JP2005264208A - Low yield ratio wide flange beam having excellent earthquake resistance and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low yield ratio hot-rolled wide flange beam having excellent earthquake resistance, and to provide its production method. <P>SOLUTION: A steel stock preferably having a composition containing, by mass, 0.01 to 0.20% C, ≤0.6% Si, 0.6 to 1.6% Mn, ≤0.030% P, ≤0.030% S and ≤0.1% Al, is reheated at 1,000 to 1,350°C, and subjected to rolling in which hot rolling finishing temperature is controlled to ≥(Ar<SB>3</SB>transformation point-100°C), so as to produce a wide flange beam. Next, the outside face or inside face of the flange is cooled to a cooling stopping temperature in the range of 20 to 650°C at a mean cooling rate of ≥5°C/s, thereafter, the cooling is stopped, and then it is heated to ≥200°C. Thus, a structure where either surface layer in the inside and outside faces of the flange has a hard layer and the other surface layer has a soft layer, the average grain size of ferrite on the side of the soft layer is 5 to 40 μm, and also, the hard phase is comprised in 20 to 80% by volume ratio on the average value in the sheet thickness direction of the flange can be formed. In this way, the rolled wide flange beam having a high strength of ≥490 MPa by TS while maintaining a low yield ratio of ≤80%, and having excellent earthquake resistance can be obtained. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a hot-rolled H-section steel frequently used for building structures and a method for producing the same, and more particularly to a reduction in yield ratio and improvement in earthquake resistance. The rolled H-section steel to which the present invention is applied is a high-strength, low-yield ratio rolled H-section steel having a tensile strength of 490 MPa to 750 MPa. It also includes the regular rolled H-section steel and web thin rolled H-section steel.

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

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

  In this situation, in 1998, the yield strength (YS) range was 120 MPa or less and the variation range of YS was narrow, and the yield ratio (YR) was 80% or less, YR was low, narrow YS, low YR. Steel for building structures was established as a JIS standard.

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

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

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

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

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

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

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

Patent Document 3 proposes a method for producing an H-section steel having a plate thickness exceeding 40 mm, which is a combination of cooling recuperation of the inner and outer surfaces of the flange and the upper and lower surfaces of the web and hot rolling. In the technique described in Patent Document 3, the process of cooling the surface layers of the inner and outer surfaces of the flange and the upper and lower surfaces of the web to just above the Ms point and immediately rough rolling is repeated twice or more, and then the surface layer of the material to be rolled is just above the Ms point. The process of cooling and reheating is performed at least once, finish rolling is performed with the surface layer temperature set to 750 ° C. or higher, and the process of cooling to the point just above the Ms point and reheating after the finish rolling is repeated one or more times. Thus, high strength and high toughness can be achieved without requiring a complicated process such as controlled rolling.
Japanese Patent No. 2965813 Japanese Patent No. 2837056 Japanese Patent No. 3241444

  However, all of the above prior arts are based on the idea of controlling the thickness direction of the steel material to a uniform structure, and as a result, a complicated casting process is required and the production efficiency of the slab is reduced. In addition, it is difficult to obtain high strength because ferrite transformation is promoted by water cooling during rolling, and the heat-affected zone (HAZ) toughness is reduced by adding an alloy more than necessary, or a heat treatment process is required. Problems such as an increase in lead time remained.

  The present invention advantageously solves the problems of the prior art and proposes a H-shaped steel made of low yield specific hot rolled steel having a high tensile strength of 490 MPa to 750 MPa and excellent earthquake resistance and a method for producing the same. The purpose is to do.

  In order to achieve the above-described problems, the present inventors diligently studied a new strength control method. As a result, the present inventors can control the strength of the H-section steel by utilizing the structural change in the plate thickness direction while maintaining the yield ratio as low as 80% or less without compromising productivity. I found out.

  First, a basic experiment conducted by the present inventors will be described.

  Using steel materials a to c (sheet thickness: 90 mm) having the composition shown in Table 1, a hot rolling experiment simulating H-shaped steel rolling as shown in FIG. did. The rolling heating temperature of the steel material was 1250 ° C, and the rolling finishing temperature was 900 ° C. In addition, the cooling after completion | finish of hot rolling was made into three types, cooling, water cooling from the front and back, or water cooling from one side. In addition, the cooling stop temperature changed to a range of 700 to 300 ° C. The cooling rate at the plate thickness average was 7 to 40 ° C./s.

  From the obtained steel sheet, a No. 1 tensile test piece specified in JIS Z 2201 was taken with the rolling direction as the tensile direction, and a tensile test was conducted in accordance with the provisions of JIS Z 2241. Yield strength YS, tensile strength We asked for TS. The obtained result is shown in FIG. 2 by the relationship between the yield strength YS and the tensile strength TS.

  One-sided cooling material (□ mark) with water cooling on one side has lower yield strength YS than double-sided cooling material (△ mark) with water cooling on both sides, and the yield ratio YR is comparable to that of the cooling material (○ mark). 70% of the total. From this, it was found that high strength can be achieved while maintaining a low yield ratio of 80% or less by single-sided water cooling.

  Next, the control factors of tensile strength and yield strength were examined.

  Regarding the cross-section in the rolling direction (L-direction cross section) of the single-sided coolant that is water-cooled on one side, the structure is observed with an optical microscope (magnification: 200 times) over 5 fields of view, and the structure distribution in the plate thickness direction is obtained using an image analyzer. It was. A hard layer mainly composed of bainite and martensite, which are hard phases, is formed on the water-cooled side surface layer. The amount of bainite and martensite in the structure, that is, the hard phase fraction, is measured by area ratio at each position in the plate thickness direction (front surface, 1 / 4t, 1 / 2t, 3 / 4t, back surface) and converted to volume ratio. The amount (volume ratio) of each position obtained was averaged to obtain the average hard phase fraction (volume%) in the steel sheet thickness direction. The relationship between the obtained average hard phase fraction in the thickness direction and the tensile strength is shown in FIG. From FIG. 3, when the average hard phase fraction in the plate thickness direction is 20% by volume or more, a high strength generally having a tensile strength TS: 490 MPa or more can be obtained. When the average hard phase fraction exceeds 80% by volume, the tensile strength increases, but at the same time, the yield strength (yield strength) increases, the yield ratio increases, and the elongation also decreases. From this, in order to maintain a high yield and a low yield ratio of 80% or less, it was found that the average hard phase fraction in the thickness direction is preferably 20 to 80% by volume.

  Next, the surface layer structure of the non-water-cooled side of the single-sided coolant that was water-cooled on one side was observed with a microscope. A soft layer mainly composed of ferrite, which is a soft phase, is formed on the non-water-cooled side surface layer. The particle size of the ferrite grains is measured at a depth of 1 to 5 mm from the surface on the soft layer side, and arranged in relation to the yield strength YS, and is shown in FIG. The ferrite grain size was observed at 5 magnifications at 200 magnifications, the average grain size in each field was determined as the equivalent circle diameter using an image analyzer, and the average value for each field was determined for the soft layer of the steel sheet. The average ferrite particle size was used. From FIG. 4, it was found that high strength of YS: 325 MPa or more and 540 MPa or less can be obtained by setting the average particle diameter of the ferrite on the soft layer side to 5 μm or more. By setting the average grain size of ferrite to 40 μm or less, YS: 540 MPa or less can be obtained by setting YS: 325 MPa or more and 5 μm or more.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) Rolled H-section steel manufactured by hot rolling, wherein one surface layer of the inner and outer surfaces of the flange of the rolled H-section steel includes bainite and / or tempered martensite having a volume ratio of 50% or more. The other surface layer has a soft layer containing 50% or more of ferrite by volume ratio, the average particle diameter of the ferrite on the soft layer side is 5 to 40 μm, and the average value in the flange thickness direction is bainite and A low yield ratio rolled H-section steel excellent in earthquake resistance characterized by having a structure containing 20 to 80% by volume of tempered martensite.
(2) In (1), in addition to the above structure, in mass%, C: 0.01 to 0.20%, Si: 0.6% or less, Mn: 0.6 to 1.6%, P: 0.030% or less, S: 0.030% or less, Low yield ratio rolled H-section steel having a composition comprising Al: 0.1% or less, the balance being Fe and inevitable impurities.
(3) In (2), in addition to the above-mentioned composition, it is further characterized by containing, in mass%, one or two selected from Cu: 1% or less and Ni: 3% or less Yield ratio rolled H-section steel.
(4) In (2) or (3), in addition to the above-mentioned composition, it is further mass%, Cr: 3% or less, Mo: 1% or less, V: 0.3% or less, Nb: 0.1% or less, B: 0.01 % Yield ratio rolled H-section steel, characterized by containing one or more selected from below.
(5) In any one of (2) to (4), in addition to the above composition, in mass%, Ti: 0.1% or less, Ca: 0.1% or less, Mg: 0.1% or less, Zr: 0.1% or less, A low yield ratio rolled H-section steel containing one or more selected from Hf: 0.1% or less and REM: 0.1% or less.
(6) Steel material having a composition of mass%, including C: 0.01 to 0.20%, Si: 0.6% or less, Mn: 0.6 to 1.6%, P: 0.030% or less, S: 0.030% or less, Al: 0.1% or less Is heated to 1000 to 1350 ° C., and the hot rolling step is performed to make the H-shaped steel of a predetermined shape by hole rolling and universal rolling with the hot rolling end temperature being (Ar ₃ transformation point−100 ° C.) or higher. Then, after cooling the outer surface of the flange or the inner surface of the flange at an average cooling rate of 5 ° C./s or more to a cooling stop temperature in the range of 20 to 650 ° C. at the surface temperature of the cooling surface, the cooling is stopped, A method for producing a low yield ratio rolled H-section steel excellent in earthquake resistance, characterized by performing a cooling reheat treatment to reheat to a temperature of 200 ° C. or higher.
(7) In (6), in addition to the above-mentioned composition, it is further characterized by containing, in mass%, one or two selected from Cu: 1% or less and Ni: 3% or less Yield ratio rolled H-section steel manufacturing method.
(8) In (6) or (7), in addition to the above-mentioned composition, it is further mass%, Cr: 3% or less, Mo: 1% or less, V: 0.3% or less, Nb: 0.1% or less, B: 0.01 A method for producing a low yield ratio rolled H-section steel, comprising one or more selected from 1% or less.
(9) In any one of (6) to (8), in addition to the above composition, in mass%, Ti: 0.1% or less, Ca: 0.1% or less, Mg: 0.1% or less, Zr: 0.1% or less, A method for producing a low yield ratio rolled H-section steel comprising one or more selected from Hf: 0.1% or less and REM: 0.1% or less.

  According to the present invention, even when using a relatively inexpensive component system, it is possible to easily produce a hot rolled H-section steel having high strength and having a low yield ratio of 80% or less and excellent earthquake resistance. It has a remarkable industrial effect. Further, according to the present invention, there is an effect that the reliability of the structure is remarkably improved.

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

  The rolled H-section steel of the present invention has a structure in which one surface layer of the inner and outer surfaces of the flange has a hard layer and the other surface layer has a soft layer. Here, the “hard layer” refers to a layer containing bainite and / or tempered martensite which are hard phases in a volume ratio of 50% or more with respect to the entire hard layer. In addition to the bainite and / or tempered martensite, there is no problem even if the hard layer contains a ferrite phase and a pearlite phase having a volume ratio of 50% or less with respect to the entire hard layer. The “soft layer” refers to a layer containing 50% or more by volume of the soft phase ferrite with respect to the entire soft layer. In addition to the ferrite, the soft layer may contain a pearlite phase, a bainite phase, and a tempered martensite phase in a volume ratio of 50% or less with respect to the entire soft layer.

  By making one surface layer of the flange a hard layer and the other surface a soft layer, as shown in FIG. 2, the tensile strength: high strength of 490 MPa or more is maintained while maintaining the yield ratio at 80% or less. Easy to secure.

  In the rolled H-section steel of the present invention, the flange has an average value in the thickness direction of the flange plate and has a structure containing 20-80% by volume of bainite and / or tempered martensite which are hard phases. If the average fraction of bainite and / or tempered martensite in the thickness direction of the flange plate (average hard phase fraction) is less than 20% by volume, the tensile strength will be less than 490 MPa, making it impossible to achieve high strength. . On the other hand, if the average hard phase fraction exceeds 80% by volume, the yield ratio exceeds 80%, and a low yield ratio cannot be secured.

  In the rolled H-section steel of the present invention, the ferrite formed on the soft layer side is a ferrite having a particle size of 5 to 40 μm on average. If the average grain size of ferrite is less than 5 μm, it is difficult to ensure a low yield ratio and ductility of 80% or less. On the other hand, when the average particle diameter of ferrite exceeds 40 μm, YS decreases and toughness deteriorates. In addition, the measurement of the average particle diameter of a ferrite shall be performed about the ferrite in a soft layer.

  In addition to the above structure, the rolled H-section steel of the present invention is mass%, C: 0.01 to 0.20%, Si: 0.6% or less, Mn: 0.6 to 1.6%, P: 0.030% or less, S: 0.030% or less. Al: 0.1% or less, or Cu: 1% or less, Ni: 1 or 2 selected from 3% or less, and / or Cr: 3% or less, Mo: 1% or less, V : 0.3% or less, Nb: 0.1% or less, B: One or more selected from 0.01% or less, and / or Ti: 0.1% or less, Ca: 0.1% or less, Mg: 0.1% or less, It preferably has a composition comprising one or more selected from Zr: 0.1% or less, Hf: 0.1% or less, and REM: 0.1% or less, with the balance being Fe and inevitable impurities.

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

C: 0.01-0.20%
C is an element that increases the strength of the rolled H-section steel, and is preferably contained in an amount of 0.01% or more in the present invention in order to ensure a strength of a predetermined value or more. On the other hand, when the content exceeds 0.20%, the welded portion is hardened, and there is a concern that a weld crack may occur in a small heat input welded portion such as a tack welded portion. For this reason, C is preferably limited to a range of 0.01 to 0.20%.

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

Mn: 0.6-1.6%
Mn, like Si, is an element that effectively works to improve the strength of rolled H-section steel, and is preferably contained in an amount of 0.6% or more in the present invention. On the other hand, if the content exceeds 1.6%, weldability decreases. For this reason, it is preferable to limit Mn to the range of 0.6 to 1.6%.

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

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

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

One or two selected from Cu: 1% or less, Ni: 3% or less
Cu and Ni are solid solution strengthening elements that can increase the strength of H-section steel without improving hardenability, and are particularly effective for increasing the strength of thin-wall flange H-section steel. It can be selected depending on the content. If contained, Cu: 0.05% or more, Ni: 0.05% or more is preferable, but inclusion of Cu exceeding 1% promotes surface cracking during rolling and causes significant Cu precipitation embrittlement. . Ni is an expensive element and is preferably limited to 3% or less.

One or more selected from Cr: 3% or less, Mo: 1% or less, V: 0.3% or less, Nb: 0.1% or less, B: 0.01% or less
Cr, Mo, V, Nb, and B are all transformation strengthening elements. In particular, in the case of a thin-walled flange H-shaped steel, the formation of a soft ferrite phase is inhibited, and the cooling rate by accelerated cooling is reduced. In the case of a thick-walled flange H-shaped steel mainly, the hardenability is improved and it contributes to increasing the strength of the H-shaped steel. Therefore, it can be selected and contained as needed for the purpose of increasing the strength of the H-shaped steel. In addition, in the case of Mo, V, and Nb, a part of them precipitates during the recuperation process after cooling, which contributes particularly to an increase in yield strength. However, if Cr exceeds 3%, Mo exceeds 1%, V exceeds 0.3%, Nb exceeds 0.1%, and B exceeds 0.01%, the toughness, weldability, and HAZ toughness are deteriorated.

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

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

First, after reheating the steel material having the above composition to 1000 to 1350 ° C., the hot rolling finish temperature is set to a temperature equal to or higher than (Ar ₃ transformation point−100 ° C.), and a predetermined shape is formed by universal rolling and universal rolling. A hot rolling process for forming H-shaped steel is performed.
In addition, the manufacturing method of a steel raw material is not specifically limited in this invention. Both normal melting methods and casting methods can be suitably applied, but it is economically advantageous to use a continuous casting method as the casting method.

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

The heated steel material is made into an H-shaped steel having a predetermined size and shape by hole rolling and universal rolling. In the hole rolling and universal rolling, it is preferable that the hot rolling end temperature is set to a temperature of (Ar ₃ transformation point−100) ° C. or higher. When the hot rolling finish temperature is less than (Ar ₃ transformation point−100) ° C., work strain is introduced into the formed ferrite, the yield strength increases more than the tensile strength, and the yield ratio increases. . For this reason, it is preferable to limit the hot rolling end temperature to (Ar ₃ transformation point−100) ° C. or higher. Incidentally, more preferably in the range of (Ar ₃ transformation point -30) ℃ ~950 ℃. The Ar ₃ transformation point is expressed by the following formula (1): Ar ₃ = 910-273C + 25Si-74Mn-56Ni-16Cr-9Mo-5Cu-1620Nb (1)
It shall be calculated using. The hot rolling end temperature is the steel sheet surface temperature.

  Next, after the hot rolling is finished, the H-shaped steel that has finished the hot rolling process is air-cooled immediately or to a desired temperature, and then subjected to a cooling reheat treatment.

  In the cooling reheat treatment, the outer surface of the flange or the inner surface of the flange is cooled at an average cooling rate of 5 ° C./s or more to a cooling stop temperature in the range of 20 to 650 ° C. at the surface temperature of the cooling surface. It is preferable that the heat treatment is reheated to a temperature of 200 ° C. or higher.

  In the present invention, it is desirable to cool from one of the inner and outer surfaces of the flange, and at this time, it is preferable to cool at an average cooling rate in the flange plate of 5 ° C./s or more. The cooling is preferably water cooling from the viewpoint of obtaining the hardened layer depth. When the cooling rate is less than 5 ° C./s, a hard layer cannot be formed on the cooling surface of the flange. In addition, Preferably it is 8 degrees C / s or more. The cooling rate is an average cooling rate in the flange plate obtained by heat transfer calculation.

The cooling stop temperature is preferably a temperature in the range of 20 to 650 ° C. as the surface temperature of the cooling surface.
When the cooling stop temperature exceeds 650 ° C., the desired average hard phase fraction in the flange plate thickness direction cannot be ensured, and the desired tensile strength cannot be ensured. On the other hand, although the lower limit of the cooling stop temperature is limited to 20 ° C. of the water temperature, it is desirable to set it to 200 ° C. or higher in consideration of the tempering effect by recuperation for reducing the surface hardness. More preferably, it is 250 ° C. or higher.

  In the cooling, it is preferable that the cooling is performed such that after cooling is stopped, the surface temperature of the cooling surface is reheated to a temperature of 200 ° C. or higher. In order to set the recuperation temperature to 200 ° C. or higher, it is preferable to set it to 250 ° C. or higher during cooling. When the recuperation temperature is less than 200 ° C., the cooling surface is excessively cured, and workability such as drilling and ductility are deteriorated. In addition, Preferably, it is 350 degreeC or more.

  In addition, the other surfaces (non-cooled surfaces) that are not subjected to the above-described cooling among the inner and outer surfaces of the flange are left to cool or slowly cooled at a cooling rate of 1 ° C./s or less in order to form a soft layer on the surface layer. It is preferable that

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

  From the H-shaped steel thus obtained, No. 1 tensile test piece defined in JIS Z 2201 was taken from the portion of 1/4 of the flange width as the rolling direction. Further, a V-notch test piece defined in JIS Z 2202 was taken from a 1/4 t part of the plate thickness at a quarter of the flange width. The tensile test was performed at room temperature and the Charpy impact test was performed at 0 ° C. Moreover, the hardness of the surface layer part (1 mm from the surface) of the water cooling surface side of the flange was measured using a Vickers hardness tester under a load of 10 kg.

  Furthermore, the structure was investigated using the optical microscope about the plate | board thickness direction cross section (L direction cross section) of a flange. The observation position is the front surface (outer surface), 1 / 4t, 1 / 2t, 3 / 4t, and back surface (inner surface). At each position, five or more fields of view are observed, and bainite and martensite (tempering) that are hard phases at each position. ) Was calculated by an image analyzer, and the hard phase fraction at each position was averaged, and the hard phase fraction at each position was averaged to obtain the average hard phase fraction in the flange plate thickness direction. In addition, from the structure observation, the presence or absence of the hard layer and the soft layer was confirmed on the surface layer of the flange, and the tissue fractions of the hard layer and the soft layer were similarly determined.

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

  In addition, a reproducible heat cycle test piece is collected from the flange part, and a reproducible heat cycle equivalent to a heat input of 35 kJ / cm (peak temperature: 1400 ° C, cooling time of 800 to 500 ° C: 70 s, temperature between welding passes: 350 ° C) After application, a Charpy impact test piece (V-notch standard size) was collected, tested at 0 ° C., the absorbed energy was determined, and HAZ toughness was evaluated.

  Table 4 shows the obtained results.

  Each of the inventive examples is a high strength, low yield ratio rolled H-section steel having a high tensile strength of 490 MPa or more, a low yield ratio of 80% or less, and excellent base material toughness. ing. In addition, all of the inventive examples are rolled H-section steels having Charpy absorbed energy at 0 ° C. of 70 J or more and excellent HAZ toughness. On the other hand, comparative examples that are outside the scope of the present invention have low strength, high yield ratio, and low ductility and toughness.

It is explanatory drawing which shows typically the heating, rolling, and cooling conditions in a basic experiment. It is a graph which shows the relationship between yield strength YS and tensile strength TS. It is a graph which shows the influence of the average hard phase fraction of the plate | board thickness direction on the tensile strength TS. It is a graph which shows the relationship between yield strength YS and the average particle diameter of a soft layer ferrite.

Claims (6)

  A rolled H-section steel produced by hot rolling, wherein one surface layer of the inner and outer surfaces of the flange of the rolled H-section steel has a hard layer containing bainite and / or tempered martensite having a volume ratio of 50% or more, The surface layer has a soft layer containing ferrite of 50% or more by volume, the ferrite has an average particle diameter of 5 to 40 μm, and an average value in the flange plate thickness direction, bainite and / or tempering A low yield ratio rolled H-section steel excellent in earthquake resistance, characterized by having a structure containing martensite in a volume ratio of 20 to 80%. In addition to the organization, mass%,
C: 0.01-0.20%, Si: 0.6% or less,
Mn: 0.6 to 1.6%, P: 0.030% or less,
2. The low yield ratio rolled H-section steel according to claim 1, comprising: S: 0.030% or less, Al: 0.1% or less, with the balance being composed of Fe and inevitable impurities.   The low yield ratio according to claim 2, further comprising one or two kinds selected from Cu: 1% or less and Ni: 3% or less in mass% in addition to the composition. Rolled H-section steel.   In addition to the above-described composition, mass%, Cr: 3% or less, Mo: 1% or less, V: 0.3% or less, Nb: 0.1% or less, B: 0.01% or less The low yield ratio rolled H-section steel according to claim 2 or 3, characterized by containing seeds or more. In addition to the above composition, mass: Ti: 0.1% or less, Ca: 0.1% or less, Mg: 0.1% or less,
The low yield according to any one of claims 2 to 4, comprising one or more selected from Zr: 0.1% or less, Hf: 0.1% or less, and REM: 0.1% or less. Specific rolled H-section steel. mass%
C: 0.01-0.20%, Si: 0.6% or less,
Mn: 0.6 to 1.6%, P: 0.030% or less,
Soil rolling with a composition containing 0.030% or less and Al: 0.1% or less is reheated to 1000-1350 ° C, and hot rolling finish temperature is set to (Ar ₃ transformation point-100 ° C) or more. And a hot rolling process to obtain a H-shaped steel with a predetermined shape by universal rolling, and then the surface temperature of the cooling surface is 20 to 650 ° C. at an average cooling rate of 5 ° C./s or more on the flange outer surface or flange inner surface. Of low yield ratio rolled H-section steel with excellent earthquake resistance, which is characterized by performing cooling reheat treatment after cooling to a cooling stop temperature of 1, after cooling is stopped and reheating to a temperature of 200 ° C. or higher at the surface temperature. Method.

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