Cr-Mn CONTAINING DUPLEX STEELS WITH EXCELLENT CRYOGENIC TOUGHNESS AND MANUFACTURING METHOD THEREOF

Abstract

The present invention provides an alloy composition having a two-phase structure with excellent cryogenic toughness, which can store various liquefied gases such as hydrogen, LNG and nitrogen more safely because it exhibits excellent cryogenic impact toughness compared to conventional cryogenic high-manganese steel, and significantly increase resistance to container damage even in accidents where external impact is applied, chromium-manganese steel and a manufacturing method thereof.

Description

TECHNICAL FIELD

[0001] The present invention relates to chromium-manganese steel alloy compositions having a two-phase structure with excellent cryogenic toughness, chromium-manganese steels and a method of manufacturing the same. With their excellent cryogenic impact toughness, the chromium-manganese duplex steels of the present invention may find applications as structural materials for vessels and containers storing various liquefied gases such as hydrogen, nitrogen and liquefied natural gas, and the containers made of the present steels may exhibit significantly higher resistance to breakage than those of conventional cryogenic steels even in cryogenic impact accidents, and thus ensure a higher level of storage safety at cryogenic temperatures.BACKGROUND ART

[0002] In general, liquefied gases such as liquefied hydrogen, liquefied oxygen, liquefied natural gas and the like essentially require cryogenic containers for their storage and transportation in a state of maintaining a liquid state. In order to achieve this, the storage container made of a material that has sufficient toughness and strength at extremely low temperatures is required.

[0003] Conventional carbon steels, however, have the disadvantage in that when the operating temperature is lowered, the yield strength increases rapidly and the toughness decreases significantly, which limits the usage thereof. Most metals consisting of the austenite phase are known to have excellent cryogenic toughness. Such metals, especially austenitic stainless steels, show no ductile-brittle transition phenomenon and therefore have quite high toughness even at cryogenic temperatures. Unlike ferritic steels, austenitic stainless steels exhibit lower yield strength at low temperatures and are more plastically deformable so that they may absorb the impact energy more efficiently. There is, however, a problem in that the manufacturing costs of austenitic stainless steels are much higher because they contain expensive nickel (Ni) which act as an austenite stabilizer.

[0004] Further, in the case of liquefied hydrogen, which has recently been in the spotlight, the storage temperature is lower than that of conventional liquefied gas, and thus, higher toughness is required at cryogenic temperatures. However, the toughness of conventional austenitic stainless steels is not sufficiently high for the storage of hydrogen. Accordingly, cryogenic steels containing a large amount of low-cost manganese (Mn) and carbon (C) as austenite stabilizers have recently been developed in place of austenitic stainless steels with high nickel contents. Their cryogenic toughness is reported to be comparable to or improved over conventional austenitic stainless steels, but there is a problem in that their corrosion resistance is poor and they still do not exhibit high enough toughness to ensure structural stability at cryogenic temperatures.

[0005] Accordingly, there is an urgent need for research on new alloys, which are capable of storing various liquefied gases more safely, significantly increasing resistance to container breakage even in the event of an accident in which an external impact is applied, and having excellent economic feasibility and excellent cryogenic toughness.DISCLOSURE OF INVENTIONTechnical Problem

[0006] The present invention has been devised to overcome the above-described problems, and the problem to be solved by the present invention is to provide alloy compositions that exhibit better cryogenic impact toughness than cryogenic metallic materials previously developed for liquefied gas storage containers, and more specifically, to provide alloy compositions of chromium-manganese steels, and a method of manufacturing the same.Solution to Problem

[0007] In order to solve the above-described problems, the present invention provides an alloy composition of chromium-manganese steel with excellent cryogenic toughness, including 15 to 25 wt. % of manganese (Mn), 5 to 15 wt. % of chromium (Cr), 4 wt. % or less of aluminum (Al), 0.05 wt. % or less of carbon (C) and the remainder on an Fe base, wherein the alloy composition has a two-phase microstructure consisting of austenite and δ-ferrite phases.

[0008] Further, in an exemplary embodiment of the present invention, silicon (Si) may be included in addition to the aluminum (Al) such that the total amount of aluminum (Al) and silicon is 6 wt. % or less.

[0009] In addition, the present invention provides chromium-manganese steel with excellent cryogenic toughness, having the above-described alloy composition, wherein the chromium-manganese steel exhibits a low stacking fault energy of 30 mJ / m2 or less such that under externally applied stress and impact conditions, phase transformation from austenite to ε-martensite occurs, and phase transformation from ε-martensite to α′-martensite occurs.

[0010] In addition, the ratio (b / a) of a length (b) in the shortest direction to a length (a) in the longest direction of δ-ferrite grains may be 0.5 or less.

[0011] In addition, the impact absorption energy obtained through an impact test on a Charpy V-notch standard specimen at liquid nitrogen temperature (approximately −196° C.) may be 200 J or more.

[0012] In addition, the chromium-manganese steel may satisfy all of Relationship Formulas (1) and (2) below:

[0013] (1) Yield strength of 200 MPa or more

[0014] (2) Ultimate tensile strength of 550 MPa or more.

[0015] In addition, the chromium-manganese steel may satisfy Relationship Formula (3) below:

[0016] (3) Total elongation of 60% or more.

[0017] In addition, the present invention provides a method for manufacturing chromium-manganese steel having excellent cryogenic toughness, including: step 1 of melting an alloy composition of chromium-manganese steel consisting of austenite and δ-ferrite phases; step 2 of heat treating for homogenization by maintaining a molten ingot in a temperature range of 900° C. to 1,200° C. for 1 hour to 10 hours, followed by air cooling or water cooling; step 3 of hot working (rolling or forging) the heat-treated ingot to 50% to 90% reduction in thickness in a temperature range of 900° C. to 1,200° C.; and step 4 of heat treating hot-worked plates or bars for recrystallization in a temperature range of 600° C. to 900° C. for 0.5 to 10 hours, followed by air cooling or water cooling.

[0018] In addition, according to an exemplary embodiment of the present invention, through step 2, a δ-ferrite phase having an area fraction of 5% or more to 30% or less may be homogeneously generated.

[0019] In addition, the method may further include the step of cold working to a sheet of the desired thickness and then performing heat treatment again, after performing step 4.Advantageous Effects of Invention

[0020] According to the present invention, the chromium-manganese duplex steels described above exhibit excellent cryogenic toughness by consuming externally imposed strain and impact energy through transformation-induced plasticity (TRIP) and twining-induced plasticity (TWIP), by grain refinement resulted from TRIP and TWIP phenomena, and further by the elongated δ-ferrite grains which efficiently block the propagation of cracks. The detailed mechanisms behind the improved impact toughness are as follows: Firstly, the externally imposed impact energy on the chromium-manganese duplex steel causes transformation of the austenite phase to the ε-martensite phase and then induces the subsequent transformation of the produced ε-martensite phase to the α′-martensite phase, through which imposed impact energy is greatly consumed and the grain structure is significantly refined. Secondly, the energy required for the propagation of cracks generated by impact is consumed through twining-induced plasticity (TWIP) and slip in the austenite phase around the cracks. Thirdly, the propagation of cracks can be suppressed effectively by the δ-ferrite grains elongated perpendicular to the direction of crack propagation.BRIEF DESCRIPTION OF DRAWINGS

[0021] FIG. 1 is a set of scanning electron microscope images of chromium-manganese steel according to an exemplary embodiment of the present invention.

[0022] FIG. 2 is a set of scanning electron microscope images of the fracture surface of chromium-manganese steel according to an exemplary embodiment of the present invention after an impact test at −196° C.

[0023] FIG. 3 is a set of spatial distribution images of the phase analyzed through electron backscattering diffraction on the microstructure around the fracture surface of a specimen fractured through an impact test at −196° C. of chromium-manganese steel according to an exemplary embodiment of the present invention.

[0024] FIG. 4 is a set of crystal orientation images for the same area as FIG. 3.

[0025] FIG. 5 is a phase spatial distribution image and a crystal orientation image at a location 5 mm inward from the fracture surface for the specimen of Example 1 fractured through an impact test at −196° C.MODE FOR THE INVENTION

[0026] Hereinafter, the exemplary embodiments of the present invention will be described in detail so that those skilled in the art can easily practice the present invention. The present invention may be implemented in many different forms and is not limited to the exemplary embodiments described herein.

[0027] As mentioned above, the use of conventional cryogenic materials is limited due to problems with corrosion resistance or structural stability at extremely low temperatures and economic issues that require expensive nickel. Furthermore, there is a problem in that they cannot be used as storage containers for liquefied hydrogen, which has recently been in the spotlight.

[0028] Accordingly, the present invention has sought to solve the above-described problems by providing alloy compositions of chromium-manganese steel with excellent cryogenic toughness, including 15 to 25 wt. % of manganese (Mn), 5 to 15 wt. % of chromium (Cr), 4 wt. % or less of aluminum (Al), 0.05 wt. % or less of carbon (C) and the remainder on an Fe base, wherein the chromium-manganese steel has a two-phase microstructure of austenite and δ-ferrite, and chromium-manganese steel exhibits excellent cryogenic toughness.

[0029] Through this, the present invention can provide chromium-manganese steels having a two-phase microstructure with excellent cryogenic toughness, which can store various liquefied gases such as hydrogen, LNG and nitrogen more safely because it exhibits excellent cryogenic impact toughness than conventional cryogenic high-manganese steel, and significantly increase resistance to container damage even in accidents where external impact is applied

[0030] The present invention will be described in detail below with reference to the drawings.Alloy Compositions of Chromium-Manganese Steels with Excellent Cryogenic Toughness and Chromium-Manganese Steels

[0031] The concept of alloy design of chromium-manganese steels with excellent cryogenic toughness according to the present invention is based on the fact that under a cryogenic environment, a significant amount of externally applied impact energy is consumed to transform an austenite phase with a face-centered cubic (fcc) structure to the ε-martensite phase with a hexagonal close-packed (hcp) structure, and subsequently to transform the phase-transformed ε-martensite phase to the α′-martensite phase with a body-centered cubic (bcc) structure. Meanwhile, in this case, some impact energy is consumed to accommodate plastic deformation by deformation twinning and slip in the crack propagation zone. Additionally, the elongated δ-ferrite grains that coexist with the austenite phase have the effect of suppressing the propagation of cracks, thereby contributing to improving cryogenic impact toughness. In addition, as a result of the above-described phase transformation, the grain refinement effect of the remaining austenite phase, and transformed ε-martensite and α′-martensite phases also contributes to the improvement of cryogenic impact toughness.

[0032] To this end, the chromium-manganese steels with excellent cryogenic toughness according to the present invention are realized with the alloy composition of 15 to 25 wt. % of manganese (Mn), 5 to 15 wt. % of chromium (Cr), 4 wt. % or less of aluminum (Al), 0.05 wt. % or less of carbon (C) and the remainder on an Fe base. The steels produced have a two-phase microstructure consisting of the austenite phase with a stacking fault energy in an appropriate range and an appropriate amount of the δ-ferrite phase, and thereby achieving the effect of improving the cryogenic impact toughness described above.

[0033] In this case, in the present invention, “manganese (Mn)” may be added as an essential element to replace expensive nickel (Ni), increase strength, increase stacking fault energy and stabilize the austenite phase. In addition, manganese may combine with sulfur (S) or oxygen (O), which are impurity elements, to form non-metallic inclusions such as manganese sulfide (MnS) or manganese oxide (MnO).

[0034] Accordingly, if the amount of manganese is less than 15 wt. %, the stabilization of the austenite phase may be difficult, and there may be a problem in that a large amount of δ-ferrite phase is generated, and if manganese is added in excess of 25 wt. %, there may be problems in that hot formability, weldability and oxidation resistance are deteriorated.

[0035] Further, in the present invention, “chromium (Cr)”, which is a representative alloying element that stabilizes ferrite, is added to steels to improve corrosion resistance. Chromium added to high-manganese steel may slightly increase the stacking fault energy of austenite, and when the content thereof increases, it may combine with carbon to form various carbides.

[0036] Accordingly, if the chromium is added at less than 5 wt. %, corrosion resistance may be significantly reduced, and if the chromium is added in excess of 15 wt. %, there may be a problem in that an excessive amount of 5-ferrite phase is generated.

[0037] Further, in the present invention, the “carbon (C)” is an element that increases the stability of austenite, and it may suppress the generation of martensite even in a high stacking fault energy region, and play a role in improving the strength of steel through a solid solution strengthening effect. In addition, the carbon may play a role in lowering the temperature at which the austenite phase starts to transform to martensite by cooling or deformation.

[0038] Accordingly, if the carbon is added by exceeding 0.05 wt. %, the stability of the austenite phase increases, making it difficult to generate the δ-ferrite phase, and at the same time, the added chromium and carbon react to form carbides at grain boundaries, and there may be a problem in that the cryogenic impact toughness is reduced.

[0039] Meanwhile, in the present invention, in order to exhibit both of transformation-induced plasticity, which is caused by transformation of the austenite phase to the ε-martensite phase, and twinning-induced plasticity, the stacking fault energy of austenite at room temperature must be about 30 mJ / m2 or less. If the stacking fault energy is more than 30 mJ / m2, transformation-induced plasticity may not occur, and thus, the cryogenic impact absorption energy may only be at the level of existing high manganese steel. That is, in the present invention, transformation from an austenite phase to the ε-martensite phase and transformation from the ε-martensite phase to the α′-martensite phase may occur under strain or impact conditions, as the stacking fault energies of the steels with the alloy compositions described above are 30 mJ / m2 or less.

[0040] To this end, silicon (Si) may be included in addition to aluminum (Al) such that the total amount of aluminum (Al) and silicon is 6 wt. % or less.

[0041] The stacking fault energy was derived through the relationship between the alloy composition as shown in Mathematical Formula 1 below and the stacking fault energy in Table 1, which will be described below.[Mathematical⁢ Formula⁢ 1]Stacking⁢ fault⁢ energy [mJ / m2]=0.5×(wt. %⁢ of⁢ manganese)+5.2×(wt. %⁢ of⁢ aluminum)⁢\[NoBreak]+40×(wt. %⁢ of⁢ carbon)-0.016×(wt. %⁢ of⁢ chromium)

[0042] In this case, the “aluminum and silicon” act as lightweight elements and have the effect of lowering the density of high manganese steel and increasing stacking fault energy, and they may improve the cryogenic toughness of high manganese steel. In other words, the present invention may control the stacking fault energy to an appropriate level through the addition of aluminum and silicon, and may additionally obtain the effect of partial strength improvement.

[0043] Further, in the present invention, in order to increase the effect of suppressing crack propagation by the δ-ferrite phase as described above, they must be distributed homogeneously in the microstructure in an elongated form, and to this end, the aspect ratio (for elongated particles, the ratio of a length in the shortest direction (DS) to a length in the longest direction (DL), i.e., DS / DL) of the elongated ferrite phase may be 0.5 or less. In this case, if the aspect ratio of the elongated ferrite phase is more than 0.5, there may be a problem in that the desired effect of suppressing crack propagation by the δ-ferrite phase cannot be obtained.Manufacturing Method of Chromium-Manganese Steel with Excellent Cryogenic Toughness

[0044] The method for manufacturing chromium-manganese steel with excellent cryogenic toughness according to the present invention will be described. However, in order to avoid duplication, the description of parts that have the same technical idea as the alloy composition of chromium-manganese steel with excellent cryogenic toughness described above will be omitted.

[0045] The method for manufacturing chromium-manganese steel with excellent cryogenic toughness according to the present invention includes step 1 of melting an alloy composition of chromium-manganese steel consisting of austenite and δ-ferrite phases; step 2 of heat treating for homogenization by maintaining a molten ingot in a temperature range of 900° C. to 1,200° C. for 1 hour to 10 hours, followed by air cooling or water cooling; step 3 of hot working (rolling or forging) the heat-treated ingot to 50% to 90% reduction in thickness in a temperature range of 900° C. to 1,200° C.; and step 4 of heat treating hot-worked plates or bars for recrystallization in a temperature range of 600° C. to 900° C. for 0.5 to 10 hours, followed by air cooling or water cooling.

[0046] Step 1 is a step for preparing and melting an alloy composition of the chromium-manganese steel with excellent cryogenic toughness described above. In this case, for the melting, the ingot may be manufactured by using a conventionally known melting method that conforms to the above-described alloy composition and can achieve the objects of the present invention, and preferably, by using vacuum induction melting (VIM), raw materials matching the alloy composition may be measured and melted, and an ingot may be manufactured therefrom.

[0047] Next, step 2 is a step of performing homogenization heat treatment by maintaining the ingot melted in step 1 in a temperature range of 900° C. to 1,200° C. for 1 hour to 10 hours, followed by air cooling or water cooling, and through step2, the present invention may uniformly form a δ-ferrite phase having an area fraction of 5% or more to 30% or less.

[0048] More specifically, in order to effectively suppress the propagation of cracks in the crack propagation zone, the area fraction of the δ-ferrite phase in the entire microstructure must be 5% or more to 30% or less. That is, if the area fraction of the δ-ferrite phase is lower than 5%, there may be a problem of not being able to obtain a sufficient crack propagation suppression effect, and conversely, if the area fraction of the δ-ferrite phase is higher than 30%, the influence of ferrite phases with low impact absorption energy may increase at extremely low temperatures, resulting in a relative decrease in impact absorption energy.

[0049] Accordingly, if the temperature of step 2 is lower than 900° C. or the maintaining time is less than 1 hour, there may be a problem in that it is not possible to obtain a sufficient crack propagation suppression effect due to the low area fraction of the δ-ferrite phase, and additionally, if the temperature of step 2 is higher than 1,200° C. or the maintaining time is longer than 10 hours, the influence of ferrite phases with low impact absorption energy may increase at extremely low temperatures, resulting in a relative decrease in impact absorption energy.

[0050] Next, step 3 is a step of hot working (rolling or forging) of the ingot at a thickness reduction rate in the range of 50% to 90% in a temperature range of 900° C. to 1,200° C. after the homogenization heat treatment in step 2, and step 4 is a step of performing recrystallization heat treatment of performing heat treatment in a temperature range of 600° C. to 900° C. for 0.5 hours to 10 hours and then air-cooling or water-cooling, after the hot working in step 3.

[0051] In this way, by performing step 3 and step 4, the present invention may manufacture chromium-manganese steel with excellent cryogenic toughness, which has an impact absorption energy of 200 J or more as obtained through an impact test on a Charpy V-notch standard specimen at a liquid nitrogen temperature (about −196° C.), and satisfies all of Relationship Formulas (1) to (3) below:

[0052] (1) Yield strength of 200 MPa or more

[0053] (2) Ultimate tensile strength of 550 MPa or more

[0054] (3) Total elongation of 60% or more

[0055] Meanwhile, the final recrystallization heat treatment is generally performed after hot rolling, but if necessary, after recrystallization heat treatment, the step of cold working to a sheet of the desired thickness and then performing heat treatment again may be performed.

[0056] Hereinafter, the present invention will be described in more detail through examples. However, the following examples do not limit the scope of the present invention, and should be interpreted to aid understanding of the present invention.Example 1—Preparation of Alloy Composition of Chromium-Manganese Steel with Excellent Cryogenic Toughness

[0057] An alloy composition of chromium-manganese steel having the composition shown in Table 1 below was prepared, and raw materials corresponding to the alloy composition were measured and melted by using vacuum induction melting (VIM) to prepare an ingot therefrom. Next, the molten ingot was subjected to homogenization heat treatment at 1,150° C. for 2 hours, then immediately hot-rolled at a thickness reduction rate of 75%, and then immediately water-cooled to produce a hot-rolled plate. Next, the hot-rolled plate was subjected to recrystallization heat treatment at 800° C. for 2 hours and then immediately water-cooled to obtain the final chromium-manganese steel.Examples 2 to 4—Preparation of Alloy Compositions of Chromium-Manganese Steel with Excellent Cryogenic Toughness

[0058] Chromium-manganese steel was manufactured in the same manner as in Example 1, but with different compositions as shown in Table 1 below.Comparative Examples 1 to 4

[0059] As comparative examples, four types of high manganese steel containing a large amount of carbon without chromium were selected. All four comparative examples consisted of a single-phase austenite phase, and their alloy composition, room temperature tensile properties, and room temperature and cryogenic temperature (−196° C.) impact absorption energy information were extracted from the previously published papers below (Comparative Examples 1 and 2: Hyunmin Kim, Jaeyoung Park, Joong Eun Jung, Seok Su Sohn, Sunghak Lee, Materials Science and Engineering A, (2015), vol. 641, pp. 340-347, Comparative Examples 3 and 4: Changsheng Li, Kun Li, Jingbo Dong, Jikai Wang, Zhibao Shao, Materials Science and Engineering A, (2021), vol. 809, pp. 140998).TABLE 1Stacking fault energyCCrMnAlFe[mJ / m2]Example 10.038.9917.141.80Bal.18.9Example 20.109.0417.591.77Bal.21.8Example 30.329.1618.011.89Bal.31.4Example 40.549.1118.211.99Bal.40.4Comparative0.4—18—Bal.25.0Example 1Comparative1.0—18—Bal.49.0Example 2Comparative0.3—204Bal.42.8Example 3Comparative0.3—274Bal.46.3Example 4Experimental Example 1—Scanning Electron Microscope Image Analysis

[0060] Scanning electron microscopic microstructures for Examples 1 to 4 are shown in FIG. 1.

[0061] In the case of Example 1 where the carbon content was 0.03 wt. %, the δ-ferrite phase was formed at an area fraction of about 10% during the homogenization heat treatment and preheating process for hot-rolling, and it can be seen that it was then elongated during the hot rolling process and homogeneously distributed throughout the austenite matrix.

[0062] In Example 2, as the carbon content increased to 0.1 wt. %, the area fraction of the δ-ferrite phase was significantly reduced to 2% or less, and it can be seen that the added chromium and carbon combined to locally form carbides.

[0063] Meanwhile, in the case of Example 3 where the carbon content was 0.32 wt. %, the δ-ferrite phase was not formed, the matrix structure was entirely composed of austenite phase, and it can be seen that carbides were precipitated along the grain boundaries.

[0064] Further, in the case of Example 4 where the carbon content was 0.54 wt. %, only a single austenite phase existed similar to Example 3, and it can be seen that a large amount of carbides was formed along the grain boundaries.Experimental Example 2—Tensile Property Analysis

[0065] The room temperature tensile properties of the examples and comparative examples were measured and summarized in Table 2.

[0066] In the examples, both of yield strength and tensile strength increased as the carbon content increased. More specifically, in the case of total elongation, Example 2 showed the lowest at 60.3%, and the remaining examples showed elongation in the range of 73.8 to 82.1%. The comparative examples with no chromium and high carbon content showed higher strength than the examples, and Comparative Examples 1 and 2 without Al added showed very excellent elongation.TABLE 2Yield strengthUltimate tensileTotal[MPa]strength [MPa]elongation [%]Example 121956877.6Example 226659160.3Example 331468773.8Example 431779682.1Comparative322977125Example 1Comparative3381058120Example 2Comparative37666058Example 3Comparative41967849Example 4Experimental Example 3—Impact Test Analysis

[0067] The Charpy impact absorption energy of the examples and comparative examples at room temperature and −196° C. was measured and summarized in Table 3. The impact test was conducted by using ASTM E23 standard V-notch specimens according to ASTM D6110 procedures.

[0068] Example 1 showed a high impact absorption energy value of 394 J at room temperature and 260 J at −196° C., which is the liquid nitrogen temperature. In Examples 2, 3 and 4, the impact absorption energy values decreased as the carbon content increased at both of room temperature and −196° C. In the case of Comparative Example 4, the impact absorption energy values were 310 J and 179 J at room temperature and −196° C., respectively, showing the highest value among the comparative examples, but it can be seen that the impact absorption energy value was significantly lower than that of Example 1TABLE 3Impact absorption energy [J]25° C.−196° C.Example 1394260Example 2225112Example 318472Example 416530Comparative19692Example 1Comparative197123Example 2Comparative166102Example 3Comparative310179Example 4

[0069] Experimental Example 4—Scanning Electron Microscope Image Analysis After Impact Test

[0070] For the examples, scanning electron micrographs of the fracture surface after the impact test at room temperature and −196° C. are shown in FIG. 2. When it was fractured through an impact test at room temperature, dimples and tearing, which are characteristics of ductile fracture, were observed in Examples 1 and 2. In Examples 3 and 4, inter-granular fractures were observed, although some dimples appeared. In addition, when it was fractured through an impact test at −196° C., dimples, which are characteristics of ductile fracture, were still observed in Examples 1 and 2, but Examples 3 and 4 showed distinct inter-granular facture. Such inter-granular fracture is caused by the addition of a large amount of carbon and thus formation of carbides along the grain boundaries.Experimental Example 5—Image Analysis Through Electron Backscattering Diffraction after Impact Test

[0071] For the examples, the phase map, obtained by the electron backscattering diffraction technique, showing the spatial distribution of the phases around the fracture surface of a specimen fractured through an impact test at −196° C. are shown in FIG. 3. In Example 1, a large amount of α′-martensite (light green) was generated not only near the fracture surface but also inside away from the fracture surface, and a small amount of ε-martensite (red) coexisted around the same. This means that the impact energy applied at cryogenic temperatures was consumed to transform austenite to ε-martensite and ε-martensite to α′-martensite. In Example 2, α′-martensite and E-martensite were also observed, but the amount of formation was significantly smaller than that in Example 1. In Examples 3 and 4, only austenite (gray) phase was observed, and it could be confirmed that cracks propagated along the grain boundaries adjacent to the fractured surface.

[0072] Meanwhile, crystallographic orientation images for the same area as in FIG. 3 are shown in FIG. 4. In Example 1, fine grains with different orientations were formed through transformation to ε-martensite and α′-martensite, which was also observed in Example 2. In the case of Examples 3 and 4, it can be seen that only the deformation pattern around the fracture surface was observed due to slip and deformation twinning.

[0073] In addition, for the specimen of Example 1 fractured through an impact test at −196° C., the maps and crystallographic orientation images at a location 5 mm inward from the fracture surface are shown in FIG. 5. As observed on the fracture surface in FIG. 3, it can be seen that the transformation of austenite to ε-martensite and the transformation of ε-martensite to α′-martensite occurred even in areas far away from the location where the impact was applied. This means that the applied impact energy was transmitted even far away from the fracture surface and was consumed to cause the transformation of austenite to ε-martensite and the phase transformation of ε-martensite to α′-martensite. Through crystallographic orientation images of the same area, it can be seen that the microstructure composed of austenite and δ-ferrite phases before the impact test changed to a multiple-phase structure of austenite, δ-ferrite, ε-martensite and α′-martensite with fine grain sizes after cryogenic impact. It is determined that this grain refinement also contributed to improving cryogenic impact toughness.INDUSTRIAL APPLICABILITY

[0074] The present invention provides an alloy composition having a two-phase structure with excellent cryogenic toughness, which can store various liquefied gases such as hydrogen, LNG and nitrogen more safely because it exhibits excellent cryogenic impact toughness compared to conventional cryogenic high-manganese steel, and significantly increase resistance to container damage even in accidents where external impact is applied, chromium-manganese steel and a manufacturing method thereof.

Examples

example 1

Preparation of Alloy Composition of Chromium-Manganese Steel with Excellent Cryogenic Toughness

[0057]An alloy composition of chromium-manganese steel having the composition shown in Table 1 below was prepared, and raw materials corresponding to the alloy composition were measured and melted by using vacuum induction melting (VIM) to prepare an ingot therefrom. Next, the molten ingot was subjected to homogenization heat treatment at 1,150° C. for 2 hours, then immediately hot-rolled at a thickness reduction rate of 75%, and then immediately water-cooled to produce a hot-rolled plate. Next, the hot-rolled plate was subjected to recrystallization heat treatment at 800° C. for 2 hours and then immediately water-cooled to obtain the final chromium-manganese steel.

examples 2 to 4

Preparation of Alloy Compositions of Chromium-Manganese Steel with Excellent Cryogenic Toughness

[0058]Chromium-manganese steel was manufactured in the same manner as in Example 1, but with different compositions as shown in Table 1 below.

experimental example 1

Scanning Electron Microscope Image Analysis

[0060]Scanning electron microscopic microstructures for Examples 1 to 4 are shown in FIG. 1.

[0061]In the case of Example 1 where the carbon content was 0.03 wt. %, the δ-ferrite phase was formed at an area fraction of about 10% during the homogenization heat treatment and preheating process for hot-rolling, and it can be seen that it was then elongated during the hot rolling process and homogeneously distributed throughout the austenite matrix.

[0062]In Example 2, as the carbon content increased to 0.1 wt. %, the area fraction of the δ-ferrite phase was significantly reduced to 2% or less, and it can be seen that the added chromium and carbon combined to locally form carbides.

[0063]Meanwhile, in the case of Example 3 where the carbon content was 0.32 wt. %, the δ-ferrite phase was not formed, the matrix structure was entirely composed of austenite phase, and it can be seen that carbides were precipitated along the grain boundaries.

[0064]Furt...

TECHNICAL FIELD

[0001] The present invention relates to chromium-manganese steel alloy compositions having a two-phase structure with excellent cryogenic toughness, chromium-manganese steels and a method of manufacturing the same. With their excellent cryogenic impact toughness, the chromium-manganese duplex steels of the present invention may find applications as structural materials for vessels and containers storing various liquefied gases such as hydrogen, nitrogen and liquefied natural gas, and the containers made of the present steels may exhibit significantly higher resistance to breakage than those of conventional cryogenic steels even in cryogenic impact accidents, and thus ensure a higher level of storage safety at cryogenic temperatures.BACKGROUND ART

[0002] In general, liquefied gases such as liquefied hydrogen, liquefied oxygen, liquefied natural gas and the like essentially require cryogenic containers for their storage and transportation in a state of maintaining a liquid state. In order to achieve this, the storage container made of a material that has sufficient toughness and strength at extremely low temperatures is required.

[0003] Conventional carbon steels, however, have the disadvantage in that when the operating temperature is lowered, the yield strength increases rapidly and the toughness decreases significantly, which limits the usage thereof. Most metals consisting of the austenite phase are known to have excellent cryogenic toughness. Such metals, especially austenitic stainless steels, show no ductile-brittle transition phenomenon and therefore have quite high toughness even at cryogenic temperatures. Unlike ferritic steels, austenitic stainless steels exhibit lower yield strength at low temperatures and are more plastically deformable so that they may absorb the impact energy more efficiently. There is, however, a problem in that the manufacturing costs of austenitic stainless steels are much higher because they contain expensive nickel (Ni) which act as an austenite stabilizer.

[0004] Further, in the case of liquefied hydrogen, which has recently been in the spotlight, the storage temperature is lower than that of conventional liquefied gas, and thus, higher toughness is required at cryogenic temperatures. However, the toughness of conventional austenitic stainless steels is not sufficiently high for the storage of hydrogen. Accordingly, cryogenic steels containing a large amount of low-cost manganese (Mn) and carbon (C) as austenite stabilizers have recently been developed in place of austenitic stainless steels with high nickel contents. Their cryogenic toughness is reported to be comparable to or improved over conventional austenitic stainless steels, but there is a problem in that their corrosion resistance is poor and they still do not exhibit high enough toughness to ensure structural stability at cryogenic temperatures.

[0005] Accordingly, there is an urgent need for research on new alloys, which are capable of storing various liquefied gases more safely, significantly increasing resistance to container breakage even in the event of an accident in which an external impact is applied, and having excellent economic feasibility and excellent cryogenic toughness.DISCLOSURE OF INVENTIONTechnical Problem

[0006] The present invention has been devised to overcome the above-described problems, and the problem to be solved by the present invention is to provide alloy compositions that exhibit better cryogenic impact toughness than cryogenic metallic materials previously developed for liquefied gas storage containers, and more specifically, to provide alloy compositions of chromium-manganese steels, and a method of manufacturing the same.Solution to Problem

[0007] In order to solve the above-described problems, the present invention provides an alloy composition of chromium-manganese steel with excellent cryogenic toughness, including 15 to 25 wt. % of manganese (Mn), 5 to 15 wt. % of chromium (Cr), 4 wt. % or less of aluminum (Al), 0.05 wt. % or less of carbon (C) and the remainder on an Fe base, wherein the alloy composition has a two-phase microstructure consisting of austenite and δ-ferrite phases.

[0008] Further, in an exemplary embodiment of the present invention, silicon (Si) may be included in addition to the aluminum (Al) such that the total amount of aluminum (Al) and silicon is 6 wt. % or less.

[0009] In addition, the present invention provides chromium-manganese steel with excellent cryogenic toughness, having the above-described alloy composition, wherein the chromium-manganese steel exhibits a low stacking fault energy of 30 mJ / m2 or less such that under externally applied stress and impact conditions, phase transformation from austenite to ε-martensite occurs, and phase transformation from ε-martensite to α′-martensite occurs.

[0010] In addition, the ratio (b / a) of a length (b) in the shortest direction to a length (a) in the longest direction of δ-ferrite grains may be 0.5 or less.

[0011] In addition, the impact absorption energy obtained through an impact test on a Charpy V-notch standard specimen at liquid nitrogen temperature (approximately −196° C.) may be 200 J or more.

[0012] In addition, the chromium-manganese steel may satisfy all of Relationship Formulas (1) and (2) below:

[0013] (1) Yield strength of 200 MPa or more

[0014] (2) Ultimate tensile strength of 550 MPa or more.

[0015] In addition, the chromium-manganese steel may satisfy Relationship Formula (3) below:

[0016] (3) Total elongation of 60% or more.

[0017] In addition, the present invention provides a method for manufacturing chromium-manganese steel having excellent cryogenic toughness, including: step 1 of melting an alloy composition of chromium-manganese steel consisting of austenite and δ-ferrite phases; step 2 of heat treating for homogenization by maintaining a molten ingot in a temperature range of 900° C. to 1,200° C. for 1 hour to 10 hours, followed by air cooling or water cooling; step 3 of hot working (rolling or forging) the heat-treated ingot to 50% to 90% reduction in thickness in a temperature range of 900° C. to 1,200° C.; and step 4 of heat treating hot-worked plates or bars for recrystallization in a temperature range of 600° C. to 900° C. for 0.5 to 10 hours, followed by air cooling or water cooling.

[0018] In addition, according to an exemplary embodiment of the present invention, through step 2, a δ-ferrite phase having an area fraction of 5% or more to 30% or less may be homogeneously generated.

[0019] In addition, the method may further include the step of cold working to a sheet of the desired thickness and then performing heat treatment again, after performing step 4.Advantageous Effects of Invention

[0020] According to the present invention, the chromium-manganese duplex steels described above exhibit excellent cryogenic toughness by consuming externally imposed strain and impact energy through transformation-induced plasticity (TRIP) and twining-induced plasticity (TWIP), by grain refinement resulted from TRIP and TWIP phenomena, and further by the elongated δ-ferrite grains which efficiently block the propagation of cracks. The detailed mechanisms behind the improved impact toughness are as follows: Firstly, the externally imposed impact energy on the chromium-manganese duplex steel causes transformation of the austenite phase to the ε-martensite phase and then induces the subsequent transformation of the produced ε-martensite phase to the α′-martensite phase, through which imposed impact energy is greatly consumed and the grain structure is significantly refined. Secondly, the energy required for the propagation of cracks generated by impact is consumed through twining-induced plasticity (TWIP) and slip in the austenite phase around the cracks. Thirdly, the propagation of cracks can be suppressed effectively by the δ-ferrite grains elongated perpendicular to the direction of crack propagation.BRIEF DESCRIPTION OF DRAWINGS

[0021] FIG. 1 is a set of scanning electron microscope images of chromium-manganese steel according to an exemplary embodiment of the present invention.

[0022] FIG. 2 is a set of scanning electron microscope images of the fracture surface of chromium-manganese steel according to an exemplary embodiment of the present invention after an impact test at −196° C.

[0023] FIG. 3 is a set of spatial distribution images of the phase analyzed through electron backscattering diffraction on the microstructure around the fracture surface of a specimen fractured through an impact test at −196° C. of chromium-manganese steel according to an exemplary embodiment of the present invention.

[0024] FIG. 4 is a set of crystal orientation images for the same area as FIG. 3.

[0025] FIG. 5 is a phase spatial distribution image and a crystal orientation image at a location 5 mm inward from the fracture surface for the specimen of Example 1 fractured through an impact test at −196° C.MODE FOR THE INVENTION

[0026] Hereinafter, the exemplary embodiments of the present invention will be described in detail so that those skilled in the art can easily practice the present invention. The present invention may be implemented in many different forms and is not limited to the exemplary embodiments described herein.

[0027] As mentioned above, the use of conventional cryogenic materials is limited due to problems with corrosion resistance or structural stability at extremely low temperatures and economic issues that require expensive nickel. Furthermore, there is a problem in that they cannot be used as storage containers for liquefied hydrogen, which has recently been in the spotlight.

[0028] Accordingly, the present invention has sought to solve the above-described problems by providing alloy compositions of chromium-manganese steel with excellent cryogenic toughness, including 15 to 25 wt. % of manganese (Mn), 5 to 15 wt. % of chromium (Cr), 4 wt. % or less of aluminum (Al), 0.05 wt. % or less of carbon (C) and the remainder on an Fe base, wherein the chromium-manganese steel has a two-phase microstructure of austenite and δ-ferrite, and chromium-manganese steel exhibits excellent cryogenic toughness.

[0029] Through this, the present invention can provide chromium-manganese steels having a two-phase microstructure with excellent cryogenic toughness, which can store various liquefied gases such as hydrogen, LNG and nitrogen more safely because it exhibits excellent cryogenic impact toughness than conventional cryogenic high-manganese steel, and significantly increase resistance to container damage even in accidents where external impact is applied

[0030] The present invention will be described in detail below with reference to the drawings.Alloy Compositions of Chromium-Manganese Steels with Excellent Cryogenic Toughness and Chromium-Manganese Steels

[0031] The concept of alloy design of chromium-manganese steels with excellent cryogenic toughness according to the present invention is based on the fact that under a cryogenic environment, a significant amount of externally applied impact energy is consumed to transform an austenite phase with a face-centered cubic (fcc) structure to the ε-martensite phase with a hexagonal close-packed (hcp) structure, and subsequently to transform the phase-transformed ε-martensite phase to the α′-martensite phase with a body-centered cubic (bcc) structure. Meanwhile, in this case, some impact energy is consumed to accommodate plastic deformation by deformation twinning and slip in the crack propagation zone. Additionally, the elongated δ-ferrite grains that coexist with the austenite phase have the effect of suppressing the propagation of cracks, thereby contributing to improving cryogenic impact toughness. In addition, as a result of the above-described phase transformation, the grain refinement effect of the remaining austenite phase, and transformed ε-martensite and α′-martensite phases also contributes to the improvement of cryogenic impact toughness.

[0032] To this end, the chromium-manganese steels with excellent cryogenic toughness according to the present invention are realized with the alloy composition of 15 to 25 wt. % of manganese (Mn), 5 to 15 wt. % of chromium (Cr), 4 wt. % or less of aluminum (Al), 0.05 wt. % or less of carbon (C) and the remainder on an Fe base. The steels produced have a two-phase microstructure consisting of the austenite phase with a stacking fault energy in an appropriate range and an appropriate amount of the δ-ferrite phase, and thereby achieving the effect of improving the cryogenic impact toughness described above.

[0033] In this case, in the present invention, “manganese (Mn)” may be added as an essential element to replace expensive nickel (Ni), increase strength, increase stacking fault energy and stabilize the austenite phase. In addition, manganese may combine with sulfur (S) or oxygen (O), which are impurity elements, to form non-metallic inclusions such as manganese sulfide (MnS) or manganese oxide (MnO).

[0034] Accordingly, if the amount of manganese is less than 15 wt. %, the stabilization of the austenite phase may be difficult, and there may be a problem in that a large amount of δ-ferrite phase is generated, and if manganese is added in excess of 25 wt. %, there may be problems in that hot formability, weldability and oxidation resistance are deteriorated.

[0035] Further, in the present invention, “chromium (Cr)”, which is a representative alloying element that stabilizes ferrite, is added to steels to improve corrosion resistance. Chromium added to high-manganese steel may slightly increase the stacking fault energy of austenite, and when the content thereof increases, it may combine with carbon to form various carbides.

[0036] Accordingly, if the chromium is added at less than 5 wt. %, corrosion resistance may be significantly reduced, and if the chromium is added in excess of 15 wt. %, there may be a problem in that an excessive amount of 5-ferrite phase is generated.

[0037] Further, in the present invention, the “carbon (C)” is an element that increases the stability of austenite, and it may suppress the generation of martensite even in a high stacking fault energy region, and play a role in improving the strength of steel through a solid solution strengthening effect. In addition, the carbon may play a role in lowering the temperature at which the austenite phase starts to transform to martensite by cooling or deformation.

[0038] Accordingly, if the carbon is added by exceeding 0.05 wt. %, the stability of the austenite phase increases, making it difficult to generate the δ-ferrite phase, and at the same time, the added chromium and carbon react to form carbides at grain boundaries, and there may be a problem in that the cryogenic impact toughness is reduced.

[0039] Meanwhile, in the present invention, in order to exhibit both of transformation-induced plasticity, which is caused by transformation of the austenite phase to the ε-martensite phase, and twinning-induced plasticity, the stacking fault energy of austenite at room temperature must be about 30 mJ / m2 or less. If the stacking fault energy is more than 30 mJ / m2, transformation-induced plasticity may not occur, and thus, the cryogenic impact absorption energy may only be at the level of existing high manganese steel. That is, in the present invention, transformation from an austenite phase to the ε-martensite phase and transformation from the ε-martensite phase to the α′-martensite phase may occur under strain or impact conditions, as the stacking fault energies of the steels with the alloy compositions described above are 30 mJ / m2 or less.

[0040] To this end, silicon (Si) may be included in addition to aluminum (Al) such that the total amount of aluminum (Al) and silicon is 6 wt. % or less.

[0041] The stacking fault energy was derived through the relationship between the alloy composition as shown in Mathematical Formula 1 below and the stacking fault energy in Table 1, which will be described below.[Mathematical⁢ Formula⁢ 1]Stacking⁢ fault⁢ energy [mJ / m2]=0.5×(wt. %⁢ of⁢ manganese)+5.2×(wt. %⁢ of⁢ aluminum)⁢\[NoBreak]+40×(wt. %⁢ of⁢ carbon)-0.016×(wt. %⁢ of⁢ chromium)

[0042] In this case, the “aluminum and silicon” act as lightweight elements and have the effect of lowering the density of high manganese steel and increasing stacking fault energy, and they may improve the cryogenic toughness of high manganese steel. In other words, the present invention may control the stacking fault energy to an appropriate level through the addition of aluminum and silicon, and may additionally obtain the effect of partial strength improvement.

[0043] Further, in the present invention, in order to increase the effect of suppressing crack propagation by the δ-ferrite phase as described above, they must be distributed homogeneously in the microstructure in an elongated form, and to this end, the aspect ratio (for elongated particles, the ratio of a length in the shortest direction (DS) to a length in the longest direction (DL), i.e., DS / DL) of the elongated ferrite phase may be 0.5 or less. In this case, if the aspect ratio of the elongated ferrite phase is more than 0.5, there may be a problem in that the desired effect of suppressing crack propagation by the δ-ferrite phase cannot be obtained.Manufacturing Method of Chromium-Manganese Steel with Excellent Cryogenic Toughness

[0044] The method for manufacturing chromium-manganese steel with excellent cryogenic toughness according to the present invention will be described. However, in order to avoid duplication, the description of parts that have the same technical idea as the alloy composition of chromium-manganese steel with excellent cryogenic toughness described above will be omitted.

[0045] The method for manufacturing chromium-manganese steel with excellent cryogenic toughness according to the present invention includes step 1 of melting an alloy composition of chromium-manganese steel consisting of austenite and δ-ferrite phases; step 2 of heat treating for homogenization by maintaining a molten ingot in a temperature range of 900° C. to 1,200° C. for 1 hour to 10 hours, followed by air cooling or water cooling; step 3 of hot working (rolling or forging) the heat-treated ingot to 50% to 90% reduction in thickness in a temperature range of 900° C. to 1,200° C.; and step 4 of heat treating hot-worked plates or bars for recrystallization in a temperature range of 600° C. to 900° C. for 0.5 to 10 hours, followed by air cooling or water cooling.

[0046] Step 1 is a step for preparing and melting an alloy composition of the chromium-manganese steel with excellent cryogenic toughness described above. In this case, for the melting, the ingot may be manufactured by using a conventionally known melting method that conforms to the above-described alloy composition and can achieve the objects of the present invention, and preferably, by using vacuum induction melting (VIM), raw materials matching the alloy composition may be measured and melted, and an ingot may be manufactured therefrom.

[0047] Next, step 2 is a step of performing homogenization heat treatment by maintaining the ingot melted in step 1 in a temperature range of 900° C. to 1,200° C. for 1 hour to 10 hours, followed by air cooling or water cooling, and through step2, the present invention may uniformly form a δ-ferrite phase having an area fraction of 5% or more to 30% or less.

[0048] More specifically, in order to effectively suppress the propagation of cracks in the crack propagation zone, the area fraction of the δ-ferrite phase in the entire microstructure must be 5% or more to 30% or less. That is, if the area fraction of the δ-ferrite phase is lower than 5%, there may be a problem of not being able to obtain a sufficient crack propagation suppression effect, and conversely, if the area fraction of the δ-ferrite phase is higher than 30%, the influence of ferrite phases with low impact absorption energy may increase at extremely low temperatures, resulting in a relative decrease in impact absorption energy.

[0049] Accordingly, if the temperature of step 2 is lower than 900° C. or the maintaining time is less than 1 hour, there may be a problem in that it is not possible to obtain a sufficient crack propagation suppression effect due to the low area fraction of the δ-ferrite phase, and additionally, if the temperature of step 2 is higher than 1,200° C. or the maintaining time is longer than 10 hours, the influence of ferrite phases with low impact absorption energy may increase at extremely low temperatures, resulting in a relative decrease in impact absorption energy.

[0050] Next, step 3 is a step of hot working (rolling or forging) of the ingot at a thickness reduction rate in the range of 50% to 90% in a temperature range of 900° C. to 1,200° C. after the homogenization heat treatment in step 2, and step 4 is a step of performing recrystallization heat treatment of performing heat treatment in a temperature range of 600° C. to 900° C. for 0.5 hours to 10 hours and then air-cooling or water-cooling, after the hot working in step 3.

[0051] In this way, by performing step 3 and step 4, the present invention may manufacture chromium-manganese steel with excellent cryogenic toughness, which has an impact absorption energy of 200 J or more as obtained through an impact test on a Charpy V-notch standard specimen at a liquid nitrogen temperature (about −196° C.), and satisfies all of Relationship Formulas (1) to (3) below:

[0052] (1) Yield strength of 200 MPa or more

[0053] (2) Ultimate tensile strength of 550 MPa or more

[0054] (3) Total elongation of 60% or more

[0055] Meanwhile, the final recrystallization heat treatment is generally performed after hot rolling, but if necessary, after recrystallization heat treatment, the step of cold working to a sheet of the desired thickness and then performing heat treatment again may be performed.

[0056] Hereinafter, the present invention will be described in more detail through examples. However, the following examples do not limit the scope of the present invention, and should be interpreted to aid understanding of the present invention.Example 1—Preparation of Alloy Composition of Chromium-Manganese Steel with Excellent Cryogenic Toughness

[0057] An alloy composition of chromium-manganese steel having the composition shown in Table 1 below was prepared, and raw materials corresponding to the alloy composition were measured and melted by using vacuum induction melting (VIM) to prepare an ingot therefrom. Next, the molten ingot was subjected to homogenization heat treatment at 1,150° C. for 2 hours, then immediately hot-rolled at a thickness reduction rate of 75%, and then immediately water-cooled to produce a hot-rolled plate. Next, the hot-rolled plate was subjected to recrystallization heat treatment at 800° C. for 2 hours and then immediately water-cooled to obtain the final chromium-manganese steel.Examples 2 to 4—Preparation of Alloy Compositions of Chromium-Manganese Steel with Excellent Cryogenic Toughness

[0058] Chromium-manganese steel was manufactured in the same manner as in Example 1, but with different compositions as shown in Table 1 below.Comparative Examples 1 to 4

[0059] As comparative examples, four types of high manganese steel containing a large amount of carbon without chromium were selected. All four comparative examples consisted of a single-phase austenite phase, and their alloy composition, room temperature tensile properties, and room temperature and cryogenic temperature (−196° C.) impact absorption energy information were extracted from the previously published papers below (Comparative Examples 1 and 2: Hyunmin Kim, Jaeyoung Park, Joong Eun Jung, Seok Su Sohn, Sunghak Lee, Materials Science and Engineering A, (2015), vol. 641, pp. 340-347, Comparative Examples 3 and 4: Changsheng Li, Kun Li, Jingbo Dong, Jikai Wang, Zhibao Shao, Materials Science and Engineering A, (2021), vol. 809, pp. 140998).TABLE 1Stacking fault energyCCrMnAlFe[mJ / m2]Example 10.038.9917.141.80Bal.18.9Example 20.109.0417.591.77Bal.21.8Example 30.329.1618.011.89Bal.31.4Example 40.549.1118.211.99Bal.40.4Comparative0.4—18—Bal.25.0Example 1Comparative1.0—18—Bal.49.0Example 2Comparative0.3—204Bal.42.8Example 3Comparative0.3—274Bal.46.3Example 4Experimental Example 1—Scanning Electron Microscope Image Analysis

[0060] Scanning electron microscopic microstructures for Examples 1 to 4 are shown in FIG. 1.

[0061] In the case of Example 1 where the carbon content was 0.03 wt. %, the δ-ferrite phase was formed at an area fraction of about 10% during the homogenization heat treatment and preheating process for hot-rolling, and it can be seen that it was then elongated during the hot rolling process and homogeneously distributed throughout the austenite matrix.

[0062] In Example 2, as the carbon content increased to 0.1 wt. %, the area fraction of the δ-ferrite phase was significantly reduced to 2% or less, and it can be seen that the added chromium and carbon combined to locally form carbides.

[0063] Meanwhile, in the case of Example 3 where the carbon content was 0.32 wt. %, the δ-ferrite phase was not formed, the matrix structure was entirely composed of austenite phase, and it can be seen that carbides were precipitated along the grain boundaries.

[0064] Further, in the case of Example 4 where the carbon content was 0.54 wt. %, only a single austenite phase existed similar to Example 3, and it can be seen that a large amount of carbides was formed along the grain boundaries.Experimental Example 2—Tensile Property Analysis

[0065] The room temperature tensile properties of the examples and comparative examples were measured and summarized in Table 2.

[0066] In the examples, both of yield strength and tensile strength increased as the carbon content increased. More specifically, in the case of total elongation, Example 2 showed the lowest at 60.3%, and the remaining examples showed elongation in the range of 73.8 to 82.1%. The comparative examples with no chromium and high carbon content showed higher strength than the examples, and Comparative Examples 1 and 2 without Al added showed very excellent elongation.TABLE 2Yield strengthUltimate tensileTotal[MPa]strength [MPa]elongation [%]Example 121956877.6Example 226659160.3Example 331468773.8Example 431779682.1Comparative322977125Example 1Comparative3381058120Example 2Comparative37666058Example 3Comparative41967849Example 4Experimental Example 3—Impact Test Analysis

[0067] The Charpy impact absorption energy of the examples and comparative examples at room temperature and −196° C. was measured and summarized in Table 3. The impact test was conducted by using ASTM E23 standard V-notch specimens according to ASTM D6110 procedures.

[0068] Example 1 showed a high impact absorption energy value of 394 J at room temperature and 260 J at −196° C., which is the liquid nitrogen temperature. In Examples 2, 3 and 4, the impact absorption energy values decreased as the carbon content increased at both of room temperature and −196° C. In the case of Comparative Example 4, the impact absorption energy values were 310 J and 179 J at room temperature and −196° C., respectively, showing the highest value among the comparative examples, but it can be seen that the impact absorption energy value was significantly lower than that of Example 1TABLE 3Impact absorption energy [J]25° C.−196° C.Example 1394260Example 2225112Example 318472Example 416530Comparative19692Example 1Comparative197123Example 2Comparative166102Example 3Comparative310179Example 4

[0069] Experimental Example 4—Scanning Electron Microscope Image Analysis After Impact Test

[0070] For the examples, scanning electron micrographs of the fracture surface after the impact test at room temperature and −196° C. are shown in FIG. 2. When it was fractured through an impact test at room temperature, dimples and tearing, which are characteristics of ductile fracture, were observed in Examples 1 and 2. In Examples 3 and 4, inter-granular fractures were observed, although some dimples appeared. In addition, when it was fractured through an impact test at −196° C., dimples, which are characteristics of ductile fracture, were still observed in Examples 1 and 2, but Examples 3 and 4 showed distinct inter-granular facture. Such inter-granular fracture is caused by the addition of a large amount of carbon and thus formation of carbides along the grain boundaries.Experimental Example 5—Image Analysis Through Electron Backscattering Diffraction after Impact Test

[0071] For the examples, the phase map, obtained by the electron backscattering diffraction technique, showing the spatial distribution of the phases around the fracture surface of a specimen fractured through an impact test at −196° C. are shown in FIG. 3. In Example 1, a large amount of α′-martensite (light green) was generated not only near the fracture surface but also inside away from the fracture surface, and a small amount of ε-martensite (red) coexisted around the same. This means that the impact energy applied at cryogenic temperatures was consumed to transform austenite to ε-martensite and ε-martensite to α′-martensite. In Example 2, α′-martensite and E-martensite were also observed, but the amount of formation was significantly smaller than that in Example 1. In Examples 3 and 4, only austenite (gray) phase was observed, and it could be confirmed that cracks propagated along the grain boundaries adjacent to the fractured surface.

[0072] Meanwhile, crystallographic orientation images for the same area as in FIG. 3 are shown in FIG. 4. In Example 1, fine grains with different orientations were formed through transformation to ε-martensite and α′-martensite, which was also observed in Example 2. In the case of Examples 3 and 4, it can be seen that only the deformation pattern around the fracture surface was observed due to slip and deformation twinning.

[0073] In addition, for the specimen of Example 1 fractured through an impact test at −196° C., the maps and crystallographic orientation images at a location 5 mm inward from the fracture surface are shown in FIG. 5. As observed on the fracture surface in FIG. 3, it can be seen that the transformation of austenite to ε-martensite and the transformation of ε-martensite to α′-martensite occurred even in areas far away from the location where the impact was applied. This means that the applied impact energy was transmitted even far away from the fracture surface and was consumed to cause the transformation of austenite to ε-martensite and the phase transformation of ε-martensite to α′-martensite. Through crystallographic orientation images of the same area, it can be seen that the microstructure composed of austenite and δ-ferrite phases before the impact test changed to a multiple-phase structure of austenite, δ-ferrite, ε-martensite and α′-martensite with fine grain sizes after cryogenic impact. It is determined that this grain refinement also contributed to improving cryogenic impact toughness.INDUSTRIAL APPLICABILITY

[0074] The present invention provides an alloy composition having a two-phase structure with excellent cryogenic toughness, which can store various liquefied gases such as hydrogen, LNG and nitrogen more safely because it exhibits excellent cryogenic impact toughness compared to conventional cryogenic high-manganese steel, and significantly increase resistance to container damage even in accidents where external impact is applied, chromium-manganese steel and a manufacturing method thereof.

Examples

example 1

Preparation of Alloy Composition of Chromium-Manganese Steel with Excellent Cryogenic Toughness

[0057]An alloy composition of chromium-manganese steel having the composition shown in Table 1 below was prepared, and raw materials corresponding to the alloy composition were measured and melted by using vacuum induction melting (VIM) to prepare an ingot therefrom. Next, the molten ingot was subjected to homogenization heat treatment at 1,150° C. for 2 hours, then immediately hot-rolled at a thickness reduction rate of 75%, and then immediately water-cooled to produce a hot-rolled plate. Next, the hot-rolled plate was subjected to recrystallization heat treatment at 800° C. for 2 hours and then immediately water-cooled to obtain the final chromium-manganese steel.

examples 2 to 4

Preparation of Alloy Compositions of Chromium-Manganese Steel with Excellent Cryogenic Toughness

[0058]Chromium-manganese steel was manufactured in the same manner as in Example 1, but with different compositions as shown in Table 1 below.

experimental example 1

Scanning Electron Microscope Image Analysis

[0060]Scanning electron microscopic microstructures for Examples 1 to 4 are shown in FIG. 1.

[0061]In the case of Example 1 where the carbon content was 0.03 wt. %, the δ-ferrite phase was formed at an area fraction of about 10% during the homogenization heat treatment and preheating process for hot-rolling, and it can be seen that it was then elongated during the hot rolling process and homogeneously distributed throughout the austenite matrix.

[0062]In Example 2, as the carbon content increased to 0.1 wt. %, the area fraction of the δ-ferrite phase was significantly reduced to 2% or less, and it can be seen that the added chromium and carbon combined to locally form carbides.

[0063]Meanwhile, in the case of Example 3 where the carbon content was 0.32 wt. %, the δ-ferrite phase was not formed, the matrix structure was entirely composed of austenite phase, and it can be seen that carbides were precipitated along the grain boundaries.

[0064]Furt...

Claims

1. A chromium-manganese steel with excellent cryogenic toughness, comprising:15 to 25 wt. % of manganese (Mn), 5 to 15 wt. % of chromium (Cr), more than 0 wt. % to 4 wt. % or less of aluminum (Al), more than 0 wt. % to 0.05 wt. % or less of carbon (C) and a remainder on an Fe base,wherein the remainder comprises Fe and inevitable impurities,wherein the chromium-manganese steel has a two-phase microstructure consisting of austenite and δ-ferrite phases,wherein the impact absorption energy obtained through an impact test on a Charpy V-notch standard specimen at liquid nitrogen temperature (approximately −196° C.) is 200 J or more, andwherein the chromium-manganese steel satisfies all of Relationship Formulas (1) and (2) below:(1) Yield strength of 200 MPa or more(2) Ultimate tensile strength of 550 MPa or more.

2. The chromium-manganese steel of claim 1, wherein silicon (Si) is comprised in addition to the aluminum (Al) such that the wt. % of aluminum (Al) and silicon is 6 wt. % or less.

3. The chromium-manganese steel according to claim 1,wherein the chromium-manganese steel exhibits a low stacking fault energy of 30 mJ / m2 or less such that under externally applied stress and impact conditions, phase transformation from austenite to ε-martensite occurs, and phase transformation from ε-martensite to α′-martensite occurs.

4. The chromium-manganese steel of claim 3, wherein the ratio (b / a) of a length (b) in the shortest direction to a length (a) in the longest direction of δ-ferrite grains is 0.5 or less.

5. (canceled)6. (canceled)7. The chromium-manganese steel of claim 3, wherein the chromium-manganese steel satisfies Relationship Formula (3) below:(3) Total elongation of 60% or more.

8. A method for manufacturing chromium-manganese steel according to claim 1, comprising:step 1 of melting an alloy composition of chromium-manganese steel with excellent cryogenic toughness having a two-phase structure of austenite and δ-ferrite;step 2 of heat treating for homogenization by maintaining the molten alloy composition in an ingot in a temperature range of 900° C. to 1,200° C. for 1 hour to 10 hours, followed by air cooling or water cooling;step 3 of hot working (rolling or forging) to 50% to 90% reduction in thickness in a temperature range of 900° C. to 1,200° C. after the heat treating for homogenization; andstep 4 of heat treating for recrystallization in a temperature range of 600° C. to 900° C. for 0.5 to 10 hours after the hot working, followed by air cooling or water cooling.

9. The method of claim 8, wherein through step 2, a δ-ferrite phase having an area fraction of 5% or more to 30% or less is homogeneously generated.

10. The method of claim 8, further comprising the step of:cold working to a sheet of the desired thickness and then performing heat treatment again, after performing step 4.

11. The chromium-manganese steel according to claim 2,wherein the chromium-manganese steel exhibits a low stacking fault energy of 30 mJ / m2 or less such that under externally applied stress and impact conditions, phase transformation from austenite to ε-martensite occurs, and phase transformation from ε-martensite to α′-martensite occurs.

12. The chromium-manganese steel of claim 11, wherein the ratio (b / a) of a length (b) in the shortest direction to a length (a) in the longest direction of δ-ferrite grains is 0.5 or less.

13. The chromium-manganese steel of claim 11, wherein the chromium-manganese steel satisfies Relationship Formula (3) below:(3) Total elongation of 60% or more.

14. A method for manufacturing chromium-manganese steel according to claim 2, comprising:step 1 of melting an alloy composition of chromium-manganese steel with excellent cryogenic toughness having a two-phase structure of austenite and δ-ferrite;step 2 of heat treating for homogenization by maintaining the molten alloy composition in an ingot in a temperature range of 900° C. to 1,200° C. for 1 hour to 10 hours, followed by air cooling or water cooling;step 3 of hot working (rolling or forging) to 50% to 90% reduction in thickness in a temperature range of 900° C. to 1,200° C. after the heat treating for homogenization; andstep 4 of heat treating for recrystallization in a temperature range of 600° C. to 900° C. for 0.5 to 10 hours after the hot working, followed by air cooling or water cooling.

15. The method of claim 14, wherein through step 2, a δ-ferrite phase having an area fraction of 5% or more to 30% or less is homogeneously generated.

16. The method of claim 14, further comprising the step of:cold working to a sheet of the desired thickness and then performing heat treatment again, after performing step 4.

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