TWI754893B - High Mn steel and its manufacturing method - Google Patents

Abstract

The high-Mn steel provided by the present invention has high strength, excellent low-temperature toughness, and excellent ductility. The high-Mn steel system of the present invention includes, in 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, Ti: 0.0050% or less, and Nb: 0.0050% or less, and satisfying Ca/S≧1.0, the rest is Fe and unavoidable impurities. The microstructure with Vostian iron as the base phase; and the yield strength is more than 400MPa; the average value of Charpy impact absorbed energy at -196°C is more than 100J when using a full-scale test piece, and when using a half-size test piece The case of the film is more than 20J.

Description

High Mn steel and its manufacturing method

The present invention relates to a preferred high Mn steel for use in structures used in extremely low temperature environments, such as tanks for liquefied gas storage tanks, and a method for producing the same.

Since the structure for liquefied gas storage tanks is used in an extremely low temperature environment, the steel sheet used for such a structure is required not only to have high strength, but also to have excellent toughness at extremely low temperatures. For example, when a hot-rolled steel sheet is used in a LNG storage tank, it is necessary to ensure excellent toughness at extremely low temperatures below -164°C, the boiling point of LNG. If the low-temperature toughness of the steel material is poor, there is a possibility that safety cannot be maintained when it is used as a structure for an extremely low-temperature storage tank, and therefore, there is a strong demand for improving the low-temperature toughness of the steel material to be used.

In response to this requirement, it is known to use Wasser-based stainless steel, 9% Ni steel, or 5000-series aluminum alloy whose main structure of the steel plate is Wasserite that is not brittle at extremely low temperatures. However, since the alloy cost and manufacturing cost of these steels and alloys are high, steel materials which are inexpensive and excellent in low-temperature toughness are desired.

Therefore, Patent Document 1 and Patent Document 2 propose that a new steel material that replaces the conventional steel for extremely low temperature is a high Mn steel that is relatively inexpensive, and that a large amount of Mn is added as a stabilizing element of Vostian iron, and is used in an extremely low temperature environment. Steel for construction below.

That is, Patent Document 1 proposes to control the carbide coverage ratio of the iron grain boundaries of the Worcestershire. In addition, Patent Document 2 proposes to control the grain size of iron crystals of Vostian by carbide coating and addition of Mg, Ca, and REM. [Prior Art Literature] [Patent Literature]

Patent Document 1: Japanese Patent Laid-Open No. 2016-84529 Patent Document 2: Japanese Patent Laid-Open No. 2016-196703

(The problem that the invention intends to solve)

The Vostian steels described in the above-mentioned Patent Document 1 and Patent Document 2 used as steels for extremely low temperatures have large work hardening from the initial stage of deformation during tensile deformation until the maximum stress (tensile strength) is reached, and the plastic deformation ability is high. Excellent, and thus excellent in ductility until the middle stage of deformation. On the other hand, after the stress measured in the tensile test reaches the maximum (tensile strength), the deformation performance in the later stage of deformation is also an important characteristic of the structural member. The reason is that the deformation performance in the later stage of deformation is the final stage performance to reach the final failure. From this point of view, it is necessary to sufficiently secure the ductility in the later stage of deformation, especially the area reduction value, and from the viewpoint of securing the ductility of the high-strength steel, a reduction area value of 50% or more is ideal.

An object of the present invention is to provide a high-Mn steel having high strength, excellent low-temperature toughness and, of course, excellent ductility, and a method for producing the same. Here, the above-mentioned "high strength" means a yield strength of 400 MPa or more and a tensile strength of 800 MPa or more at room temperature. In addition, the above-mentioned "excellent low temperature toughness" refers to the case where the Charpy impact test is carried out according to JIS Z2242 (1998) at -196°C, and a steel plate with a thickness of 10 mm or more and a full-scale test piece (10 mm × 10 mm × 55 mm) are used. , Charpy impact absorption energy (average) is more than 100J on the base metal (when using a steel plate with a thickness of less than 10mm, a half-size test piece (10mm × 5mm × 55mm), according to the Charpy V-notch half-size test Department of 20J or more). In addition, the above-mentioned "excellent ductility" means having a surface reduction value of 50% or more. (Technical means to solve problems)

The inventors and the like have obtained the following findings as a result of intensive research on a method for solving the above-mentioned problems, targeting high Mn steel. That is, in the high Mn steel, by controlling the form of Ca-based inclusions, the toughness is improved, and the ductility (reduction value) at the time of tensile deformation can be ensured, and the balance between the amount of Ca and the amount of S can be effectively balanced by this. within an appropriate range. In addition, in the production of this high Mn steel, it was found that the heating temperature of the steel material, the finishing temperature of finishing rolling, and the temperature from (finishing finishing temperature -100 ℃) or more to 300 ℃ or more and 650 ℃ or less are limited by limiting the steel material heating temperature. The average cooling rate in the temperature range can control the crystal grain size, suppress the precipitates, and improve the low temperature toughness.

However, when the high Mn steel contains Cu, the Cu system has the effect of improving the resistance to chloride stress corrosion cracking in a low chloride concentration environment. However, when Cu is in a high chloride concentration environment, the resistance to chloride stress corrosion cracking is deteriorated. In response to this problem, the inventors have found that in high-Mn steel containing Cu, by adding Ni with an appropriate balance between the amount of Cu and the amount of Ni, excellent chlorine resistance can be exhibited even in a high chloride concentration environment Chemical stress corrosion cracking. As a result, excellent resistance to chloride stress corrosion cracking can be imparted to high Mn steel containing Cu regardless of the chloride concentration. In addition, in this specification, the term "chloride stress corrosion cracking" refers to a high Mn steel in a peculiar corrosive environment, especially an environment where chloride ions exist, even if the tensile stress imparted to the high Mn steel is higher than that of the high Mn steel. Below the tensile strength, it will still lead to the phenomenon of cracking or fracture of the high Mn steel. The term "chloride stress corrosion cracking resistance" means resistance to chloride stress corrosion cracking.

The present invention has been completed by further review based on the above findings, and the gist of the invention is as follows. 1. A high Mn steel comprising: In % by mass, contains: 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, Ti: 0.0050% or less, and Nb: below 0.0050%, and satisfy the following formula (1), the rest is composed of Fe and inevitable impurities, and The tissue with Worcester iron as the base phase; And, the yield strength is more than 400MPa; The average value of Charpy impact absorbed energy at -196°C is 100J or more when using a full-size test piece, and 20J or more when using a half-size test piece. Ca/S≧1.0・・・(1)

2. The high Mn steel as described in 1 above, wherein the component composition system further contains in mass % from: Cu: less than 2.0%, Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, Mg: 0.0005% or more and 0.0050% or less, and REM (rare earth metal): 0.0010% or more and 0.0200% or less 1 or 2 or more selected from among them.

3. A method for producing a high Mn steel, comprising heating a steel material having the composition described in 1 or 2 above to a temperature range of 1100°C or higher and 1300°C or lower, and then performing a finish rolling at a temperature of 750°C or higher and not higher than 750°C. After hot rolling at 950°C, a cooling treatment with an average cooling rate of 0.5°C/s or more 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 is performed.

4. A high Mn steel comprising: In % by mass, contains: 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: below 0.0050%, And satisfy the following formulas (1) and (2), the rest is composed of Fe and unavoidable impurities, and the structure of the base phase of Worcester iron. Ca/S≧1.0・・・(1) 0＜Cu/Ni≦2・・・(2)

5. A method for producing high Mn steel, comprising heating the steel material having the composition described in the above 4 to a temperature range of 1100°C or higher and 1300°C or lower, and then performing finish rolling at a temperature of 750°C or higher and less than 950°C. ℃ hot rolling, and then a cooling treatment with an average cooling rate of 0.5°C/s or more 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. (Compared to the efficacy of the prior art)

According to one aspect of the present invention, it is possible to provide high-strength, high-Mn steel that is excellent in low-temperature toughness especially in an extremely low-temperature region, and has excellent ductility. Therefore, by using the high Mn steel of the present invention, it is possible to improve the safety and life of steel structures used in extremely low temperature environments such as tanks for liquefied gas storage tanks, and to achieve special industrial effects. Furthermore, according to another aspect of the present invention, it is possible to provide a high-Mn steel that exhibits excellent resistance to chloride stress corrosion cracking regardless of the chloride concentration.

Hereinafter, the high Mn steel of the present invention will be described in detail. [Ingredient composition] First, the chemical composition of the high Mn steel of the present invention and the reason for its limitation will be described. In addition, the "%" indication of the ingredient composition means "mass %" unless otherwise stated. C: 0.10% or more and 0.70% or less The C series is an inexpensive stabilizing element for Vostian iron, and it is an important element for obtaining Vostian iron. In order to obtain this effect, C must be contained in an amount of 0.10% or more. On the other hand, when the content of C exceeds 0.70%, Cr carbides are excessively formed, and the low-temperature toughness decreases. Therefore, the amount of C is set to 0.10 to 0.70%. The amount of C is preferably 0.20% or more, 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 The Si-based acts as a deoxidizer, which is not only necessary for steel production, but also has the effect of being dissolved in the steel to increase the strength of the steel sheet by solid solution strengthening. In order to obtain these effects, Si must be contained in an amount of 0.10% or more. On the other hand, when the Si content exceeds 0.90%, the weldability is deteriorated, and the low temperature toughness, particularly the toughness at extremely low temperature, is lowered. 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, 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 series is a relatively cheap iron stabilization element of Vostian. In the present invention, Mn is an important element that can achieve both strength and ultra-low temperature toughness. In order to obtain this effect, Mn must be contained in an amount of 20% or more. On the other hand, even if the Mn content exceeds 30%, the effect of improving the low temperature toughness has reached saturation, which will still lead to an increase in the cost of the alloy. In addition, the weldability and cuttability 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, more preferably 28% or less, and more preferably 23% or more and 28% or less.

P: 0.030% or less If the content of P-based exceeds 0.030%, it will segregate on grain boundaries and become the starting point of stress corrosion cracking. Therefore, 0.030% is set as the upper limit of the amount of P, which is ideally reduced as much as possible. Therefore, the amount of P is made 0.030% or less. In addition, since excessive reduction of P will lead to a high refining cost, which is not economical, the amount of P is preferably set to 0.002% or more. The amount of P is preferably 0.005% or more, more preferably 0.028% or less, and more preferably 0.024% or less. In addition, the amount of P is more preferably 0.005% or more and 0.028% or less.

S: 0.0070% or less The S series degrades the low-temperature toughness and ductility of the base metal, so the upper limit is made 0.0070%, which is preferably as low as possible. Therefore, the amount of S is made 0.0070% or less. In addition, excessive reduction of S will lead to high refining costs, which is not economical, so the amount of S is preferably set to 0.001% or more. The amount of S is preferably 0.0020% or more, 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 series has the function of deoxidizer and is most commonly used in the deoxidation process of molten steel of steel plate. In order to obtain this effect, Al must be contained in an amount of 0.01% or more. On the other hand, if the Al content exceeds 0.07%, the Al content is made 0.07% or less because it is mixed into the welded metal portion during welding, and the toughness of the welded metal is deteriorated. 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, 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 effective element that stabilizes the iron of the Worcester and improves the low temperature toughness and the strength of the base metal by adding an appropriate amount. In order to obtain this effect, Cr must be contained in an amount of 1.8% or more. On the other hand, when the content of Cr exceeds 7.0%, Cr carbides are formed, and the low-temperature toughness and stress corrosion cracking resistance decrease. 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, more preferably 6.7% or less, and more preferably 2.0% or more and 6.7% or less. In addition, in order to improve the stress corrosion cracking resistance, the amount of Cr 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 being solid-dissolved in the steel to increase the strength of the steel sheet by solid solution strengthening, and has the effect of improving the low-temperature toughness, especially the toughness at extremely low temperature, so it is contained in an amount of 0.01% or more. On the other hand, from the viewpoint of alloy cost, the amount of Ni is ideally set to the minimum necessary limit, and from this viewpoint, the amount of Ni added is set to less than 1.0%. The amount of Ni is preferably 0.03% or more, 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, which are excellent in low temperature toughness. These steels are alloys designed to obtain the Werstein iron structure, and a large amount of Ni is added in order to optimize the Ni equivalent or Cr equivalent. With respect to these steels, the present invention makes Ni as a necessary minimum limit, and becomes a low-cost Werther field iron material. In addition, the necessary minimum limit of this Ni is realized by optimizing the addition amount of Mn.

Ni: 0.1% or more and less than 1.0% Furthermore, when the high Mn steel contains a predetermined amount of Cu, by adding Ni to balance the amount of Cu and the amount of Ni, excellent resistance to chloride stress corrosion cracking can be exhibited regardless of the chloride concentration. From this viewpoint, as will be described later, in the high Mn steel containing Cu in the range of 0.2% or more and less than 2.0%, the amount of Ni is made 0.1% or more and less than 1.0%. If the Ni content is less than 0.1%, the effect of stress corrosion cracking will not be obtained, and if the Ni content is 1.0% or more, the cost will increase.

Ca: 0.0005% or more and 0.010% or less Ca is effective in improving toughness by controlling the morphology of inclusions described below, and ensuring ductility (reduction value) during tensile deformation. In order to obtain this effect, Ca must be 0.0005% or more. On the other hand, if the addition of Ca exceeds 0.010%, the ductility and toughness may be reduced in reverse. Therefore, the amount of Ca is made 0.0005% or more and 0.010% or less. The amount of Ca is preferably 0.0010% or more, more preferably 0.0090% or less, and more preferably 0.0010% or more and 0.0090% or less.

Ca/S≧1.0 The above-mentioned amount of Ca and amount of S is an important matter to control the form of Ca-based inclusions by further setting Ca/S within an appropriate range. That is, by setting Ca/S≧1.0, the composite precipitation of MnS is promoted in the crystal grains with the Ca-based inclusions as the nucleus, thereby suppressing the precipitation and coarsening of MnS on the grain boundaries, improving the toughness, and ensuring Specifically, the ductility at the time of tensile deformation can effectively make the area reduction value 50% or more. In order to obtain this effect, Ca/S must be set to 1.0 or more. Preferably, the Ca/S ratio is 1.7 or more.

N: 0.0050% or more and 0.0500% or less The N series belongs to the stabilizing element of the Wostian iron and is an effective element for improving the low temperature toughness. In order to obtain this effect, N must contain 0.0050% or more. On the other hand, when the content of N exceeds 0.0500%, the nitrides or nitrocarbides are coarsened, and the toughness is lowered. 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, more preferably 0.0400% or less, and more preferably 0.0060% or more and 0.0400% or less.

O: 0.0050% or less O reduces low temperature toughness by forming oxides. Therefore, O is set in the range of 0.0050% or less. The preferable amount of O is 0.0045% or less. In addition, excessive reduction of the O content leads to a high refining cost, which is not economical. Therefore, the O content is ideally set to 0.0003% or more.

The content of Ti and Nb should be kept below 0.0050%, respectively Ti and Nb form high-melting nitrocarbides in steel to suppress the coarsening of crystal grains, and as a result, they become an initiation point of failure and a path for propagation of cracks. Especially in high Mn steels, it becomes a hindrance to the structure control of improving low temperature toughness and improving ductility, so it is necessary to deliberately suppress Ti and Nb. That is, Ti and Nb are components which are inevitably mixed from raw materials and the like, and are usually mixed in the range of Ti: over 0.005 to 0.010% and Nb: over 0.005 to 0.010%. Therefore, according to the method described later, the inevitable mixing of Ti and Nb is avoided, and the contents of Ti and Nb must be respectively suppressed to 0.0050% or less. By suppressing the contents of Ti and Nb to be less than or equal to 0.0050%, the adverse effects of the above-mentioned nitride carbides 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%, and more preferably 0.003% or less.

Cu: 0.2% or more and less than 2.0% The Cu series has the effect of improving resistance to chloride stress corrosion cracking in a low chloride concentration environment. From this point of view, it is effective to contain 0.2% or more of Cu. 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 be less than 2.0%. If the amount of Cu is less than 0.2%, the effect of stress corrosion cracking cannot be obtained, and if the amount of Cu is 2.0% or more, in addition to the above problems, the cost will increase. The amount of Cu is preferably 0.3% or more, more preferably 0.8% or less, and more preferably 0.3% or more and 0.8% or less.

0＜Cu/Ni≦2 Here, in the high Mn steel containing Cu and Ni, in order to ensure excellent resistance to chloride corrosion cracking regardless of the chloride concentration, it is important not only to control the amounts of Cu and Ni within the above-mentioned ranges, but also to The balance between the amount of Cu and the amount of Ni is appropriately adjusted so as to satisfy 0<Cu/Ni≦2. If Cu/Ni>2, the amount of Ni is too small relative to the amount of Cu, and the excellent chloride stress corrosion cracking resistance cannot be volatilized in a high chloride concentration environment.

The rest of the above-mentioned essential components are iron and inevitable impurities. The unavoidable impurities here include, for example, H and the like, and the total allowable amount is 0.01% or less.

In the present invention, for the purpose of further improving strength and low-temperature toughness, the following elements may be contained as necessary in addition to the above-mentioned essential components. Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, Mg: 0.0005~0.0050%, REM: 1 or more of 0.0010~0.0200%

Mo, V, W: below 2.0% respectively Mo, V, and W series contribute to the stabilization of the Voss field iron, and contribute to the improvement of the strength of the base metal. In order to obtain this effect, Mo, V and W are preferably contained in an amount of 0.001% or more. On the other hand, when each of Mo, V, and W is contained in an amount exceeding 2.0%, coarse nitrocarbides are formed, which become the starting point of destruction, and the production cost is reduced. Therefore, when these alloy elements are contained, the content is set to 2.0% or less. The amount of each of Mo, V and W is more preferably 0.003% or more, more preferably 1.7% or less, and more preferably 1.5% or less. Moreover, each content of Mo, V and W is preferably 0.003% or more and 1.7% or less, more preferably 0.003% or more and 1.5% or less.

Mg: 0.0005~0.0050%, REM: 0.0010~0.0200% Mg and REM are elements useful for controlling the morphology of inclusions, and may be contained if necessary. "Inclusion morphology control" refers to the formation of expanded sulfide-based inclusions into granular inclusions. The ductility, toughness, and sulfide stress corrosion cracking resistance can be improved by controlling the morphology of the inclusions. In order to obtain this effect, the content of Mg is preferably 0.0005% or more, and the content of REM is preferably 0.0010% or more. On the other hand, if the content of any one of the elements is too high, the amount of non-metallic inclusions increases, and ductility, toughness, and sulfide stress corrosion cracking resistance may be reduced instead. Also, there will be uneconomical situations. Therefore, when Mg is contained, it is preferably 0.0005 to 0.0050%; when REM is contained, it is preferably 0.0010 to 0.0200%. The amount of Mg is more preferably 0.0010% or more, and more preferably 0.0040% or less, and particularly preferably 0.0010% or more and 0.0040% or less. The amount of REM is more preferably 0.0020% or more, and more preferably 0.0150% or less, and particularly preferably 0.0020% or more and 0.0150% or less.

[organization] Microstructure of Worcester Iron as the Base Phase When the crystal structure of the steel is a body-centered cubic structure (bcc), it is not suitable for use in a low-temperature environment because the steel may cause brittle failure in a low-temperature environment. Here, when used in a low temperature environment, the base phase of the steel material must be a Worcestershire iron structure in which the crystal structure is a face-centered cubic structure (fcc). In addition, the so-called "Worth field iron as the base phase" means that the area ratio of the Worth field iron phase is 90% or more. The rest other than the iron phase of the Worth field is the iron phase of fertilizer grains or the phase of loose iron in the field, but of course the iron phase of the Worth field can also 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 sheet. step of cooling. In addition, the method for producing high Mn steel of the present invention is characterized in that: the temperature range when the steel material heating step is performed is 1100°C or higher and 1300°C or lower; and the finish rolling end temperature when the hot rolling step is performed is 750°C. Above and below 950°C; and in the above-mentioned cooling treatment step, the average cooling rate from the temperature above (finish rolling end temperature - 100°C) to the temperature range of 300°C or more and 650°C or below is set to 0.5°C/s above.

In the production of the high Mn steel of the present invention, first, the steel material is melted by a known melting method such as a converter or an electric furnace to melt the molten steel having the above-mentioned composition. In addition, secondary refining can also be performed using a vacuum degassing furnace. In this case, in order to limit the Ti and Nb that hinder the proper structure control to the above-mentioned ranges, it is necessary to take measures to reduce the content of Ti and Nb by avoiding the inevitable mixing of Ti and Nb from the raw materials and the like. The Ti and Nb concentrations in the final billet product are reduced, for example, by reducing the slag basicity during the refining stage, allowing the alloys to be concentrated in the slag and discharged. In addition, a method such as blowing oxygen to oxidize, and flotation separation of an alloy of Ti and Nb at the time of reflow, etc. may be employed. Then, it is preferable to form a steel material such as a steel billet of a predetermined size by a known casting method such as a continuous casting method or an agglomeration method. In addition, a steel material may be formed by subjecting the continuously cast steel billet to block rolling.

In addition, the manufacturing conditions which can manufacture the said steel material into the steel material excellent in high strength, low temperature toughness, and ductility are specifically prescribed|regulated. Steel material heating temperature: 1100°C or more and 1300°C or less In order to make the crystal grain size of the steel microstructure coarse, the heating temperature before hot rolling is set to 1100°C or higher. However, when the heating temperature exceeds 1300°C, there is a possibility that a part will begin to melt, so the upper limit of the heating temperature is made 1300°C. The temperature control here is based on the surface temperature of the steel material.

Finish rolling end temperature: 750°C or more and less than 950°C The steel material (steel block or steel sheet) is heated and then hot rolled. In order to produce coarse crystal grains, it is preferable to increase the cumulative reduction ratio at high temperature. That is, when hot rolling is performed at a low temperature, the microstructure becomes fine, and excessive working strain occurs, resulting in a decrease in low-temperature toughness. Therefore, the lower limit of the finish rolling completion temperature during hot rolling is to set the steel sheet surface temperature to 750°C. On the other hand, if sizing is carried out in a temperature range of 950° C. or higher, the crystal grain size becomes too large, and the desired yield strength cannot be obtained. Therefore, it is necessary to perform the final finish rolling of one pass or more so as to be less than 950°C.

Average cooling rate from (finish rolling end temperature -100°C) or higher to 300°C or higher and 650°C or lower: 0.5°C/s or higher Immediately after the hot rolling, cooling is carried out. When the hot-rolled steel sheet is slowly cooled, the formation of precipitates is promoted and the low-temperature toughness is deteriorated. The formation of these precipitates can be suppressed by cooling at a cooling rate of 0.5° C./s or more in a predetermined temperature range. Moreover, if supercooling is performed, the steel sheet will be twisted, and the productivity will be lowered. Therefore, the upper limit of the cooling start temperature can be set to 900°C. In addition, the lower limit of the cooling start temperature is set to (finish rolling end temperature - 100°C). The reason for this is that when cooling is started from a temperature below the above-mentioned temperature, the formation of precipitates is accelerated after hot rolling, resulting in a decrease in low-temperature toughness. Moreover, the cooling completion temperature is made into the temperature range of 300 degreeC or more and 650 degrees C or less. The reason for this is that the precipitation of carbides, which is a factor in reducing toughness, can be suppressed by performing cooling up to the above-mentioned temperature range. For the above reasons, in the cooling treatment after hot rolling, the average cooling rate of the steel sheet surface in the temperature range from (finish rolling end temperature -100°C) or higher to a temperature range of 300°C or higher and 650°C or lower is set. is 0.5°C/s or more. On the other hand, from the viewpoint of industrial production, the average cooling rate is preferably 200° C./s or less. The cooling rate calculates the average cooling rate of the steel sheet by the simulation calculation based on the surface temperature change.

Furthermore, in the above-mentioned casting step, during cooling, it is preferable to control the cooling time within the temperature range of the steel surface temperature from 1400°C to 1300°C to be 100 s or less. By controlling the cooling time of the casting step as described above, the composite precipitation of MnS with Ca-based inclusions such as Ca(O,S) as the core can be promoted to increase the number of (Ca,Mn)S. As a result, MnS does not grow in grain boundaries or crystal grains, and the proportion of MnS that is elongated is reduced. By controlling the morphology of such Ca-based inclusions, a high-Mn steel with a good area reduction value of 51% or more can be obtained. [Example]

Hereinafter, the present invention will be described in detail using examples. In addition, the present invention is not limited to the following examples. Steel billets having the compositions shown in Table 1 were made into steel materials according to the converter-pot refining-continuous casting method. Next, according to the conditions shown in Table 2, the obtained steel billet was subjected to block rolling and hot rolling to obtain a steel plate having a maximum thickness of 32 mm. With respect to the steel sheet, the tensile properties, toughness, and microstructure were evaluated according to the following procedures.

(1) Tensile test characteristics From the obtained steel plates, if the plate thickness exceeds 15mm, a JIS No. 4 tensile test piece is taken, and if the plate thickness is less than 15mm, a round bar tensile test piece with a parallel portion diameter of 6mm and a distance between punctuation points of 25mm is taken. Tensile test to investigate tensile test properties. In the present invention, a buckling strength of 400 MPa or more and a tensile strength of 800 MPa or more are determined to be excellent in tensile properties and high strength. In addition, it was judged that the area reduction ratio value was 50% or more as an excellent ductility.

(2) Low-temperature toughness is for each steel plate with a thickness of more than 20 mm from the surface to a position of 1/4 of the thickness of the plate (hereinafter referred to as "the position of 1/4 of the thickness of the plate"), or for each steel plate with a thickness of 20 mm or less to 1/4 of the thickness of the plate 2 (hereinafter referred to as "thickness 1/2 position"), from the direction parallel to the rolling direction, take a Charpy V-notch full-scale test piece according to the regulations of JIS Z2202 (1998), and then according to JIS Z2242 (1998) ), three Charpy impact tests were performed for each steel sheet, the absorbed energy at -196°C was obtained, and the low-temperature toughness of the base metal was evaluated. In the present invention, the average value of the three absorbed energies (vE _-196 ) was 100 J or more, and the base material was rated as excellent in low-temperature toughness. In addition, the Charpy V-notch half-size test piece was taken for the steel sheet with a thickness of less than 10 mm, and the same Charpy impact test was carried out. For a steel sheet with a thickness of less than 10 mm, an average value of 20 J or more was rated as excellent in the low-temperature toughness of the base metal.

(3) Stress corrosion cracking test For samples 32 and 33, the boiling magnesium chloride stress corrosion cracking test according to ASTM G36 was performed. The test piece is a U-bend test piece made according to ASTM G30 Example a. A test piece with a thickness of 2.5 mm x 20 mm in width x 80 mm in length was taken from a position 1 mm below the surface of the steel plate in the C direction, the center of the longitudinal direction of the test piece was bent by 5R, and the test was confirmed. The test time was set to 400 hours. After the test, the test piece with no cracks on the surface was judged to be excellent in resistance to chloride stress corrosion cracking. In Table 3, the case where no cracks were visually observed on the surface was designated as "0", and the case where cracks were visually recognized on the surface was designated as "x".

According to the high Mn steel of the present invention, it was confirmed that the above-mentioned target properties (base metal yield strength of 400 MPa or more, area reduction value of 50% or more, low temperature toughness based on the average value of absorbed energy (vE _-196 ) of 100 J or more (semi- The size of the test piece is 20J or more)). On the other hand, in the comparative examples outside the scope of the present invention, any one or more of the yield strength, the reduction area value, and the low-temperature toughness cannot satisfy the above-mentioned target performance.

Furthermore, the sample 32 containing Cu and Ni so that Cu/Ni falls within a predetermined range can exhibit excellent resistance to chloride stress corrosion cracking. On the other hand, in Sample 33 in which Cu/Ni was outside the predetermined range, sufficient resistance to chloride stress corrosion cracking could not be confirmed.

[Table 1] [Table 1] Steel No. Ingredient composition (mass %) C Si Mn P S Al Cr O N Nb Ti V Cu Ni Mo W Ca Mg REM Ca/S Cu/Ni Remark A 0.189 0.44 28.5 0.015 0.0030 0.042 4.01 0.0020 0.0205 0.002 0.003 - - 0.04 - - 0.0041 - - 1.4 - Invention example B 0.652 0.18 22.4 0.011 0.0048 0.031 2.52 0.0041 0.0374 0.001 0.001 - - 0.05 - - 0.0072 - - 1.5 - Invention example C 0.435 0.38 24.1 0.023 0.0038 0.027 4.50 0.0022 0.0241 0.002 0.001 - 0.31 0.07 - - 0.0080 - - 2.1 4.4 Invention example D 0.339 0.76 20.4 0.019 0.0062 0.042 3.04 0.0031 0.0185 0.002 0.002 0.04 - 0.02 0.41 - 0.0075 - - 1.2 - Invention example E 0.285 0.35 28.2 0.024 0.0021 0.067 1.85 0.0023 0.0255 0.001 0.003 - - 0.01 - 0.07 0.0023 - - 1.1 - Invention example F 0.463 0.31 26.8 0.017 0.0045 0.038 6.52 0.0040 0.0375 0.003 0.001 - - 0.06 - - 0.0063 - - 1.4 - Invention example G 0.342 0.38 22.8 0.021 0.0019 0.047 2.52 0.0030 0.0201 0.002 0.001 - 0.45 0.08 - - 0.0040 0.0011 - 2.1 5.6 Invention example H 0.412 0.18 20.4 0.017 0.0015 0.030 2.05 0.0029 0.0075 0.002 0.001 - - 0.09 - - 0.0024 - 0.0027 1.6 - Invention example I 0.325 0.14 25.2 0.020 0.0028 0.042 5.21 0.0024 0.0152 0.003 0.002 - - 0.03 - - 0.0031 - - 1.1 - Invention example J 0.420 0.37 23.7 0.014 0.0012 0.035 4.22 0.0023 0.0214 0.002 0.004 - 0.62 0.07 - - 0.0042 - - 3.5 8.9 Invention example K 0.572 0.32 26.1 0.016 0.0029 0.032 4.04 0.0021 0.0099 0.001 0.003 - - 0.05 - - 0.0056 - - 1.9 - Invention example L 0.945 0.42 20.3 0.003 0.0026 0.034 4.00 0.0039 0.0311 0.003 0.004 - - 0.01 - - 0.0029 - - 1.1 - Comparative example M 0.109 0.04 23.9 0.019 0.0032 0.042 5.23 0.0012 0.0275 0.002 0.001 - - - - - 0.0042 - - 1.3 - Comparative example N 0.132 0.52 15.8 0.005 0.0045 0.042 2.32 0.0034 0.0455 0.003 0.003 - - 0.03 - - 0.0052 - - 1.2 - Comparative example O 0.226 0.47 23.2 0.047 0.0063 0.050 1.85 0.0041 0.0336 0.001 0.003 - - 0.01 - - 0.0068 - - 1.1 - Comparative example P 0.327 0.28 27.4 0.028 0.0095 0.021 1.99 0.0032 0.0074 0.001 0.003 - - 0.02 - - 0.0097 - - 1.0 - Comparative example R 0.280 0.41 26.8 0.017 0.0039 0.052 7.84 0.0028 0.0195 0.003 0.001 - - 0.01 - - 0.0048 - - 1.2 - Comparative example S 0.427 0.28 25.7 0.024 0.0033 0.047 6.07 0.0075 0.0205 0.003 0.002 - - 0.02 - - 0.0040 - - 1.2 - Comparative example T 0.353 0.11 24.9 0.027 0.0045 0.032 3.31 0.0028 0.0769 0.003 0.002 - - 0.03 - - 0.0051 - - 1.1 - Comparative example U 0.462 0.55 25.3 0.022 0.0043 0.027 4.22 0.0021 0.0255 0.003 0.003 - - 0.01 - - 0.0029 - - 0.7 - Comparative example V 0.440 0.29 24.5 0.019 0.0037 0.034 4.34 0.0027 0.0224 0.001 0.002 - - 0.08 - - 0.0038 - - 1.0 - Invention example W 0.384 0.82 20.9 0.027 0.0004 0.045 3.22 0.0033 0.0205 0.001 0.003 - - 0.03 - - 0.0004 - - 1.0 - Comparative example X 0.587 0.44 25.8 0.022 0.0025 0.019 3.97 0.0024 0.0128 0.002 0.001 - - 0.01 0.14 - 0.0023 - - 0.9 - Comparative example Y 0.602 0.25 23.3 0.025 0.0020 0.034 2.62 0.0032 0.0246 0.004 0.011 - - 0.02 - 0.20 0.0029 - - 1.5 - Comparative example Z 0.080 0.36 20.4 0.019 0.0046 0.042 3.85 0.0021 0.0237 0.003 0.004 - - 0.02 - - 0.0048 - - 1.0 - Comparative example AA 0.290 0.39 27.6 0.012 0.0030 0.042 4.20 0.0020 0.0190 0.002 0.003 - 0.40 0.30 - - 0.0033 - - 1.1 1.3 Invention example BB 0.280 0.39 27.8 0.012 0.0030 0.042 4.20 0.0020 0.0210 0.002 0.003 - 0.40 0.05 - - 0.0033 - - 1.1 8.0 Reference example

[Table 2] [Table 2] Sample No. Steel No. Cooling time at 1400℃~1300℃ during casting Hot rolling method Remark plate thickness Heating temperature of steel material Finishing end temperature Cooling start temperature Average cooling rate to the temperature range of 300°C or more and 650°C or less (s) mm (°C) (°C) (°C) (℃/s) 1 A 20 32 1160 863 836 8 Invention example 2 B 50 twenty three 1180 825 795 11 Invention example 3 C 60 12 1150 781 726 13 Invention example 4 D 80 twenty one 1120 792 760 10 Invention example 5 E 20 twenty four 1130 860 830 9 Invention example 6 F 30 14 1130 842 787 11 Invention example 7 G 30 8 1260 885 821 15 Invention example 8 H 40 9 1200 832 773 12 Invention example 9 I 50 14 1140 828 772 4 Invention example 10 J 70 29 1160 858 831 0.9 Invention example 11 K 30 11 1110 755 705 11 Invention example 12 J 20 12 1150 775 723 11 Invention example 13 L 30 19 1230 765 733 8 Comparative example 14 M 50 30 1180 885 858 7 Comparative example 15 N 30 13 1100 820 765 11 Comparative example 16 O 40 twenty four 1130 853 823 9 Comparative example 17 P 50 20 1190 804 761 6 Comparative example 18 R 20 20 1150 801 764 9 Comparative example 19 S 40 14 1110 814 758 11 Comparative example 20 T 30 28 1150 852 828 2 Comparative example twenty one C 50 11 1170 669 614 10 Comparative example twenty two D 20 18 1220 807 775 0.3 Comparative example twenty three E 30 twenty four 1020 853 823 10 Comparative example twenty four U 50 10 1120 760 713 11 Comparative example 25 V 30 12 1180 805 742 12 Invention example 26 W 20 twenty two 1150 773 738 10 Comparative example 27 X 30 10 1090 741 693 12 Comparative example 28 A 50 30 1290 984 952 8 Comparative example 29 Y 20 8 1130 852 796 12 Comparative example 30 Z 30 28 1150 821 788 9 Comparative example 31 A 300 32 1160 863 836 8 Invention example 32 AA 20 30 1100 900 850 10 Invention example 33 BB 30 30 1100 900 850 10 Reference example

[table 3] [table 3] Sample No. Steel No. Yield strength (MPa) Tensile strength (MPa) Shrinkage value (%) Absorbed energy at -196°C (vE _-196 ) (J) Boiling Magnesium Chloride Test Remark 1 A 421 854 55 137 - Invention example 2 B 548 940 57 121 - Invention example 3 C 568 970 60 119 - Invention example 4 D 562 956 53 116 - Invention example 5 E 483 885 52 137 - Invention example 6 F 455 855 55 141 - Invention example 7 G 421 940 59 62* - Invention example 8 H 506 967 57 54* - Invention example 9 I 449 850 51 127 - Invention example 10 J 404 842 64 133 - Invention example 11 K 569 969 56 116 - Invention example 12 J 559 953 61 127 - Invention example 13 L 625 974 51 58 - Comparative example 14 M 357 817 55 127 - Comparative example 15 N 445 869 51 36 - Comparative example 16 O 450 882 54 72 - Comparative example 17 P 523 939 50 79 - Comparative example 18 R 521 939 53 48 - Comparative example 19 S 449 849 52 83 - Comparative example 20 T 419 854 52 75 - Comparative example twenty one C 670 1061 54 42 - Comparative example twenty two D 541 942 54 67 - Comparative example twenty three E 479 879 53 69 - Comparative example twenty four U 553 942 44 108 - Comparative example 25 V 553 962 51 126 - Invention example 26 W 583 960 48 108 - Comparative example 27 X 579 982 45 112 - Comparative example 28 A 369 847 54 139 - Comparative example 29 Y 501 994 55 17* - Comparative example 30 Z 428 848 50 35 - Comparative example 31 A 421 854 50 112 - Invention example 32 AA 435 851 55 105 ○ Invention example 33 BB 437 853 60 120 × Reference example *Half size test piece

Claims (5)

A high Mn steel comprising, in 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% below, 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, Ti: 0.0050% or less, and Nb: 0.0050% or less, and satisfy the following formula (1), and the rest is composed of Fe and inevitable impurities, And the structure with Wostian iron as the base phase; and, the yield strength is more than 400MPa; the average value of Charpy impact absorption energy at -196℃ is more than 100J when using a full-scale test piece, when using half-size The case of the test piece is more than 20J; the area reduction value is more than 51%; Ca/S≧1.0‧‧‧(1). The high Mn steel according to claim 1, wherein the above-mentioned component composition further contains, in mass %, Cu: less than 2.0%, Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, Mg : 0.0005% or more and 0.0050% or less, and REM (rare earth metal): 0.0010% or more and 0.0200% or less, one or two or more selected from the group consisting of. A method for producing high Mn steel, which is to produce the high Mn steel of claim 1 or 2, wherein the steel material having the composition of claim 1 or 2 is heated to a temperature range of 1100°C or higher and 1300°C or lower. , perform hot rolling with a finish rolling end temperature of 750°C or more and less than 950°C, and then perform an average cooling rate from a temperature above (finish rolling end temperature -100°C) to a temperature range of 300°C or more and 650°C or less It is a cooling treatment of 0.5°C/s or more. A high Mn steel comprising, in 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% Below, 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% below, 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, and satisfy the following formulae (1) and (2), and the rest are Fe and Composition of unavoidable impurities, and the structure with Worcester iron as the base phase; the area reduction value is more than 51%; Ca/S≧1.0‧‧‧(1) 0<Cu/Ni≦2‧‧‧ (2). A method for producing high Mn steel, which is to obtain the high Mn steel of claim 4, wherein the steel material having the component composition of claim 4 is heated to a temperature range of 1100°C or higher and 1300°C or lower, and then finish rolling is performed Hot rolling with a finish temperature of 750°C or more and less than 950°C, followed by a temperature range from (finish rolling end temperature - 100°C) or more to a temperature range of 300°C or more and 650°C or less, the average cooling rate is 0.5°C/ s or more cooling treatment.