JPH0713249B2 - 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 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 3444), square steel plate for general structure (JIS G 346)
6) etc. are widely used.

These well-known steel materials are subjected to descaling of hot metal obtained by a normal blast furnace and then subjected to converter smelting to form steel pieces in a continuous casting or agglomeration process, and then hot plastic working to obtain desired properties. Will be commercialized as equipped with.

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 round or square steel pipes are used as the building material, and cooling water is circulated to prevent temperature rise in the event of a fire and to prevent the loading capacity from being lowered. Is being pursued.

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 when the microstructure is mainly bainite.

(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%, Al 0.1% or less, N 0.006% or less, Ni 0.05 to 0.5%, Cu 0.05 to 0.5%, Cr 0.05 to 0.5%, or one or more kinds thereof, A steel slab containing the balance Fe and unavoidable impurities and having a composition of Di ^* value less than 0.80 given by the formula (1) is reheated in the temperature range of 1150 to 1300 ° C and then hot rolled to 800 to This is a method for producing a low yield ratio steel for construction with excellent fire resistance, which is finished in the temperature range of 1000 ° C and then air-cooled to have a microstructure mainly composed of ferrite.

Formula (1) (Operation) The characteristic of the present invention is that a certain amount of Mo is added to the medium C-medium Mn steel, and the Di ^* value (where Di ^* is the effect of alloying elements contributing to quenching is quantified. Steel) having a composition of less than 0.80 is reheated at a high temperature and then the rolling is finished at a relatively high temperature. And a low yield strength accompanied by a clear yield phenomenon (the yield point is clearly recognized) and high temperature proof stress. That is,
The yield strength at a temperature of 600 ° C is large relative to the room temperature yield strength.

The reason for this is that the steel has a ferrite structure (ferrite area ratio of 60% or more) in which a considerable amount of Mo is added to the base component of medium C.

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, so the upper limit is 13
It has to be set to 00 ° 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 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 addition of Mo alone 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 material, the rolling ratio is large and the rolling temperature is likely to decrease, so that the yield ratio at room temperature tends 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 securing strength and toughness, and the 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%.

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

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 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 amount of these P and S elements decreases. Desirable P and S contents are 0.02% and 0, respectively.
It is 005% or less.

Although the components for obtaining the basic properties are as described above, the steel of the present invention is considered for application under severe conditions of use (requiring excellent toughness in the base material and weld), and is described below. The characteristics are improved by selectively adding elements, that is, Ni, Cu and Cr.

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 economic efficiency is lost, so the upper limit is 0.50%.
And

Cu has almost the same effect as Ni, and also has an effect of increasing high temperature strength, corrosion resistance, and weather resistance due to Cu precipitates. However, if the Cu content exceeds 0.5%, the YR at room temperature increases, and if it is less than 0.05%, there is no effect, so the Cu content is
Limited to 0.05-0.5%.

Cr is an element that enhances the strength of the base metal and the welded portion, but if the Cr content exceeds 0.5%, it deteriorates the weldability and HAZ toughness, and
If less than 0.05%, the effect is weak, so limit it to 0.05 to 0.50%.

(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 steel Nos. 1 to 15 in Table 2, the yield ratio (yield strength / tensile strength) at room temperature is as low as 70% or less, and the yield strength at 600 ° C is 70% or more at room temperature. ing.

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 78%. Comparative steel No. 18 has a low Mo content, so at room temperature and 6
Low yield strength at 00 ℃, 600 ℃ against yield strength at room temperature
The yield strength ratio was 62%. In addition, comparative steel No. 19
Then, although Mo is too high and the yield strength at 600 ° C is sufficient,
The yield ratio at room temperature was too high, reaching 77%. Comparative steel No. 20
, The ratio of the yield strength at 600 ° C. to the yield strength at room temperature was 67%, which did not reach 70%, because C was low. further,
In Comparative Steel No. 21, since the C content was too high, the yield strength at 600 ° C was sufficient, but the yield ratio at room temperature was too high, reaching 83%.

(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 earthquake resistance.

[Brief description of drawings]

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

Claims (1)

[Claims] 1. In weight%, C: 0.04 to 0.11%, Si: 0.6% or less, Mn: 0.3 to 0.7%, Mo: 0.5 to 0.8%, Al: 0.1% or less, N: 0.006% or less, and Ni. : 0.05-0.5%, Cu: 0.05-0.5%, Cr: 0.05-0.5%, one or more, the balance contains Fe and unavoidable impurities, and the Di ^* value given by formula (1) is less than 0.80 1 slab consisting of the composition of
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)