KR20200112950A - High Mn steel and manufacturing method thereof - Google Patents

Abstract

A method for imparting further excellent ductility in high-Mn steel having excellent low-temperature toughness in the base material and the heat-affected zone of welding is proposed. C: 0.10% or more and 0.70% or less, 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: It contains 2.5% or more and 7.0% or less, N: 0.0050% or more and 0.0500% or less, and O: 0.0050% or less, and the balance has a component composition of Fe and inevitable impurities, and austenite is used as a matrix, and the matrix is poly It has a microstructure which is a recrystallization recovery delayed region of 10% or more and 50% or less in terms of a conventional recrystallization region and area ratio, and the recrystallization recovery delayed region is composed of a plurality of crystal grains having a diameter of 5 μm or less, and the rolling direction of the steel sheet It has an ellipse as a major axis or a shape approximating the ellipse, and the aspect ratio of the ellipse is 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 suitable for use in a structural steel used in a cryogenic environment, such as a tank for a liquefied gas reservoir, and particularly to a high Mn steel having excellent toughness at low temperatures and a method for producing the same. .

In the case of using a hot-rolled steel sheet for a structure for lowering the liquefied gas, since the use environment is at a cryogenic temperature, the steel sheet is required to have high strength and excellent toughness at cryogenic temperatures. For example, in the case of using a hot-rolled steel sheet for a low tank of liquefied natural gas, it is necessary to ensure excellent toughness at the boiling point of liquefied natural gas: -164°C or lower. If the low-temperature toughness of the steel material is inferior, there is a possibility that the safety as a structure for cryogenic and low temperature may not be maintained. Therefore, there is a strong demand for improvement of low-temperature toughness for the applied steel.

In response to this demand, conventionally, austenitic stainless steel, 9% Ni steel, or 5000-based aluminum alloy having a structure of a steel sheet made of austenite that does not exhibit brittleness at cryogenic temperatures has been used. However, since these materials are expensive in alloying cost and manufacturing cost, there is a demand for a steel material that is inexpensive and excellent in cryogenic toughness.

Therefore, as a new steel material that replaces the conventional cryogenic steel, it is proposed in Patent Document 1, for example, to use a high-Mn steel containing a large amount of Mn, a relatively inexpensive austenite stabilizing element, as a structural steel in a cryogenic environment. Has been.

Patent Literature 1 proposes a technique for controlling the austenite grain size to an appropriate size so that carbides generated at grain boundaries are prevented from becoming an origin of fracture or a path of propagation of cracks. In addition, Patent Document 2 proposes a technique for improving low-temperature toughness by suppressing the segregation of Mn to a certain level or more.

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

In applications such as the structure for storing liquefied gas described above, it is necessary to provide high workability in the steel material to be used, so it is important to ensure ductility in addition to low-temperature toughness. Nothing has been verified in the techniques described in Patent Documents 1 and 2 regarding this ductility. In addition, the high Mn steel material described in Patent Document 1 has a thickness of about 15 to 50 mm, but 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, in the method of performing accelerated cooling after the completion of hot rolling, exemplified in Patent Document 1, warpage or deformation is liable to occur in the obtained steel sheet, and an extra process such as shape correction is required, thereby increasing productivity. Is inhibited. In addition, in Patent Document 2, a long time heat treatment is required to alleviate segregation, which is disadvantageous in terms of productivity.

Accordingly, an object of the present invention is to propose a method for imparting further excellent ductility in a high-Mn steel having excellent low-temperature toughness in a base material and a heat-affected zone for welding. In addition, an object of the present invention is to propose a method in which such a thin plate of high Mn steel can be manufactured without occurrence of warpage or deformation.

Here, the "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 problems, the inventors of the present invention conducted intensive research on various factors that determine the component composition and manufacturing method of the steel sheet for high Mn steel, investigated the relationship with the microstructure, and recognized the following Came to get.

First, the high Mn steel does not undergo brittle fracture even at cryogenic temperatures, and when fracture occurs, it occurs from grain boundaries. That is, it was found that the shape of the grain boundary greatly influences the toughness. Particularly, carbides or the like are formed at the grain boundaries, and it was found that the distribution of the carbides and the shape of the grain boundaries greatly influence the toughness. Specifically, in the case of fine crystal grains, carbide formation sites increase as a starting point of fracture, and the toughness value decreases. Conversely, in the case of coarse crystal grains, since there is no carbide, the starting point decreases, but the propagation of the wavefront becomes easy and the toughness value decreases.

As a method of suppressing the formation of carbides, rapid cooling (hereinafter, also referred to as rapid cooling) of the steel sheet is an effective means. However, in the case of a thin material having a thickness of the steel sheet of 20 mm or less, when the steel sheet is rapidly cooled, the sheet warpage may occur due to internal stress due to thermal deformation. In particular, in the case of high Mn steel, since the structure is austenite, there is a tendency that the plate warpage becomes large compared to that of ferritic steel. When this plate warpage occurs, it becomes difficult to insert the plate into a surface smoothing treatment line or the like, which is a step after cooling. In addition, in order to ship, it is necessary to correct the warpage of the steel sheet, and since a new process must be added to the manufacturing line, the manufacturing cost increases. It is inferred that the reason why the warpage of the austenitic steel increases is because the thermal conductivity is smaller and the temperature distribution is larger than that of 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 are summarized about the situation in which the load on the manufacturing process is generated due to the bending of the steel sheet. It is shown in 1. As shown in Table 1, when the cooling rate exceeded 5°C/s, it was found that a problem occurred in the process in a steel sheet having a thickness of 20 mm or less.

In addition, the load on the manufacturing process, which is an evaluation criterion in Table 1, means a load required for care, calibration, etc. to intermediate products or products in each process. And, in Table 1, ◎ means that the above-mentioned care or correction is unnecessary, and the post-manufacturing operation, which is the transfer to the flatness straightening device (leveler) and the material storage place such as a cooling bed, is passed online without any problems, and is returned. When it was possible, ○ means that when mail order becomes possible by performing a slight smoothing treatment such as adjusting the opening degree of the leveler for each production chance, △ means a mild calibration work in an individual work offline (worker If it is judged that it is necessary to correct the processing deformation by manual), x indicates a case where there was a problem in the product shipment itself because the correction was not possible due to manufacturing reasons.

Therefore, in the case where it is difficult to apply the quenching treatment effective for suppressing carbides as described above, a method of improving toughness by controlling the shape of the crystal grains of the mother phase even if the carbides are present was examined. That is, under the condition of the presence of carbides, the influence of the shape and toughness of the matrix crystal grains was carefully examined. As a result, it was found that by combining a region composed of fine grains and polygonal grains, it was possible to simultaneously reduce the origin of fracture and suppress wavefront propagation, thereby improving the toughness value.

As a result of analyzing the structure that contributes to the improvement of the above-described toughness value, the area made of fine grains is the area where the delay in recrystallization/recovery occurs after rolling, and the polygonal area is at a relatively fast stage after rolling. It was found that this is an area where recrystallization and recovery are occurring. For this reason, it has been found that the creation of such a structure is made possible by optimizing the temperature conditions of hot rolling and cooling conditions after rolling in addition to component adjustment. In particular, it was found that the addition of Cr facilitates control of the region in which non-recrystallization and recovery were suppressed. In addition, by setting the finish rolling end temperature to 750°C to 850°C, and performing subsequent cooling at a cooling rate of 5°C/s or less, the recrystallization recovery delayed region is formed, so that both strength and toughness can be achieved. Found something.

The present invention has been made by adding further examination to the above recognition, and the summary thereof is as follows.

1. By mass%,

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

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% or more and 7.0% or less,

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

O: 0.0050% or less

Contains, the balance has a component composition of Fe and unavoidable impurities, and austenite is a matrix phase, and the matrix phase recovers from 10% to 50% by a polygonal recrystallization region and area ratio It has a microstructure as a delayed region, and the recrystallization recovery delayed region is composed of a plurality of crystal grains having a diameter of 5 μm or less, and has an ellipse having a major axis in the rolling direction of the steel sheet or a shape approximating the ellipse, and the aspect of the ellipse High Mn steel with a ratio of 2.0 or more and the 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,

REM: 0.0010% or more and 0.0200% or less and

B: 0.0005% or more and 0.0020% or less

The high Mn steel according to 1 above containing one or two or more selected from among.

3. The steel material having the component composition described in the above 1 or 2 is heated in a temperature range of 1100°C or more and 1300°C or less, and hot rolling is performed so that the finish rolling end temperature is 750°C or more and less than 850°C, and the finish rolling. A method for producing a high-Mn steel in which a cooling treatment is performed in which the average cooling rate in the temperature range from the end temperature to 650°C is 5°C/s or less.

4. The method for producing the high Mn steel according to 3 above, wherein the average cooling rate is 3°C/s or less.

According to the present invention, a high-Mn steel excellent in low-temperature toughness can be provided. When this high-Mn steel is used for welding, the base material after welding and the heat-affected zone for welding are both excellent in low-temperature toughness. Therefore, the high Mn steel of the present invention greatly contributes to the improvement of the safety and lifespan of steel structures used in cryogenic environments, such as tanks for liquefied gas storage, and brings a special industrial effect. Further, the manufacturing method of the present invention can provide a manufacturing method excellent in economical efficiency, since the high Mn steel having excellent low-temperature toughness can be manufactured without causing a decrease in productivity and an increase in manufacturing cost.

1A is a photograph of the structure by SEM.
1B is a photograph of the structure by SEM.

(Form for carrying out the invention)

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. In addition, "%" indication in a component composition shall mean "mass%" unless otherwise stated.

C: 0.10% or more and 0.70% or less

C is an inexpensive austenite stabilizing element, and is an important element for obtaining austenite. In order to obtain the effect, C needs to contain 0.10% or more. On the other hand, if it contains more than 0.70%, Cr carbide is excessively produced|generated, and low temperature toughness falls. For this reason, C is made into 0.10% or more and 0.70% or less. Preferably, it is set as 0.20% or more and 0.60% or less.

Si: 0.05% or more and 1.00% or less

Si acts as a deoxidizing agent and is not only necessary for steelmaking, but also has an effect of increasing the strength of the steel sheet by solid solution strengthening by solid solution in steel. In order to obtain such an effect, Si needs to contain 0.05% or more. On the other hand, if it contains more than 1.00%, weldability deteriorates. For this reason, Si is made into 0.05% or more and 1.00%. Preferably, it is set as 0.07% or more and 0.50% or less.

Mn: 15.0% or more and 30.0% or less

Mn is a relatively inexpensive austenite stabilizing element. In the present invention, it is an important element in order to achieve both strength and cryogenic toughness. In order to obtain the effect, Mn needs to contain 15.0% or more. On the other hand, even if it contains more than 30.0%, the effect of improving the cryogenic toughness is saturated, resulting in an increase in alloy cost. Moreover, weldability and cutability deteriorate. In addition, segregation is promoted and stress corrosion cracking is promoted. For this reason, Mn is made into 15.0% or more and 30.0% or less. Preferably, it is 18.0% or more and 28.0% or less.

P: 0.030% or less

When P is contained in an amount exceeding 0.030%, it segregates at grain boundaries and becomes a starting point of occurrence of stress corrosion cracking. For this reason, it is preferable to use 0.030% as an upper limit and reduce it as much as possible. Therefore, P is set to 0.030% or less. Preferably, it is set at 0.028% or less, more preferably 0.24% or less. Of course, it may be 0%. Moreover, in order to reduce P to less than 0.002%, it is preferable that it is 0.002% or more from the viewpoint of economical efficiency from the point that a large cost is required for refining.

S: 0.0070% or less

Since S deteriorates the low-temperature toughness and ductility of the base material, it is preferable to reduce as much as possible with 0.0070% as the upper limit. Therefore, S is set to 0.0070% or less. Preferably it is 0.0050% or less. Of course, it may be 0%. Moreover, in order to reduce S to less than 0.0005%, it is preferable that it is 0.0005% or more from the viewpoint of economical efficiency from the point of requiring a large cost for refining.

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 deoxidation process of a steel sheet. In order to obtain such an effect, Al needs to contain 0.01% or more. On the other hand, if it contains more than 0.07%, since it mixes with a weld metal part at the time of welding and deteriorates the toughness of a weld metal, it is set as 0.07% or less. Preferably, they are 0.02% or more and 0.06% or less.

Cr: 2.5% or more and 7.0% or less

Cr is an element effective in improving the cryogenic toughness and base metal strength by stabilizing austenite by adding an appropriate amount. In addition, it is an effective element for forming a recrystallization recovery delay region described later. In order to obtain such an effect, it is necessary to contain Cr in an amount of 2.5% or more. On the other hand, when it contains more than 7.0%, low-temperature toughness and stress corrosion cracking resistance deteriorate due to generation of Cr carbide. For this reason, Cr is made into 2.5% or more and 7.0% or less. Preferably it 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 the cryogenic toughness. In order to obtain such an effect, the content of N is required to be 0.0050% or more. On the other hand, when it contains more than 0.0500%, nitride or carbonitride coarsens and toughness falls. 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 an oxide and deteriorates the 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%. Moreover, in order to reduce O to less than 0.0005%, it is preferable that it is 0.0005% or more from a viewpoint of economical efficiency from the point which requires a large cost for refining.

The balance other than the above components is iron and unavoidable impurities. Examples of the inevitable impurities here include Ca, Mg, Ti, Nb, Cu, and the like, and can be allowed as long as it is 0.05% or less in total.

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

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 of 1 type or 2 or more types can be added.

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

Mo, V, and W contribute to the stabilization of austenite and to improve the strength of the base metal. In order to obtain such an effect, it is preferable to contain Mo, V, and W in an amount of 0.001% or more, respectively. On the other hand, when it contains more than 2.0%, a coarse carbonitride may be produced|generated, and it may become the starting point of destruction, and the manufacturing cost is pressed. For this reason, when these alloying elements are contained, the content is set to 2.0% or less, respectively. Preferably it is 0.003% or more and 1.7% or less, More preferably, it is 1.5% or less.

REM: 0.0010% or more and 0.0200% or less

REM is an element useful for controlling the shape of inclusions, and may be contained as necessary. Control of the shape of inclusions refers to making whole-body sulfide-based inclusions into particle-like inclusions. The ductility, toughness, and sulfide stress corrosion cracking resistance are improved by controlling the shape of the inclusions. In order to obtain such an effect, it is preferable to contain REM 0.0010% or more. On the other hand, when the content is excessive, the amount of non-metallic inclusions increases, and ductility, toughness, and sulfide stress corrosion cracking resistance 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 the improvement of toughness due to the grain boundary strength of the material. However, since it forms coarse nitride or carbide when it is added excessively, it is preferable that the addition amount is 0.0005% or more and 0.0020% or less.

Next, a structure form for realizing low-temperature toughness will be described.

[Microstructure with austenite as a matrix]

When the crystal structure of the steel material is a body-centered cubic structure (bcc), the steel material is not suitable for use in a low-temperature environment because there is a possibility of causing brittle fracture in a low-temperature environment. Here, assuming use in a low-temperature environment, it is essential that the matrix phase in the structure of the steel material is austenite whose crystal structure is face-centered cubic structure (fcc). In addition, "the austenite is made into a matrix" indicates 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 a ferrite or martensite phase having a BCC structure, or inclusions or precipitates, but the proportion thereof is preferably 5% or less. In addition, the austenite fraction can be determined by observation by EBSD, analysis by XRD, permeability, and the like.

[Micro organization type]

The present invention realizes improvement of low-temperature toughness by performing microstructure control, particularly control of austenite structure, in hot rolling and subsequent cooling processes. To do that, it is important to control the shape of the micro-organism. In particular, in the cooling process during hot rolling or after hot rolling, recrystallization/recovery proceeds quickly to have polygonal grains, i.e., polygonal regions, and regions containing many deformations inside due to delayed recrystallization/recovery, That is, by appropriately presenting the recrystallization recovery delay region, it is important to reduce the origin of the wavefront and suppress the wavefront propagation, thereby improving the toughness. Hereinafter, the shape of each of the above-described regions will be described in detail.

[Recrystallization recovery delay area]

The recrystallization recovery delayed region is a region composed of a plurality of crystal grains containing a large amount of deformation therein, in which recrystallization and recovery from the introduction of strain in hot rolling are delayed. This region is a region in which the size of individual crystal grains is 5 µm or less, basically reflects the rolled structure and reverts in the rolling direction, and shows a shape in which a set of a plurality of crystal grains becomes an ellipse. That is, the recrystallization recovery delay region has an ellipse having the rolling direction as a major axis or a shape approximating the ellipse, when the cross section perpendicular to the rolling direction of the steel sheet (hereinafter referred to as L cross section) is observed, and the aspect ratio of the ellipse is 2.0 or more and the major axis is 10 μm or more. The method of recognizing this area will be described later, but the structure is controlled in the shape described above.

[Area ratio of delayed recrystallization recovery region: 10% or more and 50% or less]

The recrystallization recovery delayed region is formed in combination with the polygonal region, and if the area ratio of the recrystallization recovery delayed region is high, the overall structure is highly deformed, which is disadvantageous in terms of ductility. Further, since carbides formed at the grain boundaries of the recrystallization recovery delay region or the like increase and the origin of the fracture surface increases, toughness is also disadvantageous. For this reason, as for the ratio occupied by the recrystallization recovery delay region in the microstructure, the upper limit thereof was set to 50% as an area ratio. On the other hand, when this area ratio is lower than 10%, the strength of the material decreases because the other portions are formed into polygonal crystal grains. Particularly, when the area ratio is lower than 10%, the fracture surface unit in the toughness test increases, so that the propagation of the fracture surface becomes easy, and thus a fraction of the recrystallization recovery delay region needs to be 10% or more. It is preferable that the said area ratio is 20% or more and 40% or less.

[Polygonal area]

The polygonal region is a region in which the deformation region introduced by hot rolling is sufficiently recrystallized and recovered to become polygonal crystal grains. These crystal grains are also recovering from deformation and effectively contribute to the improvement of ductility. In addition, since the polygonal region has a relatively large grain boundary, the formation density of carbides is reduced, and the origin of the fracture surface decreases, which is also effective in improving toughness. As the shape of the polygonal crystal grains, when the crystal grains are elliptical approximation in observation of the L cross section of the steel sheet, and the maximum diameter of the crystal grains is defined as the long axis of the ellipse, the aspect ratio is preferably 1.0 or more and 1.8 or less. Because the aspect ratio exceeding 1.8 is under the influence of the whole body by rolling, it is disadvantageous for suppression of wavefront propagation. Further, the particle diameter is preferably 5 µm or more and 100 µm or less in terms of the long axis of the ellipse. When the particle diameter is less than 5 µm, the grain boundary, which is the site for forming carbides, increases, which is disadvantageous in reducing the origin of the fracture surface. On the other hand, when the particle diameter exceeds 100 µm, the wavefront unit increases, the wavefront propagation becomes easy, and toughness decreases. Moreover, as a ratio of the polygonal region occupied in the whole microstructure, it is preferable that it is 40% or more and 90% or less in area ratio. More preferably, they are 60% or more and 80% or less.

Accordingly, the austenite phase that forms the matrix (matrix) of the steel sheet is mainly defined in the polygonal region and the recrystallization recovery delay region. However, there is a possibility that there is a region that does not meet these regulations, for example, crystal grains with an aspect ratio of 1.0 or more and 1.8 or less and less than 5 µm, or a region that is recognized as a recrystallization recovery delayed region by the following observation method, but has an aspect ratio of less than 2.0. However, these are suppressed to 5% or less by the area ratio in the microstructure, and most of the austenite phases need to be formed as any of the polygonal regions and the recrystallization recovery delay region. That is, the matrix phase is a polygonal recrystallization region and a recrystallization recovery delay region of 10% or more and 50% or less in terms of area ratio.

Next, the identification method of these areas is described below.

Each of the above-described areas can be recognized by optimizing the method of adjusting the sample for SEM observation. Specifically, when the surface of the steel sheet is subjected to mirror polishing with colloidal silica and then ion-etched on the surface of the steel sheet by ion milling, fine irregularities are formed on the surface layer of the recrystallization recovery delay region, and thus, a low-speed SEM of 5 kV or less It becomes possible to identify by observing the in-lens structure and observing the reflected electron image by Further, even by performing mirror polishing using electrolytic polishing, it is possible to identify the recrystallization recovery delay region. As described above, as for the factor causing the contrast difference in the matrix image (mother image), a difference in hardness and deformation, a small amount of element distribution, and the like are considered, but the details are unknown. In the analysis, an area that has been recognized according to the above is binarized by image processing, and is defined as an area ratio.

In addition, it is also possible to identify each region by values such as image quality using EBSD (Electron Back Scattered Diffraction). However, in this case, since deformation due to polishing may be introduced to the sample surface during sample preparation, care must be taken when preparing the sample, and it is necessary to reliably remove the deformation of the surface layer by electrolytic polishing or ion polishing. There is.

As for the shape of the recrystallization recovery delay region, the small one is composed of a plurality of crystal grains of two or more, and the size (long axis) of the aggregate of the plurality of crystal grains is about 10 µm. On the other hand, the band structure (in the rolling direction of the plate) for a large region Accordingly, there are some that have a whole-body band-shaped structure), and the (width in the plate thickness direction (lamination)) width is 50 µm, and the length (length direction (long axis) of the belt-shaped spreading) is about 500 µm. . In Fig. 1, for three cases, the recrystallization recovery delay region can be clearly identified from the polygonal region as shown by the enclosed line in the figure, as the structure photographs of the reflective electron image are shown. That is, FIG. 1A is a photograph of a structure of 200 times, and a structure (recrystallization recovery delay region) that has been telegraphed in the rolling direction can be observed. In addition, FIG. 1B is a 500-fold image of the structure, and it can be seen that various types of non-recrystallized regions (recrystallization recovery delayed regions) are formed in the observation region.

Considering the above, the SEM structure observation is performed with an appropriate magnification for a field of view of about 300 × 500 μm per place for a depth position of 1/4 of the plate thickness (hereinafter referred to as 1/4 t part) from the surface of the steel plate. It is adjusted (200 times to 5000 times), the area of the recrystallization recovery delayed 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 locations, the average thereof is calculated, and the area ratio of the recrystallization recovery delay region is determined.

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

In the high Mn steel according to the present invention, molten steel having the above-described component composition can be dissolved by a known solvent method such as a converter or an electric furnace. Further, secondary refining may be performed in a vacuum degassing furnace. After that, it is preferable to use a known casting method such as a continuous casting method or an ingot-disintegration rolling method to obtain a steel material such as a slab having a predetermined size.

In addition, manufacturing conditions for making the steel material into a steel material having excellent low-temperature toughness will be described.

[Steel material heating temperature: 1100℃ or higher and 1300℃ or lower]

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, if it exceeds 1300°C, there is a fear that some dissolution may start, so the upper limit of the heating temperature is 1300°C. Temperature control here is based on the surface temperature of the steel material.

[Finish rolling end temperature: 750℃ or more and 850℃ or less]

The hot rolling end temperature and subsequent cooling conditions become important in controlling the recrystallization recovery delay region. When this temperature is higher than 850° C., recrystallization proceeds during final rolling and immediately after rolling, the formation of polygonal crystal grains is promoted, grain boundaries become large, and toughness becomes excessively high. Further, when the rolling temperature is lower than 750°C, the formation of polygonal crystal grains due to recrystallization is suppressed, and since a large amount of strain is introduced into the recrystallization recovery delay region, the strength increases and the toughness deteriorates.

[Cooling rate from finish rolling end temperature to 650°C: 5°C/s or less]

In order to achieve both the formation of polygonal crystal grains due to recrystallization/recovery and the remaining regions of delayed recrystallization recovery, it is very important to control cooling from the rolling end temperature to 650°C where recovery/recrystallization is remarkable. At this time, if the cooling rate is too high, the structure after rolling is frozen, the formation of sufficient polygonal grains does not occur, and toughness deteriorates, so the upper limit of the cooling rate is 5°C/s. Preferably, it is 3°C/s or less. In particular, in the case of a thin material, since plate warpage occurs as described above, which becomes a problem in the process, it is preferable to cool it 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 and recovery of the matrix (matrix), the cooling rate was regulated in the temperature range from the finish rolling end temperature to 650°C. On the other hand, cooling in a temperature range of less than 650°C may be performed arbitrarily as follows.

Here, since the cooling rate changes according to the plate thickness, it is advantageous to appropriately adjust by water cooling or the like. The cooling treatment here is performed based on the center temperature of the sheet thickness of the steel sheet. In addition, the said center temperature can be calculated|required by heat transfer calculation from the steel plate surface temperature measured with a radiation thermometer.

In addition, the lower limit of the cooling rate is not particularly set, but if a heating furnace or the like is used, the cost of the furnace, the process cost, and the manufacturing time are disadvantageous.

[For cooling below 650℃]

The present invention realizes improvement of toughness at low temperatures by combining the polygonal region and the recrystallization recovery delay region even in a situation where carbides are formed at the grain boundary. For this reason, about cooling below 650 degreeC, it does not specifically regulate. However, since carbide suppression is effective in toughness, and the influence of the above-described plate warpage is reduced in a temperature range of less than 650°C, it is preferable to perform rapid cooling of 10°C/s or more from the viewpoint of suppressing carbide formation.

Further, if necessary, after performing the cooling treatment (cooling below 650°C), a treatment of heating and cooling to a temperature range of 300°C or more and 650°C or less may be added. That is, you may perform tempering treatment for the purpose of adjusting the strength of a steel plate.

Example

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

(1) steel plate

By vacuum melting, a steel slab serving as the component composition shown in Table 1 was produced. Next, the obtained steel slab is charged into a heating furnace and heated to 1250°C, and then hot rolling is performed by varying the finish rolling end temperature, and the cooling rate in the temperature range from the finish rolling end temperature to 650°C is adjusted. It changed variously and performed cooling processing, and the steel plate of 5-20 mm thickness was produced. Here, in hot rolling, a thermocouple was installed in 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 cooling rates in the temperature range from the finish rolling finish temperature and finish rolling finish temperature to 650°C.

For the obtained steel sheet, the tensile test properties and low-temperature toughness were evaluated in the following manner, and further analyzed for the structure.

(2) Tensile test properties

From each of the obtained steel sheets, a JIS No. 5 tensile test piece was taken, and a tensile test was conducted in accordance with the regulations of JIS Z2241 (1998), and tensile test properties were investigated. In the present invention, a yield strength of 400 MPa or more and a tensile strength of 800 MPa or more were determined to be excellent in tensile properties. Further, 30% or more of elongation was determined to be excellent in ductility.

(3) low temperature toughness

At a position of 1/2 of the plate thickness from the surface of each steel plate, from a direction perpendicular to the rolling direction, in accordance with the regulations of JIS Z2202 (1998), a Charpy V notch test piece was taken, and JIS Z 2242 (1998). Three Charpy impact tests were performed on each steel sheet in accordance with the regulations of, and the absorbed energy at -196°C was calculated to evaluate the base metal toughness. In the present invention, the average value of the three absorbed energies (vE-196) was 100 J or more as excellent in base metal toughness. In addition, about a steel plate having a plate thickness of 10 mm or less, a half-size (5 mm) Charpy V-notch test piece was prepared and tested, and absorbed energy of 50 J or more was set as a pass.

(4) organizational analysis

Regarding the structure analysis, the structure was observed with a scanning electron microscope (FE-SEM) having an electrolytic emission gun and an in-lens type detector. That is, the sample prepared by embedding a steel sheet with resin was subjected to diamond polishing and mirror polishing with colloidal silica, and then sputtering of the surface with an Ar ion beam. The structure observation was performed with an acceleration voltage of 5 kV, the shape of the recrystallization recovery delayed region was evaluated, and the area ratio thereof was calculated. That is, the non-recrystallized region was extracted from each SEM image, and the region was traced. The area of the traced area was calculated using an image analysis software or the like, and the area ratio was calculated. The observation area was made into an area of 500 x 500 µm per place from a position of 1/4 of the plate thickness from the surface of the steel sheet, and this observation was performed at 10 locations to obtain an average value.

Table 3 shows the results of the evaluation and observation obtained above.

As shown in Table 3, the high Mn steel according to the present invention satisfies the above target performance (the yield strength of the base material is 400 MPa or more, and the low-temperature toughness is 100J or more as the average value of the absorbed energy (vE-196)). Confirmed. On the other hand, in the comparative example outside the scope of the present invention, any one or more of yield strength and low-temperature toughness does not satisfy the target performance described above.

Claims (4)

In mass%,
C: 0.10% or more and 0.70% or less,
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% or more and 7.0% or less,
N: 0.0050% or more and 0.0500% or less and
O: 0.0050% or less
Contains, the balance has a component composition of Fe and unavoidable impurities, and austenite is a matrix phase, and the matrix phase recovers from 10% to 50% by a polygonal recrystallization region and area ratio It has a microstructure as a retardation region, the recrystallization recovery retardation region is composed of a plurality of crystal grains having a diameter of 5 μm or less, and has an ellipse having a major axis of the rolling direction of the steel sheet or a shape approximating the ellipse, and the aspect of the ellipse High Mn steel having a ratio of 2.0 or more and the major axis of 10 μm or more. The method of claim 1,
The component composition is further mass%,
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 and
B: 0.0005% or more and 0.0020% or less
High Mn steel containing one or two or more selected from among. The steel material having the component composition according to claim 1 or 2 is heated in a temperature range of 1100°C or more and 1300°C or less, and hot rolling is performed so that the finish rolling end temperature is 750°C or more and less than 850°C, and the finishing A method for producing a high-Mn steel that performs a cooling treatment in which the average cooling rate in a temperature range from the rolling end temperature to 650°C is 5°C/s or less. The method of claim 3,
The method for producing a high Mn steel having the average cooling rate of 3°C/s or less.

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