JP2019183203A - HIGH Mn STEEL AND MANUFACTURING METHOD THEREFOR - Google Patents

Abstract

To propose a method for providing further excellent ductility in a high Mn steel excellent in low temperature toughness of a base material and a heat affected zone.SOLUTION: A high Mn steel has a component composition containing C:0.10 to 0.70%, Si:0.05 to 1.0%, Mn:15 to 30%, P:0.030% or less, S:0.0070% or less, Al:0.01 to 0.07%, Cr:2.5 to 7.0%, N:0.0050 to 0.0500% and O:0.0050% or less and the balance Fe with inevitable impurities, a micro structure in which an austenite is as a base phase, and fine crystal zone of 50 area% to 80 area% remains, and Cr carbide amount of the micro structure of 0.04% or less.SELECTED DRAWING: None

Description

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

  When a hot-rolled steel sheet is used for the structure for a liquefied gas storage tank, since the use environment is extremely low temperature, the steel sheet is required to have excellent toughness at extremely low temperature in addition to high strength. For example, when a hot-rolled steel sheet is used for a liquefied natural gas storage tank, 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, the safety as a structure for cryogenic storage tanks may not be maintained. Therefore, there is a strong demand for improving the low-temperature toughness of the applied steel material.

  In response to this requirement, conventionally, austenitic stainless steel, 9% Ni steel, or 5000 series aluminum alloy having an austenite that does not show brittleness at a very low temperature as a steel sheet structure has been used. However, since alloy costs and manufacturing costs are high, there is a demand for a steel material that is inexpensive and excellent in cryogenic toughness.

  Therefore, as a new steel material to replace the conventional cryogenic steel, it is possible to use a high-Mn steel to which a relatively inexpensive austenite stabilizing element Mn is added as a structural steel in a cryogenic environment. It is proposed in Document 1.

  Patent Document 1 proposes a technique for controlling the austenite grain size to an appropriate size to prevent the carbide generated at the grain boundary from becoming a starting point of fracture or a propagation path of cracks. Patent Document 2 proposes a technique for improving low-temperature toughness by suppressing the segregation of Mn to a certain level or more.

JP 2016-196703 JP 2017-71817

  In the use of the structure for a liquefied gas storage tank described above, it is necessary to ensure high ductility in addition to low temperature toughness because the steel material to be used needs to have high workability. Nothing about the ductility is verified by the techniques described in Patent Documents 1 and 2. The high Mn steel material described in Patent Document 1 has a thickness of about 15 to 50 mm. For example, depending on the application, a thickness of less than 15 mm, particularly 10 mm or less is required. When manufacturing such a thin plate, the method of accelerating cooling after the end of hot rolling exemplified in Patent Document 1 tends to cause warpage and distortion in the obtained steel plate, and there is an extra step such as shape correction. It becomes necessary and productivity is hindered. Moreover, in patent document 2, in order to relieve segregation, the heat processing for a long time is needed, and it is disadvantageous in terms of productivity.

Therefore, an object of the present invention is to propose a method for providing further excellent ductility in a high Mn steel excellent in low temperature toughness of a base material and a weld heat affected zone. Furthermore, an object of the present invention is to propose a method capable of manufacturing such a high Mn steel sheet without warping or distortion.
Herein, the "excellent low-temperature toughness" refers to absorbed energy vE _-196 Charpy impact test at -196 ° C. is 30J or more in Charpy test pieces half-size.

In order to solve the above-mentioned problems, the present inventors conducted intensive research on various factors that determine the composition of steel sheets and the production method for high-Mn steel, investigated the relationship with the microstructure, and found the following knowledge. I came to get.
First, high-Mn steel does not undergo brittle fracture even at extremely low temperatures, but occurs from grain boundaries when fracture occurs. That is, it has been found that the shape of the crystal grain boundary greatly affects the toughness. In particular, it has been found that carbides and the like are formed at the grain boundaries, and the formation amount, distribution and grain boundary morphology of the carbides have a great influence on the toughness. It was also found that the morphology of the fine crystal region formed in the structure affects the toughness.

  As a method for suppressing the formation of carbides, it is an effective means to rapidly cool the steel sheet (hereinafter also referred to as rapid cooling). However, in the case of a thin steel sheet having a thickness of 10 mm or less, when the steel sheet is rapidly cooled, warpage due to internal stress due to thermal strain may occur. In particular, in the case of high Mn steel, since the structure is austenite, the plate warp tends to be larger than that of ferritic steel. When this plate warpage occurs, it becomes difficult to insert the plate into a surface smoothing processing line or the like, which is a process after cooling. Moreover, in order to ship, it is necessary to correct the curvature of a steel plate, and a new process must be added to the production line, leading to an increase in production cost. The reason why the warpage of the austenitic steel is large is presumed to be because the thermal conductivity is small and the temperature distribution is large compared to the ferritic steel, but the details are unknown.

  Here, in the actual machine, when the thickness of the steel sheet and the cooling rate in the cooling after hot rolling are changed, the results of summarizing the situation in which the load on the manufacturing process occurs due to the warpage of the steel sheet are shown in Table 1. . As shown in Table 1, it was found that when the cooling rate exceeded 3 ° C./s, there was a problem in the process with a steel sheet having a thickness of 10 mm or less.

  In addition, the load on the manufacturing process, which is the evaluation standard in Table 1, means a load required for care, correction, or the like for intermediate products or products in each process. ◎ in Table 1 does not require the above-mentioned care and correction, etc., and passes and transports the post-manufacturing work to a material storage site such as a flattening device (leveler) and a cooling bed without problems. If it is possible, ○ indicates that a slight smoothing process that adjusts the opening of the leveler for each production opportunity is performed and the plate can be passed. In the case where it is necessary to make an individual judgment by the operator (execution of correction of deformation by manual operation), × indicates that correction is impossible in manufacturing and there is a problem in shipping as a product.

  Thus, when it was difficult to apply quenching treatment effective for carbide suppression, we investigated a technique that can improve toughness by controlling the crystal grain morphology of the parent phase even if carbide is present. . That is, intensive studies were conducted on the influence of the morphology and toughness of the parent phase grains under the condition where carbides exist. As a result, it has been found that by forming a region composed of fine crystal grains, a reduction in the starting point of fracture and a suppression of fracture surface propagation are realized simultaneously, and the toughness value is improved.

  As a result of the analysis of the structure that contributes to the improvement of the toughness value, a region composed of fine crystal grains (hereinafter referred to as a fine crystal region) is partially recovered and regenerated by crystal grains deformed during hot rolling. This is a region in which distortion is introduced by the subsequent rolling that is refined with crystals. About this area | region, since the structure | tissue is refined | miniaturized, it is thought that the balance of strength and toughness is excellent. It has been found that the formation of a structure having a fine crystal region can be made by optimizing the temperature and reduction ratio of hot rolling and the cooling conditions after hot rolling, in addition to adjusting the components. In particular, it has been found that the addition of Cr makes it easy to control the fine crystal region, although the reason is unknown.

  On the other hand, it was confirmed that Cr contributing to control of the fine crystal region was deposited at the starting point of fracture. That is, depending on the final finish rolling conditions, the recrystallization form changes, the matrix structure changes, the Cr diffusion form changes, and the Cr carbide form also changes, and the toughness value fluctuates. Observed that. It came to discover that it is important to suppress the amount of Cr carbide. And it discovered that by controlling recrystallization, the diffusion of Cr was suppressed and the amount of carbide of Cr was reduced. In particular, since the rolling reduction of finish rolling affects the morphology of fine crystal regions and the formation behavior of carbides, it is important to control the rolling reduction of this finishing rolling to suppress carbide formation at grain boundaries as much as possible. .

  Furthermore, the formation of a fine crystal region is very important in controlling the form of carbides formed at the grain boundaries. That is, the finish rolling finish temperature is set to 890 ° C. or more and 980 ° C. or less, and the subsequent cooling is performed at a cooling rate of 3 ° C./s or less, whereby the fine crystal region is formed, and both strength and toughness can be achieved. I found out.

The present invention has been made by further studying the above knowledge, and the gist thereof is as follows.
1. % By mass
C: 0.10 to 0.70%,
Si: 0.05 to 1.0%,
Mn: 15-30%,
P: 0.030% or less,
S: 0.0070% or less,
Al: 0.01 to 0.07%,
Cr: 2.5-7.0%
N: 0.0050 to 0.0500% and O: 0.0050% or less, with the balance having a component composition of Fe and unavoidable impurities, with austenite as the base phase, and an area ratio of 50% or more A high Mn steel having a microstructure in which fine crystal regions of 80% or less remain, and the microstructure has a Cr carbide content of 0.04% or less.

2. The component composition is further mass%,
Mo: 2.0% or less,
V: 2.0% or less,
W: 2.0% or less,
REM: 0.0010-0.0200% and B: 0.0005-0.0020%
2. The high Mn steel according to 1 above, which contains one or more selected from among the above.

3. The steel material having the composition described in 1 or 2 is heated to a temperature range of 1100 ° C. to 1300 ° C., the finish rolling finish temperature is 890 ° C. to 980 ° C. and the final reduction ratio is 13% to 17%. A method for producing high-Mn steel, which is subjected to hot rolling as described below and is subjected to a cooling treatment in which the average cooling rate in the temperature range from the finish rolling finish temperature to 650 ° C. is 3 ° C./s or less.

  ADVANTAGE OF THE INVENTION According to this invention, the high Mn steel excellent in low temperature toughness can be provided. When this high Mn steel is used for welding, both the base material after welding and the weld heat affected zone are excellent in low temperature toughness. Therefore, the high Mn steel of the present invention greatly contributes to the improvement of safety and life of a steel structure used in a cryogenic environment such as a liquefied gas storage tank, and has a remarkable industrial effect. In addition, the production method of the present invention can produce the high-Mn steel having excellent low-temperature toughness without causing a decrease in productivity and an increase in production cost. it can.

It is the organization photograph by SEM.

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 limitation will be described. The “%” in the component composition means “% by mass” unless otherwise specified.
C: 0.10 to 0.70%
C is an inexpensive austenite stabilizing element and an important element for obtaining austenite. In order to acquire the effect, C needs to contain 0.10% or more. On the other hand, if the content exceeds 0.70%, Cr carbide is excessively generated and low temperature toughness is lowered. For this reason, C is made 0.10 to 0.70%. Preferably, the content is 0.20% or more and 0.60% or less.

Si: 0.05-1.0%
Si acts as a deoxidizer and is not only necessary for steelmaking, but also has an effect of increasing the strength of the steel sheet by solid solution and solid solution strengthening. In order to acquire such an effect, Si needs to contain 0.05% or more. On the other hand, when it contains exceeding 1.0%, weldability will deteriorate. For this reason, Si is made into 0.05 to 1.0%. Preferably, it is 0.07% or more and 0.5% or less.

Mn: 15-30%
Mn is a relatively inexpensive austenite stabilizing element. In the present invention, it is an important element for achieving both strength and cryogenic toughness. In order to acquire the effect, Mn needs to contain 15% or more. On the other hand, even if the content exceeds 30%, the effect of improving the cryogenic toughness is saturated, leading to an increase in alloy cost. In addition, the weldability and cutability are deteriorated. Furthermore, segregation is promoted and stress corrosion cracking is promoted. For this reason, Mn is made 15 to 30%. Preferably, it is 18% or more and 28% or less.

P: 0.030% or less When P exceeds 0.030%, it segregates at the grain boundary and becomes the starting point of stress corrosion cracking. For this reason, it is desirable to make 0.030% an upper limit and to reduce as much as possible. Therefore, P is 0.030% or less. Preferably, it is 0.028% or less, more preferably 0.24% or less. Of course, it may be 0%. In order to reduce P to less than 0.002%, a great deal of cost is required for refining, so 0.002% or more is preferable from the viewpoint of economy.

S: 0.0070% or less Since S deteriorates the low-temperature toughness and ductility of the base material, 0.0070% is the upper limit and it is desirable to reduce it as much as possible. Therefore, S is made 0.0070% or less. Preferably it is 0.0050% or less. Of course, it may be 0%. In order to reduce S to less than 0.0005%, a large amount of cost is required for refining, so 0.0005% or more is preferable from the viewpoint of economy.

Al: 0.01 to 0.07%
Al acts as a deoxidizer and is most commonly used in the molten steel deoxidation process of steel sheets. In order to acquire such an effect, Al needs to contain 0.01% or more. On the other hand, if the content exceeds 0.07%, it is mixed in the weld metal part during welding and deteriorates the toughness of the weld metal, so the content is made 0.07% or less. Preferably, it is 0.02% or more and 0.06% or less.

Cr: 2.5-7.0%
Cr is an element that stabilizes austenite by addition of an appropriate amount and is effective in improving cryogenic toughness and base material strength. Further, it is an effective element for forming a fine crystal region described later. In order to acquire such an effect, it is necessary to contain Cr at 2.5% or more. On the other hand, if the content exceeds 7.0%, the low temperature toughness and stress corrosion cracking resistance decrease due to the formation of Cr carbide. For this reason, Cr is 2.5 to 7.0%. Preferably it is 3.5% or more and 6.5% or less.

N: 0.0050 to 0.0500%
N is an austenite stabilizing element and is an element effective for improving cryogenic toughness. In order to acquire such an effect, N needs to contain 0.0050% or more. On the other hand, if the content exceeds 0.0500%, the nitride or carbonitride becomes coarse and the toughness is lowered. For this reason, N is made into 0.0050 to 0.0500%. Preferably it is 0.0060% or more and 0.0400% or less.

O: 0.0050% or less O deteriorates the cryogenic toughness due to the formation of an oxide. For this reason, O is made into the range of 0.0050%. Preferably, it is 0.0045% or less. Of course, it may be 0%. In order to reduce O to less than 0.0005%, a large amount of cost is required for refining, so 0.0005% or more is preferable from the viewpoint of economy.

  The balance other than the above components is iron and inevitable impurities. Inevitable impurities here include Ca, Mg, Ti, Nb, Cu, and the like, and a total of 0.05% or less is acceptable.

In the present invention, for the purpose of further improving the strength and the low temperature toughness, the following elements can be contained as required in addition to the above essential elements.
One or two of Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, REM: 0.0010 to 0.0200%, B: 0.0005 to 0.0020 The above can be added.

Mo, V, W: 2.0% or less Mo, V, and W contribute to the stabilization of austenite and to the improvement of the strength of the base material. In order to acquire such an effect, it is preferable to contain Mo, V, and W at 0.001% or more. On the other hand, if the content exceeds 2.0%, coarse carbonitrides are generated, which may be a starting point of destruction, and also press production costs. For this reason, when it contains these alloy elements, the content shall be 2.0% or less. Preferably it is 0.003% or more and 1.7% or less, more preferably 1.5% or less.

REM: 0.0010 to 0.0200%
REM is an element useful for controlling the form of inclusions, and can be contained as necessary. The inclusion shape control means that the expanded sulfide inclusion is a granular inclusion. Ductility, toughness, and resistance to sulfide stress corrosion cracking are improved through shape control of the inclusions. In order to acquire such an effect, it is preferable to contain REM 0.0010% or more. On the other hand, when it is contained excessively, the amount of non-metallic inclusions increases, and on the contrary, ductility, toughness, and resistance to sulfide stress corrosion cracking may decrease. Therefore, the REM amount is preferably 0.0015% or more and 0.0200% or less.

B: 0.0005 to 0.0020%
B segregates at the grain boundary and contributes to improvement of toughness due to the grain boundary strength of the material. However, the amount added is preferably 0.0005% or more and 0.0020% or less in order to form coarse nitrides or carbides when excessively added.

Next, the structure | tissue form for implement | achieving low temperature toughness is demonstrated.
[Microstructure based on austenite]
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, and thus is not suitable for use in a low temperature environment. Here, when assumed to be used in a low temperature environment, the base phase in the steel structure is austenite whose crystal structure is a face-centered cubic structure (fcc). The phrase “having austenite as a base phase” indicates that the austenite phase is 90% or more in area ratio, and may be 100%. On the other hand, the remainder other than the austenite phase is composed of the ferrite or martensite phase of BCC structure, inclusions and precipitates, and the ratio of these is preferably 5% or less. The austenite fraction can be determined by observation by EBSD, analysis by XRD, magnetic permeability, and the like.

[Microstructure]
The present invention realizes improvement of low temperature toughness by controlling the microstructure, particularly the austenite structure, in the hot rolling and the subsequent cooling process. For this purpose, it is important to control the morphology of the microstructure. In particular, during hot rolling, after hot rolling and in the subsequent cooling process, the crystal grains deformed during hot rolling are partially recovered and refined with recrystallization, and strain is introduced by subsequent rolling. It is important to improve the toughness by reducing the starting point of the fracture surface and suppressing the progress of the fracture surface by appropriately presenting the region, that is, the fine crystal region. Below, the form of each area | region mentioned above is explained in full detail.

[Fine crystal region]
The fine crystal region is a region in which strain is introduced by hot rolling and the crystal grains are refined, and is partly restored or recrystallized by a subsequent cooling process, and thus includes a strain inside. Although the method of recognizing this region will be described later, the structure is controlled so as to include the fine crystal region.

[Area ratio of fine crystal region: 50% to 80%]
It is very important to control the area ratio of the fine crystal region as the strength-toughness balance of the material and the formation site of Cr carbide. This is a region where recovery and recrystallization in the subsequent cooling process are delayed due to the introduction of strain in hot rolling, and it is formed as fine grains having many dislocations inside. As the size of this area, a fraction of 50% or more is required. When the area ratio is low, polygonal grains (polygonal regions) increase to increase the strength of the material, and carbides are formed in the polygonal grains (polygonal regions), thereby reducing the strength-toughness balance. If the area of this region increases, the strength of the material may increase excessively, leading to a reduction in toughness.

  The austenite phase forming the base phase (parent phase) of the steel sheet is defined by the fine crystal region and the polygonal region. Inclusions, precipitates, and martensite phases are formed as other structures, but these are suppressed to 5% or less, and most of the austenite phase needs to be formed as the fine crystal regions and polygonal regions. .

Next, a method for identifying these areas will be described below.
Each region described above can be recognized by optimizing the method for adjusting the SEM observation sample. Specifically, after mirror polishing with colloidal silica, if ion etching is performed on the surface layer by ion milling, fine irregularities are formed on the surface layer of the fine crystal region, so in-lens by low acceleration SEM of 5 kV or less Identification is possible by tissue observation and reflection electron image observation. Further, it is possible to identify the fine crystal region by performing mirror polishing using electrolytic polishing. As described above, the cause of the contrast difference in the base phase (matrix) may be a difference in hardness or strain, a small amount of element distribution, or the like, but the details are unknown. In the analysis, the area recognized in accordance with the above is binarized by image processing and defined as an area ratio.

  Also, EBSD, which is often used for structure evaluation, can identify crystal grains, and it is possible to distinguish between strain remaining areas (fine crystal areas) and polygonal areas using Image Quality-Map. .

  As the form of the fine crystal region, it is composed of a plurality of grains, and the size of each crystal grain is 10 μm or less at the maximum, and they exist in a state of being strained inside. FIG. 1 shows a backscattered electron image photograph of 500 times and 1000 times of the center of the tissue. In addition, regarding the 1000 × photograph, a structure photograph of the same field of view in which the fine crystal region is hatched with a surrounding line is shown at the same time. Thus, the microcrystalline region has various shapes, and can be clearly distinguished from the peripheral fully recovered polygonal region.

  In consideration of the above, the SEM structure observation is performed by appropriately adjusting the magnification for a field of view of about 300 × 500 μm per location at a depth position of 1/4 of the plate thickness from the steel plate surface (hereinafter referred to as 1/4 t portion) ( 200 to 5000 times), the area of the fine crystal region in the same field of view is measured, and the area ratio in this field of view is calculated. This operation is performed in at least 10 places, and the average is calculated as the area ratio of the fine crystal region.

[Cr carbide content: 0.04% or less]
Since the present invention is premised on the addition of Cr, Cr carbide is formed during cooling after hot rolling. This Cr carbide precipitates preferentially at the grain boundary, and this precipitate causes the toughness of the material to be reduced. Therefore, it is necessary to suppress Cr precipitation and to reduce the Cr carbide content to 0.04% or less by mass%. There is. Here, about the precipitation of Cr, the amount of Cr diffusion can be controlled and controlled by adjusting the Cr content, the reduction ratio of finish rolling and the cooling rate after hot rolling. In particular, by suppressing recrystallization, Cr diffusion is suppressed and the amount of Cr carbide is reduced. In addition, since Cr has less adverse effect on toughness when precipitated in the fine crystal region, the fine crystal region is adjusted in the following rolling conditions to adjust the amount of Cr deposited. In addition, the precipitation amount of Cr can be evaluated by the extraction residue.

  The high Mn steel according to the present invention can be produced by melting a molten steel having the above-described composition by a known melting method such as a converter or an electric furnace. Further, secondary refining may be performed in a vacuum degassing furnace. Thereafter, it is preferable to use a steel material such as a slab having a predetermined size by a known casting method such as a continuous casting method or an ingot-bundling rolling method.

Furthermore, manufacturing conditions for building the steel material into a steel material having excellent low temperature toughness will be described.
[Steel material heating temperature: 1100 ° C to 1300 ° C]
In order to make the crystal grain size of the microstructure of the steel material coarse, the heating temperature before hot rolling is set to 1100 ° C. or higher. However, since there exists a possibility that a part of melt | dissolution may start when it exceeds 1300 degreeC, the upper limit of heating temperature shall be 1300 degreeC. The temperature control here is based on the surface temperature of the steel material.

[Finish rolling finish temperature: 890 ° C or higher and 980 ° C or lower]
The finish rolling finish temperature and the subsequent cooling conditions are important in controlling the recrystallization / recovery delay region. First, when the finish rolling finish temperature exceeds 890 ° C., recrystallization / recovery proceeds during finish rolling and immediately after finish rolling, and the formation of polygonal crystal grains is promoted by reducing the fine crystal region, resulting in reduced strength and toughness. This is a problem. Further, when the finish rolling finish temperature is lower than 980 ° C., recrystallization / recovery is suppressed, formation of polygonal crystal grains due to recrystallization is suppressed, and a lot of strain is also introduced into the fine crystal region. The strength increases and the toughness deteriorates accordingly.

[Final rolling reduction: 13% to 17%]
The rolling reduction in the final rolling is important because it affects the recrystallization process in the subsequent cooling. Here, the final rolling means the final pass of finish rolling and one pass before that. When the rolling reduction in the final rolling is higher than 17%, a large amount of strain is introduced into the material, so that strain-induced recrystallization proceeds in the subsequent cooling process, and the polygonal region increases. When the rolling reduction is lower than 13%, the recrystallization of the underlying strain structure is delayed and the fine crystal region remains, but the amount of strain in the interior is reduced, so that the strength of the material is lowered. For this reason, as suitable rolling conditions, as described above, the finish rolling finish temperature is in the range of 890 ° C. or higher and 980 ° C. or lower, and the rolling reduction in the final rolling of the finish rolling is 13% or higher and 17% or lower.

[Cooling rate from finish finish temperature to 650 ° C: 3 ° C / s or less]
In order to achieve both the formation of polygonal crystal grains by recrystallization / recovery and the formation of fine crystal regions, it is very important to control the cooling from the rolling end temperature to 650 ° C. where the progress of recovery / recrystallization is remarkable. Is important to. At this time, if the cooling rate is too high, the structure after rolling is frozen, and sufficient polygonal grains are not formed, and the toughness is deteriorated. Therefore, the upper limit of the cooling rate is set to 3 ° C./s. In particular, in the case of a thin object, as described above, the plate warpage occurs, which causes a problem in the process. In addition, since cooling in a temperature range of less than 650 ° C. does not affect the recrystallization / recovery of the base phase (parent phase), the cooling rate is regulated to the temperature range from the finish rolling finish temperature to 650 ° C. On the other hand, cooling in a temperature range of less than 650 ° C. may be optionally performed as described below.

Here, since the cooling rate changes depending on the plate thickness, it is advantageous to appropriately adjust by water cooling or the like. The cooling process here is performed on the basis of the thickness center temperature of the steel sheet. In addition, this center temperature can be calculated | required by heat-transfer calculation from the steel plate surface temperature measured with the radiation thermometer.
Further, although the lower limit of the cooling rate is not particularly set, use of a heat-retaining furnace is disadvantageous in terms of furnace cost, process cost, and manufacturing time, and therefore may be within the range of air cooling.

[Cooling below 650 ° C]
The present invention realizes improvement of toughness at a low temperature by combining the polygonal region and the fine crystal region even in a situation where carbides are formed at the grain boundaries. For this reason, there is no particular regulation for cooling below 650 ° C. However, carbide suppression is effective for toughness, and since the influence of the above-mentioned sheet warpage is reduced from a temperature range of less than 650 ° C., rapid cooling of 10 ° C./s or more is required from the viewpoint of suppressing carbide formation. It is desirable to do.

  Furthermore, you may add the process which heats and cools to the temperature range of 300 to 650 degreeC after performing the said cooling process (cooling less than 650 degreeC) as needed. That is, tempering treatment may be performed for the purpose of adjusting the strength of the steel sheet.

Hereinafter, the present invention will be described in detail with reference to examples. The present invention is not limited to the following examples.
(1) Steel plate Steel slabs having the composition shown in Table 1 were prepared by vacuum melting. Next, the obtained steel slab was charged into a heating furnace and heated to 1250 ° C., and then the finish rolling end temperature was changed variously to perform hot rolling, and in the temperature range from the finish rolling end temperature to 650 ° C. The cooling rate was changed in various ways to perform a cooling process, and a steel plate having a thickness of 5 to 10 mm was produced. Here, in hot rolling, a thermocouple was installed at the center of the thickness of the steel sheet, the temperature of the steel sheet was monitored, and the finish rolling end temperature was measured. Table 2 shows the finish rolling end temperature and the cooling rate in the temperature range from the finish rolling end temperature to 650 ° C.

About the obtained steel plate, the tensile test characteristic and low temperature toughness were evaluated in the following manner, and the structure was analyzed.
(2) Tensile test characteristics JIS No. 5 tensile test specimens were collected from each of the obtained steel sheets, and subjected to a tensile test in accordance with the provisions of JIS Z2241 (1998), and the tensile test characteristics were investigated. In the present invention, it was determined that a yield strength of 400 MPa or more and a tensile strength of 800 MPa or more are excellent in tensile properties. Furthermore, the elongation of 30% or more was determined to be excellent in ductility.

(3) Low temperature toughness Half-size (5mm) Charpy V-notch test in accordance with JIS Z2202 (1998) from the direction perpendicular to the rolling direction at a position 1/2 the thickness of the steel sheet. Pieces were collected and subjected to three Charpy impact tests for each steel sheet in accordance with the provisions of JIS Z 2242 (1998), the absorbed energy at -196 ° C was determined, and the base material toughness was evaluated. Here, the average value of three absorbed energy _{(vE -196)} is more than 30J was excellent in base metal toughness.

(4) Tissue analysis For tissue analysis, the structure was observed with a scanning electron microscope (FE-SEM) having an electrolytic emission gun and an in-lens detector. That is, a sample prepared by embedding a steel plate with resin was subjected to mirror polishing with diamond polishing and colloidal silica, and then surface sputtering was performed with an Ar ion beam. The structure was observed at an acceleration voltage of 5 kV, the morphology of the fine crystal region was evaluated, and the area ratio was calculated. The observation region was an area of 500 × 500 μm per location from a position of ¼ of the plate thickness from the surface of the steel plate, and this observation was performed at 10 locations to obtain an average value.

(5) Cr carbide amount About the amount of Cr carbide, after electrolytically extracting the precipitate with 10% acetylacetone solution, the amount of the extracted precipitate was measured by ICP (high frequency inductively coupled plasma) emission spectroscopic analysis, and the measured electrolytic amount Was converted to mass%.
Table 3 shows the evaluation and observation results obtained as described above.

As shown in Table 3, high Mn steel according to the invention, the aforementioned target performance (yield strength of the base material is more than 400 MPa, the low temperature toughness absorbed energy (more than 30J in the average value of vE _-196)) satisfying the Was confirmed. On the other hand, in the comparative example that is out of the scope of the present invention, any one or more of the yield strength and the low temperature toughness does not satisfy the above target performance.

Claims (3)

% By mass
C: 0.10 to 0.70%,
Si: 0.05 to 1.0%,
Mn: 15-30%,
P: 0.030% or less,
S: 0.0070% or less,
Al: 0.01 to 0.07%,
Cr: 2.5-7.0%
N: 0.0050 to 0.0500% and O: 0.0050% or less, with the balance having a component composition of Fe and unavoidable impurities, with austenite as the base phase, and an area ratio of 50% or more A high Mn steel having a microstructure in which fine crystal regions of 80% or less remain, and the microstructure has a Cr carbide content of 0.04% or less. The component composition is further mass%,
Mo: 2.0% or less,
V: 2.0% or less,
W: 2.0% or less,
REM: 0.0010-0.0200% and B: 0.0005-0.0020%
The high Mn steel according to claim 1, comprising one or more selected from among the above.   The steel material having the component composition according to claim 1 or 2 is heated to a temperature range of 1100 ° C to 1300 ° C, the finish rolling finish temperature is 890 ° C to 980 ° C, and the final rolling reduction is 13% to 17%. % High-Mn steel manufacturing method in which hot rolling is performed at a rate of not more than%, and cooling treatment is performed at an average cooling rate of 3 ° C./s or less in the temperature range from the finish rolling finish temperature to 650 ° C.