JPH0713250B2 - Manufacturing method of low yield ratio steel with excellent fire resistance - Google Patents

Description

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

(Prior art) As is well known, as a structural material for various structures in the fields of architecture, civil engineering, and offshore structures, rolled steel for general structure (JIS
G 3103), 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 3444), square steel plate for general structure (JIS G 346)
6) etc. are widely used.

The well-known steel material is usually de-S hot metal obtained by a blast furnace,
After de-Ping, it is subjected to converter refining to form steel pieces in a continuous casting or slabbing process, and then hot plastic working to obtain a product having desired properties.

By the way, in the case of using the well-known steel materials in buildings, offices, houses, etc., which are closely related to daily life, among various buildings, it is obliged to apply a sufficient fireproof coating in order to ensure safety in a fire. According to the building-related laws and regulations, the steel material temperature is about 350 ° C and the proof stress is 60% at room temperature.
It is up to 70%, which may cause the collapse of the building. For example, in the case of a structure that uses a section steel as a pillar material specified in general structural rolled steel (JIS G 3101), slag is applied to its surface. In addition to spreading a spraying material and felt based on wool, glass wool, asbestos, etc., a method of wrapping with fireproof mortar and a method of further protecting it with a metal thin plate, that is, aluminum or stainless steel thin plate on the heat insulating material layer, etc. Carefully apply a fireproof coating so that the steel material will not lose its loading capacity due to thermal damage in the event of a fire.

Therefore, the fireproof coating work cost becomes higher than the steel material cost,
There is an unavoidable increase in construction costs.

Therefore, a technology has been proposed in which a round or square steel pipe is used as the building material, and the cooling water is circulated to prevent the temperature from rising in the event of a fire and to prevent the loading capacity from decreasing. It is being expanded.

For example, Japanese Utility Model Publication No. 52-16021 discloses a refractory structure where a water tank is placed above a building and cooling water is supplied to a pillar made of a hollow steel pipe. Further, in Japanese Patent Application No. 2-72566, by adding a certain amount of Mo, limiting the C / Mn ratio and ensuring hardenability, the microstructure is bainite, and the high temperature strength at 600 ° C. is 70% of the room temperature strength. It has been shown that the above can be secured.

However, in this method, the yield ratio at room temperature is low,
The SS curve is a round type with no clear yield point. Although this type of steel has a low apparent yield ratio, it was clarified that it was not sufficient for seismic resistance, which included problems.

Figure 1 (a) shows S when the microstructure is mainly ferrite.
-S curve, Fig. 1 (b) is an S-S curve having a bainite-based microstructure.

(Problems to be Solved by the Invention) As a result of research on the temperature 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 provided a fireproof coating. We have developed a method for manufacturing steel that can be made thin and can be used uncoated when the fire load is small.

(Means for Solving the Problems) The present invention achieves the object by overcoming the aforementioned problems.
By weight ratio, C0.04 to 0.11%, Si0.6% or less, Mn0.3 to 0.7
%, Mo 0.5 to 0.8%, Ni 0.05 to 0.50%, Cu 0.05 to 0.50%,
Cr 0.05-0.50%, Al 0.1% or less, N 0.006% or less, plus V
0.005-0.05%, Ti0.005-0.03%, Zr0.005-0.03%, C
a 0.0005 to 0.005%, REM 0.001 to 0.005%, one or two or more, the balance containing Fe and unavoidable impurities, and a steel having a composition composition with a Di ^* value less than 0.80 given by the formula (1) After reheating the piece in the temperature range of 1150 to 1300 ℃, finish the hot rolling in the temperature range of 800 to 1000 ℃, and then air-cool it to obtain a low yield ratio for buildings with excellent microstructure mainly ferrite It is a method of manufacturing steel.

Formula (1) (Operation) Now, the feature of the present invention is that a certain amount of Mo is added to medium C-medium Mn steel and the Di ^* value given by the equation (1) (where Di ^* is the effect of alloying elements contributing to quenching). Is quantified.)
The steel and steel products manufactured by the method of the present invention (hereinafter referred to as "steel") have an appropriate room temperature proof strength and a clear yield. It has a low yield strength accompanied by a phenomenon (the yield point is clearly recognized) and high temperature proof stress.

That is, the yield strength at a temperature of 600 ° C. is higher than the yield strength at room temperature. The reason is that a large amount of Mo is added to the base component of medium C.
Steel with the addition of a ferrite composition (ferrite area ratio 60
% Or more).

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 in the case of the steel of the present invention in which the microstructure is ferrite and Nb is not added, the addition amount thereof is required to be relatively large. Therefore, the lower limit of the amount of Mo added is 0.5%. However, if the amount of Mo is too large, the weldability deteriorates and the toughness of the heat affected zone (HAZ) deteriorates. Therefore, the upper limit of the amount of Mo needs to be 0.8%.

Now, at room temperature, welded structural pressure steel (JIS G 3106)
In order to satisfy the performance specified in 1. and to maintain a high yield strength at a high temperature of 600 ° C., the conditions for reheating and rolling the steel together with the steel components are important.

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.

Further, the reason why the rolling end temperature is set to 800 ° C. or higher is to prevent precipitation of Mo carbonitride 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 their balance Is important, the rolling must be terminated at high temperature because the microstructure is mainly composed of relatively coarse-grained ferrite.

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.

The use of Mo to increase the high temperature strength is known for the steel used for conventional steel pipes for boilers, etc. It is subjected to post-heat treatment and has a different manufacturing process from the steel of the present invention.

Further, as a refractory steel material used for construction, there is Japanese Patent Application Laid-Open No. 2-77523 previously filed by the present applicant. This steel is a process in which a small amount of Mo and Nb are added, and high-temperature heating-high-temperature rolling is used to produce it. This manufacturing method is the same as that of the steel of the present invention, but in order to obtain high temperature strength, it is essential to add Mo and Nb in combination, which is different from the single addition of Mo of the present invention.

Further, it is generally known that it is difficult to reduce the yield ratio of Nb-added steel, and the reason is considered to be the effect of making the ferrite grain size fine and the precipitation of Nb during rolling.
For this reason, in a relatively thin steel plate, the rolling ratio is large and the rolling temperature is likely to be lowered, so that the yield ratio at room temperature is likely to increase for the above reason. In this invention steel, the yield ratio at room temperature is
Although it has been clarified that it can be manufactured at 75% or less, it is considered difficult to industrially manufacture a thin low yield ratio steel plate.

The steel of the present invention has a low yield ratio of 70% or less at room temperature and 600 ° C.
It is suitable for manufacturing steel sheets with a yield strength of 70% or more at room temperature and a thickness of 40 mm or less, and is suitable for industrial production.

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, and if the content is less than 0.04%, the effect is weakened, so the lower limit is made 0.04%. If the amount of C is too large, the yield ratio at room temperature rises and the low temperature toughness of HAZ is adversely affected, so 0.11% 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%.

Next, Mn is an essential element for ensuring strength and toughness, and its lower limit is 0.3%. However, if the amount of Mn is too large, the hardenability increases and the weldability and HAZ toughness deteriorate, so the upper limit of Mn was made 0.7%.

Cr content is an element that enhances the strength of the base metal and the weld, but
If the addition amount is less than 05%, the effect is small, and if the Cr amount exceeds 0.5%, the weldability and HAZ toughness deteriorate, so the upper and lower limits were made 0.05% and 0.5%, respectively.

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 the addition of more than 0.5% is extremely expensive for the purpose as a building steel. Therefore, the upper and lower limits are set to 0.50% and 0.5% respectively.

Cu has almost the same effect as Ni, and also has an effect of increasing high temperature strength, improving corrosion resistance and weather resistance due to Cu precipitates. However, if it is less than 0.05%, the effect is weak, and if it exceeds 0.5%, Cu cracking occurs during hot rolling, so the upper and lower limits were made 0.05% and 0.5%, respectively.

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 is not limited for the steel of the present invention. However, if the amount of Al increases, the cleanliness of the steel will deteriorate and the toughness of the weld will deteriorate, so the upper limit is 0.1%.
And

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 amounts thereof are not particularly limited, but generally the properties of steel such as toughness and strength in the plate thickness direction improve as the amounts of these P and S elements decrease. Desirable P and S contents are 0.02% and 0.00, respectively
It is 5% 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) The following elements, namely V, Ti, Zr, Ca and REM are taken into consideration.
The characteristics are improved by selectively adding.

It combines with V and N to form VN and improves the high temperature strength, but if less than 0.005%, no effect is observed, and if it exceeds 0.05%, H
Since it impairs AZ toughness, it was limited to the range of 0.005 to 0.05%.

Ti forms carbonitrides and improves HAZ toughness. When the amount of Al is small, it forms an oxide of Ti and improves the HAZ toughness, but if it is less than 0.005%, it has no effect, and if it exceeds 0.03%, the HAZ is toughened.
Since it has an unfavorable effect on toughness, it is limited to 0.005 to 0.03%.

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%, it has no practical effect. 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% Also, REM has the same effect as Ca, and if the addition amount is increased, the same problem as Ca occurs and the economical efficiency is deteriorated.
%, And the upper limit was 0.005%.

(Example) A steel plate was manufactured by a well-known converter, continuous casting, and thick plate process, and the room temperature and high temperature strength at 600 ° C were investigated.

In Table 1, No. 1 to No. 15 show the present invention steels, and No. 16 to No. 21 show the chemical compositions of the comparative steels.

Next, Table 2 shows manufacturing conditions of the present invention steel and comparative steel such as heating and rolling and their strength characteristics.

In the examples of the invention steels No. 1 to No. 15 in Table 2, the ferrite fraction of the microstructure exceeds 60%, and the yield ratio (yield strength / yield strength) at room temperature is
The tensile strength is as low as 70% or less, and the yield strength at 600 ° C is 70% or more at room temperature.

On the other hand, in Comparative Steel No. 16, since Mn is low,
The strength was low at 00 ° C, and the ratio of the yield strength at 600 ° C to the yield strength at room temperature did not reach 70%. In Comparative Steel No. 17, the yield strength at 600 ° C was sufficient because Mn was too high, but the yield ratio at room temperature was too high, reaching 77%. Comparative steel No. 18 had a low Mo content and thus had a low yield strength at 600 ° C, which was a level not reaching 70%. On the contrary, in Comparative Steel No. 19, although Mo was too high and the yield strength at 600 ° C was sufficient, the yield ratio at room temperature was too high, reaching 80%. In Comparative Steel No. 20, since the C content was low, the yield strength at room temperature and 600 ° C was low, and the ratio of the yield strength at 600 ° C to the yield strength at room temperature was at a level not reaching 70%. Further, in Comparative Steel No. 21, the yield strength at 600 ° C. was sufficient because C was too high, but the yield ratio at room temperature was too high, reaching 82%.

(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, the yield strength of 600 ° C is 70% or more of the room temperature yield strength, and the room temperature yield ratio is 70% or less. It is a completely new steel with low fire resistance and excellent earthquake resistance.

[Brief description of drawings]

FIG. 1 is a diagram of stress-strain. (A) shows the case where the microstructure is mainly ferrite, and (b) shows the case where bainite is the main structure.

Claims (1)

[Claims] 1. A weight ratio of C: 0.04 to 0.11%, Si: 0.6% or less, Mn: 0.3 to 0.7%, Mo: 0.5 to 0.8%, Ni: 0.05 to 0.50%, Cu: 0.05 to 0.50%, Cr : 0.05 to 0.50%, Al: 0.1% or less, N: 0.006% or less, V: 0.005 to 0.05%, Ti: 0.005 to 0.03%, Zr: 0.005 to 0.03%, Ca: 0.0005 to 0.005%, REM: One or more of 0.001 to 0.005%, the balance of which contains Fe and unavoidable impurities, and a steel piece consisting of a chemical composition with a Di ^* value less than 0.80 given by equation (1)
After reheating in the temperature range of 150-1300 ℃, hot rolling 800-1000
A method for producing a low yield ratio steel for construction having excellent fire resistance, which comprises terminating in a temperature range of ℃ and then air-cooling to make the microstructure mainly composed of ferrite. Formula (1)