TWI630277B - High manganese steel plate and manufacturing method thereof - Google Patents

Abstract

The present invention provides a high manganese steel sheet and a method of producing the same. A high manganese steel sheet containing C: 0.20% to 0.70%, Si: 0.05% to 1.0%, Mn: 15% to 30%, P: 0.028% or less, and S: 0.02% or less, by mass%; 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 Nb: 0.003% to 0.030%, V: 0.03% to 0.10%, Ti: 0.003% to 0.040% of one or two or more, and the remainder contains a composition of Fe and unavoidable impurities. The microstructure of 0.5 mm under the surface of the steel sheet is a matrix phase of the Worth field, which is in the Worth field. 25% or more of the area ratio is a circle equivalent diameter of 10 μm or more, and an aspect ratio of the long diameter to the short diameter 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 in a brine corrosion environment, which is suitable for use in a storage tank for a liquefied gas storage tank or the like in a very low temperature environment, and a method for producing the same .

When a hot-rolled steel sheet is used for the structure for a liquefied gas storage tank, the use environment becomes extremely low temperature, and therefore the strength of the steel sheet is required, and the toughness at an extremely low temperature is also required. For example, in the case of using a hot-rolled steel sheet in a storage tank of a liquefied natural gas, it is necessary to ensure excellent toughness at a boiling point (-164 ° C) or lower of the liquefied natural gas. If the low-temperature toughness of the steel material is inferior, there is a risk that the safety of the structure for the cryogenic storage tank cannot be maintained. Therefore, there is a strong demand for improving the low-temperature toughness of the applied steel material. For this requirement, a Woustian system stainless steel or a 9% nickel (Ni) steel or a 5000 series aluminum alloy in which a Worth field is a steel sheet structure which does not exhibit brittleness at an extremely low temperature has been used. However, since the alloy cost or the manufacturing cost is high, there is an urgent need for a steel material which is excellent in inexpensive and extremely low temperature toughness. Therefore, as a new type of steel material in place of the conventional ultra-low temperature steel, a large amount of high-manganese steel sheet to which a relatively inexpensive Worstian body stabilized element manganese (Mn) is added is used as a structural steel for an extremely low temperature environment.

On the other hand, in the case of using Worthfield steel in a corrosive environment, there is a problem that the grain boundary of the Worth field is eroded by corrosion, and is attached. In the case of tensile stress, stress corrosion cracking easily occurs. In particular, in the production stage of the structure for a liquefied gas storage tank, the surface of the ferrite of the steel sheet may be exposed, and if the surface of the steel material is in contact with water vapor or moisture or oil containing a corrosive substance such as salt, It causes corrosion of the steel. In the case of a high-manganese steel sheet which has been studied from the past, there is a case where the Vostian system stainless steel is of poor corrosion resistance as compared with 9% nickel steel or a usual low-alloy steel. At this time, in the corrosion reaction on the surface of the high-manganese steel sheet, iron generates an oxide (rust) due to the anode reaction, and hydrogen is generated by the cathode reaction of moisture, which promotes stress corrosion cracking due to hydrogen embrittlement. There is a risk that the stress corrosion cracking as described above may be caused by the residual stress such as bending work or welding at the time of production or the load stress in the use environment, so that the structure is broken. 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 excellent in stress corrosion cracking resistance.

For example, Patent Document 1 discloses a steel material in which Mn: 15% to 35% and Cu: 5% or less are added, and C and Cr are added in an appropriate amount to improve machinability and weld heat influence at -196 ° C. The Charpy impact feature.

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

[Prior Art Literature]

[Patent Literature]

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

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

However, the high manganese steel sheets described in Patent Document 1 and Patent Document 2 have a Charpy impact characteristic at 60 ° C to 135 J at -196 ° C for the purpose of having strength and low temperature toughness (only shown in Patent Document 1). in). However, the extremely low temperature toughness of the base material is not sufficient, and the low temperature toughness and the stress corrosion cracking resistance are not considered.

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

In order to achieve the above-mentioned problems, the present inventors have made intensive studies on the composition, manufacturing method, and various factors for determining the microstructure of a steel sheet for ensuring excellent stress corrosion cracking resistance for a high-manganese steel sheet. The following findings.

1. In order to achieve both low-temperature toughness and excellent stress corrosion cracking resistance, it is effective to reduce the amount of hydrogen permeation in the steel sheet by a corrosion reaction. It is important to improve the corrosion resistance of the steel sheet surface in a salt water corrosive environment, so it is important to strictly manage the composition of the composition based on the high manganese steel sheet. In particular, Cr and Ni are simultaneously added, and the amount of addition is appropriately controlled, whereby the rust formed at the initial stage of the corrosion reaction on the surface of the steel sheet is fine. Moreover, the amount of hydrogen permeated in the steel can be reduced by delaying the subsequent corrosion reaction.

2. Further, it has been found that a method of strictly managing the microstructure near the surface of the steel sheet is also effective in improving the stress corrosion cracking resistance. In other words, the area ratio of the Worth field is 25% or more, and the circle equivalent diameter is 10 μm or more, and the aspect ratio of the long diameter to the short diameter is 3 or more, which is important in terms of improving stress corrosion cracking. It is considered that the reason is that hydrogen which has penetrated into the inside of the steel sheet due to the corrosion reaction is trapped in the crystal grains of the non-recrystallized Worth field body, so the relative amount of hydrogen on the grain boundary of the Worthfield body is lowered, and the stress corrosion of the grain boundary of the Worthfield body is suppressed. The rupture sensitivity is reduced.

3. In addition to the above 1, 2, carbides, nitrides, and composite carbonitrides of Nb, V, and Ti in the steel sheet can be further improved in stress corrosion cracking resistance by appropriately managing the dispersion state thereof. Carbides, nitrides, and composite carbonitrides of Nb, V, and Ti act as a trapping portion of diffusible hydrogen in the steel sheet. In other words, it acts as a trapping portion of diffusible hydrogen generated by the corrosion reaction of the steel material, and has an effect of suppressing stress corrosion cracking. The heating, rolling, and cooling conditions in the hot rolling step affect the dispersion state of carbides, nitrides, and composite carbonitrides of Nb, V, and Ti in the Worth field. Therefore, it is important to manage these manufacturing conditions.

4. Further, in order to effectively suppress the grain boundary destruction of the Worthfield, it is effective to improve the grain boundary strength. P is an element which is easily segregated together with Mn during solidification of the steel sheet, and the grain boundary strength which intersects with the microsegregation portion is lowered. Therefore, it is necessary to reduce the impurity element such as P.

The present invention has been made in view of the above findings, and its gist 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 one 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 unavoidable impurities, the microstructure of 0.5 mm under the surface of the steel sheet is the matrix phase of the Worth field, and the area ratio of 25% or more in the Worth 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], wherein, in addition to the component composition, further comprising an element selected from at least one of Group A or Group B described below; Group A: In terms of mass%, one or two selected from the group consisting of Mo: 0.05% to 2.0%, W: 0.05% to 2.0%; Group B: in terms of mass%, selected from Ca: 0.0005% to 0.0050% , Mg: 0.0005% to 0.0050%, rare earth metal (REM): one or more of 0.0010% to 0.0200%.

[3] The high-manganese steel sheet according to [1] or [2], wherein the microstructure of 0.5 mm below the surface of the steel sheet further has a total of 2 × 10 ² /mm ² or more in the microstructure. The round equivalent diameter is from 0.01 μm to 0.5 μm, and one or two or more kinds of carbides, nitrides, and carbonitrides containing Nb, V, and Ti.

[4] A method for producing a high-manganese steel sheet, wherein the steel material having the chemical composition according to any one of [1] to [3] is heated to a temperature region, that is, Tx (x = Nb) When V, Ti or Ti) is at a temperature represented by the formula (1) to the formula (3), the steel material is represented by any one or more of Tx (°C) defined by the formulas (1) to (3). The surface temperature is (Tx-50) ° C or more and (Tx + 200) ° C or less, and the finish rolling temperature is 750 ° C or more and 1000 ° C or less, and the steel sheet is produced, and thereafter, the steel sheet is self-rolled. Cooling at any lower temperature of the end temperature of -50 ° C) or the cooling start temperature to the surface of the steel sheet of 650 ° C is 1.0 ° C / s or more for cooling; T _Nb ( ° C ) = 7500 / {3.0 - log ₁₀ ([ %Nb]×[%C])}-273...(1)

Tv(°C)=10800/{7.2-log ₁₀ ([%V]×[%C])}-273...(2)

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

Here, [%Nb], [%V], [%Ti], and [%C] respectively represent the content (% by mass) of Nb, V, Ti, and C in the steel; in the case of an element not contained Calculate by setting the element symbol in the formula to 0.

In the present invention, the term "high strength" means a strength having a strength of 400 MPa or more. Further, 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. Further, in the present invention, "excellent stress corrosion cracking resistance" means 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, immersed in artificial seawater (chloride ion concentration: 18000 ppm) at a temperature of 23 ° C, and subjected to a constant-speed tensile test at a strain rate of 4 × 10 ^-7 inches / sec. The breaking stress is 500 MPa or more.

According to the present invention, a high manganese steel sheet excellent in stress corrosion cracking resistance can be obtained. Further, the high manganese steel sheet of the present invention contributes greatly to the safety and life of the steel structure used in a cryogenic environment such as a storage tank for a liquefied gas storage tank, and is industrially effective. Moreover, it does not cause a decrease in productivity and an increase in manufacturing cost, and therefore is excellent in economy.

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

[component composition]

First, the component composition of the steel sheet of the present invention will be described with respect to the reason for limitation. In the present invention, in order to secure excellent stress corrosion cracking resistance, the chemical composition of the steel sheet is defined as follows. In addition, the % which shows the component composition shows the mass % unless it demonstrates especially.

C: 0.20%~0.70%

C is an inexpensive Worth field stabilization element that is used to obtain important elements of the Worth field. In order to obtain the effect, C must contain 0.20% or more. On the other hand, when it contains more than 0.70%, Cr carbide and Nb, V, and Ti-based carbide are excessively formed, and low-temperature toughness and stress corrosion cracking resistance are lowered. Therefore, C is 0.20%~ 0.70%. It is preferably 0.25% or more. It is preferably 0.60% or less.

Si: 0.05%~1.0%

Si acts as a deoxidizing material, and is not only required for steel making, but also has an effect of solid-solution strengthening in steel to strengthen the steel sheet by solid solution strengthening. In order to obtain such an effect, Si must contain 0.05% or more. On the other hand, when it contains more than 1.0%, weldability will deteriorate. Moreover, it also has an effect on Stress Corrosion Cracking (SCC). Therefore, Si is 0.05% to 1.0%. It is preferably 0.07% or more. It is preferably 0.5% or less.

Mn: 15% to 30%

Mn is a relatively inexpensive Worth 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 contain 15% or more. On the other hand, even if it contains more than 30%, the effect of improving the low-temperature toughness is saturated, resulting in an increase in alloy cost. Further, the weldability and the cutting property are deteriorated. Further, it promotes segregation and promotes the generation of stress corrosion cracking. Therefore, Mn is 15% to 30%. It is preferably 18% or more. It is preferably 28% or less.

P: 0.028% or less

When P contains more than 0.028%, it segregates to the grain boundary and becomes a starting point for stress corrosion cracking. Therefore, it is desirable to use 0.028% as the upper limit and reduce as much as possible. Therefore, P is 0.028% or less. Furthermore, excessive P reduction causes a sharp increase in refining costs and is economically disadvantageous, so it is desirable to be 0.002% or more. It is preferably 0.005% or more. It is preferably 0.024% or less.

S: 0.02% or less

Since S deteriorates the low temperature toughness or ductility of the base material, it is desirable to reduce the temperature as much as possible by 0.02%. Therefore, S is 0.02% or less. Furthermore, excessive S reduction causes a sharp increase in refining costs and is economically disadvantageous, so it is desirable to be 0.001% or more. It is preferably 0.002% or more. It is preferably 0.018% or less.

Al: 0.01%~0.1%

Al acts as a deoxidizer and is most commonly used in the steel strip deoxidation process of steel sheets. Further, it has an effect of fixing solid solution N in the steel to form AlN, thereby suppressing coarsening of crystal grains. Further, it has an effect of suppressing deterioration of toughness due to reduction in solid solution N. In order to obtain such an effect, Al must contain 0.01% or more. On the other hand, when the content exceeds 0.1%, the weld metal portion is mixed during welding, and the toughness of the weld metal is deteriorated, so that it is 0.1% or less. Therefore, Al is 0.01% to 0.1%. It is preferably 0.02% or more. It is preferably 0.07% or less.

Cr: 0.5%~7.0%

Cr is an element which is stabilized by a proper amount to stabilize the Worth field and is effective for improving the low temperature toughness and the strength of the base material. Further, in the present invention, it is an important element for improving the penetration resistance of hydrogen in the steel sheet by reducing the amount of penetration of hydrogen into the steel sheet by 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 contain 0.5% or more. On the other hand, when it contains more than 7.0%, low temperature toughness and stress corrosion cracking resistance fall by the formation of Cr carbide. Therefore, Cr is 0.5% to 7.0%. It is preferably 1.0% or more, more preferably 1.2% or more, and still more preferably 2.5%. It is preferably 6.0% or less, more preferably 5.7%, still more preferably 5.5% or less.

Ni: 0.03%~0.30%

Ni is a representative Worth field stabilization element which is an element effective for improving the extremely low temperature toughness and the strength of the base material. Further, in the present invention, it is an important element for improving the penetration resistance of hydrogen in the steel sheet by reducing the amount of penetration of hydrogen into the steel sheet by 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 contain 0.03% or more. On the other hand, when the content exceeds 0.30%, the alloy cost increases, and the effect of improving the stress corrosion cracking resistance is saturated. Therefore, Ni is 0.03% to 0.30%. It is preferably 0.04% or more. It is preferably 0.25% or less.

N: 0.0010%~0.0200%

N is a Worth field stabilization element which is an element effective for improving the extremely low temperature toughness. Further, it has an effect of binding to Nb, V, and Ti to precipitate as a nitride or a carbonitride, and suppressing stress corrosion cracking as a trapping portion of diffusible hydrogen. In order to obtain such an effect, N must contain 0.0010% or more. On the other hand, when the content exceeds 0.0200%, the nitride or carbonitride is coarsened and the toughness is lowered. Therefore, N is 0.0010% to 0.0200%. It is preferably 0.0020% or more. It is preferably 0.0150% or less.

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

Nb: 0.003%~0.030%

Nb is precipitated as a carbonitride (including carbide), and the produced carbonitride is effective at a portion where diffusible hydrogen is trapped, and has an effect of suppressing stress corrosion cracking. In order to obtain such an effect, Nb must be contained in an amount of 0.003% or more. on the other hand, When the content exceeds 0.030%, a coarse carbonitride is precipitated, which is a starting point of destruction. Further, there is a phenomenon in which precipitates are coarsened and the base material toughness is deteriorated. Therefore, in the case of containing Nb, it is 0.003% to 0.030%. It is preferably 0.005% or more, more preferably 0.007% or more. It is preferably 0.025% or less, more preferably 0.022% or less.

V: 0.03%~0.10%

V is precipitated as a carbonitride, and the produced carbonitride is effective at a portion where diffusible hydrogen is trapped, and has an effect of suppressing stress corrosion cracking. In order to obtain such an effect, V must contain 0.03% or more. On the other hand, when the content is more than 0.10%, a coarse carbonitride is precipitated, which is a starting point of destruction. Further, there is a phenomenon in which precipitates are coarsened and the base material toughness is deteriorated. Therefore, in the case of containing V, it is 0.03% to 0.10%. It is preferably 0.04% or more, more preferably 0.05% or more. It is preferably 0.09% or less, more preferably 0.08% or less.

Ti: 0.003%~0.040%

Ti is precipitated as a nitride or a carbonitride, and the produced nitride or carbonitride is effective at a portion where diffusible hydrogen is trapped, and has an effect of suppressing stress corrosion cracking. In order to obtain such an effect, Ti must be contained in an amount of 0.003% or more. On the other hand, when the content is more than 0.040%, the precipitates are coarsened and the toughness of the base material is deteriorated. Further, there is a phenomenon in which coarse carbonitrides are precipitated and become a starting point of destruction. Therefore, in the case of containing Ti, it is 0.003% to 0.040%. It is preferably 0.005% or more, more preferably 0.007% or more. It is preferably 0.035% or less, more preferably 0.032% or less.

The remainder is iron and inevitable impurities. Examples of the unavoidable impurities include O or H, and the like is acceptable if the total amount is 0.01% or less.

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 low-temperature toughness, in addition to the essential elements, the following elements may be contained as needed.

Mo: 0.05% to 2.0%, W: 0.05% to 2.0% of one or two Mo: 0.05% to 2.0%

Mo is an element useful for increasing the strength of the base material, and may be contained as needed. In order to obtain such an effect, it is preferred to contain 0.05% or more of Mo. On the other hand, when it is more than 2.0%, the toughness and the weld fracture resistance may be adversely affected. Therefore, Mo is preferably 2.0% or less. Therefore, in the case of containing Mo, it is 0.05% to 2.0%.

W: 0.05%~2.0%

W is an element useful for increasing the strength of the base material, and may be contained as needed. In order to obtain such an effect, it is preferred to contain 0.05% or more of W. On the other hand, when it is more than 2.0%, the toughness and the weld fracture resistance may be adversely affected. Therefore, W is preferably 2.0% or less. Therefore, in the case of containing W, it is 0.05% to 2.0%. More preferably, it is 0.07% or more. More preferably, it is 1.5% or less.

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

Ca: 0.0005%~0.0050%

Ca is an element useful for the morphology control of inclusions, and may be included as needed. Have. The "morphological control of inclusions" means inclusions in which elongated sulfide-based inclusions are granular. Through the morphology control of the inclusions, ductility, toughness, and sulfide stress corrosion cracking resistance are improved. In order to obtain such an effect, it is preferred to contain 0.0005% or more of Ca. On the other hand, when the content exceeds 0.0050%, the amount of non-metallic inclusions increases, and conversely, toughness, and sulfide stress corrosion cracking resistance may be lowered. Moreover, there are cases where the economy becomes unfavorable. Therefore, in the case of containing Ca, it is 0.0005% to 0.0050%. More preferably, it is 0.0010% or more. More preferably, it is 0.0040% or less.

Mg: 0.0005%~0.0050%

Mg is useful as an element which contributes to improvement of sulfide stress corrosion cracking resistance, and may be contained as needed. In order to obtain such an effect, it is preferred to contain 0.0005% or more of Mg. On the other hand, even if the content exceeds 0.0050%, the effect is saturated, and the effect corresponding to the content cannot be expected. Moreover, there are cases where the economy becomes unfavorable. Therefore, in the case of containing Mg, it is 0.0005% to 0.0050%. More preferably, it is 0.0010% or more. More preferably, it is 0.0040% or less.

REM: 0.0010%~0.0200%

REM is useful as an element that contributes to the improvement of sulfide stress corrosion cracking resistance, and may be contained as needed. In order to obtain such an effect, it is preferred to contain 0.0010% or more of REM. On the other hand, even if it contains more than 0.0200%, the effect is saturated, and the effect corresponding to the content cannot be expected. Therefore, in the case of containing REM, it is 0.0010% to 0.0200%. More preferably, it is 0.0020% or more. More preferably, it is 0.0150% or less.

[micro organization]

Next, the microstructure of the vicinity of 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 under the surface of the steel sheet is the matrix phase of the Worth field. The area ratio of the Worth field is 25% or more, the circle equivalent diameter is 10 μm or more, and the aspect ratio of the long diameter to the short diameter is 3. the above.

In the present invention, the matrix phase of the microstructure of 0.5 mm below the surface of the steel sheet is set as the Worthian body. Further, in the Voss field, a Worth field having an area ratio of 25% or more and an equivalent diameter of 10 μm or more and an aspect ratio of a long diameter to a short diameter of 3 or more is used as the diffusible hydrogen. The capture site works effectively. Thereby, the suppression of stress corrosion cracking can be particularly improved. Preferably, the area ratio is 30% or more. On the other hand, when the area ratio exceeds 95%, the toughness of the base material may be deteriorated. It is preferably 90% or less. More preferably, it is 85% or less.

If the circle equivalent diameter is less than 10 μm, or the aspect ratio of the long diameter to the short diameter is less than 3, the effect cannot be obtained. Further, the circle equivalent diameter, the area ratio, and the aspect ratio of the Vostian body can be measured by the method described in the examples below.

In the present invention, the term "0.5 mm under the surface of the steel sheet" means a position of 0.5 mm in the thickness direction from the front and back surfaces of the steel sheet. The surface which is the surface of the surface of the steel sheet, and the surface which measures the crystallographic degree of the steel sheet is not only the surface which is the surface of the steel sheet, for example, when the outermost surface of the steel sheet is covered with scales, the surface after removal is shown. .

Furthermore, in the present invention, the position is 0.5 mm from the surface of the steel sheet. The effect can be similarly obtained even in the range of 5% even if the microstructure is present.

In the microstructure of 0.5 mm under the surface of the steel sheet, one or more types of Nb, V, and Ti having a circle equivalent diameter of 2 × 10 ² /mm ² or more and having a circle equivalent diameter of 0.01 μm to 0.5 μm are further contained in the structure. Carbides, nitrides and carbonitrides.

One or two or more kinds of carbides, nitrides, and carbonitrides containing Nb, V, and Ti in the microstructure of 0.5 mm under the surface of the steel sheet of the present invention (hereinafter referred to as "Nb, V, Ti-based precipitates" The existence status of ) is explained. In addition, "one or two or more kinds of carbides, nitrides, and carbonitrides containing Nb, V, and Ti" mean one or two or more kinds of carbides containing Nb, V, and Ti, and contain Nb and V. One or two or more nitrides of Ti contain one or two or more kinds of carbonitrides of Nb, V, and Ti.

The particle diameter of the Nb, V, and Ti-based precipitates is a circle equivalent diameter of 0.01 μm to 0.5 μm. When it is less than 0.01 μm, the effect of suppressing hydrogen embrittlement cracking as a trapping portion of diffusible hydrogen is saturated. Further, management in the actual production of less than 0.01 μm causes an extremely large manufacturing load, resulting in an increase in manufacturing cost. On the other hand, when it exceeds 0.5 μm, the low temperature toughness is lowered. Further, the effect of suppressing hydrogen embrittlement cracking as a trapping portion of diffusible hydrogen cannot be obtained. It is preferably 0.03 μm or more. It is preferably 0.4 μm or less.

When the total of the Nb, V, and Ti-based precipitates having the particle diameter is less than 2 × 10 ² /mm ² in the microstructure of 0.5 mm under the surface of the steel sheet, the trapping portion as the diffusible hydrogen cannot be obtained and the hydrogen embrittlement is suppressed. The effect of rupture. Therefore, it is 2 × 10 ² /mm ² or more. It is preferably 5 × 10 ² /mm ² or more. Further, the number density and the circle equivalent diameter of the Nb, V, and Ti-based precipitates can be measured by the method described in the examples below.

Further, in the microstructure of 0.5 mm under the surface of the steel sheet, in addition to the Worth field, a structure such as a granule of the genus is mixed, and the low-temperature toughness is lowered. Therefore, it is preferable that the area ratio of the tissue such as the granules is small, and when it is mixed, it is preferable that the total area ratio is 10% or less with respect to the entire steel sheet.

[Manufacture conditions]

Next, a method of producing the steel sheet of the present invention will be described. Further, 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 according to the present invention is obtained by heating a steel material having the above-described composition to a temperature region in which Tx (x = Nb, V or Ti) is expressed as a formula (1) to be described later. When the temperature is represented by the formula (3), the surface temperature of the steel material is (Tx-50) ° C or more, as indicated by any one or more of Tx (° C.) defined by the formulas (1) to (3). (Tx + 200) °C or less; a steel sheet is produced by hot rolling at a finishing finish temperature of 750 ° C or higher and 1000 ° C or lower; and thereafter, from (finishing finish temperature -50 ° C) or cooling start temperature The average cooling rate of the surface of the steel sheet of any lower temperature to 650 ° C is 1.0 ° C / s or more for cooling.

The details are described below. In the description, the expression "°C" of the temperature means 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 melted by a known melting method such as a converter or an electric furnace to have a molten steel having the above composition. Moreover, it can also be used in a vacuum degassing furnace. 2 times of refining. It is preferable to form a steel material such as a steel slab of a predetermined size by a known casting method such as a continuous casting method or a block-block rolling method.

After casting the billet: the obtained steel material is not cooled to room temperature or cooled to room temperature and then heated to the following temperature range, that is, Tx (x=Nb, V or Ti) is set to formula (1)~ In the case of the temperature represented by the formula (3), the surface temperature of the steel material is (Tx-50) ° C or more, as indicated by any one or more of Tx (°C) defined by the formulas (1) to (3). Tx+200) Temperature range below °C T _Nb (°C)=7500/{3.0-log ₁₀ ([%Nb]×[%C])}-273...(1)

Tv(°C)=10800/{7.2-log ₁₀ ([%V]×[%C])}-273...(2)

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

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

When the heating temperature is less than (Tx - 50) ° C, the deformation resistance under hot rolling becomes high, and it becomes difficult to obtain a large amount of reduction per pass, so that the number of rolling passes increases, and the rolling efficiency is lowered, and there is It is not possible to crimp the case of casting defects in steel raw materials (steel billets). Further, in the melting stage, crystals containing Nb, V, and Ti which are not uniformly crystallized in the steel remain in the steel sheet after the rolling, and the desired precipitates containing Nb, V, and Ti are not obtained. , resistance to stress corrosion cracking is reduced.

On the other hand, when the heating temperature exceeds (Tx + 200) °C, surface flaws are likely to occur due to the scales during heating, and the dressing load after rolling increases. Moreover, the surface of the steel raw material is excessively decarburized, and the surface of the steel sheet after rolling becomes a granulated body. Bending or hydrogen embrittlement is reduced. Further, the target micro-tissue was not obtained due to the coarsening of the Worthite body.

Therefore, the heating temperature of the steel material is (Tx-50) ° C or more and (Tx + 200) ° C or less. It is preferably (Tx-30) °C or more. It is preferably (Tx + 180) ° C or less. Further, in the case of direct rolling, the steel material is hot rolled at (Tx-50) ° C or higher and (Tx + 200) ° C or lower.

Further, in the present invention, "heating to a temperature range, that is, when Tx (x = Nb, V or Ti) is set to a temperature represented by the formulas (1) to (3), Any one or more of Tx (°C) defined by the formula (3) indicates that the surface temperature of the steel material is (Tx-50) ° C or more and (Tx + 200) ° C or less. When both of Nb and V are contained as the component composition, the heating temperature satisfies (T _Nb -50) ° C or more, (T _Nb +200) ° C or less, or (Tv - 50) ° C or more, (Tv + 200). Any one or more below °C. That is, any heating temperature can be selected.

Hot rolling: After the rough rolling, the finish rolling finishing temperature of the finish rolling is set to 750 ° C or more and 1000 ° C or less to obtain a steel sheet having a desired thickness.

When the finishing rolling temperature of hot rolling exceeds 1000 ° C, recrystallization of the Worth field near the surface of the steel sheet is facilitated, and the desired microstructure is not obtained, resulting in a decrease in stress corrosion cracking resistance. On the other hand, when the finish rolling finishing temperature is less than 750 ° C, the thermal deformation resistance is excessively increased, and the load on the rolling mill is increased. Moreover, the rolling efficiency is lowered to cause an increase in manufacturing cost. Therefore, the finishing rolling temperature of hot rolling is 750 ° C or more and 1000 ° C or less. It is preferably 800 ° C or more. It is preferably 950 ° C or lower.

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

When the cumulative compression ratio of the temperature region of 850 ° C or more and (Tx-50) ° C or less is less than 10%, the target microstructure is not obtained. On the other hand, when it exceeds 50%, the efficiency at the time of rolling falls. In addition, the cumulative compression ratio is obtained by adding the compression ratios of the respective rolling passes in the temperature range of 850 ° C or more and (T x 50) ° C or less in the finish rolling.

After finishing finishing, cooling from any lower temperature (finishing finish temperature -50 ° C) or cooling start temperature to 650 ° C, cooling at an average cooling rate of the steel sheet surface of 1.0 ° C / s or more

When the average cooling rate of the surface of the steel sheet is less than 1.0 ° C / s, it stays at a high temperature for a long period of time, so that the carbide is coarsened, so the strength is lowered. Not only that, but also the formation of Cr carbides causes a decrease in toughness and stress corrosion cracking. Therefore, the average cooling rate is 1.0 ° C / s or more. It is preferably 2.0 ° C / s or more. On the other hand, if 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 120.0 ° C / s or less. More preferably, it is 100.0 ° C / s or less. Here, the "average cooling rate" is an average of the cooling rate from any lower temperature (finishing finish temperature - 50 ° C) or cooling start temperature to 650 ° C after completion of finish rolling.

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

Furthermore, the finish rolling temperature is ~ (the finish rolling temperature is -50 ° C) Although the average cooling rate of the region is not particularly limited, it is preferably 1.0 ° C / s or less from the viewpoint of promoting precipitation of Nb, V, and Ti-based precipitates. Further, the average cooling rate of less than 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. More preferably, it is 80.0 ° C / s or less.

[Examples]

Hereinafter, the present invention will be described in detail by way of examples. Furthermore, the invention is not limited to the following examples.

The billet (raw material thickness: 250 mm to 300 mm) prepared with various components shown in Table 1 is heated to (Tx-50) °C or higher (Tx + 200) °C by a converter-barrel refining-continuous casting method. After (x=Nb, V or Ti), the production conditions shown in Table 2 and the cumulative compression ratio of the temperature range of 850 ° C or more and (Tx-50) ° C or less of the finish rolling were set to 40%, and heat was applied. It was rolled and then cooled under the manufacturing conditions shown in Table 2. Further, (Tx-50) ° C and (Tx + 200) ° C of Nb, V or Ti are shown in Table 1.

Regarding the obtained hot-rolled steel sheets having a thickness of 12 mm to 80 mm, a microstructure investigation, a base material tensile test, a base material toughness, and a stress corrosion cracking test were carried out in the following manner.

(1) Micro organization

The investigation of the microstructure was performed on a section perpendicular to the rolling direction at a position of 0.5 mm below the thickness of each steel sheet obtained, and a sample for microstructural observation was taken by an aqueous sodium metabisulfite solution (10 g Na ₂ S ₂ O ₅ + After immersion etching in 95 ml of an aqueous solution, the tissue was subjected to 5-field imaging at a magnification of 500 times by an optical microscope. Thereafter, the area ratio, the circle equivalent diameter, and the aspect ratio of the Worth field body were obtained using the image analysis device for the obtained tissue image.

Area ratio of the Worth field

The area ratio of the Worth field is to etch the Worth field, take photos of the tissue 500 times, track the grain boundary of the Worthfield, and find the area of the Worth field above 10 μm by image analysis. The proportion of the total area of the area of the field.

Round equivalent diameter of the Worth field

The crystal grain size of the Worth field, that is, the circle equivalent diameter of the Worth field, is an image analysis of the tissue image, and the area of each Worth field is measured. The circle equivalent diameter is calculated from each area.

Aspect ratio of Worthfield body particles

The aspect ratio of the Wolsfield body particles is obtained by observing the structure of the Worth field grain boundary due to the corrosion by an optical microscope, and calculating the widest width orthogonal to the longest diameter (long diameter) for each Worthfield body particle ( The ratio of the short diameter to the long diameter.

The circle equivalent diameter of Nb, V, Ti precipitates

The round-equivalent diameter of the Nb, V, and Ti-based precipitates is a cross section parallel to the rolling direction at a position of 0.5 mm below the thickness surface of each steel sheet, and 50,000 times the 10 fields of view by a transmission electron microscope. Photography, the area of each Nb, V, and Ti-based precipitate was measured using image analysis on the tissue image. The circle equivalent diameter of the Nb, V, and Ti-based precipitates was calculated from the respective areas.

Number density of Nb, V, Ti precipitates

The number density of the Nb, V, and Ti-based precipitates is a cross section parallel to the rolling direction at a position of 0.5 mm below the thickness of each steel sheet, and 50,000-times of 10 fields of view is performed by a transmission electron microscope. The number of Nb, V, and Ti-based precipitates having a circle-equivalent diameter of from 0.01 μm to 0.5 μm per 1 mm ² was investigated, and the total number density of Nb, V, and Ti-based precipitates was determined.

(2) Base material tensile properties

A JIS No. 5 tensile test piece was used for each of the obtained steel sheets, and a tensile test was carried out in accordance with the regulations of JIS Z 2241 (1998) to investigate the tensile properties. In the present invention, the lodging strength of 400 MPa or more is excellent in the tensile properties of the base material (within the scope of the present invention). Further, 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) Base metal toughness

The direction perpendicular to the rolling direction of each steel sheet of the steel plate having a thickness of more than 20 mm and the plate thickness of 1/2 of the plate thickness of 20 mm or less is perpendicular to the rolling direction, in accordance with JIS Z 2202 (1998). A Charpy V-notch test piece was used, and three Charpy impact tests were performed on each steel sheet in accordance with JIS Z 2242 (1998), and the absorbed energy at -196 ° C was determined to evaluate the base material toughness. In the present invention, the average value of the three absorbed energy (vE- ₁₉₆ ) is 50 J or more, which is excellent in the base material toughness (within the scope of the present invention). Further preferably, the average value of the absorbed energy (vE - ₁₉₆ ) is 100 J or more.

(4) Stress corrosion cracking

The stress corrosion cracking test was carried out in accordance with the Slow Strain Rate Test Method of the National Association of Corrosion Engineers (NACE) Standard TM0111-2011. Using a test piece in the shape of a test piece with a type A round bar cut, immersed in artificial seawater (chloride ion concentration of 18000 ppm) at a temperature of 23 ° C at a strain rate of 4 × 10 ^-7 inches / sec. A constant velocity tensile test was carried out. In the present invention, the breaking stress is 500 MPa or more, which is excellent in stress corrosion cracking resistance (within the scope of the present invention). Further preferably, the breaking stress is 600 MPa or more.

The results obtained based on the above are shown in Table 3.

It was confirmed that the example of the present invention satisfies the target performance (the falling strength of the base material is 400 MPa or more, the low temperature toughness is the average value of the absorbed energy (vE - ₁₉₆ ) is 50 J or more, and the stress corrosion cracking resistance is the breaking stress of 500 MPa or more). On the other hand, any one or more of the base material strength, the low temperature toughness, and the stress corrosion cracking resistance of the comparative example beyond the scope of the present invention cannot satisfy the target performance. In addition, in the steel plate No. 12 of the comparative example, although the C in the component composition is outside the range of the present invention, the stable Worth is small, but the unstable Worth is large, so the average circle is The area ratio of the Worth field having an equivalent diameter of 10 μm or more and an aspect ratio of the long diameter to the short diameter of 3 or more is 70%.

Claims (3)

A high manganese steel sheet containing C: 0.20% to 0.70%, Si: 0.05% to 1.0%, Mn: 15% to 30%, P: 0.028% or less, and S: 0.02% or less, by mass%; 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 Nb: 0.003% to 0.030%, V: 0.03% to 0.10%, Ti: 0.003% to 0.040% of one or two or more, and the remainder contains a composition of Fe and unavoidable impurities, and a microstructure of 0.5 mm under the surface of the steel sheet is a matrix phase of the Worth field, the Worth field 25% or more of the area ratio in the middle is a circle equivalent diameter of 10 μm or more, and an aspect ratio of the long diameter to the short diameter is 3 or more, and the microstructure of 0.5 mm under the surface of the steel sheet is further provided in the microstructure. A total of 2 × 10 ² /mm ² or more of a circle equivalent diameter of 0.03 μm to 0.5 μm containing one or two or more kinds of carbides, nitrides, and carbonitrides of Nb, V, and Ti. The high manganese steel sheet according to claim 1, wherein, in addition to the component composition, an element selected from at least one of the group A or the group B described below is further included; In terms of mass%, one or two selected from the group consisting of Mo: 0.05% to 2.0%, W: 0.05% to 2.0%; Group B: in terms of mass%, selected from Ca: 0.0005% to 0.0050% , Mg: 0.0005% to 0.0050%, rare earth metal (REM): one or more of 0.0010% to 0.0200%. A method for producing a high-manganese steel sheet, which comprises heating a steel material having the composition described in the first or second aspect of the patent application to a temperature region, that is, Tx (x=Nb, V or Ti) When the temperature is expressed by the formula (1) to the formula (3), the surface temperature of the steel material is expressed by any one or more of Tx (°C) defined by the formulas (1) to (3). Tx-50) In a temperature range of °C or more and (Tx+200) °C or less, a steel sheet is produced by hot rolling at a finish rolling temperature of 750 ° C or higher and 1000 ° C or lower, and thereafter, the steel sheet is finished at a finishing temperature of -50 °C) or any lower temperature of the cooling start temperature to 650 ° C, the average cooling rate of 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) Tv(°C)=10800/{7.2-log ₁₀ ([%V]×[%C])}-273...(2) T _Ti (°C)=7000/ {2.8-log ₁₀ ([%Ti]×[%C])}-273...(3) Here, [%Nb], [%V], [%Ti], and [%C] respectively represent steel The content (% by mass) of Nb, V, Ti, and C; and the element not included, the calculation is performed by setting the element symbol in the formula to 0.