JP2001294983A - Rolled steel and its producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide rolled steel excellent in resistance to local buckling caused in the beam member of a steel structure and to destruction caused by welding defects in the beam edges or the like in case of a big earthquake. SOLUTION: This steel has a composition containing, by mass, 0.05 to 0.20% C, <=0.6% Si, 0.5 to 1.6% Mn, <=0.020% P, <=0.015% S and 0.01 to 0.05% Al, in which the content of Nb is controlled to <0.005%, N to <=0.0080% and H to <=4 ppm, and, further, Ceq is regulated to <=0.40%, and the balance substantially Fe with inevitable impurities, and the flange has a metallic structure of the mixed one composed of a ferritic phase as a soft phase and a hard phase containing a bainitic phase, in which the volume ratio of the soft phase is controlled to >50 to 80%, the average grain size to >=10 μm, and the aspect ratio of the hard phase is controlled to <=3, yield elongation to 0.8 to 3.0%, yield ratio which is the ratio between the upper yield point and tensile strength to <=70%, and the strain hardening index (n value) in nominal strain of 2 to 5% is controlled to >=0.20.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a rolled section steel, and more particularly, to a method for producing a steel beam having excellent local buckling resistance and fracture resistance suitable for a beam of a steel structure in a high-rise building. .

[0002]

2. Description of the Related Art In today's high-rise buildings, when a large earthquake is hit, a limit state design method that emphasizes human safety, which absorbs seismic energy by plastic deformation of a beam member and avoids a large collapse. Applied. In the event of a major earthquake,
A large tensile or compressive stress is applied to cause local buckling, and a crack may be generated from the buckled location, leading to collapse. Therefore, beam members used in the limit state design method are required to have excellent buckling resistance and toughness.

[0003] H-section steel, one of the rolled steel sections, can be manufactured stably in large quantities, and is widely used as a beam for construction and civil engineering in combination with its excellent economic efficiency.
Until now, Japanese Patent Application Laid-Open No. 5-25 / 1993
JP-A-588-58 and JP-A-5-345915 disclose a technique for improving earthquake resistance from the viewpoint of a low yield ratio. However, since local buckling of the beam member occurs on the low strain side which is far below the tensile strength of the material, the effectiveness of the yield ratio as an index of local buckling resistance has not been clarified. Also,
In addition to local buckling in the event of a large earthquake, it is unclear about the indicators related to beam end fracture, which is a problem.

On the other hand, as a method for producing a rolled H-section steel, a method for producing an H-section steel having a non-heat-treated, low yield ratio, and excellent toughness and weldability has been proposed in Japanese Patent Application Laid-Open No. Hei 6-240350. I have. Using Nb as an essential additive element, refine austenite grains during hot rolling to improve toughness, and after rolling, allow to cool until ferrite precipitates by 30% or more, t / 4 (t: plate thickness) Is cooled at a cooling rate of 0.3 ° C./s or more and 5 ° C./s or less to 600 ° C. or less to obtain a structure containing ferrite and bainite.

However, it is considered that the present technology is mainly intended for the production of an extremely thick H-section steel, and the essential element Nb
Is presumed to be added in order to secure the toughness of the center of the flange plate and the fillet portion, and does not cover a relatively thin flange thickness of 8 mm or more and 40 mm or less frequently used as a member for building beams.

[0006]

As described above, the material indexes such as the steel composition and microstructure for improving the local buckling resistance and the beam fracture resistance of the H-section steel are not sufficiently grasped. In addition, 8 mm or more and 40 mm often used as beam members
The following manufacturing method for relatively thin flange thickness is not disclosed.

Therefore, the present invention grasps the relationship between the local buckling resistance due to the compressive stress acting on the beam in the axial direction and the relationship between the beam fracture resistance and the material properties, and causes local buckling in the event of a large earthquake. Provided is a rolled section steel that is hard to break and hard to break at a beam end or the like, and has high performance of preventing a building structure from being collapsed.

[0008]

Means for Solving the Problems The present inventors have studied to understand the relationship between various characteristics obtained in a tensile test and the local buckling resistance and the beam fracture resistance. As a result, regarding the local buckling resistance, the buckling occurrence limit strain in the compression test of the H-section steel is not in a high strain region where a fracture occurs, but is approximately 2 to 2.
It was found that the strain was 5% nominal,
It was found that there is a good correlation with the work hardening index (n value) and the yield elongation at 歪 5% nominal strain.

Further, as a result of investigating H-section steels having variously changed flange and web properties, the local buckling resistance was as follows.
It was also found that the properties of the flange had a large effect, and the properties of the web had little effect.

[0010] The beam rupture resistance characteristics include the fact that the actual rupture is caused by stress concentration portions due to structural discontinuities in the beam end and the like,
Since the fracture was caused by welding defects, an analytical model was created assuming these, and the relationship with the material properties was investigated. The analysis model is a section A having a surface defect of length a and depth d.
′, A load P was applied in a direction perpendicular to the direction.

When the average stress .sigma.A 'of the section A' reaches a breaking limit stress .sigma.C (provided that .sigma.C = TS), the section A of the normal portion (all sections without surface notch) and the section A ' Force balance (σA = σA ′ × (1-ad /
Wt), (σA: stress at normal section, W: full width,
t: total thickness) and Swif's equation for the stress-strain curve: σ = (α / (1 + ε)) {β + ln (1 + ε)} ⁿ
(Where σ: nominal stress, ε: nominal strain, α, β: constant,
n: work hardening index), the following equation is obtained as a fracture condition equation.

(Α / (1 + ε)) ｛β + ln (1+
ε)｝ ⁿ = TS × (1-ad / Wt) Based on this equation, a tensile test piece having a surface defect is prepared by using a material in which material properties such as n value and yield ratio are changed in a wide range. A tensile test was performed. As a result, it was found that the fracture strain was arranged by the material properties and the defect size, and that the guideline was given as to the material factor for improving the allowable defect size.

The present invention has been made based on the above findings and further studied. That is, the present invention provides: In mass%, C: 0.05 to 0.20%, Si:
0.6% or less, Mn: 0.5 to 1.6%, Al: 0.0
1 to 0.05%, P: 0.020% or less, S: 0.01
5% or less, Nb: less than 0.005%, N: 0.
0080% or less, H: 4 ppm or less, and Ceq: 0.40% or less according to the formula (1), the balance being substantially Fe and unavoidable impurities. : 0.8-3.0%, yield ratio: 70% or less, and a rolled section steel having a work hardening index (n value) of 0.20 or more according to the equation (2) at a nominal strain of 2 to 5%.

Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 1 4 (1) n = (lnσ_Five−lnσ_Two) / (Lnε_Five−lnε_Two)... (2) σ_Five= 1.05 × σ_N5, Σ_Two= 1.02 × σ_N2, Ε_Five= Ln (1.05), ε_Two= Ln (1.02) where σ_Five: True stress at a nominal strain of 5%, σ_Two: Nominal distortion 2
True stress in%, ε _Five: True strain at a nominal strain of 5%,
ε_Two: True strain at a nominal strain of 2%, σ_N5: Nominal distortion 5%
Nominal stress at σ_N2: Nominal response at 2% nominal distortion
Force ln: natural logarithm 2. As a steel composition, further, Cu: 0.6% or less, N
i: 1% or less, Cr: 0.5% or less, one or more types
2. The rolled steel section according to 1, which contains:

3. V: 0.2% as steel composition
The rolled section steel according to 1 or 2, containing one or two kinds of Ti: 0.03% or less.

4. The metal structure of the flange is a mixed structure of a ferrite phase of a soft phase and a bainite phase of a hard phase, or a hard phase containing a bainite phase, wherein the volume fraction of the soft phase is more than 50% to 80% or less, and the average particle size is 10 μm or more. The rolled section steel according to any one of claims 1 to 3, wherein the aspect ratio of the hard phase is 3 or less.

5. A slab or a slab having the steel component according to any one of 1 to 3 is heated to 1050 to 1300 ° C or less, rolling is completed at an Ar3 point or more, and 600 to 700 at a cooling rate of 5 ° C / s or less. After cooling to ℃, the outer surface of the flange or both the outer and inner surfaces of the flange are 1/4 of the flange thickness
A method for producing a rolled section steel, wherein accelerated cooling is performed at a cooling rate of 5 ° C./s or more at a cooling rate of 550 ° C. and a cooling stop temperature is 550 ° C. or less.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the component composition, mechanical properties and production conditions specified in the present invention will be described in detail.

1. Ingredient composition C: 0.05 to 0.20% C is added in an amount of 0.05% or more in order to secure the strength of steel. However, if contained in a large amount exceeding 0.20%, toughness or weldability deteriorates. Therefore, 0.05 to 0.20% (0.
05% or more and 0.20% or less).

Si: 0.6% or less Si is inevitably contained in steel due to deoxidation and improves the strength, but if it exceeds 0.6%, the hardenability of the steel excessively increases, and the HAZ toughness increases. And 0.6% or less because it is not preferable from the viewpoint of weldability.

Mn: 0.5 to 1.6% Mn is added in an amount of 0.5% or more to improve the strength and toughness of the steel material and to suppress the production of FeS. If it exceeds 1.6%, the hardenability of the steel will increase, so that a hardened layer will appear during welding and crack susceptibility will deteriorate.
1.6%.

Al: 0.01-0.05% Al is an inexpensive and powerful deoxidizer, and is added in an amount of 0.01% or more. Since the toughness of the part is deteriorated, the content is set to 0.01 to 0.05%.

N: 0.0080% or less N is an unavoidable impurity contained in steel. When the content is increased, the HAZ toughness is deteriorated, the deterioration with time is promoted, or the generation of flaws in the continuously cast slab is promoted.

H: 4 ppm or less H is an unavoidable impurity contained in steel. If the content increases, cracking and delayed fracture occur after rolling.
pm or less.

P: 0.020% or less, S: 0.015%
Hereinafter, P and S are inevitably present as impurities mixed into steel. Since reduction of P is effective in preventing grain boundary destruction in HAZ, it is set to 0.020% or less. Since reduction of S is effective in preventing hydrogen cracking in HAZ, it is set to 0.015% or less.

Ceq: 0.40 or less Ceq (= C + Mn / 6 + Si / 24 + Ni / 40 + C
If (r / 5 + Mo / 4 + V / 14) exceeds 0.40, the toughness of the base material and the welded portion may be impaired.

In the present invention, in order to further improve the characteristics,
One, two or more of Cu, Ni, Cr, V, and Ti can be added.

One or more of Cu, Ni, and Cr Cu, Ni, and Cr strengthen steel by solid solution strengthening.
Further, Cu has an effect of improving weather resistance. But,
If added more than necessary, toughness and weldability are greatly reduced, so when added, Cu: 0.6% or less, Ni: 1%
Hereinafter, Cr: 0.5% or less.

One or two types of V and Ti V and Ti are strengthened by precipitation together with solid solution strengthening.
However, excessive addition lowers toughness or weldability, so if added, V: 0.2% or less, T
i: 0.03% or less.

The characteristics of the composition of the present invention are as follows:
In general, for the purpose of improving the strength and toughness of a steel material, it does not contain Nb, which is often added, and regulates it to less than 0.005% even when it is mixed as an unavoidable impurity. When Nb is contained, the unrecrystallized region in the hot rolling is expanded to a high temperature side. For example, in the rolling of a section steel having a flange thickness of 8 mm or more and 40 mm or less, due to a significant reduction in the unrecrystallized region, The austenite grain size is refined more than necessary, the final transformed structure is refined, and the yield point increases. Therefore, the n value at a nominal strain of 2 to 5% decreases, the buckling limit occurrence strain decreases, and the effect and effect intended by the present invention cannot be obtained.

2. Mechanical Properties In the present invention, tensile properties are defined as mechanical properties.

Yield elongation: 0.8 to 3.0% Yield elongation promotes deformation of the entire member in the process of local buckling, so that low strain buckling due to local buckling is suppressed. It increases the energy absorption of the steel as a whole, improves the local buckling resistance, and also improves the fracture resistance.

If it is less than 0.8%, the effect is insufficient, and if it is more than 3.0%, buckling may occur at the yielding stage without causing work hardening. 0.8 to 3.0%.

Yield ratio: 70% or less The yield ratio is specified because it has a strong influence on the fracture resistance. When the yield ratio is reduced, the strain at break increases. When the yield ratio is 70%, a steel material having a tensile strength of 500 MPa and an n value of 0.20 has a fracture strain due to a defect corresponding to 10% of the cross-sectional area of about 4.8%, which is an SN standard for low yield ratio steel for building. It exhibits excellent fracture resistance with respect to a fracture strain of about 2.5% at an upper limit yield ratio of 80%.

Therefore, in the present invention, the yield ratio is set to 70% or less. The yield ratio may be determined as a ratio between the upper yield point and the tensile strength according to the stress-strain diagram obtained in the tensile test.

N value: 0.20 or more The n value is specified because it affects the local buckling resistance and the fracture resistance. The buckling of the H-shaped steel beam defines the n value (work hardening index) in this range since the strain at the local buckling portion occurs at a nominal strain of 2 to 5%. When the n value is large, the hardened region of the strained portion is greatly expanded, the strain is distributed throughout the steel material, local deformation of the portion is suppressed, and the local buckling resistance is improved. Such an effect becomes remarkable when the n value at a nominal strain of 2 to 5% is 0.20. Therefore, in the present invention, the n value at a nominal strain of 2 to 5% is 0.20 or more.

Further, when the n value is increased, the breaking resistance is improved. Tensile strength: 500 MPa, surface defect cross-sectional area ratio: 3% In a steel material having a relatively small defect size, the fracture strain of a steel material having a yield ratio of 70% and an n value of 0.20 is about 10%. Has a yield ratio of 80% and an n value of 0.1.
The fracture strain of the steel material of No. 5 was about 6%, the fracture strain was reduced, and the fracture resistance was poor.

3. Microstructure Desirable metallographic structure for the H-beam for building beams according to the present invention is a soft phase of ferrite and a hard phase of bainite or a mixed structure of a hard phase containing bainite (for example, bainite + pearlite). With the mixed structure of the hard phase and the hard phase, the soft phase side is yielded at the interface between the soft phase and the hard phase, while increasing the overall strength of the hard phase and reducing the yield ratio.

The volume ratio of the soft phase is more than 50% and 80% or less. The volume ratio of the soft phase is set to 5% or less in the longitudinal tensile test of the flange because the yield ratio is 70% or less and the yield elongation is 0.8% or more.
Be more than 0%. On the other hand, if it exceeds 80%, the tensile strength becomes 4
Because it does not satisfy 90N / mm2 class, it exceeds 50%, 80
% Or less. The volume fraction was assumed to be equal to the ferrite area ratio observed in the two-dimensional cross section.

Average particle size of soft phase: 10 μm or more When the average particle size of the soft phase is less than 10 μm, the yield stress increases and the yield ratio increases, so that the average particle size is 10 μm or more.

Aspect ratio of hard phase: 3 or less When the aspect ratio of the hard phase exceeds 3, the yield elongation decreases, so that it is 3 or less. The aspect ratio was determined as a ratio of the length of each hard phase along the rolling direction to the length along the thickness direction in a cross section cut parallel to the flange rolling direction. The higher the aspect ratio, the more the structure extends in the rolling direction. 4. Manufacturing method Slab heating temperature: 1050 ° C. or more, 1300 ° C. or less When the slab heating temperature is less than 1050 ° C., the cross-sectional shape is deteriorated due to an increase in hot deformation resistance, and rolling cracks are generated. Insufficiency results in deterioration of characteristics. If the temperature exceeds 1300 ° C., the crystal grains become coarse and the toughness deteriorates.

Rolling end temperature: Ar3 point or higher If the rolling end temperature is lower than the Ar3 point, strain is accumulated in the ferrite phase, the yield point is raised, and the yield ratio is increased.
Further, in order to increase the aspect ratio of the hard phase and decrease the yield elongation, the Ar point is set to 3 or more. The Ar3 point is, for example,
Ar3 = 910-310C-80Mn-20Cu-15
Cr-55Ni-80Mo + 0.35 (t-8), where t is a plate thickness and a flange thickness (mm).

Cooling conditions Cooling is performed from the outer surface of the flange, or from both the outer surface and the inner surface. From the viewpoint of preventing distortion due to cooling and uniformity of the characteristics in the flange thickness direction, it is desirable to cool from both the inner and outer surfaces. After the rolling is completed, the cooling is performed at a low cooling rate to 700 ° C. or less and 600 ° C. or more, and then a two-stage cooling is performed at a high cooling rate to continuously cool to 550 ° C. or less.

The cooling at a low cooling rate including the natural cooling after the completion of the rolling is performed in order to obtain a uniform ferrite phase in the thickness direction and a sufficient amount of a ferrite phase and a hard phase having a sufficiently small aspect ratio. If the stop temperature is higher than 700 ° C., a sufficient amount of ferrite phase cannot be obtained as a transformed structure after water cooling, while if the stop temperature is lower than 600 ° C., pearlite transformation proceeds, and desired mechanical properties cannot be obtained. .

If the cooling rate exceeds 5 ° C./s, the structure in the thickness direction of the flange tends to be nonuniform and it is difficult to control the structure to a desired structure.
As described above, cooling at 5 ° C./s or less includes the case of natural cooling.

The subsequent cooling at a high cooling rate
If the cooling rate is lower than 5 ° C./s in order to obtain the hard phase, the hardness of the hard phase is insufficient and desired mechanical properties cannot be obtained. Cooling stop temperature is 55
If the temperature is higher than 0 ° C., the hardness of the hard phase becomes insufficient,
550 ° C. or lower. Although the lower limit of the cooling stop temperature is not particularly defined, if the temperature is lower than 200 ° C., the toughness may be impaired.

Incidentally, the flange thickness: t
2 (see FIG. 1) at (1/4) t2 on the outer surface side.

The flange thickness of the section steel in the present invention is 8 m.
m or more and 40 mm or less are most desirable for obtaining the effect. When the thickness is less than 8 mm, it is difficult to secure a good cross-sectional shape after rolling. When the thickness is more than 40 mm, it is difficult to control the metal structure to a desired metal structure at the center of the flange plate thickness. In addition, the strength is mainly targeted at a tensile strength of 490 N / mm 2 class from the desirable microstructure, but is not limited.

[0049]

EXAMPLES (Example 1) After smelting steel having the composition shown in Table 1, it was made into a slab by continuous casting. Steels A and B were subjected to finish-rolling and then water-cooled on the flange to obtain JIS G3136 SN490.
It is a component composition aimed at mechanical properties equivalent to steel B. Steel B has Nb added and is outside the scope of the present invention.

After the slab was heated to 1250 ° C., it was rolled at a rolling end temperature of 800 ° C. to obtain 300 × 300 × 10 × 15
(Mm) (H × B × t1 × t2, see FIG. 1).

After the completion of rolling, cooling was performed under cooling conditions shown in Table 2. The cooling method 1 is a method in which the water cooling start temperature is 780 ° C., the cooling stop temperature is 500 ° C., and then the mixture is naturally cooled to room temperature. After cooling is stopped, let it cool naturally to room temperature 2
It is a step cooling method. The cooling rate was 60 ° C./s at １ ／ of the flange thickness (t2) during water cooling in any cooling method. The cooling method 1 has a high water-cooling start temperature, which is outside the scope of the present invention.

After cooling, tensile test pieces and impact test pieces were collected from each of the manufactured H-section steels, and mechanical properties were examined. FIG.
Fig. 2 shows the approximate positions of the test specimens. In the tensile test, the rolling direction is set to the longitudinal direction from the position of 1/4 of the flange width direction,
JISZ2201 1A tensile test piece (parallel width: 40
mm, gauge length: 200 mm), and the tensile properties at room temperature and the n value at 2 to 5% were determined by nominal strain. Impact test is based on JISZ22 from center of thickness (1 / 2t)
Three impact strips of 02 V notch were collected, and the impact absorption energy was determined at a test temperature of 0 ° C.

Table 3 shows the results of the tensile test and the impact test.
All the samples have S in terms of tensile strength and impact properties.
Although the characteristics satisfying the standard of N490B are obtained, any one or both of the steel composition and the cooling method are out of the range of the present invention and are comparative examples. 11, 13, 1
In No. 4, the yield elongation, the yield ratio or the n value is out of the range of the present invention. Sample No. No. 12, excellent properties were obtained in both the steel composition and the cooling method within the scope of the present invention.

Next, in order to evaluate the local buckling resistance of these section steels, the strain causing buckling (buckling generation limit strain) was determined by a compression test of a section steel sample having a length of 500 mm. FIG. 2 schematically shows the state of the compression test. In order to evaluate the fracture resistance, the parallel portion was 10% of the cross section of the parallel portion.
Using a tensile test piece having a notch corresponding to, the breaking strain was measured by a tensile test. Table 4 shows the results of the compression test and the notched tensile test. Sample No. 1 in which both the steel composition and the cooling conditions are within the range of the present invention for both the buckling limit strain and the breaking strain. In the sample No. 12, a sample N as a comparative example
o. It has excellent characteristics with respect to 11, 13, and 14. The sample No. in these tests was used. The characteristics of No. 12 are superior to those obtained with the As roll material of 490 MPa grade steel. The buckling limit strain is 0.51 for As roll material of 490 MPa class steel.
%, Which is 0.77% even if it is improved by 50%. Less than 12. The breaking strain is 490MPa
In the case of As roll material of As-grade steel, it is 3.2%, and even if it is improved by 50%, it is 4.8%. Less than 12. As described above, in steel containing Nb and having a composition outside the range of the present invention, the yield elongation, the yield ratio, or the n value is out of the range of the present invention, and the local buckling resistance and the fracture resistance are reduced. Excellent characteristics cannot be obtained.

[0055]

[Table 1]

[0056]

[Table 2]

[0057]

[Table 3]

[0058]

[Table 4]

(Example 2) After smelting steel A shown in Table 1, it was made into a slab by continuous casting, heated to 1250 ° C, and then 300 × 30
It was rolled into a 0 × 10 × 15 (mm) H-section steel. After rolling,
Rolling and cooling were performed under the manufacturing conditions shown in Table 5. Thereafter, a tensile test piece and an impact test piece were collected from each H-shaped steel in the same manner as in Example 1, and the mechanical properties were examined. In the tensile test, the rolling direction is set to the longitudinal direction from the position of 1/4 of the flange width direction,
JISZ2201 1A tensile test piece (parallel width: 40
mm, gauge length: 200 mm), and the tensile properties at room temperature and the n value at 2 to 5% were determined by nominal strain. Impact test is based on JISZ22 from center of thickness (1 / 2t)
Three impact strips of 02 V notch were collected, and the impact absorption energy was determined at a test temperature of 0 ° C.

Table 6 shows the results of the tensile test and the impact test.
According to the production conditions within the scope of the present invention, 21, 22, 28
The sample No. which satisfies all the requirements of the present invention regarding the tensile properties, but is manufactured under manufacturing conditions outside the scope of the present invention. Any of 23 to 27, 29, and 30 fall outside the scope of the present invention. In addition, No. 1 of the present invention example. 21,
22,490 and Comparative Examples 24,26,29
Tensile strength satisfying the standard was obtained.

Next, No. 1 of the present invention, whose tensile strength satisfies the SN490B standard, was used. In order to evaluate the local buckling resistance of 21, 22, and 28 and Comparative Examples 24, 26, and 29, distortion that causes buckling (buckling generation limit strain) was performed by a compression test of a shaped steel sample having a length of 500 mm. I asked.
Further, in order to evaluate the fracture resistance, a tensile test piece having a notch corresponding to the parallel portion at 10% of the cross section of the parallel portion was used to measure the breaking strain by a tensile test. The compression test and the notched tensile test were the same as in Example 1. Table 7
Shows the results of the compression test and the notched tensile test. The buckling limit strain is the sample No. 1 in which all of the yield elongation, the yield ratio, and the n value satisfy the requirements of the present invention. Excellent characteristics were obtained in 21, 22, and 28. Also, Comparative Example Sample N
o. In Sample Nos. 24, 26, and 29, the yield elongation and the yield ratio were out of the range of the present invention, but the n value satisfied the definition of the present invention. 26 is sample No. 26; The buckling limit strain was better than those of Examples 24 and 29, and the same characteristics as those of the present invention were obtained. It was recognized that the influence of the n value on the local buckling resistance was large.

The breaking strain was measured for the sample No. of the present invention. 2
Sample Nos. 1, 22, and 28 are sample Nos. Of Comparative Examples. 24, 26,
29 is excellent. In all of the comparative examples, the yield ratio is 70% or more, but No. 2 having a relatively low yield ratio. In the case of the sample No. 26, it was recognized that the breaking strain was large and the yield ratio had a large effect, though it was inferior to the examples of the present invention.

As is clear from the above test results, when all of the yield elongation, the yield ratio and the n value satisfy the requirements of the present invention, excellent local buckling resistance and fracture resistance are obtained.

[0064]

[Table 5]

[0065]

[Table 6]

[0066]

[Table 7]

(Example 3) Steel having the component composition shown in Table 1 was melted and then cast into a slab by continuous casting. After the slab was heated to 1250 ° C, the rolling was completed at a rolling end temperature of 800 ° C.
An H-section steel of 300 × 10 × 15 (mm) (H × B × t1 × t2, see FIG. 1) was used. After the completion of the rolling, it was manufactured under the rolling and cooling conditions shown in Table 8. Sample No. 33 is
After the end of the rolling, it was naturally cooled to room temperature. The cooling rate of the natural cooling is about 0.8 ° C./800 to 500 ° C. on average.
s.

The effects of the microstructure on the tensile properties, impact properties, local buckling resistance, and fracture resistance of these section steels were investigated. The microstructure is 1/4 in the flange width direction
The optical microscopic structure and the scanning electron microscopic structure at the center of the flange thickness were observed in the cross section along the longitudinal direction at the position of, and the volume fraction of ferrite and the average particle size were determined by the line segment method.
Regarding the hard phase, the structure form, the length in the rolling direction and the length in the sheet thickness direction were determined, and the aspect ratio was calculated as a value of the ratio.

Table 9 shows the observation results of the microstructure. Sample No. whose steel composition and manufacturing conditions are within the scope of the present invention. 31,3
Sample Nos. 2 and 38 satisfy all the requirements of the present invention regarding the microstructure. 33, 34, 35, 36, 3
7, 39, 40 do not satisfy any of the provisions of the present invention relating to microstructure.

Next, in the same manner as in Examples 1 and 2, a tensile test
A Charpy impact test was performed. Table 10 shows the results. Sample No. does not satisfy any of the provisions of the present invention regarding the microstructure. 33, 34, 35, 36, 3
In Nos. 7, 39 and 40, any one of the yield elongation, the yield ratio and the n value is out of the range of the present invention. On the other hand, Sample No. Nos. 31, 32 and 38 satisfy the requirements of the present invention regarding the microstructure, and all of the yield elongation, the yield ratio, and the n value fall within the scope of the present invention.

Further, Sample No. having a tensile strength of 490 MPa or more. With respect to 31, 32, 34, 35, 37, and 38, the local buckling resistance and the fracture resistance were determined in the same manner as in Examples 1 and 2.

Table 11 shows the measured values of buckling limit strain and rupture strain. Sample No. having the tissue morphology defined in the present invention. In the samples 31, 32 and 38, excellent characteristics were obtained in both cases, whereas in the samples not satisfying the requirements for the soft phase and hard phase, one or both of the characteristics were inferior.

As described above, the microstructure obtained from the rolled H-section steel having the steel composition and the manufacturing conditions satisfying the requirements of the present invention exhibited characteristics excellent in local buckling resistance and fracture resistance.

[0074]

[Table 8]

[0075]

[Table 9]

[0076]

[Table 10]

[0077]

[Table 11]

[0078]

As described above, the rolled section steel of the present invention is
In the event of a major earthquake, a large tensile force applied to the building beam members,
It is hard to cause local buckling caused by compression, and has excellent resistance to fracture due to cracks in buckled parts and welding defects at beam ends, so in the event of a large earthquake, the seismic energy is absorbed by plastic deformation of the beam members It is most suitable for the critical state design method to avoid large collapse and provides a steel structure that is safe against a large earthquake, and its effect is extremely large in industrial society.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a sampling position of a tensile test specimen of a rolled H-section steel and an observation position of a microstructure.

FIG. 2 is a diagram showing an outline of a compression test for evaluating local buckling resistance.

FIG. 3 is a view showing a tensile test piece (unit: mm) with a surface notch for evaluating fracture resistance.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroyuki Kado 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Masayoshi Kurihara 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Sun F-term (reference) in this steel pipe Co., Ltd.

Claims (5)

[Claims] 1. A mass% of C: 0.05 to 0.20%,
Si: 0.6% or less, Mn: 0.5 to 1.6%, Al:
0.01-0.05%, P: 0.020% or less, S:
0.015% or less, Nb: less than 0.005%,
N: regulated to 0.0080% or less, H: regulated to 4 ppm or less,
Further, when Ceq according to the equation (1) is 0.40% or less, the balance is actually
Qualitatively made of Fe and unavoidable impurities, flange length
Tensile properties in the hand direction are as follows: yield elongation: 0.8-3.0%, yield
Yield ratio: 70% or less, and at a nominal strain of 2 to 5%
When the work hardening index (n value) according to the equation (2) is 0.20 or more
Some rolled steel sections. Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 1 4 (1) n = (lnσ_Five−lnσ_Two) / (Lnε_Five−lnε_Two)... (2) σ_Five= 1.05 × σ_N5, Σ_Two= 1.02 × σ_N2, Ε_Five= Ln (1.05), ε_Two= Ln (1.02) where σ_Five: True stress at a nominal strain of 5%, σ_Two: Nominal distortion 2
True stress in%, ε _Five: True strain at a nominal strain of 5%,
ε_Two: True strain at a nominal strain of 2%, σ_N5: Nominal distortion 5%
Nominal stress at σ_N2: Nominal response at 2% nominal distortion
Force ln: natural logarithm 2. The rolled section steel according to claim 1, wherein the steel composition further contains one or more of Cu: 0.6% or less, Ni: 1% or less, and Cr: 0.5% or less. 3. The rolled section steel according to claim 1, wherein the steel composition further contains one or two of V: 0.2% or less and Ti: 0.03% or less. 4. The metal structure of the flange is a mixed structure comprising a ferrite phase as a soft phase and a bainite phase as a hard phase, or a hard phase containing a bainite phase, and the soft phase has a volume fraction of 50.
%, The average particle size is 10 μm or more, and the aspect ratio of the hard phase is 3 or less.
4. A rolled section steel according to any one of claims 1 to 3. 5. A slab or a slab having the steel component according to any one of claims 1 to 3, which is heated to 1,050 to 1,300 ° C. or less, rolling is completed at an Ar3 point or more, and 5 ° C./s or less. After cooling at a cooling rate of 600 to 700 ° C., the outer surface of the flange or both of the outer surface and the inner surface of the flange are reduced to 1 / th of the flange thickness.
4. A method for producing a rolled section steel, wherein accelerated cooling is performed at a cooling rate of 5 ° C./s or more at a cooling rate of 4 and a cooling stop temperature is 550 ° C. or less.