JP6954475B2 - High Mn steel and its manufacturing method - Google Patents

Description

The present invention relates to high Mn steel suitable for use in structures used in a cryogenic environment, such as a tank for a liquefied gas storage tank, and a method for producing the same.

Since the environment in which the liquefied gas storage tank is used is extremely low, the steel plate used for this type of structure is required to have high strength and excellent toughness at extremely low temperatures. For example, when a hot-rolled steel sheet is used in a storage tank for liquefied natural gas, it is necessary to ensure excellent toughness at an extremely low temperature of liquefied natural gas boiling point: -164 ° C. or lower. If the low temperature toughness of the steel material is inferior, it may not be possible to maintain the safety of the structure for the cryogenic storage tank. Therefore, there is a strong demand for improving the low temperature toughness of the applied steel material.

In response to this requirement, conventionally, austenitic stainless steel, 9% Ni steel, or 5000-based aluminum alloy having austenite as the main structure of the steel sheet, which does not show brittleness at extremely low temperatures, has been used. However, since these steels and alloys have high alloy costs and manufacturing costs, there is a demand for steel materials that are inexpensive and have excellent low temperature toughness.

Therefore, as a new steel material to replace the conventional cryogenic steel, a high Mn steel which is relatively inexpensive and contains a large amount of Mn, which is an austenite stabilizing element, should be used as a structural steel in a cryogenic environment. Is proposed in Patent Document 1 and Patent Document 2.

That is, Patent Document 1 proposes to control the carbide coverage of austenite grain boundaries. Further, Patent Document 2 proposes to control the austenite crystal grain size by adding a carbide coating and Mg, Ca, and REM.

Japanese Unexamined Patent Publication No. 2016-84529 Japanese Unexamined Patent Publication No. 2016-196703

The austenite steel used as the ultra-low temperature steel described in Patent Documents 1 and 2 described above has a large work hardening from the initial stage of deformation during tensile deformation until the maximum stress (tensile strength) is reached, and is plastic. Since it has excellent deformability, it has excellent ductility until the middle stage of deformation. On the other hand, the deformation performance in the latter stage of deformation after the stress measured in the tensile test reaches the maximum (tensile strength) is also an important characteristic as a structural member. This is because the deformation performance in the latter stage of deformation is the performance at the final stage leading to the final fracture. From this point of view, it is necessary to secure sufficient ductility in the latter stage of deformation, especially the drawing value, and from the viewpoint of ensuring the ductility of high-strength steel, a drawing value of 50% or more is desirable.

An object of the present invention is to provide a high Mn steel having excellent ductility as well as high strength and excellent low temperature toughness and a method for producing the same. Here, the "high strength" means having a yield strength of 400 MPa or more and a tensile strength of 800 MPa or more at room temperature. Further, the above-mentioned "excellent in low temperature toughness" is a steel plate having a thickness of 10 mm or more obtained by performing a Charpy impact test in accordance with JIS Z2242 (1998) at -196 ° C, and is a full-size test piece (10 mm × 10 mm). When using (× 55 mm), the Charpy impact absorption energy (average value) is 100 J or more (a steel plate with a plate thickness of less than 10 mm, and when a half-size test piece (10 mm × 5 mm × 55 mm) is used, the Charpy impact absorption energy (average value) is 100 J or more. , 20J or more by Charpy V notch half size test). The term "excellent in ductility" means having a diaphragm value of 50% or more.

As a result of diligent research on ways to solve the above problems for high Mn steels, the inventors have obtained the following findings.
That is, in high Mn steel, by controlling the morphology of Ca-based inclusions, the toughness can be improved and the ductility (drawing value) at the time of tensile deformation can be secured, and for that purpose, the Ca amount and It was found that it is effective to keep the balance with the amount of S within an appropriate range.
Further, during the production of the high Mn steel, the heating temperature of the steel material, the finish rolling end temperature, and the average cooling rate from a temperature of (finish rolling end temperature −100 ° C.) or higher to a temperature range of 300 ° C. or higher and 650 ° C. or lower are measured. It has been found that by limiting the temperature, the crystal grain size can be controlled, precipitation can be suppressed, and low temperature toughness can be improved.

By the way, when the high Mn steel contains Cu, Cu has an effect of improving the chloride stress corrosion cracking resistance in a low chloride concentration environment. However, Cu adversely deteriorates the chloride stress corrosion cracking resistance in a high chloride concentration environment. In response to this problem, the inventors have solved this problem by adding Ni in a high Mn steel containing Cu by optimizing the balance between the amount of Cu and the amount of Ni, even in a high chloride concentration environment. It has been found that excellent chloride stress corrosion cracking resistance can be exhibited. As a result, excellent chloride stress corrosion cracking resistance can be imparted to high Mn steel containing Cu regardless of the chloride concentration.
In the present specification, chloride stress corrosion cracking means that the tensile stress given to a high Mn steel is the tensile strength of the high Mn steel in a corrosive environment peculiar to the high Mn steel, particularly in an environment in which chloride ions are present. Even if it is less than or equal to that, it refers to a phenomenon in which high Mn steel cracks or breaks. The chloride stress corrosion cracking resistance indicates the resistance to the chloride stress corrosion cracking.

The present invention has been made by further studying the above findings, and the gist thereof is as follows.
1. 1. By mass%
C: 0.10% or more and 0.70% or less,
Si: 0.10% or more and 0.90% or less,
Mn: 20% 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: 1.8% or more and 7.0% or less,
Ni: 0.01% or more and less than 1.0%,
Ca: 0.0005% or more and 0.010% or less,
N: 0.0050% or more and 0.0500% or less,
O: 0.0050% or less,
It contains Ti: 0.0050% or less and Nb: 0.0050% or less, satisfies the following formula (1), has a component composition of Fe and unavoidable impurities in the balance, and a structure having austenite as a base phase. ,
Yield strength is 400 MPa or more,
A high Mn steel having an average value of Charpy impact absorption energy at -196 ° C of 100 J or more when a full-size test piece is used and 20 J or more when a half-size test piece is used.
Ca / S ≧ 1.0 ... (1)

2. The composition of the components is further increased by mass%.
Cu: less than 2.0%,
Mo: 2.0% or less,
V: 2.0% or less,
W: 2.0% or less,
The high Mn according to 1 above, which contains one or more selected from Mg: 0.0005% or more and 0.0050% or less and REM (rare earth metal): 0.0010% or more and 0.0200% or less. steel.

3. 3. After heating the steel material having the component composition according to 1 or 2 to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, hot rolling in which the finish rolling end temperature is 750 ° C. or higher and lower than 950 ° C. is performed, and then ( A method for producing high Mn steel, which performs a cooling treatment having an average cooling rate of 0.5 ° C./s or more from a temperature of -100 ° C. or higher to a temperature range of 300 ° C. or higher and 650 ° C. or lower.

4. By mass%
C: 0.10% or more and 0.70% or less,
Si: 0.10% or more and 0.90% or less,
Mn: 20% 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: 1.8% or more and 7.0% or less,
Cu: 0.2% or more and less than 2.0% Ni: 0.1% or more and less than 1.0%,
Ca: 0.0005% or more and 0.010% or less,
N: 0.0050% or more and 0.0500% or less,
O: 0.0050% or less,
Ti: 0.0050% or less and Nb: 0.0050% or less, satisfying the following formulas (1) and (2), the balance is the composition of Fe and unavoidable impurities, and the structure with austenite as the base phase. High Mn steel with and.
Ca / S ≧ 1.0 ... (1)
0 <Cu / Ni ≦ 2 ... (2)

5. After heating the steel material having the component composition described in 4 above to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, hot rolling is performed in which the finish rolling end temperature is 750 ° C. or higher and lower than 950 ° C., and then (finishing). A method for producing high Mn steel, which performs a cooling treatment having an average cooling rate of 0.5 ° C./s or more from a rolling end temperature (-100 ° C.) or higher to a temperature range of 300 ° C. or higher and 650 ° C. or lower.

According to one embodiment of the present invention, it is possible to provide a high Mn steel having high strength, excellent low temperature toughness particularly in an extremely low temperature region, and excellent ductility. Therefore, by using the high Mn steel of the present invention, it is possible to improve the safety and life of the steel structure used in the cryogenic environment such as the tank for the liquefied gas storage tank, which is a remarkable industrial effect. Play.
Further, according to another embodiment of the present invention, it is possible to provide a high Mn steel exhibiting excellent chloride stress corrosion cracking resistance regardless of the chloride concentration.

Hereinafter, the high Mn steel of the present invention will be described in detail.
[Ingredient composition]
First, the component composition of the high Mn steel of the present invention and the reason for its limitation will be described. The "%" indication in the component composition shall mean "mass%" unless otherwise specified.
C: 0.10% or more and 0.70% or less C is an inexpensive austenite stabilizing element and is an important element for obtaining austenite. In order to obtain the effect, it is necessary to contain C in an amount of 0.10% or more. On the other hand, if C is contained in an amount of more than 0.70%, Cr carbides are excessively generated and the low temperature toughness is lowered. Therefore, the amount of C is set to 0.10 to 0.70%. The amount of C is preferably 0.20% or more, preferably 0.60% or less, and more preferably 0.20% or more and 0.60% or less.

Si: 0.10% or more and 0.90% or less Si acts as a deoxidizing material and is not only necessary for steelmaking, but also has the effect of dissolving in steel and increasing the strength of the steel sheet by solid solution strengthening. .. In order to obtain these effects, it is necessary to contain Si at 0.10% or more. On the other hand, if Si is contained in an amount of more than 0.90%, the weldability is deteriorated and the low temperature toughness, particularly the toughness at extremely low temperature is low. Therefore, the amount of Si is set to 0.10% or more and 0.90% or less. The amount of Si is preferably 0.12% or more, preferably 0.70% or less, and more preferably 0.12% or more and 0.70% or less.

Mn: 20% or more and 30% or less Mn is a relatively inexpensive austenite stabilizing element. Mn is an important element in the present invention in order to achieve both strength and cryogenic toughness. In order to obtain the effect, it is necessary to contain Mn in an amount of 20% or more. On the other hand, even if Mn is contained in an amount of more than 30%, the effect of improving low temperature toughness is saturated and the alloy cost is increased. In addition, weldability and cutability deteriorate. Therefore, the amount of Mn is set to 20% or more and 30% or less. The amount of Mn is preferably 23% or more, preferably 28% or less, and more preferably 23% or more and 28% or less.

P: 0.030% or less If P is contained in excess of 0.030%, it segregates at the grain boundaries and becomes the starting point for stress corrosion cracking. Therefore, it is desirable to limit the amount of P to 0.030% and reduce it as much as possible. Therefore, the amount of P is set to 0.030% or less. It is desirable that the amount of P is 0.002% or more because excessive reduction of P increases the refining cost and is economically disadvantageous. The amount of P is preferably 0.005% or more, preferably 0.028% or less, and more preferably 0.024% or less. The amount of P is more preferably 0.005% or more and 0.028% or less.

S: 0.0070% or less Since S deteriorates the low temperature toughness and ductility of the base material, it is desirable to limit it to 0.0070% and reduce it as much as possible. Therefore, the amount of S is set to 0.0070% or less. Since excessive reduction of S increases the refining cost and is economically disadvantageous, it is desirable that the amount of S is 0.001% or more. The amount of S is preferably 0.0020% or more, preferably 0.0060% or less, and more preferably 0.0020% or more and 0.0060% or less.

Al: 0.01% or more and 0.07% or less Al acts as a deoxidizing agent and is most commonly used in the molten steel deoxidizing process of steel sheets. In order to obtain such an effect, it is necessary to contain Al in an amount of 0.01% or more. On the other hand, if Al is contained in an amount of more than 0.07%, it is mixed in the weld metal portion at the time of welding and deteriorates the toughness of the weld metal. Therefore, the amount of Al is set to 0.07% or less. Therefore, the amount of Al is set to 0.01% or more and 0.07% or less. The amount of Al is preferably 0.02% or more, preferably 0.06% or less, and more preferably 0.02% or more and 0.06% or less.

Cr: 1.8% or more and 7.0% or less Cr is an element that stabilizes austenite by adding an appropriate amount and is effective in improving low temperature toughness and base metal strength. In order to obtain such an effect, it is necessary to contain Cr in an amount of 1.8% or more. On the other hand, when Cr is contained in an amount of more than 7.0%, low temperature toughness and stress corrosion cracking resistance are lowered due to the formation of Cr carbides. Therefore, the amount of Cr is set to 1.8% or more and 7.0% or less. The amount of Cr is preferably 2.0% or more, preferably 6.7% or less, and more preferably 2.0% or more and 6.7% or less. Further, in order to improve the stress corrosion cracking resistance, the Cr amount is more preferably 2.0% or more and 6.0% or less.

Ni: 0.01% or more and less than 1.0% Ni has the effect of increasing the strength of the steel sheet by solid solution strengthening by solid solution in steel, and also has the effect of improving low temperature toughness, especially toughness at extremely low temperatures. Since it has, it is contained in an amount of 0.01% or more. On the other hand, it is desirable to minimize the amount of Ni from the viewpoint of alloy cost, and from this viewpoint, the amount of Ni added is less than 1.0%. The amount of Ni is preferably 0.03% or more, preferably 0.8% or less, and more preferably 0.03% or more and 0.8% or less. Here, there are stainless steels such as SUS304 and SUS316 as austenitic steels having excellent low temperature toughness, but these steels have been optimized for Ni equivalent and Cr equivalent as an alloy design for obtaining an austenitic structure. , A large amount of Ni is added. With respect to these steels, the present invention is an austenite material that has been reduced in cost by minimizing Ni. The minimum required amount of Ni was realized by optimizing the amount of Mn added.

Ni: 0.1% or more and less than 1.0% When the high Mn steel contains a predetermined amount of Cu, the chloride concentration can be increased by adjusting the balance between the Cu amount and the Ni amount and adding Ni. Therefore, excellent chloride stress corrosion cracking resistance can be exhibited. From this point of view, in the high Mn steel containing Cu in the range of 0.2% or more and less than 2.0% as described later, the amount of Ni is set to 0.1% or more and less than 1.0%. If the amount of Ni is less than 0.1%, the effect on stress corrosion cracking cannot be obtained, and if the amount of Ni is 1.0% or more, the cost increases.

Ca: 0.0005% or more and 0.010% or less Ca improves toughness by controlling the morphology of the inclusions described below, and effectively acts to secure ductility (drawing value) during tensile deformation. In order to obtain such an effect, Ca is required to be 0.0005% or more. On the other hand, if Ca is added in an amount of more than 0.010%, the ductility and toughness may be lowered. Therefore, the amount of Ca is set to 0.0005% or more and 0.010% or less. The amount of Ca is preferably 0.0010% or more, preferably 0.0090% or less, and more preferably 0.0010% or more and 0.0090% or less.

Ca / S ≧ 1.0
It is important to control the morphology of Ca-based inclusions by further keeping Ca / S within an appropriate range in the above-mentioned Ca amount and S amount. That is, by setting Ca / S ≧ 1.0, the complex precipitation of MnS in the crystal grains is promoted with the Ca-based inclusions as nuclei, thereby suppressing the precipitation and coarsening of MnS on the grain boundaries. It is effective for improving toughness, ensuring ductility at the time of tensile deformation, specifically, setting the drawing value to 50% or more. In order to obtain such an effect, Ca / S needs to be 1.0 or more. Preferably, Ca / S is 1.7 or more.

N: 0.0050% or more and 0.0500% or less N is an austenite stabilizing element and is an element effective for improving low temperature toughness. In order to obtain such an effect, it is necessary to contain N in an amount of 0.0050% or more. On the other hand, if N is contained in an amount of more than 0.0500%, the nitride or carbonitride becomes coarse and the toughness decreases. Therefore, the amount of N is set to 0.0050% or more and 0.0500% or less. The amount of N is preferably 0.0060% or more, preferably 0.0400% or less, and more preferably 0.0060% or more and 0.0400% or less.

O: 0.0050% or less O deteriorates low temperature toughness due to the formation of oxides. Therefore, O is in the range of 0.0050% or less. Preferably, the amount of O is 0.0045% or less. It is desirable that the amount of O is 0.0003% or more because excessive reduction of the amount of O increases the refining cost and is economically disadvantageous.

Suppressing the content of Ti and Nb to 0.0050% or less, respectively Ti and Nb form high melting point carbonitrides in steel and suppress the coarsening of crystal grains, resulting in the origin of fracture and crack propagation. It becomes a route. In particular, in high Mn steel, it is necessary to intentionally suppress Ti and Nb because it hinders the structure control for increasing the low temperature toughness and improving the ductility. That is, Ti and Nb are components that are inevitably mixed from raw materials and the like, and are usually mixed in the range of Ti: more than 0.005 to 0.010% and Nb: more than 0.005 to 0.010%. Is. Therefore, it is necessary to avoid unavoidable contamination of Ti and Nb and suppress the content of Ti and Nb to 0.0050% or less, respectively, according to the method described later. By suppressing the contents of Ti and Nb to 0.0050% or less, the adverse effects of the above-mentioned carbonitride can be eliminated, and excellent low temperature toughness and ductility can be ensured. The content of Ti and Nb is preferably less than 0.0050%, more preferably 0.003% or less.

Cu: 0.2% or more and less than 2.0% Cu has an effect of improving chloride stress corrosion cracking resistance in a low chloride concentration environment. From this point of view, it is effective to contain Cu in an amount of 0.2% or more. On the other hand, Cu deteriorates the chloride stress corrosion cracking resistance in a high chloride concentration environment. Therefore, when Cu is contained, the amount of Cu is set to less than 2.0%. If the amount of Cu is less than 0.2%, the effect on stress corrosion cracking property cannot be obtained, and if the amount of Cu is 2.0% or more, the cost increases in addition to the above problems. The amount of Cu is preferably 0.3% or more, preferably 0.8% or less, and more preferably 0.3% or more and 0.8% or less.

0 <Cu / Ni ≦ 2
Here, in a high Mn steel containing Cu and Ni, in order to ensure excellent chloride corrosion cracking resistance regardless of the chloride concentration, the amounts of Cu and Ni should be controlled within the above-mentioned range. In addition, it is important to optimize the balance between the amount of Cu and the amount of Ni so as to satisfy 0 <Cu / Ni ≦ 2. When Cu / Ni> 2, the amount of Ni is too small with respect to the amount of Cu, and excellent chloride stress corrosion cracking resistance cannot be exhibited in a high chloride concentration environment.

The rest other than the essential components mentioned above are iron and unavoidable impurities. Examples of the unavoidable impurities here include H, and a total of 0.01% or less is acceptable.

In the present invention, for the purpose of further improving the strength and low temperature toughness, the following elements can be contained, if necessary, in addition to the above essential components.
Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, Mg: 0.0005 to 0.0050%, REM: 0.0010 to 0.0200% 1 or 2 More than seeds

Mo, V, W: 2.0% or less, respectively Mo, V and W contribute to the stabilization of austenite and the improvement of the strength of the base metal. In order to obtain such an effect, Mo, V and W are preferably contained in an amount of 0.001% or more. On the other hand, if Mo, V and W are contained in an amount of more than 2.0%, coarse carbonitride may be formed, which may be a starting point of fracture and put pressure on the manufacturing cost. Therefore, when these alloying elements are contained, the content thereof is set to 2.0% or less. The amount of each of Mo, V and W is more preferably 0.003% or more, preferably 1.7% or less, and more preferably 1.5% or less. The amounts of Mo, V, and W are preferably 0.003% or more and 1.7% or less, and more preferably 0.003% or more and 1.5% or less.

Mg: 0.0005 to 0.0050%, REM: 0.0010 to 0.0200%
Mg and REM are elements useful for controlling the morphology of inclusions and can be contained as needed. Morphological control of inclusions means that the expanded sulfide-based inclusions are made into granular inclusions. Through morphological control of this inclusion, ductility, toughness and sulfide stress corrosion cracking resistance are improved. In order to obtain such an effect, it is preferable that Mg is contained in an amount of 0.0005% or more and REM is contained in an amount of 0.0010% or more. On the other hand, if a large amount of any of the elements is contained, the amount of non-metal inclusions may increase, and the ductility, toughness, and sulfide stress corrosion cracking resistance may decrease. It may also be economically disadvantageous. Therefore, when Mg is contained, it is preferably 0.0005 to 0.0050%, and when REM is contained, it is preferably 0.0010% to 0.0200%. The amount of Mg is more preferably 0.0010% or more, more preferably 0.0040% or less, and further preferably 0.0010% or more and 0.0040% or less. The amount of REM is more preferably 0.0020% or more, more preferably 0.0150% or less, and further preferably 0.0020% or more and 0.0150% or less.

[Organization]
Microstructure with austenite as the base phase When the crystal structure of the steel material is a body-centered cubic structure (bcc), the steel material may cause brittle fracture in a low temperature environment, so it is suitable for use in a low temperature environment. Not. Here, assuming use in a low temperature environment, it is essential that the matrix phase of the steel material has an austenite structure in which the crystal structure is a face-centered cubic structure (fcc). Here, "using austenite as the base phase" means that the austenite phase has an area ratio of 90% or more. The rest other than the austenite phase is a ferrite phase or a martensite phase, but it goes without saying that the austenite phase may be 100%.

[Production method]
The method for producing high Mn steel of the present invention includes a step of heating a steel material having the above-mentioned composition, a step of hot-rolling the heated steel material, and a hot-rolled hot-rolled plate. Includes a step of performing cooling treatment. In the method for producing high Mn steel of the present invention, the temperature range in the step of heating the steel material is set to 1100 ° C. or higher and 1300 ° C. or lower, and the finish rolling end temperature in the step of performing hot rolling is set to 750 ° C. or higher. The average cooling rate from a temperature of (finish rolling end temperature -100 ° C) or higher to a temperature range of 300 ° C or higher and 650 ° C or lower in the step of performing the cooling treatment is 0.5 ° C / s. It is characterized by the above.

In producing the high Mn steel according to the present invention, first, as the steel material, molten steel having the above-mentioned composition can be melted by a known melting method such as a converter or an electric furnace. Further, secondary refining may be performed in a vacuum degassing furnace. At that time, in order to limit Ti and Nb that hinder suitable tissue control to the above range, measures are taken to prevent unavoidable mixing of Ti and Nb from raw materials and the like and reduce their contents. Need to take. For example, by lowering the basicity of the slag during the refining step, these alloys are concentrated into the slag and discharged to reduce the concentration of Ti and Nb in the final slab product. Alternatively, a method such as blowing oxygen to oxidize the alloy and floating and separating the alloy of Ti and Nb at reflux may be used. After that, it is preferable to use a known casting method such as a continuous casting method or an ingot forming method to obtain a steel material such as a slab having a predetermined size. The slab after continuous casting may be subjected to bulk rolling to obtain a steel material.

Further, the manufacturing conditions for incorporating the above steel material into a steel material having excellent high strength, low temperature toughness, and ductility are specifically specified.
Steel material heating temperature: 1100 ° C or higher and 1300 ° C or lower The heating temperature before hot rolling is set to 1100 ° C or higher in order to coarsen the crystal grain size of the microstructure of the steel material. However, if the heating temperature exceeds 1300 ° C., there is a concern that partial melting may start, so the upper limit of the heating temperature is set to 1300 ° C. The temperature control here is based on the surface temperature of the steel material.

Finish rolling end temperature: 750 ° C or higher and lower than 950 ° C After heating the steel material (steel ingot or steel piece), hot rolling is performed. In order to produce coarse crystal grains, it is preferable to increase the cumulative reduction rate at high temperature. That is, when hot rolling is performed at a low temperature, the microstructure becomes fine and excessive processing strain is applied, which causes a decrease in low temperature toughness. Therefore, the lower limit of the finish rolling end temperature in hot rolling is 750 ° C. at the surface temperature of the steel sheet. On the other hand, when finished in the temperature range of 950 ° C. or higher, the crystal grain size becomes excessively coarse and the desired yield strength cannot be obtained. Therefore, final finish rolling of 1 pass or more is required at a temperature lower than 950 ° C.

Average cooling rate from (finish rolling end temperature -100 ° C.) or higher to a temperature range of 300 ° C. or higher and 650 ° C. or lower: 0.5 ° C./s or higher After hot rolling is completed, cooling is performed promptly. When the steel sheet after hot rolling is gently cooled, the formation of precipitates is promoted and the low temperature toughness deteriorates. The formation of these precipitates can be suppressed by cooling at a cooling rate of 0.5 ° C./s or higher in a predetermined temperature range. In addition, excessive cooling distorts the steel sheet, reducing productivity. For that purpose, the upper limit of the cooling start temperature can be set to 900 ° C. The lower limit of the cooling start temperature is (finish rolling end temperature −100 ° C.). This is because if cooling is started from a temperature lower than the above temperature, the formation of precipitates is promoted after hot rolling and the low temperature toughness is lowered. Further, the cooling end temperature is set to a temperature range of 300 ° C. or higher and 650 ° C. or lower. This is because the precipitation of carbides, which causes a decrease in toughness, can be suppressed by cooling to the above temperature range. For the above reasons, in the cooling process after hot rolling, the average cooling rate of the steel sheet surface from the temperature above the surface temperature of the steel sheet (finish rolling end temperature -100 ° C) to the temperature range of 300 ° C or more and 650 ° C or less. Is 0.5 ° C./s or higher. On the other hand, from the viewpoint of industrial production, the average cooling rate is preferably 200 ° C./s or less. The cooling rate is calculated as the average cooling rate of the steel sheet by simulation calculation based on the temperature change of the surface.

Further, in the casting step described above, it is preferable to control the cooling time in the temperature range of 1400 ° C. to 1300 ° C. as the surface temperature of the steel to 100 s or less at the time of cooling. By controlling the cooling time in the casting step as described above, the composite precipitation of MnS centered on Ca-based inclusions such as Ca (O, S) is promoted, and the number of (Ca, Mn) S increases. As a result, MnS does not grow at the grain boundaries or within the crystal grains, and the proportion of elongated MnS decreases. By controlling the morphology of Ca-based inclusions in this way, a high Mn steel having a good drawing value of 51% or more can be obtained.

Hereinafter, the present invention will be described in detail with reference to Examples. The present invention is not limited to the following examples.
A steel slab having the composition shown in Table 1 was prepared as a steel material by a converter-ladle refining-continuous casting method. Next, the obtained steel slab was subjected to bulk rolling and hot rolling under the conditions shown in Table 2 to obtain a steel plate having a maximum thickness of 32 mm. Tensile properties, toughness and microstructure evaluation of the steel sheet were carried out as follows.

(1) Tensile test characteristics From each of the obtained steel plates, JIS No. 4 tensile test pieces were collected for steel plates with a plate thickness of more than 15 mm, and for steel plates with a plate thickness of less than 15 mm, a round bar with a parallel portion diameter of 6 mm and a distance between gauge points of 25 mm. Tensile test pieces were collected and subjected to a tensile test to investigate the characteristics of the tensile test. In the present invention, it was determined that the yield strength of 400 MPa or more and the tensile strength of 800 MPa or more were excellent in tensile properties and high strength. Further, it was determined that the aperture value of 50% or more was excellent in ductility.

(2) Low temperature toughness A position from the surface of each steel plate having a plate thickness of more than 20 mm to 1/4 of the plate thickness (hereinafter referred to as a plate thickness of 1/4 position), or 1 of the plate thickness of each steel plate having a plate thickness of 20 mm or less. Charpy V notch full size test pieces are collected in accordance with the regulations of JIS Z2202 (1998) from the direction parallel to the rolling direction at the positions up to / 2 (hereinafter referred to as the plate thickness 1/2 position), and JIS. Three Charpy impact tests were carried out on each steel sheet in accordance with the provisions of Z2242 (1998), the absorbed energy at -196 ° C. was determined, and the low temperature toughness of the base metal was evaluated. In the present invention, the average value of three absorbed energy (vE _-196) is more than 100J was excellent in low temperature toughness of the base material. For steel sheets with a thickness of less than 10 mm, Charpy V notch half-size test pieces were collected and the same Charpy impact test was carried out. For steel sheets with a plate thickness of less than 10 mm, an average value of 20 J or more was considered to be excellent in low temperature toughness of the base material.

(3) Stress Corrosion Cracking Test Samples 32 and 33 were subjected to a boiling magnesium chloride stress corrosion cracking test in accordance with ASTM G36. The test piece was a U-bending test piece conforming to ASTM G30 Example a. A test piece having a thickness of 2.5 mm, a width of 20 mm, and a length of 80 mm was collected from a position 1 mm below the surface of the steel sheet in the C direction, and the central portion in the longitudinal direction of the test piece was bent by 5R and used for the test.
The test time was 400 hours. After the test, the test piece in which no crack was confirmed on the surface was judged to have excellent chloride stress corrosion cracking resistance. In Table 3, the case where no crack was visually confirmed on the surface was shown as ◯, and the case where crack was visually confirmed on the surface was shown as x.

High Mn steel according to the invention, the aforementioned target performance (yield strength of the base material more than 400 MPa, the aperture value is 50% or more, 100 J or more at low temperature toughness average value of absorbed energy _{(vE -196)} (half-size test In the case of one piece, it was confirmed that 20J or more)) was satisfied. On the other hand, in the comparative example outside the scope of the present invention, any one or more of the yield strength, the drawing value and the low temperature toughness does not satisfy the above-mentioned target performance.

Further, in the sample 32 containing Cu and Ni so that Cu / Ni was within a predetermined range, excellent chloride stress corrosion cracking resistance was exhibited. On the other hand, in the sample 33 in which Cu / Ni was out of the predetermined range, sufficient chloride stress corrosion cracking resistance could not be confirmed.

Claims (3)

By mass%
C: 0.10% or more and 0.70% or less,
Si: 0.10% or more and 0.90% or less,
Mn: 20% 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: 1.8% or more and 7.0% or less,
Ni: 0.01% or more and less than 1.0%,
Ca: 0.0005% or more and 0.010% or less,
N: 0.0050% or more and 0.0500% or less,
O: 0.0050% or less,
It contains Ti: 0.0050% or less and Nb: 0.0050% or less, satisfies the following formula (1), has a component composition of Fe and unavoidable impurities in the balance, and a structure having austenite as a base phase. ,
Yield strength is 400 MPa or more,
The average value of the Charpy impact absorption energy at -196 ° C is, it is 100J or more in the case of using a full size test piece state, and are more 20J in the case of using half-size specimens, aperture value more than 51% Oh Ru, high-Mn steel.
Ca / S ≧ 1.0 ... (1) The composition of the components is further increased by mass%.
Cu: less than 2.0%,
Mo: 2.0% or less,
V: 2.0% or less,
W: 2.0% or less,
The high Mn steel according to claim 1, which contains one or more selected from Mg: 0.0005% or more and 0.0050% or less and REM: 0.0010% or more and 0.0200% or less. After heating the steel material having the component composition according to claim 1 or 2 to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, hot rolling is performed in which the finish rolling end temperature is 750 ° C. or higher and lower than 950 ° C. , (Finish rolling end temperature -100 ° C) or higher to a temperature range of 300 ° C or higher and 650 ° C or lower with an average cooling rate of 0.5 ° C / s or higher , according to claim 1 or 2. A method for producing a high Mn steel, which obtains the above-mentioned high Mn steel.