JPH072969B2 - Manufacturing method of thin low yield ratio steel for construction with excellent fire resistance and weldability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to the fields of architecture, civil engineering and marine structures, etc.
The present invention relates to a method for producing a low yield ratio steel having excellent fire resistance and weldability used in various structures.

(Prior Art) As is well known, as a building material for various structures in the fields of construction, civil engineering and marine structures, etc., rolled steel for general structure (JIS
G 3101), rolled steel for welded structure (JIS G 3106), weather resistant hot rolled steel for welded structure (JIS G 3114), high weather resistant rolled steel (JIS G 3125) and carbon steel pipe for general structure (JIS G
3444), square steel plates for general structure (JIS G 3466), etc. are widely used.

The well-known steel material is subjected to converter smelting after de-S and de-P of the hot metal usually obtained by a blast furnace, and is made into a steel slab in a continuous casting or slabbing process, and then hot plastic working to obtain desired characteristics. Be commercialized as equipped.

By the way, in the case of using the above-mentioned well-known steel materials for buildings, offices, houses, etc. that are closely related to daily life among various buildings, it is not obligatory to apply sufficient fireproof coating to ensure safety in case of fire. However, building-related laws and regulations stipulate that the steel material temperature should not exceed 350 ° C during a fire.

In other words, if the well-known steel is used for buildings,
Since the yield strength will be 60 to 70% of the normal temperature and it may cause the building to collapse, for example, a structure using a shaped steel as a pillar material specified in general structural rolled steel (JIS G 3101). In the above example, a slag wool, a glass wool asbestos etc. as a base material is spread on the surface and a felt is spread, and a method of covering with fire mortar and the heat insulating material layer is further applied to a metal thin plate, that is, aluminum. Use a fireproof coating, such as a method of protecting with a stainless steel thin plate, etc., so that the steel material will not lose its loading force due to thermal damage in the event of a fire. Therefore, it is inevitable that the fireproof coating work cost will be higher than the steel product cost and the construction cost will be significantly increased.

Therefore, a technology was proposed that uses round or square steel pipes as building materials so that the cooling water circulates, prevents temperature rise in the event of a fire, and does not reduce the loading capacity. Is being expanded.

For example, Japanese Utility Model Publication No. 52-16021 discloses a refractory structure in which a water tank is placed above a building and cooling water is supplied to a pillar made of a hollow steel pipe. Also, Japanese Patent Application Sho 63
-143740 specification, as a basic component of the steel material, by adding a considerable amount of Mo and Nb in combination, high temperature heating-high temperature rolling method, the microstructure as a relatively large ferrite main structure, 60
It has been proposed that the high temperature strength of 0 ° C can be secured at 70% or more of the normal temperature strength.

However, with this method, it is not
There is no choice but to rely on the addition of alloying elements such as o and Nb, and it is inevitable that the amount of alloy addition is large, which includes problems such as affecting the weldability, which is important as a steel material for construction. I was out.

Further, in Japanese Patent Application No. 139329, by using a method of water cooling the steel containing a certain amount of Mo from a certain temperature of the rolled austenite and ferrite region, the microstructure of 20-50%. By using a relatively large mixed structure of ferrite and bainite, the yield ratio at room temperature was kept low and a strength of 600 ° C was secured. However, water cooling from a constant temperature after rolling is not always industrially easy, and particularly in the case of a relatively thin steel sheet, the rate of temperature drop after rolling is fast, and it is not possible to water-cool the entire rolled sheet from an appropriate temperature. It was quite difficult.

(Problems to be Solved by the Invention) As a result of research on the strength of steel materials at the time of fire, the inventors of the present invention have found that the maximum temperature reached at the time of fire is 1000 ° C. when the target temperature is uncoated. 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.

In other words, the steel material unit price becomes higher than the well-known steel material cost and the cost of applying the fireproof coating, and such steel material cannot be practically used.

Therefore, as a result of further research, we found that the steel material with a high temperature proof stress at 600 ° C of 70% or more at room temperature was the most economical, reduced the amount of expensive additional elements, and made the fireproof coating thin We have developed a manufacturing method for steel that can be used without coating when the fire load is small.

(Means for Solving the Problems) The present invention achieves the object by overcoming the aforementioned problems.
The specific means is, by weight ratio, C0.02 to 0.10%, Si0.6% or less, Mn0.8 to 2.0%, Mo0.2 to 1.0%, Al0.1% or less, N0.00.
In addition to 6% or less, Ti0.005-0.03%, Zr0.005-0.03%, C
a 0.0005 to 0.005%, REM 0.001 to 0.005%, one or more, and the balance contains Fe and unavoidable impurities, and the C / Mn ratio is 0.05 or less, and is given by the formula (1). D _I ^{* After} reheating a steel slab with a composition of 0.80 or more in the temperature range of 1150 to 1300 ℃, finish the hot rolling in the temperature range of 800 ℃ to 1000 ℃ and make the microstructure bainite. Is a method of manufacturing a thin low yield ratio steel for construction having excellent fire resistance and weldability.

(1 set: (Operation) The feature of the present invention is that Mo is added to the low C-medium to high Mn steel, the C / Mn ratio is 0.05 or less, and the D _I ^* value given by the equation (1) is 0.80 or more. After reheating the steel composition of the composition at high temperature,
Since the rolling is completed at a relatively high temperature, the steel and the steel material (hereinafter referred to as steel) produced by the method of the present invention have an appropriate room temperature proof stress and a low yield ratio, and also have high high temperature proof stress and good weldability. It has the characteristics of

In other words, the yield strength in the temperature range of 600 ° C is higher than the yield strength at room temperature. The reason for this is that the base component of low C has a bainite structure.
If the amount is high, even if the microstructure is bainite, the yield ratio at room temperature becomes high and the seismic resistance cannot be satisfied. In addition, even if the base component is low C, if the hardenability is low, it will not exceed 600 at room temperature.
The strength at ℃ is insufficient.

In the steel of the present invention, a small amount of Ti or Ca is added in order to improve the characteristics of the welded portion, that is, the lamellar tear resistance and the toughness of the welded joint portion, in addition to these base material characteristics. The toughness of the welded joint portion required for steel for construction is not so severe at a Charpy value of about 0 ° C., but it becomes a problem when the welding heat input is relatively high. In the steel of the present invention, this problem has been solved by distributing fine TiN and ZrN in the steel.

Further, diffusible hydrogen mixed from the welding material during welding is trapped by inclusions existing in the steel and extending in the plate thickness direction, and cracks occur along the plate thickness direction. To prevent this, Mn
It is necessary to reduce S and group oxides, and addition of Ca and REM is effective. The steel of the present invention is added with an appropriate amount of Ca or REM as necessary in consideration of such applications.

Next, the characteristic component elements according to the present invention and the addition amounts thereof will be described.

Mo forms fine carbonitrides and further increases the high temperature strength by solid solution strengthening, but when the microstructure is bainite, the addition amount can secure a high temperature strength of 600 ° C with a relatively small amount. .

As a result of research, the present inventors have found that it is effective to add Mo to a low C base component to bainite the microstructure in order to secure the strength at room temperature and the high temperature strength at 600 ° C. It was However, if the Mo content is too high, the weldability deteriorates and the toughness of the heat affected zone (HAZ) deteriorates. Therefore, the upper limit of the Mo content needs to be 1.0%.

Now, at room temperature, rolled steel for welded structures (JIS G 310
In order to satisfy the performance specified in 6) and to maintain high yield strength at a high temperature of 600 ° C, the conditions for reheating and rolling the steel are important together with the steel components.

In order to increase the high temperature strength by adding Mo as described above, it is necessary to sufficiently solutionize Mo during reheating, and therefore the lower limit of the reheating temperature is set to 1150 ° C. Further, if the reheating temperature is too high, the crystal grains become large and the low temperature toughness deteriorates.
The upper limit must be 1300 ° C.

Furthermore, the reason for setting the rolling end temperature to 800 ° C. or higher is to prevent carbonitrides of Mo from being precipitated during rolling. 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, and room-temperature strength and 600 ° C strength and its When balance is important, rolling must be terminated at high temperatures to bainite the microstructure.

Further, in the present invention, the upper limit of the rolling end temperature is set to 1000 ° C. in order to secure the toughness as the building steel.

By the way, it is known that steel used for conventional steel pipes for boilers, etc., to increase the high temperature strength, is a small amount of Mo as a fire resistant steel material used for construction.
In addition, the steel material that suppresses the C / Mn ratio of the base component, secures the hardenability (D _I ^* ), and bainites the microstructure to satisfy the strength at room temperature and high temperature is not known. The steel used for the boiler is subjected to tempering heat treatment after rolling in order to obtain basic properties, and the manufacturing process is different from the steel of the present invention.

Next, the reasons for limiting the components other than Mo in the present invention will be described in detail.

C is necessary to secure the strength of the base material and the welded portion and to exert the effect of adding Mo. If the content is less than 0.02%, the effect is weakened, so the lower limit is made 0.02%. On the other hand, if the C content is too large, the yield ratio at room temperature rises, and the low temperature toughness of the HAZ is adversely affected, so 0.10% is the upper limit.

Si is an element contained in the deoxidized upper steel. Since the weldability and HAZ toughness deteriorate when the amount of Si increases, the upper limit was made 0.6%.

Mn is an essential 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 HAZ toughness deteriorate, so the upper limit of the Mn content was set to 2.0%.

Al is an element generally contained in deoxidized upper steel, but Si and Ti
Since deoxidation is also performed by the above, 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 weld deteriorates, so the upper limit is set to 0.
It was set to 1%.

N is generally contained in steel as an unavoidable impurity, but if the amount of N increases, it promotes the deterioration of HAZ toughness and the occurrence of surface flaws in the continuous cast slab, so its upper limit is 0.
It was 006%.

The steel of the present invention contains P and S as unavoidable impurities. Since the effects of P and S on the high temperature strength are small, the amount thereof is not particularly limited, but generally, the properties of steel such as toughness and strength in the plate thickness direction improve as the content of these P and S elements decreases. Desirable P and S contents are 0.02% and 0.005, respectively
% Or less.

Although the components for obtaining the basic properties are as described above, the steel of the present invention is applied under conditions where the use is severe (hydrogen cracking of the welded part is required, or welding with high heat input is applied) In consideration of the above, the characteristics are improved by selectively adding the elements described below, that is, Ti, Zr, Ca, and REM.

Ti forms carbonitrides and improves HAZ toughness. Further, when the amount of Al is small, an oxide of Ti is formed to improve the HAZ toughness, but if it is less than 0.005%, it has no effect, and if it exceeds 0.03%, it has an unfavorable influence on the HAZ toughness.

Zr has almost the same effect as Ti, but the effective range is 0.005 to 0.03%.

Ca and REM control the morphology of sulfide (MnS) and have the effect of improving the lamellar tear of the weld and hydrogen-organic cracking resistance, as well as increasing Charpy absorbed energy and improving low temperature toughness. is there. However, the amount of Ca is 0.0005%
If it is less than 0.005%, there is no practical effect, and if it exceeds 0.005%, a large amount of CaO and CaS are formed to form large inclusions, which not only impairs the toughness of steel but also the cleanliness, and also has weldability and lamellar tear resistance. It also adversely affects the
0.0005% to 0.005%.

Further, REM has the same effect as Ca, and if the addition amount is increased, the same problem as that of Ca occurs and the economical efficiency is deteriorated. Therefore, the lower limit of 0.001% and the upper limit of 0.005% were set.

(Example) A steel plate was manufactured by a well-known converter, continuous casting, and thick plate process, and room temperature strength, high temperature strength, etc. were investigated.

In Table 1, No. 1 to No. 14 show the present invention steels, and No. 15 to No. 20 show the chemical compositions of the comparative steels.

Next, Table 2 shows the mechanical properties of the steel of the present invention and the comparative steel by heating, rolling and conditions.

The examples of the invention steels No. 1 to No. 14 in Table 2 all have good room temperature and high temperature strength.

On the other hand, in Comparative Steel No. 15, since the heating temperature and finish rolling temperature are low, the yield ratio at room temperature is too high, and in No. 16, reheating after rolling, quenching, and heat treatment of tempering are performed. Therefore, the yield ratio at room temperature is too high. No. 17 has low normal temperature and high temperature strength because it is reheated and normalized after rolling.
Furthermore, in Nos. 18 and 19, the C / Mn ratio is high and the D _I ^* is low, so the strength at room temperature and high temperature is low. In No. 20, since D _I ^* is low, the strength at room temperature and high temperature is low. In No. 21, although D _I ^* is satisfied, the yield ratio at room temperature is too high because the C / Mn ratio is high. In Nos. 22 and 23, the strength at room temperature and 600 ° C is satisfied, but the HAZ toughness and the properties in the plate thickness direction are insufficient.

The results are shown in Tables 3 and 4.

Table 3 shows the characteristics of the reproduced HAZ toughness of the invention steel and the comparative steel having almost the same base composition. The steel of the present invention has good toughness because it contains Ti, but the comparative steel has very poor toughness because it has no addition. Table 4 shows the properties in the plate thickness direction of the steel of the present invention and the comparative steel having almost the same base composition.

The property in the plate thickness direction is a typical test method for evaluating anti-lamellar tear property.

Since the steel of the present invention has Ca added, it exhibits good properties, but the comparative steel has insufficient results because it has not added Ca.

(Effect of the invention) The steel material manufactured by the chemical composition and the manufacturing method of the present invention has a high yield strength of 600 ° C, a yield strength of 600 ° C is 70% or more of a room temperature yield strength, and a yield ratio (YS / TS) at room temperature. ) Is low, and the weldability, welded joint toughness, and properties in the plate thickness direction are good, and it is a completely new steel material.

Claims (1)

[Claims] 1. A weight ratio of C 0.02 to 0.10%, Si 0.6% or less, Mn 0.8 to 2.0%, Mo 0.2 to 1.0%, Al 0.1% or less, N 0.006% or less, and Ti 0.005 to 0.03%, One or more of Zr 0.005-0.03%, Ca 0.0005-0.005%, REM 0.001-0.005% The balance contains Fe and unavoidable impurities, and the C / Mn ratio is 0.05% or less, (1) After reheating a steel slab consisting of a composition with a D _I ^* value of 0.80 or more in the temperature range of 1150 to 1300 ° C, finish the hot rolling in the temperature range of 800 ° C to 1000 ° C to obtain a microstructure. A method for producing a thin low yield ratio steel for construction having excellent fire resistance, which is characterized by using bainite. (1 set: