JP5548856B2 - Ferritic cast iron with excellent heat resistance - Google Patents

Description

  TECHNICAL FIELD The present invention relates to cast iron having excellent heat resistance, and particularly to cast iron that exhibits excellent heat resistance without complicated molten metal treatment and has strength to withstand repeated rapid heating and cooling. .

  Since cast iron is excellent in castability and mechanical properties, it has been widely used as a material for cast products. For cast iron castings, gray cast iron products (JIS G5501), spheroidal graphite cast iron products (JIS G5502), austempered spheroidal graphite cast iron products (JIS G5503), austenitic cast iron products (JIS G5510), ductile cast iron pipes (JIS G5526), ductile cast iron Various types of pipes (JIS G5527), malleable cast iron products (JIS G5705), etc. are specified, and are expected to be used in machine tools, industrial machinery, automobile / vehicle parts, water pipes, molds, and various applications. ing.

  By the way, gray cast iron is sometimes used for casting molds. However, since there is a problem with heat resistance, CV (Compacted Vertical) graphite cast iron is used as a material for molds that require heat resistance. It has been. CV graphite cast iron is a cast iron with flake and spherical graphite in the form (III) shown in JIS G5502 and is known to have excellent mechanical properties, castability and heat resistance. (For example, Non-Patent Document 1).

  However, the production method of CV graphite cast iron is complicated, and in particular, it is necessary to sufficiently manage the spheroidization inhibiting elements contained in the molten metal treatment and the return material (remelting material). It is known that this increases the difficulty of manufacturing CV graphite cast iron and causes variations in casting quality (for example, Non-Patent Document 2).

  For these reasons, it is desired that a cast iron material exhibiting excellent heat resistance and castability can be manufactured at low cost and relatively easily without using CV graphite cast iron.

"Casting Engineering Handbook", April 2004, published by Maruzen Co., Ltd., pages 240-241 "Cast iron production technology", 1993.31, published by Center for Shape Materials, pages 282-292

  The present invention has been made under such circumstances, and the purpose thereof is to stabilize the quality of castings without exhibiting complicated melting material management and molten metal treatment, exhibiting excellent heat resistance, and castability. Another object of the present invention is to provide ferritic cast iron which is excellent as a material for mold castings.

  The ferritic cast iron of the present invention capable of achieving the above object is C: 2.5 to 4.0% (meaning “mass%”; the same applies to the chemical composition), Si: 2 to 3.6% And Mn: 0.1 to 1.0%, Al: 0.4 to 1.5% and Ti: 0.15 to 0.5%, the balance being iron and inevitable impurities And having a gist in that the graphite particles are dispersed. The present invention basically assumes cast iron in which 80% by area or more of the base structure is ferrite. The “base structure” means a structure excluding graphite particles. Further, “graphite particles dispersed” means a state in which the graphite particles exist independently without being connected.

In the ferritic cast iron of the present invention, the number of graphite particles having an equivalent circle diameter of 20 μm or less is preferably 5000 or more per 1 mm ^{2 of} the observation visual field. In the present invention, the “equivalent circle diameter” means the diameter when converted to a circle having the same area by paying attention to the size of the graphite particles.

  According to the present invention, by containing Al, Ti together with C, Si, Mn, the ferrite ratio of the base structure can be effectively increased, the graphite structure can be effectively finely dispersed, and the production of CV graphite cast iron can be achieved. Cast iron with excellent heat resistance and excellent castability could be realized without the management of such complicated melting materials and molten metal treatment. This cast iron is useful as a raw material for casting molds that require heat resistance.

It is the graph which showed the ferrite ratio of a base organization, and the relationship between heat resistance. 3 is a graph showing the relationship between the number of graphite particles and heat resistance. It is the graph which showed the relationship between Al content and a ferrite ratio. It is the graph which showed the relationship between Al content and the number of graphite particles. It is the graph which showed the relationship between Ti content and a ferrite ratio. It is the graph which showed the relationship between Ti content and the number of graphite particles. It is the graph which showed the relationship between Mn content and a ferrite ratio. It is the graph which showed the relationship between Mn content and the number of graphite particles.

  The present inventors have studied from various angles to realize a cast iron that exhibits excellent heat resistance without using CV graphite cast iron. As a result, if the cast iron contains aluminum (Al) and titanium (Ti) as essential components, and the basic components such as C, Si, and Mn are adjusted to an appropriate range, the metal base structure As a result, the present inventors completed the present invention by finding that cast iron in which the ratio of ferrite was increased and graphite particles were finely dispersed (particularly, the equivalent circle diameter was 20 μm or less) was obtained.

  In the field of casting molds, it is known that CV graphite cast iron is an excellent material. However, in order to produce CV graphite cast iron, the control of impurity elements and the control of the temperature and holding time of the molten metal must be performed sufficiently in a narrow range, and the quality of the casting can be stabilized even with a simple processing method. It is difficult.

  On the other hand, repeated heating and cooling of gray cast iron and CV graphite cast iron cause growth phenomena due to decomposition of pearlite structure, formation of graphite, oxidation of graphite and matrix, etc. It was a factor that lowered the sex.

  The present inventors have further studied under the idea that good heat resistance can be realized if the growth phenomenon of cast iron can be suppressed. As a result, it was found that the ferrite ratio in the base structure (hereinafter sometimes simply referred to as “ferrite ratio”) can be increased and graphite can be finely dispersed in a state in which predetermined amounts of Al and Ti are contained. . In cast iron with a high ferrite ratio and finely dispersed graphite, the occurrence of cracks due to repeated heating and cooling was suppressed, and good heat resistance was exhibited.

  The reasons why the above-described effects can be obtained by the cast iron of the present invention are not all clarified, but could probably be considered as follows.

  It is known that when gray cast iron is heated above the eutectoid temperature, the eutectoid structure decomposes into austenite, and when it is cooled, graphite may grow. This is thought to be because cementite and small graphite particles disappear due to heating and decomposition, and larger graphite grows larger due to cooling and precipitation. In the present invention, by containing Al and Ti together with C, Si, and Mn, the decomposition of the eutectoid structure by heating and the formation of the eutectoid structure by cooling are suppressed, so that the growth phenomenon of cast iron is suppressed and heat resistance is reduced. It was thought that the property improved.

  The cast iron of the present invention includes the predetermined amount of basic components as cast iron such as C, Si, Mn and the like, and exhibits the above effect by including Al and Ti as essential components. Reasons for limiting the range of Al and Ti, which are typical components, are as follows.

[Al: 0.4 to 1.5%]
Al is an element effective for increasing the ferrite ratio in the matrix structure and improving heat resistance. In order to exert such an effect, the Al content needs to be at least 0.4%, but if it exceeds 1.5% and the content is excessive, the fluidity of the molten metal is lowered. % Or less. In addition, the preferable minimum of Al content is 0.6%, and even when Mn content mentioned later is comparatively much with this content, favorable heat resistance is securable. Moreover, the upper limit with preferable Al content is about 1.2%, The fluidity | liquidity of a molten metal is favorable at this content, and manufacture of a comparatively complicated shape casting is also possible. However, when it is applied to a casting that is not so complicated, the Al content may be more than 1.2% to about 1.5%.

[Ti: 0.15 to 0.5%]
Ti is an element effective for making graphite particles fine and improving heat resistance. In order to exert such effects, the Ti content needs to be at least 0.15% or more, but if it exceeds 0.5% and excessively contained, the ferrite ratio decreases, resulting in a decrease in heat resistance. Therefore, it should be 0.5% or less. The preferable lower limit of the Ti content is 0.2%. With this content, good heat resistance can be ensured even when the C content is relatively large. Moreover, the preferable upper limit of Ti content is about 0.4%, and even when Mn content mentioned later is comparatively much with this content, favorable heat resistance is securable.

  The reasons for limiting the ranges of C, Si and Mn which are basic components as cast iron are as follows.

[C: 2.5 to 4%]
C is an element effective for crystallizing graphite particles and improving castability. In order to exert such an effect, the C content needs to be at least 2.5% or more, but if it exceeds 4%, the graphite particles become coarse and the heat resistance is lowered. In addition, the minimum with preferable C content is 2.8%, It is good to set it as 3.0% or more more preferably. Moreover, the upper limit with preferable C content is 3.7%, It is good to set it as 3.5% or less more preferably.

[Si: 2 to 3.6%]
Si is an element effective for promoting graphitization and improving castability. In order to exert such an effect, the Si content needs to be at least 2% or more. However, if the Si content exceeds 3.6%, the heat resistance is improved due to the preferential oxidation of Si and the coarsening of the graphite particles. Will be reduced. In addition, the minimum with preferable Si content is 2.4%, It is good to set it as 2.8% or more more preferably. Moreover, the upper limit with preferable Si content is 3.4%, It is good to set it as 3.2% or less more preferably.

[Mn: 0.1 to 1.0%]
Mn is an element effective for improving the mechanical properties of cast iron. In order to exert such an effect, the Mn content needs to be at least 0.1% or more. However, if the content exceeds 1.0% and is excessively contained, the ferrite ratio decreases. % Or less. The preferable upper limit of the Mn content is 0.7%. With this content, even when the Al content is relatively small, the ferrite ratio is large, and heat resistance can be secured. However, when Al is sufficiently contained, good heat resistance can be ensured even if it is over 0.7% to about 1.0%.

  The basic chemical composition of the cast iron of the present invention is as described above, and the balance is substantially made of iron (Fe). Substantially iron means that the cast iron according to the present invention may contain trace elements that do not impede its properties in addition to Fe, and such trace elements include unavoidable impurities such as P and S. .

The ferritic cast iron of the present invention assumes a structure with a ferrite ratio of 80 area% or more, and exhibits the above effect by finely dispersing graphite particles in such a structure. Such a structure is inevitably formed by appropriately adjusting the contents of Al, Ti, C, Si, and Mn and dissolving and solidifying them. The standard for the finely dispersed state of graphite particles is that there are 5,000 or more graphite particles with an equivalent circle diameter of 20 μm or less per 1 mm ² observation field, but the number of graphite particles decreases as the graphite particles become coarser. It becomes. The structure other than ferrite is substantially cementite.

  In addition, when manufacturing a casting using the cast iron of this invention, the methods generally performed until now, such as sand casting, metal mold casting, centrifugal casting, and precision casting, can be adopted.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

[Example 1]
The cast iron (No. 1), CV graphite cast iron (No. 2) and gray cast iron (No. 3) of the present invention, whose chemical composition is shown in Table 1 below, were melted and cast according to a conventional method. About the obtained cast iron casting, the ferrite ratio (ferrite ratio in the base structure) and the heat resistance were investigated. The heat resistance (thermal shock test) conditions at this time are as follows. Then, this thermal shock test was repeated, and the occurrence of cracking of the test piece and the presence or absence of peeling of the surface of the test piece were examined every 5 times, and the heat resistance was evaluated by “the number of thermal shocks at which cracking / peeling occurred”. The ferrite ratio was obtained from the area ratio of the white part in the base structure when the center of the test piece having the same shape and dimension as in the thermal shock test was transversely cut and the cross section was polished and corroded and observed with an optical microscope. In Table 1, the carbon equivalent CE (carbon equivalent as cast iron) obtained from the following formula (1) is shown at the same time.
Carbon equivalent CE = [C] +1/3 [Si] (1)
However, [C] and [Si] indicate the contents (mass%) of C and Si in the cast iron, respectively.

[Thermal shock test conditions]
Temperature increase rate: 15 ° C / min Maximum heating temperature: 850 ° C
Maximum heating temperature holding time: 10 minutes Cooling condition: quenching in water Test piece shape: φ20 × 25 (mm)

  The results are shown in Table 2 below. Further, based on this result, the relationship between the “ferrite ratio” and “the number of thermal shocks at which cracking / peeling occurs” is shown in FIG.

  As is clear from these results, it can be seen that the heat resistance tends to improve as the ferrite ratio increases. In particular, when the ferrite ratio exceeds 80 area%, it can be seen that heat resistance superior to that of CV graphite cast iron (No. 2) is exhibited. This is considered to be due to the fact that the amount of cementite that causes the growth of graphite particles has decreased (the amount of ferrite has increased).

[Example 2]
The cast iron shown in Table 1 was examined for the number of graphite particles and heat resistance. The number of graphite particles is the unit area of the particles in the black part when the center of a test piece having the same shape and dimension as in the thermal shock test of Example 1 is transversely cut and the cross section is polished and observed with an optical microscope without corrosion. It was calculated from the number of hits. At this time, image analysis was performed using image analysis software ("IP-1000" trade name, manufactured by Asahi Kasei Corporation), and the equivalent circle diameter in each case was also determined.

  The results are shown in Table 3 below. Based on this result, the relationship between “the number of graphite particles” and “the number of thermal shocks at which cracking / peeling occurs” is shown in FIG.

As is clear from these results, it can be seen that the heat resistance tends to improve as the number of graphite particles increases (the graphite becomes finer). In particular, when the number of graphite particles exceeds 5,000 particles / mm ² , it can be seen that heat resistance superior to that of CV graphite cast iron is exhibited. In addition, when the cross section of the test piece after the thermal shock test was observed with an optical microscope for the cast iron (No. 1) of the present invention, the loss of graphite due to oxidation compared to the other test pieces (No. 2, 3). It was confirmed that there was little.

[Example 3]
Cast irons having the chemical composition shown in Table 4 below were melted and cast according to a conventional method. About the obtained cast iron casting, in order to evaluate heat resistance, the ferrite ratio in a base structure was investigated by the same method as Example 1.

  The results are shown in Table 5 below. Also, based on this result, the relationship between “Al content” and “ferrite ratio” is shown in FIG.

  From these results, it can be seen that if the Al content is 0.4% or more, the ferrite ratio is 80 area% or more. This can be considered to be due to the ferrite stabilization effect of Al.

[Example 4]
For the cast iron shown in Table 4 above, test pieces were prepared in the same manner as in Example 3, and the graphite structure was investigated in the same manner as in Example 2. The results are shown in Table 6 below. Further, based on this result, the relationship between “Al content” and “number of graphite particles” is shown in FIG.

  These results can be considered as follows. First. It can be seen that the number of graphite particles decreases as the Al content increases, but the number of graphite particles becomes constant when the Al content exceeds 1.0%. This indicates that even when Al is contained in a large amount, the heat resistance is not lowered due to the coarsening of the graphite particles. Moreover, it was recognized by visual observation at the time of casting that the sample (No. 9) with the highest Al content slightly decreased the melt flowability of the molten metal.

[Example 5]
Cast irons having chemical composition compositions shown in Table 7 below were melted and cast according to a conventional method. About the obtained cast iron casting, in order to evaluate heat resistance, the ferrite ratio in a base structure was investigated by the same method as Example 1.

  The results are shown in Table 8 below. Further, based on this result, “Ti content” and “relationship of ferrite ratio” are shown in FIG.

  These results can be considered as follows. As the Ti content increases, the ferrite ratio decreases, but it can be seen that even if the Ti content is 0.50%, the ferrite ratio exceeds 85 area%. However, from the slope of the solid line in FIG. 5, if it exceeds 0.50%, the ferrite ratio is expected to be 80 area% or less.

[Example 6]
For the cast iron shown in Table 7, test pieces were prepared in the same manner as in Example 5, and the graphite structure was examined in the same manner as in Example 2.

  The results are shown in Table 9 below. Based on this result, the relationship between “Ti content” and “number of graphite particles” is shown in FIG.

  As is clear from these results, it can be seen that the number of graphite particles tends to increase as the Ti content increases.

[Example 7]
Cast irons having the chemical composition shown in Table 10 below were melted and cast according to a conventional method. About the obtained cast iron casting, in order to evaluate heat resistance, the ferrite ratio in a base structure was investigated by the same method as Example 1.

  The results are shown in Table 11 below. Based on this result, the relationship between the “Mn content” and the “ferrite ratio” is shown in FIG.

  As is apparent from these results, it can be seen that the ferrite ratio tends to decrease as the Mn content increases. In particular, when the Mn content exceeds 1.0%, the ferrite ratio becomes 80 area% or less, and the heat resistance tends to decrease.

[Example 8]
For the cast iron shown in Table 10 above, test pieces were prepared in the same manner as in Example 7, and the number of graphite particles was investigated in the same manner as in Example 2. The results are shown in Table 12 below. Based on this result, the relationship between “Mn content” and “number of graphite particles” is shown in FIG. As is apparent from this result, it can be seen that the number of graphite particles tends to increase as the Mn content increases.

Claims (3)

  C: 2.5 to 4% (meaning “mass%”; chemical composition is the same hereinafter), Si: 2 to 3.6% and Mn: 0.1 to 1.0%, respectively, and Al : 0.4 to 1.5% and Ti: 0.15 to 0.5%, the balance being iron and inevitable impurities, and graphite particles dispersed therein, heat resistance Excellent ferritic cast iron.   The ferritic cast iron excellent in heat resistance according to claim 1, wherein 80% by area or more of the base structure is ferrite. The ferritic cast iron excellent in heat resistance according to claim 1 or 2, wherein the number of graphite particles having an equivalent circle diameter of 20 µm or less is 5,000 or more per 1 mm ^{2 of} the observation visual field.

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