KR20230171665A - Extra heavy steel materials for flange having excellent strength and low temperature impact toughness, and manufacturing method for the same - Google Patents

Abstract

Provided are extra heavy steel materials for a flange having excellent strength and low temperature impact toughness, and a manufacturing method therefor. The steel materials of the present invention include, with wt.%, 0.05 % to 0.2 % of C, 0.05 % to 0.5 % of Si, 1.0 % to 2.0 % of Mn, 0.005 % to 0.1 % of Al, 0.01 % or lower of P, 0.015 % or lower of S, 0.001 % to 0.07 % of Nb, 0.001 % to 0.3 % of V, 0.001 % to 0.03 % of Ti, 0.01 % to 0.3 % of Cr, 0.01 % to 0.12 % of Mo, 0.01 % to 0.6 % of Cu, 0.05 % to 1.0 % of Ni, 0.0005 % to 0.004 % of Ca, the remaining Fe and other unavoidable impurities. Ceq in accordance with following relationship equation 1 satisfies the range of 0.35 to 0.55, has a thickness of 200mm to 500 mm, and has a steel microstructure consisting of a composite structure of ferrite and perlite with an average particle size of 30 ㎛ or less. The maximum size of the cementite present in the ferrite-ferrite and/or ferrite-pearlite grain boundaries is 5 ㎛ or less. The porosity in the center of the product, which is an area of 3/8t to 5/8t (where t means the thickness (mm) of the steel) from the surface of the steel in the thickness direction, is 0.1 mm^3 / g or less. Among the precipitates observed in the cross-section of the steel material, there are five or more NbC or NbCN precipitates with a diameter of 5 nm to 15 nm per 1㎛^2.

Description

Extra heavy steel materials for flange having excellent strength and low temperature impact toughness, and manufacturing method for the same}

The present invention relates to a steel material that can be used in wind power generation towers and systems and a method of manufacturing the same. More specifically, it relates to an extremely thick steel material for flanges with excellent strength and low-temperature impact toughness and a method of manufacturing the same.

Wind power generators are attracting attention as an eco-friendly means of producing electricity, and include parts such as tower flanges, bearings, and main shafts. Among these, tower flanges are joint parts required to connect towers, and usually 5 to 7 flanges are used in one tower. Since they are installed at sea or in extreme cold regions, high durability is required. In particular, the size of wind towers is increasing in response to the demand for large-capacity energy production and high efficiency, and accordingly, the steel materials used are also continuously required to be high-strength, high-toughness, and thicker. As the thickness of the material increases, the total amount of deformation decreases, so the microstructure increases, and the material tends to deteriorate due to defects within the material such as inclusions and segregation. Therefore, in order to improve the internal and external soundness of steel materials, there is a trend to reduce the concentration of impurities such as non-metallic inclusions and segregation or to control cracks and voids on the surface and inside the material to the limit.

In particular, in the case of extremely thick materials exceeding 200 mmt in thickness, the amount of deformation in the center of the material is not large, so if the unsolidified shrinkage voids generated during playing or casting are not sufficiently compressed during the forging process, they remain in the form of residual voids in the center of the flange.

These residual voids act as the starting point of cracks when the structure is subjected to thickness axial stress, which can eventually cause damage to the entire facility in the form of lamellar tearing. Therefore, before piercing (hole forging) and ring forging (product forming) with a small amount of deformation, it is necessary to sufficiently compress the center void to ensure that no residual void exists.

Patent Document 1 related to this is a technology for applying forced pressure in the heavy plate rough rolling process. Specifically, the technology to determine the limit reduction rate for each thickness where sheet bite occurs by thickness from the reduction rate for each pass set to be close to the design tolerance (load and torque) of the rolling mill, and the thickness ratio for each pass to secure the target thickness of the roughing mill. It utilizes technology to distribute the reduction rate by adjusting the index and to modify the reduction rate to prevent plate engagement based on the limit reduction rate for each thickness. The average reduction rate in the final 3 passes of rough rolling based on 80mmt is utilized. It provides a manufacturing method that can be applied at about 27.5%. However, in the case of the above rolling method, the average reduction rate of the entire product thickness is measured, and in the case of extremely thick materials with a maximum thickness of 200 mmt or more, it is technically difficult to apply high strain to the center where residual voids exist.

One of the other methods of manufacturing extremely thick products is to use a forging machine with a higher effective deformation per pass than a rolling mill. Patent Document 2 shows, in mass%, C: 0.08% to 0.20%, Si: 0.40% or less, Mn: 0.5% to 5.0%, P: 0.010% or less, S: 0.0050% or less, Cr: 3.0% or less, Ni: 0.1% or less. A slab containing 5.0%, Al: 0.010 to 0.080%, N: 0.0070% or less, and O: 0.0025% or less, satisfying the relationships in formulas (1) and (2), and the balance consisting of Fe and inevitable impurities. Hot forging is performed with a cumulative reduction of 25% or more, heating is performed from the Ac3 point to 1200°C, and hot rolling is performed with a cumulative reduction of 40% or more. It is rapidly cooled to a temperature, and through a tempering heat treatment process at a temperature of 450~700℃, a thick, high-toughness, high-strength material is manufactured with a plate thickness of more than 100mmt, a yield strength of more than 620MPa, and an absorbed energy of more than 70J when evaluating low-temperature impact toughness at -40℃. It clearly states that it can be done.

However, in the above manufacturing method, if the cumulative reduction amount is too high, surface defects may occur due to localized strain concentration. In particular, if surface or subsurface defects exist in the cast steel state before forging, the defects may propagate during the forging process and after rolling. The surface quality may deteriorate further in the product condition. In addition, if the amount of forging reduction per pass is insufficient, even if the cumulative reduction is high, it is difficult to sufficiently compress the voids remaining in the center, and the rolling process also has a small effective deformation at the center compared to the deformation of the surface layer, making it difficult to control the voids and structure at the center of extremely thick materials. It is not appropriate.

On the other hand, in Patent Document 3, a material provided with a predetermined alloy composition is heated to 1200 to 1350°C, hot forging is performed with a cumulative reduction of 25% or more, heated to the Ac3 point or more and 1200°C or less, and the cumulative reduction is Perform hot rolling to 40% or more, reheat to a temperature of not less than Ac3 point and not more than 1050°C, rapidly cool from a temperature of not less than Ac3 point to a temperature of 350°C or less or lower than Ar3 point, and temper to a temperature of 450°C to 700°C. It is disclosed that a thick high-strength steel plate of 100 mmt or more with a yield strength of 620 MPa or more can be manufactured through the process.

However, in the case of the above-mentioned ultra-high-strength steel sheet, the carbon equivalent (Ceq) and hardenability index (DI) are high, so not only is it vulnerable to surface cracks during casting, but in the case of flange steel manufactured through normalizing heat treatment, this process Conditions cannot be easily applied. In addition, when the carbon equivalent (Ceq) and hardenability index (DI) are high, cracks easily occur in the surface layer of the cast steel due to the creation of surface layer hard tissue during the secondary cooling process of steelmaking, and the cracks propagate during the forging process, resulting in the final product. The surface quality may deteriorate.

Therefore, a method of forging has been proposed to improve the internal integrity of the final product by compressing the central void, but no practical method has been proposed to ensure both the appropriate material and excellent surface quality of the flange steel. .

Republic of Korea Patent Publication No. 10-2012-0075246 (published on July 6, 2012) Republic of Korea Patent Publication No. 10-2017-0095307 (published on August 22, 2017) Republic of Korea Patent Publication No. 10-2017-0095307 (published on August 22, 2017)

Therefore, the purpose of the present invention is to provide an extremely thick steel material for flanges with excellent strength and low-temperature impact toughness and a method for manufacturing the same.

The object of the present invention is not limited to the above-described content. A person skilled in the art will have no difficulty in understanding the additional problems of the present invention from the overall content of the present specification.

One aspect of the present invention is,

By weight%, C: 0.05-0.2%, Si: 0.05-0.5%, Mn: 1.0-2.0%, Al: 0.005-0.1%, P: 0.01% or less, S: 0.015% or less, Nb: 0.001-0.07% , V: 0.001~0.3%, Ti: 0.001~0.03%, Cr: 0.01~0.3%, Mo: 0.01~0.12%, Cu: 0.01~0.6%, Ni: 0.05~1.0%, Ca: 0.0005~0.004%, It includes the remaining Fe and other inevitable impurities, and Ceq satisfies the range of 0.35 to 0.55 according to equation 1 below,

It has a thickness of 200~500mm,

It has a steel microstructure consisting of a composite structure of ferrite and pearlite with an average particle size of 30㎛ or less.

The maximum size of cementite present at the ferrite-ferrite and/or ferrite-pearlite grain boundary is 5㎛ or less,

The porosity at the center of the product, which is an area of 3/8t to 5/8t (where t means the steel thickness (mm)) in the thickness direction from the steel surface, is 0.1mm ³ /g or less, and

It relates to an extremely thick steel material for flanges with excellent strength and low-temperature impact toughness, in which there are more than 5 fine NbC or NbCN precipitates with a diameter of 5 to 15 nm per ^1㎛ among the precipitates observed in the cross section of the steel material.

[Relational Expression 1]

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

In equation 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] are C, Mn, Cr, Mo, V, Ni, and It refers to the Cu content (% by weight), and if these components are not intentionally added, 0 is substituted.

Additionally, the steel may have a tensile strength of 510 to 690 MPa, a yield strength of more than 370 MPa, and an absorbed energy value of -50°C Charpy impact test of more than 50 J.

The maximum surface crack depth of the steel may be 0.1 mm or less (including 0).

In addition, another aspect of the present invention is,

By weight%, C: 0.05-0.2%, Si: 0.05-0.5%, Mn: 1.0-2.0%, Al: 0.005-0.1%, P: 0.01% or less, S: 0.015% or less, Nb: 0.001-0.07% , V: 0.001~0.3%, Ti: 0.001~0.03%, Cr: 0.01~0.3%, Mo: 0.01~0.12%, Cu: 0.01~0.6%, Ni: 0.05~1.0%, Ca: 0.0005~0.004%, When manufacturing a slab using molten steel that contains the remaining Fe and other unavoidable impurities and satisfies the Ceq range of 0.35 to 0.55 according to Equation 1 below, the cast steel discharged from the mold is heated to a temperature range of 0.01 to 3 to 800 to 850°C. Manufacturing a slab by secondary cooling at a cooling rate of ℃/s;

Heating the manufactured slab to a temperature range of 1100 to 1300°C and then performing primary upsetting at a forging ratio of 1.3 to 2.4;

Bloom forging at a forging ratio of 1.5 to 2.0 after the first upsetting;

Reheating the bloom forged material to a temperature range of 1100 to 1300°C, then round forging at a forging ratio of 1.65 to 2.25, and then performing secondary upsetting at a forging ratio of 1.3 to 2.3;

Thirdly upsetting the secondly upsetting material at a forging ratio of 2.0 to 2.8 and then processing holes;

Reheating the hole-machined material to a temperature range of 1100 to 1300°C and then ring forging at a forging ratio of 1.0 to 1.6; and

A normalizing heat treatment step of heating the ring-forged material to a temperature range of 820 to 930°C, based on the center temperature measurement, maintaining it for 5 to 600 minutes, and then air cooling it to room temperature. During the normalizing heat treatment, the equation 2 below: It relates to a method of manufacturing extremely thick steel for flanges with excellent strength and low-temperature impact toughness by heat treatment so that the LMP, defined by , satisfies 20 to 33.

[Relationship 1]

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

In equation 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] are C, Mn, Cr, Mo, V, Ni, and It refers to the Cu content (% by weight), and if these components are not intentionally added, 0 is substituted.

[Relation 2] LMP = T (Logt + 20)x(1/1000)

In equation 2 above, T is the temperature based on Kelvin, t is time, and the exponent of log is 10.

The slab may be manufactured using one of a continuous casting process, a semi-continuous casting process, and ingot casting.

After manufacturing the slab, it is desirable that the grain size of the old austenite in the surface layer of the slab before forging is 1000㎛ or less, and that the microstructure is composed of a composite structure of 15% or more of polygonal ferrite and the remaining bainite.

During the normalizing heat treatment, it is preferable to heat treat so that the LMP defined by the following relational equation 2 satisfies 20 to 33.

[Relation 2] LMP = T (Logt + 20)x(1/1000)

In equation 2 above, T is the temperature based on Kelvin, t is time, and the exponent of log is 10.

If the size of the forged surface punched during the first upsetting is initially 700 mm x 1800 mm, it may be 1000 to 1200 mm x 1800 to 2000 mm.

In the case of bloom forging, when forging is completed, the size of the forging surface may be 1450-1850 mm x 2100-2500 mm when the initial size is 1000-1200 mm x 1800-2000 mm.

When the round forging and secondary upsetting are completed, the size of the product may be 1450~1850Ø × 1300~1700mm.

When the third upsetting is completed, the size of the product may be 2300~2800Ø × 400~800mm.

The maximum thickness of the flange made of the steel may be 200 to 500 mm, the inner diameter may be 4000 to 7000 mm, and the outer diameter may be 5000 to 8000 mm.

In addition, the present invention,

By weight%, C: 0.05-0.2%, Si: 0.05-0.5%, Mn: 1.0-2.0%, Al: 0.005-0.1%, P: 0.01% or less, S: 0.015% or less, Nb: 0.001-0.07% , V: 0.001~0.3%, Ti: 0.001~0.03%, Cr: 0.01~0.3%, Mo: 0.01~0.12%, Cu: 0.01~0.6%, Ni: 0.05~1.0%, Ca: 0.0005~0.004%, It contains the remaining Fe and other unavoidable impurities, satisfies the range of 0.35 to 0.55 for Ceq according to the following relational equation 1, has a surface old austenite crystal grain size of 1000㎛ or less, and is a composite of polygonal ferrite of more than 15% and the remaining bainite. Preparing a slab with a thickness of 500 mm or more having a fine texture made of tissue;

Heating the prepared slab to a temperature range of 1100 to 1300°C and then performing first upsetting at a forging ratio of 1.3 to 2.4;

Bloom forging at a forging ratio of 1.5 to 2.0 after the first upsetting;

Reheating the bloom forged material to a temperature range of 1100 to 1300°C, then round forging at a forging ratio of 1.65 to 2.25, and then performing secondary upsetting at a forging ratio of 1.3 to 2.3;

Thirdly upsetting the secondly upsetting material at a forging ratio of 2.0 to 2.8 and then processing holes;

Reheating the hole-machined material to a temperature range of 1100 to 1300°C and then ring forging at a forging ratio of 1.0 to 1.6; and

A normalizing heat treatment step of heating the ring-forged material to a temperature range of 820 to 930°C, based on the center temperature measurement, maintaining it for 5 to 600 minutes, and then air cooling it to room temperature. During the normalizing heat treatment, the equation 2 below: It relates to a method of manufacturing extremely thick steel for flanges with excellent strength and low-temperature impact toughness by heat treatment so that the LMP, defined by , satisfies 20 to 33.

[Relationship 1]

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

In equation 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] are C, Mn, Cr, Mo, V, Ni, and It refers to the Cu content (% by weight), and if these components are not intentionally added, 0 is substituted.

[Relation 2] LMP = T (Logt + 20)x(1/1000)

In equation 2 above, T is the temperature based on Kelvin, t is time, and the exponent of log is 10.

The present invention, configured as described above, can improve the internal soundness of the final product by compressing the voids in the center of the steel by optimizing the forging process, effectively providing an extremely thick steel that can be used for flanges with excellent low-temperature impact toughness as well as strength. can do.

The present invention relates to an extremely thick steel material for flanges with excellent strength and low-temperature impact toughness and a method of manufacturing the product. Preferred embodiments of the present invention will be described below. Embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as limited to the embodiments described below. These embodiments are provided to further explain the present invention to those skilled in the art.

Hereinafter, the extremely thick steel material for flanges with excellent strength and low-temperature impact toughness of the present invention will be described in more detail.

The extremely thick steel for flanges with excellent strength and low-temperature impact toughness of the present invention has, in weight percent, C: 0.05-0.2%, Si: 0.05-0.5%, Mn: 1.0-2.0%, Al: 0.005-0.1%, P : 0.01% or less, S: 0.015% or less, Nb: 0.001~0.07%, V: 0.001~0.3%, Ti: 0.001~0.03%, Cr: 0.01~0.3%, Mo: 0.01~0.12%, Cu: 0.01~ 0.6%, Ni: 0.05~1.0%, Ca: 0.0005~0.004%, contains the remaining Fe and other inevitable impurities, Ceq according to equation 1 satisfies the range of 0.35~0.55, has a thickness of 200~500mm, and has an average It has a steel microstructure consisting of a composite structure of ferrite and pearlite with a grain size of 30㎛ or less, and the maximum size of cementite present at the ferrite-ferrite and/or ferrite-pearlite grain boundary is 5㎛ or less, and 3/3 in the thickness direction from the steel surface. The porosity at the center of the product in the area of 8t to 5/8t (where t refers to the steel thickness (mm)) is 0.1mm ³ /g or less, and the precipitates observed on the cross section of the steel have a diameter of 5 to 15 nm. There are more than 5 fine NbC or NbCN precipitates per ^1㎛2 .

Hereinafter, the alloy composition of the present invention will be described in more detail. Hereinafter, unless otherwise specified, % and ppm stated in relation to alloy composition are based on weight.

·Carbon (C): 0.05~0.20%

Carbon (C) is the most important element in securing basic strength, so it needs to be contained in steel within an appropriate range. To achieve this added effect, more than 0.05% of carbon (C) can be added. Preferably, 0.10% or more of carbon (C) may be added. On the other hand, if the carbon (C) content exceeds a certain level, the fraction of pearlite increases during normalizing heat treatment, and the strength and hardness of the base material may be excessively exceeded. As a result, surface cracks occur during forging, and the final The low-temperature impact toughness and lamellar tearing resistance of the product may be reduced. Therefore, the present invention can limit the carbon (C) content to 0.20%. A more desirable upper limit of carbon (C) content may be 0.18%.

·Silicon (Si): 0.05~0.50%

Silicon (Si) is a substitutional element that improves the strength of steel through solid solution strengthening and has a strong deoxidation effect, so it is an essential element in the production of clean steel. Therefore, silicon (Si) may be added in an amount of 0.05% or more, and more preferably, 0.20% or more. On the other hand, when a large amount of silicon (Si) is added, it creates a MA (Martensite-Austenite) phase and excessively increases the strength of the ferrite matrix, which can cause deterioration in the surface quality of extremely thick products, so the upper limit of the content is 0.50%. It can be limited to . A more desirable upper limit of silicon (Si) content may be 0.40%.

·Manganese (Mn): 1.0~2.0%

Manganese (Mn) is a useful element that improves strength through solid solution strengthening and improves hardenability to create a low-temperature transformation phase. Therefore, in order to secure a tensile strength of 550 MPa or more, it is desirable to add 1.0% or more of manganese (Mn). A more desirable manganese (Mn) content may be 1.1% or more. On the other hand, manganese (Mn) forms MnS, an elongated non-metallic inclusion with sulfur (S), which reduces toughness and can act as an impact initiation point, which can be a factor that drastically reduces the low-temperature impact toughness of a product. . Therefore, it is desirable to manage the manganese (Mn) content to 2.0% or less, and a more preferable manganese (Mn) content may be 1.5% or less.

·Aluminum (Al): 0.005~0.1%

Aluminum (Al), along with silicon (Si), is one of the powerful deoxidizers in the steelmaking process, and it is preferably added in an amount of 0.005% or more to achieve this effect. A more desirable lower limit of aluminum (Al) content may be 0.01%. On the other hand, when the aluminum (Al) content is excessive, the fraction of Al ₂ O ₃ in the oxidizing inclusions generated as a result of deoxidation increases excessively, making their size coarse, and removing the inclusions during refining becomes difficult. It can be a factor that reduces low-temperature impact toughness. Therefore, it is desirable to manage the aluminum (Al) content to 0.1% or less. A more desirable aluminum (Al) content may be 0.07% or less.

· Phosphorus (P): 0.010% or less (including 0%), Sulfur (S): 0.0015% or less (including 0%)

Phosphorus (P) and sulfur (S) are elements that cause embrittlement at grain boundaries or by forming coarse inclusions. Therefore, in order to improve brittle crack propagation resistance, it is desirable to limit phosphorus (P) to 0.010% or less and sulfur (S) to 0.0015% or less.

·Niobium (Nb): 0.001~0.07%

Niobium (Nb) is an element that improves the strength of the base material by precipitating in the form of NbC or NbCN. In addition, niobium (Nb) dissolved in high-temperature reheating precipitates very finely in the form of NbC during rolling and suppresses recrystallization of austenite, which has the effect of refining the structure. Therefore, it is preferable that niobium (Nb) is added in an amount of 0.001% or more, and a more preferable niobium (Nb) content may be 0.005% or more. On the other hand, when niobium (Nb) is added excessively, undissolved niobium (Nb) is generated in the form of TiNb (C, N) and becomes a factor that impedes low-temperature impact toughness, so the upper limit of niobium (Nb) content is It is desirable to limit it to 0.07%. A more desirable niobium (Nb) content may be 0.065% or less.

·Vanadium (V): 0.001~0.3%

Since vanadium (V) is almost completely redissolved upon reheating, the strengthening effect due to precipitation or solid solution during subsequent rolling is minimal. However, in the case of extremely thick forged materials, the air cooling speed is very slow, so it is precipitated as very fine carbonitrides during the cooling process or additional heat treatment. This has the effect of improving strength. To fully obtain this effect, it is necessary to add more than 0.001% of vanadium (V). A more desirable lower limit of vanadium (V) content may be 0.01%. On the other hand, if the content is excessive, the surface hardness of the slab is excessively increased due to high hardenability, which may not only cause surface cracks during flange processing, but also cause a sharp increase in manufacturing cost, which is not commercially advantageous. Therefore, the vanadium (V) content can be limited to 0.3% or less. A more preferable vanadium (V) content may be 0.25% or less.

·Titanium (Ti): 0.001~0.03%

Titanium (Ti) is a component that precipitates as TiN when reheated and significantly improves low-temperature toughness by inhibiting the growth of old austenite grains at high temperatures. To achieve this effect, it is preferable that 0.001% or more of titanium (Ti) is added. On the other hand, if titanium (Ti) is added excessively, low-temperature toughness may be reduced due to clogging of the playing nozzle or crystallization in the center. In addition, titanium (Ti) combines with nitrogen (N) to form coarse TiN precipitates in the center of the thickness, which reduces the elongation of the product, so it can reduce the uniform elongation during the forging process and cause surface cracks. Therefore, the titanium (Ti) content may be 0.03% or less. A preferable titanium (Ti) content may be 0.025% or less, and a more preferable titanium (Ti) content may be 0.018% or less.

·Chromium (Cr): 0.01~0.30%

Chromium (Cr) is an ingredient that increases hardenability and forms a low-temperature transformation structure, thereby increasing yield strength and tensile strength. In addition, it is an ingredient that is effective in preventing a decrease in strength by slowing down the spheroidization rate of cementite. For this effect, more than 0.01% of chromium (Cr) may be added. On the other hand, when the chromium (Cr) content is excessive, the size and fraction of Cr-Rich coarse carbides such as M ₂₃ C ₆ increase, reducing the impact toughness of the product, and the solid solubility of niobium (Nb) in the product and As the fraction of fine precipitates decreases, a decrease in the strength of the product may be a problem. Therefore, the present invention can limit the upper limit of chromium (Cr) content to 0.30%. The upper limit of the desirable chromium (Cr) content may be 0.25%.

·Molybdenum (Mo): 0.01~0.12%

Molybdenum (Mo) is an element that increases grain boundary strength and has a significant solid solution strengthening effect in ferrite, effectively contributing to increasing the strength and ductility of products. In addition, molybdenum (Mo) has the effect of preventing a decrease in toughness due to grain boundary segregation of impurity elements such as phosphorus (P). For this effect, more than 0.10% molybdenum (Mo) may be added. However, molybdenum (Mo) is an expensive element and if added excessively, the manufacturing cost can increase significantly, so the upper limit of the molybdenum (Mo) content can be limited to 0.12%.

·Copper (Cu): 0.01~0.60%

Copper (Cu) is an advantageous element in the present invention because it can not only greatly improve the strength of the matrix phase through solid solution strengthening in ferrite, but also has the effect of suppressing corrosion in a wet hydrogen sulfide atmosphere. For this effect, it may contain more than 0.01% copper (Cu). A more desirable copper (Cu) content may be 0.03% or more. However, if the content of copper (Cu) is excessive, the possibility of causing star cracks on the surface of the steel sheet increases, and copper (Cu) is an expensive element, which can lead to a significant increase in manufacturing costs. Therefore, the present invention can limit the upper limit of copper (Cu) content to 0.60%. The upper limit of the desirable copper (Cu) content may be 0.35%.

·Nickel (Ni): 0.05~1.00%

Nickel (Ni) is an element that effectively contributes to improving impact toughness by increasing stacking faults at low temperatures, facilitating cross slip of dislocations, and improving strength by improving hardenability. For this effect, more than 0.05% nickel (Ni) may be added. A preferred nickel (Ni) content may be 0.10% or more. On the other hand, if excessive nickel (Ni) is added, the manufacturing cost may increase due to the high cost, so the upper limit of the nickel (Ni) content can be limited to 1.00%. The upper limit of the preferred nickel (Ni) content may be 0.80%.

·Calcium (Ca): 0.0005~0.0040%

When calcium (Ca) is added after deoxidation with aluminum (Al), it combines with sulfur (S) forming MnS inclusions to suppress the production of MnS, and at the same time forms spherical CaS to prevent cracks caused by hydrogen-induced cracking. It has the effect of suppressing its occurrence. In order to sufficiently form sulfur (S) and CaS contained as impurities, it is preferable to add 0.0005% or more of calcium (Ca). However, if the addition amount is excessive, calcium (Ca) remaining after forming CaS combines with oxygen (O) to generate coarse oxidative inclusions, which are elongated and destroyed during rolling, which can be a factor in deteriorating the lamellar tearing resistance. You can. Therefore, the upper limit of calcium (Ca) content can be limited to 0.0040%.

·Relationship 1

In the present invention, it is required that Ceq according to the following relational equation 1 satisfies the range of 0.35 to 0.55. If Ceq according to the following equation 1 is less than 0.35, the pearlite fraction is reduced, making it impossible to secure the tensile strength value of 510 to 690 MPa required in the present invention, and if it exceeds 0.55, the pearlite fraction exceeds 30%. It is not easy to secure low-temperature impact energy values of -50℃. Therefore, in the present invention, it is desirable to limit Ceq to the range of 0.35 to 0.55.

[Relational Expression 1]

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

In equation 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] are C, Mn, Cr, Mo, V, Ni, and It refers to the Cu content (% by weight), and if these components are not intentionally added, 0 is substituted.

The extremely thick steel for flanges and products thereof with excellent strength and low-temperature impact toughness of the present invention may contain remaining Fe and other unavoidable impurities in addition to the above-mentioned components. However, in the normal manufacturing process, unintended impurities may inevitably be introduced from raw materials or the surrounding environment, so this cannot be completely excluded. Since these impurities are known to anyone skilled in the art, all of them are not specifically mentioned in this specification. In addition, the addition of additional effective ingredients in addition to the above-mentioned ingredients is not completely excluded.

Meanwhile, the extremely thick steel material of the present invention has a steel microstructure consisting of a composite structure of ferrite and pearlite with an average particle size of 30 ㎛ or less. If the average ferrite grain size exceeds 30㎛, the length of the crack path during impact rupture becomes shorter, DBTT (Ductile Brittle Transition Temperature) increases, and low-temperature impact toughness deteriorates. Therefore, it is appropriate that the crystal grain size of ferrite is 30㎛ or less.

In addition, it is preferable that the maximum size of cementite present at the grain boundaries (grain boundaries between ferrite-ferrite and/or ferrite-pearlite) of the microstructure is 5㎛ or less. If the maximum size of the cementite exceeds 5㎛, the coarse cementite can act as an impact initiation point, so impact toughness is deteriorated and intergranular strength is lowered, and intergranular fracture is likely to occur, thereby lowering impact toughness. Therefore, it is desirable that the maximum size of cementite is 5㎛ or less.

More preferably, the fraction of cementite present at the grain boundary is controlled to 3 area% or less.

In addition, the extremely thick steel material of the present invention has a porosity of 0.1 mm ³ /g or less at the center of the product, which is an area of 3/8t to 5/8t (where t refers to the steel thickness (mm)) in the thickness direction from the steel surface. am.

In addition, the extremely thick steel material of the present invention preferably contains at least 5 fine NbC or NbCN precipitates with a diameter of 5 to 15 nm per 1㎛ ² among the precipitates observed in the cross section of the steel material. If the number of fine precipitates is less than 5, the precipitation strengthening effect is weakened and there may be a problem in securing the physical properties required in the present invention.

Additionally, the extremely thick steel material of the present invention may have a thickness of 200 to 500 mm.

In addition, the ultra-thick steel material of the present invention may have a tensile strength of 510 to 690 MPa, a yield strength of 370 MPa or more, and a -50°C Charpy impact test absorbed energy value of 50 J or more.

And the maximum surface crack depth of the steel may be 0.1 mm or less (including 0).

Next, a method for manufacturing an extremely thick steel material for flanges with excellent strength and low-temperature impact toughness, which is another aspect of the present invention, will be described in detail.

In the method of manufacturing extremely thick steel of the present invention, when manufacturing a slab using molten steel having the composition as described above, the cast iron discharged from the mold is cooled to a temperature range of 800 to 850 ° C. at a cooling rate of 0.01 to 3 ° C./s. Manufacturing a slab by secondary cooling with a furnace; Heating the manufactured slab to a temperature range of 1100 to 1300°C and then performing primary upsetting at a forging ratio of 1.3 to 2.4; Bloom forging at a forging ratio of 1.5 to 2.0 after the first upsetting; Reheating the bloom forged material to a temperature range of 1100 to 1300°C, then round forging at a forging ratio of 1.65 to 2.25, and then performing secondary upsetting at a forging ratio of 1.3 to 2.3; Thirdly upsetting the secondly upsetting material at a forging ratio of 2.0 to 2.8 and then processing holes; Reheating the hole-machined material to a temperature range of 1100 to 1300°C and then ring forging at a forging ratio of 1.0 to 1.6; And a normalizing heat treatment step of heating the ring-forged material to a temperature range of 820-930°C based on the center temperature measurement, maintaining it for 5-600 minutes, and then air-cooling it to room temperature.

Slab preparation

First, in the present invention, a slab is prepared.

Preferably, when manufacturing a slab using molten steel having the composition as described above, the cast slab discharged from the mold is secondary cooled at a cooling rate of 0.01 to 3°C/s to a temperature range of 800 to 850°C to form the slab. can be manufactured.

The inventor of the present invention conducted in-depth research on methods for manufacturing extremely thick steel materials with physical properties suitable for flanges and excellent strength, impact toughness, and surface quality, especially in slabs manufactured with a thickness of 500 mm or more. , in order to secure the strength, toughness, and surface quality of the final flange product, not only must the carbon equivalent (Ceq) of the slab be controlled within a certain range, but also the prior austenite grain size and microstructure fraction of the slab surface must be effective. Recognizing that this was a condition, the present invention was developed.

The casting speed of a large-section casting machine that produces slabs with a thickness of 650 mm or more is 0.06 to 0.1 m/min, which is significantly slower than a general casting machine that produces slabs with a thickness of 250 to 400 mm (casting speed: 0.4 to 1.5 m/min). Casting operations are carried out at speed. Therefore, when manufacturing a slab with a thickness of 500 mm or more, the time it is maintained in the mold is relatively long, creating an environment in which austenite can grow more coarsely.

As the initial austenite grain size increases, the segregation index of manganese (Mn) at the austenite grain boundary increases, the grain boundary strength decreases, and hardenability increases at the same time, so the surface layer of the slab contains hard bainite rather than soft ferrite and pearlite. And the fraction of martensite increases. Since hard tissue has a low uniform elongation rate, intergranular cracking can easily occur when thermal deformation, external deformation, or stress is applied. Therefore, when the prior austenite grain size of the slab surface layer is large, intergranular cracks on the slab surface may occur more actively, and the depth of crack introduction may further increase during high strain processes such as subsequent forging and rolling. Therefore, in order to suppress surface cracks of the final product, it is very important to control the grain size of the old austenite below an appropriate level and secure the ratio of grain boundary polygonal ferrite, which is a soft phase, above an appropriate level.

That is, in the present invention, it is preferable that the grain size of the old austenite in the surface layer of the slab is 1000㎛ or less, and that the microstructure is composed of a composite structure of polygonal ferrite of more than 15 area% and the balance bainite.

In order to reduce the old austenite grain size and secure a polygonal ferrite fraction of more than 15% by area, carbon (C), nickel (Ni), chromium (Cr), and molybdenum with solute dragging or pinning effects are used. There is a plan to design the (Mo) component to be high. However, when the components of carbon (C), nickel (Ni), chromium (Cr), and molybdenum (Mo) increase, the carbon equivalent (Ceq) also increases, and a low-temperature transformation structure may be generated during the cooling process of the slab. Therefore, the present invention can reduce the carbon equivalent (Ceq) of the steel slab to 0.55 or less according to the relational equation 1 below. A preferred carbon equivalent weight (Ceq) may be 0.4 to 0.53.

[Relational Expression 1]

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

In equation 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] are C, Mn, Cr, Mo, V, Ni, and It refers to the Cu content (% by weight), and if these components are not intentionally added, 0 is substituted.

Meanwhile, when manufacturing a slab from molten steel using a continuous casting process or a semi-continuous casting process, the surface layer temperature of the slab cools by 0.01 to 3°C/s during secondary cooling immediately after exiting the mold at a casting speed of 0.06 to 0.1 m/min. Water cooling is performed at a rate of 800~850℃ followed by air cooling. If the surface layer cooling rate is too slow, austenite grain growth continues, making it impossible to finely control the AGS of the surface layer, and if it exceeds 3°C/s, the surface layer hard phase is formed and the temperature inside the surface layer increases. Surface cracks may occur during the cooling process as the gradient increases. If the target temperature is also less than 800℃, local low-temperature transformation tissue may be formed, and if it exceeds 850℃, it is difficult to control the AGS to less than 1000㎛ required in the present invention, so appropriate surface quality cannot be secured. .

Heating and first upsetting

Next, in the present invention, the manufactured slab is heated to a temperature range of 1100 to 1300° C. and then first upset at a forging ratio of 1.3 to 2.4.

The manufactured slab can be heated in a temperature range of 1100 to 1300°C. As mentioned above, the thickness of the slab may be 500 mm or more, with a preferred thickness being 700 mm or more.

It is necessary to heat the slab above a certain temperature range to re-dissolve the composite carbonitride of titanium (Ti) or niobium (Nb) or coarse crystallized TiNb (C, N) formed during casting. In addition, before the first upsetting forging, the slab is heated and maintained above the recrystallization temperature to homogenize the structure, and the slab is heated above a certain temperature range to ensure that the forging end temperature is sufficiently high to minimize surface cracks that may occur during the forging process. Heating is preferred. Therefore, the slab heating of the present invention is preferably performed in a temperature range of 1100°C or higher.

On the other hand, if the slab heating temperature is excessively high, high-temperature oxidation scale may be excessively generated, and the increase in manufacturing cost may be excessive due to high-temperature heating and maintenance. Therefore, the slab heating of the present invention is preferably performed in the range of 1300°C or lower.

Meanwhile, upsetting is a method of rigidly deforming the material vertically along the longitudinal axis. The appropriate forging ratio during the first upsetting is 1.3 to 2.4, and preferably 1.5 to 2.0. Here, the forging ratio refers to the ratio of the cross-sectional area changed by forging. During this first upsetting, if the size of the forged surface to be punched is initially 700 mm × 1800 mm, it may be 1000 to 1200 mm × 1800 to 2000 mm.

If the forging ratio is less than 1.3 during the first upsetting, it is difficult to sufficiently compress the remaining porosity in the center of the slab. Therefore, since it is difficult to control the porosity required for the final product of the present invention to an appropriate level of 0.1 mm ³ /g or less, it is not easy to secure low-temperature impact toughness at the core. On the other hand, if the forging ratio exceeds 2.4 during the first upsetting, buckling occurs during the forging process, making it difficult to control the surface quality and appropriate shape required for flange products. Therefore, the appropriate forging ratio for the first upsetting is 1.3 to 2.4.

Bloom forging (top and bottom two-sided forging)

And in the present invention, bloom forging is performed on the first upsetting material at a forging ratio of 1.5 to 2.0.

Bloom forging is a method of processing the first upsetting material into a bloom shape by further compressing it. It is a method of expanding the area by processing both the upper and lower sides in a certain direction of width or length. In the case of bloom forging, when forging is completed, the size of the forging surface may be 1450-1850 mm × 2100-2500 mm when the initial size is 1000-1200 mm × 1800-2000 mm. In the case of bloom forging, the appropriate forging ratio is 1.5 to 2.0. If the forging ratio is less than 1.5, it is difficult to secure the appropriate void quality required in the present invention, as is the case with upsetting forging, and if it exceeds 2.0, surface cracks may occur.

The forging progress direction is possible in both the longitudinal and width directions, but in the longitudinal direction, the casting structure is more dense, so the elongation of the surface layer is higher, so machinability can be excellent. Therefore, longitudinal bloom forging may be more appropriate from a surface crack perspective than width direction.

Reheating and round forging - 2nd upsetting

Subsequently, in the present invention, the bloom forged material is reheated to a temperature range of 1100 to 1300°C, then round forged at a forging ratio of 1.65 to 2.25, and then subjected to secondary upsetting at a forging ratio of 1.3 to 2.3.

When the bloom forging is completed, the bloom surface temperature is below 950°C, and if processing continues, surface cracks or material fracture may occur. Therefore, after bloom forging, the material can be heated again to a temperature range of 1100~1300℃. As mentioned above, it is preferable to heat it above 1100°C for reasons such as re-dispersing the crystallized material, homogenizing the structure, and preventing surface cracks, and it is better to control it below 1300°C due to problems such as excessive scale and coarsening of grains.

In the case of bloom that has been heated, round forging is performed to shape the flange edge into a circular shape, and then secondary upsetting is applied again. When the round forging and secondary upsetting are completed, the size of the product may be 1450~1850Ø × 1300~1700mm. For round forging and secondary upsetting, the forging ratio can be 1.65 to 2.25 and 1.3 to 2.3, respectively. If the forging ratio during round forging and secondary upsetting is below the level required in the present invention, it is difficult to control the central porosity in the final product to less than 0.1mm ³ /g, so it is not easy to secure the low-temperature impact toughness at the central core, and the forging ratio If the standard is exceeded, it cannot be processed into the desired flange product shape due to problems such as buckling, surface cracks, and poor shape.

After the secondary upsetting is completed, round forging may be applied again for shape control, and then heated under the same conditions as the tactical reheating temperature.

3rd upsetting and hole processing

In the present invention, the second upsetting material is thirdly upset at a forging ratio of 2.0 to 2.8, and then hole-processed.

The material processed into the cylindrical shape can be processed to an appropriate flange thickness through third upsetting before hole processing (piercing). When the third upsetting is completed, the size of the product may be 2300~2800Ø × 400~800mm. The forging ratio of the 3rd upsetting can be 2.0 to 2.8, and if the forging ratio is insufficient or exceeded, problems such as the aforementioned residual void control and surface crack/shape control inability may occur. After the third upsetting is completed, a hole can be made in the center of the material using a 500~1000Ø punch.

Reheating and Ring Forging

Subsequently, in the present invention, the hole-machined material is reheated to a temperature range of 1100 to 1300° C. and then ring-forged at a forging ratio of 1.0 to 1.6.

The hole-machined material is reheated to the temperature range of 1100 to 1300°C described above, and can then be processed into the final flange ring shape. The maximum thickness of the flange made of the steel may be 200 to 500 mm, the inner diameter may be 4000 to 7000 mm, and the outer diameter may be 5000 to 8000 mm. Because ring forging is a process in which final shape and dimension control is more important than void compression, rigid plastic processing is not applied. Therefore, the forging ratio may be 1.0 to 1.6, and more preferably 1.2 to 1.4.

Meanwhile, the strain rate in all forging processes presented above in the present invention may be 1/s to 4/s. At a strain rate of less than 1/s, the temperature of the finish forging may drop and surface cracks may occur. On the other hand, if a high strain rate exceeding 4/s is applied in the non-recrystallized region, surface cracks may be caused due to a decrease in elongation due to excessive local work hardening.

Normalizing heat treatment

Finally, in the present invention, normalizing heat treatment can be performed by heating the forged flange product to a temperature range of 820 to 930°C based on the product center temperature measurement, maintaining it for 5 to 600 minutes, and then air cooling it to room temperature.

During the normalizing heat treatment, if the heating temperature is less than 820°C or the holding time is less than 5 minutes, the re-dissolution of carbides generated during cooling after forging or impurity elements segregated at grain boundaries does not occur smoothly, and the low-temperature toughness of the steel after heat treatment is greatly reduced. may deteriorate. On the other hand, during the normalizing heat treatment, when the heating temperature exceeds 930°C or the holding time exceeds 600 minutes, the ferrite matrix particle size of the ferrite pearlite composite structure exceeds 30㎛ required in the present invention or Nb (C, Strength and low-temperature impact toughness may deteriorate due to coarsening of precipitated phases such as N) and V(C,N).

Meanwhile, in the present invention, it is preferable to normalize heat treat the ring-forged flange material under the condition that the LMP defined by the following relational equation 2 satisfies 20 to 33.

Normalizing heat treatment and holding time can be expressed by the Larson-Miller Parameter equation 2 as follows (Reference: F.R. Larson and J. Miller: Trans. ASME, 1952, vol. 74, pp. 765-75), and pearlite colonies ( In order to satisfy the impact toughness required in the present invention by miniaturizing the size of Coloney, the LMP for normalizing temperature and time conditions may be 20 to 23.

[Relation 2] LMP = T (Logt + 20)x(1/1000)

In equation 2 above, T is the normalizing heat treatment temperature based on Kelvin, t is the heat treatment time, and the exponent of log is set to 10.

If the LMP is less than 20, there is a disadvantage in that material deviation may occur because the austenite is not heated sufficiently to the single phase region or the solute does not spread homogeneously, and if the LMP exceeds 23, the ferrite and pearlite colonies become excessively coarse. Because it is produced, it is difficult to secure the low-temperature impact toughness required in the present invention.

And in the present invention, when welding is performed after the normalizing heat treatment, Post-Weld Heat Treatment, Stress Relieving Heat Treatment, or Tempering heat treatment can be performed. Post-welding heat treatment can be performed within the range of LMP 19.3 or less, as defined by Equation 2 above. If the LMP exceeds 19.3, the grain boundary cementite size increases and exceeds the 5㎛ required in the invention, and therefore impact toughness may be deteriorated. Therefore, when welding, it is desirable that the LMP of subsequent heat treatment is 19.3 or less.

The present invention will be described in detail below through examples. However, it should be noted that the following examples are only for illustrating and explaining the present invention in more detail, and are not intended to limit the scope of the present invention.

(Example)

division C Si Mn Al P S Nb V Ti Cr Mo Cu Ni Ca Ceq Invention Lecture 1 0.14 0.35 1.39 0.03 68 8 0.025 0.023 0.003 0.21 0.08 0.23 0.4 15 0.48 Invention Lecture 2 0.17 0.27 1.43 0.02 55 10 0.011 0.031 0.02 0.08 0.05 0.21 0.28 21 0.47 Invention Lecture 3 0.16 0.31 1.38 0.01 80 13 0.023 0.021 0.005 0.12 0.09 0.08 0.15 18 0.45 Invention Lecture 4 0.13 0.29 1.34 0.03 81 11 0.017 0.08 0.013 0.19 0.1 0.12 0.19 22 0.45 Invention Lecture 5 0.18 0.3 1.18 0.02 69 14 0.007 0.031 0.01 0.27 0.04 0.19 0.25 17 0.47 Comparison lecture 1 0.03 0.33 1.11 0.03 71 6 0.015 0.043 0.009 0.05 0.06 0.12 0.23 19 0.27 Comparison lecture 2 0.14 0.32 0.8 0.05 94 11 0.012 0.023 0.019 0.17 0.08 0.27 0.3 17 0.37 Comparison lecture 3 0.11 0.36 3.54 0.04 49 8 0.031 0.025 0.007 0.12 0.05 0.15 0.3 20 0.77 Comparison lecture 4 0.25 0.29 1.45 0.03 89 13 0.007 0.03 0.012 0.25 0.11 0.43 0.89 18 0.66 Comparison lecture 5 0.17 0.35 1.29 0.05 85 17 0.0008 0.025 0.02 0.2 0.08 0.21 0.39 18 0.49

*In Table 1, the unit of content of the constituent elements is weight%, but the unit of P, S, and Ca is ppm. And the remaining components are Fe and inevitable impurities.

A 700 mm thick cast steel having the alloy components shown in Table 1 above was manufactured. Using this cast steel, a slab is prepared through cooling according to the process conditions in Table 2 below, followed by forging process (reheating and 1st upsetting, bloom forging, reheating-2nd upsetting, 3rd upsetting, reheating and ring forging) and normalizing. After heat treatment, the final 320mmt flange was manufactured. Process conditions satisfying the scope of the present invention were applied to all processes other than those listed in Table 2.

Afterwards, the physical properties of each specimen prepared above were measured and shown in Table 3 below. did. Here, the old austenite crystal grain size and polygonal ferrite (PF) fraction of the surface layer of the slab were measured using an automatic image analyzer by collecting a specimen from the surface layer structure after casting.

Additionally, the ferrite grain size of the steel was also measured using an automatic image analyzer by collecting a specimen from the final steel structure. Meanwhile, in this example, the product microstructure in both the invention example and the comparative example is a mixed structure of ferrite and pearlite.

Additionally, yield/tensile strength was evaluated through a room temperature tensile test, and a 0.2% offset was applied for yield strength. In addition, the impact toughness of each specimen was determined by using the average of the absorbed energy values measured three times at the corresponding temperature through the Charpy V-Notch Test.

In addition, the number of NbC precipitates in the cross section of the steel was measured using TEM. NbC precipitates were confirmed through NbC diffraction patterns and EDX mapping, and the number of NbC precipitates located in 1um ² was counted.

The porosity of the center of the product was measured by measuring the density (g/mm ³ ) and taking the reciprocal (mm ³ /g).

In addition, after visually observing the surface of each specimen, grinding was performed at the point where the surface crack was formed, and the grinding length until the crack disappeared was measured as the surface crack length. In the case of penetrating cracks, the cracks were not limited to the surface layer but penetrated deep into the interior. The cross-section was cut and the total length of the cracks was measured.

division steel grade Slab manufacturing heating and
1st upsetting bloom
minor reheat and
2nd upsetting 3rd upsetting reheat and
ring posing Normalizing
LMP After welding
heat treatment
LMP secondary cooling
temperature
(℃) Cooling
speed
(℃/s) heating
temperature
(℃) forging cost forging cost reheat
Temperature (℃) forging cost forging cost reheat
Temperature (℃) forging cost Invention Example 1 Invention Lecture 1 823 1.3 1237 1.83 1.8 1159 2.01 2.54 1259 1.35 21.15 Not implemented Invention Example 2 Invention Lecture 2 815 1.5 1257 1.75 1.69 1233 198 2.46 1253 1.5 21.09 Not implemented Invention Example 3 Invention Lecture 3 833 1.7 1233 1.92 1.59 1259 2.21 2.61 1240 1.41 22.01 Not implemented Invention Example 4 Invention Lecture 4 841 2.3 1195 1.86 1.83 1260 2.07 2.33 1239 1.39 21.07 Not implemented Invention Example 5 Invention Lecture 5 807 2.8 1291 1.69 1.75 1283 2.15 2.75 1255 1.25 20.58 Not implemented Comparative Example 1 Invention Lecture 1 631 1.6 1286 1.83 1.91 1245 1.88 2.65 1243 1.41 21.14 Not implemented Comparative example 2 Invention Lecture 1 894 1.8 1256 1.81 1.69 1234 1.59 2.56 1254 1.44 21.05 Not implemented Comparative example 3 Invention Lecture 1 825 5.9 1244 1.76 1.88 1193 1.59 2.45 1195 1.09 20.73 Not implemented Comparative Example 4 Invention Lecture 2 823 2.1 1012 1.75 1.75 1208 1.68 2.44 1208 1.17 21.65 Not implemented Comparative Example 5 Invention Lecture 2 840 1.8 1255 2.95 1.68 1211 2.07 2.3 1244 1.23 21.53 Not implemented Comparative Example 6 Invention Lecture 2 841 2.5 1263 1.06 1.95 1246 2.16 2.53 1254 1.34 21.09 Not implemented Comparative example 7 Invention Lecture 3 832 0.9 1237 1.91 1.13 1259 2.15 2.19 1238 1.35 21.68 Not implemented Comparative example 8 Invention Lecture 3 809 1.5 1229 1.93 1.89 1026 1.89 2.28 1198 1.29 22.54 Not implemented Comparative Example 9 Invention Lecture 3 843 1.6 1253 1.69 1.93 1253 1.12 2.54 1259 1.43 20.99 Not implemented Comparative Example 10 Invention Lecture 4 815 2.7 1218 1.59 1.88 1283 2.66 2.63 1186 1.51 20.38 Not implemented Comparative Example 11 Invention Lecture 4 814 1.3 1200 1.62 1.86 1259 1.9 1.32 1206 1.47 21.33 Not implemented Comparative Example 12 Invention Lecture 4 841 2.5 1209 1.83 1.94 1250 2.07 3.05 1255 1.54 22.54 Not implemented Comparative Example 13 Invention Lecture 5 829 2.5 1209 1.84 1.91 1264 1.85 2.75 1336 1.38 20.69 Not implemented Comparative Example 14 Invention Lecture 5 840 1.4 1255 1.66 1.69 1257 1.9 2.55 1249 2.05 21.47 Not implemented Comparative Example 15 Invention Lecture 5 832 2.3 1289 1.73 1.69 1233 2.01 2.53 1256 1.34 27 Not implemented Comparative Example 16 Comparison lecture 1 808 2.3 1283 1.69 1.63 1255 1.89 2.5 1210 1.44 20.76 Not implemented Comparative Example 17 Comparison lecture 2 817 1.9 1272 1.65 1.58 1235 1.94 2.54 1199 1.54 21.57 Not implemented Comparative Example 18 Comparison lecture 3 808 2.3 1283 1.69 1.63 1255 1.89 2.5 1210 1.44 20.76 Not implemented Comparative Example 19 Comparison lecture 4 817 1.9 1272 1.65 1.58 1235 1.94 2.54 1199 1.54 21.57 Not implemented Comparative Example 20 Comparison lecture 5 835 1.8 1243 1.82 1.54 1249 2.11 2.51 1244 1.51 21.07 Not implemented Invention Example 6 Invention Lecture 1 822 1.6 1233 1.82 1.9 1154 2.03 2.52 1254 1.32 21.13 18.1 Invention Example 7 Invention Lecture 1 840 2.1 1254 1.05 1.90 1245 2.10 2.51 1253 1.32 21.05 19.0 Comparative Example 21 Invention Lecture 1 832 2.3 1256 1.69 1.83 1239 2.01 2.40 1233 1.45 20.58 23.7 Comparative example 22 Invention Lecture 2 839 2.0 1280 1.83 1.92 1233 1.94 2.39 1219 1.40 21.02 22.5

division steel grade Slavic product porosity
( ^mm3 /g) surrender
Strength (MPa) Seal
Strength (MPa) -50℃ shock
Absorbed energy (J) Surface crack depth
(mm) Guos
Tenite particle size (㎛) PF
Fraction (%) ferrite
Particle size (㎛) Intergranular cement
tight size
(㎛) Number of precipitates Invention Example 1 Invention Lecture 1 853 16.7 26.5 1.2 24 0.033 435 576 108 Not observed Invention Example 2 Invention Lecture 2 694 17.5 24.3 3.1 31 0.016 424 554 153 Not observed Invention Example 3 Invention Lecture 3 738 18.1 22.4 2.2 25 0.031 439 535 182 Not observed Invention Example 4 Invention Lecture 4 766 15.9 25.6 1.5 41 0.023 429 541 175 Not observed Invention Example 5 Invention Lecture 5 594 16.4 26.1 2.7 29 0.015 440 529 169 Not observed Comparative Example 1 Invention Lecture 1 695 18.3 27.3 3.3 13 0.073 419 543 172 3.6 (surface crack) Comparative example 2 Invention Lecture 1 1273 18.2 40.7 2.2 18 0.065 428 564 21 Not observed Comparative Example 3 Invention Lecture 1 659 4.7 25 2.2 22 0.043 431 535 199 3.7 (surface crack) Comparative example 4 Invention Lecture 2 707 15.9 23.9 3.1 18 0.039 452 564 182 2.8 (surface crack) Comparative Example 5 Invention Lecture 2 810 16.3 21.9 1.5 33 0.025 414 535 176 21.6 (through crack) Comparative Example 6 Invention Lecture 2 905 17.4 22.4 2.0 25 0.237 440 544 11 Not observed Comparative example 7 Invention Lecture 3 865 18.1 23.4 1.0 41 0.196 419 531 33 Not observed Comparative example 8 Invention Lecture 3 889 19 26.9 1.5 38 0.029 428 581 188 2.9 (surface crack) Comparative Example 9 Invention Lecture 3 808 16.9 28.4 1.3 27 0.209 430 601 15 Not observed Comparative Example 10 Invention Lecture 4 891 20.1 29.5 2.1 30 0.007 429 583 168 19.8 (surface crack) Comparative Example 11 Invention Lecture 4 885 18.7 28.4 2.5 41 0.259 428 587 14 Not observed Comparative Example 12 Invention Lecture 4 843 18.3 28.1 2.1 35 0.005 441 532 159 30.7 (through crack) Comparative Example 13 Invention Lecture 5 817 17.8 51.9 1.1 29 0.036 439 522 8 Not observed Comparative Example 14 Invention Lecture 5 826 18.1 26.4 1.9 41 0.061 434 584 159 16.9 (through crack) Comparative Example 15 Invention Lecture 5 841 18.9 50.7 2.3 40 0.054 409 513 10 Not observed Comparative Example 16 Comparison lecture 1 839 17.6 26.5 1.2 19 0.033 258 342 389 Not observed Comparative Example 17 Comparison lecture 2 840 15.4 26.4 1.7 22 0.038 375 486 158 Not observed Comparative Example 18 Comparison lecture 3 868 16.9 28.1 1.8 31 0.045 605 725 7 Not observed Comparative Example 19 Comparison lecture 4 887 16.6 29 1.2 25 0.068 495 587 13 Not observed Comparative Example 20 Comparison lecture 5 870 16.3 28.3 2.4 One 0.043 369 488 253 Not observed Invention Example 6 Invention Lecture 1 901 17.2 24.5 1.5 33 0.037 392 543 255 Not observed Invention Example 7 Invention Lecture 1 694 18.0 25.3 2.3 25 0.019 412 551 198 Not observed Comparative Example 21 Invention Lecture 1 705 17.6 21.4 8.9 33 0.037 382 513 12 Not observed Comparative example 22 Invention Lecture 2 773 15.9 25.2 10.5 25 0.019 402 521 3 Not observed

*The number of precipitates in Table 3 refers to the number of fine NbC or NbCN precipitates with a diameter of 5 to 15 nm per ^1㎛ among the precipitates observed in the cross section of the steel, and the porosity is 3/8t to 5 in the thickness direction from the surface of the steel. It refers to the porosity at the center of the product, which is an area of /8t (where t refers to the steel thickness (mm)).

As can be seen from Table 1-3, in the case of Invention Example 1-7 that satisfies the alloy composition and manufacturing conditions proposed by the present invention, not only has excellent strength and excellent low-temperature impact toughness at -50°C, but also a flange product. It can be seen that good surface quality can be secured in this condition.

On the other hand, Comparative Examples 1-15 and 21-22 are cases where the alloy composition proposed by the present invention is satisfied but the manufacturing conditions are not satisfied, and the old austenite grain size, polygonal ferrite fraction, or polygonal ferrite fraction of the slab proposed by the present invention are satisfied. It can be seen that the strength and low-temperature impact toughness values are low as the characteristics such as center porosity and ferrite grain size in the flange product state are not satisfied. In addition, even if the material is good, if the forging ratio conditions are not met at each stage of forging, poor surface quality characteristics can be confirmed in the product condition due to the occurrence of surface cracks or penetrating cracks.

On the other hand, Comparative Examples 16-20 satisfy the manufacturing conditions proposed by the present invention, but do not satisfy the alloy composition, so it can be seen that the quality level is low, such as excessive strength (low impact toughness) or low strength.

As described above, the detailed description of the present invention has described preferred embodiments of the present invention, but those skilled in the art can make various modifications without departing from the scope of the present invention. Of course this is possible. Therefore, the scope of rights of the present invention should not be limited to the described embodiments, but should be determined not only by the claims described later, but also by their equivalents.

Claims (15)

By weight%, C: 0.05-0.2%, Si: 0.05-0.5%, Mn: 1.0-2.0%, Al: 0.005-0.1%, P: 0.01% or less, S: 0.015% or less, Nb: 0.001-0.07% , V: 0.001~0.3%, Ti: 0.001~0.03%, Cr: 0.01~0.3%, Mo: 0.01~0.12%, Cu: 0.01~0.6%, Ni: 0.05~1.0%, Ca: 0.0005~0.004%, It includes the remaining Fe and other inevitable impurities, and Ceq satisfies the range of 0.35 to 0.55 according to equation 1 below,
It has a thickness of 200~500mm,
It has a steel microstructure consisting of a composite structure of ferrite and pearlite with an average particle size of 30㎛ or less.
The maximum size of cementite present at the ferrite-ferrite and/or ferrite-pearlite grain boundary is 5㎛ or less,
The porosity at the center of the product, which is an area of 3/8t to 5/8t (where t means the steel thickness (mm)) in the thickness direction from the steel surface, is 0.1mm ³ /g or less, and
An extremely thick steel for flanges with excellent strength and low-temperature impact toughness in which there are more than 5 fine NbC or NbCN precipitates with a diameter of 5 to 15 nm ^per 1㎛ among the precipitates observed in the cross section of the steel.
[Relationship 1]
Ceq = [C] + [Mn]/6 + ([Cr] + [Mo] + [V])/5 + ([Ni] + [Cu])/15
In equation 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] are C, Mn, Cr, Mo, V, Ni, and It refers to the Cu content (% by weight), and if these components are not intentionally added, 0 is substituted.
According to claim 1, wherein the steel has a tensile strength of 510 to 690 MPa, a yield strength of 370 MPa or more, and an absorbed energy value in a -50°C Charpy impact test of 50 J or more. An extremely thick steel for a flange with excellent strength and low-temperature impact toughness.
The extremely thick steel material for flanges according to claim 1, wherein the maximum surface crack depth of the steel material is 0.1 mm or less (including 0), and has excellent strength and low-temperature impact toughness.
The extremely thick steel material for flanges with excellent strength and low-temperature impact toughness according to claim 1, wherein the fraction of cementite present in the ferrite-ferrite or ferrite-pearlite grain boundary is 3 area% or less.
By weight%, C: 0.05-0.2%, Si: 0.05-0.5%, Mn: 1.0-2.0%, Al: 0.005-0.1%, P: 0.01% or less, S: 0.015% or less, Nb: 0.001-0.07% , V: 0.001~0.3%, Ti: 0.001~0.03%, Cr: 0.01~0.3%, Mo: 0.01~0.12%, Cu: 0.01~0.6%, Ni: 0.05~1.0%, Ca: 0.0005~0.004%, When manufacturing a slab using molten steel that contains the remaining Fe and other unavoidable impurities and satisfies the Ceq range of 0.35 to 0.55 according to Equation 1 below, the cast steel discharged from the mold is heated to a temperature range of 0.01 to 3 to 800 to 850°C. Manufacturing a slab by secondary cooling at a cooling rate of ℃/s;
Heating the manufactured slab to a temperature range of 1100 to 1300°C and then performing primary upsetting at a forging ratio of 1.3 to 2.4;
Bloom forging at a forging ratio of 1.5 to 2.0 after the first upsetting;
Reheating the bloom forged material to a temperature range of 1100 to 1300°C, then round forging at a forging ratio of 1.65 to 2.25, and then performing secondary upsetting at a forging ratio of 1.3 to 2.3;
Thirdly upsetting the secondly upsetting material at a forging ratio of 2.0 to 2.8 and then processing holes;
Reheating the hole-machined material to a temperature range of 1100 to 1300°C and then ring forging at a forging ratio of 1.0 to 1.6; and
A normalizing heat treatment step of heating the ring-forged material to a temperature range of 820 to 930°C, based on the center temperature measurement, maintaining it for 5 to 600 minutes, and then air cooling it to room temperature. During the normalizing heat treatment, the equation 2 below: A method of manufacturing extremely thick steel for flanges with excellent strength and low-temperature impact toughness by heat treating so that the LMP, defined by , satisfies 20 to 33.
[Relationship 1]
Ceq = [C] + [Mn]/6 + ([Cr] + [Mo] + [V])/5 + ([Ni] + [Cu])/15
In equation 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] are C, Mn, Cr, Mo, V, Ni, and It refers to the Cu content (% by weight), and if these components are not intentionally added, 0 is substituted.
[Relation 2] LMP = T (Logt + 20)x(1/1000)
In equation 2 above, T is the temperature based on Kelvin, t is time, and the exponent of log is 10.
The method of claim 5, wherein the slab is manufactured using a continuous casting process or a semi-continuous casting process. The method of claim 5, wherein after manufacturing the slab, the old austenite crystal grain size of the surface layer of the slab before forging is 1000㎛ or less, and the microstructure is composed of a composite structure of polygonal ferrite of more than 15% and the balance bainite, and the strength and low-temperature impact resistance This excellent method of manufacturing extremely thick steel for flanges.
The method of claim 5, wherein the size of the forged surface punched during the first upsetting is 1000 to 1200 mm x 1800 to 2000 mm when the initial size is 700 mm x 1800 mm. Method for manufacturing extremely thick steel for flanges with excellent strength and low-temperature impact toughness.
The method of claim 5, wherein in the case of bloom forging, the size of the forged surface upon completion of forging is initially 1000 to 1200 mm Manufacturing method.
The method of claim 5, wherein when the round forging and secondary upsetting are completed, the size of the product is 1450 to 1850Ø × 1300 to 1700 mm.
The method of claim 5, wherein when the third upsetting is completed, the size of the product is 2300 to 2800Ø × 400 to 800 mm.
The method of claim 5, wherein the maximum thickness of the flange made of the steel may be 200 to 500 mm, the inner diameter is 4000 to 7000 mm, and the outer diameter is 5000 to 8000 mm. Method for manufacturing extremely thick steel for flanges with excellent strength and low-temperature impact toughness. .
The method of claim 5, wherein when welding is performed on the steel material after the normalizing heat treatment, it further comprises the step of performing Post-Weld Heat Treatment, Stress Relieving Heat Treatment, or Tempering heat treatment. Method for manufacturing extremely thick steel for flanges with excellent strength and low-temperature impact toughness.
The method of claim 5, wherein the heat treatment is performed in a range of LMP 19.3 or less as defined by the relational equation 2.
By weight%, C: 0.05-0.2%, Si: 0.05-0.5%, Mn: 1.0-2.0%, Al: 0.005-0.1%, P: 0.01% or less, S: 0.015% or less, Nb: 0.001-0.07% , V: 0.001~0.3%, Ti: 0.001~0.03%, Cr: 0.01~0.3%, Mo: 0.01~0.12%, Cu: 0.01~0.6%, Ni: 0.05~1.0%, Ca: 0.0005~0.004%, It contains the remaining Fe and other unavoidable impurities, satisfies the range of 0.35 to 0.55 for Ceq according to the following relational equation 1, has a surface old austenite crystal grain size of 1000㎛ or less, and is a composite of polygonal ferrite of more than 15% and the remaining bainite. Preparing a slab with a thickness of 500 mm or more having a fine texture made of tissue;
Heating the prepared slab to a temperature range of 1100 to 1300°C and then performing first upsetting at a forging ratio of 1.3 to 2.4;
Bloom forging at a forging ratio of 1.5 to 2.0 after the first upsetting;
Reheating the bloom forged material to a temperature range of 1100 to 1300°C, then round forging at a forging ratio of 1.65 to 2.25, and then performing secondary upsetting at a forging ratio of 1.3 to 2.3;
Thirdly upsetting the secondly upsetting material at a forging ratio of 2.0 to 2.8 and then processing holes;
Reheating the hole-machined material to a temperature range of 1100 to 1300°C and then ring forging at a forging ratio of 1.0 to 1.6; and
A normalizing heat treatment step of heating the ring-forged material to a temperature range of 820 to 930°C, based on the center temperature measurement, maintaining it for 5 to 600 minutes, and then air cooling it to room temperature, and during the normalizing heat treatment, the following relational equation 2 A method of manufacturing extremely thick steel for flanges with excellent strength and low-temperature impact toughness by heat treating so that the LMP, defined by , satisfies 20 to 33.
[Relationship 1]
Ceq = [C] + [Mn]/6 + ([Cr] + [Mo] + [V])/5 + ([Ni] + [Cu])/15
In equation 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] are C, Mn, Cr, Mo, V, Ni, and It refers to the Cu content (% by weight), and if these components are not intentionally added, 0 is substituted.
[Relation 2] LMP = T (Logt + 20)x(1/1000)
In equation 2 above, T is the temperature based on Kelvin, t is time, and the exponent of log is 10.