JP3371699B2 - 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 keeping the stress level in each part of the structure within the yield point of steel has been changed.
From this, steel materials for buildings to which the new seismic design method is applied are required to have a low yield ratio (YR), which is a parameter indicating the deformation performance after yield, that is, a low yield ratio.

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 a steel 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. It was unclear whether to indicate. Therefore, when the present inventors conducted a tensile test while changing the strain rate for the conventional as-rolled (ferrite + pearlite structure) SN490 grade steel, the strain rate was 10 ^-2.
It was found that YR <80% when the strain rate was around 10 sec / sec, but the YR greatly increased to a value of 80% or higher when the strain rate was around 10 sec / sec.

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
For several types of N490 grade steel, positive and negative alternating strain increasing type at a strain rate of 10 / sec (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 technologies for building steel materials is Japanese Patent Laid-Open No. 2-19.
There is a low yield ratio steel excellent in low temperature toughness such as Japanese Patent No. 7522 and Japanese Patent Laid-Open No. 5-21440. However,
In both cases, only a tensile test was performed at a normal strain rate, and the YR value at a high strain rate was 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, high strain rate (=
The YR in the tensile test at 10 / sec) was a value exceeding 80%. Further, the toughness after high-speed cyclic pre-strain was uneven, and among them, vE-5 <20J, which was markedly brittle, was observed. That is, it was found that these steels do not have sufficient seismic resistance in the case of active fault type earthquakes.

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. 4-83821 discloses a steel material for construction having both a low yield ratio and fire resistance.
Japanese Patent Application Laid-Open No. 4-56723, Japanese Patent Application Laid-Open No. 4-56362, and the like are proposed. Therefore, the present inventors prototyped steels in accordance with both of the above proposals, and these prototyped steels were also manufactured in the same manner as the above-mentioned invention steels (JP-A-2-197522 and JP-A-5-21440). The tensile properties at strain rate were investigated. As a result, it was found that the yield ratio at high strain rate (= 10 / 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 tensile strain rate and the yield ratio (= yield strength / yield strength /
In the figure, "α" is an abbreviation for ferrite, "B" is an abbreviation for bainite, "tempering M" is an abbreviation for tempered martensite, and P is an abbreviation for pearlite. "Coarse grain α + B + tempered M (●)" is A
l steel plate, "fine grain α + B + tempered M (○)" is A2 steel plate,
“Α + P (Δ)” is an A3 steel plate. As can be seen from the figure, the YR value increases in each case as the strain rate in the tensile test increases. However, the degree of increase is lower in the ferrite + bainite + tempered martensite mixed structure (●, ◯) than in the ferrite + pearlite structure (△). Further, in the mixed structure of ferrite + bainite + tempered martensite, the coarser the ferrite (●),
It was found that low YR values were obtained at high strain rates (> 0.1 / sec). By using a mixed structure of coarse-grained ferrite, bainite, and tempered martensite
R <80% 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
Using a steel that has been changed in the range of ~ 43 ppm as the test steel,
Of the oxygen content on vE-5 after cyclic prestrain of high strain rate (= 10 / sec) was applied to the mixed structure of coarse-grained ferrite (ASTM grain size No. = 9 to 11), bainite and tempered martensite. This is a study of the effects. The toughness after repeated prestrain 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, Quantity 30
vE-5 (minimum)> 100J by reducing to below ppm
It was found that stable toughness satisfying the above conditions was obtained. This is because by setting the oxygen content to 30 ppm or less, the oxide in the steel, which is a micro strain concentration source when the high-speed cyclic pre-strain is applied, is reduced and refined.

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 was subjected to a tensile test at a high strain rate (= 10 / sec) at room temperature, and at 600 ° C., a strain rate (= 0.3% / 0.3%) specified in JIS G0567. Min) was also tested. The result is shown in FIG. (Mo + 3.5V + 20Nb) amount is 0.1
When it is less than 2%, the high temperature strength (0.2% yield strength) does not satisfy 2/3 (target value) of YS at room temperature. Also, (Mo +
If the amount of 3.5 V + 20 Nb) exceeds 0.8%, the YR value at a high strain rate exceeds 80%. Therefore, (Mo
The amount of + 3.5V + 20Nb) was limited to 0.12% or more and 0.8% or less.

From the above, low YR (≤80%) even when subjected to deformation at a high strain rate, stably exhibiting excellent toughness even after repeated prestrain at a 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, bainite, and tempered martensite.

The present invention has been made based on these findings. (1)% by weight, C: 0.04 to 0.18%,
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)%
Steel satisfying ≦ 0.8%, the balance of which is Fe and unavoidable impurities, is hot-rolled in the austenite region, water-cooled after the Ar3 point, and stopped at 400 ° C. or less,
Tempered at a temperature below the Ac1 point to make the structure ASTM
Particle size No. = 9 to 11 of a coarse-grained ferrite, bainite, and tempered martensite, which is a mixed structure, is excellent in fire resistance. 0.18%, Si: 0.05 ~
0.4%, Mn: 0.6 to 1.7%, Mo: 0.1
0.6%, V: 0.005-0.1%, Al: 0.00
1 to 0.06%, N ≦ 30 ppm, O ≦ 30 ppm,
In addition to satisfying 0.12% ≦ (Mo + 3.5V)% ≦ 0.8%, Cu: 0.05-0.6%, N
i: 0.05 to 0.6%, Cr: 0.05 to 1.0%,
Ti: Steel containing 0.005 to 0.015% of 1 or 2 or more, the balance of which is Fe and inevitable impurities, is hot-rolled in the austenite region, water-cooled after the Ar3 point, and 400 ° C. After water cooling was stopped below, tempering was performed at a temperature below the Ac1 point , and the structure was changed to ASTM grain size N
o. = 9-11 coarse-grained ferrite, bainite, and tempered martensite are mixed structures, and the manufacturing method of the earthquake-resistant building steel material excellent in fire resistance, (3) Steel of the above-mentioned (1) or (2). In addition to the composition, Nb:
Contains 0.005-0.04%, and 0.12% ≦
(Mo + 3.5V + 20Nb)% ≦ 0.8% is satisfied, and it is a manufacturing method of an earthquake-resistant building steel material having excellent fire resistance.

[0015]

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 it is added in an amount exceeding 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 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, S
The i amount was 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 0.
It was limited to 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 0.005%
It is limited to 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 amount of Al was limited to 0.001% or more and 0.06% or less.

N exists as solid solution N or nitride-based 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. In addition, coarse nitrides (for example, TiN and NbN) are easily generated in the final solidified portion,
Excellent toughness cannot be obtained. Therefore, N content is 30 pp
Restricted to m or less.

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 (Mo + 3.5V + 20Nb) content 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
If it exceeds%, the YR value at a high strain rate will exceed 80%. Therefore, (Mo + 3.5V + 20Nb)
The amount was limited to 0.12% or more and 0.8% or less.

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 set to 0.6% from the viewpoint of cost. Addition of Cr in excess of 1% significantly deteriorates weldability. Therefore, Cu is 0.05% or more and 0.6% or less, N
i was limited to 0.05% or more and 0.6% or less, and Cr was limited to 0.05% 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 limited to 0.005% or more and 0.015% or less.

Although P and S have no direct relation to the earthquake resistance which is the object of this patent, it is desirable that P and S are 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, bainite, and tempered martensite.
The reason for using this mixed structure is the high strain rate (> 0.1 / sec)
This is because a low YR value can be obtained at.

In the method of the present invention, in order to obtain such a 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 work is hardened when rolled in the ferrite region and the normal strain rate (10
^This is because a low YR cannot be obtained at around ^-2 / sec).
Next, after the Ar ₃ point has passed, water cooling is performed to 400 ° C. or less. The reason for water cooling after the passage of Ar ₃ 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 released until the passage of Ar ₃ point. This is because low YR can be obtained in a very wide cooling rate range when accelerated cooling is performed after cooling and partially precipitating ferrite. FIG. 4 shows the case where the steel A was used as the test steel and was accelerated cooled from the austenite region, and was left to cool after rolling until Ar ₃ points and accelerated cooling + tempering treatment was performed from the two-phase region in which some ferrite was precipitated. The relationship between YR and the cooling rate is shown. In the latter case, low YR is obtained in a very wide cooling rate range. From the microstructure observation, it was found that in the latter case, proeutectoid ferrite was obtained in a wide cooling rate range. The reason for water cooling to 400 ° C. or lower in the present invention is that the cooling stop temperature is 40
This is because a mixed structure of ferrite + bainite + martensite can be obtained by setting the temperature to 0 ° C. or lower, and this mixed structure cannot be obtained when the temperature is higher than 400 ° C. Next, tempering treatment is performed at a temperature of Ac ₁ point or less. The reason for tempering treatment at Ac ₁ point or less in temperature, because of its low martensite significantly toughness remains transformation, performing tempering at a temperature of ₁ point Ac, in order to recover the toughness. At temperatures exceeding the Ac ₁ point, α → γ transformation occurs in part, and the toughness cannot be recovered.

By carrying out such a treatment ,
The texture is set to ASTM grain size No. A mixed structure of coarse-grained ferrite, bainite, and tempered martensite of 9 to 11 can be used.

[0031]

EXAMPLES Examples of the present invention will be described below. Table 1, Table 2
Shows the chemical composition of the test steel. A to N have a steel composition within the scope of the present invention, and O to Y have a steel composition outside the scope of the present invention. Here, steels J, K, L, S, V, and X are TS570M.
Pa grades, M, N and Y are TS400 MPa grade steels,
Others are TS490 MPa grade steel. These steels are
All were slabbed by continuous casting including a light reduction process. The above steel was used as a steel plate under the manufacturing conditions shown in Tables 3 to 5. Tables 3 to 5 show the microstructure of the obtained steel sheet, the normal strain rate (= 0.01 / sec), and the high strain rate (= 10).
± 1 + at room temperature tensile properties at room temperature / second), high temperature strength, no pre-strain, and high strain rate (= 10 / s)
The Charpy impact test results when a cyclic prestrain of 2 + 4% is applied are also shown.

The normal temperature tensile test 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. Furthermore, the high temperature tensile test piece is a 10 mmφ × 50 mmGL round bar test piece taken in the C direction from ¼t.

From Tables 3 to 5, the oxygen content is 30 pp.
0.12% ≦ (Mo + 3.5V + 20Nb) ≦ m
0.8% is satisfied, and coarse-grained ferrite (ASTM No. 9
~ 11), bainite and tempered martensite of the present invention having a mixed structure (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, F2 and M2, which are ferrite + pearlite structures, show a remarkable increase in YR at a high strain rate as compared with a normal tensile test.
The value exceeds 0%. In addition, these steels have significantly deteriorated toughness after repeated prestrain at high strain rate, and
vTs is shown. Fine-grained ferrite (ASTM N
o. 11) and A2, E2, and J2, which are a mixed structure of bainite and tempered martensite, have a YR in a high strain rate tensile test of more than 80%. In addition, since B2 that has not been tempered has martensite that remains transformed, Y2
High R and low toughness. Furthermore, even if the structure is a mixed structure of coarse-grained ferrite, bainite, and tempered martensite, (Mo + 3.5V + 20Nb) <0.12%, O1, P1, Q1, and R1 have high-temperature strength satisfying the target value. Absent. The structure is a mixed structure of coarse-grained ferrite, bainite, and tempered martensite, and (Mo + 3.5V + 20
For S1, T1, and U1 with Nb)> 0.8%, the YR at a high strain rate exceeds 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 apparent 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 the effect of cooling rate on YR at room temperature.

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) C21D 8/00-8/10 C22C 38/00-38/60

Claims (3)

(57) [Claims] 1. C: 0.04 to 0.18%, S by weight%
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)% ≦
A steel satisfying 0.8% and the balance being Fe and unavoidable impurities was hot-rolled in the austenite region, water-cooled after the Ar3 point and stopped at 400 ° C or lower.
Tempered at a temperature of c1 point or lower to make the structure ASTM grain
Degree No. = 9 to 11 is a mixed structure of coarse-grained ferrite, bainite, and tempered martensite, and is a method for producing an earthquake-resistant building steel material having excellent fire resistance. 2. C: 0.04 to 0.18%, S by weight%
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 after water cooling is stopped at 400 ° C or less, tempering is performed at a temperature of Ac 1 point or less, and the structure is AS
TM grain size No. = 9 to 11 is a mixed structure of coarse-grained ferrite, bainite, and tempered martensite, and is a method for producing an earthquake-resistant building steel material having excellent fire resistance. 3. In addition to the composition of claim 1 or 2, Nb: 0.005 to 0.04% is further contained, and 0.12% ≦ (Mo + 3.5V + 20Nb)% ≦ 0.
A method for manufacturing an earthquake-resistant building steel material having excellent fire resistance, characterized by satisfying 8%.