EP4019656A1

EP4019656A1 - Steel and method for manufacturing same

Info

Publication number: EP4019656A1
Application number: EP20853747.2A
Authority: EP
Inventors: Daichi Izumi; Koichi Nakashima; Keiji Ueda
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2019-08-21
Filing date: 2020-08-18
Publication date: 2022-06-29
Also published as: CN114302977A; WO2021033694A1; CN114302977B; TWI726798B; JPWO2021033694A1; EP4019656A4; JP6947330B2; TW202113100A; KR20220030292A

Abstract

The steel of the present disclosure includes: a chemical composition containing, in 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; Ti: 0.005% or less; and Nb: 0.005% or less with the balance being Fe and inevitable impurities; a microstructure having austenite as a base phase and an average grain size of 80µm or more; an absorbed energy of a Charpy impact test at - 269°C of 150 J or more; and a total elongation of a tensile test at -269°C of 30% or more.

Description

TECHNICAL FIELD

The present disclosure relates to steel suitable for structural steel used in environments at cryogenic temperatures, such as tanks for storing liquid hydrogen, liquid helium, liquefied gas, and the like, in particular having excellent toughness at cryogenic temperatures, and a method of manufacturing the same.

BACKGROUND

In order to use a hot-rolled steel sheet or plate for structures for liquid hydrogen, liquid helium and liquefied gas storage, the hot-rolled steel sheet or plate requires to have excellent toughness at cryogenic temperatures because the structures are used at cryogenic temperatures. For example, when the hot-rolled steel sheet or plate is used for liquid helium storage, it is necessary to ensure excellent toughness at a temperature of -269 °C or lower, which is the boiling point of helium. If the toughness at cryogenic temperatures of the steel material is inferior, the safety as the structure for cryogenic storage may not be maintained. Therefore, there is a strong demand for improving the toughness at cryogenic temperatures of the steel material for this purpose.
In response to this demand, austenitic stainless steel where austenite, which does not exhibit brittleness at cryogenic temperatures, is the microstructure of the steel sheet or plate, have conventionally been used. However, because of the high alloy cost and manufacturing cost, there has been a desire for a steel material that is inexpensive and has excellent toughness at cryogenic temperatures.
For example, JP 2018-104792 A (PTL 1) proposes using a high-Ni steel containing a large amount of Ni, which is an austenite-stabilizing element, as a structural steel for environments at -253 °C, as a new steel material which replaces conventional steel for low temperature.
PTL 1 proposes a technique to ensure toughness at cryogenic temperatures by controlling the grain size and morphology of prior austenite.

CITATION LIST

Patent Literature

PTL 1: JP 2018-104792 A

SUMMARY

(Technical Problem)

The technique described in PTL 1 can provide high-Ni steel with excellent toughness at cryogenic temperatures, but the high-Ni steel must contain 12.5 % or more Ni from the viewpoint of ensuring toughness at cryogenic temperatures, and thus a reduction in material cost has been required. In addition, it is necessary to perform heat treatment with multiple steps such as reheating quenching, intermediate heat treatment, and tempering in order to secure an austenite phase, which results in high manufacturing costs.
Therefore, it could thus be helpful to provide a steel having excellent toughness at cryogenic temperatures and excellent tensile properties, which can reduce the material and manufacturing cost. Furthermore, it could also be helpful to propose an advantageous method of manufacturing such a steel. Herein, the "excellent toughness at cryogenic temperatures" means that the absorbed energy of a Charpy impact test at -196 °C, or even -269 °C, is 150 J or more. In addition, the "excellent tensile properties" refer to a total elongation of 30 % or more in a tensile test at -269 °C.

(Solution to Problem)

Targeting steels with a relatively high Mn content of 20.0 % or more, we conducted extensive study on the chemical compositions of the steel sheets or plates and various factors determining the microstructures. As a result, we made the following discoveries a and b.
a. The main form of brittle fracture in the austenite steel is intergranular fracture originating from crystal grain boundaries. Therefore, coarsening the crystal grain size is effective to improve the toughness at cryogenic temperatures of the steel.
b. If hot rolling and heat treatment are performed under appropriate conditions with an appropriate chemical composition, the toughness at cryogenic temperatures and tensile properties can be improved with a minimum number of heat treatments, which can reduce the manufacturing cost.
The present disclosure is based on the aforementioned discoveries and further studies. We thus provide the following.

1. A steel comprising:
- a chemical composition containing (consisting of), in 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;
- Ti: 0.005 % or less; and
- Nb: 0.005 % or less, with the balance being Fe and inevitable impurities;
- a microstructure having austenite as a base phase and an average grain size of 80 µm or more;
- an absorbed energy of a Charpy impact test at -269 °C of 150 J or more; and
- a total elongation of a tensile test at -269 °C of 30 % or more.
2. The steel according to 1., wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of:
- 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;
- Mg: 0.0005 % or more and 0.0050 % or less; and
- REM: 0.0010 % or more and 0.0200 % or less.
3. A method of manufacturing a steel, comprising:
- heating a steel material having the chemical composition according to 1. or 2. to a temperature range of 1100 °C to 1300 °C;
- performing hot rolling; and
- performing heat treatment in which reheating is executed to a temperature range of 1100 °C to 1300 °C and a product of a reheating temperature (°C) and a reheating time (h) is 100 °C·h or more.

The above-mentioned temperatures refer to a surface temperature of the steel material or steel sheet or plate.

(Advantageous Effect)

According to the present disclosure, a steel having excellent toughness at cryogenic temperatures and tensile properties can be provided. Accordingly, the steel of the present disclosure makes a significant contribution to improving the safety and product life of the steel structure used in cryogenic environments, such as a tank for liquefied hydrogen, liquid helium, and liquefied gas storage, which exhibits remarkable industrial effects. In addition, the manufacturing method of the present disclosure does not cause a decrease in productivity and an increase in manufacturing cost, thus providing a method with excellent economic efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawing:
FIG. 1 is a graph illustrating the relationship between the average grain size and the absorbed energy at -269 °C for a steel satisfying the chemical composition of the present disclosure.

DETAILED DESCRIPTION

The following describes the steel of the present disclosure in detail.

[Chemical composition]

First, the chemical composition of the steel of the present disclosure and reasons for limitation will be described. Note that the unit " %" of each component is "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 achieve the effect, the C content needs to be 0.100 % or more. On the other hand, when the C content exceeds 0.700 %, Cr carbides are excessively formed and the toughness at cryogenic temperatures is deteriorated. Therefore, the C content is set to 0.100 % or more and 0.700 % or less. The C content is preferably 0.200 % or more. The C content is preferably 0.600 % or less. The C content is more preferably 0.200 % or more and 0.600 % or less.

Si: 1.00 % or less

Si acts as a deoxidizer and is a necessary element in steelmaking, so it is preferable to add 0.05 % or more. On the other hand, if the Si content exceeds 1.00 %, the non-thermal stress (internal stress) increases excessively, resulting in deterioration of toughness at cryogenic temperatures. For this reason, Si is set to 1.00 % or less. The Si content 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 important in the present disclosure to ensure low-temperature toughness. In order to achieve this effect, the Mn content needs to be 20.0 % or more. On the other hand, when the Mn content is more than 40.0 %, the toughness at cryogenic temperatures is deteriorated. Therefore, the Mn content is set to 20.0 % or more and 40.0 % or less. The Mn content is preferably 23.0 % or more. The Mn content is preferably 38.0 % or less. The Mn content is more preferably 23.0 % or more and 38.0 % or less. The Mn content is further preferably 36.0 % or less.

P: 0.030 % or less

When the P content exceeds 0.030 %, it excessively segregates at grain boundaries, resulting in a decrease in toughness at cryogenic temperatures. Therefore, the P content is desirably as low as possible with the upper limit being 0.030 %. Therefore, the P content is set to 0.030 % or less. The P content is desirably 0.002 % or more because excessive reduction of P content increases refining cost and is economically disadvantageous. The P content is more preferably 0.005 % or more. The P content is more preferably 0.028 % or less. The P content is further preferably 0.005 % or more and 0.028 % or less. The P content is still more preferably 0.024 % or less.

S: 0.0070 % or less

S deteriorates the toughness at cryogenic temperatures of the steel sheet or plate. Therefore, the S content is desirably as low as possible with the upper limit being 0.0070 %. Therefore, the S content is set to 0.0070 % or less. The S content is desirably 0.0010 % or more because excessive reduction of S content increases refining cost and is economically disadvantageous. The S content is preferably 0.0050 % or less.

Al: 0.01 % or more and 5.00 % or less

Al acts as a deoxidizer and is most commonly used in a molten steel deoxidation process of a steel sheet or plate. In order to achieve this effect, the Al content needs to be 0.01 % or more. On the other hand, an Al content exceeding 5.00 % produces a large amount of inclusions, which results in deterioration in toughness at cryogenic temperatures. Therefore, the Al content is set to 5.00 % or less. For this reason, the Al content is set to 0.01 % or more and 5.00 % or less. The Al content is preferably 0.02 % or more. The Al content is preferably 4.00 % or less. The Al content is more preferably 0.02 % or more and 4.00 % or less.

Cr: 0.5 % or more and 7.0 % or less

Cr is an effective element to improve the toughness at cryogenic temperatures because it improves the grain boundary strength. In order to achieve this effect, the Cr content needs to be 0.5 % or more. On the other hand, when the Cr content exceeds 7.0 %, the toughness at cryogenic temperatures is deteriorated due to formation of Cr carbides. Therefore, the Cr content is set to 0.5 % or more and 7.0 % or less. The Cr content is preferably 1.0 % or more and more preferably 1.2 % or more. The Cr content is preferably 6.7 % or less and more preferably 6.5 % or less. The Cr content is further preferably 1.0 % or more and 6.7 % or less. The Cr content is still 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 effective element for improving toughness at cryogenic temperatures. In order to achieve this effect, the N content needs be 0.0050 % or more. On the other hand, when the content exceeds 0.0500 %, nitrides or carbonitrides are coarsened and the toughness is deteriorated. Therefore, the N content is set to 0.0050 % or more and 0.0500 % or less. The N content is preferably 0.0060 % or more. The N content is preferably 0.0400 % or less. The N content is more preferably 0.0060 % or more and 0.0400 % or less.

O: 0.0050 % or less

O degrades toughness at cryogenic temperatures due to formation of oxides. For this reason, the O content is set to 0.0050 % or less. The O content is preferably 0.0045 % or less. The O content is desirably 0.0010 % or more because excessive reduction of O content increases refining cost and is economically disadvantageous.

Ti and Nb contents each suppressed to 0.005 % or less

Excessive Ti and Nb contents reduce toughness at cryogenic temperatures because Ti and Nb form carbonitride with a high melting point in the steel. Ti and Nb are elements that are inevitably mixed in from raw materials, etc. In most cases, Ti of more than 0.005 % and 0.010 % or less and Nb of more than 0.005 % and 0.010 % or less are mixed in. Therefore, it is necessary to intentionally limit the mixed content of Ti and Nb to suppress the content of each of Ti and Nb to 0.005 % or less according to the method described below. By suppressing the content of each of Ti and Nb to 0.005 % or less, it is possible to eliminate the above-mentioned adverse effects of carbonitrides and to ensure excellent toughness at cryogenic temperatures. The content of each of Ti and Nb is preferably set to 0.003 % or less. Of course, the content of each of Ti and Nb may be 0 %, but is desirably 0.001 % or more because excessive reduction is not preferable from the viewpoint of steelmaking cost.
In order to further improve low-temperature toughness, the following elements can be contained as necessary in addition to the above essential elements in the present disclosure:
at least one selecting from the group consisting of 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, Mg: 0.0005 % or more and 0.0050 % or less, and REM: 0.0010 % or more and 0.0002 % or less.

Cu, Ni: 1.0 % or less each

Cu and Ni are elements that improve low-temperature toughness. In order to obtain such an effect, the content of each of Cu and Ni is preferably 0.01 % or more, and more preferably 0.03 % or more. On the other hand, when the contents of Cu and Ni each exceed 1.0 %, the surface properties are deteriorated during rolling and the manufacturing cost increases. For this reason, when these alloying elements are contained, the content of each element is preferably 1.0 % or less, more preferably 0.7 % or less, and further preferably 0.5 % or less.

Mo, V, W: 2.0 % or less each

Mo, V and W contribute to stabilization of austenite. In order to obtain such an effect, the content of each of Mo, V and W is preferably 0.001 % or more and more preferably 0.003 % or more. On the other hand, when the content of each of Mo, V, and W exceeds 2.0 %, coarse carbonitrides are formed, which may be an initiation point of fractures and increases manufacturing cost. For this reason, when these alloying elements are contained, the content of each element is preferably 2.0 % or less, and more preferably 1.7 % or less. The content of each of Mo, V and W is further preferably 0.003 % or more. The content of each of Mo, V, and W is further preferably 1.7 % or less. The content of each of Mo, V, and W is still more preferably 1.5 % or less.

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 are useful elements for controlling the morphology of inclusions and can be contained as required. Controlling the morphology of inclusions means making expanded sulfide-based inclusions (mainly MnS) into granular inclusions. MnS, which is an initiation point of fracture, is reduced through the morphological control of inclusions to thereby improve toughness. In order to obtain such an effect, the content of each of Ca and Mg is preferably 0.0005 % or more and REM content is preferably 0.0010 % or more. On the other hand, when any of these elements is contained in a large amount, the amount of nonmetallic inclusions increases, and the toughness may rather be deteriorated. In addition, it may be economically disadvantageous.
For this reason, in a case where Ca and Mg are contained, the Ca and Mg contents are each preferably 0.0005 % or more. The Ca and Mg contents are each preferably 0.0050 % or less. In a case where REM is contained, the REM content is preferably 0.0010 % or more. The REM content is preferably 0.0200 % or less. The Ca content is more preferably 0.0010 % or more. The Ca content is more preferably 0.0040 % or less. The Ca content is further preferably 0.0010 % or more and 0.0040 % or less. The Mg content is more preferably 0.0010 % or more. The Mg content is more preferably 0.0040 % or less. The Mg content is further preferably 0.0010 % or more and 0.0040 % or less. The REM content is more preferably 0.0020 % or more. The REM content is more preferably 0.0150 % or less. The REM content is further preferably 0.0020 % or more and 0.0150 % or less.
REM refers to rare earth metals, and is a generic term for 17 elements, which are the 15 elements of lanthanides plus Y and Sc. At least one of these elements can be contained. The REM content means the total content of these elements.
The balance other than the above components is a chemical composition having iron and inevitable impurities. Examples of the inevitable impurities include H and B, and a total of 0.01 % or less is acceptable.

[Microstructure]

Microstructure having austenite as a base phase

When the crystal structure of a steel material is a body-centered cubic structure (bcc), the steel material is not suitable for use in environments at cryogenic temperatures because it may cause brittle fractures in environments at cryogenic temperatures. In consideration of the use in environments at cryogenic temperatures, the base phase of the steel material is preferably an austenite microstructure where the crystal structure is a face-centered cubic structure (fcc). Here, "having austenite as a base phase" means that the austenite phase has an area ratio of 90 % or more, preferably 95 % or more. The remainder other than the austenite phase is a ferrite phase or martensite phase.

Average grain size in microstructure being 80 µm or more

As a result of verifying the relationship between the average grain size and the absorbed energy of a Charpy impact test, as illustrated in FIG. 1, it is found that when the other conditions of the present disclosure are satisfied and the average grain size is 80 µm or more, the absorption energy can be 150 J or more. Here, the crystal grain in this specification mainly refers to an austenite grain, and the average grain size can be determined by randomly selecting 100 crystal grains from an image taken at 200x magnifications using an optical microscopy, calculating an equivalent circle diameter for each of the crystal grains, and obtaining the average value of the equivalent circle diameters.
The average grain size can be achieved by performing hot rolling and heat treatment according to the conditions described below under the chemical composition described above.
The steel of the present disclosure can be obtained from a molten steel having the above-described chemical composition obtained by steelmaking using a publicly-known smelting method such as a converter or an electric furnace. In addition, secondary refinement may be performed in a vacuum degassing furnace. At that time, in order to limit Ti and Nb, which hinder the control of a preferable microstructure, to the above-described ranges, it is necessary to avoid inevitable mixing from raw materials and the like to take measures to reduce the contents thereof. For example, by lowering the basicity of slag in the refining stage, these alloys are concentrated and discharged into the slag, which can reduce the concentration of Ti and Nb in a final slab product. Alternatively, a method of blowing oxygen to oxidize the Ti and Nb and floating and separating the alloy of Ti and Nb in reflux may also be used. Subsequently, it is preferable to obtain a steel material such as a slab having a predetermined size with a known casting method such as a continuous casting method, ingot casting and blooming, or the like.
The following specifies the manufacturing conditions for making the above steel material into a steel material having excellent toughness at cryogenic temperatures.

[Reheating temperature of steel material: 1100 °C or higher and 1350 °C or lower]

In order to obtain a steel having the above-mentioned microstructure, it is important to heat the steel material to a temperature range of 1100 °C to 1300 °C and then subject it to hot rolling. The temperature control here is based on the surface temperature of the steel material. In order to obtain the effect of Mn mentioned above, it is important to diffuse Mn into the steel. In detail, in order to promote Mn diffusion, the heating temperature of the steel material before hot rolling is set to 1100 °C or higher. On the other hand, when the temperature exceeds 1300 °C, melting of the steel may start. Therefore, 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. Thee heating temperature of the steel material is preferably 1270 °C or lower. The heating temperature of the steel material is more preferably 1130 °C or higher and 1270 °C or lower.

[Hot rolling]

After the heating, hot rolling is performed. The method of hot rolling is not particularly limited, but it is preferable to set the finish temperature of finish rolling to 700 °C or higher because when the finish temperature is lower, rolling efficiency decreases. The finish temperature is more preferably 750 °C or higher.

[Performing heat treatment in which reheating is executed to a temperature range of 1100 °C to 1300 °C and a product of a reheating temperature and a reheating time is 100 °C·h or more]

After the hot rolling or subsequent cooling treatment, if such is performed, a predetermined heat treatment is performed. In the heat treatment, reheating is executed to the temperature range of 1100 °C to 1300 °C and the product of a reheating temperature (°C) and a reheating time (h: hour) is set to 100 °C/h or more. By doing so, the crystal grains are coarsened to improve toughness at cryogenic temperatures, and additionally the dislocations which have been introduced during the hot rolling are recovered to thereby improve tensile properties, in particular, elongation. The temperature range for reheating is set to 1100 °C or higher and 1300 °C or lower for the following reasons. In detail, in order to diffuse Mn in the heat treatment, the heating temperature during reheating in the heat treatment is set to 1100 °C or higher. On the other hand, when the temperature exceeds 1300 °C, melting of the steel may start. Therefore, the upper limit of the reheating temperature is set to 1300 °C. The reason why the product of the reheating temperature (°C) and the reheating time (h) is specified is that there is a correlation between crystal grain growth and dislocation recovery. The upper limit of the product of the reheating temperature and the reheating time is preferably 650 °C·h for manufacturing cost, and the lower limit thereof is preferably 208 °C·h in order to coarsen all crystal grains. The reheating temperature during reheating in the heat treatment is preferably 1130 °C or higher. The reheating temperature is preferably 1270 °C or lower. The reheating temperature is more preferably 1130 °C or higher and 1270 °C or lower. The reheating time is preferably 0.1 h or more in order to promote grain growth. The reheating time is preferably 0.5 h or less in order to suppress a decrease in manufacturing efficiency. The reheating time is more preferably 0.1 h or more and 0.5 h or less. After the heat treatment, cooling is performed.
The cooling treatment may be performed after either or both of the hot rolling and subsequent heat treatment. This is to inhibit carbide precipitation. The cooling temperature after the hot rolling is preferably 300 °C or higher. The cooling temperature after the hot rolling is preferably 650 °C or lower. The cooling temperature after the heat treatment is preferably 300 °C or higher. The cooling temperature after the heat treatment is preferably 900 °C or lower. The average cooling rate is preferably 1.0 °C/s or more.

EXAMPLES

The following provides a more detailed explanation of the present disclosure through examples. However, the present disclosure is not limited to the following examples.

Steel slabs (steel materials) having the chemical compositions listed in Table 1 were prepared by a process for refining with converter and ladle and continuous casting. Next, the steel slabs thus obtained were hot rolled under the conditions listed in Table 2 to obtain steel plates having a thickness of 6 mm to 30 mm. Here, the reheating temperature during reheating in the heat treatment was set to the same temperature as the heating temperature of the steel material for each sample.
The steel plates thus obtained were evaluated as follows for microstructure and mechanical properties of toughness at cryogenic temperatures and tensile properties.
In Table 2, the "finish temperature of finish rolling" refers to the rolling finish temperature.

(1) Microstructure evaluation

- Area ratio of austenite phase

The area ratio of each phase of the microstructure was obtained from the Phase map of electron backscatter diffraction (EBSD) analysis. A test piece for EBSD analysis was collected from a cross section parallel to the rolling direction at a position of plate thickness × 1/2 of each obtained steel plate, and EBSD analysis was performed in a field of view of 500 µm × 200 µm with a measurement step of 0.3 µm, and the value indicated on the Phase map was used as the area ratio.
The area ratio of the austenite phase was 90 % or more in all the examples and comparative examples, confirming that the base phase was austenite.

- Average grain size

For the steel plate after the heat treatment, the cross section along the rolling direction was polished, and 100 crystal grains were randomly selected from an image taken at a position of plate thickness × 1/2 at 200x magnification using an optical microscopy, and the average grain size was determined from the equivalent circle diameters of the crystal grains.

(2) Toughness at cryogenic temperatures

From each steel plate having a plate thickness exceeding 10 mm at a position of plate thickness × 1/2 and in a direction parallel to the rolling direction, three test pieces for Charpy V-notch test were collected in accordance with JIS Z 2242 (2005) standard, and a Charpy impact test was conducted at -196 °C and -269 °C for each of the test pieces to measure an absorbed energy and evaluate its base material toughness. In the present disclosure, when the average of three absorbed energy values was 150 J or more, it was determined to be excellent in base metal toughness.
For each steel plate having a plate thickness of less than 10 mm, three 5 mm sub-size test pieces for Charpy V-notch test were collected at a position of plate thickness × 1/2 and in a direction parallel to the rolling direction of the steel plate in accordance with JIS Z 2242 (2005), and a Charpy impact test were conducted at -19 6 °C and -269 °C for each of the test pieces. When the average of three absorbed energy values was 100 J or more, it was determined to be excellent in base metal toughness. The Charpy impact test at -269 °C was conducted by placing the test piece in a capsule with liquid helium flowing through it.
Reference 1: T. Ogata, K. Hiraga, K. Nagai, and K. Ishikawa: Tetsu-to-Hagane, 69(1983), 641.

(3) Tensile properties

From each of the obtained steel plates, a round bar tensile test piece with a parallel portion diameter of 6 mm and a gauge length of 25 mm was collected and a tensile test was conducted at -269 °C to investigate total elongation. In the present disclosure, total elongation of 30 % or more was determined to be excellent in ductility.
The results obtained by the above tests are listed in Table 2.
It has been confirmed that the steel according to the present disclosure satisfies the above-mentioned target performances (the average value of absorbed energy in the Charpy impact test of 150 J or more and the total elongation in the tensile test of 30 % or more). On the other hand, Comparative examples, which are outside the scope of the present disclosure, do not satisfy at least one of the above-mentioned desired performances of absorbed energy and total elongation.

Claims

A steel comprising:
a chemical composition containing, in 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;

Ti: 0.005 % or less; and

Nb: 0.005 % or less, with the balance being Fe and inevitable impurities;

a microstructure having austenite as a base phase and an average grain size of 80 µm or more;

an absorbed energy of a Charpy impact test at -269 °C of 150 J or more; and

a total elongation of a tensile test at -269 °C of 30 % or more.
The steel according to claim 1, wherein
the chemical composition further contains, in mass%, at least one selected from the group consisting of:
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;

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

REM: 0.0010 % or more and 0.0200 % or less.
A method of manufacturing a steel, comprising:
heating a steel material having the chemical composition according to claim 1 or 2 to a temperature range of 1100 °C to 1300 °C;

performing hot rolling; and

performing heat treatment in which reheating is executed to a temperature range of 1100 °C to 1300 °C and a product of a reheating temperature and a reheating time is 100 °C·h or more.