JPH09170045A - Building steel for low temperature use excellent in fire resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a building steel excellent in fire resistance and having a low yield ratio at a low temp. by specifying the components and structure of an iron base alloy. SOLUTION: This iron base alloy satisfies the following requirements: by weight, 0.1 to 0.6% Mo, 0.005 to 0.1% V, <=30ppm O and also 0.12<=(Mo+3.5V)<=0.8. Among which, the regulation of the O content is particularly important, and, by this, stable low temp. toughness can be obtd. This alloy is, e.g. subjected to [low temp. heating]+[high temp. finish rolling]+[controlled cooling in which intensive cooling is executed as it goes to a low temp. region] to obtain a mixed structure of ferrite and bainite 'α+B'. This structure has an advantage of being smaller in the rising rate of the yield ratio at a low temp. than that of the ferritic-pearlitic structure 'α+P'.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a building field designed by the new seismic design method, and has a low yield ratio at a low temperature suitable for a building such as a building in a cold region where the operating environment temperature is room temperature or lower. , A steel material for construction having excellent fire resistance and low temperature toughness.

[0002]

2. Description of the Related Art The seismic design method for buildings, which was revised and implemented in 1981, was changed to an elastic design in which the stress level in each part of the structure up to that point was kept within the yield point of the steel material, and the steel material became the maximum after yielding. By utilizing the deformation capacity in the plastic range until the strength is reached, the seismic input energy is absorbed and the seismic safety of the building is secured. From this,
Steel materials for buildings to which the new seismic design method is applied are required to have a low yield ratio (YR value), which is a parameter indicating the deformability after yield, that is, a low yield ratio. T
The steel of S500 MPa class has a low yield ratio secured by finishing the hot rolling in the recrystallized region to make the structure coarse and further to form a mixed structure of ferrite and bainite or martensite. In so-called buildings such as offices and residential buildings in warm regions, the operating temperature is mainly at room temperature (0
Since the temperature is up to 30 ° C), the above-mentioned seismic design method is also premised on room temperature. Therefore, the conventional low yield ratio steel is also manufactured so that the YR value at room temperature is 80% or less or 75% or less. However, depending on the region, the temperature in winter may always be 0 to −40 ° C. or lower, as in a cold region, or the temperature at night may be temporarily 0 ° C. or lower even in a warm region. In addition, since the purpose of use is low, such as in a low temperature warehouse, it may be kept at about -55 ° C at all times. To secure the seismic safety by applying the new seismic design method to low-temperature buildings exposed to such an environment, steel materials having a low yield ratio at low temperatures are required. However, although the conventional low yield ratio steel is premised on use at room temperature, the yield ratio at room temperature is shown, but the yield ratio at low temperature is not clear.

[0003]

Therefore, the present inventors examined the tensile properties and toughness of conventional low yield ratio steels at low temperatures. Many low-yield ratio steels are coarse-grained in order to obtain low-yield ratios, which results in low low-temperature toughness and low ambient temperature.
It was found that it cannot be used for buildings in cold regions where the temperature is 20 ° C. or lower, and low-temperature warehouses that use −55 ° C. As a conventional technique relating to a low yield ratio steel excellent in low temperature toughness, Japanese Patent Application Laid-Open No. Hei 2
-197522 and JP-A-5-21440 have been reported. Examining the low temperature tensile properties of the steel made in accordance with the inventions described in both publications, for example, -6
It was found that the yield ratio was 80% or more at 0 ° C. Also, regarding toughness, not only a good value but also a variation result.

With respect to the fire of a building, a review of the fireproof design has made it possible to reduce the fireproof coating by using a fireproof steel material 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-4-83.
821, JP-A-4-56723, JP-A-4-56723
Japanese Patent Application No. 56362/1996 has been proposed. When the tensile properties at low temperature of the steels produced according to the inventions described in these documents were examined, the above-mentioned invention steels (JP-A-2-19752) were used.
2 and Japanese Patent Laid-Open No. 5-21440).
It was found that the yield ratio at 60 ° C was 80% or more.

From the above, the problem to be solved by the present invention is to provide a new seismic resistance having stable low temperature toughness, low yield ratio (YR≤80%) at low temperature, and excellent fire resistance. A low-temperature low-yield ratio building steel material having excellent fire resistance, which enables a design method combining design and fire-resistant design.

[0006]

[Means for Solving the Problems] The first invention is weight%.
And Mo: 0.1-0.6%, V: 0.005-0.1
%, O: 30 ppm or less, and 0.12% ≦ (Mo +
It is an iron-based alloy satisfying 3.5 V)% ≦ 0.8%, has a mixed structure mainly composed of ferrite and bainite, and has excellent fire resistance and is a low-temperature building steel material satisfying a low yield ratio at low temperatures.

The second invention is, by weight%, Mo: 0.1 to 0.1.
0.6%, V: 0.005-0.1%, Nb: 0.00
5 to 0.04%, O: 30 ppm or less, and 0.12
%-(Mo + 3.5V + 20Nb)% ≤0.8% is an iron-based alloy that has a mixed structure mainly composed of ferrite and bainite, and has excellent fire resistance and is a low-temperature building steel material that satisfies a low yield ratio at low temperatures. is there.

[0008] To give concrete examples of these inventions, weight%
C: 0.06 to 0.18%, Si: 0.05 to 0.
4%, Mn: 0.6 to 1.6%, Mo: 0.1 to 0.6
%, V: 0.005-0.1%, Al: 0.001-
0.06%, N: 30 ppm or less, O: 30 ppm or less, and 0.12% ≦ (Mo + 3.5V)% ≦
A balance of 0.8% is Fe and unavoidable impurities, and is a low-temperature building steel material excellent in fire resistance and excellent in low-temperature toughness and satisfying a low yield ratio at low temperatures.

Further, in% by weight, C: 0.06 to 0.18
%, Si: 0.05 to 0.4%, Mn: 0.6 to 1.6
%, Mo: 0.1 to 0.6%, V: 0.005 to 0.1
%, Al: 0.001 to 0.06%, N: 30 ppm or less, O: 30 ppm or less, Cu: 0.05 to
0.6%, Ni: 0.05 to 0.6%, Cr: 0.05
.About.1.0%, Ti: 0.005 to 0.015%, containing one or more kinds, and 0.12% .ltoreq. (Mo + 3.
5V)% ≤ 0.8%, the balance being Fe and unavoidable impurities, which is a low-temperature building steel material excellent in fire resistance and low-temperature toughness and satisfying a low yield ratio at low temperatures.

Furthermore, Nb: 0.0 is added to these steel materials.
0.05 to 0.04% and 0.12% ≦ (Mo +
It is a structural steel material for low temperature satisfying 3.5V + 20Nb)% ≦ 0.8%, which is excellent in fire resistance and excellent in low temperature toughness and which satisfies a low yield ratio at low temperature.

In the present invention, "having a mixed structure mainly composed of ferrite and bainite" is not limited to a metallic structure consisting of only ferrite and bainite, and a mixed structure of ferrite and bainite is present even if other structures are mixed. It suffices to have the characteristics (described later).

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The low-temperature building steel material excellent in fire resistance of the present invention limits the metal structure of an iron-based alloy to a mixed structure mainly composed of ferrite and bainite. The reason for this is that the mixed structure mainly composed of ferrite and bainite tends to have a smaller increase in the YR value at low temperatures than the ferrite-pearlite structure. That is, the present inventors examined the relationship between the microstructure and the YR value (yield ratio) at low temperature. The result is shown in FIG. As shown in FIG. 1, the yield ratio increases as the tensile test temperature becomes lower, but the yield ratio of the mixed structure mainly composed of ferrite and bainite at a low temperature is lower than that of the ferrite-pearlite structure. Note that "α +
“B” is a mixed structure mainly composed of ferrite and bainite, “α +
P ″ represents a ferrite / pearlite structure.

In the low temperature building steel material of the present invention having excellent fire resistance, the oxygen content of the iron-based alloy is set to 30 ppm or less. This is because stable low temperature toughness can be obtained by limiting the oxygen amount in this range, and when the oxygen content exceeds 30 ppm, the low temperature toughness varies widely.
That is, the present inventors investigated the low temperature toughness of a mixed structure mainly composed of ferrite and bainite by using a steel in which only oxygen was changed in the range of 19 to 44 ppm based on steel A (shown in Table 1). As a result, as shown in FIG. 2, the low temperature toughness has a considerable variation. It was found that the lower limit value is governed by the oxygen content, and by setting the oxygen content to 30 ppm or less, stable toughness satisfying vE-60 (lower limit value)> 100 J can be obtained. This is because by setting the oxygen content to 30 ppm or less, the oxide in the steel, which is the starting point of the generation of microcracks, was reduced and refined.

Further, the low temperature building steel material excellent in fire resistance of the present invention, when it contains Mo and V, is an iron-based alloy (Mo + 3.
5V) amount is limited to 0.12-0.8%. Also, M
When it contains o, V and Nb, (Mo + 3.5V + 2
0Nb) amount is limited to 0.12-0.8%. The reason is that (Mo + 3.5V) amount or (Mo + 3.
This is because excellent high temperature strength and low temperature low yield ratio can be obtained by limiting the amount of 5V + 20 Nb). That is, the inventors of the present invention have (Mo +
3.5V), Mo, V and Nb are added to the steel, the amount of (Mo + 3.5V + 20Nb) is changed, and a ferrite / pearlite structure and a mixed structure mainly composed of ferrite and bainite are used at −60 ° C. Yield ratio and 600 ℃
The 0.2% proof stress was examined. Compared to the ferrite-pearlite structure, the mixed structure mainly composed of ferrite and bainite is (Mo + 3.5V), (Mo + 3.5V + 20N).
b) The effect of improving the high temperature strength is large due to the increase in the amount, and the yield ratio at low temperatures is also low. In a mixed structure mainly composed of ferrite and bainite, (Mo + 3.5V), (Mo + 3.5V)
When the amount of +20 Nb) is less than 0.12%, the high temperature strength does not satisfy the target value. In addition, exceeding 0.8% (Mo +
3.5V) and (Mo + 3.5V + 20Nb), the YR value at −60 ° C. exceeds 80%. Therefore, the structure is limited to a mixed structure of ferrite and bainite, and the (Mo + 3.5V + 20Nb) content is limited to 0.12% or more and 0.8% or less.

From the above, low temperature low temperature excellent in fire resistance which enables both new seismic design having stable low temperature toughness and low yield ratio at low temperature and fire resistant design showing excellent high temperature strength. Yield ratio The requirements for building steel materials are (Mo + 3.
5V), (Mo + 3.5V + 20Nb) amount is 0.12%
It was found that the alloy has characteristics of a mixed structure mainly composed of ferrite and bainite having an oxygen content of 0.8% or less and an oxygen content of 30 ppm or less.

Next, the reason for adding the alloying element of the present invention and the reason for limiting the addition amount are as follows in order for the welded structural steel to obtain predetermined characteristics. Mo is an element effective for improving the medium / high temperature strength of steel. In order to exert such effects, it is necessary to add 0.1% or more. 0.6%
Addition of Mo in excess of 10 significantly increases the yield ratio. Therefore, Mo is limited to 0.1% or more and 0.6% or less.

Nb and V are effective elements for increasing the strength at normal temperature and high temperature by adding a trace 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 is 0.005% or more and 0.04% or less, and V is 0.00
It is limited to 5% or more and 0.1% or less.

In the concrete composition of the steel material of the present invention, other additive elements may be limited as follows. C is the cheapest element and is an element effective for improving the strength, but if it is added in an amount exceeding 0.18%, the weldability is significantly reduced. 0.06%
If the amount is less than the above range, strength is insufficient and a large amount of alloying elements are required, resulting in high cost. Therefore, C is 0.06%
It is limited to 0.18% or less.

Si is an element necessary for strength of steel materials and preliminary deoxidation of molten steel. For pre-deoxidation, addition of 0.05% or more is necessary. An excess addition of more than 0.4%
Deteriorates the toughness of steel and the toughness of the weld heat affected zone. Therefore, 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, and it is necessary to add a large amount of alloying elements, resulting in deterioration of weldability. Further, Mn is an element that tends to segregate in the center,
If it is added in excess of 1.6%, the center of the plate thickness becomes significantly brittle. Therefore, Mn is limited to 0.6% or more and 1.6% 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. When added in excess of 0.06%,
Defects are likely to occur on the surface of continuously cast slabs. Therefore, Al is limited to 0.001% or more and 0.06% or less.

N exists as solid solution N and nitride inclusions in the solid steel. Solid solution N and coarse nitride-based inclusions deteriorate the low temperature 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 easily generated in the final solidified portion,
Excellent low temperature toughness cannot be obtained. Therefore, N is 0.0
It was limited to 03% or less.

Cu, Ni, and Cr are elements effective for strengthening at room temperature by strengthening solid solution and improving hardenability.
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 1.0% significantly deteriorates weldability. Therefore, Cu
Was limited to 0.05% to 0.6%, Ni to 0.05% to 0.6%, and Cr to 0.05% to 1.0%.

Ti is an element that forms TiN, suppresses the coarsening of the structure during heating, and contributes to the improvement of toughness.
If Ti is added in an amount of less than 0.005%, the effect of improving toughness is not exhibited. If added in excess of 0.015%, TiC precipitates during the cooling process, leading to deterioration in toughness. Therefore, Ti
Was limited to 0.005% or more and 0.015% or less.

P and S are not directly related to the low temperature low yield ratio, low temperature toughness, and fire resistance aimed at by this patent, but are preferably low from the viewpoint 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 steel of the present invention has a low yield ratio at low temperature, has stable low temperature toughness, and is excellent in fire resistance, and therefore is extremely effective as a steel material for construction.

[0026]

Next, embodiments of the present invention will be described. Table 1 shows the chemical composition of the test steel sheet, and A to N are the compositions of the present invention, and
-X show the composition which deviates from the composition of this invention, respectively. All the test steels were slabed by continuous casting including a light reduction process. Steels having the above components were made into steel plates under the manufacturing conditions shown in Tables 2 and 3. Tables 2 and 3 also show the microstructures of the obtained steel sheets. The mixed structure (α + B) mainly composed of ferrite and bainite, which is a feature of the present invention, is
It can be obtained by performing [low temperature heating] + [high temperature finish rolling] + [controlled cooling with stronger cooling in lower temperature regions]. Table 4, Table 5
Table 2 shows the strength and toughness of each steel sheet. The tensile test piece is JIS No. 4 or JIS No. 14A taken in the C direction from a plate thickness of 1/4 t. All YS values are values of the upper yield point, and all YR values are values of the upper yield point / TS value. The V-notch Charpy impact test piece was sampled in the L direction from a plate thickness of 1/4 t. In addition, vE-60 (ave), v
E-60 (min) is the average value and the minimum value of N number 9.

In Tables 4 and 5, invention steels A1, B1, C1 and D having a mixed structure of ferrite and bainite.
1, E1, F1, G1, H1, I1, J1, K1, L
1, M1 and N1 have a YR value at −60 ° C. of 80% or less,
vTs of −60 ° C. or lower, high temperature strength at 600 ° C. satisfies the target value, excellent fire resistance, and excellent low temperature toughness,
It also shows a low temperature low yield ratio. Comparative steels A2, B2, C2, E with ferrite / pearlite structure (α + P)
2, F2, G2 and H2 have YR value of 80% at -60 ° C.
Is over. It has a mixed structure mainly composed of ferrite and bainite, and has (Mo + 3.5V)% <0.12%, (Mo +
Comparative steel O with 3.5V + 20Nb)% <0.12%
1, P1, Q1 and R1 do not satisfy the target values for the high temperature strength at 600 ° C. Having a mixed structure of ferrite and bainite, (Mo + 3.5V + 20Nb)%> 0.8%
Comparative steels S1, T1, and U1 having a YR value of more than 80% at -60 ° C. Comparative steels V1 and W having a mixed structure mainly composed of ferrite and bainite and having O> 30 ppm
No. 1, X1 has a low vE-60 (min), and stable low temperature toughness is not obtained.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

[Table 3]

[0031]

[Table 4]

[0032]

[Table 5]

[0033]

EFFECTS OF THE INVENTION As is clear from the above results, the steel material according to the present invention has excellent fire resistance, low temperature toughness, and a low yield ratio at low temperatures. Enables new seismic design and fireproof design. Therefore, the safety of the building is increased. In addition, mass production of steel is possible, the price is low, the refractory coating can be reduced, the construction period can be shortened, and the construction cost is low as a whole.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a tensile test temperature and a yield ratio (= upper yield ratio / tensile strength).

Fig. 2 Oxygen content in ferrite and bainite mixed structure and Charpy impact absorption energy (vE at -60 ° C)
-60) is a diagram showing a relationship with FIG.

FIG. 3 is a diagram showing the relationship between the (Mo + 3.5V + 20Nb) amount, the yield ratio at low temperature (YR at −60 ° C.), and the high temperature strength (0.2% proof stress at 600 ° C.).

Claims (2)

[Claims] 1. Mo: 0.1-0.6% by weight,
V: 0.005 to 0.1%, O: 30 ppm or less, and an iron-based alloy satisfying 0.12% ≦ (Mo + 3.5V)% ≦ 0.8%. A low temperature building steel that has excellent fire resistance and satisfies a low yield ratio at low temperatures. 2. Mo: 0.1 to 0.6% by weight,
V: 0.005-0.1%, Nb: 0.005-0.0
4%, O: 30 ppm or less, and 0.12% ≦ (Mo
+ 3.5V + 20Nb)% ≦ 0.8% is an iron-based alloy having a mixed structure of mainly ferrite and bainite,
Low temperature building steel with excellent fire resistance and low yield ratio at low temperature.