WO2021033694A1

WO2021033694A1 - Steel and method for manufacturing same

Info

Publication number: WO2021033694A1
Application number: PCT/JP2020/031165
Authority: WO
Inventors: 大地泉; 孝一中島; 植田　圭治
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2019-08-21
Filing date: 2020-08-18
Publication date: 2021-02-25
Also published as: KR20220030292A; TWI726798B; JP6947330B2; EP4019656A1; EP4019656A4; JPWO2021033694A1; CN114302977B; TW202113100A; CN114302977A

Abstract

This steel has a component composition containing, in terms of mass%, 0.100-0.700% of C, 1.00% or less of Si, 20.0-40.0% of Mn, 0.030% or less of P, 0.0070% or less of S, 0.01-5.00% of Al, 0.5-7.0% of Cr, 0.0050-0.0500% of N, 0.0050% or less of O, 0.005% or less of Ti, and 0.005% or less of Nb, the balance being Fe and unavoidable impurities, the steel moreover having austenite as a base phase, having a micro structure that has an average grain diameter of 80 μm or greater, being such that the absroption energy in a Charpy impact test at -269°C is 150 J or higher, and the total elongation in a tension test at -269°C is 30% or higher.

Description

Steel and its manufacturing method

The present invention is suitable for structural steels used in extremely low temperature environments, such as tanks for storing liquid hydrogen, liquid helium, liquefied gas, etc., and steels having excellent toughness particularly at extremely low temperatures. Regarding the manufacturing method.

In order to use hot-rolled steel sheets for structures for liquid hydrogen, liquid helium, and liquefied gas storage tanks, the usage environment is extremely low, so excellent toughness at extremely low temperatures is required. For example, when a hot-rolled steel sheet is used in a liquid helium storage tank, it is necessary to ensure excellent toughness at a temperature of -269 ° C. or lower, which is the boiling point of helium. If the cryogenic toughness of the steel material is inferior, it may not be possible to maintain the safety of the structure for the cryogenic storage tank. Therefore, there is a strong demand for improving the cryogenic toughness of the steel material used for this application.

In response to this requirement, austenite-based stainless steel having a steel plate structure of austenite that does not show brittleness at extremely low temperatures has been conventionally used. However, since the alloy cost and the manufacturing cost are high, there is a demand for an inexpensive steel material having excellent cryogenic toughness.

Therefore, as a new steel material to replace the conventional low-temperature steel, it is proposed in Patent Document 1, for example, to use a high Ni steel to which a large amount of Ni, which is an austenite stabilizing element, is added as a structural steel in an environment of -253 ° C. Has been done.

Patent Document 1 proposes a technique for ensuring cryogenic toughness by controlling the particle size and morphology of old austenite.

JP-A-2018-104792

The technique described in Patent Document 1 makes it possible to provide high Ni steel having excellent ultra-low temperature toughness, but the high Ni steel described here contains 12.5% or more of Ni from the viewpoint of ensuring extremely low temperature toughness. There was a need to reduce material costs. Further, in order to secure the austenite phase, it is necessary to perform heat treatment over a plurality of steps such as reheating quenching, intermediate heat treatment, and tempering, which is also a problem that the manufacturing cost is high.

Therefore, an object of the present invention is to provide a steel having excellent ultra-low temperature toughness and tensile properties, which can suppress the material and the cost required for manufacturing. Furthermore, it is an object of the present invention to propose an advantageous method for producing such steel. Here, the above-mentioned "excellent in cryogenic toughness" means that the absorbed energy of the Charpy impact test at -196 ° C. and further -269 ° C. is 150 J or more. Further, the above-mentioned "excellent in tensile properties" means that the total elongation of the tensile test at -269 ° C. is 30% or more.

In order to achieve the above problems, the inventors have conducted intensive research on various factors that determine the composition and structure of the steel sheet for steels having a relatively high Mn content of 20.0% or more, and the following a , B was obtained.

a. As the main form of brittle fracture in the above-mentioned austenitic steel, grain boundary fracture starting from grain boundaries can be mentioned. From this, it is effective to make the crystal grain size coarse in order to improve the extremely low temperature toughness of the steel.

b. If hot rolling and heat treatment are performed under appropriate conditions under an appropriate composition, the cryogenic toughness and tensile properties can be improved with the minimum number of heat treatments, and the manufacturing cost can be suppressed.

The present invention has been made by further studying the above findings, and the gist thereof is as follows.
1. 1. By mass%
C: 0.100% or more and 0.700% or less,
Si: 1.00% or less,
Mn: 20.0% or more and 40.0% or less,
P: 0.030% or less,
S: 0.0070% or less,
Al: 0.01% or more and 5.00% or less,
Cr: 0.5% or more and 7.0% or less,
N: 0.0050% or more and 0.0500% or less,
O: 0.0050% or less,
It contains Ti: 0.005% or less and Nb: 0.005% or less, and the balance has a component composition of Fe and unavoidable impurities.
It has a microstructure with austenite as the base phase,
A steel having an average particle size of 80 μm or more, an absorption energy of Charpy impact test at -269 ° C. of 150 J or more, and a total elongation of tensile test at -269 ° C. of 30% or more.

2. 2. The component composition is further increased by mass%.
Cu: 1.0% or less,
Ni: 1.0% or less,
Mo: 2.0% or less,
V: 2.0% or less,
W: 2.0% or less,
Ca: 0.0005% or more and 0.0050% or less,
The steel according to 1 above, which contains at least one selected from Mg: 0.0005% or more and 0.0050% or less and REM: 0.0010% or more and 0.0200% or less.

3. 3. A steel material having the component composition according to 1 or 2 above.
Heat to a temperature range of 1100 ° C or higher and 1300 ° C or lower,
Hot rolling,
Reheat to a temperature range of 1100 ° C or higher and 1300 ° C or lower,
A method for producing steel, which comprises performing a heat treatment in which the product of a heating temperature (° C.) and a heating time (h) is 100 ° C. · h or more.

Here, each of the above temperatures is the surface temperature of the steel material or the steel plate, respectively.

According to the present invention, it is possible to provide a steel having excellent cryogenic toughness and tensile properties. Therefore, the steel of the present invention greatly contributes to the improvement of safety and life of steel structures used in extremely low temperature environments such as liquid hydrogen, liquid helium, and tanks for liquefied gas storage tanks, and has a remarkable industrial effect. Play. Further, since the production method of the present invention does not cause a decrease in productivity and an increase in production cost, it is possible to provide a method having excellent economic efficiency.

It is a graph which shows the relationship between the average crystal grain size (average particle size) of the steel satisfying the component composition of this invention, and the absorbed energy at -269 ° C.

Hereinafter, the steel of the present invention will be described in detail.
[Ingredient composition]
First, the component composition of the steel of the present invention and the reason for its limitation will be described. The "%" indication in the component composition shall mean "mass%" unless otherwise specified.

C: 0.100% or more and 0.700% or less C is an inexpensive austenite stabilizing element and is an important element for obtaining austenite. In order to obtain the effect, C needs to be contained in an amount of 0.100% or more. On the other hand, if it is contained in excess of 0.700%, Cr carbides are excessively generated and the cryogenic toughness is lowered. Therefore, the amount of C is set to 0.100% or more and 0.700% or less. The amount of C is preferably 0.200% or more, preferably 0.600% or less, and more preferably 0.200% or more and 0.600% or less.

Si: 1.00% or less Si acts as an antacid and is an element necessary for steelmaking, so it is preferable to add 0.05% or more. On the other hand, if it is contained in excess of 1.00%, the non-thermal stress (internal stress) increases excessively, so that the cryogenic toughness deteriorates. Therefore, Si is set to 1.00% or less. The amount of Si is preferably 0.80% or less.

Mn: 20.0% or more and 40.0% or less Mn is a relatively inexpensive austenite stabilizing element, and is an important element for ensuring low temperature toughness in the present invention. In order to obtain the effect, Mn needs to be contained in an amount of 20.0% or more. On the other hand, if it is contained in excess of 40.0%, the cryogenic toughness deteriorates. Therefore, the amount of Mn is set to 20.0% or more and 40.0% or less. The amount of Mn is preferably 23.0% or more, preferably 38.0% or less, and more preferably 23.0% or more and 38.0% or less. The amount of Mn is more preferably 36.0% or less.

P: 0.030% or less If P is contained in excess of 0.030%, it segregates excessively at the grain boundaries, resulting in a decrease in cryogenic toughness. Therefore, it is desirable to limit the amount to 0.030% as much as possible. Therefore, P is 0.030% or less. It should be noted that excessive P reduction raises the refining cost and is economically disadvantageous, so it is desirable to set it to 0.002% or more. The amount of P is more preferably 0.005% or more, preferably 0.028% or less, further preferably 0.005% or more and 0.028% or less, still more preferably 0.024% or less.

S: 0.0070% or less Since S deteriorates the cryogenic toughness of the steel sheet, it is desirable to limit it to 0.0070% and reduce it as much as possible. Therefore, S is set to 0.0070% or less. It should be noted that excessive reduction of S increases the refining cost and is economically disadvantageous, so it is desirable to set it to 0.0010% or more. The amount of S is preferably 0.0050% or less.

Al: 0.01% or more and 5.00% or less Al acts as a deoxidizing agent and is most widely used in the molten steel deoxidizing process of steel sheets. In order to obtain such an effect, Al needs to be contained in an amount of 0.01% or more. On the other hand, if the content exceeds 5.00%, a large amount of inclusions are present and the cryogenic toughness is deteriorated, so the content is set to 5.00% or less. Therefore, the amount of Al is set to 0.01% or more and 5.00% or less. The amount of Al is preferably 0.02% or more, preferably 4.00% or less, and more preferably 0.02% or more and 4.00% or less.

Cr: 0.5% or more and 7.0% or less Cr is an element effective for improving cryogenic toughness because it improves grain boundary strength. In order to obtain such an effect, Cr needs to be contained in an amount of 0.5% or more. On the other hand, if it is contained in excess of 7.0%, the cryogenic toughness is lowered due to the formation of Cr carbides. Therefore, the amount of Cr is set to 0.5% or more and 7.0% or less. The amount of Cr is preferably 1.0% or more, more preferably 1.2% or more, preferably 6.7% or less, more preferably 6.5% or less, and more preferably 1.0% or more and 6.7%. Hereinafter, it is more preferably 1.2% or more and 6.5% or less.

N: 0.0050% or more and 0.0500% or less N is an austenite stabilizing element and is an element effective for improving cryogenic toughness. In order to obtain such an effect, N needs to be contained in an amount of 0.0050% or more. On the other hand, if it is contained in excess of 0.0500%, the nitride or carbonitride becomes coarse and the toughness decreases. Therefore, the amount of N is set to 0.0050% or more and 0.0500% or less. The amount of N is preferably 0.0060% or more, preferably 0.0400% or less, and more preferably 0.0060% or more and 0.0400% or less.

O: 0.0050% or less O deteriorates the cryogenic toughness due to the formation of oxides. Therefore, O is set to 0.0050% or less. The amount of O is preferably 0.0045% or less. It is desirable that the amount of O is 0.0010% or more because excessive reduction of O increases the refining cost and is economically disadvantageous.

Suppressing Ti and Nb content to 0.005% or less, respectively Ti and Nb form high melting point carbonitrides in steel, so excessive content reduces cryogenic toughness. Ti and Nb are components that are inevitably mixed from raw materials and the like, and in most cases, Ti: more than 0.005% and 0.010% or less and Nb: more than 0.005% and 0.010% or less. To do. Therefore, it is necessary to intentionally limit the amount of Ti and Nb mixed in according to the method described later, and suppress the content of Ti and Nb to 0.005% or less, respectively. By suppressing the contents of Ti and Nb to 0.005% or less, the adverse effects of the above-mentioned carbonitride can be eliminated and excellent cryogenic toughness can be ensured. Preferably, the contents of Ti and Nb are 0.003% or less, respectively. Of course, the contents of Ti and Nb may each be 0%, but excessive reduction is not preferable from the viewpoint of steelmaking cost, so it is desirable to set each to 0.001% or more.

In the present invention, for the purpose of further improving low temperature toughness, the following elements can be contained, if necessary, in addition to the above essential elements.
Cu: 1.0% or less, Ni: 1.0% or less, Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, Ca: 0.0005% or more 0.0050 % Or less, Mg: 0.0005% or more and 0.0050% or less, REM: 0.0010% or more and 0.0200% or less.

Cu and Ni: 1.0% or less respectively Cu and Ni are elements that improve low temperature toughness. In order to obtain such an effect, it is preferable that Cu and Ni are each contained in an amount of 0.01% or more, and more preferably 0.03% or more. On the other hand, if each content exceeds 1.0%, the surface texture deteriorates during rolling and the manufacturing cost is reduced. Therefore, when these alloying elements are contained, the content thereof is preferably 1.0% or less, more preferably 0.7% or less, still more preferably 0.5% or less.

Mo, V, W: 2.0% or less, respectively Mo, V and W contribute to the stabilization of austenite. In order to obtain such an effect, Mo, V and W are each preferably contained in an amount of 0.001% or more, and more preferably 0.003% or more. On the other hand, if each content exceeds 2.0%, coarse carbonitride is generated, which may be a starting point of fracture and puts pressure on the manufacturing cost. Therefore, when these alloying elements are contained, the content thereof is preferably 2.0% or less, and more preferably 1.7% or less. The amounts of Mo, V, and W are more preferably 0.003% or more and 1.7% or less, and even more preferably 1.5% or less, respectively.

Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, REM: 0.0010% or more and 0.0200% or less Ca, Mg and REM control the morphology of inclusions. It is a useful element and can be contained as needed. The morphological control of inclusions means that the expanded sulfide-based inclusions (mainly MnS) are made into granular inclusions. Toughness is improved by reducing MnS, which is the starting point of fracture, through morphological control of the inclusions. In order to obtain such an effect, it is preferable that Ca and Mg are each contained in an amount of 0.0005% or more and REM is contained in an amount of 0.0010% or more. On the other hand, if a large amount of any of the elements is contained, the amount of non-metal inclusions may increase and the toughness may decrease. It may also be economically disadvantageous.
Therefore, when Ca and Mg are contained, it is preferably 0.0005% or more and 0.0050% or less, respectively, and when REM is contained, it is preferably 0.0010% or more and 0.0200% or less. The amount of Ca is more preferably 0.0010% or more, more preferably 0.0040% or less, still more preferably 0.0010% or more and 0.0040% or less. The amount of Mg is more preferably 0.0010% or more, more preferably 0.0040% or less, still more preferably 0.0010% or more and 0.0040% or less. The amount of REM is more preferably 0.0020% or more, more preferably 0.0150% or less, still more preferably 0.0020% or more and 0.0150% or less.
In addition, REM refers to a rare earth metal, and is a general term for 17 elements in which Y and Sc are combined with 15 elements of lanthanoid, and one or more of these elements can be contained. The REM content means the total content of these elements.

The rest other than the above-mentioned components is a component composition containing iron and unavoidable impurities. Examples of the unavoidable impurities here include H and B, and a total of 0.01% or less is acceptable.

[Organization]
Microstructure with austenite as the base phase When the crystal structure of the steel material is a body-centered cubic structure (bcc), the steel material may cause brittle fracture in an extremely low temperature environment, so it is suitable for use in an extremely low temperature environment. Is not suitable. Here, assuming use in an extremely low temperature environment, the matrix phase of the steel material preferably has an austenite structure in which the crystal structure is a face-centered cubic structure (fcc). In addition, "using austenite as a base phase" means that the austenite phase has an area ratio of 90% or more, and more preferably 95% or more. The rest other than the austenite phase is a ferrite phase or a martensite phase.

The average crystal grain size in the microstructure is 80 μm or more. As a result of verifying the relationship between the average crystal grain size and the absorbed energy of the Charpy impact test, as shown in FIG. 1, other conditions of the present invention are satisfied, and the average crystal grain size is further satisfied. If the value is 80 μm or more, the absorbed energy can be 150 J or more. Here, the crystal grains in the present specification mainly refer to austenite grains, and the average crystal grain size is calculated by randomly selecting 100 crystal grains from an image taken at 200 times using an optical microscope and calculating the equivalent diameter of a circle. , Can be obtained from the average value.

The above average crystal grain size can be realized by performing hot rolling and heat treatment according to the conditions described later under the above component composition.

As the steel according to the present invention, molten steel having the above-mentioned composition can be melted by a known melting method such as a converter or an electric furnace. Further, the secondary refining may be performed in a vacuum degassing furnace. At that time, in order to limit Ti and Nb, which hinder suitable tissue control, to the above-mentioned range, it is necessary to avoid inevitably mixing from raw materials and take measures to reduce their contents. .. For example, by lowering the basicity of the slag in the refining step, these alloys can be concentrated and discharged into the slag to reduce the concentrations of Ti and Nb in the final slab product. Alternatively, a method such as blowing oxygen to oxidize the alloy and floating and separating the alloy of Ti and Nb at reflux may be used. After that, it is preferable to use a known casting method such as a continuous casting method, an ingot-block rolling method, or the like to obtain a steel material such as a slab having a predetermined size.

Furthermore, the manufacturing conditions for incorporating the above steel material into a steel material with excellent cryogenic toughness are specified.

[Heating temperature of steel material: 1100 ° C or higher and 1300 ° C or lower]
In order to obtain a steel having the above-mentioned structure, it is important to heat it in a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, and then perform hot rolling. The temperature control here is based on the surface temperature of the steel material. In order to exert the above-mentioned effect of Mn, it is important to diffuse Mn in the steel. That is, in order to promote the diffusion of Mn in hot rolling, the heating temperature of the steel material before hot rolling is set to 1100 ° C. or higher. On the other hand, if the temperature exceeds 1300 ° C., there is a concern that the steel will start melting, so the upper limit of the heating temperature is set to 1300 ° C. The heating temperature of the steel material is preferably 1130 ° C. or higher, preferably 1270 ° C. or lower, and more preferably 1130 ° C. or higher and 1270 ° C. or lower.

[Hot rolling]
Hot rolling is performed after heating the steel material. The method of hot rolling is not particularly limited, but the lower the finishing temperature in finish rolling, the lower the rolling efficiency. Therefore, the temperature is preferably 700 ° C. or higher. Further, it is preferably 750 ° C. or higher.

[Reheat to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, and perform heat treatment so that the product of the heating temperature (° C.) and the heating time (h) is 100 ° C. · h or higher]
After hot rolling or subsequent cooling treatment, a predetermined heat treatment is performed. In the heat treatment, the crystals are coarsened by heating again to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, and performing the heat treatment in which the product of the heating temperature (° C.) and the heating time (h: hour) is 100 ° C. · h or higher. In addition to improving the ultra-low temperature toughness, the dislocations introduced during hot rolling are restored, so the tensile properties, especially the total elongation, are improved. The temperature range for reheating is set to 1100 ° C or higher and 1300 ° C or lower for the following reasons. That is, in order to diffuse Mn in the heat treatment, the heating temperature at the time of reheating in the heat treatment is set to 1100 ° C. or higher. On the other hand, if the temperature exceeds 1300 ° C., there is a concern that the steel will start melting, so the upper limit of the heating temperature at the time of reheating is set to 1300 ° C. The product of the heating temperature (° C.) and the heating time (h) is defined because there is a correlation between grain growth and dislocation recovery. Further, the product of the heating temperature and the heating time is preferably an upper limit value of 650 ° C. · h for the reason of manufacturing cost, and a lower limit value of 208 ° C. · h is preferable for the reason of coarsening all crystal grains. The heating temperature at the time of reheating in the heat treatment is preferably 1130 ° C. or higher, preferably 1270 ° C. or lower, and more preferably 1130 ° C. or higher and 1270 ° C. or lower. The heating time is preferably 0.1 h or more in order to promote grain growth, preferably 0.5 h or less, and more preferably 0.1 h or more and 0.5 h or less in order to suppress a decrease in production efficiency. After the heat treatment, it is cooled.

Cooling treatments may be performed after hot rolling and / or after heat treatment.
This is to suppress the precipitation of carbides. The cooling temperature range is preferably 300 to 650 ° C and 300 to 900 ° C, respectively, and the average cooling rate is preferably 1.0 ° C / s or more, respectively.

Hereinafter, the present invention will be described in detail with reference to Examples. The present invention is not limited to the following examples.
A steel slab (steel material) having the composition shown in Table 1 was prepared by a converter-ladle refining-continuous casting method. Next, under the conditions shown in Table 2, the obtained steel slab was hot-rolled to obtain a steel plate having a thickness of 6 to 30 mm. Here, the heating temperature at the time of reheating in the heat treatment was set to the same temperature as the heating temperature of the steel material for each sample.
The structure of the obtained steel sheet and the mechanical property evaluation of cryogenic toughness and tensile properties were carried out as follows.
In Table 2, "finishing temperature at the time of finish rolling" indicates the finish rolling end temperature.

(1) Structure evaluation-Austenite phase area ratio The area ratio of each phase of the microstructure was determined from the Phase map of backscattered electron diffraction (EBSD) analysis. EBSD analysis test pieces were collected from a cross section parallel to the rolling direction at a plate thickness of 1/2 of the obtained steel sheet, and EBSD analysis was performed in a measurement step of 0.3 μm in a field of view of 500 μm × 200 μm to perform Phase map. The value described in 1 was taken as the area ratio.
The area ratio of the austenite phase was 90% or more in all of the invention examples and the comparative examples, and it was confirmed that the matrix phase was austenite.

-Average particle size For the heat-treated steel sheet, the cross section in the rolling direction was polished, and 100 crystal grains were randomly selected from the images taken at the plate thickness 1/2 position at a magnification of 200 times using an optical microscope. The average crystal grain size as the average grain size was determined from the equivalent circle diameter.

(2) Ultra-low temperature toughness A Charpy V notch test piece is collected in accordance with the regulations of JIS Z 2242 (2005) from a direction parallel to the rolling direction at the plate thickness 1/2 position of each steel plate having a plate thickness exceeding 10 mm. , Three Charpy impact tests were carried out for each steel sheet at -196 ° C and -269 ° C to determine the absorbed energy and evaluate the toughness of the base metal. In the present invention, the average value of the absorbed energies of the three pieces is 150 J or more, and the toughness of the base material is excellent.
For each steel plate with a plate thickness of less than 10 mm, a 5 mm sub-sized Charpy V notch test piece is used in accordance with the provisions of JIS Z 2242 (2005) from the direction parallel to the rolling direction at the plate thickness 1/2 position. The samples were sampled and three Charpy impact tests were performed on each steel sheet at -196 ° C and -269 ° C. Here, it is assumed that the average value of the absorbed energies of the three pieces is 100 J or more, which is excellent in the toughness of the base material. The Charpy impact test at -269 ° C. was carried out by putting the test piece in a capsule and flowing liquid helium.
Reference 1: T. Ogata, K. Hiraga, K. Nagai, and K. Ishikawa: Tetsu-to-Hagane, 69 (1983), 641.

(3) Tensile characteristics From each of the obtained steel sheets, a round bar tensile test piece having a parallel portion diameter of 6 mm and a distance between gauge points of 25 mm was collected and subjected to a tensile test at -269 ° C. to investigate the total elongation. In the present invention, it was determined that the total elongation of 30% or more is excellent in tensile properties.
The results obtained as described above are shown in Table 2.

It was confirmed that the steel according to the present invention satisfies the above-mentioned target performance (the average value of absorbed energy in the Charpy impact test is 150 J or more, and the total elongation in the tensile test is 30% or more). On the other hand, in the comparative example outside the scope of the present invention, any one or more of the absorbed energy and the total elongation does not satisfy the above-mentioned target performance.

Claims

By mass%
C: 0.100% or more and 0.700% or less,
Si: 1.00% or less,
Mn: 20.0% or more and 40.0% or less,
P: 0.030% or less,
S: 0.0070% or less,
Al: 0.01% or more and 5.00% or less,
Cr: 0.5% or more and 7.0% or less,
N: 0.0050% or more and 0.0500% or less,
O: 0.0050% or less,
It contains Ti: 0.005% or less and Nb: 0.005% or less, and the balance has a component composition of Fe and unavoidable impurities.
It has a microstructure with austenite as the base phase,
A steel having an average particle size of 80 μm or more, an absorption energy of Charpy impact test at -269 ° C. of 150 J or more, and a total elongation of tensile test at -269 ° C. of 30% or more.
The component composition is further increased by mass%.
Cu: 1.0% or less,
Ni: 1.0% or less,
Mo: 2.0% or less,
V: 2.0% or less,
W: 2.0% or less,
Ca: 0.0005% or more and 0.0050% or less,
The steel according to claim 1, which contains at least one selected from Mg: 0.0005% or more and 0.0050% or less and REM: 0.0010% or more and 0.0200% or less.
A steel material having the component composition according to claim 1 or 2.
Heat to a temperature range of 1100 ° C or higher and 1300 ° C or lower,
Hot rolling,
A method for producing steel, which comprises heating again to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, and performing a heat treatment in which the product of the heating temperature and the heating time is 100 ° C. · h or higher.