STEEL BASE MATERIAL

Description

TECHNICAL FIELD

The present invention relates to a steel base material used for a building.

BACKGROUND ART

In a building, a steel base material is widely used mainly as a base material for an indoor ceiling and wall. Since the steel base material is generally used, characteristics such as a shape, a material, and a strength as a member are specified in JIS A 6517:2010. This JIS standard specifies that as the material of the steel base material, a material that satisfies “hot-dip galvanized steel plate and steel strip” in JIS G 3302:2019 or “hot-dip 55% aluminum-zinc alloy plated steel plate and steel strip” in JIS G 3321:2019 is used.

Fire resistance performances required for each part of the building, such as non-damage property, heat shielding property, and flame shielding property, are determined by the Building Standards Act in order to prevent the spread of damage when a fire occurs.

In a partition wall, an outer wall, and a ceiling using a steel base material for construction, a fire resistance covering material is attached to the steel base material to form a fire resistance structure in order to secure the heat shielding property and the flame shielding property among the fire resistance performances. For example, a wall structure includes columns made of a steel base material and disposed at regular intervals, and a fire resistance covering material such as a plasterboard attached to these columns.

When the fire occurs in the building having such a fire resistance structure, the fire resistance covering material shields heat and flame to protect the steel base material, and thus to protect the building. However, when the fire resistance covering material is damaged by the fire, for example, is broken, the fire resistance performance of the fire resistance structure such as heat shielding property and flame shielding property is lost.

Therefore, as a result of studying the factors of the cracking of the fire resistance covering material when the fire resistance performance of the wall structure using the steel base material for construction is evaluated, it has become clear that the following is considered. Since heat is blocked by the fire resistance covering material in the fire resistance structure at the beginning of the fire, the steel base material for construction is kept at room temperature. However, as time elapses from the occurrence of the fire, the heat is gradually conducted from the fire resistance covering material to the steel base material, the steel base material is heated, and thermal expansion occurs. Since both the ends of the steel base material are constrained by other members in the building, it is considered that the steel base material is deformed by buckling from the other members due to the thermal expansion, and stress is applied to the fire resistance covering material attached to the steel base material to cause cracking.

Patent Literature 1 discloses a technique for preventing damage to the fire resistance covering material in the fire resistance structure using the steel base material and the fire resistance covering material. Specifically, Patent Literature 1 discloses a dry fire resistance structure for a steel column including: a steel column; a fire resistance covering material in which plate-shaped bodies disposed so as to surround the steel column are connected to each other at corner portions to form a cylindrical body; and a spacer disposed between the fire resistance covering material and the steel column to separate the fire resistance covering material and the steel column from each other, wherein the spacer is in contact with both the steel column and the fire resistance covering material and is fixed to only one of both the steel column and the fire resistance covering material, the steel column and the fire resistance covering material are movable relative to each other in the axial direction of the steel column, and both the steel column and the fire resistance covering material do not follow changes in lengths of both the steel column and the fire resistance covering material due to heating.

Patent Literature 1 describes that the spacer is fixed to only one of the steel column and the fire resistance covering material, and the steel column and the fire resistance covering material are relatively movable in the axial direction of the steel column, so that even when the steel column thermally expands, stress does not act on the fire resistance covering material, and there is no risk of damage due to the thermal expansion of the steel column.

Meanwhile, in the technique described in Patent Literature 1, the spacer is introduced between the steel base material and the fire resistance covering material, so that it is necessary to provide a space corresponding to the spacer, and the living space of the building is narrowed.

The fire resistance structure including the steel base material and the fire resistance covering material is required to reduce the amount of materials to be used from the economic viewpoint of cost reduction and the environmental viewpoint of the reduction of carbon emission, which has been attracting attention in recent years. Therefore, if the fire protection and fire resistance performance of the fire resistance structure can be secured by a thinner heat insulating material, the fire resistance structure is considered to be economically and environmentally advantageous.

Furthermore, as described above, it is considered that the factor of damaging the fire resistance covering material is that the steel base material is deformed at high temperature, and stress is applied to the fire resistance covering material attached to the steel base material to cause cracking, but the characteristics of the steel base material at high temperature are not specified in JIS A 6517:2010. That is, it is not possible to suppress the deformation of the steel base material only by satisfying the JIS standard. Here, the steel base material for construction is not generally used for a load-bearing wall that bears the structure of a building such as a wall structure and a ceiling structure, and is applied only to a non-load-bearing wall. For example, Non-Patent Literature 1 describes that a light-gauge steel frame is used as a base in the specification (non-load-bearing wall) of a partition wall. Non-Patent Literature 2 describes that a steel wall base material is a non-structural body. For this reason, the following is considered: it is not necessary to ensure non-damage property relating to building collapse in fire resistance performance, and it is assumed that there is no problem in performance even if the strength is reduced in a fire. For this reason, in building material use, the application of a fire resistance galvanized steel plate having excellent high-temperature characteristics (for example, Patent Literature 2 and the like) to a steel base material for construction has not been studied heretofore.

Therefore, an object of the present invention is to provide a steel base material which is less likely to cause buckling at a high temperature.

Citation List

Patent Literatures

• • Patent Literature 1: JP 6970381 B2
  • Patent Literature 2: JP 3267324 B2

Non-Patent Literatures

• • Non-Patent Literature 1: the Ministry of Land, Infrastructure, Transport and Tourism, “Maintenance guidelines for wooden fire resistance buildings in government facilities, References, Chapter 2 Case Study”, page 13, [online], March 2013, the Ministry of Land, Infrastructure, Transport and Tourism, [search date: Sep. 7, 2023], Internet <URL: https://www.mlit.go.jp/common/000993926.pdf>
  • Non-Patent Literature 2: Yashio Kenzaikogyo Co., ltd., “Product Catalog Steel Base Material for Construction”, page 6, [online], January 2020, Yashio Kenzaikogyo Co., ltd., [search time: Sep. 7, 2023], Internet <URL:https://www.yasio.jp/doc/catalog/catalog_seihinLT_202001.pdf>

SUMMARY OF INVENTION

A steel base material according to one aspect of the present invention includes a steel plate having a yield strength of 250 MPa or more at 500° C. and a yield strength of 125 MPa or more at 600° C.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing a relationship between a yield strength and a temperature.

FIG. 2 is a graph showing a relationship between a heating time and a strain amount in a length direction of a heated surface obtained by an FEM.

DESCRIPTION OF EMBODIMENTS

The present inventors have found that the use of a material which is less likely to cause buckling at a high temperature as a steel base material, in the fire resistance structure of a building including the steel base material and a fire resistance covering material makes it possible to secure fire resistance performance without introducing a spacer between the steel base material and the fire resistance covering material and even when the fire resistance covering material to be used is thin. Therefore, the present invention makes it possible to provide a steel base material which is less likely to cause buckling at a high temperaturc.

Hereinafter, a steel base material according to an embodiment of the present invention will be described.

The steel base material according to the present embodiment includes a steel plate having a yield strength of 250 MPa or more at 500° C. and a yield strength of 125 MPa or more at 600° C.

Such a configuration makes it possible to provide a steel base material which is less likely to cause buckling at a high temperature.

As described above, the steel base material for construction is not generally used for a load-bearing wall that bears the structure of a building such as a wall structure and a ceiling structure, and is applied only to a non-load-bearing wall. Therefore, conventionally, in building material use, the application of a steel plate capable of securing a strength even at a high temperature to the steel base material for construction has not been studied. The present inventors have found that the use of a steel plate which has a sufficient high-temperature strength and is less likely to cause buckling at a high temperature as a steel base material makes it possible to secure the fire resistance performance of the fire resistance structure of a building including the steel base material and a fire resistance covering material even when the fire resistance covering material to be used is thin. Furthermore, it has been clarified that there is an advantage that the living space of the building can be further widened since the cracking of the fire resistance covering material due to the buckling of the steel base material can be suppressed even at a high temperature, which makes it unnecessary to introduce a spacer between the steel base material and the fire resistance covering material.

The reason is considered as follows. When a fire occurs in the building, thermal expansion occurs when the steel base material constituting the fire resistance structure is heated. At this time, since both the end portions of the steel base material are constrained by other members in the building, a compressive force acts on the steel base material from the other members due to the thermal expansion. When the compressive force acting on the steel base material exceeds the yield strength after a lapse of time from the occurrence of the fire, the steel base material is deformed by buckling. When stress is applied to the fire resistance covering material attached to the steel base material to cause cracking due to the deformation of the steel base material, the fire resistance performance of the fire resistance structure is lost. Here, since the steel plate according to the present embodiment has a higher yield strength at high temperatures of 500° C. and 600° C. than that of a conventional steel plate, the steel plate is less likely to cause buckling even when heated. Therefore, in the fire resistance structure of the building, the occurrence of damage such as cracking of the fire resistance covering material can be suppressed for a long time after the fire occurs without introducing the spacer between the steel base material of the present embodiment and the fire resistance covering material according to the present embodiment. This makes it possible to secure a wider living space of the building while securing the fire resistance performance.

According to the steel base material of the present embodiment, by using the fire resistance covering material having the same fire resistance structure as the fire resistance structure using the conventional steel plate for the steel base material, a time from the occurrence of the fire to the occurrence of buckling in the steel base material can be delayed as compared with the case of using the conventional steel plate, and the fire resistance performance can be improved.

Furthermore, according to the steel base material of the present embodiment, even if the fire resistance covering material used for the fire resistance structure is made thinner or smaller than that in the case of using the conventional steel plate for the steel base material, the time from the occurrence of the fire to the occurrence of buckling in the steel base material can be made equal to that in the case of using the conventional steel plate. That is, fire resistance performance equivalent to conventional fire resistance performance can be secured even with a small amount of fire resistance covering material, which makes it possible to achieve cost reduction, carbon emission reduction, and securement of a wider living space.

Steel Plate

The steel plate used for the steel base material according to the present embodiment has a yield strength of 250 MPa or more at 500° C. and a yield strength of 125 MPa or more at 600° C. The use of the steel plate having such a configuration as the steel base material makes it possible to provide a steel base material which is less likely to cause buckling at a high temperature.

The yield strength at 500° C. is preferably 280 MPa or more, and more preferably 300 MPa or more. The upper limit of the yield strength at 500° C. is not particularly limited, but is preferably 500 MPa or less as a feasible numerical value.

The yield strength at 600° C. is preferably 150 MPa or more, and more preferably 175 MPa or more. The upper limit of the yield strength at 600° C. is not particularly limited, but is preferably 300 MPa or less as a feasible numerical value.

The yield strength at room temperature of the steel plate used for the steel base material according to the present embodiment is preferably 250 MPa or more, and more preferably 300 MPa or more. In the present embodiment, the room temperature is 20° C. The upper limit of the yield strength at room temperature is not particularly limited, but is preferably 1000 MPa or less as a feasible numerical value.

The yield strength of the steel plate according to the present embodiment is a value measured by a measurement method specified in JIS G0567:2020.

The steel base material according to the present embodiment may include the steel plate as described above and a metal plating formed on the surface of the steel plate. In general, from the viewpoint of improving corrosion resistance, the metal plating can be formed on the surface of the steel plate, but the yield strength of the above-described steel plate at 500° C., 600° C., and room temperature does not change depending on the presence or absence of the plating. That is, the plated steel plate in which the metal plating is formed on the surface of the steel plate in the present embodiment preferably has a yield strength of 250 MPa or more at 500° C. and a yield strength of 125 MPa or more at 600° C. The yield strength of the plated steel plate at 500° C. is more preferably 280 MPa or more, still more preferably 300 MPa or more, and preferably 500 MPa or less. The yield strength of the plated steel plate at 600° C. is more preferably 150 MPa or more, still more preferably 175 MPa or more, and preferably 300 MPa or less. The yield strength at room temperature is preferably 250 MPa or more, more preferably 300 MPa or more, and preferably 500 MPa or less.

The chemical composition of the steel plate according to the present embodiment is not particularly limited, but examples of elements contained in the steel plate include C (carbon), Si (silicon), Mn (manganese), Cu (copper), Ni (nickel), Cr (chromium), Mo (molybdenum), Ti (titanium), Nb (niobium), V (vanadium), P (phosphorus), S (sulfur), N (nitrogen), and B (boron).

Metal Plating

Examples of the metal plating formed on the surface of the steel plate in the steel basc material according to the present embodiment include a molten zinc plating, an alloyed molten zinc plating, a molten 55% aluminum-zinc alloy plating, and a molten zinc-aluminum-magnesium alloy plating.

Structure

The structure of the steel plate of the steel base material according to the present embodiment is not particularly limited, but the area ratio of a recrystallized structure is preferably less than 20%. When the area ratio of the recrystallized structure is less than 20%, the yield strength at a high temperature can be more reliably enhanced. The area ratio of the recrystallized structure is more preferably less than 10%. The lower limit value of the area ratio of the recrystallized structure is not particularly limited, and the yield strength at a high temperature can be more reliably enhanced as the area ratio is smaller. The area ratio of the recrystallized structure can be determined using the photograph of a steel plate portion in the cross section of the steel base material taken using an optical microscope.

Production Method

A method for producing the steel base material is performed by a general cold-rolled steel plate production method and hot-dip galvanizing method. An example of the method for producing a steel base material will be described below.

For example, steel having a desired chemical composition is first cast. The cast steel is hot-rolled, subjected to pickling and cold rolling, and then subjected to heat treatment in a process such as a continuous annealing line (CAL or CAPL) or a continuous hot-dip galvanizing line (CGL), so that a steel plate having a yield strength of 250 MPa or more at 500° C. and a yield strength of 125 MPa or more at 600° C. can be obtained. A galvanized steel plate having a yield strength of 250 MPa or more at 500° C. and a yield strength of 125 MPa or more at 600° C. can be obtained by subjecting the steel plate to the heat treatment in the process such as a continuous annealing line (CAL or CAPL), and then applying a zinc plating in a hot-dip galvanizing line or an electroplating line, or applying heat treatment and a molten zinc plating in the process of the continuous hot-dip galvanizing line (CGL). The obtained steel plate and galvanized steel plate can be processed into a predetermined size and shape and used as a steel plate base material.

The conditions for the heat treatment of the cold-rolled steel plate are not particularly limited since the conditions can be appropriately changed depending on the chemical composition of the steel and the conditions for the cold rolling, but for example, it is preferable that the cold-rolled steel plate is heated from room temperature to an annealing temperature of 650 to 900° C. and held at the annealing temperature for 1 to 1800 s. By this annealing treatment, the area ratio of the recrystallized structure can be set to be less than 20% in the structure of the steel plate. The annealing temperature is preferably 680°C. or higher, and preferably 880° C. or lower. A holding time (annealing time) at the annealing temperature is preferably 5 s or more, and preferably 1500 s or less.

The annealed steel plate is preferably cooled to 600 to 400° C. at a cooling rate of 3 to 100°C./s.

The present description discloses various modes of techniques as described above, of which the main techniques are summarized below.

As described above, the steel base material according to one aspect of the present invention includes a steel plate having a yield strength of 250 MPa or more at 500° C. and a yield strength of 125 MPa or more at 600° C.

The use of the steel plate having such a configuration as the steel base material makes it possible to provide the steel base material capable of securing fire resistance performance even if the amount of the fire resistance covering material to be used is small in the fire resistance structure of a building including the steel base material and the fire resistance covering material.

The steel plate in the steel base material having the above configuration may have a yield strength of 250 MPa or more at room temperature.

The use of the steel plate having such a configuration as the steel base material makes it possible to provide the steel base material capable of securing not only the fire resistance performance but also a more sufficient strength at room temperature.

The steel base material having the above configuration may further include a metal plating formed on the surface of the steel plate.

EXAMPLES

Samples

As samples, steel plates A and B described below were used. The steel plate A is Comparative Example, and is a commercially available hot-dip galvanized steel plate (SGCC) specified in JIS G 3302:2019, to which Nb (niobium), Ti (titanium), and V (vanadium) are not added. The steel plate B is a cold-rolled steel plate which contains, as a component composition thereof, 0.06% by mass of C (carbon), 0.02% by mass of Si (silicon), 1.45% by mass of Mn (manganese), 0.02% by mass of Nb (niobium), and 0.04% by mass of Ti (titanium), with the balance being Fe (iron) and unavoidable impurities, and which is prepared in the laboratory under the following production conditions. The steel plate B is the present inventive example satisfying the requirements of the present embodiment.

Production Conditions

• • Hot Rolling Condition, Cold Rolling Condition

A slab was heated at 1200°C., rolled to 3.6 mm, inserted into a holding furnace at 550° C. to simulate winding, held for 30 minutes, and then cooled in the furnace. The front and back surfaces of the cooled hot-rolled steel plate were ground to 1.6 mm. This is a laboratory process for simulating a 1.6 mm hot-rolled steel plate in an actual production facility. Thereafter, the 1.6 mm hot-rolled steel plate was cold-rolled to 0.8 mm.

Annealing Condition

The cold-rolled steel plate was held at 700° C. for 60 s, cooled to 200° C. or lower at a cooling rate of 10°C./s, and then allowed to cool to obtain a steel plate B. The steel plate B is not plated in order to verify the ideal state of the galvanized steel plate in the laboratory.

The area ratio of the recrystallized structure of the produced steel plate B was 0.5%. The area ratio of the recrystallized structure of the steel plate in the steel plate A was 100%. The area ratio of the recrystallized structure was determined using the photograph of the steel plate taken using an optical microscope.

Evaluation

A tensile test piece of JIS No. 13B specified in JIS Z 2241:2022 was collected from each of the steel plate A and the steel plate B, and a tensile test was performed at each temperature of 20° C. to 750° C. in accordance with JIS G 0567:2020 to measure the yield strength of the tensile test piece. At temperatures of 100° C. or higher, the tensile test piece was held at each temperature for 10 minutes, and then subjected to the tensile test. A tensile speed in a tensile tester was set to 0.004 min^-1 up to the yield strength.

FIG. 1 is a graph showing a relationship between a yield strength and a temperature. The yield strength of the steel plate A at 20° C. is 159 MPa, the yield strength at 500° C. is 123 MPa, and the yield strength at 600° C. is 101 MPa. The yield strength of the steel plate B at 20° C. is 901 MPa, the yield strength at 500° C. is 382 MPa, and the yield strength at 600° C. is 167 MPa. From FIG. 1, it can be seen that the steel plate B has a higher yield strength at each temperature than that of the steel plate A, and has a particularly excellent yield strength at high temperatures of 500° C. and 600° C.

Next, deformation behaviors due to heating when the steel plates A and B were used as steel base materials were subjected to simulation by finite element analysis (FEM). For the FEM, in addition to the above measured values, the longitudinal elastic moduli, transverse elastic moduli, and linear expansion cocfficients of the steel plates A and B were used.

The conditions of the simulation in the FEM were as follows. The reason why a heated surface is only one surface of a rectangular tube-shaped steel base material is that the surface simulates a heating state at the time of a fire.

Dimensions and shape of steel base material

• • Plate thickness: 0.8 mm
  • Cross-section: Square with side length of 45 mm
  • Length: 2000 m
  • Heated surface: Only one of four side surfaces
  • Constraint condition: Constrain both ends of steel base material in longitudinal direction
  • Temperature T (° C.) of each surface at time t (min) from heating start time
  • Heated surface: T (t)=900×log₁₀(1+0.08 t)
  • Surface facing heated surface: T(t)=900×log₁₀(1+0.045(t−5)), provided that room temperature (20° C.) is maintained from the start of heating to 5 minutes
  • Longitudinal elastic moduli E (kgf/cm²) and transverse elastic moduli G (kgf/cm²) of steel plate A and steel plate B at temperature T (° C.)
  • Longitudinal elastic modulus E: E=2.14×10⁶×exp (−0.000374 T)
  • Transverse elastic modulus G: G=8.30×10⁶×exp (−0.000409 T)
  • Linear expansion coefficients α (1/° C.) of steel plates A and B: α=0.000012

As a reference example, a perfect elastic body having no yield strength was also subjected to the FEM.

FIG. 2 is a graph showing a relationship between a heating time(s) and a strain amount in the length direction of the heated surface, obtained by the FEM. According to FIG. 2, the steel plate A of Comparative Example started plastic deformation (buckling) in about 600 s after the start of heating. Meanwhile, the steel plate B of the present inventive example started plastic deformation in about 1800 s after the start of heating. From these, it can be seen that the steel plate B has more excellent fire resistance performance than that of the steel plate A. FIG. 2 shows that plastic deformation does not occur in the perfect elastic body of the reference example.

This application is based on Japanese Patent Application No. 2022-151271 filed on Sep. 22, 2022, the contents of which are incorporated herein.

The present invention has been appropriately and sufficiently described through the embodiments with reference to specific examples and the drawings and the like in the foregoing to express the present invention, but it should be recognized that a person skilled in the art can easily change and/or improve the above-described embodiments. Therefore, unless a change or improvement made by a person skilled in the art is at a level departing from the scope of rights of the claims described in Claims, the change or improvement is interpreted to be included in the scope of rights of the claims.

Industrial Applicability

The present invention has broad industrial applicability in the technical field of steel base materials used in buildings.