JPWO2019059095A1 - Steel sheet and manufacturing method thereof - Google Patents

Abstract

Provided is a high Mn steel having excellent 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 0.0200% or less, Al: 0.010% or more and 0.100% or less, Cr: 0.5% or more and 8.0% or less, and N: 0.0010% or more and 0.0300% or less, and the balance Fe and It has a component composition of inevitable impurities, and 60% or more of the contained Cr is solid solution Cr.

Description

  The present invention relates to a steel sheet suitable for structural steel used in a cryogenic environment such as a tank for a liquefied gas storage tank, and particularly to a steel sheet excellent in corrosion resistance in a salt water corrosive environment and a method for producing the same.

  When a hot-rolled steel sheet is provided to a structure for a liquefied gas storage tank, the use environment is extremely low, so that the hot-rolled steel sheet is required to have not only strength but also toughness at a very low temperature. For example, it is necessary to ensure excellent toughness at −164 ° C. or lower, which is the boiling point of liquefied natural gas, in a hot-rolled steel sheet used in a liquefied natural gas storage tank. If the low-temperature toughness of the steel material is inferior, there is a risk that the safety as a cryogenic storage tank structure cannot be maintained. Therefore, there is a strong demand for improving the low-temperature toughness of the applied steel material. In response to this requirement, conventionally, austenitic stainless steel, 9% Ni steel, or 5000 series aluminum alloy having an austenitic structure that does not show brittleness at extremely low temperatures has been used. However, since these metal materials have high alloy costs and manufacturing costs, there is a demand for steel sheets that are inexpensive and excellent in cryogenic toughness. Therefore, as a new steel plate to replace conventional cryogenic steel, high Mn steel is applied as a structural steel plate in a cryogenic environment by adding a large amount of relatively inexpensive austenite stabilizing element Mn to austenite structure. It is being considered.

  However, when a steel sheet having an austenite structure is placed in a corrosive environment, stress corrosion cracking is likely to occur when austenite grain boundaries are eroded by corrosion and tensile stress is applied. It was. In the manufacturing stage of structures for liquefied gas storage tanks, etc., the steel plate may be exposed on the surface, and if the steel surface comes into contact with water vapor containing corrosive substances such as salt, moisture or oil, Corrosion occurs. In this corrosion reaction on the steel sheet surface, iron generates oxide (rust) by the anodic reaction, while hydrogen is generated by the cathode reaction of water, and hydrogen embrittlement occurs due to the penetration of hydrogen into the steel. . If residual stress in bending or welding at the time of manufacture or load stress in the environment of use acts there, there is a risk that stress corrosion cracking will occur and the structure will be destroyed. Conventionally studied high Mn steels may be inferior in corrosion resistance as compared to 9% Ni steel and ordinary low alloy steel as well as austenitic stainless steel. Therefore, from the viewpoint of safety, it is important that the steel material to be used has high strength and toughness at extremely low temperatures, as well as excellent corrosion resistance.

  For example, in Patent Document 1, by adding Mn in an amount of 15 to 35%, Cu in an amount of 5% or less, and further adding appropriate amounts of C and Cr, the Charpy impact characteristics at −196 ° C. of the machinability and the heat and heat affected zone are described. A steel material with improved is disclosed.

  In Patent Document 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 7.0% Or less, Cr: 0.1% or more and less than 8.0% is added to improve the low temperature toughness, and a high Mn steel material is disclosed.

  Furthermore, Patent Document 3 contains 0.001 to 0.80% C and 15 to 35% Mn, and by adding elements such as Cr, Ti, Si, Al, Mg, Ca, and REM, A high Mn steel has been disclosed which has improved cryogenic toughness of the material and welds.

JP-T-2015-508452 JP-A-2006-84529 Japanese Patent Laid-Open No. 2006-196703

  However, for the steel materials described in Patent Documents 1, 2, and 3, from the viewpoint of manufacturing cost to achieve strength and low temperature toughness, and from the viewpoint of corrosion resistance when the austenitic steel material described above is placed in a salt corrosion environment, There was still room for consideration.

  The present invention has been made in view of such problems, and an object thereof is to provide a high Mn steel having excellent corrosion resistance, particularly corrosion resistance in a salt corrosion environment.

  In order to achieve the above-mentioned problems, the present inventors conducted extensive research on various factors that determine the composition of components and production conditions of high-Mn steel, and have obtained the following knowledge. .

a. When Cr is added here based on a high Mn steel, the initial corrosion reaction on the steel sheet surface in a salt water corrosion environment can be delayed by appropriately controlling the amount of addition and solid solution. . Thereby, the amount of hydrogen entering the steel can be reduced, and the stress corrosion cracking of the austenitic steel described above can be suppressed.

b. Furthermore, in order to effectively suppress the destruction of austenite from the grain boundary, a measure for increasing the grain boundary strength is effective. In particular, P is an element that easily segregates together with Mn in the solidification process of the steel slab, and lowers the grain boundary strength of the portion that intersects with such a segregated portion. Therefore, it is necessary to reduce impurity elements such as P.

The present invention has been made by further studying the above knowledge, and the gist thereof is as follows.
1. % By mass
C: 0.20% or more and 0.70% or less,
Si: 0.05% or more and 1.00% or less,
Mn: 15.0% to 35.0%,
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 and N: 0.0010% or more and 0.0300% or less, and has a component composition of balance Fe and inevitable impurities, and 60% or more of the contained Cr A steel plate made of solute Cr.

2. The component composition is further mass%,
Nb: 0.003% to 0.030%,
2. The steel sheet according to 1 above, containing one or more selected from V: 0.01% to 0.10% and Ti: 0.003% to 0.040%.

3. The component composition is further 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,
The steel sheet according to 1 or 2 above, containing one or more selected from Mo: 0.01% to 2.0% and W: 0.01% to 2.0%.

4). The component composition is further mass%,
Ca: 0.0005% or more and 0.0050% or less,
The steel plate according to 1, 2 or 3, which contains one or more selected from Mg: 0.0005% to 0.0100% and REM: 0.0010% to 0.0200%.

5. After heating the steel material having the component composition according to any one of 1 to 4 to 1000 ° C. or more and 1300 ° C. or less, the steel is subjected to hot rolling at a reduction ratio of 3 to 30 and a rolling finish temperature of 750 ° C. or more. Residence time of rolled material in a temperature range of 950 ° C. or lower and 600 ° C. or higher: Production of a steel sheet that is performed in 30 minutes or shorter, and then the average cooling rate in the temperature range of 700 ° C. or lower and 600 ° C. or higher is 3 ° C./s or higher Method.

  In the present invention, “excellent corrosion resistance” is a test in accordance with the SLOW Standard Rate Test Method of NACE Standard TM0111-2011, and is immersed in artificial seawater (chloride ion concentration 18000 ppm) at a temperature of 23 ° C. When the constant speed tensile test is performed at a strain rate of 4 × 10 −7 inch / s, the breaking stress is 400 MPa or more.

  ADVANTAGE OF THE INVENTION According to this invention, the steel plate excellent in corrosion resistance, especially the corrosion resistance in a salt corrosion environment can be provided. Therefore, by using the steel sheet of the present invention for a steel structure used in a cryogenic environment, such as a tank for a liquefied gas storage tank, the safety and life of the steel structure are greatly improved. Will bring about the effect. Moreover, since the steel plate of this invention is cheap compared with the existing material, it also has the advantage which is excellent in economical 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 plate of this invention and the reason for limitation are demonstrated. In this invention, in order to ensure the outstanding corrosion resistance, the component composition of a steel plate is prescribed | regulated as follows. “%” Representing the component composition means “% by mass” unless otherwise specified.

C: 0.20% or more and 0.70% or less C is effective for increasing the strength, and is an inexpensive austenite stabilizing element and an important element for obtaining austenite. In order to acquire the effect, C needs to contain 0.20% or more. On the other hand, if the content exceeds 0.70%, excessive precipitation of Cr carbide and Nb, V, Ti-based carbides is promoted, so that low temperature toughness is lowered and corrosion starts. For this reason, C is made 0.20% or more and 0.70% or less. Preferably, the content is 0.25% or more and 0.60% or less.

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

Mn: 15.0% to 35.0% Mn is a relatively inexpensive austenite stabilizing element. In the present invention, it is an important element for achieving both strength and cryogenic toughness. In order to acquire the effect, Mn needs to contain 15.0% or more. On the other hand, when it contains exceeding 35.0%, the effect which improves cryogenic toughness will be saturated, and the cost of an alloy will be raised. In addition, the weldability and cutability are deteriorated. Furthermore, segregation is promoted and stress corrosion cracking is promoted. For this reason, Mn is made 15.0% or more and 35.0% or less. Preferably, the range is 18.0% or more and 28.0%.

P: 0.030% or less When P is contained in excess of 0.030%, it segregates at the grain boundary and lowers the grain boundary strength, and becomes the starting point of stress corrosion cracking. For this reason, it is desirable to make 0.030% an upper limit and to reduce as much as possible. The lower the P content, the better the characteristics. Therefore, the P content is preferably 0.024% or less, and more preferably 0.020% or less. On the other hand, in order to make it less than 0.001%, a great cost is required for steel making and the economic efficiency is impaired. Therefore, the content of 0.001% or more is allowed.

S: 0.0200% or less Since S deteriorates the low temperature toughness and ductility of the base material, 0.0200% is the upper limit, and it is desirable to reduce it as much as possible. Therefore, S is 0.0200% or less, preferably 0.0180% or less. On the other hand, if the content is less than 0.0001%, steelmaking requires a large cost and the economic efficiency is impaired. Therefore, the content of 0.0001% or more is allowed.

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 of a steel sheet. Moreover, it has the effect which suppresses the coarsening of a crystal grain by fixing the solid solution N in steel and forming AlN. At the same time, it has the effect of suppressing toughness deterioration due to the reduction of solute N. In order to acquire such an effect, Al needs to contain 0.010% or more. On the other hand, if the content exceeds 0.100%, coarse nitrides are formed, which may become a starting point for corrosion and destruction, and the stress corrosion cracking resistance may decrease. Moreover, in order to diffuse to a weld metal part at the time of welding and to deteriorate the toughness of a weld metal, it is made 0.100% or less. Preferably, the content 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 contained Cr is solid solution Cr
Cr has the effect of delaying the initial corrosion reaction on the steel sheet surface in a salt water corrosive environment when contained in an appropriate amount. This effect reduces the amount of hydrogen intrusion into the steel sheet and is important for improving stress corrosion cracking resistance. Element. Although the corrosion resistance can be improved by increasing the amount of Cr, on the other hand, Cr is unavoidably precipitated in the form of nitride, carbide, carbonitride, etc. during rolling. Stress corrosion cracking resistance may decrease as a starting point for corrosion and destruction. For this reason, the Cr content is 0.5% or more and 8.0% or less.

  Here, when the effect of Cr, which delays the initial corrosion reaction on the steel sheet surface in a salt water corrosive environment, was examined in detail, the solid solution amount of Cr is important to obtain this effect reliably. It was proved that it is reliably exerted when 0.3% or more exists in a solid solution state. On the other hand, in order to make Cr into a solid solution state, it is necessary to devise manufacturing conditions, and since the lower limit of the solid solution rate of Cr that can be stably secured by small changes in the manufacturing conditions is 60%, In order to make the amount 0.3% or more, a Cr content of at least 0.5% is required. The amount of solute Cr is preferably 1.0% or more and 6.0% or less, more preferably 1.2% or more and 5.5% or less. The solid solution state is a state in which solute atoms exist as atoms without forming precipitates or the like.

N: 0.0010% or more and 0.0300% or less N is an austenite stabilizing element and is an element effective in improving the cryogenic toughness. Moreover, it combines with Nb, V, Ti, and precipitates finely as nitride or carbonitride, and has the effect of suppressing stress corrosion cracking as a diffusible hydrogen trap site. In order to acquire such an effect, N needs to contain 0.0010% or more. On the other hand, when the content exceeds 0.0300%, the formation of excess nitride or carbonitride is promoted, and not only the amount of solid solution elements decreases and corrosion resistance decreases, but also toughness decreases. For this reason, N is made into 0.0010% or more and 0.0300% or less. Preferably it is 0.0020% or more and 0.0150% or less.

In the present invention, for the purpose of further improving the corrosion resistance, in addition to the above essential elements, if necessary,
Nb: 0.003% to 0.030%, V: 0.01% to 0.10% and Ti: 0.003% to 0.040% can be contained.

Nb: 0.003% or more and 0.030% or less Nb precipitates as carbonitride, and the generated carbonitride functions as a trap site for diffusible hydrogen, and is therefore an element having an effect of suppressing stress corrosion cracking. . In order to obtain such an effect, Nb is preferably contained at 0.003% or more. On the other hand, if the content exceeds 0.030%, coarse carbonitride precipitates and may become a starting point of fracture. Further, the precipitates may become coarse and the base material toughness may be deteriorated. 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, it is 0.005% or more and 0.025% or less, and further 0.007% or more and 0.022% or less.

V: 0.01% or more and 0.10% or less V is an element having an effect of suppressing stress corrosion cracking because the carbonitride deposited as carbonitride and functions as a diffusible hydrogen trap site. . In order to obtain such an effect, V is preferably contained at 0.01% or more. On the other hand, if the content exceeds 0.10%, coarse carbonitride precipitates and may become a starting point of fracture. Further, the precipitates may become coarse and the base material toughness may be deteriorated. For this reason, when it contains V, it is preferable to set it as 0.01% or more and 0.10% or less. More preferably, it is 0.02% or more and 0.09% or less, and further 0.03% or more and 0.08% or less.

Ti: 0.003% or more and 0.040% or less Ti precipitates as a nitride or carbonitride, and the generated nitride or carbonitride functions as a trap site for diffusible hydrogen. It is an element that has an effect. In order to obtain such an effect, Ti is preferably contained at 0.003% or more. On the other hand, if the content exceeds 0.040%, the precipitates become coarse and the base material toughness may be deteriorated. In addition, coarse carbonitrides may precipitate and become the starting point of fracture. 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, it is 0.005% or more and 0.035% or less, and further 0.007% or more and 0.032% or less.

Furthermore, in the present invention, for the purpose of further improving the corrosion resistance, if necessary,
Cu: 0.01% to 0.50%, Ni: 0.01% to 0.50%, Sn: 0.01% to 0.30%, Sb: 0.01% to 0.30% Hereinafter, one or more of Mo: 0.01% to 2.0% and W: 0.01% to 2.0% can be contained.

Cu, Ni, Sn, Sb, Mo, and W are elements that improve the corrosion resistance of a high Mn steel in a salt corrosion environment by being added in combination with Cr. Here, Cu, Sn, and Sb have the effect of suppressing the hydrogen generation reaction, which is a cathode reaction, by increasing the hydrogen overvoltage of the steel material. Ni forms a precipitate coating on the steel material surface, Cl ^- physically inhibit the transmission of the corrosive anion such as base steel. In addition, Cu, Ni, Sn, Sb, Mo and W are liberated as metal ions from the steel material surface during the corrosion, and the corrosion product is made dense, so that the steel interface (the interface between the rust layer and the ground iron) is introduced. Suppresses permeation of corrosive anions. Mo and W are liberated as Mo ₄ ^2- and WO ₄ ^2- , respectively, and are adsorbed in the corrosion products or on the steel sheet surface to give selective permeation of the cations and electrically permeate the corrosive anions into the iron. Suppress it.

  The above effects are manifested in high-Mn steel when coexisting with Cr, and are manifested at the above lower limit values or more, respectively. However, if any element is contained in a large amount, the weldability and toughness are deteriorated, which is disadvantageous from the viewpoint of cost.

  Accordingly, the Cu content is in the range of 0.01% to 0.50%, the Ni content is in the range of 0.01% to 0.50%, the Sn content is in the range of 0.01% to 0.30%, Sb amount is in the range of 0.01% to 0.30%, Mo amount is in the range of 0.01% to 2.0%, and W amount is in the range of 0.01% to 2.0%. Is preferred.

  More preferably, the amount of Cu is 0.02% to 0.40%, the amount of Ni is 0.02% to 0.40%, the amount of Sn is 0.02% to 0.25%, and the amount of Sb is 0. 0.02% to 0.25%, Mo amount is 0.02% to 1.9%, and W amount is 0.02% to 1.9%.

Similarly, in the present invention, for the purpose of further improving the corrosion resistance, if necessary,
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. it can.
Ca, Mg and REM are elements useful for controlling the form of inclusions, and can be contained as necessary. Here, the form control of inclusions means that the expanded sulfide inclusions are made into granular inclusions. Ductility, toughness, and resistance to sulfide stress corrosion cracking are improved through shape control of the inclusions. In order to obtain such an effect, Ca and Mg are preferably contained in an amount of 0.0005% or more and REM in an amount of 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 on the contrary, ductility, toughness, and resistance to sulfide stress corrosion cracking may decrease. Moreover, it may become economically disadvantageous.

  Therefore, when Ca is contained, 0.0005% or more and 0.0050% or less, when Mg is contained, 0.0005% or more and 0.0100% or less, and when REM is contained, 0.0010% It is preferable that it is 0.0200% or less. More preferably, the Ca content is 0.0010% to 0.0040%, the Mg content is 0.0010% to 0.0040%, and the REM content is 0.0020% to 0.0150%.

Next, the manufacturing conditions of the present invention will be described. In the following description, the temperature (° C.) means the temperature at the thickness center of the steel sheet.
[Reheating temperature of steel material: 1000 ° C or higher and 1300 ° C or lower]
The reason why the steel material is heated to 1000 ° C. or higher is to make the carbonitride in the structure solid, and to make the crystal grain size uniform. That is, when the heating temperature is less than 1000 ° C., the carbonitride is not sufficiently dissolved, so that desired characteristics cannot be obtained. In addition, since heating exceeding 1300 ° C requires excessive energy in addition to material deterioration due to coarsening of the crystal grain size, the upper limit of the heating temperature is set to 1300 ° C. Preferably it is 1050 degreeC or more and 1250 degrees C or less, More preferably, it is the range of 1070 degreeC or more and 1250 degrees C or less.

[Rolling ratio: 3 to 30]
In hot rolling with a reduction ratio of less than 3, the effect of promoting recrystallization and achieving grain size reduction cannot be obtained, and coarse austenite grains remain, and the portion is preferentially oxidized to deteriorate the corrosion resistance. It will be. Therefore, the reduction ratio in hot rolling is limited to 3 or more. On the other hand, the upper limit needs to be 30 for reasons described later. Here, the reduction ratio is defined by the thickness of the material to be rolled / the thickness of the steel sheet after rolling.

[Rolling finishing temperature: 750 ° C or higher]
When the rolling finish temperature is less than 750 ° C., the amount of carbide precipitation during rolling is remarkably increased, and even when the residence time at 600 ° C. or more and 950 ° C. or less is 30 minutes or less, the amount of solid solution Cr may not be ensured and the corrosion resistance is reduced. descend. Moreover, when rolling below 750 degreeC or less, since a deformation resistance becomes large and an excessive load is applied to a manufacturing facility, rolling finishing temperature shall be 750 degreeC or more. The upper limit is preferably set to 1050 ° C. or lower from the viewpoint of suppressing marked coarsening of crystal grains.

[Dwelling time in a temperature range of 950 ° C. or lower and 600 ° C. or higher: 30 minutes or less]
If the material to be rolled stays in a temperature range of 950 ° C. or lower and 600 ° C. or higher in hot rolling, if the time exceeds 30 minutes, carbonitrides and carbides are precipitated in large quantities during rolling, and the necessary amount of solute Cr is reduced. In order to decrease and cause deterioration in corrosion resistance and cryogenic toughness, the residence time in a temperature range of 950 ° C. or lower and 600 ° C. or higher is regulated to 30 minutes or shorter. In addition, since it is better that the stay time is shorter, it is not necessary to set a lower limit for the stay time.

  Here, in order to set the stay time in the temperature range of 950 ° C. or lower and 600 ° C. or higher to 30 minutes or less, the length of the material to be rolled is set to 5000 mm or less, and the reduction ratio from the material to be rolled is set to 30 or less as described above. limit. This is because when the length of the material to be rolled exceeds 5000 mm and the reduction ratio exceeds 30, the rolling time becomes long, and as a result, the residence time in the range of 950 ° C. or lower and 600 ° C. or higher exceeds 30 minutes.

[Average cooling rate at 700 ° C or lower and 600 ° C or higher: 3 ° C / s or higher]
When 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. Note that the higher the average cooling rate, the better.

  After the steel No. 1 to 28 shown in Table 1 was melted to form a slab, the steel plate having a thickness of 6 mm to 50 mm was manufactured according to the manufacturing conditions shown in Table 2, and the steel plates No. 1 to 34 were manufactured. And subjected to the following test.

The corrosion resistance test was carried out in accordance with the Narrow Standard TM0111-2011 standard Slow Strain Rate Test Method (hereinafter referred to as SSRT test). The shape of the test piece was a Type A round bar notched test piece, 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 inch / s. . Here, the fracture stress is 400 MPa or more and the stress corrosion cracking resistance is excellent.
The results obtained as described above are shown in Table 2.

  The invention examples according to the present invention (Sample Nos. 1 to 17) confirmed that the corrosion resistance satisfies 400 MPa or more as the breaking stress in the SSRT test. On the other hand, the comparative examples (sample Nos. 18 to 34) outside the scope of the present invention do not satisfy the above target performance in terms of stress corrosion cracking resistance.

Claims (5)

% By mass
C: 0.20% or more and 0.70% or less,
Si: 0.05% or more and 1.00% or less,
Mn: 15.0% to 35.0%,
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 and N: 0.0010% or more and 0.0300% or less, and has a component composition of balance Fe and inevitable impurities, and 60% or more of the contained Cr A steel plate made of solute Cr. The component composition is further mass%,
Nb: 0.003% to 0.030%,
The steel plate according to claim 1, comprising one or more selected from V: 0.01% to 0.10% and Ti: 0.003% to 0.040%. The component composition is further 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,
The steel plate according to claim 1 or 2, containing one or more selected from Mo: 0.01% to 2.0% and W: 0.01% to 2.0%. The component composition is further mass%,
Ca: 0.0005% or more and 0.0050% or less,
The steel plate according to claim 1, 2 or 3, which contains one or more selected from Mg: 0.0005% to 0.0100% and REM: 0.0010% to 0.0200%.   After the steel material having the component composition according to any one of claims 1 to 4 is heated to 1000 ° C or higher and 1300 ° C or lower, hot rolling at a rolling reduction ratio of 3 to 30 and a rolling finish temperature: 750 ° C or higher, Residual time in a temperature range of 950 ° C. or lower and 600 ° C. or higher of the material to be rolled: Production method.