Steel and method for manufacturing the same

Abstract

A steel is provided, comprising, by weight percent: 0.20% to 0.40% carbon (C), 0.10% to 0.40% silicon (Si), 1.00% to 1.40% manganese (Mn), greater than 0% and not more than 0.015% phosphorus (P), greater than 0% and not more than 0.015% sulfur (S), 0.80% to 1.00% chromium (Cr), 0.40% to 1.00% nickel (Ni), 0.20% to 0.45% molybdenum (Mo), and so on. The mixture comprises 0% and no more than 0.30% copper (Cu), 0.01% to 0.05% aluminum (Al), 0.050% to 0.100% vanadium (V), 0.008% to 0.020% titanium (Ti), 0.002% to 0.015% nitrogen (N), 0.0030% or less boron (B), and 0.0020% or less calcium (Ca), wherein the remainder contains Fe and unavoidable impurities, and satisfies the following expressions (1) and (2). Expression (1): 0.10 ≤ Al + V + Ti ≤ 0.15, Expression (2): C 当量 : 0.70 ≤ C + (1 / 6)*Mn + (1 / 15)*(Ni + Cu) + (1 / 5)*(Cr + Mo + V) ≤ 0.80 (where each element symbol represents the content of each element (weight %)).

Description

Technical Field

[0001] This disclosure relates to a type of steel and a method for manufacturing the same. Background Technology

[0002] Recently, with the active construction of floating production storage and offloading (FPSO) units for marine resource development such as crude oil or natural gas, or floating offshore wind farms representing environmentally friendly and renewable energy, the importance of mooring chains that anchor various offshore platforms and vessels below sea level has been increasing.

[0003] Because the working environment at sea is extremely harsh, the steel used to manufacture mooring chains needs to have properties such as high strength, excellent toughness, and excellent resistance to seawater corrosion, so as to ensure safety even in extreme environments.

[0004] The steel used to manufacture mooring chains is high-alloy steel, which has high production costs and produces martensite during cooling after hot working, making it prone to internal cracking due to stress. Furthermore, the high-alloy composition degrades weldability during chain manufacturing, increasing the likelihood of weld cracks, and there is a risk of cracking even during quenching.

[0005] Therefore, it is necessary to develop high-strength and high-toughness steels and their manufacturing methods through optimized design to reduce the heat treatment sensitivity of steels, improve weldability, and ensure hardenability. Summary of the Invention

[0006] Technical issues

[0007] This disclosure aims to provide a steel with improved tensile strength, low-temperature impact toughness, and corrosion resistance based on steel used in the manufacture of mooring chains, etc.

[0008] Technical solution

[0009] According to one aspect of this disclosure, the steel comprises, by weight percentage (wt%): 0.20% to 0.40% carbon (C), 0.10% to 0.40% silicon (Si), 1.00% to 1.40% manganese (Mn), greater than 0% and less than 0.015% phosphorus (P), greater than 0% and less than 0.015% sulfur (S), 0.80% to 1.00% chromium (Cr), 0.40% to 1.00% nickel (Ni), and 0.20% to 0.45%... The molybdenum (Mo), greater than 0% and less than 0.30% copper (Cu), 0.01% to 0.05% aluminum (Al), 0.050% to 0.100% vanadium (V), 0.008% to 0.020% titanium (Ti), 0.002% to 0.015% nitrogen (N), 0.0030% or less boron (B), 0.0020% or less calcium (Ca), and the remainder iron (Fe) and unavoidable impurities, and satisfying the following formulas (1) and (2).

[0010] Formula (1): 0.10 ≤ Al + V + Ti ≤ 0.15

[0011] Equation (2) C 当量 :0.70 ≤ C + (1 / 6)*Mn + (1 / 15)*(Ni + Cu) + (1 / 5)*(Cr + Mo +V) ≤ 0.80

[0012] (Each element symbol represents the content (by weight%) of each element).

[0013] According to one aspect of this disclosure, steel can satisfy equation (3).

[0014] Formula (3): 0.00 ≤ (1 - 1.4*Cu) x (1 - 0.4*Ni) + 0.3*Cr - 2.2*Mo ≤ 0.50

[0015] (Each element symbol represents the content (by weight%) of each element).

[0016] According to one aspect of this disclosure, steel may have a Charpy impact energy of 100 J or greater at -20°C and a tensile strength of 860 MPa or greater.

[0017] According to one aspect of this disclosure, steel may contain per 1 µm 2 Two or more carbonitrides with a size of 50 nm or smaller.

[0018] Steels according to one aspect of this disclosure may have an ASTM grain size number of 8.0 or greater for the original austenite.

[0019] According to one aspect of this disclosure, steel may have a corrosion-resistant diameter reduction of 1.0 mm or less.

[0020] Another aspect of this disclosure relates to a method for manufacturing steel, the method comprising: preparing a primary rolled steel billet, the primary rolled steel billet comprising, by weight percentage (wt%): 0.20% to 0.40% carbon (C), 0.10% to 0.40% silicon (Si), 1.00% to 1.40% manganese (Mn), greater than 0% and less than 0.015% phosphorus (P), greater than 0% and less than 0.015% sulfur (S), 0.80% to 1.00% chromium (Cr), 0.40% to 1.00% nickel (Ni), 0.20% to 0.45% molybdenum (Mo), greater than 0% and The steel billet contains 0.30% or less copper (Cu), 0.01% to 0.05% aluminum (Al), 0.050% to 0.100% vanadium (V), 0.008% to 0.020% titanium (Ti), 0.002% to 0.015% nitrogen (N), 0.0030% or less boron (B), 0.0020% or less calcium (Ca), and the remainder iron (Fe) and unavoidable impurities, and satisfies the following formulas (1) and (2); the primary rolled steel billet is hot rolled to produce bar steel; the bar steel is quenched by water cooling at 850°C to 1,000°C; and the water-cooled bar steel is tempered.

[0021] Formula (1): 0.10 ≤ Al + V + Ti ≤ 0.15

[0022] Equation (2) C 当量 :0.70 ≤ C + (1 / 6)*Mn + (1 / 15)*(Ni + Cu) + (1 / 5)*(Cr + Mo +V) ≤ 0.80

[0023] (Each element symbol represents the content (by weight%) of each element).

[0024] In a method for manufacturing steel according to one aspect of this disclosure, the production of bar steel may include: heating a primary rolled steel billet at 1,150°C to 1,250°C for 3 to 6 hours, and then rolling the primary rolled steel billet at 1,050°C to 1,150°C to produce a square billet; and heating the square billet at 1,150°C to 1,250°C for 1 to 2 hours, and then hot-rolling the square billet to produce bar steel.

[0025] In a method for manufacturing steel according to one aspect of this disclosure, tempering can be carried out at 600°C to 650°C.

[0026] In a method for manufacturing steel according to one aspect of this disclosure, the initial rolled steel billet may satisfy the following formula (3).

[0027] Formula (3): 0.00 ≤ (1 - 1.4*Cu) x (1 - 0.4*Ni) + 0.3*Cr - 2.2*Mo ≤ 0.50

[0028] (Each element symbol represents the content (by weight%) of each element).

[0029] Steel manufactured by a method according to one aspect of this disclosure may contain per 1 µm 2 Two or more carbonitrides with a size of 50 nm or smaller.

[0030] Steel manufactured by a method according to one aspect of this disclosure can have a corrosion-resistant diameter reduction of 1.0 mm or less.

[0031] Beneficial effects

[0032] According to this disclosure, a steel with excellent corrosion resistance, low-temperature impact toughness, and tensile strength, and a method for manufacturing the same, can be provided. The effects obtainable from this disclosure are not limited to those described above, and other unmentioned effects will be clearly understood by those skilled in the art from the following description. Detailed Implementation

[0033] Preferred embodiments of this disclosure will be described below. However, embodiments of this disclosure can be modified in various other forms, and the technical spirit of this disclosure is not limited to the embodiments described below. Furthermore, embodiments of this disclosure are provided to provide a more complete explanation of this disclosure to those skilled in the art.

[0034] The terminology used in this application is for the purpose of describing specific instances only. Therefore, singular expressions include plural expressions unless the context clearly indicates otherwise. Furthermore, it should be noted that terms such as "comprising" or "having" as used in this application are used to clearly indicate the presence of the features, steps, functions, components or combinations thereof described in the specification, and are not intended to presuppose the presence of other features, steps, functions, components or combinations thereof.

[0035] Furthermore, unless otherwise defined, all terms used herein should be considered to have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains. Therefore, unless explicitly defined in the specification, particular terms should not be interpreted in an overly idealized or formalistic sense.

[0036] Furthermore, the terms "about," "substantially," etc., used in the specification, when inherent in the manufacturing and material tolerances described herein, are used to indicate that the value is at or near that value, and are used to prevent unethical infringers from unfairly using the precise or absolute values ​​mentioned herein to aid in understanding the content of this disclosure.

[0037] Unless otherwise specified in the instructions, the percentages representing the content of each element are based on weight.

[0038] According to one aspect of this disclosure, the steel comprises, by weight percentage (wt%): 0.20% to 0.40% carbon (C), 0.10% to 0.40% silicon (Si), 1.00% to 1.40% manganese (Mn), greater than 0% and less than 0.015% phosphorus (P), greater than 0% and less than 0.015% sulfur (S), 0.80% to 1.00% chromium (Cr), 0.40% to 1.00% nickel (Ni), 0.2% carbon (C), 0.10% to 0.40% silicon (Si), 1.00% to 1.40% manganese (Mn), greater than 0% and less than 0.015% phosphorus (P), greater than 0% and less than 0.015% sulfur (S), 0.2% to 0.80% chromium (Cr), 0.40% to 1.00% nickel (Ni), and 0.2% carbon (C). 0% to 0.45% molybdenum (Mo), greater than 0% and less than 0.30% copper (Cu), 0.01% to 0.05% aluminum (Al), 0.04% to 0.10% vanadium (V), 0.008% to 0.020% titanium (Ti), 0.002% to 0.015% nitrogen (N), 0.0030% or less boron (B), 0.0020% or less calcium (Ca), and the remainder iron (Fe) and unavoidable impurities.

[0039] The role and content of each component contained in the steel according to this disclosure will be described below.

[0040] The carbon (C) content is between 0.20% and 0.40%. .

[0041] Carbon (C) is an essential element added to ensure the strength of the product and needs to be included in the steel within an appropriate range. When the C content is less than 0.20%, it is difficult to ensure sufficient strength, and the increased content of expensive alloying elements added to replace C may reduce economic efficiency. When the C content exceeds 0.40%, the hardness of the material increases excessively, which may deteriorate low-temperature impact toughness and weldability. Therefore, the C content is controlled between 0.20% and 0.40%, and preferably between 0.20% and 0.30%.

[0042] The Si content is 0.10% to 0.40%.

[0043] Si is an element used for deoxidation of steel and is added to improve wear resistance. When the added Si is less than 0.10%, the deoxidation effect on the steel is not significant, and therefore clean steel may not be obtained. When the Si content exceeds 0.40%, the toughness of the weld heat-affected zone may decrease, which may reduce low-temperature impact toughness. Therefore, the Si content is controlled between 0.10% and 0.40%, and preferably between 0.14% and 0.35%.

[0044] The Mn content is 1.00% to 1.40%.

[0045] Mn is an element added to improve strength and hardenability through solid solution strengthening. Furthermore, Mn can prevent red brittleness caused by sulfur (S) in steel by forming an appropriate amount of MnS. When the Mn content is less than 1.00%, the effect of adding Mn is not significant, which may make it difficult to ensure sufficient strength and may reduce hardenability. When the Mn content exceeds 1.40%, low-temperature impact toughness may decrease rapidly, and sufficient weldability may be difficult to ensure. Therefore, to ensure sufficient weldability, hardenability, and tensile strength without deteriorating low-temperature impact toughness, the Mn content is controlled between 1.00% and 1.40%, and preferably between 1.20% and 1.40%.

[0046] The phosphorus content is 0.015% or less (excluding 0). .

[0047] Polymer (P) is an unavoidable impurity that segregates at grain boundaries, thereby degrading the toughness of steel and reducing its resistance to delayed fracture. Therefore, it is theoretically advantageous to control the P content to 0%, as it is desirable to keep it as low as possible. However, since P is inevitably included in the manufacturing process, the upper limit for P is controlled at 0.015% or less.

[0048] The sulfur content is 0.015% or less (excluding 0).

[0049] Sulfur (S) is an unavoidable impurity that can reduce the fatigue strength and low-temperature impact toughness of steel. Therefore, it is theoretically advantageous to control the S content to 0%, as it is desirable to keep it as low as possible. However, since S is inevitably included in the manufacturing process, the upper limit for S is controlled at 0.015% or less.

[0050] The Cr (chromium) content is 0.80% to 1.00%.

[0051] Cr increases the hardenability of steel to improve tensile strength and plays a role in preventing strength loss during heat treatment processes such as tempering. When the Cr content is less than 0.80%, the effect of adding Cr is not significant, which may make it difficult to ensure the desired tensile strength. When the Cr content exceeds 1.00%, the hardness of the steel increases excessively, which may reduce machinability. Therefore, Cr is controlled between 0.80% and 1.00%, preferably between 0.84% ​​and 1.00%, and more preferably between 0.84% ​​and 0.98%.

[0052] The Ni content is 0.40% to 1.00%.

[0053] Ni is an element added to refine the grain size of steel, thereby improving hardenability and low-temperature impact toughness. When the Ni content is less than 0.40%, the effect of adding Ni is not significant, thus failing to ensure the desired low-temperature impact toughness. Furthermore, when the content exceeds 1.00%, the hardness increases excessively, which may reduce machinability and increase manufacturing costs, thereby reducing economic efficiency. Therefore, the Ni content is controlled between 0.40% and 1.00%, and preferably between 0.40% and 0.70%.

[0054] The Mo (molybdenum) content is 0.20% to 0.45%.

[0055] Mo is an element that improves the hardenability of steel, refines grain size, and provides resistance to softening during high-temperature tempering. Furthermore, Mo can prevent toughness degradation caused by grain boundary segregation of impurities such as P. To achieve these effects, the lower limit of Mo content is controlled at 0.20%. However, when the Mo content exceeds 0.45%, weldability may decrease, and manufacturing costs may increase due to the large amount of Mo as an expensive element. Therefore, the Mo content is controlled between 0.20% and 0.45%, preferably between 0.20% and 0.42%, and more preferably between 0.20% and 0.40%.

[0056] The Cu (copper) content is greater than 0% and is 0.30% or less. .

[0057] Cu is an element added to improve the strength and corrosion resistance of steel. Adding Cu to steel causes the formation of fine, nano-sized ε-Cu precipitates during the tempering process, thereby enhancing the steel's strength and corrosion resistance. However, when the addition exceeds 0.30%, thermal ductility decreases significantly, which can lead to surface defects during manufacturing; therefore, the Cu content is controlled to be greater than 0% and 0.30% or less.

[0058] The Al (aluminum) content is 0.01% to 0.05%.

[0059] Al is an element that acts as a strong deoxidizer. Furthermore, Al combines with C or N to form precipitates, which contribute to grain refinement. The effect of adding Al is not significant when the addition is less than 0.01%, therefore a lower limit is set at 0.01%. Even when Al is added at levels exceeding 0.05%, the aforementioned effects saturate despite the increased Al content. Moreover, when the Al content exceeds 0.05%, non-metallic inclusions such as Al₂O₃ may form, which can lead to a deterioration in toughness; therefore, the Al content is controlled between 0.01% and 0.05%.

[0060] The vanadium (V) content is 0.050% to 0.100%.

[0061] V forms precipitates with C and N to help improve strength and plays a role in improving low-temperature impact toughness by refining the grain size. Furthermore, it is an element that has an anti-softening effect during tempering and reduces sensitivity to hydrogen-induced cracking (HIC) caused by H (hydrogen). To achieve these effects, the lower limit of V content is controlled at 0.04%. Adding more than 0.050% increases strength, but may decrease low-temperature impact toughness, and manufacturing costs may increase with the increase in the content of the expensive element V. Therefore, the V content is controlled between 0.050% and 0.100%.

[0062] The Ti (titanium) content is 0.008% to 0.020%.

[0063] Ti combines with C and N to help improve strength and plays a role in improving low-temperature impact toughness by refining the grain size. Furthermore, since Ti forms carbonitrides before B at high temperatures, it inhibits the formation of B carbonitrides in the steel, thereby increasing the amount of free B that does not form carbonitrides. By sufficiently ensuring free B, the effect of improving the hardenability of the steel can be maximized. To achieve the above effects, the lower limit of Ti is set at 0.008%. However, adding more than 0.020% may lead to the formation of coarse carbonitrides, which may reduce the elongation of the steel; therefore, the Ti content is controlled between 0.008% and 0.020%, and preferably between 0.010% and 0.020%.

[0064] The N (nitrogen) content is 0.002% to 0.015%.

[0065] Nitrogen (N) is an element that combines with alloying elements such as V, Ti, and Al to form nitrides and contributes to grain refinement. However, excessive addition can actually reduce toughness, so the N content is controlled between 0.002% and 0.015%. Preferably, it is controlled between 0.003% and 0.015%, and more preferably between 0.003% and 0.007%.

[0066] The content of boron (B) is 0.0030% or less.

[0067] Since boron (B) is an element used to improve the strength and hardenability of steel, it can be included when necessary. However, when the B content exceeds 0.0030%, not only are the aforementioned effects saturated, but low-temperature impact toughness may also decrease. Therefore, the upper limit for B is controlled at 0.0030%.

[0068] The Ca content is 0.0020% or less.

[0069] Ca is an element added to improve corrosion resistance. Because Ca dissolves as ions and raises the pH of the corrosion interface where the pH has already decreased, thus inhibiting corrosion, it can be included when necessary. Furthermore, by forming CaS, the size and shape of inclusions can be improved, and the low-temperature impact toughness of the steel can be enhanced. However, when the Ca content exceeds 0.0020%, the effect saturates and the low-temperature impact toughness may actually decrease; therefore, the upper limit is controlled at 0.0020%.

[0070] In addition to the above components, the remaining component is iron (Fe). However, in typical manufacturing processes, unexpected impurities may inevitably be introduced from raw materials or the surrounding environment, and therefore this cannot be ruled out. Since these impurities are known to any person skilled in the art of typical manufacturing processes, not all their details are specifically mentioned in this specification.

[0071] In the aforementioned alloy composition, Al, V, and Ti are elements closely related to low-temperature impact toughness. They form fine carbonitrides in the steel. Carbonitrides of 50 nm or smaller inhibit grain boundary growth, thereby refining the grain size and improving low-temperature impact toughness. However, since excessive addition of Al, V, and Ti can actually reduce toughness, it is essential to understand the correlation between each element to obtain the optimal composition.

[0072] Based on this, the inventors of this disclosure studied the correlation of alloy composition used to improve low-temperature impact toughness in the above alloy composition, and as a result, it was found that when the value of formula (1) is 0.10 or greater and 0.15 or less, steel with excellent low-temperature impact toughness can be obtained.

[0073] Formula (1): 0.10 ≤ Al + V + Ti ≤ 0.15

[0074] In equation (1), each element symbol represents the content (by weight%) of each element.

[0075] When the value of equation (1) is less than 0.10, the carbonitride formation effect of Al, V and Ti is not obvious, so it is impossible to obtain compounds with 2 / µm 2For steels with higher carbonitride densities and ASTM grain size numbers of 8.0 or higher, it is difficult to ensure a Charpy impact energy of 100 J or higher at -20°C. These effects may saturate or decrease when the value of Equation (1) exceeds 0.15; therefore, the value of Equation (1) is controlled to be 0.10 or higher and 0.15 or lower, preferably 0.10 or higher and 0.13 or lower.

[0076] Meanwhile, the manufacturing process of the mooring chain involves quenching and tempering to improve the strength of the final product. However, it is necessary to optimize the carbon equivalent (C). 当量 To ensure the structural stability of the heat-affected zone after the heat treatment process, the inventors of this disclosure have studied a method for optimizing carbon equivalent, and as a result, optimized the carbon equivalent by controlling the correlation between Mn, Ni, Cu, Cr, Mo, V, and C. Steels with excellent low-temperature impact toughness and strength can be obtained when the carbon equivalent value represented by equation (2) is 0.70 or greater and 0.80 or less.

[0077] Equation (2) C 当量 :0.70 ≤ C + (1 / 6)*Mn + (1 / 15)*(Ni + Cu) + (1 / 5)*(Cr + Mo +V) ≤ 0.80

[0078] In equation (2), each element symbol represents the content (by weight%) of each element.

[0079] When the value of Equation (2) is less than 0.70, it is difficult to ensure a tensile strength of 860 MPa or greater, even after quenching and tempering. When the value of Equation (2) exceeds 0.80, it is difficult to ensure a low-temperature impact toughness of 100 J or greater. Therefore, Equation (2) is controlled to be 0.70 or greater and 0.80 or less, and preferably 0.71 or greater and 0.79 or less.

[0080] Furthermore, due to the nature of mooring chains being used in seawater for extended periods, excellent corrosion resistance to seawater is essential. Cu and Ni are elements closely related to corrosion resistance, and their contents need to be appropriately controlled. To improve the corrosion resistance of steel, in addition to the elements mentioned above, the interaction behavior of other alloying elements in the product's operating environment must also be considered. In particular, in the case of Cr, it is usually used to improve corrosion resistance, but it has been shown that when chlorides and the like concentrate on the surface of the mooring chain to form a film, corrosion resistance decreases with the addition of Cr. On the other hand, in the case of Mo, it has been shown that it dissolves in the steel's operating environment to form molybdate ions, thereby improving corrosion resistance.

[0081] Based on this, the inventors of this disclosure have conducted in-depth research on measures to improve corrosion resistance in the aforementioned alloy composition, and as a result, by optimizing the addition amounts of Cr and Mo in addition to Cu and Ni content, and comprehensively considering the interactions between each element, the following formula (3) was derived. When the value of formula (3) is 0.00 or greater and 0.50 or less, a steel exhibiting good corrosion resistance can be obtained. Therefore, the value of formula (3) is controlled to be 0.00 or greater and 0.50 or less, and preferably 0.00 or greater and 0.41 or less.

[0082] Formula (3): 0.00 ≤ (1 - 1.4*Cu) x (1 - 0.4*Ni) + 0.3*Cr - 2.2*Mo ≤ 0.50

[0083] In equation (3), each element symbol represents the content (by weight %) of each element.

[0084] The steel according to this disclosure can exhibit a corrosion resistance diameter reduction of 1.0 mm or less.

[0085] The corrosion resistance diameter reduction value refers to the difference between the diameter before and after the corrosion resistance assessment process ({(diameter before the corrosion resistance assessment process)} mm) measured after eight cycles of the corrosion resistance assessment process consisting of seawater immersion, drying, and wetting.

[0086] The seawater immersion process was performed by immersing the sample in artificial seawater. The composition (unit: g / L) of the prepared artificial seawater was: NaCl: 24.5, MgCl2·6H2O: 11.1, Na2SO4: 4.1, CaCl2: 1.2, KCl: 0.7. The sample was immersed in the prepared artificial seawater for 15 minutes while maintaining it at 35°C. Afterwards, drying and wetting processes were carried out.

[0087] The drying process involved exposing the sample to a drying environment simulating the dry state of the sample surface in the atmosphere, and the wetting process involved exposing the sample to a wetting environment simulating the state of the sample surface being wetted by droplets in the atmosphere. During the drying process, the temperature was maintained at 60°C and the relative humidity at 25% RH, and during the wetting process, the temperature was maintained at 60°C and the relative humidity at 100% RH.

[0088] Each process was performed by exposing the specimens to each process condition for 4 hours. These are extreme environmental conditions set with consideration of the steel's intended use, and the steel's corrosion resistance can be guaranteed if the reduction in corrosion diameter is less than 1.0 mm under these conditions.

[0089] Furthermore, the steel according to one example of this disclosure may contain carbonitrides, and the amount of carbonitrides with a size of 50 nm or smaller may be per 1 µm. 2 Two or more.

[0090] Here, carbonitrides refer to Al(C,N), V(C,N), Ti(C,N) and their composite carbides, and the size of the carbide refers to its size measured in terms of the equivalent circle diameter. The reason for limiting the size of carbonitrides to 50 nm or less is that carbonitrides with sizes exceeding 50 nm contribute little to the improvement in strength, refinement of the original austenite, and reduction of hydrogen-induced cracking. The number of carbonitrides with sizes of 50 nm or less is limited to 1 µm. 2 The reason for less than 2 is that the amount of carbonitrides per 1 µm is less than 2. 2 The above effect is insufficient when there are fewer than two.

[0091] At this point, the steel according to this disclosure may have an ASTM grain size number (G) of 8.0 or greater for the original austenite. The ASTM grain size number (G) is related to the grain size per unit area (1 inch). 2 The number of grains (N) AE They have the following relationship.

[0092] N AE = 2 (G-1)

[0093] When the ASTM grain size number is 8.0 or greater, the average grain size of the original austenite is less than about 22.5 µm, thus fully refining the grain size of the steel and ensuring low-temperature impact toughness of 100 J or greater.

[0094] Furthermore, the steel according to one example of this disclosure may have a Charpy impact energy of 100 J or greater at -20°C and a tensile strength of 860 MPa or greater.

[0095] Next, a method for manufacturing steel according to this disclosure will be described.

[0096] A method for manufacturing steel according to one embodiment of this disclosure includes: preparing a primary rolled steel billet, said primary rolled steel billet comprising, by weight percentage (wt%): 0.20% to 0.40% carbon (C), 0.10% to 0.40% silicon (Si), 1.00% to 1.40% manganese (Mn), greater than 0% and less than 0.015% phosphorus (P), greater than 0% and less than 0.015% sulfur (S), 0.80% to 1.00% chromium (Cr), 0.40% to 1.00% nickel (Ni), 0.20% to 0.45% molybdenum (Mo), and greater than 0% and less than 0.3%. 0% or less copper (Cu), 0.01% to 0.05% aluminum (Al), 0.050% to 0.100% vanadium (V), 0.008% to 0.020% titanium (Ti), 0.002% to 0.015% nitrogen (N), 0.0030% or less boron (B), 0.0020% or less calcium (Ca), and the remainder iron (Fe) and unavoidable impurities, and satisfying the following formulas (1) and (2); producing bar steel by hot rolling of a primary rolled steel billet; quenching the bar steel by water cooling at 850°C to 1,000°C; and tempering the water-cooled bar steel.

[0097] Formula (1): 0.10 ≤ Al + V + Ti ≤ 0.15

[0098] Equation (2) C 当量 :0.70 ≤ C + (1 / 6)*Mn + (1 / 15)*(Ni + Cu) + (1 / 5)*(Cr + Mo +V) ≤ 0.80

[0099] In equations (1) and (2), each element symbol represents the content (by weight%) of each element.

[0100] Each step will be described in detail below.

[0101] Steps for preparing primary rolled steel billets

[0102] A primary rolled steel billet with an alloy composition satisfying the above-described contents of this disclosure is prepared. The roles and contents of each component in the alloy composition, as well as the descriptions of equations (1) and (2), are as described above. At this time, since the general manufacturing conditions for primary rolled steel billets can be used as the manufacturing conditions for primary rolled steel billets, there are no particular limitations on them.

[0103] According to one example of this disclosure, the initial rolled steel billet can satisfy the following equation (3). Equation (3) is described as above.

[0104] Formula (3): 0.00 ≤ (1 - 1.4*Cu) x (1 - 0.4*Ni) + 0.3*Cr - 2.2*Mo ≤ 0.50

[0105] In equation (3), each element symbol represents the content (by weight %) of each element.

[0106] Steps for producing bar steel

[0107] The initial rolled steel billet is hot-rolled to produce bar steel. At this time, since the general manufacturing conditions for bar steel can be used as the manufacturing conditions for bar steel, there are no particular restrictions.

[0108] For example, the method may include heating a primary rolled steel billet at 1,150°C to 1,250°C for 3 to 6 hours, then rolling the primary rolled steel billet at 1,050°C to 1,150°C to produce a square billet, and heating the square billet at 1,150°C to 1,250°C for 1 to 2 hours, then hot rolling the square billet to produce a bar.

[0109] The steps of quenching and water cooling of bar steel

[0110] After quenching the steel bar at 850°C to 1,000°C, it is water-cooled. Preferably, the quenching step can be performed at 880°C to 1,000°C, and more preferably at 880°C to 950°C. Furthermore, the quenching step may take 30 minutes or longer to ensure sufficient reaction time.

[0111] When the quenching step is performed below 850°C, the austenitic phase transformation may not occur sufficiently, and the solution treatment in the steel may not be adequately achieved, making it difficult to fully guarantee the strength of the steel. However, when the quenching step is performed above 1000°C, oxide scale may form, and the grains may coarsen, which may reduce low-temperature impact toughness. Therefore, quenching within the above temperature range can yield steel with the target strength and low-temperature impact toughness.

[0112] Meanwhile, the quenching step can be performed using water cooling. However, if the quenching step is performed using oil cooling instead of water cooling, sufficient strength cannot be guaranteed due to the low cooling rate.

[0113] The steps of tempering water-cooled steel bars

[0114] Temper the water-cooled steel bars. The tempering process can be carried out at 600°C to 650°C for 30 minutes or longer.

[0115] When the tempering temperature is below 600°C, carbonitrides are not fully formed, resulting in reduced low-temperature impact toughness and a risk of product breakage during final product manufacturing. However, when the tempering temperature exceeds 650°C, the strength may decrease. Therefore, tempering within the above temperature range yields steel with the target strength and low-temperature impact toughness.

[0116] Steel manufactured under the manufacturing conditions according to this disclosure may contain carbonitrides, and the amount of carbonitrides with a size of 50 nm or smaller may be per 1 µm. 2 Two or more.

[0117] Furthermore, the corrosion-resistant diameter reduction of steel manufactured under the manufacturing conditions according to this disclosure can be 1.0 mm or less.

[0118] The configuration and function of this disclosure will be described in more detail below through preferred embodiments. However, this is presented as a preferred embodiment of this disclosure and should not be construed as limiting this disclosure in any way.

[0119] (Example)

[0120] The samples of Examples 1 to 7 and Comparative Examples 1 to 7 were prepared by the following method.

[0121] A rough-rolled steel billet with the composition shown in Table 1 was prepared and heated at 1200°C for 4 hours, then finished rolled at 1100°C to produce a square billet. Subsequently, the square billet was heated at 1200°C for 1 hour and 30 minutes and then hot-rolled to produce a bar steel with a thickness of 25 mm. The bar steel was then quenched at 900°C for 60 minutes and then water-cooled, followed by tempering at 640°C for 60 minutes to produce the final sample.

[0122] Table 1 below shows the component composition content (wt%) of each sample, and Table 2 shows the values ​​of equations (1) to (3) calculated from the component composition of each sample. Underlined values ​​indicate values ​​outside the scope of this disclosure.

[0123] [Table 1]

[0124]

[0125] [Table 2]

[0126]

[0127] Subsequently, the physical properties and microstructure of each sample were observed for the following items.

[0128] [tensile strength]

[0129] Tensile strength was measured according to ASTM E8M. Measurements were taken by collecting specimens at a point 1 / 2 t along the thickness direction of each specimen, and the average of five measurements is shown.

[0130] [Charpy Impact Energy at -20℃]

[0131] To evaluate the low-temperature impact toughness of each specimen, the Charpy impact energy at -20°C was measured. After fabricating specimens with a V-notch and dimensions of 10*10*55 mm, the average value of five measurements taken at -20°C using a Charpy impact tester is shown.

[0132] [Carbon and nitride density]

[0133] Carbonitride density was measured by fabricating TEM replicas. After capturing TEM images at 20,000x magnification, the density was measured per 1 µm. 2 The number of carbonitrides of 50 nm or smaller. The average value is shown after measurements were taken at 20 randomly selected locations in each sample.

[0134] [ASTM Particle Size Number]

[0135] The particle size number (ASTM particle size number, G) is indicated by measuring the particle size of the original austenite according to ASTM E112.

[0136] [Reduction in diameter due to corrosion resistance]

[0137] To assess the corrosion resistance of each specimen, the specimens were cut into pieces with a diameter of 20 mm and a length of 100 mm. For each specimen, a corrosion resistance assessment process consisting of seawater immersion, drying and wetting, and a wetting process was performed. One cycle of the corrosion resistance assessment process consisted of the following steps.

[0138] The seawater immersion process was carried out by immersing the sample in artificial seawater. The composition (unit: g / L) of the prepared artificial seawater was: NaCl: 24.5, MgCl2·6H2O: 11.1, Na2SO4: 4.1, CaCl2: 1.2, KCl: 0.7. The sample was immersed in the prepared artificial seawater for 15 minutes while maintaining it at 35°C.

[0139] Subsequently, drying and wetting processes were performed. The drying process involved exposing the sample to a drying environment simulating the dry state of the sample surface in the atmosphere, while the wetting process involved exposing the sample to a wetting environment simulating the state of the sample surface being wetted by droplets in the atmosphere. During the drying process, the temperature was maintained at 60°C and the relative humidity at 25% RH, and during the wetting process, the temperature was maintained at 60°C and the relative humidity at 100% RH. Each process was performed by exposing the sample to each process condition for 4 hours.

[0140] After eight cycles of the corrosion resistance evaluation process, consisting of seawater immersion, drying, and wetting, the reduction in diameter was measured. The average diameter was calculated after measuring the reduction at five locations—the center and both ends—for each specimen. Good corrosion resistance was determined when the reduction in diameter was within 1.0 mm.

[0141] Table 3 shows the observations for the above items. In Table 3, underlined values ​​indicate values ​​outside the scope of this disclosure.

[0142] [Table 3]

[0143]

[0144] Examples 1 to 7, which meet the alloy composition disclosed in this disclosure and all of Equations (1) to (3), have a tensile strength of 860 MPa or greater, a Charpy impact energy of 100 J or greater at -20°C, and a carbonitride density of 2 atoms / µm. 2 Or larger, ASTM particle size number 8.0 or larger, and corrosion resistance diameter reduction value 1 mm or smaller. In the case of Comparative Example 1, the low-temperature impact toughness deteriorated due to the excessively high C content and the value of Equation (2) exceeding 0.80. Therefore, the Charpy impact energy at -20°C showed to be less than 100 J.

[0145] In Comparative Example 2, all equations (1) to (3) of this disclosure are satisfied, but as a sample with excessively high Si content, the low-temperature impact toughness deteriorates. Therefore, the Charpy impact energy at -20°C is less than 100 J.

[0146] In Comparative Example 3, the tensile strength deteriorated due to the low Mn content and the fact that the value of Equation (2) did not reach 0.70. Therefore, the tensile strength was less than 860 MPa.

[0147] In Comparative Example 4, the Cr content was too high, causing Equation (3) to be greater than 0.50. As a result, corrosion resistance deteriorated, and the corrosion diameter decreased by more than 1.00 mm.

[0148] In Comparative Example 5, the Mo content was included in the sample at more than 0.45%, which led to a deterioration in low-temperature impact toughness. As a result, the Charpy impact energy at -20°C did not reach 100 J.

[0149] In Comparative Example 6, since the V content was low and the value of Equation (1) did not reach 0.10, no fine carbonitrides were formed, and therefore the carbonitride density was 1 per µm. 2 (less than 2 / µm) 2 The ASTM grain size number is 7.3 (less than 8.0). Therefore, the effect of grain refinement on improving low-temperature impact toughness is not significant, and the Charpy impact energy at -20°C is less than 100 J.

[0150] In Comparative Example 7, the alloy composition presented in this disclosure is satisfied, but the value of Equation (3) exceeds 0.50. Therefore, the improvement in corrosion resistance is not significant, and the reduction in corrosion diameter is shown to exceed 1.00 mm.

[0151] After manufacturing specimens with the same composition as Examples 1 and 7 but different quenching and tempering temperatures during manufacturing, tensile strength, Charpy impact energy at -20°C, and ASTM particle size number were measured.

[0152] Comparative Examples 8 and 9 are samples with the same alloy composition as Example 1, and Comparative Examples 10 and 11 are samples with the same alloy composition as Example 7. Comparative Examples 8 to 11 were prepared in the same manner as Example 1, except that the quenching and tempering temperatures are shown in Table 4.

[0153] Table 4 shows the observations for each sample. Each item was measured in the same manner as in Table 3.

[0154] [Table 4]

[0155]

[0156] In Comparative Examples 8 and 9, the quenching or tempering temperatures did not reach the temperature range specified in this disclosure, therefore insufficient solution treatment occurred, and the effect of improving strength or toughness was not significant. In Comparative Example 10, the quenching temperature exceeded 1,000°C. Therefore, insufficient grain refinement occurred, resulting in an ASTM grain size number less than 8.0, and the improvement in low-temperature impact toughness due to grain refinement was not significant; thus, the Charpy impact energy at -20°C was less than 100 J.

[0157] In Comparative Example 11, the tempering temperature exceeded 650°C. Therefore, the strength improvement effect was not significant, and a tensile strength of 860 MPa or greater was not guaranteed.

[0158] Although described with reference to the above embodiments, those skilled in the art will understand that various modifications and changes can be made to this disclosure without departing from the spirit and scope of the disclosure as described in the appended claims.

Claims

1. A steel comprising, by weight percentage (wt%): 0.20% to 0.40% carbon (C), 0.10% to 0.40% silicon (Si), 1.00% to 1.40% manganese (Mn), greater than 0% and less than 0.015% phosphorus (P), greater than 0% and less than 0.015% sulfur (S), 0.80% to 1.00% chromium (Cr), 0.40% to 1.00% nickel (Ni), and 0.20% to 0.45% molybdenum (Mn). o), greater than 0% and less than 0.30% copper (Cu), 0.01% to 0.05% aluminum (Al), 0.050% to 0.100% vanadium (V), 0.008% to 0.020% titanium (Ti), 0.002% to 0.015% nitrogen (N), 0.0030% or less boron (B), 0.0020% or less calcium (Ca), and the remainder iron (Fe) and unavoidable impurities, and satisfying the following formulas (1) and (2). Formula (1): 0.10 ≤ Al + V + Ti ≤ 0.15 Formula (2) C 当量 : 0.70 ≤ C + (1 / 6)*Mn + (1 / 15)*(Ni + Cu) + (1 / 5)*(Cr + Mo + V) ≤ 0.80 (Each element symbol represents the content (by weight%) of each element).

2. The steel according to claim 1 satisfies the following formula (3). Formula (3): 0.00 ≤ (1 - 1.4*Cu) x (1 - 0.4*Ni) + 0.3*Cr - 2.2*Mo ≤ 0.50 (Each element symbol represents the content (by weight%) of each element).

3. The steel according to claim 1, wherein the steel has a Charpy impact energy of 100 J or greater and a tensile strength of 860 MPa or greater at -20°C.

4. The steel according to claim 1, wherein the steel comprises carbonitrides, and the amount of said carbonitrides is per µm 2 Two or more.

5. The steel according to claim 1, wherein the steel comprises carbonitrides, and the carbonitrides have a size of 50 nm or less.

6. The steel of claim 1, wherein the steel has an ASTM grain size number of 8.0 or greater for the original austenite.

7. The steel according to claim 1, wherein the steel has a corrosion-resistant diameter reduction value of 1.0 mm or less.

8. A method for manufacturing steel, the method comprising: A preliminary rolled steel billet is prepared, wherein the preliminary rolled steel billet comprises, by weight percentage (wt%): 0.20% to 0.40% carbon (C), 0.10% to 0.40% silicon (Si), 1.00% to 1.40% manganese (Mn), greater than 0% and less than 0.015% phosphorus (P), greater than 0% and less than 0.015% sulfur (S), 0.80% to 1.00% chromium (Cr), 0.40% to 1.00% nickel (Ni), and 0.20% to 0.45% [unclear - possibly referring to a specific component or component]. Molybdenum (Mo), copper (Cu) greater than 0% and less than 0.30%, aluminum (Al) from 0.01% to 0.05%, vanadium (V) from 0.050% to 0.100%, titanium (Ti) from 0.008% to 0.020%, nitrogen (N) from 0.002% to 0.015%, boron (B) from 0.0030% or less, calcium (Ca) from 0.0020% or less, and the remainder iron (Fe) and unavoidable impurities, and satisfying the following formulas (1) and (2); The initial rolled steel billet is hot rolled to produce bar steel; The steel bar is quenched by water cooling at 850°C to 1,000°C. as well as Tempering of water-cooled steel bars; Formula (1): 0.10 ≤ Al + V + Ti ≤ 0.15 Formula (2) C 当量 : 0.70 ≤ C + (1 / 6)*Mn + (1 / 15)*(Ni + Cu) + (1 / 5)*(Cr + Mo + V) ≤ 0.80 (Each element symbol represents the content (by weight%) of each element).

9. The method according to claim 8, wherein the production of the bar steel comprises: The initial rolled steel billet is heated at 1,150°C to 1,250°C for 3 to 6 hours, and then rolled at 1,050°C to 1,150°C to produce a square billet. as well as The billet is heated at 1,150°C to 1,250°C for 1 to 2 hours, and then hot-rolled to produce bar steel.

10. The method of claim 8, wherein the tempering is performed at 600°C to 650°C.

11. The method according to claim 8, wherein the initial rolled steel billet satisfies the following formula (3). Formula (3): 0.00 ≤ (1 - 1.4*Cu) x (1 - 0.4*Ni) + 0.3*Cr - 2.2*Mo ≤ 0.50 (Each element symbol represents the content (by weight%) of each element).

12. The method of claim 8, wherein the steel comprises carbonitrides, and the amount of carbonitrides is per µm 2 Two or more.

13. The method of claim 8, wherein the steel comprises carbonitrides, and the carbonitrides have a size of 50 nm or less.

14. The method of claim 8, wherein the corrosion-resistant diameter reduction value of the steel is 1.0 mm or less.

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