EXTREMELY THICK STEEL MATERIAL FOR FLANGE HAVING EXCELLENT STRENGTH AND LOW TEMPERATURE IMPACT TOUGHNESS, AND MANUFACTURING METHOD FOR SAME
20250376747 ยท 2025-12-11
Assignee
Inventors
Cpc classification
C22C38/002
CHEMISTRY; METALLURGY
International classification
B21B1/02
PERFORMING OPERATIONS; TRANSPORTING
Abstract
The present disclosure relates to an extremely thick steel material for a flange having excellent strength and low-temperature impact toughness, and a method of manufacturing the same.
Claims
1. An extremely thick steel material for a flange comprising: in wt %, C: 0.05 to 0.2%, Si: 0.05 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.005 to 0.1%, P: 0.01% or less, S: 0.015% or less, Nb: 0.001 to 0.07%, V: 0.001 to 0.3%, Ti: 0.001 to 0.03%, Cr: 0.01 to 0.3%, Mo: 0.01 to 0.12%, Cu: 0.01 to 0.6%, Ni: 0.05 to 1.0%, Ca: 0.0005 to 0.004%, and a remainder of Fe and other unavoidable impurities, and Ceq thereof according to the following relational expression 1 satisfying a range of 0.35 to 0.55, the extremely thick steel material for a flange, having a thickness of 200 to 500 mm, having a steel microstructure composed of a composite structure of pearlite and ferrite with an average grain size of 30 m or less, having a maximum size of cementite existing in ferrite-ferrite and/or ferrite-pearlite grain boundary, being 5 m or less, having a porosity of 0.1 mm.sup.3/g or less in a central portion of a product, a region of t to t (where t means a steel thickness (mm)) in a thickness direction from a steel surface, and having 5 or more fine NbC or NbCN precipitates with a diameter of 5 to 15 nm, per 1 m.sup.2, among precipitates observed in a cross section of steel,
2. The extremely thick steel material for a flange of 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 of 50 C. Charpy impact test of 50 J or more.
3. The extremely thick steel material for a flange of claim 1, wherein a maximum surface crack depth of the steel is 0.1 mm or less (including 0).
4. The extremely thick steel material for a flange of claim 1, wherein a fraction of the cementite existing in the ferrite-ferrite or ferrite-pearlite grain boundary is 3 area % or less.
5. A method of manufacturing an extremely thick steel material for a flange, comprising: an operation of manufacturing a slab containing, in wt %, C: 0.05 to 0.2%, Si: 0.05 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.005 to 0.1%, P: 0.01% or less, S: 0.015% or less, Nb: 0.001 to 0.07%, V: 0.001 to 0.3%, Ti: 0.001 to 0.03%, Cr: 0.01 to 0.3%, Mo: 0.01 to 0.12%, Cu: 0.01 to 0.6%, Ni: 0.05 to 1.0%, Ca: 0.0005 to 0.004%, and a remainder of Fe and other inevitable impurities, the slab satisfying, Ceq according to the following relational expression 1 being in a range of 0.35 to 0.55, and having a thickness of 500 mm or more; an operation of heating the manufactured slab to a temperature within a range of 1100 to 1300 C., and then performing a first upsetting with a forging ratio of 1.3 to 2.4; an operation of bloom forging with a forging ratio of 1.5 to 2.0 after the first upsetting; an operation of reheating the bloom forged material to a temperature within a range of 1100 to 1300 C., and then performing round forging with a forging ratio of 1.65 to 2.25, and then performing a second upsetting with a forging ratio of 1.3 to 2.3; an operation of performing a third upsetting with a forging ratio of 2.0 to 2.8 on the second upsetting material, and then performing hole processing; an operation of reheating the hole-processed material to a temperature within a range of 1100 to 1300 C., and then ring-forging with a forging ratio of 1.0 to 1.6; and an operation of performing a normalizing heat treatment by heating the ring-forged material to a temperature within a range of 820 to 930 C. based on a temperature measurement standard of a central portion thereof and maintaining the temperature for 5 to 600 minutes and then performing air cooling to room temperature,
6. The method of manufacturing an extremely thick steel material for a flange of claim 5, wherein the slab is manufactured using a continuous casting process or a semi-continuous casting process.
7. The method of manufacturing an extremely thick steel material for a flange of claim 5, wherein after manufacturing the slab, a prior austenite grain size of a surface layer of the slab before forging is 1000 m or less, and a microstructure of the surface layer of the slab before forging is composed of a composite structure of polygonal ferrite of 15% or more and residual bainite.
8. The method of manufacturing an extremely thick steel material for a flange of claim 5, wherein a size of a forging surface punched during the first upsetting is 1000-1200 mm1800-2000 mm when being initially 700 mm1800 mm.
9. The method of manufacturing an extremely thick steel material for a flange of claim 5, wherein in the case of the bloom forging, when forging is completed, a size of a forging surface is 1450-1850 mm2100-2500 mm when being initially 1000-1200 mm1800-2000 mm.
10. The method of manufacturing an extremely thick steel material for a flange of claim 5, wherein when the round forging and the second upsetting are completed, a size of a product is 1450-18501300-1700 mm.
11. The method of manufacturing an extremely thick steel material for a flange of claim 5, wherein when the third upsetting is completed, a size of a product is 2300-2800400-800 mm.
12. The method of manufacturing an extremely thick steel material for a flange of claim 5, wherein the flange made of the steel has a maximum thickness of 200 to 500 mm, an inner diameter of 4000 to 7000 mm, and an outer diameter of 5000 to 8000 mm.
13. The method of manufacturing an extremely thick steel material for a flange of claim 5, wherein during the normalizing heat treatment, a heat treatment is performed such that an LMP defined by the following relational expression 2 satisfies 20 to 33,
14. The method of manufacturing an extremely thick steel material for a flange of claim 5, further comprising an operation of performing a post-weld heat treatment, a stress relieving heat treatment, or a tempering heat treatment, when welding is performed on the steel after the normalizing heat treatment.
15. The method of manufacturing an extremely thick steel material for a flange of claim 14, wherein the post-weld heat treatment is performed in a range where a value defined by the following relational expression 2 is LMP 19.3 or less,
16. A method of manufacturing an extremely thick steel material for a flange, comprising: an operation of manufacturing a slab by second-cooling a cast iron discharged from a mold to a temperature within a range of 800 to 850 C. at a cooling rate of 0.01 to 3 C./s, when manufacturing the slab using molten steel containing, in wt %, C: 0.05 to 0.2%, Si: 0.05 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.005 to 0.1%, P: 0.01% or less, S: 0.015% or less, Nb: 0.001 to 0.07%, V: 0.001 to 0.3%, Ti: 0.001 to 0.03%, Cr: 0.01 to 0.3%, Mo: 0.01 to 0.12%, Cu: 0.01 to 0.6%, Ni: 0.05 to 1.0%, Ca: 0.0005 to 0.004%, and a remainder of Fe and other inevitable impurities, Ceq thereof according to the following relational expression 1 satisfying a range of 0.35 to 0.55; an operation of heating the manufactured slab to a temperature within a range of 1100 to 1300 C., and then performing a first upsetting with a forging ratio of 1.3 to 2.4; an operation of bloom-forging with a forging ratio of 1.5 to 2.0 after the first upsetting; an operation of reheating the bloom-forged material to a temperature within a range of 1100 to 1300 C., and then performing round forging with a forging ratio of 1.65 to 2.25, and then performing a second upsetting with a forging ratio of 1.3 to 2.3; an operation of performing a third upsetting of the second-upsetting material with a forging ratio of 2.0 to 2.8, and then performing hole processing; an operation of reheating the hole-processed material to a temperature within a range of 1100 to 1300 C., and then performing ring forging with a forging ratio of 1.0 to 1.6; and an operation of performing a normalizing heat treatment by heating the ring-forged material to a temperature within a range of 820 to 930 C. based on a temperature measurement standard of a central portion thereof and maintaining the temperature for 5 to 600 minutes and then air-cooling to room temperature,
17. The method of manufacturing an extremely thick steel material having excellent strength and low-temperature impact toughness for a flange of claim 16, wherein in the normalizing heat treatment, a heat treatment is performed so that an LMP defined by the following relational expression 2 satisfies 20 to 33,
Description
BEST MODE FOR INVENTION
[0061] The present disclosure relates to an extremely thick steel for flanges having excellent strength and low-temperature impact toughness and a method of manufacturing a product. Hereinafter, preferred embodiments of the present disclosure will be described. The embodiments of the present disclosure may be modified in various forms, and the scope of the present disclosure should not be construed as being limited to the embodiments described below. These implementation examples are provided to further detail the present disclosure to a person having ordinary knowledge in the technical field to which the present disclosure belongs.
[0062] Hereinafter, extremely thick steel for a flange of the present disclosure will be described in more detail.
[0063] An extra heavy steel material for a flange of the present disclosure includes, in wt %, C: 0.05 to 0.2%, Si: 0.05 to 0.5%, Mn: 1.0 to 2.0%, Al: 0.005 to 0.1%, P: 0.01% or less, S: 0.015% or less, Nb: 0.001 to 0.07%, V: 0.001 to 0.3%, Ti: 0.001 to 0.03%, Cr: 0.01 to 0.3%, Mo: 0.01 to 0.12%, Cu: 0.01 to 0.6%, Ni: 0.05 to 1.0%, Ca: 0.0005 to 0.004%, the remainder of Fe and other unavoidable impurities, the extremely thick steel material for a flange having a Ceq according to the relational expression 1 satisfying a range of 0.35 to 0.55, having a thickness of 200 to 500 mm, having a steel microstructure composed of a composite structure of ferrite and pearlite with an average grain size of 30 m or less, having a maximum size of cementite existing in a ferrite-ferrite and/or ferrite-pearlite grain boundary, being 5 m or less, having a porosity of 0.1 mm.sup.3/g or less in a central portion of a product, which is a region of t to t (where t represents the steel thickness (mm)) in the thickness direction from the steel surface, and having five or more fine NbC or NbCN precipitates with a diameter of 5 to 15 nm per 1 m.sup.2, among the precipitates observed in the cross section of the steel.
[0064] Hereinafter, the alloy composition of the present disclosure will be described in more detail, and unless otherwise specifically indicated, the % and ppm described in relation to the alloy composition are based on weight.
Carbon (C): 0.05-0.20%
[0065] Carbon (C) is the most important element for securing basic strength, and thus it needs to be contained in steel within an appropriate range, and to obtain this addition effect, 0.05% or more of carbon (C) may be added. Preferably, 0.10% or more of carbon (C) may be added. On the other hand, if the content of carbon (C) exceeds a certain level, the fraction of pearlite increases during normalizing heat treatment, which may excessively exceed the strength and hardness of the base material, resulting in surface cracks during a forging process and deterioration of the low-temperature impact toughness and lamellar tearing resistance characteristics in the final product. Therefore, the present disclosure may limit the carbon (C) content to 0.20%, and the upper limit of the more desirable carbon (C) content may be 0.18%.
Silicon (Si): 0.05-0.50%
[0066] Silicon (Si) is a substitutional element that
[0067] improves the strength of steel through solid solution strengthening and has a strong deoxidation effect, and thus is an essential element for manufacturing clean steel. Therefore, silicon (Si) may be added at 0.05% or more, and more preferably, may be added at 0.20% or more. On the other hand, if silicon (Si) is added in a large amount, a Martensite-Austenite (MA) phase is generated and the strength of the ferrite matrix excessively increases, which may deteriorate the surface quality of the ultra-thick product, and thus the upper limit of the content may be limited to 0.50%. A more preferable upper limit of the silicon (Si) content may be 0.40%.
Manganese (Mn): 1.0-2.0%
[0068] Manganese (Mn) is a useful element that improves strength by solid solution strengthening and enhances hardenability to generate a low-temperature transformation phase. Therefore, to secure a tensile strength of 550 MPa or more, it is preferable to add 1.0% or more of manganese (Mn). A more preferable manganese (Mn) content may be 1.1% or more. On the other hand, manganese (Mn) forms MnS, a non-metallic inclusion that is elongated with sulfur (S), and reduces toughness and may act as an impact initiation point, and may thus be a factor that rapidly reduces the low-temperature impact toughness of the product. Therefore, it is preferable to manage the manganese (Mn) content to 2.0% or less, and a more desirable manganese (Mn) content may be 1.5% or less.
Aluminum (Al): 0.005-0.1%
[0069] Aluminum (Al) is one of the powerful deoxidizers in the steelmaking process along with silicon (Si), and it is preferable to add 0.005% or more to obtain this effect. The lower limit of the more desirable aluminum (Al) content may be 0.01%. On the other hand, if the aluminum (Al) content is excessive, the fraction of Al.sub.2O.sub.3 among the oxidizing inclusions generated as a result of deoxidation increases excessively, and the size thereof becomes coarse, causing a problem in which it is difficult to remove the inclusions during refining, which may be a factor that reduces the low-temperature impact toughness. Therefore, it is preferable 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%)
[0070] Phosphorus (P) and sulfur(S) are elements that cause brittleness at grain boundaries or form coarse inclusions to cause brittleness. Therefore, to improve brittle crack propagation resistance, it is preferable to limit phosphorus (P) to 0.010% or less and sulfur(S) to 0.0015% or less.
Niobium (Nb): 0.001-0.07%
[0071] 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 during high-temperature reheating is significantly finely precipitated in the form of NbC during rolling, which inhibits recrystallization of austenite, thus having the effect of refining the structure. Therefore, it is preferable that niobium (Nb) be 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, if niobium (Nb) is added excessively, undissolved niobium (Nb) is generated in the form of TiNb (C, N), which becomes a factor that inhibits low-temperature impact toughness, and thus it is preferable that the upper limit of the niobium (Nb) content be limited to 0.07%. A more desirable niobium (Nb) content may be 0.065% or less.
Vanadium (V): 0.001-0.3%
[0072] Since vanadium (V) is almost completely reused during reheating, the strengthening effect by precipitation or solid solution during subsequent rolling is minimal, but in the case of extremely thick forged products, since the air cooling speed is very slow, it has the effect of improving the strength by precipitating as very fine carbonitrides during the cooling process or additional heat treatment process. To sufficiently obtain this effect, it is necessary to add vanadium (V) of 0.001% or more. The lower limit of the more desirable vanadium (V) content may be 0.01%. On the other hand, if the content is excessive, the slab surface hardness may be excessively increased due to the high hardenability, which may act as a factor such as surface cracks or the like during flange and the manufacturing cost may also increase rapidly, which is not commercially advantageous. Therefore, the vanadium (V) content may be limited to 0.3% or less. The more desirable vanadium (V) content may be 0.25% or less.
Titanium (Ti): 0.001-0.03%
[0073] Titanium (Ti) is a component that significantly improves low-temperature toughness by precipitating as TiN during reheating and inhibiting the growth of prior austenite grains at high temperatures. To obtain this effect, it is preferable to add 0.001% or more of titanium (Ti). On the other hand, if titanium (Ti) is added excessively, low-temperature toughness may decrease due to clogging of the casting nozzle may occur or low-temperature toughness may decrease due to central crystallization. In addition, titanium (Ti) combines with nitrogen (N) to form coarse TiN precipitates at the thickness center, which reduces the elongation of the product, thereby reducing the uniform elongation during the forging process and causing surface cracks. Therefore, the titanium (Ti) content may be 0.03% or less. The preferred titanium (Ti) content may be 0.025% or less, and the more preferred titanium (Ti) content may be 0.018% or less.
Chromium (Cr): 0.01-0.30%
[0074] Chromium (Cr) is a component that increases the yield strength and tensile strength by increasing the hardenability and forming a low-temperature transformation structure. It is also a component that has the effect of preventing the decrease in strength by slowing down the spheroidization rate of cementite. For this effect, 0.01% or more of chromium (Cr) may be added. On the other hand, if the chromium (Cr) content is excessive, the size and fraction of Cr-rich coarse carbides such as M.sub.23C.sub.6 or the like increase, which reduces the impact toughness of the product, and the solubility of niobium (Nb) in the product and the fraction of fine precipitates such as NbC decrease, which may cause a decrease in the strength of the product. Therefore, the upper limit of the chromium (Cr) content in the present disclosure may be limited to 0.30%. The preferable upper limit of the chromium (Cr) content may be 0.25%.
[0075] Molybdenum (Mo): 0.01-0.12%
[0076] Molybdenum (Mo) is an element that increases grain boundary strength and has a large effect of solid solution strengthening in ferrite, and is an element that effectively contributes to increasing the strength and ductility of the product. In addition, molybdenum (Mo) has the effect of preventing the deterioration of toughness due to grain boundary segregation of impurity elements such as phosphorus
[0077] (P) or the like. For this effect, 0.10% or more of molybdenum (Mo) may be added. However, molybdenum (Mo) is an expensive element, and if added excessively, the manufacturing cost may increase significantly, and thus the upper limit of the molybdenum (Mo) content may be limited to 0.12%.
[0078] Copper (Cu): 0.01-0.60%
[0079] Copper (Cu) may significantly improve the strength of the matrix phase by solid solution strengthening in ferrite, and also has the effect of suppressing corrosion in a wet hydrogen sulfide atmosphere, and thus is an advantageous element in the present disclosure. For this effect, 0.01% or more of copper (Cu) may be included. A more preferable copper (Cu) content may be 0.03% or more. However, if the copper (Cu) content is excessive, the possibility of causing star cracks on the surface of the steel plate increases, and since copper (Cu) is an expensive element, there may be a problem of significantly increasing the manufacturing cost. Therefore, the present disclosure may limit the upper limit of the 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%
[0080] Nickel (Ni) is an element that effectively contributes to improving impact toughness and improving strength by easily providing cross-slip of dislocations by increasing stacking faults at low temperatures and improving hardenability. For this effect, 0.05% or more of nickel (Ni) may be added. The desirable nickel (Ni) content may be 0.10% or more. On the other hand, if nickel (Ni) is added excessively, it may increase the manufacturing cost due to its high cost, so the upper limit of the nickel (Ni) content may be limited to 1.00%. The upper limit of the desirable nickel (Ni) content may be 0.80%.
Calcium (Ca): 0.0005-0.0040%
[0081] When calcium (Ca) is added after deoxidation by aluminum (Al), it combines with sulfur (S) forming MnS inclusions to suppress the formation of MnS, and at the same time, it forms spherical CaS to suppress the occurrence of cracks due to hydrogen-induced cracking. To sufficiently form sulfur (S) contained as an impurity into CaS, it is preferable to add 0.0005% or more of calcium (Ca). However, if the amount added is excessive, the calcium (Ca) remaining after forming CaS combines with oxygen (O) to generate coarse oxidized inclusions, which may be elongated and destroyed during rolling, thereby deteriorating the lamellar tearing characteristics. Therefore, the upper limit of the calcium (Ca) content may be limited to 0.0040%.
Relational Expression 1
[0082] In the present disclosure, it is required that Ceq by the following relational expression 1 satisfies the range of 0.35 to 0.55. If Ceq by the following relational expression 1 is less than 0.35, the pearlite fraction decreases, so that the tensile strength value of 510 to 690 MPa required by the present disclosure cannot be secured, and if it exceeds 0.55, the pearlite fraction exceeds 30%, so that it is not easy to secure the 50 C. low-temperature impact energy value. Therefore, in the present disclosure, it is preferable to limit Ceq to the range of 0.35 to 0.55.
[0083] In the relational expression 1, [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] represent the contents (weight %) of C, Mn, Cr, Mo, V, Ni and Cu contained in the steel, respectively, and if these components are not intentionally added, 0 is substituted.
[0084] An extremely thick steel material for a flange of the present disclosure, having excellent strength and low-temperature impact toughness, and a product thereof may contain the remaining Fe and other unavoidable impurities in addition to the aforementioned components. However, since unintended impurities may inevitably be mixed in during a normal manufacturing process from raw materials or the surrounding environment, they cannot be completely excluded. Since these impurities are known to anyone with ordinary knowledge in the art, not all of their contents are specifically mentioned in this specification. In addition, the addition of additional effective components other than the aforementioned components is not completely excluded.
[0085] Meanwhile, the extremely thick steel material of the present disclosure has a steel microstructure composed of a composite structure of pearlite and ferrite with an average grain size of 30 m or less. If the average grain size of the ferrite exceeds 30 m, the length of the crack path during impact fracture becomes shorter and the Ductile Brittle Transition Temperature (DBTT) increases, thereby deteriorating the low-temperature impact toughness. Therefore, it is appropriate that the grain size of the ferrite is 30 m or less.
[0086] In addition, it is preferable that a maximum size of the cementite existing at the grain boundary of the microstructure (the grain boundary between ferrite-ferrite and/or ferrite-pearlite) is 5 m or less. If the maximum size of the cementite exceeds 5 m, the coarse cementite may act as an impact initiation point, so the impact toughness deteriorates and the grain boundary strength decreases, and accordingly, intergranular fracture easily occurs, thereby deteriorating the impact toughness. Therefore, it is preferable that the maximum size of cementite be 5 m or less.
[0087] More preferably, the fraction of cementite existing in the grain boundary is controlled to 3 area % or less.
[0088] And the extremely thick steel material of the present disclosure has a porosity of 0.1 mm.sup.3/g or less in the central portion of the product, which is a region of t to t (where t means the steel thickness (mm)) in the thickness direction from the steel surface.
[0089] In addition, it is preferable that the extremely thick steel material of the present disclosure has 5 or more fine NbC or NbCN precipitates with a diameter of 5 to 15 nm among the precipitates observed in the cross section of the steel, per 1 um.sup.2. If the number of the fine precipitates is less than 5, the precipitation strengthening effect is weakened, and there may be a problem in securing the properties required in the present disclosure.
[0090] In addition, the extremely thick heavy steel material of the present disclosure may have a thickness of 200 to 500 mm.
[0091] In addition, the extremely thick steel material of the present disclosure may have a tensile strength of 510 to 690 MPa, a yield strength of 370 MPa or more, and an absorption energy value of 50 J or more in 50 C. Charpy impact test.
[0092] And a maximum surface crack depth of the steel material may be 0.1 mm or less (including 0).
[0093] Next, a method of manufacturing an extremely thick steel material for a flange according to another aspect of the present disclosure will be described in detail.
[0094] A method of manufacturing an extremely thick steel material of the present disclosure includes, in manufacturing a slab by using molten steel having the composition as described an above, operation of manufacturing a slab by second cooling the cast iron discharged from a mold to a temperature within a range of 800 to 850 C. at a cooling rate of 0.01 to 3 C./s; an operation of heating the manufactured slab to a temperature within a range of 1100 to 1300 C., and then performing a first upsetting with a forging ratio of 1.3 to 2.4; an operation of bloom forging with a forging ratio of 1.5 to 2.0 after the first upsetting; an operation of reheating the bloom-forged material to a temperature within a range of 1100 to 1300 C., and then performing round forging with a forging ratio of 1.65 to 2.25, and then performing a second upsetting with a forging ratio of 1.3 to 2.3; an operation of performing a third upsetting of the second-upset material with a forging ratio of 2.0 to 2.8, and then performing hole processing; an operation of reheating the hole-processed material to a temperature within a range of 1100 to 1300 C., and then performing ring forging with a forging ratio of 1.0 to 1.6; and an operation of performing a normalizing heat treatment by heating the ring-forged material to a temperature within a range of 820 to 930 C. based on the temperature measurement standard of a central portion thereof, maintaining the temperature for 5 to 600 minutes, and then air cooling the same to room temperature.
Slab Preparation
[0095] First, in the present disclosure, a slab is manufactured.
[0096] Preferably, when manufacturing a slab using molten steel having the composition as described above, the cast iron discharged from the mold is secondarily cooled at a cooling rate of 0.01 to 3 C./s to a temperature within a range of 800 to 850 C., thereby manufacturing the slab.
[0097] The inventor of the present disclosure has conducted in-depth research on a method of manufacturing an extremely thick steel material having properties suitable for flanges while also having excellent strength, impact toughness, and surface quality, and in particular, to secure the strength, toughness, and surface quality of the final flange product in a slab manufactured with a thickness of 500 mm or more, it was recognized that it is necessary to control the carbon equivalent (Ceq) of the slab within a certain range, and the grain size and microstructure fraction of the prior austenite of the slab surface layer are also effective conditions, which led to the derivation of the present invention.
[0098] Since the casting speed of a large-section casting machine that manufactures slabs with a thickness of 650 mm or more is 0.06 to 0.1 m/min, the casting process is performed at a significantly slower speed than that of a general casting machine (casting speed: 0.4 to 1.5 m/min) that manufactures slabs with a thickness of 250 to 400 mm. Therefore, when manufacturing slabs with a thickness of 500 mm or more, the time maintained in the mold is relatively long, and thus an environment is created where austenite may grow more coarsely.
[0099] As the initial austenite grain size increases, the manganese (Mn) segregation index of the austenite grain boundary increases, and the grain boundary strength decreases while the hardenability increases at the same time, so the fraction of hard bainite and martensite, rather than soft ferrite and pearlite, increases in the surface layer of the slab. Since the hard structure has low uniform elongation, intergranular cracking may 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 cracking on the slab surface may occur more actively, and the depth of crack intrusion may further increase during subsequent high-strain processes such as forging, rolling and the like. Therefore, to suppress surface cracking of the final product, it is very important to control the prior austenite grain size to an appropriate level or less and secure the ratio of the soft intergranular polygonal ferrite to an appropriate level or more.
[0100] For example, in the present disclosure, it is preferable that the prior austenite grain size of the slab surface layer is 1000 m or less, and its microstructure is composed of a composite structure of polygonal ferrite of 15 area % or more and the remainder bainite.
[0101] To reduce the prior austenite grain size and secure a polygonal ferrite fraction of 15 area % or more, there is a method of designing the components of carbon (C), nickel (Ni), chromium (Cr), and molybdenum (Mo), which have a solute dragging effect or a pinning effect, to be high. However, when the components of carbon (C), nickel (Ni), chromium (Cr), and molybdenum (Mo) are high, the carbon equivalent (Ceq) also increases, which may cause low-temperature transformation structure to be generated during the cooling process of the slab. Therefore, the present disclosure may limit the carbon equivalent (Ceq) of a steel slab to 0.55 or less according to the following relational expression 1. The preferred carbon equivalent (Ceq) may be 0.4 to 0.53.
[0102] In the relational expression 1, [C], [Mn], [Cr], [Mo], [V], [Ni] and [Cu] represent the contents (in weight %) of C, Mn, Cr, Mo, V, Ni and Cu contained in the steel, respectively, and 0 is substituted if these components are not intentionally added.
[0103] Meanwhile, when manufacturing a slab from molten steel using a continuous casting process or a semi-continuous casting process, water cooling and then air cooling performed from immediately after coming out of the mold at a casting speed of 0.06 to 0.1 m/min until the slab surface layer temperature of 800 to 850 C. at a cooling rate of 0.01 to 3 C./s during second cooling. If the surface layer cooling speed is too slow, the austenite grain growth continues, so AGS of the surface layer cannot be finely controlled, and if it exceeds 3 C./s, surface layer hard phase formation and surface layer-interior temperature gradient increase may cause surface cracks during the cooling process. If the target temperature is also less than 800 C., a local low-temperature transformation structure may be formed, and if it exceeds 850 C., it is also difficult to control the AGS to 1000 m or less as required in the present disclosure, and thus appropriate surface quality cannot be secured.
Heating and First Upsetting
[0104] Next, in the present disclosure, the manufactured slab is heated to a temperature within a range of 1100-1300 C., and then first upsetting is performed at a forging ratio of 1.3-2.4.
[0105] The manufactured slab may be heated in a temperature within a range of 1100-1300 C. As described above, the thickness of the slab may be 500 mm or more, and the preferred thickness may be 700 mm or more.
[0106] It is necessary to heat the slab within a certain temperature range or more to re-dissolve the composite carbonitride of titanium (Ti) or niobium (Nb), the coarse crystallites of TiNb (C, N) formed during casting, or the like. In addition, it is preferable to homogenize the structure by heating the slab before the first upsetting forging to the recrystallization temperature or more and maintaining the same, and to heat the slab within a certain temperature range or more to secure a sufficiently high forging end temperature to minimize surface cracks that may occur during the forging process. Therefore, it is preferable that the slab heating of the present disclosure is performed in a temperature within a range of 1100 C. or higher.
[0107] On the other hand, if the slab heating temperature is excessively high, high-temperature oxide scale may occur excessively, and the manufacturing cost may increase excessively due to high-temperature heating and maintenance. Therefore, it is preferable that the slab heating of the present disclosure is performed in a range of 1300 C. or lower.
[0108] Meanwhile, upsetting is a method of performing strong plastic deformation vertically with the longitudinal axis as the axis, and the forging ratio at the time of the first upsetting is appropriately 1.3 to 2.4, and preferably 1.5 to 2.0. In this case, the forging ratio refers to the ratio of the cross-sectional area changed by forging. At the time of this first upsetting, if the size of the forging surface to be punched is initially 700 mm1800 mm, it may be 1000 to 1200 mm1800 to 2000 mm.
[0109] If the forging ratio is less than 1.3 during the first upsetting, it is difficult to sufficiently pressurize the porosity remaining in the center of the slab. Therefore, it is difficult to control the porosity required in the final product of the present disclosure to an appropriate level of 0.1 mm.sup.3/g or less, so it is not easy to secure the low-temperature impact toughness in the center. On the other hand, if the forging ratio exceeds 2.4 during the first upsetting, buckling occurs during the forging process, so it is not easy to obtain the surface quality and appropriate shape control required in the flange product. Therefore, the forging ratio is appropriately 1.3 to 2.4 during the first upsetting.
Bloom Forging (Upper and Lower Two-Sided Forging)
[0110] And in the present disclosure, bloom forging is performed on the first upsetting material with a forging ratio of 1.5 to 2.0.
[0111] Bloom forging is a method of further compressing the first upsetting material into a bloom shape, and is a method of expanding the area while processing both the upper and lower surfaces in a certain direction of width or length. In the case of the bloom forging, the size of the forged surface when forging is completed may be 1450 to 1850 mm2100 to 2500 mm if it is initially 1000 to 1200 mm1800 to 2000 mm. In the case of bloom forging, the forging ratio is appropriately 1.5 to 2.0. If the forging ratio is less than 1.5, it is difficult to secure the appropriate pore quality required in the present disclosure, like in upsetting forging, and if it exceeds 2.0, surface cracks may occur.
[0112] Forging may be performed in both the longitudinal and transverse directions, but in the longitudinal direction, since the casting structure is more densely structured, the elongation of the surface layer structure is high, which may lead to excellent workability. Therefore, longitudinal bloom forging may be more appropriate than transverse bloom forging in terms of surface cracks.
Reheating and Round ForgingSecond Upsetting
[0113] In the present disclosure, the bloom-forged
[0114] material is reheated to a temperature within a range of 1100 to 1300 C., then round forged at a forging ratio of 1.65 to 2.25, and then second upsetting is performed at a forging ratio of 1.3 to 2.3.
[0115] When the bloom forging is completed, the bloom surface layer temperature is 950 C. or lower, and if processing continues, surface cracks or material fracture may occur. Therefore, the material may be heated again to a temperature within a range of 1100 to 1300 C. after bloom forging. As mentioned above, it is preferable to heat to 1100 C. or higher for reasons such as re-dissolution of the crystallized material, homogenization of the structure, prevention of surface cracks and the like, and it is preferable to control to 1300 C. or lower due to problems such as excessive scale, grain coarsening, and the like.
[0116] In the case of a bloom after heating is completed, round forging is performed to process the flange edge into a circular shape, and then second upsetting is applied again. When the round forging and second upsetting are completed, the size of the product may be 1450-18501300-1700 mm. The forging ratio for round forging and second upsetting may be 1.65-2.25 and 1.3-2.3, respectively. If the forging ratio is lower than the level required in the present disclosure during round forging and second upsetting, it is difficult to control the center porosity in the final product to 0.1 mm.sup.3/g or less, so it is not easy to secure the center low-temperature impact toughness, and if the forging ratio standard is exceeded, the desired processed shape of the flange product may not be obtained due to problems such as buckling and surface cracks, shape defects and the like.
[0117] After the second upsetting is completed, round forging may be applied again for shape control, and then heating may be performed under the same conditions as the aforementioned reheating temperature.
Third Upsetting and Hole Processing
[0118] And in the present disclosure, the material that has been second upset is upset 3rd with a forging ratio of 2.0 to 2.8, and then a hole is processed.
[0119] The material processed into the cylindrical shape may 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 2300-2800400-800 mm. The forging ratio of the third upsetting may be 2.0-2.8, and if the forging ratio is insufficient or exceeded, problems such as residual gap pore control, surface cracks/shape control failure and the like as mentioned above may occur. After the third upsetting is completed, a hole may be made in the center of the material using a 500-1000 punch.
Reheating and Ring Forging
[0120] Subsequently, in the present disclosure, the material with the hole processed is reheated to a temperature within a range of 1100 to 1300 C., and then ring forged with a forging ratio of 1.0 to 1.6.
[0121] The material with the hole processed is reheated to the temperature within a range of 1100 to 1300 C. mentioned above, and may then be processed into a final flange ring shape. A 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. Since ring forging is a process in which final shape and dimension control are more important than pore compression, strong plastic processing is not applied. Therefore, the forging ratio may be 1.0 to 1.6, and more preferably, may be 1.2 to 1.4.
[0122] Meanwhile, the strain rate in all the forging processes presented in the present disclosure may be 1/s to 4/s. At a strain rate of less than 1/s, the temperature of the finishing forging may decrease, which may cause possibility of surface cracks. On the other hand, when a high strain rate exceeding 4/s is applied in the non-recrystallized region, surface cracks may be induced due to a decrease in elongation caused by excessive local work hardening.
Normalizing Heat Treatment
[0123] Lastly, in the present disclosure, a normalizing heat treatment may be performed by heating the flange product, which has completed the forging, to a temperature within a range of 820 to 930 C. based on the temperature measurement standard of the central portion of the product, maintaining the temperature for 5 to 600 minutes, and then air cooling to room temperature.
[0124] During the normalizing heat treatment, if the heating temperature is lower than 820 C. or the maintaining time is lower than 5 minutes, the carbides generated during cooling after forging or the impurity elements segregated at the grain boundaries do not re-dissolve smoothly, so that the low-temperature toughness of the steel after the heat treatment may be significantly reduced. On the other hand, when the heating temperature exceeds 930 C. or the holding time exceeds 600 minutes during the normalizing heat treatment, the ferrite matrix phase grain size of the ferrite-pearlite composite structure may exceed 30 m required in the present disclosure or the strength and low-temperature impact toughness may deteriorate due to the coarsening of precipitated phases such as Nb(C, N), V(C, N) and the like.
[0125] Meanwhile, in the present disclosure, it is preferable to perform normalizing heat treatment on the ring-forged flange material under the condition that LMP defined by the following relational expression 2 satisfies 20 to 33.
[0126] The normalizing heat treatment and holding time may be expressed by the Larson-Miller Parameter Equation 2 (Literature: F.R. Larson and J. Miller: Trans. ASME, 1952, vol. 74, pp. 765-75) as follows, and the LMP for the normalizing temperature and time conditions may be 20 to 23 to satisfy the impact toughness required in the present disclosure by refining the size of pearlite colonies.
[0127] In the relational expression 2, T is the normalizing heat treatment temperature in Kelvin, t is the heat treatment time, and the log exponent is 10
[0128] If LMP is less than 20, there is a disadvantage that the material may not be sufficiently heated to the austenite single-phase region or the diffusion of the solute does not occur uniformly, in material deviation, and if the LMP exceeds 23, the ferrite and pearlite colonies are formed too coarsely, and thus it is difficult to secure the low-temperature impact toughness required by the present disclosure.
[0129] And in the present disclosure, if welding is
[0130] performed after the normalizing heat treatment, post-weld heat treatment or stress-relieving heat treatment or tempering heat treatment may be performed. This post-weld heat treatment may be performed in a range where the value defined by the relational expression 2 is LMP 19.3 or less. If LMP exceeds 19.3, the grain boundary cementite size increases and exceeds 5 m required in the present disclosure, and thus the impact toughness may deteriorate. Therefore, when welding is performed, it is preferable that the LMP of the subsequent heat treatment be 19.3 or less.
Mode for Invention
[0131] The present disclosure will be described in detail through examples below. However, it should be noted that the following examples are only intended to illustrate the present disclosure in more detail and are not intended to limit the scope of the rights of the present disclosure.
EXAMPLE
TABLE-US-00001 TABLE 1 Classification C Si Mn Al P S Nb V Ti Cr Mo Cu Ni Ca Ceq Invention 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 steel 1 Invention 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 steel 2 Invention 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 steel 3 Invention 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 steel 4 Invention 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 steel 5 Comparison 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 steel 1 Comparison 0.14 0.32 0.8 0.05 91 11 0.012 0.023 0.019 0.17 0.08 0.27 0.3 17 0.37 steel 2 Comparison 0.11 0.36 3.51 0.04 49 8 0.031 0.025 0.007 0.12 0.05 0.15 0.3 20 0.77 steel 3 Comparison 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 steel 4 Comparison 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 steel 5 * In Table 1 above, 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 unavoidable impurities.
[0132] A 700 mm thick cast steel having the alloy composition of Table 1 above was manufactured. Using this cast steel, a slab was prepared by cooling according to the process conditions of Table 2 below, and then a final 320 mmt flange was manufactured through a forging process (reheating and 1st upsetting, bloom forging, reheating-2nd upsetting, 3rd upsetting, reheating and ring forging) and normalizing heat treatment. Process conditions satisfying the range of the present disclosure were applied to all processes other than the processes described in Table 2.
[0133] Thereafter, the values of physical properties of the manufactured respective specimens were measured and are illustrated in Table 3 below. In this case, the prior austenite grain size and polygonal ferrite (PF) fraction of the slab surface layer were measured using an image auto-analyzer by collecting a specimen from the surface layer structure after casting.
[0134] And the ferrite grain size of the steel was also
[0135] measured using an image auto-analyzer by collecting a specimen from the final steel structure. Meanwhile, in both the invention examples and comparative examples, the product microstructure was a mixed structure of ferrite and pearlite.
[0136] In addition, the yield/tensile strength was evaluated through a room temperature tensile test, and a 0.2% offset was applied for the yield strength. In addition, the impact toughness for each specimen used the average of the absorbed energy values measured three times at each temperature by the Charpy V-Notch Test was used.
[0137] In addition, the number and the like of NbC precipitates in the cross section of the steel was measured using TEM. The NbC precipitates were confirmed through the diffraction pattern of NbC and EDX mapping, and the number of NbC precipitates located at 1 m.sup.2 was counted.
[0138] The porosity at the central portion of the product was measured by measuring the density (g/mm.sup.3) and taking the reciprocal (mm.sup.3/g).
[0139] 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 a penetrating crack, the crack is not limited to the surface layer portion, but penetrates deep into the interior, and the total length of the crack introduced was measured by cutting the cross-section.
TABLE-US-00002 TABLE 2 Slab Heating and Reheating and manufacturing 1st upsetting Bloom 2nd upsetting 2nd cooling Cooling Heating forging Reheating Steel temperature speed temperature Forging Forging temperature Classification grade ( C.) ( C./s) ( C.) ratio ratio ( C.) Inventive Invention 823 1.3 1237 1.83 1.8 1159 Example 1 steel 1 Inventive Invention 815 1.5 1257 1.75 1.69 1233 Example 2 steel 2 Inventive Invention 833 1.7 1233 1.92 1.59 1259 Example 3 steel 3 Inventive Invention 841 2.3 1195 1.86 1.83 1260 Example 4 steel 4 Inventive Invention 807 2.8 1291 1.69 1.75 1283 Example 5 steel 5 Comparative Invention 631 1.6 1286 1.83 1.91 1245 Example 1 steel 1 Comparative Invention 894 1.8 1256 1.81 1.69 1234 Example 2 steel 1 Comparative Invention 825 5.9 1244 1.76 1.88 1193 Example 3 steel 1 Comparative Invention 823 2.1 1012 1.75 1.75 1208 Example 4 steel 2 Comparative Invention 840 1.8 1255 2.95 1.68 1211 Example 5 steel 2 Comparative Invention 841 2.5 1263 1.06 1.95 1246 Example 6 steel 2 Comparative Invention 832 0.9 1237 1.91 1.13 1259 Example 7 steel 3 Comparative Invention 809 1.5 1229 1.93 1.89 1026 Example 8 steel 3 Comparative Invention 843 1.6 1253 1.69 1.93 1253 Example 9 steel 3 Comparative Invention 815 2.7 1218 1.59 1.88 1283 Example 10 steel 4 Comparative Invention 814 1.3 1200 1.62 1.86 1259 Example 11 steel 4 Comparative Invention 841 2.5 1209 1.83 1.94 1250 Example 12 steel 4 Comparative Invention 829 2.5 1209 1.84 1.91 1264 Example 13 steel 5 Comparative Invention 810 1.4 1255 1.66 1.69 1257 Example 14 steel 5 Comparative Invention 832 2.3 1289 1.73 1.69 1233 Example 15 steel 5 Comparative Comparison 808 2.3 1283 1.69 1.63 1255 Example 16 steel 1 Comparative Comparison 817 1.9 1272 1.65 1.58 1235 Example 17 steel 2 Comparative Comparison 808 2.3 1283 1.69 1.63 1255 Example 18 steel 3 Comparative Comparison 817 1.9 1272 1.65 1.58 1235 Example 19 steel 4 Comparative Comparison 835 1.8 1243 1.82 1.54 1249 Example 20 steel 5 Inventive Invention 822 1.6 1233 1.82 1.9 1154 Example 6 steel 1 Inventive Invention 840 2.1 1254 1.05 1.90 1245 Example 7 steel 1 Comparative Invention 832 2.3 1256 1.69 1.83 1239 Example 21 steel 1 Comparative Invention 839 2.0 1280 1.83 1.92 1233 Example 22 steel 2 Reheating and Reheating and 3rd ring forging Post-weld 2nd upsetting upsetting Reheating heat Forging Forging temperature Forging Normalizing treatment Classification ratio ratio ( C.) ratio LMP LMP Inventive 2.01 2.54 1259 1.35 21.15 Not Example 1 conducted Inventive 198 2.46 1253 1.5 21.09 Not Example 2 conducted Inventive 2.21 2.61 1240 1.41 22.01 Not Example 3 conducted Inventive 2.07 2.33 1239 1.39 21.07 Not Example 4 conducted Inventive 2.15 2.75 1255 1.25 20.58 Not Example 5 conducted Comparative 1.88 2.65 1243 1.41 21.14 Not Example 1 conducted Comparative 1.59 2.56 1254 1.44 21.05 Not Example 2 conducted Comparative 1.59 2.45 1195 1.09 20.73 Not Example 3 conducted Comparative 1.68 2.44 1208 1.17 21.65 Not Example 4 conducted Comparative 2.07 2.3 1244 1.23 21.53 Not Example 5 conducted Comparative 2.16 2.53 1254 1.34 21.09 Not Example 6 conducted Comparative 2.15 2.19 1238 1.35 21.68 Not Example 7 conducted Comparative 1.89 2.28 1198 1.29 22.54 Not Example 8 conducted Comparative 1.12 2.54 1259 1.43 20.99 Not Example 9 conducted Comparative 2.66 2.63 1186 1.51 20.38 Not Example 10 conducted Comparative 1.9 1.32 1206 1.47 21.33 Not Example 11 conducted Comparative 2.07 3.05 1255 1.54 22.54 Not Example 12 conducted Comparative 1.85 2.75 1336 1.38 20.69 Not Example 13 conducted Comparative 1.9 2.55 1249 2.05 21.47 Not Example 14 conducted Comparative 2.01 2.53 1256 1.34 27 Not Example 15 conducted Comparative 1.89 2.5 1210 1.44 20.76 Not Example 16 conducted Comparative 1.94 2.54 1199 1.54 21.57 Not Example 17 conducted Comparative 1.89 2.5 1210 1.44 20.76 Not Example 18 conducted Comparative 1.94 2.54 1199 1.54 21.57 Not Example 19 conducted Comparative 2.11 2.51 1244 1.51 21.07 Not Example 20 conducted Inventive 2.03 2.52 1254 1.32 21.13 18.1 Example 6 Inventive 2.10 2.51 1253 1.32 21.05 19.0 Example 7 Comparative 2.01 2.40 1233 1.45 20.58 23.7 Example 21 Comparative 1.94 2.39 1219 1.40 21.02 22.5 Example 22
TABLE-US-00003 TABLE 3 Product Slab Grain 50 C. Prior boundary impact Surface austenite PF Ferrite cementite Number Yield Tensile absorbed crack Steel grain size fraction grain size size of Porosity strength strength energy depth Classification grade (m) (%) (m) (m) precipitates (mm.sup.3/g) (MPa) (MPa) (J) (mm) Inventive Invention 853 16.7 26.5 1.2 24 0.033 435 576 108 Not Example1 steel1 observed Inventive Invention 694 17.5 24.3 3.1 31 0.016 424 554 153 Not Example2 steel2 observed Inventive Invention 738 18.1 22.4 2.2 25 0.031 439 535 182 Not Example3 steel3 observed Inventive Invention 766 15.9 25.6 1.5 11 0.023 429 511 175 Not Example4 steel4 observed Inventive Invention 594 16.4 26.1 2.7 29 0.015 440 529 169 Not Example5 steel5 observed Comparative Invention 695 18.3 27.3 3.3 13 0.073 419 543 172 3.6 Example1 steel1 (Surface crack) Comparative Invention 1273 18.2 40.7 2.2 18 0.065 428 564 21 Not Example2 steel1 observed Comparative Invention 659 4.7 25 2.2 22 0.043 431 535 199 3.7 Example3 steel1 (Surface crack) Comparative Invention 707 15.9 23.9 3.1 18 0.039 452 564 182 2.8 Example4 steel2 (Surface crack) Comparative Invention 810 16.3 21.9 1.5 33 0.025 414 535 176 21.6 Example5 steel2 (Penetrating crack) Comparative Invention 905 17.4 22.4 2.0 25 0.237 440 544 11 Not Example6 steel2 observed Comparative Invention 865 18.1 23.4 1.0 41 0.196 419 531 33 Not Example7 steel3 observed Comparative Invention 889 19 26.9 1.5 38 0.029 428 581 188 2.9 Example8 steel3 (Surface crack) Comparative Invention 808 16.9 28.4 1.3 27 0.209 430 601 15 Not Example9 steel3 observed Comparative Invention 891 20.1 29.5 2.1 30 0.007 429 583 168 19.8 Example10 steel4 (Surface crack) Comparative Invention 885 18.7 28.4 2.5 41 0.259 428 587 14 Not Example11 steel4 observed Comparative Invention 843 18.3 28.1 2.1 35 0.005 441 532 159 30.7 Example12 steel4 (Penetrating crack) Comparative Invention 817 17.8 51.9 1.1 29 0.036 439 522 8 Not Example 13 steel5 observed Comparative Invention 826 18.1 26.4 1.9 11 0.061 131 581 159 16.9 Example14 steel5 (Penetrating crack) Comparative Invention 841 18.9 50.7 2.3 40 0.054 409 513 10 Not Example15 steel5 observed Comparative Comparison 839 17.6 26.5 1.2 19 0.033 258 342 389 Not Example16 steel1 observed Comparative Comparison 840 15.4 26.4 1.7 22 0.038 375 486 158 Not Example17 steel2 observed Comparative Comparison 868 16.9 28.1 1.8 31 0.045 605 725 7 Not Example18 steel3 observed Comparative Comparison 887 16.6 29 1.2 25 0.068 495 587 13 Not Example19 steel4 observed Comparative Comparison 870 16.3 28.3 2.4 1 0.043 369 488 253 Not Example20 steel5 observed Inventive Invention 901 17.2 24.5 1.5 33 0.037 392 543 255 Not Example6 steel1 observed Inventive Invention 694 18.0 25.3 2.3 25 0.019 412 551 198 Not Example7 steel1 observed Comparative Invention 705 17.6 21.4 8.9 33 0.037 382 513 12 Not Example21 steel1 observed Comparative steel2 773 15.9 25.2 10.5 25 0.019 402 521 3 Not Example22 Invention observed * In Table 3 above, the number of precipitates refers to the number of fine NbC or NbCN precipitates per 1 m.sup.2, with a diameter of 5 to 15 nm, among the precipitates observed in the cross-section of the steel, and the porosity refers to the porosity in the central portion of the product, which is a region of 3/8t to 5/8t (where t refers to the steel thickness (mm)) in the thickness direction from the surface of the steel.
[0140] As can be seen from Tables 1-3, in the case of all Invention Examples 1-7, which satisfy the alloy composition and manufacturing conditions proposed by the present disclosure, it can be seen that in addition to excellent strength and excellent low-temperature impact toughness at 50 C., good surface quality may be secured in the flange product state.
[0141] In contrast, Comparative Examples 1-15 and 21-22 satisfy the alloy composition proposed by the present disclosure but do not satisfy the manufacturing conditions, and thus it can be seen that the strength and low-temperature impact toughness values are low because they do not satisfy the characteristics of the slab's prior austenite grain size, polygonal ferrite fraction, or center porosity, and ferrite grain size and the like in the flange product state proposed by the present disclosure. In addition, even if the material is good, if the forging ratio conditions are not satisfied at each stage of forging, poor surface quality characteristics may also be confirmed in the product state due to the occurrence of surface cracks or penetrating cracks.
[0142] Meanwhile, Comparative Examples 16-20 satisfy the manufacturing conditions proposed by the present disclosure, but do not satisfy the alloy composition, so it can be seen that the quality level is low, such as exceeding the strength (not meeting the impact toughness) or not meeting the strength.
[0143] As described above, in the detailed description of the present disclosure, preferred embodiments of the present disclosure have been described, but it is obvious that various modifications are possible within the scope of the present disclosure by those skilled in the art. Therefore, the scope of the rights of the present disclosure should not be limited to the described embodiments, but should be defined by the claims described below as well as equivalents thereof.