Shock-resistant and corrosion-resistant structural steel

Abstract

To improve the seismic resistance and corrosion resistance of steel materials used in steel structures. [Solution] The steel grade used for structural steel materials such as H, I, and C shaped steels, and square and round steel pipes in construction will be changed from the current low-carbon steel to industrial pure iron. The yield strength will decrease by approximately 20%, but the wall thickness will be increased inward by 20% to 100% to ensure tensile strength equal to or greater than the current level. The elongation value of the steel will be more than double that of the current material (40-50%), and seismic resistance will be strengthened in conjunction with the improvement in yield strength. The surplus strength will also allow for a reduction in the number of steel materials used. Since this industrial pure iron contains an excess of oxygen, it forms a dense magnetite film on the surface when corroded, improving corrosion resistance.

Description

Technical Field

[0001] The present invention relates to structural steel such as H-beams used in steel structures, and relates to steel materials having excellent seismic resistance and durability related to the safety and durability of structures.

Background Art

[0002] Structural steel such as H-beams, I-beams, C-channels, square tubes, round tubes, etc. are widely used in steel structures. When used in buildings, etc., not only the structure but also the seismic resistance and corrosion resistance of the steel material used as its raw material become major problems. Regarding the seismic resistance of structures, regulations and improvements through revisions are in progress according to the Building Standards Law and others, but there does not seem to be a dramatic advancement regarding the steel materials themselves. Structural steel is an economical steel material manufactured by hot rolling using low-carbon steel as a raw material along with reinforcing bars. The shape, dimensions, and mechanical properties of the products are specified in detail by JIS. As seen as an example in Patent Document 1, a measure is disclosed to add metallurgical improvements to improve the seismic resistance of steel materials, partially increasing the strength and preventing the accompanying decrease in toughness. When a structure receives impact and destructive external forces, deformation occurs by plastic elongation immediately before fracture, and this toughness is an essential element for avoiding collapse, and the greater it is, the more effective it is. It can be said that the first condition for the seismic resistance of steel materials is that toughness is prioritized, not to mention strength.

[0003] Patent Document 2 discloses a method aiming to improve strength while maintaining appropriate toughness, which is a characteristic of low-carbon steel. It is modified by using special steel and appropriate control of the metal structure, but there are cost problems, and the elongation value is also about 20% of the conventional level and has not reached a dramatic improvement.

[0004] Increasing the strength of steel materials is highly beneficial, but there are also problems regarding seismic resistance. Increasing the strength means an increase in the yield stress. Since the longitudinal elastic modulus of steel is constant regardless of the steel type, an increase in the yield stress means an increase in elastic strain. That is, it means that the shaking becomes more severe, which is inconvenient for peace of mind.

[0005] Patent Document 3 discloses an economical method for manufacturing corrosion-resistant reinforcing bars. To dramatically improve corrosion resistance, it uses virtually industrial-grade pure iron with extremely low carbon content and high oxygen content. Corrosion resistance also provides long-term support for earthquake-resistant construction. The lack of strength due to carbon deficiency is compensated for by impurities and controlled rolling during hot rolling. This technology may be useful in solving the above problem. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Published Patent Application 2020-172707 [Patent Document 2] Published Patent Application No. 2019-011508 [Patent Document 3] Patent No. 6808873 [Overview of the project] [Problems that the invention aims to solve]

[0007] While structural design is the primary requirement for earthquake-resistant steel structures, improvements in the seismic resistance of the steel materials themselves are also needed. Increasing strength is one way to improve seismic resistance, but a dramatic improvement in ductility is more effective in preventing building collapse. Conventional structural steel is made of ordinary low-carbon steel, which possesses the necessary elongation, but further improvements are expected. The corrosion resistance of steel materials also contributes to earthquake resistance in the long term. The present invention aims to solve the problem of economically providing structural steel sections and the like that possess dramatically improved ductility and corrosion resistance, in order to advance the seismic and corrosion resistance of structures from the perspective of steel materials. [Means for solving the problem]

[0008] The present invention relates to a structural steel or square / round steel pipe made of an extremely low-carbon, high-oxygen steel grade whose composition, by mass%, consists of 0.02% or less of C, 0.04% to 0.13% or less of O, 0.01% or less of Si, 0.5% or less of Mn, and the rest consisting of unavoidable impurities and Fe, characterized in that the cross-sectional shape has the same external inclusion dimensions as the dimensions specified by JIS, the wall thickness is 1.2 times or more and 2.0 times or less, the yield strength is 240 MPa or more, and the elongation is 40% or more, making it earthquake-resistant and corrosion-resistant structural steel or square / round steel pipe. [Effects of the Invention]

[0009] Since the material is essentially pure iron, the tensile strength of the steel is inferior to that of conventional low-carbon steel. However, by increasing the wall thickness, the yield strength and breaking load increase in proportion to the wall thickness ratio, making it easy to achieve the same or even higher levels as conventional steel. Furthermore, as is well known to those skilled in the art, because it is essentially pure iron, the elongation increases by 40-50%, more than twice that of ordinary structural steel, steel bars, and steel pipes, thus strengthening the overall seismic resistance (allowing deformation but preventing fracture and collapse).

[0010] Increasing the wall thickness has another effect: elastic strain decreases inversely proportional to the wall thickness ratio. In other words, earthquake shaking is suppressed. As seen in the Great Hanshin Earthquake, where furniture was thrown out from the upper floors of high-rise buildings made of high-strength steel, low-strength steel is advantageous for safety and security.

[0011] The corrosiveness of steel increases and deteriorates in proportion to the amount of alloy, except for large amounts of chromium. Oxygen is the only exception; oxygen-containing steel (essentially industrial pure iron) readily forms black rust, which suppresses the progression of subsequent corrosion. The steel material with the components of this invention has excellent corrosion resistance. Maintenance costs such as painting are reduced. Because it is pure iron, it is highly resistant to low-temperature brittleness and is unaffected by H-embrittlement. It results in a stable steel structure.

[0012] Let's explain the economics. With increased wall thickness, the mass of a single steel bar increases in proportion to the wall thickness ratio. The raw material cost increases proportionally. There is a reduction in the processing cost, and the total cost is slightly lower than the wall thickness ratio. For example, if the wall thickness becomes 1.5 times, the total cost becomes about 1.4 times. Although it is difficult to accept the increase in the selling unit price (yen per piece), if the surplus of the increased strength is utilized for reducing the number of pieces used, the total steel material cost will be about 10% higher. Furthermore, from the perspective of LCA (Life Cycle Assesment), steel structures will all be disassembled, recycled, and regarded as buried resources. Due to the increase in the amount of resources, the purchase loss will be even smaller.

Brief Description of the Drawings

[0013] [Figure 1] The cross-sectional shape of the H-shaped steel is shown, where A is the current JIS and B is the invention of the present application. [Figure 2] The cross-sectional shape of the square steel pipe is shown, where A is the current JIS and B is the invention of the present application. [Figure 3] Shows the influence of alloying elements on the corrosion resistance of iron. Source of this figure: Japan Iron and Steel Federation, 'Steel Materials and Alloying Elements', P.223 [Figure 4] Shows a comparison of the cost composition of steel materials.

Embodiments for Carrying Out the Invention

[0014] The H-shaped steel used for construction according to the invention of the present application will be described according to FIG. 1. A shows the cross-sectional shape based on JIS (G3192), B is the cross-sectional shape of the invention of the present application, 1 is the web, and 2 is the flange. The outer shape inclusion line of the invention of the present application is equivalent to JIS, and the wall thickness expands inward in the flange part and expands outward on both side surfaces in the web part. The increase in the wall thickness results in an increase in the cross-sectional area, and the tensile strength and bending strength increase. The wall thickness ratio (web t1 / to1, flange t2 / to2) is set according to the following formula. It is not necessary for the web and flange to have the same ratio. t1 / to1 = 1.2 - 2.0 t2 / to2 = 1.2 = 2.0 Since the outer shape is the same, the content of the assembly work is no different from the conventional one. The I-shaped steel and C-shaped steel are also thickened in the same way.

[0015] Figure 2 shows the case of a square steel pipe for construction (commonly known as a column) 3 of JIS (G3466). In the present invention, similarly, the wall thickness is enlarged inward, and the tensile strength and bending strength increase. The round steel pipe is also thickened in the same manner.

[0016] Table 1 shows the chemical components and mechanical properties of various steel materials. In the present invention, as shown in the table, alloy elements, deoxidizers, etc. are not contained, and it is regarded as industrial pure iron characterized by extremely low carbon and high oxygen.

[0017]

Table 1

[0018] For the materials of shaped steel, usually SN400 and SN490 of JIS low-carbon steel are used. For square steel pipes, STKR400 and SYKR490 of JIS are used. They are respectively equivalent low-carbon steel and low-carbon Mn steel, with balanced strength and toughness, and are characterized by easy availability and manufacture of raw materials. In the present invention as well, low-grade scrap iron equivalent to shaped steel and steel bars for reinforcing bars is used as the main raw material. The product purity is 99.0% or more, which is not high, and the raw material cost has no difference from the above products. Regarding the mass production method of industrial pure iron, there was once Armco iron for electromagnets that was oxidized and refined for a long time in a basic open hearth furnace. Recently, vacuum decarburization and vacuum deoxidation are applied, but both have a large cost burden. In the present invention, the method described in Patent Document 3, that is, in a ladle, reduced-pressure bubbling with an oxidizing gas is performed on molten steel, and oxidation and decarburization are quickly processed, thus solving the cost problem.

[0019] Regarding the mechanical properties of various steel materials described in Table 1, the numbers 400 and 490 of JIS indicate the lower limit value of the tensile strength. The main factors related to earthquake resistance are rather the yield strength and elongation value. The standard of the yield stress of 235 MPa is loose, and in reality, it is approximately 300 MPa, which is much higher. The characteristic of pure iron compared to low-carbon steel is that its strength is about 20% lower, but its elongation is more than doubled (15-20% to over 40%), a dramatic improvement. Changing the type of steel (low-carbon steel to industrial-grade pure iron) solves one of the conditions for earthquake resistance. In this invention, the decrease in strength due to the change in steel type is compensated for by increasing the cross-sectional area of ​​the steel, that is, by increasing the wall thickness of the structural steel. Since the yield stress decreases by approximately 80% (300 MPa ⇒ 240 MPa), the wall thickness ratio needs to be at least 1.2 times the current ratio. If it exceeds 1.2 and approaches 2.0, the excess can be expected to reduce the number of structural materials (columns, beams, etc.) used, but if it exceeds 2.0, the cost problem becomes significant and undesirable.

[0020] If you wish to enhance seismic resistance by changing high-strength steel grades such as SN490 and STKP490 to the steel grade of the present invention, you should increase the wall thickness to match the increase in the strength standard value. Approximately 1.5 to 1.7 times the original thickness will be necessary.

[0021] Table 2 compares the sectional properties of H-shaped steel between JIS standards and the present invention. The second moment of area and section modulus increase almost proportionally to the increase in wall thickness ratio. An increase of approximately 20% in wall thickness ratio provides bending strength comparable to the current standards, and a wall thickness ratio exceeding 20% ​​improves the second condition for seismic resistance (yield strength).

[0022] [Table 2]

[0023] Table 3 compares the cross-sectional properties of a rectangular steel pipe between JIS standards and the present invention. The second moment of area and section modulus increase almost proportionally to the increase in wall thickness ratio, increasing bending strength and improving seismic resistance.

[0024] [Table 3]

[0025] Another requirement for steel structures is corrosion resistance, which relates to durability and aesthetics. Weathering steel is sometimes used for outdoor structures such as bridges, but this presents cost issues. The present invention is an extended application of the weather-resistant reinforcing bar described in Patent Document 3, with equivalent chemical composition, corrosion resistance, and manufacturing method. Therefore, cost issues are also resolved.

[0026] Figure 3 shows an example illustrating the effect of alloying elements on the corrosion resistance of iron. As is well known, iron itself is relatively resistant to corrosion, but when it contains carbon to become steel, or when it contains large amounts of alloying elements or impurities, the corrosion process accelerates as shown in the figure. Today, deoxidized steel is commonplace, and it is generally assumed that steel has low corrosion resistance. However, as described in Patent Document 3, when oxygen is added to pure iron, a magnetite non-conductive film is formed, improving corrosion resistance. Wrought iron (which contains a high amount of oxygen) is positioned between steel and industrial-grade pure iron, and is well known to be more resistant to corrosion than steel, which is why the Eiffel Tower's repainting cycle is approximately 7 years, longer than that of typical steel structures. The steel material of the present invention does not need to be painted, and even if it is painted, the paint life is extended compared to ordinary deoxidized steel.

[0027] We will now examine the economic aspects of the present invention. Figure 4 shows the approximate cost breakdown of one H-beam weighing one ton. The raw material cost is approximately 50,000 yen per ton, the selling price is approximately 100,000 yen, and the difference of 50,000 yen represents processing costs, fixed costs, and profit. The new steel material of this invention has a wall thickness ratio of 1.5 and a product mass of 1.5 tons. If the selling price increases by 1.5 times, the manufacturer will have no complaints. However, it lacks appeal for the user. The increased thickness is expected to improve production efficiency and streamline processing costs, making a selling price of approximately 1.4 times more reasonable.

[0028] While increasing wall thickness results in a 50% increase in strength, 20% of that is essential for reducing yield stress. If 20% of the remaining 30% of strength is used to reduce the amount of steel used (e.g., reducing the standard required number of 100 to 80), Total purchase price: 100,000 yen / ton x 100 bottles = 10,000,000 yen This invention costs 140,000 yen / ton x 80 units = 11,200,000 yen. * The total will increase by 12%.

[0029] Steel structures will eventually be dismantled, removed, and turned into scrap metal. The recovery of scrap metal has significant economic benefits. While scrap prices fluctuate, they tend to increase gradually in the macroeconomic and long term. Structural steel scrap (thick-walled scrap) is considered high-grade scrap (low impurities, efficient melting, and high yield), resulting in a high recovery cost (0.8). Thickening the walls increases the amount of scrap recovered. Even if it's a burden during construction, it represents the long-term preservation of resources. We will add the perspective of LCA (Life Cycle Assessment). Demolition profit (current): 1 ton x 100 pieces x 50,000 yen / ton x 0.8 = 4 million yen This invention: 1.5t x 80 pieces x 50,000 yen / ton x 0.8 = 480 yen * The loss is reduced from 12% to 4%. From the above, it can be seen that the present invention is effective in improving the reliability of steel structures without significant burden. [Examples]

[0030] To improve seismic resistance, the material of the current H-beams will be changed from low-carbon steel to the industrial pure iron of the present invention. The yield strength will be approximately 20% lower. In order to make the strength of the steel material equal to or greater than the current material, it will be necessary to increase the wall thickness by at least approximately 20%. A comparison will be made when the wall thickness ratio is 1.2. Current 400mm×200mm, t=6.0mm This invention: 400mm x 200mm, t=7.2mm As illustrated in Table 1, both the second moment of area and the section modulus increase to 1.2 times the original value in response to the reinforcement to a thickness ratio of 1.2, so there are no strength issues. The growth rate can be more than doubled, resulting in a dramatic improvement. Raw material costs will increase by 1.2 times, but total costs will only increase by 10-15%. When LCA (Life Cycle Assessment) is taken into account, the loss on purchase price will be halved. [Explanation of symbols]

[0031] 1; H-beam web 2; H-beam flange 3; Cross-section of rectangular steel pipe

Claims

[Claim 1] A structural steel or square / round steel pipe made of an extremely low-carbon, high-oxygen steel grade whose composition, by mass%, consists of C at 0.02% or less, O at 0.04% to 0.13%, Si at 0.01%, Mn at 0.5% or less, and the rest consisting of unavoidable impurities and Fe, characterized in that the cross-sectional shape has the same external inclusion dimensions as the dimensions specified by JIS, the wall thickness is 1.2 times or more to 2.0 times, the yield strength is 240 MPa or more, and the elongation is 40% or more, making it earthquake-resistant and corrosion-resistant.

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