Heat-resistant cast steel and fire grate

Abstract

This heat-resistant cast steel contains, by mass %, C: 0.60% to 0.80% inclusive, Si: 1.00% to 1.70% inclusive, Mn: 0.50% to 1.00% inclusive, P: 0.040% or less, S: 0.040% or less, Cr: 24.30% to 27.00% inclusive, Ni: 1.60% to 2.50% inclusive, and the balance being Fe with inevitable impurities.

Description

Heat-resistant cast steel and grates

[0001] The technologies disclosed herein relate to heat-resistant cast steel and fire grates.

[0002] In a waste incinerator, waste is burned on the hearth as it is fed out, turning into incinerated ash. The hearth is covered with numerous metal parts called grates. The grates move forward and backward repeatedly, agitating the waste on the hearth as it is sent downstream.

[0003] Patent Document 1 discloses a heat-resistant cast steel used in a grate. The corrosion resistance of this heat-resistant cast steel against intergranular corrosion and general corrosion has been investigated.

[0004] Patent No. 4742314

[0005] Incidentally, the grate is exposed to a high-temperature environment due to the combustion of waste. To prevent excessive heating of the grate, a cooling system may be placed below the grate to cool it down. Because the shape of the cooling system is complex, the shape of the grate may also be complex. If the castability of the heat-resistant cast steel is poor, there is a risk that complex-shaped grates cannot be manufactured properly. For this reason, there is a need to improve the castability of heat-resistant cast steel.

[0006] Furthermore, the incinerated waste ash is pushed out from the front edge of the grate and melts due to the heat of combustion, making it prone to adhering to the front edge of the grate. As a result, the front edge of the grate is susceptible to corrosion. Therefore, overlay welding is sometimes performed on the parts of the grate that are prone to corrosion. Laser overlay welding, which uses a laser, is attracting attention. When performing laser overlay welding, it is necessary to preheat the weld area to reduce the temperature difference between the weld metal and the base metal and to avoid various defects caused by the heat effect of the weld. Although laser overlay welding without preheating is being considered, cracks are currently occurring in the weld area. For this reason, there is a need to improve the overlay properties of heat-resistant cast steel.

[0007] The technology disclosed herein has been developed in view of these points, and its purpose is to improve both castability and build-up properties.

[0008] The heat-resistant cast steel disclosed herein contains, by mass%, C: 0.60% or more and 0.80% or less, Si: 1.00% or more and 1.70% or less, Mn: 0.50% or more and 1.00% or less, P: 0.040% or less, S: 0.040% or less, Cr: 24.30% or more and 27.00% or less, and Ni: 1.60% or more and 2.50% or less, with the remainder being Fe and unavoidable impurities.

[0009] The grate disclosed herein is a grate used in a waste incinerator, and is a grate made of the heat-resistant cast steel.

[0010] According to this disclosure, both castability and build-up properties can be improved.

[0011] Figure 1 is a schematic diagram of a waste incinerator using a grate according to the embodiment of this disclosure. Figure 2 is a perspective view of a grate according to the embodiment of this disclosure. Figure 3 is a longitudinal cross-sectional view of a grate according to the embodiment of this disclosure. Figure 4 is a graph showing the relationship between Ni content and Vickers hardness. Figure 5 is a graph showing the relationship between Cr equivalent / Ni equivalent and ferrite content. Figure 6 is an optical microscope image showing an example of the microstructure of heat-resistant cast steel. Figure 7 is an optical microscope image showing an example of the microstructure of heat-resistant cast steel. Figure 8 is an optical microscope image showing an example of the microstructure of heat-resistant cast steel. Figure 9 is a photograph showing the appearance of a test piece after penetrant testing. Figure 10 is a photograph showing the appearance of a test piece after penetrant testing. Figure 11 is a photograph showing the cross-sectional macrostructure of a cast test piece. Figure 12 is a photograph showing the cross-sectional macrostructure of a cast test piece. Figure 13 is a graph showing the relationship between ferrite content and the coefficient of linear expansion (measured value).

[0012] [Grates] First, the grates 11 using the heat-resistant cast steel of this disclosure will be described. Figure 1 is a schematic diagram of the waste incinerator 1. Figure 2 is a perspective view of the grates 11. Figure 3 is a longitudinal cross-sectional view of the grates 11. In this example, the grates 11 are used in the waste incinerator 1. The waste incinerator 1 is a stoker-type waste incinerator.

[0013] The waste incinerator 1 comprises a hopper 2 into which waste W is fed, a hearth section 3 that sends out the waste W fed from the hopper 2 and burns it to produce incinerated ash, and an outlet 7 that discharges the incinerated ash produced by burning the waste W. The hearth section 3 is sloped with the outlet 7 side lower to facilitate the sending of waste W from the hopper 2 side to the outlet 7 side. The hearth section 3 has a drying stage 4 for drying the waste W, a combustion stage 5 for burning the waste W at a position one level lower than the drying stage 4, and a post-combustion stage 6 for further burning the fixed carbon residue of the waste W, starting from the upstream side along the direction in which the waste W is sent out. Numerous grates 11 are laid out in the hearth section 3. As shown in Figures 1 and 2, each grate 11 is partially overlapped with the downstream grate 11 (grate 11a in Figure 2) and is sloped with the downstream side slightly higher. The waste W is melted into incinerated ash by combustion and pushed downstream by the reciprocating movement of the grate 11.

[0014] As shown by the white double arrows in Figure 2, the grate 11 is configured to move back and forth upstream and downstream, thereby pushing out the incinerated ash from its downstream end face. As shown in Figures 2 and 3, the grate 11 comprises a grate body 12 and a laser-coated portion 13 formed on at least a part of the surface of the grate body 12.

[0015] The grate body 12 occupies almost the entire grate 11 and is formed of the heat-resistant cast steel of this disclosure. The use of the heat-resistant cast steel of this disclosure makes it possible to create the complex shape required for the grate 11. The top surface of the grate body 12 is substantially flat. The grate body 12 includes a front portion 23 that contacts the upper surface of the downstream grate 11a, a rear portion 21 that is supported by the hearth portion 3 shown in Figure 1, and an intermediate portion 22 located between the front portion 23 and the rear portion 21. The front portion 23 has a flat tip surface 32 that is the downstream end surface and an inclined surface 31 that is formed to slope backward from the upper edge of the tip surface 32.

[0016] The laser-clad portion 13 is a clad layer formed by laser clad welding. In this example, the laser-clad portion 13 is formed to cover the tip surface 32, that is, the downstream end surface of the grate body 12, and the inclined surface 31. The tip surface 32 is a place that is prone to corrosion due to the accumulation of incinerated ash. The laser-clad portion 13 protects this surface from corrosion by covering the tip surface 32. The covering position of the laser-clad portion 13 on the surface of the grate body 12 is not limited. For example, the laser-clad portion 13 may cover a surface of the grate body 12 other than the tip surface 32. An example of a surface other than the tip surface 32 is the inclined surface 31.

[0017] [Heat-Resistant Cast Steel] Next, the heat-resistant cast steel of this disclosure will be described. The inventors have diligently studied from various angles in order to realize a heat-resistant cast steel that can improve both castability and build-up properties. Regarding build-up properties, the inventors have diligently studied a heat-resistant cast steel that does not crack even when laser build-up is performed without preheating. As a result, the inventors have found that both castability and build-up properties can be improved by controlling the component composition of the heat-resistant cast steel. Specifically, the inventors have found that castability can be improved by controlling the amount of C and Si in particular, and build-up properties can be improved by controlling the amount of Si, Cr and Ni in particular.

[0018] Furthermore, the inventors have found that by preferably controlling the ratio of Ni equivalent to Cr equivalent, as described later, the balance between ferrite and austenite structures can be appropriately adjusted, thereby effectively improving castability and build-up properties.

[0019] 1. Chemical Composition Next, the chemical composition of the heat-resistant cast steel of this disclosure will be described.

[0020] <C: 0.60% by mass or more, 0.80% by mass or less> Carbon (C) is an element that improves castability and wear resistance. If the amount of C is less than 0.60% by mass, castability and wear resistance deteriorate, so it is necessary to add 0.60% by mass or more of C. The amount of C is preferably 0.63% by mass or more, more preferably 0.65% by mass or more, and even more preferably 0.70% by mass or more. If the amount of C exceeds 0.80% by mass, it leads to a decrease in ductility, causing cracks to occur during build-up or welding, and further deteriorates corrosion resistance, so the amount of C is limited to 0.80% by mass. The amount of C is preferably 0.78% by mass or less, more preferably 0.76% by mass or less, and even more preferably 0.74% by mass or less.

[0021] <Si: 1.00 mass% or more, 1.70 mass% or less> Si (silicon) is an element that improves castability and corrosion resistance. If the amount of Si is less than 1.00 mass%, castability and corrosion resistance deteriorate, so it is necessary to add Si at a rate of 1.00 mass% or more. The amount of Si is preferably 1.10 mass% or more, more preferably 1.20 mass% or more, and even more preferably 1.30 mass% or more. If the amount of Si exceeds 1.70 mass%, it leads to a decrease in ductility, causing cracks to occur during build-up or welding, so the amount of Si is limited to 1.70 mass%. The amount of Si is preferably 1.60 mass% or less, more preferably 1.50 mass% or less, and even more preferably 1.40 mass% or less.

[0022] <Mn: 0.50% by mass or more, 1.00% by mass or less> Manganese (Mn) has a desulfurization effect on molten metal and is an austenite-forming element. If the amount of Mn is less than 0.50% by mass, the desulfurization effect on molten metal is reduced, so it is necessary to add 0.50% by mass or more of Mn. The amount of Mn is preferably 0.55% by mass or more, more preferably 0.60% by mass or more, and even more preferably 0.70% by mass or more. If the amount of Mn exceeds 1.00% by mass, the corrosion resistance deteriorates, so the amount of Mn is limited to 1.00% by mass. The amount of Mn is preferably 0.95% by mass or less, more preferably 0.90% by mass or less, and even more preferably 0.80% by mass or less.

[0023] <P: 0.040% by mass or less> Phosphorus (P) is an unavoidable impurity, and the amount of P present within the range normally included to ensure castability is acceptable. The amount of P is capped at 0.040% by mass. Preferably, the amount of P is 0.035% by mass or less, more preferably 0.030% by mass or less, and even more preferably 0.025% by mass or less. P is usually present at 0.001% by mass or more.

[0024] <S: 0.040% by mass or less> S (sulfur) is an unavoidable impurity, and the amount of sulfur present within the range normally included to ensure castability is acceptable. The amount of sulfur is capped at 0.040% by mass. Preferably, the amount of sulfur is 0.020% by mass or less, more preferably 0.010% by mass or less, and even more preferably 0.005% by mass or less. S is usually present at 0.001% by mass or more.

[0025] <Cr: 24.30% by mass or more, 27.00% by mass or less> Cr (chromium) is an element that improves corrosion resistance and oxidation resistance. If the amount of Cr is less than 24.30% by mass, corrosion resistance and oxidation resistance deteriorate, so it is necessary to add 24.30% by mass or more of Cr. The amount of Cr is preferably 24.70% by mass or more, more preferably 25.00% by mass or more, and even more preferably 25.20% by mass or more. If the amount of Cr exceeds 27.00% by mass, it increases the ferrite structure which is a cause of cracking during build-up and welding, so the amount of Cr is limited to 27.00% by mass. The amount of Cr is preferably 26.75% by mass or less, more preferably 26.50% by mass or less, and even more preferably 26.00% by mass or less.

[0026] <Ni: 1.60 mass% or more, 2.50 mass% or less> Ni (nickel) is an element that is effective in improving wear resistance and increases the austenite structure, which contributes to preventing cracking during build-up and welding. If the amount of Ni is less than 1.60 mass%, wear resistance and build-up performance deteriorate, so it is necessary to add Ni at a rate of 1.60 mass% or more. The amount of Ni is preferably 1.70 mass% or more, more preferably 1.80 mass% or more, and even more preferably 1.90 mass% or more. Figure 4 is a graph showing the relationship between the amount of Ni and Vickers hardness. Figure 4 was created using the experimental results of the examples described later. It can be seen that the Vickers hardness increases as the amount of Ni increases, but it tends to saturate when it exceeds 2.50 mass%. For this reason, considering wear resistance and cost-effectiveness, the amount of Ni is limited to 2.50 mass%. The amount of Ni is preferably 2.30% by mass or less, more preferably 2.15% by mass or less, and even more preferably 2.00% by mass or less.

[0027] <Remainder> The remainder consists of Fe (iron) and unavoidable impurities. As unavoidable impurities, the inclusion of trace elements (e.g., As, Sb, Sn, etc.) introduced depending on the raw materials, materials, manufacturing equipment, etc. is permissible. In addition, there are elements such as P and S, for example, which are generally preferable in smaller amounts and therefore unavoidable impurities, but whose composition range is separately defined as described above. For this reason, in this specification, when we refer to "unavoidable impurities" that constitute the remainder, we mean the concept excluding elements whose composition range is separately defined.

[0028] <1.350 ≤ Cr equivalent / Ni equivalent ≤ 1.500> It is preferable that the relationship between the Cr equivalent and the Ni equivalent, as defined by the following formulas (1) and (2), satisfies the following formula (3). The "Cr equivalent / Ni equivalent" is preferably 1.370 or more, more preferably 1.400 or more, and even more preferably 1.420 or more. The "Cr equivalent / Ni equivalent" is preferably 1.490 or less, more preferably 1.470 or less, and even more preferably 1.450 or less. Cr equivalent = [Cr] + 1.5 × [Si] ... (1) Ni equivalent = [Ni] + 0.5 × [Mn] + 17 ... (2) 1.350 ≤ Cr equivalent / Ni equivalent ≤ 1.500 ... (3) Where [X]: Mass % of element X

[0029] By satisfying equation (3) above, the balance between ferrite and austenite structures in the metal structure is appropriate, cracking during build-up welding is suppressed, and thermal expansion during high-temperature use of the grate is reduced. The Cr equivalent is calculated using the amounts of Cr and Si, which are elements that promote the ferrite structure. The Ni equivalent is calculated using the amounts of Ni and Mn, which are elements that promote the austenite structure. In other words, "Cr equivalent / Ni equivalent" represents the balance between the ferrite and austenite structures. The coefficients in equations (1) and (2) above were determined based on the experimental results of the examples described later. Specifically, the coefficients were determined by regression calculation using Excel based on the measurement results of the ferrite content of test specimens having a carbon content of 0.60 mass% or more.

[0030] Figure 5 is a graph showing the relationship between Cr equivalent / Ni equivalent and ferrite content. Figure 5 was created using experimental results from samples with a carbon content of 0.60 mass% or more among the examples described later. It can be seen that the ferrite content increases with increasing Cr equivalent / Ni equivalent. The inventors have found that when the ferrite content increases, cracks are more likely to occur during build-up welding. Furthermore, the inventors have found that when the ferrite content decreases, cracks during build-up welding are suppressed, but thermal expansion at high temperatures may increase. Thus, the inventors have found that thermal expansion at high temperatures and cracking are in a trade-off relationship with ferrite content as a parameter. For this reason, the inventors have diligently investigated the numerical range of "Cr equivalent / Ni equivalent" that correlates with ferrite content and have determined the numerical range of "Cr equivalent / Ni equivalent" that provides good thermal expansion and cracking at high temperatures.

[0031] 2. Metallographic Structure Next, the metallographic structure of the heat-resistant cast steel of this disclosure will be described.

[0032] <Volume fraction of ferrite: 55% or more> Figures 6, 7, and 8 are optical microscope images showing examples of the microstructure of heat-resistant cast steel. Figures 6, 7, and 8 are the microstructures of Invention Example 6, Invention Example 1, and Comparative Example 4 of the Examples described later, respectively. The microstructure of the heat-resistant cast steel of this disclosure contains ferrite (F) and may further contain austenite (A) and carbides (C). In the examples of Figures 6 and 7, the microstructure of the heat-resistant cast steel contains ferrite, austenite, and carbides. In the example of Figure 8, the microstructure of the heat-resistant cast steel contains ferrite and carbides. If the amount of ferrite is low and the amount of austenite, which has excellent ductility, is high, the crack susceptibility during build-up welding may decrease, while the shrinkage rate from the start of casting to the completion of solidification may increase, which may cause casting defects. Furthermore, if the amount of ferrite is low, the thermal expansion when the grate is used at high temperatures will increase, which may cause problems such as deformation. Therefore, it is preferable that the volume fraction of ferrite relative to the total metal structure is 55% or more. More preferably, the volume fraction of ferrite is 60% or more, even more preferably 70% or more, and even more preferably 80% or more. From the viewpoint of crack susceptibility during build-up welding, the volume fraction of ferrite is preferably 99% or less, more preferably 95% or less, and even more preferably 90% or less. The volume fraction of ferrite in this disclosure is measured with a ferrite scope as described later. Therefore, as in the example in Figure 8 (Comparative Example 4), the metal structure may contain carbides even if the volume fraction of ferrite is 100%. In other words, the "volume fraction of ferrite" in this disclosure means the volume fraction of ferrite relative to the metal structure excluding carbides from the total metal structure.

[0033] 3. Characteristics Next, the characteristics of the heat-resistant cast steel of this disclosure will be described. By satisfying the above-mentioned chemical composition, the heat-resistant cast steel of this disclosure can improve both castability and build-up properties. Furthermore, the heat-resistant cast steel of this disclosure can also reduce thermal expansion and improve corrosion resistance and wear resistance. These characteristics will be described in detail below.

[0034] <Castability: Melting point of 1450°C or lower, volume shrinkage rate from the start of casting to completion of solidification of 4.00% or lower> The melting point of the heat-resistant cast steel of this disclosure is 1450°C or lower. This increases fluidity during casting, thereby improving castability. The melting point is preferably 1430°C or lower, more preferably 1410°C or lower, and even more preferably 1390°C or lower. The volume shrinkage rate from the start of casting at 1500°C to completion of solidification is 4.00% or lower. The shrinkage rate is preferably 3.90% or lower, more preferably 3.80% or lower, and even more preferably 3.70% or lower.

[0035] <Crack-resistant properties: No cracking during laser cladding welding> The heat-resistant cast steel of this disclosure does not crack during laser cladding welding. Specifically, the heat-resistant cast steel of this disclosure does not crack even without preheating during laser cladding welding tests.

[0036] <Thermal expansion: The coefficient of linear expansion is 14.0 × 10 －６ Below / ℃ > The coefficient of linear expansion is 14.0 × 10 －６ It is preferable that the temperature is below / °C. The coefficient of linear expansion is 14.0 × 10⁻⁶. －６ By keeping the temperature below / °C, deformation caused by thermal expansion during high-temperature use of the grate can be suppressed. The coefficient of linear expansion is more preferably 13.0 × 10⁻⁶. －６ The temperature is below / ℃, and more preferably 12.0 × 10 －６ The temperature is below / ℃, and more preferably 11.5 × 10 －６ It is below / ℃.

[0037] <Wear resistance: Vickers hardness of 250 HV or higher> The Vickers hardness is preferably 250 HV or higher. More preferably, the Vickers hardness is 270 HV or higher, even more preferably 300 HV or higher, and even more preferably 320 HV or higher.

[0038] <Corrosion resistance: Thinning amount in high-temperature corrosion test is 0.5500 mm or less> Preferably, the thinning amount in the high-temperature corrosion test is 0.5500 mm or less. More preferably, the thinning amount is 0.5000 mm or less, even more preferably 0.4500 mm or less, and even more preferably 0.4000 mm or less.

[0039] Next, an example of the method for manufacturing the grate of the present disclosure will be described. The method for manufacturing the grate preferably includes forming the laser cladding portion 13 by laser cladding welding. The method for manufacturing the grate may further include forming the grate body 12 by casting before forming it by laser cladding welding. Specifically, first, the grate body 12 is cast by pouring a steel material into the mold of the grate body 12. This pouring is adjusted so that the component composition of the cast grate body 12 satisfies the above-described conditions. The pouring temperature is preferably 1500°C or higher and 1650°C or lower. Cooling after pouring is, for example, air cooling. Since welding cracks are unlikely to occur in the grate body 12 even in laser cladding welding, there is a high possibility that preheating for preventing welding cracks can be omitted. The grate 11 is manufactured by performing laser cladding welding on at least the front end surface 32 of the grate body 12. The grate body 12 is protected from corrosion by being covered with the laser cladding portion 13.

[0040] 1. Sample Preparation Using a small melting furnace, Y-shaped test specimens having the chemical component compositions shown in Table 1 were cast. The Y-shaped test specimen means a test specimen having a shape corresponding to Shape a of JIS G 0307:2014. Comparative Example 9 and Comparative Example 10 are JIS standard materials. In Table 1 and Table 2 described later, the underlined numerical values indicate that they are outside the scope of the present disclosure.

[0041]

[0042] 2. Evaluation of Metallographic Structure For each sample, the ferrite amount (i.e., the volume fraction of ferrite) was measured. The ferrite amount was measured using a ferrite scope (FMP30) manufactured by Fisher. Samples with a ferrite amount of 55% or more were evaluated as samples with an appropriate ferrite amount. The measurement results are shown in Table 2.

[0043] 3. Evaluation of Properties (1) Evaluation of Castability To evaluate castability, the melting point of each sample was calculated using JMatPro, a material simulation software developed by Sente Software. In addition, assuming a casting temperature of 1500°C, the volume shrinkage rate from the start of casting to the completion of solidification was calculated. Samples with a melting point of 1450°C or lower and a shrinkage rate of 4.00% or lower were evaluated as having excellent castability. The calculation results are shown in Table 2.

[0044] (2) Evaluation of Cladding Properties To evaluate the cladding properties, plate-shaped test pieces were taken from the cast specimens, and laser cladding was performed on their surfaces. The cladding test pieces were 70 mm long, 60 mm wide, and 30 mm thick. The laser cladding conditions were as follows: ・Beam pre-treatment (advance angle 5°) ・Powder used: Inconel 625 (Ni-based corrosion-resistant alloy) Particle size 53-150 μm ・Laser power [kW]: 7 ・Powder supply amount [g / min]: 120 ・Welding speed [cm / min]: 100 ・No preheating

[0045] Penetrant testing (PT) was performed on the test specimens after laser cladding to evaluate whether or not cracks had occurred in the specimens. Figures 9 and 10 are photographs showing the appearance of the test specimens after penetrant testing. More specifically, Figures 9 and 10 are photographs of the test specimen T viewed from above. A laser-clad area B is formed on a part of the upper surface of the test specimen T. Figure 9 is a photograph of the test specimen T of Example 1, and Figure 10 is a photograph of the test specimen T of Comparative Example 1. No cracks occurred in the test specimen T of Example 1, but an indication pattern P was observed in the test specimen T of Comparative Example 1, confirming that cracks had occurred in the base material near the laser-clad area B. The test results for each sample are shown in Table 2.

[0046] (3) Evaluation of thermal expansion To evaluate thermal expansion, the push rod type linear expansion measurement method (load: 9.8 × 10) －２ The coefficient of linear expansion was measured in the temperature range of 30°C to 600°C at N). The coefficient of linear expansion was 14.0 × 10⁻⁶. －６ Samples with a temperature of 0 / °C or lower were evaluated as having low thermal expansion. The measurement results are shown in Table 2. In Table 2, the values ​​marked with an asterisk (*) were estimated from the regression equation shown in Figure 13, which will be described later.

[0047] (4) Evaluation of wear resistance To evaluate wear resistance, Vickers hardness was measured using a Vickers hardness tester. Samples with a Vickers hardness of 250 HV or higher were evaluated as having excellent wear resistance. The measurement results for each sample are shown in Table 2.

[0048] (5) Evaluation of Corrosion Resistance To evaluate corrosion resistance, plate-shaped test specimens taken from the cast specimens were subjected to a high-temperature corrosion test in accordance with JIS Z 2293:2004. The plate-shaped test specimens were 15 mm in length, 10 mm in width, and 2 mm in thickness. In the high-temperature corrosion test, the plate-shaped test specimens taken from each cast specimen were embedded in incinerated ash collected from a waste incinerator and held at 610°C for 100 hours. After this, the amount of wall thinning (mm) of each test specimen was measured. Samples with a wall thinning of 0.5500 mm or less were evaluated as having excellent corrosion resistance. The measurement results are shown in Table 2.

[0049]

[0050] Let's discuss the results in Table 2.

[0051] Examples 1 to 6 of the invention were excellent in both castability and build-up properties because they satisfied the chemical composition specified in the embodiments of the present disclosure. Examples 1 to 5 were also excellent in thermal expansion, corrosion resistance, and Vickers hardness. Example 6 was also excellent in corrosion resistance and Vickers hardness, but had high thermal expansion due to its low ferrite content.

[0052] In Comparative Example 1, cracking occurred in the base material near the build-up area due to the high Si content. It is believed that the cracking occurred because the base material's poor ductility could not withstand the deformation caused by the build-up. In particular, in Comparative Example 1, the large Cr equivalent / Ni equivalent ratio resulted in a 100% ferrite content, which is thought to have made cracking more likely.

[0053] Comparative Examples 2 and 3 had low Cr content, resulting in high shrinkage and poor corrosion resistance. In particular, Comparative Examples 2 and 3 had a small Cr equivalent / Ni equivalent ratio, with a ferrite content of less than 55%, which likely contributed to their high shrinkage. Furthermore, Comparative Examples 2 and 3 also experienced high thermal expansion due to their ferrite content of less than 55%.

[0054] In Comparative Example 4, cracks occurred and the wear resistance was poor because the Ni content was low.

[0055] In Comparative Example 5, cracks occurred because the C and Si contents were high. In Comparative Example 5, the amount of weight loss in the high-temperature corrosion test was also large because the C content was high.

[0056] In Comparative Example 6, the shrinkage rate was large and the corrosion resistance was poor because the Cr content was low. In Comparative Example 6, the thermal expansion was large because the Cr equivalent / Ni equivalent was small and the ferrite amount was less than 55%.

[0057] In Comparative Example 7, cracks occurred because the C content was high and the Ni content was low. In Comparative Example 7, the amount of weight loss in the high-temperature corrosion test was also large because the C content was high.

[0058] In Comparative Examples 8 and 9, the melting point was high because the C content was low, cracks occurred because the Ni content was low, and the wear resistance was poor.

[0059] In Comparative Example 10, the melting point was high and the wear resistance was poor because the C content was low, and the thermal expansion was large because the ferrite amount was less than 55%.

[0060] To confirm the validity of the shrinkage rate calculation results, the cross-sectional macrostructure of the casting test piece was observed. FIGS. 11 and 12 are appearance photos showing the cross-sectional macrostructure of the casting test piece. FIG. 11 is the cross-sectional macrostructure of Example 1, and FIG. 12 is the cross-sectional macrostructure of Comparative Example 2. In Comparative Example 2 with a large shrinkage rate, a casting defect D (specifically, blowhole) occurred, but no casting defect was observed in Example 1.

[0061] FIG. 13 is a graph showing the relationship between the ferrite amount and the linear expansion coefficient (measured value). It can be seen that the linear expansion coefficient can be reduced as the ferrite amount increases. By setting the ferrite amount to 55% or more, the linear expansion coefficient can be made 14.0×10 －６ / °C or less. The reduction of the linear expansion coefficient is also effective in reducing the deformation during build-up welding on the fire grate.

[0062] [Aspect] The above embodiments are specific examples of the following aspects.

[0063] (Aspect 1) The heat-resistant cast steel contains, by mass%, C: 0.60% or more, 0.80% or less, Si: 1.00% or more, 1.70% or less, Mn: 0.50% or more, 1.00% or less, P: 0.040% or less, S: 0.040% or less, Cr: 24.30% or more, 27.00% or less, and Ni: 1.60% or more, 2.50% or less, with the remainder being Fe and unavoidable impurities.

[0064] This configuration allows for improved castability and build-up properties due to the appropriate adjustment of the chemical composition.

[0065] (Aspect 2) In the heat-resistant cast steel described in Aspect 1, the volume fraction of ferrite is 55% or more.

[0066] This configuration reduces the shrinkage rate from the start of casting to the completion of solidification, and also reduces thermal expansion when the grate is used at high temperatures.

[0067] (Aspect 3) In the heat-resistant cast steel described in Aspect 1 or Aspect 2, the relationship between the Cr equivalent and the Ni equivalent, as defined by the following formulas (1) and (2), satisfies the following formula (3): Cr equivalent = [Cr] + 1.5 × [Si] ... (1) Ni equivalent = [Ni] + 0.5 × [Mn] + 17 ... (2) 1.350 ≤ Cr equivalent / Ni equivalent ≤ 1.500 ... (3) where [X]: mass % of element X

[0068] This configuration ensures an appropriate balance between ferrite and austenitic structures in the metal structure, further suppressing cracking in the build-up and reducing thermal expansion during high-temperature use of the grate.

[0069] (Aspect 4) In the heat-resistant cast steel described in any one of aspects 1 to 3, the coefficient of linear expansion is 14.0 × 10 －６ It is below / ℃.

[0070] This configuration reduces thermal expansion during high-temperature use.

[0071] (Aspect 5) The heat-resistant cast steel described in any one of aspects 1 to 4, wherein the Vickers hardness is 250 HV or higher.

[0072] This configuration allows for improved wear resistance.

[0073] (Aspect 6) The grate 11 is a grate 11 used in a waste incinerator 1, and is a grate made of heat-resistant cast steel as described in any one of aspects 1 to 5.

[0074] With this configuration, by using heat-resistant cast steel with improved castability and build-up properties, even complex-shaped grates can be manufactured without generating casting defects, and a crack-free grate 11 can be achieved even when welding build-up is performed.

[0075] (Aspect 7) The grate 11 described in aspect 6 has a grate body 12 and a laser-coated portion 13 formed on at least a part of the surface of the grate body 12.

[0076] With this configuration, by using heat-resistant cast steel with improved build-up properties, a crack-free grate 11 can be achieved even when laser build-up welding is performed.

[0077] W Waste 1 Waste Incinerator 2 Hopper 3 Hearth 4 Drying Stage 5 Combustion Stage 6 Post-Combustion Stage 7 Discharge Port 11 Grate 12 Grate Body 13 Laser-Covered Section 21 Rear 22 Middle 23 Front 31 Inclined Surface 32 Front Surface

Claims

1. Heat-resistant cast steel containing, by mass%, C: 0.60% or more, 0.80% or less, Si: 1.00% or more, 1.70% or less, Mn: 0.50% or more, 1.00% or less, P: 0.040% or less, S: 0.040% or less, Cr: 24.30% or more, 27.00% or less, and Ni: 1.60% or more, 2.50% or less, with the remainder being Fe and unavoidable impurities.

2. The heat-resistant cast steel according to claim 1, wherein the volume fraction of ferrite is 55% or more.

3. The heat-resistant cast steel according to claim 1, wherein the relationship between the Cr equivalent and the Ni equivalent, as defined by the following formulas (1) and (2), satisfies the following formula (3): Cr equivalent = [Cr] + 1.5 × [Si] ... (1) Ni equivalent = [Ni] + 0.5 × [Mn] + 17 ... (2) 1.350 ≤ Cr equivalent / Ni equivalent ≤ 1.500 ... (3) where [X]: mass % of element X 4. In the heat-resistant cast steel according to claim 1, the coefficient of linear expansion is 14.0 × 10 －６ Heat-resistant cast steel with a temperature of 1°C or lower.

5. The heat-resistant cast steel according to claim 1, wherein the Vickers hardness is 250 HV or more.

6. A grate for use in a waste incinerator, comprising heat-resistant cast steel as described in any one of claims 1 to 5.

7. A grate according to claim 6, comprising a grate body and a laser-coated portion formed on at least a part of the surface of the grate body.

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