KR102367801B1 - High Mn steel and manufacturing method thereof - Google Patents

Abstract

We propose a method for providing more excellent ductility in high-Mn steel having excellent low-temperature toughness in the base metal and in the heat-affected zone of welding. 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, 0.0500% or less, and O: 0.0050% or less, the balance has a component composition of Fe and unavoidable impurities, and austenite as a matrix phase, and the matrix phase is poly It has a smooth recrystallization region and a microstructure that is a recrystallization recovery delayed region of 10% or more and 50% or less by area ratio, and the recrystallization recovery delayed region is composed of a plurality of grains having a diameter of 5 μm or less, and the rolling direction of the steel sheet is It has an ellipse having a long axis or a shape approximate to the ellipse, and an aspect ratio of the ellipse is 2.0 or more and the long 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 providing structural steel used in a cryogenic environment, for example, a tank for a liquefied gas storage tank, etc., particularly excellent in toughness at a low temperature, and a method for manufacturing the same .

When a hot-rolled steel sheet is used for a structure for storing liquefied gas, the use environment becomes cryogenic, so that the steel sheet is required to have high strength and excellent toughness at cryogenic temperatures. For example, when using a hot-rolled steel sheet for a tank of liquefied natural gas, it is necessary to ensure the outstanding toughness at the boiling point of liquefied natural gas: -164 degreeC or less. If the low-temperature toughness of the steel material is inferior, since there is a possibility that the safety as a cryogenic storage structure cannot be maintained, the demand for improvement of the low-temperature toughness of the applied steel material is strong.

In response to this request, conventionally, an austenitic stainless steel, a 9% Ni steel, or a 5000 series aluminum alloy in which austenite, which does not exhibit brittleness at cryogenic temperatures, is used as the structure of the steel sheet has been used. However, since alloy cost and manufacturing cost are high, these materials are cheap, and there exists a request|requirement for the steel material excellent in cryogenic toughness.

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

Patent Document 1 proposes a technique of controlling the austenite grain size to an appropriate size and avoiding that carbides generated at grain boundaries serve as a fracture origin or a path for propagation of cracks. Moreover, in patent document 2, the technique which improves low-temperature toughness by suppressing segregation of Mn to a fixed or more is proposed.

Japanese Laid-Open Patent Publication No. 2016-196703 Japanese Laid-Open Patent Publication No. 2017-71817

In applications such as the structure for storing liquefied gas described above, since it is necessary to provide high workability to the steel material to be used, it is important to secure ductility in addition to low-temperature toughness. Regarding this ductility, nothing has been verified in the techniques described in Patent Documents 1 and 2. Moreover, although the thickness of the high Mn steel material described in patent document 1 is about 15-50 mm, depending on a use, for example, the thickness of less than 15 mm, especially 10 mm or less is calculated|required. When manufacturing such a thin plate, in the method of performing accelerated cooling after completion of hot rolling as illustrated in Patent Document 1, warpage or deformation tends to occur in the obtained steel plate, and extra steps such as shape correction are required, resulting in lower productivity. hindered Moreover, in patent document 2, in order to relieve|moderate segregation, the heat processing for a long time is required, and it is disadvantageous in the point of productivity.

Then, an object of this invention is to propose about the method for providing further outstanding ductility in the high Mn steel excellent in the low-temperature toughness of a base material and a weld heat-affected zone. In addition, an object of the present invention is to propose a method capable of manufacturing such a thin plate of high Mn steel without occurrence of warpage or deformation.

Here, the said "excellent low-temperature toughness" means that the absorbed energy vE-196 of the Charpy impact test in -196 degreeC is 100 J or more.

In order to solve the above problems, the inventors of the present inventors earnestly study various factors determining the component composition and manufacturing method of a steel sheet for high Mn steel, and investigate the relationship with the microstructure, and recognize the following came to obtain

First, high Mn steel does not undergo brittle fracture even at cryogenic temperatures, and when fracture occurs, it occurs from grain boundaries. That is, it has been found that the shape of the grain boundaries greatly affects the toughness. In particular, it was found that carbides and the like are formed at grain boundaries, and the distribution of these carbides and the shape of grain boundaries have a great influence on toughness. Specifically, in the case of fine grains, the number of carbide formation sites as the starting point of fracture increases and the toughness value decreases. Conversely, in the case of coarse grains, the number of origins decreases because there is no carbide, but the propagation of the fracture surface becomes easy and the toughness value decreases.

As this method of suppressing the formation of carbides, rapidly cooling the steel sheet (hereinafter also referred to as rapid cooling) is an effective means. However, in the case of a thin material having a thickness of a steel sheet of 20 mm or less, when the steel sheet is rapidly cooled, sheet warpage due to internal stress due to thermal deformation may occur. In particular, in the case of high Mn steel, since the structure is austenite, the plate warpage tends to be large compared to that of ferritic steel. When this board|plate curvature generate|occur|produces, it becomes difficult to insert a board|board into the surface smoothing process line etc. which are a process after cooling. Moreover, in order to ship, it is necessary to correct the curvature of a steel plate, and since a new process must be added to a manufacturing line, it causes an increase in manufacturing cost. It is inferred that the cause of the increase in the warpage of austenitic steel is that the thermal conductivity is small and the temperature distribution is large compared to that of ferritic steel, but the details are unknown.

Here, in the actual manufacturing line, when the thickness of the steel sheet and the cooling rate for cooling after hot rolling are changed, the results are summarized for the situation in which the load on the manufacturing process is generated due to the bending of the steel sheet, 1 is shown. As shown in this Table 1, when the cooling rate exceeded 5 degreeC/s, it turned out that a problem arises on a process with the steel plate with a plate|board thickness of 20 mm or less.

In addition, the load on the manufacturing process, which is an evaluation standard in Table 1, means the load required for maintenance, calibration, etc. with respect to the intermediate product or product in each process. And, in Table 1, ◎ indicates that the above-mentioned care or correction is unnecessary, and the transfer to the material holder such as the flat correcting device (leveler) and the cooling bed, which is a post-production operation, passes and conveys online without any problem. If it was possible, ○ is when mail-ordering is possible by performing a slight smoothing process such as adjusting the leveler opening at every manufacturing opportunity, △ is a mild correction work (individual work by an operator) , and manual processing deformation correction) is necessary, and × indicates that there was a problem in manufacturing and shipping itself as a product because it was impossible to correct.

Therefore, when it is difficult to apply a quenching treatment effective for suppressing carbides in this way, even if carbides are present, a method for improving toughness by controlling the crystal grain shape of the matrix was studied. That is, under the condition in which carbides exist, the influence between the shape of the mother-phase grains and the toughness was intensively studied. As a result, it was found that, by combining a region made of fine grains with polygonal grains, it is possible to simultaneously reduce the origin of fracture and suppress wavefront propagation, thereby improving the toughness value.

As a result of analyzing the structure contributing to the improvement of the toughness value described above, the region made of fine grains is a region where recrystallization and recovery are delayed after rolling, and the polygonal region is at a relatively early stage after rolling. It was found that this was an area where recrystallization and recovery were taking place. For this reason, it was discovered that creation of such a structure|tissue becomes possible by optimization of the temperature condition of hot rolling, and also cooling condition after rolling in addition to component adjustment. In particular, it was discovered that by adding Cr, control of the region in which non-recrystallization and recovery was suppressed became easy. In addition, by setting the finish rolling termination temperature to 750 ° C. to 850 ° C. and cooling thereafter at a cooling rate of 5 ° C./s or less, the recrystallization recovery delay region is formed, which can achieve both strength and toughness. found that

This invention is made|formed by adding examination further to the above recognition, The summary is as follows.

1. 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 as a matrix phase, and recrystallization recovery of 10% or more and 50% or less in a polygonal recrystallization region and area ratio It has a microstructure that is 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 or a shape close to the ellipse with the long axis of the rolling direction of the steel sheet as the major axis, and the aspect of the ellipse A high Mn steel having a ratio of 2.0 or more and the long axis of 10 μm or more.

2. The component composition is further in 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, comprising one or two or more selected from among.

3. The steel material having the component composition described in 1 or 2 above is heated to a temperature range of 1100°C or higher and 1300°C or lower, hot-rolling such that the finish rolling end temperature is 750°C or higher 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 a temperature range from the end temperature to 650°C is 5°C/s or less.

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

ADVANTAGE OF THE INVENTION According to this invention, high Mn steel excellent in low-temperature toughness can be provided. When this high Mn steel is used for welding, both the base metal 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 the safety and lifespan of steel structures used in cryogenic environments, such as tanks for storing liquefied gas, and brings special industrial effects. In addition, the manufacturing 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 manufacturing cost, so that a manufacturing method excellent in economy can be provided.

1A is a tissue photograph by SEM.
Figure 1b is a tissue photograph by SEM.

(Form for implementing 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, unless otherwise indicated, "%" indication in a component composition shall mean "mass %".

C: 0.10% or more and 0.70% or less

C is an inexpensive austenite stabilizing element, and is an important element in order to obtain austenite. In order to obtain the effect, C needs to contain 0.10% or more. On the other hand, when it contains exceeding 0.70%, Cr carbide will generate|occur|produce excessively, and low-temperature toughness will fall. For this reason, C is made into 0.10% or more and 0.70% or less. Preferably, you may be 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 dissolves in steel and has an effect of strengthening the steel sheet by solid solution strengthening. In order to obtain 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 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 when it contains exceeding 30.0%, the effect of improving the cryogenic toughness is saturated, and an increase in alloy cost is caused. Moreover, weldability and cutability deteriorate. In addition, it promotes segregation and promotes the occurrence of stress corrosion cracking. For this reason, Mn is made into 15.0 % or more and 30.0 % or less. Preferably, you may be 18.0 % or more and 28.0 % or less.

P: 0.030% or less

When P contains more than 0.030 %, it will segregate at a grain boundary and will become a generation|occurrence|production origin of stress corrosion cracking. For this reason, it is preferable to make 0.030 % as an upper limit, and to reduce as much as possible. Therefore, P is made into 0.030% or less. Preferably, it is 0.028 % or less, More preferably, it is set as 0.024 % or less. Of course, 0% may be sufficient. In addition, in order to reduce P to less than 0.002%, it is preferable that it is 0.002% or more from a viewpoint of economical efficiency at the point which requires a large cost for refining.

S: 0.0070% or less

Since S deteriorates the low-temperature toughness and ductility of a base material, it is preferable to make 0.0070% as an upper limit and to reduce it as much as possible. Therefore, S is made into 0.0070% or less. Preferably it is made into 0.0050% or less. Of course, 0% may be sufficient. In addition, in order to reduce S by less than 0.0005%, it is preferable that it is 0.0005% or more from a viewpoint of economical efficiency at the point which requires large cost for refining.

Al: 0.01% or more and 0.07% or less

Al acts as a deoxidizer and is most commonly used in the 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, when it contains exceeding 0.07 %, since it mixes in 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, it is 0.02 % or more and 0.06 % or less.

Cr: 2.5% or more and 7.0% or less

Cr is an element effective for stabilizing austenite by adding an appropriate amount to improve cryogenic toughness and strength of the base material. In addition, it is an effective element for forming a recrystallization recovery delay region to be described later. In order to obtain such an effect, it is necessary to contain Cr at 2.5% or more. On the other hand, when it contains exceeding 7.0%, low-temperature toughness and stress corrosion cracking resistance will fall by generation|generation of Cr carbide. For this reason, Cr is made into 2.5 % or more and 7.0 % or less. Preferably, it is set as 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 for improving the cryogenic toughness. In order to obtain such an effect, N needs to contain 0.0050% or more. On the other hand, when it contains exceeding 0.0500 %, nitride or carbonitride will coarsen and toughness will fall. For this reason, N is made into 0.0050 % or more and 0.0500 % or less. Preferably, it is set as 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 the range of 0.0050% or less. Preferably, it is 0.0045% or less. Of course, 0% may be sufficient. In addition, 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 at the point which requires large cost for refining.

The remainder other than the above components is iron and unavoidable impurities. Ca, Mg, Ti, Nb, Cu, etc. are mentioned as an unavoidable impurity here, and if it is 0.05 % or less in total, it is permissible.

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 can be contained as needed.

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 or 2 or more types can be added.

Mo, V, W: 2.0% or less each

Mo, V, and W contribute to the improvement of the strength of the base metal while contributing to the stabilization of austenite. 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, when it contains exceeding 2.0 %, a coarse carbonitride may produce|generate and it may become a starting point of destruction, and manufacturing cost will be pressed. For this reason, when these alloy elements are contained, the content shall be 2.0 % or less, respectively. Preferably it is 0.003 % or more and 1.7 % or less, More preferably, it is set as 1.5 % or less.

REM: 0.0010% or more and 0.0200% or less

REM is an element useful for shape control of inclusions, and may be contained as needed. The shape control of inclusions refers to making the extended sulfide-based inclusions into particle-shaped inclusions. Through control of the shape of these inclusions, the ductility, toughness and resistance to sulfide stress corrosion cracking are improved. In order to obtain such an effect, it is preferable to contain 0.0010% or more of REM. On the other hand, when it contains excessively, the amount of nonmetallic inclusions may increase and ductility, toughness, and sulfide stress corrosion cracking resistance may fall on the contrary. Accordingly, the amount of REM is preferably 0.0015% or more and 0.0200% or less.

B: 0.0005% or more and 0.0020% or less

B segregates at grain boundaries and contributes to the improvement of toughness due to grain boundary strength of the material. However, when it is added excessively, coarse nitride or carbide is formed, so the amount of addition is preferably 0.0005% or more and 0.0020% or less.

Next, the structure form for realizing low-temperature toughness is demonstrated.

[Microstructure with austenite as matrix]

When the crystal structure of the steel material is a body-centered cubic structure (bcc), since the steel material may cause brittle fracture in a low-temperature environment, it is not suitable for use in a low-temperature environment. Here, when use in a low-temperature environment is assumed, it becomes essential that the matrix phase in the structure of steel materials be austenite whose crystal structure is a face-centered cubic structure (fcc). In addition, "use austenite as a matrix phase" 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 balance other than the austenite phase is composed of a ferrite or martensite phase having a BCC structure, inclusions, or precipitates, but the proportion thereof is preferably 5% or less. In addition, about the austenite fraction, it can determine by observation by EBSD, analysis by XRD, magnetic permeability, etc.

[Micro tissue morphology]

The present invention realizes improvement in low-temperature toughness by controlling the microstructure, particularly the austenite structure, in the hot rolling and subsequent cooling processes. To do so, it is important to control the morphology of the microstructure. In particular, during hot rolling or in the cooling process after hot rolling, recrystallization/recovery proceeds rapidly to have polygonal grains, that is, a polygonal region; That is, it is important to improve toughness by appropriately providing a recrystallization recovery delay region to reduce the origin of a fracture and suppress fracture growth. Hereinafter, the shape of each area mentioned above 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 strain 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, and basically reflects the rolling structure and spreads in the rolling direction, and shows a shape in which a set of a plurality of crystal grains becomes an elliptical shape. That is, the recrystallization recovery delay region has an ellipse with the long axis of the rolling direction or a shape close 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), and the aspect ratio of the ellipse is 2.0 or more Moreover, the said long axis is 10 micrometers or more. Although the method of recognizing this area will be described later, the organization control for the above-described shape is performed.

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

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 a structure with a lot of deformation, which is disadvantageous in terms of ductility. In addition, since the number of carbides formed at the grain boundaries of the recrystallization recovery delay region and the like increases, the origin of the fracture surface increases, which is also disadvantageous in terms of toughness. For this reason, in the microstructure, the upper limit of the ratio occupied by the recrystallization recovery delayed region was 50% in terms of area ratio. On the other hand, when this area ratio is lower than 10 %, since the other part is formed with polygonal crystal grains, the intensity|strength of material falls. In particular, when the area ratio is lower than 10%, since the fracture front unit increases in the toughness test and the fracture front progresses easily, the recrystallization recovery delay region needs a fraction of 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 strain region introduced by hot rolling has sufficiently recrystallized and recovered to become polygonal grains. These crystal grains are also recovering strain and effectively contribute to the improvement of ductility. In addition, since the polygonal region has a relatively large grain boundary, the density of carbide formation is reduced, and the origin of the fracture surface is reduced, which is effective for improving toughness. As the shape of this polygonal crystal grain, when observing the L section of the steel sheet, when the crystal grain is elliptical and the maximum diameter of the crystal grain is defined as the major axis of the ellipse, the aspect ratio is preferably 1.0 or more and 1.8 or less. This is because, when the aspect ratio exceeds 1.8, the whole body is affected by rolling, which is disadvantageous in suppression of wavefront propagation. Moreover, as a particle diameter, it is preferable that they are 5 micrometers or more and 100 micrometers or less in conversion of the long axis of the said ellipse. When this particle diameter becomes less than 5 micrometers, since the grain boundary which is a formation site of carbides increases, it is disadvantageous in reducing the origin of a wavefront. On the other hand, when the particle diameter exceeds 100 µm, the fracture front unit becomes large, the fracture front propagation becomes easy, and the toughness decreases. Moreover, as a ratio of the polygonal area|region which occupies for the whole microstructure, it is preferable that they are 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 forming the matrix phase (parent phase) of the steel sheet is mainly defined in the polygonal region and the recrystallization recovery delay region. However, regions that do not meet these regulations, for example, crystal grains having an aspect ratio of 1.0 or more and 1.8 or less and less than 5 μm, or a region with an aspect ratio of less than 2.0 although recognized as a recrystallization recovery delayed region by the following observation method, are unlikely to exist. However, these are suppressed to 5% or less in terms of the area ratio in the microstructure, and most of the austenite phase needs to be formed as one of the polygonal region and the recrystallization recovery delayed 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 area ratio.

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

Each of the above regions can be recognized by optimizing the adjustment method of the sample for SEM observation. Specifically, if the surface of the steel sheet is mirror polished with colloidal silica and then ion-etched on the surface layer of the steel sheet by ion milling, fine irregularities are formed in the surface layer of the recrystallization recovery delay region, so a low-acceleration SEM of 5 kV or less It becomes possible to identify by observation of in-lens tissue and observation of reflected electron images. Further, even by performing mirror polishing using electropolishing, it is possible to identify the recrystallization recovery delay region. As described above, about the factor causing the contrast difference in the matrix phase (matrix phase), differences in hardness and strain, distribution of trace elements, and the like are considered, but the details are unknown. Analysis binarizes the area|region which was recognized according to the above by image processing, and defines it as an area ratio.

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

As the shape of the recrystallization recovery delay region, a small one consists of two or more plural crystal grains, the size (long axis) of the aggregate of the plural crystal grains is about 10 μm, while for a large region, a band structure (in the rolling direction of the plate) There are also some that have a band-like structure that spreads along the body), have a width (width in the plate thickness direction (laminated)) of 50 μm, and a length (length direction (long axis) of the band that is spreading throughout the body) of about 500 μm. . As shown in Fig. 1 for three cases, the tissue photograph by the reflected electron image, the recrystallization recovery delay region is clearly distinguishable from the polygonal region as indicated by the encircling line in the figure. That is, FIG. 1A is a photograph of the tissue at a magnification of 200, and the tissue (recrystallization recovery delay region) whole body in the rolling direction can be observed. In addition, FIG. 1b is a photograph of the structure at a magnification of 500, and it can be seen that various types of non-recrystallized regions (recrystallization recovery delayed regions) are formed in the observation region.

In consideration of the above, the SEM structure observation is appropriately magnified for a field of view of about 300 × 500 µm per location at a depth position of 1/4 of the sheet thickness from the surface of the steel sheet (hereinafter referred to as 1/4 t part). It adjusts (200 times - 5000 times), the area of the recrystallization recovery delay area|region within the same visual field is measured, and the area ratio in this visual field is computed. This operation is performed in at least 10 places, the average is calculated, and it is set as the area ratio of the recrystallization recovery delay area|region.

For polygonal crystal grains (polygonal region), SEM observation is performed at a magnification of 1000, and 100 or more crystal grains are recognized. Moreover, in this case, in combination with EBSD, you may measure the size of the crystal grain in the area|region except the recrystallization recovery delay area|region recognized by SEM observation.

The high-Mn steel according to the present invention can be prepared by melting molten steel having the above-described component composition by a known melting method such as a converter or an electric furnace. Moreover, you may perform secondary refining 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-ingot rolling method to obtain a steel material such as a slab having a predetermined size.

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

[Steel material heating temperature: 1100℃ or more and 1300℃ or less]

In order to make the crystal grain size of the microstructure of steel materials coarse, the heating temperature before hot rolling shall be 1100 degreeC or more. However, when it exceeds 1300 degreeC, since there exists a possibility that some melt|dissolution may start, the upper limit of heating temperature shall be 1300 degreeC. The temperature control here is based on the surface temperature of the steel material.

[Finishing 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 and immediately after final rolling, the formation of polygonal crystal grains is promoted, and the grain boundary becomes large and toughness becomes excessively high. In addition, when the rolling temperature is lower than 750° C., the formation of polygonal grains due to recrystallization is suppressed, and since a large amount of strain is also introduced into the recrystallization recovery delayed region, the strength increases and the toughness deteriorates.

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

In order to achieve both the formation of polygonal grains by recrystallization and recovery and the remaining of the delayed recrystallization recovery region, it is very important to control cooling from the rolling end temperature to 650° C., where recovery and recrystallization progress significantly. At this time, if the cooling rate is too fast, the structure after rolling is frozen, and the formation of sufficient polygonal ribs does not occur and the toughness deteriorates. Therefore, the upper limit of the cooling rate is set to 5°C/s. Preferably, it is made into 3 degreeC/s or less. In particular, in the case of a thin material, it is preferable to cool at a rate of 3° C./s or less, because plate warpage occurs as described above and becomes a problem in the process. In addition, since cooling in a temperature range below 650 degreeC does not affect the recrystallization and recovery|recovery of a matrix phase (base phase), regulation of the cooling rate was made into the temperature range from the finish rolling completion temperature to 650 degreeC. In addition, you may perform cooling in the temperature range below 650 degreeC arbitrarily as follows.

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

Further, the lower limit of the cooling rate is not particularly set, but if a thermal insulation furnace or the like is used, it is disadvantageous in terms of furnace cost, process cost, and manufacturing time. Therefore, it may be within the range of air cooling.

[About cooling below 650℃]

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

Moreover, after performing the said cooling process (cooling below 650 degreeC) as needed, you may add the process of heating and cooling to the temperature range of 300 degreeC or more and 650 degrees C or less. That is, you may perform a tempering process for the purpose of adjusting the intensity|strength of a steel plate.

Example

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

(1) steel plate

By vacuum melting, steel slabs having the component compositions shown in Table 1 were produced. Next, the obtained steel slab is charged into a heating furnace, heated to 1250°C, and hot-rolled by varying the finish rolling end temperature in various ways, and the cooling rate in the temperature range from the finish rolling end temperature to 650°C is determined. Various changes were made and the cooling treatment was performed to produce a steel sheet having a thickness of 5 to 20 mm. Here, in the 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 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 way, and also the structure was analyzed.

(2) Tensile test properties

From each obtained steel plate, JIS No. 5 tensile test piece was extract|collected, the tensile test was implemented based on the prescription|regulation of JIS Z2241 (1998), and the tensile test characteristic was investigated. In this invention, 400 MPa or more of yield strength and 800 MPa or more of tensile strength were judged as being excellent in a tensile characteristic. Moreover, 30% or more of elongation was determined as being excellent in ductility.

(3) low temperature toughness

A Charpy V-notch test piece was taken from the surface of each steel plate at a position of 1/2 of the plate thickness from a direction perpendicular to the rolling direction in accordance with JIS Z2202 (1998), and JIS Z 2242 (1998) Three Charpy impact tests were performed on each steel sheet in accordance with the provisions of In this invention, the average value of three absorbed energies (vE-196) made 100J or more excellent in base material toughness. Moreover, about the steel plate with a plate|board thickness of 10 mm or less, a half-size (5 mm) Charpy V-notch test piece was produced and tested, and absorbed energy made 50 J or more a pass.

(4) tissue analysis

For tissue analysis, tissue observation was performed with an electrolytic emission gun and a scanning electron microscope (FE-SEM) having an in-lens detector. That is, after performing mirror polishing with diamond polishing and colloidal silica for a sample produced by embedding a steel sheet in resin, surface sputtering was performed with an Ar ion beam. The structure observation was performed with the acceleration voltage of 5 kV, the shape of the recrystallization recovery delayed area|region was evaluated, and the area ratio was calculated. That is, the non-recrystallized 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 area was made into an area|region of 500x500 micrometers per location from the position of 1/4 of the board thickness from the surface of a steel plate, this observation was performed at 10 locations, and it was made into the average value.

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

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

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 as a matrix phase, and recrystallization recovery of 10% or more and 50% or less in a polygonal recrystallization region and area ratio It has a microstructure that is 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 or a shape close to the ellipse with the long axis of the rolling direction of the steel sheet as the major axis, and the aspect of the ellipse High Mn steel with a ratio of 2.0 or more and the long axis of 10 µm or more. According to claim 1,
The component composition is further in 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
A 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 to 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 is performed. By performing a cooling treatment in which the average cooling rate in the temperature range from the rolling end temperature to 650 °C is 5 °C/s or less,
With austenite as a matrix phase, the matrix phase has a polygonal recrystallization region and a microstructure that is a recrystallization recovery delay region of 10% or more and 50% or less in area ratio, and the recrystallization recovery delay region has a diameter of 5 To obtain high Mn steel composed of a plurality of crystal grains of ㎛ or less, and having an ellipse or a shape close to the ellipse with the major axis in the rolling direction of the steel sheet, the aspect ratio of the ellipse being 2.0 or more and the long axis of 10 μm or more , a method for manufacturing high Mn steel. 4. The method of claim 3,
The method for manufacturing high Mn steel, wherein the average cooling rate is 3° C./s or less.

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