WO2021255856A1

WO2021255856A1 - Box column

Info

Publication number: WO2021255856A1
Application number: PCT/JP2020/023750
Authority: WO
Inventors: 竜一本間; 智樹浦川; 駿田中
Original assignee: 日本製鉄株式会社
Priority date: 2020-06-17
Filing date: 2020-06-17
Publication date: 2021-12-23
Also published as: JPWO2021255856A1; JP7372577B2

Abstract

Provided is a box column in which a diaphragm comprising a second steel plate is fixed, via a welding section, to the interior of a box-shaped section column (column trunk) formed of a skin plate comprising a first steel plate, wherein: the first steel plate has a prescribed chemical composition in which Mn/Ni, which is the content ratio of manganese and nickel, is 0.80 or less and the carbon equivalent CEWES is 0.430%–0.900%; the first steel plate has a tensile strength of 780 MPa–930 MPa, a yield strength of 630 MPa–750 MPa, and a yield ratio of 85% or less; the plate thickness of the first steel plate is 40 mm–120 mm; the welding section is an electroslag welding section; and the mean Charpy absorbed energy in the heat-affected zone of the welding section at 0°C is at least 27 J.

Description

Box pillar

The present invention relates to a box pillar (BOX pillar).

Welded assembly box-shaped cross-section columns (so-called four-sided box columns) are used as steel frames for large buildings. A four-sided box pillar is a pillar trunk made by welding (square welding) four thick steel plates so that the cross section is square or rectangular. As a reinforcing member (diaphragm), it is a pillar member manufactured by attaching another thick steel plate in a rectangular shape of bamboo.
In recent years, the size of buildings has been further increased, and there is a demand for higher strength and thicker four-sided box pillars. On the other hand, if the steel sheet becomes thicker, the production efficiency of small heat input welding decreases. Therefore, from the viewpoint of improving construction efficiency, it is required to apply highly efficient large heat input welding such as electroslag welding and submerged arc welding to the diaphragm welding and square welding.
From the viewpoint of improving safety against rupture during an earthquake, box columns are also required to have a low yield ratio and excellent toughness. However, conventionally, when the above-mentioned large heat input welding is applied to a high-strength thick steel sheet, it has been difficult to secure good toughness in HAZ.

For example, Non-Patent Document 1 and Non-Patent Document 2 show the HAZ toughness of an electroslag welded portion in a 780 MPa class thick steel sheet with a tensile strength. However, according to FIG. 6 of Non-Patent Document 1, Charpy when the melt line (Fusion Line, FL), FL to 1 mm (HAZ1), FL to 3 mm (HAZ3), and FL to 5 mm (HAZ5) are notch positions. The average value of absorbed energy is 40 J or less. Further, according to FIGS. 3 and 5 of Non-Patent Document 2, the average value of Charpy absorption energy when FL is set to the notch position is 50 J or less.

In the large heat input welding HAZ (HAZ formed by applying the large heat input welding), austenite (γ) grows in grains when heated to a high temperature by the welding heat input. Further, since the high-strength steel plate contains a large amount of alloying elements, the large heat-affected zone HAZ has a bainite-based structure in which the old γ grain size is coarsened after cooling. As a result, the toughness of the large heat-affected zone HAZ decreases.
In order to suppress the decrease in toughness caused by such coarsening of crystal grains, for example, Patent Documents 1 to 3 describe fine particles that pin the grain boundaries of austenite (γ) heated by welding heat input. A technique for generating particles on a thick steel plate (base material) has been proposed. The techniques proposed in Patent Documents 1 to 3 suppress the grain growth of austenite (heated γ) heated by welding heat input by the pinning effect of fine particles containing Mg.
However, although the techniques of Patent Documents 1 to 3 can suppress the grain growth of austenite, the countermeasures against other factors of the decrease in toughness are not sufficient.

Patent Document 4 discloses a high-strength thick steel plate with a low yield ratio and excellent toughness in the heat-affected zone of large heat input, and in a welded joint having a plate thickness of 65 mm manufactured by electroslag welding (welding heat input amount ≥ 400 kJ / cm). It is disclosed that the charpy absorption energy of the bond portion at 0 ° C. is 70 J. Patent Document 4 discloses that MA can be reduced by optimizing the amounts of Si, Mn, and P. However, measures are insufficient to suppress the formation of MA that remarkably embrittles the large heat-affected zone, and it has not been shown that sufficient toughness of the large heat-affected zone HAZ can be obtained with a plate thickness of more than 65 mm.

Japanese Patent Application Laid-Open No. 2006-28827 Japanese Patent Application Laid-Open No. 11-236645 Japanese Patent Application Laid-Open No. 10-298708 Japanese Patent Application Laid-Open No. 2017-155333

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a box column formed by using a high-strength thick steel plate and having excellent toughness of a large heat-affected zone HAZ. do.
In the present invention, the weld heat-affected zone (HAZ) formed by the large heat input welding may be referred to as a large heat input welding HAZ or a large heat input HAZ.

As a result of the study by the present inventors, the main causes of deterioration of the high heat-affected zone HAZ toughness of high-strength thick steel plates are (1) the embrittled phase (Martensite-Austenite constitut) due to the increase in the content of alloying elements. It was found that MA) was formed and (2) HAZ crystal grains were coarsened.
Therefore, the present inventors have increased the strength of the steel plate (base material) and ensured the toughness of the large heat-affected zone HAZ from the viewpoint of suppressing the formation of MA that significantly embrittles the large heat-affected zone of the high-strength steel plate. We conducted a study to achieve both. As a result, it was found that the formation of MA was caused by the microsegregation part formed by locally thickening alloying elements such as Mn and Ni contained in the steel sheet. Specifically, it was found that when the microsegregated zone is heated and cooled by the influence of welding heat, a part of the metal structure becomes MA due to the phase transformation and causes a decrease in toughness. As a result of further studies by the present inventors, it is caused by microsegregation that the Mn / Ni, which is the ratio of the Mn content to the Ni content, is controlled to 0.80 or less in the steel component (chemical composition). It was found that it is effective in suppressing the production of MA.
Further, after suppressing the formation of MA by the above-mentioned method, Ti, Mg, and B are utilized to control the carbon equivalent CEWES to suppress the coarsening of the old γ and the coarsening of the crystal grains of HAZ. We have obtained a new finding that it is possible to secure both the strength of the base metal and the toughness of the large heat-affected zone HAZ.

The present invention was made in view of the above findings. The gist of the present invention is as follows.

[1] In the box pillar according to one aspect of the present invention, a diaphragm made of a second steel plate is fixed to the inside of a box-shaped cross-section pillar made of a skin plate made of a first steel plate via a welded portion. The first steel plate, which is a box pillar, has a chemical composition of C: 0.03% or more, 0.18% or less, Mn: 0.3% or more, less than 1.4%, Ni in mass%. : 1.0% or more, 7.0% or less, Al: 0.005% or more, 0.20% or less, B: 0% or more, 0.0050% or less, Ti: 0% or more, 0.035% or less , Cu: 0% or more, 2.0% or less, Cr: 0% or more, 2.0% or less, Mo: 0% or more, 2.0% or less, W: 0% or more, 1.0% or less, Co : 0% or more, 1.0% or less, Nb: 0% or more, 0.10% or less, V: 0% or more, 0.10% or less, Ca: 0% or more, 0.005% or less, Mg: 0 % Or more, 0.005% or less, REM: 0% or more, 0.005% or less, Zr: 0% or more, 0.005% or less, Si: 0.30% or less, P: 0.015% or less, S : 0.005% or less, O: 0.0060% or less, N: 0.0100% or less, the balance consists of Fe and impurities, and Mn / Ni, which is the ratio of Mn and Ni contents, is 0. It is 80 or less, the carbon equivalent CEWES calculated by the following equation (1) is 0.430% or more and 0.900% or less, the tensile strength of the first steel plate is 780 MPa or more and 930 MPa or less, and the yield. The strength is 630 MPa or more and 750 MPa or less, the yield ratio is 85% or less, the plate thickness of the first steel plate is 40 mm or more and 120 mm or less, and the welded portion is an electroslag welded portion. In the HAZ of the welded portion, the average of the charpy absorption energy at 0 ° C. is 27 J or more.
CEWES = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ... (1)
Here, C, Mn, Si, Ni, Cr, Mo, and V in the formula (1) are the content [mass%] of each element, and 0 is substituted for the term of the element not contained.
[2] In the box column according to the above [1], the average crystal grain size measured by EBSD in the microstructure of the welded portion of the first steel plate is 250 μm or less, and the area ratio of MA is It may be 3.0% or less.
[3] The box pillar according to the above [1] or [2] may have an average Charpy absorption energy of 70 J or more.

According to the above aspect of the present invention, it is possible to provide a box column formed by using a high-strength thick steel plate and having excellent toughness of high heat-affected zone HAZ.

It is a figure which shows the collecting procedure of the Charpy test piece in the electroslag welded T-shaped joint. It is a figure which shows the collecting procedure of the Charpy test piece in the electroslag welded T-shaped joint.

Hereinafter, a box pillar according to an embodiment of the present invention (a box pillar according to the present embodiment) will be described.
The box pillar according to the present embodiment is a box in which a diaphragm made of a second steel plate is fixed via a welded portion inside a box-shaped cross-section pillar (pillar trunk) made of a skin plate made of a first steel plate. It is a pillar.
Further, as will be described later, the first steel sheet has a predetermined chemical composition, a predetermined plate thickness, and a predetermined mechanical property.
In the box column in the present embodiment, the welded portion is a portion that has been once melted and solidified by welding (welded metal portion) and a portion that has not been melted by welding but whose structure has changed due to the heat of welding (heat). Affected zone: HAZ), and the base metal portion refers to a portion (a portion excluding the welded portion) that is not affected by the heat of welding (the structure has not changed from that before welding).

<First steel plate>
(Chemical composition)
The chemical composition of the first steel sheet (chemical composition of the base material portion) will be described.
Hereinafter,% regarding the chemical composition is mass% unless otherwise specified.

C: 0.03% or more and 0.18% or less C is an element that enhances the hardenability of steel and contributes to high strength. In order to obtain this effect, the C content is 0.03% or more.
On the other hand, from the viewpoint of preventing excessive formation of cementite and ensuring toughness, the C content is set to 0.18% or less. The C content is preferably 0.17% or less, more preferably 0.16% or less.

Mn: 0.3% or more and less than 1.4% Mn is an element that enhances the hardenability of steel and contributes to high strength. In order to obtain this effect, the Mn content is set to 0.3% or more.
On the other hand, if the Mn content is excessively increased, the MA of the large heat-affected zone HAZ increases, and the toughness is significantly deteriorated. Therefore, the Mn content is set to less than 1.4%. The Mn content is preferably 1.3% or less, more preferably 1.2% or less, still more preferably 1.1% or less.

Ni: 1.0% or more, 7.0% or less Ni is an element that enhances the hardenability of steel and contributes to high strength, and at the same time, it is also an element that enhances the toughness of high heat input HAZ. From the viewpoint of ensuring strength and toughness, the Ni content is 1.0% or more. The Ni content is preferably 1.2% or more, more preferably 1.4% or more, still more preferably 1.5% or more.
On the other hand, Ni is an expensive element, and the Ni content is 7.0% or less from the viewpoint of suppressing an increase in manufacturing cost.

Mn / Ni: 0.80 or less, which is the ratio of the contents of Mn and Ni. Both Mn and Ni are elements that contribute to increasing the strength of steel. Easy to accelerate production. Therefore, the Mn content is preferably smaller than the Ni content. From the viewpoint of ensuring toughness while increasing the strength of the large heat-affected zone HAZ, it is the ratio obtained by dividing the Mn content in the steel by the Ni content in the first steel plate provided in the box column according to the present embodiment. Mn / Ni is 0.80 or less. Mn / Ni is preferably 0.70 or less, more preferably 0.60 or less. The lower limit of Mn / Ni may be the ratio obtained by dividing the lower limit of the Mn content by the upper limit of the Ni content, that is, it may be 0.17 or more. Mn / Ni may be 0.20 or more.

Al: 0.005% or more and 0.20% or less Al is important as a deoxidizing element. Further, when adding B, Al is contained in order to suppress the precipitation of BN and secure a solid solution B effective for hardenability of steel by forming AlN and fixing N. It is an important element. In order to exert this effect, the Al content is 0.005% or more in this embodiment. It is preferably 0.01% or more.
On the other hand, when the Al content becomes excessive, a coarse aluminum oxide that becomes a fracture starting point and lowers toughness is produced. From the viewpoint of suppressing the formation of such a coarse aluminum oxide, the Al content is 0.20% or less. The Al content is preferably 0.18% or less, more preferably 0.16% or less, still more preferably 0.15% or less.

Si: 0.30% or less Si may be contained in steel for deoxidation and ensuring strength, but it is also an element that promotes the formation of MA. As a result of investigating the harmfulness of Si on MA, the present inventors confirmed that Si has an extremely large effect on MA formation in the microsegregated portion of the large heat input HAZ. Therefore, in order to secure the toughness of the large heat-affected zone HAZ, the Si content is set to 0.30% or less. The Si content is preferably 0.25% or less, more preferably 0.20% or less, still more preferably 0.15% or less. Since the lower limit of the Si content is not particularly limited, it may be 0%, but the Si content may be 0.01% or more from the viewpoint of manufacturing cost.

The chemical composition of the first steel plate of the box column according to the present embodiment may contain the above elements and the balance may be iron (Fe) and impurities, but in order to improve strength and toughness, it may be necessary. , One selected from the group consisting of the following selective elements, B, Ti, Cu, Cr, Mo, W, Co, Nb, V, Ca, Mg, REM, Zr, instead of a part of Fe. Two or more kinds may be contained. However, since these elements do not necessarily have to be contained, the lower limit is 0%. Further, even if these elements are mixed into the steel sheet as impurities from raw materials, scraps, etc., there is no significant adverse effect as long as they are within the range described later.

B: 0% or more and 0.0050% or less B is an element that significantly improves the hardenability of steel even if it is contained in a small amount, and when the hardenability of steel is ensured while limiting the carbon equivalent CEWES. It is an effective element for. In order to obtain the above effect, the B content may be 0.0003% or more.
On the other hand, if the B content is excessive, the toughness and weldability of the large heat-affected zone HAZ deteriorate. Therefore, even when it is contained, the B content is 0.0050% or less.

Ti: 0% or more, 0.035% or less Ti is an element effective for increasing the strength of the base metal and granulating it. The Ti content may be 0%, but in order to obtain the above effect, the Ti content may be 0.005% or more.
On the other hand, if the Ti content is excessive, there is a concern that the coarse TiN adversely affects the toughness. Therefore, even when it is contained, the Ti content is 0.035% or less.

Cu: 0% or more, 2.0% or less Cu has a small adverse effect on weldability and HAZ toughness, and is also an element that improves the strength and toughness of the base metal. The Cu content may be 0%, but in order to obtain the above effect, the Cu content may be 0.1% or more.
On the other hand, when the Cu content becomes excessive, Cu cracks occur during hot rolling of the steel sheet, and the toughness and weldability of the large heat-affected zone HAZ deteriorate. Therefore, even when it is contained, the Cu content is set to 2.0% or less.

Cr: 0% or more, 2.0% or less Cr is an element that improves the strength of the base metal. The Cr content may be 0%, but in order to obtain the above effect, the Cr content may be 0.1% or more.
On the other hand, if the Cr content becomes excessive, the toughness and weldability of the large heat-affected zone HAZ deteriorate. Therefore, even when it is contained, the Cr content is set to 2.0% or less.

Mo: 0% or more, 2.0% or less Mo is an element that improves the strength and toughness of the base metal. The Mo content may be 0%, but in order to obtain the above effect, the Mo content may be 0.1% or more.
On the other hand, when the Mo content becomes excessive, the toughness and weldability of the large heat-affected zone HAZ deteriorate, and the alloy cost increases. Therefore, even when it is contained, the Mo content is set to 2.0% or less.

W: 0% or more, 1.0% or less W is an element that improves the strength and toughness of the base metal. The W content may be 0%, but in order to obtain the above effect, the W content may be 0.1% or more.
On the other hand, when the W content becomes excessive, the toughness and weldability of the large heat-affected zone HAZ deteriorate, and the alloy cost increases. Therefore, even when it is contained, the W content is 1.0% or less. The W content is preferably 0.5% or less.

Co: 0% or more, 1.0% or less Co is an element that has little adverse effect on weldability and toughness of HAZ and improves the strength and toughness of the base metal. The Co content may be 0%, but the Co content may be 0.1% or more in order to obtain the above effects.
On the other hand, when the Co content becomes excessive, the above effects are saturated and the alloy cost increases. Therefore, even when it is contained, the Co content is 1.0% or less. The Co content is preferably 0.5% or less.

Nb: 0% or more and 0.10% or less Nb is also an element that improves the strength and toughness of the base metal. The Nb content may be 0%, but in order to obtain the above effect, the Nb content may be 0.005% or more, or 0.01% or more.
On the other hand, if the Nb content becomes excessive, the toughness and weldability of the large heat-affected zone HAZ deteriorate. Therefore, even when it is contained, the Nb content is set to 0.10% or less. The Nb content is preferably 0.05% or less, more preferably 0.03% or less.

V: 0% or more, 0.10% or less V is an element that improves the strength of the base metal part of the steel. The V content may be 0%, but in order to obtain the above effect, the V content may be 0.005% or more, or 0.01% or more.
On the other hand, when the V content becomes excessive, the toughness and weldability of the large heat-affected zone HAZ deteriorate. Therefore, the V content is 0.10% or less even when it is contained. The V content is preferably 0.08% or less, more preferably 0.06% or less.

Ca: 0% or more, 0.005% or less Ca is an element that forms oxides, sulfides, and acid sulfides to suppress the formation of coarse inclusions and enhance the toughness of the base material and HAZ. The Ca content may be 0%, but in order to obtain the above effect, the Ca content may be 0.0001% or more or 0.001% or more.
On the other hand, when the Ca content becomes excessive, Ca-based inclusions that may act as a starting point for brittle fracture increase. Therefore, even when it is contained, the Ca content is 0.005% or less. The Ca content is preferably 0.004% or less.

Mg: 0% or more, 0.005% or less Mg is an element that forms oxides, sulfides, and acid sulfides like Ca to suppress the formation of coarse inclusions and enhance the toughness of the base metal and HAZ. be. The Mg content may be 0%, but in order to obtain the above effect, the Mg content may be 0.0001% or more or 0.001% or more.
On the other hand, when the Mg content becomes excessive, the amount of Mg-based inclusions that may act as a starting point for brittle fracture increases. Therefore, even when it is contained, the Mg content is set to 0.005% or less. The Mg content is preferably 0.003% or less.

REM: 0% or more, 0.005% or less REM, like Ca and Mg, forms oxides, sulfides, and acid sulfides to suppress the formation of coarse inclusions, and improves the toughness of the base metal and HAZ. It is an element that enhances. The REM content may be 0%, but in order to obtain the above effects, the REM content may be 0.0001% or more or 0.001% or more.
On the other hand, when the REM content becomes excessive, the amount of REM-based inclusions that may act as a starting point for brittle fracture increases. Therefore, even when it is contained, the REM content is 0.005% or less. The REM content is preferably 0.003% or less.
REM (rare earth element) is a general term for two elements, Sc and Y, and 15 lanthanoid elements such as La, Ce and Nd. The REM referred to in the present embodiment is composed of one or more selected from these rare earth elements, and the REM content described below is the total amount of the rare earth elements.

Zr: 0% or more, 0.005% or less Zr, like Ca, Mg and REM, forms oxides, sulfides and acid sulfides to suppress the formation of coarse inclusions, and the base material and HAZ. It is an element that enhances toughness. The Zr content may be 0%, but in order to obtain the above effect, the Zr content may be 0.0001% or more.
On the other hand, when the Zr content becomes excessive, the amount of Zr-based inclusions that may act as a starting point for brittle fracture increases. Therefore, even when it is contained, the Zr content is set to 0.005% or less. The Zr content is preferably 0.003% or less.

The balance of the chemical composition of the first steel plate of the box pillar according to the present embodiment is iron (Fe) and impurities. Impurities are components that are mixed in by raw materials such as ore and scrap and other factors when steel materials are industrially manufactured, and are allowed as long as they do not adversely affect the characteristics of the box pillars according to the present embodiment. Means something.
However, among impurities, especially for P, S, O, and N, the upper limit of the content is limited as described later.

P: 0.015% or less P is an impurity harmful to toughness. The P content needs to be limited in order to stably secure the toughness of the large heat-affected zone, and is 0.015% or less in this embodiment. The P content is preferably 0.010% or less, more preferably 0.008% or less. The lower limit of the P content is not limited, but the P content may be 0.001% or more from the viewpoint of manufacturing cost. Further, although P is an impurity harmful to toughness, it has the effect of enhancing the hardenability of the large heat-affected zone HAZ, making the crystal grain size finer, and improving the toughness of the large heat-affected zone HAZ. From the viewpoint of obtaining the effect, the P content may be 0.003% or more.

S: 0.005% or less S is an impurity, and if it is contained in a large amount, it may form coarse inclusions and reduce toughness. Therefore, in order to stably secure the toughness of the large heat-affected zone HAZ, the S content is limited to 0.005% or less. The lower limit of the S content is not particularly limited and may be 0%, but the S content may be 0.0001% or more or 0.001% or more from the viewpoint of manufacturing cost.

O: 0.0060% or less O is an impurity and an element forming an oxide. When a coarse aluminum oxide generated by combining O and Al is superimposed on the microsegregated zone of the large heat-affected zone, it acts as a fracture starting point and the toughness becomes extremely low. Therefore, the O content is set to 0.0060% or less. It is desirable that the O content is low, and it may be 0%, but from the viewpoint of manufacturing cost, the O content may be 0.0001% or more in this embodiment.

N: 0.0100% or less N is an element that forms a nitride. When the N content becomes excessive, BN is generated and the amount of solid solution B that contributes to the improvement of hardenability is significantly reduced, or coarse nitrides are formed and the toughness is lowered. From the viewpoint of suppressing the formation of coarse nitrides and suppressing the formation of BN to ensure hardenability, the N content is 0.0100% or less. The N content is preferably 0.0080% or less, more preferably 0.0060% or less.
It is desirable that the N content is low, but from the viewpoint of manufacturing cost, the N content may be 0.0001% or more, or 0.0020% or more.

Carbon equivalent CEWES is 0.430% or more and 0.900% or less In the first steel plate of the box column according to the present embodiment, the content of each element is controlled as described above, and then the carbon equivalent CEWES is further applied. It shall be 0.430% or more and 0.900% or less. The carbon equivalent CEWES is an index of hardenability that affects the strength of the steel sheet (base material) and the hardness of HAZ. In order to secure the strength of the first steel sheet, the carbon equivalent CEWES is 0.430% or more in this embodiment. The carbon equivalent CEWES is preferably 0.440% or more, more preferably 0.450% or more, still more preferably 0.500% or more.
On the other hand, when the carbon equivalent CEWES exceeds 0.900%, the large heat-affected zone becomes martensite, and the toughness of the large heat-affected zone decreases. Therefore, CEWES is set to 0.900% or less. The carbon equivalent CEWES is preferably 0.800% or less, more preferably 0.700% or less.
The carbon equivalent CEWES is calculated by the following equation (1) using the content of alloying elements.
CEWES = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ... (1)
Here, C, Mn, Si, Ni, Cr, Mo, and V in the formula (1) are the content [mass%] of each element, and 0 is substituted for the term of the element not contained.

(Micro organization)
In the welded portion of the first steel plate of the box column according to the present embodiment, the average particle size measured by EBSD is preferably 250 μm or less.
When the average particle size is 250 μm or less, the toughness of the welded portion of the first steel sheet is improved.
The average particle size of the welded portion of the first steel sheet can be achieved by suppressing crystal grain growth by the pinning effect of TiN, adjusting the hardenability by adjusting CEWES, and the like.

The average crystal grain size of the welded portion is in the range from FL to HAZ side 0.5 mm (FL to FL + 0.5 mm range) in the L cross section of the electroslag welded joint, using an EBSD (electron backscatter diffraction device). When the crystal orientation is measured and the grain boundary is defined as a 15 ° large-angle grain boundary (a large-angle grain boundary with a crystal orientation difference of 15 ° or more), it is equivalent to a circle among crystal grains with a circle-equivalent diameter of more than 1.0 μm. The average value is the circle-equivalent diameter of the crystal grains whose diameter is within the upper 0.2%. At that time, the measurement area is 0.5 mm × 0.5 mm, and the measurement pitch is 1.0 μm at the maximum.

Further, in the welded portion of the box column according to the present embodiment, from the viewpoint of ensuring toughness, it is preferable that the area ratio of MA is small, and the area ratio of MA is preferably 3.0% or less.
The structure other than MA is not limited, but from the viewpoint of ensuring strength and toughness, a mixed structure mainly composed of bainite structure and partially containing ferrite and martensite is preferable.
The MA area ratio can be achieved by adjusting Mn / Ni or the like.

The area ratio of MA (Martensite-Austenite Constant) is the same cross section as the sample for which the average crystal grain size was measured, MA is revealed by repeater etching, an optical microscope image with a magnification of 500 times is taken, and then image analysis, etc. It is obtained by the method of. As a method of measuring the area ratio, for example, there is a method of separating the part that looks white from the other part by image processing of the captured optical microscope image and measuring the area ratio of the part that looks white as the area ratio of MA. ..

(Mechanical characteristics, plate thickness)
Tensile strength: 780 MPa or more, 930 MPa or less Yield strength: 630 MPa or more, 750 MPa or less Yield ratio: 85% or less Plate thickness: 40 mm or more, 120 mm or less With the increase in size of buildings, higher efficiency of construction, and improvement of safety, welding The demand for thick steel plates for structures and box columns manufactured using the thick steel plates is increasing. In the box column according to the present embodiment, in order to meet these requirements, the plate thickness of the first steel plate is 40 mm or more and 120 mm or less, the yield strength is 630 MPa or more and 750 MPa or less, and the tensile strength is 780 MPa or more and 930 MPa or less. .. Further, from the viewpoint of earthquake resistance, the yield ratio of the first steel plate of the box pillar according to the present embodiment is 85% or less. The lower limit of the yield ratio is not limited, but for example, the yield ratio may be 70% or more.

For the mechanical properties of the first steel sheet, JIS No. 4 tensile test pieces taken from the surface of the steel sheet at a position of 1/4 of the plate thickness were prepared, and JIS Z 2241: 2011 was applied to these two test pieces. Obtained by conducting a tensile test in accordance with this. The yield strength YS (0.2% yield strength) and the tensile strength TS are the average values of the two test pieces, respectively. YR (yield ratio) is the ratio of YS to TS and is expressed as a percentage, that is, 100 × (YS / TS). The unit of YR (yield ratio) is%.

<Second steel plate>
The second steel plate fixed inside the box-shaped cross-section column (column trunk) as the diaphragm as a reinforcing member is not limited, but usually a YP440 class steel plate is used, the plate thickness is 40 mm to 70 mm, and the TS is. It is preferably 440 to 600 MPa.

<Welded part>
In the box column according to the present embodiment, from the viewpoint of high efficiency of construction, at least the welding of the first steel plate (skin plate) and the second steel plate (diaphragm) is electroslag welding, which is a large heat input welding. It is assumed that (ESW) is applied.
That is, the first steel plate and the second steel plate are joined via a welded portion (electroslag welded portion) formed by electroslag welding.
In the box pillar according to the present embodiment, from the viewpoint of the safety of the building, the average value of the Charpy absorption energy (test temperature: 0 ° C.) in the HAZ of the electroslag weld is 27J or more.
Preferably, in a box column in which the average value of the charpy absorption energy (test temperature: 0 ° C.) in the HAZ of the electroslag weld is 70 J or more, if electroslag welding is not applied, it is characteristic of electroslag welding on the skin plate. There is no start / end treatment for the start part and crater part, and there is a weld line when the pillars are joined to the skin plate near the diaphragm. That is, whether or not electroslag welding is applied (whether or not the welded portion is an electroslag welded portion) can be determined by the weld line on the surface of the skin plate.

The Charpy absorption energy in the HAZ of the weld is measured by the following method.
In the box column, as shown in FIGS. 1A and 1B, the second steel plate (diaphragm 2) is vertically fixed to the first steel plate (skin plate 1) via a welded portion, and is a T-shaped joint. Will be. Therefore, the test piece 4 passes from the weld metal portion 3 along the plate thickness center line of the second steel plate, crosses the melt line (FL), passes through the HAZ on the first steel plate side, and passes through the HAZ on the first steel plate side to the inner side of the first steel plate. It is collected from the part leading to. The position of the notch is the position of FL, which tends to have the lowest toughness in HAZ, and three test pieces are collected. For example, when the notch position is the FL position, the test piece 4 and the skin plate 1 are collected in the direction perpendicular to each other as shown in FIG. 1A, or the test piece 4 is the skin plate 1 as shown in FIG. 1B. The test piece may be collected so as to be slanted with respect to the subject. That is, as shown in FIG. 1B, the test piece may be collected so that the intersection of the line 6 mm in the plate thickness direction from the surface of the skin plate 1 and the FL is at the center of the notch.
The Charpy impact test conforms to JIS Z 2242: 2018, and the test temperature is 0 ° C. If necessary, the test may be performed at −20 ° C. The absorbed energy (KV2 (0 ° C.)) is the average value (arithmetic mean) of the absorbed energies of the three measured pieces.

<Manufacturing method>
Next, a method of manufacturing a box pillar according to the present embodiment will be described.
The box pillar according to this embodiment is
(I) A process (preparation process) for preparing the first steel sheet and the second steel sheet to be welded, and
(II) Includes a step of assembling the first steel plate and the second steel plate into a box pillar (assembly step).
Hereinafter, each step will be described.

(I) Preparation step First, a first steel plate to be a skin plate for a box pillar and a second steel plate to be a diaphragm are prepared.
The first steel sheet is not limited as long as it has the above-mentioned chemical composition and mechanical properties, and for example, a steel sheet manufactured by the following manufacturing method can be used.

(Preferable manufacturing method of the first steel sheet)
A steel piece having a thickness of 200 mm or more, which is composed of the above-mentioned chemical composition and is manufactured by a continuous casting method, is manufactured. This piece of steel is once cooled to 400 ° C. or lower, then heated to a temperature range of 900 ° C. or higher and 1250 ° C. or lower, and subjected to hot rolling to produce a steel sheet having a plate thickness of 40 mm or higher and 120 mm or lower. Will be done. The steel sheet is subjected to various heat treatments as needed.

When the steel pieces after continuous casting are charged into a heating furnace by hot charging without being cooled to 400 ° C. or lower, the coarse γ structure generated during casting remains in the steel pieces after heating, and the structure of the steel sheet. However, it may not be sufficiently refined and the low temperature toughness may deteriorate. Therefore, it is preferable that the steel pieces after continuous casting are once cooled to 400 ° C. or lower.

The heating temperature of the steel piece is preferably 900 ° C. or higher in order to dissolve the BN deposited on the steel piece after casting and promote the formation of TiN in hot rolling. N in the heated steel pieces forms TiN during hot rolling, and the formation of BN is suppressed. As a result, in the steel sheet, the solid solution B that improves the hardenability of the steel and the TiN that suppresses the grain growth are sufficiently secured.
On the other hand, the heating temperature of the steel pieces is 1250 ° C. or lower from the viewpoint of suppressing the coarsening of γ grains, refining the metal structure after hot rolling, and suppressing the deterioration of low temperature toughness. preferable. The heating temperature is more preferably 1200 ° C. or lower.

The steel sheet after hot rolling is directly quenched or once air-cooled, and then reheated to the γ single phase region and subsequently quenched (γ reheat quenching).

In the case of direct quenching after hot rolling, the end temperature (finishing temperature) of hot rolling is preferably the austenite (γ) single phase region, that is, the Ar ₃ transformation point or higher at which the ferrite transformation starts. At this time, even if the temperature of the surface layer portion of the steel sheet is in the two-phase region of austenite (γ) / ferrite (α) at the end of hot rolling, there is no problem if the temperature of the central portion in the plate thickness direction is in the γ single phase region. No. The end temperature of hot rolling may be 750 ° C. or higher. The end temperature of hot rolling is preferably 900 ° C. or lower from the viewpoint of miniaturization of the metal structure. In the present embodiment, the Ar ₃ transformation point can be obtained by the following equation (4).

Ar ₃ transformation point = 868-396 × C + 24.6 × Si-68.1 × Mn-36.1 × Ni-20.7 × Cu-24.8 × Cr + 29.1 × Mo… (4)

Here, C, Si, Mn, Ni, Cu, Cr, and Mo in the above equation (4) are the contents of each element expressed in mass% in the steel sheet, and 0 is substituted for the term of the element not contained. do.

In the case of direct quenching after hot rolling, hot rolling is finished in the γ single-phase region, and water cooling is continued in order to adjust the material of the steel sheet.
On the other hand, when the steel sheet is air-cooled after hot rolling, the steel sheet is reheated to the γ single phase region and subsequently quenched (γ reheat quenching).

After hot rolling, the steel sheet that has been directly quenched or γ-reheat-quenched is subjected to various heat treatments in order to adjust the material. Specifically, the steel sheets subjected to these quenching treatments (direct quenching or γ reheating quenching) are moved to the two-phase region where austenite (γ) and ferrite (α) coexist in order to reduce the yield ratio. Reheating and subsequent quenching (two-phase region quenching) are performed.
Here, the two-phase region is the Ac ₁ transformation point or more _{and less than the Ac 3} transformation point, and the Ac ₁ transformation point and the Ac ₃ transformation point can be obtained by the following equations (5) and (6), respectively. ..

Ac ₁ transformation point = 750.8-26.6 x C + 17.6 x Si-11.6 x Mn-22.9 x Cu-23.0 x Ni + 24.1 x Cr + 22.5 x Mo-39.7 x V -5.7 x Ti + 232.4 x Nb-169.4 x Al-894.7 x B ... (5)
Ac ₃ transformation point = 910-203 x √C + 44.7 x Si-30 x Mn-400 x Al-15.2 x Ni + 104 x V + 31.5 x Mo + 13.1 x W + 11 x Cr + 20 x Cu-700 x P-400 x Ti ... (6)

Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, Nb, Al, B, W, and P in the above equations (5) and (6) are expressed in% by mass. It is the content of the element in the steel plate, and 0 is substituted for the term of the element that does not contain.

Furthermore, the steel sheet may be tempered in order to finally adjust the strength, yield ratio, and toughness of the steel sheet. When tempering is performed, the tempering temperature is preferably 350 ° C. or higher and 600 ° C. or lower.

Here, the above-mentioned hot rolling finish temperature, γ reheating quenching temperature, two-phase region quenching temperature, and tempering temperature all refer to the temperature at the center in the plate thickness direction. The temperature of the central portion in the plate thickness direction can be obtained by heat transfer calculation from the temperature of the steel plate surface measured by a radiation thermometer.

The first steel sheet can be manufactured by the above manufacturing method (manufacturing method including direct quenching or γ reheating quenching + two-phase region quenching + tempering).

The method for manufacturing the second steel sheet is not limited, and a steel sheet manufactured by a known manufacturing method can be used. Alternatively, as the second steel sheet, a steel sheet manufactured by the same manufacturing method as the first steel sheet can also be used.

(II) Assembly process In the assembly process, the box pillars are assembled by welding so that the first steel plate becomes a skin plate and the second steel plate becomes a diaphragm. As the welding and assembling methods, known conditions may be adopted, but at least the welding between the first steel plate and the second steel plate is electroslag welding. For welding of other parts, shielded metal arc welding, carbon dioxide shielded arc welding, and submerged arc welding may be used.
For example, the diaphragm is welded to the column flange of the skin plate by electroslag welding, and the column flange and the column web are welded (square welding) so that the skin plate has four sides in a box shape, and then the diaphragm and the skin plate are welded. By manufacturing (pillar flange and pillar web) by electroslag welding, it is possible to efficiently manufacture a four-sided box pillar. Welding can be performed by selecting appropriate welding materials and welding conditions, but in principle, it is preferable to select welding materials from JIS standard products or products certified by the Minister of Land, Infrastructure, Transport and Tourism according to the welding construction conditions and the like.
Further, in a different type joint having different strengths between the skin plate and the diaphragm, a welding material satisfying the standard value on the low strength side can be used.
Regarding other matters regarding the manufacturing method of box columns, matters common to building steel frame construction and standard specifications can be referred to. For example, JASS6 steel frame construction, steel frame construction technical guideline / factory production edition, steel frame construction technical guideline / construction site construction edition, etc. of the Architectural Institute of Japan.

A first steel plate and a second steel plate used for assembling the box pillar were prepared.
As the first steel plate used as the skin plate, steel plates of steel 1-1 to steel 2-14 having the chemical composition, plate thickness and mechanical properties shown in Tables 1A to 1D were prepared. Further, as the second steel plate used as the diaphragm, known 440 MPa class steels A to D having the chemical composition and plate thickness shown in Table 2 were prepared.
The mechanical properties of the first steel sheet were evaluated by performing a tensile test in accordance with JIS Z 2241: 2011 as described above.

Further, Steel 1-1 to Steel 2-14 were produced by the following methods.
Steel pieces with a thickness of 200 to 300 mm manufactured by the continuous casting method are once cooled to 400 ° C. or lower, then heated to a temperature range of 900 ° C. or higher and 1250 ° C. or lower, and hot-rolled to a plate thickness of 40 to 120 mm. It was made into a steel plate. Some steel sheets were directly quenched from temperatures above the _{Ar 3 transformation point.} For the steel sheets that were not directly quenched, after air cooling, reheating to the γ single phase region and subsequent quenching (γ reheating quenching) were carried out.
After hot rolling, the steel sheet that had been directly quenched or γ-reheated and quenched was reheated to the two-phase region and subsequently quenched (two-phase region quenching). Further, the steel sheet after quenching in the two-phase region was tempered at 350 ° C. or higher and 600 ° C. or lower.

Subsequently, the box columns were assembled by welding so that the first steel plate became a skin plate and the second steel plate became a diaphragm. When assembling, the diaphragm is welded to the column flange of the skin plate by electroslag welding, and the column flange and the column web are welded by carbon dioxide gas shield arc welding so that the skin plate has four sides of a box shape (square welding). ), Then the diaphragm and the skin plate (pillar flange and pillar web) were electroslag welded to obtain a four-sided box pillar of 1000 mm × 1000 mm (cross section) × 13000 mm (length).
For electroslag welding, welding is performed by using JIS standard products or welding materials certified by the Minister of Land, Infrastructure, Transport and Tourism, setting the current to 380 A and the voltage to 52 V, and changing the speed according to the thickness of each plate. The heat input was 55 to 130 kJ / mm.

A sample was taken from the joint portion between the first steel plate (skin plate) and the second steel plate (diaphragm) of the obtained box column so that the plane perpendicular to the weld line could be observed, and the HAZ side from FL 0. In a crystal grain with a circle-equivalent diameter of more than 1 μm when the crystal orientation is measured using EBSD in the range up to 5 mm (FL to FL + 0.5 mm range) and the grain boundary is defined as a 15 ° large-angle grain boundary. Then, the circle-equivalent diameters of the crystal grains whose circle-equivalent diameters were within the upper 0.2% were obtained, and these were averaged to obtain the average crystal grain size.
Further, MA was revealed by repeater etching on the same cross section, an optical microscope image at a magnification of 500 times was taken, and then the area ratio of MA was obtained by image analysis.
The results are shown in Table 3.

In addition, three test pieces were collected from the position shown in FIG. 1A so that the notch position was the FL position, and the Charpy impact test was performed, the test temperature was 0 ° C., and the test temperature was 0 ° C., in accordance with JIS Z 2242: 2018. The average value (arithmetic mean) of the absorbed energies of the three measured pieces was taken as the HAZ toughness at 0 ° C.
The results are shown in Table 3.

As shown in Tables 1A to 3, the BOX column No. 1 using steel 1-1 to steel 1-16 as the skin plate. In 1 to 16, the HAZ toughness (average value of Charpy absorption energy) of the electroslag welded portion was 27J or more.
On the other hand, the BOX pillar No. which is a comparative example. At 17 to 30, HAZ toughness was inferior.

In the box column of the present invention, the yield strength of the first steel plate is 630 MPa or more, the tensile strength is 780 MPa or more, the yield ratio is 85% or less, the plate thickness is 40 to 120 mm, and the electroslag welded portion. The average value of Charpy absorption energy (test temperature 0 ° C.) in HAZ is 27J or more. Therefore, the box columns of the present invention are suitable for building steel frames, and by applying the box columns of the present invention, it is possible to promote the progress of high-rise buildings and large spans, and further improve construction efficiency and seismic safety. Can be enhanced.

1 skin plate 2 diaphragm 3 weld metal part 4 test piece 5 winnings

Claims

A box column in which a diaphragm made of a second steel plate is fixed via a welded portion inside a box-shaped cross-section column made of a skin plate made of a first steel plate.
The first steel sheet has a chemical composition of% by mass.
C: 0.03% or more, 0.18% or less,
Mn: 0.3% or more, less than 1.4%,
Ni: 1.0% or more, 7.0% or less,
Al: 0.005% or more, 0.20% or less,
B: 0% or more, 0.0050% or less,
Ti: 0% or more, 0.035% or less,
Cu: 0% or more, 2.0% or less,
Cr: 0% or more, 2.0% or less,
Mo: 0% or more, 2.0% or less,
W: 0% or more, 1.0% or less,
Co: 0% or more, 1.0% or less,
Nb: 0% or more, 0.10% or less,
V: 0% or more, 0.10% or less,
Ca: 0% or more, 0.005% or less,
Mg: 0% or more, 0.005% or less,
REM: 0% or more, 0.005% or less,
Zr: 0% or more, 0.005% or less,
Si: 0.30% or less,
P: 0.015% or less,
S: 0.005% or less,
O: 0.0060% or less,
N: Contains 0.0100% or less,
The rest consists of Fe and impurities
Mn / Ni, which is the ratio of the contents of Mn and Ni, is 0.80 or less.
The carbon equivalent CEWES calculated by the following equation (1) is 0.430% or more and 0.900% or less.
The tensile strength of the first steel sheet is 780 MPa or more and 930 MPa or less, the yield strength is 630 MPa or more and 750 MPa or less, and the yield ratio is 85% or less.
The thickness of the first steel plate is 40 mm or more and 120 mm or less.
The welded portion is an electroslag welded portion, and the average Charpy absorption energy at 0 ° C. in the HAZ of the welded portion is 27 J or more.
A box pillar that features that.
CEWES = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ... (1)
Here, C, Mn, Si, Ni, Cr, Mo, and V in the formula (1) are the contents in mass% of each element, and 0 is substituted for the term of the element not contained.
In the microstructure of the welded portion of the first steel sheet,
The average crystal grain size measured by EBSD is 250 μm or less.
The area ratio of MA is 3.0% or less,
The box pillar according to claim 1.
The average of the Charpy absorbed energy is 70 J or more.
The box pillar according to claim 1 or 2.