JP6328547B2 - Manufacturing method of large cast steel product and large cast steel product - Google Patents

Description

  The present invention relates to a method for producing a large cast steel product and a large cast steel product.

  Radioactive waste generated at nuclear facilities is stored in a container with a shielding function at the time of disposal. It is reasonable to make this storage container by reusing scrap generated in nuclear facilities. Moreover, the said storage container has a shielding body inside, and the thick large-sized cast-steel goods manufactured from the said scrap are used for this shielding body.

  The large cast steel product used for the shield is required to have high toughness in order to prevent radiation from leaking from the storage container. Since the toughness of a steel material decreases due to embrittlement due to impurities in the steel material, a cleaning material from which impurities are removed is generally used for members that require high toughness. However, when the scrap as a raw material contains a high concentration of impurities, there is an inconvenience that the refining time for removing the impurities to a low level becomes longer, and that the amount of refining slag increases and extra waste is generated. Occurs. Also, some scraps may contain impurities that cannot be removed by ordinary oxidation refining.

  Therefore, a means for ensuring toughness is required even when a high concentration of impurities is contained in steel obtained by melting scrap. As a method for producing a cast steel product having excellent toughness, a cast steel product having a two-phase structure composed of retained austenite and ferrite, acicular bainite, acicular ferrite, bainite-type ferrite, and, if necessary, at least one of martensite. There is provided a heat treatment method for adjusting the temperature (see Japanese Patent Publication No. 11-537813). However, when quenching an extremely thick large cast steel product having a thickness of 80 mm or more, it is necessary to accelerate the cooling by water cooling or the like in order to cool it to a critical temperature that suppresses the precipitation of pearlite. When the member is water-cooled, cracking occurs due to thermal stress and transformation stress during cooling.

  For low alloy steels, a method for increasing toughness by heat treatment has been devised (see JP-A-53-85710). However, it is known that temper embrittlement is remarkable in Ni—Cr steel when an impurity element such as P is excessive. Therefore, in the heat treatment described in the above publication, it may be difficult to ensure toughness for steel materials with many impurities.

  Here, the toughness of the steel material excessively containing an impurity element such as P is lowered because the impurity element segregates at the grain boundary and weakens the grain boundary. In order to suppress the segregation of impurity elements to the crystal grain boundaries, a method of speeding up the cooling after heating in the heat treatment process is generally used. It cannot be cooled at a speed that prevents field movement.

  Therefore, it is conceivable to reduce the amount of local impurity segregation in each grain boundary and alleviate grain boundary embrittlement by making the crystal grain size finer and increasing the grain boundary density in the steel. In order to reduce the crystal grain size of ferrite or pearlite in this way, it is necessary to make the austenite grains fine. In general, it is known that austenite grain refinement can be achieved by repeating austenite formation. However, when the concentration of an impurity element such as P is high, a phenomenon in which the grain is not refined by taking over the initial structure even when austenite is formed is promoted. Since the grain size immediately after casting is coarse, if this grain memory occurs, it becomes difficult to make austenite grains finer, and relaxation of embrittlement at grain boundaries cannot be achieved.

Japanese National Patent Publication No. 11-537813 JP-A-53-85710

  This invention is made | formed based on the above situations, and aims at provision of the large-sized cast-steel goods excellent in toughness, and its manufacturing method.

  As a result of intensive studies, the present inventors preliminarily treated the cast steel product before the tempering treatment to have a metal structure mainly containing ferrite and pearlite and having a residual austenite content of 1% by volume or less. The inventors have found that the crystal grain size obtained by the treatment can be refined, and have reached the present invention.

  That is, the invention made to solve the above problems is a method for producing a large cast steel product comprising a step of casting scrap as a raw material, and a step of tempering a cast after the casting step, The composition of the product is C (carbon): 0.1 mass% or more and 0.25 mass% or less, Si (silicon): 0.01 mass% or more and 0.8 mass% or less, Mn (manganese): 0.3 mass % To 2% by mass, Cr (chromium): 0.01% to 0.4% by mass, Ni (nickel): 0.01% to 0.5% by mass, Mo (molybdenum): 0. It contains a basic component of 01% by mass or more and 0.5% by mass or less, and the balance is Fe (iron) and inevitable impurities. Before the tempering step, the main structure is ferrite and pearlite, and the residual austenite content is 1 Cast the casting so that it is less than volume%. The preliminary treatment step includes a step of heating the casting to an austenitizing temperature or higher, and a step of cooling the cast after heating to 300 ° C. or lower at a rate of 60 ° C./h or lower. It is characterized by that.

  The method for producing the large cast steel product is mainly composed of ferrite and pearlite by heating the casting to the austenitizing temperature or higher before tempering and then cooling the casting to 300 ° C. or lower at a rate of 60 ° C./h or lower. The casting is pretreated so that the structure has a residual austenite content of 1% by volume or less. Thereby, even when impurity element density | concentrations, such as P, are high, grain memory can be suppressed and the crystal grain diameter after a refining process can be refined | miniaturized. Therefore, the manufacturing method of the said large cast steel goods can manufacture the large cast steel goods which have the outstanding toughness, using the scrap containing many impurities as a raw material. The “main structure” means a structure that occupies 99% by volume or more of the entire metal structure. The “large cast steel product” refers to a cast steel product having a thickness of 50 mm or more or a weight of 1000 kg or more.

  Moreover, another invention made | formed in order to solve the said subject is C: 0.1 mass% or more and 0.25 mass% or less, Si: 0.01 mass% or more and 0.8 mass% or less, Mn: 0.00%. 3 mass% to 2 mass%, Cr: 0.01 mass% to 0.4 mass%, Ni: 0.01 mass% to 0.5 mass%, Mo: 0.01 mass% to 0.5 mass It is a large cast steel product containing a basic component of less than mass%, the balance being the composition of Fe and inevitable impurities, the main structure being ferrite and pearlite, and the crystal grain size being 6 or more and 10 or less.

  The large cast steel product has ferrite and pearlite as a main structure and has a crystal grain size of 6 to 10, so that grain boundary embrittlement is mitigated by a fine crystal grain size. Therefore, the large cast steel product is excellent in toughness.

  Accordingly, the large cast steel product can be suitably used as a shield for radioactive substance storage containers.

  The “crystal grain size” means a grain size number calculated in accordance with JIS-G0551 (2013).

  As described above, the method for producing a large cast steel product of the present invention can produce a large cast steel product having excellent toughness, and the large cast steel product of the present invention has excellent toughness.

[Manufacturing method of large cast steel products]
Hereinafter, an embodiment of a method for producing a large cast steel product according to the present invention will be described.

  The manufacturing method of the large cast steel product mainly includes a step of casting scrap as a raw material, a step of pre-processing the cast before the tempering step, and a step of tempering the cast after the casting step. In addition, you may perform the process of machining a casting after a tempering process as needed.

<Casting process>
In the casting process, first, scrap as a raw material is melted using a high-frequency melting furnace, an electric furnace, a converter, or the like.

  The scrap used as a raw material in the manufacturing method of the large cast steel product is not particularly limited as long as it is a scrap mainly composed of steel. However, scrap generated due to dismantling of the apparatus in a nuclear facility is preferably used. According to the method for producing a large cast steel product, a large cast steel product having excellent toughness can be obtained even when scrap containing a large amount of impurities that are generated in a nuclear facility is used.

  The composition of the molten steel in the manufacturing method of the large cast steel product is C: 0.1% by mass or more and 0.25% by mass or less, Si: 0.01% by mass or more and 0.8% by mass or less, Mn: 0.3% by mass 2 mass% or less, Cr: 0.01 mass% or more and 0.4 mass% or less, Ni: 0.01 mass% or more and 0.5 mass% or less, Mo: 0.01 mass% or more and 0.5 mass% or less The balance is Fe and inevitable impurities.

  The lower limit of the C content of the large cast steel product is 0.1% by mass, and preferably 0.15% by mass. On the other hand, the upper limit of the C content of the large cast steel product is 0.25% by mass, and preferably 0.22% by mass. If the C content of the large cast steel product is smaller than the above lower limit, the strength of the large cast steel product may be insufficient, or a preferable metal structure may not be ensured in the tempering process. Conversely, if the C content of the large cast steel product exceeds the above upper limit, the toughness may be reduced. By setting the C content of the large cast steel product within the above range, the toughness of the large cast steel product can be appropriately ensured.

  The lower limit of the Si content of the large cast steel product is 0.01% by mass, preferably 0.1% by mass. On the other hand, the upper limit of the Si content of the large cast steel product is 0.8% by mass, and preferably 0.5% by mass. If the Si content of the large cast steel product is smaller than the lower limit, the strength may be insufficient due to insufficient deoxidation. Conversely, if the Si content of the large cast steel product exceeds the upper limit, casting defects may increase due to reverse V segregation or the like. By setting the Si content of the large cast steel product within the above range, the strength of the large cast steel product can be appropriately improved.

  The lower limit of the Mn content of the large cast steel product is 0.3% by mass, and preferably 0.5% by mass. On the other hand, the upper limit of the Mn content of the large cast steel product is 2% by mass, and preferably 1.5% by mass. If the Mn content of the large cast steel product is smaller than the lower limit, the strength may be insufficient. Conversely, if the Mn content of the large cast steel product exceeds the upper limit, coarse sulfide inclusions are generated in the segregated part due to the promotion of reverse V segregation, and the toughness may be reduced. By setting the Mn content of the large cast steel product within the above range, the strength of the large cast steel product can be appropriately improved.

  The lower limit of the Cr content of the large cast steel product is 0.01% by mass, preferably 0.05% by mass. On the other hand, the upper limit of the Cr content of the large cast steel product is 0.4 mass%, preferably 0.2 mass%. If the Cr content of the large cast steel product is smaller than the above lower limit, the strength and toughness may be insufficient. Conversely, if the Cr content of the large cast steel product exceeds the above upper limit, reverse V segregation may be promoted. By setting the Cr content of the large cast steel product within the above range, the strength and toughness of the large cast steel product can be appropriately ensured.

  The lower limit of the Ni content of the large cast steel product is 0.01% by mass, preferably 0.1% by mass. On the other hand, the upper limit of the Ni content of the large cast steel product is 0.5% by mass, preferably 0.3% by mass. If the Ni content of the large cast steel product is smaller than the above lower limit, the strength and toughness may be insufficient. Conversely, if the Ni content of the large cast steel product exceeds the upper limit, the cost effectiveness is reduced. By setting the Ni content of the large cast steel product within the above range, the strength and toughness of the large cast steel product can be appropriately ensured.

  The lower limit of the Mo content of the large cast steel product is 0.01% by mass, preferably 0.05% by mass. On the other hand, the upper limit of the Mo content of the large cast steel product is 0.5% by mass, and preferably 0.2% by mass. If the Mo content of the large cast steel product is smaller than the above lower limit, the strength and toughness may be insufficient. Conversely, when the Mo content of the large cast steel product exceeds the upper limit, the cost effectiveness is reduced. By setting the Mo content of the large cast steel product within the above range, the strength and toughness of the large cast steel product can be appropriately ensured.

  The large cast steel product contains Fe and inevitable impurities in the balance other than the basic components described above. Inevitable impurities include, for example, P (phosphorus), S (sulfur), Cu (copper), Sn (tin), As (arsenic), and Pb (lead) brought in depending on the conditions of raw materials, materials, manufacturing equipment, and the like. , Mixing of elements such as Ti (titanium) is allowed. It is also effective to positively contain these elements or other elements or compositions, and the properties of the cast steel material are further improved depending on the elements and the types of compositions contained.

  As an upper limit of the content rate of P which is an inevitable impurity of the said large sized steel product, 0.05 mass% is preferable, 0.03 mass% is more preferable, 0.01 mass% is further more preferable. When the P content of the large cast steel product exceeds the above upper limit, there is a risk of promoting grain boundary fracture due to grain boundary sieving. Moreover, the minimum of the content rate of P which is an unavoidable impurity of the said large-sized cast-steel goods is 0.001 mass%, for example, and is 0.002 mass% depending on a scrap composition.

  As an upper limit of S content which is an unavoidable impurity of the said large-sized cast-steel goods, 0.03 mass% is preferable and 0.01 mass% is more preferable. When the S content of the large cast steel product exceeds the above upper limit, ductility and toughness may be reduced. Moreover, the minimum of the content rate of S which is an unavoidable impurity of the said large-sized cast-steel goods is 0.001 mass%, for example, and is 0.005 mass% depending on a scrap composition.

  As an upper limit of Cu content which is an unavoidable impurity of the said large-sized cast-steel goods, 0.3 mass% is preferable and 0.05 mass% is more preferable. When the Cu content of the large cast steel product exceeds the above upper limit, hot brittleness may increase. Moreover, the minimum of the content rate of Cu which is an unavoidable impurity of the said large-sized cast-steel goods is 0.001 mass%, for example, and is 0.01 mass% depending on a scrap composition.

  After the vacuum treatment, the molten steel is poured into a mold, and a cast (steel ingot) is obtained by casting (casting). As this casting method, a known general method can be used. For example, a method of charging molten steel from an injection tube below the mold can be employed.

<Pretreatment process>
In the pretreatment process, the casting is pretreated before the refining process described later. Specifically, the cast product is processed so that ferrite and pearlite are main structures and the retained austenite content becomes a predetermined value or less.

  The upper limit of the retained austenite content after the pretreatment step is 1% by volume, preferably 0.5% by volume, and more preferably 0.1% by volume. If the retained austenite content exceeds the above upper limit, the proportion of ferrite and pearlite in the composition of the casting is lowered, and there is a possibility that the crystal grain size cannot be reduced.

  This preliminary treatment step includes a step of heating the casting to the austenitizing temperature or higher and a step of cooling the heated cast to 300 ° C. or lower at a rate of 60 ° C./h or lower.

(Heating process)
In the heating step, the casting is heated to the austenitizing temperature or higher, and the structure is austenitized. Here, the austenitizing temperature means the Ac3 transformation point (830 ° C.).

  As a minimum of heating temperature in this heating process, 850 ° C is preferred, 900 ° C is more preferred, and 930 ° C is still more preferred. On the other hand, the upper limit of the heating temperature is preferably 1000 ° C., more preferably 990 ° C., and further preferably 980 ° C. If the heating temperature is lower than the lower limit, austenitization may be insufficient. On the other hand, when the heating temperature exceeds the upper limit, the capital investment and running cost may be excessive.

  As a minimum of heating time in this heating process, 1 hour is preferred and 2 hours is more preferred. On the other hand, the upper limit of the heating time is preferably 5 hours, and more preferably 4 hours. If the heating time is smaller than the lower limit, there is a possibility that the inside of the casting cannot be heated uniformly. Conversely, if the heating time exceeds the upper limit, the production efficiency of the large cast steel product obtained may be reduced.

  As a minimum of a heating rate in this heating process, 10 ° C / h is preferred, 20 ° C / h is more preferred, and 30 ° C / h is still more preferred. On the other hand, the upper limit of the heating rate is preferably 100 ° C./h, more preferably 80 ° C./h, and further preferably 50 ° C./h. If the heating rate is smaller than the lower limit, the production efficiency of the resulting large cast steel product may be reduced. On the other hand, when the heating rate exceeds the upper limit, there is a risk that capital investment and running cost will be excessive.

(Cooling process)
In the cooling step, the casting heated in the heating step is furnace-cooled to a constant temperature at a constant speed. In addition, after the primary cooling by this furnace cooling, it is preferable to further cool the casting by air cooling to room temperature. As the air cooling method, for example, a fan can be used in addition to natural slow cooling.

  As a minimum of a cooling rate in primary cooling of this cooling process, 10 ° C / h is preferred, 20 ° C / h is more preferred, and 30 ° C / h is still more preferred. On the other hand, the upper limit of the cooling rate is 60 ° C./h, more preferably 55 ° C./h, and still more preferably 50 ° C./h. If the cooling rate is less than the lower limit, the production efficiency of the resulting large cast steel product may be reduced. Conversely, when the cooling rate exceeds the upper limit, the amount of retained austenite may increase.

  As a minimum of the cooling temperature in the primary cooling of this cooling process, 180 ° C is preferred, 200 ° C is more preferred, 220 ° C is still more preferred, and 240 ° C is especially preferred. On the other hand, the upper limit of the cooling temperature is 300 ° C., more preferably 280 ° C., and further preferably 260 ° C. If the cooling temperature is lower than the lower limit, the capital investment and running cost may be excessive. Conversely, if the cooling temperature exceeds the upper limit, the amount of retained austenite may increase.

  The secondary cooling in the main cooling step is performed by cooling the casting after the primary cooling to room temperature. Here, room temperature is 20 degreeC or more and 30 degrees C or less, for example.

<Refining process>
In the tempering step, the casting subjected to the preliminary treatment is tempered (final heat treatment). Specifically, the casting is normalized. Further, after normalization, tempering may be performed to remove residual stress.

(Normalizing process)
In the normalizing process, the casting is first austenitized, and after the temperature of the casting becomes uniform after austenitizing, the casting is cooled.

  The austenitization in the normalizing step is maintained at a certain temperature above the austenitizing temperature (Ac3 transformation point). In addition, when manufacturing large cast steel products as in the method for manufacturing large cast steel products, a temperature difference occurs between the inside and outside of the material during heating. It is necessary to hold for a certain period of time. This holding time depends on the diameter of the steel material and needs to be longer for larger materials.

  As a minimum of heating temperature in a regularizing process, 830 ° C is preferred, 850 ° C is more preferred, and 900 ° C is still more preferred. On the other hand, the upper limit of the heating temperature is preferably 950 ° C, more preferably 940 ° C, and further preferably 930 ° C. If the heating temperature is lower than the lower limit, the crystal grains may grow secondary and become coarse. On the other hand, when the heating temperature exceeds the upper limit, the capital investment and running cost may be excessive.

  The lower limit of the heating time in the normalizing step is preferably 1 hour and more preferably 2 hours. On the other hand, the upper limit of the heating time is preferably 5 hours, and more preferably 4 hours. If the heating time is smaller than the lower limit, there is a possibility that the inside of the casting cannot be heated uniformly. Conversely, if the heating time exceeds the upper limit, the production efficiency of the large cast steel product obtained may be reduced.

  The lower limit of the heating rate in the normalizing step is preferably 10 ° C./h, more preferably 30 ° C./h, further preferably 40 ° C./h. On the other hand, the upper limit of the heating rate is preferably 100 ° C./h, more preferably 80 ° C./h, and further preferably 60 ° C./h. If the heating rate is smaller than the lower limit, the production efficiency of the resulting large cast steel product may be reduced. On the contrary, when the heating rate exceeds the upper limit, there is a possibility that the inside of the casting cannot be heated uniformly.

  Next, the austenitized casting is cooled. In the case of large castings with a weight of several tons to several tens of tons, water cooling causes cracks due to thermal stress and transformation stress during cooling, so cooling after austenitizing can be done by methods such as air cooling that is slower than water cooling. Cooling is preferred.

  The lower limit of the cooling rate in the normalizing step is preferably 10 ° C / h, more preferably 15 ° C / h, and still more preferably 20 ° C / h. If the cooling rate is less than the lower limit, the production efficiency of the resulting large cast steel product may be reduced.

  More specifically, the cooling rate varies depending on the diameter D (mm) of the casting. For example, the cooling rate of air cooling at the D / 4 position is about 300 ° C./h at φ200 mm, about 150 ° C./h at φ500 mm, and about 70 at φ1000 mm. ° C / h.

  The cooling temperature in the normalizing process is, for example, room temperature. If the cooling is insufficient, untransformed retained austenite remains and causes variation in characteristics.

(Tempering process)
In the tempering process, the casting is held again at a constant heating temperature for a certain time after the normalizing process.

  As a minimum of heating temperature in this tempering process, 500 ° C is preferred and 600 ° C is more preferred. On the other hand, the upper limit of the heating temperature is preferably 800 ° C, more preferably 700 ° C. If the heating temperature is lower than the lower limit, the residual stress may not be sufficiently removed. Conversely, when the heating temperature exceeds the upper limit, the steel material is softened due to coarsening of carbides, recovery of dislocation structure, and the like, and there is a possibility that sufficient strength cannot be ensured.

  As a minimum of heating time in this tempering process, 2 hours are preferred and 5 hours are more preferred. On the other hand, the upper limit of the heating time is preferably 20 hours, and more preferably 10 hours. If the heating time is smaller than the lower limit, the residual stress may not be sufficiently removed. Conversely, if the heating time exceeds the upper limit, the production efficiency of the large cast steel product obtained may be reduced.

  As a minimum of a heating rate in this tempering process, 10 ° C / h is preferred and 30 ° C / h is more preferred. On the other hand, the upper limit of the heating rate is preferably 100 ° C./h, more preferably 70 ° C./h. If the heating rate is smaller than the lower limit, the production efficiency of the resulting large cast steel product may be reduced. On the other hand, when the heating rate exceeds the upper limit, there is a risk that capital investment and running cost will be excessive.

<Machining process>
In the machining process, the casting after the tempering process is machined by finishing such as forging, cutting, or grinding as necessary.

<Advantages>
The method for producing the large cast steel product is mainly composed of ferrite and pearlite by heating the casting to the austenitizing temperature or higher before tempering and then cooling the casting to 300 ° C. or lower at a rate of 60 ° C./h or lower. The casting is pretreated so that the structure has a residual austenite content of 1% by volume or less. Thereby, even when impurity element density | concentrations, such as P, are high, grain memory can be suppressed and the crystal grain diameter after a tempering process can be refined | miniaturized. Therefore, the manufacturing method of the said large cast steel goods can manufacture the large cast steel goods which have the outstanding toughness, using the scrap containing many impurities as a raw material.

[Large cast steel products]
The large cast steel product has a composition of C: 0.1 mass% to 0.25 mass%, Si: 0.01 mass% to 0.8 mass%, Mn: 0.3 mass% to 2 mass% Hereinafter, basic components of Cr: 0.01% by mass to 0.4% by mass, Ni: 0.01% by mass to 0.5% by mass, Mo: 0.01% by mass to 0.5% by mass The balance is Fe and inevitable impurities. The large cast steel product has ferrite and pearlite as a main structure, and has a crystal grain size of 6 or more and 10 or less.

<Metallic structure>
The metal structure of the large cast steel product is mainly composed of ferrite and pearlite. Thus, by making ferrite and pearlite into a main structure in a metal structure, the large cast steel product has fine crystal grains to be described later and a certain yield strength. In addition, as a measuring method of the structure fraction (volume%) of each structure, for example, a test piece for microstructural observation is cut out from a cast steel product, the surface of this test piece is mirror-polished, corroded with nital, and is examined with an optical microscope. This can be done by observing.

  Further, the large cast steel product preferably has a retained austenite of 1% by volume or less in the metal structure. The structural fraction of retained austenite can be measured by, for example, analyzing the X-ray crystal structure of the above-described specimen for microstructure observation.

  The lower limit of the crystal grain size of the large cast steel product is No. 6, and No. 7 is preferable. On the other hand, the upper limit of the crystal grain size of the large cast steel product is No. 10, and No. 9 is preferable. If the crystal grain size is smaller than the above lower limit, grain boundary embrittlement is not alleviated by grain memory, and the toughness may be insufficient. On the other hand, if the crystal grain size exceeds the above upper limit, the production cost of the large cast steel product may be excessive, or the production efficiency may be reduced.

<Mechanical properties>
The lower limit of the yield strength of the large cast steel at room temperature is preferably 250 MPa, and more preferably 300 MPa. The lower limit of the tensile strength at room temperature of the large cast steel product is preferably 400 MPa, and more preferably 450 MPa. When the yield strength and tensile strength of the large cast steel product are equal to or higher than the lower limit, the large cast steel product can be suitably used as a radioactive substance shield or the like. Yield strength and tensile strength are values measured by a tensile test based on JIS-Z2241 (2011).

  The lower limit of room temperature elongation of the large cast steel product is preferably 30%, more preferably 35%. The lower limit of the drawing at room temperature of the large cast steel product is preferably 40%, more preferably 50%. When the elongation and drawing of the large cast steel product are equal to or more than the above lower limit, it can be suitably used as a radioactive substance shield or the like. In addition, elongation and drawing are values measured by a tensile test based on JIS-Z2241 (2011).

  The lower limit of the absorbed energy at 0 ° C. of the large cast steel product is preferably 27J, and more preferably 30J. The toughness requested | required of the shielding body of a radioactive substance storage container can be satisfy | filled as the absorbed energy of the said large sized steel product is more than the said minimum. The absorbed energy is a value measured by a Charpy impact test based on JIS-Z2242 (2005).

<Advantages>
The large cast steel product has ferrite and pearlite as a main structure and has a crystal grain size of 6 to 10, so that grain boundary embrittlement is mitigated by a fine crystal grain size. Therefore, the large cast steel product is excellent in toughness. Therefore, the large cast steel product can be suitably used as a shield for a radioactive substance storage container.

[Radioactive substance storage container]
The radioactive substance storage container includes a cylindrical exterior and a shield disposed inside the exterior, and at least the shield is formed from the large cast steel product. The radioactive substance storage container stores the radioactive substance inside the shield.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these Examples.

  First, molten steel having a composition of five steel types A to E shown in Table 1 was melted from scrap as a raw material using a high-frequency melting furnace, and then cast into a 40 kg ingot.

  Next, the cast steel after casting was heated to 950 ° C. above the austenitizing temperature at a heating rate of 40 ° C./h using a heat treatment simulator, and austenitized by holding for 3 hours. Then, the furnace was cooled to 250 ° C. at a cooling rate of 50 ° C./h, further cooled from 250 ° C. to room temperature, and cast steel was pretreated.

  After the preliminary treatment, a test piece having a width of 20 mm, a length of 20 mm, and a thickness of 10 mm was cut out from the cast steel, the surface of the test piece was polished to a mirror finish, then corroded with nital, and the microstructure was observed with an optical microscope. Further, according to the method described in “X-ray analysis handbook (published by Rigaku Corporation, 1998, first edition)”, an X-ray crystal structure analysis of this test piece was performed using “RINT-1500” manufactured by Rigaku Corporation. The results of these observations and analyzes are shown in Table 2. In Table 2, “F” means a ferrite structure, and “P” means a pearlite structure. “-” Means unmeasured.

  The cast steel after the preliminary treatment was tempered using the heat treatment simulator. Specifically, first, the pretreated cast steel was heated to 920 ° C. at a heating rate of 50 ° C./h and held for 3 hours to perform austenitization. Then, it cooled to room temperature with the cooling rate of 30 degrees C / h. This heat treatment condition simulates the state of the center in the plate thickness direction when a large cast steel product having an average thickness of 500 mm or more and a weight of several tons is normalized.

  After the tempering treatment, a tensile test piece, a Charpy impact test piece, and a microstructure observation test piece were cut out from the cast steel, and the tensile properties, toughness, and microstructure were measured.

(Tensile test)
In accordance with JIS-Z2241 (2011), the yield point, tensile strength, elongation, and drawing were measured for each steel type using a No. 4 test piece at room temperature. Table 3 shows the results of the tensile test.

(Charpy impact test)
Based on JIS-Z2242 (2005), the absorption energy was measured for each of four steel types at 0 ° C. using a 2 mmV notch test piece, and the average value was obtained. Table 4 shows the results of the Charpy impact test.

(Microstructure observation)
A test piece having a width of 20 mm, a length of 20 mm, and a thickness of 10 mm was cut out from the cast steel, and the surface of the test piece was polished to a mirror finish, then corroded with nital, and the microstructure was observed with an optical microscope. Moreover, based on JIS-G0551 (2013), the crystal grain size number was evaluated by the comparison method. Table 5 shows the results of the microstructure observation. In Table 5, “F” means a ferrite structure, and “P” means a pearlite structure.

[Consideration of results]
From Table 2, it can be seen that the cast steel is adjusted to a structure composed of ferrite and pearlite by pretreatment before tempering. Further, it can be seen from this test piece that residual austenite is not detected by X-ray crystal structure analysis, that is, the residual austenite is less than 1% by volume. Here, as shown in Tables 3 and 4, good tensile test results and Charpy impact test results have been obtained for all steel types. Therefore, in the pretreatment before tempering, the microstructure of the cast steel is changed to ferrite and pearlite. It can be seen that the strength and toughness are appropriately increased by setting the residual austenite to 1% by volume or less.

  Table 4 also shows that the Charpy absorbed energy at a low temperature (0 ° C.) for all steel types satisfies a certain toughness value (the average value of Charpy absorbed energy is 27 J or more) at which the shield does not break. In particular, it can be confirmed that the toughness value can be achieved even with B and D materials containing a large amount of impurities such as P and S. As shown in Table 5, this is presumed to be because the crystal grains are fine grains in all steel types.

  As described above, when the metal structure before tempering of cast steel is made the main structure of ferrite and pearlite and the retained austenite is 1% by volume or less, the crystal grain size obtained by tempering is refined. It was found that toughness at low temperature (absorbed energy value at Charpy) can be secured above a certain value even when a large amount of impurity elements such as P is contained. That is, according to the present invention, it is possible to obtain a large cast steel product having high toughness while using scraps with many impurities as raw materials.

  As explained above, the method for producing a large cast steel product can provide a large cast steel product having excellent toughness. In addition, the large cast steel product is suitably used as a shield.

Claims (3)

A process of casting scrap as a raw material;
A method for producing a large cast steel product comprising a step of tempering a cast after the casting step,
The composition of the cast steel product is
C: 0.1 mass% or more and 0.25 mass% or less,
Si: 0.01 mass% or more and 0.8 mass% or less,
Mn: 0.3 mass% or more and 2 mass% or less,
Cr: 0.01% by mass or more and 0.4% by mass or less,
Ni: 0.01 mass% or more and 0.5 mass% or less,
Mo: 0.01% by mass or more and 0.5% by mass or less basic components, the balance being Fe and inevitable impurities,
Before the tempering step, comprising ferrite and pearlite as a main structure, comprising a step of pretreating the casting so that the residual austenite content is 1% by volume or less,
Large-sized cast steel, characterized in that the preliminary treatment step includes a step of heating the casting to a temperature above the austenitizing temperature and a step of cooling the cast after heating to 300 ° C. or less at a rate of 60 ° C./h or less. Product manufacturing method. C: 0.1 mass% or more and 0.25 mass% or less,
Si: 0.01 mass% or more and 0.8 mass% or less,
Mn: 0.3 mass% or more and 2 mass% or less,
Cr: 0.01% by mass or more and 0.4% by mass or less,
Ni: 0.01 mass% or more and 0.5 mass% or less,
Mo: 0.01% by mass or more and 0.5% by mass or less of basic components, the balance being the composition of Fe and inevitable impurities,
Large cast steel products mainly composed of ferrite and pearlite and having a grain size of 6 or more and 10 or less. The large cast steel product according to claim 2, which is used as a shield for a radioactive substance storage container.