JPH0364441A - Manufacture of low yield ratio-hot dip galvanized hot-rolled steel sheet for building, having excellent - Google Patents

Abstract

PURPOSE:To obtain the hot rolled steel sheet having excellent high temp. characteristics, in which fireproofing can be reduced or eliminated and having steel components similar to those in a common steel by specifying the chemical components in the steel sheet and specifying its hot rolling conditions and hot dip galvanizing conditions. CONSTITUTION:A steel slab constituted of, by weight, 0.02 to 0.1% C, <=0.5% Si, 0.2 to 1.0% Mn, 0.005 to 0.04% Nb, 0.3 to 0.7% Mo, <=0.05% Al, <=0.006% N and the balance Fe is hot-rolled directly at a high temp. as it is, or it is heated to 1100 to 1300 deg.C and is thereafter hot-rolled at 850 to 950 deg.C finishing end temp. In succession, the slab is cooled at 5 to 30 deg.C/s average cooling rate and is coiled at 400 to 650 deg.C; continuously, in the stage of passing the steel sheet through a continuous hot dip galvanizing line, it is heated to 500 to 700 deg.C in a reducing atmosphere, is thereafter cooled, is immersed into a hot dip galvanizing bath and is galvanized.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for producing hot-dip galvanized hot-rolled steel sheets with excellent fire resistance and fire resistance for use in various buildings in the fields of architecture, civil engineering, marine structures, etc. Concerning superior architectural steel materials.

(Conventional technology) Hot-rolled steel plates for construction include rolled steel plates for shell structures (JIS G
3101), rolled steel plates for welded structures (JIS G 31
06) Weather-resistant hot-rolled steel plates for welded structures (JIS c
3114), High Weather Resistance Rolled Steel Plate (JIS G 3], 2
5), (hereinafter referred to as well-known steel), etc. are widely used.

Fire resistance of buildings is important, and a variety of measures are being taken for everything from large buildings to general residential buildings.

However, at present, when fishing on a boat, fire prevention measures are taken by using a fireproof coating as described in Japanese Patent Application Laid-Open No. 63-47451. As a result, construction costs are rising and the usable space of buildings is becoming smaller.

Recently, fire-resistant design has been reviewed, and in 1986, the new Fire-resistant Design Law for Buildings was enacted, and the previous regulations regarding the allowable steel temperature (350℃ or less) in the event of a fire were removed.
The ability of fire-resistant coating can now be determined based on the high-temperature strength of the steel plate and the load actually applied to the building, and if the high-temperature strength of the material steel plate is ensured, it is also possible to use steel plate without coating. An invention relating to a steel plate with guaranteed high-temperature strength for use in steel sheets is disclosed in Japanese Patent Application No. 63-143740, but this invention mainly relates to thick plates.

However, in buildings, lightweight steel sections and U-shaped columns are often made of hot-rolled steel strips or steel plates. Hot-rolled steel strips or steel plates are produced by hot strip rolling, but since this process involves continuous hot rolling, it is not possible to unnecessarily lower the finishing temperature or extremely reduce the threading speed.

Furthermore, in order to produce in large quantities, there is a quenching process for the runout table and a winding process. For these reasons, imparting specified room temperature and high temperature properties requires a significant difference from the thick plate manufacturing process.

Therefore, the present inventors proposed a method for manufacturing hot-rolled steel sheets with excellent fire resistance using the technology of Japanese Patent Application No. 1-3834 (prior application invention), but corrosion resistance is also important for architectural steel, so It is effective to galvanize hot-rolled steel sheets.

When hot-dip galvanizing is applied, it is manufactured on a continuous hot-dip galvanizing line, but in this process the steel plate is heated in a reducing atmosphere, but if the temperature is raised unnecessarily, precipitation etc. will occur, ensuring the necessary high-temperature proof strength. If this temperature cannot be lowered unnecessarily, the plating adhesion will deteriorate. Furthermore, plating adhesion is also affected by the type and amount of alloying elements added. In this way, it is necessary to set unique conditions for hot-dip galvanized hot rolled steel sheets in order to ensure specified room temperature properties, high temperature properties, and plating adhesion.

The present invention further develops the technology of the prior invention and proposes a method for manufacturing hot-dip galvanized hot-rolled steel sheets with excellent fire resistance, and a technology for building steel materials imparted with fire resistance.

(Problems to be Solved by the Invention) Conventional steels have difficulty achieving high-temperature strength due to grain growth, coarsening of precipitates, and carbide melting. In addition, high-alloy heat-resistant metals include iron-based metals, but they are economically disadvantageous as they are consumed in large quantities for construction purposes. Furthermore, as a construction steel, it is desirable to have corrosion resistance as well.

The object of the present invention is to produce hot-dip galvanized hot rolled steel with excellent high-temperature properties, reduced or omitted fire-resistant coating, and low cost, with a steel composition close to that of ordinary steel, and which is galvanized with excellent corrosion resistance. The purpose of the present invention is to provide a method for manufacturing steel plates and a technology for building steel materials with fire resistance.

(Means for Solving the Problems) The present invention overcomes the above-mentioned problems and achieves the objects.
The summary is as follows.

(1) Weight ratio: C: 0.02 to 0.1%, Si: 0.5% or less, M
n20, 2-1.0%, Nb: 0.005-0.
0.04%, Mo: 0.3-0.7%, A17: 0.05% or less, N: 0.006% or less, and the balance consisting of iron and unavoidable impurities. After heating to 'C, hot rolling is carried out at a finishing temperature of 850-950°C, followed by cooling at an average cooling rate of 5-30°C/s and winding at 400-650°C.
Next, when passing through a continuous hot-dip galvanizing line, the sheet is heated to 5.00 to 700°C in a reducing atmosphere, cooled, and then immersed in a hot-dip galvanizing bath to perform hot-dip galvanizing. A method for manufacturing low yield ratio hot-dip galvanized hot-rolled steel sheets for construction with excellent fire resistance.

(2) C: 0.02 to 0.1%, Si: 0.5% or less, by weight ratio
Mn: 0, 2-1.0%, Nb: 0.0 (15-0
．． 04%, MQ: 0.3-0.7%, AI=0.05%
Hereinafter, in addition to N: 0.006% or less, Ti: 0.
005-0.10%, zr: 0.005-0.03%,
v: 0.005~0.10%, Ni: 0.05~
0.5%, Cu: 0.05-0.5%, Cr: 0.05
~1.0%, B: 0. ．． 0001-0.002%,
Ca: 0.0005-0.005%, ・REM
: A steel slab containing one or more of 1 to 0.02% of o, oo, and the balance consisting of iron and unavoidable impurities is heated directly at a high temperature or after heating to 1100 to 1300°C, and then heated to a finishing temperature of 850 to 950°C. ℃, followed by winding at an average cooling rate of 5 to 650℃, and passing it through a continuous hot-dip galvanizing line in a reducing atmosphere of 500 to 700℃.
1. A method for producing a low yield ratio hot-dip galvanized hot-rolled steel sheet for architectural use, which has excellent fire resistance and is characterized by heating it to ℃, cooling it, and then immersing it in a hot-dip galvanizing bath to apply hot-dip galvanizing.

(3) A method for producing a low-yield-ratio hot-dip galvanized hot-rolled steel sheet for construction with excellent fire resistance, which further comprises plastic working the steel plate or steel strip obtained by the method described in 1 or 2 above in a hot process.

(4) A method for producing a low-yield-ratio hot-dip galvanized hot-rolled steel sheet for construction with excellent fire resistance, which comprises plastically working the steel plate, steel strip or steel material obtained by the method described in 1, 2 or 3 above in a cold process.

(5) Previous item 1. ．． 2. An MEJ material for construction with excellent fire resistance, which is obtained by spreading an inorganic fibrous fire-resistant thin layer material on the heat-receiving surface of a steel material obtained by the method described in 3 or 4.

(6) A steel material for construction with excellent fire resistance, which is obtained by coating the heat-receiving surface of the steel material obtained by the method described in 1, 2.3 or 4 above with a highly heat-resistant paint.

(Function) As a result of research on the strength of steel plates in the event of a fire, the present inventors found that
The maximum temperature reached during a normal fire is 1000'C, and if the goal is to use it without coating, a large amount of expensive alloying elements must be added in order for the steel plate to have a yield strength of 70% or more of the room temperature yield strength at that temperature. I realized that I had to do it and would lose economic efficiency.

In other words, the price of the steel plate becomes higher than the cost of the steel plate and the cost of installing the fireproof coating, and such steel plate cannot be practically used.

Therefore, as a result of further research, we found that steel plates (i.e., 40k
For gf/mj class, the yield point strength at 600℃ is 16kg
It was revealed that steel sheets with a yield point strength of 22 kgf/- or more at 600°C are the most economical for the f/- class, 5, Okgf/- class, and the addition of expensive alloying elements can be reduced. It is also possible to reduce the need for fireproof coating.
In addition to a manufacturing method for steel plates that can be used without coating when the fire load is small, we have developed a steel material with 990 fire resistance.

The chemical composition, hot rolling conditions and molten zinc coating are listed below.

Explain the reason for limiting the key conditions.

0.02% of C is required to obtain strength at room temperature to high temperature. If it is less than this, the required structure or clusters or precipitates for strengthening cannot be obtained.

Moreover, if it exceeds 0.1%, plating adhesion will decrease.

To further exhibit these effects, C should be 0.04 to 0.
It is preferable to set it to 0.08%.

Si is an element added for solid solution strengthening, and if it exceeds 0.5%, plating adhesion will decrease. Note that Si generates a scale pattern on the surface of the steel plate. To avoid this, 0
．． It is preferably 1% or less.

Mn is an essential element for ensuring strength and toughness.
Its lower limit is 0.2%. On the other hand, if it exceeds 1.0%, the coating adhesion will decrease.

Nb and Mo produce fine clusters or precipitates. In order to obtain sufficient yield point strength at high temperatures, Nb,
Combined addition of Mo is extremely effective. Nb, M.

The lower limits of the amounts were set at 0.005% and 0.3%, respectively, as the minimum amounts that would provide a composite effect. However, if the amount added increases, the manufacturing cost increases and the economic efficiency as a construction steel is lost, so the upper limits were set at 0.04% and 20.7%, respectively.

M is an element generally contained in deoxidized steel. However, Al
If the amount exceeds 0.05%, plating adhesion will decrease.

N is generally contained in steel as an unavoidable impurity, but it forms nitrides and improves high-temperature strength. As the amount of N increases, the amount of Al for fixing increases and the ductility decreases, so the upper limit was set at 0.006%.

In the present invention, in addition to the above components, Ti, ZrV,
By appropriately adding one or more of Ni, Cu, Cr, B, Ca, and REM, the effects of the invention can be further exhibited.

Ti is an element that has almost the same effect as Nb mentioned above, and
When the amount is small, carbonitride is formed and high temperature strength is improved.
1-, but it has no effect if it is less than 0.005%, and if it is less than 0.005%, it has no effect.
If it exceeds 10%, the cleanliness will deteriorate.

Zr is an element that increases the strength of the base material, but 0.00
If it is less than 5%, the effect is weak, and if it exceeds 0.03%, the toughness decreases.

■Although the effect on high temperature strength is small compared to Nb and Ti, it has no effect at less than 0.005%.
If it exceeds 0.10%, the ductility deteriorates.

Ni improves the strength and toughness of the base metal, but it is not effective if it is less than 0.05%! <, If it exceeds 0.5%, it becomes extremely expensive and loses economic efficiency as a construction steel.

Cu has almost the same effect and weather resistance as Ni, and Cu
It is also effective in increasing high temperature strength and corrosion resistance due to precipitates. However, if the Cu amount exceeds 0.5%, Cu cracking occurs during hot rolling, making manufacturing difficult, and if it is less than 0.05%, there is no effect.

Cr is an element that increases the strength of the base metal and welded part,
When it exceeds 1.0%, weldability deteriorates, and when it is less than 0.05%, the effect is weak.

B is an element that increases the hardenability of steel and increases its strength. In order to obtain this effect of B, if it is less than 0.0001%, there is no effect, and if the amount of B exceeds 0.002%, the recrystallization temperature increases and hardening occurs.

Ca + RIEn controls the morphology of MnS, increases Charpy absorbed energy and improves low temperature toughness.

However, if the amount of Ca is less than 0.0005%, there is no practical effect, and if it exceeds 0.005%, a large amount of CaO and CaS will be generated and become large inclusions, which will deteriorate the ductility.

Furthermore, REM has the same effects as Ca, and if the amount added is large, problems similar to those of Ca will occur, and the economy will be poor, so the lower limit is set to 0.00]% and the upper limit is set to 0.02%.

Note that the present invention contains P and S as inevitable impurities. Since P and S have little influence on high temperature strength, their amounts are not particularly limited. Desirable amounts of P and B are 0.02% or less and 0.005% or less, respectively.

When hot-rolling heating, the heating temperature is 1100-1300"C.
shall be. This is necessary for solution treatment of NbC.

If it is less than the lower limit, solution treatment will be insufficient and strengthening by Nb at room temperature or high temperature cannot be expected. In order to achieve complete solution formation even more stably, the heating temperature is preferably 120034°C or higher. The upper limit may be 1300°C, which is usually the same.

The rolling end temperature is 850 to 950°C. If it is less than the lower limit, clusters or precipitates of Nb and Mo will occur during rolling. In addition, it is effective to coarsen the ferrite grain size in order to lower the yield ratio, and for this purpose it is preferable that the rolling end temperature be high, but if it exceeds 950°C, the ferrite grain size will become excessively coarse and impact Characteristics deteriorate.

The average cooling rate is 5 to 30°C/s. 30”C7
If it exceeds s, hardening will occur easily due to the addition of Mo, the room temperature strength will become too high, and the impact properties will also deteriorate. If it is less than 5°C/s, the ferrite grain size becomes excessively coarse and the impact properties deteriorate.

The winding temperature is 400 to 650°C. If the temperature exceeds 650°C, clusters or precipitates of Nb and Mo will form during winding, making it impossible to ensure high-temperature strength.

If the temperature is less than 400°C, a considerable amount of martensitic phase and quenched phase will be mixed in, the room temperature strength will become too high, and the impact properties will also deteriorate.

When applying hot-dip galvanizing to the above-mentioned steel plate or steel strip, it is heated to 500 to 700'C in a reducing atmosphere.

If the upper limit is exceeded, Mo will precipitate due to over-aging, making it impossible to obtain the necessary tensile properties. Below the lower limit, plating adhesion deteriorates. Preferably, it is desirable to heat to 550 to 650°C.

Heating in a reducing atmosphere may be achieved by either an indirect heating method using a radiant tube or a direct fire non-oxidation heating method using the reducing region of a flame.

After hot-dip galvanizing, the plating layer may be alloyed by post-heating depending on the case.

Furthermore, 0.01 to 20% of IV may be added to the Zn plating bath. As a result, Fe-
A Zn-Al ternary alloy layer can be formed to improve plating adhesion.

Similarly, in the Zn plating bath, Pb, Cd, Sn
, low melting point alloys such as Sb, or Mg in an amount of 1% or less will not impair the effects of the present invention.

In the present invention, hot-dip galvanized hot-rolled coils are manufactured as described above, but they may be used as they are in the form of coils or cut plates. At that time, it is preferable to pass the material through a skin bath or a leveler in order to adjust the yield point strength at room temperature. In that case, skin pass has an elongation rate of 0.5 to 2.
%, and for a leveler, the maximum surface strain is 0.3 to 2%.

Furthermore, the above-mentioned product may be used as a raw material to undergo secondary processing to produce products such as lightweight section steel.

Next, the mechanical properties of the steel of the present invention will be explained in detail in comparison with known steel materials.

Table 1 shows a compositional comparison between the steel of the present invention and JIS G 3106 rolled steel for welded structures (5M50A).

In addition, the steel plate of the present invention consists of a slab having the composition shown in Table '1.
Heated to 200℃, finishing temperature 920℃ 1 average cooling rate 1
It was rolled at 2°C/s at a winding temperature of 550°C, heated to 600°C in a reducing atmosphere, cooled, and hot-dip galvanized.

In FIG. 1, the vertical axis represents stress (kgf/mj) and the horizontal axis represents temperature (° C.). The solid broken line 1 shows the change in the invention steel, and the broken broken line 2 shows the change in the comparative steel ('5M50A). As is clear from Fig. 1, the difference disappears at temperatures exceeding 800°C, but the steel of the present invention has 5M50A at 600 to 700°C.
It can be seen that the steel plate has twice the proof strength as the steel plate, and has excellent fire resistance properties as a steel plate for construction.

In FIG. 2, the vertical axis represents the elastic modulus (kgf/wj), and the horizontal axis represents temperature (°C). The solid broken line ■ shows the change in the invention steel, and the broken broken line 2 shows the change in Comparative 1ii1 (5M50A).

As is clear from Fig. 2, the elastic modulus of the steel of the present invention decreases rapidly at temperatures exceeding 700°C, whereas the 5M5
For 0A, the elastic modulus decreases rapidly at around 600°C.

Figure 3 shows creep strain (χ) on the vertical axis and time (minutes) on the horizontal axis.
The stress at 600℃ (kg
Fig. 4, which shows the changes in the steel of the present invention using f/mj) as a parameter, similarly shows the changes in 5M50A. As is clear from Figures 3 and 4, the steel of the present invention has a stress of 15 kgf/- that acts on structural members such as pillars and beams of normal buildings at a temperature of 600°C, which is the maximum duration of a normal fire. Although the progress of creep strain is extremely small even after 3 hours, in 5M50A, the progress of creep strain is extremely large when a stress level of 10 kgf/md is applied at a temperature of 78 600°C. The fact that the elastic modulus does not decrease at high temperatures and that creep strain progresses is small.
To reduce the deformation of buildings in the event of a fire.

Therefore, it can be seen that the steel of the present invention has superior properties as a construction steel compared to 5M50A.

The present inventors obtained similar results in comparison with comparative steel 5S41.

From this, it is clear that the steel of the present invention requires a thinner fire-resistant coating when the fire load is equal to that of 5M50A and 5S41, and if the fire load is thicker (and there is no coating), no coating may be required. It's also obvious.

Next, examples in which an inorganic fibrous refractory thin layer material is spread on a comparative steel and a steel of the present invention will be described.

Table 2 is an example of the fireproof coating thickness of comparative steel.
This table shows the coating thickness of each refractory material required to prevent the steel plate temperature from exceeding 350°C in experiments stipulated by SA 1304.

Table 3 shows examples regarding the fireproof coating thickness of the steel of the present invention.
This table shows the coating thickness of each refractory material required to prevent the steel plate temperature from exceeding 350°C in experiments specified by IS A 1304.

In the case of the steel of the present invention, the steel sheet temperature can rise up to 600°C, so the thickness of the fireproof coating can be as thin as shown in Table 3, as described above. As is clear from Tables 2 and 3, when the steel of the present invention is used, the material cost and construction cost of the fireproof coating can be significantly reduced.

Next, Fig. 5 shows the lightweight section steel 1 (125m) according to the present invention.
FIG. 2 is a schematic elevational view and an AA cross-sectional view of a column on which sprayed Lenorock wool (wet type) 2 shown in Table 3 is spread on a column measuring 125 mm x 3.2 mm.

FIG. 6 shows the test results of heating the lightweight section steel according to JIS A 1304, applying a load normally supported by the pillars of a building, and determining the time required for the steel to break. The vertical axis shows temperature (
'C), time (minutes) is plotted on the horizontal axis, broken line 1 shown as a solid line shows the steel material temperature of the column, and broken line 2 shown as a broken line shows the change in heating temperature.

Further, in FIG. 7, the vertical axis represents deformation (CII+), the horizontal axis represents temperature (° C.) and time (minutes), and the solid broken line represents the deformation of the column. As is clear from Figures 6 and 7, 1
By applying Rock-Ul (wet method) spraying to a thickness of 0.0 mm, columns made from the steel of the present invention do not break down until temperatures exceed 600'C, demonstrating performance exceeding 1-hour fire resistance. Recognize.

Figure 8 shows a lightweight section steel beam 3 (3,2 beams x 2 beams) according to the present invention.
00+nmXnmX75mmX20, spraying in Table 3 is a schematic elevational view and an AA sectional view of a beam on which rock wool (wet type) 4 is spread.

FIG. 9 shows the test results of heating the lightweight section steel beam as specified in JIS A 1304, applying a load normally supported by a building beam, and determining the time required for the beam to break. The vertical axis represents the temperature ('C) and the horizontal axis represents the time (minutes), and the solid line 1 represents the temperature of the upper beam flange 5, the 2nd line represents the temperature of the lower flange 6 of the beam, and the 3rd line represents the temperature of the web 7. Broken line 4 shown as a broken line
indicates the change in heating temperature.

In FIG. 10, the vertical axis represents deformation (vertical deflection, cm), and the horizontal axis represents temperature (° C.) and time (minutes), and solid broken lines indicate deformation at each point of the beam. As is clear from Figures 9 and 10, the spraying with a thickness of 10 mm was made using rock wool (
A beam manufactured from the steel material of the present invention by subjecting it to 60
It can be seen that it does not break down until the temperature exceeds 0'C, demonstrating performance that exceeds fire resistance for one hour. Further, it can be seen that the amount of deformation at 600° C. is also below the deformation allowable value.

The present inventors also conducted tests on other refractory materials and obtained similar results.

Next, the steel of the present invention was coated with a highly heat-resistant paint and tested, and the results are shown in Table 4. Paint 1 and Paint 2 are foaming highly heat-resistant paints (manufactured by West German Decivac, trade name Pyrotect, types S30 and F2O), and the test steel is 3.2 mm thick, 2
A 20 mm square piece of steel according to the invention was used.

Conventional steel had a temperature of 350℃ or less, so the fourth
Even by painting with conventional paint 1 and paint 2 shown in the table, a fire resistance time of only 30 or 60 minutes could be secured, but since the steel of the present invention can secure yield strength up to 600°C, the paint 1. Painting with Paint 2 also ensures a fire resistance time of 60 minutes and 120 minutes. That is, if the conventional fire resistance time of 11 2 is to be maintained, there is an advantage that painting can be simplified.

That is, the steel of the present invention 1 coated with a highly heat-resistant paint is highly economical and can reduce construction costs.

Figure 11 shows a 1-hour fire resistance test (JIS A 1304) of a deck plate made of the steel of the present invention and a 7.5-nun-thick rock wool-based fibrous refractory material sprayed on the back side using a wet method. This shows the results obtained in accordance with the above-mentioned test method, and it was confirmed that the steel of the present invention can be used as an effective fire-resistant steel material since the temperature of the deck plate itself does not exceed 600°C.

(Example) Examples are shown below.

Steel having the components shown in Table 5 was tapped in a converter, made into a slab by continuous casting, hot-rolled immediately or after heating, and then hot-dip galvanized in a continuous hot-dip galvanizing line.

Table 6 shows hot rolling conditions, hot dip galvanizing conditions, and characteristic values of the obtained steel.

The room temperature tensile test uses a JISS No. test piece, and JIS G
Tests were conducted based on 0567. High temperature tensile test is 1
After increasing the temperature at 0℃/min and holding it at 600℃ for 15 minutes, JIS
The test was conducted according to G 0567.

The toughness test of the material was conducted in accordance with JIS Z 2242 using a JIS Z 2202 Sharpie one-notch test piece. However, since the plate thickness was 10 mm or less, a sub-size test piece closest to the original thickness was used.

In addition, the plating adhesion of the material was evaluated by an impact test. The method is to use a hemispherical punch (diameter 12.7 m) on a steel plate.
mφ) was dropped, a tape was affixed to the formed circular depression, the tape was peeled off from the steel plate, and the amount of plating attached to the tape was visually determined. The evaluation is as follows. ◎: A few dot-shaped peelings (good), ○: Slightly many peelings (good product), △ Partial peeling (unable to ship due to the tip), ×: Full-scale peeling (defective product) Table 5. As is clear from Table 6, the steel according to the present invention is
The plating adhesion is good, and the tensile strength at room temperature is 40 kg.
For tensile strength of f/- class or 50 kgf/- class, yield point strength is 25 kgf/mj, the respective standard value.
It fully satisfies the requirement of 33kgf/- or more. Katsu600
The yield point strength at ℃ is 16 kgf for 40 kgf/- class.
/- or more, and the 50 kgf/- class sufficiently satisfies the standard value of 22 kgf/- or more.

5 6 Table 2 Table 9 Table 31 (Effects of the invention) Building fire prevention is a social issue, and high-performance housing is also required for general housing, and fire prevention is an important item. . Under these circumstances, the present invention aims to manufacture iron-based materials with excellent high-temperature properties using hot slip mills that can supply large quantities of materials with compositions close to those of ordinary steel, and to perform hot-dip galvanizing with excellent corrosion resistance. This will greatly contribute to solving the social issues mentioned above.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between stress and temperature for the steel of the present invention and comparison steel, Figure 2 is a diagram showing the relationship between elastic modulus and temperature, and Figure 3 is a diagram showing the stress at 600°C as a parameter. Figure 4 is a diagram showing the relationship between creep strain and time for the steel according to the present invention, and Figure 5 (a) is a diagram showing the relationship between creep strain and time for the 5M50A steel. Steel (1
25 mm x 125 mm x 3.2 mm) 1. Spraying in Table 3 is a schematic elevational view of a pillar on which rock wool (wet type) 2 is spread, and (b) is a cross-sectional view taken along line A-A in Figure 5 (a). , Figure 6 shows the test results of heating the lightweight section steel specified in JIS A 1304 and applying a load normally supported by the pillars of a building to determine the distance between 114 and 114. The broken line 2 shows the change in heating temperature. Figure 7 shows the relationship between the deformation of the column and the test temperature and time. Figure 8 (a) shows the light weight type according to the present invention. Schematic elevational view of a beam with blown LJ rock wool (wet type) 4 in Table 3 spread on steel beam 3, also (b)
is a sectional view taken along the line A-A in FIG. 8(a), and FIG. 9 is a sectional view of the lightweight section steel beam subjected to heating specified in JIS A 1304,
Figure 10 is a diagram showing the relationship between the vertical deflection (cm) of the lightweight section steel beam and the test temperature and time; Figure 11 shows a deck plate made of the steel of the present invention, on which a fibrous fireproofing material made of rock wool is sprayed to a thickness of 7.5mm on the back side using a wet method, resulting in a one-hour fire resistance of 3AM (according to JIS A 13049). ) is a diagram showing the results obtained.

Claims (6)

[Claims] (1) Weight ratio: C: 0.02 to 0.1%, Si: 0.5% or less, Mn:
0.2-1.0%, Nb: 0.005-0.04%, M
o: 0.3 to 0.7%, Al: 0.05% or less, N: 0
．． A steel slab containing 0.06% or less and the balance consisting of iron and unavoidable impurities is hot-rolled at a finishing temperature of 850-950°C either directly at high temperature or after heating to 1100-1300°C, followed by an average cooling rate of 5.
The sheet is cooled at ~30°C/s and wound at 400-650°C, then heated to 500-700°C in a reducing atmosphere, cooled, and then hot-dip galvanized before being passed through a continuous hot-dip galvanizing line. A method for producing a hot-rolled hot-rolled steel sheet with a low yield ratio for construction purposes, which has excellent fire resistance and is characterized by applying hot-dip galvanizing by immersing it in a bath. (2) Weight ratio: C: 0.02-0.1%, Si: 0.5% or less, Mn:
0.2-1.0%, Nb: 0.005-0.04%, M
o: 0.3 to 0.7%, Al: 0.05% or less, N: 0
．． In addition to 0.006% or less, Ti: 0.005 to 0.10
%, Zr: 0.005~0.03%, V: 0.005~
0.10%, Ni: 0.05-0.5%, Cu: 0.0
5-0.5%, Cr: 0.05-1.0%, B: 0.0
001-0.002%, Ca: 0.0005-0.00
5%, REM: Contains one or more of 0.001 to 0.02%, with the balance consisting of iron and unavoidable impurities. Finishing temperature of a steel slab directly at high temperature or after heating to 1100 to 1300°C Hot rolling at 850-950°C followed by an average cooling rate of 5
The sheet is cooled at ~30°C/s and wound at 400-650°C, then heated to 500-700°C in a reducing atmosphere, cooled, and then hot-dip galvanized before being passed through a continuous hot-dip galvanizing line. A method for producing a hot-rolled hot-rolled steel sheet with a low yield ratio for construction purposes, which has excellent fire resistance and is characterized by applying hot-dip galvanizing by immersing it in a bath. (3) A method for producing a low-yield-ratio hot-dip galvanized hot-rolled steel sheet for construction with excellent fire resistance, which further comprises plastic working the steel plate or steel strip obtained by the method according to claim 1 or 2 in a hot process. (4) A method for producing a low yield ratio hot-dip galvanized hot-rolled steel sheet for construction with excellent fire resistance, which comprises plastically working a steel plate, steel strip or steel material obtained by the method according to claim 1, 2 or 3 in a cold process. . (5) A steel material for construction with excellent fire resistance, which is obtained by spreading an inorganic fibrous fire-resistant thin layer material on the heat-receiving surface of the steel material obtained by the method according to claim 1, 2, 3 or 4. (6) A steel material for construction with excellent fire resistance, which is obtained by coating the heat-receiving surface of the steel material obtained by the method according to claim 1, 2, 3, or 4 with a highly heat-resistant paint.