JPH0456722A - Production of steel with low yield ratio for construction use excellent in refractoriness - Google Patents

Abstract

PURPOSE:To produce a steel with low yield ratio for construction use excellent in refractoriness by subjecting a slab of a steel having a specific composition in which respective contents of C and Mo are specified to reheating and to hot rolling under respectively specified conditions and forming a microstructure into a structure composed essentially of ferrite. CONSTITUTION:A slab of steel having a composition which consists of, by weight ratio, 0.04-0.11% C, <=0.6% Si, 0.3-0.7% Mn, 0.5-0.8% Mo, <=0.1% Al, <=0.006% N, one or >=2 kinds among 0.05-0.5% Ni, 0.05-0.5% Cu, and 0.05-0.5% Cr, and the balance Fe with inevitable impurities and in which Di value* given by equation is regulated to <0.80 is reheated at 1150-1300 deg.C and hot rolling is finished at 800-1000 deg.C, by which the steel with low yield ratio, wherein microstructure is composed essentially of ferrite, is produced. By this method, the completely new steel which has yield strength at 600 deg.C as high as >=70% of yield strength at ordinary temp. and also has yield ratio at ordinary temp. as low as <=70% and combines refractoriness with earthquake resistance can be obtained.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is applicable to the fields of architecture, civil engineering, marine structures, etc.
This invention relates to a method for manufacturing low yield ratio steel with excellent fire resistance for use in various structures.

(Prior art) As is well known, rolled steel for membrane structures (JT
SG 3101), rolled steel materials for welded structures (JIS
G 310B), weather-resistant hot rolled steel for welded structures (J
IS G 3114), highly weather resistant rolled steel (JIS
G 3444), square steel plate for shell structure (JIS G 3
46B) etc. are widely used.

These well-known steel materials are usually produced by removing S and P from hot metal obtained in a blast furnace, then refining it in a converter furnace, turning it into billets in a continuous casting or blooming process, and then hot plastic working to achieve the desired properties. It will be commercialized as a product equipped with the following.

By the way, when using the well-known steel materials in buildings such as buildings, offices, and residences that are closely connected to daily life among various buildings, it is mandatory to apply sufficient fireproof coating to ensure safety in the event of a fire. According to construction-related laws and regulations, in the event of a fire, when the temperature of the steel material is around 350℃, the proof strength will be 60 to 70% of that at room temperature, which may cause the collapse of the building. JIS G
3101), the surface of which is sprayed with slag wool, glass wool, asbestos, etc. as a base material, or by spreading felt or fireproof mortar. A fire-resistant coating is carefully applied, such as by encasing the heat insulating material layer or by further protecting it with a thin metal plate, such as aluminum or stainless steel plate, to prevent the steel material from losing its load-bearing capacity due to thermal damage in the event of a fire. Use it like this.

As a result, the cost of fireproof coating is high compared to the cost of steel materials,
A significant increase in construction costs cannot be avoided. Therefore, a technology has been proposed that uses round or square steel pipes as construction materials to allow cooling water to circulate, preventing temperature rise in the event of a fire and not reducing loading capacity.This technology reduces building construction costs and expands the usable space. is planned.

For example, Japanese Utility Model Publication No. 52-16021 discloses a fire-resistant building in which a water tank is placed on the top of the building and cooling water is supplied to pillars made of hollow steel pipes. Also,
Japanese Patent Application No. 2-72566 discloses that by adding a certain amount of Mo, limiting the C/Mn ratio, and ensuring hardenability, the microstructure is made into bainite, and the high-temperature strength at 600°C is 70% or more of the room-temperature strength. It is shown.

However, with this method, although the yield ratio at room temperature is low,
The SS curve has a round shape with no clear yield point. Although this type of steel has a low apparent yield ratio, it has been found that its earthquake resistance is not sufficient, and it has some problems.

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

(Issue 8 to be solved by the invention) As a result of research on the temperature of steel materials in the event of a fire, the present inventors found that the maximum temperature reached in the event of a fire is 1000°C when uncoated use is the goal. It was found that in order to have a yield strength of 70% or more of the room temperature yield strength at this temperature, a large amount of expensive metal elements must be added, resulting in a loss of economic efficiency. In other words, the unit price of the steel material becomes higher than the cost of the well-known steel material and, in addition, the cost of installing a fireproof coating, and such steel material cannot be practically used.

Therefore, as a result of further research, we found that the most economical steel material had a high-temperature yield strength of 70% or more at 600℃ compared to room temperature, and we decided to reduce the amount of expensive additive elements and add a fire-resistant coating. We have developed a method for manufacturing steel that can be made thin and can be used without coating if the fire load is small.

(Means for Solving the Problems) The present invention overcomes the above-mentioned problems and achieves the objects.
Weight ratio: C0.04-0.11%, S10.6% or less, Mn0.3-0.7%, Mo0.5-0.8%, AI
o, 1% or less, N0.006% or less, plus Ni0.
05-0.5%, Cu0.05-0.5%, Cr0.0
5 to 0.5% of one or more types, the balance containing Fe and unavoidable impurities, and DX given by formula (1)
*Steel slabs with a component composition with a value of less than 0.80 are classified as 1150
After reheating in the temperature range of ~1300℃, hot rolling is performed at ~800℃.
This is a method for producing a low yield ratio steel for architectural use which has excellent fire resistance and has a microstructure mainly composed of ferrite, which is finished in a temperature range of 1000°C.

(1) Formula % Formula %) Now, the feature of the present invention is that a certain amount of Mo is added to medium C-medium Mn steel.
steel and steel materials (hereinafter referred to as steel) manufactured by the method of the present invention, which consists of reheating a steel billet having a composition with a Di value of less than 0.80 at a high temperature and then finishing rolling at a relatively high temperature.
has a low yield strength with an appropriate room temperature yield strength and a clear yield phenomenon (the yield point is clearly recognized), and
It has the characteristic of high high temperature resistance. In other words, the ratio of the proof stress at a temperature of 600° C. to the room temperature proof stress is large.

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 M□ is added to the medium C base component.

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

Mo forms fine carbonitrides and further increases high-temperature strength through solid solution strengthening, but in the case of the steel of the present invention, which has a ferrite microstructure and does not contain Nb, a relatively large amount of Mo is required. Therefore, the lower limit of Mo addition amount is 0
．． It is 5%. However, if the amount of Mo is too large, weldability deteriorates and furthermore, the toughness of the weld heat affected zone (HAZ) deteriorates, so the upper limit of the amount of Mo needs to be 0.8%.

Now, at room temperature, welded structure circumferential steel (JISG 3
In order to satisfy the performance specified in 10B) and maintain high yield strength at a high temperature of 600°C, the conditions for reheating and rolling the steel are important as well as the steel composition.

In order to increase the high temperature strength by adding MO as mentioned above, it is necessary to add Mo.
It is necessary to sufficiently dissolve the material during reheating, and therefore the lower limit of the reheating temperature is set at 1150°C. Furthermore, if the reheating temperature is too high, the crystal grains will become large and the low temperature toughness will deteriorate, so the upper limit must be set at 1300°C.

Furthermore, the reason why the rolling end temperature is set to 800° C. or higher is to prevent Mo carbonitride from precipitating 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 the strength at room temperature and 600°C and the balance thereof are required. If this is important, the rolling must be completed at a high temperature in order to make the microstructure mainly composed of relatively coarse grained ferrite.

In addition, in the present invention, the upper limit of the rolling end temperature is set to 1000.
The temperature was set at ℃ in order to ensure toughness as a building steel.

Now, the use of MO in order to increase high-temperature strength is known in steel used for conventional boiler steel pipes, etc., but this steel has to be rolled/pipe-formed in order to obtain the basic properties. It is subjected to post-refining heat treatment, and the manufacturing process is different from that of the steel of the present invention.

In addition, there is Japanese Patent Application No. 143740/1988, which was previously filed by the present applicant as a fire-resistant steel material for use in construction.

This steel is manufactured by adding trace amounts of Mo and Nb and by high-temperature heating and high-temperature rolling.

This manufacturing method is the same as the steel of the present invention, but in order to obtain high-temperature strength, combined addition of MO and Nb is essential, which is different from the sole addition of Mo of the present invention.

Furthermore, it is generally known that it is difficult to reduce the yield ratio of Nb-added steel, and the reason for this is believed to be the effect of reducing the ferrite grain size and the precipitation of Nb during rolling. For this reason, in a relatively thin steel sheet, the rolling reduction ratio is large and the rolling temperature tends to decrease, so the yield ratio at room temperature tends to increase for the above-mentioned reasons. Although it has been revealed that the invention steel can be produced with a yield ratio of 75% or less at room temperature, it is considered difficult to industrially produce 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 a yield strength of 70% or more at 600°C, and has a thickness of 40 mm.
Suitable for manufacturing the following steel plates, suitable for industrial production.

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

C is necessary to ensure the strength of the base metal and the welded part and to exhibit the effect of adding Mo. If it is less than 0.04%, the effect will be diminished, so the lower limit is set to 0.04%.

In addition, if the amount of C is too large, the yield ratio at room temperature will increase and it will also have a negative effect on the low temperature toughness of the HAZ, so 0.11%
is the upper limit.

SI is an element contained in deoxidized steel, and as SIR increases, weldability and HAZ toughness deteriorate, so the upper limit is set at 0.6%.
And so.

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

L is an element generally contained in deoxidized steel, but since deoxidation is also performed by St, there is no lower limit for the steel of the present invention. But Aj? If ffi increases, the cleanliness of the steel will deteriorate and the toughness of the weld will deteriorate, so the upper limit should be set to 0.
．． It was set at 1%.

N is generally contained in steel as an unavoidable impurity, but if the amount of N increases, it promotes deterioration of HAZ toughness and the occurrence of surface scratches on continuously cast slabs, so the upper limit was set at 0.006%. .

Note that the steel of the present invention contains P and S as inevitable impurities. Since P and S have little effect on high temperature strength,
Although there is no particular limitation on the amount, generally the properties of steel regarding toughness, strength in the thickness direction, etc. are improved as the amount of these P and S elements is smaller. Desirable amounts of p and s are 0 and 0, respectively.
0.02%, 0.005% or less.

The ingredients for obtaining the basic properties are as described above, but the steel of the present invention is designed to be used under severe conditions (requires excellent toughness in the base metal and welded part), and is described below. The characteristics are improved by selectively adding elements such as Ni, Cu, and Cr.

Ni has no adverse effect on weldability or HAZ toughness,
It improves the strength and toughness of the base metal, but if it is less than 0.05%, the effect is weak, and if it is added more than 0.5%, it becomes extremely expensive and loses economic efficiency for the purpose of building steel. The upper limit was set to 0.50%.

Cu has almost the same effect as Ni, and also has the effect of increasing high temperature strength and improving corrosion resistance and weather resistance due to Cu precipitates. However, if the Cu amount exceeds 0.5%, Y at room temperature
Curl was limited to 0.05 to 0.5% in order to increase R, and since there is no effect if it is less than 0.05%.

Cr is an element that increases the strength of the base metal and welded part, but C
If the r amount exceeds 0.5%, weldability and HAZ toughness will deteriorate, and if it is less than 0.05%, the effect will be weak, so 0.05
-0.50%.

(Example) Steel plates were manufactured using the well-known converter, continuous casting, and thick plate processes, and their high-temperature strengths at room temperature and 600°C were investigated.

The steel of the present invention was added to No. 1 to Na15 in Table 1, No. 1
Nos. 6 to 21 show the chemical compositions of comparative steels.

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

In the examples of the invention steels Nα1 to No. 15 in Table 2, the yield ratio (yield strength/tensile strength) at room temperature is as low as 70% or less;
The yield strength at 00°C is 70% or more of that at room temperature.

On the other hand, comparative steels NQ and 1B have low Mn, so
The strength at room temperature and at 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 below 70%. Furthermore, in comparison steel kL7, since the Mn content was too high, the yield strength at 600°C was sufficient, but the yield ratio at room temperature was too high, reaching 78%. For comparison steel k18,
Due to the low Mo content, the yield strength at room temperature and 600°C is low, and the ratio of the yield strength at 600°C to the yield strength at room temperature is 62
%Met. Furthermore, in comparative steel No. 19, the Mo content was too high, and although the yield strength at 600°C was sufficient, the yield ratio at room temperature was too high, reaching 77%. In 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 7096, due to the low C content. Furthermore, in comparative steel No. 21, C was too high, so 6
Although the yield strength at 00°C was sufficient, the yield ratio at room temperature was too high, reaching 83%.

(Effect of the invention) Steel products manufactured using the chemical composition and manufacturing method of the present invention can be heated to 600°C.
The yield strength is high, the yield strength at $H°C is 70% or more of the yield strength at room temperature, and the yield ratio at room temperature is low at 70% or less,
This is a completely new steel that has both fire resistance and earthquake resistance.

[Brief explanation of drawings]

Figures 1(a) and 1(b) are graphs of stress stress. (a) Cb) Agent Patent attorney Tatsuo Chanoki procedural amendment (
Method) (1) Amend the entire text of page 19, line 8 of the specification to read, ``Figure 1 is a diagram of Kerr strain, where (a) shows the case where the microstructure is mainly ferrite. (b) shows the case where the microstructure is mainly bainite.'' .

Claims (1)

[Claims] In weight percent, C0.04 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, N0. In addition to 0.06% or less, the balance of one or more of Ni0.05-0.5%, Cu0.05-0.5%, and Cr0.05-0.5% contains Fe and inevitable impurities, and ( 1) After reheating a steel piece with a component composition whose Di^* value given by the formula is less than 0.80, in a temperature range of 1150 to 1300°C,
A method for producing a low yield ratio steel for construction with excellent fire resistance, characterized in that hot rolling is completed in a temperature range of 800 to 1000° C. so that the microstructure is mainly ferrite. (1) Formula Di^*=0.316√C(1+0.7Si)(4.1
Mn+0.35)(1+3Mo)(1+0.36Ni)
(1+0.365Cu) (1+2.16Cr) (component unit: weight%)