JP5494167B2 - Cryogenic steel plate and manufacturing method thereof - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a cryogenic thick steel plate excellent in toughness, and more particularly to a cryogenic thick steel plate excellent in brittle fracture occurrence inhibiting properties after strain aging and a method for producing the same. In this case, the ultra-low temperature thick steel plate is intended for a relatively thin plate having a thickness of 3 to 50 mm, and the ultra-low temperature is intended for use at an extremely low temperature of −60 ° C. or less, particularly −165. It means the use at extremely low temperature of ℃.

  The cryogenic thick steel sheet is mainly used for storage tanks for storing liquefied gas such as LPG (Liquefied petroleum gas) and LNG (Liquefied natural gas) in a cryogenic region. An extremely low temperature of −60 ° C. or lower means a temperature at which these liquefied gases can be stored in a liquid temperature range, and an extremely low temperature of −165 ° C. means a temperature at which LNG can be stored in a liquid temperature range. .

  Therefore, an extremely low temperature thick steel plate that is intended for a comparatively thin one having a thickness of 3 to 50 mm is required to have excellent fracture toughness at an extremely low temperature from the viewpoint of ensuring safety. In particular, there is a demand for characteristics capable of suppressing the occurrence of brittle fracture even after the cold plastic working that the material receives during the production of the storage tank. In other words, both the base metal and the welded joint are required to have excellent brittle fracture occurrence suppression characteristics after strain aging at an extremely low temperature of −60 ° C. or less, particularly −165 ° C.

  A so-called 9% Ni steel added with 9% Ni is known as a steel material having excellent brittle fracture occurrence suppression characteristics in an extremely low temperature environment of -165 ° C., and is also defined in the Japanese Industrial Standard (JIS). Yes.

  And based on this 9% Ni steel, the control of the base material properties is to reduce impurities such as P and S, to reduce C, [Quenching (Q) -2 phase quenching (L)- Various improvements such as a three-step heat treatment method comprising three steps of tempering (T)] have been attempted. In addition, low-temperature steel with a reduced Ni content has been proposed in order to reduce the cost of steel materials.

  Patent Document 1 discloses a low-temperature steel containing 4.0 to 7.5% Ni and having a martensitic transformation start temperature (Ms point) of 370 ° C. or lower. Patent Document 2 discloses a steel containing 5.5 to 10% Ni and a continuous casting method thereof. Patent Document 3 discloses a method for producing a low yield ratio nickel steel sheet having excellent low temperature toughness. Patent Document 4 discloses a steel containing 1.5 to 9.5% Ni and 0.02 to 0.08% Mo.

Japanese Patent Laid-Open No. 6-136483 JP 7-90504 A JP 60-59023 A JP 2002-129280 A

  The storage tank that stores LPG and LNG has a cylindrical shape, so when manufacturing the storage tank, the steel plate as a raw material is cold-plastic processed and the ends of the steel plate are welded together to ensure the shape. It is normal to do. Thick steel plates after being subjected to such cold plastic working generally have only the properties before being subjected to strain, even though the performance may be extremely reduced due to plastic strain. It is only an evaluation, and it cannot be said that the brittle fracture occurrence suppression property after strain aging is properly evaluated. However, the above Patent Documents 1 to 4 do not refer to the brittle fracture occurrence inhibiting property after strain aging.

  In view of such circumstances, the present invention attempts to examine the brittle fracture occurrence suppression characteristics after strain aging from the viewpoint of the metal structure, and further reveals a key technology that maintains high fracture resistance even when subjected to plastic strain. Therefore, in designing alloys for steel plates for cryogenic temperatures, the parameters on the metal structure corresponding to the target property of inhibiting the occurrence of brittle fracture after strain aging are clarified, and cold plastic working is performed. It is an object of the present invention to provide a cryogenic thick steel plate excellent in brittle fracture occurrence suppression properties after strain aging even at low cost.

  In order to solve the above-mentioned problems, a wide range of prototype tests were conducted on various alloy components and metal structures, with the amount of Ni effective for cryogenic toughness being specified as 5 to 10% as an alloy design for cryogenic thick steel plates. In order to investigate the characteristics to prevent brittle fracture after strain aging as a thick steel plate for cryogenic temperatures. As a result, the following findings (a) to (d) were obtained.

  (a) The first necessary characteristic as a cryogenic tank material is a brittle fracture prevention characteristic. Furthermore, considering the manufacturing process of the tank, in order to improve this characteristic even after being subjected to strain, the residual γ (under the alloy composition range containing 5 to 10% of Ni effective for cryogenic toughness Austenite), effective crystal grain size refinement, and grain structure control are required. Since the austenite structure is a structure in which brittle fracture does not occur in principle, the brittle fracture occurrence suppressing property of the material is remarkably enhanced by the dispersion of this structure. Here, the residual γ can be evaluated by an X-ray diffraction method.

  (b) Next, the effective crystal grain size is difficult to define in the case of the high Ni-based component that is the subject of the present invention. In general, in a steel having a high hardenability such as a martensite structure, an old γ grain size or the like is often used as a structural unit.

  However, there are subdivided regions called packets and blocks inside the old γ grains, and the old γ grains are not necessarily parameters representing the size of the structure. Therefore, the present inventors diligently studied to define the crystal grain size by using an EBSP (Electron Backscatter Diffraction Pattern) method.

  Even when trying to measure the effective crystal grain size, if it is quantified based on the grain boundary recognized by an optical microscope or a scanning electron microscope, the fracture surface unit may be measured when the orientation difference between adjacent crystal grains is small. Correspondence is poor and it cannot be a value that represents the organization size. Therefore, in the present invention, as the “effective crystal grain size”, a crystal grain size of a portion surrounded by a structure boundary having an orientation difference of 15 ° or more when evaluated by EBSP is adopted. In other words, using the EBSP method, observations of 5 fields of view or more at a magnification of 2000 times are taken, and a structure boundary having an orientation difference of 15 ° or more is regarded as a grain boundary, and the area inside one crystal is obtained, and the area is equivalent to a circle. The value converted to the particle size was evaluated as the “effective crystal particle size”.

  In addition, when moderate defects are introduced into the structure in the grain, a brittle fatigue crack is less likely to occur than in a very well-organized structure. This can be quantified using the EBSP method, and as a result of examination by the inventors, it can be defined as an average value GAM (Grain Average Misorientation) of misorientation between adjacent measurement points in one crystal grain. I found out that I can do it. Here, the crystal grain refers to a region surrounded by a large-angle grain boundary of 15 ° or more, and in order to analyze the inside of one crystal grain, the observation magnification may be set to 10,000 times. I also found out.

  (c) As a result, the structural requirements necessary to ensure the brittle fracture occurrence suppression characteristics after strain aging are the thickness portion where the effective crystal grain size is 1/4 from the surface of the thick steel plate as defined above, i.e. 2. The average thickness is 5.5 μm or less at the plate thickness (1/4) t position, and the residual γ at the plate thickness portion of 1/4 from the surface of the thick steel plate, that is, the plate thickness (1/4) t position is 3. The present inventors have found that the presence of 0% by volume or more and the average value of misorientation GAM (Grain Average Misorientation) between adjacent measurement points in one crystal grain is 0.85 ° or more.

  (d) Next, the method for producing steel having the structural requirements necessary for ensuring the brittle fracture occurrence suppression characteristics after strain aging is not particularly limited, but is not limited to heating. It is desirable to control the conditions. That is, it is desirable to heat the steel ingot at a low temperature, and it is desirable that the heating time is short. This is because when the temperature is low, heating for a long time can be allowed without causing coarsening of the metal structure of the steel ingot.

As a result of various experiments conducted by the inventors to define a desirable range of the heating temperature Tr (° C.) and the heating time t (hr), the following equations (1) and (2) can be derived. It was.
^{t × exp (Tr 3/270000000} ) ≦ 580 ·········· (1) formula Ac ₃ point ≦ Tr ....................... (2 Where Tr is the heating temperature (° C.) of the steel ingot, t is the heating time (hr) after the steel ingot is stabilized with Tr, and Ac ₃ is the temperature at which the transformation from ferrite to austenite is completed. , Respectively.

  According to these formulas (1) and (2), it is possible to control the heating temperature to be low or to shorten the occupation time of the heating furnace, so that it is possible to expect a reduction in manufacturing cost. In addition, reduction of heating temperature or shortening of heating time is important from the viewpoint of suppressing greenhouse gas emissions. In addition to satisfying the equations (1) and (2), the heating temperature of the steel ingot in the heating furnace is more preferably 1000 ° C. or less.

  The present invention has been made on the basis of the above findings, and the gist thereof is as shown in the following (1) to (7).

(1) By mass%, C: 0.01 to 0.12%, Si: 0.01 to 0.3%, Mn: 0.4 to 2.0%, P: 0.05% or less, S: 0.008% or less, Ni: less than 5.0 super ~10.0%, Al: 0.002~0.05%, N: 0.005% or less, the balance being Fe and impurities, Martens Structure mainly composed of site structure, residual γ amount at plate thickness (1/4) t position is 3.0% by volume or more, and structure surrounded by large angle grain boundaries of 15 ° or more observed by EBSP method at a magnification of 2000 times The average value of the equivalent circle grain size is 5.5 μm or less at the plate thickness (1/4) t position, and misorientation between adjacent measurement points in one crystal grain observed by EBSP method at a magnification of 10,000 times An ultra-low temperature thick steel plate excellent in brittle fracture occurrence suppression characteristics after strain aging, characterized by having an average value GAM of 0.85 ° or more.

  (2) Instead of a part of Fe, by mass%, Cu: 2.0% or less, Cr: 1.5% or less, Mo: 0.5% or less, V: 0.1% or less, and B: 0 The steel plate for cryogenic temperatures excellent in the brittle fracture occurrence suppression property after strain aging of said (1) characterized by containing 1 type or 2 types or more in 0.005% or less.

  (3) Instead of a part of Fe, the composition contains one or two of Nb: 0.1% or less and Ti: 0.1% or less in mass%. ) Or (2) Cryogenic thick steel plate with excellent brittle fracture occurrence prevention properties after strain aging.

  (4) Instead of a part of Fe, by mass%, it contains one or more of Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less. A thick steel sheet for cryogenic temperature excellent in brittle fracture occurrence suppression characteristics after strain aging according to any one of (1) to (3) above.

(5) After heating the steel ingot having the chemical composition shown in any of (1) to (4) above so as to satisfy the following formulas (1) to (3) , hot rolling, And the manufacturing method of the steel plate for cryogenic temperature excellent in the brittle fracture generation | occurrence | production inhibitory property after the strain aging in any one of said (1)-(4) characterized by cooling.
^{t × exp (Tr 3/270000000} ) ≦ 580 ·········· (1) formula Ac ₃ point ≦ Tr ·········· (2) formula Tr _max ≦ Tr + 50 ·· (3) where Tr is the heating temperature (° C) of the steel ingot, t is the heating time (hr) after the steel ingot is stabilized with Tr, Tr _max is the steel ingot Represents the maximum temperature reached until it stabilizes at Tr, and the Ac ₃ point represents the temperature at which the transformation from ferrite to austenite is completed.

(6) After heating the steel ingot having the chemical composition shown in any of (1) to (4) above so as to satisfy the following formulas (1) to (3) , hot rolling, [Ac ₁ point + 80 ° C.] Tempering and cooling at a temperature of not more than ₁ point, and having excellent brittle fracture occurrence suppression characteristics after strain aging according to any one of the above (1) to (4) A method for producing a low-temperature thick steel plate.
^{t × exp (Tr 3/270000000} ) ≦ 580 ·········· (1) formula Ac3 point ≦ Tr ·················· (2) Formula Tr _max ≦ Tr + 50 (3) where Tr is the heating temperature (° C.) of the steel ingot, and t is the heating time (hr) after the steel ingot is stabilized by Tr Tr _max represents the maximum temperature that the steel ingot reaches until it stabilizes with Tr, and Ac ₃ point represents the temperature at which the transformation from ferrite to austenite is completed.

(7) After heating the steel ingot having the chemical composition shown in any of (1) to (4) above so as to satisfy the following formulas (1) to (3) , hot rolling, Then, after reheating to a temperature of Ac ₁ point to 900 ° C., quenching and further tempering at a temperature of [Ac ₁ point + 80 ° C.] or less, any of the above (1) to (4) The manufacturing method of the thick steel plate for cryogenic temperature which was excellent in the brittle fracture generation | occurrence | production suppression property after distortion aging described in 2 .
^{t × exp (Tr 3/270000000} ) ≦ 580 ·········· (1) formula Ac ₃ point ≦ Tr ....................... (2 ) Formula Tr _max ≦ Tr + 50 Equation (3) where Tr is the heating temperature (° C.) of the steel ingot, and t is the heating time after the steel ingot is stabilized by Tr (hr ), Tr _max represents the maximum temperature that the steel ingot reaches until it stabilizes with Tr, and Ac ₃ point represents the temperature at which the transformation from ferrite to austenite is completed.

  Even after cold plastic working, it is possible to provide a cryogenic thick steel plate excellent in brittle fracture occurrence suppression characteristics after strain aging at low cost.

The calculation method of the average value GAM of misorientation between adjacent measurement points in one crystal grain is shown.

  Below, the thick steel plate for cryogenic temperature concerning this invention and its manufacturing method are demonstrated in detail for every requirement. In addition, "%" regarding content means "mass%" unless otherwise indicated.

A. Regarding chemical composition C: 0.01 to 0.12%
C is an element necessary for ensuring the strength of the base material. If the content is less than 0.01%, not only the required strength cannot be secured, but also lath formation in FL (Fusion Line) becomes insufficient, and the toughness of HAZ (Heat Affected Zone) near FL decreases. , C must be contained in an amount of 0.01% or more. On the other hand, if the content exceeds 0.12%, the toughness deterioration of HAZ, particularly HAZ near FL, becomes remarkable. Therefore, the C content is set to 0.01 to 0.12%. In addition, the preferable range of content of C is 0.03 to 0.09%.

Si: 0.01 to 0.3%
Si is an element necessary as a deoxidizer. In order to acquire this effect, it is necessary to contain 0.01% or more of Si. On the other hand, in the case of the steel according to the present invention, Si is greatly related to the tempering process of martensite as it is quenched, and if the Si content exceeds 0.3%, The toughness of the weld is reduced by suppressing the self-tempering by suppressing the decomposition and precipitation reaction from cement to cementite, or by increasing island martensite. Therefore, the Si content is set to 0.01 to 0.3%. In addition, from the viewpoint of improving the toughness of the welded portion, the Si content should be as low as possible, with a preferable range of 0.02 to 0.15% and a more preferable range of 0.03 to 0.10%.

Mn: 0.4 to 2.0%
Mn is an element necessary as a deoxidizer and for ensuring the strength and toughness of the base material and the hardenability of HAZ. If the Mn content is less than 0.4%, not only these effects cannot be obtained, but also ferrite side plates are formed in the HAZ, resulting in insufficient lath formation, and the toughness of the weld is reduced. The content is 0.4% or more. On the other hand, if the Mn content exceeds 2.0%, the base material characteristics in the thickness direction are uneven due to center segregation. Therefore, the content of Mn is set to 0.4 to 2.0%. In addition, a preferable range is 0.5 to 1.5%, and a more preferable range is 0.6 to 1.1%.

P: 0.05% or less P is present in the steel as an impurity, and segregates at the grain boundary to cause a decrease in toughness. If the P content exceeds 0.05%, hot cracking is caused during welding, so the P content is 0.05% or less. In addition, it is good to make content of P as small as possible, and preferable content of P is 0.03% or less.

S: 0.008% or less S is present in the steel as an impurity, and if it is too much, it promotes center segregation or causes a large amount of stretched MnS that causes brittle fracture. If the S content exceeds 0.008% or less, the mechanical properties of the base material and the HAZ deteriorate. Since the S content should be as small as possible, there is no particular lower limit. In addition, the preferable content of S is 0.003% or less.

Ni: more than 5.0% and less than 10.0% Ni is the most basic element necessary for securing toughness as a cryogenic steel. In order to ensure toughness as a cryogenic steel, a Ni content exceeding 5.0% is required. The higher the Ni content, the higher the cryogenic toughness is obtained. However, the cost increases accordingly, so the upper limit of the Ni content is less than 10.0%. Therefore, the Ni content target is more than 5.0% and less than 10.0%. In addition, from the viewpoint of cryogenic toughness and cost reduction, a preferable range of Ni content is more than 5.5% and less than 9.0%, and a more preferable range is more than 6.0% and less than 8.0%. .

Al: 0.002 to 0.05%
Al is an element generally contained as a deoxidizer, but in the case of the steel of the present invention, it has a function of delaying self-tempering of martensite, similar to Si. The content of is preferably as low as possible. However, if the Al content is less than 0.002%, a sufficient deoxidizing effect cannot be obtained. On the other hand, when the Al content exceeds 0.05% and becomes excessive, as in the case of Si described above, the decomposition precipitation reaction from martensite, which is supersaturated with C to solid solution, in the welding cooling process to cementite is suppressed. , Reduce the toughness of the weld. Therefore, the Al content is 0.002 to 0.05%. In addition, the preferable range of content of Al is 0.005-0.040%.

N: 0.005% or less N is present in steel as an impurity and causes deterioration of HAZ toughness through the increase in solid solution N and the formation of precipitates. The amount should be low. If the N content exceeds 0.005%, the HAZ toughness deteriorates significantly, so the N content is set to 0.005% or less. In addition, the preferable content of N is 0.004% or less.

  In addition to the above components, the thick steel plate for cryogenic temperature according to the present invention comprises Fe and impurities in the balance. Here, the impurity is a component that is mixed due to various factors in the manufacturing process, including raw materials such as ore and scrap, when industrially manufacturing a steel plate for cryogenic temperature, and is included in the present invention. It means what is allowed as long as it does not adversely affect.

  The cryogenic thick steel plate according to the present invention further contains one or more of Cu, Cr, Mo, V, B, Nb, Ti, Ca, Mg and REM in addition to the above components. Also good.

Cu: 2.0% or less Cu can be contained as required. When Cu is contained, the strength of the base material can be improved. However, if this content exceeds 2.0%, the toughness of HAZ heated to a temperature of Ac ₃ points or less is deteriorated, so the Cu content is set to 2.0% or less. In order to stably develop the strength improvement effect of the base material by Cu, it is preferable to contain Cu by 0.1% or more. A more preferable range of the Cu content is 0.2 to 1.3%.

Cr: 1.5% or less Cr can be contained as necessary. When Cr is contained, the carbon dioxide gas corrosion resistance and hardenability can be improved. However, if this content exceeds 1.5%, not only is it difficult to suppress the hardening of the HAZ, but the effect of improving the corrosion resistance to carbon dioxide gas is saturated, so the Cr content is 1.5% or less. . In order to stably exhibit the effects of improving the carbon dioxide corrosion resistance and hardenability by Cr, it is preferable to contain 0.05% or more of Cr. A more preferable Cr content range is 0.1 to 1.0%.

Mo: 0.5% or less Mo can be contained as necessary. Inclusion of Mo has an effect of improving the strength and toughness of the base material. However, if this content exceeds 0.5%, the hardness of the HAZ increases and the toughness and SSC resistance are impaired, so the Mo content is set to 0.5% or less. In order to stably develop the effect of improving the strength and toughness of the base material due to Mo, it is preferable to contain 0.02% or more of Mo. A more preferable range of the Mo content is 0.05 to 0.3%.

V: 0.1% or less V can be contained as necessary. Inclusion of V has an effect of improving the strength of the base material mainly due to carbonitride precipitation during tempering. However, if the content exceeds 0.1%, the performance improvement effect of the base material strength is saturated and the toughness is deteriorated, so the V content is set to 0.1% or less. In order to stably develop the effect of improving the strength of the base material by V, it is preferable to contain V by 0.015% or more. A more preferable range of the V content is 0.02 to 0.08%.

B: 0.005% or less B can be contained if necessary. Inclusion of B has an effect of improving the strength of the base material. However, if this content exceeds 0.005%, precipitation of coarse boron compounds is caused and the toughness is deteriorated, so the B content is made 0.005% or less. In order to stably develop the effect of improving the strength of the base material by B, it is preferable to contain B by 0.0003% or more. A more preferable range of the B content is 0.001 to 0.004%.

Nb: 0.1% or less Nb can be contained as necessary. When Nb is contained, there is an effect of refining the structure and improving the cryogenic toughness. However, if this content exceeds 0.1%, coarse carbides and nitrides are formed and the toughness is lowered, so the Nb content is made 0.1% or less. In order to stably develop the effect of improving the cryogenic toughness by Nb, it is preferable to contain Nb by 0.01% or more. A more preferable range of the Nb content is 0.02 to 0.08%.

Ti: 0.1% or less Ti can be contained as necessary. When Ti is contained, it is mainly used as a deoxidizing element, but has the effect of forming an oxide phase composed of Al, Ti, and Mn to refine the structure. However, when this content exceeds 0.1%, the oxide formed is Ti oxide or Ti-Al oxide, and the dispersion density is lowered. Therefore, the Ti content is 0.1% or less. In order to stably develop the effect of refining the structure by Ti, it is preferable to contain Ti by 0.02% or more. A more preferable range of Ti content is 0.03 to 0.07%.

Ca: 0.004% or less Ca can be contained as necessary. When Ca is contained, it reacts with S in the steel to form oxysulfide (oxysulfide) in the molten steel. Unlike MnS, etc., this oxysulfide does not extend in the rolling direction during rolling, so it remains spherical after rolling, and has no weld cracks or hydrogen-induced cracks starting from cracks at the ends of the elongated inclusions. There is an inhibitory effect. However, if this content exceeds 0.004%, the toughness may be deteriorated, so the Ca content is set to 0.004% or less. In order to stably develop the effect of suppressing weld cracking and hydrogen induced cracking due to Ca, it is preferable to contain 0.0003% or more of Ca. A more preferable range of the Ca content is 0.0005 to 0.003%.

Mg: 0.002% or less Mg can be contained as necessary. When Mg is contained, a fine Mg-containing oxide is produced, which is effective in reducing the γ particle size. However, if this content exceeds 0.002%, the amount of oxide becomes too much and the ductility may be lowered, so the Mg content is set to 0.002% or less. In order to stably develop the effect of refining the γ particle diameter by Mg, it is preferable to contain 0.0002% or more of Mg. A more preferable range of Mg content is 0.0003 to 0.0010%.

REM: 0.002% or less REM (rare earth element) can be contained as required. When REM is contained, there is an effect of refining the structure of the weld heat affected zone and fixing S. By the way, when REM is excessively contained, inclusions are formed and the cleanliness is lowered. However, inclusions formed by the addition of REM have a relatively small influence on deterioration of toughness. Even if it is contained in an amount of 0.002% or less, a decrease in the toughness of the base material can be tolerated. In order to stably develop the effect of refining the structure of the heat affected zone by REM and the effect of fixing S, it is preferable to contain 0.0002% or more of REM. A more preferable range of the content of REM is 0.0003 to 0.001%.

  Here, REM is a general term for 17 elements in which Y and Sc are combined with 15 elements of lanthanoid, and one or more of these elements can be contained. Note that the content of REM means the total content of these elements.

B. Regarding metal structure B-1. The amount of residual γ at the position of thickness (1/4) t is 3.0% by volume or more. In order to obtain this effect, it is necessary that the amount of residual γ in the steel is 3.0% by volume or more. The upper limit of the amount of residual γ is not particularly specified, but if there is too much residual γ, the yield stress may decrease, so the amount of residual γ is preferably 15.0% by volume or less. More preferably, it is 10.0 volume% or less. Here, the reason why the amount of residual γ is evaluated at the position of the thickness (1/4) t is to evaluate at an average position throughout the thickness.

  In addition, since the thick steel plate according to the present invention has a high Ni content, it is easy to be baked, and therefore, it has a martensite structure in addition to the residual γ. In addition to the residual γ and martensite structure, even if a metal structure such as a bainite structure is present in an amount of 25% by volume or less, it does not affect the brittle crack propagation stopping characteristics of the thick steel plate.

B-2. The average value of the circle-equivalent grain size of the structural unit surrounded by the large-angle grain boundaries of 15 ° or more observed by the EBSP method at a magnification of 2000 times is 5.5 μm or less at the plate thickness (1/4) t position. The average value of the equivalent-circle particle size affects the brittle fracture inhibition characteristics. Brittle fracture when the average value of equivalent circle diameter of the structural unit surrounded by large-angle grain boundaries of 15 ° or more observed by EBSP method at 2000 times magnification is 5.5μm or less at the thickness (1/4) t position Occurrence suppression characteristics are improved. In addition, since this brittle fracture generation | occurrence | production suppression property becomes so good that this particle size is small, the minimum of a particle size is not prescribed | regulated. Here, the reason why the particle size is evaluated at the position of the plate thickness (1/4) t is to perform the evaluation at the average position of the entire plate thickness.

B-3. Average value GAM of misorientation between adjacent measurement points in one crystal grain observed by EBSP method at a magnification of 10,000 times is 0.85 ° or more. Destruction is likely to occur. In other words, brittle fracture is less likely to occur if defects occur in the structure within the crystal grains.

  In the present invention, GAM is used as an index of defects in the structure within the crystal grains. GAM is an average value of misorientation in one crystal grain.

FIG. 1 schematically shows crystal grains for explaining GAM. Here, the outer periphery of the region indicated by the solid line is a large-angle grain boundary of 15 ° or more, and the region surrounded by the outer periphery is defined as one crystal grain. Within this crystal grain, there is a subgrain boundary having an orientation difference of less than 15 ° (in FIG. 1, there is a subgrain boundary of m = 19 ). This subcrystal grain boundary corresponds to each solid line inside the crystal grain. Then, the orientation difference (α _i ) between adjacent subcrystal grains is measured for each subgrain boundary. The average of these is GAM. When GAM is measured, it is measured by EBSP method at a magnification of 10,000 times. It is reasonable to set the resolution to about 20000 to 50000 pixels.

  If there is turbulence in the crystal grains, the brittle fracture occurrence suppression characteristics are improved. In order to obtain this effect, the GAM needs to be 0.85 ° or more. The upper limit of GAM is not specified, but normally GAM is at most 5 °.

C. Regarding the manufacturing method The thick steel plate according to the present invention can be manufactured by the following manufacturing method. However, it is not limited to the following manufacturing method.

  The thick steel plate according to the present invention can be manufactured by sequentially performing the steps of heating, hot rolling and cooling on the steel ingot having the above-described chemical composition. You may perform the process of tempering, reheating, and quenching as needed.

  In addition, about manufacture of a steel ingot, the casting conditions are not prescribed | regulated exceptionally. An ingot-splitting slab or continuous cast slab may be used as a steel ingot, or a continuous cast slab may be used. From the viewpoint of production efficiency, yield, and energy saving, it is preferable to use a continuously cast slab.

In the heating step, the steel ingot is preferably heated so that the heating temperature Tr (° C.) and the heating time t (hr) of the steel ingot satisfy the following expressions (1) and (2).
^{t × exp (Tr 3/270000000} ) ≦ 580 ·········· (1) formula Ac ₃ point ≦ Tr ·········· (2) equation where, Tr steel The ingot heating temperature (° C.), t represents the heating time (hr) after the steel ingot was stabilized with Tr, and Ac ₃ point represents the temperature at which the transformation from ferrite to austenite is completed.

More specifically, the heating temperature Tr (° C.) of the steel ingot may be the temperature of the soaking zone in the heating furnace, and the heating time t (hr) is the time during which the ingot is in the soaking zone. That's fine. Incidentally, Ac ₃ point may be used calculated values based on the following equation (4).
Ac ₃ points = 897.3−271.1 × C + 43.7 × Si−17 × Mn + 117.8 × P + 15.95 × S-40.8 × Cu-22.3 × Ni−6.5 × Cr + 6.5 × Mo + 65.8 × V + 145.2 × Nb + 56. 9 × Al + 88.5 × Ti−17968.4 × B + 121.8 × N (4) Formula Here, the element symbol in the formula means the content (% by mass) of each element.

The heating process greatly affects the structure of the thick steel plate. As described above, the higher the heating temperature, the more coarse the structure, so a high heating temperature is not preferable. Usually, in the heating process, after charging the heating furnace, the steel ingot temperature gradually rises, and after exceeding the soaking zone temperature, so-called overshoot in which the ingot temperature becomes steady at the soaking zone temperature may occur. When the steel ingot temperature exceeds 50 ° C. from the soaking zone temperature due to the occurrence of overshoot, the steel ingot structure becomes coarser and the intended structure may not be obtained. Therefore, it is necessary to control the overshooting temperature to 50 ° C. or lower. That is, in the heating process, it is necessary to suppress the maximum ultimate temperature Tr _max of the steel ingot before the steel ingot is stabilized at Tr (° C.) to [Tr + 50] (° C.) or less. In other words, it is necessary to satisfy the following expression (3).
Tr _max ≦ Tr + 50 (3)

The heating temperature needs to be Ac ₃ points or higher in order to transform the structure into austenite. In addition, it is preferable that heating temperature shall be 850 degreeC or more. This is because a steel ingot of 850 ° C. or higher has low deformation resistance, and the load on the roll used in the subsequent hot rolling process is not so large. On the other hand, the heating temperature is preferably 1000 ° C. or lower. This is because if the heating is performed at 1000 ° C. or less, a sufficient heating time can be secured, and a more uniform steel ingot can be obtained.

  Thus, since the heating process is the process that most affects the steel structure, strict control is required. On the other hand, suppressing the heating temperature to 1000 ° C. or lower is desirable not only from the technical aspect, but also from the environmental aspect, such as the suppression of greenhouse gas emissions.

  In the hot rolling process, the heated steel ingot is rolled. Specifically, the rolling may be divided into rough rolling and finish rolling.

  In the rough rolling for the heated steel ingot, it is preferable to reduce the steel ingot thickness at the end of the rough rolling until it becomes 3 to 8 times the product thickness (thick steel plate thickness). When the steel ingot thickness after rough rolling is reduced to 3 times or more of the product thickness, sufficient reduction can be performed in the subsequent finish rolling, so that the toughness of the product thick steel plate can be improved. On the other hand, if the steel ingot thickness after rough rolling is reduced to 8 times the product thickness or less, the finish rolling temperature in the subsequent finish rolling (the temperature at which finish rolling ends) can be easily controlled to 650 ° C or higher. Become.

  In finish rolling, the steel ingot subjected to rough rolling in this way is continuously reduced without cooling to a product with a predetermined plate thickness. In this finish rolling, it is preferable to perform the rolling so that the finish rolling temperature is 650 ° C. or higher. This is because if the finish rolling temperature is 650 ° C. or higher, the deformation resistance is small and rolling is easy. In addition, what is necessary is just to measure the surface temperature of the steel ingot or thick steel plate which is a to-be-rolled material as the temperature in rolling.

  In the cooling step, the thick steel plate that has been finish-rolled is cooled. Since the steel plate is naturally cooled to some extent throughout the rolling process, the structure of the steel plate does not become coarse. It is sufficient to remove the cooling from the production line (offline) and let it cool as it is. Further, accelerated cooling may be performed on the production line. When subsequently quenching and / or tempering, it is not always necessary to cool the thick steel plate to room temperature.

The quenching is preferably performed by heating to a temperature of Ac ₁ point to 900 ° C. Moreover, in order to prevent the coarsening of a structure | tissue in reheating before quenching, it is preferable that heating temperature shall be Ac ₁ point -900 degreeC. By setting Ac to ₁ point or more, increase in residual γ can be expected, and by setting it to 900 ° C. or less, coarsening of the structure can be prevented, so that improvement of brittle crack propagation stopping characteristics can be expected. . In addition, the method of a quenching process does not ask | require means, such as a spray method. At this time, the cooling rate is preferably 5 ° C./s or more, and the cooling stop temperature is preferably 200 ° C. or less.

Tempering is preferably performed at a temperature of [Ac ₁ point + 80 ° C.] or lower. Tempering is mainly performed to remove the strain in martensite caused by quenching, but there is a sufficient strain removing effect even if it is performed on a thick steel plate not subjected to special quenching treatment. Tempering is preferably performed at a temperature of [Ac ₁ point + 80 ° C.] or less because this can increase the toughness of the as-quenched martensite structure and increase the amount of residual γ. In order to effectively obtain the distortion removal effect, the temperature is preferably 500 ° C. or higher.

  A steel ingot having a thickness of 300 mm made of 42 types of steel having the chemical composition shown in Table 1 is prepared, and finishes by heating, rolling, accelerated cooling, etc. under the conditions shown in Table 2, and then quenching and depending on the type of steel. Tempering was performed. The plate thickness is a 6-50 mm thick steel plate. From each of the obtained thick steel plates, a No. 4 test piece was taken from a No. 10 tensile test piece, a No. 5 tensile test piece or a (1/4) t portion specified in JISZ2201. The direction is the direction perpendicular to rolling. Moreover, the V notch test piece prescribed | regulated to JISZ2202 was extract | collected from the rolling direction. The subsize test was used for the plate thickness of 10 mm or less.

Tensile test at normal temperature and Charpy impact test at -196 ° C, tensile strength TS (MPa), yield strength YS (MPa) and absorbed energy vE-196 (J / mm ² ) (average value of 3) I investigated. Absorbed energy was converted to absorbed energy per unit area in order to facilitate comparison between the sub-size test piece and the full-size test piece. Furthermore, in order to evaluate the characteristics after strain aging, a 3% pre-strain was applied to the wide tensile test piece, followed by aging treatment at 250 ° C. × 1 hr, and a V-notch Charpy test was performed. The test piece was sampled from a thickness portion of 1/4 from the surface of the thick steel plate, that is, from the thickness (1/4) position. The material with a thickness of 10 mm or less was processed into a sub-size test piece similar to the V-notch. The absorbed energy is converted to a numerical value per unit area as with the V-notch test piece.

  Next, in order to investigate the brittle fracture occurrence suppression characteristics, CTOD test pieces based on BS7448 part 1 were collected from the L direction. The plate thickness was a full-thickness test piece, a B × 2B test piece was used, and the test method was also compliant with the BS standard. The characteristics of this CTOD test were also evaluated before and after strain aging treatment.

Judgment criteria for strength and brittle crack initiation inhibiting properties are as follows.
YS: 590 MPa or more, TS: 690 MPa or more, V-notch Charpy absorbed energy per unit area vE-196 (J / mm ² ): 1.25 J / mm ² or more, Limit CTOD value δ _c (mm): 0.1 mm or more Was evaluated as “good characteristics”.

  Furthermore, the “effective crystal grain size” is determined by irradiating a specimen placed in a scanning electron microscope (SEM) with an electron beam, analyzing the image of EBSP projected on the screen with a computer, and calculating an orientation difference of 15 The portion surrounded by the structure boundary of more than 0 ° was measured as the grain boundary. That is, EBSP (Electron Backscatter Diffraction Pattern) method is used to observe five or more fields of view at a magnification of 2000 times, and a structure boundary having an azimuth difference of 15 ° or more is regarded as a grain boundary. The area inside the crystal was determined, and the area converted into a circle-equivalent grain size was evaluated as the effective crystal grain size at the plate thickness (1/4) t position. The direction is a direction perpendicular to rolling.

  The amount of residual γ (volume%) was measured by X-ray diffractometry after collecting a test piece for measuring residual γ at the position (1/4) t of the steel sheet. More specifically, since all the manufactured specimens were mainly composed of a martensite structure, the difference between the lattice structure of the residual γ having a face-centered cubic structure and the martensite having a body-centered cubic structure was used to calculate X The amount of residual γ was measured from the integrated intensity ratio of the line peak.

  Regarding the average value GAM of misorientation between adjacent measurement points in one crystal grain, the structure inside the region surrounded by the structure boundary with an orientation difference of 15 ° or more was observed by the EBSD method at a magnification of × 10000. The resolution was measured as 30000 pixels.

  The above investigation results are summarized in Tables 1 to 3. As can be seen from the characteristic evaluation results shown in Table 3, the steel types having a chemical composition within the range defined by the present invention, namely, steel Nos. Thick steel plates (steel materials) made of steels 1 to 35 should be adjusted to an effective crystal grain size of 5.5 μm or less, a residual γ amount of 3% by volume or less, and a GAM of 0.85 ° or more by using an appropriate manufacturing method. The strength and brittle crack initiation characteristics (V-notch Charpy absorbed energy, critical CTOD value) satisfy the target ranges.

  The thick steel plate of Test No. 1-b has a chemical composition within the range specified in the present invention, but the structure surrounded by the large-angle grain boundary has a large equivalent-circle grain size, and the GAM is also small. Crack initiation characteristics (V-notch Charpy absorbed energy, limit CTOD value) do not satisfy the target range. In addition, the thick steel plates of Test No. 1-d and Test No. 1-f also have a chemical composition within the range specified by the present invention, but the equivalent circular grain size of the structure surrounded by the large angle grain boundaries has increased. Therefore, the strength and brittle crack initiation characteristics (V-notch Charpy absorbed energy, critical CTOD value) do not satisfy the target range.

  On the other hand, a steel type whose chemical composition is outside the range specified in the present invention, that is, steel no. For thick steel plates (steel materials) made of 36-42 steel, any of strength and brittle crack initiation characteristics (V-notch Charpy absorbed energy, limit CTOD value) does not reach the target.

  That is, since the thick steel plate of Test No. 36 has a high C content, the strength property is not a problem, but the fracture property is inferior. The steel plate of Test No. 37 and the steel plate of Test No. 38 have high Si content, Mn content, large equivalent-equivalent grain size, small residual γ content, and small GAM. Although there is no problem, the fracture characteristics are inferior. Moreover, since the thick steel plate of Test No. 39 has too high Ni content, the yield stress does not reach the target. The thick steel plate of Test No. 40 has a high B content, a small amount of residual γ, and a small GAM. The thick steel plate of Test No. 41 has a high Al content and a small amount of residual γ, so there is no problem in strength characteristics, but the fracture characteristics are inferior. Since the thick steel plate of Test No. 42 has a high N content, the strength properties are not a problem, but the fracture properties are inferior.

  Even after cold plastic working, it is possible to provide a cryogenic thick steel plate excellent in brittle fracture occurrence suppression characteristics after strain aging at low cost.

Claims (7)

In mass%, C: 0.01 to 0.12%, Si: 0.01 to 0.3%, Mn: 0.4 to 2.0%, P: 0.05% or less, S: 0.008 %: Ni: more than 5.0 to less than 10.0%, Al: 0.002 to 0.05%, N: 0.005% or less, with the balance being Fe and impurities , The main unit, the amount of residual γ at the plate thickness (1/4) t position is 3.0% by volume or more, and the unit is a circle of a structure unit surrounded by a large-angle grain boundary of 15 ° or more observed by the EBSP method at a magnification of 2000 times The average value of equivalent grain size is 5.5μm or less at the plate thickness (1/4) t position, and the average value of misorientation between adjacent measurement points in one crystal grain observed by EBSP method at a magnification of 10,000 times A steel plate for cryogenic temperature excellent in brittle fracture occurrence suppression characteristics after strain aging characterized by a GAM of 0.85 ° or more.   Instead of a part of Fe, by mass%, Cu: 2.0% or less, Cr: 1.5% or less, Mo: 0.5% or less, V: 0.1% or less, and B: 0.005% The steel plate for cryogenic temperature excellent in the brittle fracture generation | occurrence | production inhibitory property after the strain aging of Claim 1 characterized by containing 1 type, or 2 or more types of the following.   It replaces with a part of Fe and contains 1 type or 2 types of Nb: 0.1% or less and Ti: 0.1% or less in the mass%, The claim 1 or 2 characterized by the above-mentioned. Thick steel plate for cryogenic use with excellent brittle fracture occurrence suppression properties after strain aging.   Instead of a part of Fe, by mass%, it contains one or more of Ca: 0.004% or less, Mg: 0.002% or less, and REM: 0.002% or less. The thick steel plate for cryogenic temperature excellent in the brittle fracture occurrence suppression property after strain aging according to any one of claims 1 to 3. A steel ingot having the chemical composition according to any one of claims 1 to 4 is heated so as to satisfy the following formulas (1) to (3) , and then hot-rolled and cooled: The method for producing a thick steel plate for cryogenic temperature excellent in brittle fracture occurrence inhibiting property after strain aging according to any one of claims 1 to 4, wherein
^{t × exp (Tr 3/270000000} ) ≦ 580 ·········· (1) formula Ac ₃ point ≦ Tr ·········· (2) formula Tr _max ≦ Tr + 50 ·· (3) where Tr is the heating temperature (° C) of the steel ingot, t is the heating time (hr) after the steel ingot is stabilized with Tr, Tr _max is the steel ingot Represents the maximum temperature reached until it stabilizes at Tr, and the Ac ₃ point represents the temperature at which the transformation from ferrite to austenite is completed. A steel ingot having the chemical composition according to any one of claims 1 to 4 is heated so as to satisfy the following formulas (1) to (3) , and then hot-rolled, [Ac ₁ point The production of a steel plate for cryogenic use having excellent brittle fracture occurrence suppression properties after strain aging according to any one of claims 1 to 4, wherein the steel plate is tempered at a temperature of + 80 ° C or lower and then cooled. Method.
^{t × exp (Tr 3/270000000} ) ≦ 580 ·········· (1) formula Ac ₃ point ≦ Tr ....................... (2 ) Formula Tr _max ≦ Tr + 50 Equation (3) where Tr is the heating temperature (° C.) of the steel ingot, and t is the heating time after the steel ingot is stabilized by Tr (hr ), Tr _max represents the maximum temperature that the steel ingot reaches until it stabilizes with Tr, and Ac ₃ point represents the temperature at which the transformation from ferrite to austenite is completed. A steel ingot having the chemical composition according to any one of claims 1 to 4 is heated so as to satisfy the following formulas (1) to (3) , and then hot-rolled, and then Ac ₁ After strain aging according to any one of claims 1 to 4, characterized in that after reheating to a temperature of point ~ 900 ° C, quenching and further tempering at a temperature of [Ac ₁ point + 80 ° C] or less. The manufacturing method of the thick steel plate for cryogenic temperature excellent in the brittle fracture generation | occurrence | production inhibitory property.
^{t × exp (Tr 3/270000000} ) ≦ 580 ·········· (1) formula Ac ₃ point ≦ Tr ....................... (2 ) Formula Tr _max ≦ Tr + 50 Equation (3) where Tr is the heating temperature (° C.) of the steel ingot, and t is the heating time after the steel ingot is stabilized by Tr (hr ), Tr _max represents the maximum temperature that the steel ingot reaches until it stabilizes with Tr, and Ac ₃ point represents the temperature at which the transformation from ferrite to austenite is completed.

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