JP2018131671A - Heat-resistant spheroidal graphite cast iron having excellent creep resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the high-temperature creep resistance of FCD400 alloy which can be used for a checker-receiving hardware member and the like.SOLUTION: A heat-resistant spheroidal graphite cast iron having excellent creep resistance, contains, in mass%, C: 3.0%-4.3%, Si: 1.5-3.5%, Mn: 1.0% or less, a graphite spheroidizing agent comprising at least one of Mg, misch metal, Ce, and Ca: 0.010-0.15%, P: 0.20% or less, S: 0.050% or less, Ni: 0.50-1.5%, Cu: 0.20-1.0%, and Mo: 0.50-1.5%, with the balance being Fe and inevitable impurities.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a heat-resistant spheroidal graphite cast iron having excellent creep resistance.

  In addition to high strength and high toughness, heat resistance is required for a checker metal receiving member that supports checker bricks provided in a heat storage chamber of a blast furnace hot stove for iron making.

2A and 2B show a conceptual diagram of the structure of the heat storage chamber in the hot stove, and the process of storing heat and supplying hot air in the heat storage chamber. The heat storage chamber heats the checker brick 1 by blowing high-temperature checker brick heating gas 3 previously generated in a combustion chamber separate from the heat storage chamber into the checker brick 1 provided in the heat storage chamber from the upper part of the heat storage chamber. (FIG. 2 (A), heat storage process), the air 6 heated by the heat storage chamber having a low temperature to be heated is introduced into the heat storage chamber as necessary, and the heat of the stored checker brick 1 is stored. By heating the air 6 heated by the heat storage chamber, the air 6 heated by the heat storage chamber is supplied as hot air 5 to the blast furnace (FIG. 2B, hot air supply process).
During this heat storage process (FIG. 2 (A)), the checker brick heating gas 3 loses heat after heating, and passes through the checker metal receiving member 2 configured to allow the gas to pass from the lower part to the outside. The exhaust gas 4 is exhausted.

  The exhaust gas 4 exhausted in the heat storage process through the checker receiving material member 2 is at a high temperature of about 150 to 400 ° C. even though the heat is transferred to the checker brick 1 of the heat storage member and loses heat. The member 2 is required to have heat resistance.

  Conventionally, cast iron such as FC and FCD has been adopted as such a checker-receiver member. Among these, FCD (spheroidal graphite cast iron) is also called ductile cast iron or nodular cast iron, and exhibits a structure in which the carbon content in the cast iron is precipitated in a spherical shape as graphite in the parent phase iron. In order to obtain such a spheroidal graphite cast iron structure, Mg is generally added as a cast iron component and inoculated with an Fe—Si alloy. Alternatively, it can be realized by adding appropriate amounts of Ce and Ca. By forming a spherical graphite structure in which graphite precipitates in a spherical shape, stress concentration at the interface between the graphite and the parent phase is relieved, and cracks are difficult to progress. Therefore, high strength, high toughness (high ductility), and high wear resistance can be imparted as compared with flake graphite cast iron in which the precipitated graphite is flake. Furthermore, since it is for casting, shape design can be performed relatively freely. Since it can be designed freely, it can be easily used for automobile parts and building materials.

However, in recent years, in order to improve the thermal efficiency of a blast furnace or a hot stove, it is required to further increase the temperature of the hot air 5 supplied from the hot stove.
In order to increase the temperature of the hot air 5, it is necessary to increase the temperature of the heat storage chamber, and one of the means is to increase the exhaust gas temperature.
However, when the temperature of the checker brick heating gas 3 is increased, the temperature of the exhaust gas 4 that has finished heating the checker brick rises from 350 to 400 ° C., which is the conventional heat resistance temperature. Therefore, the checker metal receiving member 2 that must support the weight of the checker brick 1 while being exposed to the high temperature exhaust gas 4 is buckled by heat and weight. That is, if the heat resistance strength of the checker-receiver member 2 is improved and the temperature of the exhaust gas 4 can be increased, the amount of heat stored can be increased and the temperature of the hot air 5 can be increased.

  As a technique for raising the exhaust gas temperature, Patent Document 1 discloses that a support assembly for supporting a heat storage grid refractory brick includes a ferrite matrix and a dispersion of graphite particles, and the shape of the graphite particles is substantially the same. In particular, it is described that the exhaust temperature can exceed 600 ° C. or even 700 ° C. by using cast iron which is a cast iron structure having a worm-like or nodular shape. In terms of components, the cast iron material is carbon 2.0 to 3.8%, silicon 1.8 to 5.0%, manganese 0.1 to 1.0%, phosphorus 0.1% or less, sulfur 0 .1% or less, optionally molybdenum 1.25% or less, unavoidable impurities and balance iron.

Special table 2008-528808

  Since FCD (spheroidal graphite cast iron) is a nodule in which the base structure is ferrite and the shape of the graphite particles is spherical, the invention described in Patent Document 1 is substantially composed of FCD and Mo. Added alloy cast iron. Since this FCD is spheroidal graphite cast iron, it has high strength and high toughness, and has a certain degree of heat resistance. However, the hot air blown into the blast furnace is heated to a higher temperature to reduce the heating coke charged in the blast furnace. In order to achieve this, since it is required to improve the temperature of the heat storage chamber, further heat resistance is required. In particular, in a checker-receiver member that is loaded with a high weight of about 2000 tons f at a high temperature of the actual large-scale actual machine level, for example, by raising the exhaust gas temperature from 400 ° C. to 500 ° C. by 100 ° C., the blast furnace 1 The annual coke consumption per base can be reduced by tens of thousands of tons, and cost reduction of several hundred million yen per year can be expected.

Since the checker receiving member has a higher temperature than conventional and a load as high as 2000 tons f is applied, it is necessary to set the required durability at a high temperature. In general, the allowable stress of a material used at high temperature is the design stress specified by “ASME Code” (American Society of Mechanical Engineers), that is, “¼ of high temperature tensile strength” and “10 ⁻⁵ % / hour”. Is determined by the smaller value of “the stress at which the creep rate is generated” (“% / hour” is the deformation rate per hour).

The broken line in FIG. 1 shows the relationship between “Frequency of high-temperature tensile strength” and “Stress at which a creep rate of 10 ⁻⁵ % / hour is generated” in FCD and temperature. Further, the solid line in FIG. 1 represents the high strength characteristic targeted by the present invention. In general, FCD, which is a conventional material, reverses the "high temperature tensile strength 1/4" and the "stress at which a creep rate of ^10-5 % / hour" occurs at around 400 ° C as the temperature rises. “Stress that generates a creep rate of 10 ⁻⁵ % / hour” becomes smaller. As described above, when a material is used at a high temperature exceeding 400 ° C., it cannot be evaluated whether the material can be used only by a simple tensile strength at the temperature, and the creep strength at a high temperature needs to be high. There is. For this reason, when used as a checker-receiver member, the creep strength is rate-determining, and the maximum allowable temperature is determined by design stress and design conditions. That is, if the conventional technique shown by the broken line can be improved to the characteristic shown by the solid line, the stress at which the creep rate of 10 ⁻⁵ % / hour is improved, and the maximum allowable temperature of the checker metal receiving member can be improved.

  Patent Document 1 describes that the exhaust temperature can exceed 600 ° C. or even 700 ° C., but does not mention high-temperature creep characteristics, and does not disclose actual high-temperature strength. .

The gist of the present invention for solving the above problems is as follows.
(1) C: 3.0% to 4.3% by mass%,
Si: 1.5 to 3.5%,
Mn: 1.0% or less,
Graphite spheroidizing agent comprising at least one of Mg, Misch metal, Ce, and Ca: 0.010 to 0.15%,
P: 0.20% or less,
S: 0.050% or less,
Ni: 0.50 to 1.5%,
Cu: 0.20 to 1.0%,
Mo: 0.50 to 1.5%,
Containing
A heat-resistant spheroidal graphite cast iron excellent in creep resistance, characterized in that the balance consists of Fe and inevitable impurities.
(2) The heat resistant spheroidal graphite cast iron having excellent creep resistance according to (1), which is used for a heat storage brick support member of a hot stove.
(3) The heat-resistant spheroidal graphite cast iron having excellent creep resistance according to (2), wherein the heat storage brick support member is a checker metal receiving member.
(4) A heat storage brick support member for a hot stove comprising the heat-resistant spheroidal graphite cast iron according to (1).
(5) A checker-receiver member for a hot stove comprising the heat-resistant spheroidal graphite cast iron according to (1).

  According to the present invention, the creep strength and high-temperature tensile strength of spheroidal graphite cast iron can be improved, and by using it suitably as a heat-resistant structural material such as a heat storage brick support member of a hot blast furnace, a checker metal receiving member, a hot blast furnace, a blast furnace Thermal efficiency such as operation can be improved, and resources and costs can be reduced.

The conceptual diagram explaining the use temperature of FCD in high temperature, the relationship between tensile strength and creep strength, and the target characteristic of this invention. (A) The schematic diagram which shows the structure of the thermal storage chamber of a hot stove, and the thermal storage process in a thermal storage chamber. (B) The schematic diagram which shows the structure of the thermal storage chamber of a hot stove, and the hot air supply process in a thermal storage chamber.

  The present inventor examined various additive elements in order to improve the high temperature creep strength based on the component composition used for FCD. As a result, it was found that the high-temperature creep strength and tensile strength of spheroidal graphite cast iron are improved by adding a predetermined amount of Mo, Cu, and Ni in combination. Hereinafter, the reason for limitation of each component is demonstrated. All percentages relating to the components are mass%.

C: 3.0 to 4.3%
Cast iron originally contains about 1.7 to 4.5% by mass of carbon for improving castability. In the present invention, spheroidal graphite and carbide (such as cementite contained in pearlite structure) are sufficiently generated. Therefore, the range is defined as 3.0 to 4.3%. If it is less than 3.0%, spheroidal graphite cast iron has a poorer molten metal flow than flake graphite cast iron. Therefore, castability deteriorates and casting defects and shrinkage due to poor molten metal flow occur, resulting in poor formation of spheroidal graphite and carbide. , Lack of strength. When it exceeds 4.3%, primary crystal graphite (hypereutectic graphite) is likely to be generated because it exceeds the eutectic composition. Generation of primary graphite is not preferable because the toughness of cast iron decreases and the elongation deteriorates. On the other hand, if it exceeds 4.3%, dross and segregation occur, resulting in a casting defect, graphite is not sufficiently spheroidized and dispersed, the toughness of cast iron is reduced, and the elongation is deteriorated. A more preferable lower limit range is 3.5% or more, and a further preferable lower limit range is 3.7% or more. On the other hand, a more preferable upper limit range is 4.0% or less, and a more preferable lower limit range is 3.9% or less.

Si: 1.5 to 3.5%
Si is added in order to facilitate crystallization of graphite, and in combination with the effect of adding Mg, which will be described later, to spheroidize graphite and to improve castability. If it is less than 1.5%, the above effect is not sufficient, and sufficient graphite does not crystallize, so that the strength is insufficient. On the other hand, if it exceeds 3.5%, the amount of crystallized graphite becomes too large, and a flake crystallized product that does not spheroidize is generated. When graphite is crystallized in the form of a piece, stress concentrates on the interface with graphite and deteriorates strength, toughness, and elongation. A more preferred lower limit range is 2.0% or more, and a more preferred lower limit range is 2.5% or more. On the other hand, a more preferable upper limit range is 3.0% or less, and a more preferable lower limit range is 2.7% or less.

Mn: 1.0% or less Mn is added in a small amount to ensure deoxidation and toughness in ordinary steel smelting. If it exceeds 1.0%, hard and brittle manganese carbide is likely to be produced, and the strength, toughness and elongation are deteriorated. A more preferable upper limit range is 0.6%, and a more preferable upper limit range is 0.4% or less. On the other hand, the lower limit is not necessarily limited, but it is preferably 0.1% or more in order to reduce Mn more than necessary to increase the cost and to ensure deoxidation and toughness. More preferably 2% or more.

Graphite spheroidizing agent comprising at least one of Mg, Misch metal, Ce, and Ca: 0.010 to 0.15%
A graphite spheroidizing agent comprising at least one of Mg, Misch metal, Ce, and Ca is added to spheroidize the crystallized graphite. If it is less than 0.010%, the graphite cannot be sufficiently spheroidized, and flake graphite is produced. On the other hand, if it exceeds 0.15%, hard and brittle carbides are produced, and the toughness and elongation deteriorate. A more preferable graphite spheroidizing agent is Mg added alone or Mg and Ce and Ca are used in combination.

P: 0.20% or less P (phosphorus) is an element inevitably contained in cast iron, but if it exceeds 0.20%, castability deteriorates and casting defects tend to occur, and toughness and elongation deteriorate. Therefore, the content is limited. A more preferable range is 0.10% or less.

S: 0.050% or less S (sulfur) is an element inevitably contained in the cast iron as in the case of P. However, if it exceeds 0.05%, the spheroidization of graphite is inhibited, so the content is limited. A more preferable range is 0.02% or less.

Ni: 0.50 to 1.5%
When Ni is added together with Mo and Cu, the tensile strength at high temperature is improved. If the content is less than 0.50%, the effect of improving the strength is not sufficient. A more preferable lower limit range is 0.8% or more, and a further preferable lower limit range is 1.0% or more. On the other hand, a more preferable upper limit range is 1.3% or less, and a more preferable upper limit range is 1.1% or less.

Cu: 0.20 to 1.0%
By adding Cu together with Ni and Mo, the creep strength at high temperature is improved. If the content is less than 0.20%, the effect of improving the creep strength is not sufficient. If the content exceeds 1.0%, the toughness and elongation are reduced, and a Cu-rich phase is easily generated and the strength is also reduced. A more preferable lower limit range is 0.4% or more, and a further preferable lower limit range is 0.5% or more. On the other hand, a more preferable upper limit range is 0.8% or less, and a more preferable upper limit range is 0.6% or less.

Mo: 0.50 to 1.5%
When Mo is added together with Ni and Cu, the creep strength at high temperature is improved. If it is less than 0.50%, the effect of improving the creep strength is not sufficient, and if it exceeds 1.5%, an intermetallic compound phase composed of a hard carbide is generated, and the toughness and elongation are lowered, and the strength is also lowered. A more preferred lower limit range is 0.6% or more, and a more preferred lower limit range is 0.8% or more. On the other hand, a more preferable upper limit range is 1.2% or less, and a more preferable upper limit range is 1.0% or less.

  The balance other than the above is Fe and inevitable impurities. The inevitable impurities mentioned here include the elements and the amount of elements that are inevitably contained in the production process of producing ordinary cast iron, and each is clearly specified to the limit that is possible with the current technology. It is not a statement to the effect that impurity elements that are not used are reduced. In other words, inevitable impurities refer to elements contained rather than intentionally added elements for a specific purpose. For example, adding an expensive modifying element such as V significantly increases the cost, especially in large products such as structural materials, and is out of the scope of the present invention. As a measure of impurities, it is allowed to contain elements of the kind and amount that are allowed by the FCD standard.

  The spheroidal graphite cast iron of the present invention exhibits the effects of the invention by determining the component composition as described above. For production, it is melted by a usual method, adjusted for components, cast, and cast as it is. It can be manufactured by releasing it. As for the structure of spheroidal graphite cast iron produced by as-casting, the ferrite structure is 30 to 70% in area ratio and the remaining pearlite structure except for the spheroidal graphite part.

Creep test result The creep test was performed using a tensile test piece prepared by melting and casting cast iron, producing an ingot by as-casting, and cutting out in accordance with the provisions of JIS Z 2271. In addition, the result of having analyzed the component of the test piece was the component range shown in Table 1.

The creep test was performed at 400 ° C., 450 ° C., 500 ° C., and 550 ° C. The test time is 300 to 1200 hours as the total time including the transition zone and acceleration zone, and the elongation over time during steady-state creep is measured at various temperatures at various temperatures. Creep speed was calculated. Then, a relational expression between the minimum creep rate at each temperature and the corresponding load stress is obtained by extrapolation, and the load stress when it becomes 10 ⁻⁵ % / hour from the relational expression (at a constant load at the same temperature, The stress at the time of deformation of 10 ⁻⁵ % in one hour was calculated individually and evaluated as the creep strength. When the relational expression between the load stress and the minimum creep rate is obtained from Examples 1 to 3, the load stress = 220.65 × (minimum creep rate) is ^{0.2922. In} Examples 4 to 6, the load stress = 99.175. X (Minimum creep rate) It was ^0.2987 . In the comparative example, the relational expression was similarly obtained and the creep strength was calculated. The results are shown in Table 2.

  In Comparative Examples 1 to 12, where the material is FCD, since none of Ni, Cu, and Mo is added, the creep strength is low. Furthermore, Comparative Examples 13 to 15 in which Ni is added to FCD are not sufficient, although an improvement in creep strength is observed.

On the other hand, Examples 1 to 6, which are the present invention, have a sufficiently low minimum creep rate, and were confirmed to withstand loads even at high temperatures. Moreover, Examples 1-3 which are this invention improved to the intensity | strength 10 times or more with 7.6 kgf / mm < ² > of creep strength at the temperature equivalent to Comparative Examples 5-8. Furthermore, as shown in Examples 4 to 6, a sufficient creep strength could be maintained even at 550 ° C. In addition, it was confirmed that there was little generation of oxide scale and excellent high-temperature oxidation resistance.

Result of high-temperature tensile test The tensile strength and elongation of the slab were measured by a high-temperature tensile test specified in JIS G 0567. The test temperatures were 300, 350, 400, 450, 500, 550, and 600 ° C., two each. The results are shown in Table 3.

As shown in Table 3, Examples 1-1 to 7-2 exceeded each of Comparative Examples 1-1 to 14-2 at the same temperature in any of the high-temperature tensile tests at 300 to 600 ° C. .
Further, in Tables 2 and 3, in light of “ASME Code” regarding allowable stress of materials used at high temperatures at temperatures of 450, 500, and 550 ° C., “1/4 of high temperature tensile strength” and “10 ⁻⁵ ”. The smaller of “Stress at which creep rate of% / hour is generated” is confirmed to be “Stress at which creep rate of 10 ⁻⁵ % / hour” is used. It was confirmed that the “stress at which a creep rate of 10 ⁻⁵ % / hour is generated” must be high.

  Since the present invention is excellent in high-temperature strength, creep resistance at high temperature, and heat resistance, it can be applied to various high-temperature materials that continue to be loaded at high temperatures, such as engines, boilers, and heating furnaces.

DESCRIPTION OF SYMBOLS 1 ... Checker brick, 2 ... Checker receiving material member, 3 ... Checker brick heating gas, 4 ... Exhaust gas, 5 ... Hot air, 6 ... Air heated by a thermal storage chamber

The gist of the present invention for solving the above problems is as follows.
(1) C: 3.0% to 4.3% by mass%,
Si: 1.5 % or more, less than 3.0% ,
Mn: 0.4 % or less,
Graphite spheroidizing agent comprising at least one of Mg, Misch metal, Ce, and Ca: 0.010 to 0.15%,
P: 0.20% or less,
S: 0.050% or less,
Ni: 0.50 to 1.5%,
Cu: 0.20 % or more, less than 0.8% ,
Mo: 0.50 to 1.5%,
Containing
A heat-resistant spheroidal graphite cast iron excellent in creep resistance, characterized in that the balance consists of Fe and inevitable impurities.
(2) A regenerative brick support member for a hot stove comprising the heat-resistant spheroidal graphite cast iron according to (1).
(3) A checker-receiver member for a hot stove comprising the heat-resistant spheroidal graphite cast iron according to (1).

Claims (5)

C: 3.0% to 4.3% by mass%,
Si: 1.5 to 3.5%,
Mn: 1.0% or less,
Graphite spheroidizing agent comprising at least one of Mg, Misch metal, Ce, and Ca: 0.010 to 0.15%,
P: 0.20% or less,
S: 0.050% or less,
Ni: 0.50 to 1.5%,
Cu: 0.20 to 1.0%,
Mo: 0.50 to 1.5%,
Containing
A heat-resistant spheroidal graphite cast iron excellent in creep resistance, characterized in that the balance consists of Fe and inevitable impurities.   The heat-resistant spheroidal graphite cast iron excellent in creep resistance according to claim 1, which is used for a heat storage brick support member of a hot stove.   3. The heat-resistant spheroidal graphite cast iron with excellent creep resistance according to claim 2, wherein the heat storage brick support member is a checker metal receiving member.   A heat storage brick support member for a hot stove comprising the heat-resistant spheroidal graphite cast iron according to claim 1.   A checker-receiver member for a hot stove comprising the heat-resistant spheroidal graphite cast iron according to claim 1.

Images