KR20000019209A - Preparation method of hot rolled steel sheet with low yield ratio having good hot strength - Google Patents

Abstract

PURPOSE: A method for preparing a hot rolled steel sheet is provided, which steel sheet has the low yield ratio and the good hot strength. CONSTITUTION: A steel sheet is prepared by finish hot rolling the steel comprising carbon in arrange of 0.05-0.10 percent by weight; silicon in a range of 0.6-1.0 percent by weight; manganese in a range of 0.5-1.0 percent by weight; phosphorus less than 0.015 percent by weight; sulfur less than 0.016 percent by weight; aluminum in a range of 0.02-0.1 percent by weight; niobium in a range of 0.02-0.06 percent by weight; molybdenum in a range of 0.1-0.3 percent by weight; chromium in a range of 0.1-0.3 percent by weight; and iron and inevitable impurities being the balance at a temperature of 820-880°C; water-cooling it at a cooling velocity of 10-60°C per second; and coiling it at a temperature of 520-650°C. The steel sheet has a yield strength more than 22 kg per square millimeter at 600°C and a yield ratio less than 85%.

Description

Manufacturing method of resistive complex ratio hot rolled steel sheet with excellent high temperature strength

The present invention relates to a method for manufacturing a hot rolled steel sheet having excellent high temperature strength, and in particular, exhibits excellent material properties at high temperature, so that the high temperature strength of alloy elements and various rolling conditions can be improved in order to be used as a substitute for general steel used for construction. The present invention relates to a method for producing an excellent resistance ratio ratio type hot rolled steel sheet.

In general, general steels used for construction show a rapid decrease in strength in the event of fire, which is a risk of collapse. Therefore, in order to prevent this, the fireproof coating is applied to the steel surface. However, due to such fireproof coating construction on the steel surface, there are problems such as an increase in construction cost, a decrease in use space, and an increase in construction period.

Therefore, many efforts are being made to develop steels having excellent high temperature strength of the material itself in order to reduce the fire resistance coating applied to the steel surface in order to prevent a decrease in strength due to the temperature rise of the steel materials.

First, Japanese Patent Kobe Publication No. Hei 6-264136 discloses a method for producing a resistive-strength thick plate refractory steel having excellent weldability. The alloy component system is C: 0.04 to 0.15%, Si: 0.05 to 0.60%, and Mn. : Pcm (= C + Si / 30 + Mn / 20 + Cu containing 0.5 to 1.5%, Mo: 0.1 to 0.4%, Nb: 0.005 to 0.06%, V: 0.005 to 0.06%, Ti: 0.005 to 0.03% / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B, weldability evaluation index) After heating the steel of 0.2% or less at 1050 ℃ or more and finishing rolling at 850 ~ 950 ℃, Ar Steel is produced by accelerating cooling to 400 to 550 ° C. at a cooling rate of 3 to 20 ° C./s at a temperature of ₃ or more, and reheating again to a temperature range of Ac ₁ to Ac ₃ to produce the steel at 500 to 650 ° C. Doing.

In addition, Japanese Patent Laid-Open No. 7-173532 discloses a method for manufacturing a resistive wear-resistant medical steel having excellent weldability having a material thickness of 60 to 70 mmt. Here, the component system which is almost similar to the patent introduced before, that is, C: 0.04 to 0.15%, Si: 0.05 to 0.60%, Mn: 0.5 to 1.5%, Mo: 0.1 to 0.4% m, Nb: 0.005 to 0.06%, V: 0.005 Pcm 0.2% or less of Pcm containing -0.06%, Ti: 0.005-0.03%, and Al: 0.002-0.1% is heated at 1050 degreeC or more, air-cooled after finishing rolling at 850-950 degreeC, and is 700-750 degreeC. After cooling to 400 to 550 ° C. at a cooling rate of 3 to 20 ° C./s at a temperature of 500 ° C., it is considered at 500 to 650 ° C., if necessary, Cu is 0.05 to 0.4%, Ni is 0.05 to 0.5%, and Cr is 0.1. It is reported that steel materials of excellent material properties can be produced by adding at least one alloy component of ˜0.4% and Ca: 5 to 50 ppm.

In Japanese Patent Laid-Open No. 6-73449, C: 0.05-0.15%, Si: 0.60%, Mn: 0.5-1.8%, P: 0.03% or less, S: 0.03% or less . Al: 0.003% or less, Mo: 0.1 to 0.4%, Nb: 0.005 to 0.06%, Ti: 0.005 to 0.03%, N: 0.002 to 0.007%, Ca 5 to 50 ppm by heating to 1050 ℃ using a component system, A method for producing a high strength strength-bearing steel sheet for construction in which a rolling reduction rate of 1000 ° C. or lower is 50% or more and finishes rolling at 850 ° C. to 950 ° C. is reported, which combines high strength and good weldability at 600 ° C. By manufacturing steel materials showing yield ratio, it is possible to reduce and omit fireproof coating. This invention features sol. The addition amount of Al is limited, sol. Al: The yield strength is increased below 0.003%, but the specific cause is not reported.

In addition, Japanese Patent Laid-Open No. 6-57371 reports a method for producing a resistive wear-resistant refractory steel for construction that has a high yield strength at 600 ° C. and shows good weldability and toughness of a high heat input weld, which is appropriately added with alloying elements Mo and Cr and 200 A. It is characterized by containing the following grain size, Nb carbonitride of 106 piece / mm <^3> or more. According to the present invention, after the hot rolling, Nb precipitation is suppressed by accelerated cooling to take advantage of the transformation of solid solution by Nb, and after the base material is made of bainite, a large number of fine Nb carbonitrides are precipitated by thinning. It is characterized by ensuring high temperature strength while having a low carbon equivalent by strengthening. On the other hand, Japanese Laid-Open Patent Publication No. 3-173715 also reports that a steel material exhibiting excellent high-temperature strength is produced by carrying out a composite addition of Cr, Mo, and Nb, followed by controlled rolling.

Next, Japanese Laid-Open Patent Publication No. 7-207338 reports on a method for manufacturing a resistive wear-resistant refractory steel sheet for construction, where C: 0.04 to 0.15%, Si: 0.60% or less, and Mn: 0.8 to 1.6%. , P: 0.03% or less, S: 0.01% or less, Mo: 0.4 to 1.0%, Nb: 0.005% or less, V: 0.02 to 0.1%, Ti: 0.005 to 0.025%, Al: 0.06% or less, N: 10 to containing 60ppm, and substantially in the manufacture of almost then by not re-heating a steel material that is added to terminate the rolling at least 850 ℃ temperature air cooling as the thickness construction resistance of the yield ratio of 80% or less of 25mm or less yield ratio by refractory 490N / mm ² class steels for Nb The method is disclosed. In general, when hot rolled steel having a thin plate thickness, yield strength and tensile strength increase due to the rapid cooling of the microstructure, formation of processed structure, and air-cooling after rolling. In particular, since Nb is known to increase the yield strength of the material at room temperature, in the present invention, the amount of Nb added is suppressed as much as possible, and the amount of Mo and V is adjusted to obtain a resistance ratio with fire resistance.

In addition, in the case of Japanese Kawasaki Steel (KSC), in Japanese Patent Application Laid-Open No. 6-248334, the weldability is superior to that of the conventional high temperature low alloy steel, and the manufacturing method of the refractory steel for the construction structure, which is superior to the conventional welding structural steel, has high temperature strength. Mo: 0.15 to 0.60%, Nb: 0.005 to 0.10% to improve room temperature strength (Mn + Cr + Cu + Ni + 500B) to 0.8 ~ 1.6%, and then the steels are heated to 1050-1300 ℃. After the hot rolling is finished at 950 ° C or higher, a method for manufacturing a refractory steel for a building structure, which is air cooled after being quenched in the range of 600 to 750 ° C, is reported, which is intended to improve the high temperature strength by Mo and Nb carbides. I'm doing it.

Further, in the Japanese steel pipe (NKK), the relationship between the component system capable of manufacturing the refractory steel, the thickness, and the cooling start temperature at the time of controlled rolling / controlled cooling is made clear, and the plate of the refractory steel having the predetermined strength In order to provide the optimum manufacturing conditions for the thickness, a component system satisfying 5% ≦ Mo + V + Ceq ≦ 1.0% is heated to a thickness capable of hot rolling, followed by filamentous rolling over Ar ₃ to a predetermined sheet thickness, and then ( 1) Start of cooling in the temperature range of the temperature Ts ± 10 ℃ provided by the formula, and quench at 1-50 ℃ / s every second to a temperature of 550 ℃ or less every ^second, the building fire resistance of 490 ~ 720 N / mm ² It provides a method for manufacturing steel.

Where Ts = 400 × logK... … … … … (1) Ts: Cooling start temperature (℃), K = t / (Mo + V + Ceq), t: Thickness of final product (mm)

To summarize the above patents of Japanese steelmakers, an appropriate amount of alloying elements such as Mo, Cr, Nb, and V are added to improve the high temperature strength at 600 ° C, and the processes such as controlled rolling and accelerated cooling are appropriately performed. It is used, It is characterized by adding some Cu, Ni, B, and rare earth elements.

However, the general steel still used for building has the risk of collapse due to the rapid decrease in strength in case of fire, and in order to prevent it, the construction cost is increased due to the fireproof coating on the surface of the steel, and the space used and the construction period are increased. Because of this problem, efforts are being made to develop steel materials with excellent high temperature strength.

The present invention has been made to solve the above-mentioned problems, the steel material that can reduce or omit the thickness of the fire-resistant coating to prevent collapse in the event of a fire of the building, more precisely excellent low temperature and high temperature strength It is an object to provide a steel having a yield ratio (85%).

The present invention for achieving the above object is by weight% C: 0.05 to 0.10%, Si: 0.6% to 1.0%, Mn: 0.5 to 1.0%, P <0.015%, S <0.016%, Al: 0.02 to 0.1 %, Nb: 0.02% to 0.06%, Mo: 0.1% to 0.3%, Cr: 0.1% to 0.3%, and the remainder is hot finished rolling at 820 ° C to 880 ° C for a remainder of Fe and an unavoidable impurity element; It is characterized by consisting of a step of water cooling in the range of 60 ℃ / s, and winding at 520 ~ 650 ℃.

1 is a graph showing a change in room temperature tensile properties according to the amount of Si added according to the present invention.

2 is a graph showing a change in yield ratio according to the amount of Si added according to the present invention.

Figure 3 is a graph showing the change in tensile properties at high temperature 600 ℃ according to the amount of Si added in accordance with the present invention.

Hereinafter, the present invention will be described in detail with reference to the drawings.

In the present invention, C: 0.05 to 0.10%, Si: 0.6% to 1.0%, Mn: 0.5 to 1.0%, P <0.015%, S <0.016%, Al: 0.02 to 0.1%, Nb: 0.02 to 0.06 %, Mo: 0.1% to 0.3%, Cr: 0.1% to 0.3%, the remainder is hot-rolled and rolled at a temperature of 820 ° C to 880 ° C for a steel composed of Fe and an unavoidable impurity element, and water cooled at a cooling rate of 10 ° C to 60 ° C / s. And a step of winding at 520 to 650 ° C., a method for producing a resistance-ratio type hot rolled steel sheet having excellent high temperature strength.

Hereinafter, the reason for limiting the composition range of the inventive steel will be described in detail.

C: Elements necessary for securing the strength of the material, but excessive addition increases the carbon equivalent, which is an index for evaluating the weldability of the material, to reduce the weldability, and also limits the amount because it adversely affects the impact toughness. Needs to be. In this invention, the minimum was set to 0.05% and the upper limit to 0.10%.

Si: An element usually used as a deoxidizer. Since Si slows the diffusion rate of C in ferrite due to the addition of Si, it suppresses the formation of carbides. In the present invention, using the Si effect to maintain the state of the steel sheet to suppress the formation of carbides in the hot rolled, this precipitation suppressed hot rolled steel sheet formed a large amount of carbide in the process of high temperature tensile test at 600 ℃ to increase the high temperature strength To improve. Efforts to improve the high-temperature strength by using Si, which is usually used as a deoxidizer, have not been made yet. In the present invention, the effect of Si addition is less than 0.6%, and the Si addition is excessively added. In this case, the upper limit is limited to 1.0% because it shows defects due to the formation of a thin scale on the surface.

Mn: As an element to enhance the strength of steel, 0.5% or more must be added in order to exhibit high strength, but too much addition of Mn increases the amount of nonmetallic inclusions and increases segregation, thereby securing toughness. The upper limit is limited to 1.0% because it is difficult to perform and also adversely affects the welding phase.

P, S: P is an element that promotes the formation of ferrite and can increase the ductility without harming the strength of steel. In addition, S forms a non-metallic inclusion represented by MnS and is likely to generate defects that cause cracks during processing of steel. Therefore, it is preferable to manage P and S as low as possible, and in general, the present invention is limited in this way because 0.01 P, 0.016 or less, which is a level that can generally achieve low P and S, is preferable.

Al: It is an element mainly used for deoxidation, and it is advantageous in terms of processability improvement since it helps to form ferrite. In this case, the oxygen in the steel should be sufficiently removed at about 0.02% or more, and the formation of AlN also contributes to the refinement of the structure. In the case where the content is large, 0.1% is set as the upper limit because it is known that welding defects are easily generated by the formation of oxide during welding.

Nb: It is an element that suppresses recrystallization of austenite and refines ferrite grains produced during the cooling process after rolling, and also increases the room temperature strength and the high temperature strength of the steel by the carbonitrides produced. In this excessive case, the yield strength rise at room temperature becomes larger than the tensile strength rise, thereby rapidly increasing the yield ratio. Therefore, in the present invention, the lower limit of Nb is limited to 0.02% and the upper limit to 0.06%, respectively.

Mo: It is an element essential for high temperature strength improvement, and is an element which forms fine carbide and contributes greatly to the strength improvement at 600 degreeC. However, if the amount is less than 0.1%, the effect is small, and when 0.3% or more is added, the price of steel is increased by expensive alloying elements, so the range is limited as described above.

Like Cr: Mo, it is an element that is effective in improving high temperature strength by forming fine carbides, and its effect is difficult to expect at less than 0.1%, and its effect is saturated at 0.3% or more, and it is disadvantageous in terms of economics. do.

Looking at the hot rolling conditions related to the present invention, the hot finish rolling temperature 820 to 880 ° C, which is important for determining the material of the present invention, is preferable, and then water-cooled at a cooling rate of 10 to 60 ° C / s and the winding temperature of 520 to When working at 850 ℃ can produce a material having a yield ratio of 85% or less while having a yield strength of 22kg / mm ² or more at 600 ℃ in TS 50kg class.

Hereinafter, the present invention will be described in detail with reference to Examples.

(Example 1)

Table 1 shows the chemical composition and hot rolling conditions of the test specimens, and Table 2 summarizes the mechanical properties of each test specimen.

Steel grade C Si Mn P S Al Nb Mo Cr FDT (℃) CT (℃) Cooling rate (℃ / s) Remarks Al 0.08 0.05 0.78 0.011 0.010 0.05 0.03 0.28 0.31 853 610 27 Comparative steel A2 0.08 0.49 0.78 0.012 0.010 0.03 0.03 0.28 0.29 856 620 31 Comparative steel A3 0.08 0.86 0.83 0.012 0.011 0.06 0.03 0.26 0.31 858 690 24 Comparative steel A4 0.08 0.86 0.83 0.012 0.011 0.06 0.03 0.26 0.31 855 635 16 Invention steel A5 0.08 0.86 0.83 0.012 0.011 0.06 0.03 0.26 0.31 854 537 50 Invention steel A6 0.09 0.25 0.87 0.012 0.010 0.03 0.03 0.30 0.30 853 600 30 Comparative steel A7 0.04 0.22 1.14 0.013 0.011 0.03 0.03 0.25 0.27 852 600 31 Comparative steel A8 0.12 0.26 0.40 0.011 0.011 0.05 0.03 0.28 0.30 854 630 27 Comparative steel A9 0.09 0.25 0.80 0.013 0.012 0.02 0.03 - 0.30 849 566 36 Comparative steel A10 0.16 0.40 1.30 0.012 0.012 0.02 - - - 852 570 37 Comparative steel

(FDT: finish delivery temperature, CT: coiling temperature)

Steel grade Room temperature tensile properties High temperature tensile properties Hot Rolling Condition YS (kg / mm ² ) TS (kg / mm ² ) El (%) YR (%) YS (kg / mm ² ) TS (kg / mm ² ) EI (%) FDT (℃) CT (℃) A1 46.9 53.1 30.3 88.3 23.8 30.9 62.6 853 610 A2 48.9 56.6 31.0 86.3 25.1 32.4 59.1 856 620 A3 42.9 52.2 33.3 82.2 21.1 28.7 54.2 858 690 A4 48.3 58.3 29.3 82.9 26.0 32.4 57.1 855 635 A5 55.2 65.2 18.9 84.6 31.9 39.4 33.0 854 537 A6 49.8 56.8 30.9 87.7 25.1 33.4 52.3 853 600 A7 48.2 53.9 28.0 89.4 23.7 32.1 55.0 852 600 A8 45.2 54.2 30.2 83.3 22.5 30.5 58.7 854 630 A9 46.2 53.1 29.9 87.0 20.6 24.8 61.0 849 566 A10 38.0 49.8 36.0 76.4 15.6 21.3 88.3 852 570

(El: elongation, YR: yield ratio)

In order to investigate the change in the room temperature material according to the amount of Si addition, the tensile properties of the A1, A2, A4 steel with varying the amount of Si addition according to the present invention are shown in FIG. 1, and the room temperature tensile strength increases as Si is increased. Can be. This progress is due to the addition of Si as a solid solution strengthening element, and it can be seen that the tensile strength is increased by about 5 kg / mm ² by increasing Si from 0.8% to 0.87%. On the other hand, the yield strength can be seen to decrease slightly by increasing 2kg / mm ² as the Si is increased from 0.05% to 0.49% and further increased to 0.87%. The change in yield ratio (yield strength ÷ tensile strength x 100) of this test specimen showing such room temperature tensile characteristics according to the amount of Si added is shown in FIG. 2. In the case of refractory steel, it is usually used as a thick plate for building structure, and it is used in a place where deformation such as bending process is relatively small. However, when used as a hot rolled material like the present invention, pipe forming is performed by pipe forming. Since it is used, it is advantageous to lower the yield ratio in order to facilitate tube forming. In FIG. 2, when the Si addition amount is 0.05% (A1 steel), 0.49% (A2 steel), and 0.87% (A3 steel), the yield ratio is 88.3%, 86.3%, and 82.9%, respectively. It can be seen that the yield ratio decreases, and in order to obtain a yield ratio of 85% or less, the Si addition amount should be added more than 0.6%.

(Example 2)

In the case of general steel used in buildings, a fire resistant steel, which is a high strength steel at high temperatures, was required because of the risk of collapse due to a rapid decrease in strength in a fire. In order to evaluate the fire resistance of steels, a tensile test is usually performed at 600 ° C., and it is evaluated whether the high temperature yield strength indicates 2/3 or more of the room temperature yield strength. At higher temperature tensile tests, the yield strength shall be at least 22 kg / mm ² .

FIG. 3 shows the change in strength at 600 ° C. of A1, A2, and A4 steels with Si addition amounts of 0.05%, 0.49%, and 0.86%, respectively, in order to investigate the change of high temperature material properties according to Si addition. , A4 steel showed yield strengths of 23.8, 25.1 and 26.0 kg / mm ² , respectively, and as the Si addition amount increased, the yield strength continued to increase. In general, high temperature strength is known to increase the high temperature strength because fine Mo, Cr-based carbides are precipitated during the tensile test conducted at 600 ℃ to hinder the movement of dislocations, and therefore, a large amount of carbides are precipitated during the high temperature tensile test. For this purpose, it is advantageous to suppress the precipitation of carbides as much as possible in the material before the tensile test, and to deposit a large amount of carbide during the tensile test.

In general, Si is widely known as an element for inhibiting carbide formation. Due to the role of Si, precipitation of carbide is suppressed in the state of the material before the tensile test, and the material having the small amount of carbide deposited before the tensile test is subjected to the tensile test. Since carbides precipitate actively during the process, the high temperature strength seems to improve as the amount of Si added increases. On the other hand, the yield strength of A3 steel added 0.89% of Si was 21.1kg / mm ² , which is below TS 50kg grade, which is different from the winding temperature of A3 steel (A4, A5 steel). Because of the high rolling temperature, a large amount of carbides are precipitated in the state of the material when the coiling temperature is high, and the carbides that have already been precipitated in the state of the material coarsen in the process of being kept at a high temperature during the tensile test, and thus do not contribute significantly to the improvement of the high temperature strength. In addition, since a large amount of carbide has already been precipitated before the tensile test, the newly formed carbide fraction during the high temperature tensile test also appears to be low in strength. Therefore, in order to increase high temperature strength, it is thought that setting the winding temperature range as low as less than 650 degreeC is desirable.

On the other hand, the high temperature yield strength of A9 steel without Mo and A10 steel without Mo and Cr is shown to be 20.6 and 15.6 kg / mm ² , respectively. It can be seen that the target TS 50kg / mm ² refractory steel standards below 22kg / mm ² , and therefore, Cr, Mo is an essential element to improve the high temperature strength.

The A6, A7, and A8 steels are for limiting the amount of C and Mn, and the amount of C is reduced compared to A6 steel (0.25% Si), and the amount of C is increased and the amount of Mn is reduced compared to A7 and A6. Yield strength at high temperatures is 23.7 and 22.5kg / mm ² , respectively, and 1.4, 2.6kg / mm ^{2 is} reduced compared to A6 steel. Therefore, it is confirmed that an appropriate amount of C and Mn is required in terms of high temperature yield strength. In the present invention, the amount of C is limited to 0.05 to 0.1%, and Mn is limited to 0.5 to 1.0%.

As described above, by controlling the hot finishing rolling temperature, cooling rate, winding temperature, and controlling the Si content of the steel according to the present invention, the yield ratio is 85% or less while having a yield strength of 22Kg / mm2 or more at 600 ° C in TS 50Kg class. Since the material can be manufactured, it exhibits excellent material properties, especially at high temperatures, and thus can be used to replace general steel used for construction.

Claims (1)

By weight% C: 0.05-0.10%, Si: 0.6% -1.0%, Mn: 0.5-1.0%, P <0.015%, S <0.016%, Al: 0.02-0.1%, Nb: 0.02-0.06%, Mo : 0.1 to 0.3%, Cr: 0.1 to 0.3%, the remainder being hot-finish-rolled at 820 to 880 ° C. of steel consisting of Fe and unavoidable impurity elements, followed by water cooling at a cooling rate of 10 to 60 ° C./s, , The method of manufacturing a high-temperature resistance excellent strength ratio hot-rolled steel sheet, characterized in that consisting of winding to 520 ~ 650 ℃.