JPH1036910A - Production of earthquake-resistant building steel excellent in fire resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing an earthquake-resistant building steel which has excellent fire resistance, low yield ratio (<=80%) even in the case of receiving deformation at high distortion speed and stable and excellent toughness even after repeatedly receiving prestrain at the high distortion speed and can use a designing method combining a fire-resistant design excellent in the fire resistance and a plastic earthquake-resistant design of the structure near an active fault. SOLUTION: The steel composed, by wt., of 0.04-0.18% C, 0.05-0.4% Si, 0.6-1.7% Mn, 0.1-0.6% Mo, 0.005-0.1% V, 0.001-0.006% Al, <=30ppm N, <=30ppm O under condition of satisfying 0.12% <=(Mo+3.5V)%<=0.8% and the balance Fe with inevitable impurities is hot-rolled in the austenitic range, then is subjected to water cooling after passing the Ar3 point. Then, after stopping the water-cooling at <=400 deg.C, tempering treatment is executed at <= the Ac1 point, and the main structure is made as the mixing structure of coarse grain ferrite, bainite and tempered martensite.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art The seismic design method for buildings, which was revised and enforced in 1981, uses the deformation capability in the plastic region until the steel reaches its maximum strength after yielding to absorb the earthquake input energy. In order to ensure the seismic safety of the building, the elastic design has been drastically changed to keep the stress level of each part of the structure within the yield point of the steel material up to that point.
For this reason, steel materials of buildings to which the new seismic design method is applied are required to have a low yield ratio (YR), which is a parameter representing the deformation performance after yielding, that is, a low yield ratio.

[0003] The TS490 MPa class steel material is subjected to hot rolling in a recrystallization region, coarsening the structure, and securing a low yield ratio. In the case of a steel having a strength of TS570 MPa or higher, a low yield ratio is ensured by quenching from a two-phase region of ferrite-austenite to form a two-phase structure of ferrite and bainite or martensite.

[0004] In the Hanshin Earthquake of January 1995, unlike the marine-type earthquake in which the hypocenter was distant as assumed in the above seismic design method, it was an active fault-type earthquake whose epicenter was very close. Was. In the case of an active fault type earthquake, the speed of shaking is very high, and the strain rate of the building is 10 ^-1 to 10 /
The feature is that high-speed deformation of seconds is added. As described above, the current building steel material has a low YR, which is a value when pulled at a normal strain rate of about 10 ^-2 / sec. In the case of deformation at a high strain rate as described above, the low YR is used. Was unknown. Thus, the present inventors conducted a tensile test while changing the strain rate of the conventional as-rolled (ferrite + pearlite structure) SN490 grade steel, and found that the strain rate was 10 ^−2.
It was found that YR <80% when the strain rate was about 10 / sec, but the YR greatly increased to a value of 80% or more when the strain rate was about 10 / sec.

In the case of the Great Hanshin Earthquake, there have been cases where structural members are embrittled by repeated high-speed plastic deformation, and brittlely fracture when subjected to the next tensile deformation. If brittle fracture occurs, it may lead to a large collapse of the building, so this is a failure mode that must be avoided as steel for earthquake resistance. Conventional SN
When the 490 grade steel is not subjected to prestrain, the brittle-ductile fracture surface transition temperature has a sufficient toughness of not more than room temperature. It was not clear whether or not it exhibited deterioration. The present inventors have proposed a conventional S
For several types of N490 grade steel, positive / negative alternating strain increasing type (1% compression plastic strain → 1% tensile plastic strain → 2% compressive plastic strain → 2% tensile plastic strain → 4%
After applying a pre-strain of compressive plastic strain → 4% tensile plastic strain, and then applying this pre-strain as ± 1 + 2 + 4%), a Charpy impact test was performed. The brittle-ductile fracture surface transition temperature was higher than room temperature. Also appeared.

[0006] Among the prior art of building steel materials, Japanese Patent Laid-Open No.
Japanese Patent Application Laid-Open No. 7522 and Japanese Patent Application Laid-Open No. 5-21440 relate to a low yield ratio steel having excellent low-temperature toughness. However,
In both cases, only a tensile test at a normal strain rate was performed, and the YR value at a high strain rate was not shown. Also, the toughness is a Charpy impact test value when there is no pre-strain, and the toughness after pre-strain is applied is unknown. Therefore, the present inventors prototyped steels in accordance with the above proposals, and obtained tensile properties of these prototype steels at a high strain rate (= 10 / sec) and ± 1 + 2 + 4% at a high strain rate (= 10 / sec). The toughness after repeated prestraining was examined. As a result, a high strain rate (=
YR in the tensile test at 10 / sec) was more than 80%. In addition, the toughness after high-speed cyclic prestrain varied, and some of them were extremely brittle, showing vE-5 <20 J. That is, it was found that these steels did not have sufficient earthquake resistance in the case of an active fault type earthquake.

[0007] Further, with respect to fires in buildings, a review of fire-resistant design has made it possible to reduce fire-resistant coatings by using fire-resistant steel excellent in high-temperature strength. The use of refractory steels has led to shorter construction periods, lower construction costs, and an increase in the effective area of buildings. For construction steel materials having both low yield ratio and fire resistance, see JP-A-4-83821.
Japanese Patent Application Laid-Open No. 4-56723, Japanese Patent Application Laid-Open No. 4-56362, and the like have been proposed. Therefore, the present inventors prototyped steel in accordance with the above proposals, and as with the above-mentioned invention steels (JP-A-2-197522 and JP-A-5-21440), these trial-produced steels were also manufactured. Tensile properties at strain rate were investigated. As a result, it was found that the yield ratio at a 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 a 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.Excellent fire resistance that enables a design method combining fire resistance design and plastic seismic design of structures near active faults. It is intended to provide a method for manufacturing an earthquake-resistant steel material.

[0009]

Means for Solving the Problems In order to solve this problem, the present inventors have intensively studied the relationship between the microstructure and YR at a high strain rate, and have found the following important findings.

First, FIG. 1 shows that tensile strength and yield ratio (= yield strength /
In the figure, “α” is an abbreviation for ferrite, “B” is an abbreviation for bainite, “tempered M” is an abbreviation for tempered martensite, and P is an abbreviation for pearlite. "Coarse grain α + B + tempered M (●)" in the
l steel plate, “fine grain α + B + tempered M (○)” is A2 steel plate,
“Α + P (△)” 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, ferrite + bainite + tempered martensite mixed structure (●,）) has a lower degree of rise than ferrite + pearlite structure (△). In the ferrite + bainite + tempered martensite mixed structure, the coarser the ferrite (●),
It was found that a low YR value was obtained at a high strain rate (> 0.1 / sec). By forming a mixed structure of coarse ferrite, bainite and tempered martensite, even at a strain rate of 10 / sec, Y
R <80% or less has been achieved. In the present invention, the coarse ferrite is an ASTM particle size No. 11 or less.

FIG. 2 shows that in the same steel type as steel A, only oxygen is used.
Using steel changed in the range of ~ 43 ppm for the test steel,
The effect of oxygen content on vE-5 after repeatedly pre-straining a mixed structure of coarse ferrite (ASTM grain size No. = 9-11), bainite and tempered martensite at a high strain rate (= 10 / sec) The effect was investigated. As shown in FIG. 2, the toughness after repeated pre-straining at a high strain rate (= 10 / sec) has considerable variation, but the lower limit is governed by the oxygen content. 30 quantity
vE-5 (minimum)> 100J
It has been found that stable toughness that satisfies is satisfied. This is because by reducing the oxygen content to 30 ppm or less, the oxides in the steel, which are micro-strain concentration sources when high-speed repetitive prestrain is applied, are reduced and refined.

Further, the present inventors have found that (Mo + 3.5 V
+ 20Nb) Using a 50 kg grade steel with varying amounts,
A tensile test was performed on a steel having a mixed structure of coarse-grained ferrite and bainite at a high strain rate (= 10 / sec) at room temperature, and at 600 ° C., a strain rate defined by JIS G0567 (= 0.3% / Min) also performed a tensile test. The result is shown in FIG. (Mo + 3.5V + 20Nb) amount is 0.1
If it is less than 2%, the high temperature strength (0.2% proof stress) does not satisfy ／ (target value) of YS at normal temperature. Also, (Mo +
When the (3.5V + 20Nb) amount exceeds 0.8%, the YR value at a high strain rate exceeds 80%. Therefore, (Mo
+ 3.5V + 20Nb) is limited to 0.12% or more and 0.8% or less.

From the above, the low YR (≦ 80%) even when deformed at a high strain rate, stable and excellent toughness even after repeated prestrain at a high strain rate, and The necessary condition of the seismic steel material that enables the design combining 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 composition satisfying ≦ 0.8% and having an oxygen content of 30 ppm or less has characteristics of a mixed structure of coarse 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 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 + 3.5V)%
≦ 0.8%, the balance of steel consisting of Fe and unavoidable impurities is hot-rolled in the austenitic region, water-cooled after the lapse of _three points of Ar, and water-cooling is stopped at 400 ° C or lower.
A method for producing a seismic building steel material having excellent fire resistance, characterized by tempering at a temperature of ₁ point or less to obtain a main structure of a mixed structure of coarse ferrite, bainite and tempered martensite, (2) By weight%, C: 0.04 ~
0.18%, Si: 0.05 to 0.4%, Mn: 0.6
To 1.7%, Mo: 0.1 to 0.6%, V: 0.005
０．１0.1%, Al: 0.001 to 0.06%, N ≦ 30
ppm, O ≦ 30 ppm and 0.12% ≦ (Mo +
3.5V)% ≦ 0.8% and C
u: 0.05 to 0.6%, Ni: 0.05 to 0.6%,
Cr: 0.05 to 1.0%, Ti: 0.005 to 0.0
After hot rolling in the austenitic region a steel containing one or two or more of 15% and the balance consisting of Fe and unavoidable impurities, water cooling is performed after the lapse of _three points of Ar, and water cooling is stopped at 400 ° C or lower. , Ac a tempering treatment at a temperature of not more than _one point to produce a main structure of a mixed structure of coarse-grained ferrite, bainite and tempered martensite. ) In addition to the above steel composition (1) or (2), Nb:
0.005 to 0.04%, and 0.12% ≦
(Mo + 3.5V + 20Nb)% ≦ 0.8%.

[0015]

Next, the reason for adding each component of the steel material according to the present invention and the reason for limiting the amount of addition will be described.
C, Si, Mn, and Al were limited as follows based on the relationship between the functions and effects that have been conventionally confirmed in order to obtain required materials in ordinary welded structural steel.

C is the cheapest element and is effective for increasing the strength. However, if added in excess of 0.18%, the weldability is significantly reduced. If it is less than 0.04%, the strength of a thick material is insufficient, and a large amount of alloying elements must be added, resulting in an increase in 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 and the preliminary deoxidation of molten steel. For pre-deoxidation, addition of 0.05% or more is necessary. An excess addition of more than 0.4%
It degrades the toughness of the steel material and the weld HAZ toughness. Therefore, S
The i amount was limited to 0.05% or more and 0.4% or less.

Mn is an element necessary for securing the strength of the base material. If it is less than 0.6%, the strength is insufficient for a thick material, and a large amount of alloying elements must be added, resulting in an increase in cost. Further, Mn is an element that is easily segregated at the center. 1.7
%, The center of the sheet thickness becomes significantly embrittled. 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 exhibit such an effect, 0.1
% Or more is required. The addition of Mo in excess of 0.6% significantly increases the yield ratio. Therefore, Mo is set to 0.
It is limited to 1% or more and 0.6% or less.

Nb and V are effective elements for increasing the strength at room temperature and high temperature by adding a small amount. Nb <0.005%, V
At <0.005%, no clear increase in strength is observed. The addition of Nb exceeding 0.04% and the addition of V exceeding 0.1% significantly increase the yield ratio. Therefore, Nb
0.005% to 0.04%, V is 0.005%
It was limited to not less than 0.1%.

Al is an element necessary for deoxidation. If the Al content is less than 0.001%, a sufficient deoxidizing effect cannot be expected. When added in excess of 0.06%,
The surface of the continuous casting slab is easily scratched. Therefore,
The amount of Al was limited to 0.001% or more and 0.06% or less.

N exists in solid steel as solid solution N or nitride-based inclusions. 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, the N content is 30 pp
m or less.

O has an oxygen content of 3 as described above.
By setting the content to 0 ppm or less, oxides in the steel, which are sources of micro-strain concentration when high-speed repetitive pre-strain is given, are reduced and refined.
mum) Stable toughness satisfying> 100 J cannot be obtained.

As described above, when the (Mo + 3.5V + 20Nb) amount is less than 0.12%, the high-temperature strength (0.
2% yield strength) does not satisfy （(target value) of YS at room temperature. In addition, (Mo + 3.5V + 20Nb) amount is 0.8
%, The YR value at a high strain rate exceeds 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 high strength through solid solution strengthening and hardenability improving effects. Cu <0.05
%, Ni <0.05%, and Cr <0.05%, a clear strength increase effect is not seen. Addition of Cu exceeding 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 more than 1% of Cr significantly deteriorates weldability. Therefore, the Cu content is 0.05% or more and 0.6% or less,
i is limited to 0.05% or more and 0.6% or less, and Cr is 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 of TiN and contributes to the improvement of the HAZ toughness.
If less than 0.005% of Ti is added, the effect of improving the HAZ toughness is not exhibited. If it is added in an amount exceeding 0.015%, TiC precipitates during the cooling process of welding, leading to deterioration of HAZ toughness. Therefore, the content of Ti is limited to 0.005% or more and 0.015% or less.

Although P and S do not have a direct relationship with the seismic resistance, which is the object of the present invention, it is desirable that P and S are low from the viewpoint of weldability and ductility in the thickness direction. From the viewpoint of inclusion morphology control, it is desirable to add an appropriate amount of Ca or REM.

The main microstructure of the steel material according to the present invention is a mixed structure of coarse ferrite, bainite and tempered martensite. The reason for this mixed organization is
This is because 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 such a microstructure, a steel material is manufactured under the following manufacturing conditions. First, a steel satisfying the above component range is hot-rolled in an austenite region. The reason for hot rolling in the austenite range is that when rolling is performed in the ferrite range, work hardening occurs and the normal strain rate (10
^This is because a low YR cannot be obtained at about ^-2 / sec).
Then, after the lapse of _three points of Ar, the mixture is water-cooled to 400 ° C. or less. The reason for water cooling after Ar ₃ point elapsed, when accelerated cooling from the austenite region, the ferrite is hardly obtained unless the cooling rate was controlled according to the hardenability of the steel, release to Ar ₃ point elapsed This is because when cooling is performed and accelerated cooling is performed after some ferrite precipitates, low YR can be obtained in a very wide cooling rate range. FIG. 4 shows the case where steel A was used as a test steel and accelerated cooling was performed from the austenite region, and the case where the steel was allowed to cool down after the elapse of _three points of Ar after rolling and was subjected to accelerated cooling + tempering treatment from the two-phase region where 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 less in the present invention is that the cooling stop temperature is 40 ° C.
This is because a mixed structure of ferrite + bainite + martensite is obtained by setting the temperature to 0 ° C. or lower, and the mixed structure cannot be obtained at a temperature higher than 400 ° C. Next, tempering treatment is performed at a temperature equal to or lower than Ac ₁ point. 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 a temperature exceeding the Ac ₁ point, α → γ transformation occurs partially, and the toughness cannot be recovered.

By performing such a treatment, the main structure of the steel material can be a mixed structure of coarse ferrite, bainite, and tempered martensite.

[0031]

Next, embodiments of the present invention will be described. Table 1, Table 2
The chemical composition of the test steel is shown below. A to N have a steel composition within the scope of the invention, and O to Y have a steel composition outside the scope of the invention. Here, steels J, K, L, S, V and X are TS570M
Pa class, M, N, Y are TS400MPa class steel,
Others are TS490 MPa grade steel. These steels
All were slab cast by continuous casting, including a light reduction process. The steel was made into a steel sheet 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
/ S) at room temperature, and high temperature strength, without prestrain, and at high strain rates (= 10 / s) ± 1 +
The results of the Charpy impact test when a 2 + 4% cyclic prestrain is given are also shown.

The room-temperature tensile test is a rod test piece of 12 mm square × 100 mm square parallel part length sampled in the C direction from 1/4 t. This specimen was subjected to a tensile test at a stroke speed of 1 m / sec, that is, a strain speed of 10 / sec, using a servo-type testing machine. In addition, the same test specimen was used at a stroke speed of 1 m /
Seconds, ie 1% compression plastic deformation at a strain rate of 10 / sec →
1% tensile plastic deformation → 2% compressive plastic deformation → 2% tensile plastic deformation → 4% compressive plastic deformation → 4% tensile plastic deformation , VTs and vE-5 were measured. 9 at -5 ° C
The Charpy impact test of the book was performed, and the average value and the minimum value were obtained. Further, the high-temperature tensile test piece is a round bar test piece of 10 mmφ × 50 mmGL taken in the C direction from １ ／ t.

Referring to Tables 3 to 5, the oxygen content is 30 pp.
0.12% ≦ (Mo + 3.5V + 20Nb) ≦ m
0.8% and satisfy coarse grain ferrite (ASTM No. 9
11), a steel sheet of the present invention (A1, B1, C1, D1, E) having a mixed structure of bainite and tempered martensite.
1, F1, G1, H1, I1, J1, K1, L1, M
1, N1) has a high temperature strength of not more than 80% at a high strain rate, a high temperature strength satisfying a target value, and a minimum value of vE-5 of 150 J or more even after repeated strain. A3, C2, F2, and M2, which are ferrite + pearlite structures, show a marked increase in YR at a high strain rate as compared with a normal tensile test, and have an 8R.
The value exceeds 0%. In addition, these steels have significantly deteriorated toughness after repeated prestraining at high strain rates,
vTs. Fine grain ferrite (ASTM N
o. A2, E2, and J2, which are mixed structures of bainite and tempered martensite, have a YR of more than 80% in a high strain rate tensile test. B2, which has not been tempered, has martensite in the transformed state.
R is high and toughness is low. Further, 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 ferrite, bainite and tempered martensite, and (Mo + 3.5V + 20
For S1, T1, and U1 in which Nb)> 0.8%, the YR at a high strain rate exceeds 80%. For V1, W1, X1, and Y1 having a mixed structure of coarse-grained ferrite and bainite and having an oxygen content exceeding 30 ppm, the minimum value of vE-5 after repeated prestraining at a high strain rate is less than 47 J. .

[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 manufactured by the method of the present invention exhibits a low YR (≦ 80%) even when it is deformed at a high strain rate, and is repeatedly repeated at a high strain rate. Since it exhibits excellent toughness and excellent fire resistance even after being subjected to prestrain, it enables the design near the active fault to be combined with plastic seismic design and fire resistant design. Mass production of steel is also possible.

[Brief description of the drawings]

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

FIG. 2 shows cyclic plastic strain (± 1) at high oxygen content and high strain rate.
(+ 2 + 4%), and shows the relationship between Charpy impact absorption energy (vE-5) tested at -5 ° C.

FIG. 3 is a graph showing the relationship between the amount of (Mo + 3.5V + 20Nb), the yield ratio at a 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.

Claims (3)

[Claims] C .: 0.04 to 0.18% by weight,
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 + 3.5V)%
≦ 0.8%, the balance of steel consisting of Fe and unavoidable impurities is hot-rolled in the austenitic region, water-cooled after the lapse of _three points of Ar, and water-cooling is stopped at 400 ° C or lower.
A method for producing an earthquake-resistant building steel excellent in fire resistance, characterized in that tempering treatment is performed at a temperature of ₁ point or less, and the main structure is a mixed structure of coarse ferrite, bainite and tempered martensite. 2. In% by weight, C: 0.04 to 0.18%,
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 + 3.5V)%
≦ 0.8%, Cu: 0.05-
0.6%, Ni: 0.05 to 0.6%, Cr: 0.05
1.0%, Ti: 0.005 to 0.015%
After hot rolling in the austenitic region a steel containing one or more kinds, the balance being Fe and unavoidable impurities,
_{After three} points, water cooling is performed. After the water cooling is stopped at 400 ° C. or less, tempering is performed at a temperature of Ac ₁ point or less, and the main structure is a mixed structure of coarse ferrite, bainite, and tempered martensite. Method of manufacturing earthquake-resistant building steel with excellent fire resistance. 3. The composition according to claim 1, further comprising Nb: 0.005 to 0.04%, and 0.12% ≦ (Mo + 3.5V + 20Nb)% ≦ 0.
A method for producing an earthquake-resistant building steel excellent in fire resistance, characterized by satisfying 8%.