JP5381440B2 - Thick low-temperature steel sheet with excellent arrestability and method for producing the same - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a thick low-temperature steel sheet excellent in arrestability and a method for producing the same, and more particularly, a Ni-containing thick low-temperature steel sheet excellent in arrestability and a method for producing the same, particularly a material for a low-temperature storage tank such as LNG. The present invention relates to a 9% Ni-containing thick low-temperature steel sheet and a method for producing the same.

  The “thick steel plate” in the present invention is a plate having a thickness of 40 to 60 mm. Further, “for low temperature” is a steel plate used in a temperature range of liquids such as LPG and LNG, that is, a temperature range of −60 ° C. or less, and particularly used for a storage tank used at an LNG temperature of −165 ° C. To do.

  The “arrestability” refers to a characteristic that can stop the propagation of cracks.

  A low temperature steel sheet for producing a storage tank of a low temperature substance such as LNG is required to have excellent fracture toughness from the viewpoint of ensuring safety. A typical example of a steel sheet that meets the above requirements was a 9% Ni steel sheet.

  Conventionally, about 9% Ni steel sheet, the reduction of impurities such as P and S, the reduction of C, and further three-stage heat treatment ("quenching (Q)-two-phase region quenching (L)-tempering (T)" In the following, various improvements such as “QLT” have been performed.

  On the other hand, the inclusion of Mo as an alloy element effective for improving the strength and toughness of Ni-containing steel has been studied.

  The above-mentioned QLT method and the inclusion of Mo are for increasing the amount of retained austenite (austenite remaining without transformation) which is the basis of toughness improvement.

  Such a state of the prior art is summarized as follows based on patent literature.

  In Patent Document 1, 0.04-0 having a plate thickness of 40 mm or more produced by the QLT method or the direct quenching-2 phase region quenching-tempering (hereinafter referred to as “DQ-LT” method). 9% Ni steel containing 5% Mo is disclosed.

  Patent Document 2 discloses the production of 9% Ni steel having a thickness of 40 mm or more by a quenching-tempering (hereinafter referred to as “Q-T”) method or a direct quenching-tempering (hereinafter referred to as “DQ-T”) method. A method is disclosed. Specifically, Patent Document 2 discloses a method for producing a thick-walled 9% Ni steel having excellent low-temperature toughness by performing a QT process or a DQ-T process after controlling the rolling reduction during slab rolling. Has been.

JP-A-4-371520 JP-A-6-184630

  The manufacturing methods based on the QLT method and the DQLT method described in Patent Document 1 have the following problems.

  That is, in the QLT method, the heat treatment is a three-step heat treatment, and the number of heat treatments is increased, and there is a problem in production cost and production lead time.

  On the other hand, in the case of the DQ-LT method, it is necessary to lower the rolling finishing temperature in order to ensure toughness. At this time, although characteristic anisotropy occurs in the steel sheet, re-quenching (L) after direct quenching (DQ) heats the steel sheet only up to the two-phase region, not the austenite region, so the structure after quenching (L). Will take over part of the anisotropic structure, so that the rolling direction of the steel sheet (hereinafter sometimes referred to as “L direction”) and the direction parallel to the rolling surface and perpendicular to the L direction (hereinafter referred to as “ It is inevitable that characteristic anisotropy occurs in the “C direction”.

  The technique disclosed in Patent Document 2 is merely intended to improve the stability of austenite generated by dissociating from a finely and uniformly dispersed carbide precipitation state. For this reason, Patent Document 2 does not consider the manufacturing conditions for making the microstructure finer, such as the heating temperature and the rolling reduction per pass.

  The present invention is a thick-walled low-temperature steel plate excellent in arrestability that can be suitably used as a material for a low-temperature storage tank such as LNG, in particular, -196 ° C. in a DT test (Dynamic Tear Test) defined in ASTM E604. It is an object of the present invention to provide a 9% Ni-containing thick low-temperature steel sheet in which the absorbed energy at 1 is 1600 J or more in both the “L direction” and the “C direction”.

  It is also an object of the present invention to provide a method for producing the 9% Ni-containing thick low-temperature steel sheet by the “QT” method without performing the three-stage heat treatment of “QLT”. is there.

  In order to achieve the above-described object, the present inventors have conducted various studies. As a result, the knowledge shown in the following (a) to (f) was obtained.

  (A) The average grain size of the prior austenite grains in the “region of ± 5 mm from the center of the plate thickness” (hereinafter also referred to as “plate thickness center portion”) with respect to the arrestability of the 9% Ni-containing thick low-temperature steel plate And the area ratio of austenite in the microstructure is greatly affected.

  In the case of a steel sheet containing martensite and bainite, the prior austenite grain boundaries can be easily revealed by etching and can be identified with an optical microscope. The “particle diameter” in the present invention refers to a so-called “circle equivalent diameter”, that is, a diameter of a circle when the projected area is regarded as a circle having the same area. The projected area can be calculated as the “particle size” by taking the exposed austenite grain boundary as an electrophotographic image and processing it with an image analyzer.

  (B) If the aspect ratio of the prior austenite grains in the center portion of the plate thickness is in a specific range, the same arrestability is obtained in the L direction and the C direction of the 9% Ni-containing thick low temperature steel plate.

  The “aspect ratio of the prior austenite grains” in the center portion of the plate thickness means the rolling direction (L direction) and the plate thickness direction (hereinafter referred to as “Z direction”) of the steel plate in the “± 5 mm region of the plate thickness center”. In the plane parallel to the L direction and the Z direction and perpendicular to the C direction, the size of the prior austenite grains is determined by using an optical microscope in the Z direction and the L direction of the steel sheet. Each of the directions is measured as an average intercept length, and indicates a value obtained from “average intercept length in the L direction / average intercept length in the Z direction”. The aspect ratio can also be calculated by taking the prior austenite grain boundary as an electrophotographic image and processing it with an image analysis device, in the same manner as the above-mentioned “particle diameter”.

  Note that the anisotropy of arrestability in the “L direction” and the “C direction” is determined based on the average intercept lengths in the “L direction” and the “Z direction”. This is because the average intercept length in the Z direction and the C direction can be handled in the same way for convenience because both directions are reduced in the same way.

  (C) The ratio of coarse prior austenite grains in the central portion of the plate thickness also affects the arrestability of the 9% Ni-containing thick low-temperature steel plate.

  (D) By controlling the heating, rolling and cooling conditions of the steel sheet, dislocations as lattice defects can be introduced into the untransformed austenite, and further, the prior austenite grains which are untransformed austenite grains can be made fine. it can.

  (E) When a strong pressure is applied at a low temperature, a large amount of dislocations are introduced, and the subsequent structure becomes fine. In particular, when a small amount of Nb is contained in the steel, Nb (C, N) precipitates finely. Thus, the movement of dislocations is hindered and the dislocation density in austenite is increased, so that the structure of the steel sheet can be made fine. However, in the cryogenic temperature region equivalent to the 9% Ni-containing steel sheet for low temperature according to the present invention, if the above precipitates are present, the toughness is lowered, so that high toughness cannot be ensured. Therefore, the strong pressure at low temperature and the Nb content do not necessarily lead to the improvement of the toughness of the 9% Ni-containing steel sheet according to the present invention.

  (F) In the case of a 9% Ni-containing steel sheet, in the slab heating-rolling step before quenching, the heating temperature and the rolling finishing temperature are controlled to a specific temperature range, and the reduction rate and cumulative reduction per pass. In general low-carbon low-alloy steels, the rate is optimized and the rolling is finished, and then cooling to a specific temperature range is performed at an appropriate cooling rate, followed by heating to a specific temperature range and quenching. It is possible to develop an effect of refining the structure that is not possible. Moreover, in the case of the 9% Ni-containing steel sheet obtained under the above-mentioned proper conditions, the ratio of coarse prior austenite grains is small in the center of the sheet thickness, and the aspect ratio of the prior austenite grains is centered at 1.0. The variation becomes smaller. In addition, austenite can be secured in an appropriate ratio in the microstructure.

  The present invention has been completed on the basis of the above findings, and the gist of the present invention is the thick steel plate for low temperature and the arrestability shown in (6). It is in the manufacturing method of the steel plate for excellent thickness and low temperature.

(1) By mass%, C: 0.01 to 0.1%, Si: 0.03 to 0.60%, Mn: 0.3 to 2.0%, P: 0.005% or less, S: 0.003% or less, Ni: more than 8.5% and less than 9.5%, sol. Al: 0.005 to 0.050% and N: 0.0005 to 0.0060%, the balance is made of Fe and impurities, and the equivalent circle diameter of the prior austenite grains in the region of ± 5 mm at the center of the plate thickness The average grain size at 25 μm or less, with the plate thickness cross section parallel to the rolling direction and the plate thickness direction of the steel plate, the average austenite grain size in the rolling direction and the plate thickness direction of the steel plate using an optical microscope, respectively. Measured as the section length , the aspect ratio of the prior austenite grains determined from “average section length in the rolling direction / average section length in the plate thickness direction” is 0.7 to 1.3, and in the microstructure in the viewing contains a 3.0 to 15.0% of the austenite area ratio, it absorbed energy at -196 ° C. in DT test specified in ASTM E604 or more 1600J in both the rolling direction and thickness direction Ri, and the thickness is 40-60 mm, thick low-temperature steel sheet excellent in arrestability.

  (2) The thick low-temperature steel sheet having excellent arrestability as described in (1) above, further containing, by mass%, Mo: 0.1% or less.

  (3) The arrestability as described in (1) or (2) above, further comprising at least one of Cu: 2.0% or less and Cr: 0.6% or less in mass%. Excellent thick and low temperature steel sheet.

(4) In any one of the above (1) to (3), the ratio of the prior austenite grains having a grain size of 40 μm or more is 5% or less in an area of ± 5 mm at the center of the plate thickness A thick-walled low-temperature steel sheet with excellent arrestability as described in 1.

(5) By applying the following steps 1 to 5 to the steel pieces having the chemical composition according to any one of (1) to (3) above , the prior austenite grains in the region of ± 5 mm at the center of the plate thickness. The average grain size of the equivalent circle diameter of the steel sheet is 25 μm or less, and the thickness of the prior austenite grains is determined by using an optical microscope in the plate thickness section parallel to the rolling direction and the plate thickness direction of the steel plate. The aspect ratio of the prior austenite grains determined from the average section length in the thickness direction and determined from "average section length in the rolling direction / average section length in the plate thickness direction" is 0.7 to 1.3. The austenite is 3.0 to 15.0% in area ratio in the microstructure, and the absorbed energy at −196 ° C. in the DT test specified in ASTM E604 is both in the rolling direction and the plate thickness direction. 1600J or higher , And the and thickness to obtain a steel sheet of 40-60 mm, a manufacturing method excellent thick low-temperature steel sheet arrestability.

Process 1: The process of heating a steel slab to 800-1150 degreeC.

Step 2: A step of rolling in a temperature range of at least 800 to 650 ° C. so that the rolling reduction per pass is 5% or more and the cumulative rolling reduction is 25% or more.

Step 3: After completion of rolling, 10 ~50 ℃ / s step of cooling to a temperature of 200 ° C. or less at a cooling rate of.

Process 4: The process of heating and hardening to the temperature of 760-900 degreeC.

Process 5: The process of tempering at the temperature of 550-640 degreeC.

  The “impurities” in the remaining “Fe and impurities” refer to those inevitably mixed from ore, scrap, or the environment as raw materials when industrially producing steel materials.

  The “plate thickness center” defined in the present invention refers to “a region of ± 5 mm from the plate thickness center”. “Grain size” refers to the so-called “equivalent circle diameter”, that is, the diameter of a circle when the projected area is regarded as a circle of the same area, and “aspect ratio” refers to the “average intercept in the rolling direction” of the steel sheet. "Length / average section length in the thickness direction".

  800-1150 degreeC as a heating temperature of the steel slab prescribed | regulated by this invention says the temperature of a steel slab surface.

  Moreover, the temperature in the 800-650 degreeC temperature range in a rolling process points out the surface temperature of the steel piece or steel plate which is a to-be-rolled material.

  The rolling reduction refers to the reduction rate of the sheet thickness before and after rolling.

  The cooling rate after the end of rolling refers to the average cooling rate obtained from the surface temperature of the steel sheet, and the temperature of 200 ° C. or less that cools at the cooling rate refers to the surface temperature of the steel sheet.

  The temperature of 760 to 900 ° C. for heating the steel plate for quenching and the temperature of 550 to 640 ° C. for tempering after quenching both indicate the temperature of the steel plate surface.

  Since the steel sheet for low-temperature wall having excellent arrestability according to the present invention has an absorbed energy at −196 ° C. in the DT test specified in ASTM E604, in both the L direction and the C direction of the steel sheet, it is 1600 J or more. It can be suitably used as a material for a low temperature storage tank such as LNG. This thick low-temperature steel sheet having excellent arrestability can be produced, for example, by applying the method of the present invention.

  Hereinafter, the constituent requirements of the present invention will be described in detail. In addition, "%" display of the content of each component element means "mass%".

(A) About chemical composition:
C: 0.01 to 0.1%
C is an element effective for lowering the Mf point, which is the temperature at which martensitic transformation is completed, and stabilizing retained austenite. However, C hardens the martensite substrate itself and lowers toughness more than improvement of toughness by increasing the amount of austenite. Therefore, the C content should be more than the amount necessary to ensure strength, and an excessive amount that degrades toughness should be avoided. If the C content is less than 0.01%, the strength is insufficient, while if it exceeds 0.1%, the toughness deteriorates. Therefore, the C content is set to 0.01 to 0.1%. The C content is preferably 0.03% or more, and is preferably 0.07% or less.

Si: 0.03 to 0.60%,
Si is an element that acts as a deoxidizing element and has an effect of suppressing the precipitation of cementite to improve the stabilization of austenite during tempering, and must be contained in an amount of 0.03% or more. However, if the Si content is too large, toughness deterioration occurs, and if it exceeds 0.60%, toughness deterioration becomes significant. Therefore, the content of Si is set to 0.03 to 0.60%. Note that the Si content is desirably 0.10% or more, and desirably 0.30% or less.

Mn: 0.3 to 2.0%,
Mn is an element that is effective in stabilizing the austenite by lowering the Mf point. The greater the content, the greater the amount of austenite, so 0.3% or more is contained. However, when the Mn content is excessive, the toughness of the martensite substrate is deteriorated, and particularly when the content exceeds 2.0%, the toughness deterioration becomes remarkable. Therefore, the Mn content is set to 0.3 to 2.0%. The Mn content is preferably 0.5% or more, and more preferably 0.7% or more. Further, the Mn content is desirably 1.5% or less, and more desirably 1.0% or less.

P: 0.005% or less P is present in steel as an impurity. In the thick and low temperature steel sheet according to the present invention, the presence of P greatly affects the properties of the steel. For this reason, it is necessary to restrict | limit content of P severely compared with usual, and when it exceeds 0.005%, it not only segregates to a grain boundary and reduces toughness, but also causes a hot crack at the time of welding. Therefore, the P content is 0.005% or less. The P content is preferably 0.002% or less.

S: 0.003% or less S is present in steel as an impurity. In the thick and low temperature steel sheet according to the present invention, since the presence of S greatly influences the properties of the steel, it is necessary to strictly limit the S content as compared with usual. When the content of S is large, the center segregation is promoted or a large amount of stretched MnS is generated, so that the mechanical properties of the base material and the weld heat-affected zone deteriorate, particularly exceeding 0.003%. As a result, the deterioration of the mechanical properties of the base metal and the weld heat affected zone becomes significant. Therefore, the S content is 0.003% or less. Since the smaller the S, the more preferable the element, the lower limit of the S content is not particularly specified.

Ni: more than 8.5% and less than 9.5% Ni is the most important element in the present invention, and has an action of increasing strength and stabilizing austenite. Containing. A higher Ni content is preferred because the strength increases and the Mf point decreases and the amount of retained austenite increases. However, containing a large amount of Ni of 9.5% or more causes an increase in cost, so the Ni content is less than 9.5%.

sol. Al: 0.005 to 0.050%
Al, like Si, is an element that acts as a deoxidizing element and also has the effect of suppressing the precipitation of cementite and improving the stabilization of austenite during tempering. Furthermore, Al combines with N to become AlN, and has the effect of contributing to the refinement of austenite grains during heating. In order to obtain such an effect, sol. It is necessary to contain 0.005% or more in terms of Al (acid-soluble Al). However, sol. When the Al content in Al exceeds 0.050%, toughness deterioration occurs. Therefore, the content of Al is sol. 0.005 to 0.050% with Al. Note that sol. The Al content in Al is preferably 0.020% or more, and preferably 0.040% or less.

N: 0.0005 to 0.0060%
Since N is an element that contributes to the stabilization of austenite, it is desirable to contain N. Further, N combines with Al to become AlN, which is effective for refining austenite grains during heating. In order to obtain these effects, it is necessary to contain N in an amount of 0.0005% or more. However, N deteriorates the toughness of the martensite substrate, and when it exceeds 0.0060%, the toughness deterioration becomes remarkable. Therefore, the N content is set to 0.0005 to 0.0060%. Note that the N content is preferably 0.0020% or more, and more preferably 0.0050% or less.

  One of the thick-walled low-temperature steel sheets having excellent arrestability according to the present invention is one in which the balance is Fe and impurities in addition to the above elements.

  Another one of the thick-walled low-temperature steel sheets excellent in arrestability according to the present invention contains one or more elements selected from Mo, Cu and Cr in addition to the above elements. Hereinafter, the effect of these arbitrary elements and the reason for limiting the content will be described.

Mo: 0.1% or less An effect of preventing temper embrittlement is obtained when Mo is contained. Therefore, you may contain Mo in order to acquire this effect. However, if the Mo content exceeds 0.1%, the toughness of the martensite substrate is deteriorated. For this reason, content of Mo in the case of making it contain shall be 0.1% or less. The upper limit of the Mo content when contained is preferably 0.06%, more preferably 0.05%.

  In addition, in order to make the said effect by Mo express reliably, it is desirable to contain Mo 0.03% or more.

Cu: 2.0% or less When Cu is contained, an effect of stabilizing austenite in a solid solution state is obtained. Therefore, Cu may be contained to obtain this effect. However, since solute Cu precipitates as ε-Cu during tempering, it is effective for increasing the strength. However, when the Cu content is increased, the toughness is deteriorated. Becomes remarkable. Therefore, the Cu content in the case of inclusion is 2.0% or less. The upper limit of the Cu content when contained is preferably 1.6%, more preferably 1.2%.

  In order to ensure the effect of stabilizing austenite by Cu, it is desirable to contain 0.05% or more of Cu. More desirably, it is 0.3% or more.

Cr: 0.6% or less When Cr is contained, the effect of improving strength and toughness by improving hardenability can be obtained. Therefore, in order to acquire this effect, you may contain Cr. However, if the Cr content increases and exceeds 0.6%, the toughness is deteriorated. Therefore, the Cr content when contained is 0.6% or less. The upper limit of the Cr content when it is contained is preferably 0.5%.

  In addition, in order to make the said effect by Cr express reliably, it is desirable to contain 0.05% or more of Cr.

  In addition, said Cu and Cr can be contained only in any 1 type in them, or 2 types of composite. The total content of these elements may be 2.6%, but is preferably 2.0% or less.

(B) Microstructure in the center of the plate thickness:
(B-1) Average particle size of prior austenite grains:
When the average grain size of the prior austenite grains exceeds 25 μm at the center of the plate thickness, the arrestability deteriorates. For this reason, the average particle diameter of prior austenite grains needs to be 25 μm or less. It is desirable that the prior austenite grains have an average particle size of 15 μm or less.

  The smaller the prior austenite grains, the better. Therefore, the lower limit of the average grain size of the prior austenite grains is not particularly specified, but about 5 μm is the lower limit in the production of industrial thick steel plates.

(B-2) Aspect ratio of prior austenite grains:
Even if the average grain size of the prior austenite grains is 25 μm or less, the arrestability in the C direction is deteriorated if the aspect ratio of the prior austenite grains is less than 0.7 or exceeds 1.3. For this reason, the aspect ratio of prior austenite grains must be 0.7 to 1.3. The aspect ratio of the prior austenite grains is preferably 0.85 to 1.15, and most preferably 1.0.

(B-3) Area ratio of austenite in the microstructure:
Austenite is important as a means for improving the toughness of thick steel plates, and in order to ensure high toughness, austenite in an amount of 3.0% or more by area ratio is required. The greater the amount of austenite, the more effective for improving toughness. However, when the amount of austenite exceeds 15.0% by area ratio, the hardness (strength) is lowered. Therefore, the area ratio of austenite in the microstructure needs to be 3.0 to 15.0%. The area ratio of austenite in the microstructure is preferably 4.0% or more, and more preferably 5.0% or more.

  Note that austenite in the microstructure can be discriminated by measurement with X-rays. And the area ratio of austenite is the same as the grain size and aspect ratio calculated by taking the previous austenite grain boundary as an electrophotographic image and processing it with an image analyzer. It can be measured by taking it as a photograph and using an image analyzer.

(B-4) Ratio of prior austenite grains having a grain size of 40 μm or more:
Even if the microstructure in the central portion of the plate thickness satisfies the conditions (B-1) to (B-3), the arrestability may be deteriorated when coarse particles having a particle size of 40 μm or more are mixed. For this reason, it is desirable to suppress the formation of the coarse particles as much as possible. Specifically, when the ratio of the prior austenite grains having a grain size of 40 μm or more is 5% or less, stable arrestability can be secured stably.

(C) About manufacturing conditions:
The production conditions of the present invention described in detail below are one of the methods for economically and efficiently realizing a thick and low temperature steel sheet having excellent arrestability on an industrial scale, and the steel sheet itself. The technical scope is not defined by the manufacturing conditions.

  The thick-walled low-temperature steel sheet having excellent arrestability according to the present invention can be manufactured by sequentially treating the steel pieces having the above-described chemical composition in the following steps 1 to 5, for example.

  In addition, about manufacture of the steel slab in the case of processing sequentially in process 1-5, it is not necessary to specify the casting conditions in particular. This is because the microstructure in the central portion of the plate thickness can be controlled by sequentially processing in steps 1 to 5.

(C-1) Heating step:
In step 1 as the heating step, a steel slab as a rolling material for manufacturing the thick-walled low-temperature steel sheet of the present invention is heated to 800 to 1150 ° C.

  The reason why the steel piece is heated to 800 ° C. or more is that when the heating temperature is less than 800 ° C., untransformed ferrite remains and a uniform structure cannot be obtained. The heating temperature is more preferably 850 ° C. or higher.

  On the other hand, if a steel piece is heated exceeding 1150 degreeC, an austenite grain will coarsen and arrestability will deteriorate. For this reason, the heating temperature of a steel piece shall be 1150 degrees C or less. The upper limit of the heating temperature is more preferably 1050 ° C, and even more preferably 1000 ° C.

(C-2) Rolling process:
In step 2 as a rolling step, the heated steel slab is rolled in a temperature range of at least 800 to 650 ° C. so that the rolling reduction per pass is 5% or more and the cumulative rolling reduction is 25% or more. Reduce the thickness to the required thickness.

  In order to obtain fine austenite grains and sufficient arrestability, it is essential to perform sufficient rolling in the non-recrystallized austenite region, and for this purpose, rolling is performed under the above conditions.

  In addition, you may perform a part of rolling in the temperature range exceeding 800 degreeC.

  The rolling reduction per pass in the temperature range of 800 to 650 ° C. is more preferably 7% or more, and the cumulative rolling reduction is more preferably 50% or more.

  In addition, if it rolls in a comparatively low temperature range of 800-650 degreeC, the deformation resistance of a to-be-rolled material will become large. For this reason, the rolling reduction per pass in the temperature range of 800 to 650 ° C. is desirably 30% or less, and the cumulative rolling reduction is desirably 80% or less.

(C-3) Cooling step:
In step 3 as the cooling step, the steel plate after rolling is cooled to a temperature of 200 ° C. or lower at a cooling rate of 0.1 to 50 ° C./s.

  When the cooling rate is less than 0.1 ° C./s, the transformation from austenite to martensite becomes insufficient and untransformed austenite remains, and in this state, the structure is transferred to the heating step described in the next section (C-4). It is not uniform and the arrestability deteriorates, and when it exceeds 50 ° C./s, the strength becomes too high and the toughness decreases.

  The cooling rate of 0.1 to 50 ° C./s may be ensured by appropriate means such as cooling in the atmosphere after rolling and water cooling.

  If the cooling stop temperature when cooling at the above cooling rate exceeds 200 ° C., the transformation from austenite to martensite becomes insufficient, and untransformed austenite remains, and in this state, the following (C-4) When the process proceeds to the heating step described in (4), the tissue becomes non-uniform and the arrestability deteriorates, so cooling is performed to a temperature of 200 ° C. or lower.

  The cooling rate of the steel sheet after completion of the rolling is more preferably 10 ° C./s or more, and further preferably 40 ° C./s or less.

  The cooling may be stopped at any temperature as long as the temperature is 200 ° C. or lower, and may be cooled to room temperature.

(C-4) Step of heating and quenching after cooling:
In step 4 as a heating and quenching step, the cooled steel sheet is heated to a temperature of 760 to 900 ° C. and quenched.

  If the heating temperature is less than 760 ° C., it is not completely transformed into austenite, so that a uniform structure cannot be obtained and the arrestability deteriorates. On the other hand, when heated above 900 ° C., austenite grains become coarse and the arrestability deteriorates.

  What is necessary is just to quench by appropriate means, such as water cooling, after heating to the temperature of 760-900 degreeC, for example.

(C-5) Tempering step:
In step 5 as a tempering step, the quenched steel plate is tempered at a temperature of 550 to 640 ° C.

  When the tempering temperature is less than 550 ° C., sufficient toughness cannot be obtained, while when the temperature exceeds 640 ° C., the strength decreases.

  The tempering temperature is more preferably 575 ° C. or higher, and further preferably 620 ° C. or lower.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Steels 1 to 20 having the chemical composition shown in Table 1 were melted using an experimental furnace, then hot forged to produce blocks, and finished into steel plates having a thickness of 50 mm under the conditions shown in Table 2.

  Steels 1 to 14 in Table 1 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, steels 15 to 20 are steels of comparative examples whose chemical compositions deviate from the conditions specified in the present invention.

上記のようにして得た板厚５０ｍｍの各厚鋼板について、先ず、板厚中心部におけるミクロ組織を調査した。

For each thick steel plate having a thickness of 50 mm obtained as described above, first, the microstructure in the central portion of the thickness was investigated.

  That is, a test piece is taken from the center of the plate thickness of each steel plate so that the cross section of the plate thickness parallel to the rolling direction becomes the test surface, and then the test piece is embedded in resin and mirror polished, By using and etching to reveal old austenite grain boundaries, using an optical microscope, observing 20 fields of view at a magnification of 100, taking the old austenite grain boundaries as an electrophotographic image, and processing with an image analyzer, “Average grain size of prior austenite grains”, “ratio of former austenite grains having a grain size of 40 μm or more”, and “aspect ratio of former austenite grains” in the region of ± 5 mm in the thickness center were calculated.

  Similarly, after mirror polishing, the “area ratio of austenite” was measured by X-ray measurement.

  Furthermore, with respect to each thick steel plate having a thickness of 50 mm, a test piece having a thickness of 25 mm is sampled in two directions, L and C, centering on the position of 1/2 of the plate thickness, and a DT test prescribed in ASTM E604 is performed. The absorbed energy at -196 ° C was measured. The target of absorbed energy was 1600 J or more in both the L direction and the C direction.

  Table 3 summarizes the above survey results.

  Among the above test numbers, in the case of test numbers 1 to 13 in which the ratio of the prior austenite grains having a particle size of 40 μm or more is 5% or less, compared to the test number 14 in which the ratio of the prior austenite grains is 5.8%. It is also clear that it has better arrestability.

  On the other hand, although the steel satisfies the chemical composition conditions specified in the present invention, the test number in which the average grain size of the prior austenite grains deviates from the conditions specified in the present invention, in particular, regarding the microstructure in the center of the plate thickness. In the case of 15 to 19, the absorbed energy at −196 ° C. in the DT test does not reach the target in both the L and C directions and is inferior in arrestability.

  Similarly, although the steel satisfies the chemical composition conditions specified in the present invention, DT in the case of test number 20 in which the aspect ratio of the prior austenite grains deviates from the conditions specified in the present invention with respect to the microstructure in the center of the plate thickness. The absorbed energy at −196 ° C. in the test achieved 2194 J in the L direction, but was 1299 J far from the target in the C direction, and the arrestability in the C direction was deteriorated.

  Further, in the case of Test No. 21 and Test No. 26 of comparative examples in which the steel deviates from the chemical composition conditions specified in the present invention, the absorbed energy in the C direction at −196 ° C. in the DT test did not reach the target. Inferior to sex.

  Similarly, in the case of test numbers 22 to 25 of comparative examples in which the steel deviates from the chemical composition conditions specified in the present invention, the absorbed energy at −196 ° C. in the DT test did not reach the target in both the L and C directions. Inferior to arrestability.

Incidentally, the comparison of test numbers 1 to 14 and test numbers 15 to 20, according to the production method of the present invention, is also apparent that capable of producing thick cold steel plate having good arrestability stably and reliably .

  Since the steel sheet for low-temperature wall having excellent arrestability according to the present invention has an absorbed energy at −196 ° C. in the DT test specified in ASTM E604, in both the L direction and the C direction of the steel sheet, it is 1600 J or more. It can be suitably used as a material for a low temperature storage tank such as LNG. The thick low-temperature steel sheet having excellent arrestability can be stably and reliably manufactured by applying the method of the present invention.

Claims (5)

In mass%, C: 0.01 to 0.1%, Si: 0.03 to 0.60%, Mn: 0.3 to 2.0%, P: 0.005% or less, S: 0.003 %, Ni: more than 8.5% and less than 9.5%, sol. Al: 0.005 to 0.050% and N: 0.0005 to 0.0060%, the balance consists of Fe and impurities,
In the region of ± 5 mm at the center of the plate thickness , the average austenite grain diameter of the old austenite grains is 25 μm or less, and the plate thickness section parallel to the rolling direction and the plate thickness direction of the old austenite grains is measured using an optical microscope. Old austenite grains obtained by measuring the grain size as the average section length in the rolling direction and the thickness direction of the steel sheet, and calculating from “average section length in the rolling direction / average section length in the sheet thickness direction” the aspect ratio is 0.7 to 1.3, only contains a 3.0 to 15.0 percent of the austenite in the area ratio in the microstructure,
Thickness excellent in arrestability in which the absorbed energy at −196 ° C. in the DT test prescribed in ASTM E604 is 1600 J or more in both the rolling direction and the plate thickness direction, and the plate thickness is 40 to 60 mm. Low temperature steel sheet.   The steel sheet for thick and low temperature excellent in arrestability according to claim 1, further comprising, by mass%, Mo: 0.1% or less.   3. Thickness and low temperature excellent in arrestability according to claim 1 or 2, further comprising at least one of Cu: 2.0% or less and Cr: 0.6% or less in mass%. steel sheet. 4. The arrestability according to claim 1, wherein the ratio of the prior austenite grains having a grain size of 40 μm or more is 5% or less in an area of ± 5 mm at the center of the plate thickness. Excellent thick and low temperature steel sheet. By sequentially performing the following steps 1 to 5 on a steel piece having the chemical composition according to any one of claims 1 to 3 ,
In the region of ± 5 mm at the center of the plate thickness, the average grain size of the prior austenite grains at the equivalent circle diameter is 25 μm or less, and in the plate thickness section parallel to the rolling direction and the plate thickness direction of the steel plate, using an optical microscope The old austenite grain size was measured as the average section length in the rolling direction and the thickness direction of the steel sheet, respectively, and was obtained from “average section length in the rolling direction / average section length in the sheet thickness direction”. The aspect ratio of the austenite grains is 0.7 to 1.3, the area ratio includes 3.0 to 15.0% austenite in the microstructure, and at −196 ° C. in the DT test specified in ASTM E604. The manufacturing method of the steel plate for thick-walled low temperature excellent in arrestability which obtains the steel plate whose plate | board thickness is 40-60 mm in both the rolling direction and the plate | board thickness direction of 1600J or more .
Process 1: The process of heating a steel slab to 800-1150 degreeC.
Step 2: A step of rolling in a temperature range of at least 800 to 650 ° C. so that the rolling reduction per pass is 5% or more and the cumulative rolling reduction is 25% or more.
Step 3: After completion of rolling, 10 ~50 ℃ / s step of cooling to a temperature of 200 ° C. or less at a cooling rate of.
Process 4: The process of heating and hardening to the temperature of 760-900 degreeC.
Process 5: The process of tempering at the temperature of 550-640 degreeC.