JP2005272949A - Rolled h-section steel with low yield ratio superior in fire resistance, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rolled H-section steel having a low yield ratio and superior fire resistance, and to provide a manufacturing method therefor. <P>SOLUTION: The rolled H-section steel has a composition comprising 0.10-1.0% Mo, preferably, 0.01-0.20% C, 0.6% or less Si, 0.05-1.6% Mn and 0.1% or less Al. The manufacturing method comprises reheating a base steel material having the above composition to 1,000 to 1,350°C; hot-rolling it at a finishing temperature not lower than the Ar<SB>3</SB>transformation temperature to obtain the H-section steel; subsequently cooling the outer or inner surface of a flange to a cooling-stopping temperature in the range of 100 to 650°C with an average cooling rate of 5 to 80°C/s; then stopping cooling; and recuperating it to 200 to 700°C. Thereby, the rolled H-section steel has a flange of which the inner or outer surface layer has a hard layer thereon and the rest surface layer has a soft layer thereon, of which the soft layer includes ferrite with an average particle diameter of 5 to 40 μm, and which has a structure containing a hard phase of 20 to 80% by a volume fraction by an average value in a flange thickness direction; has such a hot strength as to show a yield strength of 176 MPa or higher at 600°C while maintaining a low yield ratio of 80% or lower; and consequently shows superior fire resistance. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to a hot-rolled H-section steel frequently used in building structures and a method for producing the same, and more particularly to a reduction in yield ratio and improvement in fire resistance. Note that the rolled H-section steel targeted by the present invention is
It is a rolled H-section steel with a tensile strength of 400 MPa or more, and includes a rolled H-section steel with a constant outer method and a rolled thin H-section steel with a constant outer method, as well as a rolled H-section steel with a constant outer method.

  In view of the occurrence of serious damage to building structures due to recent large earthquakes, further improvements in the safety of structures are required.

  From the viewpoint of plasticizing a structural member to absorb seismic energy and improving the safety of the structure, a steel material having a low yield ratio is required. In addition, for example, when the variation in the yield strength of steel applied to the beam of a building structure is large, it is necessary to thicken (or increase the strength of) the column material more than necessary, from the viewpoint of safety and economy. Is disadvantageous. For this reason, from the viewpoint of improving safety and economic efficiency as a structure, a steel material having a reduced yield strength variation range is required.

  Under such circumstances, in 1998, the yield strength (YS) range was 120MPa or less, the variation range of YS was narrow, and the yield ratio (YR) was 80% or less, YR was low, narrow YS, low YR architecture. Structural steel was established as a JIS standard.

  On the other hand, when brittle fracture occurs, the building structure may collapse without fully exhibiting the elastic performance and plastic performance of the steel material as described above. For this reason, the steel material excellent also in weld part toughness as well as base material toughness is requested | required. From recent loading test studies assuming column-beam structures, it has become clear that toughness is required to be 70 J or higher, including welds at beam ends.

  Rolled H-section steel is mainly used as a structural material for welded structures, particularly as a beam for building structures. Therefore, the rolled H-section steel is also required to have excellent toughness including a narrow variation range of YS (narrow YS), low YR (low YR), and weld heat affected zone (HAZ). ing.

  In general, HAZ uses fine inclusions such as oxides, nitrides and sulfides (or their composites) to refine HAZ crystal grains and lower the carbon equivalent, thereby reducing alloy elements. By selecting and adding, toughness is improved.

  In addition, the strength of steel materials can be controlled by conventional methods such as solid solution strengthening by solid solution strengthening elements, precipitation strengthening by adding precipitation strengthening elements, dispersion strengthening by dispersing hard phases, and control. Strength has been increased by appropriately combining strengthening methods such as grain refinement and strengthening by microstructure control such as transformation by rolling, controlled cooling or quenching-tempering treatment.

  However, in H-section steel manufactured in various shapes as rolled, it is necessary to consider deformation such as flange warpage, web waves and torsion caused by thermal stress differences. It is not easy to produce a rolled H-section steel that satisfies the above required characteristics.

For such a problem, for example, Patent Document 1 proposes a yield point controlled rolled shape steel excellent in earthquake resistance that guarantees the yield point range of the shape steel. The technique described in Patent Document 1 adjusts C, Si, Mn, P, S, N, and Al to an appropriate range, and sets the ΔS amount that is a relational expression of S, Ca, Mg, and REM to −0.005 to 0.010%. Cast molten steel to which Ca, Mg, and REM are added so as to be within the range, and gradually cool between 900 ° C from the solidification temperature to composite oxidation of Al-based double oxide, MnS, Al-based double oxide and MnS A slab in which the total number of objects is dispersed at 20 pieces / mm ² or less is formed, and the slab is heated to form a rolled section steel by hot rolling at 20 ° C. or less at 900 ° C. or less. As a result, it is said that it will be a structural steel having a low yield ratio of 80% or less and a narrow YP and lamellar resistance.

  Patent Document 2 proposes a method for producing a low carbon equivalent rolled shape steel using flange water cooling and controlled rolling. In the technique described in Patent Document 2, a steel piece having a low carbon equivalent composition is heated to 1100 to 1300 ° C. to start rolling, and the flange is 750 ° C. at the surface layer temperature between the reverse rolling passes in the intermediate rolling process. Repeat the process of water cooling to the following and rolling in the double-heated process at the flange surface layer temperature in the low temperature γ to α / γ two-phase coexistence temperature range one or more times, and the total rolling reduction when the average rolling temperature of the flange is 950 ° C or less The rolling is reduced by 20% or more and cooled at a cooling rate corresponding to the flange thickness after rolling to obtain a rolled steel.

  Patent Document 3 proposes a method for producing an H-section steel having a plate thickness exceeding 40 mm, which is a combination of cooling recuperation of the inner and outer surfaces of the flange and the upper and lower surfaces of the web and hot rolling. The technique described in Patent Document 3 repeats the process of cooling and rough rolling the inner and outer surfaces of the flange and the upper and lower surfaces of the web to the position just above the Ms point twice or more, and then bringing the surface layer of the material to be rolled directly above the Ms point. The process of cooling and reheating to 1 or more times is performed, finish rolling is performed with the surface layer temperature set to 750 ° C. or more, and the process of cooling to the point just above the Ms point and reheating after the finish rolling is repeated once or more. Thus, high strength and high toughness can be achieved without requiring a complicated process such as controlled rolling.

  However, the techniques described in Patent Document 1, Patent Document 2, and Patent Document 3 are all based on the idea of controlling the thickness direction of the steel material to a uniform structure, and as a result, complex casting The production efficiency of slabs is reduced due to the need for processes, the ferrite transformation is promoted by water cooling during rolling and high strength is difficult to obtain, and the weld heat-affected zone (HAZ) toughness is reduced by adding alloys more than necessary. In addition, problems such as an increase in lead time due to a decrease in rolling and cooling stop temperatures, hindering production efficiency and ductility, or a need for a heat treatment process still remain.

Moreover, in a steel structure building structure, in order to prevent the structure member from collapsing due to a decrease in strength of the structural member due to a temperature rise during a fire, the structural member is provided with a fireproof coating. However, since this fireproof coating leads to a rise in construction costs and a prolonged construction period, there is a demand for reduction in fireproof coating and non-covering. In order to reduce the fireproof coating or to make it uncoated, it is necessary to use a steel material having high high-temperature strength, and there is a demand for a steel material for building structures having high high-temperature strength, that is, excellent fire resistance.
In response to such a demand, for example, Patent Document 4 proposes a steel material in which Mo and Nb are added to improve fire resistance.
Japanese Patent No. 2965813 Japanese Patent No. 2837056 Japanese Patent No. 3231444 Japanese Patent Publication No. 4-50362

  To improve the fire resistance, as disclosed in Patent Document 4, when carbide is added to the steel and further carbide forming elements such as Nb and V are added to the steel, the carbides are heated when heated to a high temperature in a fire. A method based on precipitating fine precipitates such as the above and improving high-temperature proof stress is employed. Further, since fine precipitates such as these carbides are likely to precipitate on dislocations, to make the structure mainly composed of bainite or martensite mainly having a dislocation density higher than the structure mainly composed of ferrite, It is said to be effective for improving high-temperature proof stress.

  However, when a bainite-based structure including bainite as a main component, or a bainite-based structure including a part of martensite, normal temperature proof stress and tensile strength increase. For this reason, when such H-shaped steel is applied to a beam, the pillar material that supports the structure is a fire-resistant steel or thick-walled material having a high normal temperature proof stress and tensile strength that is comparable to the beam material in terms of the building structure. Is necessary, which is economically disadvantageous. Moreover, in a bainite-based structure containing bainite as a main component or partially containing martensite, there is a problem that the yield ratio is high, ductility is further decreased, and the earthquake resistance as a beam material is decreased.

The present invention solves the above-mentioned problems of the prior art, has a tensile strength of 400 MPa or more and a yield ratio as low as 80% or less, is excellent in earthquake resistance, and has a low yield ratio rolled H type excellent in fire resistance. It aims at providing steel and its manufacturing method. Here, “excellent in fire resistance” refers to a case where the proof stress YS _{600 at} 600 ° C. is 3/4 or more of the design reference strength F value (corresponding to room temperature yield strength). Therefore, when the F value is 235 MPa, YS ₆₀₀ is 176 MPa or more.

  In order to achieve the above-mentioned problems, the present inventors diligently studied a method for improving fire resistance without increasing the room temperature proof stress and tensile strength more than necessary, having a low yield ratio. As a result, a composition containing Mo, which is a carbide forming element, is essential, and one surface layer of the inner and outer surfaces of the flange is a hard layer containing bainite and / or tempered martensite with a volume ratio of 50% or more, and the other surface layer has a volume. By making the structure made of a soft layer containing ferrite at a rate of 50% or more and changing in the thickness direction of the flange plate, an increase in normal temperature proof stress can be suppressed and a low yield ratio of 80% or less can be secured, and the desired fire resistance It was found that can be secured.

  First, an experiment that is the basis of the present invention will be described.

  Using the steel material a having the composition shown in Table 1, hot rolling conditions and cooling conditions after rolling were changed to produce steel sheets having structures in which the bainite phase and martensite phase fractions were variously changed. About some steel plates, cooling after rolling was performed only from one side. In a steel sheet cooled from only one side, the surface layer on the cooling side has a bainite-based structure, and the surface layer on the non-cooling side has a structure mainly composed of ferrite, and has a structure that changes in the thickness direction. When cooled from both sides, a substantially uniform structure is exhibited in the thickness direction.

Next, the obtained steel sheet was subjected to a tensile test at room temperature and 600 ° C. to obtain a room temperature yield strength YS _RT and a yield strength YS _{600 of} 600 ° C. In addition, about the steel plate cooled only from one side, the tension test was carried out by full thickness. The yield strength YS ₆₀₀ at 600 ° C. was obtained by heating to 600 ° C. and holding for 15 minutes, and then conducting a tensile test.

  In addition, the structure of the obtained steel sheet was observed, and the amount of bainite and martensite in the structure was measured at each position in the plate thickness direction (front surface, 1 / 4t, 1 / 2t, 3 / 4t, back surface). The amount (volume ratio) of each position obtained was averaged to obtain the bainite and martensite fraction (volume%) and the average hard phase fraction of the steel sheet.

  In addition, some steel plates were heated to 600 ° C. and held for a predetermined time (30 min), and then rapidly cooled, and the amount of precipitated Mo was measured by the extraction residue method to obtain the amount of precipitated Mo when heated at 600 ° C.

  The obtained results are shown in FIG. 1 in terms of the relationship between the amount of precipitated Mo and the average hard phase fraction, and in FIG. 2 in terms of the relationship between the yield strength YS and the average hard phase fraction.

1 and 2, as the bainite and martensite fraction (average hard phase fraction) increases, the amount of precipitated Mo during heating at 600 ° C increases, resulting in an increase in yield strength (YS ₆₀₀ ) at 600 ° C. However, it can be seen that the room temperature yield strength (YS _RT ) also increases. However, when cooling after rolling is performed from only one side (marked with ☆), the increase in room temperature proof stress (YS _RT ) is suppressed compared to cooling from both sides, while the proof stress at 600 ° C (YS ₆₀₀ ) is reduced. There is almost no decline.

  That is, one surface layer on the inner and outer surfaces of the flange is a hard layer mainly composed of bainite and / or martensite, and the other surface layer is a soft layer mainly composed of ferrite. While suppressing an increase in yield strength, it is possible to promote precipitation of carbides in bainite or martensite having a high dislocation density in the hard layer on the cooling side, and it is possible to increase the high temperature yield strength and improve fire resistance. I found out.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) A rolled H-section steel produced by hot rolling, having a composition containing Mo: 0.10 to 1.0% in mass%, and one surface layer of the flange inner and outer surfaces of the rolled H-section steel It consists of a hard layer containing bainite and / or tempered martensite with a volume ratio of 50% or more, and the other surface layer consists of a soft layer containing ferrite with a volume ratio of 50% or more. The average particle diameter of ferrite on the soft layer side Having a structure containing 20 to 80% by volume of the bainite and / or tempered martensite with an average value in the flange plate thickness direction of 5 to 40 μm, and a proof stress at 600 ° C. of 176 MPa or more. Low yield ratio rolled H-section steel with excellent fire resistance.
(2) In (1), the composition is mass%, C: 0.01 to 0.20%, Si: 0.6% or less, Mn: 0.05 to 1.6%, P: 0.030% or less, S: 0.030% or less, Al: A low yield ratio rolled H-section steel comprising 0.1% or less, Mo: 0.10 to 1.0%, and the balance being composed of Fe and inevitable impurities.
(3) In (2), in addition to the above-mentioned composition, it is further characterized by containing, in mass%, one or two selected from Cu: 1% or less and Ni: 3% or less Yield ratio rolled H-section steel.
(4) In (2) or (3), in addition to the above composition, it is further selected from mass%, Cr: 3% or less, V: 0.3% or less, Nb: 0.1% or less, B: 0.01% or less A low yield ratio rolled H-section steel, characterized by containing one or more of them.
(5) In any one of (2) to (4), in addition to the above composition, in mass%, Ti: 0.1% or less, Ca: 0.1% or less, Mg: 0.1% or less, Zr: 0.1% or less, A low yield ratio rolled H-section steel containing one or more selected from Hf: 0.1% or less and REM: 0.1% or less.
(6) In mass%, C: 0.01 to 0.20%, Si: 0.6% or less, Mn: 0.05 to 1.6%, P: 0.030% or less, S: 0.030% or less, Al: 0.1% or less, Mo: 0.10 to 1.0 After reheating the steel material having a composition containing 1% to 1000 to 1350 ° C., the hot rolling finish temperature is expressed by the following formula (1): Ar ₃ (° C.) = 910−273C + 25Si−74Mn−56Ni−16Cr−9Mo−5Cu− 1620Nb (1)
(Here, C, Si, Mn, Ni, Cr, Mo, Cu, Nb: content of each element (mass%))
A hot rolling process is carried out to form a H-shaped steel having a predetermined shape by squeeze rolling and universal rolling at an Ar ₃ transformation point (° C.) or higher defined by the following, and then the outer surface of the flange or the inner surface of the flange is 5-80 ° C./s. After cooling to the cooling stop temperature in the range of 100 to 650 ° C. at the surface temperature of the cooling surface at the average cooling rate, cooling is stopped, and cooling reheat treatment is performed to reheat to 200 to 700 ° C. at the surface temperature. A method for producing a low yield ratio rolled H-section steel excellent in fire resistance.

  According to the present invention, it is possible to easily produce a low yield ratio rolled H-section steel that suppresses an increase in room temperature yield strength and is excellent in earthquake resistance at a low yield ratio, and that is improved in high temperature yield strength and excellent in fire resistance. Yes, and it has a remarkable industrial effect. In addition, according to the present invention, the reliability of the structure is remarkably improved, and it is unnecessary to increase the strength and thickness of the column material more than necessary, which is advantageous in terms of economy.

  First, the reason for limiting the structure of the rolled H-section steel of the present invention will be described.

The rolled H-section steel of the present invention has a structure in which one surface layer of the inner and outer surfaces of the flange is a hard layer and the other surface layer is a soft layer. Here, the “hard layer” refers to a layer containing bainite and / or tempered martensite which are hard phases in a volume ratio of 50% or more with respect to the entire hard layer. In addition to the bainite and / or tempered martensite, there is no problem even if the hard layer contains a ferrite phase and a pearlite phase having a volume ratio of 50% or less with respect to the entire hard layer. The “soft layer” refers to a layer containing 50% or more by volume of the soft phase ferrite with respect to the entire soft layer. In addition to the ferrite phase, the soft layer may contain a pearlite phase, a bainite phase, and a tempered martensite phase in a volume ratio of 50% or less with respect to the entire soft layer.
By forming one surface layer of the flange as a hard layer and the other surface layer as a soft layer, as shown in FIG. 2, it is possible to easily increase the high-temperature resistance while suppressing an increase in room-temperature resistance and maintaining a low yield ratio. It can be secured.

  In the rolled H-section steel of the present invention, the flange has an average value in the thickness direction of the flange plate and has a structure containing 20-80% by volume of bainite and / or tempered martensite which are hard phases. If the average fraction (average hard phase fraction) of bainite and / or tempered martensite in the thickness direction of the flange plate is less than 20% by volume, it is difficult to ensure sufficient high-temperature proof stress. On the other hand, when the average hard phase fraction exceeds 80% by volume, the normal temperature proof stress and the tensile strength increase, and the ductility and toughness decrease. In addition, a low yield ratio of 80% or less cannot be secured. For this reason, the average hard phase fraction was limited to 20 to 80% by volume. The average hard phase fraction is determined by imaging the structure of 5 fields or more at each position in the plate thickness direction (front surface, plate thickness 1 / 4t, 1 / 2t, 3 / 4t, and back surface). , And the average value at each position, and the average value in the thickness direction is used.

    In the rolled H-section steel of the present invention, the ferrite formed on the soft layer side is a ferrite having a particle size of 5 to 40 μm on average. When the average grain size of ferrite is less than 5 μm, the yield strength at room temperature increases, and it becomes difficult to secure a low yield ratio of 80% or less, and the ductility is lowered. On the other hand, if the average particle diameter of ferrite exceeds 40 μm, toughness deteriorates. In addition, Preferably it is 7-30 micrometers. Here, the measurement of the average particle diameter of a ferrite shall be performed about the ferrite in a soft layer.

  The rolled H-section steel of the present invention has a composition containing Mo: 0.10 to 1.0% in mass% in addition to the above structure, and more preferably in mass%, C: 0.01 to 0.20%, Si: 0.6 %: Mn: 0.05 to 1.6%, P: 0.030% or less, S: 0.030% or less, Al: 0.1% or less, Mo: 0.10 to 1.0%, or Cu: 1% or less, Ni: 3% One or two selected from the following and / or Cr: 3% or less, V: 0.3% or less, Nb: 0.1% or less, B: 0.01% or less One or more selected from Ti: 0.1% or less, Ca: 0.1% or less, Mg: 0.1% or less, Zr: 0.1% or less, Hf: 0.1% or less, REM: 0.1% or less Or it has 2 or more types, and the remainder has a composition which consists of Fe and an unavoidable impurity.

  Next, the reason for limiting the composition will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.

Mo: 0.10 to 1.0%
Mo is an effective element that increases the high-temperature strength. In the present invention, the content of 0.10% or more is required in order to ensure excellent fire resistance with a yield strength at 600 ° C. of 176 MPa or more. On the other hand, the content exceeding 1.0% causes a decrease in weldability. For this reason, Mo was limited to the range of 0.10 to 1.0%. In addition, Preferably it is 0.2 to 0.8%.

C: 0.01-0.20%
C is an element that increases the strength of the rolled H-section steel. In order to ensure the strength of a predetermined value or more and to precipitate special carbide effective for increasing the high temperature strength, in the present invention, 0.01% is added. % Content is required. On the other hand, if the content exceeds 0.20%, a welded portion, particularly a small heat input welded portion such as a tack welded portion, is hardened and there is a concern that a weld crack will occur. For this reason, C is preferably limited to a range of 0.01 to 0.20%. In addition, More preferably, it is 0.02 to 0.18%.

Si: 0.6% or less
Si is an inexpensive element that dissolves in steel to increase strength and acts as a deoxidizer in the melting stage. To obtain such an effect, 0.05% or more should be contained. desirable. On the other hand, the content exceeding 0.6% lowers the toughness. For this reason, it is preferable to limit Si to 0.6% or less.

Mn: 0.05-1.6%
Mn, like Si, is an element that effectively acts to improve the strength of rolled H-section steel, and it is preferably contained in an amount of 0.05% or more in the present invention. On the other hand, if the content exceeds 1.6%, weldability decreases. For this reason, it is preferable to limit Mn to the range of 0.05 to 1.6%.

P: 0.030% or less, S: 0.030% or less P and S are present as unavoidable impurities in steel and adversely affect toughness and tempering brittleness, so it is desirable to reduce them as much as possible. However, if P: 0.030% or less and S: 0.030% or less, those adverse effects are small. For this reason, it is preferable to limit P to 0.030% or less and S to 0.030% or less.

Al: 0.1% or less
Al is an element that acts as a deoxidizer, and in order to obtain such an effect, 0.005% or more is preferably contained. On the other hand, the content exceeding 0.1% lowers the cleanliness of the steel. For this reason, it is preferable to limit Al to 0.1% or less. In addition, when performing a deoxidation process with other elements, such as Si, you may not add, but in this case, it will contain less than 0.005% as an unavoidable impurity.

  In addition to the basic composition described above, if necessary, one or two selected from Cu: 1% or less, Ni: 3% or less, and / or Cr: 3% or less, V: 0.3% Hereinafter, Nb: 0.1% or less, B: One or more selected from 0.01% or less, and / or Ti: 0.1% or less, Ca: 0.1% or less, Mg: 0.1% or less, Zr: One or more selected from 0.1% or less, Hf: 0.1% or less, and REM: 0.1% or less may be selected and contained. The balance other than the components described above is Fe and inevitable impurities.

One or two selected from Cu: 1% or less, Ni: 3% or less
Cu and Ni are solid solution strengthening elements, which can increase the strength of H-section steel without greatly improving hardenability, and are particularly effective in adjusting the room temperature proof stress of thick-wall flange H-section steel. It can be selected and contained as necessary. If contained, Cu: 0.05% or more, Ni: 0.05% or more is preferable, but inclusion of Cu exceeding 1% promotes surface cracking during rolling and causes significant Cu precipitation embrittlement. . Ni is an expensive element and is preferably limited to 3% or less.

One or more selected from Cr: 3% or less, V: 0.3% or less, Nb: 0.1% or less, B: 0.01% or less
When Cr, V, Nb, and B are contained in combination with Mo, a further increase in high-temperature strength can be expected, and they can be selected and contained as necessary.

  Cr increases the high-temperature strength through an improvement in temper softening resistance. Such an effect becomes remarkable when the content is 0.1% or more. On the other hand, the content exceeding 3% lowers the weldability. For this reason, it is preferable to limit Cr to 3% or less.

  Both Nb and V are elements having a strong carbide forming ability like Mo and are effective in improving the high temperature strength, and are preferably contained in an amount of 0.010% or more. A part of V and Nb precipitates as carbides in the cooling or recuperation process after rolling, and increases the normal temperature proof stress. For this reason, it is preferable to limit to V: 0.3% or less and Nb: 0.1% or less.

  B is an element that improves hardenability by adding a small amount, and acts effectively when improving the fire resistance of an extremely thick flange having a plate thickness exceeding 50 mm that makes accelerated cooling insufficient after rolling. . For this purpose, it is desirable to contain 0.0003% or more. On the other hand, even if the content exceeds 0.01%, the effect of improving hardenability is saturated and an effect commensurate with the content cannot be expected. For this reason, it is preferable to limit B to 0.01% or less.

Ti: 0.1% or less, Ca: 0.1% or less, Mg: 0.1% or less, Zr: 0.1% or less, Hf: 0.1% or less, REM: 0.1% or less
Ti, Ca, Mg, Zr, Hf, and REM are all effective elements for refining the austenite grain size of the weld heat affected zone, and can be selected and contained as necessary. On the other hand, an excessive content exceeding 0.1% respectively reduces the cleanliness and reduces toughness and ductility. Therefore, it is preferable to limit to Ti: 0.1% or less, Ca: 0.1% or less, Mg: 0.1% or less, Zr: 0.1% or less, Hf: 0.1% or less, and REM: 0.1% or less. Ti, Ca, Mg, Zr, Hf, and REM are all strong deoxidizing elements, and can be added as a deoxidizing agent instead of Si or Al.

  Below, the preferable manufacturing method of the rolling H-section steel of this invention is demonstrated.

First, after reheating the steel material having the above composition to 1000 to 1350 ° C., it is converted into an H-shaped steel having a predetermined shape by hole rolling and universal rolling in which the hot rolling finish temperature is not less than the Ar ₃ transformation point (° C.). A hot rolling process is performed.
In addition, the manufacturing method of a steel raw material is not specifically limited in this invention. Both normal melting methods and casting methods can be suitably applied, but it is economically advantageous to use a continuous casting method as the casting method.

  The steel material is preferably reheated to 1000 ° C. or higher in order to transform it into uniform austenite with low deformation resistance. On the other hand, if the steel material is reheated above 1350 ° C., the oxidation becomes significant and the risk of increasing surface defects and scale loss increases. For this reason, it is preferable to make the reheating temperature of a steel raw material into the range of 1000-1350 degreeC.

The heated steel material is made into an H-shaped steel having a predetermined size and shape by hole rolling and universal rolling. In the hole rolling and universal rolling, it is preferable that the hot rolling end temperature is a temperature equal to or higher than the Ar ₃ transformation point. If the hot rolling end temperature is less than the Ar ₃ transformation point, the ferrite is precipitated on the subsequent flange water cooling surface side, so that the fire resistance is lowered. For this reason, it is preferable to limit the hot rolling end temperature to the Ar ₃ transformation point or higher. Incidentally, more preferably in the range of (Ar ₃ transformation point + 20 ℃) ~950 ℃. The Ar ₃ transformation point is the following formula (1): Ar ₃ (° C.) = 910−273C + 25Si−74Mn−56Ni−16Cr−9Mo−5Cu−1620Nb (1)
(Here, C, Si, Mn, Ni, Cr, Mo, Cu, Nb: content of each element (mass%))
Defined by

  Subsequently, after the hot rolling, the H-shaped steel that has finished the hot rolling process is subjected to a cooling reheat treatment.

The cooling heat treatment is preferably performed at a temperature of (Ar ₃ transformation point −100 ° C.) or higher, at an average cooling rate of 5 to 80 ° C./s on the flange outer surface or the flange inner surface, and a surface temperature of the cooling surface of 100 to 650 ° C. After cooling to a cooling stop temperature in the range, it is preferable to stop the cooling and reheat to a temperature of 200 to 700 ° C. at the surface temperature.

  In the present invention, it is desirable to perform accelerated cooling using water cooling from one of the inner and outer surfaces of the flange. This makes the water-cooled surface side a hard layer mainly composed of bainite and / or martensite having a high dislocation density and improves the fire resistance, while the non-water-cooled surface side is a soft layer mainly composed of a soft ferrite phase. And it can suppress that normal temperature tensile strength raises more than necessary.

In the accelerated cooling in the cooling reheat treatment, the cooling is preferably performed at a temperature equal to or higher than (Ar ₃ transformation point−100 ° C.) in order to suppress the precipitation of ferrite and improve the fire resistance on the water-cooled surface side. The cooling rate for accelerated cooling is preferably 5 ° C./s or higher in order to suppress ferrite precipitation and improve fire resistance. In addition, even if it cools with the cooling rate exceeding 80 degreeC / s, it does not show an effect for fire resistance improvement. For this reason, the cooling rate of accelerated cooling is preferably limited to a range of 5 to 80 ° C./s. More preferably, it is 6-50 degreeC / s. The cooling rate is an average cooling rate in the flange plate determined by heat transfer calculation.

  The cooling stop temperature for accelerated cooling is preferably in the range of 100 to 650 ° C. as the surface temperature of the cooling surface. If the cooling stop temperature exceeds 650 ° C., the water cooling surface cannot be sufficiently transformed into bainite or martensite. On the other hand, when the temperature is lower than 100 ° C., recuperation from the non-water-cooled surface becomes insufficient, and ductility is lowered. For this reason, it is preferable to limit the cooling stop temperature to a range of 100 to 650 ° C. in terms of the surface temperature of the water cooling surface. In addition, it is 150-600 degreeC more preferably.

  In the cooling, it is preferable that the cooling is performed after the cooling is stopped and the surface temperature of the water cooling surface is reheated to a temperature within a range of 200 ° C to 700 ° C. In order to set the recuperation temperature to 200 ° C. or higher, it is preferable to set the surface temperature of the cooling surface to 100 ° C. or higher during cooling.

  When the recuperation temperature is less than 200 ° C., the cooling surface is excessively cured and the ductility is lowered. On the other hand, when the recuperation temperature exceeds 700 ° C., precipitation of carbides is promoted during the recuperation process, and the increase in the high-temperature yield strength decreases, the room temperature yield strength increases, and the yield ratio increases. For this reason, the recuperation temperature is preferably in the range of 200 to 700 ° C. In addition, More preferably, it is 300-700 degreeC.

  In addition, the other surfaces (non-cooled surfaces) that are not subjected to the cooling among the inner and outer surfaces of the flange are not subjected to special cooling because they form a soft layer on the surface layer, or are allowed to cool or have a cooling rate of 1 It is preferable that the cooling is performed at a low temperature of ℃ / s or less.

    Molten steel having the composition shown in Table 2 was melted in a converter and used as a beam blank slab (steel material) by a continuous casting method. Next, after reheating these steel materials to the heating temperatures shown in Table 3, they are subjected to a hot rolling process with the conditions shown in Table 3 to form H-shaped steels, and then water-cooled on the flange outer surface under the conditions shown in Table 3 and restored. A cooling reheat treatment for heating was performed. In some H-section steels, cooling outer heat treatment was applied to the outer surface and inner surface of the flange as a comparative example. Further, in some H-section steels, the cooling inner heat treatment was not performed on the inner and outer surfaces of the flange, and it was left to cool.

  From the H-shaped steel thus obtained, No. 1 tensile test piece defined in JIS Z 2201 was taken from the portion of 1/4 of the flange width as the rolling direction. Further, a V-notch test piece defined in JIS Z 2202 was taken from a 1/4 t part of the plate thickness at a quarter of the flange width. The tensile test was performed at room temperature and the Charpy impact test was performed at 0 ° C.

  Further, a high-temperature tensile test piece (10 mmφ) was taken from the flange having a thickness of 1/2 t, and a high-temperature tensile test was performed at 600 ° C. The high temperature tension was performed after holding at 600 ° C. for 15 minutes.

  Furthermore, the structure was investigated using the optical microscope about the plate | board thickness direction cross section (L direction cross section) of a flange. The observation positions are the front surface (outer surface), 1 / 4t, 1 / 2t, 3 / 4t, and back surface (inner surface), and at least 5 visual fields are observed at each position, and bainite and martensite (tempering) that are hard phases at each position. ) Calculated by the image analysis device, the hard phase fraction at each position, and the average hard phase fraction at each position, the average hard phase fraction in the flange plate thickness direction . In addition, from the structure observation, the presence or absence of the hard layer and the soft layer was confirmed on the surface layer of the flange, and the tissue fractions of the hard layer and the soft layer were similarly determined.

  For the soft layer, the region of 1 to 5 mm from the surface was observed with 5 or more fields of view with an optical microscope, and the ferrite particle size was measured as an equivalent circle diameter using an image analyzer.

  Table 4 shows the obtained results.

In all of the examples of the present invention, the increase in room temperature yield strength (room temperature yield point) YP, 0.2YS was suppressed, the low yield ratio of 80% or less was maintained, and the high temperature yield strength YS ₆₀₀ at 600 ° C. was 176 MPa or higher. It is a low yield ratio rolled H-section steel with high strength and excellent fire resistance. On the other hand, in the comparative examples (H-section steel Nos. 5 and 13) in which the average hard phase fraction falls outside the scope of the present invention when both surfaces of the flange are cooled with water, the high temperature strength is remarkably increased, but the room temperature strength is more than that. Increased and ductility decreased. In addition, in the comparative examples (H-section steel No.14, No.19) that are out of the scope of the present invention, in which the water cooling of one side of the flange is insufficient and the average hard phase fraction is less than 20%, the high-temperature proof stress is low, On the other hand, in the comparative example (H-section steel No. 7) in which the average hard phase fraction exceeds 80% and deviates from the scope of the present invention, the normal temperature strength is greatly increased, but the ductility and toughness are significantly decreased.

  Further, in the comparative example (H-section steel No. 7) in which the recuperation temperature is lower than 200 ° C. and the average hard phase fraction is out of the range of the present invention, the ductility is lowered. In the present invention examples (H-section steel No. 6 and No. 20) in which the recuperation temperature exceeds 650 ° C. and the production conditions are outside the preferred range of the present invention, the high-temperature strength is slightly reduced.

4 is a graph showing the influence of the average hard phase fraction on the amount of precipitated Mo at 600 ° C. It is a graph showing the relationship between the yield strength YS ₆₀₀ and average hard phase fraction at room temperature yield strength YS _RT, 600 ℃.

Claims (6)

  Rolled H-section steel manufactured by hot rolling, having a composition containing Mo: 0.10 to 1.0% in mass%, and one surface layer of the flange inner and outer surfaces of the rolled H-section steel in volume ratio It consists of a hard layer containing 50% or more of bainite and / or tempered martensite, and the other surface layer consists of a soft layer containing 50% or more of ferrite by volume, and the average particle diameter of ferrite on the soft layer side is 5 to 5%. It has a structure containing 20 to 80% by volume of the bainite and / or tempered martensite at an average value in the thickness direction of the flange plate of 40 μm, and the proof stress at 600 ° C. is 176 MPa or more. Low yield ratio rolled H-section steel with excellent fire resistance. The composition is mass%,
C: 0.01-0.20%, Si: 0.6% or less,
Mn: 0.05 to 1.6%, P: 0.030% or less,
S: 0.030% or less, Al: 0.1% or less,
Mo: 0.10 to 1.0%
The low yield ratio rolled H-section steel according to claim 1, wherein the balance is a composition comprising Fe and inevitable impurities.   The low yield ratio according to claim 2, further comprising one or two kinds selected from Cu: 1% or less and Ni: 3% or less in mass% in addition to the composition. Rolled H-section steel.   In addition to the above-described composition, it may further contain at least one selected from mass%, Cr: 3% or less, V: 0.3% or less, Nb: 0.1% or less, and B: 0.01% or less. The low yield ratio rolled H-section steel according to claim 2 or 3, wherein: In addition to the above composition, mass: Ti: 0.1% or less, Ca: 0.1% or less, Mg: 0.1% or less,
The low yield according to any one of claims 2 to 4, comprising one or more selected from Zr: 0.1% or less, Hf: 0.1% or less, and REM: 0.1% or less. Specific rolled H-section steel. mass%
C: 0.01-0.20%, Si: 0.6% or less,
Mn: 0.05 to 1.6%, P: 0.030% or less,
S: 0.030% or less, Al: 0.1% or less
Mo: 0.10 to 1.0%
After reheating the steel material having the composition containing 1000 to 1350 ° C., the hot rolling finish temperature is set to Ar ₃ transformation point (° C.) defined by the following formula (1) or more by the squeeze rolling and the universal rolling. A hot rolling process is performed to form a H-shaped steel with a predetermined shape, and then the cooling of the outer surface of the flange or the inner surface of the flange is stopped at an average cooling rate of 5 to 80 ° C./s and a surface temperature of the cooling surface in the range of 100 to 650 ° C. A method for producing a low yield ratio rolled H-section steel excellent in fire resistance, characterized in that after cooling to a temperature, cooling is stopped and a cooling reheat treatment is performed to reheat to a temperature of 200 to 700 ° C. at the surface temperature.
Ar ₃ (° C.) = 910−273C + 25Si−74Mn−56Ni−16Cr−9Mo−5Cu−1620Nb (1)
Here, C, Si, Mn, Ni, Cr, Mo, Cu, Nb: Content of each element (mass%)

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