JPH0726158B2 - Manufacturing method of fireproof steel plate for building with low yield ratio - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel sheet having a low yield ratio and fire resistance used for various structures in the fields of construction, civil engineering, offshore structures and the like.

[0002]

2. Description of the Related Art Rolled steel for general structure (JIS G 31
01), rolled steel for welded structure (JISG 3106),
Weather resistant hot rolled steel for welded structures (JIS G 311
4), high weather resistant rolled steel (JIS G 3125), carbon steel pipe for general structure (JIS G 3444), square steel pipe for general structure (JIS G 3466) (hereinafter referred to as well-known steel), etc. It is widely used as a construction material for various constructions in the field of structures and the like. By the way, among various buildings, especially when using the well-known steel material for buildings such as buildings and offices and residences closely related to daily life, in order to ensure safety in a fire,
It is obliged to apply a sufficient fireproof coating, and various building-related laws and regulations stipulate that the steel material temperature should not exceed 350 ° C during a fire. In other words, the well-known material has a yield strength of 60 at room temperature at 350 ° C when used in buildings.
Since it is up to 70%, which may cause the collapse of buildings, a fireproof coating is carefully applied so that the steel material does not lose its loading force due to thermal damage during a fire. Therefore, it is inevitable that the fireproof coating construction cost will be higher than the steel material cost and the construction cost will increase significantly. Due to the recent rise of high-rise buildings, the improvement of design technology and its high reliability, the fire-resistant design has been reviewed, and the high temperature strength of steel has been improved without depending on the temperature limit of 350 ° C mentioned above. The load actually applied to the building made it possible to determine the capacity of the fireproof coating, and in some cases it was possible to use steel without coating. For example, Japanese Unexamined Patent Publication No. 02-77523 discloses a steel material that can be supplied on the market at an economical price as a structural steel material having excellent fire resistance by adding a small amount of Nb-Mo composite and high temperature rolling.

[0003]

As mentioned above, the use of well-known materials for the refractory material is inexpensive, but since the high temperature strength is low, a high-priced refractory coating must be applied, which increases the construction cost and the utilization of the building. There is a problem of narrowing the space. On the other hand, a trace amount N disclosed in JP-A-02-77523.
Although b-Mo steel has good fire resistance and low YR,
It is not suitable for the production of steel plates having a plate thickness exceeding 50 mm, and the maximum plate thickness was 50 mm in the examples. An object of the present invention is to provide a method for manufacturing a thick plate (50 mm) having a low yield ratio and excellent high temperature strength.

[0004]

Means for Solving the Problems As a result of research on the strength of steel materials that can be placed during a fire, the inventors have found that the maximum temperature reached at the time of a fire is 1000 ° C. when the uncoated use is the goal. It has been found that in order to have a yield strength of 70% or more of the room temperature yield strength at that temperature, a large amount of expensive metal elements must be added, which is economically disadvantageous. That is,
The steel unit price becomes higher than the well-known steel cost and the cost of applying the fireproof coating, and such steel cannot be practically used.

Then, as a result of research, it was found that a steel material having a high temperature proof stress at 600 ° C. of 70% at room temperature was the most economical, the amount of expensive additional elements was reduced, and a fire resistant coating was formed. In addition to the steel materials that can be thinned and can be used uncoated when the fire load is small, that is, the steel of the present invention and the steel material, and a manufacturing method thereof, a steel material having fire resistance performance has been developed.

Therefore, the gist of the present invention is that C: 0.05 to 0.15% by weight, Si: 0.6% or less, Mn: 0.8 to 1.6%, P: 0. 0.03% or less,
S: 0.005% or less, Mo: 0.35 to 0.80%,
Ti: 0.005 to 0.025%, Al: 0.06% or less, N: 0.006% or less, and if necessary, Nb: 0.
005-0.05%, V: 0.005-0.1%, N
i: 0.05 to 0.1%, Cu: 0.05 to 0.1%,
Cr: 0.05-1.0%, Ca: 0.001-0.0
850 to 10 after hot rolling a steel containing 06% of one or two or more and the balance of iron and unavoidable impurities
Reheat to a temperature range of 00 ° C, quench, then 740-
After reheating to a temperature range of 820 ° C and quenching again,
It is a method for manufacturing a refractory steel sheet for buildings having a low yield ratio, which is characterized by performing a tempering treatment in a temperature range of 450 to 700 ° C.

[0007]

The inventors of the present invention have made extensive studies as to a method for producing a steel sheet having both a low yield ratio and fire resistance. As a result, it is possible to achieve the object by applying a completely new heat treatment to a steel containing a predetermined amount of Mo. I found it. As is well known, Mo is known to increase high temperature strength. However, since the weldability is important in the field of application of the steel of the present invention, it is desirable that the addition amount be low. Due to such a situation, in a thick steel plate (50 mm or more), a large amount of Mo must be added to obtain a predetermined strength in the as-rolled manufacturing method.

The present inventors have found that Mo is 0.35 to 0.80.
By applying the newly found heat treatment method in the range of%, it has become possible to manufacture thick (50 mm or more) steel sheets having a low yield ratio and excellent fire resistance. It has been conventionally known that quenching is performed to obtain high strength, but it cannot satisfy both the low yield ratio and the fire resistance as in the case of the steel of the present invention.

The present inventors have solved this problem by carrying out a quenching treatment that has never been performed twice. That is, the first hardening quenches the fine-grained austenite, and the second heating heats the two-phase coexisting region of austenite and ferrite.
By quenching from this temperature, a mixed structure of about 30 to 70% of ferrite, martensite and bainite is obtained, and then tempering treatment is performed to obtain predetermined material characteristics.

That is, fine-grained austenite is obtained by the first quenching treatment. Here, in order to suppress the coarsening of the austenite grains, 850 ° C. or higher and 1000
Reheating must be performed in the temperature range below ℃. Also, the second quenching is about 30 by heating the α-γ two-phase region.
In order to generate ˜70% of ferrite, it is necessary to reheat in the temperature range of 740 ° C. or higher and 820 ° C. or lower.

Then, in order to stabilize the carbides by the tempering treatment and obtain the desired material characteristics, the temperature is 450 ° C. or higher and 700 ° C.
Reheating must be performed within the temperature range below. Although the main points of the method for producing a steel having a low yield ratio and fire resistance have been described above, the object of the invention cannot be achieved by this alone. That is,
It is necessary to control the other alloying elements within the proper range. The limited ranges will be described below.

C: C is necessary to ensure the base metal, welded portion, strength guarantee, and the effect of adding Nb and Mo. If the content is 0.05% or less, the effect is weakened, so the lower limit is made 0.05%. Further, if the amount of C is too large, not only the low temperature toughness of the weld heat affected zone (HAZ) is adversely affected but also the base metal toughness and weldability are deteriorated.
Is the upper limit. Si: Si is an element contained in the deoxidized upper steel. Since the weldability and HAZ toughness deteriorate when Si is increased, the upper limit was made 0.6%. In the steel of the present invention, Al deoxidation is sufficient, and Ti deoxidation may be used. From the viewpoint of HAZ toughness, it is desirable that the content of Si be about 0.15%.

Mn: Mn is an unavoidable element for ensuring strength and toughness, and its lower limit is 0.8%. However, if the Mn content is too large, the hardenability increases and the weldability and HA
Not only the Z toughness is deteriorated, but also the strength of the base material that meets the target standard cannot be obtained. Therefore, the upper limit of the amount of Mn is set to 1.6%. P: If too much P deteriorates toughness and strength in the plate thickness direction, the upper limit was made 0.03% or less. S: S, like P, too much deteriorates toughness and strength in the plate thickness direction, so the upper limit was made 0.005% or less.

Mo: Mo is an element indispensable for ensuring high temperature strength, and is particularly effective for strengthening ferrite.
Therefore, it is necessary to add 0.35% or more, but
If it exceeds 80%, the weldability and base material toughness deteriorate, so
The upper and lower limits are 0.35% and 0.80%. Ti: T
iN is effective for making the structure finer and producing very fine and uniform γ grains at the time of reheating and tempering,
If the content is 0.005% or less, the effect is diminished.
005%. On the other hand, if it exceeds 0.025%, the toughness is significantly impaired, so the upper limit is made 0.025%. A
l: Al is an element generally contained in deoxidized upper steel, but S
Since deoxidation is also performed by i and Ti, the lower limit of Al is not limited in the present invention. However, if the amount of Al increases, the cleanliness of the steel deteriorates and the toughness of the welded portion deteriorates, so the upper limit was made 0.06%. N: N is generally contained in steel as an unavoidable impurity, but combines with Ti to form a carbonitride TiN and exerts an effect of improving toughness. Therefore, the minimum amount is 0.001%,
If the amount of N is large, the HAZ toughness is deteriorated and surface defects of the continuous cast slab are promoted, so the upper limit is 0.00.
It was 6%.

The basic components of the steel of the present invention are as described above,
Although the objects can be sufficiently achieved, in order to further improve the characteristics for the purposes, the elements described below, namely Nb, V, Ni,
By selectively adding one or more of Cu, Cr and Ca, more preferable results can be obtained with respect to improvement in strength and toughness. Nb: Nb forms fine carbonitrides and is effective in securing strength and toughness at room temperature. When it is contained in an amount of 0.005% or less, it has no effect. Deteriorate. Therefore, the Nb content is 0.00
It was limited to 5 to 0.05%. V: 0.005-0.10
In the range of%, the HAZ toughness is improved. But,
If it is less than 0.005%, there is no effect, and if it exceeds 0.10%, there is an unfavorable effect on the HAZ toughness. Ni: Ni improves the strength and toughness of the base metal without adversely affecting the weldability and HAZ toughness, but the hardening is thin at 0.05% or less, and the addition of 0.5% or more is the purpose as a building steel. On the other hand, since it becomes extremely expensive and the economy is lost, the upper limit was made 0.5%.

Cu: Cu has substantially the same effect as Ni, and also has the effect of increasing the high strength due to Cu precipitates, and improving the corrosion resistance and weather resistance. However, if the Cu content exceeds 1.0%, Cu cracking occurs during hot rolling, which makes production difficult, and if it is 0.05% or less, there is no effect, so the Cu content is 0.05 to 0.1%. limit. Cr: Cr enhances the strength of the base material and the welded portion, and improves corrosion resistance and weather resistance at 0.05% or more, but deteriorates weldability and HAZ toughness at more than 1.0%, and 0.05% or less. So the effect is weak. Therefore, the Cr amount is limited to 0.05 to 0.1%. Ca:
Ca has the effect of controlling the morphology of MnS, increasing Charpy absorbed energy, and improving low temperature toughness. However, 0.001% or less has no practical effect, and
If it exceeds 0.006%, a large amount of CaO and CaS is formed to form large inclusions, which not only impairs the toughness of the steel but also the cleanliness, and also has an adverse effect on weldability. 0.001 to 0.006%.

[0017]

Example A slab having the composition shown in Table 1 was produced and produced under the conditions shown in Table 2. The mechanical properties and fire resistance properties of the obtained steel are summarized in Table 2.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

[Table 3]

[0021]

[Table 4]

Since steels 11 to 16 do not have proper chemical compositions, proper material properties cannot be obtained. First, steel 11 has a small amount of C and cannot obtain strength. In Steel 12, since the Mn content is small, strength cannot be obtained. Steel 13 has little Mo and lacks strength. Steel 14 has a large amount of C, and it is difficult to secure a low yield ratio and toughness. Steel 15 has a large amount of Mn and has deteriorated toughness. Steel 16 has a large amount of Mo, and it is difficult to secure a low yield ratio and toughness. Further, the steels 17 to 20 are out of the manufacturing conditions of the present invention, and proper material characteristics cannot be obtained. Steel 1
No. 7 has insufficient strength at room temperature due to DQT, and it is difficult to reduce yield and secure toughness. Steel 18 does not have a low yield ratio due to the QT treatment. Steel 19 cannot be secured at room temperature strength as it is rolled. Steel 20 has a low room temperature strength due to the normalizing treatment. On the other hand, the steels 1 to 10 which are steels of the present invention are at room temperature,
High temperature strength, low yield ratio and high toughness are all satisfied.

[0023]

Industrial Applicability According to the present invention, it is possible to manufacture a thick plate having a low yield ratio and excellent strength at high temperature, which is an industrially great invention.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location C22C 38/50

Claims (2)

[Claims] 1. By weight%, C: 0.05 to 0.15%, Si: 0.6% or less, Mn: 0.8 to 1.6%, P: 0.03% or less, S: 0.0. 005% or less, Mo: 0.35
0.80%, Ti: 0.005 to 0.025%, Al: 0.06% or less, N: 0.006 or less, and the balance is 850 to 1000 after hot rolling steel containing iron and inevitable impurities. Yield ratio characterized by being reheated to a temperature range of ℃, quenched, subsequently reheated to a temperature range of 740 to 820 ℃, and then quenched again, and then tempered in a temperature range of 450 to 700 ℃. Of low fire resistance steel plate for construction. 2. Nb: 0.005-0.05% by weight, V: 0.005-by weight
0.1%, Ni: 0.05 to 0.1%, Cu: 0.05 to
0.1%, Cr: 0.05 to 1.0%, Ca: 0.001 to
0.006% of 1 type (s) or 2 or more types are contained, The manufacturing method of the fireproof steel plate for buildings with a low yield ratio of Claim 1 characterized by the above-mentioned.