KR20190134704A - High Mn steel and its manufacturing method - Google Patents

Abstract

In the high Mn steel excellent in the low-temperature toughness of the base material and the weld heat-affected zone after welding, a method for providing more excellent ductility is proposed. In mass%, C: 0.30 to 0.90%, Si: 0.05 to 1.0%, Mn: 15 to 30%, P: 0.030% or less, S: 0.0070% or less, Al: 0.01 to 0.07%, Cr: 0.5 to 7.0% , N: 0.0050 to 0.0500%, O: 0.0050% or less, and the content of Ti and Nb is suppressed to less than 0.005%, respectively, and the balance has a component composition of Fe and inevitable impurities, and austenitic as a base phase. and has a microstructure, the area fraction of the non-metallic inclusions in the art microstructure less than 5.0%, a yield strength of at least 400㎫, and also equal to or higher than the absorption energy (vE _-196) 100J.

Description

High Mn steel and its manufacturing method

TECHNICAL FIELD This invention relates to the high Mn steel which is especially excellent in toughness at low temperature, and its manufacturing method suitable for providing to structural steel used in cryogenic environments, such as a tank for liquefied gas storage, for example.

In order to use a hot rolled steel sheet for a structure for liquefied gas storage, since the use environment becomes extremely low temperature, the steel sheet is required to be excellent in toughness in addition to high strength. For example, when using a hot rolled steel sheet for the storage of liquefied natural gas, it is necessary to ensure excellent toughness at the boiling point of -liquefied natural gas: -164 degrees C or less. If the low-temperature toughness of steel is inferior, there exists a possibility that safety as a cryogenic low temperature structure may not be maintained, and the demand for the improvement of low-temperature toughness with respect to the steel material applied is strong.

In response to this demand, austenitic stainless steel, 9% Ni steel, or 5000-based aluminum alloy, in which austenite which is not brittle at cryogenic temperature, is used as a steel sheet structure, has been used. However, in view of high alloying cost and manufacturing cost, there is a demand for steel materials having low cost and excellent cryogenic toughness.

Therefore, as a new steel material replacing the conventional cryogenic steel, it is proposed in Patent Document 1, for example, to use a high Mn steel containing a large amount of Mn, which is a relatively inexpensive austenite stabilizing element, as a structural steel in a cryogenic environment. It is.

Patent Literature 1 proposes a technique of controlling the austenite grain size to an appropriate size to avoid carbides generated at the grain boundaries from becoming a starting point of fracture or propagation of cracks. By this technique, it is possible to provide high Mn steel excellent in low temperature toughness of the base material after welding and the weld heat affected zone.

Japanese Laid-Open Patent Publication No. 2016-196703

By the way, it is important to ensure ductility in addition to low-temperature toughness in the use of the above-described liquefied gas storage structure. That is, when producing such a structure, the steel materials to be used need to have high workability, and excellent ductility is required for this kind of use, but nothing is verified by the technique of patent document 1 about this ductility. In addition, although the thickness of the high Mn steel material of patent document 1 is about 15-50 mm, for example, in use, such as a long paper, less than 15 mm, especially 10 mm or less of thickness is calculated | required. When manufacturing such a thin plate, in the method of performing accelerated cooling after finishing hot rolling illustrated in Patent Document 1 at 950 ° C. or higher, warpage and deformation easily occur in the steel sheet obtained, and the shape An extra process such as calibration is required, which impairs productivity.

Therefore, an object of the present invention is to propose a method for imparting more excellent ductility in high Mn steel excellent in low temperature toughness of the base metal and the weld heat affected zone. Moreover, an object of this invention is to propose about the method which can manufacture such a thin sheet of high Mn steel, without the occurrence of a curvature or a deformation | transformation. Here, the said "excellent low temperature toughness" means that absorption energy # E-196 _degreeC of the Charpy impact test in _{-196 degreeC} is 100J or more.

MEANS TO SOLVE THE PROBLEM In order to achieve the said subject, the present inventors earnestly researched about the various factors which determine the component composition and manufacturing method of a steel plate with respect to high Mn steel, and acquired the recognition of the following a-c.

a. In high Mn austenitic steels, brittle fracture does not occur even at cryogenic temperatures, and the fracture occurs from grain boundaries. From this point of view, it is effective to improve the low-temperature toughness of high Mn steel by reducing the grain boundaries as a starting point of fracture and propagation path due to coarsening of crystal grains.

b. In addition, it has been newly discovered that nonmetallic inclusions are a starting point of fracture or a path of crack propagation and adversely affect low temperature toughness and ductility. Therefore, by appropriately controlling the amount of Cr added to the high Mn steel and suppressing the amount of Ti and Nb that are inevitably mixed, the increase in the grain boundary which becomes the starting point of fracture and excessive generation of nonmetallic inclusions are avoided.

c. On the other hand, when the grain size is simply coarsened, the yield strength is lowered. Moreover, when manufacturing the thin material whose plate | board thickness is less than 15 mm, curvature and a deformation | transformation tend to remain in the obtained steel plate. Therefore, in order to ensure sufficient yield strength as structural steel and not to bend or deform the steel sheet, it is necessary to appropriately control the hot rolling conditions at the time of steel sheet production.

The present invention has been made by further examining the above recognition, and the gist thereof is as follows.

1.in mass%,

C: 0.30% or more and 0.90% or less,

Si: 0.05% or more and 1.0% or less,

Mn: 15% or more and 30% or less,

P: 0.030% or less,

S: 0.0070% or less,

Al: 0.01% or more and 0.07% or less,

Cr: 0.5% or more and 7.0% or less,

N: 0.0050% or more and 0.0500% or less,

O: 0.0050% or less

Containing Ti and Nb in an amount of less than 0.005%, and the balance has a component composition of Fe and unavoidable impurities;

Has a microstructure with an austenite matrix (基地相), and the area fraction of the non-metallic inclusions in the art microstructure less than 5.0%, the yield strength is at least 400㎫, also the absorption energy (vE _-196) 100J High Mn steel which is ideal.

Here, the said nonmetallic inclusion is a nonmetallic interference | inclusion in the structure test of JIS G0202, Specifically, it means the A type | system | group inclusion, B type | system | group inclusion, and C type inclusion which were described in the said standard.

2. The said component composition is further in mass%,

Cu: 0.01% or more and 1.00% or less,

Ni: 0.01% or more and 1.00% or less,

Mo: 2.0% or less,

V: 2.0% or less,

W: 2.0% or less,

Ca: 0.0005% or more and 0.0050% or less,

Mg: 0.0005% or more and 0.0050% or less and

REM: 0.0010% or more and 0.0200% or less

The high Mn steel of the said 1 containing 1 type or 2 or more types chosen from among.

3. The steel material having the component composition according to the above 1 or 2 is heated to a temperature range of 1100 ° C or more and 1300 ° C or less, hot rolling in which the finish rolling finish temperature is 800 ° C or more and less than 950 ° C, and then (finish rolling is finished The manufacturing method of the high Mn steel which performs the cooling process whose average cooling rate from the temperature of temperature -100 degreeC) or more to the temperature range of 300 degreeC or more and 650 degrees C or less is 1.0 degreeC / s or more.

Here, each said temperature range is surface temperature of a steel raw material or a steel plate, respectively.

4. The method for producing high Mn steel according to the above 3, wherein the cooling treatment is performed, followed by heating and cooling to a temperature range of 300 ° C or higher and 650 ° C or lower.

According to the present invention, a high Mn steel excellent in low temperature toughness and ductility can be provided, and when the high Mn steel is welded, both the base material and the welding heat affected part are excellent in low temperature toughness. Therefore, the high Mn steel of the present invention greatly contributes to the improvement of the safety and the life of the steel structure used in the cryogenic environment, such as a tank for liquefied gas storage, and brings a special effect in the industry. Moreover, in the manufacturing method of this invention, since the fall of productivity and the increase of a manufacturing cost do not occur, the method excellent in economic efficiency can be provided.

(Form to carry out invention)

Hereinafter, the high Mn steel of this invention is demonstrated in detail.

[Component Composition]

First, the component composition of the high Mn steel of this invention and the reason for limitation are demonstrated. In addition, "%" display in a component composition shall mean "mass%" unless there is particular notice.

C: 0.30% or more and 0.90% or less

C is an inexpensive austenite stabilizing element and is an important element for obtaining austenite. In order to obtain the effect, C needs to contain 0.30% or more. On the other hand, when it contains exceeding 0.90%, Cr carbide will generate | occur | produce excessively and low-temperature toughness will fall. For this reason, C amount may be 0.30% or more and 0.90% or less. In particular, 0.36% is preferable and, as for a minimum, from a viewpoint of stabilizing austenite, 0.40% is more preferable. In addition, from a viewpoint of suppressing the fall of low-temperature toughness, an upper limit becomes like this. Preferably it is 0.80%, More preferably, it is 0.66%. Preferable content of C amount can combine these upper limits and lower limits, for example, Preferably it is 0.36% or more and 0.80% or less, More preferably, you may be 0.40% or more and 0.80% or less.

Si: 0.05% or more and 1.0% or less

Si acts as a deoxidizer and is not only required in steelmaking but also has an effect of solidifying the steel sheet and increasing the strength of the steel sheet by solid solution strengthening. In order to obtain such an effect, Si needs to contain 0.05% or more. On the other hand, when it contains exceeding 1.0%, weldability will deteriorate. For this reason, Si amount is made into 0.05% or more and 1.0% or less. In particular, from the viewpoint of obtaining a high strength steel sheet, the lower limit is preferably 0.07%, more preferably 0.23%, still more preferably 0.26%, and even more preferably 0.51%. From the standpoint of suppressing deterioration of weldability, the upper limit is preferably 0.8%, more preferably 0.7%, still more preferably 0.6%, even more preferably 0.5%. Preferable content of Si can combine these upper limit and lower limit, for example, Preferably you may be 0.07% or more and 0.8% or less, 0.23% or more and 0.7% or less, More preferably, you may be 0.26% or more and 0.6% or less. Moreover, preferable content of Si amount is 0.07% or more and 0.5% or less.

Mn: 15.0% or more and 30.0% or less

Mn is a relatively inexpensive austenite stabilizing element. In the present invention, it is an important element for achieving both strength and cryogenic toughness. In order to acquire the effect, Mn needs to contain 15.0% or more. On the other hand, even if it contains exceeding 30.0%, the effect of improving cryogenic toughness is saturated, and it raises an alloy cost. In addition, weldability and cutability deteriorate. In addition, segregation is encouraged and stress corrosion cracking is encouraged. For this reason, Mn amount may be 15.0% or more and 30.0%. In particular, from the viewpoint of stabilizing austenite, the lower limit is preferably 16.0%, more preferably 18.0%, and even more preferably 19.0%. In addition, from a viewpoint of suppressing the fall of low-temperature toughness, an upper limit becomes like this. Preferably it is 29.0%, More preferably, it is 28.0%. Preferable content of Mn amount can combine these upper limits and lower limits, for example, Preferably they are 16.0% or more and 29.0% or less, More preferably, you may be 18.0% or more and 28.0% or less.

P: 0.030% or less

When P contains more than 0.030%, it will segregate in a grain boundary, and will become a starting point of a stress corrosion cracking. For this reason, it is preferable to make 0.030% an upper limit and to reduce as much as possible. Therefore, P is made into 0.030% or less. Moreover, 0.028% or less is preferable and, as for an upper limit from a viewpoint of reducing the origin of stress corrosion cracking, 0.024% or less is more preferable. In addition, since excessive P reduction raises refining cost and becomes economically disadvantageous, it is preferable to set it as 0.002% and it is more preferable to set it as 0.005%.

S: 0.0070% or less

Since S deteriorates low-temperature toughness and ductility of a base material, it is preferable to make 0.0070% an upper limit and to reduce as much as possible. Therefore, S is made into 0.0070% or less. Moreover, as for an upper limit, 0.0060% or less is preferable from a viewpoint of suppressing low-temperature toughness and ductility deterioration of a base material. In addition, since excessive reduction of S raises refining cost and becomes economically disadvantageous, it is preferable to make a lower limit into 0.001% or more. It is preferable to make the range of S amount into 0.0020% or more and 0.0060% or less.

Al: 0.01% or more and 0.07% or less

Al acts as a deoxidizer and is most widely used in the molten steel deoxidation process of steel sheets. In order to acquire such an effect, Al needs to contain 0.01% or more. On the other hand, when it contains exceeding 0.07%, since it will mix in a weld metal part at the time of welding, and will degrade the toughness of a weld metal, it is made into 0.07% or less. For this reason, Al amount may be 0.01% or more and 0.07% or less. In particular, from a viewpoint of obtaining the effect as a deoxidizer, 0.02% is preferable, 0.046% is more preferable, and its 0.052% is still more preferable. From the viewpoint of the toughness of the weld metal, the upper limit is preferably 0.065%, more preferably 0.06%. Preferable content of Mn amount can combine these upper limits and lower limits, For example, Preferably you may be 0.02% or more and 0.06% or less.

Cr: 0.5% or more and 7.0% or less

Cr is an element effective in stabilizing austenite by the addition of an appropriate amount and improving the cryogenic toughness and the base metal strength. In order to acquire such an effect, Cr needs to contain 0.5% or more. On the other hand, when it contains exceeding 7.0%, low temperature toughness and stress corrosion cracking resistance will fall by formation of Cr carbide. For this reason, Cr amount may be 0.5% or more and 7.0% or less. In particular, from the viewpoint of improving the cryogenic toughness and the base metal strength, the lower limit is preferably 1% or more, more preferably 1.2%, and even more preferably 2.0%. From the viewpoint of low temperature toughness and stress corrosion cracking resistance, the upper limit is preferably 6.7% or less, more preferably 6.5% or less, still more preferably 6.0%. Preferable content of Mn amount can combine these upper limit and lower limit, for example, Preferably it is 1.0% or more and 6.7% or less, More preferably, you may be 1.2% or more and 6.5% or less. Moreover, in order to further improve stress corrosion cracking, 2.0% or more and 6.0% or less are more preferable.

N: 0.0050% or more and 0.0500% or less

N is an austenite stabilizing element and is an element effective for improving cryogenic toughness. In order to acquire such an effect, N requires 0.0050% or more of content. On the other hand, when it contains exceeding 0.0500%, nitride or carbonitride will coarsen and toughness will fall. For this reason, N amount may be 0.0050% or more and 0.0500% or less. In particular, from the viewpoint of improving the cryogenic toughness, the lower limit is preferably 0.0060% or more, more preferably 0.0355%, even more preferably 0.0810%. In addition, from a viewpoint of suppressing the fall of toughness, an upper limit becomes like this. Preferably it is 0.0450% or less, More preferably, you may be 0.0400% or less. Preferable content of N amount can combine these upper limits and lower limits, for example, Preferably you may be 0.0060% or more and 0.0400% or less.

O: 0.0050% or less

O deteriorates cryogenic toughness by the formation of an oxide. For this reason, O is made into 0.0050% or less of range. From a viewpoint of suppressing the fall of toughness, an upper limit becomes like this. Preferably it is 0.0045% or less. Moreover, as for the minimum of O amount, 0.0023% or more is preferable. Preferable content of O amount can combine these upper limits and lower limits, For example, Preferably you may be 0.0023% or more and 0.0050% or less.

Inhibit the contents of Ti and Nb to less than 0.005%, respectively

Ti and Nb form high melting point carbonitride in steel to suppress coarsening of crystal grains, and as a result, it becomes a starting point of fracture and a path of crack propagation. In particular, in high Mn steels, it is necessary to suppress them intentionally because they increase the low-temperature toughness and hinder the control of the structure to improve the ductility. That is, Ti and Nb are components which are inevitably mixed from raw materials, etc., and it is common to mix Ti in 0.005% or more and 0.010% or less, and Nb: 0.005% or more and 0.010% or less. Therefore, according to the method mentioned later, it is necessary to avoid the unavoidable mixing of Ti and Nb, and to suppress content of Ti and Nb to less than 0.005%, respectively. By suppressing the contents of Ti and Nb to less than 0.005%, respectively, the adverse effects of the carbonitrides can be eliminated and excellent low-temperature toughness and ductility can be ensured. Therefore, from the viewpoint of the excellent low-temperature toughness and ductility, the content of Ti and Nb is preferably 0.004% or less, more preferably 0.003% or less, respectively.

Remainder other than the above-mentioned component is iron and an unavoidable impurity. Examples of the unavoidable impurity here include H and the like, which can be allowed to be 0.01% or less in total.

[group]

Microstructure Based on Austenitic

If the crystal structure of the steel is a body-centered cubic structure (bcc), the steel is likely to cause brittle fracture in a low temperature environment, and therefore is not suitable for use in a low temperature environment. Here, assuming the use in a low temperature environment, it is essential that the known phase of the steel material is an austenitic structure whose crystal structure is face-centered cubic structure (fcc). In addition, "austenitic as a base phase" means that an austenite phase is 90% or more by area ratio. Low-temperature toughness can be ensured by making an austenite phase into 90% or more by area ratio. The remainder other than the austenite phase is ferrite or martensite phase. However, when epsilon martensite is produced, even if it is a small amount, it will cause low-temperature toughness. Therefore, as a microstructure based on austenite according to the present invention, a tissue having substantially no epsilon martensite phase is preferable. That is, in order to ensure low-temperature toughness, it is preferable to make volume fraction of (epsilon) martensite less than 1.0%, It is more preferable to set it as less than 0.5%, It is more preferable to set it as less than 0.1%.

Area fraction of nonmetallic inclusions: less than 5.0%

In the non-metallic inclusions, A type means sulfides, B type clusters, and C type means inclusions taking the form of particulate oxides. When these nonmetallic inclusions exist in a large amount in steel, it becomes a starting point of breakdown, resulting in a decrease in cryogenic toughness and deterioration of ductility. For this reason, these inclusions need to be suppressed to 5% or less in area fraction as a total amount. Preferably, it is suppressed to 4% or less. For that purpose, the above-mentioned component composition control and implementation of the manufacturing method mentioned later are required.

When the austenite phase is 90% or more in area ratio and less than 5.0% in area fraction of nonmetallic inclusions, cryogenic toughness can be ensured and steel having good ductility can be provided.

By making the above requirements essential, the characteristic made into the objective of this invention is obtained. For example, when high Mn steel is used for the welding treatment, particularly low temperature toughness of the weld heat affected zone becomes a problem, when using high Mn steel that satisfies the above requirements, the microstructure of the weld heat affected zone becomes austenite. The austenite has a known shape, and the particle size of the austenite becomes 50 µm or more in a circle equivalent diameter, thereby ensuring low temperature toughness even in the weld heat affected zone.

In other words, coarsening of crystal grain size is effective to secure low-temperature toughness of austenitic steel. This also applies to the weld heat affected zone. For example, in order to obtain a value of 100 J or more as the absorption energy of the Charpy impact test at −196 ° C., the maximum crystal grain size of the microstructure is required to be 50 μm or more. This can be achieved by using satisfactory high Mn steel.

In the present invention, in order to further improve the strength and low-temperature toughness, the following elements may be included as necessary in addition to the above essential elements.

Cu: 0.01% or more and 1.00% or less, Ni: 0.01% or more and 1.00% or less, Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more 0.0050% or less or REM: 0.0010% or more and 0.0200% or less, 1 type or 2 or more types.

Cu: 0.01% or more and 1.00% or less, Ni: 0.01% or more and 1.00% or less, Mo, V, W: 2.0% or less

Cu, Ni, Mo, V, and W contribute to stabilization of austenite or contribute to improvement of base material strength. In order to acquire such an effect, Cu and Ni contain 0.01% or more, and Mo, V, and W contain 0.001% or more. On the other hand, when Cu and Ni are respectively contained in excess of 1.00% and Mo, V and W in excess of 2.0%, coarse carbonitrides are formed, which may be a starting point of destruction. Pressure. For this reason, when it contains these alloying elements, 1.00% or less is preferable in Cu and Ni, respectively, and 2.0% or less is preferable in Mo, V, and W, respectively. As for Cu amount and Ni amount, it is more preferable that they are 0.05% or more and 0.70% or less, respectively. Moreover, it is more preferable that Mo amount, V amount, and W amount are 0.003% or more and 1.7% or less, respectively.

Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, REM: 0.0010% or more and 0.0200% or less

Ca, Mg, and REM are elements useful for controlling the shape of inclusions and may be contained as necessary. The shape control of inclusions means that the sulfide-based inclusions which have been expanded are used as particle inclusions. Through morphology control of this inclusion, ductility, toughness, and sulfide stress corrosion cracking resistance are improved. In order to acquire such an effect, it is preferable to contain Ca and Mg 0.0005% or more and REM 0.0010% or more. On the other hand, when a large amount of any element is included, the amount of non-metallic inclusions increases, so that ductility, toughness, and sulfide stress corrosion cracking resistance may decrease. In addition, it may be disadvantageous economically.

For this reason, when it contains Ca and Mg, it is 0.0005% or more and 0.0050% or less, respectively, and when it contains REM, you may be 0.0010% or more and 0.0200% or less. Preferably, the amount of Ca is made 0.0010% or more and 0.0040% or less, the amount of Mg is made 0.0010% or more and 0.0040% or less, and the amount of REM is made 0.0020% or more and 0.0150% or less.

The high Mn steel which concerns on this invention can melt the molten steel which has said component composition by well-known solvent methods, such as a converter and an electric furnace. Further, secondary refining may be performed in a vacuum degassing furnace. In that case, in order to limit Ti and Nb which interfere with proper structure control to the above-mentioned range, it is necessary to avoid the inevitable mixing from raw materials etc., and to take measures to reduce these contents. For example, by lowering the basicity of slag in the refining step, these alloys are concentrated by slag and discharged, and the concentration of Ti and Nb in the final slab product is reduced. Alternatively, oxygen may be blown and oxidized, and the alloy of Ti and Nb may be separated at the time of reflux, for example. Then, it is preferable to set it as steel raw materials, such as a slab of a predetermined dimension, by well-known casting methods, such as a continuous casting method or a lump-fragment rolling method.

Furthermore, the manufacturing conditions for composition of the said steel material into the steel material excellent in low-temperature toughness are prescribed | regulated.

Steel material heating temperature: 1100 ℃ or more and 1300 ℃ or less

In order to make the crystal grain diameter of the micro structure of steel materials coarse, the heating temperature before hot rolling shall be 1100 degreeC or more. In addition, when the lower limit of the steel material heating temperature is less than 1100 ° C, the amount of the nonmetallic inclusions in the steel increases, causing the cryogenic toughness and ductility deterioration by the nonmetallic inclusions in the steel. However, since melt | dissolution may be started when it exceeds 1300 degreeC, the upper limit of heating temperature shall be 1300 degreeC. Temperature control here is based on the surface temperature of steel materials.

Finish rolling finish temperature: More than 800 degrees Celsius and less than 950 degrees Celsius

After heating a steel ingot or steel piece, hot rolling is performed. In order to produce coarse grains, it is desirable to increase the cumulative reduction ratio at high temperature. However, when finishing in the temperature range of 950 degreeC or more, a grain size will become excessively coarse and a desired yield strength will not be obtained. Therefore, the final finishing rolling of 1 pass or more is needed at less than 950 degreeC. On the other hand, when hot rolling is performed at low temperature, the microstructure becomes fine, and excessive work deformation enters, resulting in low temperature toughness. Therefore, the minimum of rolling finish temperature shall be 800 degreeC.

Average cooling rate from temperature more than (Finish rolling finish temperature -100 degreeC) to the temperature range of 300 degreeC or more and 650 degrees C or less: 1.0 degreeC / s or more

After completion of hot rolling, cooling is performed quickly. Slow cooling of the steel sheet after hot rolling promotes the formation of precipitates, resulting in deterioration of low temperature toughness. By cooling at a cooling rate of 1 ° C / s or more, generation of these precipitates can be suppressed. In addition, excessive cooling causes the steel sheet to deform, thereby lowering productivity. For that purpose, the upper limit of cooling start temperature shall be 900 degreeC. For the above reasons, cooling after hot rolling sets the average cooling rate of the steel plate surface to the temperature range of 300 degreeC or more and 650 degrees C or less at the temperature ((finish rolling finish temperature -100 degreeC)) or more to 1.0 degreeC / s or more. In addition, in a steel plate having a sheet thickness of 10 mm or less, the cooling rate is 1 ° C / s or more even in air cooling. When the sheet thickness is 10 mm or less, cooling by air cooling can prevent the deformation of the steel sheet.

Furthermore, after performing the said cooling process as needed, you may add the process which heats and cools to the temperature range of 300 degreeC or more and 650 degrees C or less. That is, you may perform a tempering process for the purpose of adjusting the strength of a steel plate.

Example

Hereinafter, an Example demonstrates this invention in detail. In addition, this invention is not limited to a following example.

By the converter-ladle refinement-continuous casting method, the steel slab used as the component composition shown in Table 1 was produced. Subsequently, the obtained steel slab was charged into a heating furnace and heated to 1150 ° C to obtain a steel sheet having a thickness of 10 to 30 mm by hot rolling. Tensile characteristics and toughness were performed about the steel plate by the following method.

(1) tensile test characteristics

From each obtained steel plate, the JIS No. 5 tensile test piece was extract | collected, the tensile test was performed based on the specification of JIS Z 2241 (1998), and the tensile test characteristic was investigated. In the present invention, the yield strength of 400 MPa or more and the tensile strength of 800 MPa or more were determined to be excellent in tensile properties. In addition, it was determined that 30% or more of total elongation at breakage was excellent in ductility.

(2) low temperature toughness

From the direction perpendicular to the rolling direction of the plate thickness 1/4 position of each steel plate exceeding plate thickness 20mm, or the plate thickness 1/2 position of each steel plate of plate thickness 20mm or less, JIS Z 2202 (1998) Charpy V notched test pieces were taken in accordance with the standard, three Charpy impact tests were carried out on each steel sheet in accordance with JIS Z 2242 (1998), and the absorbed energy at -196 ° C was obtained. Evaluated. In this invention, the average value of three absorption energy (E- ₁₉₆ ) made 100J or more into excellent base material toughness.

The result obtained by the above is shown in Table 2.

The high Mn steel according to the present invention satisfies the above-described target performance (yield strength of the base material is 400 MPa or more, total elongation 30% or more at break, and low temperature toughness is 100 J or more as an average value of absorbed energy (ｖE- ₁₉₆ )). It was confirmed. On the other hand, in the comparative example beyond the scope of the present invention, any one or more of total elongation, yield strength and low temperature toughness does not satisfy the above-described target performance.

In addition, for the purpose of evaluating the impact absorption characteristics of the welded portion, the hot steel was subjected to a thermal cycle treatment under conditions of a peak temperature of 1400 ° C. and a cooling rate of 10 ° C./s to evaluate low temperature toughness. As shown in Table 2 together, the steel material according to the present invention showed excellent low-temperature toughness similarly to the base material. That is, with respect to the welding which gives the heat input of 0.5-5 kJ / cm, the maximum crystal grain size became 50 micrometers or more, and the absorption energy of the Charpy impact test in -196 degreeC obtained the value of 100 J or more.

This application claims the priority of Japanese Patent Application No. 2017-087702 (application for April 26, 2017), and takes in the whole indication of the said application here for reference.

Claims (4)

In mass%,
C: 0.30% or more and 0.90% or less,
Si: 0.05% or more and 1.0% or less,
Mn: 15.0% or more and 30% or less,
P: 0.030% or less,
S: 0.0070% or less,
Al: 0.01% or more and 0.07% or less,
Cr: 0.5-7.0%,
N: 0.0050% or more and 0.0500% or less,
O: 0.0050% or less
Containing Ti and Nb in an amount of less than 0.005%, respectively, the balance having a component composition of Fe and unavoidable impurities,
It has a microstructure which has austenite as a matrix, and the area fraction of the nonmetallic inclusion in this microstructure is less than 5.0%,
Yield strength of not less than 400㎫, also the absorption energy (vE _-196) 100J or more high-Mn steel. The method of claim 1,
The said component composition is further in mass%,
Cu: 0.01% or more and 1.00% or less,
Ni: 0.01% or more and 1.00% or less
Mo: 2.0% or less,
V: 2.0% or less,
W: 2.0% or less,
Ca: 0.0005% or more and 0.0050% or less,
Mg: 0.0005% or more and 0.0050% or less and
REM: 0.0010% or more and 0.0200% or less
High Mn steel containing 1 type (s) or 2 or more types chosen from among. The steel raw material which has the component composition of Claim 1 or 2 is heated to the temperature range of 1100 degreeC or more and 1300 degrees C or less, hot rolling which finish finishing temperature is 800 degreeC or more and less than 950 degreeC, and after that (finish rolling The manufacturing method of the high Mn steel which carries out the cooling process whose average cooling rate from the temperature of finishing temperature -100 degreeC) to the temperature range of 300 degreeC or more and 650 degrees C or less is 1.0 degreeC / s or more. The method of claim 3,
The manufacturing method of the high Mn steel which heats and cools to the temperature range of 300 degreeC or more and 650 degrees C or less after performing the said cooling process.