Steel material and manufacturing method for same

Abstract

Provided is a steel material that contains, in wt%, 0.20-0.40% of carbon (C), 0.10-0.40% of silicon (Si), 1.00-1.40% of manganese (Mn), more than 0% and not more than 0.015% of phosphorus (P), more than 0% and not more than 0.015% of sulfur (S), 0.80-1.00% of chromium (Cr), 0.40-1.00% of nickel (Ni), 0.20-0.45% of molybdenum (Mo), more than 0% and not more than 0.30% of copper (Cu), 0.01-0.05% of aluminum (Al), 0.050-0.100% of vanadium (V), 0.008-0.020% of titanium (Ti), 0.002-0.015% of nitrogen (N), 0.0030% or less of boron (B), and 0.0020% or less of calcium (Ca), with the remainder comprising Fe and inevitable impurities, and satisfies Expressions (1) and (2) below. Expression (1): 0.10 ≤ Al + V + Ti ≤ 0.15 Expression (2) Ceq: 0.70 ≤ C + (1 / 6)*Mn + (1 / 15)*(Ni + Cu) + (1 / 5)*(Cr + Mo + V) ≤ 0.80 (where, each element symbol represents the content (wt%) of each element)

Description

Steel and its manufacturing method

[0001] The present invention relates to steel and a method for manufacturing the same.

[0002] Recently, with the increasing activity in the development of resources such as crude oil and gas in the ocean, such as floating production storage and offloading (FPSO) facilities, and the construction of floating offshore wind farms representing eco-friendly renewable energy, the importance of mooring chains, which secure various offshore platforms and ships below the sea surface, is increasing.

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

[0004] The steel used in mooring chain manufacturing is high-alloy steel, which is expensive to produce. It also forms 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 cracking. Furthermore, the quenching process also poses a risk of cracking.

[0005] Accordingly, there is a need for the development of high-strength and high-toughness steel and its manufacturing method through optimized design to reduce the heat treatment sensitivity of steel, improve weldability, and secure hardenability.

[0006] The present invention aims to provide a steel material having improved tensile strength, low-temperature impact toughness, and corrosion resistance based on steel material used in the manufacture of mooring chains and the like.

[0007] One aspect of the present invention is a composition comprising, in weight %, carbon (C): 0.20 to 0.40%, silicon (Si): 0.10 to 0.40%, manganese (Mn): 1.00 to 1.40%, phosphorus (P): more than 0% and 0.015% or less, sulfur (S): more than 0% and 0.015% or less, chromium (Cr): 0.80 to 1.00%, nickel (Ni): 0.40 to 1.00%, molybdenum (Mo): 0.20 to 0.45%, copper (Cu): more than 0% and 0.30% or less, aluminum (Al): 0.01 to 0.05%, vanadium (V): 0.050 to 0.100%, titanium (Ti): 0.008 to 0.020%, nitrogen (N): 0.002 to 0.015%, boron (B): It relates to steel containing 0.0030% or less, calcium (Ca): 0.0020% or less, and the remainder iron (Fe) and unavoidable impurities, and satisfying the following formulas (1) and (2).

[0008] Equation (1): 0.10 ≤ Al + V + Ti ≤ 0.15

[0009] Equation (2) C eq : 0.70 ≤ C + (1 / 6)*Mn + (1 / 15)*(Ni + Cu) + (1 / 5)*(Cr + Mo + V) ≤ 0.80

[0010] (Here, each element symbol represents the content (weight%) of each element)

[0011] A steel material according to one aspect of the present invention can satisfy equation (3).

[0012] Equation (3): 0.00 ≤ (1 - 1.4*Cu) x (1 - 0.4*Ni) + 0.3*Cr - 2.2*Mo ≤ 0.50

[0013] (Here, each element symbol represents the content (weight%) of each element)

[0014] A steel according to one aspect of the present invention may have a Charpy impact energy of 100 J or more at -20°C and a tensile strength of 860 MPa or more.

[0015] According to one aspect of the present invention, a steel material comprises carbonitrides having a size of 50 nm or less, the number of which is 1 ㎛. 2 It can contain more than 2 parties.

[0016] According to one aspect of the present invention, the steel may have an ASTM grain size number of 8.0 or more.

[0017] According to one aspect of the present invention, the corrosion resistance reduction diameter of the steel may be 1.0 mm or less.

[0018] Another aspect of the present invention is a composition comprising, in weight %, carbon (C): 0.20 to 0.40%, silicon (Si): 0.10 to 0.40%, manganese (Mn): 1.00 to 1.40%, phosphorus (P): more than 0% and 0.015% or less, sulfur (S): more than 0% and 0.015% or less, chromium (Cr): 0.80 to 1.00%, nickel (Ni): 0.40 to 1.00%, molybdenum (Mo): 0.20 to 0.45%, copper (Cu): more than 0% and 0.30% or less, aluminum (Al): 0.01 to 0.05%, vanadium (V): 0.050 to 0.100%, titanium (Ti): 0.008 to 0.020%, nitrogen (N): 0.002 to 0.015%, boron (B): The present invention relates to a method for manufacturing steel, comprising: a step of preparing a bloom containing 0.0030% or less of zinc, 0.0020% or less of calcium (Ca), and the remainder of iron (Fe) and unavoidable impurities, and satisfying the following formulas (1) and (2); a step of hot-rolling the bloom to manufacture a steel bar; a step of quenching the steel bar by water cooling at 850 to 1,000°C; and a step of tempering the water-quenched steel bar.

[0019] Equation (1): 0.10 ≤ Al + V + Ti ≤ 0.15

[0020] Equation (2) C eq : 0.70 ≤ C + (1 / 6)*Mn + (1 / 15)*(Ni + Cu) + (1 / 5)*(Cr + Mo + V) ≤ 0.80

[0021] (Here, each element symbol represents the content (weight%) of each element)

[0022] In a method for manufacturing steel according to one aspect of the present invention, the step of manufacturing the steel bar may include a step of heating the bloom at 1,150 to 1,250°C for 3 to 6 hours and then rolling it at 1,050 to 1,150°C to manufacture a billet; and a step of heating the billet at 1,150 to 1,250°C for 1 to 2 hours and then hot-rolling it to manufacture the steel bar.

[0023] In a method for manufacturing steel according to one aspect of the present invention, the tempering step can be performed at 600 to 650°C.

[0024] In a method for manufacturing a steel according to one aspect of the present invention, the bloom can satisfy the following equation (3).

[0025] Equation (3): 0.00 ≤ (1 - 1.4*Cu) x (1 - 0.4*Ni) + 0.3*Cr - 2.2*Mo ≤ 0.50

[0026] (Here, each element symbol represents the content (weight%) of each element)

[0027] A steel manufactured by a manufacturing method according to one aspect of the present invention comprises carbonitrides having a size of 50 nm or less and a size of 1 μm or less. 2 It can contain more than 2 parties.

[0028] The corrosion resistance reduction diameter of the steel manufactured by the manufacturing method according to one aspect of the present invention may be 1.0 mm or less.

[0029] According to the present invention, a steel material with excellent corrosion resistance, low-temperature impact toughness, and tensile strength, and a method for manufacturing the same, can be provided. The effects achieved by the present invention are not limited to those mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art to which the present invention pertains from the description below.

[0030] Preferred embodiments of the present invention are described below. However, the embodiments of the present invention may be modified in various ways, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more fully explain the present invention to those of ordinary skill in the art.

[0031] The terminology used in this application is solely for the purpose of describing specific examples. Therefore, for example, singular expressions include plural expressions unless the context clearly dictates otherwise. Additionally, it should be noted that terms such as "comprise" or "have" used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the presence of other features, steps, functions, components, or combinations thereof.

[0032] Meanwhile, unless otherwise defined, all terms used herein should be considered to have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Therefore, unless explicitly defined herein, specific terms should not be interpreted in an overly idealistic or formal sense.

[0033] In addition, the terms "about", "substantially", etc. in this specification are used in the sense of or close to the numerical value when manufacturing and material tolerances inherent to the meanings mentioned are presented, and are used to prevent unscrupulous infringers from unfairly exploiting the invention contents in which exact or absolute numerical values ​​are mentioned to aid understanding of the invention.

[0034] Unless otherwise specifically stated herein, percentages indicating the content of each element are based on weight.

[0035] According to one aspect of the present invention, the steel material contains, in wt%, carbon (C): 0.20 to 0.40%, silicon (Si): 0.10 to 0.40%, manganese (Mn): 1.00 to 1.40%, phosphorus (P): more than 0% and 0.015% or less, sulfur (S): more than 0% and 0.015% or less, chromium (Cr): 0.80 to 1.00%, nickel (Ni): 0.40 to 1.00%, molybdenum (Mo): 0.20 to 0.45%, copper (Cu): more than 0% and 0.30% or less, aluminum (Al): 0.01 to 0.05%, vanadium (V): 0.04 to 0.10%, titanium (Ti): 0.008 to 0.020%, nitrogen (N): 0.002 to 0.015%, boron (B): 0.0030% or less, calcium (Ca): 0.0020% or less, and the remainder contains iron (Fe) and unavoidable impurities.

[0036] Hereinafter, the role and content of each component included in the steel according to the present invention will be described.

[0037] The C (carbon) content is 0.20 to 0.40%.

[0038] Carbon (C) is an essential element added to ensure product strength, and must be contained within an appropriate range in steel. If the C content is less than 0.20%, it is difficult to ensure sufficient strength, and the high content of expensive alloying elements added to replace C may reduce economic feasibility. If the C content exceeds 0.40%, the hardness of the material may increase excessively, resulting in deterioration of low-temperature impact toughness and weldability. Therefore, the C content is controlled to 0.20 to 0.40%, and preferably 0.20 to 0.30%.

[0039] The content of Si (silicon) is 0.10 to 0.40%.

[0040] Silicon (Si) is an element used not only for deoxidation but also to improve wear resistance. If the Si content is less than 0.10%, the deoxidation effect of the steel is minimal, making it difficult to obtain clean steel. If the Si content exceeds 0.40%, the toughness of the weld heat-affected zone may deteriorate, resulting in lower low-temperature impact toughness. Therefore, the Si content is controlled to 0.10 to 0.40%, and preferably 0.14 to 0.35%.

[0041] The content of Mn (manganese) is 1.00 to 1.40%.

[0042] Manganese (Mn) is an element added to improve strength through solid solution strengthening while simultaneously improving hardenability. In addition, Mn can prevent red-hot embrittlement caused by S present in steel by forming an appropriate amount of MnS. When the Mn content is less than 1.00%, the effect of adding Mn is minimal, making it difficult to secure sufficient strength and hardenability may deteriorate. When the Mn content exceeds 1.40%, low-temperature impact toughness may deteriorate rapidly and it may be difficult to secure sufficient weldability. Therefore, in order to secure sufficient weldability, hardenability, and tensile strength without deteriorating low-temperature impact toughness, the Mn content is controlled to 1.00 to 1.40%, and preferably 1.20 to 1.40%.

[0043] The content of P(phosphorus) is 0.015% or less (excluding 0).

[0044] Phosphorus (P) is an unavoidable impurity that segregates at grain boundaries, lowering the toughness of steel and reducing its resistance to delayed fracture. Therefore, it is desirable to control its content as low as possible. Therefore, theoretically, it is advantageous to control the P content at 0%. However, since P is inevitably included during the manufacturing process, the upper limit of P is controlled to 0.015% or less.

[0045] The content of S (sulfur) is 0.015% or less (excluding 0).

[0046] S is an unavoidable impurity that can degrade the fatigue strength and low-temperature impact toughness of steel. Therefore, it is desirable to control its content as low as possible. Therefore, theoretically, it is advantageous to control the S content to 0%. However, since S is inevitably included during the manufacturing process, the upper limit of S is controlled to 0.015% or less.

[0047] The content of Cr (chromium) is 0.80 to 1.00%.

[0048] Cr increases the hardenability of steel, thereby enhancing tensile strength, and prevents strength loss during heat treatment processes such as tempering. If the Cr content is less than 0.80%, the effect of adding Cr is minimal, making it difficult to secure the desired tensile strength. If the Cr content exceeds 1.00%, the hardness of the steel may increase excessively, thereby reducing workability. Therefore, Cr is controlled to 0.80 to 1.00%, preferably 0.84 to 1.00%, and more preferably 0.84 to 0.98%.

[0049] The content of Ni (nickel) is 0.40 to 1.00%.

[0050] Ni is an element added to refine the grain size of steel to improve hardenability and low-temperature impact toughness. When the Ni content is less than 0.40%, the effect of Ni addition is minimal, failing to secure the desired low-temperature impact toughness. When the Ni content exceeds 1.00%, hardness may increase excessively, reducing machinability and increasing manufacturing costs, lowering economic feasibility. Therefore, the Ni content is controlled to be between 0.40 and 1.00%, and preferably between 0.40 and 0.70%.

[0051] The content of Mo (molybdenum) is 0.20 to 0.45%.

[0052] Mo is an element that improves the hardenability of steel, has a grain refinement effect, and has a softening resistance effect during high-temperature tempering. In addition, Mo can prevent the deterioration of toughness due to grain boundary segregation of impurities such as P. To obtain the above-mentioned effects, the lower limit of Mo is controlled to 0.20%. However, if the Mo content exceeds 0.45%, weldability may deteriorate, and manufacturing cost may increase due to the inclusion of a large amount of Mo, which is an expensive element. Therefore, the Mo content is controlled to 0.20 to 0.45%, preferably 0.20 to 0.42%, and more preferably 0.20 to 0.40%.

[0053] The content of Cu (copper) is greater than 0% and less than or equal to 0.30%.

[0054] Copper (Cu) is an element added to steel to improve its strength and corrosion resistance. When added to Cu steel, it forms fine nanoscale ε-Cu precipitates during the tempering process, thereby improving the steel's strength and corrosion resistance. However, if added in excess of 0.30%, hot ductility is significantly reduced, which can cause surface defects during manufacturing. Therefore, the Cu content is controlled to be greater than 0% and less than 0.3%.

[0055] The content of Al (aluminum) is 0.01 to 0.05%.

[0056] Al is an element that acts as a strong deoxidizer. In addition, Al combines with C or N to form precipitates, which plays a role in refining the crystal grains. Since the effect of adding Al is minimal when added below 0.01%, the lower limit is set at 0.01%. Even if Al is added exceeding 0.05%, the above-mentioned effect is saturated despite the increase in Al content. In addition, if Al exceeds 0.05%, it can form non-metallic inclusions such as Al2O3, which can cause a decrease in toughness, so the Al content is controlled to 0.01 to 0.05%.

[0057] The content of V (vanadium) is 0.050 to 0.100%.

[0058] V forms precipitates with C and N, contributing to the improvement of strength, and plays a role in improving low-temperature impact toughness by refining crystal grains. In addition, it is an element that has a softening resistance effect during the tempering process and has an effect of reducing the susceptibility to hydrogen induced cracking (HIC) by H (hydrogen). To achieve the above-mentioned effects, the lower limit of V is controlled to 0.04%. If V is added exceeding 0.050%, the strength increases, but the low-temperature impact toughness may decrease, and the manufacturing cost may increase as the content of V, an expensive element, increases. Therefore, the content of V is controlled to 0.050 to 0.100%.

[0059] The content of Ti (titanium) is 0.008 to 0.020%.

[0060] Ti combines with C and N to contribute to strength enhancement, and plays a role in improving low-temperature impact toughness by refining crystal grains. In addition, since Ti forms carbonitrides before B at high temperatures, it suppresses the formation of carbonitrides of B in the steel, thereby increasing the amount of free B that does not form carbonitrides. As the free B is sufficiently secured, the effect of improving the hardenability of the steel can be maximized. To obtain the above-described effect, the lower limit of Ti is set to 0.008%. However, when added in excess of 0.020%, coarse carbonitrides may be formed, which may reduce the elongation of the steel. Therefore, the Ti content is controlled to 0.008 to 0.020%, and preferably 0.010 to 0.020%.

[0061] The content of N (nitrogen) is 0.002 to 0.015%.

[0062] Nitrogen 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 to 0.002 to 0.015%. Preferably, it is controlled to 0.003 to 0.015%, and more preferably, it is controlled to 0.003 to 0.007%.

[0063] The content of B (boron) is less than 0.0030%.

[0064] B is an element that improves the strength and hardenability of steel, and thus may be included as needed. However, if the B content exceeds 0.0030%, not only will the aforementioned effects be saturated, but low-temperature impact toughness may also deteriorate. Therefore, the upper limit of B is controlled at 0.0030%.

[0065] The Ca (calcium) content is less than 0.0020%.

[0066] Ca is an element added to improve corrosion resistance. Ca can be dissolved as an ion, increasing the pH of the corrosion interface where the pH is low, thereby inhibiting corrosion. Therefore, its inclusion may be appropriate. Furthermore, by forming CaS, it can improve the size and shape of inclusions and enhance the low-temperature impact toughness of the steel. However, if the Ca content exceeds 0.0020%, the effect becomes saturated, and low-temperature impact toughness may actually decrease. Therefore, the upper limit is controlled at 0.0020%.

[0067] In addition to the above composition, the remaining component is iron (Fe). However, during the normal manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the normal manufacturing process, their full details are not specifically mentioned in this specification.

[0068] Among the alloy compositions described above, Al, V, and Ti are elements closely related to low-temperature impact toughness. These elements form fine carbonitrides within the steel. Carbonitrides smaller than 50 nm suppress grain boundary growth, resulting in finer grain sizes and improved low-temperature impact toughness. However, excessive addition of Al, V, and Ti can actually reduce toughness. Therefore, the interrelationships among these elements must be understood to determine the optimal composition.

[0069] Inspired by this, the inventors of the present invention studied the correlation of alloy compositions for improving low-temperature impact toughness within the above-described alloy compositions, and as a result, they deduced that when the value of equation (1) is 0.10 or more and 0.15 or less, a steel having excellent low-temperature impact toughness can be obtained.

[0070] Equation (1): 0.10 ≤ Al + V + Ti ≤ 0.15

[0071] In the above formula (1), each element symbol represents the content (weight%) of each element.

[0072] When the value of equation (1) is less than 0.10, the carbonitride formation effect of Al, V, and Ti is minimal, and the carbonitride density is 2 / ㎛. 2 Above, it is difficult to obtain steel having an ASTM grain size number of 8.0 or higher, and thus it is difficult to secure a -20℃ Charpy impact energy of 100 J or higher. If the value of equation (1) exceeds 0.15, this effect may be saturated or even reduced, so the value of equation (1) is controlled to be 0.10 or more and 0.15 or less, and preferably 0.10 or more and 0.13 or less.

[0073] Meanwhile, in the manufacturing process of mooring chains, quenching and tempering processes are performed to improve the strength of the final product. However, in order to secure the structural stability of the heat-affected zone after the heat treatment process, carbon equivalent (C eq) is necessary to optimize. Accordingly, the inventors of the present invention studied a method for optimizing the carbon equivalent and optimized the carbon equivalent by controlling the correlation among C, Mn, Ni, Cu, Cr, Mo, and V. When the carbon equivalent value represented by Equation (2) is 0.70 or more and 0.80 or less, a steel having excellent low-temperature impact toughness and strength can be obtained.

[0074] Equation (2) C eq : 0.70 ≤ C + (1 / 6)*Mn + (1 / 15)*(Ni + Cu) + (1 / 5)*(Cr + Mo + V) ≤ 0.80

[0075] In the above formula (2), each element symbol represents the content (weight%) of each element.

[0076] If the value of formula (2) is less than 0.70, it is difficult to secure a tensile strength of 860 MPa or more even after quenching and tempering, and if the value of formula (2) exceeds 0.80, it is difficult to secure a low-temperature impact toughness of 100 J or more. Therefore, formula (2) is controlled to be 0.70 or more and 0.80 or less, and preferably 0.71 or more and 0.79 or less.

[0077] In addition, due to the nature of mooring chains that are used in seawater for a long time, they need to have excellent corrosion resistance against seawater. Cu and Ni are elements that are closely related to corrosion resistance, so it is necessary to appropriately control the content of Cu and Ni. In order to improve the corrosion resistance of steel, in addition to the above elements, the effects of other alloying elements that interact in the product's usage environment must also be considered. In particular, although Cr is generally an element that improves corrosion resistance, it was confirmed that corrosion resistance decreases with the addition of Cr when chlorides, etc. are concentrated on the surface of the mooring chain and form a thin film. On the other hand, it was confirmed that Mo is dissolved in the steel's usage environment and forms molybdate ions, thereby improving corrosion resistance.

[0078] In view of this, the inventors of the present invention have conducted in-depth research on a method for improving corrosion resistance within the above-described alloy composition, and as a result, in addition to the contents of Cu and Ni, the addition amounts of Cr and Mo have also been optimized, and the interactions between the elements have been comprehensively considered to derive the following equation (3). When the value of equation (3) is 0.00 or more and 0.50 or less, a steel material exhibiting good corrosion resistance can be obtained. Therefore, the value of equation (3) is controlled to be 0.00 or more and 0.50 or less, and preferably 0.00 or more and 0.41 or less.

[0079] Equation (3): 0.00 ≤ (1 - 1.4*Cu) x (1 - 0.4*Ni) + 0.3*Cr - 2.2*Mo ≤ 0.50

[0080] In the above formula (3), each element symbol represents the content (weight%) of each element.

[0081] The steel according to the present invention may exhibit a corrosion resistance reduction diameter of 1.0 mm or less.

[0082] The corrosion resistance reduction diameter refers to {(diameter before corrosion resistance evaluation process) - (diameter after corrosion resistance evaluation process)} mm measured after repeating the corrosion resistance evaluation process consisting of seawater immersion process - drying and wetting process for 8 cycles.

[0083] The seawater immersion process was performed by immersing the specimens in artificial seawater. The artificial seawater was prepared to have a composition (unit: g / L) of NaCl: 24.5, MgCl2·6H2O: 11.1, Na2SO4: 4.1, CaCl2: 1.2, and KCl: 0.7. The specimens were immersed in the prepared artificial seawater for 15 minutes while maintaining the temperature at 35°C.

[0084] Afterwards, drying and wetting processes were performed. The drying process, in which the specimen was exposed to a dry environment simulating the dry state of the specimen surface in an air environment, and the wetting process, in which the specimen was exposed to a wet environment simulating the wet state of the specimen surface in an air environment, were performed alternately. The temperature was maintained at 60°C and the relative humidity was 25% RH (relative humidity) during the drying process, and the temperature was maintained at 60°C and the relative humidity was 100% RH during the wetting process.

[0085] Each process was performed by exposing the specimens to the respective process conditions for four hours. This set of extreme environmental conditions takes into account the intended use environment of the steel. Under these conditions, if the corrosion resistance reduction diameter is less than 1.0 mm, the steel's corrosion resistance can be guaranteed.

[0086] In addition, the steel according to one example of the present invention may include carbonitride, and the number of carbonitrides having a size of 50 nm or less is 1㎛. 2 There can be more than two per party.

[0087] Here, carbonitride refers to Al(C, N), V(C, N), Ti(C, N) and their composite carbides, and the size of the carbide refers to the size measured by the equivalent diameter of a circle. The reason why the size of carbonitride is limited to 50 nm or less is because when the size of carbonitride exceeds 50 nm, it has little effect on improving strength, refining old austenite and reducing hydrogen-induced cracking, and the number of carbonitrides less than 50 nm is 1㎛. 2 The number of carbon nitrides is limited to less than 2, which means that the number of carbon nitrides is 1㎛. 2 This is because the above-mentioned effect is insufficient when there are less than two per party.

[0088] At this time, the steel according to the present invention may have an ASTM grain size number (G) of old austenite of 8.0 or more. The ASTM grain size number (G) is the unit area (1 inch 2 ) Number of crystal grains (N)AE ) and has the following relationship.

[0089] N AE = 2 G-1

[0090] When the ASTM grain size number is 8.0 or higher, the average grain size of the old austenite is derived to be less than about 22.5㎛, so that the grain refinement of the steel occurs sufficiently, and the low-temperature impact toughness can be secured to 100J or higher.

[0091] In addition, the steel according to an example of the present invention may have a Charpy impact energy of 100 J or more and a tensile strength of 860 MPa or more at -20°C.

[0092] Next, a method for manufacturing steel according to the present invention is described.

[0093] A method for manufacturing a steel according to one embodiment of the present invention comprises, in wt%, carbon (C): 0.20 to 0.40%, silicon (Si): 0.10 to 0.40%, manganese (Mn): 1.00 to 1.40%, phosphorus (P): more than 0% and 0.015% or less, sulfur (S): more than 0% and 0.015% or less, chromium (Cr): 0.80 to 1.00%, nickel (Ni): 0.40 to 1.00%, molybdenum (Mo): 0.20 to 0.45%, copper (Cu): more than 0% and 0.30% or less, aluminum (Al): 0.01 to 0.05%, vanadium (V): 0.050 to 0.100%, titanium (Ti): 0.008 to 0.020%, nitrogen (N): 0.002 to 0.015%, A method for producing a bloom, comprising: a step of preparing a bloom containing boron (B): 0.0030% or less, calcium (Ca): 0.0020% or less, and the remainder iron (Fe) and unavoidable impurities, and satisfying the following formulas (1) and (2); a step of hot-rolling the bloom to produce a steel bar; a step of quenching the steel bar by water cooling at 850 to 1,000°C; and a step of tempering the water-cooled steel bar.

[0094] Equation (1): 0.10 ≤ Al + V + Ti ≤ 0.15

[0095] Equation (2) C eq : 0.70 ≤ C + (1 / 6)*Mn + (1 / 15)*(Ni + Cu) + (1 / 5)*(Cr + Mo + V) ≤ 0.80

[0096] In formulas (1) and (2), each element symbol represents the content (weight%) of each element.

[0097] Below, each step is explained in detail.

[0098] Steps to prepare bloom

[0099] A bloom satisfying the alloy composition of the present invention described above is prepared. The role and content of each component of the alloy composition, as well as the explanations for Equations (1) and (2), are as described above. In this case, the bloom manufacturing conditions are not particularly limited, as they can be applied to general bloom manufacturing conditions.

[0100] According to one example of the present invention, the bloom can satisfy the following equation (3). The explanation for equation (3) is as described above.

[0101] Equation (3): 0.00 ≤ (1 - 1.4*Cu) x (1 - 0.4*Ni) + 0.3*Cr - 2.2*Mo ≤ 0.50

[0102] In the above formula (3), each element symbol represents the content (weight%) of each element.

[0103] Steps for manufacturing steel bars

[0104] The above bloom is hot rolled to produce a bar. At this time, the manufacturing conditions for the bar can be applied in the same manner as general bar manufacturing conditions, so there are no special restrictions.

[0105] For example, the method may include a step of heating the bloom at 1,150 to 1,250°C for 3 to 6 hours and then rolling it at 1,050 to 1,150°C to produce a billet, and a step of heating the billet at 1,150 to 1,250°C for 1 to 2 hours and then hot-rolling it to produce a steel bar.

[0106] Step of inserting and water-cooling the steel

[0107] After the above-mentioned steel bar is quenched at 850 to 1,000°C, it is water-cooled. Preferably, the quenching step can be performed at 880 to 1,000°C, more preferably at 880 to 950°C. In addition, the quenching step can take more than 30 minutes to ensure sufficient reaction time.

[0108] If the quenching step is performed below 850°C, the phase transformation to austenite may not occur sufficiently, and the solution within the steel may not sufficiently occur, making it difficult to secure sufficient strength of the steel. However, if the quenching step is performed above 1,000°C, scale may occur, and the grains may coarsen, reducing the low-temperature impact toughness. Therefore, if the quenching is performed within the above temperature range, a steel having the target strength and low-temperature impact toughness can be obtained.

[0109] Meanwhile, the quenching step can be performed using water cooling. If the quenching step is performed using oil cooling instead of water cooling, sufficient strength cannot be secured due to the low cooling rate.

[0110] Steps for cooling water-cooled steel bars

[0111] The above water-cooled steel bar is tempered. The tempering step can be performed at 600 to 650°C for 30 minutes or more.

[0112] If the tempering temperature is below 600°C, carbonitride formation is insufficient, resulting in reduced low-temperature impact toughness and a risk of product breakage during final production. However, if the tempering temperature exceeds 650°C, strength may be reduced. Therefore, when tempering is performed within the above temperature range, a steel with the desired strength and low-temperature impact toughness can be obtained.

[0113] The steel manufactured under the manufacturing conditions according to the present invention may include carbonitrides, and the number of carbonitrides having a size of 50 nm or less is 1 ㎛. 2 There can be more than two per party.

[0114] In addition, the steel manufactured under the manufacturing conditions according to the present invention may have a corrosion resistance reduction diameter of 1.0 mm or less.

[0115]

[0116] Hereinafter, the structure and operation of the present invention will be described in more detail through preferred embodiments of the present invention. However, these are presented as preferred examples of the present invention and should not be construed as limiting the present invention in any way.

[0117] (Example)

[0118] Specimens of Examples 1 to 6 and Comparative Examples 1 to 6 were manufactured by the following methods.

[0119] A bloom having the same composition as Table 1 was manufactured, and the bloom was heated at 1200°C for 4 hours, and then finish-rolled at 1100°C to manufacture a billet. Thereafter, the billet was heated at 1200°C for 1 hour and 30 minutes, and then hot-rolled to manufacture a 25 mm thick bar. Subsequently, the bar was quenched at 900°C for 60 minutes, followed by water cooling, and then tempered at 640°C for 60 minutes to manufacture the final specimen.

[0120] Table 1 below shows the component composition content (weight %) for each specimen, and Table 2 shows the values ​​of formulas (1) to (3) calculated through the component composition of each specimen. Underlines indicate areas outside the scope of the present invention.

[0121]

[0122] Equation (1) Equation (2) Equation (3) Example 10.110.720.29 Example 20.100.790 Example 30.110.720.30 Example 40.130.770.41 Example 50.130.750.08 Example 60.130.710.31 Example 70.110.790.06 Comparative Example 10.130.950.21 Comparative Example 20.100.700.38 Comparative Example 30.140.600.34 Comparative Example 40.110.800.52 Comparative Example 50.100.79-0.46 Comparative Example 60.070.730.28 Comparative Example 70.120.750.65

[0123] Next, the physical properties and microstructure of each specimen were observed for the following items.

[0124] [tensile strength]

[0125] Tensile strength was measured according to ASTM E8M. Specimens were taken at the 1 / 2t point in the thickness direction of each specimen and measured, and the average value of five measurements is presented.

[0126] [-20℃ Charpy impact energy]

[0127] To evaluate the low-temperature impact toughness of each specimen, the Charpy impact energy at -20℃ was measured. After producing specimens measuring 10*10*55 mm with a V-notch, the average value of the values ​​measured five times using a Charpy impact tester at -20℃ is presented.

[0128] [Carbon nitride density]

[0129] The density of carbonitride was measured by making a TEM replica specimen. TEM images were taken at 20,000x magnification and then 1 μm 2The number of carbon nitride particles smaller than 50 nm was measured. Measurements were taken at 20 random locations on each specimen, and the average value is presented.

[0130] [ASTM particle size number]

[0131] The grain size number (ASTM grain size number, G) is expressed by measuring the grain size of old austenite according to ASTM E112.

[0132] [Corrosion resistance reduction diameter]

[0133] To evaluate the corrosion resistance of each specimen, a 20 mm diameter and 100 mm length specimen was cut. Each specimen underwent a corrosion resistance evaluation process consisting of a seawater immersion process, followed by a drying and wetting process. One cycle of the corrosion resistance evaluation process consists of the following processes.

[0134] The seawater immersion process was performed by immersing the specimens in artificial seawater. The artificial seawater was prepared to have a composition (unit: g / L) of NaCl: 24.5, MgCl2·6H2O: 11.1, Na2SO4: 4.1, CaCl2: 1.2, and KCl: 0.7. The specimens were immersed in the prepared artificial seawater for 15 minutes while maintaining the temperature at 35°C.

[0135] Afterwards, drying and wetting processes were performed. The drying process, in which the specimen was exposed to a dry environment simulating a dry state of the specimen surface in an air environment, and the wetting process, in which the specimen was exposed to a wet environment simulating a wet state of the specimen surface in an air environment, were performed alternately. The temperature was maintained at 60°C and the relative humidity was 25% RH (relative humidity), and the temperature was maintained at 60°C and the relative humidity was 100% RH in the wetting process. Each process was performed by exposing the specimen to each process condition for 4 hours each.

[0136] The corrosion resistance evaluation process, consisting of a seawater immersion process followed by a drying and wetting process, was repeated for eight cycles, after which the corrosion resistance reduction diameter was measured. The diameter reduction was measured at five locations, at the center and both ends of each specimen, and then averaged. A corrosion resistance reduction of less than 1.0 mm was considered satisfactory.

[0137] Table 3 presents the observation results for the above items. In Table 3, underlined items are outside the scope of the present invention.

[0138] Tensile strength (MPa) -20℃ Charpy impact energy (J) Carbonitride density (pieces / ㎛) 2 )ASTM particle size numberCorrosion resistanceReduced diameter (mm)Example 190410528.20.85Example 291012328.80.5Example 386312629.10.86Example 4927129390.99Example 586711738.50.59Example 686211838.40.87Example 787912828.30 .57Comparative Example 19435938.80.75Comparative Example 29227828.20.95Comparative Example 382414538.40.91Comparative Example 487510428.81.32Comparative Example 58795728.20.05Comparative Example 68808917.30.84Comparative Example 786411338.41.27

[0139] The alloy composition disclosed in the present invention and Examples 1 to 7 satisfying all of Formulas (1) to (3) have a tensile strength of 860 MPa or more, a -20°C Charpy impact energy of 100 J or more, and a carbonitride density of 2 / ㎛. 2 Above, the ASTM particle size number was 8.0 or higher, and the corrosion resistance reduction diameter was 1 mm or less. In the case of Comparative Example 1, the low-temperature impact toughness deteriorated as the C content was excessive and the value of Equation (2) exceeded 0.80. Accordingly, the -20℃ Charpy impact energy was less than 100 J.

[0140] In the case of Comparative Example 2, all of the equations (1) to (3) presented by the present invention were satisfied, but the low-temperature impact toughness was deteriorated due to the specimen having an excessive Si content. Accordingly, the -20°C Charpy impact energy was found to be less than 100 J.

[0141] In the case of Comparative Example 3, the tensile strength deteriorated as the Mn content was low and the value of Equation (2) was less than 0.70. Accordingly, the tensile strength was found to be less than 860 MPa.

[0142] In the case of Comparative Example 4, the Cr content was excessive, so that Equation (3) was greater than 0.50. Accordingly, the corrosion resistance deteriorated, and the corrosion resistance reduction diameter exceeded 1.00 mm.

[0143] In the case of Comparative Example 5, the low-temperature impact toughness was deteriorated because the Mo content exceeded 0.45%, and accordingly, the -20℃ Charpy impact energy was significantly less than 100 J.

[0144] Comparative Example 6 In this case, since the content of V is low and the value of formula (1) is less than 0.10, fine carbonitrides are not formed, and the carbonitride density is 1 / ㎛. 2 2 pieces / ㎛ 2 The ASTM particle size number was 7.3, which was less than 8.0. Accordingly, the effect of improving low-temperature impact toughness due to particle size refinement was minimal, and the Charpy impact energy at -20℃ was less than 100J.

[0145] In the case of Comparative Example 7, the alloy composition suggested by the present invention was satisfied, but the value of Equation (3) exceeded 0.50. Accordingly, the effect of improving corrosion resistance was insignificant, and the corrosion resistance reduction diameter exceeded 1.00 mm.

[0146]

[0147] After producing specimens having the same composition as Examples 1 and 7 but with different quenching and tempering temperatures during the manufacturing process, the tensile strength, -20°C Charpy impact energy, and ASTM particle size number were measured.

[0148] Comparative Examples 8 and 9 are specimens having the same alloy composition as Example 1, and Comparative Examples 10 and 11 are specimens having the same alloy composition as Example 7. Comparative Examples 8 to 11 were manufactured in the same manner as Example 1, except for the quenching temperature and tempering temperature in Table 4.

[0149] The observation results for each specimen are shown in Table 4. Each item was measured in the same way as each item in Table 3.

[0150] Quenching temperature (℃) Tempering temperature (℃) Tensile strength (MPa) -20℃ Charpy impact energy (J) ASTM particle size number Example 1 9006409041058.2 Example 7 9006408791288.3 Comparative example 8 8006408571028.3 Comparative example 9 900580946478.3 Comparative example 10 1100640892897.8 Comparative example 11 9006808351348.4

[0151] In the case of Comparative Examples 8 and 9, the quenching temperature or tempering temperature was below the temperature range suggested by the present invention, so that solution formation did not occur sufficiently, and the effect of improving strength or toughness was minimal. In the case of Comparative Example 10, the quenching temperature exceeded 1,000°C. Accordingly, grain refinement did not occur sufficiently, so that the ASTM grain size number was smaller than 8.0, and the effect of improving low-temperature impact toughness due to grain refinement was minimal, so that the Charpy impact energy at -20°C was less than 100 J.

[0152] In the case of Comparative Example 11, the tempering temperature exceeded 650°C. Accordingly, the strength improvement effect was minimal, and a tensile strength of 860 MPa or more was not secured.

[0153] Although the present invention has been described with reference to the above embodiments, it will be understood by those skilled in the art that various modifications and changes can be made to the present invention without departing from the spirit and scope of the present invention as set forth in the claims below.

Claims

1. In weight%, carbon (C): 0.20 to 0.40%, silicon (Si): 0.10 to 0.40%, manganese (Mn): 1.00 to 1.40%, phosphorus (P): more than 0% but not more than 0.015%, sulfur (S): more than 0% but not more than 0.015%, chromium (Cr): 0.80 to 1.00%, nickel (Ni): 0.40 to 1.00%, molybdenum (Mo): 0.20 to 0.45%, copper (Cu): more than 0% but not more than 0.30%, aluminum (Al): 0.01 to 0.05%, vanadium (V): 0.050 to 0.100%, titanium (Ti): 0.008 to 0.020%, nitrogen (N): 0.002 to 0.015%, boron (B): Steel containing 0.0030% or less of calcium (Ca), 0.0020% or less of iron (Fe) and unavoidable impurities, and satisfying the following formulas (1) and (2). Equation (1): 0.10 ≤ Al + V + Ti ≤ 0.15 Equation (2) C eq : 0.70 ≤ C + (1 / 6)*Mn + (1 / 15)*(Ni + Cu) + (1 / 5)*(Cr + Mo + V) ≤ 0.80 (Here, each element symbol represents the content (weight%) of each element) 2. In claim 1, Steel satisfying the following equation (3). Equation (3): 0.00 ≤ (1 - 1.4*Cu) x (1 - 0.4*Ni) + 0.3*Cr - 2.2*Mo ≤ 0.50 (Here, each element symbol represents the content (weight%) of each element) 3. In claim 1, Steel having a Charpy impact energy of 100 J or more and a tensile strength of 860 MPa or more at -20℃.

4. In claim 1, The above steel contains carbon nitride, The number of the above carbon nitrides is 1㎛ 2 Steel, with 2 or more pieces per party.

5. In claim 1, The above steel contains carbon nitride, Steel, wherein the size of the above carbon nitride is 50 nm or less.

6. In claim 1, Steel having an ASTM grain size number of the austenite of the above steel of 8.0 or higher.

7. In claim 1, The above steel is a steel having excellent low-temperature impact and corrosion resistance, with a corrosion resistance reduction diameter of 1.0 mm or less.

8. In weight%, carbon (C): 0.20 to 0.40%, silicon (Si): 0.10 to 0.40%, manganese (Mn): 1.00 to 1.40%, phosphorus (P): more than 0% but not more than 0.015%, sulfur (S): more than 0% but not more than 0.015%, chromium (Cr): 0.80 to 1.00%, nickel (Ni): 0.40 to 1.00%, molybdenum (Mo): 0.20 to 0.45%, copper (Cu): more than 0% but not more than 0.30%, aluminum (Al): 0.01 to 0.05%, vanadium (V): 0.050 to 0.100%, titanium (Ti): 0.008 to 0.020%, nitrogen (N): 0.002 to 0.015%, boron (B): A step for preparing a bloom containing 0.0030% or less, calcium (Ca): 0.0020% or less, and the remainder iron (Fe) and unavoidable impurities, and satisfying the following formulas (1) and (2); A step of manufacturing a bar by hot rolling the above bloom; A step of quenching the above-mentioned steel by water cooling at 850 to 1,000℃; and A method for manufacturing steel, comprising: a step of tempering the water-cooled steel bar. Equation (1): 0.10 ≤ Al + V + Ti ≤ 0.15 Equation (2) C eq : 0.70 ≤ C + (1 / 6)*Mn + (1 / 15)*(Ni + Cu) + (1 / 5)*(Cr + Mo + V) ≤ 0.80 (Here, each element symbol represents the content (weight%) of each element) 9. In claim 8, The steps for manufacturing the above-mentioned steel bar are: A step of heating the above bloom at 1,150 to 1,250°C for 3 to 6 hours and then rolling it at 1,050 to 1,150°C to produce a billet; and A method for manufacturing steel, comprising the step of heating the billet at 1,150 to 1,250°C for 1 to 2 hours and then hot-rolling it to manufacture a bar.

10. In claim 8, A method for manufacturing steel, wherein the above-mentioned step is performed at 600 to 650°C.

11. In claim 8, The above bloom is a method for manufacturing steel, satisfying the following equation (3). Equation (3): 0.00 ≤ (1 - 1.4*Cu) x (1 - 0.4*Ni) + 0.3*Cr - 2.2*Mo ≤ 0.50 (Here, each element symbol represents the content (weight%) of each element) 12. In claim 8, The above steel contains carbon nitride, The number of the above carbon nitrides is 1㎛ 2 A method for manufacturing steel having two or more sugars.

13. In claim 8, The above steel contains carbon nitride, A method for manufacturing steel, wherein the size of the carbon nitride is 50 nm or less.

14. In claim 8, A method for manufacturing a steel having excellent low-temperature impact and corrosion resistance, wherein the corrosion resistance reduction diameter of the above steel is 1.0 mm or less.

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