JP2018104793A - Ni steel for liquid hydrogen - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide Ni steel for liquid hydrogen having sufficient toughness at liquid hydrogen temperature (-253°C) and yield stress at room temperature of 590 MPa or more and containing Ni.SOLUTION: There is adopted Ni steel for liquid hydrogen containing, by mass%, C:0.030% to 0.070%, Si:0.03% to 0.30%, Mn:0.10% to 0.80%, Ni:12.5% to 15.4%, Mo:0.03% to 0.60%, Al:0.010% to 0.060%, N:0.0015% to 0.0060% and O:0.0007% to 0.0030% and having particle diameter of old austenitic grain of 3.0 μm to 20.0 μm and aspect ratio of 3.0 to 10.0. It may contain an austenite phase of 2.0% to 20.0% by volume fraction and effective crystal grain diameter may be 2.0 μm to 12.0 μm.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 40 mm when the tank is enlarged, so that an increase in the 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 that 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 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 590 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 steel in which the content of Ni is higher than the conventional 9% Ni and is about 14%. 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) Ni steel for liquid hydrogen according to one embodiment of the present invention is mass%, C: 0.030% or more, 0.070% or less, Si: 0.03% or more, 0.30% or less, Mn : 0.10% or more, 0.80% or less, Ni: 12.5% or more, 15.4% or less, Mo: 0.03% or more, 0.60% or less, Al: 0.010% or more, 0 0.060% or less, N: 0.0015% or more, 0.0060% or less and O: 0.0007% or more, 0.0030% or less, P: 0.008% or less, S: 0.0040% Cu: 1.00% or less, Cr: 1.00% 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, the balance is Fe and impurities, The particle size of the austenite grains is 3.0μm or more, 20.0 .mu.m or less, an aspect ratio of 3.0 or more, and wherein the at 10.0.
(2) The Ni steel for liquid hydrogen described in (1) above may contain an austenitic phase with a volume fraction of 2.0% or more and 20.0% or less.
(3) In the Ni steel for liquid hydrogen described in (1) or (2) above, the effective crystal grain size may be limited to 2.0 μm or more and 12.0 μm or less.
(4) In the Ni steel for liquid hydrogen according to any one of the above (1) to (3), the plate thickness is 12 mm or more and 40 mm or less, the yield stress at room temperature is 590 MPa or more, 710 MPa or less, room temperature The tensile strength at 690 MPa or more and 810 MPa or less may be sufficient.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide Ni steel for liquid hydrogen which has cryogenic toughness sufficient as a liquid hydrogen tank use, and has a high yield stress at room temperature.

  Steel containing 13 to 15% Ni (sometimes referred to as 14% Ni steel) contains 4 to 6% more Ni that improves low temperature toughness than 9% Ni steel. Ensuring toughness at low temperatures can be expected. However, -253 ° C, which the present invention aims at low temperature toughness, is significantly lower than the conventional evaluation temperatures of 9% Ni steel, -165 ° C and -196 ° C. For this reason, the present inventors have conducted numerous studies in order to clarify the influence of the component content and metal 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 Ni content is simply increased by 4 to 6% with respect to 9% Ni steel.

  Furthermore, the present inventors examined a method for increasing the toughness of 14% Ni steel at −253 ° C. As a result, in particular, (a) the C content is 0.030% or more and 0.070% or less, and (b) the Si content is 0.03% or more and 0.30% or less. It is necessary to satisfy the following four conditions simultaneously: (c) the content of Mn is 0.10% or more and 0.80% or less, and (d) the grain size and form of prior austenite are controlled. I understood it. Furthermore, the knowledge 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 was 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, 0.070% 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 0.070%, cementite is likely to precipitate at the prior austenite grain boundaries, fracture at the grain boundaries occurs at -253 ° C, and the cryogenic toughness decreases. 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 still more 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 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 0.30%, cementite at the prior austenite grain boundaries tends to be coarsened, and fracture at the grain boundaries occurs at −253 ° C., resulting in a reduction in cryogenic toughness. Therefore, limiting the upper limit of the Si content to 0.30% or less is extremely important in order to ensure toughness at −253 ° C. The upper limit of the Si content is preferably 0.20% or less, more preferably 0.15% or less, and still more preferably 0.10% or less.

(Mn: 0.10% 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 0.10%, the strength cannot be ensured, and the cryogenic toughness may decrease due to the formation of coarse bainite or the like, so the lower limit of the Mn content is 0.10% or more. The minimum of preferable Mn content is 0.30% or more. On the other hand, if the Mn content exceeds 0.80%, Mn segregated at the prior austenite grain boundaries and MnS that precipitates coarsely cause fractures at the grain boundaries at -253 ° C, and cryogenic toughness decreases. Therefore, limiting the upper limit of the Mn content to 0.80% or less is extremely important in order to ensure 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%, the toughness at -253 ° C cannot be sufficiently secured, so the lower limit of the Ni content is 12.5% or more. The lower limit of the preferred Ni content is 12.8% or more. However, Ni is an expensive element, and if it exceeds 15.4%, the economy is impaired, so the upper limit of Ni content is limited to 15.4% or less.

(Mo: 0.03% or more, 0.60% or less)
Mo is an element that increases the yield stress at room temperature and has the effect of suppressing grain boundary embrittlement. If the Mo content is less than 0.03%, the strength cannot be ensured, and the cryogenic toughness may decrease due to the occurrence of grain boundary fracture. Therefore, the lower limit of the Mo content is set to 0.03% or more. The minimum of preferable Mo content is 0.05% or more. However, Mo is an expensive element, and if it exceeds 0.60%, the economy is impaired, so the upper limit of the Mo content is limited to 0.60% or less.

(Al: 0.010% or more, 0.060% or less)
Al is an element mainly used for deoxidation, and also forms AlN, contributing 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 deoxidation effect, the metal structure refinement effect, and the solid solution N reduction effect 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, and 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. A more preferable upper limit of the 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%, 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 lower limit of the N content is set to 0.0015% or more, preferably 0.0020% or more. On the other hand, if the N content exceeds 0.0060%, solid solution N increases or AlN coarsens, so the toughness at −253 ° C. decreases. For this reason, the upper limit of the N content is set to 0.0060% or less, preferably 0.0050% or less, 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 be small. However, since the reduction of the O content to less than 0.0007% may involve an increase in cost, the upper limit is set to 0.0007% or more. On the other hand, when 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 0.0030% 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.

(P: 0.008% 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 0.008%, the toughness at -253 ° C may be lowered. Therefore, the upper limit of the P content is limited to 0.008% or less. The upper limit of the P content is preferably 0.005% or less, more preferably 0.004% or less, and 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 is not particularly limited, and the lower limit is 0% or more.

(S: 0.0040% 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 0.0040%, the toughness at -253 ° C may be lowered, 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, 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% or more.

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

(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, and 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 there is no need 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, and still more preferably 0.30% or less. Although Cr may be mixed as an impurity from scrap or the like during the production of molten steel, it is not necessary to particularly limit the lower limit of the Cr 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 also be contained because it has an effect of improving the cryogenic toughness by refining the metal structure. However, if the Nb content exceeds 0.020%, the toughness at -253 ° C decreases, so the upper limit is made 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 there is no need to particularly limit the lower limit of the Nb content, and the lower limit is 0% or more.

(V: 0.080% or less)
V is an element that increases the yield stress at room temperature, and may be contained. However, if the V content exceeds 0.080%, the toughness at -253 ° C decreases, so the upper limit of the V content is limited. 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 there is no need 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 form TiN and contribute to the refinement of the metal structure and the reduction of solute N that lowers the cryogenic toughness. However, if the Ti content exceeds 0.020%, 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 Ti content is preferably 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 there is no need 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 also forms BN and contributes to the reduction of solid solution N that lowers the cryogenic toughness, so B may be contained. 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 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 there is no need 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 for 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, 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 the production of molten steel, but there is no need to particularly limit the lower limit of the Ca content, and the lower limit is 0% or more.

(REM: 0.0050% or less)
REM (Rare-Earth Metal), like Ca, spheroidizes MnS as REM oxysulfide, which is stretched by hot rolling to increase the harmfulness to cryogenic toughness, and improves cryogenic toughness Since it is effective to make it contain, you may contain. 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 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 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 may contain the following alloy elements as impurities from auxiliary materials such as scrap, but the strength of the steel material itself, 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 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 the 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 As content is preferably 0.005% or less, more preferably 0.003% or less, and most preferably 0.001% or less. preferable.

  Moreover, in order to fully demonstrate the said effect of the said component, it is preferable to restrict | limit the upper limit of Co, Zn, and W content 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% or more. 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 above-described range, the cryogenic steel (steel material) is interpreted as being within the scope 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 hot rolling and water cooling, followed by heat treatment such as intermediate heat treatment and tempering. The former austenite grain boundary described here is after hot rolling and before the start of water cooling. It is a grain boundary of austenite that existed. 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 560 ° C. or more and 670 ° 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. For this reason, the grain size and aspect ratio of the prior austenite grains after hot rolling and water cooling are substantially important, and when measuring the grain size and aspect ratio of the prior austenite grains after intermediate heat treatment and tempering, It is necessary to measure prior austenite grains after hot rolling and water cooling. Whether the prior austenite grain boundary is a grain boundary after hot rolling, water cooling, or a grain boundary after intermediate heat treatment, the grain size of the former austenite grain is 2 μm or less is the grain boundary after the intermediate heat treatment. Judge. That is, when measuring prior austenite grains after intermediate heat treatment or after tempering, the prior austenite grains having a grain size of 2 μm or less are excluded and the grain size and aspect ratio of the prior austenite grains are measured.

  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 0.070% or less, (b) the Mn content is set to 0.80% or less, and (c) the P content is set to 0.008% or less. (D) Si content is 0.30% or less, (e) Mo content is 0.03% or more, (f) Old austenite particle size is 20.0 μm or less, G) When the seven conditions of suppressing the aspect ratio of prior austenite grains to 10.0 or less are satisfied at the same time, it is found that fracture at the prior austenite grain boundaries is suppressed and toughness at −253 ° C. can be secured. It was.

  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: 3.0 μm or more, 20.0 μm or less)
The prior austenite particle size needs to be 3.0 μm or more and 20.0 μm or less. Refinement of the prior austenite grain size to less than 3.0 μm is accompanied by an increase in the manufacturing cost such as increasing the number of heat treatments, so the lower limit of the prior austenite grain size is set to 3.0 μm or more. On the other hand, when the prior austenite grain size exceeds 20.0 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 20.0 μm or less.

(Aspect ratio of prior austenite grains: 3.0 or more and 10.0 or less)
The aspect ratio of the prior austenite grains is the ratio of the length and thickness of the prior austenite grains in a plane parallel to the rolling direction (L-plane), that is, the length of the prior austenite grains in the rolling direction: the thickness direction of the prior austenite grains Of the thickness. The aspect ratio increases when the prior austenite is flattened by rolling in the non-recrystallized region. If the aspect ratio is less than 3.0, the introduction of dislocations by rolling in the non-recrystallized region is insufficient, and the strength cannot be increased. For this reason, the lower limit of the aspect ratio of the prior austenite grains is set to 3.0 or more. On the other hand, when the aspect ratio exceeds 10.0, a portion where the prior austenite grain size exceeds 50 μm occurs due to excessive non-recrystallization zone rolling, and the cryogenic toughness may be lowered. In addition, cementite tends to be coarsened at the prior austenite grain boundaries along the rolling direction, and the acting stress increases and breakage is likely to occur. For this reason, the upper limit of the aspect ratio of prior austenite grains is set to 10.0 or less.

  The grain size and aspect ratio of prior austenite are preferably measured by collecting a sample from water-cooled steel after hot rolling, but may be measured by collecting a sample from 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. In the case of measuring the grain size and aspect ratio of the prior austenite by collecting a sample from the tempered steel, as described above, excluding the prior austenite grains having a grain size of 2 μm or less, the grains of the prior austenite grains Measure diameter and aspect ratio.

(Volume fraction of austenite phase: 2.0% or more, 20.0% or less)
In order to further increase the toughness at −253 ° C., it is preferable that the austenite phase is contained in 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, 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 exists even at −253 ° C., it is considered that the toughness is improved because the stress and strain applied are relaxed 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 with a volume fraction of 2.0% or more, it is considered that coarse cementite, which is a starting point of 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 to martensite at a cryogenic temperature may reduce the cryogenic toughness at −253 ° C., so 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 an X-ray diffraction method by collecting 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 where the crystal orientation is almost the same. When the crystal grain size is reduced, resistance to propagation of fracture cracks increases, and the toughness is further improved. However, when the effective crystal grain size is reduced to less than 2.0 μm, the number of heat treatments is increased, which increases the manufacturing cost. Is 2.0 μm or more. Further, when the effective crystal grain size exceeds 12.0 μm, the hard phase that causes the occurrence of brittle fracture, that is, coarse cementite in prior austenite grain boundaries and tempered martensite, coarse AlN, MnS, alumina, etc. The stress acting on the inclusions increases, 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 grain size is determined by taking a sample from the steel after tempering, and using the plane parallel to the rolling direction (L plane) at the center of the plate thickness as the observation plane, the backscattered electron diffraction pattern attached to the scanning electron microscope It measures using the method (Electron Back Scatter Diffraction: EBSD) analyzer. 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 or more and 40 mm or less, the yield stress at room temperature is 590 MPa or more and 710 MPa or less, and the tensile strength is preferably 690 MPa or more and 810 MPa or less.

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, hot-rolled, cooled with water, and then subjected to intermediate heat treatment and tempering heat treatment.

  Below, preferable manufacturing conditions are demonstrated. In addition, the conditions shown below show an example of preferable conditions. If a steel material within the scope of the present invention can be obtained, there is no particular problem even if it is outside the conditions described below.

  The heating temperature for hot rolling is preferably 950 ° C. or higher and 1180 ° 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 1180 ° C, the austenite grain size becomes coarse during heating, and the cryogenic toughness at -253 ° C may be lowered. The upper limit of the heating temperature is more preferably 1100 ° C. or lower, and further preferably 1050 ° C. or lower. 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 50%, 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 50% or more. A more preferable lower limit of the rolling reduction is 60% or more, and further preferably 70% or more. On the other hand, 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. . 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.

  When the end temperature of hot rolling falls below 650 ° C., the water cooling start temperature falls below 550 ° C., and as described later, the cryogenic toughness at −253 ° C. may decrease, or the yield stress at room temperature may decrease. . Even if the water cooling start temperature does not fall below 550 ° C., the aspect ratio of the prior austenite grains becomes large, and the cryogenic toughness at −253 ° C. may be lowered. Therefore, the lower limit of the hot rolling end temperature is preferably 650 ° C. or higher, and the more preferable hot rolling end temperature is 700 ° C. or higher, more preferably 740 ° C. or higher. On the other hand, when the end temperature of hot rolling exceeds 920 ° C., dislocations introduced by rolling are reduced by recovery, the effective crystal grain size becomes larger than 12.0 μm, and cryogenic toughness is insufficient. Therefore, the upper limit of the end temperature of hot rolling is preferably 920 ° C. or lower. A more preferable upper limit of the end temperature of hot rolling is 880 ° 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 920 ° C. or lower. When the water cooling start temperature is lower than 550 ° C., coarse bainite may be partially generated. Since this coarse bainite is likely to remain as relatively coarse bainite even after the subsequent heat treatment such as intermediate heat treatment, it becomes difficult to control the effective crystal grain size to 12.0 μm or less. Low temperature toughness may be reduced. In addition, since it contains bainite transformed at high temperature, the lower limit of the water cooling start temperature is preferably 550 ° C. or more because the yield stress at room temperature tends to decrease, and the more preferable lower limit of the water cooling start temperature is 600 ° C. or more. is there. 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. Therefore, 920 ° C., which is a preferable upper limit of the end temperature of hot rolling, is a preferable upper limit of the water cooling start temperature.

  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 560 ° C. or more and 670 ° C. or less. When the heating temperature of the intermediate heat treatment (intermediate heat treatment temperature) is less than 560 ° C., the austenite transformation is insufficient, the volume fraction of tempered martensite excessively tempered increases, and the base metal strength decreases. There is. On the other hand, 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. A more preferable lower limit of the 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, stabilization of austenite becomes insufficient, it becomes impossible to secure an austenite phase with a volume fraction of 2.0% or more, or the effective crystal grain size cannot be controlled to 12.0 μm or less, so that the cryogenic temperature is low. Toughness may decrease. A more preferable upper limit of the intermediate heat treatment temperature is 650 ° 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 480 ° C or higher and 570 ° C or lower. When the tempering heating temperature (tempering temperature) is lower than 480 ° C., the austenite phase cannot be secured in a volume fraction of 2.0% or more, and the cryogenic toughness may be insufficient. A more preferable lower limit of the tempering temperature is 520 ° C. or more. On the other hand, when 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 to low temperature toughness. Therefore, the upper limit of the tempering temperature is preferably 570 ° C. or less, more preferably 560 ° 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.

  As described above, according to the present invention, it is possible to obtain Ni steel for liquid hydrogen having a cryogenic toughness sufficient for 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 for a liquid hydrogen tank, the steel plate for the tank can be made thinner than 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 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.

  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 150 mm to 400 mm was manufactured by continuous casting. Tables 1 and 2 show chemical components of the steel materials A1 to A25. These slabs were heated, subjected to controlled rolling, cooled with water as they were, and subjected to intermediate heat treatment and tempering heat treatment to produce steel sheets. The holding time of the hot rolling heating was 30 to 120 minutes, and the holding time of the 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 460 ° 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 460 ° C., a Charpy test piece is taken from the heat-treated sample, subjected to an impact test at −196 ° C., and the former austenite grain boundaries. 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 particle size and aspect ratio of prior austenite were measured by excluding prior austenite grains having a particle size of 2 μm or less.

  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 590 MPa or more and 710 MPa or less, and the target value of tensile strength is 690 MPa or more and 810 MPa or less. Although the yield stress was a yield stress, there were many cases where no clear yield stress was observed, in which case the yield stress was 0.2% proof stress.

Cryogenic toughness is a thickness of 30 mm from the center of the plate thickness when the plate thickness of the steel plate exceeds 31 mm. CT specimens were collected in a direction perpendicular to the rolling direction (C direction), and a JR curve was prepared in liquid hydrogen (−253 ° C.) in accordance with the unloading compliance method defined in ASTM standard E1820-13. The J value was converted into a K _IC value. The target value of the cryogenic toughness is 150 MPa · √m or more.

  Tables 3 and 4 show steel plate thicknesses, manufacturing methods, and base materials manufactured using steel materials (manufacturing conditions No. 1 to 30) manufactured using slabs having chemical components of steel materials A1 to A25 of Tables 1 and 2. The characteristics and metal structure are shown.

As is apparent from Tables 3 and 4, No. The steel materials 1 to 14 satisfy the target values of yield stress at room temperature, tensile strength at room temperature, and cryogenic toughness at -253 ° C.
Production condition No. in Table 3 In Steel No. 9, the heating temperature during hot rolling is the upper limit of the preferable range, the austenite phase increases, and the balance between strength and toughness is slightly inferior.
Manufacturing condition No. Steel No. 10 has an intermediate heat treatment temperature higher than the preferred range, a small austenite phase, a large effective crystal grain size, and a slightly poor balance between strength and toughness.

On the other hand, in Table 4, No. Steel No. 15 has a low C content. Since No. 23 has a low Mo content, the yield stress and tensile strength at room temperature are low in any steel material, and the cryogenic toughness is reduced.
No. Since the steel No. 18 has a low Mn content, the cryogenic toughness is lowered.
No. Each steel material of 16, 17, 19-22, 24 has a lot of C content, Si content, Mn content, P content, S content, Cr content, Al content, and cryogenic toughness. It is falling.
No. Steel No. 25 has a high Nb content and B content, an increased aspect ratio of prior austenite grains, an increased effective crystal grain size, and a reduced cryogenic toughness.
No. The steel material No. 26 has a large Ti content and an N content, and the cryogenic toughness is lowered.

No. Each of the steel materials 27 to 30 is an example in which manufacturing conditions deviating from a preferable range are adopted.
No. Steel No. 27 has a high heating temperature during rolling, the grain size of prior austenite grains is increased, the effective crystal grain size is also increased, and the cryogenic toughness is reduced.
No. Steel No. 28 has a low rolling reduction at 950 ° C. or lower, a large prior austenite grain size, a large effective crystal grain size, and low cryogenic toughness. Further, the aspect ratio of the prior austenite grains is reduced, and the yield stress and tensile strength at room temperature are reduced.
No. Steel No. 29 has a high rolling end temperature, a large prior austenite grain size, a large effective crystal grain size, and low cryogenic toughness. Further, the aspect ratio of the prior austenite grains is reduced, and the yield stress and tensile strength at room temperature are reduced.
No. Steel No. 30 has a low rolling end temperature of hot rolling, an aspect ratio of prior austenite grains is increased, and cryogenic toughness is reduced.

Claims (4)

% By mass
C: 0.030% or more, 0.070% or less,
Si: 0.03% or more, 0.30% or less,
Mn: 0.10% or more, 0.80% or less,
Ni: 12.5% or more, 15.4% or less,
Mo: 0.03% or more, 0.60% or less,
Al: 0.010% or more, 0.060% or less,
N: 0.0015% or more, 0.0060% or less and 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,
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,
The balance consists of Fe and impurities,
A Ni steel for liquid hydrogen, wherein the prior austenite grains have a particle size of 3.0 μm or more and 20.0 μm or less and an aspect ratio of 3.0 or more and 10.0 or less. 2. The Ni steel for liquid hydrogen according to claim 1, comprising an austenite phase having a volume fraction of 2.0% or more and 20.0% or less. 3. The Ni steel for liquid hydrogen according to claim 1, wherein an effective crystal grain size is 2.0 μm or more and 12.0 μm or less. The plate thickness is 12 mm or more and 40 mm or less,
Yield stress at room temperature is 590 MPa or more, 710 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 690 MPa or more and 810 MPa or less.