JPH0548290B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to heat-resistant cast steel, and more specifically, it has performance and durability characteristics such as excellent heat resistance (high temperature strength), heat cracking resistance, and oxidation resistance, as well as excellent castability. This heat-resistant cast steel has good machinability, has good productivity, and can be manufactured at low cost, so it can be suitably applied to exhaust system parts of vehicle engines. [Prior Art] In recent years, research and development efforts have been made to improve combustion efficiency in vehicle engines such as gasoline engines and diesel engines, especially in automobile engines, as demands for higher output and lower fuel consumption have increased. It is being actively implemented. As a result, in automobile engines that meet such demands, the exhaust gas temperature tends to be significantly higher than that of conventional automobile engines. In particular, exhaust system parts such as exhaust manifolds for automobile engines, turbine housings for turbochargers, and pre-combustion chambers for diesel engines are used under particularly high temperature and harsh conditions. Heat-resistant cast iron such as cast iron, and in special cases, expensive high-alloy heat-resistant (stainless) cast steel such as ferritic or austenitic stainless cast steel were used. [Problems to be Solved by the Invention] In view of the current state of the conventional technology as described above, the problems to be solved by the present invention are as follows: Heat-resistant cast irons such as high-Si cast iron, Ni-resist cast iron, and Al cast iron have good productivity characteristics such as excellent castability and machinability, but they also have good productivity characteristics such as high-temperature strength, heat cracking resistance, and acid resistance. Due to its inferior performance and durability characteristics such as oxidation resistance, it cannot be applied to parts with strict requirements for heat resistance at high temperatures of 800℃ or higher, and it cannot be used in high-alloy heat-resistant cast steels such as stainless steel cast steel. Although it has excellent performance and durability characteristics such as heat resistance (high-temperature strength), heat cracking resistance, and oxidation resistance at temperatures above 800℃, it has poor castability and may cause "shrinkage cavities" and "oil" during casting. Productivity is poor due to easy occurrence of casting defects such as "defects in rotation" and poor machinability, but productivity characteristics such as excellent castability, machinability, and low cost as a heat-resistant component There was a strong desire to develop a heat-resistant casting material that has a well-balanced combination of performance and durability characteristics such as excellent heat resistance (high-temperature strength), heat cracking resistance, and oxidation resistance. . Therefore, the technical problem of the present invention is to
By adjusting the composition of heat-resistant cast steel and performing an annealing treatment after casting, it has achieved productivity characteristics such as castability, machinability, and low cost comparable to conventional heat-resistant cast iron, as well as productivity characteristics comparable to conventional heat-resistant cast iron. The objective is to create a heat-resistant cast steel that has a good balance of performance and durability characteristics such as heat resistance (high-temperature strength), heat cracking resistance, and oxidation resistance comparable to alloy heat-resistant cast steel. [Means for solving the problems] In view of the problems in the conventional technology,
The means for solving the problems of the conventional technology in the present invention is as follows: C; 0.5 to 2.5%; Si;
1.5% to 4.5%, Mn; 0.7% or less, P; 0.05% or less,
S: 0.1% or less, Cr: 5.0-14.0%, Ti: 0.15-4.6
%, the balance essentially consisting of Fe, and is characterized by being annealed after casting. In addition, the heat treatment after casting of the heat-resistant cast steel of the present invention includes austenitizing treatment at 900 to 950°C for 0.5 hours or more, followed by furnace cooling to 680 to 750°C, and then heating to 680 to 750°C.
A normal annealing treatment of holding for 0.5 hours or more and cooling is sufficient. [Operation] The operation of the present invention will be explained below. In the present invention, the composition of the heat-resistant cast steel of the present invention, especially Ti
By improving the heat resistance properties through the balance of additives and other alloying elements, it has productivity characteristics such as castability, machinability, and low cost that are comparable to conventional heat-resistant cast iron. Heat resistance (high temperature strength) similar to high alloy heat resistant cast steel such as cast steel, heat crack resistance,
This range provides performance and durability characteristics such as oxidation resistance, and because the base structure is made into a ferrite structure through annealing treatment after casting, it has castability and machinability comparable to conventional heat-resistant cast iron. It has productivity characteristics such as flexibility and low cost, as well as heat resistance (high temperature strength), heat cracking resistance, and heat resistance comparable to conventional high-alloy heat-resistant cast steel.
This makes it possible to create heat-resistant cast steel that has a well-balanced combination of performance and durability characteristics such as oxidation resistance. The reason for limiting the range of the amount of each alloying element added to the heat-resistant cast steel of the present invention will be explained below. In the following description, the amount of each alloying element added is expressed in percent by weight. First, C is effective in improving the strength characteristics and castability of the heat-resistant cast steel of the present invention, but
If it is less than 0.5%, the effect of improving these properties will not be sufficient, while if it is added in excess of 2.5%, it will promote graphitization of carbon and reduce the strength properties of heat-resistant cast steel, so it was set at 0.5 to 2.5%. In addition, Si is effective not only as a deoxidizing agent in the heat-resistant cast steel of the present invention, but also because it improves castability and oxidation resistance, but if it is less than 1.5%, the effect of improving these properties is insufficient. Without,
If it is added in excess of 4.5%, the balance with C (carbon equivalent) causes primary carbides to become coarser and deteriorates the machinability of heat-resistant cast steel. Excessive Si content in the ferrite base structure lowers the toughness of heat-resistant cast steel and deteriorates productivity. For these reasons, it was set at 1.5% to 4.5%. In addition, since Mn is a forming element of pearlite structure, it is an alloying element that is not preferred for heat-resistant cast steels with ferrite matrix structure as in the present invention, but it is effective as a deoxidizing agent like Si. In addition, since it is effective as an alloying element that improves "flowability" during casting and improves productivity, it is preferable to contain it in a range of 0.7% or less. Further, P was set at 0.05% or less since adding more than 0.05% would promote pearlite organization of the base structure and promote crystallization of steadite. In addition, S is usually not a particularly essential alloying element, but when manufacturing parts with strict requirements for machinability, increasing the amount of S and Mn added to crystallize MnS is necessary for machining. It was set at 0.1% or less because it can improve workability. In addition, like Si, Cr is effective because it improves oxidation resistance, but if it is less than 5.0%, the effect of improving oxidation resistance is not sufficient, and if it is added in excess of 14.0%, it will cause high hardness Cr carbide. 5.0 because the amount of precipitation increases and machinability deteriorates significantly.
~14.0%. Furthermore, Ti is a particularly important alloying element in the material of the present invention, and is effective for improving oxidation resistance and heat resistance (high temperature strength), but if it is less than 0.15%, it will not be effective in improving the heat resistance (high temperature strength). was not sufficient, and if added in excess of 4.6%, the machinability of heat-resistant cast steel would be significantly deteriorated due to crystallization of TiC, so it was set at 0.15 to 4.6%. [Embodiment] Hereinafter, one embodiment of the present invention will be described based on the accompanying drawings. In order to evaluate the oxidation resistance and heat cracking resistance of the present invention materials, three types of the present invention materials as shown in Table 1 ~
and four types of comparative materials were manufactured by casting. For casting, melting is performed in the atmosphere using a 20Kg high-frequency melting furnace, deoxidized by adding 0.3% Fe-Si (75% by weight), and then tapped at 1650℃ or higher and heated to 1550℃. The hot water was poured as described above. As the mold for casting, a JIS standard No. A Y block casting mold was used. Then, a normal annealing treatment was performed on each test material in the state of a cast rough shape that was cast as described above. FIG. 3 shows a micrograph of the metal structure of the heat-resistant cast steel according to the present invention manufactured in this manner. In addition, Fig. 3a shows the metal structure of the material of the present invention at a magnification.
This was observed at a low magnification of 100 times, and Fig. 3b shows the same metal structure observed at a high magnification of 400 times. As is clear from Figure 3a, primary carbides are observed to be crystallized in the white base ferrite structure in the form of dendrites in the material of the present invention, and it is also clear from Figure 3b. Like,
It is observed that granular secondary carbides are finely dispersed and precipitated in the white base ferrite structure. Here, the formation of dendrite-like primary carbides is presumed to ensure high-temperature strength, and the dispersion of secondary carbides such as Cr carbide and Ti carbide in fine particles, which improves oxidation resistance, provides uniform acid resistance. It is inferred that the chemical properties are ensured.

〔Effect of the invention〕

As is clear from the above, according to the heat-resistant cast steel of the present invention, the compositional adjustment of the heat-resistant cast steel and the implementation of annealing treatment after casting provide castability and machinability comparable to that of conventional heat-resistant cast iron. It has productivity characteristics such as flexibility and low cost, as well as heat resistance (high temperature strength), heat cracking resistance, and heat resistance comparable to conventional high-alloy heat-resistant cast steel.
It has the advantage of being a heat-resistant cast steel that has a well-balanced combination of performance and durability characteristics such as oxidation resistance.

[Brief explanation of the drawing]

Figure 1 shows the results of comparing the oxidation resistance at 900°C between the inventive material and the comparative material. Figure 2 shows the results of comparing the heat cracking resistance of the inventive material and the comparative material. FIG. 3 is a diagram showing a microscopic photograph of the metal structure of the material of the present invention.

Claims (1)

[Claims] 1 Weight ratio: C: 0.5-2.5%, Si: 1.5%-4.5
%, Mn; 0.7% or less, P; 0.05% or less, S; 0.1%
Hereinafter, a heat-resistant cast steel having a composition consisting of Cr: 5.0 to 14.0%, Ti: 0.15 to 4.6%, and the remainder substantially Fe, and characterized by being annealed after casting.