BR112021015919A2 - STEEL WITH HIGH MN CONTENT AND MANUFACTURING METHOD FOR THE SAME - Google Patents

Abstract

steel with high mn content and manufacturing method for the same. high mn steel with high strength and excellent low temperature toughness and ductility is supplied, the high mn steel having a chemical composition containing, in weight percentage, c: 0.10-0.70%, si: 0.10-0.90%, mn: 20-30%, p: 0.030% or less, s: 0.0070% or less, al: 0.01-0.07%, cr: 1.8 -7.0%, ni: 0.01% or more and less than 1.0%, ca: 0.0005-0.010% or less, n: 0.0050-0.0500%, o: 0.0050% or less, ti: 0.0050% or less and nb: 0.0050% or less, satisfying ca/s = 1.0, with the balance being fe and visible impurities unavoidable; a microstructure containing austenite as matrix; a yield strength of 400 mpa or more; and an energy absorbed by charpy impact on average at -196ºC of 100j or more for a full-size specimen and 20j or more for a half-size specimen.

Description

"STEEL WITH HIGH Mn CONTENT AND MANUFACTURING METHOD FOR THE SAME"

TECHNICAL FIELD

[001] This disclosure refers to high Mn steel that is suitable for a structure employed in a cryogenic environment, such as a liquefied gas storage tank and a method of fabrication for the same.

BACKGROUND

[002] A structure for a liquefied gas storage tank is used in a cryogenic environment. Therefore, a steel sheet used for this type of structure needs to have high strength and excellent toughness at extremely low temperatures. For example, when a hot rolled steel sheet is used for a liquefied natural gas storage tank, excellent toughness needs to be ensured at an extremely low temperature at or below the boiling point of liquefied natural gas, i.e. -164°C. or lower. When a steel material has low strength at low temperatures, security as a structure for a cryogenic storage tank may not be maintained. Thus, there is an increasing demand for steel materials with improved toughness at low temperature that are applied to such a structure.

[003] In view of the demand, austenitic stainless steel which has austenite as the primary microstructure of a steel sheet, austenite not showing brittleness at extremely low temperatures, 9% Ni steel or 5000 series aluminum alloy have been conventionally used. However, the alloy cost and manufacturing cost of these steels and alloys are high and therefore there is a demand for steel materials that are inexpensive and have excellent low temperature toughness.

[004] As new steel materials replacing conventional steel for extremely low temperatures, JP 2016-084529 A (PTL 1) and JP 2016-196703 A

(PTL 2) propose the use, as structural steel employed in a cryogenic environment, high Mn steel added with a large amount of Mn which is relatively cheap and an austenite stabilizing element.

[005] That is, PTL 1 proposes to control the carbide coverage at the grain boundaries of austenite crystal. Furthermore, PTL 2 proposes to control the grain size of the austenite crystal using carbide coated materials and adding Mg, Ca and REM.

LIST OF QUOTES Literature Patent PTL 1: JP 2016-84529 A PTL 2: JP 2016-196703 A

SUMMARY Technical Problem

[006] Austenite steel used as the extremely low temperature steel described in PTLs 1 and 2, which is highly strain-hardened from the initial strain-hardening stage until maximum stress (stress-strain) is applied in the tensile strain, has excellent plastic deformability and therefore exhibits excellent ductility up to the intermediate stage of deformation. On the other hand, the strain performance in the later phase of strain after the stress measured in a tensile test reaches the maximum (stress-strain) is also an important characteristic for a structural element. This is because the strain performance at the post-strain stage is the performance at the final stage leading to eventual fracture. From this perspective a sufficient reduction in area must be ensured, in particular ductility in the later stage of deformation. From the point of view of ensuring the ductility of high strength steel, a desirable area reduction is 50% or more.

[007] Thus, it could be useful to supply high Mn steel that has high strength and excellent low temperature toughness and exhibits excellent ductility and a manufacturing method for the same. As used herein, the terms "high strength" refer to having a yield strength of 400 MPa or more and a stress-strain of 800 MPa or more at room temperature. Furthermore, the terms "excellent low temperature toughness" means that when a Charpy impact test in accordance with JIS Z2242 (1998) is performed at -196°C on a full-size specimen (10 mm x 10 mm x 55 mm ) part that is sheet steel with a sheet thickness of 10 mm or more, the base metal has a Charpy impact energy absorbed of 100 J or more on average (when using a 10 mm half-size specimen x 5 mm x 55 mm which is a sheet steel with a sheet thickness of less than 10 mm, 20 J or more in a half-size Charpy V-notch test). Also, the terms "excellent ductility" refer to having an area reduction of 50% or more. Solution to the problem

[008] We did extensive studies on measures to solve the above mentioned problems in high Mn steel and found the following.

[009] That is, we found that in steel with high Mn content, toughness can be improved and ductility (area reduction) in tensile deformation can be guaranteed by controlling the morphology of Ca-based inclusions and, for that, , it is effective to control the balance between Ca content and S content within an appropriate range.

[010] Furthermore, we found that in the manufacture of steel with high Mn content, it is possible to control the crystal grain size and suppress the formation of precipitates, thus improving the toughness at low temperature, limiting the heating temperature of the material of steel, rolling finishing temperature and average cooling rate from an equal or higher temperature (the rolling finishing temperature - 100ºC) to a temperature range of 300ºC to 650ºC.

[011] When high Mn steel contains Cu, Cu has an effect of improving chloride-induced stress corrosion cracking in a low chloride environment. However, Cu, on the other hand, deteriorates the cracking resistance by chloride-induced stress corrosion cracking in an environment with high chloride concentration. To solve this problem, we found that high Mn steel containing Cu, when optimized in the balance between Cu content and Ni content and added with Ni, exhibits excellent resistance to chloride-induced stress cracking corrosion, even in an environment with a high concentration of chloride. Thus, it is possible to provide excellent resistance to chloride-induced stress corrosion cracking in high-Mn Cu-containing steel, regardless of chloride concentration. In this disclosure, the term chloride stress corrosion cracking refers to a phenomenon in which, in a corrosive environment specific to high Mn steel, in particular, in an environment with chloride ions, high Mn steel Mn leads to cracking or breakage, even when the stress-strain applied to high Mn steel is not greater than the stress-strain applied to high Mn steel. Furthermore, the terms chloride-induced stress-corrosion cracking resistance refers to the chloride-induced stress-corrosion cracking properties.

[012] Based on these findings, we conducted further investigations that eventually led to the present revelation. The main features of the present disclosure are as follows.

1. High Mn steel, comprising:

a chemical composition containing (consisting), by weight percent: C: 0.10% or more and 0.70% or less, Si: 0.10% or more and 0.90% or less, Mn: 20% or less plus and 30% or less, P: 0.030% or less, S: 0.0070% or less, Al: 0.01% or more and 0.07% or less, Cr: 1.8% or more and 7, 0% or less, Ni: 0.01% or more and less than 1.0%, Ca: 0.0005% or more and 0.010% or less, N: 0.0050% or more and 0.0500% or less , O: 0.0050% or less, Ti: 0.0050% or less, and Nb: 0.0050% or less, within a range satisfying the following formula (1), with the balance being Fe and unavoidable impurities , Ca/S ≥ 1.0 (1); a microstructure containing austenite as a matrix; a yield strength of 400 MPa or more; and a medium Charpy impact absorbed energy at -196°C of 100 J or more for a full-size specimen and 20 J or more for a half-size specimen.

[013] 2. Steel with a high content of Mn according to 1, in which the chemical composition still contains, in percentage by weight, at least one selected from the group consisting of: Cu: less than 2.0%, Mo : 2.0% or less, V: 2.0% or less,

W: 2.0% or less, Mg: 0.0005% or more and 0.0050% or less, and REM (rare earth metal): 0.0010% or more and 0.0200% or less.

[014] 3. Method of manufacturing steel with high Mn content, comprising: heating a steel material containing the chemical composition according to 1 or 2 at a temperature range of 1,100ºC to 1,300ºC; then hot rolling the steel material with a rolling finishing temperature of 750ºC or higher and lower than 950ºC, to obtain a hot rolled sheet; and then subjecting the hot-rolled sheet to a cooling treatment at an average cooling rate of 0.5°C/s or more, from a temperature equal to or higher (the rolling finishing temperature - 100°C) to a temperature range of 300°C to 650°C.

[015] 4. Steel with a high Mn content comprising: a chemical composition containing (consisting), in percentage by weight, C: 0.10% or more and 0.70% or less, Si: 0.10% or plus and 0.90% or less, Mn: 20% or more and 30% or less, P: 0.030% or less, S: 0.0070% or less, Al: 0.01% or more and 0.07% or less, Cr: 1.8% or more and 7.0% or less, Cu: 0.2% or more and less than 2.0% Ni: 0.1% or more and less than 1.0%, Ca: 0.0005% or more and 0.010% or less, N: 0.0050% or more and 0.0500% or less, O: 0.0050% or less, Ti: 0.0050% or less and

Nb: 0.0050% or less, within a range satisfying the following formulas (1) and (2), with the balance being Fe and unavoidable impurities, Ca/S ≥ 1.0 (1), and 0 < Cu /Ni ≤ 2 (2); and a microstructure containing austenite as a matrix.

[016] 5. Method of manufacturing high Mn steel, comprising: heating a steel material having the chemical composition according to 4, at a temperature range of 1,100ºC to 1,300ºC; then hot rolling the steel material with a rolling finishing temperature of 750ºC or higher and lower than 950ºC to obtain a hot rolled sheet; and then subjecting the hot-rolled sheet to a cooling treatment at an average cooling rate of 0.5°C/s or more, from a temperature equal to or higher (the rolling finishing temperature - 100°C) to a temperature range of 300°C to 650°C. Advantageous Effect

[017] According to one embodiment of this disclosure, it is possible to provide high Mn steel that has high strength, excellent low-temperature toughness, especially in a cryogenic range, and excellent ductility. Therefore, the safety and lifespan of a steel structure used in a cryogenic environment such as a liquefied gas storage tank can be improved by using our high Mn steel which provides significant industrial effects.

[018] Furthermore, according to another embodiment of this disclosure, it is possible to provide high Mn steel that exhibits excellent resistance to chloride-induced stress corrosion cracking, regardless of chloride concentration.

DETAILED DESCRIPTION

[019] Our high Mn steel will be described in detail below. Chemical composition

[020] The chemical composition of our high Mn steel and the reasons for its limitations are described first. In the chemical composition description, “%” indicates “weight percentage” unless otherwise noted.

[021] C: 0.10% or more and 0.70% or less

[022] C is an inexpensive austenite stabilizing element and an important element to obtain austenite. To achieve this effect, the C content needs to be 0.10% or more. On the other hand, a C content above 0.70% generates excess Cr carbides, deteriorating the toughness at low temperatures. Therefore, the C content is defined as 0.10% or more and 0.70% or less. The C content is preferably 0.20% or more. The C content is preferably 0.60% or less. The C content is more preferably 0.20% or more and 0.60% or less.

[023] Si: 0.10% or more and 0.90% or less

[024] Si acts as a deoxidizer, the same is necessary for steel making and is effective in increasing the hardness of a steel sheet by strengthening a solid solution when dissolved in steel. To obtain such effects, the Si content needs to be 0.10% or more. On the other hand, an Si content above 0.9% deteriorates weldability and also reduces toughness at low temperatures, in particular toughness at extremely low temperatures. Therefore, the Si content is defined as 0.10% or more and 0.90% or less. The Si content is preferably 0.12% or more. The Si content is preferably 0.70% or less. The Si content is more preferably 0.12% or more and 0.70% or less.

[025] Mn: 20% or more and 30% or less

[026] Mn is a relatively inexpensive austenite stabilizing element. In this revelation, Mn is an important element in achieving cryogenic toughness and toughness. To obtain these effects, the Mn content needs to be 20% or more. On the other hand, a Mn content above 30% does not increase the low-temperature toughness-enhancing effect, but increases the cost of the alloy. In addition, this high Mn content deteriorates weldability and cutability. Therefore, the Mn content is defined as 20% or more and 30% or less. The Mn content is preferably 23% or more. The Mn content is preferably 28% or less. The Mn content is more preferably 23% or more and 28% or less.

[027] P: 0.030% or less

[028] When a P content is above 0.030%, P segregates to the grain boundaries and becomes a source of stress corrosion cracking. Therefore, the upper limit of the P content is 0.030% and, desirably, the P content is kept as low as possible. Therefore, the P content is defined as 0.030% or less. Excessive P reduction, however, involves high refining costs and is economically disadvantageous. Therefore, the P content is desirably set to 0.002% or more. The P content is preferably 0.005% or more. The P content is preferably 0.028% or less, and more preferably 0.024% or less. Furthermore, the P content is more preferably 0.005% or more and 0.028% or less.

[029] S: 0.0070% or less

[030] S deteriorates the hardness and ductility of the base metal at low temperatures. Therefore, the upper limit of the S content is 0.0070% and, desirably, the S content is kept as low as possible. Consequently, the S content is set to 0.0070% or less. Excessive S reduction, however, involves high refining costs and is economically disadvantageous. Therefore, the S content is preferably adjusted to 0.001% or more. The S content is preferably 0.0020% or more. The S content is preferably 0.0060% or less. The S content is more preferably 0.0020% or more and 0.0060% or less.

[031] Al: 0.01% or more and 0.07% or less

[032] Al acts as a deoxidizer and is most commonly used in molten steel deoxidation processes to obtain a steel sheet. To obtain such an effect, the Al content needs to be 0.01% or more. On the other hand, when the Al content is above 0.07%, Al is mixed with a portion of the weld metal during welding, deteriorating the toughness of the weld metal. Therefore, the Al content is defined as 0.07% or less. Therefore, the Al content is defined as 0.01% or more and 0.07% or less. The Al content is preferably 0.02% or more. The Al content is preferably 0.06% or less. The Al content is more preferably 0.02% or more and 0.06% or less.

[033] Cr: 1.8% or more and 7.0% or less

[034] Cr is an element that stabilizes austenite with an appropriate amount of addition and is effective in improving low temperature toughness and base metal strength. To obtain such effects, the Cr content needs to be 1.8% or more. On the other hand, a Cr content above 7.0% generates Cr carbides, deteriorating low temperature toughness and stress corrosion cracking resistance. Therefore, the Cr content is defined as 1.8% or more and 7.0% or less. The Cr content is preferably 2.0% or more. The Cr content is preferably 6.7% or less. The Cr content is more preferably 2.0% or more and 6.7% or less. Furthermore, to improve stress corrosion cracking resistance, the Cr content is further preferably 2.0% or more. The content is further preferably 6.0% or less.

[035] Ni: 0.01% or more and less than 1.0%

[036] Ni is effective in increasing the hardness of a steel sheet through solid solution strengthening when dissolved in steel and in improving low temperature toughness, especially extremely low temperature toughness. Thus, Ni is contained in an amount of 0.01% or more. On the other hand, from an alloy cost point of view, the Ni content is desirably minimized and therefore the amount of Ni added is set to less than 1.0%. The Ni content is preferably 0.03% or more. The Ni content is preferably 0.8% or less. The Ni content is more preferably 0.03% or more and 0.8% or less. In the present document, stainless steels such as SUS304 and SUS316 as austenitic steel, which are excellent in low temperature toughness, optimize Ni equivalent and Cr equivalent as an alloy design to obtain an austenitic microstructure and, therefore, they are added with a large amount of Ni. In contrast to these steels, the steel in this revelation is an austenitic material that is produced at a lower cost, minimizing the need for Ni. The need for Ni is minimized by optimizing the amount of Mn addition.

[037] Ni: 0.1% or more and less than 1.0%

[038] Furthermore, when high Mn steel contains a predetermined amount of Cu, addition of Ni with a proper balance between the amount of Cu and the amount of Ni can exhibit excellent resistance to induced stress corrosion cracking. per chloride, irrespective of the chloride concentration. From this point of view, in high Mn steel containing Cu in an amount of 0.2% or more and less than 2.0% as described later, the Ni content is defined as 0.1% or more and less than 1.0%. A Ni content of less than 0.1% has no effect on stress corrosion cracking. A Ni content of 1.0% or more incurs a cost increase.

[039] Ca: 0.0005% or more and 0.010% or less

[040] Ca improves toughness through morphological control of the inclusions described below and acts effectively to ensure ductility (area reduction) during tensile deformation. To obtain such effects, the Ca content must be 0.0005% or more. On the other hand, addition of Ca in excess of 0.010% can reduce ductility and toughness. Therefore, the Ca content is defined as

0.0005% or more and 0.010% or less. The Ca content is preferably 0.0010% or more. The Ca content is preferably 0.0090% or less. The Ca content is more preferably 0.0010% or more and 0.0090% or less.

[041] Ca/S ≥ 1.0

[042] With the Ca content and S content described above, it is important to control the morphology of Ca-based inclusions, keeping even more Ca/S within an appropriate range. That is, setting Ca/S to ≥ 1.0 to promote composite precipitation of MnS using Ca-based inclusions as nuclei in crystal grains is effective in suppressing precipitation and thickening of MnS at crystal grain boundaries, thereby improving toughness and ensuring ductility during tensile deformation, specifically, achieving an area reduction of 50% or more. To obtain such effects, Ca/S must be 1.0 or higher. Ca/S is preferably 1.7 or more.

[043] N: 0.0050% or more and 0.0500% or less

[044] N is an austenite stabilizing element and an element that is effective in improving toughness at low temperatures. To achieve this effect, the N content needs to be 0.0050% or more. On the other hand, the N content above 0.0500% makes the nitrides or carbonitrides thicker, deteriorating the toughness. Therefore, the N content is defined as 0.0050% or more and 0.0500% or less. The N content is preferably 0.0060% or more. The N content is preferably 0.0400% or less. The N content is more preferably 0.0060% or more and 0.0400% or less.

[045] O: 0.0050% or less

[046] Oxygen (O) deteriorates toughness at low temperatures through the formation of oxides. Therefore, the O content is defined as 0.0050% or less. The O content is preferably 0.0045% or less. Excessive reduction of O content involves high refining cost and is economically disadvantageous. Therefore,

the O content is desirably 0.0003% or more.

[047] Ti: 0.0050% or less and Nb: 0.0050% or less

[048] Ti and Nb form carbonitrides with high melting point in steel to prevent the crystal grains from thickening, thus becoming a fracture origin and a crack propagation path. In particular, in high Mn steel, Ti and Nb impede microstructural control to increase low temperature toughness and improve ductility and therefore need to be intentionally limited. Specifically, Ti and Nb are components that are inevitably introduced with raw materials and other sources, and Ti of more than 0.005% and 0.010% or less and Nb of more than 0.005% and 0.010% or less are typically introduced into steel. Thus, according to the method described below, it is necessary to avoid the unavoidable introduction of Ti and Nb and to limit the content of each of Ti and Nb to 0.0050% or less. By limiting the content of each of Ti and Nb to 0.0050% or less, the adverse effect of carbonitrides can be eliminated and excellent low temperature toughness and ductility can be guaranteed. The content of each of Ti and Nb is preferably adjusted to less than 0.0050%, and more preferably to 0.003% or less.

[049] Cu: 0.2% or more and less than 2.0%

[050] Cu has an effect of improving chloride-induced stress corrosion cracking in an environment with low chloride concentration. From this point of view, a Cu content of 0.2% or more is effective. On the other hand, Cu deteriorates the cracking resistance by chloride-induced stress corrosion cracking in an environment with high chloride concentration. Therefore, when Cu is contained, the Cu content is set to less than 2.0%. A Cu content of less than 0.2% does not have an effect on stress corrosion cracking properties, and a Cu content of 2.0% or more causes the above problem and incurs increased cost. The Cu content is preferably 0.3% or more. The Cu content is preferably 0.8% or less. The Cu content is more preferably 0.3% or more and 0.8% or less.

[051] 0 < Cu/Ni ≤ 2

[052] In the present document, in high Mn steel containing Cu and Ni, in order to ensure excellent resistance to chloride-induced corrosion cracking, regardless of the chloride concentration, it is important to control the Cu and Ni contents within the range described above and, additionally, optimize the balance between the Cu content and the Ni content in order to satisfy 0 < Cu/Ni ≤ 2. In the case of Cu/Ni> 2, the Ni content is excessively small in relation to to the Cu content and excellent resistance to chloride-induced stress corrosion cracking cannot be achieved in a high chloride environment.

[053] The different balance of the above mentioned essential components is Fe and unavoidable impurities. Inevitable impurities include H, and a total content of 0.01% or less may be allowed.

[054] In this disclosure, in order to further improve low temperature strength and toughness, the following elements may be contained as needed in addition to the above essential components: at least one selected from the group consisting of Mo: 2.0 % or less, V: 2.0% or less, W: 2.0% or less, Mg: 0.0005% or more and 0.0050% or less, REM: 0.0010% or more and 0.0200 % or less.

[055] Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less

[056] Mo, V and W stabilize the austenite and improve the strength of the base metal. To obtain such effects, Mo, V and/or W is preferably contained in an amount of 0.001% or more. On the other hand, when the content of each added element is beyond 2.0%, thick carbonitrides are generated, which can become a source of fracture and additionally increase the manufacturing cost. Therefore, when any of these alloying elements is/are added, the content of each added element is preferably 2.0% or less. The content of each element added is more preferably 0.003% or more. The content of each element added is preferably 1.7% or less, and more preferably 1.5% or less. Furthermore, the content of each element added is preferably 0.003% or more and 1.7% or less, and more preferably 0.003% or more and 1.5% or less.

[057] Mg: 0.0005% or more and 0.0050% or less, REM: 0.0010% or more and 0.0200% or less

[058] Mg and REM are useful elements for morphological control of inclusions and can be contained as needed. The morphological control of the inclusions means granulating elongated sulfide inclusions. Morphological control of inclusions improves ductility, toughness and resistance to sulphide-induced stress corrosion cracking. To obtain such effects, Mg is preferably contained in an amount of 0.0005% or more and REM is preferably contained in an amount of 0.0010% or more. On the other hand, when these elements are contained in a large amount, not only can the amount of non-metallic inclusions be increased, eventually deteriorating ductility, toughness, and sulphide-induced stress corrosion cracking resistance, but also a economic disadvantage may result. Therefore, when Mg is contained, the Mg content is preferably 0.0005% or more. The Mg content is preferably 0.0050% or less. When REM is contained, the REM content is preferably 0.0010% or more. The REM content is preferably 0.0200% or less. The Mg content is more preferably 0.0010% or more. The Mg content is more preferably 0.0040% or less. The Mg content is further preferably 0.0010% or more and 0.0040% or less. The REM content is more preferably 0.0020% or more. The REM content is more preferably 0.0150% or less. The REM content is further preferably 0.0020% or more and 0.0150% or less. Microstructure Microstructure containing austenite as matrix

[059] When a steel material has a body-centered cubic (bcc) crystal structure, the steel material may cause brittle fracture in a low-temperature environment and is therefore not suitable for use in a low-temperature environment. When the steel material is assumed to be used in a low temperature environment, it is necessary that the steel material has, as a matrix, an austenite microstructure, which has a face-centered cubic (fcc) crystal structure. As used in this document, "containing austenite as a matrix" means that the area ratio of the austenite phase is 90% or more. The remainder, in addition to the austenite phase, is the ferrite or martensite phase. Of course, the area ratio of the austenite phase can be 100%. manufacturing method

[060] The high Mn steel fabrication method of this disclosure comprises: heating a steel material having the aforementioned chemical composition; hot rolling the heated steel material to obtain a hot rolled sheet; and subjecting the hot-rolled sheet to a cooling treatment. Furthermore, in the high Mn steel fabrication method of this disclosure, the temperature range in heating the steel material is from 1100°C to

1300°C, the hot rolling finishing temperature in hot rolling is 750°C or higher and lower than 950°C, and the average cooling rate from a temperature equal to or higher (the rolling finishing temperature -100°C) at a temperature range from 300ºC to 650ºC in the cooling treatment is 0.5ºC/s or more.

[061] In making the high Mn steel of this disclosure, steel material can be obtained from molten steel with the aforementioned chemical composition which is prepared by steel making using a publicly known method such as the use of a converter or an electric heating furnace. In addition, high Mn steel can also be subjected to secondary refinement in a vacuum degassing furnace. During secondary refinement, in order to limit the Ti and Nb contents that prevent adequate microstructural control within the aforementioned range, it is necessary to avoid that Ti and Nb are inevitably introduced with raw materials and other sources, in order to reduce the Ti and Nb contents. You and Nb. For example, by reducing the basicity of the slag during the refining stage, these alloying elements are concentrated in the slag to be discharged, thus reducing the concentrations of Ti and Nb in a final ingot product. Alternatively, the molten steel can be oxidized by blowing oxygen, and the Ti and Nb alloys can be suspended and separated during reflux. Subsequently, it is preferable to transform the steel into a steel material, such as an ingot of a certain size, by a publicly known steelmaking method, such as continuous casting or casting. The ingot after continuous casting can be subjected to wear to obtain a steel material.

[062] Furthermore, the manufacturing conditions are specifically defined below to transform the aforementioned steel material into a steel material exhibiting high strength, excellent low temperature toughness and ductility.

[063] Heating temperature of a steel material: 1,100ºC or higher and 1,300ºC or lower.

[064] In order to thicken the crystal grain size in the microstructure of the steel material, the heating temperature before hot rolling is set to 1100°C or more. However, the heating temperature beyond

1300ºC can trigger local melting. Thus, the upper limit of the heating temperature is set to 1300°C. The temperature is controlled based on the surface temperature of the steel material.

[065] Lamination finishing temperature: 750ºC or higher and lower than 950ºC.

[066] The steel material (steel ingot or billet) is heated and then subjected to hot rolling. In order to obtain thick crystal grains, it is preferable to increase the lamination reduction rate accumulated at high temperature. That is, low-temperature hot rolling refines the microstructure and excessively introduces work deformation, deteriorating low-temperature toughness. Therefore, the lower limit of hot rolling mill finish temperature is set to 750°C in terms of a steel sheet surface temperature. On the other hand, finishing the lamination in a temperature range of 950°C or higher will result in an excessively thick grain size and therefore the desired yield point cannot be achieved. Therefore, final finish lamination needs to be performed in one or more passes at less than 950°C.

[067] Average cooling rate from a temperature equal to or greater (the lamination finishing temperature -100ºC) at a temperature range of 300ºC to 650ºC: 0.5ºC/s or more.

[068] After hot rolling, cooling is carried out immediately. Slow cooling of the steel sheet after hot rolling promotes the formation of precipitates, deteriorating toughness at low temperatures. Cooling the steel sheet at a cooling rate of 0.5°C/s or more over a predetermined temperature range can prevent these precipitates from forming. In addition, excessive cooling distorts the steel sheet, deteriorating productivity. Therefore, the upper limit of the initial cooling temperature can be set to 900°C. Also, the lower limit of the initial cooling temperature is set at (laminating finish temperature -

100°C). The start of cooling from a temperature below this temperature promotes the formation of precipitates after hot rolling, deteriorating the toughness at low temperatures. Furthermore, the final cooling temperature is within a temperature range of 300°C to 650°C. This is because cooling to the above temperature range can suppress the precipitation of carbides that reduce toughness. For the reasons mentioned above, in cooling after hot rolling, the average rate of cooling on a steel sheet surface from a temperature equal to or greater than (rolling finish temperature -100°C) in terms of a surface temperature of the sheet steel, for a temperature range of 300°C to 650°C is defined as being 0.5°C/s or more. On the other hand, from an industrial production point of view, the average cooling rate is preferably 200°C/s or less. The cooling rate is calculated as an average rate of cooling of the steel sheet by simulation calculation based on its surface temperature change.

[069] Furthermore, in the aforementioned casting process, during cooling, the cooling time in the temperature range of 1400°C to 1300°C in terms of a steel surface temperature is preferably controlled to 100 s or less. Controlling the cooling time in the casting process, as described above, promotes precipitation composed of MnS containing Ca-based inclusions such as Ca (O, S) as the core and the number of (Ca, Mn) S is increased. As a result, MnS does not grow on crystal grain boundaries or within crystal grains, and therefore the proportion of elongated MnS is reduced. Through this morphological control of Ca-based inclusions, high Mn steel with a good area reduction of up to 51% or more can be obtained.

EXAMPLES

[070] This revelation will be described in more detail below by way of examples. Note that this revelation is not limited to the following examples.

[071] Steel ingots with the chemical compositions listed in Table 1 were obtained as steel materials by a converter and ladle refining process and continuous melting. Then, the steel ingots obtained were subjected to wear and hot rolling to obtain steel sheets with a thickness of up to 32 mm under the conditions listed in Table 2. The tensile properties, toughness and microstructures of the steel sheets were evaluated according to Described below.

[072] (1) Tensile test properties

[073] JIS NO. 4 tensile test specimens were collected from steel sheets obtained with a sheet thickness of more than 15 mm. Round bar tensile specimens with a parallel portion diameter of 6 mm and a reference length of 25 mm were collected from steel sheets obtained with a sheet thickness of less than 15 mm. Then, the specimens were subjected to a tensile test to investigate the properties of the tensile test. In this disclosure, when a specimen had a yield strength of 400 MPa or more and a stress-strain of 800 MPa or more, the corresponding steel sheet was determined to have excellent tensile properties and high strength. Furthermore, when a specimen had an area reduction of 50% or more, the corresponding steel sheet was determined to have excellent ductility.

[074] (2) Low temperature toughness

[075] Full-size Charpy V-notch specimens were taken from the direction parallel to the rolling direction at a position on the surface of the steel plate at a depth of 1/4 of the plate thickness (hereinafter referred to as position 1 /4 of the plate thickness) of each steel plate more than 20 mm thick, or in a position on the surface of the steel plate at a depth of 1/2 of the plate thickness (hereinafter referred to as 1/2 position sheet thickness) of each steel sheet not exceeding 20 mm in thickness, in accordance with JIS Z 2202 (1998). Then, the specimens were submitted to the Charpy impact test in accordance with JIS Z 2242 (1998), where three specimens were used for each steel plate, to determine the energy absorbed at -196ºC and to evaluate the toughness of the material. base metal. In this disclosure, when the three specimens had an average absorbed energy (vE-196) of 100 J or more, the corresponding steel sheet was determined to have excellent base metal toughness at low temperature. For steel sheets with a sheet thickness of less than 10 mm, half-sized Charpy V-notch specimens were collected from the steel sheets and similarly subjected to the Charpy impact test. For steel sheets with sheet thickness less than 10 mm, when the specimens had an average absorbed energy of 20 J or more, the corresponding steel sheet was determined to have excellent base metal toughness at low temperature.

[076] (3) Stress corrosion cracking test

[077] The stress corrosion cracking resistance of samples 32 and 33 was evaluated in a boiling magnesium chloride solution in accordance with ASTM G36 (hereinafter referred to as "Boiling Magnesium Chloride Solution Test "). As a specimen, a U-bend specimen conforming to ASTM G30 was used Example a. A specimen 2.5 mm thick x 20 mm wide x 80 mm long was taken in the C direction from a position 1 mm below the surface of each steel plate. The specimen was bent in its center, in the longitudinal direction with 5R and submitted to the test.

[078] The test time was 400 hours. After the test, the specimens without cracks observed on the surface were considered to have excellent resistance to chloride-induced stress corrosion cracking. In Table 3, cases where no cracking was observed on the surface are indicated as “good” and cases where cracking was visually observed on the surface are indicated as “bad”.

[079] High Mn steel from this disclosure has been confirmed to meet the aforementioned desired performance (a base metal yield strength of 400 MPa or more, an area reduction of 50% or more, an average absorbed energy ( vE-196) of 100 J or more (20 J or more for a half-size specimen) for low temperature toughness). On the other hand, comparative examples outside the scope of this disclosure do not satisfy the aforementioned desired performance in terms of one or more of yield stress, area reduction, and low temperature toughness.

[080] Also, sample 32, in which Cu and Ni were contained so that Cu/Ni was within the predetermined range. exhibited excellent resistance to chloride-induced stress corrosion cracking. On the other hand, in sample 33 where Cu/Ni were outside the predetermined range, sufficient resistance to chloride-induced stress corrosion cracking could not be confirmed.

Table 1 Chemical Composition (Percent by Weight) Steel Number C Si Mn P S Al Cr O N Nb Ti V Cu Ni Mo W Ca Mg REM Ca/S Cu/Ni Remarks

A 0.189 0.44 28.5 0.015 0.0030 0.042 4.01 0.0020 0.0205 0.002 0.003 - - 0.04 - - 0.0041 - - 1.4 - Example

B 0.652 0.18 22.4 0.011 0.0048 0.031 2.52 0.0041 0.0374 0.001 0.001 - - 0.05 - - 0.0072 - - 1.5 - Example

C 0.435 0.38 24.1 0.023 0.0038 0.027 4.50 0.0022 0.0241 0.002 0.001 - 0.31 0.07 - - 0.0080 - - 2.1 4.4 Example

D 0.339 0.76 20.4 0.019 0.0062 0.042 3.04 0.0031 0.0185 0.002 0.002 0.04 - 0.02 0.41 - 0.0075 - - 1.2 - Example

E 0.285 0.35 28.2 0.024 0.0021 0.067 1.85 0.0023 0.0255 0.001 0.003 - - 0.01 - 0.07 0.0023 - - 1.1 - Example

F 0.463 0.31 26.8 0.017 0.0045 0.038 6.52 0.0040 0.0375 0.003 0.001 - - 0.06 - - 0.0063 - - 1.4 - Example

23/27 G 0.342 0.38 22.8 0.021 0.0019 0.047 2.52 0.0030 0.0201 0.002 0.001 - 0.45 0.08 - - 0.0040 0.0011 - 2.1 5.6 Example

H 0.412 0.18 20.4 0.017 0.0015 0.030 2.05 0.0029 0.0075 0.002 0.001 - - 0.09 - - 0.0024 - 0.0027 1.6 - Example

I 0.325 0.14 25.2 0.020 0.0028 0.042 5.21 0.0024 0.0152 0.003 0.002 - - 0.03 - - 0.0031 - - 1.1 - Example

J 0.420 0.37 23.7 0.014 0.0012 0.035 4.22 0.0023 0.0214 0.002 0.004 - 0.62 0.07 - - 0.0042 - - 3.5 8.9 Example

K 0.572 0.32 26.1 0.016 0.0029 0.032 4.04 0.0021 0.0099 0.001 0.003 - - 0.05 - - 0.0056 - - 1.9 - Example

Example L 0.945 0.42 20.3 0.003 0.0026 0.034 4.00 0.0039 0.0311 0.003 0.004 - - 0.01 - - 0.0029 - - 1.1 - Comparative Example M 0.109 0.04 23, 9 0.019 0.0032 0.042 5.23 0.0012 0.0275 0.002 0.001 - - - - - 0.0042 - - 1.3 - Comparative Example N 0.132 0.52 15.8 0.005 0.0045 0.042 2.32 0 .0034 0.0455 0.003 0.003 - - 0.03 - - 0.0052 - - 1.2 - Comparative Example O 0.226 0.47 23.2 0.047 0.0063 0.050 1.85 0.0041 0.0336 0.001 0.003 - - 0.01 - - 0.0068 - - 1.1 - Comparative Example P 0.327 0.28 27.4 0.028 0.0095 0.021 1.99 0.0032 0.0074 0.001 0.003 - - 0.02 - - 0, 0097 - - 1.0 - Comparative Example R 0.280 0.41 26.8 0.017 0.0039 0.052 7.84 0.0028 0.0195 0.003 0.001 - - 0.01 - - 0.0048 - - 1.2 - Comparative Example S 0.427 0.28 25.7 0.024 0.0033 0.047 6.07 0.0075 0.0205 0.003 0.002 - - 0.02 - - 0.0040 - - 1.2 - Comparative Example T 0.353 0.11 24, 9 0.027 0.0045 0.032 3.31 0.0028 0.0769 0.003 0.002 - - 0.03 - - 0.0051 - - 1.1 - Comparative Example U 0.462 0.55 25.3 0.022 0.0043 0.027 4, 22 0.0021 0, 0255 0.003 0.003 - - 0.01 - - 0.0029 - - 0.7 - Comparative

Chemical Composition (Percent by weight) Steel number C Si Mn P S Al Cr O N Nb Ti V Cu Ni Mo W Ca Mg REM Ca/S Cu/Ni Remarks

V 0.440 0.29 24.5 0.019 0.0037 0.034 4.34 0.0027 0.0224 0.001 0.002 - - 0.08 - - 0.0038 - - 1.0 - Example

Example W 0.384 0.82 20.9 0.027 0.0004 0.045 3.22 0.0033 0.0205 0.001 0.003 - - 0.03 - - 0.0004 - - 1.0 - Comparative Example X 0.587 0.44 25, 8 0.022 0.0025 0.019 3.97 0.0024 0.0128 0.002 0.001 - - 0.01 0.14 - 0.0023 - - 0.9 - Comparative Example Y 0.602 0.25 23.3 0.025 0.0020 0.034 2.62 0.0032 0.0246 0.004 0.011 - - 0.02 - 0.20 0.0029 - - 1.5 - Comparative Example Z 0.080 0.36 20.4 0.019 0.0046 0.042 3.85 0.0021 0.0237 0.003 0.004 - - 0.02 - - 0.0048 - - 1.0 - Comparative AA 0.290 0.39 27.6 0.012 0.0030 0.042 4.20 0.0020 0.0190 0.002 0.003 - 0.40 0.30 - - 0.0033 - - 1.1 1.3 Example Example of BB 0.280 0.39 27.8 0.012 0.0030 0.042 4.20 0.0020 0.0210 0.002 0.003 - 0.40 0.05 - - 0.0033 - - 1.1 8.0 Reference

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Table 2 Hot Rolling Method Temperature Time Specs Temperatur Cooling Temperat Cooling Temperat Initial ure number 1400°C average ure to finish 1300°C heat range Observations sample sheet metal cooling steel during steel melting rolling temperature to 300°C to 650°C (s) (mm) (°C) (°C) (°C) (°C/s)

1 A 20 32 1160 863 836 8 Example

2 B 50 23 1180 825 795 11 Example

3 C 60 12 1150 781 726 13 Example

4 D 80 21 1120 792 760 10 Example

5 E 20 24 1130 860 830 9 Example

6 F 30 14 1130 842 787 11 Example

7 G 30 8 1260 885 821 15 Example

8 H 40 9 1200 832 773 12 Example

9 I 50 14 1140 828 772 4 Example

10 J 70 29 1160 858 831 0.9 Example

11 K 30 11 1110 755 705 11 Example

12 J 20 12 1150 775 723 11 Example Example 13 L 30 19 1230 765 733 8 Comparative Example 14 M 50 30 1180 885 858 7 Comparative Example 15 N 30 13 1100 820 765 11 Comparative Example 16 O 40 40 24 1 17 P 50 20 1190 804 761 6 Comparative Example 18 R 20 20 1150 801 764 9 Comparative Example 19 S 40 14 1110 814 758 11 Comparative Example 20 T 30 28 1150 852 828 2 Comparative Example 21 C 50 11 11 Comparative Example 20 T 30 28 1150 852 828 2 Comparative Example 21 C 50 11 1 1 22 D 20 18 1220 807 775 0.3 Comparative Example 23 E 30 24 1020 853 823 10 Comparative Example 24 U 50 10 1120 760 713 11 Comparative 25 V 30 12 1180 805 742 12 Example Example 26 W 1 50 27 73 Example 27 X 30 10 1090 741 693 12 Comparative

Hot Rolling Method Temperature Time Specs Temperatur Cooling Temperat Cooling Temperat Number of initial temperature of 1,400°C to average temperature of finish of 1,300°C heating range Remarks sample steel cooling plate during melting of the steel rolling temperature to 300°C to 650°C (s) (mm) (°C) (°C) (°C) (°C/s) Example 28 A 50 30 1290 984 952 8 Comparative Example 29 Y 20 8 1130 852 796 12 Comparative Example 30 Z 30 28 1150 821 788 9 Comparative 31 A 300 32 1160 863 836 8 Example 32 AA 20 30 1100 900 850 10 Example Example of 33 BB 100 30 Reference

Table 3

Solution Energy Test Stress Limit- Absorbed Reduction a - Chloride Number Flow Deformation Area 196°C of Steel Observations (vE-196) Magnesium sample in (MPa) (MPa) (%) (J) boiling

1 A 421 854 55 137 - Example

2 B 548 940 57 121 - Example

3 C 568 970 60 119 - Example

4 D 562 956 53 116 - Example

5 E 483 885 52 137 - Example

6 F 455 855 55 141 - Example

7 G 421 940 59 62* - Example

8 H 506 967 57 54* - Example

9 I 449 850 51 127 - Example

10 J 404 842 64 133 - Example

11 K 569 969 56 116 - Example

12 J 559 953 61 127 - Example Example 13 L 625 974 51 58 - Comparative Example 14 M 357 817 55 127 - Comparative Example 15 N 445 869 51 36 - Comparative

Example 16 O 450 882 54 72 - Comparative Example 17 P 523 939 50 79 - Comparative Example 18 R 521 939 53 48 - Comparative Energy Test solution Number Limit of Voltage - Absorbed reduction a - of chloride Flow number deformation of the area 196° C of Observations of sample steel (MPa) (MPa) (%) (vE-196) boiling magnesium (J) Example 19 S 449 849 52 83 - Comparative Example 20 T 419 854 52 75 - Comparative Example 21 C 670 1061 54 42 - Comparative Example 22 D 541 942 54 67 - Comparative Example 23 E 479 879 53 69 - Comparative Example 24 U 553 942 44 108 - Comparative 25 V 553 962 51 126 - Example Example 26 W 583 960 48 108 - Comparative Example 27 X 579 982 45 112 - Comparative Example 28 A 369 847 54 139 - Comparative Example 29 Y 501 994 55 17* - Comparative Example 30 Z 428 848 50 35 - Comparative 31 A 421 854 50 112 - Example

32 AA 435 851 55 105 Good Example Example of 33 BB 437 853 60 120 Bad Reference

* half-size specimen

Claims (5)

1. Steel with a high content of Mn, CHARACTERIZED in that it comprises: a chemical composition containing, in percentage by weight: C: 0.10% or more and 0.70% or less, Si: 0.10% or more and 0.90% or less, Mn: 20% or more and 30% or less, P: 0.030% or less, S: 0.0070% or less, Al: 0.01% or more and 0.07% or less, Cr: 1.8% or more and 7.0% or less, Ni: 0.01% or more and less than 1.0%, Ca: 0.0005% or more and 0.010% or less, N: 0.0050% or more and 0.0500% or less, O: 0.0050% or less, Ti: 0.0050% or less, and Nb: 0.0050% or less, within a range satisfying the following formula (1), with the balance being Fe and unavoidable impurities, Ca/S ≥ 1.0 (1); a microstructure containing austenite as a matrix; a yield strength of 400 MPa or more; and a medium Charpy impact absorbed energy at -196°C of 100 J or more for a full-size specimen and 20 J or more for a half-size specimen. 2. Steel with a high Mn content according to claim 1, CHARACTERIZED by the fact that the chemical composition also contains, in percentage by weight, at least one selected from the group consisting of: Cu: less than 2.0%, Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, Mg: 0.0005% or more, and 0.0050% or less, and REM: 0.0010% or more and 0.0200% or less. 3. Method of manufacturing steel with a high content of Mn, CHARACTERIZED by the fact that it comprises: heating a steel material containing the chemical composition according to 1 or 2 at a temperature range of 1,100ºC to 1,300ºC; then hot rolling the steel material with a rolling finishing temperature of 750ºC or higher and lower than 950ºC, to obtain a hot rolled sheet; and then subjecting the hot-rolled sheet to a cooling treatment at an average cooling rate of 0.5°C/s or more, from a temperature equal to or higher (the rolling finishing temperature - 100°C) to a temperature range of 300°C to 650°C. 4. Steel with a high content of Mn, CHARACTERIZED in that it comprises: a chemical composition containing, in percentage by weight: C: 0.10% or more and 0.70% or less, Si: 0.10% or more and 0.90% or less, Mn: 20% or more and 30% or less, P: 0.030% or less, S: 0.0070% or less, Al: 0.01% or more and 0.07% or less, Cr: 1.8% or more and 7.0% or less, Cu: 0.2% or more and less than 2.0% Ni: 0.1% or more and less than 1.0%, Ca: 0.0005% or more and 0.010% or less, N: 0.0050% or more and 0.0500% or less, O: 0.0050% or less, Ti: 0.0050% or less and Nb: 0.0050% or less, within a range satisfying the following formulas (1) and (2), with the balance being Fe and unavoidable impurities, Ca/S ≥ 1.0 (1), and 0 < Cu/Ni ≤ 2 (2); and a microstructure containing austenite as a matrix. 5. Method of manufacturing steel with a high content of Mn, CHARACTERIZED by the fact that it comprises: heating a steel material having the chemical composition according to claim 4, at a temperature range of 1,100ºC to 1,300°C; then hot rolling the steel material with a rolling finishing temperature of 750ºC or higher and lower than 950ºC to obtain a hot rolled sheet; and then subjecting the hot-rolled sheet to a cooling treatment at an average cooling rate of 0.5°C/s or more, from a temperature equal to or higher (the rolling finishing temperature - 100°C) to a temperature range of 300°C to 650°C.