JP6856129B2 - Manufacturing method of high Mn steel - Google Patents

Description

The present invention relates to high Mn steel, which is suitable for structural steel used in an extremely low temperature environment, such as a tank for a liquefied gas storage tank, and which is particularly excellent in toughness at low temperature, and a method for producing the same.

In order to use a hot-rolled steel sheet for a structure for a liquefied gas storage tank, the operating environment is extremely low, so that the steel sheet is required to have high strength and excellent toughness at low temperature. For example, when a hot-rolled steel sheet is used in a storage tank for liquefied natural gas, it is necessary that excellent toughness is ensured at a boiling point of liquefied natural gas: -164 ° C. or lower. If the low temperature toughness of the steel material is inferior, it may not be possible to maintain the safety of the structure for the cryogenic storage tank. Therefore, there is a strong demand for improving the low temperature toughness of the applied steel material. Hereinafter, it is generically referred to as a low temperature including an extremely low temperature range of -164 ° C.

In response to this requirement, austenitic stainless steels, 9% Ni steels, or 5000-based aluminum alloys having austenitic stainless steel having a structure of steel sheets that do not show brittleness at low temperatures have been conventionally used. However, since the alloy cost and the manufacturing cost are high, there is a demand for a steel material that is inexpensive and has excellent low temperature toughness.

Therefore, as a new steel material to replace the conventional low-temperature steel, it is possible to use a high-Mn steel to which a large amount of Mn, which is a relatively inexpensive austenite stabilizing element, is added as a structural steel in a low-temperature environment, for example, Patent Document 1. Has been proposed to.

Patent Document 1 proposes a technique for controlling the Mn segregation ratio to prevent carbides generated at grain boundaries from becoming the starting point of fracture.

Japanese Unexamined Patent Publication No. 2017-7817

The technique described in Patent Document 1 makes it possible to provide a high Mn steel having excellent low temperature toughness. However, the high Mn steel described here must contain Ni from the viewpoint of ensuring toughness, and the material cost. Was required to be reduced. Further, in order to reduce the Mn segregation ratio, it is necessary to perform diffusion heat treatment in which the product of the heating temperature (° C.) and the heating time (hr) is 30,000 ° C. hr or more, so that the manufacturing cost is high. ..

Therefore, an object of the present invention is to provide a high Mn steel having excellent low temperature toughness, which can suppress the material and the cost required for manufacturing. Furthermore, it is an object of the present invention to propose an advantageous method for producing such high Mn steels. Here, the above-mentioned "excellent in low temperature toughness" means that the absorbed energy vE _-196 of the Charpy impact test at -196 ° C. is 100 J or more.

In order to achieve the above problems, the inventors conducted intensive studies on various factors that determine the composition and structure of steel sheets for high Mn steels, and obtained the following findings a to d.
a. In the high Mn austenitic steel, since the diffusion of Mn is slow, the Mn segregated portion having a low Mn concentration generated during continuous casting is present even after hot rolling. When the Mn concentration of the Mn segregated portion is less than 16%, work-induced martensite is generated at a low temperature, which causes deterioration of low temperature toughness. From this, it is effective to increase the Mn concentration of the Mn segregated portion in order to improve the low temperature toughness of the high Mn steel.

b. In the high Mn austenitic steel, since the diffusion of Mn is slow, the Mn segregated portion having a high Mn concentration generated during continuous casting exists even after hot rolling. If the Mn segregated portion exceeds 38%, the grain boundary fracture is caused, which also causes deterioration of low temperature toughness. Therefore, in order to improve the low temperature toughness of high Mn steel, it is effective to reduce the Mn concentration in the Mn segregated portion.

c. If hot rolling is performed under appropriate conditions, the above a or b can be realized without performing diffusion heat treatment, and the manufacturing cost can be suppressed.

d. Giving a high dislocation density by hot rolling under appropriate conditions is effective in increasing the yield strength.

The present invention has been made by further studying the above findings, and the gist thereof is as follows.
1. 1. By mass%
C: 0.100% or more and 0.700% or less,
Si: 0.05% or more and 1.00% or less,
Mn: 20.0% or more and 35.0% 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,
N: 0.0050% or more and 0.0500% or less,
O: 0.0050% or less,
It contains Ti: 0.0050% or less and Nb: 0.0050% or less, and the balance has a component composition of Fe and unavoidable impurities and a microstructure having austenite as a matrix phase, and the Mn segregated portion in the microstructure. The Mn concentration is 16% or more and 38% or less, the average KAM (Kernel Average Misorientation) value is 0.3 or more, the absorption energy of the Charpy impact test at -196 ° C is 100 J or more, and the yield strength is 400 MPa or more. Mn steel.

The KAM value is the average value of the orientation difference between each pixel (0.3 μm pitch) in the crystal grain and the adjacent pixel. From the results of EBSD (Electron Backscatter Diffraction) analysis in a field of view of 500 μm × 200 μm for the steel sheet after hot rolling over any two fields of view, the average value of all the measured regions was taken as the average KAM value.

2. The composition of the components is further increased by mass%.
Mo: 2.0% or less,
V: 2.0% or less,
W: 2.0% or less,
Ca: 0.0005% or more and 0.0050% or less,
The high Mn steel according to 1 above, which contains one or more selected from Mg: 0.0005% or more and 0.0050% or less and REM: 0.0010% or more and 0.0200% or less.

3. 3. A steel material having the component composition described in 1 or 2 above is heated to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, and hot rolling is performed with a rolling end temperature of 800 ° C. or higher and a total rolling reduction of 20% or higher. Method for manufacturing Mn steel.

4. In 3 above, hot rolling with a finish rolling end temperature of 700 ° C. or higher and lower than 950 ° C. is further performed, and then 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. A method for producing high Mn steel, which is subjected to a cooling treatment having an average cooling rate of 1.0 ° C./s or more.
Here, each of the above temperature ranges is the surface temperature of the steel material or the steel plate, respectively.

According to the present invention, it is possible to provide a high Mn steel having excellent low temperature toughness. Therefore, the high Mn steel of the present invention greatly contributes to the improvement of safety and life of steel structures used in a low temperature environment such as a tank for a liquefied gas storage tank, and exerts a remarkable industrial effect. Further, since the production method of the present invention does not cause a decrease in productivity and an increase in production cost, it is possible to provide a method having excellent economic efficiency.

It is a graph which shows the relationship between the Mn concentration of the Mn segregation part and the Charpy absorption energy (vE _-196). It is a graph which shows the relationship between the Mn concentration of the Mn segregation part and the Charpy absorption energy (vE _-196).

Hereinafter, the high Mn steel of the present invention will be described in detail.
[Ingredient composition]
First, the component composition of the high Mn steel of the present invention and the reason for its limitation will be described. The "%" indication in the component composition shall mean "mass%" unless otherwise specified.
C: 0.100% or more and 0.700% or less C is an inexpensive austenite stabilizing element and is an important element for obtaining austenite. In order to obtain the effect, C needs to be contained in an amount of 0.100% or more. On the other hand, if it is contained in excess of 0.700%, Cr carbides are excessively generated and the low temperature toughness is lowered. Therefore, C is set to 0.100% or more and 0.700% or less. Preferably, it is 0.200% or more and 0.600% or less.

Si: 0.05% or more and 1.00% or less Si acts as a deoxidizing material and is not only necessary for steelmaking, but also has the effect of dissolving in steel and increasing the strength of the steel sheet by solid solution strengthening. .. In order to obtain such an effect, Si needs to be contained in an amount of 0.05% or more. On the other hand, if it is contained in excess of 1.00%, the weldability deteriorates. Therefore, Si is set to 0.05% or more and 1.00% or less. Preferably, it is 0.07% or more and 0.50% or less.

Mn: 20.0% or more and 35.0% or less Mn is a relatively inexpensive austenite stabilizing element. In the present invention, it is an important element for achieving both strength and low temperature toughness. In order to obtain the effect, Mn needs to be contained in an amount of 20.0% or more. On the other hand, if it is contained in excess of 35.0%, the low temperature toughness deteriorates. In addition, weldability and cutability deteriorate. Furthermore, it promotes segregation and promotes the occurrence of stress corrosion cracking. Therefore, Mn is set to 20.0% or more and 35.0% or less. Preferably, it is 23.0% or more and 30.0% or less. More preferably, it is 28.0% or less.

P: 0.030% or less If P is contained in excess of 0.030%, it segregates at the grain boundaries and becomes the starting point for stress corrosion cracking. Therefore, it is desirable to limit the amount to 0.030% as much as possible. Therefore, P is 0.030% or less. It should be noted that excessive P reduction increases the refining cost and is economically disadvantageous, so it is desirable to set it to 0.002% or more. It is preferably 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 desirable to limit it to 0.0070% and reduce it as much as possible. Therefore, S is set to 0.0070% or less. It should be noted that excessive reduction of S increases the refining cost and is economically disadvantageous, so it is desirable to set it to 0.001% or more. It is preferably 0.0020% or more and 0.0060% or less.

Al: 0.01% or more and 0.07% or less Al acts as a deoxidizing agent and is most commonly used in the molten steel deoxidizing process of steel sheets. In order to obtain such an effect, Al needs to be contained in an amount of 0.01% or more. On the other hand, if it is contained in excess of 0.07%, it is mixed in the weld metal portion during welding and deteriorates the toughness of the weld metal, so the content is set to 0.07% or less. Therefore, Al is set to 0.01% or more and 0.07% or less. It is 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 metal strength. In order to obtain such an effect, Cr needs to be contained in an amount of 0.5% or more. On the other hand, if it is contained in excess of 7.0%, low temperature toughness and stress corrosion cracking resistance are lowered due to the formation of Cr carbides. Therefore, Cr is set to 0.5% or more and 7.0% or less. It is preferably 1.0% or more and 6.7% or less, and more preferably 1.2% or more and 6.5% or less. Further, in order to further improve the stress corrosion cracking resistance, 2.0% or more and 6.0% or less are more preferable.

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, N needs to be contained in an amount of 0.0050% or more. On the other hand, if it is contained in excess of 0.0500%, the nitride or carbonitride becomes coarse and the toughness decreases. Therefore, N is set to 0.0050% or more and 0.0500% or less. It is preferably 0.0060% or more and 0.0400% or less.

O: 0.0050% or less O deteriorates low temperature toughness due to the formation of oxides. Therefore, O is in the range of 0.0050% or less. Preferably, it is 0.0045% or less. It should be noted that excessive reduction of O increases the refining cost and is economically disadvantageous, so it is desirable to set it to 0.0010% or more.

Suppressing the content of Ti and Nb to 0.005% or less, respectively Ti and Nb form high melting point carbonitrides in steel and suppress the coarsening of crystal grains, resulting in the origin of fracture and crack propagation. It becomes a route. In particular, in high Mn steel, it is necessary to intentionally suppress it because it hinders the structure control for increasing the low temperature toughness and improving the ductility. That is, Ti and Nb are components that are inevitably mixed from raw materials and the like, and are mixed in the range of Ti: more than 0.005% and 0.010% or less and Nb: more than 0.005% and 0.010% or less. Is customary. Therefore, it is necessary to avoid unavoidable mixing of Ti and Nb and suppress the content of Ti and Nb to 0.005% or less, respectively, according to the method described later. By suppressing the contents of Ti and Nb to 0.005% or less, the adverse effects of the above-mentioned carbonitride can be eliminated, and excellent low temperature toughness and ductility can be ensured. Preferably, the content of Ti and Nb is 0.003% or less. Of course, the content of Ti and Nb may be 0%.
The rest other than the above components are iron and unavoidable impurities. Examples of the unavoidable impurities here include H and the like, and a total of 0.01% or less is acceptable.

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

As described above, in the high Mn steel containing Mn of 20.0% or more and 35.0% or less, an segregation part having a low Mn concentration and a segregation part having a high Mn concentration are generated as compared with the Mn content in the component composition. .. As shown below, it was found that the portion having a difference in the concentration of Mn becomes a factor for deteriorating the low temperature toughness.
That is, with respect to the steel sheet obtained by hot rolling the steel material having the above-mentioned composition under various conditions, the Mn concentration of the Mn segregated portion and the absorbed energy of the Charpy impact test at -196 ° C. were measured. Here, the Mn segregation portion is a region where the Mn concentration between the Mn segregation bands is low or high, and specifically, EBSD (Electron Backscatter Diffraction) analysis on the polished surface of the rolling direction cross section of the steel sheet after hot rolling. It is represented by the region where the Mn concentration measured by is the lowest or the highest.

The Mn concentration of the Mn segregated portion in the microstructure is 16% or more and 38% or less. First, for the Mn segregated portion having a low Mn concentration, the results of measuring the Mn concentration and the absorbed energy of the Charpy impact test at -196 ° C. are shown in FIG. As described above, it can be seen that when the Mn concentration of the Mn segregated portion is 16% or more, the absorbed energy: 100 J or more is realized. The Mn concentration of the Mn segregated portion is preferably 17% or more.

Further, as shown in FIG. 2, the result of measuring the Mn concentration and the absorbed energy of the Charpy impact test at -196 ° C. for the Mn segregated portion having a high Mn concentration is such that the Mn concentration of the Mn segregated portion is 38% or less. , It can be seen that the absorbed energy: 100 J or more is realized. The Mn concentration of the Mn segregated portion is preferably 37% or less.

The average of KAM (Kernel Average Misorientation) values is 0.3 or more. As described above, the KAM value is obtained by performing EBSD (Electron Backscatter Diffraction) analysis in a field of view of 500 μm × 200 μm for the steel sheet after hot rolling over any two fields of view. From the results, it is a value obtained as the average value of the orientation differences between each pixel (0.3 μm pitch) in the crystal grain and the adjacent pixel. This KAM value reflects the local change in crystal orientation due to dislocations in the structure, and the higher the KAM value, the larger the orientation difference between the measurement point and the adjacent portion. That is, the higher the KAM value, the higher the degree of local deformation in the grain, and therefore, the higher the KAM value in the rolled steel sheet, the higher the dislocation density. When the average of the KAM values is 0.3 or more, a large amount of dislocations are accumulated, so that the yield strength is improved. Preferably, it is 0.5 or more. On the other hand, if the average KAM value exceeds 1.3, the toughness may deteriorate, so it is preferably 1.3 or less.

The above Mn concentration of the Mn segregated portion: 16% or more and 38% or less and the average KAM value: 0.3 or more can be realized by hot rolling according to the conditions described later under the above-mentioned component composition. it can.

In the present invention, in addition to the above essential elements, the following elements can be contained, if necessary, for the purpose of further improving the strength and low temperature toughness.
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, REM: 0.0010% or more and 0.0200% or less One or two types Mo, V, W: 2.0% or less Mo, V and W contribute to the stabilization of austenite and improve the strength of the base metal. Contribute to. In order to obtain such an effect, Mo, V and W are preferably contained in an amount of 0.001% or more. On the other hand, if it is contained in excess of 2.0%, coarse carbonitride is formed, which may be a starting point of fracture and puts pressure on the manufacturing cost. Therefore, when these alloying elements are contained, the content thereof is set to 2.0%. It is preferably 0.003% or more and 1.7% or less, and 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 control the morphology of inclusions. It is a useful element and can be contained as needed. Morphological control of inclusions means that the expanded sulfide-based inclusions are made into granular inclusions. Through morphological control of this inclusion, ductility, toughness and sulfide stress corrosion cracking resistance are improved. In order to obtain such an effect, it is preferable that Ca and Mg are contained in an amount of 0.0005% or more and REM is contained in an amount of 0.0010% or more. On the other hand, if a large amount of any of the elements is contained, the amount of non-metal inclusions may increase, and the ductility, toughness, and sulfide stress corrosion cracking resistance may decrease. It may also be economically disadvantageous.
Therefore, when Ca and Mg are contained, the content is 0.0005% or more and 0.0050% or less, respectively, and when REM is contained, the content is 0.0010% or more and 0.0200% or less. Preferably, the Ca amount is 0.0010% or more and 0.0040% or less, the Mg amount is 0.0010% or more and 0.0040% or less, and the REM amount is 0.0020% or more and 0.0150% or less.

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

Further, the manufacturing conditions for incorporating the above steel material into a steel material having excellent low temperature toughness are specified.
[Steel material heating temperature: 1100 ° C or higher and 1300 ° C or lower]
In order to obtain a high Mn steel having the above-mentioned structure, it is important to heat it in a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, and perform hot rolling with a rolling end temperature of 800 ° C. or higher and a total rolling reduction of 20% or higher. Is. The temperature control here is based on the surface temperature of the steel material.
That is, in order to promote the diffusion of Mn in hot rolling, the heating temperature before rolling is set to 1100 ° C. or higher. On the other hand, if the temperature exceeds 1300 ° C., there is a concern that the steel will start melting, so the upper limit of the heating temperature is set to 1300 ° C. Preferably, it is 1150 ° C. or higher and 1250 ° C. or lower.

[Rolling end temperature is 800 ° C or higher and total rolling reduction is 20% or higher]
Further, it is also important to shorten the distance between the Mn segregated portion and promote the diffusion of Mn by increasing the total rolling reduction ratio at the time of rolling to 20% or more. Similarly, from the viewpoint of promoting the diffusion of Mn during rolling, the rolling end temperature is set to 800 ° C. or higher. This is because below 800 ° C., Mn cannot be sufficiently diffused because it is far below two-thirds of the melting point of Mn. It is preferably 950 ° C. or higher, and more preferably 1000 ° C. or higher and 1050 ° C. or lower. The total reduction rate is preferably 30% or more. The upper limit of the total rolling reduction ratio does not need to be set in particular, but it is preferably 98% from the viewpoint of improving the rolling efficiency.

Further, if necessary, it is advantageous to add a second hot rolling that satisfies the following conditions after the hot rolling described above in order to increase the KAM value. At that time, if the end temperature of the first hot rolling described above is 1100 ° C. or higher, the second hot rolling may be continued as it is, but if it is less than 1100 ° C., reheating of 1100 ° C. or higher is performed. I do. Here, too, the upper limit of the heating temperature is set to 1300 ° C. because there is a concern that the steel will start melting if the temperature exceeds 1300 ° C. The temperature control is based on the surface temperature of the steel material. Preferably, it is 1150 ° C. or higher and 1250 ° C. or lower.

[Finish rolling end temperature: 700 ° C or more and less than 950 ° C]
The second hot rolling requires one or more passes of final finish rolling at 700 ° C. or higher and lower than 950 ° C. That is, by performing rolling of preferably 10% or more at a temperature lower than 950 ° C. for one pass or more, the dislocations introduced in the first rolling are difficult to recover and tend to remain, so that the KAM value can be increased. Further, when finished in a temperature range of 950 ° C. or higher, the crystal grain size becomes excessively coarse and a desired yield strength cannot be obtained. Therefore, it is preferable to perform final finish rolling at a temperature of less than 950 ° C. for one pass or more. The finishing temperature is preferably 900 ° C. or lower, more preferably 850 ° C. or lower.

On the other hand, if the finishing temperature is less than 700 ° C., the toughness deteriorates, so the temperature is set to 700 ° C. or higher. Further, it is preferably 750 ° C. or higher. The reduction rate below 950 ° C. is preferably 20% or more, more preferably 50% or more. However, if the reduction is more than 95%, the toughness deteriorates, so 95% or less is preferable.

[Average cooling rate from (finish rolling end temperature -100 ° C) or higher to a temperature range of 300 ° C or higher and 650 ° C or lower: 1.0 ° C / s or higher]
After the hot rolling is completed, it is cooled immediately. When the steel sheet after hot rolling is gently cooled, the formation of precipitates is promoted and the low temperature toughness deteriorates. The formation of these precipitates can be suppressed by cooling at a cooling rate of 1.0 ° C./s or higher. In addition, excessive cooling distorts the steel sheet, reducing productivity. In particular, steel materials with a plate thickness of less than 10 mm are preferably air-cooled. Therefore, the upper limit of the cooling start temperature is 900 ° C. For the above reasons, the average cooling rate of the steel sheet surface from the temperature of (finish rolling end temperature -100 ° C) or higher to the temperature range of 300 ° C or higher and 650 ° C or lower is 1.0 ° C / s. That's all. Since the range of Mn concentration in the Mn segregated portion is narrowed as it is rolled, no subsequent heat treatment is required.

Hereinafter, the present invention will be described in detail with reference to Examples. The present invention is not limited to the following examples.
A steel slab having the composition shown in Table 1 was prepared by a converter-ladle refining-continuous casting method. Next, the obtained steel slab was subjected to bulk rolling (first hot rolling) and hot rolling (second hot rolling) under the conditions shown in Table 2 to obtain a steel sheet having a thickness of 10 to 30 mm. Tensile properties, toughness and microstructure evaluation of the obtained steel sheet were carried out as follows.

(1) Tensile test characteristics Tensile test pieces of JIS No. 5 were collected from each of the obtained steel sheets, and a tensile test was carried out in accordance with the provisions of JIS Z 2241 (1998) to investigate the tensile test characteristics. In the present invention, it was determined that the yield strength of 400 MPa or more and the tensile strength of 800 MPa or more are excellent in tensile properties. Further, it was determined that the elongation of 40% or more was excellent in ductility.

(2) Low temperature toughness JIS Z 2242 (2005) from the direction parallel to the rolling direction at the 1/4 position of the plate thickness of each steel plate exceeding 20 mm or the 1/2 position of the plate thickness of each steel plate with a plate thickness of 20 mm or less. ), And three Charpy impact tests were conducted on each steel sheet in accordance with JIS Z 2242 (2005) to obtain the absorbed energy at -196 ° C. The toughness of the base metal was evaluated. In the present invention, the average value of the three absorbed energies (vE _-196 ) of 100 J or more is considered to be excellent in the base metal toughness. For each steel plate with a plate thickness of less than 10 mm, a 5 mm sub-sized Charpy V notch test piece is used in accordance with JIS Z 2242 (2005) from a direction parallel to the rolling direction at the plate thickness 1/2 position. The samples were sampled and three Charpy impact tests were performed on each steel sheet at −196 ° C. in accordance with JIS Z 2242 (2005). Here, it is assumed that the average value of the three absorbed energies (vE _-196 ) is 67 J or more, which is excellent in the toughness of the base metal.

Brittle fracture surface ratio After the Charpy impact test at -196 ° C, SEM observation (10 fields of view at 500 times) was performed to measure the brittle fracture surface ratio. A brittle fracture surface ratio of 0% was defined as having excellent low temperature toughness.

(3) Structure evaluation KAM value For the steel sheet after hot rolling, EBSD (Electron Backscatter Diffraction) analysis (measurement step: 0.3 μm) in the field of view of 500 μm × 200 μm on the polished surface of the cross section in the rolling direction can be performed in any two fields (measurement step: 0.3 μm). The average value of all the regions measured over the plate thickness of 1/4 position and the plate thickness of 1/2 position was taken as the average KAM value.

Process-induced martensite After the Charpy impact test, the test piece is driven to the bottom of the notch and polished, and the presence or absence of process-induced martensite is measured by observing 5 fields of 100 μm × 100 μm by EBSD analysis (measurement step: 0.08 μm). did.

Mn concentration Further, the Mn concentration was determined by performing EPMA (Electron Probe Micro Analyzer) analysis at the EBSD measurement position of the KAM value, and the place where the Mn concentration was the lowest and the place where the Mn concentration was the highest were set as the segregation part.
The results obtained as described above are shown in Table 3.

It was confirmed that the high Mn steel according to the present invention satisfies the above-mentioned target performance (yield strength of the base material is 400 MPa or more, low temperature toughness is 100 J or more on average of _{absorbed energy (vE -196)).} On the other hand, in the comparative example outside the scope of the present invention, any one or more of the yield strength and the low temperature toughness does not satisfy the above-mentioned target performance.

Claims (2)

C: 0.100% or more and 0.700% or less,
Si: 0.05% or more and 1.00% or less,
Mn: 20.0% or more and 35.0% 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,
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
A steel material containing Fe and an unavoidable impurity component composition is heated to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, and the rolling end temperature is 800 ° C. or higher and the total rolling reduction ratio is 20% or higher. no line rolling,
Further, hot rolling with a finish rolling end temperature of 700 ° C. or higher and lower than 950 ° C. is performed, and then the average cooling rate from a temperature of (finish rolling end temperature of -100 ° C.) or higher to a temperature range of 300 ° C. or higher and 650 ° C. or lower is obtained. A method for producing high Mn steel , which is cooled at 1.0 ° C./s or higher.
The high Mn steel has a microstructure having austenite as a matrix phase, the Mn concentration of the Mn segregated portion in the microstructure is 16% or more and 38% or less, and the average KAM (Kernel Average Misorientation) value is 0.3. The method for producing a high Mn steel, wherein the absorption energy of the Charpy impact test at -196 ° C. is 100 J or more and the yield strength is 400 MPa or more. The composition of the components is further increased by mass%.
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
The method for producing a high Mn steel according to claim 1, which contains one or more selected from the above.

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