KR20240102651A - Thick steel plate and method of manufacturing the same - Google Patents

Abstract

The present invention, in weight percent, carbon (C): 0.04% ~ 0.065%, silicon (Si): 0.20% ~ 0.30%, manganese (Mn): 1.60% ~ 1.80%, soluble aluminum (S_Al): 0.02% ~ 0.05%, copper (Cu): 0.05% ~ 0.15%, chromium (Cr): 0.2% ~ 0.3%, molybdenum (Mo): 0.15% ~ 0.25%, nickel (Ni): 0.15% ~ 0.25%, niobium (Nb) ): 0.045% ~ 0.055%, vanadium (V): more than 0% but less than 0.01%, titanium (Ti): 0.008% ~ 0.018%, boron (B): more than 0% ~ 0.0005% or less, phosphorus (P): 0 % to less than 0.012%, sulfur (S): more than 0% to 0.003% or less, and the balance includes iron (Fe) and inevitable impurities, tensile strength (TS): 625 MPa to 825 MPa, yield strength (YS) ): 555 MPa to 705 MPa, Elongation (EL): 40% to 50%, Yield Ratio (YR): 0.70 to 0.90 and Average Drop Weight Test Value (DWTT) at temperatures from 0°C to -20°C: 90% to We provide thick steel plates that meet 100% requirements.

Description

Thick steel plate and method of manufacturing the same }

The technical idea of the present invention relates to steel materials and their manufacturing methods, and more specifically, to thick steel plates with excellent low-temperature fracture toughness and their manufacturing methods.

Recently, as the problem of resource depletion has emerged, oil drilling and transportation work in the deep sea or polar regions is increasing. Likewise, as oil pipeline lines move from general areas to special areas such as permafrost and earthquake zones, the demand for energy resource transport stability is increasing. Accordingly, there is a need to change the design of pipe materials to secure deformation resistance, for example, from stress-based in the circumferential direction to based on strain in the longitudinal direction of the pipe.

To meet these requirements, a low yield ratio compared to the standard, high and uniform elongation, tensile properties in the longitudinal direction, and low-temperature fracture stability are required, but it is difficult to satisfy the above conditions with steel manufactured by existing TMCP rolling. Therefore, in order to develop thick steel plates applicable to extreme environments, research on API-X80 steel is necessary.

Korean Patent Registration No. 10-2122643

The technical problem to be achieved by the technical idea of the present invention is to provide a thick steel plate with excellent low-temperature fracture toughness and a manufacturing method thereof.

However, these tasks are illustrative, and the technical idea of the present invention is not limited thereto.

According to one aspect of the present invention, a thick steel plate with excellent low-temperature fracture toughness and a manufacturing method thereof are provided.

According to an embodiment of the present invention, the thick steel plate has, in weight percent, carbon (C): 0.04% to 0.065%, silicon (Si): 0.20% to 0.30%, manganese (Mn): 1.60% to 1.80%. , Soluble Aluminum (S_Al): 0.02% ~ 0.05%, Copper (Cu): 0.05% ~ 0.15%, Chromium (Cr): 0.2% ~ 0.3%, Molybdenum (Mo): 0.15% ~ 0.25%, Nickel (Ni) : 0.15% ~ 0.25%, Niobium (Nb): 0.045% ~ 0.055%, Vanadium (V): More than 0% but less than 0.01%, Titanium (Ti): 0.008% ~ 0.018%, Boron (B): More than 0% ~ 0.0005% or less, phosphorus (P): more than 0% to less than 0.012%, sulfur (S): more than 0% to 0.003% or less, and the balance includes iron (Fe) and inevitable impurities, tensile strength (TS): 625 MPa to 825 MPa, Yield Strength (YS): 555 MPa to 705 MPa, Elongation (EL): 40% to 50%, Yield Ratio (YR): 0.70 to 0.90 and average drop at temperatures from 0°C to -20°C. Weight test value (DWTT): Satisfies 90% to 100%.

In the thick steel plate, the final microstructure may include ferrite and bainite.

In the thick steel plate, the area fraction of the bainite is 5 to 10%, and the area fraction of the ferrite may have the remaining area fraction.

In the thick steel plate, the area fraction of polygonal ferrite in the final microstructure is 15 to 25%, the area fraction of acicular ferrite is 65 to 75%, and the area of bainitic ferrite is The fraction may be 5-10%.

In the thick steel plate, the carbon equivalent (C _eq ) may be 0.45 or less, and the weld crack susceptibility index (P _cm ) may be 0.18 or less.

According to an embodiment of the present invention, the method for manufacturing the thick steel plate is, in weight percent, carbon (C): 0.04% ~ 0.065%, silicon (Si): 0.20% ~ 0.30%, manganese (Mn): 1.60% ~ 1.80%, soluble aluminum (S_Al): 0.02% ~ 0.05%, copper (Cu): 0.05% ~ 0.15%, chromium (Cr): 0.2% ~ 0.3%, molybdenum (Mo): 0.15% ~ 0.25%, nickel ( Ni): 0.15% ~ 0.25%, Niobium (Nb): 0.045% ~ 0.055%, Vanadium (V): More than 0% but less than 0.01%, Titanium (Ti): 0.008% ~ 0.018%, Boron (B): 0% Exceeding ~ 0.0005% or less, Phosphorus (P): Exceeding 0% ~ less than 0.012%, Sulfur (S): Exceeding 0% ~ 0.003% or less, and the balance is steel containing iron (Fe) and inevitable impurities at 1,000℃ ~ Reheating at a temperature of 1,200°C; Hot rolling the heated steel material through rough rolling and finishing rolling; A step of accelerated cooling of the hot-rolled steel; wherein, in the hot rolling step, the finishing temperature (FRT) of the finishing rolling is 750 ℃ to 850 ℃, and in the accelerated cooling step, the accelerated cooling termination temperature (FCT) ) is 450℃ to 550℃.

In the method of manufacturing a thick steel plate, the accelerated cooling step may be performed at a cooling rate of 20°C/sec to 40°C/sec.

In the method of manufacturing the thick steel plate, after performing the cooling step, the thick steel plate has tensile strength (TS): 625 MPa to 825 MPa, yield strength (YS): 555 MPa to 705 MPa, and elongation (EL). : 40% ~ 50%, yield ratio (YR): 0.70 ~ 0.90, and average drop weight test value (DWTT): 90% ~ 100% can be satisfied at a temperature of 0℃ ~ -20℃.

In the method of manufacturing a thick steel plate, after performing the cooling step, the final microstructure of the thick steel plate includes ferrite and bainite, the area fraction of bainite is 5 to 10%, and polygonal ferrite ), the area fraction may be 15 to 25%, the area fraction of acicular ferrite may be 65 to 75%, and the area fraction of bainitic ferrite may be 5 to 10%.

According to the technical idea of the present invention, a thick steel plate with excellent low-temperature fracture toughness and a manufacturing method thereof can be implemented. Specifically, according to the technical idea of the present invention, in order to secure low-temperature fracture toughness for The component design conditions were derived. The rolling conditions refined the overall structure by lowering the heating and rolling temperatures compared to existing materials, and produced products with fine bainite and ferrite bases through optimal accelerated cooling conditions (cooling end temperature, cooling rate). As a result of the material test, a product was implemented that maintains the performance of the same X80 grade and has excellent low-temperature fracture toughness (DWTT). In addition, as a low pcm material, weldability is superior to that of existing materials, and a high-strength API steel plate of 65kg class and 1.25 inches thick was implemented that can guarantee fracture toughness in extreme cold places.

The effects of the present invention described above have been described as examples, and the scope of the present invention is not limited by these effects.

Figure 1 is a process flow chart schematically showing a method of manufacturing a thick steel plate according to an embodiment of the present invention.
Figure 2 is a graph showing the drop weight test value (DWTT) of the thick steel plate according to the comparative example among the experimental examples of the present invention.
Figure 3 is a graph showing the drop weight test value (DWTT) of a thick steel plate according to an example among the experimental examples of the present invention.
Figure 4 is a photograph of the microstructure of a thick steel plate according to a comparative example among the experimental examples of the present invention.
Figure 5 is a photograph of the microstructure of a thick steel plate according to an example among the experimental examples of the present invention.
Figure 6 is a diagram showing the fracture surface observed in a drop weight test in a thick steel plate according to an experimental example of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. The embodiments of the present invention are provided to more completely explain the technical idea of the present invention to those skilled in the art, and the following examples may be modified into various other forms, and the embodiments of the present invention may be modified. The scope of the technical idea is not limited to the following examples. Rather, these embodiments are provided to make the present disclosure more faithful and complete and to fully convey the technical idea of the present invention to those skilled in the art. In this specification, like symbols refer to like elements throughout. Furthermore, various elements and areas in the drawings are schematically drawn. Accordingly, the technical idea of the present invention is not limited by the relative sizes or spacing drawn in the attached drawings.

Hereinafter, a thick steel plate with excellent low-temperature fracture toughness, which is one aspect of the present invention, will be described.

Heavy steel plate with excellent low-temperature fracture toughness

The thick steel plate with excellent low-temperature fracture toughness, which is one aspect of the present invention, has, in weight percent, carbon (C): 0.04% to 0.065%, silicon (Si): 0.20% to 0.30%, manganese (Mn): 1.60% to 1.80. %, soluble aluminum (S_Al): 0.02% ~ 0.05%, copper (Cu): 0.05% ~ 0.15%, chromium (Cr): 0.2% ~ 0.3%, molybdenum (Mo): 0.15% ~ 0.25%, nickel (Ni) ): 0.15% to 0.25%, niobium (Nb): 0.045% to 0.055%, vanadium (V): more than 0% but less than 0.01%, titanium (Ti): 0.008% to 0.018%, boron (B): more than 0% ~ 0.0005% or less, phosphorus (P): more than 0% ~ less than 0.012%, sulfur (S): more than 0% ~ 0.003% or less, and the balance consists of iron (Fe).

Hereinafter, the role and content of each component included in the thick steel plate with excellent low-temperature fracture toughness according to the present invention will be described as follows. At this time, the content of the component elements all means weight%.

Carbon (C): 0.04% to 0.065%

Carbon is added to ensure the strength of steel. If the carbon content is less than 0.04%, it is difficult to secure strength. If the carbon content exceeds 0.065%, low-temperature impact toughness and weldability may be reduced. Therefore, it is preferable that carbon is added at 0.04% to 0.065% of the total weight of the steel.

Silicon (Si): 0.20% to 0.30%

Silicon contributes to increasing the strength of steel. Additionally, as a ferrite stabilizing element, it is effective in improving the toughness and ductility of steel by inducing ferrite formation. If the silicon content is less than 0.20%, the addition effect is insufficient. If the silicon content exceeds 0.30%, red scale is generated in the heating furnace during the hot rolling process, which may deteriorate the surface quality of the steel and deteriorate weldability. Therefore, it is preferable that silicon is added at 0.20% to 0.30% of the total weight of the steel.

Manganese (Mn): 1.60% ~ 1.80%

Manganese is an element that contributes to solid solution strengthening and improving the hardenability of steel. If the manganese content is less than 1.50%, the addition effect is insufficient. If the manganese content exceeds 1.70%, the synergistic effect of increasing the addition amount is minimal, and MnS inclusions and oxides may be formed, impairing the weldability of the steel when making line pipes. Therefore, it is preferable that manganese is added at 1.50% to 1.70% of the total weight of the steel.

Soluble Aluminum (S_Al): 0.02% to 0.05%

Soluble aluminum acts as a deoxidizer to remove oxygen in steel. If the aluminum content is less than 0.02%, the addition effect is insufficient. If the aluminum content exceeds 0.05%, non-metallic inclusions, Al ₂ O _{3 ,} may be formed, thereby reducing low-temperature impact toughness. Therefore, it is preferable that aluminum is added at 0.02% to 0.05% of the total weight of the steel.

Copper (Cu): 0.05% to 0.15%

Copper is an element effective in increasing strength and improving toughness. When the copper content is less than 0.05%, the effect of adding copper is insufficient. If the copper content exceeds 0.15%, it may cause surface defects. Therefore, copper is preferably added at 0.05% to 0.15% of the total weight of the steel.

Chromium (Cr): 0.2% to 0.3%

Chromium, like manganese, lowers the equilibrium temperature, so it is added to ensure strength. When added in large amounts, chromium can combine with carbon to form coarse carbides, which slightly increases strength, but is weak in toughness, so adding large amounts should be avoided. If the chromium (Cr) content is less than 0.20%, it is difficult to achieve the above-mentioned effect. If the chromium (Cr) content exceeds 0.30%, weldability and toughness of the heat-affected zone (HAZ) may decrease, and red heat embrittlement may occur.

Molybdenum (Mo): 0.15% to 0.25%

Molybdenum is a substitutional element and contributes to improving the strength of steel through solid solution strengthening. Additionally, it improves the hardenability and corrosion resistance of steel. If the molybdenum (Mo) content is less than 0.15%, it is difficult to achieve the above-mentioned effect. If the molybdenum (Mo) content exceeds 0.25%, weldability and toughness of the heat-affected zone (HAZ) may decrease, and red heat embrittlement may occur.

Nickel (Ni): 0.15% to 0.25%

Nickel refines the grains and is dissolved in austenite and ferrite to strengthen the matrix. In particular, nickel is an effective element in improving low-temperature impact toughness. If the nickel (Ni) content is less than 0.15%, it is difficult to achieve the above-mentioned effect. If the nickel (Ni) content exceeds 0.25%, weldability and toughness of the heat-affected zone (HAZ) may decrease, and red heat embrittlement may occur.

Niobium (Nb): 0.045% to 0.055%

Niobium plays a role in pinning grain boundaries by precipitating carbonitride (NbC) in steel, and can improve strength by interfering with grain boundary sliding (GBS) and dislocation movement that occur at high temperatures. . If the niobium content is less than 0.045%, the addition effect is insufficient. If the niobium content exceeds 0.055%, the synergistic effect of increasing the addition amount is minimal, and excessive precipitation may reduce playability, rollability, and elongation. Therefore, niobium is preferably added in an amount of 0.045% to 0.055% of the total weight of the steel.

Vanadium (V): More than 0% but less than 0.01%

Vanadium (V) contributes to strength improvement through solid solution strengthening and formation of complex precipitates with niobium (Nb) at low temperatures. The vanadium fully exerts its effect when added in an amount greater than 0% and less than or equal to 0.01% of the total weight of the hot rolled steel sheet according to the present invention. On the other hand, if vanadium is added in excess of 0.01% by weight, manufacturing costs increase, weldability decreases, and problems may occur during winding due to excessive precipitation at low temperatures.

Titanium (Ti): 0.008% to 0.018%

Titanium produces Ti(C,N) precipitates with excellent high-temperature stability, thereby hindering austenite grain growth during welding and refining the structure of the weld zone, thereby improving the toughness and strength of hot-rolled products. If the titanium content is less than 0.008%, the addition effect is insufficient. If the titanium content exceeds 0.018%, coarse precipitates may be formed and the toughness of the steel may be reduced. Therefore, titanium is preferably added at 0.008% to 0.018% of the total weight of the steel.

Boron (B): More than 0% ~ 0.0005% or less

Boron (B) is an element that increases hardenability when solidified, and improves the toughness of HAZ by lowering dissolved N when precipitated as BN. In order to maintain a good balance between strength and toughness, it is desirable to set the addition amount to more than 0% and less than or equal to 0.0005%.

Phosphorus (P): More than 0% ~ Less than 0.012%

Phosphorus is limited to more than 0% ~ 0.012% of the total weight of the steel. Phosphorus is a representative element that reduces impact toughness, and the lower its content, the better. If phosphorus is added in amounts of 0.012% or more, weldability and toughness may decrease.

Sulfur (S): More than 0% ~ 0.003% or less

Sulfur is limited to more than 0% ~ 0.003% of the total weight of the steel. Sulfur is an element that is inevitably contained in the production of steel along with phosphorus, and can impair the toughness and weldability of steel. If the sulfur content exceeds 0.003%, sulfur-based inclusions (MnS) may be formed to worsen resistance to stress corrosion cracking, which may cause cracks to occur during processing of the steel, and as a result, the corrosion resistance of the steel may be reduced. there is.

The remaining component of the present invention is iron (Fe). However, in the normal manufacturing process, unintended impurities may inevitably be introduced from raw materials or the surrounding environment, so this cannot be ruled out. Since these impurities are known to anyone skilled in the ordinary manufacturing process, all of them are not specifically mentioned in this specification.

The carbon equivalent (C _eq ) and weld crack susceptibility index (P _cm ) of the steel are as shown in Equation 1 and Equation 2, respectively.

[Equation 1]

C _eq = [C] + [Mn]/6 + ([Ni] + [Cu])/15 + ([Cr] + [Mo] + [V])/5

[Equation 2]

P _cm = [C] + [Si]/30 + ([Mn] + [Cu] + [Cr])/20 + [Ni]/60 + [Mo]/15 + [V]/10 + 5[B ]

In the above equations 1 and 2, [C], [Mn], [Ni], [Cu], [Cr], [Mo], [V], [Si] and [B] are included in the steel. It is the content of carbon (C), manganese (Mn), nickel (Ni), copper (Cu), chromium (Cr), molybdenum (Mo), vanadium (V), silicon (Si), and boron (B). The unit is weight%.

The steel may have a carbon equivalent (C _eq ) according to Equation 1, for example, 0.28 to 0.45, for example, 0.30 to 0.45. If the carbon equivalent (C _eq ) according to Equation 1 above exceeds 0.45, the weldability of the present invention may be reduced.

The steel may have a weld crack susceptibility index (P _cm ) according to Equation 2, for example, 0.11 to 0.18, for example, 0.12 to 0.18. If the weld crack susceptibility index (P _cm ) according to Equation 2 above exceeds 0.18, weldability may deteriorate.

The thick steel plate manufactured through the steel manufacturing method described later by controlling the specific components and content ranges of the alloy composition described above has tensile strength (TS): 625 MPa ~ 825 MPa, yield strength (YS): 555 MPa. ~ 705 MPa, elongation (EL): 40% ~ 50%, yield ratio (YR): 0.70 ~ 0.90 and average drop weight test value (DWTT): 90% ~ 100% at temperatures from 0℃ to -20℃. You can.

For reference, the Drop Weight Tear Test (DWTT) performed in the present invention is one of the methods for evaluating the fracture propagation transition temperature (FPTT) of an oil pipeline, and unlike the Charpy impact test, it is used on actual oil pipelines. By using a specimen with the same thickness, the transition temperature change due to the specimen thickness change is well explained, and unlike the results of the Charpy impact test, the fracture path is long, making it suitable for showing the pattern of fracture propagation in an oil pipeline.

The final microstructure of the thick steel plate may include ferrite and bainite. The area fraction of the bainite is 5 to 10%, and the area fraction of the ferrite may have the remaining area fraction. In the final microstructure, the area fraction of polygonal ferrite is 15 to 25%, the area fraction of acicular ferrite is 65 to 75%, and the area fraction of bainitic ferrite is 5 to 10. It may be %.

The area fraction refers to the area ratio derived from a photo of the microstructure of the steel through an image analyzer. The thick steel plate may include ferrite as a soft phase and bainite as a hard phase.

Another aspect of the present invention provides a method for manufacturing a thick steel plate with excellent low-temperature fracture toughness. According to this, reheating the steel material made of the above-described alloy composition at a temperature of 1,000°C to 1,200°C; Hot rolling the heated steel material through rough rolling and finishing rolling; It includes; a step of accelerated cooling of the hot rolled steel.

In the hot rolling step, the finish rolling temperature (FRT) is 750°C to 850°C, and in the accelerated cooling step, the accelerated cooling end temperature (FCT) is 450°C to 550°C, and the accelerated cooling step is It can be performed at a cooling rate of 20℃/sec to 40℃/sec.

Hereinafter, a method for manufacturing a thick steel plate with excellent low-temperature fracture toughness according to the present invention will be described with reference to the attached drawings.

Manufacturing method of steel

Figure 1 is a process flow chart schematically showing a method of manufacturing a thick steel plate according to an embodiment of the present invention.

In the method for manufacturing steel materials according to the present invention, the semi-finished product subject to the hot rolling process may be, for example, a slab. Slabs in a semi-finished state can be obtained through a continuous casting process after obtaining molten steel of a predetermined composition through a steelmaking process.

The steel contains, in weight percent, carbon (C): 0.04% ~ 0.065%, silicon (Si): 0.20% ~ 0.30%, manganese (Mn): 1.60% ~ 1.80%, soluble aluminum (S_Al): 0.02% ~ 0.05%, copper (Cu): 0.05% ~ 0.15%, chromium (Cr): 0.2% ~ 0.3%, molybdenum (Mo): 0.15% ~ 0.25%, nickel (Ni): 0.15% ~ 0.25%, niobium (Nb) ): 0.045% ~ 0.055%, vanadium (V): more than 0% but less than 0.01%, titanium (Ti): 0.008% ~ 0.018%, boron (B): more than 0% ~ 0.0005% or less, phosphorus (P): 0 % to less than 0.012%, sulfur (S): more than 0% to less than 0.003%, and the balance includes iron (Fe) and inevitable impurities.

Referring to Figure 1, the method of manufacturing a thick steel plate according to an embodiment of the present invention includes a reheating step (S110), a hot rolling step (S120), and a cooling step (S130).

Reheating step (S110)

In the reheating step (S110), steel having the above composition, for example, a slab plate, is reheated at a reheating temperature (Slab Reheating Temperature, SRT) of 1,000°C to 1,200°C. Through this reheating, re-dissolution of components segregated during casting and re-dissolution of precipitates may occur. Strictly, the reheating temperature is controlled relatively low at 1,100 ~ 1,150℃ to reduce the austenite average grain size (AGS), refine the structure, improve low-temperature toughness (DWTT), and increase fracture propagation resistance.

When the reheating temperature is less than 1,000°C, there is a problem in that impurities and precipitate forming elements are not sufficiently dissolved, and components segregated during casting are not sufficiently evenly distributed. If the reheating temperature exceeds 1,200°C, very coarse austenite grains are formed, making it difficult to secure strength. In addition, as the reheating temperature increases, there is a problem that increases manufacturing costs and reduces productivity due to heating costs and additional time required to adjust the hot rolling temperature.

Hot rolling step (S120)

The heated steel is first heated and then hot rolled to adjust its shape. The hot rolling may be performed continuously through width rolling, rough rolling, and finishing rolling. By the hot rolling step, the steel material can form a steel plate.

In the hot rolling, the rolling temperature is managed lower than before to reduce the non-recrystallized region by more than 80%, thereby refining the structure and improving low-temperature toughness (DWTT). At the end of rough rolling, a reduction rate of 15% or more is applied.

During the hot rolling, the starting temperature (FST) of the finishing rolling and the finishing temperature (FRT) of the finishing rolling may be performed at a temperature higher than Ar3. Temperatures higher than Ar3 correspond to the austenite single phase region. The finishing temperature (FRT) of the finishing rolling may be performed at a temperature higher than Ar3, and may be 750°C to 850°C in this composition range. If the finishing temperature (FRT) of the finishing rolling is less than 750°C, toughness and strength may be reduced due to the formation of a pearlite structure. If the finishing temperature (FRT) of the finishing rolling exceeds 850°C, the area fraction of ferrite, which is a soft phase, is reduced and a mixed structure occurs, making it difficult to secure the resistance compound ratio and low-temperature fracture resistance toughness.

The thickness of the hot rolled steel after hot rolling may be, for example, less than 30 mm.

Cooling step (S130)

The hot-rolled steel can be acceleratedly cooled. Since accelerated cooling is performed immediately after hot rolling without waiting for air cooling, the starting temperature (SCT) of accelerated cooling is the same as the finishing temperature (FRT) of finishing rolling. The accelerated cooling step is performed at a cooling rate of 20°C/sec to 40°C/sec, and the end temperature (FCT) of accelerated cooling may be 450°C to 550°C.

When cooling in the above cooling rate range, the low-temperature microstructure can be sufficiently secured while preventing the phenomenon of lowering of low-temperature toughness due to an increase in hardness. Since phase transformation must occur intensively during cooling, it is better to cool as quickly as possible after hot rolling, and a cooling rate of 20°C/sec to 40°C/sec may be appropriate to form acicular ferrite during cooling.

Demand for high-strength API-X80 for heavy plate LSAW (Longitudinal Submerged Arc Welding) continues to increase to reduce installation and transportation costs. Conventional API-X80 steel had a lot of alloying elements that were unfavorable in securing toughness, such as carbon (C) and molybdenum (Mo), added to ensure high strength. As the use of steel materials is moving to extreme cold regions, there is a need to develop high-strength thick materials with low-temperature toughness. Recently, requests for the development of heavy plate materials with a low pcm component system are increasing to ensure pipe forming weldability for actual users. Changes in the strength/low-temperature fracture toughness properties of high-strength API TMCP steels are closely related to the phase transformation of the microstructure and the formation of precipitates. In the present invention, in terms of components, the standards for addition of nickel (Ni) element and molybdenum (Mo), a hardenable element, are lowered to ensure low-temperature toughness, and the standards for carbon (C) and phosphorus (P) are lowered to reduce central segregation. did. In addition, solid solution strengthening elements manganese (Mn) and copper (Cu) were added to compensate for strength. Therefore, low-temperature toughness was guaranteed and high-strength material was secured through optimization of alloy elements. In terms of the process, the reheating temperature was lowered to prevent coarsening of the austenite average grain size (AGS), the rolling temperature was lowered to refine the grains, and accelerated cooling was performed to secure a fine bainite structure. An increase in low-temperature fracture toughness was realized by optimizing the fraction of fine bainite and ferrite through precise control of low-temperature rolling and accelerated cooling.

Hereinafter, the present invention will be described in detail through experimental examples, but these are only preferred embodiments of the present invention and the scope of the present invention is not limited by the scope of the experimental examples. Any information not described here can be technically inferred by anyone skilled in the art, so description thereof will be omitted.

Experiment example

Table 1 shows the composition of the thick steel plates of comparative examples and examples. The remainder is iron (Fe). The unit of content of each ingredient is weight%.

ingredient C Si Mn P S T-Al Cu Cr Experimental Example 1 0.078 0.22 1.55 0.012 0.001 0.036 0.022 0.02 Experimental Example 2 0.078 0.22 1.55 0.012 0.001 0.036 0.022 0.02 Experimental Example 3 0.056 0.25 1.642 0.01 0.002 0.034 0.091 0.24 Experimental Example 4 0.053 0.24 1.655 0.011 0.003 0.036 0.086 0.25 ingredient Mo Ni Nb V Ti B Ceq Pcm Experimental Example 1 0.2 0.01 0.051 0.029 0.016 0.0003 0.388 0.183 Experimental Example 2 0.2 0.01 0.051 0.029 0.016 0.0003 0.388 0.183 Experimental Example 3 0.17 0.18 0.052 0.002 0.014 0.0003 0.430 0.179 Experimental Example 4 0.17 0.17 0.05 0.002 0.012 0.0003 0.430 0.176

Referring to Table 1, Experimental Examples 3 and 4 are examples of the present invention, and in weight%, carbon (C): 0.04% ~ 0.065%, silicon (Si): 0.20% ~ 0.30%, manganese (Mn) ): 1.60% ~ 1.80%, soluble aluminum (S_Al): 0.02% ~ 0.05%, copper (Cu): 0.05% ~ 0.15%, chromium (Cr): 0.2% ~ 0.3%, molybdenum (Mo): 0.15% ~ 0.25%, Nickel (Ni): 0.15% ~ 0.25%, Niobium (Nb): 0.045% ~ 0.055%, Vanadium (V): More than 0% but less than 0.01%, Titanium (Ti): 0.008% ~ 0.018%, Boron ( B): more than 0% ~ 0.0005% or less, phosphorus (P): more than 0% ~ less than 0.012%, sulfur (S): more than 0% ~ 0.003% or less, and the remainder satisfies the composition range of iron (Fe) . Furthermore, in Experimental Examples 3 and 4, the carbon equivalent (C _eq ) according to Equation 1 above satisfies the range of 0.45 or less, for example, the range of 0.28 to 0.45. In addition, in Experimental Examples 3 and 4, the weld crack susceptibility index (P _cm ) according to Equation 2 above satisfies the range of 0.18 or less, for example, satisfies the range of 0.11 to 0.18.

On the other hand, Experimental Examples 1 and 2 are comparative examples of the present invention, and are not satisfactory as they exceed the range of carbon (C): 0.04% to 0.065% and phosphorus (P): more than 0% to less than 0.012%, Manganese (Mn): 1.60% to 1.80%, copper (Cu): 0.05% to 0.15%, nickel (Ni): 0.15% to 0.25% and therefore not satisfactory. In addition, Experimental Examples 1 and 2 are not satisfied because the weld crack susceptibility index (P _cm ) according to Equation 2 above exceeds the range of 0.18 or less.

Table 2 shows process condition values for forming thick steel plates of comparative examples and examples. The process condition values are reheating temperature (SRT), finish rolling temperature (FRT), accelerated cooling start temperature (SCT), accelerated cooling end temperature (FCT), and accelerated cooling cooling rate (CR).

SRT
(℃) FRT
(℃) SCT
(℃) FCT
(℃) CR
(℃/s) Experimental Example 1 1163 850 825 488 25 Experimental Example 2 1152 849 812 504 25 Experimental Example 3 1132 781 753 491 22 Experimental Example 4 1136 783 749 493 21

Referring to Table 2, Experimental Examples 1 to 4 have a reheating temperature (SRT): 1,000°C to 1,200°C, finishing temperature of finishing rolling (FRT): 750°C to 850°C, and ending temperature of accelerated cooling (FCT): Satisfies 450℃ ~ 550℃ and accelerated cooling cooling rate (CR): 20℃/sec ~ 40℃/sec.

Table 3 shows the strength, elongation, and yield ratio of the thick steel plates of Comparative Examples and Examples, and Table 4 shows the drop weight test value (DWTT) values of the thick steel plates of Comparative Examples and Examples. In Table 3, YS represents yield strength, TS represents tensile strength, EL represents elongation, and YR represents yield ratio. In Table 4, drop weight test values (DWTT) were evaluated using two specimens each at 0°C, -10°C, and -20°C. The required drop weight test value (DWTT) must be 85% or higher on average, and each individual drop weight test value must be 75% or higher.

YS (Mpa) TS(Mpa) YR(%) EL(%) Experimental Example 1 567 672 84 43 Experimental Example 2 569 673 85 46 Experimental Example 3 587 680 86 50 Experimental Example 4 594 683 87 47

DWTT test temperature (S.A, %) 0℃ -10℃ -20℃ Experimental Example 1 50, 100 50, 85 30, 30 Experimental Example 2 100, 100 55, 90 35, 35 Experimental Example 3 100, 100 100, 100 100, 100 Experimental Example 4 100, 100 100, 100 90, 100

Referring to Table 3, Experimental Examples 1 to 4 have tensile strength (TS): 625 MPa ~ 825 MPa, yield strength (YS): 555 MPa ~ 705 MPa, elongation (EL): 40% ~ 50%, yield Ratio (YR): 0.70 ~ 0.90 and average drop weight test value (DWTT): 90% ~ 100% at a temperature of 0℃ ~ -20℃.

Figure 2 is a graph showing the drop weight test value (DWTT) of the thick steel plate according to the comparative example among the experimental examples of the present invention, and Figure 3 is the drop weight test value (DWTT) of the thick steel plate according to the embodiment among the experimental examples of the present invention. ), and Figure 6 is a diagram showing the fracture surface observed in the drop weight test in the thick steel plate according to the experimental example of the present invention.

Referring to Figures 2, 3, 6 and Table 4, Experimental Examples 3 and 4 are examples of the present invention, and the average drop weight test value (DWTT) at a temperature of 0℃ to -20℃: 90% ~ 100% are all satisfied, and the individual drop weight test values (DWTT) are all above 75%.

On the other hand, Experimental Example 1, which is a comparative example of the present invention, is not satisfied as it falls below the average drop weight test value (DWTT): 90% ~ 100% at a temperature of 0 ℃ ~ -20 ℃, and the individual drop weight test value ( DWTT) was also found in many cases below 75%. In addition, Experimental Example 2 is also a comparative example of the present invention, and is not satisfied as it falls below the average drop weight test value (DWTT): 90% to 100% at a temperature of -10 ℃ to -20 ℃, and the individual drop weight test value ( DWTT) was also found in many cases below 75%.

Figure 4 is a photograph of the microstructure of a thick steel plate according to a comparative example (Experimental Example 1) among the experimental examples of the present invention, and Figure 5 is a photograph of a thick steel plate according to an example (Experimental Example 3) among the experimental examples of the present invention. This is a photo of microstructure.

Referring to FIG. 5, the microstructure of the thick steel plate according to Example (Experimental Example 3) among the experimental examples of the present invention includes ferrite and bainite as the final microstructure, and the area fraction of bainite is 5 to 10%. The area fraction of polygonal ferrite ranges from 15 to 25%, the area fraction of acicular ferrite ranges from 65 to 75%, and the area fraction of bainitic ferrite ranges from 5 to 10%. It can be confirmed that it is satisfied. Specifically, the area fraction of bainite is 7.8%, the area fraction of polygonal ferrite is 16.1%, the area fraction of acicular ferrite is 68.2%, and the area fraction of bainitic ferrite is 68.2%. was measured at 7.9%.

Meanwhile, it can be seen that the structure in the example shown in FIG. 5 is relatively finer than in the comparative example shown in FIG. 4. For example, the particle size measured based on the ASTM E112 standard is FGS No.: 13.1 in FIG. 5, and FGS No.: 11.6 in FIG. 4, which quantitatively confirms that the structure is finer in the examples than in the comparative examples. You can.

In the case of the developed steel (Example) compared to the conventional steel (Comparative Example), the low-temperature toughness (DWTT) was significantly improved while having similar strength, which is due to the smaller size of polygonal ferrite compared to the existing structure, resulting in resistance to crack propagation. This is because it has increased. Fine polygonal ferrite, like acicular ferrite, has excellent resistance to crack propagation, so the unit crack propagation path is short.

According to the technical idea of the present invention, in order to secure low-temperature fracture toughness for Conditions were derived. The rolling conditions refined the overall structure by lowering the heating and rolling temperatures compared to existing materials, and produced products with fine bainite and ferrite bases through optimal accelerated cooling conditions (cooling end temperature, cooling rate). As a result of the material test, a product was implemented that maintains the performance of the same X80 grade and has excellent low-temperature fracture toughness (DWTT). In addition, a 65kg, 1.25-inch-thick, high-strength API steel sheet was implemented as a low pcm material that has better weldability than existing materials and can guarantee fracture toughness in extreme cold temperatures.

The technical idea of the present invention described above is not limited to the above-described embodiments and the accompanying drawings, and various substitutions, modifications, and changes are possible without departing from the technical idea of the present invention. It will be clear to those skilled in the art.

Claims (9)

By weight, Carbon (C): 0.04% to 0.065%, Silicon (Si): 0.20% to 0.30%, Manganese (Mn): 1.60% to 1.80%, Soluble Aluminum (S_Al): 0.02% to 0.05%, Copper (Cu): 0.05% ~ 0.15%, Chromium (Cr): 0.2% ~ 0.3%, Molybdenum (Mo): 0.15% ~ 0.25%, Nickel (Ni): 0.15% ~ 0.25%, Niobium (Nb): 0.045% ~ 0.055%, Vanadium (V): More than 0% but less than 0.01%, Titanium (Ti): 0.008% ~ 0.018%, Boron (B): More than 0% ~ 0.0005% or less, Phosphorus (P): More than 0% ~ 0.012 %, sulfur (S): more than 0% ~ 0.003% or less, and the balance includes iron (Fe) and inevitable impurities,
Tensile Strength (TS): 625 MPa ~ 825 MPa, Yield Strength (YS): 555 MPa ~ 705 MPa, Elongation (EL): 40% ~ 50%, Yield Ratio (YR): 0.70 ~ 0.90 and 0℃ ~ -20 Average drop weight test value (DWTT) at a temperature of ℃: 90% ~ 100%,
Thick steel plate. According to claim 1,
The final microstructure includes ferrite and bainite,
Thick steel plate. According to claim 2,
The area fraction of the bainite is 5 to 10%,
The area fraction of the ferrite has the remaining area fraction,
Thick steel plate. According to claim 2,
In the final microstructure, the area fraction of polygonal ferrite is 15 to 25%, the area fraction of acicular ferrite is 65 to 75%, and the area fraction of bainitic ferrite is 5 to 10. %person,
Thick steel plate. According to claim 1,
The carbon equivalent (C _eq ) is 0.45 or less, and the weld crack susceptibility index (P _cm ) is 0.18 or less,
Thick steel plate.
(where C _eq = [C] + [Mn]/6 + ([Ni] + [Cu])/15 + ([Cr] + [Mo] + [V])/5;
P _cm = [C] + [Si]/30 + ([Mn] + [Cu] + [Cr])/20 + [Ni]/60 + [Mo]/15 + [V]/10 + 5[B ]) By weight, Carbon (C): 0.04% to 0.065%, Silicon (Si): 0.20% to 0.30%, Manganese (Mn): 1.60% to 1.80%, Soluble Aluminum (S_Al): 0.02% to 0.05%, Copper (Cu): 0.05% ~ 0.15%, Chromium (Cr): 0.2% ~ 0.3%, Molybdenum (Mo): 0.15% ~ 0.25%, Nickel (Ni): 0.15% ~ 0.25%, Niobium (Nb): 0.045% ~ 0.055%, vanadium (V): more than 0% but less than 0.01%, titanium (Ti): 0.008% ~ 0.018%, boron (B): more than 0% ~ 0.0005% or less, phosphorus (P): more than 0% ~ 0.012 %, sulfur (S): more than 0% to 0.003% or less, and the balance is reheating the steel containing iron (Fe) and inevitable impurities at a temperature of 1,000°C to 1,200°C;
Hot rolling the heated steel material through rough rolling and finishing rolling;
Including; a step of accelerated cooling of the hot-rolled steel,
In the hot rolling step, the finish rolling temperature (FRT) is 750°C to 850°C, and in the accelerated cooling step, the accelerated cooling end temperature (FCT) is 450°C to 550°C.
Manufacturing method of thick steel plate. According to claim 6,
The accelerated cooling step is performed at a cooling rate of 20°C/sec to 40°C/sec.
Manufacturing method of thick steel plate. According to claim 6,
After performing the cooling step,
The thick steel plate has tensile strength (TS): 625 MPa ~ 825 MPa, yield strength (YS): 555 MPa ~ 705 MPa, elongation (EL): 40% ~ 50%, yield ratio (YR): 0.70 ~ 0.90, and Average drop weight test value (DWTT): 90% to 100% at a temperature of 0℃ to -20℃,
Manufacturing method of thick steel plate. According to claim 6,
After performing the cooling step,
The final microstructure of the thick steel plate includes ferrite and bainite,
The area fraction of bainite is 5 to 10%, the area fraction of polygonal ferrite is 15 to 25%, the area fraction of acicular ferrite is 65 to 75%, and bainitic ferrite. The area fraction is 5 to 10%,
Manufacturing method of thick steel plate.

Images