HIGH MANAGEMENT STEEL WITH EXCELLENT LOWER TEMPERATURE Toughness and Yield Strength AND MANUFACTURING PROCESS

Description

[Technical Field]

[0001] The present invention relates to a high strength and high toughness steel material used in various parts of LNG fueled vehicles and ships for LNG transport, and a method for manufacturing the same, and more particularly, to high manganese steel having superior low-temperature toughness and yield strength and a manufacturing method thereof.[Background Art]

[0002] There has been an increased interest in energy sources such as LNG, due to exhaustion of conventional energy sources such as petroleum, and the like. As the consumption of fuels such as natural gas, transferred in a cryogenic liquid state at -100°C or lower, has increased, demand for manufacturing devices for storing and transferring such fuels and materials of such devices have been increased.

[0003] In the case of general carbon steel, toughness of the material may rapidly degrade in the cryogenic state, such that the problem of fracture of the material may occur even by small external impact. To address the problem, materials having excellent impact toughness even in a low temperature have been used. Representative materials may be an aluminum alloy, austenitic stainless steel, 35% invar steel, 9% Ni steel, and the like.

[0004] However, there may be the problem in which prices of such materials are high as a content of nickel is high. Thus, it has been necessary to develop a steel material having a low manufacturing price and excellent low temperature toughness.

[0005] There may be limitation in use of a general carbon steel product because, when a use temperature decreases, yield strength may rapidly increase, such that toughness may greatly degrade. Also, stainless steel, a representative material having excellent toughness, has low yield strength, and thus, it may not be suitable to use stainless steel as a structural member.

[0006] Meanwhile, the method for making a material having high low temperature toughness is to allow the material to have a stable austenite structure at a low temperature. In the case of a ferrite structure, a ductility-brittleness transition phenomenon may appear at a low temperature, such that toughness may rapidly decrease in a low temperature brittleness region, whereas an austenite structure does not have a ductile- brittle transition phenomenon even in an extremely low temperature and has high low temperature toughness. That is because, unlike ferrite, as austenite has low yield strength at a low temperature, plastic deformation may easily occur such that impacts caused by external deformation may be absorbed.

[0007] A representative element which may increase austenite stability in a low temperature is nickel, but a price of nickel may be expensive, which is a disadvantage.

[0008] References 2 to 5 relate to low temperature steels that can be used in, inter alia, liquefied gas storage tanks manufacturing, and production methods thereof.(Prior Art)

[0009] (Reference 1) Japanese Laid-Open Patent Publication So No. 60-077962 (Reference 2) EP 3 088 555 A1 (Reference 3) KR 2015 0075305 A (Reference 4) WO 2016 / 105002 A1 (Reference 5) KR 2016 0078664 A. [Disclosure][Technical Problem]

[0010] An aspect of the present invention is to provide high manganese steel having superior low temperature toughness and yield strength.

[0011] Another aspect of the present invention is to provide a method for manufacturing high manganese steel having superior low temperature toughness and yield strength.[Technical Solution]

[0012] The invention is defined in the claims.[Advantageous Effects]

[0013] According to the present invention, high manganese steel having an impact toughness value of 100J or higher, measured by a charpy impact test at -196°C, and room temperature yield strength of 380MPa or higher.[Best Mode for Invention]

[0014] The present invention will be described in greater detail.

[0015] The present invention is based on the result obtained by research and experimentation on high manganese steel having superior low temperature toughness and yield strength, and the main ideas are as follows. 1) In a steel composition, particularly, contents of manganese and carbon may be controlled.

[0016] Accordingly, a highly uniform and stable austenite phase may be secured.

[0017] In a steel composition, particularly, appropriate contents of Cr (selectively added), a carbonitride formation element, and of Cu, Al, and the like, solid solution strengthening elements, are added.

[0018] Accordingly, yield strength may increase.

[0019] 3) Among manufacturing conditions, a hot-rolling condition is controlled.

[0020] Accordingly, strength and impact toughness may increase.

[0021] The invention is defined in the appended claims. In the description below, austenitic high manganese steel used in an extremely low temperature will be described according to an aspect of the present invention.

[0022] High manganese steel having superior low-temperature toughness and yield strength according to an aspect of the present invention includes, by wt%, 0.3 to 0.6% of C, 20 to 25% of Mn, 0.01 to 0.3% of Mo, 3% or less of Al, including 0%, 0.1 to 3% of Cu, 0.06% or less of P, , and 0.005% or less of S, one or more selected from between 8% or less of Cr, including 0%, and 0.1 to 3% of Ni, and a remainder of Fe and other inevitable impurities, and Mo and F satisfy Relational Expression 1 below. 1.5 ≤ 2 * Mo / 93 / P / 31 ≤ 9

[0023] The microstructure is formed of austenite having a grain size of 50 µm or less.

[0024] A steel composition and composition ranges will be described.Carbon (C) : 0.3 to 0.6 wt% (hereinafter, referred to as %)

[0025] C is an element which may be required to stabilize austenite in steel and to secure strength by being solute to steel. However, when a content of C is less than 0.3%, austenite stability may be insufficient, such that ferrite or martensite may be formed, which may degrade low temperature toughness. When a content of C exceeds 0.6%, carbide may be formed such that a surface defect may occur, and toughness may degrade. Thus, a content of C is controlled to be 0.3 to 0.6%.

[0026] A preferable content of C is 0.35 to 0.55%, and a more preferable content of C is 0.4 to 0.5%.Manganese (Mn): 20 to 25%

[0027] Mn is an important element which may stabilize an austenite structure. To secure low temperature toughness, the formation of ferrite should be prevented, and austenite stability may need to be increased. Thus, in the present invention, a minimum content of Mn is 20% or higher. When a content of Mn is less than 20%, a α'-martensite phase may be formed, which may decrease low temperature toughness. When a content of Mn exceeds 25%, manufacturing costs may greatly increase, and internal oxidation may excessively occur during heating in a hot-rolling process in terms of process such that the problem of degradation of surface quality may be caused. Thus, a content of Mn is controlled to be 20 to 25%.

[0028] A preferable content of Mn is 21 to 24%, and a more preferable content of Mn is 22 to 24%.Molybdenum(Mo): 0.01 to 0.3%

[0029] Mo may be effective for improving impact toughness by generating an effect of preventing P grain boundary segregation by forming a Fe-Mo-P compound. To this end, a content of Mo needs to be 0.01% or higher. However, as Mo is an expensive element, a content of Mo is controlled to be 0.3% or less to prevent a decrease of impact energy caused by an increase of strength due to the formation of Mo carbonitride.Aluminum (Al) : 3% or less (including 0%)

[0030] Al has an effect of, by increasing stacking fault energy, enabling plastic deformation by facilitating movement of dislocation in a low temperature. When a content of Al exceeds 3%, manufacturing costs may greatly increase, and cracks may be created in a consecutive casting process in terms of process, which may cause the problem of degradation of surface quality. Thus, a content of Al is controlled to be 3% or less (including 0%). A preferable content of Al is 0 to 2%, and a more preferable content of Al is 0.5 to 1.5%.Copper (Cu):0.1 to 3%

[0031] Cu is required to increase strength by being solute in steel.

[0032] When a content of Cu is less than 0.1%, it may be difficult to obtain an effect of addition of Cu. When a content of Cu exceeds 3%, cracks may easily be created on a slab. Thus, a content of Cu is controlled to be 0.1 to 3%.

[0033] A preferable content of Cu is 0.5 to 2.5%, and a more preferable content of Cu is 0.5 to 2%.Phosphorus (P): 0.06% or less

[0034] P is an element which may be inevitably added when manufacturing steel. When P is added, P may be segregated in a central portion of a steel sheet, and may be used as a crack initiation point or a crack growth path. It may be preferable to control a content of P to be 0% theoretically, but in terms of manufacturing process, P may be inevitably included as impurities. Thus, it is important to control an upper limit content. In the present invention, an upper limit content of P is controlled to be 0.06%.Sulfur (S): 0.005% or less

[0035] S is an impurity element present in steel. S may be combined with Mn, and the like, and may form a non-metal inclusion, which may degrade toughness of steel. Thus, it may be preferable to decrease a content of S as possible, and thus, an upper limit content of S is controlled to be 0.005%.

[0036] In the steel composition, Mo and P satisfy Relational Expression (1) below. 1.5 ≤ 2 * Mo / 93 / P / 31 ≤ 9

[0037] Relational Expression (1) is to prevent grain boundary segregation of P. When a value of Relational Expression (1) is less than 1.5, the effect of preventing P grain boundary segregation by forming an Fe-Mo-P compound may not be sufficient. When a value of Relational Expression (1) exceeds 9, strength may increase by formation of Mo carbonitride, which may decrease impact energy.One or more selected from between 8% or less of Cr (including 0%) and 0.1 to 3% of Ni

[0038] In addition to the above-described composition, one or more selected from between 8% or less of Cr (including 0%) and 0.1 to 3% of Ni is included.Chromium (Cr) : 8% or less (including 0%)

[0039] An appropriate range of a content of Cr may stabilize austenite such that impact toughness at a low temperature may improve, and Cr may be solute in austenite and may increase strength of a steel material. Cr is also an element which may improve corrosion-resistance of a steel material. Cr, however, is a carbide-forming element, which may form carbides at an austenite grain boundary and may decrease low temperature impact. Thus, in the present invention, it is preferable to determine a content of Cr in consideration of relationships with C and other elements to be included. When a content of Cr exceeds 8%, it may be difficult to effectively prevent the formation of carbide in an austenite grain boundary, and accordingly, impact toughness at a low temperature may decrease. Thus, a content of Cr is controlled to be 0 to 8%. A preferable content of Cr is 0 to 6%, and a more preferable content of Cr is 0 to 5%.Nickel (Ni): 0.1 to 3%

[0040] Ni is an element which is required to stabilize austenite in steel. When a content of Ni is less than 0.1%, it may be difficult to obtain an effect of addition of Ni. When a content of Ni exceeds 3%, there may be the problem of an increase in manufacturing costs.

[0041] Thus, a content of Ni is controlled to be 0.1 to 3%.

[0042] A preferable content of Ni is 0.5 to 2.5%, and a more preferable content of Ni is 0.5 to 2%.

[0043] High manganese steel according to the present invention has a microstructure formed of austenite having a grain size of 50 µm or less.

[0044] When the grain size exceeds 50 µm, there may be the problem of decrease of yield sensitivity and impact energy.

[0045] High manganese steel in the present invention has an impact toughness value of 100J or higher, measured by a charpy impact test at -196°C, and room temperature yield strength of 380MPa or higher.

[0046] In the description below, a method of manufacturing high manganese steel having superior low temperature toughness and yield strength will be described according to the present invention.

[0047] The method of manufacturing high manganese steel having superior low temperature toughness and yield strength include reheating a slab at 1000 to 1250°C, the slab comprising, by wt%, 0.3 to 0.6% of C, 20 to 25% of Mn, 0.01 to 0.3% of Mo, 3% or less of Al, including 0%, 0.1 to 3% of Cu, 0.06% or less of P, and 0.005% or less of S, one or more selected from between 8% or less of Cr, including 0%, and 0.1 to 3% of Ni, and a remainder of Fe and other inevitable impurities, where Mo and P may satisfy the following Relational Expression (1), 1.5 ≤ 2*(Mo / 93) / (P / 31) ≤ 9, obtaining a hot-rolled steel sheet by primarily hot-rolling the heated slab, terminating the primary hot-rolling at 980 to 1050°C, secondarily hot-rolling the primarily hot-rolled steel sheet in a non-recrystallization region at a rolling reduction rate of 3% or less, and terminating the secondary hot-rolling at 800 to 960°C, water-cooling the hot-rolled steel sheet to a cooling terminating temperature of 350 to 600°C, and coiling the cooled hot-rolled steel sheet.Reheating Slab

[0048] Before hot-rolling, a slab is reheated at 1000 to 1250°C.

[0049] The slab reheating temperature is important in the present invention. The slab reheating process may be performed for a casting structure and segregation thereof, and solid solution and homogenization of secondary phases, formed in a slab manufacturing process. When the reheating temperature of a slab is less than 1000°C, deformation resistance may increase during hot-rolling as homogenization is insufficient or a temperature of a heating furnace is too low. When the reheating temperature exceeds 1250°C, surface quality may be deteriorated. Thus, the slab reheating temperature is controlled to be 1000 to 1250°C.Hot-rolling

[0050] A hot-rolled steel sheet is obtained by primarily hot-rolling the heated slab, terminating the primary hot-rolling at 980 to 1050°C, secondarily hot-rolling the hot-rolled steel sheet in a non-recrystallization region at a rolling reduction rate of 3% or less, and terminating the secondary hot-rolling at 800 to 960°C.

[0051] It is important to obtaining a hot-rolled steel sheet by primarily hot-rolling the heated slab, terminating the primary hot-rolling at 980 to 1050°C, secondarily hot-rolling the primarily hot-rolled steel sheet in a non-recrystallization region at a rolling reduction rate of 3% or less, and terminating the secondary hot-rolling at 800 to 960°C.

[0052] That is because, if the rolling finish temperature is too high, a final structure may be coarse such that desired strength and impact toughness may not be obtained. If the rolling finish temperature is too low, there may be the problem of facility load in a finish rolling device. Also, if a reduction amount of a non-recrystallization region is too high, impact toughness may decrease. Thus, it is important to control the rolling reduction rate to be 3% or less.Cooling and Coiling

[0053] After finishing the hot-rolling, the hot-rolled steel sheet is water-cooled, and is coiled at 350 to 600°C. When the cooling terminating temperature is higher than 600°C, surface quality may degrade, and coarse carbide may be formed such that toughness may decrease. When the cooling terminating temperature is less than 350°C, a large amount of cooling water may be required during the coiling, and a coiling force during the coiling may greatly increase.

[0054] The high manganese steel manufactured by the method of manufacturing high manganese steel in the present invention has an impact toughness value of 100J or higher, measured by a charpy impact test at -196°C, and yield strength at a room temperature of 380MPa or higher.[Mode for Invention]

[0055] In the description below, the present invention will be described in greater detail according to an example embodiment. The example embodiment below is merely an example for describing the present invention in detail, and may not limit the scope of rights of the present invention.(Embodiment)

[0056] An inventive steel having a chemical composition as in Table 1 below was manufactured as a slab by a consecutive casting method, and the slab was hot-rolled as in Table 2, thereby manufacturing a steel material.

[0057] A grain size, room temperature yield strength, and an impact energy value of the steel material manufactured as above were examined, and the results were listed in Table 2.

[0058] As indicated in Table 2, the inventive steel manufactured by the manufacturing method of the present invention using inventive steel satisfying the composition ranges of the present invention had high strength and high toughness after rolling.

[0059] While exemplary embodiments have been shown and described above, the scope of the present invention is not limited thereto, and it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims.

Claims

1. High manganese hot-rolled steel sheet having superior low temperature toughness and yield strength, comprising: by wt%, 0.3 to 0.6% of C, 20 to 25% of Mn, 0.01 to 0.3% of Mo, 3% or less of Al, including 0%, 0.1 to 3% of Cu, 0.06% or less of P, and 0.005% or less of S, one or more selected from between 8% or less of Cr, including 0%, and 0.1 to 3% of Ni, and the remainder being Fe and inevitable impurities, wherein Mo and P satisfy the following Relational Expression (1), 1.5 ≤ 2 * Mo / 93 / P / 31 ≤ 9 where a microstructure is formed of austenite having a grain size of 50 µm or less, wherein the high manganese hot-rolled steel sheet has an impact toughness value of 100J or higher, measured by a charpy impact test at -196°C, and wherein the high manganese hot-rolled steel sheet has room temperature yield strength of 380MPa or higher.

2. A method of manufacturing high manganese hot-rolled steel sheet having superior low temperature toughness and yield strength, the method comprising: reheating a slab at 1000 to 1250°C, the slab comprising, by wt%, 0.3 to 0.6% of C, 20 to 25% of Mn, 0.01 to 0.3% of Mo, 3% or less of Al, including 0%, 0.1 to 3% of Cu, 0.06% or less of P, and 0.005% or less of S, one or more selected from between 8% or less of Cr, including 0%, and 0.1 to 3% of Ni, and the remainder being Fe and inevitable impurities, where Mo and P satisfy the following Relational Expression (1), 1.5 ≤ 2 * Mo / 93 / P / 31 ≤ 9 obtaining a hot-rolled steel sheet by primarily hot-rolling the heated slab, terminating the primary hot-rolling at 980 to 1050°C, secondarily hot-rolling the primarily hot-rolled steel sheet in a non-recrystallization region at a rolling reduction rate of 3% or less, and terminating the secondary hot-rolling at 800 to 960°C; water-cooling the hot-rolled steel sheet to a cooling terminating temperature of 350 to 600°C; and coiling the cooled hot-rolled steel sheet, wherein a microstructure of the high manganese steel is formed of austenite having a grain size of 50 µm or less, wherein the high manganese hot-rolled steel sheet has an impact toughness value of 100J or higher, measured by a charpy impact test at -196°C, and wherein the high manganese hot-rolled steel sheet has room temperature yield strength of 380MPa or higher.