TWI653343B - High-manganese steel plate and manufacturing method thereof - Google Patents

Abstract

The invention provides a high-manganese steel plate and a manufacturing method thereof. A high-manganese steel sheet having C: 0.20% to 0.70%, Si: 0.05% to 1.0%, Mn: 15% to 30%, P: 0.028% or less, S: 0.02% or less, and Al: 0.01% to 0.1%, Cr: 0.5% to 7.0%, Ni: 0.03% to 0.30%, N: 0.0010% to 0.0200%, and contains Nb: 0.003% to 0.030%, V: 0.03% to 0.10%, Ti: One or two or more of 0.003% to 0.040%, the remaining part contains the composition of Fe and unavoidable impurities. The microstructure of 0.5 mm below the surface of the steel plate uses the Voss field as the matrix phase. A ratio of 25% or more means that the circle equivalent diameter is 10 μm or more, and the aspect ratio of the major axis to the minor axis is 3 or more.

Description

High-manganese steel plate and manufacturing method thereof

The present invention relates to a high-manganese steel sheet excellent in stress corrosion cracking resistance, particularly to a steel for structural use, which is suitable for supply to a storage tank for a liquefied gas storage tank and the like in an extremely low temperature environment, particularly in a salt water corrosive environment, and a manufacturing method thereof .

When a hot-rolled steel sheet is used in a structure for a liquefied gas storage tank, the use environment becomes extremely low temperature, so not only the strength of the steel sheet but also the toughness at extremely low temperature is required. For example, when a hot-rolled steel sheet is used in a storage tank of liquefied natural gas, it is necessary to ensure excellent toughness below the boiling point (-164 ° C) of the liquefied natural gas. If the low-temperature toughness of the steel material is poor, there is a risk that the safety as a structure for an extremely low-temperature storage tank cannot be maintained, and therefore, there is a strong demand for improving the low-temperature toughness of the applied steel material. For this requirement, previously used Vossat system stainless steel or 9% nickel (Ni) steel, or 5000 series aluminum alloy, which does not show brittleness at extremely low temperatures, uses Vossat body as a steel plate structure. However, due to high alloy or manufacturing costs, there is an urgent need for steel materials that are inexpensive and excellent in extremely low-temperature toughness. Therefore, as a new type of steel for replacing the steel for the extremely low temperature, a high-manganese steel plate to which a relatively inexpensive Voss body stabilizing element manganese (Mn) is added as a structural steel for the extremely low temperature environment has been studied.

On the other hand, in the case of using Worsted body steel in a corrosive environment, there is a problem that the grain boundary of the Worsted body is corroded due to corrosion, In the case of tensile stress, stress corrosion cracking is likely to occur. In particular, in the manufacturing stage of the structure for the liquefied gas storage tank, the ferrite surface of the steel plate may be exposed. If the surface of the steel material is in contact with water vapor containing corrosive substances such as salt, or water or oil, Corrosion of the steel occurs. If it is a high-manganese steel sheet that has been studied before, there is a case where, compared with 9% nickel steel or ordinary low-alloy steel, Vostian system stainless steel is of course poor in corrosion resistance. At this time, in the corrosion reaction on the surface of the high-manganese steel sheet, iron generates oxides (rust) due to the anodic reaction, and hydrogen is generated due to the cathodic reaction of moisture, which promotes stress corrosion cracking due to hydrogen embrittlement. There is a danger that stress corrosion cracking caused as described above may cause damage to the structure due to residual stresses such as bending processing or welding during production, or load stress in the use environment. Therefore, from the viewpoint of safety, it is important that the steel material used is excellent in strength and extremely low-temperature toughness, and is also excellent in stress corrosion cracking resistance.

For example, Patent Document 1 discloses a steel material having Mn: 15% to 35% and Cu: 5% or less, and further adding C and Cr in appropriate amounts to improve machinability and welding heat-affected zone at -196 ° C. Charpy impact characteristics.

Furthermore, Patent Document 2 discloses a high-manganese steel material to which C: 0.25% to 0.75%, Si: 0.05% to 1.0%, Mn: more than 20% to 35%, and Ni: 0.1% to less than 7.0. %, Cr: 0.1% or more and less than 8.0%, improving low temperature toughness.

[Prior Art Literature] [Patent Literature]

Patent Document 1: Japanese Patent Publication No. 2015-508452

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

However, the high-manganese steel sheets described in Patent Documents 1 and 2 are intended to have strength and low-temperature toughness. The Charpy impact characteristics of the welded heat-affected zone at -196 ° C are 60J to 135J (only shown in Patent Document 1). in). However, the extremely low-temperature toughness of the base material is not sufficient, and there is no balance between the extremely low-temperature toughness and the stress corrosion cracking resistance.

The present invention has been made in view of this problem, and an object thereof is to provide a high-manganese steel sheet excellent in extremely low-temperature toughness and excellent resistance to stress corrosion cracking and a method for manufacturing the same.

In order to achieve the above-mentioned problem, the present inventors made an intensive study on the composition, manufacturing method, and various factors that determine the microstructure of a high-manganese steel plate as a target to ensure excellent stress corrosion cracking resistance, and obtained The following findings.

1. In order to achieve both low temperature toughness and excellent resistance to stress corrosion cracking, it is effective to reduce the amount of hydrogen infiltration into a steel sheet through a corrosion reaction. It is important to improve the corrosion resistance of the steel sheet surface in a saltwater corrosive environment. Therefore, it is important to strictly manage the component composition based on the high-manganese steel sheet. In particular, by simultaneously adding Cr and Ni, and appropriately controlling the amount of Cr and Ni added thereto, the rust formed at the initial stage of the corrosion reaction on the surface of the steel sheet becomes fine. Moreover, the amount of hydrogen infiltrated into the steel can be reduced by delaying the subsequent corrosion reaction.

2. Furthermore, it has been found that a method for strictly managing the microstructure near the surface of the steel sheet is also effective in improving the stress corrosion cracking resistance. That is, an area ratio of 25% or more in a Voss field is a circle equivalent diameter of 10 μm or more, and an aspect ratio of a long diameter to a short diameter of 3 or more is important in terms of improving stress corrosion cracking resistance. . The reason is considered to be that the hydrogen infiltrated into the steel plate due to the corrosion reaction was trapped in the crystal grains of the unrecrystallized Voss field, so the relative amount of hydrogen at the grains boundary of the Voss field decreased, and the stress corrosion of the Voss field grain boundary Fracture susceptibility is reduced.

3. In addition to the above 1 and 2, the carbides, nitrides, and composite carbonitrides of Nb, V, and Ti in the steel sheet can further improve the stress corrosion cracking resistance by appropriately controlling the dispersion state. The carbides, nitrides, and composite carbonitrides of Nb, V, Ti, and the like function as capture sites for diffusible hydrogen in the steel sheet. That is, it functions as a capture site of diffusible hydrogen generated by a corrosion reaction of a steel material, and has the effect of suppressing stress corrosion cracking. The heating, rolling, and cooling conditions of the hot rolling step affect the dispersion state of Nb, V, Ti carbides, nitrides, and composite carbonitrides in the Voss field. Therefore, it is important to manage these manufacturing conditions.

4. Furthermore, in order to effectively suppress the grain boundary destruction of the Vostian body, it is effective to improve the strength of the grain boundary. P is an element that is easy to co-segregate with Mn during the solidification of the steel sheet, and reduces the strength of the grain boundary that intersects the microsegregated portion. Therefore, it is necessary to reduce impurity elements such as P.

The present invention has been made by studying the above findings, and the gist thereof is as follows.

[1] A high-manganese steel sheet having C: 0.20% by mass% 0.70%, Si: 0.05% to 1.0%, Mn: 15% to 30%, P: 0.028% or less, S: 0.02% or less, Al: 0.01% to 0.1%, Cr: 0.5% to 7.0%, Ni: 0.03 % ~ 0.30%, N: 0.0010% ~ 0.0200%, and contains one or two or more of Nb: 0.003% ~ 0.030%, V: 0.03% ~ 0.10%, Ti: 0.003% ~ 0.040%, and the remainder contains Fe and The composition of the inevitable impurities. The microstructure of 0.5 mm below the surface of the steel plate uses the Voss field as the matrix phase. More than 25% of the area ratio in the Voss field is a circle equivalent diameter of 10 μm or more, and the long diameter. The aspect ratio to the short diameter is 3 or more.

[2] The high-manganese steel sheet according to [1], in addition to the component composition, further containing an element selected from at least one of the following group A or group B;

Group A: in mass%, selected from one or two of Mo: 0.05% to 2.0%, W: 0.05% to 2.0%;

Group B: In mass%, one or two or more selected from Ca: 0.0005% to 0.0050%, Mg: 0.0005% to 0.0050%, and rare earth metal (REM): 0.0010% to 0.0200%.

[3] The high-manganese steel sheet according to [1] or [2], wherein the microstructure is 0.5 mm below the surface of the steel sheet, and further, the microstructure has a total of 2 × 10 ² pieces / mm ² or more. Carbide, nitride, and carbonitride containing one or two or more kinds of Nb, V, and Ti having a circle equivalent diameter of 0.01 μm to 0.5 μm.

[4] A method for manufacturing a high-manganese steel sheet, which comprises heating a steel raw material having the component composition described in any one of [1] to [3] to a temperature range as follows, that is, Tx (x = Nb (V, Ti or Ti) is set to the temperature shown in formulas (1) to (3), and is expressed by any one or more of Tx (° C) defined by formulas (1) to (3), and the surface temperature of the steel material In the temperature range of (Tx-50) ° C or higher and (Tx + 200) ° C or lower, hot rolling is performed at a finishing rolling temperature of 750 ° C or higher and 1000 ° C or lower to produce a steel sheet. -50 ℃) or any lower temperature from the cooling start temperature to 650 ℃, the average cooling rate on the surface of the steel plate is 1.0 ℃ / s or more for cooling; T _Nb (℃) = 7500 / {3.0-log ₁₀ ([% Nb ] × [% C])}-273 ... (1)

T _V (℃) = 10800 / {7.2-log ₁₀ ([% V] × [% C])}-273… (2)

T _Ti (℃) = 7000 / {2.8-log ₁₀ ([% Ti] × [% C])}-273… (3)

Here, [% Nb], [% V], [% Ti], and [% C] respectively represent the content (mass%) of Nb, V, Ti, and C in the steel; in the case of elements that are not contained The calculation is performed by setting the element symbol in the formula to 0.

Furthermore, in the present invention, the "high strength" refers to a person having a strength of 400 MPa or more. In the present invention, the "very low temperature toughness" means that the low temperature toughness, that is, the absorbed energy vE _-196 of the Charpy impact test at -196 ° C is 50 J or more. Furthermore, in the present invention, the term "excellent stress corrosion cracking resistance" means that a slow strain rate test method (Slow Strain Rate) according to the National Association of Corrosion Engineers (NACE) standard TM0111-2011 Test Method), that is, when the sample is immersed in artificial seawater (with a chloride ion concentration of 18000 ppm) at a temperature of 23 ° C and subjected to a constant velocity tensile test at a strain rate of 4 × 10 ^-7 inch / sec. The breaking stress is 500 MPa or more.

According to the present invention, a high-manganese steel sheet having extremely low-temperature toughness and excellent resistance to stress corrosion cracking can be obtained. In addition, the high-manganese steel sheet of the present invention greatly contributes to improving the safety or life of steel structures used in an extremely low-temperature environment, such as a tank for a liquefied gas storage tank, and is particularly effective industrially. In addition, it does not cause a reduction in productivity and an increase in manufacturing costs, and is therefore excellent in economic efficiency.

Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the following embodiments.

[Ingredient composition]

First, the component composition of the steel sheet of the present invention and the reasons for its limitation will be described. In the present invention, in order to ensure excellent stress corrosion cracking resistance, the component composition of the steel sheet is specified as follows. In addition,% which shows a component composition shows mass% unless there is particular notice.

C: 0.20% ~ 0.70%

C is a cheap Voss field stabilization element, which is an important element to obtain a Voss field. In order to obtain the effect, C must be 0.20% or more. On the other hand, if the content exceeds 0.70%, Cr carbides and Nb, V, and Ti-based carbides are excessively formed, and low-temperature toughness and stress corrosion cracking resistance are reduced. Therefore, C is 0.20% ~ 0.70%. It is preferable that C is 0.25% or more. It is preferable that C is 0.60% or less. More preferably, C is 0.30% or more. More preferably, C is 0.55% or less.

Si: 0.05% ~ 1.0%

Si functions as a deoxidizing material, and is not only necessary for steel making, but also has the effect of solid-solubilizing the steel by solid-solution strengthening to increase the strength of the steel sheet. In order to obtain such an effect, Si must be contained in an amount of 0.05% or more. On the other hand, when the content exceeds 1.0%, the weldability deteriorates. In addition, it also affects the stress corrosion cracking resistance (SCC). Therefore, Si is 0.05% to 1.0%. It is preferable that Si is 0.07% or more. The Si content is preferably 0.50% or less. More preferably, Si is 0.15% or more. More preferably, Si is 0.45% or less.

Mn: 15% ~ 30%

Mn is a relatively inexpensive Voss field stabilization element. In the present invention, it is an important element for achieving both strength and extremely low temperature toughness. In order to obtain this effect, Mn must be contained in an amount of 15% or more. On the other hand, even if the content exceeds 30%, the effect of improving the extremely low temperature toughness is saturated, resulting in an increase in the cost of the alloy. In addition, weldability and cutting properties are deteriorated. Furthermore, it promotes segregation and promotes the occurrence of stress corrosion cracking. Therefore, Mn is 15% to 30%. It is preferable that Mn is 18% or more. The Mn is preferably 28% or less. More preferably, Mn is 20% or more. More preferably, Mn is 27% or less.

P: 0.028% or less

If the P content exceeds 0.028%, it is segregated at the grain boundaries, and becomes a starting point for stress corrosion cracking. Therefore, it is desirable to set the upper limit to 0.028% and reduce it as much as possible. Therefore, P is 0.028% or less. In addition, excessive P reductions have caused refining costs to skyrocket. It is economically disadvantageous, so it is desirable to be 0.002% or more. It is preferable that P is 0.005% or more. It is preferable that P is 0.024% or less.

S: 0.02% or less

S degrades the low-temperature toughness or ductility of the base material. Therefore, it is desirable that the upper limit be 0.02% and decrease as much as possible. Therefore, S is 0.02% or less. Furthermore, an excessive reduction in S causes a sharp increase in refining costs and is economically unfavorable. Therefore, it is preferably 0.001% or more. It is preferable that S is 0.002% or more. It is preferable that S is 0.018% or less. More preferably, S is 0.010% or less.

Al: 0.01% ~ 0.1%

Al functions as a deoxidizing agent, and is most commonly used in the dissolution deoxidation process of steel plates. In addition, there is an effect that the solid solution N in the steel is fixed to form AlN, thereby suppressing the coarsening of crystal grains. In addition, it has the effect of suppressing the deterioration of toughness due to the decrease in solid solution N. In order to obtain such an effect, Al must be contained in an amount of 0.01% or more. On the other hand, if the Al content exceeds 0.1%, it is mixed into the weld metal portion during welding and the toughness of the weld metal is deteriorated, so it is 0.1% or less. Therefore, Al is 0.01% to 0.1%. The Al content is preferably 0.02% or more. The Al content is preferably 0.07% or less.

Cr: 0.5% ~ 7.0%

Cr is an element effective for stabilizing the Voss field by adding it in an appropriate amount, and is effective for improving the extremely low temperature toughness and the strength of the base material. Furthermore, in the present invention, it is an important element that reduces the penetration of hydrogen into the steel sheet and improves the stress corrosion cracking resistance through the effect of densifying the rust generated on the surface of the base material in a salt water environment. In order to obtain such an effect, Cr must be contained in an amount of 0.5% or more. On the other hand, if it contains more than 7.0%, Due to the formation of Cr carbides, low-temperature toughness and stress corrosion cracking resistance are reduced. Therefore, Cr is 0.5% to 7.0%. Cr is preferably 1.0% or more, more preferably 1.2% or more, and still more preferably 2.5% or more. Cr is preferably 6.0% or less, more preferably 5.7% or less, and still more preferably 5.5% or less.

Ni: 0.03% ~ 0.30%

Ni is a typical Voss field stabilizing element, and it is an element effective for improving the extremely low temperature toughness and the strength of the base metal. Furthermore, in the present invention, it is an important element that reduces the penetration of hydrogen into the steel sheet and improves the stress corrosion cracking resistance through the effect of densifying the rust generated on the surface of the base material in a salt water environment. In order to obtain such an effect, Ni must be contained in an amount of 0.03% or more. On the other hand, if it exceeds 0.30%, the cost of the alloy increases and the effect of improving the stress corrosion cracking resistance is saturated. Therefore, Ni is 0.03% to 0.30%. The Ni content is preferably 0.25% or less. It is preferably 0.04% or more. More preferably, Ni is 0.23% or less. More preferably, Ni is 0.05% or more. Still more preferably, Ni is 0.21% or less.

N: 0.0010% ~ 0.0200%

N is a Vostfield body stabilizing element, which is an element effective for improving the extremely low temperature toughness. In addition, it has the effect of binding to Nb, V, and Ti to precipitate as a nitride or carbonitride, and to suppress stress corrosion cracking as a site for diffusing hydrogen. In order to obtain this effect, N must be 0.0010% or more. On the other hand, if the content exceeds 0.0200%, the nitride or carbonitride becomes coarse and toughness is reduced. Therefore, N is 0.0010% to 0.0200%. It is preferable that N is 0.0020% or more. It is preferable that N is 0.0150% or less. More preferably, N is 0.0030% or more. More preferably, N is 0.0170% or less.

Nb: 0.003% ~ 0.030%, V: 0.03% ~ 0.10%, Ti: 0.003% ~ 0.040%, one or two or more

Nb: 0.003% ~ 0.030%

Nb is an element that precipitates as carbonitrides (including carbides), and the generated carbonitrides are effective at the site of diffusive hydrogen capture and have the effect of suppressing stress corrosion cracking. In order to obtain this effect, Nb must be contained in an amount of 0.003% or more. On the other hand, if the Nb content exceeds 0.030%, coarse carbonitrides are precipitated and become a starting point of destruction. In addition, there is a phenomenon that the precipitates become coarse and the toughness of the base material is deteriorated. Therefore, when Nb is contained, it is 0.003% to 0.030%. Nb is preferably 0.005% or more, and more preferably 0.007% or more. Nb is preferably 0.025% or less, and more preferably 0.022% or less.

V: 0.03% ~ 0.10%

V is an element that precipitates as a carbonitride, and the generated carbonitride is effective at the site of diffusible hydrogen capture and has an effect of suppressing stress corrosion cracking. To obtain this effect, V must be 0.03% or more. On the other hand, if the V content exceeds 0.10%, coarse carbonitrides are precipitated and become a starting point of destruction. In addition, there is a phenomenon that the precipitates become coarse and the toughness of the base material is deteriorated. Therefore, when V is contained, it is 0.03% to 0.10%. V is preferably 0.04% or more, and more preferably 0.05% or more. V is preferably 0.09% or less, more preferably 0.08% or less, and still more preferably 0.07% or less.

Ti: 0.003% ~ 0.040%

Ti is precipitated as a nitride or a carbonitride, and the generated nitride or carbonitride is an element effective at the site of diffusing hydrogen capture and has an effect of suppressing stress corrosion cracking. In order to obtain this effect, Ti must contain 0.003% or more. On the other hand, if the Ti content exceeds 0.040%, there is a phenomenon that the precipitates become coarse and the toughness of the base material is deteriorated. In addition, there is a phenomenon that coarse carbonitrides are precipitated and become a starting point of destruction. Therefore, when Ti is contained, it is 0.003% to 0.040%. Ti is preferably 0.005% or more, and more preferably 0.007% or more. Ti is preferably 0.035% or less, and more preferably 0.032% or less.

The balance is iron and unavoidable impurities. Examples of the unavoidable impurities include O, H, and the like, which are allowable if the total content is 0.01% or less.

Furthermore, from the viewpoint of lowering the low-temperature toughness, O and S are preferably defined as follows.

O: 0.0005% ~ 0.0070%

When O content exceeds 0.0070%, coarse inclusions are formed with Al, and low-temperature toughness is reduced. Therefore, it is desirable that O be as low as possible with an upper limit of 0.0070%. It is preferable that O is 0.0060% or less. Furthermore, the excessive reduction of O increases the refining cost so that it is economically unfavorable, so it is 0.0005% or more. It is preferable that O is 0.0008% or more.

O / S <1

The balance of O and S forms oxides, sulfides, and composite precipitates with Al, Ti, and Mn, and functions effectively as a site for diffusing hydrogen to improve the stress corrosion cracking resistance. In order to obtain this effect, it is set to O / S <1. If O / S ≧ 1, coarse oxysulfide may be formed, and low-temperature toughness may be reduced. Therefore, in the present invention, in order to ensure the low-temperature toughness, it is set to O / s <1.

The target characteristics of the present invention are obtained by the above essential elements. In the present invention, in order to further improve the strength and the low-temperature toughness, in addition to the essential elements described above, the following elements may be optionally contained.

Mo: 0.05% ~ 2.0%, W: 0.05% ~ 2.0%

Mo: 0.05% ~ 2.0%

Mo is an element useful for increasing the strength of the base material, and may be contained as necessary. In order to obtain such an effect, it is preferable to contain Mo in an amount of 0.05% or more. On the other hand, if it exceeds 2.0%, Mo may adversely affect the toughness and welding crack resistance. Therefore, Mo is preferably 2.0% or less. Therefore, when Mo is contained, it is 0.05% to 2.0%. More preferably, Mo is 0.07% or more. More preferably, Mo is 1.7% or less.

W: 0.05% ~ 2.0%

W is an element useful for increasing the strength of the base material, and is contained as needed. In order to obtain such an effect, W is preferably contained in an amount of 0.05% or more. On the other hand, if the content exceeds 2.0%, it may adversely affect toughness and resistance to welding cracking. Therefore, W is preferably 2.0% or less. Therefore, when W is contained, it is 0.05% to 2.0%. More preferably, it is 0.07% or more. It is more preferably 1.5% or less.

Ca: 0.0005% to 0.0050%, Mg: 0.0005% to 0.0050%, REM: 0.0010% to 0.0200%, one or two or more

Ca: 0.0005% ~ 0.0050%

Ca is an element useful for controlling the morphology of inclusions, and is contained as needed. The "morphological control of inclusions" refers to making elongated sulfide-based inclusions into granular inclusions. By controlling the morphology of the inclusions, ductility, toughness, and resistance to sulfide stress corrosion cracking are improved. In order to obtain such an effect, it is preferable to contain Ca of 0.0005% or more. On the other hand, if the content exceeds 0.0050%, the amount of non-metallic inclusions may increase, and the ductility, toughness, and sulfide stress corrosion cracking resistance may be reduced. Moreover, there are situations where it becomes economically disadvantageous. Therefore, when Ca is contained, it is 0.0005% to 0.0050%. More preferably, it is 0.0010% or more. It is more preferably 0.0040% or less.

Mg: 0.0005% ~ 0.0050%

Mg is useful as an element that contributes to improvement of sulfide stress corrosion cracking resistance, and is contained as needed. In order to obtain such an effect, it is preferable to contain Mg of 0.0005% or more. On the other hand, even if the content exceeds 0.0050%, the above-mentioned effects may be saturated, and an effect according to the content may not be expected. Moreover, there are situations where it becomes economically disadvantageous. Therefore, when Mg is contained, it is 0.0005% to 0.0050%. More preferably, it is 0.0010% or more. It is more preferably 0.0040% or less.

REM: 0.0010% ~ 0.0200%

REM is useful as an element that contributes to improvement of sulfide stress corrosion cracking resistance, and is contained as needed. In order to obtain such an effect, it is preferable to contain 0.0010% or more of REM. On the other hand, even when the content exceeds 0.0200%, the above-mentioned effects may be saturated, and an effect according to the content may not be expected. Therefore, when REM is contained, it is 0.0010% to 0.0200%. More preferably, it is 0.0020% or more. Better 0.0150% or less.

[Micro-organization]

Next, the microstructure near the surface of the steel sheet, which is an important requirement of the steel sheet of the present invention, will be described.

The microstructure of 0.5 mm below the surface of the steel plate uses the Voss field as the matrix phase. More than 25% of the area ratio in the Voss field is a circle equivalent diameter of 10 μm or more, and the aspect ratio of the major axis to the minor axis is 3 the above.

In the present invention, the matrix phase of the microstructure with a thickness of 0.5 mm below the surface of the steel plate is defined as a Voss field. In addition, the Voss field has a Voss field with an area equivalent to 25% or more of a circle equivalent diameter of 10 μm or more, and an aspect ratio of a long diameter to a short diameter of 3 or more. Outside the crystal grain boundary, the deformation band in the crystal grain also functions effectively as a site for diffusing hydrogen, and effectively acts on stress corrosion cracking resistance. As a result, the suppression of stress corrosion cracking can be particularly improved. In addition, the drop intensity is also increased. The area ratio is preferably 30% or more. On the other hand, if the area ratio exceeds 95%, the strength of the steel material may become too large, and the toughness of the base material may be deteriorated. It is preferably 95% or less, and more preferably 94% or less. It is more preferably 90% or less. Furthermore, it is more preferably 85% or less.

If the equivalent circle diameter is less than 10 μm, or the aspect ratio of the major axis to the minor axis is less than 3, the desired drop strength cannot be obtained, and the deformation in the crystal grains that effectively functions as a site for diffusing hydrogen cannot be obtained. The belt has reduced stress corrosion cracking resistance, and the above-mentioned effect cannot be obtained. The circle equivalent diameter, area ratio, and aspect ratio of the Voss field can be measured by a method described in Examples described later.

In the present invention, "0.5 mm below the surface of the steel sheet" means a cross section parallel to the rolling direction, which indicates a position of 0.5 mm in the thickness direction from the front and back surfaces of the steel sheet. Furthermore, in the present invention, the above-mentioned effect can be obtained similarly even in the cross section parallel to the rolling direction in a range of ± 5% from a position 0.5 mm below the steel sheet surface. Therefore, in the present invention, the term "0.5 mm below the surface of the steel sheet" means that it is within a range of ± 5% from the position of 0.5 mm in the thickness direction from the front and back surfaces of the steel sheet, in the direction of rolling. The microstructure is present in parallel sections. In addition, it indicates not only the simple surface of the finished product, but also the surface treated with the surface capable of measuring the crystal density of the steel sheet. For example, when the outermost surface of the steel sheet is covered with scales, it means removing it. After the noodles.

The microstructure of 0.5 mm below the surface of the steel sheet, and further has a total equivalent of 2 × 10 ² / mm ² in the structure, and the equivalent diameter of 0.01 μm to 0.5 μm containing one or two or more carbonized Nb, V, and Ti Compounds, nitrides and carbonitrides.

Carbides, nitrides, and carbonitrides (hereinafter referred to as "Nb, V, Ti-based precipitates" containing one or two or more kinds of Nb, V, and Ti in a microstructure 0.5 mm below the surface of the steel sheet of the present invention ). In addition, "the one or two or more kinds of carbides, nitrides, and carbonitrides containing Nb, V, and Ti" means one or two or more kinds of carbides containing Nb, V, and Ti, and Nb, V One or two or more nitrides of Ti, and one or two or more carbonitrides of Nb, V, and Ti.

The particle size of the Nb, V, and Ti-based precipitates is a circle-equivalent diameter of 0.01 μm to 0.5 μm. If it is less than 0.01 μm, hydrogen embrittlement is suppressed as a diffusible hydrogen capture site. The effect of chemical cracking is saturated. In addition, if the management is less than 0.01 μm in actual manufacturing, the manufacturing load is extremely increased, and the manufacturing cost is increased. On the other hand, if it exceeds 0.5 μm, the low-temperature toughness decreases. Furthermore, the effect of suppressing hydrogen embrittlement and cracking as a site for diffusing hydrogen cannot be obtained. It is preferably 0.03 μm or more. It is preferably 0.4 μm or less.

If the total of the Nb, V, and Ti-based precipitates having the above-mentioned particle size is less than 2 × 10 ² pieces / mm ² in the microstructure of 0.5 mm below the surface of the steel sheet, the precipitates function as a site for capturing diffusible hydrogen. Insufficient, the effect of suppressing hydrogen embrittlement and cracking as a site for diffusing hydrogen cannot be obtained. Therefore, it is 2 × 10 ² pieces / mm ² or more. It is preferably 5 × 10 ² pieces / mm ² or more. The number density and circle equivalent diameter of the Nb, V, and Ti-based precipitates can be measured by a method described in Examples described later.

In addition, if microstructures 0.5 mm below the surface of the steel plate are mixed with other structures such as Asada body in addition to the Vostian body, the low-temperature toughness is reduced. Therefore, the Voss field is more than 90%. Furthermore, from the viewpoint of lowering the low-temperature toughness, it is preferable that the area ratio of a structure such as Asada powder is small. The tissues such as the Mata powder are Mata powder, bainite, ferrous iron, and pearlite. When a structure such as Mata powder is mixed, it is desirable that the total area ratio of each structure to the entire steel plate is 10% or less.

[Manufacturing conditions]

Next, the manufacturing method of the steel plate of this invention is demonstrated. The steel sheet of the present invention is preferably a high-manganese steel sheet having a thickness of 4 mm or more.

The steel sheet of the present invention is obtained by having The steel material having the composition is heated to a temperature range, that is, when Tx (x = Nb, V, or Ti) is set to a temperature shown by the formulas (1) to (3) described later, the formula (1) to Any one or more of Tx (° C) defined by the formula (3) indicates that the surface temperature of the steel raw material is a temperature range of (Tx-50) ° C or more and (Tx + 200) ° C or less; the finishing rolling finishing temperature is 750 ° C The steel sheet is manufactured by hot rolling above 1000 ° C or lower; thereafter, the average cooling rate of the surface of the steel sheet from any lower temperature (finish rolling end temperature -50 ° C) or cooling start temperature to 650 ° C is 1.0 ° C Cool over s.

The details are described below. In the description, the expression "° C" regarding the temperature refers to the temperature of the surface of the steel sheet or the surface of the steel material.

The high-manganese steel sheet of the present invention can be used to melt molten steel having the above-mentioned component composition by a known melting method such as a converter or an electric furnace. Furthermore, it is also possible to perform refining twice in a vacuum degassing furnace. It is preferable that a steel material such as a billet having a predetermined size is subsequently produced by a known casting method such as a continuous casting method or a block-and-roll rolling method.

Steel slab after casting: The obtained steel raw material is not cooled to room temperature or heated to the following temperature range after cooling to room temperature, that is, Tx (x = Nb, V or Ti) is set to formula (1) ~ At the temperature shown in formula (3), it is expressed by any one or more of Tx (° C) defined by formulas (1) to (3), and the surface temperature of the steel material is (Tx-50) ° C or more, 200) ℃ temperature range below

T _Nb (℃) = 7500 / {3.0-log ₁₀ ([% Nb] × [% C])}-273… (1)

T _V (℃) = 10800 / {7.2-log ₁₀ ([% V] × [% C])}-273… (2)

T _Ti (℃) = 7000 / {2.8-log ₁₀ ([% Ti] × [% C])}-273… (3)

Here, [% Nb], [% V], [% Ti], and [% C] represent Nb, V, and Content (mass%) of Ti and C. When there are no elements, calculation is performed by setting the element symbol in the formula to 0.

If the heating temperature is lower than (Tx-50) ° C, the deformation resistance under hot rolling becomes high, and it becomes difficult to obtain a large reduction amount per pass. Therefore, the number of rolling passes increases, resulting in a reduction in rolling efficiency. When the casting defect in the steel raw material (slab) cannot be crimped. Furthermore, in the melting stage, crystals containing Nb, V, and Ti that were unevenly crystallized in the steel also remained in the steel sheet after the rolling was completed, and desired precipitates containing Nb, V, and Ti were not obtained. , Reduced stress corrosion cracking resistance.

On the other hand, if the heating temperature exceeds (Tx + 200) ° C, surface defects are liable to occur due to the scales during heating, and the dressing load after rolling is increased. In addition, the surface of the steel raw material was excessively decarburized, and the surface of the steel sheet after rolling became Asa Intermediate, and the bendability or hydrogen embrittlement decreased. Furthermore, due to the coarsening of body size in Vostian, the target microstructure was not obtained.

Therefore, the heating temperature of the steel material is (Tx-50) ° C or higher and (Tx + 200) ° C or lower. It is preferably (Tx-30) ° C or higher. The temperature is preferably (Tx + 180) ° C or lower. Furthermore, in the case of direct-feed rolling, the steel raw material starts hot rolling at (Tx-50) ° C or higher and (Tx + 200) ° C or lower.

In addition, in the present invention, the so-called "heating to the following temperature range, that is, when Tx (x = Nb, V, or Ti) is set to the temperature shown in formulas (1) to (3), is expressed by formula (1) ) ~ Tx (℃) defined by the formula (3) is more than one, and the surface temperature of the steel material is (Tx-50) ° C or more and (Tx + 200) ° C or less. It means, for example, that it contains Nb When V and V are used as the component composition, if the heating temperature satisfies (T _Nb -50) ° C or higher, (T _Nb +200) ° C or lower, or (T _V -50) ° C or higher, (T _V +200) ) Any one or more below ℃. That is, an arbitrary heating temperature can be selected.

Hot rolling: After rough rolling, the finish rolling finish temperature of finish rolling is set to 750 ° C or higher and 1000 ° C or lower to produce a steel plate having a desired thickness.

When the finish rolling temperature of hot rolling exceeds 1000 ° C., it is easy to recrystallize the Voss body near the surface of the steel sheet, fail to obtain a desired microstructure, and reduce the stress corrosion cracking resistance. On the other hand, if the finishing rolling temperature is set to less than 750 ° C, the thermal deformation resistance becomes excessively high, and the load on the rolling mill becomes large. In addition, the reduction in rolling efficiency leads to an increase in manufacturing costs. Therefore, the finish rolling finish temperature of hot rolling is 750 ° C or higher and 1000 ° C or lower. The temperature is preferably 800 ° C or higher. The temperature is preferably 950 ° C or lower. It is more preferably 940 ° C or lower.

The cumulative compression ratio in the temperature range of 850 ° C or higher (Tx-50) ° C or lower for finishing rolling is 10% or more and 50% or less (suitable conditions)

If the cumulative compression ratio in a temperature range of 850 ° C or higher and (Tx-50) ° C or lower is less than 10%, the target microstructure may not be obtained. On the other hand, if it exceeds 50%, the efficiency at the time of rolling will fall. In addition, there is a possibility that the strength becomes too large and the low-temperature toughness decreases. It should be noted that the cumulative reduction ratio is a total of the reduction ratios of rolling in the temperature range of 850 ° C. or higher and (Tx-50) ° C. or lower in the finish rolling.

The cumulative reduction ratio in the non-recrystallization temperature range of the finish rolling (960 ° C or lower) is 5% or more and 60% or less (more suitable conditions)

If the cumulative compression ratio in the non-recrystallization temperature region is less than 5%, the target strength may not be obtained. On the other hand, if it exceeds 60%, there is a possibility that the drop strength becomes too large and the low-temperature toughness may decrease. It should be noted that the cumulative reduction ratio is a total of the reduction ratios of the rolling passes in the non-recrystallization temperature range during the finish rolling.

After finishing rolling, cooling is performed from the lower temperature (finish rolling end temperature -50 ° C) or the cooling start temperature to 650 ° C, and the average cooling rate on the surface of the steel plate is 1.0 ° C / s or more.

If the average cooling rate on the surface of the steel sheet is less than 1.0 ° C./s, the steel sheet stays at a high temperature for a long time, and therefore, the carbides are coarsened and the strength is reduced. Not only this, but also the formation of Cr carbides, resulting in reduced toughness and stress corrosion cracking resistance. Therefore, the average cooling rate is preferably 1.0 ° C / s or more. It is more preferably 2.0 ° C / s or more. On the other hand, when the average cooling rate exceeds 150.0 ° C / s, it becomes difficult to secure the shape of the steel sheet. Therefore, the average cooling rate is preferably 150.0 ° C / s or less. The average cooling rate is more preferably 120.0 ° C / s or less. It is more preferably 100.0 ° C / s or less. Here, the "average cooling rate" is an average of the cooling rate from arbitrarily lower temperature (finish rolling end temperature -50 ° C) or cooling start temperature to 650 ° C after finishing rolling.

In the present invention, it is newly discovered that controlling the average cooling rate during cooling can effectively suppress the precipitation of Cr carbides during cooling, thereby improving the stress corrosion cracking resistance.

The average cooling rate in the temperature range from the finish rolling end temperature to (finish roll end temperature -50 ° C) is not particularly specified, but it is preferably 1.0 ° C in view of promoting the precipitation of Nb, V, and Ti-based precipitates. / s or less. In addition, the average cooling rate below 650 ° C is not particularly limited, but from the viewpoint of preventing deformation of the steel sheet, it is preferably Less than 100.0 ° C / s. It is more preferably 80.0 ° C / s or less.

[Example]

Hereinafter, the present invention will be described in detail through examples. The present invention is not limited to the following examples.

By the converter-barrel refining-continuous casting method, slabs (raw material thickness: 250mm ~ 300mm) prepared with various components shown in Table 1-1 and Table 1-2 are heated to (Tx-50) ° C or more , (Tx + 200) ° C or lower (x = Nb, V or Ti), hot rolling is performed under the manufacturing conditions shown in Table 2-1, Table 2-2, and then in Table 2-1, Table 2- Cooling was performed under the manufacturing conditions shown in 2. The (Tx-50) ° C and (Tx + 200) ° C of Nb, V, or Ti are shown in Table 1-1 and Table 1-2, respectively.

Regarding the obtained hot-rolled steel sheet having a plate thickness of 12 mm to 80 mm, a microstructure investigation, a base metal tensile test, a base metal toughness, and a stress corrosion cracking test were performed in the following manner.

(1) Micro organization

The microstructure investigation is about a section parallel to the rolling direction at a position of 0.5 mm below the plate thickness surface of each steel plate obtained. A sample for microstructure observation was taken, and a sodium metabisulfite aqueous solution (10 g Na ₂ S ₂ O ₅ + 95 ml of aqueous solution) was immersed and etched, and then the tissue was photographed in a 5-field at a magnification of 500 times by an optical microscope. Thereafter, the obtained tissue image was determined for the area ratio, circle equivalent diameter, and aspect ratio of the Voss field using an image analysis device.

Area ratio of Voss field

The area ratio of the Voss field is to etch the Voss field to 500 times Photographs were taken, the grain boundaries of the Voss field were tracked, and the ratio of the area of the Voss field over 10 μm to the total area of the Voss field area was determined by image analysis.

Circle equivalent diameter

The crystal grain size of the Voss fields, that is, the circle equivalent diameter of the Voss fields, is an image analysis of the tissue image to determine the area of each Voss field. The circle equivalent diameter is calculated from each area.

Aspect ratio of Vostian body grains

The aspect ratio of the Vostian body grains was observed by an optical microscope. The microstructures appearing at the grain boundaries of the Vossian body due to the corrosion were calculated. For each Vossian body grain, the widest width (normally longest) (Minor diameter) to major diameter.

Circle equivalent diameter of Nb, V, Ti system precipitates

The investigation of the circle equivalent diameter of Nb, V, and Ti-based precipitates was performed on a cross section parallel to the rolling direction at a position 0.5 mm below the thickness surface of each steel plate. Photography was performed, and the area of each Nb, V, and Ti-based precipitate was measured using image analysis on the tissue image. The circle equivalent diameter of Nb, V, and Ti-based precipitates was calculated from each area.

Number density of Nb, V, Ti-based precipitates

The investigation of the number density of Nb, V, and Ti-based precipitates is a section parallel to the rolling direction at a position 0.5 mm below the plate thickness surface of each steel plate. The number of Nb, V, and Ti-based precipitates having a circle equivalent diameter of 0.01 μm to 0.5 μm per 1 mm ² was investigated, and the total number density of the Nb, V, and Ti-based precipitates was determined.

(2) Tensile properties of base material

From each of the obtained steel plates, a Japanese Industrial Standards (JIS) No. 5 tensile test piece was used, a tensile test was performed in accordance with JIS Z 2241 (1998), and the tensile characteristics were investigated. In the present invention, it is assumed that the drop-out strength is 400 MPa or more so that the base material has excellent tensile properties (within the scope of the present invention). In addition, the excellent tensile properties of the base material of the present invention means that the tensile strength is 800 MPa or more and the total elongation is 30% or more.

(3) toughness of base material

The direction perpendicular to the rolling direction from the 1/4 position of the plate thickness of each steel plate with a plate thickness exceeding 20mm, or the 1/2 position of the plate thickness of each steel plate with a plate thickness of 20mm or less, according to JIS Z 2202 (1998) Charpy V-notch test pieces were taken, and three Charpy impact tests were performed on each steel plate in accordance with JIS Z 2242 (1998). The absorbed energy at -196 ° C was determined to evaluate the toughness of the base metal. In the present invention, an average value of three absorbed energy (vE- ₁₉₆ ) of 50 J or more is considered as excellent base material toughness (within the scope of the present invention). Still more preferably, the average value of the absorbed energy (vE- ₁₉₆ ) is 100 J or more.

(4) Stress corrosion cracking

The stress corrosion cracking test is performed in accordance with the Slow Strain Rate Test Method of the National Association of Corrosion Engineers (NACE) standard TM0111-2011. A test piece in the shape of a test piece with a Type A round bar cut was used, and the test piece was immersed in artificial seawater (chloride ion concentration: 18000 ppm) at a temperature of 23 ° C and a strain rate of 4 × 10 ^-7 inch / sec. A constant speed tensile test was performed. In the present invention, a fracture stress of 500 MPa or more is considered to be excellent in stress corrosion cracking resistance (within the scope of the present invention). It is more preferable that the breaking stress is 600 MPa or more.

The results obtained from the above are shown in Tables 3-1 and 3-2.

It was confirmed that the examples of the present invention satisfy the above-mentioned target performance (the undulation strength of the base material is 400 MPa or more, the average value of the low-temperature toughness is the absorbed energy (vE-196) is 50 J or more, and the stress corrosion cracking resistance is 500 MPa or more). On the other hand, any one or more of the base material strength, low-temperature toughness, and stress corrosion cracking resistance of the comparative examples that are outside the range of the present invention cannot satisfy the target performance. Furthermore, in Tables 3-1 and 3-2, steel plate No. 12 and steel plate No. 36 as comparative examples have stable Vostfield bodies due to the fact that C in the component composition exceeds the range of the present invention, but has a stable number of vowel fields. There are many unstable Voss fields, so the area ratio of the Voss fields with an average circle equivalent diameter of 10 μm or more and an aspect ratio of the long and short diameters of 3 or more is 70%.

Claims (4)

A high-manganese steel sheet having C: 0.20% to 0.70%, Si: 0.05% to 1.0%, Mn: 15% to 30%, P: 0.028% or less, S: 0.02% or less, and Al: 0.01% ~ 0.1%, Cr: 0.5% ~ 7.0%, Ni: 0.03% ~ 0.30%, N: 0.0010% ~ 0.0200%, and contains Nb: 0.003% ~ 0.030%, V: 0.03% ~ 0.10%, Ti: One or two or more of 0.003% to 0.040%, and the remainder contains Fe and inevitable impurities. The microstructure of 0.5 mm below the surface of the steel plate uses the Voss field as the matrix phase. An area ratio of 25% or more means that the circle equivalent diameter is 10 μm or more, and the aspect ratio of the major axis to the minor axis is 3 or more. The high-manganese steel sheet according to item 1 of the scope of patent application, further comprising, in addition to the component composition, an element selected from at least one of the following group A or group B, group A: Based on mass%, selected from one or two of Mo: 0.05% to 2.0%, W: 0.05% to 2.0%; Group B: calculated from mass%, selected from Ca: 0.0005% to 0.0050%, Mg: 0.0005 % ~ 0.0050%, rare earth metal: one or two or more of 0.0010% ~ 0.0200%. The high-manganese steel sheet according to item 1 or item 2 of the patent application scope, wherein the microstructure is 0.5 mm below the surface of the steel sheet, and further, the microstructure has a total of 2 × 10 ² pieces / mm ² or more. Carbide, nitride, and carbonitride containing one or two or more kinds of Nb, V, and Ti having a circle equivalent diameter of 0.01 μm to 0.5 μm. A method for manufacturing a high-manganese steel plate is to heat a steel raw material having the composition as described in any one of the scope of claims 1 to 3 of a patent application to a temperature range as follows, that is, Tx (x = When Nb, V or Ti) is set to the temperature shown in formulas (1) to (3), it is expressed by any one or more of Tx (° C) defined by formulas (1) to (3). The surface of the steel raw material The steel sheet is produced by hot rolling at a temperature range of (Tx-50) ° C to (Tx + 200) ° C and below, and a finish rolling finish temperature of 750 ° C to 1000 ° C. Temperature -50 ° C) or any lower temperature from the cooling start temperature to 650 ° C. The average cooling rate on the surface of the steel plate is 1.0 ° C / s or more for cooling, T _Nb (° C) = 7500 / {3.0-log ₁₀ ([% Nb] × [% C])}-273… (1) T _V (℃) = 10800 / {7.2-log ₁₀ ([% V] × [% C])}-273… (2) T _Ti (℃ ) = 7000 / {2.8-log ₁₀ ([% Ti] × [% C])}-273… (3) Here, [% Nb], [% V], [% Ti], and [% C] are respectively The content (mass%) of Nb, V, Ti, and C in the steel is shown. When the element is not contained, the element symbol in the formula is set to 0 and calculated.