JP6645107B2 - H-section steel and manufacturing method thereof - Google Patents

Description

  The present invention relates to an H-section steel suitable for a steel structure such as a building and a method for producing the same.

  Steel structures such as pillars and beams of buildings are required to prevent collapse of buildings at high temperatures for a certain period of time so that users can evacuate when exposed to fire. It is required to have fire resistance performance that exhibits the required strength. Generally, steel materials decrease in strength when exposed to high temperatures. Therefore, conventionally, a technique has been adopted in which a steel material is covered with a refractory coating to suppress a rise in the temperature of the steel material during a fire. However, in recent years, steel materials having high strength (high-temperature strength) at 600 ° C. (hereinafter, may be referred to as fire-resistant steel materials) capable of simplifying or reducing the refractory coating have been used.

  Conventional refractory steel materials are those to which Mo that increases the high-temperature strength is positively added. However, since the price of Mo tends to fluctuate and the cost increases when the price of Mo rises, a refractory steel plate based on an alloy design that does not necessarily rely on the addition of Mo has been proposed (for example, see Patent Document 1). . The refractory steel plate disclosed in Patent Literature 1 is a refractory steel plate in which V carbonitride is precipitated on bainite and martensite having a high dislocation density to improve high-temperature strength.

  By the way, at present, demand for H-section steel having high high-temperature strength (hereinafter, sometimes referred to as fire-resistant H-section steel), particularly fire-resistant H-section steel for buildings, is increasing. Conventional refractory H-section steels have alloy elements added to them to enhance high-temperature strength by precipitation strengthening or solid solution strengthening. On the other hand, although the fire resistance performance is unknown, an H-section steel having a steel material structure mainly composed of bainite by applying accelerated cooling to secure toughness has been proposed (for example, see Patent Documents 2 and 3). Further, for a fire-resistant H-section steel, a technique using pseudopolygonal ferrite generated by accelerated cooling has been proposed (for example, see Patent Document 4).

JP 2007-212278 A International Publication No. 2014-142060 International Publication No. 2014-080818 JP 2013-224460 A

  In recent years, with the increase in the size of welded structures such as buildings, the thickness of steel materials has increased, and large heat input welding has often been adopted in order to increase welding efficiency. In large heat input welding, the temperature of the heat-affected zone (hereinafter, sometimes referred to as HAZ) during welding rises and the cooling rate decreases, so that the austenite (γ) grain size is coarsened and the old HAZ γ Precipitation of carbides and the like at grain boundaries is promoted. In particular, when a large amount of Nb, V, Mo, B, or the like is added in order to increase the high-temperature strength, carbonitrides are generated in the welded HAZ, and reheating embrittlement of the HAZ may occur.

  As described in Patent Document 4, in a fire-resistant H-section steel using pseudo-polygonal ferrite, although a certain high-temperature strength can be ensured, further improvement in characteristics is required. In view of such circumstances, an object of the present invention is to provide an H-section steel in which the high-temperature strength of a steel material is improved without adding a large amount of alloying elements, and a method of manufacturing the same.

  The present inventors did not add a large amount of Mo and B, which were conventionally used positively to ensure high-temperature strength, to prevent reheat embrittlement of large heat input welding HAZ, to ensure toughness, and The chemical components and manufacturing conditions for ensuring high-temperature strength were studied repeatedly. As a result, by optimizing the contents of C, Si, Mn, Ti, Al, and N and accelerated cooling conditions after hot rolling, a steel material mainly composed of bainite and pseudopolygonal ferrite satisfying fire resistance performance is obtained. It has been found that a refractory steel having a structure can be obtained stably. This refractory steel material exhibited excellent refractory performance such that the tensile strength at room temperature was 490 to 610 MPa and the yield strength at 600 ° C. was 217 MPa or more. Furthermore, it has been found that the addition of Ti and the limitation of the contents of Mo, Nb, V, and B are effective in suppressing reheat embrittlement of the large heat input welding HAZ.

The present invention has been made based on such findings, and according to an aspect of the present invention, it is an H-section steel, which is 0.05 to 0.15% by mass, C: 0.05% by mass, and Si: 0% by mass%. 0.01 to 0.50%, Mn: 0.50 to 1.60%, Ti: 0.005 to 0.030%, Al: 0.01 to 0.100%, N: 0.0010 to 0.0050 %, P: 0.030% or less, S: 0.020% or less, Mo: 0.05% or less, Nb: 0.005% or less, V: 0.01% or less, B: 0.0003 % or less, O: limited to 0.010% or less, having a steel composition and the balance being Fe and impurities, not more than 0.46% by weight carbon equivalent _{C eq} obtained by the following formula (1), wherein In the section of the H-section of the H-section steel, the thickness of the flange is 1/6 from the surface in the longitudinal direction of the flange. In the metal structure at a position １ ／ from the surface in the direction, the area ratio of bainite is 20% or more, and the H-section steel having a total area ratio of bainite and pseudopolygonal ferrite of 90% or more is obtained. Provided.

  The H-section steel is, by mass%, one or two of Cr: 0.50% or less, W: 0.50% or less, Cu: 0.50% or less, and Ni: 0.50% or less. The above may be contained.

  The H-section steel is, by mass%, Zr: 0.010% or less, Mg: 0.005% or less, Ca: 0.005% or less, Y: 0.050% or less, La: 0.050% or less, Among them, one or more kinds may be contained.

  The H-shaped steel has a flange thickness of 80 mm or less, at a position 1/6 from the surface in the length direction of the flange, and at a position 1/4 from the surface in the thickness direction of the flange. The yield strength or 0.2% proof stress at room temperature may be 325 MPa or more, the tensile strength may be 490 MPa or more, and the 0.2% proof stress at 600 ° C. may be 217 MPa or more.

According to another aspect of the present invention, there is provided a method for manufacturing an H-section steel, comprising: a hot rolling step of hot rolling a steel slab at 1000 to 1350 ° C. at 800 ° C. or higher. And after the hot rolling step, an accelerated cooling step of accelerating and cooling the slab to 300 ° C. or less, wherein the slab is 0.05 to 0.15% by mass, C: 0.05 to 0.1% by mass. 01 to 0.50%, Mn: 0.50 to 1.60%, Ti: 0.005 to 0.030%, Al: 0.01 to 0.100%, N: 0.0010 to 0.0050% , P: 0.030% or less, S: 0.020% or less, Mo: 0.05% or less, Nb: 0.005% or less, V: 0.01% or less, B: 0.0003% Hereinafter, O is limited to 0.010% or less, and the balance has a steel composition composed of Fe and impurities, and is determined by the following formula (1). Carbon equivalent C _eq is der less 0.46 mass% to be is, the H-shaped steel, in the cross section of the H-shaped, a position from the surface 1/6 the length direction of the flange, the thickness direction of the flange at 1/4 position from the surface in the metal structure is a area ratio of bainite of 20% or more, and, the production of bainite and quasi polygonal ferrite total area ratio of 90% or more der Ru H-beams A method is provided.

According to another aspect of the present invention, there is provided a method for manufacturing an H-section steel, comprising: a hot rolling step of hot rolling a steel slab at 1000 to 1350 ° C. at 800 ° C. or higher. And an accelerated cooling step of accelerating and cooling the slab to 300 to 600 ° C. after the hot rolling step, wherein the slab is represented by mass%, C: 0.05 to 0.15%, Si: 0. 0.01 to 0.50%, Mn: 0.50 to 1.60%, Ti: 0.005 to 0.030%, Al: 0.01 to 0.100%, N: 0.0010 to 0.0050 %, P: 0.030% or less, S: 0.020% or less, Mo: 0.05% or less, Nb: 0.005% or less, V: 0.01% or less, B: 0.0003 %, O: 0.010% or less, with the balance being a steel composition consisting of Fe and impurities. Carbon obtained equivalent C _eq is Ri der less 0.46 mass%, the H-shaped steel, in the cross section of the H-shaped, a position of 1/6 from the surface in the length direction of the flange, the thickness of the flange at the position of 1/4 from the surface in the direction, the metal structure is a area ratio of bainite of 20% or more, and, the bainite and quasi polygonal ferrite total area ratio of 90% or more der Ru H-beams A manufacturing method is provided.

  The method of manufacturing the H-section steel may include a cooling step of allowing the steel slab to cool as it is, after an accelerated cooling step of accelerating and cooling the steel slab to 300 to 600 ° C.

  The method for manufacturing an H-section steel may include a tempering heat treatment step of performing a tempering heat treatment while maintaining the steel slab in a temperature range of 400 to 650 ° C. after the accelerated cooling step.

  The steel slab is, by mass%, one or more of Cr: 0.50% or less, W: 0.50% or less, Cu: 0.50% or less, and Ni: 0.50% or less. May be contained.

  The steel slab is expressed by mass% of Zr: 0.010% or less, Mg: 0.005% or less, Ca: 0.005% or less, Y: 0.050% or less, La: 0.050% or less. Among them, one or more kinds may be contained.

  The H-shaped steel of the present invention has a tensile strength at room temperature of 490 to 610 MPa, a yield strength at 600 ° C. of 217 MPa or more, and a drawing reduction value in a welded HAZ of 600 ° C. tensile test of 50% or more, Excellent reheat embrittlement resistance. Furthermore, toughness is ensured even in HAZ by welding at a heat input of 10 kJ / mm.

It is a figure which shows an example of the pseudo polygonal ferrite observed with the optical microscope. It is a figure showing an example of polygonal ferrite observed with an optical microscope. It is a figure showing an example of bainite observed by an optical microscope. It is a figure showing the manufacturing line of the hot rolling process in an example. It is sectional drawing of the H-shaped steel manufactured in the Example.

  Hereinafter, embodiments for carrying out the present invention will be described. Conventionally, accelerated cooling after hot rolling has been employed to increase the tensile strength of steel at room temperature. On the other hand, accelerated cooling causes defective shape and residual stress of the steel sheet. Therefore, from the viewpoint of reducing the load of the manufacturing process, when manufacturing a steel material having a tensile strength at room temperature of about 490 to 610 MPa, there is a tendency that accelerated cooling is avoided. However, as a result of the study by the present inventors, a steel material having a room temperature strength of 490 to 610 MPa manufactured by performing accelerated cooling has a higher room temperature strength than a steel material to which accelerated cooling is not applied, even if the room temperature strength is about the same. It was found that high temperature strength was obtained.

  When a steel material manufactured by applying accelerated cooling and a steel material manufactured without applying accelerated cooling were compared, there was a difference in dislocation density. That is, it has been clarified by the present inventors that the change in high-temperature strength due to the presence or absence of accelerated cooling is caused by the difference in the amount of dislocation enhancement. Then, it was concluded that the application of accelerated cooling is preferable in the case of manufacturing an H-section steel requiring excellent high-temperature strength without excessively increasing the room-temperature strength.

  Furthermore, the present inventors studied what kind of change occurs in the steel material structure depending on whether or not the accelerated cooling is applied. As a result, in the steel material to which accelerated cooling is applied and high high-temperature strength is obtained, the metal structure has a total area ratio of bainite and pseudopolygonal ferrite of 90% or more and an area ratio of bainite of 20% or more. I got the knowledge. On the other hand, even when the accelerated cooling was applied, it was also found that when the content of C was insufficient or when the temperature at which the accelerated cooling was stopped was high, polygonal ferrite was formed and high high-temperature strength could not be obtained. .

  In this manner, when accelerated cooling is applied to steel containing 0.05% by mass or more of C, bainite and pseudopolygon having a high dislocation density can be obtained without adding an alloy element that contributes to precipitation strengthening or solid solution strengthening. It has been found that the metal structure is mainly composed of null ferrite, and high-temperature strength can be secured by dislocation strengthening.

  FIG. 1 shows an example of pseudo-polygonal ferrite observed by an optical microscope, FIG. 2 shows an example of polygonal ferrite, and FIG. 3 shows an example of bainite. As shown in FIGS. 1 and 2, the shape of the crystal grains of pseudo-polygonal ferrite is more angular than that of polygonal ferrite. The present inventors analyzed various steel materials by the X-ray diffraction method, and confirmed that the diffraction peaks of bainite and pseudopolygonal ferrite have a large half width and a high dislocation density.

  Furthermore, the present inventors have studied in detail the effect of the alloy element added to the steel material on the reheat embrittlement of the welded HAZ through experiments and analysis. Specifically, test pieces were sampled from various steel materials, and a heat history assuming welding at a heat input of 10 kJ / mm was given, and then the ductility at 600 ° C. was evaluated.

  The heat history assuming welding with a heat input of 10 kJ / mm means that a steel material is heated from room temperature to 1400 ° C. at 20 ° C./s, held at 1400 ° C. for 2 seconds, and then cooled to 800 ° C. to 500 ° C. This is a heat history in which the cooling rate up to 3 ° C./sec. Thereafter, the temperature was raised from room temperature to 600 ° C. for 60 minutes, and after maintaining at 600 ° C. for 30 minutes, a tensile test was performed at a stress increase rate of 1 MPa / s while maintaining at 600 ° C., and the test piece was broken. Then, the aperture value of the broken portion of the test piece was measured. The reduction value was used as an index of reheat embrittlement of the HAZ, and the case where the reduction value was 50% or more was defined as good.

  As a result, it was found that reheating embrittlement was promoted by containing Mo, Nb, V, and B, and reheating embrittlement was significantly improved by containing Ti. Ti is effective in suppressing reheat embrittlement because Ti has the effect of fixing C and N at locations other than the prior austenite grain boundaries, thereby preventing reheat embrittlement, which is embrittlement of grain boundaries. It is thought that it is possible. Further, when 0.05% by mass or more of C is contained, it is necessary to limit the content of strengthening elements, particularly Mo, Nb, V and B, which degrade reheat embrittlement of the large heat input welding HAZ. It became clear that it became.

  Hereinafter, embodiments of the present invention will be described in more detail.

  The steel composition of the H-section steel of the present invention is, in mass%, C: 0.05 to 0.15%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.60%, Ti : 0.005 to 0.030%, Al: 0.01 to 0.100%, N: 0.0010 to 0.0050%, P: 0.030% or less, S: 0.020% or less , Mo: 0.05% or less, Nb: 0.005% or less, V: 0.01% or less, B: 0.0003% or less, O: 0.010% or less, with the balance being Fe and impurities. Become. Hereinafter, components of the steel composition of the H-section steel of the present invention will be described. In the following description, “%” of the content of each element is represented by mass%.

(C: 0.05-0.15%)
C is an element effective for improving the hardenability of the steel material. In order to obtain a metal structure mainly composed of bainite and pseudopolygonal ferrite, the content of C is set to 0.05% or more. In order to enhance the hardenability and increase the high-temperature strength, the content of C is preferably set to 0.07% or more. On the other hand, when C is contained in excess of 0.15%, a large amount of martensite-austenite mixed structure (hereinafter sometimes referred to as MA phase) and carbides are generated in HAZ at the time of large heat input welding, In some cases, the toughness of the HAZ is reduced, and the reheat embrittlement of the welded HAZ becomes significant. Therefore, the upper limit of the content of C is set to 0.15%.

(Si: 0.01 to 0.50%)
Si is a deoxidizing element and also an element contributing to improvement of hardenability, and the content is set to 0.01% or more. In order to increase the strength of the H-section steel, the content of Si is preferably set to 0.05% or more. On the other hand, when the content of Si is excessive, generation of MA of HAZ at the time of large heat input welding may be promoted, and the toughness of HAZ may be reduced. Therefore, it is necessary to set the upper limit of the Si content to 0.50%. In order to increase the toughness of the HAZ, the upper limit of the Si content is preferably set to 0.30%.

(Mn: 0.50 to 1.60%)
Mn is effective in improving hardenability, and in order to obtain a metal structure mainly composed of bainite and pseudopolygonal ferrite, the content of Mn is set to 0.50% or more. In order to increase the high-temperature strength of the H-section steel, the content of Mn is preferably set to 0.70% or more. On the other hand, Mn tends to segregate at the grain boundaries and promote reheat embrittlement of the welded HAZ. In consideration of this, the upper limit of the content of Mn is set to 1.60%.

(Ti: 0.005 to 0.030%)
Ti is an element that precipitates carbides and nitrides and suppresses the formation of carbides and nitrides of other elements at grain boundaries, thereby contributing to suppression of reheat embrittlement of the welded HAZ. Further, the nitride of Ti suppresses the growth of austenite grains in the HAZ at the time of welding by pinning, and contributes to the improvement of the toughness of the weld HAZ. In consideration of this, the content of Ti is set to 0.005% or more. A preferable lower limit of the content of Ti is 0.008%. On the other hand, if the Ti content exceeds 0.030%, the toughness of the welded HAZ decreases due to the formation of coarse nitrides. In consideration of this, the upper limit of the content of Ti is limited to 0.030%.

(Al: 0.01 to 0.100%)
Since Al is an element necessary for deoxidizing steel materials, the lower limit of the content is 0.01%. A preferred lower limit of the Al content is 0.02%, and more preferably 0.03%. On the other hand, when the Al content exceeds 0.100%, coarse oxide clusters are formed, and the toughness of the steel material may be impaired. In consideration of this, the upper limit of the Al content is set to 0.100%. A preferred upper limit of the Al content is 0.050%.

(N: 0.0010 to 0.0050%)
N forms nitrides with various alloying elements and contributes to improvement in high-temperature strength. Also, by forming a nitride with Ti, the pinning effect in the HAZ at the time of welding causes the austenite grains to be refined, thereby contributing to the improvement of the toughness of the welded HAZ. Therefore, the content of N is set to 0.0010% or more. A preferred lower limit of the N content is 0.0020%. However, when the content of N is excessive, nitrides precipitated at the grain boundaries of the HAZ may become coarse and reheat embrittlement of the HAZ may become remarkable. Therefore, the upper limit of the N content is limited to 0.0050%. A preferred upper limit of the N content is 0.0040%.

  The H-section steel of the present invention contains the above-mentioned elements, and further restricts each of P, S, Mo, Nb, V, B, and O as follows.

(P: 0.030% or less)
P is an impurity and may reduce the toughness of the base metal and the weld HAZ. Therefore, the upper limit of the content of P is limited to 0.030%. A preferable upper limit of the content of P is 0.020%. The lower limit of the content of P is not specified, but is preferably 0.001% or more in order to suppress an increase in cost in the steel making process.

(S: 0.020% or less)
S is an impurity, and when coarse MnS is generated, the toughness of the base metal and the welded HAZ may be reduced. Therefore, the upper limit of the S content is limited to 0.020%. A preferable upper limit of the content of S is 0.010%. The lower limit of the S content is not specified, but is preferably 0.0001% or more to suppress an increase in cost in the steel making process.

(Mo: 0.05% or less)
Mo is an element conventionally added to increase the high-temperature strength by precipitation strengthening. However, if the Mo content exceeds 0.05%, reheat embrittlement of the welded HAZ may become significant. Therefore, the content of Mo is limited to 0.05% or less. The lower limit of the Mo content is not specified, and may be 0%.

(Nb: 0.005% or less)
Nb is an element conventionally added to increase the high-temperature strength by solid solution strengthening. However, if the Nb content exceeds 0.005%, reheat embrittlement of the welded HAZ may become significant. Therefore, the content of Nb is limited to 0.005% or less. The lower limit of the Nb content is not specified and may be 0%.

(V: 0.01% or less)
V is an element conventionally added to increase the high-temperature strength by precipitation strengthening. However, if the V content exceeds 0.01%, reheat embrittlement of the welded HAZ may become significant. Therefore, the content of V is limited to 0.01% or less. The lower limit of the V content is not specified and may be 0%.

(B: 0.0003% or less)
B is an element that contributes to the improvement of hardenability when added in a small amount, promotes the formation of a metal structure having a high dislocation density, and is effective in improving the high-temperature strength. However, since B may promote reheat embrittlement of the welded HAZ, B is not intentionally added, and the upper limit is limited to 0.0003% of the impurity level. The lower limit of the B content is not specified, and may be 0%.

(O: 0.010% or less)
O is an impurity, and when it is combined with other elements to form a coarse oxide, the toughness of the base material and the welded HAZ may be reduced. Therefore, the upper limit of the O content is limited to 0.010%. The preferable upper limit of the O content is 0.0050%, and more preferably 0.0030%. The lower limit of the O content is not specified, but is preferably 0.0001% or more in order to suppress an increase in deoxidation cost in the steelmaking process.

  In the present invention, in addition to the above elements, one or more of the following elements can be selectively contained.

(Cr: 0.50% or less)
Cr is an element that improves the hardenability and contributes to the improvement in room temperature strength and high temperature strength. In order to obtain this effect, the content of Cr is preferably set to 0.01% or more. In addition, Cr forms fine Cr carbide, has an effect of suppressing generation of carbide at the grain boundary of the welding HAZ, and suppressing reheat embrittlement of the welding HAZ. In order to obtain this effect, the content of Cr is preferably set to 0.10% or more. On the other hand, if Cr is added excessively, MA may increase in the weld HAZ and the toughness may decrease. Therefore, the upper limit of the Cr content is set to 0.50%. More preferably, the content of Cr is 0.40% or less, further preferably 0.30% or less.

(W: 0.50% or less)
W is an element that contributes to improvement in room temperature strength and high temperature strength by improving hardenability. In order to obtain this effect, the content of W is preferably set to 0.01% or more. On the other hand, if W is added excessively, MA increases in the weld HAZ, and the toughness may decrease. Therefore, the upper limit of the W content is set to 0.50%. More preferably, the content of W is 0.40% or less, further preferably 0.30% or less.

(Cu: 0.50% or less)
Cu is an element effective for improving the room temperature strength and the high temperature strength by improving the hardenability. In order to obtain this effect, the content of Cu is preferably set to 0.01% or more. More preferably, the content of Cu is 0.10% or more. On the other hand, Cu is also an element that promotes reheat embrittlement of the welded HAZ. Therefore, it is preferable to set the upper limit of the Cu content to 0.50%. A more preferred upper limit of the Cu content is 0.30%.

(Ni: 0.50% or less)
Ni is an element effective for improving the room temperature strength and the high temperature strength by improving the hardenability. In order to obtain this effect, the content of Ni is preferably set to 0.01% or more. More preferably, the content of Ni is set to 0.10% or more. On the other hand, if the content of Ni exceeds 0.5%, the formation of MA in the welded HAZ may be promoted and the toughness may be reduced. Therefore, the upper limit of the Ni content is set to 0.50%. A more preferred upper limit of the Ni content is 0.35%, and a still more preferred upper limit is 0.20%.

(Zr: 0.010% or less)
Zr has the effect of controlling the form of sulfide in the steel material and reducing the decrease in base metal toughness due to sulfide. In order to obtain such an effect, the content of Zr is preferably set to 0.002% or more. On the other hand, if the Zr content exceeds 0.010%, segregation at the grain boundaries may promote reheat embrittlement of the welded HAZ. Therefore, the upper limit of the Zr content is set to 0.010%. A more preferable upper limit of the Zr amount is 0.005%.

(Mg: 0.005% or less, Ca: 0.005% or less)
Mg and Ca have the effect of controlling the form of sulfide in the steel material and reducing the decrease in base metal toughness due to sulfide. In order to obtain such effects, the contents of Mg and Ca are preferably set to 0.0005% or more. On the other hand, even if Mg and Ca each contain more than 0.005%, the effect is saturated. Therefore, the upper limits of the contents of Mg and Ca are each 0.005%.

(Y: 0.050% or less, La: 0.050% or less)
Y and La have the effect of controlling the form of sulfide in the steel material and reducing the decrease in base metal toughness due to sulfide. In order to obtain this effect, the contents of Y and La are each preferably 0.001% or more. On the other hand, the effect is saturated even if Y and La each contain more than 0.050%. Therefore, the upper limits of the contents of Y and La are each 0.050%.

  In addition, in the H-beam of the present invention, in the manufacturing process, elements other than the above may be mixed as unavoidable impurities due to raw materials such as scrap and refractory materials, but do not affect the properties of the H-beam. A level of content is acceptable.

In the present invention, in order to improve the weldability and HAZ characteristics, the carbon equivalent C _eq determined by the following equation (1) is set to 0.46% by mass or less. If C _eq is more than 0.46% by mass, there may be a problem that the toughness of the HAZ decreases due to the hardening of the HAZ and an increase in the amount of the MA. On the other hand, although the lower limit value of C _eq is not particularly defined, C _eq is preferably set to 0.24% by mass or more from the viewpoint of securing hardenability and securing room-temperature strength and high-temperature strength. More preferably, C _eq is at least 0.30% by mass.

The carbon equivalent C _eq is an index of _{hardenability and} is obtained by a known formula (1). Here, C, Mn, Si, Ni, Cr, Mo and V are the contents (% by mass) of each element in the steel, and the elements not contained are 0.

  The H-section steel of the present invention preferably has a flange thickness of 80 mm or less. When the thickness of the flange is greater than 80 mm, the cooling rate is reduced, and the effect of accelerated cooling may be insufficient, which may cause a problem such as insufficient room temperature strength or high temperature strength. From the viewpoint of securing room temperature strength and high temperature strength, the thickness of the flange is preferably 60 mm or less. On the other hand, the lower limit of the thickness of the flange is not particularly defined, but is preferably 12 mm or more from the viewpoint of securing the flatness of the shape of the flange of the H-beam after cooling. More preferably, the thickness of the flange is 15 mm or more.

  In the present invention, the yield strength or 0.2% proof stress at room temperature is 325 MPa or more, and the tensile strength at room temperature is in the range of 490 to 610 MPa due to the limitation of the steel composition by the elements as described above. Even in the case of having a high yield strength at a temperature of 600 ° C., at the same time, reheat embrittlement in the weld heat-affected zone of the welded joint is suppressed, and the refractory material has excellent low-temperature toughness of the base metal and the welded joint. An H-beam is obtained.

  Next, the metal structure of the H-section steel of the present invention will be described. In the present invention, the position where the mechanical properties have an average numerical value in the section of the H-section steel, that is, in the H-section of the H-section steel, is 1/6 from the surface in the longitudinal direction of the flange. Then, the evaluation is made at a position 1/4 from the surface in the thickness direction of the flange. Therefore, the steel structure is also evaluated at the same position.

  In general, it is considered that the high-temperature strength of a steel material is generated by dislocation strengthening due to dislocations present in the steel material, and precipitates and grain boundaries that hinder the dislocation motion. When the temperature of the steel material exceeds 550 ° C. and the dislocation coalescence disappears due to the dislocation rising motion, the high-temperature strength may decrease rapidly. For this reason, in order to ensure high high-temperature strength, the steel material must have a sufficient amount of dislocations at a time before being exposed to a fire, that is, at room temperature, and further, the movement of the dislocation motion. It is effective to include a number of obstacles, specifically, a large number of precipitates and grain boundaries.

  However, in the present invention, there is a strict restriction on the addition of the alloy in order to suppress the reheat cracking of the welding HAZ. Therefore, since the content of the alloy contributing to strengthening is limited, high-temperature strength is secured mainly by strengthening by dislocations introduced by accelerated cooling. For this reason, the steel material structure of the H-section steel of the present invention, that is, the metal structure, has an area ratio of bainite of 20% or more and a total area ratio of bainite and pseudopolygonal ferrite when observed with an optical microscope. It shall be 90% or more. Bainite and pseudo-polygonal ferrite have higher high-temperature strength than polygonal ferrite even when the room-temperature strength is almost the same. Therefore, if the area ratio of bainite is less than 20%, or the total area ratio of bainite and pseudopolygonal ferrite is less than 90%, for example, the area ratio of polygonal ferrite having poor high-temperature strength increases, and the high-temperature strength increases. May decrease.

  The balance of bainite and pseudopolygonal ferrite is one or more of polygonal ferrite, acicular ferrite, martensite, and retained austenite. When these area ratios are suppressed to less than 10%, the room temperature strength and the high temperature strength of the H-section steel are hardly affected.

  Next, the mechanical properties of the steel material of the present invention will be described. The H-section steel of the present invention has a yield strength or 0.2% proof stress at room temperature of 325 MPa or more, a tensile strength at room temperature of 490 to 610 MPa, and a yield stress at a temperature of 600 ° C. of 217 MPa or more. Furthermore, the H-section steel of the present invention is also excellent in reheat embrittlement resistance. As a result, in building applications, it is possible to realize an H-section steel that satisfies fire resistance and has various requirements for building design and a sufficient safety margin against fire.

  The reheat embrittlement resistance of the H-section steel of the present invention is obtained by applying a thermal history assuming welding of 10 kJ / mm heat to a test piece collected from the H-section steel, and then performing a tensile test at a temperature of 600 ° C. Then, the breaking reduction value is measured and evaluated by the value. The H-section steel according to the present invention has an HAZ of a welded joint having a sufficient deformability when re-heated to 600 ° C, which is an assumed temperature at the time of fire, at a temperature of 600 ° C at a temperature of 600 ° C of 50% or more. Shaped steel.

Next, the method for producing the H-section steel of the present invention will be described.
The H-section steel of the present invention includes a hot rolling step of hot rolling a steel piece at 1000 to 1350 ° C. at 800 ° C. or more, and an accelerated cooling step of accelerated cooling the steel piece after the hot rolling step. Hereinafter, each step will be described.

(Hot rolling process)
The hot rolling step is a step of performing rough rolling, intermediate rolling, and finish rolling. In each rolling, a rough rolling can use a breakdown rolling mill, an intermediate rolling can use a universal rolling mill and an edging rolling mill, and a finish rolling can use a universal rolling mill and the like. The temperature of the billet is set to 1000 ° C. or higher in order to promote solid solution of the alloy element. In order to promote the solid solution of carbides and nitrides, the temperature of the steel slab is preferably 1100 ° C. or higher. On the other hand, when the temperature of the slab exceeds 1350 ° C., a problem such as an increase in the amount of scale generated or a problem such as dissolution of the scale may occur. Therefore, the upper limit of the temperature of the billet is set to 1350 ° C. Preferably, the temperature of the billet is 1300 ° C. or less.

  Hot rolling is performed at 800 ° C. or higher. If hot rolling is completed at 800 ° C. or higher, accelerated cooling can be started at a high temperature, and as a result, bainite and pseudopolygonal ferrite having a high dislocation density can be generated. In order to facilitate the formation of bainite and pseudo-polygonal ferrite by accelerated cooling, it is preferable to perform hot rolling at 830 ° C. or higher so that the end temperature of hot rolling is 830 ° C. or higher. The upper limit of the end temperature of the hot rolling is set to 1000 ° C. from the viewpoint of securing the base material toughness.

  By the hot rolling step, the thickness of the flange can be reduced to 80 mm or less. If the thickness of the flange is greater than 80 mm, the cooling rate is reduced and the effect of accelerated cooling is insufficient, so that a problem such as insufficient room temperature strength and high temperature strength may occur. Preferably, the thickness of the flange is 60 mm or less. On the other hand, the lower limit of the thickness of the flange is not particularly defined, but is preferably 12 mm or more from the viewpoint of securing the flatness of the shape of the flange of the H-section steel after accelerated cooling. More preferably, the thickness of the flange is 15 mm or more.

(Accelerated cooling process)
The accelerated cooling step is a step of accelerated cooling the steel slab after the hot rolling step. In the accelerated cooling, cooling is performed at a higher cooling rate than air cooling by using various cooling control means such as an accelerated cooling device. The high-temperature strength of the H-section steel is ensured by the dislocation density introduced by the accelerated cooling, so that the cost can be reduced by alloy saving, and further, the reheat embrittlement of the welded HAZ can be prevented. If the temperature of the steel slab is higher, the accelerated cooling effect increases, so it is preferable to perform accelerated cooling immediately after the hot rolling step. If the stop temperature of the accelerated cooling is too high, the production amount of bainite and pseudopolygonal ferrite decreases. Therefore, in the present invention, the stop temperature of the accelerated cooling is set to 600 ° C. or less. The stop temperature of the accelerated cooling is preferably 550 ° C or lower, more preferably 500 ° C or lower, and the accelerated cooling may be performed to room temperature. That is, the steel slab can be accelerated and cooled to 300 ° C. or less by the accelerated cooling process. Further, the steel slab can be accelerated and cooled to 300 to 600 ° C.

  After the accelerated cooling step, the H-section steel is cooled to room temperature. For example, cooling can be controlled by air cooling, mist cooling, or the like, and the H-section steel after the accelerated cooling step can be allowed to cool as it is. For example, after the accelerated cooling step of accelerating and cooling the steel slab to 300 to 600 ° C, a cooling step of allowing the steel piece to cool as it is can be provided.

(Tempering heat treatment step)
In the present invention, a tempering heat treatment step may be performed after the accelerated cooling step. By this tempering heat treatment, it is possible to obtain an effect that the room temperature strength is greatly reduced and the high temperature strength is not significantly reduced. By this effect, an H-section steel which secures high-temperature strength higher than room-temperature strength can be obtained, and sufficient fire resistance can be obtained. The temperature range of the tempering heat treatment is set to 400 ° C. or higher for the purpose of lowering the room temperature strength, and the upper limit is set to 650 ° C. for suppressing the lowering of the high temperature strength. More preferably, the upper limit is set to 600 ° C.

  The hot rolling step can include steps other than those described above. For example, in order to set the temperature of the slab to 1000 to 1350 ° C., a heating step of heating the slab using a heating furnace or the like can be provided before hot rolling. However, this does not apply when the slab after casting is directly fed at a high temperature and rolled. Further, the method for producing an H-section steel according to the present invention can include not only the hot rolling step but also other steps. For example, a sawing step of cutting the H-section steel to adjust the length in the longitudinal direction after the hot rolling step, a straightening step of correcting distortion, deformation, and the like of the H-section steel after cooling can be provided.

  Further, the billet is prepared by a normal procedure. For example, it can be obtained by casting a steel that has passed through a blast furnace or a converter after adjusting the chemical composition of the molten steel in a steelmaking process. For casting, continuous casting is preferable from the viewpoint of productivity, but a beam blank having a shape close to the H-beam to be manufactured may be used. In addition, the thickness of the billet is preferably 200 mm or more from the viewpoint of productivity, and considering the reduction of segregation and the uniformity of the heating temperature when heating the billet before the hot rolling step, The thickness is preferably 350 mm or less.

  In the method for producing an H-section steel according to the present invention, the steel slab is represented by mass%: C: 0.05 to 0.15%, Si: 0.01 to 0.50%, Mn: 0.50 to 1.60. %, Ti: 0.005 to 0.030%, Al: 0.01 to 0.100%, N: 0.0010 to 0.0050%, P: 0.030% or less, S: 0. 020% or less, Mo: 0.05% or less, Nb: 0.005% or less, V: 0.01% or less, B: 0.0003% or less, O: 0.010% or less, the balance being Fe And a steel composition comprising impurities. The steel composition is as described for the H-section steel according to the present invention. The steel slab is, by mass%, one or more of Cr: 0.50% or less, W: 0.50% or less, Cu: 0.50% or less, and Ni: 0.50% or less. Can be contained. Furthermore, in mass%, Zr: 0.010% or less, Mg: 0.005% or less, Ca: 0.005% or less, Y: 0.050% or less, La: 0.050% or less, Species or two or more species may be contained.

In the method for producing an H-section steel according to the present invention, the billet has a carbon equivalent C _eq of 0.46% by mass or less. The carbon equivalent is as described for the H-section steel according to the present invention.

  Hereinafter, the present invention will be described in detail based on examples.

The steel making process and Steels, performs deoxidation and desulfurization of the molten steel, by adjusting the chemical composition, to produce a steel slab having the component composition and the carbon equivalent C _eq shown in Table 1 by continuous casting. The components shown in Table 1 were determined by chemical analysis of a sample collected from an H-beam after the manufacture.

  FIG. 4 is a diagram illustrating a production line in a hot rolling step in the example. The hot rolling was performed in a production line 1 of a universal rolling mill line including a heating furnace 2, a rough rolling mill 3a, an intermediate rolling mill 3b, a finishing rolling mill 3c, and water cooling devices 4a and 4b. The hot rolling is water cooling between passes, and water cooling between rolling passes is performed using a water cooling device 4a provided on the front and rear surfaces of an intermediate rolling mill (intermediate universal rolling mill) 3b, and spray cooling and reverse rolling of the flange outer surface are performed. Was done. Water cooling after the controlled rolling was performed by a cooling device (water cooling device) 4b installed on the rear surface after finishing rolling was completed in a finishing rolling mill (finishing universal rolling mill) 3c.

  After the hot rolling step, an accelerated cooling step was immediately performed so that the temperature of the slab did not decrease. The accelerated cooling was performed by using an on-line water cooling device 4b in FIG. 4, specifically, by cooling the flange and the web by spray cooling.

  After the accelerated cooling step, it was allowed to cool from the temperature at which the accelerated cooling was stopped to room temperature (cooling step).

  After the accelerated cooling step or the cooling step, a part of the steel slab was subjected to a tempering treatment in a tempering step. Specifically, a steel slab was inserted into a gas-fired furnace installed offline, and the temperature of the furnace was controlled to heat the steel slab to a predetermined temperature.

  Table 2 shows the type of slab used, the thickness of the flange of the manufactured H-section steel, the heating temperature of the slab heated by the heating furnace 2, the rolling finish temperature, the stop temperature of accelerated cooling, the temperature of tempering heat treatment, Are respectively shown. The finish rolling temperature is the surface temperature of the H-section steel after the finish rolling.

  FIG. 5 is a cross-sectional view of the H-shaped steel manufactured in the example, which is H-shaped perpendicular to the longitudinal direction. The H-section steel 10 includes a flange 11 and a web 12. F is the length of the flange in the cross section, and H indicates the height of the H-section steel. Further, t1 is the thickness of the web in the cross section, and t2 is the thickness of the flange in the cross section. The fractions of bainite and pseudopolygonal ferrite were obtained from the evaluation part 13 which is 1/6 from the surface in the length direction of the flange and 1/4 from the surface in the thickness direction of the flange. Then, it was determined and calculated by observation with an optical microscope. Using a tissue photograph taken at a magnification of 200 times with an optical microscope, measurement points are arranged in a lattice shape with one side of 50 μm, and it is determined whether 400 measurement points are bainite or pseudopolygonal ferrite. The fraction was calculated as the ratio of the number of measurement points counted as pseudopolygonal ferrite.

  Next, a tensile test piece is collected from the evaluation site 13 and subjected to a tensile test in accordance with JIS Z 2241. If an upper yield point appears on the stress-strain curve, the upper yield point is set to YS (yield strength) at room temperature. When it did not appear, the YS at room temperature was 0.2% proof stress. The target of the room temperature YS is 325 MPa or more, and the target of the room temperature TS (tensile strength) is 490 to 610 MPa.

  Further, a tensile test piece was collected from the evaluation site 13 and a high-temperature tensile test was performed at a temperature of 600 ° C. in accordance with JIS G 0567. The measured 0.2% proof stress was defined as 600 ° YS. The target value of YS at 600 ° C. is 217 MPa or more.

  As a Charpy test for evaluating the toughness of the base material, a 2 mm V-notch Charpy test specimen was collected from the evaluation site 13 and subjected to a Charpy impact test at 0 ° C. in accordance with JIS Z2242. At this time, the target value of the absorbed energy was set to 100 J in consideration of the earthquake resistance of the building structure.

  In order to evaluate the welding HAZ toughness, a heat history was assumed assuming welding at a heat input of 10 kJ / mm, a 2 mm V notch Charpy test piece was sampled, a Charpy test was performed, and the absorbed energy at 0 ° C. was measured. The thermal history was measured under the conditions of heating from room temperature to 1400 ° C. at 20 ° C./sec, holding at 1400 ° C. for 2 seconds, and then cooling when cooling from 800 ° C. to 500 ° C. at 3 ° C./sec. is there. The target value of the absorbed energy was set to 100 J similarly to the base material.

  Further, the tensile drawing value at 600 ° C. of the welded HAZ was determined by applying the same heat history as the above-mentioned welded HAZ toughness, then taking a tensile test specimen, and raising the temperature from room temperature to 600 ° C. for 60 minutes. After maintaining the temperature for 600 minutes, a tensile test was carried out while maintaining the temperature at 600 ° C. at a stress increase rate of 1 MPa / s, and the evaluation was made by measuring the aperture value at the broken portion of the test piece. The target value of the drawing, which is an index of reheat embrittlement resistance of the welded HAZ, was set to 50% or more.

  Table 3 shows the area fraction of bainite and pseudopolygonal ferrite, room temperature YS, room temperature TS, 600 ° C. YS, toughness of base material (absorbed energy in Charpy test at 0 ° C.), toughness of welded HAZ, and reheat embrittlement. The results of the drawing reduction values in the 600 ° C. tensile test of the welded HAZ for measuring the degree of are shown below.

  Production No. in Tables 2 and 3 H-beams 1-5, 7-12 and 14-19 are examples within the scope of the present invention. These H-section steels have a steel composition, rolling conditions, accelerated cooling conditions, and the like within the scope of the present invention, and have a steel structure in which bainite and pseudopolygonal ferrite have an area ratio of 90% or more. And, room temperature YS, room temperature TS, 600 ° C. YS, base metal toughness, weld HAZ toughness, and 600 ° C. tensile draw value of the weld HAZ all satisfied the target.

  Production No. The H-shaped steels of Nos. 6 and 13 are comparative examples in which the manufacturing conditions are out of the range of the present invention. Production No. The H-shaped steel of No. 6 is an example in which the rolling finish temperature is too low, the hardenability is insufficient, and the amount of bainite and pseudopolygonal ferrite generated is insufficient. As a result, the tensile properties were inferior. Production No. No. 13 is an example in which the cooling stop temperature is too high, and as a result, the steel structure was not quenched and the tensile properties were inferior.

  Production No. The H-section steels of Nos. 20 to 39 are comparative examples in which the steel component composition was out of the range of the present invention. Production No. The H-beam of No. 20 had an excessive C content, resulting in reduced weld HAZ toughness and reheat embrittlement of the weld HAZ. Production No. The H-beam of No. 21 was insufficient in the content of C, and the formation of bainite and pseudopolygonal ferrite was insufficient. As a result, the tensile properties were inferior. Production No. The H-beam of No. 22 had a high Si content and caused an embrittlement phase in the weld HAZ, resulting in a decrease in the toughness of the weld HAZ. Production No. The H-shaped steel of No. 23 lacked the content of Si, and the formation of bainite and pseudopolygonal ferrite was insufficient. As a result, the tensile properties were inferior. Production No. The H-section steel No. 24 had an excessive Mn content, and as a result, reheat embrittlement became remarkable. Production No. In No. 25, the content of Mn was small, and the formation of pseudopolygonal ferrite was insufficient. As a result, the tensile properties were inferior.

  Production No. The H-section steel No. 26 is an example in which the content of Ti is excessive, and as a result, the toughness of the welded HAZ is reduced. Production No. The H-section steel No. 27 is an example in which the content of Ti is insufficient, and as a result, the reheat embrittlement of the welded HAZ and the HAZ toughness are reduced. Production No. The H-shaped steel No. 28 had an excessive Al content, and the Al oxide was coarsened to lower the toughness of the base metal and the welded HAZ. Production No. The H-shaped steel of No. 29 lacked the Al content, the amount of dissolved oxygen in the steel increased, and the toughness of the base metal and the welded HAZ decreased. Production No. In the H-beam of No. 30, the content of P was excessive, and the toughness of the base metal and HAZ was reduced. Production No. The H-shaped steel No. 31 had an excessive S content, generated a large number of coarse MnS in the steel, and reduced the toughness of the base metal and the welded HAZ. Production No. In the H-beam of No. 32, the N content was excessive, and the amount of nitride precipitation at the former austenite grain boundary in the welded HAZ increased, resulting in reheat embrittlement as a result. Production No. In the H-beam of No. 33, the N content was insufficient, the precipitation amount of TiN was insufficient, the effect of reducing the austenite grains in the weld HAZ was insufficient, and the toughness of the weld HAZ was reduced. Production No. In the H-section steel No. 34, the O content was excessive, the oxides of Al and Ti were coarsened, and the toughness of the base metal and HAZ was reduced. No. In the H-section steel No. 35, the Nb content was excessive, and as a result, the reheat embrittlement of the welded HAZ became remarkable. Production No. The H-beam of No. 36 had an excessive V content, and as a result, reheat embrittlement of the welded HAZ became remarkable. Production No. The H-section steel No. 37 had an excessive Mo content, and as a result, reheat embrittlement of the welded HAZ became remarkable. Production No. The H-beam of No. 38 had an excessive B content, and reheat embrittlement of the welded HAZ became remarkable. Production No. In the case of the H-section steel No. 39, the Ceq was too high, and as a result, the room temperature TS was excessively high, and the weld HAZ toughness was lowered.

  Advantageous Effects of Invention According to the present invention, it is possible to provide a fire-resistant H-section steel suitable for architectural applications and a method for manufacturing the same, which can secure requirements in architectural design and obtain a sufficient safety margin in fire, and contribute to industry. Very remarkable.

DESCRIPTION OF SYMBOLS 1 Manufacturing line 2 Heating furnace 3a Rough rolling mill 3b Intermediate rolling mill 3c Finish rolling mill 4a Water cooling device of the front and back of an intermediate rolling mill 4b Water cooling device of the rear of a finishing rolling mill 10 H-section steel 11 Flange 12 Web 13 Evaluation site F Flange in section Length H Height of H-section steel t Thickness of web in _one section t Thickness of flange in _two sections

Claims (10)

H-section steel,
In mass%,
C: 0.05 to 0.15%,
Si: 0.01 to 0.50%,
Mn: 0.50 to 1.60%,
Ti: 0.005 to 0.030%,
Al: 0.01 to 0.100%,
N: 0.0010 to 0.0050%
Containing
P: 0.030% or less,
S: 0.020% or less,
Mo: 0.05% or less,
Nb: 0.005% or less,
V: 0.01% or less,
B: 0.0003% or less,
O: limited to 0.010% or less, the balance having a steel composition consisting of Fe and impurities,
The carbon equivalent C _eq determined by the following formula (1) is 0.46% by mass or less;
In the H-shaped cross section of the H-shaped steel, the metal structure at a position 1/6 from the surface in the length direction of the flange and 1/4 from the surface in the thickness direction of the flange has an area of bainite. An H-shaped steel having a ratio of not less than 20% and a total area ratio of bainite and pseudopolygonal ferrite of not less than 90%.
C _eq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 Equation (1)
(In the formula (1), C, Mn, Si, Ni, Cr, Mo, and V are the contents (% by mass) of each element, and are 0 when they are not contained.) The H-shaped steel is expressed in mass%,
Cr: 0.50% or less,
W: 0.50% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
The H-section steel according to claim 1, wherein one or more types are contained. The H-shaped steel is expressed in mass%,
Zr: 0.010% or less,
Mg: 0.005% or less,
Ca: 0.005% or less,
Y: 0.050% or less,
La: 0.050% or less,
The H-section steel according to claim 1, wherein the H-shaped steel contains one or more kinds. The thickness of the flange is 80 mm or less,
Yield strength at room temperature or 0.2% proof stress of 325 MPa or more at a position 1/6 from the surface in the length direction of the flange and 1/4 from the surface in the thickness direction of the flange. The H-section steel according to any one of claims 1 to 3, having a strength of 490 MPa or more and a 0.2% proof stress at 600 ° C of 217 MPa or more. A method for producing an H-section steel,
A hot rolling step of hot rolling a billet of 1000 to 1350 ° C. at 800 ° C. or higher,
After the hot rolling step, including an accelerated cooling step of accelerated cooling the steel slab to 300 ° C. or less,
The billet is, in mass%,
C: 0.05 to 0.15%,
Si: 0.01 to 0.50%,
Mn: 0.50 to 1.60%,
Ti: 0.005 to 0.030%,
Al: 0.01 to 0.100%,
N: 0.0010 to 0.0050%
Containing
P: 0.030% or less,
S: 0.020% or less,
Mo: 0.05% or less,
Nb: 0.005% or less,
V: 0.01% or less,
B: 0.0003% or less,
O: limited to 0.010% or less, the balance having a steel composition consisting of Fe and impurities,
Formula (1) Ri carbon equivalent C _eq is der less 0.46 mass% as determined by, the H-shaped steel, in the cross section of the H-shaped, there at the position of 1/6 from the surface in a length direction of the flange In the metal structure at a position 1/4 from the surface in the thickness direction of the flange, the area ratio of bainite is 20% or more, and the total area ratio of bainite and pseudopolygonal ferrite is 90% or more. method of manufacturing Oh Ru H-shaped steel.
C _eq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 Equation (1)
(In the formula (1), C, Mn, Si, Ni, Cr, Mo and V are the contents (% by mass) of each element, and are 0 when they are not contained.) A method for producing an H-section steel,
A hot rolling step of hot rolling a billet of 1000 to 1350 ° C. at 800 ° C. or higher,
After the hot rolling step, the steel slab includes an accelerated cooling step of accelerated cooling to 300 to 600 ° C,
The billet is, in mass%,
C: 0.05 to 0.15%,
Si: 0.01 to 0.50%,
Mn: 0.50 to 1.60%,
Ti: 0.005 to 0.030%,
Al: 0.01 to 0.100%,
N: 0.0010 to 0.0050%
Containing
P: 0.030% or less,
S: 0.020% or less,
Mo: 0.05% or less,
Nb: 0.005% or less,
V: 0.01% or less,
B: 0.0003% or less,
O: limited to 0.010% or less, the balance having a steel composition consisting of Fe and impurities,
Formula (1) Ri carbon equivalent C _eq is der less 0.46 mass% as determined by, the H-shaped steel, in the cross section of the H-shaped, there at the position of 1/6 from the surface in a length direction of the flange In the metal structure at a position 1/4 from the surface in the thickness direction of the flange, the area ratio of bainite is 20% or more, and the total area ratio of bainite and pseudopolygonal ferrite is 90% or more. method of manufacturing Oh Ru H-shaped steel.
C _eq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 Formula (1)
(In the formula (1), C, Mn, Si, Ni, Cr, Mo and V are the contents (% by mass) of each element, and are 0 when they are not contained.)   The method for producing an H-section steel according to claim 6, further comprising a cooling step of allowing the steel slab to cool as it is after the accelerated cooling step.   The method for manufacturing an H-section steel according to any one of claims 5 to 7, further comprising a tempering heat treatment step of holding the steel slab in a temperature range of 400 to 650 ° C and performing a tempering heat treatment after the accelerated cooling step. The billet is, in mass%,
Cr: 0.50% or less,
W: 0.50% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
The method for producing an H-section steel according to any one of claims 5 to 8, wherein at least one of them is contained. The billet is, in mass%,
Zr: 0.010% or less,
Mg: 0.005% or less,
Ca: 0.005% or less,
Y: 0.050% or less,
La: 0.050% or less,
The method for producing an H-section steel according to any one of claims 5 to 9, wherein the method comprises one or more types.