JP3371712B2 - Manufacturing method of earthquake resistant building steel with excellent fire resistance - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the field of construction designed with emphasis on seismic resistance, and more particularly to a method of manufacturing seismic resistant building steel material having excellent fire resistance, which is mainly used for important structures near active faults.

[0002]

2. Description of the Related Art The seismic design method for buildings, which was revised and implemented in 1981, absorbs earthquake input energy by utilizing the deformability in the plastic region until the steel material reaches its maximum strength after yielding. In order to ensure the seismic safety of a building, the elastic design of retaining the stress level generated in each part of the structure within the yield point of steel has been changed significantly. From this, the steel materials for buildings to which the new seismic design method is applied are
Yield ratio (Y
R) is low, that is, a low yield ratio is required.

The steel material of TS490 MPa class is subjected to hot rolling in a recrystallization region to coarsen the structure and secure a low yield ratio. Further, in the case of high-strength steel of TS570 MPa class or higher, by quenching from a two-phase region of ferrite-austenite, a low yield ratio is secured by forming a two-phase structure of ferrite and bainite or martensite.

The Great Hanshin Earthquake of January 1995 was an active fault type earthquake whose epicenter was very close, unlike an ocean type earthquake in which the hypothesis assumed by the above seismic design method was far away. It was In the case of an active fault type earthquake, the shaking speed is very fast, and the strain rate of the building is 10 ^-1 to 10 /
It has the feature that high-speed deformation of seconds is added. The current building steel has low YR as described above, but it is a value when pulled at a normal strain rate of 10 ^-2 / sec or so, and in the case of deformation at a high strain rate as described above, low YR. Was wondering what to show. When the present inventors conducted a tensile test while changing the strain rate for a conventional as-rolled (ferrite + pearlite structure) SN490 grade steel, when the strain rate was around 10 ^-2 / sec, YR <80% was obtained. There was a strain rate of 10 /
It was found that YR greatly increased and reached a value of 80% or more in the case of about 2 seconds.

Further, in the Great Hanshin Earthquake, there have been cases in which structural members become brittle due to high-speed cyclic plastic deformation and become brittle when subjected to the next tensile deformation. Since brittle fracture can lead to major collapse of buildings, it is a fracture mode that must be avoided for seismic steel. Conventional SN
The 490 grade steel also has sufficient toughness that the brittle-ductile fracture surface transition temperature is room temperature or less when not subjected to prestrain, but to what extent when high-speed cyclic prestrain is applied. It was unclear whether or not it showed deterioration. We use the conventional S
Strain increase type of positive and negative steel plates at a strain rate of 10 / sec for several types of N490 grade steel (1% compressive plastic strain application → 1% tensile plastic strain application → 2% compressive plastic strain application → 2% tensile plastic strain application → 4%
Compressive plastic strain application → 4% tensile plastic strain application, and after applying a prestrain of ± 1 + 2 + 4%), a Charpy impact test was conducted, and the brittle-ductile fracture surface transition temperature was above room temperature. Something that has become.

Among the conventional construction steel materials, Japanese Patent Laid-Open No.
There is a low yield ratio steel excellent in low temperature toughness such as Japanese Patent Laid-Open No. 197522 and Japanese Patent Laid-Open No. 5-21440.
However, both of them perform only a tensile test at a normal strain rate, and the YR value at a high strain rate is not shown. Further, the toughness is also a Charpy impact test value when there is no prestrain, and the toughness value after the prestrain is applied is unknown.
Therefore, the present inventors prototyped steels according to both of the above proposals, and obtained tensile properties of these prototype steels at high strain rates (= 10 / sec) and ± 1 + 2 + 4% at high strain rates (= 10 / sec). The toughness after repeated prestraining was investigated. As a result, the YR of the tensile test at a high strain rate (= 10 / sec) was 80.
The value exceeded%. Further, the toughness after high-speed cyclic pre-strain was uneven, and among them, vE-5 <20J, which was markedly brittle, was observed. In other words, it was found that in the case of an active fault type earthquake, it does not have sufficient earthquake resistance.

Further, with respect to the fire of the building, the review of the fire resistant design has made it possible to reduce the fire resistant coating by using the fire resistant steel excellent in high temperature strength. Since the use of refractory steel leads to shortening the construction period, reducing the construction cost, and expanding the effective area in the building, such new design methods are becoming popular. Japanese Patent Application Laid-Open No. 4-83 discloses a steel material for construction having a low yield ratio and fire resistance.
821, JP-A-4-56723, and JP-A-4
No. 56362 is proposed. Therefore, the present inventors prototyped steels according to both of the above proposals, and regarding these prototype steels, the invention steels described above (Japanese Patent Laid-Open No. 2-197522).
The tensile properties at a high strain rate were examined in the same manner as in Japanese Patent Application Laid-Open No. Hei 5-214440. As a result, high strain rate (= 1
It was found that the yield ratio at 0 / sec) was 80% or more.

[0008]

From the above, the problem to be solved by the present invention is to exhibit low YR (≤80%) even when subjected to deformation at a high strain rate, and to repeat at a high strain rate. Stable and excellent toughness even after being subjected to pre-strain, and also excellent in fire resistance, which enables a design method that combines a fire resistant design with a plastic seismic design of a structure near an active fault. A method for manufacturing earthquake-resistant steel material is provided.

[0009]

Means for Solving the Problems In order to solve this problem, the present inventors have made extensive studies on the relationship between the microstructure and YR at a high strain rate, and have found the following important findings. First, FIG. 1 shows the relationship between the tensile strain rate and the yield ratio (= yield strength / tensile strength) using the A1 to A3 steel sheets in Table 2 as test materials, and “α” in the figure is Ferrite abbreviation "B" is an abbreviation for bainite, "P" is an abbreviation for pearlite, "coarse grain α + B" in the figure is Al steel plate, "α + P"
Is an A2 steel plate, and "fine grain α + B" is an A3 steel plate. As can be seen from the figure, the YR value increases as the strain rate in the tensile test increases. However, the degree of increase is lower in the ferrite + bainite structure than in the ferrite + pearlite structure. It was found that, in the ferrite + bainite structure, the coarser the ferrite particles, the higher the strain rate (> 0.1 / sec), the lower the YR value. YR <8 even at a strain rate of 10 / sec by using a mixed structure of coarse-grained ferrite and bainite.
0% or less is achieved. In the present invention, coarse-grained ferrite means ASTM grain size No. Say 11 or less.

FIG. 2 shows that in the same steel type as steel A, only oxygen is 19
By using the steel changed in the range of ~ 44 ppm as the test steel,
The effect of oxygen content on vE-5 after cyclic prestrain of coarse grain ferrite (ASTM grain size No. = 9 to 11) and bainite mixed structure with high strain rate (= 10 / sec) was investigated. Is. The toughness after repeated pre-strain of high strain rate (= 10 / sec) has a considerable variation as shown in FIG. 2, but the lower limit is controlled by the oxygen content, By reducing the amount to 30 ppm or less, vE
It was found that stable toughness satisfying -5 (minimum)> 100J was obtained. It has an oxygen content of 30
This is because the oxide content in the steel, which serves as a micro strain concentration source at the time of the high-speed cyclic strain, is reduced and refined by setting the content to be ppm or less.

Furthermore, the present inventors have found that (Mo + 3.5V
+ 20Nb) 50kg class steel grade with different amount,
A steel having a mixed structure of coarse-grained ferrite and bainite is subjected to a tensile test at a high strain rate (= 10 / sec) at room temperature and a strain rate (= 0.3%) specified in JIS G0567 at 600 ° C. / Min) was also tested. The result is shown in FIG. (Mo + 3.5V + 20Nb) amount is 0.
When it is less than 12%, the high temperature strength (0.2% yield strength) does not satisfy 2/3 (target value) of YS at room temperature. Also, 0.8
If the amount of (Mo + 3.5V + 20Nb) is increased to exceed 80%, the YR value at a high strain rate exceeds 80%. Therefore, the amount of (Mo + 3.5V + 20Nb) is 0.12%.
It is limited to 0.8% or less.

From the above, low YR (≤80%) even when subjected to deformation at high strain rate, stable and excellent toughness after repeated prestrain at high strain rate, and The requirement of seismic resistant steel material that enables the design that combines the plastic seismic design and the fire resistant design of the structure near the active fault with excellent fire resistance is 0.12% ≦ (Mo + 3.5V + 20Nb)%
It was found that the content of ≦ 0.8% was satisfied, the oxygen content was 30 ppm or less, and it had a characteristic of a mixed structure of coarse-grain ferrite and bainite.

The present invention has been made based on these findings. (1) C: 0.04 to 0.18% by weight ratio,
Si: 0.05-0.4%, Mn: 0.6-1.7%,
Mo: 0.1-0.6%, V: 0.005-0.1%,
Al: 0.001 to 0.06%, N ≦ 30 ppm, O ≦
30 ppm and 0.12% ≦ (Mo + 3.5V)%
Satisfy ≦ 0.8%, after hot rolling the steel balance of Fe and unavoidable impurities in the austenite region, then water cooling from after Ar3 point, to stop the water cooling at 650 ° C. or less 400 ° C. or higher, tissue ASTM grain size No. = 9-11 coarse-grained ferrite and bainite two-phase structure is used, the manufacturing method of the earthquake-resistant building steel material excellent in fire resistance.

(2) C: 0.04 to 0.18 by weight ratio
%, Si: 0.05 to 0.4%, Mn: 0.6 to 1.7
%, Mo: 0.1 to 0.6%, V: 0.005 to 0.1
%, Al: 0.001 to 0.06%, N ≦ 30 ppm,
O ≦ 30 ppm and 0.12% ≦ (Mo + 3.5
V)% ≦ 0.8%, in addition to Cu: 0.
05-0.6%, Ni: 0.05-0.6%, Cr:
0.05-1.0%, Ti: 0.005-0.015%
Steel containing 1 or 2 or more of them, with the balance being Fe and unavoidable impurities, is hot-rolled in the austenite region, water-cooled after the Ar3 point, and 650 ° C or less 400 ° C
With the above, the water cooling is stopped and the structure is set to ASTM grain size No. = 9
A method for producing an earthquake-resistant building steel material having excellent fire resistance, characterized by having a two-phase structure of coarse-grained ferrite and bainite of No. 11 to No. 11 .

(3) The steel contains Nb: 0.005 to 0.04% in addition to the above-mentioned components (1) or (2), and 0.12% ≦ (Mo + 3.5V + 20Nb).
% Is less than 0.8%, which is a method for producing an earthquake-resistant building steel material having excellent fire resistance.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Next, the reason for adding each component of the steel material according to the present invention and the reason for limiting the addition amount will be explained.
C, Si, Mn, and Al are limited as follows based on the relationship of action and effect that has been conventionally confirmed in order to obtain a required material in ordinary welded structural steel.

C is the cheapest element and is an element effective for increasing the strength, but if added in excess of 0.18%, the weldability is remarkably reduced. If it is less than 0.04%, the material is thick and the strength is insufficient, and it is necessary to add a large amount of alloying elements, resulting in high cost. Therefore, C is limited to 0.04% or more and 0.18% or less.

Si is an element necessary for the strength of steel materials and preliminary deoxidation of molten steel. For preliminary deoxidation, addition of 0.05% or more is necessary. Excessive addition over 0.4%
It deteriorates the toughness of steel materials and the weld HAZ toughness. Therefore, the amount of Si is limited to 0.05% or more and 0.4% or less.

Mn is a necessary element for ensuring the strength of the base material. If it is less than 0.6%, the material is thick and the strength is insufficient, so that it is necessary to add a large amount of alloying elements, resulting in high cost. Further, Mn is an element that tends to segregate in the center. 1.7
If added in excess of%, the center of the plate thickness becomes significantly brittle. Therefore, the range of Mn is limited to 0.6% or more and 1.7% or less.

Mo is an element effective in improving high temperature strength in steel. In order to exert such effects, 0.1
% Or more must be added. Addition of Mo in excess of 0.6% significantly increases the yield ratio. Therefore, Mo is limited to 0.1% or more and 0.6% or less.

Nb and V are elements effective for increasing the strength at normal temperature and high temperature when added in a small amount. Nb <0.005%, V
At <0.005%, no clear strength increasing effect is observed. Addition of Nb in excess of 0.04% and addition of V in excess of 0.1% significantly increase the yield ratio. Therefore,
Nb 0.005% to 0.04%, V 0.00
It was limited to 5% or more and 0.1% or less.

Al is an element necessary for deoxidation. If the Al content is less than 0.001%, a sufficient deoxidizing effect cannot be expected. Also, if added in excess of 0.06%,
Scratches are likely to occur on the surface of continuously cast slabs. Therefore, the Al amount is limited to 0.001% or more and 0.06% or less.

N exists as solid solution N or nitride inclusions in the solid steel. Solid solution N and coarse nitride-based inclusions deteriorate the toughness of steel. When N is contained in excess of 30 ppm, solid solution N exists. Further, coarse nitrides (for example, TiN and NbN) are likely to be generated in the final solidified portion, and excellent toughness cannot be obtained. Therefore, N content is 30p
It was regulated below pm.

O has an oxygen content of 3 as described above.
This is because by setting the content to 0 ppm or less, the oxide in the steel, which is a micro strain concentration source, is reduced and refined when a high-speed cyclic pre-strain is applied, and when it exceeds 30 ppm, vE-5 (mini
Stable toughness satisfying mum)> 100J cannot be obtained.

As described above, when the content of (Mo + 3.5V + 20Nb) is less than 0.12%, the high temperature strength (0.
2% proof stress) does not satisfy 2/3 (target value) of YS at room temperature. In addition, the amount of (Mo + 3.5V + 20Nb) is 0.8
When it exceeds%, the YR value at high strain rate exceeds 80%. Therefore, the amount of (Mo + 3.5V + 20Nb) is 0.
It is limited to 12% or more and 0.8 or less. When Nb is not added, the amount of (Mo + 3.5V + 20Nb) is (Mo +
3.5V) amount.

Cu, Ni and Cr contribute to higher strength through solid solution strengthening and hardenability improving effects. Cu <0.05
%, Ni <0.05%, Cr <0.05%, no clear strength increasing effect is observed. Addition of Cu in excess of 0.6% significantly increases the risk of Cu cracking. Ni
Is an expensive element, and the upper limit is 0.6% from the viewpoint of cost.
And Addition of Cr in excess of 1% significantly deteriorates weldability. Therefore, Cu is 0.05% to 0.6%, Ni is 0.05% to 0.6%, and Cr is 0.0%.
It was limited to 5% or more and 1% or less.

Ti is an element that suppresses the coarsening of the structure of the welded HAZ portion of TiN and contributes to the improvement of HAZ toughness.
If the Ti content is less than 0.005%, the effect of improving the HAZ toughness is not exhibited. If added in excess of 0.015%, TiC precipitates during the cooling process of welding, resulting in deterioration of HAZ toughness. Therefore, Ti is 0.005% or more, 0.015
% Or less.

P and S have no direct relation to the earthquake resistance targeted by the present patent, but are preferably low in terms of weldability and ductility in the plate thickness direction. From the viewpoint of inclusion morphology control, it is desirable to add an appropriate amount of Ca or REM.

The microstructure of the steel material according to the present invention has the ASTM grain size No. = 9 to 11 is a mixed structure of coarse-grained ferrite and bainite. The reason why this mixed structure is used is that a low YR value can be obtained at a high strain rate (> 0.1 / sec).

In the method of the present invention, in order to obtain this microstructure, a steel material is manufactured under the following manufacturing conditions. First, steel satisfying the above composition range is hot-rolled in the austenite region.
The reason for hot rolling in the austenite region is that when it is rolled in the ferrite region, work hardening occurs and low YR cannot be obtained at a normal strain rate (around 10 −2 / sec). Then,
Water cooling after Ar3 point. The reason for water cooling after the Ar3 point is that when accelerated cooling from the austenite region, ferrite is difficult to obtain unless the cooling rate is controlled according to the hardenability of the steel, but it is allowed to cool until after the Ar3 point passes. If some ferrite is precipitated and then accelerated cooling is performed,
This is because low YR can be obtained in a very wide cooling rate range. Fig. 4 shows the YR when using A steel as the test steel, when accelerated cooling from the austenite region, and when left to cool until after the Ar3 point after rolling and accelerated cooling from the two-phase region where some ferrite was precipitated. The relationship between the cooling rates is shown. In the latter case, low TR is obtained in a very wide cooling rate range.
From the microstructure observation, it was found that in the latter case, a mixed structure of ferrite and bainite was obtained in a wide cooling rate range. Then 650 ℃ or less 400
Water cooling is stopped at a temperature of ℃ or more, and the structure becomes a two-phase structure of ferrite and bainite. The reason for limiting to this temperature range is
When the stopping temperature is higher than 650 ° C, the structure becomes a ferrite + pearlite structure, and when the stopping temperature is lower than 400 ° C, martensite is mixed and the toughness is significantly deteriorated. This is because it ends up. By performing such processing, the steel of the tissue ASTM
Particle size No. A mixed structure of coarse-grained ferrite and bainite of 9 to 11 can be obtained.

[0031]

EXAMPLES Examples of the present invention will be described below. Table 1, Table 2
Shows the chemical composition of the test steel. Steels A to N have a steel composition within the scope of the present invention, and steels OY have a steel composition outside the scope of the present invention. Here, steels J, K, L, S, V, and X are TS570.
The MPa class and the steels M, N, and Y are TS400 MPa class steels, and the others are TS490 MPa class steels. all,
It was made into a slab by continuous casting including a light reduction process. The above steel was used as a steel plate under the manufacturing conditions shown in Table 3. Tables 4 and 5 show the microstructure of the obtained steel sheet, normal strain rate (= 0.01 / sec), normal temperature tensile properties at high strain rate (= 10 / sec), high temperature strength, and prestrain. The results of the Charpy impact test are also shown in the case of none, and after the repeated prestrain of ± 1 + 2 + 4% is applied at a high strain rate (= 10 / sec).

The normal temperature tensile test piece is a bar test piece of 12 mm square × 100 mm square parallel part length, which is sampled in the C direction from 1/4 t. A tensile test was performed on this test piece with a servo type tester at a stroke speed of 1 m / sec, that is, a strain rate of 10 / sec. In addition, the test piece is the same as above with a stroke speed of 1 m /
Seconds, that is, strain rate of 10 / sec, compressive plastic deformation of 1% →
1% tensile plastic deformation → 2% compressive plastic deformation → 2% tensile plastic deformation → 4% compressive plastic deformation → 4% tensile plastic deformation After repeated prestraining, a Charpy impact test piece was taken. , VTs and vE-5 were measured. 9 at -5 ° C
The Charpy impact test of the book was implemented, and the average value and the minimum value were calculated. Further, the high temperature tensile test piece is a 10φ × 50GL round bar test piece taken in the C direction from ¼t.

As shown in Table 4, 0.12% ≦ (Mo + 3.5V + 20Nb) ≦ 0.8 when the oxygen content is 30 ppm or less.
% To satisfy coarse grain ferrite (ASTM No. 9 to 1).
1) and bainite mixed structure (bainite ratio 40-70
%) Of the present invention (A1, B1, C1, D1, E)
1, F1, G1, H1, I1, J1, K1, L1, M
1, N1) has a YR at a high strain rate of 80% or less, high temperature strength satisfies the target value, and has a toughness of vJ-5 of 150 J or more even after repeated strain. A3, C2, M2, which is a ferrite + pearlite structure, shows a remarkable increase in YR at a high strain rate compared to a normal tensile test, and is 80%.
The value exceeds. Moreover, the toughness of these steels deteriorates significantly after repeated prestrain at high strain rate, and vT near room temperature
s is shown. Fine-grained ferrite (ASTM No. 1
A2, E2, J which is a mixed structure of bainite
The steel sheet No. 2 has a high strain rate tensile test YR of more than 80%. Furthermore, the B2, F2 steel sheet having a mixed structure of coarse-grained ferrite, bainite, and martensite has a YR of 8 in a tensile test at a normal strain rate and a tensile test at a high strain rate.
When it exceeds 0%, the toughness is low. Further, even if the structure is a mixed structure of coarse-grained ferrite and bainite (Mo +
3.5V + 20Nb) <0.12% O1, P1,
The high temperature strength of Q1 and R1 does not satisfy the target value. The structure is a mixed structure of coarse-grained ferrite and bainite.
3.5V + 20Nb)> 0.8% S1, T1, U
No. 1 had a YR at a high strain rate of more than 80%. The structure is a mixed structure of coarse-grained ferrite and bainite, and the oxygen content exceeds 30 ppm. For V1, W1, X1 and Y1, the minimum value of vE-5 after repeated pre-strain at high strain rate is less than 47J. .

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

[Table 4]

[0038]

[Table 5]

[0039]

As is clear from the above examples, the steel material produced by the method of the present invention exhibits low YR (≤80%) even when subjected to deformation at a high strain rate, and repeats at a high strain rate. It exhibits stable toughness even after being subjected to pre-strain, and also exhibits excellent fire resistance, enabling a design that combines the plastic seismic design and fire resistant design of the structure near the active fault. Also, mass production of steel materials is possible.

[Brief description of drawings]

Figure 1: Tensile strain rate and yield ratio (= yield strength / tensile strength)
FIG.

[Fig. 2] Cyclic plastic strain (± 1) at high oxygen content and high strain rate
The figure which showed the relationship of the Charpy impact absorption energy (vE-5) tested at -5 degreeC after giving + 2 + 4%).

FIG. 3 is a diagram showing the relationship between the (Mo + 3.5V + 20Nb) amount, the yield ratio at high strain rate (= 10 / sec), and the high temperature strength (0.2% proof stress at 600 ° C.).

FIG. 4 is a diagram showing a relationship between YR and a cooling rate at room temperature.

Continuation of front page (56) Reference JP-A-9-170044 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 8/00-8/10 C22C 38/00-38 / 60

Claims (3)

(57) [Claims] 1. A weight ratio of C: 0.04 to 0.18%, S
i: 0.05 to 0.4%, Mn: 0.6 to 1.7%, M
o: 0.1-0.6%, V: 0.005-0.1%, A
1: 0.001 to 0.06%, N ≦ 30 ppm, O ≦ 3
0 ppm and 0.12% ≦ (Mo + 3.5V)% ≦
Steel satisfying 0.8% and the balance being Fe and unavoidable impurities is hot-rolled in the austenite region, water-cooled after the Ar3 point has passed, and water-cooling is stopped below 650 ° C and above 400 ° C to form a microstructure . ASTM grain size No. = 9-11 coarse-grained ferrite and bainite two-phase structure is used, the manufacturing method of the earthquake-resistant building steel material excellent in fire resistance. 2. A weight ratio of C: 0.04 to 0.18%, S
i: 0.05 to 0.4%, Mn: 0.6 to 1.7%, M
o: 0.1-0.6%, V: 0.005-0.1%, A
1: 0.001 to 0.06%, N ≦ 30 ppm, O ≦ 3
0 ppm and 0.12% ≦ (Mo + 3.5V)% ≦
In addition to satisfying 0.8%, Cu: 0.05-
0.6%, Ni: 0.05 to 0.6%, Cr: 0.05
~ 1.0%, Ti: 0.005 to 0.015%, 1
Steel containing at least one kind or two or more kinds, with the balance being Fe and unavoidable impurities, is hot-rolled in the austenite region, and then Ar
After 3 points have passed, water cooling is performed, and water cooling is stopped at 650 ° C or lower and 400 ° C or higher, and the structure is set to ASTM grain size No. = 9 to 11 A method for producing a seismic resistant building steel material having excellent fire resistance, characterized by having a two-phase structure of coarse-grained ferrite and bainite. 3. The steel further contains Nb: 0.005 to 0.04% in addition to the components described in claim 1 or 2, and 0.12% ≦ (Mo + 3.5V + 20Nb).
% Of 0.8% is satisfied, The manufacturing method of the earthquake-resistant building steel material excellent in fire resistance characterized by the above-mentioned.