JP6328968B2 - Spheroidal graphite cast iron and method for producing spheroidal graphite cast iron - Google Patents

Description

  The present invention relates to a spheroidal graphite cast iron used for, for example, cast iron pipes, and a method for producing the spheroidal graphite cast iron.

  Common spheroidal graphite cast irons include high toughness types such as JIS standards FCD350, FCD400, and FCD450, and high strength types such as FCD600, FCD700, and FCD800. For ductile cast iron mainly cast for water pipes, FCD450 (tensile strength of 450 MPa or more and elongation of 10% or more) having a relatively good balance between strength and elongation is selected. On the other hand, high hardness materials are used for transporting slurry-like materials and hard materials with high wear resistance, and high strength and high strength materials are used for materials such as automobile parts and construction machinery parts. Selected.

  In the casting of ductile cast iron, the matrix (base) of the as-cast structure is pearlite. For example, when ductile iron pipes are cast by centrifugal mold casting, the cooling rate during casting is high. Has a plaque structure in which a large amount of metastable cementite is crystallized simultaneously with stable graphite. Since this cementite becomes an obstructive factor for elongation, annealing for the purpose of decomposition of cementite and ferrite formation of the matrix is required in order to achieve both strength and elongation required for the FCD450 type.

  Ductile cast iron is generally annealed in a continuous furnace or a batch furnace. In the annealing furnace, the ductile cast iron is heated to an austenitizing temperature range or higher (870 ° C. or higher). As a result, cementite is completely decomposed and the base structure is austenitized. This decomposition of cementite depends on the processing temperature and processing time, and the processing time can be shortened as the processing temperature is higher, while the processing time is longer as the processing temperature is lower. In an annealing furnace, uniform temperature control in the furnace is often difficult. For this reason, in order to make a cementite austenite reliably, it is necessary to determine processing temperature and processing time.

  When the austenite of the base structure is completed, in order to precipitate ferrite from this austenite, the temperature range near the eutectoid transformation point (about 680 to 750 ° C.) is maintained for a certain time, or the vicinity of this eutectoid transformation point is gradually cooled. Heat treatment is performed. The ferrite precipitation amount is determined by the holding time and the cooling rate at this time. That is, as the holding time is longer or the cooling rate is lower, the ferrite precipitation amount is increased. On the other hand, as the holding time is shorter or the cooling rate is higher, the ferrite precipitation amount is reduced, and the matrix is mainly pearlite.

  In this heat treatment, it is difficult to finely control the amount of ferrite and pearlite by strictly controlling the temperature. Therefore, annealing is basically performed under conditions mainly composed of ferrite to ensure toughness. .

  When ductile cast iron is required to have a high strength type such as FCD600, FCD700, and FCD800, the matrix needs to be made pearlite. This pearlite method may be performed only by controlling the heat treatment conditions, or may be added with Mn, Cr, Cu, Sn or the like that promotes pearlite. The general spheroidal graphite cast iron produced in this way has a yield ratio (yield / tensile strength) of about 0.5 to 0.6 regardless of its strength, and does not improve compared to FCD450. .

  In recent years, spheroidal graphite cast iron used for materials of automobile parts and construction equipment parts is required to have not only high strength but also high proof stress, that is, high proof stress ratio (0.7 or more). In order to improve the yield strength ratio, special heat treatment such as austempering, quenching or tempering for the purpose of forming low carbon martensite may be performed. In addition, as shown in Patent Document 1, the matrix may be improved by adding rare metals such as Ni.

Japanese Patent No. 3823347

  The annealing furnace generally used is difficult to strictly control the temperature as described above, and there is a problem that the heat treatment cost increases when applied to the special heat treatment. Moreover, since rare metals, such as Ni, are expensive, there also exists a problem which leads to soaring material cost.

  Therefore, an object of the present invention is to construct a spheroidal graphite cast iron having high strength and high yield strength without performing special heat treatment and without using an expensive rare metal.

  In order to solve the above-described problems, the present invention is based on weight%, C: 3.20 to 4.00%, Si: 1.40 to 3.00%, Mg: 0.02 to 0.08%, Cr : 0.01 to 0.20%, Mn: 1.80 to 3.00%, Cu: 0.50 to 2.00%, the balance from Fe and inevitable impurities Thus, a spheroidal graphite cast iron having an area ratio of pearlite in the base structure after annealing of 80% or more and an area ratio of undecomposed cementite in the range of 5 to 15% was formed.

  Here, the area ratio of pearlite means the ratio (%) of the area of pearlite when the area of the matrix in the field of view of a predetermined size is 100%, and the area ratio of cementite is a predetermined area ratio. This means the percentage (%) of the cementite area when the entire area of the size field of view is 100%.

  Next, the reason why the content of each alloy element is limited to the above range will be described.

  C is contained in an amount of at least 3.20% in order to ensure the amount of graphite and castability (fluidity of the molten metal) necessary for the present invention. On the other hand, if the content is too high, crystallization of graphite becomes excessive and high strength cannot be obtained, so the upper limit was made 4.00%.

  Si is contained in an amount of at least 1.40% in order to ensure the effect of increasing the fluidity of the molten metal and the effect of promoting the crystallization of graphite. On the other hand, if the content is too high, the crystallization of graphite becomes excessive and the effect of suppressing the pearlite formation of the matrix structure becomes large and high strength cannot be obtained, and roughness such as pin holes occurs on the outer surface of the product Therefore, the upper limit was made 3.00%.

  Mn is an element effective for fixing and detoxifying S, making pearlite stably present, and improving the strength of pearlite, and is contained at least 1.80% in order to sufficiently obtain the effect. I did it. On the other hand, if the content is too high, cementite remains remarkably and the strength and elongation decrease, so the upper limit was made 3.00%.

  Mg is an element necessary for spheroidizing graphite, and is contained at least 0.02% in order to sufficiently obtain the effect. On the other hand, if the content is too high, the improvement of the effect cannot be seen so much, so the upper limit was made 0.08%.

  Cu, like Mn, is an element effective for stably presenting pearlite, and is contained in an amount of at least 0.50% in order to sufficiently obtain the effect. On the other hand, even if the content is increased more than necessary, the effect is limited, so the upper limit was made 2.00%.

  Usually, Cr is inevitably contained in an amount of 0.01% or more, but the effect is small if the content is 0.20% or less.

  In addition to the above alloy elements, unavoidable impurities such as P and S are contained, but the smaller the content, the better. For example, it is preferable that P is 0.08% or less and S is 0.015% or less.

  Thus, each alloy element has a sufficient pearlite area ratio (80% or more) by containing Mn and Cu within the above concentration range, in particular, within the above concentration range, in which pearlite structure is stably present. At the same time, it is possible to obtain spheroidal graphite cast iron in which the area ratio of undecomposed cementite is within a predetermined area ratio range (5 to 15%). Spheroidal graphite cast iron cast by adjusting the concentration of each alloy element in this way does not require special heat treatment such as austempering treatment, and has both strength and proof strength in a good balance. 7 or more) can be used without problems in applications such as materials for automobile parts and construction equipment parts. In addition, since relatively inexpensive Mn and Cu are used instead of expensive rare metal Ni or the like, the manufacturing cost can be reduced.

  Alternatively, by weight, C: 3.20 to 4.00%, Si: 1.40 to 3.00%, Mg: 0.02 to 0.08%, Cr: 0.01 to 0.20% In addition, Mn: 1.80 to 3.00%, Cu: 0.90 to 2.00%, the balance is made of Fe and inevitable impurities, the pearlite in the base structure after annealing Spheroidal graphite cast iron having an area ratio of 80% or more and an area ratio of undecomposed cementite in the range of 5 to 15% can also be configured.

  In this way, by increasing the lower limit of the Cu content, the stability of pearlite is further increased, and therefore an improvement in strength can be expected.

  In each of the above structures, it is preferable that the graphite crystallized in the base structure is in a fine state.

  As described above, by crystallization of graphite having a fine size (for example, 15.0 μm or less) in the matrix, the spheroidal graphite cast iron can be further enhanced in strength and strength.

  Moreover, the manufacturing method of the spheroidal graphite cast iron according to the present invention is, by weight, C: 3.20 to 4.00%, Si: 1.40 to 3.00%, Mg: 0.02 to 0.08% , Cr: 0.01 to 0.20%, Mn: 1.80 to 3.00%, Cu: 0.50 to 2.00%, the balance is Fe and unavoidable After casting a semi-finished product having a predetermined shape at a cooling rate of 2.0 to 8.0 ° C./second using a molten metal made of impurities and holding the semi-finished product within a temperature range of 900 to 1100 ° C. for 5 to 30 minutes. Nodular cast iron is produced by cooling at a cooling rate of 1 to 8 ° C./min.

  As described above, by making the range of the content of each alloy element as described above, it is possible to produce spheroidal graphite cast iron having both high strength and high yield strength. In addition, since the above heat treatment does not require strict temperature control, the heat treatment can be performed using a general annealing furnace.

  In this production method, when pouring the molten metal into a mold, an Fe-Si inoculum containing 45 to 75% by weight of Si is inoculated with 0.1 to 0.5% by weight of the molten metal. Is preferred.

  In this way, the number of graphite grains crystallized in the base structure can be increased, and high strength and proof stress can be obtained more reliably.

  According to the present invention, the melt of spheroidal graphite cast iron includes Mn and Cu within a predetermined concentration range, thereby providing high strength and proof strength (proof strength ratio of 0.7 or more) without performing special heat treatment. Spheroidal graphite cast iron can be constructed. Moreover, since relatively inexpensive Mn and Cu are used instead of expensive rare metals such as Ni, the material cost can be reduced.

The micrograph of the material structure of spheroidal graphite cast iron is shown, (a) is Example 1, (b) is Example 2, and (c) is Example 3. The micrograph of the material structure of spheroidal graphite cast iron is shown, (a) is Example 4, (b) is Example 5. The micrograph of the material structure of spheroidal graphite cast iron is shown, (a) is Comparative Example 1, (b) is Comparative Example 2, and (c) is Comparative Example 3.

  Prior to the characteristic evaluation experiment of the spheroidal graphite cast iron according to the present invention, spheroidal graphite cast iron serving as an example of the present invention and a spheroidal graphite cast iron serving as a comparative example of the present invention were respectively cast. Table 1 shows the chemical components of the spheroidal graphite cast iron melts according to the examples and comparative examples (the remainder not shown in this table is inevitable impurities such as Fe, P, and S). The chemical composition data shown in Table 1 is a value obtained by analyzing a white birch sample prepared from each molten metal with an emission spectroscopic analyzer. As shown in this table, the elements of C, Si, Mg, and Cr are substantially the same in the examples and comparative examples, while the elements of Mn and Cu are different in the examples and comparative examples. Preparation is performed so as to be in the concentration range.

  In this embodiment, each molten metal having the chemical composition shown in Table 1 was poured into a mold having a predetermined shape at 1300 ° C. to cast a semi-finished product (as-cast product) having a predetermined shape. In the case of this pouring, 0.1-0.5 wt% pouring was inoculated with an Fe-Si inoculum containing 45 to 75 wt% Si. For example, when casting a tubular semi-finished product by pouring into a cylindrical mold of a mold centrifugal casting apparatus, the cooling rate is about 2.0 to 8.0 ° C./second. This cooling rate varies depending on the shape of the mold, the amount of pouring, the thickness of the cast product, etc. For example, when casting a tubular semi-finished product having a thickness of 12.0 mm, the cooling rate is 4.0-6. 0 ° C./second.

Next, this semi-finished product was annealed under the following annealing conditions to finish into a spheroidal graphite cast iron as a product.
(Annealing conditions)
-Heating temperature: 900-1100 ° C
・ Heat holding time: 5 to 30 minutes ・ Cooling rate: 1 to 8 ° C./minute

  Test specimens were collected from each of the castings thus obtained, and each was subjected to structure observation after corrosion treatment with 5% nital liquid and measurement of mechanical properties (tensile strength, proof stress). FIG. 1 to FIG. 3 show the results of tissue observation with a microscope, Table 2 shows the results of image analysis of the tissue surface, and Table 3 shows the results of measurement of mechanical properties. Image analysis is performed at the center of the casting in the thickness direction.

  As shown in Table 2, by setting the contents of Mn and Cu within a predetermined range (Mn: 1.80 to 3.00%, Cu: 0.5 to 2.0%), a high pearlite area ratio A cementite area ratio within a predetermined range (5 to 15%) (95% or more) could be secured. If this pearlite area ratio is 80% or more, sufficient strength can be secured (see Examples 1 to 5). On the other hand, when the contents of Mn and Cu were below the predetermined range, the pearlite area ratio and the cementite area ratio were reduced as compared with Examples 1 to 5 (see Comparative Examples 1 to 3).

  Here, the area ratio of pearlite means the ratio (%) of the area of pearlite when the area of the matrix in the field of view of a predetermined size is 100%, and the area ratio of cementite or graphite is The ratio (%) of the area of cementite or graphite when the total area of the field of view of a predetermined size is 100%. In addition, regarding graphite, measurement is performed except for particles having a particle size of 3 μm or less.

  In addition, as shown in Table 3, by setting the contents of Mn and Cu within the above predetermined range, a high tensile strength of 700 MPa or more (equivalent to FCD700 or more) is secured, and a high value of 0.7 or more. It was confirmed that the yield strength ratio (equivalent to FCD450 or higher) could be achieved (see Examples 1 to 5). In addition, when the Mn content is 2.6% or more, the tendency that the tensile strength is slightly reduced due to the increase in the cementite area ratio was confirmed (see Examples 3 to 5), but high. The yield strength ratio was sufficiently secured. On the other hand, when the contents of Mn and Cu are below the predetermined range, the yield strength is significantly reduced as compared with Examples 1 to 5, and a sufficient yield ratio (0.7 or more) cannot be ensured. It was confirmed (see Comparative Examples 1 to 3).

  As described above, the content of each additive element in the molten metal, particularly the contents of Mn and Cu are within a predetermined range (Mn: 1.80 to 3.00%, Cu: 0.50 to 2.00%). No special heat treatment is performed on the as-cast product by setting the area ratio of pearlite in the base structure after annealing to 80% or more and the area ratio of undecomposed cementite to 5 to 15%. In addition, spheroidal graphite cast iron having both high strength and high yield strength and having a yield strength ratio of 0.7 or more can be configured without using expensive rare metals such as Ni.

  In the above embodiment, the Fe-Si type inoculum was used as the inoculum, but Bi type inoculation containing 0.5 to 5.0% by weight of Bi and 45 to 75% by weight of Si, respectively. An agent can also be used. In addition, these inoculants are used to crystallize more graphite, but it is acceptable to omit the use of the inoculants as long as necessary proof stress is ensured.

Claims (5)

In mass %, C: 3.20 to 4.00%, Si: 1.40 to 3.00%, Mg: 0.02 to 0.08%, Cr: 0.01 to 0.20% further Mn: 1.80~3.00%, Cu: contains at 0.50 to 2.00% scope, balance of Fe and unavoidable impurities, the area ratio of pearlite in groups ground weave 80 % Or more, and the area ratio of undecomposed cementite is in the range of 5 to 15%. Spheroidal graphite cast iron. In mass %, C: 3.20 to 4.00%, Si: 1.40 to 3.00%, Mg: 0.02 to 0.08%, Cr: 0.01 to 0.20% further Mn: 1.80~3.00%, Cu: contains at 0.90 to 2.00% scope, balance of Fe and unavoidable impurities, the area ratio of pearlite in groups ground weave 80 % Or more, and the area ratio of undecomposed cementite is in the range of 5 to 15%. Spheroidal graphite cast iron. The spheroidal graphite cast iron according to claim 1 or 2, wherein the graphite crystallized in the matrix structure is refined to 15.0 µm or less . In mass %, C: 3.20 to 4.00%, Si: 1.40 to 3.00%, Mg: 0.02 to 0.08%, Cr: 0.01 to 0.20% Further, Mn: 1.80 to 3.00%, Cu: 0.50 to 2.00% contained in the range, the balance is Fe and unavoidable impurities, the cooling rate 2.0 ~ After casting a semi-finished product of a predetermined shape at 8.0 ° C./second, holding the semi-finished product within a temperature range of 900 to 1100 ° C. for 5 to 30 minutes, and then cooling at a cooling rate of 1 to 8 ° C./min .
A method for producing spheroidal graphite cast iron in which the area ratio of pearlite in the base structure is 80% or more and the area ratio of undecomposed cementite is in the range of 5 to 15% . The spherical shape according to claim 4, wherein when pouring the molten metal into a mold, 0.1 to 0.5% by mass of a Fe-Si inoculum containing 45 to 75% by mass of Si is inoculated. A method for producing graphite cast iron.