ULTRA-THICK STEEL MATERIAL HAVING EXCELLENT STRENGTH AND LOW TEMPERATURE IMPACT TOUGHNESS FOR FLANGE, AND MANUFACTURING METHOD THEREFOR
20250229310 ยท 2025-07-17
Assignee
Inventors
Cpc classification
C22C38/002
CHEMISTRY; METALLURGY
International classification
B21B1/02
PERFORMING OPERATIONS; TRANSPORTING
Abstract
Provided are an ultrathick steel material having excellent strength and low-temperature impact toughness for flanges and a method for manufacturing same. The steel material of the present disclosure comprises, by wt %, 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%, and the balance of Fe and inevitable impurities, has Ceq satisfying the range of 0.35-0.55 as calculated by the following equation, has an average ferrite grain size of 25 m or less in the central portion thereof, and contains a microstructure including 5-30 area % of pearlite and the balance of ferrite.
Claims
1. An ultrathick steel material for a flange, comprising: by 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 the balance of Fe and inevitable impurities, has Ceq satisfying a range of 0.35 to 0.55 as calculated by the following equation 1, and the ultrathick steel material has a microstructure comprising of 5 to 30 area % of pearlite and the balance of ferrite, and the ferrite has an average grain size of 25 m or less in a central portion of the steel material, a porosity of the central portion of the steel material, which is an area of t to t in a thickness direction from a surface of the steel material, is 0.05 mm.sup.3/g or less, and the number of strain-induced NbC precipitates of 5 to 50 nm observed in a cross-section of the steel material is 10 or more, and the number of coarse precipitates of 100 nm or more is 5 or less, per 1 m.sup.2.
2. The ultrathick steel material of claim 1, wherein the steel material has a thickness of 200 to 500 mm.
3. The ultrathick steel material of claim 1, wherein the steel material has a tensile strength of 500 to 700 MPa, a yield strength of 350 MPa or more, and a Charpy impact test absorption energy value of 50 J or more at 50 C.
4. The ultrathick steel material of claim 1, wherein a maximum surface crack depth of the steel material is 0.1 mm or less (including 0).
5. A manufacturing method for an ultrathick steel material for a flange, comprising: preparing a slab comprising, by 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 the balance of Fe and inevitable impurities, and Ceq satisfying a range of 0.35 to 0.55 as calculated by the following equation 1 and then heating the slab in a temperature within a range of 1100 to 1300 C.; performing primary upsetting on the heated slab at a forging ratio of 1.3 to 2.4 and then bloom forging on the heated slab at a forging ratio of 1.5 to 2.0; reheating the bloom-forged material to a temperature within a range of 1100 to 1300 C.; performing secondary upsetting on the reheated bloom-forged material to a forging ratio of 1.3 to 2.3 and then round forging on the reheated bloom-forged material to a forging ratio of 1.65 to 2.25; performing tertiary upsetting on the round-forged material at a forging ratio of 2.0 to 2.8 so that a cumulative reduction amount is 10% or more at a temperature of recrystallization temperature or lower defined by the following equation; performing hole processing on the tertiary upset material, reheating the tertiary upset material to a temperature within a range of 1100 to 1300 C., and then performing ring forging on the reheated tertiary upset material at a forging ratio of 1.0 to 1.6; and performing normalizing heat treatment by heating the ring-forged material to a temperature within a range of 820 to 930 C. that is a temperature measured based on the central portion thereof, maintaining the heated ring-forged material for 5 to 600 minutes, and then air cooling the heated ring-forged material to room temperature.
6. The manufacturing method of claim 5, wherein the slab is manufactured using one of a continuous casting process, a semi-continuous casting process, and an ingot casting process.
7. The manufacturing method of claim 5, wherein a size of a forged surface punched during the primary upsetting is 1000 to 1200 mm1800 to 2000 mm when an initial size is 700 mm1800 mm.
8. The manufacturing method of claim 5, wherein, for the bloom forging, the size of the forged surface upon the completion of forging is 1450 to 1850 mm2100 to 2500 mm when an initial size is 1000 to 1200 mm1800 to 2000 mm.
9. The manufacturing method of claim 5, wherein, when the secondary upsetting and round forging end, a size of the product is 1450 to 18501300 to 1700 mm.
10. The manufacturing method of claim 5, wherein, when the tertiary upsetting ends, a size of the product is 2300 to 2800 400 to 800 mm.
11. The manufacturing method of claim 5, wherein a maximum thickness of the flange made of the steel material is 200 to 500 mm, an inner diameter is 4000 to 7000 mm, and an outer diameter is 5000 to 8000 mm.
Description
BEST MODE
[0040] The present disclosure relates to an ultrathick steel material for a flange having excellent strength and low-temperature impact toughness and a manufacturing method for a product. Preferred implementation embodiments of the present disclosure will be described below. Implementation embodiments of the present disclosure may be modified into several forms, and it is not to be interpreted that the scope of the present disclosure is limited to exemplary embodiments described in detail below. The present implementation embodiments are provided to explain the present disclosure in more detail to those skilled in the art to which the present disclosure pertains.
[0041] Hereinafter, the ultrathick steel material for flanges having excellent strength and low-temperature impact toughness of the present disclosure will be described in more detail.
[0042] According to the present disclosure, an ultrathick steel material for a flange having excellent strength and low temperature impact toughness includes, by 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 the balance of Fe and inevitable impurities, has Ceq satisfying a range of 0.35 to 0.55 as calculated by the following equation 1, the ultrathick steel material has a microstructure comprising of 5 to 30 area % of pearlite and the balance of ferrite, and the ferrite has an average grain size of 25 m or less in a central portion of the steel material, a porosity of the central portion of the steel material, which is an area of t to t in a thickness direction from a surface of the steel material, is 0.05 mm.sup.3/g or less, and the number of strain-induced NbC precipitates of 5 to 50 nm observed in a cross-section of the steel material is 10 or more, and the number of coarse precipitates of 100 nm or more is 5 or less, per 1 m.sup.2.
[0043] Hereinafter, alloy compositions of the present disclosure will be described in detail. Hereinafter, unless otherwise stated, % and ppm described in relation to alloy compositions are based on weight.
Carbon (C): 0.05 to 0.20%
[0044] 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, 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, when the carbon (C) content exceeds a certain level, a pearlite fraction may excessively increase during normalizing heat treatment and strength and hardness of a base material may be excessively exceeded, so surface cracks may occur during the forging processing and properties of low-temperature impact toughness and Lamellar tearing of a final product may be reduced. Therefore, the present disclosure may limit the carbon (C) content to 0.20%. A more preferable upper limit of the carbon (C) content may be 0.18%.
Silicon (Si): 0.05 to 0.50%
[0045] Silicon (Si) is a substitutional element, and is an essential element in manufacturing of clean steel since it improves the strength of the steel material through solid solution strengthening and has a strong deoxidation effect. Therefore, the 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, the silicon (Si) may generate a martensite-austenite (MA) phase and excessively increases ferrite matrix strength, which may cause deterioration in surface quality of the ultrathick material products, so the upper limit of the silicon (Si) 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 to 2.0%
[0046] Manganese (Mn) is a useful element that improves strength through solid solution strengthening and improves hardenability to generate low-temperature transformation phase. Therefore, in order 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, the manganese (Mn) may form MnS which is an elongated non-metallic inclusion with sulfur(S) to reduce toughness and act as an impact initiation point, which may be a factor that drastically reduces the low-temperature impact toughness of a product. Therefore, it is preferable 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 to 0.1%
[0047] Aluminum (Al) is one of the powerful deoxidizers in the steelmaking process along with the silicon (Si), and preferably added in an amount of 0.005% or more to achieve this effect. A more preferable lower limit of the aluminum (Al) content may be 0.01%. On the other hand, when the aluminum (Al) content is excessive, the fraction of Al203 in the oxidizing inclusions generated as results of deoxidation increases excessively to make the size of the inclusions coarsen and make it difficult to remove the inclusions during the refining, which may be a factor of reducing the low-temperature impact toughness. Therefore, it is preferable to manage the aluminum (Al) content to 0.1% or less. A more preferable 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 Cause embrittlement by forming coarse inclusions. Therefore, in order to improve embrittlement 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 to 0.07%
[0048] Niobium (Nb) is an element that improves strength of a base material by precipitating in the form of NbC or NbCN. In addition, the niobium (Nb) dissolved during the reheating at high temperature precipitates very finely in the form of Nbc during the rolling to suppress the recrystallization of austenite, thereby refining the structure. Therefore, it is preferable that the niobium (Nb) is added in an amount of 0.001% or more, and a more preferable niobium (Nb) content may be 0.05% or more. On the other hand, when the niobium (Nb) is excessively added, non-dissolved niobium (Nb) is generated in the form of TiNb (C, N), which is a factor that inhibits the low-temperature impact toughness, so it is preferable to limit the upper limit of niobium (Nb) content to 0.07%. A more preferable niobium (Nb) content may be 0.065% or less.
Vanadium (V): 0.001 to 0.3%
[0049] Since vanadium (V) t completely re-dissolved during the reheating, the strengthening effect is insignificant due to precipitation or solid solution strengthening during the subsequent rolling. However, in the case of forging the ultrathick material, an air cooling rate is very slow, so very fine carbonitrides precipitates during the cooling or additional heat treatment process, which has the effect of improving the strength. To fully obtain this effect, it is necessary to add 0.001% or more of vanadium (V). A more preferable lower limit of the vanadium (Al) content may be 0.01%. On the other hand, when the vanadium content is excessive, the hardness of the surface layer of the slab may excessively increase due to high hardenability to not only cause a factor of surface cracks, etc., during the flange processing, but also cause a sharp increase in manufacturing cost, which is not commercially advantageous. Therefore, the vanadium (V) content may be limited to 0.3% or less. A more preferable vanadium (V) content may be 0.25% or less.
Titanium (Ti): 0.001 to 0.03%
[0050] Titanium (Ti) is a component that precipitates as TiN during the reheating and significantly improves low-temperature toughness by suppressing the growth of spherical 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, when the titanium (Ti) is excessively added, the low-temperature toughness may be reduced due to clogging of a continuous casting nozzle or crystallization in the central portion thereof. In addition, the titanium (Ti) combines with nitrogen (N) to form coarse TiN precipitates in the central portion of the thickness, which may reduce the elongation of the product, reduce the uniform elongation during the forging process and cause the 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 to 0.30%
[0051] Chromium (Cr) is a component that increases yield strength and tensile strength by increasing hardenability and forming a low-temperature transformation structure. In addition, the chromium (Cr) is a component that is effective in preventing a decrease in strength by slowing down a spheroidization rate of cementite. For this effect, 0.01% or more 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.sub.23C.sub.6 increase, which may reduce the impact toughness of the product, and reduce the solid solubility of niobium (Nb) in the product and the fraction of fine precipitates such as NbC, so there may be the problem of reducing the strength of the product. Therefore, the present disclosure may limit the upper limit of the chromium (Cr) content to 0.30%. The preferable upper limit of the chromium (Cr) content may be 0.25%.
Molybdenum (Mo): 0.01 to 0.12%
[0052] Molybdenum (Mo) is an element that increases grain boundary strength, increases solid solution strengthening in ferrite, and is an element that effectively contributes to increasing the strength and ductility of products. In addition, the 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, 0.10% or more of molybdenum (Mo) may be added. However, the molybdenum (Mo) is an expensive element and when the molybdenum (Mo) is excessively added, the manufacturing costs may increase significantly, so the upper limit of the molybdenum (Mo) content may be limited to 0.12%.
Copper (Cu): 0.01 to 0.60%
[0053] Copper (Cu) is an advantageous element in the present disclosure since it may not only greatly improve the strength of the matrix phase through the solid solution strengthening in ferrite, but also suppress corrosion in a wet hydrogen sulfide atmosphere. For this effect, 0.01% or more of copper (Cr) may be added. A more preferable copper (Cu) content may be 0.03% or more. However, when the copper (Cu) content is excessive, the possibility of causing star cracks on the surface of the steel sheet increases, and as the copper (Cu) is an expensive element, there may be a problem of significantly increasing manufacturing costs. Therefore, the present disclosure may limit the upper limit of copper (Cu) content to 0.60%. The preferable upper limit of the copper (Cu) content may be 0.35%.
Nickel (Ni): 0.05 to 1.00%
[0054] Nickel (Ni) is an element that effectively contributes to improving impact toughness by increasing stacking defects at low temperature to facilitating cross slip of dislocations, and improving strength by improving hardenability. For this effect, 0.05% or more of nickel (Ni) may be added. A preferred nickel (Ni) content may be 0.10% or more. On the other hand, when the nickel (Ni) is excessively added, the manufacturing costs may increase due to the high cost, so the upper limit of the nickel (Ni) content may be limited to 1.00%. The preferable upper limit of the nickle (Ni) content may be 0.80%.
Calcium (Ca) 0.0005 to 0.0040%
[0055] When calcium (Ca) is added after deoxidation with aluminum (Al), the calcium (Ca) combines with sulfur(S) forming MnS inclusions to suppress the generation of MnS, and at the same time form spherical Cas, thereby suppressing cracks caused by hydrogen-induced cracking from occurring. In order to sufficiently form the sulfur(S) contained as an impurity into CaS, it is preferable to add 0.0005% or more of calcium (Ca). However, when the added amount is excessive, calcium (Ca) remaining after forming CaS combines with oxygen (O) to generate coarse oxidative inclusions, which may be a factor in reducing properties of Lamellar tearing due to elongation and destruction during the rolling. Therefore, the upper limit of calcium (Ca) content may be limited to 0.0040%.
Equation 1
[0056] Ceq according to the following Equation 1 is required to satisfy the range of 0.35 to 0.55. When the Ceq according to the above Equation 1 is less than 0.35, the pearlite fraction is reduced, so the tensile strength value of 500 to 700 MPa required in the present disclosure may not be secured, and when the Ceq exceeds 0.55, the pearlite fraction exceeds 30%, so it is not easy to secure the low-temperature impact energy values at 50 C. Therefore, in the present disclosure, the Ceq is preferable in the range of 0.35 to 0.55.
[0057] In the above equation 1, [C], [Mn], [Cr], [Mo], [V], [Ni], and [Cu] mean a content (wt %) of C, Mn, Cr, Mo, V, Ni, and Cu contained in the steel material, respectively, and when these components are not intentionally added, 0 is substituted.
[0058] The ultrathick steel material for a flange having excellent strength and low-temperature impact toughness of the present disclosure and products thereof may contain the balance being Fe and other inevitable impurities in addition to the above-described components. However, since the unintended impurities from raw materials or the surrounding environment may inevitably be mixed in a normal manufacturing process, the unintended impurities may not be completely excluded. Since these impurities are known to those skilled in the art, all thereof are not specifically mentioned in this specification. In addition, the additional addition of the effective components in addition to the above-described components is not completely excluded.
[0059] Meanwhile, the ultrathick steel material of the present disclosure preferably has a microstructure comprising of 5 to 30 area % of pearlite and the balance of ferrite, and the ferrite has an average grain size of 25 m or less in a central portion of the steel material.
[0060] When the average ferrite grain size present in the central portion of the steel material exceeds 25 m, the length of the crack path during the impact fracture is shortened, and ductile brittle transition temperature (DBTT) increases, SO the low-temperature impact toughness deteriorates. Therefore, it is appropriate that the average ferrite grain size is 25 m or less.
[0061] In addition, when the pearlite fraction in the steel material microstructure is less than 5 area %, it is not appropriate to secure a tensile strength value of 500 MPa or more, and when the pearlite fraction exceeds 30 area %, the low-temperature impact toughness value at 50 C. may deteriorate due to the increase in hard structure.
[0062] In addition, a porosity of the central portion of the steel material, which is an area of t to t in a thickness direction from a surface of the steel material, may be 0.05 mm.sup.3/g or less.
[0063] In addition, the steel material of the present disclosure may have 10 or more strain-induced NbC precipitates of 5 to 50 nm and 5 or less coarse precipitates of 100 nm or more per 1 m.sup.2 in its matrix structure. When the number of strain-induced NbC precipitates of 5 to 50 nm is less than 10, the precipitation strengthening effect is weakened, and when the number of coarse precipitates of 100 nm or more exceeds 5, the pinning effect and precipitation strengthening effect are lost, so it is not easy to secure the tensile strength of 500 to 700 MPa required in the present disclosure.
[0064] In addition, the ultrathick steel material of the present disclosure may have a thickness of 200 to 500 mm.
[0065] In addition, the steel material of the present disclosure may have a tensile strength of 500 to 700 MPa, a yield strength of 350 MPa or more, and a Charpy impact test absorption energy value of 50 J or more at 60 C.
[0066] In addition, the maximum surface crack depth of the steel material may be 0.1 mm or less (including 0).
[0067] Next, a manufacturing method for an ultrathick steel material for a flange having excellent strength and low-temperature impact toughness according to another aspect of the present disclosure will be described in more detail.
[0068] The manufacturing method of an ultrathick steel material of the present disclosure includes: preparing a slab having the composition components as described above and then heating the slab in a temperature within a range of 1100 to 1300 C.; performing primary upsetting on the heated slab at a forging ratio of 1.3 to 2.4 and then bloom forging on the heated slab at a forging ratio of 1.5 to 2.0; reheating the bloom-forged material to a temperature within a range of 1100 to 1300 C.; performing secondary upsetting on the reheated bloom-forged material at a forging ratio of 1.3 to 2.3 and then round forging on the reheated bloom-forged material at a forging ratio of 1.65 to 2.25; performing tertiary upsetting on the round-forged material at a forging ratio of 2.0 to 2.8 so that a cumulative reduction amount is 10% or more at a temperature of recrystallization temperature or lower defined by the following equation 2; performing hole processing on the tertiary upset material, reheating the tertiary upset material to a temperature within a range of 1100 to 1300 C., and then performing ring forging on the reheated tertiary upset material at a forging ratio of 1.0 to 1.6; and performing normalizing heat treatment by heating the ring-forged material to a temperature within a range of 820 to 930 C. that is a temperature measured based on the central portion thereof, maintaining the heated ring-forged material for 5 to 600 minutes, and then air cooling the heated ring-forged material to room temperature.
Heating Slab
[0069] First, in the present disclosure, a slab having the composition as described above is prepared and then heated to a temperature within a range of 1100 to 1300 C.
[0070] It is necessary to heat the slab above a certain temperature range to re-dissolve the composite carbonitride of titanium (Ti) or niobium (Nb), coarse crystallized TiNb (C, N), etc., formed during the casting. In addition, before the primary upsetting forging, the slab is heated and maintained above the recrystallization temperature to homogenize the structure, and is heated above a certain temperature range so that the forging end temperature is sufficiently high to minimize surface cracks that may occur during the forging process. Therefore, it is preferable to heat the slab of the present disclosure in a temperature within a range of 1100 C. or higher.
[0071] On the other hand, when the heating temperature of the slab is excessively high, the high-temperature oxidation scale may be excessively generated, and the increase in manufacturing costs may be excessive due to the high-temperature heating and maintenance. Therefore, it is preferable that the primary heating of the slab of the present disclosure is performed in the range of 1300 C. or lower.
[0072] Meanwhile, in the present disclosure, the thickness of the slab may be 500 mm or more, and a preferred thickness of the slab may be 700 mm or more.
Primary Upsetting and Bloom Forging
[0073] Next, in the present disclosure, the heated slab to a forging ratio of 1.3 to 2.4 is subjected to the primary upsetting and then subjected to the bloom forging to a forging ratio of 1.5 to 2.0.
[0074] The upsetting is a method for rigidly deforming a material vertically along a longitudinal axis, and the forging ratio during the primary upsetting may be preferably 1.3 to 2.4, and more preferably 1.5 to 2.0. Here, the forging ratio refers to the ratio of the cross-sectional area changed by the forging. A size of a forged surface punched during the primary upsetting may be 1000 to 1200 mm1800 to 2000 mm when an initial size is 700 mm1800 mm.
[0075] When the forging ratio is less than 1.3 during the primary upsetting, it is difficult to sufficiently compress the porosity remaining in the central portion of the slab. Therefore, since it is difficult to control the porosity required for the final product of the present disclosure to an appropriate level of 0.05 mm.sup.3/g or less, it is not easy to secure the low-temperature impact toughness in the central portion thereof. On the other hand, when the forging ratio exceeds 2.4 during the primary upsetting, buckling occurs during the forging process, making it difficult to control the surface quality and appropriate shape required for flange products. Therefore, during the primary upsetting, the forging ratio is preferably 1.3 to 2.4.
[0076] In the present disclosure, the bloom forging is performed on the primary upsetting material to a forging ratio of 1.5 to 2.0.
[0077] The bloom forging is a method for processing a material subjected to primary upsetting into a bloom shape by further compressing the material subjected to the primary upsetting, and is a method for expanding an area by processing both upper and lower surfaces in a certain direction of width or length. For the bloom forging, the size of the forged surface upon the completion of forging may be 1450 to 1850 mm2100 to 2500 mm when an initial size is 1000 to 1200 mm1800 to 2000 mm. In the case of bloom forging, the forging ratio is preferably 1.5 to 2.0. This is because, when the forging ratio is less than 1.5, it is difficult to secure the appropriate void quality required in the present disclosure as in the upsetting forging, and when the forging ratio exceeds 2.0, the surface cracks may occur.
[0078] The forging progress direction is possible in both the longitudinal and width directions, but in the longitudinal direction, the casting structure is configured to be denser, so the elongation of the surface layer may increase and processability may be excellent. Therefore, the longitudinal bloom forging may be more appropriate than width direction in view of the surface crack.
Reheating
[0079] In the present disclosure, the bloom-forged material is reheated to a temperature within a range of 1100 to 1300 C.
[0080] When the bloom forging ends, the bloom surface temperature is 950 C. or lower, and when the processing continues, the surface cracks or material fracture may occur. Therefore, after the bloom forging, the material may be heated again to a temperature within a range of 1100 to 1300 C. As described above, it is preferable to heat the material at 1100 C. or higher for reasons such as re-dissolving the crystallized material, homogenizing the structure, and preventing surface cracks, and it is better to control the material to 1300 C. or lower due to problems such as excessive scale and coarsening of grains.
Secondary Upsetting-Round Forging
[0081] Next, the reheated bloom-forged material is subjected to the secondary upsetting at a forging ratio of 1.3 to 2.3 and then round forged at a forging ratio of 1.65 to 2.25.
[0082] That is, in the present disclosure, the heated bloom material is subjected to the secondary upsetting at a forging ratio of 1.3 to 2.3, and then round-forged at a forging ratio of 1.65 to 2.25 in order to process the bloom into a circular shape of the flange border. When the secondary upsetting and round forging end, the size of the product may be 1450 to 18501300 to 1700 mm.
[0083] During the secondary upsetting and round forging, when the forging ratio is below the level required in the present disclosure, it is difficult to control the central porosity in the final product to 0.05 mm.sup.3/g or less, making it difficult to secure the low-temperature impact toughness in the central portion of the steel material. On the other hand, when the forging ratio level of the present disclosure is exceeded, it cannot be processed into the desired flange product shape due to the problems such as buckling, the occurrence of surface cracks, and the poor shape.
Tertiary Upsetting (Generation of Strain-Induced Precipitates)
[0084] Subsequently, in the present disclosure, the tertiary upsetting is performed on the round-forged material at a forging ratio of 2.0 to 2.8 so that the cumulative reduction amount is 10% or more at a temperature of recrystallization temperature or lower defined by the following equation 2 below.
[0085] The material processed into the cylindrical shape may be processed to an appropriate thickness of the flange through the tertiary upsetting before the hole processing (piercing). When the tertiary upsetting ends, the size of the product may be 2300 to 2800400 to 800 mm.
[0086] The forging ratio of the tertiary upsetting may be 2.0 to 2.8, and when the forging ratio is insufficient or exceeded, problems such as the above-mentioned residual void control and surface crack/shape control inability may occur.
[0087] What is important in this tertiary upsetting process is the cumulative reduction amount at a temperature of the recrystallization temperature (Rst) or lower of the steel material, and the forging and rolling is performed so that the cumulative reduction amount is 10% or more. In this case, the recrystallization temperature may be calculated by the following Equation 2.
[0088] When the cumulative reduction amount is less than 10% at a temperature of the recrystallization temperature or lower, it is not easy to generate the strain-induced ultrafine NbC or NbCN precipitates of 5 to 50 nm, and the number of the precipitates is less than 10 or the number of coarse precipitates with a size of 100 nm or more may exceed 5, per 1 m.sup.2. When the amount of fine precipitates is reduced or the size is increased, the precipitation strengthening effect is insignificant, and the pinning effect is reduced when the normalizing temperature is increased, so it is not easy to secure the average ferrite grain size in the central portion of the product below 25 m. Therefore, it is preferable to control the cumulative reduction amount to 10% or more, more preferably 15% or more, and most preferably 20% or more, at a temperature of the recrystallization temperature or lower.
Hole Processing and Ring Forging
[0089] Next, in the present disclosure, after the hole processing is performed on the material subjected to the tertiary upsetting, the material subjected to the tertiary upsetting is reheated to a temperature within a range of 1100 to 1300 C., and then subjected to performing the ring forging at a forging ratio of 1.0 to 1.6.
[0090] After the tertiary upsetting ends, the hole may be machined in the central portion of the material using a 500 to 1000 punch.
[0091] The hole-processed material is reheated to the temperature within a range of 1100 to 1300 C. described above, and may then be processed into the final flange ring shape. The maximum thickness of the flange made of the steel material 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. The ring forging does not apply rigid plastic processing because it is more important to control the final shape and dimensions rather than compressing voids. Therefore, the forging ratio may be 1.0 to 1.6, and more preferably 1.2 to 1.4.
[0092] Meanwhile, the strain rate in all forging processes presented above in the present disclosure may be 1/s to 4/s. At the strain rate of less than 1/s, the temperature of the finish forging may drop and surface cracks may occur. On the other hand, when a high strain rate exceeding 4/s is applied in the non-recrystallized region, the surface cracks may occur due to the decrease in elongation due to excessive local work hardening.
Normalizing Heat Treatment
[0093] Finally, in the present disclosure, the ring-forged material is subject to the normalizing heat treatment. Specifically, the ring-forged material is heated to a temperature within a range of 820 to 930 C. that is a temperature measured based on the central portion thereof, maintained for 5 to 600 minutes, and then air cooling to room temperature.
[0094] The forged flange product may be subjected to the normalizing heat treatment. Specifically, the forged flange product is heated to a temperature within a range of 820 to 930 C. which is the temperature measured based on the central portion (t/2) of the product, maintained for 5 to 600 minutes, and then air cooling to room temperature.
[0095] During the normalizing heat treatment, when the heating temperature is less than 820 C. or the holding time is less than 5 minutes, the carbides generated during the cooling after the forging or the impurity elements segregated at the grain boundaries are not re-dissolved smoothly, so the low-temperature toughness of the steel material after the heat treatment may greatly 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 grain size of the ferrite matrix of the composite structure of the ferrite and pearlite exceeds 30 m required in the present disclosure or the precipitated phases such as Nb (C, N) and V (C, N) become coarse, so the strength and low-temperature impact toughness may deteriorate.
MODE FOR INVENTION
[0096] Hereinafter, the present disclosure will be described in more detail with reference to Examples. However, it should be noted that the following Examples are only for illustrating the present disclosure in more detail and are not intended to limit the scope of the present disclosure.
Example
TABLE-US-00001 TABLE 1 Division C Si Mn Al P Nb V Ti Cr Mo Cu Ni Ca Ceq Inven- 0.17 0.37 1.44 0.01 83 0.021 0.023 0.005 0.17 0.09 0.19 0.35 20 0.50 tive steel 1 Inven- 0.16 0.29 1.35 0.02 62 0.02 0.031 0.013 0.09 0.07 0.12 0.29 18 0.45 tive steel 2 Inven- 0.15 0.3 1.39 0.03 88 0.017 0.021 0.008 0.15 0.08 0.25 0.18 17 0.46 tive steel 3 Inven- 0.17 0.33 1.36 0.02 75 0.015 0.08 0.011 0.16 0.11 0.16 0.22 18 0.49 tive steel 4 Inven- 0.18 0.3 1.2 0.03 65 0.009 0.031 0.013 0.23 0.06 0.38 0.29 22 0.49 tive steel 5 Com- 0.04 0.35 1.33 0.05 73 0.016 0.043 0.009 0.08 0.08 0.43 0.24 21 0.35 parative steel 1 Com- 0.16 0.36 0.65 0.04 77 0.011 0.023 0.016 0.11 0.07 0.25 0.18 18 0.34 parative steel 2 Com- 0.21 0.38 3.64 0.01 69 0.027 0.025 0.009 0.15 0.05 0.16 0.76 20 0.92 parative steel 3 Com- 0.15 0.36 1.37 0.03 81 0.002 0.021 0.009 0.16 0.1 0.15 0.29 21 0.46 parative steel 4 Com- 0.24 0.28 1.83 0.04 81 0.009 0.03 0.01 0.29 0.12 0.41 0.3 19 0.68 parative steel 5 *In Table 1, the unit of content of component elements is wt %, but the unit of P, S, and Ca is ppm. The residual components are Fe and inevitable impurities.
[0097] A 700 mm thick slab having the alloy components shown in Table 1 above was manufactured. The slab was subjected to slab preparation, forging process (reheating and primary upsetting, bloom forging, reheating, secondary upsetting-round forging, tertiary upsetting, reheating and ring forging), and normalizing heat treatment that are the process conditions in Table 2 below to manufacture a flange of a final 320 mmt. In this case, after the completion of the bloom forging, the reheating temperature for the secondary upsetting was 1230 C.10 C., and forging ratio of the round forging after the secondary upsetting was applied equally at 2.0. In addition, process conditions satisfying the scope of the present disclosure were applied to all processes other than those listed in Table 2 below.
[0098] Thereafter, the physical values of each specimen manufactured above were measured and shown in Table 3 below. Here, the ferrite grain size and ferrite phase fraction were measured using an automatic image analyzer by collecting a specimen from the tissue specimen of the central portion after the normalizing heat treatment. Meanwhile, in this example, the product microstructure in both the Inventive Example and Comparative Example is a mixed structure of the ferrite and pearlite.
[0099] In addition, the yield/tensile strength was evaluated through a room temperature tensile test according to ASTM A370, and a 0.2% offset was applied for the yield strength. In addition, the impact toughness of each specimen used the average of the absorption energy values measured three times at the corresponding temperature through the Charpy V-notch test.
[0100] In addition, the number of strain-induced NbC precipitates of 5 to 50 nm observed in the cross-section of the steel material was measured using TEM. NbC precipitates were confirmed through NbC diffraction patterns and EDX mapping, and the number of NbC precipitates located in 1 m.sup.2 was counted.
[0101] The porosity of the central portion of the product was measured by measuring the density (g/mm.sup.3) and taking the reciprocal (mm.sup.3/g).
[0102] In addition, after visually observing the surface of each specimen, the grinding was performed at the point where the surface crack was formed, and the grinding length until cracks disappeared was measured as the surface crack length.
TABLE-US-00002 TABLE 2 Tertiary upsetting Cumulative Heating Primary Bloom Secondary reduction temper- upsetting forging upsetting rate Normalizing ature forging forging forging Forging below temperature Division Steel type ( C.) ratio ratio ratio ratio Rst (%) ( C.) Inventive Inventive 1263 1.83 1.8 2.01 2.54 23 883 Example 1 steel 1 Inventive Inventive 1234 1.75 1.69 1.98 2.46 19 889 Example 2 steel 2 Inventive Inventive 1195 1.92 1.59 2.21 2.61 25 890 Example 3 steel 3 Inventive Inventive 1241 1.86 1.83 2.07 2.33 24 813 Example 4 steel 4 Inventive Inventive 1269 1.69 1.75 2.15 2.75 15 890 Example 5 steel 5 Comparative Inventive 981 1.62 1.83 2.05 2.56 18 915 Example 1 steel 1 Comparative Inventive 1196 1.1 1.85 2.14 2.65 17 877 Example 2 steel 1 Comparative Inventive 1256 1.92 3.51 1.95 2.42 23 918 Example 3 steel 2 Comparative Inventive 1268 1.62 1.85 3.1 2.68 29 925 Example 4 steel 2 Comparative Inventive 1266 1.58 1.81 1.2 2.43 29 921 Example 5 steel 3 Comparative Inventive 1273 1.59 1.84 2.15 1.3 28 910 Example 6 steel 3 Comparative Inventive 1250 1.81 1.77 2.2 2.65 4 908 Example 7 steel 4 Comparative Inventive 1264 1.76 1.76 1.95 0.19 24 957 Example 8 steel 4 Comparative Comparative 1242 1.81 1.75 1.89 2.28 24 887 Example 9 steel 1 Comparative Comparative 1200 1.7 1.81 1.87 2.64 23 901 Example 10 steel 2 Comparative Comparative 1269 1.84 1.88 1.91 2.54 25 903 Example 11 steel 3 Comparative Comparative 1168 1.88 1.76 1.95 2.35 29 905 Example 12 steel 4 Comparative Comparative 1153 1.66 1.86 2.01 2.45 27 910 Example 13 steel 5
TABLE-US-00003 TABLE 3 Mechanical properties Number of Impact Ferrite Pearlite strain- absorption Surface grain fraction induced Yield Tensile energy crack size (area precipitates Porosity strength strength at 50 C. depth Division Steel type (m) %) a+ b+ (mm.sup.3/g) (MPa) (MPa) (J) (mm) Inventive Inventive 16.5 14.3 35 0 0.021 410 551 257 No Example 1 steel 1 observation Inventive Inventive 17.2 15.2 24 1 0.025 389 535 268 No Example 2 steel 2 observation Inventive Inventive 20 14.2 20 1 0.031 390 513 238 No Example 3 steel 3 observation Inventive Inventive 15.9 13.9 31 2 0.019 408 539 218 No Example 4 steel 4 observation Inventive Inventive 16.5 15.1 28 3 0.023 399 541 298 No Example 5 steel 5 observation Comparative Inventive 15.9 13.8 31 0 0.024 405 528 208 19.5 Example 1 steel 1 Comparative Inventive 18.1 16.5 32 2 0.03 403 529 12 No Example 2 steel 1 observation Comparative Inventive 18.5 14.2 29 1 0.018 388 534 254 23.9 Example 3 steel 2 Comparative Inventive 17.3 16.7 18 2 0.015 389 529 236 18.9 Example 4 steel 2 Comparative Inventive 21.3 15.5 38 3 0.014 394 534 15 No Example 5 steel 3 observation Comparative Inventive 15.6 14.9 30 2 0.024 390 530 18 No Example 6 steel 3 observation Comparative Inventive 16.6 15.2 5 23 0.033 340 485 8 No Example 7 steel 4 observation Comparative Inventive 18.2 16.1 27 2 0.018 390 559 27 No Example 8 steel 4 observation Comparative Comparative 20.5 14.1 30 1 0.023 273 451 254 No Example 9 steel 1 observation Comparative Comparative 21.4 16.2 45 3 0.02 350 489 253 No Example 10 steel 2 observation Comparative Comparative 18.9 15.1 40 2 0.015 425 720 21 No Example 11 steel 3 observation Comparative Comparative 18.2 14.3 3 1 0.014 359 501 51 No Example 12 steel 4 observation Comparative Comparative 17.8 15.3 39 0 0.019 597 751 12 No Example 13 steel 5 observation *In Table 3, a* represents the number of strain-induced organic NbC precipitates of 5 to 50 nm per 1 m.sup.2 observed in the cross-section of the steel material, and b* represents the number of coarse precipitates of 100 nm or more.
[0103] It can be seen from Tables 1 to 3 that all of Inventive Examples 1 to 5 satisfying the alloy compositions and manufacturing conditions proposed by the present disclosure have excellent strength and low-temperature impact toughness at 50 C. as well as good surface quality in the flange product state.
[0104] On the other hand, Comparative Examples 1 to 8 are cases where the alloy compositions proposed by the present disclosure are satisfied but the manufacturing conditions are not satisfied, and it can be seen that the strength and low-temperature impact toughness values are low as the characteristics proposed by the present disclosure, such as the porosity of the central portion and the ferrite grain size in the flange product state, are not satisfied. In addition, even if the material is good, even when the forging ratio conditions are not met at each step of forging, poor surface quality properties may be confirmed in the product state due to the occurrence of surface cracks or penetrating cracks.
[0105] Meanwhile, Comparative Examples 9 to 13 satisfy the manufacturing conditions proposed by the present disclosure, but do not satisfy the alloy compositions, so it can be seen that the quality level is low, such as exceeding the strength (less than impact toughness) or less than the strength.
[0106] As described above, exemplary embodiments in the present disclosure have been described in the detailed description of the present disclosure, but those of ordinary skill in the art to which the present disclosure pertains may be variously modified without departing from the scope of the present disclosure. Therefore, the scope of the present disclosure is not construed as being limited to the embodiments described above, but should be defined by the following claims as well as equivalents thereto.