JP6206423B2 - High strength stainless steel plate excellent in low temperature toughness and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength stainless steel plate and a method for producing the same, and more particularly to improvement of low-temperature toughness.

  In recent years, due to problems such as soaring energy prices such as crude oil and depletion of oil resources, oil fields with deeper depths that were not previously excluded, and oil fields under severe corrosive environments (so-called sour environments) including hydrogen sulfide, etc. Development of oil fields and gas fields under severe weather conditions such as those in the extreme north has been actively conducted. Steel materials used in such an environment are required to have high strength, excellent corrosion resistance (sour resistance), and excellent low temperature toughness.

Conventionally, 13% Cr martensitic stainless steel has been frequently used as a steel material in oil fields and gas fields under corrosive environments containing carbon dioxide CO ₂ , chlorine ions Cl ^{− and the} like. However, under the sour environment, 13% Cr martensitic stainless steel has insufficient corrosion resistance, and recently, the use of duplex stainless steel with reduced C content and increased Cr content and Ni content has been expanded. .

For example, Patent Document 1 describes a method for producing martensitic stainless steel having excellent corrosion resistance and sulfide stress corrosion cracking resistance. In the technique described in Patent Document 1, by weight%, C: 0.005-0.05%, Si: 0.05-0.5%, Mn: 0.1-1.0%, Cr: 10-15%, Ni: 4.0-9.0%, Cu : Steel containing 0.5 to 3%, Mo: 1.0 to 3%, Al: 0.005 to 0.2%, N: 0.005 to 0.1%, with Nieq adjusted to -10 or more, hot-worked to room temperature After natural cooling, heat treatment is performed at a temperature not lower than an Ac ₁ point and a temperature at which the austenite fraction is 80% or less, and further, heat treatment is performed at a temperature or less at which the austenite fraction is 60%. Thereby, it consists of an above-described composition, a tempered martensite phase, a martensite phase, and a retained austenite phase. The total fraction of the tempered martensite phase and the martensite phase is 60 to 90%, and the balance is the retained austenite phase. It is said that martensitic stainless steel (steel plate) having a structure and excellent in corrosion resistance and sulfide stress corrosion cracking resistance in a wet carbon dioxide environment and a wet hydrogen sulfide environment is obtained.

Patent Document 2 describes a method for producing a high-strength stainless steel seamless steel pipe for oil wells that has excellent corrosion resistance. In the technique described in Patent Document 2, in mass%, C: 0.005 to 0.05%, Si: 0.05 to 0.5%, Mn: 0.2 to 1.8%, Cr: 15.5 to 18%, Ni: 1.5 to 5%, Mo : 1 to 3.5%, V: 0.02 to 0.2%, N: 0.01 to 0.15%, O: 0.006% or less, Cr + 0.65Ni + 0.6Mo + 0.55Cu-20C ≧ 19.5 and Cr + Mo + 0.3Si-43.5C-0.4Mn -Ni-0.3Cu-9N≥11.5 steel pipe material is heated, piped by hot working, and then piped and then cooled to room temperature at a cooling rate equal to or higher than that of air cooling to seamlessly pass through the specified dimensions. Then, after reheating the seamless steel pipe to a temperature of 850 ° C. or higher, cooling it to 100 ° C. or lower at a cooling rate of air cooling or higher, and then performing a quenching and tempering treatment to heat it to a temperature of 700 ° C. or lower. It is said. As a result, it has a structure including a ferrite phase of 10 to 60% by volume and the balance being a martensite phase, yield strength: 654 MPa or more, Charpy impact test absorption energy at a test temperature: −40 ° C. is 50 J It is said that a high-strength stainless steel pipe for oil wells having the above-mentioned high toughness and sufficient corrosion resistance in a severe corrosive environment up to 230 ° C. containing CO ₂ and Cl ⁻ is obtained.

Japanese Patent Laid-Open No. 10-1755 Japanese Patent No. 5109222

  In addition to seamless steel pipes for oil wells, recently, steel sheets with high strength and excellent corrosion resistance have been required for UOE steel pipes, ERW welded steel pipes, spiral steel pipes, etc., and steel structures. As a material for steel pipes from the standpoint of improving the pressure resistance and the fracture resistance, and as a material for the structure from the viewpoint of increasing the size of the structure, a thick and large diameter material is required.

  However, the techniques described in Patent Documents 1 and 2 target steel materials having a thickness of up to 12.7 mm, and there is no mention of thick steel materials exceeding a thickness of 15 mm. Moreover, recently, oil wells have been deepened, and thick steel materials are often used as steel materials used for oil wells. As the thickness increases, it is difficult to apply a desired processing strain to the thickness center portion with a normal hot working method, and the structure of the thickness center portion tends to become coarse. Therefore, the thick steel material has a problem that the low temperature toughness at the central portion of the wall thickness is likely to be lower than that of the thin steel material. In addition, when the thickness is increased, the influence of the surface per thickness is reduced, and the influence of the structure at the central portion of the thickness on the toughness is increased. In addition, recently, oil wells under severe weather conditions are increasing, and steel materials used for oil wells are required to have excellent low temperature toughness. However, Patent Documents 1 and 2 do not mention particularly the improvement of low-temperature toughness in thick-walled steel materials.

The object of the present invention is to solve the problems of the prior art and to provide a high-strength stainless steel plate excellent in low-temperature toughness and a method for producing the same. Here, “high strength” refers to the case where the yield strength YS is 600 MPa or more, and “excellent in low temperature toughness” refers to the absorbed energy vE ₋₄₀ of the Charpy test at a test temperature of −40 ° C. Shall be the case where is over 100J. Further, the term “thick steel plate” used herein refers to a case where the steel plate thickness is 15 mm or more, preferably 50 mm or less.

  In order to achieve the above-mentioned object, the present inventors first conducted intensive research on various factors affecting the low temperature toughness of a martensitic stainless steel plate. As a result, in order to improve the low temperature toughness of the stainless steel plate, it is effective to make the structure contain an appropriate amount of a stable austenite phase in addition to the martensite phase and the ferrite phase. I came up with it.

As a result of further studies by the present inventors, the ratio of the Ni concentration in the austenite phase to the Ni concentration in the surrounding martensite phase, (C _Ni ) _γ / (C _Ni ) _M, is the stability of the austenite phase. I found out that it controls. Then, it was found that a predetermined amount of stable austenite phase can be secured by adjusting (C _Ni ) _γ / (C _Ni ) _M to an appropriate range (1.15 or more) by combining thermomechanical treatment. Furthermore, the present inventors have found that in order to further improve the low temperature toughness of the stainless steel plate, it is necessary to make the ferrite phase finer.

  In order to obtain the structure as described above, the material of the martensitic stainless steel composition is heated to a high temperature (1100 to 1350 ° C) at which the ferrite phase fraction becomes high, and then hot-rolled to obtain a thick steel plate. Before the hot rolling, in the middle of the hot rolling, or after the hot rolling, an accelerated cooling treatment is performed to adjust the structure in a state where the ferrite → austenite transformation is suppressed, and the cooling is once performed to 500 ° C. or less. After that, it has been found that it is effective to further heat and cool at least once to a two-phase temperature (temperature in the temperature range of 550 to 980 ° C.) of (ferrite + austenite).

  As a result, Ni is concentrated in the austenite phase, and a predetermined amount of austenite phase can be secured stably. In addition, an appropriate amount of granular martensite phase is formed in the ferrite phase, and the ferrite phase is divided and refined. The obtained knowledge that it can be set as the thick steel plate which has the structure | tissue was obtained.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.050% or less, Si: 0.50% or less, Mn: 0.20 to 1.80%, Cr: 15.5 to 18.0%, Ni: 1.5 to 5.0%, Mo: 1.0 to 3.5%, V: 0.02 -0.20%, Al: 0.05% or less, N: 0.01-0.15%, O: 0.006% or less, the composition consisting of the balance Fe and unavoidable impurities, the martensite phase of 50% or more by volume, and 3 -15% of austenite phase and the balance being composed of ferrite phase, and Ni concentration (C _Ni ) _γ (mass%) in the austenite phase and Ni concentration (C _Ni ) _M in the martensite phase A high-strength stainless steel plate excellent in low-temperature toughness, characterized in that the ratio to (% by mass), (C _Ni ) _γ / (C _Ni ) _M, is 1.15 or more.
(2) In (1), the structure is a structure in which a part of the martensite phase is in a granular form having a particle size of 10 μm or less and includes 3.0 or more per 100 μm ^{2 of} ferrite grains in the ferrite phase. High strength stainless steel plate.
(3) A high-strength stainless steel plate according to (1) or (2), characterized in that, in addition to the above composition, the composition further includes Cu: 3.5% or less by mass%.
(4) In any one of (1) to (3), in addition to the above-mentioned composition, in mass%, Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, W: 3.0% or less A high-strength stainless steel plate having a composition containing one or more selected from among them.
(5) In any one of (1) to (4), in addition to the above composition, the composition further includes one or two selected from Ca: 0.01% or less and REM: 0.01% or less by mass%. A high-strength stainless steel plate having a composition.
(6) When the steel material is subjected to a heating step and a hot rolling step to obtain a thick steel plate having a predetermined plate thickness, the steel material is, in mass%, C: 0.050% or less, Si: 0.50% or less, Mn : 0.20 to 1.80%, Cr: 15.5 to 18.0%, Ni: 1.5 to 5.0%, Mo: 1.0 to 3.5%, V: 0.02 to 0.20%, Al: 0.05% or less, N: 0.01 to 0.15%, O: 0.006 %, And the remaining Fe and inevitable impurities are used as a steel material, and the heating step is a step of heating the steel material to a heating temperature of 1100 to 1350 ° C., and the hot rolling step is hot Rolling is interrupted in the middle, and cooling is started from a temperature of 900 ° C. or higher at the plate thickness central temperature, and the center of the plate thickness is reached from the temperature at which the cooling is started to a cooling stop temperature of at least 50 ° C. and 800 ° C. or higher. After performing an accelerated cooling step of cooling at an average cooling rate of 2.0 ° C./s or higher at a temperature, the hot rolling is resumed to obtain a thick steel plate having the predetermined plate thickness Then, after the hot rolling step, the steel sheet is cooled to a temperature of the plate thickness center temperature of 500 ° C. or lower, and then heated to a temperature in the range of 550 to 980 ° C., and then subjected to a heat treatment step of performing at least one heat treatment for cooling. A method for producing a high-strength stainless steel plate having excellent low-temperature toughness.
(7) In (6), the hot rolling is interrupted in the middle, and when the accelerated cooling step is performed, the steel plate temperature in the middle of hot rolling, when the plate thickness center temperature is less than 900 ° C, A method for producing a high-strength stainless steel plate, characterized by performing a heat treatment during hot rolling in which the steel sheet is heated to a temperature of 900 ° C. or more at a center thickness, and then performing the accelerated cooling step.
(8) When the steel material is subjected to a heating step and a hot rolling step to obtain a thick steel plate having a predetermined plate thickness, the steel material is, in mass%, C: 0.050% or less, Si: 0.50% or less, Mn : 0.20 to 1.80%, Cr: 15.5 to 18.0%, Ni: 1.5 to 5.0%, Mo: 1.0 to 3.5%, V: 0.02 to 0.20%, Al: 0.05% or less, N: 0.01 to 0.15%, O: 0.006 %, And the heating material is a step of heating the steel material to a heating temperature of 1100 to 1350 ° C., and the hot rolling step is the heating material. Immediately start cooling the steel material subjected to the process, the temperature at which the cooling is started as a cooling start temperature, from the cooling start temperature to a cooling stop temperature at least 50 ° C. and 800 ° C. or higher, After performing an accelerated cooling step of cooling at an average cooling rate of 2.0 ° C./s or more at the plate thickness center temperature, the hot rolling is performed, and the predetermined plate After the hot rolling step, the steel plate is cooled to a temperature not exceeding 500 ° C. at the center thickness, and then heated to a temperature in the range of 550 to 980 ° C. A method for producing a high-strength stainless steel plate, characterized by performing a heat treatment step performed once.
(9) When a steel material is subjected to a heating step and a hot rolling step to obtain a thick steel plate having a predetermined thickness, the steel material is mass%, C: 0.050% or less, Si: 0.50% or less, Mn : 0.20 to 1.80%, Cr: 15.5 to 18.0%, Ni: 1.5 to 5.0%, Mo: 1.0 to 3.5%, V: 0.02 to 0.20%, Al: 0.05% or less, N: 0.01 to 0.15%, O: 0.006 %, Including the balance Fe and inevitable impurities as a steel material, and the heating step is a step of heating the steel material to a heating temperature of 1100 to 1350 ° C., and after performing the heating step, The hot rolling step is a step of performing hot rolling at a rolling end temperature of 900 ° C. or higher, and after the hot rolling step, cooling is started from a temperature of 900 ° C. or higher at the plate thickness center temperature of the thick steel plate. From the temperature at which the cooling is started to the cooling stop temperature at least 50 ° C. and 800 ° C. or more, at an average cooling rate of 2.0 ° C./s or more at the plate thickness center temperature. After the completion of the accelerated cooling process, the sheet is further cooled to a temperature at a sheet thickness center temperature of 500 ° C. or lower, and then heated to a temperature in the range of 550 to 980 ° C. A method for producing a high-strength stainless steel plate excellent in low-temperature toughness, characterized by performing a heat treatment step performed once.
(10) The method for producing a high-strength stainless steel plate according to any one of (6) to (9), further comprising Cu: 3.5% or less by mass% in addition to the above composition.
(11) In any one of (6) to (10), in addition to the above composition, in terms of mass%, Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, W: 3.0% or less A method for producing a high-strength stainless steel plate having a composition containing one or more selected from among them.
(12) In any one of (6) to (11), in addition to the above composition, the composition further includes one or two kinds selected from Ca: 0.01% or less and REM: 0.01% or less by mass%. A method for producing a high-strength stainless steel plate having a composition.

  According to the present invention, a high-strength stainless steel plate excellent in low-temperature toughness can be easily manufactured, and an industrially remarkable effect is achieved. In addition, according to the present invention, since the structure can be easily refined even in the center portion of the plate thickness, there is an effect that the low temperature toughness of the thick steel plate can be easily improved.

It is explanatory drawing which shows an example of the manufacturing equipment row | line for implementing this invention.

  First, the manufacturing method of this invention high strength thick steel plate is demonstrated.

  In the present invention, the steel material is subjected to a heating step and a hot rolling step to obtain a thick steel plate having a predetermined plate thickness. The steel materials used are mass%, C: 0.050% or less, Si: 0.50% or less, Mn: 0.20 to 1.80%, Cr: 15.5 to 18.0%, Ni: 1.5 to 5.0%, Mo: 1.0 to 3.5%, V: 0.02 to 0.20%, Al: 0.05% or less, N: 0.01 to 0.15%, O: 0.006% or less, and a steel material having a composition composed of the remaining Fe and inevitable impurities.

  First, the reasons for limiting the composition of the steel material used will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.

C: 0.050% or less
C is an important element related to the strength of martensitic stainless steel. In the present invention, C is preferably contained in an amount of 0.005% or more in order to ensure a desired strength. On the other hand, if the content exceeds 0.050%, sensitization during tempering due to Ni inclusion increases. From the viewpoint of corrosion resistance, it is desirable that C is less. For these reasons, C is limited to 0.050% or less. In addition, Preferably it is 0.030 to 0.050%.

Si: 0.50% or less
Si is an element that acts as a deoxidizer, and in order to obtain such an effect, it is desirable to contain 0.05% or more. On the other hand, if the content exceeds 0.50%, the corrosion resistance is lowered and the hot workability is also lowered. For this reason, Si was limited to 0.50% or less. In addition, Preferably it is 0.10 to 0.30%.

Mn: 0.20 to 1.80%
Mn is an element having an action of increasing the strength, and in order to obtain such an effect, it needs to be contained in an amount of 0.20% or more. On the other hand, if the content exceeds 1.80%, the toughness is adversely affected. For this reason, Mn was limited to 0.20 to 1.80%. In addition, Preferably it is 0.20 to 1.00%.

Cr: 15.5-18.0%
Cr is an element that has a function of forming a protective film to improve corrosion resistance and further increasing the strength of the steel by solid solution. In order to obtain such an effect, the content of 15.5% or more is required. On the other hand, if the content exceeds 18.0%, the hot workability is lowered and the strength is further lowered. For this reason, Cr was limited to 15.5-18.0%. In addition, Preferably it is 16.6 to 18.0%.

Ni: 1.5-5.0%
Ni is an element that has an action of strengthening the protective film and improving the corrosion resistance, and further increasing the strength of the steel by solid solution and further improving the toughness. In order to obtain such an effect, the content of 1.5% or more is required. On the other hand, if the content exceeds 5.0%, the stability of the martensite phase decreases and the strength decreases. For this reason, Ni was limited to 1.5 to 5.0%. In addition, Preferably it is 2.5 to 4.5%.

Mo: 1.0-3.5%
Mo is an element that increases resistance to pitting corrosion caused by Cl ⁻ . In order to acquire such an effect, it is necessary to contain 1.0% or more. On the other hand, if the content exceeds 3.5%, the strength decreases and the material cost increases. For this reason, Mo was limited to 1.0 to 3.5%. In addition, Preferably it is 2.0 to 3.5%.

V: 0.02 to 0.20%
V is an element that increases strength and improves corrosion resistance. In order to obtain such an effect, a content of 0.02% or more is required. On the other hand, if the content exceeds 0.20%, toughness decreases. For this reason, V was limited to 0.02 to 0.20%. In addition, Preferably it is 0.02 to 0.08%.

Al: 0.05% or less
Al is an element that acts as a deoxidizer, and in order to obtain such an effect, it is desirable to contain 0.002% or more. On the other hand, if it exceeds 0.05%, the toughness is adversely affected. For this reason, Al was limited to 0.05% or less.

N: 0.01-0.15%
N is an element that remarkably improves the pitting corrosion resistance. In order to obtain such an effect, N is required to be contained in an amount of 0.01% or more. On the other hand, if the content exceeds 0.15%, various nitrides are formed and the toughness is lowered. For this reason, N was limited to the range of 0.01 to 0.15. In addition, Preferably it is 0.02 to 0.08%.

O: 0.006% or less
O (oxygen) exists as an oxide in steel and adversely affects various properties. For this reason, it is desirable to reduce as much as possible. In particular, when O is contained in a large amount exceeding 0.006%, the hot workability, toughness and corrosion resistance are remarkably deteriorated. For this reason, O was limited to 0.006% or less.

  In the present invention in which the above-described components are basic components, in addition to the basic composition, if necessary, as an optional element, Cu: 3.5% or less and / or Nb: 0.2% or less, Ti: 0.3 %, Zr: 0.2% or less, W: 3.0% or less, and / or one selected from Ca: 0.01% or less, REM: 0.01% or less Or you may contain 2 types.

Cu: 3.5% or less
Cu strengthens the protective film and suppresses the penetration of hydrogen into the steel, improving the resistance to sulfide stress corrosion cracking. Such an effect becomes remarkable when the content is 0.5% or more. On the other hand, if the content exceeds 3.5%, CuS grain boundary precipitation occurs, and hot workability decreases. For this reason, when it contains, it is preferable to limit Cu to 3.5% or less. In addition, More preferably, it is 0.8 to 1.2%.

One or more selected from Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, W: 3.0% or less
Nb, Ti, Zr, and W are all elements that increase the strength, and can be selected to contain one or more as required. In order to obtain such an effect, it is desirable to contain Nb: 0.03% or more, Ti: 0.03% or more, Zr: 0.03% or more, and W: 0.2% or more. On the other hand, inclusions exceeding Nb: 0.2%, Ti: 0.3%, Zr: 0.2%, and W: 3.0% respectively reduce toughness. For this reason, when it contains, it is preferable to limit to Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, and W: 3.0% or less, respectively.

One or two selected from Ca: 0.01% or less, REM: 0.01% or less
Both Ca and REM are elements that have the effect of spheroidizing sulfide inclusions, reducing the lattice strain of the matrix surrounding the inclusions, and reducing the hydrogen trapping capacity of the inclusions. Yes, it can contain 1 type or 2 types as needed. In order to acquire such an effect, it is desirable to contain Ca: 0.0005% or more, REM: 0.001% or more. On the other hand, when it contains exceeding Ca: 0.01% and REM: 0.01%, toughness will fall. For this reason, when it contains, it is preferable to limit to Ca: 0.01% or less and REM: 0.01% or less.

  The balance other than the components described above consists of Fe and inevitable impurities. Inevitable impurities are P: 0.03% or less and S: 0.005% or less. Further, B is acceptable if it is 0.0010% or less.

  The method for producing a steel material having the above composition need not be particularly limited. Using a conventional melting furnace, such as a converter, electric furnace, etc., the molten steel having the composition described above is melted, and a cast material such as a slab is obtained as a steel material by a conventional casting method such as a continuous casting method. It is preferable to do. In addition, it is good also as a steel raw material as steel slabs, such as a slab of a predetermined dimension, hot-rolling a slab. Moreover, it is set as a steel slab by the ingot-making-slab rolling method, and there is no problem as a steel raw material.

  FIG. 1 shows an example of a production equipment row suitable as equipment used in the method for producing a high-strength stainless steel plate of the present invention. Examples of equipment for manufacturing thick steel plates include a heating device 1, a hot rolling device 2, a cooling device 3, a heat treatment device 4, and a heating means 5. The hot rolling device 2 may be divided into a rough rolling device 2a and a finish rolling device 2b as shown in FIG. 1 (b). Further, the cooling device 3 may be arranged separately from the cooling device 3a and the cooling device 3b according to the time when the accelerated cooling process is performed as shown in FIG.

  In the present invention, a steel material (slab) S having the above composition is charged into a heating device (heating furnace) 1 and heated to a heating temperature of 1100 to 1350 ° C.

Heating temperature: 1100-1350 ° C
When the heating temperature is less than 1100 ° C., the ferrite phase is small, and it is not possible to achieve remarkable microstructure refinement using the ferrite → austenite transformation. On the other hand, if the heating temperature is less than 1100 ° C., the deformation resistance becomes too high and subsequent hot working becomes difficult. On the other hand, at 1350 ° C. or higher, deformation due to its own weight occurs, or accumulation of strain due to molding (processing) becomes difficult. For this reason, the heating temperature of the steel material was limited to a temperature in the range of 1100 to 1350 ° C. Note that the temperature is preferably 1150 to 1300 ° C. from the viewpoint that the deformation resistance is small and processing is easy, and that the temperature difference can be increased during cooling.

  In the steel material having the composition range described above, a ferrite single phase is formed in a temperature range of 1200 ° C. or higher. In the temperature range from 1100 ° C to less than 1200 ° C, it exhibits a two-phase structure of ferrite and austenite, but in this temperature range, the ferrite is the majority, so the subsequent processing (hot rolling) can sufficiently refine the structure. Can be achieved.

  The heated steel material is then subjected to a hot rolling process by the hot rolling device 2 to obtain a thick steel plate having a predetermined size.

  In the present invention, since a desired microstructure can be achieved even at a relatively low rolling reduction, the hot rolling condition is not particularly limited as long as it can be a thick steel plate having a predetermined dimension. For miniaturization, it is preferable to set the rolling reduction to be 70% or more cumulatively. In the present invention, it is necessary to adjust the hot rolling conditions according to the conditions of the accelerated cooling process in order to refine the structure mainly by combining accelerated cooling and hot rolling.

  In the present invention, in the hot rolling process, hot rolling is interrupted halfway, and after performing a predetermined accelerated cooling process using the cooling device 3 (3a, 3b), the hot rolling device 2 again performs hot rolling. It is preferable to carry out rolling to obtain a thick steel plate having a predetermined thickness. The accelerated cooling treatment may be performed before hot rolling or after completion of hot rolling. However, it is better to perform cooling in the middle of hot rolling, cooling efficiency, and further refinement of the structure, and further the steel plate shape. From the viewpoint of the above.

  Here, the accelerated cooling process starts cooling from a temperature of 900 ° C. or higher at the plate thickness center temperature, and stops cooling at a temperature of 50 ° C. or higher and 800 ° C. or higher from the temperature at which the cooling started (cooling start temperature). It is set as the process cooled to the temperature by the average cooling rate of 2.0 degrees C / s or more by sheet thickness center temperature.

Cooling start temperature for accelerated cooling: Temperature at the center thickness of 900 ° C or higher “Cooling start temperature” here refers to the temperature of steel materials (including steel plates or steel plates in the middle of rolling) when accelerated cooling starts. It is. In the present invention, it is intended to refine the structure by hot rolling with as much ferrite phase as possible. Therefore, accelerated cooling is performed so that the structure before hot rolling becomes a structure containing as many supercooled ferrite phases as possible. Therefore, it is preferable that the start temperature of accelerated cooling be as high as possible. For this reason, in the present invention, the cooling start temperature of accelerated cooling is limited to a temperature of 900 ° C. or more at the plate thickness center temperature. In addition, Preferably it is 950 degreeC or more.

Accelerated cooling temperature range: 50 ° C or higher The temperature range of accelerated cooling, that is, the temperature difference between the cooling start temperature and the cooling stop temperature is at least 50 ° C or higher. If the temperature range of accelerated cooling is less than 50 ° C, the phase fraction in the non-equilibrium state cannot be increased, the effect of refining the structure by subsequent processing cannot be expected, and the coarsening of the structure proceeds with subsequent cooling, etc. There is a problem in that a predetermined amount of a granular ferrite phase cannot be formed by transformation, and it becomes difficult to adjust the distribution of elements to each phase within an appropriate range. For this reason, the temperature range of accelerated cooling was limited to 50 ° C. or higher. The larger the temperature range of accelerated cooling, the higher the phase fraction in the non-equilibrium state can be maintained. In addition, Preferably it is 100 degreeC or more.

Average cooling rate of accelerated cooling: 2.0 ° C./s or more at the plate thickness center temperature The average cooling rate of accelerated cooling is 2.0 ° C./s or more at the plate thickness center portion. When the average cooling rate is less than 2.0 ° C./s, the phase fraction in the non-equilibrium state cannot be kept high, and the effect of refining the structure by subsequent processing cannot be expected. For this reason, the average cooling rate of accelerated cooling was limited to 2.0 ° C./s or more. The upper limit of the average cooling rate is determined by the capacity of the cooling device and is not particularly limited, but is preferably 50 ° C./s or less from the viewpoint of preventing cracking and bending. In addition, Preferably it is 5-20 degrees C / s.

  The temperature at the center of the plate thickness is determined by heat transfer calculation from the surface temperature measured with a radiation thermometer. Needless to say, measurement may be performed by inserting a thermocouple into the center of the plate thickness.

Cooling stop temperature for accelerated cooling: 800 ° C or higher The cooling stop temperature for accelerated cooling shall be 800 ° C or higher. When the cooling stop temperature is less than 800 ° C., the diffusion of the alloy elements is delayed, and the subsequent phase transformation (α → γ transformation) by hot rolling is delayed, and the desired structure refinement effect cannot be expected. For this reason, the cooling stop temperature of accelerated cooling is limited to 800 ° C. or higher. In addition, Preferably it is 1000 degrees C or less.

  When hot rolling is interrupted halfway and the accelerated cooling process is performed, the steel plate temperature when hot rolling is interrupted is 900 ° C. at the center thickness of the plate thickness, which is a preferred cooling start temperature for accelerated cooling. The above temperature is preferable. When the sheet thickness center temperature is less than 900 ° C., the heating means 5 performs a heat treatment during hot rolling to heat the sheet thickness center temperature to 900 ° C. or higher, and then performs an accelerated cooling process. preferable. If the steel sheet temperature before the accelerated cooling step is less than 900 ° C., the structure fraction of the ferrite phase before accelerated cooling becomes too low, and the desired accelerated cooling effect cannot be expected. It is also possible to continue hot rolling after accelerated cooling.

  In the rapid cooling when the steel plate temperature is less than 900 ° C. at the plate thickness center temperature, alloy distribution is small, so that the transformation does not proceed by the subsequent heat treatment, and it becomes difficult to obtain a desired structure.

  Moreover, if an accelerated cooling process is performed after completion of hot rolling, the structure at the time of hot rolling can be frozen, and a martensite phase transformed from a supercooled ferrite phase and a part of austenite is formed. Therefore, it is not necessary to limit the conditions for hot rolling in particular, but the hot rolling process is a process for performing hot rolling with a rolling end temperature of 900 ° C. or higher. It is preferable from the viewpoint of further miniaturization. If the rolling end temperature is less than 900 ° C., the structure fraction of the ferrite phase before accelerated cooling is low, and a desired accelerated cooling effect cannot be expected.

  Further, when the accelerated cooling step is performed before the hot rolling step, that is, after the heating step, the cooling start temperature is high, and a large accelerated cooling effect can be expected. However, the thickness of the material to be cooled is thick, and in order to ensure a desired cooling rate, a facility problem that requires a cooling device having a large cooling capacity, and further hot rolling after the accelerated cooling process There is a problem that the rolling efficiency is reduced and the productivity is lowered.

  After performing the above-described hot rolling step or accelerated cooling step, the present invention further cools to a temperature of the plate thickness center temperature of 500 ° C. or lower.

  The cooling rate after finishing the above hot rolling process or accelerated cooling process is not particularly limited, and if it can be cooled to a temperature of the sheet thickness center temperature of 500 ° C. or less (cooling stop temperature), it can be allowed to cool. Alternatively, rapid cooling (water cooling, gas cooling) may be used. By cooling to a temperature of 500 ° C or less at the sheet thickness center temperature, a part of the austenite phase is transformed into the martensite phase, and in the subsequent heat treatment process, the diffusion of Ni into the remaining austenite phase is promoted, and the austenite phase Can be stabilized. Note that if the cooling stop temperature is higher than 500 ° C., it becomes difficult to stabilize the austenite phase during subsequent heating. For this reason, it is preferable that the cooling stop temperature in cooling after the hot rolling process or the accelerated cooling process is limited to 500 ° C. or less.

  In the present invention, after the cooling described above, a heat treatment step is performed in which the heat treatment is performed at least once by heating to a temperature in the range of 550 to 980 ° C. and cooling.

  This heat treatment is performed for the purpose of accelerating the concentration of alloy elements such as Ni in the austenite phase, stabilizing the austenite phase, and increasing its structural fraction. In this heat treatment, an austenite phase is formed by transformation from the martensite phase during heating. And by subsequent cooling, a part of formed austenite phase remains, and others transform into a martensite phase. Also, an austenite phase (granular) is formed from the ferrite phase during heating, and a part of the austenite phase (granular) is transformed into a martensite phase (granular) during the subsequent cooling, and is dispersed in the ferrite phase. The heating temperature in this heat treatment is set to a temperature in the range of 550 to 980 ° C.

Heating temperature in the heat treatment process: Temperature in the range of 550 to 980 ° C When the heating temperature is lower than 550 ° C, the non-equilibrium ferrite phase rarely transforms into the austenite phase, and the diffusion of the alloy elements is slow and the desired austenite phase Since the concentration of the alloying element cannot be promoted, a desired amount of stable austenite phase cannot be secured. On the other hand, when heated to a high temperature exceeding 980 ° C., the austenite phase formed increases, the stability is lowered, and a desired amount of austenite phase cannot be left. Moreover, it becomes a coarse austenite grain and causes the fall of toughness. For this reason, the heating temperature of the heat treatment was limited to a temperature in the range of 550 to 980 ° C.

  As for the heating conditions, if the ultimate temperature (heating temperature) is in the above-described temperature range, the heating rate is not particularly limited, but is preferably 2 ° C./min or more from the viewpoint of workability. . The holding time is preferably 3 min or more at the ultimate temperature (heating temperature). The upper limit of the holding time is not particularly limited as long as it is within a time that does not cause a problem in the process, but is preferably 180 min or less.

  In this heat treatment, the cooling rate after heating and holding is not particularly limited, and may be allowed to cool, gradually cooled, or rapidly cooled (water cooled). Rapid heating (quenching) is preferable when the heating temperature is on the high temperature side of the above-described heating temperature range, and cooling is preferably performed on the low temperature side from the viewpoint of ensuring a desired structure stably.

  This heat treatment is preferably repeated at least once, preferably a plurality of times. By repeating this heat treatment a plurality of times, the stability of the austenite phase is increased, the amount of stable austenite phase during deformation is increased, and the low temperature toughness is improved. For example, as the first stage treatment, the heating temperature is heated to 750 to 950 ° C. and held and then rapidly cooled (water cooling), and as the second stage treatment, the heating temperature is heated to a range of 550 to 680 ° C. and allowed to cool. Processing may be performed. Thereby, the distribution of Ni into the austenite phase further proceeds, and the stable austenite phase can be increased. From the viewpoint of productivity, it is up to about 4 times.

  In addition, this heat processing may be performed using the heat processing apparatus 4, such as an induction heating furnace, and the heating means 5 in a steel plate production line, or may be performed off-line using the heat processing apparatus 4, such as a batch furnace. Moreover, you may implement hot rolling again after heat processing. Moreover, you may carry out after forming steel materials and a steel strip in a pipe etc.

  As equipment for manufacturing thick steel plates used in the present invention, as shown in FIG. 1, there are a heating device 1, a hot rolling device 2, a cooling device 3, a heat treatment device 4, and a heating means 5. It can be illustrated. Next, functions that the devices should have will be described.

  The heating apparatus 1 may be any heating furnace that can heat a steel material such as a normal slab to a predetermined temperature, and is not particularly limited, and any ordinary heating apparatus (heating furnace) can be applied.

  Moreover, the hot rolling apparatus 2 should just be a hot rolling apparatus which can roll a thick steel plate, and any conventional hot rolling apparatus can be applied. As a hot rolling device, a rough rolling device 2a and a finish rolling device 2b may be arranged in two units, or only one finishing rolling device 2b may be arranged, and rough rolling and finish rolling may be performed. There is no problem even if you share.

  The cooling device 3 includes cooling means such as water injection, water spray, and gas injection that can control the flow rate, and is a thick steel plate during hot rolling, a thick steel plate after hot rolling, or before hot rolling. This steel material can be cooled at a predetermined cooling rate. The cooling device 3 used in the present invention has a cooling capacity capable of obtaining an average cooling rate of 2.0 ° C./s or more at the center position of the thickness of the material to be cooled (thick steel plate, steel material) having a stainless steel composition. It is preferable to use a cooling device. The upper limit of the average cooling rate is not particularly limited, but is preferably 50 ° C./s from the viewpoint of preventing cracking and bending. The cooling device 3 is not particularly problematic as long as it can cool a material having a thickness of 180 mm or less.

  The cooling device 3 described above is provided upstream and / or downstream of the hot rolling device 2 so that an accelerated cooling process can be performed during hot rolling, after hot rolling is completed, or before hot rolling. It is preferable to arrange. In the case where the hot rolling device 2 is composed of two units of a rough rolling device 2a and a finishing rolling device 2b, the upstream side or the downstream side of the finishing rolling device 2b as shown in FIG. More preferably, it is arranged on the side. Note that one cooling device 3 may be provided. In that case, it can be coped with by repeatedly conveying between the hot rolling device 2 and the cooling device 3 according to desired cooling conditions.

  The heat treatment apparatus 4 may be a furnace (apparatus) capable of heating a thick steel plate having a thickness of about 50 mm or less to a predetermined heating temperature. If heating in a non-oxidizing atmosphere is possible, the heating type is particularly It is not limited. There is no problem whether the installation location is online or offline.

  The heating means 5 is used to heat the steel sheet to a predetermined temperature or higher when the steel sheet temperature becomes too low when hot rolling is interrupted and accelerated cooling is performed. Therefore, it is preferable to arrange | position so that conveyance between the hot rolling apparatuses 2 can be carried out online so that subsequent hot rolling can be performed. The heating method is not particularly limited, and an induction heating device, a direct heating gas furnace, and the like can be exemplified, but an induction heating device is preferable from the viewpoint of productivity.

The thick steel plate obtained by the above-described manufacturing method has the above-described composition and volume ratio, a martensite phase of 50% or more, an austenite phase of 3 to 15%, and a structure in which the balance is a ferrite phase, and The ratio of Ni concentration (C _Ni ) _γ (mass%) in the austenite phase to Ni concentration (C _Ni ) _M (mass%) in the martensite phase, (C _Ni ) _γ / (C _Ni ) _M is 1.15. As described above, or in addition, it is a high-strength stainless steel plate excellent in low-temperature toughness in which a part of the martensite phase is in a granular form having a particle size of 10 μm or less and 3.0 or more per 100 μm ^{2 of} ferrite particles in the ferrite phase.

  Next, the reason for limiting the structure of the high strength stainless steel plate of the present invention will be described.

Martensite phase: 50% or more in volume ratio In the present steel plate, a martensite phase having a volume ratio of 50% or more is a main phase. The martensite phase is an important phase for securing a desired high strength. If the volume ratio is less than 50%, the strength is lowered and the desired high strength cannot be secured. For this reason, the martensite phase was limited to 50% or more by volume ratio. A part of the martensite phase is dispersed in the form of ferrite grains and contributes to the refinement of the structure.

Austenitic phase: 3-15% by volume
The austenite phase is rich in toughness and is dispersed to ensure excellent low temperature toughness. In order to obtain such an effect, it is necessary to contain 3% or more of an austenite phase by volume ratio. On the other hand, if the content exceeds 15%, the strength is lowered and the desired high strength cannot be ensured. For this reason, the austenite phase was limited to a range of 3 to 15% by volume ratio.

(C _Ni ) _γ / (C _Ni ) _M : 1.15 or more Ni concentration in the austenite phase is important for the stabilization of the austenite phase. In particular, in the austenite phase formed in the martensite phase, Ni is concentrated as compared with the surrounding martensite phase, so that the stability is increased and the low temperature toughness is improved. Therefore, the ratio of the Ni concentration (C _Ni ) _γ (mass%) in the austenite phase and the Ni concentration (C _Ni ) _M (mass%) in the martensite phase, (C _Ni ) _γ / (C _Ni ) _M , Was limited to 1.15 or more.

  The Ni concentration of the austenite and martensite phases is determined by the energy dispersive X-ray spectrometer (EDS) and wavelength dispersive X-ray spectrometer attached to the scanning electron microscope (SEM) and transmission electron microscope (TEM). (WDS) can be used for measurement. When the austenite phase (grains) is 1 μm or less, it is preferable to measure by TEM-EDS from the viewpoint of measurement time and spatial resolution.

Granular martensite particles having a particle size of 10 μm or less: 3.0 or more per 100 μm ^{2 of} ferrite particles Part of the martensite phase is dispersed as granular martensite particles in the ferrite phase. By dispersing the granular martensite grains in the ferrite grains, the ferrite grains are divided, the apparent miniaturization is achieved, and the low temperature toughness is improved. From the viewpoint of refining ferrite grains among the granular martensite grains, 3.0 or more granular martensite grains having a particle diameter of 10 μm or less are dispersed per 100 μm ² of ferrite grains. In the present invention, the particulate martensitic grains having a particle size of more than 10 [mu] m, and does not contribute to the grain refining of a ferrite grain of shed too large, the particle size is limited to granular martensite grains 10 [mu] m, the ferrite grains 100 [mu] m ² It is assumed that 3.0 or more per unit are dispersed. When the dispersion of granular martensite grains having a particle size of 10 μm or less is less than 3.0 per 100 μm ^{2 of} ferrite grains, it does not contribute to the desired subdivision of the ferrite phase. In addition, granular martensite grains often exhibit a substantially elliptical shape, and the “particle diameter” herein refers to the major axis.

  In the present invention, an appropriate amount of granular martensite grains can be dispersed in the ferrite phase by the above-described heat treatment, so that the low-temperature toughness of the central portion of the plate thickness, which is difficult to add strain by hot working, is remarkably improved. be able to.

  Hereinafter, based on an Example, this invention is demonstrated further.

  Molten steel having the composition shown in Table 1 was melted in a high-frequency vacuum melting furnace and cast into a small steel ingot (150 kg steel ingot). The obtained small steel ingot was made into a slab (wall thickness 150 mm) by hot rolling as a steel material. The obtained steel material was then charged into a heating device and subjected to a heating step of heating to a heating temperature of 1250 ° C., followed by various hot rolling steps and accelerated cooling steps with the conditions shown in Table 2. The combination was applied to obtain a thick steel plate having the thickness shown in Table 2, and after cooling to a temperature of 500 ° C. or less, a heat treatment step for performing the heat treatment shown in Table 3 was performed.

  As the rolling cooling process, the accelerated cooling process is performed in the middle of the hot rolling process, the hot rolling is interrupted during the hot rolling, and the accelerated cooling is performed before the hot rolling process. Three types of accelerated cooling after hot rolling performed after the hot rolling step were used. Moreover, in some thick steel plates, hot rolling was interrupted in the middle, and the heat treatment during hot rolling which heated with a heating means was performed, and then accelerated cooling was performed.

  Note that the heat treatment in the heat treatment step was performed at least once using a heating furnace in a protective atmosphere. In some thick steel plates (No. 7, No. 8), heat treatment was performed using an induction heating device. In addition, the plate | board thickness center part temperature of the steel plate in hot rolling, accelerated cooling, and heat processing measured by inserting a thermocouple in the plate | board thickness center part.

From the obtained thick steel plate (heat-treated plate), a test piece was collected and subjected to a structure observation, a tensile test, and an impact test. The test method was as follows.
(1) Structure observation From the obtained thick steel plate (heat treated plate), a structure observation specimen was collected so that a cross section (C cross section) perpendicular to the rolling direction was an observation surface, and mechanical polishing and electrolytic polishing were performed. The structure | tissue was observed using the scanning electron microscope (SEM) (magnification: 500-5000 times). The tissue in the center of the plate thickness was imaged, and the type of tissue and the tissue fraction (volume%) of each phase were determined by image analysis (image processing) using the obtained tissue photograph. In addition, in the observation by SEM (magnification: 500 times), the structure fraction of the ferrite phase is measured, and in the observation by SEM (magnification: 2000 times, 5000 times), granular martensite whose major axis in ferrite is 10 μm or less. The number of sites was measured and converted to the number in 100 μm ^{2 of} ferrite. In addition, when 80% or more of the interfaces of granular martensite are interfaces with ferrite, it was determined that granular martensite is present in the ferrite grains.

In addition, a transmission electron microscope (TEM) test piece (for a thin film) was collected from a 1/2 ton portion of the obtained thick steel plate (heat-treated plate), subjected to mechanical polishing and electrolytic polishing, and a transmission electron microscope ( TEM) Observation specimen was used, and the structure was observed by TEM. In TEM observation, electron diffraction is performed to identify austenite grains, and by using an energy dispersive X-ray spectrometer (EDS) attached to the TEM, Ni concentration (C _Ni ) _γ in austenite and the surrounding martens The Ni concentration (C _Ni ) _M of the site was measured, and (C _Ni ) _γ / (C _Ni ) _M was calculated.

Further, from the obtained thick steel plate (heat treated plate), an X-ray diffraction test piece is collected so that the center of the plate thickness becomes the measurement surface, mechanically polished and electropolished, and subjected to X-ray diffraction to obtain austenite. The volume fraction V _γ of the phase was calculated. In X-ray diffraction, the X-ray integrated intensities of diffraction on the (220) plane of the austenite phase (γ) and the (211) plane of the ferrite phase (α) are measured.
_{V γ = 100 / {1+ (} I α R γ / I γ R α)}
(Where I _α : integrated intensity of α,
I _γ : Integral intensity of γ,
R _α : crystallographically calculated value of α,
R _γ : Conversion was performed using a crystallographic theoretical calculation value of γ. The fraction of the martensite phase was the remainder other than these phases.
(2) Tensile test Take a round bar tensile test piece (parallel part 6mmφ × GL20mm) so that the direction perpendicular to the rolling direction is the tensile direction from the thickness center position of the obtained thick steel plate (heat treated plate). A tensile test was carried out in accordance with the provisions of JIS Z 2241 to determine tensile properties (yield strength YS, tensile strength TS). The yield strength YS was the strength at 0.2% elongation.
(3) Impact test A V-notch test piece was sampled from the plate thickness center position of the obtained thick steel plate (heat treated plate) so that the direction perpendicular to the rolling direction (C direction) was the test piece longitudinal direction, and JIS A Charpy impact test was performed in accordance with Z 2242. The test temperature was −40 ° C., and the absorbed energy vE ₋₄₀ (J) was determined. The number of test pieces was three each, and the average value thereof was defined as the absorbed energy of the thick steel plate.

  Table 4 shows the obtained results.

In all of the examples of the present invention, even in the central portion of the thickness exceeding 15 mm, YS: 600 MPa or more and vE ₋₄₀ : 100 J or more, a thick steel plate exhibiting high strength and high toughness. On the other hand, in the comparative examples that are outside the scope of the present invention, the desired high strength is not obtained or the desired high toughness is not obtained.

1 Heating device (heating furnace)
2 Hot rolling device 3 Cooling device 4 Heat treatment device 5 Heating means

Claims (11)

% By mass
C: 0.050% or less, Si: 0.50% or less,
Mn: 0.20-1.80%, Cr: 15.5-18.0%,
Ni: 1.5-5.0%, Mo: 1.0-3.5%,
V: 0.02 to 0.20%, Al: 0.05% or less,
N: 0.01 to 0.15%, O: 0.006% or less, the composition comprising the balance Fe and inevitable impurities,
By volume, more than 50% of martensite phase, 3% to 15% of the austenite phase, Ri Do balance being ferrite phase, the ferrite phase, some particle size 10μm or less granular the martensite phase And having a structure including 3.0 or more per 100 μm ^{2 of} ferrite grains , and Ni concentration (C _Ni ) _γ (mass%) in the austenite phase and Ni concentration (C _Ni ) _M (mass%) in the martensite phase. ) And (C _Ni ) _γ / (C _Ni ) _M is 1.15 or more, a high-strength stainless steel plate excellent in low-temperature toughness. The high-strength stainless steel plate according to claim 1, further comprising Cu: 3.5% or less by mass% in addition to the composition. In addition to the above composition, the composition further includes one or more selected from the group consisting of Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, and W: 3.0% or less. The high-strength stainless steel plate according to claim 1 or 2 . In addition to the above composition, by mass%, Ca: 0.01% or less, REM: claims 1, characterized in that a composition comprising one or two species selected from among 0.01% or less any 3 The high-strength stainless steel plate described in Crab. The steel material is subjected to a heating process and a hot rolling process to obtain a thick steel plate with a predetermined thickness.
The steel material in mass%,
C: 0.050% or less, Si: 0.50% or less,
Mn: 0.20-1.80%, Cr: 15.5-18.0%,
Ni: 1.5-5.0%, Mo: 1.0-3.5%,
V: 0.02 to 0.20%, Al: 0.05% or less,
N: 0.01 to 0.15%, O: 0.006% or less, the steel material of the composition consisting of the balance Fe and inevitable impurities,
The heating step is a step of heating the steel material to a heating temperature: 1100 to 1350 ° C.,
The hot rolling step, the hot rolling is interrupted halfway, and cooling is started at a sheet thickness center temperature of 900 ° C. or higher, at least 50 ° C. from the temperature at which the cooling was started, and 800 ° C. or higher. After the accelerated cooling process of cooling at an average cooling rate of 2.0 ° C./s or more at the sheet thickness center temperature to the cooling stop temperature, the hot rolling is restarted to form a thick steel sheet having the predetermined sheet thickness. ,
After heat-rolling process, it cooled to a temperature below 500 ℃ at thickness center temperature, heated to a temperature in the range of five hundred and fifty to nine hundred and eighty ° C. to after accordingly, and facilities the heat treatment step of performing a heat treatment for cooling at least once,
By volume ratio, a martensite phase of 50% or more, an austenite phase of 3 to 15%, and the balance is composed of a ferrite phase. In the ferrite phase, a part of the martensite phase is granular with a particle size of 10 μm or less. And having a structure containing 3.0 or more per 100 μm ^{2 of} ferrite grains , and
The ratio of the Ni concentration (C _Ni ) _γ (mass%) in the austenite phase and the Ni concentration (C _Ni ) _M (mass%) in the martensite phase , (C _Ni ) _γ / (C _Ni ) _M is A method for producing a high-strength stainless steel plate excellent in low-temperature toughness, characterized by being a stainless steel plate having a thickness of 1.15 or more . When the hot rolling is interrupted and the accelerated cooling step is performed, the steel plate temperature during hot rolling is 900 ° C. or more at the plate thickness central temperature when the plate thickness central temperature is less than 900 ° C. 6. The method for producing a high-strength stainless steel plate according to claim 5 , wherein a heat treatment is performed during hot rolling to be heated to a temperature of 5 mm, and then the accelerated cooling step is performed. The steel material is subjected to a heating process and a hot rolling process to obtain a thick steel plate with a predetermined thickness.
The steel material in mass%,
C: 0.050% or less, Si: 0.50% or less,
Mn: 0.20-1.80%, Cr: 15.5-18.0%,
Ni: 1.5-5.0%, Mo: 1.0-3.5%,
V: 0.02 to 0.20%, Al: 0.05% or less,
N: 0.01 to 0.15%, O: 0.006% or less, the steel material of the composition consisting of the balance Fe and inevitable impurities,
The heating step is a step of heating the steel material to a heating temperature: 1100 to 1350 ° C.,
The hot rolling step immediately starts cooling the steel material that has been subjected to the heating step, and the temperature at which the cooling is started is set as a cooling start temperature, is at least 50 ° C. from the cooling start temperature, and 800 After performing an accelerated cooling step of cooling at an average cooling rate of 2.0 ° C./s or higher at a sheet thickness center temperature to a cooling stop temperature that is equal to or higher than 0 ° C., the hot rolling is performed to obtain a thick steel plate having the predetermined plate thickness Process,
After heat-rolling process, it cooled to a temperature below 500 ℃ at thickness center temperature, heated to a temperature in the range of five hundred and fifty to nine hundred and eighty ° C. to after accordingly, and facilities the heat treatment step of performing a heat treatment for cooling at least once,
By volume ratio, a martensite phase of 50% or more, an austenite phase of 3 to 15%, and the balance is composed of a ferrite phase. In the ferrite phase, a part of the martensite phase is granular with a particle size of 10 μm or less. And having a structure containing 3.0 or more per 100 μm ^{2 of} ferrite grains , and
The ratio of the Ni concentration (C _Ni ) _γ (mass%) in the austenite phase and the Ni concentration (C _Ni ) _M (mass%) in the martensite phase , (C _Ni ) _γ / (C _Ni ) _M is A method for producing a high-strength stainless steel plate excellent in low-temperature toughness , characterized by being a stainless steel plate having a thickness of 1.15 or more . The steel material is subjected to a heating process and a hot rolling process to obtain a thick steel plate with a predetermined thickness.
The steel material in mass%,
C: 0.050% or less, Si: 0.50% or less,
Mn: 0.20-1.80%, Cr: 15.5-18.0%,
Ni: 1.5-5.0%, Mo: 1.0-3.5%,
V: 0.02 to 0.20%, Al: 0.05% or less,
N: 0.01 to 0.15%, O: 0.006% or less, the steel material of the composition consisting of the balance Fe and inevitable impurities,
The heating step is a step of heating the steel material to a heating temperature: 1100 to 1350 ° C,
After performing the heating step, the hot rolling step is a step of performing hot rolling at a rolling end temperature of 900 ° C. or higher,
After completion of the hot rolling process, cooling is started from a temperature of 900 ° C. or higher at the plate thickness center temperature of the thick steel plate, and from the temperature at which the cooling is started to a cooling stop temperature of at least 50 ° C. and 800 ° C. or higher. , Subjected to an accelerated cooling process of cooling at an average cooling rate of 2.0 ° C./s or more at the plate thickness center temperature,
After the pressurized-speed cooling step is completed, further cooled to a temperature below 500 ℃ at thickness center temperature, heated to a temperature in the range of five hundred fifty to nine hundred and eighty ° C. to after accordingly, and facilities the heat treatment step of performing a heat treatment for cooling at least once ,
By volume ratio, a martensite phase of 50% or more, an austenite phase of 3 to 15%, and the balance is composed of a ferrite phase. In the ferrite phase, a part of the martensite phase is granular with a particle size of 10 μm or less. And having a structure containing 3.0 or more per 100 μm ^{2 of} ferrite grains , and
The ratio of the Ni concentration (C _Ni ) _γ (mass%) in the austenite phase and the Ni concentration (C _Ni ) _M (mass%) in the martensite phase , (C _Ni ) _γ / (C _Ni ) _M is A method for producing a high-strength stainless steel plate excellent in low-temperature toughness, characterized by being a stainless steel plate having a thickness of 1.15 or more . The method for producing a high-strength stainless steel plate according to any one of claims 5 to 8 , wherein the composition further comprises Cu: 3.5% or less by mass% in addition to the composition. In addition to the above composition, the composition further includes one or more selected from the group consisting of Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less, and W: 3.0% or less. A method for producing a high-strength stainless steel plate according to any one of claims 5 to 9 . The composition according to any one of claims 5 to 10 , wherein the composition further comprises one or two selected from Ca: 0.01% or less and REM: 0.01% or less in mass% in addition to the composition. A method for producing a high-strength stainless steel plate according to claim 1.

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