JP2016196703A - HIGH Mn STEEL MATERIAL FOR CRYOGENIC USE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high Mn steel material for cryogenic use, which materializes a base metal and a heat affected zone each having an excellent toughness.SOLUTION: The high Mn steel material for cryogenic use is provided that has composition containing C: 0.001 to 0.80%, Mn: 15.0 to 35.0%, S: 0.0001 to 0.01%, Cr: 0.01 to 10.0%, Ti: 0.001 to 0.05%, N: 0.0001 to 0.10%, and O: 0.001 to 0.010%;, P: limited to 0.02% or less; further containing either or both of Si: 0.001 to 5.0% and Al: 0.001 to 5.0%; further containing one or two or more kinds of Mg: 0.01% or less, Ca: 0.01% or less, and REM: 0.01% or less in total of 0.0002% or more, the composition satisfying 30C+0.5Mn+Ni+0.8Cr+1.2Si+0.8Mo≥25 --(Equation 1) and O/S≥1 --(Equation 2), the remainder being composed of Fe and inevitable impurities. The high Mn steel material for cryogenic use is characterized in that the volume fraction of austenite is 95% or more, the austenite has a crystal grain size of 20 to 200 μm, and the carbide coverage ratio at the crystal grain boundary of the austenite is 50% or less.SELECTED DRAWING: None

Description

  The present invention relates to a high Mn steel material suitable for use in a cryogenic environment, such as a storage container for liquefied gas such as liquefied natural gas (LNG), liquid hydrogen, and liquid helium.

  As materials that can be used in a cryogenic environment, such as liquefied natural gas (boiling point: −164 ° C.), liquid hydrogen (boiling point: −249 ° C.), liquid helium (boiling point: −269 ° C.), Ni—such as SUS304 has been used. Cr-based austenitic alloys and 9% Ni steel sheets have been used. However, since alloy steel containing a large amount of Ni is expensive, high Mn austenitic alloys in which Ni is replaced with Mn have been proposed (see, for example, Patent Documents 1 to 4).

  Patent Document 1 proposes a high Mn steel in which Cr is limited to 0.3% or less, and Patent Document 2 proposes a high Mn steel containing 10% or more of Cr. Further, Patent Document 3 proposes suppression of carbide generation by addition of Cu and regulation of the lower limit value of the cooling rate of the welded portion. Patent Document 4 contains 0.01 to 0.25% C, 15 to 40% Mn, and the parameter represented by X = 30 × P + 50 × (S + N) + 300 × O is 3.0% or less. High Mn steel having high strength and high toughness even at extremely low temperatures by satisfying

JP 47-154 JP 59-104455 A International Publication No. 2013/100614 JP 2007-126715 A

  In general, refinement of the crystal grain size is effective for increasing the toughness of steel. However, when carbides such as Cr carbide and cementite are generated at the grain boundaries, if the crystal grain size is made finer, the starting point of fracture increases and cracks tend to propagate, so the low temperature toughness does not necessarily improve There is. On the other hand, when the crystal grain size of the weld heat-affected zone (HAZ) becomes coarse when the steel material is welded, the low temperature toughness of the welded joint decreases.

  In view of such circumstances, the present invention provides a high-Mn steel material for cryogenic use that is excellent in the toughness of the base material and the weld heat-affected zone.

  In the present invention, the austenite grain size is controlled to an appropriate size so that carbides generated at the crystal grain boundaries do not become the starting point of fracture or the propagation path of cracks. And this invention utilizes the pinning effect by adding the 1 type (s) or 2 or more types of Mg, Ca, and REM appropriately controlling the addition amount and balance of an alloy element, and also O amount and S amount. Thus, the austenite grain size is appropriately controlled, and the coarsening of the HAZ crystal grain size can be suppressed. The gist of the present invention is as follows.

(1) In mass%,
C: 0.001 to 0.80%,
Mn: 15.0-35.0%,
S: 0.0001 to 0.01%,
Cr: 0.01-10.0%,
Ti: 0.001 to 0.05%,
N: 0.0001 to 0.10%,
O: 0.001 to 0.010%
Containing
P: 0.02% or less,
In addition to
Si: 0.001 to 5.0%,
Al: 0.001 to 5.0%
One or both of
Mg: 0.01% or less,
Ca: 0.01% or less,
REM: containing 0.0002% or more in total of one or more of 0.01% or less,
30C + 0.5Mn + Ni + 0.8Cr + 1.2Si + 0.8Mo ≧ 25 (Formula 1)
O / S ≧ 1 (Formula 2)
And the balance consists of Fe and inevitable impurities,
The volume ratio of austenite is 95% or more,
The austenite has a crystal grain size of 20 to 200 μm,
A high Mn steel material for cryogenic use, characterized in that a carbide coverage at a crystal grain boundary of the austenite is 50% or less.
However, when Ni, Si, and Mo are not included, these terms are set to 0 in the above (Formula 1).

(2) Furthermore, in mass%,
Nb: 0.05% or less,
Ta: 0.05% or less,
Zr: 0.05% or less,
V: The high Mn steel material for cryogenic temperature as described in said (1) characterized by containing 1 type or 2 types or less of 0.10% or less.
(3) Furthermore, in mass%,
Cu: 3.0% or less,
Ni: 3.0% or less,
Co: 3.0% or less,
Mo: 3.0% or less,
W: High Mn steel material for cryogenic temperature as described in said (1) or (2) characterized by containing 1 type or 2 types or less of 3.0% or less.
(4) Furthermore, in mass%,
B: The high-Mn steel material for cryogenic temperature according to any one of (1) to (3) above, containing 0.010% or less.
(5) Furthermore, the contents of C, Si, Mn, P, S, Al, N, Cr, Cu, Ni, Co, Mo, W are
CI = -2C + 0.8Si-0,2Mn + 3.3Cr + 9 (Mo + W / 2)
+1.5 (Cu + Ni + Co) + 60N + 0.8Al-90P-90S
≧ 6.5 (Formula 3)
The high Mn steel material for cryogenic temperature according to any one of the above (1) to (4), characterized in that:
However, when Cu, Ni, Co, Mo, and W are not included, these terms are set to 0 in the above (Formula 3).

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the high Mn steel material for cryogenic temperatures excellent in the toughness of a base material and a welding heat affected zone. Specifically, high strength welding with a tensile strength of 800 MPa or more and large heat input welding up to about 150,000 J / cm is possible, and at each use temperature at a cryogenic temperature of −164 ° C. or lower, and −269 ° C. or lower, 100 J It is possible to provide a high-Mn steel material for cryogenic temperatures that has the above Charpy absorbed energy, and preferably also has excellent corrosion resistance, and the present invention has an extremely significant industrial contribution.

A steel material whose crystal structure is a body-centered cubic structure (bcc) is not suitable for use at an extremely low temperature because it brittlely breaks at a low temperature of, for example, about minus several tens of degrees. Accordingly, the cryogenic steel material is required to stably maintain austenite whose crystal structure is a face-centered cubic structure (fcc), for example, at a temperature of −196 ° C. or lower. This is because if austenite is unstable, it transforms into martensite at a very low temperature and the toughness may be lowered. In order to stabilize the austenite at an extremely low temperature, the present inventors are effective to add C, Mn, Ni, Cr, Si, and Mo, and it is necessary to satisfy the following (Formula 1). I got the knowledge.
30C + 0.5Mn + Ni + 0.8Cr + 1.2Si + 0.8Mo ≧ 25 (Formula 1)

  In order to improve toughness, it is important to control the austenite grain size. If the austenite grain size is too large, stress concentration on the austenite grain boundary occurs, and fracture starts from the grain boundary. In general, refinement of the austenite grain size is effective in improving toughness, but when hard Cr carbide or cementite is generated at the austenite grain boundaries, the origin of fracture increases due to refinement, and , The propagation of cracks may be promoted and the toughness may be reduced. According to the study by the present inventors, it has been found that the austenite grain size needs to be 20 to 200 μm in order to ensure cryogenic toughness.

Furthermore, when manufacturing a liquefied gas storage container using a cryogenic steel material, welding is performed. In particular, when high heat input welding is performed in order to increase the welding efficiency, the weld heat affected zone (HAZ) is heated to 1400 ° C., the crystal grains become coarse and the toughness decreases. A pinning effect using fine particles that are stable at high temperatures is effective in reducing the crystal grain size of the base material and the coarsening of the crystal grain size of HAZ.
In order to suppress the coarsening of the particle size of the high heat input HAZ, it is possible to add one or more of Mg, Ca, and REM to generate fine oxides and oxysulfides in the steel. It is valid. However, the present inventors have found that when the amount of S is excessive with respect to the amount of O, the thermal stability of the oxysulfide decreases, and an effective pin capable of suppressing the grain growth of HAZ austenite heated to a high temperature. Since it does not become a stop particle, the knowledge that it is necessary to satisfy the following (Formula 2) was obtained.
O / S ≧ 1 (Formula 2)

  Hereinafter, the present invention will be described in detail. First, the reason why each composition of the steel material is limited will be described.

C: 0.001 to 0.80%
C is an important element that stabilizes austenite and increases strength, and 0.001% or more is added. Preferably, the C content is 0.01% or more, more preferably 0.05% or more, and still more preferably 0.10% or more. When the intensity | strength in normal temperature is calculated | required, suitable C amount is 0.20% or more. On the other hand, if the amount of C is too large, the toughness decreases due to the precipitation of Cr carbide and cementite, which is the starting point of ductile fracture, so the upper limit is made 0.80% or less. Preferably, the C content is 0.70% or less, more preferably 0.60% or less, and still more preferably 0.50% or less.

Mn: 15.0-35.0%
Mn is added in an amount of 15.0% or more in order to stabilize austenite. Preferably, the Mn content is 17.0% or more, more preferably 20.0% or more, and further preferably 22.0% or more. On the other hand, if the amount of Mn exceeds 35.0%, corrosion resistance of the steel material and fume generation at the time of welding are caused, and hot workability is deteriorated and the surface of the steel material may be cracked. The upper limit of the amount is 35.0% or less. Preferably, the Mn content is 33.0% or less, more preferably 30.0% or less.

Cr: 0.01-10.0%
Cr is an element that stabilizes austenite, contributes to improvement of corrosion resistance, and increases the strength of the steel material by solid solution strengthening, and is added in an amount of 0.01% or more. Preferably, the Cr content is 0.10% or more, more preferably 0.50% or more, and still more preferably 1.0% or more. On the other hand, if the Cr content exceeds 10.0%, Cr carbide is formed and the toughness is deteriorated, so the upper limit is made 10.0% or less. Preferably, the Cr content is 8.0% or less, more preferably 6.0% or less, and still more preferably 5.0% or less. Further, in order to suppress stress corrosion cracking (SCC) due to precipitation of Cr carbide, it is preferable to add so as to satisfy Cr ≦ 0.3 / C + 2.

Ti: 0.001 to 0.05%
Ti is an element that forms TiN and N in steel, and TiN is generated by using Ca, Mg, and REM oxides and oxysulfides as precipitation nuclei. A composite precipitate of Ca, Mg, REM oxide or oxysulfide and TiN is excellent in thermal stability and contributes to suppression of coarsening of the HAZ particle size. In order to obtain such an effect, it is necessary to add 0.001% or more of Ti. Preferably, the Ti amount is 0.005% or more, more preferably 0.010% or more. On the other hand, if Ti is added excessively, coarse TiN is generated and the toughness is lowered, so the upper limit of the Ti amount is made 0.05% or less. Preferably, the upper limit of the Ti amount is 0.040% or less, more preferably 0.030% or less, and still more preferably 0.025% or less.

N: 0.0001 to 0.10%
N is contained in an amount of 0.0001% or more in order to form composite precipitates of oxides of Ca, Mg, REM or oxysulfides and TiN to improve HAZ toughness. N is an element effective for stabilizing austenite and strengthening particularly at low temperatures. Preferably, the N content is 0.0010% or more, more preferably 0.0020% or more, and still more preferably 0.0030%. That's it. On the other hand, if the N content exceeds 0.10%, the toughness deteriorates due to an increase in strength or the influence of nitrides, so the upper limit is made 0.10% or less. Preferably, the N content is 0.03% or less.

O: 0.001 to 0.010%
O is an element forming an oxide or oxysulfide of Ca, Mg, or REM, and the content is 0.001% or more. On the other hand, if the amount of O exceeds 0.010%, the deterioration of toughness due to inclusions becomes significant, so the upper limit is made 0.010% or less. Preferably, the amount of O is 0.0070% or less, more preferably 0.0050% or less, and still more preferably 0.0030% or less.

S: 0.0001 to 0.01% or less S is an impurity, and if the content is excessive, MnS starts as a starting point to promote ductile fracture and reduce toughness. Restrict to: Further, if the amount of S is excessive, S may be excessively dissolved in Ca, Mg, and REM oxysulfides to lower the thermal stability, so the upper limit is limited to 0.005% or less. It is preferable. More preferably, the S amount is 0.003% or less, and further preferably 0.002% or less. The lower limit of the amount of S is 0.0001% from the viewpoint of steelmaking cost.

P: 0.02% or less P is an impurity, and if the content is excessive, P segregates at the grain boundaries, and the toughness decreases due to grain boundary embrittlement, so the P content is limited to 0.02% or less. To do. The amount of P is preferably 0.015% or less, more preferably 0.010% or less, and still more preferably 0.008% or less.

Si, Al: 0.001 to 5.0% each
Si and Al are usually added as deoxidizing elements or for strengthening in many cases, but in the present invention, one or both are added in order to suppress the formation of Cr carbide and cementite. The suppression of the formation of carbides by the addition of Si and Al is a new finding obtained by the present inventors, and in order to obtain the effect, the lower limits of the Si amount and the Al amount are both 0.001% or more. Preferably, the Si amount and the Al amount are 0.01% or more alone or in total of both. On the other hand, the upper limits of the Si content and the Al content are both set to 5.0% or less in order to prevent a decrease in toughness due to the formation of coarse inclusions. The upper limit of the Si amount and the Al amount is preferably 2.0% or less, more preferably 1.0% or less, and still more preferably 0.05% or less.

Mg, Ca, and REM: 0.01% or less each, 0.0002% or more in total Mg, Ca, and REM form fine oxides and oxysulfides, and coarse crystal grains of the base material and HAZ It is an important element that suppresses crystallization, and one or more are added. In order to obtain an effect, the amount of Mg, the amount of Ca, and the amount of REM are set individually or in total of two or more to 0.0002% or more. Preferably it is 0.0005% or more, More preferably, it is 0.0010% or more. On the other hand, the upper limits of the amount of Mg, the amount of Ca, and the amount of REM are all set to 0.010% or less in order to prevent a decrease in toughness due to the formation of coarse inclusions. The upper limit of the Mg amount, Ca amount, and REM amount is preferably 0.0070% or less, more preferably 0.0050% or less, and still more preferably 0.0040% or less.

  Furthermore, in order to improve the strength, one or more of Nb, Ta, Zr, and V can be contained as necessary.

Nb, Ta, Zr: each 0.05% or less, V: 0.10% or less Nb, Ta, Zr, V is an element that forms carbides and nitrides and contributes to improving the strength by precipitation strengthening, It is preferable to add 1 type (s) or 2 or more types. Among these, Zr, like TiN, forms composite precipitates of Ca, Mg, REM oxides and oxysulfides and ZrN in the steel, contributing to the suppression of coarsening of the HAZ particle size. To do. The lower limits of the Nb amount, Ta amount, Zr amount, and V amount are all preferably 0.005% or more. On the other hand, if the Nb content, Ta content, and Zr content exceed 0.05% and the V content exceeds 0.10%, the toughness may decrease due to the coarsening of the precipitates, so the Nb content, Ta content, and Zr content. Are preferably 0.05% or less, and the V amount is preferably 0.10% or less.

  Furthermore, in order to increase the stability of austenite and suppress the precipitation of Cr carbide and cementite at grain boundaries, one or more of Cu, Ni, Co, Mo, and W may be included as necessary. Can do.

Cu: 3.0% or less Cu is an element that contributes to stabilization and strengthening of austenite and further suppression of precipitation of Cr carbide and cementite, and addition of 0.01% or more is preferable. More preferably, the amount of Cu is 0.10% or more. On the other hand, if the Cu content exceeds 3.0%, the hot workability may deteriorate, so the upper limit is preferably 3.0% or less. More preferably, the Cu amount is 1.0% or less.

Ni and Co: 3.0% or less, respectively Ni and Co are elements that contribute to stabilization and strengthening of austenite, and further suppression of precipitation of Cr carbide and cementite, both of which are preferably added in an amount of 0.01% or more. . More preferably, the Ni amount and the Co amount are both 0.10% or more. On the other hand, if Ni and Co are added excessively, martensite is likely to be generated, and the toughness and permeability of the welded portion may be deteriorated. Therefore, both the Ni content and the Co content should be 3.0% or less. Is preferred. More preferably, both the Ni amount and the Co amount are 1.0% or less.

Mo and W: 3.0% or less respectively Mo and W are elements that contribute to stabilization and strengthening of austenite, and further suppression of precipitation of Cr carbide and cementite, and both are preferably added in an amount of 0.01% or more. . More preferably, the Mo amount and the W amount are both 0.10% or more. On the other hand, since the effect is saturated even if Mo and W are added excessively, it is preferable that both the Mo amount and the W amount be 3.0% or less from the viewpoint of cost. More preferably, the Mo amount and the W amount are both 1.0% or less, and more preferably 0.80% or less.

B: 0.010% or less B is an element that segregates at the austenite grain boundaries and prevents intergranular fracture to improve toughness and yield strength. Addition of 0.0002% or more is preferable. More preferably, the B amount is 0.0003% or more, and further preferably 0.0010% or more. On the other hand, when B is contained excessively, the toughness may be lowered, so the amount of B is preferably 0.01% or less. More preferably, the B amount is 0.005% or less.

Essentially contained C, Mn, Cr, and optionally contained Si, Ni, and Mo are added so that the content satisfies the following (formula 1) in order to stabilize austenite. It is necessary. The following (Formula 1) is obtained from the relationship between the element and the amount of austenite at an extremely low temperature by experiment. When Si, Ni, and Mo are not included, the content is calculated as 0.
30C + 0.5Mn + Ni + 0.8Cr + 1.2Si + 0.8Mo ≧ 25 (Formula 1)

In order to increase the thermal stability of oxysulfides of Mg, Ca, and REM and to suppress the grain growth of HAZ austenite heated to a high temperature, it is necessary to satisfy the following (Formula 2). (Expression 2) is obtained from the relationship between the effect of suppressing the coarsening of the particle size of the HAZ by the oxides and oxysulfides of Mg, Ca, and REM, and the amounts of S and O by experiments.
O / S ≧ 1 (Formula 2)

Among the above-mentioned elements, there are elements that are effective for improving the corrosion resistance of high-Mn austenitic steel materials containing Cr and elements that have an adverse effect on the formation tendency of carbides and the state of formation of a protective oxide film of Cr. If the effect of each element is quantified based on the level of corrosion weight loss in atmospheric exposure (1 year), and the obtained corrosion resistance index (Corrosion Index: CI) is 6.5 or more, preferably 11.5 or more, The corrosion of the steel material can be remarkably suppressed, and in particular, excellent corrosion resistance that hardly rusts even when exposed to the atmosphere (60 days) can be obtained. In the following (Equation 3) for obtaining CI, an element having a positive coefficient sign means that the corrosion resistance is improved, and an element having a negative coefficient sign means that the corrosion resistance is lowered. The magnitude of the coefficient represents the magnitude of the influence.
CI = -2C + 0.8Si-0,2Mn + 3.3Cr + 9 (Mo + W / 2) +1.5 (Cu + NI + Co) + 60N + 0.8Al-90P-90S (Formula 3)
However, when Cu, Ni, Co, Mo, and W are not included, these terms are set to 0 in the above (Formula 3).

Next, the metal structure of the cryogenic high Mn steel material will be described.
In order to prevent brittle fracture in a cryogenic environment, the metal structure of the high-temperature Mn steel material for cryogenic use of the present invention has a volume ratio of austenite of 95% or more. The balance of austenite is ferrite, martensite, bainite, pearlite, etc., but all of them are brittle in a cryogenic environment, so it is preferable to limit it to less than 5%. The volume ratio of austenite may be 100%. The volume ratio of austenite can be measured by an X-ray diffraction method or a magnetic induction method in addition to observation of a metal structure by an optical microscope.

  The crystal grain size of austenite is set to 200 μm or less in order to ensure toughness in a cryogenic environment. Preferably, it is 150 μm or less. On the other hand, if the crystal grain size of austenite is too small, hard Cr carbide and cementite precipitated at the grain boundaries become the starting point of fracture, and in order to promote the propagation of cracks, the crystal grain size of austenite is 20 μm or more. And Preferably, it is 50 μm or more. The crystal grain size of austenite can be determined by structural observation using an optical microscope.

  When the carbides present in the austenite grain boundaries are close to each other, cracks are generated between and around the carbides, and cracks easily propagate through the grain boundaries, resulting in a decrease in toughness. Therefore, in order to suppress a decrease in toughness, it is preferable to control the coverage of the austenite grain boundaries with carbide (grain boundary carbide coverage) to 50% or less. The carbide grain boundary coverage is preferably as low as possible, but may be 1% or more or 5% or more. The grain boundary carbide coverage of austenite can be determined by structural observation using a transmission electron microscope.

Next, the manufacturing method of the high Mn steel material for cryogenics of this invention is demonstrated.
Generally, high-Mn steel is inferior in hot workability compared to carbon steel and low alloy steel, and therefore it is preferable to perform rolling under appropriate conditions. If it deviates from appropriate conditions, cracks occur on the surface of the steel slab, the steel ingot, or the steel sheet, which may lead to a decrease in yield. A steel piece or a steel ingot is obtained by melting and casting by a conventional method.

  If the heating temperature of the steel slab or the steel ingot is less than 1000 ° C., the deformation resistance during rolling is large, and the load on the rolling mill becomes excessive. On the other hand, if the temperature is higher than 1250 ° C. and heated to a high temperature, there is concern about a decrease in yield due to surface oxidation. Therefore, the upper limit of the heating temperature is preferably 1250 ° C. or less.

  After the steel slab or ingot is heated, hot rolling is performed. In order to refine the crystal grain size, it is necessary to increase the cumulative reduction rate at 1100 ° C. or less to promote recrystallization, and preferably the cumulative reduction rate from 1100 ° C. to the finishing temperature is 30% or more. . The finishing temperature of the hot rolling is preferably 950 ° C. or higher so that the crystal grain size of austenite is not excessively refined and is 20 μm or more.

When using a high Mn steel material for cryogenic temperatures as it is in hot rolling, it is preferable to perform forced cooling after hot rolling. In order to suppress the precipitation of Cr carbide and cementite after hot rolling and increase the low temperature toughness, it is effective to increase the cooling rate in the temperature range of 500 to 900 ° C. Therefore, after completion of hot rolling, it is preferable to perform forced cooling by water cooling or the like from a temperature of 900 ° C. or higher, and the cooling rate is preferably 3 ° C./s or higher. The forced cooling stop temperature is preferably 600 ° C., more preferably 500 ° C.
When forced cooling is not performed after hot rolling, it is preferable to perform solution treatment. In the solution treatment, the heating temperature is set to 950 to 1250 ° C., and the cooling rate is set to 3 ° C./s or higher from a temperature of 900 ° C. or higher in order to solidify the Cr carbide and cementite precipitated by hot rolling in the steel. It is preferable to cool to a temperature of 600 ° C. or lower.

Hereinafter, the present invention will be described in more detail by way of examples.
Using steel slabs having the chemical composition shown in Table 1, high Mn steel materials were produced under the production conditions shown in Table 2. The metal structure of each steel material (base material) was observed with an optical microscope, and the volume ratio (γ volume ratio) and crystal grain size (γ particle diameter) of austenite were measured. Further, the carbides (Cr carbide and cementite) were observed with a transmission electron microscope at 20 magnifications at a magnification of 10,000 times, and the grain boundary carbide coverage of austenite was measured. Further, the equivalent circle diameter of the carbide was measured, and the volume of each carbide was calculated to obtain the volume ratio.

  Furthermore, a tensile test was performed at room temperature in accordance with JIS Z 2241, and yield strength (YS) and tensile strength (TS) were measured. Moreover, the Charpy absorbed energy in liquid nitrogen temperature (-196 degreeC) and liquid He temperature (-269 degreeC) was measured using the V notch test piece of JISZ2242. Corrosion resistance was evaluated by emulsifying a 200 mm × 500 mm steel plate and performing an atmospheric exposure test for 60 days. The evaluation of rusting property was evaluated by measuring the area ratio (rusting rate) at which rust was generated after the exposure period was completed. A rusting rate of 5% or less is very good (◎), more than 5% to 10% or less is good (O), more than 10% to 25% or less is slightly bad (△), and more than 25% is bad (X). did. The results are shown in Table 3.

  In addition, in order to evaluate the metal structure and characteristics of the weld heat affected zone, heat treatment for reproducing the thermal cycle of welding was performed by a thermal cycle simulator under the conditions shown in Table 4. The crystal grain size (γ grain size) of austenite was measured by structural observation with an optical microscope. The carbide coverage and carbide volume ratio at the austenite grain boundaries were determined using a transmission electron microscope, as with the base material. Moreover, the Charpy absorbed energy in liquid nitrogen temperature (-196 degreeC) and liquid He temperature (-269 degreeC) was measured using the V notch test piece of JISZ2242. The results are shown in Table 4.

  From Tables 3 and 4, it can be seen that the high Mn steel materials according to the examples of the present invention are excellent in the properties of the base material and the HAZ and are excellent as a cryogenic material. On the other hand, in the comparative example that does not satisfy the conditions defined in the present invention, the desired characteristics cannot be obtained in one or both of the strength and the Charpy characteristics. Moreover, as shown in Table 3, the steel materials with high CI whose values are listed in Table 1 have good corrosion resistance.

  The high Mn steel material according to the present invention can be used as a steel material suitable for tanks used at cryogenic temperatures such as liquid helium, liquid hydrogen, LNG, liquid nitrogen using inexpensive alloy components, and is an expensive aluminum alloy, Ni-based Since it can be used as an alternative to austenitic stainless steel and 9% Ni steel, it contributes to saving of Ni resources and enables the tank construction cost to be reduced.

Claims (5)

% By mass
C: 0.001 to 0.80%,
Mn: 15.0-35.0%,
S: 0.0001 to 0.01%,
Cr: 0.01-10.0%,
Ti: 0.001 to 0.05%,
N: 0.0001 to 0.10%,
O: 0.001 to 0.010%
Containing
P: 0.02% or less,
In addition to
Si: 0.001 to 5.0%,
Al: 0.001 to 5.0%
One or both of
Mg: 0.01% or less,
Ca: 0.01% or less,
REM: containing 0.0002% or more in total of one or more of 0.01% or less,
30C + 0.5Mn + Ni + 0.8Cr + 1.2Si + 0.8Mo ≧ 25 (Formula 1)
O / S ≧ 1 (Formula 2)
And the balance consists of Fe and inevitable impurities,
The volume ratio of austenite is 95% or more,
The austenite has a crystal grain size of 20 to 200 μm,
A high Mn steel material for cryogenic use, characterized in that a carbide coverage at a crystal grain boundary of the austenite is 50% or less.
However, when Ni, Si, and Mo are not included, these terms are set to 0 in the above (Formula 1). Furthermore, in mass%,
Nb: 0.05% or less,
Ta: 0.05% or less,
Zr: 0.05% or less,
The high-Mn steel material for cryogenic use according to claim 1, characterized by containing one or more of V: 0.10% or less. Furthermore, in mass%,
Cu: 3.0% or less,
Ni: 3.0% or less,
Co: 3.0% or less,
Mo: 3.0% or less,
The high-Mn steel material for cryogenic use according to claim 1 or 2, which contains one or more of W: 3.0% or less. Furthermore, in mass%,
B: 0.010% or less is contained, The high Mn steel material for cryogenic temperatures of any one of Claims 1-3 characterized by the above-mentioned. Furthermore, the contents of C, Si, Mn, P, S, Al, N, Cr, Cu, Ni, Co, Mo, W are
CI = -2C + 0.8Si-0,2Mn + 3.3Cr + 9 (Mo + W / 2)
+1.5 (Cu + Ni + Co) + 60N + 0.8Al-90P-90S
≧ 6.5 (Formula 3)
The high-Mn steel material for cryogenic use according to any one of claims 1 to 4, wherein:
However, when Cu, Ni, Co, Mo, and W are not included, these terms are set to 0 in the above (Formula 3).