JP2020204084A - Steel sheet - Google Patents

Abstract

To provide a steel sheet having excellent strength and low temperature toughness.SOLUTION: A steel sheet contains, by mass%, C: 0.03 to 0.12%, Mn: 6.0 to 13.0%, Ni: exceeding 1.0% and 5.0% or smaller, Si: 1.50% or smaller, Al: 0.30% or smaller, P: 0.010% or smaller, S: 0.0050% or smaller, N: 0.0100% or smaller, O: 0.0050% or smaller, as needs arise, one or more kinds of Cu, Co, Cr, Mo, W, B, Nb, V, Ti, Zr, Hf, Ta, Mg, Ca, and REM, and has a metallographic structure made of, by vol%, one or more kinds of α'martensite: 80% or larger, a residual γ: 10 to 20%, the balance made of bainite: 5% or smaller, ferrite: 5% or smaller, and ε martensite :10% or smaller, a circle equivalent diameter of α'martensite: 0.1 to 5.0 μm, a residual γ circle equivalent diameter: 0.01 to 2.50 μm, an old γ particle circle equivalent diameter: 200 μm or smaller, an old γ particle aspect ratio : 1 to 50, and an old γ interface occupation rate due to the residual γ : 40 to 100%.SELECTED DRAWING: None

Description

The present invention relates to a steel sheet.

Mn contained in steel exhibits the effect of stabilizing austenite and enhancing hardenability. Further, Mn is a relatively inexpensive element, and steel containing Mn as a substitute for relatively expensive Ni that exhibits the same effect has been conventionally proposed (for example, Patent Documents 1 to 3). ,reference). Patent Documents 1 to 3 disclose steel sheets having excellent strength at room temperature and toughness at low temperature.

Special Table 2014-501848 Japanese Unexamined Patent Publication No. 5-195156 Japanese Unexamined Patent Publication No. 4-346636

Patent Document 1 discloses a steel sheet having excellent low temperature toughness, but it is desirable to further increase the tensile strength from the viewpoint of thinning the steel sheet. On the other hand, Patent Documents 2 and 3 disclose steel sheets having excellent tensile strength, but it is desirable to further improve the toughness at low temperatures from the viewpoint of expanding applications. In view of such circumstances, it is an object of the present invention to provide a steel sheet having excellent strength and low temperature toughness.

As a result of the studies by the present inventors, in the manufacturing process of the steel sheet containing Mn of 6.0% or more, a processing heat treatment (hereinafter, may be referred to as direct quenching) in which accelerated cooling is performed as it is after hot rolling is adopted. , It was found that the low temperature toughness of the steel sheet is improved. On the other hand, in the case of reheating quenching in which air cooling is performed after hot rolling and then reheating and quenching are performed, it was found that crystal grains grow during heating during reheating quenching, and as a result, grain boundaries become embrittlement. Next, in hot rolling, it was found that low-temperature toughness is further improved by rolling in a temperature range in which recrystallization of austenite is suppressed, that is, a so-called unrecrystallized temperature range (hereinafter, may be referred to as controlled rolling). .. It is presumed that this is mainly due to the effect that the stress applied or the strain introduced by the controlled rolling promotes the transformation progress from ε-martensite to α'martensite. Further, it was found that in hot rolling, the metallographic structure is refined by ensuring the rolling reduction ratio of rough rolling before finish rolling, and a steel sheet having excellent strength and low temperature toughness can be obtained. Further, it was found that the low temperature toughness is remarkably improved by increasing the Ni content.

The present invention has been completed based on such findings, and the gist thereof is as follows.
[1] By mass%
C: 0.03% or more, 0.12% or less,
Mn: 6.0% or more, 13.0% or less,
Ni: Over 1.00%, 5.00% or less,
Si: 0% or more, 1.50% or less,
Al: 0% or more, 0.30% or less,
Cu: 0% or more, 2.00% or less,
Co: 0% or more, 2.00% or less,
Cr: 0% or more, 2.00% or less,
Mo: 0% or more, 2.00% or less,
W: 0% or more, 2.00% or less,
B: 0% or more, 0.0100% or less,
Nb: 0% or more, 0.100% or less,
V: 0% or more, 0.100% or less,
Ti: 0% or more, 0.100% or less,
Zr: 0% or more, 0.100% or less,
Hf: 0% or more, 0.100% or less,
Ta: 0% or more, 0.100% or less,
Mg: 0% or more, 0.0100% or less,
Ca: 0% or more, 0.0100% or less, and REM: 0% or more, 0.0100% or less,
P: 0.010% or less,
S: 0.0050% or less,
N: 0.0100% or less, O: 0.0050% or less, and the balance consists of Fe and impurities.
The metallic structure contains 80% or more of α'martensite by volume and 10% or more and 20% or less of retained austenite, and if the residual structure is present, 5% or less of bainite by volume. Consists of less than% ferrite and less than 10% ε martensite
The circle-equivalent diameter of the α'martensite is 0.1 μm or more and 5.0 μm or less.
The equivalent circle diameter of the retained austenite is 0.01 μm or more and 2.50 μm or less.
The circle-equivalent diameter of the old austenite is 200 μm or less, and the aspect ratio of the old austenite is 1 or more and 50 or less.
A steel sheet having a grain boundary space occupied by the retained austenite of 40% or more and 100% or less.
[2] The steel sheet according to the above [1], wherein the former austenite has an aspect ratio of 4 or more and 50 or less.
[3] By mass%
Cu: 0.10% or more, 2.00% or less,
Co: 0.10% or more, 2.00% or less,
Cr: 0.10% or more, 2.00% or less,
Mo: 0.10% or more, 2.00% or less,
W: 0.10% or more, 2.00% or less,
B: 0.0002% or more, 0.0100% or less,
Nb: 0.005% or more, 0.100% or less,
V: 0.005% or more, 0.100% or less,
Ti: 0.005% or more, 0.100% or less,
Zr: 0.005% or more, 0.100% or less,
The above-mentioned [1] or [2], wherein Hf: 0.005% or more and 0.100% or less, and Ta: 0.005% or more and 0.100% or less contain one or more kinds. Steel plate.
[4] By mass%
Mg: 0.0001% or more, 0.0100% or less,
Any of the above [1] to [3], which contains one or more of Ca: 0.0001% or more and 0.0100% or less, and REM: 0.0001% or more and 0.0100% or less. Steel plate described in Crab.

According to the present invention, it is possible to provide a steel sheet having excellent strength and low temperature toughness.

In general, when steel is heated to a high temperature, the grain boundaries move and the crystal grain size becomes large. The phenomenon in which the grain boundaries move at high temperatures is called grain boundary movement, and the phenomenon in which the grain size increases is called grain growth. As a result of the examination by the present inventors, a steel containing 6.0% or more of Mn (hereinafter, may be referred to as medium Mn steel) crystallizes when the grain boundaries move when heated to a high temperature. It was found that the concentration of Mn at the grain boundaries is promoted. Furthermore, it was found that as the crystal grain size increases, the concentration of Mn at the grain boundaries becomes higher than the average content, and grain boundary fracture occurs and the low temperature toughness decreases. Therefore, from the viewpoint of suppressing the decrease in low-temperature toughness of medium-Mn steel, as a manufacturing process, direct quenching is performed after hot rolling without reheating, rather than reheating quenching in which the medium Mn steel is reheated to a high temperature after hot rolling. It is considered desirable to adopt quenching.

Further, in hot rolling, from the viewpoint of suppressing grain boundary movement, it is preferable to employ rolling in the unrecrystallized temperature range (controlled rolling) in finish rolling. The present inventors have also found that the formation of ε-martensite is remarkably suppressed by controlled rolling. ε-Martensite has a hexagonal close-packed structure (hcp structure), has low ductility, and adversely affects the toughness of steel. Therefore, the low temperature toughness of the steel sheet is significantly improved by suppressing the formation of ε-martensite. The effect of such controlled rolling is considered to be the result of the stress applied by rolling or the strain introduced by the rolling affecting the transformation behavior of the medium Mn steel. Although details are unknown, controlled rolling may promote the transformation of ε-martensite to α'martensite. Here, α'martensite is a martensite having a body-centered square structure (bct structure).

It was also found that the retained austenite was increased by controlled rolling. The retained austenite is an austenite that does not transform into another phase by accelerated cooling after hot rolling and remains in the steel after being cooled to room temperature, and may be referred to as residual austenite below. Residual austenite is also thought to improve the low temperature toughness of the steel sheet. In general, the formation of retained austenite is due to the enrichment of the elements that stabilize austenite. As described above, it is considered that the Mn concentration at the grain boundaries of austenite in the medium Mn steel becomes high, and the carbon atoms are also likely to be concentrated at the grain boundaries of austenite. It is considered that when hot rolling is performed at a low temperature, the concentrations of Mn and carbon at the grain boundaries of austenite increase, and the austenite is cooled in a stabilized state, and as a result, the formation of retained austenite is promoted. ..

Further, the miniaturization of the metal structure is extremely effective for increasing the strength and toughness of the steel sheet, and the miniaturization of the former austenite is desirable. The reason for this is based on the following findings. The austenite is austenite before it is transformed into another phase by accelerated cooling after hot rolling, and may be referred to as old γ below. In medium Mn steel, transformation proceeds from austenite to ε-martensite and from ε-martensite to α'martensite. At this time, it was found that by controlling the Mn content within a certain range, the blocks of α'martensite were remarkably miniaturized. Further, in order to refine the block of α'martensite, granulation of austenite before metamorphosis, so-called old austenite, is effective. Therefore, due to the miniaturization of the old austenite, the block of α'martensite becomes finer and the low temperature toughness is improved. By controlling the rolling conditions of rough rolling, recrystallization of austenite before controlled rolling is promoted, and the grain size of the old austenite becomes finer.

<Chemical composition>
Hereinafter, the steel sheet according to this embodiment will be described in detail. In the present embodiment, the "steel plate" has a plate thickness of 3 mm or more, for example, 5 mm or more, 10 mm or more, 15 mm or more, 18 mm or more, 20 mm or more, 25 mm or more, 30 mm or more, or 50 mm or more and is hot. It is a rolled steel sheet manufactured by rolling. First, the chemical composition contained in the steel sheet according to the present embodiment will be described. In addition, "%" regarding the content of an element means "mass%" unless otherwise specified.

[C: 0.03% or more, 0.12% or less]
C is an element that increases the strength of steel, and on the other hand, if it is contained in an excessive amount, the low temperature toughness deteriorates. In the present embodiment, the content of C is 0.03% or more in order to secure the strength. The content of C is preferably 0.04% or more, more preferably 0.05% or more. On the other hand, in order to ensure low temperature toughness, the content of C is 0.12% or less in this embodiment. The content of C is preferably 0.10% or less, more preferably 0.08% or less.

[Mn: 6.0% or more and 13.0% or less]
Mn is an element that stabilizes austenite and enhances hardenability of steel. In the medium Mn steel according to the present embodiment, Mn is an extremely important element that affects the block size and grain boundary embrittlement of α'martensite. In the present embodiment, Mn is contained in order to increase the volume fraction of α'martensite to improve the strength and to miniaturize the block of α'martensite to ensure low temperature toughness. The Mn content required to obtain such an effect is 6.0% or more in this embodiment. The Mn content is preferably 7.0% or more, more preferably 8.0% or more. On the other hand, in order to suppress grain boundary embrittlement due to Mn and to miniaturize the martensite block to ensure low temperature toughness, the Mn content is 13.0% or less in this embodiment. The Mn content is preferably 12.0% or less, more preferably 11.0% or less.

[Ni: more than 1.00%, less than 5.00%]
Ni is an element that stabilizes austenite and improves toughness, and is contained to promote the formation of retained austenite and improve low temperature toughness. In the present embodiment, the Ni content is more than 1.00%, preferably 1.50% or more, and more preferably 2.00% or more. From the viewpoint of manufacturing cost, the Ni content is 5.00% or less in this embodiment. The Ni content is preferably 4.00% or less, more preferably 3.50% or less.

[Si: 0% or more, 1.50% or less]
Si is a deoxidizing element. However, a deoxidizing element such as Al or Ti may be contained, and in the present embodiment, the Si content may be 0% or more. The Si content is preferably 0.01% or more from the viewpoint of strengthening the solid solution, suppressing the formation of carbides, and increasing the residual γ. The Si content is more preferably 0.10% or more. On the other hand, from the viewpoint of suppressing the formation of coarse inclusions and ensuring low temperature toughness, the Si content is 1.50% or less in this embodiment. The Si content is preferably 1.20% or less, more preferably 0.50% or less.

[Al: 0% or more, 0.30% or less]
Al is a deoxidizing element. However, a deoxidizing element such as Si or Ti may be contained, and in the present embodiment, the Al content may be 0% or more. The Al content is preferably 0.01% or more in order to ensure deoxidation. Further, from the viewpoint of suppressing the formation of carbides and increasing the residual γ, the Al content is more preferably 0.03% or more. On the other hand, from the viewpoint of suppressing the formation of coarse inclusions and ensuring low temperature toughness, the Al content is 0.30% or less in this embodiment. The Al content is preferably 0.10% or less, more preferably 0.05% or less.

[P: 0.010% or less]
P is an impurity that segregates at grain boundaries and reduces toughness. In order to ensure low temperature toughness, the content of P is 0.010% or less in this embodiment. The content of P is preferably 0.008% or less, more preferably 0.006% or less. The smaller the P content, the more preferable, but from the viewpoint of manufacturing cost, it may be 0.001% or more.

[S: 0.0050% or less]
S is an impurity that produces MnS and reduces ductility and toughness. In order to ensure low temperature toughness, the content of S in this embodiment is 0.0050% or less. The content of S is preferably 0.0030% or less, more preferably 0.0010% or less. The smaller the S content, the more preferable, but from the viewpoint of manufacturing cost, it may be 0.0001% or more.

[N: 0.0100% or less]
Although N is generally contained as an impurity, it may be positively contained in the medium Mn steel according to the present embodiment because it is an element that stabilizes austenite and improves the strength. However, since the effect of N is the same as that of C and it is not always necessary to include it, the lower limit of the content of N is not limited. From the viewpoint of manufacturing cost, the content of N may be 0.0010% or more. Further, N is an element that forms a nitride, and the fine nitride dispersed in the steel is effective in suppressing the coarsening of the structure. In the present embodiment, the N content is preferably 0.0020% or more, more preferably 0.0030% or more, from the viewpoint of securing retained austenite, refining the former austenite, and improving the strength. .. On the other hand, when N atoms dissolved in steel are bonded to dislocations to cause age hardening, the toughness may decrease. Therefore, in the present embodiment, the N content is 0.0100% or less from the viewpoint of ensuring low temperature toughness. The content of N is preferably 0.0080% or less, more preferably 0.0060% or less.

[O: 0.0050% or less]
O is an impurity and forms an oxide. In this embodiment, the O content is 0.0050% or less in order to suppress the formation of coarse oxides and ensure toughness. The content of O is preferably 0.0040% or less, and more preferably 0.0030% or less. The smaller the O content, the more preferable, but from the viewpoint of manufacturing cost, it may be 0.0005% or more.

In the steel sheet according to the present embodiment, Cu: 0% or more, 2.00% or less, Co: 0% or more, 2.00% or less, Cr: 0% or more, if necessary, in order to improve the mechanical properties. , 2.00% or less, Mo: 0% or more, 2.00% or less, W: 0% or more, 2.00% or less, B: 0% or more, 0.0100% or less, Nb: 0% or more, 0 .100% or less, V: 0% or more, 0.100% or less, Ti: 0% or more, 0.100% or less, Zr: 0% or more, 0.100% or less, Hf: 0% or more, 0.100 % Or less, and Ta: 0% or more and 0.100% or less, one or more of them are contained.

[Cu: 0% or more, 2.00% or less]
Cu is an element that stabilizes austenite and is contained as necessary to promote the formation of retained austenite and improve low temperature toughness. In the present embodiment, the Cu content is 0% or more, preferably 0.10% or more, and more preferably 0.20% or more. From the viewpoint of manufacturing cost, the Cu content in this embodiment is 2.00% or less. The Cu content is preferably 1.00% or less, more preferably 0.50% or less.

[Co: 0% or more, 2.00% or less]
Co is an element that stabilizes austenite and is contained as necessary to promote the formation of retained austenite and improve low temperature toughness. In the present embodiment, the Co content is 0% or more, preferably 0.10% or more, and more preferably 0.20% or more. From the viewpoint of manufacturing cost, the Co content is 2.00% or less in this embodiment. The Co content is preferably 1.00% or less, more preferably 0.50% or less.

[Cr: 0% or more, 2.00% or less]
Cr is an element that enhances the hardenability of steel, and is contained as necessary in order to form carbides and improve the strength. In the present embodiment, the Cr content is 0% or more, preferably 0.10% or more, and more preferably 0.20% or more. On the other hand, since the toughness deteriorates as the strength increases, the Cr content is 2.00% or less in this embodiment in order to improve the low temperature toughness. The Cr content is preferably 1.00% or less, more preferably 0.50% or less.

[Mo: 0% or more, 2.00% or less]
Mo is an element that enhances the hardenability of steel, and is contained as necessary in order to improve the strength. In the present embodiment, the Mo content is 0% or more, preferably 0.10% or more, and more preferably 0.20% or more. On the other hand, from the viewpoint of manufacturing cost, the Mo content is 2.00% or less in this embodiment. The Mo content is preferably 1.00% or less, more preferably 0.50% or less.

[W: 0% or more, 2.00% or less]
W is an element that enhances the hardenability of steel, and is contained as necessary in order to improve the strength. In the present embodiment, the W content is 0% or more, preferably 0.10% or more, and more preferably 0.20% or more. On the other hand, from the viewpoint of manufacturing cost, the W content is 2.00% or less in this embodiment. The W content is preferably 1.00% or less, more preferably 0.50% or less.

[B: 0% or more, 0.0100% or less]
B is an element that remarkably enhances the hardenability of steel, and is also an element that segregates at the grain boundaries of austenite and suppresses grain boundary fracture. B is contained as needed, and in the present embodiment, the content of B is 0% or more. In particular, from the viewpoint of improving low temperature toughness, the content of B is preferably 0.0002% or more, more preferably 0.0005% or more. On the other hand, B is also an element that forms nitrides and carbon dioxide, and the content of B is 0.0100% or less from the viewpoint of ensuring low temperature toughness. The content of B is preferably 0.0050% or less, more preferably 0.0030% or less.

[Nb: 0% or more, 0.100% or less]
Nb is an element that produces precipitates such as carbides and nitrides. Nb is contained as necessary in order to improve the strength and toughness by refining the crystal grain size and strengthening precipitation. In the present embodiment, the Nb content is 0% or more, preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, in order to suppress the coarsening of the precipitate and secure the low temperature toughness, the content of Nb is 0.100% or less in this embodiment. The Nb content is preferably 0.050% or less, more preferably 0.040% or less.

[V: 0% or more, 0.100% or less]
V is an element that produces precipitates such as carbides and nitrides. V is contained as necessary in order to improve the strength and toughness by refining the crystal grain size and strengthening precipitation. In the present embodiment, the V content is 0% or more, preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, in order to suppress the coarsening of the precipitate and secure the low temperature toughness, the V content is 0.100% or less in this embodiment. The V content is preferably 0.050% or less, more preferably 0.040% or less.

[Ti: 0% or more, 0.100% or less]
Ti is an element that produces precipitates such as carbides and nitrides. Ti is contained as necessary in order to improve the strength and toughness by refining the crystal grain size and strengthening precipitation. In the present embodiment, the Ti content is 0% or more, preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, in order to suppress the coarsening of the precipitate and secure the low temperature toughness, the Ti content is 0.100% or less in this embodiment. The Ti content is preferably 0.050% or less, more preferably 0.040% or less.

[Zr: 0% or more, 0.100% or less]
Zr is an element that produces precipitates such as carbides and nitrides. Zr is contained as necessary in order to improve the strength and toughness by refining the crystal grain size and strengthening precipitation. In the present embodiment, the Zr content is 0% or more, preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, in order to suppress the coarsening of the precipitate and secure the low temperature toughness, the content of Zr is 0.100% or less in this embodiment. The Zr content is preferably 0.050% or less, more preferably 0.040% or less.

[Hf: 0% or more, 0.100% or less]
Hf is an element that produces precipitates such as carbides and nitrides. Hf is contained as necessary in order to improve the strength and toughness by refining the crystal grain size and strengthening precipitation. In the present embodiment, the Hf content is 0% or more, preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, in order to suppress the coarsening of the precipitate and secure the low temperature toughness, the content of Hf is 0.100% or less in this embodiment. The Hf content is preferably 0.050% or less, more preferably 0.040% or less.

[Ta: 0% or more, 0.100% or less]
Ta is an element that produces precipitates such as carbides and nitrides. Ta is contained as necessary in order to improve the strength and toughness by refining the crystal grain size and strengthening precipitation. In the present embodiment, the Ta content is 0% or more, preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, in order to suppress the coarsening of the precipitate and secure the low temperature toughness, the Ta content is 0.100% or less in this embodiment. The content of Ta is preferably 0.050% or less, more preferably 0.040% or less.

In the steel sheet according to the present embodiment, in order to control the morphology of inclusions, Mg: 0% or more, 0.0100% or less, Ca: 0% or more, 0.0100% or less, and REM: Of 0% or more and 0.0100% or less, one type or two or more types are contained.

[Mg: 0% or more, 0.0100% or less]
Mg is an element that forms oxides and sulfides. Mg is contained as necessary in order to refine the crystal grain size with fine oxides and sulfides. In the present embodiment, the Mg content is 0% or more, preferably 0.0001% or more, and more preferably 0.0005% or more. On the other hand, in order to suppress the coarsening of inclusions and ensure low temperature toughness, the Mg content is 0.0100% or less in this embodiment. The Mg content is preferably 0.0050% or less, more preferably 0.0040% or less.

[Ca: 0% or more, 0.0100% or less]
Ca is an element that forms oxides and sulfides. Ca is contained as necessary in order to prevent the MnS from being stretched in the rolling direction and to improve the toughness. In the present embodiment, the Ca content is 0% or more, preferably 0.0001% or more, and more preferably 0.0005% or more. On the other hand, in order to suppress the coarsening of inclusions and ensure low temperature toughness, the Ca content is 0.0100% or less in this embodiment. The Ca content is preferably 0.0050% or less, more preferably 0.0040% or less.

[REM: 0% or more, 0.0100% or less]
REM (rare earth element) is a general term for two elements, Sc and Y, and 15 lanthanoid elements such as La, Ce and Nd. The REM referred to in the present embodiment is composed of one or more kinds selected from these rare earth elements, and the REM content described below is the total amount of the rare earth elements.
REM is an element that forms oxides and sulfides. REM is contained, if necessary, in order to prevent the MnS from being stretched in the rolling direction and to improve the toughness. In the present embodiment, the content of REM is 0% or more, preferably 0.0001% or more, and more preferably 0.0005% or more. On the other hand, in order to suppress the coarsening of inclusions and ensure low temperature toughness, the content of REM is 0.0100% or less in this embodiment. The content of REM is preferably 0.0050% or less, more preferably 0.0040% or less.

In the steel sheet according to the present embodiment, the balance other than the above chemical components is composed of Fe and impurities. Here, the impurity is a component that is mixed due to various factors in the manufacturing process, including raw materials such as ore and scrap, when the steel sheet is industrially manufactured, and is the component of the steel sheet according to the present embodiment. It means that the content is allowed within the range that does not adversely affect the characteristics.

<Metal structure>
Next, the metal structure of the steel sheet according to the present embodiment will be described. The metallographic structure of the steel sheet according to the present embodiment contains α'martensite and retained austenite, and the residual structure thereof, if present, is composed of one or more of bainite, ferrite, and ε-martensite. Alternatively, the metallographic structure is composed only of α'martensite and retained austenite. Unless otherwise specified, "%" relating to the volume fraction of α'martensite, retained austenite, bainite, ferrite, and ε martensite means "volume%". Here, the volume ratios of α'martensite, retained austenite, bainite, ferrite and ε-martensite are electron backscatter diffraction (EBSD) at a position 1/4 of the plate thickness from the surface of the steel sheet. The area ratio of each phase measured by. The circle-equivalent diameter of α'martensite and retained austenite is calculated from the area and number of each phase measured by EBSD.

[α'martensite: 80% or more]
α'martensite is the main structure having the largest volume fraction in the steel sheet according to the present embodiment. α'martensite is a low-temperature transformation structure generated by accelerated cooling after hot rolling, has a high dislocation density, and significantly improves the strength of steel. The volume fraction of α'martensite is 80% or more in this embodiment in order to secure the strength. The volume fraction of α'martensite is preferably 83% or more, more preferably 85% or more. The higher the volume fraction of α'martensite is, the more preferable it is. However, in the present embodiment, since the volume fraction of retained austenite is 10% or more, the volume fraction of α'martensite is 90% or less.

[Circle-equivalent diameter of α'martensite: 0.1 μm or more, 5.0 μm or less]
The circle-equivalent diameter of α'martensite is the circle-equivalent diameter of the block of α'martensite and can be measured by EBSD. The smaller the circle-equivalent diameter of α'martensite, the higher the toughness of the steel. From the viewpoint of ensuring low temperature toughness, the equivalent circle diameter of α'martensite is 5.0 μm or less in this embodiment. The equivalent circle diameter of α'martensite is preferably 4.0 μm or less, and more preferably 3.0 μm or less. The equivalent circle diameter of α'martensite is preferably small, but in this embodiment, it is 0.1 μm or more. The equivalent circle diameter of α'martensite may be 0.5 μm or more. The equivalent circle diameter of α'martensite can be made finer by increasing the hardenability of steel, increasing the cooling rate of accelerated cooling, and making old austenite finer.

[Residual austenite: 10% or more, 20% or less]
The retained austenite is austenite that remains in the steel sheet after cooling without being transformed by accelerated cooling after hot rolling, and significantly improves the low temperature toughness of the steel. The volume fraction of retained austenite is 10% or more in this embodiment in order to ensure low temperature toughness. The volume fraction of retained austenite is preferably 12% or more, more preferably 14% or more. On the other hand, as the volume fraction of retained austenite increases, the concentration of carbon contained in retained austenite decreases. Retained austenite with a reduced carbon concentration is cooled to a low temperature, and when further deformed, it is easily transformed into α'martensite, which may reduce toughness. From such a viewpoint, in order to ensure low temperature toughness, the volume fraction of retained austenite is 20% or less in this embodiment. The volume fraction of retained austenite is preferably 18% or less, more preferably 16% or less.

[Circle-equivalent diameter of retained austenite: 0.01 μm or more, 2.50 μm or less]
Although retained austenite improves low temperature toughness, coarse retained austenite is susceptible to transformation into α'martensite when cooled to low temperatures and further deformed. Therefore, from the viewpoint of ensuring low temperature toughness, the equivalent circle diameter of retained austenite is 2.50 μm or less. The equivalent circle diameter of the retained austenite is preferably 2.00 μm or less, more preferably 1.50 μm or less. The equivalent circle diameter of the retained austenite is preferably small, but in the present embodiment, it is 0.01 μm or more. The equivalent circle diameter of the retained austenite may be 0.50 μm or more. The circle-equivalent diameter of retained austenite can be made finer by increasing the cooling rate of accelerated cooling, making old austenite finer, and so on.

[Bainite: 0% or more and 5% or less]
Bainite is a lath-like low-temperature metamorphosis structure, but cementite is precipitated and the crystal grain size is larger than that of α'martensite. Bainite has a softer structure than α'martensite and is likely to be a starting point of fracture. In order to ensure the strength and low temperature toughness of steel, it is preferable that the volume ratio of bainite is small. In the present embodiment, the volume fraction of bainite is 0% or more and 5% or less. The volume fraction of bainite is preferably 3% or less, more preferably 0%.

[Ferrite: 0% or more and 5% or less]
Ferrite has a softer structure than the low temperature transformation structure and has a large crystal grain size. In the present embodiment, in order to secure the strength and low temperature toughness of the steel, it is preferable that the volume fraction of ferrite is small. In the present embodiment, the volume fraction of ferrite is 0% or more and 5% or less. The volume fraction of ferrite is preferably 3% or less, and more preferably 0%.

[ε Martensite: 0% or more and 10% or less]
In the medium Mn steel according to the present embodiment, ε-martensite may be generated because Mn lowers the stacking defect energy. When ε-martensite is formed in the process of transformation from austenite to α'martensite, the metallographic structure becomes finer. However, since ε-martensite has low ductility, it is desirable to retransform it into α'martensite, and in the present embodiment, the volume fraction of ε-martensite is 0% or more. Since ε-martensite may become a starting point of fracture and reduce the toughness of steel, the volume ratio of ε-martensite is 10% or less in this embodiment in order to secure low temperature toughness. The volume fraction of ε-martensite is preferably 5% or less, more preferably 1% or less. The volume fraction of ε-martensite is preferably 0%.

To measure the volume ratio of the metal structure and the equivalent diameter of the circle, a sample is used with the observation surface perpendicular to the rolling width direction of the steel sheet and the center of the observation site at the position of 1/4 of the plate thickness from the surface in the plate thickness direction. Will be done. The observation surface is electropolished. The volume fraction of ferrite is calculated by EBSD as the area fraction of a region where the grain average misorientation (GAM) in the local orientation with respect to the surrounding measurement points is 1 ° or less. The volume fractions of retained austenite and ε-martensite are determined by measuring a region of 300 × 300 μm in 0.1 μm steps by EBSD, identifying each phase from the difference in crystal structure, and determining the area fraction. The volume fraction and the area fraction are the same from the viewpoint of quantitative metallographic histology. The circle-equivalent diameter of retained austenite is calculated from the area and number of retained austenite obtained by measuring the region determined to be retained austenite by EBSD in 0.02 μm steps. When the retained austenites are adjacent to each other, the number of crystal grains is measured with the boundary having a 15 ° orientation difference as the grain boundary.

The portions excluding the regions determined by EBSD as ferrite, retained austenite and ε-martensite are α'martensite and bainite. Further, the field of view discriminated by EBSD is observed with a scanning electron microscope (SEM) to discriminate between α'martensite and bainite. In a 20-field photograph showing a lath structure taken by SEM at a magnification of 5000 times, the part where the long axis direction of cementite is oriented in two or more directions in the block is α'martensite, and the other part is It is determined to be bainite, and the area of each is calculated. Further, the circle-equivalent diameter of α'martensite is calculated from the number and area of crystal grains measured with the boundary having a 15 ° orientation difference as the grain boundary.

[Old austenite circle-equivalent diameter: 200 μm or less]
Old austenite is austenite after hot rolling and before accelerated cooling. Since the volume ratio of α'martensite is 80% or more in the steel sheet of the present embodiment, the circle-equivalent diameter of the former austenite is polished and etched at a position 1/4 of the plate thickness from the surface of the steel sheet. The sample is observed with an optical microscope and measured using the photograph taken. The above-mentioned block size of α'martensite is almost the same region within a few degrees of crystal orientation difference, and as the diameter corresponding to the circle of old austenite becomes smaller, the block size of α'martensite also becomes smaller. Therefore, the equivalent circle diameter of the old austenite is preferably small in order to ensure low temperature toughness, and is 200 μm or less in the present embodiment. The equivalent circle diameter of the austenite is preferably 100 μm or less, more preferably 50 μm or less. The lower limit of the circle-equivalent diameter of the former austenite is not limited, but may be 10 μm or more, and may be 20 μm or more. In order to reduce the circle-equivalent diameter of the old austenite, it is recommended to secure the rolling reduction in rough rolling of hot rolling and to make the austenite finer before controlled rolling.

[Aspect ratio of old austenite: 1 or more, 50 or less]
The aspect ratio of the old austenite is measured as the ratio of the minor axis to the major axis from the shape of the metal structure that appears by polishing and etching. In finish rolling, the larger the rolling reduction in the unrecrystallized temperature range, the finer the grain size of α'martensite and the better the strength and toughness of the steel. The rolling reduction of rough rolling is obtained from the thickness of the steel piece before rough rolling and the thickness of the steel piece after the rough rolling. The thickness of the steel piece after the rough rolling is the thickness of the steel piece transferred from the rough rolling to the finish rolling, and is called the transfer thickness. The thickness of the steel piece before rough rolling is sometimes referred to as the piece thickness.

Rough rolling reduction rate = [(thickness of steel pieces before rough rolling-transfer thickness) / thickness of steel pieces before rough rolling] x 100

In the present embodiment, the aspect ratio of the former austenite may be 1 or more, but the aspect ratio of the former austenite is preferably more than 1 from the viewpoint of improving the strength and low temperature toughness of the steel. In the finish rolling, when rolling is performed in the unrecrystallized temperature range, the aspect ratio of the old austenite becomes more than 1. The aspect ratio of the old austenite is preferably 2 or more, more preferably 4 or more. By setting the rolling reduction ratio in the unrecrystallized temperature range to 2 or more, it is possible to manufacture a steel sheet having an aspect ratio of old austenite of 4 or more. On the other hand, when the aspect ratio of the old austenite is increased, the anisotropy of the mechanical properties becomes remarkable. It is desirable that the mechanical properties of the steel sheet be isotropic, and from this viewpoint, the aspect ratio of the former austenite is 50 or less. The aspect ratio of the old austenite is preferably 40 or less, more preferably 30 or less.

[Granular space occupancy of old austenite due to retained austenite: 40% or more, 100% or less]
When retained austenite is generated at the grain boundaries of the former austenite, the grain boundary fracture is suppressed and the low temperature toughness is improved. The space factor of the former austenite grain boundary by retained austenite (hereinafter, may be referred to as the residual γ space factor) is the ratio of retained austenite in the grain boundary of the former austenite. The grain boundary space factor of austenite due to retained austenite, which is required to improve low temperature toughness, is 40% or more. The grain boundary space occupied by the retained austenite of the former austenite is preferably 50% or more, more preferably 60% or more. The higher the grain boundary space factor of the former austenite due to the retained austenite, the better the low temperature toughness, and the lower the temperature toughness may be 100% or less.

The equivalent circle diameter and aspect ratio of the old austenite are measured by an optical microscope at a position 1/4 of the thickness of the steel sheet from the surface. The surface perpendicular to the rolling width direction of the steel sheet is used as the observation surface, and is corroded by nital after alumina polishing. On the observation surface of the sample, the 1 mm square field of view is magnified 100 times by an optical microscope, and the number and area of crystal grains of the former austenite are measured. At this time, the region determined to be ferrite is excluded. The circle-equivalent diameter of the former austenite is calculated from the number and area of crystal grains. Next, the aspect ratio of the old austenite is obtained as a ratio obtained by measuring the major axis and the minor axis of each crystal grain and dividing the major axis by the minor axis. Here, the major axis is the length in the rolling direction of the former austenite, and the minor axis is the length in the plate thickness direction of the former austenite. When the rolling direction is unknown, the length in the direction in which the crystal grains of the old austenite are stretched is the major axis, and the length in the direction orthogonal to the major axis is the minor axis. The residual γ space factor is measured in combination with SEM and EBSD at a position of 1/4 of the plate thickness from the surface of the steel plate. The sample used to locate the retained austenite by EBSD is corroded by nital and observed and photographed by SEM. The position of the grain boundary of the former austenite specified by SEM is compared with the position of the retained austenite specified by EBSD, and the residual austenite is occupied as the ratio of the residual austenite to the grain boundary length of the former austenite having a total length of 5 mm or more. The product ratio is measured.

Next, an example of a method for manufacturing a steel sheet according to the present embodiment will be described. The following description is intended to illustrate the method for manufacturing the steel sheet of the present invention, and intends to limit the steel sheet of the present invention to those manufactured by the manufacturing method as described below. It's not a thing.

The steel sheet according to the present embodiment is manufactured by melting steel, casting it to produce a steel piece, and hot rolling the obtained steel piece. The method for producing the steel piece is not limited, and the steel piece may be produced by a known method. For example, steel pieces are melted by a normal refining process such as a converter or an electric furnace, and then manufactured by a method such as a continuous casting method or a ingot-breaking method. For example, the steel piece may have a thickness of 100 to 300 mm. The steel pieces are hot-rolled and then subjected to controlled cooling such as water cooling as they are. In addition, heat treatment may be applied to adjust the mechanical properties.

Hereinafter, preferable manufacturing conditions for the steel sheet according to the present embodiment will be described.

[heating]
A steel piece having a thickness of 200 mm or more, which is composed of the above-mentioned chemical components and is produced by a continuous casting method, may be once cooled to 400 ° C. or lower. After that, the steel pieces are preferably heated to 1250 ° C. or higher at the Ac3 transformation point or higher. In order to make the metal structure of the steel piece into an ausnite single-phase structure, the heating temperature is preferably equal to or higher than the Ac3 transformation point. From the viewpoint of solid-solving the carbides present in the steel pieces before heating in the steel, the heating temperature is more preferably 1000 ° C. or higher, still more preferably 1050 ° C. or higher. On the other hand, from the viewpoint of suppressing oxidation of the surface of the steel piece and coarsening of austenite, the heating temperature is preferably 1250 ° C. or lower. The heating temperature is preferably 1200 ° C. or lower, more preferably 1100 ° C. or lower. The Ac3 transformation point is the temperature at which the transformation to austenite is completed by raising the temperature, and can be obtained from the volume change during heating.

[Rough rolling]
The hot rolling process comprises rough rolling followed by finish rolling. Rough rolling is performed in a temperature range equal to or higher than the recrystallization temperature of austenite, and the crystal grain size of the old austenite of the steel sheet according to the present embodiment is controlled by the starting temperature and rolling reduction of rough rolling. In order to reduce the grain size of the old austenite, the starting temperature of rough rolling is preferably low. The starting temperature of rough rolling does not exceed the heating temperature of the steel pieces, and is preferably 1100 ° C. or lower. The starting temperature of rough rolling may be, for example, 900 ° C. or higher. Further, in order to make the crystal grain size of the old austenite finer, the reduction ratio of rough rolling is set to 20% or more. The rolling reduction of rough rolling is preferably 25% or more, more preferably 30% or more. Further, the rolling reduction ratio of rough rolling is preferably 90% or less, more preferably 80% or less, still more preferably 70% or less, from the viewpoint of securing the rolling reduction ratio of finish rolling.

[Finish rolling]
After rough rolling, finish rolling is performed. The start temperature of finish rolling does not exceed the end temperature of rough rolling, and is preferably low from the viewpoint of refining the crystal grain size of α'martensite and ensuring retained austenite. The starting temperature of finish rolling is preferably 1000 ° C. or lower. The start temperature of finish rolling is more preferably 900 ° C. or lower from the viewpoint of rolling in the unrecrystallized temperature range. The starting temperature of finish rolling may be, for example, 700 ° C. or higher. On the other hand, the end temperature of finish rolling is equal to or higher than the Ar3 transformation point from the viewpoint of suppressing the anisotropy of the mechanical properties of the steel sheet. The finish temperature of the finish rolling is preferably 700 ° C. or higher. However, the end temperature of finish rolling is preferably low from the viewpoint of refining the crystal grain size of α'martensite, suppressing the formation of ε-martensite, and securing retained austenite. When the finish rolling end temperature is 900 ° C. or higher, the aspect ratio of the old austenite becomes close to 1, and when the finish rolling end temperature decreases, the aspect ratio increases. Therefore, it is preferably less than 900 ° C. The Ar3 transformation point is the temperature at which the transformation from austenite to ferrite starts due to the temperature decrease, and can be obtained from the volume change during the temperature decrease after heating.

The rolling reduction of finish rolling is preferably 30% or more from the viewpoint of refining the crystal grain size of α'martensite, suppressing the formation of ε-martensite, and securing retained austenite. The rolling reduction of finish rolling is more preferably 40% or more. Further, the rolling reduction ratio of finish rolling is preferably 90% or less, more preferably 80% or less, still more preferably 80% or less, from the viewpoint of limiting by the thickness of the steel piece and the plate thickness of the product and securing the rolling reduction ratio of rough rolling. Is 70% or less. On the other hand, as the reduction rate in the unrecrystallized temperature range increases, the aspect ratio of the old austenite increases. As a rough guide, when the start temperature of finish rolling is 900 ° C. or lower and the rolling reduction is about 50%, the aspect ratio of the old austenite is about 4. The rolling reduction of the finish rolling is obtained from the thickness of the intermediate body before the finish rolling and the plate thickness of the steel plate after the finish rolling. The thickness of the intermediate before finish rolling is synonymous with the transfer thickness.

Rolling reduction rate of finish rolling = [(transfer thickness-steel plate thickness) / transfer thickness] x 100

[Direct quenching]
After the completion of hot rolling, direct quenching by water cooling is performed immediately. Direct quenching can promote the transformation of austenite to α'martensite. The starting temperature of direct quenching is equal to or higher than the Ar3 transformation point from the viewpoint of suppressing the formation of ferrite. The starting temperature of direct quenching is preferably 600 ° C. or higher, more preferably 650 ° C. or higher. On the other hand, the starting temperature of direct quenching is, for example, 1000 ° C. or lower, or 950 ° C. or lower. Further, it is desirable that the end temperature of direct quenching is equal to or lower than the Ms point at which the transformation to α'martensite starts. The end temperature of direct quenching is 350 ° C. or lower in this embodiment. The end temperature of direct quenching is more preferably 200 ° C. or lower, still more preferably 100 ° C. or lower. The end temperature of direct quenching may be room temperature. Further, the cooling rate is controlled by the water amount density of the cooling water in consideration of the plate thickness of the steel plate, and may be 100 ° C./sec or less. The cooling rate of direct quenching is 3 ° C./sec or more from the viewpoint of suppressing the formation of bainite and ferrite. The cooling rate of direct quenching is more preferably 5 ° C./sec or higher, and even more preferably 10 ° C./sec or higher. The Ms point is the temperature at which the transformation from austenite to martensite starts due to rapid cooling after heating, and can be obtained from the volume change.

[Intermediate heat treatment]
In order to improve the mechanical properties of the steel sheet, heat treatment can be performed after direct quenching to adjust the volume fraction, stability, and mechanical properties of retained austenite. Specifically, it is an intermediate heat treatment performed at a two-phase region temperature equal to or higher than the Ac1 transformation point and lower than the Ac3 transformation point, and a tempering treatment. Cooling after the intermediate heat treatment is preferably water cooling in order to suppress the formation of bainite and ferrite, and the cooling rate may be 3 ° C./sec or more and 100 ° C./sec or less. The cooling rate of the intermediate heat treatment is more preferably 5 ° C./sec or more, and even more preferably 10 ° C./sec or more. Further, it is desirable that the cooling stop temperature of the intermediate heat treatment is equal to or lower than the Ms point at which the transformation to α'martensite starts. The cooling stop temperature of the intermediate heat treatment is preferably 350 ° C. or lower in this embodiment. The cooling stop temperature of the intermediate heat treatment is more preferably 200 ° C. or lower, still more preferably 100 ° C. or lower. The cooling stop temperature of the intermediate heat treatment may be room temperature. The Ac1 transformation point is the temperature at which transformation to austenite starts when the temperature rises, and can be obtained from the volume change during heating.

[Tempering]
Tempering can be performed after direct quenching or intermediate heat treatment. The tempering process adjusts the mechanical properties of the steel sheet. The temperature of the tempering treatment is preferably 100 ° C. or higher in order to obtain the effect. The temperature of the tempering treatment is more preferably 400 ° C. or higher. On the other hand, the temperature of the tempering treatment is preferably less than Ac1 because the change in characteristics becomes large when the phase transformation occurs due to the heating of the tempering treatment. The temperature of the tempering treatment is more preferably 550 ° C. or lower.

Next, the mechanical properties of the steel sheet according to this embodiment will be described.

[Yield strength (YS)]
The steel sheet according to the present embodiment preferably has a yield strength of 700 N / mm ² or more in order to secure the strength required for a structure or the like. Yield strength is more preferably 750 N / mm ² or more, further preferably 800 N / mm ² or more. The upper limit of the yield strength is not particularly limited, but the yield strength is preferably 1100 N / mm ² or less in order to obtain excellent low temperature toughness.

[Tensile strength (TS)]
The steel sheet according to the present embodiment preferably has a tensile strength of 800 N / mm ² or more in order to secure the strength required for a structure or the like. The tensile strength is more preferably 900 N / mm ² or more, still more preferably 1000 N / mm ² or more. The upper limit of the tensile strength is not particularly limited, but in order to obtain excellent low temperature toughness, the tensile strength is preferably 1500 N / mm ² or less.

[Elongation (EL)]
From the viewpoint of workability, the steel sheet according to the present embodiment preferably has an elongation of 35% or more in order to secure sufficient ductility. The elongation is more preferably 38% or more, still more preferably 40% or more. The upper limit of elongation is not particularly limited, but for example, the elongation may be 60% or less.

[Absorbed energy (KV ₂ )]
The steel sheet according to the present embodiment preferably has a Charpy absorption energy of 100 J or more at -196 ° C. in order to secure the low temperature toughness required for low temperature applications such as liquid fuel tanks. The Charpy absorption energy at -196 ° C. is more preferably 150 J or more, still more preferably 200 J or more. The upper limit of the Charpy absorption energy at -196 ° C. is not particularly limited, but may be, for example, 500 J or less.

Examples of the present invention will be shown below, but the examples shown below are examples of the present invention, and the present invention is not limited to the examples described below.

Steel pieces produced by melting and continuous casting of steel in a converter were cooled to room temperature, reheated, and hot-rolled. The chemical components shown in Table 1 were obtained by performing a chemical analysis using a sample collected from a steel sheet. The Ar3 transformation point, Ac1 transformation point, Ac3 transformation point, and Ms point shown in Table 1 were determined from the volume change due to heating and cooling using a sample collected from a steel plate. The Ac1 transformation point and the Ac3 transformation point were measured under the condition of heating to 1100 ° C. at a heating rate of 10 ° C./s. After holding at 1100 ° C. for 600 s, the Ar3 transformation point was measured at a cooling rate of 5 ° C./s. The measurement of the Ms point was carried out at a cooling rate of 50 ° C./s after holding at 1100 ° C. for 600 s. Table 2 shows the manufacturing conditions. In Tables 2 and 3, when "direct quenching" is described, it means that accelerated cooling is applied as it is after hot rolling, and when it is described as "reheating quenching", it means hot rolling. It means that after that, it was once air-cooled, cooled to room temperature, and then reheated and hardened.

To measure the volume ratio of the metal structure and the equivalent diameter of the circle, a sample is used with the observation surface perpendicular to the rolling width direction of the steel sheet and the center of the observation site at the position of 1/4 of the plate thickness from the surface in the plate thickness direction. Was done. The observation surface was electropolished, and the volume fractions of ferrite, retained austenite, and ε-martensite were determined from the area fraction of each phase by EBSD. The circle-equivalent diameter of retained austenite was calculated from the area and number of retained austenite determined by EBSD. α'martensite and bainite are the remnants of ferrite, retained austenite and ε-martensite. SEM was used to discriminate between α'martensite and bainite, and the volume fraction of α'martensite and bainite was determined from the area ratio of each phase. Further, the equivalent circle diameter of α'martensite was determined from the number and area of crystal grains measured with the boundary having a 15 ° orientation difference as the grain boundary. The circle-equivalent diameter and aspect ratio of the old austenite are such that the surface perpendicular to the rolling width direction of the steel sheet is the observation surface at a position 1/4 of the plate thickness from the surface of the steel sheet, and the observation surface is corroded by alumina polishing and nital. It was measured using the applied sample. The equivalent circle diameter and aspect ratio of the old austenite were measured by light microscopy as described above. The space factor of the old γ grain boundary due to the residual γ was measured as described above by using SEM and EBSD in combination at a position of 1/4 of the plate thickness from the surface of the steel sheet.

[Tensile test]
The tensile test for evaluating the tensile properties of a steel sheet was taken from a portion of 1/2 of the sheet thickness in the plate thickness direction from the surface of the steel sheet, with the plate width direction of the steel sheet as the longitudinal direction, in accordance with JIS Z 2241: 2011. It was carried out using two No. 4 test pieces. The yield strength (YS), tensile strength (TS), and elongation (EL) are the average values (arithmetic mean) of the two test pieces, respectively.

[Charpy impact test]
The Charpy impact test was performed in accordance with JIS Z 2242: 2018 using three V-notch test pieces, and the absorbed energy was measured. The test piece was formed with a V notch at a position of 1/2 of the plate thickness in the plate thickness direction from the surface of the steel plate so that the rolling direction was the longitudinal direction and cracks propagated in the plate width direction. The test temperature is -196 ° C. The absorbed energy (KV ₂ ) is the average value (arithmetic mean) of the absorbed energies of the three test pieces measured in this way.

The steel of the invention example has a tensile strength of 800 N / mm ² or more and a Charpy absorption energy at -196 ° C. of 100 J or more.

The present invention provides a steel sheet having excellent strength and toughness by appropriately controlling the chemical composition and the metallographic structure. The steel plate can be manufactured in various thicknesses from about 3 mm to about 200 mm to a width of about 5 m and a length of about 50 m, and can be used as an extremely large structural member. The high-strength and high-toughness low-temperature thick steel sheet of the present invention can be used for land-based LNG storage tanks, marine LNG storage tanks, and storage tanks for cryogenic fuels such as liquid hydrogen, ethane, butane, and LPG. It is also possible to manufacture steel pipes and shaped steels having similar characteristics.

Claims (4)

By mass%
C: 0.03% or more, 0.12% or less,
Mn: 6.0% or more, 13.0% or less,
Ni: Over 1.00%, 5.00% or less,
Si: 0% or more, 1.50% or less,
Al: 0% or more, 0.30% or less,
Cu: 0% or more, 2.00% or less,
Co: 0% or more, 2.00% or less,
Cr: 0% or more, 2.00% or less,
Mo: 0% or more, 2.00% or less,
W: 0% or more, 2.00% or less,
B: 0% or more, 0.0100% or less,
Nb: 0% or more, 0.100% or less,
V: 0% or more, 0.100% or less,
Ti: 0% or more, 0.100% or less,
Zr: 0% or more, 0.100% or less,
Hf: 0% or more, 0.100% or less,
Ta: 0% or more, 0.100% or less,
Mg: 0% or more, 0.0100% or less,
Ca: 0% or more, 0.0100% or less, and REM: 0% or more, 0.0100% or less,
P: 0.010% or less,
S: 0.0050% or less,
N: 0.0100% or less, O: 0.0050% or less, and the balance consists of Fe and impurities.
The metallic structure contains 80% or more of α'martensite by volume and 10% or more and 20% or less of retained austenite, and if the residual structure is present, 5% or less of bainite by volume. Consists of less than% ferrite and less than 10% ε martensite
The circle-equivalent diameter of the α'martensite is 0.1 μm or more and 5.0 μm or less.
The equivalent circle diameter of the retained austenite is 0.01 μm or more and 2.50 μm or less.
The circle-equivalent diameter of the old austenite is 200 μm or less, and the aspect ratio of the old austenite is 1 or more and 50 or less.
A steel sheet having a grain boundary space occupied by the retained austenite of 40% or more and 100% or less. The steel sheet according to claim 1, wherein the austenite has an aspect ratio of 4 or more and 50 or less. By mass%
Cu: 0.10% or more, 2.00% or less,
Co: 0.10% or more, 2.00% or less,
Cr: 0.10% or more, 2.00% or less,
Mo: 0.10% or more, 2.00% or less,
W: 0.10% or more, 2.00% or less,
B: 0.0002% or more, 0.0100% or less,
Nb: 0.005% or more, 0.100% or less,
V: 0.005% or more, 0.100% or less,
Ti: 0.005% or more, 0.100% or less,
Zr: 0.005% or more, 0.100% or less,
The invention according to claim 1 or 2, wherein Hf: 0.005% or more and 0.100% or less, and Ta: 0.005% or more and 0.100% or less contain one or more of them. Steel plate. By mass%
Mg: 0.0001% or more, 0.0100% or less,
Any one of claims 1 to 3, which contains one or more of Ca: 0.0001% or more and 0.0100% or less, and REM: 0.0001% or more and 0.0100% or less. The steel plate according to item 1.