TW201839152A - High-Mn steel and production method therefor - Google Patents

Abstract

The present invention proposes a method for imparting better ductility to high-Mn steel that has superior cryogenic toughness in both a welding heat-affected part and a matrix after being welded. This high-Mn steel has a component composition comprising, in mass%, 0.30-0.90% of C, 0.05-1.0% of Si, 15-30% of Mn, not more than 0.030% of P, not more than 0.0070% of S, 0.01-0.07% of Al, 0.5-7.0% of Cr, 0.0050-0.0500% of N, not more than 0.0050% of O, the respective contained amounts of Ti and Nb being at suppressed levels of less than 0.005%, with the remaining portion being Fe and unavoidable impurities, wherein the high-Mn steel has a microstructure including austenite as a matrix phase, the area fraction of non-metal inclusion in the microstructure is less than 5.0%, and the high-Mn steel has a yield strength of at least 400 MPa and an absorption energy (vE-196 DEG C) of at least 100 J.

Description

High manganese steel and manufacturing method thereof

The present invention relates to a high-manganese steel excellent in toughness especially at low temperatures, and a method for producing the same, which is preferably used in structural steels that can be used in extremely low-temperature environments, such as tanks for liquefied gas storage tanks.

In order to use a hot-rolled steel sheet in a structure for a liquefied gas tank, since the use environment becomes extremely low temperature, in addition to high strength, the steel sheet is also required to have excellent toughness at extremely low temperatures. For example, when a hot-rolled steel sheet is used in a storage tank of a liquefied natural gas, it is necessary to ensure excellent toughness when the boiling point of the liquefied natural gas is -164 ° C or lower. If the low-temperature toughness of the steel material is poor, there is a possibility that the safety as a structure for an extremely low-temperature storage tank cannot be maintained. Therefore, it is strongly required to improve the low-temperature toughness of the applied steel material.

To meet the requirements, a Vosstian iron-based stainless steel or 9% Ni steel, or a 5000-series aluminum alloy having a structure having Vostian iron that does not show brittleness at a very low temperature as a steel plate has been previously used. However, due to the high cost of alloys and manufacturing costs, there is a need for a steel material that is inexpensive and excellent in extremely low-temperature toughness.

Therefore, as a new steel material that replaces the previous extremely low temperature steel, for example, Patent Document 1 proposes a structure using a high manganese steel to which a large amount of Mn, which is a relatively inexpensive Vostian iron stabilizing element, is added as an extremely low temperature environment. With steel.

Patent Document 1 proposes a technique for controlling the particle size of Vosstian iron to an appropriate size so as to prevent carbides generated in grain boundaries from becoming a starting point of destruction or a propagation path of cracks. According to the above-mentioned technology, a high-manganese steel having excellent low-temperature toughness of the base metal and the heat-affected zone after welding can be provided. [Prior Art Literature] [Patent Literature]

Patent Document 1: Japanese Patent Laid-Open No. 2016-196703

[Problems to be Solved by the Invention] In applications such as the structure for a liquefied gas storage tank, it is important to ensure ductility in addition to low-temperature toughness. That is, when manufacturing such a structure, the steel material used must have high workability, and excellent ductility is required for such applications, but the ductility is not verified in the technology described in Patent Document 1. In addition, although the thickness of the high-manganese steel material described in Patent Document 1 is about 15 mm to 50 mm, a thickness of less than 15 mm, particularly 10 mm or less is required for applications such as a longitudinal material. When manufacturing such a thin plate, in the method exemplified in [0040] of Patent Document 1, the method of accelerated cooling after hot rolling at 950 ° C or higher is performed, and the resulting steel sheet is liable to warp or strain, and extra steps such as shape correction are required. Obstructing productivity.

Therefore, an object of the present invention is to propose a method for imparting more excellent ductility to a high-manganese steel having excellent low-temperature toughness in a base material and a welded heat-affected zone. Furthermore, an object of the present invention is to provide a method for manufacturing such a thin plate of high-manganese steel without warping or strain. Here, the "excellent low-temperature toughness" means that the absorbed energy vE of the Charpy impact test at _{-196 ° C} is -100 J or higher. [Means for solving problems]

In order to achieve the above-mentioned problems, the present inventors have studied various factors that determine the composition and manufacturing method of steel plates for high-manganese steels, and have obtained the following findings a to c. a. In high manganese vostian iron steel, brittle failure does not occur even at extremely low temperatures, and the damage occurs from the grain boundary. Therefore, it is effective to improve the low-temperature toughness of the high-manganese steel by reducing the grain boundary which becomes the origin of the damage and the propagation path by coarsening the crystal grains. b. In addition, it has been newly discovered that non-metallic inclusions become the starting point of failure or the path of crack propagation, which adversely affects the low-temperature toughness and ductility. Therefore, by appropriately controlling the amount of Cr added to the high-manganese steel and suppressing the amounts of unavoidably mixed Ti and Nb, the increase of grain boundaries and the excessive generation of non-metallic inclusions, which are the starting points of destruction, are avoided. c. On the other hand, if the crystal grain size is simply coarsened, the yield strength decreases. In addition, when a thin object having a thickness of less than 15 mm is manufactured, the obtained steel sheet is liable to remain warped or strained. Therefore, in order to sufficiently secure the yield strength of the structural steel and to prevent warpage or strain from remaining on the steel sheet, it is necessary to appropriately control the hot rolling conditions during the production of the steel sheet.

The present invention has been made by further studying the above findings, and the gist thereof is as follows. 1. A high manganese steel containing C: 0.30% or more and 0.90% or less, Si: 0.05% or more and 1.0% or less, Mn: 15% or more and 30% or less, P: 0.030% or less, S : 0.0070% or less, Al: 0.01% or more and 0.07% or less, Cr: 0.5% or more and 7.0% or less, N: 0.0050% or more and 0.0500% or less, O: 0.0050% or less, and suppress the contents of Ti and Nb to Less than 0.005%, the remaining part has the composition of Fe and unavoidable impurities, and has a microstructure with Vosstian iron as the base phase, and the area fraction of non-metallic inclusions in the microstructure is less than 5.0%, yield strength is above 400 MPa, and absorbed energy (vE _{-196 ℃} ) is above 100 J. Here, the non-metallic inclusions are non-metallic inclusions in the structure test of Japanese Industrial Standards (JIS) G0202, and specifically refer to A-type inclusions and B-type inclusions described in the same specification. And C-based inclusions.

2. The high-manganese steel according to the above 1, wherein the component composition further contains, by mass%, selected from Cu: 0.01% or more and 1.00% or less, Ni: 0.01% or more and 1.00% or less, and Mo: 2.0%. Below, V: 2.0% or less, W: 2.0% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0010% or more and 0.0200% or less . 3. A manufacturing method of high-manganese steel, which heats a steel material having the component composition as described in 1 or 2 to a temperature range of 1100 ° C or higher and 1300 ° C or lower, and performs a final finishing rolling temperature of 800 ° C or higher and Hot rolling below 950 ° C is followed by cooling treatment with an average cooling rate of 1.0 ° C / s or higher from a temperature range of (final finishing rolling temperature -100 ° C) or higher to a temperature range of 300 ° C or higher and 650 ° C or lower. Here, each of the temperature regions is a surface temperature of a steel material or a steel plate, respectively.

4. The method for producing a high-manganese steel according to the above 3, wherein the cooling is performed after heating to a temperature range of 300 ° C or higher and 650 ° C or lower after performing the cooling treatment. [Effect of the invention]

According to the present invention, a high-manganese steel having excellent low-temperature toughness and ductility can be provided. When the high-manganese steel is welded, both the base material and the welding heat-affected zone are excellent in low-temperature toughness. Therefore, the high-manganese steel of the present invention significantly contributes to improving the safety or life of a steel structure that can be used in an extremely low-temperature environment, such as a tank for a liquefied gas storage tank, and exerts a special industrial effect. In addition, in the manufacturing method of the present invention, a reduction in productivity and an increase in manufacturing costs are not caused, and therefore a method excellent in economic efficiency can be provided.

Hereinafter, the high-manganese steel of the present invention will be described in detail.

[Component composition] First, the component composition of the high-manganese steel of the present invention and the reasons for its limitation will be described. The expression "%" in the component composition means "mass%" unless otherwise specified.

C: 0.30% or more and 0.90% or less C is a cheap Vosstian iron stabilizing element and is an important element for obtaining Vosstian iron. In order to obtain the effect, C must be 0.30% or more. On the other hand, if it exceeds 0.90%, Cr carbide is excessively formed, and low-temperature toughness is reduced. Therefore, the amount of C is set to 0.30% or more and 0.90% or less. In particular, from the viewpoint of stabilizing Vosstian Iron, the lower limit value is preferably 0.36%, and more preferably 0.40%. In addition, from the viewpoint of suppressing the decrease in the low-temperature toughness, the upper limit value is preferably 0.80%, and more preferably 0.66%. The upper limit and the lower limit may be combined with a preferable content of the amount of C, for example, it is preferably set to be 0.36% or more and 0.80% or less, and more preferably set to be 0.40% or more and 0.80% or less.

Si: 0.05% or more and 1.0% or less Si acts as a deoxidizing material and is required not only for steelmaking, but also has the effect of solid-solving in steel and strengthening the steel sheet by solid-solution strengthening. In order to obtain the effect, Si must be contained in an amount of 0.05% or more. On the other hand, when the content exceeds 1.0%, the weldability deteriorates. Therefore, the amount of Si is set to 0.05% to 1.0%. In particular, from the viewpoint of obtaining a high-strength steel sheet, the lower limit value is preferably 0.07%, more preferably 0.23%, still more preferably 0.26%, and still more preferably 0.51%. From the viewpoint of suppressing deterioration of weldability, the upper limit value is preferably 0.8%, more preferably 0.7%, still more preferably 0.6%, and even more preferably 0.5%. The upper and lower limits of Si can be combined, for example, it is preferably set to be 0.07% or more and 0.8% or less, 0.23% or more and 0.7% or less, and more preferably 0.26% or more and 0.6% or less. The preferable Si content is 0.07% or more and 0.5% or less.

Mn: 15.0% or more and 30.0% or less Mn is a relatively inexpensive Vosstian iron stabilizing element. The present invention is an important element for coexisting strength and extremely low temperature toughness. In order to obtain the effect, Mn must be contained in an amount of 15.0% or more. On the other hand, even if the content exceeds 30.0%, the effect of improving the extremely low temperature toughness is saturated, and the cost of the alloy increases. In addition, weldability and cutting properties are deteriorated. Furthermore, it promotes segregation and the occurrence of stress corrosion cracking. Therefore, the amount of Mn is set to 15.0% or more and 30.0% or less. In particular, from the viewpoint of stabilizing Vosstian Iron, the lower limit value is preferably 16.0%, more preferably 18.0%, and even more preferably 19.0%. In addition, from the viewpoint of suppressing reduction in low-temperature toughness, the upper limit value is preferably 29.0%, and more preferably 28.0%. The upper limit and the lower limit may be combined with a preferable content of the Mn amount. For example, the upper limit and the lower limit are preferably 16.0% to 29.0%, and more preferably 18.0% to 28.0%.

P: 0.030% or less When P is contained in an amount exceeding 0.030%, grain boundaries segregate and become a starting point of stress corrosion cracking. Therefore, 0.030% is set as an upper limit, and it is desirable to reduce it as much as possible. Therefore, P is set to 0.030% or less. From the viewpoint of reducing the occurrence of stress corrosion cracking, the upper limit value is preferably 0.028% or less, and more preferably 0.024% or less. In addition, excessive P reduction may increase the refining cost and is economically disadvantageous. Therefore, the lower limit value is preferably set to 0.002%, and more preferably set to 0.005%.

S: 0.0070% or less S degrades the low-temperature toughness or ductility of the base material. Therefore, 0.0070% is the upper limit, and it is desirable to reduce it as much as possible. Therefore, S is set to 0.0070% or less. In addition, from the viewpoint of suppressing deterioration of the low-temperature toughness and ductility of the base material, the upper limit value is preferably 0.0060% or less. Furthermore, an excessive reduction in S may increase the refining cost and is economically disadvantageous. Therefore, the lower limit value is preferably 0.001% or more. The range of the amount of S is preferably 0.0020% or more and 0.0060% or less.

Al: 0.01% or more and 0.07% or less Al functions as a deoxidizer, and is most commonly used in the deoxidation process of molten steel of a steel sheet. In order to obtain the effect, Al must be contained in an amount of 0.01% or more. On the other hand, if the content exceeds 0.07%, the weld metal portion is mixed during welding and the toughness of the weld metal is deteriorated. Therefore, it is set to 0.07% or less. Therefore, the amount of Al is set to 0.01% to 0.07%. In particular, from the viewpoint of obtaining an effect as a deoxidizer, the lower limit value is preferably 0.02%, more preferably 0.046%, and still more preferably 0.052%. From the viewpoint of the toughness of the weld metal, the upper limit value is preferably 0.065%, and more preferably 0.06%. A preferable content of the Mn content may be a combination of these upper and lower limits. For example, it is preferably set to 0.02% or more and 0.06% or less.

Cr: 0.5% or more and 7.0% or less Cr is an element that is effective in stabilizing Vosstian iron by adding an appropriate amount and is effective for improving the low-temperature toughness and the strength of the base metal. In order to obtain the effect, Cr must be contained in an amount of 0.5% or more. On the other hand, if the content exceeds 7.0%, low-temperature toughness and stress corrosion cracking resistance are reduced by the formation of Cr carbides. Therefore, the amount of Cr is set to 0.5% to 7.0%. In particular, from the viewpoint of improving the extremely low-temperature toughness and the strength of the base material, the lower limit value is preferably 1% or more, more preferably 1.2%, and even more preferably 2.0%. From the viewpoint of low-temperature toughness and stress corrosion cracking resistance, the upper limit value is preferably 6.7% or less, more preferably 6.5% or less, and even more preferably 6.0%. The upper limit and the lower limit may be combined with a preferable content of the Mn amount. For example, it is preferably 1.0% or more and 6.7% or less, and more preferably 1.2% or more and 6.5% or less. In addition, in order to further improve the stress corrosion cracking resistance, it is more preferably 2.0% or more and 6.0% or less.

N: 0.0050% or more and 0.0500% or less N is a Wastfield iron stabilizing element and is an element effective for improving the extremely low temperature toughness. In order to obtain the effect, N must be 0.0050% or more. On the other hand, if the content exceeds 0.0500%, the nitride or carbonitride becomes coarse, and the toughness decreases. Therefore, the amount of N is set to be 0.0050% or more and 0.0500% or less. In particular, from the viewpoint of improving the extremely low-temperature toughness, the lower limit value is preferably 0.0060% or more, more preferably 0.0355%, and still more preferably 0.0810%. From the viewpoint of suppressing the decrease in toughness, the upper limit value is preferably 0.0450% or less, and more preferably 0.0400% or less. A preferable content of the N amount may be a combination of these upper and lower limits. For example, it is preferably set to be 0.0060% or more and 0.0400% or less.

O: 0.0050% or less O is caused by deterioration of the extremely low temperature toughness due to the formation of oxides. Therefore, O is set to a range of 0.0050% or less. From the viewpoint of suppressing the decrease in toughness, the upper limit value is preferably 0.0045% or less. The lower limit of the amount of O is preferably 0.0023% or more. The preferable content of the amount of O may be a combination of these upper and lower limits. For example, it is preferably set to 0.0023% or more and 0.0050% or less.

The contents of Ti and Nb are suppressed to less than 0.005%. Ti and Nb form high-melting carbonitrides in the steel to suppress the coarsening of the crystal grains. As a result, they become the starting point of failure or the path of crack propagation. In particular, in high-manganese steels, it is an obstacle to microstructure control for improving low-temperature toughness and improving ductility, and therefore it must be intentionally suppressed. That is, Ti and Nb are components unavoidably mixed from raw materials and the like, and are usually mixed in a range of Ti: 0.005% or more and 0.010% or less and Nb: 0.005% or more and 0.010% or less. Therefore, it is necessary to avoid the inevitable mixing of Ti and Nb in accordance with the method described later, and to suppress the contents of Ti and Nb to less than 0.005%, respectively. By suppressing the contents of Ti and Nb to less than 0.005%, the adverse effects of the carbonitride can be eliminated, and excellent low-temperature toughness and ductility can be ensured. Therefore, from the viewpoint of the excellent low-temperature toughness and ductility, the contents of Ti and Nb are each preferably 0.004% or less, and more preferably 0.003% or less.

The balance other than the components is iron and unavoidable impurities. Examples of the inevitable impurities here include H and the like, and a total of 0.01% or less is allowable.

[Organization] In the case where the microstructure of Vostian iron is the base phase and the crystal structure of the steel is a body-centered cubic (bcc) structure, the steel may cause brittle failure in a low temperature environment. It is not suitable for use in low temperature environment. Here, when the use in a low temperature environment is assumed, the base phase of the steel must be a Vosstian iron structure with a crystal structure of a face center cubic (fcc) structure. In addition, the so-called "vostian iron base phase" means that the Vostian iron phase has an area ratio of 90% or more. By setting the Vostian iron phase to 90% or more in terms of area ratio, low-temperature toughness can be ensured. The remainder other than the Vostian iron phase is the ferrite phase or the Asada scattered iron phase. Among them, if ε-Matian loose iron is generated, even if it is a small amount, the low-temperature toughness will be reduced. Therefore, as the microstructure of Vostian iron as the base phase of the present invention, it is preferable that ε-Matian loose iron is substantially not included. Phase organization. That is, in order to ensure the low-temperature toughness, it is preferable that the volume fraction of ε-Matian loose iron is less than 1.0%, more preferably less than 0.5%, and even more preferably less than 0.1%.

Area fraction of non-metallic inclusions: less than 5.0% In non-metallic inclusions, A refers to inclusions in the form of sulfides, B refers to inclusions in the form of clustered oxides, and C-based Inclusions in the form of granular oxides. If a large amount of these non-metallic inclusions are present in the steel, it will become the starting point of failure, resulting in a decrease in the extremely low-temperature toughness or a deterioration in ductility. Therefore, the total amount of these inclusions must be suppressed to 5% or less in terms of area fraction. The suppression is preferably 4% or less. Therefore, it is necessary to control the component composition and implement a manufacturing method described later.

In addition, if the Vostian iron phase is 90% or more in terms of area ratio and the area fraction of non-metallic inclusions is less than 5.0%, it is possible to provide an extremely low-temperature toughness and exhibit good ductility. steel.

By making the above requirements necessary, the characteristics that are the object of the present invention can be obtained. For example, when supplying high-manganese steel to a welding process, especially the low-temperature toughness of the welded heat-affected zone becomes a problem, but if a high-manganese steel that satisfies the above requirements is used, the microstructure of the welded heat-affected zone will The field iron is a base phase, and the particle diameter of the field iron is 50 μm or more in terms of a circle-equivalent diameter, and low-temperature toughness can be ensured even in a welding heat affected portion.

That is, in order to ensure the low-temperature toughness of Vosstian iron steel, it is effective to coarsen the crystal grain size. This is the same in the welding heat-affected zone. For example, in order to obtain a value of 100 J or more as the absorbed energy of the Charpy impact test at -196 ° C, the maximum crystal grain size of the microstructure must be 50 μm or more. This is achieved using high-manganese steels that meet the above requirements.

In the present invention, in order to further improve the strength and low-temperature toughness, in addition to the essential elements described above, the following elements may be optionally contained. Cu: 0.01% or more and 1.00% or less, Ni: 0.01% or more and 1.00% or less, Mo: 2.0% or less, V: 2.0% or less, W: 2.0% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less or REM: one or two or more of 0.0010% and 0.0200% or less.

Cu: 0.01% or more and 1.00% or less, Ni: 0.01% or more and 1.00% or less, Mo, V, W: 2.0% or less Cu, Ni, Mo, V, and W contribute to the stabilization of Vosstian iron, or Contributes to the improvement of base material strength. In order to obtain the effect, Cu and Ni are preferably contained in an amount of 0.01% or more, and Mo, V, and W are preferably contained in an amount of 0.001% or more. On the other hand, if Cu and Ni are each contained more than 1.00%, and Mo, V, and W are each contained more than 2.0%, in addition to generating coarse carbonitrides and starting points of destruction, the manufacturing cost is increased. Therefore, when these alloy elements are contained, Cu and Ni are each preferably 1.00% or less, and Mo, V, and W are each preferably 2.0% or less. The amount of Cu and the amount of Ni are more preferably 0.05% to 0.70%, respectively. The amount of Mo, the amount of V, and the amount of W are more preferably 0.003% to 1.7%, respectively.

Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, REM: 0.0010% or more and 0.0200% or less Ca, Mg, and REM are elements useful for controlling the morphology of inclusions, and may be contained as necessary . The morphology control of the inclusions refers to making the extended sulfide-based inclusions into granular inclusions. The morphology control of the inclusions improves the ductility, toughness, and resistance to sulfide stress corrosion cracking. In order to obtain the effect, it is preferable that Ca and Mg are contained in an amount of 0.0005% or more, and REM is preferably contained in an amount of 0.0010% or more. On the other hand, if a large amount of an arbitrary element is also contained, the amount of non-metallic inclusions may increase, and instead, ductility, toughness, and sulfide stress corrosion cracking resistance may decrease. In addition, it is sometimes economically disadvantageous.

Therefore, when Ca and Mg are contained, it is set to be 0.0005% or more and 0.0050% or less, and when REM is contained, it is set to be 0.0010% or more and 0.0200% or less. The amount of Ca is preferably 0.0010% to 0.0040%, the amount of Mg is 0.0010% to 0.0040%, and the amount of REM is 0.0020% to 0.0150%.

The high-manganese steel of the present invention can be melted by a known melting method such as a converter, an electric furnace, or the like with a molten steel having the above-mentioned composition. Alternatively, the refining may be performed twice in a vacuum degassing furnace. In this case, in order to limit Ti and Nb, which are obstacles to better structural control, to the above-mentioned range, measures must be taken to avoid inevitable mixing of raw materials and the like and to reduce these contents. For example, by reducing the basicity of the slab during the refining stage, these alloys are concentrated and discharged in the slab, thereby reducing the concentrations of Ti and Nb in the final slab product. In addition, a method such as oxidizing by blowing oxygen and floating separation of the alloy of Ti and Nb at the time of reflow can also be used. Thereafter, it is preferable to produce a steel material such as a slab of a predetermined size by a known casting method such as a continuous casting method or a block-and-roll rolling method.

Furthermore, the manufacturing conditions for manufacturing the steel material into a steel material having excellent low-temperature toughness are specified.

Steel material heating temperature: 1100 ° C or higher and 1300 ° C 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. In addition, if the lower limit value of the heating temperature of the steel material is less than 1100 ° C., the amount of non-metallic inclusions in the steel increases, thereby causing deterioration of the extremely low-temperature toughness and ductility due to the non-metallic inclusions in the steel. Among them, if it exceeds 1300 ° C, there is a possibility that part of the melting will start. Therefore, the upper limit of the heating temperature is set to 1300 ° C. The temperature control here is based on the surface temperature of the steel material.

Final finishing rolling temperature: 800 ° C or higher and less than 950 ° C After heating the steel block or steel sheet, hot rolling is performed. In order to produce coarse crystal grains, it is preferable to increase the cumulative reduction ratio at a high temperature. Among these, when processing is performed in a temperature range of 950 ° C. or higher, the crystal grain size becomes excessively coarse, and a desired yield strength cannot be obtained. Therefore, a final finish rolling of less than 950 ° C and one pass or more is required. On the other hand, when hot rolling is performed at a low temperature, the microstructure becomes finer and excessive processing strain is generated, which results in a decrease in low-temperature toughness. Therefore, the lower limit of the finish rolling temperature is set to 800 ° C.

Average cooling rate from a temperature of (final finishing rolling temperature -100 ° C) or more to a temperature range of 300 ° C or more and 650 ° C or less: 1.0 ° C / s or more After the hot rolling is completed, cooling is performed rapidly. When the hot-rolled steel sheet is slowly cooled, the formation of precipitates is promoted and the low-temperature toughness is deteriorated. By cooling at a cooling rate of 1 ° C / s or more, the generation of these precipitates can be suppressed. In addition, if excessive cooling is performed, the steel sheet is strained and productivity is lowered. Therefore, the upper limit of the cooling start temperature is 900 ° C. For the above reasons, regarding cooling after hot rolling, the average cooling rate on the surface of the steel sheet from a temperature range of (final finishing rolling temperature -100 ° C) to a temperature range of 300 ° C to 650 ° C is 1.0 ° C / s the above. Furthermore, in a thick steel plate having a plate thickness of 10 mm or less, the cooling rate is 1 ° C./s or more even if air cooling is performed. When the plate thickness is 10 mm or less, the steel plate is prevented from straining by cooling by air cooling.

Further, if necessary, the cooling treatment may be performed after the above-mentioned cooling treatment is performed until the temperature is increased to a temperature range of 300 ° C to 650 ° C. That is, tempering may be performed for the purpose of adjusting the strength of the steel sheet. [Example]

Hereinafter, the present invention will be described in detail through examples. The present invention is not limited to the following examples.

The converter-barrel refining-continuous casting method was used to produce a steel slab having the composition shown in Table 1. Next, the obtained steel slab was put into a heating furnace and heated to 1150 ° C, and then a steel sheet having a thickness of 10 mm to 30 mm was produced by hot rolling. Regarding the steel sheet, tensile properties and toughness were implemented by the following methods.

(1) Tensile test characteristics A JIS No. 5 tensile test piece was taken from each of the obtained steel plates, a tensile test was performed in accordance with JIS Z 2241 (1998), and the tensile test characteristics were investigated. In the present invention, it is judged that the tensile strength is 400 MPa or more and the tensile strength is 800 MPa or more. Furthermore, it was judged that the total elongation at the time of fracture was 30% or more, and the ductility was excellent.

(2) Low-temperature toughness is a direction perpendicular to the rolling direction from the 1/4 position of the plate thickness of each steel plate with a plate thickness exceeding 20 mm, or the 1/2 position of the plate thickness of each steel plate with a plate thickness of 20 mm or less, in accordance with JIS Z. In accordance with 2202 (1998), Charpy V-notch test pieces were used. According to JIS Z 2242 (1998), three Charpy impact tests were performed on each steel plate. The absorbed energy at -196 ° C was determined and evaluated. Base material toughness. In the present invention, an average value of the absorbed energy (vE _{-196 ° C} ) of the three roots is 100 J or more, and the base material is excellent in toughness.

The results obtained by the above are shown in Table 2.

[表 1] Table 1 Note: The underline is outside the scope of the invention.

[表 2] Table 2 Note: The underline is outside the scope of the invention. (*) Average cooling rate from a temperature of (final finishing rolling temperature -100 ° C) to a temperature range of 300 ° C to 650 ° C. Regarding the high-manganese steel according to the present invention, it was confirmed that the target performance (parent The yield strength of the material is 400 MPa or more, the total elongation at break is 30% or more, and the low-temperature toughness is 100 J or more as the average value of absorbed energy (vE _{-196 ℃} ). On the other hand, in the comparative examples which deviate from the scope of the present invention, any one or more of the total elongation, yield strength, and low-temperature toughness cannot satisfy the target performance.

Further, the steel was subjected to thermal cycle treatment under conditions of a peak temperature of 1400 ° C. and a cooling rate of 10 ° C./s for the purpose of evaluating the impact absorption characteristics of the welded part to evaluate the low-temperature toughness. As shown in Table 2 together, the steel material according to the present invention shows the same excellent low-temperature toughness as the base material. That is, for welding that provides a heat input of 0.5 kJ / cm to 5 kJ / cm, a maximum crystal grain size of 50 μm or more and a value of 100 J or more at the Charpy impact test at −196 ° C. can be obtained. .

This application claims the priority of Japanese Patent Application No. 2017-087702 (filed on April 26, 2017), and the disclosure of the application is incorporated in this application as a whole for reference.

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Claims (4)

A high manganese steel containing C: 0.30% or more and 0.90% or less, Si: 0.05% or more and 1.0% or less, Mn: 15.0% or more and 30% or less, P: 0.030% or less, and S: 0.0070% Below, Al: 0.01% or more and 0.07% or less, Cr: 0.5% to 7.0%, N: 0.0050% or more and 0.0500% or less, O: 0.0050% or less, and suppressing the contents of Ti and Nb to less than 0.005%, The remaining part has the composition of Fe and unavoidable impurities, and has a microstructure with Vosstian iron as the base phase. The area fraction of non-metallic inclusions in the microstructure is less than 5.0%, yield strength It is 400 MPa or more and the absorbed energy (vE _{-196 ℃} ) is 100 J or more. The high-manganese steel according to item 1 of the scope of patent application, wherein the component composition further comprises, by mass%, selected from Cu: 0.01% or more and 1.00% or less, Ni: 0.01% or more and 1.00% or less, Mo: 2.0 % Or less, V: 2.0% or less, W: 2.0% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0010% or more and 0.0200% or less, one of them or More than two. A manufacturing method of high-manganese steel, which heats a steel material having a component composition as described in item 1 or 2 of the patent application scope to a temperature range of 1100 ° C to 1300 ° C, and performs a final finishing rolling temperature of 800 Hot rolling at a temperature of not less than 950 ° C but not more than 950 ° C, followed by an average cooling rate of 1.0 ° C / s or more from a temperature range of (final finishing rolling temperature -100 ° C) to a temperature range of 300 ° C to 650 ° C Cooling treatment. The method for manufacturing a high-manganese steel according to item 3 of the scope of application for a patent, wherein after performing the cooling treatment, heating is performed to a temperature range of 300 ° C or higher and 650 ° C or lower and cooling is performed.