KR20040003317A - API-X80 hot steel plate manufacturing method for ductile fracture ratio excellent - Google Patents

Abstract

PURPOSE: A method for manufacturing high strength API-X80 hot rolled steel plate having superior ductile fracture ratio which is used in intense cold area such as Siberia and Alaska and secures increase of strength and toughness at low temperature at the same time is provided. CONSTITUTION: In a manufacturing method of strip for pipe comprising processes of manufacturing a slab by refining molten steel comprising 0.06 to 0.09 wt.% of C, 1.30 to 1.50 wt.% of Mn, 0.15 to 0.30 wt.% of Si, 0.016 wt.% or less of P, 0.003 wt.% or less of S, 0.040 to 0.060 wt.% of Nb, 0.02 to 0.04 wt.% of V, 0.2 to 0.40 wt.% of Mo and a balance of Fe and inevitable impurities; heat treating the manufactured slab in a heating furnace; rough rolling the heat treated slab in a rough rolling mill; finish rolling the rough rolled strip in a finish rolling mill; and cooling the finish rolled strip, the method for manufacturing high strength API-X80 hot rolled strip having superior ductile fracture ratio is characterized in that the refining process includes gas injection and vacuum degassing treatment processes, a heating time of the slab in the heating furnace is 210 to 300 min, a slab delivery temperature in the heating furnace is 1,120 to 1,180 deg.C, roughing delivery temperature is 880 to 920 deg.C, and finish delivery temperature is 780 to 820 deg.C.

Description

API-X80 hot steel plate manufacturing method for ductile fracture ratio excellent

The present invention relates to a method for producing a high strength API-X80 hot rolled steel sheet having an excellent ductile fracture ratio, and more particularly, the general properties of the steel in order to be suitable for use in the cold (Siberia, Alaska, etc.) The present invention relates to a method for manufacturing a high strength API-X80 line pipe sheet having excellent ductility, which is a hot rolled steel sheet having a yield strength of 552 mpa or more by preventing brittleness due to an increase in phosphorus strength.

API (American Petroleum Institue), a line pipe material used for oil and natural gas pipes, is a standard established by the American Petroleum Association. There are a total of 10 standards for line pipes from API-B to API-X80. The hot rolled steel sheet for API is shipped in the form of a coil in the hot rolling process, and then manufactured in the form of a pipe by welding the electric resistance welding or latent welding type as shown in FIG. After cutting to the extent that it is passed through the tensile test and the impact test according to the internationally required standard and the use required by the customer, it is finally delivered to the customer and used in the ground and underground of the extreme ground.

Pipes for transporting oil and natural gas as described above are increasingly required to increase the strength of materials in order to pursue the economics of transportation, and in particular, oil field development in extreme regions (Siberia, Alaska, etc.) due to exhaustion of petroleum resources. In recent years, API-X60 has been the dominant model for smooth transportation, but recently API-X70 is the dominant model.

On the other hand, Plate is a relatively thick hot rolled steel sheet (6 ~ 150mm) manufactured mainly by four-stage reversible rolling mill and accelerated cooling, and it is API- by Thermomechanical Control Process (TMCP). It is reported to have been developed up to X80. However, in the case of hot rolled steel, API-X80 is in the development stage, and commercial production will be possible soon.

However, steel generally has a low temperature brittle region where toughness decreases as strength increases, and particularly impact toughness decreases rapidly as temperature decreases. Thus, hot-rolled steel sheets for API-X80 used for extreme paper are shown in Table 1 below. Since high strength and low temperature impact toughness are required, a method of manufacturing a hot rolled steel sheet satisfying both mechanical properties and impact toughness required for API-X80 is required.

Yield strength Tensile strength Impact absorption energy (CVN, +10 ℃) Ductility Rate (DWTT, + 10 ℃) ≥552mpa ≥621mpa Minimum 41 j Average 54 j Minimum 50% Average 60%

The conventional method for manufacturing hot rolled steel sheet for line pipes (Patent Application No. 1998-58744) is, by weight, C: 0.09% or less, Si: 0.25% or less, Mn: 1.65% or less, P: 0.020% or less, S: 0.010% or less, Mo: 0.10-0.50%, Nb: 0.010 to 0.055%, V: 0.010 to 0.055%, C equivalent: 0.43% or less [C equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15], balance Fe and other unavoidable impurities, hot-rolled steel under conditions such that the exit temperature of finishing rolling is 830 ° C or higher, and then cooled to 550-650 ° C for 15-45 ° C / sec. After cooling at a speed, it is a method of winding up, but this method is not suitable for heating in refining and heating furnaces and rolling temperature in rolling mills (crude rolling, finishing rolling), etc. There was a problem that can not produce API-X80 hot rolled steel sheet.

Another conventional method for producing a hot rolled steel sheet for pipe, in weight%, C: 0.07-0.09%, Si: 0.20-0.50%, Mn: 1.40-1.60%, P: 0.025% or less, S: 0.005% or less, Sol -Al: 0.015 ~ 0.050%, Cr: 0.05 ~ 0.15%, Mo: 0.03 ~ 0.07%, Nb: 0.035 ~ 0.045%, V: 0.040 ~ 0.050%, Ti: 0.005 ~ 0.015%, N: 20 ~ 70ppm, rest After heating the slab of Fe and other unavoidable impurities at a temperature of 1100 to 1200 ° C, hot rolling is performed under conditions of finishing rolling start temperature 850 ± 20 ° C, rolling reduction rate: 62 ± 2%, finishing rolling temperature: 700 ± 20 ° C. After rolling, it was cooled down to a cooling rate of 7 ± 3 ℃ / sec up to a temperature range of 540 ± 20 ℃, which is a stress relief heat treatment guarantee API-X65 grade steel pipe for line pipe, which does not meet the strength required by the present invention. There is a problem.

The present invention has been made in order to solve the above problems, the object is to contain C, Mn, Si, P, S and precipitation hardening elements Nb, V in Fe, and to add Mo, a transformation hardening element In the process of refining molten steel, gas injection + vacuum degassing, and the entire surface of the slab are performed during the refining process. It is to provide a high strength API-X80 hot rolled steel sheet manufacturing method excellent in the ductility wave rate that can secure the rise and low temperature toughness at the same time.

1 is a schematic diagram of a rolling process for manufacturing a hot rolled steel sheet according to the present invention;

Figure 2a and 2b is a perspective view showing a welding method for making a pipe using a steel sheet produced by the present invention,

Figure 3 is a schematic diagram showing a gas injection + vacuum degassing and playing process in the refining process according to the present smoke,

Figure 4 is a schematic diagram for explaining the process of cooling the finishing rolling finished steel sheet in a run out table according to the present invention,

Figure 5 is a comparison picture showing the ductile fracture rate through the drop crack test of the steel sheet prepared by the present invention,

Figure 6 is a comparison picture showing the structure of the steel sheet produced by the present invention.

In order to achieve the above object, a high strength API-X80 hot rolled steel sheet manufacturing method having excellent ductility of the present invention, C: 0.06 to 0.09 wt%, Mn: 1.30 to 1.50 wt%, Si: 0.15 to 0.30 wt%, P: 0.016wt% or less, S: 0.003wt% or less, Nb: 0.040 ~ 0.060wt%, V: 0.02 ~ 0.04wt%, Mo: 0.2 ~ 0.40wt%, the rest is slab by refining molten steel composed of Fe and unavoidable impurities In the manufacturing method of the steel sheet which is made, and the slab made by the heat treatment in a heating furnace, subjected to the process of rough rolling, finishing rolling, cooling sequentially

The refining treatment is a gas injection + vacuum degassing treatment, the slab heating time in the furnace is 210 ~ 300 minutes, the slab extraction temperature in the furnace is 1120 ~ 1180 ℃, the crude rolling is finishing temperature of 880 ~ It is 920 degreeC, and the finishing rolling is the finishing temperature of 780-820 degreeC.

When supplying the slab to the heating furnace, the entire surface of the slab is subjected to the entire scarfing process, and the finishing of the finishing rolled steel sheet is rapid cooling using cooling water, and the rapid cooling is shear cooling at the runout table, and the finishing The rolled steel sheet is wound, and the winding temperature is 560 to 620 ° C.

Hereinafter, the specific reasons for the limitation of the component range and the rolling conditions according to the present invention will be described in detail.

If the content of carbon (C) is less than 0.06wt%, the strength of the second phase is lowered by lowering the fraction of tissue (Pearlite Friction), and if it is more than 0.09wt%, pearlite is increased inside the steel, thereby increasing the strength but causing cracking. It is desirable to limit it within 0.06 to 0.09% because it increases the impact (Crack Source) to damage the impact toughness and weldability.

Silicon (Si) is effective as a deoxidizer in the steelmaking process, and the reinforcing mechanism is solid solution strengthening, and it is known that solid solution strengthening is caused by lattice distortion between the element and iron (Fe). Therefore, when 0.15wt% or less is added, there is a problem that the tensile strength due to solid solution strengthens sharply, and when 0.30wt% or more is added, carbon (C) is accelerated to promote pearlite formation, thereby rapidly decreasing impact transition characteristics. There is a problem. Therefore, it is desirable to regulate 0.15 ~ 0.30wt% in terms of impact toughness and tensile strength.

Manganese (Mn) is a solid solution strengthening element that delays the phase transformation from austenite to ferrite and promotes acicular ferrite, thereby improving strength and toughness. It becomes fine. Therefore, if it is less than 1.30wt%, it is difficult to secure high strength, and if it is added to more than 1.50wt%, not only the weldability is impaired, but also segregation is performed in the center of thickness such as P, S, etc., thereby promoting the banded structure to damage the impact toughness. Therefore, it is desirable to regulate 1.30 ~ 1.50wt%.

Phosphorus (P) together with Mn, S (MnS, FeS) is a representative center segregation element, which is segregated into Fe3P type in the center and grain boundary when playing, which impairs impact toughness greatly. Since it is an element that greatly impairs the impact toughness by increasing the transition temperature, it is preferable to limit it to the maximum as permitted by the steelmaking operation technology. However, in the present invention, the manufacturing cost and processing time are increased when the ultra low-lining steel is manufactured.

Sulfur (S) is segregated into FeS, MnS, etc. type, stretches in the rolling direction during hot rolling to promote anisotropy of steel sheet properties, and greatly increases impact toughness by causing surface cracks, internal cracks, and center segregation during performance. Deterioration of the furnace is preferable to completely remove during the refining and secondary refining, but it is desirable to minimize to 0.003wt% or less because it is impossible in the steelmaking process.

Niobium (Nb) and vananium (V) are initially precipitated as Nb.V (C, N) type in austenite and ferrite phases (600 ~ 1000 ℃), which are relatively lower than Ti, which is a precipitation strengthening element. It is known that Nb is greater than V as the element that suppresses austenite grain growth and recrystallization, and also increases the cumulative reduction rate by expanding the microcrystalline region to increase tensile strength and impact toughness through microstructure. have. In the present invention, mainly precipitated at a high temperature, when the precipitate is coarse, Ti is not added at the same time to decrease the strength and impact toughness, Nb content is greatly reduced, while the increase in strength when excessively added, the ductility and toughness is greatly reduced, When under-added, the strength increase effect is less, so it is preferable to limit it to 0.040 ~ 0.060wt%.

In addition, V is known to have the least strength increase effect due to precipitation, and for that reason, it is usually managed lower than the Nb content. When the V content exceeds 0.04 wt%, the weld ductility and toughness decreases and the strength increases when added below 0.02%. Since the effect is lost, it is preferable to limit the content to 0.02 to 0.04 wt%.

Molybdenum (Mo) increases the thermal dynamic driving force of the ferrite transformation and decreases the nucleation rate by lowering the Ar3 temperature, which is the temperature at which the ferrite is deposited in the austenite region, as shown in the following Ar3 temperature regression equation. ) Is the most effective element to increase the tensile strength and low temperature toughness by inhibiting the growth of pre-generated nuclei and miniaturizing tissue (Grain Size) and generating acicular ferrite due to cold winding. . Mo content is greatly increased by hardenability (Hardenability) of the steel when excessively added to decrease the toughness, and when under-added it is preferable to manage at 0.2 ~ 0.4% because needle-like ferrite formation, which is a low-temperature structure is unstable. The Ar3 temperature regression equation of molybdenum-added steel is as follows.

Ar ₃ (° C) = 910 ° C-310 [C]-80 [Mn]-20 [Cu]-20 [Cr]-20 [Ni]-80 [Mo]

In order to secure the impact toughness of the steel for high tension line pipes having the above chemical composition, the present invention provides the first slag in the ladle as shown in [Fig. 3] to secure the internal quality and the surface quality in the steelmaking process. After the formation of the top slag, the degassing operation and the large inclusions are controlled by gas injection, which carries out steel agitation for the reaction between the molten steel and the top slag, and is carried out by the RH (vacuum degassing) treatment. It is necessary to control the discharge of gas (H, O) in the inclusions and steels, and the slab front side scarfing in order to remove fine defects such as fine cracks and pin holes generated in the slab surface layer during the performance process. Carry out Scarfing.

In the hot rolling process, when the temperature of reheating in the furnace is set to a high temperature, impact toughness due to coarsening of austenite decreases, and when heated to a low temperature, the strength decreases due to incomplete re-use of precipitates such as NbC. Therefore, it is desirable to maintain the extraction temperature within 1120 ~ 1180 ℃, and if the ash time is too short, it causes the material, impact toughness deviation, mailing failure due to the lack of aging of the slab, if too long, the austenite It is preferable to limit the amount to within 210 to 300 minutes because it causes an increase in the amount of dialogue, scale, and the like, and a decrease in productivity.

Roughing Delivery Temperature (RDT) in rough rolling suppresses the dynamic recrystallization that occurs during R2 pass (5 passes with a 4Hi-reversible rolling mill), and increases the tensile strength and Since low-temperature toughness improves, it is preferable to set it as low temperature as possible. However, if it is too low, it is difficult to secure Finishing Delivery Temperature (FDT), so it is desirable to manage the temperature within 880 to 920 ° C. In general, the equation for dynamic recrystallization in rough rolling is represented by the Zener-Hollomon parameter below. It can be seen that the dynamic recrystallization decreases as the crude rolling temperature is lowered.

Z = εexp (Qd / RT)

ε = strain rate

Qd = Strain Activation Energy

R = gas constant

T = temperature

Finishing temperature of finishing rolling (FDT) prevents increase of austenite coarsening during high temperature operation and prevents increase of hardenability during accelerated cooling, and increases the rolling amount in the unrecrystallized region of austenite. It is preferable to roll at a temperature just above Ar ₃ (about 800 ° C.), which is the temperature at which the cornerstone ferrite is precipitated, for the microstructure according to the present invention.

The coiling temperature (CT) is an important control factor that determines the final structure of the hot rolled steel sheet and has a great influence on the tensile and impact toughness. If it is too high, the transformation hardening effect is decreased, and if it is too low, the precipitation hardening effect is reduced, and the telescope, Since winding shapes, such as loose winding, will become bad, it is preferable to limit to within 560-620 degreeC.

RUN OUT TABLE (ROT) is a process of forcibly water-cooling by using cooling water from each cooling nozzle of a laminar flow as shown in FIG. 4, and the cooling pattern is shear cooling (cooling nozzle # of cooling equipment #). 1 ~ # 5), post-cooling (cooling nozzles # 12 ~ # 17 of cooling equipment), step cooling, etc., to precisely control the phase ratio and the size and shape of the microstructure to obtain the desired material. However, it is preferable to perform shear cooling to refine the grains by increasing the degree of subcooling and suppressing grain grain growth.

Hereinafter, an embodiment of the present invention will be described.

(Example)

Example of the present invention by weight of C: 0.073%, Mn: 1.46%, Si: 0.20%, P: 0.014%, S: 0.002%, Nb: 0.050%, V: 0.031%, Mo: 0.29% In order to have a composition that satisfies the component design criteria of the present invention, after the molten iron dephosphorization and molten iron desulfurization in the pretreatment process and the converter is blown at a hot metal ratio of 97% low P, Manganese metal was used to reduce the S steel and inclusions, and heavy dehydration and slag cut were performed.

In the secondary refining, gas injection was used for 32 minutes to separate the floating of large inclusions in the bubbling field, and in the vacuum degassing plant, vacuum was used to fully discharge the small inclusion floating and gas gas (HO). Degassing was treated for 25 minutes.

The molten steel was controlled at a temperature of 1551 ° C. in the casting process and cast at a rate of 0.90 M / min, while complete cooling was performed using air mist in a secondary cooling zone. Slab was also subjected to cold charge rolling (CCR), which was then scarfed to remove surface microcracks and surface inclusions.

Subsequently, it was charged into a furnace to extract ash at a time of 260 minutes and an extraction temperature of 1150 ° C., followed by rolling to maintain a finishing temperature (RDT) of 900 ° C. in the rough rolling process, and finishing temperature in the finishing rolling process. (FDT) was rolled to maintain a low temperature of 795 ℃, and after finishing finishing the cooling pattern, water spray nozzles # 2, # 3, # 4, # 5, # 6 of cooling equipment to prevent coarsening of crystal grains. Rapid cooling was conducted at the front end using, # 7, and the winding temperature was maintained at 580 ° C. for the formation of acicular ferrite.

The mechanical properties and impact toughness of the invention and the comparative material prepared as described above were evaluated, respectively, and the impact toughness was carried out by the impact test (Charpy Impact Test) and the drop weight tear test (DWTT).

The evaluation results are shown in Tables 2 to 4 below. In the case of the invention, it is economical design with no addition of Ti and Ni added in the API-X70 component, and the tensile strength and impact toughness satisfies the specifications and are superior to those of the comparative material. It can be seen.

Table 5 below shows the transition curve of the drop crack test (DWTT) test, but the comparative material shows a sudden brittle transition (DBTT) curve near -35 ℃, but in the case of the invention material exhibits a good ductile fracture rate in the vicinity of -50 ℃ have.

5 and 6 show the structure photograph (X100) by the optical microscope of the invention material and the comparative material. The invention material and the comparative material have acicular ferrite as the base structure, and the ferrite grain size (Ferite Grain Size, The larger the value, the finer)), the invention material is 12.5, which is finer than the comparative material 12.0, and in the case of the invention material, there is no banded structure at all. Due to such excellent microstructure, the tensile strength and low temperature impact toughness of the inventive material can be seen to be superior to those of the comparative material.

The present invention as described above is a gas injection + vacuum degassing during the refining process of a conventional molten steel containing C, Mn, Si, P, S and Fe, Nb, V, precipitated reinforcement elements, and Mo, a transformation reinforcement element. The slab heating time is 210 ~ 300 minutes in the furnace, the slab extraction temperature in the furnace is 1120 ~ 1180 ℃, and the rough rolling finish temperature is 880. It is maintained at ~ 920 ℃, finishing rolling temperature is maintained at 780 ~ 820 ℃, shear rapid cooling on the run-out table after the end of rolling, and the strength increase and low temperature by maintaining the winding temperature at 560 ~ 620 ℃ There is an effect that can produce a high strength API-X80 hot rolled steel sheet excellent in the ductile wavefront ratio (DWTT) that can secure toughness at the same time.

Claims (4)

C 0.06 ~ 0.09wt%, Mn 1.30 ~ 1.50wt%, Si 0.15 ~ 0.30wt%, P 0.016wt% or less, S 0.003wt% or less, Nb 0.040 ~ 0.060wt%, V 0.02 ~ 0.04wt%, Mo 0.2 ~ 0.40wt%, the remainder is made of slab by refining molten steel composed of Fe and unavoidable impurities, and the slabs are heat-treated in a heating furnace, followed by rough rolling, finishing rolling, and cooling for pipes. In the steel sheet manufacturing method, The refining treatment is a gas injection + vacuum degassing treatment, the slab heating time in the heating furnace is 210 ~ 300 minutes, the slab extraction temperature in the heating furnace is 1120 ~ 1180 ℃, the crude rolling is a finishing temperature of 880 ~ 920 ℃, the finishing rolling is a high strength API-X80 hot rolled steel sheet manufacturing method having excellent ductility, characterized in that the finishing temperature is set to 780 ~ 820 ℃. The method according to claim 1, wherein when the slab is supplied to the heating furnace, the slab surface is subjected to the entire scarfing process. The high strength API-X80 hot rolled steel sheet having excellent ductility rate according to claim 1, wherein the finishing of the finishing-rolled steel sheet is rapid cooling using cooling water, and the rapid cooling is shear cooling at a runout table. Way. The method of claim 1, wherein the finishing-rolled steel sheet is wound, and the winding temperature is 560 ~ 620 ℃ excellent high ductility API-X80 hot rolled steel sheet manufacturing method.