JPH0545646B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a method for producing low yield ratio steel with excellent fire resistance for use in various structures in the fields of architecture, civil engineering, marine structures, and the like. (Prior art) As is well known, rolled steel for general structures (JIS G 3101), rolled steel for welded structures (JIS G 3106), and welded are used as construction materials for various structures in the fields of architecture, civil engineering, marine structures, etc. Structural weather resistant hot rolled steel materials (JIS G 3114), high weather resistant rolled steel materials (JIS G
3444), general structural square steel plates (JIS G 3466), 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, and then refining it in a converter furnace.
A steel billet is produced in a continuous casting or blooming process, and then subjected to 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 construction-related laws and regulations, in the event of a fire, if the temperature of the steel material is around 350℃, the yield strength will be 60 to 70% of that at room temperature.
For example, in the case of structures whose columns are made of section steel stipulated by general structural rolled steel materials (JIS G 3101), slag wool, glass wool, and asbestos may In addition to spreading sprayed materials or felt based on materials such as the The steel should be used in such a way that it will not lose its load-bearing capacity due to thermal damage during a fire. Therefore, the cost of fireproof coating becomes higher than the cost of steel materials, and it is unavoidable that construction costs will rise significantly. 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. Sho 52-16021 discloses a fireproof structure in which a water tank is placed on top of the building to supply cooling water to pillars made of hollow steel pipes. In addition, in Japanese Patent Application No. 2-72566 (Japanese Unexamined Patent Publication No. 3-271342), by adding a certain amount of Mo, limiting the C/Mn ratio, and ensuring hardenability, the microstructure is changed to bainite, It has been shown that the high temperature strength at ℃ can be 70% or more of the room temperature strength. However, in this method, although the yield ratio at room temperature is low, the SS curve has a round shape without a clear yield point. Also, although this type of steel has a low apparent yield ratio, it has been found that its seismic resistance is not sufficient, and it has some problems. Figure 1a shows S- whose microstructure is mainly ferrite.
S curve, FIG. 1b shows an S-S curve in which the 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 using uncoated steel as a goal, the maximum temperature reached during a fire is 1000°C. It was 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. As a result of further research, we found that the most economical steel material was one whose high-temperature yield strength at 600°C was 70% or more of that at room temperature.We reduced the amount of expensive additive elements and added 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.
Mn0.3~0.7%, Mo0.5~0.8%, Al0.1% or less,
After reheating a steel slab with a composition of N0.006% or less, the balance containing Fe and unavoidable impurities, and a Di ^* value given by equation (1) of less than 0.80, in a temperature range of 1150 to 1300 °C, This is a method for producing a low yield ratio steel for construction with excellent fire resistance and a microstructure mainly composed of ferrite by completing hot rolling in a temperature range of 800 to 1000°C and then air cooling. (1) Equation Di ^* = 0.316√C (1 + 0.7Si) (0.35 + 4.1Mn) (1
+3Mo) (component unit: weight%) Furthermore, the present invention is added to C0.04 to 0.11% and Si0.6% by weight.
Below, Mn0.3~0.7%, Mo0.5~0.8%, Al0.1% or less, N0.006% or less, and V0.005~0.05%.
Ti0.005~0.03%, Zr0.005~0.03%, Ca0.0005~
0.005% and REM 0.001 to 0.005%. (Function) The feature of the present invention is that after reheating a steel slab with a composition of medium C-medium Mn steel with a certain amount of Mo added, the rolling is finished at a relatively high temperature. The steel and steel materials (hereinafter referred to as steel) produced by the invented method have appropriate room temperature yield strength and low yield strength with a clear yield phenomenon (the yield point is clearly recognized), as well as high high temperature yield strength. We are prepared. In other words, the ratio of the yield strength at a temperature of 600°C is greater than the yield strength at room temperature. The reason for this is that it is a steel with a considerable amount of Mo added to the medium C base component, resulting in 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 is required. . Therefore, the lower limit of the amount of Mo added is 0.5%. However, if the amount of Mo is too large, weldability will deteriorate,
Furthermore, since the toughness of the weld heat affected zone (HAZ) deteriorates, the upper limit of the amount of Mo needs to be 0.8%. Now, at room temperature, pressure steel materials for welded structures (JIS
G 3106) and satisfies the performance specified in 600
In order to maintain high yield strength at high temperatures of .degree. 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 sufficiently dissolve Mo during reheating, and therefore the lower limit of the reheating temperature is set at 1150°C. In addition, if the reheating temperature is too high, the crystal grains will become larger and the low-temperature toughness will deteriorate, so the upper limit is
Must be heated to 1300℃. 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℃ 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 ferrite. Further, in the present invention, the upper limit of the rolling end temperature is set to 1000° C. in order to ensure toughness as a construction steel. The Di ^* value of the present invention is a parameter related to the hardenability of steel components, and the higher the value, the harder the structure of martensite or bainite becomes. In addition, the cooling rate after rolling also affects the hardenability; if the Di ^* value is the same, the faster the cooling rate, the harder the structure of martensite or bainite becomes. Therefore, in order to obtain a ferrite-based microstructure, it is essential to control the Di ^* value to less than 0.80 and to perform cooling after rolling by air cooling as shown in the Examples below. 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. Additionally, there is Japanese Patent Application No. 1-139328 (Japanese Patent Publication No. 4-50362) 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, the combined addition of Mo and Nb is essential, which is different from the sole addition of Mo in the present invention. Furthermore, it is generally known that it is difficult to reduce the yield ratio of Nb-added steel, and this is thought to be due to 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 is suitable for manufacturing steel plates with a thickness of 40 mm or less, which has a low yield ratio at room temperature and a yield strength of 70% or more of that at room temperature at 600°C, and is 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 welded part and to exhibit the effect of Mo addition, and is 0.04%.
The lower limit is set at 0.04%, as the effect will be diminished if it is less than that.
Also, if the amount of C is too large, the yield ratio at room temperature will increase,
Furthermore, since it has a negative effect on the low temperature toughness of HAZ,
The upper limit is 0.11%. Si is an element contained in deoxidized steel, and as the amount of Si increases, weldability and HAZ toughness deteriorate, so the upper limit was set at 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 will increase and weldability and HAZ toughness will deteriorate, so the upper limit of Mn has been set to 0.7.
%. Al is an element generally included in deoxidized steel,
Since deoxidation is also carried out by Si and Ti, there is no lower limit for the steel of the present invention. But Al
If the amount 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 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 a small effect on high-temperature strength, their amounts are not particularly limited, but generally the properties of steel, such as toughness and strength in the thickness direction, improve as the amounts of these P and S elements decrease. Desirable amounts of P and S are 0.02% and 0.005, respectively.
% 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 characteristics are improved by selectively adding the elements described below, namely V, Ti, Zr, Ca, and REM. V combines with N to form VN and improves high temperature strength. However, if it is less than 0.005%, the effect will be weak, and if it exceeds 0.05%, it will damage the toughness of the weld, so it is limited to 0.005 to 0.05%. Ti forms carbonitrides and improves HAZ toughness. When the amount of Al is small, Ti oxides are formed.
Although it improves HAZ toughness, it is ineffective if it is less than 0.005% and has an unfavorable effect on HAZ toughness if it exceeds 0.03%, 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, REM controls the morphology of sulfide (MnS),
In addition to being effective in improving lamellar tear and hydrogen organic cracking resistance in welds, it also increases shear py absorption energy and improves low-temperature toughness. However, if the amount of Ca is less than 0.0005%, it has no practical effect, and if it exceeds 0.005%, a large amount of CaO and CaS will be generated and become large inclusions, which will impair not only the toughness but also the cleanliness of the steel. Since this also has a negative effect on weldability and lamellar tear resistance, the range of Ca addition is set to 0.0005 to 0.005%. In addition, REM has the same effect as Ca, and when added in a large amount, it causes the same problems as Ca.
Furthermore, since the economic efficiency becomes worse, the lower limit of REM amount is
0.001%, with an upper limit of 0.005%. (Example) A steel plate was manufactured using a well-known converter, continuous casting, and thick plate process, and its strength at room temperature and 600°C was investigated. Inventive steel is used in No. 1 to No. 10 in Table 1, and No. 11 to No. 16
shows the chemical composition of comparative steel. 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 invention steels No. 1 to No. 10 in Table 2, the ferrite fraction in the microstructure is over 60%, the yield ratio (yield strength/tensile strength) at room temperature is low at 70% or less, and the
Its yield strength is 70% or more of that at room temperature. On the other hand, Comparative Steel No. 11 had a low strength at both 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%. In addition, for comparative steel No. 12,
Because 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 77%.
Comparison steel No. 13 has a low Mo content, so it can be used at room temperature and 600℃.
yield strength is low, 600% compared to yield strength at room temperature
The percentage of yield strength at °C was at a level below 70%. On the contrary, in comparative steel No. 14, 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 81%. Comparative steel No. 15 had a low yield strength at room temperature and at 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 was at a level below 70%. moreover,
Comparative steel No. 16 has too high a C content, so the yield strength at 600℃ is sufficient, but the yield ratio at room temperature is too high.
It reached 78%.

【table】

[Table] (Effects of the invention) Steel products manufactured using the chemical composition and manufacturing method of the present invention are
The yield strength at 600℃ is high, and the yield strength at 600℃ is more than 70% of the yield strength at room temperature, and the yield ratio at room temperature is 70%.
This is a completely new steel with excellent fire resistance and earthquake resistance.

[Brief explanation of drawings]

FIG. 1 is a stress-strain chart, where a shows the case where the microstructure is mainly ferrite and b shows the case where the microstructure is mainly bainite.

Claims (1)

[Claims] 1 wt%: 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, balance contains Fe and unavoidable impurities, and
After reheating a steel slab with a composition with a Di ^* value of less than 0.80 given by formula (1) in a temperature range of 1150 to 1300℃,
A method for producing a low yield ratio steel for construction with excellent fire resistance, characterized by completing hot rolling at a temperature range of 800 to 1000°C, and then air cooling to make the microstructure mainly ferrite. Di ^* =0.316√C(1+0.7Si)(0.35+4.1Mn)(1
+3Mo)...(1) The steel billet according to claim 1 containing 2 wt% and further V: 0.005 to 0.05% is reheated in a temperature range of 1150 to 1300°C, and then hot rolled to 800 to 800°C. A method for producing a low yield ratio steel for construction with excellent fire resistance, which is characterized by finishing in a temperature range of 1000°C and then air cooling to make the microstructure mainly ferrite. The steel billet according to claim 1 or claim 2 further containing one or two of Ti: 0.005 to 0.03% and Zr: 0.005 to 0.03% at a temperature of 1150 to 1300°C. After reheating, hot rolling is finished at a temperature range of 800-1000℃,
A method for producing a low yield ratio steel for construction with excellent fire resistance, which is then air-cooled to make the microstructure mainly ferrite. 4 wt% and further containing one or two of Ca: 0.0005~0.005% REM: 0.001~0.005%, the steel billet according to claims 1 to 3 is recycled in a temperature range of 1150~1300°C. After heating,
A method for producing a low yield ratio steel for construction with excellent fire resistance, characterized by completing hot rolling at a temperature range of 800 to 1000°C, and then air cooling to make the microstructure mainly ferrite.