KR20200088469A - High Mn steel and its manufacturing method - Google Patents

Abstract

It provides a high Mn steel with high strength and excellent low-temperature toughness, as well as excellent low-temperature CTOD properties. In mass%, C: 0.10% or more and 0.70% or less, Si: 0.05% or more and 0.50% or less, Mn: 20% or more and 30% or less, P: 0.030% or less, S: 0.0070% or less, Al: 0.01% or more and 0.07% Or less, Cr: 0.5% or more and 7.0% or less, Ni: 0.01% or more and less than 0.1%, Ca: 0.0005% or more and 0.0050% or less, N: 0.0050% or more and 0.0500% or less, O: 0.0050% or less, Ti: less than 0.0050% and Nb: contains less than 0.0050%, the balance has a component composition of Fe and inevitable impurities, and a structure in which austenite is known. The austenite has a particle size of 1 µm or more and a standard deviation of 9 µm or less. do.

Description

High Mn steel and its manufacturing method

The present invention relates to a high Mn steel and a method for manufacturing the same, which are preferable for providing structures used in cryogenic environments, such as tanks for liquefied gas storage, for example.

Since the liquefied gas storage structure has an extremely low temperature environment, the steel sheet used for this type of structure is required to have high strength and excellent toughness at low temperatures. For example, when a hot rolled steel sheet is used for the storage of liquefied natural gas, it is necessary to ensure excellent toughness at a boiling point of liquefied natural gas: -164°C or less. If the low-temperature toughness of the steel material falls, there is a possibility that the safety as a cryogenic low-rise structure may not be maintained, and thus there is a strong demand for improvement of the low-temperature toughness for the applied steel material.

In response to this demand, austenitic stainless steel, 9% Ni steel, or 5000-based aluminum alloy, which has austenite which does not show brittleness at cryogenic temperatures as the main structure of a steel sheet, has been conventionally used. However, from the viewpoint of high alloying cost and manufacturing cost, there is a demand for steel materials that are inexpensive and have excellent low-temperature toughness.

Therefore, as a new steel material that replaces the conventional cryogenic steel, the use of high Mn steel in which a large amount of relatively inexpensive austenite stabilizing element Mn is added as a structural steel in a cryogenic environment is used in Patent Document 1 or Patent Document 2 Is proposed.

That is, Patent Document 1 proposes to control the carbide coverage of austenite grain boundaries. In addition, Patent Document 2 proposes to control the austenite grain size by adding a carbide coating and Mg, Ca, and REM.

Japanese Patent Publication No. 2016-84529 Japanese Patent Application Publication No. 2016-196703

By the way, in applications such as tanks for liquefied gas storage, it is required from the viewpoint of securing the safety of the tank to be excellent in fracture resistance under severe fracture conditions in which the initial crack is sharper, specifically CTOD in the low temperature region. have. In the above-mentioned Patent Documents 1 and 2, the low-temperature toughness by the Charpy impact test was evaluated, but it did not lead to the guarantee of excellent CTOD properties.

An object of the present invention is to provide a high Mn steel having high strength and excellent low-temperature toughness as well as excellent low-temperature CTOD properties. Here, the "high strength" means that the yield strength is 400 MPa or more, and the "excellent low temperature toughness" means that the absorption energy vE-196 of the Charpy impact test at -196°C is 100 J or more, and the "low temperature" "CTOD characteristics are excellent" means that the CTOD value at -165°C is 0.25 mm or more.

The inventors conducted high-level studies on the methods for solving the above problems, targeting high Mn steels, and have reached the following findings of a to b.

a. High Mn steel does not undergo brittle fracture even at extremely low temperatures, and when fracture occurs, it is generated from grain boundaries. From this point, in order to improve the fracture resistance of a high Mn steel, it is effective to regulate the diameter of the grain by pursuing an area reduction of the grain boundary that is the starting point of fracture.

b. In addition, it is more effective to improve the fracture resistance of the high Mn steel by purifying crystallization in accordance with the regulation of the grain size.

c. As a means of achieving the above a and b, it is appropriate to perform hot rolling and cooling under appropriate manufacturing conditions.

The present invention has been made by further examining the above findings, and the gist is as follows.

1. In mass%,

C: 0.10% or more and 0.70% or less,

Si: 0.05% or more and 0.50% or less,

Mn: 20% or more and 30% or less,

P: 0.030% or less,

S: 0.0070% or less,

Al: 0.01% or more and 0.07% or less,

Cr: 0.5% or more and 7.0% or less,

Ni: 0.01% or more and less than 0.1%,

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

N: 0.0050% or more and 0.0500% or less,

O: 0.0050% or less,

Ti: less than 0.0050% and

Nb: less than 0.0050%

And the remainder has a component composition of Fe and unavoidable impurities, and a structure in which austenite is known, and the austenite has high particle size of 1 µm or more and a standard deviation of 9 µm or less.

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

Cu: 1.0% or less,

Mo: 2.0% or less,

V: 2.0% or less,

W: 2.0% or less,

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

REM: 0.0010% or more and 0.0200% or less

The high Mn steel according to 1 above, which contains one or two or more selected from the above.

3. After heating the steel material having the component composition described in 1 or 2 to a temperature range of 1100°C or more and 1300°C or less, the finish rolling end temperature is 750°C or more and less than 950°C, and the average rolling reduction per pass is 9%. High Mn steel which performs the above-mentioned hot rolling, and then performs a cooling process in which the average cooling rate from a temperature above (finish rolling end temperature-100°C) to a temperature range of 300°C to 650°C is 1.0°C/s or more. Manufacturing method.

According to the present invention, it is possible to provide a high Mn steel having excellent CTOD characteristics and low-temperature toughness, particularly in a cryogenic region. Therefore, by using the high Mn steel of the present invention, it is possible to realize safety and lifespan improvement of a steel structure used in a cryogenic environment, such as a tank for liquefied gas storage, and exhibits special effects in industry.

Hereinafter, the high Mn steel of the present invention will be described in detail.

[Ingredient composition]

First, the composition of the high Mn steel of the present invention and the reason for its limitation will be described. In addition, "%" display in a component composition shall mean "mass %", unless otherwise specified.

C: 0.10% or more and 0.70% or less

C is an inexpensive austenite stabilizing element and is an important element for obtaining austenite. In order to obtain the effect, it is necessary to contain C in an amount of 0.10% or more. On the other hand, when it exceeds 0.70%, Cr carbide is excessively generated, and low-temperature toughness is lowered. Therefore, the C content is 0.10% or more and 0.70% or less, preferably 0.20% or more and 0.60% or less.

Si: 0.05% or more and 0.50% or less

Si acts as a deoxidizing material, and not only is required for the steelmaking phase, but also has the effect of being solidified in the steel to strengthen the steel sheet by strengthening the solid solution. In order to obtain such an effect, it is necessary to contain Si in an amount of 0.05% or more. On the other hand, when it contains more than 0.50%, the weldability deteriorates and the low-temperature toughness, particularly the low-temperature toughness, becomes low. Therefore, the Si content is 0.05% or more and 0.50% or less, preferably 0.07% or more and 0.50% or less.

Mn: 20% or more and 30% or less

Mn is a relatively inexpensive austenite stabilizing element. Mn is an important element in the present invention in order to achieve both strength and cryogenic toughness. In order to obtain the effect, it is necessary to contain Mn in 20% or more. On the other hand, even if it contains more than 30%, the effect of improving low-temperature toughness is saturated, leading to an increase in alloy cost. Moreover, weldability and cutability deteriorate. In addition, segregation is promoted and stress corrosion cracking is promoted. Therefore, the Mn content is 20% or more and 30% or less, preferably 23% or more and 28% or less.

P: 0.030% or less

When P is contained in excess of 0.030%, segregation occurs at the grain boundary, and is a starting point for stress corrosion cracking. For this reason, it is preferable to set 0.030% as an upper limit and reduce it as much as possible. Therefore, P is made 0.030% or less. Moreover, since excessive P reduction raises a refining cost and becomes economically disadvantageous, it is preferable to set it as 0.002% or more. Preferably, it is 0.005% or more and 0.028% or less, and more preferably 0.024% or less.

S: 0.0070% or less

Since S deteriorates the low-temperature toughness and ductility of the base material, it is preferable to set 0.0070% as the upper limit and reduce it as much as possible. Therefore, S is made 0.0070% or less. Moreover, since excessive reduction of S raises the refining cost and becomes economically disadvantageous, it is preferable to set it as 0.001% or more. Preferably, it is 0.0020% or more and 0.0060% or less.

Al: 0.01% or more and 0.07% or less

Al acts as a deoxidizer and is most widely used in the steel sheet deoxidation process. In order to obtain such an effect, it is necessary to contain Al in an amount of 0.01% or more. On the other hand, if the content exceeds 0.07%, it is mixed with the weld metal portion during welding and deteriorates the toughness of the weld metal, so it is set to 0.07% or less. Therefore, Al is 0.01% or more and 0.07% or less, preferably 0.02% or more and 0.06% or less.

Cr: 0.5% or more and 7.0% or less

Cr is an element effective for stabilizing austenite by adding an appropriate amount and improving low-temperature toughness and base material strength. In order to obtain such an effect, it is necessary to contain Cr in 0.5% or more. On the other hand, when it contains more than 7.0%, low-temperature toughness and stress corrosion cracking resistance are lowered by the formation of Cr carbide. For this reason, Cr is made 0.5% or more and 7.0% or less. Preferably, it is 1.0% or more and 6.7% or less, more preferably 1.2% or more and 6.5% or less. Moreover, in order to further improve stress corrosion cracking, 2.0% or more and 6.0% or less is more preferable.

Ni: 0.01% or more and less than 0.1%

Ni has an effect of improving low-temperature toughness, but it is an important aspect in the component design of the present invention to minimize the need in terms of alloy cost. From this point of view, the amount of Ni is 0.01% or more and less than 0.1%. Here, there are stainless steels such as SUS304 and SUS316 as austenite steels having excellent low-temperature toughness, but these steels are alloy designs for obtaining an austenite structure, whereby Ni or Cr equivalents are optimized, and a large amount of Ni Is being added. For these steels, the present invention is an austenite material that is made inexpensive by making Ni the minimum required. In addition, the necessary minimum of Ni was realized by optimizing the amount of Mn added. The preferable amount of Ni is 0.03% or more and 0.07% or less.

Ca: 0.0005% or more and 0.0050% or less

Ca improves the ductility, toughness and sulfide stress corrosion cracking property by controlling the shape of the inclusions described below, and suppresses the decrease in hot ductility and effectively acts to reduce the occurrence of cracks in the cast. In order to obtain such an effect, Ca is required 0.0005% or more. On the other hand, if it is added in excess of 0.0050%, ductility, toughness, and sulfide stress corrosion cracking may decrease, and the effect of suppressing the decrease in hot ductility is also saturated. For this reason, the amount of Ca is made 0.0005% or more and 0.0050% or less. Preferably, it is 0.0010% or more and 0.0045% or less.

N: 0.0050% or more and 0.0500% or less

N is an austenite stabilizing element and is an element effective for improving low-temperature toughness. In order to obtain such an effect, it is necessary to contain N in an amount of 0.0050% or more. On the other hand, when it contains more than 0.0500%, nitride or carbonitride becomes coarse, and toughness falls. Therefore, N is 0.0050% or more and 0.0500% or less, preferably 0.0060% or more and 0.0400% or less.

O: 0.0050% or less

O deteriorates low-temperature toughness by the formation of an oxide. For this reason, O is made into 0.0050% or less of range. Preferably, it is 0.0045% or less. Moreover, since the excessive reduction of O raises a refining cost and becomes economically disadvantageous, it is preferable to set it as 0.0003% or more.

The content of Ti and Nb is suppressed to less than 0.005% each.

Ti and Nb form carbonitrides of high melting point in steel to suppress coarsening of crystal grains, and as a result, serve as a starting point for fracture or a path for crack propagation. Particularly, in high Mn steel, it is necessary to suppress it intentionally because it increases the low-temperature toughness and hinders the control of the structure for improving ductility. That is, Ti and Nb are components that are inevitably incorporated from raw materials and the like, and it is customary to mix in the range of Ti: 0.005 to 0.010% and Nb: 0.005 to 0.010%. Therefore, it is necessary to avoid the inevitable incorporation of Ti and Nb according to the method described later, and to suppress the contents of Ti and Nb to less than 0.005%, respectively. By suppressing the content of Ti and Nb to less than 0.005%, respectively, the above-mentioned adverse effects of carbonitride can be excluded, and excellent low-temperature toughness and ductility can be secured. Preferably, the content of Ti and Nb is made 0.003% or less.

The remainder other than the above essential ingredients are iron and unavoidable impurities. Examples of the inevitable impurities include H and the like, and a total of 0.01% or less is acceptable.

In the present invention, for the purpose of further improving the strength and low-temperature toughness, in addition to the above essential components, the following elements may be included as necessary.

Cu: 1.0% or less, Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, Mg: 0.0005 to 0.0050%, REM: 0.0010 to 0.0200% of one or more types

Cu: 1.0% or less, Mo, V, W: 2.0% or less, respectively

Cu, Mo, V and W contribute to the stabilization of austenite and to the improvement of the base material strength. In order to obtain such an effect, it is preferable to contain Cu, Mo, V and W at 0.001% or more. On the other hand, when Cu contains more than 1.0%, Mo, V, and W respectively exceed 2.0%, coarse carbonitrides are generated, and there is a case where it may be a starting point for destruction, and pressure on manufacturing costs. For this reason, when containing these alloying elements, the content of Cu is 1.0% or less, and Mo, V, and W are 2.0% or less. Preferably, it is 0.003% or more. Moreover, about Mo, V, and W, Preferably it is 1.7% or less, More preferably, it is 1.5% or less.

Mg: 0.0005 to 0.0050%, REM: 0.0010 to 0.0200%

Mg and REM are useful elements for controlling the shape of inclusions and can be contained as necessary. The shape control of inclusions refers to the use of a sulfide-based inclusion formed as a whole body as a granular inclusion. Through the shape control of this inclusion, ductility, toughness and sulfide stress corrosion cracking properties are improved. In order to obtain such an effect, it is preferable to contain Ca and Mg in an amount of 0.0005% or more and REM in an amount of 0.0010% or more. On the other hand, when a large amount of any of the elements is included, the amount of non-metal inclusions increases, and ductility, toughness, and sulfide stress corrosion cracking may decrease. Moreover, it may become disadvantageous economically. For this reason, when it contains Mg, it is 0.0005 to 0.0050%, and when it contains REM, it is 0.0010% to 0.0200%. Preferably, the amount of Mg is 0.0010% or more and 0.0040% or less, and the amount of REM is 0.0020% or more and 0.0150% or less.

[group]

Microstructure based on austenite

When the crystal structure of the steel material is a body-centered cubic structure (bcc), the steel material is not suitable for use in a low-temperature environment because it is likely to cause brittle fracture in a low-temperature environment. Here, when the use in a low-temperature environment is assumed, it is essential that the matrix phase of the steel material is an austenitic structure having a face-centered cubic structure (fcc). Here, "make austenite a matrix" means that the austenite phase is 90% or more in an area ratio. The remainder other than the austenite phase is a ferrite phase or martensite phase, although it goes without saying that the austenite phase may be 100%.

Austenitic particle size: 1 μm or more

Since high Mn steel has austenite-based structure, brittle fracture does not occur even at an extremely low temperature, and when fracture occurs, it occurs from grain boundaries. Reducing the area of the grain boundaries serving as the starting point for this fracture is advantageous for improving the fracture resistance of high Mn steel. For this purpose, it is important that the austenite particle size is 1 µm or more. This is because when the particle diameter is less than 1 µm, the amount of increase in the grain boundary area increases and the point of occurrence of destruction increases. Preferably, it is 2 µm or more.

Standard deviation of austenite is 9 μm or less

It is effective for further improvement of the fracture resistance property of a high Mn steel in order to achieve crystallization in addition to the regulation of the grain size. That is, when it becomes a mixed structure, it becomes a wide grain size distribution from coarse grains to fine grains, and includes grains of less than 1 µm. Especially, when the standard deviation exceeds 9 µm, the tendency becomes remarkable. Mixed tissues with a deviation of more than 9 μm need to be avoided.

[Manufacturing method]

In producing the high Mn steel according to the present invention, first, the steel material can be melted by a known solvent method, such as a converter or an electric furnace, for molten steel having the above-described component composition. Moreover, you may perform secondary refinement in a vacuum degassing furnace. At that time, in order to limit Ti and Nb, which interfere with desirable tissue control, to the above-described range, it is necessary to avoid inevitably mixing from raw materials or the like and take measures to reduce these contents. For example, by lowering the basicity of the slag in the refining step, these alloys are concentrated and discharged into slag to reduce the concentration of Ti and Nb in the final slab product. In addition, a method such as blowing oxygen to oxidize and floating and separating the alloy of Ti and Nb at reflux may be used. Thereafter, it is preferable to use a known casting method such as a continuous casting method or an ingot method to form a steel material such as a slab having a predetermined size. Further, the slab after continuous casting may be subjected to crushing rolling to form a steel material.

In addition, manufacturing conditions for making the steel material into a steel material having excellent low-temperature toughness are defined.

Steel material heating temperature: 1100 ℃ to 1300 ℃

In order to make the grain size of the microstructure of the steel material coarse, the heating temperature before hot rolling is set to 1100°C or higher. However, if it exceeds 1300°C, some melting may start, so the upper limit of the heating temperature is 1300°C. The temperature control here is based on the surface temperature of the steel material.

Finish rolling finish temperature: 750 ℃ to less than 950 ℃

After the steel material (steel ingot or steel piece) is heated, hot rolling is performed. In order to make coarse grains, it is desirable to increase the cumulative rolling reduction at high temperatures. That is, when hot rolling is performed at a low temperature, the microstructure becomes fine, and excessive processing strain enters, resulting in a decrease in low-temperature toughness. Therefore, the lower limit of the finish rolling end temperature is 750°C. On the other hand, when finishing at a temperature range of 950°C or higher, the grain size becomes excessively coarse, so that the desired yield strength cannot be obtained. Therefore, final finishing rolling of 1 pass or more is required at less than 950 degreeC. Preferably, it is 900 degrees C or less.

Average rolling reduction in one pass: 9% or more

In the above hot rolling, in order to achieve a grain size of austenite and to control the grain size to 1 µm or more, it is effective to promote recrystallization of austenite, and the average rolling reduction per pass during hot rolling is It is important to make it 9% or more. It is preferably 11% or more.

(Finishing rolling end temperature-100 °C) Average cooling rate from a temperature above 300 °C to 650 °C below: 1.0 °C/s or higher

After the hot rolling is finished, cooling is performed quickly. When the steel sheet after hot rolling is gently cooled, the formation of precipitates is promoted, resulting in deterioration of low-temperature toughness. The production of these precipitates can be suppressed by cooling at a cooling rate of 1.0° C./s or more. Moreover, when excessive cooling is performed, the steel sheet is deformed and productivity is reduced. Therefore, the upper limit of the cooling start temperature is 900°C. For the above reasons, the cooling after hot rolling sets the average cooling rate of the surface of the steel sheet from a temperature of (finish rolling end temperature-100°C) or higher to a temperature range of 300°C or higher and 650°C or lower at 1.0°C/s or higher. On the other hand, from the viewpoint of industrial production, it is preferable to set the average cooling rate to 200°C/s or less.

Example

Hereinafter, the present invention will be described in detail by examples. In addition, the present invention is not limited to the following examples.

A steel slab having a component composition shown in Table 1 was produced by a converter-ladle refining-continuous casting method. Subsequently, the obtained steel slab was made into a steel plate having a thickness of 10 to 30 mm by fracture rolling and hot rolling under the conditions shown in Table 2. The obtained steel sheet was evaluated for tensile properties, toughness, and texture in the following manner.

(1) Tensile test properties

A tensile test piece of JIS No. 5 was taken from each obtained steel sheet, and a tensile test was conducted in accordance with the provisions of JIS Z 2241 (1998) to investigate tensile test characteristics. In the present invention, the yield strength of 400 MPa or more and the tensile strength of 800 MPa or more were judged to be excellent in tensile properties. Moreover, it was judged that elongation 40% or more was excellent in ductility.

(2) Low temperature toughness

Positions from the surface of each steel sheet exceeding the sheet thickness of 20 mm to 1/4 of the sheet thickness (hereinafter referred to as the 1/4 sheet thickness position), or 1/2 of the sheet thickness of each steel sheet having a sheet thickness of 20 mm or less. Charpy V notch test specimens were collected in accordance with the provisions of JIS Z 2202 (1998) from the direction parallel to the rolling direction of the position up to (hereinafter referred to as 1/2 the plate thickness), and JIS Z 2242 (1998) ), the three Charpy impact tests were performed on each steel sheet, the absorption energy at -196°C was determined, and the base material toughness was evaluated. In the present invention, the average value of the three absorbed energy (vE-196) of 100 J or more was considered to be excellent in base material toughness.

(3) Evaluation of CTOD value

CTOD test pieces were taken from the direction parallel to the rolling direction at the position of 1/2 of the sheet thickness of the steel sheet, and 2-3 samples were tested at -165°C to evaluate the average value. In the present invention, a CTOD value of 0.25 mm or more was considered to have excellent fracture resistance.

(4) Organizational evaluation

With respect to the L cross section at a position of 1/4 of the sheet thickness of the steel sheet, an EBSD (Electron Backscatter Diffraction) analysis was performed to observe a 200 to 200 μm field of view in any of 2-3 views, and the minimum value of the austenite grain size in each field of view was determined. It was measured. In addition, the standard deviation of the austenite particle diameter was evaluated from the distribution of the area ratio of each grain size using the above EBSP analysis results. Using all the grain sizes obtained above as a population, the variance which is the sum of squares of the difference between each individual value and the average value was obtained, and the square root of the variance was taken to obtain a standard deviation.

Table 3 shows the evaluation results obtained by the above.

The high Mn steel according to the present invention has the above-described target performance (the yield strength of the base material is 400 MPa or more, the low-temperature toughness is 100 J or more as the average value of the absorbed energy (vE-196), and 0.25 mm or more as the average value of the CTOD value) It was confirmed to satisfy. On the other hand, in the comparative example outside the scope of the present invention, any one or more of yield strength, low temperature toughness, and CTOD value did not satisfy the above-described target performance.

Claims (3)

In mass%,
C: 0.10% or more and 0.70% or less,
Si: 0.05% or more and 0.50% or less,
Mn: 20% or more and 30% or less,
P: 0.030% or less,
S: 0.0070% or less,
Al: 0.01% or more and 0.07% or less,
Cr: 0.5% or more and 7.0% or less,
Ni: 0.01% or more and less than 0.1%,
Ca: 0.0005% or more and 0.0050% or less,
N: 0.0050% or more and 0.0500% or less,
O: 0.0050% or less,
Ti: less than 0.0050% and
Nb: less than 0.0050%
High residual Mn steel having a component composition of Fe and unavoidable impurities, and a structure in which austenite is known, wherein the austenite has a particle diameter of 1 µm or more and a standard deviation of 9 µm or less. According to claim 1,
The component composition is, in addition to mass%,
Cu: 1.0% or less,
Mo: 2.0% or less,
V: 2.0% or less,
W: 2.0% or less,
Mg: 0.0005% or more and 0.0050% or less and
REM: 0.0010% or more and 0.0200% or less
High Mn steel containing one or two or more selected from. After heating the steel material having the component composition according to claim 1 or 2 to a temperature range of 1100°C or more and 1300°C or less, the finish rolling end temperature is 750°C or more and less than 950°C, and the average rolling reduction per pass is 9 High Mn after performing hot rolling of% or more, and then performing an average cooling rate of 1.0°C/s or more from a temperature above (finish rolling end temperature-100°C) to a temperature range of 300°C or more and 650°C or less Method of manufacturing steel.