BR112019022088A2 - high mn steel and production method - Google Patents

Abstract

a method is provided to provide excellent ductility to high-mn steel, which exhibits excellent toughness at low temperature in a base metal and a thermally affected zone after welding. high mn steel has a chemical composition that contains, in% by mass, c: 0.10% to 0.70%, si: 0.05% to 1.0%, mn: 15% to 30%, p: 0.030% or less, s: 0.0070% or less, al: 0.01% to 0.07%, cr: 0.5% to 7.0%, n: 0.0050% to 0.0500%, o: 0.0050% or less, ti: less than 0.005% and nb: less than 0.005%, with the remainder being unavoidable and impurities, it has a microstructure containing austenite as the matrix phase and, in the microstructure, non-metallic inclusions with a fraction of area less than 5.0% and exhibits an elastic limit equal to or greater than 400 mpa and absorbed energy (ve-196) equal to or greater than 100 j.

Description

HIGH STEEL Μη AND THE SAME PRODUCTION METHOD

Technical Field [001] The invention relates to high Mn steel which is suitable for structural steel used in an extremely low temperature environment, such as a liquefied gas storage tank, in particular, high Mn steel excellent in low toughness temperature and production method.

Background of the Invention [002] To use a hot-rolled steel sheet in a structure for a liquefied gas storage tank, the steel sheet must have high strength and excellent toughness at extremely low temperature, because the structure is used at temperature extremely low. For example, when hot rolled steel sheet is used for a liquefied natural gas storage tank, it is necessary to ensure excellent toughness at the boiling point of liquefied natural gas, that is, -164 ° C or lower. When a steel material has poor toughness at low temperature, safety such as an extremely low temperature storage tank structure may not be maintained. Thus, there is an increasing demand for steel materials with improved low temperature toughness that is applied to this structure.

[003] In view of the demand, austenitic stainless steel, which has austenite as a steel plate structure, austenite does not show fragility at extremely low temperature, 9% Ni steel or five thousand aluminum alloys in the series, have conventionally used. However, the cost of the alloy and the cost of production are high and therefore there is a demand for steel materials that are cheap and excellent in toughness at extremely low temperature.

Petition 870190106645, of 10/21/2019, p. 59/88

2/25 [004] As new steel materials have been replacing conventional steel for extremely low temperature, for example, patent document JP 2016-196703 A (PTL 1) proposes the use, as structural steel used in a temperature environment extremely low, high Mn steel added with a large amount of Mn which is relatively inexpensive and an austenite stabilizing element.

[005] PTL 1 proposes a technique to adequately control the size of austenite grains to prevent carbides generated in crystal grain outlines from becoming a fracture source and a crack propagation path. The technique can provide steel with a high Mn content, which exhibits excellent low temperature toughness in a base metal and an area affected by heat after welding.

List of Citations

Patent Literature [006] PTL1: JP 2016 196703 A

summary

Technical Problem [007] In an application such as the structure mentioned above for a liquefied gas storage tank, it is important to guarantee ductility, in addition to low temperature toughness. Specifically, when this structure is made, the steel materials to be used in the structure must have high workability. In addition, excellent ductility is required in such an application. The PTL 1 technique, however, does not verify ductility. In addition, the high steel material Mn of PTL 1 has a thickness of about 15 mm to 50 mm, but, for example, in an application as a longitudinal member, it is necessary that the thickness be less than 15 mm, in particular 10 mm or less. When a thin sheet is produced, the technique

Petition 870190106645, of 10/21/2019, p. 60/88

3/25 exemplified in paragraph 0040 of PTL 1, in which hot rolling at 950 ° C or more and subsequent accelerated cooling are performed, it tends to cause deflection and deformation in a obtained steel sheet, which requires an extra process, such as shape adjustment, thereby deteriorating productivity.

[008] Thus, it could be useful to propose a method to provide excellent ductility to high Mn steel, which exhibits excellent toughness at low temperature of the base metal and the zone affected by heat. In addition, it could be useful to propose a method of producing a thin sheet of this steel with a high Mn content without deflection or deformation. As used in this document, exhibiting excellent toughness at low temperature means that the energy absorbed vE-i96 ° c in a Charpy impact test at -196 ° C is 100 J or more.

Solution to the Problem [009] To achieve the aforementioned objectives, we carried out an extensive study on high Mn steel regarding several factors that determine the chemical composition of a steel sheet and a method of producing it to discover the following:

The. Austenite steel with high Mn content does not present brittle fractures at extremely low temperature. This fracture occurs from the grain boundaries. This suggests that in order to improve the low temperature toughness of high Mn steel, it is effective to thicken crystal grains to decrease grain contours, which become a fracture source and a propagation path.

B. In addition, we recently discovered that non-metallic inclusions become a source of fracture and a crack propagation pathway, adversely affecting low temperature toughness and ductility. Thus, by properly controlling the content of Cr added to high Mn steel and limiting the content of Ti and Nb inevitably mixed, the increase in crystal grain contours and the formation of excessive non-metallic inclusions that become

Petition 870190106645, of 10/21/2019, p. 61/88

4/25 a fracture origin are avoided.

ç. On the other hand, the simple thickening of the crystal grains deteriorates the elastic limit. In addition, when a thin sheet with a thickness of less than 15 mm is produced, a steel sheet obtained tends to show deflection and deformation. Therefore, in order to sufficiently guarantee the elastic limit as structural steel and not to leave deflection or deformation in a steel plate, it is necessary to adequately control the conditions of hot rolling in the production of a steel plate.

[010] The present invention is based on these discoveries and further investigations carried out by the inventors. The main aspects of this invention are as follows.

1. Mn high steel including:

a chemical composition containing (consisting of), in% by mass,

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, and

O: 0.0050% or less,

Ti: less than 0.005%, and

Nb: less than 0.005%, with the remainder being Fe and unavoidable impurities;

a microstructure containing austenite as a matrix phase and non-metallic inclusions with an area fraction of less than 5.0%; and an elastic limit of 400 MPa or more and an absorbed energy (vE-i96) of

Petition 870190106645, of 10/21/2019, p. 62/88

5/25

100 J or more.

[Oil] Non-metallic inclusions are non-metallic inclusions in a JIS G0202 structure test and refer specifically to type A inclusions, type B inclusions and type C inclusions described in document JIS G0202.

2. High Mn steel, according to 1., where the chemical composition still contains, in% by mass, at least one selected from among

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.

3. High Mn steel production method, comprising: heating a steel material with the chemical composition according to 1. or 2. up to a temperature range of 1100 ° C or more and 1300 ° C or less; hot rolling of the steel material with a rolling finish temperature of 800 ° C or more and less than 950 ° C to obtain a hot-rolled steel sheet; and then subjecting the hot-rolled steel sheet to the cooling treatment at an average cooling rate of 1.0 ° C / s or more from a temperature equal to or greater (rolling finish temperature -100 ° C) at a temperature range of 300 ° C or more and 650 ° C or less.

[012] Each temperature range is a surface temperature of the steel material or steel plate.

Petition 870190106645, of 10/21/2019, p. 63/88

6/25

4. High steel production method Μη, according to 3., comprising: after the cooling treatment, heating the steel sheet to a temperature range of 300 ° C or more and 650 ° C or less and then , cooling of the steel plate.

Advantageous Effects [013] According to the present invention, it is possible to provide high Mn steel excellent in low temperature toughness and ductility. When high Mn steel is welded, both a base metal and a thermally affected zone have excellent toughness at low temperature. Therefore, our high Mn steel contributes greatly to improving the safety and service life of a steel structure used in an extremely low temperature environment, such as a liquefied gas storage tank, and has industrially significant effects. In addition, our production method does not decrease productivity or increase production costs and is therefore excellent in economic efficiency.

Detailed Description [014] Our Mn high steel will be described in detail below.

[015] [Chemical Composition]

The chemical composition of our Mn high steel and the reasons for its limitations will be described first. In the description of the chemical composition,% indicates% by mass, unless otherwise indicated.

[016] C: 0.30% or more and 0.90% or less

C is an inexpensive austenite stabilizing element and an important element for obtaining austenite. To achieve this effect, the C content must be 0.30% or more. On the other hand, a C content above 0.90% generates excess Cr carbides, deteriorating the toughness at low temperature. Therefore, C content is defined as 0.30% or more and 0.90% or less. In particular, the

Petition 870190106645, of 10/21/2019, p. 64/88

7/25 From the point of view of austenite stabilization, the lower limit of the C content is preferably 0.36%, and more preferably 0.40%. In addition, with a view to preventing deterioration of toughness at low temperature, the upper limit of the C content is preferably 0.80% and more preferably 0.66%. For the preferable C content, the upper and lower limits can be combined arbitrarily. Thus, for example, the C content is preferably set at 0.36% or more and 0.80% or less, and more preferably 0.40% or more and 0.80% or less.

[017] Si: 0.05% or more and 1.0% or less

Si acts as a deoxidizer, is necessary for the production of steel and is effective in increasing the hardness of a steel plate, by strengthening the solid solution when dissolved in steel. To achieve this effect, the Si content must be 0.05% or more. On the other hand, a Si content above 1.0% deteriorates weldability. Therefore, the Si content is defined as 0.05% or more and 1.0% or less. In particular, from the perspective of obtaining a steel plate with increased hardness, the lower limit of the Si content is preferably 0.07%, more preferably 0.23%, still preferably 0.26% and even more preferably 0.51% . In addition, from the perspective of preventing deterioration of weldability, the upper limit of the Si content is set at 0.8%, more preferably 0.7%, still preferably 0.6% and even more preferably 0.5%. For the preferable Si content, the upper limit and the lower limit can be combined. Thus, for example, the Si content is preferably defined as 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. In addition, the Si content is preferably 0.07% or more and 0.5% or less.

[018] Mn: 15.0% or more and 30.0% or less

Mn is a relatively inexpensive austenite stabilizer. At

Petition 870190106645, of 10/21/2019, p. 65/88

8/25 invention, ο Μη is an important element to obtain strength and toughness at extremely low temperature. To obtain such effects, the Mn content needs to be 15.0% or more. On the other hand, an Mn content above 30.0% does not increase the effect of improving toughness at extremely low temperature, but it does increase the cost of the alloy. In addition, such a high content of Mn deteriorates weldability and cutting capacity, in addition to promoting segregation, as well as the occurrence of cracking by stress corrosion. Therefore, the Mn content is defined as 15.0% or more and 30.0% or less. In particular, from the point of view of stabilizing austenite, the lower limit of the Mn content is preferably 16.0%, more preferably 18.0% and still preferably 19.0%. In addition, with a view to preventing deterioration of toughness at low temperature, the upper limit of the Mn content is preferably 29.0% and more preferably 28.0%. For the preferable Mn content, the upper and lower limits can be combined arbitrarily. Thus, for example, the Mn content is preferably defined as 16.0% or more and 29.0% or less, and more preferably 18.0% or more and 28.0% or less.

[019] P: 0.030% or less

When a P content exceeds 0.030%, P secretes in the grain boundaries and becomes a source of stress corrosion cracking. Therefore, the upper limit of the P content is 0.030% and, desirably, the P content is kept as low as possible. Therefore, the P content is defined as 0.030% or less. In addition, from the perspective of decreasing the origin of stress corrosion cracking, the upper limit of the P content is preferably 0.028% or less, and more preferably 0.024% or less. Excessively reducing P, however, involves high refining costs and is economically disadvantageous. Therefore, the lower limit of the P content is preferably defined as 0.002%,

Petition 870190106645, of 10/21/2019, p. 66/88

9/25 and more preferably 0.005%.

[020] S: 0.0070% or less

S deteriorates the toughness at low temperature and ductility of the base metal. Therefore, the upper limit of the S content is 0.0070% and, desirably, the S content is kept as low as possible. Therefore, the S content is defined as 0.0070% or less. In order to prevent deterioration of the low temperature toughness and ductility of the base metal, the upper limit of the S content is preferably 0.0060% or less. Excessively reducing S, however, involves high refining costs and is economically disadvantageous. Therefore, the lower limit of the S content is preferably defined as 0.001% or more. The S content is preferably defined as 0.0020% or more and 0.0060% or less.

[021] Al: 0.01% or more and 0.07% or less

Al acts as a deoxidizer and is most commonly used in deoxidation processes of molten steel to obtain a steel plate. To achieve this effect, the Al content needs to be 0.01% or more. On the other hand, when the Al content exceeds 0.07%, Al is mixed in a part of the weld metal during welding, deteriorating the toughness of the weld metal. Therefore, the content of Al is defined as 0.07% or less. Therefore, the content of Al is defined as 0.01% or more and 0.07% or less. In particular, from the perspective of obtaining an effect as a deoxidizer, the lower limit of the Al content is preferably 0.02%, more preferably 0.046% and still preferably 0.052%. In addition, from the perspective of the toughness of the weld metal, the upper limit of the Al content is preferably defined as 0.065% and, more preferably, 0.06%. For the Al content, the upper limit and the lower limit can be combined. Thus, for example, the content of Al is preferably defined as 0.02% or more and 0.06% or less.

Petition 870190106645, of 10/21/2019, p. 67/88

10/25 [022] Cr: 0.5% or more and 7.0% or less

Cr is an element that stabilizes austenite with an adequate amount of addition and is effective in improving the toughness at extremely low temperature and the resistance of the base metal. To achieve this effect, the Cr content needs to be 0.5% or more. On the other hand, a Cr content above 7.0% generates Cr carbides, deteriorating toughness at low temperature and resistance to cracking under stress corrosion. Therefore, the Cr content is defined as 0.5% or more and 7.0% or less. In particular, from the perspective of improving the toughness at extremely low temperature and the strength of the base metal, the lower limit of the Cr content is preferably 1% or more, more preferably 1.2% and still preferably 2.0%. In addition, from the point of view of low temperature toughness and resistance to cracking under stress corrosion, the upper limit of the Cr content is preferably defined as 6.7% or less, more preferably 6.5% or less and still preferably 6.0%. For the preferred Cr content, the upper and lower limits can be arbitrarily combined. Thus, for example, the Cr content is preferably defined as 1.0% or more and 6.7% or less, and more preferably 1.2% or more and 6.5% or less. To further improve the resistance to cracking by stress corrosion, the Cr content is still preferably 2.0% or more and 6.0% or less.

[023] N: 0.0050% or more and 0.0500% or less

N is an austenite stabilizing element and an element that is effective in improving toughness at extremely low temperature. To achieve this effect, the N content must be 0.0050% or more. On the other hand, the N content above 0.0500% thickens nitrides or carbonitrides, deteriorating the toughness. Therefore, the N content is defined as 0.0050% or more and 0.0500% or less. In particular, from the perspective of improving the toughness to

Petition 870190106645, of 10/21/2019, p. 68/88

At an extremely low temperature, the lower limit of the N content is preferably 0.0060% or more, more preferably 0.0355% and still preferably 0.0810%. In addition, with a view to preventing deterioration of toughness, the upper limit of the N content is preferably defined as 0.0450% or less and, more preferably, 0.0400% or less. For the preferable N content, the upper and lower limits of the N content can be combined arbitrarily. Thus, for example, the N content is preferably defined as 0.0060% or more and 0.0400% or less.

[024] O: 0.0050% or less

O deteriorates toughness at extremely low temperature due to the formation of oxides. Therefore, the O content is defined as 0.0050% or less. In order to prevent deterioration of toughness, the upper limit of the O content is preferably 0.0045% or less. In addition, the lower limit of the O content is preferably 0.0023% or more. For the preferable O content, the upper and lower limits of the O content can be combined arbitrarily. Thus, for example, the O content is preferably defined as 0.0023% or more and 0.0050% or less.

[025] Ti: less than 0.005% and Nb: less than 0.005%

Ti and Nb form carbonitrides with a high melting point in steel to prevent a thickening of the crystal grains, becoming a fracture source and a crack propagation path. In particular, in high steel Μη, Ti and Nb prevent control of the structure to increase toughness at low temperature and improve ductility and therefore need to be intentionally limited. Specifically, Ti and Nb are components inevitably mixed from raw materials in steel, and Ti of 0.005% or more and 0.010% or less and Nb of 0.005% or more and 0.010% or less are typically mixed. Thus, according to the method described below, it is

Petition 870190106645, of 10/21/2019, p. 69/88

12/25 necessary to prevent the inevitable mixing of Ti and Nb and to limit the content of each Ti and Nb to less than 0.005%. By limiting the content of each Ti and Nb to less than 0.005% to eliminate the adverse effect of carbonitrides, excellent toughness at low temperature and ductility can be guaranteed. Therefore, from the point of view of excellent toughness at low temperature and ductility, the content of each Ti and Nb is preferably defined as 0.004% or less and, more preferably, 0.003% or less.

[026] The remainder other than the components mentioned above includes Fe and unavoidable impurities. Unavoidable impurities include H, and a total content of 0.01% may be allowed.

[Structure] [027] Microstructure that has austenite as its matrix phase [028] When a steel material has a cubic crystalline body-centered structure (ccc), the steel material can cause brittle fractures in a low temperature environment, therefore, it is not suitable for use in a low temperature environment. When it is assumed that the steel material is used in a low temperature environment, it is necessary that the steel material has, as its matrix phase, an austenite structure that has a cubic crystalline structure with a centered face (cfc). As used here, having austenite as its matrix phase means that the area fraction of the austenite phase is 90% or more. When the area rate of the austenite phase is 90% or more, low temperature toughness can be guaranteed. The remainder other than austenite is the ferrite or martensite phase. However, even a small amount of ε martensite deteriorates toughness at low temperatures. Thus, like the microstructure that has austenite as a matrix phase according to the invention, it is preferably a structure that substantially has no ε martensite phase. Specifically, to ensure

Petition 870190106645, of 10/21/2019, p. 70/88

Low temperature toughness, the volume fraction of martensite ε is preferably less than 1.0%, more preferably less than 0.5% and still preferably less than 0.1%.

Non-metallic inclusions area fraction: less than 5.0% [029] As for non-metallic inclusions, type A means sulphide inclusions, type B means cluster type inclusions and type C means granular oxide inclusions. When a large number of these non-metallic inclusions exist in steel, they become a source of fracture, deteriorating toughness at extremely low temperature and ductility. Therefore, the area fraction of inclusions in the total needs to be limited to 5% or less and, preferably, 4% or less. Consequently, the chemical composition control as indicated above and the following production method need to be carried out.

[030] In addition, when the area rate of the austenite phase is equal to or greater than 90% and the area fraction of non-metallic inclusions is less than 5.0%, steel exhibiting extremely low temperature toughness and good ductility can be provided.

[031] When the above requirements are met, the properties desired in the invention can be obtained. For example, when Mn high steel is subjected to welding treatment, the low temperature toughness of a thermally affected zone is of particular concern. However, when Mn high steel that meets the requirements mentioned above is used, the microstructure of the thermally affected zone has austenite as its matrix phase, and the grain size of the austenite is an equivalent circular diameter of 50 μίτι or more. Thus, low temperature toughness is guaranteed in the thermally affected area.

[032] Specifically, the thickening of the crystal grains is effective

Petition 870190106645, of 10/21/2019, p. 71/88

14/25 to guarantee low temperature toughness in austenite steel. The same applies to a thermally affected area. For example, to obtain a value of 100 J or more as energy absorbed from a Charpy impact test at 196 ° C, the largest crystal grain size of the microstructure must be 50 μίτι or more, which is achieved with use high Mn steel that meets the requirements mentioned above.

[033] In the invention, to further improve low temperature toughness and toughness, in addition to the essential elements described above, the following elements can be present as needed.

[034] At least one of 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.

[035] Cu: 0.01% or more and 1.00% or less, Ni: 0.01% or more and 1.00% or less, Mo, V and W: 2.0% or less [036] Cu, Ni, Mo, V and W stabilize austenite and improve the strength of the base metal. To achieve this 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, when the content of each Cu and Ni is above 1.00%, or the content of each Mo, V and W is above 2.0%, thick carbonitrides are generated, which can become a source of fracture and increase production costs. Therefore, when these alloying elements are contained, the content of each Cu and Ni is preferably 1.00% or less, and the content of each Mo, V and W is preferably 2.0% or less. The content of each Cu and Ni is more preferably 0.05% or more and 0.70% or less. The content of each Μη, V and W is more preferably 0.003% or more and 1.7% or less.

Petition 870190106645, of 10/21/2019, p. 72/88

15/25 [037] 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 [038] Ca, Mg and REM are useful elements for the morphological control of inclusions and can be contained as needed. The morphological control of the inclusions means elongated granular inclusions based on sulfide. The morphological control of inclusions improves ductility, toughness and resistance to cracking by corrosion under stress by sulfide. To obtain such effects, Ca and Mg are preferably contained in an amount of 0.0005% or more and REM is preferably present in an amount of 0.0010% or more. On the other hand, when these elements are present in large quantities, not only can the amount of non-metallic inclusions be increased, ending the deterioration of ductility, toughness and resistance to cracking by corrosion under tension by sulfide, but it can also be caused by economic disadvantage.

[039] Therefore, when Ca and Mg are present, the content of each element is defined as 0.0005% or more and 0.0050% or less. When REM is contained, the content is defined as 0.0010% or more and 0.0200% or less. Preferably, the Ca content is defined as 0.0010% or more and 0.0040% or less, the Mg content is defined as 0.0010% or more and 0.0040% or less, and the REM content is defined as 0.0020% or more and 0.0150% or less.

[040] Our Mn high steel can be obtained from cast steel with the chemical composition mentioned above, which is prepared by steel making, using a publicly known method, such as the use of a converter and an electric heating oven. In addition, Mn high steel can also be subjected to secondary refinement in a vacuum degassing furnace. During secondary refinement, to limit the content of Ti and Nb that

Petition 870190106645, of 10/21/2019, p. 73/88

16/25 makes it difficult to adequately control the structure in the range mentioned above, it is necessary to prevent Ti and Nb from being inevitably mixed from raw materials or similar in steel and decrease the content of Ti and Nb. For example, by decreasing the basicity of the slag in the refining stage, these alloy elements are concentrated in the slag to be discharged, thus decreasing the concentrations of Ti and Nb in the final plate product. In addition, a method can be used in which oxygen is injected to oxidize, float and separate Ti and Nb alloys during circulation. Thereafter, it is preferable to transform the steel into a steel material, such as a size plate determined by a publicly known steel production method, such as continuous casting or ingot and rough casting.

[041] In addition, production conditions are defined to transform the steel material mentioned above into a steel material that exhibits excellent toughness at low temperature.

[042] Heating temperature of a steel material: 1100 ° C or more and 1300 ° C or less

To thicken the crystal grains of the microstructure of the steel material, the heating temperature before hot rolling is 1100 ° C or more. In addition, when the lower limit of the heating temperature of the steel material is below 1100 ° C, the amount of non-metallic inclusions in the steel increases, thus deteriorating the toughness at extremely low temperature and ductility. However, the heating temperature above 1300 ° C can cause local melting. Thus, the upper limit of the heating temperature is defined as 1300 ° C. The temperature is controlled based on the surface temperature of the steel material.

[043] Finishing lamination temperature: 800 ° C or more and less than 950 ° C

Petition 870190106645, of 10/21/2019, p. 74/88

A steel billet or billet is heated and subsequently subjected to hot rolling. In order to produce coarse crystal grains, it is preferable to increase the ratio of the cumulative reduction of the high temperature lamination. However, the crystal grains become excessively coarse over a temperature range of 950 ° C or more and therefore the desired elastic limit cannot be achieved. Therefore, final finishing lamination of at least one pass at less than 950 ° C is required. On the other hand, hot rolling at low temperature refines the microstructure and introduces excessively deformation in the processing, deteriorating the toughness at low temperature. Therefore, the lower limit of the lamination finish temperature is set to 800 ° C.

[044] Average cooling rate from a temperature equal to or greater than (lamination finishing temperature - 100 ° C) at a temperature range of 300 ° C or more and 650 ° C or less: 1.0 ° C / s or more [045] After hot rolling, cooling is performed immediately. The slow cooling of the steel sheet after hot rolling promotes the formation of precipitates, deteriorating the toughness at low temperature. Cooling the steel sheet to a cooling rate of 1 ° C / s or more can prevent the formation of these precipitates. In addition, excessive cooling distorts the steel sheet, deteriorating productivity. Therefore, the upper limit of the initial cooling temperature is defined as 900 ° C. For the reasons mentioned above, in cooling after hot rolling, the average rate of cooling of a steel sheet surface from a temperature equal to or greater (rolling finishing temperature - 100 ° C) to a temperature range 300 ° C or more and 650 ° C or less is set to 1.0 ° C / s or more. In the case of air cooling, a thick steel plate with a thickness of 10 or less

Petition 870190106645, of 10/21/2019, p. 75/88

18/25 mm, the cooling rate is 1 ° C / s or more. When air-cooling a steel sheet with a thickness of 10 mm or less, the steel sheet can be free of deformation.

[046] In addition, after the cooling treatment, the heating treatment to a temperature range of 300 ° C or more and 650 ° C or less and then cooling can be added as needed. Specifically, tempering treatment can be carried out to adjust the strength of the steel sheet.

Examples [047] The present invention will be described in more detail below by way of Examples. Note that the present invention is not limited to the following Example.

[048] The steel plates with the chemical compositions listed in Table 1 were made by a refining process with converter and melting pot and continuous casting. Then, the obtained steel plates were loaded in a heating oven and heated to 1150 ° C and, later, hot rolled in steel sheets with a thickness of 10 mm to 30 mm. The tensile and toughness properties of steel sheets were assessed as described below.

(1) Tensile Test Properties [049] JIS NO. 5 tensile test pieces were collected from each steel plate. Then, the test pieces were subjected to a tensile test according to J IS Z 2241 (1998) to investigate the properties of the tensile test. In the invention, when a test piece had an elastic limit of 400 MPa or more and a tensile strength of 800 MPa or more, the corresponding steel sheet was determined to have excellent tensile properties. In addition, when a test piece had a total elongation of 30% or more after breaking, it was determined that the corresponding steel sheet

Petition 870190106645, of 10/21/2019, p. 76/88

19/25 had excellent ductility.

(2) low temperature toughness [050] Charpy V-notch test pieces were collected from each steel sheet with a sheet thickness greater than 20 mm at position 1/4 of the sheet thickness or from each steel sheet with plate thickness equal to or less than 20 mm in the 1/2 position of the plate thickness, in a direction orthogonal to the lamination direction in accordance with J IS Z 2202 (1998). Then, the test pieces were subjected to the Charpy impact test, in accordance with J IS Z 2242 (1998), where three test pieces were used for each steel plate, to determine the energy absorbed at -196 ° C and assess base household toughness. When the three test pieces had an average absorbed energy (vE-i96) of 100 J or more, it was determined that the corresponding steel plate had good toughness of the base metal.

[051] The results of the assessment described above are listed in Table 2.

Petition 870190106645, of 10/21/2019, p. 77/88

Table 1

Steel No. Chemical composition (% by mass) Comments Ç Si Mn P s Al Cr O N Nb V You Ass Ni Mo w Here Mg REM 1 0.600 0.45 27.3 0.028 0.0066 0.029 4.52 0.0028 0.0084 0.002 - 0.002 - - - - - - - Invented steel 2 0.369 0.32 21.8 0.015 0.0037 0.026 2.26 0.0036 0.0073 0.001 - 0.002 - - 0.25 - - - - Invented steel 3 0.552 0.38 21.0 0.023 0.0037 0.047 5.71 0.0027 0.0307 0.001 - 0.002 - - - - - - - Invented steel 4 0.554 0.07 24.7 0.019 0.0028 0.028 6.00 0.0018 0.0192 0.002 - 0.002 - - - - - - - Invented steel 5 0.374 0.65 19.5 0.011 0.0046 0.027 5.82 0.0020 0.0211 0.002 - 0.004 - - - - - - - Invented steel 6 0.481 0.42 26.6 0.006 0.0019 0.016 6.23 0.0033 0.0060 0.003 - 0.003 0.4 - - - - - - Invented steel 7 0.454 0.51 24.5 0.020 0.0030 0.042 1.80 0.0022 0.0280 0.003 0.07 0.003 - - - - - - - Invented steel 8 0.633 0.43 24.2 0.016 0.0064 0.035 5.27 0.0035 0.0389 0.004 - 0.001 - - - - - - - Invented steel 9 0.753 0.45 29.0 0.015 0.0039 0.029 3.20 0.0034 0.0104 0.001 - 0.001 0.5 0.5 - - - - - Invented steel 10 0.545 0.50 22.1 0.010 0.0057 0.020 6.72 0.0030 0.0357 0.002 - 0.001 - - - - - - - Invented steel 11 0.412 0.38 24.4 0.016 0.0036 0.070 4.64 0.0015 0.0205 0.001 - 0.001 - - - - - - - Invented steel 12 0.799 0.38 27.7 0.014 0.0050 0.054 4.80 0.0029 0.0276 0.001 - 0.001 - - 0.05 - - - - Invented steel 13 0.435 0.48 18.9 0.027 0.0042 0.040 5.65 0.0027 0.0400 0.002 - 0.001 - - - - - - - Invented steel 14 0.686 0.47 20.1 0.026 0.0022 0.049 5.16 0.0021 0.0339 0.001 - 0.001 - - - 0.05 - - - Invented steel 15 0.669 0.38 28.5 0.022 0.0035 0.046 3.83 0.0029 0.0175 0.002 - 0.001 - - - - 0.0021 - - Invented steel 16 0.629 0.28 18.5 0.008 0.0041 0.060 2.83 0.0040 0.0225 0.002 - 0.001 - - - - - 0.0020 - Invented steel 17 0.356 0.39 27.3 0.011 0.0034 0.038 2.02 0.0026 0.0121 0.002 - 0.001 - - - - - - 0.0015 Invented steel

Note: Underlines are outside the scope of the invention.

20/25

Petition 870190106645, of 10/21/2019, p. 78/88

Table 1 (continued)

Steel No. Chemical composition (% by mass) Comments Ç Si Mn P s Al Cr O N Nb V You Ass Ni Mo w Here Mg REM 18 0.924 0.49 17.0 0.009 0.0065 0.033 3.92 0.0035 0.0326 0.002 - 0.001 0.2 - 0.10 - - - - Comparative Steel 19 0.080 0.54 27.8 0.015 0.0034 0.034 6.11 0.0038 0.0126 0.004 - 0.003 - - - - - - - Comparative Steel 20 0.582 1.20 26.9 0.013 0.0057 0.040 1.80 0.0019 0.0291 0.002 0.005 0.001 - - - - - - 0.0055 Comparative Steel 21 0.514 0.01 27.1 0.010 0.0028 0.070 5.34 0.0025 0.0144 0.003 - 0.003 - - - - - - - Comparative Steel 22 0.488 0.45 32.3 0.015 0.0042 0.052 6.64 0.0032 0.0129 0.002 - 0.002 - - - - - - - Comparative Steel 23 0.680 0.64 14.5 0.002 0.0036 0.049 6.83 0.0026 0.0123 0.001 - 0.001 - - 0.21 0.12 - - - Comparative Steel 24 0.479 0.47 15.8 0.040 0.0045 0.051 3.48 0.0024 0.0268 0.001 - 0.001 - - - - 0.0020 - - Comparative Steel 25 0.265 0.31 18.5 0.017 0.0085 0.035 4.45 0.0024 0.0311 0.001 - 0.001 - - - - - - - Comparative Steel 26 0.498 0.28 26.6 0.019 0.0044 0.009 5.32 0.0072 0.0189 0.002 - 0.002 - - - - - - - Comparative Steel 27 0.621 0.42 26.8 0.014 0.0039 0.080 5.91 0.0029 0.0201 0.003 - 0.003 - - - - - - - Comparative Steel 28 0.270 0.66 26.7 0.021 0.0046 0.034 4.16 0.0065 0.0202 0.001 - 0.001 - - 0.08 - - - 0.0032 Comparative Steel 29 0.569 0.58 25.9 0.018 0.0051 0.045 7.50 0.0039 0.0162 0.002 - 0.003 - - - - - - - Comparative Steel 30 0.612 0.48 18.2 0.008 0.0010 0.039 0.30 0.0034 0.0396 0.001 - 0.001 - - - 0.08 0.0018 - - Comparative Steel 31 0.556 0.45 21.6 0.025 0.0041 0.022 1.43 0.0021 0.0523 0.001 - 0.001 - - - - - - - Comparative Steel 32 0.588 0.47 27.4 0.021 0.0038 0.047 5.53 0.0027 0.0029 0.003 - 0.002 - - - - - - - Comparative Steel 33 0.318 0.49 20.4 0.013 0.0024 0.044 3.43 0.0029 0.0090 0.010 - 0.001 - - 0.35 - - 0.0025 - Comparative Steel 34 0.518 0.53 25.5 0.017 0.0038 0.048 3.99 0.0037 0.0123 0.005 - 0.003 - - - - - - - Comparative Steel 35 0.603 0.59 27.1 0.018 0.0044 0.037 5.18 0.0041 0.0089 0.002 - 0.005 - - - - - - - Comparative Steel 36 0.432 0.71 21.8 0.023 0.0032 0.047 3.47 0.0022 0.0214 0.001 - 0.010 - - - - - - - Comparative Steel

21/25

Note: Underlines are outside the scope of the invention

Petition 870190106645, of 10/21/2019, p. 79/88

Table 2

Sample to N- B.C 0 N- Sheet thickness (mm) Production method Base metal properties Tenacity and base metal Part ZTA Comments Plate heating temperatures (Ç) Finishing rolling temperatures (Ç) Cooling start temperature 0 (-Ç) Cooling rate (*) (C / s) Reheat temperature 0 (-Ç) Inclusion area ratio s (%) Volume fraction of martensit at ε (%) Elastic limit (MPa) Tensile strength (MPa) Lengthening the total (%) Energy absorbed at -196 ° C (vE-196 ° c) (J) Energy absorbed at -196 ° C (VE-I96 ° c) (J) Larger grain size (pm) 1 1 10 1110 945 872 13 450 0.2 0.0 480 912 60 100 145 165 Example 2 2 15 1110 835 768 7 - 4.0 0.0 510 934 62 108 100 175 Example 3 3 20 1100 890 815 12 - 2.0 0.0 487 912 61 113 112 150 Example 4 4 25 1100 910 845 10 - 0.2 0.0 446 891 61 124 134 187 Example 5 5 10 1150 860 795 12 500 0.2 0.1 467 905 62 125 121 50 Example 6 6 15 1150 890 808 5 550 0.3 0.0 442 899 61 133 129 102 Example 7 7 20 1150 825 785 7 - 0.2 0.0 445 903 61 122 128 100 Example 8 8 5 1200 825 740 4 - 1.0 0.0 598 987 51 106 108 50 Example 9 9 10 1200 810 730 10 450 0.1 0.0 484 858 65 112 115 100 Example 10 10 15 1200 870 798 10 - 0.3 0.0 466 884 64 127 124 150 Example 11 11 15 1250 860 795 9 500 2.0 0.0 487 840 66 114 106 100 Example 12 12 20 1250 915 860 13 550 0.5 0.0 435 850 67 113 113 168 Example 13 13 25 1250 860 805 6 - 1.5 0.2 441 852 65 109 110 125 Example 14 14 30 1250 815 744 3 300 0.1 0.0 426 853 61 132 127 109 Example 15 15 10 1300 870 821 1 - 0.1 0.0 422 874 61 104 108 115 Example 16 16 15 1300 854 784 7 450 1.0 0.0 501 926 57 116 114 89 Example 17 17 20 1300 884 810 20 - 1.5 0.0 512 934 52 112 116 97 Example

22/25

Petition 870190106645, of 10/21/2019, p. 80/88

Table 2 (continued)

Sample to N- B.C 0 N- Sheet thickness (mm) Production method Base metal properties Tenacity and base metal Part ZTA Comments Plate heating temperatures (Ç) Finishing rolling temperatures 0 (Ç) Cooling start temperature 0 (° C) Cooling rate (*) (C / s) Reheat temperature 0 (° C) Inclusion area ratio s (%) Volume fraction of martensit at ε (%) Elastic limit 0 (MPa) Tensile strength (MPa) Lengthening the total (%) Energy absorbed at -196 ° C (vE-196 ° c) (J) Energy absorbed at -196 ° C (VE-I96 ° c) (J) Larger grain size (pm) 18 18 10 1150 895 810 5 - 2.0 0.0 512 922 58 56 61 125 Comparative Example 19 19 15 1150 815 735 15 - 0.1 l, o 471 879 57 121 61 95 Comparative Example 20 20 20 1150 862 805 5 - 0.5 0.0 361 701 51 45 44 80 Comparative Example 21 21 25 1150 871 820 13 - 0.1 0.0 425 871 61 24 26 80 Comparative Example 22 22 10 1150 900 815 10 - 4.0 0.0 497 922 34 46 48 105 Comparative Example 23 23 15 1150 850 815 5 - 0.2 0.5 455 906 60 51 53 65 Comparative Example 24 24 20 1200 840 770 11 - 0.2 0.0 375 716 61 78 65 60 Comparative Example 25 25 5 1200 920 850 10 - 5.5 0.0 467 900 30 42 41 126 Comparative Example 26 26 10 1200 875 805 5 - 4.5 0.0 525 927 54 θζ 68 78 Comparative Example 27 27 15 1200 855 790 12 - 0.3 0.0 511 916 58 65 41 65 Comparative Example 28 28 15 1150 896 853 5 - 3.5 0.0 456 897 29 70 66 100 Comparative Example 29 29 15 1150 916 848 4 - 0.5 0.0 432 875 25 43 46 125 Comparative Example 30 30 15 1150 898 805 9 - 0.1 0.0 465 912 60 60 58 110 Comparative Example 31 31 15 1150 908 821 11 - 0.2 0.0 448 905 35 47 51 105 Comparative Example 32 32 15 1150 850 785 4 - 0.1 0.0 487 932 55 79 68 75 Comparative Example 33 33 15 1150 806 726 6 - 0.2 0.2 512 936 45 59 61 35 Comparative Example

23/25

Petition 870190106645, of 10/21/2019, p. 81/88

34 34 15 1150 827 768 4 - 0.2 0.0 487 909 45 78 76 40 Comparative Example 35 35 15 1150 878 804 3 - 0.1 0.0 439 887 51 69 69 40 Comparative Example 36 36 15 1150 851 776 7 - 0.1 0.0 466 889 43 51 51 35 Comparative Example 37 1 20 1050 815 724 8 - 5.5 0.0 560 1064 42 78 91 165 Comparative Example 38 2 25 1250 960 885 9 - 3.5 0.1 367 724 61 116 108 175 Comparative Example 39 3 30 1250 780 695 14 - 2.0 0.0 598 987 52 54 81 150 Comparative Example 40 4 10 1250 835 715 10 - 0.2 0.0 437 879 28 65 20 187 Comparative Example 41 5 10 1250 975 889 8 - 0.2 0.0 375 786 64 126 124 50 Comparative Example 42 6 15 1250 850 800 05 - 0.3 0.0 412 861 28 55 67 102 Comparative Example 43 7 20 1250 915 865 9 800 0.2 0.0 316 704 45 119 112 100 Comparative Example

Note: Underlines are outside the scope of the invention (*) an average cooling rate from a temperature equal to or greater (laminating finish temperature -100 ° C) to a temperature range of 300 ° C or more and 650 ° C or less

24/25

Petition 870190106645, of 10/21/2019, p. 82/88

25/25 [052] It has been confirmed that the high Mn steel of the invention satisfies the desired performance mentioned above (an elastic limit of base metal equal to or greater than 400 MPa, total elongation after breaking 30% or more, an average of the energy absorbed (vE-i96) of 100 J or more for low temperature toughness). On the other hand, comparative examples outside the scope of the invention did not satisfy the desired performance mentioned above in terms of one or more of total elongations, elastic limit and low temperature toughness.

[053] In addition, to evaluate the impact absorbing properties of a welded part, the steel materials were subjected to heat cycle treatment under peak temperature conditions of 1400 ° C and cooling rate of 10 ° C / s . The results are listed in Table 2. As shown in Table 2, the steel materials according to the invention were excellent in the toughness of the welded part, as well as in the toughness of the base metal at low temperature. Specifically, in welding that provided input heat from 0.5 kJ / cm to 5 kJ / cm, the maximum crystal grain size was 50 μηη or more, and the energy absorbed in the Charpy impact test at -196 ° C was 100 J or more.

[054] The present invention claims priority to patent application JP 2017-087702 A filed on April 26, 2017, which is incorporated here for reference in its entirety.

Claims (4)

1. High Mn steel characterized by the fact that it comprises: chemical composition containing, in% by mass, 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, and O: 0.0050% or less, Ti: less than 0.005%, and Nb: less than 0.005%, with the remainder being Fe and unavoidable impurities; a microstructure containing austenite as a matrix phase and non-metallic inclusions with an area fraction of less than 5.0%; and an elastic limit of 400 MPa or more and an absorbed energy (vE-i9g) of 100 J or more. 2. High Mn steel, according to claim 1, characterized by the fact that the chemical composition still contains, in% by mass, at least one selected from among 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, Petition 870190106645, of 10/21/2019, p. 87/88 2/2 Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0010% or more and 0.0200% or less. 3. High Mn steel production method characterized by the fact that it comprises: heating a steel material with the chemical composition defined in claims 1 or 2, to a temperature range of 1100 ° C or more and 1300 ° C or less; hot rolling of the steel material with a rolling finish temperature of 800 ° C or more and less than 950 ° C to obtain a hot-rolled steel sheet; and then subjecting the hot-rolled steel sheet to the cooling treatment at an average cooling rate of 1.0 ° C / s or more from a temperature equal to or greater than (the rolling finishing temperature - 100 ° C) at a temperature range of 300 ° C or more and 650 ° C or less. 4. High Mn steel production method, according to claim 3, characterized by the fact that it comprises: after the cooling treatment, heating the steel sheet to a temperature range of 300 ° C or more and 650 ° C or less and then cooling the steel sheet.