KR102497359B1 - Steel plate and method of producing same - Google Patents

Abstract

Provided is a high Mn steel excellent in corrosion resistance, particularly corrosion resistance in a salt corrosion environment. C: 0.20% or more and 0.70% or less, Si: 0.05% or more and 1.00% or less, Mn: 15.0% or more and 35.0% or less, P: 0.030% or less, S: 0.0200% or less, Al: 0.010% or more and 0.100% or less, Cr: It contains 0.5% or more and 8.0% or less and N: 0.0010% or more and 0.0300% or less, the balance has a component composition of Fe and unavoidable impurities, and 60% or more of the Cr is solid solution Cr.

Description

Steel plate and its manufacturing method {STEEL PLATE AND METHOD OF PRODUCING SAME}

The present invention relates to a steel sheet suitable for use in structural steel used in a cryogenic environment such as a liquefied gas reservoir tank and the like, and particularly excellent in corrosion resistance in a saltwater corrosion environment, and a method for manufacturing the same.

Attempts have been made to use hot-rolled steel sheets for structures such as tanks for storing liquefied gas. Since the use environment of such a structure is extremely low temperature, a hot-rolled steel sheet applied to the structure is required not only to have high strength but also to have excellent toughness at a very low temperature. For example, when a hot-rolled steel sheet is used for boiling of liquefied natural gas, it is necessary to ensure excellent toughness at -164°C or lower, which is the boiling point of liquefied natural gas. If the low-temperature toughness of steel materials is low, there is a risk that safety as a cryogenic low-temperature low-temperature structure may not be maintained, so there is a strong demand for improved low-temperature toughness of steel materials to be applied.

In response to this demand, conventionally, austenitic stainless steel, 9% Ni steel, or 5000 series aluminum alloy having an austenite structure that does not show brittleness at cryogenic temperatures has been used. However, since these metal materials have high alloy costs and high manufacturing costs, there is a demand for inexpensive steel sheets with excellent cryogenic toughness. Therefore, as a new steel sheet in place of conventional steel for cryogenic use, application of high Mn steel, which has an austenite structure by adding a large amount of Mn, a relatively inexpensive austenite stabilizing element, as a structural steel sheet in a cryogenic environment has been studied.

However, when a steel sheet having an austenite structure is placed in a corrosive environment, there is a problem that austenite grain boundaries are eroded by corrosion, and stress corrosion cracking is likely to occur when tensile stress is applied. In the manufacturing stage of a structure for storing liquefied gas, there are cases where the base iron of the steel sheet is exposed to the surface, and when the surface of the steel material comes into contact with water vapor, moisture or oil containing corrosive substances such as salt, corrosion occurs in the steel material. . At this time, in the corrosion reaction on the surface of the steel sheet, iron generates oxides (rust) by an anode reaction, while hydrogen is generated by a cathode reaction of moisture, and hydrogen embrittlement occurs due to hydrogen entering the steel. . When residual stress in bending or welding during manufacturing or load stress in the use environment acts thereon, stress corrosion cracking occurs and there is a risk of structural failure. High Mn steels that have been studied in the past sometimes have poor corrosion resistance compared to austenitic stainless steels as well as 9% Ni steels and ordinary low-alloy steels. Therefore, from the viewpoint of safety, it is important that steel materials used for structures have high strength and toughness at extremely low temperatures as well as excellent corrosion resistance.

For example, in Patent Literature 1, machinability and Charpy impact characteristics of the heat-affected zone at -196°C are improved by adding 15 to 35% of Mn, 5% or less of Cu, and appropriate amounts of C and Cr. is disclosed.

Further, in Patent Literature 2, C: 0.25 to 0.75%, Si: 0.05 to 1.0%, Mn: more than 20% to 35% or less, Ni: 0.1% to less than 7.0%, Cr: 0.1% to less than 8.0% added Thus, a high Mn steel with improved low-temperature toughness is disclosed.

Further, in Patent Literature 3, by containing 0.001 to 0.80% of C and 15 to 35% of Mn, and adding elements such as Cr, Ti, Si, Al, Mg, Ca, and REM, the cryogenic toughness of the base material and the weld zone is improved. An improved, high Mn steel is disclosed.

Japanese Patent Publication No. 2015-508452 Japanese Unexamined Patent Publication No. 2016-84529 Japanese Unexamined Patent Publication No. 2016-196703

However, the steel materials described in Patent Literatures 1, 2 and 3 still have room for improvement in terms of manufacturing cost for achieving strength and low-temperature toughness, and corrosion resistance when the austenitic steel materials described above are placed in a salt corrosion environment. there is

The present invention has been made in view of these problems, and an object thereof is to provide a high Mn steel excellent in corrosion resistance, particularly corrosion resistance in a salt corrosion environment. Here, “excellent corrosion resistance” is a test based on the Slow Strain Rate Test Method of NACE Standard TM0111-2011, immersed in artificial seawater (chloride ion concentration 18000 ppm) at a temperature of 23 ° C, strain rate: 4 × When a constant velocity tensile test is conducted at 10 ^-7 inch/s, it means that the breaking stress is 600 MPa or more.

In order to achieve the above object, the inventors of the present invention, as a result of diligent investigation of various factors that determine the component composition and production conditions of high Mn steel, have reached the following findings.

a. The initial corrosion reaction on the steel sheet surface in a salt water corrosion environment can be delayed by using high Mn steel as a base, adding Cr thereto, and appropriately controlling the addition amount and the high amount. Thereby, the amount of hydrogen penetrating into the steel can be reduced, and the above-mentioned stress corrosion cracking of the austenitic steel is suppressed.

b. Further, in order to effectively suppress fracture of austenite from grain boundaries, countermeasures for increasing grain boundary strength are effective. In particular, P, as an element that tends to segregate together with Mn during the solidification process of steel slabs, reduces the grain boundary strength of the portion intersecting with such a segregated portion. Therefore, it is necessary to reduce impurity elements such as P. On the other hand, B is an element that increases the strength of austenite grain boundaries, and by adding B in addition to reducing impurity elements such as P, it becomes possible to further effectively suppress grain boundary fracture.

This invention was made by adding examination further to the above knowledge, and the summary is as follows.

1. In mass %,

C: 0.20% or more and 0.70% or less;

Si: 0.05% or more and 1.00% or less;

Mn: 15.0% or more and 35.0% or less;

P: 0.030% or less;

S: 0.0200% or less;

Al: 0.010% or more and 0.100% or less;

Cr: 0.5% or more and 8.0% or less;

N: 0.0010% or more and 0.0300% or less; and

B: 0.0003% or more and 0.0100% or less

A steel sheet containing, the balance being Fe and unavoidable impurities, wherein 60% or more of the Cr is solid-solution Cr.

2. The above component composition is further expressed in mass%,

Nb: 0.003% or more and 0.030% or less;

V: 0.01% or more and 0.10% or less; and

Ti: 0.003% or more and 0.040% or less

The steel sheet excellent in corrosion resistance as described in the said 1 containing 1 type(s) or 2 or more types selected from.

3. The above component composition is further expressed in mass%,

Cu: 0.01% or more and 0.50% or less;

Ni: 0.01% or more and 0.50% or less;

Sn: 0.01% or more and 0.30% or less;

Sb: 0.01% or more and 0.30% or less;

Mo: 0.01% or more and 2.0% or less; and

W: 0.01% or more and 2.0% or less

The steel sheet as described in said 1 or 2 containing 1 type(s) or 2 or more types selected from.

4. The above component composition is further expressed in mass%,

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

Mg: 0.0005% or more and 0.0100% or less; and

REM: 0.0010% or more and 0.0200% or less

The steel sheet according to 1, 2 or 3 above, containing one or two or more selected from the above.

5. After heating the steel material having the component composition described in any one of 1 to 4 above to 1000 ° C. or more and 1300 ° C. or less, hot rolling is performed, finishing temperature: 750 ° C. or more, material temperature to be rolled: 950 ° C. or less 600 ° C. or more A method for producing a steel sheet in which the residence time in is 30 minutes or less, and then cooling is performed at an average cooling rate of 3°C/s or more in a temperature range of 700°C or less to 600°C or more.

ADVANTAGE OF THE INVENTION According to this invention, the steel plate excellent in corrosion resistance, especially corrosion resistance in a salt corrosion environment can be provided. Therefore, by using the steel plate of the present invention for a steel structure used in a cryogenic environment, such as a liquefied gas storage tank, for example, the safety and lifespan of the steel structure is greatly improved, resulting in a special industrial effect. do. In addition, since the steel sheet of the present invention is cheaper than conventional materials, it also has an advantage of excellent economic efficiency.

Hereinafter, the steel sheet of the present invention will be described in detail. In addition, this invention is not limited to the following embodiment.

[Ingredient Composition]

First, the component composition of the steel sheet of the present invention and the reason for its limitation will be described. In the present invention, in order to ensure excellent corrosion resistance, the component composition of the steel sheet is specified as follows. In addition, "%" indicating a component composition shall mean "mass %" unless otherwise indicated.

C: 0.20% or more and 0.70% or less

C is an important element for obtaining austenite as an inexpensive austenite stabilizing element that is effective for high strength and inexpensive. In order to obtain the effect, C needs to contain 0.20% or more. On the other hand, if the content exceeds 0.70%, excessive precipitation of Cr carbides and Nb, V, Ti-based carbides is promoted, and these precipitates serve as a starting point for corrosion and lower low-temperature toughness. For this reason, C is made into 0.20 % or more and 0.70 % or less. Preferably, it is 0.25% or more and 0.60% or less.

Si: 0.05% or more and 1.00% or less

Si acts as a deoxidizer and is not only necessary for steelmaking, but also has an effect of increasing the strength of the steel sheet by solid solution strengthening in steel. In order to obtain such an effect, Si needs to contain 0.05% or more. On the other hand, when it contains more than 1.00%, weldability and surface properties may deteriorate and stress corrosion cracking resistance may decrease. For this reason, Si is made into 0.05 % or more and 1.00 % or less. Preferably, it is 0.07% or more and 0.50% or less.

Mn: 15.0% or more and 35.0% or less

Mn is a relatively inexpensive austenite stabilizing element. In the present invention, it is an important element for achieving both strength and toughness at cryogenic temperatures. In order to obtain the effect, Mn needs to contain 15.0% or more. On the other hand, when it contains more than 35.0%, the effect of improving the toughness at cryogenic temperatures is saturated, resulting in an increase in alloy cost. Moreover, weldability and cutability deteriorate. In addition, it causes segregation of Mn and promotes the occurrence of stress corrosion cracking. For this reason, Mn is 15.0% or more and 35.0% or less. Preferably, it is set as the range of 18.0% or more and 28.0% or less.

P: 0.030% or less

When P is contained in an amount exceeding 0.030%, it is segregated at the grain boundaries to reduce grain boundary strength, and becomes a starting point for stress corrosion cracking. For this reason, it is preferable to make 0.030 % an upper limit and reduce it as much as possible. P is preferably set to 0.024% or less, more preferably 0.020% or less, since the lower the content of P, the better the characteristics. On the other hand, if P is to be less than 0.001%, a great amount of cost is required for steelmaking and economic efficiency is impaired. Therefore, from the viewpoint of economic efficiency, a content of 0.001% or more is acceptable.

S: 0.0200% or less

Since S deteriorates the low-temperature toughness and ductility of the base material, it is preferable to reduce it as much as possible with 0.0200% as the upper limit. Therefore, S is 0.0200% or less, preferably 0.0180% or less. On the other hand, if the content is less than 0.0001%, a large amount of cost is required for steelmaking and economic efficiency is impaired. Therefore, from the viewpoint of economic efficiency, a content of 0.0001% or more is acceptable.

Al: 0.010% or more and 0.100% or less

Al acts as a deoxidizing agent and is most commonly used in a molten steel deoxidizing process. In addition, by fixing solid solution N in steel to form AlN, it has an effect of suppressing coarsening of crystal grains. Moreover, it has the effect of suppressing the toughness deterioration by the reduction of solid solution N. In order to obtain such an effect, Al needs to contain 0.01% or more. On the other hand, when it contains more than 0.100%, coarse nitrides may be formed, and the stress corrosion cracking resistance may decrease as a starting point of corrosion or fracture. In addition, it diffuses into the weld metal portion during welding and deteriorates the toughness of the weld metal. Therefore, Al is 0.100% or less. Preferably, it is 0.020% or more and 0.070% or less.

Cr: 0.5% or more and 8.0% or less, and 60% or more of Cr is solid solution Cr

Cr has an effect of retarding the initial corrosion reaction on the surface of the steel sheet in a salt water corrosion environment by inclusion of an appropriate amount. This effect reduces the amount of hydrogen penetration into the steel sheet and is an important element for improving stress corrosion cracking resistance. In order to obtain such an effect, 0.5% or more of containing is required. On the other hand, when Cr exceeds 8.0%, the obtained effect is saturated, and economical efficiency is rather impaired. Therefore, the amount of Cr is 0.5% or more and 8.0% or less. Preferably, it is 1.0 % or more.

Here, among the added Cr, the solid solution contributes to the improvement of the stress corrosion cracking resistance, but the precipitated content may conversely hinder the improvement of the stress corrosion cracking resistance. It is important. That is, when the dissolved Cr is 60% or more of the amount of Cr contained, the above effects can be obtained, and the stress corrosion cracking resistance can be improved by adding Cr. It is preferable that solid solution Cr is 70% or more of the amount of Cr contained, and it is more preferable that it is 100%.

In addition, solid solution Cr is a state in which solute atoms exist in an atomic state without forming precipitates or the like. Specifically, the amount of solid solution Cr was determined by taking a test piece for electrolytic extraction from a steel sheet and extracting the precipitate by an electrolytic extraction method using a 10% AA (10% acetylacetone-1% tetramethylammonium chloride-methanol) solution. ICP emission It can be obtained by measuring the amount of Cr in the precipitate by an analytical method and subtracting it from the total Cr in the test piece.

N: 0.0010% or more and 0.0300% or less

N, as an austenite stabilizing element, is an element effective in improving cryogenic toughness. In addition, it bonds with Nb, V and Ti, precipitates finely as a nitride or carbonitride, and has an effect of suppressing stress corrosion cracking as a trap site for diffusible hydrogen. In order to obtain such an effect, N needs to contain 0.0010% or more. On the other hand, when it contains more than 0.0300%, formation of excessive nitrides or carbonitrides is promoted, the amount of solid solution elements decreases, and not only corrosion resistance decreases, but also toughness decreases. For this reason, N is 0.0010% or more and 0.0300% or less. Preferably it is 0.0020% or more and 0.0150% or less.

B: 0.0003% or more and 0.0100% or less

B is an element that increases the strength of the austenitic grain boundary, and is effective in improving stress corrosion cracking resistance by suppressing cracking at grain boundaries. In order to obtain such an effect, B needs to contain 0.0003% or more. Preferably, it is 0.0005% or more, more preferably more than 0.0007%, and more than 0.0010%. On the other hand, when it contains exceeding 0.0100%, this effect is saturated. Therefore, B was limited to 0.0100% or less. Preferably, it is 0.0070% or less.

In the present invention, for the purpose of further improving corrosion resistance, in addition to the above essential elements, Nb: 0.003% or more and 0.030% or less, V: 0.01% or more and 0.10% or less, and Ti: 0.003% or more 0.040%, as needed. May contain the following.

Nb: 0.003% or more and 0.030% or less

Nb is an element that is precipitated as a carbonitride and has an effect of suppressing stress corrosion cracking because the precipitated carbonitride functions as a trap site for diffusible hydrogen. In order to obtain such an effect, it is preferable to contain Nb at 0.003% or more. On the other hand, when it contains more than 0.030%, coarse carbonitrides may precipitate and become the starting point of destruction. In addition, precipitates may be coarsened to deteriorate base material toughness. For this reason, when it contains Nb, it is preferable to set it as 0.003 % or more and 0.030 % or less. More preferably, they are 0.005% or more and 0.025% or less, and also 0.007% or more and 0.022% or less.

V: 0.01% or more and 0.10% or less

V is an element that is precipitated as a carbonitride and has an effect of suppressing stress corrosion cracking because the produced carbonitride functions as a trap site for diffusible hydrogen. In order to obtain such an effect, it is preferable to contain V at 0.01% or more. On the other hand, when it contains more than 0.10%, coarse carbonitrides may precipitate and become the starting point of destruction. In addition, precipitates may be coarsened to deteriorate base material toughness. For this reason, when containing V, it is preferable to set it as 0.01 % or more and 0.10 % or less. More preferably, they are 0.02% or more and 0.09% or less, and also 0.03% or more and 0.08% or less.

Ti: 0.003% or more and 0.040% or less

Ti is an element that has an effect of suppressing stress corrosion cracking because it is precipitated as a nitride or carbonitride and the nitride or carbonitride produced functions as a trap site for diffusible hydrogen. In order to obtain such an effect, it is preferable to contain Ti at 0.003% or more. On the other hand, when it contains more than 0.040%, precipitates may become coarse and the base material toughness may be deteriorated. Coarse carbonitrides may also precipitate and become the starting point of destruction. For this reason, when it contains Ti, it is preferable to set it as 0.003 % or more and 0.040 % or less. More preferably, they are 0.005% or more and 0.035% or less, and also 0.007% or more and 0.032% or less.

In addition, in the present invention, for the purpose of further improving corrosion resistance, as necessary,

Cu: 0.01% or more and 0.50% or less, Ni: 0.01% or more and 0.50% or less, Sn: 0.01% or more and 0.30% or less, Sb: 0.01% or more and 0.30% or less, Mo: 0.01% or more and 2.0% or less, W: 0.01% or more 2.0 % or less of 1 type or 2 or more types can be contained.

That is, Cu, Ni, Sn, Sb, Mo, and W are elements that improve corrosion resistance in a salt water corrosion environment of high Mn steel by adding Cr in combination. Here, Cu, Sn, and Sb have an effect of suppressing the hydrogen generation reaction, which is a cathode reaction, by increasing the hydrogen overpotential of the steel material. Ni forms a precipitate film on the steel surface and physically suppresses permeation of corrosive anions such as Cl ⁻ into the base iron. In addition, during corrosion, Cu, Ni, Sn, Sb, Mo, and W are released as metal ions from the surface of the steel material, and by making the corrosion product dense, the corrosive anions to the steel interface (interface between long layer and base iron) inhibit permeation. Mo and W are released as Mo ₄ ^2- and WO ₄ ^2- , respectively, and are adsorbed in corrosion products or on the surface of the steel sheet, thereby imparting cation selective permeability and electrically suppressing the permeation of corrosive anions into the base iron. .

The above effects are manifested when Cr and Cr coexist in high Mn steel, and are expressed when each element is added in an amount of 0.01% or more. However, when a large amount of any element is contained, weldability and toughness are deteriorated, and the viewpoint of cost is also disadvantageous.

Therefore, the amount of Cu is in the range of 0.01% or more and 0.50% or less, the amount of Ni is in the range of 0.01% or more and 0.50% or less, the amount of Sn is in the range of 0.01% or more and 0.30% or less, the amount of Sb is in the range of 0.01% or more and 0.30% or less, It is preferable that the amount of Mo is in the range of 0.01% or more and 2.0% or less, and the amount of W is in the range of 0.01% or more and 2.0% or less.

More preferably, the amount of Cu is 0.02% or more and 0.40% or less, the amount of Ni is 0.02% or more and 0.40% or less, the amount of Sn is 0.02% or more and 0.25% or less, the amount of Sb is 0.02% or more and 0.25% or less, and the amount of Mo is 0.02% 0.40% or less, and the amount of W is 0.02% or more and 0.40% or less.

Similarly, in the present invention, for the purpose of further improving corrosion resistance, as necessary,

One or two or more of Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0100% or less, and REM: 0.0010% or more and 0.0200% or less may be contained.

That is, Ca, Mg, and REM are useful elements for controlling the shape of inclusions, and may be contained as necessary. Here, the control of the shape of inclusions refers to making the sulfide-based inclusions that have undergone transformation into granular inclusions. Through controlling the shape of this inclusion, ductility, toughness and sulfide stress corrosion cracking resistance can be improved. In order to obtain such an effect, it is preferable to contain Ca and Mg at 0.0005% or more, and REM at 0.0010% or more. On the other hand, when a large amount of any element is contained, the amount of non-metallic inclusions increases, and ductility, toughness, and sulfide stress corrosion cracking resistance may rather decrease. In addition, there are cases where it becomes economically disadvantageous.

For this reason, when containing Ca, it is 0.0005% or more and 0.0050% or less, When containing Mg, it is 0.0005% or more and 0.0100% or less, When containing REM, it is preferable to set it as 0.0010% or more and 0.0200% or less. More preferably, the amount of Ca is 0.0010% or more and 0.0040% or less, the amount of Mg is 0.0010% or more and 0.0040% or less, and the amount of REM is 0.0020% or more and 0.0150% or less.

Next, the manufacturing conditions of this invention are demonstrated. In addition, the temperature of the material to be rolled in the hot rolling process and the cooling rate in the cooling process thereafter mean the temperature and cooling rate measured on the surface of the rolled material. That is, after heating the steel material having the above component composition to 1000 ° C. or more and 1300 ° C. or less, hot rolling is performed, the reduction ratio: 3 or more and 30 or less, the finishing temperature: 750 ° C. or more, and the material temperature to be rolled: 950 ° C. or less 600 ° C. A steel sheet is manufactured by carrying out the residence time in the above to 30 minutes or less, and then performing cooling at an average cooling rate of 3°C/s or more in a temperature range of 700°C or less to 600°C or more.

[Heating temperature of steel material: 1000 ° C or more and 1300 ° C or less]

Heating the steel material to 1000 ° C. or higher is to make the carbonitride in the structure into solid solution and to homogenize the crystal grain size and the like. That is, when the heating temperature is less than 1000°C, the desired characteristics are not obtained because the carbonitride is not sufficiently dissolved. In addition, when heated at a temperature exceeding 1300°C, in addition to material deterioration due to coarsening of the crystal grain size, excessive energy is required and productivity decreases. Therefore, the upper limit of the heating temperature is set at 1300°C. It is preferably 1050°C or higher and 1250°C or lower, more preferably 1070°C or higher and 1250°C or lower. Further, as the steel material, it is preferable to use a steel material such as a slab or billet by a conventionally known method such as an ingot method other than a continuously cast slab. In addition, it goes without saying that processing such as blow refining or vacuum degassing may be added to molten steel.

[Finishing temperature of hot rolling: 750 ℃ or more]

When the finishing temperature of hot rolling is less than 750 ° C, the amount of carbides precipitated during the rolling significantly increases. may be lost, and corrosion resistance is lowered. In addition, when rolling at less than 750 ° C., the deformation resistance increases and an excessive load is applied to the manufacturing equipment, so the rolling finish temperature is set to 750 ° C. or higher.

[Average cooling rate at 700 ° C or lower and 600 ° C or higher: 3 ° C / s or higher]

For cooling after hot rolling, if the average cooling rate at 700°C or lower and 600°C or higher is less than 3°C/s, a large amount of precipitates such as Cr carbides are formed, so the average cooling rate is limited to 3°C/s or higher. . Preferably, it is the range of 10 °C/s or more and 150 °C/s or less.

[Residence time in the temperature range of 950 ° C or lower and 600 ° C or higher: 30 minutes or less]

In hot rolling, if the time during which the material to be rolled remains in the temperature range of 950 ° C. or less and 600 ° C. or more exceeds 30 minutes, carbonitrides and carbides precipitate in large quantities during rolling, and the required amount of solid solution Cr decreases, and corrosion resistance deteriorates. And since it causes a decrease in cryogenic toughness, the staying time in the temperature range of 950 ° C. or less and 600 ° C. or more is regulated to 30 minutes or less. Preferably, it is the range of 5 minutes or more and 25 minutes or less.

Here, in order to make the stay time in the temperature range of 950 ° C. or less and 600 ° C. or more to 30 minutes or less, the length of the material to be rolled is 5000 mm or less, and the reduction ratio in hot rolling from the material to be rolled is 30 or less it is desirable That is, when the length of the material to be rolled is 5000 mm or less and the reduction ratio is 30 or less, the rolling time is shortened, and as a result, the staying time in the range of 950 ° C. or less and 600 ° C. or more can be 30 minutes or less.

As described above, the upper limit of the reduction ratio in hot rolling is preferably 30 or less. On the other hand, when the reduction ratio in hot rolling is less than 3, the effect of accelerating recrystallization and sizing is reduced, and as a result, coarse austenite grains remain and the corrosion resistance is deteriorated because the portion is preferentially oxidized. there is Therefore, it is preferable to make the reduction ratio in hot rolling into 3 or more.

Here, the reduction ratio is defined as (the sheet thickness of the raw material to be rolled for hot rolling)/(the sheet thickness of the steel sheet after hot rolling).

Example

After melting the steels No. 1 to 57 shown in Table 1 into slabs, steel plates of sample Nos. 1 to 65 having a plate thickness of 6 mm or more and 50 mm or less were manufactured according to the manufacturing conditions shown in Table 2. . Next, the obtained steel sheet was subjected to the following corrosion resistance test. In addition, the results of measuring the amount of solid solution Cr by the electrolytic extraction method described above are also described in Table 2.

The corrosion resistance test was conducted based on the NACE Standard TM0111-2011 Standard Slow Strain Rate Test Method (hereinafter referred to as SSRT test). That is, the shape of the test piece was immersed in artificial seawater (chloride ion concentration 18000 ppm) at a temperature of 23 ° C. using a Type A round bar notch formation test piece, and a constant velocity tensile test was performed at a strain rate: 4 × 10 ^-7 inch / s. . Here, a fracture stress of 600 MPa or more was regarded as excellent in stress corrosion cracking resistance.

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

It was confirmed that the steel sheets according to the present invention (Sample Nos. 1 to 42) had corrosion resistance of 600 MPa or more as a breaking stress in the SSRT test. On the other hand, the comparative examples (sample Nos. 43 to 65) outside the scope of the present invention were unable to satisfy the above-described target performance in stress corrosion cracking resistance.

Claims (5)

in mass %,
C: 0.20% or more and 0.70% or less;
Si: 0.05% or more and 1.00% or less;
Mn: 15.0% or more and 35.0% or less;
P: 0.030% or less;
S: 0.0200% or less;
Al: 0.010% or more and 0.100% or less;
Cr: 0.5% or more and 8.0% or less;
N: 0.0010% or more and 0.0300% or less; and
B: 0.0003% or more and 0.0100% or less
A steel sheet containing, the balance being Fe and unavoidable impurities, wherein 60% or more of the Cr is solid-solution Cr. According to claim 1,
The above component composition is further expressed in mass%,
Nb: 0.003% or more and 0.030% or less;
V: 0.01% or more and 0.10% or less;
Ti: 0.003% or more and 0.040% or less;
Cu: 0.01% or more and 0.50% or less;
Sn: 0.01% or more and 0.30% or less;
Sb: 0.01% or more and 0.30% or less;
Mo: 0.01% or more and 2.0% or less;
W: 0.01% or more and 2.0% or less;
Ca: 0.0005% or more and 0.0050% or less;
Mg: 0.0005% or more and 0.0100% or less; and
REM: 0.0010% or more and 0.0200% or less
A steel sheet containing one or two or more selected from In the manufacturing method of the steel plate according to claim 1 or 2,
After heating the steel material having the above component composition to 1000 ° C. or higher and 1300 ° C. or lower, hot rolling is performed, finishing temperature: 750 ° C. or higher, temperature of the material to be rolled: 950 ° C. or lower, 600 ° C. or higher, the residence time is 30 minutes or less A method for producing a steel sheet in which cooling is performed, and then cooling is performed at an average cooling rate of 3° C./s or more in a temperature range of 700° C. or less and 600° C. or more. delete delete