JP2016084529A - HIGH Mn STEEL MATERIAL AND PRODUCTION METHOD THEREFOR - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high Mn steel material excellent in yield strength at room temperature and toughness at ultralow temperature without being subjected to heat treatment after hot-rolled, and to provided a production method for the high Mn steel material.SOLUTION: There is provided the high Mn steel material which has a chemical composition containing by mass%, C:0.25 to 0.75%, Si:0.05 to 1.0%, Mn:over 20% and 35% or less, Ni:0.1% or more and less than 7.0%, Cr:0.1% or more and less than 8.0%, Al:0.005 to 0.10%, N:0.005% or more and less than 0.05% and having a parameter X (%) (=C+10×Si+2×Ni,C, Si and Ni are contents of each element) of 6.0 to 15.0% and in which carbide coverage factor in a grain boundary contained in the steel material is 30% or less. One or more kind of Cu, Mo, Nb, V, Ti, B, Ca, Mg, REM may be contained. There is provided the production method in which the high Mn steel material is cooled at a cooling rate of 5°C/s or more in a temperature range from 750°C to 600°C after hot rolling the steel material and then naturally cooling the steel material as it is.SELECTED DRAWING: None

Description

  The present invention relates to a high Mn steel material suitable for a material for storing a liquefied gas and a method for producing the same.

  Examples of materials that can be used in a cryogenic environment such as liquefied natural gas (boiling point: -164 ° C.) include aluminum alloys such as 5000 series (Al—Mg series), Ni—Cr austenite alloys such as SUS304, and 9 % Ni steel sheet has been used. However, the yield stress is not as high as that of a low-alloy high-strength steel, and in addition to having to increase the plate thickness, the weldability is not high, and the material cost is high due to containing a large amount of Ni. There has been a demand for materials that are problematic and inexpensive and that are excellent in strength, weldability, and weld toughness. The strength required for pressure vessel materials is increasing as the size of the tank increases.

  Therefore, a high-Mn austenitic alloy in which Ni contained in a Ni-based austenitic alloy is replaced with Mn has been proposed as a low-temperature material that does not use expensive Ni and Al. In fusion reactors, superconducting generators and linear motor cars, It has been studied as a nonmagnetic material to be used.

  For example, Patent Document 1 shows that a high Mn steel having excellent low temperature toughness and magnetic properties can be obtained by containing less than 0.5% C and 16 to 40% Mn. Patent Document 2 discloses a high-Mn steel containing 26-30% Mn in a range where the C content is 0.10% or more, the N content is 0.05% or more, and C + 2N is 1.0% or less. Has been.

  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. Patent Document 4 contains 0.01 to 0.25% C, more than 15% to 40% Mn, and a 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

Japanese Examined Patent Publication No.59-11661 Japanese Patent Publication No. 5-18887 Japanese Patent Laid-Open No. 9-41087 Japanese Patent No. 4529872

  The high Mn steel materials according to Patent Documents 2 and 3 contain a large amount of N that increases the strength of the steel at low temperatures, but the strength at room temperature is not considered. The high Mn steel material according to Patent Document 1 is manufactured by performing reheating treatment such as solution treatment after hot rolling. The high Mn steel material according to Patent Document 4 needs to limit the content of impurities in order to ensure toughness. These were not able to provide a thick material with high strength and excellent base material toughness at low cost, and did not satisfy the requirements for a large steel material for low temperature tanks. In addition, it cannot cope with the increase in strength of the materials used with the increase in size of the tank. In addition, no mention is made of the form of carbides that affect the toughness of high-Mn steel.

  The present invention solves such a conventional problem, and it is 400 MPa or more at room temperature (25 ° C.) only by accelerated cooling without performing reheating treatment such as solution treatment after hot rolling. Yield stress of liquefied natural gas (boiling point: −164 ° C.) and liquid nitrogen (boiling point: −196 ° C.), etc. An object of the present invention is to provide a high Mn steel material capable of securing a JIS No. 4 Charpy absorbed energy value at −196 ° C. of 50 J or more as a base material at a maximum plate thickness of 50 mm, and a manufacturing method thereof.

  The present inventors examined high Mn steel materials that can be used in liquefied gas storage tanks and the like.

  As a result, regarding the chemical composition of the steel material, based on the high Mn steel, not only the amount of each alloy element such as C, Si, P, S, Ni, Cr, Al, N, etc. is defined within an appropriate range, but also X (%) = Parameter X defined by C + 10 × Si + 2 × N (where C, Si and N indicate the content (unit: mass%) of each element in the steel) from 6% to 15% It was found that the above object can be achieved by controlling the carbide coverage at the austenite grain boundaries in the steel material to an appropriate range.

  In other words, by controlling the chemical composition of the high Mn steel material and the carbide coverage at the austenite grain boundaries to an appropriate range, it has been found that the strength and low temperature toughness value of the base metal as a low temperature steel can be secured as hot rolled. It was.

  The present invention has been completed based on such findings. The gist of the present invention is as follows.

(1) In mass%, C: 0.25 to 0.75%, Si: 0.05 to 1.0%, Mn: more than 20% and 35% or less, Ni: less than 0.1 to 7.0% Cr: 0.1% or more and less than 8.0%, Al: 0.005 to 0.10%, N: 0.005% or more and less than 0.05%, P: 0.04% or less, S : It is limited to 0.02% or less, and consists of the balance Fe and impurities.
A high Mn steel having a parameter X (%) defined by the formula of 6.0 to 15.0% and a carbide coverage at a grain boundary of 30% or less.
X (%) = C + 10 × Si + 2 × Ni (1) Formula where C, Si and Ni 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, Mo: 3.0% or less, Nb: 0.5% or less, V: 0.5% or less, Ti: 0 0.5% or less, B: 0.001% 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 (1) above, characterized by

(3) After heating the steel slab or ingot having the chemical composition described in (1) or (2) above to 950 to 1200 ° C, the cumulative rolling reduction in the temperature range of 800 to 1100 ° C was 30% or more. In addition, after performing hot rolling with a rolling finishing temperature of 750 to 950 ° C., the temperature range from 750 ° C. to 600 ° C. is cooled at a cooling rate of 5 ° C./s or more, and left to cool as it is. Manufacturing method of high Mn steel.

  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. In addition, this high Mn steel material can be used as an alternative to aluminum alloys, Ni-based austenitic stainless steels, and 9% Ni steel materials used for tank materials in LNG tanks, and contributes to saving of Ni resources. The tank construction cost can be reduced. Without requiring heat treatment such as solution treatment after hot rolling, the yield stress at room temperature is 400 MPa or more, the tensile strength is 800 MPa or more, and the base material Charpy absorbed energy at liquid nitrogen temperature (−196 ° C.) is 50 J or more. The present invention has a remarkable industrial contribution, such as being able to provide a high Mn steel material and a method for producing the same.

  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) Chemical composition [C: 0.25 to 0.75%]
C is an element effective for securing the strength required for low-temperature steel such as a liquefied gas tank through stabilization of austenite. In particular, in order to ensure the strength at room temperature, the C content is set to 0.25% or more. Preferably, the C content is 0.35% or more. On the other hand, if the C content exceeds 0.75%, Cr carbide precipitates in a large amount on the austenite grain boundaries, which may deteriorate the toughness and corrosion resistance of the base material and the low temperature toughness of the weld heat affected zone. Therefore, the C content is 0.75% or less. Preferably it is 0.65% or less, More preferably, it is 0.50% or less.

[Si: 0.05-1.0%]
Si is an effective element for deoxidation and is an effective element for increasing the strength. However, if it is less than 0.05%, deoxidation may be insufficient, and the Si content is set to 0.05% or more. Preferably, the Si content is 0.4% or more. Further, if the Si content exceeds 1.0%, the ductility and toughness may be deteriorated. Preferably, the Si content is 0.8% or less.

[Mn: more than 20% and 35% or less]
Mn is an element effective for increasing yield stress and improving low temperature toughness through the stabilization of austenite. However, if the content is 20% or less, not only the yield stress and the low temperature toughness are lowered, but also austenite is destabilized and α 'martensite is precipitated to deteriorate the toughness. Therefore, the Mn content exceeds 20%. And Preferably, the Mn content is 23% or more. On the other hand, if the Mn content exceeds 35%, workability and weldability deteriorate, so the content is made 35% or less. Preferably, the Mn content is 30% or less, more preferably 27% or less.

[Ni: 0.1% or more and less than 7.0%]
Ni is an extremely effective element for stabilizing austenite and improving toughness, and the Ni content is 0.1% or more. However, even if 7.0% or more of Ni is contained, the effect is saturated and α ′ martensite is easily generated, and there is a possibility that weld toughness and magnetic permeability are deteriorated. Less than 7.0%. The Ni content is preferably less than 3.0%, more preferably 2.0% or less.

[Cr: 0.1% or more and less than 8.0%]
Cr is an element that stabilizes austenite and improves yield strength. In the present invention, this effect is obtained when the Cr content is 0.1% or more in relation to other alloy elements. Preferably, the Cr content is 1.0% or more, more preferably 3.0% or more, and still more preferably 4.0% or more. However, when the Cr content is 8.0% or more, Cr carbide tends to precipitate on the grain boundaries, and the toughness is reduced and heat treatment such as solution treatment is required. Therefore, the Cr content is less than 8.0%. Preferably, the Cr content is 6.0% or less.

[Al: 0.005 to 0.10%]
Al is an element having an effect of improving the properties of steel by deoxidation of steel and refinement of crystal grains. However, if less than 0.005%, a sufficient effect cannot be obtained, so the Al content is made 0.005% or more. Preferably, the Al content is 0.01% or more. On the other hand, if the Al content exceeds 0.10%, the toughness deteriorates, so the upper limit is made 0.10% or less. Preferably, the Al content is 0.05% or less.

[P: 0.04% or less, S: 0.02% or less]
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 limited to 0.04% or less, and the S content is limited to 0.02% or less. Preferably, the P content is 0.02% or less, and the S content is 0.003% or less. The smaller the P and S contents, the better. However, from the viewpoint of manufacturing cost, the P content may be 0.003% or more, and the S content may be 0.001% or more.

[N: 0.005% or more and less than 0.05%]
N is an element effective for stabilizing austenite and improving proof stress. N as an austenite stabilizing element has an effect equivalent to that of C, has no adverse effects such as deterioration of toughness due to grain boundary precipitation, and has an effect of increasing the strength at extremely low temperatures. 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 is set to 0.005% or more. On the other hand, if the N content is 0.05% or more, the toughness deteriorates remarkably, so the content is made less than 0.05%. Preferably, the N content is 0.03% or less.

[Parameter X: 6.0 to 15.0%]
The parameter X defined by the above-mentioned formula (1), that is, X (%) = C + 10 × Si + 2 × Ni, is a Charpy at −196 ° C. in particular from the viewpoint of improving the base material strength, carbide formation suppression and base material toughness. It is a parameter that needs to be controlled from the viewpoint of improving characteristics. Here, C, Si, and Ni of the parameter X indicate the content (unit: mass%) of each element in the steel material. The high-Mn steel material in the present invention is mainly made of an austenite phase, and thus is a material that is less likely to cause so-called cleavage fracture. However, carbides precipitated at the austenite crystal grain boundaries may be the starting point of fracture and may reduce Charpy characteristics.

  The present inventors have examined this point in detail, and found that the lower limit of the parameter X has a correlation with strength, and the upper limit has a correlation with toughness, particularly fracture starting from carbides at grain boundaries. It was. And, by controlling the parameter X within an appropriate range, the present invention succeeded in both ensuring the strength of the base material and suppressing the decrease in toughness due to carbide formation. The parameter X is 6.0% or more, preferably 7.0% or more in order to ensure strength. On the other hand, if the parameter X exceeds 15.0%, Charpy characteristics cannot be obtained due to the grain boundary carbide, so the content is made 15.0% or less, preferably 11.0% or less.

The high Mn steel material according to the present invention is further made of Cu, Mo,
One or more selected from Nb, V, Ti, B, Ca, Mg, and REM can be contained. Hereinafter, these optional elements will be described.

[Cu: 3.0% or less]
Cu strengthens austenite and is effective in increasing the yield strength, so it may be contained as necessary. However, if the content exceeds 3.0%, the workability deteriorates. Therefore, when Cu is contained, the content is 3.0% or less, more preferably 1.0% or less, and still more preferably 0. 0.7% or less. In order to increase the strength, the Cu content is preferably 0.01% or more.

[Mo: 3.0% or less]
Mo is not only effective in increasing the strength, but also effective in preventing toughness deterioration due to grain boundary precipitation of Cr carbide and increasing the strength of steel. May be. However, if the content exceeds 3.0%, the effect is saturated. Therefore, when Mo is contained, the content is 3.0% or less, more preferably 2.0% or less, still more preferably 1.0% or less, and still more preferably 0.8% or less. In order to increase the strength, the Mo content is preferably 0.01% or more.

[Nb: 0.5% or less]
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, its content is 0.5% or less, more preferably 0.2% or less. In order to increase the strength, the Nb content is preferably 0.005%, more preferably 0.01%.

[V: 0.5% or less]
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 0.5%, the toughness deteriorates. Therefore, when V is contained, its content is 0.5% or less, more preferably 0.2% or less. In order to increase the strength, the V content can be 0.01% or more.

[Ti: 0.5% or less]
Ti is an element effective for bonding carbon and N to precipitate carbonitride and improving the proof stress of the steel by precipitation strengthening. Therefore, Ti may be included as necessary. However, if the content exceeds 0.5%, the toughness deteriorates. Therefore, when Ti is contained, its content is 0.5% or less, more preferably 0.3% or less. In order to increase the strength, the Ti content can be 0.005% or more.

[B: 0.001% or less]
B has the effect of preventing grain boundary breakage by segregating at the austenite grain boundaries and improving the proof stress, so B may be contained as required. However, if the content exceeds 0.001%, the toughness deteriorates. Therefore, when it contains B, the content shall be 0.001% or less. In order to suppress intergranular fracture, the B content is preferably 0.0005% or more.

[Ca: 0.01% or less]
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 may be deteriorated and toughness may be lost, and the Ca content is preferably 0.01% or less. More preferably, the Ca content is 0.003% or less. In order to improve toughness, the Ca content is preferably 0.0003% or more.

[Mg: 0.01% or less]
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%, the cleanliness may be deteriorated and the toughness may be lost, and the Mg content is preferably 0.01% or less. More preferably, the Mg content is 0.003% or less. In order to improve toughness, the Mg content is preferably 0.0002% or more.

[Rare earth element (REM): 0.05% or less]
The rare earth element (REM), like Ca, brings about the effect of spheroidization of inclusions and has the effect of improving toughness, and may be contained as necessary. However, if the content exceeds 0.05%, the cleanliness may be deteriorated and the toughness may be lost. The content of REM is preferably 0.05% or less. More preferably, the REM content is 0.003% or less. In order to improve toughness, the rare earth element (REM) content is preferably 0.0002% or more, and more preferably 0.0003%. 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.

[Carbide coverage of austenite grain boundaries contained in high Mn steel: 30% or less]
The metal structure of the high Mn steel material of the present invention is austenite. In the present invention, since heat treatment such as solution treatment is not performed after hot rolling, carbides are precipitated at austenite crystal grain boundaries (austenite grain boundaries). In order to impart sufficient low-temperature toughness as a low-temperature material with a high Mn-based steel material, it is important to control the above-described coverage to 30% or less. In high-Mn steel, fine carbides are generated mainly at the austenite grain boundaries, but these are hard phases and can be the starting point of fracture, so it is necessary to control the carbide coverage. The lower limit of the carbide coverage is preferable, but it may be 1% or more, or 5% or more. The austenite grain boundary carbide coverage in the steel can be determined by structural observation.

  As described above, the high Mn steel material according to the present invention can be used in a low temperature range without performing heat treatment after rolling by controlling carbides at the austenite grain boundaries.

(B) Production conditions Generally, high-Mn steel is inferior in hot workability to carbon steel and low alloy steel, and therefore, it is necessary to perform rolling 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 the steel ingot is important.

[Heating temperature: 950-1200 ° C]
First, when the heating temperature of the steel slab or the steel ingot is less than 950 ° C., the deformation resistance during rolling is large, and an excessive load is applied to the rolling mill. On the other hand, when heated to a high temperature exceeding 1200 ° C., there is a concern about a decrease in yield due to oxidation of the surface, and the austenite grains become coarse, so that even after hot rolling, they cannot be easily refined, It shall be 1200 degrees C or less.

[Cumulative rolling reduction: 30% or more in a temperature range of 800 to 1100 ° C.]
After heating the steel slab or the steel ingot, it is necessary to perform hot rolling with a cumulative rolling reduction of 30% or more in a temperature range of 800 to 1100 ° C. This is to destroy the cast structure of the steel slab or the steel ingot, and to make the austenite grains in the steel material finer and flattened. In order to further enhance the effect of hot rolling with a cumulative rolling reduction of 30% or more in a temperature range of 800 to 1100 ° C. and obtain fine austenite crystal grains, the rolling finishing temperature of hot rolling is important. The cumulative rolling reduction is obtained by dividing the difference between the plate thickness at 1100 ° C. and the plate thickness at 800 ° C. by the plate thickness at 1100 ° C., and is expressed as a percentage. When hot rolling is finished at over 800 ° C., the plate thickness at 800 ° C. is calculated as the plate thickness after rolling.

[Rolling finishing temperature: 750-950 ° C]
The rolling finishing temperature of hot rolling needs to be 750 to 950 ° C. 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.

[Cooling rate: 750 to 600 ° C. at 5 ° C./s or higher, then allowed to cool]
Then, in order to suppress the production | generation of a precipitate and to improve low temperature toughness, the accelerated cooling which makes the cooling rate of the temperature range from 750 degreeC to 600 degreeC 5 degrees C / s or more is performed. When the cooling rate is less than 5 ° C./s, the effect of accelerated cooling is not sufficient, and in particular, the carbide coverage of the austenite grain boundaries is increased. 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. or higher. 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. After accelerating cooling is stopped, it is allowed to cool as it is and reheating treatment such as solution treatment is not performed, but accelerated cooling may be continued to a temperature of 600 ° C. or lower. As a result, a steel sheet excellent in both strength and fracture resistance can be obtained. This steel sheet has properties suitable for the tank material in the LNG tank.

  Hereinafter, the present invention will be described in more detail by way of examples.

  Using steel slabs of steel types 1 to 38 having the chemical composition and parameter X shown in Table 1, production conditions shown in Table 2 (heating temperature, cumulative rolling reduction in the temperature range of 800 to 1100 ° C, rolling finish temperature, from 750 ° C A high Mn steel material having a thickness of 10 to 50 mm was produced by variously controlling the cooling rate to 600 ° C.). Then, by observing the metal structure with an optical microscope to measure the carbide coverage of the austenite grain boundaries contained in the steel (measured values are shown in Table 2), as the base material properties, tensile properties (yield strength, tensile strength) ), Charpy impact properties were measured. The obtained measured values are shown in Table 2. The tensile properties were evaluated by conducting a tensile test at room temperature in accordance with JIS Z 2241. The Charpy impact property was evaluated by conducting a Charpy impact test at -196 ° C in accordance with JIS Z 2242.

  From Table 2, it can be seen that the high Mn steel materials according to the examples of the present invention are hot-rolled and excellent in both base material strength and toughness, and are excellent as low-temperature materials.

  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 strength and the Charpy characteristics.

  The high-Mn steel material according to the present invention can be provided as it is without being subjected to heat treatment after hot rolling, and can be provided as it is, such as aluminum alloy, Ni-based austenitic stainless steel, 9% It can be used as a substitute for Ni steel, contributes to saving of Ni resources, and enables the tank construction cost to be reduced.

Claims (3)

In mass%, C: 0.25 to 0.75%, Si: 0.05 to 1.0%, Mn: more than 20% to 35% or less, Ni: 0.1% or more and less than 7.0%, Cr : 0.1% or more and less than 8.0%, Al: 0.005 to 0.10%, N: 0.005% or more and less than 0.05%, P: 0.04% or less, S: 0 0.02% or less, the balance being Fe and impurities, the parameter X (%) defined by the following formula (1) is 6.0 to 15.0%, and the carbide coverage at the grain boundary is A high Mn steel material characterized by being 30% or less.
X (%) = C + 10 × Si + 2 × Ni Equation (1) where C, Si and Ni are the contents of each element in the steel (unit: mass) %).   Instead of a part of Fe, by mass%, Cu: 3.0% or less, Mo: 3.0% or less, Nb: 0.5% or less, V: 0.5% or less, Ti: 0.5% In the following, B: 0.001% or less, Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.05% or less are included, The high Mn steel material according to claim 1.   After heating the steel slab or steel ingot having the chemical composition according to claim 1 or 2 to 950 to 1200 ° C, the cumulative rolling reduction in the temperature range of 800 to 1100 ° C is 30% or more and the rolling finishing temperature is A method for producing a high Mn steel material, characterized in that after hot rolling at 750 to 950 ° C., the temperature range from 750 ° C. to 600 ° C. is cooled at a cooling rate of 5 ° C./s or more and left to cool. .