WO2019156179A1

WO2019156179A1 - HIGH-Mn STEEL AND PRODUCTION METHOD THEREFOR

Info

Publication number: WO2019156179A1
Application number: PCT/JP2019/004470
Authority: WO
Inventors: 仲道　治郎; 植田　圭治; 大地泉; 孝一中島
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2018-02-07
Filing date: 2019-02-07
Publication date: 2019-08-15
Also published as: KR20200112950A; JP6590120B1; CN111684093A; JPWO2019156179A1; KR102367801B1; CN111684093B

Abstract

Proposed is a method for further imparting excellent ductility to a high-Mn steel having excellent cryogenic toughness in a base material and welding-heat-affected portion thereof. This high-Mn steel has a compositional makeup containing 0.10-0.70% of C, 0.05-1.00% of Si, 15.0-30.0% of Mn, 0.030% or less of P, 0.0070% or less of S, 0.01-0.07% of Al, 2.5-7.0% of Cr, 0.0050-0.0500% of N, and 0.0050% or less of O, the remaining portion being Fe and incidental impurities, and includes austenite as a base phase, wherein the base phase includes a polygonal recrystallization region and a microstructure which is a recrystallization recovery delay region having an area ratio of 10-50%, the recrystallization recovery delay region includes a plurality of crystal grains having a diameter of 5 μm or less, and is in the shape of an ellipse having a major axis in the rolling direction of a steel sheet or a shape similar to said ellipse, and the ellipse has an aspect ratio of 2.0 or more and the major axis is 10 μm or more.

Description

High Mn steel and manufacturing method thereof

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

When using a hot-rolled steel sheet for a 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 in a liquefied natural gas storage tank, it is necessary to ensure excellent toughness 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 these materials have high alloy costs and manufacturing costs, there is a demand for steel materials that are inexpensive and have excellent 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 and preventing the carbides generated at the grain boundaries from becoming the starting point of fracture and the propagation path of cracks. Patent Document 2 proposes a technique that improves low-temperature toughness by suppressing the segregation of Mn to a certain level or more.

Japanese Unexamined Patent Publication No. 2016-196703 Japanese Unexamined Patent Publication No. 2017-71817

In the use of the structure for a liquefied gas storage tank described above, it is important to ensure ductility in addition to low temperature toughness because the steel material to be used must 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. However, depending on the application, for example, 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.
Here, “excellent in low temperature toughness” means that the absorbed energy vE-196 in the Charpy impact test at −196 ° C. is 100 J or more.

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 distribution of the carbides and the form of the grain boundaries have a great influence on the toughness. Specifically, when fine crystal grains are used, carbide forming sites increase as a starting point of fracture, and the toughness value decreases. On the other hand, when the crystal grains are coarse, the starting point is reduced because there is no carbide, but the propagation of the fracture surface is facilitated and the toughness value is lowered.

As a method for suppressing the formation of carbides, it is an effective means to rapidly cool a steel plate (hereinafter also referred to as rapid cooling). However, in the case of a thin steel sheet having a thickness of 20 mm or less, when the steel sheet is rapidly cooled, a sheet warp accompanying 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 production line, when the thickness of the steel sheet and the cooling rate in the cooling after hot rolling are changed, the results of the summary of the situation in which the load on the manufacturing process occurs due to the warpage of the steel sheet It is shown in 1. As shown in Table 1, it was found that when the cooling rate exceeded 5 ° C./s, a problem occurred in the process with a steel plate having a thickness of 20 mm or less.

In addition, the load on the manufacturing process, which is the evaluation standard in Table 1, means the load required for care and correction for intermediate products and 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.

Therefore, when it was difficult to apply quenching treatment effective for carbide suppression, a method for improving toughness by controlling the crystal grain morphology of the matrix even when carbide was present was studied. In other words, the inventors studied diligently about 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 combining a region composed of fine crystal grains and polygonal crystal grains, the fracture starting point can be reduced and the propagation of the fracture surface can be suppressed simultaneously, and the toughness value can be improved.

As a result of analyzing the structure contributing to the improvement of the toughness value, the region composed of fine crystal grains is a region where recrystallization and recovery are delayed after rolling, and the polygonal region is a region after rolling. It was found that recrystallization / recovery occurred in a relatively early stage. For this reason, it has been found that such a structure can be formed by optimizing the temperature conditions for hot rolling and the cooling conditions after rolling in addition to adjusting the components. In particular, it has been found that addition of Cr makes it easy to control a region in which non-recrystallization and recovery are suppressed. In addition, by setting the finish rolling finish temperature from 750 ° C. to 850 ° C., and performing subsequent cooling at a cooling rate of 5 ° C./s or less, the recrystallization recovery delay region is formed, and both strength and toughness are achieved. Found to get.

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% or more and 1.00% or less,
Mn: 15.0% to 30.0%,
P: 0.030% or less,
S: 0.0070% or less,
Al: 0.01% or more and 0.07% or less,
Cr: 2.5% to 7.0%,
N: 0.0050% or more and 0.0500% or less and O: 0.0050% or less, with the balance being a component composition of Fe and inevitable impurities, and austenite as a base phase, the base phase being polygonal And a microstructure that is a recrystallization recovery delay region of 10% to 50% in terms of area ratio, the recrystallization recovery delay region is composed of a plurality of crystal grains having a diameter of 5 μm or less, and A high-Mn steel having an ellipse whose major axis is the rolling direction of a steel sheet or a shape close to the ellipse, having an aspect ratio of 2.0 or more and a major axis of 10 μm or more.

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

3. The steel material having the component composition according to 1 or 2 is heated to a temperature range of 1100 ° C. or higher and 1300 ° C. or lower, and subjected to hot rolling at a finish rolling end temperature of 750 ° C. or higher and lower than 850 ° C., and the finish rolling A method for producing high-Mn steel, in which an average cooling rate in the temperature range from the end temperature to 650 ° C. is 5 ° C./s or less.

4). 4. The method for producing a high Mn steel as described in 3 above, wherein the average cooling rate is 3 ° C./s or less.

According to the present invention, high Mn steel having excellent 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. 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% or more and 0.70% or less. Preferably, the content is 0.20% or more and 0.60% or less.

Si: 0.05% or more and 1.00% or less Si acts as a deoxidizer and is not only necessary for steelmaking, but also has the effect of increasing the strength of the steel sheet by solid solution strengthening by solid solution in steel. . In order to acquire such an effect, Si needs to contain 0.05% or more. On the other hand, when it contains exceeding 1.00%, weldability will deteriorate. For this reason, Si is made 0.05% or more and 1.00%. Preferably, the content is 0.07% or more and 0.50% or less.

Mn: 15.0% to 30.0% 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 15.0% or more. On the other hand, even if the content exceeds 30.0%, the effect of improving the cryogenic toughness is saturated, resulting in 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 15.0% or more and 30.0% or less. Preferably, the content is 18.0% or more and 28.0% 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% or more and 0.07% or less Al acts as a deoxidizing agent, and is most commonly used in a molten steel deoxidizing process of a steel sheet. 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% to 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 recrystallization recovery delay 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% or more and 7.0% or less. Preferably, the content is 3.5% or more and 6.5% or less.

N: 0.0050% or more and 0.0500% or less N is an austenite stabilizing element, and is an element effective in 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% or more and 0.0500% or less. Preferably it is 0.0060% or more and 0.0400% or less.

O: 0.0050% or less O forms oxides and degrades cryogenic toughness. For this reason, O is made into 0.0050% or less of range. 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.
Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, REM: 0.0010% or more and 0.0200% or less, B: 0.0005% or more and 0.0020% or less 1 type (s) or 2 or more types can be added.

Mo, V, W: each 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, respectively. 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, respectively. Preferably it is 0.003% or more and 1.7% or less, more preferably 1.5% or less.

REM: 0.0010% or more and 0.0200% or less 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% or more and 0.0020% or less 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). “Austenite is the base phase” means that the austenite phase is 90% or more in terms of the area ratio in the microstructure, 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, in the cooling process during hot rolling and after hot rolling, recrystallization / recovery progresses rapidly and has regions with polygonal grains, that is, polygonal regions, and recrystallization / recovery is delayed, causing many strains inside. 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 including the region including the recrystallization recovery delay region. Below, the form of each area | region mentioned above is explained in full detail.

[Recrystallization recovery delay region]
The recrystallization recovery delay region is a region composed of a plurality of crystal grains containing a lot of strain inside, in which recrystallization and recovery from the introduction of strain in hot rolling are delayed. This region has an individual crystal grain size of 5 μm or less, basically extends in the rolling direction reflecting the rolling structure, and has a shape in which a set of a plurality of crystal grains is elliptical. It is. That is, the recrystallization recovery delay region has an ellipse whose major axis is the rolling direction or an approximate shape to the ellipse in observation of a cross section perpendicular to the rolling direction of the steel sheet (hereinafter referred to as L cross section), The aspect ratio of the ellipse is 2.0 or more and the major axis is 10 μm or more. Although the method of recognizing this area will be described later, the tissue control having the shape described above is performed.

[Area ratio of recrystallization recovery delay region: 10% to 50%]
The recrystallization recovery delay region is formed in combination with the polygonal region, and when the area ratio of the recrystallization recovery delay region is high, the entire structure becomes strained, which is disadvantageous in terms of ductility. Furthermore, since carbides formed at the grain boundaries and the like in the recrystallization recovery delay region increase and the starting point of the fracture surface increases, this is also disadvantageous for toughness. For this reason, the upper limit of the ratio of the recrystallization recovery delay region in the microstructure is 50% in terms of area ratio. On the other hand, when the area ratio is lower than 10%, the other portions are formed of polygonal crystal grains, so that the strength of the material is lowered. In particular, if the area ratio is lower than 10%, the fracture surface unit in the toughness test increases and the progress of the fracture surface is facilitated. Therefore, the recrystallization recovery delay region requires a fraction of 10% or more. is there. The area ratio is preferably 20% or more and 40% or less.

[Polygonal area]
The polygonal region is a region in which the strain region introduced by hot rolling is sufficiently recrystallized and recovered to become polygonal crystal grains. The crystal grains are also subjected to strain recovery and contribute to the improvement of ductility. Further, since the polygonal region has a relatively large grain boundary, the formation density of carbide is reduced and the starting point of the fracture surface is reduced, which is effective in improving toughness. As the form of polygonal crystal grains, the aspect ratio is 1.0 or more and 1.8 or less when the crystal grain is elliptically approximated and the maximum diameter of the crystal grain is defined as the major axis of the ellipse in the observation of the L cross section of the steel sheet. It is desirable that This is because if the aspect ratio exceeds 1.8, it is affected by the expansion due to rolling, which is disadvantageous for the suppression of fracture surface propagation. The particle diameter is preferably 5 μm or more and 100 μm or less in terms of the major axis of the ellipse. When this particle size is less than 5 μm, the grain boundary which is a carbide forming site increases, which is disadvantageous in reducing the starting point of the fracture surface. On the other hand, when the particle diameter exceeds 100 μm, the fracture surface unit becomes large, the fracture surface progresses easily, and the toughness decreases. The ratio of the polygonal region in the entire microstructure is preferably 40% or more and 90% or less in terms of area ratio. More preferably, it is 60% or more and 80% or less.

Therefore, the austenite phase forming the base phase (parent phase) of the steel sheet is mainly defined by the polygonal region and the recrystallization recovery delay region. However, a region that does not satisfy these regulations, such as a crystal grain having an aspect ratio of 1.0 to 1.8 and less than 5 μm, a region that is recognized as a recrystallization recovery delay region by the following observation method, but has an aspect ratio of less than 2.0, etc. However, the area ratio in the microstructure is suppressed to 5% or less, and most of the austenite phase needs to be formed as one of the polygonal region and the recrystallization recovery delay region. There is. That is, the matrix phase is a polygonal recrystallization region and a recrystallization recovery delay region of 10% to 50% in area ratio.

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 performing mirror polishing with colloidal silica on the steel sheet surface, if ion etching is performed on the steel sheet surface layer by ion milling, fine irregularities are formed on the surface layer of the recrystallization recovery delay region, so that it is 5 kV or less. Identification is possible by in-lens texture observation and backscattered electron image observation by low acceleration SEM. The recrystallization recovery delay region can also be identified 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, it is possible to identify each region with values such as image quality using EBSD (“Electron” Back “Scattered” Diffraction). In this case, however, strain due to polishing may be introduced into the sample surface during sample preparation, so care must be taken when preparing the sample, and surface strain must be removed by electrolytic polishing, ion polishing, etc. There is.

As for the form of the recrystallization recovery delay region, the small one is composed of two or more crystal grains, and the aggregate size (major axis) of the plurality of crystal grains is about 10 μm, while the large region is a band. It has a structure (a belt-like structure stretched in accordance with the rolling direction of the plate), (width in the plate thickness direction (lamination)) width is 50 μm, and length (the longitudinal direction of the belt in the stretched direction (major axis) )) Is about 500 μm. As shown in FIG. 1 for the three cases, the recrystallization recovery delay region can be clearly distinguished from the polygonal region, as indicated by the outline in the figure, as shown in the structure photographs of the reflected electron images. That is, FIG. 1A is a 200 × structure photograph, and a structure (recrystallization recovery delay region) stretched in the rolling direction can be observed. Moreover, FIG. 1B is a 500-fold structure photograph, and it can be confirmed that various forms of non-recrystallized regions (recrystallization recovery delay regions) are formed in the observation 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) ( Measure the area of the recrystallization recovery delay region within the same field of view, and calculate the area ratio in this field of view. This operation is performed at at least 10 places, and the average is calculated as the area ratio of the recrystallization recovery delay region.

For polygonal crystal grains (polygonal region), SEM observation is performed at 1000 times, and 100 or more crystal grains are recognized. In this case, in combination with EBSD, the crystal grain size may be measured in a region excluding the recrystallization recovery delay region recognized by SEM observation.

The high Mn steel according to the present invention can be obtained 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, a steel material such as a slab having a predetermined size is preferably formed 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: 750 ° C or higher and 850 ° C or lower]
The hot rolling finish temperature and the subsequent cooling conditions are important in controlling the recrystallization recovery delay region. When this temperature is higher than 850 ° C., recrystallization proceeds during the final rolling and immediately after rolling, the formation of polygonal crystal grains is promoted, the grain boundary becomes large, and the toughness becomes too high. Further, when the rolling temperature is lower than 750 ° C., formation of polygonal crystal grains due to recrystallization is suppressed, and a lot of strain is also introduced into the recrystallization recovery delay region, so that the strength increases and the toughness deteriorates. .

[Cooling rate from finish rolling finish temperature to 650 ° C: 5 ° C / s or less]
In order to achieve both the formation of polygonal crystal grains by recrystallization / recovery and the remaining recrystallization recovery delay region, the cooling from the rolling end temperature to 650 ° C. in which the progress of recovery / recrystallization is remarkable should be controlled. Is very important. 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 deteriorates. Therefore, the upper limit of the cooling rate is set to 5 ° C./s. Preferably, it shall be 3 degrees C / s or less. In particular, in the case of a thin object, as described above, the plate warpage occurs and causes a problem in the process, so that it is preferable to cool at a rate of 3 ° C./s or less. 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 recrystallization recovery delay 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, if necessary, after performing the cooling process (cooling below 650 ° C.), a process of heating and cooling to a temperature range of 300 ° C. or higher and 650 ° C. or lower may be added. 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 steel sheet having a thickness of 5 to 20 mm was prepared by performing a cooling process while changing the cooling rate. 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 Charpy V-notch specimens were sampled in accordance with JIS Z2202 (1998) from the direction perpendicular to the rolling direction at a position 1/2 the thickness from the surface of each steel sheet. Three Charpy impact tests were performed on each steel sheet in accordance with the provisions of Z 2242 (1998), the absorbed energy at -196 ° C. was determined, and the base metal toughness was evaluated. In the present invention, the average value of the three absorbed energies (vE-196) is 100 J or more, and the base metal toughness is excellent. For steel plates with a thickness of 10 mm or less, half-size (5 mm) Charpy V-notch test pieces were prepared and tested, and the absorbed energy was 50 J or more.

(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 recrystallization recovery delay region was evaluated, and the area ratio was calculated. That is, an unrecrystallized region was extracted from each SEM image and the region was traced. About the area | region which performed the trace, the area was calculated | required using image analysis software etc., 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.
Table 3 shows the evaluation and observation results obtained as described above.

As shown in Table 3, the high-Mn steel according to the present invention satisfies the above-mentioned target performance (base material yield strength of 400 MPa or more, low-temperature toughness is 100 J or more in terms of the average value of absorbed energy (vE-196)). 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

% By mass
C: 0.10% to 0.70%,
Si: 0.05% or more and 1.00% or less,
Mn: 15.0% or more and 30.0% or less P: 0.030% or less,
S: 0.0070% or less,
Al: 0.01% or more and 0.07% or less,
Cr: 2.5% to 7.0%,
N: 0.0050% or more and 0.0500% or less and O: 0.0050% or less, the balance having a component composition of Fe and inevitable impurities, and austenite as a base phase, Having a microstructure which is a polygonal recrystallization region and a recrystallization recovery delay region of 10% to 50% in area ratio, and the recrystallization recovery delay region is composed of a plurality of crystal grains having a diameter of 5 μm or less, And the high Mn steel which has an ellipse which makes the rolling direction of a steel plate a major axis, or the shape approximate to the said ellipse, the aspect ratio of the said ellipse is 2.0 or more, and the said major axis is 10 micrometers or more.
The component composition is further mass%,
Mo: 2.0% or less,
V: 2.0% or less,
W: 2.0% or less,
The high Mn steel according to claim 1, comprising one or more selected from REM: 0.0010% to 0.0200% and B: 0.0005% to 0.0020%.
The steel material having the component composition according to claim 1 or 2 is heated to a temperature range of 1100 ° C or higher and 1300 ° C or lower and subjected to hot rolling at a finish rolling finish temperature of 750 ° C or higher and lower than 850 ° C, and the finish A method for producing high-Mn steel, wherein a cooling treatment is performed at an average cooling rate of 5 ° C./s or less in a temperature range from the rolling end temperature to 650 ° C.
The method for producing high-Mn steel according to claim 3, wherein the average cooling rate is 3 ° C / s or less.