JP6760055B2 - Ni steel for liquid hydrogen - Google Patents

Description

The present invention relates to Ni steel for liquid hydrogen, that is, a steel containing Ni, which is mainly intended for use at around -253 ° C., such as a tank for storing liquid hydrogen.

In recent years, expectations for the use of liquid hydrogen as clean energy have increased. Austenitic stainless steel, which is required to have excellent cryogenic toughness and is resistant to brittle fracture, is used for the steel sheet used for the tank for storing and transporting liquefied gas such as liquid hydrogen. Austenitic stainless steel has sufficient cryogenic toughness, but the yield stress of general-purpose materials at room temperature is about 200 MPa.

When applying austenitic stainless steel with insufficient strength to a liquid hydrogen tank, there is a limit to the increase in size. Further, when the yield stress of the steel material is about 200 MPa, it is necessary to increase the plate thickness to more than 40 mm when increasing the size of the tank, which causes problems such as an increase in tank weight and an increase in manufacturing cost. To solve such a problem, an austenitic high Mn stainless steel having a 0.2% proof stress at room temperature of 450 MPa and a plate thickness of 5 mm has been proposed (see, for example, Patent Document 1).
Further, a ferrite-based 9% Ni steel is used for a tank (sometimes referred to as an LNG tank) for liquefied natural gas (LNG), which is a typical liquefied gas. Although LNG has a higher temperature than liquid hydrogen, 9% Ni steel has excellent cryogenic toughness, and various 9% Ni steels and 7% Ni steels suitable for LNG tanks have been conventionally used. Have been proposed (see, for example, Patent Documents 2-4). Further, the 9% Ni steel can have a yield strength of 590 MPa or more at room temperature, and can be applied to a large LNG tank.

For example, Patent Document 2 contains 5 to 7.5% Ni, has a yield stress of more than 590 MPa at room temperature, and a brittle fracture surface ratio of 50% or less in a Charpy test at -233 ° C. A low temperature steel having a plate thickness of 25 mm is disclosed. In Patent Document 2, low temperature toughness is ensured by setting the volume fraction of retained austenite stable at -196 ° C to 2 to 12%.

Further, Patent Document 3 contains 5 to 10% Ni, has a yield stress of 590 MPa or more at room temperature, and has excellent low temperature toughness at -196 ° C. after strain aging for low temperature of 6 to 50 mm. Steel is disclosed. In Patent Document 3, the volume fraction of retained austenite is 3% or more, the effective crystal grain size is 5.5 μm or less, and appropriate defects are introduced into the structure inside the grain to ensure low temperature toughness after strain aging. ing.

Further, Patent Document 4 discloses a thin nickel steel sheet for low temperature having a thickness of 6 mm, which contains 7.5 to 12% Ni and is excellent not only in the base material but also in the low temperature toughness of the weld heat affected zone. .. In Patent Document 4, the contents of Si and Mn are reduced to ensure low temperature toughness at -196 ° C. so that island-shaped martensite is not generated in the weld heat affected zone.

Japanese Patent No. 5709881 Japanese Unexamined Patent Publication No. 2014-210948 Japanese Unexamined Patent Publication No. 2011-219849 Japanese Unexamined Patent Publication No. 3-223442

The austenitic high Mn stainless steel disclosed in Patent Document 1 has a larger coefficient of thermal expansion than the ferrite-based 9% Ni steel. For a large liquid hydrogen tank, 9% Ni steel having a small coefficient of thermal expansion is advantageous because of problems such as fatigue. On the other hand, the 9% Ni steel and the 7% Ni steel disclosed in Patent Documents 2 to 4 have not obtained sufficient toughness at the liquid hydrogen temperature of -253 ° C. in the study by the present inventors.

In view of such circumstances, the present invention has been made to provide a Ni-containing cryogenic steel having sufficient toughness at -253 ° C. and a yield stress of 460 MPa or more at room temperature. is there.

The present inventors have conducted a number of studies on the toughness at -253 ° C. and the yield stress at room temperature of a steel having a Ni content of about 14%, which is higher than that of the conventional 9% Ni. As a result, in order to ensure cryogenic toughness, it is necessary to limit the Si content, strictly limit the Mn content, and optimally control the particle size and aspect ratio of the former austenite. I found it. The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) The Ni steel for liquid hydrogen according to one aspect of the present invention has C: 0.020% or more, 0.070% or less, Si: 0.03% or more, 0.30% or less, Mn in mass%. : 0.20% or more, 0.80% or less, Ni: 12.5% or more, 15.4% or less, Al: 0.010% or more, 0.060% or less, N: 0.0015% or more, 0 .0060% or less, O: 0.0007% or more, 0.0030% or less, P: 0.008% or less, S: 0.0040% or less, Cu: 1.00% or less, Cr: 1. 00% or less, Mo: 0.60% or less, Nb: 0.020% or less, V: 0.080% or less, Ti: 0.020% or less, B: 0.0020% or less, Ca: 0.0040% Below and REM: Limited to 0.0050% or less, the balance is composed of Fe and impurities, the particle size of the old austenite grains is 3.0 μm or more and 20.0 μm or less, and the aspect ratio is 1.0 or more and 2.9 or less. der Ri, _{K IC} values at -253 ° C. is at 150 MPa · √m or higher, the yield stress at room temperature, characterized in der Rukoto least 460 MPa.
(2) The Ni steel for liquid hydrogen described in (1) above may contain an austenite phase having a volume fraction of 2.0% or more and 20.0% or less.
(3) In the Ni steel for liquid hydrogen according to the above (1) or (2), the effective crystal particle size may be limited to 2.0 μm or more and 12.0 μm or less.
(4) The Ni steel for liquid hydrogen according to any one of (1) to (3) above has a plate thickness of 12 mm or more and 40 mm or less, a yield stress at room temperature of 460 MPa or more and 580 MPa or less, and room temperature. The tensile strength at 560 MPa or more and 680 MPa or less may be used.

According to the present invention, it is possible to provide a Ni steel for liquid hydrogen having sufficient cryogenic toughness for use in a liquid hydrogen tank and having a high yield stress at room temperature.

Steel containing about 13 to 15% Ni (sometimes referred to as 14% Ni steel) contains 4 to 6% more Ni, which improves low temperature toughness, as compared with 9% Ni steel. It can be expected to secure toughness at low temperatures. However, -253 ° C., which is the object of the present invention in terms of low temperature toughness, is significantly lower than the conventional evaluation temperatures of -165 ° C. and -196 ° C. for 9% Ni steel. Therefore, the present inventors have conducted a number of studies to clarify the influence of the component content and the metallographic structure on the toughness of 14% Ni steel at -253 ° C. As a result of the study by the present inventors, it was found that the toughness at -253 ° C. may be insufficient even if the amount of Ni is simply increased by 4 to 6% with respect to 9% Ni steel.

Furthermore, the present inventors have investigated a method for increasing the toughness of 14% Ni steel at -253 ° C. As a result, in particular, (a) the content of C should be 0.020% or more and 0.070% or less, and (b) the content of Si should be 0.03% or more and 0.30% or less. , (C) The Mn content must be 0.20% or more and 0.80% or less, and (d) the particle size and morphology of the old austenite must be controlled at the same time. I understand. Furthermore, it was also found that the low temperature toughness at -253 ° C. is further improved by (e) controlling the volume fraction of the austenite phase and (f) controlling the effective crystal grain size.

Hereinafter, the present invention will be described. First, the composition of Ni steel will be described. In addition,% of content means mass% unless otherwise specified.

(C: 0.020% or more, 0.070% or less)
C is an element that increases the yield stress at room temperature and also contributes to the formation of martensite and austenite. If the C content is less than 0.020%, the strength cannot be ensured, and the cryogenic toughness may decrease due to the formation of coarse bainite or the like. Therefore, the lower limit is 0.020% or more. The lower limit of the preferable C content is 0.025% or more. On the other hand, when the C content exceeds 0.070%, cementite is likely to be precipitated at the old austenite grain boundaries, fracture occurs at the grain boundaries at -253 ° C., and the cryogenic toughness is lowered. The upper limit is 0.070% or less. The upper limit of the preferable C content is 0.060% or less, more preferably 0.050% or less, and further preferably 0.045% or less.

(Si: 0.03% or more, 0.30% or less)
Si is an element that increases the yield stress at room temperature. If the Si content is less than 0.03%, the effect of improving the yield stress at room temperature is small, so the lower limit is 0.03% or more. The lower limit of the preferable Si content is 0.05% or more. On the other hand, when the Si content exceeds 0.30%, the cementite at the former austenite grain boundary tends to be coarsened, fracture occurs at the grain boundary at -253 ° C., and the cryogenic toughness is lowered. Therefore, limiting the upper limit of the Si content to 0.30% or less is extremely important for ensuring toughness at -253 ° C. The upper limit of the preferable Si content is 0.20% or less, more preferably 0.15% or less, and further preferably 0.10% or less.

(Mn: 0.20% or more, 0.80% or less)
Mn is an element that increases the yield stress at room temperature. If the Mn content is less than 0.20%, the strength cannot be ensured, and the cryogenic toughness may decrease due to the formation of coarse bainite or the like. Therefore, the lower limit of the Mn content is set to 0.20% or more. The lower limit of the preferable Mn content is 0.30% or more. On the other hand, when the Mn content exceeds 0.80%, Mn segregated at the old austenite grain boundaries and MnS coarsely precipitated cause fracture at the grain boundaries at -253 ° C., resulting in a decrease in cryogenic toughness. Therefore, limiting the upper limit of the Mn content to 0.80% or less is also extremely important for ensuring toughness at -253 ° C. The upper limit of the preferable Mn content is 0.60% or less, more preferably 0.50% or less.

(Ni: 12.5% or more, 15.4% or less)
Ni is an essential element for ensuring cryogenic toughness. If the Ni content is less than 12.5%, sufficient toughness at -253 ° C. cannot be sufficiently ensured, so the lower limit of the Ni content is set to 12.5% or more. The lower limit of the preferable Ni content is 12.8% or more. However, Ni is an expensive element, and if it is contained in excess of 15.4%, economic efficiency is impaired. Therefore, the upper limit of Ni content is limited to 15.4% or less.

(Al: 0.010% or more, 0.060% or less)
Al is an element mainly used for deoxidation, and also contributes to the miniaturization of metal structure and the reduction of solid solution N, which lowers the cryogenic toughness, by forming AlN. If the Al content is less than 0.010%, the deoxidizing effect, the finening effect of the metal structure, and the effect of reducing solid solution N are small, so the lower limit of the Al content is set to 0.010% or more. The lower limit of the Al content is preferably 0.015% or more, more preferably 0.020% or more. However, if the Al content exceeds 0.060%, the toughness at -253 ° C. decreases, so the upper limit of the Al content is set to 0.060% or less. The upper limit of the more preferable Al content is 0.040% or less.

(N: 0.0015% or more, 0.0060% or less)
N contributes to the formation of nitrides, and when the N content is reduced to less than 0.0015%, fine AlN that suppresses the coarsening of the austenite particle size during heat treatment is insufficient, and the austenite grains are coarsened to a cryogenic temperature. Toughness may decrease. Therefore, the lower limit of the N content is 0.0015% or more, preferably 0.0020% or more. On the other hand, when the N content exceeds 0.0060%, the solid solution N increases and the AlN becomes coarse, so that the toughness at -253 ° C decreases. Therefore, the upper limit of the N content is 0.0060% or less, preferably 0.0050% or less, and more preferably 0.0040% or less.

(O: 0.0007% or more, 0.0030% or less)
O is an impurity, and it is desirable that the O content is small, but reducing the O content to less than 0.0007% may accompany an increase in cost, so 0.0007% or more is set as the lower limit. On the other hand, if the O content exceeds 0.0030%, the clusters of Al ₂ O ₃ may increase and the toughness at -253 ° C may decrease. Therefore, the upper limit of the O content is set to 0.0030% or less. .. The upper limit of the preferable O content is 0.0025% or less, more preferably 0.0020% or less, still more preferably 0.0015% or less.

(P: 0.008% or less)
P is an element that causes grain boundary embrittlement at the former austenite grain boundaries and is harmful to cryogenic toughness. Therefore, it is desirable that the P content is low. If the P content exceeds 0.008%, the toughness at -253 ° C may decrease. Therefore, the upper limit of the P content is limited to 0.008% or less. The upper limit of the P content is preferably 0.006% or less, more preferably 0.004% or less, still more preferably 0.003% or less. P may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof does not need to be particularly limited, and the lower limit is 0% or more.

(S: 0.0040% or less)
S may be the starting point of brittle fracture as MnS, and is an element harmful to cryogenic toughness. If the S content exceeds 0.0040%, the toughness at -253 ° C. may decrease, so the upper limit of the S content is limited to 0.0040% or less. The upper limit of the S content is preferably 0.0030% or less, more preferably 0.0020% or less, still more preferably 0.0010% or less. S may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof does not need to be particularly limited, and the lower limit thereof is 0% or more.

In the present invention, one or more of Cu, Cr, Mo, Nb, V, Ti, B, Ca and REM described below may or may not be selectively contained.

(Cu: 1.00% or less)
Since Cu is an element that increases the yield stress at room temperature, it may be contained. However, if the Cu content exceeds 1.00%, the toughness at -253 ° C. decreases, so the upper limit of the Cu content is set to 1.00% or less. The upper limit of the Cu content is preferably 0.70% or less, more preferably 0.50% or less, still more preferably 0.30% or less. Cu may be mixed as an impurity from scrap or the like during the production of molten steel, but it is not necessary to particularly limit the lower limit of the Cu content, and the lower limit is 0% or more.

(Cr: 1.00% or less)
Since Cr is an element that increases the yield stress at room temperature, it may be contained. However, if the Cr content exceeds 1.00%, the toughness at -253 ° C. decreases, so the upper limit of the Cr content is set to 1.00% or less. The upper limit of the Cr content is preferably 0.70% or less, more preferably 0.50% or less, still more preferably 0.30% or less. Cr may be mixed as an impurity from scrap or the like during the production of molten steel, but it is not necessary to particularly limit the lower limit of the Cr content, and the lower limit is 0% or more.

(Mo: 0.60% or less)
Mo is an element that increases the yield stress at room temperature and may be contained because it also has an effect of suppressing grain boundary embrittlement. However, Mo is an expensive element, and if the Mo content exceeds 0.60%, economic efficiency is impaired. Therefore, the upper limit of the Mo content is limited to 0.60% or less. Mo may be mixed as an impurity from scrap or the like during the production of molten steel, but it is not necessary to particularly limit the lower limit of the Mo content, and the lower limit is 0% or more.

(Nb: 0.020% or less)
Nb is an element that increases the yield stress at room temperature, and may be contained because it also has an effect of improving cryogenic toughness by miniaturizing the metal structure. However, if the Nb content exceeds 0.020%, the toughness at -253 ° C. decreases, so the upper limit is set to 0.020% or less. The upper limit of the Nb content is preferably 0.015% or less, more preferably 0.010% or less. Nb may be mixed as an impurity from scrap or the like during the production of molten steel, but it is not necessary to particularly limit the lower limit of the Nb content, and the lower limit is 0% or more.

(V: 0.080% or less)
Since V is an element that increases the yield stress at room temperature, it may be contained, but if the V content exceeds 0.080%, the toughness at -253 ° C decreases, so the upper limit of the V content is set. It shall be 0.080% or less. The upper limit of the V content is preferably 0.060% or less, more preferably 0.040% or less. V may be mixed as an impurity from scrap or the like during the production of molten steel, but it is not necessary to particularly limit the lower limit of the V content, and the lower limit is 0% or more.

(Ti: 0.020% or less)
Ti may be contained because it forms TiN and contributes to the miniaturization of the metal structure and the reduction of the solid solution N that lowers the cryogenic toughness. However, if the Ti content exceeds 0.020%, the toughness at -253 ° C. decreases, so the upper limit of the Ti content is set to 0.020% or less. The upper limit of the preferable Ti content is 0.015% or less, more preferably 0.010% or less. Ti may be mixed as an impurity from scrap or the like during the production of molten steel, but it is not necessary to particularly limit the lower limit of the Ti content, and the lower limit is 0% or more.

(B: 0.0020% or less)
B is an element that increases the yield stress at room temperature, and may be contained because it also contributes to the reduction of solid solution N that forms BN and lowers the cryogenic toughness. However, if the B content exceeds 0.0020%, the toughness at -253 ° C. decreases, so the upper limit of the B content is set to 0.0020% or less. The upper limit of the B content is preferably 0.0015% or less, more preferably 0.0012% or less, and further preferably 0.0010% or less. B may be mixed as an impurity from scrap or the like during the production of molten steel, but it is not necessary to particularly limit the lower limit of the B content, and the lower limit is 0% or more.

(Ca: 0.0040% or less)
Ca may be contained because it is effective in improving the cryogenic toughness by spheroidizing MnS as CaS, which is easily stretched by hot rolling to increase the harmfulness to the cryogenic toughness. However, when the Ca content exceeds 0.0040%, the acid sulfide containing Ca becomes coarse and the toughness at -253 ° C. decreases. Therefore, the upper limit of the Ca content is limited to 0.0040% or less, preferably 0.0030% or less. Ca may be mixed as an impurity from scrap or the like during the production of molten steel, but it is not necessary to particularly limit the lower limit of the Ca content, and the lower limit is 0% or more.

(REM: 0.0050% or less)
Similar to Ca, REM (rare-earth metal) improves ultra-low temperature toughness by spheroidizing MnS, which is easily stretched by hot rolling and is likely to be harmful to ultra-low temperature toughness, as an acid sulfide of REM. It may be contained because it is effective for rolling. However, when the REM content exceeds 0.0050%, the acid sulfide containing REM becomes coarse and the toughness at -253 ° C. decreases. Therefore, the upper limit of the REM content is limited to 0.0050% or less, preferably 0.0040% or less. REM may be mixed as an impurity from scrap or the like during the production of molten steel, but it is not necessary to particularly limit the lower limit of the REM content, and the lower limit is 0% or more.

The cryogenic steel according to the present embodiment contains or limits the above components, and the balance contains iron and impurities. Here, the impurity is a component mixed by various factors in the manufacturing process, including raw materials such as ore and scrap, when steel is industrially manufactured, and does not adversely affect the present invention. Means what is acceptable in the range. However, in the present invention, among impurities, it is necessary to specify the upper limit for P and S as described above.

Further, the cryogenic steel according to the present embodiment may contain the following alloying elements as impurities from auxiliary raw materials such as scrap in addition to the above components, but the strength of the steel material itself and the cryogenic toughness It is preferable to limit the content for the purpose of further improving the above.

Since Sb impairs cryogenic toughness, the upper limit of the Sb content is preferably 0.005% or less, more preferably 0.003% or less, and most preferably 0.001% or less. preferable.

Since Sn impairs cryogenic toughness, the upper limit of Sn content is preferably 0.005% or less, more preferably 0.003% or less, and most preferably 0.001% or less. preferable.

Since As impairs cryogenic toughness, the upper limit of the As content is preferably 0.005% or less, more preferably 0.003% or less, and most preferably 0.001% or less. preferable.

Further, in order to fully exert the above effects of the above components, it is preferable to limit the upper limit of the Co, Zn and W contents to 0.01% or less or 0.005% or less, respectively.

It is not necessary to limit the lower limit of Sb, Sn, As, Co, Zn and W, and the lower limit of each element is 0% or more. Further, even if alloying elements having no lower limit (for example, P, S, Cu, Cr, Mo, Nb, V, Ti, B, Ca, and REM) are intentionally added or mixed as impurities. Even so, if the content is within the above range, the cryogenic steel (steel material) is interpreted as being within the range of the present invention.

Next, the metallographic structure of the Ni steel of the present invention will be described.
The present inventors have newly found that at an extremely low temperature of -253 ° C, fracture occurs at the former austenite grain boundaries and the toughness tends to decrease. The Ni steel of the present invention is produced by hot rolling, water-cooling or air-cooling, and then undergoing heat treatments such as reheat quenching, intermediate heat treatment, and tempering. The former austenite grain boundaries described here are reheat quenching. It is the grain boundary of austenite that existed at the time of heating. Mn, P, and Si are segregated at the former austenite grain boundaries, and it is considered that these elements reduce the binding force of the former austenite grain boundaries and cause fracture at the former austenite grain boundaries at -253 ° C. ..

Although old austenite grain boundaries are newly generated during the intermediate heat treatment, the temperature of the intermediate heat treatment of the present invention is as low as 560 ° C. or higher and 670 ° C. or lower, and Mn, P, Si diffused into the new old austenite grain boundaries. Since the amount of these new austenite grains is relatively small, it is considered that destruction from these new austenite grains is relatively unlikely. For this reason, the particle size and aspect ratio of the old austenite grains after reheating and quenching are practically important, and when measuring the grain size and aspect ratio of the old austenite grains after intermediate heat treatment or tempering, reheating and quenching is performed. It is necessary to measure the later austenite grains. When determining whether the old austenite grains are the grain boundaries after reheating and quenching or the grain boundaries after the intermediate heat treatment, it is judged that the grain size of 2 μm or less is the grain boundaries after the intermediate heat treatment. That is, the particle size and aspect ratio of the former austenite grains are measured by excluding the former austenite grains having a particle size of 2 μm or less.

The present inventors have conducted a number of studies on means for suppressing fracture at the prior austenite grain boundaries at a cryogenic temperature of -253 ° C. As a result, (a) C content should be 0.070% or less, (b) Mn content should be 0.80% or less, and (c) P content should be 0.008% or less. , (D) Si content should be 0.30% or less, (e) old austenite grain size should be 20.0 μm or less, and (f) aspect ratio of old austenite grains should be suppressed to 2.9 or less. It was found that when the six conditions were satisfied at the same time, the fracture at the old austenite grain boundaries was suppressed and the toughness at -253 ° C could be ensured.

The mechanism by which fracture from the old austenite grain boundaries is likely to occur at the cryogenic temperature of -253 ° C is not clear, but at the cryogenic temperature of -253 ° C, the effect of the stress concentration relaxation mechanism due to the movement of dislocations in the tempered martensite structure is effective. It is limited. In addition, as a result of suppressing the miniaturization of cementite in tempered martensite and the generation of fracture origins such as hard high-C martensite and coarse inclusions, the former austenite grain boundaries are more likely to be the fracture origins. there is a possibility.

As described above, it is presumed that at an extremely low temperature of -253 ° C, fracture is likely to occur selectively in a portion having a relatively weak binding force such as the former austenite grain boundary. Therefore, it is considered that the decrease in the binding force of the former austenite grain boundary can be suppressed by suppressing the segregation of cementite and Mn and P that weaken the binding force of the former austenite grain boundary. Further, the increase in the C content and the Si content and the coarsening of the former austenite grains promote the coarsening of the grain boundary cementite. Therefore, suppression of C content and Si content and fine graining of the old austenite grain size are required to suppress fracture at the old austenite grain boundaries at -253 ° C.

(Old austenite particle size: 3.0 μm or more, 20.0 μm or less)
The particle size of the old austenite needs to be 3.0 μm or more and 20.0 μm or less. Since reducing the particle size of the old austenite to less than 3.0 μm involves an increase in manufacturing cost such as increasing the number of heat treatments, the lower limit of the particle size of the old austenite is set to 3.0 μm or more. On the other hand, when the particle size of the old austenite exceeds 20.0 μm, the cementite precipitated at the grain boundaries of the old austenite becomes coarse, and the concentrations of the grain boundaries of Mn and P increase. Coarse cementite precipitation and Mn and P concentration weaken the binding force of the former austenite grain boundaries and lead to fracture at the former austenite grain boundaries, or become the starting point for brittle fracture, and the pole at -253 ° C. Since the low temperature toughness is lowered, the upper limit of the old austenite grain size is set to 20.0 μm or less.

(Aspect ratio of old austenite grains: 1.0 or more and 2.9 or less)
The aspect ratio of the former austenite grains is the ratio of the length and thickness of the former austenite grains on the plane parallel to the rolling direction (L plane), that is, the length in the rolling direction of the former austenite grains: the plate thickness direction of the former austenite grains. The thickness of. Therefore, the case where the length and the thickness of the old austenite grains are the same is the lower limit of the aspect ratio, and the aspect ratio is 1.0 or more. Further, in the present invention, the aspect ratio of the old austenite grains needs to be 2.9 or less in order to secure the cryogenic toughness at -253 ° C.

When the aspect ratio exceeds 2.9, even if the grain size of the old austenite is 20.0 μm or less, there is a high probability that a coarse austenite grain boundary exceeding 30 μm is included in the rolling direction. Destruction at grain boundaries is likely to occur. In addition, the aspect ratio increases when the old austenite is flattened by rolling in the unrecrystallized region. When the aspect ratio exceeds 2.9, the introduction of dislocations by rolling in the unrecrystallized region becomes excessive, and as a result, it remains without disappearing even after subsequent reheating quenching, intermediate heat treatment, and tempering. Since the amount of dislocations present is excessive, the cryogenic toughness is reduced. Therefore, the upper limit of the aspect ratio of the old austenite grains is set to 2.9 or less.

The particle size and aspect ratio of the old austenite are preferably measured by collecting a sample from the steel after reheating and quenching, but may be measured by collecting a sample from the steel after tempering. The particle size and aspect ratio of the old austenite are measured by using the plane (L plane) parallel to the rolling direction at the center of the plate thickness as the observation plane. As for the particle size of the old austenite, after the observation surface is corroded with a saturated aqueous solution of picric acid to reveal the old austenite grain boundaries, a scanning electron microscope (SEM) is used to take a picture of 5 or more fields at 1000 times or 2000 times. And measure. After identifying the former austenite grain boundaries using SEM photographs, the circle-equivalent particle size (diameter) of at least 20 former austenite grains is determined by image processing, and the average value thereof is taken as the particle size of the former austenite. As for the aspect ratio of the former austenite, the ratio (aspect ratio) of at least 20 former austenite grains to the length in the rolling direction and the thickness in the thickness direction is measured by using an SEM photograph in the same manner as the measurement of the particle size. , Let the average value of these be the aspect ratio of the old austenite. When a sample is taken from tempered steel and the particle size and aspect ratio of the former austenite are measured, as described above, the particle size and aspect ratio of the former austenite grains are excluded by excluding the former austenite grains having a particle size of 2 μm or less. Measure the ratio.

(Volume fraction of austenite phase: 2.0% or more and 20.0% or less)
Further, in order to further enhance the toughness at -253 ° C., it is preferable to contain the austenite phase at a volume fraction of 2.0% or more. Therefore, it is preferable that the lower limit of the volume fraction of the austenite phase is 2.0% or more. However, unlike the old austenite, this austenite phase is an austenite phase present in Ni steel after heat treatment, and the volume fraction is measured by an X-ray diffraction method. In the presence of an austenite phase that is stable even at -253 ° C, it is considered that the toughness is improved because the applied stress and strain are relaxed by the plastic deformation of the austenite. In addition, the austenite phase is formed relatively uniformly and finely at the former austenite grain boundaries, the block boundary of tempered martensite, and the lath boundary.

That is, it is considered that the austenite phase exists in the vicinity of the hard phase, which is likely to be the starting point of the occurrence of brittle fracture, relaxes the concentration of stress and strain around the hard phase, and contributes to the suppression of the occurrence of brittle fracture. Be done. Furthermore, as a result of generating an austenite phase having a volume fraction of 2.0% or more, it is considered that coarse cementite, which is the starting point of brittle fracture, can be significantly reduced. On the other hand, when the volume fraction of the austenite phase increases, the concentration of C and the like in the austenite phase becomes insufficient, and the possibility of transformation to martensite increases at -253 ° C. Unstable austenite, which transforms into martensite at cryogenic temperatures, may reduce cryogenic toughness at -253 ° C. Therefore, the upper limit of the volume fraction of the austenite phase is preferably 20.0% or less. The volume fraction of the austenite phase may be measured by X-ray diffraction after taking a sample from the tempered steel.

(Effective crystal grain size: 2.0 μm or more and 12.0 μm or less)
The effective crystal grain size is preferably 2.0 μm or more and 12.0 μm or less. The effective crystal grain size is a region in which the crystal orientations are substantially the same, and when miniaturized, the resistance to propagation of fracture cracks increases and the toughness is further improved. However, reducing the effective crystal grain size to less than 2.0 μm involves an increase in manufacturing costs such as increasing the number of heat treatments. Therefore, when considering the effective crystal grain size, the lower limit of the effective crystal grain size is required. Is 2.0 μm or more. Further, when the effective crystal grain size exceeds 12.0 μm, the hard phase that is the starting point of brittle fracture, that is, coarse cementite in the former austenite grain boundaries and tempered martensite, coarse AlN, MnS, alumina, etc. The stress acting on the inclusions may increase and the cryogenic toughness at -253 ° C may decrease. Therefore, when considering the effective crystal grain size, the upper limit of the effective crystal grain size is set to 12.0 μm or less.

The effective crystal particle size is the backscattered electron beam diffraction pattern attached to the scanning electron microscope, with a sample taken from the tempered steel and the plane (L plane) parallel to the rolling direction at the center of the plate thickness as the observation plane. Measurement is performed using a method (Electron Backscatter Diffraction) analyzer. Observations of 5 visual fields or more are performed at a magnification of 2000 times, the boundary of the metal structure having an orientation difference of 15 ° or more is regarded as a grain boundary, and the crystal grains surrounded by these grain boundaries are regarded as effective crystal grains, and a circle is formed from their area. The equivalent grain size (diameter) is obtained by image processing, and the average value of those circle-equivalent grain sizes is taken as the effective crystal grain size.

As an example, the characteristics of the cryogenic steel in the present invention are preferably 12 mm or more and 40 mm or less, the yield stress at room temperature is 460 MPa or more and 580 MPa or less, and the tensile strength is 560 MPa or more and 680 MPa or less.

Next, a method for producing cryogenic steel according to this embodiment will be described.
The method for melting steel is, for example, after adjusting the element content in a state where the molten steel temperature is 1650 ° C. or less, the molten steel O concentration is 0.01% or less, and the molten steel S concentration is 0.02% or less. , Manufacture steel pieces by continuous casting. The obtained steel pieces are heated, hot-rolled, water-cooled or air-cooled, and then reheat-quenched, intermediate heat-treated, and tempered in sequence.

Hereinafter, preferable manufacturing conditions will be described. The conditions shown below are examples of preferable conditions. As long as a steel material within the scope of the present invention can be obtained, there is no particular problem even if the conditions described below are not met.

The heating temperature for hot rolling is preferably 950 ° C. or higher and 1180 ° C. or lower. If the heating temperature is lower than 950 ° C., AlN may become coarse and the cryogenic toughness may decrease. If the heating temperature exceeds 1180 ° C., the austenite particle size becomes coarse during heating, and the cryogenic toughness at -253 ° C. may decrease. A more preferable upper limit of the heating temperature is 1150 ° C. or lower, and even more preferably 1100 ° C. or lower. The heating holding time is preferably 30 minutes to 180 minutes.

In hot rolling, when the rolling reduction at 950 ° C or lower is less than 50%, the austenite grains are not sufficiently finely divided by recrystallization of austenite during rolling, and coarse grains are partly formed in the austenite grains after rolling. May reduce cryogenic toughness. Therefore, the lower limit of the reduction rate at 950 ° C. or lower is preferably 50% or more. The lower limit of the more preferable reduction rate is 60% or more, and more preferably 70% or more. On the other hand, if the reduction rate at 950 ° C or lower exceeds 95%, the rolling time becomes long and productivity may be problematic. Therefore, the upper limit of the reduction rate at 950 ° C or lower is preferably 95% or less. .. Homogeneous granulation of old austenite grains by recrystallization of rolling is particularly important for ensuring the cryogenic toughness of the present invention, and strict regulation of rolling temperature and rolling reduction is preferable.

When the end temperature of hot rolling is lower than 650 ° C., the deformation resistance increases and the load on the rolling mill increases. Therefore, the lower limit of the end temperature of hot rolling is preferably 650 ° C. or higher, and the lower limit of the end temperature of hot rolling is more preferably 700 ° C. or higher, still more preferably 740 ° C. or higher. On the other hand, when the end temperature of hot rolling exceeds 920 ° C., the dislocations introduced by rolling decrease due to recovery, the effective crystal grain size exceeds 12.0 μm, the toughness at extremely low temperature becomes insufficient, or the room temperature Since the yield stress of the hot rolling may be insufficient, the upper limit of the end temperature of hot rolling is preferably 920 ° C. or lower. The upper limit of the more preferable hot rolling end temperature is 880 ° C. or lower.

The cooling after hot rolling may be either water cooling or air cooling, but water cooling to around room temperature is preferable. The water cooling start temperature when water cooling to around room temperature is preferably 550 ° C. or higher and 920 ° C. or lower. When cooling as it is after hot rolling, the water cooling start temperature is equal to or lower than the hot rolling end temperature. When the water cooling start temperature is set to 550 ° C. or higher, the formation of coarse bainite is suppressed, and the effective crystal particle size can be made finer. The lower limit of the more preferable water cooling start temperature is 600 ° C. or higher. The upper limit of the water cooling start temperature does not need to be particularly regulated, and water cooling may be started immediately after the completion of hot rolling. Therefore, 920 ° C. or lower, which is a preferable upper limit of the end temperature of hot rolling, is a preferable upper limit of the water cooling start temperature.

Reheating quenching is effective for refining old austenite, and the heating temperature is preferably 700 ° C. or higher and 880 ° C. or lower. When the heating temperature of reheating quenching (reheating quenching temperature) falls below 700 ° C, a part that does not transform into austenite remains, and the untransformed austenite part does not have a martensite structure even after quenching, and the strength of the base metal decreases. Or, the effective crystal grain size becomes coarse, so that the austenite toughness may decrease. The lower limit of the more preferable reheating quenching temperature is 720 ° C. or higher. On the other hand, when the reheating quenching temperature exceeds 880 ° C., the austenite particle size becomes coarse and AlN becomes coarse, so that the cryogenic toughness may decrease. A more preferable upper limit of the reheating quenching temperature is 860 ° C. or lower, and even more preferably 840 ° C. or lower. The retention time during reheating and quenching is preferably 20 to 180 minutes. For cooling at the time of reheating and quenching, a method such as water cooling to 200 ° C. or lower can be adopted.

The intermediate heat treatment is effective in reducing the effective crystal grain size and securing the austenite phase, which contributes to the improvement of cryogenic toughness, and the heating temperature is preferably 560 ° C. or higher and 670 ° C. or lower. When the heating temperature of the intermediate heat treatment (intermediate heat treatment temperature) is lower than 560 ° C., the austenite transformation is insufficient, the volume fraction of the tempered martensite that has been excessively tempered increases, and the strength of the base metal decreases. There is. Further, when the intermediate heat treatment temperature is lower than 560 ° C., the effective crystal grain size may exceed 12.0 μm and the cryogenic toughness may decrease. The lower limit of the more preferable intermediate heat treatment temperature is 610 ° C. or higher. On the other hand, when the temperature of the intermediate heat treatment exceeds 670 ° C., the austenite transformation proceeds excessively. As a result, the stabilization of austenite becomes insufficient, it becomes impossible to secure an austenite phase of 2.0% or more in volume fraction, or the effective crystal grain size cannot be controlled to 12.0 μm or less, and the cryogenic temperature is extremely low. Toughness may decrease. The upper limit of the more preferable intermediate heat treatment temperature is 650 ° C. or lower. The holding time of the intermediate heat treatment is preferably 20 minutes to 180 minutes. As a cooling method during the intermediate heat treatment, it is desirable to perform water cooling in order to avoid tempering and embrittlement.

Tempering is also effective in securing the austenite phase, and the heating temperature is preferably 480 ° C. or higher and 570 ° C. or lower. If the tempering heating temperature (tempering temperature) is lower than 480 ° C., the austenite phase cannot be secured at a volume fraction of 2.0% or more, and the cryogenic toughness may be insufficient. The lower limit of the more preferable tempering temperature is 520 ° C. or higher. On the other hand, if the upper limit of the tempering temperature exceeds 570 ° C, the austenite phase at room temperature exceeds 20.0% in volume fraction, and when this is cooled to -253 ° C, some austenite becomes high C martensite. May transform into austenite and reduce cryogenic toughness. Therefore, the upper limit of the tempering temperature is preferably 570 ° C. or lower, more preferably 560 ° C. or lower. The tempering retention time is preferably 20 to 180 minutes. As a cooling method at the time of tempering, it is desirable to perform water cooling in order to avoid tempering embrittlement.

As described above, according to the present invention, it is possible to obtain a Ni steel for liquid hydrogen having sufficient cryogenic toughness for use in a liquid hydrogen tank and having a high yield stress at room temperature.
If the Ni steel for liquid hydrogen of the present invention is used in a liquid hydrogen tank, the thickness of the steel plate for the tank can be reduced as compared with the austenitic stainless steel. Therefore, according to the present invention, it is possible to increase the size and weight of the liquid hydrogen tank, improve the heat insulating performance by reducing the surface area relative to the volume, effectively use the site of the tank, and improve the fuel efficiency of the liquid hydrogen carrier. Further, since the Ni steel for liquid hydrogen of the present invention has a smaller coefficient of thermal expansion than the austenitic stainless steel, the design of a large tank is not complicated and the tank manufacturing cost can be reduced. As described above, the present invention has an extremely remarkable industrial contribution.

Examples of the present invention are shown below, but the examples shown below are examples of the present invention, and the present invention is not limited to the examples described below.

Steel was melted by a converter, and slabs with a thickness of 150 mm to 400 mm were produced by continuous casting. Tables 1 and 2 show the chemical components of the steel materials A1 to A24. These slabs were heated, controlled-rolled, water-cooled or air-cooled as they were, and subjected to reheat quenching, intermediate heat treatment, and tempering heat treatment to produce a steel sheet. The holding time for heating in hot rolling was 30 to 120 minutes, and the holding time for reheating quenching, intermediate heat treatment, and tempering heat treatment was 20 to 60 minutes. Samples were taken from the steel sheet and evaluated for metallographic structure, tensile properties and toughness.

The particle size of the old austenite was measured with the plane (L plane) parallel to the rolling direction at the center of the plate thickness as the observation plane. The particle size of the former austenite was determined in accordance with JIS G 0551. First, the observation surface of the sample was corroded with a saturated aqueous solution of picric acid to reveal the old austenite grain boundaries, and then photographs of 5 or more fields of view were taken with a scanning electron microscope at 1000 or 2000 times. After identifying the prior austenite grain boundaries using microstructure photographs, the circle-equivalent particle size (diameter) of at least 20 former austenite grains was determined by image processing, and the average value thereof was taken as the particle size of the former austenite.

Further, in the steel of the present invention, the former austenite grain boundaries are difficult to identify due to corrosion because the former austenite grain boundaries are made finer and the P content is suppressed so that the grain boundaries of the former austenite are less likely to be destroyed. There is. In such a case, after heating to 430 ° C. to 460 ° C., performing a heat treatment for holding for 1 hour or more, the particle size of the old austenite was measured by the above method.

If it is difficult to identify the old austenite grain boundaries even after heat treatment at 430 ° C to 460 ° C, a Charpy test piece is taken from the heat-treated sample and an impact test is performed at -196 ° C to perform an impact test at the old austenite grain boundaries. The sample destroyed in was used. In this case, a cross section of the fracture surface is prepared on a plane parallel to the rolling direction (L plane), and after corrosion, the former austenite grain boundaries of the fracture surface cross section at the center of the plate thickness are identified with a scanning electron microscope, and the former austenite is identified. The particle size was measured. When the former austenite grain boundaries are embrittled by heat treatment, minute cracks are generated in the former austenite grain boundaries due to the impact load during the Charpy test, so that the former austenite grain boundaries can be easily identified.

The aspect ratio of the austenite grains was determined as the ratio between the maximum value (length in the rolling direction) and the minimum value (thickness in the thickness direction) of the length of the austenite grain boundaries identified as described above. The aspect ratios of at least 20 former austenite grains were measured, and the average value thereof was taken as the aspect ratio of the former austenite grains. The particle size and aspect ratio of the former austenite were measured excluding the former austenite grains having a particle size of 2 μm or less.

The volume fraction of the austenite phase was measured by X-ray diffraction method by taking a sample parallel to the plate surface at the center of the plate thickness. The volume fraction of the austenite phase was determined from the ratio of the integrated intensities of the X-ray peak austenite (face-centered cubic structure) and tempered martensite (body-centered cubic structure).

The effective crystal grain size was determined by using the EBSD analyzer attached to the scanning electron microscope with the plane (L plane) parallel to the rolling direction at the center of the plate thickness as the observation plane. Observations of 5 visual fields or more are performed at a magnification of 2000 times, the boundary of the metal structure having an orientation difference of 15 ° or more is regarded as a grain boundary, and the crystal grains surrounded by these grain boundaries are regarded as effective crystal grains, and a circle is formed from their area. The equivalent grain size (diameter) was determined by image processing, and the average value of those circle-equivalent grain sizes was taken as the effective crystal grain size.

For strength (yield stress and tensile strength), take a No. 1A full-thickness tensile test piece specified in JIS Z 2241 with the direction parallel to the rolling direction (L direction) as the longitudinal direction, and use the method specified in JIS Z 2241. Was evaluated at room temperature. The target value of the yield stress is 460 MPa or more and 580 MPa or less, and the target value of the tensile strength is 560 MPa or more and 680 MPa or less. The yield stress was set to the downward yield stress, but in many cases no clear yield stress was observed. In that case, the yield stress was set to 0.2% proof stress.

Cryogenic toughness is a CT test piece with a total thickness obtained by grinding the front and back surfaces by 0.5 mm each when the steel plate thickness is 31 mm or less, and when the steel plate thickness exceeds 31 mm, the thickness is 30 mm from the center of the plate thickness. CT test piece is sampled in the direction perpendicular to the rolling direction (C direction), and a JR curve is created in liquid hydrogen (-253 ° C) according to the unloading compliance method specified in ASTM standard E1820-13. and, by converting the J value in the _{K IC} value. The target value of cryogenic toughness is 150 MPa · √ m or more.

Tables 3 and 4 show the plate thickness, manufacturing method, base material properties, and metallographic structure of the steel materials (No. 1-32) manufactured using the slabs having the chemical components of the steel materials A1 to A24 of Tables 1 and 2. Shown. In Table 3, No. 2, 4, 6 and No. Since the steel material of No. 14 was cooled after hot rolling by air cooling, the water cooling start temperature after hot rolling is described as "-".

As is clear from Tables 3 and 4, No. The yield stress at room temperature, the tensile strength at room temperature, and the cryogenic toughness at -253 ° C of the steel materials 1 to 16 satisfy the target values.
No. in Table 3 The steel material of No. 4 has a large amount of austenite phase because the tempering temperature is higher than the preferable range, and the reheat quenching temperature is higher than the preferable range, so that the old austenite particle size is large and the effective crystal particle size is also large. As a result, the cryogenic toughness is slightly low.
No. In the steel material No. 7, the end temperature of hot rolling is higher than the preferable range, the effective crystal grain size is large, and the cryogenic toughness is slightly low.
No. Each of the steel materials 10 and 11 has an intermediate heat treatment temperature outside the preferable range, a small amount of austenite phase, and a slightly low cryogenic toughness.
No. Since the tempering temperature of the steel material 12 is higher than the preferable range, the austenite phase is increased and the cryogenic toughness is slightly low.

On the other hand, No. in Table 4 The steel material of No. 17 has a low C content, and No. Since the steel material of No. 20 has a low Mn content, its strength is low and its cryogenic toughness is also low. No. Each of the steel materials 18, 19, 21 to 25 has a large C content, Si content, Mn content, P content, S content, Cr content, and Al content, respectively, and the cryogenic toughness is lowered. ing.
No. The steel material of No. 26 has a large Nb content and B content, the aspect ratio of the former austenite grains is large, and the cryogenic toughness is lowered.
No. The steel material of No. 27 has a large Ti content and N content, and the cryogenic toughness is lowered.

No. The steel materials 28 to 32 are examples in which the old austenite particle size or aspect ratio deviates from the range of the present invention as a result of adopting manufacturing conditions deviating from the preferable range, and the extremely low temperature toughness is lowered.
No. The steel material of No. 28 has a low heating temperature during hot rolling, a large aspect ratio of old austenite grains, and a low cryogenic toughness.
No. The steel material of No. 29 has a high heating temperature during hot rolling, a large austenite grain size, a large effective crystal grain size, and a low cryogenic toughness.
No. The steel material of No. 30 has a low reduction rate at 950 ° C. or lower, a large austenite grain size, a large effective crystal grain size, and a low cryogenic toughness.
No. The steel material of No. 31 has a high end temperature of hot rolling, a large old austenite grain size, a large effective crystal grain size, and a low cryogenic toughness.
No. The steel material of No. 32 has a high reheating quenching temperature, a large old austenite grain size, a large effective crystal grain size, and a low cryogenic toughness.

Claims (4)

By mass%
C: 0.020% or more, 0.070% or less,
Si: 0.03% or more, 0.30% or less,
Mn: 0.20% or more, 0.80% or less,
Ni: 12.5% or more, 15.4% or less,
Al: 0.010% or more, 0.060% or less,
N: 0.0015% or more and 0.0060% or less and O: 0.0007% or more and 0.0030% or less.
P: 0.008% or less,
S: 0.0040% or less,
Cu: 1.00% or less,
Cr: 1.00% or less,
Mo: 0.60% or less,
Nb: 0.020% or less,
V: 0.080% or less,
Ti: 0.020% or less,
B: 0.0020% or less,
Ca: 0.0040% or less and REM: 0.0050% or less, limited to
The rest consists of Fe and impurities
The particle size of prior austenite grains is 3.0μm or more, 20.0 .mu.m or less, an aspect ratio of 1.0 or more state, and are 2.9 or less, the _{K IC} values at -253 ° C. is 150 MPa · √m or higher, yield stress at room temperature, characterized in der Rukoto least 460 MPa, Ni steel for liquid hydrogen. The Ni steel for liquid hydrogen according to claim 1, which contains an austenite phase having a volume fraction of 2.0% or more and 20.0% or less. The Ni steel for liquid hydrogen according to claim 1 or 2, wherein the effective crystal particle size is 2.0 μm or more and 12.0 μm or less. Plate thickness is 12 mm or more, 40 mm or less,
Yield stress at room temperature is 460 MPa or more and 580 MPa or less,
The Ni steel for liquid hydrogen according to any one of claims 1 to 3, wherein the tensile strength at room temperature is 560 MPa or more and 680 MPa or less.