JPH0665675A - High tensile strength steel for building excellent in fire resistance and its production - Google Patents

Abstract

PURPOSE:To produce high tensile strength steel for building excellent in fire resistance by preparing high tensile strength steel having a specified componental compsn. in which the contents of Cu, Mo and V are prescribed. CONSTITUTION:High tensile strength steel contg., by weight, 0.03 to 0.10% C, <=0.60% Si, 0.8 to 1.6% Mn, 0.5 to 1.5% Ni, 0.8 to 1.5% Cu, 0.8 to 1.5% Mo, 0.005 to 0.05% V, 0.005 to 0.02% Ti, <=0.10% Al and 0.001 to 0.006% N and furthermore contg. at need, 0.05 to 0.50% Cr and one ore more minds among 0.0005 to 0.003% Ca are 0.001 to 0.005% rare earth elements, and the balance Fe with inevitable impurities is produced. In this way, the high tensile strength steel for building having high strength and excellent in high temp. strength can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel having excellent fire resistance used for various structures in the fields of construction, civil engineering, etc., and a method for producing the same.

[0002]

2. Description of the Related Art As various building steel materials in the fields of construction, civil engineering, etc., steel materials standardized by JIS etc. are widely used. By the way, when using the above-mentioned steel materials for buildings, offices, houses, multi-storey parking lots, etc., in order to ensure safety in the case of fire, it is obligatory to apply a sufficient fireproof coating. The law stipulates that the steel material temperature should not exceed 350 ° C during a fire.

That is, the yield strength of the steel material is about 2/3 at room temperature at about 350 ° C., which is less than the required strength. When a steel material is used for a building, it is used with a fireproof coating so that the temperature of the steel material does not reach 350 ° C. in case of fire. Therefore, the fireproof coating work cost becomes higher than the steel material cost, and the construction cost is unavoidably increased. Recently, in order to solve the above problems, for example,
Japanese Patent Application No. 01-139328 and Japanese Patent Application No. 01-13932
There is a proposal according to the specification.

However, Japanese Patent Application No. 01-139328
In No. 6, steel with a considerable amount of Mo and Nb added, 600 ° C
The yield strength is 70% or more of the room temperature yield strength, but even the highest strength (tensile strength) is 65 kgf / mm ²
It was difficult to manufacture steel having higher strength and higher fire resistance.

Also, in Japanese Patent Application No. 01-139329,
The steel is strengthened by rolling with water containing a considerable amount of Mo and then water-cooling it.
It was 3 kgf / mm ² , and it was difficult to manufacture steel with high strength and excellent fire resistance.

[0006]

When a steel material is used for a building, ordinary steel cannot be used as an uncoated or light coating because of its low high-temperature strength, and an expensive fireproof coating must be applied. There wasn't. Also, with newly developed steel,
The strength of steel materials is up to 60 kg class, and it has been difficult to apply steel of higher strength to large structures and structures with large spans. An object of the present invention is to provide a steel material having high strength and excellent high temperature strength, and a method for manufacturing the steel material.

[0007]

The specific means of the present invention will be described below.

(1) C: 0.03 to 0.10.
%, Si: 0.60% or less, Mn: 0.8 to 1.6%,
Ni: 0.5-1.5%, Cu: 0.8-1.2%, M
o: 0.8-1.5%, V: 0.005-0.05%,
Ti: 0.005 to 0.02%, Al: 0.10% or less, N: 0.001 to 0.006%, the balance being Fe and inevitable impurities.

(2) C: 0.03 to 0.10.
%, Si: 0.60% or less, Mn: 0.8 to 1.6%,
Ni: 0.5-1.5%, Cu: 0.8-1.2%, M
o: 0.8-1.5%, V: 0.005-0.05%,
After reheating a steel slab containing Ti: 0.005 to 0.02%, Al: 0.10% or less, N: 0.001 to 0.006% and the balance Fe and inevitable impurities to 1100 to 1300 ° C. , The hot plastic working is finished at 800 to 1000 ° C. and left to cool as it is, or water cooled and cooled to room temperature, and then reheated to a temperature in the range of 750 to 850 ° C. Water cooled to 500 ° C
A method for producing a high-strength steel for construction having excellent fire resistance, which comprises reheating to a temperature of 700 ° C. or higher and tempering.

(3) Cr: 0.05 to 0.50 by weight ratio
% And Ca: 0.0005 to 0.003%, REM:
The high-strength architectural steel having excellent fire resistance according to the above (1) or (2), which contains one or two elements from each group of 0.001 to 0.005%, and a method for producing the same.

[0011]

The present inventors have found that a steel material having a tensile strength of more than 70 kgf / mm ² and used for a large structure requiring fire resistance and having excellent fire resistance must also have weldability and a low yield ratio. I knew that. That is, in large-scale structures that require fire resistance, earthquake resistance is almost always required, and low YR of the steel material used is an essential property.

Further, even if the YR is low and the fire resistance is good, if the weldability is poor, the welding construction cost increases significantly. In such cases, no matter how high the fire resistance of the steel material is used, the construction cost becomes high and the advantage of using the high strength steel is reduced by half. In addition, in such high strength steel, a method of using a large amount of alloying elements that enhance hardenability in order to improve high temperature strength has been generally used. For this reason,
When the high temperature strength was increased, the weldability was almost inevitably deteriorated.

Now, the feature of the present invention is that a low C steel contains a considerable amount of C.
After reheating a steel slab having a composition composition in which u, Mo and V are added together at a high temperature, rolling is finished at a relatively high temperature, and the steel is cooled to room temperature by air cooling or water cooling, and then the γ-α two-phase region. By reheating to water and water cooling, the microstructure is a mixture of bainite mainly to bainite-tempered martensite, which has a room temperature proof stress of 55 kgf / mm ² or more, and 6
The yield strength at 00 ° C is ⅔ or more of room temperature, and it has low YR and good weldability.

It is known that a method of increasing Mo or Cr is effective for improving the proof stress at 600 ° C. with respect to the proof stress at room temperature. However, this method deteriorates the weldability, and for the reasons described above, the merit is not sufficient.

In addition to the addition of Mo, the present inventors have found that C
It has been found that by adding u and V in combination, it is possible to realize a high-strength steel with excellent weldability and less deterioration in weldability. However, the addition of Cu and V leads to an increase in YR, and the amount added should be limited. YR
Can be achieved by combining the above alloy components with the above manufacturing method.

Next, the characteristic component elements according to the present invention and the addition amounts thereof will be described. Mo forms solid solution strengthening and forms fine carbides and increases high temperature strength, but addition of Mo alone requires addition of more than 2% to secure high strength and 600 ° C strength. There is a problem that the property deteriorates and the manufacturing cost of the steel material increases.

As a result of intensive studies, the present inventors have found that it is extremely effective to add Cu and V together in addition to Mo. However, Mo, Cu, V
If the addition amount of is too large, the weldability and the toughness of the weld heat affected zone deteriorate, and YR also increases, so the upper limits of Mo, Cu, V contents are Mo: 1.5%, Cu: 1.2%, V: 0.05%
And need to. In addition, the lower limit is Mo: 0.8%, Cu: 0.8%, V: 0.
It is necessary to contain 005%.

In order to add Mo, Cu and V together to secure the strength at room temperature and 600 ° C., it is important that the conditions for reheating, rolling and cooling the steel as well as the steel components are important.

6 by the composite addition of Mo, Cu and V described above
In order to increase the 00 ° C. strength, these elements must be sufficiently solution-treated at the time of reheating. Therefore, the lower limit of the reheating temperature is 1100 ° C. If the reheating temperature is too high, the austenite grains become coarse and the low temperature toughness deteriorates, so the upper limit must be set to 1300 ° C.

Further, the reason why the rolling end temperature is set to a high temperature of 800 ° C. or higher is to prevent precipitation of carbides of Mo and Cu and carbonitrides of V during rolling. The size of the product increases and the high temperature strength decreases significantly. Further, at the end of rolling in the temperature range exceeding 1000 ° C, the toughness is extremely lowered, so 1000 ° C is the upper limit of the rolling end temperature.

In order to lower the YR and optimize the strength at room temperature and 600 ° C., the following method is essential. After rolling, air or water cool to room temperature, then 750-85
It was found that reheating to a temperature in the range of 0 ° C., water cooling from that temperature to room temperature, reheating to a temperature of 500 ° C. to 700 ° C. or lower, and tempering were effective.

That is, after the completion of rolling, it is a method of air-cooling or water-cooling to room temperature and then reheating to a γ-α two-phase coexisting region of 750 to 850 ° C. In this two-phase coexistence region, the solid solubility limit of C is different, so diffusion of C from α to γ is promoted, and
An extremely low C bainite is formed in a portion where C is decreased, and martensite or bainite is formed in a portion where C is concentrated.

Since the extremely low C portion has a low local strength,
Yield strength (proof stress) decreases. On the other hand, the portion where C is concentrated has a locally high strength, so the tensile strength does not decrease. As a result, YR decreases. Moreover, the steel produced by such a method has a high strength ratio of 600 ° C. to room temperature.

Further, when reheating at 750 to 850 ° C., Cu
A part of the precipitates and the precipitates such as V ₄ C ₃ and VN are dissolved (solid solution) and re-precipitated during the subsequent tempering, which serves to increase the strength. This is because the solid solution of Cu and V occurs at a relatively low temperature, and in the case of Nb, the solid solution is much less at the same temperature, so that there is no effect of increasing the strength. That is, in the manufacturing method of the present invention, in order to secure high temperature strength,
Cu, V addition, reheating to γ-α two-phase temperature range and water cooling are essential.

Next, the reasons for limiting the components other than Mo, Cu and V will be described in detail. C is necessary to secure the strength of the base material and the welded portion and to exert the effects of Mo and V. If the content is 0.03% or less, the effect is diminished, so the lower limit is 0.
It is set to 03%. However, if the amount of C is too large, the weldability is impaired and the toughness of the base metal and the weld heat affected zone (hereinafter HAZ) is adversely affected, so 0.10% is the upper limit.

Si is an element contained in steel for deoxidation, but if too much, it deteriorates the weldability and HAZ toughness.
The upper limit was set to 0.60%.

Mn is an important 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 deteriorates, so the upper limit was made 1.6%.

Cu is an important element that does not increase in hardenability and forms Cu precipitates to improve the strength at room temperature and 600 ° C. Therefore, the lower limit is 0.8%, which is effective for strength. If the amount of Cu is too large, the amount of Cu precipitates increases, which increases the yield strength and YR, so the upper limit is 1.2%.

Ni is an important element for improving the strength and toughness, but it also increases the cost. In the steel of the present invention,
Since a considerable amount of Cu is added, it is an important element for preventing hot cracking of Cu. Therefore, the lower limit is 0.5
%, The upper limit is 1.5%.

Ti produces Ti nitride and improves the HAZ toughness, but if less than 0.005%, the effect is small, and
If it exceeds 02%, TiC is generated and the toughness is deteriorated.
The upper limit was 0.02%.

Al is an element contained in steel for deoxidation, but since deoxidation is also performed by Si and Ti, the lower limit is not limited. If the amount of Al increases, the cleanliness of the steel deteriorates and the HAZ toughness is impaired, so the upper limit was made 0.10%.

N is generally contained in steel as an unavoidable impurity, but when the amount of N is large, it promotes the generation of surface flaws in the continuous casting slab and also impairs the HAZ toughness, so the upper limit is set to 0.
It was set to 006%.

The steel of the present invention contains P as an unavoidable impurity.
And S are contained. Since P and S have little influence on the high temperature strength, their amounts are not limited. In general, the properties of steel such as toughness and strength in the plate thickness direction are improved as P and S are reduced. Desirable P and S amounts are 0.02 each
%, 0.005% or less.

The basic components of the steel of the present invention are as described above, and the object can be sufficiently achieved. Further, the strength and toughness can be improved by selectively adding Cr, and the toughness and the strength in the plate thickness direction can be improved by selectively adding Ca and REM.

Cr is an element that enhances the strength of the base metal and the welded portion and the high temperature strength. If it exceeds 0.5%, the weldability and H
Since the AZ toughness is deteriorated and the effect is small at less than 0.05%, the Cr content is limited to 0.05 to 0.5%.

Ca and REM control the morphology of sulfide (MnS), improve the low temperature toughness, and also exhibit the effect of improving the hydrogen-induced cracking resistance. However, the Ca amount of 0.
If it is less than 0005%, there is no practical effect, and 0.00
If it exceeds 3%, a large amount of large inclusions of CaO and CaS will be generated, impairing toughness and adversely affecting weldability, so 0.0005 to 0.003% was made the limiting range. R
EM has the same effect as Ca, and if the addition amount is large, the same problem as Ca occurs. Therefore, the limited range of REM was set to 0.001 to 0.005%.

Even if the steel of the present invention is manufactured and then reheated to a temperature below the Ac ₁ transformation point for the purpose of dehydrogenation, the characteristics of the steel of the present invention will not be impaired.

[0038]

[Examples] Various steels were manufactured by a converter, continuous casting, and a thick plate process, and room temperature strength, high temperature strength, etc. were investigated. Table 1 No. 1 to
No. No. 15 is the steel of the present invention. 16-No. 20 shows the chemical composition of comparative steel. The continuation of Table 1 shows the mechanical properties of the steel of the present invention and the comparative steel under heating and rolling / cooling conditions.

[0039]

[Table 1]

[0040]

[Table 2]

Steel No. of the present invention 1-No. In Example 15, all have good room temperature and high temperature strength, and the impact value at −20 ° C. is as high as 25 kgf-m 2. Also, YR is 84
%, The ratio of 600 ° C. strength to room temperature strength (YS ratio) was 69% or more.

On the other hand, comparative steel No. In 16, the amount of C is too high and the YS ratio is satisfactory, but the impact value is 1.6 kgf-m.
The YR is 91%, which is less than 85%.
No. In No. 17, since the amount of Mo is low, the YS ratio is as low as 58%, and 2/3 or more at room temperature cannot be satisfied. Also, YR is 85
It was a value exceeding%. No. In No. 18, since V was not added, the YS ratio was as low as 63%, and 2/3 or more at room temperature could not be satisfied. No. In 19, since the amount of Cu is too low,
The YS ratio is as low as 65%, which cannot satisfy 2/3 or more at room temperature. No. In 20, since the C content is too low, YS ratio is high room temperature YS low as 54kgf / mm ^2, can not satisfy the 55 kgf / mm ² or more, YR also reached 90 percent.

[0043]

EFFECTS OF THE INVENTION The steel material produced by the chemical composition and the production method of the present invention has a yield strength (YS) at room temperature of 55 kgf / mm ² or more and YR
Is 85% or less, YS at 600 ° C. is 2/3 or more of YS at room temperature, the impact value is good, etc., and Pcm is 0.275% or less, which has good weldability. It is a completely new steel material that has never been seen before and has the properties required for a steel material.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Seiji Isoda 1 Kimitsu, Kimitsu City Nippon Steel Corporation Kimitsu Works

Claims (4)

[Claims] 1. By weight ratio, C: 0.03 to 0.10% Si: 0.60% or less Mn: 0.8 to 1.6% Ni: 0.5 to 1.5% Cu: 0.8 To 1.2% Mo: 0.8 to 1.5% V: 0.005 to 0.05% Ti: 0.005 to 0.02% Al: 0.10% or less N: 0.001 to 0. 006% High-strength steel for construction with excellent fire resistance, the balance being Fe and inevitable impurities. 2. One or two kinds of elements in each group of Cr: 0.05 to 0.50% and Ca: 0.0005 to 0.003% REM: 0.001 to 0.005% by weight. The high-strength steel for construction with excellent fire resistance according to claim 1, comprising: 3. By weight ratio, C: 0.03 to 0.10% Si: 0.60% or less Mn: 0.8 to 1.6% Ni: 0.5 to 1.5% Cu: 0.8 To 1.2% Mo: 0.8 to 1.5% V: 0.005 to 0.05% Ti: 0.005 to 0.02% Al: 0.10% or less N: 0.001 to 0. 006% 110 of a steel slab containing the balance Fe and unavoidable impurities
After reheating to 0 to 1300 ° C, hot plastic working is 800 to 1
Finish at 000 ° C and allow to cool as it is, or cool with water and cool to room temperature, then reheat to a temperature in the range of 750 to 850 ° C, and then water cool from that temperature to room temperature,
Furthermore, reheat to a temperature of 500 ° C or higher and lower than 700 ° C,
A method for producing a high-strength steel for construction having excellent fire resistance, which is characterized by tempering. 4. One or two elements selected from the group consisting of Cr: 0.05 to 0.50% and Ca: 0.0005 to 0.003% REM: 0.001 to 0.005% by weight. The method for producing a high-strength steel for construction having excellent fire resistance according to claim 3, characterized by containing.