Heat-resistant cast steel and grates

Abstract

To improve both castability and build-up properties. [Solution] 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.

Description

[Technical Field]

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

[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. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 4742314 [Overview of the project] [Problems that the invention aims to solve]

[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. [Means for solving the problem]

[0008] The heat-resistant cast steel disclosed herein is In 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, Ni: 1.60% or more, 2.50% or less It contains [a certain substance], with the remainder consisting of 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. [Effects of the Invention]

[0010] According to this disclosure, both castability and build-up properties can be improved. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a schematic configuration diagram of a garbage incinerator in which a fire grate according to an embodiment of the present disclosure is used. [Figure 2] Figure 2 is a perspective view of a fire grate according to an embodiment of the present disclosure. [Figure 3] Figure 3 is a longitudinal sectional view of a fire grate according to an embodiment of the present disclosure. [Figure 4] Figure 4 is a graph showing the relationship between the Ni content and the Vickers hardness. [Figure 5] Figure 5 is a graph showing the relationship between the Cr equivalent / Ni equivalent and the ferrite content. [Figure 6] Figure 6 is an optical micrograph showing an example of the metal structure of heat-resistant cast steel. [Figure 7] Figure 7 is an optical micrograph showing an example of the metal structure of heat-resistant cast steel. [Figure 8] Figure 8 is an optical micrograph showing an example of the metal structure of heat-resistant cast steel. [Figure 9] Figure 9 is a photograph showing the appearance of a test piece after a penetration flaw detection test. [Figure 10] Figure 10 is a photograph showing the appearance of a test piece after a penetration flaw detection test. [Figure 11] Figure 11 is an appearance photograph showing the cross-sectional macrostructure of a casting test piece. [Figure 12] Figure 12 is an appearance photograph showing the cross-sectional macrostructure of a casting test piece. [Figure 13] Figure 13 is a graph showing the relationship between the ferrite content and the linear expansion coefficient (measured value).

Embodiments for Carrying Out the Invention

[0012] [Fire Grate] First, the fire grate 11 using the heat-resistant cast steel of the present disclosure will be described. Figure 1 is a schematic configuration diagram of the garbage incinerator 1. Figure 2 is a perspective view of the fire grate 11. Figure 3 is a longitudinal sectional view of the fire grate 11. In this example, the fire grate 11 is used in the garbage incinerator 1. The garbage incinerator 1 is a stoker-type garbage 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 most of the 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 is in contact with 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 build-up laser portion 13 is a build-up layer formed by laser build-up welding. In this example, the build-up laser portion 13 is formed so as 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 location where incineration ash easily adheres and corrodes. The build-up laser portion 13 has the role of protecting this surface from corrosion by covering the tip surface 32. Incidentally, on the surface of the grate body 12, the covering position of the build-up laser portion 13 is not limited. For example, the build-up laser portion 13 may be covered on a surface other than the tip surface 32 of the surface of the grate body 12. Examples of surfaces other than the tip surface 32 include the inclined surface 31.

[0017] [Heat-resistant cast steel] Next, the heat-resistant cast steel of the present disclosure will be described. The inventor earnestly studied from various angles in order to realize a heat-resistant cast steel capable of improving both the casting property and the build-up property. Regarding the build-up property, the inventor earnestly studied a heat-resistant cast steel in which cracks do not occur even in laser build-up without preheating. As a result, the inventor found that by controlling the component composition of the heat-resistant cast steel, both the casting property and the build-up property can be improved. Specifically, the inventor found that the casting property can be improved by controlling particularly the amount of C and the amount of Si, and the build-up property can be improved by controlling particularly the amount of Si, the amount of Cr, and the amount of Ni.

[0018] Furthermore, the inventor found that by controlling preferably the ratio of the Ni equivalent to the Cr equivalent described later, the balance between the ferrite structure and the austenite structure becomes appropriate, and the casting property and the build-up property can be effectively improved.

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

[0020] <C: 0.60 mass% or more and 0.80 mass% or less> C (carbon) is an element that improves castability and wear resistance. If the C content is less than 0.60% by mass, the castability and wear resistance deteriorate, so it is necessary to add C at 0.60% by mass or more. The C content 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 C content exceeds 0.80% by mass, it causes a decrease in ductility, resulting in cracks during build-up welding or welding, and furthermore, the corrosion resistance deteriorates. Therefore, the C content is capped at 0.80% by mass. The C content 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% by mass or more, 1.70% by mass or less> Si (silicon) is an element that improves castability and corrosion resistance. If the Si content is less than 1.00% by mass, the castability and corrosion resistance deteriorate, so it is necessary to add Si at 1.00% by mass or more. The Si content is preferably 1.10% by mass or more, more preferably 1.20% by mass or more, and even more preferably 1.30% by mass or more. If the Si content exceeds 1.70% by mass, it causes a decrease in ductility, resulting in cracks during build-up welding or welding. Therefore, the Si content is capped at 1.70% by mass. The Si content is preferably 1.60% by mass or less, more preferably 1.50% by mass or less, and even more preferably 1.40% by mass or less.

[0022] <Mn: 0.50% by mass or more, 1.00% by mass or less> Mn (manganese) is an austenite-forming element in addition to having a desulfurization effect on the molten metal. If the Mn content is less than 0.50% by mass, the desulfurization effect on the molten metal etc. is reduced, so it is necessary to add Mn at 0.50% by mass or more. The Mn content 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 Mn content exceeds 1.00% by mass, the corrosion resistance deteriorates. Therefore, the Mn content is capped at 1.00% by mass. The Mn content 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> P (phosphorus) is an inevitable impurity, and the amount of P incorporated within the range normally incorporated to ensure castability is acceptable. The amount of P is capped at 0.040 mass%. The amount of P is preferably 0.035 mass% or less, more preferably 0.030 mass% or less, and even more preferably 0.025 mass% or less. P is normally contained at 0.001 mass% or more.

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

[0025] <Cr: 24.30 mass% or more, 27.00 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 mass%, the corrosion resistance and oxidation resistance will deteriorate, so it is necessary to add Cr at 24.30 mass% or more. The amount of Cr is preferably 24.70 mass% or more, more preferably 25.00 mass% or more, and even more preferably 25.20 mass% or more. If the amount of Cr exceeds 27.00 mass%, it will increase the ferrite structure, which is a cause of cracking during build-up welding or welding, so the amount of Cr is capped at 27.00 mass%. The amount of Cr is preferably 26.75 mass% or less, more preferably 26.50 mass% or less, and even more preferably 26.00 mass% or less.

[0026] <Ni: 1.60 mass% or more, 2.50 mass% or less> Nickel (Ni) 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 above 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 mass or less, more preferably 2.15 mass or less, and even more preferably 2.00 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, and manufacturing equipment is permissible. However, 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 Ni equivalent, as defined by the following formulas (1) and (2), satisfies the following formula (3). The ratio of "Cr equivalent / Ni equivalent" is preferably 1.370 or higher, more preferably 1.400 or higher, and even more preferably 1.420 or higher. The ratio of "Cr equivalent / Ni equivalent" is preferably 1.490 or lower, more preferably 1.470 or lower, and even more preferably 1.450 or lower. Cr equivalent=[Cr]+1.5×[Si] (1) Ni equivalent = [Ni] + 0.5 × [Mn] + 17 ... (2) 1.350≦Cr equivalent / Ni equivalent≦1.500 (3) However, [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 based on the amounts of Cr and Si, which are elements that promote the ferrite structure. The Ni equivalent is calculated based on 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 with 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, cracking is more likely to occur during build-up welding. Furthermore, the inventors have found that when the ferrite content decreases, cracking during build-up welding is 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.Metal structure Next, the microstructure 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 represent the microstructures of Invention Example 6, Invention Example 1, and Comparative Example 4, respectively, in the examples described later. 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 small and the amount of austenite, which has excellent ductility, is large, 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, potentially causing casting defects. Furthermore, if the amount of ferrite is small, the thermal expansion when the grate is used at high temperatures may increase, potentially causing problems such as deformation. For this reason, it is preferable that the volume fraction of ferrite to the total microstructure is 55% or more. The volume fraction of ferrite is more preferably 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. For this reason, 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 properties 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 properties 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 the 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] <Build-up properties: No cracking during laser build-up welding> The heat-resistant cast steel of this disclosure does not crack during laser overlay welding. More specifically, the heat-resistant cast steel of this disclosure does not crack during laser overlay welding tests even without preheating.

[0036] Thermal expansion: The coefficient of linear expansion is 14.0 × 10⁻⁶. -6 Below / ℃> The coefficient of linear expansion is 14.0 × 10⁻⁶. -6 It is preferable that the temperature is below / ℃. 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 It is below / ℃, and more preferably 11.5 × 10 -6 It is below / ℃.

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

[0038] <Corrosion resistance: Wall thickness reduction in high-temperature corrosion tests is 0.5500 mm or less> In the high-temperature corrosion test, the amount of wall thinning is preferably 0.5500 mm or less. More preferably, the amount of wall thinning 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 a method for manufacturing a grate according to this disclosure will be described. The method for manufacturing a grate preferably includes forming the laser-clad portion 13 by laser cladding welding. The method for manufacturing a 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 molten steel material into a mold for the grate body 12. This pouring is adjusted so that the component composition of the cast grate body 12 satisfies the above-mentioned conditions. The pouring temperature is preferably 1500°C or higher and 1650°C or lower. Cooling after pouring is, for example, by air cooling. Since the grate body 12 is less prone to welding cracks even with laser cladding welding, there is a high possibility that preheating to prevent welding cracks can be omitted. The grate 11 is manufactured by laser cladding welding at least to the tip surface 32 of the grate body 12. The grate body 12 is protected from corrosion by being covered with the laser-clad portion 13. [Examples]

[0040] 1. Sample preparation Using a small melting furnace, Y-shaped test specimens with the chemical composition shown in Table 1 were cast. A Y-shaped test specimen refers to a test specimen with a shape corresponding to shape a of JIS G 0307:2014. Comparative Examples 9 and 10 are JIS standard materials. In Table 1 and Table 2 described later, underlined values ​​indicate that they are outside the scope of this disclosure.

[0041] [Table 1]

[0042] 2. Evaluation of the metal structure The ferrite content (i.e., the volume fraction of ferrite) was measured for each sample. The ferrite content was measured using a Fisher ferrite scope (FMP30). Samples with a ferrite content of 55% or more were evaluated as having an appropriate ferrite content. The measurement results are shown in Table 2.

[0043] 3. Evaluation of characteristics (1) Evaluation of castability To evaluate the castability, the melting point of each sample was calculated using JMatPro, a material simulation software developed by Sente Software. Furthermore, 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 the thickness To evaluate the build-up properties, plate-shaped test specimens were taken from the cast specimens, and laser build-up was performed on their surfaces. The shape of the build-up test specimens was 70 mm in length, 60 mm in width, and 30 mm in thickness. The laser build-up conditions were as follows: • Beam pre-installation (advancing 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 required

[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) is used. -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 values ​​below / °C 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 discussed later.

[0047] (4) Evaluation of abrasion 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 superior wear resistance. The measurement results for each sample are shown in Table 2.

[0048] (5) Evaluation of corrosion resistance To evaluate corrosion resistance, high-temperature corrosion tests were conducted on plate-shaped test specimens taken from cast specimens, 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] [Table 2]

[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 and 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] Comparative Example 4 exhibited cracking and poor wear resistance due to its low Ni content.

[0055] Comparative Example 5 experienced cracking due to its high carbon and silicon content. Comparative Example 5 also exhibited significant wall thinning in the high-temperature corrosion test due to its high carbon content.

[0056] Comparative Example 6 had a low Cr content, resulting in a high shrinkage rate and poor corrosion resistance. Comparative Example 6 also had a small Cr equivalent / Ni equivalent ratio and a ferrite content of less than 55%, which led to high thermal expansion.

[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, and cracks occurred and the wear resistance was poor because the Ni content was low.

[0059] In Comparative Example 10, the melting point was high and the wear resistance was poor because the C content was low. In Comparative Example 10, 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-described embodiments are specific examples of the following aspects.

[0063] (Aspect 1) The heat-resistant cast steel, in 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, 27.00% or less, Ni: 1.60% or more, 2.50% or less It contains [a certain substance], with the remainder consisting of 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 Embodiment 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 Embodiment 1 or Embodiment 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) However, [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 embodiments 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] (Appendix 5) In the heat-resistant cast steel described in any one of embodiments 1 to 4, 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 used in the waste incinerator 1, and is a grate made of heat-resistant cast steel as described in any one of embodiments 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 according to embodiment 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. [Explanation of Symbols]

[0077] W Garbage 1. Waste Incinerator 2 Hopper 3 Hearth part 4 Drying stage 5 Combustion Stages 6 Post-combustion stage 7 Outlet 11 Fire grates 12. Grill body 13 Laser build-up area 21 Rear 22 Middle section 23 Front 31 Slope 32 Tip surface

Claims

1. In mass percent, 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, Ni: 1.60% or more, 2.50% or less A heat-resistant cast steel containing [a certain substance], with the remainder being Fe and unavoidable impurities.

2. In the heat-resistant cast steel according to claim 1, Heat-resistant cast steel with a ferrite volume fraction of 55% or more.

3. In the heat-resistant cast steel according to claim 1, A heat-resistant cast steel in which the relationship between the Cr equivalent and Ni equivalent, as defined by the following equations (1) and (2), satisfies the following equation (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) However, [X]: Mass percentage of element X

4. In the heat-resistant cast steel according to claim 1, The coefficient of linear expansion is 14.0 × 10⁻⁶. －６ Heat-resistant cast steel with a temperature of 1°C or lower.

5. In the heat-resistant cast steel according to claim 1, Heat-resistant cast steel with a Vickers hardness of 250 HV or higher.

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. In the grate according to claim 6, A grate 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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