JP4529872B2 - High Mn steel material and manufacturing method thereof - Google Patents

Description

  The present invention relates to a steel material for a welded structure that is exposed to a wide range of temperatures from a low temperature to a room temperature in a liquefied gas atmosphere, such as a liquefied gas tank, and has a high strength, a low thermal conductivity, a low magnetic permeability, a low thermal expansion coefficient, and a wide temperature range. The present invention relates to a high manganese steel having excellent base metal toughness and weld heat-affected zone toughness and a method for producing the same. In particular, without requiring heat treatment such as solution treatment after hot rolling, the yield stress at room temperature is 300 MPa or more, and the Charpy absorbed energy of the base material and the weld heat affected zone at the liquid nitrogen temperature (−196 ° C.) is The present invention relates to a high Mn steel material having a value of 80 J or more and a manufacturing method thereof.

  As materials that can be used at low temperatures in a liquefied gas atmosphere such as liquefied natural gas (boiling point: -164 ° C), liquid oxygen (boiling point: -183 ° C), liquid nitrogen (boiling point: -196 ° C), JIS SUS304, etc. Ni-Cr austenitic alloys and aluminum alloys such as JIS 5000 series have been used. However, these materials are expensive, and the yield stress is not as high as that of low-alloy high-strength steel. In addition to having to increase the plate thickness, the weldability is not high. There is a problem with application to large welded structures such as large tanks, and there is a demand for materials that are relatively inexpensive and excellent in strength, weldability, and welded portion toughness.

  Therefore, a high-Mn austenitic alloy in which Ni, which is an expensive element in Ni-based austenitic alloys, is replaced with Mn is proposed as a low-temperature material that does not frequently use expensive Ni and Al. Fusion reactors, superconducting generators and linear It is being studied as a non-magnetic material used in motor cars.

  For example, in Patent Document 1, the C + N content is limited to 0.20% or less, and the Mn + 20 (C + N) content is set to 27% or more, so that excellent low temperature toughness, magnetic properties, and machinability are achieved. It has been shown that a high Mn steel with can be obtained. In Patent Document 2, a solution treatment is performed by adding 8 to 16% Ni to high Mn steel containing 18 to 30% Mn and finishing hot rolling at a temperature of (1050-12.5Ni) ° C or higher. A method for producing a high Mn steel excellent in high strength and low temperature toughness without being subjected to the above has been disclosed. Furthermore, in Patent Document 3, the parameter represented by X = Ni-30C + 0.5Mo containing 10-30% Mn and 10-25% Cr satisfies 5.50 or more, and 0.0005-0. A high Mn steel having high strength and high toughness even at an extremely low temperature of 4K is disclosed by containing .0050% Ca and 0.15 to 0.24% N.

JP-A-60-204864 JP-A-6-264135 JP-A-9-41087

  The high Mn steel materials according to these conventional technologies need to contain a large amount of Ni or require special heat treatment after rolling, and have high strength and excellent strength at a low cost. It was not possible to provide the toughness of the base metal, and it did not satisfy the requirements necessary for a large steel material for low temperature tanks. In addition, the toughness in the low temperature region of the weld heat affected zone, which is indispensable as a material for large welded structures, could not be guaranteed.

  The present invention solves such conventional problems, and an object of the present invention is to provide a high-Mn steel material that satisfies the following requirements (a) to (c) and a method for producing the same.

  (a) Yield stress of 300 MPa or more and liquefied natural gas (boiling point: −) at room temperature (25 ° C.) only by air cooling or accelerated cooling after rolling without performing reheating treatment such as solution treatment after hot rolling. 164 ° C.) and liquid nitrogen (boiling point: −196 ° C.), etc., and sufficient base material toughness can be secured even in thick wall materials. It can be secured at a maximum thickness of 65 mm.

  (b) When welding steel plates with a maximum thickness of 65 mm by welding, the welding heat-affected zone is sufficient even in operating temperature ranges such as liquefied natural gas (boiling point: -164 ° C) and liquid nitrogen (boiling point: -196 ° C). Toughness can be secured, specifically, an energy value of 70 J or more can be secured by JIS4 Charpy absorbed energy.

  (c) In addition, the material properties required as a low-temperature material can be achieved. Specifically, a low relative magnetic permeability of 1.02 or less and a low thermal conductivity of 17 W / m · K or less can be achieved simultaneously.

  The inventors repeated diligent examination and experiment on low-temperature steel that can be used in a liquefied gas storage tank or the like.

  As a result, regarding the chemical composition of the steel material, not only the amount of each alloy element such as C, Si, P, S, Cr, Al, N, O (oxygen) is specified within an appropriate range based on high Mn steel. , X (%) = 30 × P + 50 × (S + N) + 300 × O defined by parameter X (where P, S, N, and O indicate the content (unit: mass%) of each element in the steel material) .) Is defined as 3.0% or less, and the above object is achieved by controlling the grain size in the thickness direction of the austenite crystal in the steel material and the volume fraction of the ε-martensite amount within an appropriate range. I found out that I can.

  That is, by controlling the chemical composition of the high Mn steel material, the grain size in the thickness direction of the austenite crystal in the steel material, and the volume fraction of the ε martensite amount within an appropriate range, the strength and low temperature of the base metal as a low temperature steel Not only can the toughness value be maintained in the hot-rolled state, but also the toughness value in the low-temperature region can be secured for the weld heat affected zone, and the thermal expansion coefficient is achieved by a synergistic effect with the control of the amount of ε martensite in the steel. The inventors have found that characteristic values such as magnetic permeability and thermal conductivity can be controlled to desired numerical values.

  The present invention has been completed based on such findings. The gist of the present invention is the following high-Mn steel materials (1) and (2) and high-Mn steel materials (3) and (4). Hereinafter, the present invention (1) to the present invention (4), respectively. The present invention (1) to the present invention (4) may be collectively referred to as the present invention.

(1) By mass%, C: 0.01 to 0.25%, Si: 0.01 to 0.5%, Mn: more than 15% and 40% or less, Cr: 0.5% or more and less than 10%, Al: 0.005-0.10%, P: 0.03% or less, S: 0.01% or less, N: 0.001% or more and less than 0.05%, and O (oxygen): 0.003% or less A balance of Fe and impurities, and a parameter X defined by the following formula (1) having a chemical composition of 3.0% or less, the thickness of the austenite grain boundary contained in the steel A high Mn steel material having an average section length in the direction of 40 μm or less and an amount of ε martensite in the range of 0.1 to 30% in terms of volume fraction.
X (%) = 30 × P + 50 × (S + N) + 300 × O (1) Formula where P, S, N and O are the contents of each element in the steel (unit: mass%) ).

(2) Instead of a part of Fe, in mass%, Cu: 3.0% or less, Ni: 5.2% or less, Mo: 3.0% or less, Nb: 0.5% or less, V: 1 0.0% or less, Ti: 0.8% or less, B: 0.003% or less, Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.05% or less Or the high Mn steel materials as described in said (1) characterized by containing 2 or more types.

  (3) After heating the steel slab or steel ingot having the chemical composition defined in (1) or (2) above to 950 to 1200 ° C, the cumulative reduction amount in the temperature range of 1000 to 800 ° C is 30% or more. A method for producing a high Mn steel material, characterized in that after hot rolling with a finishing temperature of 950 to 750 ° C., air cooling is performed.

  (4) After heating the steel slab or steel ingot having the chemical composition defined in (1) or (2) above to 950 to 1200 ° C, the cumulative reduction amount in the temperature range of 1000 to 800 ° C is 30% or more. And after performing hot rolling which makes rolling finishing temperature 950-800 degreeC, the temperature range of 750-600 degreeC is cooled with the cooling rate of 2 degrees C / sec or more, The high Mn steel material characterized by the above-mentioned Production method.

  ADVANTAGE OF THE INVENTION According to this invention, the high Mn steel materials excellent not only in low temperature toughness and weldability but also in characteristics such as thermal expansion coefficient, magnetic permeability and thermal conductivity can be provided as hot rolled. Moreover, this high Mn steel material can be used as an alternative to the Ni-based austenitic stainless steel material used for LNG tank skirts and the like, and greatly contributes to the saving of Ni resources.

  Below, the high Mn steel material and its manufacturing method which concern on this invention are demonstrated. Hereinafter, “%” display of the content of each chemical component means “mass%”.

(A) About chemical composition C:
C is an element effective for securing the strength required for low-temperature steel such as a liquefied gas tank through stabilization of austenite. However, if the content is less than 0.01%, such an effect is poor. On the other hand, if the C content exceeds 0.25%, Cr carbide precipitates at the austenite grain boundaries, and the toughness and corrosion resistance of the base material. Furthermore, the low temperature toughness of the weld heat affected zone may be deteriorated. Therefore, the C content is in the range of 0.01 to 0.25%. A preferable range is 0.05 to 0.18%.

Si:
Si is an effective element for deoxidation and is an effective element for increasing the strength. However, if it is less than 0.01%, deoxidation may be insufficient, and if it exceeds 0.5%, ductility and toughness may be deteriorated. Therefore, the Si content is in the range of 0.01 to 0.5%. A preferable range is 0.02 to 0.3%.

Mn:
Mn is an element effective for increasing yield stress and improving low temperature toughness through the stabilization of austenite. However, if the content is 15% or less, not only the low temperature toughness is lowered, but also α ′ martensite is precipitated, the magnetic permeability is increased, non-magnetism is lost, and the thermal conductivity is increased. Moreover, when it exceeds 40%, workability and toughness deteriorate. Therefore, the Mn content is in the range of more than 15% and 40% or less. A preferable range is 20 to 37%, and a more preferable range is 25 to 35%.

Cr:
Cr is an element that stabilizes austenite and improves yield strength. This effect is obtained when the content is 0.5% or more in relation to other alloy elements. However, if it is 10% or more, Cr carbide tends to precipitate on the grain boundaries, and the toughness is lowered, and heat treatment such as solution treatment is required. Therefore, the Cr content is in the range of 0.5% or more and less than 10%. A preferred range is 1.5-7%.

Al:
Al is an element having an effect of improving the properties of steel by deoxidation of steel and refinement of crystal grains. However, if it is less than 0.005%, a sufficient effect cannot be obtained, while if it exceeds 0.10%, the toughness deteriorates. Therefore, the Al content is in the range of 0.005 to 0.10%. A preferable range is 0.01 to 0.05%.

P and S:
P and S are both impurity elements that impair hot workability. In austenitic steel, by reducing the contents of both elements P and S at the same time, a greater effect of improving the toughness value of the base metal and the weld heat-affected zone can be obtained than when it is reduced solely. Therefore, the P content is 0.03% or less, and the S content is 0.01% or less. Preferably, the P content is 0.01% or less, and the S content is 0.003% or less.

N:
N is an element effective for stabilizing austenite and improving proof stress. C is also used as a stabilizing element for austenite, but C causes toughness deterioration due to grain boundary precipitation of Cr carbide, whereas N not only has such an adverse effect, but also yields in high Mn steel. The stress increasing effect is larger than C. Further, N coexists with the nitride-forming element, thereby having the effect of dispersing fine nitrides in the steel. In order to express these effects, the N content needs to be 0.001% or more. However, when the content is 0.05% or more, the toughness is significantly deteriorated. Therefore, N is set to a range of 0.001% or more and less than 0.05%. A preferable range is 0.001 to 0.03%.

O (oxygen):
O is inevitably mixed during steel making, but if its content increases, it causes internal defects in the steel and deteriorates the properties of the steel. If the O content exceeds 0.003%, the low temperature toughness, particularly the low temperature toughness of the heat affected zone of the welding, is significantly reduced, α ′ martensite is likely to be generated, and magnetic properties such as permeability are likely to deteriorate. Become. Therefore, the O content is 0.003% or less. Preferably, it is 0.002% or less. Note that the smaller the amount of O, the better, but considering the manufacturing cost, it is usually sufficient to deoxidize to about 0.0005%.

Parameter X:
Parameter X defined by the above formula (1), that is, X (%) = 30 × P + 50 × (S + N) + 300 × O (where P, S, N and O are the contents of each element in the steel material) (Unit: mass%) is a parameter that needs to be controlled from the viewpoint of improving the base metal toughness and the low temperature toughness of the weld heat affected zone, particularly from the viewpoint of improving the Charpy characteristics at -196 ° C. . The high-Mn steel material in the present invention is mainly made of an austenite phase, so that it is a material that is difficult to cause so-called cleavage fracture. There is a case where brittle fracture is caused by a decrease in the strength of the boundary or generation of acid / sulfide.

  As a result of detailed examination and research on this point, the present inventors have determined that if the parameter X is controlled to 3.0% or less, the grain boundary strength decreases due to microsegregation on the austenite grain boundaries due to P and S, It is extremely effective in suppressing the occurrence of brittle fracture because it can suppress the decrease in the strength of the boundary between the austenite crystal and ε martensite crystal due to N and O and the formation of inclusions due to S, N and O. In addition, the present inventors have found that it is also effective for improving the characteristics of the weld heat affected zone structure. However, if the parameter X exceeds 3.0%, sufficient fracture resistance cannot be obtained. A preferable range of the parameter X is 2.0% or less, and a more preferable range is 1.5% or less.

Average intercept length in the thickness direction of austenite grain boundaries contained in high Mn steel:
In order to impart sufficient low-temperature toughness as a low-temperature material in a high Mn-based steel material, the above-mentioned parameter X is controlled to 3.0% or less, and the austenite crystal grains in the plate thickness direction are further made finer. It is extremely important to do. In high-Mn steel, plate-like and strip-like martensite is formed in austenite, and since the length thereof is determined by the size of austenite grains, it is necessary to control the grain size of austenite crystals. The purpose of the present invention is to obtain good characteristics in the as-rolled state, so that the refinement of the crystal structure in the thickness direction is specified.

  This refinement of the crystal structure in the plate thickness direction substantially reduces not only the austenite structure but also various martensite structures. Resistance increases significantly. Here, in order to provide sufficient low temperature toughness including crack propagation stop characteristics, the austenite grain size in the thickness direction of the high Mn steel material, that is, the average section length of the austenite grain boundary in the thickness direction needs to be 40 μm or less. There is. The average intercept length of the austenite grain boundary in the thickness direction is preferably 30 μm or less, and more preferably 20 μm or less.

Ε martensite content in high Mn steel:
In high Mn steel materials, in addition to α 'martensite having a body-centered cubic (bcc) crystal structure often found in low alloy steel materials as martensite, hexagonal crystals (hcp) characteristic of high Mn steel materials Ε-martensite having the following crystal structure is included. The volume fraction of each martensite in the steel material can be obtained by measuring the X-ray intensity as follows. That is, an X-ray diffraction pattern of a sample is measured by a normal X-ray diffraction method, and the diffraction intensity ratio of austenite (fcc), α ′ martensite (bcc), and ε martensite (hcp) on the diffraction pattern is anisotropic. The volume fraction of each phase can be calculated by correcting the intensity ratio of each diffractive surface due to the property.

  Although ε-martensite has some adverse effects on the fracture resistance of steel, it has a good effect on the strength of steel. By making the amount of ε martensite 0.1% or more by volume% in the high Mn steel material, it is possible to achieve both a yield stress of 300 MPa or more and a tensile strength of 600 MPa or more. On the other hand, if the content exceeds 30%, the fracture resistance decreases, which is not preferable. The content of ε martensite is preferably 2 to 15%.

  As described above, the high Mn steel material according to the present invention is a large-sized weld that can be used in a low temperature region without heat treatment after rolling for the first time by controlling the average section length in the thickness direction of the austenite grain boundary and the amount of ε martensite. Steel material is obtained.

  The high-Mn steel material according to the present invention includes one or more selected from Cu, Ni, Mo, Nb, V, Ti, B, Ca, Mg, and REM as necessary for improving the yield strength. It can be included. Hereinafter, these optional elements will be described.

Cu:
Since Cu is effective in strengthening austenite and increasing the yield strength, it may be contained if necessary. However, if the content exceeds 3.0%, the workability deteriorates. Therefore, when Cu is contained, the content is 3.0% or less. A preferable range is 0.01 to 3.0%, and a more preferable range is 0.2 to 2.0%.

Ni:
Ni is an element effective for stabilizing austenite and improving toughness, and may be contained if necessary. However, even if the content exceeds 10%, the effect is saturated and α ′ martensite is easily generated, and there is a possibility that weld toughness and magnetic permeability are deteriorated. Therefore, when it contains Ni, the content shall be 10% or less. A preferable range is 0.01 to 5%, and a more preferable range is 0.01 to 1%.

Mo:
Mo is effective not only in increasing the strength, but also in preventing toughness deterioration caused by grain boundary precipitation of Cr carbide and increasing the strength of steel. You may let them. However, if the content exceeds 3.0%, the effect is saturated. Therefore, when Mo is contained, the content is set to 3.0% or less. A preferable range is 0.01 to 2%.

Nb:
Nb is an element effective for bonding carbon and N to precipitate carbonitride and improving the yield strength of the steel by its precipitation strengthening. Therefore, Nb may be included as necessary. However, if the content exceeds 0.5%, the toughness deteriorates. Therefore, when Nb is contained, the content is 0.5% or less. A preferable range is 0.005 to 0.5%, and a more preferable range is 0.01 to 0.2%.

V:
V is an element effective for binding carbon and nitride to precipitate carbonitride and improving the proof stress of the steel by precipitation strengthening, so it may be included as necessary. However, if the content exceeds 1.0%, the toughness deteriorates. Therefore, when V is contained, the content is set to 1.0% or less. A preferable range is 0.01 to 1.0%, and a more preferable range is 0.05 to 0.3%.

Ti:
Ti is an element effective for bonding carbon and N to precipitate carbonitride and improving the yield strength of the steel by precipitation strengthening. Therefore, Ti may be contained if necessary. However, if the content exceeds 0.8%, the toughness deteriorates. Therefore, when Ti is contained, the content is set to 0.8% or less. A preferred range is 0.005 to 0.3%, and a more preferred range is 0.008 to 0.1%.

B:
B has the effect of preventing grain boundary breakdown and improving proof stress by segregating at austenite grain boundaries, and therefore B may be contained as necessary. However, if the content exceeds 0.003%, the toughness deteriorates. Therefore, when it contains B, the content shall be 0.003% or less. A preferable range is 0.0005 to 0.003%, and a more preferable range is 0.0005 to 0.002%.

Ca:
Ca brings about the effect of spheroidizing inclusions and has the effect of improving toughness. Therefore, Ca may be contained as necessary. However, if the content exceeds 0.01%, cleanliness is deteriorated and toughness is lost. Therefore, when Ca is contained, its content is set to 0.01% or less. A preferable range is 0.0003 to 0.01%, and a more preferable range is 0.0003 to 0.004%.

Mg:
Mg, like Ca, brings about the effect of spheroidization of inclusions and has the effect of improving toughness, so it may be contained as necessary. However, if the content exceeds 0.01%, cleanliness is deteriorated and toughness is lost. Therefore, when Ca is contained, its content is set to 0.01% or less. A preferable range is 0.0002 to 0.01%, and a more preferable range is 0.0002 to 0.002%.

Rare earth elements (REM):
The rare earth element (REM), like Ca, brings about the effect of spheroidization of inclusions and has the effect of improving toughness. Therefore, it may be contained as necessary. However, if the content exceeds 0.05%, cleanliness is deteriorated and toughness is lost. Therefore, when it contains REM, the content shall be 0.05% or less. A preferable range is 0.0002 to 0.05%, and a more preferable range is 0.0003 to 0.001%. When REM is contained, a misch metal containing La or Ce as a main component may be used. In addition, the rare earth element as used in the field of this invention is a general term of the total 17 elements of Sc, Y, and a lanthanoid, and the content of rare earth elements refers to the total content of these elements.

(B) Manufacturing conditions (1)
In general, high-Mn steel is inferior in hot workability to carbon steel and low alloy steel, and therefore must be rolled under appropriate conditions. If it deviates from an appropriate condition, a crack occurs on the surface of a steel piece, a steel ingot, or a steel plate, resulting in a decrease in yield. Therefore, strict management of the heating condition and rolling condition of the steel slab or steel ingot is important.

  First, the heating temperature of a steel piece or a steel ingot needs to be 950-1200 degreeC. If it is less than 950 degreeC, the deformation resistance at the time of rolling is large, and an excessive load will be applied to a rolling mill. On the other hand, when heated to a high temperature exceeding 1200 ° C., the oxidation loss on the surface becomes large and the austenite grains become coarse, and even after hot rolling, they cannot be easily made fine.

  After heating to 950 to 1200 ° C., it is necessary to perform hot rolling with a cumulative reduction amount of 30% or more in a temperature range of 1000 to 800 ° C. This destroys the cast structure of the steel slab or steel ingot, and also refines and flattens the austenite grains in the steel material, along with the appropriate heating temperature, in the thickness direction of the austenite grain boundaries contained in the steel material. This is because the average section length is 40 μm or less.

  And the rolling finishing temperature of a hot rolling needs to be 950-750 degreeC. Combined with the effect of hot rolling with a cumulative reduction amount of 30% or more in the temperature range of 1000 to 800 ° C., a microstructure with an average section length in the thickness direction of the austenite grain boundary contained in the steel is 40 μm or less. This is because When the rolling finishing temperature exceeds 950 ° C., the austenite crystal grain growth after rolling becomes too large, and thus a desired microstructure cannot be obtained. On the other hand, when the rolling finishing temperature is less than 750 ° C., the deformation resistance during rolling is large, and an excessive load is applied to the rolling mill. Furthermore, the rolling texture develops and the anisotropy of the steel sheet increases, which is not preferable.

  Thereafter, when air-cooled, the amount of ε-martensite is 0.1 to 30% in volume% in the steel, so that a steel sheet excellent in both strength and fracture resistance can be obtained. This steel sheet has properties suitable for LNG tank skirt materials.

(C) Manufacturing conditions (2)
Among the production conditions of (B) above, after changing the hot rolling rolling finish temperature to 950 to 800 ° C., instead of air cooling after hot rolling, a temperature range of 750 to 600 ° C. Accelerated cooling at a cooling rate of 2 ° C / sec or more. Even if it manufactures in this way, since the amount of epsilon martensite will produce | generate 0.1 to 30% in volume% in steel, the steel plate excellent in both intensity | strength and fracture resistance will be obtained. This steel sheet has properties suitable for LNG tank skirt materials.

  Here, the lower limit of the hot rolling rolling finish temperature is defined as 800 ° C., unlike the case of air cooling, in order to suppress the formation of precipitates that become noticeable at about 800 ° C. or less and increase the low temperature toughness.

  Moreover, the reason why the temperature range of 750 to 600 ° C. is accelerated and cooled at a cooling rate of 2 ° C./sec or more is to suppress the formation of precipitates and increase the low temperature toughness. However, at a cooling rate of less than 2 ° C./sec, the effect of accelerated cooling is not sufficient, and a desired structure cannot be obtained. In this accelerated cooling, since the effect of accelerated cooling cannot be obtained if the rolling structure is changed, it is necessary to start accelerated cooling at 750 ° C. The reason why the lower limit of the accelerated cooling range is 600 ° C. is that a predetermined accelerated cooling effect can be obtained by cooling to at least 600 ° C. Note that the accelerated cooling may be continued 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 of steel types A to P having the chemical composition and parameter X shown in Table 1, the production conditions shown in Table 2 (heating temperature, cumulative rolling amount in the temperature range of 1000 to 800 ° C., rolling finishing temperature, cooling rate A high Mn steel material having a thickness of 60 mm was prepared. Then, the average intercept length in the thickness direction of the austenite (γ) grain boundary contained in the steel material and the amount of ε martensite are measured (measured values are shown in Table 2), and tensile properties (yield strength) are used as the base material properties. , Tensile strength), Charpy impact properties, relative permeability and thermal conductivity. The obtained measured values are shown in Table 3.

  Further, the high Mn steel material thus obtained was subjected to test piece processing with a plate thickness of 55 mm and a groove angle of 30 ° (V groove), then Root Gap: 10 mm, welding heat input: 29.6 KJ / GTAW welding was performed under the conditions of cm, welding material: DW308LP (manufactured by Kobe Steel), and shielding gas: carbon dioxide. Charpy specimens were collected at a notch position (fusion line: FL) of 50% weld metal / HAZ 50%, and the absorbed energy value when the test was conducted at -196 ° C was measured. The measured values are as shown in Table 3.

表３から、本発明例に係る高Ｍｎ鋼材は、熱間圧延ままで、母材特性（強度、靭性、比透磁率及び熱伝導度）と溶接熱影響部靭性のいずれにおいても優れており、低温材料として優れていることが分かる。

From Table 3, the high Mn steel material according to the present invention is hot-rolled, and is excellent in any of the base material properties (strength, toughness, relative permeability and thermal conductivity) and weld heat-affected zone toughness, It turns out that it is excellent as a low-temperature material.

  On the other hand, in the comparative example that does not satisfy the conditions defined in the present invention, it can be seen that the intended characteristics cannot be obtained in one or both of the base material characteristics and the weld heat affected zone toughness.

The high Mn steel material according to the present invention is excellent not only in low temperature toughness and weldability but also in characteristics such as thermal expansion coefficient, magnetic permeability and thermal conductivity. Moreover, this high Mn steel material can be provided hot-rolled, and can be used as an alternative to the Ni-based austenitic stainless steel material used for LNG tank skirts, etc. It contributes to.

Claims (4)

In mass%, C: 0.01 to 0.25%, Si: 0.01 to 0.5%, Mn: more than 15% to 40% or less, Cr: 0.5% or more and less than 10%, Al: 0 0.005 to 0.10%, P: 0.03% or less, S: 0.01% or less, N: 0.001% or more and less than 0.05%, and O (oxygen): 0.003% or less , Balance Fe and impurities, and a steel material having a chemical composition with a parameter X defined by the following formula (1) of 3.0% or less, an average in the thickness direction of austenite grain boundaries contained in the steel material A high Mn steel material having a section length of 40 μm or less and an amount of ε martensite in the range of 0.1 to 30% in terms of volume fraction.
X (%) = 30 × P + 50 × (S + N) + 300 × O (1) Formula where P, S, N and O are the contents of each element in the steel (unit: mass%) ). Instead of a part of Fe, in mass%, Cu: 3.0% or less, Ni: 5.2% or less, Mo: 3.0% or less, Nb: 0.5% or less, V: 1.0% 1 or 2 types selected from Ti: 0.8% or less, B: 0.003% or less, Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.05% or less The high Mn steel material according to claim 1, comprising the above.   The steel slab or steel ingot having the chemical composition defined in claim 1 or 2 is heated to 950 to 1200 ° C, the cumulative reduction amount in the temperature range of 1000 to 800 ° C is 30% or more, and the rolling finishing temperature. A method for producing a high-Mn steel material, characterized in that after hot rolling at 950 to 750 ° C., air cooling is performed.   The steel slab or steel ingot having the chemical composition defined in claim 1 or 2 is heated to 950 to 1200 ° C, the cumulative reduction amount in the temperature range of 1000 to 800 ° C is 30% or more, and the rolling finishing temperature. A method for producing a high Mn steel material, characterized in that, after hot rolling at 950 to 800 ° C., a temperature range of 750 to 600 ° C. is cooled at a cooling rate of 2 ° C./sec or more.