Charpy value at −10 ° C. of the corner part was lowered, and the HAZ part toughness of the square steel pipe-diaphragm welded joint was lowered.

Comparative Example R7 is an example in which the amount of S in the steel sheet component is increased, and the values of the carbon equivalent Ceq, the weld crack sensitivity composition P _CM , and the weld heat affected zone toughness index f _HAZ are within the scope of the present invention. The 2 mmV notch Charpy value at −40 ° C. and the 2 mmV notch Charpy value at −10 ° C. of the steel sheet were below the provisions of the present invention. Further, in the square steel pipe using the steel plate of this comparative example R7, the 2 mm V notch Charpy value at −10 ° C. of the corner part was lowered, and the HAZ part toughness of the square steel pipe-diaphragm welded joint was lowered.

Comparative Example R8 is an example in which the amount of Al of the steel plate component is increased, and in this case, the values of carbon equivalent Ceq, weld crack sensitivity composition P _CM , and weld heat affected zone toughness index f _HAZ are within the scope of the present invention. However, the 2 mmV notch Charpy value at −40 ° C. of the steel sheet was below the regulation of the present invention.

Comparative Example R9 is an example in which the Ti content of the steel sheet component is low. In this case, the values of the carbon equivalent Ceq, the weld crack susceptibility composition P _CM , and the weld heat affected zone toughness index f _{HAZ in} the steel sheet are Although within the range, the yield ratio YR in the steel sheet was too high.

Comparative example R10 is an example in which, as a result of increasing the amount of Ti of the steel plate component, the weld heat affected zone toughness index f _HAZ in the steel plate is too low.

Comparative Example R11 is an example in which the N amount of the steel plate component is too high, and the values of the carbon equivalent Ceq, the weld crack susceptibility composition P _CM , and the weld heat affected zone toughness index f _{HAZ in} the steel plate are within the scope of the present invention. However, the 2 mmV notch Charpy value at −40 ° C. and the 2 mmV notch Charpy value at −10 ° C. of the steel sheet were below the regulation of the present invention. Further, in the square steel pipe using the steel plate of this comparative example R11, the 2 mmV notch Charpy value at −10 ° C. of the corner part was lowered, and the HAZ part toughness of the square steel pipe-diaphragm welded joint was lowered.

  The preferred embodiments and examples of the present invention have been described above. However, these embodiments and examples are merely examples within the scope of the present invention and do not depart from the spirit of the present invention. Thus, addition, omission, replacement, and other changes of the configuration are possible. That is, the present invention is not limited by the above description, is limited only by the scope of the appended claims, and can be appropriately changed within the scope.

11 Column 12 Diaphragm 14 Beam 20 Square Steel Pipe 20A Corner 22 Weld Joint 23 Weld Metal 24 HAZ (Welding Heat Affected Zone)
25 Base material part 27 Crack

Claims (7)

In mass%, C: 0.06 to 0.16%, Si: 0.05 to 0.50%, Mn: 0.90 to 2.00%, P: 0.015% or less, S: 0.006 %: Al: 0.005 to 0.060%, Ti: 0.005 to 0.025%, N: 0.001 to 0.006%, with the balance being Fe and inevitable impurities, 0.43% carbon equivalent Ceq is 0.33% or more, which is calculated from the steel component (mass%) or less, the weld crack susceptibility composition _{P CM} 0.24% 0.15% or less, the weld heat-affected zone toughness index f _HAZ has a composition of 0.30% or more and 0.47% or less, and the metal structure observed by an optical microscope at a 1/4 thickness position of the cross section in the thickness direction is composed of ferrite and the remaining second phase, and ferrite And the second phase is bainite and / or pearlite. Rannahli, yet made of steel circle-equivalent mean diameter of the ferrite is 12μm or less,
Yield strength in the tensile test at the steel plate original thickness is 325 to 505 MPa, tensile strength is 490 to 670 MPa, yield ratio is 80% or less, uniform elongation with the steel plate original thickness is 13% or more,
Cold press forming RHS for steel plate characterized in that 2mmV notch Charpy value taken from the steel plate front and rear surfaces is in at even -10 ° C. only at 90 J / cm ² or more at -40 ℃ 180J / cm ² or more.
However, the carbon equivalent Ceq, the weld crack susceptibility composition P _CM , and the weld heat affected zone toughness index f _HAZ are values defined as follows.
Ceq (%) = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14
P _CM (%) = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B
f _HAZ (%) = C + Mn / 8 + 6 × (P + S) + 12 × N−4 × Ti
(However, N is total nitrogen, and Ti = 0 when Ti ≦ 0.005%.) In the thick steel plate for cold press-formed square steel pipe according to claim 1,
Thickness for cold press-formed square steel pipe, characterized in that the steel is contained by mass%, further contains Ca: 0.0005-0.005%, and the mass ratio of Ca / S is 0.15 or more. steel sheet. In the thick steel plate for cold press-formed square steel pipe according to any one of claims 1 and 2,
A steel plate for cold press-formed square steel pipes, characterized in that the steel contains, by mass%, Nb: 0.005 to 0.030%. In the thick steel plate for cold press-formed square steel pipe according to any one of claims 1 to 3,
Cold press-molded square, wherein the steel contains at least one of Cu, Ni, Cr, and Mo by mass%, each being 0.04% or less, and a total of 0.10% or less Thick steel plate for steel pipes. In the thick steel plate for cold press-formed square steel pipe according to any one of claims 1 to 4,
A thick steel plate for cold press-formed square steel pipe, wherein the thickness t is in the range of 9 to 60 mm. A cold press-formed square steel pipe using the thick steel plate for cold-pressed square steel pipe according to claim 5 as a base plate ,
Outer corner bend radius (mm) is in the range of 2.0T～4.0T,
Furthermore, the Charpy value by a 2 mmV notch Charpy test piece taken along the steel pipe length direction from directly below the bending outer surface of the corner portion is 90 J / cm ² or more at −10 ° C. A cold-pressed square steel pipe characterized by a uniform elongation of 4% or more. A welding method for creating a welded joint formed by welding the end of the cold press-formed square steel pipe according to claim 6 to a plate surface of a diaphragm made of a steel plate,
Forming a reed groove at an end of the square steel pipe, and welding the reamed groove of the square steel pipe and the plate surface of the diaphragm by multilayer welding with an average heat input of 7 to 40 kJ / cm;
A welding method characterized by producing a welded joint in which a ² mmV notch Charpy value of an outer surface side heat-affected zone of the square steel pipe is 90 J / cm ² or more at 0 ° C.

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