JPH0747771B2 - Manufacturing method of structural steel with excellent fire resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention is intended for a steel plate used for manufacturing a structure, and has sufficient strength in a fire even if the coating of the refractory material is simplified or omitted. Involved in steel manufacturing methods.

(Prior art) For structures such as steel structures, in order to ensure sufficient strength even in the event of a fire, the steel is coated with a refractory material such as rock wool to prevent the temperature of the steel from rising above 350 ° C. It was mandatory to take measures.

In recent years, it has been permitted to simplify or omit the refractory coating depending on the strength of steel materials at high temperatures. That is,
It is said that if the steel has sufficient strength at high temperatures such as 600 ° C (2/3 or more of the normal yield strength at room temperature), the refractory coating can be omitted and bare use can be performed.

The strength of steel materials at high temperatures has been well investigated so far, and the developed materials with excellent high temperature strength have been standardized as steel for boilers or steel for pressure vessels. In addition, various improvements and developments are still in progress, as in Japanese Patent Publication No. S51-15188. These are steel materials having high strength when used for a long time such as tens of thousands or hundreds of thousands of hours at high temperatures, that is, high creep strength.

(Problems to be Solved by the Invention) In a structure such as a steel frame structure, a steel material having improved strength at 600 ° C., which is an important characteristic when omitting a fireproof coating, is required. It is intended to provide a method for manufacturing a steel material having a high strength within a few hours at the time of a fire.

(Means for Solving the Problems) As a result of various studies on the effects of chemical components and manufacturing processes on the strength of structural steel materials at 600 ° C., the present inventors have found that Cr, M
It has been found that it is extremely effective to limit the contents of o, V, Nb and B and appropriately select the heating-rolling-heat treatment conditions.

The present invention has been made based on this finding. (1) Cr, Mo, V, Nb and B (% Cr) + 2% by weight
(% Mo) +10 (% V) +20 (% Nb) +200 (% B) = 0.2
~ 3% content and carbon equivalent (Ceq = C + Mn / 6 + Si / 24 +
Ni / 40 + Mo / 4 + Cr / 5 + V / 14) steel with 0.30 to 0.50% is heated to 1000 to 1280 ° C, hot rolling is completed at Ar _{3 to} 1000 ° C, and Ar ₃ −50 ° C to Ar ₃ +50 Start water cooling at a temperature of ℃, 20
After cooling with water to 0 ℃ or less, tempered at 400 ~ 600 ℃, and a method of manufacturing a structural steel material having excellent fire resistance strength,
(2) Cr, Mo, V, Nb and B (% Cr) + 2 in% by weight
(% Mo) +10 (% V) +20 (% Nb) +200 (% B) = 0.2
~ 3% content and carbon equivalent (Ceq = C + Mn / 6 + Si / 24 +
Ni / 40 + Mo / 4 + Cr / 5 + V / 14) 0.30 to 0.50% steel is heated to 1000 to 1280 ℃, hot rolling is completed at Ar _{3 to} 1000 ℃, and then Ar ₃ -50 ℃ to Ar ₃ A method for producing a structural steel material having excellent fire resistance strength, which is characterized by starting water cooling at a temperature of + 50 ° C, stopping water cooling at a surface temperature of 400 to 600 ° C, and then allowing to cool.

(Operation) Hereinafter, the present invention will be described in more detail.

0.1% C-0.15% Si-1.2% Mn-0.015% P-0.005% S-
After heating 0.55% Cr-0.32% Mo-0.05% V steel at 1250 ℃
Fig. 1 shows the ratio of the proof stress (0.2% proof stress) at room temperature to the refractory strength when rolled to a thickness of 35 mm, water-cooled at various temperatures, cooled to below 100 ° C, and tempered at 500 ° C for 30 minutes. Show.

In this case, when determining the fireproof strength at 600 ℃, considering the rising behavior of the steel material temperature during a fire, the test piece was heated in the temperature rising pattern as shown in Fig. 3, and after heating and holding at 600 ℃ for 15 minutes. 0.1
It was deformed at a tensile rate of 5% / min, and the strength at a plastic strain of 0.2% was determined as the fire resistance strength.

As shown in Fig. 1, the water cooling start temperature is Ar ₃ -50 ℃ or more Ar ₃
When the temperature is below + 50 ° C, the ratio of fireproof strength / room temperature yield strength improves,
Excellent fire resistance is obtained with respect to room temperature proof stress.

Here, Ar ₃ (℃) is Ar ₃ (℃) = -396C% + 24.6Si% -6
8.1Mn% -36.1Ni% -20.7Cu% -24.8Cr% + 29.6Mo% + 8
I got it from 68.

From the results shown in Fig. 1, the water cooling start temperature is Ar ₃ −50 ° C. or higher Ar ₃ +
Limit to 50 ° C or less.

Further, as shown in FIG. 2, Cr% + 2Mo% + 10V% + 20Nb% + 100B calculated by Cr, Mo, V, Nb and B in the steel.
When the value of% is 0.3% or more and 3% or less, the effect of the manufacturing process of the present invention is remarkable. Therefore, Cr, Mo, V,
The amount of Nb and B is Cr% + 2Mo% + 10V% + 20Nb% + 100B%
Value is limited to 0.3% or more and 3% or less. Cr, M
At least one kind of o, V, Nb and B may be included in an amount regulated by the above calculation formula, but two or more kinds may be added.

Ceq is Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Mo / 4 + Cr / 5 + Mo / 4
Defined as + V / 14, it is an index for weldability and has a strong correlation with strength at room temperature. Depending on the manufacturing heat treatment conditions, if Ceq is less than 0.30%, the strength as a structural steel material cannot be obtained, and if Ceq is more than 0.50%, the strength becomes too high, and ductility, toughness, and weldability deteriorate. Become.

Therefore, Cr% + 2Mo% + 10V% + 20 to secure fire resistance
In addition to regulating the value of Nb% + 100B% as described above, C, Si, Mn, Ni, etc. are regulated so that Ceq is 0.30 to 0.50%.

Each element is preferably within the following range.

C is an element effective in increasing the room temperature strength and the fire resistance strength, and is preferably added in an amount of 0.05% or more. However, if the addition amount is too large, the weldability is impaired, so the upper limit of the addition amount is preferably 0.15%.

Since Si is deoxidized, 0.02% or more is added, but if the addition amount is large, the toughness decreases, so the upper limit is preferably 0.5%.

Mn is an element effective for fixing S and increasing the strength, but if the addition amount is large, segregation in the material becomes remarkable and the anisotropy of toughness is increased, so Mn is preferably 0.1 to 1.5%. .

Ni is an element that improves the toughness of steel, and when such effects are required, it is added in an amount of 0.05% or more. However, if it exceeds 0.5%, the addition cost is too high and it is unsuitable as a structural steel material. Therefore, it is preferable to set the upper limit to 0.5%.

P is an element that lowers toughness and also causes microsegregation to impede weldability, so the upper limit is preferably made 0.03%.

S forms a non-metallic inclusion MnS in the steel, increases the toughness direction difference, and lowers the upper shelf energy in the Charpy test, so the upper limit is preferably made 0.02%.

Cu is an element that enhances the hardenability of steel and also improves the corrosion resistance. When such effects are required, 0.05% or more is added. However, addition of more than 0.5% impairs hot workability. Therefore, the upper limit of the amount of Cu added is preferably 0.5%.

Ti forms carbonitrides and has the effect of improving the fire resistance of steel. If you need such effects, 0.005%
The above additions are necessary. However, if it exceeds 0.05%, Ti
The upper limit is preferably set to 0.05% because C increases too much and adversely affects toughness.

Al is an element that is indispensable for deoxidation of steel, and also effectively fixes nitrogen, which is a factor that hinders the hardenability improvement effect of B.
Prevent the formation of. For these purposes, 0.01% or more is added. However, addition of more than 0.10% is unnecessary, so
01 to 0.10% is preferable.

N increases the refractory strength of the steel, but if the addition amount is too large, the weldability is impaired, so the addition amount is preferably 0.02% or less.

Next, heating-rolling-heat treatment conditions will be described.

Steel having the above-mentioned chemical components is obtained by subjecting it to melting in a converter or an electric furnace, and then subjecting it to ladle refining or vacuum degassing treatment if necessary, and ingoting it in a normal mold or a unidirectionally solidified mold. Later, it is made into a slab in chunks. Further, the slab may be directly manufactured from molten steel by a continuous casting method.

Any soaking and rolling in the slab may be used. That is, the slab may be cooled and then subjected to soaking, or the slab may be charged into the soaking furnace as a slab with hot pieces. After soaking at 1000 to 1300 ℃, slab is made by rolling or forging. The slab thickness is preferably about 1.3 to 2.5 times the product plate thickness.

The heating temperature before final rolling is 1000 because of the solid solution of the added elements.
℃ or above. However, if the temperature exceeds 1280 ° C, the austenite grains become too coarse and it becomes difficult to reduce the grain size by rolling. Therefore, the temperature is preferably 1280 ° C or less.

The rolling end temperature is not less than Ar ₃ temperature and not more than 1000 ° C. That is, when the temperature is lower than Ar ₃ temperature, the strength is rather decreased because rolling becomes a two-phase region rolling, and when it exceeds 1000 ° C., the austenite grains are not sufficiently refined by rolling and the structure becomes rough and it becomes difficult to secure toughness, which is preferable. Absent.

Next, as the cooling condition after rolling, accelerated cooling is adopted as described above. Water cooling initiation temperature is the Ar ₃ -50 ° C. or more Ar ₃ + 50 ° C. Temperature below. The water amount density depends on the plate thickness, but is preferably 0.5 m ³ / cm ² / s or more and 1.0 m ³ / cm ² / s. If it is 1.0 m ³ / cm ² / s or more, the cooling effect is saturated and unnecessary.

Water cooling is performed up to a temperature of 200 ° C or less, and tempering is performed at 400 to 600 ° C to adjust strength and toughness. If it is less than 400 ° C, the strength at room temperature is high and it is difficult to obtain low temperature toughness. When it exceeds 600 ° C, the toughness is improved but the strength required as a structural material cannot be obtained. During the water cooling after rolling, the surface temperature is
It is also possible to stop the water cooling at 0 to 600 ° C. and then allow it to cool to substitute for tempering.

Here, when the water cooling stop temperature is less than 400 ° C., the strength at normal temperature is high and it is difficult to obtain low temperature toughness. When it exceeds 600 ° C, the toughness is improved but the strength required as a structural material cannot be obtained.

The steel sheet produced in this way can be used as a structural material after processing such as cutting and welding.

(Example) Steel having the chemical composition shown in Table 1 was subjected to controlled rolling and controlled cooling under the conditions shown in Table 2, and tempered according to the water cooling end temperature. Second tensile test and Charpy impact test results
Shown in the table.

The steel numbers A1, B1, C1, D1, D2, E1 and F1 which are steels of the present invention show excellent room temperature strength, and the ratio of fire resistance strength to room temperature proof strength (PS ₆₀₀ / PS _RT ) is as high as 0.75 to 0.83. Toughness is also vRo> 10kgf
-M is excellent.

On the other hand, A2, B2, D3 and F3 have low rolling end temperature (A2), low water cooling stop temperature (B2) or too high tempering temperature (D3), so they not only have low strength at room temperature but also fire resistance. The strength is also low. Steel plate C2 has a sufficiently high rolling end temperature, and therefore has sufficient strength and fire resistance, but has poor toughness.
Steel plate F3 has a low tempering temperature, so its strength at room temperature is high, but its fire resistance is low and its toughness is poor. Steel plate G1 is Cr% ＋ 2Mo% ＋ 1
The value of 0V% + 20Nb% + 200B% is less than 0.2%, so the fire resistance is poor. Steel plate H1 is Cr% + 2Mo% + 10V% + 20Nb% +
The value of 200B% is larger than 0.2% and the fire resistance is sufficient, but C
eq is higher than 0.5% and toughness is poor.

(Effect of the invention) The steel sheet produced by this method not only satisfies the yield strength, tensile strength and toughness of the welded structural steel material (JIS G3106) at room temperature, but also has a high fire resistance strength, which is important as a fire resistant steel. It is excellent, and the refractory coating can be simplified or omitted in the production of buildings such as steel structures, which has great industrial value.

[Brief description of drawings]

Fig. 1 is a chart showing the change in the ratio of fireproof strength to room temperature proof strength depending on the water cooling stop temperature, and Fig. 2 is Cr% + 2Mo% + 10V% + 20Nb%.
Fig. 3 is a chart showing the change in the ratio of fireproof strength to room temperature yield strength by the value of + 200B%, and Fig. 3 is a chart showing the temperature rise pattern of the test piece when the fireproof strength is obtained.

Claims (2)

[Claims] 1. Cr, Mo, V, Nb and B (% C
r) +2 (% Mo) +10 (% V) +20 (% Nb) +200 (%
B) = 0.2 to 3%, and carbon equivalent (Ceq = C + Mn / 6
+ Si / 24 + Ni / 40 + Mo / 4 + Cr / 5 + V / 14) steel with 0.30 to 0.50% is heated to 1000 to 1280 ° C, hot rolling is finished at Ar _{3 to} 1000 ° C, and Ar ₃ −50 ° C to Ar _A method for producing a structural steel material having excellent fire resistance, which is characterized by starting water cooling at a temperature of ₃ + 50 ° C, cooling to 200 ° C or less, and then tempering at 400 to 600 ° C. 2. Cr, Mo, V, Nb and B (% C
r) +2 (% Mo) +10 (% V) +20 (% Nb) +200 (%
B) = 0.2 to 3%, and carbon equivalent (Ceq = C + Mn / 6
The + Si / 24 + Ni / 40 + Mo / 4 + Cr / 5 + V / 14) is from .30 to 0.50% steel, and heated to from 1,000 to 1,280 ° C., to exit the hot rolled at _{Ar 3 ~1000 ℃, Ar 3 -50} ℃ ~Ar _A method for manufacturing a structural steel material having excellent fire resistance, which is characterized by starting water cooling at a temperature of ₃ + 50 ° C, stopping water cooling at a surface temperature of 400 to 600 ° C, and then allowing to cool.