JPH06104856B2 - Manufacturing method of low yield ratio steel plate with excellent fire resistance for construction - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a low yield ratio steel plate for construction having excellent fire resistance, which is mainly used for various constructions in the fields of construction, civil engineering, offshore structures and the like.

(Prior Art) As a building material for various structures in the fields of construction, civil engineering, marine structures, etc., rolled steel for general structure (JIS G 310
1), Welded structural rolled steel (JIS G 3106), Welded structural weather resistant hot rolled steel (JIS G 3114), High weather resistant rolled steel (JIS G 3125), (hereinafter known as steel plate), etc. are widely used. Has been done.

In order to ensure safety in the case of using the well-known steel material in buildings, offices, general houses, etc., which are closely related to daily life, among various kinds of buildings, for example, in order to ensure safety in
Sufficient fireproof coating is applied as in the technique described in Japanese Patent Laid-Open No. 63-47451. Therefore, the construction cost of fireproof coating becomes higher than the steel material cost, and the construction cost rises significantly. This is because the building-related laws and regulations stipulate that the steel material temperature should not exceed 350 ° C during a fire.

In recent years, the number of buildings has increased, and due to the improvement in design technology and its high reliability, the fireproof design has been reviewed.
In 1987, the new fireproof design method for buildings was legalized,
It is possible to determine the capacity of the fireproof coating by the high temperature strength of the steel material and the load actually applied to the building without depending on the temperature limit of 350 ° C described above, and when the high temperature strength of the material steel sheet is secured, etc. It became possible to use steel sheets without coating.

In conventional low alloy high strength steel, it is difficult to secure high temperature strength due to grain growth, coarsening of precipitates, dissolution of carbides, etc.In order to secure high temperature strength, a large amount of expensive metal element is added. It must be done and loses economic efficiency as a building steel.

Therefore, the inventors have developed a technology for manufacturing a low yield ratio high tensile strength steel sheet for buildings that has a relatively small amount of expensive alloying elements and is excellent in fire resistance and can make the fireproof coating thin (patent application 63-143740).

According to this method, Nb: 0.005-0.04%, and Mo: 0.4-0.7%
By the combined addition of Nb and Mo, fine carbonitrides of Nb and Mo can be formed, and the high temperature strength can be increased by solid solution strengthening of Mo.

However, when Mo is added in an amount of 0.4 to 0.7%, it is expensive and therefore economically difficult.In addition, since Mo is an element that enhances hardenability, it also increases the strength at room temperature, which reduces weldability and reduces welding heat. Affected zone (HAZ) causes deterioration of toughness. Therefore, there is a demand for a steel plate for construction that is cheaper, has good weldability, and has excellent fire resistance.

(Problems to be Solved by the Invention) An object of the present invention is to provide a method for producing a low yield ratio steel plate for construction, which has excellent high temperature characteristics and weldability, and which can be supplied to the market at an economical price and has excellent fire resistance. It is a thing.

(Means for Solving the Problem) The gist of the present invention is, by weight%, C: 0.04 to 0.12%, Si: 0.6%.
Below, Mn: 0.5-1.6%, Cu: 0.8-1.5%, Ti: 0.005-0.10
%, Al: 0.1% or less, N: 0.001 to 0.006%, the balance being Fe and inevitable impurities, or Nb:
0.005-0.04%, V: 0.005-0.10%, Mo: 0.05-0.4%, N
i: 0.05-0.5%, Cr: 0.05-1.0%, Ca: 0.0005-0.005%
One or two or more of these are contained, and the balance is a steel slab containing Fe and unavoidable impurities at 900 to 1100 ° C.
After heating in the temperature range of 1, the rolling is completed at 800 ° C. or higher, and then air cooling is performed.

(Function) In order to use it as a steel material for construction, it is important to secure weldability as well as fire resistance. This is because if weldability is poor, a large increase in construction cost cannot be avoided.
Therefore, the balance between room temperature strength and high temperature strength is particularly important from the viewpoint of weldability and fire resistance. That is, it is important to keep the room temperature strength relatively low from the viewpoint of weldability and to secure the high temperature strength from the viewpoint of fire resistance.

As a result of further intensive research, the inventors have found that the use of Cu precipitation strengthening is effective from the viewpoint of the balance between room temperature strength and high temperature strength and from the viewpoint of ensuring room temperature strength. That is, when Cu-added steel is rolled and then air-cooled, Cu precipitation does not occur at room temperature, so that the strength can be suppressed low and excellent weldability can be secured.

Also, at the time of fire, high temperature strength can be secured by Cu precipitation strengthening, and excellent fire resistance can be secured. As described above, it is the main object of the present invention to provide a steel sheet for construction in which the room temperature strength and the high temperature strength are well balanced.

Furthermore, by using Cu precipitation strengthening, the amount of expensive Mo added can be greatly reduced to 0.05-0.4%, which is excellent in economic efficiency.
In addition, the weldability and HAZ toughness can be dramatically improved.

In order to obtain such an excellent effect of Cu addition, the addition amount thereof needs to be 0.8 to 1.5%. The upper limit of the Cu content is to prevent deterioration of HAZ toughness due to coarsening of Cu precipitates and increase in hardenability. The lower limit is the minimum amount for obtaining sufficient precipitation strengthening.

The reasons for limiting the other basic components will be described below.

The lower limit of 0.04% of the amount of C is the minimum amount for ensuring the strength of the base material and the welded portion and exerting these effects when Nb, V and the like are added. However, if the C content is too large, not only the HAZ toughness is adversely affected but also the base metal toughness and weldability are deteriorated, so the upper limit was made 0.12%.

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 is also acceptable. From the viewpoint of HAZ toughness, it is desirable that the content of Si be about 0.15%.

Mn is an essential element for ensuring strength and toughness, and its lower limit is 0.5%. However, if the amount of Mn is too large, not only the hardenability increases and the weldability and HAZ toughness deteriorate, but also the base metal strength that meets the target specifications cannot be obtained. Therefore, the upper limit of the amount of Mn is set to 1.6%.

Ti is an element that forms fine precipitates and increases high-temperature strength, and forms an oxide or carbonitride of Ti when the amount of Al is small at 0.005 to 0.1%, and improves the HAZ toughness.
If it is less than 05%, there is no effect, and if it exceeds 0.1%, it is unfavorable because it adversely affects the weldability for the purpose.

Al is an element generally contained in deoxidized upper steel, but Si and
Since Ti is also deoxidized, 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 HAZ toughness deteriorates, so the upper limit was made 0.1%.

Although N is generally contained in steel as an unavoidable impurity, it combines with Nb to form a carbonitride Nb (CN), which is effective in improving high-temperature strength. Therefore, the minimum amount is 0.00
Although 1% is required, the HAZ toughness deteriorates when the N content increases, so the upper limit was made 0.006%.

The steel of the present invention contains P and S as unavoidable impurities. Since P and S have little effect on high temperature strength,
Although the amount thereof is not particularly limited, the properties of steel such as toughness and strength in the plate thickness direction are generally improved as the amount of these P and S elements decreases. Desirable P and S contents are 0.02% and 0.0, respectively.
It is less than 05%.

The basic components of the steel of the present invention are as described above, and although the object can be sufficiently achieved, in order to further improve the properties for the purpose,
By selectively adding the elements described below, that is, Nb, V, Mo, Ti, Ni, Cr, and Ca, more preferable results can be obtained with respect to improvement in strength and toughness.

Next, the above-mentioned additional element and its addition amount will be described.

Nb is an element that forms fine precipitates and increases high temperature strength. The lower limit for obtaining this effect is 0.005%. N
If the amount of b is too large, the weldability deteriorates and the HAZ toughness deteriorates, so the upper limit is 0.04%.

V is an element having almost the same effect as Vb, and although the effect on high temperature strength is smaller than that of Nb, it improves the HAZ toughness in the range of 0.005 to 0.10%. However, if it is less than 0.005%, it has no effect, and if it exceeds 0.10%, it has an unfavorable effect on the HAZ toughness.

Mo forms fine precipitates like Nb, and further increases high temperature strength by solid solution strengthening. The lower limit for obtaining this effect is 0.05%. If the amount of Mo is too large, the weldability deteriorates and the HAZ toughness deteriorates, so the upper limit is 0.4%. Ni improves the strength and toughness of the base metal without adversely affecting the weldability and HAZ toughness, but the effect is weak at less than 0.05%, and for the purpose as a structural steel exceeding 0.5%,
The upper limit was set to 0.5% because it becomes extremely expensive and loses economy.

Cr is an element that enhances the strength of the base metal and the weld, but Cr
If the amount exceeds 1.0%, weldability and HAZ toughness are improved, and it also becomes 0.
If less than 05%, the effect is weak. Therefore, the Cr content is 0.05 to 1.0.
%.

Ca controls the morphology of sulfides, increases Charpy absorbed energy, improves low temperature toughness, and is also effective in improving hydrogen-induced cracking resistance. However, if the amount of Ca is less than 0.0005%, there is no practical effect, and if it exceeds 0.005%,
Since a large amount of O and CaS are generated and become large inclusions, which not only impairs the toughness of steel but also the cleanliness and also has an adverse effect on weldability, the range of Ca addition is set to 0.0005 to 0.005%.

The effects of the present invention are of course not brought about by specifying only the above component systems. That is, the rolling conditions are also extremely important.

First, the heating temperature of the slab needs to be 900 to 1100 ° C. The reason for setting the upper limit to 1100 ° C is that heating above this temperature causes hot embrittlement called Cu embrittlement. Also
The lower limit was set to 900 ° C because it is necessary to sufficiently solutionate elements such as Cu. Immediately after slab casting (so-called C
C-Direct rolling) Rolling may be performed.

The reason for setting the rolling end temperature to 800 ° C. or higher is that when rolling is performed at a temperature lower than this temperature, Cu is strain-induced precipitation due to rolling, and the high temperature strength is significantly reduced. The upper limit of the rolling end temperature is preferably 1000 ° C or lower. This is to ensure the toughness of steel for construction. It is necessary to air-cool the steel sheet after rolling, and the cooling rate is preferably 5 ° C / sec or less.

Well-known low-temperature rolling (controlled rolling) is an essential requirement for steel materials that require low-temperature toughness, such as line pipes, but there is no high requirement for low-temperature toughness like the steel of the present invention, rather the balance between room-temperature strength and high-temperature strength is important. In that case, the rolling must be finished at high temperature. This is also important as a condition for reducing the yield ratio.

The tensile strength of the steel sheet according to the present invention is 40 to 60 kg f / mm ² ,
The plate thickness range is preferably a steel plate obtained by rolling a plate having a thickness of 10 mm or more.

(Examples) Steel plates (thickness: 12 to 50 mm) of various steel components were manufactured by a well-known converter, continuous casting, and thick plate process, and room temperature strength, high temperature strength, etc. were investigated.

Table 1 shows the composition comparison of the steel of the present invention and the comparative steel, and then Table 2 shows the mechanical properties under heating and rolling cooling conditions.

As is clear from Table 2, the steels according to the present invention all have good room temperature and high temperature strength. On the other hand, the comparative steels are all unsuitable as fire-resistant building materials because their strength at room temperature is too high or their strength at high temperature is insufficient, and their strength ratio at room temperature is 600 ° C (70% or less).

In Comparative Steel, Steel 31 does not have high temperature strength because Cu is not added. Steel 32 has Cu added, but the added amount is less than 0.8%, so high temperature strength cannot be secured. Steel 33 cannot secure high-temperature strength because the amount of Cu added is too large. Since the heating temperature of Steel 34 is low, the solution of Cu is not sufficient and high-temperature strength cannot be secured. Since Steel 35 has a high heating temperature, Cu cracks are generated and the characteristics of the steel sheet are not sufficiently satisfied. Since Steel 36 has a low rolling finish temperature, Cu precipitation occurs and the high temperature strength is not satisfied.

(Effect of Invention) Building fire countermeasures are a social issue, and fire countermeasures are an important item among them. Under the circumstances, the present invention makes it possible to inexpensively mass-produce a steel sheet having excellent earthquake resistance, fire resistance, and weldability. It is considered that it will greatly contribute to the solution of the above social issues.

Claims (2)

[Claims] 1. By weight%, C: 0.04 to 0.12%, Si: 0.6% or less, Mn: 0.5 to 1.6%, Cu: 0.8 to 1.5%, Ti: 0.005 to 0.10%, Al: 0.1% or less, N : 0.001 to 0.006%, 900 to 110 for steel bill with balance of Fe and unavoidable impurities
A method for manufacturing a low yield ratio steel sheet for buildings having excellent fire resistance, which comprises heating in a temperature range of 0 ° C, finishing rolling at 800 ° C or higher, and then air cooling. 2. By weight%, Nb: 0.005 to 0.04%, V: 0.005 to 0.10%, Mo: 0.05 to 0.4%, Ni: 0.05 to 0.5%, Cr: 0.05 to 1.0%, Ca: 0.0005 to 0.005% Further contains any one of the above, or two or more thereof,
The method for producing a low yield ratio steel sheet for construction having excellent fire resistance according to claim 1, wherein the balance is a steel slab composed of Fe and unavoidable impurities.