JP2017197790A - Ni steel for liquid hydrogen - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Ni steel for liquid hydrogen which has toughness sufficient at liquid hydrogen temperature (-253°C), has a yield stress at room temperature of 460 MPa or higher, and contains about 11% of Ni.SOLUTION: A Ni steel for liquid hydrogen that contains C:0.030% or higher and 0.060% or lower, Si:0.03% or higher and 0.19% or lower, Mn:0.30% or higher and 0.50% or lower, and Ni:10.5% or higher and 11.4% or lower by mass%, has a particle size of old austenite particles of 3 μm or larger and 8 μm or smaller, and an aspect ratio of 1.0 or larger and 2.4 or smaller. An austenite phase may be contained by 2% or larger and 6% or smaller by volume ratio, and an effective crystal particle size may be 2 μm or larger and 5 μm or smaller.SELECTED DRAWING: None

Description

  The present invention relates to Ni steel for liquid hydrogen, that is, a steel containing Ni mainly intended for use in the vicinity of −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. An austenitic stainless steel that is required to have excellent cryogenic toughness and is difficult to brittle fracture is used for a steel plate used for a tank for storing and transporting a liquefied gas such as liquid hydrogen. Although austenitic stainless steel has sufficient cryogenic toughness, the yield stress at room temperature of a general-purpose material is about 200 MPa.

  When austenitic stainless steel with insufficient strength is applied to a liquid hydrogen tank, there is a limit to enlargement. Further, if the yield stress of the steel material is about 200 MPa, it is necessary to make the plate thickness more than 30 mm when the tank is enlarged, so that an increase in tank weight and an increase in manufacturing cost become a problem. For such a problem, an austenitic high Mn stainless steel having a thickness of 5 mm and a 0.2% proof stress at room temperature of 450 MPa has been proposed (for example, see Patent Document 1).

  Further, a ferritic 9% Ni steel is used for a tank for liquefied natural gas (LNG), which is a typical liquefied gas (sometimes referred to as an LNG tank). Although LNG is hotter than liquid hydrogen, 9% Ni steel has excellent cryogenic toughness, and various types of 9% Ni steel and 7% Ni steel suitable for LNG tanks have been used in the past. Has been proposed (see, for example, Patent Documents 2 to 4). Moreover, 9% Ni steel can have a yield strength at room temperature of 590 MPa or more, and can be applied to a large LNG tank.

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

  Patent Document 3 contains 5 to 10% Ni, has a yield stress of 590 MPa or more at room temperature, and excellent low temperature toughness at −196 ° C. after strain aging. Steel is disclosed. In Patent Document 3, low volume toughness after strain aging is ensured by setting the volume fraction of retained austenite to 3% or more, the effective crystal grain size to 5.5 μm or less, and introducing appropriate defects into the grain structure. ing.

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

Japanese Patent No. 5709882 JP 2014-210948 A JP 2011-219849 A Japanese Patent Laid-Open No. 3-223442

  The austenitic high Mn stainless steel disclosed in Patent Document 1 has a larger thermal expansion coefficient than that of ferritic 9% Ni steel. For large liquid hydrogen tanks, 9% Ni steel with a small coefficient of thermal expansion is advantageous due to problems such as fatigue. On the other hand, the 9% Ni steel and 7% Ni steel disclosed in Patent Documents 2 to 4 do not have sufficient toughness at the liquid hydrogen temperature of −253 ° C. as studied by the present inventors.

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

  The present inventors conducted a number of studies on the toughness at −253 ° C. and the yield stress at room temperature of a steel in which the Ni content is about 11% higher than the conventional 9% Ni. As a result, to secure cryogenic toughness, it is necessary to limit the Si content, strictly limit the Mn content, and to optimally control the grain size and aspect ratio of the prior austenite. I found it.

  The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) The cryogenic steel according to one embodiment of the present invention is in mass%, C: 0.030% or more and 0.060% or less, Si: 0.03% or more, 0.19% or less, Mn: 0.30% or more, 0.50% or less, Ni: 10.5% or more, 11.4% or less, Al: 0.010% or more, 0.050% or less, N: 0.0015% or more, 0050% or less, O: 0.0007% or more, 0.0030% or less, P: 0.0060% or less, S: 0.0030% or less, Cu: 0.50% or less, Cr: 0.35 %: Mo: 0.40% or less, Nb: 0.015% or less, V: 0.060% or less, Ti: 0.015% or less, B: 0.0020% or less, Ca: 0.0040% or less , REM: limited to 0.0050% or less, the balance is Fe and impurities, the particle size of the prior austenite grains 3μm or more, 8 [mu] m or less, an aspect ratio of 1.0 or more, characterized in that 2.4 or less, Ni steel for liquid hydrogen.
(2) The Ni steel described in (1) above may contain an austenitic phase with a volume fraction of 2% or more and 6% or less.
(3) In the Ni steel described in (1) or (2) above, the effective crystal grain size may be limited to 2 μm or more and 5 μm or less.
(4) In the Ni steel according to any one of (1) to (3), the plate thickness is 12 mm or more and 20 mm or less, the yield stress at room temperature is 460 MPa or more and 580 MPa or less, and the tensile strength at room temperature. May be 560 MPa or more and 680 MPa or less.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide Ni steel for liquid hydrogen which has a cryogenic toughness sufficient as a liquid hydrogen tank use, and has a high yield stress at room temperature. Therefore, if the Ni steel for liquid hydrogen of the present invention is used for a liquid hydrogen tank, the thickness of the tank steel plate can be made thinner than that of 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 insulation performance by reducing the surface area with respect to the volume, effectively use the tank site, and improve the fuel efficiency of the liquid hydrogen carrier. In addition, since the Ni steel for liquid hydrogen of the present invention has a small coefficient of thermal expansion compared to austenitic stainless steel, the design of a large tank is not complicated and the tank manufacturing cost can be reduced. As described above, the industrial contribution of the present invention is extremely remarkable.

  Although steel containing about 11% Ni (sometimes referred to as 11% Ni steel) contains 2% more Ni that improves low-temperature toughness compared to 9% Ni steel, the present invention relates to low-temperature toughness. The target -253 ° C is significantly lower than the conventional evaluation temperatures of 9% Ni steel, -165 ° C and -196 ° C. The present inventors conducted a number of studies in order to clarify the influence of the component content and metal structure on the toughness of 11% 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 Ni content is simply increased by 2% with respect to 9% Ni steel.

  Furthermore, the present inventors examined a method for increasing the toughness of 11% Ni steel at −253 ° C. As a result, in particular, (a) the Mn content is 0.30% or more and 0.50% or less, (b) the Si content is limited to 0.19% or less, (c) prior austenite It was found that the following three conditions must be satisfied simultaneously: Furthermore, the knowledge that the low temperature toughness at -253 ° C. is improved by (d) controlling the volume fraction of the austenite phase and (e) controlling the effective crystal grain size was also obtained.

  The present invention will be described below. First, the component composition of Ni steel will be described. In addition,% of content means the mass% unless there is particular description.

(C: 0.030% or more, 0.060% or less)
C is an element that increases the yield stress at room temperature and contributes to the formation of martensite and austenite. If the C content is less than 0.030%, the strength cannot be ensured, and the cryogenic toughness may decrease due to the formation of coarse bainite or the like, so 0.030% or more is made the lower limit. The minimum of preferable C content is 0.035% or more. On the other hand, if the C content exceeds 0.060%, cementite is likely to precipitate at the prior austenite grain boundaries, fractures occur at the grain boundaries at -253 ° C, and the cryogenic toughness decreases. 0.060% or less. A preferable upper limit of the C content is 0.055% or less, more preferably 0.050% or less, and a further preferable upper limit of the C content is 0.045% or less.

(Si: 0.03% or more, 0.19% 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 0.03% or more is made the lower limit. The lower limit of the preferable Si content is 0.05% or more. On the other hand, when the Si content exceeds 0.19%, cementite at the prior austenite grain boundaries tends to be coarsened, fracture at the grain boundaries occurs at −253 ° C., and cryogenic toughness decreases. Therefore, limiting the upper limit of the Si content to 0.19% or less is extremely important in order to ensure toughness at −253 ° C. The upper limit of the preferable Si content is 0.16% or less, more preferably 0.13% or less, and the further preferable upper limit of the Si content is 0.10% or less.

(Mn: 0.30% or more, 0.50% or less)
Mn is an element that increases the yield stress at room temperature. If the Mn content is less than 0.30%, the strength cannot be ensured, and the cryogenic toughness may decrease due to the formation of coarse bainite or the like, so 0.30% or more is made the lower limit. The minimum of preferable Mn content is 0.35% or more. On the other hand, if the Mn content exceeds 0.50%, Mn segregated at the prior austenite grain boundaries and MnS that precipitates coarsely cause fractures at the grain boundaries at -253 ° C., thereby reducing the cryogenic toughness. Therefore, limiting the Mn content to 0.50% or less is also extremely important in order to ensure toughness at -253 ° C. The upper limit of the preferable Mn content is 0.45% or less, more preferably 0.40% or less.

(Ni: 10.5% or more, 11.4% or less)
Ni is an essential element for ensuring cryogenic toughness. If the Ni content is less than 10.5%, the toughness at −253 ° C. is insufficient, so 10.5% or more is set as the lower limit. The lower limit of the preferred Ni content is 10.8% or more. However, Ni is an expensive element, and if it exceeds 11.4%, the economy is impaired, so the Ni content is limited to 11.4% or less.

(Al: 0.010% or more, 0.050% or less)
Al is an element mainly used for deoxidation, and also forms AlN and contributes to the refinement of the metal structure and the reduction of solid solution N that lowers the cryogenic toughness. If the Al content is less than 0.010%, the effect of deoxidation, the effect of refining the metal structure and the effect of reducing the solid solution N are small, so 0.010% or more is set as the lower limit. 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 0.050%, the toughness at -253 ° C decreases, so the upper limit is made 0.050% or less. A more preferable upper limit of the Al content is 0.040% or less.

(N: 0.0015% or more, 0.0050% or less)
N contributes to the formation of nitrides, and when the N content is reduced to less than 0.0015%, there is a shortage of fine AlN that suppresses the coarsening of the austenite grain size during heat treatment, and the austenite grains become coarse, resulting in extremely low temperatures. Toughness may decrease. For this reason, the N content may be 0.0015% or more, preferably 0.0020% or more. on the other hand. If the N content exceeds 0.0050%, solid solution N increases or AlN coarsens, so the toughness at −253 ° C. decreases. Therefore, the N content is set to 0.0050% or less, preferably the upper limit is set to 0.0040% or less, more preferably 0.0030% or less.

(O: 0.0007% or more, 0.0030% or less)
O is an impurity, and if the O content exceeds 0.0030%, Al ₂ O ₃ clusters increase and the toughness at −253 ° C. may decrease, so the upper limit of the O content is 0.00. 0030% or less. The upper limit of the preferable O content is 0.0025% or less, more preferably 0.0020% or less, and still more preferably 0.0015% or less. Although it is desirable that the O content is small, the reduction of the O content to less than 0.0007% may involve an increase in cost, so 0.0007% or more is set as the lower limit.

(P: 0.0060% or less)
P is an element that causes grain boundary embrittlement at the prior austenite grain boundaries and is harmful to cryogenic toughness. Therefore, it is desirable that the P content is small. If the P content exceeds 0.0060%, the toughness at -253 ° C may be lowered. Therefore, the P content is limited to 0.0060% or less. The upper limit of the P content is preferably 0.0050% or less, more preferably 0.0040% or less, and still more preferably 0.0030% or less. P may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit is not particularly limited, and the lower limit is 0%.

(S: 0.0030% or less)
S, as MnS, may be a starting point for brittle fracture, and is an element harmful to cryogenic toughness. If the S content exceeds 0.0030%, the toughness at -253 ° C may be lowered, so the S content is limited to 0.0030% or less. The upper limit of the S content is preferably 0.0020% or less, more preferably 0.0015% or less, and 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 is not particularly limited, and the lower limit is 0%.

(Cu: 0.50% or less)
Cu is an element that increases the yield stress at room temperature, and may contain Cu. However, if the Cu content exceeds 0.50%, the toughness at -253 ° C decreases, so the upper limit is made 0.50% or less. The upper limit of the Cu content is preferably 0.40% or less, more preferably 0.30% or less, and still more preferably 0.20% or less. Cu may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof is not particularly limited, and the lower limit is 0%.

(Cr: 0.35% or less)
Cr is an element that increases the yield stress at room temperature, and may contain Cr. However, if the Cr content exceeds 0.35%, the toughness at -253 ° C decreases, so the upper limit is made 0.35% or less. The upper limit of the Cr content is preferably 0.30% or less, more preferably 0.25% or less, and still more preferably 0.20% or less. Although Cr may be mixed as an impurity from scrap or the like during the production of molten steel, there is no need to particularly limit the lower limit thereof, and the lower limit is 0%.

(Mo: 0.40% or less)
Mo is an element that increases the yield stress at room temperature, and since it also has an effect of suppressing embrittlement at grain boundaries, it may contain Mo. However, Mo is an expensive element, and if the Mo content exceeds 0.40%, the economy is impaired, so the Mo content is limited to 0.40% or less. Mo may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof is not particularly limited, and the lower limit is 0%.

(Nb: 0.015% or less)
Nb is an element that increases the yield stress at room temperature, and may also contain Nb because it has an effect of improving the cryogenic toughness due to the refinement of the metal structure. However, if the Nb content exceeds 0.015%, the toughness at -253 ° C decreases, so the upper limit is made 0.015% or less. The upper limit of the Nb content is preferably 0.012% 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 the lower limit thereof is not particularly limited, and the lower limit is 0%.

(V: 0.060% or less)
V is an element that increases the yield stress at room temperature, and may contain V, but if it exceeds 0.060%, the toughness at −253 ° C. decreases, so the upper limit is 0.060% or less. To do. The upper limit of the V content is preferably 0.050% 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 the lower limit is not particularly limited, and the lower limit is 0%.

(Ti: 0.015% or less)
Ti forms TiN and contributes to the refinement of the metal structure and the reduction of solute N that lowers the cryogenic toughness. However, if the Ti content exceeds 0.015%, the toughness at -253 ° C decreases, so the upper limit is made 0.015% or less. The upper limit of the preferable Ti content is 0.012% or less, and the more preferable upper limit is 0.010% or less. Ti may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof is not particularly limited, and the lower limit is 0%.

(B: 0.0020% or less)
B is an element that increases the yield stress at room temperature, and also contributes to the reduction of solute N that forms BN and decreases the cryogenic toughness. However, if B is contained in excess of 0.0020%, the toughness at −253 ° C. decreases, so the upper limit is made 0.0020% or less. The upper limit of the B content is preferably 0.0015% or less, more preferably 0.0012% or less, and still more preferably 0.0010% or less. B may be mixed as an impurity from scrap or the like during the production of molten steel, but the lower limit thereof is not particularly limited, and the lower limit is 0%.

(Ca: 0.0040% or less)
Ca is effective for improving the cryogenic toughness by spheroidizing MnS as CaS, which is easily stretched by hot rolling to increase the harmfulness to cryogenic toughness, and may contain Ca. However, if the Ca content exceeds 0.0040%, the Ca-containing oxysulfide becomes coarse, and the toughness at -253 ° C decreases. For this reason, 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 manufacturing of molten steel, but there is no need to particularly limit the lower limit, and the lower limit is 0%.

(REM: 0.0050% or less)
REM (rare earth metal element) is effective for improving cryogenic toughness by spheroidizing MnS as an oxysulfide of REM, which is likely to increase the harmfulness to cryogenic toughness by hot rolling, like Ca. Therefore, REM may be contained. However, when the REM content exceeds 0.0050%, the oxysulfide containing REM becomes coarse and the toughness at −253 ° C. decreases. For this reason, 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 the lower limit thereof is not particularly limited, and the lower limit is 0%.

  The cryogenic steel according to this embodiment contains or restricts the above components, and the balance contains iron and impurities. 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 steel, and does not adversely affect the present invention. It means what is allowed in the range. However, in the present invention, it is necessary to define an upper limit for P and S among impurities as described above. In addition to the above components, the cryogenic steel according to the present embodiment includes the following alloys for the purpose of further improving the strength of the steel material itself, the cryogenic toughness, etc., or as impurities from secondary materials such as scrap. An element may be contained.

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

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

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

  In order to sufficiently exhibit the above-described effects of the above components, it is preferable to limit Co, Zn and W to 0.01% or less or 0.005% or less, respectively.

  There is no need to limit the lower limit of Sb, Sn, As, Co, Zn and W, and the lower limit of each element is 0%. Moreover, even if an alloy element (for example, P, S, Cu, Cr, Mo, Nb, V, Ti, B, Ca, and REM) with no lower limit is intentionally added or mixed as an impurity Even so, if the content is within the claims, the cryogenic steel (steel) is interpreted as being within the claims of the present invention.

  Next, the metal 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 prior austenite grain boundaries and the toughness tends to decrease. The Ni steel of the present invention is manufactured by subjecting it to hot rolling and water cooling, followed by heat treatment such as reheating quenching, intermediate heat treatment, and tempering. The former austenite grain boundaries described here are reheat quenching heating. It is a grain boundary of austenite that existed at the time. Mn, P, and Si are segregated in the prior austenite grain boundaries, and these elements are considered to reduce the bonding strength of the prior austenite grain boundaries and cause fracture at the former austenite grain boundaries at -253 ° C. .

  In addition, although a prior austenite grain boundary is newly generated during the intermediate heat treatment, the temperature of the intermediate heat treatment of the present invention is as low as 600 ° C. or more and 650 ° C. or less, and Mn, P, Si diffused to the new prior austenite grain boundary. Since the amount of is relatively small, the breakage from these new prior austenite grain boundaries is considered relatively unlikely.

  The inventors of the present invention have made many studies on means for suppressing fracture at the prior austenite grain boundaries at an extremely low temperature of −253 ° C. As a result, (b) the C content is set to 0.060% or less, (b) the Mn content is set to 0.50% or less, and (c) the P content is set to 0.0060% or less. (D) the Si content is 0.19% or less, (e) the prior austenite grain size is 8 μm or less, (f) the aspect ratio of the prior austenite grains is suppressed to 2.4 or less, It was found that when the above six conditions were satisfied at the same time, fracture at the prior austenite grain boundaries was suppressed and toughness at −253 ° C. could be secured.

  Although the mechanism at which fracture from prior austenite grain boundaries is likely to occur at an extremely low temperature of −253 ° C. is not clear, at the very low temperature of −253 ° C., the effect of the stress concentration mitigation mechanism due to dislocation movement in the tempered martensite structure It is limited. In addition, the refinement of cementite in tempered martensite and the generation of fracture starting points such as hard high-C martensite and coarse inclusions were suppressed, and as a result, the prior austenite grain boundaries became the starting point of fracture. there is a possibility.

  Thus, at an extremely low temperature of −253 ° C., it is presumed that breakage is likely to occur selectively in a portion having a relatively weak bonding force, such as a prior austenite grain boundary. Therefore, it is considered that the decrease in the bonding strength of the prior austenite grain boundaries can be suppressed by suppressing cementite that weakens the bonding strength of the prior austenite grain boundaries and segregation of Mn and P. Moreover, the increase in C content and Si content and the coarsening of prior austenite grains promote the coarsening of grain boundary cementite. Therefore, the suppression of the C content and the Si content and the refinement of the prior austenite grain size are necessary for the suppression of the fracture at the prior austenite grain boundary at -253 ° C.

(Old austenite particle size: 3 μm or more, 8 μm or less)
The prior austenite grain size needs to be 3 μm or more and 8 μm or less. When the prior austenite grain size exceeds 8 μm, cementite precipitated at the prior austenite grain boundaries becomes coarse, and the concentration of Mn and P grain boundaries increases. Precipitation of coarse cementite and enrichment of Mn and P weaken the bond strength of the prior austenite grain boundaries, leading to fracture at the former austenite grain boundaries, or the origin of brittle fracture. Since the low temperature toughness is lowered, the upper limit of the prior austenite grain size is set to 8 μm or less. In order to reduce the prior austenite grain size to less than 3 μm, the number of heat treatments is increased, which increases the manufacturing cost. Therefore, the lower limit is set to 3 μm or more.

(Aspect ratio of prior austenite grains: 1.0 or more and 2.4 or less)
Furthermore, in the present invention, in order to ensure cryogenic toughness at −253 ° C., the aspect ratio of prior austenite grains needs to be 2.4 or less. The aspect ratio of the prior austenite grains is the ratio of the length and thickness of the prior austenite grains in the plane parallel to the rolling (L plane), that is, the rolling direction length of the prior austenite grains: the thickness direction of the prior austenite grains. Thickness. Therefore, the lower limit of the aspect ratio is when the length and thickness of the prior austenite grains are the same, and the aspect ratio is 1.0 or more.

  When the aspect ratio exceeds 2.4, even if the prior austenite grain size is 8 μm or less, there is a high probability that a coarse prior austenite grain boundary exceeding 12 μm is included in the rolling direction. It is easy for destruction to occur. Also, the aspect ratio increases when the prior austenite is flattened by rolling in the non-recrystallized region. When the aspect ratio exceeds 2.4, the introduction of dislocations due to rolling in the non-recrystallized region becomes excessive, so that it remains without disappearing even after the subsequent reheating quenching, intermediate heat treatment, and tempering. Since the amount of dislocations becomes excessive, the cryogenic toughness is lowered. For this reason, the upper limit of the aspect ratio of prior austenite grains is set to 2.4 or less.

  The particle size and aspect ratio of prior 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. Measurement of the grain size and aspect ratio of the prior austenite is performed using the plane (L plane) parallel to the rolling direction at the center of the plate thickness as the observation plane. For the prior austenite particle size, the observation surface was corroded with a saturated aqueous solution of picric acid to reveal the prior austenite grain boundaries, and then a scanning electron microscope (SEM) was used to shoot a photograph of five or more fields at 1000 or 2000 times. And measure. After identifying the prior austenite grain boundaries using SEM photographs, the circle equivalent particle diameter (diameter) of at least 20 prior austenite grains is determined by image processing, and the average value of these is set as the prior austenite grain size. As for the aspect ratio of prior austenite, the ratio (aspect ratio) between the length in the rolling direction and the thickness in the thickness direction of at least 20 prior austenite grains was measured using SEM photographs in the same manner as the grain size measurement. These average values are taken as the aspect ratio of the prior austenite.

(Volume fraction of austenite phase: 2% or more, 6% or less)
Moreover, in order to improve the toughness at -253 degreeC, it is preferable to contain an austenite phase 2% or more by a volume fraction. However, this austenite phase is different from old austenite and is an austenite phase present in Ni steel after heat treatment, and the volume fraction is measured by X-ray diffraction. When a stable austenite phase is present even at −253 ° C., it is considered that the toughness is improved because the applied stress and strain are alleviated by plastic deformation of austenite. In addition, the austenite phase is generated relatively uniformly and finely at the prior austenite grain boundaries, the block boundaries of the tempered martensite, the lath boundaries, and the like.

  In other words, the austenite phase is present in the vicinity of the hard phase, which is likely to be the starting point of brittle fracture, and it is thought that it contributes to the suppression of the occurrence of brittle fracture by alleviating stress and strain concentration around the hard phase. It is done. Furthermore, as a result of generating an austenite phase of 2% or more, it is considered that coarse cementite, which is the starting point for the occurrence of brittle fracture, can be greatly reduced. On the other hand, when the volume fraction of the austenite phase increases, the concentration of C or the like into the austenite phase becomes insufficient, and the possibility of transformation to martensite increases at -253 ° C. Unstable austenite that transforms into martensite at a cryogenic temperature may lower the cryogenic toughness at -253 ° C., and the volume fraction of the austenite phase is preferably 6% or less.

(Effective crystal grain size: 2 μm or more and 5 μm or less)
The effective crystal grain size is preferably 2 μm or more and 5 μm or less. The effective crystal grain size is a region in which the crystal orientation is almost the same. When the crystal grain size is reduced, resistance to propagation of fracture cracks increases and toughness is improved. However, in order to reduce the effective crystal grain size to less than 2 μm, the number of heat treatments is increased to increase the manufacturing cost. Therefore, the lower limit is set to 2 μm or more. In addition, when the effective crystal grain size exceeds 5 μm, the hard phase that causes the occurrence of brittle fracture, ie, coarse cementite in the prior austenite grain boundaries and tempered martensite, coarse AlN, MnS, alumina, etc. The stress acting on the object may increase, and the cryogenic toughness at -253 ° C may decrease.

  Effective crystal grain size is a backscattered electron diffraction pattern (EBSD) analyzer attached to a scanning electron microscope with a plane (L plane) parallel to the rolling direction at the center of the plate thickness as an observation plane. Use to measure. Observe at least 5 fields of view at a magnification of 2000 times, consider the boundary of the metal structure having an orientation difference of 15 ° or more as a grain boundary, and use the crystal grain surrounded by this grain boundary as an effective crystal grain, from the area to the circle The equivalent particle diameter (diameter) is obtained by image processing, and the average value of the equivalent circle particle diameters is defined as the effective crystal particle diameter.

  As an example of the characteristics of the cryogenic steel in the present invention, the plate thickness is preferably 12 mm to 20 mm, the yield stress at room temperature is preferably 460 MPa to 580 MPa, and the tensile strength is preferably 560 MPa to 680 MPa.

  Next, a method for manufacturing the cryogenic steel according to this embodiment will be described.

  The steel melting method 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. Steel slabs are manufactured by continuous casting. The obtained steel slab is heated, subjected to hot rolling, cooled with water, and then subjected to reheating quenching, intermediate heat treatment, and tempering sequentially.

  Below, preferable manufacturing conditions are demonstrated.

  The heating temperature for hot rolling is preferably 950 ° C. or higher and 1100 ° C. or lower. When the heating temperature is lower than 950 ° C., AlN becomes coarse and the cryogenic toughness may be lowered. When the heating temperature exceeds 1100 ° C, the austenite grain size becomes coarse during heating, and the cryogenic toughness at -253 ° C may be lowered. A more preferable upper limit of the heating temperature is 1050 ° C. or higher. The heating holding time is desirably 30 minutes to 180 minutes.

  In hot rolling, if the rolling reduction at 950 ° C. or less is less than 80%, the austenite grains are not sufficiently refined by recrystallization of austenite during rolling, and coarse grains are formed in a part of the austenite grains after rolling. May be included, and the cryogenic toughness may decrease. Therefore, the lower limit of the rolling reduction at 950 ° C. or less is preferably 80% or more. A more preferable lower limit of the rolling reduction is 85% or more, and a further preferable lower limit is 90% or more. Homogenous refinement of prior austenite grains by recrystallization of rolling is particularly important for securing the cryogenic toughness of the present invention, and strict regulation of rolling temperature and rolling reduction is preferable. If the rolling reduction at 950 ° C. or lower exceeds 95%, the rolling time becomes long, and problems may occur in productivity. Therefore, the upper limit of the rolling reduction at 950 ° C. or lower is preferably 95% or lower.

  When the end temperature of hot rolling is below 650 ° C., the water cooling start temperature falls below 550 ° C., and the cryogenic toughness at −253 ° C. may be lowered as described later. Therefore, the lower limit of the end temperature of hot rolling is preferably 650 ° C. or higher. More preferably, the lower limit of the end temperature of hot rolling is 700 ° C. or higher, and more preferably 740 ° C. or higher. On the other hand, when the end temperature of hot rolling exceeds 800 ° C., the dislocations introduced by rolling decrease due to recovery, the effective crystal grain size becomes larger than 5 μm, and cryogenic toughness is insufficient. Therefore, the upper limit of the end temperature is preferably 800 ° C. or lower. A more preferable upper limit of the end temperature of hot rolling is 770 ° C. or less.

  After hot rolling, it is preferable to perform water cooling to near room temperature. The water cooling start temperature is preferably 550 ° C. or higher and 800 ° C. or lower. When the water cooling start temperature is lower than 550 ° C., coarse bainite may be partially generated. This coarse bainite becomes a comparatively coarse tempered martensite even if a subsequent heat treatment such as reheating and quenching is performed, so that it is difficult to control the effective crystal grain size to 5 μm or less. A more preferable lower limit of the water cooling start temperature is 600 ° C. or higher. The upper limit of the water cooling start temperature is not particularly limited, and water cooling may be started immediately after the end of hot rolling. A preferable upper limit of the water cooling start temperature is 800 ° C. or less, which is a preferable upper limit of the rolling end temperature.

  Reheating and quenching is effective for refinement of prior austenite, and the heating temperature is preferably 740 ° C. or higher and 780 ° C. or lower. When the reheating and quenching heating temperature (reheating and quenching temperature) is below 740 ° C., a portion that does not transform to austenite remains, and the untransformed austenite portion does not yield a martensite structure even when quenched, resulting in a decrease in the strength of the base metal. Or the effective crystal grain size becomes coarse, so that the cryogenic toughness may decrease. A more preferable lower limit of the reheating quenching temperature is 750 ° C. or higher. When the reheating quenching temperature is higher than 780 ° C., the austenite grain size is coarsened, and since AlN is coarsened, the cryogenic toughness may be lowered. A more preferable upper limit of the reheating quenching temperature is 770 ° C. or less. The holding time at the time of reheating and quenching is desirably 20 minutes to 180 minutes. Cooling at the time of reheating and quenching can employ a method such as water cooling to 200 ° C. or lower.

  The intermediate heat treatment is effective for reducing the effective crystal grain size contributing to the improvement of the cryogenic toughness and securing the austenite phase, and the heating temperature is preferably 600 ° C. or higher and 650 ° C. or lower. When the heating temperature of the intermediate heat treatment (intermediate heat treatment temperature) is below 600 ° C., the austenite transformation is insufficient, the fraction of tempered martensite excessively tempered increases, and the strength of the base material may decrease. is there. On the other hand, when the intermediate heat treatment temperature is lower than 600 ° C., the effective crystal grain size may exceed 5 μm and the cryogenic toughness may be lowered. A more preferable lower limit of the intermediate heat treatment temperature is 620 ° C. or higher.

  When the temperature of the intermediate heat treatment exceeds 650 ° C., the austenite transformation proceeds excessively. As a result, the austenite becomes insufficiently stabilized, it becomes impossible to secure an austenite phase with a volume fraction of 2% or more, or the effective crystal grain size cannot be controlled to 5 μm or less, and the cryogenic toughness decreases. Sometimes. A more preferable upper limit of the intermediate heat treatment temperature is 640 ° C. or less. The holding time of the intermediate heat treatment is desirably 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 temper embrittlement.

  Tempering is also effective for securing the austenite phase, and the heating temperature is preferably 530 ° C. or higher and 560 ° C. or lower. When the tempering heating temperature (tempering temperature) is lower than 530 ° C., the austenite phase cannot be secured by 2% or more in volume fraction, and the cryogenic toughness may be insufficient. A more preferable lower limit of the tempering temperature is 540 ° C. or higher. When the upper limit of the tempering temperature exceeds 560 ° C., the austenite phase at room temperature exceeds 6% in volume fraction, and when this is cooled to −253 ° C., some austenite is transformed into high C martensite, Low temperature toughness may be reduced. For this reason, the upper limit with preferable tempering temperature is 560 degrees C or less, and a more preferable upper limit is 550 degrees C or less. The holding time for tempering is desirably 20 minutes to 180 minutes. As a cooling method during tempering, it is desirable to perform water cooling to avoid temper embrittlement.

  Examples of the present invention will be described below. However, the following examples 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 a slab having a thickness of 100 mm to 240 mm was manufactured by continuous casting. Tables 1 and 2 show chemical components of steel types A1 to A24. These slabs were heated, subjected to controlled rolling, cooled with water as they were, and subjected to reheating quenching, intermediate heat treatment, and tempering heat treatment to produce steel sheets. The holding time for the hot rolling heating was 30 to 120 minutes, and the holding time for the reheating quenching, intermediate heat treatment, and tempering heat treatment was 20 to 60 minutes. Samples were taken from the steel plates and evaluated for metal structure, tensile properties, and toughness.

  The grain size of the prior austenite was measured using 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 prior austenite was performed in accordance with JIS G 0551. First, the observation surface of the sample was corroded with a picric acid saturated aqueous solution to reveal the prior austenite grain boundaries, and then photographs of five fields or more were taken with a scanning electron microscope at 1000 times or 2000 times. After identifying the prior austenite grain boundaries using the structure photograph, the equivalent-equivalent particle diameter (diameter) of at least 20 prior austenite grains was determined by image processing, and the average value of these was used as the prior austenite grain size.

  In addition, in the steel of the present invention, the prior austenite grain boundaries are difficult to identify due to corrosion in order to reduce the prior austenite grain size and to suppress the P content so that the prior austenite grain boundaries are not easily destroyed. There is. In such a case, after heating to 450 ° C. to 490 ° C., a heat treatment for holding for 1 hour or more was performed, and then the prior austenite particle size was measured by the method described above.

  In addition, when it is difficult to identify the prior austenite grain boundaries even after heat treatment at 450 ° C. to 490 ° C., a Charpy test piece is taken from the heat-treated sample and subjected to an impact test at −196 ° C. The sample destroyed by using was used. In this case, a cross-section of the fracture surface is prepared by a plane parallel to the rolling direction (L-plane), and after corrosion, the old austenite grain boundary of the fracture surface cross-section at the center of the plate thickness is identified with a scanning electron microscope. The particle size was measured. When the prior austenite grain boundaries are embrittled by heat treatment, minute cracks are generated in the prior austenite grain boundaries due to the impact load during the Charpy test, so that the prior austenite grain boundaries are easily identified.

  The aspect ratio of the prior austenite grains was determined as a ratio between the maximum value (length in the rolling direction) and the minimum value (thickness in the thickness direction) of the length of the prior austenite grain boundary identified as described above. The aspect ratio of at least 20 prior austenite grains was measured, and the average value thereof was taken as the aspect ratio of the prior austenite grains.

  The volume fraction of the austenite phase was measured by the X-ray diffraction method by collecting 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 austenite (face centered cubic structure) of the X-ray peak and the tempered martensite (body centered cubic structure).

  The effective crystal grain size was measured using an 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. Observe at least 5 fields of view at a magnification of 2000 times, consider the boundary of the metal structure having an orientation difference of 15 ° or more as a grain boundary, and use the crystal grain surrounded by this grain boundary as an effective crystal grain, from the area to the circle The equivalent particle diameter (diameter) was determined by image processing, and the average value of the equivalent circle particle diameters was taken as the effective crystal particle diameter.

  The strength (yield stress and tensile strength) is a method specified in JIS Z 2241 by collecting a No. 1A full-thickness tensile test piece specified in JIS Z 2241 whose longitudinal direction is parallel to the rolling direction (L direction). At room temperature. The target value of yield stress is 460 MPa or more and 580 MPa or less, and the target value of tensile strength is 560 MPa or more and 680 MPa or less. Although the yield stress was a yield stress, there were many cases where no clear yield stress was observed, and in that case, the yield strength was 0.2%.

For cryogenic toughness, CT specimens of full thickness with the front and back surfaces ground by 0.5 mm each were sampled in a direction perpendicular to the rolling direction (C direction) and in liquid hydrogen (−253 ° C.), ASTM standard E1820. A JR curve was prepared according to the unloading compliance method specified in -13, and the J value was converted to a K _IC value. The target value of the cryogenic toughness is 150 MPa · √m or more.

  Tables 3 and 4 show the plate thickness, manufacturing method, base material characteristics, and metal structure of steel materials (steel materials No. 1 to 30) manufactured using slabs having chemical components of steel types A1 to A24.

  As apparent from Tables 3 and 4, the steel material No. 1 to 14 satisfy the target values of the yield stress at room temperature and the cryogenic toughness at −253 ° C. Production No. In No. 9, the intermediate heat treatment temperature is higher than the preferred range, the austenite phase is small, the effective crystal grain size is large, and the balance between strength and toughness is slightly inferior.

  On the other hand, the steel material No. No. 15 has a low C content, and steel material No. Since No. 18 has a low Mn content, the cryogenic toughness is lowered. Steel No. 16, 17, and 19 to 23 have a high C content, Si content, Mn content, P content, S content, Cr content, and Al content, respectively, and the cryogenic toughness is reduced. Steel No. No. 24 has a large Nb content and B content, an aspect ratio of prior austenite grains is increased, and cryogenic toughness is reduced. Steel No. No. 25 has high Ti content and N content, and cryogenic toughness is reduced.

  Steel No. 26-30 is an example which employ | adopted the manufacturing conditions which deviate from a preferable range. Steel No. In No. 26, the heating temperature is low, the aspect ratio of the prior austenite grains is increased, and the cryogenic toughness is reduced. Steel No. Nos. 27, 29, and 30 have a high heating temperature, rolling end temperature, and reheat quenching temperature, respectively, and the prior austenite grain size is increased, the effective crystal grain size is also increased, and the cryogenic toughness is reduced. Steel No. No. 28 has a low rolling reduction at 950 ° C. or lower, an increase in the prior austenite grain size, and a decrease in cryogenic toughness.

Claims (4)

% By mass
C: 0.030% or more, 0.060% or less,
Si: 0.03% or more, 0.19% or less,
Mn: 0.30% or more, 0.50% or less,
Ni: 10.5% or more, 11.4% or less,
Al: 0.010% or more, 0.050% or less,
N: 0.0015% or more, 0.0050% or less,
O: 0.0007% or more, containing 0.0030% or less,
P: 0.0060% or less,
S: 0.0030% or less,
Cu: 0.50% or less,
Cr: 0.35% or less,
Mo: 0.40% or less,
Nb: 0.015% or less,
V: 0.060% or less,
Ti: 0.015% or less,
B: 0.0020% or less,
Ca: 0.0040% or less,
REM: limited to 0.0050% or less,
The balance consists of Fe and impurities,
A Ni steel for liquid hydrogen, wherein the prior austenite grains have a particle size of 3 μm or more and 8 μm or less, and an aspect ratio of 1.0 or more and 2.4 or less. 2. The Ni steel for liquid hydrogen according to claim 1, comprising an austenitic phase having a volume fraction of 2% or more and 6% or less. 3. The Ni steel for liquid hydrogen according to claim 1, wherein an effective crystal grain size is 2 μm or more and 5 μm or less. The plate thickness is 12 mm or more and 20 mm or less,
Yield stress is 460 MPa or more, 580 MPa or less,
The Ni steel for liquid hydrogen according to any one of claims 1 to 3, wherein the tensile strength is 560 MPa or more and 680 MPa or less.