KR20030053122A - A method for manufacturing TMCP heavy plate using soft reduction - Google Patents

Abstract

PURPOSE: A method for manufacturing TMCP steel plate is provided to manufacture TMCP steel plate with 350 MPa class yield strength having superior weldability and internal quality by controlling steel constituents, and applying soft reduction and optimizing hot rolling and cold rolling conditions during continuous casting. CONSTITUTION: The method for manufacturing TMCP (thermo-mechanical controlled process) steel plate with 350 MPa class yield strength comprises a step of preparing molten steel having a composition comprising 0.06 to 0.11 wt.% of C, 0.20 to 0.50 wt.% of Si, 1.30 to 1.50 wt.% of Mn, 0.020 wt.% or less of P, 0.005 wt.% or less of S, 0.015 to 0.050 wt.% of Al, 0.005 to 0.025 wt.% of Nb, 0.1 to 0.2 wt.% of Cu, 0.1 to 0.2 wt.% of Ni, 0.005 to 0.015 wt.% of Ti, 50 ppm or less of N, and a balance of Fe and other inevitable impurities; a step of manufacturing a steel slab by soft reducing the slab to 2.43 mm after continuously casting molten steel having the composition in a continuous casting machine; a step of reheating the air cooled steel slab to a temperature of 1,050 to 1,150 deg.C after air cooling the steel slab to an ordinary temperature; a step of second finish rolling the first finish rolled steel slab at a temperature of 720 to 780 deg.C after first finish rolling the reheated steel slab at a temperature of 780 to 850 deg.C to obtain a residual reduction ratio of 45%; and a step of air cooling the resulting steel plate after cooling the rolled steel plate to the temperature range of 500 to 580 deg.C.

Description

A method for manufacturing TMCP heavy plate using soft reduction} using light pressure under reduced pressure (S method for manufacturing TMCP steel)

The present invention relates to a YP 350Mpa TMCP type shipbuilding steel manufacturing method used for the Hatch Coaming of upper deck of a ship (container ship), and more specifically, to control the steel components and apply light pressure during continuous casting. In addition, the present invention relates to a method of manufacturing 350 Mpa thick TMCP steel having a yield strength of weldability and excellent internal quality by optimizing hot rolling and cold rolling conditions.

Recently, as container ships are accelerated to reduce logistics costs and economies of scale, domestic and overseas shipbuilders have low CEQ (carbon equivalent index) and low Pcm (welding crack susceptibility) in the container deck to improve dry productivity. It is required to apply TMCP (Thermo-Meechanical Controlled Process) thick steel that can be made exponentially.

However, in the case of thick materials, inclusions, central segregation, internal pores, and internal cracks in the material are not sufficiently compressed or mitigated during the rolling process, which acts as a starting point for embrittlement at low temperature after processing, causing various problems in using the material. However, there is a problem that the existing steel that performs the external furnace refining treatment to reduce the P, S and impurities after the component control has a problem in satisfying the required quality. Therefore, the manufacturing technology of the thick TMCP steel material which has drastically improved the internal quality is required.

Therefore, the present invention is to solve the problems of the prior art, by applying light pressure during continuous casting, while squeezing the pores generated by the shrinkage, by optimizing the hot rolling conditions and cold rolling conditions continuous weldability and excellent internal quality Its purpose is to provide a method for producing thick TMCP steel with excellent internal quality during casting.

The present invention for achieving the above object, in weight%, C: 0.06 ~ 0.11%, Si: 0.20 ~ 0.50%, Mn: 1.30 ~ 1.50%, P: 0.020% or less, S: 0.005% or less, Al: 0.015 ~ 0.050%, Nb: 0.005 ~ 0.025%, Cu: 0.1 ~ 0.2%, Ni: 0.1 ~ 0.2%, Ti: 0.005 ~ 0.015%, N: 50ppm or less, to provide molten steel composed of residual Fe and other unavoidable impurities step; Manufacturing molten steel slab by continuously casting molten steel of the composition in a continuous casting machine and then reducing the cast slab to 2.43 mm; Cooling the steel slab to room temperature and then reheating it to a temperature of 1050 to 1150 ° C .; Completing the primary finishing rolling at the reheated steel slab at 780 to 850 ° C. such that the residual pressure reduction rate is 45%, and then completing the secondary finishing rolling at 720 to 780 ° C .; It relates to a method of producing a yield strength of 350Mpa thick TMCP steel, including; and the step of cooling the rolled steel sheet to a temperature range of 500 ~ 580 ℃ and air-cooled.

Hereinafter, the steel composition component of this invention is demonstrated.

Carbon [C] is an element for securing the strength of steel, and has an effect of increasing strength by increasing the fraction of Pearlite in the steel structure during thick plate rolling. However, when excessively added, hardened materials such as needle martensite, upper bainite, cementite, etc., formed after cooling of the base material or cooling of the welded part, form toughness of the base material and weld part. In the present invention to apply the accelerated cooling (TMCP) manufacturing method after rolling to limit the carbon content is limited to 0.06 ~ 0.11%.

Silicon [Si] is an element that forms carbide to form a solid solution on a ferrite to increase hardness, elastic limit, and tensile strength. In the present invention, the Si content is limited to 0.20 to 0.50%. If the content exceeds 0.50%, the ferrite fraction decreases and the nonmetallic inclusions are excessively formed, thereby reducing the toughness and deteriorating the plastic workability, while to compensate for the decrease in tensile strength due to tissue degradation after welding. It is because it should contain 0.20% or more.

Manganese [Mn] is an element that forms carbide (Mn ₃ C) in the steel to suppress grain growth and refine the structure. In addition, by expanding the austenite region and strengthening the Ar ₃ temperature to enlarge the region of the control rolling, it helps the particle fineness by rolling and contributes to the improvement of toughness and strength.

In the present invention, Mn is added in a range of 1.30 to 1.50%, which is less than 1.30%, which is insufficient to form a two-phase (ferrite + pearlite) and thus does not contribute to the improvement of strength. This is because the inclusion of MnS and the like can deteriorate the impact toughness and the internal quality of the weld.

Phosphorus [P] is an element that forms Fe ₃ P in steel to reduce impact resistance, thereby causing room temperature and low temperature brittleness and thus impair impact toughness. In addition, since the pearlite band structure (Pearlite Band Structure) formed during rolling does not easily diffuse even at high temperature is extended in the longitudinal direction to degrade the internal quality, the content is limited to less than 0.020%.

Sulfur (S) embrittles the steel by forming segregation zones by inclusions such as MnS, Fe ₃ P in the steel, and lowers the low-temperature toughness and weldability, so the content is limited to 0.005% or less.

Aluminum (Al) is a strong deoxidizer, which precipitates AlN and shows a grain refinement effect. However, when excessively added, dendritic carbides are formed, which makes the steel brittle, generates surface cracks of the slabs, and impairs impact toughness, thereby limiting the addition amount to 0.015 to 0.050%.

In the present invention, when the niobium (Nb) addition amount is 0.005% or more, Nb (C, N) precipitates segregated at grain boundaries during rolling inhibit grain growth and thus play a role of securing strength by grain refinement. However, excessive addition may impair the impact toughness of the weld zone, so the amount of Nb added is limited to 0.005% to 0.025%.

When copper (Cu) is added 0.10% or more, it has a solid solution strengthening effect to increase the strength, hardness and corrosion resistance of the steel. However, if excessively added, the oxidation rate is lower than that of Fe during hot working, so that the surface penetrates into the interior after being scattered on the surface, causing red brittleness.

Nickel (Ni) is an effective ingredient to refine the structure of the steel and solidify the matrix by solidifying with austenite or ferrite, and in particular, to improve the toughness at low temperatures to prevent low temperature embrittlement. In addition, since it serves to prevent the thermal embrittlement of Cu, it is preferable to set the content to 0.10 to 0.20% of the same content as Cu.

Titanium (Ti) is an element contributing to the refinement of ferrite by forming fine carbides and nitrides during the solidification process of steel to suppress grain growth of austenite. However, when the content of steel increases, it is difficult to suppress grain growth of austenite due to coarsening of nitride (TiN) precipitated particles and precipitation of carbide (TiC) that is coarse precipitated compared to nitride, and excessive tissue due to TiC precipitated at grain boundaries. Since embrittlement of is generated, the upper limit of the content is preferably limited to 0.015%.

In addition, it is preferable to add at least 0.005% because TiN is difficult to precipitate when a small amount is added.

Nitrogen (N) generates inclusions in the steel, impairing the internal quality of the steel and workability during welding, so it is extremely low in management, but it is managed within the range of 50ppm or less because of excessive management cost and difficulty in management. It is preferable.

Molten steel of the composition as described above is preferably subjected to an outside furnace refining process before continuous casting in a continuous casting machine.

In the present invention, the molten steel of the composition is blown in a Tallinn furnace for Tallinn, and then subjected to a spheroidizing treatment. In other words, when the quicklime (CaO) in molten steel is added to make the basicity (CaO / SiO ₂ ) an appropriate value, and then inert gas (argon: Ar) is blown therein and stirred, sulfur in the molten steel is mostly separated and injured. However, some remaining S forms MnS inclusions and is elongated in the rolling direction during hot rolling to deteriorate impact toughness at low temperatures. In order to prevent this, it is preferable to add Ca-Si, which is a Ca-based spheroidizing agent, in the range of 0.8 to 1.3 kg / tonton in order to spheroidize the MnS inclusions.

The spheroidized molten steel is refluxed in a vacuum facility for 20 to 25 minutes, which is necessary to increase the cleanliness of the molten steel.

Next, in the present invention, the molten steel of the composition is cast in a continuous casting machine to produce a cast, and the steel slab is manufactured by reducing the pressure to 2.43 mm to the produced cast.

In the present invention, in order to apply such light pressure at the end of the casting, it is preferable to set the combustion casting speed at 0.95 to 1.0 m / min. This is because if the casting speed is less than 0.95m / min, it is impossible to control the equipment specifications under light pressure, and if it exceeds 1.0m / min, the light pressure effect due to increased segregation of the center of slab may be reduced.

In the present invention, by applying the ASTC (Automatic Strand Taper Control) system to compress the center of the cast slab at the end of continuous casting to reduce internal pores and segregation, after applying a 3mm Taper, a predetermined soft-reduction ) To prepare a steel slab of proper thickness.

In the present invention, the ASTC gives a low pressure of 2.43 mm, which is a condition in which the amount of taper is a fixed condition in the performance equipment under the condition of low pressure, and the amount of low pressure is applied to 7 mm or more, including the amount of taper, to control the equipment specifications. This is because it is preferable to set a pressure reduction amount of 2.43 mm since it is impossible.

The steel slabs prepared above are air-cooled to room temperature after being placed in multiple stages and then reheated again.

In general, in the case of heat treatment material, after reheating to recrystallization temperature (900 ℃) zone after rolling, carbonitride is precipitated in steel and the strength is secured by refining the grains of ferrite after cooling, so the reheating temperature before rolling is usually about 1250 ℃. Although heated at a high temperature of the present invention, since the additional heat treatment is not performed after rolling, the reheating temperature is limited to the range of about 1050 ~ 1150 ℃. If the reheating temperature is less than 1050 ° C, the steel slab is insufficient to recrystallize, if it exceeds 1150 ° C it is not preferable because the productivity decreases due to excessive rolling waiting time for temperature control during rolling.

After the reheated slab is extracted and roughly rolled, followed by filament rolling, the present invention is characterized in controlling such filamentous rolling conditions.

Specifically, in the present invention, the finishing finishing is composed of two stages. First, the primary finishing rolling is performed at 780 to 850 ° C. so that the residual reduction ratio is 45% in order to carry out the secondary finishing rolling in the unrecrystallized zone. Terminate at. If the first finishing rolling end temperature is out of the range of 780 ~ 850 ℃ it is impossible to secure the target temperature of the end of rolling after the second rolling.

Thereafter, the secondary finishing rolling is finished at 720 to 780 ° C, which is 40 to 70 ° C lower than the recrystallization stop temperature. This is because when the temperature for finishing the secondary finishing rolling is less than 720 ° C., an increase in strength and a decrease in toughness occur due to excessive generation of hardened tissue, and when the temperature exceeds 780 ° C., the yield strength is excessively lowered.

After the end of the controlled rolling, the grain growth after rolling is prevented and the residual austenite is transformed into bainite by accelerated cooling (water cooling) to cool to a temperature range of 500 to 580 ° C to obtain high strength.

In the present invention, the accelerated cooling end temperature is defined as 500 ~ 580 ℃, if the temperature is less than 500 ℃ is limited in the cooling capacity of the equipment, if it exceeds 580 ℃ it is difficult to secure the required tensile strength (Tensile Strength) Because.

At this time, it is preferable to cool the rolled steel sheet at a rate of 4 ~ 7 ℃ / sec to the end temperature. If the cooling rate is faster than 7 ℃ / sec, the bainite fraction is increased to contribute to the strength increase effect, but the low temperature impact toughness is deteriorated, if it is slower than 4 ℃ / sec grain growth is promoted to decrease the strength is undesirable Because.

As described above, it is possible to effectively manufacture YP 350Mpa thick TMCP steel having excellent weldability and internal quality by removing the emphasis component and controlling the continuous casting conditions and hot rolling conditions.

Hereinafter, the present invention will be described in detail through examples.

(Example)

Molten steels prepared as shown in Table 1 were prepared and blown in a Tallinn furnace, and then Ca-Si was added to perform spheroidization treatment of MnS inclusions. The molten steel thus refined was continuously cast at a rate of 0.98 m / min, and then steel slabs having a thickness of 244 mm were manufactured by varying the requirements under light pressure (2.43 mm) as shown in Table 2. On the other hand, in Table 1, steel grades 1 to 7 are steel grades belonging to the scope of the present invention, and steel grades 8 to 9 are steel grades outside the scope of the present invention.

Next, after reheating the prepared steel slab at 1130 ℃, as shown in Table 2 to prepare a thick steel sheet having a thickness of 60mm by varying the rolling and cooling conditions. The mechanical strength and impact toughness of the steel sheets manufactured as described above are shown in Table 3, and all the manufactured steel sheets were subjected to nondestructive testing (UST) to measure internal quality defect levels. Also shown in Table 3. Meanwhile, in Table 2, T4 is the secondary finishing rolling start temperature, T5 is the finishing rolling temperature, SCT is the cooling start temperature, FCT is the cooling end temperature, and CR is the cooling rate.

Steel grade C Si Mn P S Sol-Al Cu Ni Nb Ti N One 0.084 0.25 1.33 0.017 0.003 0.035 0.16 0.14 0.02 0.009 37 ppm 2 0.096 0.25 1.34 0.015 0.002 0.033 0.14 0.13 0.02 0.011 33 ppm 3 0.086 0.24 1.39 0.015 0.002 0.033 0.15 0.14 0.02 0.012 40 ppm 4 0.086 0.24 1.39 0.015 0.002 0.033 0.15 0.14 0.02 0.012 40 ppm 5 0.086 0.24 1.39 0.015 0.002 0.033 0.15 0.14 0.02 0.012 40 ppm 6 0.086 0.24 1.39 0.015 0.002 0.033 0.15 0.14 0.02 0.012 40 ppm 7 0.086 0.24 1.39 0.015 0.002 0.033 0.15 0.14 0.02 0.012 40 ppm 8 0.101 0.45 1.44 0.019 0.002 0.043 0.13 0.14 0.018 0.012 51 ppm 9 0.116 0.26 1.30 0.018 0.005 0.035 0.01 0.03 0.002 0.011 -

division Thickness (mm) Control Rolling (℃) Accelerated Cooling (℃) Under light pressure T4 (Residual Pressure Reduction Rate) T5 SCT FCT CR Inventive Steel (1) 60 820 (45%) 750 740 550 5 ℃ / s Conduct (2.43mm) Comparative Steel (2) 60 820 (45%) 750 740 550 5 ℃ / s Not carried Comparative Steel (3) 60 950 (50%) 754 709 614 2 ℃ / s Conduct (2.43mm) Comparative Steel (4) 60 807 (45%) 758 715 601 2.4 ℃ / s Conduct (2.43mm) Comparative Steel (5) 60 723 (40%) 700 688 490 5.1 ℃ / s Conduct (2.43mm) Comparative Steel (6) 60 973 (50%) 800 749 533 4.1 ℃ / s Conduct (2.43mm) Comparative Steel (7) 60 779 (45%) 747 706 561 4 ℃ / s Conduct (2.43mm) Comparative Steel (8 ~ 9) 60 830 (45%) 750 720 570 5 ℃ / s Conduct (2.43mm)

division Mechanical strength (Mpa) Impact Toughness (Ve-40 ℃, J) UST Defective Rate (%) YP TS EL (%) Inventive Steel (1) 395 519 30 324 0 Comparative Steel (2) 414 517 24 247 27.9 Comparative Steel (3) 284 484 28 308 - Comparative Steel (4) 374 481 33 258 - Comparative Steel (5) 416 543 18 112 - Comparative Steel (6) 441 595 28 38 - Comparative Steel (7) 496 628 22 159 - Comparative Steel (8) 403 556 30 39 - Comparative Steel (9) 388 510 24 46 -

As shown in Table 2 and Table 3, the invention steel (1) having optimal control of not only the steel composition but also the light rolling condition and the hot rolling condition is excellent in mechanical properties such as yield strength, tensile strength and elongation, and its impact toughness. Not only is this excellent but also the internal quality of the material is excellent.

On the contrary, the compositional components and rolling conditions are within the scope of the present invention, but the comparative steel (1) which is not subjected to light rolling has poor impact toughness and high UST defect rate, which indicates that the internal quality of the material is poor. .

In addition, the composition is within the scope of the present invention, but the comparative steels (3 to 7) in which the rolling conditions or cold rolling conditions are outside the scope of the present invention, the impact toughness is generally deteriorated, the mechanical properties are also deteriorated, and the composition itself is Comparative Examples 8 to 9 outside the scope of the invention can be seen that the impact toughness is not good.

As described above, the present invention can provide a thick TMCP steel with excellent weldability and internal quality by controlling steel components and optimizing hot rolling conditions and cold rolling conditions. have.

Claims (3)

By weight%, C: 0.06 to 0.11%, Si: 0.20 to 0.50%, Mn: 1.30 to 1.50%, P: 0.020% or less, S: 0.005% or less, Al: 0.015 to 0.050%, Nb: 0.005 to 0.025% Preparing a molten steel composed of Cu: 0.1 to 0.2%, Ni: 0.1 to 0.2%, Ti: 0.005 to 0.015%, N: 50 ppm or less, balance Fe and other unavoidable impurities; Manufacturing molten steel slab by continuously casting molten steel of the composition in a continuous casting machine and then reducing the cast slab to 2.43 mm; Cooling the steel slab to room temperature and then reheating it to a temperature of 1050 to 1150 ° C .; Completing the primary finishing rolling at the reheated steel slab at 780 to 850 ° C. such that the residual pressure reduction rate is 45%, and then completing the secondary finishing rolling at 720 to 780 ° C .; And Yield strength 350Mpa grade TMCP steel production method comprising the step of cooling the rolled steel sheet to a temperature range of 500 ~ 580 ℃ and air-cooled. The method of claim 1, wherein the continuous casting is performed at a casting speed of 0.95 to 1.0 m / min. The method of claim 1, wherein the hot rolled steel sheet is cooled to a temperature range of 500 to 580 ° C at a rate of 4 to 7 ° C / sec.