JPH0456723A - 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 steel, having a specific composition in which respective contents of C and Mo are specified, to reheating and hot rolling under respectively specified conditions and forming microstructure into a structure composed essentially of ferrite. CONSTITUTION:A slab of a 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, 0.05-0.5% Ni, 0.05-0.50% Cu, 0.05-0.50% Cr, <=0.1% Al, <=0.006% N, one or >=2 kinds among 0.005-0.05% V, 0.005-0.03% Ti, 0.005-0.03% Zr, 0.0005-0.005% Ca, and 0.001-0.005% REM, 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 is excellent in refractoriness and earth quake 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, general structural rolled steel (JI
SG 3101), rolled steel materials for welded structures (JIS
G 3106), weather-resistant hot-rolled steel for welded structures (J
IS G 3114), highly weather resistant rolled steel (JIS
G 3444), - square steel plate for membrane structure (JIS G 3
468) etc. are widely used.

The above-mentioned well-known steel materials are usually made by removing S from hot metal obtained in a blast furnace.
After dephosphorization, the steel is refined in a converter furnace and made into a steel billet in a continuous casting or blooming process, followed by hot plastic working to produce a product with desired properties.

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 various 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 coated with slag wool, glass wool, asbestos, etc. as a base material or felt, as well as fire prevention mortar. A fire-resistant coating is carefully applied, such as encasing the insulation layer with a metal layer, or protecting it with a thin metal plate, such as an 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 in such a way that it doesn't.

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 reducing loading capacity. Expansion is being 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 whose microstructure is mainly bainite.

(Problem to be Solved by the Invention) As a result of research on the temperature of steel materials during a fire, the present inventors found that when uncoated use is aimed at, the maximum temperature reached during a fire is 1000°C. It has been learned that in order to provide 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 manufacturing method for steel that can be made thin and 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: C 0.04-0.11%, SiO, 6% or less, Mn 0.3-0.7%, Mo 0.5-0.8%, N
10.05-0.50%, Cu 0.05-0.50%
, Cr0.05-0.50%, All 0.1% or less, N 0.006% or less, and Vo, 005-0.0
5%, Ti0.005-0.03%, Zr0.005
~0.03%, Cao, oo05~0.005%, RE
1150 A steel billet consisting of a component composition containing one or more kinds of M0.001 to 0.005%, the balance containing Fe and unavoidable impurities, and having a Di value given by equation (1) of less than 0.80. After reheating in a temperature range of ~1300°C, hot rolling is completed in a temperature range of 800~1000°C, thereby producing a low yield ratio steel for construction with excellent fire resistance and a microstructure mainly composed of ferrite.

(1) Formula % Formula %) Now, the feature of the present invention is that a certain amount of M□ is added to medium C-medium Mn steel.
The steel and steel materials (hereinafter referred to as steel) produced by the method of the present invention have an appropriate room temperature yield strength and a clear property. It has a low yield strength accompanied by a yield phenomenon (a clearly visible yield point) and a high high temperature yield strength.

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 medium C base component with a considerable amount of Mo added, and has a ferrite structure (ferrite area ratio of 60% or more).

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 it is necessary to set the upper limit of Moff1 to 0.8%.

Now, at room temperature, pressed copper material for welded structures (JISG 3
In order to satisfy the performance specified in 106) 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 described 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 steels used for conventional boiler steel pipes, etc., and this steel is used after rolling/pipemaking to obtain basic properties. It has been subjected to 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 small amounts of M□ and Nb and by high-temperature heating and high-temperature rolling.

This manufacturing method requires the combined addition of Mo and Nb in order to obtain hardness or high-temperature strength with the steel of the present invention, 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 this 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 plate thickness of 40+U.
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 weakened, so the lower limit is set to 0.04%.

In addition, if Cff1 is too large, the yield ratio at room temperature will increase, and furthermore, it will have a negative effect on the low temperature toughness of HAZ, so 0.1
The upper limit is 1%.

St is an element contained in deoxidized steel, and as the amount of Si increases, weldability and HAZ toughness deteriorate, so the upper limit is set to 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, hardenability increases and weldability and HAZ toughness deteriorate, so the upper limit of Mn was set at 0.7%.

The amount of Cr is an element that increases the strength of the base metal and welded part,
If the amount of addition is less than 0.05%, the effect will be weak (if the amount of Cr is 0.05%).
If it exceeds 5%, weldability and HAZ toughness will deteriorate.
The upper and lower limits were set to 0.05% and 0.5%, respectively.

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 lower limits were set to 0.50% and 0.5%, respectively.

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 it is less than 0.05%, the effect is weak;
If it exceeds 5%, Cu cracking will occur during hot rolling, so the upper and lower limits are set at 0.059fi, respectively.

It was set to 0.5%.

Although Al is generally an element contained in deoxidized steel, deoxidation is also performed by St and Ti, so there is no lower limit 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 was set at 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 scratches on continuous casting slabs, so the upper limit was set as o, ooe%. .

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 each 0.
0.02%, 0.005% or less.

The ingredients for obtaining the basic properties are as described above, but the steel of the present invention can be used under severe conditions (hydrogen cracking resistance is required in the weld, or welding with large heat input is applied). The elements described below, namely V, TI, 2r
, Ca, and REM are selectively added to improve the characteristics.

V combines with N to form VN and improves high temperature strength, but if it is less than 0.005%, no effect is observed;
If it exceeds 0.005 to 0.05, it will damage HAZ toughness.
% range.

T1 forms carbonitrides and improves HAZ toughness. A
If the ll amount is small, T1 oxide is formed and the HAZ toughness is improved, but if it is less than 0.005%, there is no effect and 0.
．． If it exceeds 0.03%, it will have an unfavorable effect on HAZ toughness, so 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 are effective in improving lamellar tear and hydrogen organic cracking resistance in welds, as well as increasing Charpy absorbed energy and improving low-temperature toughness. effective. However, if the amount of Ca is less than 0.0005%, there is no practical effect;
．． If it exceeds 0.005%, a large amount of Cab and CaS will be generated and become large inclusions, which will impair not only the toughness but also the cleanliness of the steel, and will also have an adverse effect on weldability and lamellar tear resistance. Addition amount ranges from 0.0005 to 0.005
%.

In addition, REM has the same effect as Ca, and increasing the amount added causes the same problems as Ca, and is also less economical, so the lower limit of REM amount is set at 0.001% and the upper limit is set at 0.005%. And so.

(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.

In Table 1, Nchl ~ No, I5 is the steel of the present invention, No,
Nos. 18 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 present invention *No. 1 to No. 15 in Table 2, the ferrite fraction in the microstructure is over 80%, the yield ratio (yield strength/tensile strength) at room temperature is as low as 70% or less, and the yield ratio at 600 ° C.
Its yield strength is 70% or more of that at room temperature.

On the other hand, Comparative Steel No. 16 had a low strength at room temperature and 600°C due to its low Mn content, and the ratio of the yield strength at 600°C to the yield strength at room temperature did not reach 70%. Furthermore, in comparative steel No. 17, since the Mn content was too high, the yield strength at 800° C. was sufficient, but the yield ratio at room temperature was too high, reaching 77%. Comparative steel No.

In No. 18, the yield strength at 600° C. was low and did not reach 70% due to the low Mo content. On the contrary, in comparative steel No. 19, the Mo content is too high and the yield strength at 600°C is sufficient, but the yield ratio at room temperature is too high, and the yield ratio at room temperature is 80%.
It also reached Comparative steel Nα20 had a low yield strength at room temperature and 600° C. due to its low C content, and the ratio of the yield strength at 600° C. to the yield strength at room temperature did not reach 70%. Furthermore, in comparison steel No. 21, since the C content was too high, the yield strength at 600° C. was not sufficient, or the yield ratio at room temperature was too high, reaching 82%.

/ / / / / / (Effects of the invention) Steel products manufactured using the chemical composition and manufacturing method of the present invention have a temperature of 600°C.
It is a completely new steel that has a high yield strength, the yield strength at 600°C is 70% or more of the yield strength at room temperature, and the yield ratio at room temperature is as low as 70% or less, and has excellent fire resistance and earthquake resistance.

[Brief explanation of the drawing]

FIGS. 1(a) and 1(b) are graphs of stress strain. Representative Patent Attorney Tatsuo Chanoki (a> (b) Procedural amendment form),! j September 28, 1990

Claims (1)

[Claims] By weight: C 0.04-0.11%, Si 0.6% or less, Mn 0.3-0.7%, Mo 0.5-0.8%, Ni 0.05-0.50% , Cu0.05~0.50%, Cr0.05~0.50%, Al0.1% or less, N0.006% or less, V0.005~0.05%, Ti0.005~0.03% , Zr0.005~0.03%, Ca0.0005~0.005%, and REM0.001~0.005%, the balance of which contains Fe and inevitable impurities, and which is given by formula (1). After reheating a steel piece having a composition with a Di^* value of 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%)