JPH10204572A - 700×c fire resistant rolled shape steel and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a 700 deg.C fire resistant rolled shape steel having high strength and excellent in toughness for the structural member of a building at a low cost by specifying the compsn. composed of C, Si, Mn, Ti, Mg, Nb, V, Mo, N, O and Fe and the contents of B and Al in impurities and forming its microstructure into the specified one contg. fine bainite. SOLUTION: A slab having a compsn. contg., by weight, 0.01 to 0.08% C, 0.05 to 0.25% Si, 0.8 to 1.6% Mn, 0.05 to 0.025% Ti, 0.0005 to 0.0050% Mg, 0.05 to 0.15% Nb, 0.05 to 0.20% V, 0.7 to 1.0% Mo, 0.002 to 0.006% N and 0.003 to 0.006 O, contg., at need, prescribed amounts of Cr, Ni and Cu, and the balance Fe with inevitable impurities in which the content of B is limited to <=0.0003% and the content Al is limited to <=0.005% is subjected to controlled water cooling rolling. Its microstructure is formed into the one composed of, by area, 40 to 80% beinite, and the balance ferrite-pearlite and <=5% high carbon insulator martensite, and in which the grain size of old γimmediately after the completion of hot rolling is regulated to <=40μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rolled section steel having excellent fire resistance and toughness used as a structural member of a building and a method for producing the same.

[0002]

2. Description of the Related Art Due to the increase in height of buildings and the advancement of building design techniques, review of fire-resistant design was conducted by the Ministry of Construction's comprehensive project, and the "New Fire-resistant Design Law" was enacted in March 1987. . According to this regulation, the restriction on fire-resistant coating to keep the temperature of steel at 350 ° C or less in the event of a fire under the old law is lifted, and the fire-resistant coating method adapted to the high-temperature strength of steel and the actual load of the building is taken into account. Can now be determined. That is, when the design strength at 600 ° C. can be secured, the refractory coating can be reduced accordingly.

In response to such a trend, Japanese Patent Laid-Open No.
No. 77523 proposes a low yield ratio steel and a steel material for building having excellent fire resistance and a method for producing the same. In this prior art, high-temperature strength is improved by adding Mo and Nb so that the yield ratio at 600 ° C. becomes 2/3 or more of that at normal temperature. The reason for setting the design high-temperature strength of the steel material to 600 ° C. is based on the finding that it is the most economical in view of a balance between the increase in the cost of the steel material due to the alloying element and the construction cost for the refractory coating of the conventional steel material.

[0004] In addition, as buildings become ultra-high-rise, due to stricter safety standards and the like, steel materials used for columns, for example, H-shaped steel having a particularly large plate thickness (hereinafter referred to as extra-thick H-shaped steel). ) Requires higher strength, higher toughness, and lower yield ratio. In order to satisfy such required characteristics, conventionally, a heat treatment such as a normalizing process has been performed after the completion of rolling. The addition of heat treatment causes a significant increase in cost, such as a decrease in heat treatment cost and production efficiency, and has a problem in economy.
In order to solve this problem, it is necessary to develop a slab and a manufacturing method by a new alloy design that can obtain high-performance material properties as rolled.

In general, a section steel having a flange, for example, H
When a section steel is manufactured by universal rolling, the rolling conditions (temperature, rolling reduction) from the roll molding and the uniqueness of the shape limit the rolling finish temperature, rolling reduction, and cooling rate at each part of the web, flange, and fillet. Make a difference. As a result, variations in strength, ductility and toughness occur between the parts,
For example, there are portions that do not meet the standards such as rolled steel materials for welded structures (JIS G3106). In particular, when rolling an extremely thick H-beam using a continuous cast slab as a raw material, there is a limit to the maximum thickness of a slab that can be produced by a continuous casting facility, and a sufficient slab cross-sectional area required for molding can be obtained. Therefore, the rolling is a low reduction ratio rolling. Furthermore, since high-temperature rolling is performed in order to obtain the dimensional accuracy of the product by rolling molding, the flange portion having a large thickness is subjected to high-temperature rolling, and the steel material after rolling is also gradually cooled. As a result, the microstructure becomes coarse and the strength and toughness are reduced.

As a method of refining the structure in the rolling process, T
Although there is MCP (Thermo-Mechanical-Controlled Process), since rolling conditions are limited in section steel rolling, it is difficult to apply low-temperature / high-pressure rolling such as TMCP to a steel sheet. In the field of heavy steel plates, high strength and high toughness steels are produced by utilizing the precipitation effect of VN. For example, Japanese Patent Publication No. Sho 62-505
No. 48, and Japanese Patent Publication No. 62-54862 have been proposed. However, this method requires 490MPa (50kg
When applied to the production of f / mm ² ) grade, since it contains a high concentration of solute N, high-carbon island-like martensite (hereinafter referred to as M *) is generated in the generated bainite structure, There was a problem that it was difficult to clear the standard value because the toughness was significantly reduced.

[0007]

In order to solve the above-mentioned problems, it is necessary to form a low-carbon bainite with a small amount of M * generated as it is in the form of rolled steel to refine the structure. For this purpose, in order to reduce the γ grain size at the time of rolling and heating, MgO and Ti oxide (Ti-
In order to finely crystallize O), finely precipitate TiN using these as nuclei, and further reduce the carbon content, it is necessary to produce a slab to which a small amount of microalloy, which has a small amount and high strength, is added. The fillet at the joint between the flange of the H-section steel and the web coincides with the central segregation zone of the CC slab, and MnS in this segregation zone is significantly elongated by rolling. The high-concentration elemental segregation zone and stretched MnS here significantly reduce the drawing value and toughness in the sheet thickness direction, and may cause lamella tear cracking during welding.
It is also a major challenge to prevent the generation of. As described above, it is difficult for the conventional technology to manufacture a rolled section steel having high reliability and high strength with high reliability on-line and to provide it at low cost.

[0008] In addition, the conventional fire-resistant steel guarantees high-temperature strength at 600 ° C. However, if the range in which the fire-resistant coating of a building can be omitted can be further expanded, it is very advantageous in terms of construction cost reduction and the like. There has been a demand for a refractory rolled steel capable of guaranteeing high-temperature strength at 700 ° C. Therefore, the present invention
It is possible to produce high-strength rolled section steel at low cost without performing heat treatment such as conventional normalizing treatment, that is, as-rolled, and has fire resistance at 700 ° C. and 490 MPa used for structural members of buildings. 700 ° C with high strength and excellent toughness
An object of the present invention is to provide a refractory rolled steel and a method for producing the same.

[0009]

The feature of the present invention is different from the conventional idea, in that Mo is added to secure the high-temperature strength at 700 ° C. by the precipitation of Mo carbide, and that Mg and T are added.
i, and the resulting fine Mg-based oxide (hereinafter abbreviated as “MgO”), Ti oxide and T
The point is that a 700 ° C. refractory rolled steel of high strength and high toughness is realized by making the structure fine by the fine dispersion of iN and the formation of a low carbon bainite structure by adding a small amount of microalloy.

In addition, the feature of the TMCP adopted is that the structure can be efficiently refined even in the hot rolling under light reduction in the shape steel rolling instead of the large reduction rolling performed in the thick steel plate. There is a method of cooling with water between passes and repeating rolling and water cooling. The present invention casts a slab in which a low-carbon bainite microstructure with a low M * content is obtained as-rolled, and using this slab, performs efficient TMCP in rolling of a section steel to achieve high strength and high toughness. It is characterized by producing a shaped steel having.

In the steelmaking process, in the steelmaking process, Mg and Ti are added to the slab to finely crystallize MgO and Ti oxides and finely disperse TiN in order to reduce γ during rolling and heating. At the same time, M
Mo carbide is generated by the addition of o, and in addition, in order to secure appropriate hardenability that can reduce M * in the structure after rolling, the alloy element is replaced with a small amount of Nb and V, and the extremely low B
And manufacture.

Next, the cast slab is rolled and shaped to produce a shaped steel. In this rolled shaped steel rolling process, the temperature difference between the surface layer and the inside of the steel is reduced by water-cooling the steel between hot rolling passes. In addition, even under light rolling conditions, the rolling penetration into the steel material at a higher temperature is increased, and working dislocations that become bainite forming nuclei in γ grains are introduced to increase the number of formed nuclei.
According to this method, the microstructure can be refined, and the above-mentioned problem has been solved based on the finding that it is possible to produce a controlled rolled steel with a high efficiency and a low production cost. It is as follows.

In weight%, C: 0.01-0.08%, Si: 0.05-0.
25%, Mn: 0.8 to 1.6%, Ti: 0.005 to 0.025%, Mg: 0.0005 to
0.0050%, Nb: 0.05-0.15%, V: 0.05-0.20%, Mo:
0.7-1.0%, N: 0.002-0.006%, and O: 0.003-0.006
%, The balance being Fe and unavoidable impurities, the unavoidable impurities having a chemical composition in which the B content was limited to 0.0003% or less and the Al content to 0.005% or less, and the area ratio of bainite was 40%. ~ 80%, the remainder is ferrite
Consisting of perlite and high carbon island martensite,
700 ° C. refractory rolled steel having a microstructure in which the area ratio of the high-carbon island martensite is 5% or less and the old γ grain size immediately after completion of hot rolling is 40 μm or less.

In weight%, C: 0.01 to 0.08%, Si: 0.05 to 0.
25%, Mn: 0.8 to 1.6%, Ti: 0.005 to 0.025%, Mg: 0.0005 to
0.0050%, Nb: 0.05-0.15%, V: 0.05-0.20%, Mo:
0.7-1.0%, N: 0.002-0.006%, O: 0.003-0.006%, and Cr: 0.1-1.0%, Ni: 0.1-1.0% and Cu: 0.1-1.0%
And the balance consists of Fe and inevitable impurities, and the B content of the inevitable impurities is 0.00
It has a chemical composition in which the content of Al is limited to not more than 03% and the content of Al is not more than 0.005%, and the area ratio of bainite is 50 to 90%.
The balance is composed of ferrite / pearlite and high carbon island martensite, the area ratio of the high carbon island martensite is 5% or less, and the old γ grain size immediately after completion of hot rolling is 40 μm or less. 700 ° C. refractory rolled section steel having a microstructure.

In weight%, C: 0.01-0.08%, Si: 0.05-0.
25%, Mn: 0.8 to 1.6%, Ti: 0.005 to 0.025%, Mg: 0.0005 to
0.0050%, Nb: 0.05-0.15%, V: 0.05-0.20%, Mo:
0.7-1.0%, N: 0.002-0.006%, and O: 0.003-0.006
%, And the balance consisted of Fe and inevitable impurities, and among the inevitable impurities, a slab having a B content of 0.0003% or less and an Al content of 0.005% or less was heated to a temperature range of 1200 to 1300 ° C. Rolling is started later, and the flange surface of the section steel is water-cooled to 700 ° C. or less in a rolling process, and a water-cooling / rolling cycle of rolling in a recuperation process is performed at least once, and then cooled after finishing the rolling. Method of manufacturing refractory rolled steel.

By weight%, C: 0.01-0.08%, Si: 0.05-0.
25%, Mn: 0.8 to 1.6%, Ti: 0.005 to 0.025%, Mg: 0.0005 to
0.0050%, Nb: 0.05-0.15%, V: 0.05-0.20%, Mo:
0.7 to 1.0%, N: 0.002 to 0.006%, O: 0.003 to 0.006%, and Cr: 0.1 to 1.0%, Ni: 0.1 to 1.0%, and Cu: 0.1 to 1.0% Consists of Fe and inevitable impurities, and the B content of the inevitable impurities is 0.0003.
% Or less and the slab whose Al content is limited to 0.005% or less,
Rolling is started after heating to a temperature range of 1200 to 1300 ° C, and at least one water-cooling / rolling cycle is performed, in which the flange surface of the section steel is water-cooled to 700 ° C or less in the rolling process and rolled in the reheating process. A method for producing a 700 ° C. refractory rolled steel, wherein the steel is allowed to cool later.

The 700 ° C. refractory rolled steel according to the present invention has
Desirably, the 0.2% proof stress at 00 ° C. is 220 MPa or more, and the surface hardness at a depth of 3 mm or less from the surface at room temperature is Hv 210 or less.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. Strengthening of steel is achieved by refinement of ferrite crystals, solid solution strengthening by alloying elements, dispersion strengthening by hardened phases, precipitation strengthening by fine precipitates, and the like. Also,
Higher toughness is achieved by miniaturization of crystal, reduction of solid solution N and C of mother phase (ferrite), reduction and miniaturization of high carbon martensite and coarse oxides and precipitates of hardened phase which is a starting point of fracture. And so on.

In general, the toughness is reduced by increasing the strength of the steel, and it is necessary to contradict high strength and toughness.
The only metallurgical factor that satisfies both at the same time is crystal refinement. A feature of the present invention is that Mg and T
MgO and Ti oxide and TiN by adding i
It achieves high strength and high toughness by microstructural refinement by low carbon bainite microstructure based on the dispersion and microalloying alloy design.

The high-temperature strength of steel is about 1 of the melting point of iron.
At 700 ° C. or less, which is equivalent to / 2, it can be obtained by the above-described strengthening mechanism at room temperature. Hitherto, improvement of high-temperature strength has been generally performed by adding Mo and Cr to increase the high-temperature softening resistance due to precipitation strengthening and suppression of dislocation loss. However, the addition of Mo and Cr significantly increases the hardenability, and as described above, the rolling and cooling conditions differ between the sections of the section steel, which greatly changes the amount of bainite formation, and increases the strength and ductility at ordinary and high temperatures. However, the required value cannot be stably secured due to the variation in toughness between the portions, and the toughness of the weld heat affected zone is reduced due to remarkable hardening of the welded portion.

In the present invention, high-temperature strength is ensured by precipitation of Mo carbide. However, by limiting the amount of Mo added, there is no variation in the strength and toughness due to the variation in hardenability between the parts as described above and no decrease in the toughness of the heat affected zone. In addition, in the present invention, in the hot rolling step, the flange surface is water-cooled between hot rolling passes, and the rolling process is repeated at the time of reheating to impart a rolling reduction effect to the center of the flange thickness. The microstructure refinement effect of the TMCP is also enhanced in the parts, and the refinement of the microstructure improves the mechanical properties of the base material in each part of the H-section steel, reduces the variation, and achieves homogeneity.

The reasons for limiting the composition range and the control conditions of the section steel of the present invention will be described below. First, C is added to strengthen the steel. If it is less than 0.01%, the strength required for structural steel cannot be obtained, and if it exceeds 0.08%, base metal toughness, weld crack resistance, The heat affected zone (hereinafter abbreviated as HAZ) significantly lowers the toughness and the like, so the lower limit is 0.01%.
The upper limit is set to 0.08%.

Next, Si is necessary for securing the strength of the base material, pre-deoxidizing the molten steel, etc. When it exceeds 0.25%, high-carbon island-like martensite is formed in the hardened structure of the base material and HAZ, It significantly reduces the toughness of the base metal and the welded joint. If the content is less than 0.05%, the preliminary deoxidation of the molten steel cannot be sufficiently performed, so the Si content is limited to the range of 0.05 to 0.25%. Mn must be added at 0.8% or more to secure the strength of the base material, but the upper limit is set to 1.6% from the allowable concentration for the toughness, cracking, and the like of the base material and the welded portion.

Ti forms Ti oxides to refine γ grains by grain boundary pinning, and suppresses generation of M * by precipitating TiN to reduce solid solution N. The finely precipitated TiN also contributes to the refinement of the γ phase.
By the action of these Tis, the structure is refined and the strength and toughness are improved. Therefore, if the content is less than 0.005%, the precipitation amounts of Ti oxide and TiN are insufficient, and these effects cannot be exerted. Therefore, the lower limit of the Ti content is set to 0.005%. However, if it exceeds 0.025%, excessive Ti precipitates TiC, and the precipitation hardening deteriorates the toughness of the base metal and the weld heat affected zone.
Limited to 5% or less.

Mg is necessary to reduce the old γ grain size immediately after hot rolling to 40 μm or less by the grain boundary pinning action of the Mg-based oxide. M used for Mg addition
The g alloy is preferably Si-Mg-Fe and Ni-Mg. The reason for using the Mg alloy is to reduce the Mg content by alloying, suppress the deoxidation reaction at the time of addition to molten steel, ensure the safety at the time of addition, and improve the yield of Mg. Mg 0.00
The reason for limiting the content to 05 to 0.0050% is that Mg is a strong deoxidizing element, and the crystallized MgO is easily floated and separated in molten steel. Without
On the other hand, if it is less than 0.0005%, the dispersion density of the Mg-based oxide for refining the prior γ particle size becomes insufficient. In this specification, the Mg-based oxide is also abbreviated as MgO, but according to electron microscopic analysis and the like, it is mainly composed of MgO, Ti, and a small amount of A.
1 and a composite oxide with Ca and the like contained as impurities.

Nb is added for the purpose of increasing hardenability and increasing strength. To achieve this effect, the Nb content needs to be 0.05% or more. However, if it exceeds 0.15%, Nb
Since the amount of precipitated carbonitride increases and the effect as solid solution Nb saturates, it is limited to 0.15% or less. The addition of a small amount of V can make the rolling structure finer and strengthening it by precipitation of vanadium carbonitride, so that a lower alloy can be obtained and welding characteristics can be improved. To achieve this effect, the V content must be 0.05% or more. However, excessive addition of V hardens the weld and
Since the base material has a high yield point, the upper limit of the content is set to V: 0.
20%.

Mo is an element necessary to secure normal-temperature strength by improving hardenability and to increase high-temperature strength by precipitation of Mo carbide. In particular, for the precipitation of an effective amount of Mo carbide at 700 ° C.,
The content must be 0.7% or more. However, if it exceeds 1.0%, the precipitated Mo carbides are coarsened and the toughness is reduced.
Limited to:

N forms a solid solution in α and increases the strength.
In the upper bainite structure, M * is generated and the toughness is degraded, so that the solute N must be reduced as much as possible. However, in the present invention, N is combined with Ti to finely precipitate TiN in steel to reduce solid solution N, and is added for the purpose of suppressing crystal grain growth by TiN and exerting a structure refinement effect. doing. Therefore, in order for this effect to be manifested, the amount of N should be 0.
If it is less than 002%, the amount of TiN precipitated is insufficient, and if it exceeds 0.006%, the amount of precipitation is sufficient. However, coarse TiN is precipitated and the toughness is impaired, so N was limited to 0.002 to 0.006%.

B, when added in a small amount, increases the hardenability and contributes to an increase in strength. However, it has been found that when B is contained in excess of 0.0003%, M * is formed in the upper bainite structure and the toughness is significantly reduced.
%. The reason why the content of Al is set to 0.005% or less is that Al
Is a strong deoxidizing element, and if its content exceeds 0.005%, the formation of Ti oxide is inhibited and fine dispersion cannot be performed.
l was also limited to 0.005% or less as an impurity.

O (oxygen) is indispensable for the formation of Ti oxide, and its content needs to be more than 0.003%.
When the content exceeds 0.006%, the generated Ti oxide particles are coarsened, and the toughness is reduced.
Limited to 0.006%. P and S contained as unavoidable impurities
The amounts of P and S are not particularly limited, but may cause welding cracks and decrease in toughness due to solidification segregation. Therefore, the contents of P and S should be reduced as much as possible.

In addition to the above elements, at least one of Cr, Ni and Cu can be contained for the purpose of increasing the strength of the base material and improving the toughness of the base material. Cr is effective in strengthening the base material by improving the hardenability. In order to achieve this effect, the Cr content must be 0.1% or more. However, an excessive addition exceeding 1.0% is harmful from the viewpoint of toughness and curability, so the upper limit was made 1.0%.

Ni is an extremely effective element for increasing the toughness of the base material. To achieve this effect, the Ni content must be 0.1% or more. However, the addition of more than 1.0% increases the alloy cost and is not economical, so the upper limit was made 1.0%. Cu forms C on dislocations in the α phase by holding in the α temperature range and slowly cooling.
The u phase is precipitated, and the room temperature strength of the base material is increased by the precipitation hardening. However, if the precipitation of the Cu phase in α is less than 0.1%, it is within the solid solubility limit of Cu in α, and since precipitation does not occur, strengthening by Cu precipitation cannot be obtained. At 1.0% or more, the precipitation strengthening saturates, so Cu was limited to 0.1 to 1.0%.

The rolled section steel of the present invention has a capacity of 490 MPa (50 kg).
f / mm ² ) class tensile strength and toughness at the same time,
It is necessary to have a microstructure in which the area ratio of bainite is 50 to 90% and the balance is composed of ferrite pearlite and high carbon island martensite, and the area ratio of the high carbon island martensite is 5% or less. is there. Further, although the fire-resistant rolled section steel of the present invention has a tensile strength of 490 MPa class, the surface hardness within a depth of 3 mm from the surface is not more than Hv210 in order to drill bolt holes when used as a beam. desirable.

The above microstructure and surface hardness can be realized by the method of the present invention. That is, the slab having the above chemical composition is reheated to a temperature range of 1200 to 1300 ° C. The reason for limiting the reheating temperature to this temperature range is that in the production of shaped steel by hot working, plastic deformation is easily performed.
Heating of 200 ° C or more is required, and Mo, V, Nb
Since it is necessary to sufficiently dissolve such elements as a solid solution, the lower limit of the reheating temperature is set to 1200 ° C. The upper limit was set to 1300 ° C. in view of the performance and economy of the heating furnace.

Water-cooling between hot rolling passes, the surface temperature of the flange is reduced to 700 ° C. or less during rolling, and a water-cooling / rolling cycle of rolling in the reheating process between the next rolling passes is performed at least once. Water cooling between the rolling passes gives a temperature difference between the surface layer and the inside of the flange to penetrate the processing strain into the interior even under light rolling conditions. This is for performing the TMCP efficiently.

Rolling in the reheating process after the flange surface temperature is cooled to 700 ° C. or lower is performed in order to suppress quenching and hardening of the surface due to accelerated cooling after finish rolling and to soften the surface. The reason is that the flange surface temperature is 700 ° C
Once cooled, the γ / α transformation temperature is temporarily cut off, and the surface layer is reheated and heated by the next rolling. Rolling is performed in the γ / α two-phase coexisting temperature range. A mixed structure with the fine α thus formed is formed. Thereby, the hardenability of the surface layer can be significantly reduced, and the hardening of the surface layer caused by accelerated cooling can be prevented.

[0037]

EXAMPLE A prototype steel was melted in a converter, an alloy was added, preliminary deoxidation was performed, and the oxygen concentration of the molten steel was adjusted.
Alloys were sequentially added and cast into 250-300 mm thick slabs by continuous casting. The cooling of the slab was controlled by selecting the amount of water in the secondary cooling zone below the mold and the speed of drawing the slab. The slab was heated and rolled into an H-beam by a universal rolling mill row shown in FIG. 1, although illustration of the rough rolling step was omitted. Water cooling between rolling passes is provided with a water cooling device 5 before and after the intermediate universal rolling mill 4, and spray cooling and reverse rolling are repeatedly performed on the outer surface of the flange. Accelerated cooling after rolling is finished by the finishing universal rolling mill 6 and then cooled after finishing rolling. did.

The mechanical characteristics are shown in FIG. 2 at the center (1 / 2t2) of the thickness t2 of the flange 2 at 1 / 4,1 / of the overall flange width (B).
2 From the width (1 / 4B, 1 / 2B), it was determined using test specimens collected.
The properties of these parts were determined as follows: The flange 1 / 4F shows the average mechanical properties of the H-section steel, and the flange 1 / 2F has the lowest properties. This is because it was judged that the mechanical test characteristics of the H-section steel could be represented by the above.

Tables 1 and 2 show the chemical composition values of the steels of the present invention and the comparative steels, and Tables 3 and 4 show the water cooling times and rolling finishing temperatures of these steels during rolling. Next, Tables 5 and 6
Shows the mechanical test characteristic values of these H-section steels, the surface hardness of the flange side surfaces, and the bainite and M * area ratios. Note that the rolling heating temperature was adjusted to 1300 ° C. It is well-known that the temperature is set to a value that generally lowers the heating temperature so that the γ grains are refined to improve the mechanical test characteristics. This is because it was determined that this value could represent the mechanical test characteristics at a lower heating temperature. The underlined numerical values in each table are outside the scope of the present invention.

As shown in Tables 5 and 6, H according to the present invention was
Shape steels 1 to 3 and H-shapes A1 and A2 both meet the JIS standard values of 490 MPa grade steel for both yield strength and tensile strength. In other words, the yield strength exceeds the lower limit of 320 MPa, and the tensile strength is 490 MPa.
And their yield ratio (YS / TS) satisfies the low YR value of 0.8 or less. The Charpy impact value also exceeds 47 (J) at -10 ° C, sufficiently meeting the JIS standard value.

On the other hand, in the H-section steel 4, since the Mo content and the oxygen content exceed the respective upper limits, they exceed the upper limits of the room temperature strength and the surface hardness, and in addition, the upper limit of the bainato area ratio of 80%. -10 ° C because M * area ratio exceeds the upper limit of 5%
Can not meet the target 47J or more. In the H-section steel 5, the contents of carbon, Si, nitrogen, boron and Ti exceed the respective upper limits, so that the M * area ratio exceeds the upper limit and the Charpy absorbed energy value at −10 ° C. cannot be cleared. In addition, since the Mo content is less than the lower limit, the high temperature strength cannot be cleared.

In the H-section steel 6, the Nb, V and Al contents are
Since the respective upper limits are exceeded, the precipitation hardening of Nb and V carbonitrides exceeds the upper limit of the room temperature strength, and since Mg is not added, it is not possible to achieve the structure refinement by the Mg-based fine oxide, The Charpy absorbed energy value cannot be cleared. H
In the section steel A3, the Mo content, in the H section steel A4, the Nb content,
Each is less than the lower limit, and precipitation strengthening at high temperatures cannot be performed, and the high-temperature strength is insufficient. In addition, since the H-section steel A4 is not subjected to water cooling during rolling, the hardenability of the surface increases, and the surface hardness exceeds the upper limit.

That is, when all the requirements of the production method of the present invention are satisfied, H-section steels 1 to 3 and H
Like the section steels A1 and A2, high strength rolled section steels with sufficient strength and low temperature toughness even at the flange plate thickness 1/2 and width 1/2 where the mechanical test characteristics of the rolled section steel are the least guaranteed. Production becomes possible. The rolled section steel to which the present invention is applied is not limited to the H section steel of the above embodiment, but is also applicable to section steels having flanges such as I section steel, angle steel, channel steel, and unequal thickness angle steel. Of course, you can.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

[Table 3]

[0047]

[Table 4]

[0048]

[Table 5]

[0049]

[Table 6]

[0050]

According to the present invention, the alloy-designed cast slab and the rolled section steel to which the controlled rolling method is applied guarantee the fire resistance at 700 ° C., and the flange thickness 1/100 at which the mechanical test characteristics are most difficult to guarantee. 2. It has sufficient strength even in the width of 1/2 part, and it is possible to produce shaped steel with excellent toughness as it is rolled, improving the reliability of large steel structures, ensuring safety, economy, etc. The industrial effects of are extremely remarkable.

[Brief description of the drawings]

FIG. 1 is a schematic view of an example of an apparatus arrangement for performing the method of the present invention.

FIG. 2 is a diagram illustrating a cross-sectional shape of an H-section steel and a sampling position of a mechanical test piece.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... H-shaped steel 2 ... Flange 3 ... Web 4 ... Intermediate rolling mill 5 ... Water cooling device of the front and rear surface of intermediate rolling mill 6 ... Finishing rolling mill

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/58 C22C 38/58

Claims (6)

[Claims] (Claim 1) C: 0.01 to 0.08%, Si: 0.05 to 0.25%, Mn: 0.8 to 1.6%, Ti: 0.005 to 0.025%, Mg: 0.0005 to 0.0050%, Nb: 0.05 to 0.15% by weight%, V: 0.05 to 0.20%, Mo: 0.7 to 1.0%, N: 0.002 to 0.006%, and O: 0.003 to 0.006%, with the balance being Fe and inevitable impurities, and the B content of the inevitable impurities 0.0003%
And the chemical composition of which the Al content is limited to 0.005% or less, the bainite area ratio is 40 to 80%, and the balance is composed of ferrite / pearlite and high carbon island martensite. The area ratio of martensite is 5% or less, and the old γ grain size immediately after completion of hot rolling is 40%.
7 having a microstructure of not more than μm
00 ° C refractory rolled section steel. 2. The 0.2% proof stress at 700 ° C. is 217 MPa.
The 700 ° C. refractory rolled steel according to claim 1, wherein the surface hardness at a depth of 3 mm or less from the surface at room temperature is Hv210 or less. 3% by weight C: 0.01 to 0.08%, Si: 0.05 to 0.25%, Mn: 0.8 to 1.6%, Ti: 0.005 to 0.025%, Mg: 0.0005 to 0.0050%, Nb: 0.05 to 0.15%, V: 0.05-0.20%, Mo: 0.7-1.0%, N: 0.002-0.006%, O: 0.003-0.006%, and Cr: 0.1-1.0%, Ni: 0.1-1.0% and Cu: 0.1-1.0% At least one of them, and the balance consists of Fe and inevitable impurities, and the B content of the inevitable impurities is 0.0003%.
And a chemical composition in which the Al content is limited to 0.005% or less, the area ratio of bainite is 50 to 90%, and the balance is composed of ferrite / pearlite and high carbon island martensite. The area ratio of martensite is 5% or less, and the old γ grain size immediately after completion of hot rolling is 40%.
7 having a microstructure of not more than μm
00 ° C refractory rolled section steel. 4. The 0.2% proof stress at 700 ° C. is 217 MPa.
The 700 ° C. refractory rolled steel according to claim 1, wherein the surface hardness at a depth of 3 mm or less from the surface at room temperature is Hv210 or less. 5% by weight C: 0.01 to 0.08%, Si: 0.05 to 0.25%, Mn: 0.8 to 1.6%, Ti: 0.005 to 0.025%, Mg: 0.0005 to 0.0050%, Nb: 0.05 to 0.15%, V: 0.05 to 0.20%, Mo: 0.7 to 1.0%, N: 0.002 to 0.006%, and O: 0.003 to 0.006%, with the balance being Fe and inevitable impurities, and the B content of the inevitable impurities 0.0003%
Below and the slab with the Al content limited to 0.005% or less is 1200
Rolling is started after heating to a temperature range of ~ 1300 ° C, and at least one water-cooling / rolling cycle is performed, in which the flange surface of the section steel is water-cooled to 700 ° C or less in the rolling process and rolled in the recuperation process. A method for producing a 700 ° C. refractory rolled section steel, which is left to cool. 6% by weight C: 0.01 to 0.08%, Si: 0.05 to 0.25%, Mn: 0.8 to 1.6%, Ti: 0.005 to 0.025%, Mg: 0.0005 to 0.0050%, Nb: 0.05 to 0.15%, V: 0.05-0.20%, Mo: 0.7-1.0%, N: 0.002-0.006%, O: 0.003-0.006%, and Cr: 0.1-1.0%, Ni: 0.1-1.0% and Cu: 0.1-1.0% At least one of them, and the balance consists of Fe and inevitable impurities, and the B content of the inevitable impurities is 0.0003%.
Below and the slab with the Al content limited to 0.005% or less is 1200
Rolling is started after heating to a temperature range of ~ 1300 ° C, and at least one water-cooling / rolling cycle is performed, in which the flange surface of the section steel is water-cooled to 700 ° C or less in the rolling process and rolled in the recuperation process. A method for producing a 700 ° C. refractory rolled section steel, which is left to cool.