CN109715842B

CN109715842B - H-shaped steel and manufacturing method thereof

Info

Publication number: CN109715842B
Application number: CN201780057895.4A
Authority: CN
Inventors: 沟口昌毅; 市川和利; 杉山博一; 清家彻哉
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2016-12-21
Filing date: 2017-12-21
Publication date: 2020-03-06
Anticipated expiration: 2037-12-21
Also published as: EP3533893A1; US20190203309A1; CN109715842A; KR20190032625A; WO2018117228A1; JP6468408B2; EP3533893A4; KR102021726B1; JPWO2018117228A1; PH12019500350A1

Abstract

An H-shaped steel contains C, Si, Mn, Nb, V, Ti, N as chemical components, and has a metal structure containing 60% or more and less than 100% by area of ferrite, the ferrite having an average grain diameter of 1 to 30 μm, a flange thickness of 20 to 140mm, a tensile yield stress of 385 to 530MPa, and a Charpy impact absorption energy at-20 ℃ of 100J or more.

Description

H-shaped steel and manufacturing method thereof

Technical Field

The present invention relates to a thick H-shaped steel having excellent strength and low-temperature toughness, and a method for producing the same. This application is based on the priority claim of patent application 2016-.

Background

In recent years, the size and height of buildings such as high-rise buildings have been increased, and thick steel materials have been used as strength members required for the structure. However, in general, in steel materials, as the thickness of products increases, it becomes more difficult to ensure strength and toughness.

In order to solve such a problem, patent document 1 proposes a technique for obtaining a steel material in which toughness is ensured by utilizing the effect of refining prior austenite grains by Ca — Al-based oxides, and high strength is ensured by applying accelerated cooling.

Patent document 2 proposes a technique for obtaining a steel material in which toughness is ensured by utilizing the effect of refining prior austenite grains due to Mg — S inclusions, and high strength is ensured by applying accelerated cooling.

However, when accelerated cooling is applied after hot rolling in the production of a thick steel sheet, the cooling rate in the interior of the steel sheet is slower than that in the surface, a large difference occurs in the temperature history in cooling the surface and the interior, and depending on the location of the steel, a difference occurs in the mechanical properties such as strength, ductility, and toughness.

In addition, for large buildings, thick H-section steel is desired, but the shape of the H-section steel is peculiar. In order to form a billet into an H-shape, universal rolling or the like is used, but the universal rolling is limited in rolling conditions (temperature and reduction ratio). Therefore, in the case of manufacturing H-section steel, particularly in the case of manufacturing H-section steel having a flange (flange) thickness of 20mm or more, it is difficult to control mechanical properties as compared with a general thick steel plate (thick steel plate).

In order to solve such a problem, patent documents 3 and 4 propose a method of reducing the amount of C and adding B to a steel slab, hot rolling the steel slab, and then naturally cooling the steel slab to ensure homogeneous mechanical properties.

Patent documents 5 to 8 disclose thick H-shaped steels or methods for producing H-shaped steels for the purpose of high strength, high toughness, and the like.

Prior art documents

Patent document

Patent document 1 Japanese patent No. 5655984

Patent document 2 Japanese patent No. 5867651

Patent document 3 Japanese laid-open patent application No. 2003-328070

Patent document 4 Japanese patent application laid-open No. 2011-

Patent document 5 Japanese patent application laid-open No. Hei 11-158543

Patent document 6 Japanese patent application laid-open No. Hei 11-335735

Patent document 7 Japanese laid-open patent publication No. 2016-84524

Patent document 8 Japanese patent application laid-open No. Hei 10-68016

Disclosure of Invention

Conventionally, an H-shaped steel having a flange thickness of 20mm or more is difficult to control mechanical properties, and therefore, it is only required to satisfy toughness at room temperature or at 0 ℃ at best for such a thick H-shaped steel. However, in recent years, in consideration of use in cold regions and the like, a thick H-section steel is required to have excellent toughness at lower temperatures. Further, in consideration of the strength per unit weight as a structural material, the yield stress (specifically, yield strength or σ) is also required for thick H-section steel_0.2(conditional yield strength: stress value at which 0.2% residual strain occurs) is 385MPa or more.

The present invention has been made in view of such circumstances, and an object thereof is to provide a thick H-shaped steel excellent in strength and low-temperature toughness, and a method for producing the same.

The gist of the present invention is as follows.

(1) The H-section steel according to one aspect of the present invention contains, as chemical components of the steel, in mass%, C: 0.05 to 0.160%, Si: 0.01-0.60%, Mn: 0.80-1.70%, Nb: 0.005-0.050%, V: 0.05 to 0.120%, Ti: 0.001-0.025%, N: 0.0001-0.0120%, Cr: 0-0.30%, Mo: 0-0.20%, Ni: 0-0.50%, Cu: 0-0.35%, W: 0-0.50%, Ca: 0-0.0050%, Zr: 0 to 0.0050%, and is limited to Al: 0.10% or less, B: 0.0003% or less, the balance being Fe and impurities, wherein when Ceq is C + Mn/6+ (Cr + Mo + V)/5+ (Ni + Cu)/15, the chemical components C, Mn, Cr, Mo, V, Ni, and Cu satisfy 0.30. ltoreq. Ceq.ltoreq.0.48, the steel has a microstructure including 60% or more and less than 100% of ferrite in terms of surface area fraction, a mixed structure MA of martensite and austenite is restricted to 3.0% or less, a structure other than the ferrite and MA is restricted to 37% or less, and the ferrite has an average particle diameter1 to 30 μm, and a flange having a thickness of 20 to 140mm, wherein the steel has an H-shape when viewed in a cross-section orthogonal to the rolling direction, the flange has a tensile yield stress of 385 to 530MPa, a tensile maximum strength of 490 to 690MPa, and a thickness t of the flange, when the flange has a length F in the width direction, at a position away from an end face (1/6) F in the width direction of the flange₂At the time, the flange is positioned on a surface (1/4) t away from the end surface (1/6) F in the width direction of the flange and from the outer side in the thickness direction of the flange₂The position (2) has a Charpy impact test absorption energy of 100J or more at-20 ℃.

(2) The H-shaped steel according to the above (1), which may contain, as the chemical components of the steel, in mass%: more than 0.02% and less than 0.050%.

(3) The H-shaped steel according to the above (1) or (2), wherein the chemical components of the steel may include, in mass%: more than 0.005% and less than 0.0120%.

(4) In the H-shaped steel according to any one of (1) to (3), the chemical components of the steel may be limited to Cu: less than 0.03%.

(5) In the H-section steel according to any one of (1) to (4), the chemical components of the steel may be limited to, in mass%, Al: less than 0.003%.

(6) In the H-shaped steel according to any one of the above (1) to (5), the thickness of the flange may be 25 to 140 mm.

(7) A method for producing an H-shaped steel according to an aspect of the present invention is a method for producing an H-shaped steel according to any one of (1) to (6) above, including:

a steel-making step of obtaining a molten steel having the chemical composition according to any one of the above (1) to (5);

a casting step of casting the molten steel after the steel-making step to obtain a billet;

a heating step of heating the billet after the casting step to 1100 to 1350 ℃;

a hot rolling step of rolling the slab after the heating step under the following conditions so that a shape of the slab when viewed as a cut plane perpendicular to a rolling direction is an H shape: a cumulative reduction ratio at a position from an end face (1/6) F in the width direction of the flange is 20% or more in a temperature region of more than 900 ℃ and 1100 ℃ or less, the cumulative reduction ratio at the position is 15% or more in a temperature region of 730 to 900 ℃, and the rolling is finished at a temperature of 730 ℃ or more; and

and a cooling step of naturally cooling the hot rolled material after the hot rolling step.

According to the above aspect of the present invention, it is possible to provide thick H-shaped steel excellent in strength and low-temperature toughness, and a method for manufacturing the same.

Drawings

FIG. 1 is a schematic sectional view illustrating a position where a sample of H-shaped steel according to an embodiment of the present invention is prepared.

FIG. 2 is a flowchart illustrating a method for producing H-shaped steel according to an embodiment of the present invention.

Detailed Description

Preferred embodiments of the present invention will be described in detail below. However, the present invention is not limited to the configurations disclosed in the present embodiment, and various modifications can be made without departing from the scope of the present invention. In the following numerical limitation ranges, the lower limit value and the upper limit value are included in the range. Numerical values expressed as "greater than" or "less than" are not included in the numerical range. The "%" relating to the content of each element means "% by mass".

As described above, heretofore, for H-shaped steel having a flange thickness of 20mm or more, toughness at room temperature or at most 0 ℃ has been required. However, at present, in consideration of use in cold regions and the like, a thick H-shaped steel is required to have excellent toughness at a lower temperature of about-20 ℃. Further, in consideration of the strength per unit weight as a structural material, the yield stress (specifically, yield strength or σ) is also required for thick H-section steel_0.2) Above 385 MPa.

Therefore, the present inventors have studied the influence of the steel composition (chemical composition of the steel) and the steel structure (metal structure of the steel) on the strength and the low-temperature toughness of the thick H-section steel (hereinafter sometimes referred to as steel material), particularly, the flange which is a structurally important part of the H-section steel, and have obtained the following findings. In the present embodiment, the strength means the tensile yield stress and the tensile maximum strength, and the low-temperature toughness means the absorption energy in the charpy impact test at-20 ℃.

First, an excessive increase in hardenability due to the addition of alloy elements promotes the formation of a martensite-austenite mixed structure (hereinafter referred to as MA) in the steel material, resulting in a decrease in low-temperature toughness. In particular, B among the alloying elements has a significant tendency to promote the generation of MA, and therefore, it is effective to limit B to an impurity level or less without actively adding B.

In addition, in order to achieve a high yield stress (yield strength or σ)_0.2) At the same time, the toughness at-20 ℃ is improved, and Nb addition is effective. Since Nb increases the strength of the steel material by precipitation strengthening, the hardenability is not excessively increased, and the strength of the steel material can be increased without promoting the generation of MA. In addition, Nb has an effect of suppressing recrystallization of austenite during hot rolling, accumulating strain in the steel material due to rolling, and causing grain refining of ferrite after transformation.

In addition, V is effective for improving the toughness at-20 ℃. V precipitates as carbonitride (VC, VN, or a complex thereof) and acts as a nucleus generation site of ferrite, thereby providing an effect of making ferrite fine-grained.

In addition, the strength and low-temperature toughness are further improved by adding Mn. Further, in addition to controlling the steel composition, it is important to control the surface area fraction of ferrite, the surface area fraction of MA, the average grain size of ferrite, and the like as the steel structure in order to achieve both high strength and low temperature toughness.

In order to stably control the steel structure, it is necessary to impart sufficient rolling strain to the austenite recrystallization temperature region and the non-recrystallization temperature region when hot rolling a steel slab whose steel composition is controlled. Specifically, hot rolling with a cumulative reduction of 20% or more is performed in a temperature range of more than 900 ℃ and 1100 ℃ or less, and hot rolling with a cumulative reduction of 15% or more is performed in a temperature range of 900 ℃ or less. By rolling at a temperature of more than 900 ℃, austenite grains are made fine to reduce hardenability, the amount of MA formation and the like are suppressed to be low, and by rolling at a temperature of 900 ℃ or less, a large strain is imparted to a steel material to increase the frequency of ferrite nucleus formation, thereby making ferrite fine.

In order to stably control the steel structure, it is preferable that the difference in cooling rate between the surface and the inside of the steel material is small in cooling after hot rolling. When the steel material is cooled naturally without accelerated cooling after hot rolling, the cooling rate of the surface and the cooling rate of the inside of the steel material are both reduced, and the difference therebetween is also reduced. For example, in the case of H-shaped steel having a flange thickness of 20mm, when the steel is naturally cooled after hot rolling, the average cooling rate of the surface and the inside of the steel from 800 ℃ to 500 ℃ is 1 ℃/sec or less.

When the cooling rate after hot rolling is slow, it is generally not easy to ensure both the yield stress and the low-temperature toughness. However, by optimally controlling the steel composition and the production conditions, it becomes possible to achieve both yield stress and low-temperature toughness. For example, as the steel component, the content of C is set to 0.05% to 0.160%, B is not added but limited to an impurity level or less, Nb and V are actively added, the content of alloying elements such as Mn, Ti, and N is appropriately controlled, and the carbon equivalent Ceq is controlled to be in the range of 0.30 to 0.48. Further, the production conditions were optimally controlled, and the surface area fraction of ferrite, the surface area fraction of MA, the average grain size of ferrite, and the like were examined and determined as the steel structure. As a result, a thick H-shaped steel having excellent strength and low-temperature toughness can be obtained.

The H-section steel according to the present embodiment will be described below. First, the steel composition and the reason for the limitation thereof will be described in detail.

The H-shaped steel according to the present embodiment contains basic elements, optional elements as necessary, and the balance (the remainder) of Fe and impurities as chemical components.

C, Si, Mn, Nb, V, Ti, and N among the chemical components of the H-shaped steel according to the present embodiment are basic elements (main alloying elements).

(C：0.05～0.160％)

C (carbon) is an element effective for strengthening steel. Therefore, the lower limit of the C content is set to 0.05%. The lower limit of the C content is preferably 0.060%, 0.070%, or 0.080%. On the other hand, when the C content is more than 0.160%, a decrease in low-temperature toughness is incurred. Therefore, the upper limit of the C content is set to 0.160%. In order to further improve the low-temperature toughness, the upper limit of the C content is preferably 0.140%, 0.130%, or 0.120%.

(Si：0.01～0.60％)

Si (silicon) is a deoxidizing element and also contributes to improvement of strength. Therefore, the lower limit of the Si content is set to 0.01%. The lower limit of the Si content is preferably set to 0.05%, 0.10%, or 0.15%. On the other hand, if the Si content is more than 0.60%, the formation of MA is promoted, resulting in a decrease in low-temperature toughness. Therefore, the upper limit of the Si content is set to 0.60%. In order to further improve the low-temperature toughness, the upper limit of the Si content is preferably 0.40% or 0.30%.

(Mn：0.80～1.70％)

Mn (manganese) is an element contributing to the improvement of strength. Therefore, the lower limit of the Mn content is set to 0.80%. In order to further improve the strength, the lower limit of the Mn content is preferably set to 1.0%, 1.1%, or 1.2%. On the other hand, if the Mn content is more than 1.70%, hardenability is excessively increased, and the formation of MA is promoted, thereby impairing low-temperature toughness. Therefore, the upper limit of the Mn content is set to 1.70%. The upper limit of the Mn content is preferably set to 1.60% or 1.50%.

(Nb：0.005～0.050％)

Nb (niobium) is an element that suppresses recrystallization of austenite during hot rolling, contributes to grain refinement of ferrite by accumulating work strain in the steel, and contributes to improvement of strength by precipitation strengthening. Therefore, the lower limit of the Nb content is set to 0.005%. The lower limit of the Nb content is preferably 0.010%, more than 0.020%, 0.025%, or 0.030%. However, if the Nb content is more than 0.050%, the low-temperature toughness may be significantly reduced. Therefore, the upper limit of the Nb content is set to 0.050%. The upper limit of the Nb content is preferably set to 0.045%, 0.043%, or 0.040%. When Nb is not intentionally added, the content of Nb contained as an impurity is less than 0.005%. In order to make the Nb content 0.005% or more, Nb is intentionally added to the steel.

(V：0.05～0.120％)

V (vanadium) is an element having the effect of precipitating as carbonitrides in austenite grains, acting as transformation nuclei into ferrite, and refining ferrite grains. Therefore, the lower limit of the V content is set to 0.05%. The lower limit of the V content is preferably set to more than 0.05%, 0.06%, or 0.07%. However, if the V content is more than 0.120%, the low-temperature toughness may be impaired due to coarsening of precipitates. Therefore, the upper limit of the V content is set to 0.120%. The upper limit of the V content is preferably set to 0.110% or 0.100%.

(Ti：0.001～0.025％)

Ti (titanium) is an element that forms TiN and fixes N in steel. Therefore, the lower limit of the Ti content is set to 0.001%. In order to further refine the austenite by the pinning effect of TiN, the lower limit of the Ti content is preferably set to 0.005%, 0.007%, or 0.010%. On the other hand, if the Ti content is more than 0.025%, coarse TiN is formed, and the low-temperature toughness is impaired. Therefore, the upper limit of the Ti content is set to 0.025%. The upper limit of the Ti content is preferably 0.020%, 0.015%, or 0.012%.

In addition, when Al is not actively added, Ti acts as a deoxidizing element, and therefore N is generated which is not bonded to Ti. However, this N precipitates as V carbonitride with Ti oxide as a nucleus. That is, Ti acts as a deoxidizing element to precipitate Ti oxide, thereby promoting precipitation of V carbonitride and improving low-temperature toughness.

(N：0.0001～0.0120％)

N (nitrogen) is an element that forms TiN and VN and contributes to grain refinement and precipitation strengthening of the structure. Therefore, the lower limit of the N content is set to 0.0001%. The lower limit of the N content is preferably 0.0020%, 0.0035%, more than 0.0050%, or 0.0060%. However, if the N content is more than 0.0120%, the low temperature toughness is lowered, which causes surface cracking during casting and material defects due to strain aging of the produced steel. Therefore, the upper limit of the N content is set to 0.0120%. The upper limit of the N content is preferably set to 0.0110%, 0.0100%, or 0.0090%.

The H-shaped steel according to the present embodiment contains impurities as chemical components. The term "impurities" refers to components mixed from ores and scraps as raw materials or mixed from a production environment or the like in the industrial production of steel. For example, elements of Al, B, P, S, O, and the like are meant. In order to sufficiently exhibit the effects of the present embodiment, Al and B among these impurities are preferably limited as follows. Further, since the content of impurities is preferably small, the lower limit is not necessarily limited, and the lower limit of impurities may be 0%.

(Al: 0.10% or less)

Al (aluminum) is an element used as a deoxidizing element, but if the Al content is more than 0.10%, the oxide coarsens, becomes a base point of brittle fracture, and the low-temperature toughness decreases. Therefore, the upper limit of the Al content is limited to 0.10%. In addition, when Al is not actively used as a deoxidizing element, Ti functions as a deoxidizing element and a Ti oxide precipitates in the steel. The Ti oxide functions as a nucleation site of V carbonitride, miniaturizes ferrite grain size, and contributes to improvement of low-temperature toughness. Therefore, the upper limit of the Al content may be limited to less than 0.003%, 0.002%, or 0.001% by using Al as an impurity instead of Al as a deoxidizing element. In general, Al is intentionally contained in steel so that the Al content is 0.003% or more.

(B: 0.0003% or less)

B (boron) improves hardenability, promotes the formation of MA, and reduces low-temperature toughness. Therefore, in the present embodiment, B is not actively added but is limited to the impurity level or less. The upper limit of the B content is limited to 0.0003%. The upper limit of the B content is preferably limited to less than 0.0003%, 0.0002%, or 0.0001%. In general, B is intentionally contained in steel so that the B content is more than 0.0003%.

(P is 0.03% or less, S is 0.02% or less, O is 0.005% or less)

P (phosphorus), S (sulfur), and O (oxygen) are impurities. P and S are solidified and segregated, which promotes weld cracking and lowers low-temperature toughness. The upper limit of the P content is preferably limited to 0.03%, 0.02%, or 0.01%. In addition, the upper limit of the S content is preferably limited to 0.02% or 0.01%. O is dissolved in steel to lower the low-temperature toughness, and coarsening of oxide particles causes the low-temperature toughness to be lowered. The upper limit of the O content is preferably limited to 0.005%, 0.004%, or 0.003%.

The H-shaped steel according to the present embodiment may contain a selective element in addition to the basic elements and impurities described above. For example, Cr, Mo, Ni, Cu, W, Ca, Zr, Mg, and/or REM may be contained as optional elements in place of a part of the balance (the rest) of Fe. These optional elements may be contained depending on the purpose. Therefore, the lower limit of these selection elements need not be limited, and the lower limit may be 0%. Further, even if these optional elements are contained as impurities, the above effects are not impaired.

(Cr：0～0.30％)

Cr (chromium) is an element contributing to an increase in strength. The Cr content may be set to 0 to 0.30% as required. In order to further improve the strength, the lower limit of the Cr content is preferably set to 0.01%, 0.05%, or 0.10%. On the other hand, if the Cr content is more than 0.30%, the formation of MA is promoted, and the low-temperature toughness may be lowered. Therefore, the upper limit of the Cr content is preferably set to 0.30%, 0.25%, or 0.20%.

(Mo：0～0.20％)

Mo (molybdenum) is an element that is solid-dissolved in steel and contributes to improvement of strength. The Mo content may be 0 to 0.20% as required. In order to further improve the strength, the lower limit of the Mo content is preferably set to 0.01%, 0.05%, or 0.10%. However, when the Mo content is more than 0.20%, the generation of MA is promoted, and the low-temperature toughness may be lowered. Therefore, the upper limit of the Mo content is preferably set to 0.20%, 0.17%, or 0.15%.

(Ni：0～0.50％)

Ni (nickel) is an element that is solid-dissolved in steel and contributes to improvement of strength. The Ni content may be set to 0 to 0.50% as required. In order to further improve the strength, the lower limit of the Ni content is preferably set to 0.01%, 0.05%, or 0.10%. However, if the Ni content is more than 0.50%, hardenability is improved, and the formation of MA is promoted, which may lower low-temperature toughness. Therefore, the upper limit of the Ni content is preferably set to 0.50%, 0.30%, or 0.20%.

(Cu：0～0.35％)

Cu (copper) is an element contributing to the improvement of strength. The Cu content may be 0 to 0.35% as required. However, the addition of Cu promotes the formation of MA, and the low-temperature toughness may be lowered. Therefore, the Cu content may be preferably limited to 0.30% or less, 0.20% or less, 0.10% or less, or to less than 0.03% or less than 0.01% of the impurity level.

(W：0～0.50％)

W (tungsten) is an element that is solid-dissolved in steel and contributes to improvement of strength. The W content may be 0 to 0.50% as required. The lower limit of the W content is preferably 0.001%, 0.01%, or 0.10%. However, if the W content is more than 0.50%, the formation of MA is promoted, and the low-temperature toughness may be lowered. Therefore, the upper limit of the W content is preferably set to 0.50%, 0.40%, or 0.30%. When W is not intentionally added, the content of W contained as an impurity is less than 0.001%. W is intentionally added to the steel so that the W content is 0.001% or more.

(Ca：0～0.0050％)

Ca (calcium) is an element effective for controlling the form of sulfides, suppressing the formation of coarse MnS, and contributing to the improvement of low-temperature toughness. The Ca content may be 0 to 0.0050% as required. The lower limit of the Ca content is preferably set to 0.0001%, 0.0005%, or 0.0010%. On the other hand, if the Ca content is more than 0.0050%, the low-temperature toughness may be lowered. Therefore, the upper limit of the Ca content is preferably set to 0.0050%, 0.0040%, or 0.0030%.

(Zr：0～0.0050％)

Zr (zirconium) is an element that precipitates as a carbide, a nitride, or a composite thereof and contributes to precipitation strengthening. The Zr content may be 0 to 0.0050% as required. The lower limit of the Zr content is preferably set to 0.0001%, 0.0005%, or 0.0010%. On the other hand, if the Zr content is more than 0.0050%, carbides, nitrides and the like of Zr are coarsened, and the low-temperature toughness may be lowered. Therefore, the upper limit of the Zr content is preferably set to 0.0050%, 0.0040%, or 0.0030%. In the case where Zr is not intentionally added, the content of Zr contained as an impurity is less than 0.0001%. Zr is intentionally added to the steel so that the Zr content is 0.0001% or more.

(Mg：0～0.0050％、REM：0～0.0050％)

Mg (magnesium) and REM (rare earth element) are elements contributing to improvement in toughness of the base metal and toughness of the weld Heat Affected Zone (HAZ). If necessary, the Mg content may be 0 to 0.0050% and the REM content may be 0 to 0.0050%. The lower limit of the Mg content is preferably set to 0.0005%, 0.0010%, or 0.0020%, and the lower limit of the REM content is preferably set to 0.0005%, 0.0010%, or 0.0020%. On the other hand, the upper limit of the Mg content is preferably 0.0040%, 0.0030%, or 0.0025%, and the upper limit of the REM content is preferably 0.0040%, 0.0030%, or 0.0025%.

(Ceq：0.30～0.48)

The H-shaped steel according to the present embodiment has a carbon equivalent Ceq controlled from the viewpoint of ensuring strength. Specifically, when Ceq is expressed by the following formula 1, the chemical composition of the H-shaped steel contains C, Mn, Cr, Mo, V, Ni and Cu in mass% and satisfies 0.30-Ceq 0.48. If Ceq is less than 0.30, the intensity is insufficient. Therefore, the lower limit of Ceq is set to 0.30. The lower limit of Ceq is preferably set to 0.32%, 0.34%, or 0.35%. On the other hand, if Ceq is more than 0.48, the low temperature toughness decreases. Therefore, the upper limit of Ceq is set to 0.48. The upper limit of Ceq is preferably 0.45%, 0.43%, or 0.40%. When Ceq is calculated by the following formula 1, elements whose content in the steel is equal to or less than the detection limit may be calculated by substituting 0 as the value into the formula 1.

Ceq ═ C + Mn/6+ (Cr + Mo + V)/5+ (Ni + Cu)/15 · (formula 1)

The above-mentioned steel components may be measured by a usual analysis method for steel. For example, the steel composition may be measured by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured by a combustion-infrared absorption method, N may be measured by an inert gas melting-thermal conductivity method, and O may be measured by an inert gas melting-non-dispersive infrared absorption method.

Next, the steel structure of the H-section steel according to the present embodiment and the reasons for the limitation thereof will be described in detail.

In the H-shaped steel according to the present embodiment, the steel structure contains 60% or more and less than 100% of ferrite in terms of surface area fraction, the MA of the mixed structure of martensite and austenite is limited to 3.0% or less, and the structure other than ferrite and MA is limited to 37% or less. The average grain size of ferrite is set to be 1 μm or more and 30 μm or less

(area fraction of ferrite: 60% or more and less than 100%)

Ferrite is a main constituent phase in the steel structure of the H-shaped steel according to the present embodiment. If the area fraction of ferrite is less than 60%, the low-temperature toughness is lowered. Therefore, the lower limit of the ferrite fraction is set to 60%. The lower limit of the ferrite fraction is preferably 65%, 70%, or 75%. On the other hand, it is physically difficult to control the area fraction of ferrite to 100% due to the generation of pearlite or bainite. Therefore, the upper limit of the ferrite fraction is set to less than 100%. In order to desirably control the strength and the low-temperature toughness, the upper limit of the ferrite fraction is preferably set to 90%, 85%, or 80%. (area ratio of MA: 3.0% or less)

If the formation of MA is promoted, the low-temperature toughness is lowered. In the H-shaped steel according to the present embodiment, the strength of the steel material is increased without promoting the generation of MA. Therefore, the MA fraction is limited to 3.0% or less. The upper limit of the MA fraction is preferably 2.5%, 2.0%, or 1.5%. Since the smaller the MA fraction is, the better the MA fraction is, the lower limit of the MA fraction may be 0%.

(surface area ratio of the structure excluding ferrite and MA: 37% or less)

The steel structure of the H-shaped steel according to the present embodiment includes bainite, pearlite, and the like as structures other than the ferrite and MA described above. If the structure other than ferrite and MA is contained excessively, the low-temperature toughness decreases. Therefore, the area fraction of the structure other than ferrite and MA (the rest other than ferrite and MA described above) is limited to 37% or less. The fraction of the structure other than ferrite and MA is preferably 35% or less, 30% or less, or 25% or less. Since the fraction of the structure other than ferrite and MA is preferably as small as possible, the lower limit thereof may be 0%.

(average grain size of ferrite: 1 to 30 μm)

The average grain size of ferrite is preferably fine. If the ferrite grain size is more than 30 μm, the low temperature toughness is lowered. Therefore, the upper limit of the ferrite grain size is set to 30 μm. The upper limit of the ferrite grain size is preferably 25 μm, 22 μm, or 18 μm. On the other hand, it is industrially difficult to control the ferrite grain size to less than 1 μm. Therefore, the lower limit of the ferrite grain size is set to 1 μm. The lower limit of the ferrite grain size is preferably 3 μm, 5 μm, or 10 μm.

The steel structure may be determined by observation with an optical microscope. For example, fig. 1 is a schematic cross-sectional view of an H-shaped steel perpendicular to the rolling direction, and the steel structure is observed with the vicinity of the evaluation portion 7 shown in fig. 1 as an observation plane. Specifically, in fig. 1, the width-direction end face 5a of the flange is (1/6) F, and the thickness-direction outer side face 5b of the flange is (1/4) t₂The vicinity of the evaluation site 7 at the position of (2) is used as an observation surface to observe the steel structure. The observation surface is a surface parallel to the width direction end surface 5a of the flange.

The above observation surface was polished and corroded to observe the steel structure. Until the observed surface becomes a mirror surface, the polishing was performed using an etching solution suitable for the identification of the constituent phases. For example, if the steel structure is developed by etching the mirror-finished observation surface with a nital solution, the pearlite and bainite are colored, and therefore ferrite, martensite, and austenite can be identified. Further, if the steel structure is developed by etching the mirror-finished observation surface with a Lepera etching solution, the martensite and austenite constituent phases other than martensite and austenite are colored and blackened, and therefore the martensite and austenite mixed structure MA can be identified.

In the H-shaped steel according to the present embodiment, the fraction of ferrite and MA is obtained from the observation surface corroded by the nital corrosion solution, and the MA fraction is obtained from the observation surface corroded by the Lepera corrosion solution with the balance being the structure fraction of pearlite and bainite. Specifically, on an optical microscope photograph (optionally, a plurality of visual fields) of 200 magnifications taken on an observation surface corroded by a nital corrosion solution, measurement points are arranged in a lattice shape with one side of 25 μm, whether ferrite or MA is present is determined at least at 1000 measurement points, and a value obtained by dividing the number of measurement points determined to be ferrite or MA by the total number of measurement points is defined as the fraction of ferrite or MA.

Similarly, on an optical micrograph (a plurality of visual fields as necessary) of 200 magnifications taken on an observation surface corroded by a Lepera corrosive solution, measurement points are arranged in a lattice shape with one side of 25 μm, whether MA is present or not is determined at least 1000 measurement points, and a value obtained by dividing the number of measurement points determined to be MA by the total number of measurement points is defined as an MA fraction. The fraction of ferrite is obtained by subtracting the total fraction of pearlite, bainite and MA fractions obtained in the above from 100%.

In addition, the H-shaped steel according to the present embodiment uses an optical microscope photograph of 200 × taken on the observation surface corroded with the nital etching solution, and the average grain size of ferrite is determined by a cutting method in accordance with JIS G0551 (2013).

Next, the mechanical properties of the H-section steel according to the present embodiment will be described in detail.

The H-shaped steel according to the present embodiment is prepared by sampling a sample from a region including an evaluation site 7 shown in fig. 1 as a position where average mechanical properties (strength and low-temperature toughness) are obtained, and evaluating the mechanical properties.

First, the evaluation portion 7 in fig. 1 will be explained. FIG. 1 is a schematic cross-sectional view of an H-shaped steel orthogonal to the rolling direction. In fig. 1, the X-axis direction is defined as the width direction of the flange, the Y-axis direction is defined as the thickness direction of the flange, and the Z-axis direction is defined as the rolling direction.

As shown in FIG. 1, the length of the flange in the width direction is represented by F and the thickness of the flange is represented by t at the center of the evaluation portion 7₂At this time, the distance from the end face (1/6) F in the width direction of the flange to the outer side (1/4) t in the thickness direction of the flange is₂The position of (a). The surface on the outer side in the thickness direction of the flange is a surface in the thickness direction of the flange, is not in contact with the web 6, and is a surface 5b shown in fig. 1. The width-direction end surface of the flange is an end surface 5a shown in fig. 1.

The samples for evaluation of low-temperature toughness were prepared by the charpy impact test so that the longitudinal direction of the samples was parallel to the rolling direction from the position of the evaluation site 7. The surface of the sample on which the notch is formed is any surface parallel to the width-direction end surface 5a of the flange. The sample is set to have a surface 5b (1/4) t from the outer side in the thickness direction of the flange, the surface 5a being (1/6) F from the end surface 5a in the width direction of the flange₂The position of (2) can be obtained from any position.

Evaluation of yield stress (yield strength or. sigma.) by tensile test_0.2) And the tensile strength (tensile maximum strength) were obtained so that the position (1/6) F from the widthwise end face 5a of the flange in fig. 1 was the center in the thickness direction of the sample. The sample may be cut in such a manner that the longitudinal direction of the sample is parallel to the rolling direction and the thickness direction of the flange is all cut. The sample may be obtained from any position as long as it is located at (1/6) F from the width-direction end face 5a of the flange.

The H-shaped steel according to the present embodiment has mechanical properties such that the yield stress at room temperature is 385MPa or more, the tensile strength is 490MPa or more, and the charpy impact absorption energy at-20 ℃ is 100J or more. If the strength is too high, the low-temperature toughness may be impaired, and therefore, it is preferable that the upper limit of the yield stress is 530MPa and the upper limit of the tensile strength is 690 MPa. Further, since it is industrially difficult to make the Charpy impact absorption energy at-20 ℃ higher than 500J, the upper limit of the Charpy impact absorption energy at-20 ℃ can be 500J. Further, the normal temperature means 20 ℃.

In evaluating the mechanical properties of the H-section steel according to the present embodiment, the tensile test is performed in accordance with JIS Z2241(2011) and the charpy impact test is performed in accordance with JIS Z2242 (2005). When a yield phenomenon is observed on the stress-strain curve obtained by the tensile test, the yield strength is determined as the yield stress, and when no yield phenomenon is observed on the stress-strain curve, σ is determined_0.2As yield stress.

Next, the shape of the H-section steel according to the present embodiment will be described in detail.

The thickness t of the flange of the H-shaped steel according to the embodiment₂Set to 20 to 140 mm. For example, in high-rise building structures, thick H-section steel is required as a strength member. Therefore, the lower limit of the flange thickness is set to 20 mm. The lower limit of the flange thickness is preferably set to 25mm, 40mm, or 56 mm. On the other hand, if the thickness t of the flange is₂If the thickness is more than 140mm, the amount of working at the time of hot working is insufficient, and it is difficult to achieve both strength and low-temperature toughness. Therefore, the upper limit of the flange thickness is set to 140 mm. The upper limit of the flange thickness is preferably set to 125mm, 89mm, or 77 mm. For example, the thickness t of the flange₂Preferably 25 to 140 mm. Further, the thickness t of the web of H-section steel₁Although not particularly limited, the thickness is preferably 20 to 140mm, and more preferably 25 to 140 mm.

In addition, in the case of manufacturing H-shaped steel by hot rolling, the ratio of flange thickness/web thickness (t)₂/t₁) Preferably 0.5 to 2.0. When the ratio of flange thickness/web thickness (t)₂/t₁) When the amount exceeds 2.0, the web may be deformed into a wavy shape. On the other hand, in the ratio of flange thickness/web thickness (t)₂/t₁) If the thickness is less than 0.5, the flange may be deformed into a wavy shape.

In the prior art, the H-shaped steel with the flange thickness of more than 20mm has difficulty in having both strength and toughness. However, the H-shaped steel according to the present embodiment is an H-shaped steel having a flange thickness of 20mm or more, but can have both strength and low-temperature toughness because the steel composition and steel structure are optimally controlled.

Next, a preferred method for producing the H-section according to the present embodiment will be described in detail.

The method for producing H-shaped steel according to the present embodiment includes: a steel making process, a casting process, a heating process, a hot rolling process and a cooling process.

In the steel making process, the chemical composition of the molten steel is adjusted so as to have the above steel composition. In the steel-making step, molten steel produced by converter refining and/or secondary refining may be used, or molten steel melted in an electric furnace may be used as a raw material. In the steel-making step, deoxidation treatment and vacuum degassing treatment may be performed as necessary.

In the casting step, the molten steel after the steel-making step is cast to obtain a billet. The casting is performed by a continuous casting method, an ingot casting method, or the like. From the viewpoint of productivity, continuous casting is preferred. The shape of the billet is preferably a beam blank (beam blank) having a shape close to that of the H-beam to be produced, but is not particularly limited. The thickness of the slab is preferably 200mm or more from the viewpoint of productivity, and is preferably 350mm or less in consideration of reduction in segregation, homogeneity of the heating temperature before hot rolling, and the like.

In the heating step, the billet after the casting step is heated to 1100 to 1350 ℃. When the heating temperature of the billet is less than 1100 ℃, the deformation resistance at the time of finish rolling becomes high. Therefore, the lower limit of the heating temperature is set to 1100 ℃. In order to sufficiently dissolve the carbide-forming elements such as Nb and the nitride-forming elements, the lower limit of the heating temperature is preferably 1150 ℃. On the other hand, if the heating temperature is higher than 1350 ℃, the scale on the billet surface liquefies, and a trouble occurs during production. Therefore, the upper limit of the heating temperature is 1350 ℃. In the heating step, a billet that has not been cooled to room temperature after the casting step may be used.

In the hot rolling step, the billet after the heating step is subjected to rough rolling, intermediate rolling, and finish rolling. In rough rolling, forming is performed so that the shape when viewed in a cross section perpendicular to the rolling direction becomes substantially H-shaped. The substantially H-shaped steel slab is hot-rolled at a cumulative reduction of 20% or more in a temperature range where the surface temperature of the steel is more than 900 ℃ and 1100 ℃ or less, and further hot-rolled at a cumulative reduction of 15% or more in a temperature range where the surface temperature of the steel is 730 to 900 ℃. In this hot rolling, forming is performed so that the shape when viewed in the above-described cut surface finally becomes an H shape.

In a temperature region of more than 900 ℃ and 1100 ℃ or less, the cumulative reduction ratio is set to 20% or more in order to reduce the amount of bainite and MA produced by grain refining of austenite grains. The lower limit of the cumulative reduction in the temperature range of more than 900 ℃ and 1100 ℃ or less is preferably set to 25%, 30%, or 35%. The upper limit of the cumulative reduction in the temperature range of more than 900 ℃ and 1100 ℃ or less may be set to 60% as necessary.

The cumulative reduction ratio is set to 15% or more for grain refining of ferrite in a temperature range of 730 to 900 ℃. The lower limit of the cumulative reduction in the temperature range of 730 to 900 ℃ is preferably set to 20%, 25%, or 30%. The upper limit of the cumulative reduction in the temperature range of 730 to 900 ℃ may be set to 50% as necessary.

Further, when rolling is performed at a temperature lower than 730 ℃, the low-temperature toughness may be lowered. Therefore, the rolling finish temperature (finish rolling temperature) is set to 730 ℃ or higher in accordance with the surface thermometer of the steel. The upper limit of the finish rolling temperature is preferably 750 ℃.

In the hot rolling step, rough rolling, intermediate rolling, and finish rolling are performed, but for example, rolling in a temperature range of more than 900 ℃ and 1100 ℃ or less may be performed by any of rough rolling, intermediate rolling, and finish rolling. Similarly, the rolling in the temperature range of 730 to 900 ℃ may be performed by any of rough rolling, intermediate rolling, and finish rolling. In the method of producing H-shaped steel according to the present embodiment, the cumulative reduction ratio in the temperature region may be controlled.

The cumulative reduction ratio in the temperature region is determined based on the flange thickness at the position corresponding to (1/6) F from the width-direction end face 5a of the flange shown in fig. 1. For example, the cumulative reduction ratio in a temperature region of more than 900 ℃ and 1100 ℃ or less is set as a reduction ratio calculated from the difference between the flange thickness at the time point when the surface temperature of the steel is 1100 ℃ and the flange thickness immediately before 900 ℃. Similarly, the cumulative reduction ratio in the temperature region of 730 ℃ to 900 ℃ is set to a reduction ratio calculated from the difference between the flange thickness at the time point when the surface temperature of the steel is 900 ℃ and the flange thickness at the time point when it is 730 ℃.

The method of rough rolling, intermediate rolling, and finish rolling in the hot rolling step is not particularly limited. For example, the steel sheet may be formed so that the shape when viewed in a cross section perpendicular to the rolling direction is H-shaped by performing cogging (breakdown) rolling as rough rolling, performing universal rolling or widening (edging) rolling as intermediate rolling, and performing universal rolling as finish rolling.

In the hot rolling step, water cooling may be performed between rolling passes. The water cooling between the rolling passes is cooling for the purpose of temperature control in a temperature region higher than the temperature at which austenite undergoes phase transformation. Bainite and MA are not formed in the steel material by water cooling between rolling passes.

In the hot rolling step, two hot rolling steps may be performed. The second hot rolling is a rolling method in which after the first rolling, the slab is cooled to 500 ℃ or lower, and then the slab is heated again to 1100 to 1350 ℃ to perform the second rolling. In the second hot rolling, since the amount of plastic deformation in the hot rolling is small and the temperature decrease in the rolling step is small, the second heating temperature can be set to be slightly lower.

In the cooling step, the hot rolled material after the hot rolling step is cooled. In the method for producing H-shaped steel according to the present embodiment, after the hot rolling is completed, the hot rolled material is naturally cooled in the air as it is. When the hot rolled material is naturally cooled in the air, the average cooling rate of the surface and the inside of the steel material from 800 ℃ to 500 ℃ is 1 ℃/sec or less. By naturally cooling the hot rolled material in the atmosphere, the cooling rate of the surface and the inside of the steel material becomes uniform, and thus variation in mechanical properties due to the portion of the steel material is suppressed. In the method for producing H-shaped steel according to the present embodiment, the natural cooling means cooling in the atmosphere without forced cooling until the steel temperature becomes 400 ℃.

Conventionally, in order to achieve both strength and toughness, a hot rolled material is subjected to accelerated cooling, and therefore, variations in mechanical properties occur on the surface and inside of a steel material. On the other hand, in the method for producing H-shaped steel according to the present embodiment, although the hot rolled material is naturally cooled in the atmosphere, since the steel composition and the steel structure are optimally controlled, it is possible to achieve both of the strength and the low-temperature toughness without causing variation in the mechanical properties on the surface and inside of the steel material.

The method for producing H-shaped steel according to the present embodiment does not require advanced steel-making techniques and accelerated cooling, and therefore can reduce the production load and shorten the construction period. Therefore, the H-shaped steel according to the present embodiment can improve the reliability of large structures without impairing the economical efficiency.

Examples

Next, the effects of one embodiment of the present invention will be described in more detail by way of examples, but the conditions in the examples are only one example of conditions adopted to confirm the feasibility and the effects of the present invention, and the present invention is not limited to this one example of conditions. The present invention can employ various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Steels having chemical compositions shown in tables 1 to 3 were melted and continuously cast to produce billets having a thickness of 240 to 300 mm. The steel is melted in a converter, subjected to primary deoxidation, added with alloying elements to adjust the composition, and subjected to vacuum degassing treatment as needed. The obtained billet was heated and hot-rolled to produce H-shaped steel. The steel components shown as component nos. 1 to 48 were determined by chemical analysis of samples collected from each H-shaped steel after production. Although not shown in the table, in either embodiment: p is 0.03% or less, S is 0.02% or less, and O is 0.005% or less. The open columns in the chemical components in the table indicate that the steel was not actively added or the content was below the detection limit.

The production process of the H-shaped steel is shown in FIG. 2. The slab heated in the heating furnace 1 is hot-rolled in a universal rolling train including a roughing mill 2a, an intermediate rolling mill 2b, and a finishing mill 2 c. After the hot rolling is completed, the hot rolled material is naturally cooled as it is until it becomes 400 ℃ or less. The average cooling rate of the surface and the inside of the hot rolled material from the hot rolling end temperature to 500 ℃ is 1 ℃/sec or less. When water cooling is performed between passes of hot rolling, the water cooling apparatuses 3 provided before and after the intermediate universal rolling mill (intermediate rolling mill) 2b are used to perform spray cooling of the outer surface of the flange. At this time, reversible rolling was performed.

Table 4 to table 6 show the production conditions and the production results. The rolling reductions in hot rolling shown in tables 4 to 6 are cumulative rolling reductions in the respective temperature ranges at the positions corresponding to (1/6) F from the widthwise end face 5a of the flange shown in fig. 1.

As described above, the produced H-shaped steel was subjected to the Charpy impact test at-20 ℃ using the sample obtained from the evaluation site 7 shown in FIG. 1, and the low-temperature toughness was evaluated. Further, a tensile test was performed at normal temperature (20 ℃) using a sample whose position (1/6) F from the widthwise end face 5a of the flange was the center in the thickness direction, and the tensile properties were evaluated. Further, the steel structure was evaluated by observing the structure using a sample having the vicinity of the evaluation site 7 shown in fig. 1 as an observation surface.

The tensile test was carried out in accordance with JIS Z2241 (2005). The yield stress is the yield point in the case where the stress-strain curve of the tensile test shows yield behavior, and the yield stress is σ in the case where the yield behavior is not shown_0.2. The charpy impact test was performed in accordance with JIS Z2242 (2005). The Charpy impact test is carried out at-20 ℃.

For the structure observation, the ferrite fraction, MA fraction, and the fractions of the structures other than ferrite and MA were measured by the above-described methods using an optical micrograph. The structure other than ferrite and MA is bainite or pearlite. The average grain size of ferrite was determined by a cutting method in accordance with JIS G0551(2013) using an optical micrograph.

As the tensile properties, a steel material having a Yield Stress (YS) of 385MPa or more and a Tensile Strength (TS) of 490MPa or more at ordinary temperature was judged as a pass. Further, as the low temperature toughness, a steel material having a Charpy impact energy absorption at-20 ℃ (vE-20) of 100J or more was judged as a pass.

As shown in tables 1 to 6, production Nos. 1 to 8, production Nos. 11 to 18, and production Nos. 34 to 43 as examples of the present invention satisfy the scope of the present invention in terms of steel composition, steel structure, and mechanical properties.

On the other hand, production Nos. 9 to 10, 19 to 33, and 44 to 50 as comparative examples do not satisfy the scope of the present invention in some of the steel components, steel structures, and mechanical properties.

Production No.9 is an example in which the ferrite fraction in the steel structure is insufficient, the fractions of ferrite and MA other structures are excessive, and the charpy absorption energy at-20 ℃ is insufficient because the reduction ratio in the temperature region of greater than 900 ℃ and 1100 ℃ or less is insufficient.

Production No.10 is an example in which the ferrite grain size becomes coarse because the reduction ratio in the temperature range of 730 to 900 ℃ is insufficient, and the charpy impact absorption energy at-20 ℃ becomes insufficient.

Production No.19 is an example in which the ferrite fraction is insufficient, the MA fraction is excessive, the fractions of ferrite and structure other than MA are excessive, and the charpy absorption energy at-20 ℃ is insufficient because the rolling reduction in the temperature region of more than 900 ℃ and 1100 ℃ or less is insufficient.

Production No.20 was an example in which the charpy impact absorption energy at-20 ℃ was insufficient because of the high C content, production No.25 was a high Nb content, production No.26 was a high V content, production No.28 was a high Al content, production No.29 was a high Ti content, production No.30 was a high N content, and production No.31 was an excessive Ceq content.

Production No.21 was an example in which the content of C was small, production No.24 was an example in which the content of Mn was small, production No.32 was an example in which Ceq was insufficient, and production No.46 was an example in which the content of Si was small, and thus YS and TS were insufficient.

Production No.22 is an example in which the Charpy impact absorption energy at-20 ℃ is insufficient because of a large Si content, production No.23 is an example in which the Mn content is large and the MA fraction is excessive.

Production No.27 is an example in which the ferrite grain size becomes coarse due to a small V content, and the Charpy impact energy at-20 ℃ becomes insufficient.

Production No.33 is an example in which the B content and Ceq were excessive, and production No.49 is an example in which the B content was large, the MA fraction was excessive, and the Charpy impact absorption energy at-20 ℃ was insufficient.

Production nos. 44 and 45 are examples in which the ferrite grain size is coarse due to a small V content, and the charpy impact absorption energy at-20 ℃ is insufficient.

Production No.47 is an example in which the ferrite grain size becomes coarse due to a small Nb content, YS and TS become insufficient, and the Charpy impact energy at-20 ℃ becomes insufficient.

Production No.48 is an example in which the ferrite grain size becomes coarse due to a small Ti content, and the Charpy impact energy at-20 ℃ becomes insufficient.

Production No.50 is an example in which the Charpy impact energy at-20 ℃ is insufficient because the finish rolling temperature is low.

Industrial applicability

According to the above aspect of the present invention, since a thick H-shaped steel excellent in strength and low-temperature toughness and a method for manufacturing the same can be provided, industrial applicability is high.

Description of the reference numerals

1 heating furnace

2a roughing mill

2b intermediate rolling mill

2c finishing mill

3 front and rear water cooling device of intermediate rolling mill

4H-shaped steel

5 Flange

5a width direction end face of flange

5b outer surface of flange in thickness direction

6 web

7 parts to be evaluated for tensile characteristics, low-temperature toughness, and steel structure

Length of flange F in width direction

Height H

t₁Thickness of web

t₂Thickness of the flange

Claims

1. An H-shaped steel characterized by comprising, as a chemical component of the steel, in mass%

C：0.05～0.160％、

Si：0.01～0.60％、

Mn：0.80～1.70％、

Nb：0.005～0.050％、

V：0.05～0.120％、

Ti：0.001～0.025％、

N：0.0001～0.0120％、

Cr：0～0.30％、

Mo：0～0.20％、

Ni：0～0.50％、

Cu：0～0.35％、

W：0～0.50％、

Ca：0～0.0050％、

Zr：0～0.0050％，

Further, Al is limited to 0.10% or less, B is limited to 0.0003% or less,

the balance of Fe and impurities,

when Ceq is C + Mn/6+ (Cr + Mo + V)/5+ (Ni + Cu)/15, C, Mn, Cr, Mo, V, Ni and Cu in the chemical components meet the condition that Ceq is more than or equal to 0.30 and less than or equal to 0.48,

the steel has a microstructure including 60% or more and less than 100% ferrite by area fraction, a mixed structure MA of martensite and austenite of 3.0% or less, a structure other than the ferrite and the MA of 37% or less,

the ferrite has an average particle diameter of 1 to 30 μm,

the steel has an H-shape when viewed from a cut plane orthogonal to the rolling direction, a flange thickness of 20 to 140mm,

when the width direction length of the flange is expressed as F, the tensile yield stress is 385-530 MPa, the maximum tensile strength is 490-690 MPa at the position away from the width direction end face 1/6F of the flange,

when the thickness of the flange is denoted as t₂At the outer side of the flange in the thickness direction from the end face 1/6F in the width direction of the flange 1/4t₂The position (2) has a Charpy impact test absorption energy of 100J or more at-20 ℃.

2. The H-shaped steel according to claim 1, characterized by containing, as the chemical components of the steel, in mass%: more than 0.02% and less than 0.050%.

3. The H-shaped steel according to claim 1, characterized by containing, as the chemical components of the steel, in mass%: more than 0.005% and less than 0.0120%.

4. The H-beam according to claim 1, characterized in that as the chemical component of the steel, Cu is limited to less than 0.03% by mass%.

5. The H-shaped steel according to claim 1, characterized in that as the chemical component of the steel, Al is limited to less than 0.003% in mass%.

6. The H-beam according to claim 1, wherein the thickness of the flange is 25 to 140 mm.

7. A method for producing an H-shaped steel according to any one of claims 1 to 6, comprising:

a steel-making step of obtaining a molten steel having the chemical component described in any one of claims 1 to 5;

a hot rolling step of rolling the slab after the heating step under the following conditions such that the shape of the slab when viewed in a cross section perpendicular to the rolling direction is an H shape: a cumulative reduction ratio at a position 1/6F from the widthwise end face of the flange is 20% or more in a temperature region of more than 900 ℃ and 1100 ℃ or less, and a cumulative reduction ratio at the position is 15% or more in a temperature region of 730 to 900 ℃ and rolling is terminated at a temperature of 730 ℃ or more; and

and a cooling step of naturally cooling the hot-rolled material after the hot-rolling step.