TW201831702A - Heat resistant spheroidal graphite cast iron with excellent creep resistance - Google Patents

Abstract

Heat resistant spheroidal graphite cast iron with excellent creep resistance that contains, in mass%, C: 3.0%-4.3%, Si: 1.5% to less than 3.0%, Mn: 0.4% or less, a graphite spheroidizing agent made of at least one of Mg, mischmetal, 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% to less than 0.8%, and Mo: 0.50-1.5%, the balance being made of Fe and unavoidable impurities.

Description

Heat-resistant spheroidal graphite cast iron excellent in resistance to latent denaturation

The present invention relates to a heat-resistant spheroidal graphite cast iron excellent in resistance to latent denaturation.

In addition to high strength and high toughness, heat-resistant properties are required for the lattice-shaped metal supporting members of the lattice-shaped bricks provided in the regenerator of the hot blast stove for blast furnaces for producing pig iron.

2A and 2B show a schematic view of the structure of the regenerator in the hot blast stove and a process of storing heat and hot air in the regenerator. The regenerator is an upper portion of the regenerator, and the grid brick 1 installed in the regenerator is blown into the high-temperature grid brick heating gas 3 which is generated in advance in the combustion chamber different from the regenerator, and the heat is first stored in the grid. In the brick 1 (Fig. 2A, heat storage process), and if necessary, the air 6 to be heated and heated by the regenerator is introduced into the regenerator, and is heated by the heat energy of the heat-removed lattice brick 1 The air 6 heated by the regenerator is used to cause the air 6 to be heated by the regenerator to be the hot air 5 and supplied to the blast furnace (FIG. 2B, hot air supply process).

During the above-described heat storage process (Fig. 2A), after the grid-shaped heating gas 3 finishes heating and loses thermal energy, it passes through the lattice-shaped metal supporting member 2 which is formed by the passage of gas, and is discharged from the lower portion as the high-temperature exhaust gas 4. External exhaust.

The exhaust gas 4 that is exhausted during the heat storage process by the lattice metal supporting member 2 loses thermal energy even if the thermal energy has been moved to the heat storage member, that is, the lattice brick 1, and has a high temperature of about 150 to 400 ° C. Heat resistance is required for the lattice metal supporting member 2.

The above-described lattice metal supporting member has conventionally used cast iron such as FC or FCD. Among them, FCD (spherical graphite cast iron) can also be called ductile cast iron or ductile cast iron, and its structure is carbon in cast iron, which is precipitated into a spherical shape by graphite in the parent phase iron. To form the above 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 achieved by further adding Ce and Ca in an appropriate amount. By forming a spherical spheroidal graphite structure in which graphite is precipitated, the stress concentration at the interface between the graphite and the parent phase can be alleviated, and the crack becomes difficult to perform. Therefore, compared with the flake graphite in which the precipitated graphite is flake-shaped, it can impart high strength, high toughness (high ductility), and high wear resistance. Moreover, since it is used for casting, the shape design can be performed relatively freely. Since the shape can be freely designed, it becomes easy to apply to automobile parts or building materials.

However, in recent years, in order to increase the thermal efficiency of a blast furnace or a hot blast stove, it is required to increase the temperature of the hot air 5 supplied from the hot blast stove.

In order to raise the temperature of the hot air 5, it is necessary to raise the temperature of the regenerator, and one of the means is an increase in the temperature of the exhaust gas.

However, if the temperature of the grid-shaped brick heating gas 3 is increased, the temperature of the exhaust gas 4 after the heating of the grid-shaped bricks is increased to a temperature higher than the conventional heat-resistant temperature of 350 to 400 °C. Therefore, the lattice-shaped metal supporting member 2 exposed to the high-temperature exhaust gas 4 and having to support the weight of the lattice brick 1 is bent by heat energy and weight. That is, as long as the heat resistance of the lattice metal supporting member 2 can be raised and the temperature of the exhaust gas 4 is increased, the heat of heat storage can be increased and the temperature of the hot air 5 can also be raised.

As a technique for increasing the temperature of the exhaust gas, Patent Document 1 (Japanese Patent Laid-Open Publication No. 2008-528808) describes the following: The support member for supporting the heat storage lattice refractory brick is a ferrite-containing iron matrix and graphite. The graphite particles are formed of a dispersion of particles, and the shape of the graphite particles is substantially a worm-like or agglomerated cast iron structure, whereby the temperature of the exhaust gas can exceed 600 ° C and more than 700 ° C. Moreover, the following items are also described: in terms of composition, the cast iron material is 2.0 to 3.8% carbon; 1.8 to 5.0% niobium; 0.1 to 1.0% manganese; 0.1% or less phosphorus; 0.1% or less sulfur; , less than 1.25% molybdenum; and inevitable impurities and the remainder of the iron.

In FCD (spheroidal graphite cast iron), since the base layer structure is ferrite iron and the shape of the graphite particles is spherical or agglomerate, the invention described in Document 1 is substantially FCD and alloy cast iron to which Mo is added. The FCD is a spheroidal graphite cast iron, so it has high strength, high toughness, and a considerable degree of heat resistance. However, in order to make the hot air blown into the blast furnace higher, to reduce the heating coke charged in the blast furnace, it is required to raise the regenerator. The temperature will therefore require further heat resistance. In particular, for a large-scale actual mechanical grade of the current state, a high-strength lattice-shaped metal supporting member that is loaded at a high temperature of 2,000 tons f can be made to have a discharge gas temperature of 400 ° C by 100 ° C to 500 ° C. The annual coke usage of a blast furnace is reduced by tens of thousands of tons, and the cost of several hundred million yuan per year can be expected to be reduced.

The lattice metal support member will load a high load of even 2000 tons f at a higher temperature than before, so it is necessary to set the durability strength required at a high temperature. In general, the allowable stress for materials used at high temperatures is determined by the design stress specified in the "ASME Code" (American Society of Mechanical Standards), ie, "1/4 of the high temperature tensile strength" and "will The stress that produces a creep rate of 10 ^-5 % / hour ("% / hour" is the deformation ratio per hour) is determined by the smaller value.

The broken line in Fig. 1 shows the relationship between the "high temperature tensile strength 1/4" and "the stress which causes a creep rate of 10 ^-5 % / hour" and the temperature in the FCD. Further, the solid line of Fig. 1 is a high-strength property which is the object of the present invention. In general, the FCD of a conventional material will reversal at a temperature of 400 ° C near the "high temperature tensile strength of 1/4" and "the stress that will produce a creep rate of 10 ^-5 % / hour". The stress that produces a creep rate of 10 ^-5 % / hour will become smaller. As a result, when a material is used at a high temperature of more than 400 ° C, it is not possible to evaluate whether or not the material can be used only by the simple tensile strength at this temperature, and the creep strength at a high temperature must be high. Therefore, when used as a lattice metal supporting member, the creep strength becomes a bottleneck, and the maximum allowable temperature is determined depending on design stress or design conditions. That is, as long as the conventional technique shown by the broken line can be improved to the characteristics of the solid line, the stress of the creep velocity of 10 ^-5 % / hour is increased, and the maximum allowable temperature of the lattice metal supporting member can be raised. .

In Document 1, although the exhaust gas temperature exceeds 600 ° C and further exceeds 700 ° C, the high temperature creep characteristics are not mentioned, and the actual high temperature strength is not disclosed.

SUMMARY OF THE INVENTION The gist of the present invention for solving the above problems is as follows. (1) A heat-resistant spheroidal graphite cast iron excellent in resistance to latent densification, characterized by containing: C: 3.0% to 4.3%, Si: 1.5% or more and less than 3.0%, and Mn: 0.4% or less by mass% A graphite spheroidizing agent composed of any one or more of Mg, a rare earth metal alloy, 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% Above and below 0.8%, Mo: 0.50 to 1.5%, and the remainder is composed of Fe and unavoidable impurities.

(2) A heat storage brick supporting member for a hot blast stove, which is characterized in that it is composed of the heat-resistant spheroidal graphite cast iron of (1).

(3) A lattice-shaped metal supporting member for a hot blast stove, which is characterized by comprising the heat-resistant spheroidal graphite cast iron of (1).

According to the present invention, the creep strength and the high-temperature tensile strength of the spheroidal graphite cast iron can be improved, and it is suitable for use as a heat-retaining brick supporting member of a hot blast stove and a heat-resistant structural member such as a lattice-shaped metal supporting member, thereby improving the hot blast stove and the blast furnace. The thermal efficiency of operations and the like can reduce resources and costs.

In order to carry out the invention, the present inventors have studied various additive elements in order to improve the high-temperature creep strength based on the component composition for FCD. As a result, it was found that the high-temperature creep strength and the tensile strength of the spheroidal graphite cast iron were improved by using and adding a predetermined amount of Mo, Cu and Ni in combination. The reason for limiting each component will be described below. The % mark of the relevant ingredients is % by mass. C: 3.0 to 4.3%

In the present invention, the cast iron contains 1.7 to 4.5% by mass of carbon to improve the castability. However, in the present invention, the spheroidal graphite and the carbide (such as ferritic carbon and iron contained in the Borne iron structure) are sufficiently produced. The range is 3.0 to 4.3%. If it is less than 3.0%, the spheroidal graphite cast iron has poor fluidity compared to flake graphite cast iron, so that the castability is deteriorated, and casting defects or shrinkage cavities accompanying poor fluidity are generated, and spheroidal graphite or carbide is poorly formed. And the strength is not enough. If it exceeds 4.3%, primary crystal graphite (hypereutectic graphite) is easily formed due to exceeding the eutectic composition. When primary crystal graphite is formed, the toughness of cast iron is lowered and the elongation is deteriorated, which is not preferable. On the other hand, if it exceeds 4.3%, scum or segregation may occur and casting defects may occur, and graphite may not be sufficiently spheroidized and dispersed, and the toughness of the cast iron may be lowered and the elongation may be deteriorated. The preferred lower limit range is 3.5% or more, and the lower limit range is preferably 3.7% or more. On the other hand, the preferred upper limit range is 4.0% or less, and the lower limit range is 3.9% or less. Si: 1.5 to 3.5%

In order to facilitate the crystallization of graphite, Si is added to the graphite in order to complement the effect of adding Mg as described later, and to improve the castability. If it is less than 1.5%, the above effects are insufficient, and sufficient graphite is not crystallized, so the strength is insufficient. On the other hand, when it exceeds 3.5%, the amount of crystallized crystals becomes too large to form an unspheroidized sheet crystal. If graphite is crystallized in a sheet form, the stress concentrates on the interface with graphite, which deteriorates strength and toughness and elongation. Therefore, the preferred lower limit range is 2.0% or more, and the lower limit range is preferably 2.5% or more. On the other hand, the preferred upper limit range is 3.0% or less, and the lower limit range is 2.7% or less. Mn: 1.0% or less

Mn is added in a small amount in order to ensure deoxidation and toughness in ordinary steel refining. If it exceeds 1.0%, it becomes easy to form a hard and brittle manganese carbide, which deteriorates strength, toughness, and elongation. The preferred upper limit range is 0.6%, and the upper limit range is preferably 0.4% or less. On the other hand, although the lower limit is not necessarily limited, the cost is increased by reducing Mn to the extent necessary, and it is preferably 0.1% or more, and more preferably 0.2% or more in terms of ensuring deoxidation and toughness. A graphite spheroidizing agent composed of any one or more of Mg, a rare earth metal alloy, Ce, and Ca: 0.010 to 0.15%

A graphite spheroidizing agent composed of any one or more of Mg, a rare earth metal alloy, and Ce and Ca is added in order to spheroidize the crystallized graphite. When it is less than 0.010%, graphite cannot be sufficiently spheroidized to form flake graphite. On the other hand, when it exceeds 0.15%, hard and brittle carbides are formed, and toughness and elongation are deteriorated. A more suitable graphite spheroidizing agent is added Mg alone or in combination with Ce and Ca. P: 0.20% or less

P (phosphorus) is an element which is inevitably contained in the cast iron. However, if it exceeds 0.20%, the castability deteriorates and casting defects are likely to occur, and the toughness and elongation are deteriorated, so that it is necessary to restrict the inclusion. The preferred range is 0.10% or less. S: 0.050% or less

S (sulfur) and P are also inevitably contained in cast iron, but if it exceeds 0.05%, it will hinder the spheroidization of graphite, so it is necessary to limit the content. The preferred range is 0.02% or less. Ni: 0.50 to 1.5%

Ni can be increased in tensile strength at high temperatures by being added together with Mo and Cu. If it is less than 0.50%, the effect of improving the strength is not sufficient; if it exceeds 1.5%, the toughness and elongation are lowered, and casting defects are easily generated. Therefore, the preferred lower limit range is 0.8% or more, and the lower limit range is preferably 1.0% or more. On the other hand, the preferred upper limit range is 1.3% or less, and the upper limit range is 1.1% or less. Cu: 0.20 to 1.0%

Cu can be added to increase the creep strength at high temperatures by adding it together with Ni and Mo. If it is less than 0.20%, the effect of increasing the creep strength is not sufficient; if it exceeds 1.0%, the toughness and elongation are lowered, and the phase rich in Cu is easily generated, and the strength is also lowered. Therefore, the preferred lower limit range is 0.4% or more, and the lower limit range is preferably 0.5% or more. On the other hand, the upper limit is preferably 0.8% or less, and the upper limit is preferably 0.6% or less. Mo: 0.50 to 1.5%

Mo can be used to increase the creep strength at high temperatures by adding it together with Ni and Cu. If it is less than 0.50%, the effect of increasing the creep strength is not sufficient; if it exceeds 1.5%, an intermetallic compound phase composed of a hard carbide is formed, and the toughness and elongation are lowered, and the strength is also lowered. Therefore, the preferred lower limit range is 0.6% or more, and the lower limit range is preferably 0.8% or more. On the other hand, the upper limit is preferably 1.2% or less, and the upper limit is preferably 1.0% or less.

The remainder other than the above is Fe and unavoidable impurities. Here, the unavoidable impurities are elements including the kind and amount which are inevitably contained in the manufacturing steps of the usual cast iron, and it is not described that the unspecified impurity elements have been reduced to the possible use by the current state of the art. The essence of the limit. In other words, the inevitable impurity refers to an element that is intentionally added without a specific purpose but contains elements. For example, the addition of an expensive modified element such as V, especially for a larger article such as a structural material, causes a significant increase in cost and is therefore not an object of the present invention. As a standard on the basis of the impurity, an element containing the type and amount of the degree allowed by the FCD specification can be tolerated.

The spheroidal graphite cast iron of the present invention exhibits the effects of the invention by the composition of the above-mentioned determinants, and can be produced by melting a usual method, performing component adjustment, casting, and directly forming an as-cast or the like. The spheroidal graphite cast iron structure obtained by forming the blank embryo casting has an area ratio of 30 to 70% except for the spherical graphite portion, and the remainder is a Borne iron structure.

EXAMPLES Latent Test Results The creep test was carried out by using a tensile test piece prepared by melting and casting cast iron according to JIS Z 2271, and then producing an ingot by making a blank casting and then cutting it. Further, the results of analyzing the components of the test piece were the ranges of the components shown in Table 1. [Table 1]

The creep test was carried out at 400 ° C, 450 ° C, 500 ° C and 550 ° C. Moreover, the test time is set to be the total time of the time including the migration domain or the acceleration domain, and is performed for 300 to 1200 hours. For each load stress, the time relative to the steady state creep is measured at each temperature. Elongation and calculate the minimum creep speed. Then, using the extrapolation to calculate the relationship between the minimum creep speed and the corresponding load stress at each temperature, the load is calculated to be 10 ^-5 % / hour by the individual calculation of the relationship (at the same temperature and constant Under load, the stress at 10 ^-5 % deformation occurs in one hour) and is evaluated as the creep strength. If the relationship between the load stress and the minimum creep speed is calculated from Examples 1 to 3, the load stress = 220.65 × (minimum creep speed) ^0.2922 , and in Examples 4 to 6, the load stress = 99.175 × (minimum Latent speed) ^0.2987 . The comparative example also calculates the relationship and calculates the creep strength. The results are shown in Table 2. [Table 2]

In Comparative Examples 1 to 12 in which the material was FCD, none of Ni, Cu, and Mo was added, so the creep strength was low. Further, in Comparative Examples 13 to 15 in which Ni was added to the FCD, the creep strength was observed to be improved but insufficient.

On the other hand, it was confirmed that the minimum creep speeds of Examples 1 to 6 of the present invention were very slow, and the load could be withstand at a high temperature. Further, in Examples 1 to 3 of the present invention, the creep strength was 7.6 kgf/mm ^{2 at the} same temperature as Comparative Examples 5 to 8, and the strength was increased by ten or more. Further, as shown in Examples 4 to 6, sufficient creep strength was maintained even at 550 °C. In addition, it was confirmed that the occurrence of oxidized scale was small, and the high-temperature oxidation resistance was also excellent. High temperature tensile test results

The tensile strength and elongation of the cast piece were measured by a high temperature tensile test as specified in JIS G 0567. The test temperature was carried out at 300, 350, 400, 450, 500, 550 and 600 ° C, respectively. The results are shown in Table 3. [table 3]

As shown in Table 3, any of the high temperature tensile tests of Examples 1-1 to 7-2 at 300 to 600 ° C were higher than Comparative Examples 1-1 to 14-2 at the same temperature, respectively.

In addition, in Tables 2 and 3, when referring to the "ASME Code" regarding the allowable stress of the material used for high temperature, it is confirmed that the temperature of 450, 500, and 550 ° C is "1/4 of the high temperature tensile strength" and the smaller "creep speed produces stress ^10-5% / h" to "produces ^10-5% / hour stress creep speed", and may be used when confirmed at the above temperature, "The stress that will produce a creep rate of 10 ^-5 % / hour" must be high.

1‧‧‧ lattice brick

2‧‧‧ lattice metal support members

3‧‧‧ lattice brick heating gas

4‧‧‧Exhaust gas

5‧‧‧ hot air

6‧‧‧ Air

Fig. 1 is a schematic view showing the relationship between the use temperature of FCD at high temperature, the tensile strength and the creep strength, and the target characteristics of the present invention.

Fig. 2A is a schematic view showing the configuration of the regenerator of the hot blast stove and the heat storage process in the regenerator.

Fig. 2B is a schematic view showing the configuration of the regenerator of the hot blast stove and the hot air supply process in the regenerator.

Claims (3)

A heat-resistant spheroidal graphite cast iron excellent in resistance to latent densification, characterized by containing: C: 3.0% to 4.3%, Si: 1.5% or more and less than 3.0%, Mn: 0.4% or less, and Mg, rare earth A graphite spheroidizing agent composed of any one or more of a metal alloy, 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, and low At 0.8%, Mo: 0.50 to 1.5%, and the remainder is composed of Fe and unavoidable impurities. A heat storage brick supporting member for a hot blast stove, which is characterized in that it is composed of the heat-resistant spheroidal graphite cast iron of claim 1. A lattice metal support member for a hot blast stove, characterized by comprising the heat-resistant spheroidal graphite cast iron of claim 1.