JPWO2019156179A1 - High Mn steel and method for producing the same - Google Patents

Abstract

We propose a way to give more excellent ductility to high Mn steel with excellent low temperature toughness of base metal and weld heat affected zone. C: 0.10% to 0.70%, Si: 0.05% to 1.00%, Mn: 15.0% to 30.0%, P: 0.030% or less, S: 0 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% And the remainder has a component composition of Fe and unavoidable impurities, and has austenite as a base phase, and the base phase has a polygonal recrystallization region and a recrystallization recovery of 10% or more and 50% or less in area ratio. The recrystallization recovery delay region has a microstructure that is a delay region, and the recrystallization recovery delay region is formed of a plurality of crystal grains having a diameter of 5 μm or less, and has an ellipse or a shape similar to the ellipse whose major axis is the rolling direction of the steel sheet. The aspect ratio of the ellipse is 2.0 or more and the major axis is 10 μm or less. To.

Description

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

  When a hot-rolled steel sheet is used for a structure for a liquefied gas storage tank, the use environment is extremely low. Therefore, the steel sheet is required to have not only high strength but also excellent toughness at an extremely low temperature. For example, when a hot-rolled steel sheet is used for a liquefied natural gas storage tank, excellent toughness must be ensured at a liquefied natural gas boiling point of -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 a cryogenic storage tank may not be maintained, and 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 having a structure of a steel sheet of austenitic material that does not show brittleness at cryogenic temperatures, 9% Ni steel, or 5000-type aluminum alloy 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, a high-Mn steel to which a large amount of relatively inexpensive austenite stabilizing element Mn is added is used as a structural steel in a cryogenic environment. Proposed in reference 1.

  Patent Literature 1 proposes a technique in which the austenite grain size is controlled to an appropriate size to prevent carbides generated at crystal grain boundaries from becoming fracture starting points or crack propagation paths. Patent Document 2 proposes a technique in which low-temperature toughness is improved by suppressing Mn segregation to a certain level or more.

JP 2016-196703 A JP 2017-71817 JP

  In the use of the above-mentioned structure for a liquefied gas storage tank or the like, since it is necessary to provide a steel material having high workability, it is important to ensure ductility in addition to low-temperature toughness. Nothing has been verified on the ductility with the techniques described in Patent Documents 1 and 2. The high Mn steel material described in Patent Document 1 has a thickness of about 15 to 50 mm. For example, depending on the application, a thickness of less than 15 mm, particularly 10 mm or less is required. When such a thin plate is manufactured, the method of performing accelerated cooling after the completion of hot rolling, as exemplified in Patent Document 1, tends to cause warpage or distortion in the obtained steel sheet, and requires extra steps such as shape correction. It becomes necessary and productivity is hindered. In Patent Document 2, a long-time heat treatment is required to reduce segregation, which is disadvantageous in terms of productivity.

Therefore, an object of the present invention is to propose a method for giving more excellent ductility to a high Mn steel excellent in low-temperature toughness of a base material and a weld heat affected zone. A further 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 absorption 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 have conducted intensive studies on the composition of steel sheets and various factors that determine the manufacturing method for high Mn steels, investigated the relationship with the microstructure, and obtained the following findings. Came to get.
First, a high Mn steel does not become brittle even at a very low temperature, and when a fracture occurs, it is generated from a grain boundary. That is, it was found that the shape of the crystal grain boundaries greatly affected the toughness. In particular, it has been found that carbides and the like are formed at the grain boundaries, and that the distribution of the carbides and the form of the grain boundaries greatly affect the toughness. Specifically, when fine crystal grains are formed, the number of carbide forming sites increases as a starting point of fracture, and the toughness value decreases. Conversely, in the case of coarse crystal grains, the starting point is reduced due to the absence of carbides, but the propagation of the fracture surface is facilitated and the toughness value is reduced.

  As a method for suppressing the formation of carbides, rapid cooling (hereinafter, also referred to as rapid cooling) of a steel sheet is an effective means. However, when the thickness of the steel sheet is 20 mm or less, if the steel sheet is rapidly cooled, the sheet may be warped due to internal stress due to thermal strain. In particular, in the case of a high-Mn steel, since the structure is austenite, the warpage tends to be larger than that of a 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. Further, in order to ship the steel sheet, it is necessary to correct the warpage of the steel sheet, and a new process must be added to the production line, which leads to an increase in the production cost. The reason why the warpage of the austenitic steel is increased is presumed to be that the thermal conductivity is small and the temperature distribution is large as compared with 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 cooling after hot rolling are changed, the results obtained by summarizing the situation in which the load in the production process occurs due to the warpage of the steel sheet are shown in the table. It is shown in FIG. As shown in Table 1, it was found that when the cooling rate exceeded 5 ° C./s, a problem occurred in the process for a steel sheet having a thickness of 20 mm or less.

  The load in the manufacturing process, which is an evaluation criterion in Table 1, means the load required for care, correction, and the like for intermediate products and products in each process. In Table 1, ◎ indicates that the above-mentioned maintenance and straightening is not required, and the work after production, which is a transfer to a flattening device (leveler) and a material storage place such as a cooling bed, is passed and transferred online without any problem. If successful, ○ indicates a slight smoothing process that adjusts the opening of the leveler for each manufacturing opportunity, and if it becomes possible to pass through the plate. In the case where it is necessary to perform (manually judge the work and correct the work deformation manually), x indicates that the work cannot correct the work and there is a problem in shipping itself as a product.

  Therefore, when it is difficult to apply a quenching treatment effective for suppressing carbides, a method for improving the toughness by controlling the crystal grain morphology of the parent phase even when carbides are present was studied. That is, intensive studies were made on the influence of the morphology and toughness of the matrix crystal grains under the conditions in which carbides were present. As a result, it has been found that by combining a region composed of fine crystal grains and polygonal crystal grains, it is possible to simultaneously reduce the starting point of fracture and suppress the propagation of a fracture surface, thereby improving the toughness value.

  As a result of analyzing the structure contributing to the improvement of the toughness value described above, a region composed of fine crystal grains is a region where recrystallization / recovery delay has occurred after rolling, and a polygonal region is a region after rolling. It was found that this was a region where recrystallization and recovery occurred at a relatively early stage. For this reason, it has been found that incorporation of such a structure can be achieved by optimizing the temperature conditions of hot rolling and the cooling conditions after rolling, in addition to component adjustment. In particular, it has been found that the addition of Cr makes it easier to control the region where non-recrystallization / recovery is suppressed. Further, the finish rolling finish temperature is set to 750 ° C. to 850 ° C., and the subsequent cooling is performed at a cooling rate of 5 ° C./s or less, whereby the recrystallization recovery delay region is formed, and both strength and toughness are achieved. I found that I got it.

The present invention has been made by further studying the above findings, and the gist thereof 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, the balance has a component composition of Fe and unavoidable impurities, and austenite is a base phase, and the base phase is polygonal. A recrystallization region and a microstructure that is a recrystallization recovery delay region having an area ratio of 10% or more and 50% or less, wherein 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 the steel sheet or a shape similar to the ellipse, wherein the aspect ratio of the ellipse is 2.0 or more and the major axis is 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,
2. The high Mn steel according to the above 1, which contains 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 described in 1 or 2 above is heated to a temperature range of 1100 ° C. or more and 1300 ° C. or less, and subjected to hot rolling such that the finish rolling end temperature becomes 750 ° C. or more and less than 850 ° C. A method for producing a high Mn steel, wherein a cooling process is performed at an average cooling rate of 5 ° C./s or less in a temperature range from an end temperature to 650 ° C.

4. 4. The method for producing a high Mn steel according to the above item 3, 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 the high Mn steel is used for welding, both the base metal and the heat affected zone after welding have excellent low-temperature toughness. Therefore, the high Mn steel of the present invention greatly contributes to the improvement of safety and life of steel structures used in an extremely low temperature environment, such as a tank for a liquefied gas storage tank, and has a remarkable industrial effect. Further, the production method of the present invention can produce a high Mn steel excellent in the low-temperature toughness without causing a decrease in productivity and an increase in production cost. it can.

It is a structural photograph by SEM. It is a structural photograph by SEM.

Hereinafter, the high Mn steel of the present invention will be described in detail.
[Component composition]
First, the component composition of the high Mn steel of the present invention and the reasons for the limitation will be described. In addition, “%” in the component composition means “% by mass” unless otherwise specified.
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 be contained at 0.10% or more. On the other hand, when the content exceeds 0.70%, Cr carbide is excessively generated, and the low-temperature toughness decreases. Therefore, C is set to 0.10% or more and 0.70% or less. Preferably, it is at least 0.20% and at most 0.60%.

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 the effect of forming a solid solution in steel and strengthening the steel sheet by solid solution strengthening. . To obtain such an effect, the content of Si needs to be 0.05% or more. On the other hand, if the content exceeds 1.00%, the weldability deteriorates. Therefore, Si is set to 0.05% or more and 1.00%. Preferably, it is 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 for achieving both strength and cryogenic toughness. In order to obtain the effect, Mn needs to be contained at 15.0% or more. On the other hand, if the content exceeds 30.0%, the effect of improving the cryogenic toughness is saturated, leading to an increase in alloy cost. Also, the weldability and the cuttability deteriorate. Further, it promotes segregation and promotes the occurrence of stress corrosion cracking. Therefore, Mn is set to 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 excess of 0.030%, it segregates at the grain boundary and becomes a starting point of stress corrosion cracking. For this reason, it is desirable to set the upper limit to 0.030% and reduce as much as possible. Therefore, P is set to 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%, refining requires a great deal of cost. Therefore, from the viewpoint of economy, it is preferably 0.002% or more.

S: 0.0070% or less Since S deteriorates the low-temperature toughness and ductility of the base material, the upper limit is 0.0070%, and it is desirable to reduce as much as possible. Therefore, S is set to 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 cost is required for refining. Therefore, from the viewpoint of economic efficiency, it is preferably 0.0005% or more.

Al: 0.01% or more and 0.07% or less Al acts as a deoxidizing agent and is most commonly used in the molten steel deoxidizing process of steel sheets. To obtain such an effect, Al needs to be contained at 0.01% or more. On the other hand, if the content exceeds 0.07%, it is mixed into the weld metal portion during welding and deteriorates the toughness of the weld metal. 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 that stabilizes austenite by adding an appropriate amount thereof and is effective for improving cryogenic toughness and base metal strength. Further, it is an element effective for forming a recrystallization recovery delay region 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 the content exceeds 7.0%, low-temperature toughness and stress corrosion cracking resistance are reduced due to generation of Cr carbide. For this reason, Cr is set to 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 for improving cryogenic toughness. To obtain such an effect, N needs to be contained at 0.0050% or more. On the other hand, if the content exceeds 0.0500%, nitrides or carbonitrides are coarsened and toughness is reduced. Therefore, N is set to 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 cryogenic toughness. For this reason, O is in a range of 0.0050% or less. Preferably, it is 0.0045% or less. Of course, it may be 0%. In order to reduce O to less than 0.0005%, refining requires a large cost. Therefore, from the viewpoint of economy, it is preferably 0.0005% or more.

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

In the present invention, for the purpose of further improving strength and low-temperature toughness, the following elements can be contained as necessary 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 One or two or more can be added.

Mo, V, W: 2.0% or less each Mo, V, and W contribute to stabilization of austenite and also to improvement of base metal strength. In order to obtain such an effect, Mo, V and W are preferably contained 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 impose a manufacturing cost. Therefore, when these alloying elements are contained, their contents are each set to 2.0% or less. Preferably it is 0.003% or more and 1.7% or less, more preferably 1.5% or less.

REM: 0.0010% 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 morphological control of inclusions refers to making expanded sulfide-based inclusions into granular inclusions. Through this morphological control of inclusions, the ductility, toughness and sulfide stress corrosion cracking resistance are improved. In order to obtain such an effect, REM is preferably contained at 0.0010% or more. On the other hand, if it is contained excessively, the amount of nonmetallic inclusions increases, and on the contrary, ductility, toughness, and sulfide stress corrosion cracking resistance may decrease. Therefore, it is preferable that the amount of REM is 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 in toughness due to the grain boundary strength of the material. However, if excessively added, coarse nitrides and carbides are formed, so the amount of addition is preferably 0.0005% or more and 0.0020% or less.

Next, a description will be given of a microstructure for realizing low-temperature toughness.
[Microstructure with austenite as base phase]
When the crystal structure of a steel material is a body-centered cubic structure (bcc), the steel material is not suitable for use in a low-temperature environment because it may cause brittle fracture in a low-temperature environment. Here, assuming use in a low-temperature environment, it is essential that the base phase in the structure of the steel material be austenite whose crystal structure is a face-centered cubic structure (fcc). The phrase "austenite as the base phase" indicates that the austenite phase has an area ratio of 90% or more 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, inclusions and precipitates, and the ratio thereof is preferably 5% or less. The austenite fraction can be determined by observation with EBSD, analysis by XRD, magnetic permeability, and the like.

[Microstructure morphology]
The present invention achieves improvement in low-temperature toughness by controlling microstructure, particularly austenite structure, in the hot rolling and the subsequent cooling process. For that purpose, it is important to control the morphology of the microstructure. In particular, in the cooling process during and after hot rolling, recrystallization / recovery progresses rapidly, and the region having polygonal crystal grains, that is, the polygonal region, recrystallization / recovery is delayed, and many strains are generated inside. It is important to improve the toughness by reducing the starting point of the fractured surface and suppressing the propagation of the fractured surface by appropriately providing the region including the, that is, the recrystallization recovery delay region. Hereinafter, the form of each region described above will be described in detail.

[Recrystallization recovery delay region]
The recrystallization recovery delay 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. In this region, each crystal grain has a 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 has an elliptical shape. It is. That is, the recrystallization recovery delay region has an ellipse whose major axis is the rolling direction or a shape similar to the ellipse when observed in a cross section orthogonal to the rolling direction of the steel sheet (hereinafter, referred to as an L cross section). The aspect ratio of the ellipse is 2.0 or more and the major axis is 10 μm or more. The manner of recognizing this area will be described later, but tissue control is performed to obtain the above-mentioned shape.

[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. If the area ratio of the recrystallization recovery delay region is high, the entire structure has a structure with much distortion, which is disadvantageous in terms of ductility. Furthermore, the amount of carbides formed at the grain boundaries and the like in the recrystallization recovery delay region increases, and the starting point of the fracture surface increases, which is disadvantageous for toughness. Therefore, the upper limit of the ratio of the recrystallization recovery delay region in the microstructure is 50% in area ratio. On the other hand, if the area ratio is lower than 10%, the other portions are formed of polygonal crystal grains, and the strength of the material is reduced. In particular, when the area ratio is lower than 10%, the unit of the fracture surface in the toughness test increases and the development of the fracture surface becomes easy, so that 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 a strain region introduced by hot rolling is sufficiently recrystallized and recovered to form polygonal crystal grains. These crystal grains are also subjected to strain recovery and effectively contribute to improvement of ductility. Further, since the polygonal region has a relatively large grain boundary, the formation density of carbides is reduced, and the starting point of the fracture surface is reduced, so that it is effective in improving the toughness. As the form of the polygonal crystal grains, when observing the L section of the steel sheet and approximating the crystal grains to an ellipse and defining the maximum diameter of the crystal grains as the major axis of the ellipse, the aspect ratio is 1.0 or more and 1.8 or less. It is desirable that This is because the aspect ratio exceeding 1.8 is affected by the elongation by rolling, which is disadvantageous for suppressing the propagation of the fracture surface. The particle diameter is desirably 5 μm or more and 100 μm or less in terms of the major axis of the ellipse. If the particle size is less than 5 μm, the number of grain boundaries as carbide formation sites increases, which is disadvantageous in reducing the starting point of the fracture surface. On the other hand, if the particle size exceeds 100 μm, the unit of the fracture surface becomes large, the fracture surface is easily developed, and the toughness is reduced. The ratio of the polygonal region to 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 (mother phase) of the steel sheet is mainly defined by the polygonal region and the recrystallization recovery delay region. However, regions that do not satisfy these rules, for example, a crystal grain having an aspect ratio of 1.0 or more and 1.8 or less 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, May be present, but these are suppressed to an area ratio of 5% or less in the microstructure, 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 base phase is a polygonal recrystallization region and a recrystallization recovery delay region having an area ratio of 10% or more and 50% or less.

Next, a method for identifying these regions will be described below.
Each of the above-mentioned regions can be recognized by optimizing the method of adjusting the sample for SEM observation. Specifically, after the surface of the steel sheet is mirror-polished with colloidal silica and then ion etching is performed on the surface layer of the steel sheet by ion milling, fine irregularities are formed on the surface layer of the recrystallization recovery delay region. The discrimination becomes possible by the observation of the in-lens structure and the observation of the backscattered electron image by the 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 difference in contrast in the base phase (mother phase) is considered to be a difference in hardness and strain, a small amount of element distribution, and the like, but the details are unknown. In the analysis, the region recognized as described above is binarized by image processing and defined as an area ratio.

  Further, it is also possible to identify each area with a value such as image quality using EBSD (Electron Back Scattered Diffraction). However, in this case, strain due to polishing may be introduced into the sample surface during sample preparation, so care must be taken when preparing the sample, and it is necessary to reliably remove surface layer strain by electrolytic polishing or ion polishing. There is.

  As for the form of the recrystallization recovery delay region, the smaller one is composed of two or more crystal grains, and the size (long axis) of the aggregate of the plurality of crystal grains is about 10 μm. It has a structure (a strip-shaped structure expanded in the rolling direction of the sheet), a width in the sheet thickness direction (lamination), a width of 50 μm, and a length (longitudinal direction (long axis of the expanded strip). )) Of about 500 μm. As shown in FIGS. 1A and 1B, the recrystallized recovery delay region can be clearly distinguished from the polygonal region as shown by the encircling line in the three cases, as shown by the micrographs of the reflected electron images. That is, FIG. 1A is a photograph of the structure at a magnification of 200 times, and a structure (recrystallization recovery delay region) extended in the rolling direction can be observed. FIG. 1B is a photograph of the structure at a magnification of 500 times, and it can be confirmed that various forms of unrecrystallized regions (recrystallization recovery delay regions) are formed in the observation region.

  Taking the above into consideration, the SEM microstructure 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 sheet thickness from the steel sheet surface (hereinafter, referred to as a 1/4 t portion) ( (200 times to 5000 times), the area of the recrystallization recovery delay region in the same visual field is measured, and the area ratio in this visual field is calculated. This operation is performed in at least 10 places, the average is calculated, and the average is used as the area ratio of the recrystallization recovery delay region.

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

  The high Mn steel according to the present invention can be produced by melting a molten steel having the above-mentioned composition by a known method such as a converter and 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-bulking rolling method.

Further, manufacturing conditions for forming the above steel material into a steel material having excellent low-temperature toughness will be described.
[Steel material heating temperature: 1100 ° C or higher and 1300 ° C or lower]
The heating temperature before hot rolling is 1100 ° C. or higher in order to increase the crystal grain size of the microstructure of the steel material. However, if the temperature exceeds 1300 ° C., there is a concern that partial melting may start. Therefore, the upper limit of the heating temperature is 1300 ° C. The temperature control here is based on the surface temperature of the steel material.

[Finishing finish temperature: 750 ° C or higher and 850 ° C or lower]
The hot rolling end temperature and the subsequent cooling conditions are important in controlling the recrystallization recovery delay region. If this temperature is higher than 850 ° C., recrystallization proceeds during and immediately after the final rolling, the formation of polygonal crystal grains is promoted, the grain boundaries become large, and the toughness becomes too high. If the rolling temperature is lower than 750 ° C., the formation of polygonal crystal grains due to recrystallization is suppressed, and a large amount of strain is introduced into the recrystallization recovery delay region, so that 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 by recrystallization / recovery and the remaining recrystallization recovery delay region, the cooling from the rolling end temperature to 650 ° C. at which the progress of recovery / recrystallization is remarkable is controlled. Is very important. At this time, if the cooling rate is too high, the structure after rolling is frozen and sufficient to form polygonal grains does not occur and the toughness deteriorates. Therefore, the upper limit of the cooling rate is set to 5 ° C./s. Preferably, it is 3 ° C./s or less. In particular, in the case of a thin material, the warpage occurs as described above, which causes a problem in the process, and therefore, it is preferable to cool at a rate of 3 ° C./s or less. Since cooling in a temperature range of less than 650 ° C. does not affect recrystallization and recovery of the base phase (mother phase), the cooling rate is restricted to a temperature range from the finish rolling end temperature to 650 ° C. On the other hand, cooling in a temperature range lower 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 perform adjustment by water cooling or the like. The cooling process here is performed based on the center temperature of the thickness of the steel sheet. The center temperature can be determined by heat transfer calculation from the steel sheet surface temperature measured by the radiation thermometer.
Although the lower limit of the cooling rate is not particularly set, the use of a heat retaining furnace or the like is disadvantageous in terms of furnace cost, process cost, and manufacturing time.

[About cooling below 650 ° C]
The present invention achieves improvement in toughness at low temperatures by a combination of the polygonal region and the recrystallization recovery delay region even in a situation where carbides are formed at grain boundaries. For this reason, cooling below 650 ° C. is not particularly specified. However, carbide suppression is effective for toughness, and from the temperature range of less than 650 ° C., the above-described effect of sheet warpage is reduced. Therefore, 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 more and 650 ° C. or less may be added. That is, tempering 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. Note that the present invention is not limited to the following embodiments.
(1) Steel plate A steel slab having the composition shown in Table 1 was produced by vacuum melting. Next, the obtained steel slab was charged into a heating furnace and heated to 1250 ° C., and then subjected to hot rolling by changing the finish rolling end temperature in various ways, and in a temperature range from the finish rolling end temperature to 650 ° C. The cooling process was performed while changing the cooling rate of the steel sheet in various ways to produce a steel plate having a thickness of 5 to 20 mm. Here, in the 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.

The tensile properties and low-temperature toughness of the obtained steel sheet were evaluated in the following manner, and the structure was analyzed.
(2) Tensile test characteristics A JIS No. 5 tensile test piece was sampled from each of the obtained steel sheets, and a tensile test was performed in accordance with the provisions of JIS Z2241 (1998) to investigate the tensile test characteristics. 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. Furthermore, elongation of 30% or more was judged to be excellent in ductility.

(3) Low-temperature toughness A Charpy V-notch test piece was sampled from a surface perpendicular to the rolling direction at a position の of the plate thickness from the surface of each steel plate in accordance with JIS Z2202 (1998). Three Charpy impact tests were performed on each steel sheet in accordance with the rules of Z 2242 (1998), the absorbed energy at -196 ° C was determined, and the base metal toughness was evaluated. In the present invention, an average value of three absorbed energies (vE-196) of 100 J or more is considered to be excellent in base metal toughness. For a steel plate having a thickness of 10 mm or less, a half-size (5 mm) Charpy V notch test piece was prepared and tested, and the absorbed energy was judged to be 50 J or more.

(4) Tissue analysis For the tissue analysis, the tissue 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 in a resin was mirror-polished with diamond polishing and colloidal silica, and then the surface was sputtered with an Ar ion beam. The structure observation was performed 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. The area of the traced area was determined using image analysis software or the like, and the area ratio was calculated. The observation area was an area of 500 × 500 μm per spot from a position 1/4 of the sheet thickness from the surface of the steel sheet, and this observation was performed at 10 spots 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 (yield strength of the base material is 400 MPa or more, and low-temperature toughness is 100 J or more in average value of absorbed energy (vE-196)). Was confirmed. On the other hand, in Comparative Examples outside the scope of the present invention, at least one of the yield strength and the 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, the balance has a component composition of Fe and unavoidable impurities, and austenite is used as a base phase. It has a polygonal recrystallization region and a microstructure that is a recrystallization recovery delay region having an area ratio of 10% or more and 50% or less, and the recrystallization recovery delay region is composed of a plurality of crystal grains having a diameter of 5 μm or less; A high Mn steel having an ellipse whose major axis is the rolling direction of the steel sheet or a shape similar to the ellipse, wherein the aspect ratio of the ellipse is 2.0 or more and the major axis is 10 μm or more. The component composition is further in mass%,
Mo: 2.0% or less,
V: 2.0% or less,
W: 2.0% or less,
2. 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 more and 1300 ° C. or less, and hot-rolled so that a finish rolling end temperature is 750 ° C. or more and less than 850 ° C. A method for producing a high-Mn steel, wherein a cooling process is performed at an average cooling rate of 5 ° C./s or less in a temperature range from a rolling end temperature to 650 ° C.   The method for producing a high Mn steel according to claim 3, wherein the average cooling rate is 3 ° C / s or less.

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